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Three principal modes of formation are apparent for authigenic smectites in Recent marine sediments: alteration of volcanic rocks and glass, low-temperature combination of biogenic silica and Fe-oxyhydroxides, and direct precipitation from hydrothermal fluids. The latter two mechanisms are discussed with reference to new evidence from studies of sediments from the Bauer Deep of the equatorial eastern Pacific and the Atlantis II Deep in the Red Sea. In the Atlantis II Deep sediments, three sub-environments of smectite formation from hydrothermal fluids are recognized. In two of them nontronites are formed, whilst in the third an Fe-poor smectite, intermediate in composition between beidellite and montmorillonite, occurs.

Sediments from a Quaternary beach ridge complex along the coastal plain of north-west Egypt have been examined by X-ray diffraction and optical and scanning electron microscopy. Palygorskite was identified in the caliche capping the beach ridges and in the gypsiferous marls between the ridges; petrographic evidence indicated that this had formed authigenically in both environments. Examples are thus provided of the pedogenic and evaporative in situ formation of palygorskite.The area studied is part of a coastal beach dune complex to the west of Alexandria which extends for several hundred kilometres along the northern coast of Egypt. The complex consists of a series of carbonate beach dune ridges separated by longitudinal depressions interfilled with lagoonal-sabkha sediments. The coastal beach dune ridge is composed largely of incohesive to lightly-cemented calcareous oolitic and biogenic sands; parallel to this on the landward side are a series of ten older carbonate beach ridges (Zeuner, 1952). These show an increasing degree of cementation with increasing distance from the present coastline (Fig. 1).The cemented dune ridges are capped by caliche crusts which increase in thickness from 0.01 to > 1 m with increasing age. The caliche also thickens from west to east where the ridges interfinger with the alluvial plain deposits of the Nile Delta.The work discussed in this paper is confined to the area of Ridges Ill and IV. The depression between these two ridges shows a succession of two gypsum bands interbedded with fossiliferous marls. The marls increase in thickness to the east towards the suggested site of an ancient lagoon inlet in Ridge III. The marls thin to the west and become more gypsiferous, with lensoid gypsum crystals up to 2 mm in length growing displacively or poikilitically, suggesting a highly evaporative environment (Fig. 2). SAMPLING AND ANALYTICAL METHODSSamples were taken of marl sediments from the depression between Ridges III and IV, the caliche capping these ridges, and the underlying oolite skeletal calcarenite parent rock. These were examined mineralogically using standard X-ray powder diffraction techniques and petrographically using optical and scanning electron microscopy. Mineralogy RE SU LT SThe fossiliferous marls in the depression between Ridges III and IV are composed of

A detailed fluid‐inclusion study of post‐Variscan quartz ± carbonate ± sulphide ± anhydrite ± fluorite veins hosted by Palaeozoic basement (Porthleven, Menheniot, Cornwall) and Permo‐Triassic sediments (Western Approaches Basin) has identified two broad fluid types. A high salinity (20–27 wt% NaCl), low temperature (80–150°C) brine is present in base metal mineralized veins hosted by Palaeozoic rocks onshore and in sulphate‐bearing veins within the Permo‐Triassic offshore succession. Microthermometric data indicate that saline fluids found in the Permo‐Triassic basinal sequences have similar compositions to the base metal mineralizing fluids and confirms that such basins are likely sources for the mineralizing fluids. A second, hotter (200°C) more dilute (0–5 wt%) fluid has been identified in fault‐hosted east–west‐trending veins in the Porthleven area. It is likely that the temperatures of both the dilute and saline fluids have been affected by the local high heat producing Cornubian granite batholith. There is also limited evidence for fluid mixing of these two fluids which may have been responsible for base metal precipitation.