Most gold deposits in the Red Lake area can be classified as orogenic gold deposit (“shear zone hosted”, “mesothermal”, “greenstone-hosted quartz-carbonate vein” deposit; Figure 8-1). These deposits occur in deformed greenstone belts, particularly those that are characterized by tholeiitic basalts and ultramafic komatiites intruded by intermediate to felsic porphyritic intrusions (Dubé and Gosselin, 2007). They are located along major compressional to transtensional crustal-scale fault zones marking convergent margins between major units but ore is typically hosted by second- and third order shears and faults and at jogs and changes in strike (Goldfarb et al., 2005). In Canada, these vein deposits are often associated with conglomerates (e.g. the Timiskaming conglomerate). They are a major source of gold in the greenstone belts of the Superior and Slave provinces of the Canadian Shield.
Orogenic gold deposits are characterized by a network of auriferous, laminated quartz-carbonate veins and locally hydrothermal breccias. The dominant sulfides are pyrite and arsenopyrite but W-, Bi- and Tebearing phases are also common. Sulfides also occur disseminated in the wall rock. Typical alteration includes iron-carbonate, silicification, muscovite, chlorite, K-feldspar, biotite, tourmaline and albite.
Orogenic deposits formed from metamorphic fluids (Dubé and Gosselin, 2007) that were rich in CO2, low in salinity and generated during prograde metamorphism where the fluids were channelled along major crustal deformation zones. Drastic pressure changes (and resulting unmixing and desulfidation) and wall rock interaction caused the precipitation of the sulfides (and gold).
World-class ore bodies are between 2 and 10 km long, approximately 1 km wide and extend to depths of 2 to 3 km (Goldfarb et al., 2005). Canadian examples include the Timmins, Kirkland Lake, Val d’Or and Rouyn-Noranda districts of the Abitibi greenstone belt and the Pickle Lake and Rice Lake greenstone belts of the Uchi subprovince.
Schematic presentation of the geological environment and crustal depth of orogenic gold deposits (from Dubé and Gosselin, 2007)