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Nigerian Tin Province - Jos Plateau, Sabon Gida Valley, Liruei Lode |
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Nigeria |
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Sn
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
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The secondary tin deposits of the Jos Plateau, including the Sabon Gida Valley, and the hard rock Liruei Lode, are located in the Tin Province of northern Nigeria. It has historically produced substantial tonnages of residual / eluvial cassiterite derived from Jurassic ring dyke complexes.
Tin is believed to have been mined on the Jos Plateau for as long as 2000 years, with the product being taken north across the Sahara Desert. Portuguese traders are known to have purchased 'straw tin' in southern Nigeria, supposedly originating from the Jos area, during the seventeenth century. Straw tin is a primitive smelted tin produced on the Jos Plateau bearing the imprint of the straw in the mud brick moulds.
At the beginning of the twentieth century the first European controlled mining heralded the beginning of the concerted extraction of Nigeria's tin deposits. During the early part of the century a large number of small operations worked the Jos Plateau accumulations, but by the mid 1930s these had been amalgamated into a smaller number of larger companies.
During the second world war peak annual productions of up to 17 000 tonnes of concentrates were attained with another peak in the early 1970s when up to 10 000 tonnes of concentrate were being produced per year. Nigeria's total production to 1978 totalled ~500 000 tonnes of contained metal.
However, with the increase in the use of non-corrosive substitutes, by 1984 production had decreased to 1284 t and with the collapse of the International Tin Agreement in 1985, tin mining in Nigeria virtually ceased.
Regional Setting and Distribution of Mineralisation
The geology of Nigeria is dominated by extensive areas of Precambrian metamorphics. These comprise granulitic quartzo-feldspathic gneisses of psammitic and semi-pelitic origin. They grade into granite gneisses and embrace significant migmatite developments. In some areas extensive schist belts are recorded. The schists are mainly mica schists, but associated quartzites and quartz-mica schists are also common with minor altered limestone and andesitic volcanics. Garnet, sillimanite, magnetite and staurolite are frequently components of the schists. Bands of amphibolite and garnetiferous granulite sometimes accompany the schists.
Exposures of 'Older Granites' with accompanying pegmatites and aplites are found in central and northern Nigeria. These have associated intermediate intrusives.
Much of the basement gives metamorphic age dates of from 1700 to 2000 Ma, corresponding to Eburnean folding and suggesting a Palaeoproterozoic to Archaean age for this sequence. The 'Older Granites' yield dates of 500 to 650 Ma with a peak at 610 Ma and are believed to have been the result of reactivation during the Ratangan (Pan African) Cycle. Only minor exposures of Precambrian basement are found in Niger, mainly in the far north.
The only Palaeozoic sedimentary rocks found in the belt are in far northern Niger where they occur as a wedge between a Mesozoic sequence and Precambrian metamorphic rocks. A group of Jurassic granitic intrusives were emplaced in a roughly north-south line parallel to and along a postulated extension of, the structure influencing the splitting of South America from Africa during the Cretaceous. These granitic bodies form a belt some 1200 km long and 200 km wide. The granites appear to have been emplaced at shallow depths within the core of sub-volcanic cone sheet and ring dyke complexes. Remnants of rhyolitic to dacitic extrusives and associated cone sheets and ring dykes are found in these complexes as are minor basic dykes. The complexes to the north in Niger are older than those further to the south, with a probable age range of Late Triassic in northern Niger to Late Jurassic to the south of the Jos Plateau.
It is likely that this granite belt is relatively continuous over its whole length. However, subsequent Mesozoic sedimentation and the Sahara sands have masked large sections of the belt.
Mesozoic continental sedimentary rocks are widespread in central Niger while a 'Y-shaped' trough, closely coincident with the current Niger and Benue River valleys in Nigeria, was infilled with Cretaceous sediments. The Cretaceous sediments are both marine and continental, and comprise sandstones, grits and conglomerates. In southern Nigeria these are overlain by a strip of Tertiary continental clastics, while similar sediments are also extensively developed in western Niger.
Tertiary and Quaternary basaltic outpourings are evident in the eastern half of Nigeria while the Quaternary sands of the Sahara mask most of the older rocks in eastern Niger and north eastern Nigeria.
