Gold Exploration in West Africa

[FL]

Gold prospects in Rio Muni, Equatorial Guinea

The following is taken from:
http://www.miningreview.com/archive/032/34_1.htm

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Equatorial Guinea
- new exploration opportunities on the north Congo Craton

By Kevin Leahy, Exploration Consultants Ltd, UK.

Much of Equatorial Guinea's main enclave Rio Muni is currently available for minerals exploration, and taken with the new mining law in the pipeline and an accessible archive of previous exploration data, must qualify as the major greenfields exploration opportunity in the region for gold, diamonds and a host of other commodities.

The territory of Equatorial Guinea includes the islands of Bioko and Annobon, and the mainland enclave of Rio Muni, which is bordered to the north by Cameroon and to the south and east by Gabon. The total surface area of the country is 28,051 km2, with mainland Rio Muni accounting for about 26,000 km2. Rio Muni has a coastal plain and a mountainous interior, and the climate is tropical with four seasons (two wet and two dry).

The economy is now dominated by hydrocarbon production, and forestry to a diminishing degree, which together account for nearly all of the export market. Hydrocarbon production includes oil, natural gas and condensate, most of which is exported, or converted to methanol at a new processing facility on Bioko. Continuing discoveries offshore Rio Muni have massively raised the profile of the Gulf of Guinea as a major new petroleum province, with it being apparent that Equatorial Guinea has a major share of the hydrocarbon endowment. These new discoveries, as well as the more established Niger Delta fields, have hoisted current production rates to over 200,000 bbl/d during 2002. Revenues from hydrocarbons are already leading to major infrastructural improvements, including a new deep-water freeport and processing facilities on Bioko, and road improvement and electrification programmes.

Geological background

The geology of Rio Muni is poorly understood, and much of the following tectonic evolution has been interpolated from surrounding Cameroon and Gabon. Rio Muni comprises the Archean terranes of the Ntem complex and the Monts de Cristal Massif, both of which were partly reworked during the Palaeoproterozoic Eburnian orogeny. Exploration in Rio Muni has indicated the presence of greenstone belts and major shear zones, and possible Eburnian terrane boundaries. Pan African transpressive structures are common in the west and are associated with granitic intrusions and pegmatite bodies, and these also occur across the interior. Low metamorphic-grade shales, dolomites and quartzites occur in the south-west, representing the northernmost extension of the Niari foreland basin of the Pan African-age West Congolian orogeny. Higher grade sedimentary packages, also attributed to the Pan African, are found along the northern border of the country where they are associated with major strike-slip and thrust faults.

The coastal strip of Rio Muni comprises cretaceous sands, shales and carbonates with basal conglomerates, developed in the early rifting phases of Atlantic opening. Significant transcurrent structures (such as the Fang and Bata fracture zones) link to major onshore lineaments, at least one of which shows evidence of Cainozoic rifting (the Benito Rift).

New Initiatives

The Ministry of Mines and Energy has several projects underway to improve the background knowledge of the geology and mineral endowment of Rio Muni. This includes mapping, prospect evaluation and a study of outcropping granites. The latter is a very important project as it includes trace element and isotopic analyses that will not only yield high quality age dates (from zircon U-Pb), but also reveal crustal growth and modification processes. The results from this will allow complete revision of the tectonic evolution presented above, for example, confirming or otherwise the degree of Pan African crustal reworking. The initial results have already produced some surprises - of the 36 samples taken from right across Rio Muni, all are from the calc-alkaline trend, incompatible with Pan African intrusives in Cameroon, suggesting that this area of the craton escaped reworking in this episode. Results from age dating and other isotopic geochemistry will elucidate these histories further, and are expected to be out to publication in the third quarter 2003.

Current metals production is negligible other than from minor artisanal gold mining activities in Rio Muni. Nevertheless, several studies have demonstrated the potential for gold, columbo-tantalite and diamond deposits, with previous exploration highlighting several areas of immediate interest. There is also potential for platinoids, dimension stone, base metals, and bauxite amongst other commodities.

Gold

The rivers of Rio Muni are worked for gold by traditional artisans using simple panning and wooden sluice technologies in three main areas - Coro, Aconibe, and Mongomo, along with numerous lesser sites. The alluvials are currently being worked far less that in recent decades, possibly due to recent low gold prices and the relative prosperity in other sectors (oil industry and construction). Of the three main areas, Coro is the most accessible and has been most explored (1980s and '90s). Historical records are incomplete, but at least 2,166 kg of alluvial gold were produced from the Coro area in the mid-'70s. The area comprises Archean upper amphibolite charnockites and gneisses with Eburnian deformation (circa 2.6 - 2.3 Ga), intruded by calc-alkaline mafic (gabbros, norites and diorites) and granitic bodies (including aplite dykes) that may have been overprinted by Pan African deformation and greenschist metamorphism. The alluvial gold is relatively coarse grained, occurring as either dendritic or rounded nuggets, mostly in the 0.4 - 0.2-mm size range, although larger nuggets up to 4.0 mm x 8.0 mm are relatively common. The common occurrence of the gold with vein-quartz, clays, and lateritic minerals attests to the proximity and variable types of bedrock gold mineralisation. Exploration to locate bedrock source is incomplete, but the findings from limited trenching suggests three main associations:

• Coarse, yellow-red gold - possible association with hydrothermal alteration (potas sic, argillic and kaolinitic), mylonitic shear zones and minor quartz veining around calc- alkaline gabbroic dykes

• Fine, pale yellow gold - possible association with hydrothermal alteration (argillic) and minor quartz veining around narrow (zircon-rich) aplite dykes. This gold type is frequently associated with zircons in regolith and alluvial occurrences

• Gold in regolith and alluvials associated with the strongly faulted boundary between a granodiorite and a gabbro intrusive suite.

