Ore Mineralogy and Mineral Chemistry of the Ashanti Gold Deposit at Obuasi, Ghana

Abstract: The gold deposit at Ashanti occurs in the Proterozoic Birimian formation of Ghana. Two main ore types mined from the deposit are gold-bearing quartz veins, and gold-sulfide disseminations in metasediments and metavolcanics. The main sulfide minerals in the gold-sulfide disseminated ores are arsenopyrite, pyrite and pyrrhotite, and to a very minor extent, sphalerite and tetrahedrite. Carbonate alteration and sericitization are prominent in the metavolcanics and the metasediments, respectively. In the quartz veins, pyrite and arsenopyrite commonly occur in small amounts, but gold mostly occurs in contact with tetrahedrite, chalcopyrite, galena, aurostibite, and sphalerite. Pyrrhotite is absent in the quartz veins.
Microprobe studies indicate that As content of homogeneous arsenopyrite grains ranges from 27. 0 to 31. 7 atm%, and gives mineralization temperatures from 170 to 430°C, although mostly from 300 to 400°C. Chlorite geothermometry using temperature dependence of substitution of Al for Si in the tetrahedral site gives formation temeratures of 330 to 400°C, comparable to the arsenopyrite temperatures. Applying sphalerite–pyrite–pyrrhotite geobarometry to sphalerite with FeS contents from 13. 6 to 12. 5 mol%, the pressure was estimated to be in a range from 5. 9 to 7. 0 kb at the stage of elevated temperatures.
Mineralogical observations, especially absence of pyrrhotite in the quartz veins, together with microprobe data for gold and associated minerals suggest that the fluids having ascended through fissures in the Ashanti deposit were reduced by the reaction with carbonaceous materials in the metasediments during the declining stage of the regional metamorphism.     

Metals in sediments of springs and cave streams as environmental indicators in karst areas

The content of heavy metals and arsenic in sediments of karst streams in southern Missouri was investigated for its potential use as an indicator of pollution. A three-step sequential extraction procedure was utilized for this purpose. The amount of trace elements bound to each extraction phase gives insight of its availability and geochemical dependence. These results were complemented with analyses of correlation and spatial variability. Although sediments collected in this study remained below EPAs critical value guidelines, concentration in the mobile phases and higher normalized Mn values successfully identified sites with concentrations higher than background levels. Correlation among elements was poor in most cases; among the trace metals only Cu and Zn correlated while Pb correlated with Ca, Al, Mn and Fe. Spatial variability analysis confirmed that natural variation among adjacent sediment samples is a common occurrence. The urban spring Ward Branch showed the highest levels of Cr, Zn, Pb and As.     

Partitioning of major and trace inorganic contaminants in fly ash acid mine drainage derived solid residues

Acid mine drainage was reacted with coal fly ash over a 24 h reaction time and species removal trends evaluated. The evolving process water chemistry was modeled by the geochemical code PHREEQC using WATEQ4 database. Mineralogical analysis of the resulting solid residues was done by X-ray diffraction analysis. Selective sequential extraction was used to evaluate the transfer of species from both acid mine drainage and fly ash to less labile mineral phases that precipitated out. The quantity of fly ash, volume of acid mine drainage in the reaction mixture and reaction time dictated whether the final solution at a given contact time will have a dominant acidic or basic character. Inorganic species removal was dependent on the pH regime generated at a specific reaction time. Sulphate concentration was controlled by precipitation of gypsum, barite, celestite and adsorption on iron-oxy-hydroxides at pH > 5.5. Increase of pH in solution with contact time caused the removal of the metal ions mainly by precipitation, co-precipitation and adsorption. PHREEQC predicted precipitation of iron, aluminium, manganese-bearing phases at pH 5.53–9.12. An amorphous fraction was observed to be the most important in retention of the major and minor species at pH > 6.32. The carbonate fraction was observed to be an important retention pathway at pH 4–5 mainly due to initial local pockets of high alkalinity on surfaces of fly ash particles. Boron was observed to have a strong retention in the carbonate fraction.     

