Geology, mineralogy and genesis of the world-class Mahd Adh Dhahab epithermal Au-(Ag)-telluride deposit, Kingdom of Saudi Arabia

Abstract

Mahd Adh Dhahab polymetallic Au-telluride mine in Kingdom of Saudi Arabia is a world-class deposit. The mineralization is hosted by subalkaline bimodal rhyolite-basalt arc succession part of the late Proterozoic Mahd group. The mineralized quartz veins mostly occupy fractures that formed in response to local folding of the Mahd volcanic-sedimentary sequence. The ore predominantly consists of massive sulfides (sphalerite, chalcopyrite, pyrite, and galena) and a diverse range of Au-(Ag)-tellurides (sylvanite, petzite, hessite, tetradymite, tellurobismuthite and altaite). Gangue mineral assemblage comprises chlorite, epidote, sassurite and quartz. Tellurides occur as subhedral grains dispersed within chalcopyrite, sphalerite and Fe-rich chlorite. Geological, structural and mineralogical observations support a model invoking a multi-stage process of mixed magmatic-meteoric fluids and boiling as a cause of telluride precipitation. The metal-bearing magmatic fluid was exsolved from a crystallizing arc alkaline melt produced by partial melting of a subducted slab containing ocean floor sediments. Precipitation from solution occurs as a result of the condensation of Te-rich vapor phase and mixing of auriferous fluids with cooled oxygenated meteoric water at a shallow depth.