Invisible gold in the Archean detrital sulphides of the Witwatersrand tailings dumps: A large and under-exploited gold resource

doi:10.21203/rs.3.rs-1986949/v2

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Invisible gold in the Archean detrital sulphides of the Witwatersrand tailings dumps: A large and under-exploited gold resource

https://doi.org/10.21203/rs.3.rs-1986949/v2

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published 22 Feb, 2023

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The tailings dumps originating from gold mining in South Africa’s Witwatersrand still contain notable gold endowments. Most tailings reprocessing operations target a native gold fraction using re-milling and carbon-in-leach extraction; however, up to 50-70% of the remaining gold is still not recoverable and instead reports to a discard stream along with abundant sulphides. The mineralogical deportment of this unrecoverable gold requires detailed investigation. Using in situ laser ablation mineral chemistry measurements, we show that this gold that is ‘invisible’ to conventional recovery is hosted preferentially in pyrite and arsenian pyrite. Importantly, complementary optical and electron microscopies reveal that the rounded detrital forms of these minerals contain the highest gold concentrations (0,01–2730 ppm), showing some similarity to values reported for sulphides from primary orogenic gold deposits found in surrounding Archean-aged granite-greenstone belt remnants. We suggest that detrital auriferous sulphides have been overlooked by historical primary and secondary beneficiation, and thus represent a large (up to 420 tons) and under-exploited Au resource currently residing in easily-mined (surficial) Witwatersrand tailings dumps. We further suggest that targeted re-mining of sulphide mineral fraction has the potential to improve gold recovery, recuperate ‘sweetener’ by-product metals (e.g., Cu, Co, Ni) and directly eliminate heavy metal pollution and acid mine drainage issues associated with surficial tailings dumps.

Earth and environmental sciences/Solid earth sciences/Geochemistry

Earth and environmental sciences/Solid earth sciences/Geology

Earth and environmental sciences/Solid earth sciences/Mineralogy

Earth and environmental sciences/Solid earth sciences/Sedimentology

Earth and environmental sciences/Environmental sciences

Earth and environmental sciences/Solid earth sciences

Physical sciences/Engineering

Physical sciences/Engineering/Chemical engineering

Tailings dumps

invisible gold

pyrite

detrital

The Witwatersrand Supergroup is a 2,9 to 2,7 Ga sedimentary basin located on the Kaapvaal Craton (Fig. 1) and is world-renowned for its super-giant gold endowment. The gold in the basin is largely associated with a series of quartz-pebble conglomerate beds (reefs) that formed during continental sedimentation in alluvial fans and braid-plain depositional environments (Frimmel, 2019). Historical gold production from the Witwatersrand sedimentary basin accounts for ~ 32% of the total gold that has ever been mined globally (Frimmel, 2014). Despite the quantitative importance of the Witwatersrand deposits, the ultimate provenance of the gold is still a topic of scientific debate, in part because any obvious hinterland gold source has been removed from the geological record through erosional processes. Genetic models for the formation of the mineralisation include solution transport models with algal mat ‘traps’ (Heinrich, 2015), placer models (Kirk et al., 2001), hydrothermal models (Barnicoat et al., 1997; Phillips & Law, 2000), and the ‘modified-placer’ model (Frimmel et al., 2005; Hayward et al., 2005). Perhaps the most broadly-accepted theory is this modified-placer model since it combines the microscale textural observations of the hydrothermal model with the observation that mined gold grades follow spatial distributions that are better explained by the placer model. Briefly, this model suggests that native gold grains, likely sourced from continental-scale chemical and mechanical weathering and erosion of granite-greenstone dominated hinterlands (Frimmel, 2014; Koglin et al., 2010), were initially concentrated in conglomerates along with other dense detrital minerals. Subsequent hydrothermal fluid overprints resulted in local-scale remobilisation of gold within these conglomeratic units, thus accounting for the observed crystalline gold textures ( e.g., Minter et al., 1993) and sulphide morphologies (e.g., da Costa et al., 2020).

