The age, metal source and genesis of the Nifty copper deposit in the context of the geological evolution of the Paterson Province, Western Australia

Abstract

The Nifty Cu deposit, hosted in Neoproterozoic shales and dolomitic mudstones of the Paleo- to Neoproterozoic Paterson Province, Western Australia, has features that have led to both syngenetic (pre-tectonic) and syn-tectonic genetic models. Here, we report Sm-Nd, Rb-Sr and U-Pb isotope data for vein apatite coeval with Cu minerals which provide absolute age constraints on apatite deposition and, therefore, Cu mineralization. Fluorapatites from two structurally and mineralogically similar vein sets have similar 87Sr/86Sr (0.7213–0.7238) and 208Pb/204Pb (37.90–38.04) ratios, unusual bell-shaped rare-earth element patterns and very high Sm/Nd ratios. A pooled Sm-Nd isochron for the vein apatites yields an age of 791 ± 42 Ma (eNdi = + 0.6 ± 3.7), consistent with a 207Pb-206Pb isochron age of 822 ± 23 Ma for galena and U-rich apatites from one of the samples. These mineral ages, combined with independent age constraints on the sedimentary host rocks and textural observations, constrain the timing of Cu mineralization to a period of ca. 850–800 Ma, that is during basin formation or early stages of basin deformation. Relatively high eNd values in vein apatite suggest that ore fluids were derived from, or reacted with, high-eNd mafic igneous rocks, as represented by a suite of gabbros and basalts (840–815 Ma) emplaced into the Neoproterozoic sequence. By contrast, 87Sr/86Sr ratios (~ 0.723) imply extensive re-equilibration of ore fluids with local sedimentary host rocks. The hypothesis of mafic igneous rocks as the source of copper at Nifty is consistent with inferred Cu sources for other sediment-hosted Cu deposits.