GEOCHEMISTRY OF EPITHERMAL MINERALIZATION IN THE VOLCANIC DISTRICTS OF WESTERN YEMEN

Much of the western half of Yemen is covered by epithermally-altered volcanic rocks, thus providing a large area hosting potential for epithermal mineralization. These anomalous regions are defined by three broad, E-W to NW-SE trending alteration and mineralization trends. These trends correlate with strong ENE-WSW to WNW-ESE magnetic domains that are transverse to NNW-SSE Tertiary rifting, and are probably related to Precambrian basement structures. The alteration trends are spatially related to large magnetic lineaments and boundaries of magnetic domains probably reflecting megashears of terrane sutures in the basement. The most common epithermal alteration styles are argillization and silicification related to rhyolite domes. Propylitic alteration is mostly pyrite-poor. Most argillic zones are hypogene, although some supergene clays appear to be derived from hypogene argillic zones. Advanced argillic alteration is also present in some regions. Silicic alteration consists of silica flooding or silica replacement, with the latter common in the upper parts of systems. Sulfide mineralization observed was almost all pyritic or limonitic (after pyrite). Gangue minerals either are quartz (veins, stockworks, or epithermal breccia matrices), calcite, or clay. Opaline silica or opalite also occurs indicating near-surface epithermal conditions. Gold in concentrations up to 1 ppm or more occur in quartz veins and stockwork veinlets, hosted mostly in mafic rocks and spatially associated with shallow felsic and mafic intrusions and normal faults. Higher gold grades are associated with higher topographic elevations probably due to metal zonation. Associated anomalous trace elements are Ag, As and Mo. Sub-economic gold grades in other anomalies are associated with felsic domes and silicic alteration. Most of the prospects identified to date are adularia-sericite-type. The upper-level acidic alteration and mineralization are produced by boiling of near-neutral solutions. Many altered systems are related to small, endogenous rhyolite domes emplaced within volcaniclastics and flows. Weakly mineralized, autobrecciated contacts are common at the margins of the altered domes. Both preliminary geochemistry and geology suggest good potential for the presence of Nevada-type heap- leach, epithermal gold deposits in Yemen.