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ES3210 - Gold Deposits Stephen J. Piercey ES 3210 ECONOMIC MINERAL DEPOSITS • Relevant Chapters in Mineral Deposits of Canada: ⇨Dube & Gosselin, Greenstone-Hosted Quartz-Carbonate Vein Deposits ☑ ⇨Taylor - Epithermal Gold Deposits (out of date, but…) • Optional ⇨Hart, Reduced Intrusion-Related Au Systems • Optional DEFINITIONS Original terminology (archaic) by Lindgren (1933): • EPITHERMAL • • MESOTHERMAL • • Shallow depth, 50°-200°C, moderate P Intermediate depth, 200°-300°C, high P HYPOTHERMAL • Great depth, 300°-500°C, very high P DEFINITIONS More current usage (e.g., Robb, 2005) • EPITHERMAL • • • Lower T (~50°- ~200°C) MESOTHERMAL • • • Shallow depth (<1500 m) Intermediate depth (1500-4500 m) T ~200°- ~400°C [HYPOTHERMAL] • • Great depth (>4500 m ) T ~400°-~600°C COMMON Au-BYPRODUCT DEPOSITS Au is often produced as a byproduct from base metal deposits, especially: • • • VMS Porphyry Cu-(Au) Ni-Cu Magmatic Sulphide (e.g. Sudbury) ⇨Byproduct Au accounts for ~5-10% of current world production (Misra, 2002) PRIMARY AU DEPOSITS - CLASSES • (Shallow) Intrusion-Related (incl. “RIRG”) • • CLOSELY RELATED TO THE PORPHYRY-Cu, SKARN, MANTO, BRECCIA CONTINUUM Quartz-Pebble Conglomerate-Hosted Paleo-Placers • • ASSOCIATED WITH URANIUM (WITWATERSRAND AND ELIOT LAKE) Banded Iron Formation (BIF) –Hosted • • MANY ARE A SUB-TYPE of orogenic Au Young Placer Deposits • FLUVIAL RECONCENTRATION OF AU FROM A WIDE VARIETY OF CRUSTAL SOURCES WITSWATERSRAND/ ELIOT LAKE VMS EPITHERMAL Au OROGENIC PORPHYRY after Dubé & Gosselin, 2007 PRIMARY AU DEPOSITS - CLASSES Next few lectures will emphasize these three most important types: • “Orogenic” = GQC = [“Mesothermal Lode” ] • • • GREENSTONE-HOSTED QUARTZ-CARBONATE VEIN (GQC) DEPOSITS (Piercey says “I hate this term.”) Volcanic-Associated Epithermal Carlin-type Sediment-Hosted Epithermal Orogenic Gold Deposits • Also known as (aka): • • • • • • GREENSTONE-HOSTED QUARTZ CARBONATE (GQC) OROGENIC MESOTHERMAL LODE GOLD SHEAR ZONE-RELATED QUARTZ- CARBONATE “GOLD-ONLY” DEPOSITS DEFINITIONS Metamorphic water (metamorphic fluids): • Produced by metamorphic dehydration (and decarbonation) reactions • • Example: Chlorite (12% H2O) Amphibole (2% H2O) Alternatively, any fluid that equilibrated with metamorphic rocks at T > 300 ºC White, 1974; Skinner, 1979 Orogenic Gold Orogenic gold are characterized by: • • Quartz-carbonate veins with valuable amounts of Au & Ag Localized within faults & shear zones • Generally splays (i.e., secondary or tertiary) structures of major regional structures DEFINITIVE CHARACTERISTICS (LOCATION) • Hosted by greenschist- to amphibolite-facies rocks (formed at implied 5-10 km depth) • • Dominantly mafic composition. However, more and more deposits are being found in other types of rocks (e.g. Metasedimentary) Generally found in: • • Greenstone belts (predominantly Archean) • Ultramafic belt (e.g., Kerr-Addison; Superior Province) in listwanite (carbonated ultramafic rock) Slate belts (e.g., Meguma, Muruntau, Otago) (postArchean) after Dubé & Gosselin, 2007 L. Montessi 2005 Timmins, Ontario – host to many famous orogenic Au deposits Dome Mine, Timmins St. Ives Gold Camp, Yilgarn Craton Victory Mine et al. (Australia) http://www.hughbrown.com Can also be found as cluster of deposits Secular Distribution: Orogenic Gold Groves et al. (2005) DEFINITIVE CHARACTERISTICS (STRUCTURE) • Structurally controlled epigenetic deposits hosted in deformed metamorphosed terranes • Mineralization is syn- to late-deformation. Typically postpeak greenschist facies or syn-peak amphibolite facies metamorphism. • Simple to complex networks of Au-bearing, laminated quartz-carbonate fault-fill veins after Dubé & Gosselin, 2007 DEFINITIVE CHARACTERISTICS (STRUCTURE) • Localized in moderately/steeply dipping, compressional brittle-ductile shear zones & faults • Associated shallow-dipping extensional veins & hydrothermal breccias • Deposits distributed along major compressional to transtensional crustal-scale fault zones in deformed greenstone terranes Structurally complex!! after Dubé & Gosselin, 2007 Athena Deposit, Yilgarn, Australia Source: https://www.goldfields.co.za DEFINITIVE CHARACTERISTICS (FLUID SOURCE) • Typically associated with Fe-carbonate alteration (calcite, dolomite, siderite, ankerite) • Genetically associated with low salinity (typically < 3 wt% NaCl equiv.), CO2-H2O-rich hydrothermal fluids (+ CH4, N2, K, S, Na) • “Chameleon” deposit -> alteration minerals tend to inherit their