Fluid inclusion studies of the Abawso gold prospect,near the Ashanti Belt,Ghana

The major Ghanaian lode gold deposits are preferentially aligned along the western and eastern contacts of the Kumasi Basin with the Ashanti and Sefwi Belts, respectively. The investigated area of the Abawso small-scale concession, covering the workings of the old Ettadom mine, is situated 3 km west of the lithological contact of the Birimian metavolcanic rocks of the Akropong Belt in the east with the Birimian metasedimentary rocks of the Kumasi Basin in the west. The rocks of the Abawso concession represent a steeply NW-dipping limb of a SE-verging anticline with an axis plunging to the SW. Quartz veining occurs predominantly in the form of en échelon dilatational veins along NNE–SSW-striking shear zones of a few metres width and shows evidence of brittle and ductile deformation. Also stockwork-style quartz veining occurs in the vicinity of the main shaft of the old Ettadom mine. Hydrothermal alteration includes sericitisation, sulphidation and locally carbonatisation. The auriferous quartz veins mainly follow the trend of brittle to ductile deformed quartz veins; however, some occur in stockwork. Fluid inclusion studies reveal a large number of H₂O inclusions along intragranular trails in auriferous quartz vein samples, as well as an overall dominance of H₂O and H₂O-CO₂ inclusions over CO₂ inclusions. Textural observations and physico-chemical fluid inclusion properties indicate post-entrapment modifications for all quartz vein samples due to grain boundary migration recrystallisation. This process is interpreted to be responsible for the generation of the CO₂ inclusions from a H₂O-CO₂ parent fluid. In comparison with mineralisation at the Ashanti and Prestea deposits, which are characterised by CO₂±N₂ inclusions, the observed inclusion assemblage may be due to a shallower crustal level of mineralisation, or different degrees and styles of recrystallisation, or a less pronounced development of laminated quartz veins due to comparably restricted pressure fluctuations. Furthermore, the microthermometric observations allow the reconstruction of a possible retrograde P-T path, depicting near-isothermal decompression in the P-T range of the brittle/ductile transition.Editorial handling: E. Frimmel     

Evolution of gold mineralization in the Ashanti Gold Belt,Ghana: Evidence from carbonate compositions and parageneses

Summary Ankerite, siderite, calcite and magnesite occur in variable proportions within all host and mineralized rocks of the Bogosu and Prestea mining districts of the Ashanti Gold Belt, Ghana. The compositions of coexisting ankerite-siderite grains establish that complex rhythmically zoned growth banding and replacement textures are present. This compositional variation is attributed to episodic fluctuation in the temperature and composition of fluids in the Bogosu-Prestea mesothermal gold system. Temperatures derived from the ankerite-siderite composition geothermometer are generally consistent with those from calcite-dolomite, arsenopyrite, carbon and oxygen stable isotope, and fluid inclusion geothermometers, and are about 360°C for the metamorphic peak, 400 to 350°C for carbonate alteration of mafic dikes, and 340 to 140°C for gold deposition. The latter range occurs on a thin-section scale and represents separate pulses of fluid in the ore conduit.

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Entstehung der Goldvererzung im Ashanti Gold Belt, Ghana: Rückschlüsse aus Karbonat zusammensetzungen und Paragenesen

Zusammenfassung In allen Wirtsgesteinen und mineralisierten Gesteinen der Bergbaureviere von Bogosu und Prestea im Ashanti Gold Belt, Ghana treten Ankerit, Siderit, Calcit und Magnesit in unterschiedlichen Verhältnissen auf. Die Zusammensetzung von koexistierenden Ankerit-Siderit-Körnern zeigt eine komplexe, rhythmisch zonierte Wachstumsstreifung und Verärdngungsstrukturen. Diese Änderungen in der Zusammensetzung sind auf episodische Fluktuationen der Temperatur und der Zusammensetzung der Fluide im mesothermalen Goldsystem von Bogosu-Prestea zurückzuführen. Temperaturen nach dem Ankerit-Siderit-Geothermometer stimmen im allgerneinen mit jenen aus Geothermometern, die auf Calcit-Dolomit, Arsenopyrit, den stabilen Isotopen von Kohlenstoff and Sauerstoff und auf Flüssigkeitseinschlüssen beruhen, überein. Sie liegen bei rund 360°C für den Höhepunkt der Metamorphose, bei 400 bis 350°C für die Karbonat Alteration der matischen Gänge and bei 340 bis 140°C für die Gold-Fällung. Der letztgenannte Bereich tritt in Dünnschlif Maßstab auf and repräsentiert einzelne Schübe von Fluid in den Erzgängen.

