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North Mara - Gokona, Nyabigena, Nyabirama

Tanzania

Main commodities: Au
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The North Mara gold mine is located within the Lake Victoria Goldfields, ~100 km east of Lake Victoria and 20 km south of the Kenyan border, in the Tarime district of the Mara region, northwestern Tanzania. It is a combined open pit and underground mine, exploiting the Gokona and Nyabigena and Nyabirama deposits respectively.

North Mara lies towards the southeastern extremity of the belt of Archaean greenstone belts that occupy a WNW trending 2000 x 350 to 550 km corridor within the Eastern Congo Craton, extending across the Central African Republic, NE Democratic Republic of Congo (DRC), SW Uganda, SW Kenya and northern Tanzania. This corridor has been variously termed the 'Ule-Nyanza granite-greenstone belt' (Cahen et al., 1976) or the 'Nyanza-Kibali granite-greenstone terrane' (Lavreau, 1984). It has been disrupted by a number of Proterozoic mobile belts which have divided it into three segments, the C.A.R., NE Congo and Tanzania cratons. Never-the-less its continuity and similarities of the constituent greenstone belts are interpreted to indice they formed as part of a single mass in Neoarchaean times (Cahen et al., 1976). In addition, the relative stability of the corridor from the Neoproterozoic to the present is indicated by the presence of flat-lying Proterozoic cover sequences along its margins and locally in its interior.

The Tanzanian craton, in which North Mara is located, is flanked by younger mobile belts, including the i). East African Orogen (Mozambique belt) to the east which is dominated by Neoproterozoic Pan-African deformation overprinting the West Nile Gneiss; ii). deformational fronts related to the Palaeoproterozoic (Ubendian-Usugaran and Ruwenzorian) and Mesoproterozoic (Karagwe-Ankolean, Kibaran) fold belts to the SW and south; iii). Ruwenzori and Karagwe-Ankolean re-entrant which separates the Tanzanian and NE Congo segments of the granite-greenstone terrane across a major structural corridor north of Lake Victoria, possibly reflecting a major Precambrian crustal structure (McConnell, 1980).

The Lake Victoria Goldfields are within the northern part of the Tanzanian Craton. The southern part of the craton is dominated by the felsic Dodoma gneisses and granitoid rock with only local supracrustal Dodoma schist relicts (Kabete et al., 2012). The northern part of the craton however, comprises a Neoarchaean granite-greenstone terrane which hosts major gold deposits including North Mara, Bulyanhulu, Geita, Golden Pride and Tulawaka.

The Nyabirama and Nyabigena-Gokona deposits lie within the northwestern portion of the Mara Musoma Greenstone Belt of the Lake Victoria Goldfield. Three main time stratigraphic units have been differentiated within this belt (Stockley, 1934; Allibone et al., 2000; Tripp et al., 2007), namely the:
Dodoman basement gneisses, composed of interleaved ~2.93 to 2.85 Ga concordant granitoids, migmatites and schist bands in widespread belts of tonalite-trondhjemite-granodiorite (TTG) and their gneissic equivalents. They include a great variety of high-grade metamorphic rocks, including biotite- hornblende gneisses, amphibolites and hematite-bearing quartzites, that locally underwent intense migmatisation and granitisation to granulite facies assemblages, with zones of retrograde talc-chlorite, sericite and corundum-bearing schists.
Nyanzian volcanic and sedimentary rocks, deposited between ~2.9 to 2.7 Ga, comprising mafic, intermediate and felsic volcanics, metamorphosed greywackes, pelites and banded ironstones that constitute the main greenstone belt lithologies. These rocks may be in part time equivalents of the Dodoman gneisses.
Kavirondian sedimentary rocks which unconformably overlie the preceding series and are composed of molassic conglomerates, arenaceous and argillaceous sedimentary and volcanic rocks.
Intrusive granites occurring within the Dodoman granitoids and metamorphic rocks, and in the Nyanzian greenstone belts, are not always clearly distinguishable from the granitoid rocks of the craton, range in composition from diorites and granodiorites to granites and feldspar-porphyries (Gabert, 1990). Such granitoids extensively intrude the greenstone belt at several stages throughout its development.

East trending regional faults separate thrusted and folded segments of stratigraphy. These structures have, in places, been reactivated as normal faults during extension related to the East African Rifting, producing prominent escarpments. Locally, Proterozoic sedimentary rocks and rift related Tertiary phonolitic lava flows cover the Archaean rocks.

