Determination of Genesis of Orogenic Gold and Sulfide Prospects at Ashashire Western Ethiopia
No Thumbnail Available
Date
2020-09-29
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Addis Ababa University
Abstract
Orogenic gold also known as (“shear zone hosted”, “Mesothermal”, “greenstone-hosted quartz-carbonate vein” deposit). The Ashashire area is also orogenic gold prospect area and source of placer deposit that used as an income source for artisanal gold miners.
The study area is part of Proterozoic terrains that related to the East African Orogeny (EAO), a N–S elongated mega collisional structure stretching from Israel to Madagascar and found between West and East Gondwana. It is located in western Ethiopia in Benishangul Gumuz region at Kurmuk Woreda within the Western Greenstone belt of Precambrian volcano-sedimentary belt, near the Sudan border locally called Ashashire.
Gold is an important commodity of the world. Due to this exploration work in gold including its genesis made worldwide by many scholars and well developed in developed countries. But not well developed in developing countries including Ethiopia. Some reports and exploration works show that Ethiopia is endowed by precious metals including gold in different parts of the country. As a result, the main objective of this research work is to characterize the genesis of the gold and the associated sulfides. To accomplish this objective geological field observation and data analysis including 15 doubly polished and 3 thin section petrographic samples and 20 geochemical samples both for ore and host rock characterization has been conducted. The geochemistry held by fire assay and ICP-MS. The petrographic investigation involves characterization of host rock and ore body. The host rock is mainly composed of greenschist to amphibolite facies metamorphic minerals such as chlorite, carbonate, sericite, amphibole, epidote, and quartz; the ore petrography investigation shows pyrite, pyrrhotite, magnetite, sphalerite, chalcopyrite, galena and gold. The ore microscopic and geochemical analysis indicates that gold mineralization has got strong spatial association with alteration features, such as carbonitization, silcification, sulfidation and sercitization. The concentration of gold increases with sericitized, carbonitized silicified and sulfidized host rocks. The host rock geochemistry Zr vs TiO2 shows that the tectonic setting lays within volcanic arc setting means that the formation of the deposit is related to subduction related activities. The ore geochemistry result shows the Ashashire area is target area for gold deposit ranges upto 10.3ppm that hosted in chlorite-sericite-carbonate schist in carbonate-quartz veins. The ore components possibly originated from dehydration, decarbonation and devolatilization of hydrated mafic rocks and carbonated rocks during prograde regional metamorphism and precipitates in retrograde conditions. The strong association of gold and carbonate-quartz vein with intense wallrock alteration indicates that gold is precipitated from hydrothermal solutions. The source of solution is still in debating, however the geological, petrographic and geochemistry result of the study indicates that hydrated mafic volcanic rocks and carbonated sedimentary rocks are possible sources.
Description
Keywords
Orogenic Gold, Hydrothermal Process, Alteration, Sulfide