PECHENGA AREA, RUSSIA .2. NICKEL-COPPER DEPOSITS AND RELATED ROCKS

The Pechenga Ni-Cu deposits are located within the early Proterozoic P echenga Rift Zone in the northwestern part of the Russian Kola Peninsu la close to the Norwegian border. Differentiated, Ni-Cu ore-bearing ga bbro-wehrlite intrusions and associated ultramafic flows occur in the Productive Formation, the fourth and youngest sedimentary formation of the North Pechenga Group. Similar rock assemblages are present in the correlative Norwegian Pasvik Group. The differentiated, nickel-copper -bearing gabbro-wehrlite intrusions have similar chemical characterist ics and are genetically related to iron-rich, picritic, spinifex-textu red ultramafic volcanic rocks known as 'ferropicrites' and to ferropic ritic tuffs and acidic tuffs and flows. The enriched geochemical natur e of the ferropicritic intrusive and volcanic suite suggests that it m ay have formed from, and represent the earliest evidence of, enriched mantle that was contaminated by subducted lithosphere and brought to t he surface by a mantle plume. All economic Ni-Cu deposits are located in the Western Rift Graben of the Pechenga rift. The graben is bounded by two major synsedimentary faults that control the location of the l argest and richest Ni-Cu ores as well as of ferropicritic and rhyoliti c volcanic centres. The ores occur mainly in differentiated gabbro-weh rlite intrusions and, to a lesser extent, in ferropicritic flows. The Pechenga Ni-Cu deposits formed as a result of the contamination of fer ropicritic magmas by sulphur that was derived from both consolidated a nd unconsolidated sediments of the Productive Formation. Two phases of deformation have localized the deposits into their present position i n the troughs of asymmetric synclines.