Sahlström, and 4 co-authors, reported in 2017 that germanium, gallium and indium are in high demand due to their growing usage in high-tech and green-tech applications. However, the mineralogy and the mechanisms of concentration of these critical elements in different types of hydrothermal ore deposits remain poorly constrained. The authors investigated the mineralogical distribution of Ge, Ga and In at the Mt Carlton high-sulfidation epithermal deposit in NE Australia, using electron probe microanalysis and laser ablation inductively-coupled plasma mass spectrometry. Parageneses from which selected minerals were analyzed include: Stage 1 acid sulfate alteration (alunite), Stage 2A high-sulfidation enargite mineralization (enargite, argyrodite, sphalerite, pyrite, barite), Stage 2B intermediate-sulfidation sphalerite mineralization (sphalerite, pyrite, galena) and Stage 3 hydrothermal void fill (dickite).
Moderate to locally high concentrations of Ga were measured in Stage 1 alunite (up to 339 ppm) and in Stage 3 dickite (up to 150 ppm). The Stage 2A ores show enrichment in Ge, which is primarily associated with argyrodite (up to 6.95 wt % Ge) and Ge-bearing enargite (up to 2189 ppm Ge). Co-existing sphalerite has comparatively low Ge content (up to 143 ppm), while Ga (up to 1181 ppm) and In (up to 571 ppm) are higher. Sphalerite in Stage 2B contains up to 611 ppm Ge, 2829 ppm Ga and 2169 ppm In, and locally exhibits fine colloform bands of an uncharacterized Zn-In mineral with compositions close to CuZn2(In,Ga)S4. Barite, pyrite and galena which occur in association with Stage 2 mineralization were found to play negligible roles as carriers of Ge, Ga and In at Mt Carlton.
Analyzed reference samples of enargite from seven similar deposits worldwide have average Ge concentrations ranging from 12 to 717 ppm (maximum 2679 ppm). The deposits from which samples showed high enrichment in critical elements in this study are all hosted in stratigraphic sequences that locally contain carbonaceous sedimentary rocks. In addition to magmatic-hydrothermal processes, such rocks could potentially be important for the concentration of critical elements in high-sulfidation epithermal deposits.
The authors concluded that the evidence from this study and from the literature highlight the potential for various styles of Ge, Ga and In mineralization in high-sulfidation epithermal deposits, as well as in the closely related intermediate-sulfidation epithermal deposits. This is something that is seldom considered during exploration and mining in these environments. In addition to their well-known resources of precious and base metals, the potential for critical elements could increase the prospectivity
of these deposits.
Holistic mineralogical studies of similar deposits elsewhere, including more trace element data for common ore and alteration minerals, can help improve our understanding of the distribution of Ge, Ga and In in the epithermal environment. Data are particularly limited for deposits affected by supergene processes. To improve exploration for these commodities, a better understanding of the source(s) as well as the mechanisms of enrichment, transport and deposition of Ge, Ga and In in epithermal systems is also needed. The authors frond the potential link between critical element enrichment and carbonaceous sedimentary rocks particularly interesting and deserving of more detailed investigation.
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