the rotokawa geothermal system new zealand an active epithermal gold depositing environment the rotokawa geothermal system is less than 20 000 yrs old and is situated in a sequence of pleistocene and recent volcanic rocks and underlying mesozoic graywacke basement in the taupo volcanic zone a region dominated by extensional tectonics at lease 12 different hydrothermal explosition breccias are preserved in surface outcrops the largest eruptive site being a crater of 1 5 km in diameter which formed about 6 060 years ago and is presently occupied by an acid lake lake rotokawa with a ph approximately 2 significant bedded sulfur deposits which formed in an earlier crater lake exist beneath the present lake the deep fluids are dilute chloride 0 022 mole kg waters having higher reduced sulfur concentrations h 2 s approximately 7x10 super 3 mole kg than any previously studied active geothermal system a maximum temperature of 320 degrees c has been measured in the system water rock interaction has given rise to a zone of propylitic alteration at depth which is overlain by a region of advanced and intermediate argillic alteration extensive boiling occurs at depth an sulfide minerals such as sphalerite galena chalcopyrite and argentite occur sporadically with ubiquitous pyrite argentiferous alunite occurs in the advanced argillic alteration zone clasts of silicified lake beds in the hydrothermal explosion breccias commonly contain pyrite and carry subeconomic gold values of up to 0 1 g metric ton thermal waters discharging on the northeast shore of the lake are depositing gold ore grade arsenic and antimony sulfide rich muds which also contain high concentrations of tungsten thallium mercury silver and gallium this surface deposit is estimated to contain about 250 kg 8 000 oz of gold and has been deposited during the past 1 800 yrs since the formation 6 060 yrs ago of the large hydrothermal explosion crater in which lake rotokawa is now situated about 12 0 million oz 370 metric tons of gold may have been transported into the environment beneath the crater it is estimated that 1 7 to 3 3 million oz of gold may have been deposited in the upper 300 to 400 m underlying lake rotokawa from hydrothermal fluids initially saturated with gold

extreme alteration by hyperacidic brines at kawah ijen volcano east java indonesia ii metasomatic imprint and element fluxes abstract the hyperacidic brines of the kawah ijen crater lake and banyu pahit river east java indonesia induce an intense alteration on their magmatic host rock this alteration is a proxy for water rock interaction in magmatic hydrothermal systems and associated high sulphidation mineralizing environments as well as for how these systems translate changes in the magmatic system to surface emissions which are used in volcanic hazard monitoring detailed bulk chemical study of altered and unaltered samples shows that alteration is characterised by near complete leaching of all major and trace elements except for pb sn and sb which are progressively enriched pb up to 15 fold absolute enrichment the resulting element release is complementary to the observed changes in composition of the banyu pahit water downstream when corrected for dilution indicating that alteration progressively increases the element load the signature of the change in water chemistry is best explained by complete alteration of fresh rock rather than mature alteration which might be expected given the advanced altered state of the riverbed together with mass balance considerations this indicates that the dominant element source is material falling into the river from the valley flanks the chemical signature of the crater lake is inconsistent with the observed alteration in samples from the hydrothermal system and likewise is best explained by surface input of cations from rocks falling in from the crater walls this indicates that the lake water cation chemistry is not a direct reflection of the underlying magmatic hydrothermal system and that its cation content is therefore not an appropriate monitor of changes in volcanic activity

the geochemistry of trace elements in geothermal fluids iceland abstract trace element geochemistry was studied in geothermal fluids in iceland the major and trace element compositions of hot springs sub boiling and two phase liquid and vapor wells from 10 geothermal areas were used to reconstruct the fluid composition in the aquifers at depth aquifer fluid temperatures ranged from 4 to 300 c ph values between 4 5 and 9 3 and fluids typically contained total dissolved solids

gold remobilisation and formation of high grade ore shoots driven by dissolution reprecipitation replacement and ni substitution into auriferous arsenopyrite abstract both gold rich sulphides and ultra high grade native gold oreshoots are common but poorly understood phenomenon in orogenic type mineral systems partly because fluids in these systems are considered to have relatively low gold solubilities and are unlikely to generate high gold concentrations the world class obuasi gold deposit ghana has gold rich arsenopyrite spatially associated with quartz veins which have extremely high localised concentrations of native gold contained in microcrack networks within the quartz veins where they are folded here we examine selected samples from obuasi using a novel combination of quantitative electron backscatter diffraction analysis ion microprobe imaging synchrotron xfm mapping and geochemical modelling to investigate the origin of the unusually high gold concentrations the auriferous arsenopyrites are shown to have undergone partial replacement 15 by au poor nickeliferous arsenopyrite during localised crystal plastic deformation intragranular microfracture and metamorphism 340 460 c 2 kbars our results show the dominant replacement mechanism was pseudomorphic dissolution reprecipitation driven by small volumes of an infiltrating fluid that had relatively low s 2 and carried aqueous nicl 2 we find that arsenopyrite replacement produced strong chemical gradients at crystal fluid interfaces due to an increase in s 2 during reaction which enabled efficient removal of gold to the fluid phase and development of anomalously gold rich fluid potentially 10 ppm or more depending on sulphur concentration this process was facilitated by precipitation of ankerite which removed co 2 from the fluid increasing the relative proportion of sulphur for gold complexation and inhibited additional quartz precipitation gold re precipitation occurred over distances of 10 m to several tens of metres and was likely a result of sulphur activity reduction through precipitation of pyrite and other sulphides we suggest this late remobilisation process may be relatively common in orogenic belts containing abundant mafic ultramafic rocks which act as a source of ni and co scavenged by chloride bearing fluids both the preference of the arsenopyrite crystal structure for ni and co rather than gold and the release of sulphur during reaction can drive gold remobilisation in many deposits across broad regions

sulphur dioxide particle and elemental emissions from mount etna italy during july 1987 sulphur dioxide emission rates were measured at mount etna italy during july 1987 while the volcano was undergoing relatively quiet activity the so2 flux averaged 930 587 1 mg d excluding 19 july when the flux was 3200 1730 1 mg d rising magma and or an influx of less degassed magma could explain the increased so2 flux the high so2 flux did not correlate with changes in observed volcanic activity this suggests that so2 monitoring may be useful as an indicator of shallow magmatic activity but not as a predictor of future eruptions of mount etna

fluid inclusion characteristics and molybdenite re os geochronology of the qulong porphyry copper molybdenum deposit tibet the qulong porphyry copper and molybdenum deposit is located at the southwest margin of the lhasa terrane and in the eastern region of the gangdese magmatic belt it represents china s largest porphyry copper system with 2200 million tonnes of ore comprising 0 5 cu and 0 03 mo the mineralization is associated with miocene granodiorite monzogranite and quartz diorite units which intruded into jurassic volcanic units in a post collisional indian asian tectonic setting field observations and core logging demonstrate the alteration and mineralization at qulong are akin to typical porphyry copper systems in subduction settings which comprise similar magmatic hydrothermal potassic propylitic and phyllic alteration assemblages molybdenite re os geochronology confirms the relative timeframe defined by field observations and core logging and indicates that the bulk copper and molybdenum at qulong were deposited within 350 000 years between 16 10 0 06 0 08 without and with decay constant uncertainty and 15 88 0 06 0 08 ma this duration for mineralization is in direct contrast to a long lived intrusive episode associated with mineralization based on previous zircon u pb data our fluid inclusion study indicates that the ore forming fluid was oxidized and contained na k ca fe cu mo cl and s the magmatic hydrothermal transition occurred at 425 c under lithostatic pressure while potassic propylitic and phyllic alteration occurred at hydrostatic pressure with temperature progressively decreasing from 425 to 280 c the fluid inclusion data presented here suggests that there has been 2 3 km of erosion at qulong after its formation and this erosion may be related to regional uplift of the lhasa terrane

