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Sierra Rutile - Lanti, Gangama, Mogbweno, Pejebu, Ndendemoia, Mosavi, Gbeni, Taninahun, Gambia, Nyandehun, Jagbahun, Sembehun, Benduma, Kibi, Dodo, Kamatipa, Komende, Gbap, Boka, Taninahun |
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Sierra Leone |
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The main Sierra Rutile mining operations comprise the Lanti wet and dry mines and the Gangama dry mine in the Moyamba and Bonthe Districts of the Southern Province, Sierra Leone, located ~30 km inland from the Atlantic Ocean and 135 km south east of Freetown (#Location - Gangama: 7° 43' 32"N, 12° 21' 4"W).
These deposits are located within the Sierra Rutile Area 1 Mine Lease of ~290 km2. At least 16 deposits have also been identified within the surrounding district. Other deposits within Area 1 include the previously mined Mogbweno, Pejebu and Ndendemoia deposits in the eastern half of the area, Gbeni to the immediate north of Lanti which is in the SW corner of Area 1; Taninahun which is immediately to the west of the Gangama deposit in the NW corner of the area. The Gambia, Nyandehun and Jagbahun deposits are 3 km north, 14 km north and 12 km NE of Area 1 respectively. The Sembehun group of deposits are ~22 km NW of Area 1 and comprise from south to north, Benduma, Kibi, Dodo, Kamatipa, Komende and Gbap. The Boka and c deposits are a further 10 km to the north.
Titanium-bearing mineral sands were first discovered in south western Sierra Leone in the early 1920s by an exploration geologist of the Gold Coast Geological Survey. It was not until the 1950s that exploration and drill testing was commenced by British Titan Products at Gangama and the Lanti-Teso-Gbeni deposits. In 1957, British Titan and the Pittsburgh Plate Glass confirmed reserve estimates and formed Sherbro Minerals in 1961. Following an intervening recession in the rutile market, Sherbro Minerals started mining operations, concentrating on the Mogbwemo deposit. Poor production due to technical difficulties associated with the suction dredge resulted in the liquidation of the Company in 1971. Nord Resources Corporation and Armco Steel acquired the Sherbro Minerals leases in 1971 and formed Sierra Rutile Limited, which after further drilling constructed a new bucket line dredge and began production at Mogbwemo in March 1979. After a temporary closure in 1982, the mine operated continuously from 1983 to 1995, producing up to 154 000 tonnes per annum (tpa) of rutile. Average annual production for the five years between 1990 and 1994 was 148 360 t rutile and 58 650 t of ilmenite. During this period dredge mining exploited the Bamba Belebu deposit from 1986 to 1989; the Pejebu North deposit from 1989 to 1990; and the Pejebu South deposit from 1990 to 1992. Dry mining was employed at the Mokula deposit from 1989 to 1990. Mining of the Lanti North deposit was from 1992 to 1995. In 1993, Nord sold 50 % of Sierra Rutile to Consolidated Rutile Limited of Australia and the decision was made to expand the facilities to a two-dredge operation. The second dredge was under construction when operations were suspended in January 1995 by civil war and the mine was shut down and placed on care and maintenance. In January 1995, the Revolutionary Union Front, an anti-government military group, overran the mine with the destruction of much of the equipment and infrastructure. Between 1999 and 2004, Sierra Rutile Limited changed ownership and in 2004 the European Union provided a grant of €25 million to the government of Sierra Leone to restart its mining operations. Refurbishment commenced in 2005 funded by the EU loan and cash from shareholders. Sierra Rutile Limited was listed on the London AIM in 2005 and Dredge D1 resumed mining operations in the Lanti North Deposit in February 2006 after an 11-year shutdown. Thereafter, construction of Dredge D2 resumed, to be completed in 2008 when it began mining the Gangama Deposit in January 2008, before capsizing in July of the same year. Between 2007 and 2011, the shareholding of Sierra Rutile Limited changed and with Pala Investments Limited investing in September 2010 and ultimately increasing its stake to 54%. In 2016, Iluka Resources Limited of Australia completed the acquisition of Sierra Rutile Limited as a wholly owned subsidiary.
