The evolution of the ore-forming system in the low sulfide horizon of the Noril'sk 1 intrusion, Russia

Abstract

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 vincentite, 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 spinel; (2) the composition of the Pt–Fe alloys with a Fe/Fe + Pt range of 0.26–0.37 (logfO2 ≈ – (9–10); and (3) crystallisation of high-temperature sperrylite from silicate melt (at >800°C and logfS2 < –10.5), which is possible under fO2 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 сhromian 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≈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).

Atypical Mineralization Involving Pd-Pt, Au-Ag, REE, Y, Zr, Th, U, and Cl-F in the Oktyabrsky Deposit, Norilsk Complex, Russia

“…In addition, synthetic specimens of kotulskite, merenskyite PdTe 2 , atokite Pd 3 Sn and paolovite Pd 2 Sn are stable at 400 • C in the systems Pd-Sn and Pd-Te [70]. It is likely that a high-temperature crystallization (>800 • C) of sperrylite can proceed directly from a silicate melt [71], as was proposed in the case of this PGM in a low-sulfide horizon of the Noril sk 1 intrusion, Russia [10]. However, this mode of origin appears highly unlikely in our case.…”

“…They are considered to be feeders of trap basalts, which attain 3.5 km in thickness. Understandably, the Norilsk complex was (and still is) a Mecca for international teams of investigators; indeed, thousands of research articles describe various characteristics of its mineralogy, geology, petrology and geochemistry, and consider models of origin [1][2][3][4][5][6][7][8][9][10]. At the same time, there are mineralized zones of relatively low-grade ores, especially those deposited in peripheral areas or in the vicinity of external contacts, which are much less thoroughly investigated, and thus deserve more attention.…”