Detrital clinopyroxene and amphibole from the Kazanian-Stage chromite-bearing sandstones in the South Pre-Urals as an indicator of chromian spinel source

Abstract

Introduction: chromite paleoplacers of Kazanian Age were found in the Southern Pre-Urals. The problem of the genesis of these placers is debatable, since they are distal and far away from the chromite-bearing complexes of the Ural Folded Belt. As sources of detrital chromites, not only ophiolitic, but also other (no ophiolitic) sources are assumed. In chromite-bearing sandstones, detrital silicate minerals were found, the study of which makes it possible to clarify the provenance. Methodology: rock samples were studied using optical (Carl Zeiss Axioskop 40) and electron (Tescan Vega Compact) microscopes. The chemical composition of minerals was determined in the EDS mode on a Tescan Vega Compact scanning electron microscope. Results and Discussion: A morphological and geochemical study of grains of detrital clinopyroxene and amphibole in chromite-bearing sandstones was carried out. Inclusions of clinopyroxene and amphibole in detrital chromian spinels were also studied. All grains of clinopyroxene have a consistent composition and correspond to low-titanium diopside. Most grains of detrital amphibole correspond to high magnesian calcium amphiboles of igneous origin. The compositions of the studied detrital clinopyroxene and amphibole on the classification and discrimination diagrams completely or almost completely overlap with the compositions of these minerals from ultrabasic-basic rocks of the Southern Ural ophiolites. Conclusion: It is concluded that during the formation of chromite placers in the Southern Pre-Urals, the ophiolitic source could be the only one in the source area. There is no need to attract additional sources of chromites.