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