Petrogenesis of the Sheba lithology
Ore chemistryand genesis of the gold deposits and associated sulphides at Sheba
Elements distribution on pyrite as indicative criteria for mineralisation stages
The Barberton Greenstone Belt (BGB) is surrounded by large areas of granitic gneisses, which range in age from about 3.5-2.7 Ga (Ward, 1995, 1999; Brandl
(1) The Onverwacht Group (ca. 3.49-3.3 Ga) consists of komatiites, komatiitic basalts, basalts and pillow lavas, minor felsic volcanics, and sedimentary rocks (such as chert beds), which are formed in a deep to shallow marine environment (Eriksson
(2) The Fig Tree Group (3.260–3.226 Ga) is a meta-turbiditic succession (Hofmann, 2005), up to 2000 m thick (Eriksson
(3) The Moodies Group is characterized by arenaceous rocks (Heubeck, and Lowe, 1994; Hofmann, 2005; Van Kranendonk
The Moodies Group (3.2 Ga, Kamo and Davis, 1994) was deposited in a shallow marine to fluvial environment, prior to the emplacement of the Kaap Valley tonalite plutons. Eriksson,
The Moodies Group, which includes syn-orogenic sediments, may have been deposited during amalgamation of the four terrains just after 3.2 Ga (Heubeck, 1993; Van Kranendonk
Several structural investigations have been carried out at the BGB, e.g., (Robertson
The NE-trending and SE-dipping main shear zones, which are sub-parallel to the bedding planes of the Sheba Formation.(2) The cross-shear zones, which are a set of shallow and southerly dipping structures, (3) the shear zones that truncated the earlier structures.
The structural interpretation model of Robertson
Archaean greenstone belts, such as the Barberton Green-stone Belt (BGB), represent essential contributor in world gold production. In these belts (Fig. 1), the geological setting of the mineralization source, and host rocks, coupled with high sensitivity of these rocks to metamorphism and hydro-thermal alterations, led to a wide range of difficulties and complications, to determine the genesis and characteristics of the enclosed gold bearing-ore deposits. Understanding of the impacts of mineral associations, metamorphism, deformation, and alteration on these lengthily re-worked and deformed rocks (subjected to multiple phases of tectonism and hydrothermal activities) is very important in order to describe gold and associated sulphides deposits.
The main host lithology for gold mineralization of Sheba deposit are quartzites (arkoses), shales, sandstones, meta-mafic and meta-ultramafic, and greywackes, belonging to the Fig Tree and the Moodies Groups (Van Kranendonk
The gold-bearing mineralized zones are either hosted in quartz and/or carbonate-rich veins along the shear zones and fractures, also with disseminated sulphides (mainly pyrrhotite, arsenopyrite, and pyrite) within the adjacent wall-rock alteration zones (Altigani
Kamo and Davis (1994) suggest that gold deposition occurred in the late tectonic development of the BGB, and has been interpreted to be temporally linked to the intrusion of late potash-granites and syenites of ca. 3.14–3.1 Ga age, and during the D3 stage. Nevertheless, gold formation could be related to one long-tectonic evolution, which comprises of different stages (Altigani
Gold mineralization at the BGB is structurally controlled by E-W trending and strike-slip shear zones (Dirks
In this study, we applied EMP and LA-ICP-MS techniques in order to determine the elemental relationships in the pyrite Type 2 of Sheba deposit. This guide to better understanding of the formation conditions of the hosted gold in associated sulphides. The compositional fingerprints of inclusive gold that associated with sulphides should reflect the related genetic processes. Based on this hypothesis, it can be said that the differences or changes in the minerals forming processes/conditions will also be reflected in their trace element contents and distribution patterns.
Several analytical techniques were utilized to accomplish the purpose of this study, including X-ray diffraction (XRD), X-ray florescence (XRF), scanning electron microscopy (SEM), electron microprobe (EMP), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). For XRD and XRF analyses, the samples were reduced to chips of 4–8 mm by jaw crusher, and were subsequently pulverized in a tungsten carbide disc mill.
