Major elements oxides, trace elements and REE data on some dolerites and dioritic rocks from the ... more Major elements oxides, trace elements and REE data on some dolerites and dioritic rocks from the Lower Benue rift indicate that they are mainly basic in composition and belong to the alkali basalt magma series. They are characterized by enrichments in incompatible elements and high fractionation indices, (La/Yb)N, 5.7–14.72 which reflect LREE enrichment and indicate the presence of garnet in the source of the rocks. The ratios (Tb/Yb)N (1.63–2.6)and Dy/Yb (2.37–3.34) also indicate garnet-bearing lherzolite sources. The HFSE ratios Zr/Nb (0.88–2.6) and Y/Nb (0.57–1.57) suggest that the rocks were derived from enriched source(s) with dominant HIMU signature. The rocks were most likely formed in a within-plate setting of the intracontinental rift-type, similar to the Kenyan rift. Crustal contamination and interactions of the rocks with aqueous fluids are suggested by depletions in the more mobile LILE (Ba, Rb, K and Sr). The major element (K), some of the trace elements (Th, Co) and the REE (La, Ce) were heavily mobilized in some of the rocks (highly altered dolerites and dioritic rocks) during a low-grade metamorphic alteration. This is thought to have been caused by an intense hydrothermal activity involving a CO2-rich hydrous phase contained in the crustal materials. This hydrous phase is possibly related to the saline groundwater common in the area with high concentrations of Ca2+, Na+, Cl−, Sr+, Ba+ and K+, thought to have been pressed out from the calcareous marine sediments during their burial.
This study presents the field, petrography and geochemical data of the late Neoproterozoic pegmat... more This study presents the field, petrography and geochemical data of the late Neoproterozoic pegmatites and host rocks in northcentral basement complex of Nigeria. Wamba pegmatites are emplaced within the reworked belt of the Nigerian basement complex. The pegmatites are composed of complex partly zoned (rare-metal pegmatites) and sparse simple (barren) pegmatites. They occur mostly as dyke (NE-SW) in migmatite-gneiss complex composed of mainly migmatitic gneisses and muscovite-biotite gneisses. The complex pegmatites are generally composed of albite, muscovite and quartz with accessory of garnet, tourmaline, beryl, ilmenite, apatite and cassiterite-columbite-tantalite minerals. The minerals show overlapping variations across the zones with albite-tourmaline wall zone, albite-beryl intermediate and quartz-spodumene core zone. The simple pegmatite constitutes of quartz, microcline and biotites. Muscovites and albite occur as minor components. The host rocks are composed of quartz, plagioclase, microcline, muscovite and biotite. Cordierite, garnet, sillimanite and hornblende are minor. The pegmatites and paragneisses are highly peraluminous with the complex pegmatites showing internal fractionation. The complex pegmatites exhibit low K/Rb, Nb/Ta and Mg/Li with relative enrichment in Rb, Li, Cs, Be, Sn, Nb and Ta from wall to core zone. The pegmatites and host rocks show similar geochemical signatures; equilibrium batch melting model further indicated crustal origin with the pegmatites as anatexis products of post-collisional reactivation of the high-grade metamorphic terrain. The rare elements formed by low-temperature partial melting of biotite-bearing pelitic rocks and contribution of hydrothermal fluids which led to albitization and metasomatism that result to high concentration of Sn.
