Freda Rebecca Mine, Zimbabwe

Freda Rebecca gold mine, located 90km north-east of Harare in Zimbabwe, has proven reserves of 2.4mt. The mine was closed in March 2007 for a refurbishment programme that will increase its production rate. Phase one of the programme was completed in September 2009. The mine poured its first gold on 13 October 2009, producing 180oz.

The mine will undergo further refurbishment. Phase two, scheduled to be completed by September 2010, is expected to increase production to 50,000oz of gold per year.

Refurbishment

A phased refurbishment programme was announced in March 2009. Phase one, intended to increase production capacity to 30,000oz of gold a year, was commissioned in mid-August. It included resumption of mining, hot commissioning of the mill and processing plant and installation and commissioning of smelting equipment and new switchgear.

Approximately $6m was invested in the first phase. Phase two, currently in the planning stage, will develop the mine further and expand the mine fleet. It will also involve the renovation of the second mill and increase the capacity of the remaining plant. The investment for phase two is expected to be about $4m.

Geology

The mine is situated on the Mazowe-Bindura greenstone belt. The belt's surrounding area is characterised by Shamvaian sediments, diorite and granodiorite that are traversed by dolerite dykes. Ore bodies are located within two major mineralised envelopes.

The envelopes are characterised by shears. The two shears unite in the south-west and flatten at a height of about 850m before extending into metasediments. They are defined by a number of anastomosing shears that are separated by under-formed rocks.

The mine is next to Phoenix Prince, an open pit 2km south-east of Freda Rebecca. Phoenix Prince is included in the mineral resources of Freda Rebecca and lies on an east-west trending shear zone that dips towards the north at 75°.

Reserves

The mine contains 2.4mt of proven reserves with an assumed grade of 2.68%. At the surface it has 270,000t of minerals. It has approximately 2.1mt underground.

Mineralisation

The minerals are hosted largely within Prince of Wales diorite and Bindura granodiorite, with limited extensions into the metasediments. At Freda Rebecca two types of mineralisation occur. Old and widespread sulphides that differentiate in intensity and grade are hosted in driorite and granodiorites. Young and high-grade sulphides including arsenopyrite, pyrite, pyrrhotite and chalcopyrite are limited to shear zones.

To a large extent the minerals are characterised by uneven micaceous chloritic alteration. It contains slight silification and carbonatisation. The quartz veins carry small amounts of sulphide without gold value. The ore zones are assayed to determine visible payable gold.

Minerals in the Phoenix Prince pit occur within diorite. Pyrite minerals are defined by minor chalcopyrite while the region's zone is characterised by strong shearing.

Exploration and processing

The mine is processed using traditional carbon in leach methods. Ore is excavated and delivered via dump trucks to the plant site. Two jaw crushers are used to crush the ore before it is stored in front of single-stage semi-autogenous grinding mills in two parallel and identical mill circuits. The grinding mills are closed with hydrocyclone.

Part of the cyclone underflow is delivered to Knelson centrifugal gravity concentrators. Before the process of gold recovery begins, gravity concentrates are processed in an inline reactor using cyanidation. Almost 20% of gold is recovered using this process.

The cyclone overflow is pumped to a set of de-watering cyclones to augment the percentage solids, a requirement in conventional pre-leach and carbon in-leach type facilities.

Before this lime is added during the process at the mill feed. Calcium cyanide and oxygen are also added in a controlled manner to maintain appropriate levels of lixiviant. To extract the minerals through electro-winning, loaded carbon is separated from the adsorption circuit.

Loaded cathodes are absorbed in hydrochloric acid prior to washing, drying and smelting the ore. Eluted carbon is treated through thermal regeneration before being returned to the adsorption circuit. Adsorption leach residue is delivered to the tailings.