Sedimentation of the Witwatersrand Supergroup occurred under a reducing Archean atmosphere, as evidenced by the presence of detrital pyrite (FeS₂) and detrital uraninite (UO₂) within the auriferous quartz pebble conglomerates (England et al., 2002). Given the possibility that Witwatersrand gold was derived from orogenic-type primary gold deposits in an Archean granite-greenstone hinterland, it is reasonable to infer that a proportion of the detrital pyrite may similarly have been sourced from such deposits. Remnant vestiges of Archean greenstone belts located on the Kaapvaal craton commonly show elevated gold endowments (e.g., Barberton Greenstone belt = 376 t gold; Amalia-Kraaipan Greenstone belt = 32 t gold; Giyani Greenstone belt = 12 t gold; Pietersburg Greenstone belt = 4 t gold (Pearton and Viljoen, 2017)). Much of this gold can be classified as ‘invisible gold’, a loosely-defined term reserved for gold that behaves in a refractory manner to direct cyanidation (Coetzee et al., 2011) and which encompasses both gold dissolved in the sulphide mineral structures (generally arsenopyrite and arsenian pyrites) as a cation substituent (Merkulova et al., 2019; Reich et al., 2005), and gold occurring as submicron and nano-scale inclusions within sulphide mineral hosts (Paleniket al., 2004). For example, arsenopyrite and arsenian pyrite mined as part of the run-of-mine from orogenic gold shoots in the Barberton greenstone belt commonly host high concentrations of invisible gold (up to 1000 ppm (Agangi et al., 2014)), and the prevalence of this ore type has warranted the use of biological oxidation (BIOX®) beneficiation strategies (Van Aswegen & Marais, 1999). Similarly, fine and invisible gold mineralization is reported in the Kraaipan Greenstone Belt, strongly associated with pyrite and pyrrhotite (Hammond & Moore, 2006; Morishita et al., 2019).

The mining of Witwatersrand conglomerates and their contained gold and associated detrital sulphides dates back to 1885 (Frimmel & Nwaila, 2021). This extended mining legacy has resulted in a massive accumulation of tailings material (6 billion tons; Wymer, 2001) which, largely on account of historical processing inefficiencies, is currently being re-mined as a secondary gold resource (Bosch, 1987; Sibanye-Stillwater Limited, 2021). Most of the dedicated plants that reprocess Witwatersrand tailings material apply re-milling followed by carbon-in-leach extraction as the beneficiation strategy. This strategy targets native gold and results in a gold recovery that ranges between 30–50% (Bosch, 1987; Harmony Gold, 2021; Sibanye-Stillwater Limited, 2021). The present contribution seeks to better understand the mineralogical deportment of the remaining 50–70% of the gold that is deemed refractory to this beneficiation strategy, with particular emphasis on the role of Archean detrital pyrites as viable mineral hosts for the ‘missing’ recovery.

Tailings material from four regionally-distributed Witwatersrand tailings dumps (Carletonville, Central Rand, Evander and Klerksdorp goldfields; Fig. 1) was pre-concentrated into density-defined mineral fractions, providing the first step toward developing a statistical understanding of the main gold carriers. Results from aqua regia digestion and gold assay consistently show that the heavy mineral fraction (ρ > 2,71 g.cm^− 3; size 15–75 µm) hosts the highest measured gold concentrations which range between 0,71 and 10,12 ppm (Fig. 2). This accounts for 0,89% − 13,23% for the mass of gold in the samples. The light mineral fraction (d < 2,71 g.cm^− 3; size 15–75 µm) and slimes fraction (< 10 µm) have measured gold concentrations that are an order of magnitude lower, measuring across the four goldfields between 0,05 − 0,22 ppm and 0,17-0.76 ppm respectively. This preferential partitioning of gold into the heavy mineral fraction may be explained by the presence of free gold particles, or by a chemical association between gold and heavy gangue minerals such as the sulphides and oxides (Goodall, 2008; Coetzee et al., 2011; Goodall & Butcher, 2012). Our detailed microscopic investigation of the heavy mineral fraction using optical microscopy (40X magnification; resolution = 20 µm), and electron microscopies (1345 X magnification; resolution = 40 − 10 µm) did not reveal any free gold particles. This suggests that the assayed gold must reside as ‘invisible’ gold within the mineralogy of the heavy mineral fraction, which comprises 13–44% silicates; 35–76% sulphides; 9–20% oxides and 0.01–0.34% carbonates and phosphates.

