South Bay Mine: a geological update

Abstract

South Bay Mine is located 50 miles east of Red Lake in the Uchi­Confederation greenstone belt. The deposit was discovered in 1968 by airborne geophysics and went into production in early 1971. Production since that time has averaged 500 tons per day, from reserves of l ,600,000 tons (including past production) averaging 2.3% copper, 14.5% zinc and 3.5 oz./ton silver. These reserves extend to the 1500 foot level with underground exploration continuing at deeper levels. South Bay belongs to the family of stratiform base metals massive sulphide deposits associated with felsic volcanic rocks. It occurs near the top of the youngest of three mafic to felsic volcanic cycles identified in the area. The ore horizon overlies quartz feldspar porphyry. The quartz feldspar porphyry thickens in the area of the mine where it probably formed the core of a rising felsic dome. The porphyry is believed to have intrusive and extrusive phases. The ore lenses, 5-70 feet wi􀀗e,contain massive fine pyrite, banded to interstitial chalcopyrite, and finely banded to coarse massive sphalerite. Sulphides constitute 70-95% of the ore, the balance being quartz, carbonate, chlorite, chert, and argillite. The highly irreg­ular ore lenses occur along a strike length of up to 700 feet. The upper portions of some of the ore zones are zinc rich, reflecting original metal zoning. Argillite underlying and intercalated with these sulphide bodies indicate quiescent conditions of deposition. Elsewhere, preconsolidation slumping has destroyed any zoning. Primary textures of sphalerite and chalcopyrite have been largely destroyed by dynamic and thermal metamorphism. The hanging wall consists of 70-300 feet of dacite breccia, in turn overlain by rhyolite. Irregular felsic dikes commonly intrude the ore zone and enclosing rocks. The dominant alterati0n consists of chloritization and brecciation of the quartz feldspar porphyry to depths of 50-300 feet below the ore, and chloritization in the hanging wall dacite breccia. It is believed that the wall rock alteration and mineralization resulted from the sa􀀽e event.