archaeal lipid biomarkers and isotopic evidence of anaerobic methane oxidation associated with gas hydrates in the gulf of mexico abstract anaerobic oxidation of methane aom occurs in the gulf of mexico gas hydrate systems here we show lipid biomarker and isotopic evidence that archaea are involved in aom the estimated abundance of total archaeal lipids ranges from 44 8 to 60 4 g g dry sediment in hydrate bearing samples but is below detection limit in the hydrate free sample the 13c values of archaeal lipids range from 69 to 99 in hydrate bearing samples these results suggest that biomass of archaea is significantly enhanced through aom at the gas hydrate deposits these data also support a currently acknowledged mechanism of aom mediated by a consortium of sulfate reducing bacteria and archaea observed in a variety of methane rich marine settings anaerobic oxidation of oil hydrocarbons also occurs in the gulf of mexico gas hydrate systems as shown by degradation of n alkanes c15 in the anoxic sediments these processes convert hydrocarbons to carbon dioxide and increase pore water alkalinity which promote the precipitation of enormous volumes of authigenic carbonate rock depleted in 13c this long term geologic sequestration of carbon may affect models of global climate change

first whiffs of atmospheric oxygen triggered onset of crustal gold cycle lack of a suitable gold source has long been regarded as a major argument against a palaeoplacer model for the exceptionally well endowed witwatersrand gold province by comparing worldwide witwatersrand type deposits occurrences ranging in age from 3 1 to 1 8 ga we propose that the primary concentration of gold in the continental crust resulted from atmospheric and biological evolution in the mesoarchaean a high au flux off the archaean land surface orders of magnitude greater than today s was a consequence of the chemistry of the mesoarchaean atmosphere and hydrosphere when early life gradually changed from anaerobic anoxygenic to oxygenic photosynthesizers at around 3 0 ga the first whiffs of free oxygen produced by photosynthesis under an overall reducing atmosphere provided the ideal trap for au dissolved in the huge reservoir in meteoric and shallow sea water oxidative precipitation of gold on the surface of the o2 producing microbes probably cyanobacteria could fix huge amounts of gold over large areas some of this microbially mediated gold is still preserved in thin kerogen layers carbon seams that typically developed on erosional unconformities scour surfaces and bedding planes in near shore environments at around 2 9 ga this gold provided the principal source for the very rich placer deposits that formed by the subsequent sedimentary reworking of the delicate microbial mats on aeolian deflation surfaces into fluvial channels and delta deposits represented by meso to neoarchaean auriferous conglomerates tectonic reworking of these first gold rich sediments by orogenic processes explains the temporal peak of orogenic type gold formation at around 2 7 to 2 4 ga with time the strongly gold enriched archaean sediments became progressively eroded covered or tectonically reworked and their role as potential source of younger placer deposits diminished this explains why witwatersrand type deposits younger than 2 4 ga are rare and far less well endowed and effectively missing in the rock record after 1 8 ga

widespread fluid infiltration during metamorphism of the witwatersrand goldfields generation of chloritoid and pyrophyllite chloritoid and pyrophyllite occur together in all major goldfields of the witwatersrand basin and are widespread in virtually all rock types of the upper witwatersrand supergroup including metaconglomeratic reefs and altered mafic rocks both minerals are particularly characteristic of the pelitic horizons intimately associated with reef packages but they are also developed locally in the regionally persistent metapelites that have basin wide extent pyrophyllite is particularly common in foliated zones adjacent to quartz veins and near unconformably overlying auriferous conglomerates the wide distribution of chloritoid and pyrophyllite in metapelites of the witwatersrand basin is attributed to alteration of chlorite rich shales rather than to unusual premetamorphic starting materials this alteration event involved the redistribution of many elements with up to 40 volume loss mainly due to removal of silica removal of most of the mg and some fe accounts for the stabilization of chloritoid and pyrophyllite relatively immobile elements included al ti nb cr v p la and ce whereas si fe mn zn co ni cu mg and ca were lost and k rb and ba were introduced by an infiltrating fluid the alteration event is inferred to have been within the chloritoid and pyrophyllite stability field and thus syn metamorphic as bulk chemical changes in metapelites are from chlorite directly towards chloritoid and then pyrophyllite rather than to lower grade minerals such as kaolinite muscovite chlorite chloritoid and muscovite chloritoid pyrophyllite assemblages are attributed to fluid buffering along appropriate curves as their production by metamorphism of lower grade mineral mixes is considered unlikely based on the present bulk rock compositional data a metamorphic timing for the alteration accounts for the correlation of strongly foliated areas with greater degrees of inferred alteration the transitions from chlorite to chloritoid to pyrophyllite define zones of increasing alteration widespread infiltration as part of peak metamorphism is suggested by the distribution of chloritoid and pyrophyllite quartz veining and textures fluid rock ratios calculated from a silica budget in one metapelitic horizon exceed 100 1 over many square kilometres these values need not imply multi pass fluid flow as much of the silica migration may be redistribution on a scale of a few metres from source rocks into veins although infiltration during metamorphism may have affected much of the upper witwatersrand succession channelized fluid flow within reef packages along faults and unconformities and in certain metaconglomerates and metapelites is inferred

