ROQUESITE AND ASSOCIATED INDIUM-BEARING SULFIDES FROM A PALEOPROTEROZOIC CARBONATE-HOSTED MINERALIZATION: LINDBOM’S PROSPECT, BERGSLAGEN, SWEDEN

The present study describes a new discovery of the copper-indium sulfide mineral roquesite (nominally CuInS2) together with indium-bearing sulfides associated with magnetite in a carbonate-hosted, polymetallic sulfide mineralization. This occurrence, Lindbom’s Prospect, is located in the western part of the Paleoproterozoic Bergslagen ore province, Sweden. Here, roquesite occurs in indium-bearing bornite, characteristically associated with indium and copper-bearing sphalerite, as well as chalcopyrite, cuproan galena, late-stage chalcocite-digenite and covellite, variable amounts of bismuth minerals, abundant magnetite, and locally cassiterite. The indium-bearing ore mineral assemblages were studied by a combination of optical and field emission scanning electron microscopy and field emission electron probe microanalyzer (FE-EPMA) techniques. FE-EPMA analyses of roquesite yield an average composition corresponding to Cu0.93Fe0.02Zn0.06In1.00S2. It occurs as ca . 4–30 micron-sized subhedral to anhedral, often angular crystals in bornite, typically in direct contact, or in close association with, indium-bearing sphalerite. The latter has variable indium content, ranging from below detection limit to at least 1.5 wt.% In, and exhibits an average of 0.03 wt.% In. Notably, sphalerite grains show a slight enrichment of indium towards the rims and the adjacent bornite, where roquesite occurs. The associated bornite, as well as minor chalcopyrite, mostly exhibit low to very low indium contents. These are mostly below detection limit, but locally higher in bornite; average 0.01 wt.% In and maximum 0.2 wt.% In. The highest indium content observed in chalcopyrite is 0.02 wt.% In. Combining textural evidence and high-resolution mineral chemical data, we suggest that roquesite formed as a consequence of reactions between diffusion-driven indium from sphalerite, and the surrounding bornite, during regional metamorphism. Based on available evidence, it is most likely that the studied assemblages represent part of a metamorphically overprinted, in part remobilized ca . 1.89–1.88 Ga volcanic-subvolcanic hydrothermal mineralization.