Diamonds

The diamond potential relates in part to post-Eburnian structural lineaments containing basic intrusives that extend along strike from the Mitzic diamond workings in Gabon (actively being evaluated by De Beers) into the Nsork area of Rio Muni, 50 km to the north and west. Heavy mineral sampling results have identified zinc-rich chromites found in the Nsork area, which are the main indicator mineral for the meta-kimberlites at Mitzic.

Recent exploration in northern Gabon by Southern Era for both gold and diamonds has identified trends right up to the borders of Rio Muni. These are unlikely to be related to the Mitzic play and are an indication of the various diamond mineralisation scenarios that this part of the north Congo Craton can accommodate.

Other exploration potential

Columbo-tantalite mineralisation is known in at least two areas (Aconibe and Ayamiken) defined by niobium (Nb) and tantalum (Ta) soil anomalies, and heavy minerals associated with Nb-Ta-rich pegmatites. Neither area has been explored in great detail and thus they represent early-stage exploration prospects for pegmatite and/or skarn systems associated with Pan African granitic intrusions. The Aconibe occurrence comprises discrete, laterally extensive pegmatites, which are also overlain by eluvial and alluvial deposits yielding grades of 3.0 to 7.5 kg/m3. Sample assays have demonstrated niobium-rich columbo-tantalite.

Widespread lateritisation and indications of bauxitic laterite with grades up to 58.3% Al2O3, and 2.1% to 5.3% SiO2, indicate some potential for bauxite, particularly in eastern Rio Muni. Anomalous values of uranium, nickel, cobalt, lead, zinc, copper, arsenic, silver, manganese and molybdenum have been detected in laterite above black shales which occur in the red-bed sequence near Cogo. The red-bed sequence is part of the West Congolian Niari foreland basin, with known base metal deposits immediately to the south in Gabon, and is also equivalent in age to the Katangan sequences of the Democratic Republic of Congo and Zambia.

Serpentinised ultramafics and other basic intrusives along the footwall of the Benito Rift constitute an untested exploration prospect with some potential for base metals and platinoid elements. Similar basic intrusives have also been reported in southern Rio Muni, which are directly along strike of the Kinguélé ultrabasics trend that is currently undergoing evaluation by Southern Era.

Previous campaigns

Between 1980 and 1986, BRGM and a Spanish exploration company undertook regional stream sediment surveys utilising heavy mineral separates and sediment geochemistry. These highlighted the potential for gold production from alluvial deposits at Coro, as well as the occurrences of columbo-tantalite, diamond indicators, iron ore, radioactive minerals, rare earths and base metals in other areas. Other activities included a side-looking radar survey and a 1970s vintage aeromagnetic survey of Rio Muni.

In the late '90s the exploration licence for all of Rio Muni was held by United Meridian Corp (later renamed Ocean Energy) and BoMc Holdings Inc. These companies undertook a wide range of exploration activities including regional and prospect geological mapping, reconnaissance evaluation of the gold and bauxite potential of laterites, sampling and prospect evaluation of the artisanal workings, stream sediment sampling, interpretation of high-resolution radar imagery, and the generation of a comprehensive GIS database. At the end of 2000, the exploration contract lapsed and the entirety of Rio Muni became open for exploration. So far two foreign companies have taken out ground, both in southeastern Rio Muni, one for diamonds and one for columbo-tantalite and gold. The ministry has also enjoyed a high level of interest in exploration from both major and junior exploration companies.

Current licensing situation

The Ministry of Mines and Energy is keen to encourage an exploration-friendly environment, and to this end the mining law is in the process of ratification in parliament. Exploration contracts permit exclusivity to explore in areas up to 500 km2 to be explored over a two-to-three-year period, and the right to select smaller prospect areas within that. Smaller prospect areas (up to 200 ha) can be contracted to permit detailed evaluation of individual prospects which also grant exclusivity and the right to move to an exploitation contract. The exploitation contracts are valid for 20 years (with extensions by application) and are granted upon submission of a mining programme that must include environmental management strategies. All contracts require an outline of a work programme and yearly reporting to the ministry.

Archived data
In 2001 Exploration Consultants Ltd (ECL) was appointed as technical advisers on minerals to the ministry of mines and energy, having been technical advisers for hydrocarbons for six years. ECL is now actively involved in the promotion of exploration opportunities, contract negotiations, and compilation and sales of archive data.
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