See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/277371308 Magmatic-hydrothermal processes within an evolving Earth: Iron oxidecopper-gold and porphyry Cu Mo Au deposits Article in Geology · June 2013 DOI: 10.1130/G34275.1 CITATIONS READS 101 2,267 2 authors, including: Hamid Mumin Brandon University 24 PUBLICATIONS 1,125 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: Central Neotethyan Porphyry-Epithermal Metallogeny View project Origin of world-class Gangdese porphyry metallogenic belt in Tibet View project All content following this page was uploaded by Hamid Mumin on 05 April 2020. The user has requested enhancement of the downloaded file. Magmatic-hydrothermal processes within an evolving Earth: Iron oxide-copper-gold and porphyry Cu ± Mo ± Au deposits Jeremy P. Richards1* and A. Hamid Mumin2* 1 Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada Department of Geology, Brandon University, 270 18th Street, Brandon, Manitoba R7A 6A9, Canada 2 surrounded and partially overprinted by broad zones of lower-temperature (<350 °C) more acidic alteration (sericite-pyrite, clay) generated by disproportionation of dissolved SO2 to form H2S and H2SO4. Porphyry deposits also form temporally and spatially separated from active arcs in post-subduction settings, from partial melting of previously subduction-modified lithosphere (Solomon, 1990; Hou et al., 2009; Richards, 2009; Shafiei et al., 2009; Pettke et al., 2010), a process also recently proposed for MH-IOCG deposits (Groves et al., 2010). Porphyry systems formed in such settings range to more alkaline compositions and may be Au rich, leading to alkaline porphyry Cu-Au and alkalic-type epithermal Au deposits (Richards, 2009). Such deposits are typically relatively S poor and Fe oxide rich, and show the closest overlap with MH-IOCG deposits in terms of tectonic setting, magma composition, and ore and alteration geochemistry. Porphyry deposits occur most commonly in Cenozoic and Mesozoic rocks, are less abundant in the Paleozoic, and are very rare in the ABSTRACT Iron oxide-copper-gold (IOCG) deposits formed by magmatic-hydrothermal fluids (MHIOCG) share many similarities with, but have important differences from, porphyry Cu ± Mo ± Au (porphyry) deposits: MH-IOCG deposits predominantly occur in Precambrian rocks, are Fe oxide rich, and have volumetrically extensive high-temperature alteration zones, whereas porphyry deposits occur almost exclusively in Phanerozoic rocks, are Fe sulfide rich, and have narrower high-temperature alteration zones. We propose that these deposit types are linked by common subduction-modified magmatic sources, but that secular changes in oceanic sulfate content and geothermal gradients at the end of the Precambrian caused a transition from the predominance of S-poor arc magmas and associated S-poor MH-IOCG systems, to S-rich arc magmas and associated S-rich porphyry deposits in the Phanerozoic. Phanerozoic MH-IOCG and rare Precambrian porphyry deposits are explained by local or periodic fluctuations in oceanic oxidation state and sulfate content, or remobilization of previously subduction-modified lithosphere in post-subduction tectonic settings. INTRODUCTION Porphyry Cu ± Mo ± Au (porphyry) and iron oxide-copper-gold deposits of magmatic-hydrothermal origin (MH-IOCG; sensu Groves et al., 2010) contain some of the largest concentrations of Cu, Au, U, Fe, and other metals on Earth, and are targets of choice for mineral exploration. These deposit types have many similarities but also some important differences, and their genetic relationships, if any, have been debated extensively (Barton and Johnson, 2000; Williams et al., 2005; Pollard, 2006; Groves et al., 2010; Mumin et al., 2010). Key differences are that MH-IOCG deposits are S poor and are common in Precambrian rocks, whereas porphyry deposits are S rich and predominate in the Phanerozoic. We suggest below that a continuum exists in processes and time between porphyry and MH-IOCG deposits formed in orogenic and post-orogenic settings, primarily reflecting the recycling of volatiles into the lithosphere via subduction, coupled with a progressive decrease in lithospheric geotherms and increase in deepseawater sulfate abundance since the Archean. *E-mails: [email protected]; Mumin @BrandonU.ca. GEOLOGY, July 2013; v. 41; no. 7; p. 767–770 | Volcanic edifice may or may not be present Magmatic-hydrothermal IOCG Intermediate-low sulfidation Supracrustal sequence High-sulfidation Intermediate-sulfidation Sinter, breccia or vein Au, Ag (As,Hg,Cu,Zn,Pb) Vein Cu,Ag,U,Co Ni,Au,Bi Propylitic Py,Hem Pyrite Clay-Chl Sericite Cu Au Mo Hematite Na-Ca Sericite K-feldspar Albite 1 km Propylitic Hem Pyrite Magnetite 1 km Py Hem Clay-Chl Vein Cu,Pb,Zn,Ag,Au Cu,Au Co Ag U Porphyry Breccia pipe Advanced argillic or vent Na-Ca Ser K-fsp FORMATION OF PORPHYRY CU ± MO ± AU DEPOSITS The formation of porphyry deposits has been described extensively in the literature (e.g., Sillitoe, 1972, 2010; Richards, 2003, 2011; Seedorff et al., 2005), and only key points are noted here (Fig. 1; Table 1). Porphyry deposits are large geochemical anomalies of Fe and S, with lesser but economically important enrichments in Cu ± Mo ± Au. They consist of veins and disseminations with variable proportions of chalcopyrite, bornite, molybdenite, pyrite, and magnetite, and are formed from magmatichydrothermal fluids exsolved from relatively oxidized (fayalite-magnetite-quartz buffer [ΔFMQ] = +1 to +2), S-rich, calc-alkaline to mildly alkaline, arc-related magmas. Mineralization is mainly associated with central zones of high-temperature (≥350 °C) potassic alteration (biotite–K-feldspar–amphibole–magnetite), Mag+Py K-fsp Na Na-Ca Basement rocks Batholith Vein Au,Ag Propylitic Chl = Chlorite Hem = Hematite K-fsp = Potassium feldspar Mag = Magnetite Na = Sodic alteration Na-Ca = Sodic-calcic alteration Py = Pyrite Ser = Sericite Figure 1. Schematic model for magmatic-hydrothermal systems illustrating relationship between S-rich porphyry Cu ± Mo ± Au deposits (right side) and S-poor magmatic-hydrothermal iron oxide-copper-gold (MH-IOCG) deposits (left side). Approximate hydrothermal mineral transitions and relative spatial footprint are shown for albite–K-feldspar, K-feldspar–sericite, and magnetite-hematite transitions. Prograde and retrograde overprinting of alteration assemblages and mineralization is common. Spatial relationships for porphyry deposits after Seedorff et al. (2005), Sillitoe (2010), and Richards (2011); for MH-IOCG, after Hitzman et al. (1992), Williams et al. (2005), and Mumin et al. (2010). doi:10.1130/G34275.1 | Published online 16 May 2013 © 2013 Geological America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY 2013 | of www.gsapubs.org | July Society 767 TABLE 1. COMPARISON BETWEEN PORPHYRY AND MAGMATIC-HYDROTHERMAL IOCG DEPOSITS Characteristic Major metal association Minor metal association Sulfur content Ore minerals Ore fluid Fluid oxidation state; acidity Source of fluid Source of metals Alteration geochemistry Width of high-temperature (>350–400 °C) alteration Depth of formation Geothermal gradient Regional metamorphism Magma association; composition; oxidation state Tectonic setting Kinematic setting Age range Porphyry Cu ± Mo ± Au [post-subduction porphyry] Cu, Mo, Au Ag, Sn, W High: sulfides and sulfates [lower] Chalcopyrite, bornite, molybdenite, magnetite; abundant pyrite with sericite H2O-NaCl-KCl Oxidized (ΔFMQ = +3 to +5); neutral to acidic Magmatic Subducted slab and asthenosphere [subductionmodified lithosphere] K-(Na)-Fe-S-SiO2 1–2 km 1–5 km Normal Minimal to low grade Calc-alkaline [mildly alkaline]; intermediate to felsic [mafic to felsic]; ΔFMQ = +1 to +2 Subduction [post-subduction] Transpression or transtension Dominant in Phanerozoic, rare in Precambrian Magmatic-hydrothermal IOCG Fe, Cu, Au U, REE, Co, Ag Low: minor sulfides Magnetite, hematite, chalcopyrite, bornite, chalcocite H2O-CO2-NaCl-KCl Likely ΔFMQ = +3 to >+5; neutral to mildly acidic, rarely acidic Magmatic ± crustal fluids Subduction-modified lithosphere and fluxing from host rocks Na-K-Fe-P-Ca-CO2-SiO2 1 to ≥7 km Surface to ~10 km Elevated Low to high grade Calc-alkaline to mildly alkaline; mafic to felsic; uncertain, but likely ΔFMQ = 0 to +2 Distal, back-arc, or post-subduction Extension to transtension Dominant in Precambrian, important in Mesozoic Note: Characteristics of post-subduction porphyry deposits shown in square brackets. IOCG—iron oxide-copper-gold; ΔFMQ—fayalite-magnetite-quartz buffer; REE— rare earth element. Precambrian. One explanation