For the microscopic investigations, polished and thin-sections were prepared at the Geology Department-University of Pretoria. These sections were studied using a conventional transmitted and reflected light microscope in order to determine the mineralogical assemblages, textural, and micro structural characteristics, the degree of mineral alterations, and the metamorphic grade for the rock associations. Representative fresh samples were selected for chemical analyses after microscopic examinations. Finally, the mineral para-geneses, textures, and structures of the ores were also investigated.
For the XRD analysis, ten grams from each sample were ground following the standard procedure. They were examined using the X-Ray diffractometer model ‘
The data was collected in the angular range 5° ≤ 2θ ≤ 90° with a step size 0.008° 2θ. The phases were identified using X’Pert Highscore Plus software. Errors are on the 3-sigma level, amorphous phases (if present) were not taken into consideration in the quantification
The relative phase amounts (weight %), were estimated by the Rietveld method using Autoquan/BGMN software (GE Inspection Technologies; Kleeberg & Bergamnn) employing Fundamental Parameter Approach. In the Rietveld Method, an observed data pattern is compared to a calculated pattern. By variation of all parameters, the difference between the calculated and observed pattern is then minimized by a least square procedure, until the best possible fit is obtained.
The samples were analysed using the pressed powder technique. Unpublished research in the department of Geology at the University of Pretoria demonstrated that major elements analysis of the same samples using lithium tetra-borate glass and powder briquettes were indistinguishable within standard analytical errors.
Ten to 12 grams (75% <63 µm) were taken from each sample and mixed with ten drops of polyvinyl alcohol (48-80) saturated solution (as binder) added to each sample powder, then loaded into collapsible aluminium cups (diameter 40 mm) and pressed by a manual oil-hydraulic press under 15 kPa. The Cr-steel piston (diameter 40 mm) was levelled by polishing. The analyses were performed using the wavelength dispersive X-Ray Fluorescence Spectrometer (PANalytical) model “
The scanning electron microscope (SEM) is a type of electron microscope that takes images of the sample surface by passing a high-energy beam of electrons across the surface of the sample (Reed, 2005). The electrons interfere with the atoms of the sample, producing signals that contain information about the samples surface topography and its chemical composition (Goldstein, 2003). The energy-dispersive X-ray spectroscopy (EDS) is an analytical technique used for the elemental analysis or chemical characterisation of the samples (Goldstein, 2003; Reed, 1983; Reed, 2005).
The polished sections used in this study were coated with carbon with a thickness of approximately 40Å, which allows dispersion of charging during the SEM analyses. A silver strip used for conduction between carbon coating on the sample and the sample holder during the analysis. The scanning microscope instrument, model ‘JEOL
Electron microprobe analysis (EMP) is a technique used for chemically analysing small selected areas in solid samples, in which a focused electron beam (Reed, 1983; Reed, 2005) excites X-rays. EMP is suitable for analysing elemental composition down to the level of a few tens of µg/g (0.010-0.001 % m/m) (Humayun
In this study, imaging and elemental analysis were performed (using the ‘
Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is favoured in many geological studies including ore mineral identification, because of its high range of sensitivity, high precision, low detection limits, and relatively a large analysed sample volume compared with EMP, which mainly exadeposits samples surface (Sylvester
In this research, an Agilent Technologies model ‘
Hessler and Lowe (2006) proposed that the Sheba rocks are composed of meta-psammitic and quartz-rich arkoses. Muscovite is found in the matrix, which formed a weak fabric approximately normal to compositional layering. The XRD results of these arkoses show a high content of quartz and biotite, ranging from 50.1 to 56.4 wt. % and 31.6 to 43.3 wt. % respectively.3.1.2. Metapelites
These rocks consist of biotite, garnet, and quartz. Biotite is the dominant mineral. Quartz is found as a sub-dominant phase in these rocks. Garnet forms augen micro-textures, zoned, and has developed a new generation of garnet at its boundaries (Fig. 2). Garnet is poikiloblastic and contains abundant inclusions of biotite and quartz. These minerals assemblage is indicative for low to medium-grade metamorphism that could be related to the late granitic intrusions, which implies at least two main metamorphic episodes affected the study area. Dynamic metamorphism is associated with the shearing in the late stages of the two main episodes.
