Supergene gold in silcrete as a vector to the Scuddles volcanic massive sulfides, Western Australia

Abstract

This study reports data on the first occurrence of economic supergene Au in a near-surface silcrete horizon over the Scuddles volcanic massive sulfide (VMS) deposit in the Golden Grove district, Western Australia. A deep weathering profile was developed on dacite, rhyodacite, siltstone, and breccia that host primary Cu, Zn, and Pb sulfides with Au-Ag ore. From the base, the weathering profile at Scuddles is subdivided into five main zones: (1) supergene sulfide enrichment zone; (2) supergene oxide enrichment zone; (3) ferruginous saprolite; (4) leached zone of kaolinitic saprolite and silcrete; and (5) lateritic zone of mottled clays, ferruginous duricrust, and gravels. Silcrete at Scuddles hosts supergene Au deposit that formed in two generations: the first is intimately associated with Ag halides during supergene enrichment of the primary VMS, and the second is associated with kaolinite in dissolution cavities inside Ag halides during lateritic weathering. These two Au generations imply more than one mechanism of Au remobilization and formation, multiple fluid pathways, and superimposed episodes of weathering under variable timing and climatic conditions. Gold grains are pure, nanocrystalline (up to 10 nm) and clustered together forming microcrystalline aggregates. A few Au grains are residual in silcrete with Ag-Sb-rich cores and Ag-poor rims possibly formed during dealloying of Ag and Sb. Chemically, Au in silcrete is associated with a multi-element concentration of Ag, I, Br, Cl, Sb, Sn, Bi, Hg, Mo, W, Te, and Ge. Gold and Ag in the supergene weathering profile were mobilized to silcrete as a halide complex under acidic and saline conditions generated during the oxidation of massive sulfides at depth. The precipitation of Au-Ag halides in the silcrete may have taken place in response to a rise in pH. Gold was likely remobilized with kaolinite from the surface lateritic zone, facilitated by decays of plant roots and bioturbation. The clustered spongy, cube-octahedral, platy (six-sided), dendritic-, and reniform-like morphologies of Au in cavities inside Ag halides may indicate biogenic-related processes in its precipitation. Recognizing Au-Ag-rich silcrete over the buried VMS at Scuddles highlights the significance of the silcrete in finding buried VMS, particularly if the gossan is absent.