ECONOMIC GEOLOGY AND BULLETIN VOL. 80 OF THE SOCIETY THE OF ECONOMIC AUGUST, GEOLOGISTS 1985 NO. 5 Analysisof SupergeneOre-FormingProcessesand Ground-Water Solute Transport Using MassBalancePrinciples GEORGEH BRIMHALL, CHARLESN. ALPERS,AND ARIC B. CUNNINGHAM* Departmentof Geologyand Geophysics, Universityof California,Berkeley,California94720 Abstract Analysisof metal distributionin weatheringprofilesdevelopedabove ore deposits providesa simpleanddirectmethodto determinethe geologiccontrolson surficialleaching and enrichmentof metals by supergene(descending)fluids. This approachoffers an understandingof the interrelationshipsof metal concentration,zone thicknesses,and bulkrock densityof the variouszoneswithin a weathered-rockcolumn.Analyticalsolutionsto massbalanceequationsprovide expressions describingthe chemicalevolutionpaths of enrichmentblanketmetal grade,b, in termsof originalprotoremetal grade,p, leachedzone metal grade,1, andthe ratio of total leached-zonecolumnheightto blanketthickness, LPr/B. Neglecting densitytermsfor the sakeof simplicity, thisfunctionis expressed as: b= p+ (p- 1). Linear data arraysin the coordinatesystemb versus(p - 1) for groupsof drill holesin individualgeologicdomains,e.g., wall-rocktypes,stronglysuggest that enrichmentblanket metalgradesevolvefroma protoremetalgrade,p, with a fairlyconstant slopeof I-Pr/Bto a final value,b, determinedby the extentof metalleachingin the zoneof oxidation.The constancy of theratioI-Pr/B withingeologic domains indicates a simultaneous deepening of the zone of oxidationand thickeningof the enrichmentblanket under conditionsof a descendingground-watertable. Analysis of metalmassbalancein drillingprofilesalsoprovidesa meansof reconstructing paleosurficial topographyat the time of activeoxidationeven in highly erodedterrains. Applicationof these methodsusingrock densityand assaydata from the Butte district of Montanashowsthe palco-oxidation surfaceto havebeenerodedby asmuchas450 ft down to the presentunconformity betweenweatheredbedrockand overlyingTertiary gravels. Geomorphic stabilityandslowerosionratesoftenthoughtnecessary for effectivechemical weatheringare thereforequestioned.We proposeinsteada generalbalanceof rates of erosionand descentof the ground-watertable for optimalenrichment.Preservationof the fossilweatheringprofilesoccursby submergence of the zone of oxidationbelow the palcoground-water table resultingin cessation of supergeneoxidationand preservation of the blanket.The massbalancemethodsderivedin this studyalsorelate overallleaching e•ciency to protorecharacteristics, showinga stronghypogenealterationcontrolat Butte asdescribedby McClave(1973). Lateralcoppertransportis evaluatedby identifyinglocal anomalies in the computedoxidationsurface.Zonesof copperintroductionnearfaultsand zonesof highpermeabilityappearasapparenttopographic highswhereaszonesof depletion appear as topographicdepressions. Preliminaryanalysisof this type at the porphyrycopperdepositat La Escondida,Chile, indicatesdeeperoxidationandleachingeffectsthan at Butte.The assumption of vertically homogeneous protorecoppergradesleadsto the conclusionthat erosionof at least 200 m * Presentaddress: SohioAlaskaPetroleum Company, Development Geoscience Division,TudorParkBuilding,Anchorage, Alaska 99502. 0361-0128/85/425/1227-30$ 2..50 122 7 1228 BRIMHALL, ALPERS, AND CUNNINGHAM of leachedcappinghasoccurredthroughoutmuch of the La Escondidadistrict. Geologic domainsdefinedby computedvaluesof LPr/Bcorrelatewell with hypogenealteration zoning. Computationof the magnitudeand direction of overall subsurfacelateral metal fluxesindicatestransportdistancesof at least 1 km in directionsconsistentwith present surfacetopographyand fracturepatterns.There is a strongcorrelationof the thickestand highest grade enrichmentzoneswith inferred positive lateral fluxes,whereasthe most thoroughlyleachedcap rocksmake up zonesof inferred negativelateral flux which are likely sourceregions.Alternativeexplanations to the lateral flux hypothesisare evaluated, includingassessment of variationsin protore metal-gradeprojections,both in vertical and horizontaldirections.Despite the approximations involvedin the presentcalculations,the lateral flux pattern appearsto be the most likely interpretationof the present metal distributionin the supergenesystemat La Escondida. Introduction present understandingof surficial enrichment systemsis ironicallyincomplete. THE knowledgethat supergeneoxidationprocesses A numberof questionsremainwhich are addressed affect a wide variety of metals has long been an in this paper. What are the interrelationshipsbeintegral part of successfulbaseand preciousmetal tween metal concentrationsin the variousweathering exploration.For the mostpart though,concernhas zones?How is the depth of leachingrelated to the beenwith interpretationof gossans, limoniticleached thicknessof an enrichmentblanket?Why do enrichoutcrops,and relict sulfidesto deduce the nature ment blanketsattain a maximummetal grade and and zoningof originalhypogenesulfideassemblageshow is thisgraderelatedto the efficiencyof leaching (Locke,1926; Blanchard,1968; LowellandGuilbert, in various rock types? How can one determine 1970; Loghry, 1972; Gustafsonand Hunt, 1975; rigorouslythe magnitude and direction of lateral Blain and Andrew, 1977; J.P. Hunt and J. A. Bratt, metal transport?How much erosionhasoccurredin unpub. data, 1981; Titley and Beane, 1981; Ander- a given area?This last questionis perhapsthe most son, 1982). Relatively few studieshave addressed fundamentalfor purposesof reconstructingthe surthe broader scientificproblemsof enrichmentpro- face environment. A far-reachingsuggestionwas made by Emmons cessesby treating geological,hydrological,and climatological factors as well as postoxidationand in 1918 (p. 153) on how to estimate the vertical erosionalhistory (Emmons,1918; Lindgren, 1933; extentof the portionof an ore depositthat hasbeen Bateman, 1950; Titley, 1978; Bladh, 1982; Sanga- eroded since oxidation. This technique utilized a meshwarand Barnes,1983). Despitesomeadvances, fundamentalprinciple in geochemistry,i.e., consermanyof the factorscontrollingand limitingoxidation vationof massof chemicalelements.By considering proposedby Emmons(1918) and Bateman(1950) the metal gradesof copper in the leachedzone, the as the result of elegant thought experimentsstill enrichmentblanket, and the protore in conjunction remain.They have gonelargelyuntestedfor lack of with the present thicknessof blanket and leached analyticalmethodswith which to assess metal leach- zones,Emmonspresenteda formula to computethe ingandredeposition in termsof weatheringprocesses amount of erosionin a given vertical profile as a and known patternsand rates of geomorphicevo- "rude approximation,"using the principle of the of mass.In retrospect,however,without lution. Somenotableexceptionsinclude a deduction conservation of rates of erosion in the Silver Bell district of taking into considerationrock density variationsin Arizona assumingan age of enrichment(Graybeal, weatheringprofilesnor primary vertical-zoningef1982) and the qualitativeestimationof lateralcopper fects,the estimatesof erosionandof originalleachedfluxesat the CastleDome district of Arizona (Peter- zone columnheight would necessarilybe crude, but son et al., 1951). Besideserosional effects, other the concept was neverthelessvalid and definitely controllingfactorsaffecting enrichmentas well as worth pursuing. A somewhat analogousapproach profilesof nickel, the stateof preservationof enrichmentblanketsare: usingmassbalanceof geochemical redox effectsat the water table (Sato, 1960; Ander- cobalt, and iron has been usedin a study of nickel son, 1982), climate, time, wall-rock characteristics, laterite deposits(Golightly, 1981). By consideringa structure, burial, uplift, and submergenceof the large group of adjacentprofiles,massbalancecalground-watertable by fluvial sedimentation(Bate- culationsgive an indication of the original amount man, 1950). Assessmentof the dominant controls of rock representedby the in situ weatheredchemon supergenebehavioris thereforelargelydependent ical profiles. In the present study we develop a upon successfulreconstructionof the surficialenvi- generalanalyticalmethodalsoutilizing massbalance ronment as it existed during enrichment at some principles. Sufficient geologic information that actime in the geologicpast. Notwithstandingoppor- countsfor complexities in primarydistrictzoningis tunities to observesurficialprocessesdirectly, the incorporated into the equationsin order to make MASS BALANCE PRINCIPLES APPLIED TO ORE-FORMING PROCESSES the results as accurate as possible.Using drilling data, reconstructionsare made of paleosurficialtopographyat the time of active oxidationand enrichment, thus providing the necessaryframework in which to assessthe questionsposedabove. Once the effects of postoxidationerosion are accountedfor, the most significantquestionsaddressedwith this approachdeal with the nature and level of exposureof the fossilhydrothermalsystem when supergeneoxidation,leaching,and enrichment were initiated. For example,in the caseof porphyry copper deposits,considerthe following questions. Is an altered sulfide-poorvolcanicedifice removed by erosionalprocesses of an essentiallymechanical nature, thus ultimately exposingprotore sulfidesto oxidizing ground water and thereby initiating a transition to composite chemical and mechanical weatheringand subsurfaceredistributionof metals? If so, what initiatessulfideoxidation?Is it simplya function of exposingsulfide-grainsurfacesto sufficientamountsof oxygenatedfluids?Once enrichment begins,is there a necessarybalancebetween rates of surface erosion, surface runoff, ground-water fluxes, and rates of chemical reaction? What roles do the ground-watertable and overlyingcapillary fringe zone play in oxidationand enrichment?What factorsdeterminethe efficiencywith which supergene processestransport metals? To beginto answersuchbroadquestionswe must considerthe presentstateof remobilizationof chemical elementsin weathering profiles in relation to 1229 A great number of elementsmay be treated in this manner,regardlessof whether they occurasprimary sulfidesor simply as constituentsin rock-forming minerals, e.g., silicates.For example, it is known that deep lateritic weathering can concentrateCr, Co, Cu, A1, and Ni regardlessof the presence of primary sulfides (Grubb, 1979; Butt and Nickel, 1981; Elias et al., 1981). Although the approach taken here is general, we concentratein this effort on weatheringof coppersulfide-bearing protores. Analytical Method Metal mass balance argumentsoffer the most direct evidence that the nature of supergeneoreforming processesis a redistributive phenomena. There is no doubt as to the source of water involved nor to the presupergenesourceof metalsthat have undergonetransport and redistribution.The fluids are exclusivelymeteoric in origin. The metals are emplacedin a protore by primary processes which may be igneous,sedimentary,or hydrothermalin nature. In this paper we are concernedonly with the secondaryredistributionof a metal or group of metalsamongthe variousweatheringzonesin an ore deposit.For this purpose,we must develop a terminologyand formalismfor variablesin the mass balanceanalysisbecausenone existsin the current literature. Table 1 is a list of symbols.The massof a given metal in each of the zones is a function of: (1) concentration,e.g., wt percent copper in a leachedcapping,(2) the thickness of eachweathering zone, and (3) the bulk-rock density of each zone, their initial or antecedentstate before supergene modification.In thiscontextwe usethe term protore givenin g per cm3. A verticalcrosssection(Fig. 1) (proto-ore)to describeall the previousmineralization showsa referencecolumnin whichsupergenemetal effectswhichoccurredbeforeweatheringprocesses. redistributionhasoccurred.For purposesof discusTwo field areashave been chosenfor this purpose: sion, its cross section is i cm2. Since we are conButte, Montana, and La Escondida, Chile. These cerned with the principle of conservationof mass, two porphyry copper districts present ideal and we are necessarilyinterested also in density, complementaryopportunitiesto gain insight into defined as massper unit volume. The volume is copper-enrichment processes. Butteis a largedistrict givenby the cross-sectional area (1 cm2) timesthe in which the geologyhas been well described.The heightof a column.Thus the massof rock per cm• geochronology,zoning,protore characteristics, and of column is equal to rock densitytimes column structure are well understood. This effort is facilitated height. To evaluatethe massbalance of a given by the homogeneousnature of the Butte quartz chemical element in a rock we must consider the monzonite wall rock. In contrast, La Escondida, a concentrationof that element in each weathering newly discovereddeposit also of world-classsize, zone of the rock column.As shownin Figure 2, has only been drilled at the explorationand pre- which depicts a leached zone, enrichmentblanket, developmentstage.By applyingour methodsearly and protore, the necessaryvariablesto describe in the history of developmentof this orebody,we metal massare: three metal gradesin wt percent(1, have an ideal opportunity to realize the practical b, and p, for leachedzone, enrichmentblanket,and utility of applyingsuchtechniquesasan exploration protore, respectively),three densityterms (t•, project develops.Our methodsalsoprovide a basis t•p)in g per cm3, andtwo distance parameters (I-,r for focusingfurther researchand explorationin the and B) where I-,r refers to the total leachedzone of La Escondidadistrictand they contributetoward a the columnabovethe top of secondary sulfidesand conceptualexplorationmodelfor supergenedeposits B refers to the enrichment blanket thickness. In this in other settings. simplest exampleit is assumed thatthe metalgrades The approachdevelopedis applicableto a variety 1,b, andp aswellasthedensities t•, t•b,andt•vare of rocksthat haveundergonechemicalweathering. constants in a singlevertical columnbut may vary 1230 BRIMHALL, ALPERS, AND CUNNINGHAM TABLE1. Symbol 1 b P• Pb Po B L L½ L• List of Symbols Description Units Average metal grade within leached capping Average metal grade within enrichment blanket Average metal grade within protore Averagerock densityof leached capping Average rock density of enrichment blanket Averagerock densityof protore Weight percent metal Weight percent metal Weight percent metal Enrichment Distance, e.g., feet or meters blanket thickness Leach cappingzone Current leachedcapping-zonecolumnheight Eroded leached-zonecolumn height Open-systemtotal leached-zonecolumnheight Total leached-zone column height assuminga closed system,i.e., zero lateral flux Gramsof rock per cma Gramsof rock per cma Gramsof rock per cma Distance Distance Distance Distance Topographic surfaceconstructed fromLPrcolumn Distance (elevation) heightsplus elevationof top of blanket Length ratio of total leached-zonecolumn height to Unitless number blanket thickness EGRAV ETB EBB EBH TDS PWT TSO4 EL Bmax Closedor "no-flux" system Open system flux flUX0 Elevation at base of gravels Elevation at top of enrichment blanket Elevation Elevation at base of enrichment blanket at bottom of a drill hole Elevation at top of