An AMF-PGC International Study Tour
Developed & Managed by Porter GeoConsultancy
Africa 2000
The Gold Deposits of East & West Africa
23 Oct - 9 Nov, 2000 - In Two Modules
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DEPOSIT DESCRIPTIONS

This tour, which was developed, organised and managed by Mike Porter of Porter GeoConsultancy Pty Ltd (PGC), as a joint venture with the Australian Mineral Foundation Inc. (AMF), included:

MODULE 1 - EAST AFRICA
Monday 23 to Saturday 28 October 2000
For information on the remainder of the tour, see: Module 2



MODULE 1 - EAST AFRICA

This MODULE commenced in Dar es Salaam, Tanzania, at 6:00 pm on the evening of Monday 23 October, 2000 with an Introductory Meeting and Dinner. Module 1 included detailed briefings and visits to the following gold mines and projects, in the order listed below:

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Overview Briefings

The following topics were covered by briefings at each of the mines and at the final wrap-up and de-briefing session at the end of the module at Geita:
  • The Geological and Tectonic Framework of East Africa.
  • The Distribution, Geological Controls and Characteristics of Gold Mineralisation in East Africa.
  • An Overview and Summary of Gold to the South of Lake Victoria.
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Golden Pride - Tanzania

The Golden Pride deposit is located in central northern Tanzania, some 200 km to the south of the Lake Victoria coastal town of Mwanza and 750 km north-west of Dar es Salaam. It is within the 2800 to 2500 Ma Nzega Greenstone Belt which is part of the granite-greenstone terrane of the Archaean Tanzanian Craton. The Tanzanian Greenstones are divided into the older Nyanzian System which hosts all of the major deposits of the craton, and the younger Kavirondian System. The Nyanzian System is inturn divided into a Lower Series of basalt, andesite and dacite with subordinate BIF and clastic sediments. These are overlain by an Upper Series of characterised by felsic volcaniclastics, ferruginous chert, BIF and shale. Unconformably over these are the conglomerates, coarse arkosic grits and quartzites of the Kavirondian System. The extrusives of the Nyanzian System are from 2.8 to 2.9 Ga, while the granites are variously between 2.57 and 2.53 Ga. Golden Pride and the other significant deposits are hosted by the Nyanzian System.

The Golden Pride deposit is located near the southern margin of the Nzega Greenstone Belt, within felsic to intermediate volcaniclastics and argillites, comprising a thick basal sequence of interbedded quartzose arkosic sandstones and pelites intruded by a massive medium grained dacite body with minor intercalated mafics near the top. These are overlain by a 100 to 200 m thick dolerite sill that is followed by 500 to 700 m of rhyolitic volcanics. The upper most unit is a 1000-1500 m succession of argillite and arenite separated from the underlying rhyolites by a thin chlorite schist. These sediments have intercalated volcaniclastics increasing upwards. The east west trending, south-west dipping Golden Pride Structure cuts the stratigraphy near the top of the Upper sequence at a low angle, near the transition from dominantly siliciclastics lithologies to the felsic/intermediate volcaniclastics. The sequence in the immediate vicinity of mineralisation comprises laminated chert and oxide facies BIF interbedded with volcaniclastics, primarily sub-aerial volcaniclastics with intercalated terriginous sediments, all overlying a locally thick (20 m) conglomerate. The BIFs are overlain by felsic tuffites and argillites. In the 80m thick ore zone the primary rock types have been obliterated by intense shearing accompanied by strong silica and carbonate alteration. The tuffaceous units form the main host, with the coarser members having undergone the most brittle deformation. Rocks within the shear zone have been totally oxidised to a depth of 75m.

The hydrothermally emplaced mineralisation is accompanied by carbonate alteration, silicification and chlorite-biotite-pyrrhotite alteration. Carbonatisation is the most widespread and comprises calcite with minor ankerite and dolomite. It is more intense in the immediate footwall of the ore, although there is no apparent correlation with gold mineralisation. There is however, a positive correlation between gold and silicification, particularly when the alteration is accompanied by arsenopyrite and pyrrhotite. It is best developed in the core of shear zones, and post dates carbonatisation. In its late stages it is accompanied by hydraulic brecciation and locally high grades of gold. Chlorite-biotite-pyrrhotite alteration is more intense in intervals of strong shearing in the hangingwall intermediate to mafic volcanics and is cut by quartz veining coeval with the silica alteration.

Approximately 80% of the gold in reserves is oxide ore, two thirds of which is in the saprolite zone, with gold being associated with characteristic yellow-brown limonitic clays. The non-mineralised wall rocks are generally strongly hematitic and variably sericitic. The primary mineralisation is clearly sympathetic with the host shear, associated with strong alteration of all three types. Arsenopyrite is an important correlative of gold, occurring as fine grained late stage crystals overprinting all of the other alteration.

