The Shoshonite-Absarokite-Picrite Karashoho Pipe, Uzbekistan: An Unusual Diamond Deposit in an Atypical Tectonic Environment

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.