Palaeoenvironmental significance of rounded pyrite in siliciclastic sequences of the Late Archaean Witwatersrand Basin: oxygen‐deficient atmosphere or hydrothermal alteration?

Petrographic and sulphur isotope studies support the long‐held contention that rounded grains of pyrite in siliciclastic sequences of the Late Archaean Witwatersrand Supergroup originated as placer grains. The grains are concentrated at sites where detrital heavy minerals are abundant within quartz‐pebble conglomerates and quartzose sandstones. Depositional sites with abundant pyrite are: (1) within the matrix of bar‐type, clast‐supported conglomerates; (2) on scoured or winnowed surfaces; and (3) on stratification planes. The grains are internally compact or porous, with truncation of internal structure at outer margins indicating fragmentation and rounding of pyritic source‐rocks during erosion and sediment transport. A large range in textures reflects source‐rock lithologies, with known varieties linked to sedimentary‐hosted diagenetic pyrite, volcanic‐hosted massive sulphide deposits and hydrothermal pyrite. Laser ablation sulphur isotope analysis of pyrite reveals a broader range in δ³⁴S values (? 5·3 to + 6·7‰) than that of previously reported conventional bulk‐grain analyses (? 1 to + 4‰). Rounded pyrite from the Steyn Reef has significant variation in δ³⁴S values (? 4·7 to + 6·7‰) that establishes heterogeneous sulphur compositions, with even adjacent grains having diverse isotopic signatures. The heterogeneity supports a placer origin for rounded pyrite. Euhedral pyrite and pyrite overgrowths which are undoubtedly authigenic have restricted δ³⁴S values (? 0·5 to + 2·5‰), are chemically distinct from rounded pyrite and are probably the products of metamorphism or hydrothermal alteration. The placer origin of rounded pyrite indicates that pyrite was a stable heavy mineral during erosion and transport in the early atmosphere. Its distribution in three sequences (Witwatersrand Supergroup, Ventersdorp Contact Reef and Black Reef), and in other sequences not linked to Witwatersrand‐type Au‐U ore deposits, implies deposition of redox‐sensitive detrital heavy minerals during the Late Archaean. Consequently, rounded grains of detrital pyrite are strong indicators of an oxygen‐poor atmosphere. While not confirming a placer origin for gold in Witwatersrand Au‐U ore deposits, the palaeoenvironmental significance of rounded pyrite negates its link to hydrothermal mineralization.     

Detrital pyrite in Witwatersrand gold reefs: X-ray diffraction evidence and implications for atmospheric evolution

Pyrite is easily oxidized, and therefore unambiguous evidence of detrital pyrite grains in metasediments is a significant constraint on when an oxygenated atmosphere developed. Compact rounded pyrite in the Witwatersrand gold reefs of South Africa has a detrital habit and is texturally equivalent to and spatially associated with detrital zircon and chromite. X-ray precession photography reveals that petrographically featureless As-poor grains are untwinned single crystals of high diffraction quality. This new evidence from crystallography is consistent with mechanically abraded pyrite from primary lode gold deposits, and excludes an origin by replacement of a pre-existing detrital phase. Further evidence of a detrital origin for the compact rounded pyrite is afforded by isolated grains of arsenian pyrite displaying truncated As-rich growth bands. The geographically extensive Witwatersrand fluvial conglomerates evidently had a matrix of quartz and pyrite sand and pyritic mud in their unconsolidated state and, thus, the late Archean atmosphere of Earth was likely essentially anoxic.     

