The Sawayaerdun gold deposit (3.7 Moz at 1.38 g/t Au), located in the South Tianshan orogen, Northwest China, is distinguished by deformed quartz-sulfide veins and veinlets overprinted by extensive quartz-sulfide stockworks. The paragenesis is delineated into four distinct stages: (0) preore framboidal pyrite (Py0) within carbonaceous slate interlayers, (1) early-ore deformed, porous pyrite (Py1) and arsenopyrite aggregates (Apy1), (2) main-ore zoned pyrite (Py2) and rhombic arsenopyrite (Apy2), and (3) late-ore anhedral pyrite (Py3). Diagenetic Py0 is notably enriched in gold (mean of 4.62 ppm), As, Co, Ni, Ti, Ba, and W. Py1 formed during the brittle−ductile transition and exhibits reduced gold concentrations (mean = 0.35 ppm). Subsequent dissolution−reprecipitation processes introduced low invisible gold (mean of 0.66 ppm in Py2), along with elevated As, Co, W, Ti, and Ba contents. Py3 displays negligible or undetectable gold content. Almost all pyrite types exhibit low Co/Ni ratios (1), and arsenopyrite (Apy1 and Apy2) contains higher Au concentrations than pyrite (Py1 and Py2). Sulfur isotope values (δ34S = −1.4‰ to +0.61‰) across all sulfide types indicate a homogeneous, auriferous fluid source. The δ34S signatures of Py0 (−1.14‰ to +0.16‰) are comparable to those of the upper Paleozoic clastic-carbonate host sediments, suggesting that sulfur and auriferous fluids were mobilized from these units during regional metamorphism. This interpretation is further supported by Pb isotope compositions, which likewise indicate sedimentary sources for the ore metals. Textural, chemical, and isotopic data indicate that the recrystallization of Au-rich Py0 during regional metamorphism generated auriferous fluids, which through fluid−rock reactions, such as sulfidation and subsequent brittle−ductile transition, formed gold-poor, deformed Py1. Crystallization of Au-poor Py2 resulted from the dissolution−reprecipitation of early Py1 during fluid boiling. These findings, combined with gold contribution estimates, underscore the critical role of early-stage gold enrichment in diagenetic pyrite, and fluid phase separation in concentrating gold in orogenic deposits, thus providing a fundamental exploration target in the South Tianshan region and elsewhere in the Central Asian Orogenic Belt.
Ma et al. (Tue,) studied this question.