chemistry from surrounding host rocks Mainly metamorphic fluids after Dubé & Gosselin, 2007 Mineral Assemblage Quartz-‐Carbonate ± Pyrite-‐Pyrrho*te-‐Albite in “reduced” systems ± Magne*te-‐Hema*te-‐Pyrite-‐Kspar in “oxidised” systems “Oxidised”) “Reduced”( Bt(Amp$ Qtz(Ab(Carb$ Po$ 5cm$ 1cm$ Qtz$ Hem$ 1cm$ St. Ives Gold Camp, Australia HOST ROCKS • May be hosted by all lithologies present in the local environment, especially ⇨ mafic & ultramafic volcanic rocks ⇨ Fe-rich tholeiitic gabbroic sills ⇨ granitoid intrusions (Archean) - “porphyries” ⇨ felsic volcaniclastic and sedimentary rock of “successor basins” Dubé & Gosselin, 2007 HOST ROCKS • Occasionally deposits are hosted by and/or centred within (or next to) felsic intrusive complexes ⇨syenite porphyry complex (Kirkland Lake) • District-specific lithological associations, implying chemical and/or structural traps for the fluid , e.g., ⇨ Golden Mile Dolerite Sill (Kalgoorlie) ⇨ Balmer Basalt (Red Lake) Dubé & Gosselin, 2007 HOST ROCKS – BIF • Many deposits referred to as “Banded Iron Formation (BIF)– hosted” type are likely GQC deposits localized in a BIF host lithology • For example, this is probably the origin of the famous Homestake Mine deposit in Black Hills, SD Homestake Mining Company From 1876-2002 Homestake Gold Mine produced 40 M troy ounces of Au 1244 T of Au A wide variety of host rocks are observed for orogenic deposits. On a global basis, the common theme is the structural control rather than a lithological one. Dubé & Gosselin, 2007 GOLD HOST • Au predominantly confined to quartz-carbonate vein networks (mainly contained in silicates and sulphides) • Significant Au often present within Fe-rich sulfidized wallrock selvages, or silicified and arsenopyrite-rich replacement zones after Dubé & Gosselin, 2007 VEIN TEXTURES • Moderately to steeply dipping, shear zone-hosted, laminated fault-fill, quartz-carbonate veins in brittle-ductile shear zones • With, or without, fringing shallow-dipping extensional veins & breccias Dubé & Gosselin, 2007 Arrays of extensional quartz veins, Pamour Mine, Timmins Laminated fault-fill veins, Pamour Mine, Timmins Extensional quartz-tourmaline “flat-vein" showing multiple stages of mineral growth perpendicular to vein walls, Sigma Mine (from Poulsen et al, 2000) Dubé & Gosselin, 2007 VEIN MORPHOLOGY • Au-bearing shear zones and faults commonly display complex geometries ⇨ Anastomosing and/or conjugate array structures • • Individual fault-fill veins extend 10 m -100s of m • Recognizing and delineating arrays of auriferous veins (“ore shoots” ) represents a critical element in defining & exploiting richest part of a orogenic Au orebody An entire vein array can extend 1-2 km in its longest (vertical) dimension Dubé & Gosselin, 2007 VEIN MORPHOLOGY • Stockworks and hydrothermal breccias may also represent a main host to the mineralization ➡ especially when developed in competent units such as granophyric facies of gabbroic sills (e.g. San Antonio Mine, MB; Argo Mine, Australia) • Ore shoots/ore zones are commonly controlled by intersection between: 1. different vein sets or host structures, or 2. auriferous structures and especially reactive and/or competent rock types such as Fe-rich gabbro Dubé & Gosselin, 2007 Athena Deposit, Yilgarn, Australia Source: https://www.goldfields.co.za MINERALOGY • • • Main gangue minerals • • quartz + carbonate (calcite, dolomite, ankerite, siderite) Ankerite: Ca(Fe2+,Mg)(CO3)2 Variable amounts of: • sericite + chlorite + amphibole + biotite ± scheelite (W) ± tourmaline (B ) Main ore minerals: • • • native Au Py ± Po ± Cpy ± Arspy ± telluride sometime hosted in silicate (biotite, amphibole) MINERALOGY (Continued) • • • Sulphides typically constitute <10% of ore No significant vertical mineral zoning Arsenopyrite often is the main sulphide in terranes of amphibolite facies (e.g., Con, Giant and Campbell-Red Lake deposits) • Trace molybdenite or sellurides appear in some deposits (e.g., syenite-hosted Kirkland Lake deposits) VG VG Syenite-hosted high-grade quartz veins. These contain visible gold (VG), disseminated pyrite and trace tellurides (Main Break, Kirkland Lake, ON). Dubé & Gosselin, 2007 High-grade zone. quartz carbonate vein. Visible Au & Asp-rich replacement of host basalt. Red Lake Mine, ON Arsenopyrite-rich auriferous silicification overprinting low grade to barren carbonate±quartz veins is the main host association of Campbell-Red Lake (Goldcorp) deposit. (This deposit is in amphibolite facies terrane). Dubé & Gosselin, 2007 Dubé & Gosselin, 2007 C. “Green-carbonate rock” showing fuchsite-rich replacement with Fecarbonate veining in highly deformed ultramafic rocks, Larder Lake. D. “Green-carbonate” alteration showing abundant green micas replacing chromite-rich ultramafic rocks, Baie Verte, NL. Chromium-rich micas (esp. fuchsite) are common indicators of potential orogenic Au mineralization in ultramafic terranes. Fuchsite K(Al,Cr)3Si3O10(OH)2 [Cr-Muscovite] ALTERATION - GREENSCHIST • Altered host rocks proximal to veins show • • Enrichment in CO2, S and K2O Depletion of Na2O • Further from veins, alteration characterized by chlorite + calcite (± magnetite) • Dimensions of alteration haloes vary widely with composition of the host rocks (m - km) • Haloes may fully envelope entire deposits where hosted by mafic and ultramafic rocks Dubé & Gosselin, 2007 ALTERATION - AMPHIBOLITE • Alteration may be more complex & varied in amphibolite facies terranes • Common assemblages associated with Au mineralization include: • • biotite, amphibole, pyrite, pyrrhotite & arsenopyrite biotite/phlogopite, diopside, garnet, pyrrhotite ± arsenopyrite ± K-feldspar ± calcite ± clinozoisite Dubé & Gosselin, 2007 ALTERATION - AMPHIBOLITE • Canadian examples of amphibolite facies deposits • • Madsen-Red Lake (Dubé et al 2000, 2001b) Eau Claire-James Bay (Cadieux, 2000) GEOCHEMICAL SIGNATURE OF ORE • Orogenic gold ore is enriched in: • • • As, Sb, W, Mo, B and Te Typically relatively low concentrations of: • • • Au, Ag Cu, Pb and Zn No vertical metallic zoning in ore shoots Au : Ag ratio > 1; Typically 5 to 10 Dubé & Gosselin, 2007 TRANSPORT OF GOLD Reduced sulphur complex at near neutral pH (From Mickuki, 1998) Another look at Au Put these two slides in as they are useful. Use them if you see fit. AuCl2- associated with what types of assemblages? Au(HS)2- associated with what types of assemblages? From Williams-Jones et al. (2009) Au Solubility So, how do we get gold to precipitate? From Williams-Jones et al. (2009) PRECIPITATION OF GOLD 2 principal methods: • Wallrock sulphidation (Mikucki, 1998 and Williams-Jones et al. (2009) FeOrock + 2H2S = FeS2 + H2O+H2 and Au(HS)2-+H++1/2H2 = Au + 2H2S • Earthquake • Rapid expansion in fracture could cause the fluids to evaporate, triggering almost instantaneous gold deposition - seismic pumping (Sibson et al., 1987; Craw, 2013) Genetic Model - Orogenic Au • Ore-forming fluids are typically: • • • • 1.5 ± 0.5 kbar 350° ± 50°C Low-salinity H2O-CO2 ± CH4 ± N2 • Gold predominantly transported as a reduced sulfur complex (Mikuki, 1998; Goldfarb et al., 2001; and Groves et al., 2003). • Current models emphasize a deep source for gold and fluids related to metamorphic devolatilization. Dubé & Gosselin, 2007 Genetic Model Kerrich et al. (2005) Genetic Model From Groves et al. (1998) and Goldfarb et al. (2001) Genetic Model (Review) Transport Deposition Source Location: splay structures off larger regional fault systems (2nd-3rd order) Mechanisms: - wallrock sulphidation in Fe-rich rocks - rapid P decrease (earthquake) Au: as reduced S complexes (mostly Au(HS)2pH: near neutral (5-7) Fluids: metamorphic fluids => low salinity, CO2-H2O-rich Au: metamorphic fluids Depth: deep source (5-10km) T (°C): moderate temperature (350° ± 50°C) COEXISTING Au DEPOSIT TYPES • In Archean greenstone belts, other types of Au deposits (formed at different crustal levels) have become juxtaposed against orogenic gold deposits as a consequence of progressive long term tectonism and metamorphism. • Au-rich VMS (formed earlier) or intrusion-related Au deposits (often formed later) now co-exist along major regional faults. • For example, the Bousquet-LaRonde Au-rich VMS deposits lie along the Larder Lake-Cadillac Fault, near several orogenic gold deposits east of Noranda in the Abitibi greenstone belt. Greenstone-Hosted Orogenic (quartz-carbonate vein (GQC) vein) Deposits • Canadian examples: • • • • • • Sigma-Lamaque (PQ) Dome and Kerr Addison (ON) Giant and Con (NWT) San Antonio (MB) Hammerdown and Deer Cove (NL) Bralorne-Pioneer (BC) Distribution of Canadian orogenic Au deposits with respect to structural province Dubé & Gosselin, 2007 Modified from Poulsen et al (2000) * mainly in secondary or tertiary structures Simplified geological map of the Abitibi Greenstone Belt. Orogenic Au deposits are typically associated with large scale (crustal) transpersonal faults. ⇨ Note also the spatial association with Au-rich VMS deposits Dubé & Gosselin, 2007 TROY OUNCES & GRAMS • Au commodities price is commonly