The Discovery of Diamonds in Chromitites of the Hegenshan Ophiolite, Inner Mongolia, China
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Abstract Diamond, moissanite and a variety of other minerals, similar to those reported from ophiolites in Tibet and northern Russia, have recently been discovered in chromitites of the Hegenshan ophiolite of the Central Asian Orogenic Belt, north China. The chromitites are small, podiform and vein‐like bodies hosted in dunite, clinopyroxene‐bearing peridotite, troctolite and gabbro. All of the analysed chromite grains are relatively Al‐rich, with Cr # [100Cr/(Cr+Al)] of about 47–53. Preliminary studies of mainly disseminated chromitite from ore body No. 3756 have identified more than 30 mineral species in addition to diamond and moissanite. These include oxides (mostly hematite, magnetite, rutile, anatase, cassiterite, and quartz), sulfides (pyrite, marcasite and others), silicates (magnesian olivine, enstatite, augite, diopside, uvarovite, pyrope, orthoclase, zircon, sphene, vesuvianite, chlorite and serpentine) and others (e.g., calcite, monazite, glauberite, iowaite and a range of metallic alloys). This study demonstrates that diamond, moissanite and other exotic minerals can occur in high‐Al, as well as high‐Cr chromites, and significantly extends the geographic and age range of known diamond‐bearing ophiolites.Keywords:
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The Oman ophiolite is one of the largest and best exposed ophiolites in the world and has >450 chromitite deposits. We report here a newly identified chromitite deposit in the Wadi Rajmi, Oman. This deposit occurs within a dunitic envelope, which is surrounded by harzburgite, and consists of both massive and disseminated chromitite types. The Rajmi peridotites represent depleted upper mantle rocks that underwent >20% partial melting and experienced metasomatism by melts and fluids derived from a subducting slab. They demonstrate geochemical affinities similar to those of the Izu–Bonin–Mariana forearc peridotites, supporting their formation in a forearc environment. The Rajmi chromitites have low Cr# values and are classified as high-Al chromitites. They have geochemical compositions comparable with those of chromitites crystallized from mid-ocean ridge basalt (MORB)-type melts. However, the chromites in these high-Al chromitites contain various silicate inclusions (e.g. amphiboles and micas), indicating the hydrous and atypical MORB nature of their parental magmas. Combined with the mineralogical and geochemical characteristics of the country rocks, we posit that the parental melts of the Rajmi high-Al chromitites have a MORB-like affinity derived from partial melting of a nascent forearc mantle. Supplementary material: The whole-rock and mineral data of the Rajmi harzburgites, dunites and chromitites are available at https://doi.org/10.6084/m9.figshare.c.6795689 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists
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The Tunceli Ophiolite, which represents the eastern part of the Izmir-Ankara-Erzincan Suture Belt (IAESB), hosts several chromite mines. The petrological characteristics of these deposits and their host rocks were investigated here for the first time. This ophiolite includes serpentinized peridotites, harzburgites, dunites, gabbros, sheeted dykes, pillow lavas and mudstones. The chromite occurrences are hosted in the mantle peridotites. The spinel group minerals in the ore and in ultramafic samples were sampled in seven different chromite mines (Yildirim, Aksu, Hasangazi, Atilla, Eskigedik, Isikvuran and Oymadal). The chromitite spinels display podiform characteristics. The Cr# = (0.39-0.89) and Mg# = values (0.43-0.74 ) of these spinel group minerals mainly correspond to magnesiochromite and chromite compositions. The silicate assemblage of the cpx-poor harzburgites include Mg-rich olivine (Fo91), enstatite and diopside. The occurrence of Ti-enriched secondary clinopyroxenes likely reflect refertilization processes which may have originated by interaction between Ti-rich melts and highly depleted peridotites. The average Cr2O3 (15.46 %wt) and ΣPGE (92.10 pbb) contents of the chromitites from the Tunceli ophiolite are lower than those of other chromitite-bearing ophiolites in Turkey. The consistent geochemical and mineral chemistry data from the mantle peridotites and chromitites within the Tunceli Ophiolite suggest formation in an intra-oceanic back-arc basin which may have been active during the closure of northern branch of the Neotethyan ocean.
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