for this temporal distribution is that arc-related porphyry deposits form in environments of active uplift and erosion, commonly followed by collision, and as such they are highly susceptible to loss through erosion (Kesler and Wilkinson, 2006). However, it is difficult to explain the extreme rarity of Precambrian deposits by erosion alone, and we find no credible evidence that porphyry deposits were in fact formed in abundance prior to the Phanerozoic. We note that many Precambrian supracrustal sequences are preserved around the world, including arc successions, but indisputable porphyry-type deposits in these rocks are almost nonexistent (Seedorff et al., 2005). FORMATION OF MH-IOCG DEPOSITS Major MH-IOCG systems are found in continental orogenic to post-orogenic settings from the late Archean (e.g., Carajas district, Brazil) and Proterozoic (e.g., Olympic Dam and Cloncurry districts, Australia; Norrbotten, Sweden; Great Bear, Canada; Williams et al., 2005; Groves et al., 2010; Mumin et al., 2010; Skirrow, 2010), to the Mesozoic (e.g., Candelaria– Punta del Cobre and Manto Verde in Chile; Raul-Condestable and Mina Justa in Peru; Marschik and Fontboté, 2001) (Table 1). MH-IOCG deposits also constitute large geochemical enrichments in Fe, but mainly as Fe oxides (± Fe silicates, Fe carbonates) with relatively minor Fe sulfides; they contain economically important enrichments in Cu ± Au ± U ± REE (rare earth elements) ± Co (Table 1). They are commonly associated with relatively oxidized but apparently S-poor, calc-alkaline to mildly alkaline magmas. They display broad zones of high-temperature (~600–400 °C) 768 Na (albite-amphibole-pyroxene), Na-Ca-Fe (magnetite-actinolite-apatite), or K-Fe (K-feldspar–magnetite–biotite–amphibole) alteration (Fig. 1). Unlike porphyry systems, lowertemperature alteration zones (<400 °C; e.g., hematite-chlorite-sericite-carbonate) are characterized by persistence of near-neutral to only mildly acidic pH conditions, with rare highly acidic alteration, reflecting lower abundances of H2SO4 in the hydrothermal fluids. Whereas fluids and metals in porphyry deposits are derived primarily from underlying magmatic sources (see the review in Richards, 2011), the origin of IOCG fluids has been widely debated, ranging from metamorphic and crustal sources to magmatic hydrothermal fluids. Here, we restrict our discussion to magma-driven systems involving primary magmatic fluids, albeit with variable crustal contributions (Barton and Johnson, 2000; Pollard, 2006; Groves et al., 2010; Skirrow, 2010). MH-IOCG systems form primarily during extensional or transtensional deformation periods, in tectonic settings ranging from back-arc or distal arc (e.g., Olympic Dam; Skirrow, 2010) to intra-arc (e.g., Candelaria–Punta del Cobre, Marcona; Marschik and Fontboté, 2001; Chen et al., 2010), and to post-collision extensional settings (e.g., Great Bear; Mumin et al., 2013). These geotectonic environments are shared by arcrelated and post-subduction porphyry deposits. GENERATION OF FERTILE MAGMAS IN ARC AND POST-SUBDUCTION ENVIRONMENTS A key factor in the generation of magmatic hydrothermal porphyry and IOCG deposits is the recycling of H2O into the mantle via subduction of seafloor-altered oceanic lithosphere. Dehydration of the subducting slab releases aqueous fluids that metasomatize the overlying asthenospheric mantle wedge and cause partial melting to form hydrous basaltic magmas. Fractionation and interaction of these magmas with the upper plate lithosphere generates evolved, hydrous, relatively oxidized (ΔFMQ = +1 to +2), calcalkaline magmas. Upon emplacement at shallow crustal depths, volatiles are released due to cooling, and these fluids may go on to form magmatic-hydrothermal ore deposits (Hedenquist and Lowenstern, 1994; Richards, 2003). Richards (2009, 2011) proposed that magmatic-hydrothermal ore deposits can also form temporally and/or spatially distal to active subduction zones by remelting of lower crustal amphibolitic cumulates and/or metasomatized mantle lithosphere residual from precursor arc magmatism. A significant amount of the chalcophile (e.g., Cu) and highly siderophile element (e.g., Au and platinum group elements [PGE]) content of the arc magma flux may be left as residual sulfides in these deep lithospheric