Abstract The pegmatites in southern Akwanga occur within the reactivated belt of the basement com... more Abstract The pegmatites in southern Akwanga occur within the reactivated belt of the basement complex of Nigeria. The pegmatites consist of dominantly albite–muscovite pegmatites (rare-metal), southern parts of the map areas and biotite-microcline pegmatites (barren) central parts of the map. The pegmatites intruded gneiss-migmatitic complex consist of metasedimentry rocks; granitic gneisses and biotite gneisses and rarely meta-igneous, amphibolites. The rare metal pegmatites are composed of quartz, albite and muscovites and tourmaline. Garnets, ilmenites and minor tin–columbite–tantalite mineralization constitute accessory minerals in contrast to the biotite-microcline pegmatites. The host rocks are composed of quartz, plagioclase (An 5–21 ; albites–oligoclase), microcline and muscovite. Minor constituents include biotites, cordierites and hornblendes. Ilmenite occurs as opaques. The pegmatites and their host rocks are corundum and hypersthene normative, highly peraluminous, exhibiting similar geochemical signatures; however, the rare metal pegmatites are more fractionated than the host rocks and the biotite-microcline pegmatites. The rare metal pegmatites are relatively enriched in Rb, Li, Cs, B, Be, Nb and Ta, low in K/Rb and Al/Ga ratios than the biotite–microcline–pegmatites and their host rocks. The pegmatites are products of crustal anatexis of sedimentary origin. This indicates that the rare metal pegmatites are source rock controlled (product of post-collision activities) rather than fractional crystallization. Research highlights Rare metal pegmatites occur in Southern Akwanga, north central Nigeria Highly peraluminous and LCT pegmatites Sn–Nb–Ta mineralization potentials in the pegmatite They are related to the migmatitic-gneiss complex by anatexis
... Petrography of some altered intrusive rocks from the Lower Benue Trough, Nigeria.Smart C Obio... more ... Petrography of some altered intrusive rocks from the Lower Benue Trough, Nigeria.Smart C Obiora, Alphonso C Umeji. Abstract. No Abstract. Journal of Mining and Geology 2005, Vol 41(1): 1-9. Full Text: DOWNLOAD FULL TEXT ...
Lead, zinc, and associated minerals have been mined in the Enyigba, Nigeria area for over 92 year... more Lead, zinc, and associated minerals have been mined in the Enyigba, Nigeria area for over 92 years, leaving accumulated sulphide dumps and contaminated mine water. Stream sediment, mine tailings, mine water, stream water, and groundwater were sampled and analysed to determine the sources of potentially harmful elements in domestic water. The mine water contained mean concentrations of Cd, As, Co, Cr, Mn, Pb, and Ni that greatly exceeded those of the stream water, but samples from the streams and groundwater exceeded Nigerian drinking water and WHO thresholds for Al, Fe, and Pb, except for acceptable Pb concentrations in the borehole. Mn concentrations in one stream also exceeded Nigerian and WHO threshold for drinking water. In addition to the Pb–Zn mining, geological weathering also contributed to Pb, Al, Fe, and Mn contamination in the area.ZusammenfassungIn der Enyigba-Region (Nigeria) werden Blei, Zink und weitere Minerale seit 92 Jahren bergbaulich gewonnen. Die Rückstände werden auf sulfidhaltigen Halden gelagert, was die Bildung von schadstoffhaltigen Grubenwasser begünstigt. In dieser Studie wurden Sedimentproben aus Vorflutern und Tailings sowie Wasserproben des Grubenabwassers und des Oberflächen- und Grundwassers untersucht, um mögliche Schadstoffquellen im genutzten Wasser zu identifizieren. Die mittleren Cd-, As-, Co-, Cr-, Mn-, Pb- und Ni- Konzentrationen des Grubenwassers liegen weit über den Werten des Oberflächenwassers. Die Proben aus dem Oberflächen- und Grundwasser liegen sowohl über den nigerianischen Trinkwasser- als auch über den WHO-Grenzwerten für Al, Fe und Pb. Ausnahmen existieren für Pb-Konzentrationen in Bohrlöchern. Die Mn-Gehalte in einem der Vorfluter übersteigen ebenfalls die gültigen Trinkwassergrenzwerte Nigerias und der WHO. Neben bergbaubedingten Freisetzungsprozessen aus dem Pb-Zn-Bergbau spielen auch geogene Verwitterungsprozesse eine Rolle, welche zusätzlich zu einer Erhöhung der Pb-, Al-, Fe- und Mn-Werte in der Region beitragen.ResumenEl plomo, el zinc y los minerales asociados han sido extraídos en el área de Enyigba, Nigeria, durante más de 92 años, dejando relaves con sulfuro y agua de mina contaminada. Los sedimentos fluviales, los relaves de la