Late Pan-African peri-Gondwana orogens: Extending the Proto-Himalayan Belt

第2回極域科学シンポジウム/第31回極域地学シンポジウム 11月17日（木） 国立極地研究所　2階大会議

Mineralization controls and petrogenesis of the rare metal pegmatites of nasarawa area, central nigeria

The pegmatites of Nasarawa area occur in the central part of Nigeria. They are mainly hosted by phyllonites in a NNE-SSW trending shear zone lying east of some foliated Pan-African and West of Jurassic Afu Complex Younger Granites. A geological mapping of the area was followed by petrographic and mineralogical studies of selected rock and mineral samples. A total of 72 samples consisting of 25 rocks, 22 feldspars and 25 white micas were analyzed for various elements. The pegmatites are peraluminous and are genetically linked to the late Pan-African leucogranite with the shear zone. The Pan-African granites have very low REE abundances and non-chondritic ratios of Zr/Hf and Y/Ho and low Nb/Ta ratios indicating crystallization from a liquid-rich melt. Barren pegmatites are closely associated with the primitive hornblende biotite Pan-African synorogenic granites while Sn- Nb - Ta mineralized granites are correspondingly enriched in pegmatites spatially associated with Pan-African synorogenic granites with enhanced values of rare lithophile elements such as Rb, Cs, Mn, Sn and Nb-Ta. The primary control of rare metal mineralization in the pegmatites is the composition of the source rock since the Ta-Nb-Sn-Li-Be-W mineralized pegmatites crystallized from fluid (H2O-B-P-F) rich melts. It is hereby proposed that the late Pan-African tectonic granite which is parental to the highly mineralized pegmatites in this area originated from anatexis of undepleted mica-rich metasediments at depth, followed by a magmatic fractionation of the fluid rich melt as it ascended through reactivated ancient fractures. The heat for the partial melting might have been supplied mainly by the reactivation of ancient fractures, which controlled the emplacement of the fertile granites and the related pegmatite

Ferruginised metapelites of the Kazaure Schist belt, northwestern Nigeria

Field, petrographic and geochemical evidence shows the occurrence of  ferruginised metapelites (FMP) in the Kazaure Schist Belt (KZSB). The KZSB consists predominantly of quartzites, pelitic metasediments, and  metaconglomerates. Minor rocks includes metavolcanics and iron  formations (Banded Iron Formation and ferruginised metapelites). The FMP occurs as narrow bands associated with quartzites and phyllites. Magnetite as euhedral to subhedral crystals together with subordinate anhedral  hematite forms the Fe band. The Fe content of the FMP (14.54 – 82.8 %) is comparable to that of banded iron formations in the schist belt. It contains lesser amounts of Mn (0.07 – 17.83 %), Ca (0.067 – 1.25%), and the lithophile elements average values: K (1.21 ppm); Rb (89.89 ppm); Sr (190.78 ppm). It is envisaged that the FMP developed from the alteration of the metasediments entrapped as a kind of tectonic mélange as a result of late tectonic brittle deformation and activity of fluids rich in iron.Keywords: Banded Iron Formation, ferruginised metapelites, Oxide facies, Tectonic mélange, Silicate facies

Post-collisional Pan-African granitoids and rare metal pegmatites in western Nigeria: age, petrogenesis, and the ‘pegmatite conundrum’

The Minna area of western Nigeria lies within a Pan-African orogenic belt that extends along the margin of the West African Craton, from Algeria southwards through Nigeria, Benin and Ghana, and into the Borborema Province of Brazil. This belt is characterised by voluminous post-collisional granitoid plutons that are well exposed around the city of Minna. In this paper we present new information about their age and petrogenesis. The Pan-African plutons around Minna can be divided into two main groups: a group of largely peraluminous biotite–muscovite granites that show varying levels of deformation in late Pan-African shear zones; and a younger group of relatively undeformed, predominantly metaluminous hornblende granitoids. Pegmatites, including both barren and rare-metal types, occur at the margins of some of the plutons. New U–Pb zircon dating presented here, in combination with published data, indicates an early phase of magmatism at c. 790–760 Ma in the Minna area. This magmatism could be related either to continental rifting, or to subduction around the margins of an existing continent. The peraluminous biotite–muscovite granites were intruded at c. 650–600 Ma during regional shearing in the orogenic belt, and are likely to have formed largely by crustal melting. Subsequent emplacement of metaluminous granitoids at c. 590 Ma indicates the onset of post-orogenic extension in this area, with a contribution from mantle-derived magmas. The rare-metal pegmatites represent the youngest intrusions in this area and thus are likely to have formed in a separate magmatic episode, post-dating granite intrusion

