Cobaltite-rich mineralisation in the iron skarn deposit of Traversella (Western Alps, Italy)

Cobaltite-rich mineralisation from the iron skarns of the Traversella magnetite mine (Western Alps, Italy) was studied by reflected-light microscopy, scanning electron microscopy and electron microprobe analysis. Cobaltite is found in carbonate–chlorite-rich rocks at the margins of the main magnetite masses, where it forms disseminations and metasomatic veinlets that postdate the formation of magnetite. The paragenesis includes cobaltite (± arsenopyrite), bismuthinite, pyrrhotite and/or pyrite, chalcopyrite, carbonates, talc, chlorite, and native gold, and is indicative of a low-sulphidation environment. The sulpharsenides show oscillatory and sector zoning, which indicates disequilibrium during crystal growth. Compositional variations are mainly due to variations in the Co/Fe ratio of arsenopyrite and in either the Co/Fe or the Ni/(Fe + Co) ratios of the coexisting cobaltite. The Ni contents are low to very low in the cobaltites (< 2.4 wt%) and very low in the arsenopyrites (< 0.16 wt%). The As/S molar ratios in the cobaltites are highly variable (0.59–1.00) and show a broad negative correlation with the Fe contents. The formation of cobaltite is related to circulation of relatively low-temperature (< ca. 300 °C), (Co, As, Bi)-rich fluids during the retrograde sulphidation stage which followed the formation of magnetite. The apparent restriction of cobaltite (+ bismuthinite ± arsenopyrite) to the margins of the main magnetite columns may reflect the establishment of thermo-chemical gradients around the main direction of infiltration of the retrograde metasomatic fluids.