The Jos Plateau is the expression of a major up-doming which commenced in the Mesozoic and is centred at the intersection of major north-south and north east-south west lineaments. Granitic intrusives are elongated parallel to these two directions.
Tin mineralisation within the Nigerian Tin Province is associated with both the 'Younger' or Jurassic granites of Nigeria and Niger, and the pegmatites of the 'Older Granites'. The older pegmatite associated deposits are almost insignificant, accounting for <5% of Nigeria's total production. The main deposits of this type are alluvial and residual and are found some 170 km NE of, and 120 km to the west of the Benue-Niger confluence. Other deposits of this type are found in south western Nigeria.
The tin deposits associated with the Jurassic 'Younger Granites' are largely alluvial accumulations with lesser probable eluvial deposits. The deposits of the Jos Plateau in north-central Nigeria are by far the most significant. Tin occurrences are found in both southern and northern Niger associated with similar intrusive bodies.
Of the at least 40 individual complexes on the Jos Plateau, two have been responsible for the majority of the tin production of the Nigerian Tin Province. The tin found in the alluvial accumulations is believed to have originated from the only obvious primary mineralisation observed in the area to date, namely the small tin bearing veinlets and disseminations found in restricted alteration zones in the roof sections of the 'Younger Granites'. These occurrences are far too rare and small to have accounted for the volume of tin found in the Jos Plateau alluvials. It is believed in Nigeria that the roof section of certain of the 'Younger Granites' had much higher tin concentration, but that these sections have now been completely removed. It is widely held that while erosion had reached the critical level of the granites in the Jos area, in the other areas, denudation had either not reached or had passed the primary tin source when conditions were favourable for the accumulation of placer tin deposits. It has also been pointed out that the granites of the belt vary in age and that this may have some bearing on the distribution of primary tin mineralisation.
In some parts of the Jos Plateau, minor primary tin mineralisation and probable eluvial deposits are found over the Precambrian basement several kilometres from outcropping 'Younger Granites'. These showings have been attributed to concealed 'Younger Granites'. It is possible however, that tin rich zones may occur within the basement metamorphics and these could have some influence on the abundance of primary tin in the 'Younger Granites'.
Columbite is found in alluvial accumulations associated with the 'Younger Granites', although not always in the same deposits as tin. Columbite also occurs in economically mineable proportions as a primary constituent of certain of the biotite 'Younger Granites' of the Jos Plateau.
Geology
The main components of the regional geology of the Jos Plateau are:
Basement complex of Lower Proterozoic to Archaean, mainly 1700 to 2000 Ma metamorphics, comprising:
• Granulitic gneisses - these are the oldest rocks of the complex and are psammitic to semi-pelitic gneisses occurring as fine grained sugary, foliated, grey to brown quartzo-feldspathic granulitic gneisses with biotite.
• Intermediate rocks - these are hypersthene and quartz diorites and are believed to have intruded the granulitic gneisses prior to migmatisation.
• Migmatites - a varied group of migmatites ranging from coarsely mixed gneisses with leuco and basic bands, to diffuse textured rocks with variable grain sizes. These were derived from the pre-existing gneisses by in situ granitisation. Migmatite bands are developed parallel to the lithological layering and are best developed adjacent to the principal granite gneiss and granitic bodies.
• Granite Gneiss and migmatitic granite gneiss - these are mainly granodioritic in composition with lesser granitic phases. They are usually weakly foliated, but some grade to well foliated varieties with recognisable relict granulitic gneiss bodies.
• Older Granites - these yield dates of from 500 to 650 Ma with a peak at 610 Ma, in contrast to the preceding rocks. There are three main types:
i). Porphyritic biotite and biotite hornblende granites - which are medium to coarse grained granitic to granodioritic in composition and range from poorly to well foliated with feldspar phenocrysts up to 5 cm long.
ii). Non-porphyritic biotite hornblende granite and quartz syenites - which are medium to coarse grained.
iii). Fine and medium grained biotite and biotite muscovite granite. These are widespread but mainly to the west of the Jos Plateau, occurring as elongate bodies with numerous dykes and sheets of associated microgranite and aplite. Most are of granodioritic composition and are foliated parallel to the long axis of the body.
