Liberia Diamonds

Historical exploration

Western Liberia is considered as under-explored for primary sources of diamond. The only significant exploration occurred in the 1960’s and 1970’s by De Beers and the UNDP respectively. These surveys led to the discovery of numerous NNE trending ilmenite bearing kimberlite dykes and one small pipe at Mano Godua. Some of these kimberlites were bulk sampled and determined to be either very low grade or barren of diamond. There has been no modern systematic diamond exploration programme in Liberia for the past 30 years and the abundance of gem quality diamonds in certain areas points to undiscovered diamondiferous kimberlites. It is possible that these diamond-bearing kimberlites have chrome spinel as a main indicator mineral and not ilmenite. Chromite was not recognised as a kimberlitic indicator until the late 1980’s and therefore would not have been recovered in the earlier exploration efforts. Considering that garnet is easily destroyed in the tropical weathering environment it is entirely possible that chromite would be the only indicator mineral to survive in any abundance.

It is the aim of Mano, through the implementation of thorough diamond exploration programmes, to identify diamondiferous kimberlite clusters that can lead to the first commercial scale production of a kimberlite diamond mine in the country.

Target Selection

All the main river systems in western Liberia carry diamonds, and all of Mano’s permits host extensive areas of artisanal diamond workings. A study of the major source areas for diamonds in western Liberia, in particular the source areas of >50 carat gem quality stones, has been combined with an initial assessment of tectonic events in this part of West Africa. Four areas, which have high potential for primary diamond deposits, have been defined. The areas targeted are: -

1. Mano Godua; in the centre of the Bea Mountains licence
2. Mabong and Yambessi Valleys in the Bea Mountains licence.

Mano’s original Bea Mountains and Kpo Range licences formed an ENE trending line. The line traces a major zone of crustal scale shearing, on the southeastern edge of an area of thickened Archaean crust, which roughly follows the Sierra Leone / Liberia border. The same group of crustal scale structures extends north-eastwards into the Nimba and N’Zerekore region on the Guinea-Liberia border; part of which area has been targeted by Mano for primary deposits through a reconnaissance permit in Guinea.

In November 1999, a study commissioned from the Witwatersrand University in Johannesburg entitled "Appraisal of the Diamond and Gold Potential of Mano River Resources’ Concessions in NW Liberia" was received. It covered the two concessions held by Mano plus surrounding areas, for a total of about 12,000 square kilometres. The study was based largely on the integration and interpretation of regional geological information and mineral occurrence maps with a variety of remotely sensed data. Detailed structural interpretation of specially processed Landsat imagery and the interpretation of airborne magnetic data using an edge enhancement technique to map faults and fractures, recently developed in the School of Earth Science at Wits, were particularly important in the investigation.

All interpretations were integrated into an overall composite map and from this product five target areas for gold and four target areas for kimberlite diamonds were selected and prioritised. The study concluded that the areas controlled by Mano are very favourable for the location of gold mineralisation and kimberlitic intrusions, and recommended high precision aeromagnetic / radiometric surveys, together with soil and indicator mineral sampling, in the target areas.

Kpo Range MDA (200 km²)

The main alluvial diamond workings are found all along the northwestern edge of the Kpo schist belt with primary deposits in the area east of Weasua. The major drainages, including the Lofa River, appear to be structurally controlled in this area. In 1999 all of the alluvial workings were visited and mapped. It became apparent that the main focus of the local artisans was not on the Lofa River, but along smaller tributaries to it. Gravels along the Butulu, Wuegbeni and Wuedakena Creeks have all been exploited and continue to be so. The gravels were mapped and in many cases were classified as angular, suggesting that they have not travelled any significant distance and hence the diamonds potentially have a local source. A further field visit in February 2000 was made to collect a selection of heavy mineral samples as a pre-cursor to the onset of a detailed diamond exploration programme orientated towards the location of primary kimberlitic occurrences. Four samples were collected downstream of gravel diggings and all returned positive results. The most interesting samples were taken along to Butulu Creek and a small tributary to the Butulu called the Wuejua Creek. Here abundant kimberlitic chromites and garnets were recovered from samples, and one chrome diopside was also picked. Mineral chemistry analysis of these indicator minerals determined that a very large proportion of these are classified as G10 garnets, being derived from harzburgitic mantle (after Gurney, 1984). Furthermore, a large proportion of the have >12wt% MgO and >60wt% Cr₂O₃ and are classified as diamond inclusion chromites (after Fipke et al. 1995). The angular nature of the gravels coupled with the recovery of abundant diamond inclusion indicator minerals and a chrome diopside indicates that potentially diamondiferous kimberlites are present in the area to the east of Weasua in a localised area.

During May and June 2000 detailed sediment sampling was conducted around Weasua, through the collection of 72 samples. Many of these samples yielded abundant kimberlitic chromite supported by garnet, chrome diopside and ilmenite. A large number of these indicators have been probed and the mineral chemistry of the chromites and garnets is considered excellent. Many chromites are classified as diamond inclusion type chromites, whereas there are a very large percentage of G10 garnets from those garnets probed. These data indicate that some of the sources of the indicator minerals are likely to be diamond bearing. One macrodiamond in the +0.71mm size fraction was picked from one sample that also contains diamond inclusion chromites.

Follow up of a number of these anomalies (including 660 loam samples) has led to the discovery of 6 kimberlites in the Weasua area, of which 4 are confirmed pipes, 1 is a suspected pipe and 1 is of insignificant size. Small samples have been collected from K001 (1.6ha pipe), K002 (2 to 4ha pipe) and K003 (4ha pipe), which yielded both micro and macrodiamonds. The diamondiferous nature of these thee pipes is also confirmed by the fact that local artisans are mining diamondiferous gravels associated with, and downstream of, these pipes.

Kpo MDA Stream Sediment Sampling

Initial work on K004 and K006 has yielded encouraging results (including a diamond recovery from K4) and again both these kimberlites show significant artisanal diamond mining. A sample of K006 was processed for microdiamonds at Lakefield Research in Canada, but with negative results. This somewhat contrasts the garnet chemistry where all of the garnets probed (25 grains) were determined to be G10 in composition. Clearly a larger and more representative sample of K006 is required for diamond analysis. K005 has been determined to be of no interest and will not be investigated further.

It is clear from the regional anomalies though that many more kimberlites remain to be discovered in this new diamondiferous cluster and a work programme has been designed to bring these anomalies to account. The mineral chemistry of these anomalies is excellent and it is more than likely that some of these kimberlites will be diamondiferous.

Bea Mountains MDA (1,000 km²)

There has been a great deal of diamond artisanal activity concentrated on the triangle within the centre of the Bea Mountains permit, between Lofa Bridge, Weaju and Mano Godua. At this point major fault zones intersect the schist-belt lithologies. Much of the mining occurs in areas sourced from the mountains, suggesting primary source rocks are present within the schist-belt lithologies.

Reconnaissance and detailed stream sampling (193 samples) has been conducted throughout most of the licence and many high interest indicator mineral anomalies have been identified that warrant follow up to locate the kimberlite sources. Particular anomalies in the Mabong Valley are associated with extensive alluvial mining and based on their mineral chemistry it is clear that some of the source kimberlites will be diamondiferous.