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Review

Evolution of Auriferous Fluids in the Kraaipan-Amalia Greenstone Belts: Evidence from Mineralogical and Isotopic Constraints

by
Kofi Adomako-Ansah
1,
Napoleon Q. Hammond
2,*,
Yuichi Morishita
3 and
Daizo Ishiyama
4
1
Department of Geological Engineering, University of Mines and Technology, Tarkwa P.O. Box 237, Ghana
2
Department of Geology and Mining, School of Physical and Mineral Sciences, University of Limpopo, Private Bag X1106, Sovenga 0727, South Africa
3
Faculty of Science, Centre for Integrated Research and Education of Natural Hazards, Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka 422-8529, Japan
4
Graduate School of International Resource Sciences, Akita University, 1-1 Tegata-gakuen machi, Akita 010-8502, Japan
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(11), 1171; https://doi.org/10.3390/min14111171
Submission received: 25 August 2024 / Revised: 11 November 2024 / Accepted: 12 November 2024 / Published: 18 November 2024
(This article belongs to the Special Issue Geochemistry and Genesis of Hydrothermal Ore Deposits)

Abstract

:
The Kraaipan and Amalia greenstone belts in South Africa occur in the western part of the Kaapvaal Craton. The two belts stretch discontinuously in an approximately north–south orientation over a distance of about 250 km from southern Botswana in the north to the Vaal River near Christiana in the south and are separated by a distance of about 90 km. Gold mineralization is hosted in banded iron formation at both the Kalahari Goldridge deposit (Kalgold) in the Kraaipan greenstone belt in the north and the Amalia deposit in the Amalia greenstone belt in the south, with the mineralization associated with quartz–carbonate veins. The footwalls of these deposits are generally composed of mafic volcanic schist and the hanging walls consisting of graywackes, schist and shale units. The Kalgold and Amalia gold deposits show some variation in the redox condition of the mineralizing system and fluid chemistry. The ore mineral assemblage is characterized by magnetite–pyrrhotite–pyrite at Kalgold, which is indicative of reducing conditions, and a magnetite–hematite–pyrite assemblage at Amalia that suggests a relatively oxidizing environment. Average mineralizing temperatures determined from chlorite geothermometry were relatively higher at the Kalahari Goldridge deposit ranging from 350 to 400 °C compared to the slightly cooler range of 330 to 390 °C at Amalia. The composition of the fluids derived from fluid inclusions is indicative of low salinity H2O--CO2±CH4-rich fluids at Kalgold against relatively H2O-CO2-rich fluids at Amalia. Evidence from strontium–carbon–oxygen isotopic ratios from carbonates suggests that differences in redox conditions in the deposits could be attributed to different flow pathways by an evolving fluid from a common source (with minimum 87Sr/86Sr = 0.70354) to the sites of gold deposition, with a significant ore fluid interaction with a thick sequence of carbonaceous meta-pelitic rock units at the Kalahari Goldridge deposit that is absent in the Amalia deposit.

1. Introduction

Several studies on Archaean orogenic gold deposits (e.g., [1,2,3,4,5]) are documented to have been formed from ore-forming fluids that originated from deep within the crust and migrated along crustal-scale faults or fissures to their final depositional sites. The stable isotope data associated with most of the previous studies were unable to clearly define the origin of ore-forming fluids associated with such deposits due to the overlap of isotopic signatures among fluid sources. In recent years, the application of radiogenic isotopes, particularly strontium (Sr), have been widely used as tracers by analyzing strontium-rich gangue minerals to define the source reservoir and flow pathways of these fluids (e.g., [6,7]), because the 87Sr/86Sr ratios can identify the origin of Sr from continental, marine or mantle reservoirs as these have characteristic signatures (e.g., [8,9,10,11,12,13,14]). The flow pathways can provide significant insight into the nature of water–rock interactions between the fluid and the rocks encountered during hydrothermal fluid migration, thus providing important information on the nature and evolution of the hydrothermal fluid in ore deposit systems.
The Kraaipan and Amalia greenstone belts in South Africa are N-S trending, laterally discontinuous structures that are spatially associated with granitoids of similar petrological characteristics and age (Figure 1). The basement rocks of these greenstone belts are Archaean-aged tonalite–trondhjemite–granodiorite (TTG) gneisses [15]. Epigenetic banded iron formation (BIF)-hosted gold deposits within the Kraaipan-Amalia terrane include the Kalahari Goldridge deposit in the Kraaipan greenstone belt (to the north) and the Amalia deposit in the Amalia greenstone belt (to the south) (Figure 2) and are separated about 90 km from each other. The hanging wall units in both deposits are made up of Archean supracrustal metasedimentary rocks of shales and schists, and the footwall comprises mafic volcanic schists (Figure 2 and Figure 3). However, local lithological variations occur between the BIF and footwall at the Kalahari Goldridge deposit [16]. Thin carbonaceous- and pyrite-bearing metamorphosed pelites (meta-pelites) ranging between 1 and 2 m in thickness occur discontinuously between the contacts of the orebody and footwall and also between the orebody and immediate hanging wall units at the Kalahari Goldridge deposit. The meta-pelites have been subjected to intense deformation as evidenced by the tight to isoclinal folding associated with the unit.
Despite similar geological settings, the geochemistry of the mineralization in these deposits exhibits some local variations [18,19]. It is therefore not clear if the mineralizing events in these deposits were spatially linked to a unique fluid source that were modified along different flow pathways to the depositional sites, or if they were discrete mineralizing events with distinct sources. A combination of Sr-C-O isotopic ratios from previous studies on the Kalahari Goldridge [18,20], the C-O isotope on Amalia [19] and our new Sr isotope data for the Amalia deposit (this paper) help to unravel and evaluate the nature and evolution of the fluids associated with gold mineralization in the Kraaipan-Amalia terrane.

