GENETIC CLASSIFICATION OF BRECCIAS Introduction Epigenetic breccia bodies are a not uncommon feature of numerous geological environments (especially magmatic arcs), and may show a spatial and indirect or direct genetic relationship to ore formation. Commonly, breccias are usually just one part of a protracted history of magmatic-hydrothermal activity. Some examples of large gold deposits hosted within large epigenetic breccia bodies are Cripple Creek (alkalic low sulfidation epithermal), Olympic Dam (IOCG), Grasberg (Au-rich porphyry) and numerous high sulfidation epithermal deposits. Breccia Classification Summary 5 Despite showing an important association with numerous types of gold deposits, epigenetic breccias remain an enigmatic topic for many geologists. This classification of breccias is genetic and based on the inferred role of magmas, magmatic volatiles and their interaction with groundwaters. It is taken from a classification by Sillitoe (1985) and a modified and expanded classification in Lawless et al (1998). Diagram Energy Code Breccia Category Source 4 3 2 1 1 5 7 2 Mobile Phase Geometry Diameter (m) Magmatic volatiles Single or multiple subvertical pipes AngularLocal to subrounded, common up to 2,000 locally rounded (<50%) Magmatichydrothermal Magma (directly) Phreatic Magma (through circulating groundwaters) Groundwater 2 Before assigning a breccia body to a breccia type within this classification, numerous pieces of evidence need to be observed. These include: dominant breccia texture and composition, particular important textural features (e.g. accretionary lapilli, ragged juvenile clasts, etc), distribution and relationships between breccia facies and the overall geometry of the breccia body. By Ned Howard with help from Andrew Ford and David Brookes Fragment form Pipe-like, irregular, pebble Angular to dikes up to ~500 rounded Rock flour matrix Commonly present <50% 1 3 Magma Magmatic-phreatic (indirectly) Groundwater Diatreme up to ~3,000 Subrounded to Present rounded (<90%) 4 4 Magma Phreatomagmatic (directly) 5 5 • Sillitoe, R.H., 1985, Ore-related breccias in volcanoplutonic arcs, Economic Geology, v. 80, p. 1467-1514. • Lawless, J.V., White, P.J., Bogie, I., Paterson, L.A., Cartwright, A.J., 1998, Appendix 1: Genetic Classification of Breccias, Ore Deposits and MagmaticHydrothermal Processes (Workshop manual), Kingston Morrison consulting, pp. 20. 66 77 6 1. Magmatic Hydrothermal Magmatic Intrusion Tectonic Groundwater Diatreme Magma (directly) Magmatic volatiles Intrusion Regional tectonism Magma Diatreme, volcanic vent Irregular patches Steep tabular bodies N/A 1,000 3,000 Subrounded to Present rounded (<90%) Subrounded to 500 - 5,000 rounded Present up to ~100 Angular Angular to up to ~50 subrounded Absent Present (<100%) None No juvenile clasts, wall-rock blocks, base surge deposits, Maar, tuff accretionary lapilli ring, domes Minor Tuff matrix, cognate lithics Tuff matrix, pumice, cognate lithics Wall-rock blocks, base surge deposits, locally exfoliated fragments, Maar, tuff accretionary lapilli ring, domes Pyroclastic Wall-rock blocks, fall & flow locally exfoliated deposits, fragments domes Minor None Intrusive rock matrix None Variable (minor) None Slickensides, gouge None Variable (minor) Pebble dyke, Mt Bischolf Sn mine, Tasmania, Australia Low Sulphidation vein breccia with minor milling to sub-angular fragments, evidence of re-brecciation, Woolgar Australia Breccias caused by the emplacement of an intrusive body, but not associated magmatichydrothermal fluids. Accretionary lapilli, Wau gold district, PNG Breccias caused by the flashing /expansion of groundwater heated by a magma. No direct contact between the magma and the water. Geologically and genetically similar to phreatomagmatic breccias. Schematic X Section of Kerkil low sulphidation breccia system (Kalimantan, Indonesia) showing alteration zonation and breccia distribution Type 1. Magmatic-Hydrothermal Also referred to as “carapace breccias”, these are the product of juvenile hydrothermal fluids exsolved from magmas. • Genesis: