Indium mineralisation in A-type granites in southeastern Finland: Insights into mineralogy and partitioning between coexisting minerals

Several promising exploration targets in the western parts of the Wiborg batholith, southeastern Finland, have been studied with respect to Zn, In, Ag, As, Sn and Cu. Ores occur both as massive In-bearing magnetite–sphalerite (Getmossmalmen), as greisen-style veins (Jungfrubergen) and as Cu-dominant polymetallic quartz veins (Korsvik-1 and -2, Sarvlaxviken area). The Cu-dominant (chalcopyrite–bornite), Zn-poor quartz veins are characterised by high In/Zn ratios (> 3000); roquesite (CuInS2) being a major indium-carrier, alongside sphalerite, chalcopyrite and arsenopyrite. In contrast, sphalerite is the dominant In-carrier in the greisen veins and massive ores characterised by lower In/Zn ratios (< 100). In such cases, it appears that all indium partitioned into sphalerite at concentrations of 500–1500 ppm, and no roquesite formed. In the veins of the Sarvlaxviken area, we observe the exsolution of a high-temperature precursor sulphide to form co-existing sphalerite and roquesite. These exsolution textures extend down to the sub-microscopic scale and there is no evidence of extensive solid solution along the ZnS–CuInS2 join beyond a couple of wt.% In (and Cu) in sphalerite. The co-existing roquesite is close to an ideal composition and contains μm-scale exsolutions of sphalerite, indicating negligible solid solution at the CuInS2 end of the join. The new data are compared with the published data on the mineralogical distribution of the indium in sulphide ores.

Research highlights
► First description of new styles of Cu-In-rich mineralisation within the Wiborgrapakivigranite, Finland.
► Indium is currently in high demand and attracting considerable exploration interest. We review what is known about indium distribution and geochemical behaviour.
► Our mineralogical and mineral-chemical study assists in understanding partitioning among In-bearing minerals, how and why the discrete phase roquesite (CuInS2) sometimes appears in ores, and the significance of the Zn/In ratio in determining vein mineralogy.
► In-situ LA-ICPMS analysis confirms wt.% level concentrations of In within sphalerite and associated minerals.