Composition and Textual Characterization of Platinum-group Mineral(PGM) in Chromites from Zedang Ophiolite in Tibet, China

Platinum-group elements （PGE） are mainly concentrated in some specific minerals known as PGMs, which commonly occur in podifbrm chromites of ophiolites. In-situ PGM assemblages in chromites can provide valuable information on the physico-chemical nature of the parental melt（s） from which chromitite crystallized （Melcher et al., 1997）, and used as a significant petrogenetic indicator.

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,

How Depth is the Tibetan Chromite: Experimental Study

Podiform chromitites are generally lenticular bodies of massive to disseminated chromite typically hosted in mantle peridotites in ophiolites. Most such chromitites are surrounded by envelopes of dunite, a few centimeters to a few meters wide. Based on their textures and compositions, podiform chromitites have been interpreted as magmatic rocks formed as partial melting of mantle peridotite under low-pressure conditions in the upper mantle or by magma mingling and melt-rock reaction.

Distinctive melt activity and chromite mineralization in Luobusa and Purang ophiolites, southern Tibet: constraints from trace element compositions of chromite and olivine

To investigate the factors controlling the mineralization in ophiolites we systematically compared the petrology and mineral compositions of the harzburgites/lherzolites, dunites and chromitites in the Luobusa and Purang ophiolites. Generally, the petrological features and trace element compositions of chromite and olivine in peridotite and chromitite are distinctly different between the two ophiolites.In Luobusa, boninitic melts are inferred to have interacted with the harzburgites and modified the distributions of some trace elements(e.g., Ni, Mn and V) in chromite and olivine. The subsequently formed dunites and chromitites experienced significant elemental exchange. In contrast, the Purang ophiolite contains a wider range of chromitite compositions and records diverse melt activities, such as the growth of relatively abundant secondary clinopyroxene. The metasomatic melts were enriched in Al and depleted in Si, Na and highly incompatible trace elements(e.g., Nb, Zr). Such melts resemble MORBlike melts proposed in the literature but are assumed to be more hydrous than typical MORB because of presence of hydrous minerals. The parental magmas of the Purang dunites and intermediate chromitites are inferred to be compositionally intermediate between boninitic and MORB-like melts. In addition,the more refractory nature of the Luobusa harzburgites facilitated a high Cr concentration gradient with the interacting melts, making it easier to increase Cr in the melts. Crystallization of clinopyroxene and amphibole in the Purang ophiolite accommodated significant amounts of Cr and water, respectively,and negatively affected Cr concentration and chromite crystallization. The concentration of chromite to form chromitites requires the presence of focused melt channels.