Magondi Belt - Mangula, Mhangura, Miriam, Norah, Shamrocke, Alaska, Avondale-Shackleton-Avonshack
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The Mangula (or Mhangura), Miriam, Norah, Shamrocke, Alaska and Avondale-Shackleton-Avonshack group of deposits are among a number copper mines that define the Magondi Copper Belt on the margin of the Magondi Mobile Belt in Zimbabwe (#Location: Mangula -16° 53' 16"S, 30° 9' 42"E).
The Magondi Mobile Belt outcrops in a number of windows surrounded by the overlying Phanerozoic cover, over an area of some 500 x 150 km, within north western Zimbabwe. The Palaeoproterozoic sequences within the Mobile Belt also laps onto the adjacent Zimbabwe Craton, where the Cu deposits of the Mangula (Mhangura) and Avondale/Shackleton areas ore found. Other less significant base metal occurrences are hosted by the same sequence within the mobile belt, as are large pegmatite hosted Sn deposits. This belt is older than the Zambian and Congolese copperbelts that constitute the Central African Copperbelt to the north in Zambia and the DRC, and the Kalahari Copperbelt to the SW.
The stratigraphic succession in the Magondi Belt may be summarised as follows, from the base (after Master 1991, and Leyshon and Tennick 1988):
Basement of Late Archaean granitic and greenstone belt rocks.
Magondi Supergroup, subdivided from the base into,
Dewaris Group, up to 1300 m thick - This group is not continuously developed throughout the basin, but occurs as isolated elongate cores surrounded by the overlying Lomagundi Group or Archaean basement. The succession differs in the two main areas in which it is found, the 'southern' and 'northern' outcrop areas as described below. While each includes in general a 'Lower Arenaceous', a 'Volcanic' and an 'Upper Arenaceous' Formation, which unconformably overlies an Archaean basement and are overlain by the Lomagundi Formation, it appears unlikely that the 'Volcanic Formation' in each area is directly correlatable. The two sequences are as follows:
In the Northern Outcrop Area, where it hosts the Shamrocke, Mangula/Mhangura, Silverside, Norah and Avondale-Shackleton Cu deposits, the Dewaris Group comprises, from the base:
• Mhangura Formation, up to 200 m thick - this is the basal portion of the 'Lower Arenaceous Formation' of the area and comprises a clastic sedimentary sequence consisting mainly of red trough and planar crossbedded arkoses, together with lesser conglomerates, wackes, argillites and pelitic matrix boulder conglomerates. These are arranged as two fining upwards cyclothems, each commencing with rudites/granulestones and ending with calc-argillites. These facies are interpreted as alluvial to braided stream deposits. In the far north, although the Dewaris Group is up to 2200m thick, the 'Lower Arenaceous Formation' is only thin and discontinuous, comprising conglomeratic arkoses and grits.
• Norah Formation, up to 500m thick - this is the upper section of the 'Lower Arenaceous Formation' and consists mainly of thinly bedded anhydrite bearing dolomites and argillites with interbeds of pink, graded bedded and ripple marked, arkosic, arenites. The impersistent Norah Orebody Sub-formation, which is intercalated with the dolomitic argillites, consists of pink gritty to conglomeratic trough crossbedded and plane bedded arkosic granulestone, overlain by grey, reduced, crossbedded, chloritic quartz-wackes and chlorite-calcite schists with thin interbedded evaporitic beds (anhydrite, barite, celestite, chlorite, tourmaline and copper sulphides). These sedimentary rocks are interpreted as alluvial plain and playa flat/lake deposits. The grey reduced beds also carry graphite, magnetite and pyrite. This formation is absent from the northern part of the area.
• Suiwerspruit Formation, probably 600 m thick - an approximately 130 m thick sequence of mafic pyroclastics (tuffs, volcaniclastics and agglomerates) overlain by a series of mafic lavas with thin interflow sedimentary rocks. These rocks are cut by sub volcanic dolerites to meta-gabbro sills, which have the same textural expression, suggesting that they are related. In the southern sections of this area the 'Volcanic Formation' consists of sheared, schistose, carbonated and silicified greenstones, with massive or amygdaloidal varieties, while in the northern extremities conformable units of tremolite schist up to 30 m thick are taken to be the equivalent.
• Chimsenga Formation, up to 2000 m thick - this is the 'Upper Arenaceous Formation', represented in the south and central part of the area as a poorly exposed sequence (whose thickness is unknown) of white, fine to medium grained arkoses and well bedded pelites and siltstone (metamorphosed to coarse grained slaty phyllites), overlying the Suiwerspruit Volcanics. In the northern section of the area, the 'Upper Arenaceous Formation' which may here be up to 2000 m thick, has great thickness and facies variations, and is characterised by massive to flaggy and schistose arkose, interbedded with narrow bands of grey, fine grained, massive quartzites, biotite and anthophyllite-tremolite-actinolite schists and local conglomerates.
In the Silverside area, to the east of the exposures described above and immediately adjacent to the Archaean basement, sheared tholeiitic lavas rest with pronounced angular unconformably on amphibole schists and felsic schists of the basement and are overlain by Dewaris Group conglomerates which transgress over the volcanic rocks onto the basement further south. These volcanic rocks may represent an earlier phase than the Suiwerspruit Formation.
In the Southern Outcrop Area, the Dewaris Group comprises, from the base:
• Njerere Formation, up to 20 m thick - this unit comprises the 'Lower Arenaceous Formation' in the southern outcrop area and consists of an impersistent sequence of pink, brown or red conglomerates, arkosic arenites, grits and interbedded silty mudstones. In the northern part of the outcrop area, this unit is absent and the volcanic rocks of the Munyati Formation rest directly upon basement.
• Munyati Formation, 300 to 600 m thick - this is the 'Volcanic Formation', and comprises ophitic, porphyritic non-ophitic, amygdaloidal and vesicular basalts with columnar jointing, volcanic breccia, agglomerates and a few interflow sedimentary rocks. The composition is tholeiitic to andesitic.
