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.     

Porphyry,epithermal, and orogenic gold prospectivity of Argentina

This paper presents a review of the available information on the significant porphyry, epithermal, and orogenic gold districts in Argentina, including the tectonic, geological, and structural settings of large deposits or deposits that have been exploited in the past. Based on this review of the geology and mineralization, targeting models are developed for epithermal and orogenic gold systems, in order to produce GIS-based prospectivity models. Using publically available digital geoscience data, weights of evidence and fuzzy logic prospectivity maps were generated for epithermal and orogenic gold mineralization in Argentina. The results of the prospectivity mapping highlight existing gold deposits within known mineralized districts throughout Argentina, as well as other highly prospective areas with no known deposits within these districts. Additionally, areas within Argentina that have no known gold mineralization (based on publically available information) were highlighted as being highly prospective based on the models used.     

Predictive mapping of prospectivity for orogenic gold,Giyani greenstone belt (South Africa)

We present a mineral systems approach to predictive mapping of orogenic gold prospectivity in the Giyani greenstone belt (GGB) by using layers of spatial evidence representing district-scale processes that are critical to orogenic gold mineralization, namely (a) source of metals/fluids, (b) active pathways, (c) drivers of fluid flow and (d) metal deposition. To demonstrate that the quality of a predictive map of mineral prospectivity is a function of the quality of the maps used as sources of spatial evidence, we created two sets of prospectivity maps — one using an old lithologic map and another using an updated lithological map as two separate sources of spatial evidence for source of metals/fluids, drivers of fluid flow and metal deposition. We also demonstrate the importance of using spatially-coherent (or geologically-consistent) deposit occurrences in data-driven predictive mapping of mineral prospectivity. The best predictive orogenic gold prospectivity map obtained in this study is the one that made use of spatial evidence from the updated lithological map and spatially-coherent orogenic gold occurrences. This map predicts 20% of the GGB to be prospective for orogenic gold, with 89% goodness-of-fit between spatially-coherent inactive orogenic gold mines and individual layers of spatial evidence and 89% prediction-rate against spatially-coherent orogenic gold prospects. In comparison, the predictive gold prospectivity map obtained by using spatial evidence from the old lithological map and all gold occurrences has 80% goodness-of-fit but only 63% prediction-rate. These results mean that the prospectivity map based on spatially-coherent gold occurrences and spatial evidence from the updated lithological map predicts exploration targets better (i.e., 28% smaller prospective areas with 9% stronger fit to training gold mines and 26% higher prediction-rate with respect to validation gold prospects) than the prospectivity map based on all known gold occurrences and spatial evidence from the old lithological map.     

Paleoproterozoic,turbidite-hosted,gold deposits of the Ashanti gold belt (Ghana,West Africa): Comparative analysis of turbidite-hosted gold deposits and an updated genetic model

The Paleoproterozoic gold deposits of the Ashanti gold belt, Ghana, occur mainly as auriferous quartz veins in graphitic shears proximal to contacts between deformed and metamorphosed turbidites and tholeiitic volcanics (alternately block-faulted Tarkwaian sediments). They are suggested in this article to belong to that class of gold deposits recognized as ‘turbidite-hosted’. Hence, evidence from previous investigations of turbidite-hosted and turbidite-associated gold deposits occurring in circumstances roughly similar to the Ashanti deposits are used, where relevant, to produce an updated genetic model for the latter.Given the relatively significant amounts of graphite associated with the major Ashanti deposits, the role of non-carbonate carbon is assessed in the light of conclusions reached by work on other turbidite-hosted and turbidite-associated gold belts. Conclusions as to the origins of mineralizing fluids and temporary repositories of gold (auriferous interflow sediments) reached by work on these other belts are also analysed.A concept of the tectonic development of the Ashanti belt-Kumasi basin area, introduced in this article, is deemed to be critical to the origin of the Ashanti gold deposits. It encompasses the role of non-carbonate carbon, turbidites, and the primary sources and secondary repositories of Au and As in the genesis of the major Ashanti gold deposits. It examines the significance of the intimate relationship between calc alkaline volcanism and the basin sediments, the occurrences of felsic metasandstones in transition-zone sediments, and TTG granitic material and fragments of chemical sediments in turbidites: all of which are incompatible with one or the other of existing concepts of tectonic development in the area.     