Gold and sulphide minerals in Tertiary quartz pebble conglomerate gold placers, Southland, New Zealand

Auriferous quartz pebble conglomerates (QPC) formed during Tertiary sedimentary recycling in the Waimumu district, Southland, New Zealand. These sediments contain fine-grained gold of detrital origin with abundant surface textures and gold-forms associated with authigenic gold remobilisation. Most authigenic gold contains no detectable silver and occurs as overgrowths on detrital Au–Ag and Au–Ag–Hg alloys that contain up to 13 wt.% Ag, and 9 wt.% Hg. Fine-grained Au–Ag and Au–Ag–Hg alloys are compositionally heterogeneous, exhibiting both well-defined silver-depleted and silver-enriched rims. Rare coarse Au–Ag alloy is intergrown with quartz and is homogenous. Discrete grains of authigenic, porous, sheet-like gold occur in carbonaceous mudstone within a QPC sequence. Some QPC contain abundant sulphide minerals. Some of these sulphides (pyrite and arsenopyrite) are of long-distance detrital origin, presumably from the Otago Schist, whereas the bulk of the sulphide suite is marcasite of variably transported diagenetic origin, derived from the erosion of QPC and underlying Tertiary sediments. There has also been authigenic deposition of sulphide minerals in the QPC themselves. These diagenetic sulphides include framboidal and anhedral marcasite, and framboidal and euhedral pyrite. Sulphur isotope data for the sulphide minerals range from − 45‰ to + 18‰ (relative to VCDT). Sulphur isotope data for euhedral detrital pyrite and arsenopyrite range from − 9‰ to − 1‰ and are most likely derived from the Otago Schist to the north. Both framboidal and anhedral marcasite have lower values (< − 20‰) reflecting microbial sulphate reduction as a source for the precursor hydrogen sulphide. Anhedral marcasite contains elevated concentrations of Ni, Co, As and Cr, commonly with compositional banding of these metals.Both the gold and diagenetic sulphides from the Belle-Brook QPC are compositionally similar to gold and sulphides from Archaean QPC. Porous, sheet-like authigenic gold is morphologically similar to gold associated with carbonaceous material in the Witwatersrand. In addition, Southland marcasite textures resemble the rounded and banded pyrite in Witwatersrand QPC placers. There is abundant evidence from these Tertiary QPC in southern New Zealand for sedimentary transport of sulphide minerals and post-depositional sulphide mineralisation in the surficial environment despite an oxygen-rich atmosphere. These young deposits thus provide an example of authigenic gold and sulphide textures formed during diagenesis in unmetamorphosed placers. Many of these textures are similar to those commonly ascribed to metamorphic processes in Archaean auriferous QPC.     

Trace element distribution in uraninite from Mesoarchaean Witwatersrand conglomerates (South Africa) supports placer model and magmatogenic source

The first systematic analyses of the trace and rare earth element (REE) distribution in uraninite from various gold-bearing conglomerates of the Mesoarchaean Central Rand Group in South Africa’s Witwatersrand Basin by in situ laser ablation-inductively coupled plasma-mass spectrometry confirms a placer origin for the uraninite and a magmatogenic provenance thereof. The chemistry of commonly rounded to sub-rounded uraninite is highly variable from grain to grain but generally marked by elevated Th, W, Bi, Mo, Ta, Y, REE contents and unusually high Au concentrations. Especially, the high Th contents and the chondrite-normalised REE patterns are incompatible with post-sedimentary hydrothermal genetic models for the U mineralisation and point to derivation of the detrital uraninite from a high-temperature, magmatogenic, presumably granitic to pegmatitic source. The elevated Au concentrations (of as much as 67 ppm) in this uraninite are unique to the Witwatersrand and hint at a granitic hinterland that was enriched in both U and Au, thus presenting a potential source domain for some of the detrital gold in the Witwatersrand conglomerates. Minute fracture fills of brannerite in close proximity to the larger, rounded uraninite grains are devoid of detectable Bi, Mo, REE and Au and have only very low concentrations of Th, W, Ta and Y. This is explicable by crystallisation from a low-temperature hydrothermal fluid. Thus, Witwatersrand U phases show, analogous to many other ore constituents, such as pyrite and gold, clear evidence of partial, short-range mobilisation of originally detrital particles by post-sedimentary fluids.     

