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

Published online by Cambridge University Press:  05 July 2018

P. Nimis ,
L. Dalla Costa  and
A. Guastoni
P. Nimis*
Affiliation:
Dipartimento di Geoscienze, Università di Padova, via Gradenigo 6, 35131 Padua, Italy
L. Dalla Costa
Affiliation:
Amici Museo "G. Zannato", Piazza Marconi, 15, 36075 Montecchio Maggiore (VI), Italy
A. Guastoni
Affiliation:
Museo di Mineralogia, Università di Padova, via Giotto 1, 35121 Padua, Italy
*
*E-mail: [email protected]
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Abstract

Cobaltite-rich mineralization 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-sulfidation environment. The sulfarsenides 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 (<∼300°C), (Co,As,Bi)-rich fluids during the retrograde sulfidation 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 thermochemical gradients around the main direction of infiltration of the retrograde metasomatic fluids.

Keywords

cobaltiteskarnmagnetiteore depositsTraversellaWestern Alps
Type
Research Article
Information
Mineralogical Magazine , Volume 78 , Issue 1 , February 2014 , pp. 11 - 27
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2014

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