The Twangiza property can be divided into three distinct litho-structural terrains which can be seen in the figure below. The eastern terrain is characterised by N-S trending Neoproterozoic sediments, which are part of the Itombwe synclinorium, a regional-scale fold which extends southwards from the Twangiza area for about 150 km.

The sediments in the Neoproterozoic terrain are generally very weakly metamorphosed. The dominant lithology is mudstone, often with a significant amount of carbonaceous material. Subordinate units of siltstone are commonly interbedded with the mudstone, being slightly coarser, more siliceous and harder. Quartz wacke and sandstone occur locally, but are usually confined to relatively thin beds or lenses which lack continuity.

In the vicinity of the Twangiza Main and North deposits, the Neoproterozoic sediments have been intruded by porphyritic sills, ranging in thickness from less than 1 m to over 50 m. The sills have undergone extensive hydrothermal alteration and the original composition is difficult to determine. However, it is possible that the sills are part of a suite of alkaline intrusive rocks that were emplaced along the line of the present-day Western Rift, at about 750 Ma. Small granitic intrusions have been found in the Neoproterozoic rocks, and have been locally exploited for tin by colonial prospectors and artisanal miners. It is believed these granites are younger than the G4 tin granites which were emplaced at 975 Ma, and may also be related to the same 750 Ma intrusive event as the porphyry sills.

The Twangiza Main ore body consists of a wide (up to 200 m) zone of pervasively altered mudstone, siltstone and porphyry sills, with abundant sulphidic veins. The veins form a complex irregular network, although veining parallel to bedding is relatively common. Hydrothermal fluids have exploited both the fracture system which developed during folding due to competency contrasts between the lithologies, and dilational zones between bedding planes to form saddle reefs.

The style of mineralisation in the sediments and sills varies, but can be sub-divided into two main types as discussed below:
Mineralisation in the sills is characterised by the presence of pyrite and arsenopyrite (approximately 65% pyrite: 35% arsenopyrite). The total abundance of sulphide is variable, averaging about 3% of the rock, but locally comprising up to about 30% of a 1 m sample. There is a positive correlation between grade and sulphide content. The sulphides occur in a variety of habits: (a) disseminated crystals, (b) stringers, (c) coarsely crystalline veins up to 10 cm in width, but usually 1 – 3 cm across, and often with intergrown quartz, and (d) irregular massive patches.

Mining operations are based on conventional open cast techniques. Excavation of the ore and waste rock on 2.5m high mining benches will be performed by hydraulic excavators in backhoe configuration loading out to 40 tonne nominal capacity articulated dump trucks (Bell B40D or Cat equiv.). Mining is free dig in the oxide zone with any harder transition and fresh materials drilled and blasted on 5.0 m benches.

In order to control the size of ROM fed to the Plant, mobile crushers and screens organized under the outside section or ore re-handling section in the pit are used for the realization of pre-crushing and pre-screening of ROM ore which is then selectively routed either to the ROM pad (-100mm to +25mm) or the secondary feed point (-25 mm) for plant feed.

Blended ore from either the ROM storage pad or direct from the pit are fed into the in-plant crushing circuit. The front end of the plant will treat ore at a rate of up to 300 wet t/h to a product of size 100% passing 100 mm, through the ALP mineral sizer. The blended ore is tipped into the ROM bin using rear dump trucks or fed from the ROM pad stockpile by front end loader. The ROM bin has a capacity of 50m³. A variable speed apron feeder withdraws the ore from the bin at a controlled rate and discharges it into the mineral sizer. A small amount of oversize ore (rejects) from the crusher are collected in a sizer rejects stockpile. The mineral sizer reduces the ore to a product size of 100% passing 100 mm. The crushed product is discharged onto the crusher discharge conveyor which, in turn feeds into a rotary scrubber.

Process water is added to the scrubber to produce discharge slurry with a 50% solids density by weight. The scrubber discharge slurry overflows onto a vibrating double deck scrubber discharge screen. The upper screen deck is set at 50mm x 10 mm slotted. The lower deck is 2 mm.

The secondary crusher feed conveyor transfers the screen top deck oversize to the secondary crusher. The bottom deck oversize is conveyed into the mill feed bin by means of a scrubber screen discharge conveyor, transfer conveyor and crusher product conveyor. The screen bottom deck undersize slurry gravitates into a 12m³ scrubber discharge sump, from where it is pumped to the mill discharge sump.

The secondary crusher reduces the ore size to a product of 100% passing 37 mm. The crusher discharge conveyor transfers the crushed ore to the mill feed bin which has a capacity of 480 m3.

The milling circuit consists of one primary ball mill (Mill 1) and one secondary ball mill (Mill 2), which are capable of treating up to 250 t/h of blended ore.

Crushed ore, from either or both primary and secondary feed points is fed into the No.1 ball mill using a variable speed reclaim conveyor. The dimensions of this ball mill are 3.638mØ (inside liners) x 4.660m EGL and its installed power 1300kW. The ball mill discharge slurry overflows onto No.1 mill trommel screen with a screen size of 10mm. The oversize from the screen is collected in No. 1 ball mill scats bunker. The undersize slurry gravitates into a 17 m³ mill discharge hopper, where it combines with the slurry from the No.2 ball mill. The combined mill discharge slurry, with a solids density of 52% by weight, is pumped by one of the cyclone feed pumps to the mill discharge cyclone cluster.

The cyclone underflow, at 67% solids density by weight, gravitates to the 2m³ cyclone underflow splitter box 1, where there is an off take to the gravity circuit which is currently not used. The cyclone underflow then reports to a 2 m³ cyclone underflow splitter box 2, from where 33% of the stream is sent to the No. 2 ball mill via a velocity break box. The dimensions of the second ball mill are 3.088 m Ø (inside liners) x 3.035 m EGL and its installed power 550 kW. 67% of the stream from the splitter box 2 gravitates back to the no. 1 ball mill.

Milling and cyclone spillage is contained in a bunded area that has a sloping floor to direct spillage to two sumps. One sump is allocated to the mill feed spillage and the other the mill discharge and cyclone spillage. Each sump has a vertical spindle spillage pump. Both spillage pumps discharge into the mill discharge sump. There are two more spillage pumps, one for the No.1 ball mill scats bunker and another for the No.2 mill bunker scats bunker area.

The processing plant comprises portions of two second-hand plants which were purchased in Australia, respectively a crushing, screening and grinding section and a leach, adsorption and elution section. The proposed Twangiza Process Plant design is based on a well known and established Gravity/Leach & CIP technology, which consisted of three-stage crushing, ball milling, gravity recovery of free gold followed by leaching/adsorption of gravity tailings, elution & gold smelting and tailings disposal. However during further testing CIP tanks were changed to CIL which result in better recovery of the ore.

Gravity and Intensive Cyanidation
A portion of the cyclone underflow can be diverted from splitter box No.1 over the 1.12 mW x 2.40 mL gravity feed screen to remove the +2mm material. Dilution water is added onto the screen to dilute the feed slurry to a solids density of 45% by weight. The screen oversize is recycled back to the no.1 ball mill. The screen undersize is fed ........