quoted in $US per troy ounce: 1 Troy Ounce = 31.1 g • • At $1230/oz, 1g of Au is worth US$40 Ore that is 1g/t of Au contains the equivalent of only 1ppm Au WORLD PRODUCTION & RESERVES • Historical production + reserves for the entire orogenic gold deposit subtype is ~16,585 T Au (Dubé and Gosselin, 2004) - equivalent to 13 % of the world total for Au • 41 “world-class” orogenic gold deposits contain >100 T of Au - including 12 giant deposits with > 250 T • 7 of these 41 are from the Archean Superior Province - 6 from the Abitibi GB and 1 from the Uchi Sub-Province (Campbell-Red L. Deposit) • Superior Province is the single largest well-preserved Archean craton in Au endowment, followed by the Yilgarn craton of Australia WORLD PRODUCTION & RESERVES • Historical production + reserves for the entire orogenic Au deposit subtype ~16,585 T Au (Dubé and Gosselin, 2004) ⇨ Equivalent to 13 % of the world’s total historical production of Au ⇨ US Government reports a holding of 8,133.5 tonnes as a bullion reserve (June 2013 - World Gold Council data) Dubé & Gosselin, 2007 1 oz/T Tonnage and grade for all global Au deposits containing >30 T Au WORLD PRODUCTION & RESERVES • At 13% of historical production, orogenic gold are second only to the Witwatersrand “paleoplacers” of South Africa • • Orogenic gold deposits typically 5 - 15 g/t Au Tonnage highly variable (104 - >107 T ) ⇨ typically a few million T of ore WORLD PRODUCTION & RESERVES • Largest greenstone orogenic gold deposit (total Au content) is the Golden Mile complex, Kalgoorlie, W. Australia (Norseman-Wiluna GB, Yilgarn Block) ⇨ 1821 T Au • The Hollinger-McIntyre deposit in Timmins, ON (Abitibi GB) is second largest orogenic gold known ⇨ 987 T Au Dubé & Gosselin, 2007 Tonnage vs grade of Canadian and all world-class size (≥100 T Au) orogenic deposits. GRADE AND TONNAGE CHARACTERISTICS • In Canada, the discovery (Slave, Yellowknife GB) and Campbell-Red Lake (Superior, Uchi –Red Lake GB) deposits have had the highest average grades ⇨ 34 g/T & 23 g/T Au, respectively • The Goldcorp high-grade zone of the Campbell-Red Lake deposit has an average production grade of 88 g/T Au since mining began (Dubé et al, 2002) GSC, 2007 DEFINITIONS In reality, Fluid Temperatures for Epithermal Au & Mesothermal Au environments largely overlap (100 - 400ºC) ⇨ Depth (P) Regimes are a more fundamental difference between GQC & other Au deposit types – as is fluid chemistry • EPITHERMAL ⇦ Epithermal Au • • • Shallow depth (<1500m)!! Lower T (~50°- ~200°C) MESOTHERMAL ⇦ Orogenic (GQC) Au • • Intermediate depth (>1500m) T ~200°- ~400°C Distribution of Canadian Orogenic Au deposits ⇨ by structural province Dubé & Gosselin, 2007 GSC Open File 4668 All Au Producing Districts of Canada. “Lode Gold” in this GSC report refers to all hydrothermal deposits whose principal commodity is gold ⇨ i.e., orogenic & epithermal types EPITHERMAL Au DEPOSITS • “Epithermal” Au deposits account for < 5% of Canadian production, but a substantially higher percentage of total global production. • In Canada, they occur predominantly in extensional terranes in Mesozoic & Tertiary rocks of the Cordillera • Important Canadian deposit examples: • • • Mt. Skukum, YT Blackdome, Cinola, Toodoggone, BC Hope Brook, NL [Neoproterozoic - Avalon Zone] EPITHERMAL Au DEPOSITS • Often amenable to open-pit & to heap-leach extraction of Au • Almost all well-studied examples of epithermal Au deposits are located in circum-Pacific volcanic belt • In particular: • • • Basin & Range Province, western USA Western Pacific island arcs Andean continental arcs WITSWATERSRAND/ ELIOT LAKE VMS EPITHERMAL Au OROGENIC PORPHYRY after Dubé & Gosselin, 2007 Distribution of epithermal Au deposits in the Circum-Pacific region Hedenquist et al, 2000 EPITHERMAL Au DEPOSITS • Individual deposits characterized by marked variations in temperature & pressure across ore forming regime • Ore deposition provoked by marked changes in physicochemical conditions of ore-forming fluids over short distances (meter scale) ⇨ Often vertically zoned DEFINITIVE CHARACTERSITICS • Typically hosted in calc-alkaline volcanic pile, most commonly andesitic volcanics & pyroclastics (sub-caldera environment) • Often localized within fractures associated with caldera structures • • Epigenetic mineralization in quartz veins Vein systems flare upwards into wedge- or cone-like features DEFINITIVE CHARACTERSITICS • • Principal economic metals: Au & Ag Characteristic ore-associated wallrock alteration types: ⇨ Adularia-sericite (ADS) ⇨ Quartz-kaolinite-alunite (QAL) • Other classification based on sulfidation state (most common): • • • High sulfidation (~QAL; high sulfur) Low sulfidation (~ADS; low sulfur) Also have intermediate sulfidation NB: Subsidiary (aka) terms involving “sulphur” are misleading. Those involving “sulphidation” are widely used, and have an explicit physicochemical meaning (i.e., sulphide assemblage). B Abundance of Au-Ag-base metals in major ore deposit types Poulsen, 1996 Meaning of the term “sulfidation” High or low sulfidation state is reflected by the stable sulfide mineral assemblage and is fundamentally controlled by T and ƒS2 Einaudi et al, 2003 Low vs. Intermediate vs. High Sulfidation Advanced Argillic Sillitoe and Hedequist (2003) A simpler table Simmons et al. (2005) EPITHERMAL Au - MINERALOGY • • Au typically as native Au and/or electrum (Au-Ag) • • Most have Au:Ag <1 Au sometimes in tellurides or dissolved within sulphides (e.g., in arsenious Py) Hot spring, Carlin-type & some others have a characteristic Au, As, Sb, Hg, Tl association EPITHERMAL Au DEPOSITS • Hot spring-type • May comprise part of high and low sulfidation types; more common in the latter. • Essentially any significant (subaerial) surface expression of an epithermal system • Near surface silicified zones, sinters or phreatic breccias ±(Au/Ag) mineralization • Explicitly identified by sinters (only stable in low sulfidation systems), or other evidence of nearsurface (100ºC) boiling MINERALOGY-Low Sulfidation Ore minerals : • • • Pyrite ± Pyrrhotite, arsenopyrite, Fe-rich sphalerite chalcopyrite,tetrahedrite/tennantite, Fe-poor sphalerite (galena, acanthite (argentite)) and: • Quartz (sugary & crystalline), chalcedony as colloform, crustiform, cockscomb veins • • • • Adularia Calcite ± rhodochrosite ± barite (often bladed in nature) Chlorite Fluorite MINERALOGY- Low Sulfidation Many low sulfidation eposits contain an ore assemblage with one or more of the following: Native gold Au Electrum Au-Ag Native silver Ag Acanthite (argentite) Ag2S Naumannite Ag2Se Aguilarite Ag4SeS Native silver + one or more of the Ag-Se-S minerals is the “ginguro“ (silverblack) assemblage described at Hishikari and elsewhere. • Inner zone (silicification) • • Outer Zone (potassic-phyllic Alteration) • • Replacement of wallrocks by quartz/chalcedony quartz ± Kfeldspar(adularia) ± sericite Distal/overprinting zone (argillic alteration) • Kaolinite/Smectite/ • Chlorite + carbonate ± epidote are often present in a broad surrounding envelope (Propylitic Alteration) • Sinter/steam-heated zone • Kaolite, alunite, native S, opaline silica. From Hedenquist et al. (2000) and Tosdal et al. (2009) Crustiform Quartz-Adularia Veins (Low Sulfidation) M. Thirnbeck A typical example of crustiform banded quartz-adularia. Indonesia Etoh et al, 2002 Quartz pseudomorphs after bladed calcite, Hishikari, Japan Bladed calcite, and quartz pseudomorphs, are commonly formed in the boiling portions of ADS/Low-Sulfidation Au deposits SPECIAL MINERALOGY/LITHOLOGY Hot Spring Type Deposits • Sinter* * A fine-grained chemical sedimentary rock deposited by precipitation from mineral waters, especially siliceous sinter and calcareous sinter . May contain clays, sulphate minerals, minor pyrite. Silica sinter is only stable in ADS-Type fluids Often a result of magmatic fluids/vapours interacting with the water table. Sinter Crowfoot-Lewis deposit, Nevada Champagne Pool, NZ http://waiotapu.co.nz/wp-content/plugins/doptg/uploads/ e96E95D8hMm9YFfmd7RwYd8TnF6fLRQpt6rAfZ8jX3zKjS25OsTNMzhLn92qwHK 2y.jpg Mud Pools, Torotoro National Park, Bolvia Sinters show columnar structures not typical of other naturally occurring silicas White et al, 1989 Native Sulfur, Alunite Crowfoot-Lewis, Nevada KAl3(SO4)2(OH)6. • Inner zone (silicification) • • Outer Zone (potassic-phyllic Alteration) • • Replacement of wallrocks by quartz/chalcedony quartz ± Kfeldspar(adularia) ± sericite Distal/overprinting zone (argillic alteration) • Kaolinite/Smectite/ • Chlorite + carbonate ± epidote are often present in a broad surrounding envelope (Propylitic Alteration) • Sinter/steam-heated zone • Kaolite, alunite, native S, opaline silica. From Hedenquist et al. (2000) and Tosdal et al. (2009) MINERALOGY-High Sulfidation “High sulfidation” ore minerals: • • Pyrite + enargite/luzonite + covellite ± bornite, chalcocite, bismuthinite and: • • • • Quartz (“vuggy silica”) Alunite and Al-silicate (kaolinite-dickite) Barite ± native sulphur Bladed calcite is NOT Characteristic High Sulfidation deposits