柴达木盆地北缘造山型金矿地质、成矿流体及成矿时代特征

柴达木盆地北缘是一条著名的超高压变质带,带内矿产资源丰富.造山带金矿广泛分布于柴北缘带内,本文着重对柴北缘金矿的地质特征、成矿流体的温度和同位素及成矿时代进行研究.结果显示:柴北缘造山型金矿主要赋存在中元古界、寒武系和奥陶系变质岩发育的剪切带中,且多数矿体的展布与NW向的构造相关.大多数金矿成矿温度呈双峰态分布,少部分...     

Mapping of prospectivity and estimation of number of undiscovered prospects for lode gold,southwestern Ashanti Belt,Ghana

In the southwestern part of the Ashanti Belt, the results of fractal and Fry analyses of the spatial pattern of 51 known mines/prospects of (mostly lode) gold deposits and the results of analysis of their spatial associations with faults and fault intersections suggest different predominant structural controls on lode gold mineralisation at local and district scales. Intersections of NNE- and NW-trending faults were likely predominantly involved in local-scale structural controls on lode gold mineralisation, whilst NNE-trending faults were likely predominantly involved in district-scale structural controls on lode gold mineralisation. The results of the spatial analyses facilitate the conceptualisation and selection of spatial evidence layers for lode gold prospectivity mapping in the study area. The applications of the derived map of lode gold prospectivity and a map of radial density of spatially coherent lode gold mines/prospects results in a one-level prediction of 37 undiscovered lode gold prospects. The applications of quantified radial density fractal dimensions of the spatial pattern of spatially coherent lode gold mines/prospects result in an estimate of 40 undiscovered lode gold prospects. The study concludes finally that analysis of the spatial pattern of discovered mineral deposits is the key to a strong link between mineral prospectivity mapping and assessment of undiscovered mineral deposits.     

Land use and urban travel in Kumasi,Ghana

Globally, transport literature indicates a strong effect of land use on urban travel as people living in low density suburban areas tend to travel more by car than people living in high density urban areas. This is because in dense areas, public transport is organised more efficiently and travellers tend to travel shorter distances. However, this assertion is frequently based on locations with efficient integration of transport within the land use planning framework. In Ghana and many African countries, it remains unknown whether the effect of land use on urban travel is strong as reported in developed countries and elsewhere. This research examines the effect of land use on urban travel in Ghana using Kumasi, the second largest city in Ghana, as a case study. Simple questionnaire survey with urban residents, semi-structured interviews with agencies and secondary data analysis were used for this research. Results indicate negative effect of land use on urban travel as there has been increased congestion in all the major road arterials in the city resulting in difficulty in commuting using motorised transport. Findings further show a weak effect of land use on urban travel, as areas experiencing change of land use have poor locational accessibility. The paper recommends innovative ways of meeting the growing travel demand of residents in the city such as the development of a light rail and bus rapid transit systems to help ease congestion and improve public transportation.     

Exploration targeting for orogenic gold deposits in the Granites-Tanami Orogen: Mineral system analysis,targeting model and prospectivity analysis

