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Barberton Mountains, New Consort, Sheba, Fairview, Agnes |
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Mpumalanga, South Africa |
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Au
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Super Porphyry Cu and Au
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The gold deposits of the Barberton Greenstone Belt, which include the Sheba Group, New Consort, Fairview and Agnes are located in Barberton Mountain Land in Mpumalanga Province of South Africa, approximately 350 km east of Johannesburg
(#Location: Sheba - 25° 42' 53"S, 31° 8' 4"E;
New Consort - 25° 39' 6"S, 31° 4' 37"E;
Fairview - 25° 44' 0"S, 31° 4' 33"E;
Agnes - 25° 49' 57"S, 30° 58' 59"E).
The pre 3100 Ma greenstones of Barberton Mountain Land are part of the pre-Witwatersrand basement in the eastern Kaapvaal Craton, occurring as an elongate structural inlier within the larger terrane of Archaean granites, gneisses and migmatites. The granitoids both surround and intrude the greenstones.
Within the Barberton Greenstone Belt komatiitic basalts dominate the three lower formations representing the Tjakastad Subgroup of the Onverwacht Group. In addition to the pillowed and massive komatiitic, magnesian and tholeiitic basalt lava flows and sills there are subordinate layers of felsic pyroclastic rocks and discontinuous beds and lenses of carbonaceous shaly chert, banded chert, quartz-diopside-plagioclase-garnet calc-silcates and magnetite-quartz-amphibole banded iron formations. These rocks are locally cut by sodium rich quartz-feldspar porphyries and have been extensively metamorphosed, dislocated and partially granitised by the surrounding larger 3500 to 2700 Ma tonalitic to trondhjemitic gneiss intrusives.
A persisitent meta-sedimentary marker unit marks the top of the ultra-mafic to mafic Tjakastad Subgroup and separates it from the overlying mafic to felsic Geluk Subgroup which constitutes the upper part of the Onverwacht Group. The Geluk Subgroup is composed of a calc-alkaline series of cyclically alternating mafic and intermediate to acid volcanic and pyroclastic rocks. Each cycle generally consists of a thick lower zone of tholeiitic basalt overlain by thinner zones of dacitic to rhyodacitic lavas and pyroclastic rocks with intercalated siliceous tuffs and banded cherts. Pelagic carbonate-chlorite and chemical (banded carbonaceous shales, cherts and siderite-carbonate) sediments are also interlayered with the lavas and tuffs of the subgroup.
The Onverwacht Group is disconformably to onconformably overlain by a sedimentary sequence with lesser volcanic-pyroclastic members. The Fig Tree Group, an argillaceous sequence of greywackes, shales and chemical sediments (banded ferruginous chert and banded iron formation) with lesser trachyandesite lava, agglomerate and tuff, is overlain by the predominantly arenaceous Moodies Group of conglomertate, quartzite, sub-greywacke sandstone and shale as well as minor volcanics, jaspilite and banded iron formation.
The sediments of the Fig Tree and Moodies Groups form the core of the greenstone belt. The total sequence of the Barberton Greenstones is estimated to be around 20 km in thickness.
Approximately 95% of the gold produced has been derived from the north-western flank of the overall NE trending 150 km long and 30 to 60 km wide greenstone belt. Most of the gold deposits are clustered in subregions that have been subjected to complex structural modification of the host greenstones, while most of the more significant deposits are within 6 km of granitic contacts or are adjacent to major regional faults which have undergone several stages of reactivation.
Production from the greenstone belt deposits to 1985 totalled 250 t Au from over 350 mines, although 70% came from just 4 operations. These are the Sheba Group (69 t Au), New Consort (55 t Au), Fairview (37 t Au) and Agnes (19 t Au). All others yielded less than 6 t. In 1997 the East Transvaal Consolidated operation produces around 3.5 t Au per annum at an average grade of 15 g/t Au, with reserves in 1994 of 15 t Au. It is unusual for significant production to emanate from pre-3100 Ma greenstone belts.
