Geology and footwall alteration of the South Bay massive sulphide deposit, northwestern Ontario, Canada

Abstract

South Bay is an Archean volcanogenic massive Cu–Zn sulphide deposit having many features in common with the Kuroko deposits of Japan. The ore lenses overlie a quartz–feldspar porphyritic rhyolite (QFP) lava dome and are covered by or occur within rhyolitic tuff breccia that, together with rhyolite tuffs and lavas, is contained within a caldera-like structure.Footwall hydrothermal alteration at South Bay is detectable for several hundred metres from ore. "Unaltered" footwall QFP lava dome contains a mineral assemblage of quartz + two feldspars + two micas + epidote + calcite + ilmenite. Closer to the orebodies, K-feldspar, epidote, and biotite disappear first, followed by consumption of calcite, ilmenite, and albitized plagioclase. The most altered QFP has an assemblage of quartz + paragonite + phengitic muscovite + chlorite + dolomite + sphene + rutile. The ratio Fe/(Fe + Mg) in dolomite, muscovite (phengite), and chlorite decreases consistently towards the orebodies. Neither bulk chemistry (except for Na2O) nor oxygen isotopic ratio shows consistently systematic lateral changes within the alteration halo. Quartz from the stringer zone, from lenses in massive ore, and from ore-horizon chert all have a very narrow δ18O range of +9.0 to +11.3‰. The δ18O of the QFP is +9.3 to +9.4‰, regardless of the degree of alteration.The temperature of ore formation is estimated to have been around 300 °C based on the paragonite–muscovite geothermometer and the carbonate geothermometer. The δ18O value of the ore-forming solution at 300 °C would have been between +2.1 and +4.4‰, which is similar to that of the Kuroko deposits.