pyrite zoning as a record of mineralization in the ventersdorp contact reef witwatersrand basin south africa the archean metasedimentary succession of the witwatersrand basin south africa hosts the largest au deposit in the world gold mineralization is mostly concentrated in conglomerate horizons or reefs and is tightly associated with pyrite trace element zoning of pyrite from the ventersdorp contact reef studied by x ray elemental as ni co and pb maps electron microprobe analysis and laser ablation inductively coupled plasma mass spectrometry indicates successive stages of pyrite formation each characterized by different textures and trace element composition as 2 2 wt ni 1 37 wt co 1 98 wt four generations have been distinguished generation 1 is detrital and includes compact nonporous porous and laminated pyrite generations 2 to 4 are postsedimentary authigenic generation 4 pyrite formed at near peak metamorphic conditions t 270 350 c chlorite geothermometry porous and concentrically laminated pyrite grains generation 1 are particularly enriched in au average 6 4 ppm maximum 70 ppm in addition to sb tl pb mn mo cu and ag in comparison with compact pyrite types of all generations in these grains au occurring as invisible gold and other trace elements might be finely dispersed with the phyllosilicates filling the pyrite pores trace element composition of porous and concentrically laminated pyrite is reminiscent of pyrite known to form in suboxic to anoxic environments black shales the presence of au in detrital pyrite indicates an early introduction of au in the ventersdorp contact reef gold is also present as secondary inclusions of electrum associated with the last pyrite generation generation 4 together with sphalerite chalcopyrite galena and pyrrhotite

trace element and upb isotope compositions of pyrite types in the proterozoic black reef transvaal sequence south africa implications on genesis and age abstract the proterozoic black reef is in most cases indistinguishable from the gold uranium and pyrite bearing archaean witwatersrand reefs however the prevailing opinion that the black reef gold mineralization was largely derived from reworking of the underlying kimberley reef is not supported by the morphology trace element geochemistry and u pb isotope data on detrital pyrite in these reefs the grade of metamorphism recorded in the black reef quartzite formation is significantly lower than the 300 400 c that has been proposed for the sediments of the witwatersrand supergroup the u pb isotope and trace element geochemical signatures in pyrite have therefore been preserved in the black reef the morphological and compositional differences retained by the detrital and synsedimentary pyrite types both occurring together in the black reef imply that they were formed by different processes and in contrasting geochemical environments in the black reef the data for the detrital pyrite are consistent with their derivation from a 3000 2800 ma granitic probably hydrothermally altered source region by contrast the geochemical and pb isotopic composition of the concretionary pyrite is interpreted to reflect its synsedimentary formation in a lagoonal or tidal flat environment prior to 2500 ma the morphology primary mineral inclusions and the unradiogenic pb isotopic compositions preserved in the black reef detrital pyrite are irreconcilable with models proposed for the formation of witwatersrand pyrite by a process of pyritization of fe bearing detrital minerals fluids present in the black reef depositional diagenetic environment were anomalously enriched in radiogenic pb and any transformation process would not be able to escape acquiring this enriched pb isotopic signature

metamorphism of the witwatersrand gold fields a review abstract there is widespread evidence for greenschist facies metamorphism in different rock types within all the witwatersrand gold mines the silicate mineral assemblages suggest near uniform peak metamorphic conditions equivalent to the chlorite zone in all the major witwatersrand gold fields assemblages of coexisting pyrophyllite chloritoid have been recorded in most of the gold mines and the critical assemblage of pyrophyllite chloritoid muscovite chlorite quartz tourmaline rutile pyrite is found in most gold fields kaolinite paragonite biotite pyrrhotite and arsenopyrite have more local distribution pelites shales quartzites meta conglomerates metabasalts and some dykes record these metamorphic assemblages albeit in different mineral proportions and pyrophyllite and chloritoid are recorded from many gold producing reefs andalusite kyanite garnet and or cordierite bearing assemblages indicate higher metamorphic grades in three discrete areas along the nw margin of the witwatersrand basin to the ne of the evander gold field and around the collar of the vredefort dome concentric metamorphic facies around the vredefort dome culminate in granulite facies assemblages near the centre it is important to note that there are no definitive published data constraining the temporal relationships between the granulites and amphibolites of the vredefort dome greenschist facies assemblages in the vredefort collar rocks thermal metamorphism related to intrusion of the bushveld complex and the regional greenschist facies metamorphism documented in the gold fields metamorphic conditions of 350 50 c and up to 3 kbars are inferred for the gold fields with temperatures increasing to around 700 c in granulites of the vredefort dome other geothermometric methods including illite crystallinity fluid inclusions carbon elemental ratios and vitrinite reflectance are compatible with the above estimates but are each associated with considerable temperature uncertainties fluids in equilibrium with pyrophyllite muscovite quartz pyrite paragonite are inferred during metamorphism suggesting slightly acid conditions and elevated sulphur activities regional greenschist facies metamorphism documented in the witwatersrand gold mines was accompanied by widespread deformation large fluxes of aqueous fluid and considerable element mobility the low intermediate pressure of this metamorphic terrain and the lack of large scale tectonic thickening favours underplating by mafic magmas as the likely cause of the metamorphic pattern no model for gold genesis in the witwatersrand basin is complete without accommodating post burial processes including metamorphism deformation and alteration any placer gold would be overprinted by these processes and any epigenetic gold could potentially be introduced during these periods of thermal and fluid activity it is also important to note at least two quite unusual features of the witwatersrand metamorphic assemblages the virtual absence of feldspar from the 3 km thick central rand group over 300 km and the narrow range of metamorphic grade inferred for the gold mines over this same distance

