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

Abstract

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 °C and 11.7–15.5 °C wt% NaCl equiv), through the oxide stage (230–280 °C and 6.5–12.2 wt% NaCl equiv), the sulfide stage (190–230 °C and 1.3–10.3 wt% NaCl equiv), and eventually to the post-ore stage (120–180 °C and 1.6–4.6 wt% NaCl equiv). Correspondingly, the δ¹⁸O_fluid values decrease from 1.8–7.1‰ to 1.0–6.4‰, -1.0 to 1.3‰, and -3.6 to -1.4‰. 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 δ¹⁸O_fluid 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 δ¹³C_CO2 values in equilibrium with the hydrothermal calcite (-5.2 to -1.6‰) 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.