2. Geological Setting

The evolutional history of the Kaapvaal craton was initiated by the amalgamation of the ~3.7-Ga-old Witwatersrand block (to the east) and the 3.2-Ga-old Kimberly block (to the west) along the Colesberg Lineament (CL) suture (Figure 1) followed by subduction-accretion and continent-continent collision processes at about 3.0–2.9 Ga ago [21,22,23,24,25,26,27]. The Kraaipan-Amalia greenstone belts are located in the Kimberly block of the Kaapvaal craton and aligned parallel to the north-trending CL. The regional thermo-tectonic processes are believed to have resulted in the formation of highly deformed, north-trending subvertical volcano-sedimentary rock units in the Kraaipan-Amalia greenstone belts that are generally fault-bounded and partially engulfed by abundant intrusive syn- to post-tectonic granitoids [22,25]. The Kraaipan-Amalia greenstone belt is flanked to the west by 3.08–2.93 Ga-old reworked TTGs and to the east by the 2.93–2.85 Ga-old Kraaipan group of post-collisional and intrusive granodioritic and magnetite-bearing quartz monzonitic plutons (Figure 1; [15,27,28]). The spatial association between the Kraaipan-Amalia greenstone belts and the granodioritic-monzonitic rocks aroused speculations by earlier researchers on a genetic link between the gold deposits within the greenstone belts and felsic igneous activity e.g., [15,27,28]. The greenstone belts are poorly exposed due to the limited outcrop in the region. A few BIF deposits were reported to be associated with a suite of agglomerates and accretionary lapilli tuffs [15,29]. A geophysical investigation conducted across the CL by [30] revealed that the Kraaipan-Amalia greenstone belt terrane was obducted unto the Kaapvaal craton from the east as an allochthonous unit by accretionary tectonics [30].

3. Hydrothermal Alteration and Gold Mineralization

The host rocks to gold mineralization in the Kalahari Goldridge and Amalia deposits are oxide facies BIFs with variable amounts of chert and magnetite (Figure 3) [16,31]. The BIFs are intercalated with a variety of schists [16,31].
Gold ore in both deposits is associated with shallow-dipping quartz-carbonate veins that cut across the oxide facies BIF layers (Figure 4), with the mineralization concentrated in the altered oxide-rich units. Carbonate minerals in the veins are dominated by ankerite and siderite. At Kalahari Goldridge, the mineralizing quartz–carbonate veins were further classified as Group II (A and B) based on their sizes and cross-cutting relation on the magnetite mesobands (Figure 4B). Field and textural evidence in both deposits indicate pervasive carbonate alteration of the BIF with intense sulfidation. Sulfidation haloes commonly occur at the contacts and selvages between the quartz–carbonate veins and the host BIF units. The replacement of magnetite by pyrite and pyrrhotite is prominent at Kalahari Goldridge (Figure 5A–C), indicating a paragenetically later episode of sulfidation. However, there is an extensive replacement of magnetite by hematite and pyrite at the Amalia deposit with minor chalcopyrite and arsenopyrite (Figure 5D–F). The hematization of magnetite was generally absent at Kalahari Goldridge. Field and geochemical observations showed that gold mineralization in both deposits is closely associated with the altered host rocks; typically, the carbonate-altered and highly sulfidized BIF. These observations are clearly consistent with contrasting ambient redox conditions in the ore-forming fluids during mineralization in these deposits: a more reduced fluid system at Kalahari Goldridge deposit and more oxidized conditions at Amalia.
The varied redox state of these deposits is further evidenced from microthermometric and Raman analyses of fluid inclusions in quartz veins that indicated low salinity H2O-NaCl-CO2 ± CH4 compositions at the Kalahari Goldridge [18] and H2O-NaCl-CO2 at Amalia [19] (Figure 6). Thus, a relatively ‘pure’ CO2-rich composition characterizes the ore-forming fluid at Amalia compared to the Kalahari Goldridge, which contains significant CH4 in some inclusions.
Chlorite occurs in all the lithological units at the Kalahari Goldridge deposit. In the pelitic sediments, chlorite typically occurs as fine- to medium-grained schistose or felt-like mass. In the altered BIF units, the chlorite occurs as tabular to equigranular grains intergrown with magnetite and other alteration minerals. Chlorite is rare in the unaltered to less altered BIF units. Chlorite associated with the mineralized quartz veins occurs as sheaves, rosettes or felt-like equigranular aggregates, which may be intergrown with carbonate and quartz in a mosaic texture. Chlorite occurrence at Amalia exhibits similar characteristic features as the Kalahari Goldridge deposits, occurring in all the litholigies. They occur as fine-to-tabular and fibrous grains showing intergrowth with magnetite, siderite, ankerite and pyrite in the highly altered BIF. In the quartz–carbonate veins, it occurs as fine, equigranular grains with a felt-like mass appearance, texturally associated with quartz, ankerite and pyrite. Chlorite also occurs as veinlets cutting magnetite-rich bands in the Amalia BIF.