These rocks are composed of 53.3-56.4 wt. % quartz, 25.9-31.6 wt. % muscovite, 12.6-16.3 wt. % dolomite, 11.5-12.6 wt. %, pyrite 2.7-11.5 wt. %, and 2.5-4.4 wt. % chlorite. The rocks display alternating bands of quartz and dolomite, intercalated with sub-parallel sulphides veins. Two types of pyrite are distinguished: the first one is well-developed porphyroblast, which are spatially situated close to the quartz-carbonate veins (Altigani,
The other pyrite type has anhedral shape. Minor amounts of muscovite, chlorite, and quartz polygons are commonly associated with these sulphides. Carbonate-rich rocks of the Sheba deposit consist of a fine-grained quartz matrix, which is crosscut by sub-parallel veins of calcite and muscovite. The groundmass contains disseminated sulphide porphyroblasts.
The majority of the studied samples from Sheba deposit are dominated by quartz, calcite, biotite, muscovite, garnet, and chlorite. This mineral assemblage indicates low green-schist metamorphic facies (Miyashiro, 1973; Stiegler
Hessler and Lowe (2006) suggested that the basal conglomerates of the Moodies Group (which the Sheba Deposit represents part of it), were derived from volcanic and sedimentary rocks of the Swaziland Supergroup. They indicated that felsic volcanic rocks, cherts, silicified ultramafic fragments, and quartz veins dominate the sedimentary clasts of the Moodies Group.
The dissolution and re-crystallization of the quartz during the prograde metamorphism formed a new generation of small (120o triple-junction) quartz polygons. This texture indicates textural equilibrium during the re-crystallization at higher temperatures coupled with lack of deformation (Barrie
Quartz and carbonate veins are common in the majority of the Sheba rocks. Most of these veins were deposited concordantly with either relicts of primary bedding, or the foliation planes, while some of these veins filled the shearing planes. The shape, symmetry, and the interaction of these veins with the country rocks suggest late injection. These quartz veins may have formed due to a recent magmatic hydrothermal activity (De Vries and Touret, 2007) or due to dehydration that was caused by prograde meta-morphism (Dziggel
The mineralogical relationships in the Sheba rocks indicate multi-metamorphism episodes affected the lithologies of the BGB. The first, is regional type affected the mafic and ultrabasic units, and associated with early tectonism, and; producing greenschist metamorphic facies. The second is a prograde thermal and/or regional type that delivers higher-grade metamorphic facies (high greenschist, low amphibolite facies), which found occasionally close to the granitic intrusions. The third is dynamic metamorphic event was associated with shearing, which caused brecciating, fracturing and recrystallization of these rocks and associated ores, forming structural porosity and pathways for the syn-to late orogenic mineralized fluids. These metamorphic events produced changes to the mineralogy and morphology of the ore-minerals of Sheba deposit, reflected in heterogeneity, zoning, alteration processes, and trace elements (Au) remobilization (Altigani
Several geochemical investigations were carried out on the pelites of the Sheba Formation (e.g., Danchin, 1967; Condie
The XRF whole rock analyses indicate that most of the studied arkoses of the Sheba Formation display a felsic rock composition. SiO2 values explain the relatively high quartz contents for the majority of the samples. The relatively low values of Al2O3 suggest low clay contents, which indicate chemical weathering and short transporting distance from the original material of the Sheba rocks (Hofmann, 2005).
