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

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 Re-187-Os-187 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 sigma 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 (Ad(96) Gr(2 similar to 3)Py(0 similar to 1)) and clinopyroxene (two series: Hd(6)Di(94) and Hd(86)Di(13)Jo(1)) 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 degrees - 600 degrees 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 degrees and 389 degrees C (average = 350 degrees +/- 24.7 degrees 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 similar to 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 degrees to 290 degrees C (average = 249 degrees +/- 27 degrees 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 delta S-34 values from -1.9 to 2.6 parts per thousand, consistent with a magmatic origin. Calcite in the late ore stage has delta C-13 values ranging from -3.2 to -5.9 parts per thousand and delta O-18 from 7.2 to 18.0 parts per thousand, distinct from the host-rock marble compositions (delta C-13 = 1.2-4.3 parts per thousand, delta O-18 = 10.8-23.9 parts per thousand). When corrected for temperature (250 degrees C, estimated from fluid inclusion analysis in calcites), these calcite data correspond to ore fluid delta C-13(fluid) values of -2.0 to -4.6 parts per thousand and delta O-18(fluid) values of 0.5 to 11.2 parts per thousand (clustering at similar to 10 parts per thousand), which are consistent with a magmatic origin. Lead isotope compositions of sulfides (Pb-206/Pb-204 = 18.273-18.369, Pb-207/Pb-204 = 15.627-15.677, and Pb-208/Pb-204 = 38.445-39.611) are similar to those of the granitic intrusions and sedimentary wall rocks, but distinct from those of basalts (Pb-206/Pb-204 = 18.282-19.133, Pb-207/Pb-204 = 15.564-15.665, and Pb-208/Pb-204 = 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.