THE KHANNESHIN URANIUM DEPOSIT AT THE CARBONATITE VOLCANO MARGIN (AFGHANISTAN)

The Khanneshin deposit is located in the south of Afghanistan in the R egistan Desert in the center of the vast (about 80 000 square km) Seis tan neotectonical basin. It is confined to the marginal part of the Ea rly Quaternary carbonatite volcano-plutonic complex. The deposit is th e first example of pure uranium deposit associated with carbonatite an d free of thorium, titanium, and niobium. The deposit was discovered a nd preliminarily evaluated by Soviet and Afghan specialists of the Geo logical Department of Afghanistan and then studied in detail by the au thors of this paper. The deposit is of Early Quaternary age (not more than 700000 years) and has barely eroded (not more than 300 m). It is composed of uranylsilicate ores unusual for uranium vein deposits. The desert conditions of the area undoubtedly favoured the preservation o f slightly stable uranylsilicate minerals. The deposit is situated in the center of the Seistan neotectonic basin and is confined to the mar gin of the volcano-cupola structure. The basin is filled with Neogene sediments. The main volcano vent is filled with calcite, ankerite, aci d ankerite-siderite carbonatites with increased contents of niobium, r are earth metals of the cerium series, and thorium. The younger satell ite vents are composed of calcite carbonates that differ in their abun dance of inclusions of apatite-magnetite-pyroxene and K-feldspar-garne t rocks and increased (except phosphorus) contents of titanium and zir conium. The deposit is controlled by a radial (relative to the volcano -cupola structure) fault in the Neogene sandstones. Erruptive breccias developed within it, and host sandstones were subjected to bleaching, chloritization, and dolomitization. Ores are concentrated in these al tered rocks. At first, ores with uranium-bearing fluorapatite and an a dmixture of zirconium were developed, and then uranylsilicate and week site-boltwoodite ores were formed.