Africa-B 2001 - Mineral Deposits of Southern & Central Africa - Deposit Descriptions

Copper Belt Overview

The Zambian Copper Belt and the Cupriferous Arc of the neighbouring Democratic Republic of Congo (DRC) closely coincide with a complex structural zone, the Lufilian Arc, which is normal to the NE-SW trend of, and near the NE extremity of, the elongate, 2000 km long Damaran-Katangan belt of Meso- to Neo Proterozoic sediments that extends all the way to the Atlantic Ocean to the SW.

Traditionally the ore deposits of the Zambian Copper Belt have been interpreted to lie within the Neo-Proterozoic Lower Roan Group (up to 1000 m thick) composed principally of coarse silici-clastics (conglomerate to arkose and siltstone, with lesser carbonates). Some 65% of the mineralisation lies within a finer 0 to 100 m thick unit of generally carbonaceous argillites, carbonatic argillites and interbedded arenites (the Ore Formation) within the coarser clastic succession. A further 25% lies within coarser footwall clastics and the remaining 10% within the coarse hangingwall clastics. Lithologically 60% of the ore is hosted by argillites and 40% in arkose, quartzites and conglomerates.

The Lower Roan is overlain by Neo-Proterozoic carbonates of the Upper Roan (up to 600 m thick) which hosts the Cupriferous Arc deposits of the neighbouring DRC. This unit is in turn followed by up to 600 m of carbonaceous argillites and dolomitic argillites (Mwashia Group) and a very thick sequence (6000 m+) of finer clastics and carbonates, the Kundelungu, with two prominent conglomerate/diamictite units.

The Zambian Copper Belt comprises two NW-SE trending parallel lines of Cu mineralisation some 20 km apart, separated by the Palaeo-Proterozoic basement gneisses, granitoids and schists, and Meso-Proterozoic conglomerates, quartzites and granitoids that make up the Kafue Anticline. The majority of the deposits are in the SW band. Each of these two belts is 5 to 20 km wide and up to 150 km long. The ore grade mineralisation however tends to occupy a linear, often more structurally complex band up to 2 km wide on the SW limb, interrupted by narrow barren gaps and cross folded anticlinal basement cores.

Within the two belts there are some 7 major and 25 minor stratabound deposits. The larger orebodies range from 90 to 580 mt @ 2.4 to 3.6% Cu. Between 1930 and 1987 some 24 million tonnes of copper metal was produced from 1070 mt of ore averaging 2.71% Cu and containing 29 mt Cu. The total mined ore plus reserves/resources has been calculated at 3000 mt @ 2.9% Cu.

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Kitwe Seminar - The Setting, Geology & Metallogeny of the Zambian Copper Belt

The seminar will include presentations covering the following topics:

The setting & geology of the Damaran-Katangan System in Africa and the place of the Lufilian Arc & Copper-Belt within it.
The tectonics, geology, structure, metallogeny and mineralisation of the Copper Belt in Zambia and the Cupriferous Arc in the neighbouring Democratic Republic of the Congo (DRC) and how they are related.
Descriptions of the important DRC deposits and their characteristics.
The stratigraphy, structure, occurrence and controls of copper mineralisation within the Copper Belt in Zambia.
Details of the main deposits that are not to be visited and how they compare to those orebodies on the itinerary.
Aspects of exploration on the Copper Belt - the expression of mineralisation in outcrop, geochemistry, geophysics, etc.

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Mufulira - Cu-Co

The Mufulira sediment hosted copper-cobalt deposit is located in the Zambian Copper Belt of eastern Zambia. The main deposit was discovered in 1923, with production commencing 10 years later.

The total production + resource was estimated at 335 Mt @ 3.3% Cu in 1988 (Freeman). The proven+probable reserve was estimated at 27.2 Mt @ 3.04% Cu in 1999.

