Base metal mineralization in the Kanmantoo Group, South Australia; a regional sulfur isotope study

Abstract

A number of base metal sulfide deposits are hosted by multiply deformed metagraywackes of the Kanmantoo Group, South Australia. Sulfide mineralization is of three main types: (1) copper deposits which include the Kanmantoo and Bremer mines, South Hill prospect, and several minor occurrences; (2) lead-zinc deposits at the Aclare, Wheal Ellen, and Strathalbyn mines; (3) pyrite-pyrrhotite mineralization of the Nairne pyrite deposit and other pyritic schists within the Kanmantoo Group.Copper deposits are discordant and pipelike and contain chalcopyrite, pyrrhotite, magnetite, and pyrite as the major opaque minerals. Lead-zinc deposits lack magnetite and are apparently concordant. Beds of pyritic schist reach an overall thickness of 100 m and persist for as much as 32 km.Sulfur isotope ratios for ore shoots from the Kanmantoo mine, excluding one sample, range from 3.5 to 12.4 per mil and average 8.0 + or - 2.3 per mil. Average values for individual ore shoots differ by as much as 4 per mil. 34 S enrichment is noted at Kanmantoo among some low-grade sulfide mineralization (delta 34 S = 16ppm) and supergene pyrite from the main orebody (delta 34 S = 39.6ppm). The minor deposits of copper and lead-zinc all show a similarity in sulfur isotope composition. The delta 34 S values for sulfides range from -4.7 to 6.5 per mil and averages for these deposits lie close to zero per mil. The pyritic schists are strongly depleted in 34 S and contain biogenic sulfur formed in an anoxic marine environment; individual sulfides range from -19.9 to -11.9 per mil. Isotopic disequilibrium is a characteristic feature of coexisting sulfide pairs from many of the deposits, despite subsequent amphibolite facies metamorphism (3.8 kb and 500 degrees -600 degrees C). Sphalerite-galena pairs at the Wheal Ellen mine yield consistent calculated temperatures (379 degrees C) and may have reequilibrated below the metamorphic peak.Base metal mineralization is accounted for by subsurface (copper) and sea-floor (lead-zinc) accumulation from hydrothermal fluids circulating to or expelled from depths of at least 3 to 4 km within the sedimentary basin. This minimum depth estimate for fluid extraction corresponds to the stratigraphic interval occupied by pyritic schists, which underlies the horizon of base metal mineralization. Sulfur isotope evidence suggests that sulfur was extracted by hydrothermal leaching from the pyritic schists. Sulfur within the base metal mineralization shows a mixed Cambrian seawater and biogenic (pyritic schist) source. Variations in mixing proportions of the two forms of sulfur are responsible for differences among ore lenses at the Kanmantoo mine and individual deposits within the region.