Fractionation of intrusive magma may lead to the exsolution of an immiscible volatile phase (‘second boiling’), which exceeds lithostatic pressure resulting in varying degrees of hydraulic fracturing. This process may occur multiple times as further magmatic fractionation and exsolution occur. • Geometry: Commonly sub- vertical pipe to tabular bodies. Single or multiple bodies and phases. • Diameter: 50-300m, locally >1,000m. • Breccia Characteristics: Angular to sub-rounded (locally rounded) clasts of country rock ± intrusions within a matrix of hydrothermal infill with local minor clastic matrix. Infill minerals commonly indicative of high temperature and salinity (e.g. tourmaline, feldspar). • Geological Setting and Relationships: Spatially associated with intrusions but extending sub-vertically away. May grade downwards into cupolas of intrusives with or without intrusion breccias or pegmatites. May grade upwards into breccia pipes and then to veins through decreasingly fractured country rocks. May occur at any depth from >5km to ~1-2km depth. • Surface Expression: None • Associated Ore Deposits: Commonly closely spatially ± genetically associated with intrusion-related deposits and porphyry Cu-(Au/Mo) deposits (e.g. Kidston breccia-hosted Au, Australia; Los Bronces porphyry Cu-Au, Chile; Ok Tedi porphyry Cu-Au, PNG; Galore Creek porphyry Cu-Mo, Canada). Brecciation is typically preto inter-mineral and may be genetically associated with mineralisation. Mineralisation in breccia-pipe hosted deposits commonly occurs near the margins, while in porphyry systems, mineralisation is more common within the breccia itself. Type 2. Phreatic Breccias Breccias caused by the expansion of steam and gas from circulating groundwater, but driven by magmatic heat. Involvement of magmatic volatiles is unimportant. Also referred to as hydrothermal eruption breccias. • Genesis: Ground waters circulating above a cooling magma are heated and accent to a shallow subsurface level where flashing can occur, causing fracturing and brecciation. Often deposition of silica and other minerals decreases permeability, allowing pressure to build up again and re-brecciation to occur. • Geometry: Commonly irregular but usually pipelike. Also pebble dikes. • Diameter: up to ~500m • Breccia Textures: Degree of clast mixing, rounding and proportion of matrix depend on longevity of brecciation. Commonly sub-surface breccias are monomictic to polymictic, clast-supported with angular to rounded clasts. Near surface and surface (eruptive) breccia products tend to contain more matrix, and more polymictic and generally thin and small volume relative to volcanic eruption products. Accretionary lapilli are common near surface. Clasts and matrix are commonly highly altered, generally to low temperature, low pH minerals, such as silica, clays and sulphur. • Distinguishing Features: Common low temperature hydrothermal alteration, accretionary lapilli, association with hotspring and fumarolic activity. Exfoliated fragments and sinter fragments may also occur. • Geological Setting: Typically occurring close to surface (<1-200m below ground) and associated with surface hydrothermal activity such as hotsprings and solfatars. • Surface Expression: Blind deposits may occur but are not common. Small eruption vents flanked by surface breccia deposits, surface hydrothermal activity and low temperature (steam heated) alteration define the surface expression of these breccias. Phreatic breccias are poorly reserved within the geological record. • Associated Ore Deposits: As hydrothermal fluid circulation is involved in the formation of phreatic breccias, it is not surprising that both clasts and matrix of phreatic breccias are generally highly altered. Phreatic breccias are not generally associated with porphyry deposits, due to their near surface location, or post-date them. However, epithermal mineralisation commonly occurs genetically and spatially associated with phreatic breccias. Mineralisation commonly occurs within the breccia cement itself (e.g. Red Mountain high sulfidation