Mufulira is the largest of the orebodies found on the NE margin of the Kafue Anticline, and is principally hosted by arkose/quartzites. At Mufulira the Lower Roan Group is 200 to 250 m thick, comprising a lower 0 to 150 m thick Footwall Formation of up to 20 m of basal conglomerate overlain by medium to fine grained grey, cross bedded arkosic to argillaceous sandstone. This unit is followed by the 30 to 80 m thick Ore Formation, which is composed of three "Orebody Quartzites", each being from 3 to 20 m thick, separated by two 0 to 20 m thick units of dolomite, dolomitic siltstone and interbedded quartzite and argillite bands. The Ore Formation is followed by argillaceous quartzite, grit and quartzites of the 65 to 75 m thick Hangingwall Formation.

The main ore is concentrated within the three "Orebody Quartzites" where it occurs as either 'fly speck' disseminations, 1 to 2 mm blebs or very thin lenses, all following particular beds, or as irregular clots of weak to strong disseminations up to 10 cm across. The dominant sulphide is bornite, with chalcocite increasing in the uppermost quartzite. The lowest and most extensive of the "Orebody Quartzites", C Orebody, has a lateral extent of 5800 m, by 1500 m down dip and is up to 15 m thick. Generally there is little mineralisation in the finer and dolomitic inter-orebody units, except in the coarser intercalated lithologies. In places however these may be mineralised and at one location where they thin, the three "Orebody Quartzites" are connected and the total thickness of ore is 65 m. Anhydrite is common in the Footwall and Ore Formation, locally comprising up to 35% as cements and thin discontinuous lenses. Anhydrite and sulphides have an anti-pathetic relationship, with interstitial dolomite and sulphides being found in the place of anhydrite in the ore zones.

A number of basement rises cut through the sequence to the orebodies, with the highest grades tending to flank these basement hills. Where the Footwall Formation is absent and the ore rests on basement, that may also be mineralised. The base of each of the "Orebody Quartzites" corresponds to an increase in the silicification of the hosts sediments, and the disappearance of banding and argillaceous and arkosic content. The upper limit of each orebody is usually marked by the appearance of fine dolomites and argillites.

Following the recent privatisation, Mufulira is now operated by Mopani Copper Mines plc, a joint venture between First Quantum Minerals Ltd (44%), Glencore International AG (46%) and Zambian Consolidated Copper Mines Ltd (10%).

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Nkana - Cu-Co

The Nkana mine exploits mineralisation within the Lower Roan Group Ore Formation, more or less continuously over a strike length of 14 km on the SW margin of the Kafue Anticline. In 1988 Freeman estimated the total production + resource was 690 Mt @ 2.5% Cu. In 1999 the proven+probable reserve was estimated at 117 Mt @ 2.3% Cu, 0.11% Co. Mineralisation at Nkana was first shown to Europeans in 1899, although mining did not commence until 1927, and has continued to the present.

The Lower Roan at Nkana commences with the Footwall Formation which is up to 170 m thick, subdivided into an intermittently developed 0 to 24 m thick Basal Conglomerate; a more extensive 0 to 150 m thick Footwall Quartzite; Footwall Transition, a 20 m thick mixture of alternating arenites, grits & conglomerates; the 10 to 14 m thick Lower Conglomerate; the 15 to 20 m thick Footwall Sandstone comprising almost black interbedded arenites and argillites, with abundant anhydrite, including one local 3 m thick seam; and the impersistent 1 to 4 m thick Footwall Conglomerate.

The Ore Formation is variable along strike. From Mindola in the north, the Mindola Succession passes rapidly into a 600 m wide Barren Gap of 50% thicker massive siliceous dolomite, then back into the Mindola Succession, which persists for another 2 km before gradually grading into the Southern Orebody Succession. The Mindola succession is up to 25 m thick and comprises a basal dolomitic schist, followed by interbedded dolomitic mudstones, dolomitic argillite and siltstones, cherty ore, leached and silicified anhydrite-dolomitic rock, capped by a grey dolomitic argillite to siltstone. The Southern Orebody Succession is composed of a 0 to 4 m thick band of laminated to massive tremolitic dolomite and argillites, followed by the 5 to 20 m thick Orebody Member of black carbonaceous shale to mudstone with lesser dolomite veins and sulphides, which is in turn capped by the 5 to 10 m Hangingwall Argillite as in the Mindola Succession. The ore is primarily chalcopyrite and bornite with only minor chalcocite. Extensive zones have associated pyrite and little bornite. In the structurally simpler Mindola ores the sulphides are present as disseminations, commonly aggregated into clots and blebs, while in the more structurally complex areas it occurs as large streaks, lenses and crosscutting veins.

