Genesis of the Maweishan Pb–Zn deposit, eastern China and controls on the distribution and formation of sphalerite-hosted critical metal (Cd, Ga, and In) mineralization

Abstract

Skarn-type Pb (lead)–Zn (zinc) polymetallic deposits are not only important sources of Pb and Zn but also critical metals such as Cd, In, Ge, and Ga. Significant numbers of skarn-type (or magmato-hydrothermal) Pb–Zn deposits have been identified within the Xuancheng ore district of the Middle and Lower Yangtze River metallogenic belt of eastern China. However, these Pb–Zn deposits are understudied compared to the well-documented porphyry-type Cu–Au, skarn-type Cu–W and Cu–Mo deposits within this district. In addition, the controls and nature of the deportment of critical metals within Pb–Zn deposits also remain poorly constrained, including identification of substitution processes and the sources of these metals within these Pb-Zn systems. The potential genetic link between the Maweishan deposit and the adjacent Qiaomaishan deposit also remains uncertain, with some research considering that the Maweishan deposit is a distal part of the Qiaomaishan skarn system whereas other research suggests that these are independent skarn systems. This study addresses these issues using new evidence of the timing of mineralization obtained by the in-situ U–Pb dating of the garnet from the Maweishan deposit. This garnet yields a well-defined lower-intercept U–Pb age of 132.3 ± 1.4 Ma, consistent with a zircon U–Pb age for the proximal Maweishan quartz diorite (132.4 ± 0.9 Ma). This, combined with the spatial relationships of alteration and mineralization within the deposit, strongly suggests that the proximal quartz diorite is genetically related to the formation of the Maweishan deposit and this deposit is not a distal manifestation of the Qiaomaishan skarn system. Sphalerite from the Maweishan deposit also contains high levels of the critical metals Cd [3480–5107 parts per million (ppm)], In (78.3–407 ppm) and Ga (22.4–348 ppm). The Cd²⁺ within this sphalerite was incorporated into the mineral lattice by direct substitution for Zn²⁺, whereas the uniform concentrations of In³⁺ in all three types of sphalerites is indicative of incorporation by the adsorption or capture of homogeneous In nanoparticles. The Ga³⁺ within the Maweishan deposit was predominantly incorporated into sphalerite as a result of coupled substitution with Cu⁺ replacing 2Zn²⁺, with the Ag⁺ + Ga³⁺ coupled substitution for 2Zn²⁺ also involved in the incorporation of Ga into the Sp2 sphalerite. The narrow range of sulfur isotopes (δ³⁴S from 4.9 ‰ to 8.3 ‰, average of 5.8 ‰) for sulfides within the Maweishan deposit combined with the new garnet trace element data and elemental maps presented in this study suggests that the metals and fluids that formed the Maweishan were primarily derived primarily from the proximal quartz diorite. These sulfur isotopic data also indicate that a small amount of metals or sulfur were derived from the surrounding country rocks. These data provide new insights into critical metal mineralization associated with Pb–Zn deposits in the Xuancheng ore district, where regional folding concentrated ore-forming fluids that generated both 139 Ma Cu–W and 132 Ma Pb–Zn mineralization events, indicating that this structural zone is an ideal area for future exploration within the Xuancheng district and in other areas with similar characteristics.