Au-Ag-Cu, USA; Hasbrouck Mountain low sulfidation Au-Ag, USA) but can also occur within clasts within breccias (e.g. Wau Au-Ag stockworks, PNG) or cross-cut breccias (e.g. Buckskin vein-stockwork Au-Ag, USA). Tuff apron of pyroclastic material including surrounding maar volcano, Ukinrek Maar, Alaska USA Polymictic, milled diatreme breccia, Ntina pit, Placer Au mine, Philippines Contact zone between porphyritic andesite (top) and a more mafic later intrusive with brittle fracturing and plucking of wall rock into the later magma. Vent explosion, Ukinrek Maar, Alaska, 1964 Schematic X section showing progressive development of a maar, with intrusion following existing zone of weakness (fault) 5. Magmatic Schematic cross section through typical maar-diatreme showing diatreme breccia pipe capped by maar lake sediments and surrounding tuff apron Minor magma and external water. Commonly form diatreme breccias. 3. Magmatic Phreatic Accretionary lapilli, cryptic rounded shapes thought to form from accretion of wet ash onto rock fragments in eruption clouds, or within ‘muddy’ breccia pipes, Lepanto, Philippines Characteristic “shingle texture” breccia, Wheal Remfry clay mine, Cornwall, UK Energite breccia, Lepanto Cu–Au mine, Philippines Silicification, clay 6. Intrusion Breccias Schematic Diagram showing end-member Breccia Environments of Formation from circulating groundwater, but driven by magmatic heat. Involvement of magmatic volatiles is unimportant. Also referred to as hydrothermal eruption breccias. Tourmaline breccia, Kidston Breccia Pipe, Queensland Australia None None Sericite, tourmaline, Ksilicate 4. Phreatomagmatic Breccias formed due to the direct interaction of Also referred to as “carapace breccias”, these are the product of juvenile hydrothermal fluids exsolved from magmas. 2. Phreatic Breccias caused by the expansion of steam and gas Surface connection Sheeted contacts, shingle breccia, exfoliated fragments None Explosion crater, breccia apron, Exfoliated fragments, hydrothermal sinter fragments activity 3 Clearly, the below classification system is not suitable for field use, and considerable field work is required before it is used. In breccia hosted hydrothermal systems, it is important that variations in breccia facies are recognised and their distribution determined. This can help to vector towards prospective zones within the breccia system. Field classification of breccia facies should be based on features such as clast composition (mono/polymictic), degree of rounding, clast:matrix ratio (clast vs matrix supported), matrix composition (hydrothermal infill vs milled rock) and the presence/absence of important clast types (e.g. soft-deformed sediments, accretionary lapilli, juvenile ‘whispy’ clasts). Quartz tourmaline breccia with exfoliated fragments, Wheal Remfry clay mine, Cornwall, UK Juvenile component Other features Alteration (temporally associated with brecciation) Breccias generated by the explosive decompression of magmatic volatiles. These breccias include vent breccias and magmatic diatremes. Altered rhyolitic autoclastic breccia, with clay alteration of matrix and clast margins, Oga Peninsula, Japan 7. Tectonic Breccias associated with regional and local tectonism, brittle and ductile faulting. Pronounced brittle fracture of existing quartz vein by fault, with later hydrothermal alteration, Kangaroo Hills Tin field, Qld Australia Tectonic breccia, Malibu fault, California USA Type 3. Magmatic-phreatic Breccias Breccias caused by the flashing/expansion of groundwaters that is heated by a magma. No direct contact between the magma and the water (unlike phreatomagmatic breccias) and the groundwater does not circulate to a shallower level before brecciation can occur (unlike phreatic breccias). Magmatic-phreatic breccias are difficult to differentiate from phreatomagmatic breccias and some bodies identified as the latter may actually be magmatic-phreatic breccias. • Genesis: Similar to phreatomagmatic breccias but not involving direct magma-water contact. An intruding magma leads to an increase in the