Jurassic ring dyke complexes - some 40 individual complexes are known, stretching from the Niger border to the Benue (Cretaceous) Trough in the south. They range in areal extent from <2 to 1500 km2. Acid to intermediate extrusives accompany the complexes, particularly in the northern section of the Plateau where they have been less thoroughly eroded in comparison with those to the south. The volcanic rocks, even where they are reasonably well preserved, are restricted to the peripheral ring faults. The acid varieties are of rhyolitic to dacitic composition and are of two types namely:
• Early lava flows, vent fillings and ignimbrites, and
• Later intrusive and cauldron rhyolites.
The complexes have outer cone sheets and ring dykes of, in particular, quartz porphyry and granite porphyry.
These early stages of the complex are intruded by circular granitic bodies, often concentrically zoned, believed to represent repeated phases of intrusion. Individual circular structures seldom occur. In most cases complexes comprise a group of overlapping circular structures eg. the Sara Fier structure. The age of the granitic intrusives ranges from Early Jurassic, as in the Liruei Complex in the north, to Middle Jurassic at the Sara Fier Complex to Late Jurassic at Afu. There 'Younger Granites' are present as three types, namely:
• Hornblende, pyroxene, fayalite granite,
• Riebeckite rich granites,
• Biotite rich granite.
Tin mineralisation is associated with the contact zones of granites belonging to all three types, although columbite only occurs in albite-biotite granites.
Minor mafic igneous rocks are associated with the ring complexes, occurring as restricted basic flows interlayered with the rhyolitic extrusives and as doleritic dykes and small gabbroic intrusives.
Basaltic extrusives, which were probably deposited during two or three main phases, and are found on the western and southern sides of the Jos Plateau. They have been subdivided into two groups, namely, the Older and Newer basalts. However, it seems there were a number of extrusive events spread over a range of time. The oldest basalts are from 10 to 11 Ma, while the youngest are less than 1 Ma. old. They are subdivided as follows:
• Older Basalts - which occur in two forms:
i). Lateritised Older Basalts - ie. lavas which are now decomposed to clays and are usually protected by thick lateritic ironstone caps. These sometimes overlie tin bearing fluviatile sediments. The greatest thickness of this basalt type encountered is 60 m. They are usually found on flat ridges and water sheds.
ii). Fresh Older Basalts - which comprise small eroded and partly decomposed remnants of basalt. Some grade upwards into lateritised basalt while others overlie lateritised basalts.
• Newer Basalt - which are not well decomposed and are usually thicker than the older basalts. They cover a cumulative area of some 150 km2 on the Jos Plateau and are associated with two lines of cones aligned in NNE and NNW directions respectively.
The older basalts do not have associated cones although a number of plugs and doleritic dykes are associated with them. The Older Basalts are tholeiitic while the Newer Basalts are alkaline. Both the Newer and Older Basalts were deposited in old valleys. The Newer Basalts in particular are known to have dammed some valleys forming lakes thus altering the depositional environment. Subsequent valleys were formed around the basalt flows, leading to the development of new drainage patterns with the basalt flow protecting the previous mineralised leads from erosion.
Tertiary and Quaternary Sedimentary Rocks
• Tertiary Sedimentary Rocks which are found below the Older Basalts. A typical section from Kinnaird (1977) is as follows from base to top:
Basal red sandstone, which is 75 to 100 cm thick and was deposited on an undulating weathered granite surface. Cassiterite is found on the bedrock in depressions. This unit is capped by a 1 cm thick ironstone band.
Yellow-green mudstone, 60 to 100 cm thick. This sometimes grades from yellow-green to grey and has several ironstone bands.
Conglomeratic red sandstone, which is up to 10 cm thick, and comprises poorly bedded sandstone with poorly sorted angular and subangular quartz grains in a coarse feldspathic sandstone and reddish clay matrix.
Red conglomerate and laterite, a few metres thick - the conglomerate has pebbles of lateritised sandstone and ironstone.
