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Directorate of Geological Survey Sierra Leone This report summarizes a desk top study from historic, published and unpublished sources. It is intended to only hi-lights background information with particular reference of Gold occurrences relating to Sierra Leone. Definition: Gold is a chemical element with symbol Au (from Latin: Aurum) and atomic number 79. In its purest form, it is a bright slightly reddish yellow, dense soft, malleable and ductile metal. Chemically, gold is a transition metal and a group 11 element. It is one of the least reactive chemical elements, and is solid under standard conditions. The metal there occurs often in free elemental (native) form, as nuggets or grains, in rocks, in veins and in alluvial deposits. It occurs in a solid solution series with the native element silver (as electrum) and also naturally alloyed with copper and palladium. Less commonly, it occurs in minerals as gold tellurides. Gold deposits in Sierra Leone are formed via a wide variety of geological processes. In this process the mineral is believed to exist initially in a molten state in which it solidified to form the deposit. But it is understood that the occurrence of gold in the country is actually re-deposited by circulating magma that cools as it reaches the outer surface, and ends up in specific areas that exhibit the correct conditions. Intensely hot magma deep within the earth brings element such as gold into a solution form during eruption. As gold is moved and the magma cools, gold nuggets and veins form as the metal solidifies. Various deposits in the country are classified as either primary, alluvial or placer and residual or laterite. But some deposit within the country contains a mixture of all three types of ore. The majority of primary gold deposits in the country are purely referred to as lode gold deposits. Lode gold deposits in the country are generally high-grade, thin, vein and fault hosted. They are primarily made up of quartz veins also known as lodes or reefs, which contain either native gold or gold sulfides or telluride. January, 2016 Lode gold deposits in the country are usually hosted in silicified banded ironstones and schistose quartzites intercalated with amphibolites and intruded by muscovite granite and associated tourmalinised pegmatites. Most lode gold deposits are sourced from metamorphic rocks because it is thought that the majority are formed by dehydration of these rocks during metamorphism. The gold is transported up faults by hydrothermal waters and deposited when the water cools too much to retain gold in solution as shown in the diagram below: Figure 1 Primary Gold Formation Table 1 Selected Properties of Gold Symbol Au Atomic Number 79 Appearance Metallic Yellow Period Period 6 Phase Solid Element Category Transition Metal Melting Point Boiling Point Density near r.t Heat of Vaporization Crystal Structure ( °C, °F) 3243 K ( °C, °F) 19.30 g/cm3 Heat of Fusion Hardness 1337.33K 12.55 KJ/Mol 342 KJ/Mol 25.418J Moh-K Face Centered Cube DGS- SIERRA LEONE 1 Placer or alluvial deposits are the most commonly found gold in the country and are sourced from pre-existing gold deposits and are said to be secondary deposits. Almost all of the gold mined in the country is from alluvial/placer deposits. Placer deposits in the country are formed by alluvial processes within rivers and streams. The deposits are formed via gravity, with the density of gold causing it to sink into trap sites within the river bed, or where water velocity drops, such as bends in rivers and behind boulders. Figure 3 Gold Bars Figure 2 Primary Gold Formation Gold Mineralization in Sierra Leone Although gold mining activities in Sierra Leone has a long history but the occurrence of extractable gold was first discovered in 1930 by Junner of SLGS in the Makoke River near Masumbiri. Present investigation coupled with data obtained from previous works revealed that the country is a potentially significant gold country. Gold deposit in the country exhibits a high degree of variability in form and distribution corresponding to different lithological, geochemical and structural controlling aspects. Therefore, gold sulphide, gold-quartz vein and gold ferruginous quartzite formations is hosted by intensely sheared and hydrothermal altered granitoid rocks of the greenstone belt of the country. Gold mineralization in the country is considered to occur in structurally controlled sites, commonly within iron-rich garnet-cummingtonite schists. Potential ore zones develop in dilatants zones related to fold and shear deformation of the schist and their wall rocks. Gold ores occur in a series of sub-parallel or en echelon ore shoots and have been recorded in mineralized zone in silicified banded ironstones and schistose quartzites, intercalated with amphibolites and intruded by muscovite granite and associated tourmalinised pegmatite. From previous reports, Gold deposit is said to be a N-S trending, steeply dipping veins or zones parallel to the axial plane of different folds. Subsequent evaluation of the arsenic geochemistry and the geology led to the conclusion that the mineralization of gold in the country is structurally controlled. The mineralization is generally confined to the more iron formation-rich portions of the garnet-cummingtonite schist and the adjacent biotite quartzite and amphibolites. The main ore minerals are asenopyrite, pyrrhotite, pyrite, and chalcopyrite; and two types of mineralization are observed, both of which may be of ore grade with respect to gold: 1) Arsenopyrite-rich zones, +/- pyrhotite, in garnet-cummingtonite schists. 