dominant sulfides (baseof significantoxidation) Elevation of present water table Elevation at top of Ca sulfates(gypsum,anhydrite) Leachingefficiency:percent removal of original protore metal grade Maximum attainable closed-systemblanket grade Chemicalsystemdefined assumingno material exchange with surroundings,i.e., no lateral fluxesor basal discharge Chemicalsystemdefined with provisionfor material exchangewith surroundings Flux of ore metal transportedlaterally Flux of ore metal transportedlaterally, assumingno leachingof rocksother than preservedleached Distance (elevation) Distance (elevation) Distance (elevation) Distance (elevation) Distance (elevation) Distance (elevation) Distance (elevation) Percent Weight percent metal Gramsmetalper cm2 Gramsmetalper cm2 capping flUX3,300 0 Pete Flux of ore metal transportedlaterally, assuming leaching of rocks from 3,300om elevation to ETB Protore metal grade necessaryto explain present metal distributionwith no lateral flux and no leachingof rocksother than preservedleachedcapping;calc Gramsmetalper cm• Weight percent metal -- calculated Protoremetal gradenecessaryto explainpresentmetal distribution with no lateral flux and leaching of rocks Weight percent metal from 3,300-m elevation to ETB bo b• Co Cl d• eo el pø Opø Regressioncoefficientsto drill hole data; subscript0 refers to y-intercept, 1 refers to slope Ore metal grade at sealevel Ore metal rate of changewith elevation Blanket density at sealevel Blanket densityrate of changewith elevation Leached-zonedensity at sea level Leached-zonedensityrate of changewith elevation Protore density at sealevel Protore densityrate of changewith elevation Leached-zoneore metal grade at sea level Leached-zonemetal grade rate of changewith elevation Projected protore metal grade at EBB usingregression Projectedprotore densityat EBB usingregression Projected leached-zonedensityat EGRAV using regression a and • Two-cycle enrichmentprocesses,a being the first,/• the second;they apply to all time-dependentvariablesin massbalance expressions See below Weight percent metal Weight percent metal per distance Gramsper cma Gramsper cma per distance Gramsper cma Gramsper cma per distance Gramsper cma Gramsper cma per distance Weight percent metal Weight percent metal per distance Weight percent metal Gramsper cma Gramsper cma MASS BALANCE PRINCIPLES APPLIED TO ORE-FORMING PROCESSES 12 31 TABLE1--(Continued) Symbol Description hc Units Capillary fringe zone height -dz E, dt Erosion rate Distance perunittime t Time Time U Uplift rate W Ground-water Distance per unit time Wo Initial elevationof the ground-watertable at start of table Distance (elevation) oxidation dW Time rate of changeof the ground-watertable Distanceper unit time Elevation Distance(elevation) dt z or Z (in figures) laterally in adjacent columns.Obviously, geologic reality and zoning patternsmay demand that some of theseparameterswill vary aswell with elevation, but thisinitialsimplifiedmodel1 suffices for purposes of illustration.Additionalgeologiccomplexitywill be incorporatedin other models,discussed below. As depictedin Figures1 and2, substantial erosion mayhaveoccurredbeforemetalsoriginallycontained in the protore are redistributed by percolating groundwater. The extent of suchpreoxidationerosion and mechanicalsurface transport of sulfides cannotbe determineddirectly by the chemicalmass balance methods described herein. However, once chemicalweatheringand downwardchemicaltransport is initiated, metal conservationby subsequent reprecipitationof solublemetal speciesin the subsurface, supposedlyat or near the palcogroundwater table, does provide a direct meansfor computing an estimateof the total leached-zonecolumn WEATHERING : MECHANICAL I CHEMICAL -+ MECHANICAL OXIDATION SURFACE height, LT, for a given metal. This distancemay then be added to the elevation of the top of an enrichment blanket to estimate the approximate elevation of the land surface when soluble metals first began to descendtoward the water table. It is possiblethat differentelementsbegintheir chemical remobilization at different times, so that calculated oxidationsurfacesfor variouselementsmay be at differentelevationscorresponding to differentstages in topographic evolution. This problem will be treated in a subsequentpaper as it relates to the generalproblemof weatheringand soil formation. Oxidation surface In a temporalframework,the surfacereferred to as an oxidationsurface may be depicted (Fig. 1) usingtime sinceinitiation of sulfideoxidationas an independentvariable. The thicknessof the enrichment zone, B, grows with time. The total leachedzone column height, LT, also varies in time, as do the eroded leached-zonecolumn height, Lz, and the current leached-zonecolumn height, Lc, such that LT is equal to Lc plus Lz. The erosionrate, dz dZ E- %- dt'where z iselevation andt istimesince initiation of oxidation. Thus, the amount of erosion r,q dz Z ZONE O --ENRICHMENT LIJ ..J ofleached capping isLE= -t • --tE.Conservation LEACHED BLANKET GROUNDWATER PROTORE- FLOWLI NE TIME SINCE INITIATION OF SUPERGENE REMOBILIZATION FIG. 1. Conceptualframework of a vertical control volume showingleached,enriched,and protore zones.Erosionrate is indicated as dz/dt. Mass balance analysismay be performed only after chemicalweatheringbegins with the descentand fixationof oremetalsin the subsurface at or nearthe palcogroundwater table (long dashedline) extendingfrom initial elevation of the ground-watertable (W0). of a metal, e.g., copper, in a vertical column is expressedin equation(1) and is showngraphically in Figure 3. Most simplystated,the leachedcopper must equal the fixed copper in a closedsystem. Equation (1) is a more rigorous statement about mass balance: (1) (2) (3) (4) (5) dz pppLT q-flux= B(bpb - POp) q-lp•Lc - lplt-•. (1) Term (1) in equation(1) is the total massof copper originally in the leached zone up to the height of LT abovethe presenttop of the enrichmentblanket. Thisterm isrepresentedin Figure3 by the diagonally 1232 BRIMHALL, ALPERS, ANDCUNNINGHAM VERTICAL CONTROL VOLUME leached-zone columnheight,LPr,asgivenin equation (3): (NO N- DEFORMATIONALSYSTEM) DISTANCE METAL ROCK GRADE DENSITY I cm -- OXIDATION -- PRESENT LEACHED LPr =B(bob - POp) (POp --lpl) SURFACE " (3) CAPPING It shouldbe kept in mindthat manyof the termsin equation(3) are functionsof time, includingLPr,B, ENRICHMENT BLANKET b, pl, and 1. Equation(3) is simplya statementof _ PROTOR E massbalancein a closedchemical system at a given time sincethe initiationof oxidation.Equations (2) and (3) are generalfor any element,keepingin enrichmentmay ROCK MASS, m=p.Vwhere V=Z'(llcm 2) sorn=pZ mindthat the zoneof subsurface be at different depths for various elements. CalcuFIG. 2. Statementof variablesdescribinga vertical control lated leached-zone column height (LPr) when added volume,includingdistances, metal grades,and rock density at the topof the enrichment terms. For a control volume with a unit cross-sectionalarea, to the presentelevation b.... -- massof metal equalsrock density times metal concentration times delta z (zone thickness). blanket (ETB) providesthe basicinformationnecessaryto reconstructa topographicoxidationsurface (SlA): SLPr:I•r + ETB, (3a) ruled zone. Term (2) refers to fluxesin or out of the controlcolumn,eitherby lateralflow in or out correspondingto the surfaceconditionswhen leachof the leachedand enrichedzonesor by flow of ing beganfor a singlechemicalelement,as shown copper in solution below the enrichment blanket in Figure 3 and equation(3a). (basaldischarge).Thesefluxesare shownwith small L•/B ratios arrowsin contrastto the dominantdownwardflow, LPris relatedin a simplewayto othervariables. shownwith a largearrow.We will assume initially Rewriting equation(3) givesequation(4) showing thatall ground-water flowwaslocallyin a perfectly verticaldirectionwith completereprecipitation of the similarityof L•r to B in that both are measures all leachedcopperwithin the enrichmentblanket of length: andwe will startby solvingequation(1) assuming no lateralfluxesor lossof metalin basaldischarge. (1) These assumptions will then be evaluatedin detail. Term (3) is the excesscopper in the enrichment blanket(dottedpattern,Fig. 3) which represents the differencein coppergradebetweenthe blanket (b) and the protore(p) multipliedby the blanket thickness(B) and the appropriatedensityterms. (4) ENRICHMENT PROTORE I BLANKETI LEACHED CAPPING Term (4) is the copper containedin the current leachedcapping(patternof verticalrulesoverlying diagonalrules)and term (5) is the coppereroded from the surfacecappingwith metal grade,1, over time, t. Giventhat LT is equalto the sumof Lc plus (2) LPr(ppp -- lp.)= B(bpb -- POp). ,.•LUXE$ ,,,, ' • •m C•PER M•S (-I [COPPER ::• LE, equation(1) may be rewritten in terms of LPr where the superscriptsignifieszero fluxesin or out + VLUXE$ , of the control column. This assumes that eroded leachedcappinghadthe sameaveragemetalgrade asthe cappingleft in place: (•) (2) (3) pppLPr = B(bpb -- POp) 4-lplLP r. (2) ELEY•T•0 N Z FIG. 3. Plot of wt percentore metal (copper)vs. elevation (z) showing protoregrade(p), averageenrichment blanketgrade (b), andpresentleached-zone grade(1).Thisfigurerepresents In equation(2), term (1) is the total coppermetal the concept of metal massbalance, i.e., the metal leached from in the originalprotorezoneup to heightLPr;term the oxidizedzonemustequalthe excessor fixedcopperin the (2) is the excesscopperin the enrichmentblanket, zone of enrichment. The main metal flux is downward from zoneto enrichment blanket,althoughlateralfluxesmay andterm (3) is the residualcoppercontainedin the leached occurlocally.Oxidationsurfaceelevation(SLPr) is calculated by entireleachedzone,includingerodedmaterial,with addingtotal leached-zonecolumnheight (LPr)to the elevation grade,1. Equation(2) may be solvedfor the total of the top of the enrichmentblanket(ETB). MASS BALANCE PRINCIPLES APPLIED TO ORE-FORMING PROCESSES Term (1) in equation (4) is simply the product of total leached-zonecolumn height times the difference in metal grade between the protore and the leached capping,modified by density terms. The metalgradefor thisleachedcopperis p - 1asshown in Figure 3. The rectangulararea correspondingto term (1) mustequal,in a closedsystem,the area of excessor fixed copper added to the enrichment blanket, term (2). The ratio of the widths of these two areas,the unitlessnumberLPr/B,is a function which simultaneously relatesb to p as well as p to 1, and thus carriesinformationconcerningboth the extent of enrichmentand the extent of leachingfor a given control volume. b 1233 b=ppp +L(ppp-lpl ) (A) B pb bmoximum ....... g O SAy --FLUX PPp 0 LEACHING PP,-IP, AXIS (B) Blanket metal grade In additionto findingthe approximatepositionof the former oxidation surface, it is important to considerthe factorscontrollingenrichmentblanket metal grade, b. Solving equation (4) for b gives equation (5): (]) bp-pp. (2) ENRICHMENT Pb AXIS pI Pb FIG. 4. Interrelationshipsof metal grades, distances,and rock densitiesshowingevolution of enrichment blanket metal In this form we see that b may be expressedas a grade (b) as the extent of leachingincreaseswith 1, ultimately linear functionof the form y = ao q- alx with two approachingzero. Closed-systempath (eq. 5) of b is shown, terms.Term (1) is the y-intercept,or ao,of a straight reaching a maximumvalue when leaching is complete (eq. 6). of b path is LPr/Bwhich appearsto be constantwithin line. Term (2) is a productof two terms.Considering Slope geologicdomains,e.g., singlerock typesor hypogenealteration the ratio LøT/Bas the slope,a•, the independent facies. Data from a given domain may plot above or below the variablebecomes(POp-- 101)/0b. The systematicline, indicatingpositive fluxes(sinks)or negative fluxes(source behaviorof blanketmetal grade,b, in sucha coor- regions).Path beginsat protore grade (p) modifiedby density dinate systemis shownin Figure 4A. The abscissa, terms. (POp -- 101)/0b, maybe imaginedasessentially being equalto p - 1. Initially, before leachingbegins,1 is equalto p, sop - 1equalszero.The startingposition of the protorebefore oxidationbeginsmay thusbe shownin Figure 4A alongthe y-axis.As weathering andoxidationproceedfromthispoint,an enrichment path is followedasthe blanket metal gradeincreases from an initial valueof p and finallyto b, and asthe leachedzonebecomesincreasingly depletedin metal and p - I increases.The path is a straightline with a slopeLPr/B. We will presentevidencethatL•/B is essentiallya constantwithin a givengeologicdomain of a weatheringore depositat a giventime.Typically, we have observed this ratio to be between 1.5 and 4.0. Giventhe domainconstancy of LPr/B in a closed b.... = Pl) (6) We see that, except for a minor densityeffect, b... is essentiallyequal to protore metal grade, p, plus p(LPr/B). This explainsthe attainmentof a maximum enrichment grade in supergeneporphyry copper systems;its apparentlimitation is about 5 percent totalcopperfor LPr/Bratioslessthan4.0 andaverage protore metal gradesless than about 1.0 percent Cu. Continuedlowering of the ground-watertable, therebyextendingthe zoneof oxidation,LPr,could allow for the grade of the enrichmentblanket to chemicalsystem(no lateral fluxes),metal massbalance producesa blanket metal-gradecurve, b, that increase further, if the blanket thickness, B, were would attain ideally a maximumvalue at complete to remain relatively constant. However, because leaching,or when 1 = 0. We refer to this maximum LPr/Bappearsto be limited to a maximumvalue of blanket metal grade as bmaxwhich is expressedin 4 or 5 in most areas, the concept of a maximum equation (6): averageenrichmentblanketmetal gradeseemsvalid. 1234 BRIMHALL, ALPERS, ANDCUNNINGHAM We haveshowna closed-system analysis of blanket metal-gradeevolutionin Figure 4A as a straight path proceedingawayfrom the initial protorestate to a final,rarely attainedstateof completeleaching for zoneswhich are nearly completelyleached,but they cannotexceed100 becausethe productlpl will alwaysbe finite andpositive.It is, however,possible for EL to be negativefor the specialcasewhere lpl in the oxidationzone. Open-systemcharacteristics > ppp,aswill be seenbelowfor oneanomalous area are also shown. Positive lateral metal fluxes into the at La Escondida.This anomaloussituation may be blanketzone (metalsink areas)would occurabove interpreted in terms of vertically inhomogeneous the closed-system line, with negativefluxesof metal protore metal gradesor alternativelyin termsof a source areas below. positivelateral flux into the leachedzone, perhaps most commonlyexpressedin the form of exotic Leached-zonemetal grade chrysocollaand other copperoxide mineralization. It is possibleto derive an expression for leachedSubstitutingequation(8a) into equation(5) gives zone metal grade, 1, from equation (4) which is equation(9), an expressionfor blanket metal grade, analogousto the expressionfor blanket grade,b, in b, as a function of leaching efficiency,EL, shown that chemicalevolutionpathsare shownfor geologic graphicallyin Figure 5: domainswith constantLPr/B.Equation(7) shows b=PPv PPp pb+(I-fir'l( •,-•-] •,l'•pb ]']EL ' leached-zonemetal grade, 1, as a function of a startinggrade (p) or y-intercept, a slope which is the negativereciprocalof I_fir/B, andan independent variable, essentially(b - p), which representsthe excess metal in the zone of enrichment: (9) This functionshowsa similarityto the linearbehavior in Figure 4 with maximumblanket metal grade at 1 equalto zero and leachingefficiencyequalto 100. 