Genesis of Magmatic Phosphate Mineralization in Gabbro Intrusive Rocks of Bikilal-Ghimbi area, Western Ethiopia
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Date
2013-12
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Addis Ababa Universty
Abstract
Western Ethiopia is underlain by proterozoic metamorphic belts that form parts of exposed Arabian Nubian shield. The metamorphic belt consists of high-grade biotite gneisses, low-grade volcanogic sediments and mafic ulteramafic complexes. Bikilal-Ghimbi gabbroic intrusive is an elliptical mafic body that intruded into the metamorphic gneissic body during Neoproterozoic time. It consists of olivine gabbro at its center, hornblende gabbro and hornblendite at its periphery. The olivine gabbros are very fresh and un deformed, while hornblende gabbro and hornblendites show slight weathering and with minor deformational structures. The Bikilal-Ghimbi gabbro intrusive has deserved wide attention due to its ore mineralization. Apatite, magnetite and ilmenite mineralization occurs in all this intrusive lithotypes with varying grades. Apatite occurs in the disseminated, lense and lenticular forms. Two main zones of phosphate mineralization, the upper and lower zones, were identified and delineated entirely based on phosphate (P2O5) assay values. The strike length of the upper zone is 1600m, while the strike length of the lower zone is 3000m having a thickness of 60m _200m.The mineable reserve of Soji-Bikilal phosphate deposit is estimated to be 181 million tons, at a grade of 3.5% P2O5. The major element geochemical shows that despite the difference in olivine and hornblende gabbro, there is no chemical contrast between the lithotypes, except for fluid mobile elements suggesting an origin from the same parental magma. Only the perimeter is affected by moderate metasomatism. An estimation of the parental magma composition by using trace element abundance analysia from fresh clinopyroxene and olivine gabbro bulk rock suggests intraplate-type tholiitic magma. The REE element analysis derived from apatite and host rocks show the same magmatic origin for the ores and silicate host rocks. The source of the magma may be linked to mantle degassing or to basic magmas derived by partial melting of the upper mantle. The ores were formed during magmatism as immiscible liquids which was separated from strongly differentiated magmas, aided by large volatile and alkali elements content. In this magma chamber, the phosphorous and alkali content probably led to the formation of a Fe–F, CO2–H2O–P–Na dominated immiscible melt which separated from a silica-rich melt. The breakdown of this late immiscible phase has resulted in the formation of magnetite–apatite melt and segregation of dense Fe-Ti-(P) rich melts which settled downward in a silicate crystal mush form net-textured and massive Fe-Ti oxide and phosphate ores as part of the cumulate sequence at this Bikilal-Ghimbi region. The immiscible phases of silicate and oxide- phosphates separation, breakdown, crystallization and Segregation might have occurred for more than two episodes resulting in the formation of different ore zones. The host rocks might have crystallized and segregated from the silicate rich melt which then invaded by the dense Fe-Ti-(P) rich melts which settled downward into a silicate crystal to crystallize and segregate together. The apatite free lithologies were crystallized during the less saturation of the Fe-Ti-(P) immiscible melt phase.
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Bikilal-Ghimbi area