In-situ UPb geochronology, geochemistry, and spatial-temporal evolution of multi-generational garnet from the Muzuke FeCu polymetallic skarn deposit, Chinese Western Tianshan, NW China

Abstract

Spatial and temporal processes of variations in garnet composition and corresponding fluid evolution during the prograde skarn stage are matters of ongoing controversy. Here, we combine in situ UPb geochronology and major- and trace-element compositions of garnet (Grt) in the Muzuke FeCu polymetallic skarn deposit to address the above debate. Three generations of garnets were identified concerning their locations in the contact zone, colors, textural and optical characteristics, and chemical compositions. They belong to the grossular-andradite solid solution and are characterized by a temporal and spatial chemistry zonation, which ranges greatly in composition from nearly pure grossular (Grs₉₈Adr_0.6) during early prograde stage in endoskarn and proximal exoskarn to almost pure andradite (Grs_1.7Adr₉₇) during late prograde stage in distal exoskarn. We observed a clear correspondence between the major-element compositions and optical characteristics of garnet from Muzuke, and the formation of the birefringent garnets is probably induced by the symmetry reduction due to trivalent cation ordering varieties at the Y site in the regular octahedron. From Grt-1 to Grt-3, the overall REE patterns shift from HREE enrichment to LREE enrichment, accompanied by negative to positive Eu anomalies, which can be attributed to the relative proportion of grossular and andradite end-members, while the chemical composition of fluid in the late prograde stage also has some effects on the REE patterns of garnet. The transitions of geochemical characteristics between three generations of garnets suggest that despite the existence of multiple pulses of fluid flux in the prograde stage, the overall trend of fluids evolution indicates that from the early to terminal prograde stage, from endoskarn to distal exoskarn, the formation mechanism of skarn transitions from diffusion metasomatism under a closed system to advective metasomatism under an open system, accompanied by a gradual decrease in temperature of hydrothermal fluids and an increase in fO₂ and W/R ratios. The above process inhibited premature sulfide precipitation and may be a precursor to the precipitation of magnetite in the later stage. In situ UPb dating on garnet yields an age of 364.9 ± 7.6 Ma for Grt-2 and of 363.2 ± 4.5 Ma to 363.0 ± 2.0 Ma for Grt-3 from the Muzuke deposit, suggesting a genetic connection between the polymetallic mineralization and the emplacement of the nearby granodioritic intrusion. The garnet spatial-temporal evolution model established in this study could assist in localizing and exploring regional skarn ore bodies with similar causative intrusions and wall rock properties.