Geology, geochemistry and mineralogy of indium resources at Pitkäranta Mining District, Ladoga Karelia, Russia

This study is aimed at substantiating the prospects of the Pitkäranta Mining District for indium, assessing its resources and elucidating the mineralogical and geochemical features of the distribution of indium in ores. The skarn deposits genetically related to the Salmi anorthosite-rapakivi granite batholith of Early Riphean age were studied. The deposits were divided into two groups, depending on the degree of the greisen alteration of skarns: 1) Sn-Zn-Cu-Fe deposits (the Old Ore Field, Kitelä, Hopunlampi, Heposelkä and Kulismajoki) with little or no greisenization and 2) Be-Sn-Zn deposits with fluorite and magnetite (the New Ore Field, Lupikko, Uksa and Ristiniemi) with well-defined greisenization. Deposits of group 1 are located in the external zone of the Salmi Batholith with a steeply plunging roof (Kitelä), and deposits of group 2 commonly lie above LiF granite domes unexposed by erosion, and are spatially associated with the dykes of these granites and stockscheiders. The distribution of In and associated elements in skarn ores and major ore minerals (sphalerite, chalcopyrite, magnetite and cassiterite) were studied by ICP MS, LA ICP MS and electron probe microanalysis. A total of over 200 samples were analyzed. The results obtained show that In is closely associated geochemically with Zn and Cd, less closely with Cu, Bi and Te is not associated practically with Mo, W, As, Sn and Ag. The content of In in ores from various mines varies from <1 ppm to 0.33 %, and the average In content is 66 ppm. These variations are mainly due to different sphalerite contents in ores and the degree of manifestation of propylitic and greisen alteration of ore-bearing skarns. The main indium-bearing mineral is sphalerite (average In content is 1927 ppm). Other high-In minerals (stannoidite and mawsonite) are uncommon (<0.01 %) and, therefore, do not markedly affect the In balance in ore. The same applies with rare exception to roquesite, the only indium's mineral known here. In-richest (up to 1.5 %) sphalerites with 3–6 % Fe, up to 300–400 ppm Mn and 0.1–0.2 % Co were identified in ore mineral associations of propylitized skarns, which contain no or small amounts of chalcopyrite. Sphalerite-chalcopyrite association is characteristic of roquesite. It occurs mainly in low-tin aposkarn greisens with abundant arsenopyrite-löllingite-sphalerite mineralization. The formation of roquesite seems to have been supported by indium, which was released upon the decomposition of its solid solutions in chalcopyrite and sphalerite and upon the hydrothermal-temperature effect of the late intrusion phases of the Salmi Batholith, LiF granites, on ores. The indium distribution pattern in sphalerite, which is also rich Cd, Ag and Au, shows that roquesite-free propylitized skarns with Sn-Zn-Fe mineralization have the greatest potential for indium and these metals in the zones where the roof of the Salmi Batholith plunges steeply (40–50°) and LiF granites do not occur (Kitelä, Kulismajoki and Hopunlampi deposits). Predicted indium resources in the sphalerite ores of the Pitkäranta Mining District are estimated at 1585–2536 t (as shown by various calculations), cadmium resources at 18424 t, silver resources at 226 t and gold resources at 2 t.