NI-CU-PGE DEPOSITS OF NORILSK REGION, SIBERIA - THEIR FORMATION IN CONDUITS FOR FLOOD-BASALT VOLCANISM

Two principal associations are recognized in the Permo-Triassic volcan ism at Noril'sk, Siberia, the second of which is divided into two sub- types: (association I) high-TiO2, sub-alkalic, tholeiitic and picritic basalts (Ivakinsky (iv), Syverminsky (sv) and Gudchichinsky (gd) Form ations); and (association IIA) low-TiO2, tholeiitic and olivine-rich b asalts (Tuklonsky (tk) Formation), which are succeeded by 500 m of cru stally contaminated, low-TiO2 basalts (Nadezhdinsky (nd1 and nd2) Form ations) that are marked by very low contents of Ni, Cu and platinum-gr oup elements (PGE). These are succeeded, in turn, by a transitional se ries 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. Concent rations of Ni, Cu and PGE increase progressively with the increasing a dmixture of mr2-mk-type magma. The ore-bearing (Noril'sk-type) and rel ated (Lower Talnakh-type) intrusions each consist of a central, elonga ted 'main body' (MB), 100-300 m thick, that is flanked by 'peripheral sills' 10-30 m thick. Ratios of critical incompatible trace elements a nd isotopic data for the intrusions (La/Sm versus Gd/Yb, Th/U versus C e/Yb and epsilon(CHUR)Nd) indicate that the weakly mineralized Lower T alnakh type display aspects of both the nd2 and nd3 volcanic formation s, whereas the ore-bearing (Noril'sk-type) intrusions correlate with t he mr2 stage. The average composition of the sill peripheral to the No ril'sk-type Northwest Talnakh intrusion corresponds to the average of the mr2-mk volcanic formations, and that for the sill peripheral to th e Lower Talnakh intrusion is, in most respects, between the averages f or the nd2 and nd3. It is concluded that tk magma ascended along a tra nscrustal fault (the Noril'sk-Kharayelakh fault) to occupy a mid- to u pper-crustal level magma chamber, where it fractionated, became contam inated and segregated immiscible sulphide to produce the nd magma type . This magma then rose beneath the keel of the developing volcanic bas in, where its direct ascent to surface was obstructed; it was forced t o pond and then spread out within sedimentary strata immediately benea th the volcanic sequence, where it encountered and reacted with crusta l sulphur to form additional sulphide liquid. It is suggested that the main bodies of the ore-bearing intrusions have served as the principa l exit conduits from this chamber and feeders for the tk to mr2 volcan ic formations. The liquid sulphides produced by reaction removed Ni, C u and PGE from this magma and remained trapped in the MB. The magma co ntinued to surface along routes that are now occupied by the periphera l sills to erupt as the nd1 Formation. Subsequent magma (nd2, nd3, mr1 , mr2) came into contact with the initial sulphides, losing Ni, Cu and PGE and enriching the sulphides in the process. Interpreting the intr usions as part of the plumbing system for the overlying lavas explains their high proportion of ore and the PGE-rich nature of the ore, thei r extensive metamorphic aureole, the chalcophile element depletion in the overlying volcanic rocks, the sulphur isotopic composition of the ore and the nearsurface development of the orebodies.