Browsing by Author "Muluwork, Eyoel"

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    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