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Nampundwe |
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Zambia |
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S Cu
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Nampundwe (previously the King Edward) pyrite-copper deposit is located ~45 km WSW of Lusaka in Zambia, and is ~300 km SSW of Ndola in the Zambian Copper Belt (#Location: 15° 29' 31"S, 27° 54' 35"E).
Ancient workings were known at Nampundwe prior to the first significant copper production during 1913 and 14 as the King Edward mine. After various other small-scale mining activity, the main Nampundwe mine commenced operations in 1970 to produce a bulk concentrate of pyrite. That concentrate is trucked to Lusaka, where part is used in a local fertiliser plant, while the balance, including the cupriferous pyrite concentrate, is railed north to the Zambian Copperbelt for use as a sulphur source in the copper smelters and roast leach electrowin plants.
Regional Setting
The Nampundwe deposit is hosted within supracrustal metasedimentary rocks towards the northern margin of the of the Proterozoic Zambezi Belt. The Zambezi Belt is a narrow, east-west trending orogenic strip immediately to the north of, and overlapping, the Zimbabwean craton to the south. It is separated from the Lufilian Arc by the major trans-continental WSW-ENE trending, sinistral, Mwembeshi Shear Zone to the NW, and is predominantly composed of Archaean to Mesoproterozoic granulites, gneisses and schists, and unconformably overlying metamorphosed Neoproterozoic metasedimentary and metavolcanic rocks.
The Neoproterozoic metasedimentary and metavolcanic rocks which are host to the Nampundwe deposit are located along the northern margin of the belt, and have been correlated with Katangan metasedimentary sequences to the north of the Mwembeshi Shear Zone (Drysdall et aI., 1972; Cahen and Snelling, 1984; Hanson et aI., 1990). These younger Zambezi belt metasedimentary rocks were deposited in a series of transtensional basins extending from Namibia in the SW to Malawi in the NE, which formed during early to mid-Neoproterozoic intracontinental rifting (Porada, 1989). This sequence comprises pelitic and semipelitic schists, interbedded with quartzites, calc-silicates, marbles and scapolitic marbles (Simpson, 1962; Simpson et aI., 1963; Drysdall et aI., 1972; Thieme and Johnson, 1981), and includes rhyolitic, dacitic and basaltic metavolcanic rocks in the south, and isolated metadolerite plugs and dykes throughout the belt (Vrana, 1979; Munyanyiwa, 1988). Rare eclogite and gabbroic bodies associated with metalherzolites, troctolites and serpentinised equivalents are found in the north, interpreted to be at least in part, tectonically emplaced (Munyanyiwa, 1988).
The Neoproterozoic supracrustal metasedimentary and metavolcanic rocks have been intensely deformed and are metamorphosed to amphibolite to upper greenschist facies, with the higher grades being located in the northern section of the belt. The pretectonic Rufunsa granite (Johns et aI., 1989) and syntectonic Lusaka granite (Barr et aI., 1978), have been respectively radiometrically dated (U/Pb concordia on zircon) at 927 and 854±12 Ma (Barr et aI., 1977), although K/Ar and Rb/Sr determinations (on whole rock samples and mineral separates) suggest younger ages of 807±20 and 800±35 Ma respectively (Cahen and Snelling, 1984). Johnson et al. (2007) dated the Lusaka Granite at 820±15 Ma (U/Pb zircon). The Lusaka Granite is intrusive into basement, and is tectonically overlain by Neoproterozoic rocks.
The basal section of the Neoproterozoic sequence includes a syn-rift, bimodal volcanic suite, the Kafue Rhyolite Formation and Nazingwe Formation metabasalts. The Kafue Rhyolite Formation, which have been dated at 880 Ma (Johnson et al., 2007), comprise a 2500 m thick sequence of folded, variably metamorphosed felsic volcanic flows and tuffs with subordinate tuffaceous sediments and extremely rare thin mafic horizons. The Kafue Rhyolite Formation passes up into the Nazingwe Formation, an ~500 m thick sequence of tuffaceous semipelites with intercalated acid and minor mafic volcanic horizons. Mafic volcanic rocks of the Nazingwe Formation are within error of the same age as the Kafue Rhyolite Formation (Johnson et al., 2007). The Kafue Rhyolite and Nazingwe formations are overlain by the quartzite-dominated sequence of the Mulola Formation, the base of which is marked by a variable thickness of undeformed conglomerates that are in part composed of rhyolitic pebbles (Mallick 1966). The Mulola Formation, in turn, passes up into kyanite-bearing, biotite-rich schists and semipelites of the Chipongwe Formation (these two formations and the basal bimodal volcanic suites are amalgamated into the Chunga Formation by Porada and Berhorst, 2000) and subsequently into the monotonous dolomitic marbles of the Cheta Formation. The latter contains abundant, randomly oriented, isolated blocks of variably metamorphosed, gabbroic and ultramafic blocks that have N-MORB chemistries (Vrána et al., 1975; John and Schenk 2003; John et al., 2003, 2004).
