Ni, Cu, Au, and platinum-group element contents of sulphides associated with intraplate magmatism: a synthesis

Abstract

The tectonic setting of intraplate magmas, typically a plume intersecting a rift, is ideal for the development of Ni -Cuplatinum-group element-bearing sulphides. The plume transports metal-rich magmas close to the mantle-crust boundary. The interaction of the rift and plume permits rapid transport of the magma into the crust, thus ensuring that no sulphides are lost from the magma en route to the crust. The rift may contain sediments which could provide the sulphur necessary to bring about sulphide saturation in the magmas. The plume provides large volumes of mafic magma; thus any sulphides that form can collect metals from a large volume of magma and consequently the sulphides will be metal rich. The large volume of magma provides sufficient heat to release large quantities of S from the crust, thus providing sufficient S to form a large sulphide deposit. The composition of the sulphides varies on a number of scales: (i) there is a variation between geographic areas, in which sulphides from the Noril'sk-Talnakh area are the richest in metals and those from the Muskox intrusion are poorest in metals; (ii) there is a variation between textural types of sulphides, in which disseminated sulphides are generally richer in metals than the associated massive and matrix sulphides; and (iii) the massive and matrix sulphides show a much wider range of compositions than the disseminated sulphides, and on the basis of their NiICu ratio the massive and matrix sulphides can be divided into Cu rich and Fe rich. The Cu-rich sulphides are also enriched in Pt, Pd, and Au; in contrast, the Fe-rich sulphides are enriched in Fe, Os, lr, Ru, and Rh. Nickel concentrations are similar in both. Differences in the composition between the sulphides from different areas may be attributed to a combination of differences in composition of the silicate magma from which the sulphides segregated and differences in the ratio of silicate to sulphide liquid (R factors). The higher metal content of the disseminated sulphides relative to the massive and matrix sulphides may be due to the fact that the disseminated sulphides equilibrated with a larger volume of magma than massive and matrix sulphides. The difference in composition between the Cu-and Fe-rich sulphides may be the result of the fractional crystallization of monosulphide solid solution from a sulphide liquid, with the Cu-rich sulphides representing the liquid and the Fe-rich sulphides representing the cumulate.RCsumC : Le contexte tectonique des magmas d'intraplaque, caracterise par un panache croisant un rift, fournit les conditions ideales pour le dkveloppement de sulfures de Ni-Cu et d'element du groupe du platine. Le panache transporte les magmas riches en metaux prks de la limite separant le manteau de la croQte. L'interaction du rift et du panache permet un transport rapide du magma dans la croQte, ce qui attinue la perte des sulfures du magma en route vers la croGte. Le rift peut contenir des sediments susceptibles de fournir le soufre necessaire pour atteindre le point de saturati...

The Uitkomst intrusion and Nkomati Ni-Cu-Cr-PGE deposit, South Africa: trace element geochemistry, Nd isotopes and high-precision geochronology

“…Their Pt/Pd is 0.7, in sharp contrast to the internal Bushveld PGE reefs (Merensky Reef, UG2 chromitite) which have Pt/Pd significantly above unity (Maier et al 2013), but resembling the lower portions of the Platreef that have Pt/Pd <1 (Kinnaird 2005). Barnes et al (1997) have shown that certain sulphide deposits have lower Pt/Pd than their parent magmas and suggested that this is due to higher D Pd than D Pt into sulphide liquid. Based on this argument, one could propose that the relatively low Pt/Pd at Uitkomst is a result of the relative sulphide enrichment of the intrusion.…”

Genèse des dépôts de Fe-Ti-P associés aux intrusions litées (exemples l intrusion mafique de Sept-Iles au Québec complexe de Duluth aux États-Unis)

“…Des chercheurs (Lesher 1993;Ripley, 1981;Thériault et al, 1997;Barnes et al, 1997) ont mis en évidence la contamination comme étant le principal processus provoquant la saturation du magma en soufre et par conséquent, la formation de gisements de sulfures magmatiques. Les magmas qui proviennent du manteau sont, de L'assimilation du matériel crustal peut se faire : 1) Par la fusion du matériel solide (Watson 1982;Thériault et al, 1997), 2) Par dégazage lié à la mise en place du magma dans l'encaissant et les xénolithes lors du métamorphisme de contact, (Ripley, 1981), 3) La contamination peut se faire aussi par mélange de magmas (Emslie, 1978).…”

“…La teneur en forstérite des olivines des roches riches en oxydes de fer de Boulder Lesher (1993), Ripley, (1981), Thériault et al, (1997 et Barnes et al, (1997) …”

“…Des chercheurs (Lesher, 1993;Ripley, 1981;Thériault et al, 1997;Barnes et al, 1997) ont mis en évidence la contamination comme étant le principal processus provoquant la saturation du magma en soufre et par conséquent la formation de gisements de sulfures magmatiques. Les magmas qui proviennent du manteau sont de toute évidence une source de chaleur.…”

Fluid Transport of Sulfur and Metals Between Sulfide Melt and Basaltic Melt

“…1. Conceptual model for fluid transport of sulfur from country rocks into magma, after Barnes et al (1997). The hypothesis is that the intrusion of magma raises country-rock temperatures above the stability of pyrite (white squares), which breaks down to pyrrhotite + S 2 .…”