The Ore Formation is followed by the approximately 60 m thick Hangingwall Formation of alternating quartzites, argillite and dolomitic argillite. These are in turn overlain by the Upper Roan carbonates and the Mwashia carbonaceous argillites.

The orebodies have been folded into a steep asymmetric syncline, with a complex series of en echelon drag folds and decollement faults between lithologies of varying competence.

The mine is operated by Mopani Copper Mines plc (see Mufulira for ownership details).

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Nchanga - Cu-Co

The Nchanga mine is located on the SW margin of the Kafue Anticline near its northern extremity. The first Nchanga mineralisation was investigated in 1923. Testing continued until the main operation commenced in 1937 and has continued until the present. In 1988 the total mined and remaining resource was 580 Mt @ 3.6% Cu (Freeman). Reserves in 1999 were 111 Mt @ 3.8% Cu. The ores at Nchanga are hosted by both arenites and argillites of the Lower Roan Group, which is composed of a thick Footwall Formation and the overlying Ore Formation (which includes rocks that elsewhere are put into the Hangingwall Formation, but as they are ore bearing at Nchanga are locally included in the Ore Formation).

The Footwall Formation comprises 0 to 200 m of basal conglomerate followed by 0 to 300 m of arkose, overlain by a transitional kaolinitic sandstone to dolomite-shale unit that is brecciated and oxidised and is up to 15 m thick. The overlying Ore Formation commences with a 12 to 27 m thick Lower Banded Shale, which is a black, carbonaceous, laminated, pyritic, dolomitic shale to silty argillite which is the host to the "Lower Orebody". This is followed by the 0 to 6 m Brown Chert Marker, 12 to 30 m Lower Banded Sandstone, 0.5 to 2 m Shale Marker, 4.5 to 6 m Pink Quartzite (which hosts some ore lenses), the 15 to 30 m Upper Banded Sandstone, the 15 to 40 m thick TFQ (The Feldspathic Quartzite) - which is the main host to the "Upper Orebody"), and the 15 to 37 m thick Upper Banded Shale (which is mineralised in its basal section where ore is found below in the TFQ).

Ore is present as follows: i). The Footwall Orebody in the arkose and transitional sandstone (30% of Nchanga ore); ii). The Lower Orebody in the Lower Banded Shale and upper sections of the Transitional Sandstone, constituting 25% of the total ore, and present as chalcopyrite and bornite disseminations, laminae accumulations and veinlets; iii). The Intermediate Orebody in the Shale Marker and Pink Quartzite; iv). The Hangingwall Orebody in The Feldspathic Quartzite and the lower sections of the Upper Banded Shale, accounting for 30% of the ore and occurring as fine to dense disseminations to clots of bornite-chalcocite; and v). the refractory Copper Micas which comprise 14% of the mineralisation with 286 mt of 1.2% Cu occurring as selvages to the main orebodies in the micas of the banded sandstones and the dolomitic schists of the Upper Banded Shale. Oxide mineralisation (malachite, azurite, chrysocolla, etc.) is important in those sections of the ore within 300 m of the surface.

The hosts and ore have been strongly deformed in the mine area. The more brittle granites and gneisses of the basement have been dislocated by thrusts, which in part pass into the Lower Roan as decollement faults between units of differing competence. In the main however the basement thrusting is accommodated by strong asymmetric folding of the Lower Roan with vertical to overturned limbs and nappes.

Nchanga, which comprises both a large open pit and underground mines, is operated by Konkola Copper Mines plc (KCM) which is owned 65% by Zambia Copper Investments (ZCI), 20% by Zambian Consolidated Copper Mines Ltd, and 7.5% each by the World Bank's International Finance Corporation (IFC) and the Commonwealth Development Corporations Financial Services. ZCI is a 51% owned subsidiary of Anglo American plc.