     

Constrained gravity models and structural evolution of the Ashanti Belt, southwest Ghana

The Palaeoproterozoic terrane of southwest Ghana comprises a sequence of folded sedimentary/volcaniclastic rocks which separate a series of 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 along which most of the major epigenetic Au deposits in Ghana are located. Early models of the tectonic evolution of this belt, which generally invoked two phases of deformation, are inconsistent with more recent structural and isotopic evidence (Eisenlohr and Hirdes, 1992). Modelling of regional gravity data by Hastings (1982) acknowledged the tectonic significance of the belt margins but lacked the benefit of constraints provided by more detailed mapping. This paper re-examines existing gravity data over the Ashanti Belt and presents constrained cross-sectional models which honour the mapped geology along the length of the belt and are consistent with the concept of a single, continuous deformational event. A synoptic scenario for the large-scale structural evolution of the belt is proposed which has implications for the location and timing of Au mineralisation.     

3D modeling of the Ashanti Belt,southwest Ghana: Evidence for a litho-stratigraphic control on gold occurrences within the Birimian Sefwi Group

A three-dimensional model of the gold-rich Paleoproterozoic Ashanti Greenstone Belt has been built with a “hybrid” geological modeling method using Geomodeller and Gocad, then inverted using VPmg software. The model helps to identify the main regional-scale controls leading to abundant gold mineralization in the southeast of the belt by highlighting the lithological distribution and geometry of geological structures. The three-dimensional geological model shows that 85% of gold occurrences in the Ashanti Greenstone Belt are proximal to a specific tholeiitic metabasaltic horizon “BV1”. This result is noteworthy given that the relatively thin, 300 m thick, BV1 layer represents only 10%, by volume, of the modeled Sefwi Group stratigraphy. A map of distance between the topography and the BV1 layer is produced and identifies areas showing a high probability to host gold deposits in the southeast of the Ashanti Belt. Possible structural controls performed by the BV1 tholeiitic metabasaltic rocks on the location of gold occurrences are discussed, and we hypothesize that the BV1 layer acted as a mineralization trap or source of gold. Both hypotheses agree with examples from the literature and explain the observed distribution of the mineralized sites.     

Geology,geodynamics and orogenic gold prospectivity modelling of the Paleoproterozoic Kumasi Basin,Ghana, West Africa

This paper describes the geology and tectonics of the Paleoproterozoic Kumasi Basin, Ghana, West Africa, as applied to predictive mapping of prospectivity for orogenic gold mineral systems within the basin. The main objective of the study was to identify the most prospective ground for orogenic gold deposits within the Paleoproterozoic Kumasi Basin. A knowledge-driven, two-stage fuzzy inference system (FIS) was used for prospectivity modelling. The spatial proxies that served as input to the FIS were derived based on a conceptual model of gold mineral systems in the Kumasi Basin. As a first step, key components of the mineral system were predictively modelled using a Mamdani-type FIS. The second step involved combining the individual FIS outputs using a conjunction (product) operator to produce a continuous-scale prospectivity map. Using a cumulative area fuzzy favourability (CAFF) curve approach, this map was reclassified into a ternary prospectivity map divided into high-prospectivity, moderate-prospectivity and low-prospectivity areas, respectively. The spatial distribution of the known gold deposits within the study area relative to that of the prospective and non-prospective areas served as a means for evaluating the capture efficiency of our model. Approximately 99% of the known gold deposits and occurrences fall within high- and moderate-prospectivity areas that occupy 31% of the total study area. The high- and moderate-prospectivity areas illustrated by the prospectivity map are elongate features that are spatially coincident with areas of structural complexity along and reactivation during D4 of NE–SW-striking D2 thrust faults and subsidiary structures, implying a strong structural control on gold mineralization in the Kumasi Basin. In conclusion, our FIS approach to mapping gold prospectivity, which was based entirely on the conceptual reasoning of expert geologists and ignored the spatial distribution of known gold deposits for prospectivity estimation, effectively captured the main mineralized trends. As such, this study also demonstrates the effectiveness of FIS in capturing the linguistic reasoning of expert knowledge by exploration geologists. In spite of using a large number of variables, the curse of dimensionality was precluded because no training data are required for parameter estimation.     

High CO2 content of fluid inclusions in gold mineralisations in the Ashanti Belt, Ghana: a new category of ore forming fluids?