witwatersrand gold deposits formed by volcanic rain anoxic rivers and archaean life the witwatersrand gold deposit is the largest in the world thermodynamic calculations show that such rich accumulations of gold could be linked to abundant volcanism primitive life and the oxygen free atmosphere of the archaean the witwatersrand basin in south africa is one of the best preserved records of fluvial sedimentation on an archaean continent1 the basin hosts the worlds biggest gold resource in thin pebble beds but the process for gold enrichment is debated2 3 4 5 mechanical accumulation of gold particles from flowing river water is the prevailing hypothesis2 6 yet there is evidence for hydrothermal mobilization of gold by fluids invading the metasedimentary rocks after their burial3 4 7 earth s atmosphere three billion years ago was oxygen free8 but already sustained some of the oldest microbial life on land9 here i use thermodynamic modelling and mass balance calculations to show that these conditions could have led to the chemical transport and precipitation of gold in anoxic surface waters reconciling the evidence for fluvial deposition with evidence for hydrothermal like chemical reactions i suggest that the release of sulphurous gases from large volcanic eruptions created acid rain that enabled the dissolution and transport of gold in surface waters as sulphur complexes precipitation of the richest gold deposits could have been triggered by chemical reduction of the dissolved gold onto organic material in shallow lakes and pools i conclude that the witwatersrand gold could have formed only during the archaean after the emergence of continental life but before the rise of oxygen in the earth s atmosphere

morphology of witwatersrand gold grains from the basal reef evidence for their detrital origin thousands of rounded particles of fine grained gold released from a crossbedded gold specimen of the basal reef placer in the welkom gold field after acid digestion of the host quartz arenite have revealed morphological features and abrasion textures that are characteristic of detrital gold the size range which is between 0 038 and 0 473 mm has a lognormal distribution with a mean of 2 88 phi units 0 136 mm and a standard deviation of 0 48 this is a characteristic of well sorted sediment the shapes are predominantly disclike with toroidal forms being most common a corey shape factor of 0 08 indicates extreme flattening the preponderance of these shapes is interpreted to be the result of selective sorting by entrainment of particular shapes during extensive transport a knobby surface texture similar to placer grains recovered from both recent and devonian placers and produced during laboratory controlled tumbling with sand and cobbles in water is considered distinctive of sediment abrasion secondary gold comprising 25 percent of the total gold content is finer grained has an interstitial jagged form and displays unabraded smooth to crystalline surfaces study of this remarkable specimen has produced unequivocal evidence of the detrital origin of gold in the extensive basal reef placer and led to research concerning its metamorphic overprints

the growth and concentration of uranium and titanium minerals in hydrocarbons of the carbon leader reef witwatersrand supergroup south africa abstract uranium and gold bearing pyrobitumen from the carbon leader reef in the witwatersrand basin south africa was investigated by high resolution transmission electron microscopy this study provides evidence for the in situ growth of uraninite and anatase nanocrystals in the pyrobitumen implying mobilization and concentration of uranium and titanium by formerly mobile liquid hydrocarbons individual nanocrystals of uraninite and anatase are pervasively distributed and locally isolated within the pyrobitumen matrix crystallization of uraninite and anatase led to the formation of complex nanocrystal aggregates by oriented attachment in which anatase generally provided nuclei for the growth of uraninite single nanocrystals of curite occur locally in channel ways within masses of uraninite nanocrystals consistent with later auto oxidation of uraninite and limited release of water during hydrocarbon maturation on the basis of evidence for the migration of liquid hydrocarbons in the witwatersrand basin and the presence of abundant uraninite and anatase nanoparticles in pyrobitumen a new model is proposed for the transport and concentration of uranium and titanium in the carbon leader reef according to this model liquid hydrocarbons that were circulating in the witwatersrand basin dissolved detrital u ti bearing minerals and transported the uranium and titanium until thermal degradation immobilized the hydrocarbons by solidifying them as pyrobitumen the latter process involved the release of volatiles and the destruction of bonds that may have held the uranium and titanium in solution thereby inducing the growth of individual uraninite and anatase nanocrystals and the formation of complex nanocrystal aggregates within the pyrobitumen

the black reef quartzite formation in the western transvaal sedimentological and economic aspects and significance for basin evolution a sedimentological study of the early proterozoic black reef quartzite formation in the south western parts of the transvaal province of south africa was undertaken with the primary aim of examining the sedimentological controls of gold mineralization in the black reef placer which occurs at the base of this formation a second aim of the study was to investigate the early history of the basin in which the transvaal sequence of south africa was deposited the thin siliciclastic black reef quartzite formation which is informally subdivided into a lower conglomerate unit and an upper quartzite unit is underlain by archaean rocks belonging to the basement complex and the witwatersrand and ventersdorp supergroups and is overlain by a thick succession of carbonate rocks of the malmani subgroup the pre transvaal palaeosurface is characterised by elongated northeast to southwest trending grabens and partly eroded horst blocks the black reef quartzite formation which has a maximum thickness of about 30 m in the study area typically comprises a succession of interbedded arenites and mudstones with a sporadically developed basal conglomerate unit thickness trends are similar to the dominant structural trend of the pre transvaal palaeosurface at localities where the conglomerate unit is absent the formation invariably overlies quartzites of the witwatersrand supergroup directly the palaeocurrent distribution of the conglomerate unit is unimodal with modes towards the southwest in the southern parts of the study area and towards the north in the northern regions most of the palaeocurrent distributions of the quartzite unit are unimodal too but bimodal distributions were found at three localities pebble size of the black reef placer is largest in the northeastern parts of the study area but no orderly lateral size variation was found pebble roundness too varies greatly and apparently randomly the composition of the pebble assemblage is not constant but no systematic lateral change could be detected a petrographic study of the arenites of the formation reveals a remarkable textural and mineralogical maturity especially for the upper beds it is concluded that the pre transvaal palaeosurface had a palaeorelief of up to 30 m and that the topography of the palaeo landscape was the dominant factor controlling early sedimentation in the basin the palaeo grabens probably constituted the valleys of shallow braided stream systems that drained south westwards and northwards from a palaeo drainage divide in the northern parts of the study area sediment including detrital gold was derived from erosion of witwatersrand rocks and fed to the graben valleys via several alluvial fans during a subsequent transgression the fluvial systems became drowned and transgressive estuarine conditions ensued during the final stages of siliciclastic sedimentation the upper quartzite beds of the formation were probably reworked by shallow marine processes before carbonate precipitation commenced the cause of the marine transgression is not known beyond doubt it is suggested however that lithospheric rifting which initiated the extrusion of the underlying ventersdorp lavas resumed during early transvaal times resulting in complete severing of the continental crust and the creation of a linear sea

