Platinum-group elements in alpine-type ultramafic rocks and related chromite ores of the main ophiolite belt of the Urals

Abstract

Ultramafic magmas made the main contribution to PGE mineralization of ophiolite associations. An orebearing dunite-chromitite complex intruded from the mantle depth is widespread in the southern Kempirsai massif, where a large chromite deposit of Os-Ir specialization has been formed. The PGE contents (mg/t) in this type of chromitites (290 Os, 265 Ir, 200 Ru, 60 Pt, 55 Pd,18 Rh) reflect the specific character of Cr-bearing melt as an efficient carrier of PGE, while host dunite is practically free of PGE (about 20 mg/t in total). The geochemical specialization of ophiolite chromitites is displayed only for refractory PGE (Os, Ir, Ru), prevailing over fusible PGE (Rh, Pd, Pt). Therefore, the ophiolite association was classified as chromite-bearing and unpromising for PGE mineralization, as was exemplified in the Urals by Zavaritsky and Betekhtin, who opposed this rock association to PGE-bearing zonal and layered intrusions. This difference is caused by reductive conditions and high PT parameters dominating in magmatic chambers of ophiolite associations. The main conclusion that can be drawn from this study Abstract -On the basis of a representative collection of ultramafic rocks and chromite ores and a series of technological samples from the largest (Central and Western) deposits in the Rai-Iz massif of the Polar Urals and the Almaz-Zhemchuzhina and Poiskovy deposits in the Kempirsai massif of the southern Urals, the distribution and speciation of platinum-group elements (PGE) in various type sections of mafic-ultramafic massifs of the Main ophiolite belt of the Urals have been studied. Spectral-chemical and spectrophotometric analyses were carried out to estimate PGE in 700 samples of ultramafic rocks and chromite ores; 400 analyses of minerals from rocks, ores, and concentrates and 100 analyses of PGE minerals (PGM) in chromite ores and concentrates were performed using an electron microprobe. Near-chondritic and nonchondritic PGE patterns in chromitebearing sections have been identified. PGE mineralization has been established to occur in chromite ore from all parts of the mafic-ultramafic massifs in the Main ophiolite belt of the Urals. The PGE deposits and occurrences discovered therein are attributed to four types (Kraka, Kempirsai, Nurali-Upper Neiva, and Shandasha), which are different in mode of geological occurrence, geochemical specialization, and placer-forming capability. Fluid-bearing minerals of the pargasite-edenite series have been identified for the first time in the matrix of chromite ore of the Kempirsai massif (the Almaz-Zhemchuzhina deposit) and Voikar-Syn'ya massif (the Kershor deposit). The PGE grade in various types of chromite ore ranges from 0.1-0.2 to 1-2 g/t or higher. According to technological sampling, the average PGE grade in the largest deposits of the southeastern ore field of the Kempirsai massif is 0.5-0.7 g/t. Due to the occurrence of most PGE as PGM 10-100 mm in size and the proved feasibility of their recovery into nickel alloys, chromites of the Kempirsai massif can be con...

Unnamed Pt(Cu0.67Sn0.33) from the Bolshoy Khailyk River, Western Sayans, Russia, and a Review of Related Compounds and Solid Solutions

“…The maximum levels of Sb and Hg (#14, 15, Table 7) are similar: 0.15 and 0.17 a.p.f.u., respectively [17,38]. The incorporation of Hg is unusual for a Pt-Fe alloy mineral, though it is consistent with the compositions of potarite, PdHg, synthetic PtHg or NiHg, also having the AuCu-type structure [64,65].…”

“…Levels of Pd attain 0.3 Pd a.p.f.u. (#1, 4 in Table 7) [38,55]. A value greater than 0.35 Ir a.p.f.u.…”

“…The major trend extends along the PtFe-PtCu join; numerous compositions are Cu-dominant. In contrast, the PtFe-PtNi series is much more limited, with relatively few alloy samples having a Ni-dominant compositions (#1, 12, 13, Table 7), reported from the Soldzhersky complex, Tuva, Russia, the Bushveld layered complex, South Africa, and from the Butyrinskoye deposit, Kytlym complex, Urals, Russia [38,52,57]. Interestingly, the PtNi-PtFe join is totally free of data points in spite of a large number of compositions examined from these complexes (Figure 5).…”

“…Potarite, PdHg, is involved in two solid-solution series (Figure 9): the PdHg-PdCu series, which is present in the Kytlym complex, Urals [38,66], and the PdHg-PdAu series, reported in association with Pd-Pt alloys [67] from Córrego Bom Sucesso, Minas Gerais, Brazil [23,68]. Note that pure "PdCu" presumably does not represent the end-member component in those series because it corresponds to skaergaardite, PdCu, a cubic species crystallizing in space group Pm3m, with a = 3.0014(2) Å [69].…”

“…On the other hand, according to the suggestion of [18], these species may represent intermediate members, i.e., Pt(Cu 0.5 Fe 0.5 ) and Pt(Ni 0.5 Fe 0.5 ) (#2, 6, Table 7). In addition, the Ni-dominant phases reported (#1, 12, 13, Table 7; [38,52,57]) are likely related to synthetic PtNi (#7, Table 9; [16]). The unnamed mineral [Pt(Cu 0.67 Sn 0.33 )] described here may represent the Cudominant member of the group; by analogy, different compositional variants could occur in the systems PtCu-PtSn and PtNi-PtFe-PtCu (cf.…”