Fluid inclusions were studied in samples from the Ashanti, Konongo-Southern Cross, Prestea, Abosso/Damang and Ayanfuri gold deposits in the Ashanti Belt, Ghana. Primary fluid inclusions in quartz from mineralised veins of the Ashanti, Prestea, Konongo-Southern Cross, and Abosso/Damang deposits contain almost exclusively volatile species. The primary setting of the gaseous (i.e. the fluid components CO₂, CH₄ and N₂) fluid inclusions in clusters and intragranular trails suggests that they represent the mineralising fluids. Microthermometric and Raman spectroscopic analyses of the inclusions revealed a CO₂ dominated fluid with variable contents of N₂ and traces of CH₄. Water content of most inclusions is below the detection limits of the respective methods used. Aqueous inclusions are rare in all samples with the exception of those from the granite-hosted Ayanfuri mineralisation. Here inclusions associated with the gold mineralisation contain a low salinity (<6 eq.wt.% NaCl) aqueous solution with variable quantities of CO₂. Microthermometric investigations revealed densities of the gaseous inclusions of 0.65 to 1.06 g/cm³ at Ashanti, 0.85 to 0.98 g/cm³ at Prestea, up to 1.02 g/cm³ at Konongo-Southern Cross, and 0.8 to 1.0 g/cm³ at Abosso/Damang. The fluid inclusion data are used to outline the PT ranges of gold mineralisation of the respective gold deposits. The high density gaseous inclusions found in the auriferous quartz at Ashanti and Prestea imply rather high pressure trapping conditions of up to 5.4 kbar. In contrast, mineralisation at Ayanfuri and Abosso/Damang is inferred to have occurred at lower pressures of only up to 2.2 kbar. Mesothermal gold mineralisation is generally regarded to have formed from fluids characterized by H₂O > CO₂ and low salinity ( ±  6 eq.wt.%NaCl). However, fluid inclusions in quartz from the gold mineralisations in the Ashanti belt point to distinctly different fluid compositions. Specifically, the predominance of CO₂ and CO₂ >> H₂O have to be emphasized. Fluid systems with this unique bulk composition were apparently active over more than 200␣km along strike of the Ashanti belt. Fluids rich in CO₂ may present a hitherto unrecognised new category of ore-forming fluids. Received: 30 May 1996 / Accepted: 8 October 1996     

Sulfide Oxidation and Production of Gossans,Ashanti Mine,Ghana

At the Justice mine, in the Ashanti goldfields of southwestern Ghana, chemical weathering of gold- bearing sulfide-rich lodes has produced a series of characteristic mineralogical and geochemical features that are diagnostic. In this type of gold mineralization, the most abundant sulfides are arsenopyrite, pyrite, pyrrhotite, and chalcopyrite with minor bornite and sphalerite. Gold occurs predominantly as native gold, spatially associated and chemically bound with arsenopyrite. Elsewhere gold-silver tellurides are present in quartz veins. During sulfide oxidation, arsenopyrite is replaced by amorphous and crystalline Fe-Mn arsenates, goethite, hematite, and arsenolite in box- and ladderwork textures. In the extremely weathered gossans exposed at surface or in exploration pits, goethite, hematite, and scorodite are present as pseudomorphs of oxidized arsenopyrite, which can be used as a visual pathfinder for gold-arsenic mineralization. As with arsenopyrite, pyrite and pyrrhotite alteration produces boxwork and ladderwork textures with the sulfide replaced by goethite, hematite, and a complex limonite. Copper sulfides and goethite replace bornite and chalcopyrite in ladder-type textures. With more intensive weathering, this assemblage is replaced by cuprite, goethite, and hematite. Gold mineralogy in the gossan is complex, with evidence of in situ precipitation of supergene gold as well as alteration of hypogene native gold. The concentration of pathfinder elements decreases in the gossan as a result of supergene leaching. Mass- balance calculations confirm that gossan production largely is in situ and, consequently, the hypogene geochemical dispersion patterns are preserved even though the proportion of many elements decreases as intensity of weathering increases.

The problem remains of discriminating between auriferous and non-auriferous gossans, or those produced by pedological concentration of iron. Although mineral textures such as box-and ladderwork replacement and mineral pseudomorphs are useful field criteria, the most reliable guide for evaluation still is trace-element geochemistry. By use of multi-element discriminant analysis, gossans of different origins can be distinguished (along with their surface expression) from ironstones and barren lateritic soils. In regional reconnaissance studies, the evaluation of trace-element geochemistry as a discriminant along with field mapping may indicate gold potential of even extremely altered products of mineralization and, in so doing, provide a basis for the classification of weathered samples.     

