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Econ. Environ. Geol. 2021; 54(2): 213-232

Published online April 30, 2021

https://doi.org/10.9719/EEG.2021.54.2.213

© THE KOREAN SOCIETY OF ECONOMIC AND ENVIRONMENTAL GEOLOGY

Studies on the Ore Mineralogy and Litho-geochemistry of the Sheba Deposit, Barberton Greenstone Belt, South Africa

Mohammed Alnagashi Hassan Altigani*

Department of Geology of Minerals Wealth, Faculty of Petroleum and Minerals, Alneelain University, 11121, Sudan

Correspondence to : m.alnagashi@gmail.com

Received: January 8, 2021; Revised: March 7, 2021; Accepted: March 10, 2021

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided original work is properly cited.

Abstract

Ore criteria at the Sheba Deposit indicate orogenic mineralization type. Rocks and mineral assemblages suggest low formationtemperature of green-schist facies. Pyrite found in two generations; Type1 is irregular grains, contains higher arsenic and gold contents, compared to the relatively younger phase Type2 pyrite, which is composed of euhedral grains, found adjacent to late quartzcarbonate veins or at rims of type1 pyrite. Two gold generations were identified; type1 found included in sulphides (mainly pyrite). The second gold type was remobilized (secondary) into free-lodes within silicates (mainly quartz). Gold fineness is high, as gold contains up to 95 wt. % Au, Ag up to 3.5 wt. %, and traces of Cu, Ni, and Fe. Pyrite type2 contains tiny mineral domains (rich in Al, Mn, Hg, Se, Ti, V, and Cr). Zoning, and replacement textures are common, suggesting multiple mineralization stages. The distribution and relationships of trace elements in pyrite type2 indicate three formation patterns: (1) Al, Mn, Hg, Se, Ti, V, Cr, and Sn are homogeneously distributed in pyrite, reflecting a synchronous formation. (2) As, Ni, Co, Zn, and Sb display heterogeneous distribution pattern in pyrite, which may indicate post-formation existence due to other activities. (3) Au and Ag show both distribution patterns within pyrite, suggesting that gold is found both in microscopic phases and as chemically bounded phase.

Keywords Sheba, pyrite mapping, zoning, LA-ICP-MS, EMP

Article

Research Paper

Econ. Environ. Geol. 2021; 54(2): 213-232

Published online April 30, 2021 https://doi.org/10.9719/EEG.2021.54.2.213

Copyright © THE KOREAN SOCIETY OF ECONOMIC AND ENVIRONMENTAL GEOLOGY.

Studies on the Ore Mineralogy and Litho-geochemistry of the Sheba Deposit, Barberton Greenstone Belt, South Africa

Mohammed Alnagashi Hassan Altigani*

Department of Geology of Minerals Wealth, Faculty of Petroleum and Minerals, Alneelain University, 11121, Sudan

Correspondence to:m.alnagashi@gmail.com

Received: January 8, 2021; Revised: March 7, 2021; Accepted: March 10, 2021

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided original work is properly cited.

Abstract

Ore criteria at the Sheba Deposit indicate orogenic mineralization type. Rocks and mineral assemblages suggest low formationtemperature of green-schist facies. Pyrite found in two generations; Type1 is irregular grains, contains higher arsenic and gold contents, compared to the relatively younger phase Type2 pyrite, which is composed of euhedral grains, found adjacent to late quartzcarbonate veins or at rims of type1 pyrite. Two gold generations were identified; type1 found included in sulphides (mainly pyrite). The second gold type was remobilized (secondary) into free-lodes within silicates (mainly quartz). Gold fineness is high, as gold contains up to 95 wt. % Au, Ag up to 3.5 wt. %, and traces of Cu, Ni, and Fe. Pyrite type2 contains tiny mineral domains (rich in Al, Mn, Hg, Se, Ti, V, and Cr). Zoning, and replacement textures are common, suggesting multiple mineralization stages. The distribution and relationships of trace elements in pyrite type2 indicate three formation patterns: (1) Al, Mn, Hg, Se, Ti, V, Cr, and Sn are homogeneously distributed in pyrite, reflecting a synchronous formation. (2) As, Ni, Co, Zn, and Sb display heterogeneous distribution pattern in pyrite, which may indicate post-formation existence due to other activities. (3) Au and Ag show both distribution patterns within pyrite, suggesting that gold is found both in microscopic phases and as chemically bounded phase.

Keywords Sheba, pyrite mapping, zoning, LA-ICP-MS, EMP

    Fig 1.

    Figure 1.Geologic map of the Barberton Greenstone Belt and surrounding granitoid terrane (Kisters, et al., 2010), also shown the locations of the studied gold deposit.
    Economic and Environmental Geology 2021; 54: 213-232https://doi.org/10.9719/EEG.2021.54.2.213

    Fig 2.

    Figure 2.Photomicrographs of metapelite from the Sheba Deposit, (A) poikiloblastic garnet (cross-polarized light), (B) Biotite occurred after decomposed garnet (plain polarized light).
    Economic and Environmental Geology 2021; 54: 213-232https://doi.org/10.9719/EEG.2021.54.2.213

    Fig 3.