tectonic and geochronological constraints on late archaean and palaeoproterozoic stratigraphic correlation within and between the kaapvaal and pilbara cratons recent radiometric dating of carbonates and banded iron formations on the kaapvaal craton southern africa has suggested that late archaean and palaeoproterozoic carbonate sedimentation was diachronous across the craton we present new shrimp u pb zircon ages of 2583 5 ma and 2588 7 ma for two samples of a tuff bed 320 km apart in the upper oak tree formation transvaal supergroup these ages in conjunction with published data support previously established sequence stratigraphic correlations that show the temporal equivalence of late archaean carbonates in the region a review of the sequence stratigraphy and geochronology of the mount bruce megasequence set on the pilbara craton western australia and the newly named highveld megasequence set on the kaapvaal craton indicates a remarkably similar geohistory both cratons are characterized by a divergent megacycle hamersley range and vaal megasequences followed by a convergent megacycle chichester range and drakensberg megasequences our data show that the megasequence boundary is marked by drowning of both cratons at approximately the same time c 2590 ma which permits more meaningful geotectonic correlations between them

the origin of the gold and uranium ores of the black reef formation transvaal supergroup south africa abstract the black reef formation is a laterally extensive sedimentary succession at the base of the transvaal supergroup the quartz pebble conglomerates host erratic concentrations of gold and uranium however in areas that are located spatially above the gold and uranium bearing reefs of the witwatersrand supergroup the concentrations can reach ore grade although the depositional environment was similar to that of the witwatersrand supergroup the processes that lead to the high gold and uranium concentrations in the black reef were quite different the black reef experienced intense post depositional hydrothermal alteration by circulating aqueous and hydrocarbon fluids oils that deposited large volumes of native gold uranium minerals and pyrobitumen the gold which is filamentous is concentrated along as ni rich surfaces of pyrite and with uraninite in pyrobitumen in both environments the gold is interpreted to be the product of chemically triggered precipitation from hydrothermal fluid s the uraninite occurs either as precipitates in pyrobitumen or is invisible in the pyrite occurring as nanoparticles in this mineral this indicates that conditions were sufficiently oxidizing for the hydrothermal transport of uranium and contrasts with the case for the witwatersrand reefs where much of the uranium is detrital luminescent radiation damage associated with vein quartz and pyrobitumen as well as the diffuse distribution pattern of uranium in the pyrobitumen may indicate that some uranium was transported by liquid hydrocarbons pyrite in the black reef sedimentary detrital and hydrothermal pyrite is highly enriched in gold uranium and other redox sensitive trace elements provenance analyses using the trace element compositions of the pyrite show that it was not the result of erosional reworking of the underlying witwatersrand reefs however the close spatial association of high gold and uranium concentrations in the black reef with ore bearing witwatersrand reefs suggests strongly that gold and uranium whether in pyrite or occurring as native gold and uraninite were hydrothermally recycled uranium may have been recycled by hydrocarbon fluids from the witwatersrand reefs

multiple sulphur and iron isotope composition of detrital pyrite in archaean sedimentary rocks a new tool for provenance analysis abstract multiple s 34 s and 33 s and fe 56 fe isotope analyses of rounded pyrite grains from 3 1 to 2 6 ga conglomerates of southern africa indicate their detrital origin which supports anoxic surface conditions in the archaean rounded pyrites from meso to neoarchaean gold and uranium bearing strata of south africa are derived from both crustal and sedimentary sources the latter being characterised by non mass dependent fractionation of s isotopes 33 s as negative as 1 35 and large range of fe isotope values 56 fe between 1 1 and 1 2 most sediment sourced pyrite grains are likely derived from sulphide nodules in marine organic matter rich shales sedimentary exhalites and volcanogenic massive sulphide deposits some sedimentary pyrite grains may have been derived from in situ sulphidised fe oxides prior to their incorporation into the conglomerates as indicated by unusually high positive 56 fe values sedimentary sulphides without significant non mass dependent fractionation of s isotopes were also present in the source of some conglomerates the abundance in these rocks of detrital pyrite unstable in the oxygenated atmosphere may suggest factors other than high p o 2 as the cause for the absence of significant non mass dependent fractionation processes in the 3 2 2 7 ga atmosphere rounded pyrites from the c 2 6 ga conglomerates of the belingwe greenstone belt in zimbabwe have strongly fractionated 34 s 33 s and 56 fe values the source of which can be traced back to black shale hosted massive sulphides in the underlying strata the study demonstrates the utility of combined multiple s and fe isotope analyses for provenance reconstruction of archaean sedimentary successions

