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

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 °C. 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 °C, 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 °C.