The metapelites of the Sheba are rich in MgO and Fe2O3 rather than Na2O, K2O, and CaO. The Mg content ranges between 1.4-7.8 wt. % (average 3.7 wt. %). Fe2O3 values are higher than 10 wt. %, which is consistent with values from other Archaean greywackes terrains (McLennan, 1984). The CaO values in most of these samples are very low, with an average of 2 wt. %. However, two of the samples from the Sheba Deposit have relatively high values of CaO (6.1 and 3.3 wt. %). The positive correlation between K2O and TiO2 suggests that these elements are located within the micas of the Sheba pelites (Hofmann, 2005)
Greywackes at Sheba show a dacitic to rhyodacitic composition in the major-element chemical composition (Toulkeridis
The Na2O values are less than 1%. Fe2O3 and MgO show high values (Table 1). High Mg and relatively high iron contents suggest a significant mafic to ultramafic component in the source of the Sheba greywackes (Hofmann, 2005), and intermediate to basic rock fragments (McLennan, 1984). Thus, the chemical compositions and chemical affinities(calc-alkaline - tholeiitic) for the Sheba Formation implies mixed mafic–felsic (volcano-sedimentary) sources of different proportions (Toulkeridis
In these rocks, the main sulphides are pyrite (2.7-11.5 wt. %, XRD analyses), and arsenopyrite (3.5-7.7 wt. %, XRD analyses). Pyrite forms poikiloblast large grains, which are consistent with the host rocks general foliation. Small amounts of gold electrum are enclosed within the pyrite. Based on the textural evidence obtained from the reflected microscope, subhedral to euhedral crystals of arsenopyrite were formed after the pyrite.3.3.2. Greywacke
Pyrite represents the dominant sulphide phase in these rocks (up to 11.5 wt. %, XRD analyses). Two phases of pyrite exist in the studied deposit (Figs. 4 A and B); the early pyrite type 1 (core) phase is porous, irregular shape, and includes arsenopyrite, pyrrhotite, chalcopyrite, and rare gold inclusions. The later pyrite phase type 2 (rims) occurs as large idiomorphic, seems to be clear, very compact, and concentrates close to the late quartz-carbonate veins. In some samples pyrite type 2 (rim) overgrowing pyrite type 1 (core).
In these rocks, the dominant ore mineral is arsenopyrite (up to 7.7 wt. %, XRD analyses). Gold in these rocks is found as small grains located between arsenopyrite and pyrrhotite, intergrowth textures are very common between different sulphide phases such as arsenopyrite-pyrite, pyrite-pyrrhotite, arsenopyrite-pyrrhotite, and chalcopyrite-pyrrhotite. Arsenopyrite is found close to the pyrite and is restricted to parallel bands concordant with the general foliation. Minor sphalerite is found located close to pyrite. Chalcopyrite is found enclosed by the early pyrite phases.
Spot analyses (131 points - Table 2) were performed on the sulphides of the Sheba Deposit, using the electron microprobe (EMP). The principal sulphide phases in the Sheba ores are pyrite, pyrrhotite, arsenopyrite, and rare chalcopyrite and sphalerite.
The EMP results show that there are differences in the elemental contents of the analysed pyrite grains, coupled with the textural variations gives an indication that there are two types of pyrite within the Sheba ores (Table 2). These are:
A) Pyrite type 1 (core), which is considered as the older type. This type of pyrite varies in its S + Fe contents due to the substitution of Fe and S by both Ni, Co, and As respectively, to form small domains of arsenian pyrite, and arsenopyrite. The EMP results show that gold content in pyrite type 1 increases with increasing iron and sulphur rather than arsenic. However, nickel content increases with arsenic concentration in the pyrite composition. Bismuth content is higher in this type compared to pyrite type 2. However, nickel content increase with arsenic concentration and replaced iron (Fig. 5). This also seen in LA-ICP-MS results, in which Ni is positively correlated with As, but has no systematic relationship with Co (Fig. 6).
