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X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDX) are applied to investigate the properties of fine-grained concentrates on artisanal, small-scale gold mining samples from the Kubi Gold Project of the Asante Gold Corporation near Dunwka-on-Offin in the Central Region of Ghana. Both techniques show that the Au-containing residual sediments are dominated by the host elements Fe, Ag, Al, N, O, Si, Hg, and Ti that either form alloys with gold or with inherent elements in the sediments. For comparison, a bulk nugget sample mainly consisting of Au forms an electrum, i.e., a solid solution with Ag. Untreated (impure) sediments, fine-grained Au concentrate, coarse-grained Au concentrate, and processed ore (Au bulk/nugget) samples were found to contain clusters of O, C, N, and Ag, with Au concentrations significantly lower than that of the related elements. This finding can be attributed to primary geochemical dispersion, which evolved from the crystallization of magma and hydrothermal liquids as well as the migration of metasomatic elements and the rapid rate of chemical weathering of lateralization in secondary processes. The results indicate that Si and Ag are strongly concomitant with Au because of their eutectic characteristics, while N, C, and O follow alongside because of their affinity to Si. These non-noble elements thus act as pathfinders for Au ores in the exploration area. This paper further discusses relationships between gold and sediments of auriferous lodes as key to determining indicator minerals of gold in mining sites.
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minerals
Article
Elucidating Pathfinding Elements from the Kubi Gold Mine
in Ghana
Gabriel K. Nzulu 1, 2, * , Babak Bakhit 1, Hans Högberg 1, Lars Hultman 1and Martin Magnuson 1


Citation: Nzulu, G.K.; Bakhit, B.;
Högberg, H.; Hultman, L.; Magnuson,
M. Elucidating Pathfinding Elements
from the Kubi Gold Mine in Ghana.
Minerals 2021,11, 912. https://
doi.org/10.3390/min11090912
Academic Editors:
Gioacchino Tempesta and
Giovanna Agrosì
Received: 29 July 2021
Accepted: 20 August 2021
Published: 24 August 2021
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Copyright: © 2021 by the authors.
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distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
1Department of Physics, Chemistry and Biology (IFM) Linköping University, SE-58183 Linköping, Sweden;
babak.bakhit@liu.se (B.B.); hans.hogberg@liu.se (H.H.); lars.hultman@liu.se (L.H.);
martin.magnuson@liu.se (M.M.)
2Gold Corporation, No. 1, Yapei Link, Airport Residential Area, P.O. Box 9311, Accra 00233, Ghana
*Correspondence: gabriel.nzulu@liu.se
Abstract:
X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDX)
are applied to investigate the properties of fine-grained concentrates on artisanal, small-scale gold
mining samples from the Kubi Gold Project of the Asante Gold Corporation near Dunwka-on-Offin
in the Central Region of Ghana. Both techniques show that the Au-containing residual sediments
are dominated by the host elements Fe, Ag, Al, N, O, Si, Hg, and Ti that either form alloys with
gold or with inherent elements in the sediments. For comparison, a bulk nugget sample mainly
consisting of Au forms an electrum,i.e., a solid solution with Ag. Untreated (impure) sediments,
fine-grained Au concentrate, coarse-grained Au concentrate, and processed ore (Au bulk/nugget)
samples were found to contain clusters of O, C, N, and Ag, with Au concentrations significantly
lower than that of the related elements. This finding can be attributed to primary geochemical
dispersion, which evolved from the crystallization of magma and hydrothermal liquids as well as the
migration of metasomatic elements and the rapid rate of chemical weathering of lateralization in
secondary processes. The results indicate that Si and Ag are strongly concomitant with Au because
of their eutectic characteristics, while N, C, and O follow alongside because of their affinity to Si.
These non-noble elements thus act as pathfinders for Au ores in the exploration area. This paper
further discusses relationships between gold and sediments of auriferous lodes as key to determining
indicator minerals of gold in mining sites.
Keywords:
gold; garnet; X-ray photoelectron spectroscopy; electron dispersive X-ray spectroscopy;
chemical bonding
1. Introduction
Gold mineralization in the Kubi Gold Project of the Asante Gold Corporation near
Dunkwa-on-Offin in the Central Region of Ghana is mainly controlled by the silicate
type of the mineral garnet (A
3
B
2
(SiO
4
)
3
), where the Asite represents the divalent cations
(Ca, Mg, Fe, Mn)
2+
and the Bsite has trivalent cations in either an octahedral or tetra-
hedral milieu, with (SiO
4
)
4
occupying the tetrahedral sites [
1
,
2
]. Sulfide minerals such
as pyrite/marcasite (FeS
2
), arsenopyrite (FeAsS), pyrrhotite (Fe
(1x)
S), and chalcopyrite
(CufeS
2
) are other important pathfinder minerals for the location of the Au ore and help to
understand the geology of Kubi.
Traditionally, coarse-grained concentrates are used in gold production in most large-
scale mining, while the fine-grained powder sediments are considered part of mining
waste (tailings). Artisanal and small-scale gold mining, without the use of mercury, also
depends on the crushing or milling of Au particles into similar grain sizes, which can
easily be separated by using gravity concentration methods, i.e., panning, sluicing, shaking
tables, spiral concentration, vortex concentration, and centrifuges [
3
]. It will therefore be
economically viable to extract gold and other minerals from fine-grained materials under
realistic conditions.
Minerals 2021,11, 912. https://doi.org/10.3390/min11090912 https://www.mdpi.com/journal/minerals
Minerals 2021,11, 912 2 of 22
Indicator and pathfinder theories are deployed by geologists and explorers to gain
information on the location of ore deposits, using methods such as gold grain morphology
(changes in Au surface shape via weathering and erosion as sediments are transported
from far distances), gold grain inclusion (presence of other minerals in gold grains are used
to provide information about the deposit type and mineralization associated), composition
studies (to gather and examine mineral samples for information on distances and direction
of transport), and geochemistry (identifying and analyzing minerals such as silver, plat-
inum, palladium, copper, lead, iron, telluride) to test clues and patterns in order to locate
ore bodies for gold [4,5].
To date, researchers have tried to understand the nature and properties of the various
elements associated with gold as well as their chemical bonds and reaction mechanisms in
relation to gold deposition and agglomeration [
6
9
]. Several studies indicate that during
ore deposition in hydrothermal systems, there are surface reductions or adsorption or both
in gold precipitation [1013].
From a geological perspective, it is important to study the diffusion, flow mechanism,
and chemical bonding of host (indicator) minerals to understand the dynamics and evo-
lution of the Earth and other terrestrial planets. These features can influence the mantle
convection, earth processes (erosion, weathering, landslide, volcanic eruption, earthquake,
etc.), and also give information on the Earth’s thermo-chemical structure. This will enable
us to understand the geodynamics of the Earth, its rheological properties (material defor-
mation), kinetics of phase transformation, and the thermal conductivity of minerals on the
planet.
Gold-associated minerals such as the sulfide group FeS
2
, FeAsS, CuFeS
2
, Fe
(1x)
S, and
galena (PbS) make the tiny particles of gold invisible or cause them to attach to the metallic
sulfide lattices [
14
18
]. Pyrite is one of the most common minerals that conglomerate
gold in various ore deposits; it is also part of the gold-bearing minerals (as free milling or
refractory) in the Au exploration areas, alongside garnet and the sulfide group [6,7,14].