• Quaternary Sedimentary Rocks which are usually much better developed than those of the Tertiary sequence, although still confined to old river channels. In mineralised areas the basal unit comprises a coarse gravel developed directly on an undulating bedrock with undulations of 1.5 to 3 m. These gravels were deposited in a high energy environment and comprise ~20% cobbles and pebbles from 1 to 3 cm and up to 5 x 10 cm in diameter, set in an unsorted subrounded to subangular sand of 1 to 3 mm quartz grains. Up to 10% feldspar is found in the sand on the channel margins. This layer varies from zero to a maximum of 3 m in thickness with the tin mineralisation usually occurring in the bottom 30 cm or less.
The basal gravel usually grades upwards into progressively lower energy sediments over the succeeding few metres. Above the gravel the sediments usually comprise poorly sorted sands, grading upwards into sandy clay and clay.
These deposits were probably formed by fast flowing braided streams. In the Sabon Gida Valley, deposition was in the old valley of the N' Gell River. The river valley appears to have comprised a series of broad basin shaped valley segments separated by bedrock bars which were cut by narrow gorges. The broad basins were up to 500 m wide and 500 to 1000 m long. The separating rock bars were from 50 to 500 m wide, while the connecting gorges cutting them were around 100 m or so wide. The sides of both the basins and the gorges were sharp, beginning abruptly and having slopes of from 1 in 1 to 1 in 2, often rising 30 m vertically in a 30 to 60 m lateral distance.
In the southern part of the district, thick deposits are not always found. However, in the Sabon Gida Valley area the system silted up following the main sand/gravel units resulting in a thick stiff black carbonaceous mudstone to clay being developed. Large tree trunks are found at the base of this clay which is thought to have been deposited in a series of lakes dammed up by Newer Basalt flows. Similar, but thinner ,stiff black clays are found in the southern areas.
The black clays are overlain by either 'Newer Basalt' or recent river gravel and sand of a new cycle. The basal coarse gravels of the new cycle often carry low order tin mineralisation.
Following the extrusion of the Newer Basalts into the old river channels, the diverted river channel often formed new valleys cutting into both granite basement and older Quaternary sediments. Infilling by basalt and sediments has resulted in a mature surface at the present day with sharp inselbergs of Younger Granite. No trace remains at the surface of the once sharply dissected topography of the earlier Quaternary.
It should be noted that the Older Basalts had a similar effect in capping older sediments as the Newer Basalts have, but to a much lesser extent.
Mineralisation
Both primary and secondary tin and columbite mineralisation are known on the Jos Plateau. Although both types of columbite deposit are significant, only one primary tin deposit of any consequence had been found by 1978.
Primary Deposits - Small primary tin occurrences are found on the margins of a number of the granite bodies of the Younger Granite complexes. Mineralisation does not appear to be related to any specific phase. These occurrences comprise zones of one or more of chlorite, fluorite, green mica or topaz alteration developed over widths of from 1 to 3 m and lengths of a few to a few tens of metres. These zones embrace small vein like accumulations of cassiterite, sometimes with accompanying white quartz. Lesser cassiterite, sphalerite, chalcopyrite, pyrite and occasionally galena is dispersed through this alteration zone. Only a few tens of deposits of this type are known. It has been assumed that a much higher density of similar but bigger deposits once existed in the roof zones of the granitic complexes and that these have been eroded with their tin content now being in the alluvial accumulations.
• The Liruei Lode is developed in the Liruei Complex to the north west of Jos, and is the only primary tin occurrence of any substance. It comprises a vertically dipping quartz vein system developed sporadically over a length of 5 km. The mineralised zone is from 0.6 to 8 m in width, averaging 2.5 m and encloses up to 10 quartz veins. These veins range from 5 mm to a maximum of 75 cm in thickness, but are rarely continuous for more than 20 m laterally or vertically. They carry cassiterite and sphalerite with minor galena, chalcopyrite and wolframite. Cassiterite and sphalerite are also disseminated in the altered granite halo of the mineralised zone. An inferred resource of > 3 Mt @ 2% Zn and 0.5% Sn has been published prior to 1978.
