Textural, trace elemental and sulfur isotopic signatures of arsenopyrite and pyrite from the Mandongshan gold deposit (west Junggar, NW China): Implications for the conditions of gold mineralization

[Display omitted] •The occurrence of titanite instead of rutile reflects a relatively reduced condition.•The local dissolution and re-precipitation processes control the Au(2b) precipitation.•Calculations show solubility of Au decreases with increase of fH2 in auriferous fluid. The Mandongshan gold deposit, located in the west Junggar region (Xinjiang Province, NW China), is hosted by Carboniferous basalt, tuff, and diabase. The gold ores at Mandongshan gold deposit take the form of hydrothermal veins cutting and disseminations within the altered rocks that host the deposit. A characteristic feature of the Mandongshan is the presence of abundant arsenopyrite. Two gold events have been recorded at Mandongshan: (I) an initial quartz-sulfide stage, and (II) a subsequent calcite-sulfide stage. In situ LA-ICP-MS analyses suggest that arsenopyrite of stage I has higher Te and lower Pb, Bi, Au, Ag, and Cu concentrations than arsenopyrite of stage II. Nano-SIMS δ34S values of pyrite associated with arsenopyrite of stage I and II fall within two ranges of −7.6 to −1.5‰ and −7.8 to −1.6‰. The presence of titanite and lack of rutile in the gold ores were taken to imply that the Mandongshan gold deposit formed from reduced fluids. Titanite crystals recorded in the gold-bearing quartz-sulfide veins show strongly positive Eu anomalies (Eu* = 2.15–3.99) and weakly positive Ce anomalies (Ce* = 1.04–1.58). The δ34S values of pyrite provide evidence for a magmatic sulfur source, which is consistent with the geochemical data obtained from other nearby gold deposits as well as the spatial relationship in the west Junggar region between gold mineralization and widespread felsic intrusions. In addition, our analyses suggest that the native gold of stage I was deposited due to phase separation of the ore-forming fluid. Native gold deposition of stage II can be further subdivided into stages IIa and IIb. The former stage is represented by coarse-grained gold that co-precipitated with arsenopyrite, while stage IIb is represented by fine-grained gold that precipitated as a result of dissolution and re-precipitation process occurring in earlier-formed stage IIa arsenopyrite grains. This model is consistent with our phase equilibrium calculations for the As-Au-Cu-Fe-S-O system of the Mandongshan gold deposit.