The Fluid Evolution in the Skarn Stages of the Baoshan Skarn Cu-Polymetallic Deposit, South China

Abstract

Baoshan is a world-class skarn Cu-polymetallic deposit located at the junction of the Nanling and Qin-Hang metallogenic belts in China. While there has been extensive research on the mineralogy and geochemistry of skarn deposits, studies on the fluid characteristics and evolutionary history from the early to late skarn stages in such deposits are still limited. In this study, we analyzed garnet and pyroxene from the early skarn stage and scheelite from the late skarn stage of the Baoshan deposit. We distinguished two generations of garnet (Grt1 and Grt2), one generation of pyroxene, and three generations of scheelite (Sch I, Sch II, and Sch III) on the basis of mineral assemblages and microscopic characteristics. Grt1 appears coarse-grained, and Grt2 cuts through Grt1 as veinlets. In Grt1, the andradite end-member increases from the core to the rim, while the grossular portion decreases (Ad35–36Gr59–61Sp3–4 to Ad59–61Gr36–37Sp2–3), and in Grt2, the andradite end-member significantly increases (Ad41–73Gr25–55Sp2–3). Grt1 and Grt2 have similar trace element compositions, with enrichment in Zr and depletion in Nb and Hf, depletion in LREE, enrichment in HREE, and weak negative Eu anomalies. Pyroxene coexists with Grt1 and is similarly cut by Grt2, with its composition mainly being diopside (Di82–99Hd0.6–15Jo0–3.2). Sch I and Sch II appear as anhedral to subhedral grains, while Sch III is predominantly found in veinlets. In Sch I and Sch II, most REEs enter the scheelite lattice via the Na-REE coupled substitution mechanism, with a smaller portion substituting Ca vacancies. In Sch III, the substitution mechanism involving Ca site vacancies may dominate. During the early skarn stage, the oxygen fugacity of the fluid gradually decreased from Grt1 and pyroxene to Grt2. In the late skarn stage, fluid oxygen fugacity remains stable from Sch I and Sch II to Sch III in shallow parts but significantly decreases in deeper parts. The garnet and pyroxene from the Baoshan deposit align with typical skarn Cu deposit compositions, while scheelite in the late skarn stage shows Sch I, Sch II, and shallow Sch III as skarn-type and deep Sch III as vein-type scheelite. Early skarn stage fluids were weakly acidic. Sch I, Sch II, and Sch III originated from fluids related to the Baoshan granite porphyry, with Sch III also showing evidence of water–rock interaction. This study reconstructed the fluid evolution history from the early to late skarn stages at the Baoshan deposit, providing insights into the ore-forming processes of other skarn deposits.