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Konkola - Cu-Co

The Konkola mine is hosted by the Lower Roan Group on the SW alignment of deposits of the Zambian Copper Belt, near its northern extremity, some 25 km to the NW of Nchanga. The mine is operated by Konkola Copper Mines (for ownership see Nchanga). The deposit occupies a strike length of near 10 km on the rim of the Konkola Dome of basement rocks and extends across the border into the DRC where it is exploited by the Musoshi mine.

The total production + resource in 1988 (Freeman) was 685 Mt @ 3.1% Cu (including the adjacent and probably connected ores at the Kirilabombwe mine to the SE, but not the Musoshi mine section in the DRC). The undeveloped Konkola Deep orebody currently contains 100 Mt @ 4.33% Cu.

The Lower Roan at Konkola-Musoshi commences with the Footwall Formation as a 0 to 300 m thick Basal Conglomerate, followed by a 0 to 300 m Footwall Quartzite, 15 to 50 m of Porous Conglomerate - a leached conglomerate unit with a dolomite and anhydrite cement, capped by a thin argillaceous band, the 0 to 3.5 m thick Footwall Conglomerate - a highly porous arkose with rounded feldspar clasts.

The 60 m thick Ore (or Banded Shale & Sandstone) Formation has been divided into a series of units designated A to F. These are as follows, from the base:

* Unit A (0.6-1.0m) - thin bedded, calcareous sandy siltstone with erratic mineralisation, averaging 1.5% Cu as chalcocite & oxides;
* Unit B (1.4-2.0m) - thick bedded massive grey siltstone, fairly uniformly mineralised with 5.5% Cu as disseminated chalcocite;
* Unit C (0.9-2.0m) - alternating 7 to 12 cm layers of grey siltstone & pink calcareous sandstone, with wavy bedding and sulphide laminae and veins averaging 3 to 6% Cu;
* Unit D (1.0-1.8m) - thinly laminated grey siliceous siltstone with erratic lenses and bedding plane sulphides assaying up to 8% Cu;
* Unit E (0.6-1.5m) - thinly bedded grey to reddish brown sandstone with mica flakes and very erratic mineralisation, generally up to 1.3% Cu in the oxide zone;
* Unit F - the remainder of the Formation, composed of banded shale and sandstone.

The Hangingwall Formation comprises 12 m of arkose, generally a finely bedded argillaceous quartzite (the equivalent of the TFQ at Nchanga), followed by 140 m of feldspathic sandstones, grits & shales, which become more dolomitic upwards and grade into the Upper Roan Group.

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Chambishi - Cu-Co

The Chambishi sediment hosted copper-cobalt deposit is hosted by the Lower Roan Group in the Zambian Copper Belt and is located on the SW margin of the Kafue Anticline, approximately half way between Nkana and Nchanga in northern Zambia.

For details of the geological setting and sequence consult the Zambian Copper Belt record.

Locally the Lower Roan is up to 200 m thick, followed by 350 m of Upper Roan Group carbonates, 550 m of Mwashia Group carbonaceous argillites, and 1000 m of Kundelungu diamictite, dolomite and shale. Basement is granite and Meso-Proterozoic conglomerate, quartzite and schist. The Lower Roan is composed of a similar sequence to that described for Nkana (see above), followed by the Ore Formation which varies from 0 to 30 m in thickness, and around 50 m of the Hangingwall Formation.

The Ore Formation averages 30 m in thickness near the surface but thins to 20 m down dip and to as little as 6 m over granite basement highs. It is composed mainly of fine grained biotite-quartz argillite, with a basal zone that is strongly contorted, schistose, rich in carbonate and cut by quartz-dolomite-anhydrite veins. The basal schistose band contains coarse aggregates of bornite and chalcopyrite, with associated quartz veins carrying the same sulphides. Above this the ore occurs as finely disseminated bornite and chalcopyrite, grading down dip and up section into pyrite. The Lower Roan is strongly deformed in the vicinity of Chambishi, the main structure being the Chambishi Monocline, with the most intense folding being in the Ore and Hangingwall Formations.

In 1988 Freeman estimated the total production + resource was 240 mt @ 2.3% Cu.

The mine has been bought by China Non Ferrous Metal Industries Foreign Engineering Construction. The open pit mine remains closed and provides an excellent site for observing mineralisation and geology.

The current reserve of the Chambishi group of mines is 33.5 mt @ 2.55% Cu.