Cyanide from gold mining and its effect on groundwater in arid areas,Yanqul mine of Oman

The use of cyanide (CN), which is characterized by volatility, toxicity and high odor, in gold mining is scarcely addressed in the literature and remain controversial. Environmentalists oppose CN usage as it potentially poses serious environmental threats, whereas economic and mining geologists are in favor of its usage for its extracting capacity and economic feasibility. The present study investigates the possible dispersion of CN into groundwater resources caused by a gold mine (ca. 15 years old) located in the arid area of Yanqul, North Oman. The gold is hosted in gossan deposits associated with ophiolitic rocks and sulfide deposits. Sodium cyanide is mixed with 0.5 m³ of water and then added to a tonne of crushed ore rock to extract 6 g of gold mineral. The final residues are dumped in engineered, lined and uncovered tailing dams. Subsequent to rainfall water draining the mine plateau flows along the wadies and percolates into the shallow Quaternary alluvium aquifer. Hence, groundwater samples were collected from 16 piezometers adjacent to and around the mine. The samples were analyzed for CN using the revised phenolphthalin method and they all show CN concentration below the detection limit (5 ppb). The samples were also analyzed for heavy metals to investigate the potential of CN complexation. Most of heavy metals indicated very trace concentration. The absence of CN in groundwater is attributed to volatilization of CN (converted to HCN), lined dam structure, high evapotranspiration rate and deeper water table. This finding is consistent with the historical CN analysis in the groundwater and solid wastes. It can be pointed out that within few years of operation well engineered tailing dams can provide safe structure preventing CN-groundwater pollution in arid areas. Potential threats to the air and soil are not addressed in this article.     

Release of toxic elements from rocks and mine wastes at the Furtei gold mine (Sardinia, Italy)

The Furtei gold mine in Sardinia (Italy) exploits a volcanic-hosted high-sulphidation epithermal deposit. Large amounts of materials derived from exploitation are present in open pits, waste rock dumps and cyanidation tailings impoundment. Mineralized rocks in outcrops and waste dumps contain significant amounts of sulphides (mainly pyrite and enargite). These materials have a high potential for acid drainage generation and release of toxic elements (notably Cu and As, but also Al, Ni, Co and Cd) as pointed out by laboratory leaching tests and in agreement with chemical composition of waters draining the mining area, that show pH as low as 2, up to 180 mg/L Cu, up to 5 mg/L As, and up to 788 mg/L Al. On the other hand, leaching solutions and waters interacting with mineral assemblages of the propylitic alteration zone (mainly composed of chlorite, quartz, and calcite, with relic magmatic plagioclase) show higher pH, and lower metal loads. Leachates from cyanidation tailings show variable pH (between 6.2 and 9.7, depending on sulphide content in tailings); cyanide concentration varies between 110 µg/L and about 3 mg/L, whereas contents of toxic elements in leachates are, with the exception of Hg, within the limits of Italian regulations for non-dangerous industrial wastes. Reclamation plans provide for confinement of tailings within specific repositories. This measure should effectively reduce the environmental impact of these materials. Reclamation plans should also include an adequate management of other high-sulphide wastes.     

Phosphorus,silicon, and some trace contaminants in the Ganges Estuary

Dissolved silica and phosphorous show a highly nonconservative behavior in the Ganges Estuary. In addition, phosphorous also shows seasonal variations, with winter levels higher than the levels in the monsoon. Mineral composition inducates enrichment of coarse quartz grains due to selective removal of finer clays in the estuary, either due to flocculation or due to dredging effects of the Port of Calcutta. Particulate P and Si show strong opposite trends, possibly indicating the presence of P in predominently nondetrial fractions in the sediments. Certain trace contaminants, such as Fe, Mn, Cu, Pb and Zn, show seasonal variability as well as enrichment in suspended particulate material relative to bed sediments. Levels of all these contaminents are still well below those reported for rivers such as the Rhine and other highly man-influenced systems.     