mina, el agua de mina y las aguas superficiales y subterráneas fueron muestreados y analizados para determinar las fuentes de elementos potencialmente dañinos en el agua de consumo doméstico. El agua de la mina contenía concentraciones medias de Cd, As, Co, Cr, Mn, Pb y Ni que superaban en gran medida a las de las aguas superficiales y subterráneas pero éstas superaban los niveles aceptados para el agua potable por la legislación nigeriana y por la OMS para Al, Fe y Pb, excepto por las concentraciones aceptables de Pb en el pozo. Las concentraciones de Mn en el agua superficial también superaron los mínimos aceptados para el agua potable. Además de la minería Pb-Zn, la meteorización geológica también contribuyó a la contaminación por Pb, Al, Fe y Mn en el área.抽象尼日利亚Enyigba的铅、锌及其它矿物开采历史已经92年,遗留大量含硫化物的固体废物和污染的采矿废水。采集和测试了研究区河底污泥、尾矿、采矿废水、河水和地下水以识别生活用水的潜在有害元素污染源。采矿废水的Cd、As、Co、Cr、Mn、Pb和Ni平均浓度远大于河水;除了钻孔水Pb浓度处于可容许范围,河水和地下水的Al、Fe和Pb浓度已经超过尼日利亚和世界卫生组织饮用水标准。一条河水的Mn浓度也超过了尼日利亚和世卫组织饮用水标准。除了Pb-Zn开采,地质风化也使Pb、Al、Fe和Mn浓度提高。
This work presents updates on the tectonomagmatic origin of igneous rocks in the lower Benue rift... more This work presents updates on the tectonomagmatic origin of igneous rocks in the lower Benue rift based on recent available geochemical data. The igneous rocks occur as volcanic and plutonic rock series. The rock types are basalts and basaltic sills, pyroclastic rocks (tuffs and lapilli tuff), trachytes, trachybasalts, gabbros (stocks), dolerite sills, and dioritic rocks (stocks). The rocks were emplaced within folded sedimentary sequence of shales, subordinate sandstones, and siltstone and carbonate rocks of Asu-River and Eze-Aku Group. Most of the rocks are moderately to highly altered, with the pyroclastic rocks showing higher effects of alteration (LOI 3 to 22 %). Incompatible and immobile elements and less altered rocks with LOI < 3% show that the rocks were derived from mantle enriched source and possibly HIMU signatures. The high ratios of (La/Yb)N 4.40 to 31.55 in the basic rocks and 23.59 to 135.35 in the trachytic rocks show alkaline affinity with sparse tholeiites and garnets in the source region. The rocks were formed by plume uprising in a within-plate setting of intra-continental rift system similar to the South Atlantic Ocean rocks and East African rift. The magnesium number (Mg#) and differences of the spidergrams and rare earth patterns indicate that the rocks are cogenetic, and source is heterogeneous. The depletion of K, P, Th, and Rb and formation of calcites and sericites in the rocks show mobilization probably by interaction with calcareous host rocks in prevailing conditions.
This study presents the field, petrography and geochemical data of the late Neoproterozoic pegmat... more This study presents the field, petrography and geochemical data of the late Neoproterozoic pegmatites and host rocks in northcentral basement complex of Nigeria. Wamba pegmatites are emplaced within the reworked belt of the Nigerian basement complex. The pegmatites are composed of complex partly zoned (rare-metal pegmatites) and sparse simple (barren) pegmatites. They occur mostly as dyke (NE-SW) in migmatite-gneiss complex composed of mainly migmatitic gneisses and muscovite-biotite gneisses. The complex pegmatites are generally composed of albite, muscovite and quartz with accessory of garnet, tourmaline, beryl, ilmenite, apatite and cassiterite-columbite-tantalite minerals. The minerals show overlapping variations across the zones with albite-tourmaline wall zone, albite-beryl intermediate and quartz-spodumene core zone. The simple pegmatite constitutes of quartz, microcline and biotites. Muscovites and albite occur as minor components. The host rocks are composed of quartz, plagioclase, microcline, muscovite and biotite. Cordierite, garnet, sillimanite and hornblende are minor. The pegmatites and paragneisses are highly peraluminous with the complex pegmatites showing internal fractionation. The complex pegmatites exhibit low K/Rb, Nb/Ta and Mg/Li with relative enrichment in Rb, Li, Cs, Be, Sn, Nb and Ta from wall to core zone. The pegmatites and host rocks show similar geochemical signatures; equilibrium batch melting model further indicated crustal origin with the pegmatites as anatexis products of post-collisional reactivation of the high-grade metamorphic terrain. The rare elements formed by low-temperature partial melting of biotite-bearing pelitic rocks and contribution of hydrothermal fluids which led to albitization and metasomatism that result to high concentration of Sn.
