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The Great Dyke |
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Zimbabwe |
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Cr PGE PGM
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Super Porphyry Cu and Au
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The Great Dyke intruded at 2575.4±0.7 Ma into Archaean granites and greenstone belts of the Zimbabwe Craton in central Zimbabwe. It extends in a NNE-SSW direction for approximately 550 km, with a slightly sinuous width of 4 to 12 km, and hosts a number of PGE deposits.
The Dyke is divided vertically into three major successions, a lower mafic sequence consisting mainly of steeply-dipping, fine-grained rocks of variable composition, including pyroxenites and norites, an overlying ultramafic sequence, dominated from the base upwards by cyclic repetitions of dunite, harzburgite and bronzitite, and an upper mafic sequence consisting mainly of gabbro and gabbro-norite. The compositional banding dips steeply inwards at depth into the feeder dyke which comprises the lower of the three sequences, but flattens upwards towards the surface, to form a 'basin-like' flat chamber above the feeder dyke, containing the upper two ultramafic and mafic sequences. Overall, the 'dyke' has a 'T' shaped cross section, with the layering near the margins of the Dyke dipping inwards towards the axis of the intrusion and flattening out near the centre to form a flat-lying floor above the feeder dyke. Much of the upper mafic sequence has been removed by erosion.
The Dyke developed as a series of initially discrete upper magma compartments, which coalesced as the chambers were filled, fed by the steep feeder dyke. Each of these compartments can be divided into two stratigraphic successions, the upper two of the three detailed above, comprising,
i). a lower ultramafic sequence, consisting of dunites, harzburgites, olivine bronzitites, pyroxenites, and thin layers of chromitites at the base of a series of cyclic units - there are 14 thin layers (each ~10 cm thick) in sections of the dyke, half of which have 'economic' Cr grades;
ii). an upper mafic sequence made up of plagioclase-rich norites, gabbronorites, and olivine gabbros (Wilson 1991).
The uppermost part of the lower ultramafic sequence contains two major zones of sulphide concentration, 5 to 50 metres below the transition from the Ultramafic to the overlying Mafic Sequence:
i). the Lower Sulphide Zone (LSZ).
ii). the overlying Main Sulphide Zone (MSZ), which is the major PGE resource, subdivided into a lower PGE-rich subzone and an upper Cu- and Ni-rich subzone, and consists of bronzitites with average sulphide contents of about 2 vol.% (locally up to 7 vol.%). The most common sulphides are pyrrhotite, pentlandite, chalcopyrite and pyrite that mainly form polyphase aggregates. The main platinum-group minerals (PGM) in the MSZ are sperrylite [PtAs2], (Pt-Pd)-bismuthotellurides, cooperite, and braggite [(Pt,Pd,Ni)S], and Rh-Ir-Pt sulpharsenides (Oberthür 2002). Platinum-group elements, especially Pd, are also present in solid solution in the crystal lattices of the sulphides, particularly pentlandite (Locmelis et al., 2010).
The magma chambers coalesced below the MSZ and before erosion, the MSZ would have been continuous along the length of the Dyke, although much of the MSZ has been removed by erosion, with four remnants preserved. At its present plane of erosion, the Great Dyke is longitudinally subdivided into a series of narrow contiguous layered complexes or chambers, composed of two main magma chambers: the North Chamber, subdivided from north to south into the Musengezi, Darwendale and Sebakwe sub-chambers, and the South Chamber made up by the Selukwe and the Wedza sub-chambers. The Darwendale and Sebakwe sub-chambers are known as the Hartley Complex. The Darwendale and Sebakwe sub-chambers contain 80% of the known PGM mineral resources (Zimplats, 2012).
The MSZ is a continuous layer, 2 to 10 m thick that forms an elongate basin, with layers dipping inwards at between 5 and 20° near the margins, flattening near the centre to form a flat-laying floor. The MSZ is typically composed of a 2 to 10 m thick zone of 2 to 8% Fe-Ni-Cu sulphides, as detailed above, disseminated in pyroxenite. The base of this Ni-Cu-rich zone is straddled by a 1 to 5 m thick zone of elevated precious metal (Pt, Pd, Au and Rh) content. The precious metals zone where it overlaps the sulphide-rich interval contains up to 5% sulphide, while the underlying PGM zone has <0.5%. The change in sulphide content has a consistent relationship to the metal distribution and can be used as a reliable marker. The PGM content and distribution is consistent over large areas and can be readily correlated between drill holes. In each sub-chamber, the MSZ is vertically and gradationally zoned around a high grade core, with the most rapid transition in the footwall (Zimplats, 2012).
