Economically Significant Gold Deposits: Characteristics and Distribution

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1 Economically Significant Gold Deposits: Characteristics and Distribution C. Jay Hodgson, Consultant Annual KEGS Meeting, March 6, 2010

2 Outline 1. Although gold deposit can be classified into many types, almost all economically important deposits can be grouped into five types: Precambrian paleoplacer, gold-rich (>.25 g/t) porphyry, epithermal, greenstone, and sediment-hosted. Only the last five are considered here. 2. Although the relative abundance of types varies, the size-distribution among types is very similar, so that the contribution of each to total world gold resources is proportional to their abundance. 3. Most gold deposits occur in gold belts, and are clustered within these belts in gold districts and camps characterized by deposits of a variety of types and ages. 4. Most gold deposits, perhaps all, are formed in association with felsic magmatic activity in the upper few kilometers of the earth s crust, but at least some greenstone deposits probably formed at ~ 10km depth. 5. Variations in ore fluid source (magmatic, meteoric, metamorphic, mixed), temperature, pressure, pressure-temperature gradients, boiling, extent of fluid- wall rock interaction, wall rock type and nature and distribution of permeability control the variations among deposits.

3 Main Geological Types of Gold Deposits (F. Robert, pers. com., 2010) Epithermal Sediment-hosted Porphyry Greenstone

Most gold deposits occur in clusters ranging in scale from regional metal provinces or belts, to district to camp scale clusters within these belts World distribution of Big gold deposits and high

4 Most gold deposits occur in clusters ranging in scale from regional metal provinces or belts, to district to camp scale clusters within these belts World distribution of Big gold deposits and high endowment areas 1-3 Moz 3-20 Moz >20 Moz Rand Some post-precambrian subduction & continental collision zones Endowed area, with 1 or more >20 Moz deposits, showing total ounces and Oz in >20 Moz deps/no.dep number of >20 Moz deposits Endowed belt?

5 Gold deposit distribution in the Superior Province clearly illustrates clustering on the regional to mining camp scale Distribution of gold deposits in the Superior Province

6 Deposits of different type and/or significantly different age are concentrated (and thus help define) the same belts, districts and camps. Syn- to post D2 veins (~2660 Ma) Post D1 disseminated (~2675 Ma) Syn-volcanic VMS, veins (~2700 Ma) > ~1 Moz. > ~5 Moz.

7 Endowment of the Timmins-Val d Or gold belt is ~200 Moz., with over 95% of it in deposits located in these 7 camps Gold camps of the Southern Abitibi Belt >~1 Moz. > ~5 Moz Porcupine 2. Kirkland-Larder L. 3. Harker-Holloway 4. Duparquet 5. Noranda 6. Doyon-LaRonde-Cadillac 7. Malartic-Val d Or

8 Million ounces The size distribution in all types is remarkably similar, Size distribution of gold deposit types Porphyry Greenstone Sediment-hosted Epithermal Percent of deposits ep porp CSH gnstn total popul

9 Differences in the contribution of types to total gold is the result of differences in deposit abundance among the types. Percent of total gold in each type Subpopulation size vs percent of total gold in all > 3Moz deposits Porphyry Greenstone Epithermal Sed-hosted Number of deposits of each type

10 Gold-rich porphyry deposits Two subtypes: 1. Associated with high-level oxidized intrusive complexes; 2. Associated with intermediate level reduced intrusive complexes. Large stockwork, breccia, sheeted veinlet zones spatially and genetically related to multiphase felsic to intermediate intrusive complexes Grade: >0.25g/t Au (>$6-10), with % Cu ($25-50) in oxidized systems and no Cu, or much less, in reduced systems; Mineralogy: Inverted cup zoned from bornite/chalcopyrite to chalcopyrite to pyrite, with lithocap HS bornite-pyrite-enargite overprint. Early magnetite (M veins) in some deposits, especially those enriched in Au Gold associations: in bornite-chalcopyrite deep zone and/or bornite-enargite-pyrite in high-level advanced argillic overprint. Alteration: zoned from potassic to quartz-sericite-pyrite to prophyllitic, with late high-level over-print of advanced argillic associated with high sulfidation Cu- Ag or Au+Ag mineralization (normally veinlet to vein systems) in oxidized systems.

