Critical metal enrichment in carbonate-hosted Pb-Zn systems: Insight from the chemistry of sphalerite within the Hehuashan Pb-Zn deposit, Middle-Lower Yangtze River Metallogenic Belt, East China

Abstract

Carbonate-hosted Pb–Zn deposits frequently contain significant amounts of critical metals such as Cd, Ga, Ge, Tl, and In. However, the characteristics of critical metal enrichments and the geochemical controls of the critical metal enrichments present within different types of sphalerite in individual deposits remain poorly constrained. Textural observations and in-situ laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) and associated elemental mapping allows the investigation of the controls on the incorporation of critical metals into sphalerite within the Hehuashan carbonate-hosted Pb–Zn deposit, East China, which contains 639.36 t Cd at an average grade of 0.0020 % and 161.23 t Ga at an average grade of 0.0161 %. Two ore-formation stages (early and late) and five textural types of sphalerite are present in the deposit, namely fine-grained brown sphalerite (FCS), irregular sphalerite (IS), oolitic sphalerite (OS), banded sphalerite (BS), and coarse-grained sphalerite (CCS), all of which are present within breccia-, vein-, and massive ores. The FCS, IS, OS, and BS formed during early stage mineralization whereas the CCS formed during the later stages of mineralization. Large variations of Cd, Ga, Ge, Tl, and In concentrations are present within these sphalerites with an average Cd of 5997 ppm in OS, 14003 ppm in FCS, 962 ppm in IS, 2565 ppm in BS, and 5250 ppm in CCS; Ga of 1047 ppm in OS, 5 ppm in FCS, 3 ppm in IS, 15 ppm in BS, and 33 ppm in CCS; Ge of 166 ppm in OS, 21 ppm in FCS, 530 ppm in IS, 13 ppm in BS, and 4 ppm in CCS. The Cd within this sphalerite is incorporated by direct substitution between Cd2+ and Zn2+ whereas Ga concentrations are correlated to monovalent cation concentrations (Cu and Ag) and Ge concentrations are not correlated to monovalent cation concentrationss, indicative of an unusual vacancy-related substitution of Ge for Zn. Trace and minor element distribution variations and the calculated temperatures of formation of the deposit suggest that the ore-forming fluids associated with the system evolved significantly over the multiple stages of mineralization recorded within the study area. In addition, the deposit records two critical metal enrichment events associated with the early and late stages of mineralization within the Hehuashan Pb–Zn deposit, respectively; an early event associated with enrichments in Cd, Ge, and In, and a late event associated with enrichments in Cd and Ga, indicating that carbonate-hosted Pb–Zn deposits should be considered significant potential sources for critical metals such as Cd, Ga, Ge. However, the concentrations of these critical metals vary within the different types of sphalerite present in in individual Pb–Zn deposits, suggesting that thhe Hehuashan deposit, and other similar deposits within the MLYRMB, may have undergone at least two critical metal enrichments events that should be targeted for future critical metal exploration.