Regularity Analysis of Airborne Natural Gamma Ray Data Measured in the Hoggar Area (Algeria)

The airbone Gamma Ray (GR) measurements have been used since decades in geophysical researc

The Cu-Mo±Au Mineralizations Associated to the High-K Calc-Alkaline Granitoids from Tifnoute Valley (Siroua massif, Anti-Atlas, Morocco): an Arc-Type Porphyry in the Late Neoproterozoic Series

The Tifnoute valley Cu-Mo±Au mineral occurrences are located NW of Siroua massif in the central Moroccan Anti-Atlas. This mineralization appears to be associated to the Imourkhssan granite and the Asskaoun granodiorite dated respectively 561 ± 3 and 558 ± 2 Ma. These highly potassic granitoids show an arc signature and are in-placed in an extensional tectonic setting typical of the post-collision Late Pan-African period. In places, these rocks are strongly to slightly affected by hydrothermal alterations of phyllic, propylitic and argillic types with development of a pyritic facies around the Imourkhssan granite. The mineralization can be divided into two types: A molybdenite mineralization and some sulphides spatially linked to the Imourkhssan granite. It is disseminated in the granite or appears in lamellar or pluri-millimetric fragments. It also occurs as spots and nets along chloritized fractures planes oriented NNE-SSW and dipping 40 to 60 SE. The paragenesis consists of molybdenite, pyrite, chalcopyrite and sphalerite. Talat N’Lbnour Cu-Au mineralization linked to NS fractures affecting Askaoun granodiorite. The vein is about 0.5 to 2 m thick that extends about 400 m. The mineralized fractures are filled essentially with quartz, siderite and chlorite. Metallographic study reveals a diverse paragenesis that consists of pyrite, chalcopyrite, gold, bornite and chalcocite. The secondary paragenesis consists of chalcopyrite, covellite, bornite, malachite, azurite and hematite. These features of these mineralizations are discussed in the context of an arc-type Cu-Mo±Au porphyry mineralization. Keywords: Anti-Atlas, Siroua, Molybdenum, Copper, Gold, Porphyry

Geological and Induced Polarization Geophysical Investigations of the Quartz-Vein Gold Mineralization in Some Parts of Ayegunle Sheet 226nw, North Central, Nigeria

The study area is predominated by granite-gneiss, mica schist, leucocratic micro granite and quartzites.  The Pan African orogeny left an imprint of structural similarity upon the rock units. The granite-gneisses are found as small belts within the study area, and hosting the quartz-veins. The metasediments are fractured and folded in places. The dominant fold axes and quartz vein intrusions trend NNE-SSW parallel to the regional foliation of the rocks. The emplacement of the quartz vein is associated with the generalized strike direction of quartz vein and host rocks. The structural settings suggest that the emplacement of gold mineralization occurred during Late Pan African orogeny. Geophysical method using the Electrical Resistivity Tomography (ERT) has been applied in some parts of Ayegunle sheet 226NW, North Central, Nigeria aimed at investigating the electrical properties and probable exploration targets for quartz-vein gold mineralization. Electrical Resistivity Tomography (ERT), using the measuring unit ‘MER124’ and generator unit ‘ASTRA’ Imaging System was adopted to map the electrical properties as an aid to characterizing the subsurface conditions of the study area. Nine (9) profile lines were marked out for ERT investigations. For each profile, the layout for the electrical imaging was such that two reels of cables were used with 5m intervals between the takeout electrodes. However, the two innermost electrodes adjoining the two reels of cables were connected together thus yielding a total of 41 electrodes and a spread length of 200m. The data obtained using MER124 measuring unit were processed using Zond2dinv and Oasis Montaj 8.2 software.  Chargeability and resistivity values varies due to the inhomogeneity of the subsurface rock hence, chargeability values in percentage (%) ranges from 0.17% around the alteration zones and relatively low sulphide mineralization to 6.22% with high sulphide mineralization and resistivities in ohms-meter ranges from as low as 5ohm-m along the alteration zones to >10,000 ohms-meter around the competent bedrock and probable zones of quartz-veins gold mineralization. Keywords: Granite-gneiss, Mica schist, Leucocratic micro granite and Quartzite, Electrical Resistivity Tomography, Quartz-vein gol