11 Gold-rich Epithermal Deposits Two types: 1. High Sulfidation (enargite; <1 mole % FeS in sphalerite); 2. Intermediate Sulfidation (tennantite, 20-1 mole % FeS in sphalerite) to Low Sulfidation (arsenopyrite-chalcopyrite; Mole% FeS in sphalerite); Advanced argillic alteration with HS, sericite-carbonate-chlorite with IS and LS Highly variable geometry: Veins, veinlet systems, stockwork/disseminated zones and breccias, formed within a few hundred meters of the paleosurface in subarea system, and exhalative massive sulfide accumulations at the paleosurface in submarine systems. Invariably show mineralogical and compositional zoning related to paleosuface. Limited vertical extent of ore ( m), with narrower low grade base and barren top Grade ranges from bonanza (multi-ounce/tonne) to <0.5g/t, with Ag/Au typically high. Proportions of main minerals, quartz, carbonate, basemetal sulfides are highly variable. High sulfide content ore typical of deeper parts of subareal vein systems, or VMS deposits where mineral precipitation occurs at high temperature due to pressure of overlying water column.

12 Sediment-hosted Deposits Two sub-types: 1. Calcareous siltstone-hosted (Carlin-type) and 2. Clastic sediment-hosted (Muruntau/Sukhoi Log-type) Stratiform (and less commonly, cross-cutting) zones of disseminated replacement, veinlet stockwork/breccia mineralization. Grade variable, but no-oxidized deposits typically 3-6g/t Mineralogy: pyrite, arsenical pyrite, arsenopyrite, quartz. Ores commonly refractory due to significant part of Au occurring in arsenical pyrite. Alteration: decalcification of calcareous units; silification, quartz-pyrite veins and breccia matrix, illite.

13 Greenstone Deposits Defined as all except VMS deposits in Precambrian greenstone belts, and are mainly quartz veins and vein systems formed in association with late-tectonic reduced porphyry dikes and plugs. However, in the group are low sulfidation epithermal deposits and sediment-hosted deposits. Two age subtypes: 1. Post D1-Pre D2 and peak metamorphism + fabric vein and disseminated replacement (Malartic type); 2. Syn- to post D2 vein (Sigma-type) Large geometrical variety of shear-hosted veins, stockwork, breccias and siliceous replacement zones; commonly partly controlled by favourable units (iron formation, mafic to felsic dikes and plugs, interflow sedimentary horizons, etc.). Almost invariably associated with felsic porphyry dikes and plugs. Mineralogy: Quartz, Fe-Mg carbonate, pyrite, scheelite, tourmaline; minor chalcopyrite, sphalerite. Pyrrhotite and arsenopyrite particularly common in host sequences dominated by sediments Grade: open pits may mine rock grading less than 1g/t, but underground mine grades typically 4-10 g/t, with high grade zones > 30g/t. Alteration: pervasive quartz-fe-mg carbonate in mafic and ultramafic rocks, with sericite-carbonate-chlorite and silicification directly associated with mineralization

14 Main Geological Types of Gold Deposits (F. Robert, pers. com., 2010) Epithermal Sediment-hosted Porphyry Greenstone

Bingham Cu-Au-Mo porphyry with quartz-sericite-pyrite halo,

Carlin-like deposits (Melco and Barney s Canyon).

15 Bingham Cu-Au-Mo porphyry with quartz-sericite-pyrite halo, surrounding zone of Pb-Zn skarns and mantos, and outer zone of Au-As Carlin-like deposits (Melco and Barney s Canyon). Apatite fission-track reset to 40 Ma to outer edge Au-As deposit zone (Cunningham et al., 2004). Similar thermal anomaly on Carlin Trend, shown in comparison to total residual field magnetic anomaly (Cline et al., 2005) >0 nt

16 Yerington porphyry deposits, Nevada showing depth of ore-related granitoid below paleosurface (contours at 6, 5, 4 and 3 kms) (dash black lines), Ca-Na silicate alteration with magnetite (yellow), potassic alteration (green) and dikes (red lines). From Seedorff et al., 2005)

17 Vertical and lateral zoning in LS-IS and HS epithermal deposits. From Simmons et al., 2005 LS-IS deposits HS deposits

18 Simplified map of Lepanto Cu-Au-Ag HS epithermal, Victoria Au-Ag LS epithermal and FSE porphyry Cu-Au deposits, Philippines. From Simmons et al., 2005