• Copper Pot Formation, approximately 150 m thick - commences with a purple dolomitic argillite, followed by purple (in the south and lower in the sequence) to green-grey (in the north and the upper section) medium grained graded bedded greywacke, with common intercalated argillites and lensoid conglomerates and grits. These greywackes include basal boulder and cobble conglomerates and cross bedded grits. The conglomerate clasts were derived from the Archaean basement and from the Dewaris Group lavas and are matrix supported. The unit, which is interpreted as having been deposited within an alluvial fan and braided stream environment, forms the lower section of the 'Upper Arenaceous Formation'. The Copper Pot, Nyachechene and Nyamachena Formations exhibit an upward increase in granitic components, with better sorted, more mature arkoses and a decrease in the abundance of Dewaris mafic clasts and fragments.
• Nyachechene Formation, unstated thickness - a varied succession of steel-grey to mauve to purplish-red argillites interbedded with mudstone, and greywacke type siltstones and sandstones as well as coarse grits with numerous argillite fragments. These beds exhibit complex diapiric structures, disruption of bedding and sand filled mud cracks. Fine grained, ripple marked, impure arkosic quartzites are common, while in some areas thin beds (30cm thick) of fine grained pink dolomite are interbedded with silty argillites and thin grit bands. In the south the formation only consists of grey-green and red fine grained shales and siltstones. Overall it is interpreted as having been deposited in a playa lake environment.
• Nyamachena Formation, up to 510 m thick or more - comprising a lower zone (200 m thick) of flaky bedded grey arkose, a middle zone (100 m thick) of mauve flaggy bedded gritty arkose which contains thin argillite intercalations and a few beds of pebble conglomerate. These are overlain by at least 210 m of purplish-pink, coarse grained, cross bedded arkosic grits with pebbly and cobbly horizons and thin argillite intercalations. In the northern section of the area pinkish, medium to coarse grained, massive to cross bedded, granitoid, arkosic wackes predominate. Deposition is believed to have been in a braided stream and alluvial fan environment.
Unconformity, that separates the Dewaris Group sequences from the overlying Lomagundi Group, which is also locally transgressive onto the Archaean basement. In some other places the contact is a thrust plane.
Lomagundi Group, approx. 1200 m thick - comprising from the base,
• Mcheka Formation, approx. 700 m thick - comprising basal pebbly grits up to 30m thick (ranging from a conglomerate to a grit to a schistose quartzite) derived from the Dewaris Group and the Archaean basement to the east; overlain by the lower dolomite which are up to 200 m thick (whitish-pink mottled dolomite with thin argillaceous and arenaceous interbeds and some stromatolites); sandy phyllites (pinkish-grey with a schistose, and sometimes sandy texture, occasional narrow lenses of pebble conglomerate, iron stained layers and an isolated hematitic lava lens); a thick quartzite unit (two massive quartzites separated by a pock marked quartzite with iron stained pockmarks, possibly after anhydrite or carbonate nodules); upper dolomite (with great variability in texture, crystallinity and colour, contains stromatolites, oolites, sedimentary breccia and bands of biotitic phyllites, sericitic and feldspathic grit and chloritic quartzite); sandy argillites (light coloured, fine grained, laminated to massive with local grits and both iron oxide and tourmaline accessories). Iron formation bands and lenses from 2 to 50 m wide, both oxide (magnetite-quartz) and silicate (grunerite-garnet-quartz) facies are recorded from sections of the unit.
• Nyagari Formation, approx. 400 m thick - comprises a lower sequence of striped grey and purple slates and graphitic (also often pyritic) shale with subordinate horizons of graded, upward fining sandy argillite and phyllite; intercalated with dark grey massive, poorly bedded, medium grained quartzite which often contains pyrite and clay pellets, with local coarse grained arkosic grit and associated argillite or dolomite lenses; overlain by porphyritic andesitic lavas, agglomerates and tuffs.
• Sakurgwe Formation, approx. 100 m thick - a monotonous succession of poorly bedded medium to fine grained greywackes, with occasional narrow intercalated shale or feldspathic quartzites beds.
Piriwiri Group, regarded as being a facies equivalent of the Lomagundi Group towards the basin centre, although it may alternatively conformably overlie that group, either in part or fully. It has been subdivided into,
• Umfuli Formation, approx. 1000 m thick - commencing with a very fine, soft, black graphitic shale, brecciated in places with associated stilpnomelane; followed by cream, grey and black cherty quartzites, commonly containing manganese or iron oxides and graphite, interbedded with reddish-grey to greyish-green to black manganiferous or graphitic and pyritic slates, with local lenses of thinly bedded phosphatic (collophane) shale; thin discontinuous bands of schistose conglomerate (which strongly resemble those of the Nyagari Formation of the Lomagundi Group), local grit bands and dark grey ferruginous to feldspathic quartzites are interbedded with the chert/quartzites and pyritic slates; overlain by a thicker succession of fine grained argillites, phyllites and slates with intercalated fine grained greywacke, and calcareous and feldspathic quartzites. The predominant argillaceous rocks of the upper section have a wide variation in colour, while the arenites are greyish-green and poorly sorted and exhibit load casting, flame structures and sand volcanoes.
• Chenjiri Formation, approx. 800 m thick - commences with a thin bed of pyritic slate. It comprises mainly phyllites and greywackes with minor quartzite. chert, felsite, tuffs and agglomerates. The phyllites are fine grained, greyish-green to brown, with associated black, fine grained massive or schistose graphitic rocks, which are identical to those of the Umfuli and Copper Queen Formations. The clastic textured recrystallised quartzites and grits, finer feldspathic quartzites and fine grained black or banded black and white cherts are intercalated with the argillites, and all contain variable amounts of graphite and iron oxides. A volcanic member is represented by an 85 km linear zone of agglomeratic, tuffaceous and fine grained pyritic felsites forming the keel of a doubly plunging synclinal structure. The exposures of this line of volcanic rocks appears to be partly within and at the top of the formation, and partly within the striped slates of the Mcheka Formation of the Lomagundi Group.
• Copper Queen Formation, approx. 300 m thick - consists of a monotonous sequence of fine grained greyish-green phyllites and coarse grained, vitreous, micaceous, dark grey-green, feldspathic quartzites with a ferruginous marble (tremolite, actinolite, ankerite and calcite) near the base.
Makuti-Rushinga Group - paragneiss, meta-sedimentary rocks and amphibolites.
Tengwe River Group - limestone, dolomite and orthoquartzite.
Sijarira - red grits, sandstones, shales and conglomerates.
Permian to Jurassic Karoo System - Permian glacial beds, mudstone, coal measures and sandstones; Triassic grits sandstones and silstones; Jurassic basalt.