Previously, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy
(SEM), electrochemical, and other techniques have been deployed in the study of gold de-
position on pyrite and other gold-bearing minerals in aqueous solutions [
7
,
8
,
16
]. Pathfinder
minerals for Au such as chalcopyrite and pyrite minerals in their natural states were stud-
ied using cyclic voltammetry, electrochemical impedance spectroscopy, XPS, and SEM,
which also included microanalysis (SEM/EDX). In both minerals, oxygen containing Fe
3+
and Cu
2+
were found, while the measurements also showed alteration in both minerals at
different potential values [
19
]. Harmer et al. [
20
23
] studied the rate of leaching of Cu and
Fe in chalcopyrite when percolated in perchloric acid and found the leach rate to be the
same after a long period of time; the final analysis was based on the surface speciation and
oxidation steps in the release of Cu and Fe into solutions and their polymerization from
mono-sulfide to polysulfide. Wang et al. [
24
] used high-resolution transmission electron
microscopy and XPS to characterize the morphological change in Au nano-rods (NRs)
from artisanal small-scale mining in order to understand the concentration of mercury.
It was found that the low concentration of mercury that formed and was deposited onto
the gold surface could be reversibly removed by an electrochemical stripping process
without causing any change in terms of size or shape to the Au NRs. However, at high
concentrations, Hg did not only deposit onto the gold surface, but also entered the interior
of Au NRs and transformed them into irreversible gold nanospheres (Au NSs) due to the
amalgamation of Au with Hg. Since the assembling of Au with Hg by small-scale artisanal
miners has significant negative impacts on the health of individuals and the environment,
there is a need for a more environmentally sustainable separation mechanism.
XPS has been a contributory technique [
25
] in the study of the products of oxida-
tion [
26
] formed on sulfide minerals as well as in the use of synchrotron radiation [
27
];
it is an added advantage in explaining mineral surface chemistry (mineral geochem-
istry)
[2830]
, predominantly the primary surface conditions of materials formed by de-
formations and cracks [
30
32
]. The main advantage of XPS and resonant photoemission
Minerals 2021,11, 912 3 of 22
using synchrotron radiation in mineralogy is their ability to define comprehensive surface-
sensitive information from high-resolution spectra of samples of macroscopic sizes in
different mineral phases. The distribution of oxidation products provides heterogeneity to
mineral surfaces, and because most gold-bearing sulfide minerals contain other minerals
and impurities with varying texture, it will be of interest to reveal the individual elements
by XPS, which can then be assigned to the original host minerals. It is therefore essential to
identify the existence of the different types of elements on both the bulk and impure sample
surfaces using high resolution XPS, and determine the distribution of those elements across
the sample surfaces [3335].
Mikhlin et al. [
36
] used XPS, AFM, and scanning tunnelling microcopy/scanning tun-
nelling spectroscopy (STM/STs) to characterize arsenopyrite samples oxidized in Au (III)
chloride solutions at ambient temperatures in order to explain the moderation of oxidation
in chloride solutions. Murphy and Strongin [
37
] conducted surface reactivity studies on
pyrite (FeS
2
) and pyrrhotite (Fe
x
S
1x
) using different experimental techniques such as
vacuum type experiments, where electron and photon spectroscopies were applied and
further analyzed microscopically using infra-red spectroscopy. XPS and X-ray absorption
spectroscopy techniques were also used to investigate the structure of the original pyrite
and sulfide surfaces [
37
,
38
]. Acres et al. [
39
] applied synchrotron-based XPS, near-edge
X-ray Absorption Florescence Spectroscopy, and time of flight secondary ion mass spec-
trometry studies on smooth and rough fracture surfaces of chalcopyrite and pyrite samples.
The results showed an increase in the chalcopyrite surface roughness and an intensification
in the formation of the sulfur surface in the pyrite as well as the typical chalcopyrite grain
size formation [39].
Other studies on Au and associated minerals have concentrated on the valence band
electronic structure using nanoparticle sizes on NiO, TiO
2
, and Al
2
O
3
[
24
,
27
,
36
,
37
] using
ultraviolet photoelectron spectroscopy (~40 eV). Likewise, conventional Al K
α
X-ray XPS
(1486 eV) was used to study the electronic structure of Au nanoparticles on carbon [
40
44
].
The results from these studies indicate that a metal-insulator transition occurs as a function
of size between Au atoms, which is attributed to quantum size effects.
In order to understand the relationships between Au and associated minerals, there
is the need to study the chemical bonding and properties of elemental species in these
indicator minerals. This will aid in determining the features of the elements’ concentration
and diffusion during ore formation and the flow mechanism in hydrothermal systems
ascribed to Au host minerals. Despite increasing knowledge that indicator minerals can
host numerous elements, most geologists in the study area have mainly concentrated
on host minerals for Au such as arsenopyrite, pyrite, garnet, and quartz, which occur
in orogenic and sediment-hosted and disseminated gold deposits known as the Carlin
type [45].
Bayari et al. [
46
] used X-ray florescence spectroscopy, XRD, and inductively coupled
plasma mass spectrometry to identify pathfinder elements and their associated pathfinder
minerals in the mineralized regolith profiles at the Bole–Nangoli gold belt in north-eastern
Ghana. Moreover, Nude et al. [
47
] used a multivariate statistical approach to identify
pathfinder elements for gold from the north-west of Ghana. In both cases, the trace
elements such as Fe, Mn, Ag, As, Cu, Zn, Ni, Pb, etc., were identified and appeared to be
associated with Au and suitable as pathfinder elements of Au in the study areas.
Up until now, there is little or no work on pathfinder elements and minerals in
Ghana using XPS and EDX, and in particular, in the Kubi Gold mining area. Rock and
mineral identifications leading to Au discovery in this exploration area have been based on
geophysical techniques and geochemical analysis (soil/sediments sampling) in laboratories
as well as geologists’ experiences in mineral identification. However, these procedures
are sometimes misleading and may cause delays in projects and high financial loss to
mineral exploration activities. Therefore, there is a need to introduce faster and more
robust techniques for the effective identification of pathfinder elements and minerals in the
mining industry.
Minerals 2021,11, 912 4 of 22
In this present work, fine-grained sediments (mining waste) from artisanal small-scale
mining sites are investigated by surface sensitive XPS and bulk-sensitive EDX to quantify
their Au content and other important elements that act as indicator minerals of gold. We
investigate Au samples from alluvia deposits to identify individual elements in order to
gain corresponding insight into surface reaction mechanisms as well as detailed properties
of precipitated Au particles observed during the hydrothermal formation of the ore deposit.
These identifiable pathfinder elements are then utilized to infer the original indicator host
minerals in the area.
2. Experimental Details
2.1. Description of Study Area
Samples for the study were taken from the alluvia small-scale mining site located in
the Dunkwa-on-Offin district in the central region of Ghana. Figure 1shows the district
that is located within latitudes 5
30
0
and 6
02
0
north of the equator and longitudes between
1
W and 2
W of the Greenwich Meridian [
48
]. Close to the vicinity of the Kubi Gold
Barbados is the alluvia mining site, where the crushed and washed samples for the project
were collected. The area is drained by a number of rivers and streams, including the Offin
River as the main river source, which is 90 m above sea level, forming a boundary between
the Ashanti region and the Central region with the Pra River [
49
]. The area consists of
volcanic belts, basement rock, and sedimentary basins with a lot of igneous (volcanic)
activity and two orogenic events, genuinely following the geology of Ghana and leading
Au-hosted quartz veins to dip steeply in shear zones with Birimian basins of sulfur-rich
minerals. The gold at the Kubi mining area is free milling (open cast mining) and occurs
within a near vertical 1 km by 1 m, to 15 m thick shear confined garnet zone. The ore is
estimated to be hosted by 30% garnet, 15% sulfide mineralization, and also occurs as coarse
gold in late quartz veins.
In addition, the Offin river has alluvia placer Au deposits in gravels and in quartz-
pebble conglomerates of the Tarkwaian deposits [
48
]. Within the alluvia regime are oxides
or weathered rocks of the host minerals, which are mostly iron oxides and mineralized
quartz [50].
