The genetic significance of gold-bearing sulphides from Atalla gold mine, Eastern Desert, Egypt

Citation

Osman A & Smillie Z (1996) The genetic significance of gold-bearing sulphides from Atalla gold mine, Eastern Desert, Egypt. 9th Annual meeting of the Mineralogical Society of Egypt, Cairo, 01.1996-01.1996.

Abstract
Atalla gold deposit is situated at about 30 kilometres NNW of the mouth of W. Atalla in the Central Eastern Desert. The basement rocks exposed in the area include an ophiolitic melange intruded by felsite cut by calc-alkaline granite. Gabbro is found as a small body some 7 km north of the mine area. Structurally, the area is traversed by a left lateral NNW-SSE shear zone along which the Atalla felsite was intruded. The deposit is a vein type aligned along the ENE-WSW direction and filling the second order gashes perpendicular to the main shear Zone of Wadi Atalla. Petrographic investigations, Atomic Absorption Analysis, SEM and the EDX Analysis of the sulfide-bearing quartz veins and granites at Atalla Gold Mine revealed two successive stages of sulphide mineralization. The first stage is represented by euhedral disseminated arsenopyrite filling the intercrystalline spaces between the feldspars. It is believed that this arsenopyrite have been developed from a hydrothermal residual solution rich in Fe, As, Sb, Cu, Ag and Au. The gold content of arsenopyrite as revealed by atomic absorption analysis is 21.6 ppm. The second stage is represented by the introduction of quartz veins into the granite. The hydrothermal solutions in this stage underwent gradual decrease in temperature and a continuous enrichment in sulphur. This is evidenced by the early formation of pyrrhotite followed by the precipitation of pyrite and its intergrowth. The overgrowth bands of pyrite are remarkably mineralized by galena and gold. The disseminated suIphides in the quartz include sphalerite, chalcopyrite and galena which represent the latest stage of sulphide mineralization. The replacement of sphalerite by galena and the absence of chalcopyrite disease indicate that these sulphides were precipitated from an epithermal solution. Four types of gold are recognized; gold inside pyrite which is connected with galena, gold inside pyrite free of galena, gold inside quartz, and gold inside either pseudomorphic goethite after pyrite or collomorphic goethite in fissures. The first type is found as inclusions (5-10 microns) accumulated in pyrite. Ag-contents in this gold (either simple or composite inclusions with galena) exceed 30 wt.% indicating a hypogene origin. These inclusions are bound to the outer parts of the pyrite crystals or to the overgrowth pyrite crystals and are believed to be related to the gradual decrease in temperature and increase of sulphur. The second gold is related to supergene alteration where native gold (rounded grains > 50 J..UIl, Ag-content 10-15 wf>,Io) is trapped inside fractures in pyrite and quartz (third type). Upon oxidation of pyrite, gold was remobilized and precipitated in the pseudomorphic or collomorphic goethite as residual grains with very low Ag-content (fourth type). From the economic point of view, pyrite in Atalla deposit is considered as gold indicator but not as economic gold carrier.