Sulfide mineralogy and chalcophile and siderophile element abundances in the Ivrea-Verbano mantle peridotites (Western Italian Alps)

Abstract

Sulfide mineralogy as well as chalcophile and siderophile element geochemistry have been investigated in samples of the peridotite massifs of Baldissero, Balmuccia and Finero (Western Italian Alps). These massifs are supposed to represent a suite of mantle fragments affected by various degrees of partial melting beneath an area of plate collision. In the relatively primitive (undepleted) mantle rocks of Baldissero and Balmuccia, “primary” sulfide phases occur associated with Al Cr spinel. They consist of pentlandite and chalcopyrite, with exsolutions of Cu phases (cubanite, bornite and digenite), and minor amounts of pyrrhotite. In the strongly depleted Finero peridotites, sulfides are less abundant and consist mainly of pentlandite, with exsolution of mackinawite, and heazlewoodite. The content of sulfur and of chalcophile elements (Cu, Se, Hg) in the primitive Baldissero and Balmuccia peridotites is higher than previous mantle estimates and it tends to decrease with increasing extent of mantle partial melting. Siderophile trace elements (Ag, Au, Pt, Pd, Rh) are rather inhomogeneously distributed within each single mantle fragment. Average Pt contents in the most primitive Baldissero and Balmuccia lherzolites approach mantle estimates, while Ag, Au and Pd are probably in excess. Most of the siderophile elements seem to concentrate in the liquid during partial melting processes, thus behaving as incompatible elements. Mineralogy and phase relationships in the sulphide component as well as siderophile and chalcophile element geochemistry distinguish the Ivrea-Verbano mantle fragments from other assumed mantle materials such as ultramafic xenoliths in kimberlites and alkaline volcanics. The presence of a sulfide component consisting of a monosulfide Fe Ni Cu solid solution is postulated in the primitive mantle under conditions of the spinel-lherzolite facies. During mantle melting, this would fractionate, concentrating Fe-rich sulfide phases in the liquid. The possible role of the sulfide component as carrier of chalcophile and siderophile elements in the upper mantle is stressed. The abundances of these elements in the mantle were controlled by their affinity for the sulfide phase present in the primordial mantle after core-mantle differentiation.