Unique Ore–Silicate Pegmatite of Monchepluton with High Contents of Ni, Cu, and PGE (Murmansk Region)

Abstract

At the beginning of the 1960s, a large body of funnel-shaped ore pegmatite composed of Cu–Ni sulfide ores and a gabbronorite matrix was discovered at the Nittis–Kumuzhya–Travyanaya ore vein field of the Northern Chamber of Monchepluton (Kola Peninsula). In terms of its localization conditions, size, structure, and enrichment in sulfides and platinum-group elements (PGEs), it is a unique formation that has no analogues among other Paleoproterozoic layered complexes of the Fennoscandian Shield. Ore pegmatite occurs in the upper part of the layered zone of Mt. Nittis, which is composed of harzburgites and orthopyroxenites. Its horizontal size is 9 × 16 m, and the vertical one is 15 m. Three zones are distinguished in the structure of the body: I, a core of continuous sulfides; II, coarse- and giant-grained gabbronorites, enriched in interstitial sulfides; and III, a contact zone with sideronite sulfides with gradual transitions into host orthopyroxenites. Petro- and geochemical, mineralogical, and isotope studies of rocks and ores were performed using modern analysis methods. Ore pegmatite is most recent product in the processes of fractional crystallization of a magmatic melt, which derived as a large schlier with a sulfide core and a silicate matrix with a high concentration of fluids. The hypsometric level of the termination of the melt upwelling was determined by the equilibrium of the internal pressure of the volatiles and the external pressure of the overlying rocks. The δ¹⁸O values (+4.9…+6.1‰) are close to mantle marks (δ¹⁸O = +5.7‰) that correspond to basic igneous rocks. Early, late, and postmagmatic stages of mineral formation with a consistent increase in the role of fluid components (H₂O, CO₂, Cl, F) are distinguished in the formation history of the ore pegmatite. According to the results of calculations performed using various mineral geothermometers, the melt crystallization at the magmatic stage occurred in the range of ∼1100–900°C at a pressure of about 5 kbar. The separation (liquation) of the immiscible sulfide liquid began at temperatures of 1100–1000°C. As the temperature decreases, the main silicate minerals (clino- and orthopyroxenes, plagioclase) were first to crystallize. In the interstitial space between these minerals, residual melt and sulfide liquid enriched in PGEs, Au, Ag, and chalcophile elements (As, Sn, Sb, Te, Bi, Pb, Zn) accumulated. A late magmatic association (pargasite, magnesian hornblende and phlogopite) was formed from the residual melt. During the cooling of the sulfide liquid to a temperature of ∼1000°C and below, a copper-bearing monosulfide solid solution (Mss) is detached. As a result of its solid-phase transformations, pyrrhotite, pentlandite, and chalcopyrite were formed. An intermediate solid solution (Iss) was formed from the residual sulfide liquid, which was enriched with Cu and noble metals. Under a temperature below 550°C, this solution was successively decomposed into chalcopyrite, pyrrhotite, and cubanite. The dominant PGE minerals are the following: michenerite PdBiTe, sobolevskite Pd(Bi,Te), froodite PdBi₂, merenskyite PdTe₂, and moncheite PtTe₂. Au and Ag minerals are represented by electrum (AuAg) and hessite (Ag₂Te). A rare mineral—cervelleite (Ag₄TeS)—has been discovered. The Pd content in ore pegmatite varies in the range of 64.13–0.09 ppm, and that of Pt in the range of 2.70–0.004 ppm. The ore potential of Monchepluton is far from exhausted. Therefore, clarifying the genetic features of the origin of ore pegmatites and their connection with copper–nickel mineralization is of not only petrological, but also important practical significance, including for the purpose to develop prospecting indicators for the vein type of PGE–Cu–Ni ores.