4. Analytical Procedure

Two independent studies using the same analytical techniques were undertaken for the Kalahari Goldridge and Amalia deposits. These studies are summarized below.

4.1. Mineral Chemistry of Chlorite

The mineral composition of chlorites from the Kalahari Goldridge deposit were obtained by WDS at a 15kV operating voltage and 20 nA beam current using a JEOL CXA-733 Superprobe (JEOL, Tokyo, Japan) at the Department of Geology, Rhodes University. The counting time was maintained at 20 s. The composition of chlorite from Amalia was acquired by a wavelength dispersive spectrometer (WDS at a 15 kV operating voltage and 20 nA beam current using a JEOL JXA-8800R Superprobe (JEOL, Tokyo, Japan) at the Department of Earth Science and Technology, Akita University, Japan. The peak acquisition time was maintained at 20 s for all elements. In both cases, calibrations were performed using international standards, and data were corrected using ZAF corrections.

4.2. Carbon and Oxygen Isotope Analysis of Carbonates

Carbon and oxygen isotopes analyses for Kalahari Goldridge and Amalia deposits were conducted following the conventional method of McRea (1950). Pulverized carbonate samples were decomposed in orthophosphoric acid (H3PO4). The sequential extraction of CO2 was performed for samples containing more than one carbonate phase. CO2 gas from carbonates samples from Kalahari Goldridge was liberated from ankerite and siderite. The CO2 initially evolved at 25° and 50 °C for calcite and ankerite/dolomite, respectively, for at least 4 h. Subsequently, CO2 was liberated from siderite in a paraffin bath at 100 °C for at least 6 h. The evolved CO2 was analyzed for C- and O-isotopic ratios using a Finnigan MAT 252 mass spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) at the Department of Geological Sciences, University of Cape Town. The C- and O-isotopic data are presented in the standard notation of δ13C and δ18O, respectively. In samples from Amalia, CO2 gas was liberated from ankerite. This step was carried out in a water bath maintained at 100 °C for at least 12 h. The released CO2 gas was collected in a liquid N2 trap and then separated from water vapor by the substitution of a dry-ice-acetone trap. The pure CO2 was analyzed for carbon and oxygen isotopic ratios using a mass spectrometer (Iso-Prime EA-IRMS) at the Research Center for Engineering Sciences, Akita University, Japan. In both cases, carbon and oxygen isotopic ratios are reported as per mil (%) with respect to the Pee Dee Belemnite (PDB) for carbon and Standard Mean Ocean Water (SMOW), respectively.

4.3. Strontium Isotopic Measurements

Nine carbonate samples from mineralized quartz–carbonate veins from the Kalahari Goldridge deposit were selected for Sr isotope studies. Selected samples were further analyzed using X-ray diffraction for high-purity carbonate (siderite and ankerite) separates for Sr isotopic measurements. About 50 to 100 mg of the carbonates were dissolved for 2 h using 0.8 mol−1 hydrochloric acid in a Teflon vessel. Strontium was separated by a standard ion exchange technique. 87Sr/86Sr ratios were measured on a Finigan MAT 261 mass spectrometer at the Research School of Earth Sciences, Australian National University. Strontium isotopic ratios were normalized to a value of 0.1194 for 86S/88Sr. During the analytical run, the mean value of nine analyses of the NBS SRM-987 standard was 0.71022 ± 10 (±2σ) for 87Sr⁄86Sr. The nine samples were also analyzed for Sr and Rb concentrations using a Quadrupole-ICP-MS at the Research School of Earth Sciences, Australian National University.
From the Amalia gold deposit, strontium isotopic signatures (87Sr/86Sr ratios) were determined from six vein carbonates (ankerite) using a standard ion exchange technique in accordance with the procedure described by [32]. The 87Sr/86Sr isotopic measurements were performed on these separates, using a Finnigan MAT261 mass spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) at the Department of Earth Science and Technology, Akita University, Japan. 86Sr/88Sr results were normalized to 86Sr/88Sr = 0.1194. A mean value of the NBS SRM-987 standard of 0.710244 ± 5 (±2σ) for 87Sr/86Sr was used during analytical runs. The concentrations of Sr and Rb were determined from the same vein carbonate samples using an Agilent 7500i Quadrupole ICP-MS instrument (Agilent, Santa Clara, CA, USA) at the Mining Museum of Akita University, Japan.

