A model for the Ni-Cu-PGE ores of the Noril'sk region and its application to other areas of flood basalt

Abstract

Five principal magma types characterize the 3.8-km vertical thickness of flood basalt of the Noril9sk region. From base to top these are (1) a high TiO 2 type with alkalic affiliations (Ivakinsky and Syverminsky suites), (2) a TiO 2 -rich picrite-bearing magma (Gudchichinsky suite), (3) a picrite-bearing TiO 2 -poor magma (Tuklonsky suite), (4) a crustally contaminated TiO 2 -poor magma (Lower Nadezhdinsky suite), and (5) a picrite-free magma (Mokulaevsky, Kharayelakhsky, Kumginsky, and Samoyedsky suites) similar but not identical in its chemistry to the Tuklonsky and separated from the Lower Nadezhdinsky by transitional suites (Upper Nadezhdinsky and Morongovsky).The Ni-Cu ores are associated with olivine-rich intrusions that are geochemically similar to the Mokulaevsky magma. They are associated with about 1/15 to 1/10 of their mass in PGE-rich sulfides; this requires that these sulfides equilibrated with 15 to 200 times more magma than is represented by the intrusions themselves.Following the model of Naldrett et al. (1992), it is proposed that crustal contamination of the original Tuklonsky-type magma occurred at the top of a vertically extensive, fault-controlled magma chamber and caused segregation of immiscible sulfides with a low R factor. These depleted this magma in chalcophile elements and settled deeper in the chamber. As they settled, they reacted with less contaminated magma, scavenging additional chalcophile elements. They finally came to rest near the base of the chamber to form a zone of sulfide- (and thus chalcophile element-) enriched magma. Eruption, and in some instances intrusion, of progressively stratigraphically lower, less contaminated and thus less depleted layers in the chamber gave rise to the observed sequence of basalts and intrusions. The answer to the strong enrichment in Ni, Cu, and PGE associated with the ore-bearing intrusions is that these metals came from the magma that is now represented by the basalt magma with which the sulfides have come into contact and which shows chalcophile element depletion.Key metallogenic characteristics of the Noril9sk region are (1) a hot region in the mantle giving rise to vast amounts of basalt, (2) continental-scale rifting, (3) major faults serving as controls for extrusive and intrusive magmatism, (4) crustal contamination of magma, (5) chalcophile element depletion of contaminated magma, (6) intrusion of magma that has inherited sulfide removed from the depleted magma, and (7) the presence of evaporites in the sedimentary sequence underlying the basalts.The simplest way to recognize chalcophile element depletion is to plot Ni/MgO vs. Mg number [= at. % Mg/(Mg + 0.9Fe total )]. A strong indication that basalts have undergone crustal contamination is the correlation of high La/Sm ratios with high SiO 2 contents.The Lake Superior region shows many of the key metallogenetic characteristics of the Noril9sk region, including continental rifting, a triple junction, huge thicknesses of basalt, and Cu-Ni sulfide-bearing intrusions (Duluth Complex, Crystal Lake gabbro). Some of the Keweenawan basalts of the region show Ni depletion correlated with chalcophile element depletion. The region is an example of the potential application of the Noril9sk model to exploration.