multistage sedimentary and metamorphic origin of pyrite and gold in the giant sukhoi log deposit lena gold province russia gold mineralization at sukhoi log in eastern siberia is hosted in a deformed neoproterozoic organic bearing and pyritic black shale and siltstone sequence that is folded into a tight overturned anticline the deposit contains about 30 million ounces of gold at an average grade of 2 0 g t au and is one of the largest known undeveloped gold resources the high grade gold zone forms a gently dipping tabular body in the core of the anticline the best gold grades occur in narrow bedding parallel pyrite quartz veinlets that have been folded during the main deformation event lower grade gold is associated with disseminated pyrite developed in and around the high grade core of the deposit detailed paragenetic studies of the mineralization and host rocks have defined six stages of pyrite development in the carbonaceous sediments the two earliest forms of pyrite termed py1 and py2 are commonly developed in stratiform layers of micron sized crystals framboids and fine euhedra which are interpreted as synsedimentary to early diagenetic in origin coarser grained bedding parallel aggregates of inclusion rich pyrite termed py3 contain inclusions of arsenopyrite native gold and gold tellurides and are interpreted to form during late diagenesis and earliest deformation coarse euhedral pyrite py4 overgrows the earlier pyrite py1 py2 and py3 and the slaty cleavage developed in the host rocks indicating a syndeformation timing late stage inclusion free pyrite py5 overgrows and replaces earlier sulfides and is considered to be syn to late deformation laser ablation inductively coupled plasma mass spectrometry la icpms analyses of the various pyrite types indicate that the synsedimentary py1 contains the highest levels of invisible gold varying from 0 4 to 12 1 ppm with a mean of 3 22 ppm au and 1 900 ppm as py1 is also enriched in a suite of trace elements mo sb ni co se te ag cu pb zn mn ba cr u v which are similar to those concentrated by organic processes in euxinic sedimentary environments later generations of pyrite from py2 to py5 including pyrite in bedding parallel pyrite quartz veinlets contain progressively lower contents of invisible gold and most other trace elements however this metamorphic and postmetamorphic pyrite contains microinclusions of free gold arsenopyrite pyrrhotite sphalerite and chalcopyrite the paragenetic textural and chemical relationships at sukhoi log suggest that gold was clearly initially introduced prior to cleavage development accompanying sedimentation of the organic rich shales and fixed during diagenesis within the structure of diagenetic arsenian pyrite subsequently accompanying deformation gold was liberated from recrystallized diagenetic pyrite to become concentrated as free gold and gold tellurides within metamorphic pyrite and folded bedding parallel pyrite quartz veinlets two key processes are considered vital to the formation of the sukhoi log deposit original synsedimentary and early diagenetic concentration of gold dissolved within arsenian pyrite in organic rich black shales and metamorphic processes that liberated gold from the early forms of arsenian pyrite to be concentrated as free gold and gold tellurides within late diagenetic and metamorphic pyrite and associated pyrite quartz veinlets in the core of an overturned anticline these ore forming processes are unlikely to be unique to sukhoi log other black shale and turbidite hosted deposits that occur in rifted continental margin environments which have undergone collision and basin inversion may form by similar processes

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

a marine biogeochemical perspective on black shale deposition abstract deposition of marine black shales has commonly been interpreted as having involved a high level of marine phytoplankton production that promoted high settling rates of organic matter through the water column and high burial fluxes on the seafloor or anoxic sulfidic water column conditions that led to high levels of preservation of deposited organic matter or a combination of the two processes here we review the hydrography and the budgets of trace metals and phytoplankton nutrients in two modern marine basins that have permanently anoxic bottom waters this information is then used to hindcast the hydrography and biogeochemical conditions of deposition of a black shale of late jurassic age the kimmeridge clay formation yorkshire england from its trace metal and organic carbon content comparison of the modern and jurassic sediment compositions reveals that the rate of photic zone primary productivity in the kimmeridge sea based on the accumulation rate of the marine fraction of ni was as high as 840 g organic carbon m 2 yr 1 this high level was possibly tied to the maximum rise of sea level during the late jurassic that flooded this and other continents sufficiently to allow major open ocean boundary currents to penetrate into epeiric seas sites of intense upwelling of nutrient enriched seawater would have been transferred from the continental margins their present location onto the continents this global flooding event was likely responsible for deposition of organic matter enriched sediments in other marine basins of this age several of which today host major petroleum source rocks bottom water redox conditions in the kimmeridge sea deduced from the v mo ratio in the marine fraction of the kimmeridge clay formation varied from oxic to anoxic but were predominantly suboxic or denitrifying a high settling flux of organic matter a result of the high primary productivity supported a high rate of bacterial respiration that led to the depletion of o2 in the bottom water a high rate of burial of labile organic matter albeit a low percentage of primary productivity in turn promoted anoxic conditions in the sediment pore waters that enhanced retention of trace metals deposited from the water column

zircon ion microprobe studies bearing on the age and evolution of the witwatersrand triad abstract the ventersdorp and witwatersrand supergroups and the dominion group collectively referred to as the witwatersrand triad represent some of the oldest and economically most important volcano sedimentary basins within the kaapvaal craton of southern africa precise age constraints which are vital to models aimed at understanding the formation mineralisation and evolution of these basins have been hampered by analytical and geological problems these problems have been overcome by this ion microprobe study on zircons from volcanic rocks within the three basins felsic and mafic volcanics from the ventersdorp supergroup yield ages of 2709 4 ma and 2714 8 ma respectively constraining the timing of the closure of the witwatersrand basin a maximum age for the witwatersrand basin is indicated by the age of 3074 6 ma for volcanics from the underlying dominion group thus a period of c 350 ma was available for the accumulation of the full witwatersrand basin succession unfortunately dating within the witwatersrand basin itself is not complete and future work could show that extended time breaks may exist within the basin these new data show that previous age determinations on the ventersdorp witwatersrand and dominion basins were inaccurate and that the economically important witwatersrand basin must now be recognised as archaean