The Cheta Formation may be overlain across a structural break, possibly representing a thrust (Simpson et al., 1963; Porada and Berhorst 2000), by the Lusaka Dolomite, which was apparently deposited after the intrusion of the Lusaka Granite, dated as detailed above. The Lusaka Dolomite has been included in the Cheta Formation (e.g., Johnson et al., 2007). As all of these lithologies have been subjected to the same shearing, folding and metamorphism that affected the Kafue Rhyolite and Nazingwe formations, the upper sequence of metasediments was most likely deposited before the regional Pan-African event at ~600 to 530 Ma (Johnson et al., 2007).
A similar sequence some 40 to 100 km to the SW has been divided into the equivalent dominantly siliciclastic Nega Formation composed of basal quartzites with intercalated metapelites and minor calcareous rocks, passing up into mica schists, conformably overlain by the Muzuma Formation calc-silicate rocks with minor pure quartzites, marbles, and mica schists (Smith, 1963).
The Neoproterozoic metasedimentary rocks of the Zambezi Belt have been correlated with the Roan Group of the Lufillian arc, based upon broad lithological similarities and available age data (e.g., Cahen and Snelling, 1984; Hanson et aI., 1990), although no direct correlation is possible due to the lack of stratigraphic continuity and the metamorphic grade (Burnard et al., 1993).
According to Johnson et al. (2007), there is a close similarity between the Upper Roan Subgroup of the Lufilian Arc in Zambia and the Cheta Formation of the Zambezi Supracrustal sequence in the presence of abundant isolated mafic blocks that sit within the marble units and near the contact with the upper pelite beds (Vrána et al., 1975). In the Upper Roan Subgroup, these blocks have tholeiitic intraplate geochemical signatures and are interpreted to be syn-rifting dykes and sills (Tembo et al., 1999), which have undergone brittle dislocation and redistribution in the plastic regime of the enclosing carbonates. In the Cheta Formation, they have N-MORB and depleted mantle-like geochemical and isotopic signatures (John et al., 2003, 2004). A preliminary U-Pb SHRIMP zircon crystallisation age of 852±22 Ma (S. P. Johnson, unpublished data) for the Munali Gabbro intrusion to the SW, suggests these Zambezi Belt mafic rocks may also represent rift-related rocks similar to those in the Roan Group (Tembo et al., 1999). Apart from the Lusaka Dolomite, which may or may not be younger than the Lusaka Granite (Simpson et al., 1963; Porada and Berhorst 2000), there is little evidence for sedimentation after 820 Ma in the Zambezi Belt.
Johnson et al. (2007) compared the stratigraphy of the Lufilian Arc and Zambezi Belt and concluded that the Zambezi supracrustal sequence and the Roan Group in the Zambian Copperbelt are temporally correlatable, and suggest both successions formed in discrete rift basins between the southern margin of the composite Congo-Tanzania-Bangweulu and Kalahari cratons, with extension probably not resulting in complete continental separation.
Numerous base metal occurrences and deposits are found within a 10 km radius of Nampundwe, including six copper, zinc and gold, and several iron oxide deposits, which are typically stratabound (Freeman, 1988). Some 4 km to the ESE of Nampundwe, a substantial plug of hydrothermal magnetite and specular hematite carries 67% Fe at the Sanje iron mine where it forms a prominent hill that protrudes 120 m above the surrounding terrane (Burnard et al., 1993; Lobo-Guerrero, 2010).
Deposit Geology
The Nampundwe deposit is composed of eight separate, conformable sulphide bearing sheets, each 10 to 12 m thick, which have sharp upper and lower contacts, but laterally pinch-out and bifurcate over strike lengths of several kilometres. These bands persist to depths of at least 300 m. Not all are at separate stratigraphic levels, although at least three mineralised horizons are known, whilst others may be developed along strike from another lens. The ores are dominantly fine-grained, semi-massive to massive pyrite sulphidic horizons containing >30% sulphide. Two of these sheet-like orebodies have been mined, orebodies 1 and 2, which are separated by a 10 and 90 m thick poorly mineralised zone, comprising a sequence of sparsely mineralised massive dolostones. These orebodies, are locally isoclinally folded and overturned, dipping between 70° E to 80°W (overturned), and striking near north-south (Burnard et al., 1993).
The lodes have been complexly folded, with a strike that varies from NW to the west, to east-west in the east, with a steep dip punctuated by tight parasitic folds and NE-SW axial cross-folding that reverses the plunge of these structures (Freeman, 1988).
