Flood Basalts and Magmatic Ni, Cu, and PGE Sulphide Mineralization: Comparative Geochemistry of the Noril'sk (Siberian Traps) and West Greenland Sequences

Abstract

The geological settings of the continental flood basalts at Noril'sk in the northwestern Siberian Traps and in West Greenland have a number of parallels. These include (1) a thick (0-3.5 km) sequence of tholeiitic and picritic lavas; (2) the inferred presence of a mantle plume or hot spot; (3) comagmatic intrusions, some of which are picritic; (4) faults which penetrate the upper mantle and acted as conduits for magmatism; and (5) an epicontinental setting with sulphidic sediments. The intrusions at Noril'sk are known to host giant deposits (>555 million tonnes with 2.7 wt% Ni, 3.9 wt% Cu, 3 ppm Pt, 12 ppm Pd) of sulphide mineralization, whereas those in West Greenland contain only small known showings of nickel sulphide mineralization. The lavas at both Noril'sk and West Greenland exhibit an empirical relation between crustal contamination and depletion in Ni and Cu, and at Noril'sk such lavas are highly depleted in the platinum group elements (PGE). Different styles of crustal contamination are recognized, but the observed depletion in Ni, Cu, and the PGE is similar irrespective of the nature of the contaminant. Moreover, the main trends are controlled by igneous crustal components that are likely to have had low S contents. Thus, we argue that initial sulphur saturation occurred largely in response to the increase in SiO 2 (typically 5 wt%) brought about by 20-25% crustal contamination, rather than to the addition of significant quantities of crustal-derived S. At'Noril'sk, minor- and trace-element arguments indicate that the contaminated lavas contain <0.5% of shallow-level evaporite-rich sediments, and so although such sediments may have contributed significant quantities of S, they were not responsible for the crustal contamination that is associated with the distinctive Ni, Cu, and PGE depletions in the Nadezhdinsky lavas. Mass-balance considerations suggest that as little as 1% of the S available in the mantle-derived magmas and assimilated evaporite-rich sediments at Noril'sk is locked up in the known deposits. The sulphides at Noril'sk have unusually high metal contents, and so they must have interacted with large volumes of magma. In one model the intrusions acted as open-system conduits, or chonoliths, in which the sulphides were continually upgraded as more magma passed through. The earlier magmas were stripped of Ni, Cu and PGE, but as the system evolved the sulphides became saturated and, in some cases, isolated, and so the later magmas were progressively less depleted in metals. In another model the sulphides formed at much deeper levels and were emplaced into the intrusions as sulphide magmas. Whichever model proves to be correct, the lavas at Noril'sk are characterised by a progressive upward recovery in Ni, Cu, and PGE abundances (over about 700 m) as the degree of contamination falls, but those in West Greenland are not. This may in turn have implications for exploration in other large igneous provinces.