Age dating suggests that the Magondi Supergroup was deposited between 2150 Ma (from a dating of the Dewaris Group lavas) to 2000 Ma (from granitic rocks cutting the Lomagundi-Piriwiri Groups). Master (1991) estimates deposition between 2160 and 2100 Ma. This was during the early stages of the Eburnian tectonic event (2200 to 1800 Ma). It appears that the Dewaris Group represents rift phase continental deposition developed in an initial broad subsidence stage, followed by the bulk of the group being laid down in a series of narrow fault bounded graben/half grabens (hence the variability in continuity of the sequence) as extensional tectonic development proceeded, associated with listric faulting. Widespread mafic volcanism was probably related to this rifting and is largely concentrated along the eastern margin of the basin of deposition as volcanic rocks and dykes. This rifting may have represented the margins of a Lower to Middle Proterozoic precursor of the Damaran/Katangan Rift Zone, with the Lower Proterozoic sequences in Zambia and Malawi and the reworked Lower Proterozoic metamorphics in Zambia, Malawi and Mozambique being equivalents of the Magondi Supergroup in the same or related rifts.
The rifting was followed by the onset of the sag (or thermal subsidence) phase, with the deposition of the Lomagundi Group on the margins of the Zimbabwe Craton, over stepping the Dewaris Group in places, and the Piriwiri Group in the main mobile belt/rift basin.
The sequence has been subjected to at least three stages of deformation, including the later stages of the Eburnian (2000 to 1800 Ma), the Kibaran and the Pan African. Deformation and metamorphism is less intense where the sequence laps onto the Zimbabwe Craton, but increases in degree to the northwest, such that the Piriwiri Group is of a higher metamorphic grade than the Lomagundi and Dewaris (previously used as an argument to suggest it was older). Metamorphic rocks within the Piriwiri Group in this zone grade from garnet-mica schists, to staurolite schist, to staurolite-kyanite schist, to sillimanite schist to sillimanite gneiss. Granitic rocks and pegmatites within this belt are dated as Pan African (400 to 650 Ma). Further to the SE, also within Piriwiri Group there is a NE-SW trending zone of metamorphism forming garnet-mica schists in the Copper Queen and Copper King belt, accompanied by a string of elongate granite gneiss domes. The metamorphics of the Dete-Kamativi Inlier are of a similar high grade, ranging from muscovite and biotite schists to garnet gneisses, with 2000 Ma to 1900 Ma granitoids and pegmatoids (some dated at 1600 to 1900, and 970 Ma).
The most intense deformation and metamorphism affecting the Magondi Supergroup was during the first stage (i.e. 'Eburnian'), producing both folding and the development of a series of NW dipping thrusts slices, nappes and klippes. Thrust belts are well developed along the southeastern margins of the sequence, often with thrust contacts between the Magondi Supergroup and Archaean basement. Thrusting has also led to differences in tectonic style, with in some areas poorly deformed packages being preserved in thrust slices, as at Avondale and Shackleton, while not far distant the same rocks are more deformed and metamorphosed as at Mangula. Further to the north and northwest the younger Upper Proterozoic sequences have been more intensely metamorphosed by the Pan Africa metamorphism of the Zambezi Mobile and in association with the major structural dislocation on the southeastern margin of the Damaran/Katangan Rift zone.
Introduction - The Mangula orebody, which is the largest of the copper deposits of the Magondi Supergroup, has been exploited as part of the Mhangura Mining Complex, and is some 140 km to the north west of Harare in Zimbabwe. Historically the mine and its sections have been referred to as the Mangula, Miriam-Mangula, Miriam and Molly mines.
Unlike some of the other deposits in the area, Mangula was not marked by ancient workings. Initial exploration drilling and trenching was undertaken in the area in 1930 by Anglo American Corporation, who were looking for a continuation of the Zambian Copper Belt in rocks at that stage correlated with the Zambian Copperbelt hosts. Two diamond holes were drilled. The first cut two zones of mineralisation and stopped in a third, which assayed 2.75% Cu, reputedly due to loss of water. The second hole encountered three bands of mineralisation which totalled 60 m cumulative thickness @ 2.2% Cu. These results were regarded as discouraging, bearing in mind the then low copper price, and the project was abandoned. In 1947, Rhodesian Copper Ventures was formed to explore for copper in the Mhangura area. In 1949 Messina (Transvaal) Development Co Ltd acquired a minor interest in the project. A program of more than 30 drill holes totalling 12 000 m of drilling was undertaken, spread over a strike length of 3 km, but mainly concentrated in the Molly North section. The results of this work were inconclusive and the project was placed on a caretaker basis in 1953. Subsequent detailed work by Messina involving a further 6000 m of drilling and significant underground development, indicated viable copper mineralisation. As a consequence the company acquired a controlling interest in the project in 1954 and assumed management responsibility. In 1955 plans were initiated for a production of 3000 tonnes of ore per day by 1959, and in 1956 the name of the company was changed to MTD (Mhangura) Ltd. In 1957, 18 months ahead of schedule the mine commenced operations. Since 1985 the company has operated as Mhangura Copper Mines Limited, a subsidiary of the state owned Zimbabwe Mining Development Corporation.
The Mangula mine closed in 2007, but has been the subject of plans to reopen in recent years.
The original pre-mining orebody is believed to have totalled some - 60 Mt @ 1.2% Cu, 20 g/t Ag (Maiden et al., 1984). Other smaller deposits mined have included Norah which is adjacent to Mangula (10 Mt @ 1 to 1.5% Cu), Alaska (4 Mt @ 1.4% Cu) and Shamrocke (possibly 10 Mt @ 1.5% Cu).
Jacobsen (1964) reports that the concentrates contained 430 g/t Ag, 2 g/t Au, 0.5 g/t Pt, 1 g/t Pd, 300 ppm Bi, 300 ppm Pb, 'nil' Zn, 'nil' ppm Co, 'nil' to trace As, 'nil' Sb, 100 ppm Se, 'nil' to trace Ni, 100 ppm P and trace to 'nil' Sn. Master (1992) quotes 0.4% MoS2 in concentrates.