Extensive in-situ laser ablation analyses (800 spots, spot size = 30 µm) and gold mineralogical balancing reveal that arsenian pyrite and pyrite account for most of the gold within the heavy mineral fraction. Specifically, these two minerals account for approximately 65% of gold in the Klerksdorp heavy mineral fraction, 78% of gold in the Carletonville heavy mineral fraction and 85% of gold in the Evander heavy mineral fraction (Fig. 2). Measurements of the Central Rand heavy mineral fraction indicate that it is an outlier showing low gold deportment to the sulphides (37%), which may be an artefact of the low total gold concentration and a limited number of sulphide grains (two factors that may be codependent in tailings material). Gold deportment into oxide minerals and other sulphide minerals is quantitatively less important (i.e., typically accounting for ~ 1–6% of total gold in the heavy mineral fraction), with the largest concentrations observed in hematite (0,64 ppm) and pyrrhotite (0,34 ppm) respectively.

The observation that pyrite is the preferred mineral host for unrecovered gold in Witwatersrand tailings material demands a further level of classification to discern between auriferous and non-auriferous pyrite. Based on morphology, texture, and degree of oxidation (from surface weathering) six classes of pyrite were designated: viz. 1. inclusion-bearing anhedral 2. inclusion-free anhedral, 3. oxidized inclusion-bearing anhedral 4. oxide rim anhedral 5. rounded anhedral and 6. Fresh inclusion-free euhedral/subhedral (Fig. 3a-f). Given that the tailings materials are ex-situ samples that have been subject to extensive comminution, processing and surface weathering, this coarse classification allows only limited comment on the ultimate provenance of the pyrite (e.g., detrital). Notwithstanding this limitation, we believe that the distinctive rounding of pyrite particles from the ‘rounded pyrite’ class (Fig. 3d, h-j) is a natural feature developed during fluvial processing and sedimentation of the Witwatersrand conglomerates. In previous in-situ morphological studies of Witwatersrand conglomerate-hosted pyrite, these distinctively rounded textures have been used to diagnostically characterize pyrites as detrital in origin (e.g., Agangi et al., 2015; da Costa et al., 2020).

Figure 3j presents a binary plot of arsenic concentration versus gold concentration measured for individual LA ICP-MS ablation spots located within pyrite grains from all six different pyrite classes (Fig. 3a-g). The pyrite morphological class that hosts the highest gold concentrations is the rounded, nominally-detrital pyrite class, which records an average spot concentration value of 21,36 ppm gold and a maximum of 2700 ppm gold. This maximum data point and the second most auriferous pyrite data point both plot above the gold saturation line of Reich et al. (2005), indicative that the gold is occurring within these pyrites as nanoparticles (or sub-micron particles) of free gold. Importantly, a further eleven detrital pyrites plots below the gold saturation line but above the concentration value of 10 ppm gold, representing 5,74% of all the detrital pyrites analyzed, and 2,43% of the total dataset. Laser ablation mapping of five individual detrital pyrite grains shows that the gold may be associated with pyrite rims (e.g., Fig. 3h-j) or more variably distributed throughout the grains, and that there is commonly a strong association with arsenic (fig. h-j, (Babedi et al., 2022; Deditius et al., 2014; Reich et al., 2005)). In contrast to the detrital pyrite population, only one pyrite particle from the oxide rim pyrite class and only one particle from the inclusion-bearing pyrite class fall above the arbitrarily-assigned 10 ppm concentration cut-off. Our data, collected from the representative tailings samples, compare well with data derived from LA ICP-MS measurements collected for pyrites located in situ in Witwatersrand conglomerate-hosted gold reefs (Morishita et al., 2019; Large & Maslennikov, 2020; SI 1). Moreover, considering published data for pyrites sampled from orogenic gold shoots in the Barberton greenstone belt (assumed to be representative of a possible granite-greenstone hinterland gold source), ~ 5% of the detrital pyrites show an overlap in terms of their gold/arsenic concentration signatures (Fig. 3j; Agangi et al., 2014). The possible linkages between Witwatersrand pyrites and an Archean greenstone orogenic gold source region were further tested using multivariate analyses of pyrite mineral chemistry, revealing some overlap relationships within a principal component biplot (SI 2).