The stratigraphy may be summarised as follows (after Burnard et al., 1993):
• Footwall - dominantly composed of grey laminated dolostones with intercalated bands of biotitic limestone. A thin spotty laminated biotitic limestone with significant disseminated pyrrhotite and/or magnetite occurs within 0.5 m below the base of Orebody 1. The quantity of pyrrhotite within this unit increases along strike to the SE, where it is locally extensively replaced by magnetite.
• Orebody No. 1 - is between 8 and 12 m thick, increasing to the south, where the Cu, Co, Mn and Zn values are the highest. It commences with a variable 0.3 to 1.1 m thick basal unit of massive pyrrhotite that is found throughout the mine. This basal unit is essentially non-cupriferous and comprises a matrix of massive, fine-grained pyrrhotite with lesser chalcopyrite and occasional sphalerite, enveloping coarse (up to 25 cm) clasts of brecciated footwall dolomite and coarse porphyroblasts of pyrite. This unit is overlain by a highly variable, well-cleaved and occasionally crenulated phyllitic dolomite with significant, fine-grained pyrrhotite disseminated along bedding planes, while pyrrhotite is also commonly finely veined with microveinlets of mackinawite and/or iron oxides. The more phyllitic bands contain minor quartz, feldspar and chloritised amphiboles. The uppermost 3 to 4 m of this orebody are made up of interbedded massive pyritic sulphides and dolostones.
• Poorly Mineralised Zone - which is narrowest in the north with a thickness of ~10 m, but progressively expands southward to a maximum width of ~90 m. It is dominantly composed of massive or flaggy, grey and black, fine-grained dolostones, with occasional, thin (<30 cm thick), but often laterally continuous, pyritic sulphidites. These sulphidites are especially common toward the top of the unit, where overall sulphur grades are as high as 8%, compared to the 3 to 5% S typical of the Poorly Mineralized Zone. Two shaley dolomite bands containing numerous, 0.5 to 1 m nodules of coarse, interlocking calcite grains that form a "chicken-mesh" texture are found at the very top of the zone, 1 to 2 m below the base of Orebody 2.
• Orebody No. 2 - is between 13 and 18 m thick, with consistent high grade of between 16 and 18 wt.%S. It also carries chalcopyrite in a more copper-rich section that occupies about half of the lode width (Freeman, 1988). The orebody commences with comparatively well bedded shaley black dolomite with numerous intercalated <0.3 m thick pyritic sulphidites that become progressively thicker and more massive toward the top of the orebody. The sulphidites are characterised by soft-sediment structures, e.g., slumping, channel infill, rip-up clasts and soft-sediment boudinage, and commonly contain skeletal pyrite grains and annealed framboids, frequently either within a matrix of, or associated with veins of, fine-grained pyrrhotite and chalcopyrite. An ~50 cm thick band of massive pyrrhotite and chalcopyrite with clasts of brecciated dolomite, similar to that at the base of Orebody 1 is found at one locality, and can be traced down dip where it rapidly grades into bedded pyrite sulphidite. Occasional, commonly highly altered, coarse amphibole laths and alkali feldspars occur, particularly near the base of Orebody 2, associated with slightly more micaceous units. A few highly altered garnets have also been recognised at one locality.
The host rocks to the orebody are fine-grained dolostones, which range from shaly near the base, but more massive toward the top. The upper section of Orebody 2 is typically composed of massive pyritic sulphidites up to 2 m thick, separated by lenses or thin bands of black dolomite. Close to the hanging wall, the sulphidites have a pyrrhotite-chalcopyrite groundmass with fine, euhedral grains of pyrite.
• Hanging wall - Orebody 2 is generally directly overlain by 1 to 3 m of flaggy grey and black phyllitic dolostones, which grade into massive, unmineralised scapolitic dolostones. Locally, particularly to the north of the mine, a <1 m thick, well-banded, coarse, green amphibolite occurs, either immediately above orebody 2 or separated from it by 1 to 2 m of mineralised dolomite. This amphibolite is composed of a fine-grained chloritic groundmass enclosing coarse, euhedral blades of tremolite or hornblende and coarse garnets with up to 5 cm alteration halos. Elsewhere, a small amphibolite dyke is found in the hanging-wall dolomites, although its relationship to the banded amphibolite is not clear. However, this dyke appears to be related to brown chlorite and hematite alteration in the top part of orebody 2, and to several phases of pyrite. The dominant amphiboles are generally tremolite or cummingtonite, although minor magnesian hornblende also occurs in the banded amphibolites. The amphibolite is overlain by massive, grey unmineralised dolomites.
A third orebody is found 20 to 50 m above Orebody 2 in the central to southeastern section of the field, SE from the B shaft (Freeman, 1988).
