Role of Pyrite in the Formation and Localization of Gold Mineralization at the Owl Creek Mine, Timmins, Ontario
Ian R. Jonasson, David M. Kingston, David H. Watkinson, Sally R. Elliott, 1999. "Role of Pyrite in the Formation and Localization of Gold Mineralization at the Owl Creek Mine, Timmins, Ontario", The Giant Kidd Creek Volcanogenic Massive Sulfide Deposit, Western Abitibi Subprovince, Canada, Mark D. Hannington, C. Tucker Barrie
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The Owl Creek gold mine, now closed, lies in the Archean Abitibi greenstone belt of the Superior structural province of the Canadian shield, 18 km northeast of the city of Timmins, Ontario. The major rock types observed at the Owl Creek mine are tholeiitic basalt, carbonaceous shale, arkosic graywacke, and mudstone. All rocks have been subjected to middle greenschist facies regional metamorphism. Most gold mined occurs in the form of inclusions in vein and wall-rock pyrite in steeply dipping, highly strained tholeiitic basalt and carbonaceous shale; however, considerable free gold was also mined from quartz veins located mainly within brittle, altered basalt.
Most of the buff to gray basalt is high magnesium tholeiite that is massive to locally pillowed and intensely sheared. Basalt is intensely carbonatized (ankerite-dolomite; up to 16.7% CO2) and the gray color is the result of postcarbonatization addition of chlorite and carbon. Sericite and pyrite alteration of basalt followed this addition. Most auriferous pyrite is between 0.1 and 1.0 mm and resides in basalt, where it forms 5 percent of the rock; such pyrite averages 58 ppm gold (n = 21). Carbonaceous shale contains as much as 14.8 percent amorphous carbon, and carbon content is highest adjacent sheared tholeiitic basalt. Carbonaceous shale contains fine-grained disseminated euhedral, nodular, and skeletal pyrite, in order of decreasing gold content. Local accumulations of larger pyrite nodules (up to 3 cm) and small lenses of massive pyrite (up to 2 m thick) occur close to shale-basalt contacts. Gold content of shale pyrite averages 3.63 ppm (n = 19).
Nodules of diagenetic and hydrothermal origins are recognized. Although the former type contains low levels of gold (x = 1.72 ppm, n = 13), sporadic zone enrichment is not uncommon (up to 80 ppm). Hydrothermal nodules are significantly enriched in gold (x = 31 ppm, n = 4). Mudstone has a much lower amorphous carbon content (<2%) than shale; pyrite occurs as scattered euhedra (<1 mm) and as disseminations in bands up to 5 cm thick which may make up as much as 15 percent of the rock. However, both types are relatively devoid of gold (x = 1.47 ppm, n = 12). Two major exposures of arkosic graywackes and a 5-m-thick interflow unit within basalt make up the bulk of enveloping sedimentary rocks at Owl Creek. Individual beds range from centimeters to tens of meters in thickness. These are interbedded with mudstone laminae which are ripped up, indicating overturned strata which young to the south. Pyrite is present as well-preserved cubes up to 10 mm in size. Graywacke pyrite contains no appreciable quantities of gold. Widespread, diffuse, but sustained hydrothermal activity in the district led to accumulation of strongly arsenical and nickeliferous pyrite in sediments that bear low but significant levels of gold; however, later epigenetic activity induced pervasive carbonate, sericite, and chlorite alteration in tholeiites and introduced auriferous pyrite via myriad thin quartz veinlets.
Development of a heterogeneous deformation zone was responsible for juxtaposing of altered, brec-ciated basalt against carbonaceous sediments and promoted increased permeability in these rocks. This deformation zone was the principal conduit for gold-rich fluids, and its development controlled auriferous wall-rock pyrite and gold-bearing quartz vein precipitation at Owl Creek. Quartz veins in the highly strained rocks exposed in the Owl Creek open pit are (1) subvertical; (2) south dipping, en echelon ten-sional; and (3) more shallowly dipping to subhorizontal. All three types of quartz veins contain ore-grade concentrations of gold. Subhorizontal quartz veins may themselves be brecciated, resulting in subvertical quartzose-breccia veins consisting of angular quartz fragments and a matrix of either chlorite and auriferous pyrite or goethite and chalcedony. The presence of nonfragmented auriferous pyrite confirms that there was a gold-precipitating event which postdated emplacement of gold-bearing quartz veins.