are characterized by advanced argillic alteration: • • • Quartz + kaolinite* + alunite + dickite* + pyrite • • Diaspore* may also occur Pyrophyllite [Al2(Si4O10) (OH)2 ], instead of kaolinite in deeper deposits Carbonates absent !! (very low pH) Zones of silica replacement & vuggy (residual) silica are common *Kaolinite Al2Si2O5(OH)4 Triclinic *Dickite Monoclinic Al2Si2O5(OH)4 *Diaspore AlO(OH) Orthorhombic From Hedenquist et al. (2000) and Tosdal et al. (2009) Henley, 1991 Ore Mineralization classically vertically below argillic alteration & contains the high sulfidation ore assemblage Ore mineralization often replaces pre-existing vuggy silica Advanced argillic alteration forms an ore envelope Propylitic alteration constitutes a broad underlying host envelope – dominantly chlorite ± epidote Schematic cross-section of high sulfidation deposit. Showing alteration, mineralogy & general location of ore zones Vuggy Silica Vuggy silica is a residue of heavy leaching of the host rocks by acid hydrothermal fluids Stratex – Altintepe, Turkey geocosas.wordpress.com Vuggy Silica, Cerro Rico, Potosi, Bolivia SPECIAL MINERALOGY/LITHOLOGY • Quartz-kaolinite-alunite deposits • • • • • Alunite KAl3[(OH)3|SO4]2 Natroalunite (Na,K)Al3[(OH)3|SO4]2 Jarosite KFe3+3[(OH)3|SO4]2 Enargite Cu3AsS4 Orthorhombic Luzonite Cu3AsS4 Tetragonal Dyet.net KAl3 [(OH)3|SO4]2 Alunite. Marysvale, UT mindat.org Alunite on quartz (20mm sample) ALUNITE Heves Co., Hungary Enargite and Pyrite (8mm sample). Red Mountain Deposit, San Juan Co., CO Cu3AsS4 mindat.org ENARGITE Enargite (5mm sample). Julcani Mine, Angaraes Pr., Peru mindat.org Although it is a consistent component of many examples, not all epithermal Au systems conform to a simple, structurally controlled, “stockwork feeder” geometry. Inhomogeneities in country rock permeability, and hydrothermally induced porosity, can also act to localize alteration and ore zones. Hedenquist et al, 2000 From Hedenquist et al. (2000) and Tosdal et al. (2009) Epithermal Au - SIZE & GRADE • • Grades typically 2.5 – 25 g/t • • Au:Ag typically < 1 (unlike GQC) Tend to be smaller than orogenic gold deposits – many epithermal Au deposits have been exploited at < 1Mt of ore Some deposits contain “Bonanza” zones that have grades well in excess of 30 g/t Hishikari Au Deposit Kyushu Island, Japan 5.2 Mt @ 60g/T Au (Hedenquist et al, 2000) Metal Mining Agency of Japan, 1990 Ultra high grade Au ore with quartz-adularia in rhyolite breccia Sleeper Deposit, Humboldt Cty., Nevada Long dimension is 10.4 cm James St. John, OSU-Newark Reserves & production statistics-epithermal Au deposits of BC Panteleyev, 1988 Taylor, 1996 Grade vs tonnage for Canadian epithermal Au deposits SIZE & GRADE - High Sulfidation • High sulfidation deposits of magmatic-hydrothermal origin tend to be restricted to areas proximal/above the implied heat source • For example at Summitville, CO, altered rocks outcrop over an area of 1.5 km x 1.0 km (produced 3.5 T of Au) • At the Al Deposit (Toodoggone) quartz-clay-alunite(+barite +dickite) alteration is exposed over 250m x 1500m after Taylor 1996 Schematic cross-section of the HS deposit at Summitville, CO. Note close spatial relationship of deposit to underlying porphyry stock, and implied position of deposit within an original resurgent rhyolite dome within a parent caldera complex. Hedenquist, 2008 SIZE & GRADE - Low Sulfidation • Low sulfidation deposits can cover very large areas, even though the host-rock alteration is generally restricted to a narrow envelope enclosing veins and breccias • Blackdome Mine (BC) - quartz veins up to 0.7m thick and 2200m long, within a 2 km x 5 km area, contain ~8.9 T Au • Creede, CO, - mineralized veins have been mined over strike lengths exceeding 5 km after Taylor 1996 Commodore Mine, Creede, CO. Circa 1900 GEOLOGICAL SETTING • Intraoceanic island arc volcanoes ⇨ e.g., Papua New Guinea • Continental arcs ⇨ e.g., Silverton & Creede calderas, CO after Taylor 1996 STRUCTURAL CONTROLS • Regional strike-slip faulting ⇨ e.g., Eocene of BC • Features of caldera volcanism ⇨ e.g. Colorado deposits: • • • Ring fractures Radial faults Extensional faults from resurgent doming after Taylor 1996 An example of laterally extensive veins in an low sulfidation-type deposit Structural setting and vein distribution. Blackdome Mine, BC Taylor, 1996 Thickness x grade cross-section. Cirque Vein, Mt. Skukum, YT Taylor, 1996 Secular Distribution Goldfarb et al. (2010) AGE SIGNIFICANCE? • Most deposits are Tertiary-age or younger • • Since