Geology - The Mangula Orebody is hosted by the Mhangura Formation of the Lower Proterozoic Dewaris Group, which is in turn a member of the Magondi Supergroup. The orebody is located on the eastern rim of the Lomagundi Basin where the Mhangura Formation laps onto late Archaean metamorphics of the Zimbabwe Craton. This location also corresponds with the eastern margin of the Magondi Mobile Belt. The basement in this area is composed of granitic gneiss of the 'Doma Batholith' which formed a resistant rise protruding into the Lomagundi Basin. This basement ridge forms the core of an anticlinal structure over which the arenites and rudites of the lower Mhangura Formation are draped, dipping down to the west, southwest and south and apparently being overstepped by younger argillites, calc-argillites and lesser dolomites to the northeast.
The host sequence within the mine leases is as follows, from the base,
Archaean Basement - in the mine area this comprises the Mangula Granite, a pink, medium grained, massive and equigranular rock, very poor in mafic minerals. It comprises mainly quartz, albite and microcline with hematite coating the grains and as an accessory.
Unconformity, evident in places underground and at the surface. Over much of the surface exposure, the contact is a shear zone occupied by a sericitic-quartz schist with associated quartz blows. There is a complete gradation from undeformed granite through sheared granite to quartz-sericite schists on the eastern margin. There are zones within the schist, which consist predominantly of vein quartz with a few sericite stringers. The foliation in the schist dips steeply to the east towards the Mangula Granite and there is a strong down dip quartz mineral stretching lineation. Well developed mylonitic 'S-C' fabrics indicate a thrust movement and sinistral shear, while thin quartz veins are developed into sheath folds. Thin pyritic dolerite dyke have followed this shear zone and are in part also sheared.
Basal Conglomerate up to 1 m thick - a thin and impersistent 'unit', mainly seen underground where the Mhangura Formation overlies the granite basement, without the intervening shear, and at a single outcrop. The contact is relatively clean with slab like fragments of 'exfoliating' granite at the immediate contact separated from the basement by a thin silty matrix. This is followed by pebbles to bounders of rounded to angular granite within a silty matrix. There is controversy as whether this is an intrusive or sedimentary contact. The overall opinion gained from inspection of several sites is of the latter. The granite fragments and slabs appear to be isolated and too irregular to be intrusive, with very sharp margins with no chilling. On the other hand there is no apparent palaeo-weathering on the basement surface.
Arkose, up to 200 m thick - the chief host rock to copper mineralisation at Mangula is a fine grained, well jointed, hard, splintery, pinkish, metamorphosed arkose of the Mhangura Formation. The arkose unit comprises two upward fining cycles of conglomerates to arkose to semi-pelitic schists. To the south the lower cycle laps onto the granite basement and only the upper cycle is present.
The hard arkose unit tends to form the high ground on the mine leases, and is flanked to the north and south by softer arkose and to the west by strongly foliated overlying chloritic arkose and schists.
The host arkose consists essentially of quartz, microcline and plagioclase. Magnetite is always present, averaging 1% or more, as is common within the Dewaris Group. Accessory minerals are chlorite, muscovite and biotite, which appear to result from modification of the original constituents of the rock. Within the ore zone calcite, apatite, sphene and megacrysts of magnetite occur, particularly where there are pegmatite and chlorite veins. The fine hard arkose is sometimes banded where its magnetite content has been concentrated into fine bands/laminae of minute grains. The banding dips steeply and is seldom contorted.
The conglomerate bands within each of the cycles are generally lenticular and have associated grits. They usually occur as intermixed conglomeratic to pebbly arkoses with subordinate feldspathic and micaceous sandstones/quartzites. The conglomerates have a similar matrix to the arkose with pebbles and cobbles chiefly of granite. The schists are mainly a dark well foliated chlorite-sericite-quartz schists which are carbonatic in part.
A traverse through the upper cycle from the capping schists of the lower cycle, commences with bedded to cross bedded pink arkose with occasional thin pelitic schist bands and occasional isolated to discontinuous layers of rounded 1 to 2 cm pink granite pebbles with minor grey-green pelitic schist pebbles. This then passes upwards into more even grained banded arkose. The banding in the arkose is seen through the pinkish hematite alteration of the magnetite of the arkose, giving alternating grey-green and pink layers. This banding shows structures interpreted as cross bedding, trough cross bedding and planar bedding. Towards the top of the cycle, bands of schist increase in frequency with schists separated by a few cm's to a few metres of arkose, then to a finely interbanded schist and arkose alternating on a 1 to 5 mm basis. Both lithologies are grey and are hard to differentiate. Where the arkose and schist are interbedded carbonate cement and veining is common, mainly as calcite.
In places within the arkose there are silt-arkose breccias, where ragged randomly oriented clasts of black siltstone are set in a massive matrix of fine pink arkose. Away from the breccias, which only persist for a few metres along strike, the siltstone band becomes continuous and the banding of the arkose resumes. This infers that pore water within the arkose formed a slurry in that rock type, breaching and brecciating the lithified siltstone.
The arkose is believed to have been derived from the Archaean granites that form the basement. This is supported by the composition of the arkose and the presence of identical granite clasts in the interbedded conglomeratic layers. A number of substantial quartz veins, up to 5 km long by hundreds of metres in width, within the granite carry strong magnetite, a characteristic component of the arkose. In contrast, the equivalent rocks in the Avondale-Shackleton area to the south have a different character. These only have Archaean Greenstone basement in the hinterland.
Chlorite-Carbonate Schist, 0 to 25 m thick - occurs towards the top of the second cycle of the arkose unit. It is generally a lensoid, complexly deformed chlorite-sericite-carbonate schist with local quartzite horizons and is found in association with the West and Far West Orebodies.
Upper Schists and Arkoses - comprises a lower speckled to streaky calcitic sericitic and chloritic schist, followed by interbanded schistose arkoses, quartzites, chloritic and sericitic schists, minor conglomerates and a few dolomite/carbonate schist bands. In contrast to the arkose unit these meta-sedimentary rocks are generally complexly deformed and contorted.
The host sequence has been interpreted as having been deposited in a braided stream to sub aqueous environment. The overlying Norah Formation includes evaporite bands.
Substantial dykes of epi-diorite, a local term for meta-dolerite, cut the ore zone, both within the shear zone that separates the Archaean granite of the basement from the host sequence, and within the country rock further to the west, semi-parallel to that shear. In places these dykes form a wall to the ore and have assimilated it or been mineralised. While intruding the shear zone, they have been in part sheared themselves, and are folded with the Dewaris Group.