A major challenge for mineral exploration geologists is the development of a transparent and reproducible approach to targeting exploration efforts, particularly at the regional to camp scales, in terranes under difficult cover where exploration and opportunity costs are high. In this study, a three-pronged approach is used for identifying the most prospective ground for orogenic gold deposits in the Paleoproterozoic Granite-Tanami Orogen (GTO) in Western Australia.A key input to the analyses is the recent development of a 4D model of the GTO architectural evolution that provides new insights on the spatio-temporal controls over orogenic gold occurrences in the area; in particular, on the role of pre-mineralization (pre-1795 Ma) D_GTOE–D_GTO1–D_GTO2 architecture in localization of gold deposits and the spatial distribution of rock types in 3D. This information is used to build up a model of orogenic gold minerals system in the area, which is then integrated into the three mutually independent but complementary mineral prospectivity maps namely, a concept-driven “manual” and “fuzzy” analysis; and a data-driven “automated” analysis.The manual analysis involved: (1) generation of a process-based gold mineral systems template to aid target selection; (2) manual delineation of targets; (3) manual estimation of the probability of occurrence of each critical mineralization process based on the available information; and (4) combining the above probabilities to derive the relative probability of occurrence of orogenic gold deposits in each of the targets. The knowledge-based Geological Information System (GIS) analysis attempts to replicate the expert knowledge used in the manual approach, but queried in a more systematic format to eliminate human heuristic bias. This involves representing the critical mineralization processes in the form of spatial predictor maps and systematically querying them through the use of a fuzzy logic model to integrate the predictor maps and to derive the western GTO orogenic gold prospectivity map. The data-driven ‘empirical’ GIS analysis uses no expert knowledge. Instead it employs statistical measures to evaluate the spatial associations between known deposits and predictor maps to establish weights for each predictor layer then combines these layers into a predictive map using a Weights of Evidence (WofE) approach.Application of a mineral systems approach in the manual analysis and the fuzzy analysis is critical: potential high value targets identified by these approaches in the western GTO lie largely under cover, whereas traditional manual targeting is biased to areas of outcrop or sub-crop amenable to direct detection technology such as exploration geochemistry, and therefore towards areas that are data rich.The results show the power of combining the three approaches to prioritize areas for exploration. While the manual analysis identifies and employs human intuition and can see through incomplete datasets, it is difficult to filter out human bias and to systematically apply to a large region. The fuzzy method is more systematic, and highlights areas that the manual analysis has undervalued, but lacks the intuitive power of the human mind that refines the target by seeing through incomplete datasets. The empirical WoE method highlights correlations with favorable host stratigraphy and highlights the control of an early set of structures potentially undervalued in the knowledge driven approaches, yet is biased due to the incomplete nature of exploration datasets and lack of abundant gold deposits due to the extensive cover.The results indicate that the most prospective areas for orogenic gold in western GTO are located in the central part of the study area, largely in areas blind to previous exploration efforts. According to our study, the procedure to follow should be to undertake the analyses in the following order: manual prospectivity analysis, followed by the conceptual fuzzy approach, followed by the empirical GIS-based method. Undertaking the manual analysis first is important to prevent explorationists from being biased by the automated GIS-based outputs. It is however emphasized that all of the prospectivity outputs from these three methods are possible, and they should not be treated as ‘treasure maps’, but instead, as decision-support aids. Therefore, a final manual prospectivity analysis redefined by the mutual consideration of output from all of the methods is required.The strategy employed in this study constitutes a new template for best-practice in terrane- to camp-scale exploration targeting that can be applied to different terranes and deposit types, particularly in terranes under cover, and provides a step forward in managing uncertainty in the exploration targeting process.     

Conflicting evidence on the timing of mesothermal and paleoplacer gold mineralisation in early Proterozoic rocks from Southwest Ghana,West Africa

Rocks of early Proterozoic age (ca. 2100 Ma) host the major gold deposits in Ghana. The deposits are either located in mesothermal quartz vein systems or hosted in a quartz pebble conglomerate that represents a paleoplacer. Both types of mineralisation are largely confined to the Ashanti Belt, one of four parallel northeast-trending volcanic belts. While the stratigraphy and structure of the belts are similar, the Ashanti belt is characterised by a more tectonised northwest margin where most of the epigenetic gold deposits are located. In these deposits, gold mineralisation is located in faults that parallel the regional trend of the belts and were active late in the deformation history of the terrane. The auriferous quartz pebble conglomerate is part of a clastic sequence that is largely derived from the adjacent volcanic and plutonic rocks with the gold widely regarded as having originated from eroded vein deposits. Structural data, however, show that both the volcanic rocks and clastic sequence were deformed jointly prior to epigenetic gold mineralisation. Thus, the quartz vein deposits could not have been the source of the paleoplacer mineralisation. The paleoplacer gold could have originated from one of several possible sources but none has been unequivocally identified.     

