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Bulyanhulu

Tanzania

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The Bulyanhulu gold deposit is located in central northern Tanzania, some 127 km to the south-west of the Lake Victoria coastal town of Mwanza, and 125 km north-west of the Golden Pride deposit.
(#Location: 3° 13' 17"S, 32° 29' 13"E).

The Tanzania Craton, covers an area of ~500 000 km2, and comprises of a series of volcano-sedimentary greenstone sequences deposited between ~2.8 Ga and at least 2.5 Ga that were affected by multiple phases of deformation and pulses of granitoid emplacement dated from ~2.7 to 2.5 Ga (Pinna et al., 2000). The granite-greenstone sequences has many features typical of Archaean cratons, including the existence of numerous gold deposits, granitoids within and around the greenstones, a predominance of greenschist facies regional metamorphism, and regional deformation. The basement rocks of the Craton is the 'Dodoman System', a series of elongate ESE trending outcrops of a variety of high-grade metamorphic rocks, including banded and ferruginous quartzites, biotite-hornblende gneisses, and ironstones. These rocks include granulite-facies assemblages and pyroxene gneisses, along with lower grade talc-chlorite, sericite and corundum-bearing schists (Quennel et al., 1956; Harpum, 1970). Post-migmatitic aplites, pegmatites, olivine-monzonites and monzodiorites intrude the unit. The Archaean supracrustal rocks are referred to as the 'Nyanzian Supergroup' (Quennel et al., 1956), which Stockley (1943) and Clifford (1970) sub-divided into the Nyanzian System, dominantly a greenstone belt volcanic assemblage, and the unconformably overlying Kavirondian System which is dominantly greenstone belt derived molasse. Eight distinct belts of the Nyanzian System have been recognised within the Tanzania Craton (Borg and Shackleton, 1997), extending through SE Uganda, SW Kenya and NW Tanzania, collectively referred to as the 'Lake Victoria Goldfields'. These belts comprise typical greenstone belt assemblages of mafic and felsic volcanics, banded iron formation (BIF) and associated low-grade metasediments. They occur as irregularly shaped lenses surrounded by granitoids, and are characterised by intense folding and structural repetition.

Bulyanhulu lies within the Sukumaland Greenstone Belt, one of the 'Lake Victoria Goldfields, 'an arcuate terrane of Archaean age greenstones of the Archaean Tanzanian Craton, which have a similar setting and are a comparable sequence to that described at the Golden Pride mine (see the Golden Pride record).

The Sukumaland Greenstone Belt has a distinct eye or kidney shape, comprising two concentric, intermittently developed bands of supracrustal greenstone rocks, enclosed and surrounded by granitoid intrusions. The westernmost part of the megastructure is covered unconformably by Proterozoic rocks. The belt is ~200 x 100 km, with east-west trending long axis. Important shear zones in the greenstone belt follow this east-west trend, occurring both within the greenstones and at greenstone-granite contacts. Two broad subdivisions of the belt have been established. The inner comprises an older (Lower Nyanzian) sequence characterised by basaltic and andesitic lavas and tuffs, whilst the outer, younger (Upper Nyanzian) succession consists of banded iron formation and tuffs. Regional mapping and limited structural data (Barth, 1990) suggest a general younging trend from the centre outwards (Chamberlain, 2003).

At Bulyanhulux, rocks of the host Lower Nyanzian sequence are well preserved below ~80 m of weathering. The mine sequence is composed of a steep NE dipping volcano-sedimentary sequence largely comprising bimodal extrusive and intrusive rocks, subdivided into mafic and felsic volcanic rocks and argillaceous sedimentary units. The felsic rocks include ash, lapilli and agglomerate tuffs. A significant granitic intrusion is located in the south-western corner of the deposit area, whilst a series of porphyritic intrusions occur within the volcanic pile. All volcanic units are metamorphosed to lower greenschist facies. Contacts between units are poorly defined, except over the main zone of mineralisation, where a structural contact is confirmed by drilling (Chamberlain, 2003).

The Bulyanhulu deposit is predominantly hosted within a thin argillaceous unit between a mafic volcanic footwall and a calc-alkaline rhyodacitic hangingwall, intruded by a number of sub-volcanic quartz-feldspar porphyry bodies. A pyritic zone, situated At the base of the felsic sequence there is a pyritic zone which is interpreted to be the distal expression of a syn-volcanic hydrothermal system (Chamberlain, 2003).