    Figure 3.Ternary diagram of molecular Al2O3 (CaO+Na2O+K2O) (FeOt+MgO), showing-weathering trends of granitic source (A) and basaltic source (B). C= digenetic and/or metasomatic transformation of kaolinite (Ka) into illite (Ill) with fluids characterized by high K+/H+ ratios. D= digenetic and/or metasomatic transformation of kaolinite into chlorite (Chl) with fluids characterized by high Mg+2 /H+ ratios. Bt= biotite; Kfs= feldspar; Hb= hornblende; Ms= muscovite; Sm= smectite, (Toulkeridis et al., 1999).
    Economic and Environmental Geology 2021; 54: 213-232https://doi.org/10.9719/EEG.2021.54.2.213

    Fig 4.

    Figure 4.A: Pyrite Type 1 in metapelite, sample 125941, section B, Sheba Mine. Shows rotation in the pyrite grains. B: Pyrite Type 2 in arenaceous rocks, FSC-307917, Sheba. The pyrite grains are located close to a carbonate vein. Fibrous quartz is located in the pressure shadows (Altigani, et al., 2016).
    Economic and Environmental Geology 2021; 54: 213-232https://doi.org/10.9719/EEG.2021.54.2.213

    Fig 5.

    Figure 5.Compositional variation in the different types of pyrite from the Sheba Deposit using EMP data, showing different major and trace elements relationships.
    Economic and Environmental Geology 2021; 54: 213-232https://doi.org/10.9719/EEG.2021.54.2.213

    Fig 6.

    Figure 6.LA-ICP-MS results from pyrite type 2 of the Sheba Deposit illustrating the relationship between Ni and As, Co.
    Economic and Environmental Geology 2021; 54: 213-232https://doi.org/10.9719/EEG.2021.54.2.213

    Fig 7.

    Figure 7.Ni distribution around pyrite from Sheba. Concentration in the low ppm range, showing several generations of crystal growth of this composite grain. Concentration shown as counts per second.
    Economic and Environmental Geology 2021; 54: 213-232https://doi.org/10.9719/EEG.2021.54.2.213

    Fig 8.

    Figure 8.Back-scattered electron images for pyrite grain from the adjacent Fairview Deposit, indicating micro-nuggets of chromite and rutile (zoned!) occur within the pyrite. The appearance of chromite micro-grains is indicating mafic to ultramafic provenance (Altigani, 2021).
    Economic and Environmental Geology 2021; 54: 213-232https://doi.org/10.9719/EEG.2021.54.2.213

    Fig 9.

    Figure 9.a, b, c distribution of trace elements on Sheba pyrite grains.
    Economic and Environmental Geology 2021; 54: 213-232https://doi.org/10.9719/EEG.2021.54.2.213

    Fig 10.

    Figure 10.(A) Homogenous distributions of some trace elements throughout three pyrite grains of type 2 from the Sheba Deposit in the, suggesting synchronous formation of these elements with pyrite, could possibly be due to background effects as well; indicating the presence of very small mineral inclusion. (B): The skewed distributions of some elements in the three pyrite grains of type 2 from the Sheba Deposit, indicating later accumulation of these elements in zones inside the pyrite by later hydrothermal solutions. These distributions reflect heterogeneity in the pyrite type 2, which is not observed by microscopy, EMPA, or LA-ICP-MS single shots.
    Economic and Environmental Geology 2021; 54: 213-232https://doi.org/10.9719/EEG.2021.54.2.213

    Fig 11.

    Figure 11.Geological map around Sheba area, showing the distribution of major mineralised shear zones and associated gold activities. Details of the mineralised shear zones and their envelopes (grey) in the Golden Quarry area (Dirks, et al., 2009).
    Economic and Environmental Geology 2021; 54: 213-232https://doi.org/10.9719/EEG.2021.54.2.213

    Table 1 . Selected whole rock analyses using XRF powder pellets of fresh Sheba Deposit rock samples.

    Sample125208
    (metapelites)
    125490
    (metapelites)
    125941
    (metapelites)
    FSC-307917
    (greywacke)
    FSC-307919
    (metapelites)
    FSC-307929
    (muscovite arkoses)
    SiO263.8960.5167.0437.4056.2975.34
    Al2O319.412.319.415.819.514.8
    Na2O0.150.160.800.230.180.15
    MgO0.991.485.111.397.782.93
    P2O50.030.020.020.020.060.01
    K2O5.094.561.527.182.981.78
    CaO0.100.240.360.086.063.32
    Fe2O34.266.659.1910.609.603.18
    Cr2O30.140.140.260.230.300.12
    TiO20.640.610.230.970.500.19
    V0.040.040.020.060.050.02
    Mn0.010.040.050.010.280.15
    Co0.020.010.040.020.030.03
    Ni0.040.040.080.060.080.04
    Cu0.010.020.020.030.010.58
    Zn0.010.020.010.040.010.11
    Ga0.000.000.000.000.000.00
    As2.024.304.476.950.001.30
    Br0.000.000.000.000.410.00
    Rb0.020.000.010.010.000.00
    Sr0.000.020.010.020.010.01
    Y0.010.000.000.000.020.01
    Zr0.020.010.000.010.010.00

    Table 2 . Representative Electron Microprobe analyses for the Sheba Deposit ore in wt. %.

