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 ne-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 postmagnatic (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 caiplex. The Saiya Shokobo complex forms a distinctive hill mass in north central Nigeria which covers an area of approximately 250km. 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

Geology and mineralization of the Cu-rich Mumbwa district, a potential IOCG-type system at the eastern margin of the Pan-African Hook batholith, Zambia

The Mumbwa mineralized district is located approximately 200 km west of Lusaka, at the north-eastern margin of the Pan-African granitic Hook batholith. Polymetallic sulphide occurrences in the area have been known for hundreds of years, but more recent geophysical and geochemical investigations led to the discovery of a copper-rich hydrothermal system, mostly associated with late-stage syenite intrusions of the Hook batholith. The extent of the hydrothermal system is not known, but to date two main centres have been identified, the Sugar Loaf and Mutoya. Sulphide mineralization occurs along regional-scale lineaments, following a 25 km-long NNW-trending corridor. Mineralized host rocks are characterized by brecciation, often pervasively replaced by magnetite-hematite, and by strong metasomatism with multiple - and often superimposed - alteration cycles, from potassic, to carbonate, sericite-chlorite and amphibole-apatite-carbonate. Sulphur isotopes suggest that a mixture of magmatic and sedimentary-derived evaporitic fluids were critical in providing sulphur and metals. Late syenitic intrusions triggered the relevant hydrothermal circulation and favoured the mineralization processes. Diagrams plotting key element geochemistry and alteration indexes highlight vectors to alteration and mineralization and suggest that the sulphides preferably concentrated in rocks affected by hydrothermal iron oxides in association with sericite-chlorite alteration. At the Kitumba prospect (in the Sugar Loaf mineralized centre), granitic to syenitic bodies host a hypogene copper mineralization (mostly chalcopyrite), that was subsequently overprinted by pervasive and deep supergene mineralization (malachite, chalcocite, chalcosiderite, cuprite, digenite, chrysocolla, bornite, native copper). Supergene mineralization has been identified in boreholes to depths exceeding 700 m. Reserves at Kitumba are estimated at 27.9 Mt with an average grade of 2.2% copper at a 1.0% copper cut-off grade. Gold is present, although generally at low grades. In the Mutoya centre, metasedimentary rocks alternate with felsic intrusions. Two prospects have been identified, characterized by large areas of magnetite-hematite breccias hosting sulphide mineralization with predominantly pyrite and minor chalcopyrite. An affiliation to the iron oxide copper gold (IOCG) category is discussed. Many, but not all of the distinctive IOCG features, are present in the studied area.This work is part of a two year collaborative research project involving the University of the Witwatersrand and RioTinto Exploration Ltd., in an attempt to correlate the Damara Belt in Namibia with the Lufilian Arc and Zambezi belt in Zambia.South African THRIPhttp://www.elsevier.com/locate/jafrearsci2020-10-01hj2020Geolog