Sheba Gold Mine
Between1886 to the present day, the Sheba mine has exploited more than 25 fracture-hosted ore bodies, in a variety of structural and lithological settings. These bodies are hosted by steeply-dipping, isoclinally-folded, metasediments and metavolcanics, which have undergone several phases of deformation and only low-grade metamorphism. The ore bodies are steeply-plunging, sulphide-impregnated zones of limited width (1 to 2 m) and strike, which are developed to considerable depth. The ore shoots are irregularly developed over a strike distance of approximately 6 km, in a 2 km wide corridor within which ore-bearing fractures are associated with anticlinal structures, contiguous with a regional strike-slip fault. The cores of the anticlines generally comprise a sequence of fuchsitic quartz schists, carbonate rocks and cherts. The auriferous zones are characterised by sulphides, quartz veins and wall rock discolouration. Inflections of the fracture planes or intersections with other planes are often preferred sites of mineralisation.
New Consort Gold Mine
The New Consort Gold Mine exploited steeply plunging auriferous sulphide shoots associated with the intensely folded and silicified contact (the Consort Bar) between fine-grained metasediments of the Fig Tree Group in the hanging wall, and fine-grained mafic and ultramafic schists, lavas and serpentinite of the Onverwacht Group in the footwall. Gold mineralisation associated with sulphide occurs below this contact. Locally, this strongly laminated, siliceous horizon (Consort Bar) hosts auriferous, apparently interlayered, sulphide bands, but is barren of sulphide or gold where exposed elsewhere. The Consort Bar and associated ore shoots have been affected by later faulting and folding, while numerous granitic pegmatite bodies which are devoid of sulphide or gold, have been intruded along fractures and faults, some of which have displaced the ore shoots.
Fairview Gold Mine
The Fairview Gold Mine area includes 41 known gold-bearing fractures, 22 of which are payable. It covers a variety of rocks of the Barberton Greenstone Sequence and is located over the central and southern portions of both the Eureka Syncline and Ulundi Synclinorium which are separated by the Sheba Fault, bounded to the north by the Lily Fault and to the south by the Barbrook Fault. These synclinal structures were refolded about a north-west axis resulting in the development of fractures which served as hosts for the main mineralising phase.
Two styles of gold mineralisation are mined at Fairview, namely:
i). Sulphidic reefs which include disseminated to massive pyrite and arsenopyrite in concordant and discordant fractures in greywackes and shales of the lower Fig Tree Group. These fractures were mainly the result of tangential shear movement during the major folding periods. They vary from 2 cm to 2 m in width and may persist for up to 500 m along strike, although payable gold values are restricted to several discrete payshoots, seldom >60 m in strike length. The principal constituents of the sulphidic ore are pyrite and arsenopyrite with small to trace amounts of chalcopyrite, sphalerite, pyrrhotite, stibnite, native antimony, ullmannite, galena, enargite, pentlandite, niccolite, safflorite, skutterudite and graphite. Calcite, quartz and sericite are the most important gangue minerals. It is estimated that 50 per cent of the gold is included in pyrite, 20 per cent in arsenopyrite and associated minerals and 30 per cent occurs as free gold.
ii). Quartz reefs which consist of discrete quartz-filled fractures in the brittle quartzite horizons of the Moodies Group. These pale-grey to blue-black quartz veins occur at right angles to both the strike of the host rocks and the Sheba Fault, and were formed during the refolding of the synclines around a north-west-trending fold axis. They vary in thickness from 1 cm to 1.5 m and normally carry very little pyrite and arsenopyrite and often contain visible gold as disseminated specks and plates. Two generations of quartz have been recognized, often with free gold along their contacts.
Agnes Gold Mine
The Agnes Gold Mine has exploited two of a number of near vertical, easterly plunging, ore bodies hosted in a zone of shaly, shallow water elastic sediments within the Moodies Group of the Barberton Greenstone Sequence. These sediments have been folded locally to assume near vertical dips, and have only been subjected to low-grade metamorphism, and consequently show have well preserved sedimentary textures. Two types of ore are known, namely: i). Swarms of generally strata-conformable (and less commonly crosscutting), calcareous quartz veins; and ii).Strata-conformable concentrations of pyrite and minor quartz veining. Individual shoots have strike lengths of 200 to 500 m and persist down the steep easterly plunge for up to 1000 m or more.