The Burkinian orogenic cycle,precursor of the Eburnian orogeny in West Africa

In the best known areas of the West African craton structural, petrographic, and geochronological data provide a distinction between two separate units of the Lower Proterozoic in West Africa. A lower unit was metamorphosed from low to medium grade around 2170 Ma by horizontal shearing. This unit is mainly composed of dominantly basic bimodal magmatic rocks, and some trondhjemitic to tonalitic anatectic gneisses, and locally mesozonal mica schists known in Ivory Coast as the ‘Kounoukou formation’ which has been dated as 2183 Ma old. An upper unit, which frequently begins with polygenic conglomerates, also shows important bimodal vulcanism, but in contrast to the lower unit is dominantly silicic. This unit is affected by lower grade conditions of metamorphism and is weakly deformed. These two units correspond to the classic Eburnian I/Eburnian II succession. However, a clearer distinction is possible, with the recognition of two successive orogenic cycles. In the basal part of each of the two units a major episode of tholeiitic magmatism is evidence for separate periods of lithospheric thinning and fracturing. This was followed by the deposition of various lithological sequences, then by one or more tectonometamorphic events. Later uplift led to the emplacement of anorogenic subvolcanic granitic massifs which are displayed occasionally as ring complexes. This suggests that ‘Eburnian I’ is actually an independent orogenic cycle appearing in the West African formations between 2400 and 2150 Ma, for which we propose the name ‘Burkinian cycle’. This cycle has affected the lower magmatic and sedimentary formations, which we designate Dabakalian. This implies a restricted time span for the Eburnian cycle from 2100–2150 to 1800 Ma. The stratigraphic term ‘Birimian’ is applied to the sedimentary and magmatic formations of the upper unit. This scheme proposed for the Lower Proterozoic in West Africa is probably applicable to other Lower Proterozoic terranes in Africa.     

Spatial plans and urban development trajectory in Kumasi,Ghana

This research uses empirical data to explore the link between spatial plans and ‘actual development’ occurring in Kumasi, Ghana. The research found that urban development is determined by both spatial plans and spontaneous informal development patterns (i.e. self-organisation). However, self-organisation is more widespread compared to spatially planned neighbourhoods. This phenomenon was expressed in a context of uncertainty created by weak institutional planning system which promote and reinforce haphazard development disregarding urban planning requirements. These findings provide a perspective on urban development that differs from that occurring in cities of developed countries, where planning institutions in consultation with urban residents determine the patterns of urban development. Unfortunately in Kumasi, self-organisation is not viewed as important in mainstream planning system. The paper concludes that more needs to be done in terms of understanding self-organisation processes, and the way they could be integrated into mainstream planning process to respond more fully to the urban development challenges in Kumasi.     

西秦岭造山带天子坪金矿床成矿时代及成因

西秦岭天子坪金矿床为含金石英细脉、网脉的蚀变岩型金矿,以含金石英脉中的热液锆石作为研究成矿时代的对象,挑选的锆石部分呈局部溶蚀状,无环带或弱环带,Th、U、Pb含量较高,Th/U值较高,属成矿期热液锆石。LA-ICP-MS锆石U-Pb分析获得了110±7 Ma的谐和年龄,与矿区所在地区早白垩世晚期的推覆构造事件吻合,代表了矿床的成矿时代。分析认为,柴家庄矿集区在燕山晚期有一次强度较大的与大型构造有关的构造蚀变岩型金的成矿作用。     

A review of the orogenic events in the Precambrian basement of Nigeria,West Africa

The Basement complex of Nigeria is polycyclic and retains memories of events dating back to about 3000 m. y. As in other parts of Africa, it suffered its most pronounced deformation and remobilisation during the Pan African Orogeny (about 650-450 m. y. ago).A review of the geology of this terrain is presented following recent field and geochronological data. It is also shown that the basement may have participated in the 1300-900 m. y. Kibaran Orogeny.In addition, an attempt is made at discussing the evolution of the Precambrian rocks in Nigeria and the conclusion is made that most of the rocks evolved by ensialic rather than plate tectonic processes.

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Zusammenfassung Das Basement Nigerias ist polyzyklisch aufgebaut mit Ereignissen, die bis zu 3000 m. y. zurückliegen. Wie in anderen Teilen Afrikas erfolgte die stÄrkste Deformation und Remomobilisation wÄhrend der Panafrikanischen Orogenese (vor etwa 650-450 m. y.).Es wird eine übersicht über die Geologie dieses Gebietes gegeben, die jüngste GelÄndeaufnahmen und geochronologische Daten berücksichtigt. Auch wird gezeigt, da\ das Basement von der Kibarischen Orogenese vor 1300-900 m. y. erfa\t worden sein könnte.ZusÄtzlich wird der Versuch unternommen, die Evolution der prÄkambrischen Gesteine Nigerias zu diskutieren mit dem Schlu\, da\ die meisten Gesteine sich eher aus ensialischen als aus plattentektonischen Prozessen entwickelten.

