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Great Dyke - Hartley, Ngezi, Mimosa, Unki

Zimbabwe

Main commodities: PGE PGM Pt Pd
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The 550 km long, 2575 ±0.7 Ma Great Dyke in Zimbabwe contains four geological complexes which are known to contain PGM and base metal deposits. These are the Hartley Geological Complex, exploited by the Hartley and Ngezi mines (Zimplats), 60 km SW of Harare; the Wedza Complex exploited by the Mimosa mine (Aquarius and Implats), the Selukwe Complex exploited by the Unki mine (Anglo Platinum), the and the Musengezi Complex. Ngezi, Unki and Mimosa are 75, 190 and 280 km SSW of Hartley respectively. (#Location: Hartley - 18° 2' 44"S, 30° 25' 37"E; Ngezi - 18° 41' 17"S, 30° 20' 40"E; Unki - 19° 37' 28"S, 30° 5' 38"E; Mimosa - 20° 19' 35"S, 29° 49' 24"E).

For background on the setting and composition of the Great Dyke see the seperate The Great Dyke record.

The Hartley platinum mines exploit mineralisation of the Main Sulphide Zone (MSZ) of the Great Dyke in the Darwendale sub-chamber of the main North Chamber.   Economic PGE, Ni and Cu disseminations as predominantly intercumulus sulphides which occur within pyroxenites some metres below the transition from the Ultramafic to the overlying Mafic Sequence (the two sequences that make up the Great Dyke).

At the Hartley Mine the MSZ is several metres thick and comprises a basal PGE rich sub-zone which overlaps slightly with an overlying sulphide rich base metal sub-zone at the top. The subzones are further divided into geochemically distinct layers, based on the degree of sulphide mineralization and PGE ratios. The MSZ displays metal profiles, stratigraphically upwards, which are characterised by increasing Cu:Ni, Pt:Pd and PPGE:IPGE ratios, accompanied by a general element decoupling in the order Pd to Pt to base metals, more specifically IPGE to Pd to Pt to Ni,Cu,Co,S(sulphides), Au, Te, Bi. The MSZ has a finely banded structure, regarded to reflect primary magmatic features of consecutive batches of sulphide accumulation, concomitant scavenging of PGE, and fractionation. Sulphide deposition was succeeded by a second, limited subsolidus stage of PGE redistribution. Most of the Pd and Rh is hosted in pentlandite, while Pt is dominantly in the form of discrete platinum-group minerals (PGM). Within the MSZ sequence, sperrylite occurs throughout the PGE subzone, cooperite/braggite are found mainly in its basal part, and the (Pt,Pd)-bismuthotellurides concentrate at the top. These observations suggest a large proportion of the PGE, primarily concentrated in sulphide under magmatic conditions, was redistributed in the subsolidus stage and formed discrete PGM with available reactant partners. Chemical gradients and magmatic-hydrothermal fluids probably led to small-scale redistribution of PGE within the MSZ (Oberthur et al., 2003).

At Hartley the MSZ outcrops on the western flank of the Great Dyke with a NNE trend and 18°E dip.   Mining commenced at 100 m below surface, below the base of oxidation.   In the supergene/oxide interval Pd has been preferentially removed, thus substantially lowering the grade.

The Hartley mine opened in 1995, but following a string of geological and metallurgical problems, underground operations were suspended in June 1999. Following a change in ownership, a new open pit mine 75 km south of Hartley, at Ngezi was opened in 2001, followed by an underground section, with operations continuing. Ngezi is on the Sebake sub-chamber of the North Chamber.

Ore reserves at Hartley prior to mining were stated (Brown 1998) as:
    51 Mt @ 2.64 g/t Pt, 1.81 g/t Pd.
The total reserves for oxide ores at the "Zimplats concession" (Hartley and Ngezi) are given as:
    138 Mt at 3.4 g/t (Pt+Pd+Rh+Au) with an average thickness of 230 cm (Zimplats, Annual Report 2005).
Resources and reserves at Hartley and Ngezi at June 30, 2012 (Zimplats, 2012) were:
    Total measured + indicated + inferred mineral resources - 1904 Mt @ 3.6 g/t 4E (Pt+Pd+Rh+Au), including;
    Total proven + probable or reserves - 227.2 Mt @ 3.35 g/t 4E (Pt+Pd+Rh+Au).
The resource grade is made up of: 1.75 g/t Pt, 1.42 g/t Pd, 0.15 g/t Rh, 0.29 g/t Au, 0.13% Ni, 0.11% Cu, over an average thickness of 2.0 m.

