METAMORPHOSED ARCHEAN EPITHERMAL AU-AS-SB-ZN-(HG) VEIN MINERALIZATIONAT THE CAMPBELL MINE, NORTHWESTERN ONTARIO

The Campbell mine and the adjacent Red Lake mine are developed in an A rchean gold deposit hosted within volcanic rocks of the Red Lake green stone belt, situated in the western part of the Uchi subprovince of th e Superior province. Geochronological evidence suggests that the Campb ell-Red Lake deposit formed after volcanism but prior to late felsic p lutonism and regional metamorphism between 2722 and 2710 Ma. The orebo dies at Campbell mine are largely hosted in epithermal-style veins and vein stockworks associated with strike-slip faults. Complex, superimp osed phases of wall-rock alteration occurred prior to the emplacement of main stage veins and were controlled by both primary and secondary (contact, fault, and fracture induced) permeability in mafic and ultra mafic rocks as well as secondary permeability) in rhyolite and diorite . Widespread pervasive carbonatization and potassic (biotite) alterati on with distal chloritic alteration is well developed in basalt and ul tramafic rocks. In basalt, this alteration was in turn partly overprin ted by silicification and aluminous alteration consisting of zoned min eral assemblages of ndalusite-sericite-chloritoid-cordierite-margarite occurring within bleached zones, and quartz-chlorite-chloritoid-garne t-cummingtonite assemblages occurring in flanking chloritic zones. Sim ilarly, zoned aluminous mineral assemblages also occur in diorite and rhyolite. In ultramafic rocks, the aluminous alteration is represented by sericite-fuchsite-cordierite assemblages, which grade out toward c hlorite-anthophyllite-cordierite assemblages. This zoning appears to r eflect the bulk composition of zoned premetamorphic aluminous (argilli c) alteration. Main-stage carbonate (dolomite to ankerite)-quartz vein s exhibit well-developed open-space-filling textures characterized by colloform and crustiform banding and cockade breccia infill textures t ypical of veins which formed in a near-surface environment. Fissure ve ins occur within fault zones themselves, whereas large snowbank (bande d carbonate-quartz) veins formed in dilatant sites associated with a r eleasing bend in the faults. Fissure veins and snowbank veins are best developed in basalt, whereas veinlet stockwork and sheeted veinlet zo nes occur in ultramafic rocks adjacent to main-stage vein sites in bas alt. The main-stage veins partially sealed remaining permeable zones, and proximal early alteration assemblages were overprinted by distal c hloritic alteration as the hydrothermal system collapsed inward. The m ain-stage veins were then overprinted by silicification +/- tourmaline within breccias and veinlet stockworks. Mineralization is associated with microstockwork sulfide veinlets, strongly disseminated sulfide, a nd sulfide cemented breccias which cut silicified zones in veins and w all rocks. Native gold occurs on its own or with sulfides in fractures cutting silicification. Arsenopyrite is commonly associated with gold and occurs in ore zones with pyrite, pyrrhotite, and magnetite. Stibn ite and sphalerite commonly occur in high-grade zones. Mineralized zon es hosted in fissure veins and wall-rock replacement zones tend to be narrow (<1 m nide) and in narrow fault zones. Mineralization associate d with brecciated snowbank veins are typified by multiple brecciation and fracturing events and are formed in dilatant sites at a high angle to fault zones. Mineralized zones are cut by late hydrothermal brecci a dike and pipelike bodies as well as late quartz-carbonate veins and faults. The zoned aluminous alteration, open-space-filling textures of the main-stage veins, sheeted veinlet zones and stockwork fracturing, multiple phases of hydrothermal breccias, and anomalous Au-Ag-As-Sb-H g-Zn-K at the Campbell mine are characteristics similar to those of Ph anerozoic low sulfidation epithermal deposits. The mineralized environ ment has been deformed (flattened and stretched) and metamorphosed to middle to upper greenschist facies during the Kenoran orogeny (ca. 265 0 Ma).