Ore-forming processes in the Wang’ershan gold deposit (Jiaodong, China): Insight from microtexture, mineral chemistry and sulfur isotope compositions

Abstract

The microtexture, trace element characteristics and sulfur isotopic features of ore-related minerals provide important clues to trace physico-chemical conditions and source characteristics of ore formation. In this study, we investigate the Wang’ershan gold deposit in the Jiaodong gold province of eastern North China Craton where three hydrothermal stages were distinguished. Stage I is characterized by K-feldspar, sericite, quartz with minor pyrite precipitation (Py-I); sericite, quartz (Qtz-a) and pyrite (Py-II) with minor gold precipitated during stage II; and stage III involved quartz (Qtz-b), gold, and polymetallic sulfides with minor pyrite (Py-III). In situ δ34S results show a decreasing trend from 21.7 to 14.6‰ (Py-I) to 9.2–8.3‰ (Py-II) and 8.9–8.6‰ (Py-III). The Py-II and Py-III are poor in mineral inclusions and porous microtextures, and have no oscillatory composition zoning. The Co content is 1–2 order of magnitudes higher in Py-I (371.2 – 823.2 ppm, av. 597.2 ppm) than late stage pyrites (0.4 – 524.2 ppm, av. 95.4 ppm). Qtz-a show cataclastic breccia cemented by Qtz-b with little dissolution during the injection of stage III ore-forming fluids. The Al contents decrease from Qtz-a (49.0 – 426.6 ppm) to Qtz-b (18.2 – 46.8 ppm). Based on the sulfur isotope, trace element and microtextural characteristics in Py-II and Py-III, combined with previous fluid inclusion studies, we infer that the ore-forming fluids of stage II and III experienced gentle immiscibility and were suggested to be of different evolutionary stages of a single ore-forming fluid sharing common sources with similar and near-equilibrium physicochemical conditions. The trace element characteristics with lack of oscillatory compositional zoning in the two stages of quartz suggest gentle cooling with lack of any abrupt change of physicochemical conditions during fluid immiscibility. The decoupling of Au and As contents in Py-II and Py-III was likely controlled by chemisorption of gold in pyrite structure. Complex variation in geochemical features of ore-forming fluids from early alteration to main mineralization period is indicated by the large range of δ34S and Co contents in Py-I and late stage pyrites, and the differences in alteration processes. Based on a comparison of the Co/Ni, Ni and As among pyrites from the Wang’ershan gold deposit, typical porphyry deposits and orogenic gold deposits, together with the large positive deviation of δ34S relative to mantle values, we propose that ore-forming materials of the Wang’ershan gold deposit were derived from mixed sources involving magmatic hydrothermal-related and sedimentary strata-related components.