PETROLOGY AND STRUCTURE OF THE BARRAMIYA AREA WITH SPECIAL I tr EMPHASIS ON THE GOLD MINERALIZATION, EASTERN DESERT, EGYPT.

Abstract

The present study deals mainly with the petrology and structures of the basement rock units outcropping nearby the Barramiya old gold mine between latitudes 25° 02' 36" and 25° 06' 20" N and longitudes 33° 44' 30" and 33o 50' 05" E. This thesis aims at studying the genesis of the gold deposit in the Barramiya mine (25° 04' 24"N - 33° 47' 15"E) as well as the role of structures affecting the mine site in the process of mineralization.

Comprehensive field, petrographical and geochemical studies revealed that the study area is generally covered by assorted rock units which mainly include; serpentinites and talc­ carbonates including quartz - carbonate dyke-like bodies, metagabbros, metapyroclastics, granodiorites, perthitic leucogranites and dykes. In the mine, quartz- feldspar porphyry dyke-like bodies were also recorded.

Major folding affects most of the basement rock units encountered in the area except granitic intrusions. Five alternating syn- and antiforms were encountered extending for about 0 km. The axial planes of these folds strike N70°E and the limbs dip steeply 70°-80°. They are plunging >80° in N70°E direction. Major faults can be classified into older ductile and younger brittle faults. The ductile faults are mainly represented by the major thrust faults that are related to early stages of subduction and collision tectonics during the formation of the ancient oceanic crust and volcanic island arcs. Major brittle faults in the study area are mainly represented by inferred NNE left-lateral strike-slip faults recorded in the mine site. Major ductile shear zones of E-W to ENE strike occur in the central part of the study area. They reactivate older thrust faults by convergent right lateral movement (transpression) as indicated from the shear sense indicators which comprises S-C fabric, sheath folding, sigmoidal tension veins filled by quartz, asymmetry of boudinaged quartz veins and anastomosing shear planes. These transpressional ductile shear zones were formed during the later stages of collision. The study area is mainly cut by numerous types of dykes; the most important types are the quartz - carbonate dykes and the quartz - feldspar dyke-like bodies. Quartz- carbonate dykes have a general major E-W trend with various length and thickness, mainly associated with serpentinites. Quartz - feldspar porphyry dyke-like bodies, recorded only in the mine site, occur in four main trends E-W to ENE, NE, WNW and NW. The dykes of the first three trends are thinner than that of the fourth trend. The E-W to ENE trending dykes are affected by strong shearing whereas the NW trending dykes are affected by extension fracturing and still show most of their primary textures. The rock units of the study area have undergone a polyphase deformation history. Each single deformational event is defined by a group of structures showing the same strain axis, the same bulk kinematics and the same igneous and/or metamorphic event (s). Three successive deformations called Dl, D2 and D3 have been recognized. The P-T conditions of each deformational event were concerned.