Geochemical and temperature controls on ore mineralization at the Emperor gold mine, Vatukoula, Fiji

Emperor is a large epithermal gold-tellurium deposit which occurs at the margin of the Tavua caldera and is associated with high-level mineralized tuffs, breccias and silicious sinters. The deposit consists of mainly narrow, steep “shear” and shallow dipping “flatmake” mineralized vein structures. Mineralization spanned a period during which different veins and dyke intrusions occurred so that some early structures are offset by later ones. High Au values and high Au/Ag ratios, corresponding to the intersection of shear and flatmake structures, generally correlate with high Ag, Te and Ba values in some structures (Crown Shear) and high Hg in others (166 Flatmake). Other elements commonly anomalous in the high-grade ore zones are As, Mo and Cu. Thallium, Se and Sb are commonly high below ore zones while Hg may be high above (e.g., Crown Shear), in (166 Flatmake) or even below (e.g., Prince of Wales Shear) ore zones.Gold precipitated in a temperature interval of 180–210°C; higher temperatures commonly occur at depth in some flatmakes (166 N, 608) and lower temperatures nearer the surface. In the two steep shears analyzed, higher temperatures correspond to where they are intersected by flatmakes. Temperatures may decrease below such intersections in which case Hg, Sb, etc., commonly increase with depth.Evidence of boiling, characterized by coexisting gas- and liquid-rich fluid inclusions, is minor and spatially sporadic. Quartz and carbonate found in ore containing bonanza concentrations of Au contain few to no gas-rich fluid inclusions. The intensity of wall-rock alteration is unrelated to gold grades.Ore deposition is inferred to have been caused mainly in response to fluid mixing rather than extensive boiling and/or wall-rock alteration, although these processes occurred. The source of the Au, Ag, Te, etc. is inferred to be near a neutral, relatively reduced, bisulphide-rich, ore solution at near 300°C and derived in some way from the shoshonitic volcanics or associated monzonitic intrusions. The second non-ore solution necessary for mixing, is inferred to have been an acid, oxidized solution at 150°C and having a large meteoric component. The result of fluid mixing was ore genesis, and a telluride, minor sulphide and very minor sulphate (barite) and anhydrite ore assemblage.     

金在地壳、岩石和沉积物中的丰度

金在地壳，岩石和沉只物中的丰度一直是地球化学研究和金矿勘查中非常重要的参数。随着20世纪80年代金分析测试技术的突破性进展以及目前积累的大量可靠的金的分析数据，发现引用20世纪70年代前发表的有关金的地壳丰度和金在岩石，沉积物中的平均含量作为衡量标准已不适宜。根据近15年来发表的大量有关金在地壳，岩石和沉积物中含量的文献，结合作者在地壳与岩石研究中积累的金的大量可靠数据，综合给出金的地壳丰度为1.0ng/g，出露出壳丰度为0．8ng/g，金在岩石，火成岩，沉积岩和变质岩中的平均含量分别为0．9，0．6，1．0和1．0ng/g，金在中国水纱沉积物，泛滥平均沉积物，浅海沉积物和土壤中的平均含量分别为1．4，1．6，1．2和2．0ng/g。这种对金在地壳，岩石和沉积物中丰度值的新认识，对地球化学理论研究，勘查地球化学甚至金的矿产勘查工作都有重大意义。     

Stratigraphy, fossils, and age of sediments at the upper pit of the Lost Chicken gold mine: new information on the late Pliocene environment of east central Alaska

The “upper pit” at the Lost Chicken placer gold mine in east central Alaska contains fossils that provide information on the flora and insect fauna of interior Alaska just before the onset of global cooling at 2.5 myr. Fossils come from sediments interbedded with the Lost Chicken tephra (dated at 2.9 ± 0.4 myr—early Late Pliocene) and portray the floodplain and valley of a small creek within a region dominated by a coniferous forest richer in genera and species than the present one. Climate was wetter and less continental, and there was probably little or no permafrost. At least one other Pliocene tephra (the Fortymile tephra) occurs at the site and is also associated with plant and insect fossils. Among these fossils are extinct plants and insects like those found at other Tertiary sites in northern Canada and Alaska. The Lost Chicken sequence is the same age as the Beaufort Formation on Meighen Island, more than 1000 km to the north. Like Lost Chicken, Meighen Island sediments contain fossils representing a diverse boreal environment. This shows that the latitudinal climate gradient during early Late Pliocene time was shallower than at present and the boreal forest had a far greater latitudinal span than now.     