Structure - The dominant structural direction in the Mangula Mine area is north-south, including the thrust/shear along which the Mangula Granite is overthrust onto the Magondi Supergroup. This shear does not parallel the contact exactly, so that to the north it is entirely within the granite and the sedimentary contact is observable at surface.
Folding also follows this direction, with a tight anticline over the orebody and a tighter syncline to the east before the basement. Part of the eastern synclinal limb is truncated by the thrust/shear. A further syncline and anticline are mapped to the west of the orebody. The main anticlinal axis over the orebody doubly plunges, forming an elongated domal structure centred on the main orebody.
Within the orebody there is a network of anastomosing, generally steeply west dipping north-south shears and faults that cut the host rocks and ore. These shears become more intensely deformed and conspicuous with depth. In the southernmost part of the mine the downward extension of these shear zones tends to converge with the western flank of the basement granite. Although they displace the host rocks and sometimes form the margins of the ore, they dont necessarily always displace the ore. They also often form the lower extensions of the orebodies and contain carbonate-quartz-feldspar veinlets associated with coarse sulphide mineralisation. This set of shears may form a slippage zones caused by the over riding of the basin sediments during folding against the irregular surface of the resistant granite basement (Machiri 1990).
Where traversing the arkoses these shears and faults are represented by increasing chloritisation and sericitisation until the rock becomes densely schistose. Brecciated veins, pressure lenses and tortuous blows of coarse quartz and pink to blood red feldspar and occasional calcite are often seen in these structures. Minor shear zone of this type often display little displacement, although major ones, showing in place strong brecciation, with fragments of arkose cemented by quartz may offset the geology by up to 50m or more. These shears appear to be contemporaneous with the folding (Jacobsen 1964).
A further younger set of shears are obvious within the mine area, striking NW-SE and dipping to the NE, cutting the mine area over a width of 150 to 200 m. These structures have a thrust and a dextral component.
Four phases of deformation are recognised at Mhangura. The first two are Eburnian (2000 to 1800 Ma) which accompanied the main metamorphism which is greenschist facies at a maximum temperature of 420°. The remaining deformation was Kibaran (1200 to 1000 Ma) and Pan African (650 to 500 Ma).
Distribution of Ore - Ore mineralisation occurs in a range of rock types distributed over the 200 m thickness of the Mhangura Formation. It has been divided into eight sub-parallel orebodies which are separated by sub-grade mineralisation or barren zones and extend over a strike length of 2.25 km. The main mineralisation appears to have been preferentially concentrated in antiformal and synformal folds in the upper levels of the mine, although most peter out down dip on the limbs. This gives rise locally to good thicknesses of ore as 'saddle like' bodies in the upper parts of the mine and transfer down dip to steep limbs which tend to thin with depth to form numerous stringers of ore. Most of the orebodies coalesce at depth and extend down dip for about 900 m. The exceptions are the two prominent western ones, the West and Middle Orebodies which are persistent in both strike and depth, but combine below the 16 level to form the Combined Orebody comprising a broad zone of weak mineralisation with local high grade bands. Locally in the south, one of the eastern orebodies extends downwards following around the curve of the underlying granite basement to coalesce with the Combined Orebody where it is coincident with a prominent sheared zone marginal to the granite contact..
In general the eight orebodies show an en echelon distribution, diverging northwards from the eastern shear zone and dyke in the extreme south at roughly 15° to the dyke. It has been suggested that this en echelon pattern is the reflection of the onlapping nature of the arkose unit to the south, with the lower and progressively higher mineralised positions being terminated against the unconformity towards the south.
The western most orebody, the Far West, occurs in the fringe of the hangingwall mantle of sericite/chlorite schist, which locally seems to have formed a shear zone of bedding dislocation during folding.
While there is a strong structural influence on the distribution of ore, there is similarly a marked lithological control, generally being tabular and overall, concordant. Although mineralisation is found in most lithologies, many of the orebodies are formed in association with, and follow, particular rock types. Specifically, good grades are always present at the contact of finer pelitic beds within the coarser arkoses, particularly below thicker developments of such finer intercalations and in zones of intercalated arkose and pelitic schist which occur in these positions.
Significant orebodies are developed near the top of each of the two fining upwards cycles of the Mhangura Formation, generally in the upper part of the arkoses and the lower sections of the overlying schistose bands. All of the ore is within the arkose or the lowermost sections of the overlying capping 'schist' sequence.
These relationships, and those detailed in the 'Structure' section above infer that the mineralisation is stratabound, but has been substantially remobilised by later metamorphism. The Avondale/Shackleton orebodies to the south (see below) are found in the same sequence which is far less deformed and metamorphosed. In these the stratigraphic control is paramount, with virtually no apparent structural modification. It is reasonable to suggest that Mangula was initially similar to these deposits, but the differences are the result of the deformation, the other main difference between the two examples.
Alteration - Alteration in and around the orebodies is characterised by the development of quartz, microcline and hematite. A major zone of intense alteration is found in arkosic beds between the two largest orebodies. In this zone sedimentary features such as cross bedding are completely destroyed and replaced by generally concordant alternating 1 to 20cm thick bands of hematite stained and unaltered magnetite bearing arkose. The margins of the pink hematised bands are generally diffuse, grading into a pale buff, very quartzose halo 5 to 20 mm thick, which passes in turn into dense orange arkose which has a dark grey to black dispersed colouration and/or banding due to its contained magnetite. In the areas of less developed alteration the ends of these magnetite rich (ie. unaltered) bands have a rounded nose. As the alteration proceeds, the hematisation breeches the magnetite bands, resulting in isolated lozenges of unaltered arkose. The final product is a series of aligned ellipsoids of unaltered magnetite bearing arkose, each with a narrow diffuse margin, set in a mass of featureless hematised pink arkose. These bands and ellipsoids are frequently transgressive in detail.
There is a suggestion that the orientation of the long axes of these ellipsoids is parallel to the stretching direction of deformation. This has been taken to infer that they are pre-folding and pre-metamorphism.
As ore is approached the magnetite in the ellipsoids becomes chalcocite.
The hematisation is accompanied by silicification and microcline (i.e. potassic alteration) development within the arkose. This style of alteration accounts for the resistant nature of the host rocks and their preservation as a positive topographic feature.
