The Bisie Project is located within the Walikale District roughly 140 km west-northwest of the regional centre of Goma (Figure 1), in one of the world’s principal Precambrian orogenic-metallogenic provinces in the Democratic Republic of Congo.
Alphamin Resources Corp (Alphamin), through its wholly owned subsidiary, Mining and Processing Congo sprl (MPC) has full legal title (100% ownership) over 5 exploration permits which cover 1,470km2 of highly prospective ground in the North Kivu Province.
Tin was first discovered at Bisie in 2004 and was mined by artisanal miners from two main target areas, Gecomines and Golgotha. Both targets are located along 1.5km of a ridge which extends over more than 9km on PR 5266. Artisanal mining from Bisie in the past has dominated the DRC’s production by supplying up to 70% of the cassiterite exported from the country.
No systematic exploration has been carried out in the area since independence in the early 1960’s.
Alphamin completed a 2400m drilling programme between June and December 2012 to test the true width and grades of tin mineralisation at both target areas. Best results include:
- 25.85m @ 2.26% Sn from 47.65m including 5.8m @ 8.55% Sn
- 12m @ 3.15% Sn from 53m & 3m @ 6.14% Sn from 80m
- 18.5m @2.21% Sn from 53m including 2.9m @ 6.27% Sn
- 11m @ 1.48% Sn from 71m including 2.5m @ 5.76% Sn
Copper and rare earth element (Cerium and Lanthanum) mineralisation is commonly associated with the tin rich zone. Significant results included:
- 9.4m @ 1.01% Cu from 67m including 3.5m @ 2.15% Cu
- 35m @ 0.77% Cu from 53m including 10m @ 1.67% Cu
- 10.1m @ 1042g/t Ce from 162m
Drilling further identified a zone rich in silver, zinc and lead mineralisation. Best results included:
- 19m @ 197g/t Ag
- 17.7m @ 14.11% Zn including 13m @ 18.08% Zn
- 14.75m @ 10.82% Pb
Tin and base and precious metal mineralisation is thought to be derived from residual fluids from granites exposed adjacent to the Bisie ridge
Figure 1: Location Map showing the Infrastructure and Alphamin tenements (Bisie located on PR 5266)
Tin was first discovered at Bisie in 2004 and was mined illegally until recent times. The deposit is unique with historic reported grades of 50-95% SnO2 (tin oxide – cassiterite) which attracted a large informal mining and support community. Official recorded production figures for Bisie for 2009 were 6,840 tons of tin concentrate although 10,000 tons of concentrate are more likely due to under declaration to avoid high taxes imposed by the Government of DRC. The Government imposed a moratorium on tin sales in 2010 which saw a mass movement of people off the property thereby creating a favorable environment for the Company to commence with detailed exploration programmes at Bisie.
The Bisie Project area is underlain in part by the Kibaran Belt lithologies, interpreted as an inter-cratonic collision zone with different periods of extension and compression. The lower Paleo-proterozoic unit comprises Rusizian and Ante-Rusizian units, composed mainly of poorly exposed dolomites, quartzites, amphibolites, mica schists and migmatite gneisses. The Meso-proterozoic rocks that overly the Rusizian Basement in the eastern DRC, Rwanda and Burundi are loosely termed the “Burundian”. These are composed of dominant micaceous schists and arenaceous phyllites with minor interbedded quartzites and amphibolites. Shales and conglomerates are also found in the upper parts of this sequence.
Granite intrusions started mid Proterozoic (1,375 Ma) and continued up until the last so-called “tin granite” intrusion at about 986 Ma (late Proterozoic). These intrusions are usually interpreted as being the source of the numerous tin occurrences in the region and the granites themselves show elevated levels of tin.
Figure 2: Regional Geology with Tenement Locations
The metamorphic units in the region generally have a northwesterly trend, although the ridge housing Bisie has a north south orientation up to the Oso river in the north, after which it trends to the northwest (Figure 3). While satellite imagery clearly shows large scale folding in the area, it has not been ascertained whether the Bisie ridge lies on a tight fold. There is strong evidence of a fault/shear zone which runs along the western slope of the ridge that probably gives rise to the elevation of the Bisie ridge. Small cliffs in the Golghota area and further to the south expose a mineralized and highly sheared and altered amphibolite.