Uranium pollution of South African streams – An overview of the situation in gold mining areas of the Witwatersrand

During more than a century of gold mining in South Africa large amounts of tailings were produced, which now cover vast areas in densely populated regions. These dumps contain elevated levels of uranium and other toxic heavy metals associated with gold in the mined ore. Large-scale extraction of uranium from auriferous ore only took place during the cold war, leaving tailings with high uranium concentrations that were deposited before and after this period. Recent studies found elevated levels of the radioactive heavy metal in groundwater and streams, mainly attributed to the discharge of contaminated water from mines. In this paper the contribution of seepage from slimes dams to the uranium pollution of adjacent streams is analysed. Based on geochemical analyses of samples, field observations and long-term in situ measurements of hydraulic and hydrochemical parameters at selected mining sites across the Witwatersrand goldfields, the extent, mechanisms and dynamics of diffuse stream contamination by tailings seepage is characterised. Temporal and spatial variations of the process and the associated hazard potential are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

新疆哈密红山金矿区样品加工流程实验探讨

新疆哈密地区红山金矿区的样品中，有品位的样品大多含有明金．但含明金的样品又多为难加工的金矿样品，由此带来的测试结果不稳定等问题，严重影响了分析测试质量．笔认为中碎缩分粒度和细碎时间，是影响加工质量的重要因素，而中碎缩分粒度的确定又成为制定合理的样品加工流程的关键．笔采用重砂鉴定和样品加工流程试验相结合的方法，进行综合分析判断后，确定了红山金矿区样品中碎缩分粒度，从而制定了合理的加工流程，保证了加工完毕的样品有较好的代表性和均匀性，进一步保证了测试结果的准确性和重现性．该流程在2003年度红山矿区金矿样品加工中使用，样品双份分析的一次性合格率为98．0％，内检合格率达88．7％，外检合格率达96．8％。达到规范要求．     

Mineralogical and sulfur isotopic characteristics of Archean greenstone belt-hosted gold mineralization at the Tau deposit of the Mupane gold mine,Botswana

The Mupane gold deposit, which is one of the numerous gold occurrences in the Tati Greenstone Belt in the northeastern part of Botswana, consists of four orebodies, namely Tau, Tawana, Kwena, and Tholo deposits. The present research, which focuses on the genesis of the Tau deposit, was based on ore petrography, mineral chemistry of sulfides, and sulfur isotope data. Mineralogical characteristics of the host rocks indicate that banded iron formation at the Tau deposit includes iron oxides (magnetite), carbonates (siderite and ankerite), silicates (chlorite and amphibole), and sulfides (arsenopyrite and pyrrhotite). The deposit features arsenopyrite-rich zones associated with biotite-chlorite veins, which are indicative of the precipitation of arsenopyrite concomitant with potassic alteration. The replacement of magnetite by pyrrhotite in some samples suggests that sulfidation was likely the dominant gold precipitation mechanism because it is considered to have destabilized gold-thiocomplexes in the ore-forming fluids. Based on textural relationships and chemical composition, arsenopyrite is interpreted to reflect two generations. Arsenopyrite 1 is possibly early in origin, sieve textured with abundant inclusions of pyrrhotite. Arsenopyrite 1 was then overgrown by late arsenopyrite 2 with no porous textures and rare inclusions of pyrrhotite. Gold mineralization was initiated by focused fluid flow and sulfidation of the oxide facies banded iron formation, leading to an epigenetic gold mineralization. The mineralogical assemblages, textures, and mineral chemistry data at the Tau gold deposit revealed two-stage gold mineralizations commencing with the deposition of invisible gold in arsenopyrite 1 followed by the later formation of native gold during hydrothermal alteration and post-depositional recrystallization of arsenopyrite 1. Laser ablation inductively coupled plasma mass spectrometric analysis of arsenopyrite from the Tau deposit revealed that the hydrothermal event responsible for the formation of late native gold also affected the distribution of other trace elements within the grains as evidenced by varying trace elements contents in arsenopyrite 1 and arsenopyrite 2. The range of δ³⁴S of gold-bearing assemblages from the Tau deposit is restricted from +1.6 to +3.9‰, which is typical of Archean orogenic gold deposits and indicates that overall reduced hydrothermal conditions prevailed during the gold mineralization process at the Tau deposit. The results from this study suggest that gold mineralization involved multi-processes such as sulfidation, metamorphism, deformation, hydrothermal alteration, and gold remobilization.     

