Genesis and hydrothermal evolution of the Zhazigou skarn W (Mo) deposit, East Qinling, China: Constraints from fluid inclusions and H–O–S–Pb isotopes

Abstract

The Zhazigou is a typical redox-intermediate skarn W (Mo) deposit, located in the Luanchuan ore district within the East Qinling orogenic belt, central China. Skarn and W (Mo) orebodies are hosted mainly in the contact zones between the Yuku porphyritic granite and carbonaceous sedimentary rocks of the Luanchuan and Guandaokou groups. This paper presents detailed fluid inclusion (FI) and isotope data that are used to reconstruct the hydrothermal processes and explore the role of CO2 during scheelite precipitation at the Zhazigou. Four mineralization and alteration stages are recognized: anhydrous skarn (I), hydrous skarn (II), quartz–sulfide (III), and carbonate (IV). Systematic analysis of the FIs elucidates the evolution of hydrothermal fluids through these stages. Four primary types of FIs are recognized: two-phase liquid-rich inclusions (L-type), two-phase vapor-rich inclusions (V-type), halite-bearing (hypersaline) inclusions (H-type), and two- or three-phase CO2-bearing inclusions (C-type). In stage I, coexisting H-type (36.19–46.37 wt% NaCl equiv.) and low salinity V-type FIs have similar homogenization temperatures, indicating that fluid boiling during the formation of anhydrous skarn minerals occurred at 500–550 °C and 550–700 bar (2.0–2.5 km lithostatic depth). In stage II, coexisting V- and L-type FIs in quartz and scheelite within quartz–scheelite ± actinolite ± tremolite veins have different salinities but similar homogenization temperatures, indicating they were also trapped in the boiling process at 350–380 °C and ~240 bar (1.8–2.8 km hydrostatic depth). In stage III, L-, V-, and minor H-type FIs have homogenization temperatures indicating continued fluid boiling at 300–340 °C and <100–200 bar (1.1–1.8 km hydrostatic depth). In stage IV, only minor primary L-type FIs were observed in quartz and calcite, which have lower salinities (0.88–11.34 wt% NaCl equiv.) and homogenization temperatures of 150–200 °C, reflecting typical post-ore conditions. The O and H isotope data from garnet, scheelite, and quartz indicate that the ore-forming fluids at the Zhazigou deposit have a dominantly magmatic water signature, with δ18Ofluid values from 7.9 to 1.8 ‰ and δ18Dfluid values from −99.1 to −84.4 ‰. The S isotopic compositions, which range from 1.8 to 5.6 ‰, and the relatively homogeneous Pb isotopic compositions, demonstrate that the ore-forming materials were dominantly derived from magmatic sources with only a minor contribution from sedimentary sources. Scheelite was precipitated as a result of fluid–rock interactions and/or fluid boiling, and the former released abundant Ca2+ by altering calcium-rich minerals (e.g., andradite, diopside, hedenbergite, and plagioclase) into the mineralizing fluids. Abundant C-type FIs occur in quartz within the hydrous skarn stage, providing evidence of CO2-rich fluids that would have decrease the pH and generated relatively acidic conditions to help precipitate scheelite.