Abstract The pyroclastic rocks in the Cretaceous Abakaliki basin occur mostly as oval-shaped bodi... more Abstract The pyroclastic rocks in the Cretaceous Abakaliki basin occur mostly as oval-shaped bodies, consisting of lithic/lava and vitric fragments. They are commonly characterized by parallel and cross laminations, as well contain xenoliths of shale, mudstone and siltstones from the older Asu River Group of Albian age. The rocks are basic to ultrabasic in composition, comprising altered alkali basalts, altered tuffs, minor lapillistones and agglomerates. The mineral compositions are characterized mainly by laths of calcic plagioclase, pyroxene (altered), altered olivines and opaques. Calcite, zeolite and quartz represent the secondary mineral constituents. Geochemically, two groups of volcaniclastic rocks, are distinguished: alkaline and tholeiitic rocks, both represented by fresh and altered rock samples. The older alkali basalts occur within the core of the Abakaliki anticlinorium while the younger tholeiites occur towards the periphery. Though most of the rocks are moderate to highly altered [Loss on ignition (LOI, 3.43–22.07 wt. %)], the use of immobile trace element such as Nb, Zr, Y, Hf, Ti, Ta and REEs reflect asthenospheric mantle source compositions. The rocks are enriched in incompatible elements and REEs (∑REE = 87.98–281.0 ppm for alkaline and 69.45–287.99 ppm for tholeiites). The ratios of La/Yb n are higher in the alkaline rocks ranging from 7.69 to 31.55 compared to the tholeiitic rocks which range from 4.4 to 16.89 and indicating the presence of garnet-bearing lherzolite in the source mantle. The spidergrams and REEs patterns along with Zr/Nb, Ba/Nb, Rb/Nb ratios suggest that the rocks were generated by a mantle plume from partial melting of mixed enriched mantle sources (HIMU, EMI and EMII) similar to the rocks of the south Atlantic Ocean such as St. Helena (alkaline rocks) and Ascension rocks (tholeiitic rocks). The rocks were formed in a within-plate setting of the intra-continental rift type similar to other igneous rocks in the Benue Rift and are not related to any subduction event as previously suggested.
Major elements oxides, trace elements and REE data on some dolerites and dioritic rocks from the ... more Major elements oxides, trace elements and REE data on some dolerites and dioritic rocks from the Lower Benue rift indicate that they are mainly basic in composition and belong to the alkali basalt magma series. They are characterized by enrichments in incompatible elements and high fractionation indices, (La/Yb)N, 5.7–14.72 which reflect LREE enrichment and indicate the presence of garnet in the source of the rocks. The ratios (Tb/Yb)N (1.63–2.6)and Dy/Yb (2.37–3.34) also indicate garnet-bearing lherzolite sources. The HFSE ratios Zr/Nb (0.88–2.6) and Y/Nb (0.57–1.57) suggest that the rocks were derived from enriched source(s) with dominant HIMU signature. The rocks were most likely formed in a within-plate setting of the intracontinental rift-type, similar to the Kenyan rift. Crustal contamination and interactions of the rocks with aqueous fluids are suggested by depletions in the more mobile LILE (Ba, Rb, K and Sr). The major element (K), some of the trace elements (Th, Co) and the REE (La, Ce) were heavily mobilized in some of the rocks (highly altered dolerites and dioritic rocks) during a low-grade metamorphic alteration. This is thought to have been caused by an intense hydrothermal activity involving a CO2-rich hydrous phase contained in the crustal materials. This hydrous phase is possibly related to the saline groundwater common in the area with high concentrations of Ca2+, Na+, Cl−, Sr+, Ba+ and K+, thought to have been pressed out from the calcareous marine sediments during their burial.
