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Renco |
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Zimbabwe |
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Au
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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.
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The Renco gold mine is located ~75 km southeast of Masvingo in Zimbabwe. It lies within the northern marginal zone of Limpopo Mobile Belt where it cuts the south-eastern margin of the Zimbabwe Craton. It is notable in that mineralisation is hosted within a granulite facies metamorphic terrane.
Gold mining in the Renco area started as early as the 15th century when artisanal miners worked near surface outcrops. Claims were first pegged in 1938 after a local inhabitant brought the area to the attention of prospectors. Production commenced in 1939, and by 1940 numerous small mines were operating in what became known as the Nyajena Goldfield. Mining by small syndicates continued intermittently until 1963 when Gold Fields of South Africa Ltd purchased the mine and began an exploration programme. From 1964 until 1972 the area was examined by a succession of major companies but results were generally disappointing. The mine was offered to the Rio Tinto group in 1972. Rio Tinto Zimbabwe negotiated an option to purchase, which was exercised in 1975, followed by an exploration program and a mine expansion in 1980. In 2004, Rio Tinto Zimbabwe was separated from Rio Tinto plc to become RioZim Limited, a wholly owned Zimbabwean company.
The Late Archaean to Mesoproterozoic granulitic facies Limpopo Belt separates the predominantly greenschist facies terranes of the Zimbabwe Craton to the north, from those of the Kaapvaal Caton, to the south. It has been subdivided into three zones, the northern and southern marginal, and the central zones. The Renco deposit is located in the northernmost parts of the Northern Marginal Zone, ~5km south of the boundary with the Zimbabwe Craton. This boundary is marked by a series of shallow southerly dipping, mylonitic thrust zones, known as the North Limpopo Thrust Zone System. Lithologically, the Northern Marginal Zone is predominantly composed of intrusive charnockites and enderbites and their gneissic equivalents, along with porphyritic K-rich granites of the Razi Suite and volumetrically minor supracrustal assemblages. The porphyritic granites form prominent elongate corridors parallel to the North Limpopo Thrust Zone (after Kisters Kolb and Meyer, 1998).
Four reefs groups have historically been exploited, namely the 'steep reefs', named after their subvertically inclined, shallow easterly plunging pipe-like orientation, and the orange, green and red reefs, which were arbitrarily given these names to distinguish them. The red, green and orange reefs have a shallow southeasterly dip of ~25° and are anastomosing, NNE- to ENE- trending tabular lodes, known as the 'shallow reefs'. The red reef, which is the most persistent, as detailed below, has been exploited to the greatest extent. The kinematics and orientation of the mineralised shear zones are consistent with a lateral and frontal thrust zone geometry that formed during the Late Archaean thrusting of the northern marginal zone of the Limpopo Belt onto the Zimbabwe Craton (Kisters et al., 1998).
The gold bearing reef zones also contain bismuth, copper and iron sulphides and are found in granulite facies rocks that are highly contorted. The reefs, which are associated with mylonites, are transgressed by the foliation and isoclinal folding of the Archaean Limpopo Mobile Belt. The reefs are associated with a very fine grained brown to white, laminated quartz-magnetite granulite. The average reef width overall is 126 cm, with a grade of 9.4 g/t Au, 37 ppm Bi and 0.16 Cu. Production in 1994 was 2.1 tonnes of gold.
Within the shallow reefs, gold mineralisation is contained within mylonitic shear zones which transgress lithological contacts and wall-rock fabric but have sharp wall rock contacts. The average thickness of the shallow reef ore zones is ~80 cm, but range from <10 cm to 3 m, due to pinch and swell structures both downdip and along strike, which are common. The spacing between the reef structures is commonly 25 to 40 m, although the reefs may also coalesce so that, in places, only two reefs are developed. The red reef is the most continuous structure, and can be followed throughout the mine workings over a strike length of ~2 km and its downdip extent is intersected to at least 500 m below surface (Kisters et al., 1998).
The steep reefs are subvertical quartz sulphide lodes which are spatially closely associated with the shallow reefs. They have a short, easterly strike extent of 50 to 70 m, with widths that range from <0.5 to >2.5 m, but have considerable down plunge extents of >250 m, resulting in a pronounced elongated, blade like geometry. Plunges are consistently at shallow to moderate angles of 20 to 30°E. In plan view, steep reefs appear to occur in three east-west trending clusters. The spacing between each cluster is 400 to 450 m. Within each cluster, steep reefs have a right-stepping en echelon pattern with a spacing of 20 to 50 m (Kisters et al., 1998).
