Summary:
The Bisie tin deposit is a cassiterite-bearing stock-work or vein system adjacent and possibly distal to underlying source granite. The mineralization at Bisie is unusual and different from other classic tin deposits. The deposit has up to 0.5 % rare earth elements (REE) and very high grade tin (with some sample assays reaching greater than 60 % Sn).
Mpama North
At Mpama North deposit, delineated one main continuous discrete tin (with minor copper) mineralized zone, named the Main Vein, along with two subordinate smaller mineralized zones named the Footwall Vein (restricted to the southern areas) and the Hangingwall Vein (in the northern areas).
- The Main Vein mineralization consists of a number of uncorrelated cassiterite veins within pervasively chloritised schist. This zone generally occurs over thicknesses of between 2 m and 22 m with an average thickness of approximately 9 m. This zone is generally the highest grade and most consistent.
- Hangingwall Vein (HW Vein) mineralization occurring within partly chloritised schist and micaceous schist between 1 m and 20 m above the Main Vein. This zone of mineralization is generally between 0.5 m and 4 m wide and occurs in the northern area of the deposit and tapers out northwards. The middling between the Hanging Wall Vein and the Main Vein decreases in areas. It is possible that this vein merges into the Main Vein in some parts of the deposit.
- Footwall Vein (FW Vein) mineralization, occurring within the micaceous schist and amphibolite schist between 1 m and 10 m below the Main Vein. This zone is restricted to the southern areas, is very narrow (<50 cm) and high grade in its most northern occurrences. Towards the south it thickens to approximately 6 m and is lower grade. It is possible that this vein merges into the Main Vein in some parts of the deposit.
The strike length of mineralization at Mpama North is 600 metres. Depth of mineralization at Mpama North is from surface at an RL of ~800 metres, as observed in weathered outcrop, to an RL elevation of ~200 metres in drilling, or approximately over 600 metres vertical extent.
Mineralization
Tin mineralization at Bisie is hosted within a north-south striking, eastward dipping shear zone hosting multiple cassiterite veins, stringers and disseminated cassiterite in a structurally controlled zone. Structural and mineralogical evidence indicates cassiterite was emplaced first, followed by copper mineralization in the form of chalcopyrite and bornite, then by lead and zinc mineralization occurring as sphalerite and galena, likely a function of retrograde metasomatism. There is also evidence of late-stage quartzchalcopyrite veining which cross-cuts the mineralization with veins trending northnorth-west.
Chlorite alteration is widespread and appears to be the result of later stage fluids entering the system. The host rocks are predominantly highly chloritised fine to medium grained, mica and mica-garnet schists. The tin and copper mineralization are almost exclusively found in zones dominated by intense chlorite alteration, however, cassiterite mineralization with no chlorite alteration has rarely been intersected in the hangingwall and footwall vein zones hosted in Mica schist (MSCH).
Tin mineralization can on occasions be more highly concentrated in high-grade chutes, such as at Mpama North where two are referred to by Alphamin as the upper high-grade chute and lower high-grade chute. Mineralization between these two chutes is lower grade and these areas contain narrower, more widely spaced cassiterite veins. Both chutes run parallel to each other and plunge to the north at approximately 35°.
The highest tin grades appear to show a “chocolate-tablet” or “eggbox” geometry.
The highest-grade tin mineralization has so far been found at Mpama North where the widest and highest grade cassiterite veins were intersected in the deepest most northerly portion of the deposit. At Mpama South, mineralization is also characterised by multiple sets of cassiterite veins in the same host lithology and orientation as Mpama North, however, at relatively lower grades.
Dimensions
The area defined as a Mineral Resource extends approximately 750 m in the down plunge direction. It extends for a width of approximately 300 m in the plane of mineralisation perpendicular to the plunge. The main zone of the Mineral Resource, which accounts for 97% of the Mineral Resource, is on average approximately 9 m thick, although is narrower (less than 1 m) at the margins and up to 20 m thick in the central areas.
The minor zones that occur several metres above and below the main zone are considerably narrower than the main zone and cover areas of between 100 m and 200 m in the dip and strike directions.
Mpama South
The Mpama South deposit is immediately to the south of the Mpama North deposit and the Mineral Resources currently extends from surface to 310 m below surface. The Mineral Resource is interpreted to occur as irregular tabular mineralized zones, dipping 65-70° to the east, containing several narrow veins and disseminations of cassiterite. The deposit is made up of seven lodes with similar strike direction and varying in width and strike length. The vast majority of the mineralized material is contained in lodes 1, 2 and 3.
At Mpama South deposit, delineated seven continuous discrete tin (with minor copper) mineralized zones in a stacked/parallel configuration. MZ1 and MZ2 represent the two main zones of the Mineral Resource and are continuous for 900 m and 650 m along strike respectively, and both have a down dip extent of 350 m and vary from as little as 1 metre up to 14 metres thick. Four zones are below the main zones and towards the south (MZ3 – MZ6) and one is in the hangingwall. They range in thickness from 1 – 9 metres with average thicknesses of typically 1.0 – 1.5 metres for MZ3 – MZ5, 4.0 metres for MZ6 and 0.5 metres for MZ7.
The two main zones of mineralization (MZ1 and MZ2) have similar average tin grades at a 0.5% Sn cut-off of (2.22% and 2.11% respectively). The smaller, footwall zones (MZ3 to MZ5) are higher in tin grade with averages ranging from 3% to 4.41% while MZ6 and MZ7 are lower in tin grade, with an average of 0.63% and 1.07% respectively.