Assessment of the radiological hazards of sand sediments collected from streams and streamlets of the uranium deposit areas in West Khasi Hills District,Meghalaya, India

Primordial radionuclides in sand sediments that are often used as constructing materials are one of the sources of radiation hazard in dwellings. Activity concentrations of the primordial radionuclides of ⁴⁰K, ²²⁶Ra and ²³²Th have been measured in sand sediments collected from streams and streamlets lying within and around the uranium mineralization deposit blocks of Kylleng-Pyndensohiong, Mawthabah Areas of West Khasi Hills District, Meghalaya, India. The technique of gamma-ray spectroscopy using a NaI(Tl) detector with a PC-based multi channel analyser was applied for determination of the activity concentrations. The activity of the sand sediments obtained in this study ranges from 95.3 to 1,088.8 Bq kg⁻¹ for ⁴⁰K; 38.3 to 784.1 Bq kg⁻¹ for ²²⁶Ra and 78.0 to 316.1 Bq kg⁻¹ for ²³²Th. Sand sediments from two sampling locations lying within the mineralization zone show highest concentrations of these radionuclides. The radiological hazards of the sand sediments were calculated using various models given in the literature. The radium equivalent activity was found to be higher than the accepted standard criterion value of 370 Bq kg⁻¹ and the values of external and internal hazard indices were also found to be higher than unity in these two sampling locations. Besides these two sampling locations, a sampling location lying at a nearby distance from the mineralization zone also exhibits hazard indices values greater than unity.     

Geology and geochemical patterns of the Birimian gold deposits, Ghana, West Africa

A major gold province of the world exists in the Proterozoic Birimian and Tarkwaian supracrustal rocks of West Africa. The bulk of the gold comes from the primary lode occurrences of the Birimian rocks of Ghana (formerly The Gold Coast). Birimian lithofacies is characterised by subaqueous fine-grained sediments with bimodal volcanic material. Metasedimentary rocks include phyllites and metawackes. Metavolcanic rocks are predominantly tholeiitic basalts. Komatiites and banded iron formations (BIF) are absent.Gold is in 5 parallel, evenly spaced, more than 300 km long, northeast-trending volcanic belts separated by basins containing pyroclastic and meta-sedimentary units. The most prominent is the Ashanti volcanic “greenstone” belt, which hosts the Ashanti Goldfields Corporation mines at Obuasi (more than 800,000 kg Au since 1896), the Billiton Bogosu Gold mine at Bogosu, and the State Gold Mining Corporation mines at Prestea, Bibiani and Konongo.Gold, ranging from 2 to 30 ppm, is in quartz veins of laterally extensive major orebodies which deeply penetrate fissures and shear zones at contacts between metasedimentary and metavolcanic rocks. The veins consists mainly of quartz with carbonate minerals, green sericite, carbonaceous partings and metallic sulfides and arsenides of Fe, As, Zn, Au, Cu, Sb, and Pb. Gold occurs in carbonate fillings in fractured quartz veins. Country rocks, which contain rutile, anatase and granular masses of leucoxene, along ore channels, have been hydrothermally altered to carbonates, sericite, silica and sulfide minerals. Fluid inclusion evidences suggest that mineral deposition took place at about 350°C and 140 bar from dilute aqueous solutions. Timing deduced from ore textures, however, show complex multi-stage mineralization events, with higher temperature minerals commonly having formed later than lower temperature ones. Geochemical studies of materials produced by tropical processes, especially soils, are essential in prospecting poorly exposed terranes of west Africa. Trace and major element distributions at mines and mineral occurrences can indicate mineralization otherwise difficult to detect.This paper highlights the features of the Ghanaian gold deposits that may aid the current search for new deposits along the gold belts. Exploration based on geochemistry is highly important, but should be integrated with data from accompanying geological, lithologic, mineralogical, and structural studies.     