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.     

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.     

Mineralogical siting and distribution of gold in quartz veins and sulfide ores of the Ashanti mine and other deposits in the Ashanti belt of Ghana: genetic implications

The Ashanti belt of Ghana constitutes a gold province which has produced a total of about 1500 t of gold historically. Gold mineralization is found in steep, NNE-SSW to NE-SW trending shear zones predominantly transecting metasediments of the Palaeoproterozoic Birimian Supergroup (2.2–2.1 Ga), disseminated in ca. 2.1 Ga granitoids, in paleo-conglomerates of the Tarkwaian Group (< 2135 Ma), and in recent placers. The distribution of gold, its chemistry, paragenesis and mineralogical siting in the mesothermal ores of the major mines in the Ashanti belt, namely Konongo, Ashanti, Bogosu and Prestea mine, are the subject of this study. At the localities studied, gold is present in two main types of ores: 1. Quartz veins with free-milling gold. The gold is relatively silver-rich (true fineness values from 730 to 954) and is accompanied by a distinct suite of Cu, Pb, Sb sulfides. 2. Sulfide ores, consisting of arsenopyrite, pyrite and rarer pyrrhotite and marcasite, with refractory gold. The ores have apparent fineness values larger than 910. Arsenopyrite and locally (at Bogosu) pyrite were identified as the hosts of submicroscopic gold. Mean concentrations of gold in arsenopyrite in various samples from the different mines, obtained by secondary ion mass spectrometry (SIMS), range from 67 to 314 ppm Au. Gold concentration mapping in individual arsenopyrite crystals from the different deposits revealed similar patterns of gold distribution: the grains possess a gold-poor core, and elevated gold contents are present along distinct crystal growth zones towards their rims. The outermost crystal layer is usually gold-poor. The well-preserved distribution patterns also indicate that remobilization of gold from the sulfides played an insignificant role in the ores of the Ashanti belt. Multiple quartz veining and growth zoning of the sulfides are interpreted as manifestations of multiple episodes of fluid infiltration, fluid flow and mineral deposition. The bimodal occurrence of gold in spatially closely associated quartz vein and sulfide ores indicates a genetic link between these ore types. A model implying a grossly coeval formation of the ores from mesothermal fluids is proposed. Received: 28 September 1995 / Accepted: 11 June 1996     

机器学习与成矿预测:以闽西南铁多金属矿预测为例

作为近年来爆炸式发展的方法模型,机器学习为地质找矿提供了新的思维和研究方法.本文探讨矿产预测研究的理论方法体系,总结机器学习在矿产预测领域的特征信息提取和信息综合集成两个方面的应用现状,并讨论机器学习在矿产资源定量预测领域面临的训练样本稀少且不均衡、模型训练中缺乏不确定性评估、缺少反哺研究、方法选择等困难和挑战.进一步...     

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.     

Archaean lode gold mineralisation in banded iron formation at the Kalahari Goldridge deposit, Kraaipan Greenstone Belt, South Africa

The Kalahari Goldridge Mine is located within the Archaean Kraaipan Greenstone Belt, about 60 km southwest of Mafikeng in the North West Province, South Africa. The ore body thickness varies from 15 to 45 m along a strike length of about 1.5 km within approximately N–S striking banded iron formation (BIF). The stratabound ore body is hosted primarily by BIF, which consists of alternating chert and magnetite–chlorite–stilpnomelane–sulphide–carbonate bands of millimetre- to centimetre scale. A footwall of sericite–carbonate–chlorite schist underlain by mafic amphibolite occurs to the west and carbonaceous metapelites in the hanging wall to the east. Overlying the hanging wall, carbonaceous metapelites, units of coarse-grained metagreywackes fining upwards, become increasingly conglomeratic up the stratigraphy. Small-scale isoclinal folds, brecciation, extension fractures and boudinage of cherty BIF units reflect brittle-ductile deformation. Fold axial planes have foliation, with subvertical plunges parallel to prominent rodding and mineral lineation in the footwall rocks. Gold mineralisation is associated with two generations of quartz–carbonate veins, dipping approximately 20° to 40° W. The first generation consists of ladder-vein sets (group IIA) preferentially developed in centimetre-scale Fe-rich mesobands, whereas the second generation consists of large quartz–carbonate veins (group IIB), which locally crosscut the entire ore body and extend into the footwall and hanging wall. The ore body is controlled by mesoscale isoclinal folds approximately 67° E, orthogonal to the plane of mineralised, gently dipping veins, defining the principal stretching direction and development of fluid-focussing conduits. The intersections of the mineralised veins and foliation planes of the host rock plunges approximately 08° to the north. Pervasive hydrothermal alteration is characterised by chloritisation, carbonatisation, sulphidation and K-metasomatism. Gold is closely associated with sulphides, mainly pyrite and pyrrhotite, and to a lesser extent, with bismuth tellurides and carbonate minerals. Mass balance transfer calculations indicate that hydrothermal alteration of BIF involved enrichment of Au, Ag, Bi, Te, S and CO₂ (LOI), MgO, Ba, K and Rb, but significant depletion of SiO₂ and, to a lesser extent, Fe₂O₃. Extensive replacement of magnetite and chlorite in BIF and other pelitic sedimentary rocks by sulphide and carbonate minerals, both on mesoscopic and microscopic scales, is evidence of interaction of CO₂- and H₂S-bearing fluids with the Fe-rich host rocks. The fineness of gold grains ranges from 823 to 921, similar to that of other epigenetic Archaean BIF-hosted gold deposits, worldwide.     