Minerals 2021,11, 912 5 of 22
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Figure 1. (a) Map of Kubi village and surrounding Townships and (b) Picture of sampling area of
“Kubi Gold Barbados”, Dunkwa-on-Offin. (Map and photo by G.K. Nzulu).
2.2. Behavior of Gold in Hydrothermal Systems
Figure 1.
(
a
) Map of Kubi village and surrounding Townships and (
b
) Picture of sampling area of
“Kubi Gold Barbados”, Dunkwa-on-Offin. (Map and photo by G.K. Nzulu).
Minerals 2021,11, 912 6 of 22
2.2. Behavior of Gold in Hydrothermal Systems
Figure 2a shows a schematic profile of the sampling area (Kubi soils). The saprolite
(altered by hematite and limonite/goethite) that dominates the profile grades upwards
into the transition and continues to the clay zone, which consist of ferruginous materials,
mostly of goethite, kaolinite, and chlorite. Within the stone line horizon are fragments
of quartz (pebble and cobbles) and gravels mixed with ferruginous clay that have been
formed by the weathering profile. The Au in this soil is due to weathering activities and in
association with the sulfide group of minerals and Fe-oxides. The smaller Au grains found
a few meters from the surface occur beyond the stone line lateritic zone through chemical
dissolution [
51
]. Thus, beyond the stone line, Au mineralization occurs with much coarser
Au grains as a result of pedoturbation (soil horizon mixing) that goes downward due to the
migration of fragments from the stone line, through the transition zone and to the saprolite,
and vice versa.
Minerals 2021, 11, x FOR PEER REVIEW 6 of 22
Figure 2a shows a schematic profile of the sampling area (Kubi soils). The saprolite
(altered by hematite and limonite/goethite) that dominates the profile grades upwards
into the transition and continues to the clay zone, which consist of ferruginous materials,
mostly of goethite, kaolinite, and chlorite. Within the stone line horizon are fragments of
quartz (pebble and cobbles) and gravels mixed with ferruginous clay that have been
formed by the weathering profile. The Au in this soil is due to weathering activities and
in association with the sulfide group of minerals and Fe-oxides. The smaller Au grains
found a few meters from the surface occur beyond the stone line lateritic zone through
chemical dissolution [51]. Thus, beyond the stone line, Au mineralization occurs with
much coarser Au grains as a result of pedoturbation (soil horizon mixing) that goes down-
ward due to the migration of fragments from the stone line, through the transition zone
and to the saprolite, and vice versa.
Figure 2. (a) Schematic section of the Kubi soil profile; (b) Schematic section of ground profile showing Au auriferous
lodes.
Figure 2b shows the oxidation and reduction processes that occur when pathfinder
minerals undergo chemical dissolution to deposit gold above and below the water table.
Au formed on the oxides (near surface) and on external particles are usually from a mix-
ture of chemical reactions and microbiological activity in groundwater of sedimentary
origin, with a pH ranging from 6 to 9. During the flow regime within the hydrothermal
system (zone of aeration), the rich sulfide minerals (pyrite group) and the oxide groups
can release metastable thiosulphate ions (S2O32) during the oxidation process to form a
composite material with Au under near-neutral-alkaline pH conditions to trigger a disso-
lution of Au particles within the sediments/minerals [52,53]. These thiosulphate ions can
decompose after a short period to aid in the oxidation of sulphate ions formed above the
water table, or to aid in the reduction of dissolved hydrogen sulfide ions formed below
the water table (zone of saturation) [53]. Sulphate minerals found in underground water
undergo a reduction process to form bisulphide ions (HS) near the water table in order
to assemble Au in a sedimentary environment under near-neutral pH conditions. This
same chemical process can facilitate the precipitation of Ag with Au under favorable pH
Figure 2.
(
a
) Schematic section of the Kubi soil profile; (
b
) Schematic section of ground profile showing Au auriferous lodes.
Figure 2b shows the oxidation and reduction processes that occur when pathfinder
minerals undergo chemical dissolution to deposit gold above and below the water table.
Au formed on the oxides (near surface) and on external particles are usually from a mixture
of chemical reactions and microbiological activity in groundwater of sedimentary origin,
with a pH ranging from 6 to 9. During the flow regime within the hydrothermal system
(zone of aeration), the rich sulfide minerals (pyrite group) and the oxide groups can release
metastable thiosulphate ions (S
2
O
32
) during the oxidation process to form a composite
material with Au under near-neutral-alkaline pH conditions to trigger a dissolution of Au
particles within the sediments/minerals [
52
,
53
]. These thiosulphate ions can decompose
after a short period to aid in the oxidation of sulphate ions formed above the water table,
or to aid in the reduction of dissolved hydrogen sulfide ions formed below the water
table (zone of saturation) [
53
]. Sulphate minerals found in underground water undergo a
reduction process to form bisulphide ions (HS
) near the water table in order to assemble
Au in a sedimentary environment under near-neutral pH conditions. This same chemical
process can facilitate the precipitation of Ag with Au under favorable pH conditions, to
deposit gold together with a small quantity of Ag [
52
,
53
]. Thus, under alkaline conditions,
Minerals 2021,11, 912 7 of 22
the most soluble complex of the Au-Ag-S-O system is the Ag(S
2
O
3
)
23
ion, such that
the separation of Ag and other elemental species from Au at high pH levels is always a
possibility [54].
2.3. Sample Preparation
Sediment extracted from a depth of 10 m and containing Au samples from the artisanal
small-scale mining site in Dunkwa-on-Offin was investigated in this study. The sample was
collected by one of the authors (G.K.N.) with the aid of a geological hammer and placed
into a sample collection bag. The composite “concentrate sample” weighing 1.90 kg, as
shown in Figure 3a, was divided into two parts. One portion (1.20 kg) was refined into pure
solid Au (bulk Au), while the other powder sample (0.70 kg) was subjected to panning,
i.e., washing (Figure 3c) and extraction of residual sediments of Fe
3
O
4
, Fe
2
O
3
, and other
minerals to obtain an untreated Au powder concentrate with a weight of 80 g
(Figure 3d)
.
This untreated powder sample was later separated into coarse-grained (0.85 g) and fine-
grained (1.20 g) with the aid of a tweezer and water panning (via gravity), respectively (the
residual 77.95 g containing sand and gravel was disposed). The impure powder sample
(Figure 3d) was first investigated by XPS, after which a tweezer was used to pick the
average-grain-sized particles as coarse-grained powder sample. The final residual sample
was water-panned for tiny particles of Au, quartz, and the pyrite group as fined-grained
powder samples.
The final three samples, namely solid Au (bulk), untreated Au concentrate powder,
fine and coarse-grained Au concentrates in Figure 3b,e,f, respectively, in addition to a
standard thin film Au sample in Figure 3g, were examined by XPS to reveal distinct
features of the materials. Particle/grain sizes of the powder samples ranged from 0.05 cm
to about 0.2 cm in maximum dimensions of which most were hoppered single crystals. The
nature of materials indicates that most Au from this artisanal area come from weathered
SiO
2
veins hosted by coarse-grained igneous and meta-volcanic rocks, with mineralization
controlled by a garnetiferous horizon of fine-grained Au associated with sulfide minerals
and occasional carbonates [50].
Minerals 2021,11, 912 8 of 22
Minerals 2021, 11, x FOR PEER REVIEW 8 of 22
Figure 3. (a) Dry residual sample (tailing) from an artisanal mine site at Dunkwa-On-Offin. (b) Part of the sample refined
into pure Au nugget of 1 gram and 22 carats (measured with a digital electronic Au purity Analyzer D H 300 K from
VTSYIQ). (c) Panned sample of Au with other impurities, (d) untreated ore powder, (e) fine-grained concentrate Au, (f)
coarse-grained concentrate powder, and (g) Au film on wafer substrate. (Photo by G.K. Nzulu).
