Fluid-Induced Formation of Podiform Ophiolitic Chromitite: Implications from Inclusions and Isotope Systematics, Kempirsai Massif, Urals
Frank Melcher Institute of Geological Sciences, Mining University, A-8700 Leoben, Austria
Walter Grum Institute of Geological Sciences, Mining University, A-8700 Leoben, Austria
The southern part of the Kempirsai ultramafic massif in the southeastern Urals contains exceptionally large orebodies of podiform chromite. They represent part of a 400 Ma old ophiolitic assemblage which has been obducted over sequences of the Russian craton during the Variscan orogeny (Melcher et al., 1994, 1995). Giant orebodies (up to 150 m thick and 1500 m long) of Al-poor chromite containing 55-60 wt.% Cr2O3 occur in depleted mantle rocks (harzburgite) and are characteristically enveloped by dunite halos of variable thickness. Chromite orebodies are mainly concordant to the foliation in harzburgite, and are aligned along two parallel zones in a NNE-SSW direction over a distance of more than 25 km.
Chrome spinel in the orebodies is characterized by 100Cr/Cr+Al from 77 to 84, 100Mg/Mg+Fe from 55 to 85, low Ti, and elevated Fe3+ contents. Serpentine minerals and brucite generally are matrix minerals formed during extensive serpentinization processes. In small pods and veins, chromite is hosted by Cr-rich amphibole and secondary chlorite. Chromite carries a number of solid inclusions of less than 1 up to 100 µm size: silicate minerals are most abundant (pargasite, forsterite, diopside, enstatite, sodium phlogopite, uvarovite, antigorite), followed by platinum-group minerals, and base metal sulfides and arsenides. Inclusions may be either monophase or polyphase. Mg, Cr, and Ni values in silicate inclusions are uniformly higher than in unaltered mantle rocks hosting the orebodies. Chromite-olivine geothermometry and oxygen fugacity barometry indicate temperatures of formation of 850 to 1080°C and DfO2 (FMQ) of +2.9 to +3.9 in the orebodies.
Fluid inclusions are rare in massive and in disseminated chromite. Chromites in amphibole-chromite rocks in places contain numerous, primary and secondary, fluid inclusions up to 50 µm in size which form trails or occur as solitary inclusions. Fluids are moderately saline (2-15 wt.% NaCl equ.) aqueous fluids and homogenize between 302 and 314°C (V->L). Mass spectrometric gas analyses of small pieces of crushed chromite revealed high contents of water (27-96 mol.%), besides H2 (3-70 %), CO2 (<1.5 %), CH4 (<0.2 %), N2 (<1 %), and less than 0.2 wt.% other gases such as C2H6, higher hydrocarbons, and a sulfur species.
A number of isotopic systems are used to describe the chemical conditions of chromite-, silicate, and PGM formation. These comprise: (1) Sm-Nd, (2) Rb-Sr, (3) Re-Os,
(4) Ar-Ar, (5) oxygen, and (6) hydrogen isotopes. An Ar-Ar cooling age of 385.4±9.2 Ma was obtained for amphibole coexisting with chromite.
Transformation of harzburgitic mantle to dunite (Johan, 1986) in a mantle wedge above a subducted slab was induced by dehyration of subducted, serpentinized oceanic crust releasing Na, Ca, and H2O into a metasomatizing fluid phase; this took place in an extensional regime underneath a back-arc basin. Chromium was liberated from orthopyroxene in an oxidizing process, transported as complex ions, and focussed into structurally favorable zones, e.g. shear zones. Precipitation of massive chromitite was then induced by the appearance of a reducing (e.g., hydrogen-bearing) fluid phase, causing reduction of Cr-bearing complexes. Formation of hydrous, and Na-Ca-rich silicates preceded and accompanied chromite crystallization, as did the formation of Ru-Os-Ir bearing alloys and sulfides.
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