Metamorposed Archean epithermal Au-As-Sb-Zn-(Hg) vein mineralization at the Campbell Mine, northeastern Ontario

Abstract

The Campbell mine and the adjacent Red Lake mine are developed in an Archean gold deposit hosted within volcanic rocks of the Red Lake greenstone belt, situated in the western part of the Uchi subprovince of the Superior province. Geochronological evidence suggests that the Campbell-Red Lake deposit formed after volcanism but prior to late felsic plutonism and regional metamorphism, between 2722 and 2710 Ma. The orebodies at Campbell mine are largely hosted in epithermal-style veins and vein stockworks associated with strike-slip faults.Complex, superimposed 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 ultramafic rocks as well as secondary permeability in rhyolite and diorite. Widespread pervasive carbonatization and potassic (biotite) alteration with distal chloritic alteration is well developed in basalt and ultramafic rocks. In basalt, this alteration was in turn partly overprinted by silicification and aluminous alteration consisting of zoned mineral assemblages of quartz-andalusite-sericite-chloritoid-cordierite-margarite occurring within bleached zones, and quartz-chlorite-chloritoid-garnet-cummingtonite assemblages occurring in flanking chloritic zones. Similarly, 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 chlorite-anthophyllite-cordierite assemblages. This zoning appears to reflect the bulk composition of zoned premetamorphic aluminous (argillic) alteration.Main-stage carbonate (dolomite to ankerite)-quartz veins exhibit well-developed open-space-filling textures characterized by colloform and crustiform banding and cockade breccia infill textures typical of veins which formed in a near-surface environment. Fissure veins occur within fault zones themselves, whereas large snowbank (banded carbonate-quartz) veins formed in dilatant sites associated with a releasing bend in the faults. Fissure veins and snowbank veins are best developed in basalt, whereas veinlet stockwork and sheeted veinlet zones occur in ultramafic rocks adjacent to main-stage vein sites in basalt. The main-stage veins partially sealed remaining permeable zones, and proximal early alteration assemblages were overprinted by distal chloritic alteration as the hydrothermal system collapsed inward. The main-stage veins were then overprinted by silicification + or - tourmaline within breccias and veinlet stockworks.Mineralization is associated with microstockwork sulfide veinlets, strongly disseminated sulfide, and sulfide cemented breccias which cut silicified zones in veins and wall 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. Stibnite and sphalerite commonly occur in high-grade zones. Mineralized zones hosted in fissure veins and wall-rock replacement zones tend to be narrow (<1 m wide) and in narrow fault zones. Mineralization associated 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 breccia 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-Hg-Zn-K at the Campbell mine are characteristics similar to those of Phanerozoic low sulfidation epithermal deposits. The mineralized environment has been deformed (flattened and stretched) and metamorphosed to middle to upper greenschist facies during the Kenoran orogeny (ca. 2650 Ma).