The final three samples, namely solid Au (bulk), untreated Au concentrate powder,
fine and coarse-grained Au concentrates in Figure 3b,e,f, respectively, in addition to a
standard thin film Au sample in Figure 3g, were examined by XPS to reveal distinct fea-
tures of the materials. Particle/grain sizes of the powder samples ranged from 0.05 cm to
about 0.2 cm in maximum dimensions of which most were hoppered single crystals. The
nature of materials indicates that most Au from this artisanal area come from weathered
SiO2 veins hosted by coarse-grained igneous and meta-volcanic rocks, with mineralization
Figure 3.
(
a
) Dry residual sample (tailing) from an artisanal mine site at Dunkwa-On-Offin. (
b
) Part of the sample refined
into pure Au nugget of 1 gram and 22 carats (measured with a digital electronic Au purity Analyzer D H 300 K from
VTSYIQ). (
c
) Panned sample of Au with other impurities, (
d
) untreated ore powder, (
e
) fine-grained concentrate Au, (
f
)
coarse-grained concentrate powder, and (g) Au film on wafer substrate. (Photo by G.K. Nzulu).
2.4. XPS Measurements
XPS was used to analyze the elemental composition and chemistry of the powder
samples (untreated, fine, and coarse-grained Au ore) and reference materials (bulk nugget
and a 300 nm thick Au film with a 2.5 nm Ti buffer layer on a Si wafer substrate). The
analyses were performed in an Axis Ultra DLD instrument from Kratos Analytical (UK),
employing monochromatic Al K
α
radiation (h
ν
= 1486.6 eV) and operated at a base pressure
lower than 1.1
×
10
9
Torr (1.5
×
10
7
Pa) during spectra acquisition. XPS depth profiles
Minerals 2021,11, 912 9 of 22
were acquired by sputter-etching with 0.5-keV Ar
+
ions incident at an angle of 70
relative
to the sample surface normal. The low Ar
+
energy and shallow incidence angle were chosen
to minimize the effect of sputtering damage in the spectra. The sample areas analyzed
by XPS were 0.3
×
0.7 mm
2
and located in the center of 3
×
3 mm
2
ion-etched regions.
The binding energy (BE) scale was calibrated using the ISO-certified procedure, with
the spectra referenced to the Fermi edge in order to circumvent discrepancies associated
with employing the C 1s peak from adventitious carbon [
55
]. Prior to the XPS core and
valence band measurements, the structural properties were investigated [
56
]. After XRD
analysis [
56
], Au, quartz (SiO
2
), magnetite (Fe
3
O
4
), and hematite (Fe
2
O
3
) were identified
in these samples.
2.5. EDX Measurements
The bulk, untreated, fine- and coarse-grained concentrate samples were analyzed
using SEM/EDX operated at 20 kV. The EDX used in this study is of the Zeiss Supra 35 VP
field emission SEM type whose field emission scanning electron microscopy (FESEM)
is equipped with an energy dispersive X-ray spectroscopy (EDX) detector, which was
used for the elemental mapping of the samples to understand the elemental distribution.
Samples were prepared by mounting the macrocrystalline samples onto aluminum stubs
with carbon sticky tabs.
3. Results and Discussion
3.1. Core-Level XPS
Figure 4shows the XPS survey spectra of Au in the form of thin film, bulk, and powder
(untreated, fine-grained, and coarse-grained) concentrate samples, which are vertically
offset for clarity. The observed peaks of the thin film are identified as Au 4f
7/2,5/2
at 84
and 87 eV, Au 4d
5/2,3/2
at 335 and 353 eV, Au 4p
3/2,1/2
at 546 and 642 eV, and Au 4s at 762 eV.
The impure powder sample was first investigated by XPS, after which a tweezer was
used to pick the average-grain-sized particles as coarse-grained powder sample. The final
residual sample was water-panned for tiny particles of Au, quartz, and the pyrite group as
fined-grained powder samples.
Minerals 2021,11, 912 10 of 22
Minerals 2021, 11, x FOR PEER REVIEW 10 of 22
Figure 4. XPS survey spectra of film, bulk, and powder (untreated, fine, and coarse-grained concen-
trate) Au. The broken line connects the band edges as well as the apparent spin-orbit-splitting max-
ima. VB denotes valence band.
In the Au bulk sample, we can also identify all the Au peaks as well as peaks arising
from Ag 3d5/2,3/2 at 368 and 374 eV, Ag 3p3/2,1/2 at 573 and 604 eV, and Ag 3s at 719 eV. In
addition, there are C 1s and O 1s peaks at 285 and 532 eV, respectively. In the untreated
concentrate powder sample, in addition to the Au peaks, we find a Ti 3p peak at 37.50 eV,
Ti 2p3/2,1/2 peaks at 458 and 461 eV, respectively, and an N 1s peak at 397 eV, as listed in
Table 1 and consistent with reference data [57].
Table 1. Elements identified in the analysis of XPS core levels for Au bulk and powder (untreated, fine-grained, and coarse-
grained) Au concentrate samples.
Element Spectral Line Formula Energy (eV) Parent Mineral
Au 4s Au 762.0 Gold
Au 4p3/2,1/2 Au 546.0/642.0 Gold
Au 4d Au 335.0 Gold
Au 5p3/2 Au 57.20 Gold
Au 4f Au 84.0/87.0 Gold
Ti 3p TiO2 37.50 Hematite, garnet, and
other silicate minerals
Ti 2s TiO2 561.00 Hematite, garnet, and
other silicate minerals
Ti 2p3/2, 1/2 TiO2 458.0/464.19 Hematite, garnet, and
other silicate minerals
Si 2p SiO2 102 Quartz
Si 2s SiO2 153.0 Quartz
S 2p S 163 Au-S/FeS2
S 2s S 231 Au-S/FeS2
Figure 4.
XPS survey spectra of film, bulk, and powder (untreated, fine, and coarse-grained con-
centrate) Au. The broken line connects the band edges as well as the apparent spin-orbit-splitting
maxima. VB denotes valence band.
In the Au bulk sample, we can also identify all the Au peaks as well as peaks arising
from Ag 3d
5/2,3/2
at 368 and 374 eV, Ag 3p
3/2,1/2
at 573 and 604 eV, and Ag 3s at 719 eV. In
addition, there are C 1s and O 1s peaks at 285 and 532 eV, respectively. In the untreated
concentrate powder sample, in addition to the Au peaks, we find a Ti 3p peak at 37.50 eV,
Ti 2p
3/2,1/2
peaks at 458 and 461 eV, respectively, and an N 1s peak at 397 eV, as listed in
Table 1and consistent with reference data [57].
Table 1.
Elements identified in the analysis of XPS core levels for Au bulk and powder (untreated, fine-grained, and
coarse-grained) Au concentrate samples.
Element Spectral Line Formula Energy (eV) Parent Mineral
Au 4s Au 762.0 Gold
Au 4p3/2,1/2 Au 546.0/642.0 Gold
Au 4d Au 335.0 Gold
Au 5p3/2 Au 57.20 Gold
Au 4fAu 84.0/87.0 Gold
Ti 3p TiO237.50 Hematite, garnet, and
other silicate minerals
Ti 2s TiO2561.00 Hematite, garnet, and
other silicate minerals
Ti 2p3/2, 1/2 TiO2458.0/464.19 Hematite, garnet, and
other silicate minerals
Si 2p SiO2102 Quartz
Si 2s SiO2153.0 Quartz
S 2p S 163 Au-S/FeS2
Minerals 2021,11, 912 11 of 22
Table 1. Cont.
