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

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.

Geology of the Ar Rahail ancient gold mine, Kingdom of Saudi Arabia

“…Regionally, the Ar Rahail ancient mine is in the lower part of the upper portion of the Margham formation of the Murdama Group (Johnson and Williams, 1984, p. 12) (C. A. Wallace, 1986)…”

Volcanosedimentary Basins in the Arabian-Nubian Shield: Markers of Repeated Exhumation and Denudation in a Neoproterozoic Accretionary Orogen

“…Because of the predominance of planar-bedded, poorly sorted sandstone, it is proposed [50] that the Murdama group was deposited in an alluvial to deltaic environment. The thick, locally stromatolitic Farida carbonate suggests a dominantly marine setting in the eastern Maslum sub-basin, whereas lagoonal, lacustrine, or shallow-marine conditions are inferred for the Maraghan sub-basin on the basis of sandstone, siltstone, and shale that are interpreted as deposits in near-shore mud-flat and broad-channel environments with carbonate mud and microbial buildups [63].…”

Stratigraphy of the late Proterozoic Murdama Group, Saudi Arabia

“…An Najady Area Wallace (1986) studied the lithofacies and depositional environments in part of the Murdama group in the An Najady area north of Uqlat as Suqur. He assigned these rocks to the Maraghan formation, a name introduced by du Bray (1983a).…”