Two old gold mines are most important, and are within the Maru schist belt. Duki Mine is hosted by a shear zone traversing a quartzite-schist series, often exploiting the S1 schistosity planes. The mineralisation apparently made up of gold-quartz veins, was exploited by past miners for over 1km of strike length, leaving behind series of collapsed N-S trending workings without any surface exposures of the mineralisation. Recent exploration drilling by the Nigerian Mining Corporation have shown the continuity of the gold-squartz-sulphide veins below the old workings.
The other old gold mine (Maraba) is a two sub-parallel quartz vein (-300m long) system. Tourmaline and chlorite in quartz floats and altered wall rocks are the only indicators of mineralisation as there are no in situ exposures due to intensity of past mining.
In this area, there are a number of old gold mines, such as kwali, Jameson, Zuzzurfa and Kuba, hosted by schists, phyllites and quartzitesof the schist belt. Individual veins or reefs seldom exceed 0.5km of strike lenth as indicated by the extent of past workings, and are concordant with the host rock foliation. These mineralised wall rocks show metal values in the following range: Cu = 647-5410 ppm, Pb =7550-22600 ppm. Ag = 1.5-8.6 ppm, and Au =123-6320 ppb.
The Malele area is in the extreme southern part of the Maru schist belt. Gold mineralisation occur in a NNE trending gold quartz vein series cutting both biotite gneiss and chlorite schist, now represented by a series of sub-parallel surface working (pits, trenches and water ponds) with the vein exposure seldom observed. The mineralisation was discovered in 1934 with the most prominent of the veins being 1.5m thick, 370m long and a grade of about 30 ppm Au.
Tshohon Birnin Gwari and Kwaga
The Tsohon Birnin Gwari and Kwaga gold sites are within the Kushaka schist belt of north-western Nigeria. While the Tsohon Birnin Gwari was an active gold mine in the 1930s where over 600 ounces of gold were produced, the Kwaga site is a recent discovery by artisanal miners following the extensions of the Tsohon Birnin Gwari mine.
The Tsohon Birnin Gwari gold reef system is about 7 km long, while the Kwaga reef system extends for about 3 km. Gold is mainly associated with pyrite and minor sphalerite, chalcopurite, pyrrhotite, galena and magnetite. The gangue is predominantly quartz, but K feldspar and graphitic matter (from wall rocks) ae also constituents. Grades re very variable within the reef system and between oxide and sulphide ore zones, but generally 5-100 g/t Au are found.
Recent core drilling and near-surface mine exposures have provided relatively fresh samples of the gold-quartz reefs and their alrered wall rocks from which the geochemical characteristics of the Tshohon Brnin Gwari mineralistion were studied.
Gold-sulphide-carbonate quartz veins occur in a brittle fault zone cutting hornfels of the contact of a Pan-African grandiorite batholiths intruding phyllites and tourmalinites of Zuru schist belt in northwesterm Nigeria. Mineralisation has been uncovered over a strike length of 1.5km by past miners (1920s-1940s)
Gold occurs associated with pyrite, chalcopyrite, galena,and minor sphalerite, magnetite and bismuth telluride in a gangue of mainly quartz with some carbonates, sericite, chlorite and tourmaline. The gold veins are surrounded by a narrow zone of hydrothermal alteration in which a choritetourmaline –pyrite- carbonates association overprints a dominantly sericite fabric of the hornfels wall rocks. Past mine records, reconnaissance exploration and studies have shown gold grades in the range 5-15ppm.
The Gurmana area of gold mineralisation is situated in the extreme southern part of the Kushaka schist belt. Gole mineralisation is in the form of quartz-sulphide veins and stockworks hosted by amphibolites and gneisses. The gold-quartz-sulphide veins seldom extend more than a few tens of meters. Gold occurs with pyrite, chalcopyrite, galena, covelite and chalcosite.