Locally where rocks of this the sequence are cut by faults within the mine they are generally more strongly altered.
It is believed that the hematisation of the magnetite is the result of oxidised, potassic, cupriferous brines being reduced by the magnetite (or pre-metamorphism pyrite) bearing arkoses, which resulted in the decrease in the Cu carrying capacity of the brines, the subsequent precipitation of copper to form the orebodies and the oxidation of the magnetite/pyrite to hematite in the footwall.
Ore Mineralogy and Form - Copper mineralisation is present in a number of forms, including,
i). Even, banded or cloudy disseminations in arkose and schist, accounting for the bulk of the ore.
ii). Replacement of detrital iron-titanium oxides on crossbed foreset laminae.
iii). Quartz-microcline-sulphide (-hematite-carbonate) veins occupying brittle fractures in competent lithologies of the shear zones. Mineralisation is erratically distributed as coarse aggregates and veinlets. This style of mineralisation is pegmatitic and present both in the sedimentary rocks and the basement granite, generally adjacent to sheared contacts, but also at the normal unconformity. In an example of the latter case that was visited brittle fractures extended 1 to 2 m into the granite near a lamprophyre dyke. Both the granite and lamprophyre are altered. The veins were seen to be very irregular to isolated islands in a face, coarsely crystalline with quartz, microcline, bornite and chalcopyrite. The mineralogy changed progressively from bornite near the sediment contact to chalcopyrite at the extremities of the vein, furthest from the sedimentary rocks.
iv). Cleavage parallel quartz-microcline-sulphide veins in semi-pelitic schists.
In general the only sulphide minerals present within the ore are the copper sulphides. As with most other sediment hosted copper deposits, there is a zonation from outer pyritic zones to chalcopyrite, then through bornite and chalcocite to small cores of native copper. While the better grade generally corresponds to the higher Cu minerals, the zonation does not necessarily parallel the ore grade boundaries. Low grade zone may also be within the chalcocite zone.
The main minerals occur as follows (after Master 1991),
• Chalcocite - occurs as disseminated irregular grains, most abundant in the central Cu rich parts of the orebodies. In the East, Molly South and East Plate Orebodies chalcocite is the chief Cu mineral, while it is second in abundance in the West and Far West Orebodies. It is commonly intergrown with bornite. In the upper part of the orebodies some masses of supergene chalcocite are found.
• Bornite - is the most important sulphide after chalcocite. It occurs as disseminated irregular grains which are generally interstitial to silicate grains; as stringers along schistosity planes, cracks and grain boundaries; and as coarse blebs in veins and on faults. It is commonly found as myrmetitic intergrowths with chalcocite. On the fringes of the orebodies where chalcopyrite appears, the two may be intergrown, usually with a mutual boundary texture. In some cases there are exsolution lamellae of chalcopyrite within bornite grains. Bornite commonly replaces Fe oxides, especially magnetite grains, which are concentrated on crossbedding foresets.
• Chalcopyrite - is a minor Cu sulphide at Mangula, mainly disseminated on the margins of the orebodies and commonly occurs intergrown with bornite on the fringes of the ore. Where present it occurs in interstitial positions and on grain boundaries between silicate minerals. Chalcopyrite only occurs on its own on the 1 to 2 m thick chalcopyrite zones fringing the orebodies. On the outer edges of this fringe it grades into a chalcopyrite-pyrite zone where the two minerals coexist in a mutual boundary texture.
• Pyrite - occurs as fine, sparsely disseminated grains in a broad envelope surrounding the ore zone at Mangula.
• Molybdenite - is a common accessory of the Cu mineralisation.
• Pyrrhotite - is very rare, and almost exclusively within the meta-dolerite dykes.
• Native Copper - is very rare, generally found within the core of the chalcocite zone.
• Native Silver - is the main form of Ag at Mangula, occurring chiefly as tiny (a few microns) rounded blebs within chalcocite and bornite and occasionally within and surrounding magnetite grains. The magnetic concentrate at Mangula has high Ag values, up to 24 g/t Ag.
• Native Gold - only one sample exhibited any gold. The mineral was present as a tiny inclusion in an euhedral grain of magnetite, accompanied by native Ag. The magnetic concentrate at Mangula assays 0.2 to 0.5 g/t Au.
• Magnetite - is a common mineral, present as,
i). Rounded, 0.5 to 1mm detrital grains, disseminated in varying amounts throughout the un-mineralised arkose. However where the same rocks are mineralised, the Cu sulphides are usually clustered around much smaller grains of magnetite up to 0.1mm in diameter. The sulphides frequently invade the magnetite grains along cracks or form interlacing stringers of magnetite crystals and bornite which surround grains of quartz and feldspar, invading larger grains of these minerals in fine cracks, sometimes as mere stringers of fine dust. Pseudomorphic replacement of larger rounded magnetite grains has been observed sporadically (Jacobsen 1964).
ii). Octahedra of metamorphic origin in schists and shear zones;
iii). Fine (0.03 to 0.1mm) groundmass martised magnetite (intergrown with and replaced by hematite) disseminated in dark laminated bands. This style is only present in restricted areas, generally in more structurally compacted zones, and are not persistent in strike. The best developments are to the west just below the sericitic and chloritic schist mantle capping the arkoses. The proportion of dark magnetite rich layers increases downwards (to the east) across the gradational contact until well defined alternating dense pink (hematitic) and dark grey to black (magnetitic) bands 5 to 10 mm thick are evident. From this position there is a gradual decrease in the intensity of banding to a dense widely banded arkose with pale grey magnetitic bands. This mode then gives way to the ellipsoids of the type under the next point and described in the 'Alteration' section above.
iv). Similar fine groundmass martised magnetite within ellipsoids surrounded by oxidised arkoses. The ellipsoids and associated gradational bands are apparently remnants left after pervasive oxidation as described in the 'Alteration' section above.
According to Jacobsen (1964) there is close a relationship between the magnetite and Cu contents within the orebody, specifically with respect to the disseminated and laminated magnetite zones. In general the magnetite bearing ellipsoids are barren, except in the northern section of the mine. The coarse octahedra of magnetite have no associated Cu, although the shears and schists in which they occur may carry high Cu grades.
• Hematite - is common and ubiquitous in the metamorphics at Mangula. It occurs as a very fine grained dissemination, as intergranular cement and in tiny cracks in silicate grains. It commonly occurs as a martite replacement of magnetite.