geochemistry of archean shales from the witwatersrand supergroup south africa source area weathering and provenance with a few exceptions shales from the archean witwatersrand supergroup 2800 ma in south africa are depleted in na ca lile ree and hfse compared to phanerozoic shales cr co and ni are enriched in all witwatersrand shales and fe and mg are high in shales from the west rand groups wrg and lower central rand group crg shales from the crg and uppermost wrg are enriched in na al lile ree hfse and transition metals relative to shales from the lower wrg chondrite normalized ree patterns for all witwatersrand shales are enriched in light ree and exhibit small to moderate negative eu anomalies a positive correlation of ree and al2o3 contents in the shales suggests that ree are contained principally in clay minerals relative to shales from the crg shales from the wrg exhibit depletions of na ca and sr a feature probably reflecting intense chemical weathering of their source rocks cia indices in witwatersrand shales are variable chiefly 70 98 even within the same shale unit such variations reflect chiefly variable climatic zones or rates of tectonic uplift in source areas with perhaps some contribution from provenance and element remobilization during metamorphism compared to present day upper continental crust all but the orange grove roodepoort and k8 shales appear to have been derived from continental sources depleted in lile ree and hfse and enriched in transition metals computer mixing models based on six relatively immobile elements th hf yb la sc co and four source rocks indicate that the relative proportions of granite basalt and komatiite increased with time in sediment source areas at the expense of tonalite the contributions of basalt and komatiite appear to reach a maximum during deposition of the booysens shale and granite during deposition of the k8 shales and possibly during deposition of the orange grove shales

a carbonaceous sedimentary source rock model for carlin type and orogenic gold deposits this paper presents evidence and arguments that carbonaceous sedimentary rocks were a source for au and as in sediment hosted orogenic and carlin type gold deposits and develops a corresponding genetic model in this two stage basin scale model gold and arsenic are introduced early into black shale and turbidite basins during sedimentation and diagenesis stage 1 and concentrated to ore grades by later hydrothermal structural or magmatic processes stage 2 in reduced continental margin basin settings organic matter sedimented under anoxic to euxinic conditions immobilizes and concentrates gold arsenic and a range of trace elements particularly v ni se ag zn mo cu u present in marine bottom waters into fine grained black mudstone and siltstone of slope and basin facies during early diagenesis gold and certain other trace elements ni se te ag mo cu pge are preferentially partitioned into arsenian pyrite that grows in the muds these processes produce regionally extensive black shale and turbidite sequences enriched in syngenetic gold and arsenic commonly from 5 to 100 ppb au and 10 to 200 ppm as rare organic and sulfide rich metalliferous black shales may contain up to 1 to 2 ppm au and over 1 000 ppm as present as refractory gold in arsenian pyrite and nanoparticles of free gold during late diagenesis and early metamorphism stage 2 the diagenetic arsenian pyrite is recrystallized to form coarser grained pyrite generations and the organic matter is cooked to bitumen under higher grade metamorphism lower greenschist facies and above arsenian pyrite in carbonaceous shales is converted to pyrrhotite these processes release gold arsenic sulfur and other elements sb te cu zn mo bi tl and pb from the source rocks to become concentrated by hydrothermal processes locally to produce gold ores in structural sites such as fold hinge zones shear or breccia zones within or above the black shale sequence la icp ms analyses of diagenetic pyrite in carbonaceous sediments both associated and not associated with gold deposits suggests that invisible gold contents of greater than 250 ppb in diagenetic pyrite are indicative of carbonaceous shale source rocks with the potential to produce economic gold deposits application of this sedimentary source rock model enables a systematic exploration approach for sediment hosted gold deposits based on the distribution composition and structure of carbonaceous shale sequences and their contained diagenetic pyrite

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 quadrilatero ferrifero 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

a major archean gold and crust forming event in the kaapvaal craton south africa the 2 89 to 2 76 gigayear old conglomerates of the central rand group of south africa host an immense concentration of gold the gold and rounded pyrites from the conglomerates yield a rhenium osmium isochron age of 3 03 0 02 gigayears and an initial 187 os 188 os ratio of 0 1079 0 0001 this age is older than that of the conglomerates thus the gold is detrital and was not deposited by later hydrothermal fluids this middle archean gold mineralization event corresponds to a period of rapid crustal growth in which much of the kaapvaal craton was formed and is evidence for a significant noble metal flux from the mantle

the geochemistry of black shales from the chuniespoort group transvaal sequence eastern transvaal south africa geochemical and mineralogical data have been obtained from black carbonaceous shales interbedded with dolomite in the chuniespoort group of the late archean early proterozoic transvaal sequence the black shales are fine grained and generally argillaceous and contain on average 1 2 percent total c organic 0 7 percent s 7 5 ppb au 60 ppm cu 330 ppm cr and 125 ppm ni these metal concentrations are similar to or slightly higher than the values given for the u s geological survey black shale standard sdo 1 all other heavy and transition metals however are depleted in the chuniespoort shales relative to sdo 1 it is well known that high concentrations of cr and ni occur in archcan shales the enhanced cr and ni values found in the chuniespoort shales are a reflection of their antiquity and provenance and are not linked to preferential uptake of these metals during sedimentation and or diagenesis the chuniespoort shales are characterized by excess fe with respect to that required to form pyrite and have extremely variable c s ratios as well this together with the wide ranging s isotope ratios is taken as an indication that biogenically produced sulfide was augmented by sulfide introduced from an extraneous source whole rock s isotopes further indicate that s could have been derived from biogenic and inorganic reduction of sulfate as well as from hydrothermal fluids interelement correlations suggest that organic carbon does not play a role in hosting significant concentrations of base and transition metals rather element associations indicate that v and cr are preferentially contained in illite hydromuscovite and that ni co zn cu fe pb and mo probably reside in pyrite and or chalcopyrite gold is only weakly correlated with other elements which may indicate that its distribution is not solely determined by the presence of sulfide minerals the fact that au cu and s c ratios show an increase in samples with delta 34 s values around 0 per mil suggests that these elements are likely to have an epigenetic hydrothermal origin the limited syngenetic metal accumulation can be explained by the stable cratonic environment in which the sediments were deposited and the absence of submarine hydrothermal springs venting metalliferous fluids into the basin other factors such as the low metal potential of the provenance the rate of sedimentation and the degree of oxygenation of the bottom waters of the basin may have been additional factors limiting the concentration in and extraction of metals from seawater the fact that the chuniespoort shales host significant epigenetic gold occurrences particularly in the sabie pilgrim9s rest gold field is largely related to their chemical characteristics which promoted permeability and high absorption capacities for metals in hydrothermal fluids because of their low metal concentrations elsewhere in the basin the shales cannot however be considered the source rocks for gold mineralization