The main zone of mineralisation has a strike length of 2.7 km and remains open at depth. The operation is expected to produce over 5.5 t (180 000 oz) of gold per year at a cash cost of USD200/oz. The proven+probable reserve in 1998 was 10.9 Mt @ 3.1 g/t Au within a global resource of 33.4 Mt @ 2.56 g/t Au, for a total of 85 t (2.7 Moz) of gold. The mine is operated by Resolute Tanzania Ltd, a 100% owned subsidiary of the Australian company Resolute Limited.

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Bulyanhulu - Tanzania

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 Golden Pride. It also lies within greenstones of the Archaean Tanzanian Craton, which have a similar setting and are a comparable sequence to that described at Golden Pride (see above). Gold occurs in a set of parallel to sub-parallel quartz-sulphide shear controlled veins within the Archaean Sukumaland Greenstone Belt. These structure strike north-west and dip at about 80 degrees NE. Almost all (95%) of the resource is contained within the Reef 1 System which is developed over a strike length of 1.5 km and has been tested to a depth of at least 1.1 km. The Reef 1 System, which 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. There are a series of other reefs in the immediate and general vicinity, as well as geochemical and geophysical targets and abandoned artisanal workings. 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. The project is now controlled and operated by Barrick Gold Corporation. Projected annual production after the mine commences operation in late 2000 is of the order of 7.5 t (270 000 oz) of gold.

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Geita - Tanzania

The Geita gold project is located in central northern Tanzania, some 90 km to the -west of the Lake Victoria coastal town of Mwanza. It also lies within the Geita Greenstone Belt, the northern arm of the regional Sukumaland Greenstone Belt of the Archaean Tanzanian Craton, which has a similar setting and a comparable sequence to that described at Golden Pride (see above). The Sukumaland Greenstone Belt comprises two intermittent roughly oval shaped domes with outer radii of from 80 to 140 km. The core is composed of the Lower Nyanzian mafic volcanics and sediments (see the Golden Pride description for more detail) and hosts the Bulyanhulu deposit (see above). The peripheries of the domes are dominated by the Upper Series of the Nyanzian (see the Golden Pride description for more detail) made up of BIF and intermediate and felsic volcanics. The Geita deposits are within the Upper Series. The western section of the belt, in the Geita area, is cut by regional scale Proterozoic quartz-gabbro dykes with a strong north-east trend. The area is dominated by both NW and NE structural trends, with a weaker NNW trend occupied by Karoo dolerite dykes. The greenstone belts of the Tanzanian Craton are not characterised by major regional structures as is the case in the Yilgarn and Superior Cratons, but by more localised and discontinuous structures.

Mineralisation in the belt is present in a number of forms, including 1). BIF associated ores (both near the contact with felsic pyroclastics or intrusives, and in the main body of the BIF sequence), 2). mineralisation in footwall of hangingwall felsic tuffs, 3). shear hosted mineralisation in Lower Nyanzian mafic volcanics, or as volcanogenic massive sulphides (pyrite), or 4). ferricrete enrichments over bedrock mineralisation or from erosion of the BIFs.

Historically the Geita mine has operated from earlier in the 20th century, having produced just under 30 t (0.94 Moz) of gold from 5.5 mt of ore between 1936 and 1966. In early April 2000 the previous manager Ashanti Goldfields, was joined by AngloGold and the project is now a 50:50 joint venture. The combined project has a resource of the order of 430 t (14 Moz) of contained gold with open pit grades averaging around 3.5 g/t Au. Reserves have been drilled below the open pit limits where higher grade intersections of more than 10 m @10 g/t have been encountered and the ore deposits are not closed. Production from the new Geita operation is expected to commence in late 2000.

The Geita Project now revolves around a series of deposits which includes the 1). Geita Hill and adjacent Lone Cone deposits from which the historical production was derived, 2). Nyankanga immediately to the SW, 3). the Kukuluma Hill and Matandani deposits 9 km to the NE of Geita and 4). Nyamulilima Hill some 15 km to the WSW of Geita. All of these deposits are associated with BIF rich units with intercalated andesite and felsic tuff. They occur in complex transgressive sheared zones, which at Geita and Nyankanga strike ENE and dip at 30 to 55 degrees to the north. At Kukuluma the host sequence comprises a NE dipping footwall argillaceous sequence passing up into a finely bedded, 100-150 m thick BIF-chert unit which is overlain in turn by felsic tuffs. Gold lies within two parallel shears which follow steeply dipping axial fold and limb structures, with mineralisation preferentially developed in the BIF-chert units. At the Geita deposit economic gold s found in the oxide, transition and primary sulphide intervals.

Two days were spent at Geita visiting Geita itself, Nyankanga, Nyamulilima and Kukuluma, followed by an overview and de-briefing session.

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For more information contact:   T M (Mike) Porter, of Porter GeoConsultancy   (mike.porter@portergeo.com.au)

This was another of the International Study Tours designed, developed, organised and escorted by T M (Mike) Porter of Porter GeoConsultancy Pty Ltd (PGC) in joint venture with the Australian Mineral Foundation (AMF).  While the reputation and support of the AMF contributed to the establishment of the tours, after it ceased trading at the end of 2001, PGC has continued to develop, organise and manage the tour series.

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