Chemical Composition and Genesis Implication of Garnet from the Laoshankou Fe-Cu-Au Deposit, the Northern Margin of East Junggar, NW China

In order to reveal the formation mechanism of different garnets and its implications for fluid evolution in Laoshankou Fe-Cu-Au deposit northern margin East Junggar (NW China), three types garnet have been investigated detail this study. (1) Type 1 grossular, formed at Ca-silicate stage (stage I, pre-mineralization stage), was replaced by 2 magnetite, displays a compositional range Grs44–53Adr44–53, which has relatively lower total REE (rare earth elements) contents (8.14–32.8 ppm) markedly depleted LREE (light rare with distinctive positive Eu anomaly (1.36–9.61). (2) Al-rich andradite, early sub-stage amphibole-epidote-magnetite II, main magnetite mineralization can be divided into two sub-types, i.e., 2a 2b. exhibit polysynthetic twinning narrow variations Adr63–66Grs31–34 HREE-(heavy rare-earth enrichment anomalies (3.22–3.69). 2b own wide Adr55–77Grs21–43 higher (49.1–124 ppm), (2.11–4.61). (3) 3 andradite (Adr>91) associated sulfide III, copper-gold stage) is from other Laoshankou, are characterized lowest (1.66–91.1 flat HREE patterns, LREE-enrichment strongest (3.31–45.48). Incorporation largely controlled external factors, such as chemistry, pH, ƒO2 water-rock ratios well crystal chemistry. mainly follow creation X2+ (e.g., Ca2+) site vacancy, e.g., [X2+]?3VIII[]+1VIII[REE3+]+2VIII. The REE3+ substitution Na+-REE3+ coupled substitutions, [X2+]?2VIII[X+]+1VIII[REE3+]+1VIII, without evaluation vacancy. indicate significant differences compositions physicochemical conditions, used trace fluid–rock interaction hydrothermal garnet. grossular magmatic under low water–rock ƒO2, neutral pH environment diffusion metasomatism nearly closed system preferential incorporation HREE. As long pore residence continuing infiltration closed-system fluids high water/rock were increased more active Fe3+ REE, andradite. contrast, oxidizing mildly acidic highest ratios, influenced externally derived salinity Ca-rich an open system. Thus, geochemical features generations provide important information evolution, revealing transition addition non-magmatic stage. above proved process further indicates that prefers IOCG-like (iron oxide-copper-gold) rather than typical skarn deposit.