The petrology and mineralization of the Yubdo-Daleti-Tulu Dimtu mafic-ultramafic rocks and the surrounding basement rocks in Western Ethiopia

Research project P 13643 Petrology and mineralization in Western Ethiopia Aberra MOGESSIE 28.06.1999 From 1993 to 1996 a cooperative project between the Institute of Geology and Paleontolog and the Institute of Mineralog -Crystallography and Petrology of the Karl-Franzens University Graz on the "Geodynarnic setting of the Panafrican Orogenesis in Northeast Africa" (funded by the Austrian Science Foundation, FWF project P9703-Geo, project leader, Prof. E. Wallbrecher) was undertaken. This resulted in an integrated study of the Panafrican evolution of the Central Eastern Desert of Egypt. Results of this project indicated tectonic processes dominated by island arc accretion and transpression during oblique convergence in the Egyptian part of the Panafrican mobile belt which forms a N-S striking belt in Eastern Africa and stretches from Egypt to Mozambique. Previous studies in the southern parts of the Panafrican mobile belt (e.g., in Tanzania and Kenya), suggest that magmatic underplating, probably connected with crustal thickening during continent collision are the dominant tectonic processes which contrasts with the Panafrican evolution in Egypt. Structural and metamorphic studies of the Mozambique belt in Kenya and Tanzania with a reported collisional-type setting in combination with results collected in the Arabian Nubian Shield of Egypt offer the unique opportunity to compare different tectonic processes along the strike of one mobile belt, and to gain most pertinent data on the evolution of Gondwana and the collision of E- and W- Gondwana during Panafrican times. A research proposal for an integrated structural and petrological study of the mobile belt of Tanzania and Kenya in comparison with the results from the Panaffican mobile belt of Egypt with the aim of developing an integrated tectonic model has been approved by the Austrian Research Council (FWF - Project P12375-GEO, project leader, Prof Wallbrecher) and started on 1.1.98. Although a large amount of data is available on the regional geology of western Ethiopia detailed petrological, structural, geochernical, isotopic, and economic geological studies are lacking. These studies are basic requirements for characterizing the metamorphic and magmatic evolution of different tectonic units locally, and lead to a better understanding of the P-T-t conditions of metamorphism and the geologic history of the region. Detailed structural investigations, field data and microfabrics, are necessary to understand the type and style of collision in this part of the Mozambique Belt. Further to the north, in the Eastern Desert of Egypt, a lot of recent data could proof earlier stacking processes of ophiolitic nappes and following exhumation in a transpressional regime. In the south, in Kenya and Tanzania, deeper parts of continental crust are predominant and ophiolitic material is not yet really known. Pronounced strike slip shear zone and mylonites were found in Egypt which continue into Sudan. Shear zones have also been described in Northern Kenya. Therefore, it seems to be of great importance to receive more structural information about Ethiopia and to localize the possible Suture zone between East- and West-Gondwana in this region and to analyse the style of convergence. Additionally, the economic mineral potential in the different metamorphic units, and in the mafic: and ultramafic rocks has been considered, but quantitative investigations are still lacking. The Yubdo ultramafic rocks are one -of the few known platinum - bearing bodies world wide and the type locality for erlichmanite (OsS2). However, except for the work of Cabri et al. (1981), a detailed modem study of the platinum-group minerals at Yubdo has not been undertaken up to now. Scientific and economic significance is attached to this work. Therefore, the present proposal will deal with the petrological and structural study of the basement and enclosed mafic-ultramafic rocks in the western belt of Ethiopia and, intends to fill the gap between the already studied Pan- African belt of the Eastern Desert of Egypt and the ongoing research work in Tanzania and northern Kenya. In addition to the geological, structural and petrological studies of the Pan-African basement in western Ethiopia, a detailed study of the ultramafic rocks (ophiolites ?) in the Yubdo area will be made.

The 9 km long and 4 - 5 km wide ultramafic intrusion of Yubdo which strike NNE-SSW are underlain by Precambrian basement consisting mainly of gneisses, mica schists, quartzites, amphibolites and chlorite schists. Acidic rocks (syn-tectonic granodiorites, hybrid granites, quartz-diorite) intrude the ultramafic rocks. Several Pt-Fe primary grains were identified enclosed in chromite phases and along alteration zones. Nuggets contain a wide variety of inclusions ranging from hollingworthite, irarsite, erlichmanite, laurite, genkinite, stibio- palladinite, and osmium laths with iron oxide coatings.The embayed and sculpted surfaces of the platinum-iron nuggets and their intimate association with fine grained iron oxides in the cavities of the nuggets documented from Yubdo are also features of gold grains considered to have grown in a lateritic environment indicating that the processes of formation of gold in laterites are likely to be paralleled by comparable development of platinum-group minerals (PGM). Based on the investigation made one can conclude that 1) the occurrence of droplets of PGM in chromites from bore hole ultramafic samples at depth suggests a magmatic origin; and 2) a remobilization and transport of the Pt-Fe alloys have taken place from a possible dunitic source and concentrated them in the laterites. The Western Ethiopian Shield comprises three lithotectonic units: the Birbir domain, an assemblage of deformed and metamorphosed ultramafic to felsic intrusive, extrusive and volcano-sedimentary rocks, is bounded by the dominantly orthogneissic Baro and Geba domains. Bulk chemical analyses show that the metamorphosed plutonic and volcanic rocks of the Birbir domain are predominantly calc-alkaline and similar to those generated by subduction in modern magmatic arcs. They belong in part to the low-K series, suggesting an oceanic environment. This interpretation is supported by Sm-Nd isotope data which indicate an early crust-forming event at ca. 1050 Ma which is interpreted to be an oceanic island arc complex. A primitive source is supported by low initial 87Sr/86Sr ratios. This complex was accreted and formed the core and source area for subsequent plutonism and metamorphism. The earliest recorded deformation event resulted in the formation of a sub-horizontal gneissosity within the gneissic terrains which was synchronous with an early metamorphic event (M 1 ) which record amphibolite facies conditions (600-800 C and ca. 6-8 kbar) in the gneissic terrains and greenschist facies conditions in the Birbir domain. The igneous protoliths to the orthogneisses (i.e. pre-dating M 1 ) are dated at 830-785 Ma. All terrains were subsequently deformed in the D2 event which was the result of severe E-W directed crustal shortening and an anticlockwise P-T-t path is implied. Subsequent D3 deformation was concentrated within mylonitised domain boundaries which record major transcurrent movement. A second metamorphic event, M 2 , at around 635 Ma, resulted in localised rehydration of M 1 parageneses and granite genesis. The peak of M 2 was synchronous with D3 shearing. Reactivation and associated fluid incursion was concentrated within sheared domain boundaries and caused protracted isotopic re-equilibration until 410 Ma. The WES records a long-lived, episodic development. Its evolution can be explained in terms of closure of an arc- back-arc setting. Melting of a subducting slab and associated intrusion caused metamorphism and the formation of an oceanic island arc complex. Continued plate convergence caused severe E-W shortening and basin closure. Further attenuation gave rise to transcurrent shearing, fluid influx and a second thermal event during the accretion of microcontinents and closure of the orogen.