• Primary Columbite - occurs as an accessory mineral within albite-biotite granites. Primary grades of 1200 g/m3 have been worked while a resource of 15 million m3 of 360 g/m3 of columbite has been released. As tantalum is more valuable than niobium the Ta:Nb ratio of the columbite is critical. Other accessory minerals accompanying columbite are-zircon and minor thorite. The columbite:cassiterite ratio in the columbite granites is usually between 10:1 and 20:1.
Secondary Accumulations - The current topography of the Jos Plateau is very mature, interrupted by prominent inselbergs representing the Jurassic ring dyke complexes. Secondary accumulations of tin and columbite are found peripheral to these inselbergs and in valleys cutting their outer margins. Almost all significant accumulations are found in coarse gravels within 25 to 30 cm above bedrock. These are known locally as 'Gaba' deposits. The cobbles of these deposits are usually of two main types, either of Younger Granite or of a cream to white siliceous material from the siliceous bands of the Precambrian Basement Complex. The former usually accompany mineralised zones, the latter being found to predominate in barren gravel.
A number of deposit types are encountered. The principal of these are:
• The 'Main type' - These are developed in the coarse gravel layers immediately above bedrock and were laid down either as sheet-like deposits by high energy braided streams or as ribbon-like deposits by fast flowing streams in more restricted valleys. The sheet like deposits are best represented in the pre-Newer Basalt sediments of the Sabon Gida Valley in the 'broad basins' described in the 'Tertiary and Quaternary Sediments' section above. These accumulations are usually limited only by abrupt valley floor changes in the form of the valley walls or ridges of bedrock projecting into the valley. The deposits display irregularities and 'holes' locally, but range from 150 to 250 x 200 to 300 m laterally. The actual mineralised layer is usually <1 m thick with the majority of the cassiterite being in the bottom 25 to 30 cm. The 1 m layer commonly carries from 10 to 50 kg/m3 cassiterite and is sufficient to give an average grade of greater than the economic limit of 250 g/m3 required when overlying barren thicknesses of up to 50 m have to be removed.
One of the largest of these deposits known in 1978 had dimensions of the order of 1.2 x 0.3 km and carried some 6000 tonnes of cassiterite.
The more ribbon like accumulations are found in tributaries of the major palaeo-channels or where the main streams cut rock bars and have widths of from 50 to 200 m and lengths of 200 m to 1 km. In these deposits the tin grades are also concentrated in the bottom 1 m of coarse gravels, as outlined above, with overburdens of 10 to 50 m.
• Terrace Deposits that were formed during the rejuvenation of stream systems often following basalt extrusions when the channel changed its course and eroded into bedrock on the margin of the old valley. Cassiterite accumulations are found in higher energy gravels or coarse sands developed immediately above bedrock. In most cases, when the lateral equivalent of this layer passes over older alluvium of the channel the grade falls-off rapidly. These deposits are usually developed immediately adjacent to, but 10 to 30 m higher than older 'main type' deposits, suggesting a common local source of cassiterite. Occasionally terrace deposits comprise up to 6 m of coarse, unsorted, granitic sand, with grades of 500 to 600 g/m3 throughout. In other cases significant grades persist when the lateral equivalent passes over older alluvium and older 'main type', deposits allowing two layers to be mined in the one section. This however is rare. Terrace type deposits can cover areas of up to 200 x 500 m and are usually mined as a continuation of the operation extracting adjacent 'main type' deposits.
• Russo Deposits which are usually relatively low grade and thin. They occur as thin sheets on ridges and water sheds and are covered by resistant laterite layers which protect the wash from erosion. The deposits are usually only a few metres thick with grades of only a few hundred g/m3 and comprise red granitic sand. Their origin is uncertain. They may be older alluvials formed in broad flat pans, or residual deposits. Minor vein or disseminated mineralisation is sometimes encountered in the migmatites above which they are often found. Russo deposits are mainly found to the south, adjacent to the Ropp Complex.