1-ppp (--•o) (bpb - ppp) (7)Estimationof subsurfacelateralfluxes Pl Pl This functionis shownin Figure 4B, illustratingthe In principle,by the use of equation(3), it should endstateof a closedsystemwhichattainsa maximum be possibleto reconstructthe original oxidation blanket metal grade, bmax,when 1 is equal to zero surfaceusing drilling data and surfacecontouring even in highly eroded areas. In certain instances, and leachingis complete. after reconstructionof the calculatedoriginal oxiLeachingefficiency dationsurface(SLPr), we recognizetopographicfeaThe efficiencyof the leachingprocessfor a given turesthat are geologicallyunrealistic.Thesemay be control volume (e.g., drill hole) may be evaluated evaluatedconsideringthe possiblecausesof such in terms of the percent removalof a certain metal apparenttopographicanomalies(Fig. 6). There are from the leached capping zone. Of course, this four possibleexplanations:(1) real physicaltopogpresumesknowledgeof the concentrationof that raphy on the top of coppersulfidesat the time of metal before the leachingtook place.Thus,we must initiation of chemicalweathering,(2) lateral metal examinethe deepestpenetrationsin that area to get fluxes into or out of these anomalousareas, (3) an idea of protore metal-gradevariationsbeforewe can properlyinterpretthe leachedcappingzonefor that area. The leachingefficiency,E•., can be expressedas: bmaxlmum metal leached zone EL= 1001- m-•--•al oretained rig• in •-n 1ea-•--h-d-ed z-•-ne =100(1 IplLT.'•. pppLT/ ..................... (s) b= ppp+ LgPP.EL p• B IOOp• This expression can be simplifiedasshownin equa- -- tion (8a): EL-I \ •p Note that the distanceterm • . (8a) cancelsout, so that EL is solelya functionof protore and leached-zone gradesand densities.Valuesfor EL approach100 o El_ LEACHING I00 % AXIS FIG. 5. Interrelationships of metal grades, distances,and rock densities showing evolution of the enrichment blanket grade (b) asthe leachingefficiency(EL) approaches100 (eq. 9). MASS BALANCE PRINCIPLES APPLIED TO ORE-FORMING PROCESSES PHASE I: ASSUME NO LATERAL FLUX • • shown schematicallyin Figure 6. It is a simple COMPUTEDOXIDATION SURFACE ........... matterto computethe signandmagnitudeof appar•'•._ _•'?•--, / ..... '."] ENRICHMENT ZONE ---• '1' LLI .•. O__?:5•--• -'-"----- // ,•[ 1235 B(bp-p.) - L (,p-Ip) i' FLUXFLUX:B •bJ' T J' h ent lateral fluxesusingequations(11) or (12). These fluxesare negativefor sourceareasand positivefor sinks.The value of Dr used to solveequations(11) or (12) is not the LPrcalculatedfrom the initial (or phase1) effort assumingno suchlateral fluxes,but instead,is the projected value of Dr from the relatively smoothsurfaceimmediatelysurroundingthe apparenttopographicanomaly. Data base LATERAL ; TRANSPORT Requisite data (assaysand rock properties) to solvethe analyticalexpressions maybe derivedfrom FIG. 6. Upper figure presents a block diagram showing drilling coresusing standardmethods.In situ rock leached zone, enrichment blanket, and protore. Computed density data may be obtainedthrough application '•..DISTAN CE values of L• are added to each ETB and are contoured. Local anomaliesas shownin this surface(elevation of top of blanket of a number of methods. However, in order to plusIPr) may be interpretedas being due to subsurface lateral obtainbulk-rockdensitiesincludingporesand open mustbe effectivelysheathed fluxesof metalsfrom sourceareasto sink areas.Such regions fractures,core samples are identified by matched pairs of anomaliesin the computed if a water immersionmethodis employed.Alterna- surface,SLUr.The regular surface,with no lateral fluxes,is interpreted as the topography on the oxidation surface when leachingbegan. tively, densitiesmay be obtainedfrom weighted lengths of unsplit core, if the theoretical core diameterand recoveryare known.Trays of core may be examined with this method. errors in vertical protore metal-gradeprojections, It is importantto considerthe sizeof eachsample, and/or (4) rapid lateral variationin metal grades, e.g., a core sampleusedfor a densitymeasurement e.g., in and aroundwell-mineralizedbrecciacolumns. or a samplingintervalover whichmetalsare assayed. Consideringlateral fluxesfirst, we use equations Since densitiesare usuallymeasuredon individual handsamples, the deviationwill be quitehighsimply (10), (11), and (12): becausethe protore characteristicsare heteroge(10) pOpL-r + flux= B(bob- POp)+ 10]Lr, neouson that length scale.In contrast,assaysare flux= B(bpb- pop)- L.r(p,op-10,), (11) and (]) (2) flux = [Bb0b+ LT10,]--[(B + LT)P01,].(12) Note that term (1) in equation(12) is the current amountof copperpresentin the blanketandleached cappingzonesincludingeroded cappingand that term (2) is the originalamountof copperpresentin these two zones. Thus, the flux term accountsfor typicallymadeoverlongerintervalsandwill typically have a much smaller deviation. It is necessaryto keep these points in mind when one is concerned with the scatter in the data. Besidesassays androckdensities,drill-holesurvey data(holedepthandazimuth)are necessary in order to fix samplesin a three-dimensionalframework using northing, casting,and elevationparameters. Drill-hole survey data are especiallyimportant in analysisof massbalancefor inclineddrill holes. any and all fluxesin or out of the Dr or B portion Computercalculationsand delimitationof of the controlcolumnas well as for copperin the enriched-zone boundaries In each of the two districts studied, tens of thoubasaldischarge.Any basaldischargeof copperis likely to be exceedinglysmallbecauseof the low sandsof assaysand thousands of densitymeasuresolubilityof copper sulfidesunder the reducing ments were used from hundreds of diamond drill conditions of the protore(Cunningham, 1984). holes. Computer programshave been written to Once an initial oxidation surface is determined, facilitatedata handlingand computation, including topographicanomaliesare evaluatedin termsof the graphicaldisplay. Grade profileswere plotted for lateralflux hypothesis. Possible sourceregionsfor each availabledrill hole, i.e., wt percentore metal metal are representedas regionswith anomalouslyas a functionof elevation(e.g., Fig. 7). From these low SI?rvaluesandcorresponding sinkregionswith plots,in combinationwith the geologists'drill logs, anomalously high SI?rvalues.If lateralfluxeswere the top and bottom elevationsof the enrichment responsiblefor the anomaloussurface, then metal blanketswere chosenfor each hole. Many holes wouldhavebeentransported fromthe sourceregion were excluded from consideration in the mass balalongground-water flowlinesto the sinkregionas ancecalculations becauseof insufficientpenetration 1236 BRIMHALL, ALPERS, AND CUNNINGHAM i PROTORE BLANKET LEACHED FRESH'•'"'•'Q•S, - • • INCIPIENT ' ST'--'"'"""•ONG ß CAPPING PROTOREI PROTORE ENRICHMENT I ENRICHMENT - 4.0 absent from the superadjacentstrongly enriched zone (Fig. 7). An attemptwasmadeto excludefrom the present calculationsany holes which bottom in either of these two zones, because reliable estimates I ß3.0 i ß .2.0 leee ', 1.0 . ee ,, ', ß I . ß "/ • • eee I . ß .II ' 0.5 I EBH I EBB ETB COLLAR ELEVATION FIC. 7. Plot of wt percent metal (copper) vs. elevation (z) for La Escondida diamond drill hole 159, showing various weathering zonesdescribedin the text. Each dot represents2 m of diamond drill core. into unenriched protore zones or incomplete assay data for the leached zone. Owing to the lack of accurate quantitative mineralogicaldata for mostholes,the boundariesof the enrichedzone were assignedon the basisof slope changesin grade profiles.The elevationat the top of the enrichmentblanket (ETB) is extremelywell defined for most holes both at Butte and La Escon- of protore metal grade cannot be made for these areas. Work is currently in progressto gather accurate modal data for sulfide mineralogyfrom such holes. Plots of percent total copper vs. percent supergenecopper will be usedto estimateoriginal protoremetal gradesin zonesfor which little or no protorehasbeen drilled, asdoneby Brimhall(1979) for the analogousproblem of hypogeneenrichment. Beneath the zone of incipient enrichment is a zone referred to as quasi-protorewhich has not been enrichedsignificantlyin copper(lessthan 10% total Cu in supergenephases)but neverthelesshas been affectedby supergenefluids,as manifestedby the hydration,hydrolysis,and/or dissolutionof various hypogene gangue minerals and the attendant lowering of bulk density (see Figs. 12 and 13 and discussion of model 3 densityvs. depth regressions). Supergene alteration effects in the quasi-protore zone include hydrationof anhydriteto gypsumand ultimatedissolutionandalterationof plagioclaseand K-feldsparto mineralsat the kaolinite group and (at La Escondida)alteration of biotite to a green phyllosilicate(chlorite?).The possibleformationof supergene sericite is a matter of some controversy (Titley and Beane, 1981; Graybeal, 1982; Marozas, 1982), the resolutionof which must await a careful study of stable isotopessuch as the work at Santa Rita, New Mexico,by Sheppardet al. (1969, 1971). The elevation of the bottom of the enrichment dida by an abrupt rise in copper grade, usually coincidentwith the horizon mapped as the top of blanket is defined for this study by the usually dominant sulfides(TDS), which in general marks distinct change in slope of the copper grade vs. the bottom of significant oxidation. As shown in depth profile which separatesthe zone of incipient Figure 7, zonesof partially oxidizedor "mixed" ore enrichment from the quasi-protorezone. Because may occurwithin the leachedzone.These "perched the quasi-protorezone is consideredtogetherwith blankets"may representformer topsof enrichment, the protorefor the purposesof protoremetal-grade or ETB horizons,whosesignificance will be discussed estimates,the presenceof small amountsof superin a later sectionon multistageleaching. gene copper in the quasi-protorezone must ineviDelimiting the bottomof the enrichedzone (EBB) tably introducea small systematicerror in the calis considerablymore problematicfor most holes, culation of the no-flux leached-zonecolumn height yet it is critical in terms of influencingthe results (LPr).If protore metal-gradeestimatesare slightly of the massbalancecalculations.As shownin Figure too high becauseof supergenecopper in the quasi7, the enrichment blanket can be divided into two protore, the amountof excesscoppercalculatedto zones:an upper zone of strongenrichment,where be in the enriched zone will be slightly too low, as greater than 50 percent of the copper resides in will the calculatedheightof the leached-zonecolumn supergenephases(usuallydigenite, djurleite, chal- needed to accountfor that excesscopper. This can cocite,idaitc, and/orcovellite),and a lower zone of be seenby inspectionof equation(3). An increase incipient enrichment, where only 10 to 50 percent in p will decreasethe excesscopperin the numerator of the copperresidesin supergene phases. In addition and increasethe leachingterm in the denominator. to the relatively higher abundance of hypogene Thus, the results presented below may, in some copper sulfides(mostly chalcopyriteand bornite), instances, be considered minimum values for the the zone of incipient enrichmentis distinguishedby total leached-zonecolumnheight(LPr). Once the elevationsof the top and bottom of the the presenceof apparently unenrichedintervals of approximately protoregradewhichare conspicuously enrichment blanket are selected for a given drill MASSBALANCE PRINCIPLESAPPLIED TO ORE-FORMING PROCESSES hole, the computerprogramcalculatesaveragemetal grades,densities,and all the derivative functions such as LPr,B, lateral fluxes, etc. In addition, the program performs linear least-squaresregressions and standardstatisticalmeasuresof fit for grade and densitydata within the three zones,L½, B, and the protore.SeeTable i for definitionof symbols. Results for the Butte District, Montana OXIDATION SURFACE RECONSTRUCTION •..,,_-,- MODELI 1983). An excellentsummaryof supergeneeffects is given by McClave (1973). •'•'"';'J F TR • • TERTIARY GRAVELS [•] LEACHED CAPPING -----'"r'•"•ENRICHMENT BLANKET SL;=ETB +L•=ETB,B(bpFpp • E•--"[] VE,N-BE•,NG ,•RO•RE ppFIp, [] D'SSEM,NAT• OI I000FEET ß SCALE bJ L?oo•"• The Butte districtof Montanais a porphyrycopper-molybdenumdepositin which large, high-grade o baseand preciousmetal veinshave been mined for • over a century.The geologicsettinghasbeen well i described(Meyer et al., 1968; Brimhall,1977, 1979, 1980; Brimhall and Ghiorso, 1983; Brimhall et al., 1237 ...:.:.:.:.: .......... :.:.:.:.::::!:i -•.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:,....v.-.'.' :,:.:,x.:.:.ßIN PROTORE ..... :':'':.... :'' ':':':':g':':':':': .... :.:.:.:':,:':' ':. FIG. 9. .Generalizedeast-westcrosssection (looking north) throughthe Butte districtof Montanashowingresultsof model 1 calculations. Model I results in the leachedzone of the originalcolumn(term 1, eq. 2) above the top of the enrichmentblanket is equal to the area below the protore metal-grade curvep up to the heightof LPr.By equation(2) this area is equal to the excesscopper in the blanket, i.e., the copper leachedfrom the leachedcapping sentative diamond drill hole in the Butte district. zone (term 2 in eq. 2; dotted pattern in Fig. 8A) Data points for assaysrepresent 5- to 10-ft core plusthe relict coppercontainedin the leachedzone intervals.Elevationshavebeen computedusinghole (term 3 in eq. 2; verticallined pattern in Fig. 8A). surveyandsampledepthdata.Mean valuesof copper The areas depicted in Figure 8A present only a gradesfor leached zone, 1, enrichment blanket, b, partial assessment of the copper massbalanceand and protore, p, are shown.Figure 8A representsa must be multiplied by densitiesgiven in Figure 8B simple,geometricinterpretationof the closed-system for each zone. Results of model 1 calculations are shown in an copper massbalanceequation(2). The validity of model 1 asa reasonable approximation hingeslargely east-westvertical crosssectionthrough the eastern upon the assumptionof a constantaverageprotore half of the Butte district(Fig. 9). Shownare Tertiary metal grade (p) throughout the column (a column gravelsoverlying the leached capping, the enrichmay be constructedfor each drill hole). Average ment blanket, and underlyingprotorespreviously valuesfor the other five gradeand densityterms(1, describedby Brimhall(1977, 1979, 1980). LPrsob, pl, p•,,andpp),asshownin Figure8A andB, are lutionsto massbalanceequationsare shownassolid alsousedasinput datato model 1. The total copper dots,onefor eachdiamonddrill hole. A majornorthsouth-strikingfault divides the district protore do- The massbalanceequationsdevelopedaboveare only a first approximation(model1) to the variations in metal gradesand rock densitieswhich are known to existin weatheringzones.Figure 8A showsmetalgrade (copper)variationwith elevationfor a repre- mains. To the west of the Continental fault, in the (A) 1.81 ,,,-..-- MODEL, , . •le ! .1.z •e<•o , i •N LLI•"•/ UII.U•' 0 It ) .r -:! / , I: .