geochronology and geochemistry of the max porphyry mo deposit and its relationship to pb zn ag mineralization kootenay arc southeastern british columbia canada the max porphyry mo deposit is located at the northern end of the kootenay arc in southeastern british columbia canada molybdenum mineralization is genetically related to the variably altered trout lake granodiorite stock which has lithogeochemical affinities to the low fluorine calc alkaline granodiorite hosted porphyry mo deposit type three molybdenite samples from early and late mo bearing veins yield re os dates that overlap within analytical error with a weighted average age of 80 3 0 2 ma these dates are in close agreement with two 206pb 238u weighted average zircon ages for phases of the trout lake stock at 80 3 1 6 and 79 2 1 0 ma indicating that crystallization of the stock and hydrothermal molybdenite formation were coeval and likely cogenetic 40ar 39ar plateau ages for primary biotite in igneous dikes secondary biotite in early potassic alteration and muscovite in late phyllic alteration range from 80 to 76 ma the younger 40ar 39ar plateau ages may reflect unusually slow cooling of the magmatic hydrothermal system or a late thermal disturbance pb zn ag vein deposits are common throughout the kootenay arc and several such veins are located within and proximal to the max porphyry mo deposit the presence of rare molybdenite associated with sphalerite and argentiferous galena in some of these pb zn ag veins suggests that they represent a late stage event related to the max magmatic hydrothermal system fluid inclusions in quartz from mo and pb zn ag veins are similar and can be divided into two compositional groups type 1 aqueous liquid rich fluid inclusions and type 2 aqueous carbonic liquid rich fluid inclusions pressure corrected 1 4 1 7 kbar trapping temperatures of typical type 1 inclusions from mo bearing veins range from 430 to 460 c and for pb zn ag veins from 335 to 365 c the ranges of salinity 0 12 wt nacl equiv in fluid inclusions from both vein types are almost identical and type 2 inclusions from both vein types contain similar amounts of co2 avg xco2 0 08 with traces of ch4 the similarity of chemistry in the two vein types suggests a genetic link between porphyry mo mineralization and adjacent pb zn ag deposits however muscovite from the alteration halo around a pb zn ag vein within the max deposit yielded a 40ar 39ar plateau age of 72 2 0 5 ma this date is significantly younger than the 80 ma age of porphyry magmatism and mo mineralization suggesting either that this event is in fact unrelated to porphyry mo mineralization at 80 ma or that the k ar isotope system has been disturbed by a later thermal event

extreme winds and waves in the aftermath of a neoproterozoic glaciation the most severe excursions in the earth s climatic history are thought to be associated with proterozoic glaciations according to the snowball earth hypothesis the marinoan glaciation which ended about 635 million years ago involved global or nearly global ice cover at the termination of this glacial period rapid melting of continental ice sheets must have caused a large rise in sea level here we show that sediments deposited during this sea level rise contain remarkable structures that we interpret as giant wave ripples these structures occur at homologous stratigraphic levels in australia brazil canada namibia and svalbard our hydrodynamic analysis of these structures suggests maximum wave periods of 21 to 30 seconds significantly longer than those typical for today s oceans the reconstructed wave conditions could only have been generated under sustained high wind velocities exceeding 20 metres per second in fetch unlimited ocean basins we propose that these extraordinary wind and wave conditions were characteristic of the climatic transit and provide observational targets for atmospheric circulation models

evidence of zn isotopic fractionation in a soil plant system of a pristine tropical watershed nsimi cameroon abstract the objective of this study is to determine for the first time the zinc zn isotopic composition of different soil horizons parent rocks litter and plants from a single pristine watershed three soil profiles from nsimi zoetele site south cameroon africa have been investigated the 66zn 66zn 66zn 64zn sample 66zn 64zn jmc 3 0749l 1 103 measured in the soils and rocks range from 0 05 to 0 64 in a typical hillslope soil profile developed in situ from a granitic parent rock we observe two important and systematic trends i a weathering regime in the saprolite horizon that involves strong zn depletion with no or slight fractionation compared to the fresh igneous rock and ii a shallower weathering regime with significant depletion in heavy zn isotopes in the most superficial soil horizons in contrast the soil profile of the swamp zone does not exhibit isotopic fractionation between the soil and the parent rock six plants and two litter samples were analyzed for their zn isotopic compositions 66zn varied from 0 75 to 0 91 among all samples plant roots and shoots are generally enriched in heavy isotopes 66zn of 0 42 to 0 76 relative to the litter layer 0 12 to 0 25 and most superficial soils similar to the controlled plant growth study of weiss et al weiss d j mason t f d zhao f j kirk g j d coles b j and horstwood m s a 2005 isotopic discrimination of zinc in higher plants new phytologist 165 703 710 negative 66zn values were obtained for tree leaves the most aerial part of the tree based on biomass data for the tropical forest vegetation is on average enriched in light isotopes compared to the litter and the superficial soil horizons which are the nutrients reservoirs due to the age of these soils and the complexity of pedogenetic processes it is difficult to explain the isotopic composition of the soils at present however the transformation of the saprolite horizon into a ferruginous horizon is associated with an enrichment in light zn isotopes this observation may be due to the adsorption of zn onto clays surfaces the uptake of zn by roots in the soils leads to an enrichment in heavy isotopes once the zn is in the xylem it will be transported within the plant during this transport both diffusion and active uptake of heavy isotopes by cells out of the xylem favour the mobility of light isotopes to the most aerial parts of the plants the difference in zn isotopic composition between the deep horizons and the superficial horizons of soils suggests that zn isotopes can be used to discriminate the source of zn and other metals leaving the catchment by river water

late quaternary changes of western equatorial atlantic surface circulation and amazon lowland climate recorded in ceara rise deep sea sediments today the western tropical atlantic is the most important passage for cross equatorial transfer of heat in the form of warm surface water flowing from the south into the north atlantic circulation changes north of south america may thus have influenced the global thermohaline circulation system and high northern latitude climate here we reconstruct late quaternary variations of western equatorial atlantic surface circulation and amazon lowland climate obtained from a multiproxy sediment record from ceara rise variations in the illite smectite ratio suggest drier climatic conditions in the amazon basin during glacials relative to interglacials the 230thex normalized fluxes and the 13c 12c record of organic carbon indicate that sea level fluctuations shelf topography and changes of the surface circulation pattern controlled variations and amplitude of terrigenous sediment supply to the ceara rise we attribute variations in thermocline depth reconstructed from vertical planktic foraminiferal oxygen isotope gradients and abundances of the phytoplankton species florisphaera profunda to changes in southeast trade wind intensity strong trade winds during ice volume maxima are associated with a deep western tropical atlantic thermocline strengthening of the north brazil current retroflection and more vigorous eastward flow of surface waters

hydrothermal alteration and veins at the epithermal au ag deposits and prospects of the waitekauri area hauraki goldfield new zealand the waitekauri area of the hauraki goldfield new zealand contains several adularia sericite epithermal au ag deposits and prospects from west to east the area contains the sovereign jubilee scimitar scotia teutonic and jasper creek deposits and prospects which are hosted by andesitic and dacitic flows breccias and localized pyroclastic and air fall deposits drill core reveals spatial and temporal zonation of alteration and vein minerals along a 3 km long composite cross section through the area most host rocks are intensely altered with 100 percent of the igneous minerals replaced by hydrothermal minerals although the alteration intensity becomes more variable and weaker toward the east alteration minerals include quartz adularia albite chlorite pyrite illite interstratified illite smectite smectite calcite hematite and minor epidote many of these minerals have zoned distributions adularia is widespread at sovereign but is restricted to shallow levels at both scotia and jasper creek albite occurs in a discrete zone below adularia at scotia and minor epidote is restricted to sovereign and jubilee illite occurs throughout sovereign and jubilee and at the western margin of scotia and scimitar where it grades eastward into interstratified illite smectite and smectite at teutonic and jasper creek veins are typically less than 10 cm wide but have diverse mineralogy with zoned distributions quartz veins dominate at sovereign and jubilee whereas calcite veins are more abundant at scotia scimitar and jasper creek laumontite occurs at scotia and locally at scimitar whereas veins of clinoptilolite and mordenite calcite occur at jasper creek and stilbite veins occur at teutonic fluid inclusions in quartz and calcite homogenized between 132 and 310 c and trapped a dilute solution with an apparent salinity of less than 2 6 wt percent nacl equiv homogenization temperatures are highest at sovereign avg 241 c jubilee avg 239 c and scimitar avg 236 c lower at scotia avg 204 c and lowest at teutonic avg 168 c and jasper creek avg 162 c estimated positions of the paleowater table above sovereign jubilee scimitar scotia jasper creekg and teutonic relative to present elevations was at least 690 750 575 450 225 and 150 m above sea level respectively the deposits and prospects therefore span a 600 m vertical interval individual deposits and prospects have undergoen at least 35 to more than 455 m of erosion with the greatest erosion to the west alteration intensity alteration and vein mineral distributions and fluid inclusion microthermometry are interpreted to indicate that sovereign and jubilee formed at relatively high temperatures whereas teutonic and jasper creek formed at relatively cooler temperatures several hydrologic reconstructions are possible including 1 a single hydrothermal system with an inclined water table and significant lateral outflow to the east or 2 a single low relief hydrothermal system with a flat lying water table that has subsequently been displaced by postmineral faults or tilted approximately 10 to the east regardless of the preferred reconstruction the sovereign and jubilee deposits appear to have formed in the main zone of fluid upflow whereas the teutonic and jasper creek prospects appear to have formed toward the margin moreover the greatest erosion has occurred at the jubilee and sovereign deposits 300 400 m and these may represent the roots of a more extensive vein network that has largely been eroded

