Geology topics

A.M. Afifi

Publications and source records attributed to A.M. Afifi.

3 recordsLinked to original sources

USGS1990

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

Approximately 200 km 2 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.

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USGS1987

Rock geochemistry in the Mahd adh Dhahab district, Kingdom of Saudi Arabia

Ten sets of geochemical samples from country rock, altered country rock, and quartz veins were collected to determine metal abundances and distributions in the Mahd adh Dhahab district. Gold, silver, copper, lead, and zinc are present throughout the district in veins and altered country rock. Although four generations of quartz veins are defined, there is complete gradation between types. The metals present are the same in all four, only the abundances vary. The concentrations of gold, silver, copper, lead, and zinc in altered country rock are zoned around known or projected mineralized-vein zones. Similarities in metal populations of all quartz veins, altered rock, and mineralized zones, plus the distribution of metals and some major elements, suggest that all formed as part of a single hydrothermal system. Gold in abundances greater than 1 ppm does not correlate with any other element and its distribution is very erratic. The variance in gold values between replicate samples collected in 0.5 m square panels is very high. Radio-element analyses (radium-equivalent uranium, thorium, and potassium) for 40 widely distributed sample localities indicate little variation in these elements throughout the district. If the hydrothermal system had an effect on the distribution of these elements, the system must have extended beyond the limits of the sampled area. Anomalous values of gold, silver, lead, and to a lesser extent copper and zinc in surface rock samples clearly delineated the northern mineralized zone in the upper agglomerate, and an east-vein area and west-vein area of the southern mineralized zone in the lower agglomerate. A third geochemically anomalous area occurs farther to the west in the lower agglomerate, suggesting that mineralization may have extended at least to this area along the lower agglomerate-lower tuff contact, and possibly even further to the west.

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USGS1983

Preliminary mineralogic, fluid inclusion, and stable isotope study of the Mahd adh Dhahab gold mine, Kingdom of Saudi Arabia

The Mahd adh Dhahab mine, located about 280 km northeast of Jiddah, Kingdom of Saudi Arabia, has yielded more than 2 million ounces of gold from periodic production during the past 3,000 years. A new orebody on the southern side of the ancient workings, known as the South orebody, is being developed by Gold Fields-Mahd adh Dhahab Limited. A suite of samples was collected from the newly exposed orebody for preliminary mineralogic, stable isotope, fluid inclusion, and geochemical studies. The Mahd adh Dhahab deposit is in the carapace of a Proterozoic epizonal rhyolite stock that domed pyroclastic and metasedimentary rocks of the Proterozoic Halaban group. Ore of gold, silver, copper, zinc, tellurium, and lead is associated with north-trending, steeply dipping quartz veins in a zone 1,000 m long and 400 m wide. The veins include an assemblage of quartz-chlorite-pyrite-hematite-chalcopyrite-sphalerite-precious metals, which is similar to the mineral assemblage at the epithermal deposit at Creede, Colorado. The primary ore contains abundant chalcopyrite, sphalerite, and pyrite in addition to a complex precious metal assemblage. Gold and silver occur principally as minute grains of telluride minerals disseminated in quartz-chlorite-hematite and as inclusions in chalcopyrite and sphalerite. Telluride minerals include petzite, hessite, and sylvanite. Free gold is present but not abundant. All of the vein-quartz samples contained abundant, minute inclusions of both low-density, vapor-rich fluids and liquid-rich fluids. Primary fluid inclusions yielded homogenization temperatures of from 110? to 238? C. Preliminary light-stable isotope studies of the sulfide minerals and quartz showed that all of the d34S values are between 1.2 and 6.3 per mil, which is a typical range for hydrothermal sulfide minerals that derive their sulfur from an igneous source. The data-suggest that the sulfide sulfur isotope geochemistry was controlled by exchange with la large sulfur isotope reservoir at depth. The d18O values of all stages of vein quartz in the South orebody range between 8.5 and 11.1 per mil. This range is similar to that for quartz from the North orebody and indicates that the hydrothermal system consisted of dominantly exchanged meteoric water, which was uniform in temperature and d18O content throughout the area during the entire period of mineralization. Lead isotope analyses of two galena samples indicate that the lead in the South orebody is less radiogenic than that from the North orebody and confirm that the lead was derived from oceanic crust approximately 700 Ma ago.

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