•.: downdroppedblock, oxidationand enrichmenthave occurredin a phyllicallyalteredvein-bearing protore, above a deeper protore massconsistingof disseminatedpyrite andchalcopyrite.Noticethat the current I -- J.'J --AVERAGE b/ (B) leached-zone Z ZONE GRADE Z o .... leached-zone column region eastof the Continentalfault where a disseminated sulfide protore with potassicalteration has been enriched. 2.0 PROJECTED/ The enrichment blanket is corre- i : J Average"' "' I 500(•- - 6OO0 ELEVATION(feet) and the total heightsare both substantiallygreater than in the 6000 4000 ELEVATION (feet) FIG. 8. A. Plot of wt percent metal (copper) vs. elevation (z), for a typical Butte drill hole. B. Variation in bulk-rock density with elevation (z). spondinglythinner over this disseminatedsulfide protoreand leachingefficiencyis lower. In order to check the validity of model i calculationsfor the resultantpresupergeneoxidationsurface, additional models 2 and 3 have been devised. These are offered simplyto place likely boundson solutionsto the mass balance equations and to 1238 BRIMHALL,ALPERS,AND CUNNINGHAM bracket the position and shape of the oxidation surface.In addition,by incorporatingmore realistic geologicvariationintothe models,the relativeeffects of the different variables on the solutionsmay be OXIDATION SURFACE RECONSTRUCTION $1•T MODEL 2 p=%+Ci, Z • TERTIARY GRAVELS [•]} LEACHED CAPPING ETB--•'."•,ENRICHMENT BLANKET [] VEIN-BEARING PROTORE • DISSEMINATED PY-CP assessed. 1 IOO0 FEET SCALE Model 2 (unboundedlinear protore metal-gradevariation) An improvementto the first-orderapproximation offered in model 1 is to incorporate variation in protore metal grade with elevation. This is accomplished in a secondmodel in which protore metal gradep is regressedin drillingdatafor eachhole as a linear fit of elevation with two coefficients, ao and .....ß ......'-:.........:. • .•,.,,...'•. ::z.• •...... ! ....::!:i:i:i:i:i:i:i:i:i:i:i:i:i: :':':':':':':':':IN PROTORE,:.:.:.:.:,:-:,:.:.:.:.:.:,:,:.:.:': oø,:.:.:.:.:.:.:,:,:,:,:,:.:,:.:.:.:. FIG. 11. Generalized east-westcross section through the a•. Sucha fit is shownin Figure 10, which may be Butte district showingresults oœmodel 9. calculations(p comparedto the projection shown previouslyin + a]z). See Tables œ and 3 œormassbalance expressions. Faultis t•e •s•e• vertical ]i•e e•st o• t•e Co•ti•e•t•] Figure 8A for model 1, where p = ao (constant). •lepper F•ult. Notice that in Figure 10, becausethe protoremetalgraderegressionslope(a]) is positive,the resultant LPris significantlysmallerthan in model 1. This seemsintuitively correct in that a shorter leached column of richer grade would provide essentially the sameamountof excesscopperto the blanketas and in model 1. P = • OE• (a0 + a,z)dz (13a) 1fETS+Le Figure 11 presentsin crosssectionthe resultsof : (a0+ a•z)dz. (13b) model2 calculations, takinginto accountunbounded linearvariationof protoremetalgradewith elevation. ETB is defined as the elevation of the top of the The closed-system massbalanceequationfor model enrichment blanket and EBB as the elevation at the 2 is given in Table 2. It is analogousto equation(2) bottom of the enrichment blanket. (model1) exceptthat a protoremetal-gradefunction In general,the computedmodel 2 oxidationsurhasbeen substitutedfor p in the expressionfor total face is at lower elevations than the model 1 surface. protore metal initially in the enriched zone (eq. In fact, the model 2 surfaceapproximatesthe base 13a), as well as in the expressionfor metal initially of the Tertiary gravel. The geologicimplicationof in the total leached-zonecolumnheight (eq. 13b): the model 2 result is that the oxidation zone of the columnup to the originalheight, IPr, has been MODEL essentiallypreservedwith little if any erosion.Were 2 this conclusioncorrect, it would be consistentwith I i•--'ø•T• a state of geomorphicstabilityduring enrichment. One may hypothesizethat a late vertical displacement on the Continental fault with the east side movingup occurred,sheddingdetritustoward the west. This burial by Tertiary sedimentsmay have causedan ascension of the ground-watertable which would have submergedthe previouszonesof oxidation in a reductive state. This may have resulted in deactivationand preservationof the formerly : o reactive supergene system. 5000 GOO0 ELEVATION Z FIG. 10. Model 2, showing protore metal-grade variation with elevation (z) suchthat p -- a0 -t- alz. Coefficientsa0 and al are determinedby least-squares regressionof drill hole data for each profile. See Tables 2 and 3 for massbalanceexpressions. Model 3 (boundedregressionof protore metal grade and rock densities) In additionto protore metal-gradevariationwith elevation, the geologicallymore complexmodel 3 accountsfor variationin densitiesof the protore and leached zones with elevation. Numerous drill hole densityprofilesshowin generalthatleachedcappings have densities well below fresh protore and that enrichment blanket rocks have intermediate densities MASS BALANCE PRINCIPLES APPLIED TO ORE-FORMING PROCESSES 1239 exceptfor very high gradeintervalsin which secondarysulfidesaccountfor increasedrock density. Overall, the enrichment blanket density represents a transitionzonebetweenthe highlyaltered(weathered) leached capping and the protore rocks in which no or very little supergeneflow has penetrated. As shownin Figure 12, however, we have recognizeda hydrolyzedquasi-protore at the base of enrichment blankets, in which there has been 4- little secondary sulfideformationbut whereprotore silicates,sulfates,and carbonategangue minerals are neverthelessaltered by descendingsurficial 4- fluids, as discussedabove. We see, therefore, indi- cationsof supergenepercolationwell below the base of copper enrichment(Sillitoe et al., 1984). Imagining the downwardencroachmentof the leachedzone upon the enrichmentblanket, which itself descendsinto quasi-protorerocks, one must include in the density projectionsthe effectsof progressivemineral replacementand dissolution. This complexityis treatedas shownin Figure 13B, in which densityis regressedwithin the protoreto yield the functionop= do+ dl. z andis projected upwardat the densityvaluegivenat the baseof the enrichment blanket elevation (EBB). In doing so, projected protore density is given as a constant abovethis elevationequalto o• -- do + d•.EBB. Similarly, there is an upward minimum or limit in the surfaceleachedcappingdensity.Thisis regressed as p•' = Co+ c•' EGRAV where EGRAV is the elevation of the top of the bedrock or the base of overlyinggravels.Protoremetal gradeis projected upwardat a value of p* -- a0 + a•. EBB in order to r--1 account tbr the effects of incipient enrichment im- mediatelyat the base of the enrichmentblanket (shownin Fig. 13A). The widespreadoccurrence of this deep low-gradesecondaryenrichment,both at Butte and La Escondida,is a concern of considerable importance.It will be discussed in more detail in a later section. L__J The model 3 oxidation surface is presented in Figure 14 in crosssection.Notice that in general • this surface occurs at elevations intermediate 4-4-++ be- tween those of models I and 2. This is essentially due to the protoremetal-gradebounds,givenasp* = ao + a•-EBB, which keep the upward-projected protore gradefrom attainingexcessively high or low values.Given the scatterin the data, the quality of the fit, and the depth of drilling penetration into the protore, this bounded regressionprovides a reasonablecompromisebetween averagevaluesand unboundedextrapolation. We feel that model 3 offers the most accurate estimationof the originaloxidationsurface,as constructedin Figure 14. Significantlymore erosionis indicatedby model3 thanby model1, approaching as much as 400 ft of leached cappingremoval. Of 1240 BRIMHALL, ALPERS,AND CUNNINGHAM I •(• -3,.2 I PROTORE BLANKET LEACHED _ QUASiCAPPING PROTORE •03 -5.0 <[ I ?. _'-2-' .ß :iiiih --2.8 • FRESH v ßßß'::;":ß z r, -Z Z MAXIMUM , • DEPTHOF , u o _2.0 |:::!11 •iiI•'At• . .......... FLOW , In summary,evaluationof the three modelsdescribing massbalance indicates that a substantial amount of erosion of leached capping occurred before depositionof the Tertiary gravelsat Butte. Efforts using palynology to date these sediments have not been successful,but we anticipate their age to be substantiallyyounger than supergene oxidation.The fault block east of the Klepper fault may have been exposedmuch more recently than the main Berkeley zone. Essentiallyno erosionhas occurred in this area. • I EBH I ETB I Comparisonof models 1, 2, and 3 I For the sake of comparisonof the analytical assumptionsin the three models,we have presented •C. •. Bu}E-roc••ebsity v•iatiob wffh elevatiobi, typical in Tables2, 3, and 4, respectively,the massbalance Batte •ill hole. Note •i•tarb•ce i. rock •e.•itie• equation, the analytical solution for Dr, and the reactiobsib the quasi-protorezobe, more thab 500 •t below the expression for leachingefficiency,EL. 400 0 5000 ELEVATION 6000 (FEET) In Table 3 noticethe simplesolutions for I_• in models 1 and 3. Model 2 results in a quadratic expression for I_• with two roots,one positivereal all the models, it seems to be most sensitive to root and a second,generallynegative,unreal solu- geologicdetails.For examplein Figure 14, wherever the enrichment blanket is unusuallythick, as indicatedby the dark arrows,the model3 I-firis anomalouslyhigh, indicatingpositivelateral fluxes.This can be saidwith certainty as there is little indication of incipient enrichmentin theseparticularzones.In fact, ax is negative in these cases.These thick portionsof the enrichmentblankethavebeen shown by mapping to be due to the existenceof pyritic tion. The accuracyof each model dependsupon the validity of the assumptionsmade in each geologic environment in which our methods are applied. Furthermore, limitationsmay exist in the available data.For example,at La Escondida,drill holesrarely penetrate sufficientlyfar in the protore zone to provide an accurate regressionof protore metal gradewith elevation.In thisinstance,model 1, with veins, which upon oxidation, produce zones of ex- a constantprotore metal grade for each column,is ceptionally high permeability without contributing most appropriate. Results of these mass balance calculationsare presentedbelow, after a brief introsignificantlyto the copperaddedto the blanket. The significance of the calculatedoxidationsurface ductionto the geologyat La Escondida. may be understoodgeologicallyby consideringthe region just east of the Continental fault. A broad area exists in which no oxidation or enrichment (B) phenomenahave been found. A debate has existed as to whether or not a blanket ever formed there, becauseif it had, it musthave been entirely eroded away.Our I-firsolutionimpliesthat at least300 ft of o •' • '•z •_50 -- postoxidationerosionhasoccurred,and in this area, even the blanket has been eroded. To check this hypothesis,we have conducteda ground magnetic traverseacrossthis area. A 200-gammadrop in the total magnetic field occurs as one traversesthe steeply dipping Klepper fault which juxtaposes unoxidized material with the eastern leached capping. This implies that just west of this fault, magnetite is present very near the surface.It is likely, therefore, that the erosionalhypothesisis correct for this east block between the Continental fault 2.0 .•, • P=a +o,EBB ß •e• 5000 , • 6000 ELEVATION(feet) 4000 ELEVATION(feet) FIG. 13. Model 3 (boundedregressionmodel)showingmetal grade and density profiles as functions of elevation (z). See and the Klepper fault. East of the Klepper fault, in Tables 2 and 3 for mass balance expressionsand limiting a grabenstructure,the amountof erosionof leached (bounded)valuesof protore metal grade and densitiesfor the leachedand protore zones. cappingis lessthan 100 ft. MASS BALANCE PRINCIPLES APPLIED TO ORE-FORMING OXIDATION SURFACE RECONSTRUCTION $ [•T........... MODEL 3 • • • [] TERTIARY GRAVELS LEACHED CAPPING ENRICHMENT BLANKET VEIN-BEARING PROTORE [] DISSEMINATED PY-CP ooo , I • z •-6ooa--,I-_ • L• • • ,,,I,• ,- ,........•' '• 1241 hypogenealteration of this rock type has resulted in a variety of facies with dramatically different leachingand enrichmentpropertiesin the supergene environment, as will be discussedbelow. A dacitic unit probablyoverliesthe andesitein the western 01 IOO0 FEET part of the district and lies in fault contactwith a . ,TF -157o00 sedimentarysequencewhich includes sandstones, a • PROCESSES SCALE .t ,IF" .... "•.-'"'•. "..'.':::..'•:i'::;:..'•' ßßß• ,,J• :::::::::::::::::::::::::::::: :::¾::::::: siltstones, limestones, shales, and interbedded vol- canicsandconglomerates (Perell(•,1983b).Tentative identification of these sediments as Lower Cretaceous (Chong, 1977; Perell6, 1983b; Ford, 1983) and ::::::::::::::::::::::::::::::::::::::::::::::::::::: interpretation of their contact with the dacite unit structural disconformitysuggesta minimum Li•L_ I// INC PROTORE. Ill[•o-......... '--'•----:..... '•••' '" "''•• "'•'•..'i:')i:: ß [ :':':':':':':';':IN ß.:..:..... '• :, .:.,:.:.:.:. :.:.:.:. :.:.:.:.:.:.:.: ß as a Lower Cretaceousage for the andesitewall rock. A sequenceof hornblende andesite |apilli tuffs and volcanicbrecciaswhich unconformably overliesthe sedimentarysequenceis tentativelycorrelatedby Ford (1983) and Perell6(1983b) with the Augusta the enrichment zone. Victoria Formation of Upper Cretaceousto lower Tertiary age (Garcia,1967). This unconformitymay correlatewith the Middle Cretaceoussub-Hercynian Geologic Setting at La Escondida,Chile tectonic phasedocumentedin Peru, central Chile, La Escondidais a large, relatively high-grade and Argentina (Coira et al., 1982). porphyry copper deposit located in the Atacama Intrusiverocksand youngervolcanics desert of northern Chile, approximatelymidway At leastthree intrusivepulsesof porphyriticquartz between the Chuquicamataand E1 Salvadormines diorite with associated brecciationandhydrothermal (Fig. 15). Since discoveryof the depositin March 1981, explorationand developmentdrilling hasout- alterationare thoughtto have been responsiblefor lined an orebody of world-classproportions.The the bulk of the hypogeneCu-Au-Ag-Momineraliza112 diamonddrill holes consideredin the present tion at Escondida(Ford, 1982, 1983; Ojeda, 1982; studywere chosenbasedon completenessof assay Perell6, 1983a). The distribution of these units is profiles,with respectto adequatepenetrationinto shown in Figure 16. Breccias and pebble dikes protore or quasi-protorezones (e.g., Fig. 7) and crosscuttheseplugsand generallytrend northwestcontinuityof assayprofilesfor all zones,including southeast,as shownin Figure 16. A younger (postmineral?)rhyolitic porphyry, the leached zone. shownin the light gray area, formsa major, northMineralization at La Escondida is associated with a complexof porphyriticquartz diorite to granitic trending dike 0.5 to 1.5 km wide which may have intrusions,extrusions(?), and igneousbrecciasset truncated the easternmargin of the La Escondida in a structurally complicatedzone of Lower Creta- orebody(Ford, 1983). A subvolcanicneck, as eviceoussedimentaryrocks and Upper Cretaceousto dencedby vertical flow bandingand coarseigneous lower Tertiary (?) volcanic rocks. The following brecciation,of the rhyoliticporphyryis presentnear discussionof the geologyat La Escondidawill be hill C in the southeastportion of the district (P. dos limited to that neededasbackgroundto understand