Abstract
The discrimination diagrams: Rb versus Nb +Y and Nb versus Y, and Th/Ta ratio combined with field observations and empirical information from the literature were used to infer the tectonic environments and sources of the rare metal mineralization in the pegmatites of Komu area, southwestern Nigeria. The discrimination diagrams show that the pegmatites were mainly emplaced in tectonic environments similar to those of syn-collisional granites (Syn COG) and within plate granites (WPG). Volcanic arc environment was not pronounced. The emplacement of the pegmatites was structurally controlled by the predominantly Northeast (NE)– Southwest (SW) and North northeast (NNE)-South southwest (SSW) fault systems that were developed during the Pan-African orogeny. The upwelling of the mantle at the beginning and faulting systems associated with the active and closing stages of the Pan-African orogeny elevated rare metals from the mantle to higher crustal levels where they were incorporated into the pegmatitic fluids. The Th/Ta ratios of less than 1.00 which resemble that of the oceanic ridge tectonic environment is attributable to the upwelling of mantle material. The variation in the tectonic environments of the pegmatites may be attributed to the long span of the Pan-African orogeny (750-450 Ma).KEY WORDS: Komu. Nigeria. Pegmatites. Rare metals. Tectonic environments.
Highlights
In the last sixty years, studies of the Nigerian rare metal occurrence have established two distinct episodes of mineralization
The graphic granite and the alkali feldspar pegmatite wholly plot in the field of granites formed in syn-collisional environment (Syn COG), the quartzalbite-muscovite-tourmaline pegmatite spreads over the fields of granite formed in syn-collisional, volcanic arc (VAG) and within plate (WPG) tectonic environments while the fine-grained rock plots solely in the field of granite formed in ocean ridge tectonic environment (ORG)
Th/Ta ratios and the discrimination diagrams were used to infer the tectonic environments of the pegmatites at the time of their emplacement and sources of their rare metals mineralization
Summary
In the last sixty years, studies of the Nigerian rare metal occurrence have established two distinct episodes of mineralization. The younger of these is the predominantly columbite-cassiterite mineralization associated with the anorogenic Mesozoic granite of central Nigeria. The older episode is tantalite-columbitecassiterite mineralization associated with pegmatites of the main phase granite magmatism of the Pan-African orogeny (600±150 Ma). Most studies on these pegmatites were focused on the petrology, mineralization trends and factors that governed the degree of mineralization. Published work on the tectonic environments and sources of rare metals mineralization in the pegmatites is scarce. In sharp contrast to these observations, Wright (1970) suggested that pegmatite mineralization must have been leached from continental rocks by hydrothermal fluids ejected from
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