natural gold nanoparticles abstract the nanoparticulate gold component in ore deposits and exploration samples is yet to be fully realised but new methods of characterisation and analysis access to high grade gold samples and a strong focus on au nanoparticles in the manufacturing sector provide a new impetus to quantify its significance in geology nanoparticles are increasingly recognised as a fundamental step in geochemical reactions a critical component in weathering processes in biomineralisation and metal migration in ore systems the nanoparticulate population of au is a significant factor in the formation of economic deposits as a refractory component locked up in sulphides in secondary supergene enrichments the formation of surface geochemical anomalies and in the formation of high grade accumulations a better understanding and characterisation of this nanoparticulate gold could unlock hidden resources in known deposits through easier beneficiation and processing but also assist in the discovery of new deposits through advanced exploration methods

interpreting carbon isotope excursions strangelove oceans large negative excursions in marine carbonate delta sup 13 c are commonly associated with period boundaries and mass extinctions explanations for these events must be consistent with limitations imposed by carbon isotope mass balance at steady state i e for excursions lasting more than 10 sup 5 yr the surface ocean delta sup 13 c is set by the organic fraction of the total carbon burial rate and the magnitude of the photosynthetic isotope effect the delta sup 13 c of the deep ocean and the surface to deep isotope gradient are set by both the organic fraction of the ocean s remineralized particulate flux and the magnitude of the isotope effect thus it is the carbon isotope composition of the deep ocean that is most reflective of internal oceanic processes the surface ocean records changes in the longer term throughput of carbon in the system the cessation of organic export from the surface ocean such as is presumed to have caused the strangelove ocean condition of the cretaceous tertiary k t boundary leads to an isotopically homogeneous ocean in decades to centuries if this condition persists the ocean s isotopic composition approaches that of the riverine weathering input in 10 sup 5 yr failure to approach this value during the k t event suggests continued production andmore burial of organic carbon dominantly in either terrestrial or shallow marine environments less

sources and ore forming fluid pathways of carbonate hosted pb zn deposits in southwest china implications of pb zn s cd isotopic compositions the sichuan yunnan guizhou metallogenic province of southwest china contains more than 200 mt of pb zn ore in carbonate hosted pb zn deposits representing 27 of the total pb and zn resources of china sources of metals and pathways of ore forming fluids were elucidated through a study of the pb zn s cd isotopic compositions of sphalerite from the tianbaoshan fusheng maozu jinshachang and daliangzi carbonate hosted pb zn deposits 34s values indicate that s in the deposits is derived mainly from evaporites in cambrian to triassic sedimentary strata sulfates coexisting with sulfides and meso neoproterozoic folded basement 66zn values and pb isotopic ratios indicate that these metals originate mainly from sinian to lower permian sedimentary rocks and proterozoic basement there is a trend towards isotopically heavier cd and zn compositions of sphalerite from maozu to daliangzi jinshachang and tianbaoshan which are all hosted in the upper sinian dengying formation previous studies showed that there would be a trend of enrichment in heavier zn and cd isotopes following the migration of fluids and precipitation of minerals the above observations suggest that the ore forming fluids of these deposits are probably derived from the same hydrothermal fluid system the fluids most likely flowed through maozu first migrating along the xiaojiang and anninghe fault belts and their branch faults to daliangzi jinshachang and tianbaoshan respectively zn and cd isotopes could be useful tools in tracing the pathways of ore forming fluids in this district and heavier zn and cd isotopic compositions could provide a geochemical fingerprint for detecting remote orebodies in large hydrothermal fluid systems

gold enrichment in active geothermal systems by accumulating colloidal suspensions the origins of high grade hydrothermal ore deposits are debated but active geothermal systems provide important clues to their formation the highest concentrations of gold are found in geothermal systems with direct links to island arc magmatism yet similar concentrations have also been found in the absence of any input from arc magmas for example in the reykjanes geothermal field iceland here we analyse brine samples taken from deep wells at reykjanes and find that gold concentrations in the reservoir zone have increased over the past seven years from an average of 3 ppb to 14 ppb the metal concentrations greatly exceed the maximum solubility of gold in the reservoir under saturated conditions and are now nearly two orders of magnitude higher than in mid ocean ridge black smoker fluids the direct analogues of reykjanes deep liquids we suggest that ongoing extraction of brine the resulting pressure drop and increased boiling have caused gold to drop out of solution and become trapped in the reservoir as a colloidal suspension this process may explain how the stock of metal in the reservoirs of fossil geothermal systems could have increased over time and thus become available for the formation of gold rich ore deposits

hydrothermal transport of ag au cu pb te zn and other metals and metalloids in new zealand geothermal systems spatial patterns fluid mineral equilibria and implications for epithermal mineralization in order to analyze the concentrations of ag as au cd cu hg mn mo ni pb se sb te tl w and zn in deep hydrothermal chloride waters at 195 to 320 c and in shallow boiled chloride waters at 160 to 230 c we sampled wells drilled to 3 km depth in geothermal systems of the north island new zealand six of the systems are located in a segment of volcanic arc in the central taupo volcanic zone and the seventh system is associated with an intraplate mafic felsic volcanic center the concentrations of metals range widely from 0 1 to 1 000 g kg with a large degree of intersystem variability some of the largest contrasts in au ag pb and te concentrations are observed in the two nearest systems rotokawa and wairakei which are only 10 km apart the correlations between metals are poor except for ag au pb te and as sb the correlations between metals cl and h2s are also poor with the exception of rotokawa where the highest concentrations of ag au cu and te correlate with the highest concentration of aqueous h2s speciation calculations indicate that the dominant aqueous species of ag au cu pb and zn involve hs complexes the calculations also show that the states of saturation range from undersaturated conditions for acanthite arsenopyrite and gold to oversaturated conditions for chalcopyrite sphalerite and tellurides notably the au and ag transporting capacities of the deep chloride waters are much larger than the measured aqueous concentrations these results suggest that fluid mineral equilibria and the concentrations of ligands exert weak influence on metal concentrations at the temperatures and depths of sampling the complex trends in hydrothermal metal concentrations strongly suggest that the deep seated sources of metals comprising magmatic intrusions deep country rock and their related fluids limit the hydrothermal supplies of metals between the geothermal systems hydrothermal fluxes of ag 6 8 000 kg y au 0 9 66 kg y cu 30 23 500 kg y and te 2 10 400 kg y are variable the highest concentrations and fluxes of ag au and te in rotokawa and mokai are attributed to direct fluid inputs from intrusions of andesitic and basaltic magmas respectively compared to their deep counterparts boiled chloride waters are strongly depleted in ag au cd cu pb and te because these metals deposit in sharp response to gas loss and cooling in the well by comparison the as mn mo ni sb tl and zn concentrations are measurably less depleted in boiled waters making them available to form metal anomalies at shallow depths and in the peripheral parts of the epithermal environment periods of strong metal flux through the geothermal system combined with deep boiling favor epithermal ore formation and the development of large precious metal deposits however even moderate metal fluxes can produce au and ag mineralization as long as the duration of focused fluid flow and boiling can be sustained there is no evidence that the compositions of deep magmatic intrusions mafic or intermediate limit the ore forming potential of a geothermal system

tracing of anthropogenic zinc sources in coastal environments using stable isotope composition abstract the use of zinc isotopes to trace anthropogenic sources in coastal areas has been tested in this study we determined the stable isotopic composition of zinc in sediment cores suspended particulate matter spm and rocks collected at the sepetiba bay southeastern brazil an estuarine lagoon heavily impacted by metallurgic activities these isotopic signatures were compared with those from willemite ore which represent the main mineral refined by the major industrial source of zinc the aim was to test if this tracer system enables to identify sources and sinks of anthropogenic zinc and to reconstruct the temporal and spatial evolution of zinc contamination the zinc isotopic compositions expressed using the 66zn notation relative to the jmc 3 0749 l solution showed significant variations in the sediment cores the spm and willemite ore minerals ranging between 0 01 and 1 15 spatial and temporal analysis of sediments samples fit well in a model of mixing involving three main end members i terrestrial background 66znjmc 0 28 0 12 2 ii marine detrital material 66znjmc 0 45 0 03 2 and iii a major anthropogenic source associated with electroplating wastes released into the bay 66znjmc 0 86 0 15 2 sediment cores collected in the mud flats showed high correlation between 66zn and zinc enrichment factors suggesting good preservation of the isotopic records of natural and anthropogenic sources the sediment core sampled from a mangrove wetland located in a zone impacted by the metallurgy presented levels of zinc up to 4 sediment dry weight and preserved the isotopic signatures of electroplating wastes despite evidences that post depositional processes slightly changed the isotopic signatures in some layers from this core toward heavier 66znjmc values above 1 0 a two component mixing model suggests contributions of this major anthropogenic zn source up to nearly 80 during periods of electroplating activities our work suggests that zn isotope compositions of sediments are reliable tracers of anthropogenic sources and therefore can be useful to improve environmental monitoring efforts in coastal systems

