Geology of the Mahd Adh Dhahab District, Kingdom of Saudi Arabia

Open-File Report 90-315




Approximately 200 km2 around the Mahd adh Dhahab mine were mapped at 1:18,000 scale, with emphasis on establishing the volcanic stratigraphy. The Mahd Group, which rests over a basement of metamorphosed tonalite, is divided into the Lahuf formation and Tuwal formation, which are separated by an unconformity. The Lahuf is divided into a lower felsic pyroclastic member, a middle felsic tuff member, and an upper mafic member. The overlying Tuwal formation is divided into the Mine member, which consists predominantly of subaqueous felsic pyroclastic rocks with minor dolostone and chert, and the Ghuwayshat member, which consists predominantly of subaerial ignimbrites. The Mahd Group also contains numerous basaltic and rhyolitic subvolcanic intrusions, some of which were extruded as lava flows. The Mahd Group was folded, intruded by rocks of the Ramram cauldron, and eroded prior to deposition of the overlying Ghamr group. Remnants of the latter consist of fanglomerate, volcaniclastic sediments, and dacitic lava flows. Burial of the region under rocks of the Ghamr group probably accounts for prehnite-pumpelleyite facies metamorphism. Vein mineralization at Mahd adh Dhahab and other occurrences is probably unrelated to the magmatism that produced the Mahd and Ghamr groups. The Wadi Sayilah-Wadi Ghadayrah fault system was active during deposition of the Mahd and Ghamr groups and accounts for about 2.6 km of cumulate uplift of the basement block relative to the Mahd basin.

Major-element data show that the Mahd Group was produced from separate basaltic and dacitic-rhyolitic magmas that overlapped without mixing. The alkalis and alkaline-earth elements were particularly mobile during metamorphism (which caused widespread albitization of feldspars) and also during hydrothermal alteration (which added secondary microcline). This mobility adversely affected rubidium-strontium whole-rock systematics, which makes whole-rock isochron dates obtained from these rocks questionable. The new geological data presented here are combined with the geochronologic data of Calvez and Kemp (1982) to re-interpret the geologic history of this area.

Study Area

Additional publication details

Publication type:
Publication Subtype:
USGS Numbered Series
Geology of the Mahd Adh Dhahab District, Kingdom of Saudi Arabia
Series title:
Open-File Report
Series number:
Year Published:
U.S. Geological Survey
Publisher location:
Reston, VA
Report: iii, 49 p.; Map: 28 cm.
Saudi Arabia
Online Only (Y/N):
Additional Online Files (Y/N):