The key controlling factors on Sn–Cu mineralization: A case study from the world-class Gejiu Sn–Cu-polymetallic deposit

Gejiu, a globally prominent Sn–Cu-polymetallic district, contains an extensive suite of magmatic rocks. Magmatic rocks in the southeastern portion of the Gejiu district comprise the Masong and Laoka equigranular and porphyritic granites, and basalts. Previous studies have suggested that Sn mineralization in this region is primarily associated with the equigranular granite, whereas Cu mineralization is predominantly associated with the basalts. Despite this, the major factors controlling the formation and distribution of Sn–Cu mineralization in this region remain poorly constrained. This contribution characterizes the mechanisms of formation of the Gaofengshan Sn–Cu and Zhuyeshan Cu–Sn deposits, focusing on the petrogenesis of the host Masong and Laoka equigranular granites, and the evolution of the ore-forming magmatic–hydrothermal fluids. This is accomplished by combining bulk-rock geochemistry of the Masong and Laoka equigranular granites, and basalt, with biotite geochemistry from both granites and pyrite geochemistry from associated skarns. The Masong and Laoka equigranular granites crystallized from hybrid crust–mantle-derived magmas. Reduced granites like the Masong that underwent high degrees of fractional crystallization, and have elevated halogen are prospective for Sn mineralization. The abnormally high concentrations of Cu in pyrite in the Zhuyeshan deposit suggest that the Cu was primarily derived from sources external to the granite. Garnets from skarn-type ores and stratiform ores at Gaofengshan and Zhuyeshan yielded U–Pb ages of 88.2 ± 1.4 Ma and 81.8 ± 3.7 Ma, respectively, while vesuvianite from the skarn-type ores at Zhuyeshan yield a U–Pb age of 84.1 ± 0.5 Ma. These ages confirm that both the Sn and Cu mineralizing events occurred during the Late Cretaceous, coeval with emplacement of the granitic intrusions. Notably, there is no direct geochronological link between Cu mineralization and the basalt (ca. 244.4 Ma), which has a close spatial relationship with the Laoka equigranular granite. Combined with previous S–Pb isotope data, we propose that large-scale Cu mineralization in Gejiu resulted from the extraction of Cu from basalt by fluids exsolved from the Late Cretaceous equigranular granitic magmas.