Geochronology, Petrogenesis, and Geological Significance of the Longchahe Granite, Gejiu Sn Polymetallic Ore District, SW China

Longchahe porphyritic granite is the largest pluton within the western Gejiu complex, a series of mainly intermediate to felsic alkali intrusions in southwestern China. Our research indicates that the pluton intruded during the Late Cretaceous (82–84 Ma). The pluton is primarily a medium- to coarse-grained porphyritic granite, which shows weakly peraluminous (A/CNK = 0.92–1.82, with an average of 1.09) and alkali (shoshonitic) characteristics, exhibiting an affinity with highly differentiated I-type granite. The porphyritic granite is enriched in K and Rb, but depleted in Ba, P, and Ti, and displays significant enrichment of light rare earth elements with minor negative Eu anomalies (Eu/Eu* = 0.46–0.66). It has elevated (87Sr/86Sr)i ratios (0.71243–0.71301), negative εNd(t) values (−8.42–−6.46), and a broad range of εHf(t) values (−13.80–9.17). These geochemical characteristics indicate that the formation of Longchahe granite involved both crust–mantle assimilation and strong crystal fractionation. Additionally, the pluton demonstrates a significant enrichment of W. A factor analysis study suggests that the formation of granites is associated with F1 (Nb–Ta–Th–LREE–HREE–[W]), whilst F2 represents Sn–Pb–U–[Zn] polymetallic mineralisation in western Gejiu. Further, a score diagram indicates that the granites exhibit a high abundance of ore-forming elements, with potential for Pb and Zn mineralisation. Our study favours that the Longchahe granites likely formed within a continental arc–tectonic setting, related to subduction and subsequent rotation processes experienced by the Paleo-Pacific plate.