GERMANIUM AND GALLIUM ENRICHMENT OF ZNS POLYMORPHS IN ZN ORE: IMPLICATIONS FOR BYPRODUCT CRITICAL MINERAL RECOVERY FROM MVT DEPOSITS

Germanium (Ge) and gallium (Ga) are critical minerals essential to advanced and energy transition technologies. Zinc (Zn) ores from some Mississippi Valley Type deposits (MVT) are enriched in Ge and Ga, yet these are only occasionally recovered as byproducts. We investigated the trace element chemistry and crystalline structure of ZnS from MVT deposits in the United States in order to gain insight into ore-formation processes and to inform exploration and byproduct recovery efforts.

Sphalerite, the cubic polymorph of ZnS, from the Metaline mining district (Josephine and Yellowhead ores, Washington), Central Tennessee (TN) district, and the Tar Creek Superfund Site (Oklahoma) in the Tri-State district contains Ge in concentrations that can exceed 1,000 mg kg^-1. On average, Josephine and Central TN ores are more enriched than Yellowhead and Tar Creek. Gallium reaches the highest concentrations in ZnS from Central TN (maximum 3,000 mg kg^-1) but is also enriched in ZnS from Tar Creek and the East TN mining district (maximum ~1,000 mg kg^-1).

On the grain scale, the concentration of copper (Cu) correlates with Ge, with a molar ratio of ~2:1 in most sphalerites from Metaline, Central TN, and Tar Creek. The molar ratio and the occurrence of mainly Cu⁺¹ and Ge⁺⁴ are consistent with co-substitution of 2Cu⁺¹ + Ge⁺⁴ = 3Zn⁺² within ZnS. Interestingly, ZnS from Central TN also contains mm-sized Ga-enriched domains which exhibit a roughly 1:1 Cu:Ga molar ratio consistent with the co-substitution Cu⁺¹ + Ga⁺³ = 2Zn⁺². In these domains, the Ge⁺² observed suggests direct substitution of Ge⁺² for Zn⁺². The Ga-rich domains are generally optically anisotropic and, based on synchrotron-based XRD, contain both cubic sphalerite and hexagonal wurtzite. Optical observations indicate fine-scale interlayering of the hexagonal and cubic close-packed layers.

MVT deposits in the United States contain Ge-enriched sphalerite (Metaline, Tar Creek), Ga-enriched sphalerite (East TN), or both Ge-enriched sphalerite and Ga-enriched wurtzite (Central TN) suggesting a range of mineralization conditions can lead to Zn ore critical-mineral endowment. Due to the structural associations of Cu with Ge and Ga, byproduct recovery should focus on Cu-enriched ore zones in these deposits.