The sulphides have been oxidised to a depth of ~40 m, and occur at surface as iron gossans capping low hills over a strike length of >5 km. An oxide copper resource has been estimated from trenching of ~0.9 Mt @ 1.87% Cu total, and 0.96% Cu acid soluble. The oxide copper occurs as malachite, wad, cupriferous mica and subordinate chrysocolla and cuprite, occurring as patches and disseminations in the porous iron oxide. At a 0.5% Cu cut-off, the Cu-rich oxide sections of the lode in the gossan varies from 2 to 15 m in thickness (Freeman, 1988).
The mode of occurrence of the pyrrhotite- and pyrite-rich sulphidites are different, with the former being more massive with numerous brecciated clasts of dolomite and pyrite. In one locality, massive to brecciated pyrrhotite-dominated sulphidite can be seen to laterally grade into a well-bedded pyrite-dominated sulphidite over a distance of 15 to 20 m. Frequently, sulphides account for >90% of the rock in sulphidite bands, the remainder being carbonate or mica.
Sulphur isotope determinations by Burnard et al. (1993) on pyrite, pyrrhotite, chalcopyrite and sphalerite show δ34S values ranging from -10 to +9.8‰. Orebody 1 exhibits a distinct linear variation
of δ34S values across the ore unit, from a positive correlation in the south of the
mine (from -10 at the base to +5‰ at the stratigraphic top of the orebody), whereas 1.2 km along strike to the north, the correlation is reversed (from +9.8 at the base to -3.5‰ at the top of the orebody). Within Orebody 2, there is a general trend to increasingly heavy sulphur from the base to the top of the orebody, both for the north and the south of the mine, although at the very top of the orebody, the trend is reversed with the sulphur isotope composition falling sharply by 4.6‰.
Burnard et al. (1993) regard this wide range in δ34S values to suggest mixing between two isotopically distinct fluids. The lighter fluid, thought to be broadly hydrothermal in origin, would have to have a sulphur isotope composition of -10‰ or less and is apparently responsible for around 30% of the 19.2 Mt of sulphide sulphur in the deposit. The isotopically heavy sulphur was thought to be derived from reduced marine sulphate, although no remaining sulphates have been encountered at Nampundwe. However, nodules of coarse "chicken-mesh" calcite found below Orebody 2 have been interpreted as anhydrite nodules subsequently replaced by calcite, suggesting a high sulphate activity, and possibly evaporative conditions at the time of deposition (Burnard et al., 1993; Hanson et al., 1991).
The same researchers noted a broad spread in the Pb isotope composition of sulphides (in chalcopyrite, pyrrhotite and pyrite) from the ores, specifically 206Pb/204Pb = 17.89 to 19.34, 207Pb/204Pb = 15.69 to 15.74, 208Pb/204Pb = 37.62 to 38.97, although pyrite and pyrrhotite have less radiogenic Pb than chalcopyrite. Burnard et al. (1993) suggest this large spread in Pb isotope compositions is not compatible with a single common Pb source, but implies a more complex evolution, which they interpreted to reflect mixing of fluids from at least two upper crustal reservoirs and possibly introduction of radiogenic Pb into the ores since deposition, most likely during Lufilian metamorphism.
Burnard et al. (1993) concluded that the sulphides were deposited from metal-rich brines into a marine basin during or soon after precipitation of the host Mg-rich carbonate rocks. They indicate that the carbon isotope compositions of the carbonates are compatible with an inorganic formation of these dolomites. However, Lobo-Guerrero (2005; 2010), points to the presence of magnetite within the immediate footwall rocks, increasing to the SE towards the large, massive, hydrothermal magnetite-specular hematite plug exploited at the Sanje iron mine, which is <1 km east of the known gossans at Nampundwe. That author suggests the Sanje deposit and the cluster of Cu, Cu-Au, Au, Fe and Zn-Pb occurrences in the surrounding district are part of a semi-regional "iron oxide copper-gold system" (IOCG), similar to that at Mubwa, 125 km to the NW. This may imply that the Cu sulphides, and at least part of the pyrite accumulation, is the result of an overprint of an IOCG mineralised sytem onto pre-existing diagenetic pyrite accumulations.
The two orebodies being worked in 1993 contained an estimated 11 Mt @ 13.5% S, 0.78% Cu.
The total geological resource of the whole has been estimated to be 115 Mt @ 8% S (Burnard et al., 1993).
For more detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 2010.
Record last updated: 9/11/2015
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Nampundwe
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Burnard P G, Sweeney M A, Vaughan D J, Spiro B and Thirlwall M F, 1993 - Sulfur and lead isotope constraints on the genesis of a southern Zambian massive sulphide deposit: in Econ. Geol. v88 pp 418-436
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