Microstructural evolution and trace element mobility in Witwatersrand pyrite

Microstructural analysis of pyrite from a single sample of Witwatersrand conglomerate indicates a complex deformation history involving components of both plastic and brittle deformation. Internal deformation associated with dislocation creep is heterogeneously developed within grains, shows no systematic relationship to bulk rock strain or the location of grain boundaries and is interpreted to represent an episode of pyrite deformation that predates the incorporation of detrital pyrite grains into the Central Rand conglomerates. In contrast, brittle deformation, manifest by grain fragmentation that transects dislocation-related microstructures, is spatially related to grain contacts and is interpreted to represent post-depositional deformation of the Central Rand conglomerates. Analysis of the low-angle boundaries associated with the early dislocation creep phase of deformation indicates the operation of <010>{100} slip systems. However, some orientation boundaries have geometrical characteristics that are not consistent with simple <010>{100} deformation. These boundaries may represent the combination of multiple slip systems or the operation of the previously unrecognized <001>{120} slip system. These boundaries are associated with order of magnitude enrichments in As, Ni and Co that indicate a deformation control on the remobilization of trace elements within pyrite and a potential slip system control on the effectiveness of fast-diffusion pathways. The results confirm the importance of grain-scale elemental remobilization within pyrite prior to their incorporation into the Witwatersrand gold-bearing conglomerates. Since the relationship between gold and pyrite is intimately related to the trace element geochemistry of pyrite, the results have implications for the application of minor element geochemistry to ore deposit formation, suggest a reason for heterogeneous conductivity and localized gold precipitation in natural pyrite and provide a framework for improving mineral processing.     

Witwatersrand gold particle chemistry matches model of metamorphosed, hydrothermally altered placer deposits

Analyses of gold particles from the Ventersdorp Contact Reef in the Klerksdorp and Carletonville goldfields of the Late Archaean Witwatersrand Basin reveal a wide range in Au, Ag and Hg concentrations of 80.9 to 92.9, 6.0 to 17.6, and 0.6 to 5.8 wt.%, respectively. However, individual gold particles are generally homogeneous. This observation is predicted by modelling of Ag and Hg diffusion through gold at the peak metamorphic temperature of about 300 °C. The only exception is gold in hydrothermal quartz veins, the formation of which can be ascribed to the mobilisation of originally detrital gold particles during a chloritisation event that was triggered by tectonically expelled fluids, as a consequence of the 2023 Ma Vredefort meteorite impact event. Inter-particle homogenisation on a hand-sample scale was generally not achieved. The mean gold composition for the same reef in different mines (kilometre scale) can vary drastically. These results confirm the model of a metamorphosed and hydrothermally altered placer deposit. Furthermore they indicate that, in spite of a significant post-depositional modification of the composition of individual gold particles, the mean gold composition at a given reef locality may be used to distinguish different source areas of the originally detrital particles. Received: 3 December 1996 / Accepted: 19 May 1997     

Trace-element characteristics of different pyrite types in Mesoarchaean to Palaeoproterozoic placer deposits