Gold in altered basalt is most closely associated with pyrite, pyrrhotite, sphalerite, chalcopyrite, and ar-senopyrite in order of decreasing abundance; however, pyrite represents no less than 95 percent of the total sulfides present. With the exception of rare euhedral arsenopyrite, all inclusions in gold-bearing pyrite are anhedral. Gold grains occur as isolated blebs or as fracture fill in pyrite but may be found adjacent to chalcopyrite-pyrrhotite inclusions or at grain boundaries between arsenopyrite and pyrite. Gold was also observed as inclusions in arsenopyrite and other sulfarsenides. Altaite and rare gold tellurides were observed only with gold inclusions in hydrothermal pyrite nodules. Distinctive assemblages of accessory minerals in pyrite separates from basalt and shale suggest that there is no clear genetic relationship between gold in euhedral pyrite in carbonatized basalt, gold in crystalline pyrite nodules in carbonaceous shale, or gold in quartz veins. Rather, a succession of discrete hydrothermal events is envisaged.
Sulfur isotope ratios of all types of pyrite were measured and permit the following observations. Ore-grade auriferous pyrite has a distinctive narrow range of S34S signatures (1.8-3.5%) which is independent of rock type. However, a narrow positive range is not exclusively indicative of auriferous pyrite (e.g., certain types of nodules). Weakly positive correlations between gold and S34S values for pyrite from basalt and graywacke (in contrast to negative correlations between gold and shale pyrite) support a direct hydrothermal source for epigenetic gold in the Owl Creek ore zone.
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The Giant Kidd Creek Volcanogenic Massive Sulfide Deposit, Western Abitibi Subprovince, Canada
ARCHEAN Cu-Zn deposits are among the most important mineral deposit types in Canada. The Superior province of Canada contains nearly 80 percent of the known Archean Cu-Zn deposits in the world (about 100 of 125 deposits). These deposits are concentrated in 10 separate mining camps, including Sturgeon Lake, Manitouwadge, Mattagami Lake, Chibougamau, Joutel, Val d’Or, Bous-quet, Noranda, Kidd Creek, and Kamiskotia (Fig. 1 and Table 1). A few deposits in rocks of similar age and composition are also known in the Slave province, the Churchill province, and in the Archean of Western Australia, southern Africa, China, and Brazil. Known deposits of this age worldwide account for more than 650 million metric tons (Mt) of massive sulfides, containing 10 Mt of Cu metal, 29 Mt of Zn, 1 Mt of Pb, 33 Mkg Ag, and 750,000 kg Au. The giant Kidd Creek volcanogenic massive sulfide deposit in the western Abitibi subprovince of Canada is the largest known deposit of this age currently in production. The Superior province is the world’s largest exposed Archean craton, occupying an area of more than 1.5 million km2, bounded by the Trans-Hudson orogen to the west and the Grenville province to the east. A number of distinct subprovinces are recognized, assembled into east-west-trending granite-greenstone terranes and metasedi-mentary belts (Fig. 1). The granite-greenstone terranes are composed of gneissic rocks of plutonic origin, supracrustal rocks of dominantly volcanic origin, and a variety of syn- to late kinematic granitoids. Volcanic rocks comprise about 12 percent of the total area. The greenstone belts have been described variously as successive lateral accretions of volcano-plutonic arcs, oceanic islands, oceanic plateaus, and rift-related assemblages (e.g., Langford and Morin, 1976; Percival and Card, 1985; Ludden and Hubert, 1986; Ludden et al., 1986; Card, 1990; Jackson and Sutcliffe, 1990; Williams, 1990; Corfu, 1993; Heather et al., 1995; Jackson and Cruden, 1995). The metallogenic history of the Superior province has been described in detail by Franklin and Thorpe (1982) and Poulsen et al. (1992).
The Abitibi subprovince (94,000 km2) is the largest of the greenstone belts. It contains the major gold and base metal mining camps in Canada (Fig. 2), with production and reserves totaling more than 480 Mt of massive sulfide and 4,700 t of Au. Metal production in the western portion of the Abitibi greenstone belt is dominated by the Timmins region, which historically has accounted for 37 percent of the total gold production