The Evolution Of The Ore-Forming System In The Low Sulfide Horizon Of The Noril'Sk 1 Intrusion, Russia

We present here new data on the low-sulfide mineralisation in the upper endocontact of the Noril'sk 1 intrusion. Twenty four mineral species of platinum-group elements and their solid solutions, as well as numerous unnamed phases, including an Sb analogue of vin-centite, As and Sn analogues of mertieite-I and a Sn analogue of mertieite-II have been found. It is shown that the features of the mineral association: (1) the atypical trend of TiO2 and Fe2+ in chromian spinet; (2) the composition of the Pt-Fe alloys with a Fe/Fe + Pt range of 0.26-0.37 (logf(O2), approximate to - (9-10); and (3) crystallisation of high-temperature sperrylite from silicate melt (at >800 degrees C and logf(S2) < -10.5), which is possible under f(O2) of FMQ to FMQ-2 in mafic magma, are due to the reducing conditions of their formation and evolution. Droplet-like inclusions of silicate-oxide minerals in chromian spinels and sulfides in platinum-group minerals are interpreted to be trapped droplets of co-existing sulfide melt. The captured sulfide melt has evolved in the direction of increasing the fugacity of sulfur: troilite + pentlandite (Fe>Ni) - in sperrylite (paragenesis I) to monoclinic pyrrhotite + pentlandite (Ni approximate to Fe) + chalcopyrite - in Pt-Fe alloys (paragenesis II). Paragenesis from the sulfide aggregates in the silicate matrix are more fractionated: pyrrhotite + pyrrhotite (Ni>Fe) + chalcopyrite (III) and pyrite + pentlandite (Ni>>Fe) + millerite (IV). Pd arsenides and antimonides crystallised later than sperrylite and isoferroplatinum, as a result of the evolution of a sulfide melt with an increased activity of the element ligands (Te, Sn, Sb and As).