Geochemical Exploration for Epithermal Gold and Associated Base Metals in the Gedemsa Caldera
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Date
1999-08
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Addis Ababa Universty
Abstract
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
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Keywords
Gedemsa Caldera