4.4. Fluid Inclusion Analysis

Microthermometric measurements of fluid inclusions were conducted on primary or pseudo-secondary inclusions in mineralized quartz veins from Kalahari Goldridge and Amalia deposits employing standard procedures. In these studies, five phase transition temperatures were measured during heating runs: (i) the final melting temperature of CO2 (TmCO2), (ii) the final melting temperature of ice (Tmice), (iii) the dissociation temperature of clathrate (Tmclathrate), (iv) the homogenization temperature of CO2 and CH4 (ThCO2/ThCH4) and (v) the final homogenization of mixed aqueous–carbonic (H2O–NaCl–CO2 ± CH4) and aqueous (H2O–NaCl) inclusions (Thtotal).
Microthermometric measurements on fluid inclusions in quartz from the Kalahari Goldridge deposit was performed in the Department of Geology at Rhodes University, South Africa using a Linkam THMS 600 heating-freezing (Linkam, Redhill, UK) stage attached to a Nikon microscope. Freezing runs were performed using liquid nitrogen aided by an LNP cooling pump connected to the stage, while heating runs were performed by a thermal resistor with a TM-93 control unit. Microthermometric measurements of fluid inclusions in quartz veins from Amalia were also conducted on a Linkam THMSG 600 heating-freezing stage at the Department of Earth Science and Technology, Akita University. In both cases, mixed aqueous–carbonic inclusions were initially frozen below 100°C and then heated slowly at a rate of 0.5 °C to 1 °C/min for temperatures below 31°C (critical temperature of CO2) and increased to 0.5 °C to 10 °C/min until total homogenization temperatures. TmCO2 and ThCO2 measurements were used to evaluate the purity of the CO2 phase and to ascertain the density of CO2, respectively.

5. Mineralization Temperatures Based on Chlorite Geothermometry

Chlorite occurs in a wide range of geological environments such as in diagenetic, low- to medium-grade metamorphism and hydrothermally altered rocks. It displays a wide range of non-stoichiometric compositional variations depending on the bulk composition of host rocks, particularly the Fe/(Fe + Mg) ratio, prevailing physico-chemical conditions such as temperature, pressure and the redox state during formation [33,34,35] and fluid chemistry in systems during the mineralization. The variation in chemical composition in chlorite serve as a useful tool to obtain information on the physico-chemical conditions during the evolution of mineralizing hydrothermal fluids. The geothermometer is generally applied to chlorite in diagenetic, hydrothermal and metamorphic settings (e.g., [36,37,38]).
The mineralization temperatures for the Amalia and Kalahari Goldridge deposits were determined using the chlorite geothermometry method of [39,40]. Chlorite in mineralizing veins and hydrothermally altered BIFs was analyzed from both deposits. Figure 7 illustrates overlapping temperatures determined from the chlorite geothermometry for the Kalahari Goldridge and Amalia deposits ranging from 350° to 400 °C at the Kalahari Goldridge [18,20] and Amalia from 330° to 390 °C at Amalia [31]. However, there was an overall decrease in temperature with a corresponding decreasing Fe/(Fe+Mg) ratio, with the lowest Fe/(Fe + Mg) ratios associated with chlorites in late fractures in Amalia BIF and the highest Fe/(Fe + Mg) ratio occurring in the sulfidized altered BIF at Kalahari Goldridge. Reference [36] demonstrated factors that cause Fe2+/(Fe2+ + Mg) compositional variations in chlorites in hydrothermal ore deposits. They documented in their study that the exchange of Fe2+ and Mg2+ between the chlorite and hydrothermal solution is a function of physicochemical parameters (e.g., temperature, oxygen fugacity, pH, total dissolved sulfur and activity of Mg2+ and Fe2+ ions in aqueous solution), as well as the extent of water–rock interactions. The observed positive correlation between the temperature and Fe/(Fe + Mg) ratios at the Kalahari Goldridge and Amalia deposits is inferred to be a regional trend in the Kraaipan-Amalia greenstone terranes, which could possibly be associated with fluid movement along the flow path of mineralizing fluids from the Kalahari Goldridge in the north to the Amalia deposit in the south.

6. Isotopic (Sr, C and O) Signatures and Rb Concentrations

Rubidium–strontium, carbon and oxygen isotopic systematics in carbonates from the alteration carbonates at the Amalia and Kalahari Goldridge deposits were undertaken to deduce the nature and evolution of fluids in these deposits in an attempt to define the fluid origin in the Kraaipan-Amalia terrane. The Rb-Sr isotopic technique was based on the premise that minerals with relatively low concentrations of Rb (<1 ppm) but high concentrations of Sr, such as in carbonates are commonly subjected to Sr-Ca substitution due to similar ionic radii of the two elements. Hence, the 87Sr/86Sr ratio can remain relatively unchanged with time due to limited or no influence of radiogenic Sr from the in situ radioactive decay of Rb. 87Sr/86Sr ratios represent the approximate initial compositions of fluids at the time of carbonate crystallization. On the basis of this premise, diagnostic fluid–rock interaction trends between evolving fluid systems and crustal components can be constructed to deduce this assertion. The Rb-Sr isotope data for Kalahari Goldridge deposit reported earlier by [20] and compared with new strontium isotopic signatures (87Sr/86Sr ratios) for the Amalia gold deposit are shown in Table 1. Results of the carbonate separates yielded very radiogenic 87Sr/86Sr ratios that range from 0.70354 to 0.73914 for the carbonates from Kalahari Goldridge. The Sr and Rb concentrations vary from 1.12 to 168 ppm and 0.04 to 0.4 ppm, respectively. In the Amalia gold deposit, the 87Sr/86Sr ratios of the vein carbonate is from 0.703023 to 0.706643, while the Sr and Rb concentrations vary from 3.4 to 157.2 ppm and 0.6 to 2.1 ppm, respectively. The δ13C-δ18O data of carbonates from quartz–carbonate veins from the Amalia and Kalahari Goldridge deposits are derived from [18,19], respectively, and are also summarized in Table 1.