these are shallow phenomena, older deposits are more likely to have been removed by erosion However, examples of epithermal Au type mineralization are found Late Proterozoic through Recent: • • • Toodoggone, BC – Jurassic Queensland, AUSTRALIA – Paleozoic Avalonian (NL) - NeoProterozoic Genetic Models From Hedenquist et al. (2000) and Tosdal et al. (2009) Epithermal Systems From Robb (2005) after Hedenquist et al. (2000) GENETIC MODEL-High Sulfidation Au • Formed at shallow (“epithermal”) depths in the core of a volcanic edifice • • Overlie intermediate to felsic intrusions of porphyry-type Implied dominant role of magmatic water – especially during initial stages GENETIC MODEL-Low Sulfidation Au • Formed in upper few 100 m of a large hydrothermal system • System driven by volcanic heat - but dominated by variable contributions from meteoric water (more in LS than HS) • Extensive lateral fluid flow may result in ore deposition over wider area, sometimes displaced laterally from proposed heat source Epithermal Systems From Robb (2005) after Hedenquist et al. (2000) Epithermal Systems From Robb (2005) after Hedenquist et al. (2000) Au Solubility Boiling 1) Au(HS)2-+ = Au + H2S + S 2) AuCl2- +H+ = Au + 2HCl Mixing 1) 4Au(HS)2-+ 2H2O+ 4H+ = 4Au + 8H2S + O2(g) 2) 4AuCl2- + 2H2O = 4Au + 8Cl- + 4H+ + O2(g) From Williams-Jones et al. (2009) Epithermal Systems 4Au(HS)2-+ 2H2O+ 4H+ = 4Au + 8H2S + O2(g) 4AuCl2- + 2H2O = 4Au + 8Cl- + 4H+ + O2(g) From Robb (2005) after Arribas et al. (1995) “Intermediate Sulfidation” Deposits • Recent coinage of intermediate sulfidation (IS) deposits refers to a subgroup that displays many of the general characteristics of ADS (LS) deposits. • IS are characterized by qz-sericite with barite, rhodochrosite, anhydrite • • • Creede, Co is considered a typical example of IS IS are associated with andesitic continental arc volcanism IS are deeper (300-800m) & slightly hotter (>225°C) vs LS (<300-400m; <225°C) (Hedenquist 2008) Epithermal Systems From Robb (2005) after Hedenquist et al. (2000) Carlin-type gold deposits READ: Robb (2005) Section 3.9.2 Carlin-type Gold Deposits, p.192-195. Posted on web Carlin-Type Au Deposits • • Exact genesis still controversial !! • Significant potential for additional discoveries of this deposit type beyond Nevada/Western USA. Generally recognized as a separate Type of epithermal Au deposit – but possess characteristics of both orogen and epithermal Au deposits. ⇨ Carlin-type deposits ostensibly recognized in the 1980s in Guizhou Province, SE China. ⇨ Belt of Au mineralization with strong Carlin-type features currently being explored in YT. The Carlin Deposit • • • Discovered 1961 – Newmont Mining Co • Numerous subsequent discoveries of this “type” in the Carlin Trend and four related districts in Nevada • Other examples exist in similar terranes from N. Mexico through Montana; also China and YT. Production began 1965 Estimated to contain 3440 T of recoverable Au (Teal & Jackson, 1997) @ < 5g/T after Berger & Bagby, 1991 Carlin-type Au deposits in Nevada (white circles) Hofstra et al, 2003 T. Moore, USGS(2012) Battle Mountain Mining District, Nevada DEFINITIVE CHARACTERISTICS • Most favourable host rocks: ⇨ Silty carbonaceous rocks (dirty carbonates) • • Fine-grained, detritus-rich lithologies are often most susceptible to broad scale replacement/Au-mineralization Major host structures: ⇨ High-angle normal faults • • Related to tectonic doming of autochthonous* rocks. Important in channeling fluids/localizing mineralization * In a tectonic context: found where they and their constituents were formed [opposite of allochthonous = tectonically emplaced suites] after Berger & Bagby, 1991 Schematic Cross Section: Carlin from Ridley (2013) Meikle Mine from Ridley (2013) DEFINITIVE CHARACTERISTICS • Granitic igneous rocks occur (or are suggested by geophysics) in the vicinity of all the Nevada Carlin-type deposits • these intrusions themselves often display intensive hydrothermal alteration • overlying ore mineralization commonly occurs in fractures parallel to the intrusions • to date, no direct genetic link established between intrusions and Au deposits • genetic link is implied, however (e.g., Ressell et al., 2006; Muntean et al., 2011) after Berger & Bagby, 1991 Regional Relationships from Ressell and Hendry (2006), Dickinson (2004) and Ridley (2013) DEFINITIVE CHARACTERISTICS • Pre-Au stage jasperoid often replaces carbonate rocks along/adjacent to fault structures • • barite a common accessory of this stage Syn-Au