Native antimony in the Baogutu gold deposit (west Junggar, NW China): Its occurrence and origin

摘要

The Baogutu gold deposit, West Junggar (Xinjiang, NW China), is composed of quartz-sulfide veins and their stockworks and is hosted within an Early Carboniferous volcanic-sedimentary sequence. Three ore-forming paragenetic stages can be identified: coarse-grained quartz-sulfide vein (stage I), gold-bearing fine-grained quartz-sulfide vein (stage II), and native antimony-bearing calcite-sulfide veinlets (stage III). The estimated formation temperatures (approximately 360 to 220℃),fS2 (-7 to —15 log units), and fO_2 (-26 to -43 log units) decrease from stage I, through stage II, to stage III. The nature of the hydrothermal fluid changed from weakly acidic (pH: 5 to 6 at stage I) to alkaline (pH: >7 at stage III). Two different occurrences of native antimony could be identified: one coexists with chalcopyrite andpyrrhotite, whereas the other occurs as isolate grains within calcite veins. Native antimony (up to 100 pm insize) contains minor amounts of As (2.0 to 2.7 wt.%); empirical formula Sb_(0.95-0.96)AS_(0.03-0.04). Decrease oftemperature and fO_2 at high pH and low fS_2 conditions favored co-precipitation of native antimony-ullmannite assemblages and the decomposition of tetrahedrite. Native antimony crystallized later than thenative gold, suggesting fractionation between Au and Sb during the hydrothermal evolution of the deposit.