The Geita Greenstone Belt (GGB) hosts several world-class shear-hosted Archaean lode gold deposits and forms the northern portion of the regional Sukumaland Greenstone Belt, itself one of several belts that comprise the Lake Victoria Goldfields. Other gold mines hosted in the Lake Victoria Goldfields include Golden Pride, Bulyanhulu, Tulawaka, Buzwagi and North Mara.

The east-west oriented GGB is 60km in length, up to 15km wide. The Geita terrain is comprised of upper- to mid-Nyanzian greenschist facies units, made up of clastic sediments, black shales, banded iron formation (BIF) and volcaniclastics. These have been intruded by a variety of felsic to mafic intrusive bodies, dykes and sills. Regional north-northeasterly structures hosting Proterozoic gabbro dykes are also prominent geological features in the area.

North-west trending deformation corridors divide the GGB into three distinct sub-terrains, namely the Nyamulilima Terrain in the west (hosting the Star and Comet, Ridge 8 and Roberts deposits), the Central Terrain in the central part (hosting the Nyankanga, Geita Hill, Lone Cone and Chipaka deposits) and the Kukuluma Terrain to the north-east (hosting the Matandani, Kukuluma and Area 3 West deposits).

Geita’s gold mineralisation is preferentially hosted in BIF, cherts and ironstones that have been affected by both ductile and brittle deformation associated with shear zones. The shears exploit fold axial planes as well as the contacts between the supracrustal and intrusive rocks.

The GGB has been through a protracted history of deformation, which resulted in a large-scale synformal configuration in the Central Terrain, with west-northwest trending limbs connected by a north-east trending hinge zone. The deposits of the Central Terrain are mainly located within the relatively low-strain hinge zone. The Nyankanga deposit is hosted in a BIF-dominated supracrustal package that is extensively intruded by, and locally form a roof-pendant within the dioritic Nyankanga Intrusive Complex. At Geita Hill, dioritic rocks are present as sills and dykes intruded into a supracrustal sequence that has been subject to extensive polyphase folding.

To the west, the Nyamulilima Terrain comprises a semi-circular structure surrounding intrusive centers, which internally encompasses structural systems of variable scale that locally control gold mineralisation. At Star and Comet, a folded sedimentary package of BIF intercalated with clastic and tuffaceous meta-sediments is intruded by a tonalitic complex.

The Kukuluma Terrain trends west-northwesterly, with sub-vertical limbs being dominant over compressed, multiphase folded zones. The three major deposits in the area (Kukuluma, Matandani and Area 3) are located along a 5km long east southeast mineralisation trend. The geology of the deposits is dominated by volcano-sedimentary rocks that are polydeformed and intruded by syn- to latefolding diorite bodies. Host rocks for mineralisation are fine-grained iron-rich clastic sediments, cherts, BIF and tuffaceous rocks, with local intercalated carbonaceous shales.

Gold mineralisation at Nyankanga occurs within a northeast-trending and northwest-dipping anastomosing shear system, typically along the lowermost shears, with higher grade mineralisation mainly proximal to the basal contact of BIF packages. Mineralisation is associated with chlorite-carbonate-silica alteration and pyrite-dominant sulphide in the damage zones surrounding the shear surfaces as veins, veinlets, local breccias and sulphide replacement of magnetite layers. At Geita Hill, mineralisation at the deposit scale is controlled by a narrow NE-trending and NW-dipping shear zone that exploits the axial surfaces of F3 folds. The bulk of the ore is also carried by damage zones adjacent to the main shear.

At Star and Comet, a major mineralised shear zone runs NNW-SSE through the deposit where it is localised along the contact of BIF and tonalite. An envelope of mostly brittle deformation up to 10m thick affecting both lithologies occurs either side of the shear zone and controls distribution of mineralisation. Most of the gold mineralisation is hosted in pyrrhotite patches associated with strong silicification together with carbonate alteration.

Within the Kukuluma Terrain, steeply dipping ductile/brittle gold-fertile shear zones are developed along, or close to, the edges of an elongate diorite body, hosted in iron-rich host rocks and locally exploiting axial surfaces of tight folds. Gold mineralisation in the Kukuluma terrain is strongly associated with pyrrhotite, pyrite and arsenopyrite concentrations, accompanied by strong carbonate and silica alteration of host rocks. Gold is present in gold minerals and sulphides, dominantly in arsenopyrite.

The Geita gold deposits are mined as a multiple open pit and underground operation, with ore production from Star and Comet, Nyankanga and Geita Hill underground mines, and from Nyamulilima open pit. The mine is currently serviced by a CIL processing plant with an annual capacity of 5.2Mt.

Geita has been an open pit mining operation from 1999, with underground operations commencing at Star and Comet in 2016, at Nyankanga in 2017 and at Geita Hill in 2020. Underground ore is now a significant part of the feed to the plant. The Nyankanga open pit was completed in late 2020, with the new Nyamulilima open pit commencing in April 2021, providing four sources of ore to the Geita processing plant.

Open pit mining is by conventional truck and shovel methods, where production mining equipment is operated by GGM with Capital Mining Services Tanzania Limited providing production and grade control drilling services, and Orica providing blasting and explosives services. Underground mining commenced at Star and Comet in 2016 and subsequently at Nyankanga in 2017 and most recently Geita.

Hill in 2020. Star and Comet underground has successfully transitioned to owner mining and the mining contractor African Underground Mining Services is used at Nyankanga and Geita Hill for underground development and stoping. The underground mining method is a combination of LOS and TOS. Cemented aggregate fill backfill is used at Nyankanga to fill the primary stopes. Ore is hauled from the Nyamulilima open pit (22km) and from Star and Comet (17km), Nyankanga (4km) and Geita Hill (2km) underground operations to the central run of mine (ROM) pad by the Geita surface mining fleet.

The majority of the ore from the pit is dumped onto the run-of-mine (ROM) stockpile, and then fed into the gyratory crusher by a front end loader. The plant design is such that all ore passes through this crusher. The crusher feed rate is higher than the plant design rate which is currently targeted at around 640 tonnes per hour, and there is a buffer stockpile with a total capacity of around 100,000 tonnes. Reasonable stock of critical crusher spares is always maintained to ensure minimum delays in the crushing operation.

The ore is crushed in a three stage crushing circuit and the crushed product is deposited on the fine ore stockpile. A conveyor belt transfers the ore form the fine ore stockpile directly into the SAG Mill feed.

The SAG mill is 9.14m in diameter, and 5.5m long. The underflow from the primary cyclones passes into the, 6.71m in diameter and 9.6m in length, Ball Mill. This mill operates in closed circuit with the cyclones. Two 48 inch Knelson Concentrators are used for gravity gold recovery.

The process plant includes:
- Primary, secondary and tertiary crushing plants - Semi-autogenous grinding (SAG) mill and ball mill
- SAG mill scats recycle line - (Scats/pebble crusher was decommissioned)
- G-max Cyclones
- High rate thickener
- Pre-oxidation in the first 2 tanks followed by a ten stage carbon-in-leach (CIL) process
- Gold recovery circuit (Elution, Electrowinning and Smelting)
- Gravity concentration section within the milling and classification circuit, where the gravity gold is recovered using Knelson concentrators and an intensive cyanidation process.

Two 48 inch Knelson Concentrators are used for gravity gold recovery. The concentrate from the gravity circuit (Knelson concentrators) enters a closed circuit Acacia Reactor from where the solution is pumped to the gold room where it is passed through a dedicated electro winning cell.

The overflow material from the cyclones passes through linear screens, before ........