Extensive faulting has modified the shape of the basins on all scales, while post-mineralisation intrusion of various types and scales has further disrupted the mineralisation.
In addition, the PGE-rich sulphide ores of the MSZ are usually weathered to depths of 20 to 30 m, as seen in the open pits at the Hartley and Ngezi mines. Oxidised ores near the surface constitute a resource of ~400 Mt, although mining of this ore type has so far been hampered by low recovery rates. During the oxidation/weathering of the fresh hypogene ores, S and Pd are depleted, whereas Cu and Au are enriched. The concentrations of most other elements (including the other PGE) remain relatively constant. In the oxidised MSZ, PGE occur in a number of modes: i). as relict primary PGM (mainly sperrylite, cooperite, and braggite); ii). in solid solution in relict sulphides (dominantly Pd in pentlandite, up to 6,500 ppm Pd and 450 ppm Pt); iii). as secondary PGM neoformations (i.e., Pt-Fe alloy and zvyagintsevite); iv). as PGE oxides/hydroxides that replace primary PGM as the result of oxidation; v). hosted in weathering products, i.e., iron oxides/hydroxides (up to 3600 ppm Pt and 3100 ppm Pd), manganese oxides/hydroxides (up to 1.6 wt.% Pt and 1150 ppm Pd), and in secondary phyllosilicates (up to a few hundred ppm Pt and Pd) (Locmelis et al., 2010).
Production of PGE from the MSZ commenced at the Hartley Platinum Mine in the Darwendale Subchamber in 1996, and has taken place at the Mimosa mine (Wedza Subchamber), 280 km to the south of Hartley, near Zvishavane, since 1994. The Ngezi mine (Sebakwe Subchamber), ~50 km south of Hartley, started production in 2002, and exploration work is performed at the Unki mine in the Selukwe Subchamber (Oberthür et al., 2003).
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: 12/11/2012
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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Selected References:
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Li C , Ripley E M, Oberthur T, Miller J D and Joslin G D, 2008 - Textural, mineralogical and stable isotope studies of hydrothermal alteration in the main sulfide zone of the Great Dyke, Zimbabwe and the precious metals zone of the Sonju Lake Intrusion, Minnesota, USA: in Mineralium Deposita v43 pp 97-110
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Locmelis M, Melcher F and Oberthur T, 2010 - Platinum-group element distribution in the oxidized Main Sulfide Zone, Great Dyke, Zimbabwe: in Mineralium Deposita v.45 pp. 93-109
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Oberthur T, Weiser T W, Gast L 2003 - Geochemistry and mineralogy of platinum-group elements at Hartley Platinum Mine, Zimbabwe. Part 2: Supergene redistribution in the oxidized Main Sulfide Zone of the Great Dyke, and alluvial platinum-group minerals: in Mineralium Deposita v38 pp 344-355
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Oberthur T, Weiser T W, Gast L 2003 - Geochemistry and mineralogy of platinum-group elements at Hartley Platinum Mine, Zimbabwe. Part 1: Primary distribution patterns in pristine ores of the Main Sulfide Zone of the Great Dyke: in Mineralium Deposita v38 pp 327-343
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Schoenberg R, Nagler Th F, Gnos E, Kramers J D, Kamber B S 2003 - The source of the Great Dyke, Zimbabwe, and its tectonic significance: Evidence from Re-Os isotopes: in J. of Geol. v111 pp 565-578
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Stribrny B, Wellmer F K, Burgath K P, Oberthur T, Tarkian M, Pfeiffer T 2000 - Unconventional PGE occurrences and PGE mineralization in the Great Dyke: metallogenic and economic aspects: in Mineralium Deposita v35 pp 260-280
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Wilson A H and Prendergast M D, 2001 - Platinum-Group Element Mineralisation in the Great Dyke, Zimbabwe, and its Relationship to Magma Evolution and Magma Chamber Structure : in S. Afr. J. Geol. v104 pp 319-342
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Wilson A H, Tredoux M 1990 - Lateral and vertical distribution of Platinum-group elements and petrogenetic controls on the Sulfide mineralization in the P I pyroxenite layer of the Darwendale Subchamber of the Great Dyke, Zimbabwe: in Econ. Geol. v85 pp 556-584
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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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