The article encompasses the analysis of the mineral composition and geochemistry of weathering crusts of the Svetloborsky and Nizhnetagilsky massifs of the Ural Platinum Belt. Weathering crusts are represented by a partial profile and composed of disintegrated lizardite-chrysotile and loose leached lizardite serpentinite, which are overlapped by clays of nontronite zone at the Svetloborsky massif. The supergene process reveals accumulation of trace elements and rare earth elements (REE) upwards within the weathering profile. Rocks of the Svetloborsky massif con-tacting with dike complexes show high levels of REE content and their accumulation coefficients. In weathering profile of the Svetloborsky and Nizhnetagilsky massifs, the average content of precious metals is low; palladium and platinum prevail, in contrast to primary substrate rocks, where the main platinum group metals are platinum and iridium.
Isotope ratio 87Sr/86Sr was determined for the first time for the Ufalei and Sakhara supergene nickel deposits. The average obtained 87Sr/86Sr ratio in the Sakhara deposit (0,70838) is higher then in the Ufalei deposit (0,70697). In both deposits 87Sr/86Sr ratio increases from low-altered serpentinite rocks of the lower part of the deposits (0,70583 and 0,70687) to exogenous iron-oxide rocks of the upper part of the deposits (0,70917 and 0,71004).
Chromitites of Nizhni Tagil massif veins in length from a few centimeters to several meters. The contents of rare earth elements in the platinum-veined chromitites characterized by reduced compared with the enclosing dunite values. In quantitative terms, is dominated by light rare earth elements. The positive correlation between the rare-earth elements and platinum group metals in the samples with normal contents. High and extremely high content of PGE in the chromite-platinum ores Nizhni Tagil massif are not accompanied by a significant increase in the concentrations of REE.
Platinum mineralization in Svetly Bor massif is represented by two promising mineral assemblages: chromite-platinum and platinum-type dunite. Body platinum of chromites lie within the fields of small-and medium-grained dunite central part of the array. Spinel epigenetic vein platinum chromites of Svetly Bor clinopyroxenite-dunite massif have some geochemical features such as high iron content, low chromium and titanium. Changing the chemical composition of the constituent minerals chromites is the result of processes of serpentinization of dunites host and is accompanied by the emergence of new mineral phases. Noble metal mineralization is represented by fine of up to 50 microns, mostly idiomorphic grains isoferroplatinum, tetraferroplatinum, osmiridium.
Contents of platinum group elements (PGE), gold and silver in oxide-silicate nickel ores of the Buruktal, Ufalei and Elov supergene nickel deposits are determined in relation to their ophiolitic dunite-harzburgite bedrock.
Weathering processes and infiltration metasomatic processes in the weathering crusts hyperbasite arrays have a positive effect on the accumulation of rare earth elements. The content of rare earth elements is steadily increasing bottom-up on the profile of weathering. Metasomatic upper profile characterized by a high content of rare earth elements, which leads to the appearance of rare earth elements phosphate mineral phase – xenotime found in this laterite for the first time. The composition of the rare earth elements in incorporating their metasomatic and minerals demonstrates chondritic distribution.
We have analysed chemical of nepouites. NiO content in then ranges from 13,00 to 35,18 %, MgO from 18,29 to 44,61 %, i.e. the composition of nepouite corresponds to the composition of Mg-nepouite. General row of chemical element mobility in lizardite – nepouite ores of the Elov supergene nickel deposit is the following: (Mo, Sb, Se, W)20-35 > (Sn, As, Ni)10-20 > (Pb, Be, U)3-7 > (Ti, Ga, Mn)1-3 > (Th, Rb, Si)~1 > (Al, V, Co, Tm, Zn, Mg)0,6-0,9 > (Yb, Ca, Cu)0,4-0,5 > (Sc, Cr, Zr, Sr, Ba, Y, Ta, TR)0,1-0,4 > (Cs, Nb; Ag, Te, Bi, Au)< 0,01-0,1.
The main geochemical barrier in supergene Buruktal nickel deposit is oxygen oxidized barrier in upper ferrous-oxide zone of the deposit. It makes sharp decrease of chemical element migration. Nevertheless, ore mineral concentrations present oft in complex geochemical barriers: absorbed-oxidized, carbonate-reducing and others. Every type of the geochemical barriers is able to concentrate specific association of migrated elements. That reflected on the different values of coefficients of enrichment in different types of Buruktal metasomatites. Oxidized barrier is more effective for elements with different valence (Fe, Mn), and absorbed clay, ferrous-oxide und manganese-oxide barriers are more effective for the main part of microelements.
Tectonic fractures of meridian spread, masked by block system of neotectonic breaks, play the leading role in structural control of nickel mineralization in supergene nickel deposits in the Urals. The deposits have long-term genesis and polygenic character. They are characterized by intensive tectonic and hydrothermal workup of Paleozoic substrate and block structure with small amplitude of vertical displacement. All of them have a two-floor structure, where upper supergene floor has a «background» of lower hydrothermal floor. This fact considerably increases the field of geological prospecting and searching of new oxide-silicate nickel deposits in the Uralian region.
In the Buruktal supergene nickel deposit, iron oxides possess vertical mineralogical zoning (bottom-up): magnetite-maghemite-goethite-hematite. The main rock- and ore-forming mineral in the iron-oxide zone of the deposit is magnetite, presented by three generations: primary relic magnetite, surviving from ultramafic rocks; secondary magnetite, forming at serpentinization process and neogenic supergene magnetite. Supergene magnetite, like a goethite, is nickel ore mineral, containing about 1 % NiO. Under the complex thermal analysis data, maghemite-magnetite and goethite have two main diagnostic maximums: exothermal effect of magnetite, caused by magnetite oxidation to maghemite in the interval 317‑340 °С, displays maximum at 327 °С («magnetite» point), and endothermic effect of goethite, connected with loss of constitutional water of the mineral and its transition to hematite in the interval 269‑296 °С, displays maximum at 288 °С («goethite» point).
