Abstract
The mantle tectonite of the Kraka ophiolite contains several chromite deposits. Two of them consisting of high-Cr podiform chromitite—the Bolshoi Bashart located within harzburgite of the upper mantle transition zone and Prospect 33 located in the deep lherzolitic mantle—have been investigated. Both deposits are enveloped in dunite, and were formed by reaction between the mantle protolith and high-Mg, anhydrous magma, enriched in Al2O3, TiO2, and Na2O compared with boninite. The PGE mineralization is very poor (<100 ppb) in both deposits. Laurite (RuS2) is the most common PGM inclusion in chromite, although it is accompanied by erlichmanite (OsS2) and (Ir,Ni) sulfides in Prospect 33. Precipitation of PGM occurred at sulfur fugacity and temperatures of logƒS2 = (−3.0), 1300–1100 °C in Bolshoi Bashart, and logƒS2 = (−3.0/+1.0), 1100–800 °C in Prospect 33, respectively. The paucity of chromite-PGM mineralization compared with giant chromite deposits in the mantle tectonite in supra-subduction zones (SSZ) of the Urals (Ray-Iz, Kempirsai) is ascribed to the peculiar petrologic nature (low depleted lherzolite) and geodynamic setting (rifted continental margin?) of the Kraka ophiolite, which did not enable drainage of the upper mantle with a large volume of mafic magma.
Highlights
Podiform chromitite associated with ophiolites ranks second among sources of chromite for industrial use, after the Bushveld-type stratiform deposits
Two of them consisting of high-Cr podiform chromitite—the Bolshoi Bashart located within harzburgite of the upper mantle transition zone and Prospect 33 located in the deep lherzolitic mantle—have been investigated
There are, significant differences in total PGE content and platinum group mineral (PGM) mineralogy between ophiolitic chromite deposits, showing total PGE concentrations up to more than 1 ppm or low PGE contents not exceeding a few hundred ppb. The reasons for this discrepancy have been matter of debate and are not completely understood, they are seemingly related to the different thermodynamic conditions of the chromite forming system. In this contribution we present the unusual occurrence of high-Cr, low-Ti podiform chromitite in the mantle tectonite of the lherzolite-ophiolite-type (LOT) of Kraka (Southern Urals, Russia)
Summary
Podiform chromitite associated with ophiolites ranks second among sources of chromite for industrial use, after the Bushveld-type stratiform deposits. Economic chromite deposits occur preferentially in the mantle sections of harzburgite-ophiolite-type complexes (HOT) ([18,19] and references therein) They are characterized by high-Cr and low-Ti composition, and are interpreted as the results of metasomatic reactions between residual mantle and percolating boninite magma [20] in supra-subduction-zones (SSZ). The reasons for this discrepancy have been matter of debate and are not completely understood, they are seemingly related to the different thermodynamic conditions of the chromite forming system In this contribution we present the unusual occurrence of high-Cr, low-Ti podiform chromitite in the mantle tectonite of the lherzolite-ophiolite-type (LOT) of Kraka (Southern Urals, Russia). The mineral chemistry of chromite, and the compositions of accessory silicates and PGM inclusions have been studied and compared with analogous deposits of the Urals in order to identify the metallogenic processes that led to the formation of high-Cr chromitite in a poorly depleted mantle slab
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