Platinum Group Element Enrichment of Natural Quenched Sulfide Solid Solutions, the Norilsk 1 Deposit, Russia

Abstract

The deepest terminations of the Mount Rudnaya subvertical massive sulfide offshoots of the Norilsk 1 orebody are composed of exceptionally fine grained sulfides that are believed to be natural quenched sulfide solid solutions. Copper-rich intermediate solid solution (ISS) and Fe-rich monosulfide solid solution (MSS) form an equigranular and lamellar matrix hosting MSS- and ISS-dominant globules. The nonstoichiometric chemical compositions of the solid solutions plot within their high-temperature fields known from experiments. MSS contains 19 to 35 wt % Ni, 0.09 to 0.45 wt % Co, and up to 0.6 wt % Cu and is heterogeneously enriched in Rh (up to 32 ppm), Ir (up to 0.6 ppm), Pt (up to 65 ppm), and Pd (up to 168 ppm). ISS occurs as the lamellar intergrowths of the chalcopyrite (Ccpss) and cubanite (Cubss) solid solutions, which bear up to 4.74 wt % Ni and 0.2 wt % Co and are heterogeneously enriched in Zn, Ag, and In. The assemblage of platinum group minerals (PGMs) is hosted mostly in the ISS and is dominated by Pt-Fe alloys and minerals of the rustenburgite-atokite series, like the set of PGMs at the Norilsk 1 deposit. Similar Pt-Pd-Sn compounds in the laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) spectra of profiles through MSS and ISS are interpreted to be trapped microinclusions. The pentlandite contains up to 0.13 wt % Pt, up to 4.62 wt % Pd, <0.53 wt % Co, and <0.4 wt % Cu according to electron microprobe analysis. LA-ICP-MS data and mapping show that Pd content in the pentlandite increases toward contacts with ISS and decreases toward contacts with MSS, supporting a reaction origin of pentlandite. The wide variations of the concentrations of major and trace elements in the solid solutions, as well as the coexistence of Pd-poor (a few ppm Pd) and Pd-rich (over 4.62 wt % Pd) pentlandite within a single sample, seem to characterize the different generations of the MSS to MSS-ISS globules, antecrysts, and phenocrysts with the distinct histories of enrichment due to exchange with fractionated Cu-platinum group element-rich residue. The directional distribution of Pd of high-temperature primary magmatic origin is preserved due to rapid quenching of the sulfides from ~650°C.

Origin of the Pd-Rich Pentlandite in the Massive Sulfide Ores of the Talnakh Deposit, Norilsk Region, Russia

“…The authors proposed that Pd-rich pentlandite formed by a peritectic reaction of MSS + Liquid = PnSS, where PnSS is a high-temperature solid solution from which pentlandite is obtained upon cooling [43,45]. In addition, Brovchenko and coauthors found 0.04-4.62 wt.% Pd in pentlandite from Norilsk-1 deposit and similarly came up with a peritectic reaction as plausible explanation for this atypical pentlandites [12]. Accordingly, there are four main hypotheses, which have been proposed to explain the high concentrations of palladium in pentlandite.…”

“…Pentlandite is the only palladium concentrator among the base metal sulfide minerals of the nickel-copper ores, since palladium is a compatible element with pentlandite while incompatible to coexisting pyrrhotite, chalcopyrite and cubanite [1][2][3][4][5][6][7][8][9][10][11][12]. Pentlandite is considered a late-forming mineral, within the base metal sulfides assemblage, that may be formed as a result of exsolution of MSS and ISS.…”

“…The development of ICP-MS LA methods has made it possible to determine very low concentrations of palladium and other PGE in common sulfide minerals of PGE-Ni-Cu deposits such as chalcopyrite, pyrrhotite and pentlandite. Several studies have demonstrated to a great extent that pentlandite is the only one of the three common sulfide minerals that concentrates Pd [1][2][3][4][5][6][7][8][9][10][11][12]. Nonetheless, pentlandite typically contains low to moderate value ranges (ppb to ppm) of the PGE, mostly Pd and Rh [13].…”

“…A relatively low palladium content (0.1-6 ppm) was recorded in the pentlandite from the Jinchuan [3], and even reached up to 1.5 wt.% in Stillwater ores [1,19]. Values ranging from 0.04-4.62 wt.% of Pd in the ores of the Norilsk−1 deposit were published by [4,12]. However, the highest Pd content in natural pentlandite (up to 9.8 wt.%) is reported in the J-M reef in the Stillwater Complex [20].…”

“…However, the highest Pd content in natural pentlandite (up to 9.8 wt.%) is reported in the J-M reef in the Stillwater Complex [20]. Thus, in the scientific literature, there are only a few references to even higher Pd concentrations in natural pentlandites [4,12,20]; however, these cases are extremely rare, and hence, special conditions must have been required.…”