Abstract: Evaporites, Sulfides, and Petroleum

By

Dr. Robert F. Schmalz

Many ancient evaporite deposits exhibit features which are difficult to explain in the framework of a shallow-water saline or sabkha genetic model. Most of these difficulties can be overcome by postulating evaporite deposition in a deep-barred basin filled to sill depth with salt-saturated brine. Such a deep-basin model appears oceanographically and geologically reasonable, even though no such basin is known today.

The deep-basin hypothesis has been applied primarily to the study of ancient "saline giants," but it has geologic implications which are of almost greater interest. A period of stagnation and euxinic sedimentation must develop in the basin prior to inception of salt deposition. During this period, large, quantities of organic material may settle to the basin floor where reducing conditions ensure their preservation, providing a potential petroleum source bed. Sulfate reduction and proteolytic decay in the anoxic bottom water yields hydrogen sulfide which chemically will "strip" the overlying water of dissolved base metals, precipitation extremely insoluble sulfide of copper, lead, zinc, and iron. Because the volume of seawater which must be concentrated in the basin by evaporation before salt precipitation can begin must be at least ten times the volume of the basin, this chemical stripping action may produce an economically significant deposit of sulfide minerals. The salts in a deep evaporite basin thus may provide the seal above a petroleum source bed or the cap over an important ore deposit. 

This paragenetic model is shown to conform closely to several well-known sedimentary basins in which salt, petroleum, and sulfide ores are associated, and offers an essential guide in exploration for new reserves of petroleum, natural gas, and base-metal ores.

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