THE GENETIC RELATIONSHIP BETWEEN THE NAIRNE PYRITE MEMBER AND METAMORPHOSED SEDIMENT-HOSTED CU-AU AND PB-ZN-AG DEPOSITS, KANMANTOO GROUP, SOUTH AUSTRALIA

Pyrite is common in nature and ubiquitous in most hydrothermal ore deposits. Its open structure allows it to incorporate significant quantities of trace elements (e.g. Cu, Pb, Zn, As, Te, Co, Ni). Such trace element-enriched pyrite has been used to explore for hydrothermal ore deposits and to understand ore-forming processes. Here, we address the question as to whether metamorphosed pyrite can be used as an exploration guide and whether metals expelled during the recrystallization of pyrite can accumulate to form sulfides and, subsequently, be remobilized to form metallic mineral deposits.

The current study evaluates the genetic relationship between the Nairne Pyrite Member (NPM), one of the world’s most extensive pyrite/pyrrhotite units, and metamorphosed Pb-Zn-Ag and Cu-Au deposits. These deposits occur in a package of Cambrian quartzites, greywackes, pyritic units, and siltstones metamorphosed to the amphibolite facies in the Kanmantoo Group, South Australia. The NPM occurs near the base of the Kanmantoo Group, and hosts the Mt. Torrens Pb-Zn-Ag deposit, whereas pyritic schists and other base metal deposits occur higher in the stratigraphic sequence. Mineralization at Mt. Torrens is associated with calc-silicate rocks composed of quartz-muscovite-plagioclase-calcite ± (scapolite, chlorite, Ca-Mn-garnet, titanite, phlogopite, tremolite, hyalophane (3-11 wt. % BaO)). Although sulfur isotopes studies suggest sulfur in the Cu-Au and Pb-Zn-Ag deposits was, in part, derived from the pyritic units, it is unknown whether these same units were the source of base and precious metals. Whole rock analyses of the NPM show elevated base metal contents with averages of 362 ppm Cu, 265 ppm Pb, and 3,067 ppm Zn. Although these values are partly manifested via the presence of minor amounts of sphalerite, chalcopyrite, and galena, trace element studies of pyrite also show that it contains up to 189 ppm Cu, 310 ppm Zn, and 560 ppm Pb, whereas pyrrhotite contains up to 804 ppm Cu, 1,943 ppm Zn, and 88 ppm Pb. The elevated trace element contents of the iron sulfides support the concept that the NPM is a source of base metals to the overlying deposits. The most likely scenario is that metal migration was associated with the formation of large hydrothermal cells that tapped the NPM prior to metamorphism to form the Cu-Au and Pb-Zn-Ag deposits.