Using garnet geochemistry discriminating different skarn mineralization systems: Perspective from Huangshaping W-Mo-Sn-Cu polymetallic deposit, South China

Abstract

Important Mid-Late Jurassic W-Mo-Sn and Cu polymetallic skarn deposits are widely distributed in South China, yet differences in ore-forming processes and physiochemical conditions between these two skarn deposit types are poorly constrained. The large Huangshaping deposit in South China developed both W-Mo-Sn skarn (named 301 system) and Cu skarn (named 304 system) mineralization systems, which are genetically linked to granite porphyry and quartz porphyry, respectively. Both skarn systems are characterized by abundant garnet. Three types of garnet were distinguished in the 301 system, i.e., the texturally homogeneous Grt301a and veined Grt301b from deep-level (−176 m), and oscillatory-zoned Grt301c from shallow-level (−96 m). Moreover, Grt301a is commonly cut by Grt301b vein. Meanwhile, two types of garnet were identified in the 304 system, i.e., the texturally anisotropic Grt304a and oscillatory zoned Grt304b, with the former being commonly replaced by the latter.Trace element geochemistry shows that REE3+ entered garnet mainly by YAG-type substitution ([Ca2+]VIII −1[REE3+]VIII + 1[Si4+]IV −1[Z3+]IV + 1) in the 301 system and menzerite substitution ([Ca2+]VIII −1[REE3+]VIII + 1[Al3+]VI −1[Y2+]VI + 1) in the 304 system. Grt301a, Grt301b and Grt304a are HREE-enriched, suggesting that they were formed under nearly-neutral pH and low water/rock (W/R) ratios in closed and stable systems via diffusive metasomatism. Meanwhile, Grt301c and Grt304b are LREE-enriched, suggesting that they were formed under mildly-acidic pH and high W/R ratios in open and fluctuating systems via infiltration metasomatism. The elevated Sn content in Grt301c (median 5039 ppm) indicates its higher oxygen fugacity (fO2) than Grt301a (median 1861 ppm) and Grt301b (median 1776 ppm). Similarly, the high Sn content in Grt304b (median 1444 ppm) also suggests higher fO2 than that of Grt304a (median 338 ppm). The different formation conditions between deep-level garnet (Grt301a and Grt301b) and shallow-level garnet (Grt301c) in the 301 system are not consistent with garnet in the 304 system that shows similar texture, trace element geochemistry and formation conditions at different depths. This reveals differences of formation environments between two skarn systems at Huangshaping. Partial least squares-discriminant analysis (PLS-DA) suggests that W in garnet can discriminate the 301 and 304 skarn systems, which may be related to the lower fO2 and higher fractionation of the ore-causative granite porphyry in the former, which favors W enrichment in corresponding magmatic-hydrothermal fluids in 301 system. Moreover, Mo can discriminate garnet from the deep- and shallow-levels of the 301 system, which may be related to the higher fO2 in the latter. We highlight the importance of garnet trace element geochemistry on tracing hydrothermal fluid evolution and discriminating between different skarn systems.