Browsing by Author "Tadesse, Solomon (PhD)"

Now showing 1 - 2 of 2
  • No Thumbnail Available
    Item
    Geochemical Exploration for Epithermal Gold and Associated Base Metals in the Gedemsa Caldera
    (Addis Ababa Universty, 1999-08) Muluwork, Eyoel; Tadesse, Solomon (PhD)
    Several central volcanoes are present along the axial portion of the rift, which offered opportunities for potential geothermal resource investigations. The Gedemsa caldera, which is a Holocene volcanic complex in the central sector of the Main Ethiopian Rift (MER), is among those promoted for detailed investigation. Results of these investigations have, inturn, generated great interest in studying epithermal phenomena related to these volcanics. However, studies aimed at delineating epithermal-type mineralization of precious metals (mainly gold) within the MER is a relatively recentphenomena. The present work deals with preliminary geological and geochemical exploration for these occurrences within the Gedemsa caldera. Propylitic, potassic and silicification with minor argillic alteration characterize the Gedemsa Caldera. In addition to field observations, alteration is diagnosed by the presence of mineral assemblages including epidote/chlorite, calcite, quartz, k-feldspar, and oxides with some clay minerals. Ore minerals observed include pyrite, chalcopyrite with malachite/azurites, magnetite and hematite. Features indicative of epithermal activities include deposition of fine silica in the oxidized pumiceous deposits, and the thin veins of chalcedonic quartz and k-feldspar, which are intensely brecciated together with the enclosing wall rock. Occurrences of several active thermal manifestations are also known along NNE fault lines immediately outside the caldera. A total of 79 samples (stream sediment, soil, rock/chip and drill cutting) have been assayed (ICP-MS) for Au, Ag, As, Sb, Tl, Te, Pb, Zn, Cu, Mn, U and other trace elements. Statistical parameters were computed in order to define the threshold value and to separate background from anomalous ones. Anomalous values for most elements are concentrated along the E-W running central chain of hills and at the vicinity of NNESSW trending faults both in the western and eastern part within the caldera . In terms of average crustal abundance, the surveyed region is of relatively higher background content for Au, Ag, Zn and Mn. Analyses of rock/chip and drill cutting samples (ICP-MS) fromGedemsa range in value from 0.1 to 4.16 g/t Ag and from hundreds of ppb to 0.375 g/t Au, respectively. Gold values between 100-200 ppb are quite common throughout the geological profile as observed from drill hole section. The occurrence of many high gold intervals (zone) within the geologic sections reflect-different levels of mineralization. On the basis of alteration mineral assemblages and the Ag /Au ratios, a low sulphidation type of epithermal system is suggested. A preliminary reserve estimation of the gold disseminated in the upper porous pumice is discussed. From the results obtained in this work, evidence for the presence of ore-forming phenomena is positively identified, and the study, the first of its kind, offers the preliminary results of the investigation on the occurrences of epithermal type precious(Au and Ag) metallic mineralizations in the Gedemsa caldera
  • No Thumbnail Available
    Item
    Sulfide. Mineralization in the Lega Dembi Primary Gold Deposit, Sidamo, Southern Ethiopia
    (Addis Ababa Universty, 1994-06) Getaneh, Worash; Tadesse, Solomon (PhD)
    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.