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Résumé Le complexe du soubassement nigérien est polycyclique, et porte la marque d'événements remontant à 3.000 millions d'années. Il a subi sa déformation et sa remobilisation les plus marquées, comme ce fut le cas dans le reste du continent Africain, au cours de l'orogénie Pan Africaine, c-a-d il y a entre 650-450 millions d'années.Dans ce travail, nous présentons la géologie de ce terrain d'après des données cartographiques et géochronologiques récentes. Nous démontrons également que ce soubassement peut très bien avoir participé à l'orogénie kibarienne d'il y a 1.3000 à 900 millions d'années.En outre, nous essayons de discuter l'évolution de roches précambriennes de la Nigérie, et nous en tirons la conclusion que pour la plupart des roches, cette évolution répond à une formation ensialique plutÔt qu'à des processus de tectonique de plaques.

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Intrusive rocks and tectono-metamorphic evolution of the Mako Paleoproterozoic belt (Eastern Senegal,West Africa)

The Kedougou Kenieba Inlier (KKI) (Paleoproterozoic of Eastern Senegal) is a portion of the West African Craton (WAC) containing a granite-greenstone terrain that experienced three distinct periods of magmatic activity, peaking at 2200, 2160–2130 and 2100–2070 Ma. In the Inlier, Paleoproterozoic granitoids and large-scale transcurrent shear zones are spatially associated, suggesting a genetic link between magma bodies and shear zones. Granitic intrusions are associated with all the volcanic episodes and phases of deformation, and have been used to constrain the age of many of these events. Our structural data and deformation sequence indicate that the Mako Greenstone Belt and the adjacent granitoid complexes have undergone a multi-phase evolutionary history that is spread over a prolonged period. The available geochronological data and field studies allowed classification of the granitoids of the KKI into four generations GI, GII, GIII and GIV.The current data suggest that the oldest rocks in the KKI, the Badon granites (2198 ± 2 Ma) and the tonalitic gneisses from Tonkouto (2200–2198 Ma) (GI), could be correlated with an early Birimian magmatic event. The gneisses, crystallized at depth, record the earliest deformation and in contrast to other tonalites, do not appear to have intruded volcanic rocks. The second manifestation of magmatism was intrusion of mafic diorite – the Gabbro Sandikounda Layered Igneous Complex type (GII) and development of the Laminia Kaourou Plutonic Complex (LKPC) (2160–2130 Ma). These bodies pre-date or are sometimes synchronous with a major deformational episode, and may, therefore, have formed very early in convergent Birimian orogenesis.The third major peak of magmatic activity occurred after the above major episode with the development of the oval shaped Diombalou and Bouroumbourou plutons (GIII). The orientation of these plutons parallel to the regional strike of the schistosity indicates structural control on granite emplacement. Eburnean magmatism was terminated in the Mako Belt following compressional Eburnean deformation, with the emplacement of the Tinkoto, Mamakono plutons (GIV) in the east of the complex and continued in the Dialé–Daléma supergroup with the syntectonic emplacement of the Saraya batholith. Garnitiferous granites of crustal derivation were emplaced in the final period of extensional activity around 2080 Ma.Field observations suggest the early plutons of the complex granitic (Kakadian) batholith intruded during convergent deformation whereas later igneous activity accompanied regional orogen-parallel extension, followed by exhumation. In the Mako Belt, thickening of the crust was proposed to have caused heating and the ‘apparent diapiric rise’ of the Diombalou and Bouroumbourou plutons.     

