干旱区生态环境知识资源中心

Gold deposits and occurrences located in the Nubian Shield have been known in Egypt since Predynastic times. Despite the fact that these deposits were long under exploitation and investigated many times, they are still insufficiently classified in harmony with the crustal evolution models suggested for the evolution of the Nubian Shield. Several plate tectonic models were proposed for the development of the Nubian Shield and the present classification relies heavily on the model that implies collision of arc-inferred continent through subduction and obduction of oceanic lithosphere. A three-fold classification of gold deposits of Egypt is offered here in harmony with this evolutionary model. These are stratabound deposits and non-stratabound deposits hosted in igneous and metamorphic rocks, as well as placer gold deposits.

The stratabound deposits are hosted in island arc volcanic and volcaniclastic rocks of comparable composition formed in ensimatic island arcs. They are thought to have formed by exhalative hydrothermal processes during the waning phases of sub-marine volcanic activity. Stratabound deposits are sub-divided into three main types: gold-bearing Algoma-type Banded Iron Formation, gold-bearing tuffaceous sediments and gold-bearing volcanogenic massive sulphide deposits.

Non-stratabound deposits occur in a wide range of igneous and metamorphic rocks. They were formed during orogenic and post-cratonization periods by mineralizing fluids of different sources. Non-stratabound deposits are divided into vein-type mineralization, which constituted the main target for gold in Egypt since Pharaonic times, and disseminated-type mineralization hosted in hydrothermally altered rocks (alteration zones) which are taken recently into consideration as a new target for gold in Egypt.

Placer gold deposits are divided into modem placers and lithified placers. The former are sub-divided into alluvial placers and beach placers. Conglomerates occurring on or near ancient eroded surfaces represent lithified placers. (C) 2003 Published by Elsevier B.V.