tracking the onset of phanerozoic style redox sensitive trace metal enrichments new results from basal ediacaran post glacial strata in nw canada abstract a global rise in oxygen levels has been proposed to coincide with the ediacaran to cambrian radiation of animals yet the precise timing and nature of this change remains unresolved one hypothesis is that the ocean atmosphere system became temporarily well oxygenated in the earliest ediacaran directly following the marinoan snowball earth glaciation 635 ma the evidence for oxygenation is based on large enrichments of redox sensitive trace elements in black shale from south china presumably deposited under a euxinic water column these enrichments meet or exceed the bulk concentrations of redox sensitive trace elements found in phanerozoic shale deposited under euxinic water columns here we test the early ediacaran post snowball oxygenation hypothesis with new data from a high resolution multi proxy geochemical and sedimentological study of three stratigraphic sections in the earliest ediacaran sheepbed formation of the mackenzie and wernecke mountains nw canada iron speciation data from all sections suggest that the local water column was dominantly ferruginous with a notable exception of probable euxinic conditions recorded in part of one section redox sensitive elements show no appreciable enrichments in the basal sheepbed formation with maximum concentrations of molybdenum vanadium uranium and chromium only slightly above world average shale values the lack of substantial elemental enrichments within the sheepbed formation is consistent with results from coeval ferruginous strata in svalbard the dominance of local ferruginous conditions in nw canada cannot alone account for the muted redox sensitive element enrichments as v u and cr are thought to be enriched under these conditions and samples deposited under euxinic conditions also lack phanerozoic style enrichments these contrasting results from different localities highlight a need to further investigate the veracity of trace metal enrichments in all localities as representing global redox conditions as the results have important implications for the timing of oxygenation with respect to early animal evolution

cu rich scales in the reykjanes geothermal system iceland seawater dominated fluids discharge from the subaerial reykjanes geothermal system iceland there is a sharp pressure decrease in surface pipes at an orifice throttle point and cu rich scales deposit at this orifice that consist largely of bornite and digenite along with sphalerite and other sulfides the bornite and digenite form complex intergrowths with sphalerite and galena accompanied by high concentrations of gold and silver up to 590 ppm and 2 3 wt respectively the precipitates form in response to rapid boiling flashing of the hydrothermal fluids due to the sharp decrease in pressure 37 22 bars at the orifice downstream of the well head the boiling and concomitant separation of the vapor phase results in a temperature decrease from 252 to 220 c across this throttle over distances of centimeters there is a nearly quantitative deposition of metals from solution compared to concentrations in reservoir liquids resulting in formation of the metastable cu fes solid solution rich in silver and gold plus sphalerite and galena a high degree of supersaturation of metals in the quenched liquid is indicated by the very fine grain size of the sulfides and dendritic textures postdepositional cooling likely caused exsolution of bornite and digenite from the high temperature solid solution in addition silver was expelled from the cu fe s solid solution and remobilized into late fractures in the scales whereas the gold appears to have remained in solid solution or as submicroscopic inclusions in the bornite and digenite the observed mineral assemblage at reykjanes contrasts with that of sea floor black smokers despite similar fluid compositions the sea floor hydrothermal vents characteristically have cu rich linings composed predominantly of chalcopyrite which is precipitated by a combination of conductive cooling of high temperature 350 c hydrothermal fluids and mixing with seawater the difference in sulfide mineralogy between the reykjanes orifice scales with cu sulfides dominated by bornite and digenite and many sea floor vents may provide an indication of the subsea floor mineralization where fluids boil before discharging at sea floor vents

how accurately can we date the duration of magmatic hydrothermal events in porphyry systems an invited paper determining the absolute duration of magmatic hydrothermal events leading to the formation of porphyry systems i e including porphyry copper skarn and epithermal deposits is one of the key questions in ore geology this is so because the duration of magmatic hydrothermal events in porphyry systems is instrumental to the development of genetic models necessary to explore a category of mineral deposits that provide most of the copper and significant amounts of base and precious metals to our economy the problem of determining the absolute duration of magmatic hydrothermal events in porphyry systems has been addressed through thermal modeling of cooling intrusions and time needed to precipitate specified metal amounts from active hydrothermal systems with known metal concentrations and fluid fluxes both these methods have shown that the likely duration of hydrothermal systems is on the order of a few tens of kilo annum ka isotopic dating in contrast is the only possible way to determine the life span of magmatic hydrothermal events in fossil porphyry systems analytical and methodological developments during the last decade in the fields of the most robust isotopic systems commonly used for absolute dating u pb 40ar 39ar re os allow us to date minerals with internal precisions 0 2 2 for a 10 ma old mineral this corresponds to a 20 ka uncertainty which is marginally sufficient to discriminate the duration of hydrothermal systems at the tens of kilo annum scale however many geochronological studies on fossil porphyry systems have shown that these are most often formed through repeated cycles of several intrusion events which extend the overall life of the porphyry systems to a few 0 x and up to 2 ma in some cases internal precisions of the above mentioned dating methods allow us in theory to comfortably discriminate events at the 0 x scale and the combination of u pb 40ar 39ar and reos geochronology is a tool widely used by ore geologists to bracket the duration of cyclic magmatic hydrothermal events in porphyry systems in this review we discuss some fundamental problems that are systematically overlooked in most geochronological studies trying to bracket the life span of porphyry systems we show that if these problems are not adequately taken into account and tackled the result will be that fundamentally wrong life spans of porphyry systems will be estimated we also provide basic guidelines to follow when trying to resolve the duration of magmatic hydrothermal events in porphyry systems with the highest accuracy and precision currently achievable

ngatamariki geothermal system magmatic to epithermal transition in the taupo volcanic zone new zealand the ngatamariki geothermal system is one of more than 20 high enthalpy 225 c geothermal systems in the taupo volcanic zone north island new zealand at shallow levels 2 km they are analogous to low intermediate sulfidation state epithermal ore forming environments unique to ngatamariki is the occurrence of an intrusive complex with an associated magmatic hydrothermal alteration halo that was intersected by deep geothermal drilling 3 km depth and that resembles hydrothermal alteration associated with high intermediate sulfidation state epithermal mineralization this paper describes the results of a study involving alteration petrography x ray diffraction shortwave infrared swir reflectance spectroscopy backscattered electron scanning electron microscopy energy dispersive spectroscopy bse sem eds plus whole rock and trace element geochemistry to document and characterize distinct hydrothermal alteration found nowhere else in the taupo volcanic zone two separate phases of hydrothermal activity are distinguished old and modern as defined by a paleosurface that is dated at 0 68 ma which occurs 500 m below sea level a composite plutonic body comprising intrusions of diorite to tonalite was encountered in three adjacent drill holes nm4 nm8 and nm9 between 2 000 and 3 200 m below sea level 2 300 3 500 m depth below the surface in the northern part of the ngatamariki system the associated hydrothermal alteration is zoned and made up of potassic advanced argillic phyllic and propylitic mineral assemblages that occur between 500 and 2 500 m below sea level subtle potassic alteration consisting of biotite magnetite k feldspar mantles the intrusive complex it is crosscut by a hypogene advanced argillic alteration containing pyrophyllite minor andalusite topaz anhydrite rare aluminophosphates ap and fluorine bearing minerals but lacks alunite the deep formed advanced argillic alteration appears in some samples to be overprinted by phyllic alteration made up of quartz muscovite pyrite between 500 and 1 000 m below sea level the intense phyllic alteration is less pervasive and the hydrothermal alteration is dominated by kaolinite rare dickite and localized occurrences of highly silicified rocks that resemble vuggy quartz which is bounded at the top by the paleosurface defined as the unconformity at the base of the overlying whakamaru group ignimbrite in the central and southern part of the system below the paleosurface propylitic hydrothermal alteration consisting of chlorite calcite epidote wairakite actinolite along with albite and illite is widespread and could equally have formed during old hydrothermal activity associated with emplacement of the intrusive complex or in the deep hot parts of the modern hydrothermal system the mineralogy and geochemistry of advanced argillic altered rock indicates that acidic magmatic hydrothermal fluids leached base cations resulting in the loss of elements typically considered immobile al ti y zr nb as well as rare earth elements to form cation depleted minerals such as pyrophyllite andalusite and topaz the strongest enrichments in au 0 6 g t ag 4 6 g t te as sb and bi coincide with intense acid alteration at 250 m depth beneath the paleosurface the results of this study reveal a complex history of intrusion and hydrothermal activity that provides a modern example of successive development 600 000 years apart of acid and neutral ph hydrothermal alteration assemblages which are associated with the two end member types of epithermal mineralization