This study presents the field, petrography and geochemical data of the late Neoproterozoic pegmat... more This study presents the field, petrography and geochemical data of the late Neoproterozoic pegmatites and host rocks in northcentral basement complex of Nigeria. Wamba pegmatites are emplaced within the reworked belt of the Nigerian basement complex. The pegmatites are composed of complex partly zoned (rare-metal pegmatites) and sparse simple (barren) pegmatites. They occur mostly as dyke (NE-SW) in migmatite-gneiss complex composed of mainly migmatitic gneisses and muscovite-biotite gneisses. The complex pegmatites are generally composed of albite, muscovite and quartz with accessory of garnet, tourmaline, beryl, ilmenite, apatite and cassiterite-columbite-tantalite minerals. The minerals show overlapping variations across the zones with albite-tourmaline wall zone, albite-beryl intermediate and quartz-spodumene core zone. The simple pegmatite constitutes of quartz, microcline and biotites. Muscovites and albite occur as minor components. The host rocks are composed of quartz, plagioclase, microcline, muscovite and biotite. Cordierite, garnet, sillimanite and hornblende are minor. The pegmatites and paragneisses are highly peraluminous with the complex pegmatites showing internal fractionation. The complex pegmatites exhibit low K/Rb, Nb/Ta and Mg/Li with relative enrichment in Rb, Li, Cs, Be, Sn, Nb and Ta from wall to core zone. The pegmatites and host rocks show similar geochemical signatures; equilibrium batch melting model further indicated crustal origin with the pegmatites as anatexis products of post-collisional reactivation of the high-grade metamorphic terrain. The rare elements formed by low-temperature partial melting of biotite-bearing pelitic rocks and contribution of hydrothermal fluids which led to albitization and metasomatism that result to high concentration of Sn.
Abstract The pegmatites in southern Akwanga occur within the reactivated belt of the basement com... more Abstract The pegmatites in southern Akwanga occur within the reactivated belt of the basement complex of Nigeria. The pegmatites consist of dominantly albite–muscovite pegmatites (rare-metal), southern parts of the map areas and biotite-microcline pegmatites (barren) central parts of the map. The pegmatites intruded gneiss-migmatitic complex consist of metasedimentry rocks; granitic gneisses and biotite gneisses and rarely meta-igneous, amphibolites. The rare metal pegmatites are composed of quartz, albite and muscovites and tourmaline. Garnets, ilmenites and minor tin–columbite–tantalite mineralization constitute accessory minerals in contrast to the biotite-microcline pegmatites. The host rocks are composed of quartz, plagioclase (An 5–21 ; albites–oligoclase), microcline and muscovite. Minor constituents include biotites, cordierites and hornblendes. Ilmenite occurs as opaques. The pegmatites and their host rocks are corundum and hypersthene normative, highly peraluminous, exhibiting similar geochemical signatures; however, the rare metal pegmatites are more fractionated than the host rocks and the biotite-microcline pegmatites. The rare metal pegmatites are relatively enriched in Rb, Li, Cs, B, Be, Nb and Ta, low in K/Rb and Al/Ga ratios than the biotite–microcline–pegmatites and their host rocks. The pegmatites are products of crustal anatexis of sedimentary origin. This indicates that the rare metal pegmatites are source rock controlled (product of post-collision activities) rather than fractional crystallization. Research highlights Rare metal pegmatites occur in Southern Akwanga, north central Nigeria Highly peraluminous and LCT pegmatites Sn–Nb–Ta mineralization potentials in the pegmatite They are related to the migmatitic-gneiss complex by anatexis
... Petrography of some altered intrusive rocks from the Lower Benue Trough, Nigeria.Smart C Obio... more ... Petrography of some altered intrusive rocks from the Lower Benue Trough, Nigeria.Smart C Obiora, Alphonso C Umeji. Abstract. No Abstract. Journal of Mining and Geology 2005, Vol 41(1): 1-9. Full Text: DOWNLOAD FULL TEXT ...
