Structural setting, wall rock alteration and gold mineralisation of the Mt. Percy gold deposit, Kalgoorlie, Western Australia

Abstract

Field mapping, drill-core logging, petrography, laser ICP-MS analysis on pyrite, chlorite geothermometry and geochemical analysis of hydrothermally altered rocks reveal two distinct styles of gold mineralisation at Mt. Percy. The first style is D2 shear zone-hosted and correlates with Fimiston-style mineralisation documented in the Golden Mile. Laser ICP-MS analyses on pyrite indicate high As concentrations (average of 5823 ppm for pyrite in Hannans Lake Serpentinite and 635 ppm in feldspar-quartz porphyry). Chlorite geothermometry reveals average temperatures of 262 ± 50 °C and 357 ± 50 °C in distal and proximal alteration zones, respectively. Geochemical analyses reveal (1) positive gold-silver and gold-antimony relationships; (2) lack of a base metal-gold relationship; (3) systematic increase of carbonatisation and alkali saturation indices from distal to proximal alteration zones; and (4) addition of K, Ca, Na and V, with corresponding depletion of Mg, Fe, P and REE. The second style is related to D4 sheeted quartz vein sets and correlates with Mt. Charlotte-style mineralisation documented in the Mt. Charlotte deposit. Laser ICP-MS analyses on pyrites reveal low As concentrations (average of 51 ppm for pyrites in feldspar-quartz porphyry and 870 ppm in Devon Consols Basalt). Chlorite geothermometry reveals an average temperature of 325 ± 50 °C on vein selvage in the Devon Consols Basalt. High-grade gold mineralisation (> 50 g/t Au) and extensive (< 50 m wide) hydrothermal alteration zones coincide with D2 shear zones and D4 fault-controlled sheeted quartz vein sets, and particularly in areas where both of these structures intersect porphyry stocks.