Paul T. Robinson

Diamond Discovered in High-Al Chromitites of the Sartohay Ophiolite, Xinjiang Province, China

In recent years diamonds and other exotic minerals have been recovered from mantle peridotites and high-Cr chromitites of a number of ophiolites of different age and different tectonic environments. Here we report a similar collection of minerals from the Sartohay ophiolite of Xinjiang Province, western China, which is characterized by having high-Al chromitites. Several samples of massive podiform chromitite with an aggregate weight of nearly 900 kg yielded diamonds, moissanite and other highly reduced minerals, as well as common crustal minerals. Thus far, more than 20 grains each of diamond and moissanite have been recovered from heavy mineral separates of the chromitites. The diamonds are all 100–200 μm in size and range in color from pale yellow to reddish-orange to colorless. Most of the grains are anhedral to subhedral octahedra, commonly with elongate forms exhibiting well-developed striations. They all display characteristic Raman spectra with shifts between 1325 cm−1 and 1333 cm−1, mostly 1331.51 cm−1 or 1326.96 cm−1. The moissanite grains are light blue to dark blue, broken crystals, 50–150 μm across, commonly occurring as small flakes or fragments. Their typical Raman spectra have shifts at 762 cm−1, 785 cm−1, and 966 cm−1. This investigation extends the occurrence of diamonds and moissanite to a Paleozoic ophiolite in the Central Asian Orogenic Belt and demonstrates that these minerals can also occur in high-Al chromitites. We conclude that diamonds and moissanite are likely to be ubiquitous in ophiolitic mantle peridotites and chromitites.

The Discovery of Diamonds in Chromitites of the Hegenshan Ophiolite, Inner Mongolia, China

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.

Tectonic Evolution of the Western Yarlung Zangbo Ophiolitic Belt, Tibet: Implications from the Petrology, Mineralogy, and Geochemistry of the Peridotites

The northern subbelt in the western segment of the Yarlung Zangbo suture zone, Tibet, China, includes the Dajiweng, Kazhan, Baer, Cuobuzha, Jianabeng, and Zhalai ophiolitic massifs. These ophiolites are strongly dismembered, typically 1–2 km wide and 10–20 km long, and composed chiefly of peridotites with minor volcanic and siliceous sedimentary rocks. No cumulates have been observed in the northern ophiolitic belt. Harzburgites of the Dajiweng and Zhalai ophiolites have prominent light rare earth element (LREE)–enriched (U-shaped or spoon-shaped) chondrite-normalized rare earth element (REE) patterns. Such patterns have generally been interpreted as the result of modification by suprasubduction zone (SSZ) melts/fluids. However, the abundance of peridotites sampled from mid-ocean ridge with similar LREE-enriched REE patterns suggest that this feature is not unique to SSZ peridotites. The U-shaped REE patterns of the Dajiweng harzburgites, combined with their low heavy rare earth element (HREE) contents and their mineral chemistry, indicate that these rocks most likely have been modified by SSZ melts (e.g., boninitic melts) in a forearc setting. In contrast, the Zhalai harzburgites, which also have U-shaped REE patterns but are characterized by high HREE contents, high Al2O3/SiO2 ratios, low MgO/SiO2 ratios, and relatively fertile mineral compositions, most likely have been refertilized in a mid-ocean ridge setting. The Zhalai, Kazhan, Baer, and Cuobuzha peridotites are similar to abyssal and back-arc peridotites in mineral chemistry and whole-rock geochemistry. Combining the mafic intrusions from Jianabeng, Baer, and Cuobuzha massifs, we propose that the ophiolites in the northern belt of the western segments have been trapped in an intraoceanic forearc–arc–back-arc system. According to the zircon U-Pb age of mafic intrusions, the geochemical characteristics of both mafic and ultramafic rocks, a detrital zircon study of Zhongba terrane, and the klippen structure of ophiolitic massif in the southern belt, we conclude that the northern and southern ophiolitic belts were developed in the same intraoceanic subduction system.

Platinum‐group Mineral (PGM) and Base‐metal Sulphide (BMS) Inclusions in Chromitites of the Zedang Ophiolite, Southern Tibet, China and their Petrogenetic Significance

Voluminous platinum-group mineral（PGM） inclusions including erlichmanite（Os,Ru）S2, laurite（Ru,Os）S2, and irarsite（Ir,Os,Ru,Rh）As S, as well as native osmium Os（Ir） and inclusions of base metal sulphides（BMS）, including millerite（NiS）, heazlewoodite（Ni3S2）, covellite（CuS） and digenite（Cu3S2）, accompanied by native iron, have been identified in chromitites of the Zedang ophiolite, Tibet. The PGMs occur as both inclusions in magnesiochromite grains and as small interstitial granules between them; most are less than 10 μm in size and vary in shape from euhedral to anhedral. They occur either as single or composite（biphase or polyphase） grains composed solely of PGM, or PGM associated with silicate grains. Os-, Ir-, and Ru-rich PGMs are the common species and Pt-, Pd-, and Rh-rich varieties have not been identified. Sulfur fugacity and temperature appear to be the main factors that controlled the PGE mineralogy during crystallization of the host chromitite in the upper mantle. If the activity of chalcogenides（such as S, and As） is low, PGE clusters will remain suspended in the silicate melt until they can coalesce to form alloys. Under appropriate conditions of ?S2 and ?O2, PGE alloys might react with the melt to form sulfides-sulfarsenides. Thus, we suggest that the Os, Ir and Ru metallic clusters and alloys in the Zedang chromitites crystallized first under high temperature and low ?S2, followed by crystallization of sulphides of the laurite-erlichmanite, solid-solution series as the magma cooled and ?S2 increased. The abundance of primary BMS in the chromitites suggests that ?S2 reached relatively high values during the final stages of magnesiochromite crystallization. The diversity of the PGE minerals, in combination with differences in the petrological characteristics of the magnesiochromites, suggest different degrees of partial melting, perhaps at different depths in the mantle. The estimated parental magma composition suggests formation in a suprasubduction zone