We provide results of a comprehensive mineralogical and microstructural study of relict lherzolites of the main ore field and fresh rocks from a deep structural borehole drilled in the south-eastern part of the Kempirsay massif. Olivine and orthopyroxene from lherzolites contain numerous pieces of evidence of material redistribution at different scales caused mainly by solid-state processes, such as plastic flow of mantle, syntectonic recrystallization, and annealing. The results of deformation-induced processes at the submicron scale are recorded by optical and electronic microscopy. In olivine, the plastic deformation caused segregation of impurities at structural defects. As a result, abundant tiny rods of newly formed Cr-spinels occurred inside its grains. Moreover, in enstatite, deformation caused partial or complete chemical decomposition with exsolution of diopside, pargasite and spinel lamellae up to the formation of a “fibrous” structure. In other cases, it provided partial or complete recrystallization to form new phases of enstatite-2, forsterite, diopside, pargasite, and spinel. Petrographic observations are validated by geochemical data, i.e., regularly decreasing concentrations of minor elements in neoblasts compared to large grains (porphyroclasts). Further redistribution of spinel grains with the formation of chromitite bodies is witnessed by their permanent association with the most mobile phase of the upper mantle, i.e., olivine, which is the only mineral that remains stable under the intense plastic flow. An increased concentration of Cr-spinel grains during formation of massive chromitites could appear under conditions close to pressure sintering, as evidenced by stressed textures of ores and an increased grain size compared to disseminated chromitites. The formation of unique chromitite deposits is associated with integration of numerous disparate podiform bodies into “ore bunches” due to the tectonic impact in the shear-compression regime. This was most likely associated with transition of the rifting (spreading) regime to that of the upper mantle of the fore-arc basin.
The paper provides results of a detailed mineralogical study of some chromitite ores from two deposits in the Southern Urals of Kazakhstan: Almaz-Zhemchuzhina and Geofizicheskoe-VII. It is revealed that the main ore minerals are Cr-spinel with high Cr# (Cr/(Cr + Al) = 0.8–0.83), as well as serpentine and chlorite, replacing primary olivine. Chromium spinel grains contain mineral inclusions, which are distributed rather unevenly. The most common mineral inclusions are olivine (serpentine) and amphibole; phlogopite, pyroxenes, and base metal sulfides are rare. Olivine from inclusions in chromite is the highest in magnesium (Fo97–98), and is anomalously high in nickel (up to 1.8 wt.% NiO). The closure of exchange reactions between olivine and chromite occurred in the temperature range of 700–850 °С and in the oxygen fugacity range of −1.04 … +2.8 ΔFMQ, which most likely corresponds to the upper mantle settings of the fore-arc basin. A few tens of monomineral grains and polymineral intergrowths of platinum group minerals (PGMs) were found in chromite aggregates. Notably, monomineral grains are mainly represented by Ru, Os, and Ir disulfides, while in polymineral inclusions, iridium prevails (with widespread native phases, sulfides, and sulfoarsenides). PGM grains included in chromite are often associated with hydrous silicates: amphibole, and less often with phlogopite or chlorite. Discussed in the paper is the possible genesis of ores and inclusions. As a preliminary conclusion, we suggest that the solid-phase processes played the most significant role in the crystallization of Cr-spinel in the investigated chromitite ores.
In the paper data of morphology, textural and structural features of chromitites from deposits of south-east part of Kempirsay massif (South Urals, Kazakhstan)are summarized. It is showed that formation of unique chromium deposits is closely related with formation processes of wall dunite-harzburgite association and that chromitite localization occur abidingly in olivine monomineralic rock – dunite. Superimposed low-T processes altered primary mineralogical composition of wall peridotites completely but these affected weakly their structure on the micro and macro scale. Mesh serpentine replaced olivine and pyroxene grains but pseudomorphosis of both are survive. Addition, significant displacements of mineral aggregates in the massive peridotite blocks are not observed and it allow to study textural and structural characteristic of chromitites and primary wall ultramafic rocks. We have found some major features of building of ore-bearing associations as follow: (i) increasing chromite grain size according to increasing concentration of chromite, (ii) widespread of deformational structures – ore folding and boudinage, extrusion of solid dunite into massive chromitite, break of ore veinlets. We have performed retrospective analysis of papers about Kempirsay chromitite which in present day are not available for wide readers. Based on this analysis and our observations, we propose a modified dynamic model of chromitite formation as result rheomorphic differentiation of upper mantle matter during its upwelling from deep zone of rift structure with later transformation in the upper mantle of fore-arc setting.
Solid inclusions in chromite grains of Geofizicheskoe VII deposit located in Dzharlybutak area hosting the largest ophiolitic chromitite resources in the world are described. We show that mineralogical composition of inclusions (olivine, serpentine, chlorite, pargasite, PGM, base metal sulfides) is more various in comparison to that of host ultramafic rocks (olivine, enstatite, serpentine). PGM are predominantly disulfides of refractory PGE, rarely – native Ru-Os-Ir solid solutions. PGM inclusions always locate in chromite grains and they often form intergrowth with chlorite. We infer that the inclusions in chromite grains would been formed by solid state processes (deformation-induced breakdown of pyroxenes, impurity segregation in spinel, etc.) during high-T plastic flow and recrystallization at upper mantle conditions.
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