assessing the role of bitumen in the formation of stratabound cu ag deposits insights from the lorena deposit las luces district northern chile abstract stratabound cu ag deposits located in north central chile have been grouped into two ns parallel belts according to the age of their volcanic and volcanoclastic host rocks deposits from the cretaceous belt are characterized by an unusual assemblage of cu sulfides with migrated hydrocarbons whereas the presence of organic matter has not yet been recognized in deposits from the jurassic belt here we report the first evidence of pyrobitumen in a jurassic stratabound cu ag deposit and evaluate its role in ore formation we present a micro analytical study of pyrobitumen associated with cu fe sulfides in the lorena deposit located in northern chile our approach involved a combination of surface and drill core sample characterization fe sem observations empa and xrf mapping of pyrobitumen in addition we determined the speciation of sulfur in pyrobitumen by using synchrotron xanes two main events of hypogene mineralization were identified at lorena deposit i e a pre ore and a main cu ore stage the pre ore stage is characterized by globularly shaped pyrobitumen and minor pyrite and a widespread albite alteration of the andesite host rock pre ore pyrite was replaced by chalcopyrite bornite and chalcocite providing evidence for a late cu inception in the main cu ore stage chalcocite is often associated with sharp edged angular pyrobitumen most likely indicating remobilization or a later pulse of hydrocarbons empa data show that pyrobitumen at lorena is enriched in fe 0 02 1 74 wt cu 0 02 0 87 wt s 0 01 0 43 wt and cl 0 01 0 4 wt other elements detected include v co ni cr sb as ag and au both ag and au were only detected in a few spots with au reaching up to 600 ppm desktop and synchrotron xrf mapping shows that s is homogeneously distributed within pyrobitumen with lower s concentrations observed at grain rims on the other hand empa wds x ray maps show that cu precipitated on pyrobitumen grain surfaces filling fractures and forming veinlets in addition textures showing the presence of micro to nano sized pyrobitumen inclusions within both pre ore pyrite and main ore chalcocite grains point unequivocally to the interaction of hydrothermal fluids with hydrocarbons during the formation of this deposit the high s concentrations measured in pyrobitumen and the presence of organosulfur compounds detected by xanes can be attributed to an organic source the pre ore assemblage is interpreted as formed from a petroleum aqueous fluid connate waters mixture that migrated from the sedimentary source rock into the jurassic host rocks where pyrite and pyrobitumen precipitated in the andesite late cu bearing hydrothermal fluids interacted with this pyrobitumen pyrite assemblage to form cu sulfides either by replacement of pyrite or precipitation we conclude that some of the s contained in pyrobitumen could have been released during interaction with the cu bearing hydrothermal fluid contributing at least partially to the s budget required for sulfide mineralization in the main ore stage our results provide evidence supporting pyrobitumen fluid interaction as a key process in the formation of the lorena deposit and possibly other stratabound cu ag deposits in chile

multi stage and multi sourced fluid and gold in the formation of orogenic gold deposits in the world class mana district of burkina faso revealed by la icp ms analysis of pyrites and arsenopyrites abstract mana is a world class paleoproterozoic orogenic gold district 8 0 moz that includes five gold deposits fofina nyafe siou wona kona and yaho hosted in various rocks in the northern part of the birimian hounde greenstone belt western burkina faso the gold deposits recorded different styles of mineralization associated with sulfides during polyphase deformation events detailed petrographic and la icp ms analyses of pyrites and arsenopyrites from the main mineralization of each deposit provide new information on the potential gold and arsenic sources and timing of fluids responsible for mineralization at mana four hydrothermal fluid events associated with two distinct source reservoirs and at least two hydrothermal systems were recognized the first hydrothermal fluid event was generated during the eoeburnean d1md md for mana district shortening event 2172 ma evidence of which is preserved in the fofina and siou deposits and occurred under greenschist metamorphic facies conditions economic gold deposits at fofina are associated with metamorphic disseminated pyrite py1f in black shale zoned pyrite py2f enriched in as ni cu ag sb w au and pb and arsenopyrite in folded v1f quartz carbonate veins in contrast the siou deposit exhibits au in porous pyrite cores py1s which are associated with ag bi te and w sb in v1s quartz veins hosted in the siou granodiorite these two trace element suites are interpreted as having been induced by a metamorphic source in the fofina deposit and by a local contribution of a magmatic source in the siou deposit the second hydrothermal fluid event occurred during the eburnean d3md transpressional event 2113 2090 ma in the nyafe deposit this event is defined by au as rich rims in hydrothermal pyrite py3n and by the co precipitation of au rich arsenopyrite in the v3n quartz veinlets v3n controlled by a second order dextral shear zone the third and main hydrothermal fluid event during the d4md transpressional event 2090 ma is characterized by the development of strong silicification and by zoned pyrites py3wk and py2y and au rich arsenopyrite in the wona kona and yaho deposits however the same late au as rich rims in hydrothermal pyrite are observed in both deposits and in the nyafe deposit and these rims are interpreted to be related to the same economic gold source related to the lower birimian metamorphic reservoir composed of black shale diagenetic framboidal pyrite and tholeiitic basalts the diagenetic framboidal py1wk in black shale lower birimian group is enriched in au median of 0 8 ppm low in as median of 70 ppm with other trace elements ni cu zn ag sb and pb the last hydrothermal event is associated with free gold remobilization in late microfractures in the wona kona and siou deposit during the reactivation of major transcurrent shear zones during the d4md event or the d5md shortening event as late as 2022 ma a binary diagram of as ag vs sb bi appears to be effective at separating the diagenetic magmatic and metamorphic pyrites and could be used in orogenic gold exploration to ascertain potential gold endowment at the district scale

