Silicates from Lherzolites in the South-Eastern Part of the Kempirsay Massif as the Source for Giant Chromitite Deposits (the Southern Urals, Kazakhstan)

Abstract

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.

Genesis of the Sartohay Podiform Chromitite Based on Microinclusions in Chromite

“…The solid-phase mechanism [6,[59][60][61] provides a rational genesis model for silicate inclusions and host chromite grains. This hypothesis suggests that the formation of chromites is caused by plastic deformation of rock-forming silicates, as evidenced by typical petrographic facts like chromite, pyroxene, and pargasite lamellae or neoblasts in silicate matrix of peridotites.…”

“…Furthermore, the mechanism of chromite concentration as ore bodies is substantiated in the melt-peridotite reaction model insufficiently [90]. This has been interpreted by the rheomorphic model [7,8,61], which provides thermodynamic reasons for the compaction or sintering mechanism of chromitites. The lack of petrographical evidence for plastic deformation in Sartohay chromitites is possibly due to the unattained high pressure required for pressure sintering [8], which may have enabled the retention of primary mineral inclusions in the chromite.…”

Mantle peridotites of ophiolites rarely preserve reliable records of paleo-oceanic lithospheric mantle

“…Olivine in mantle peridotites of ophiolites has been extensively studied by numerous authors, particularly regarding deformation textures (e.g., Linckens et al, 2011aLinckens et al, , 2011bHiggie and Tommasi, 2012;Hidas et al, 2021;Saveliev et al, 2021Saveliev et al, , 2022Shao et al, 2021), serpentinization (e.g., Etiope et al, 2011;Frost et al, 2013;Huang et al, 2017a;Gahlan et al, 2018;Nozaka, 2018), and major and trace elemental compositions (e.g., Su et al, 2019;. Although our knowledge of these olivine crystals is still mostly restricted to high-temperature partial melting and low-temperature alteration, elemental distributions in these olivine grains might have been modified by sub-solidus elemental exchange with spinel or reaction with melts/ fluids (e.g., Zhou et al, 1996;Xiao et al, 2016;Rollinson, 2019;Su et al, 2019;Xiong et al, 2020a;Xu et al, 2023).…”