temperature of ambient groundwater at a shallower level than the magma, causing the groundwater to flash/expand and overcome lithostatic pressure. The fluid pressure generated is enough to cause brecciation at the site of heating, rather than driving the circulation of groundwater to shallower levels were confining pressures are low enough that boiling and brecciation can occur. Multiple stages of brecciation may occur. • Geometry: Subvertical pipe-like bodies (similar to phreatomagmatic breccias), magmatic-phreatic diatremes. Breccia complexes may be formed by the coalescing and overprinting of adjacent breccia bodies • Diameter: up to >2,000m • Breccia Textures: Similar to phreatomagmatic breccias (i.e. generally polymictic, matrix supported with rounded to subrounded clasts) but lacking any juvenile clasts. • Distinguishing Features: As for phreatomagmatic breccias. Distinguished from magmatic breccias by the lack of juvenile clasts and the separation of the root of the breccia body from coeval intrusions. Generally larger or deeper-seated and with evidence of higher temperatures than phreatic breccias. • Geological Setting: Similar to phreatomagmatic breccias. Base of the breccia body is separated from coeval intrusions. The breccia body gives way laterally to decreasingly fractured wallrock, though boundaries between milled breccia and fractured wallrock may be sharp. • Surface Expression: Similar to phreatomagmatic breccias. Maars or post-breccia domes may occur at surface. Blind breccia bodies are known. • Associated Ore Deposits: Similar relationship to porphyry and epithermal deposits as phreatomagmatic breccias. Gold deposits at Boulder County, USA and Cerro Violeta and Cerro Colorado, Chile may be associated with magmatic-phreatic breccias. Gold mineralisation at Kelly gold mine, Philippines is associated with an event interpreted as magmatic-phreatic in origin. Type 4. Phreatomagmatic Breccias Breccias formed due to the direct interaction of magma and external water. Also known as diatreme breccias. • Genesis: Contact between a rising magma and groundwater results in flashing of water to steam and the explosive fragmentation of country rock. Fluidisation of material may occur, resulting in mixing of clasts and a high degree of milling. Diatremes are commonly the products of multiple stages of magma-water interaction. • Geometry: Pipe to upwardly flared cone shape • Diameter: up to >1500m horizontally and up to >2,500m vertically. • Breccia Characteristics: Typically subrounded to rounded polymictic clasts of wallrock matrix supported in rock flour with or without hydrothermal cement. . Clasts are commonly hydrothermally altered. Accretionary lapilli (fragments coated in a concentric rim of rock flour) are diagnostic of the involvement of water in formation of the breccia. Near the base of the diatreme ‘whispy’ juvenile clasts may occur and are indicative of the involvement of magma. Towards the top of the diatreme, inclusion of blocks/fragments of fine grained surface sediments and wood may occur within the breccia and suggest a nearsurface position. However, wallrock fragments can undergo considerable vertical transport and large blocks of near-surface material or basement can occur within the breccia body 100’s m below their original position. • Geological Setting and Relationships: Phreatomagmatic diatremes directly involve magma and commonly terminate down into dikes. The breccia body gives way laterally to decreasingly fractured wallrock, though boundaries between milled breccia and fractured wallrock may be sharp. • Surface Expression: Diatremes vent out at maar or tuff ring volcanoes, maars where the vent floor has been excavated below the surrounding ground level. Tuff rings occur above small diatremes that do not penetrate significantly into country rocks. • Associated Ore Deposits: Mineralisation is not directly associated with diatremes, but are commonly spatially associated with ore deposits in the porphyry-epithermal environments. Diatremes commonly post-date porphyry-style (magmatic