ice marginal thrusting of drift and bedrock thermal regime subglacial aquifers and glacial surges glacial thrust systems composed of blocks of drift and bedrock associated with hummocky stagnation moraine along the margin of the rainy lobe of the laurentide ice sheet in minnesota are used in

evidence for an intrabasinal source and multiple concentration processes in the formation of the carbon leader reef witwatersrand supergroup south africa laser ablation inductively coupled plasma mass spectrometer la icp ms trace element maps of pyrite and gold from the carbon leader reef in the witwatersrand basin provide new evidence that a significant proportion of the pyrite and gold was intrabasinal derived from the west rand group or equivalent shales stratigraphically below this reef rounded detrital pyrite grains in the carbon leader reef vary from compact to porous to sooty with similar textures and composition to diagenetic pyrites reported from sedimentary rocks in the west rand group detrital porous and sooty pyrites contain 0 4 to 11 3 ppm solid solution gold sulfur isotope 34s values from 17 to 16 and high levels of as and te plus a wide range of other trace elements in decreasing order ni co sb cr cu u pb bi mo zn and ag the sooty pyrite is the most au te and mo rich and is intergrown with alumino silicates and organic matter the detrital pyrite textures s isotopes and geochemistry resemble diagenetic pyrite developed under suboxic to anoxic bottom water conditions the laicp ms maps also show that the detrital pyrite grains have euhedral hydrothermal pyrite overgrowths containing micro inclusions of gold brannerite and ni as sulfides the pyrite overgrowths are also enriched in solid solution gold tellurium and other trace elements including in decreasing order as co ni cr cu mn pb bi and ag fractured and brecciated pyrite associated with brittle bedding parallel fracture zones in the carbon leader reef are also enriched in these elements due to alteration of pyrite surrounding the fractures laser icp ms pb isotope determinations on the cores of detrital pyrite indicate an age between 2750 and 2950 ma with hydrothermal overgrowths originating between 2100 and 2020 ma incorporating highly radiogenic pb this study demonstrates that both of the two competing theories for the origin of the witwatersrand gold reefs are likely to be correct we suggest that the hydrothermal event was widespread kilometer scale and involved basinal fluids that scavenged gold tellurium arsenic and trace elements co ni cr cu mn pb bi ag from gold bearing sedimentary units in the central rand group 2013 society of economic geologists inc

hydrothermal gold mineralization in the witwatersrand basin the origin of the gold mineralization in the witwaterstrand basin of south africa the largest known gold province has been controversial for decades with arguments favouring detrital1 2 placer modified placer3 4 and hydrothermal5 6 origins here we present the results of an extensive geological study of witwatersrand rocks which show that the gold and associated uranium mineralization is hydrothermal in origin and postdates a regional high temperature alteration event alteration processes identified on a small scale can be mapped out regionally as roughly strata bound zones of acid metasomatism extending far into the basin the fluid flow responsible for this alteration was concentrated in small scale structures localized along lithological boundaries we find that the gold precipitated as a consequence of interactions of the fluid with shale derived hydrocarbons present within the basin

geochemical and geochronological constraints on the nature of the immediate basement next to the mesoarchaean auriferous witwatersrand basin south africa acombined petrological geochemical and geochronological rb srand sm nd whole rock u pb and lu hf zircon and ar arhornblende study on a section of pre witwatersrand basementdrilled at the northwestern margin of thewitwatersrand basin hasrevealed new insights into the nature and tectonic setting of thelikely source area for some of the mesoarchaean auriferouswitwatersrand sediments the protolithsof intersected alteredgran ite and hornblende metagabbro are of indistinguishable age 3062 5 ma and have very similar geochemical signatures trace element characteristics typical of calc alkaline magmatismand evidence of variable contamination with older crust subchon dritic e

a revised classification scheme of pyrite in the witwatersrand basin and application to placer gold deposits abstract pyrite is the most abundant ore mineral in gold bearing quartz pebble conglomerates qpcs of the witwatersrand basin and similar archaean sedimentary units much of the pyrite in archaean conglomerates is detrital in origin implying that it survived weathering transport and reworking under anoxic conditions detrital pyrite is generally found together with authigenic pyrite formed in situ as a result of syn to post depositional processes as pyrite is frequently associated with gold mineralization many authors have developed different pyrite classification schemes in order to better understand the nature and origin of the mineralization although the different classifications cannot always be easily compared as they are largely based on inferred pyrite genesis we propose a revised entirely descriptive classification scheme for detrital and authigenic pyrite in archaean qpcs the scheme was created in order to provide a consistent framework for the petrographic description of pyrite in clastic sedimentary rocks detrital pyrite is subdivided into massive inclusion bearing and coarsely crystalline types authigenic pyrite includes euhedral overgrowth infill aggregate and pseudomorphic types we evaluate the methods used in pyrite classification propose formation pathways for detrital and authigenic pyrites and apply the classification scheme to some well known occurrences of detrital pyrite bearing conglomerates