Unusual mantle mineral group from chromitite orebody Cr-11 in Luobusa ophiolite of Yarlung-Zangbo suture zone, Tibet

Abstract

A wide variety of unusual mantle has been reported from podiform chromitite orebodies Cr-31 and Cr-74 in the Luobusa (罗布莎) ophiolite, Tibet. A detailed investigation of chromitite orebody Cr-11, located in the Kangjinla (康金拉) district at the eastern end of the ophiolite, has revealed many of the same minerals, including diamond, moissanite, and some native elements, alloys, oxides, sulphides, silicates, carbonates, and tungstates. This orebody is particularly rich in diamonds, with over 1 000 grains recovered from about 1 100 kg sample of chromitite. More detailed studies and experiments are needed to understand the origin and significance of these unusual minerals because they have not been found in situ. It is a great breakthrough in mineralogical research that we have picked up more than 40 kinds of minerals from the Kangjinla chromite deposit in Luobusa. It is notable that a large amount of diamonds were firstly discovered from the Kangjinla chromite deposit as well as many other unusual minerals, such as moissanites, rutiles, native irons, and metal alloys. Especially, that diamond was found again in different chromitites in the same ophiolite belt provided new key evidence for discussing the origin of the diamond and the hosted chromitite and ophiolite. The mantle mineral group in Tibet has great significance in mineralogy and geodynamics.

Petrogenesis of the Kangjinla peridotite in the Luobusa ophiolite, Southern Tibet

“…Diamonds and serpentine pseudomorphs after octahedral olivine were discovered many years ago in podiform chromitite of the Luobusa ophiolite (IGCAGS, 1981;Yang et al, 1981) and have been confirmed by subsequent workers (Hu, 1999;Bai et al, 2000a;Robinson et al, 2004;Yang et al, 2007Yang et al, , 2008Yang et al, , 2009. Similar UHP and highly reduced minerals have also been found recently in both the chromitites and peridotites of Luobusa, including coesite (possibly pseudomorphing stishovite) (Yang et al, 2007), kyanite, a diamond inclusion in Os-Ir alloy (Yang et al, 2007), diamond in peridotite , inclusions of silicon spinel in Os-Ir alloy (Bai et al, 2005), and reduced phases such as moissanite, wüstite, native elements and base-metal alloys (Hu, 1999;Bai et al, 2000bBai et al, , 2006Robinson et al, 2004;Fang et al, 2009;Shi et al, 2009;Xu et al, 2009). Diamond and moissanite have now been confirmed as insitu phases in the chromitites .…”

“…Abundant diamonds and moissanite have been recovered from mineral separates of the chromitites (Hu, 1999;Bai et al, 2001;Robinson et al, 2004;Xu et al, 2009). Diamonds and moissanite also occur in-situ as inclusion in chromite grains ).…”

“…These include moissanite (SiC), wüstite (FeO) and many native elements such as Si, Ti, Fe, Al, W, Cr, and Ni, along with a range of base-metal and PGE alloys (Bai et al, 2000a(Bai et al, , 2001(Bai et al, , 2005(Bai et al, , 2006Robinson et al, 2004;Yang et al, 2003Yang et al, , 2007Xu et al, 2008Xu et al, , 2009Fang et al, 2009;Shi et al, 2009). There is no evidence, either in the ophiolite or surrounding rocks, of a meteorite impact, so we conclude that the UHP minerals must have came from minimum depths of 120-280 km, and possibly deeper.…”

High-pressure highly reduced nitrides and oxides from chromitite of a Tibetan ophiolite

“…The material that makes up the subject of both ref. 8 and this article has been extracted from massive chromite ore 93 of the Luobusa ophiolite, located on the collisional boundary between Asia and India (13,14).…”

Timing of Secondary Hydrothermal Alteration of the Luobusa Chromitites Constrained by Ar/Ar Dating of Chrome Chlorites

“…These studies include: major and trace elemental and/or isotopic compositions of different textural chromitites and their host peridotites (e.g., [1][2][3][4][5][6][7][8]), and unusual ultrahigh-pressure (UHP), highly reduced and crustally derived minerals found either in situ or as separates in the chromitites and peridotites (e.g., [9][10][11][12][13][14][15][16][17][18][19][20][21]). [42]), and the distribution of magmatism on the Lhasa terrane (modified from Chung et al [43]).…”