9.6 EVAPORITES
455
to magnesium. Brines of this composition could generate the restricted mineral assemblage found in the potash zones of the Devonian of Saskatchewan. The Middle Devonian evaporites of the Elk Point basin are separated from the open marine Mackenzie
limestone and shale basin that lay to the north by a limestone dolomite complex, the
Presqu'ile Formation. It is a matter of opinion whether the Presqu'ile Formation was
present as a physical barrier (i.e., a leaky dam with a deep-water evaporite basin behind)
or whether the carbonate complex built up as shoal banks coevally with the accumulation of shallow-water evaporites behind. Whichever of the alternatives applied, the
seawater that entered the Elk Point evaporite basin had to do so through the Presqu'ile
carbonate barrier. The Presqu'ile Formation is largely dolomite with a complex of dolomites and anhydrite rocks behind it, and these in turn pass back mainly into halite.
The possibility has to be considered that the incoming seawater was conditioned chemically by dolomitization and associated precipitation of sulfate as it passed through the
barrier. In consequence the brines that passed on into the distal parts of the basin would
only have been capable of generating halite and the observed potash assemblage of silvite and carnallite.
Some potash deposits evidently accumulated in highly saline lakes of residual brine
that remained after the virtual drying out of the evaporite "basin." In other instances
the lakes may have been formed after complete drying out, by the bleeding of interstitial brines into tectonic depressions. However, in some potash deposits the crystal fabrics are not those of precipitates, but of diagenetic replacements, for example, the Permian potash of Texas and New Mexico, and the Zechstein potash of northwest England.
The evidence suggests that the potash minerals were emplaced by reaction between interstitial potassium and magnesium chloride brines and earlier formed minerals.
9.6.5 Economic Significance of Evaporites
Evaporite minerals are of great economic importance for three reasons. They are an
economic material in their own right, they are closely related to the genesis and entrapment of hydrocarbons, and there is a strong presumption that evaporite associated
brines play an important role in the genesis of certain metallic ores (Melvin, 1991).
These three aspects are now examined.
Evaporites are a natural resource of great importance. They supply a large proportion of the world's requirements for the rare earth elements, notably sodium and potassium, for the halogens, principally chlorine and bromine, and for sulfur. Chemical
industrial complexes thus tend to be situated adjacent to economic evaporite bodies.
The crests of salt domes develop a diagenetic cap rock of limestone, dolomite, anhydrite, gypsum native sulfur, and diverse sulfide minerals (Kyle and Posey, 1991). Sometimes the sulfur is in commercial quantities (Fig. 9.42).
Evaporites are of importance in the search for oil and natural gas for three reasons:
source, structure and seal (Buzzalini et al., 1969). The conditions that favor evaporite
genesis are unfavorable to biological decay. In a basin with brine in its lower depths, organic matter may be preserved on the basin floor interbedded with evaporites because
the conditions are hostile to bacteria. Similarly, in the sabkha environment, algal laminae are preserved interbedded with evaporites and carbonates. Organic laminae are
thus a common constituent of evaporites (be they basinal or sabkha in origin) and there
is a strong presumption that evaporites are often potential hydrocarbon-source rocks
455
to magnesium. Brines of this composition could generate the restricted mineral assemblage found in the potash zones of the Devonian of Saskatchewan. The Middle Devonian evaporites of the Elk Point basin are separated from the open marine Mackenzie
limestone and shale basin that lay to the north by a limestone dolomite complex, the
Presqu'ile Formation. It is a matter of opinion whether the Presqu'ile Formation was
present as a physical barrier (i.e., a leaky dam with a deep-water evaporite basin behind)
or whether the carbonate complex built up as shoal banks coevally with the accumulation of shallow-water evaporites behind. Whichever of the alternatives applied, the
seawater that entered the Elk Point evaporite basin had to do so through the Presqu'ile
carbonate barrier. The Presqu'ile Formation is largely dolomite with a complex of dolomites and anhydrite rocks behind it, and these in turn pass back mainly into halite.
The possibility has to be considered that the incoming seawater was conditioned chemically by dolomitization and associated precipitation of sulfate as it passed through the
barrier. In consequence the brines that passed on into the distal parts of the basin would
only have been capable of generating halite and the observed potash assemblage of silvite and carnallite.
Some potash deposits evidently accumulated in highly saline lakes of residual brine
that remained after the virtual drying out of the evaporite "basin." In other instances
the lakes may have been formed after complete drying out, by the bleeding of interstitial brines into tectonic depressions. However, in some potash deposits the crystal fabrics are not those of precipitates, but of diagenetic replacements, for example, the Permian potash of Texas and New Mexico, and the Zechstein potash of northwest England.
The evidence suggests that the potash minerals were emplaced by reaction between interstitial potassium and magnesium chloride brines and earlier formed minerals.
9.6.5 Economic Significance of Evaporites
Evaporite minerals are of great economic importance for three reasons. They are an
economic material in their own right, they are closely related to the genesis and entrapment of hydrocarbons, and there is a strong presumption that evaporite associated
brines play an important role in the genesis of certain metallic ores (Melvin, 1991).
These three aspects are now examined.
Evaporites are a natural resource of great importance. They supply a large proportion of the world's requirements for the rare earth elements, notably sodium and potassium, for the halogens, principally chlorine and bromine, and for sulfur. Chemical
industrial complexes thus tend to be situated adjacent to economic evaporite bodies.
The crests of salt domes develop a diagenetic cap rock of limestone, dolomite, anhydrite, gypsum native sulfur, and diverse sulfide minerals (Kyle and Posey, 1991). Sometimes the sulfur is in commercial quantities (Fig. 9.42).
Evaporites are of importance in the search for oil and natural gas for three reasons:
source, structure and seal (Buzzalini et al., 1969). The conditions that favor evaporite
genesis are unfavorable to biological decay. In a basin with brine in its lower depths, organic matter may be preserved on the basin floor interbedded with evaporites because
the conditions are hostile to bacteria. Similarly, in the sabkha environment, algal laminae are preserved interbedded with evaporites and carbonates. Organic laminae are
thus a common constituent of evaporites (be they basinal or sabkha in origin) and there
is a strong presumption that evaporites are often potential hydrocarbon-source rocks