The Gokona and Nyabigena orebodies lie immediately north of the Nyarwana Fault. Mineralisation essentially occurs as disseminated sulphide-gold (2 to 3% sulphide) within pervasively altered andesitic volcanic rocks and quartz veinlets. Both of the deposits are of similar size, having strike lengths of 500 to 600 m and widths of 150 to 200 m, persisting to depths of >300 m. Both are hosted by Nyanzian greenstones that dip and face south and are composed of fine-grained and distinctively feldspar-phyric intermediate volcanic and intrusive rocks. Much of the deposit area is masked by Tertiary phonolite lavas. Mineralisation within the two deposits occurs as moderate to steeply SW dipping, stacked tabular lenses, approximately concordant with the contacts of the local stratigraphy. Individual lodes pinch, swell and bifurcate, but are typically 30 to 60 m thick. The host sequence has been subjected to early ductile isoclinal folding and transposition, producing a consistent penetrative cleavage parallel to bedding. NNE striking brittle faults offset the local stratigraphy and mineralised systems by up to several hundred metres. Two types of gold carrying veins have been recognised; i). steeply dipping grey quartz veinlets; and ii). sulphidic stockwork stringer veins (e.g. Stewart et al., 2001; Smith and Anderson, 2003; Tripp et al., 2007). Higher gold grades occur in the steeply dipping quartz veinlets which commonly contain coarse visible gold. Higher gold grades are also commonly a result of increased vein density. Pyrrhotite occurs within the grey quartz veins whilst arsenopyrite and pyrite are associated with stringer veins. The texture of the volcanic host rocks have been destroyed to varying degrees by K feldspar, silica and sericite alteration.

The Nyabirama deposit is located 7 km along the Mara Shear to the SW of Gokona and Nyabigena. Mineralisation in this deposit is predominantly disseminated within intensively deformed, pervasively altered granitoid intrusive rocks associated with intensive silicious alteration and about 1% sulphide minerals. The Mara Shear, with which the mineralisation is associated, follows the northerly dipping contact between tonalite and granodiorite bodies, with the contrasting rheology interpreted to be responsible for the formation of a set of splays with intervening link structures that formed a dilatant zone amenable to trapping mineralised hydrothermal fluids. The major Nyabirama ore shoots are consistently located in dextral jogs along these structures. Flatter segments of the main shears and intervening link structures also host ore shoots. Faults that are sub-parallel to the Mara Shear are common throughout the mineralised sequence and mark the hanging wall of the deposit. Crosscutting west and NW trending faults are also mapped. Gold occurs within disseminated pyrite, silica gangue and quartz veinlets, with the former accounting for the bulk of the mineable deposit. Gold bearing quartz veins range from a few millimetres to several metres in thickness. K feldspar, silica, albite, sericite and pyrite alteration associated with gold mineralisation increases in intensity towards the centre of the deposit. Minor arsenopyrite, leucoxene, chlorite and calcite consistently accompany the albitic alteration assemblage. Significant silver and weakly anomalous antimony and molybdenum are also asscoaued with the gold mineralisation.

The North Mara mine commenced operation in 2002, and by the end of 2014 had produced a total of 89 tonnes of gold and had a combined Ore Reserves of 23.7 Mt @ 2.69 g/t Au for 63.76 t of contained gold and total Mineral Resources of 18 Mt @ 2.87 g/t Au for 51.6 t of contained gold (Henckel et al., 2016). This suggests a total endowment of 204 t of contained gold as known in 2016.

Remaining NI 43-101 compliant Ore Reserves and Mineral Resources at 31 December, 2018 were as follows (Acacia Mining plc, Annual report, 2018):
  Open pit.
    Proved + Probable reserves - 20.068 Mt @ 1.65 g/t Au for 33.1 t of contained Au;
    Measured + Indicated resources - 7.451 Mt @ 1.83 g/t Au for 13.6 t of contained Au;
    Inferred resources - 0.446 Mt @ 1.26 g/t Au for 0.56 t of contained Au;
  Underground.
    Proved + Probable reserves - 6.182 Mt @ 5.62 g/t Au for 34.75 t of contained Au;
    Measured + Indicated resources - 5.299 Mt @ 3.54 g/t Au for 18.76 t of contained Au;
    Inferred resources - 3.992 Mt @ 5.27 g/t Au for 21 t of contained Au;
  TOTAL contained gold in both underground and open pit - 122 tonnes.
  NOTE: Ore Reserves are additional to Mineral Resources.

The most recent source geological information used to prepare this summary was dated: 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.


  References & Additional Information
   Selected References:
Henckel, J., Poulsen, K.H., Sharp, T. and Spora, P.,  2016 - Lake Victoria Goldfields: in    Episodes,   v.39, pp. 135-154


Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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