The advent of Witwatersrand gold mining (137 years ago) greatly predates the development of modern analytical techniques capable of quantifying mineral chemistry at ppm concentration levels (e.g., LA ICP-MS). This temporal disconnect may have contributed to the entrenched paradigm that the Witwatersrand is purely a native gold resource despite 1) a dominant hypothesis that greenstone belt orogenic gold mineralization may represent the ultimate provenance of Witwatersrand gold (Frimmel et al., 2005; Koglin et al., 2010); 2) a firmly established paragenetic relationship between gold and pyrite/arsenopyrite in orogenic gold mineralization studied in greenstone belt vestiges on the Kaapvaal craton (Agangi et al., 2014; Cabri et al., 1989); and 3) knowledge that these sulphides behaved as stable heavy minerals during sedimentation under the reducing Archean paleo-environmental conditions (e.g., England et al., 2002; Frimmel, 2005). Here we highlight the presence of auriferous detrital pyrites identified within the Witwatersrand tailings material, which commonly show geochemical signatures that are comparable to those reported for pyrites associated with orogenic gold in Kaapvaal greenstone belts. Implications of this are two-fold: firstly, exploration and mining have specifically targeted free gold (19.3 g.cm^− 3) in conglomeratic units, whereas our results suggest that there may be exploration potential for an accumulation of detrital auriferous pyrite (~ 5 g.cm^− 3) in alternative (discrete) sedimentary settings. The gold grades of such a sedimentary setting may however be restrictively low to warrant mining. Secondly, historically mined detrital auriferous pyrites have been overlooked during original processing as well as during more recent reprocessing. This ‘invisible’ gold is known to be inaccessible through direct cyanidation (Coetzee et al., 2011), and may thus be a significant contributor to the 50–70% recovery inefficiencies associated with secondary beneficiation of the Witwatersrand tailings. From an economic standpoint, extraction of auriferous pyrite from these tailings is deemed more favourable due to fine grain sizes and surficial location, respectively negating comminution and the high overheads associated with underground mining.

Metallurgical flowsheets that target the sulphide fraction during tailings reprocessing (e.g., by froth flotation, pre-oxidative processing and gold leaching (see recommendations by (Fleming et al., 2010; Janse van Rensburg, 2016)) would recover this invisible gold fraction along with other metals and metalloids that may be of economic interest or environmental concern. Such a beneficiation strategy would be highly beneficial to the Witwatersrand region, where acid mine drainage (AMD), and concomitant release and mobility of deleterious elements (e.g., Cu, Cd, Co, Pb, Zn, As, Ni, Hg, U and SO₄²⁻), is an environmental issue (García et al., 2005; Naicker et al., 2003; Rösner & Van Schalkwyk, 2000; Tutu et al., 2008). This is because sulphide oxidation under surface weathering conditions (e.g., Fig. 3b, f) is the primary cause of acid mine drainage pollution in the Witwatersrand region (Tutu et al., 2008).

Most deleterious elements in Witwatersrand tailings are found preferentially in the heavy mineral fraction (Fig. 4), whereas the light mineral fractions have deleterious metal concentrations that are relatively low with the majority complying with established re-dump criteria (Canadian Council of Ministers of the Environment, 2007; Department of Environmental Affairs, 2014). Results from the in situ LA ICP-MS analyses and mass balance calculations reveal that within the heavy mineral fraction, most of the deleterious elements deport to the sulphide mineral fraction (Fig. 4). For example, the sulphides host approximately 95% of Zn, Cu, As and Pb; 55–97% of Ni; 64–99% of Co; 77–99% of Se and 84–99% of Sb within the heavy mineral fraction. This observed deportment of the high deleterious element concentrations specifically within the tailings sulphides strongly underpins an environmental motivation towards sulphide removal during tailings reprocessing. The value proposition of such an approach would be further enhanced by the recovery of high-demand heavy metals such as Co, Cu and Ni as by-products, which respectively have average calculated concentrations of 9755,88 ± 2330 ppm Co, 8771,40 ± 1817 ppm Cu and 9822,57 ± 1810 ppm Ni within the sulphide mineral fraction.