Origin and age of Paleoproterozoic conglomerates and sandstones of the Tarkwaian Group in Burkina Faso, West Africa

The Tarkwaian rocks of Burkina Faso overlie the Birimian Supergroup which is considered to be part of a juvenile crust formed during the Eburnean orogen. They comprise fluviatile pebbles (quartz, rhyolite, chert and schist), embedded in an acid volcaniclastic matrix. During a sinistral transpressive shearing they underwent a low-grade metamorphism characterized by a paragonitic muscovite assemblage formed during isoclinal folding. Fold axes are nearly horizontal, parallel to a stretching lineation and oriented 035–215°.A typologic study of zircons in the volcaniclastics, coupled with radiometric datings, enables the volcanism to be characterized and its age to be determined.The typologic study has shown that: (1) zircons from the rhyolitic pebbles, the matrix of the conglomerates and from the sandstone are basically the same; (2) the zircons' morphology characterizes a calc-alkaline to alkaline evolutionary trend implying three main episodes of volcanism; (3) the morphology of this zircon population is very similar to those usually observed in post-orogenic metasediments or sediments. The tectonic setting is one of intracontinental extension associated with rifting and crustal thinning in the course of a major period of crustal growth around 2.1 Ga. The continental volcaniclastics accumulated in the rift at the same time as the sediments.The radiometric study has shown that there are no reworked Archean rocks. The ages cover the period 2170 to 2124 Ma. Three main volcanic episodes may be defined 2170, 2150 and 2124 Ma; the last one was probably contemporaneous with the sedimentation. Deformation of Tarkwaian rocks took place between 2124 ± 9 Ma (the age of the youngest inherited zircon) and 1991 ± 12 Ma (the age of a post-Tarkwaian granite). This period, 2170 to 2124 Ma, fits well with the main Birimian crustal growth and the ages reported for the Tarkwaian rocks of Ghana. The same period of accretion is also recognized in South America.The Tarkwaian conglomeratic rocks of Burkina Faso do not have gold concentrations similar to those in Ghana and Guyana. With respect to the Tarkwaian rocks of Ghana, where gold is concentrated in the quartz pebbles, they also differ because: (1) there is a volcaniclastic contribution; (2) the zircons from the Tarkwaian rocks in Burkina Faso are euhedral, indicating that transport distances were short; and (3) they have been deformed by shearing.With respect to Guyana, the rocks are very similar and probably have the same origin; the short transport distance may not allow a secondary concentration of gold. These differences, at least, may help explain the lack of economic gold concentration in the Tarkwaian metasediments.     

新疆西天山查汗萨拉金矿地质、金赋存状态及同位素地球化学研究

查汗萨拉金矿是近年在新疆西天山新发现的一处金矿床,处于依连哈比尔尕构造带西端.矿体旱不规则脉状产于细品闪长岩构造破碎蚀变带及其接触带附近的上石炭统奇尔古斯套组蚀变围岩中,围岩蚀变较弱.矿石中硫化物主要为黄铁矿,并含少量磁黄铁矿、黄铜矿、方铅矿等.硫化物矿物呈自形粗晶或半自形结构,斑杂状分布在构造蚀变岩石中.金矿物以自然金和银金矿为主,还发现有硫(碲)银金矿和金铀化物等独特矿化线索,金矿物多赋存在黄铁矿中,以包体金、裂隙金和少量粒间金形式存在.金矿物形态以粒状和长角状为主,多为细、微细粒金(粒度<10 μm).矿石中矿物流体包裹体均一温度为220～340℃.热液脉三石矿物石英流体包裹体的δD为-92‰～-74‰,δ18Ov-SMOW为11.8‰～12.6‰,成矿流体显尔岩浆热液和变质建造水混合的特征.热液方解石脉的占δ13Cv-PDB为-8.92‰～-8.06‰,δ18Ov-SMOW为13.45‰～17.18‰,反映成矿流体中CO2主体米源于岩浆.硫化物206pb/204Pb为18.036～18.173,207pb/204pb为15.536～15.612,208pb/204pb为37.940～38.097,成矿金属具岩浆来源特征.矿石中硫化物δ34Sv-CDT为-9.8‰～-7.3‰,显示其可能与地层有关.查汗萨拉金矿为构造蚀变岩型中温岩浆热液矿床.小同于本区阿希金矿,是西天山金矿勘查中值得关注的新类型.     

