Search

Mineralogical features of two orebodies, lenses (C-3 and C-4), at the central part of the Oktyabr'sky deposit have been investigated. Multidirectional mineralogical zoning in the northern and southern orebodies is shown, confirming the hypothesis of their formation from various magmatic flows, which have individual features and their own modes of formation. The southern C-3 and northern C-4 orebodies differ in their mineralogical associations: C-3 is characterised by a high-sulfur association of sulfides; and C-4 contains minerals with a sulfur deficit (talnakhite, sugakiite). Variations in Fe and Ni ratios in pentlandite are controlled by the volatility of sulfur during ore crystallisation. Direct crystallisation zoning is observed in the disseminated ores of the C-4 orebody (borehole RT-107), where the fugacity of sulfur ($f_{S_2}$) increases from bottom to top. In contrast in orebody C-3 (borehole RT-30) $f_{S_2}$ decreases in the same direction. This reverse zoning coincides with the vectors of the evolution of ore systems in various blocks of the Main Ore Body of the Oktyabr'sky deposit. The difference in typomorphic features of disseminated ores of the southern and northern orebodies is confirmed by differences in the associations of platinum-group element minerals (PGMs). Disseminated ores in picritic gabbro-dolerites and massive pyrrhotite ores in the exocontact of the intrusion within the southern orebody differ in the specialisation of PGMs: the former is characterised by minerals of the Pd–Bi–Sb–Te system, the latter by only Pt minerals. The similarity of these types of ores lies in the similar reverse mineralogical and geochemical zoning from top to bottom along the sections, caused by the evolution of the sulfide melt in this direction. The formation of reverse zoning of disseminated ores (zones of 'marginal reversion') is probably due to the action of a mechanism of repeated influx of a melt of an increasingly primitive composition into the upper parts of the crystallising flow. Unidirectional trends in massive and disseminated ores are more likely to be due to the action of the same type of mechanism. [ABSTRACT FROM AUTHOR]

This document is an editorial for a special issue of the journal Minerals, focusing on the geochemistry and mineralogy of basic-ultrabasic and alkaline intrusions and related magmatic deposits. The special issue includes articles on various deposits in Russia, such as PGE-Cu-Ni deposits in the Norilsk district, rare earth element deposits in the Eastern Sayan mountains, and layered intrusions in the Karelo-Kola area. The articles discuss the geological characteristics, formation mechanisms, and mineral compositions of these deposits, providing valuable insights into their origins. The editorial expresses gratitude to the authors, organizations, and reviewers involved in the research. [Extracted from the article]

Large igneous provinces (LIPs) are known for their rapid production of enormous volumes of magma (up to several million cubic kilometres in less than a million years) (1), for marked thinning of the lithosphere (2,3), often ending with a continental break-up, and for their links to global environmental catastrophes (4,5). Despite the importance of LIPs, controversy surrounds even the basic idea that they form through melting in the heads of thermal mantle plumes (2,3,6-10). The Permo-Triassic Siberian Traps (11)--the type example and the largest continental LIP (1,12)--is located on thick cratonic lithosphere (1,12) and was synchronous with the largest known mass-extinction event1. However, there is no evidence of pre-magmatic uplift or of a large lithospheric stretching (7), as predicted above a plume head (2,6,9). Moreover, estimates of magmatic CO2 degassing from the Siberian Traps are considered insufficient to trigger climatic crises (13-15), leading to the hypothesis that the release of thermogenic gases from the sediment pile caused the mass extinction (15,16). Here we present petrological evidence for a large amount (15 wt%) of dense recycled oceanic crust in the head of the plume and develop a thermomechanical model that predicts no pre-magmatic uplift and requires no lithospheric extension. The model implies extensive plume melting and heterogeneous erosion of the thick cratonic lithosphere over the course of a few hundred thousand years. The model suggests that massive degassing of [CO.sub.2] and HCl, mostly from the recycled crust in the plume head, could alone trigger a mass extinction and predicts it happening before the main volcanic phase, in agreement with stratigraphic and geochronological data for the Siberian Traps and other LIPs (5)., Petrological studies of Siberian Traps and associated alkaline rocks reveal high temperatures (1,600-1,650°C) (14,17) in their mantle sources. Olivine compositions in samples from lower units of the Norilsk lava section [...]