Supergene and hypogene alteration in the dual-use kaolin-bearing coal deposit Angren, SE Uzbekistan

摘要

The Jurassic Angren coal-kaolin deposit Uzbekistan, is one of the largest producers of coal and kaolin suitable for refractories and industrial ceramics in central Asia. The Major coal seam, attaining a thickness between 4 and 24 m, is encased by kaolin-bearing bedsets which have been derived from supergene pre- and hypogene post coal kaolinization. Joint clay-mineralogical and coal petrographic analyses formed the basis of the environment analysis of this coal-kaolin series and constrained the physico-chemical conditions existing during the Triassic through Jurassic period of time. Massive kaolin I underneath the coal seam is a typical residual kaolin or underclay with arsenic Fe-disulfides and siderite indicative of a reducing swampy depositional environment developing under moderately hot climatic conditions. Towards the top, kaolin I became reworked fluvial by processes. The Major coal seam developed in swamps interfingering with a fluvial drainage system of suspended to mixed-load deposits. The maximum temperature for the post-depositional alteration of the carbonaceous material is 70 ℃. Post-coal kaolinization (kaolin II) affecting trachyandesites and trachytes is of low-temperature origin and low-sulphidation-type. The temperature of formation was well below 200 ℃, deduced from the absence of dickite in the clay mineral assemblage. Basaltic dykes intersected the coal-kaolin series and account for contact metamorphic reactions in the proximal parts of the aluminum-bearing wall rocks reaching sanidinite-facies conditions with temperatures around 1000 ℃.