Indium distribution in sulfides from a Sn-poor skarn system: A case study of the Lawu Cu–Pb–Zn deposit, Tibet

Abstract

The Lawu Cu–Pb–Zn deposit is situated in the eastern part of Nyainqêntanglha metallogenic belt, Tibet. It represents a rare indium (In)-bearing and tin (Sn)-poor skarn system, however, the occurrence, distribution, and endowment of In, along with their geochemical controls, remain poorly constrained. In this contribution, we conducted a comprehensive mineralogical study of sulfides using microscopic observation, electron probe micro-analyzer (EPMA), and laser ablation inductively coupled plasma mass spectrometer (LA–ICP–MS). Based on mineral assemblage and textural relationships, four types of sphalerite are identified: sphalerite I as micro-inclusions within chalcopyrite and pyrrhotite, sphalerite II with abundant mineral inclusions, sphalerite III exhibiting chalcopyrite disease with watermelon texture, and sphalerite IV displaying a relatively clean texture with minimal inclusions. Sphalerite I is enriched in In, Cu, Cd, and Mn, whereas sphalerite II and sphalerite IV exhibit variable contents of In, Sn, Cu, Mn, Co, Se, and Ag. Trace elements, such as In, commonly occur in sulfides via isomorphic substitution, with minor amounts present as micron-scale mineral inclusions (e.g., Cu in sphalerite; Pb and Bi in sphalerite, chalcopyrite, and pyrite). From the perspective of In content in minerals, In is mainly hosted in sphalerite (average >400 ppm). Chalcopyrite also shows relatively elevated In content (average ∼200 ppm). Pyrrhotite, pyrite, and arsenopyrite typically contain negligible In content, generally below 10 ppm. According to the GGIMFis geothermometer calculations, the crystallization temperatures of sphalerite II to sphalerite IV are as follows: 378 to 427 °C (average = 398 °C) for sphalerite II, 389 to 399 °C (395 °C) for sphalerite III, and 338 to 375 °C (356 °C) for sphalerite IV. The In, Cu, and Co contents in sphalerite increase with crystallization temperatures, implying that their incorporation is primarily controlled by the temperature of ore-forming fluids. Combined with the published sphalerite dataset, a strong correlation is inferred between In and Sn, Cu, or Fe. Notably, Sn-poor Zn-polymetallic skarn systems represent strategic targets for In resource exploration. This finding refines the understanding of In occurrence and enrichment in Sn-poor skarn systems, providing insights applicable to similar geological environments.