Selective extraction of organically bound gold in soils, lake sediments and stream sediments

The extraction of organically bound gold from surficial materials is accomplished by using a solution of 5% sodium hypochlorite. A 10-g sample is digested at least twice with 50 mL of hypochlorite solution. The recovery of gold from the organic phase is nearly quantitative for lake sediments and stream sediments high in organic content. For stream sediments and soils low in organics, the recovery of gold is complicated by the sorption of released gold onto inorganic sample components. For these materials, recoveries as low as 70% of the organically bound gold are possible. For a stream sediment sample containing 53.4 ppb gold, the precision of the method is approximately 6% for a 10-g sample and 14% when a 5-g sample of stream sediment is used. The results obtained for all materials tested indicate that a substantial fraction of the gold found in sediments and soils exists as organically bound gold.     

Interaction of acid mine drainage with waters and sediments at the Corona stream, Lousal mine (Iberian Pyrite Belt, Southern Portugal)

This study investigates the geochemical characteristics of the acid mine drainage discharged from the abandoned mine adits and tailing piles in the vicinity of the Lousal mine and evaluates the extent of pollution on water and on the stream sediments of the Corona stream. Atmospheric precipitation interacting with sulphide minerals in exposed tailings produces runoff water with pH values as low as 1.9–2.9 and high concentrations of (9,249–20,700 mg l⁻¹), Fe (959–4,830 mg l⁻¹) and Al (136–624 mg l⁻¹). The acidic effluents and mixed stream water carry elevated Cu, Pb, Zn, Cd and As concentrations that exceed the water quality standards. However, the severity of contamination generally decreases 4 km downstream of the source due to mixing with fresh waters, which causes the dilution of dissolved toxic metals and neutralization of acidity. Some natural attenuation of the contaminants also occurs due to the general reduced solubility of most trace metals, which may be removed from solution, by either co-precipitation or adsorption to the iron and aluminium precipitates.     

Impact of mining activities on soils and sediments at the historical mining area in Podljubelj,NW Slovenia

The abandoned Hg mine in Podljubelj was in operation with interceptions from 1557 to 1902. The entire operating period yielded about 110 000 tons of ore, from which 360 tons of Hg was produced. The objective of the research project was to establish the contents and spatial distribution of Hg in soils and stream sediments in the vicinity of the mine. On an area of 88 ha the soil was sampled in a 100 × 100 m grid. Two soil horizons (0–5 cm and 20–30 cm) were sampled in order to distinguish between geogenic and anthropogenic Hg sources. It was established that on an area of about 9 ha Hg content in soil exceeds The New Dutchlist action value for Hg (10 mg/kg). Total Hg concentrations in soil samples vary between 0.17 and 719 mg/kg, with a mean of 3.0 mg/kg. Mercury contents in stream sediments range from 0.065 to 1.4 mg/kg, with a mean of 0.64 mg/kg. The highest determined value in soils was found in the area around the former roasting furnace, where the ore was processed. Increased Hg concentrations were also found on the mine waste dump (108 mg/kg). Mercury contents in soils generally decrease with soil profile depth and with the distance from the mine and from the roasting furnace location. Mercury also appears in higher concentrations along the road that runs through the valley, which results from the use of Hg-bearing ore residues in road construction. The average enrichment factor (EF) of Hg in topsoil with respect to subsoil is 3.3. Calculated enrichment factors show higher values also for Cd (3.2), Pb (2.7), Ca (2.4) and P (1.9). The average enrichment factor of Hg in topsoil with regard to the established Slovenian soil averages (EF_slo) is 19. EF_slo of other determined chemical elements do not exceed 3.0.