cumulate or metasomatized zones. Second-stage partial melting of these amphibolitic rocks may occur at any later time due to tectonic processes that increase temperature or reduce pressure, such as crustal thickening or lithospheric mantle delamination during arc or continent collision, post-collisional extension, or back-arc rifting. Lacking a fresh supply of subduction-sourced sulfur, derivative partial melts will be relatively S poor, and will dissolve a significant proportion of any residual Cu ± Au–rich sulfides; such magmas may go on to form relatively S-poor magmatic-hydrothermal Cu ± Au deposits (Richards, 2009). Porphyry and MH-IOCG www.gsapubs.org | July 2013 | GEOLOGY GEOLOGY | July 2013 | www.gsapubs.org ~ Paleo heat production (% present value) B basins where conditions were oxidizing; Fig. 2), but favoring the development of S-poor MHIOCG systems. Conversely, S-rich Phanerozoic arc magmas favor the formation of porphyrytype deposits, although periodic swings back to anoxic deep-ocean conditions, particularly evident in the Mesozoic (Holland, 2006), could lead to the intermittent formation of S-poor magmas favorable for MH-IOCG formation. The Role of Decreasing Geothermal Gradients Not all differences between MH-IOCG and porphyry deposits can be explained on the basis of sulfur content, including the full nature and extent of high-temperature alteration in MHIOCG deposits, and their different minor-element contents (e.g., U and REE). Given that the source magmas and therefore initial fluid temperatures for both deposit types are similar (~700 °C), one explanation for these further differences may be that ambient rock temperatures in the upper crust were higher under steeper Precambrian geothermal gradients, due to higher radiogenic heat output and higher primordial heat loss (Araki et al., 2005). Some estimates suggest that geothermal gradients were approximately twice current values at 2 Ga (Richter, 1988; Plant and Saunders, 1996), while Hand et al. (1999) argued for highly variable paleogeotherms depending on the concentration of radioactive elements (U, K, Th) in Proterozoic rocks. They calculated that high-heat-producing granites such as those associated with the Olympic Dam and Cloncurry mining districts could have had geothermal gradients double the 2 2 2 Deep ocean O2 (mM) Fe-Oxide Versus Fe-Sulfide Deposits: The Role of Seawater Sulfate A key difference between Fe oxide–rich MHIOCG deposits and Fe sulfide–rich porphyry deposits reflects a difference in magmatic sulfur content in otherwise similar hydrous, relatively oxidized, calc-alkaline to mildly alkaline source magmas. Phanerozoic arc magmas and related porphyry and high-sulfidation epithermal deposits are S rich, with the bulk of the sulfur being derived from seawater via the subduction cycle (De Hoog et al., 2001; Wallace and Edmonds, 2011). In contrast, Prouteau and Scaillet (2013) have shown that arc magmas generated in the Precambrian were relatively S poor, and this correlates with the scarcity of S-rich Precambrian porphyry deposits. A secular increase in the sulfur content of arc magmas may relate to a parallel increase in the abundance of dissolved sulfate in seawater over time. Seawater sulfate concentrations are estimated to have been ~0.2 mM from the late Archean to earliest Proterozoic, with a gradual increase to the early Phanerozoic, followed by a rapid increase to modern day values of ~28 mM (Fig. 2; Kah et al., 2004; Gill et al., 2007). The increase in seawater sulfate content is directly linked to the availability of biogenic oxygen and the onset of oxidative weathering, which released continent-derived sulfate to the oceans beginning at ca. 2.3 Ga (Kasting, 2001). A dramatic spike in both seawater and seafloor sulfate content occurred in the Phanerozoic due to sulfide oxidation by ocean ventilation and marine bioturbation (Canfield and Farquhar, 2009; Shields-Zhou and Och, 2011). Subduction of this oxidized material would introduce abundant sulfate to the mantle wedge for the first time, and generate S-rich arc magmas. Thus, we propose that subduction-related magmas would have been S poor in the Precambrian, inhibiting the formation of S-rich porphyry-type deposits (except in locally anomalous environments, such as subduction of shallow marine A Seawater sulfate concentration (mM) DISCUSSION Despite the similarities noted above, porphyry and MH-IOCG deposits vary somewhat in both economic and trace-element signatures, are distinct in their alteration footprints, and vary in abundance over geological time. We argue below that these differences may be explained by secular changes in the abundance of seawater sulfate entering magmatic-hydrothermal systems, and decreasing lithospheric geothermal gradients since the Precambrian. Figure 2. Global changes Present-day sulfate concentration 28 affecting magmatic-hydrothermal ore deposits from Archean to present. 