A comparative in situ LA-ICP MS trace-element study on pyrite from three different, variably auriferous, Archaean to Palaeoproterozoic palaeoplacer deposits in the Ouro Fino Syncline (Quadrilátero Ferrífero; Brazil), the Elliot Lake area north of Lake Huron (SE Canada) and several deposits within the Witwatersrand Basin (South Africa) revealed systematic differences between morphologically different pyrite types and between the various palaeoplacer deposits. Especially the Ni and Au concentrations as well as Co/Ni and Mo/Ni ratios were found to be systematically different in detrital compact, detrital porous and post-sedimentary/hydrothermal pyrite grains from different source areas. High Co/Ni ratios and low Au concentrations are typical of post-sedimentary pyrite, which is hydrothermal in origin. In contrast, relatively low Co/Ni ratios and high Au contents characterise detrital porous banded and concentric pyrite grains (Au > 1 ppm), which are syn-sedimentary in origin. In the Elliot Lake area and the Witwatersrand Basin, detrital compact rounded pyrite is characterised by high Co/Ni ratios, which is in agreement with derivation from a hydrothermal source. Low Au concentrations in this pyrite type support the contention of the gold and the pyrite in these deposits coming from different source rocks. In contrast, derivation from an originally diagenetic pyrite is suggested for the detrital compact pyrite in the Ouro Fino Syncline because of low to intermediate Co/Ni ratios. High Au contents may indicate a genetic relationship between pyrite and gold there. Systematic differences exist between the three areas with respect to Au, Ni, Co, Mo and Cu distributions in detrital pyrite, which reflects differences in the provenance. A predominantly mafic/ultramafic source is indicated for the Ouro Fino, a felsic source for the Elliot Lake, and a mixed felsic–mafic provenance for the Witwatersrand pyrite populations. Independently of pyrite type, the higher Au endowment of the studied Witwatersrand and Ouro Fino conglomerates are also reflected by an overall higher Au concentration in the respective pyrite grains compared to the relatively Au-poor samples from Elliot Lake. In general, a strong positive correlation between Au and Pb levels in the various pyrite grains is noted. Analogous to Pb, which is well known for not being easily accommodated in the pyrite crystal lattice but occurring as discrete PbS phases, Au is considered to be present mainly in the form of discrete Au phases in minute pores and interstices of the pyrite grains rather than within the pyrite lattice.     

大平沟金矿床矿石特征与金的赋存状态

大平沟金矿床是受韧性剪切带控制的中温动力变质热液矿床，金矿石主要为蚀变糜棱岩型，夹少量钾长石石英脉型，矿石结构有变晶结构、交代－充填结晶结构两主要类型，矿石构造以块状构造、团块状构造、细脉状构造和浸染状构造为主。金呈独立金矿物（主要为自然金）出现，以包体金、裂隙金、连生金和粒间金等形式嵌布于黄铁矿、黄铜矿、石英、钾长石及方解石等主要载金矿和中，金矿物形态多样，粒度以中细粒为主。上述特点与我国东部地区产于太古变质岩（绿岩带）中的金矿床具有可对比性，也与矿床成因研究的认识相吻合。     

The Discovery of Native Ruthenium in Nuanquanzi and Tuling—Shihu Gold Deposits：Implications for Source of Gold

Native ruthenium and platinum-bearing hedleyite were recognized two gold deposits contained in Archaean metamorphic rocks in northern China.They are coexistent with native gold,quartz and pyrite.The high W content of native ruthenium may reflect the precipitation of ores in a W-rich hydrothermal system at moderate to high temperatures,The presence of platinum-group minerals(PGM)in the two deposits suggests that Au and PGE were both derived from mantle-source rocks.     

Chemical and structural relations of epitaxial xenotime and zircon substratum in sedimentary and hydrothermal environments: a TEM study

Xenotime overgrowths on detrital zircon in siliciclastic sediments have been reported in numerous studies. However, in natural samples, solid solution of zircon and xenotime is limited to near-end-member compositions. In order to characterize the interface region between both minerals and to draw inferences on the growth mechanisms of authigenic xenotime, we studied xenotime overgrowths on detrital zircon grains from two Phanerozoic sandstone samples with contrasting post-depositional histories. In one sample, the small (≤10 μm), pyramidal xenotime overgrowths are of diagenetic origin and grew without major discontinuity on the detrital zircon grain. The second sample shows up to >50-μm-wide, porous and inclusion-rich, hydrothermal xenotime overgrowths on detrital zircon, whereas the transition zone between both minerals is accompanied by large pore volume. Chemical compositions of the xenotime precipitates from the two samples differ particularly in Y, REE, Th and Sc concentrations, whereas high MREE availability in the diagenetic sample and the presence of Sc in the hydrothermal sample, respectively, appear to have promoted xenotime growth. Transmission electron microscopy on electron-transparent foils cut from the interface region shows that both the diagenetic xenotime and the hydrothermal xenotime are crystalline and grew in optical and crystallographic continuity to their detrital zircon substrata. Only a narrow transition zone (≤90 nm—diagenetic sample, 200–300 nm—hydrothermal sample) between zircon and xenotime is in part made up of nanometre-scale crystalline domains that are slightly distorted and may have formed from dissolution–re-precipitation processes at the zircon rim along with precipitation from the respective fluid.     

