Lithofacies and depositional environment of the Maraghan Formation, and speculation on the origin of gold in ancient mines, An Najady area, Kingdom of Saudi Arabia
Year: 1987
Type: report
Abstract: The Upper Proterozoic Maraghan formation of the Murdama group is subdivided into five lithofacies units over an area of about 750 sq km near An Najady, in the northern Arabian Shield.The Maraghan formation is composed mainly of sandstone, siltstone, shale, and lesser amounts of limestone, dolomite, and carbonate-cemented sandstone, and becomes finer grained upward.The base of the Maraghan is not exposed in the study area.The lowest exposed unit, lithofacies A, is dominated by coarse-, medium-, and fine-grained sandstone and lesser amounts of siltstone, shale, and interbedded limestone and dolomite.Beds in lithofacies A form fining-upward sequences that decrease in maximum grain size upward.Lithofacies B, which gradationally overlies lithofacies A, is dominantly a fine-grained sequence that is characterized by siltstone and shale, with lesser amounts of fine-grained sandstone.Beds of limestone and dolomite are rare; some zones of medium-grained sandstone occur locally.Lithofacies C overlies lithofacies B with a gradational contact and shows a west-to-east facies change.In the western part of the area, lithofacies C is fine-grained sandstone, siltstone, and shale, containing locally thick zones of brecciated limestone and dolomite; a pinkish tan dolomite is at the base, and a thick gray dolomite pod at Jabal Rahail forms a local carbonate unit that may have been tectonically thickened.In the eastern part of the area, sandstones are coarse-and medium-grained; fining-upward cycles prevail, and carbonate occurs as thin, lenticular beds at the base of cyclic beds.Locally, sections as thick as 10 m are dominated by coarse-or medium-grained sandstone.Lithofacies C appears to thicken toward the east.Lithofacies D, which gradationally overlies lithofacies C, is composed mainly of medium-and fine-grained sandstone, siltstone, and shale.Carbonate-bearing beds are rare, although carbonate pods along the bounding thrust fault in the northwest part of the basin may be part of lithofacies D. Fining upward cycles dominate clastic rocks.In western exposures siltstone and shale are dominant, but in eastern exposures medium-and fine-grained sandstone are more common.Lithofacies E, the uppermost of the facies units mapped, is composed mainly of graded sequences of coarse-and medium-grained sandstone, fine-grained sandstone, siltstone, and shale.Limestone and dolomite occur as cement in sandstone beds or as lenticular concretions.The depositional environment of rocks of the Maraghan Formation was probably an extensive mudflat drained by braided channels.Carbonate mounds and possibly algal reefs developed between distributary channels.Barrier islands may have bordered the mudflat to the east and southeast.I/ USGS, Denver, CO Ancient gold mines occur as gold-quartz vein deposits mainly in fine-grained rocks of lithofacies B and D. The ancient workings at An Najady occur in a fine-grained zone within Lithofacies A. Five samples of shale analyzed for gold, contained concentrations that ranged from less than 4 ppb to 11 ppb.These preliminary results suggest that hydrothermal fluids may have mobilized gold from shale of Maraghan rocks during intrusion of small stocks.The potential for gold occurrences of unknown size and concentration appears to be highest near silicic stocks and buried intrusive bodies, or within fine-grained carbonaceous rocks, and syngenetic volcanic rocks of the Maraghan formation.
DOI: 10.3133/ofr87177
Author: C.A. Wallace
Language: English
Subfield: Earth-Surface Processes
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: bronze
OpenAlex ID: W178806646