B) Pyrite type 2 (rim) is relatively younger and is found overgrowing pyrite type 1. It forms well-developed cubes and eight-sided crystals. This pyrite type is homogenous and not showing any zoning or inclusions when using microscopy or back-scattered electron imaging. Nevertheless, the use of LA-ICP-MS mapping technique (Table 3) revealed that this type contains micro-zoning and very minute inclusions.
The iron content of pyrite is slightly variable; indicating the replacement of Fe by Ni, As and Co. The substitution and possible metasomatism, which occur due hydrothermal activities causes a development, occasionally, of new generations of Ni and Co rich pyrite and Ni-arsenides.
Both types of pyrite in this deposit commonly contains invisible gold (up to 1 wt. %), which was thought by many authors (e.g., Vaughan and Kyin, 2004) to be related to the arsenic content in the pyrite itself. Analysed gold in pyrite grains does not illustrate any systematic relationships between Au and Fe, S, As, Ag, Ni, Bi, and Hg (Fig. 5).
Sphalerite of the Sheba Deposit contains considerable amounts of Co, Ni, Bi, and Cd. The chemical composition of sphalerite is variable, indicating a chain of mineral, which may form between sphalerite to Fe-rich member, which might be due to the effects of associated chalcopyrite and arsenopyrite. Chalcopyrite and pyrrhotite are usually form intergrowth textures, suggesting corresponding formation.
Arsenopyrite of the Sheba Deposit contains up to 0.26 wt. % Ni, 0.15 wt. % Au, 0.24 wt. % Hg and 0.41 wt. %B (Table 2). Found in rhombic domains, long laths, and irregular shaped with oscillatory growth zones.
Gold in pyrite and arsenopyrite and with large, pyrrhotite grains of the Sheba Deposit occurs in two chemical forms: invisible and elemental (Cabri
The ore microscopic investigations demonstrated that the dominant sulphides in Sheba are pyrite, arsenopyrite, pyrrhotite, chalcopyrite, and, to a lesser degree, sphalerite. Nevertheless, the primary ore-forming process is difficult to distinguish in such assemblages, because they were subjected to long period of metasomatism and deformation. Ores and host rocks were intensively tectonically reworked. Native gold occurs in many of these samples as inclusions associated with haematite. This reflects hydrothermal (orogenic) origin rather than magmatic for these sulphides (Hutchinson, 1993). The origin of hydrothermal solutions plays a major distinguished role appeared in textures and trace elements distributions among the ores (Altigani
In most of the Sheba samples, two (generations) types of pyrite were identified based on textural and chemical variations. However, Agangi
The early pyrite type 1 is partially decomposed to pyrrhotite, which may possibly have happened as result of increasing the temperature, and lowering of the sulphur fugacity (Hu
This type 2 is found adjacent to quartz-carbonate veins, reflecting the fundamental effects of these late hydrothermal veins in remobilizing and reforming pyrite and other sulphides of the Sheba Deposit (Cook
The contacts between pyrite type 2 and pyrrhotite suggest relatively later formation of pyrite type 2. In some samples, arsenopyrite is found close to sphalerite, but is more commonly inter-grown with pyrite than with pyrrhotite. Arsenopyrite is commonly forms oscillatory zones, which reflect different timing of arsenopyrite formation, fluctuation of trace elements, or reactions. The effects of the late hydrothermal solutions are obvious in the Sheba Deposit ores. It contributes by a wide range of textural variations and chemical reactions between the solutions and the pre-existent gold-bearing sulphides, which obviously inherited in their element’s distribution, forming elemental zoning or heterogeneities (Hammond and Tabata, 1997; Hammond,
These solutions are responsible of the formation of many textures, such as, atoll texture in the pyrite type 1. The rounded pyrite, found in these ores, is seems to be a detrital, which may have been derived from different sources including: (1) sulphides of magmatic-hydrothermal or metamorphism-related hydrothermal origin, hosted in granitoid–greenstone regions of sedimentary basins, (2) Older sedimentary rock successions, or (3) syn-depositional to a digenetic intra-formational sulphides signifying primary chemical precipitates, early digenetic products or secondary replacements (Hofmann
The differences in the gold size and shapes represented in the Sheba Deposit indicate the variation in the geological processes (Cook
Pyrite can incorporate Au through different processes, which include solid solution, or containing very small inclusions of gold or gold bearing mineral (Simon
LA-ICP-MS mapping results of three pyrite grains disclosed that the pyrite type 2 from the Sheba Deposit is compositionally zoned, and heterogeneous (Fig. 7). These zones are due to differences in elemental distributions in the pyrite grains (Dixon,
Trace elements, such as Sn, Mn, Cu, Hg are displaying homogenouse distributions in the structure of the pyrite type 2 of the Sheba Deposit, which implies synchronous incorporation of these elements with the pyrite. In contrast, other trace elements, such as As, Zn, Ni, Co, and Sb show positevly skewed distributions in the pyrite, suggesting that these elements were hosted by the pyrite due to exchanges with late hydrothermal solutions. These elemets are rather concentrated in certain zones.
