Microchemical signature of alluvial gold from two contrasting terrains in Cameroon (original) (raw)
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Microchemical characterization of placer gold grains from the Meyos- Essabikoula area, Ntem complex, southern Cameroon, 2019
Gold occurs as a native metal, usually containing silver, and in some cases mercury, copper, and palladium. It may also occur as inclusions within sulfur-rich minerals, such as pyrite and arsenopyrite. The style and variety of gold mineralization is influenced by the geological setting, chemistry of the ore fluids, and the nature of their interactions with rocks. Gold grains liberated from bedrock into surficial sediments during weathering and erosion are chemically stable and may be characterized according to their mineralogy: i.e the alloy composition and suite of mineral inclusions revealed within polished sections, characteristics faithful to gold from the hypogene source. This approach has been applied to placer gold grains from the Meyos-Essabikoula area, Cameroon, where the source of gold is not yet confirmed due to poor outcrop exposure. A total of 221 alluvial gold grains from 10 sites, tributaries of Sing and Bivele River over the Ntem Complex have been studied using Electron Probe Micro-Analysis (EMPA) to determine the concentration of minor alloying metals, (notably Au, Ag, Cu, and Hg) and Scanning Electron Microscopy (SEM) in order to evaluate the assemblage of mineral inclusions within the gold. Most of the grains are sub-rounded with pitted surfaces and inclusions of pyrrhotite, acanthite, and chalcopyrite were observed. The grains are AueAg alloys ranging from 54.4 to 99.8 wt% Au, 0.1-48.4 wt% Ag, 0.1-0.8 wt% Hg and 0-0.3 wt% Cu. The presence of Fe oxide (magnetite) inclusions containing Cr and V (to around 5 wt %) has not been reported elsewhere and suggests a strong interaction between hot reducing ore fluids and local mafic lithologies.
OGR, 2021
The Bétaré Oya district has a substrate composed of the Neoproterozoic metavolcanic-metasedimentary rocks of the Lom Belt in eastern Cameroon. The district is well-known for alluvial gold mining activities, however, the primary gold mineralization has received little attention. In the current study, we newly report geological, ore mineralogy, fluid inclusion microthermometric, laser Raman spectroscopy, and stable isotope data from auriferous veins of the Lom Belt to characterize the style of gold mineralization and to constrain the origin of the oreforming fluids. The auriferous quartz veins are laminated, fractured, N-to NE-trending, and spatially associated with the Bétaré Oya Shear Zone. Fieldwork coupled with microscopic examination and the textural relationships of ore minerals revealed two stages of mineralization. The first stage is characterized by the presence of pyrite, sphalerite, galena, chalcopyrite, pyrrhotite, hematite, petzite, hessite, wolframite, electrum, and gold, while the second stage is characterized by the presence of a later deposition of galena and pyrite, as well as minor greenockite. The gangue minerals are quartz, sericite, muscovite, chlorite, calcite, ankerite, and barite, whereas the supergene assemblage (stage 3) includes goethite, hematite, covellite, and enargite. Two fluid inclusion assemblages containing three types of inclusions are identified in mineralized quartz veins. Assemblage 1 consists of gold-related types 1 and 2 with daughter minerals consisting of nahcolite, magnesite, and arsenolamprite (black native arsenic), whereas assemblage 2 consists of post-gold type 3 fluid inclusions. Type 1 has H 2 O-NaCl-CO 2 ± N 2 ± CH 4 primary fluid inclusions (FI), with a Tm CO2 ranging from − 59.8 to − 56.6 • C, salinities from 0.5 to 10.8 wt% NaCl eq., densities from 0.87 to 1.00 g.cm − 3 , and total homogenization temperatures between 280 and 360 • C. Type 2 contains CO 2 (±H 2 O-NaCl) ± N 2 ± CH 4 FI that exhibit Tm CO2 ranging between − 60.0 and − 56.7 • C, Th CO2 from 13 to 25 • C, and densities between 0.73 and 0.85 g.cm − 3. Type 3 shows H 2 O-NaCl FI with salinities between 0.2 and 10.1 wt% NaCl eq., densities between 0.82 and 0.98 g.cm − 3 , and total homogenization temperatures from 160 to 235 • C. Measured δ 18 O for gold-bearing quartz (+11.5 to +16.0‰), δD from FI (− 50.6 to − 21.8‰), δ 13 C from FI (− 5.8 to − 5.5‰), and δ 34 S from galena and pyrite grains (+5.3‰ and +8.2‰, respectively) suggest a metamorphic source as most likely for the ore-forming fluids and sulfur, although a mantle CO 2 contribution cannot be ruled out. The gold deposition probably took place by fluid-rock interaction and fluid unmixing at ~310 • C and at a depth of about 6-9 km. The ore-forming fluid was a low salinity (~6.2 wt% NaCl eq.) H 2 O-NaCl-CO 2 ± N 2 ± CH 4 solution. These data revealed that the Lom Belt gold mineralization is best classified as a mesozonal orogenic gold deposit.
