Ore microscopic characterization of mineralized rocks at the Bi’r Tawilah gold prospect, Saudi Arabia

Abstract

Ore microscopic investigation of core samples from the Bi’r Tawilah gold prospect enables to distinguish three main groups of ore paragenesis from oldest to youngest, namely magmatic, hydrothermal and supergene. The ore minerals are encountered in sheared and mineralized ophiolitic serpentinites and a variety of hydrothermally altered granitic rocks that include quartz–diorite, granodiorite, monzogranite and quartz–feldspar porphyry. Magmatic ore minerals are chromite, magnetite, ilmenite and pyrrhotite. Hydrothermal ore minerals are the most common (>90%) and they follow the paragenetic sequence: rutile, sphalerite, chalcopyrite, pyrite (with visible gold and electrum inclusions) and arsenopyrite. Supergene ore minerals are ferric Ca-arsenate and ferric oxyhydroxide (±goethite) which represent the weathering products of pyrite and arsenopyrite, respectively. The non-arsenian nature of pyrite and late formation of arsenopyrite all together suggest mobility of As in the hydrothermal fluids until late stages. There are no remarks of ore metamorphism as all events took place post-orogenically despite zonal arrangement of inclusions in pyrite that can result from paucity of crystallization. Ore textures indicate change from early relatively reducing hydrothermal fluids at low fS2 that crystallize pyrrhotite which is converted into pyrite when the fluid becomes relatively more oxidizing at high fS2 and pH <5. On the other hand, the supergene enrichment zone at the uppermost 40m is dominated by exclusively oxidizing fluids (pH >7) at low temperature up to ∼75°C. Supergene ferric Ca-arsenate is a cellular-structured hydrous phase with distinct As5+↔Fe3++Ca2+ ionic substitution in three distinct domains.