metals in deep liquid of the reykjanes geothermal system southwest iceland implications for the composition of seafloor black smoker fluids seafloor hydrothermal systems precipitate cu zn and fe sulfides at and below black smoker vents on the seafloor as a result the metal concentrations in the vent fluids are minimum values we sampled deep unboiled liquids from the reykjanes geothermal reservoir iceland and measured the metal concentrations this active seawater dominated system situated on the mid atlantic ridge is the subaerial equivalent to mid ocean ridge hydro thermal systems the liquids collected at 1350 1500 m depth and 284 295 c contain 154 2431 m fe 9 140 ppm 207 261 m cu 14 17 ppm 79 393 m zn 5 27 ppm 0 6 1 4 m pb 120 290 ppb 6 31 n m au 1 6 ppb and 250 960 n m ag 28 107 ppb fluids discharged at surface from the same wells have orders of magnitude lower metal concentrations due to precipitation caused by boiling and vapor loss during depressurization the concentrations of cu zn and pb in the high temperature reservoir liquids at reykjanes are similar to those in the highest temperature black smoker discharges whereas au and ag concentrations are one to two orders of magnitude higher at reykjanes lower temperature seafloor fluids have lower metal contents suggesting subseafloor deposition before discharge the reykjanes heat flux of 130 mw requires a liquid flux of 100 kg s over 104 yr the minimum life of the system 0 5 mt each of cu and zn may have precipitated at depth

the trace metal chemistry of deep geothermal water palinpinon geothermal field negros island philippines implications for precious metal deposition in epithermal gold deposits palinpinon geothermal field negros island philippines is a high temperature liquid dominated geothermal system in an active volcanic island arc setting parallels have been drawn between the geologic setting and hydrothermal alteration styles recognized at palinpinon with those that characterize several types of magmatichydrothermal ore deposits e g porphyry skarn and high and intermediate sulfidation epithermal systems the neutral chloride palinpinon reservoir water chemistry is affected by boiling mixing and conductive cooling the occurrence of base and precious metal scale deposits in several geothermal wells demonstrates that the deep reservoir fluid is capable of transporting and depositing base and precious metals however trace metal analysis of the deep reservoir fluids shows they are undersaturated with respect to gold 1 4 ig kg chemical modeling of palinpinon geothermal water predicts that boiling should be the most effective mechanism for base and precious metal deposition producing sulfide assemblages similar to those present in well scale deposits mixing with acid bearing sulfate waters can also produce these sulfide assemblages but less efficiently and with gold and silver deposition occurring at lower temperatures i e 110 c despite indications that modern hydrothermal fluids are capable of metal transportation base and precious metal deposition is not strongly developed at palinpinon base and precious metal concentrations in drill core and drill cuttings are one to two orders of magnitude below ore grade 0 02 wt cu 0 03 wt pb 0 01 wt zn 0 01 wt mo 8 g t ag and 0 05 g t au and important features that characterize high grade ore zones of porphyry i e quartz stockwork veining and epithermal deposits i e residual vuggy quartz massive crustiform veins and or hydrothermal breccia complexes are apparently absent at palinpinon such features intimate that high grade ore deposition is promoted by large volumes of fluid and highly permeable wall rock their absence at palinpinon indicates that permeability fluid flux and or metal transport have been insufficient to form significant zones of base or precious metal enrichment

the iron isotope composition of microbial carbonate abstract in a comparative study of fossil and recent stromatolites and microbialites we have investigated whether reefal stromatolitic limestones are able to preserve fe isotope compositions that potentially serve as proxies of seawater chemistry at the time of their formation it was found that 56 fe values of archean stromatolites vary between 2 1 and 0 5 devonian and carboniferous microbiallites vary between 0 and 1 and modern microbialites from the great barrier reef show a range from 0 12 to 0 15 five lines of evidence support the possibility that these compositions are potentially pristine 1 fe concentrations and fe isotope ratios are not correlated 2 the concentrations of other elements that are potentially mobile during carbonate diagenesis mg sr do not correlate with 56 fe 3 mn concentrations are inversely correlated with 56 fe which might hint at meteoric alteration however if mn concentrations and fe isotope signatures were pristine features they might serve as a record of the reduction of these two metals in seawater 4 dolomitisation of two devonian limestones does not shift their fe isotope compositions 5 rare earth element and yttrium rey patterns in all limestones are similar to seawater rey sources this suggests that the bulk of the fe in the samples occurs in hydrogenous carbonates not in primary or secondary fe and mn hydroxides because the latter have different rey patterns the modern microbialites are unfractionated in fe relative to crustal rocks thus these hydrogenous precipitates either incorporated unfractionated fe of the ambient seawater composition or they modified the fe composition upon incorporation from a fractionated ocean fe reservoir in either case provided diagenetic alteration has not modified the fe isotope composition in a major way a potential open ocean fe isotope proxy may exist that may allow reconstruction of secular variations in seawater fe isotope compositions our preliminary results suggest that neoarchean microbial carbonate fe may have been lighter than today s microbialite fe and that the carbonates fe isotopes potentially record the efficiency of transition metal reduction in ocean basins through geologic time

an early diagenetic deglacial origin for basal ediacaran cap dolostones abstract the beginning of the ediacaran period 635 ma is marked by conspicuous dolostone units that cap marinoan glacial deposits worldwide the extent and sedimentary characteristics of the cap dolostones indicate that anomalous carbonate over saturation coincided with deglacial sea level rise and ocean warming however the geochemical variability within cap dolostones both between continents across single continental margins and within individual stratigraphic sections has been difficult to reconcile with depositional models using a compilation of new calcium and magnesium isotope measurements in marinoan cap dolostone successions worldwide we show that the geochemical variability can be explained by early diagenetic dolomitization of aragonite along a spectrum of fluid and sediment buffered conditions dolostones from the outer platform formed under fluid buffered conditions whereas dolostones on the inner platform and foreslope environment formed under sediment buffered conditions this spatial pattern of dolomitizing conditions is consistent with buoyant recirculation of glacial seawater within carbonate platforms driven by the deglacial sea level rise and development of a meltwater surface ocean using a numerical diagenetic model to evaluate the geochemical differences between sediment and fluid buffered cap dolostone units we constrain the chemical and isotopic composition of both the dolomitizing fluid glacial seawater 13 c 0 2 the meltwater lens 13 c 11 and the primary aragonite sediment 13 c 6 to 3 these model end members do not imply that primary geochemical variability did not exist but demonstrates that it is not necessary to change the chemistry of seawater to explain the global stratigraphic variability in the geochemistry of basal ediacaran cap dolostones our results provide a novel framework for understanding the geochemical variability of cap dolostone units including large excursions in carbon isotopes and how this variability is the product of local diagenetic processes expressed globally in continental margin environments following the last snowball earth

genesis of middle miocene yellowstone hotspot related bonanza epithermal au ag deposits northern great basin usa epithermal deposits with bonanza au ag veins in the northern great basin ngb are spatially and temporally associated with middle miocene bimodal volcanism that was related to a mantle plume that has now migrated to the yellowstone national park area the au ag deposits formed between 16 5 and 14 ma but exhibit different mineralogical compositions the latter due to the nature of the country rocks hosting the deposits where host rocks were primarily of meta sedimentary or granitic origin adularia rich gold mineralization formed where glassy rhyolitic country rocks host veins colloidal silica textures and precious metal colloid aggregation textures resulted where basalts are the country rocks clay rich mineralization with silica minerals adularia and carbonate developed oxygen isotope data from quartz originally amorphous silica and gels from super high grade banded ores from the sleeper deposit show that ore forming solutions had 18o values up to 10 heavier than mid miocene meteoric water the geochemical signature of the ores including their se rich nature is interpreted here to reflect a mantle source for the epithermal suite elements au ag se te as sb hg and that signature is preserved to shallow crustal levels because of the similar volatility and aqueous geochemical behavior of the epithermal suite elements a mantle source for the gold in the deposits is further supported by the pb isotopic signature of the gold ores apparently the host rocks control the mineralization style and gangue mineralogy of ores however all deposits are considered to have derived precious metals and metalloids from mafic magmas related to the initial emergence of the yellowstone hotspot basalt derived volatiles and metal loid s are inferred to have been absorbed by meteoric water dominated geothermal systems heated by shallow rhyolitic magma chambers episodic discharge of volatiles and metal loid s from deep basaltic magmas mixed with heated meteoric water to create precious metal ore forming fluids colloidal nanoparticles of au ag alloy electrum naumannite ag2se silica and adularia likely nucleated at depth traveled upward and deposited where they grew large enough to aggregate along vein walls silica and gold colloids have been reported in hot springs from yellowstone national park suggesting that such processes may continue to some extent to the present however it is possible that the initial development of the mantle plume led to a major but short lived distillation process which led to the mid miocene bonanza ore forming event

