Paleovolcanic facies and exhalite geochemistry; guides for selecting exploration areas in volcano-sedimentary complexes

Abstract

About a half of the 620,000-km 2 upper Proterozoic Arabian Shield comprises weakly meta-morphosed and tectonized volcano-sedimentary rocks. Prospecting for precious and base metals in this large area is likely to continue for some time. A pilot study of a typical area around Wadi Abennah involved the mapping of paleovolcanic facies and the geochemical analysis of contained cherts, gossans, and carbonates. Portions of three deformed stratovolcanoes were identified, one of which (the Iktinah paleovolcano) is mineralized. A characteristic feature of the Iktinah paleovolcano is that the entire central facies and adjacent parts of the proximal facies contain disseminated pyrite in argillized host rocks indicating an extensive original hydrothermal core. Base metal occurrences are recorded immediately outside this core, and associated geophysical anomalies suggest other hidden occurrences. Other base metal occurrences are known in the distal facies. Most cherts and gossans of the central facies of the Iktinah paleovolcano also reflect well-developed hydrothermal core geochemistry since they contain significant amounts of the precious metal pathfinder elements As, Sb, Mo, Bi, and Te. Such geochemistry suggests that precious metal-bearing cherts and/or epithermal quartz veins may also occur. Results of this study suggest exploration areas can be rapidly selected within extensive deformed volcano-sedimentary complexes. Such target areas correspond to the central, proximal, and distal facies of paleovolcanoes with large pyritized-argillized cores such as Iktinah. Others lacking such cores are rejected as unlikely to have had well-developed hydrothermal systems.