Characteristics and origin of Archean premetamorphic hydrothermal alteration at the Campbell gold mine, northwestern Ontario, Canada

The Campbell mine, located in northwestern Ontario, Canada, is developed wi thin a metamorphosed and deformed fault-controlled Archean gold deposit hos ted predominantly within basalt and ultramafic rocks with lesser quantities of rhyolite and diorite. The sequence of hydrothermal alteration has been subdivided into three partially overlapping phases consisting of an early a lteration phase, followed by the main-stage vein phase, and culminating in the mineralization phase. Early alteration consists of two substages represented by carbonatization a nd pervasive biotite (potassic) alteration, and early silicification and al uminosilicate-bearing alteration. Carbonate alteration is relatively widesp read, whereas biotitic alteration occurs proximal to alteration-controlling structures in highly carbonatized and/or silicified rocks, Carbonate-bioti te alteration grades into carbonate-chlorite alteration laterally outward. Carbonate-biotite- and carbonate-chlorite-altered rocks are locally overpri nted by early silicification and aluminosilicate-bearing alteration. These aluminous mineral assemblages are zoned, consisting of a proximal bleached zone (with quartz-andalusite-sericite +/- margarite +/- chloritoid +/- cord ierite), which grades outward to a more chloritic intermediate zone (quartz -chloritoid-chlorite +/- sericite), followed by a distal chloritic garnetif erous zone (quartz-chlorite-garnet +/- chloritoid +/- sericite +/- cordieri te +/- cummingtonite). Alluminosilicate- bearing alteration is well develop ed in basalt, diorite, and rhyolite, whereas similar alteration assemblages in ultramafic rocks are represented by proximal quartz-sericite-(green mic a)-cordierite with distal chlorite +/- anthophyllite assemblages. Compared to relatively unaltered basalt, the proximal bleached zone in altered basal t is characteristically depleted in Ca, Na, Fe, Mg, and Mn. The adjacent ga rnetiferous zone is enriched in Fe and Mn and depleted in Ca, Na, and Mg. S i has been added to both alteration zones. Both the bleached and garnetifer ous zones exhibit anomalously high Al2O3/(CaO + Na2O + K2O). The zoned alum inous mineral assemblages reflect the bulk composition of premetamorphic al teration precursors, possibly consisting of proximal clay-rich assemblages which grade outward to chlorite-rich assemblages (zoned intermediate argill ic alteration). Early alteration is crosscut by barren colloform and crustiform-banded carb onate (dolomite to ankerite)-quartz main-stage veins, cockade breccias, and sheeted to stockwork veinlet zones. Chloritic alteration is associated wit h main-stage veins and varies in mode of occurrence from narrow selvages ar ound individual veins to a wide pervasive to veinlet-controlled alteration. Chloritic alteration consisting of chlorite(+/- cordierite)-rich assemblag es overprint zones of pervasive biotite alteration. Silicification and mineralization (consisting of arsenopyrite-pyrite-pyrrho tite +/- gold +/- magnetite +/- sphalerite +/- stibnite) locally replace an d brecciate main-stage veins and carbonatized wall rocks. Alteration associ ated with mineralization is localized and consists of quartz-sericite I cor dierite in pyrite and arsenopyrite-rich wall-rock mineralization. Biotite /- tourmaline veinlets and microstockworks occur in mineralized zones and c ontain sulfides. Many of the features of wall-rock alteration at the Campbell mine, includin g primary permeability control, selective alteration of specific minerals, veinlet and microstockwork control of alteration, and zoned (intermediate a rgillic) alteration, are common to nonmetamorphosed deposits which have for med in the epizonal environment (i.e., volcanogenic massive sulfide, epithe rmal, and porphyry deposits). The mineralogy of aluminosilicate-bearing alt eration at the Campbell mine is similar to assemblages resulting from the m etamorphism of intermediate to advanced argillic alteration in some metamor phosed epithermal and volcanogenic massive sulfide deposits. These features , together with the presence of open space-filling textures of veins, silic ification textures, hydrothermal breccias, and anomalous Au-Ag-As-Sb-Hg-Zn- K suggest a low-sulfidation epithermal origin for the Campbell-Red Lake dep osit.