Geochemistry and Genesis of Base Metal-Rich Mn–Fe Mineralization in Volcaniclastic Sediments, Asfar Thwelil Area, Saudi Arabia

Abstract

The Asfar Thwelil area, the northern Arabian Shield, contains a base metal-rich manganese-iron mineralization hosted by volcaniclastic sediments and ignimbrites of a volcanic caldera. These rock associations belong to the Shammar Group, a youngest acidic volcanic and volcaniclastic unit in the Precambrian rocks of the Arabian Shield. The Mn–Fe oxides form veins and horizontal dense stratabound layers along bedding planes and within beds. The mineralized samples contain up to 49.2 wt% MnO, 47.6 wt% FeO, 10.71 wt% SO3, 14,000 ppm Zn, 6,560 ppm Pb and 484ppm Cu. The high concentration of sulfur and base metals may be a good indicator for proximity to the heat source or to massive sulfide deposits formed at depth. Cryptomelane, coronadite, jacobsite and chalcophanite are the main Mn minerals identified, associated with goethite and hematite as Fe-bearing minerals. Pyrite, galena, sphalerite, gold and silver are the main sulfides and precious metals observed. The geological and geochemical characteristics of the Mn–Fe oxides suggest a hydrothermal origin. The hydrothermal criteria of the Mn–Fe mineralization include strong fractionation of Fe and Mn, remarkably high contents of base metals, and common abundance of fresh euhedral to subhedral sulfide minerals. The possible source of hydrothermal fluids was most probably metal-rich magmatic fluids, mixed with seawater, that penetrated through fractures, faults, and the permeable volcaniclastic pile to precipitate the Mn–Fe mineralization. Chemical characteristics of the Mn–Fe mineralization most probably indicate leaching of sulfide-bearing intermediate to acidic volcanic rocks. The results obtained would be the base for further exploration of base-metal sulfides.