'Main type' deposits are usually found as sheet-like accumulations in larger channels, with ribbon like deposits in smaller tributaries and sometimes erratically between the sheet deposits. The confluence zone of the main channels and tributaries is usually barren. Terrace deposits are found on the margin of the main channels, usually adjacent to 'main type' deposits. Russo deposits are not common and are largely restricted to the southern district. Here they occur on ridges between the main channels, being cut by the head waters of the tributaries. In some areas older 'main type' mineralised channels and terrace deposits have been cut by younger channels which rework and disperse the ore accumulations and re-deposit barren sand.
Some 75% of Nigeria's tin production to 1978 has been derived from accumulations developed adjacent to Jurassic granites of the Jos-Bukuru and Ropp ring dyke complexes.The main operations in 1878 were:
• Sabon Gida Valley, (or more fully the Sabon Gida Kanan) is located on the western margin of the Jos-Bukuru Complex, over a basement of Precambrian metamorphic rocks with minor concealed 'Younger Granite' dykes. It is part of the old valley of the N'Gell River whose headwaters are in the centre of the Jos-Bukuru complex. Minor mineralisation is found throughout much of the length of the N'Gell River, although major deposits are restricted to the 3 to 4 km section where thicker, more extensive, alluvials were formed. The deposits of the Sabon Gida have thicker overburdens, but carry greater tonnages of cassiterite at higher grades than those of the Southern Areas. The Sabon Gida Valley embraces substantial reserves, but has a greater thickness of overburden. The general range of overburden in the Sabon Gida Valley is from 15 to 50 m. Individual blocks of ore grade wash carrying more than 1000 tonnes of cassiterite account for nearly half of the reserves of the Sabon Gida, with a significant number of others having >400 tonnes. The remainder in general have more than 100 tonnes. Due to the deeper overburden in this area, small deposits cannot be worked due to the increased volume of barren material that must be mined to form a batter on the pit. The average grade in the Sabon Gida Valley ranges from 350 to 600 g/m3 with few blocks being <350 g/m3. These are “natural grades” not calculated to a cut off, but to the deposit margin limited by bedrock topography.
• Southern Areas. The deposits on the margin of the Ropp Complex to the south are spread over an area of some 15 x 10 km and are developed in a number of drainage systems. Individual deposits are in general small, although some large occurrences have been worked. One of these in the Mongu Valley was 1 km x 300 m x 20 m thick with an average grade of 600 g/m3. This deposit had adjacent tributary deposits 100 to 300 m long, 20 to 60 m wide and 5 to 15 m thick with lower grades. A small dredge in the Ladi Valley has worked a string of deposits developed almost continuously over a length of 5 km and width of 50 to 200 m. In general, the deposits being worked in the Southern District in 1978 individually carried around, or <100 tonnes of cassiterite. The smallest deposits have as little as 50 tonnes of SnO2. However, individual deposits often occur in groups and can usually be worked together or progressively. Those with 1000 tonnes are considered large in this area. Grades are in general between 300 and 450 g/m3 although some deposits as low as 180 g/m3 can be worked profitably where the development has been paid for by adjacent higher grade blocks. Overburden thickness are usually <15 m.
The cassiterite of the Jos Plateau deposits is relatively coarse with 95% of the Sabon Gida product being >75 mesh (i.e., 0.34 mm), while 78% is >52 mesh (i.e., 0.4 mm). However, in the Mongu Valley where deposits are further removed from 'Younger Granite' outcrops, the cassiterite is finer, being mainly <100 mesh (i.e., <0.25 mm). The Russo deposits in contrast, have 95% >52 mesh (i.e., >0.4 mm).
Alluvial columbite is found in relatively small accumulations close to primary sources. It is usually separate from significant alluvial tin, but occasionally the two co-exist. In these cases the columbite grade usually falls off rapidly downstream such that in a distance of 1 km tin deposits may be worked which are entirely devoid of columbite.
Most of the information above was assembled from publicly available sources during and following a visit to the tin deposits of the Jos Plateau in 1978.
The most recent source geological information used to prepare this decription was dated: 1980.
Record last updated: 30/12/2019
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Olade M A 1980 - Geochemical characteristics of tin-bearing and tin-barren granites, Northern Nigeria: in Econ. Geol. v75 pp 71-82
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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