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Title: Lithogeochemistry and spinel compositions in the ultramafic complexes of Western Ethiopia : criteria for the identification of Alaskan-type intrusions
Author: Jackson, Matt
ISNI:       0000 0004 2748 851X
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2006
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This thesis covers two separate but complimentary themes. Firstly, work on the spinel composition of the ultramafic rocks of has provided evidence that they formed as Alaskan-type intrusions. This has implications both for prospectivity and for the understanding of neoproterozoic plate movements. Additionally, the study of the rock and soil geochemistry has been used to identify exploration targets and evaluate the prospectivity of the major complexes. It is proposed here that the ultramafic complexes Tulu Dimtu, Kingy, Daleti, Ankori and Yubdo, in the Western Ethiopian Shield are Alaskan type intrusions. Alaskan-type intrusions are concentrically zoned ultramafic intrusions thought to be the feeder pipes of volcanoes. They have dunite at the core and grade outwards to clinopyroxenite and sometimes hornblendite. These intrusions typically occur in continental arc settings such as Alaska, British Colombia and the Urals. This compares with ophiolite complexes which are thought to be obducted oceanic upper mantle and crust. The two types of complex may be difficult to distinguish because if ophiolites are dismembered and deformed fragments, they may resemble Alaskan-type intrusions. This thesis documents several features of these Ethiopian complexes that are typical of Alaskan type intrusions. Mapping of the Yubdo complex has shown a circular out crop pattern with concentric zones of clinopyroxenite at the edge to dunite at the centre. New mapping of the Tulu Dimtu area has shown a similar zoned circular body. Both the Yubdo and Tulu Dimtu complexes show concentric zones of Cr values, where the greatest values occur at the edge and the lowest in the centre. These mafic and ultramafic intrusions are very altered by surface weathering but contain chrome- spinels which have a geochemistry which is unlike ophiolites and similar to Alaskan-type intrusions. The analysis of these spinels demonstrates how the compositions of spinel Fe2+ and Cr values may be used to distinguish between an Alaskan type or ophiolite complexes. In Alaskan-type intrusions, spinels with Fe2+ values greater than 0.85 frequently have Cr values which are lower than 0.5. Such low Cr values in spinels with high Fe2+ values are rare in ophiolite complexes. The variations in Ni, Cu, Cr and Al contents of highly altered ultramafics and fresher ultramafics have been used to investigate the magmatic and post-magmatic ore forming processes that have influenced the Pt and Pd content of the complexes. It is expected that medium to low temperature hydrothermal activity may have had a significant impact on the geochemistry of even the fresher rocks. The discovery of high Pd values and low Pt in lithologies such as talc-schists and quartzite support the idea that Pd is more mobile than Pt in medium to low temperature conditions. Furthermore, in the Tulu Dimtu Main Intrusion, the altered rocks indicate that Pd has been removed from the magmatic sites of concentration. In a few places it is possible to see through the extensive alteration and potential magmatic processes can be considered. Within the Tulu Dimtu Main Intrusion and the Main Yubdo Intrusion, the most primitive rocks occur at the flanks. Furthermore, it is indicated that sulphide segregation may have occurred in the Tulu Dimtu Main Intrusion and Daleti Ultramafic. In the Daleti Ultramafic, regardless of the presence of sulphides, the Pt and Pd values remain low - it is therefore unlikely that the complex hosts economic grades. However, at Tulu Dimtu and Yubdo, it is recommended that any future exploration be targeted at the flanks of the complexes, where magmatic and post-magmatic processes may potentially co-incide to elevate the grade. Much work has been published to document the nature of platinum-group minerals (PGM) in the alluvial and eluvial placers around the Yubdo area and some PGM have been discovered in the serpentinsed dunites of the main Yubdo intrusion. In this thesis further discoveries of PGM have been made in the serpentinised dunites and chromites from the Yubdo area. In the course of this work new base maps covering the ultramafic complexes and the surrounding basement have been produced. These have revealed many smaller ultramafic bodies which are referred to here as the Lensoid Ultramafics. Both spinel and whole rock geochemistry supports the hypothesis that these are slivers of rock "sheared-off" the outside of larger complexes. The conclusion that these complexes have an Alaskan-type origin has consequences for the understanding of plate movements in this part of the Neoproterozoic Western Ethiopian Shield. Additionally, the identification of exploration targets will help focus efforts to uncover any potential economic mineralisation.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available