• Uraninite - is present as fine disseminations in sedimentary rocks independent of Cu minerals, particularly within the magnetite ellipsoids and in the arkoses below the orebody. The U3O8 level is almost invariably <100 ppm, generally 1 or 2, and up to 20 ppm.
• Chromite and PGE are very rare. It has been suggested that the provenance is from the Great Dyke some 75 km to the east which carries substantial Cr and PGE. The arkose has detrital chromite which in some examples are replaced on their outer margins by magnetite, which is in turn replaced by Cu sulphides (Master 1992).
• Other heavy minerals include sphene, the commonest Ti bearing mineral in both the meta-sedimentary and meta-igneous rocks; zircon which is a common detrital mineral; apatite which occurs as sub-rounded detrital grains in the arkoses; and rutile which is a rare replacement of Fe-Ti minerals.
• Dolomite which is mainly found in the hangingwall of the Far West Orebody near the top of the upper Mangula cyclothem, where thin beds of pink dolomite have been formed. The grains are xenoblastic with serrated boundaries which show deformation lamellae and annealed subgrains.
• Anhydrite is very rare in the Mangula, only occurring as a cement within the arkoses associated with the Far West Orebody. It also is found as purple crystalline anhydrite in veins between the Far West and West orebodies and at Norah.
The distribution and occurrence of the various minerals, the alteration and relationship to structure suggest that the Cu mineralisation was introduced by oxidised, potassic, cupriferous brines subsequent to deposition. These brines were reduced by the magnetite or possibly pyrite produced from an earlier phase of sulphidation from reaction with sulphate rich brines. An alternative would be the encounter of the cupriferous brines and sulphate rich ground water related to the evaporite beds in the overlying Norah Formation. Upon reduction, resulting in the hematisation of magnetite and pyrite within the arkose, the copper carrying capacity of the brines would have been substantially lowered, leading to the deposition of Cu sulphides, nucleated on the original magnetite. Deformation and shearing during the Eburnian, and to a lesser extent in the Kibaran and Pan African events resulted in structural remobilisation of mineralisation into shear zones.
Surface and Geophysical Expression - Surface showings of copper mineralisation are known over a strike length of 3 km, with the payable ore being within the central 2km. While mineralisation is obvious on the slopes of the two main hills of silicified arkose, mining has shown the best ore to be under the flats between these two topographic highs on the northern and southern lateral strike flanks (Jacobsen 1964).
The depth of weathering at Mangula is very variable, ranging from rare sulphides in outcrop to oxidation to depths of as much as 200 m below the surface, but in general is around 50 m. Within the oxidised zone the mineralisation is present as malachite and lesser chrysocolla, disseminated throughout the arkose as films and streaks on joint and fracture planes and cementing breccias of the larger faults. Where the oxidation is complete there does not appear to have been much leaching with only minor zones of secondary enrichment which are restricted to the less compact Western Orebody. In general the oxides and sulphide zone grades are comparable. Oxidation is present to depths of up to 40 m below surface. The lack of secondary enrichment is taken to be a reflection of the hard, siliceous, compact and low sulphide character of the ore.
Avondale, Shackleton and Avonshack Deposits
Introduction - The Avondale, Shackleton and Avonshack orebodies have been mined by Lomagundi Smelting and Mining (Pvt) Ltd, a subsidiary of Zimbabwe Mining Development Corporation. These operations, with the old Alaska mine and the Angwa and Hans orebodies, are centred on the town of Alaska, some 50 km to the south of Mhangura in north western Zimbabwe. Lomagundi Smelting and Mining also operates a refinery with a capacity of 20 000 tpy of Cu and a smelter, both located at Alaska.
While the old Alaska deposit had been worked by the 'Ancients' with around 1mt of ore having been extracted prior to the arrival Europeans, the Avondale and Shackleton deposits were unknown. Alaska is hosted by sheared carbonates, while Avondale and Shackleton are within silicate arkoses and siltstones.
Avondale and Shackleton, which are virtually blind, were discovered as a result of an extensive geochemical soil sampling program and drilling follow up, conducted by Rhodesian Selection Trust Ltd in the late 1950's and early 1960's. The deposits were acquired by MTD (Mhangura) Ltd in 1964 when Rhodesian Selection Trust withdrew from the then Rhodesia (Zimbabwe). The Shackleton mine was opened in 1971. Ownership passed to the Zimbabwe Mining Development Corporation in 1985.
The proven reserves and historic production at Shackleton, Avondale and the nearby Angwa deposits is stated at around 10 Mt with probable and possible resources of another 20 Mt. Angwa contributes around 0.5 Mt to the total. This would result in an overall resource of,
30 Mt @ 1.1 to 1.5% Cu, 10 to 20 g/t Ag.
Geology and Structure - The Avondale, Shackleton and Avonshack orebodies are hosted by two upward fining cycles of clastic sedimentary rocks which are believed to belong to the Copper Pot Formation of the Lower Proterozoic Deweras Group. The Copper Pot Formation is found in the southern outcrop belt of the Deweras Group in the Lomagundi Basin.
While the Avondale and Avonshack mineralisation is hosted by the lower cycle sedimentary rocks, the Shackleton orebody is in the upper cycle. All three are elongate stratabound bodies, each composed of a number of lenses, stacked one above the other in the crests of two gently south west plunging anticlines. The Shackleton orebody is approximately 350 m directly above Avonshack, in the upper cycle, centred on the intersection of the same anticline axial plane where it cuts the host lithologies of the respective cycle.
The orebodies continue down plunge to the SW until they intersect a NE dipping wrench fault zone, now occupied by an epi-diorite dyke. This fault zone dips at around 60° to the NE.
Unlike Mangula (see above), the hosts at these deposits are not lapping onto basement, which is not seen in the mine area. The rudaceous rocks in each of the cycles contain mainly rip up clasts of argillites and greenstones of the Archaean basement. In the southern outcrop area, the principal basement, which is found to the east, is made up of Archaean greenstones, in contrast to the granites of the northern outcrop area.