white island new zealand volcanic hydrothermal system represents the geochemical environment of high sulfidation cu and au ore deposition the white island volcanic hydrothermal system new zealand is thought to closely represent the chemical conditions that lead to the formation of high sulfidation cu au ore deposits the amounts of cu and au produced over a 10 ka period of activity largely from degassing magma are calculated to be 106 and 45 t respectively altered andesite blocks ejected from recent vents contain alunite anhydrite and pyrite their s isotopic composition indicates vein filling at 380 c at this temperature cu and au are highly soluble in acid solutions which may explain the depletion of cu and absence of au in the ejecta mass balance calculations however suggest that cu and au are precipitated in cooler zones before the acid solutions discharge at the surface

source of gold in neoarchean orogenic type deposits in the north atlantic craton greenland insights for a proto source of gold in sub seafloor hydrothermal arsenopyrite in the mesoarchean abstract given that gold au mostly remained in the incipient earth mantle until ca 3 9 3 8 ga a proto source of gold may have been present in the dominantly mafic crust precursor born through first stage melting of the early earth mantle in south westernmost greenland a fragment of the north atlantic craton is characterised by greenstone belts comprising mafic volcanic and magmatic rocks and harzburgite cumulates that were emplaced at ca 3 0 ga prograde amphibolite facies metamorphic assemblages and caused local dissolution of arsenopyrite during this retrograde tectono metamorphic stage in gold rich shear zones the re os geochronometer in arsenopyrite was reset to a neoarchean age while invisible gold was liberated and deposited as free gold with 2 66 ga pyrite re os isochron ages the initial os isotope ratios of neoarchean arsenopyrite 187os 188osi 0 13 0 02 and gold bearing pyrite 0 12 0 02 overlap with the estimated 187os 188os ratio of the mesoarchean mantle 0 11 0 01 and preclude contribution of radiogenic crustal os from evolved lithologies in the accretionary arc complex but instead favour a local contribution in os from basaltic rocks and serpentinised harzburgite protoliths by metamorphic fluids thus the ca 2 66 ga lode gold mineralisation identified in the north atlantic craton may illustrate a gold endowment in shear zones in earth s stabilizing continental crust at the time of the 2 75 2 55 ga global gold event through metamorphic upgrading of bulk gold which had originally been extracted from the mesoarchean mantle and concentrated in hydrothermal arsenopyrite deposits in oceanic crust beneath the overall reduced mesoarchean ocean

epithermal deposits in north xinjiang nw china the north xinjiang region nw china is an important part of the central asia orogenic belt situated at the junction of siberia tarim and kazakhstan plates it is an area characterized by multiple stages of phanerozoic continental growth during which several porphyry and epithermal systems were formed the relationship of these mineral systems to the geodynamic evolution of the region has not yet been well understood in this paper we list the main geological characteristics of 21 significant epithermal precious and base metal deposits in north xinjiang and classify them into high sulfidation and low sulfidation styles with the latter being predominant we have selected seven epithermal deposits representing different styles formed under different tectonic regimes and discuss their geology and geochemistry in some detail the deposit scale geology and geochemistry of epithermal systems in north xinjiang are essentially similar to those in other parts of the world all epithermal deposits in north xinjiang are hosted in volcanic rocks with ages ranging from devonian to triassic with the early carboniferous volcanic sequences being the most important followed by the permian and triassic the devonian early carboniferous host rocks belong to the calc alkaline series that developed in pre collisional arc back arc basin systems whereas the permian triassic host volcanic rocks of shoshonite series formed in post subduction regimes available isotopic ages of these epithermal systems cluster in two periods early carboniferous 320 ma and late carboniferous triassic 320 220 ma reflecting two metallogenic episodes that occurred during subduction related accretion and post subduction collision regimes respectively accordingly three groups of epithermal deposits in north xinjiang can be recognized as 1 pre collisional deposits without or with negligible collisional related modification 2 deposits formed in collision regime and 3 ore systems strongly overprinted by fluid flow in post subduction collision regime

zinc isotope variations in deep sea carbonates from the eastern equatorial pacific over the last 175 ka the carbonate fraction of sediment core odp 849 leg 138 located in the eastern equatorial pacific mostly consisting of coccoliths was separated and analyzed for its zn isotopic composition the overall variation in zn isotopic composition as determined by multiple collector magnetic sector inductively coupled plasma mass spectrometry was found to be on the order of 1 expressed in 66zn where xzn xzn 64zn sample xzn 64zn standard 1 103 and x 66 67 or 68 over the last 175 ka the analytical precision was 0 04 and the overall reproducibility was usually better than 0 07 the zn isotopic composition signal exhibits several marked peaks and a high frequency variability a periodogram of the 66zn signal showed two periodicities of 35 2 and 21 2 ka we suggest that the latter is caused by the precession of the earth s axis of rotation the periodogram exhibits a minimum at 41 1 ka thus showing that the zn isotopic composition is independent of the obliquity in the eastern equatorial pacific the range of 66zn values observed for the carbonate fraction of odp 849 overlaps with the range observed for fe mn nodules in the world s oceans which suggests that seawater carbonate zn isotope fractionation is weak we therefore assume that most of the zn isotope variability is a result of the selective entrainment of the light isotopes by organic matter in the surface ocean the odp 849 66zn record seems to follow the changes in the insolation cycles changes in the late summer fall equatorial insolation modulate the intensity of the equatorial upwelling hence the mixing between deep and surface waters we propose that during decreased summer fall equatorial insolation when a steep thermocline can develop el nino like conditions the surface waters cannot be replenished by deep waters and become depleted in the lighter zn isotopes by biological activity thus resulting in the progressive increase of the 66zn values of the carbonate shells presumably in equilibrium with surface seawater

from 2d to 3d prospectivity modelling in the taupo volcanic zone new zealand abstract a 2d prospectivity model of epithermal gold mineralisation has been completed over the taupo volcanic zone tvz using the weights of evidence modelling technique this study was used to restrict a 3d geological interpretation and prospectivity model for the ohakuri region the tvz is commonly thought of as a present day analogue of the environment in which many epithermal ore deposits such as in the hauraki goldfield coromandel volcanic zone are formed the models utilise compiled digital data including historical exploration data geological data from the institute of geological and nuclear sciences ltd quarter million mapping programme recent glass earth geophysics data and historic exploration geochemical data including rock chip and stream sediment information spatial correlations between known deposits and predictive maps are determined from the available data which represent each component of the currently accepted mineral system model for epithermal gold the 2d prospectivity model confirms that the tvz has potential for gold mineralisation however one of the weaknesses of this weights of evidence model is that the studies are carried out in 2d with an approximation of 3d provided by geophysical and drilling data projected to a 2d plane consequently a 3d prospectivity model was completed over the ohakuri area constrained by the results of the 2d model and predictive maps the 3d model improved the results allowing more effective exploration targeting however the study also highlighted the main issues that need to be resolved before 3d prospectivity modelling becomes standard practise in the mineral exploration industry the study also helped develop a work flow that incorporates preliminary 2d spatial data analysis from the weights of evidence technique to more effectively restrict and develop 3d predictive map interpretation and development

aftermath of a snowball earth 1 using a simple 3 box model of the ocean atmosphere system we simulate the cycling of carbon and strontium in the aftermath of a global glaciation model simulations include the delivery of alkalinity to seawater from intense carbonate and silicate weathering under high pco2 conditions as well as ocean mixing air sea gas exchange and biological productivity the 13c of the first carbonate precipitated after the glaciation depends on the pco2 temperature the saturation state of the surface ocean and kinetic effects associated with mineral precipitation with no biological productivity the model produces 13c values between 1 and 3 consistent with observations this is in direct contradiction with arguments by kennedy et al 2001a who suggest that the 13c value of dissolved carbon in a snowball ocean and directly afterward must be 5 kennedy et al assume the carbon isotope cycle is in steady state which does not apply to a global glaciation and also neglect any effect of high pco2 on the carbonate chemistry of seawater a major difference between our findings and the qualitative predictions of hoffman et al 1998 is our interpretation of the cap dolostone as representing an interval dominated by carbonate weathering of exposed continental shelves as a result the 2 drop in the 13c observed in the cap dolostone is unlikely to be the product of rayleigh distillation of atmospheric co2 via silicate weathering instead we interpret the 2 drop in the 13c values as indicative of an increase in sea surface temperature which lowers the fractionation between co2 and carbonate kinetic isotope effects associated with rapid precipitation from a highly supersaturated surface ocean may also be important rayleigh distillation of atmospheric co2 via silicate weathering is a viable explanation for the continued drop in the 13c values in the limestone sequence above the cap dolostone with biological productivity and carbonate weathering driving a slow increase in 13c values once pco2 levels decline our study also simulates the cycling of strontium in seawater in contrast to the finding of jacobsen and kaufman 1999 and kennedy et al 2001a model simulations show a drop in 87sr 86sr of less than 0 001 during 5 million years of global glaciation and an increase of less than 0 001 over the entire episode of silicate weathering our calculations emphasize the importance of considering the changes in seawater chemistry due to high pco2 in evaluating the snowball earth hypothesis