2) Pyrhotite/arsenopyrite-rich micaceous quartzite. DGS- SIERRA LEONE 2 Gold Occurrences in Sierra Leone The overall geological setting, presence of extensive artisanal gold workings, and evidence from historic work and recent work by consultants demonstrates the significant potential of host gold mineralization. However, to date there appears to be little understanding of the source area, style of mineralization or conceptual exploration models. Figure 4 Visible Gold embedded in Rock Matrix However, some domains within the basement complex rocks of Sierra Leone are well known as host for substantial gold mineralization occurring in different geological environment in the country. Structures of gold in Sierra Leone display a northeast trending lineament, locally cross-cut and offset by small northwest-southeast and east-west trending faults. A number of primary gold occurrence as well as alluvial or placer gold deposits were identified from various areas in the country. In spite of the country’s long exploration history, there are still many gold occurrences remain undiscovered due to inappropriate prospecting methods. Primary gold mineralization in the country has been historically identified in quartz veining with disseminated to massive sulphides within amphibolites units. The country’s gold mineralization is hosted by shear zones or their subsidiary structures with gold precipitated preferentially in the presence of magnetite rich lithologies as a result of the competency and chemical boundaries that they form. Gold mineralization in Sierra Leone occurs in different styles corresponding to different geological environment and is genetically associated with the country’s schist belt. Most gold occurrences in the country are linked to a hydrothermal activity that accompanied emplacement of Proterozoic to early Paleozoic calc-alkaline granitoid. The occurrence of gold mineralization is in the greenstone belt which is made up of metasedimentary pelites and amphibolites units as well as some syn to late-kinematic granites. The gold occurs in different styles corresponding to different geological environment. Some gold deposits were related to a shearing episode that post-dated emplacement of major intrusions and others are thought to have formed by exhalative hydrothermal processes during the break phases of submarine volcanic activity. Greenstone related gold deposits are typically proximal to regional in scale, deep seated faults, lineaments and shear zones. Gold mineralization often occurs in relation to shoots and lenses on second and third order structures at low angles to the main structure in the country. In Sierra Leone, Gold mineralization is also associated with sulphides and follows the contact between magnetite rich garnet-cummingtonite schists and cordierite schists. The contact zone is somewhat several km wide. Gold is known to occur in all Schist belts in Sierra Leone e.g. around Kamakwie-Laminaia, Sula Mountains, Kangari Hills, Nimini Hills, Gori Hills, Kambui Hills and Gola Forest. Gold was mainly mined from placer deposits and only in two areas around Baomahun in the southern Kangari Hills and Komahun in the eastern Nimini Hills where gold has been discovered to occur as potential underground workings from quartzveins. The two companies operating in the area have since secured a large scale mining lease and are yet to commence operation. Gold was first discovered in 1926 in the Makoke River near Masumbiri. Further prospecting between 1928 and1930 proved the existence of an extensive alluvial gold field in the Sula Mountains/Kangari Hills. Gold mining started in 1930 and it occurrence were found in the following areas: DGS- SIERRA LEONE 3 Gold at Kangari Hills The gravels of almost all of the streams and rivers in the Kangari hills are gold bearing. In March 1929 Mr. J.D Pollet discovered gold in a tributary of the kwifuifui stream west of Makong. Later in the same year Dr. Junner and Mr. Pollet discovered gold at Baomahun and Dr Fowler Lunn found gold in the Mapoko stream at Makele. These are the three main areas in the Kangari schist belt where gold occur. Gold at Makong To the west of Makong in the upper reaches of the kwifuifui stream somewhat sugary quartz of veins has been found and contains megascopically visible gold. Around 