The Area 1 mining lease encloses of a series of prominent hills and slight undulations with an elevation range of from 0.9 m above sea level (masl) in the coastal mangrove swamps, to 322 masl at the highest point, Gbangbama Hill.
Sierra Leone is divided into two major tectonostratigraphic subdivisions, the
i). eastern, stable, Precambrian West African Craton which is composed ofhigh-grade metamorphic rocks and granitic gneisses; and
ii).western units that contain elements of the Rokelide Orogenic Belt, made up of Archaean rocks that were deformed during the ~550 Ma Pan-African tectonothermal event. This belt extends ~600 km from western Guinea along coastal Sierra Leone into Liberia.
The older basement rocks are unconformably overlain to the west by a 20 to 40 km wide strip along the west coast of Sierra Leone composed of Tertiary to Recent unconsolidated sediments of the Bullom Group. The Bullom Group comprises sediments deposited in alluvial, fluvial, coastal marine and estuarine environments, deposited following a late Tertiary marine regression, which exposed the basement to chemical and mechanical erosion. These basement rocks include the Kasila Group, a linear belt, no more than 30 km wide, composed of high-grade supracrustal Archaean rocks that were reworked during the Pan-African Orogeny. It mostly consists of granulite facies acid gneisses, charnockites, garnet-hornblende and garnet-plagioclase gneiss, and, in places, hypersthene gneiss, hornblendite and pyroxenite. To the east of the Kasila Group, low-grade supracrustal rocks of the recumbently folded Archaean to Palaeoproterozoic >2.1 Ga Marampa Group overlie the granitic terrane and are probably in fault contact with Kasila Group. The Marampa Group contains ironstone, mafic to felsic volcanic rocks and derived volcanogenic sediments that are similar to greenstone belt lithologies found to the east. These are, in turn, overlain by a syncline of sedimentary rocks of the Rokel River Group ~40 km east of Area 1, possibly deposited in a fault-bounded intracratonic rift basin. The latter group forms a ~225 x 30 km, NNW-SSE trending belt of marls, quartzites, sandstones and volcanic rocks, which although not dated, has a basal glaciogenic unit correlated with similar strata in Senegal and Mauritania that are generally accepted to be of Neoproterozoic age. Dolerite is common, both as predominantly east-west trending dykes within the basement complex, and as extensive sills above the Rokel River Group.
Rutile and other heavy minerals were liberated via erosion of chemical and mechanically weathered, topographically elevated Kasila Group rocks, and subsequently deposited in structurally controlled pre-incised channels, erosional valleys or as alluvial fans on a topographically benign coastal plain. Rubbly surficial laterite development is prevalent through the near surface material of the Bullom Group. Unconsolidated estuarine and marine sediments occur to the west of the major watershed located within the centre of Area 1, typically poorly sorted sandy clay and sandy clays. In contrast, sub-aerial alluvial and elluvial sediments were deposited to the NE of the watershed, where several cyclic sequences of poorly sorted clastic gravels overlain by sands and clayey silts are preserved. Hard lateritic inclusions are common but are generally confined to the upper portions of the sequence.
The heavy minerals within the Sierra Leone rutile deposits are typically angular, suggesting minimal transport and re-working. The spatial distribution of heavy minerals along the length of the palaeo-channels also reflects this, with mineral grades typically decreasing with distance from the source, accompanied by an increase in sand content, replacing argillaceous material within the matrix.
The base mineralised of the Bullom Group ranges from 9 to 34 m below surface with an average depth of 23.44 m. The mineral resources have a wide variation in physical dimensions, varying from a few metres to over 20 m in thickness. They vary in width from 100 to over 2000 m in places, with the Sembehun group of deposits, including the leading edges considered as a single body have a width of >5000 m. The deposits length varies from about 1000 to over 6000 m. In general, mineralisation is present from surface, although some poorly mineralised interburden layers are present towards the south/west of the Benduma, Dodo and Kibi sub-deposits.
