Ni-Cu-PGE deposits of Noril'sk region, Siberia: their formation in conduits for flood basalt volcanism

摘要

Two principal associations are recognized in the Permo-Triassic volcanism at Noril'sk, Siberia, the second of which is divided into two sub-types: (association I) high-TiO_2, sub-alkalic, tholeiitic and picritic basalts (Ivakinsky (iv), Syverminsky (sv) and Gudchichinsky (gd) Formations); and (association IIA) low-TiO_2, tholeiitic and olivine-rich basalts (Tuklonsky (tk) Formation), which are succeeded by 500 m of crustally contaminated, low-TiO_2 basalts (Nadezhdinsky (nd1 and nd2) Formations) that are marked by very low contents of Ni, Cu and platinum-group elements (PGE). These are succeeded, in turn, by a transitional series of units comprising the upper Nadezhdinsky and lower Morongovsky Formations (nd3, mr1), which were produced by mixing of nd2 with new (association IIB) upper Morongovsky-Mokulaevsky (mr2-mk) magma. Concentrations of Ni, Cu and PGE increase progressively with the increasing admixture of mr2-mk-type magma. The ore-bearing (Noril'sk-type) and related (Lower Talnakh-type) intrusions each consist of a central, elongated 'main body' (MB), 100-300 m thick, that is flanked by 'peripheral sills' 10-30 m thick. Ratios of critical incompatible trace elements and isotopic data for the intrusions (La/Stn versus Gd/Yb, Th/U versus Ce/Yb and implied by~(CHUR) Nd) indicate that the weakly mineralized Lower Talnakh type display aspects of both the nd2 and nd3 volcanic formations, whereas the ore-bearing (Noril'sk-type) intrusions correlate with the mr2 stage. The average composition of the sill peripheral to the Noril'sk-type Northwest Talnakh intrusion corresponds to the average of the mr2-mk volcanic formations, and that for the sill peripheral to the Lower Talnakh intrusion is, in most respects, between the averages for the nd2 and nd3. It is concluded that tk magma ascended along a transcrustal fault (the Noril'sk-Kharayelakh fault) to occupy a mid- to upper-crustal level magma chamber, where it fractionated, became contaminated and segregated immiscible sulphide to produce the nd magma type. This magma then rose beneath the keel of the developing volcanic basin, where its direct ascent to surface was obstructed; it was forced to pond and then spread out within sedimentary strata immediately beneath the volcanic sequence, where it encountered and reacted with crustal sulphur to form additional sulphide liquid. It is suggested that the main bodies of the ore-bearing intrusions have served as the principal exit conduits from this chamber and feeders for the tk to mr2 volcanic formations. The liquid sulphides produced by reaction removed Ni, Cu and PGE from this magma and remained trapped in the MB, The magma continued to surface along routes that are now occupied by the peripheral sills to erupt as the nd1 Formation. Subsequent magma (nd2, nd3, mrl, mr2) came into contact with the initial sulphides, losing Ni, Cu and PGE and enriching the sulphides in the process. Interpreting the intrusions as part of the plumbing system for the overlying lavas explains their high proportion of ore and the PGE-rich nature of the ore, their extensive metamorphic aureole, the chalcophile element depletion in the overlying volcanic rocks, the sulphur isotopic composition of the ore and the near-surface development of the orebodies.