7. Discussion

The analytical results indicate very low Rb/Sr ratios (<1) for the vein carbonates from both deposits, suggesting that the corresponding 87Sr/86Sr ratios remained unchanged through time, making the measured values a good approximation for the initial 87Sr/86Sr ratios. Consequently, the 87Sr/86Sr ratios are suitable for monitoring the nature and evolution of the ore-forming fluid(s) in the Kraaipan-Amalia terrane.
The basement rocks to the BIF-hosted Kalahari Goldridge and Amalia gold deposits are Archean TTGs of similar age and petrographic characteristics. If these ore-forming fluids interacted with only the basement TTG rocks and/or BIF units, it is expected that the deposits will exhibit similar variations or tendencies in the radiogenic Sr isotopic ratios of their respective vein carbonates. However, this is not the case. Inconsistency in the trend to high Sr isotopic ratios between the two deposits is illustrated in Figure 8, by comparing the Sr isotopic data with the δ13C values from both deposits (e.g., [41,42]).
The vein carbonates from the Amalia gold deposit are characterized by relatively homogeneous and less radiogenic 87Sr/86Sr ratios. On the other hand, the 87Sr/86Sr ratios in the vein carbonates from Kalahari Goldridge are relatively heterogenous, more radiogenic and widespread: they range from low values that are similar to that of the Amalia gold deposit to much higher radiogenic values not recorded in the Amalia gold deposit. Given that the basement rocks are the same for both deposits, we postulate that the wider spread to relatively higher radiogenic Sr isotopic values in the precipitating vein carbonates at the Kalahari Goldridge deposit resulted from the fluid–rock interaction between ore-forming fluids and the graphite-bearing metasedimentary rock surrounding the host BIF unit. In addition, the heterogeneous nature of the 87Sr/86Sr ratios at Kalahari Goldridge can be attributed to the larger volume of fluid interaction with more crustal rocks and relatively more intensive fluid–rock interaction. The large fluid volume is reflected by the large quartz veins (Group IIB veins) and the multiple ladder veins (Group IIA) [20]. Conversely, the homogeneous 87Sr/86Sr ratios in the vein carbonates from Amalia, as illustrated in Figure 8, can be attributed to the interaction of ore-forming fluids with the limited or restricted variety of crustal sedimentary host rocks at Amalia in comparison with Kalahari Goldridge. The observations at the Kraaipan-Amalia deposits are consistent with several orogenic gold deposits of variable geological settings worldwide (e.g., [4,41]). For example, a review by [4] on fluid and metal sources for orogenic gold deposits reported that strontium was derived from basement rocks below Archean greenstone belts that host these deposits or reflect a significant mantle component, with values altered along the flow path and at the site of gold deposition by host metasedimentary rock sequences. Their study also documented local wall rock sources for Sr or multiple strontium sources in host rocks distal to the gold deposits. In a study by [41], it was noted that the uniformity of initial 87Sr/86Sr ratios of mafic volcanic rocks in gold deposit-hosted terranes in the Archean Abitibi sub-province of Canada was consistent with a homogenous upper mantle reservoir. In a similar study in an epithermal gold deposits in the Kyushu region of Japan by [42], they noted that variations in 87Sr/86Sr ratios hosted in both the basement carbon-rich metasedimentary rocks of the Shimanto Group and the overlying andesitic volcanic rocks showed relatively high 87Sr/86Sr ratios in ore-related calcite, which were inferred to indicate hydrothermal ore-forming fluid interactions with the carbon-rich metasedimentary rocks that contained a much higher 87Sr/86Sr composition than the surrounding shallower volcanic rocks hosting the deposits.
Like most Archean orogenic gold deposits, the deposits in the Kraaipan-Amalia terrane exhibit similarities in δ18Owater values in quartz from the quartz–carbonate veins, in addition to the pervasive carbonatization and sulfidation mentioned earlier [18,19]. However, variations exist in the δ13C values of the associated carbonates from both deposits (Figure 9). The δ13C values of ore-related carbonates from the Kalahari Goldridge deposit (−7.6 to −5.4%; [18]) are lower than the δ13C values of the Amalia gold deposit (−5.8 to −3.5%; [19]). On the basis of geochemical mass balance calculations, the variation in δ13C values above or below average mantle/igneous values of −5 ± 3%; [43,44,45] has been attributed to basement rocks of heterogeneous TTG compositions [41] or fluid–rock interactions between ore-forming fluids and carbon-rich sedimentary rocks (e.g., [42,46]). In addition, the CO2-bearing ore-forming fluid at the Amalia contains a negligible CH4 concentration compared to the appreciable concentration in the ore-forming fluid at Kalahari Goldridge (Figure 6), which can be attributed to the lack of host carbon-rich metasedimentary rocks at Amalia [47] and reasonably explains the absence of CH4 in the Amalia ore-forming fluid. Therefore, the occurrence of carbon-rich metasedimentary rocks surrounding the Kalahari Goldridge deposit could have partly buffered the ore-forming fluid to reduced conditions, thereby resulting in lower δ13C values and, consequently, the observed difference in redox conditions at the deposits.
The δ13C and δ18O of vein carbonates from the Kalahari Goldridge and Amalia gold deposits show a generally positive correlation (Figure 9) with the Amalia deposit exhibiting relatively higher δ13C and δ18O values between the oxygen and carbon isotope ratios. References [43,48] documented such positive variation in several studied individual deposits that showed increasing trend of enrichment of 18O and 13C in carbonates in the paragenetic sequence of the mineralization. Reference [43] attributed this trend to (i) decreasing temperature, (ii) increasing CO2/CH4 ratios in an evolving fluid system resulting from the local fluid interaction with graphitic rocks or immiscible separation of CO2 + CH4 in the hydrothermal fluid and/or (iii) the contribution of CO2 from other sources, resulting in a progressive increase in the 13C/12C and 18O/16O ratios. Therefore, the general positive variation between δ13C and δ18O in the two deposits is attributed to the increasing oxidation state in an evolving fluid system from a unique homogenous origin that is conformable with the observed 87Sr/86Sr-Sr variation illustrated in Figure 10. Additionally, this finding is also consistent with the observed regional temperature trend illustrated in Figure 7 where fluid migration is inferred from the Kalahari Goldridge deposit in the north to the Amalia deposit in south. Figure 10 shows a binary plot of 87Sr/86Sr ratios and their corresponding Sr concentrations in carbonates veins from the Kalahari Goldridge and Amalia gold deposits, which demonstrate that the carbonates veins from both deposits have a common minimum value characterized by the 87Sr/86Sr isotopic ratio of 0.70354. The minimum value indicates that the ore-forming fluids for both deposits possibly originated from a common fluid reservoir source of uniform isotopic composition (87Sr/86Sr ratio = 0.70354), which is consistent with a mantle or mafic igneous signature [8,9,11,13,14]. The plot also illustrates a diverging trend towards increasing radiogenic Sr values along different evolutionary flow paths for each of the quartz–carbonate veining events in these deposits. The increasing radiogenic Sr trend can be attributed to the isotopic exchange resulting from the mixing of multiple fluids of isotopically different signatures or by the fluid–rock interaction between ore-forming fluids and supracrustal wall rocks with higher radiogenic Sr concentrations [8,10,12,49]. However, fluid inclusion and stable isotope data [17,18] are inconsistent with the involvement of multiple fluids in both deposits. Hence, the effect of fluid mixing is consequently ruled out. Thus, the trend to higher 87Sr/86Sr ratios illustrated in Figure 8 can be related to isotopic exchange between ore-forming fluids and the supracrustal rocks that surround the deposits. This finding is also consistent with the observed variation of CH4 and CO2 concentrations (Figure 6), which suggests a progressive enrichment of 13C and 18O in the fluids during the fluid interaction with host rocks from Kalahari Goldridge to Amalia (Figure 9), as well as the decreasing regional trend in the mineralization temperatures from Kalahari Goldridge to Amalia (Figure 7).