For detail consult the reference(s) listed below.

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Chibuluma - Cu-Co

The Chibuluma copper-cobalt deposit of the Zambian Copper Belt is located some 10 km to the SW of Nkana in northern Zambia.

For details of the host sequence refer to the Zambian Copper Belt record.

Mineralisation is entirely within the Footwall Formation, which is overlain directly by the Upper Roan Group, the Ore and Hangingwall Formations being absent. Chibuluma is an example of a Footwall Orebody, which are usually characterised by lower tonnages, but higher grades. The Chibuluma, Chibuluma South and Chibuluma West bodies contained 28 mt @ 4.1% Cu, 11 mt @ 4.3% Cu, 8.4 mt @ 3.2% Cu, 0.19% Co respectively. Current reserves are Chibuluma West are 0.45 mt @ 3.5% Cu, 0.2% Co, plus indicated resources of 0.9 mt @ 5.3% Cu, 0.11% Co, and at Chibuluma South, some 11 km to the south, 7.8 mt @ 4.3% Cu, 0.03% Co. Footwall orebodies on the Copper Belt are generally characterised by being markedly more transgressive, not confined to any particular stratigraphy and may even extend into the basement.

The Footwall Formation at Chibuluma commences with a thin basal conglomerate, then up to 130 m of gross-scale cross bedded quartzite (aeolian ?), a thin aqueous arkose/quartzite and the 0 to 7 m thick Ore Member. The latter is an arkose to quartzite with scattered quartzite pebbles, with a marked schistose nature and mica content where it contains ore. The orebody at Chibuluma West has been interpreted to have formed in a channel of this unit cutting down into the underlying, lithologically similar quartzites. At the base of the orebody there are 'sulphidites' with up to 33% sulphide, each 7 to 120 cm thick comprising cobaltiferous pyrite, carrollite and minor chalcopyrite in a quartz and feldspar matrix. These bands extend over the whole orebody and grade up into disseminated chalcopyrite. The disseminated sulphides are evenly distributed along any one bed, but vary considerably across the sequence. There is a general zoning across the orebody parallel to strike of barren to chalcopyrite to pyrite to chalcopyrite to barren. In some sections the chalcopyrite zone is separated from the barren interval by a bornite-chalcopyrite phase. The mineralisation is capped by up to 15 m of quartzites and conglomerates, followed by the Upper Roan carbonates. The orebody at Chibuluma South persists to a depth of 600 m, with a dip of 40 degrees and is 10 to 40 m thick. Mining is to commence with an open pit.

Chibuluma is currently held by Metorex (85%) and Zambian Consolidated Copper Mines Ltd, and 15%.

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Kalulushi Core

An extensive core storage facility at Kalulushi houses a collection of drill core that may be used to illustrate the stratigraphy of the Copper Belt. Although surface exposures will be inspected during the tour, outcrop in general is poor and the core provides a convenient substitute for surface traverses.

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MODULE 1 - PART B - Zinc, Lead & Copper in South Africa & Namibia

Geological Overview

A series of large zinc, lead and copper deposits are known over an interval of 500 km on the south-western and western margins of the Kaapvaal/Kalahari Craton in the Northern Cape Province of South Africa and in southern Namibia. These are found in both the late Palaeo- and Meso-Proterozoic metamorphics of the Namaqua Mobile Belt and in the late Neo-Proterozoic sediments and volcanics of the Gariep Province in Namibia.

The Namaqua Mobile Belt is part of an extensive U-shaped fringe that extends around the southern, western and northern margins of the Kaapvaal/Kalahari Province, from the Indian Ocean, to the Atlantic and back into central Africa and is composed of late Palaeo-Proterozoic and Meso-Proterozoic gneisses, schists and granitoids after volcanics, sediments and intrusives that were metamorphosed in the late Meso-Proterozoic from 1200-1000 Ma. These rocks in part form the basement to the Damara-Katangan that hosts the deposits of Module 1 Part A.

The Gariep, an inlier on the western margin of the Kalahari Craton along the Atlantic coast, is part of the Damaran-Katangan System, resting unconformably on Namaqualand metamorphics and intrusives. These comprise a variety of sediments from mixtites through arenites and argillites to carbonates, with variable but generally lesser mafic rocks, and felsite-rhyolite volcanics.