Mapping mineral prospectivity for Cu polymetallic mineralization in southwest Fujian Province,China

In this study, both the fuzzy weights of evidence (FWofE) and random forest (RF) methods were applied to map the mineral prospectivity for Cu polymetallic mineralization in southwestern Fujian Province, which is an important Cu polymetallic belt in China. Recent studies have revealed that the Zijinshan porphyry–epithermal Cu deposit is associated with Jurassic to Cretaceous (Yanshanian) intermediate to felsic intrusions and faulting tectonics. Evidence layers, which are key indicators of the formation of Zijinshan porphyry–epithermal Cu mineralization, include: (1) Jurassic to Cretaceous intermediate–felsic intrusions; (2) mineralization-related geochemical anomalies; (3) faults; and (4) Jurassic to Cretaceous volcanic rocks. These layers were determined using spatial analyses in support by GeoDAS and ArcGIS based on geological, geochemical, and geophysical data. The results demonstrated that most of the known Cu occurrences are in areas linked to high probability values. The target areas delineated by the FWofE occupy 10% of the study region and contain 60% of the total number of known Cu occurrences. In comparison with FWofE, the resulting RF areas occupy 15% of the study area, but contain 90% of the total number of known Cu occurrences. The normalized density value of 1.66 for RF is higher than the 1.15 value for FWofE, indicating that RF performs better than FWofE. Receiver operating characteristics (ROC) were used to validate the prospectivity model and check the effects of overfitting. The area under the ROC curve (AUC) was greater than 0.5, indicating that both prospectivity maps are useful in Cu polymetallic prospectivity mapping in southwestern Fujian Province.     

Mossman orogenic gold province in north Queensland,Australia: regional metallogenic controls and undiscovered gold endowment

The Hodgkinson and Broken River provinces of the Mossman Orogen in north Queensland host numerous orogenic gold deposits and still remain under-explored. This paper discusses regional metallogenic controls and results of a probabilistic quantitative assessment of undiscovered gold potential in the region. Significant orogenic gold deposits in the region occur only within relatively small well-endowed metallogenic zones, likely to be controlled by the eastern margin of the Paleoproterozoic continental crust underlying the western Mossman Orogen. Three distinct styles of primary orogenic gold deposits are present in the area: gold–quartz veins, refractory gold associated with quartz–pyrite–arsenopyrite veins and stockworks and stibnite–quartz±gold veins. Refractory gold deposits are estimated to have the highest potential for significant undiscovered resources in the region. The Hodgkinson Province is estimated to host between one and ten significant undiscovered refractory gold ore fields, with a 50 % probability of at least 20 t of total contained gold and a 90 % probability of at least 1 t. The Broken River Province is estimated to host up to five significant undiscovered refractory gold ore fields, with a 50 % probability of at least 12 t of contained gold.     