8. Conclusions

The combination of Sr, C and O isotopic data from the Kalahari Goldridge and Amalia BIF-hosted gold deposits were used to evaluate the nature and evolution of the ore-forming fluids associated with orogenic gold mineralizing systems in the Kraaipan-Amalia region of South Africa. A schematic model of the Kraaipan-Amalia gold mineralization system is illustrated in Figure 11. The two gold deposits had a common source for the ore-forming fluids on the basis of Sr isotopic data acquired on carbonates associated with gold mineralization, irrespective of their contrasting ambient redox conditions, fluid chemistry and temperatures of gold formation, which are best explained as local variations within similar mineralizing systems. The vein-forming fluids interacted with sedimentary rocks having organic materials at Kalahari Goldridge in comparison with the host rocks at Amalia, which lacked or had limited organic materials. This finding suggests that upward-migrating deep crustal ore-forming fluids having mantle-like signatures were associated with the gold mineralization in the deposits. However, the ore-forming fluids interacted with the supracrustal rocks of high isotopic composition en route to the depositional sites in the Kraaipan-Amalia terrane.
The absolute timing of the gold mineralization in these greenstone belts is not constrained; however, evidence from fluid composition data and Sr-C-O isotopic ratios suggests that observed differences in redox conditions in these deposits could be attributed to different local flow pathways by similar evolving fluids from a common source (with minimum 87Sr/86Sr = 0.70354) to the sites of the final gold deposition. Fluid–rock interactions between ore-forming fluid and carbonaceous meta-pelitic rock units partly resulted in reducing conditions and heterogeneity in the observed Sr-C isotopic distribution at the Kalahari Goldridge deposit. The oxidized character and homogeneous Sr-C isotopic distribution observed at the Amalia gold deposit is attributed to the lesser fluid–rock interaction between the ore-forming fluid and limited amount of (carbonaceous) supracrustal rocks. The results of this study amplify the fact that, although Archean orogenic gold deposits formed from fluids of similar composition in similar tectonic environments, minor differences in the deposits could be linked to variable host rock composition, redox conditions of gold formation and/or other physico-chemical parameters at individual deposits.

