Abstract

The diamondiferous Karashoho pipe was discovered in the Bukantau Mountains, Uzbekistan, in the 1990s and evaluated from 2002 to 2007. It is an elongate, twinned, subvertical-dipping body with an approximate surface expression of 530 × 65–155 m (~45,600 m2). Its northern part is composed of eruptive breccias and shoshonite, whereas the southern part is composed of micaceous shoshonite-absarokite-picritic rocks and shonkinite. The pipe is located within the southern Tian-Shan Hercynian fold system enclosed between the Karakum-Tajik Massif in the south and the middle Tian-Shan system to the north. The Rb-Sr isochron age of the pipe is 353 ± 13 Ma. By petrographic, mineralogical, and chemical compositions the Karashoho rocks belong to a potassic shoshonite-absarokite-picrite rock series, similar to Permian shoshonitic series rocks from the Tian-Shan volcanic belt in north Xinjang, China. However, they differ significantly from common shoshonites and absarokites because of the high magnesian index of rock-forming minerals in the pipe and the presence of some high-pressure mineral phases, such as chromian spinel (including high-chromian varieties of the diamond association) and manganoilmenite. This indicates that the rocks originated under high-pressure conditions. Bulk sampling of the Karashoho rocks yielded a diamond grade in the range of 0.1 to 5 cpht (carats per hundred tons), with the weighted-mean for different rock varieties ranging from 0.87 to 2.55 cpht. The highest grades are in eruptive breccias of the first generation and shoshonitic-absarokitic rocks. The Karashoho pipe is the first example of diamond potential in a shoshonite-absarokite-picrite rock series; and the first primary diamond deposit located within a Phanerozoic fold system.

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