Unique ore-silicate pegmatite of monchepluton with high contents of Ni, Cu and PGE (Murmansk region)

Abstract

At the beginning of the 60s of the last century at the Nittis-Kumuzhya-Travyanaya (NKT) ore vein field of the Northern Chamber of Monchepluton (Kola Peninsula), was discovered a large body of funnel-shaped ore pegmatite, composed of Cu-Ni sulfide ores and a gabbronorite matrix. In terms of localization conditions, its size, structure, enrichment in sulfides and PGE, it is a unique formation that has no analogues among other layered complexes of the Paleoproterozoic age of the Fennoscandian shield. Ore pegmatite occurs in the upper part of the layered zone of Nittis mont., composed of harzburgites and orthopyroxenites. Its horizontal size is 9 × 16 m, vertical – 15 m. Three zones are distinguished in the structure of the body: I – core of continuous sulfides, II – coarse- and giant-grained gabbronorites, enriched in interstitial sulfides, and III – 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 separated in the form of a large schlier with a sulfide core and a silicate matrix with a high concentration of fluids. The hypsometric level of melt stop 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‰) and correspond to basic igneous rocks. In the history of the formation of the ore pegmatite, early magmatic, late- and post-magmatic stages of mineral formation with a consistent increase in the role of fluid components (H2O, CO2, Cl, F) are distinguished. According to the results of calculations performed using various mineral geothermometers, crystallization of the melt at the magmatic stage occurred in the range of ~1100–900оC at a pressure of about 5 kbar. At temperatures of 1100–1000оC, separation (liquation) of the immiscible sulfide liquid began. As the temperature drop, the main silicate minerals (clino- and orthopyroxenes, plagioclase) initially crystallized, in the interstices of which residual melt and sulfide liquid enriched in PGE, Au, Ag and chalcophile elements (As, Sn, Sb, Te, Bi) accumulated, Pb, Zn). A late magmatic association (pargasite, magnesian hornblende and phlogopite) was formed from the residual melt. When the sulfide liquid cooled at a temperature of ~1000оC and below, a copper-containing a monosulfide solid solution (Mss) separated from it. As a result of its solid-phase transformations, pyrrhotite, pentlandite, chalcopyrite, precious metal minerals and an intermediate solid solution (Iss) were formed, which at temperatures below 550оC successively decomposed into chalcopyrite, pyrrhotite and cubanite. The dominant PGE minerals are: maychenerite PdBiTe, sobolevskite Pd(Bi,Te) frudite PdBi2, merenskite PdTe2 and moncheite PtTe2. Au and Ag minerals are represented by electrum (AuAg) and hessite (Ag2Te). A rare mineral in nature – servelleite (Ag4TeS) – has been discovered. The Pd content in ore pegmatite varies in the range (64.13–0.09 ppm), Pt – (2.70–0.004 ppm). The ore potential of Monchepluton is far from being exhausted, therefore, elucidating the genetic features of the origin of ore pegmatites and their connection with copper-nickel mineralization is not only petrological, but also of important practical significance, including to develop search signs on the vein type of PGE–Cu–Ni ores.