Silver and mercury in single gold grains from the Witwatersrand and Barberton,South Africa

The contents of silver and mercury in 323 spots on gold grains from seven localities of the Witwatersrand palaeo-placer and Archaean vein deposits from Barberton were measured using an electron microprobe. The objective was to obtain information on the extent of gold alteration during fluvial transport and post-depositional geological processes. The results, however, show that Ag and Hg are distributed homogeneously in the gold grains studied. No indications were found that the gold was transported in solution nor that leaching took place in an oxidizing fluvial environment. This strongly suggests that the Ag and Hg contents in Witwatersrand gold grains represent geochemical ‘fingerprints’ inherited from their eroded primary sources. Combined analysis for Ag and Hg in Witwatersrand gold grains by electron microprobe can therefore be a valuable tool in establishing the types of primary sources for the gold.     

Metamorphism of the Witwatersrand gold fields: conditions during peak metamorphism

Abstract Pelitic assemblages from all major Witwatersrand gold fields record metamorphic conditions of the greenschist facies, with minimal regional grade changes over at least 200 km strike length. Diagnostic metamorphic assemblages are less common in the volumetrically dominant quartzites, the actively-exploited auriferous conglomerates and some of the regionally persistent metapelitic horizons. Bulk rock composition has been a major control on assemblage development.
Key metapelitic assemblages include pyrophyllite, chloritoid, chlorite and muscovite in each gold field, with less common metamorphic biotite. Accessory minerals are pyrite, tourmaline, rutile and zircon. The abundance of chloritoid and pyrophyllite in thin shaly units, together with their minor, but widespread, distribution in quartzites and conglomerates, indicate that metamorphic temperatures reached 350°C ± 50°C in all the gold fields. Pressures are less-well constrained, 1–2 kbar being inferred. Outside the gold fields, higher grades are indicated by andalusite and kyanite near granitoid domes and later intrusions.
The temperatures during peak metamorphism and the abundance of pyrite provide ideal conditions to (re)mobilize gold and may explain its secondary textural features.     

Relation between U-series nuclide migration and microstructural properties of sedimentary rocks

Natural radionuclide migration in Miocene sedimentary rocks has been studied at the Tono U deposit in Japan, which is a potentially useful analogue of radioactive waste isolation in geological environments.Uranium series disequilibrium studies have revealed that natural radionuclides have been redistributed during the past 3.5 × 10⁵ a in the U-mineralized zone. Permeability tests using core samples of the U-mineralized zone have shown that the microfabrics of sedimentary rocks, such as the connectivity of pores which control the groundwater movement, directly influence the redistribution of nuclides.Detailed observations using cathodoluminescence (CL) and electron probe microanalysis (EPMA) were carried out after a dye-impregnation test in order to improve the understanding of the importance of microfabrics in relation to nuclide migration. Results of the observations show that U has migrated within detrital grains, such as biotite and quartz, as well as between these grains. Uranium-series disequilibrium studies of each detrital quartz and biotite grain have been shown that these detrital minerals behave differently in respect of the migration of U, owing to their different textural properties. The detrital biotite flakes, especially along the (001)-cleavage planes, appear to have fixed U for a long period of time, whereas the U has remained mobile within the microfractures in detrital quartz grains.     

