Indium mineralization and genesis of the Bainiuchang Ag–Sn–polymetallic deposit in southeast Yunnan, China: Evidence from mineral chemistry and U–Pb geochronology

Abstract

The Bainiuchang Ag–Sn–polymetallic deposit is located in the world-calss Southeast Yunnan Indium Metallogenic Belt (SYIMB). The deposit contains approximately 1000 tons of In, but the mechanisms of mineralization and hosts of In in this deposit remain unclear. To clarify these ambiguities, we carried out fieldwork, major- and trace-element analyses of In-bearing minerals, and cassiterite and zircon U–Pb geochronology. Five ore zones were identified, which, from contact of the granitic intrusion upwards, are the In-poor magnetite–cassiterite ore zone, the In-poor Cu ore zone, the In-rich Zn–Sn ore zone, the In-rich Sn–Pb–Zn ore zone, and the In-poor Pb–Zn ore zone. Cassiterite and zircon U–Pb geochronology demonstrate that mineralization at Bainiuchang occurred at ca. 85.80±0.45 Ma (MSWD = 1.5), which is similar to the 85.34±0.42 Ma (MSWD = 1.7) age of the granitic intrusion. This temporal relationship suggests that there is a genetic link between In mineralization and granitic magmatism in this deposit. Ore petrography and mineral chemical analyses indicate that sphalerite, chalcopyrite, and stannite are main In-bearing minerals. From a mass balance perspective, sphalerite is the most important In-bearing mineral, accounting for ∼85% of the In budget at Bainiuchang. Indium is incorporated into sphalerite via the coupled substitution 2Zn2+ ↔ Cu+ + In3+ and/or 3Zn2+ ↔ Ag+ + Sn2+(+In3+). Two generations of sphalerite occur in the deposit, SpI and SpII, which formed during the cassiterite–sulfide and carbonate–sulfide stages, respectively. Based on textural relationships, SpI is further subdivided into SpIa, SpIb, and SpIc. These temporally and texturally distinct sphalerite have variable In contents, with SpIa having the highest In content (0.08–0.44 wt%), followed by SpIb (0.02–0.39 wt%), SpIc (187–236 ppm), and SpII (0.64–7.14 ppm). It is suggested here that the variations in In contents of the sphalerite reflect the evolution of the magmatic–hydrothermal fluids from which the sphalerite precipitated under different fS2 and temperature. By comparing the Bainiuchang deposit to other In-bearing deposits in the southwestern Yangtze Block, it is evident that that the nature and source of the associated granites, and the degree of fractionation of the granitic magmas are likely important controls on In mineralization in the SYIMB and other regions.