The paper presents results of comprehensive studies of the rock association composing the lower zone of the northeastern part of the Monchetundra mafic massif in the Monchegorsk ore district. The zone consists of orthopyroxenite, plagioorthopyroxenite, and norite, which are variably amphibolized and host numerous gabbroid injections from the upper zone of the massif. The chemical composition of the dominant rock-forming minerals of the rocks was studied. The orthopyroxenes of the lower zone of the Monchetundra massif (LZMM) notably differs from those of the Monchegorsk pluton (Monchepluton) in containing lower aluminum concentrations because of differences in the P–T parameters of their crystallization. According to mineral geothermobarometric data, the LZMM rocks crystallized at 1200–1000°C and approximately 6 kbar, at a depth of about 20 km, whereas the Monchepluton rocks crystallized at an average temperature of 1230°C and a pressure of 3 kbar in shallow conditions. The LZMM rocks are characterized by moderate magnesium and silica contents and by elevated iron and low titanium concentrations, are enriched in the light rare earth elements (LREE) and LILE (Rb, Ba, and Sr) but are depleted by HFSE (Nb and Ta). The behavior of major components in the rocks is similar to that in analogous rocks in the bottom axial part of the Monchetundra massif, lying at its basement. Moreover, all petro- and geochemical parameters of the LZMM rocks are close to those of the analogical rocks of the Monchepluton. The paper presents new U–Pb isotope-geochronological data on individual zircon grains from the LZMM rocks. The age of the orthopyroxenite is 2496.3 ± 2.7 Ma, and that of the norite is 2500 ± 2 Ma, which indicates that these rocks crystallized simultaneously (within the error). The orthopyroxenite has an age of 2452 ± 85 Ma and positive ε_Nd(T) = +1.7, and the model age of the protolith T(DM) is 2.76 Ga according to data on the Sm–Nd of isotopic systematics. The possible parental melt of the rocks may have been komatiite contaminated with crustal material. The obtained results indicate that the Monchetundra massif is a composite intrusion, whose lower zone is similar to the layered intrusions and the upper one is made up of rocks belonging to the gabbro–anorthosite complex. The massif was formed by at least three pulses of magmatic activity (at 2.50, 2.47, and 2.45 Ga).

The geology of the basal-structural Loypishnyun low-sulfide Pt–Pd deposit is characterized, including its mineral composition and the peculiarities of its PGE and chalcophile-element distribution in...

The internal structure of the Volchetundra gabbro-anorthosite massif is considered, including localization of low-sulfide PGE mineralization and its mineralogy. The Volchetundra massif 24 km long and 0.5–4.0 km wide occupies the middle part of the Main Range complex, which extends for 75 km in the nearly meridional direction. The main and marginal zones are distinguished in the massif. The marginal zone 20–400 m wide extends along the entire eastern contact of the massif and is primarily composed of mediumgrained meso- and leucocratic norite, gabbronorite, plagioclasite, and less fequent orthopyroxenite. The main zone consists of coarse-grained leucogabbro and gabbronorite with an anorthosite zone in the axial part of the massif. The PGE mineralization of the Volchetundra massif is distinctly subdivided into two types substantially differing in localization, mineralogy, geochemistry, and economic importance. Mineralization of the first type is localized in the marginal zone and characterized by the highest resource potential. Mineralization hosted in the main zone belongs to the second type. The PGE ore of marginal zone is spatially and genetically related to the pyrite-pentlandite-chalcopyrite-pyrrhotite sulfide mineralization (1–5%) in the form of fine inequigranular interstitial disseminations, and less frequent larger grains and pockets localized within two ore zones each up to 2 km in extent. The thickness of separate mineralized layers varies from 0.5 to 3.0 m and up to 45 m in bulges. The average Pt + Pd grade is 1.37 gpt at Pd/Pt = 3.1. The mineralization of the second type has been penetrated by boreholes. Separate intersections do not correlate with one another and are limited in extent both along the strike and down the dip. The PGE mineralization is related to finely dispersed pentlandite-pyrite-pyrrhotite-chalcopyrite sulfides, sulfide emulsions, and less abundant stringer-disseminated sulfide ore. The orebodies vary from 2 to 7 m in thickness. The average Pt + Pd grade is 1.61 gpt; Pd/Pt = 1.3. The PGE mineralization includes 22 mineral species. PGE sulfides (cooperite-braggite-vysotskite; laurite and erlichmanite in insignificant amounts) are predominant. Bismuthotellurides (moncheite-kotulskite-merenskyite) and arsenides (sperrylite, palladoarsenite, arsenopalladinite, atheneite) are subordinate in abundance. In addition, sulfoarsenides (platarsite, hollingworthite), tellurides (telargpalite, sopcheite, keithconnite, melonite, hessite), paolovite, and Pt-Fe alloy have been identified. An admixture of native gold and electrum occur constantly.

The article presents the results of studying the chemical composition of ore-bearing rocks, as well as the mineralogy of base metal sulfides and platinum group elements (PGE), of the Nyud-II sulfide PGE–Cu–Ni deposit in the southwestern part of the Nyud massif of the Monchegorsk Pluton (Monchepluton). The ores of the deposit are represented by vein-disseminated and nest-schlieren types. They are characterized by significant predominance of Pd over Pt, fractionation of low-melting PGE (PPGE subgroup) with respect to high-melting ones (IPGE subgroup), and close correlations of Ni and Cu with S in the presence of increased As, Se, Te, and Bi contents. The S/Se ratios in ore (3470−3530) correspond to the mantle values. Among the platinum group minerals (PGM), the most widespread are Pt and Pd bismuth–tellurides and tellurides (merenskyite, michenerite, and moncheite), subordinate amounts of Pt–Fe alloys and sperrylite, and native osmium and Ir, Rh, and Pt sulfoarsenides (irarsite, hollingworthite, and platarsite). The formation of ore sulfide concentrations resulted from separation of an immiscible sulfide liquid upon cooling of a sulfur-saturated silicate magma with a mafic composition. Subsequent fractional crystallization of the sulfide liquid contributed to the uneven distribution of Ni, Cu, and PGE. PGE–sulfide ore formation took place in a fairly wide temperature range, starting at 1100−1000°C and ending at 600−400°C. At an early stage, IPGE minerals (native osmium and erlichmanite) separated. Upon cooling to a temperature of 1000−900°C, the sulfide liquid fractionated with the formation of monosulfide solid solution (mss), in which compatible IPGE were concentrated, and a residual sulfide liquid enriched in Ni, Cu, PtPGE, and chalcophile elements. With a further decrease in temperature (to 600°C), Pt–Fe alloys, sperrylite, and IPGE + Pt sulfoarsenides crystallized, with separation of the residual sulfide melt enriched in Cu, PPGE and chalcophile elements. At 600−400°C, ore formation ended with the complete crystallization of base metal sulfides and the formation of Pt and Pd bismuth–tellurides and tellurides.