Genesis and Magmatic-Hydrothermal Evolution of the Yangla Skarn Cu Deposit, Southwest China

Abstract

The Yangla skarn Cu deposit (150 Mt at 1.03% Cu) is located in the central segment of the Jinshajiang metallogenic belt within the Sanjiang (Three Rivers) region, southwest China. Skarn orebodies are mainly developed between different units of Devonian carbonate and quartz sandstone rocks with stratiform-like shapes, or within the contact zone between granitoids and marbles. Re-Os dating of molybdenite intergrowth with chalcopyrite yielded a well-constrained 187Re-187Os isochron age of 232.0 ± 1.5 Ma with a weighted average age of 231.8 ± 1.3 Ma, both coeval with the related intrusions (233.1 ± 1.4 and 231.0 ± 1.6 Ma at 2 σ by zircon U-Pb dating) from our previously published work. Field and textural relationships indicate three hydrothermal stages characterized by assemblages of prograde skarn (pre-ore stage), retrograde skarn and Cu-Fe-Mo-Bi sulfides (main ore stage), and Pb-Zn sulfides associated with calcite and quartz (late ore stage), as well as one supergene stage marked by secondary Cu mineralization (malachite and azurite). Skarns contain garnets with andraditic compositions (Ad96 Gr2~3Py0~1) and clinopyroxene (two series: Hd6Di94 and Hd86Di13Jo1) with low Mn/Fe ratios (<0.1), typical of Cu skarn deposits worldwide. Three stages of fluid evolution were observed by a detailed fluid inclusion study: (1) Early fluids were trapped under two-phase conditions, as evidenced by the coexistence of brine (homogenization temperatures = 560°–600°C, average salinity = 49.4 ± 1.7 wt % NaCl equiv, n = 33) and vapor-rich inclusions in pre-ore stage andradite and diopside (trapped at ~600 bars, or a depth of approximately 2 km assuming lithostatic pressure conditions). (2) Main ore stage fluid inclusions in quartz were also trapped under two-phase conditions (boiling), as identified by the coexistence of vapor- and liquid-rich fluid inclusions; liquid-rich inclusions homogenized between 312° and 389°C (average = 350° ± 24.7°C, n = 20), with salinities of 2.4 to 5.6 wt % NaCl equiv (average = 4.2 ± 0.9 wt % NaCl equiv, n = 13) and a depth of ~2 km (~200 bars, hydrostatic pressure conditions). (3) Late ore stage fluids are represented by inclusions in calcite, characterized by homogenization temperatures ranging from 220° to 290°C (average = 249° ± 27°C, n = 14) and salinities between 2.1 and 8.0 wt % NaCl equiv (average = 4.6 ± 1.5 wt % NaCl equiv, n = 11). Sulfur isotope compositions of sulfide minerals (pyrite, chalcopyrite, pyrrhotite, and molybdenite) from the main and late ore stage have a narrow range of δ 34S values from −1.9 to 2.6‰, consistent with a magmatic origin. Calcite in the late ore stage has δ 13C values ranging from −3.2 to −5.9‰ and δ 18O from 7.2 to 18.0‰, distinct from the host-rock marble compositions ( δ 13C = 1.2–4.3‰, δ 18O = 10.8–23.9‰). When corrected for temperature (250°C, estimated from fluid inclusion analysis in calcites), these calcite data correspond to ore fluid δ 13Cfluid values of −2.0 to −4.6‰ and δ 18Ofluid values of 0.5 to 11.2‰ (clustering at ~10‰), which are consistent with a magmatic origin. Lead isotope compositions of sulfides (206Pb/204Pb = 18.273–18.369, 207Pb/204Pb = 15.627–15.677, and 208Pb/204Pb = 38.445–39.611) are similar to those of the granitic intrusions and sedimentary wall rocks, but distinct from those of basalts (206Pb/204Pb = 18.282–19.133, 207Pb/204Pb = 15.564–15.665, and 208Pb/204Pb = 38.367–38.942) in the mining area. Taken together, these geologic, geochemical, and isotopic data confirm that Yangla is a typical Cu skarn deposit.