Origin of the gypsum-rich silica nodules, Moghra Formation, Northwest Qattara Depression, Western Desert, Egypt

Gypsum rich-silica nodules appear in two shale horizons of the Moghra Formation (early Miocene) northwestern Qattara Depression, Western Desert, Egypt. These nodules are gray to milky white in colour, mostly botroidal and rose-like in shape and range in diameter from 2 to 7.5 cm. The silica nodule-bearing shale is composed mainly of smectite with a little minor kaolinite. The silica nodules consist mainly of quartz and are composed of gypsum-free matrix and gypsum-rich megacrystalline quartz. The matrix consists of microflamboyant quartz (less than 36 mu m in diameter) and chalcedony. The megacrystalline quartz occurs as lenticular and prismatic forms (length: 90-250 mu m; width: 30-90 mu m). The microprobe, petrographic and SEM examinations confirmed the occurrence of gypsum relics (diameter; 2-16 mu m) within the megacrystalline quartz. The chalcedony and mosaic microcrystalline quartz occurs as pore-lining and pore-filling cements. The structure of the silica nodules begins with quartzine in its outer rim, then gypsum-free microcrystalline quartz in the middle part and ends with gypsum-rich lenticular to prismatic megaquartz in the center. Field study, petrographic examination and microprobe analysis reveal that the silica nodules were formed by silicification of precursor gypsum nodules deposited in a marginal sabkha environment under an and climate. The silicification selectively affected the gypsum nodules rather than the surrounding shale and occurred both through gypsum replacement and void filling. Transformation of isopachous chalcedony into mosaic microcrystalline quartz also occurred. The texture of the silica minerals reflects the different physico-chemical conditions under which they crystallized. Spherical nodules grew chiefly by the diffusive supply of the silica, and elongated ones grew by pore water advection. \The integrated effect of climate, pH, salinity, crack systems within the sediment and oscillation in the groundwater level and its chemical composition contributed to the formation of the nodules. (c) 2005 Elsevier B.V All rights reserved.