Element Spectral Line Formula Energy (eV) Parent Mineral
S 2s S 231 Au-S/FeS2
Ag 3p1/2, 3/2 Ag 604.0–573.0 Silver
Ag 3d3/2, 5/2 Ag 368.0–374.0 Silver
Ag 3s Ag 719.0 Silver
N 1s N 397.0 Nitrogen
O 1s O 532.0 Oxides
C 1s C 285.0 Carbon/graphite
Fe 2p3/2 Fe/Cu 706.90 Iron/FeS2
Fe 2p1/2 Fe/Cu 720.0 Iron/FeS2
Hg 2p Hg 104 Hg
Hg 4p3/2 Hg 577 Hg
In the present experiment, sulfur is very diluted in the samples due to the preparation
method that makes the detection of sulfur by XPS very challenging (low sensitivity). Only
a very weak S 2p peak around 165 eV was observed in untreated powder, while in the
fine-grained sample it was hardly discernable. In the other samples, it was below the
signal-to-noise level. The S 2s cross-section at 231 eV is even lower than the S 2p signal.
The Fe in the powder (untreated, fine, and coarse-grained concentrates) and Au bulk
samples has 2p
3/2,1/2
peaks located at 706.90 and 720 eV, respectively, consistent with the
literature [58,59] and in agreement with reference data [57]. The Fe 2p1/2 peaks are closely
located at the same BE of 719 eV as the Ag 3s peak [57].
The measured XPS data of Au 4p
3/2
, O 1s, and C 1s in Figure 4provide complementary
information on the cleanliness of the sample surfaces as well as the state of oxidation
and carboniferous (carbonic) contamination [
60
] as these alluvia samples may contain
mine discharge, dissolution of carbonate minerals at a well-buffered pH, different calcite
saturation indices, and dissolved in oxygen [61].
Figure 5shows the core-level Au 4fXPS spectra of the Au film, Au bulk, and untreated
concentrate powder of Au. For the Au film and bulk samples, the 4f
5/2
peaks are located
at 87.63 eV and the 4f
7/2
peaks are located at 83.95 eV, with a 3.6 eV spin-orbit splitting.
These binding energies are similar to the literature values of 87.60 and 84.0 eV [
57
] for pure
Au.
For comparison, in the Au powder sample, the 4f
5/2
is located at 87.77 eV and the 4f
7/2
peak at 84.09 eV. The Au 4fpeaks are asymmetric, while the shape of the untreated Au
powder concentrate, and the coarse-grained Au powder concentrate spectra broadens with
tails toward higher BE. The chemical shift of 0.14 eV towards higher BE for the powder
samples suggests that there is a charge transfer from Au towards the other elements. The
high-energy shift can be attributed to the screening effect in comparison to the pure bulk
Au and suggests total charge-transfer from Au towards certain elements.
From the spectra in Figure 5, the Au 4fregion has well separated spin-orbit compo-
nents of 3.6 eV, which is close to the Au metal (
= 3.7 eV) and is in agreement with the
literature [
62
]. The Au 4f
7/2
peak at 84.0 eV serves as a useful BE reference for the Au
metal [
57
] and is expected to shift (BE shift) to Au nanoparticle sizes for smaller cluster
samples.
Minerals 2021,11, 912 12 of 22
Minerals 2021, 11, x FOR PEER REVIEW 11 of 22
Ag 3p1/2, 3/2 Ag 604.0–573.0 Silver
Ag 3d3/2, 5/2 Ag 368.0–374.0 Silver
Ag 3s Ag 719.0 Silver
N 1s N 397.0 Nitrogen
O 1s O 532.0 Oxides
C 1s C 285.0 Carbon/graphite
Fe 2p3/2 Fe/Cu 706.90 Iron/FeS2
Fe 2p1/2 Fe/Cu 720.0 Iron/FeS2
Hg 2p Hg 104 Hg
Hg 4p3/2 Hg 577 Hg
In the present experiment, sulfur is very diluted in the samples due to the preparation
method that makes the detection of sulfur by XPS very challenging (low sensitivity). Only
a very weak S 2p peak around 165 eV was observed in untreated powder, while in the
fine-grained sample it was hardly discernable. In the other samples, it was below the sig-
nal-to-noise level. The S 2s cross-section at 231 eV is even lower than the S 2p signal.
The Fe in the powder (untreated, fine, and coarse-grained concentrates) and Au bulk
samples has 2p3/2,1/2 peaks located at 706.90 and 720 eV, respectively, consistent with the
literature [58,59] and in agreement with reference data [57]. The Fe 2p1/2 peaks are closely
located at the same BE of 719 eV as the Ag 3s peak [57].
The measured XPS data of Au 4p3/2, O 1s, and C 1s in Figure 4 provide complementary
information on the cleanliness of the sample surfaces as well as the state of oxidation and
carboniferous (carbonic) contamination [60] as these alluvia samples may contain mine
discharge, dissolution of carbonate minerals at a well-buffered pH, different calcite satu-
ration indices, and dissolved in oxygen [61].
Figure 5 shows the core-level Au 4f XPS spectra of the Au film, Au bulk, and un-
treated concentrate powder of Au. For the Au film and bulk samples, the 4f5/2 peaks are
located at 87.63 eV and the 4f7/2 peaks are located at 83.95 eV, with a 3.6 eV spin-orbit
splitting. These binding energies are similar to the literature values of 87.60 and 84.0 eV
[57] for pure Au.
Figure 5.
Au 4fcore-level XPS spectra of the Au film, Au bulk, fine-grained Au concentrate, and
coarse-grained Au concentrate. The dashed line connects the 4fpeaks as well as the apparent spin-
orbit-splitting maxima. The spectra containing lower Au concentrations were scaled-up by factors of
3 and 30 for comparison.
The spectra from Figure 6shows the Ag 3d
5/2,3/2
core level XPS of the bulk and
untreated Au powder, fine-grained Au, and coarse-grained Au concentrate samples. The
peaks for the Ag 3d
3/2,5/2
levels are located at 374.0 eV and 368.0 eV, respectively, with
respect to the Fermi level (EF) and in reasonable agreement with the literature data [57].
Minerals 2021, 11, x FOR PEER REVIEW 12 of 22
Figure 5. Au 4f core-level XPS spectra of the Au film, Au bulk, fine-grained Au concentrate, and
coarse-grained Au concentrate. The dashed line connects the 4f peaks as well as the apparent spin-
orbit-splitting maxima. The spectra containing lower Au concentrations were scaled-up by factors
of 3 and 30 for comparison.
For comparison, in the Au powder sample, the 4f5/2 is located at 87.77 eV and the 4f7/2
peak at 84.09 eV. The Au 4f peaks are asymmetric, while the shape of the untreated Au
powder concentrate, and the coarse-grained Au powder concentrate spectra broadens
with tails toward higher BE. The chemical shift of 0.14 eV towards higher BE for the pow-
der samples suggests that there is a charge transfer from Au towards the other elements.
The high-energy shift can be attributed to the screening effect in comparison to the pure
bulk Au and suggests total charge-transfer from Au towards certain elements.
From the spectra in Figure 5, the Au 4f region has well separated spin-orbit compo-
nents of 3.6 eV, which is close to the Au metal (Δ = 3.7 eV) and is in agreement with the
literature [62]. The Au 4f7/2 peak at 84.0 eV serves as a useful BE reference for the Au metal
[57] and is expected to shift (BE shift) to Au nanoparticle sizes for smaller cluster samples.
