CONCENTRATION MECHANISMS OF THE PLATINUM-GROUP ELEMENTS IN LAYERED INTRUSIONS OF THE KOLA-KARELIA REGION

The example of Early Paleoproterozoic (2.5-2.44 Ga) layered intrusions (Monchetundra, Lukkulaisvaara, Burakovsk, and others) of the Baltic S hield and materials on classical layered intrusions elsewhere and on o phiolitic associations are employed to discuss some petrological aspec ts of the genesis of magmatic deposits of the platinum-group elements (PGE). It was established that PGE accumulations in the intrusions wer e produced by a variety of processes from magmatic to low-temperature metasomatic, with the main role most probably played by magmatic proce sses, which were responsible for the origin of the primary PGE concent ration in certain portions of the massifs. Since the initial PGE conte nts in the ancient boninite-like melts were not high, the fractional c rystallization of these melts did not serve as an ore-forming process. Judging from available data, an important part was then played by inf luxes of fresh melt portions into the partly solid magmatic chambers. However, this function could be fulfilled only by melts originally hig h in aqueous-salt, sulfur-bearing fluids and PGE. Both of these groups of components seem to have been extracted from corresponding crustal rocks (perhaps in supracrustal complexes) in the course of their assim ilation during the ascent of the intermediate magma chambers. Simultan eously, the melt became enriched in Bi, Te, As, Sb, Sn, Ta, Te, and ot her elements extracted from crustal granitic rocks. These elements for med easily soluble compounds with volatiles and PGE, which all migrate d together during all mineralization stages, The volatile-brine fluid components played an important role in PGE transport, and ''granitic'' components caused their deposition in the rocks. We determined the ro le of the crustal protoliths in which the magmatic systems developed: when the protoliths involved no rocks with valuable components, the re spective intrusions were barren. During the magmatic stage, fluid comp onents dissolved in the magma extracted PGE from the melt and redeposi ted them at physical barriers at cooling surfaces (crystallization zon es and the inner contacts of magmatic bodies). The leading ore-forming mechanism was thermodiffusion at low temperature gradients, when the diffusion rates of matter and heat were the closest. After solidificat ion, intrusions were affected by metasomatic processes, which proceede d over a wide temperature interval and were also participated by aqueo us-salt fluids. The latter redistributed PGE and caused their secondar y concentration. The development of magmatic systems in ophiolitic ass ociations was similar to the evolution of Early Paleoproterozoic magma tic systems. The primary PGE fractionation already occurred during the melting stage. In layered ophiolitic complexes and large layered intr usions, newly formed melts were accumulated, and their crystallization products gradually evolved from the HPGE-bearing products of the high est temperature partial mantle melts to Pd-dominated LPGE-bearing mine ralization related to the relatively low-temperature derivatives of cr ustal-mantle magmas.