Trace Element Content of Pyrite from the Kapai Slate, St. Ives Gold District, Western Australia

Abstract

The Kapai Slate is a continuous, pyrite-rich carbonaceous shale horizon within the St. Ives Au district that is spatially related to high-grade Au mineralization. In situ laser ablation-inductively coupled mass spectrometry (LA-ICPMS) trace element analyses, in situ sensitive high resolution ion microprobe, stable isotope (SHRIMPSI) S isotope analyses, and optical microscopy pyrite texture analyses were used to examine the different pyrite types in the Kapai Slate and Au deposits. These data were also used to confirm that the trace element signature of sedimentary pyrite can be preserved in rocks that underwent upper to mid-greenschist facies metamorphism and significant hydrothermal overprint. The data were further utilized to gain a more detailed understanding of the ocean conditions during deposition of the Kapai Slate and determine whether some of the Au and S in the St. Ives district could have been sourced from the Kapai Slate. Seven different types of pyrite were identified: fine-grained sedimentary pyrite (Py1), nodular sedimentary pyrite (Py2), remobilized sedimentary pyrite (Py3), coarse-grained, inclusions poor late pyrite (Py4), inclusionrich magnetite series pyrite (Py5), ore stage pyrite (Py6), and pyrite associated with the mafic units (Py7). Each type of pyrite was found to have distinctive trace element compositions and S isotope signatures. The results of the LA-ICPMS analyses provide evidence for early trace element enrichment in the Kapai Slate sedimentary pyrite (median values of 158 ppm Ni, 387 ppm Co, 82 ppm Cu, 727 ppm As, 1.91 ppm Mo, 13 ppm Se, 0.25 ppm Au, 7.72 ppm Te and 3.36 ppm Ag for Py1 and 223 ppm Ni, 158 ppm Co, 99 ppm Cu, 856 ppm As, 1.27 ppm Mo, 10.2 ppm Se, 0.57 ppm Au, 10.09 ppm Te, and 6.62 ppm Ag for Py2). Concentrations of Ni and Co are low, relative to other late Archean sedimentary pyrite (median of 813 and 465 ppm, respectively) and Mo levels are near that of the euxinic shales of the similar-aged Jeerinah Formation in the Hamersley Basin, Western Australia. These data suggest that the Kapai Slate was deposited in an anoxic to euxinic basin with relatively low biological productivity. The Δ33S and δ34S signatures of the sedimentary pyrite suggest two different sources of S. Positive δ34S and negative Δ33S signatures indicate bacterial reduction of SO4 2– from seawater, whereas positive δ34S and positive Δ33S signatures indicate an elemental S8 source, indicating the pyrite formed later during diagenesis. This S isotope signature is consistent with a transition between a near-sediment environment to a more distal environment source. Analyses of the ore-phase pyrite yield weakly positive Δ33S values. This suggests there was a minor contribution of sedimentary S to the more significant oxidized ore-forming fluids, which is consistent with a small contribution of Au from a sedimentary source. Approximations of the degree of sedimentary pyrite destruction in the pyrrhotite/pyrite dominated zones and pyrrhotite/magnetite/pyrite zones of the northern part of the St. Ives district were used to calculate the amount of Au released from the early sedimentary pyrite. The calculation suggests that a minor, though possibly locally significant, amount of Au could have been sourced from the Kapai Slate.