Auriferous Precambrian conglomerates of Witwatersrand

Geological and mineralogical specificity of the world’s largest Witwatersrand gold deposit was defined by geodynamic processes in the Late Archean. The primary crust composed of felsic rocks (“gray gneisses”) was intruded 2.9–3.1 Ga ago by mafic melts that gave rise to greenstone belts. This was followed by the appearance of long-lived granulite mobile zones that promoted the formation of hydrosphere and atmosphere accompanied by an intense acidic leaching (weathering) of rocks in the greenstone belts. Numerous conglomerate interbeds were formed in the Witwatersrand Basin section due to vigorous eolian processes and floodwater washouts that produced a fan channel system mainly filled with quartz pebbles. At present, most researchers support a modified paleoplacer model of the origin of gold mineralization in Witwatersrand, suggesting a hydrothermal-metamorphic redistribution of the primary placer gold. According to various hydrothermal models, gold was introduced into sedimentary rocks in a water medium from outer deep sources or during the filtration of postsedimentary hydrothermal fluids. The present communication suggests that a significant portion of gold contained in reefs could be delivered to the sedimentation basin by the auriferous hydrothermal quartz of pebble or sand dimension that was metamorphosed at approximately 350–400°C. Metamorphism of gold was accompanied by its purification, transfer to matrix, and hydrothermal intrastratal redistribution.     

Detrital origin of hydrothermal Witwatersrand gold—a review

The Witwatersrand 'basin' is the largest known gold province in the world. The gold deposits have been worked for moren than 100 years but there is still controversy about the ore forming process. Detailed petrographic studies often reveal that the gold is late in the paragenetic sequence, which has led many researchers to propose a hydrothermal origin for the gold. However, observations, such as the occurrence of rounded, disc-like gold particles next to irregularly shaped or idiomorphic secondary gold particles in the same sample, suggest an initial detrital gold source within the Witwatersrand strata. Mineral chemical and isotopic data, together with SEM cathodoluminescence imaging and fluid inclusion studies, provide evidence for small-scale variations in the fluid chemistry – a requirement for the short-range mobilization of the gold. The existing data and observations on the Witwatersrand rocks support a model of hydrothermally altered, metamorphosed placer deposits, with at least two subsequent gold mobilization events: hydrothermal infiltration in early Transvaal time (2.6–2.5 Ga) and during the 2.020 Ga Vredefort impact event.     

Progress in application of cathodoluminescence (CL) in sedimentary petrology

Summary ¶Significant progress has been made in cathodoluminescence (CL) studies in the last three decades due to the application of enhanced methods such as high-resolution spectroscopy. The luminescence of all detrital and diagenetic minerals such as quartz, feldspar, phyllosilicates, carbonates, apatites, zircons etc. can now be quantitatively characterized by spectroscopy. For siliciclastic sediments, the potential for provenance studies has been explored because varying conditions at the time of formation (e.g. of quartz and zircons) might be encoded within the CL and therefore provide fingerprints for particular grain populations. Different cement generations allow a cement stratigraphy to be established and the deconvolution of the diagenetic history of carbonate and clastic sediments. CL investigations of carbonate, apatite and – with some limitations – siliceous biogenic skeletons provide evidence of growth zonation (ontogenetic cyclicities and changes in spatial distribution) and diagenesis (intra-skeleton cementation and recrystallization).However, further research using CL is required to fully understand detrital components, primary and secondary mineralization in sedimentary deposits and biogenic materials. We present here an, admittedly incomplete, overview of these developments.Received March 6, 2002; revised version accepted January 30, 2003 Published online June 12, 2003     

石英砂岩的硅质胶结作用及其对储集性的影响

作者应用阴极发光显微镜、扫描电镜、电子探针及包体测温等手段,对华北太原组石英砂岩的硅质胶结现象、形成温度、物质来源及其对储集性的影响作了研究。结果表明:太原组石英砂岩有两期硅质胶结,第二期规模大;其形成温度为130°-140℃;砂岩处于中成岩阶段晚期;硅质胶结作用是使该砂岩丧失良好储集性能的主要因素。     

Paragenesis of mineralization within fractured pebbles in Witwatersrand conglomerates

