Sediments silicification

Pitchblende is the dominant uranium mineral in all these vein-like deposits in sedimentary rocks. It occurs as small veinlets along fractures within and surrounding the structures and as finely disseminated crystals in porous breccia fragments within the structures. Pitchblende concentrations may be distributed zonally within the ore, and in some deposits may be concentrated in the upper levels of the structure. Associated minerals may include sulphides and sulpharsenides. Calcite and quartz are the most common gangue. Types of alteration include bleaching of red sediments, silicification, carbonatization and argillization. 

Silicification with evaporites Nodular Lacustrine/playa sediments Simple, chert like Marginal to evaporitic depressions, arid/semi-arid... 

Thiry, M. (1978) Silicification des sediments sablo-argileux de l Ypresien du sud-est du bassin de Paris. Genese et evolution des dalles quartitiques et silcretes. Bulletin du B.R.G.M. (deuxieme serie) 1, 19 46. 

Thiry, M. (1981) Sedimentation continentale et alterations associees calcitisations, ferruginisations et silicifications. Les ArgUes Plastiques de Spamacien du Bassin de Paris. Memoire 64. Sciences Geologiques. 

The silicification of sediments little affected by diagenesis under surface conditions is a pronounced frequent phenomenon entailing the dissolution of silica by surface waters, as well as hydrolysis of silicates with the subsequent redeposition of the silica in the discharge zone of the surface waters (Taylor 1950 Thompson 1959 Weyl 1959 Trumit 1968). These cases of silicification are widespread under geochemical conditions favourable for the dissolution of silicates. 

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