Under the surface - mining in Liberia

Materials World magazine
,
1 Jan 2012
Hummingbird geologist

The mining industry in Liberia is undergoing a renaissance. The civil war ended in 2003 and in November 2010, President Ellen Johnson was re-elected and has sworn to continue her inclusive policies and to promote economic reforms for the benefit of the population.

Mining is not new to Liberia. In the 1960s and 1970s, the country was Africa’s largest iron ore producer, moving high grade magnetite resources in the Bomi Hills using a heavy-duty rail line built to the port of Monrovia. In September 2010, mining returned in the form of Arcellor Mittal’s Greenfield iron ore project, its first shipment of iron ore.

The Liberian Government is encouraging investment in mining and is in the process of revising its mining code. At present, licences are subject to mineral concessions covering reconnaissance and exploration, with the latter initially valid for three years over a maximum surface area of 1,000km2 and with an optional two-year renewal period. The licence also allows pilot mining during exploration. Minerals development agreements (MDA) are equivalent to mining licences and are valid for 25 years, renewable until reserves are exhausted. Other features of the agreements include free patriation of profits and dividends, duty free privileges on mining equipment, tax not exceeding 30% and a mining land rental of US$3 per acre.

A range of possibilities

Liberia is prospective for all those commodities that are found in greenstone belts. These belts are typically found in Archean- and Proterozoic-aged domains, and represent an important stage in crustal development. The green refers to the basic and ultrabasic rocks that usually occur in linear belts in association with granitic rocks. Greenstone belts have been the host to 30-40% of global gold production, and also to significant nickel and iron ore resources. Other significant features within the belts include high magnesium lavas (komatiites), shear zones and hydrothermal alteration. The main distinction between Archean and Proterozoic greenstones in West Africa is component lithology, with banded ironstone formations and relatively lower basic ultrabasic volumes in the Archean while the Birimian-aged greenstones have a higher volcanic component of tholeiitic composition and a lack of banded iron formations. Within the region, both Archean and Proterozoic greenstones are highly prospective for gold as illustrated by the Archean gold deposits of Sierra Leone and the Proterozoic Birimian gold mines of Mali, Guinea, and Ghana.

The next new mine, due in the second half of 2013, will probably be the New Liberty mine of Aureus Mining Inc. The Toronto-listed company is a spin-off from African Aura Inc (now Afferro Mining Inc) that had iron ore and gold interests in Liberia, Ivory Coast and Cameroon. The new company is working towards completion of a definitive feasibility. New Liberty is typical Archean greenstone deposit with the gold hosted in ultramafic and mafic rocks within a metamorphosed granitic host, and controlled by a shear zone that defines the mineralised zone. The gold grade is relatively high for this environment at 4g/t and is free milling, although there is associated sulphide mineralisation, mainly pyrrhotite and arsenopyrite. The main mineralised zone is 1.8km in length and contains 1.5 million ounces of gold equally split between indicated and inferred classes within the steeply dipping ultramafic host. Extensions to both to the east and west were confirmed by drilling results that will be incorporated in the definitive feasibility. The planned mining rate is 100,000oz per year for the first five years, with a 93% recovery rate.

Licence to explore

Within Aureus licence areas are a number of lookalike deposits to New Liberty, and the company is drilling a number of them. They are up to 50km away, and display similar characteristics of: strong magnetic signature related to magnetite bearing ultramafic, low resistivity due to alteration, and high chargeability due to a high percentage of sulphides. Perhaps more importantly the structural control in the shear zone is the most important criteria for this style of mineralisation. Within the exploration licences there are a number of shears only constrained to the south by a mobile belt. According to David Reading, CEO of Aureus, this style of mineralisation is extensive and is representative of a regional gold province. ‘It has strong similarities in terms of host rock, structure, and geophysical signature to the Archean gold deposits of Western Australia.’

Hummingbird Resources, an AIM-listed company, has licences in the eastern part of the country that are defined by the western extension of Birimian terrain of West Africa that underlies the Ivory Coast, Ghana and Mali. The company has been undertaking exploration since 2005 and has extensive licences over a series of shear zones. They have, according to technical director David Pelham, first mover advantage as a private company until December 2010 before listing. At that time, the company had a Maiden Resource of 812,000oz of gold primarily in its F zone within the Dugbe shear zone that now stands at 1.76Moz, 78% in the indicated category. The company has utilised stream sediment sampling followed by soil surveys, trenching, scout drilling and finally diamond core drilling. All the cores are orientated to ensure location. Pelham explains, ‘Soil sampling works well in this terrain as the land has not been cleared and there is little laterite development. We also employ locals equipped with digital cameras and GPS devices to locate outcrop in the forested terrain that enables our geologists to map with greater efficiency.’

The shear zones are the main target and Hummingbird has more than 150km under licence, with the resources identified on the Dugbe zone that extends over 150km parallel to the regional trend. Three other shear zones are mapped.

At the Dugbe F zone, the gold mineralisation is found within shear zones trending NNE adjacent to granodiorites that have intruded pre-existing folding and structures in quartz biotite schists resulting in a sinuous outcrop. The intrusions are not foliated (similar to the intrusions in Ghana) and are thought to be the heat source and origin of the gold that migrated into the shear structures. The gold is fine-grained, averaging around 30 microns, free milling and averages 1.3 g/t at a cut-off grade of 0.5 g/t. Hummingbird says Pelham has a considerable number of targets within eastern Liberia and expects to bring these into resource categories as exploration continues.

Click here to see a graphic showing magnetic signatures of ultramafics in New Liberty region in a popup window