Malage skarn-type Sn-polymetallic deposit in the vast Gejiu ore field: New data on its age and form

Abstract

The Malage skarn-type deposit in the world’s largest Gejiu Sn-polymetallic ore field contains ore bodies and associated skarns, which typically occurring near the contact zone between the granitic intrusions and the carbonate host rock. In this study, detailed textural analyses and chemical composition data of garnet, combined with U–Pb geochronology of different garnet generations and cassiterite, have been systematically investigated to constrain the evolution of hydrothermal fluids. Three generations of garnets (Grt I, Grt II, and Grt III) are detected in the deposit based on the petrographic characteristics. The U–Pb ages for Grt I, Grt II and Grt III are 81.8 ± 1.1 Ma, 81.1 ± 1.8 and 80.4 ± 2.0 Ma, while the cassiterite show more precise U–Pb age of 80.3 ± 0.2 Ma and 80.2 ± 0.5 Ma. This chronological coherence indicates these minerals formed during the same magmatic-hydrothermal event. Magnetite and hematite inclusions-bearing Grt I exhibits a right-skewed rare earth element (REE) pattern, with light REE/heavy REE (LREE/HREE) ratios (1.04–468.74) and negative Eu anomalies (0.06–0.26). Grt II shows a similar REE pattern, with high LREE/HREE ratios (1.03–416.99) and negative Eu anomalies (0.02–0.21). Grt III exhibits a relatively flat REE distribution, lower LREE/HREE ratios (0.48–25.96) and negative Eu anomalies (0.01–0.10). From Grt I to Grt III, the fluid with high temperature, oxygen fugacity and low fluid-rock transitioned into a fluid with low-oxygen fugacity and high fluid-rock ratio, thereby cooling the magmatic fluid with low-oxygen fugacity and high water–rock ratio. Futhermore, Sn precipitation occurs due to the early high temperature and oxygen fugacity fluid, and the high Sn content (644–16,920 ppm) in garnet may be associated with magma properties.