Garnets in porphyry-skarn systems: A LA-ICP-MS, fluid inclusion, and stable isotope study of garnets from the Hongniu-Hongshan copper deposit, Zhongdian area, NW Yunnan Province, China

摘要

The Late Cretaceous Hongniu-Hongshan porphyry-skarn copper deposit is located in the Zhongdian area of northwestern Yunnan Province, China. Garnets from the deposit have compositions that range from Adr(14)Grs(36) to almost pure andradite (Adr(98)Grs(2)) and display two different styles of zoning. The garnets are predominantly of magmatic-hydrothermal origin, as is evidenced by their O-18(fluid) (5.4-6.9 parts per thousand) and low D-fluid (-142 parts per thousand to -100 parts per thousand) values, both of which likely result from late-stage magmatic open-system degassing. Three generations of garnet have been identified in this deposit: (1) Al-rich garnets (Grt I; Adr(22-57)Grs(78-43)) are anisotropic, have sector dodecahedral twinning, are slightly enriched in light rare earth elements (LREEs) compared with the heavy rare earth elements (HREEs), have negative or negligible Eu anomalies, and contain high concentrations of F. Fluid inclusions within these Al-rich garnets generally have salinities of 12-39 wt.% NaCl eq. and have liquid-vapor homogenization temperatures (Th) of 272-331 degrees C. The Grt I are most likely associated with low- to medium-salinity fluids that were generated by the contraction of an ascending vapor phase and that formed during diffusive metasomatism caused by pore fluids equilibrating with the host rocks at low W/R (water/rock) ratios. These garnets formed as a result of the high F activity of the system, which increased the solubility of Al within the magmato-hydrothermal fluids in the system. (2) Fe-rich garnets (Adr(75-98)Grs(25-2)) have trapezohedral faces, and are both anisotropic with oscillatory zoning and isotropic. These second-generation Fe-rich garnets (Grt II) have high Sigma REE concentrations, are LREE-enriched and HREE-depleted, and generally have positive but variable Eu anomalies. All of the Fe-rich garnets contain high-salinity fluid inclusions with multiple daughter minerals with salinities of 33-80 wt.% NaCl eq. Some of them show higher temperatures of halite dissolution (465-591 degrees C) than liquid-vapor homogenization temperatures (319-473 degrees C), and several Fe-rich garnets contain inclusion groups indicative of boiling. The Grt II are associated with high-temperature, hypersaline fluids that were segregated from magma at a depth of at least 5.6 km and reacted with carbonates at depths shallower than 2.0 km. (3) Al-rich garnet veins (Adr(14-60)Grs(86-40)) contain allotriomorphic crystals, have lower HFSE (high field strength element) and REE concentrations than the other garnets, and have HREE-enriched and LREE-depleted patterns with small Eu anomalies that are typical of the majority of garnets. The Grt III most likely formed from residual metasomatic fluids. (C) 2014 Elsevier Ltd. All rights reserved.