The western segment of the Kalahari Craton to the north and east of the Namaqua Mobile Belt and Gariep Complex is occupied by a thick sequence of poorly deformed volcanics and intrusives, the Rehoboth Complex of granitic, intrusives cutting volcanics of from andesitic to acid composition with intercalated sediments ranging from 1700 to possibly 1050 Ma in age. These are in part equivalent to the metamorphosed volcanics and sediments of parts of the Namaqua Mobile Belt

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Black Mountain/Swartberg - Cu-Pb-Zn

The Black Mountain or Swartberg deposit is some 6 km west of Broken Hill within the Aggeneys District of the Northern Cape Province of the Republic of South Africa.

It had an original geological resource of 82 Mt @ 0.75% Cu, 2.7% Pb, 0.6% Zn, 30 g/t Ag. It occurs on Swartberg (or Black Mountain), a hill characterised by a black colouration due to the magnetite (and to a lesser extent the manganese) content of the exposed mineralisation. The regional geology is as described for the Broken Hill (Aggeneys) record. Black Mountain is also hosted by the Aggeneys Ore Formation, which at Black Mountain is separated from the underlying schists and quartzites by an inferred shear and from the base is composed of:

i). a 5 m thick lower "orebody" of baritic schist with disseminated magnetite and sulphide;
ii). a 30 to 65 m thick package of mixed schists (banded quartz-aluminous-schist, and garnet-quartz-schist) and garnet-quartzite (quartz-almandine);
iii). 0 to 25 m of barite-magnetite quartzite (20-75% barite with quartz & 10% magnetite) and barite-quartz schist (with more muscovite, biotite and chlorite);
iv). 0 to 20 m of magnetite-quartzite (with minor garnet);
v). 0 to 25 m of amphibole-magnetite (magnetite with cummingtonite/grunerite and lesser pyroxmangite, quartz, hedenbergite, garnet, fayalite and apatite); and
vi). 1 to 30 m of amphibolite and medium grained quartzo-felspathic rocks.

All of these lithologies have been deformed into a tight, isoclinal F2 fold nose in the vicinity of the orebody. Higher grade, coarser, mineralisation is present as an elongate body concentrated down the plunge of the F2 fold axis and occurs in garnet-quartzite, magnetite-quartzite and amphibole-magnetite rocks. In this zone ore occurs as heavily disseminated pyrite and ore sulphides (galena, sphalerite and chalcopyrite) in magnetite and amphibole rich quartzose metamorphics, with finer grained, banded sulphides on the limbs aligned with the foliation of the magnetite-quartzite and amphibole-magnetite hosts.

There is a zoning from a Cu bearing garnet-quartzite, to chalcopyrite-galena rich magnetite-quartzite to an upper galena-sphalerite dominant amphibole-magnetite in the core of the fold hinge,

The deposit has been mined for a period in the past, but is not currently being worked. It is currently (2001) owned by Anglo American plc - Base Metals Division.

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Broken Hill - Pb-Zn-Cu

The Broken Hill mine at Aggeneys has exploited one of the three main ore deposits (Black Mountain, Broken Hill and Gamsberg) within the Aggeneys District of the Northern Cape Province of the Republic of South Africa. It lies some 16 km to the west of Gamsberg and 6 km east of Black Mountain. All three are hosted by the Bushmanland Group of the Namaqualand Metamorphic Complex, which in turn lies within the broader Namaqua Mobile Belt on the south-western and southern margin of the Kaapvaal/Kalahari Craton. This complex comprises an older metamorphic suite (locally a quartzo-feldspathic augen gneiss) dated at around 2000 to 1900 Ma, over lain by a 1700-1600 Ma (?) supra-crustal succession. This latter succession comprises a lower intrusive to extrusive l euco-gneiss, the Hoogoor Suite, and an upper schist/quartzite succession, the Bushmanland Group. The latter is extensive, and is largely preserved as infolded enclaves or thrust slivers, and is relatively thin, generally less than 1000m thick. At Aggeneys the Bushmanland Group comprises a lower 80 m thick aluminous schist, the Namies Schist composed of quartz-muscovite-k feldspar, locally with up to 25% sillimanite and biotite. This is overlain by a 5 to 900 m thick white crystalline white to grey quartzite, followed by the up to 200 m thick Aggeneys Ore Formation and amphibolite, leucocratic grey gneiss and conglomerates.

