Salt domes occur in many sedimentary basins throughout the world. Within ancient sedimentary basins) salt domes have been reported to form some of the well known traps for oil and hydrocarbons accumulations. In addition to trapping hydrocarbons, the occurrences of sulfur deposits) metallic sulfide minerals, barite and calcite in salt dome cap rocks have been knwon for many years.
The salt dome environment provides one of the most diverse and prolonged records of fluid-rock interactions in sedimentary basins. During their development, the salt domes go through a complex sequence of geochemical reactions and hydrological modifications that could lead to cap rock formation and later accumulation of economic deposits such as hydrocarbons and lead-zinc. Understanding of the genesis of lead-zinc sulfide deposits located in sedimentary basins has established the close association of hydrocarbons and lead-zinc mineralization and their genetic relationships. However the importance of salt domes as a host for Zn-Pb mineralization is alrgely neglected. The salt dome environment is an ideal place to study the genetic relationship between oil accumulation and sulfide mineralization. Thus salt domes provide an interesting geologic framework for investigation metallic mineralization as an aspect of fluid rock interactions in sedimentary basin.
In spite of the fact that the sequence of geologic events leading to diapirism has been discussed in many studies, cap rock formation and the origin of the fluids that participate at various stages of salt dome development have not been clearly characterized. The present work focuses on the development of cap rocks which are commonly found overlaying salt diapirs and discusses possible mechanism(s) for emplacement of various cap rock components, sources of cap rock material and possibly timing of cap rock formation.
Most mineral-forming processes in salt dome cap rocks appear to form due to the interaction of salt with basinal fluids of diverse origin. Although the fluids involved in cap rock formation have not been clearly identified, they probably range from deep basinal brines migrating updip and upward along the outer margins of the diapir to shallow circulating marine or meteoric waters. Cap rock minerals could record the com- position of the fluids that were driven up to shallow levels. Thus the composition of the salt dome cap rocks can provide valuable information on the origin, migration, and interaction of the fluids within developing sedimentary salt basins. Geologic materials have characteristic stable and radiogenic isotopic signatures which can reveal much about their origin and evolution.. The strontium and stable isotopic composition of minerals precipitated as cap rocks are powerful geochemical tool in identifying the nature of the fluids which have reacted with salt domes and could place significant constraints on their origin. Isotopic information in conjunction with fluid inclusion data provide information not only about fluid composition but sources and temperatures of calcite and other cap rock minerals.
Within sedimentary basins salt domes and related structures are important tectonic features which affect the basinal configuration, the thermal regime and the geochemistry of the basinal fluids. Salt movements tend to create depressions or basins which are ideal loci for deposition of marine organic matter. Salt dissolution increases salinity of the waters leading to anoxic conditions. This enhances organic matter preservation in the basin. Due to high thermal conductivity, salt domes act as conduits for thermal flux which effect chemical reactions, dissolution, precipitation, hydrocarbon maturity and fluid flow. It is the interaction of the basinal fluids and associated hydrocarbons with the salt domes which is greatly involved in the formation of cap rocks containing metal sulfide and economic sulfur deposits.
Salt structure regions often show geothermal gradient which are much higher than would be expected from the associated sediment thicknesses in the area between the structures. Because of this, organic rich sediments will Eeach thermal maturity much faster in a diapiric basin.
Maturation of hydrocarbon and oxidation of organic matter produce large quantities of CO2 , CH4 and H2O. In source rocks, as hydrocarbons are produced, the pore water is likely to be partially saturated with CO and CH while water derived from the hydrocarbon generation and oxidation is added to the pore water system. The most important hydrocarbon formed during diagenesis is CH4 with its formation attributed to decomposition of organic matter. Interaction of methane with sulfate in the salt dome environment produces CO2, H2S and other molecules which are important chemical consitituent during sulfate reduction and methane oxidation. Upward migration of the CO2-bearing fluids through sedimentary sequences probably results in carbonate dissolution followed by precipitation in salt dome cap rock due to the drop in CO2 pressure. As this process continues calcite precipitates in layers in reverse stratigraphic sequence. The growth of calcite cap rock probably continues as long as there is supply of upward migrating hydrocarbons.
By the post diapir stage, the salt once again is just below the surface and in this environment the probability of dominantly chemical reactions as described above appears to be highly unlikely. Under such physiochemical conditions bacterially meditated sulfate reduction is dominant according to the following general reactions.
The end product of these reactions are chemical constituents which greatly contribute to calcite precipitation within cap rock (reaction,1) and sulfur deposits according to reaction (2). A number of minerals may precipitate if the respective cations are available in sufficient amount.
The fluids involved in cap rock mineralization in salt domes could have originated from either sea water or meteoric water that had been modified by interaction with basinal sediments. Extensive interaction of these fluids with salt domes results in their enrichment with metals and various ions. During such interactions the fluids would acquire the brine characteristics which could be involved in mineralization within basinal sediments (e.g. dolomitization). Mass transport by these migrating diagenetic fluids is an important factor in hydrocarbon transport and accumulation, ore fluids migration and precipitation. In addition, these fluids could have profound effect on the diagenetic modification of basinal sediments by interaction with salt domes including. secondary porosity development, porosity occlusion and reservoir and host capabilities.
The relationships between sulfide mineralization and diapirism and genesis of the sedimentary hosted lead-zinc deposits is also examined here. The location of the sulfide bodies may reflect some biological, chemical, or textural properties of the cap rocks.
The geochemistry and nature of the ore fluids responsible for the formation of sediment-hosted lead-zinc deposits remain problematic. The sources of sulfur and metals, their concentration in the ore-forming fluids, mechanisms for their transport, deposition of the ore minerals and the specific chemical reaction is unclear. Apparently, the source of the ore fluids responsible for metallic sulfide mineralization is sedimentary in nature; thus regional development and basinal dewatering provide an appealing ore fluide source. The salt stock itself could provide the necessary metal ions to produce sulfide deposit. Similarities of ore fluids responsible for sediment hosted lead-zinc mineralization and nearby oil field brine have been noted by various workers. This similarity suggests that the hydrothermal lead-zinc deposits are genetically related to oil formation and accumulation and are formed at the same time.
A hydrothermal origin for salt deposits interbedded with volcanogenic sediments involving heated waters of magmatic origin, a process called "exhalation-sedimentary saltification", has been suggested for ancient salt deposits in rift and tectonic basin settings (see Haridie,1990, Am. Jour. Sci. ). Such processes could have been involved in forming the salts associated with volcanics in the Hormoz series in Iran.
Using the geochemical information, a comprehensive model could be developed in order to explain the genesis of these deposits. Extensive geochemical investigations in conjunction with other geologic data on cap rocks and associated mineral occurrences, using stable and radiogenic isotopes, can assist in developing agenetic model for their emplacement and association with salt diapirism.