Discovery and Significance of Diamonds and Moissanites in Chromitite within the Skenderbeu Massif of the Mirdita Zone Ophiolite,West Albania

In recent years diamonds and other unusual minerals （carbides, nitrides, metal alloys and native elements） have been recovered from mantle peridotites and chromitites （both high-Cr chromitites and high-Al chromitites） from a number of ophiolites of different ages and tectonic settings. Here we report a similar assemblage of minerals from the Skenderbeu massif of the Mirdita zone ophiolite, west Albania. So far, more than 20 grains of microdiamonds and 30 grains of moissanites （SIC） have been separated from the podiform chromitite. The diamonds are mostly light yellow, transparent, euhedral crystals, 200-300μm across, with a range of morphologies; some are octahedral and cuboctahedron and others are elongate and irregular. Secondary electron images show that some grains have well-developed striations. All the diamond grains have been analyzed and yielded typical Raman spectra with a shift at -1325 cm^-1. The moissanite grains recovered from the Skenderben chromitites are mainly light blue to dark blue, but some are yellow to light yellow. All the analyzed grains have typical Raman spectra with shifts at 766 cm^-1, 787 cm^-1, and 967 cm^-1. The energy spectrums of the moissanites confirm that the grains are composed entirely of silicon and carbon. This investigation expands the occurrence of diamonds and moissanites to Mesozoic ophiolites in the Neo-Tethys. Our new findings suggest that diamonds and moissanites are present, and probably ubiquitous in the oceanic mantle and can provide new perspectives and avenues for research on the origin of ophiolites and podiform chromitites.

Diamonds Discovered from High–Cr Podiform Chromitites of Bulqiza,Eastern Mirdita Ophiolite,Albania

Various combinations of diamond, moissanite, zircon, corundum, rutile and titanitehave been recovered from the Bulqiza chromitites. More than 10 grains of diamond have been recovered, most of which are pale yellow to reddish–orange to colorless. The grains are all 100–300 μm in size and mostly anhedral, but with a range of morphologies including elongated, octahedral and subhedral varieties. Their identification was confirmed by a characteristic shift in the Raman spectra between 1325 cm-1 and 1333 cm-1, mostly at 1331.51 cm-1 or 1326.96 cm-1. This investigation extends the occurrence of diamond and moissanite to the Bulqiza chromitites in the Eastern Mirdita Ophiolite. Integration of the mineralogical, petrological and geochemical data of the Bulqiza chromitites suggests their multi–stage formation. Magnesiochromite grains and perhaps small bodies of chromitite formed at various depths in the upper mantle, and encapsulated the ultra–high pressure, highly reduced and crustal minerals. Some oceanic crustal slabs containing the magnesiochromite and their inclusion were later trapped in suprasubduction zones, where they were modified by tholeiitic and boninitic arc magmas, thus changing the magnesiochromite compositions and depositing chromitite ores in melt channels.

Petrology and PGE Abundances of High-Cr and High-Al Podiform Chromitites and Peridotites from the Bulqiza Ultramafic Massif, Eastern Mirdita Ophiolite, Albania

The Bulqiza ultramafic massif, which is part of the eastern Mirdita ophiolite of northern Albania, is world renowned for its high-Cr chromitite deposits. High-Cr chromitites hosted in the mantle section are the crystallized products of boninitic melts in a supra-subduction zone(SSZ). However,economically important high-Al chromitites are also present in massive dunite of the mantle-crust transition zone(MTZ). Chromian-spinel in the high-Al chromitites and dunites of the MTZ have much lower Cr~# values(100 Cr/(Cr+Al))(47.7-55.1 and 46.5-51.7, respectively) than those in the high-Cr chromitites(78.2-80.4), harzburgites(72.6-77.9) and mantle dunites(79.4-84.3). The chemical differences in these two types of chromitites are reflected in the behaviors of their platinum-group elements(PGE).The high-Cr chromitites are rich in IPGE relative to PPGE with 0.10-0.45 PPGE/IPGE ratios, whereas the high-Al chromitites have relatively higher PPGE/IPGE ratios between 1.20 and 7.80. The calculated melts in equilibrium with the high-Cr chromitites are boninitic-like, and those associated with the high-Al chromitites are MORB-like but with hydrous, oxidized and TiO-poor features. We propose that the coexistence of both types of chromitites in the Bulqiza ultramafic massif may indicates a change in magma composition from MORB-like to boninitic-like in a proto-forearc setting during subduction initiation.

An Overview of the Albanian Ophiolite and Related Ore Minerals

Albanian ophiolite （or Mirdita ophiolite） represents a remnant of the Tethyan oceanic lithosphere in the territory of Albania, as an elongated belt from south to the north, about 250 km long and 30-50 km wide. It covers an area of about 4300 km2 or 1/7 of the territory of Albania and is characterized by a relatively good up to the excellent outcropping.

Petrology of Cr-rich Podiform Chromitites of Bulqiza, Eastern Ophiolitic Belt, Albania

The ultramafic massif of Bulqiza, which belongs to the eastem ophiolitic belt of Albania, is a major source of metallurgical chromitite ore. The massif consists of a thick （〉4 km） sequence, composed from the base upward of tectonized harzburgite with minor dunite, a transitional zone of dunite, and a magmatic sequence of wehrlite,

Petrology of Cr-rich Ophiolitic Chromitites of Bulqiza, Eastern Ophiolitic Belt, Albania

The ultramafic massif of Bulqiza, which belongs to the eastern ophiolitic belt of Albania, is the most important area for metallurgical chromitite ores. The massif consists of a thick （〉4 km） rock sequence, with a generalized profile from the bottom to the top as follows. The tectonite sequence composed mainly of harzburgites with subordinate intercalations of dunites,