stage with continued silicification: • • • quartz veins and silicified breccias additional silicification of host rocks some clay alteration of detrital grains in hosts after Berger & Bagby, 1991 DEFINITIVE CHARACTERISTICS • Post-Au calcite veins are common • • ±barite ±fluorite ±orpiment ±realgar ±stibnite ±cinnabar ±Tl minerals after Berger & Bagby, 1991 DEFINITIVE CHARACTERISTICS • Evidence of dissolution of carbonate minerals & precipitation of silica in host rocks • Au deposition associated with low-salinity, high-CO2, high-H2S fluids (similar to orogenic fluids) • Available data for Au-deposition stages give 200 - 300°C range for ore formation • No apparent vertical zonation in ore mineralogy (similar to orogenic Au) after Berger & Bagby, 1991 Schematic Cross of Carlin-type Deposit from Hofstra and Cline (2000) and Ridley (2013) AGE • Absolute age has been difficult to establish for the Nevada deposits: • Likely >28Ma (Oligocene)- based on radiometric dating of supergene alunite (Gold Quarry deposit) • Other deposits give radiometric ages in the Cretaceous (<144Ma) – but these may be preserved ages of minerals in host rocks • Arehart et al (2003) argue for 33-42 Ma based on compilation of what they deem most reliable geochronological data available after Berger & Bagby, 1991; Arehart et al 2003 REGIONAL GEOLOGY/TECTONICS • Host rocks for Nevada deposits are allochthonous Paleozoic through Early Mesozoic sedimentary rocks (+lesser volcanic rocks) ⇨Originally deposited along complex evolving continental margin • Sediments have subsequently been affected by three major orogenic events: • • • • L. Dev.-E. Miss. Antler Orogen (~360Ma) L. Permian Sonoma Orogen (~240 Ma) L. Jurassic - E. Cretaceous Sevier Orogen (~135Ma) Only this last (Mesozoic) orogeny was accompanied by magmatism after Berger & Bagby, 1991 Roberts Mountain Thrust Regional Tectonic Context of the Nevada Deposits Berger & Bagby, 1991 gray/Stipple pattern shows extent of Paleozoic allochthon Roberts Mountain Rocks MINERALOGY • Native Au – extremely fine-grained ⇨ often ≤1 micron (almost sub-microscopic) • • ±native Ag ±tellurides Pyrite the most common sulphide • • • Py often the host for native Au Au also commonly occurs as coatings/inclusions on/ within arsenian (As-rich) zones in Py ±marcasite ±arsenopyrite after Berger & Bagby, 1991 Upper frame is a map of As concentration. Bands labelled 3a-3c in Lower frame show the deportment of Au in zones of arsenious pyrite consequent to a specific series of hydrothermal episodes – referred to by the authors as the “Carlin event” for this deposit. From Barker et al, 2009 ALTERATION • Highest Au grades at Carlin are associated with zones of co-extensive silicification and K-metasomatism characterized by: • quartz + dolomite + illite/sericite • Highest Au grades at Carlin are not closely associated with pre-Au Jasperoid or most intensive silicification • Other alteration zones may include: • dickite, kaolinite, calcite, K-feldspar after Berger & Bagby, 1991 Simplified regional cross-section illustrating perceived fundamental structural and lithologic controls on the location of Carlin-type gold deposits in Nevada Robb 2007: after Hofstra & Cline 2000 Ck District n Trend ?? After Heitt et al 2003 SOME PROPOSED GENETIC MODELS • Distal magma-related replacement deposits (Berger & Bagby, 1991) • • • Magmatic CO2 + H2S + H2O(+ Contact Metamorphic CO2). Mixing with low-CO2 low-T meteoric water causing Au ore deposition (Rose & Kuehn (1987)) Distal W (Au)-skarns (e.g. Sawkins, 1984) SEDEX model for “stratabound” portions of Carlin Deposits (Berger & Theodore, 2005) after Berger & Bagby, 1991 Muntean et al.’s Model Muntean et al.’s Model A newly recognized belt of Carlin-style Au mineralization in Yukon Territory: The Rau-Nadaleen Trend ATAC Resources, 2013 Corporate Presentation ATAC Resources, 2013 Corporate Presentation OSIRIS AREA – CONRAD ZONE ATAC Resources, 2013 Corporate Presentation What is the single largest use of gold? (BLING BLING) Sector Usage of Gold – Au(T) galmarley.com A staggering 320 tons of gold and more than 7,500 tons of silver are now used annually to make PCs, cell phones, tablet computers and other new electronic and electrical products worldwide BullionStreet.com Monday, July 9th, 2012 Largest Gold Consumers galmarley.com Largest Primary Gold Producers(2002) – Au (t) goldsheetlinks.com Officially Reported Gold Holdings June 2013 • • • • • United States 8,133.5 T China 1,054.1 T Russia 1,015.4 T Japan Canada 765.2 T 3.2 T World Gold Council