Xu‑Feng Hu

Diamonds and an extensive assemblage of associated minerals were discovered in podiform chromitites of the Luobusa ophiolite, southern Tibet, China. Thus far, 25 diamonds have been recovered from the heavy mineral separates of samples collected in 1996.

All the diamonds are colourless and transparent. Most of them are euhedral crystals, showing sharp-edged octahedral morphology. Others are broken fragments. Euhedral crystals are 150 x 150 µm to 400 x 400 µm in size, whereas broken fragments range from 200 x 250 µm to 900 x 1000 µm. Both Raman spectra and X-ray diffraction techniques have been used to confirm the identification of the diamonds. One diamond fragment contains 3 discrete silicate mineral inclusions, which have Raman spectra patterns most similar to serpentine. One of the inclusions was analysed with the electron microprobe, and has an unusual composition, high in MgO (30.7 wt%) and SiO₂ (64.1 wt%). Its formula best fits that of clinoenstatite. SEM images show that the inclusion possesses an octahedral morphology typical of that imposed on many syngenetic inclusions by contemporaneous growth of the surrounding diamond.

Minerals associated with the diamonds include chromite, forsterite, enstatite, Cr-diopside, PGE minerals, graphite, SiC (moissanite), gehlenite, Si-Fe and Cr-C alloys, zircon, sphene, rutile, apatite, corundum, sillimanite, plagioclase, K-feldspar, amphibole, biotite, phlogopite, chlorite, serpentine, sulphides, carbonites, celestite, uvarovite, almandine, wollastonite, quartz, and Fe-Ni and Au-Ag alloys. All the minerals were recovered from heavy mineral separates and confirmed by either electron microprobe or X-ray diffraction studies. Most of the chromites are rich in magnesian and chromium, with Cr#s [100 Cr/(Cr+Al)] ranging from 77 to 84 and Mg#s [100 Mg/(Mg + Fe)] between 62 and 76, suggesting crystallisation from a boninitic melt. Olivines have Fo contents ranging from 91 to 98%, with NiO contents varying from 0.3 to 1.35 wt%, correspondingly. Enstatites contain 88-95% En end-member, with Cr₂O₃ contents varying from 0.15 to 0.90 wt%. Chromium diopsides show uniform compositions (En 46-48%, Wo 48-50% and Cr₂O₃ 0.99-1.75 wt%). SiC crystals are 0.1 to 1.1 mm in size, and transparent (if not deeply coloured) with a strong brilliant adamantine luster. Colours range from colourless to grey-blue to pale green to yellow to yellow-blue to bluish-green to blue-black. Many SiC grains are colour-zoned with graduations between zones. The Cr-C alloy is steel gray and displays well-developed acicular form. It has a Cr/C ratio of 1:1. The Si-Fe grains large up to 1 mm in size are black, very shiny, and fractured. The Fe/Si ratio averages 2.76/7, very similar to ferrosilicite (Fe3Si7) inclusions in SiC from the Yakutia kimberlite. The hydrous minerals are Ti rich and distinct in composition from those of secondary origin in Oman chromitites.

The sampling and separation procedures were designed to minimize any possibility of natural or anthropogenic contamination of the samples. The well-developed crystal morphology of diamond, SiC (moissanite) and Cr-C alloys signifies that they crystallized from melts/fluids, and they are interpreted as original phases in the Luobusa chromitites. The well-preserved character of the diamonds indicates they have not undergone any resorption after formation. Based on the available data, 2 models are proposed for formation of the Luobusa diamonds: they may have formed metastably in a superasubduction zone environment where the chromitites formed, or they may be xenocrysts formed at greater depths and later incorporated into the chromitites.

Keywords:
Pages:
Supervisor: Paul Robinson