1962, largest nugget found was about 1oz and large excavation in the tremolite schist remaining. Mr. Pollet observed that in the kwifuifui stream the best values of alluvial gold were obtained where granite contained patches of tremolite schist. Soil sampling, pitting, trenching and drilling have been carried out by Axmin Mining (SL) Ltd. as they own the Makong Licenses. Gold at Makele The gold in this area was probably derived from pegmatites and the area is tectonically disturbed like the molybdenite occurrence. The main gold locality was 276.9 arcs in area and includes the Mapoko stream near Makole and deposits appeared to contain in the present river beds. Gold at Baomahun The gold at Baomahun is associated with a mineralized zone carrying sulphides and this area is situated within a tectonically disturbed area different in geology from that of Makong. The gold of these deposits derived from the below mentioned rocks, especially from a zone of heavy asenopyrite mineralization with garnet cummingtonite schist intermingle with strips of quartzite and magnetite quartzite. Gold was found in the heavy mineral concentrates of a tributary adjacent to the asenopyrite zone. Through drilling, geochemistry and large scale mapping, three main areas of gold mineralization occurred in the West, Central and Eastern ridge. Gold deposit at Baomahun suggested that the mineralization in the country is associated with disseminated sulphides and lithologies with varying resistivity. Figure 5 Topographical Sheets with Known Gold Mineralization (hi lighted) in Sierra Leone – (Data Sources: GIMS- NMA- 2016) Cluff Gold has raised its mineral resource at the Baomahun Gold Project in Sierra Leone to over the 1million-ounce bracket and plans scoping study for underground redesigning. Cluff Gold reported a 40% increase in total mineral resources at its flagship Baomahun gold project after an extensive drilling campaign with investigation into possible mineralization extensions set to be launched shortly. Gold at Nimini Hills Placer gold was discovered in 1930 near Titambaia in the Wongo stream in the Nimini plateau west of Njaiama Nimikoro. In 1958 to 1962 lode gold was investigated near Komahun by Laing. Extensive fracturing and minor faulting occur along the Wongo River on the western margin of the schist belt, both in gneisses and schist. Very extensive alluvial gold mining operations have carried out in the Valley of Wongo River. Bands of banded iron stone occur in the ultrabasic schist, close to it contact with banded amphibolite. An extensive programme of geochemical sampling, loaming, trenching and drilling are carried out at Komahun by Axmin Mining (SL) Ltd. An assay of this drilled holes proved that lode mineralization is in the sulphide zone. The mineralization consists of asenopyrite with some pyrite in banded iron stone. DGS- SIERRA LEONE 4 reliable on their banks if they have broken through the duricrust but the main guides to mineralization are boulders of lateralized gossan. Molybdenite with galena and other metallic minerals occurs in the kinematic pegmatite Gold in the Gori Hills Gold is common in streams gravels of the schist belt which has several fairly wide bands of aluminous mica-quartz gneisses with numerous bands of quartzite. Gold is also present in some of the streams on the Western side of the schist belt. Extensive pitting and mining has been carried out with alluvial gravels in the flood plains of the western flowing streams between Koidu and Bendu along the upper part of the Sunya River and its tributaries North of Bendu. Figure 6 Geology Showing Schist Belts (yellow circles) in Sierra Leone (Data Sources: GIMS- NMA 2016) Gold at Sula Mountains. The first gold found in Sierra Leone was in this area in 1926 in the Makoke River near Masumbiri by N. R. Junner. In the 30s several companies under went exploration in the area including Coastal Exploration Syndicate Ltd., Maroc Ltd, Balia Gold Mining company etc. Gold at Dalakuru Area Dalakuru is the major gold production area in the Sula Mountain and valuable deposits of alluvial gold occur here. The area lies to the Sende River and north of the Fundiburu River. The ground surface is cover by duricrust boulders and soil. Amphibolite and late kinematic granite outcrop in the Sende River and banded iron formation occur to the west of this area. Lake Sonfon Area The lake Sonfon country produces most gold in the Sula mountain, but intense laterization hinders the recognition of mineralization outcrops. Outcrops of un weathered rock around Lake Sonfon down to Dalakuru are exceptional and loaming is unsatisfactory on duricrust surface but fairly The greenstone belt of the country also serves as a source for alluvial gold. Gold in the Gola Forest Area. Between 1940 -1941 gold was mined near Nongowa village and the Pepe-Yama village and lowrima village between 1942 to 1945. All the rivers are drained from the southern Kambui hills to the western side. Stream near Beodu village