Funding

Funding for the strontium isotope research was provided by Akita University and Japan Society for the Promotion of Science (JSPS).

Acknowledgments

Masatsugu Yamamoto, formerly of Akita University, Japan, is specially thanked for his immense support during the Sr isotope analytical runs. We also thank Toshio Mizuta for his advice during the course of this research.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Geological map of the Kaapvaal Craton showing the location of the Kraaipan and Amalia greenstone belt in the western part of the craton. (Modified after [17]).
Figure 1. Geological map of the Kaapvaal Craton showing the location of the Kraaipan and Amalia greenstone belt in the western part of the craton. (Modified after [17]).
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Figure 2. Regional geological map of the Kraaipan and Amalia greenstone belts with the location of the Kalahari Goldridge and Amalia gold deposits.
Figure 2. Regional geological map of the Kraaipan and Amalia greenstone belts with the location of the Kalahari Goldridge and Amalia gold deposits.
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Figure 3. Local geological maps of the Kalahari Goldridge, D Zone orebody (A) and the Amalia gold deposit (not to scale) (B) as shown in Figure 2. Chl = chlorite; Musc = muscovite; Sid = siderite; Qz = quartz; and BIF = banded iron formation.
Figure 3. Local geological maps of the Kalahari Goldridge, D Zone orebody (A) and the Amalia gold deposit (not to scale) (B) as shown in Figure 2. Chl = chlorite; Musc = muscovite; Sid = siderite; Qz = quartz; and BIF = banded iron formation.
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Figure 4. Photos of quartz–carbonate veins in the BIF at the Kalahari Goldridge deposit (A) and Amalia gold deposit (B).
Figure 4. Photos of quartz–carbonate veins in the BIF at the Kalahari Goldridge deposit (A) and Amalia gold deposit (B).
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Figure 5. Textural relationships between ore minerals in the BIF at the Kalahari Goldridge (AC) and Amalia gold deposits (DF). Po = pyrrhotite; Py = pyrite; Mgt = magnetite; Hmt = hematite; Au = gold.
Figure 5. Textural relationships between ore minerals in the BIF at the Kalahari Goldridge (AC) and Amalia gold deposits (DF). Po = pyrrhotite; Py = pyrite; Mgt = magnetite; Hmt = hematite; Au = gold.
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Figure 6. TmCO2 versus ThCO2 of fluid inclusions in quartz veins from the Kalahari Goldridge and Amalia gold deposits. The inclusions show relatively pure CO2-rich inclusions at Amalia compared to the Kalahari Goldridge, which have mixed CO2±CH4-rich inclusions.
Figure 6. TmCO2 versus ThCO2 of fluid inclusions in quartz veins from the Kalahari Goldridge and Amalia gold deposits. The inclusions show relatively pure CO2-rich inclusions at Amalia compared to the Kalahari Goldridge, which have mixed CO2±CH4-rich inclusions.
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Figure 7. Temperature distribution based on chlorite geothermometry in quartz–carbonate veins and altered BIF at the Kalahari Goldridge and Amalia gold deposits showing a relatively increasing temperature variation from Amalia to the Kalahari Goldridge deposit.
Figure 7. Temperature distribution based on chlorite geothermometry in quartz–carbonate veins and altered BIF at the Kalahari Goldridge and Amalia gold deposits showing a relatively increasing temperature variation from Amalia to the Kalahari Goldridge deposit.
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Figure 8. Plot of 87Sr/86S versus δ13C of carbonates in mineralizing quartz–carbonate veins at the Kalahari Goldridge and Amalia gold deposits. The green shaded area depicts the range for the mantle source [8,9,11,13,14]. The gray shaded area depicts the range for supracrustal rocks with high 87Sr/86Sr ratios [8,10,12]. The arrow depicts increasing 87Sr/86Sr values during the fluid–rock interaction in the Kalahari Goldridge gold deposit.
Figure 8. Plot of 87Sr/86S versus δ13C of carbonates in mineralizing quartz–carbonate veins at the Kalahari Goldridge and Amalia gold deposits. The green shaded area depicts the range for the mantle source [8,9,11,13,14]. The gray shaded area depicts the range for supracrustal rocks with high 87Sr/86Sr ratios [8,10,12]. The arrow depicts increasing 87Sr/86Sr values during the fluid–rock interaction in the Kalahari Goldridge gold deposit.
