The hydrothermal Bonanza-grade native Ag deposit Elizabeth Hill (Western Australia): an As-depleted five-element endmember or just supergene enrichment?

Novel petrographic, mineral chemical and microthermometric data combined with hydrothermal allanite U-Pb ages of the unusual high-grade Elizabeth Hill hydrothermal native Ag vein-type deposit in Western Australia reveal similarities to five element veins. Mineralized veins are hosted by fault-controlled quartz-carbonate breccias in Archean basement rocks of the West Pilbara Craton tied to basement contact with the layered ultramafic-mafic Munni Munni Complex hosting stratiform magmatic sulfides. Unconformity-based Fortescue Group siliciclastic-volcanic cover rocks are partly eroded. Host rocks close to the mineralization are silicified, calcified and chloritized. Pre-existing magmatic sulfides comprise chalcopyrite-pyrrhotite-pentlandite and are intersected by veins of stage I characterized by sphalerite-chalcopyrite-galena with minor pyrite, pyrrhotite and gersdorffite, and stage II Ag mineralization hosting argentopentlandite-acanthite-native Ag. Supergene remobilization of stage I and II minerals produced multiple generations of native Ag encompassing wires, veins and Ag-Hg amalgam ≤35.7 wt.% Hg. Chalcopyrite and acanthite allomorphs indicate high temperatures for stage I >557 °C and II >179 °C. Two phase (vapor-liquid) inclusions of stage I record the ternary CaCl₂-NaCl-H₂O system with salinities of 8.1–16.6 wt.% (CaCl₂+NaCl) and T_h of 107–147 °C, which are interpreted as a signature of retrograde re-equilibration. Two allanite age groups (2400–2370 and 2050–1990 Ma) coincide with episodes of craton-wide hydrothermal activity and orogenic and epithermal Au vein formation. Compared to most five-element veins, Ni, Co and As are rare; hence, Elizabeth Hill may represent an As-depleted, native Ag-dominated, Kongsberg-type five-element deposit. Repeated supergene Ag enrichment probably involved reactivation of the hosting fault system and long-term weathering.