Natural and artificial radioactivity distribution in soil, rock and water of the Central Ashanti Gold Mine, Ghana

Direct gamma spectrometry study was carried out within and around the operational area of the Central Ashanti Gold of Ghana to ascertain the baseline radioactivity levels of naturally occurring radioactive materials (NORM), prior to processing of gold ore at the mine. The average activity concentrations of ²³⁸U, ²³²Th, ⁴⁰K and ¹³⁷Cs in the soil/rock samples were 64.3, 68.4, 1243.9 and 3.5 Bqkg^?1, respectively. For the water samples, the average activity concentrations were 2.5, 2.6, 14.7 and 0.7 BqL^?1 for ²³⁸U, ²³²Th, ⁴⁰K and ¹³⁷Cs, respectively. The total annual effective dose to the public was estimated to be 0.141 mSv which is below the ICRP recommended level of 1 mSv for public exposure control. The average radium equivalent activity value was 257.8 Bqkg^?1 in the range of 72.9–402.3 Bqkg^?1. The average values of external and internal indices were 0.7 and 0.8, respectively. The concentrations of U, Th and K were variable in soil and rock samples taken from different locations in the study area with values varying in the range of 0.8–10.9, 2.4–39.4 μg/g and 0.7–6.3 %, respectively. The concentration values of gross-α and gross-β for all the water samples were below the Ghana Standards Board and Who Health Organisation recommended guideline values for drinking water quality. The results obtained in this study shows that radiation levels are within the natural background radiation levels found in literature and compared well with similar studies for other countries.     

Geochemistry and mineral chemistry of lode gold mineralisation, SE Egypt: implications for ore genesis and exploration

Orogenic, lode gold mineralisation in the South Eastern Desert of Egypt is related to quartz veins spatially and temporally associated with conjugate NW- and NE-trending brittle–ductile shear zones. These structures are assumed to be linked to a regional transpression deformation which occurred late in the tectonic evolution of the area. In the Betam deposit, gold is confined to quartz(±carbonate) veins cutting through tectonised metagabbro and metasedimentary rocks in the vicinity of small granite intrusions. The ore bodies contain ubiquitous pyrite and arsenopyrite, in addition to minor disseminated chalcopyrite, pyrrhotite, galena, tetrahedrite and rare gold/electrum. New ore microscopy and electron microprobe studies indicate that most free-milling Au is intimately associated with the late-paragenetic galena–tetrahedrite–chalcopyrite assemblage. An early Fe–As sulphide assemblage, however, shows minor traces of refractory gold. New mineralogical and geochemical data are used to better constrain on possible element dispersions for exploration uses. This study indicates that parameters that most consistently define primary dispersion of gold in the mine area include pervasive silicification, sericite and carbonate alteration. The trace element data of gold lodes reflect a systematic dispersion of gold and certain base metals. Low-cost, extensive exploration programs may use elevated concentrations of Ag, Sb, Cu and Pb as tracers for Au ore zones in the Betam mine area and surroundings.     

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.     

Relationship of paleosubduction regime and prospectivity of an epithermal gold field,coromandel, new zealand

The volcanic rocks of the Coromandel Peninsula, North Island, New Zealand, were formed during a late Miocene convergence of the Pacific and Indo-Australian plates. The consequence of varying convergence velocity at the subducting plate margin is tensional regimes and basement collapse between 17–21 Ma and again at 3 Ma. The regions of basement collapse may be expected to be more fractured and might be anticipated to allow the penetration of late-stage magmatic fluids more easily. The distribution of high- chloride hot springs in the region is consistent with this notion. Recent discovery of economic quantities of gold deposited in geothermal wells at Broadlands (Ohaaki) and Wairakei in the nearby Taupo volcanic zone suggests, by analogy, that Coromandel gold may have been similarly transported. If so, then gold mineralization prospectivity may be expected to vary in a way similar to the regional variation in the chloride content of hot springs. On this basis the northernmost Coromandel would appear to be even more highly mineralized than the southern portion, where most of the large producing mines have been sited to date.     

Orebody geometry in lode gold deposits from Zimbabwe: implications for fluid flow,deformation and mineralization

The geometry of some orebodies can be described simply and accurately by three orthogonal axes, U≥V≥W. The ratios between these axes can be expressed as a parameter j=(U/V−1)/(V/W−1), and represented by a graph of U/V plotted against V/W, analogous to the treatment of strain ellipsoids. The orientations of orebodies can be plotted simply on projections using the UVW axes. Measurements of ore bodies from two examples of lode gold deposits from the Zimbabwe craton show that most of these orebodies are oblate. However, orebodies can have significant U/V ratios, implying a component of pipe-like fluid flow during mineralization. Pipe flow is demonstrated to be orders of magnitude more conductive than flow in planar veins and faults. There are significant variations in orebody geometry between deposits and within different sections of a single deposit. W values appear to be influenced by host rock: more permeable rocks have higher W. A negative trend of j value with orebody volume indicates that orebodies do not evolve in a self-similar way, but tend to more oblate shapes with increasing volume.