Gold and silver show both distribution styles, which indicates two generations (episodes) of gold mineralization s, or could be due to existence of silver as solid solution or micro-inclusions in the pyrite (Cook
(a) Distribution of elements in the inclusions enclosed by the pyrite grains
the distribution pattern, obtained from some elements like Al and Ti reflects apperence of micro to nano-scale particles of Al and Ti-rich mineral (rutile, sphene!). Discrete chromite and rutile grains were also found inside the pyrite of the adjacent Fairview Deposit, which obtained by using high quality SEM (Fig. 8). Gold and other sulphides could also be found as nano-particles within the pyrite structure (Chenery
The elements distribution patterns of S and Fe reflect variations in the ablation rate rather than substitution. Sulphur and iron distributions in pyrite show zigzagging pattern (Fig. 9A), which reflects zonation. Nickel and cobalt utilize increasing at the pyrite boundaries (Figs. 9B&C) that could be related to later hydrothermal activities. The elements (Al, Ti, V, Ge and Cr) have homogenouse distribution inside the pyrite, while others are showing hetergenouse distribution patteren (As, Ni, Co).
LA-ICP-MS mapping reveals those very minute inclusions rich in such trace elements (Sn, Mn, Cu, and Hg) display homogenous distributions (Fig. 10A) in the structure of the pyrite type 2 of the Sheba Deposit, which implies synchronous incorporation of these elements with the pyrite. In contrast, other rich-inclusions in (As, Zn, Ni, Co, and Sb) show positively skewed distributions in the pyrite (Fig. 10B), suggesting these elements were hosted by the pyrite due to exchanges by late hydrothermal solutions. These elements are rather concentrated in certain zones.
(b) The zonation of trace-elements in the pyrite grains
The pyrite type 2 of the Sheba Deposit is zoned, this revealed by the distribution patterns of the trace elements in the pyrite.
(c) The distribution of the trace-elements in structure of the pyrite grains
Some of these elements show normal distributions (Sn, Mn, Cu, Hg), however; others are diplaying hetergenouse distribution patterns in the structure of the pyrite itself (As, Zn, Ni, Co, Sb). gold and silver show the both distribution types.
(d) The relationships of trace-elements in the pyrite grains
The data set was cleaned from the zones results to examin the elemental relationships only in the pyrite. No systematic relationship was found between gold and arsenic in the structure of the pyrite. Nickel has a possitive corelation with arsenic. While cobalt does not indiate any clear relationship with nickel. the decreasing of As and Ni in single pyrite grains from core to rim, indicating these ores formed in reduction conditions (Hammond and Tabata, 1997).