The Aggeneys Ore Formation is strongly deformed and in general comprises a footwall schist of sillimanite-quartz-biotite with minor garnet. This is followed by a ferruginous garnet-quartzite composed of bands of garnet and magnetite in a matrix of quartz. This zone is in turn followed by an interval containing magnetite-quartzite (medium grained magnetite & quartz) and amphibole-magnetite (quartz, spessartine, magnetite, ortho-pyroxene, grunerite, cummingtonite & fayalite) with some bands of ferruginous garnet-quartzite. The lower massive sulphide falls within this interval. Above this amphibole-magnetite is the main massive sulphide body and sulphide-quartzite. This is in turn overlain by magnetite-quartzite and amphibole-quartzite, another zone of ferruginous garnet-quartzite and the hangingwall sillimanite-quartz schist.

The massive sulphides (>25% sulphide) generally contain <10% magnetite and range from banded to brecciated textures to heavy disseminations. Pyrrhotite and galena dominate, followed by sphalerite, chalcopyrite and pyrite. The gangue is predominantly quartz, with variable garnet and some barite.

Initially the resource comprised 85 mt @ 0.34% Cu, 3.6% Pb, 1.8% Zn, 48 g/t Ag, including the reserve of 38 mt @ 0.45% Cu, 6.4% Pb, 2.9% Zn, 82 g/t Ag. Reserves are currently 8.7 mt @ 0.5% Cu, 5.5% Pb, 2.9% Zn, 78 g/t Ag. The mine is owned and operated by Anglo American plc - Base Metals Division.

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Gamsberg - Zn

The Gamsberg deposit is located some 16 km to the east of Broken Hill/Aggeneys in the Northern Cape Province of South Africa.

Economic reserves are quoted as 90 Mt @ 6.3% Zn, 0.5% Pb, amenable to open cu extraction. The current feasibility study (2000) is considering the development of an integrated mine and zinc refinery, potentially the largest such complex in the world.

The project is owned and managed by Anglo American plc - Base Metals Division.

Mineralisation occurs within the Gams Ore Formation, which is a direct correlative of the Aggeneys Ore Formation that hosts the ore at Broken Hill, and is a member of the Bushmanland Group (for detail see the Aggeneys/Broken Hill records). The Gams Ore/Iron Formation comprises three units, namely:

i). a lower member composed of a diverse suite of quartz-felspar-garnet-clinopyroxene rock, garnet-clinopyroxene-feldspar marble and garnet-clinopyroxene-quartz-magnetite rocks;
ii). a middle sulphide zone with quartz-garnet-amphibole rocks and graphitic quartz-sillimanite-muscovite-feldspar containing major amounts of pyrite, pyrrhotite, sphalerite and galena; and
iii). an upper unit of diverse garnet, pyroxenoid, clinopyroxene, orthopyroxene, amphibole, quartz, apatite, carbonate, magnetite, hematite and barite bearing rocks.

It is underlain by a thick, massive recrystallised white quartzite with minor schists and conglomerate which succeeds the Hoogoor Suite leuco-gneisses.

Deformation is complex, with the Gamsberg mineralisation falling within a major 'sheath fold' several kilometres across, on whose margin the deposit is located at the contact between the underlying quartzite and overlying mafic gneisses of the Nousees/Koeris Formation.

The mineralised sulphide zone of the Gamsberg Iron Formation is intermittently present within the sheath fold structure and is generally weakly mineralised containing 1 to 4% Zn. The intervals of weaker mineralisation include a number of higher grade 'ore shoots' with +7% Zn embracing smaller cores of +10% Zn.

The measured and indicated resource is 140 Mt @ 5.8% Zn, 0.5% Pb, within a geological resource of 170 Mt.