Lead, zinc, and associated minerals have been mined in the Enyigba, Nigeria area for over 92 year... more Lead, zinc, and associated minerals have been mined in the Enyigba, Nigeria area for over 92 years, leaving accumulated sulphide dumps and contaminated mine water. Stream sediment, mine tailings, mine water, stream water, and groundwater were sampled and analysed to determine the sources of potentially harmful elements in domestic water. The mine water contained mean concentrations of Cd, As, Co, Cr, Mn, Pb, and Ni that greatly exceeded those of the stream water, but samples from the streams and groundwater exceeded Nigerian drinking water and WHO thresholds for Al, Fe, and Pb, except for acceptable Pb concentrations in the borehole. Mn concentrations in one stream also exceeded Nigerian and WHO threshold for drinking water. In addition to the Pb–Zn mining, geological weathering also contributed to Pb, Al, Fe, and Mn contamination in the area.ZusammenfassungIn der Enyigba-Region (Nigeria) werden Blei, Zink und weitere Minerale seit 92 Jahren bergbaulich gewonnen. Die Rückstände werden auf sulfidhaltigen Halden gelagert, was die Bildung von schadstoffhaltigen Grubenwasser begünstigt. In dieser Studie wurden Sedimentproben aus Vorflutern und Tailings sowie Wasserproben des Grubenabwassers und des Oberflächen- und Grundwassers untersucht, um mögliche Schadstoffquellen im genutzten Wasser zu identifizieren. Die mittleren Cd-, As-, Co-, Cr-, Mn-, Pb- und Ni- Konzentrationen des Grubenwassers liegen weit über den Werten des Oberflächenwassers. Die Proben aus dem Oberflächen- und Grundwasser liegen sowohl über den nigerianischen Trinkwasser- als auch über den WHO-Grenzwerten für Al, Fe und Pb. Ausnahmen existieren für Pb-Konzentrationen in Bohrlöchern. Die Mn-Gehalte in einem der Vorfluter übersteigen ebenfalls die gültigen Trinkwassergrenzwerte Nigerias und der WHO. Neben bergbaubedingten Freisetzungsprozessen aus dem Pb-Zn-Bergbau spielen auch geogene Verwitterungsprozesse eine Rolle, welche zusätzlich zu einer Erhöhung der Pb-, Al-, Fe- und Mn-Werte in der Region beitragen.ResumenEl plomo, el zinc y los minerales asociados han sido extraídos en el área de Enyigba, Nigeria, durante más de 92 años, dejando relaves con sulfuro y agua de mina contaminada. Los sedimentos fluviales, los relaves de la mina, el agua de mina y las aguas superficiales y subterráneas fueron muestreados y analizados para determinar las fuentes de elementos potencialmente dañinos en el agua de consumo doméstico. El agua de la mina contenía concentraciones medias de Cd, As, Co, Cr, Mn, Pb y Ni que superaban en gran medida a las de las aguas superficiales y subterráneas pero éstas superaban los niveles aceptados para el agua potable por la legislación nigeriana y por la OMS para Al, Fe y Pb, excepto por las concentraciones aceptables de Pb en el pozo. Las concentraciones de Mn en el agua superficial también superaron los mínimos aceptados para el agua potable. Además de la minería Pb-Zn, la meteorización geológica también contribuyó a la contaminación por Pb, Al, Fe y Mn en el área.抽象尼日利亚Enyigba的铅、锌及其它矿物开采历史已经92年,遗留大量含硫化物的固体废物和污染的采矿废水。采集和测试了研究区河底污泥、尾矿、采矿废水、河水和地下水以识别生活用水的潜在有害元素污染源。采矿废水的Cd、As、Co、Cr、Mn、Pb和Ni平均浓度远大于河水;除了钻孔水Pb浓度处于可容许范围,河水和地下水的Al、Fe和Pb浓度已经超过尼日利亚和世界卫生组织饮用水标准。一条河水的Mn浓度也超过了尼日利亚和世卫组织饮用水标准。除了Pb-Zn开采,地质风化也使Pb、Al、Fe和Mn浓度提高。
This work presents updates on the tectonomagmatic origin of igneous rocks in the lower Benue rift... more This work presents updates on the tectonomagmatic origin of igneous rocks in the lower Benue rift based on recent available geochemical data. The igneous rocks occur as volcanic and plutonic rock series. The rock types are basalts and basaltic sills, pyroclastic rocks (tuffs and lapilli tuff), trachytes, trachybasalts, gabbros (stocks), dolerite sills, and dioritic rocks (stocks). The rocks were emplaced within folded sedimentary sequence of shales, subordinate sandstones, and siltstone and carbonate rocks of Asu-River and Eze-Aku Group. Most of the rocks are moderately to highly altered, with the pyroclastic rocks showing higher effects of alteration (LOI 3 to 22 %). Incompatible and immobile elements and less altered rocks with LOI < 3% show that the rocks were derived from mantle enriched source and possibly HIMU signatures. The high ratios of (La/Yb)N 4.40 to 31.55 in the basic rocks and 23.59 to 135.35 in the trachytic rocks show alkaline affinity with sparse tholeiites and garnets in the source region. The rocks were formed by plume uprising in a within-plate setting of intra-continental rift system similar to the South Atlantic Ocean rocks and East African rift. The magnesium number (Mg#) and differences of the spidergrams and rare earth patterns indicate that the rocks are cogenetic, and source is heterogeneous. The depletion of K, P, Th, and Rb and formation of calcites and sericites in the rocks show mobilization probably by interaction with calcareous host rocks in prevailing conditions.