The study highly suggests that the fraction of gold inaccessible to conventional processing technologies in the Witwatersrand tailings ore material is attributed to a percentage of invisible gold, predominately within the arsenian pyrites and pyrites. Preserved rounded detrital pyrite classes within the tailings dump material exhibit the highest concentration of gold, with maximum gold concentration similar to auriferous pyrites from existing granite-greenstone belt Au deposits. The residual sulphides in the tailings dump potentially represent a large under-exploited gold resource of up to 420 tons, in which a pre-treatment beneficiation can directly improve gold recovery by ~ 22% during secondary reprocessing. This will further alleviate the growing ramifications of AMD and effluents with deleterious elements in the Witwatersrand mining region, while potentially recovering additional high-demand by-product metals.

Bulk Sampling and sample preparation

Duplicates of 150 kg of sample material were acquired from four tailings dams storage facilities associated with mining in the Evander, Carletonville, Central Rand and Klerksdorp Goldfields of the Witwatersrand Basin (fig.1). To ensure sample representivity, sub-sampling comprised blending via a cone and quarter, rifle and rotary splitting protocols.

Pre-Concentration and mount preparation.

Approximately 1 kg from each of the bulk samples underwent a series of modified density separation techniques as prescribed by Zhou & Cabri, (2004) to create density-defined mineral fractions. The applied techniques were decantation (desliming), superpanning and a two-stage heavy liquid separation using tetrabromoethane at 2.95 g/cm³ density and lithium metatungstate at 2.71 g/cm³ density. The resultant density-defined mineral fractions were a slimes fraction (particle size < 25 μm), a light mineral fraction and a heavy mineral fraction. Approximately 0.3 g of representative material from each of the density-defined mineral fractions was placed as a monolayer in an epoxy mount, and subsequently diamond polished to a 1 μm finish.

Sample analyses.

Aqua regia digestion & ICP-MS analysis

Aqua regia digestion and inductively coupled plasma mass spectrometry (ICP-MS) analyses were conducted on approximately 0.55 g duplicates from each bulk sample, each density fraction and on three blank samples at the Central Analytical Facilities (CAF), Stellenbosch University. Aqua regia digestion comprised of ± 1g of sample material partially digested in a (3:1) acid mixture of 6 ml nitric acid and 2 ml of supra pure hydrochloric acid. The sample material is digested with certified reference material WQB-1 and PTM-1a The sample solution was then heated in a MARS microwave digester at 1600W power level for 4-6 hours at temperatures of approximately 60 °C to ensure the near-total breakdown of aqua regia soluble minerals [see aqua regia digestible minerals (Smith, 1946; Lorenzen & Tumilty, 1992)]. After the containers cooled, they underwent dilution to a volume of 50 ml before quantitative analysis. A total of 20 elements were analysed using an Agilent 7900 ICP -MS at 1600 W power level, 0.83 L/min argon as the carrier gas, 0,15 L/min make-up gas flow rate, 5-6 ml/min of helium and hydrogen flow rate and a multi-elemental calibration standard for confidence in ICP-MS readings.

Image analysis via QEMSCAN, SEM & Optical microscopy

Optical microscopy was used for scanning visible gold grains, describing the minerals reporting to each of the density-defined operational mineral fractions and classifying the different pyrite classes hosted in the heavy mineral fraction. Preliminary electron microscopy was conducted at Stellenbosch University’s Central Analytical Facilities (CAF) using a Zeiss Merlin FE-SEM was utilized at conditions of 10 kV voltage, stage distance of 9.55 mm, probe current of 11 nA and equipped with Oxford INCA software.

Each heavy mineral fraction mount underwent analysis with the QEMSCAN at the University of Cape Town. The operation involved Bruker energy dispersive spectroscopy (EDS) detectors at 20 kV accelerating voltage and a 10 nA beam current. Particle mineral analyses were done at 5 μm and 211 x magnification. A false image colour map was generated to assist with the identification of ‘visible gold’ and potential gold carriers, as well as a visual aid for subsequent LA ICP-MS analysis. The bulk mineral percentages of the concentrate mount were determined, and the derived chemical information was used to validate the assay data derived from acid digestion.