JORC compliant Mineral Resources in the other deposits at 31 December 2016 (Iluka Resources Mineral Resources Report to the ASX, Feb 2017) at a 0.25% rutile cut-off, were:
• Gambia deposit
Inferred Resource - 27.870 Mt @ 1.03% rutile;
• Gangama deposit
Measured Resource - 13.132 Mt @ 1.993% rutile, 0.31% zircon;
Indicated Resource - 32.100 Mt @ 1.28% rutile, 0.22% zircon;
Inferred Resource - 14.300 Mt @ 0.93% rutile, 0.09% zircon;
• Gbap deposit
Inferred Resource - 68,000 Mt @ 1.00% rutile;
• Gbeni North deposit
Measured Resource - 16.717 Mt @ 1.30% rutile, 0.17% zircon;
Indicated Resource - 26.900 Mt @ 1.19% rutile, 0.13% zircon;
• Jagbahun deposit
Inferred Resource - 2.100 Mt @ 0.970% rutile;
• Mogbwemo Tails
Indicated Resource - 12.300 Mt @ 0.72% rutile;
Inferred Resource - 0.610 Mt @ 0.92% rutile;
• Mogbwemo Virgin
Indicated Resource - 0.700 Mt @ 0.96% rutile;
• Mosavi deposit
Indicated Resource - 47.400 Mt @ 0.72% rutile, 0.40% ilmenite, 0.15% zircon;
• Ndendemoia East deposit
Indicated Resource - 14.800 Mt @ 0.88% rutile, 0.03% zircon;
• Ndendemoia West deposit
Indicated Resource - 4.000 Mt @ 0.63% rutile, 0.07% zircon;
• Nyandehun deposit
Inferred Resource - 5.630 Mt @ 1.93% rutile;
• Sembehun Group
Benduma Indicated Resource - 168.500 Mt @ 0.88% rutile, 0.17% ilmenite;
Dodo Indicated Resource - 74.800 Mt @ 1.14% rutile, 0.22% ilmenite, 0.04% zircon;
Kamatipa Indicated Resource - 46.300 Mt @ 1.56% rutile, 0.14% ilmenite;
Kibi Indicated Resource - 48.800 Mt @ 0.92% rutile;
Komende Indicated Resource - 20.800 Mt @ 0.87% rutile, 0.09% ilmenite;
• Taninahun deposit
Benduma Indicated Resource - 5.900 Mt @ 0.79% rutile, 0.07% ilmenite;
• Taninahun Boka deposit
Inferred Resource - 3.350 Mt @ 1.65% rutile;
• Combined resources
Measured Resource - 59.649 Mt @ 1.26% rutile, 0.12% ilmenite, 0.16% zircon;
Indicated Resource - 537.923 Mt @ 1.02% rutile, 0.14% ilmenite, 0.07% zircon;
Inferred Resource - 121.860 Mt @ 1.06% rutile, 0.00% ilmenite, 0.01% zircon;
TOTAL Resources - 719.432 Mt @ 1.04% rutile, 0.11% ilmenite, 0.07% zircon.
JORC compliant Ore Reserves for all Sierra Rutile deposits at 31 December 2016 (Iluka Resources Mineral Resources Report to the ASX, Feb 2017) were:
Proved Reserve - 34.251 Mt @ 1.45% rutile;
Probable Reserve - 271.395 Mt @ 1.24% rutile;
TOTAL Reserve - 305.646 Mt @ 1.27% rutile.
The JORC compliant Indicated + Inferred Mineral Resource at the Pejebu deposit alone at 25 January 2019 (Iluka Resources ASX Release) totalled:
23.4 Mt @ 0.95% rutile, 1.0% Ilmenite, 0.1% zircon.
JORC compliant Indicated + Inferred Mineral Resources in the Sembehun group of deposits alone at 30 June 2018 (Iluka Resources ASX Release) totalled:
463 Mt @ 1.1% rutile, 0.6% Ilmenite, 0.1% zircon.
The information in this summary was drawn from various Iluka Resources ASX Releases and JORC Section 1 Sampling Techniques and Data table 2012 as well as Lake, J., 2018 - Sierra Rutile Project Area 1 – Environmental, Social and Health Impact Assessment: Mine Closure Plan; prepared by SRK Consulting (South Africa) Limited for Sierra Rutile Limited, 97p.
The most recent source geological information used to prepare this decription was dated: 2018.
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.
Sierra Rutile - Gangama
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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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