CONTROLS ON THE COMPOSITION OF NI-CU SULFIDE DEPOSITS AS ILLUSTRATED BY THOSE AT NORILSK, SIBERIA

The Ni-Cu-PGE ore deposits of the Noril'sk region have developed in fe eder channels for Permo-Triassic continental flood basalt volcanism, p articularly within 100- to 300-m-thick elongate, stratified intrusions that merge with an extensive system of peripheral sills, through whic h the magma is thought to have continued to surface. The mineralizatio n is centered within two ore junctions, the Noril'sk and Talnakh ore j unctions. Principal are types include massive ore, which occurs typica lly below or at the lower margin of the intrusions, disseminated ore i n picritic and taxitic gabbrodolerite, which constitute the Ion er par t of the intrusions, and copper ore which either forms an impregnation in hornfelsic country rock adjacent to massive ore or occupies the ma trix of breccia zones that have developed at the front and upper conta ct of the intrusions. Although showing a close spatial relationship to the intrusions, the massive ore, particularly that underling the Nort hwest Talnakh intrusion, appears to have been injected separately. Ove r 500 samples have been analyzed for Ni, Cu, S, Pt, Pd, Rh, Ru, Ir, Os , and Au by NiS fire assay, acid dissolution, and neutron activation. The massive ore of the Kharayelakh orebody beneath the Northwest Talna kh intrusion has fractionated as injection proceeded from east to west , with the fractionated liquid moving progressively farther west. This liquid has also soaked into the surrounding hornfels to produce the C u-rich mantle, and above the western part of the Kharayelakh orebody, where the Cu enrichment is most pronounced, has also affected the lowe r part of disseminated ore zones within the intrusion. Study of the di sseminated ore has revealed that this has not fractionated to a great degree and that adequate sampling gives an average composition close t o that of the primary sulfide liquid. Disseminated ore from different parts of the Talnakh ore junction are characterized by very different metal tenors, with Ph ranging (all figures are metal contents recalcul ated for 100% sulfides) from 100 to 300 ppb in the western part of the Northwest Talnakh intrusion to 1,000 to 3,000 ppb in the central and eastern Northwest Talnakh intrusion and Main Talnakh intrusion. The te nor of sulfides associated with disseminated ore from three separate i ntrusions of the Noril'sk ore junction are similar, with Rh ranging fr om 4,000 to 10,000 ppb, The composition of the disseminated ore is mod eled as the result of initial low tenor sulfides becoming trapped in t he ore-bearing intrusions, and then interacting with successive. surge s of new magna passing along the feeder channels represented by their host intrusions on the way to surface. The different tenors of the dif ferent ore junctions and parts of ore junctions are the consequence of the effective number of times the magma passing through a particular channel exceeds the amount of sulfide within the channel, higher ratio s leading to higher tenors.