The dominant rock types at the Sheba deposit are arkoses, metapelites, and greywackes. Most of these rocks have the same lithology with minor variations in the quartz and mica contents (up to 56 and 31 wt. %) respectively. At least two generations of quartz, muscovite, and sulphides are distinguished in this deposit. This suggests two episodes of metamorphism and/or remobilization affected these rocks. The lithology of this deposit reflects sequence of immature, and impure sandstones beds (which intercalated with shale and carbonate seams), and intermediate to basic metamorphosed rocks that intercalated with the sedimentary sequence. All of the Sheba rock assemblage is metamorphosed generally under green schist facies conditions, however, some metapelite shows medium to high metamorphic assemblages, which localized adjacent to the granitic intrusions, there are also indications for retrograde metamorphism that could be seen in the formation of secondary biotite after the garnet porphyroblast.
The two main types of metamorphism in this deposit caused very faint effects on the primary sedimentary structures. The second episode of metamorphism led to a retrogression for the micas and feldspars in the Sheba rocks, creating new generations of carbonates, chlorites, and epidote-mineral group. Graphite (organic materials) is very common in the arkoses and metapelite of this deposit; it may suggest an existence of an Archaean life within these sediments. The chemistry of the Sheba rocks indicates an intermediate to acidic provenance of sediments. The high values of SiO2 and Al2O3 (up to 75-19 % respectively), represent the dominance of quartz and mica in this deposit lithology. Low values of Fe2O3 and MgO indicate the low contents of mafic mineral in these rocks. Ores of this deposit are greatly affected by the metasomatism, shearing, and hydrothermal alterations caused by repeated metamorphism episodes and solutions pulses, which are very clear in the ores textures (oscillatory) and trace-elements content. Remobilization of gold under green schist metamorphic facies is very significant in the Sheba ores, where the two types of gold were observed: a) associated and enclosed by sulphides, especially pyrite, which represents the principal sulphide mineral in this deposit. b) Found as free-lode grains within the silicates (mainly quartz).
Pyrite, pyrrhotite, arsenopyrite, and chalcopyrite are the main sulphides in the Sheba Deposit. There are significant variations in the chemistry of the different generations of these minerals. Pyrite was found in two generations: Pyrite 1 contains more minor and trace elements compared to pyrite 2. The substitution of Fe and S by (Ni, Co) and arsenic respectively is common in this type. Au (2 wt. %), Bi, Ni, Mn, Co, Au, Ag, As, and Pb show a significant increase in content in this type. These replacements cause the development of new generations of Ni-sulphides and Ni-arsenides in some parts inside this pyrite type. Pyrite 2 is relatively younger, found overgrowing pyrite 1, and consists of well-developed cubic and eight-side crystals.
Textural and trace element data may not be able to identify stage margins or sharp sequential boundaries of genetic evolution of the deposits in Sheba; however, they provide clear evidences for the different tectonic and metamorphism events, hydrothermal pulses, and their impact on Au distribution.
LA-ICP-MS mapping technique applied on three pyrite grains from this deposit, revealed an existence of very tiny mineral in nano scale within the pyrite type 2. It also shows an elemental zoning within these pyrite grains. The distribution and relationships of the trace elements in these pyrite grains indicate three genetic behaviours: 1) Al, Mn, Hg, Se, Ti, V, Cr, and Sn distributed equally within pyrite, reflecting a synchronous formation of these domains with pyrite. 2) As, Ni, Co, Zn, and Sb distributed heterogeneous in the pyrite, indicating post-interference due to late solutions. Moreover 3) Au and Ag show both homogenous and heterogonous distribution within pyrite, which suggests the possibility of (1) and (2) formations.
This work was part of author’s PhD thesis that sponsored by Alneelain University and conducted at University of Pretoria. The author declared no conflict-of-interest present in this study. Deep thanks to Alneelain University- Sudan, and the Geology department at the University of Pretoria for facilitating their laboratories and instruments to achieve the purposes of this research. Special thanks extended to Late Dr. RD Dixon and the South African Police Forensic Laboratory in Pretoria for allowing us to use their LA-ICP-MS instrument.