Geology and geochemistry of intrusions and flood basalts of the Noril'sk region, USSR, with implications for the origin of the Ni-Cu ores

Abstract

Geochemical studies indicate that 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 fractionated type with alkalic affiliations (Ivakinsky and Syverminsky), (2) an Ni-rich suite including picritic basalt (Gudchichinsky), (3) a primitive but Ni-depleted suite with fiat rare earth element (REE) profiles and also including picritic basalts (Tuklonsky), (4) a light REE- enriched, crustally contaminated suite (Lower Nadezhdinsky, nd 1 ). Above (4) there is a transitional suite (Upper Nadezhdinsky, nd 2 , and Morongovsky, mr 1 and mr 2 ) which is overlain by the fifth type, a second primitive suite without picrites (Mokulaevsky) similar but not identical in its chemistry to the Tuklonsky type. This is followed by tuffs and basalts of the Kharayelakhsky, Kumginsky, and Samoyedsky suites which are largely composed of similar uncontaminated tholeiitic magma. The latter three suites have not been studied by us.Intrusions of the region fall into five main groups, (1) alkalic intrusions, (2) Ti-rich dolerite dikes, (3) dolerite sills and dikes, (4) differentiated bodies not related to ore junctions, and (5) differentiated bodies related to ore junctions.The Ni-Cu ores are associated with Ni- and Cr-rich type 5 olivine-rich intrusions with flat REE profiles that are geochemicaly similar to the Mokulaevsky magma. Light REE-enriched, Ni- and Cr-poor olivine-rich type 5 intrusions with uneconomic mineralization occur closely associated with the strongly mineralized variants and resemble geochemically the basalts transitional between the Lower Nadezhdinsky and Upper Morongovsky. They are cut by the strongly mineralized bodies, indicating that the latter are of late Morongovsky to Mokulaevsky age. The mineralized intrusions are associated with about 1/5 to 1/10 of their mass in sulfides; this is a factor of 25 to 50 more than could have been carried in solution and deposited by in situ liquation from the magma of these intrusions. The platinum-group element (PGE) concentrations in the ores require that the sulfides giving rise to them equilibrated with 15 to 200 times more magma than is represented by the intrusions. Much of the ore is very massive and crosscuts the host intrusions.The sequence of igneous activity and mineralization in the immediate vicinity of Noril9sk and Talnakh from the base of the Nadezhdinsky upward is thus interpreted to be (1) extrusion of highly crustally contaminated, Ni- and Cu-poor basalt, (2) extrusion of a series of basalt flows, each progressively less contaminated and less Ni and Cu depleted than the preceding one, accompanied by intrusion of contaminated Ni-depleted magma with weak mineralization, (3) extrusion of uncontaminated, Ni- and Cu-rich basalt accompanied by intrusion of uncontaminated, Ni-rich magma with strong mineralization followed very closely by introduction of associated massive ore, and (4) continued extrusion of Ni- and Cu-rich basalt.It is proposed that crustal contamination of the original Ni- and Cu-rich 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 depleted basalts.