Accumulation zone of spinel peridotite mantle nodules enclosed in Cenozoic alkali basalts of the Hannuoba basalt field, which also contain abundant clinopyroxene xenocrysts up to 10 cm in size. The mine also works adjacent placers.

The alkaline basalt and tholeiite basalt in the area are transitionally interbedded, and no ultramafic xenoliths have been observed at the ground surface. The basalt contains a considerable number of ultramafic/mafic inclusions with yellow, olive-green, and grass-green olivine. Generally, the high-quality of olivine mainly occurs in the ultrabasic/basic rock bodies in the alkaline basalt area.
Olivine is mainly found in lherzolite xenoliths and spinel-bearing lherzolite xenoliths with porphyritic-like structures. Most of these xenoliths range from 50 to 100 mm in diameter, with individual xenoliths reaching 50 cm. The olivine-bearing zones are lenticular, layered, and irregular in shape, ranging from a few metres to several hundred metres in length. However, gem-grade olivine exhibiting granular texture and a grain size of at least 4–7 mm is characterised by a vitreous to greasy lustre, high transparency with fewer internal impurities and fractures, and good processing performance. Clinopyroxene generally shows an emerald-green colour, and orthopyroxene exhibits a yellowish-brown colour. Internal cracks are locally present in the gem-grade olivine (discoid cracks like “Lily pad”), resulting in film interference, which can be seen as strong inclusion evidence for the identification of olivine.

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References