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

Mohammed Alnagashi Hassan Altigani

doi:10.9719/EEG.2021.54.2.213

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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

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

Mohammed Alnagashi Hassan Altigani^*

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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

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

Abstract

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

Fig 1.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

Sample	125208 (metapelites)	125490 (metapelites)	125941 (metapelites)	FSC-307917 (greywacke)	FSC-307919 (metapelites)	FSC-307929 (muscovite arkoses)
SiO₂	63.89	60.51	67.04	37.40	56.29	75.34
Al₂O₃	19.4	12.3	19.4	15.8	19.5	14.8
Na₂O	0.15	0.16	0.80	0.23	0.18	0.15
MgO	0.99	1.48	5.11	1.39	7.78	2.93
P₂O₅	0.03	0.02	0.02	0.02	0.06	0.01
K₂O	5.09	4.56	1.52	7.18	2.98	1.78
CaO	0.10	0.24	0.36	0.08	6.06	3.32
Fe₂O₃	4.26	6.65	9.19	10.60	9.60	3.18
Cr₂O₃	0.14	0.14	0.26	0.23	0.30	0.12
TiO₂	0.64	0.61	0.23	0.97	0.50	0.19
V	0.04	0.04	0.02	0.06	0.05	0.02
Mn	0.01	0.04	0.05	0.01	0.28	0.15
Co	0.02	0.01	0.04	0.02	0.03	0.03
Ni	0.04	0.04	0.08	0.06	0.08	0.04
Cu	0.01	0.02	0.02	0.03	0.01	0.58
Zn	0.01	0.02	0.01	0.04	0.01	0.11
Ga	0.00	0.00	0.00	0.00	0.00	0.00
As	2.02	4.30	4.47	6.95	0.00	1.30
Br	0.00	0.00	0.00	0.00	0.41	0.00
Rb	0.02	0.00	0.01	0.01	0.00	0.00
Sr	0.00	0.02	0.01	0.02	0.01	0.01
Y	0.01	0.00	0.00	0.00	0.02	0.01
Zr	0.02	0.01	0.00	0.01	0.01	0.00

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

Mineral	S	Fe	As	Co	Ni	Cu	Zn	Pd	Ag	Cd	Au	Hg	Pb	Bi	Mn	Total
pyrite-core	51.91	45.22	2.06	0.01	0.8	0	0	0.13	0	0.03	0.02	0.07	0	0.01	0	100.27
pyrite-core	52.92	46.4	0.37	0	0.19	0	0.03	0	0.01	0.01	0	0.06	0	0	0.01	99.99
pyrite-core	52.83	46.21	1.18	0.01	0.35	0.02	0.03	0	0	0	0.03	0	0	0	0	100.66
pyrite-core	52.73	46.25	0.81	0.16	0.3	0.02	0	0.04	0	0.02	0	0.09	0	0	0	100.4
pyrite-core	52.64	46.33	0	0	0	0	0.01	0.04	0.01	0.01	0	0.08	0	0	0	99.11
pyrite-rim	52.16	46.46	0.86	0	0.04	0.01	0	0	0.01	0	0	0	0	0	0	99.54
pyrite-rim	53.25	46.91	0	0	0.03	0.03	0	0	0.01	0.03	0.11	0.09	0	0	0	100.47
pyrite-rim	53.12	46.61	0.24	0	0.08	0	0.01	0	0	0	0	0.07	0	0.06	0	100.2
pyrite-rim	53.03	46.77	0.04	0	0.09	0	0	0.01	0	0	0	0.14	0	0	0	100.09
pyrite-rim	53.15	46.95	0.12	0	0.05	0	0.03	0	0	0.01	0	0.02	0	0.22	0.01	100.55
arsenopyrite	19.91	32.76	46.21	0	0.1	0	0	0	0	0.03	0.42	0	0.01	0.23	0	99.67
arsenopyrite	19.66	34.84	45.94	0	0.11	0	0	0	0	0	0	0	0	0.2	0.01	100.76
arsenopyrite	19.99	35.04	45.24	0	0.1	0.02	0	0.07	0.01	0	0.04	0	0.03	0.07	0	100.62
arsenopyrite	20.42	34.95	44.75	0	0.06	0.02	0.02	0.01	0	0.09	0.15	0.12	0.03	0.36	0.01	101.01
arsenopyrite	19.46	34.26	46.58	0	0.24	0.01	0	0	0	0	0.09	0.13	0	0.41	0.02	101.2
pyrrhotite	38.08	58.78	1.39	0	0.17	0	0	0.01	0	0	0.06	0.08	0	0.03	0	98.62
pyrrhotite	38.34	58.69	1.82	0	0.15	0	0	0.04	0.03	0	0	0.01	0	0.14	0.03	99.25
pyrrhotite	38.71	59.01	2.26	0	0.21	0.02	0.01	0	0	0.04	0	0	0	0.09	0	100.36
pyrrhotite	38.51	60.29	0.75	0	0.42	0	0	0	0	0.01	0.11	0.27	0	0.03	0.01	100.41
pyrrhotite	33.97	54.73	5.22	0	0.35	0.05	0.01	0.02	0	0	0.03	0.02	0	0.17	0.02	94.59
gold	0.05	0.37	0	0	0	0.07	0	0	3.91	0	96.24	0	0	0	0	100.64
gold	0.01	0.24	0	0	0.01	0.09	0	0	3.89	0.06	95.53	0	0.29	0	0	100.13