Morphology and composition of gold grains in a lateritic profile
This study describes the morphological evolution of gold grains in a lateritic weathering profile in an equatorial rainforest climate. Primary sources of gold are quartz veins associated with shallow granophyric intrusion. Gold grains were found in fresh ore, saprolite, transition zones, ferruginous duricrust, red latosol, and yellow latosol. Irregularly shaped grains predominate, with smaller proportions of dendritic and prismatic forms. Gold grains are weathered in the uppermost 10 m of the regolith. Mean gold grain size is maximum in the duricrust (>125 lm) and decreases progressively upward into the yellow latosol (<90 lm). Voids and corrosion pits appear on grain surfaces, and progressive rounding is observed from the bottom of the profile to the top. Gold grains can be classified as either homogeneous or zoned with respect to their chemical composition. Homogeneous grains contain 2-15% Ag (mean 8.3%). Zoned grains have more variable Ag contents; grain cores have means of approximately 10% or 23% Ag, with Ag-poor zones of approximately 3.7% Ag along internal discontinuities and/or outer rims. Formation of Ag-poor rims is due to preferential depletion of silver. Processes responsible for duricrust formation may preserve some grains as large aggregates, but subsequent transformation into latosol further modifies them.
Base metal-enriched gold-quartz veins in the eastern Cameroon goldfields, West-Central Africa
Episodes, 2021
The Colomine and Bétaré Oya gold districts are among the main gold-producing districts in the reworked Paleoproterozoic terrain of eastern Cameroon. The characteristics of the mineralized and barren veins from these areas were investigated with the aim of helping artisanal gold miners and Small and Medium Size Enterprises (SME) active in the region to identify new gold targets. This could help boost the country's annual gold production and also generate local revenue for the indigenous community. Gold mineralization occurs in gently dipping NNE-SSW to NE-SW-trending quartz veins hosted by hydrothermally altered granitic rocks in the Colomine gold district and weathered mica schist in the Bétaré Oya gold district. The hydrothermally altered granite wallrock hosts millimetric-sized quartz lamellae/veinlets characterised by a quartz-sericite-muscovite-biotite±chlorite±albite assemblage. The mineralized quartz veins in both districts are fractured, brecciated, sheared, and vuggy, characterised by a quartz±gold±sulphide±hematite±limonite±goethite ±carbonate assemblage. Hematite occurs in a botryoidal form, ind icating the strong influence of supergene processes. This alteration phase is associated with supergene gold enrichment in the veins and constitutes the most attractive part to be exploited by artisanal and SMEs. The barren veins trend NW-SW to N-S and are generally massive to foliated. Gold ranges between 5 and 32 ppb in the wall-rock and up to 2070 and 4600 ppb in hematite-bearing quartz veins. Bulk geochemistry reveals Au-Mo (±Ag) element association in Colomine and Au-Ag-Mo-Cu in Bétaré Oya indicative of a sulfide-bearing granitic source.