Federal Capital Territory
Recent investigations and analytical geochemical data indicated the occurrence of anomalous gold concentration in samples of Ibbaban Tsauni reaching 7ppm and 27ppm in lead ores and pebbly soils respectively. The primary gold mineralisation occurs as sulphides (AuS) associated with Ag probably as Au-Ag-Pb sulphides base metal mineralisation in solid solution. Alluvial gold occurs in gravelly soils along river channels and as eluvial gold in pebbly soils. These mineralisations have resulted possibly from weathering of lead ores and gold bearing quartz vein found in shear and breccia zones found around the lead bearing quartz veins. Angular gold grains and nuggets weighing between 7-12gm have been recovered from several workings in the area indicating proximity to the source area.
The Okolom Dogondaji area of gold mineralisation is in the Egbe-Isanlu schist belt of south-western Nigeria, and the primary gold-quartz veinsandeluvial/alluvial placers have been mined extensively in the period 1930s-1950s. A series of gold-quartz veins is hosted by N-S and NNE-trending shear zones, which cut gneisses, schists and amphibolites.
The most prominent site is the Okolom old mine, which a gold sulphide –quartz reef system with a total strike length of about 3km hosted by gneiss, amphiibolite and talc schist. Other sites in the (Dogonodaji) area have relatively smaller veins hosted by amphibolites, gneisses, mica schists and phyllites. Gold is associated with pyrite, marcasite, pyrrhotite, chalcopyrite, argentite and galena, with a gangue of quartz, tourmaline, sericite and cholorite.
New Developments and exploration information on gold mineralisation in Nigeria
Recent investigation by the Nigerian Geological Survey Agency and private exploration/mining companies has continued to shed more light on the endowments and the epotentials of the country in gold mineralisation. Between the years 2006 and 2008 the Nigerian Geological Survey Agency (NGSA) has carried out evaluation and assessment of key commodities nationwide. Gold is one of the commodities given priority. Beyond the known areas of gold mineralisation and some six other states with favourable geological setting for fold mineralisation, two other states (Ogun and Ondo) are here highlighted with Ogun state showing prospect for the resource.
Gold mineralisation in Nigeria appears to by the orogenic type which is controlled by deep seated curvilinear transcrustal fracture syste. These deep seated fracture systems, Anka-Yauri-Iseyin (AYI) and Kalangai-Zuru-Ifewara (KZI) are believed to serve as conduits to the subsidiary fractures which are linked to these major fractures and form sites of gold deposition. A summary of other known gold occurrences in Nigeria is also given to serve as guide to investors.
Exploration for gold in Ajegunle-Awa area of Ogun State shows minor gold occurrence in the residual soils at the contact between amphibolites and pegmatites. These rocks are highly fractured and noticeable in the drill core samples. Analytical results of panned concentrates of soil samples from the C-horizon gave an average of 26ppm Au content. These finding suggests the possibility of gold mineralisation in the southern most portion of the curvilinear transcrustal fractures which are believed to be linked with gold mineralisation in Nigeria.
OSUN – Iperindo
The iperindo gold mineralisation is in the llesha schist belt of south-western Nigeria which comprises a series of gold bearing (Au-quartz carbonate) veins localised by subsidiary faults hosted within bioties gneiss and mica schist. These are hitherto defined by sub parallel old working extending for about 900m in NE direction. Gold occurs with pyrite, pyrhotite and minor chalacopyrite, galena, sphalerite, magnetite and ilmenite. Wall rock alteration associated with the gold mineralisation shows sericite-chlorite-epidote assemblage heamatite and pyrite. Recently (December 2009), Segilola Gold Ltd. A wholly-owned subsidiary of CGA Mining Ltd (listed on Toronto stock exchange and Australian Securities Exchange) announced the maiden resources of 620,000 ounces of gold on its Iperindo project. Segilola Gold Ltd generated the resources from first pass diamond drilling campaign of 12,166 metres in 119holes ranging from 40-220 metres. Major findings show the tenorof gold in their mining lease at between <1g/ton 136 g/ton gold. Also in the course of the exercise the strike length of the mineralised zone has been increased to 1500m.
Geological studies by the company confirms the widely held view that the regional geological setting of the area is similar to the Ashanti and Salwi gold systems in Ghana.