Hydrothermal alteration and mineralisation of the Nigerian anorogenic ring complexes : with special reference to the Saiya Shokobo complex

The Nigerian anorogenic province is characterised by more than 50 complexes. In some complexes, the outer limits of the complexes are defined partly or completely by a ring dyke. This intrusion, generally composed of granite porphyry, was the chief structural element of the complex and controlled the distribution of both volcanic and subvolcanic magmatism at high levels in the crust. The complexes are exposed at different erosional levels: in some complexes, particularly in the north, the volcanic products are preserved through caldera collapse. The majority of the volcanics are dominantly rhyolitic, some alkaline centres have preserved occasional successions of hawaiites, mugearites and trachytes confirming the dominantly alkaline trend from transitional ne-normative or hy-normative basalts. In other centres the volcanic cover has been removed so that the subvolcanic granites and syenites have been exposed. The subvolcanic assemblages include minor gabbros, monzogabbros and syenites. However granitic rocks are overwhelmingly dominant with fayalite hedenbergite granites, amphibole granites and syenites, albite-rich and albite-poor aegirine arfvedsonite granites and biotite granites. As far as the granites are concerned there was a natural progression from volcanic feeder intrusions to subvolcanic intrusions with fayalite and hedenbergite. The volcanic feeder intrusions were an important link during the caldera-forming stage between the subvolcanic roots and the overlying volcanic pile. They are represented by quartz and granite porphyries. The mineralogical assemblages of many of these complexes however, is often the result of interaction with residual fluids. Fluids affected the late magmatic and particularly the postmagmatic (subsolidus) crystallisation history of a cooling subvolcanic pluton and to some extent the overlying volcanic pile. Such hydrothermal alteration has undoubtedly modified the original granite chemistry. The residual fluids which were responsible for hydrothermal alteration also carried ore metals which were variously deposited at different stages of hydrothermal alteration. Mineralisation of a pluton generally occurred in the apical or marginal zones or in satellite dykes. Different styles Of mineralisation tend to characterise different parts of a granite pluton and five separate zones have been recognised; the roof, marginal and contact zones of a pluton, the enclosing or overlying rocks and the surrounding ring dyke. Nine different styles of mineralisation have Been identified including pervasive metasomatic disseminations; pegmatitic pods and lenses; stockworks and lodes. No particular type of hydrothermal alteration is restricted to one particular style of mineralisation and it is possible to have disseminated sodic, potassic, acid or silica metasomatism and altered ring dykes may show the effects of all these processes. The complexity of the subsolidus mineralogical re-equilibration has been appreciated by a detailed study of the Rishi area of the Saiya Shokobo complex. The Saiya Shokobo complex forms a distinctive hill mass in north central Nigeria which covers an area of approximately 250km2. The main structural feature of the complex is the elliptical fracture 17km in diameter which has largely determined the outline of the central massif of the Saiya Shokobo Hills and has controlled the emplacement of several of the intrusions of the complex. The complex is dominated by volcanic rocks which are confined within this elliptical fracture. During the latest stage of magmatic activity, the emplacement of a granite pluton moved beyond the confines of the main fracture and was intruded eccentrically to the north east of the main part of the complex. Thus the biotite granite intruded both its own volcanic pile and the basement hornblende biotite monzogranite. The volcanic rocks are composed of crystal-rich ignimbrites with fractured and embayed quartz phenocrysts. The main feeder dyke, which occupies an early ring fracture, is also porphyritic. However, the phenocrysts are typically in synneusis-type aggregates and are modally less abundant. High in the volcanic pile arfvedsonite with minor amounts of aegirine developed in the groundmass of the ignimbrite. In samples collected close to the granite roof, the matrix consists of quartz and alkali feldspar with the development of sieve-textured brown biotite and with no amphibole. The Rishi biotite granite was the final intrusive phase, emplaced to a high level within the Saiya Shokobo complex. The main granite mass covers an area of 13km2 with small windows of granite exposed in the volcanics to the south. The main granite is commonly coarseto medium-grained and cream or pink in colour when fresh. However there is a wide range of subtle coloration and grain size ranging from coarseto fine-grained, and from equigranular to porphyritic and granophyric. The essential minerals are perthitic feldspar surrounded by clusters of glassy quartz crystals and ragged discrete flakes of biotite irregularly distributed throughout the granite. Accessory minerals include zircon, fluorite, and rutile which are widely distributed with columbite, cassiterite, sphalerite and other minerals locally. Such textural and mineralogical variations are interpreted to be the result of rock-fluid subsolidus re-equilibration. In addition to the mineralogical and textural variations in the Rishi biotite granite, a sequence of alteration assemblages has been recognised which are indicative of continued fluid mobility at successively lower temperatures: small patches of saccharoidal white feldspar and lenticular zinnwaldite; local development of microclinite; chloritisation of feldspar or biotite, greisen formation, silicification and late-stage alteration processes. The processes of hydrothermal alteration are complex. They depend not only on the temperature and composition of the fluid, but also on the initial rock type that was being affected, the extent of earlier rock-fluid reaction and on how long the fluids were retained. The different mineral assemblages indicate that alteration took place within an Na20-K20-Si02-H20-F system which produced albitisation, then microclinisation of the feldspars during sodic and potassic alteration, and ultimately a destruction of the feldspars during acid metasomatism. This was followed by lower temperature silicification and late-stage alteration processes. In addition to the alteration of the Rishi pluton itself, the host rocks which surround and overlie the pluton have also been affected, Thus the Pan-African basement monzogranite country rock which surrounds the Rishi biotite granite and the volcanic pile which overlies it have also been hydrothermally altered by volatiles escaping from the biotite granite cupola. For the first time, a sequence of alteration processes can also be recognised for the basement and to a more limited extent for the volcanic pile although the effects are not always as clearly defined in the basement or volcanic pile as they are for the biotite granite. Geochemically the basement monzogranites, the volcanic rocks and the Rishi biotite granite are distinctive in terms of their trace element assemblage such that each can be distinguished even when there has been intense hydrothermal alteration. Thus the basement monzogranite has higher Ba, V,Sr,Zr and Ce, slightly higher La with lower Nb, Y, Zn and Rb than the Rishi biotite granite. The volcanics can be distinguished by their enhancement of Ce,La,Zn and Zr contents compared with other rock types. The PVTX nature of the fluids which were responsible for hydrothermal alteration have been monitored by a study of fluid inclusion characteristics in selected samples. The earliest fluids that were trapped in the quartz phenocrysts of the volcanic pile indicate that the fluids that separated from the silicate melt at a depth of around 6km, were highly saline, probably at temperatures in excess of 750°C. Frequent boiling and loss of CO? occurred in these early stages. Loss of C02 appears to have been important for the deposition of columbite during sodic metasomatism. As the fluids cooled and lost C02, the condensate was responsible for potash metasomatism and the vapour phase caused acid metasomatism with later, cooler fluids responsible for silica and late-stage alteration. The importance of boiling in the formation of the ore-deposits within the roof of the Rishi pluton must be emphasised.