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Fluid Evolution: Migration and Precipitation of Hydrocarbons and Metals |
Fluid Processes Group, British Geological Survey, Keyworth, Nottingham NG12 5GG, UK
1 Intera, 47 Burton Street, Melton Mowbray, Leics LE13 1AF, UK
Continental red beds are first-cycle, immature sediments which are deposited in oxidizing conditions and owe their red colouration to the early diagenetic development of hematite. Previously published work on the fluid/rock interactions which occur during the diagenesis of such sediments highlights that pH and redox are critical fluid parameters which control diagenesis and ore formation. However, except in modern surface water and shallow groundwater systems, these parameters cannot be measured directly, and must be estimated from theoretical considerations. These suggest that mixing between fluids of different redox states is likely to be a critical control on heavy metal mobility. Many diagenetic features can be explained largely by the chemical heterogeneity of red beds, and by the diagenetic ranges of redox conditions and pH which are among the greatest for any type of sediment. Such features include: the formation of red bed-hosted ore deposits; the extensive development of hematite; and the development of early diagenetic non-ferroan carbonate and late diagenetic ferroan carbonate cements. In order to develop theoretical models of fluid/rock interactions during such diagenesis, it is important to consider the interrelationships between fluid flow, mineral dissolution and precipitation, and sorption. At the present time such models are at an early stage of development.
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