    MineralSFeAsCoNiCuZnPdAgCdSbAuHgPbBiMnTotal
    pyrite-core51.9145.222.060.010.8000.1300.0300.020.0700.010100.27
    pyrite-core52.9246.40.3700.1900.0300.010.01000.06000.0199.99
    pyrite-core52.8346.211.180.010.350.020.0300000.030000100.66
    pyrite-core52.7346.250.810.160.30.0200.0400.02000.09000100.4
    pyrite-core52.6446.3300000.010.040.010.01000.0800099.11
    pyrite-rim52.1646.460.8600.040.01000.01000000099.54
    pyrite-rim53.2546.91000.030.03000.010.0300.110.09000100.47
    pyrite-rim53.1246.610.2400.0800.01000000.0700.060100.2
    pyrite-rim53.0346.770.0400.09000.0100000.14000100.09
    pyrite-rim53.1546.950.1200.0500.03000.01000.0200.220.01100.55
    arsenopyrite19.9132.7646.2100.100000.0300.4200.010.23099.67
    arsenopyrite19.6634.8445.9400.110000000000.20.01100.76
    arsenopyrite19.9935.0445.2400.10.0200.070.01000.0400.030.070100.62
    arsenopyrite20.4234.9544.7500.060.020.020.0100.0900.150.120.030.360.01101.01
    arsenopyrite19.4634.2646.5800.240.01000000.090.1300.410.02101.2
    pyrrhotite38.0858.781.3900.17000.010000.060.0800.03098.62
    pyrrhotite38.3458.691.8200.15000.040.030000.0100.140.0399.25
    pyrrhotite38.7159.012.2600.210.020.01000.0400000.090100.36
    pyrrhotite38.5160.290.7500.4200000.0100.110.2700.030.01100.41
    pyrrhotite33.9754.735.2200.350.050.010.020000.030.0200.170.0294.59
    gold0.050.370000.07003.910096.240000100.64
    gold0.010.24000.010.09003.890.06095.5300.2900100.13

    Table 3 . Selected LA-ICP-MS qualitative analyses for the Sheba Deposit ores, in count per second (cps).

    MineralSFeMnCoNiCuZnAsAgSbAuHgPbBi
    Pyrite-rim225381.34231599352840.57680.0515196.15130.02490.19195668.60146.686.670116.670
    Pyrite-rim159633.491697246780015582.88446.73077841.820026.67000
    Pyrite-rim482341.331229229280890.152944.24956.940041054.2824336.1101874.7580.01
    Pyrite-rim465099.661154712600184179370886.100063629.6320043.3310
    Pyrite-rim676483.2295567132.8840.57680.0515196.15130.02490.19195668.60146.686.670116.670
    Pyrite-core255598.742640747521550.69403.3622555.15483.92193.4249102.8820023.34658933.330
    Pyrite-core237884.24235613652480.567705.2323939.5818330.13413.46703464.780880.12964.422900.3686.7713.34
    Pyrite-core209421.14228641152937.0586228.6418426.5890.02553.52225692.18013.34500200
    Pyrite-core179210.39169634078043622.1837235.9100135835.2000000
    Pyrite-core165636.491503864780036200.680099918.21000000
    Pyrite-core184889.791690179780027463.148755.710422058.2100486.7000
    Pyrite-core182040.79169491578001186272981.620336172.5100203.34000
    Pyrite-core1498761.7527050094786.711086.892073.96730.15273.41033777.1112228.50836.8660.01
    Pyrite-core1200742.942414365710015582.88446.73077841.820026.67000
    Pyrrhotite66251.871027396900200.0120690.85496.95433.622504.75076.6713.3301600.42183.35
    Pyrrhotite64029.481047811900360.0320000.75170.01067388.220173.3516.660363.36230.02
    Pyrrhotite28883.58237813380001586329002358.35000000
    Sphalerite20361.3632248980002736820324029.1412724342661.28908022.670000
    Sphalerite24059.333852101.700021193197383165.0439966.13697027.710950330.670000
    Sphalerite26703.284123606.700019985491340788.9444460.01350643.510356800.670000
    Arsenopyrite69033.051199071520890.152944.24956.94021514550041054.2824336.1101874.7580.01
    Arsenopyrite66307.0793026362.886.711086.892073.96730.15273.4119547570033777.1112228.50836.8660.01
    Arsenopyrite66541.961128794900184179370886.160029801965063629.6320043.3310
    Gold000006972.930586.693059111.6024383633.28000
    Gold000000002989561028967353000
    Gold000000003234967024813073000

    KSEEG
    Feb 29, 2024 Vol.57 No.1, pp. 1~91

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