zinc and cadmium stable isotopes in the geological record a case study from the post snowball earth nuccaleena cap dolostone abstract zinc and cadmium are important biologically cycled nutrients in the modern ocean recent analytical advances and sampling efforts such as geotraces have led to measurements of the global distribution of zn and cd and their stable isotope ratios 66zn and 114cd providing an understanding of how biological processes cycle these elements and their isotopes in the modern ocean this opens the door to better application of 66zn and 114cd as tracers of biogeochemical processes archived in the geological record here we present new measurements of cd and 114cd on samples from the nuccaleena cap dolostone in south australia which was deposited during deglaciation of the ca 635 ma old marinoan snowball earth our new data are combined with previously measured 66zn data from the same samples the combined record of 114cd and 66zn is used to explore various hypotheses about the sources and sinks of cd and zn in the post marinoan ocean key features of this record are that zn and cd remain relatively similar throughout that 66zn and 114cd change at the same stratigraphic level and in the same direction and that 66zn changes are about four times larger than 114cd changes even though both quantities reflect a similar fractional mass difference we suggest that both zn and cd concentration and isotopic data are consistent with a change in the sink by which zn and cd are removed from the oceans we propose that no isotope fractionation is expressed during removal of zn and cd as sulfides while zn and cd removed with biological material are about 0 6 and 0 2 lighter than deep ocean 66zn and 114cd respectively many previous studies attribute the burial of isotopically light biological zn to preferential assimilation of isotopically lighter zn by phytoplankton in contrast we propose that lighter biological 66zn occurs mostly because isotopically heavy zn was scavenged by organic matter which causes dissolved 66zn in the surface ocean to be lighter than deep ocean 66zn the organic matter originated from primary producers that thrived in the surface ocean during deglaciation for cd whose isotope cycling in the modern ocean is dominated by preferential assimilation of isotopically lighter cd by phytoplankton much less biological fractionation is expressed the combined analyses of 66zn and 114cd provide insight into the biogeochemical processes which occurred after the marinoan snowball earth glaciation and may be a useful new tool to explore the history of life on earth in other geological records as well

the gold conveyor belt large scale gold mobility in an active orogen the southern alps of new zealand are part of an active collisional orogen where metamorphism hydrothermal fluid flow and the formation of orogenic gold deposits are ongoing the southern alps are forming due to transpressional collision between continental crust fragments on the pacific and australian tectonic plates the plate tectonic rates and geometries the sources of fluid and broad scale fluid pathways in the hydrogeological system and the geochemical compositions of the torlesse terrane rock that is being advected through the orogen are well defined so that a mass balance of metal mobility during active orogenic processing in the southern alps of new zealand can be calculated advection of a 10 km wide x 5 km deep section of torlesse rock through the orogen at tectonic rates 0 01 m yr that is then metamorphosed up to amphibolite facies causes mobilisation of over 1127 t au 10 1mt as 47000 t hg 560000 t sb and 14000 mt h2o in 1 myrs the masses of elements mobilised at the same rate along the length of the southern alps 200 km for 5 myrs would be more than 100 times greater the metals were mobilised by the metamorphic fluid produced during the orogenic processing of the torlesse terrane rocks and the concentrations of au as hg and sb in this fluid are calculated to be 0 08 711 3 and 40 mg kg respectively the mobilised metals form the orogenic gold deposits that occur in the southern alps different styles of gold deposits form contemporaneously during the active orogenesis of the southern alps including those with a fluid temperature rock temperature that may appear have formed after the peak of metamorphism but are instead just the product hydrothermal fluid mineralising rocks on their retrograde metamorphic path the mass balance shows that there has been orders of magnitude more metal mobilised in the orogen than resides in the currently known deposits there is clear potential for large gold deposits occurring in the yet to be uplifted parts of the southern alps if there have been efficient enough fluid focussing and metal precipitation mechanisms occurring under the southern alps

fluid evolution in a volcanic hosted epithermal carbonate base metal gold vein system alto de la blenda farallon negro argentina alto de la blenda is a 6 6 ma intermediate sulphidation epithermal vein system in the farallon negro volcanic complex which also hosts the 7 1 ma porphyry cu au deposit of bajo de la alumbrera the epithermal vein system is characterised by a large extent and continuity 2 km 400 m open to depth 6 m maximum width and an average gold grade of 8 g t the vein is best developed within an intrusion of a fine grained equigranular monzonite interpreted as the central conduit of a stratovolcano whose extrusive activity ended prior to porphyry cu au emplacement at bajo de la alumbrera which is in turn cut by minor epithermal veins the alto de la blenda vein consists predominantly of variably mn rich carbonates and quartz with a few percent of pyrite sphalerite galena and other sulphide and sulphosalt minerals four phases of vein opening hydrothermal mineralisation and repeated brecciation can be correlated between different vein segments stages 2 and 3 contain the greatest fraction of sulphide and gold they are separated by the emplacement of a polymictic breccia containing clasts of quartz feldspar porphyry as well as basement rocks fluid inclusions in quartz related to stages 2 to 4 are liquid rich with 2 4 wt nacl eq they homogenise between 160 and 300 c with very consistent values within each assemblage vapour inclusions are practically absent in the epithermal vein quartz fragments in the polymictic breccia contain inclusions of intermediate to vapour like density and similar low salinity 3 wt nacl eq besides rare brine inclusions containing halite laser ablation inductively coupled plasma mass spectrometry la icp ms analyses of epithermal inclusions indicate high concentrations of k fe as sb cs and pb that significantly vary within and through subsequent vein stages careful consideration of detection limits for individual inclusions shows high gold concentrations of 0 5 to 3 ppm dissolved in the ore fluid which contains variably high sulphur concentrations in excess over fe and other chalcophile metals compositional variations are interpreted to reflect cooling and contraction of lower density magmatic fluids at depth like those preserved in porphyry clasts that were mechanically transported up by the polymictic breccia ore mineral precipitation from the magmatic fluid occurred by further cooling and possibly minor mixing with surface derived water leading to sulphide saturation de sulphidation of the magmatic fluid and consequent gold precipitation the absence of flash boiling and or reduction by carbonaceous host rocks has led to relatively modest but constant gold grades in the carbonate base metal gold veins of alto de la blenda

abundance of zinc isotopes as a marine biogeochemical tracer 1 the zn isotopic compositions of ferromanganese nodules sediment trap samples sediments and organic reference samples have been analyzed by multiple collector inductively coupled plasma mass spectrometry icp ms the range of isotopic variations 1 for 66zn 64zn is significant with respect to the analytical precision 0 04 at the 95 confidence level marine argillaceous sediments show rather constant values consistent with those of continental shales and loess and with the value of a basalt from reunion island we infer that the depletion of the light zn isotopes in marine particles and in ferromanganese nodules presumably equilibrated with seawater results from biological activity in the upper water column the 66zn values of sediment trap samples collected near the upwelling off the coast of mauritania central atlantic show a seasonal isotopic fluctuation consistent with biological pumping during the high productivity period higher values of 66zn in nodules appear to be associated with the amplitude of seasonal variations rather than with the mean values of the biological productivity

sulfur cycling in the aftermath of a 635 ma snowball glaciation evidence for a syn glacial sulfidic deep ocean we have analyzed 34 s sulfate and pyrite 18 ocarbonate and 13 ccarbonate and major and trace elemental concentrations including extractable fe and mn phases in four sections of the maieberg formation the cap carbonate sequence to the marinoan glaciation in northern namibia 34 ssulfate profiles and other geochemical characteristics in the basal transgressive cap dolostone keilberg member are nearly identical in all sections and indicate deposition from a water mass with very low sulfate concentrations in the overlying interval that consists of rhythmites deposited during the transgressive high stand large geochemical disparities occur between sections that we interpret to have been deposited in open ocean versus restricted settings in the former a large negative shift in 34 ssulfate of 20 above the cap dolostone accompanies a change in mineralogy from dolomite to limestone a sharp decrease in 18 o and 13 c and a positive spike in fe and mn concentrations in the latter dolomite persists above the cap dolostone 34 ssulfate increases abruptly 18 o and 13 c are invariant and a spike in mn and fe concentrations is present but subtle these contrasting geochemical signatures in coeval sections can be explained by strong lateral chemical gradients that developed as cold euxinic deep water and a cap of warm oxic brackish water flooded the continental shelf during the post glacial transgression the geochemical di e rences between open ocean and restricted sections diminishes upsection presumably recording the gradual mixing and homogenization of the waters along the otavi platform

