Gallium distribution in the Dulong Sn polymetallic deposit, SW China

Abstract

The Dulong Sn polymetallic deposit is located in the SE Yunnan-West Guangxi ore province, and represents one of the three largest cassiterite-sulfide deposits in China. The deposit is a potential gallium (Ga) deposit in China with abundant tin and zinc resources. However, the occurrence of Ga in the deposit and the control on its mineralogy are still poorly understood. Here, we provide a detailed account of the gallium distribution in silicate, oxide, and sulfide minerals at Dulong, based on field geological and petrographic observations and geochemical analysis. Gallium is mainly hosted in Al-rich minerals, such as muscovite (avg. 139 ppm), biotite (avg. 86.8 ppm), and feldspar (avg. 21.3 ppm), in the ore-related granite. This distribution behavior of gallium is consistent with its predicted partitioning between the phenocrysts and melt. In the Sn ore, chlorite has high Ga content (avg. 104 ppm), probably caused by the gallium tends to form hydroxyl complexes. In contrast, pyroxene and sphalerite have low Ga content (avg. 1.24 and 1.50 ppm, respectively). Meanwhile, two types of magnetite (Mag-I and Mag-II) have been identified based on mineral paragenesis: Mag-I is mainly associated with sphalerite. It is porous, with the pores filled with Ga-rich aluminosilicates (e.g., chlorite), and has high Ga content (avg. of 18.3 ppm). Mag-II is mainly associated with sphalerite, pyrrhotite, and chalcopyrite, and has low Ga (avg. 1.34 ppm). The dissolution of Ga-rich aluminosilicates in magnetite likely provided gallium to magnetite, resulting in higher Ga content in Mag-I. The magnetite Ga content decrease with increasing distance from the mineralization center, and the magnetite mineralization temperature also shows a similar trend. This study confirms that the dominant substitution for Ga in muscovite is Ga3+ ↔ Al3+, and in magnetite it is Ga3+ ↔ Mn2+ + Me+ and Ga3+ ↔ Cu+ + Zn2+. Most of the Ga is hosted in chlorite and magnetite in the highly chloritized sulfide ore at Dulong, which is a critical factor that can significantly affect the estimation of economically recoverable by-product elements in the sulfide ore.