矿床地球化学国家重点实验室知识库

The Baoshan Cu–Pb–Zn deposit, located in the central part of the Qin–Hang belt in South China, is closely related to the granodiorite-porphyry. However, the characteristics and the source of the ore-forming fluid are still ubiquitous. According to the crosscutting relationships between veinlets and their mineral assemblages, three stages of hydrothermal mineralization in this deposit were previously distinguished. In this contribution, two different colored fluorites from the major sulfide mineralization stage are recognized: (1) green fluorites coexisting with Pb–Zn ores; and (2) violet fluorites coexisting with pyrite ores. Y/Ho ratios verify the green fluorites and violet fluorites were co-genetic. The fluorites display elevated (La/Yb)_N ratios, which decrease from 1201 to 5710 for green fluorites to 689–1568 for violet fluorites, indicating that they precipitated at the early hydrothermal sulfide stage, and Pb–Zn ores crystallized earlier than pyrite ores. The similar Tb/La ratios of the fluorites also indicate that they precipitated at an early stage within a short time. From the green fluorites to violet fluorites, the total rare earth element (ΣREE) concentrations decrease from 1052–1680 ppm to 148–350 ppm, indicating that the green fluorites precipitated from a more acidic fluid. The Eu/Eu* ratios increase from 0.17 to 0.30 for green fluorites to 0.29–0.48 for violet fluorites, and the Ce/Ce* ratios decrease from 1.08–1.13 to 0.93–1.11, suggesting a gradual increase in oxygen fugacity (fO₂) and pH value of the mineralization fluid. Though the fluorites display similar REE patterns to the granodiorite-porphyry and limestone, the ΣREE concentrations of the fluorites are significantly higher than those of limestone and the granodiorite-porphyry, suggesting that an important undetected non-magmatic source is involved to provide sufficient REE for fluorites. The most plausible mechanism is fluid mixing between magma fluid and an undetected non-magmatic fluid.