The Otjihase deposit comprises five sub parallel, spatula-shaped, mineralised zones namely Shoot A, Shoot B, Shoot 2, Shoot 3 and Shoot 4. Shoots A and B are known collectively as the Main Shoot in mine terminology because of their close proximity, but geologically they are separate entities. The shoots consist of massive sulphides composed primarily of chalcopyrite and pyrite, hosted in a magnetite rich quartzite. Of the five, only Shoot A is consistently mineralised and forms the bulk of the mineral resources.

With the exception of Shoot B, which is slightly elevated above the southern margin of Shoot A, the westerly-plunging mineralised shoots occur at approximately the same stratigraphic level and are separated by more than 150m of barren or weakly mineralised quartz-mica schists that dip north westerly at 16°.

The northern margins of the shoots are abrupt, whereas the southern margins are gradational, consisting of thin bands and lenses of mineralisation, or disseminated mineralisation, in the quartz-mica schist country rocks. The footwall contact between the lowermost mineralised band and the underlying quartz-mica schists is usually sharp. The hangingwall contact is often gradational due to the presence of zones of disseminated mineralisation.

Each of the shoots has a gossanous outcrop and extends down plunge for more than eight kilometres. The mineralised shoots are cut by a series of sub-vertical northerly trending normal faults that down-throw the shoots progressively deeper towards the west. The faults separate the deposit into a number of blocks (or compartments) named after the faults that occur along the western margin of each block.

The Otjihase deposit lies in quartz-biotite chlorite and quartz-biotite schists and consists of a shoot of massive and disseminated sulphides about 200m wide plunging to the west at about 6°. The deposit dips to the northwest at about 16°. The shoot thickness varies between 4m and 12m with an average between 6m and 8m. The hanging wall is a quartzbiotite chlorite schist with distinct schistosity which sometimes spalls. It is competent if supported. The declines and other development are usually excavated in competent quartzmica schists.

The mineralisation stretches from near surface to depths of 800m and more and is divided into compartments by north-south trending faults. These faults are water bearing and contain flowing material and are thus difficult to traverse.

Historically, mining has been by means of room and pillar with primary extraction rates ranging from around 80% to 85% in the upper Otjihase compartment to 45% in the Kuruma compartment.

The Otjihase compartment extends from the surface down to a depth of approximately 260m below surface. The upper area, down to around 180m below surface was mined by JCI using a room and pillar system based on 5m by 5m square pillars with 15m wide rooms on dip and strike.

The ore is trucked to an underground crusher station immediately east of the Hoffnung East fault from where it is transported by means of 10 conveyor belt sections to a point where it is tipped into kibbles pulled by locomotives which take the ore the last 1.5km to the mill receiving bins on surface.

Ore from the Otjihase mine will be crushed underground through a jaw crusher to a nominal 150mm prior to being transported to surface, to a 3,200t receiving bin at the plant. Ore from Matchless mine is road hauled and similarly crushed at Otjihase ahead of the same receiving bin. The receiving bin has 14 off take points feeding a single conveyor.

The different ore types are stored separately in the receiving bin with no mention or plan of blending practice. The ore ratio is expected to be 35,000t:15,000t Otjihase: Matchless, and some consideration may be given to blending is based on the flotation response and the quantity of ores available.

The crusher plant consists of a primary grizzly with oversize being crushed in the 5½ft Standard Symons secondary crusher. The grizzly fines are screened on a vibrating screen to reduce the load on the tertiary crusher. The screen oversize and the secondary crusher product are conveyed to a surge bin then to the tertiary screen and tertiary crusher, a 5½ft Shorthead Symons. The screen undersize is directed to three fine ore silos whilst the screen oversize is crushed in the tertiary crusher and recycled back to the screens where the fines are removed. This is a very conventional crusher circuit. The final crusher product size will nominally be 100% passing 10mm.

Crushed product is conveyed to three mill feed silos each with 1,100t capacity. Crusher plant spares observed include mantles, bowls and screen parts. The condition of this circuit is considered to be reasonable but some maintenance and checking is necessary before commissioning. The dust extraction system at the crusher plant should be checked prior to the planned start-up, particularly all ducting.

Milling plant
The milling circuit consists of three closed circuits Vecor 10ft x 12ft ball mills with 20” hydrocyclones. Mill motors are 670Kw with 11Kv supply. Historically the grind was approximately 80% passing 250 micron. The milling circuit configuration is very conventional, and considered suitable for the type of ore to be processed. The three old Ramsey weightometers measuring mill feed tonnages will be replaced with new process control units which may be configured to give some degree of automation and control.

The ball mills operate with rubber liners and lifters with 56mm plus 64mm cylpebs as grinding media. While the mills have not been run over the past year, barring gear units have been used to turn the mills to ensure that bearings and lubrication are functional. As with other parts of the plant, all equipment should be checked, cleaned and motors turned before startup.

The Otjihase concentrator is designed as a sequential flotation plant treating a base metal sulphide mineral deposit. The economic metals in the ore are copper and silver with pyrite and small amounts of gold.

During the last 20 years, the plant has been operated by TCL, Ongopolo and latterly Weatherly. No major plant modifications were implemented over the operating period other than a Maxwell cell which was installed in the pyrite circuit, the backfill plant, and some equipment refurbishment to ensure continuity of operations.

Copper flotation
The flotation circuit consists of three rougher and scavenger flotation banks with two banks of cells for the cleaning operation. Wemco flotation cells are installed. Rougher and cleaner concentrate report as final concentrate and the various froth products being recycled.

The use of the limited capacity within the cleaner circuit will result in improved recoveries but could tend to produce a lower quality ........