Santos,pers.commun.).This rhyoliticunit alsomakes the supergenemassbalancecalculationspresented up much of hill B. Its eraplacementwas probably below. Spacedoes not permit an exhaustiveintro- accompaniedby intensesilicificationas well as perduction to La Escondidageology,which might in- vasiveadvancedargillicalteration,perhapscogenetic clude the results of ongoingstudiesconcerning with hypogeneleaching of primary sulfidesfrom geochronology,petrology, geochemistry,and para- previouslymineralizedadjacentunits, especiallyin genesisof hypogeneandsupergeneores(Alpersand the northern part of the district (J. H. Courtright, pers. commun.). Brimhall,in prep.; Ojeda and Burns,in prep.). FIG. 14. Generalized east-westcross section through the Butte district showing results of model 3 calculations.Note general similarity to model 1. Anomalousdata points on the surface, indicated by arrows, are interpreted as regions of subsurface lateral copperflux into anomalously thick sectionsof Host rocks Alteration and mineralization The principal wall rock for the mineralizationat The hypogenealterationzoning is dominatedby Escondidais an aphaniticto finely porphyritican- a northwest-trendingtrough of deep sericitic alterdesitcwith rarelypreservedvesicularandtuffaceous ation, closelyassociatedwith the highly fractured, texture (Ford, 1983). The distribution of this is brecciated,and well-mineralizedquartz diorite porshown by the blank area in Figure 16. Intense phyries.This sericitic zone was superimposedupon 1242 BRIMHALL,ALPERS,AND CUNNINGHAM a preexistingK silicatestageof alterationwhich is preservedonly locallyat depthsof presentdrilling in the sericiticzone.In the westernportionof the district,strongK silicatealterationis manifestedin the andesiticwall rock by intense biotitization of the groundmass and former mafic phenocrystsaccompaniedby K-feldsparveining and replacement of plagioclase. A properpropyliticzone(i.e., epidote, chlorite, magnetite,calcite, etc.) has been distinguishedonly at the marginsof the drilled area; however, chlorite is abundantas a "retrograde" replacementof biotite at the margins of the K silicate zone. Advanced argillic alteration (pyrophyllite, alunite,diaspore,kaolinite)is pervasiveto substantialdepths in the northern portion of the districtand is probablyof predominantlyhypogene origin. However, becauseof the similarchemistry of hypogeneacid-sulfate fluidsandsupergene fluids, determiningthe origin of these mineralsunambiguouslymust await the resultsof stableisotopeinvestigations. In addition,a supergeneorigin and/or + alterationof sericitecannotbe ruled out (Titley and Beane, 1981; Graybeal, 1982; Marozas,1982). The leached and enriched zones at La Escondida containabundantsupergenekaolinite-groupminerals, which have formed as replacementproductsof hypogenefeldspars,clays,and micas,as well as in abundant fractures. Some of the abundant pyrophyllite,alunite,anddiaspore in theadvanced argillic zone may be of supergeneorigin, as mentioned above. The hydration of hypogene anhydrite to gypsumoccursat an elevationgenerallyat or below the baseof the zone of incipient enrichment.In the intrusiverocks,gypsumtends to preservedover a 5- to 50-m vertical interval, above which it has mostlybeen dissolvedoutrightand is presentonly rarely. The top of the Ca sulfatesurfacedefinesa northwest-tending troughwhichrepresentsa useful indicatorof the depth of penetrationof acid supergene fluids(Gustafson and Hunt, 1975; Sillitoeet al., 1984). Hypogenemineralization zoningat La Escondida remainspoorlyunderstood in detaildue to the lack of extensivedrilling into true protore-zonerocks and the lack of detailed mineralogicalwork. Nevertheless,an overallpatternemergesfrom existing estimatesof sulfidemineralogy,basedon staffgeologists'drill logs.A pyrite-richhalo (pyrite greater than 50% of total sulfides)occursarounda central zonewhere chalcopyriteplusborniteexceedspyrite (Minera Utah de Chile, Inc., unpub. data). Two bornite-rich areas have been identified which are virtually pyrite free; at least one of these contains relatively high Au and Ag values similar to the bornite-chalcopyritezone at E1 Salvador, Chile (Gustafsonand Hunt, 1975). An extensivepetrographicstudyof relict sulfidestrappedin quartz in MASS BALANCE PRINCIPLES APPLIED TO ORE-FORMING PROCESSES TABLE4. Solutionsto MassBalanceEquationsGiving Formulasfor the Efficiencyof Leaching in Each of the Three Models Model number Analytical solution for EL 1. EL= 1001-• 2. EL = 100B(pbb- (do + dx' EBB)(ao+ ax' EBB)) 1243 Figure 17B is a contourmap of the elevationat the top of the enrichmentblanket, as definedby grade profilesandmineralogicdata.This top surfaceshows several linear discontinuitieswhich may indicate faultoffsets of postsu, pergeneage,especially in the southeastern,eastern,and north-centralportionsof the map area. The bottom surface (not shown) roughly parallels the top in terms of its general shape;however, the slopesare generally steeper. Thus, the bottom surface also defines a northwesttrending trough whose position correspondswell with the distributionof deep (hypogene?)sericitic alterationas well as with the top surfaceof the Ca sulfates, The relationshipbetween the top surfaceof the enrichmentblanket and the presentwater table was investigatedby measurementof staticwater levels in more than 150 open vertical drill holes within both the mineralizedarea and the outlyingareas, surfacesamplesfrom La Escondida(A. Aguilar, covering a total areaof about15 km•. Thesedata unpub.data)showsa patternsimilarin manyrespects to that mappedat depth from drill intercepts,sug- indicatethat the water table is a virtually fiat surface gestingat least qualitativelythat hypogenesulfide at 2,988 ñ 3 m above sea level, or about 10 m below mineralogymay have been consistentover vertical the surfaceelevationin the nearbySalarHamburgo, intervals as long as the deepest drilling in the district.This evidenceis particularlyusefulin terms of placing possibleconstraintson the original elevation of the top of copper sulfides as well as constrainingverticalextrapolationof protorecoppergradetrendsobservedat depth. (do+ d•. EBB)(ao+ ax'EBB)L• The top of the enrichmentblanketfor mostdrill holesis a sharpcontactbetween oxidized(leached) and unoxidized(enriched)zones.Mineralogically, the uppermostsulfidesin the enriched zone are "sootychalcocite"andresidualpyrite.Smallamounts of chalcopyriteandbornite are presentin the zone of strongenrichmentascoresin grainsof supergene sootychalcociteaswell astiny inclusions(lessthan 5 #) of chalcopyriteand bornite plus digenite in pyrite grainswhich have effectively shieldedthese inclusions from supergenereplacement.Mostpyrite grainsin thiszoneare rimmed,veined,and/orpartly replacedby the sootychalcocite,the precisemineralogyof which remainsundetermined.Deeper in a typical profile, chalcopyriteoccursrimmed concentricallyoutwardby blue digeniteandthen white digeniteof distinctlylower Fe content(asnoted at MantosBlancos,Chile, by Chfivez,1983). Presentgeometryand hydrologicsettingof the enrichment blanket The collars of the 112 drill holes considered in the presentstudyare plottedon a topographic base map in Figure 17A. The three mostprominenthills in the district, labeled A, B, and C, have summit elevationsof 3,436, 3,355,and 3,194 m above sea FIG. 15. Locationmap of northern Chile, showingprincipal level, respectively.These summitsare labeled for porphyrycopperminesandprospects(afterStoertzandEricksen, reference on all appropriatemapsin this paper. 1974). 1244 BRIMHALL, ALPERS,AND CUNNINGHAM presentlyexposedto unsaturated,activelyoxidizing conditions.Conversely,throughoutmostof the district (unshadedin Fig. 17B) completely saturated conditionsnow exist in the lower portion of the leached zone. In the central part of the district (southof hill B), 100 to 200 vertical m of oxidized leached-zone rocks are now saturated below the presentwater table. The top surfaceof the enrichmentblanket in a givenplacemostlikely representsa formerinterface of saturatedand unsaturatedconditions(i.e., former top of capillary fringe). The present hydrologic setting indicatesthat some important climatic and geemorphologicchangeshave occurred since the time of active supergeneleaching and enrichment during the middle Miocene (Alpers et al., 1984). Submergenceof the zone of active oxidationin the centralportionof the districtappearsto havebeen AGE ROCK TYPE responsiblefor preservationof mostof the enrich•km ment blanket as sulfides.This is in sharpcontrastto the Chuquicamatadeposit, located approximately ] LatePorphyries 200 km to the north (Fig. 15), where decadesof MID-TERTIARY Rhyolitic Porphyry [30 - 35 m.y.I • important copper productionhave come from oxidized oreswhich representa former sulfide-enrich.. . [] Dacltic Ignimbrite ment blanket which oxidized essentiallyin place U.CRETACEOUS [] DecItc [?) [] Andesitc dueto prolongedexposureto unsaturatedconditions L.CRETACEOUS [] Sandstone, Limestone (Lindgren, 1917; Jarrell, 1944). In several porphyry copper districts, important FIG. 16. Generalized geologic map of the La Escondida reserves of copper occur as exotic copper oxide district, basedon mappingand core loggingby Minera Utah de I •-]B .... ia N I •[]Quartz Diorite Porphyries Chile, Inc., staff geologists,summarizedby Ford (1983) and J. M. Ojeda (written commun., 1984). Oligocene ages (30-35 m.y.) basedon K-Ar dating of hydrothermalbiotites and sericites associatedwith hypogene mineralization. Raw data for these and other age date samplesfrom La Escondidawill be detailed by A!pers and others in subsequentpublications.A, B, and C represent summits of principal hills in the district (see Fig. 17A). Dash-dot lines represent principal fault and fracture 1^•;-2-• ' • • '/•• PRESENT' • SURFACE zones. a local topographicdepression(or playa) located 1 to 3 km south and east of the map area shown in Figure 17A and B. This essentiallyfiat water table indicatesa nearlystatichydrologicsystem,consistent with the present hyper-arid climate in the Atacama desert. Present precipitation rates at La Escondida FIC. 17. A. Plan view of portion of the La Escondidadistrict are estimatedto be approximately0.5 cm/yr (Fuen- showingpresentsurfacetopographyin metersabovesealevel; zalida, 1967). contour interval = 40 m. Dots show locations of collars of 112 In Figure 17B, the shadedareas indicate where diamond drill holes considered as individual control volumes for the top surfaceat the enriched zone is above the mass balance calculations discussed in text. The summits of hills B, and C are shownby open trianglesin this and subsequent present water tables. In general, capillary fringe A, figuresfor reference.B. Elevationof top of enrichmentblanket effects cause saturated conditions to be maintained to heightsabovethe presentwater table, depending on pore-sizedistribution(Daviesand deWest, 1966). Therefore, at La Escondida,2,988 m representsa minimumelevationfor the presenttop of saturated conditions.The shadedareasin Figure 17B represent maximum possible areas over which sulfides are (ETB); contour interval = 40 m. Based on grade profiles for individual drill holes as well as geologic logs. Shaded areas representzoneswhere sulfidesare preservedabovethe present water table (2,988_ 3 m above sea level) and oxidation is presumably active. The oxide-sulfideinterface is submerged throughout most of the district. Contours for this and all subsequent figuresgeneratedby ISM softwarepackage,Dynamic Graphics,2855 TelegraphAvenue,Berkeley,California94705. MASSBALANCE PRINCIPLESAPPLIED TO ORE-FORMING PROCESSES mineralization, often discoveredat considerabledis- tancesfrom the oxidizingsulfidesourceareas.Most notably,the Exoticadeposit(Mina Sur), 2.5 to 6.5 km southof the main pit at Chuquicamata,hasbeen shownto havebeen derivedfrom the main deposit by lateral flow of copper-bearingground waters whichfollowedpalcoriverchannels(Newberg,1967; Mortimer et al., 1977). The genesisof other exotic copper depositsin northernChile (e.g., E1 Tesoro, Sagasea)and in Arizona (e.g., EmeraldIsle, Jerome) is lesswell documented.In the next portion of this paper, we present the results of massbalance calculationsin an attempt to quantifythe magnitude, direction, and distanceof lateral copper transport at La Escondida,in terms of both exotic copper oxide mineralizationand coppersulfidemineralization within the presentenrichmentblanket. Resultsof SupergeneMassBalanceCalculations at La Escondida 1245 will be presentedhere. Future field and laboratory work concerningLa Escondidawill provide additionalgeologicconstraints whichmay allow for reasonableextrapolationof observedprotore metalgradetrendsand calculationof morerealisticgrade projections. Model 1: No lateral flux, vertically homogeneous protore metal grade For each drill hole considered, a total leached zone thickness(I-Pr)was calculatedto accountfor the amountof copperin the enrichedzonein excess of the averageprotoremetal gradefor that hole, assuming no lateralfluxes.Valuesfor I-Prwerecalculated for each drill hole using equation(3) and constantaveragevaluesfor the variables1,b, p, ol,,andovfor eachhole(model1). A contoured plot of the resultingIPrvaluesis shownin Figure18A. This no-flux total leached-zone thickness for each hole is added to the elevation at the top of the surface, In general,there are three possibleinterpretations enrichedzoneto yield a valuefor the SI_Pr for topographyon calculatedI_• surfaces:(1) the contouredin Figure 18B.The simplestinterpretation calculatedtopographyis real, i.e., all assumptions of the topographyin Figure 18B is that the two are more or less valid, (2) the assumptionof no major assumptions made to this point (no lateral lateral fluxis incorrectandthe apparenttopography fluxesand vertically constantprotore metal grade) representssource and sink regions, or (3) the as- are approximatelycorrect and that the calculated sumedprotoremetalgradesare incorrect,resulting topographyrepresentsthe actual elevationof hyin spurioustopography.Any combinationof these pogenecoppersulfides beforethe onsetof significant three factors could account for an anomalousI-Pr oxidationand supergeneleaching.However, this value for a givendrill hole. By holdingconstantthe surfacerepresentsa minimum elevation for some alternatepairsof thesethree factors,it is possible drill holes for the reason discussedabove, i.e., that to evaluatethe magnitudeof eachfactor as an end- copperaddedto the weakly enrichedquasi-protore memberinterpretation.The resultsfor La Escondida zone near the bottoms of these holes is not considwill be discussed in termsof eachof thesepossibil- ered part of the enrichmentblanket. ities by itself, followedby a discussion of the most The surfacein Figure 18B exhibitsgentletopogreasonableoverall interpretationfor differentpor- raphy over much of the mineralized area at an tions of the district. averageelevation of roughly 3,300 m. The major Thesesupergenemassbalancecalculations account exceptionto this low relief is an anomalouslyhigh for lateral variationsin protore metal grade by consideringeach drill hole as a separatecontrol volume.Thus,the protoremetal-gradeestimatefor each hole is based on assaysat depth for that O•ID•TION SURFACE' individual drill hole. In the case of inclined drill holes,the northingand castingcoordinatesof the midpoint of the