In-situ laser ablation inductively coupled mass spectrometry (LA ICP-MS)

LA ICP-MS was carried out on 800 grains of oxides and sulphides from the heavy minerals fraction mounts using a grid method at CAF, Stellenbosch University with a Resolution 193 nm Excimer laser from Applied spectra, coupled with an Agilent 7700 Q ICP MS. The concentrations of thirteen trace elements (Co, Ni, Cu, Zn, As, Se, Ag, Sb, Te, Au, Hg, Pb and Bi) were quantified under analysis conditions of a pure helium (He) atmosphere (0,4 L/min), 2.8 J/cm² laser energy, 7 Hz frequency, before transportation to the ICP-MS mixed with argon [Ar (0,95 L/min)] plus nitrogen [N (0,003 L/min)]. Po725 and a synthetic sulphide (³⁴S) MASS-1 were used for quality control and as a calibration standard. The matrix optimized ablation parameters were set at 2.8 J/cm² for laser energy and 7 Hz for the frequency to create a stable signal for suitable sensitivity and to combat laser-induced element fractionation. Due to the fine-grained nature of the material the ablation spot size used was 20-30 µm at an ablation time of 45 seconds and a background acquisition of 20 seconds.

Resultant data were processed using LADR from Norris Scientific (Norris & Danyushevsky, 2018). For the correction of the variations in the ablation yielded between the grains and the standards, an internal standard element with a known concentration was utilized (Longerich et al., 1997). The sulphur (S) content for each sample grain was acquired from SEM analysis beforehand. Determination of gold mineralogical percentages was calculated using the assay results from aqua regia digestion ICP-MS analysis, bulk mineral percentages from the QEMSCAN analysis and the in-situ ablation analysis. LA ICP-MS elemental maps on the detrital grain were done with a spot size of 15 µm at a scan speed of 10 µm/sec. The raw data generated was processed and interpolated by Inverse distance weighting (IDW) in ArcMap.

Multivariate analysis or principal component analysis was done using the rounded detrital pyrite trace element data and pyrite trace element data from Archean greenstone belt vestiges within the Kaapvaal craton (Agangi et al., 2014; Altigani, 2021a,b). The biplots were generated on R and XLSTAT.

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Acknowledgements

This research was funded by the DST-NRF Centre of Excellence for Integrated Mineral and Energy Resource Analysis (CIMERA) for a master’s of Science degree. Additional analytical funding was provided by the Society of Economic Geologists (SEG) Tim Nutt Fund. Thank you to the analytical staff of the ICP-MS & XRF unit at CAF, Stellenbosch University; Mrs R. Rossouw, Mrs C. Anderson-Small, Mrs M. Moolman and Mr H. Achilles for the help in sample preparation and analyses. Thank you to the analytical staff at the Electron microscopy unit at CAF, Stellenbosch University; Mrs M. Frazenburg and Dr A. Botes. Thank you to Dr A. Molifie and Mrs L. Nkemba at the QEMSCAN unit, University of Cape Town for the assistance in sample analyses. Thank you to the radiation safety officer at Stellenbosch University, Dr R Botha for measuring the radioactivity of the sample material and making sure it was safe to work with. We further extend our thanks to the mining houses Harmony Gold limited, Katoro Gold, DRD Gold Limited and Pan African Resources for generously providing the tailings dam material for the study. Finally, we thank Prof A. Roychoudhury, Prof G. Stevens and Prof G. Nwaila for their critical comments on an early version of this manuscript.

Data availability

This study was carried out for the framework of a master’s of science degree at Stellenbosch University. All the datasets utilized, analyses done and results obtained are accessible from the corresponding author upon reasonable request.

Author contributions

The order of authors is given with the contributor roles as follows: Corresponding author Steve Jason Chingwaru contributed the methodology, data validation, formal analysis, investigation, data curation, writing of the original draft, visualization and review & editing of the draft. The other authors of the manuscript Dr Bjorn P. von der Heyden and Dr Margreth Tadie contributions include conceptualization, resources, review & editing of the original draft, supervision, project administration & funding acquisition.

No competing interests reported.

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Invisible gold in the Archean detrital sulphides of the Witwatersrand tailings dumps: A large and under-exploited gold resource

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Abstract

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Introduction

Gold Deportment In Density-defined Mineral Fractions And Minerals

Petrographic And Chemical Characterization Of Auriferous Pyrite

Auriferous Detrital Pyrites In Tailings As A Surface Gold Resource

Environmental Benefits Of Targeted Re-mining Of Detrital Pyrites

Conclusion

Methods

References

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Additional Declarations

Supplementary Files

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