The sphalerite rich pyrrhotitic-pyritic ore is found towards the centre of the Gams Iron/Ore Formation, flanked by iron sulphides, predominantly pyrite below and pyrrhotite above. These sulphides are in turn sandwiched by two magnetite to hematite rich zones towards the outer margins of the Gams Iron Formation.

The ore zone sphalerite occurs as intergranular disseminations within a quartz-sericite-sillimanite grading up with increasing zinc grade to a quartz-garnet-grunerite host. An impediment to the development of Gamsberg is the high Mn content of the sphalerite, which contains 2 to 3% Mn within its lattice.

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Rosh Pinah - Zn-Pb

The Rosh Pinah Zn, Pb, Ag deposit is located in the Namib Desert of south western Namibia, in southern Africa.

It is basically a stratabound deposit, hosted by the Rosh Pinah Formation arkoses and quartzites, in the lower sections of the late Proterozoic to early Palaeozoic Gariep Complex, an equivalent of the Damaran Supergroup. In this area the basement to the Gariep Complex is made up of Richtersveld Sub-province (Vioolsdrif Terrane) igneous rocks of the Namaqualand Metamorphic Complex.

The type section of the Rosh Pinah Formation, which forms the base of the Gariep Complex, comprises a thin basal mixtite/conglomerate overlain by a thin unit of mafic volcanics. These are succeeded by thicker felsite and rhyolite lavas, pyroclastics and epiclastics, followed by a thin dark argillitic arkose, more felsic volcanics and then the main thick sequence of arkoses, comprising arkose, graded quartzite, argillite and dolomite. A further felsic volcanic unit is found near the top of these arkoses. These rocks of the Rosh Pinah Formation are overlain by further formations composed of limestones, conglomerates, grit, sandstone and argillite.

In the mine area the Rosh Pinah Formation is 1220m thick. The ore zone is in the lower sections of the Formation, underlain by at least 200m of quartzites. The ore occurs just below the dark argillitic arkose and felsites which occupy the middle and upper section of the formation.

The ore bed consists of a well banded to massive carbonaceous cherty zone or micro-quartzite, in places grading into an argillite; various carbonate bearing rocks; sugary quartzite; lenses and bands of massive mixed pyrite, sphalerite and galena; argillite and intercalations of generally poorly mineralised quartzite. The microquartzites are fine grained and dark due to their carbonaceous content. Barium rich carbonate is an important constituent in places. The lower sections of the ore bed are generally Zn rich micro-quartzite, overlain by further micro-quartzites or carbonates with a higher Pb:Zn ratio, while the hangingwall is another micro-quartzite grading to argillite. Most ore is within the micro-quartzites and seldom in the argillites.

The ore minerals are generally present as intergranular disseminations and discrete blebs associated with a fine grained sugary quartz-carbonate matrix, or as thin bands from 1 mm to a few cm's thick of massive sulphide. Irregular barite-carbonate or dolomite lenses are present in the central or lower part of the ore bed.

Massive sulphide bands may be up to a few metres thick in sections of the mine within micro-quartzites and occasionally argillites, and may grade laterally into disseminated ore within the micro-quartzites or carbonates.

In contrast to the hangingwall quartzite which is generally little fractured, the footwall quartzite is intensely fractured forming a breccia, which is silicified and carries sulphide and carbonate veining.

The ore deposit comprises a number of lenses distributed over an area of some 2 x 3 km, each of 150 000 t to 2 to 3 mt. The structure is variable with orebodies ranging from steeply dipping to flat to folded.

The operation is owned (in 2001) and managed by Iscor Limited Base Metals strategic business unit and lies near the southern border of Namibia with South Africa.

Production since 1969 has been 14.5 Mt @ 7% Zn, 2% Pb, 0.1% Cu, 11 g/t Ag. In 1999 proven and probable reserves were quoted at 6.59 Mt @ 8.7% Zn, 2.5% Pb, within a resource of 15 Mt @ 7.5% Zn, 2.2% Pb.

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Field Inspection

A half field traverse of the host lithologies/sequence of the Rosh Pinah District is planned.

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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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		An AMF-PGC International Study Tour Developed & Managed by Porter GeoConsultancy Africa-B 2001 Mineral Deposits of Southern & Central Africa 10 June - 5 July 2001 - In Two Separate Modules
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