Sources and formation conditions of sulfide-silicate magmas in the Noril’sk district

Abstract

Geology, tectonomagmatic reactivation of the Noril’sk district, as well as stratigraphy and geochemistry of the volcanic sequence are considered. Sources and formation mechanism of ore-bearing magma and the scope of ore formation are discussed. The Permian-Triassic flood-basalt magmatism of the Noril’sk district developed in part of the Siberian Platform with Archean-Paleoproterozoic basement broken into blocks and overlapped by a sedimentary cover up to 13 km thick and a volcanic sequence reaching 3.7 km in thickness. The geophysical data show that remnants of the subducted ancient oceanic crust exist in the mantle and fragments of transitional magma chambers and conduits are retained at different levels of the Earth’s crust. The cyclic tectonomagmatic evolution of the territory was characterized by alternation of extension with intense volcanic activity and compression accompanied by waning of volcanic eruptions. The early rifting, transitional stage, and late dispersed spreading are distinguished. The associations of volcanic (lavas and tuffs) and intrusive rocks were formed during each stage. The volcanic sequence is subdivided into 11 formations. The intrusions of the Talnakh and Noril’sk ore fields are distinguished by two-level structure with the Upper Noril’sk ore-bearing intrusions above and the Lower Noril’sk barren intrusions below. Two types of primary magmas differ in geochemistry of lavas and intrusions: (1) OIB-type high-Ti magma (iv, sv, gd formations of the first stage from bottom to top) and (2) low-Ti magma (hk, tk, nd formations of the second stage and mr-mk formations of the third stage). The nd formation depleted in ore elements and the ore-bearing cumulus composed of silicate and sulfide melts in combination with early silicate minerals and chromite are products of the fractionation of the primary low-Ti magma. As follows from geochemical parameters, intrusions of the Lower Noril’sk type are comagmatic to the evolved lavas of the nd3 subformation, whereas intrusions of the Upper Noril’sk type are comagmatic to the lavas of the mr-mk formations. Geochemical similarity with volcanic rocks provides evidence for the composition of the initial magma and the time of intrusion emplacement. The ore-bearing intrusions of the Upper Noril’sk type were formed at the onset of the third stage, when the primitive low-Ti magma similar to the lavas of mr-mk formations in composition was emplaced. When intruding, this melt captured and transported ore-bearing cumulus (drops of sulfide melt, early olivine and chromite grains) into the magma chamber. Separate portions of sulfide liquid were involved into movement as a self-dependent intrusive subphase during formation of the Talnakh and Kharaelakh intrusions. An extremal effect of pressure on sulfur concentration in fluid-bearing and sulfide-saturated mafic magmas has been established in experiments to be P = 1−2 GPa. In this interval of pressure, the S concentration in sulfide-saturated magmas increases in the following sequence: dry magma ≤(H2O + CO2)-bearing magma 2.5 GPa) pressures, the S contents (0.1–0.2 wt %) are commensurable. The extremal baric relationship of S concentration in fluid-bearing and sulfide-saturated mafic magmas may be important for the formation of ore-bearing magmas. The calculation results show that the amount of sulfides in the known deposits does not exceed 2% of geological resources of the sulfides separated from the flood basalts. Therefore, the chance of discovery of new deposits remains rather high. Proceeding from the conditions of ore-bearing magma formation and geological setting of the known deposits, criteria for recognition of potentially ore-bearing areas are proposed and such areas are outlined.