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

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 degrees C and 357 +/- 50 degrees 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 degrees 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.

Automated summary

Research at Mt. Percy has identified two distinct styles of gold mineralisation, one correlated with the Fimiston-style mineralisation and the other with the Mt. Charlotte-style mineralisation. Geochemical analysis highlights the differences between these two styles.