2.1 Regional Geology of Ghana
The southwestern part of Ghana is underlain by the Paleoproterozoic Birimian Supergroup comprising thick sequences of northeasterly-southwesterly trending belts of steeply dipping metavolcanic rocks that alternate with metasedimentary rocks units [10]. The metavolcanic rocks consist of metamorphosed, basic, and intermediate lavas and pyroclastic rocks and the metasedimentary rocks comprise phyllites, schist and greywackes [11]. Both the metavolcanic and the metasedimentary units have been intruded by syn- and post-tectonic granitoids [12]. These granitoids can be grouped into two main types; namely the Basin granitoids, which intrude the metasedimentary basins and the Belt granitoids that are usually restricted to the volcanic and volcano-sedimentary assemblages. The Tarkwaian group which represent erosional deposits of earlier lithologies [11, 12] occupy the troughs within the Birimian in Ghana. All the major gold deposits in Ghana are hosted by Birimian and Tarkwaian rocks and are usually associated with shearing, deformation, and granite intrusions mostly along the margins of metavolcanic and the metasedimentary rocks.
2.2 Description of the chosen field site
The Kubi deposit lies in a rippling and topographically low zone with a weathering profile between 15-20 m thick. The Au mineralization profile, which is dominated by saprolite, grades upwards into the transition and ferruginous clay zone mainly consisting of garnet, goethite, and kaolinite minerals that are heavily altered by hematite and limonite [1]. Samples from diamond drill core show that native Au is associated with the garnet and the sulfide group of minerals, and are disseminated within the conglomerates, quartz felsite, phyllites, and gabbroic rocks. Historically, and even to date, numerous alluvial artisanal small scale mining activities occurred in several localities within the Proterozoic unit of the Ashanti belt, that received the most gold-rush attention along the Offin river.
Table 1: Logging codes assigned to different lithology, mineralogy, alteration, veining types, and structures, relevant to the present study.
|
Lithology
|
|
Mineralogy
|
|
Alteration
|
Code
|
Description
|
Code
|
Description
|
Code
|
Description
|
LAT
|
Laterite
|
PYRT
|
Pyrite
|
OXDZ
|
Oxidized
|
SAP
|
Saprolite
|
PYRH
|
Pyrrhotite
|
HMTT
|
Hematite
|
GWKE
|
Greywacke
|
ASPY
|
Arsenopyrite
|
LMTT
|
Limonite
|
ARG
|
Argillite
|
GOLD
|
Gold
|
CLAY
|
Clay
|
GR
|
Granite
|
HMTT
|
Hematite
|
CHL
|
Chlorite
|
FS
|
Felsite
|
LMTT
|
Limonite
|
CAL
|
Calcite
|
VLCC
|
Volcano clastic
|
CLAY
|
Clay
|
CARB
|
Carbonate
|
PHYL
|
Phyllite
|
CHL
|
Chlorite
|
GRAPH
|
Graphite
|
QTZT
|
Quartzite
|
CAL
|
Calcite
|
|
|
TRANS
|
Transition
|
CARB
|
Carbonate
|
|
Structure
|
QZVN
|
Quartz Veining
|
CHPY
|
Chalcopyrite
|
Code
|
Description
|
PYVN
|
Pyrite Veining
|
GARN
|
Garnet
|
FALT
|
Fault/Fault Zone
|
QZCARB
|
Quartz>Carbonate Veining
|
|
Veining
|
SHZN
|
Shear/ Shear Zone
|
Ga
|
Gabbro
|
Code
|
Description
|
BEDD
|
Bedding
|
|
|
QZVN
|
Quartz Veining
|
MASS
|
Massive
|
|
|
PYVN
|
Pyrite Veining
|
VN
|
Vein
|
2.3 Sample collection and preparation
The samples were collected from a drill hole with identification number KV10-522 (schematically shown in Figure 2) with azimuth of 290° and dip of -60° at three different depths: namely, the saprolite zone (0.0 - 43.5 m), moderately-slightly weathered (59.0 - 78.85 m), and slightly weathered Au rich zone (78.85 - 90 m). The saprolite sample contains quartz fragments within which most are anhedral-euhedral to rounded pebbles and cobbles. A piece of pyrite was taken from a moderately weathered zone, while two other dried samples as collected were crushed into smaller fragments and ground to a mesh size of fine and coarse- grained. The dried samples were not subjected to any form of pre-treatment to chemically remove carbonates, iron oxides, silt, clay, organic matter, and other impurities. Prior to XRD, SEM, and EDX measurements, these samples were tested with acid and the reactions showed weak (in the pyrite sample) to strong (in the quartz and garnet-gabbro samples) carbonate (CaCO3)alterations; hence all results are indicative of the as-received samples.
Figure 2 schematically shows the drill hole profile of which the saprolite extends to a depth of ~43.5 m. From the surface at the top down to ~10 m, lateritic, clay, conglomerates, and silt profiles are found containing debris of roots, stones, and decayed materials. The highly weathered saprolite profile containing gravels, quartz pebbles, and cobbles stretches to about ~43.5 m; hosted by hematite, limonite, and chlorite with occasional alterations of graphitic-phyllite. Moderately weathered greywacke with weak carbonate alterations, Fe stains, and spotty pyrites continues the drill profile to ~59 m. Between ~59 m and ~78.85 m, moderate-slightly weathered greywacke bedding is occasionally found with phyllite and contains pyrite, pyrrhotite, chalcopyrite, and spotty garnet. Slightly weathered to fresh gabbro-rich garnet (garnet zone) containing gold, pyrite, pyrrhotite, chalcopyrite, arsenopyrite, and carbonates continue down to 90 m. From this garnet zone to the end of the drill hole ~126 m, greywacke dominates with quartz veins, pyrite veining, carbonates, and occasional garnet.
2.4 Measurements
Figure 3shows the images of (a) quartz, (b) pyrite, and (c) garnet-gabbro samples from the Kubi mining concession area. XRD analyses were performed on a PANAnalyical X’pert powder diffractometer with a θ-2θ configuration using a Cu-Kα radiation wavelength of 1.5406 Å (45 kV and 40 mA) with a scan step size of 0.0082°, counting time of 19.68 s per step, and a scan range of 10 and 100 degrees in 2θ scans. During the measurement, the samples were constantly rotated on a spinner stage. The XRD data were quantitatively analyzed using Rietveld refinement with the aid of the MAUD [13] software where analytical functions were used for the modeling. The background parameters were refined simultaneously with the lattice parameters, peak shape, crystal structure, microstructure, strain, and texture in the refinement while considering the least squares for the iteration [13, 14].
The samples were also analyzed using a Zeiss Supra 35 VP field emission SEM operated at 20-30 kV. This SEM is equipped with an EDX detector, used for elemental mapping to investigate the elemental distribution of the samples. The samples were prepared by fitting the crystalline solid samples into aluminum stubs with the aid of carbon sticky tabs.