The spectra from Figure 6 shows the Ag 3d5/2,3/2 core level XPS of the bulk and un-
treated Au powder, fine-grained Au, and coarse-grained Au concentrate samples. The
peaks for the Ag 3d3/2,5/2 levels are located at 374.0 eV and 368.0 eV, respectively, with re-
spect to the Fermi level (EF) and in reasonable agreement with the literature data [57].
Figure 6. Ag 3d5/2,3/2 core level XPS spectra of the bulk Au and powder (unprocessed and processed)
Au samples. The dashed line connects the peaks as well as the apparent spin-orbit-splitting maxima.
The Ag 3d5/2,3/2 spectra in Figure 6 show well-separated spin-orbit components (Δmetal
= 6.0 eV); the peaks display asymmetric peak shapes for the Ag metal and also show loss
features in the higher BE side of each spin-orbit component of the Ag metal [63]. The bulk
Au and fine-grained Au powder concentrate, show higher intense peaks due to the high
number of Ag atoms present in the samples, interacting with unfilled electron levels of
the valence band above EF. The lower intense peaks and the loss of features for the coarse-
Figure 6.
Ag 3d
5/2,3/2
core level XPS spectra of the bulk Au and powder (unprocessed and processed)
Au samples. The dashed line connects the peaks as well as the apparent spin-orbit-splitting maxima.
Minerals 2021,11, 912 13 of 22
The Ag 3d
5/2,3/2
spectra in Figure 6show well-separated spin-orbit components
(metal = 6.0 eV)
; the peaks display asymmetric peak shapes for the Ag metal and also
show loss features in the higher BE side of each spin-orbit component of the Ag metal [
63
].
The bulk Au and fine-grained Au powder concentrate, show higher intense peaks due to
the high number of Ag atoms present in the samples, interacting with unfilled electron
levels of the valence band above E
F
. The lower intense peaks and the loss of features for the
coarse-grained Au concentrate sample suggest a negligible number of Ag atoms present in
the material. The smaller BE shifts observed in the spectra suggest the presence of oxides
in the materials causing the Ag 3d
5/2,3/2
peaks to broaden with respect to metal peaks as
a result of a difference in electronegativity (lattice potential, work function changes, and
extra-atomic relaxation energy), which then causes a charge transfer from the metal atoms
to the ligands [6466].
The reason that the Au and Ag alloy forms an “electrum” in the bulk and powder
samples is due to their partly filled d bands (d-hole count effect) and having the same fcc
structure makes it possible to exhibit relativistic effects where both bands display a correct
spin-orbit-splitting signature. The structural changes that occur during the formation of
the electrum affect the d-hole count in both Ag and Au and increase with decreasing Au
concentration, which provides a strong correlation with the ostensible Au 5d spin-orbit
splitting, lower d-band energy, and the Au 4f7/2 BE shift [67].
From the XPS data, the Fe 2p in the sample corresponds to pure Fe metal, with the
2p
3/2
peak at 710.9 eV and the spin-orbit splitting at 13.6 eV. These values are close to
those obtained from the coarse-grained Au concentrate sample, with 2p
3/2
= 706.9 eV and
spin-orbit components (
metal
= 13.1 eV). The Fe 2p peaks in Figure 7show an asymmetric
shape for metal, but the Fe 2p
3/2
spectrum is not well-resolved to exhibit multiplet splitting.
A high BE shift due to screening in comparison to pure metal shows that there is charge
transfer from the metal atoms to the ligands. If the Fe in the samples originated from
the oxide zone (FeO and Fe
2
O
3
, and Fe
3
O
4
), the peaks would have significantly shifted
towards higher BE, but the actual shift towards lower BE (~710.9 eV) suggests that the
peaks originated from pure Fe metal or pyrite (FeS
2
), as opined by Biesinger et al. [
68
]. Iron
compounds may possess high or low spin, with Fe (III) compounds exhibiting high-spin,
which leads to a complex multiplet-split [
68
]. Fe (II) compounds, however, have both
high and low spin, with spectra from the low spin lacking multiplet splitting and are
attributable to the marcasite (FeS
2
) or pyrite group of compounds [
68
]. According to Uhlig
et al. (2001), for air-exposed FeS
2
, the Fe
2+
is in a low-spin configuration and the broad
structure towards higher BE (710.5 eV) of the main Fe 2p
3/2
peak is due to the oxidation of
the Fe
3+
states of the surface [
69
]. For the smaller marcasite peak, the higher BE (708.4 eV)
is due to the electronic-deficiency of Fe
2+
sites, which are formed by the breaking of Fe–S
bonds [
69
]. This higher BE (708.4 eV) and the lower BE for marcasite FeS
2
is in close
agreement with the peak at (706.9 eV) from the XPS spectra in Figure 7, which is consistent
with reference data [
70
] for FeS
2
. It can be deduced that the deficient main Fe 2p
3/2
peak in
the samples from the Kubi Gold project contains pure metal Fe and FeS
2
, since FeS
2
is one
of the main indicator minerals in the mining area [
56
]. As telluric iron is extremely rare, we
assume that the pure metallic Fe observed by our XPS measurements originates from iron
oxides that are readily reduced in the argon monomer sputter cleaning process.
Minerals 2021,11, 912 14 of 22
f
Figure 7.
Fe 2p core-level XPS spectra of the fine-grained Au concentrate and coarse-grained Au
concentrate samples. The broken line connects the peaks as well as the apparent spin-orbit splitting
maxima.
The assigned peak values of Ti in Table 1are in good agreement with reference data [
62
]
and consistent with the literature [
71
74
]. The abundant titanium in the powder sample is
believed to be contained in fine-grained aggregates of hematite [
75
77
], which is a gangue
mineral in this mining area [
56
]. The silicate garnet minerals that control Au mineralization
in Kubi contain some percentage of TiO
2
[
59
]; on the other hand, other occasionally
encountered silicate minerals such as biotite, hornblende, and the abundant silica (quartz),
as well as some oxide groups in this area containing some TiO
2
are responsible for the
significant Ti in the untreated Au sample. The assigned peak values of Si in Table 1are
in close agreement with reference data [
62
] and consistent with the literature [
78
,
79
]; it is
also believed to be contained in the SiO
2
, which is another indicator mineral in the mining
area. Finally, the assigned peak positions of Hg, a trace element in the untreated powder
sample that occurs at binding energies of 104 eV and 577 eV, overlap with Si 2p and Ag
3p peaks. These values are in agreement with reference data [
62
] and consistent with the
literature [80,81].
As shown in Table 2, quantitative XPS analysis shows that the Ag content in the Au
bulk nugget is about 6 at.%, while the C, O, and Fe contents are 3 at.%, 2 at.%, and 1 at.%,
respectively. In the fine-grained Au powder concentrate sample, the Ag content is at.% half
of that of the bulk sample, while the levels of C and O are significantly higher.
Table 2.
Results of quantitative analysis of XPS core levels for bulk Au, fine-grained, and coarse-grained Au concentrate
samples.
System Au4f(at.%) Ag3d (at.%) C1s (at.%) O1s (at.%) Fe2p (at.%) Mn (at.%)
Au bulk 88 6 3 2 1 -
Fine-grained Au 32 4 7 32 24 3
Coarse-grained Au 6 1 27 34 26 6
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3.2. Chemical Bonding
Figure 8shows valence band XPS spectra of an Au film, Au bulk, unprocessed ore
powder, and processed (coarse and fine grained) ore powder of Au in comparison to
calculated Au spectra. The agreement with the calculated peak positions in the valence
band is improved when experimental lattice parameters (a= 4.078 Å) are used, while the
theoretical (relaxed) lattice parameter of 4.156 Å is larger and results in peak positions that
have extremely low BE.