Figure 3: Landsat Image showing limits of the Granite Pluton adjacent to the Bisie Ridge
Slickenslides are particularly common in the ore mined by artisanal miners and the movement has resulted in partial brecciation of cassiterite veins. Timing of emplacement and proximity to later structures determines the extent of this brecciation and disaggregation. The intensity of the lead, zinc and silver mineralisation also seems to have a structural control where lead, zinc and silver have replaced iron in massive pyrite units. Massive pyrite units generally show more galena and sphalerite replacement in areas of intense deformation related to shearing and faulting.
Structural interpretation from Landsat imagery shows numerous cross-cutting ENE-WSW faults with possible offset of the mineralized structure along the ridge (Figure 4). Several of these structures can be followed over several kilometers. The massive plutonic intrusion to the west of the ridge shown in Figure 2 is thought to be the source of the mineralizing fluids
Figure 4: Structural Interpretation over the Bisie Ridge
The mineralisation at Bisie is quite clearly multi-phased and appears to be very similar in character to the San Rafael tin deposit in Peru. Using San Rafael as a guide and structural and mineral evidence visible in ore samples and drill core, it appears that the cassiterite was emplaced first, followed by copper, in the form of chalcopyrite and bornite, which was followed by the lead and zinc mineralisation. Chlorite alteration is extensive in parts and thought to be a result of the last stage of fluids entering the system. Generally tin and copper mineralisation is found in zones dominated by chloritic alteration, although mineralized zones with no chlorite have also been intercepted.
The cassiterite was emplaced in massive veins ranging in thickness from 2mm to 0.4m (as seen in drill core), or in chutes of 20m x 8m mined in the past. The dominant host units are highly chlorite-altered amphibolite, chlorite schists and to a lesser extent the juxtaposing mica and quartz schists. The cassiterite is massive, pinkish brown in colour, fine grained and often botryoidal, and shows compositional layering likely due to variations in the iron content. Historically this form of cassiterite has been referred to as wood tin as shown in Figure 5.
Figure 5: Photograph of drill core showing botryoidal texture of pink cassiterite and copper sulphides
Copper mineralisation is usually associated with cassiterite zones, which are likely a result of continued tectonic movement which allowed copper rich fluids to move along the same mineralized structures. It is common to find chalcopyrite and bornite filling fractures within the cassiterite; however the majority of the copper mineralisation is normally in the form of blebs, lenses and veins, with the latter two being parallel to schistosity. Chalcopyrite is also common in quartz veins together with pyrite and to a lesser extent arsenopyrite. Generally copper mineralisation is wide spread but good grades are somewhat sporadic. Good copper intercepts are usually adjacent to and overlap high grade tin intercepts, if not directly correlating to them.
The zinc mineralisation is often present in small quantities together with the tin and copper mineralisation although the majority of zinc is hosted separately in a massive pyrite unit together with lead and silver. Structure appears to control the intensity of the galena and sphalerite replacement of the pyrite. In drill hole BGH001 the pyrite unit was crosscut by a shear/fault zone and replacement was almost complete over an interval of 14m, with at least 6m of those being massive sulphides. The massive zinc and lead mineralisation is shown in Figure 6.
Figure 6: Photograph of Drill Core showing Massive Sphalerite (Zinc) and Galena (Lead)
The two main areas of artisanal workings and drilling are Golghota in the South and Gecomines in the north as shown in Figure 4. Tin mineralisation is better developed at Gecomines with thicker and more numerous cassiterite veins and substantial copper mineralisation in parts. Golghota has less cassiterite and veins are normally thinner, however the lead and zinc mineralisation is more prevalent, likely due to the presence of the massive pyrite unit.
Soil samples were collected on lines spaced 200m apart with samples collected every 50m along the Bisie ridge. All samples were dried and sieved to -2mm and then -180µm before being analysed on site using the Niton hand held XRF analyser.
The results indicate an important zinc in soil anomaly that extends north and south of the artisanal workings shown in Figure 7. Zinc is one of the path finder elements consistently associated with the tin mineralisation, and is a good indicator of the true mineralised potential along the ridge.