enrichmentblanket are used as a point of projection. Forty-nine of the 112 holes consideredin this study (44%) were collared as inclinedholes,plungingtypically50ø to 70ø from horizontal.The distanceof projectionis typically lessthan 200 m, which introducesuncertaintiesless than thoseinherentin projectingconstantprotore metalgradesverticallyfor up to 1 km, asis necessary for model 1 calculations. Regressions of protore metal grade vs. depth for individualdrill holesare FIc. 18. A. CalculatedI-•r; contour interval = 200 m. Data not well enough constrainedat La Escondidato contouredat northingandcastingcoordinates of blanketmidpoint presentconsistentlymeaningfulresultsfor models2 for each drill hole. B. CalculatedSI-•r(=L• + ETB) for each and3. Therefore,resultsof onlymodeli calculations drill hole; contour interval = 100 m. 1246 BRIMHALL, ALPERS, AND CUNNINGHAM zone center in the right center of the map area, whichcorresponds to the thickestandrichestportion of the enrichmentblanket.CalculatedSLUr elevations in this anomalous zone are greaterthan 3,800 m for sevendrill holesin this area andgreaterthan 3,400 m for ten others.An additionalhigh portion of the ceivablybe explainedentirely by locallateralfluxes of copperin the supergeneenvironment.To evaluate the lateral flux interpretationwe solvethe complete massbalanceexpression(eq. 1) for the flux term, yielding equation(12). Given that one mustassume a value for • to evaluate lateral flux, two different SLPr(surface)is presentin the northwestern part of casesare tested:(1) no erosionof mineralizedrock, the district, where four holes show calculated ele- i.e., • -- L½, where L½is the presentthicknessof vationsgreater than 3,400 m. The anomalouslylow the leachedzone,and (2) leachingfrom the average regionpresentin the southwest portionof the map elevationof the LPrsurface(3,300 m) to the present area is discussed below. top of enrichment,i.e., I_q-= 3,300 - ETB. Contourmapsof calculatedlateral copperflux (in The calculatedLPrsurface,SLPr,in Figure 18B can be interpretedby comparingit to the present g/cm2) for thesetwo casesare shownin Figure19A surfacetopography(Fig. 17A) and the topography andB, respectively.Negativelateralfluxcorresponds of the top of the enrichmentblanket (Fig. 17B). to possiblesourceregions,and positivelateral flux The high areas on the surface,particularly the to possiblesupergenesinks.As would be expected southeastzone, are 400 to 1,000 m higher than the presentland surface,indicatingthat at least this much erosion of mineralized and leached rock must LAT.FLUXo have taken place if the assumptions of constant protoremetal gradeand no lateral flux are valid. The low areason the surfaceare locally below the present topographicsurface, indicating apparent negativeerosion,a geologicalimpossibilitywhich indicatesthat at leastone of the two major assumptions is not valid for these areas.These areas(e.g., the area near hill B) are siteswhere the amountof copperpresumablyleachedfrom the presentthickness of leached capping does not appear in the subjacentenrichedzone.To evaluatewhichof the basicassumptions is more likely to be incorrectin these cases,we have considereddata concerning relict sulfidestrappedin quartz in surfacesamples (A. Aguilar, unpub. data). The area near hill B happensto contain pyrite but no copper sulfide relicts in rhyoliticporphyrywith advancedargillic alteration,suggesting that the originalprotoredistributionwasinhomogeneous in a verticalsensedue either to the change in rock type or hypogene leachingeffects. Several holes in the southwestpart of the map areain Figure18B showSLPr valuesfor the surface at elevationsactuallybelowthoseof the top of the enrichmentblanket. Of course,this alsorepresents a physicalimpossibility andagainindicatesthat one or both of the major assumptions mustbe incorrect. These holes are anomalousin that their average leached-zonemetal gradesare greater than their averageprotoregrades(i.e., 1> b), whichrepresents evidenceeither of strong vertical inhomogeneities in protoremetal-grade distribution or positivelateral copperflux now manifestedascopperoxides. •'•C. 19. A. Calculated lateral flux of copper, assumingno erosion, based on model I calculations; contour interval = 100 g per cmz. Arrowsindicatepossibledirectionsand distances of implied subsurfacetransport of copper by ground water. B. SameasA, but assuming leachingfrom a fiat surfaceat 3,300m elevation, the approximateaverage value for the surface, SI-Prin Figure 18B. C. p•ø,•c/p for model 1; contourinterval = 0.25 wt percent.Representsvariationin protore metal grade relative to observedprotore metal grade necessaryto explain Model 1: Lateralflux interpretationusingassumed presentdistributionof copper, assumingno lateral flux and no testvaluesfor LT The anomalous topographyin certainareasof the leaching of rocks other than preserved leached capping (Lc calculatedsurface,SI_Pr, in Figure 18B couldcon- elevation; contour interval = 0.25 wt percent. = I-Pt;seetext and Table 1). D. p•00/p for model 1, assuming no lateral flux and leaching from a fiat surface at 3,300-m MASS BALANCE PRINCIPLES APPLIED TO ORE-FORMING PROCESSES 1247 for a closedchemicalsystem,the contourfor zero lateral flux in Figure 19B coincidescloselywith the contourfor SL9r= 3,300 m in Figure 18B. Possible directions andmagnitudes of lateralfluxesareshown schematically with arrows.Despitebeingspeculative to evaluatepotential vertical protore metal-grade in nature, these directions are consistentboth with the case of no erosion of mineralized rock, where variation. As in the evaluation of lateral flux, two casesare evaluated in terms of finding p•½ for assumedvaluesof IPr for eachhole. This methodof solutionis shownschematicallyin Figure 20A for o is the grade which allowsthe excesscopper district-widefractureorientationsand with present p½,•c o stippledareain Fig. 20) to equal topographicgradients.Two-dimensional polygons area(B(b - Pca•½); were constructed to calculate an area of influence the leachedcopperarea (LPr(p•ø• - 1); diagonally for each of the 112 drill holes, allowing the flux ruled minusverticallyruled area),the densityterms surfacesto be integratedby a finite elementapprox- are left out for simplicity.Similarly, in Figure 20B, p3 300 imation to evaluate the overall net flux within the •.• is the protore metal grade which equalizes regionconsidered. The integratedsum,or net lateral the amounts(areas)of excesscopper and leached flux, in the no-erosioncase (Fig. 19A) is a large copperfor the arbitraryassumption of erosionfrom positivenumber,which indicatesthat we havepos- 3,300 m. Thus,a givenlow anomalyon the surface, tulated too little leached material for the gradesas SLPr,couldbe explainedeither in termsof a negative stated. Because this result contradicts the fundamenlateralflux (sourceregion)or alternativelyby having lessthanp, i.e., havinga lower originalprotore tal principle of massconservation,at least within Pca•½ the boundariesof the systemas defined, we must metalgradein the leachedandenrichedzonesthan therefore reject the assumptionof no erosionof is presentlyobservedat depth. leachedprotore for the simplemodel 1 case.In the Figure 19C and D showcontourmapsof P•a•dP secondcase,leachingfrom 3,300 m, the net lateral flux balanceis a relatively small negativenumber, which is much more realistic.One would expectthe (A) : net lateralfluxto be zero only for a closedchemical systemin whichno copperwaslostat the boundaries, definedas the map area shownand the bottom of the blanket. To confirmthat the computedlateral fluxesrepresentreal phenomenawe must consider an additionalpossiblecontributionto anomalieson SlPrsurfaces. i I..... ,• Model 1: Evaluationof vertical protore metalgrade variation Elevation (z) An alternativeto the abovelateral flux interpretation for anomaloustopographyon the calculated surface,SL9ris to considererrorsin the projected values of protore metal grade. Errors in protore metal-gradepredictionmay be divided into three kinds: (1) errors in estimatingaveragevalues, (2) errors due to gradual variation in protore metal grade with depth due to primary sulfide-zoning features, and (3) errors due to abrupt changesin gradedue to rock-typecontactswhichcrossa control volumeof interest.To evaluatein a generalway the effectsof the possibleerrorsin protoremetal-grade estimation,equation(2) is invertedto solvefor the Elevation quantity pca•c,the calculatedprotore metal grade FIG. 20. A. Schematicgrade profile showingapproximate necessary to accountfor the presentdistributionof o Once an arbitrary original surface ore metal in each hole, given an assumedvalue for graphicalsolutionfor pc•c. EBB bobB+ lp•LPr Pca• = Op(B +LPr) ETB elevationis chosen(in this case,the presentsurface),pc• is the calculated protore metal grade which would equate excess copper and leached copper terms with no lateral flux. Actual areas as shown would not be equal due to modificationby densityterms (see eq. 14). B. Same as A but with a different assumptionfor original surfaceelevation,e.g., 3,300 m. Note (14) Note that the superscript"0" is retained for the IPr term becausewe need to assumeno lateral flux that in this casep•3;•oo < p whereasp•a;•oo > p in A for the arbitrary grade profile selected, but this is by no means a generalrelationship. 1248 BRIMHALL,ALPERS,AND CUNNINGHAM and p•;•00/p,respectively.Not surprisingly,the dence that the thickest and highest metal-grade overall geometricpatterns of anomaliesare quite similarto the lateral flux interpretationsin Figure 19A and B. To explainthe presentdistributionof copperwith no lateralfluxandno erosionof leached protore,averageprotoremetal gradesin the blanket portion of the enrichment blanket is in this zone. Convergenceof deep lateral ground-waterflow into this zone may havebeen influencedby a numberof physicalrock properties.As previouslymentioned, the indicateddirectionsof lateral flux (Fig. 19B) are and leached zones would have to have been at least consistent with dominantmappedtrendsof fracturing 2.5 times higher than the averagegradesobserved (Fig. 16). Furthermore, the geometry of resistant, at depthfor three separateareasin Figure 19C, and silicifiedareas (hills A, B, and C) may have been at least 1.5 times higher for a large portion of the responsiblefor controllingthe geometryof the pamaparea.The lowestvaluesof P•aic/P in Figure19C leoground-watertable in such a way as to focus are from areaswhere protore metal gradesof less subsurface flow into this zone. In principle, one could help to verify the lateral than half of that observedat depth would be necessaryto explainthe presentdistributionof copper flux interpretationfor this zone if it couldbe shown with these assumptions.For erosionfrom 3,300 m rigorouslythat averageprimary metal gradeswere (Fig. 19D), the areasof p•.•00/plessthan 0.5 are lessthan p•lc over the total leached and enriched very similar to the no erosion case; however, the interval. Detailed mappingand loggingof limonite areasgreater than 1.5 are substantiallysmallerbe- mineralogy, texture, and abundancehas yielded causeof the generallythicker hypotheticalleached constraintswe can useto showthat averageprotore in the high-grade column.It is possiblethat our estimatesof protore metalgradeswere lessthanp•3;•00 metal gradeare in error by as much as 50 percent, southeastern zone. Using quantitative mapping or perhapseven more in somegeologicallycomplex methods to calculate preleaching (i.e., enriched) areas(e.g., brecciasor zonesof lithologiccontact). coppergrades,asoutlinedby Loghry (1972, p. 97), One test of the lateral flux interpretation is to estimatesof former maximumblanket metal grades consider observed trends in protore metal grade were made throughoutthe leachedcappingzone, with depth,particularlyin areasof anomalous SI_Pr. in surfaceexposuresas well as downholeprofiles. of thesemapping Drill holesat La Escondidawhich penetratesuf- Spacedoesnot permit discussion ficient protore to permit well-constrainedlinear techniqueshere (AlpersandBrimhall,in prep.). The least-squares regressions (p = a0 q- aiz, where z is resultsfrom this mappingin the high-gradesouthelevation)generallyshowthat protore metal grades eastern zone show that estimated former maximum vary only slightlyover verticaldistancesof several blanketmetal gradesat the presentsurfaceare less hundred meters. The sign of the slope of these than p•.•00in this area. Given that there are no regressions,ai, is a useful parameterin assessingknownexceptionsto the observationthat b > p, we the validity of lateralflux interpretations.A negative concludethat Loghry'smappingprovidesadditional slope (al < 0) indicatesthat gradesare increasing evidencefor the lateral flux hypothesis. However, it mustbe emphasizedthat due to the with depth in the protore zone. There can be little doubt that this representsa real hypogenetrend complicatednature of rock-type distributionand zone(Fig. due either to primarysulfidezoningor to late, high- brecciationin the high-gradesoutheastern level hypogene leaching. On the other hand, a 16), it is perhapsnot unreasonableto imaginethat positivea1 is more difficult to interpret becauseit the upper portions of these brecciaswere well could representeither a real protore trend or evi- mineralized and could conceivablyaccountfor the dence of incipient enrichment, i.e., inappropriate abundantsupergenecopper in this zone by domiselection of the enrichment blanket base elevation nantly vertical supergenetransport.The fact that for that hole. This ambiguity could be resolved the brecciazonesin the presentorebodyare geneasily with accurate quantitative mineralogic data erally of lower grade than the surroundingquartz for sulfides near the enrichment base for these holes. diorite porphyriesand andesitesmay lead one away An analysisof al valuesindicatesthat predominantly from thiskind of speculation; however,it is possible with late negativeslopesoccurin the regionof highestpositive that hypogeneleachingeffectsassociated lateral flux (tip of large arrow, Fig. 19A and B). brecciation and advanced argillic alteration could copperandothermetalsat higher These negativeslopesare inconsistentwith Pealalp haveconcentrated values greater than 1, which are also observedin levels of the system (e.g., Butte; Brimhall, 1980) this region (Fig. 19C and D). Thus, the errors in whichare now erodedat La Escondida.Also,lacking protoremetal-gradeprojectionimpliedby the p•a•/ p surfacesare not supportedby the observedgradientsin protore grade.Therefore,we feel that the indicatedpositivelateral flux into this region is the most logicalinterpretation.It is perhapsno coinci- geomorphological controlon the I-Prsurface,suchas a regional erosionalsurface(e.g., Mortimer, 1973), we have no basison which to reject all large values of I-Pr,as it is well known that porphyrycopper depositscanbe mineralizedover substantialvertical MASS BALANCE PRINCIPLES APPLIED TO ORE-FORMING PROCESSES intervals(Lowell and Guilbert, 1970). Nevertheless, it is the evidencefrom anomalouslylow valuesof the surface,Slfr, which tells us that at least one of our originaltwo assumptions wasincorrectand that lateral flux and/orverticalprotoremetal-gradevariation must,in fact, havebeen importantfor at least some areas at La Escondida. Leachingej•iciency One way to expresszone propertiesis the parameter