Mineralogy, fluid inclusion and stable isotope study of the Jinchanghe Zn-Pb-Fe-Cu skarn deposit in southwestern China

Jinchanghe is a Zn-Pb-Fe-Cu skarn deposit in the northern Baoshan block, southwestern China. It is a typical distal skarn deposit with orebodies in the Cambrian Hetaoping Formation limestone and calcareous siltstone. The skarn minerals display a vertical zonation with garnet skarn in the lower part and pyroxene skarn in the upper part. Economic metals are also zoned with Fe at the base, Cu in the middle, and Zn-Pb in the upper part. The skarn formation and Zn-Pb-Fe-Cu mineralization is divided into four paragenetic stages: a pre-ore stage dominated by prograde garnet and pyroxene, an oxide stage represented by Fe mineralization associated with retrograde ilvaite, actinolite and epidote alteration, a sulfide stage characterized with Cu-Zn-Pb sulfides, and a post-ore stage with barren calcite, quartz and chlorite.Fluid inclusion microthermometry indicates that the hydrothermal fluids of the Jinchanghe skarn system evolved from the pre-ore stage (450-480 degrees C and 11.7-15.5 degrees C wt% NaCl equiv), through the oxide stage (230-280 degrees C and 6.5-12.2 wt% NaCl equiv), the sulfide stage (190-230 degrees C and 1.3-10.3 wt% NaCl equiv), and eventually to the post-ore stage (120-180 degrees C and 1.6-4.6 wt% NaCl equiv). Correspondingly, the delta 18Ofluid values decrease from 1.8-7.1 parts per thousand to 1.0-6.4 parts per thousand, -1.0 to 1.3 parts per thousand, and -3.6 to -1.4 parts per thousand. This indicates that the pre-ore fluids comprise a magmatic component but mixed with some meteoric water, and in the later stages meteoric water has become dominant in the hydrothermal system. Zinc and sulfur isotope compositions reveal that the Zn and S forming the sulfides have a dominantly magmatic origin.The coupled decreases of fluid temperature, salinity, and delta 18Ofluid values during the mineralization indicate simultaneous mixing with meteoric water and ore precipitation, suggesting that fluid mixing was critical in ore deposition. The gradual increase of delta 13CCO2 values in equilibrium with the hydrothermal calcite (-5.2 to -1.6 parts per thousand) from the sulfide stage to the post-ore is attributed to the reaction between the fluids and the carbonate wallrocks, implying a role that fluid-rock interaction has taken in the sulfide deposition. Fluid mixing and fluid-carbonate reaction are the two major factors controlling the formation of the Jinchanghe deposit.