An anomalous copper in soil anomaly which extends to the north of Gecomines was drill tested 150m north of current drilling. The hole intercepted chalcopyrite and chalcocite mineralisation in quartz veins and a 5mm cassiterite vein confirming presence of tin and copper mineralisation in areas between the high grade zones of mineralisation.
Figure 7: Zinc in Soil Anomaly (>50g/t) over the Bisie Ridge
The initial drilling programme was designed to test 700m of delineated tin mineralisation down to 100m vertically below surface and was stepped out at 100m intervals thereafter to define the true strike extent of the zone. During the initial programme eighteen holes (and two re-drill, as the original holes had to be abandoned) were completed. Results received to date are summarized in Table 1.
Figure 8: Drill Hole Locality Map with Artisanal Workings
Half core samples for all drillholes were submitted to accredited ALS Chemex laboratory in Johannesburg where samples were analyzed using ME-XRF05 conducted on a pressed pellet with 10% precision and an upper limit of 10 000ppm. Over limit samples were sent to Vancouver for ME-XRF10 which uses a Lithium Borate 50:50 flux with an upper detection limit of 60% and precision of 5%. ME-ICP61, HF, HNO3, HCL04 and HCL leach with ICP-AES finish was used for 33 elements including base metals. ME-OG62 a four acid digestion was used on ore grade samples for Pb, Zn, Cu & Ag. Industry accepted QA/QC checks were applied including use of duplicates and standards.
Although the thickness and grade of mineralisation varies, the style of mineralisation for each of the Sn/Cu and the Ag/Zn/Pb zones remains consistent. The Sn/Cu zone is well developed at both the Gecomines and Golgotha target areas while the Ag/Zn/Pb zone is better developed to the south at Golgotha.
Figure 9: Section showing drill holes BGC001 and BGC005 at Gecomines Target Area
Figure 10: Section showing drill holes BGH001 and BGC006 at Golgotha Target Area
OTHER EXPLORATION TARGETS
Figure 11: Main licence boundaries on the Landsat7 composite image showing priority targets
In addition to the Bisie deposit, other targets are present in the licence areas. The most interesting of these comprise a second ridge which follows the western limit of the same granite intrusion which is a possible source of mineralisation at Bisie. Extensive alluvial workings were observed in the area where artisanal miners are reported to be recovering gold from the stream sediments. Geochemical sampling has been planned for 2013 and will target base metals and gold with stream sediment sampling used to assess the tin potential.
The Umate gold alluvial workings were mined up to the early 1960’s by the Belgians and more recently by small scale miners. The gold deposits are located on PR 5267 currently under force majeure. The project has excellent potential for primary gold resources which will be explored in early 2013. A number of primary workings along a ridge were observed from the air recently confirming the hard rock potential.
All indications are that the Bisie system is extensive and has the potential to host very high grade tin, base metal and REE mineralisation. The structural influences and immobility of tin in soils complicates the exploration somewhat, however no more than with most deposits. The copper and lead path finder elements provide a good indicator of the polymetallic subsurface mineralisation and therefore allow geochemical targeting for the tin mineralisation by association. High grade chutes of cassiterite can be limited in strike extent but also house substantial amounts of tin as demonstrated by past artisanal mining. While almost impossible to target with drilling, they will provide a significant upside when mining. Copper is nearly always associated with the cassiterite and will be recovered in future exploitation.
Regional exploration has plenty of potential in numerous areas. Volcano-sedimentary rocks hosted in a favorable structural setting to the west, where artisanal miners are exploiting alluvial gold, will be targeted in 2013. There are numerous other known tin occurrences on the tenements held by MPC as well as the potential for new finds among the granites yet to be explored. The Umate gold prospect has proven to be a significant source of gold in both modern and Belgian times and will be a future drilling target in the already identified hard rock mineralisation. Airborne geophysics will likely provide additional structural targets and a greater understanding of the local geology.
Table 1: Summary of Drill Holes and Significant Intersections Received, Bisie Prospect, DRC
|Hole ID||GPS Easting||GPS Northing||EOH||Azi-muth||Inclina-tion||Element||From (m)||To (m)||Width (m)||Grade (%)|