leachingefficiency(EL),definedabovein equation (8) asthe percentof originalcopperremoved. Of course, this presumesknowledge of protore metalgradein the zonein question.To be consistent with othermodelI calculations, averagemetalgrades for the leached and protore zoneswere used to calculatethe leachingefficiencyfor eachof the 112 Ea Escondida drill holes, and the results are shown in a contourplot in Figure 21A. Note the extensive zone with EL > 96 percent, an area which includes a largeportionof the hills markedA, B, and C. This extensive area, which has been referred to as "su- 1249 copperprovince.Evidencefromlimonitemineralogy and texturesand the presenceof abundant(hypogene?)alunite and pyrophylliteindicatethe likelihoodof hypogeneor "hydrothermal"leachinghaving playedan importantrole (J. H. Courtright,pers. commun.,1983). The distribution of the low-grade capping (generallylessthan 0.02% Cu) corresponds in part to the aerial distributionof the rhyolitie porphyry, which supportsthe hypothesisthat this unit representsa postmineralor late-stagedike-sillwhich has truncatedthe hypogenedepositlaterally as well as verticallyin the easternandnorthernzones,respectively.Of course,anyverticalinhomogeneity of rock type of this kind would require modificationof the initialassumption of verticallyconstant protoremetal grades,which wasthe basisfor the model 1 calculations.If thishypothesis is correct,oxidationof the pyritie rhyolitie porphyrymay havebeen an importantsourceof sulfurieacidfor the supergene leaching of adjacentunits. The presenceof relict copper sulfidesin otherwisecompletelyleached surface samplesfrom hill A suggests that perhapstwo-stage (hypogene andsupergene) leachingtookplacethere, with certainsulfidegrainseffectivelyprotectedfrom both.leachingeventsdue to encapsulation in quartz. perleached"(D. Lowell, pers.commun.,1983), representsmostof the area of outcropin the district, which undoubtedlyhelpedto contributeto the fact that the La Escondidaprospectwasnot drilled until 1981 despiteits relativeaccessibility and favorable geologicsetting in a known high-gradeporphyry Alteration mineralogy Alterationmineralogyof the leachedcappingzone has been determinedusingstandardpetrographic A ALTERATION and X-ray diffractiontechniques for approximately ' ' OF Lc 500 samplesfrom 36 diamond drill holes, plus severalsurfacesamples.As shownin Figure 2lB, hillsA, B, andC are dominatedby aluniticcappings, whereaspyrophyllite, kaolinitc, sericite, and lesser alunite make up the cappingthroughoutmuch of the remainderof the map.area.Potassicand propylitic assemblages dominate the southwesternand extremenorthernportionsof the map area. The differentialability of these assemblages to ß neutralize supergeneacids is a critical factor in determiningtheir leachingbehavioraswell astheir . ability to fix copperin the reducingzone (Locke, 1926). Strong neutralizersor "reactive gangue" FIG. 21. A. Leachingefficiency(EL)from model1 calculations; (Blanchard,1968) will lead to retention of copper contourinterval = 4 percent;bold contourinterval = 20 percent. Data for several anomalousdrill holes (EL< 0%) located in in the leached zone, usually as tenoritc, cuprite, southwesternzone not included in computer-generatedcon- native copper, etc. (e.g., Kwong et al., 1982). In touring. B. Alteration mineralogyof leachedcappingzone at La addition, a silicate .alteration assemblagewhich Eseondidabasedon surfacemappingandsemiquantitative inter- readily neutralizesacids by consuminghydrogen pretation of X-ray diffractiondata for more than 500 samples ions in hydrolysisreactionscould also serve to fix from drill core and surfaceoutcrops.1 = K silieate/serieitie: copper introducedto a particular zone by lateral abundant biotite, ehlorite, plagioelase,and/or serieite, with minor K-feldspar, gypsum,calcite, anhydrite, and little to no flux. Under certain conditions,this may lead to kaolinitc;2 = serieitie/argillie:abundantserieiteand kaolinitc- formationof chrysocolla (Newberg,1967). Biotite group minerals, with minor pyrophy!!ite and alunite; 3 = ad- is a particularlyeffectiveacidneutralizer,andthereo vancedargi!lie:abundantaluniteand pyrophy!!ite,with diaspore, fore the K silicatealterationin the wall-rockandesitc, kaolinRe-groupminerals,and/or minor serieite. Dots indicate groupsof five to twentycompositesamplesfrom individualdrill which hasbeen extensivelybiotitized, is characterholesor groupsof one to five surfacesamples. ized by incompleteleachingand a low leaching 1250 BRIMHALL,ALPERS,AND CUNNINGHAM efficiency.Note that severalanomalousdrill holes Loghry,1972). Jarositeveiningis commonthroughzone,andsupergene with a highly negativeleachingefficiencywere not out thisextensivesuperleached includedin the contouringof Figure 21A. These jarositecommonlyreplaceshypogene(?)alunite in holesaremostlylocatedin the southwestern portibn former feldsparsites.Leached-cappingstudiescurof the map area and contain 1 greater than p, rently in progress(Alpersand Brimhall,in prep.) resultingin the apparentnegativeleachingmafii- include detailed mappingof limonite mineralogy, festedin !-Ptlessthan0 (seeFig. 18A andB). This textures, and modes of occurrence in addition to anomalous result can be attributed to one of several possibilities: (1) lateralfluxcouldhaveaddedcopper to the oxidized zone, (2) inhomogeneous protore metal-gradedistributioncould have led to an underestimation of p for that zone,or (3) the relatively high-gradeoxide mineralizationcould representa former chalcociteblanket which is now completely massbalances for sulfur and iron. Interrelationsof metalgradesand thicknesses of leachedand enriched zones The interrelationship of metalgrades,densities, and thicknesses of the variouszonesas expressed in equation(5) providesa basisfor groupingthe 112 oxidizedbut onlypartiallyremobilized. This final drill holesconsideredin this studyinto spatialdopossibilityonly could lead to a situationwhere. mains.Figure 22 is a plot 'of the 112 holesusing for averageleached-zonemetal grades,1, are greater the sameaxesas Figure4. The valueof I_Pr[B thanthe protoremetalgrade,p•,if the leachedzone eachdrill holeisthe slopefromthe y-intercept itselfwere heterogeneous in gradedistribution(per- /•0 (not shownin Fig. 22) to the point plotted. The hapsdue to verticallyheterogeneous hypogeneal- four shadeddomainsin Figure22 are distinguished in calculated I_Pr/B, asnoted teration or rock-type distribution)and the lower primarilyby differences gradeportionof the cappinghad been eroded (see in the figure caption and the key to Figure 23. ComparingFigure 22 to the schematicFigure 4, sectionon two-cycleleaching,below). There is no texturalor mineralogicalevidencein oneseesthat domainH, with I-Pr/Bbetween1.5 and the southwestern zone to suggestthat the oxide copperthereis an oxidationproductor replacement of chalcocite, exceptperhapsjust abovethe present top of .theenrichmentblanket.Most of the copper in the oxidized zone in that area is present as chrysocolla andbrochantitewhichprobablyformed due to lateral flux into the presentleachedzone either on a large scale(derivedfrom hill A?) or on a smallscaledue to locallyhigh-gradehypogene veinswhich maybe lessabundantat depth in that area (J. Bratt, pers. commun.,1983). In the easternportionof the studyarea:the zone of reducedleachinge•ciency is probablydue to partialdestruction of a formerhigh-grade chalcocite zone. Leaching efficienciesare low in this zone SINK DOMAINS SULFIDE OXIDE '1 ß ** •C_• * .•* • = '/:/FLUX ./DOMAIN ß • . • d **• ß SOURCE I• • • DOMAIN • '' • ' o (locallylessthan60%) but are nowherenegativeas in the southwestern area.Copperoxidemineralogy in thiszoneincludesbrochantite,antleritc,atacamite, FIG. 22. Orthogonalplot of blanketmetal grade(b) vs. (p and pseudomalachite (Cus(PO4)2(OH)4'H20); the -!), modified by densi.ty terms,asin Figure4, showing data phosphorus wasprobablyderivedfrom the super- for 112 La Eseondida drill holes. Four domains are identified gene acid destructionof apatite.The incomplete on the basisof valuesof I-•r/B,the slopewhich eormeetseach asplotted,with a uniqueintercept,ppffp•(notshown) leachingin thiszoneis probablydueto its relatively point, on the y-axis.Symbols andshadingare explainedin Figure23. low pyrite content. The remainder of the leached zone is dominated For domainI, I•r/B is lessthan or equalto 1.5; for domainH L•/B is greaterthanor equalto 1.5 andlessthanor equalto by kaolinitc,sericite,andlocalalunite-pyrophyllite-2.5; domainHI, L•/B is greaterthan2.5 andlessthan10; and domain IV, I•r]Bisnegative orgreater than10.Foursubdomains diasporealteration.This nonreactive gangueis ex- within domainHI are basedon spatialdistribution,asshownin tremelywell leached,asmanifested by the extensive Figure 23. Interpretationin terms of lateral fluxesof copper area with a leachingefficiencygreater than 96 (open-systemevolution;see Fig. 4) indicatesthat domain I percent.One findspredominantly exoticasopposed representssourceareasand domainsHI and IV representsink to indigenouslimonitesin this zone due to original areas.Domain HI sinksare manifestedprincipallyby sulfide in the enriched zone, and the domain IV sink, by or pyrite/chalcocite ratiosgreaterthan 2 to 1 and copper abundant copperoxides,wherelt• is greaterthanor approxithe inabilityof the ganguemineralsto neutralize matelyequalto ppp.DomainH represents areasof little lateral acidsproducedby pyriteoxidation(Blanchard, 1968; copper'flux(closed-system trend). MASS BALANCE PRINCIPLES APPLIED TO ORE-FORMING PROCESSES . ß: ',. LT/ • ß 1251 holesat the easternmarginof the map area,which have relativelyhigh LPr/Bvalues(greaterthan or equalto 3.5) andrelativelylowprotoremetalgrades. The rock type for this subdomainis the rhyolitie porphyry,whichhostsa thin enrichedzoneappar- entlycreatedby positivelateralfluxof copperfrom ß the west and/or northwest. DomainIV is manifestedprimarilyin the southwesternportion of the study area where biotite- alteredandesitchostsabundant ehrysocolla andbrochantitc in the oxide zone, to the extent that several [•]I ß J-J•] "m'A 'Jr /'"'• lrT,'O,o.o / 0 4 • * o• k .'-• • ß 1.0KM ß • -- holesexhibitI greaterthanp'andcalculatedvalues for lfr < 0. The enrichment blanketin thiszoneis relativelythin but is locallyhighgrade.In addition to the highlyreactivenature of the biotitie assemblagetOacidicwaters,the thin blanketin thiszone is probablydueto relativelylowpermeabilitywithin this rock and alterationtype. DomainIV probably represents areasof positivelateralfluxintotheoxide FIG. 23. Plan view of spatialdomainsbasedon contoured zone, or alternatively, areas of extreme vertical valuesof l.•r/B,showingindividualdrill holesplottedat northing protore-grade variation.Detailedmappingandcore and e.astingcoordinatesof blanket midpoints.Subdomains of domain HI are shownin central (A), southern(B), northwestern (C), and eastern (D) portions of map area. Arrows indicate possibledirectionsand distanceof implied subsurfacecopper loggingin theseareaswill makeit possible to make a reasonablechoicebetween thesecompetinginter- pretations. transport, basedon Figure19B.Solidarrowsdenotetransport to sulfidesinks;open arrow denotestransportto oxide sink (see Fig. 22 and text). Summaryof resultsfrom La Escondida At La Escondida,we are_facedwith the problem 2.5,corresponds besttotheclosed-system linein ofcharacterizing thesupergene-enriehment. process Figure 4. DomainI representsa copperdeficiency, which can be interpreted either as a zone with negativelateral flux (sourceregion) or as a zone where protoremetalgradeswere overestimated. In contrast, domains !IIA, B, C, and D include holes at a large,newlydiscovered porphyrycopperdistrict with abundantavailableassaydata but relatively little geologiccontrol.The resultsof the massbalance calculationspresentedherein representonly a first attemptat understanding how supergeneprocesses have affectedthis districtand producedone of the world'slargestcopperdeposits.By lookingat the present distributionof a single element (copper) and makingvariousassumptions regardingits initial distribution,we havebeen able to identifydomains withinthe districtwith e0nsistent anddisriflerprop- with a coppersurplus,interpretableeither as areas of positive lateral flux (sink regions)or as areas whereprotoregradeswereunderestimated. Domain IH is divided into .four subdomains (A-D), correspondingto the spatialdistributionof the drill holes, which canbe seenin Figure 23. Subdomain !IIA represents the highestgradeand erties which can be related to both chemical and thickestportion of the enrichmentblanket as well physicalparameters. Supergenemassbalancecalculationsfor La Esas the zone of highestprotore copper grades.The aeri.aldistributionof domainIHA corresponds well eondida,assumingno lateral flux and vertically hoto that of the southeastern zone of intrusions and mogeneous protore metal grades(model 1), yield regular, breccias(see Fig. 16). As discussed above,the an oxidationsurfacewhichis predominantly apparentsurplusof copperin thiszonecanperhaps at an averageelevationof roughly3,300 m above bestbe explainedby a largepositivelateralfluxinto sea level. This is our preliminaryestimatefor the thiszone,althoughanextremelythicktotalleached- original averagesurfaceelevationat the onsetof zone column(e.g., from a brecciapipe) cannotbe significantsupergeneprocesseswithout respectto ruled out. Subdomains IHB and C are located at the anyregionalpostsupergene tectoniceffects.In realextreme southernand northern parts of the map ity, this surfacewould probablyhave been gently area, respectively, and are predominantlyin inclined,perhapsdippingasmuchas 15ø southeast; K silicate-chlorite-sericite-alteredandesitc wall rock. thisconfiguration wouldprovidea surfacecompatible These areasalso have apparent surpluscopper, as with observations of relict sulfides near the summits manifested by valuesof LPr/Bgreaterthanor equal of hills A and B in the district as well as inferred to 2.5. Subdomain IHD is represented by threedrill lateral flux vectorsfor ground-watertransport. 