Platinum-group elements in magmatic sulfides from the Ivrea Zone; their control by sulfide assimilation and silicate fractionation

Abstract

The Ivrea zone consists of continental crust made up of amphibolitic to granulitic metasediments and metavolcanics intruded by a mafic-ultramafic complex which is divided into five parts: the peridotite massifs of Balmuccia, Baldissero, and Finero that are interpreted as fragments of upper mantle, the cyclic units consisting of a layered series of peridotites, pyroxenites, and gabbros, the main gabbro, the ultramafic sill of La Balma-Monte Capio, and a series of ultramafic pipes.Small-sized deposits of Ni-Cu sulfides with associated platinum-group minerals are found in all units of the mafic-ultramafic complex of the Ivrea zone except the mantle fragments. Distinct types of ores are associated with different units of the complex. In the cyclic units the sulfide concentrations mostly occur in pyroxenite layers close to, or in contact with metasediments. In the main gabbro, the sulfides are associated with pyroxenite layers close to the base. In the La Balma-Monte Capio sill, sulfides occur close to the base of the intrusion in contact with metasediments and also well inside the ultramafic sequence. In the ultramafic pipes, nodular aggregates of sulfide typically associated with hydrous silicate minerals are the most common type of mineralization.Deposits in the cyclic units have low overall concentrations of platinum-group elements (Pt CN 0.05; Ir CN 0.005), low Ni and Cu (1-2 and 0.1-0.5 wt %, respectively, in massive sulfides), and low Ni/Co ratios close to or below 10. The average is (Rh/Ir) CN = 6.5 and (Os/ Ir) CN = 2.2. The platinum-group element concentrations in the Sella Bassa deposit, located in the main gabbro, are approximately an order of magnitude higher than those in the cyclic units, and (Os/Ir) CN ratios range from 0.48 to 0.65 in contrast to ratios of close to or greater than 2 found elsewhere. In addition, the Pd/Pt ratio and Ni content of sulfides (3.5-4.5 wt%) is much higher. Platinum-group element profiles of deposits within the La Balma-Monte Capio intrusion resemble those of the cyclic units, although levels of both Ir and Os are distinctly higher, resulting in an average (Rh/Ir) CN ratio of about 2.9 instead of 6.5, and levels of Ni and Cu are also distinctly higher. Deposits at the base of the intrusion have generally very low concentrations of Ni, Cu, and Co in addition to low levels of platinum-group elements. The richer pipes have considerably higher concentrations of Os, Ir, and Ru (0.15 X chondritic abundance) than all other deposits from the Ivrea zone and have Pt and Pd concentrations on the order of those at Sella Bassa.The low platinum-group element values of the deposits of the cyclic units are attributed to these having experienced a low R value (magma/sulfide ratio) during their segregation. Contrasts between the La Balma-Monte Capio deposit and those of the cyclic units are attributed to variable fractionation of a similar magma. The main gabbro hosting the Sella Bassa deposit appears to have been deficient in platinum-group elements and to represent a different magma.