Sulfide. Mineralization in the Lega Dembi Primary Gold Deposit, Sidamo, Southern Ethiopia

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Addis Ababa Universty


Sulfides in the LDPGD are found in small quantity. The deposit is a lowsulfide- gold quartz deposit which specifically belongs to the pyrite-arsenopyrite-galena assemblage type of "Volcanic precious metals tellurides association" which is typical of volcanic to subvolcanic settings. It is hosted in volcano sedimentary succession metamorphosed to greenschist to lower amphibolite facies. It spatially coincides with the nearly N-S trending shear zone restricted to its hanging wall. Types and details of features of the enclosing rocks are ·discussed. Structures dominating in the area and their possible relation to the mineralizing process are described. The ore zone and ore accommodating structures (fractures, veins, lenses, stringers and stockworks) are localized to the shear zone and generally trends parallel to major and minor structures in the area. The mineralization seems to be highly influenced by structures than lithology and other physico-chemical factors. Peculiarities related to the ore zone and ore bodies are elaborated. Extensive metasomatic alterations are developed in the enclosing rocks. Their types, disposition and relations to the mineralized zone and mineralization process are explained. Different ore minerals sulfides, sulfosalts, tellurides, tungestate and gold are identified. Their structure, texture, parageneses and zonal arrangement (especially the vertical one) are treated oased on field and laboratory (using polished sections) studies. Since sulfides are scarcely encountered, the treatment of geochemical data of sulfide forming metals was critical. This study was employed to systematically explain the nature of distribution of the sulfides. The most important sulfide forming elements (Pb, Zn, Cu, Ag, As, Sb, Mo, Co, Ni), their distribution (which can reasonably be explained by the presence or absence of their sulfides or sulfides of other metals which can host them) in relation to different lithologies and environments, their association in different environments and lithologies and their zonal arrangement along dip, strike and across the strike of the ore zone are explained. The distribution of the elements in relation to the distribution of the sulfides, the significance of each element as a tracer or best associate of gold are given. TIle details about spatial distribution and character of gold and sulfide rich zones are treated in this work. The mineralization is believed to be the result of regional metamorphism furnishing hydrothermal fluids which resulted in extensive metasomatic alterations and subsequent emigration, remobilization and redeposition of metals in the form of their sulfides. The reprecipitation was facilitated by the formation of low poT regime within and around dilatant zones created by movements along the eastern shear zone. Recommendations as what is to be done concerning further studies on the deposit and possible prospecting guides for gold in Adola area are suggested.