Geological Journal, 2017
Lode-gold mineralization in the Bétaré Oya gold district is related to a series of quartz-sulphide veins that define a steeply dipping NNE-SSW-trending brittle-ductile shear zone. The veins transect metasedimentary sequences in the vicinity of small granitic intrusions. Quartz veins range from barren (massive quartz) to mineralized with textures that vary from foliated, sheared, vuggy, fractured, and sugary to brecciated with inclusions of altered wall rock. They contain gold, pyrite, and galena in addition to chalcopyrite and both hypogene and supergene haematite and covellite. Parameters that consistently define primary gold mineralization in the area include silicification, sulphidation, sericitization, K-feldspar alteration, haematitization, and carbonatization. Gold grains recovered from heavy mineral concentrate and grains that occur as inclusions in pyrite were characterized by microchemical methods. The gold grains are zoned and exhibit a simple internal structure of partly deformed and leached rims and high-purity intragranular veinlets. Gold is alloyed with Ag; gold fineness varies between 859 and 877 in the core and from 958 to 997 in the leached rims. Inclusions of gold in pyrite cluster between 875 and 900. Pyrite has up to 0.59 wt% Au, and other trace elements occur in low quantities but for Pb (0.13 wt%), Ag (1.36 wt%), and Sb (1.63 wt%). The bulk trace element signature of the veins is charac
Heliyon, 2021
The morphology and quantitative chemical analyses of fifty alluvial gold grains from fourteen studied sites were used to constraint gold deposit type and its implications to exploration in the Abiete-Toko Gold District in South Cameroon. The main results revealed that the gold grains show a core-rim zonation marked by Ag depleted rims as a result of leaching during transportation. The fineness of grains ranges from 826 to 1000 and their composition is almost binary gold-silver. Gold fineness refers to the relative amounts of Ag and Au present, given as a number out of 1000 and defined by Au/(Ag þ Au)*1000. The chemical composition of the gold grains are range as thus: 83.40-100 wt.% Au, 0.07-17.45 wt.% Ag, 0-0.96 wt.% Cu, 0-0.01 wt.% As, 0-0.02 wt.% Ni, 0-0.02 wt.% Co, 0-0.01 wt.% Se, 0-0.08 wt.% Hg and 0.003-0.03 wt.% S. The high sulphur concentrations of the gold grains probably imply primary deposition of gold by sulphidation; The ranges of Ni and Co concentrations suggest an interaction with greenstones. Such findings indicate that the Abiete-Toko area hosts a mesothermal-orogenic gold deposit. Exploration operations must target fractures and contact zones between ironstones, felsic gneiss and greenstones. Soil sampling is not recommended in this orogenic gold district due to the very thick and transported nature of the weathering mantle. Detailed mapping coupled to geophysical surveys are the recommended key exploration methods prior to drilling program.
Journal of Geochemical Exploration, 2006
The style of gold mineralization (e.g., mesothermal, porphyry, intrusion related, high-sulphidation epithermal, low sulphidation epithermal) is normally established at an early stage in the gold exploration process from the study of the mineralogical and lithological examination of outcrop, but this approach is problematic in areas of poor exposure. However, placer gold is frequently present in local drainages, and may be characterized according to the alloy composition and the suite of mineral inclusions. In the temperate areas which are the subject of this study, this microchemical signature of the cores of placer gold grains reflects that of gold from the hypogene source. Thus, informed speculation on the nature of the source bedrock mineralization is possible through comparison of the microchemical signature of placer grains with the generic characteristics of gold from different styles of mineralization.
2019
The Bigorne gold deposit (northern Portugal) located in the European Variscan belt is a system of goldbearing quartz veins, which crosscut late-to post-D3 biotite granites. The mineralized structures correspond to sheet-veins system parallel to the late-Variscan strike-slip fault Penacova-Régua-Verín (NNE-SSW). In Bigorne veins, it was possible to define two types of gold occurrence: I Native gold, as free particles; IIAuBi minerals in association with native-Bi and Bi-Te minerals. The gold minerals from mineralized veins and the gold grains sampled in superficial environments reveal similar chemical composition where the Au occurs in association with Ag, Cu, and Cd. This fact provides evidence of very limited transport and therefore a restricted and single local source for gold primary particles from the mineralized veins. It is very important to improve the geological and mineralogical knowledge about gold occurrences in Portugal to improve the knowledge of the Phanerozoic gold sy...
Open Journal of geology, 2023
The Elogo region has been subjected to craft industry and semi-industrial mining for gold-bearing placers, since the colonial times. It is actually undergoing an intensive exploration for a primary gold deposit. The goal of this study is to contribute to the exploration of the primary gold deposit in the Elogo Region (North-West of Congo Republic). The methodology consisted of characterization of placers deposits by their lithology, mineralogy and the gold grains shape, in order to constrain the source of gold with the respect to the local geology. The results obtained show that alluviums are polygenic and yields seventeen mineral species composed of zircon, olivine, magnetite, ilmenite, gold, garnet, rutile, coltan, cassiterite, monazite, apatite, amphibole, tourmaline, pyrite, limonite, chromite, and amphibole. The morphoscopy of gold grain shows single grains and grains with quartz inclusions that suggest their relationship with quartz veins. The gold grains are flattened, sub-flattened, and rounded. The northern region of Elogo characterized by a dominance of coarser quartz-included gold grains indicates a proximal proparte origin (less than 50 m), while the southern region, showing less coarse and more evolved grains with choc marks, appears to be associated with a distal pro-parte origin (more than 300 m). The multivariate statistical analysis shows seven classes of samples corresponding to the mineralogical paragenesis suggesting various sources, consisting of high-grade metamorphic, granitoids, pegmatites, basic and ultrabasic rocks, and BIFs that provided minerals to the placers.