The most recent source geological information used to prepare this decription was dated: 1997.
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.
Sheba New Consort Fairview Agnes
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Selected References:
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Anhaeusser C R 1986 - Archaean gold mineralisation in the Barberton Mountain Land: in Anhaeusser C R, Maske S, (eds), Mineral Deposits of Southern Africa Geol. Soc. of South Africa, Johannesburg v1 pp 113-154
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Argapadmi, W., Toth, E.R., Fehr, M.A., Schonbachler, M. and Heinrich, C.A., 2018 - Silver Isotopes as a Source and Transport Tracer for Gold: A Reconnaissance Study at the Sheba and New Consort Gold Mines in the Barberton Greenstone Belt, Kaapvaal Craton, South Africa: in Econ. Geol. v.113, pp. 1553-1570.
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de Beer J H, Stettler E H, du Plessis J G and Blume J, 1988 - The deep structure of the Barberton greenstone belt; a geophysical study : in S. Afr. J. Geol. v91 pp 198-206
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De Ronde C E J, Spooner E T C, De Witt M J, Bray C J 1992 - Shear zone-related, Au Quartz vein deposits in the Barberton Greenstone Belt, South Africa: field and petrographic characteristics, fluid properties, and light stable isotope geochemistry: in Econ. Geol. v87 pp 366-402
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Gloyn-Jones, J. and Kisters, A., 2019 - Ore-shoot formation in the Main Reef Complex of the Fairview Mine - multiphase gold mineralization during regional folding, Barberton Greenstone Belt, South Africa: in Mineralium Deposita v.54, pp. 1157-1178
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Groves, D.I., Santosh, M., Goldfarb, R.J. and Zhang, L., 2018 - Structural geometry of orogenic gold deposits: Implications for exploration of world-class and giant deposits: in Geoscience Frontiers v.9, pp. 1163-1177.
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Heubeck C, Wendt J I, Toulkeridis T, Kroener A and Lowe D R, 1993 - Timing of deformation of the Archaean Barberton greenstone belt, South Africa; constraints from zircon dating of the Salisbury Kop Pluton : in S. Afr. J. Geol. v96 pp 1-8
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Otto A, Dziggel A, Kisters A F M and Meyer F M, 2007 - The New Consort Gold Mine, Barberton greenstone belt, South Africa: orogenic gold mineralization in a condensed metamorphic profile : in Mineralium Deposita v42 pp 715-735
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Schouwstra R P, 1995 - Wall-rock alteration as a guide to gold-bearing fracture zones in the Zwartkoppie Section, Sheba gold mine, South Africa : in S. Afr. J. Geol. v98 pp 399-414
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Voges F D, 1986 - The New Consort gold mine, Barberton Greenstone Belt: in Anhaeusser C R, Maske S, (Eds.), 1986 Mineral Deposits of South Africa Geol. Soc. South Africa, Johannesburg v1 pp 163-168
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Wagener J H F 1986 - The Agnes gold mine, Barberton Greenstone Belt: in Anhaeusser C R, Maske S, (Eds.), 1986 Mineral Deposits of South Africa Geol. Soc. South Africa, Johannesburg v1 pp 181-185
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Wagener J H F, Wiegand J 1986 - The Sheba gold mine, Barberton Greenstone Belt: in Anhaeusser C R, Maske S, (Eds.), 1986 Mineral Deposits of South Africa Geol. Soc. South Africa, Johannesburg v1 pp 155-161
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Ward J H W, 1987 - Genesis of Barberton gold; the exhalite source-bed concept : in S. Afr. J. Geol. v90 pp 326-331
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Wiggett B S A, Brink W C J, Vorster M A 1986 - The Fairview Gold mine, Barberton Greenstone Belt: in Anhaeusser C R, Maske S, (Eds.), 1986 Mineral Deposits of South Africa Geol. Soc. South Africa, Johannesburg v1 pp 169-179
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Williams P J 1997 - A metamorphosed, stratabound-epigenetic origin for a gruneritic Archean gold deposit, Barberton, South Africa : in Ore Geology Reviews v13 pp 135-151
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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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