Mineralogical hosts of platinum group elements (PGE) and rhenium in the magmatic Ni-Fe-Cu sulfide deposits of the Ivrea Verbano Zone (Italy): An electron microprobe study

We determined by electron microprobe the concentration of platinum group elements (PGE), Re, Te, As, Bi and the presence of specific platinum group minerals (PGM) in base metals sulfides (BMS) from Ni-Fe-Cu deposits of the Ivrea Verbano Zone, Italy. The investigated deposits can be divided into three main groups: 1) mineralized layers in different cyclic units of the basic complex, 2) mineralization in an ultramafic sill that intruded into metasediments, and 3) mineralization in several ultramafic pipes that intruded gabbros and metasediments. Platinum group elements were analyzed in pyrrhotite, pentlandite, chalcopyrite and pyrite. The analyses of Os in chalcopyrite were excluded because of the presence of an interference from Cu, whereas Ru was below detection limit (35 ppm) in all cases. The data show that 71 % of the analyzed base metal sulfides (total analyses = 695) from the three types of mineralization contain Ir, Rh, Pt, Pd and Re above their detection limits. Several analyses obtained on the same grains revealed that the PGE are randomly distributed on the grain-scale and that the PGE contents do not correlate with Te and As. Platinum and Pd are hosted mainly in pyrrhotite and pentlandite, in about the same proportion; a minor amount of Pd also occurs in chalcopyrite. Sixty-two percent of Rh occurs in pyrrhotite and only the 28 % and 10 % in pentlandite and chalcopyrite, respectively. Nearly all of the iridium is hosted by pyrrhotite (58 %) and pentlandite (40 %); only 2 % is present in pyrite. Several platinum group minerals, with a size generally less than 10 mu m, were found included in the base metal sulfides. They comprise, in decreasing order of abundance: Pd-rich melonite (NiTe2), merenskyite ((Pd,Pt)(Te,Bi)(2)), moncheite ((Pt,Pd)(Te,Bi)(2)), sperrylite (PtAs2) and irarsite (IrAsS). Also a rare sulfide of Re, probably rheniite (ReS2), was found in the sill mineralization. Our observations suggest that the PGE, together with chalcogens, were initially dissolved in and collected by an immiscible sulfide liquid. Subsequently, they were exsolved along with Te and As to crystallize the discrete PGM described in the investigated samples. A Re-sulfide, possibly rheniite, was found only in the deposit that occurs in contact with metasediments. Therefore, it is argued that its precipitation is directly related to a contamination process from the country rocks.