Mineralized conglomerates in the Witwatersrand goldfields consistently exhibit intense fracturing of quartz pebbles. The fractures are coated with quartz, phyllosilicates, organic matter, uraninite, gold, and base metal sulfide minerals, showing that all of these phases were at least partially epigenetic. Samples from five goldfields (Evander, West Rand, Carletonville, Klerksdorp, Welkom) across the Witwatersrand Basin show comparable paragenetic sequences of fracture-fillings. This implies that fracturing and several episodes of mineral deposition were basinwide events, rather than the result of local processes. Fluid inclusion data for the quartz indicates precipitation from these basinwide fluids at relatively low temperatures (<250 °C), which may be consistent with other evidence for mineralization following the Vredefort Event at 2025 Ma.     

Cathodoluminescence characterization of quartz particles in mature arenites

A combination of cathodoluminescence (CL) microscopy with digital image analysis and spectroscopic analysis allows the detailed investigation of the commonly occurring colour shift from initial to final CL‐colours in detrital quartz with increasing irradiation. A method for the quantification of the colour shift is presented and applied to different quartz sands: unlithified sands of the Lower Cretaceous Kuhfeld Formation from Ottenstein (W Germany) and Holocene sands from the barrier island of Wangerooge (N Germany). The colour shift depends on the defect structure of the quartz grains. The defect structure is determined by the physico‐chemical conditions during crystallization and therefore can help trace the source of quartz grains. A total of 25 different types of quartz grains in the Ottenstein‐sands indicates a highly variable southern provenance. Many of the grains have recycled quartz cement overgrowths indicating an at least second cycle origin. In contrast, only 16 different quartz types have been observed in the Wangerooge‐sands reflecting a less varied plutonic to metamorphic Scandinavian provenance. The study demonstrates the potential of quantified CL initial colour and colour shift in assessing the provenance of quartz‐rich sands and sandstones.     

Gold mineralization in As-rich mesothermal gold ores of the Bogosu-Prestea mining district of the Ashanti Gold Belt,Ghana: remobilization of “invisible” gold

The Bogosu-Prestea mining district of southwestern Ghana is a 33 km section of the Early Proterozoic Ashanti Gold Belt. Greenschist facies carbonaceous and carbonate-bearing turbidites and greywackes, and mafic dikes host numerous economic mesothermal gold deposits. Structurally higher ores in the Bogosu concession have brittle deformation and consist of disseminated-sulphide lodes in tectonically-disrupted sedimentary rocks and carbonate-altered mafic dikes. Most gold occurs as micrometre-size particles in arsenian pyrite, and as invisible gold in arsenian pyrite and arsenopyrite. The structurally deeper ores of the adjoining Prestea concession are associated with brittle-ductile deformation and consist of extensive crack-seal quartz-veins and graphitic shear zones. Only minor amounts of invisible gold were detected; in these deeper lodes, gold occurs dominantly as abundant microscopic and larger particles in sulphide/arsenide minerals and in gangue. The gold distribution patterns revealed by SIMS microprobe analysis and ion maps, EMP and colour staining suggest that most of the primary gold in the Bogosu-Prestea system precipitated in solid-solution with sulphide/arsenide minerals. However, post-depositional concentration and redistribution occurred, in increasing degree with: 1) increase in metamorphic/hydrothermal gradients in the gold system (depth), 2) decrease in the refractory properties of the host mineral, and 3) increase in the amount of post-depositional, host-mineral recrystallization and deformation. Gold evolved from primary solid-solution within sulphide/arsenide minerals, to colloidal and micrometre-size particles concentrated in voids, fractures and internal grain boundaries, and finally to microscopic and larger particles at sulphide/arsenide grain margins and in the gangue assemblage. The general conclusions presented here are applicable to As-rich gold deposits of all ages, worldwide. The presence of gold in late fractures is insufficient evidence for late-stage introduction of gold in mesothermal gold systems.