Chromite Paleoplacer in the Permian Sediments at the East Edge of the East European Platform: Composition and Potential Sources

Abstract

A chromite occurrence called the Sabantuy paleoplacer was discovered in the Southern Pre-Ural region, at the east edge of the East-European Platform in the transitional zone to the Ural Foredeep. A ca. 1 m-thick chromite-bearing horizon is traced at a depth of 0.7–1.5 m from the earth’s surface for the area of ca. 15,000 m2. The chromspinel content in sandstones reaches 30–35%, maximum values of Cr2O3 are 16–17 wt.%. The grain size of detrital chromspinel ranges from 0.15 to 0.25 mm. Subangular octahedral crystals dominate; rounded grains and debris are rare. The composition of detrital chromspinel varies widely and is constrained by the substitution of Al3+ and Cr3+, Fe2+ and Mg2+ cations. Chemically, low-Al (Al2O3 = 12 wt.%) and high-Cr (Cr2O3 = 52–56 wt.%) chromspinel prevail. The compositional analysis using discrimination diagrams showed that most chromites correspond to mantle peridotites of subduction settings. Volcanic rocks could be an additional source for detrital chromites. It is confirmed by compositions of monomineralic, polymineralic and melt inclusions in chromspinels. The presented data indicates that ophiolite peridotites and related chromite ore associated with oceanic and island-arc volcanic rocks, widespread in the Ural orogen, could be the main sources of the detrital chromspinel of the Sabantuy paleoplacer.

Chromian Spinels from Kazanian-Stage Placers in the Southern Pre-Urals, Bashkiria, Russia: Morphological and Chemical Features and Evidence for Provenance

“…The most popular among these diagrams are Al 3+ -Cr 3+ -Fe 3+ , Cr#-Mg#, Cr#-Fe#, Al 2 O 3 -TiO 2 , TiO 2 -Fe 2+ /Fe 3+ [60,61]. Analysis of geochemical signatures of detrital chromian spinels using these discrimination diagrams is a critical tool for interpretation of sources for terrigenous sediments [21,[66][67][68].…”

“…The upper sedimentary cover of EEP is composed of Middle Permian sediments in almost all of the Southern Pre-Urals. Among the sediments, those of the Kazanian stage (corresponding with the Late Roadian and Wordian in International Chronostratigraphic Chart) [21] deserve particular attention. The Sabantuy chromite paleoplacer has been recently found in Kazanian-stage sandy sediments in the Southern Pre-Urals.…”

“…The Sabantuy chromite paleoplacer has been recently found in Kazanian-stage sandy sediments in the Southern Pre-Urals. Ophiolitic and volcanogenic complexes of the Southern Urals are considered the main source for this placer [21,74].…”

“…The Sabantuy paleoplacer was discovered first and studied most comprehensively [21,74]. The ore bed has the stable thickness (0.9-1.0 m) and the greatest area (16,500 m 2 , 330 m long and 50 m wide).…”

“…The indicative role of chromian spinel is associated with its varying chemical composition dependent on the magma composition and P-T-fO 2 conditions of crystallization [9,[12][13][14][15][16][17][18][19]. High resistance of chromian spinel to supergene corrosion and mechanical wear determines its accumulation in placers [20,21].…”