Pyrite nodules in black shales: recorders of craton-scale mineralization and basin fertility indicators, Eastern Goldfields, Yilgarn Craton, Western Australia

In the Eastern Goldfields Superterrane of Western Australia (A and B, right), pyrite nodules are present in sulfidic and carbonaceous black shales at the Golden Mile Au deposit, Kalgoorlie; the Nimbus Ag-ZnPbVHMS deposit, 17 km E of the Golden Mile; the Lucky Bay polymetallic prospect (70 km SE of Kalgoorlie); and the Erayinia (King) Cu-Zn VHMS deposit (150 km SE of Kalgoorlie). In general, all pyrite nodules analyzed via LA-ICP-MS in this study are enriched in many trace elements, with up to 5 ppm Au in Oroya Shale nodules from the Golden Mile. Nodules from the Golden Mile are highest in Au, while the nodules from Nimbus and Erayinia are higher in Zn and lower in Au than the Golden Mile examples. Nodules from Lucky Bay compositionally fall in between the Golden Mile and Nimbus/Erayinia nodules. In addition, many nodules from all four localities have pronounced pressure shadows that are predominantly filled with quartz but also contain chalcopyrite, sphalerite, and cubic pyrite in many instances. Trace element imaging of these features shows that the chemical signature of the cubic pressure shadow pyrite is not the same as that of the nodules, but that the signature of the cubic pressure shadow pyrite is uniform across the region. From this study, it is evident that diagenetic pyrite nodules faithfully record the chemistry of the fluid(s) that formed them, which, in the case of the Golden Mile (Au) and Nimbus (Zn), is dominated by the economic metal in those districts. They may therefore be used as a first-pass indicator of metallogenic fertility in a given basin.