This study presents the field, petrography and geochemical data of the late Neoproterozoic pegmat... more This study presents the field, petrography and geochemical data of the late Neoproterozoic pegmatites and host rocks in northcentral basement complex of Nigeria. Wamba pegmatites are emplaced within the reworked belt of the Nigerian basement complex. The pegmatites are composed of complex partly zoned (rare-metal pegmatites) and sparse simple (barren) pegmatites. They occur mostly as dyke (NE-SW) in migmatite-gneiss complex composed of mainly migmatitic gneisses and muscovite-biotite gneisses. The complex pegmatites are generally composed of albite, muscovite and quartz with accessory of garnet, tourmaline, beryl, ilmenite, apatite and cassiterite-columbite-tantalite minerals. The minerals show overlapping variations across the zones with albite-tourmaline wall zone, albite-beryl intermediate and quartz-spodumene core zone. The simple pegmatite constitutes of quartz, microcline and biotites. Muscovites and albite occur as minor components. The host rocks are composed of quartz, plagioclase, microcline, muscovite and biotite. Cordierite, garnet, sillimanite and hornblende are minor. The pegmatites and paragneisses are highly peraluminous with the complex pegmatites showing internal fractionation. The complex pegmatites exhibit low K/Rb, Nb/Ta and Mg/Li with relative enrichment in Rb, Li, Cs, Be, Sn, Nb and Ta from wall to core zone. The pegmatites and host rocks show similar geochemical signatures; equilibrium batch melting model further indicated crustal origin with the pegmatites as anatexis products of post-collisional reactivation of the high-grade metamorphic terrain. The rare elements formed by low-temperature partial melting of biotite-bearing pelitic rocks and contribution of hydrothermal fluids which led to albitization and metasomatism that result to high concentration of Sn.
Abstract The pyroclastic rocks in the Cretaceous Abakaliki basin occur mostly as oval-shaped bodi... more Abstract The pyroclastic rocks in the Cretaceous Abakaliki basin occur mostly as oval-shaped bodies, consisting of lithic/lava and vitric fragments. They are commonly characterized by parallel and cross laminations, as well contain xenoliths of shale, mudstone and siltstones from the older Asu River Group of Albian age. The rocks are basic to ultrabasic in composition, comprising altered alkali basalts, altered tuffs, minor lapillistones and agglomerates. The mineral compositions are characterized mainly by laths of calcic plagioclase, pyroxene (altered), altered olivines and opaques. Calcite, zeolite and quartz represent the secondary mineral constituents. Geochemically, two groups of volcaniclastic rocks, are distinguished: alkaline and tholeiitic rocks, both represented by fresh and altered rock samples. The older alkali basalts occur within the core of the Abakaliki anticlinorium while the younger tholeiites occur towards the periphery. Though most of the rocks are moderate to highly altered [Loss on ignition (LOI, 3.43–22.07 wt. %)], the use of immobile trace element such as Nb, Zr, Y, Hf, Ti, Ta and REEs reflect asthenospheric mantle source compositions. The rocks are enriched in incompatible elements and REEs (∑REE = 87.98–281.0 ppm for alkaline and 69.45–287.99 ppm for tholeiites). The ratios of La/Yb n are higher in the alkaline rocks ranging from 7.69 to 31.55 compared to the tholeiitic rocks which range from 4.4 to 16.89 and indicating the presence of garnet-bearing lherzolite in the source mantle. The spidergrams and REEs patterns along with Zr/Nb, Ba/Nb, Rb/Nb ratios suggest that the rocks were generated by a mantle plume from partial melting of mixed enriched mantle sources (HIMU, EMI and EMII) similar to the rocks of the south Atlantic Ocean such as St. Helena (alkaline rocks) and Ascension rocks (tholeiitic rocks). The rocks were formed in a within-plate setting of the intra-continental rift type similar to other igneous rocks in the Benue Rift and are not related to any subduction event as previously suggested.
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