Low-Sulfide Platinum Group Element Ores of the Norilsk-Talnakh Camp

Abstract

Low-sulfide platinum group element (PGE) mineralization of the Norilsk-type intrusions is located within the Upper Gabbroic Series, which comprises rocks heterogeneous in texture and composition. The highest grade of 10 to 50 g/t PGEs is confined primarily to chromitiferous taxitic gabbrodolerite, which forms irregular lens- and vein-like bodies that interfinger with contact gabbrodolerite, intrusion breccia, leucogabbro, and gabbrodolerite variably enriched in olivine, from olivine free up to picritic compositions. The abundant amygdules and pegmatoidal textures in Upper Gabbroic Series taxitic rocks, as well as the high enrichment of halogen in minerals (e.g., ≤4.6 wt % Cl in apatite), indicate a higher volatile content of the local magma compared to the magma that precipitated the Main Series. The observed diversity in spinel compositions, which evolve from chromite to Cr magnetite as well as toward hercynite, titanomagnetite, and ulvöspinel, is also indicative of crystallization from a fluid-saturated mush that subsequently reacted, to varying degrees, with contaminated trapped melt and immiscible fluid. The high PGE/S ratio is a primary feature of this mineralization style, albeit the ratio partly increased during sulfide replacement and resorption. The PGE tenor of bulk sulfides calculated as ΣPGE (g/t) in 100% sulfides exceeds 160 and may reach up to 1,400 to 2,500 in low-S ores (0.2–3 wt % S), whereas the value does not exceed 42 in the Talnakh disseminated ore and ranges from 35 to 120 in the Norilsk disseminated ore (1–10 wt % S). Several PGE peaks in the vertical sections correlate well with Cu, Ni, S, and Cr peaks, as well as with observed elevated proportion of amygdules. Low-sulfide ores are composed of two primary sulfide assemblages of pyrrhotite + pentlandite + chalcopyrite and pentlandite + pyrrhotite. The primary sulfides are depleted in the heavier 34S isotope relative to sulfides of the corresponded main orebodies (e.g., mean δ34S = 8.9‰ versus δ34S = 12.3‰, respectively, in the Kharaelakh intrusion). A secondary pyrite + millerite + chalcopyrite assemblage has isotope composition enriched in 34S by 2 to 6‰ δ34S with respect to primary sulfides. The directly measured PGE content in sulfides (e.g., 11–2,274 g/t Pd in pentlandite and 0.10–33.3 g/t Rh in pyrrhotite) is within the range of the typical Norilsk-type magmatic sulfide compositions. The textural setting and diversity of platinum group minerals (PGMs) favor the hypothesis of fluid-controlled crystallization. However, the distinct PGM assemblages in Norilsk 1 and Talnakh-Kharaelakh low-sulfide ores are comparable with those of the corresponding presumably magmatic disseminated and massive orebodies. The most remarkable characteristic is the widespread Pt-Fe alloys in Norilsk 1 and their absence in Talnakh-Kharaelakh, which is interpreted to reflect better preservation of the high-temperature PGMs in Norilsk 1 in contrast to their substantial replacement in more oxidized fluid-enriched environments in Talnakh-Kharaelakh.

The state of platinum in pyrrhotite: X-ray absorption spectroscopy study and implications for the role of Fe sulfides as platinum carriers

“…In particular, the concentration of Pt in pyrrhotite in the ores of the Norilsk group deposits varies from ∼0.1 to 40 ppm depending on the ore type and mineral assemblage (e.g. Sluzhenikin et al, 1994;Distler et al, 1999;Sluzhenikin et al, 2015;Sluzhenikin et al, 2020). These values of Pt concentration are consistent with Pt content in pyrrhotites from various other PGE deposits worldwide (Table 1), where the measured Pt content in pyrrhotite commonly ranges from a few tenths of ppm to several dozen ppm.…”

The critical role of magma degassing in sulphide melt mobility and metal enrichment

“…From top to bottom in ( b ), schematic drawings of: the subspherical shapes described in low-sulphide PGE ores within the Upper Gabbroic Series (as observed in refs. 39 , 40 ); the subspherical shapes in globular ores within the picritic rocks of the Main Layered Series and the taxitic rocks of the Lower Gabbroic Series (as observed in refs. 15 , 16 ); massive sulphide ores within the metasedimentary host rocks (as observed in refs.…”

“…6 ). Noril’sk-Talnakh three main ore types are: (i) low-sulphide PGE ores in the upper part of the intrusion within the Upper Gabbroic Series 22 , 37 – 39 , (ii) disseminated sulphides, or globular ores inside picritic and taxitic rocks, in the lower part of the intrusion 2 , 15 , 16 , 40 , 41 , and (iii) massive sulphide ores also in the lower part of the intrusion, largely in the country rocks and less frequently in the intrusion itself, occasionally accompanied by Cu-rich “cuprous ores” 2 , 22 , 42 (Fig. 6b, c ).…”

“…In the first two types of ores subspherical structures within the crystalline framework (Fig. 6b ) have been interpreted as fluid bubbles filled with late magmatic phases or hydrothermal minerals 15 , 16 , 37 – 39 . In the lower part of the intrusion, these structures are systematically associated with sulphide minerals suggesting they represent compound drops that were frozen in place in the olivine-rich rocks 15 , 16 , 37 .…”

“…In the lower part of the intrusion, these structures are systematically associated with sulphide minerals suggesting they represent compound drops that were frozen in place in the olivine-rich rocks 15 , 16 , 37 . In the upper part of the intrusion, these subspherical structures are even more common and generally contain lower amounts of sulphide minerals but abundant PGMs and a very high PGE/S ratio 38 , 39 , 43 . In this ore type, the PGE-rich minerals are displaced from the residual sulphide droplets, which is consistent with sulphide dissolution 12 .…”