The Otjikoto deposit is located within the Damara Mobile Belt, within the northern portion of the northeasterly-striking “Intracratonic Branch” of the belt. It is considered to be an example of an orogenic-style gold deposit.

The Otjikoto area is predominantly underlain by lithologies belonging to the Neoproterozoic Swakop Group. The Okonguarri Formation hosts the gold mineralization and is overlain and underlain by glacial diamictite horizons of the Ghaub and Chuos Formations, respectively. The Okonguarri Formation consists primarily of thick units of dark grey carbonaceous marble, biotite-schist, graphitic schist and calc-silicate horizons. The schist units are derived from semi-pelitic, pelitic, marl and psammitic units in a turbiditic sedimentary package. The rocks in the Otjikoto area have experienced at least three phases of moderate to tight folding and some thrust faulting. They have also been affected by extensive metasomatism, followed by prograde regional metamorphism that has reached upper greenschist to lower amphibolite facies.

Mineralization in the main Otjikoto deposit is hosted by a north–northeast striking sheeted sulphide (+ magnetite)–quartz + carbonate vein system that has a strike length of about 2.6 km, and extends at depth to at least 475 m below surface. The gold occurs in a series of thin (commonly <10 cm) sheeted veins in the Upper and Middle Okonguarri Formation. The veins and associated mineralization form a series of en-echelon zones oriented at approximately 010–020° north–northeast and plunging at 10–15° (average 12°) to the south–southwest. Vein concentrations range from one to 30 veins per metre, with a higher vein concentration within the Central and West shoots. Gold occurs within the vein system as coarse native gold particles that can vary from 5–400 µm, averaging about 100 µm in size. Mineralization remains open down plunge as presently tested.

The mining operations use conventional open pit mining methods and equipment. Mining is based on a phased approach with stockpiling to bring high-grade forward and provide operational flexibility.

Ten geotechnical domains have been defined in two oxidation domains (calcrete and oxidized; fresh rock), and pit slope angles vary by geotechnical domain. Inter-berm angles range from 30–60º. Beginning at the 1465 RL, 15 m wide geotechnical berms are included in the design on 60 m intervals. These criteria were the basis for the OSA applied in the optimization analysis.

Groundwater is actively extracted ahead of mining from a single dewatering borehole situated between the Otjikoto and Wolfshag pits. Excess water that accumulates in the pit due to groundwater seepage and rainwater accumulation is collected in sumps located in low spots in each pit and pumped to the return water dam.

Where possible, ramps were located in the east wall of both pits to mitigate potential geotechnical hazards associated with planar and wedge-forming structures present in footwall structures along the west wall. A nominal ramp and road width of 27 m, including drainage and safety berms, was used for dual lane truck operation. Ramp widths were reduced to 20 m in the lower levels of the phase designs to allow for single lane haulage on the final benches. Ramp grades were designed to a maximum of 10%. Temporary ramps will be used, as needed, for initial access to stages.

Ore is hauled by truck from the pit to a stockpile, the ROM pad or direct-tipped into the crusher. The highest-grade material is direct-tipped or placed on the ROM pad depending on the plant feed requirements. Production drilling and blasting is done on 10 m benches with patterns and powder factors varying by material type and geological conditions.

Mining operations are scheduled for 365 days per year with a 15% decrease in the production rate during the rainy season, December through March. Vertical advance is limited to two operating benches per pit phase. This will involve the movement of an average of 38.5 Mt/a of material to sustain processing of 3.5 Mt/a. Mine and mill production are scheduled for eight years with the mining rate dropping the last two years with material from the low-grade stockpiles supplementing the process feed.

Ore Receiving and Crushing
The crushing circuit has a 42 in x 65 in (1,067 mm x 1,651 mm) gyratory crusher, with a nominal throughput rate of 440 t/h. Run-of-mine ore from the open pit operations is delivered by 100 t trucks to the crusher. The crushed product at a P80 of 150 mm, is conveyed to a 7,000 t (live capacity) mill feed stockpile.

Grinding Circuit
The grinding circuit consists of a 24 ft diameter (Ø) x 14.5 ft effective grinding length (EGL) (7.11 m Ø x 4.42 m EGL) SAG mill operated in closed circuit with a trommel for pebble screening on the mill discharge and a vibrating screen to produce ball mill circuit feed. The SAG mill product is combined with tailings from the gravity concentration circuit before being pumped to a classification cyclone cluster. Cyclone underflow is fed to a 16.5 ft diameter x 28 ft EGL (5.03 m Ø x 8.53 m EGL) ball mill and the entire ball mill discharge is fed to a gravity concentration circuit for recovery of coarse free gold.

Both grinding mills are powered with 4,000 kW (5,364 HP) synchronous motors. The circuit is designed to produce an 80% passing product size (P80) of 75 µm.

A pebble crusher was added to the SAG mill circuit for the plant expansion to 3 Mt/a.

The original design of the Otjikoto mill was based on a gravity/whole ore leach flow sheet with a nominal treatment rate of 2.5 Mt/a and a plant availability of 94%. A 25% design factor was included for sizing the primary crusher, conveyors, ball mill, thickeners, cyanide destruction circuit, reagent systems and mainstream pumps which would facilitate a future expansion. A pebble crusher was installed in the SAG mill circuit and two leach tanks were added to the leach circuit in the second half of 2015 to expand the mill capacity from 2.5 to 3.1 Mt/a.

Gold is recovered by gravity concentration/intensive leaching and by a cyanide leach/CIP process for treatment of gravity tailings. The Otjikoto mill design is robust and able to process the three major ore types (XR1 – oxide, XR2 – pyrite-dominant, XR3 – pyrrhotite- dominant) and now Wolfshag over the range of ore grades mined, and with variable materials handling and metallurgical characteristics.

The proces ........