Sampling Method
The aim of the first year’s work in Liberia was to start with regional scale stream sediment sampling of the concession blocks. Although the rainy season in Liberia starts in May, it generally gets heavier towards September, and for this reason the sampling programme aimed to take as many samples as possible before September 2006. The sampling programme has concentrated on obtaining heavy sediment samples from the drainage courses, with particular emphasis on second and third order drainages (this is because the first order streams are too full of water, even in the dry season, to be safe or practicable to sample). Large samples (40 litre, or >50 kg) were taken on each site, separated into two (effectively a diamond and a gold sample), and each sample sieved and jigged down to form a heavy mineral concentrate of 1-2 kg. GPS coordinates of sample points, and the presence of heavy mineral grains and evidence of artisanal diggings were observed and noted for each site.
Analysis
The concentrate samples were taken to Monrovia and treated in the following ways:
Diamond samples were treated in the Western Minerals laboratory in Monrovia. There, each sample underwent magnetic separation, acidizing and screening, water column separation (where heavy mineral grains were separated by allowing light minerals to ‘float off’ in a water column of adjustable pressure), and hand picking of kimberlite indicator minerals under a binocular microscope. Each sample was screened into 5 size fractions: +1 mm, 0.71 - 1.0 mm, 0.5 – 0.71 mm, 0.3 – 0.5 mm, and – 0.3 mm. A total of 11 heavy mineral categories were identified, viz. garnets (kimberlitic and possibly kimberlitic categories), ilmenites (kimberlitic and possibly kimberlitic), spinels (high, medium and low interest categories), chrome diopsides / clinopyroxenes, diamonds and gold.
Gold samples were first sieved and split down to a sample of approx 50g each at the Alex Stewart prep lab in Monrovia. These samples were then airfreighted to the OMAC laboratory in Ireland for the following analyses:
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Homogenisation of sample followed by standard Au geochem analysis, including pre-ignition at 6500 C.
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n Aqua regia digestion followed by ICP – AR / ES 46 element scan, which included rare earths (eg. Ce, La, Th, Y, V), base metals and other indicators (including Cu, Pb, Zn, As, Ba, Fe, Cr, Mn, Sr), and other heavy minerals (eg. Ti, Zr).
So far, over 1000 stream sediment samples have been taken, with over 600 from the Nimba area, over 250 from the Dugbe area and nearly 150 from the Cestos area. In many ways it has been a race to complete as much of the sampling programme as possible, before the worst of the rains made sampling prohibitively difficult. Many areas were so remote and inaccessible that they involved many hours of walking to collect each sample. Original plans were to sample on a density of approximately 1 sample per sq Km. However, due to difficult access and unsuitability of many sample sites (ie. not enough alluvial material available), the first stage of sampling has achieved an overall density of approximately 0.65 samples per sq Km.
Although the first results are still being received for all areas, there are some very encouraging results with regard to indicator minerals, with substantial showings of garnets (some of which are orange and appear to be kimberlitic), ilmenites and spinels. Best results to date have been seen from the northern part of the Nimba concession area, where large white diamonds (up to 40 ct) have been found by artisanal miners in the past.
The following comments were made by Mr. Jenkins Dorbor, manager of the Western Minerals Laboratory and a Liberian geologist with decades of local experience. The comments were based on the early batches of samples, which were from the northern parts of the Nimba concession:
“A number of samples yielded abundant ilmenites, high-, mediumand low-interest spinel, and three possible kimberlitic orange garnets. Most of the ilmenites were shiny, abraded and anhedral, had sculptured surfaces, but none had a perovskite mantle. It can be deduced from the results obtained thus far, that areas hosting kimberlite source rock have been sampled.“
However, while this is obviously very encouraging, what is somewhat unexpected and equally encouraging is that some of the areas of anomalous kimberlite indicator minerals are also showing elevated levels of Rare Earth Elements (REE), specifically Ce [up to 2000ppm], La [up to 1100 ppm], Y [up to 100 ppm] and Th [up to 700 ppm]. At this stage, with many more sample results to be received, processed and interpreted, it is only possible to comment that, while kimberlites are certainly relatively enriched in REE, so are the related carbonatites, lamproites, lamprophyres and others of the alkali group. Apart from these, REE enrichments may occur in pegmatites, acid vein systems, and metasomatic skarns (any of which might be associated with other types of economic mineral deposits). For these reasons, it is too early to make definitive interpretations regarding the REE results.
Besides the diamond indicator and REE results described above, a number of samples showed anomalous gold values, with the highest being up to 25 g/t Au in the north of the Nimba area. These represent extremely encouraging results which will be followed in the next stage of work.
Mapping and Imagery
In addition to the heavy mineral sampling work, satellite imagery studies have been undertaken to help unravel the geological structure of the areas concerned. This work is very important, since geological structure is a major controlling feature in kimberlite-associated diamonds and shear-hosted and greenstone-hosted gold deposits. Digital Terrain Model (DTM) images have been acquired and processed using the Hill Shade function of the software programme ER Mapper. This has enabled the acquisition of cloud-free, black and white images, of exceptional clarity and usefulness for regional scale structural work. These images are especially useful for delineating regional and semi-regional scale geological structures.
In addition to the satellite work, described above, literature studies and map searches have been undertaken for the areas in question. While it is true that Liberia is significantly under-explored in comparison to the majority of west African countries, it is also true that some systematic geological work has been done in the past. For example, the country-scale geological mapping and airborne geophysical surveys in the 1970s (eg. by the USGS), which were of good quality, but were very regional in scale (mapping at 1:250,000 scale). All 10 of the USGS geological maps (1:250,000 scale) have been acquired and scanned, to be used as base maps for the interpretation of sample results etc.
The results of the heavy mineral sample work are being received, processed and interpreted in the UK. Sample results are being plotted onto GIS-based maps, in order to display and further interpret the results. The sample results are being interpreted, in the context of the known geology of the area and the structural work described above. Based on this interpretation, the follow up field programme for the next field season will be designed.
Summary
In conclusion, it should be stressed that these are remote areas of difficult access, with variable to dense forest cover. Weathering is deep, with little or no outcrop at surface. To our knowledge, no routine exploration of any sort has ever been conducted over these areas (apart from some early iron ore exploration, probably in the 1950s / 1960s, in the northern parts of the Nimba block). Yet these areas are known to be highly prospective, both for kimberliteassociated diamonds and for shear zone / greenstone type gold deposits. In view of this, the first season’s work of over 1000 samples, and with more samples to come, amounts to a very successful first phase of work. This is even more the case in view of the encouraging results obtained so far, especially in the northern parts of the Nimba concession.