24 300 A: Increase in seawater sulfate concentration ~ De 20 crea and decrease in heat prose in pale duction relative to preso he at pr ent-day values. Sulfur 16 Porphyry 200 oduc tion concentrations from Kah Cu most abundant et al. (2004) and Gill et al. Porphyry Cu rare 12 (2007); paleo-heat production from Plant and 8 IOCG Deposits Saunders (1996), Hand 100 et al. (1999), and Araki Initial et al. (2005). IOCG—iron 4 biospheric ate oxide-copper-gold. B: er sulf oxygen eawat S Atmospheric O2 levels as 0 0 a percentage of present 0 1.5 Age Ga 1.0 2.0 0.5 2.5 atmospheric level (%PA; Oxic upper ocean blue dashed line) super0.5 imposed on correspond100 0.4 Anoxic deep ocean with widespread ing ocean redox condisulfidic bottom waters 0.3 tions (after Canfield, 10 Anoxic Oxic deep O Max O -%PA ? 2005; Shields-Zhou and 0.2 ocean deep Och, 2011). Following the 1 ocean O Min 0.1 Neoproterozoic Oxygen0.0 Sulfate accumulation Ocean basalt and sediments ation Event, deep ocean 0.1 O2 levels (black short dashed lines) varied rapidly over short time intervals (Holland, 2006), resulting in variable sulfate additions to oceanic crust. O2 (% present atmosphere) deposits have now been recognized from all of these post-subduction tectonic settings (Pollard, 2006; Hou et al., 2009; Richards, 2009; Groves et al., 2010; Skirrow, 2010), and it is here that we observe the closest overlap between these two broad deposit types. Proterozoic average at the time of mineralization (1.6 Ga). Most MH-IOCG deposits are associated with extensional environments, and many are close to their source batholiths, which places them in host rocks with significantly higher-thannormal thermal gradients (e.g., Hitzman, 2000). In contrast, most porphyry deposits are formed at shallow levels (~1–5 km depth) above deeper batholiths (~5–10 km), and thus in cooler country rocks. High ambient temperatures in the host rocks to MH-IOCG deposits would facilitate the formation of extensive zones of high-temperature alteration, from which a significant flux of crustally derived lithophile elements such as Fe, K, Ca, U, and REE could be derived. Temporal Overlap between MH-IOCG and Porphyry Deposits This model does not preclude the formation of rare porphyry deposits in the Precambrian where magmatic sulfur contents were locally high, for example due to subduction of oxidized shallow-marine sediments. It also does not preclude the formation of MH-IOCG deposits in the Phanerozoic from S-poor magmas generated from partial melting of relatively S-poor asthenospheric or subduction-modified lithospheric sources that may be linked to periods of deep ocean anoxia, or in extensional zones with high thermal gradients. CONCLUSIONS Porphyry Cu ± Mo ± Au and magmatichydrothermal IOCG deposits share many attributes, such as their association with calc-alkaline 769 to mildly alkaline magmas, broad geotectonic relationships, and metal contents; and yet they remain distinctive. The closest overlap between these two deposit types is found in post-subduction alkalic porphyry Cu-Au deposits, which are magnetite rich and relatively S poor. We propose that higher geothermal gradients and lower seawater sulfate contents in the Precambrian favored the formation of S-poor arc magmas and MH-IOCG deposits during that period, whereas the reverse was true in the Phanerozoic, favoring S-rich porphyry formation. Exceptions exist, however, and we explain the overlap by local variations in geothermal gradients and source sulfur contents. ACKNOWLEDGMENTS This paper is dedicated to the memory of Rob Kerrich, an innovative thinker and inspiring mentor. 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