Alteration Zones: Are They a Good Target for Gold Deposits in Egypt? PDF

Keywords

Egypt, alteration zones, geology, rock, gold deposits

Abstract

Extensive rock alterations are a clearly visible characteristic of most Egyptian gold deposits and occurrences. The alterations occur either surrounding the auriferous quartz veins and/or structurally controlled by specific structural features, such as fractures and shear surfaces. Some samples of these alteration zones have proved to be anomalously enriched in gold while others are completely barren. Accordingly there is a controversy on the merit of alteration zones as a good lead to gold. Here, the various types of wall-rock alterations are reviewed with a discussion on the possible reactions that could have generated them. It is concluded that two main styles of alterations could be recognized in the field. The first results during the liberation of gold from the source rocks, and is characterized by being widely distributed and spatial relation to major structures. The second style, however, is related to the deposition of gold and is recognizable only within a few meters of the auriferous quartz veins. The potentiality of each style is discussed and some applications of the concept are offered. In general, alterations accompanying the liberation of gold are not completely devoid of gold, but may still retain some gold depending on the mineralogical siting of gold in the source rocks. Moreover, this type of alteration is a good criterion for the presence of gold in nearby sites. Alterations accompanying deposition of gold, on the other hand, constitute a good target for gold. particularly the portions that are dissected by minor quartz veins, veinlets and stock works (silicification) where gold is believed to migrate to such sites with silica liberated during the different types of alterations. The presence of some efficient precipitants, such as sulphides, carbonates, clay minerals, sericite, iron oxides, chlorite and graphite in the alteration zones is a good indicator of the potentiality of the alteration zone.