Chromite-PGM Mineralization in the Lherzolite Mantle Tectonite of the Kraka Ophiolite Complex (Southern Urals, Russia)

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.

Origin of Ru-Os Sulfides from the Verkh-Neivinsk Ophiolite Massif (Middle Urals, Russia): Compositional and S-Os Isotope Evidence

“…The composition of the laurite-erlichmanite series, plotted on the Os-Ir-Ru diagram (Figure 7a), shows common Os substitution for Ru (Ru# between 12 and 89), typical of mantle chromitites from Kempirsai and Rai-Iz (Urals), Kraubath, Eastern Alps, and Unst, Shetland Isles [5,9,11,84]. A high-temperature origin of euhedral inclusions of laurite type 2 in Os-Ir-(Ru) alloys are supported by recent experimental data [18][19][20][21] that quantitatively evaluated the effects of T and f (S 2 ) for laurite + alloy mineral pairs.…”

“…Dominance of Ru-Os-Ir minerals over other platinum-group minerals (PGM) is considered a typical feature of podiform chromitites within the residual mantle in ophiolite-type dunite-harzburgite massifs worldwide [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15], among others. According to equilibrium phase diagrams, experimental results, and empirical data, Ru-Os-Ir alloys and Ru-Os sulfides are considered to be formed during the very early stages of magmatic differentiation, under low fugacity of sulfur, and high-temperature conditions [11,[16][17][18][19][20][21][22][23].…”

Mineralogy and mineral chemistry of detrital platinum-group minerals and gold particles from the Elbe, Germany

“…Native osmium has also been described in the Guli ultramafic massif in northern Siberia (Malich and Lopatin, 1997;Malitch et al, 2002;Merkle et al, 2012), in ophiolites and Uralian-Alaskan-type complexes (Garuti et al, 1999;González-Jiménez et al, 2009;Melcher et al, 1997;Zaccarini et al, 2005Zaccarini et al, , 2018. Native osmium also been described from placer deposits such as the Witwatersrand in South Africa (Cousins, 1973;Feather, 1976;Malitch and Merkle, 2004;Merkle and Franklyn, 1999) and from various other placers worldwide (Barkov et al, 2018Cabri et al, 1996;Stepanov et al, 2019;Weiser, 2002;Weiser and Bachmann, 1999).…”

Research on the Production of Pigments Based on Composite Pellets in the Recycling of Industrial Waste

“…The chromite ore of the Kempir say massif is a fine-grained rock with a massive texture. Porphyritic crystals of amphibole and laths of plagioclase stand out against the fine-grained background of the main ore mass [20,21].…”