Geology, S–Pb isotopes, and 40 Ar/ 39 Ar geochronology of the Zhaxikang Sb–Pb–Zn–Ag deposit in Southern Tibet: implications for multiple mineralization events at Zhaxikang

Abstract

Several Au, Sb, Sb–Au, Pb–Zn, and Sb–Pb–Zn–Ag deposits are present throughout the North Himalaya in southern Tibet, China. The largest Sb–Pb–Zn–Ag deposit is Zhaxikang (18 Mt at 0.6 wt% Sb, 2.0 wt% Pb, 3.5 wt% Zn, and 78 g/t Ag). Zhaxikang veins are hosted within N–S trending faults, which crosscut the Early–Middle Jurassic Ridang Formation consisting of shale interbedded with sandstone and limestone deposited on a passive continental margin. Ore paragenesis indicates that Zhaxikang mineralization occurred in two main phases composed of six total stages. The initial phase was characterized by assemblages of fine-grained Mn–Fe carbonate + arsenopyrite + pyrite + sphalerite (stage 1), followed by relatively coarse-grained Mn–Fe carbonate + Fe-rich sphalerite + galena + pyrite (stage 2). The second phase was marked by assemblages of quartz + pyrite + Fe-poor sphalerite and Ag-rich galena + tetrahedrite + sericite (stage 3), quartz + Sb–Pb sulfosalt minerals mainly composed of boulangerite and jamesonite (stage 4), quartz + stibnite ± cinnabar (stage 5), and quartz ± calcite (stage 6). Sulfides of stage 2 have δ³⁴S_V–CDT of 8.4–12.0‰, ²⁰⁶Pb/²⁰⁴Pb ratios of 19.648 to 19.659, ²⁰⁷Pb/²⁰⁴Pb ratios of 15.788 to 15.812, and ²⁰⁸Pb/²⁰⁴Pb ratios of 40.035 to 40.153. Sulfides of stage 3 have similar δ³⁴S_V–CDT of 6.1–11.2‰ and relatively more radiogenic lead isotopes (²⁰⁶Pb/²⁰⁴Pb = 19.683–19.792). Stage 4 Sb–Pb sulfosalt minerals have δ³⁴S_V–CDT of 5.0–7.2‰ and even more radiogenic lead (²⁰⁶Pb/²⁰⁴Pb = 19.811–19.981). By contrast, stibnite of stage 5 has δ³⁴S_V–CDT of 4.5–7.8‰ and less radiogenic lead (²⁰⁶Pb/²⁰⁴Pb = 18.880–18.974). Taken together with the geological observations that the Pb–Zn-bearing Mn–Fe carbonate veins were crosscut by various types of quartz veins, sphalerite and galena of stage 2 underwent dissolution and remobilization, and that Sb–Pb(−Fe) sulfosalts formed at the expense of Pb from stage 2 galena and of Fe from stage 2 sphalerite, we argue that the early Pb–Zn veins were overprinted by later Sb-rich fluids. Stage 2 fluids were likely acidic and oxidized and leached lead from high-grade metamorphic rocks of the Greater Himalayan crystalline complex (GHC) and sulfur from reduced rocks, such as slate of the Ridang Formation, along N–S trending faults, leading to precipitation of Pb–Zn sulfides and Mn–Fe carbonate and formation of solution collapse breccias. Later Sb-rich fluids leached Pb from the GHC and the pre-existing sulfides and deposited Fe-poor sphalerite, Ag-rich galena, tetrahedrite, Sb–Pb sulfosalts, and stibnite in quartz veins that cut pre-existing Pb–Zn-bearing Mn–Fe carbonate veins. The Sb-rich fluids also likely leached Pb from Early Cretaceous gabbro and formed stibnite at shallow levels where early Pb-Zn-bearing Mn-Fe carbonate veins are absent. A sericite ⁴⁰Ar–³⁹Ar plateau age of 17.9 ± 0.5 Ma from stage 3 veins represents the timing of the onset of stage 3 mineralization.