fluid-related) mineralisation (e.g. Braddon Pipe at El Teniente porphyry Cu-Mo, Chile; Dizon porphyry Cu-Au, Philippines; Guinaoang porphyry Cu-Au, Philippines), and are pre- to inter-mineral to meteoric fluid-related (epithermal) mineralisation (e.g. Cripple Creek alkalic low sulfidation Au, USA; Kelian intermediate sulfidation Au, Indonesia; Martabe high sulfidation Au, Indonesia). In breccia-hosted deposits, mineralisation commonly occurs at the margins of the breccia (e.g. Acupan Au, Phillipines; Wau lode and stockwork Au, PNG) but may also occur within diatremes (e.g. Cripple Creek, USA; and Montana Tunnels Au-Ag-base, USA). Subaerial rhyolitic lava dome with carapace of autobreccia Altered polymict volcanic breccia with strong qtzser-chl alteration, Hellyer Mine, Tas, Australia Tectonic breccia, showing fragmentation and imbrication of wallrock clasts into foliation Type 5. Magmatic Breccias Type 6. Intrusion Breccias Breccias generated by the explosive decompression of magmatic volatiles. These breccias include vent breccias and magmatic diatremes. • Genesis: Exsolution of volatiles from a hydrous magma results in explosive pressure release at the top of a near-surface magma chamber and overlying rock. Magmatic breccias are essentially the near-surface equivalents of magmatic-hydrothermal breccias. • Geometry: Subvertical diatremes/pipes to upwardly-flaring funnels. • Diameter: 500 to 5,000m, up to >1,000m vertical extent. • Breccia Characteristics: Subrounded to rounded clasts of dominantly juvenile material within a matrix of variably comminuted vitric and lithic material (i.e. lapilli to tuffaceous rock flour). Generally breccias are clast supported. Near-surface breccias may contain recycled volcanic bombs, slumped blocks of vent-wall material. • Geological Setting and Relationships: Magmatic breccias occur within volcanic vents and magmatic diatremes (i.e. excavative volcanic vents). They are intimately associated with the source magma chamber (below) and may grade into coherent intrusive rock. Passive ascent of magma after brecciation may result in cross-cutting dikes and domes. • Surface Expression: Volcanic vent with local depression within a composite volcanic cone or a tuff ring or maar volcano above a magmatic diatreme. • Associated Ore Deposits: Not directly genetically associated with mineralisation, but can be spatially associated with pre- or postbreccia porphyry-style mineralisation or post-breccia epithermal mineralisation. E.g. Rio Blanco-Los Bronces, Chile; Toquepala, Peru; Ashio, Japan and Casino, Yukon, Canada. Breccias associated with the emplacement of an intrusive body, but not associated magmatic-hydrothermal fluids. • Genesis: Passive, mechanical brecciation associated with movement (intrusion) of magma and incorporation of country rock. • Geometry: Variably oriented lenses and patchy zones at intrusive margins • Diameter: up to ~100m • Breccia Textures: Angular fragments of country rock (metamorphics, earlier intrusions, early crystallised intrusion) within a crystalline igneous matrix. Gradational to fractured wallrock (± dykes) on one side and intrusive rock (± xenoliths). • Geological Setting: Closely spatially associated with margins of causative intrusion. May occur at any depth below the surface where intrusions occur. • Surface Expression: None • Associated Ore Deposits: Not genetically related to ore deposits. May be spatially associated with any intrusion-related mineralisation. Type 7. Tectonic Breccias Breccias associated with regional and local tectonism. • Genesis: mechanical brecciation as a result of fault movement and fragmentation of country rock; “break–up breccias” • Geometry: Diameter: up to ~50m • Breccia Textures: Angular to sub-rounded as a result of milling, with variable rock flour matrix, clasts of local country rock, imbrication of fragments, slickensides • Surface Expression: As pods or linear zones following fault trace, can be recessive or resistant to weathering depending on fault matrix. • Associated Ore Deposits: Orogenic gold deposits, often as overprint to other styles.
