The effects of amphibolite facies metamorphism on the trace element composition of pyrite and pyrrhotite in the Cambrian Nairne Pyrite Member, Kanmantoo Group, South Australia

Abstract

The trace element composition of pyrite has been used to explore for hydrothermal ore deposits and to understand ore-forming processes. However, the effects of metamorphism on the trace element distribution in pyrite have received relatively limited attention. In this study, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of pyrite and pyrrhotite, along with minor amounts of sphalerite, chalcopyrite, and galena, are used to evaluate the effects of amphibolite facies metamorphism on the trace element distribution and remobilization of metals in iron sulfides in the clastic sediment-hosted Cambrian Nairne Pyrite Member (NPM), South Australia. The NPM and the Mt. Torrens Pb-Zn-Ag prospect, which occur near the base of the Kanmantoo Group, preserve irregularly zoned subhedral to euhedral metamorphic pyrite (Py1) and anhedral pyrrhotite (Po1), along with relatively minor quantities of remobilized anhedral pitted and cataclastic pyrite (Py2a) in quartzofeldspathic rocks and anhedral inclusion-poor pyrite (Py2b) in calc-silicate rocks that armor or cross-cut earlier formed Py1. Rare anhedral secondary melnikovite pyrite (Py3) locally formed on the margins of Py1 and Py2a. Trace element studies show that Py1 in the NPM at Brukunga and Ironstone Ridge contains mean values of 1254 ppm Co, 123 ppm Ni, 2167 ppm As, 16 ppm Se, 10 ppm Cu, 25 ppm Zn, and 15 ppm Pb, whereas Py1 in the Mt. Torrens prospect contains mean values of 2312 ppm Co, 263 ppm Ni, 1835 ppm As, 95 ppm Se, 9 ppm Cu, 6 ppm Zn, and 9 ppm Pb. Rare inclusion-rich cores of Py1 show higher concentrations of trace elements than inclusion-free rims with minor amounts of chalcopyrite, galena, and sphalerite forming along grain boundaries or in fractures within pyrite. This is interpreted to be the result of the release of Cu, Pb, and Zn from pyrite as it recrystallized. Remobilization of these elements then formed discrete sulfides at the millimeter to centimeter scale, with some exceptions at the meter scale where chalcopyrite, sphalerite, and galena, along with other sulfides and sulfosalts, formed in veins and tension gashes. These observations suggest that remobilization of trace metals, including Cu, Pb, and Zn, did not migrate more than a few meters at most. Moreover, despite the recrystallization of pyrite and pyrrhotite and subsequent remobilization of some trace elements (i.e., Co, Ni, As), which were structurally bound in these Fe sulfides, still retain elevated concentrations at amphibolite facies conditions.Remobilization of metals from the NPM during metamorphism to form Cu-Au (e.g., Kanmantoo) and Pb-Zn-Ag-(Cu-Au) (e.g., Angas, Wheal Ellen) deposits in the Tapanappa Formation stratigraphically higher in the Kanmantoo Group seems unlikely. However, it is possible that one source of metals for these deposits could have been leached from the NPM and carried in large hydrothermal cells prior to metamorphism. Such a scenario is consistent with previously published sulfur isotope data for sulfides from the NPM, and Cu-Au and Pb-Zn-Ag-(Cu-Au) deposits, which indicate that sulfur derived from the NPM and pyritic schists in the Kanmantoo Group was a likely source of sulfur for the base and precious metal deposits.