Environmental and diagenetic analyses of Lower Permian epiclastic and pyroclastic fan deposits—their role for coal formation and uranium metallogeny in the Stockheim Trough (F.R.G.)

Abstract

Conglomerate-silt/mudstone successions are of widespread occurrence within Permian riftogenic troughs from mid Europe. Commonly these beds are abundant in volcanic detritus which was partly derived by weathering of volcanic cones, and partly from directly related certain eruptive pulses. The volcanic influence on sedimentation processes in the course of basin evolution decreases. The lowermost units contain abundant ignimbrites and lahars, whereas the overlying strata exhibit a gradual change from debris flow into more stream-dominated fan deposits. The outer-fan depositions built up by silty sediments, locally mixed with air-fall tuffs, contain coal seams. The coal-bearing host rocks may be categorized into four classes: (1) grey siltstones made up of prevalently non-volcanic detritus; (2) grey to black layers consisting of epiclastic and pyroclastic components; (3) silicified coal seams; and (4) brick-red petrified wood. Ore concentrations in these carbonaceous rocks result from the mobilization of elements from parent material containing large quantities of labile constituents. Four modes of strata-bound U concentration may be encountered: (1) U-bearing petrified wood remains; (2) U-bearing coal lenses; (3) U-bearing carbonaceous volcaniclastic conglomerates; and (4) U-bearing carbonaceous fine-grained tuffites (“fish eyes”). These U accumulations are produced during the course of early diagenesis of volcaniclastic material and coal-bearing beds. The reason for the very high prospectivity of these Permian mid-European sediments is the close intertongueing of carbonaceous matter-bearing rocks and volcaniclastic rocks necessary for the U supply (“uraniferous dirty coal”), in basins of rapid subsidence, which causes the bimodality of those clastic deposits.