Minerals 2021, 11, x FOR PEER REVIEW 15 of 22
3.2. Chemical Bonding
Figure 8 shows valence band XPS spectra of an Au film, Au bulk, unprocessed ore
powder, and processed (coarse and fine grained) ore powder of Au in comparison to cal-
culated Au spectra. The agreement with the calculated peak positions in the valence band
is improved when experimental lattice parameters (a = 4.078 ) are used, while the theo-
retical (relaxed) lattice parameter of 4.156 is larger and results in peak positions that
have extremely low BE.
Figure 8. Experimental and calculated valence band spectra of film, bulk, fine-grained, and coarse-
grained powder. The broken line connects the d-band edges and the apparent spin-orbit splitting
maxima.
The valence band spectra in Figure 8 show the coordination complex Au (transition
metal) with an octahedral geometry where the d orbitals split into a t2g set (lower energy)
and an eg set (higher energy) [82,83]. From the spectra, the eg set moves towards higher BE
due to the high involvement of the d orbitals in the eg set in a metal–ligand sigma (σ) in-
teraction. These orbitals in the eg are directed towards the axis where they are opposed by
strong repulsive forces from ligands; hence, they acquire higher energy than the t2g, thus
favoring covalent bonding in eg towards higher energy [83]. The shift in BE along the eg set
for the powder sample (coarse-grained) is an indication of the covalent bonding of Au
with the oxides (ligands) in the powder samples and these extend beyond the eg to higher
BE. Au has a face-centered cubic (fcc) crystal structure, and its bonding is a mixture of
covalent, ionic, and metallic bonds. Due to symmetry considerations, the covalent contri-
bution consists of Au eg-ligand 2p (pd-σ), Au t2g-ligand 2p (pd-), and Au-Au t2g (dd-sigma)
bond regions that can be observed in the valence band.
The lower energy t2g orbitals do not favor sigma interactions with ligands. The t2g
symmetry favors the participation of interactions with ligands due to their weaker in-
teractions in metal complexes [83]. This means that the t2g orbitals are completely metal
based and contribute to metallic bonding. From the valence band spectra, the pronounced
Figure 8.
Experimental and calculated valence band spectra of film, bulk, fine-grained, and coarse-
grained powder. The broken line connects the d-band edges and the apparent spin-orbit splitting
maxima.
The valence band spectra in Figure 8show the coordination complex Au (transition
metal) with an octahedral geometry where the dorbitals split into a t
2g
set (lower energy)
and an e
g
set (higher energy) [
82
,
83
]. From the spectra, the e
g
set moves towards higher
BE due to the high involvement of the dorbitals in the e
g
set in a metal–ligand sigma (
σ
)
interaction. These orbitals in the e
g
are directed towards the axis where they are opposed by
strong repulsive forces from ligands; hence, they acquire higher energy than the t
2g
, thus
favoring covalent bonding in e
g
towards higher energy [
83
]. The shift in BE along the e
g
set
for the powder sample (coarse-grained) is an indication of the covalent bonding of Au with
the oxides (ligands) in the powder samples and these extend beyond the e
g
to higher BE.
Au has a face-centered cubic (fcc) crystal structure, and its bonding is a mixture of covalent,
ionic, and metallic bonds. Due to symmetry considerations, the covalent contribution
consists of Au e
g
-ligand 2p (pd-
σ
), Au t
2g
-ligand 2p (pd-
π
), and Au-Au t
2g
(dd-sigma) bond
regions that can be observed in the valence band.
The lower energy t
2g
orbitals do not favor sigma interactions with ligands. The t
2g
symmetry favors the participation of
π
interactions with ligands due to their weaker
interactions in metal complexes [
83
]. This means that the t
2g
orbitals are completely metal
based and contribute to metallic bonding. From the valence band spectra, the pronounced
Minerals 2021,11, 912 16 of 22
peaks at t
2g
for Au film, bulk, and fine-grained powder indicate metallic bonding with
other metals, while the coarse-grained powder show weaker peaks for metallic bonding
due to the high level of oxidizing materials.
The spectra results in Figure 8for the Au film, bulk, fine, and coarse-grained indicate
that the BEq of the Au 5d
3/2
component of the valence band remains almost unaffected,
while the Au 5d
5/2
shifted away from the E
F
for a decreased Au sample size, and the
top of the d-band is observed to move away from the E
F
. Thus, the Au 5d
5/2
of fine and
coarse-grained samples due to the small grain size shifted away from the E
F
as well as the
top of the d-band [44,84,85].
The spectra show that for smaller Au powder samples (fine and coarse-grained
concentrates), there is an increase in the BE side of the asymmetry component, while the
Auger yield tends to decrease [
55
]. The spectra of the powder samples show disturbance
peaks towards the Auger yield that are attributed to the state of oxidation the samples are
subjected to. This shift towards higher BE of the Au 4f
7/2
core-level for smaller cluster size
(powder samples) is related to valence band narrowing, ascribes a general shift, and is
interpreted to be dependent on the initial electronic state structure and the final relaxation
state processes as a result of photoemission [84,86,87].
For the Au (film, bulk, fine, and coarse-grained) samples, the valence band width
and the binding energies of the two Au 5d and 4f
7/2
peaks depend solely on the mean
coordination number [
44
,
83
,
84
]. The 4fphotoemission from Au split into two distinct
peaks, namely Au 4f
7/2
and Au 4f
5/2
with different BE as a result of spin-orbit coupling
effects corresponding to final states, with angular momentum of j+ = + 1/2 = 7/2 and
j
= + 1/2 = 5/2, respectively [
88
,
89
]. It is seen from the Au 4fspectra that the Au 4f
7/2
-to-4f
5/2
peak ratios are not the same but vary between the four different samples. The
useful information one can obtain from the study of the BE shift of Au 4f
7/2
, full-width
at half-maximum (FWHM) of the Au 4f
7/2
peak and valence band width, is the spectra
response to average sample size in terms of peak broadening and shift towards higher BE,
as seen in the Au bulk and powder samples [9092].
3.3. EDX Results
Examination under the EDX of the Au powder samples (fine and coarse-grained) show
that more than 50 at.% of the Au is present in the grains and these are associated with the
pathfinder minerals garnet, hematite, and quartz. The EDX results in Figure 9and
Table 3
also reveal other elements such as Al and Na whose peaks are not easily identified by
the surface-sensitive XPS measurements since light elements (N, O, C, and surface oxides
including Fe
2
O
3
, Fe
3
O
4
, and SiO
2
) are removed by the differential sputtering of Ar
+
ions,
while bulk-sensitive EDX does not require any surface cleaning.
Table 3. Elements identified in the EDX analysis of bulk Au and Au powder (fine-grained and coarse-grained) concentrate samples.
Sample Bulk Au (at%) Powder Fine-Grained (at.%) Powder Coarse-Grained (at.%)
Element
C 11.7 9.50 23.3
O 7.44 14.4 22.1
Si 1.17 0.58 2.55
Fe 1.82 4.39 3.40
Ag 6.39 6.40 4.58
Au 71.2 64.2 42.6
Al 0.38 0.57 -
Hg - - 1.55
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Figure 9. EDX spectra of bulk Au, fine-grained, and coarse-grained Au powder concentrate samples.
Table 3. Elements identified in the EDX analysis of bulk Au and Au powder (fine-grained and coarse-grained) concentrate
samples.