Each of the cycles comprises a basal rudaceous to arenite zone that grades up through finer arkose and sandstones to a thick upper zone of argillites and calc-argillites that have sharp unconformable contacts with the coarse basal clastic rocks of the next cycle. The arkoses are generally a fine to medium grained pink arkose, similar to that at Mangula, with a vitreous, siliceous texture. The calc-argillites comprises interbedded pale grey carbonate beds 5 to 20 mm thick with thinner dark laminae and thin arkose intercalations which contain rip-up clasts of the adjacent finer sedimentary rocks. The bedding is disrupted by micro-faults, and occasional, wavy, irregular anhydrite bands up to 10 mm thick are observable.
In the lower cycle, the ore zone comprises a sequence of intercalated arkoses and subordinate shales to siltstones that are confined between the distinct and persistent Upper and Lower Shale Marker Horizons. In the upper cycle there are 16 individual small scale cycles within the ore zone, each with sharp contacts. Of these, the lowest three, which overlie a persistent large scale cross bedded sandstone, host the bulk of the ore at Shackleton.
The basal conglomerates are commonly polymictic and usually persistent, whereas the arenites and argillites higher in the cycle pinch and swell to varying degrees. The argillites commonly only persisting for 1 to 2m along strike, being truncated by channels filled with cross bedded sandstone and rip-up clasts of the argillites cut be the channel. These features suggest deposition as braided streams.
Distribution of Ore - As described above the orebodies have both a stratigraphic and structural control. They are confined to particular stratigraphic units within the crest zones of shallowly plunging anticlines, up dip from the intersection with a significant wrench fault. The ore is hosted by the 'dirtier' arkosic rocks which are enclosed by argillites. Ore commonly also occurs within the capping argillites, where grades are higher at Shackleton, decreasing downwards through the arkose and even to the basal conglomerate.
The Avondale deposit consists of a single body that is locally overlain by a number of small discontinuous 'saddle lodes' in favourable anticlinal structures. Shackleton in contrast comprises a complex arrangement of multiple stacked orebodies within the main shoot, reflecting the multiple coarse to fine mini-cycles within the main upper cycle. Note the influence of secondary cross cutting axes which formed local domal structures. Individual ore lenses range from 1 to 4 or 5 m in thickness.
The Shackleton deposit is interrupted to the south west by the Shackleton Dyke which fills the main fault. In this area there are a larger number ore lenses, most of which lens out up dip, with only a few persisting to the surface. To the SW of the dyke/fault zone there are a number of thicker, stratabound pods within the down dip extensions of the favourable hosts. These bodies however have much smaller lateral dimension away from the dyke/fault compared to those up-dip.
The better developed and most extensive ore lenses generally appear to be associated with the mini-cycles with the thicker and more persistent conglomerates (Newham 1986).
The copper sulphides are generally disseminated, but occur locally as seams, streaks and as veinlets in the more sheared areas. Locally thin dolomitic bands in the hangingwall contact of the individual ore lenses show a pronounced concentration of sulphides along their lower contacts, but are virtually barren on their upper margins.
Locally high grade mineralisation shows a distinct preference for argillite contacts, being concentrated along the margins of argillite interbeds and forming rims of dense chalcocite on the fringes of rip-up argillite clasts within the arenites. In examples sighted underground 1 to 5 cm thick grey argillite bands were seen to be rimmed by chalcocite bands several mm's thick, while whispy argillite clasts spread through the ore bearing arkose were surrounded by similar rims.
At Shackleton there is a distinct increase in grade laterally towards the Shackleton Dyke/Fault zone, with grades ranging from 1% Cu and 0.1 g/t Au up dip, to 4% Cu and 3 g/t Au adjacent to the fault.
Alteration and Mineralogy - A variable red hematite alteration is associated with most of the individual orebodies, and commonly constitutes the up-dip, tapering, barren extension of individual orebodies. However towards the dyke/fault zone the intensity of this alteration increases dramatically and commonly spreads to pervade the lithologies between the orebodies also. This alteration predates the emplacement of the Shackleton Dyke, which is not altered, but contains slivers of ferruginised and locally mineralised arkose within its sheared margins.
At Shackleton there is a reciprocal relationship between the K and Na contents of the host rocks, with Fe roughly paralleling the K, but at a lower level. These relationships are reflected petrographically by the presence of authigenic overgrowths of microcline and clear perthitic alkali feldspar on the detrital cloudy plagioclase. Hematite is significantly concentrated in the ore zone, where it constitutes up to 5% of the rock and is present as disseminated flakes that commonly occur within the authigenic feldspar overgrowths as well as locally enclosing them as armour films. The hematite is frequently intimately associated with the copper sulphides, which exhibit similar relationships to the feldspar overgrowths. It is significant that in the zones studied the Cu is concentrated above permeable breccia and conglomerate zones rather than showing a sympathetic association with the K or Fe contents. The above is largely taken from Newham (1986).
On the faces sighted underground there was a clear association between hematite and Cu sulphides, with the lower Fe sulphides being accompanied by more intense hematite development. For example, the upper chalcocite-bornite zones are accompanied by strong hematite, while the underlying chalcopyrite zone is noticeably less ferruginised. The boundary between the two zones is often marked by an irregular red Fe front from a few mm's to 3 cm thick.
At Shackleton there is a pronounced zonation of sulphides away from the Shackleton Dyke/Fault zone, grading from hematite, through chalcopyrite, bornite and chalcocite (Newham 1986). This statement is somewhat ambiguous. However as the grade increases towards the fault it is assumed that chalcocite with associated hematite predominates there, with chalcopyrite being furthest removed. There is also a bilateral zonation within individual orebodies, with increasing iron content of sulphides outwards.
The observations recorded in the 'Geology and Structure', 'Distribution of Ore' and the 'Alteration and Mineralogy' sections above suggest that the ore was emplaced in these deposits subsequent to the deposition of the host sedimentary rocks, but prior to their loss of permeability. It would seem logical to suggest that the mineralisation emanated from the Shackleton Fault zone, prior to the emplacement of the Dyke and travelled up the crests of anticlines, channelled through coarser clastic rocks, but contained between impervious argillites. The ore bearing medium was probably oxidised and K rich, reacting with the country rocks to produce authigenic K-feldspar overgrowths and hematisation. The reduction of the medium would have released Cu, progressively growing from chalcopyrite to chalcocite. As this process progressed hematite would have been produced as observed. The dyke appears to have been emplaced in the pre existing channelway of the Shackleton Fault.
The most recent source geological information used to prepare this summary was dated: 1992.
This description is a summary from published sources, the chief of which are listed below.
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