Genesis of the lead-zinc orebody in the Yangla Cu polymetallic deposit, northwestern Yunnan—constraints from in situ analyses of trace elements and sulfur isotope of sphalerites

The lead-zinc ore body in the Yangla Cu-polymetallic deposit was recently discovered. Previous studies primarily focused on analyzing characteristics of its rare earth elements, single-mineral isotopes, and ore-forming fluids, but lacked evidences from in-situ micro-scale analyses. In order to investigate the genesis of the lead-zinc ore body, in this study, by applying the in situ LA-ICP-MS analytical technique, we have analyzed trace element contents and sulfur isotopic compositions of sphalerites from the lead-zinc ore body in the Yangla Cu-polymetallic deposit. The results show that sphalerite of the early mineralization stage (Sp I) occurred as dark reddish-brown to brown irregular clumps and discontinuous veins/veinlets, whereas sphalerite of the late mineralization stage (Sp II) occurred as reddish-brown to yellowish-brown veinlets with obvious growth zones. From Sp I to Sp II, their colors are changed from dark to light, and their concentrations of Mn, Fe, Cu, Cd, and In are decreased, while those of Ge, As, and Tl are increased. Trace elements occurred in sphalerite not only in the form of simple isomorphic substitution but also in the form of complicated coupling superimposed substitution. The Sp I has δ 34S values ranging from +1.26‰ to +3.20‰, with an average of +2.47‰, whereas the Sp II has δ 34S values ranging from -1.06‰ to +0.10‰, with an average of -0.37‰, indicating that the sulfur was primarily originated from the mantle or magmatic source. The compositional characteristics of trace elements, such as enriched Fe and In but depleted Ga, Ge, and Tl, and the Ga/In and Zn/Fe ratios of sphalerites suggest that the Sp I was formed under the condition of medium- to high-temperature, while the Sp II was formed under condition of medium- to low-temperature. Additionally, the Cd/Fe and Zn/Cd ratios of sphalerite in this deposit are similar to those of sphalerites in the skarn-type deposits. Based on the geological characteristics of the lead-zinc ore body, it is concluded that the lead-zinc ore body in the Yangla Cu-polymetallic deposit is of the skarn-type origin.