Gold in both reef types is spatially and temporally closely associated with sulphide mineralisation, with pyrrhotite being the dominant sulphide accompanied by minor chalcopyrite and pyrite. Sphalerite, molybdenite, cubanitc and gold, together with magnetite, ilmenite and rutile-leucoxene occur as accessory phases whereas native bismuth and gold bismuth alloys are only present as traces (Tabcart, 1989). Magnetite is a common constituent, typically having a light brownish tinge and forming discrete euhedral grain. Sulphides occur as fine disseminations, as fracture infillings, drawn out in the mylonitic foliation of the reefs or as massive ores. Locally the ore contains subangular to rounded wall- rock fragments that are set within a massive sulphide matrix resembling 'durchbewegungs' textures of deformed massive sulphide ore. Gold is fine grained with grain sizes of typically <10µm. It occurs predominantly in macrolithons, mainly as small ovoid inclusions of native gold associated with pyrrhotite and early chalcopyrite along fracture planes within the garnet-biotite-quartz alteration paragenesis (Kisters et al., 1998).
Wall-rock alteration adjacent to and within the reefs comprises an assemblage of garnet-biotite-quartz ±siderite. Mineral textures within the host mylonites, combined with garnet-biotite thermometry, indicate gold was deposited at ~600°C under mid-amphibolite conditions, slightly post-dating regional peak metamorphic conditions. Transient episodes of brittle fracturing occurred during conditions of close to lithostatic fluid pressures, promoted by a pronounced strain partitioning within the narrow shear zones into ductile mylonite bands and brittle-ductile 'lithons' that contain the bulk of the gold sulphide mineralisation. The high-grade metamorphic ore and alteration mineral assemblages are overprinted by lower greenschist facies parageneses along brittle faults and cataclasites that are related to the Mesoproterozoic tectonism of the northern marginal zone (Kisters et al., 1998).
Remaining Ore Reserves and Mineral Resources as at 31 December, 2020 (RioZim Ltd Annual Report) were:
Proved + Probable Ore - 0.290753 Mt @ 6.03 g/t Au, for 1.754 tonnes of gold;
Measured Resource - 0.143980 Mt @ 5.38 g/t Au, for 0.775 t of gold;
Indicated Resource - 1.939105 Mt @ 5.34 g/t Au for 10.348 t of gold;
Inferred Resource - 2.633242 Mt @ 8.39 g/t Au for 22.098 t of gold;
Production between 2014 and 2018 varies from 0.59 to 0.75 t of gold per annum.
The most recent source geological information used to prepare this decription was dated: 1998.
Record last updated: 9/12/2021
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.
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Blenkinsop T G, Frei R 1996 - Archean and Proterozoic mineralization and tectonics at the Renco mine (Northern Marginal Zone, Limpopo Belt, Zimbabwe): in Econ. Geol. v91, No. 7 pp 1225-1238
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Blenkinsop T G, Kroner A and Chiwara V, 2004 - Single stage, late Archaean exhumation of granulites in the Northern Marginal Zone, Limpopo Belt, Zimbabwe, and relevance to gold mineralization at Renco mine : in S. Afr. J. Geol. v107 pp 377-396
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Bohmke F C, Varndell B J 1986 - Gold in granulites at Renco mine, Zimbabwe: in Anhaeusser C R, Maske S, (eds), Mineral Deposits of Southern Africa Geol. Soc. of South Africa, Johannesburg v1 pp 221-230
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Kisters A F M, Kolb J, Meyer F M 1998 - Gold mineralization in high-grade metamorphic shear zones of the Renco mine, southern Zimbabwe: in Econ. Geol. v93 pp 587-601
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Kolb J, Kisters A F M, Hoernes S, Meyer F M 2000 - The origin of fluids and nature of fluid-rock interaction in mid-crustal auriferous mylonites of the Renco mine, southern Zimbabwe: in Mineralium Deposita v35 pp 109-125
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Kolb J, Meyer F M 2002 - Fluid inclusion record of the hypozonal orogenic Renco Gold deposit (Zimbabwe) during the retrograde P-T evolution: in Contrib. to Mineralogy & Petrology v143 pp 495-509
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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