Firghamu, Koksha Valley, Jurm District, Badakhshan, Afghanistan

The Dassu pegmatite region, in the valley of the Braldu Shigar River, a northern tributary of the Indus River, is related to the Asian plate. It is in the southern part of the Main Karakorum thrust among schists, gneisses, and marbles of the Proterozoic - lower Paleozoic Minapin formation near a contact with the Miocene-Pliocene Karakorum granodiorite batholith. The pegmatite veins, lenses, and irregular bodies are up to 45 m long and 1 to 8 m thick.

The beryl-bearing pegmatites contain euhedral aquamarine crystals, 5 to 15 cm long and 1 to 7.5 cm across. Transparent crystals attain 5 to 7.5 cm in length and 1 to 3 cm across. Colourless and yellowish brown prismatic topaz crystals to 200 g in weight, and green semi-transparent hydroxyl-herderite may accompany beryl. Some pegmatites have a pinc quartz core with accumulations of opaque beryl, muscovite, and schorl adjacent to cavities. Thin veinlets with garnet, galena, and pyrite are locally observed in the pegmatites.

Ref: E.Ya. Kievlenko, Geology of gems, 2003, p. 110

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