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Figure 9. Variation of carbon and oxygen isotope composition in carbonates from mineralizing quartz–carbonate veins at the Kalahari Goldridge and Amalia gold deposits (From [4,17,18,19,30,44,45]).
Figure 9. Variation of carbon and oxygen isotope composition in carbonates from mineralizing quartz–carbonate veins at the Kalahari Goldridge and Amalia gold deposits (From [4,17,18,19,30,44,45]).
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Figure 10. A plot of 87S/86S versus 1/Sr depicting the evolutionary trend of ore-forming fluids at the Kalahari Goldridge and Amalia gold deposits.
Figure 10. A plot of 87S/86S versus 1/Sr depicting the evolutionary trend of ore-forming fluids at the Kalahari Goldridge and Amalia gold deposits.
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Figure 11. Schematic diagram showing BIF-hosted gold mineralization model in the Kraaipan-Amalia greenstone belts.
Figure 11. Schematic diagram showing BIF-hosted gold mineralization model in the Kraaipan-Amalia greenstone belts.
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Table 1. (A). Oxygen, carbon, rubidium and strontium isotopic data of carbonates from the Amalia gold deposit, Amalia Greenstone Belt, South Africa (this study). (B). Oxygen, carbon, rubidium and strontium isotopic data for carbonates from the Kalahari Goldridge, Kraaipan Greenstone Belt, South Africa [20].
Table 1. (A). Oxygen, carbon, rubidium and strontium isotopic data of carbonates from the Amalia gold deposit, Amalia Greenstone Belt, South Africa (this study). (B). Oxygen, carbon, rubidium and strontium isotopic data for carbonates from the Kalahari Goldridge, Kraaipan Greenstone Belt, South Africa [20].
(A)
Mineralδ18OSMOW (per mil)δ13CPDB (per mil)87Rb/86SrRb (ppm)Sr (ppm)Rb/Sr1/Sr (ppm−1)87Sr/86SrStd
C17-20Ank17.2−4.00.0100.55157.200.0040.0060.703023(±7; 2σ)
C11-5A-3Ank16.3−4.50.0761.0038.150.0260.0260.703747(±6; 2σ)
C17-15BAnk13.5−5.61.231.463.430.4260.2920.706643(±6; 2σ)
C17-23Ank16.3−3.90.3710.896.940.1280.1440.704895(±6; 2σ)
N23-7AAnk17.4−3.50.3821.8814.220.1320.0700.705781(±12; 2σ)
C17-6Ank16.3−4.60.0782.1178.450.0270.0130.704318(±7; 2σ)
(B)
Mineralδ18OSMOW (per mil)δ13CPDB (per mil)87Rb/86SrRb (ppm)Sr (ppm)Rb/Sr1/Sr (ppm−1)87Sr/86SrStd
ARC 236/11AAnk11.3−5.60.0340.123.30.0040.0430.71042(±1; 2σ)
ARC 236/11XSid10.1−5.80.1880.34.50.0670.2220.72325(±1; 2σ)
ARC 236/16Sid15.1−5.40.040.043.50.0110.2860.71138(±1; 2σ)
MSH/W-3Ank9.8−7.60.0050.31680.0020.0060.70354(±1; 2σ)
GDP 531/9CSid11.5−6.90.5130.21.10.1820.9090.72907(±2; 2σ)
DZ 40/1Sid10.3−6.70.3890.42.90.1380.3450.73914(±2; 2σ)
DZ 40/2Sid10.6−6.70.0470.212.10.0170.0830.71583(±1; 2σ)
DZ 40/3Sid10.8−6.60.0470.318.20.0160.0550.71235(±1; 2σ)
GDP 531/16BSid10.8−6.10.0060.118.10.0060.0550.70564(±2; 2σ)
Std = standard deviation; Ank = ankerite; Sid = siderite.
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Adomako-Ansah, K.; Hammond, N.Q.; Morishita, Y.; Ishiyama, D. Evolution of Auriferous Fluids in the Kraaipan-Amalia Greenstone Belts: Evidence from Mineralogical and Isotopic Constraints. Minerals 2024, 14, 1171. https://doi.org/10.3390/min14111171

AMA Style

Adomako-Ansah K, Hammond NQ, Morishita Y, Ishiyama D. Evolution of Auriferous Fluids in the Kraaipan-Amalia Greenstone Belts: Evidence from Mineralogical and Isotopic Constraints. Minerals. 2024; 14(11):1171. https://doi.org/10.3390/min14111171

Chicago/Turabian Style

Adomako-Ansah, Kofi, Napoleon Q. Hammond, Yuichi Morishita, and Daizo Ishiyama. 2024. "Evolution of Auriferous Fluids in the Kraaipan-Amalia Greenstone Belts: Evidence from Mineralogical and Isotopic Constraints" Minerals 14, no. 11: 1171. https://doi.org/10.3390/min14111171

APA Style

Adomako-Ansah, K., Hammond, N. Q., Morishita, Y., & Ishiyama, D. (2024). Evolution of Auriferous Fluids in the Kraaipan-Amalia Greenstone Belts: Evidence from Mineralogical and Isotopic Constraints. Minerals, 14(11), 1171. https://doi.org/10.3390/min14111171

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