19 Unconformity

20 Collahuasi porphyry Cu-Mo district, associated HS Cu-Ag and Au- Cu veins, and IP anomalies. From Masterman et al., 2005a and b Cu-Ag Au-Cu

21 Typical vein patterns in quartz-adulariacalcite-illite LS-IS deposits

22 Vertical extent of ore in Pachuca, Mexico LS-IS epithermal basemetal-silver veins

23 Main Geological Types of Gold Deposits (F. Robert, pers. com., 2010) Epithermal Sediment-hosted Porphyry Greenstone

24 Muruntau, Uzbekistan clastic sediment-hosted deposit Sandstone Bedded siltstone-shale, variably carbonaceous Stratif auriferous qtz vns Auriferous bio-fs-qtz rock Fine grey phyllite Diorite-granodiorite Biotote-musc phyllite Syenite-diorite porphry dikes

25 West African Precambrian gold belt, showing clastic sedimenthosted deposits Sadiola 7.6Moz Loulo 4Moz Morila 5Moz Ashanti 45Moz

Qtz-ab-chlcc-ep qtz-abep-chlqtz-ank-seric act

qtz-alb-ank-seric (From Hodgson, 1993, modified

26 Qtz-ab-chlcc-ep qtz-abep-chlqtz-ank-seric act (chl-cc) Greenstone Deposits: Alteration, Hollinger- McIntyre, Timmins (From Piroshco & Hodgson, 1988) qtz-alb-ank-seric (From Wood et al, 1986) qtz-alb-ank-seric (From Hodgson, 1993, modified after Smith & Kesler, 1985) qtz-ank-seric (chl-cc) 9% 2% Mole% CO 2 in F.I.

27 Area Selection Criteria for gold deposits (in approximate order of importance) 1. Endowment: Within gold belt, and in particular, in gold district or camp 2. Fault Structure: On or near a continental-scale structure with associated syndeformational fault-related sediments; almost invariably near intersection of cross structure or lineament. 3. Domal uplift: unconformity or unconformities; absence or thinning of coeval volcanics 4. Alteration: Large-scale alteration zone or zone of thermal resetting 5. Magmatic center: cluster, especially linear zone of dikes, stocks, domes, bx pipes, etc. Best reference: Econ Geol 100 th Anniversary Volume

28 Fluid evolution paths due to variable effect of cooling and wall rock reaction Fig.3. Sulfidation state and temperature range of formation of different deposit types of mineralized magmatic centers, after Einaudi et al, Fluids originate as brines in magmas in which fo 2 is slightly higher than that defined by the reaction 2Fe 3 O 4 + 3SiO 2 = 3Fe 2 SiO 4 + O 2, and fs 2 slightly below that defined by the reaction FeS + ½ S 2 = FeS 2. Rapid cooling associated with fluid expansion and boiling due to the transition from lithostatic to hydrostatic pressure in the zone of mineralization results in the fs 2 of fluids going out of equilibrium with wall rocks (the rock buffer zone) and crossing the various mineral reaction lines to deposit progressively higher sulfidation state sulfide assemblages until wall rock reaction eventually brings fluids back to the rock buffer field. El Indio and Yanacocha-type deposits are formed from moderate salinity brines, whereas porphyry deposits are formed from hypersaline brines: these fluids in some cases represent fluids released at hydrostatic and lithostatic pressure, respectively, from source magmas, but in others, high salinity brines are the result of boiling of a moderate salinity brine that exsolves from a magma El Indio-Lepanto-Butte-type veins and replacement lodes HS: Yanacocha- Pierina type fso 2 /fh 2 S = 5 (sulfur gas buffer in oxidized magmas) Subduction zone-related magma LS: Santo Toribio- Quirvilca- Tres Cruces type Rock buffer

Magmatic-Hydrothermal Gold Systems in the Archean of Northern Ontario, Canada: Examples of Syenite-Associated and Porphyry-Type Au-(Cu) Deposits

Magmatic-Hydrothermal Gold Systems in the Archean of Northern Ontario, Canada: Examples of Syenite-Associated and Porphyry-Type Au-(Cu) Deposits By Dr. Daniel J. Kontak Department of Earth Sciences Laurentian