1252 BRIMHALL, ALPERS, AND CUNNINGHAM Local anomalies on the calculated oxidation surface are presumedto be due either to lateral fluxesof copper in the supergeneenvironment,as indicated by arrows in Figure 19A and B, or to errors in prediction of protore metal grades, as shown in Figure 19C and D. The lateral flux interpretation appearsmostlogicalfor the highestgradeandthickest portion of the enrichmentblanket (subdomain IIIA) as well as an important zone of oxide mineralization(domainIV), a conclusionreachedthrough considerationof possibleerrors in protore metalgrade estimationconstrainedby detailed mapping andobservedgradientsin protoremetalgradeswith depth. More often than not, the detailedstudyof a newly discovered district such as La Escondida results in the generationof many unansweredquestionsand stimulatingdirections for further research. Some In this expression, we see that the initial statefor the secondcycleof blanketenrichmentis essentially equalto the protoremetal gradeasbefore (term 1) but has an additional metal contributiongiven by term (2), the excessmetal in the first cycle of blanket enrichment.Also there is a coefficient,B"/ B• which is the thickness ratio of the two blankets. In term (3), LDisthe verticaldistanceof the groundwater table drop in the leachedzone. Note that the ratio LD/B• is analogous to LPr/Bin model 1 for single-cycleenrichment.Term (4) is a measureof the extentof the leachingprocess; the morecomplete it is, the smallerthe valueof 1• and the greaterthe value of term (4). The rate of increasein b• is givenby term (3). In other words, a more profound ground-watertable drop producesa steeperrate at which the secondcycle blanket metal grade increasesalongwith the extent of leachingof the overlyingcompositezones (first-cycleof enrichmentblanket, interveningprotore, and originalleachedzone). From this analysis, we see that the ratio L•/B • for a secondcycle of other questionswhich shouldbe addressedin future work at La Escondidainclude: (1) what was the extent of hypogeneleachingand was it controlled by the intrusionof the rhyoliticporphyry,(2) how homogeneouswas the protore mineralizationand enrichment is exactlyanalogous to LøT/B for a single with what confidencecan it be extrapolatedverti- oxidationevent.In thiscontext,LPrmaybe thought cally, (3) is the supergeneprocesscontinuingin the of as simplya vertical distancecorresponding to a presentclimateand what wasthe total flux of water necessaryto carry out the observedmasstransfer, and (4) at what scaleis the district an open vs. a closedsystem? Progresstoward a Dynamic Model Two-cycleenrichment change in the position of the ground-watertable froman initial state.We maythen think of a dynamic supergenesystemas being the result of an infinite numberof smalldropsin the ground-watertable in an overall state of descent.There may be minor up and down fluctuations,but only the net descent intervalscausemetal transportin this continuous process. Perchedenrichmentblanketsthat are presently in a state of oxidation are not uncommon. Sudden Domainconstancy of L•/B drops in the ground-watertable due to uplift or Within individualgeographicor geologicdomains, tiltinghavebeenproposedto explainsuchphenomthere is goodevidencethat the ratio LPr/Bis now a ena or their limoniticequivalents,for example,at constant and we have postulatedthat this ratio has Toquepala,Peru (Richard and Courtright, 1958; Anderson,1982). In order to understandthe controls essentiallyremained constant during geochemical on two-cycleenrichmentphenomenawe have de- evolutionof the compositeleachingand enrichment rived the expression for blanketmetal grade,b, in system.That is, as the total distanceof the leached terms of mass balance variables. The first and second cycles of enrichment are referred to as a and •, respectively.Equation(15) is analogous to equation (5), givingthe second-cycle blanketmetalgrade,b•, in terms of a y-intercept(terms 1 plus 2), and a zone increased, the thickness of the enrichment blanket increasedproportionallyso that the ratio LPr/Bremainedconstant. Accordingto equation(5), the enrichmentblanketmetal grade,b, will increase linearlyin a closedsystemfrom protoremetal grade, slope(term 3) timesan independentvariable(term p, to a maximumvalue when leaching is nearly complete.The slopeof the b evolutionline is LPr/B; 4): the greater this slope, the steeper the increasein the value of b. We have just seen in the case of two-cycleenrichmentthat an analogous functionfor second-cycle blanketmetalgradeexists.In thiscase, b•=p?___•p (B") (b%og - p•Op) (3) + (4) oi (15) the slopeof b• dependson the water table drop distance,L•, dividedby the thickness of the secondcycle blanket, B•. We may envisionan evolving oxidation-enrichment systemas an infinite number MASS BALANCE PRINCIPLES APPLIED TO ORE-FORMING PROCESSES of small dropsin the water table, includingannual seasonalfluctuationsoccurringas up and down oscillationson a generallydescendingtrend in time. By thismechanism, copperliberationandenrichment would only occur during times of descentof the ground-watertable. Once a stable ground-water table were established,any perchedblanketswould slowlybe oxidizedand solubleelementstransported downwardinto the underlyingzone of reduction within the deepestenrichmentblanket. There is evidencewhich suggeststhat the hydrologic domain which is most characteristicof the blanketinterval is the capillaryfringe zonebetween the unsaturatedzone (porespartially filled with air) and the actual water table, below which the pores are filled with water. The height of this capillary fringe zone is inverselyproportionalto pore radius (Davies and DeWest, 1966). For example, a 0.5-t• pore radiuswould resultin a capillaryrise of 30 m, accordingto a simpletube calculation.It is likely that the average pore size decreaseswith time Evolutionof supergenesystemsin time Massbalancerelationshipsderived above to describethe present state of metal distributionmay be extendedin a simplifiedfashionto include variation with elapsedtime since initiation of chemical weathering. As stated above, we assumethat the top of the capillaryfringe correspondsroughly to the top of enrichmentand the palcoground-water table to the base of enrichment. We include in our formulation a function to expressthe elevation of the ground-watertable through time. W0 is the initial elevation of the ground-watertable at the start of oxidation and dW/dt is the time rate of changeof the water table, such that W(t) = W0 dW + -•--t. Theleached column height expressed asa functionof time is givenby equation(16), where z0 is the elevation of the land surface at the start of oxidation: dW I_2r(t) = z0- W0 • t - B(t)+ Ut. (16) within the blanket zone due to continued influx and reprecipitation of secondaryminerals. Thus, one would expectthat the capillaryfringe heightwould increaseas the depth and extent of leaching increases.Pore-sizedata on the Butte district clearly show predominantlysubmicron-sizepores in the blanket zone (Cunningham, 1984). We propose, 1253 In equation(16), B(t) is the thicknessof the enrichment blanket at time, t. Initially B(0) may be very smallor equalto zero and is likened to the capillary fringe. U is the uplift rate. With these additions to our treatment of mass balance,we deriveexpressions for the leached-zone generalway a direct functionof the capillaryfringe columnheight,I_•(t), andenrichmentblanketmetal height, he, and that hc is proportionalto the depth grade,b(t), equivalentto equations(3) and (5) and given in equations(17) and (18): of leachingthrougha constant. therefore, that the blanket thickness, B, is in a It appearsthat the value of IPr/B within welldefined domainsis simply a function of the local wall-rockcharacteristics, e.g., alterationtype, chemical weathering effects, and the manner in which pore-sizedistributionevolveswith time. Justas the enrichmentblanket metal grade,b, reachesa maximum value,it is possiblethat the blanketthickness, B, also approachesa maximumvalue as effective pore radii diminishto the point of precludingfurther downward flow. This assumesthat the capillary fringe is roughlycoincidentwith the zone of enrich- and - ppp) Lg(t) = B(t)- (b(t)pb ppp-- l(t)p• (17) b(t) =ppp +fzø -W0 --dw •-.t+ ut B(t) -1 Xppp - l(t)p•(18) ment and that the base of enrichment is more or In theseexpressions, l(t) is the leached-zonemetal lesscoincidentwith the ground-watertable, defined gradeexpressed asa functionof time. Althoughrock where air pressureequalspore water pressure.This densities of the leached zone and enrichment blanket end stateof blanketgrowthmay representa transi- must also vary in time, we expressthese densities tion from vertical flow to a terminal state with a here as constants for simplicity. significant horizontalflowcomponentdueto reduced Of major importancein equations(17) and (18) permeabilitywithin the blanket zone. Lateral redis- is the functionalsimilarityof thesetime-dependent tribution of metalsmay then result. Metals leached expressionswith their counterparts,equations(3) from the zone of oxidationmay migrate downward, and (5), derived for a given instantin time. This is encounteran impermeablezone, and then migrate particularlysignificantwhen we considerthat prolaterally,being fixedat somedistancein a permeable vision has been included for a changinggroundportionof the zoneof enrichment.A morecomplete water table and a variation in total leached-zone analysisof this subjectis the main focusof a subse- columnheight with time. In other words,even in a quent paper. situation in which there has been a descending 1254 BRIMHALL,ALPERS,AND CUNNINGHAM ground-watertable exposinga protore-zonecolumn amount of erosion of leached capping, may yield height of increasinglength to deep oxidation, we reliable long-termerosionratespresentlylackingin may still solvefor the originalleached-zonecolumn the literature of geomorphology. The methodsdeveloped,althoughappliedto copheightand therebyreconstructthe originaloxidation surface. per profiles,are applicableto manyother elements. Furthermore,we seein equation(18), that at any Other metalsenrichedby solutionand reprecipitatime, t, the blanket metal grade, b(t), in a closed tion in supergeneenvironmentsincludegold, silver, chemicalsystemis a linear functionof the extentof zinc, and nickel, presentingnumerouspossibilities leaching(p - l(t)) asin equation(5), with an initial for further studies.The methodsare applicable to such value given by the protore metal grade and a slope ore depositsthat are moderatelyhomogeneous givenby the leached-zonecolumnheight to blanket as nickel-cobaltlaterites or porphyry copper-gold thicknessratio. Linear data arrays in these coordi- deposits.More heterogeneous depositssuchasveinnates for several control volumes from a spatial controlled copper or lode gold depositspresent domain may be interpreted as representing the obviousproblemsin estimationof lateral and vertical metal massdistributionat a specificand spatially protore metal-gradeprojections.Another group of uniform time, t, since oxidationbegan. Thus, ob- elements with a different mechanism of concentra- servedvariationof LPr/Bbetweenspatialdomains tion, i.e., removal of nonore metals, also warrants which exhibit a linear relation between b and p - 1 (apparentlyclosedsystems)indicatesthat between such domainsthere are: (1) intrinsic differences between the protore characteristics and consequent weatheringreactionsin each zone, causingdiffer- entialeffectson B(t) andLPr(t)thatproducedifferent characteristicslopes,and/or (2) different valuesof elapsedtime, t, in the variouszones,or (3) differing ratesof movementof the ground-watertable, dW/ dt, betweendomainswithout a compensating effect in B(t), or (4) systematiclateral fluxesfrom one domainto another(open chemicalsystem). General Conclusionsand Implicationsfor Exploration The massbalancemethodsoutlinedhere provide quantitativeunderstandingof geochemicalprofiles designed to complement existing mapping, core logging,and assaypractice.We have derived equations describingredistributionof metals from the zone of near-surfaceoxidation and leaching to a deeper zone of enrichmentby reprecipitation. Calculatedtopographicsurfacesusing massbalance of geochemicalprofiles can approximatethe land surface at the onset of chemical weathering processeswhen metals were first transportedand fixed in the subsurfaceby supergenefluids. Local anomalies on these calculated oxidation surfaces probablycorrespondto open-systemlateral flux regions. Such anomalies occur in matched pairs (sourcesandsinksaligned)alongtrendsof fracturing and faulting.At La Escondida,thesesourceand sink regions are separatedover distancesexceeding 1 km. Both copper sulfide-and copper oxide-enrichment sink regionshave been identified.Continued work on oxidation surfacescould indicate regional unconformitiesassociatedwith major periods of chemical weathering and enrichment of potential use in mineral exploration.Dating of suchsurfaces, when done in conjunctionwith estimatesof the considerationin this respect. This group includes iron, gold, silver, zinc, lead, antimony,tin, nickel, and aluminum. In the case of lateritic weathering andresidualmetalconcentration, it maybe necessary to accountfor major changesin density resulting from rock and soil deformationby collapse. Metal distributionstudiesperformedduring active explorationcould better explain ore patterns and geologiccontrolswith the goal of refiningdrilling strategies.Overall mass balance summing of all lateral fluxescouldbe a very usefulmeansof identifying exoticor transportedmetal orebodiessince our methodsindicateboth the magnitudeand directionsof possiblelateral metal fluxes.In either context, a critical need existsfor deep drilling to penetrate far enoughinto protore massesto definethe antecedenthypogenemetal variationbefore weathering and remobilization. Our first-order massbalance expressionsinclude rate expressionsfor erosion, uplift, and groundwater table movementwith time. It is apparentfrom theseconsiderations that it is necessaryto reevaluate the prevalentidea of the necessityof geomorphic stability for supergeneprocesses.Instead of this staticcondition,we proposethat a stateof dynamic balanceof ratesof surficialprocesses is requiredfor optimalsupergeneenrichmentto be initiated,maintained, and ultimately preserved over geological time spans.Our analysisshowsthat a critical parameter in this regard is the height of the oxidizing protore,LPr,in the unsaturatedzone betweenthe top of the capillaryfringe and the elevationof the originaltop of the ore sulfides.This distanceis a functionof both the rate of changeof elevationof the ground-watertable, W, and the regionaluplift rate, U. Leaching and enrichment processesare enhancedwhen both rates are high. Climatic variation, specificallywet goingto more arid conditions, maycausedescentof the ground-water tablethereby exposinga longer part of the protore column to MASS BALANCE PRINCIPLES APPLIED TO ORE-FORMING PROCESSES 1255 of multiple-stageporphyry copper mineralizationat Butte, oxidizingconditions.Similarly,regionaluplift may Montana:Vein formationby hypogeneleachingandenrichment have the sameeffect.In combination,high valuesof of potassium-silicate protore: ECON. GEOL., v. 74, p. 556both rates,W and U, may be an importantreason 589. for the extent of supergeneenrichmentin active -1980, Deep hypogeneoxidationof porphyry copper potassium-silicate protore at Butte, Montana:A theoreticalevalorogeniczones,i.e., the arid desertsof the Chile in contrastto the relative paucity of well-enriched uationof the copperremobilizationhypothesis:ECON.GEOL., v. 75, p. 384-409. copperdepositsin the tropics. Brimhall, G. H, and Ghiorso, M. S., 1983, Origin and ore- Acknowledgments The support of NSF grant EAR-81-15907 has madethe development and field applicationof the principles outlinedin thispapera reality.In addition, a researchgrantfromUtah InternationalandGetty Mineralshassupportedthe Chileanfield program. The Anaconda Companyhaskindlyprovidedmine access,drilling data, and core samplesfrom the Butte district. Sustainedinterest and cooperationof a number of individualshave contributedimmenselyto this effort includingGeorge Burns and Rich Ramseier (AnacondaCompany),JamesBratt, JohnHoyt, Don Rebal, and PatrickBurns(Utah International),Richard Berg and Lester Zeihen (Montana College of Mineral Scienceand Technology),Ulrich Petersen (Harvard University), Fred Graybeal (Asarco),and especiallyT. Narasimhanand Bill Dietrich of the Universityof California-Berkeley. 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