which is about halfway between Lahuhum and Beledu low gold values were found with younger granite intruding the granulite and quartz magnetite hypersthenes gneisses. Gaura Chiefdom drained a band of pyritised hypersthene granulite rich in quartz and magnetite but sample contains silver but no gold. Low gold values were seen fairly east of the Moa River draining granulite and ironstone bands that are intruded by younger granite. YEAR Quantity(Oz) Export Value (Le) 2009 5,361.46 14,500,799,025.53 2010 8,690.22 34,445,337,511.41 2011 5,374.62 27,512,074,885.67 2012 4,534.54 24,694,746,147.75 2013 3,099.41 15,763,548,045.74 2014 1,474.99 7,496,114,636.69 2015 3,460.28 18,332,608,452.51 Table:2 Gold Export 2009 to 2015 – Data Courtesy of Precious Minerals Trading (PMT) -NMA DGS- SIERRA LEONE 5 Uses of Gold Of all the minerals mined from the earth, there is none with more practical uses than gold. Its usefulness is derived from a combination of truly unique properties. One such example of its exceptional flexibility comes from the fact that it is the most malleable and ductile of all metals. Modern science techniques have discovered how truly important this precious substrate is in helping solve age-old and future challenges. The following is a list of Gold’s most popular modern day application and uses; Production of Jewelry Gold has been used in the production of ornamental objects for several years now. While expert opinion varies on this point, many believe that this was probably the first use of gold. Specifically, it is a combination of the following attributes or qualities that give it its elevated status among other precious metals: Very High Lustre Desirable yellow color Resistance to tarnishing Ability to be manipulated into wires, hammered into sheets or cast into shapes. In Electronics A small amount of gold is used in almost every sophisticated electronic device as conductors carrying tiny currents and thus remain free of corrosion. This includes: cell phones, calculators, personal digital assistants, global positioning system units and other small electronic devices. Most large electronic appliances such as television sets also contain gold. In Computers Like other electronic devices, gold is used in modern desktops or laptop computers. Its ability to rapidly and accurately transmit digital information through the computer and from one component to another requires an efficient and reliable conductor and gold meets these requirements better than any other metal. The importance of high quality and reliable performance justifies its higher cost. Edge connectors used to mount microprocessor and memory chips onto the motherboard and the plug-and-socket connectors used to attach cables all contain gold. The gold in these components is generally electroplated onto other metals and alloyed with small amounts of nickel or cobalt to increase durability. Coinage, Currency and Bullion Because gold is so rare and highly valued, this precious metal makes a natural currency, and has for at least 6,000 years. The U.S. used to hold all its monetary currency to a gold standard, and since the turn of the millennium, gold value has been on the rise in the stock market. In an uncertain economy, gold has emerged as a possible financial staple. One of the most common ways to hold or invest in gold is in gold coins, gold bars, also gold bullion DGS- SIERRA LEONE 6 Space and Aeronautics In the aerospace industry where reliable and effective technologies are key to survival, gold plays an essential role. Gold is used to lubricate mechanical parts, conduct electricity and coat the insides of space vehicles to protect people inside from infrared radiation and heat. Environmental Uses Due to its unique chemical and metallurgical properties, gold has a vital role to play in future technologies including water purification, reducing pollution, energy consumption, and diesel emission control. Medals and Awards As a highly esteemed precious metal, gold makes a natural appearance in crowns, awards and religious statues. Because of its unparalleled beautiful qualities and rarity, gold is one of the highest status symbols. In everything from Academy Awards to Olympic medals, gold is recognized for its admirable qualities and it holds a permanent place of value in humanity’s eyes. DGS- SIERRA LEONE 7 Further Reading and Selected References. Useful Contacts for further information. The Mineral Potentials of Sierra Leone by Dr Klaus Peter-Freitag (1974) Mineral Potentials in Sierra Leone Vol 1 and 2. Geology of sierra Leone http://www.nma.gov.sl/home/library-services/ http://www.recyclefrog.com/resource-centre/modern-uses-of-gold/ This commodity profile was compiled by Joseph Lebbie for Directorate of Geological Survey in Sierra Leone (January 2016) For further information, please contact Directorate of Geological Survey Library Sierra Leone 13 Wilkinson Road, Freetown Sierra Leone +232 79 999980 +232 79 252455 [email protected] DGS- SIERRA LEONE 8