The genesis of the Changtian scheelite-fluorite deposit in the southeastern Yunnan: Constraints from the rutile U–Pb dating data and in-situ sulfur isotopes of pyrites

The southeastern Yunnan is one of the important tin-tungsten production areas in China. The Laojunshan tungsten–tin polymetallic ore concentration district is an important part of it. The Changtian hydrothermal vein-type scheelite-fluorite deposit, located in the north side of the Laojunshan granite, is a recent breakthrough achievement of the mineral exploration within the Laojunshan ore concentration district. The genesis of this deposit has not been well constrained due to its relatively simple mineral assemblage, the lack of suitable dating minerals, and the absence of exposed granite within this deposit. This hindered to deeply understand the regional metallogenic regulation and to make effective breakthrough in mineral exploration and prospecting in this area. In this study, the rutile, associated with scheelite, quartz, and pyrite, was identified in the deposit based on detailed field survey and mineralogical works. The LA-ICP-MS U–Pb dating of rutiles yields an age of (88.7±7.0) Ma (MSWD=1.6, 2δ), which is consistent with the timing of the Laojunshan granite, indicating that the deposit formed in Late Cretaceous is genetically associated with the magmatic intrusive activity of the Laojunshan granite. Furthermore, the in-situ sulfur isotope analyses of pyrites from the scheelite–fluorite veins reveal that the core and rim of obviously core-rim textured pyrite grains have significantly different sulfur isotopic values. The cores of pyrites (Py-1) have δ 34S values ranging from 15.9‰ to 16.9‰ (average of 16.4‰), which are close to those of pyrites from the Cambrian strata (from 17.5‰ to 25.8‰). This indicates that the sulfur in the core of pyrite could be originated from the Cambrian strata. By contrast, the rims of pyrites (Py-2) have δ 34S values ranging from 1.90‰ to 9.80‰ with an average of 5.82‰, which are consistent with those of pyrites in typical granite-related deposits within the Laojunshan district. This indicates that the sulfur in rims of pyrites could be mainly originated from the magma. The sulfur of rims for minor pyrites with relatively high δ 34S values could be sourced from the mixed magmatic and stratigraphic sulfurs. Based on the comprehensive study of the above analytical results and regional metallogenic regulation, it is suggested that there is close genetic relationship between the Changtian scheelite–fluorite deposit and the Yanshanian Laojunshan granite. The Changtian deposit is a typical magmatic hydrothermal vein-type deposit. The clarification of genesis of the Changtian deposit genesis is of great significance for the exploration of new skarn-type tungsten (tin, copper) resources in the deep parts of the Laojunshan ore concentration district.