SCIENCE

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Browsing SCIENCE by Author "ABAA, SOLOMON IORKOSO"

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    GEOCHEMISTRY, PETROLOGY AND MINERALISATION AT RIRIWAI, GINDI AKWATI AND DUTSEN WAI IN THE NIGERIAN YOUNGER GRANITE PROVINCE.
    (1976) ABAA, SOLOMON IORKOSO
    The Nigerian Younger Granite Province is composed of anorogenic Mesozoic ring complexes which were subjected to two distinct processes of differentiation, resulting in two series; the hornblende - fayalite granite to hornblende - biotite - granite to biotite-granite and the hornblende-fayalite-granite to arfvedsonite - fayalite - granite to riebeckitic - arfvedsonite - granite. Of the two end members; biotite granite is peraluminous while the riebeckitic - arfvedsonite - granite is peralkaline Both rocks can be closely associated and have been Mineralised. The geochemistry and petrology of three selected mineralised areas of this Younger Granite Province have been studied. Detailed maps of the mineralised zones have been produced. Samples were selected from the surface as well as from cores at depth and both altered and unaltered samples have been studied mineralogically and by chemical analyses. The results of the chemical analyses and petrological studies have been used to suggest the composition of the mineralising fluids, the alteration processes and the ore minerals formed. At Ririwai, two phases of mineralisation have been found; a prejoint autometamorphic mineralisation in which K-feldspars replaced earlier perthites, while new albite developed. Silicification and recrystallisation changed the original rock texture and introduced dispersed mineralisation with thorite, columbite, xenotime and hafnium-uranium rich zircon as well as enrichment in some trace elements. The second phase of mineralisation was a post-joint replacement greisen-mineralisation which took place in the roof zones of the cooling consolidated biotite granite.. It involved a metasomatic introduction of cassitorito and sulphide ores into the crystalline biotite granite along cooling joints, fissures and fractures. The sequence of replacement has been found to be haematisation and kaolinisation - chloritisation and sericitisation - greisenisation - silicification. At Gindi Akwati, only post-joint replacement mineralisation occurred in the Older Granite with introduction of cassiterite, sulphide ores and some trace elements which were also enriched in the associated dolerite, dykes. The alteration started by shearing and mylonitisation - argillisation and haematisation -chloritisation and museovitisation - greisenisation - silicification and feldspathisation - feldspathisation. At Dutson Wai, the roof zones of the biotite granite were affected by post joint mineralisation, and weathering of the roof zones of the granite concentrated cassiterite in the alluvials. In the per-alkaline granite, there was recrystallisation with introduction of albite, microcline and the accessory minerals cryolite, pyrochlore, amblygonite and astrophyllite. .From the ore minerals formed, and the chemical and mineralogical composition of the altered wall rocks and the unaltered rocks, it is shown that in the peralkaline granite, at the time of crystallisation, residual fluids became increasingly enriched in volatiles as temperature dropped and crystallisation continued. The fluids continued to change continuously from an agpaitic magma to a sodium - silicate rich hydrothermal solution without supercritical phenomena, and the magmatic fluids progressed to hydrothermal fluids without an aqueous phase seperating off. At late stages, and near surface conditions, the confining pressure decreased to below the hydrostatic pressure, causing an immiscible liquid consisting predominantly of salts and volatiles to separate. This led to riebeckitisation of arfvedsonite, introduction of albite, quartz and microcline, mineralisation with pyrochlore, cryolite and astrophyllite and enrichment in some trace elements. In the biotite granites, as fractional crystallisation continued, the concentration of volatiles increased in the residium until saturation when an aqueous phase separated. The cooler crystalline portions of the granites in the roof zones of the intrusions were invaded by the then immiscible phases from the still crystallising granites at slightly deeper levels, causing albitisation, dispersed mineralisation with thorite, columbite, xenotime as well as enrichment in some trace elements. The late hydrothermal fluids introduced cassiterite with sulphide minerals into crystalline host rocks along cooling joints, fissures, fractures and even faults and the regional foliation of the Basement rocks were utilised. This form of mineralisation was accompanied by wall-rock alteration, while the cassiterite was transported in the ore fluids in the form of alkali-thio-stannate or/and in the form of hydroxofluorstannate complex of tin of the [SnFx (OH)6- X] type.