Fault-induced gold saturation of a single auriferous fluid is a key process for orogenic gold deposit formation

In orogenic gold deposits, the mechanism by which a hydrothermal fluid precipitates gold in laminated quartz veins remains elusive. The Kanowna Belle deposit hosts gold mineralization in structurally controlled quartz-pyrite laminated veins that cut volcano-sedimentary and granitic rocks of the Kalgoorlie terrane, Australia. Veins show microtextural evidence for multiphased emplacement, corresponding to three distinct pyrite growth zones in which Au enrichment in the pyrite is attributed to high fluid influx. To monitor the chemical and isotopic evolution of the auriferous fluid leading to gold precipitation, we combine in situ multiple sulfur isotope analyses with trace element composition of gold-bearing pyrite growth zones: cores (Au ≤149 ppm; δ34S –3.3 to +4.2%; As ≤2.5 wt %; Ni ≤4,022 ppm; Te ≤416 ppm), Au-rich oscillatory zoned mantles (Au ≤2,251 ppm; δ34S –8.4 to +0.1%; As ≤4.5 wt %; Ni ≤1,111 ppm; Te ≤829 ppm), and rims (Au ≤264 ppm; δ34S –6.0 to +2.8%; As ≤1.4 wt %; Ni ≤2,113 ppm; Te ≤229 ppm). The positive and consistent D33S of each zone (D33S = +0.3 ± 0.2%; n = 160) indicates that one single reservoir was tapped during the evolution of the mineralizing system. The gold-rich pyrite mantle zones precipitated from a fluid undergoing SO42–/H2S fluctuations associated with phase separation due to rapid and repeated releases in fluid pressure. This study demonstrates that the “fault-valve” process applies a first-order control on the precipitation of gold from a single auriferous fluid reservoir.