Mineralisation is structurally controlled in a set of parallel to sub-parallel quartz-sulphide shear controlled veins. There are two principal, steeply-dipping, reverse-oblique shear systems, Reef 1 within the interflow sedimentary unit and Reef 2 within tholeiitic basalts. These structures strike north-west and dip at about 80° NE (Chamberlain, 2003).

These structures strike north-west and dip at about 80° NE. The Reef 1 System which originally contained the bulk of the resource, is developed over a strike length of 1.5 km and has been tested to a depth of at least 1.1 km. It is un-deformed and has an average width of 4.4 m, is stratabound and very regular, being localised at the contact of andesitic volcanics and pyritic felsic pyroclastics. The Reef is sheared, carbonaceous and contains some sulphides and is in part graphitic (Chamberlain, 2003).

The ore consists of dark grey quartz lenses that contain an early assemblage of pyrite with minor chalcopyrite and microscopic gold, followed by a later stage of quartz and carbonate in brittle cracks containing abundant chalcopyrite and coarse gold. These veins brecciate and recrystallise an earlier white quartz that does not have associated sulphides (Chamberlain, 2003).

There are a series of other reefs in the immediate and general vicinity, as well as geochemical and geophysical targets and abandoned artisanal workings.

Fluid inclusion data measured by Chamberlain (2003) indicate the pre-ore quartz in the Reef 1 System formed from saline (12 to 48 wt.% NaCl
 equiv.), CaCl2-bearing brines that had δD (-79‰) and δ18O (+7.2‰) compositions, which that author concluded was suggestive of a magmatic origin. Ore-stage fluids are carbonic, with homogenisation temperatures of 300 to 450°C and elevated CH4 contents with evidence for phase separation. These were interpreted to be of metamorphic origin, released during the late stages of the 2640 to 2550 Ma Kahaman orogeny. Sulphur isotope data from ore-stage sulphides (δ34S of +2 to 7‰) suggest sulphur was primarily remobilised and homogenised from the volcanogenic pyrite zone (δ34S of +1 to 12‰).

Reef 1 is characterised by a narrow alteration halo with most indicators (CaO, Fe
2O3, K2O, S, As and B) decreasing to background values within ~10 m outward. Basalts are characterised by carbonatisation whereas rhyodacites are dominated by sericitisation, carbonatisation and silicification. Chamberlain (2003) concluded that extreme strain and fluid focusing within the argillite, coupled with phase separation of relatively high temperature fluids are the key controls of the high gold grades and the structural and fluid overprint of Cu(-Au) pre-concentrated in a 'porphyry' system is responsible for the elevated base metal content.

Reserves in 1998 totalled some 17.6 Mt @ 13.1 g/t Au for 230 t (7.4 Moz) of gold. In late 1998 additional resources were announced in a blind extension of the existing mineralised zone. When controlled and operated by Barrick Gold Corporation, projected annual production after the mine commenced operation in late 2000 was of the order of 7.5 t (270 000 oz) of gold.

Reserves and resources at the end of 2008 totalled:
    Proven + probable reserves: 36.8 Mt @ 10.4 g/t Au for 375 t Au
    Measured + indicated resources: 1.55 Mt @ 13.25 g/t Au for 20 t Au.

Remaining ore reserves and mineral resources at the end of 2015 (Acacia Mining plc) totalled:
    Proven + probable reserves: 20.82 Mt @ 8.851 g/t Au for 184 t Au
    Measured + indicated resources: 22.158 Mt @ 7.033 g/t Au for 156 t Au.
    Inferred resources: 19.90 Mt @ 9.227 g/t Au for 183 t Au.

The most recent source geological information used to prepare this summary was dated: 2003.     Record last updated: 19/12/2016
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


Bulyanhulu

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Anonymous  1999 - Bulyanhulu: in    Register of African Gold 1999/2000    pp 183-184
Davidson A  2000 - The Bulyanhulu Gold Project: in    a speech delivered to the Prospectors & Developers Association of Canada Conference, Tuesday March 7, 2000,   Un-pub. 3p
Foster R P  1999 - Destination Africa: New Frontiers, New Mineral Exploration Opportunities: in   African Mining: Exploration and Investment Opportunities Conference, Perth, Australia: 16-17 November, 1999 AJM, Sydney    pp 1-11


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