A magnetic and gravity investigation of the Liberia Basin,West Africa

Gravity and magnetic analysis provide an opportunity to deduce and understand to a large extent the stratigraphy, structure and shape of the substructure. Euler deconvolution is a useful tool for providing estimates of the localities and depth of magnetic and gravity sources. Wavelet analysis is an interesting tool for filtering and improving geophysical data. The application of these two methods to gravity and magnetic data of the Liberia Basin enable the definition of the geometry and depth of the subsurface geologic structures. The study reveals the basin is sub-divided and the depth to basement of the basin structure ranges from about 5 km at its North West end to 10 km at its broadest section eastward. Magnetic data analysis indicates shallow intrusives ranging from a depth of 0.09 km to 0.42 km with an average depth of 0.25 km along the margin. Other intrusives can be found at average depths of 0.6 km and 1.7 km respectively within the confines of the basin. An analysis of the gravity data indicated deep faults intersecting the transform zone.     

红安造山带南缘古元古代杂岩体的发现对扬子板块古元古代造山事件的约束

识别并研究扬子板块古元古代的岩浆-变质-沉积事件,是探讨扬子板块古元古代构造演化的基础,也是重建该陆块在Columbia超大陆中位置的前提.新发现的金盆杂岩体为进一步揭示扬子板块古元古代岩浆事件和造山过程提供了新的制约信息.锆石U-Pb定年结果表明,金盆片麻状二长花岗岩、奥长花岗岩和基性岩脉的形成年龄分别为～ 2478...     

Assessing social vulnerability to biophysical hazards in the Kumasi metropolis,Ghana

This study assesses social vulnerability to hazards by analyzing the nature of biophysical hazards, and the characteristics of exposures, susceptibilities and adaptive capacities in Kumasi. A mixed method approach was used to collect primary data from farms and nearby markets. Water, soil, and plant samples were collected and tested for total coliforms and cadmium. Primary dataset was also collected from farmers and vendors using structured interview guide and from key informants through in-depth interviews. The analysis shows that several pressures residing within the coupled social-ecological system are threats to the health and livelihood of the farmers and vendors. The concentration of cadmium and total coliform in irrigation water, soil, and plants exceeded the WHO/FAO acceptable standards. Bio-toxins in plants were also found to be potentially high. However, the degree of exposure to these stressors and the strength of response and adaptive capacities were determined to be dependent on position within the urban agriculture value chain, type of plant, and socio-demographic characteristics. The study concludes that pollution of irrigation water, soil, and plant is an issue of environmental concern in the urban agriculture system and requires concerted efforts of all stakeholders to address. It is recommended that there should be frequent monitoring of the quality of irrigation water, soil, and produce from urban farms and markets to assess their physical, chemical, and microbial properties and possibly initiate remedial measures where necessary. Government policies must also focus on building the resilience of urban farmers and vendors to reduce their vulnerability to biophysical hazards.     

Contaminant leaching from gold mining tailings dams in the Witwatersrand Basin,South Africa: A new geochemical modelling approach

In this study a new geochemical reaction modelling methodology is used to shed light on the geochemical processes within Witwatersrand tailings impoundments and the evaluation of the geochemical impacts for future mining projects is evaluated. Information from international and local studies on tailings sulphidic tailings impoundments is used to develop a conceptual understanding on a typical Witwatersrand gold tailings impoundment. The tailings impoundments consist of 3 distinct geochemical zones: Oxidation Zone (OZ), a Transition Zone (TZ) and a Reduction Zone (RZ). Individual reaction models are developed for each of the 3 zones. The output of one model is used as an input to the next in spatial order. The results of the final model represent the tailings basal seepage characteristics, which indicate the most likely impacts on groundwater resources. The model results agree with existing information on AMD in the Witwatersrand. The results indicate that the tailings basal seepage is likely to be acidic (pH of ∼3.5), containing elevated concentrations of SO₄ and trace metals (Al, Co, Cr, Cu, Mn, Ni, Zn and U). Predicted Fe concentrations are low, due to the low $f_{{\text{O}}_2}$ in the TZ and RZ and the buffering effect of the precipitation of K-jarosite in the OZ and TZ. This study therefore indicates that the methodology employed produces results that can be correlated to existing information and can thus be used as a methodology in the assessments of impacts from sulphidic tailings material for future mining projects.