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

Mineral	S	Fe	Mn	Co	Ni	Cu	Zn	As	Ag	Sb	Au	Hg	Pb	Bi
Pyrite-rim	225381.34	231599352	840.57	680.05	15196.15	130.02	490.19	195668.6	0	146.68	6.67	0	116.67	0
Pyrite-rim	159633.49	169724678	0	0	15582.88	446.73	0	77841.82	0	0	26.67	0	0	0
Pyrite-rim	482341.33	122922928	0	890.15	2944.24	956.94	0		0	41054.28	24336.11	0	1874.75	80.01
Pyrite-rim	465099.66	115471260	0	184179	370886.1	0	0		0	63629.63	20	0	43.33	10
Pyrite-rim	676483.22	95567132.8	840.57	680.05	15196.15	130.02	490.19	195668.6	0	146.68	6.67	0	116.67	0
Pyrite-core	255598.74	264074752	1550.69	403.36	22555.15	483.92	193.4	249102.88	20	0	23.34	6589	33.33	0
Pyrite-core	237884.24	235613652	480.56	7705.23	23939.58	18330.13	413.46	703464.78	0	880.12	964.42	2900.3	686.77	13.34
Pyrite-core	209421.14	228641152	937.05	86228.64	18426.58	90.02	553.52	225692.18	0	13.34	50	0	20	0
Pyrite-core	179210.39	169634078	0	43622.18	37235.91	0	0	135835.2	0	0	0	0	0	0
Pyrite-core	165636.49	150386478	0	0	36200.68	0	0	99918.21	0	0	0	0	0	0
Pyrite-core	184889.79	169017978	0	0	27463.14	8755.71	0	422058.21	0	0	486.7	0	0	0
Pyrite-core	182040.79	169491578	0	0	118627	2981.62	0	336172.51	0	0	203.34	0	0	0
Pyrite-core	1498761.75	270500947	86.71	1086.89	2073.96	730.15	273.41		0	33777.11	12228.5	0	836.86	60.01
Pyrite-core	1200742.94	241436571	0	0	15582.88	446.73	0	77841.82	0	0	26.67	0	0	0
Pyrrhotite	66251.87	102739690	0	200.01	20690.85	496.95	433.62	2504.75	0	76.67	13.33	0	1600.42	183.35
Pyrrhotite	64029.48	104781190	0	360.03	20000.75	170.01	0	67388.22	0	173.35	16.66	0	363.36	230.02
Pyrrhotite	28883.58	23781338	0	0	0	15863290	0	2358.35	0	0	0	0	0	0
Sphalerite	20361.36	3224898	0	0	0	2736820	324029.14	12724	342661.2	8908022.67	0	0	0	0
Sphalerite	24059.33	3852101.7	0	0	0	21193197	383165.04	39966.13	697027.7	10950330.67	0	0	0	0
Sphalerite	26703.28	4123606.7	0	0	0	19985491	340788.94	44460.01	350643.5	10356800.67	0	0	0	0
Arsenopyrite	69033.05	119907152	0	890.15	2944.24	956.94	0	21514550	0	41054.28	24336.11	0	1874.75	80.01
Arsenopyrite	66307.07	93026362.8	86.71	1086.89	2073.96	730.15	273.41	19547570	0	33777.11	12228.5	0	836.86	60.01
Arsenopyrite	66541.96	112879490	0	184179	370886.16	0	0	29801965	0	63629.63	20	0	43.33	10
Gold	0	0	0	0	0	6972.93	0	586.69	3059111.6	0	24383633.28	0	0	0
Gold	0	0	0	0	0	0	0	0	2989561	0	28967353	0	0	0
Gold	0	0	0	0	0	0	0	0	3234967	0	24813073	0	0	0

Aug 30, 2024 Vol.57 No.4, pp. 353~471

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

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

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