Sample Bulk Au (at%) Powder Fine-Grained (at.%) Powder Coarse-Grained (at.%)
Element
C 11.7 9.50 23.3
O 7.44 14.4 22.1
Si 1.17 0.58 2.55
Fe 1.82 4.39 3.40
Ag 6.39 6.40 4.58
Au 71.2 64.2 42.6
Al 0.38 0.57 -
Hg - - 1.55
The Fe in these alluvia Au samples is believed to be contributions from the presence
of pyrite, pyrrhotite, chalcopyrite, and the “gangue mineral” magnetite [56]. According to
Moslemia et al. [93] and Corkhill et al. [94], the oxidation of minerals such as Fe3O4 and
CuFeS2 (pyrite group of minerals) under conditions such as pressure, temperature, pH,
particles size, concentration, and electrochemical mechanisms results in the release of fer-
rous ions and other constituents [93,94]. Hence, it can be inferred that the Fe in these sam-
ples is from pure elemental Fe and FeS2. In our analysis, we have prepared the samples
(fine and coarse-grained) from the untreated powder so that almost all sulfur-containing
grains were discarded. This is due to the fact that the sulfide minerals are washed away
Figure 9.
EDX spectra of bulk Au, fine-grained, and coarse-grained Au powder concentrate samples.
The Fe in these alluvia Au samples is believed to be contributions from the presence
of pyrite, pyrrhotite, chalcopyrite, and the “gangue mineral” magnetite [
56
]. According
to Moslemia et al. [
93
] and Corkhill et al. [
94
], the oxidation of minerals such as Fe
3
O
4
and CuFeS
2
(pyrite group of minerals) under conditions such as pressure, temperature,
pH, particles size, concentration, and electrochemical mechanisms results in the release of
ferrous ions and other constituents [
93
,
94
]. Hence, it can be inferred that the Fe in these
samples is from pure elemental Fe and FeS
2
. In our analysis, we have prepared the samples
(fine and coarse-grained) from the untreated powder so that almost all sulfur-containing
grains were discarded. This is due to the fact that the sulfide minerals are washed away
during panning and washing since they are much lighter than the Fe and Au metals that
settle at the bottom of the pan.
In contrast to previous observations by XRD [
56
] where the crystal structures were
identified, we find that there are significant changes in the chemical bonds in the investi-
gated materials. The XPS analysis confirms that there is covalent bonding between the Au
atoms and ligands in particular for the coarse-grained, while the Au bulk and fine-grained
powder indicate metallic bonding with other metals, as shown in the valence band spectra
in Figure 8. The ratio between metallic and covalent bonding depends on the level of
oxidizing materials present in the vicinity of the Au atoms. These oxidizing materials from
the near surface (oxide zones) consist of the gangue minerals Fe
2
O
3
, Fe
3
O
4
, SiO
2
, garnet,
and others from the silicate group of minerals that, under favorable conditions, undergo
oxidation and reduction processes to release metastable thiosulphate ions (S
2
O
32
) and
bisulphide ions (HS
) to trigger the dissolution of Au, Ag, and other metals within the
hydrothermal systems [52,54].
Minerals 2021,11, 912 18 of 22
These identified elemental species Si, Ag, Fe, Al, N, O, Hg, and Ti can act as pathfinder
elements in the host minerals of the Kubi Au ore and can lead to exploration success in
this mining area. Despite similarities in geology, mineralogy, and the structural setting
of the Kubi Gold project with that of Northern belts of Ghana, there exist differences in
pathfinder elements due to weathering, erosion, chemical, and landscape processes leading
to differences in regolith (loose materials covering solid rock material) [
46
,
47
]. In the Kubi
Gold area, the association of Ti and Fe with Au is unique and differs from what exists in
northern Ghana. The saprolitic and ferruginous clay that have been formed by weathering
profile are dominated by garnet (that contains TiO
2
), Fe-oxides, and iron sulfide minerals
through secondary processes and are capable of hosting Au mineralization.
The most interesting part of this study is the scope of economic benefits to be consid-
ered from the 1.90 kg artisanal small-scale sediments (soil samples or mining wastes), which
produced 1.0 g of Au bulk nugget (22 carats measured with a digital electronic Au purity
Analyzer DH 300 K from VTSYIQ), 0.85 g of coarse-grained, and 1.20 g of fine-grained
concentrate samples. These samples are found to contain a high percentage of Au as well
as metallic and semi-metallic elements such as Ag, Fe, Al, and Ti that are important mineral
commodities of economic value.
These new findings from the near surface sediments call for further investigations
into the ore-body of deep drilled holes where materials have been subjected to high
temperatures and pressures. We anticipate that there will be depth-dependent composition
with a lot of deformations and defects due to hydrothermal and physio-chemical processes,
among others, in the ore body.
4. Conclusions
By combining core-level and valence band X-ray photoelectron spectroscopy with
EDX electronic structure calculations, we investigated the electronic structure and chemical
bonding in various Au powder samples that remain after Au panning, in comparison to
bulk Au metal and Au film. For the impure Au powder (fine-grained and coarse-grained
concentrate) samples, there is a chemical shift of 0.14 eV for the Au 4f
7/2
and 4f
5/2
peak
positions towards higher energies as compared to the metal values of 83.95 and 87.60 eV
due to a screening effect and charge transfer, indicating the bonding of Au atoms with the
surrounding ligands in the indicator minerals.
This study demonstrates that indicator and pathfinder methods have a broader ap-
plication, which includes the Kubi Gold project, a fact that is not commonly known for
artisanal small-scale mining wastes (alluvia small-scale mining sampling). The indicator
minerals in this alluvia small-scale mining are part of a large group of indicator minerals
that can be used to explore a broad range of Au deposits and other commodities in this
mining area.
From the study, we identified that Si, Ag, Fe, Al, N, O, and Ti act as pathfinder elements
and can be used to infer indicator minerals such as quartz, hematite, pyrite (marcasite),
garnet, and other occasional silicate minerals such as biotite and hornblende.
The results also demonstrate that the mobility of Au and pathfinder elements are
very high within the oxide zones (near surface) and these elements bond to the indicator
minerals hosting the Au in the mining site.
Quantitatively, we found that the fine-grained Au concentrate has a relatively high
amount of pure Au that forms electrum with a significant amount of Ag; this finding makes
mining waste an economically viable option to be considered. The significant percentage
of oxygen in the powder samples are due to contaminations from the fresh alluvia samples,
while the C is attributed to the carbonic or graphitic alterations in the Kubi mining site.
Finally, we conclude that fine-grained sediments considered to be “mining waste”
could be of economic importance to miners and investors, and are important materials for
exploration. Mining geologists may also apply XPS, EDX, geochemistry analysis, and other
techniques in order to elucidate the indicator elements associated with Au host minerals.
Minerals 2021,11, 912 19 of 22
Author Contributions:
Individual contributions are as follows: Conceptualization, G.K.N. and M.M.;
methodology, G.K.N.; software, B.B. and M.M.; validation, M.M., H.H., and L.H.; formal analysis,
G.K.N.; investigation, G.K.N.; resources, G.K.N. and M.M.; data curation, B.B. and M.M.; writing—
original draft preparation, G.K.N.; writing—review and editing, G.K.N., B.B., M.M., H.H., and L.H.;
visualization, H.H.; supervision, M.M.; project administration, M.M.; funding acquisition, M.M.,
H.H., and L.H. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Data Availability Statement: Data available on request due to privacy/ethical restrictions.
Acknowledgments:
We acknowledge support from the Swedish Government Strategic Research
Area in Materials Science on Advanced Functional Materials at Linköping University (Faculty Grant
SFO-Mat-LiU No. 2009 00971). The computations were enabled by resources provided by the Swedish
National Infrastructure for Computing (SNIC) at the National Supercomputer Centre (NSC), partially
funded by the Swedish Research Council through Grant Agreement No. 2016-07213. M.M. also
acknowledges financial support from the Swedish Energy Research (Grant No. 43606-1) and the Carl
Tryggers Foundation (CTS20:272). Asante Gold Corporation is acknowledged for funding G.K.N.’s
industrial PhD studies at Linköping University, Sweden.
Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or
in the decision to publish the results.
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