Lake Victoria advances gold feasibility - mine development
David Kalenuik, CEO of Lake Victoria Mining Company, tells Michael
Forrest about the company’s first steps in developing a gold mine in
Tanzania might be the third largest gold producer in Africa (after South Africa, Ghana and Mali), but it has not always been that way. Mechanised large-scale gold production did not begin here until the 1990s, following extensive exploration in the country.
Today, the largest producers in Tanzania are AngloGold Ashanti Geita mine followed by Africa Barrick Gold’s Bulyanhulu mine. Together, their combined annual production totals around 750,000 ounces (23.3t). According to the US Geological Survey (USGS), overall gold resources in Tanzania amount to 1,400t.
David Kalenuik, CEO of US-listed Lake Victoria Mining Company (trading symbol LVCA), has been exploring and developing in Tanzania over the past six years and now faces the challenge of finding and developing a mine that will produce a return for investors. The company has a good address in the Archean greenstone belt of the Tanzania craton, which outcrops in the north of the country to form the southern shores of Lake Victoria.
‘The location is right,’ says Kalenuik, whose company has a wide-ranging portfolio of licences over prospective ground, the most advanced properties being in the northeast of the country. These are based on the Kinyambwiga-Murangi-Suguti (the Musoma Bunda project) properties in the south of Mara-Musoma greenstone belt where, in the past, exploration has been limited. The challenge in moving forward, says Kalenuik, is to ensure that the project has the potential to host economic mineralisation, and that each stage of exploration and development leads towards this objective.
Early work in the Kinyambwiga area dates back to 1961 when the UK overseas geological survey undertook mapping over the area. During the 1970s, airborne geophysical work outlined the regional geology, which consists of Archean (>2,500 million years) mafic volcanics, sedimentary rocks and banded ironstone formations metamorphosed and intruded by contemporaneous and later granites. These form greenstone belts of the Nyanzian Supergroup, which are estimated to be 5,000m thick and contain the majority of known gold resources. Structurally, the belts, which are marked by steeply dipping folds with an easterly to north-easterly trend, are cut by southwest trending faults and shear zones of Nyanzian age.
Exploration follows a defining programme covering a licence area, which brings focus to the most promising region. This usually follows a mapping and geochemical sampling programme to establish background values as well as any rock and soil anomalies indicating mineralisation. At Musoma Bunda, however, the licence areas are covered with variable depths of Mbuga clay soils – lacustrine sediments related to the ebb and flow of Lake Victoria over geological time that result in clays and other sediments being deposited. Very little outcrop is present in the project area, and can only be found in incised streams or where artisanal mining has exposed bedrock. ‘This has impacted on the exploration over the project area and has required a review of a normally geochemically-led programme,’ says Kalenuik.
A solution to the masking overburden has been to use a combination of geophysical techniques. These include ground magnetic, induced polarisation (IP) and Schlumberger vertical electrical profiling (VES) surveys to identify suitable sub-surface structures and, in particular, mineralised shear zones. To the east of the artisanal workings at the Kanunga 1 prospect, investigation of high chargeability/resistivity anomalies via pitting and soil sampling beneath the Mbuga cover returned soil values ranging from 80–1,260ppb, along a north–northeast shear zone for a strike length of some 500m.
Subsequent trenching to the east of the Kanunga school along a north–northeast shear zone for a strike length of 500m, revealed gold values up to 2.12g/t (ppm). On one north–south drill fence located 1.7km from the Kanunga prospect, eight auger drill holes returned values averaging 77ppb over 40m. They also hit a stone layer at the contact point of the Mbuga and underlying granite, with one hole returning a value of 2.56g/t.
A reverse circulation drill programme was undertaken once trenching had been completed across the known area of artisanal workings. Based on 40m spacing, it followed the strike extensions of known quartz mineralisation for 380m. A second reverse circulation drill programme followed the strike extension of the gold-bearing quartz veins for 700m. Overall, some 4,070m of drilling comprising 56 holes to target the quartz veins confirmed the presence of at least four mineralised structures.
At Kinyambwiga, this is associated with steeply dipping narrow veins, often traceable over 200m and varying in width from 1–12m. VES profiling has extended the outline of these veins over 700m along the strike under the Mbuga. With a structural trend to the northeast transecting the underlying granitic rocks, these veins can be correlated with significant strikes and dips. Although exploration drilling is at an early stage, late last year a conceptual resource made by independent geological consultants was released. Based on block modeling (6m x 1.5m x 3m blocks) of the drilling and on three of the veins using a cut-off grade of 0.5g/t, the resource was estimated at 578,000t at an average grade of 1.67g/t for 31,000 ounces of gold. As of late last year, the company has spent around US$800,000 in sampling analysis and reverse circulation drilling.
But feasibility is not just about defining a mineable resource. Converting these resources to proven reserves entails testing mineralisation to find the optimum method for recovering the valuable contained metal. ‘We have already submitted samples to a certified laboratory in Harare, Zimbabwe, in order to understand how the gold is held and how it could be recovered,’ says Kalenuik. From experience elsewhere in greenstone-hosted gold terranes, the gold is held as free gold, as well as being associated with sulphide minerals as coatings and fine-grained inclusions. The initial sample was crushed to 100% passing 1mm (1,000μ) using an impact crusher, followed by gravity and cyanidation tests. The initial head grade of the material supplied assayed at 7.24g/t.
Using a Knelson centrifugal concentrator on a 20kg sample, the majority of the gold was found to report to the Knelson concentrator tailings, with only 24.2% recovered in the concentrate. Of that, 22.5% was free gold while the remaining 1.6% was associated with heavy minerals. When the grind size was reduced to 80% passing 75 microns, the Knelson concentrator recovered 41% of the test feed, with free gold making up 30.6% and 9.4% associated with heavy minerals. Clearly, other recovery techniques will be required and to this end, the gravity tailings were subjected to cyanide leaching over 24 hours, which resulted in recovery of 84% of the gold (82% over eight hours), with sodium cyanide consumption of 0.84kg/t and lime 1.5kg/t. The combination of gravity and cyanide leaching gave an overall gold recovery of 92.7%.
‘These results support LVMC investment in the Musoma-Bunda project and the Musoma Mara greenstone belt,’ says Kalenuik, although he admits that ‘there is still a long way to go in terms of resource definition, metallurgy and all the other components of a feasibility study, including social and environmental impact. We are working hard to reduce risk for our investors, and we are encouraged by the results from the geochemical and metallurgical reviews.’