Mimosa Zimbabwe's oldest platinum mine, has been in operation since 1984, and is located in the southern part of the Great Dyke on the Wedza sub-chamber of the South Chamber, ~280 km to the south of Hartley, 125 km east of Bulawayo and 30 km from Zvishavane. PGM Mineral Resources at Mimosa are located in four erosionally isolated and fault bounded blocks.

The Wedza-Mimosa platinum deposit,as at the other deposits of the Great Dyke, is the generally stratiform PGE-rich Main Sulphide Zone (MSZ) which is exposed as a complexly layered, shallow "synclinal" sequence of pyroxenites and gabbros. Both the MSZ and its host rocks have a systematic stratigraphic, lithological and compositional variations between the axis and the margins of the deposit. A well preserved marginal facies has discordant layering, stratigraphic complexity, evidence of non-equilibrium crystallisation, magmatic erosion and extensive pegmatoids, as well as irregular development of the MSZ and metal distrinution. The principal transverse variations and marginal phenomena are interpreted as the result of high transverse heat flux and heat gradient caused by the narrow trumpet-like cross-sectional structure of the original magma chamber, the magma replenishment process, the angular relationship between the chamber floor and the stratified magma, and the increase in magma density caused by the onset of plagioclase crystallisation (Prendergast, 2009).

Ore reserves and mineral resources at Mimosa, as at 30 June 2012 (Aquarius Platinum, 2012) were:
    Total measured + indicated + inferred mineral resources - 67.34 Mt @ 3.18 g/t 6E (Pt+Pd+Rh+Ru+Ir+Au);
    Total proven + probable or reserves - 14.69 Mt @ 3.50 g/t 4E (Pt+Pd+Rh+Au).

Mineral resources at Mimosa are quoted at a 2 m mining cut and include the upper and lower chromitite layers

The Unki deposit is within the Selukwe sub-chamber of the South Chamber, 90 km north of Mimosa and 190 km south of Hartley. The PGE-rich MSZ at Unki is unique in having a shear, locally referred to as the Footwall Shear, developed at or close to its base . It is however, in other respects, isimilar to the other PGE occurrences on the Great Dyke MSZ, in having its hanging-wall restricted to within 1 metre of the websterite/bronzitite contact. A slight axial tilting to the west is indicated by steeper dips on the eastern flank. The sulphide concentration within the MSZ are broadly coincident with the PGE-rich zone, but is not sufficiently so to be used in mine control. Based on petrological evidence, the bulk of the sulphides associated with the PGE are cumulates, providing evidence of an orthomagmatic origin for the MSZ. The dominant platinum group mineral (PGM) phase is the arsenide/sperrylite group that is most commonly found at the contact zones between base metal sulphides (BMS) and gangue. The geochemistry of the cumulate orthopyroxenes across the 2 m thick PGE-rich MSZ interval at Unki shows a trend of marked Fe enrichment upwards, corresponding to an enrichment in sulphide. This indicates that precipitation of sulphides was the result of fractionation with lowering of temperature in the magma. The Fe enrichment is followed by a reversal in the Mg content of orthopyroxene, corresponding to the decrease in sulphide content. This suggests the termination of the PGE-rich MSZ was due to an increase in temperature associated with an influx of new magma (Murahwi, 2012).

Ore reserves and mineral resources at Unki, as at 31 December 2011 (Anglo American, 2012) were:
    Total measured + indicated + inferred mineral resources - 79.5 Mt @ 4.13 g/t 4E (Pt+Pd+Rh+Au);
    Total proven + probable or reserves - 38.7 Mt @ 3.79 g/t 4E (Pt+Pd+Rh+Au).

For more detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 2014.     Record last updated: 2/11/2012
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.


Ngezi

Unki

Mimosa

Hartley

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
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
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
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
Pina, R., Gervilla, F., Barnes, S.-J., Oberthur, T. and Lunar, R.,  2016 - Platinum-group element concentrations in pyrite from the Main Sulfide Zone of the Great Dyke of Zimbabwe: in    Mineralium Deposita   v.51, pp. 853872


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