zn isotope evidence for immediate resumption of primary productivity after snowball earth the ediacaran period began with the deglaciation of the ca 635 ma marinoan snowball earth and the deposition of cap dolostones on continental shelves worldwide during post glacial sea level rise these carbonates sharply overlie glacial sediments deposited at low paleolatitudes and preserve negative carbon isotope excursions the snowball earth hypothesis invokes an almost complete cessation of primary productivity in the surface ocean because assimilatory uptake of zn appears to fractionate its isotopes zn isotope ratios measured in carbonate precipitated in the surface ocean should track fluctuations in primary productivity here we report the first zn isotopic data together with carbon and oxygen isotopic profiles from a neoproterozoic cap dolostone the nuccaleena formation in the flinders ranges south australia we interpret the zn isotopic data in terms of a two stage evolution of the deglacial ocean slightly 66 zn enriched values at the base of the cap dolostone indicate immediate resumption of the biological pump upon melting of the surface ocean but this signal was diluted by intense surface runoff that drove 66 zn 66 zn 64 zn versus the jmc lyon reference values down to the composition of continentally derived zn a subsequent rise in 66 zn records a vigorous increase in primary production and export from a nutrient laden surface ocean

pulsed magmatic fluid release for the formation of porphyry deposits tracing fluid evolution in absolute time from the tibetan qulong cu mo deposit the magmatic hydrothermal evolution of porphyry style mineralization in the shallow crust that is linked to magmatic processes at depth has been extensively studied using bulk sample isotopic analysis combined with relative timing constraints however a lack of evaluation of the fluid evolution process against an absolute time frame limits further understanding of the ore forming process here we quantify the fluid evolution process within an absolute time frame for the first time by integrating new in situ oxygen isotope data from the qulong porphyry cu mo deposit tibet with existing fluid inclusion data and high precision re os dates of co precipitated hydrothermal quartz and molybdenite respectively we demonstrate that vein quartz records primary oxygen isotopic compositions and reached oxygen isotope equilibrium with ore forming fluids and therefore is an archive of the isotopic composition and source of the ore forming fluids the 18oquartz and 18ofluid values in absolute time show periodic fluctuations that indicate the presence of three intermittent pulses of magmatic fluid flux which have been balanced by meteoric water as such the flux of magmatic fluid during ore formation was pulsed rather than being continuous the overall highest 18ofluid in the first pulse of mineralization with a gradual decrease to the second and third pulses is suggestive of a progressive reduction in the magmatic component of the hydrothermal fluids and by inference the mineralizing potential of the hydrothermal fluids this view is supported by a decrease in sulfide bearing fluid inclusions and metal grade through time our findings favor multiple fluid release events from a single cooling magmatic reservoir although multiple fluid melt recharge events remain a competitive alternative an additional implication is that the magmatic reservoir may have a lifespan of hundreds of thousands of years with fluid release events occurring over tens of thousands of years

the snowball earth aftermath exploring the limits of continental weathering processes article i nfo carbonates capping neoproterozoic glacial deposits contain peculiar sedimentological features and geochemical anomalies ascribed to extraordinary environmental conditions in the snowball earth aftermath it is commonly assumed that post snowball climate dominated by co2 partial pressures several hundred times greater than modern levels would be characterized by extreme temperatures a vigorous hydrological cycle and associated high continental weathering rates however the climate in the aftermath of a global glaciation has never been rigorously modelled here we use a hierarchy of numerical models from an atmospheric general circulation model to a mechanistic model describing continental weathering processes to explore characteristics of the earth system during the supergreenhouse climate following a snowball glaciation these models suggest that the hydrological cycle intensifies only moderately in response to the elevated greenhouse indeed constraints imposed by the surface energy budget sharply limit global mean evaporation once the temperature has warmed sufficiently that the evaporation approaches the total absorbed solar radiation even at 400 times the present day pressure of atmospheric co2 continental runoff is only 1 2 times the modern runoff under these conditions and accounting for the grinding of the continental surface by the ice sheet during the snowball event the simulated maximum discharge of dissolved elements from continental weathering into the ocean is approximately 10 times greater than the modern flux consequently it takes millions of years for the silicate weathering cycle to reduce post snowball co2 levels to background neoproterozoic levels regarding the origin of the cap dolostones we show that continental weathering alone does not supply enough cations during the snowball melting phase to account for their observed volume

siliciclastic prelude to elatina nuccaleena deglaciation lithostratigraphy and rock magnetism of the base of the ediacaran system

a red algal bloom in the aftermath of the marinoan snowball earth organic matter from the cap dolostones overlying the marinoan age glacigenic diamictites of the araras group amazon craton has been studied to reconstruct the post glacial ecosystem molecular fossils indicate that the post marinoan ecosystem was marked by an apparent decline in marine algal diversity the proliferation of red algae may be explained by environmental changes such as a massive nutrient input accompanying continental weathering after the ice thaw and a dimer light penetrating sea water due to the drowning of the platform in addition the presence of green sulphur bacteria indicates that sea water was stratified with an anoxic possibly euxinic i e sulphidic layer at the water sediment interface sulphur cycling probably occurred at the redox boundary as suggested by the recognition of active sulphate reduction this observation supports a microbially induced model for the formation of the cap dolostones

the flux of gold and related metals through a volcanic arc taupo volcanic zone new zealand we determined the concentrations of gold silver arsenic antimony and mercury in deep hydrothermal solutions 1 km depth 200 to 300 c from six geothermal systems and calculated their fluxes for a 250 km long segment of volcanic arc in the taupo volcanic zone new zealand the three orders of variation in the concentrations

volatility of se and te during subduction related distillation and the geochemistry of epithermal ores of the western united states subduction partial melting of mantle and magma generation and ultimately magma degassing can lead to the preferential mobilization of volatile elements and compounds research indicates that devolatilization reactions in a subducted slab can cause the overlying mantle to be enriched in volatile ore forming constituents water co2 sulfur metal loid s this process of mantle enrichment plays an important role in subsequent magma generation and initiation of ore genesis furthermore the relative volatility of metal loid s appears to play a role in the ultimate geochemistry of shallow epithermal epizonal hydrothermal ores in the western united states for example there is a general west to east geographic zonation of tertiary ores from se rich to te rich it is proposed that this geographic geochemical zonation is the direct result of the higher geochemical volatility of se as compared to te previously au ag te se hg as sb and tl have been referred to as the epithermal suite of elements it is suggested here that their geochemical cycles are linked by their similar volatile behavior through subduction partial melting in the mantle and subsequent magma degassing processes essentially specific tectonomagmatic events that affected the western united states e g oligocene eocene magmatism in the great basin and southern rocky mountains mid miocene magmatism associated with the inception of the yellowstone hotspot in the pacific northwest were distillation events that drove devolatilization of fertile mantle after the laramide shallow slab event furthermore the metal loid volatility behavior likely occurs in other regions affected by subduction and may play a fundamental role in the metal endowment of these regions

isotope signatures associated with early meteoric diagenesis the environments in which carbonate diagenesis proceeds have been documented in previous studies of holocene and late pleistocene sediments and limestones on barbados west indies variations in the carbon and oxygen isotopic composition of limestones produced during early freshwater diagenesis have been observed in this study to occur in specific patterns six potentially useful patterns emerge when one views stable isotope data within a stratigraphic framework 1 the subaerial exposure surface is characterized by strongly 12c enriched limestones 13c compositions of underlying limestones grow progressively heavier with increasing depth 2 the subaerial exposure surface may also be marked by slight 18o enrichment 3 an abrupt shift in 18o values may differentiate sediments above the exposure surface from those below 4 sediments altered in the marine meteoric mixing zone may be characterized by positive covariance between their 18o and 13c compositions 5 the vadose phreatic boundary may be marked by a sharp increase in 13c values in the seaward portions of a fresh groundwater system and 6 samples contemporaneously altered in a single fresh groundwater system within an areally restricted region should display a narrow range of 18o and a wide range of 13c compositions analysis of samples from five palaeozoic and mesozoic formations which contained petrographic evidence of early freshwater diagenesis showed that isotope patterns similar to those observed in barbados limestones have been preserved in rocks as old as mississippian these isotope patterns could prove to be useful for identifying diagenetically induced porosity trends in carbonate rocks they might be used to identify limestones diagenetically altered in meteoric environments to identify mixing zone cements and dolomites and to trace the regional and vertical distributions of early meteoric groundwater systems in ancient carbonate formations

zinc isotope evidence for intensive magmatism immediately before the end permian mass extinction the end permian extinction is typically ascribed to massive volcanic eruptions but direct geochemical evidence linking the two independent events is generally lacking zinc is an important micronutrient of marine phytoplanktons and zn isotope 66zn ratios of seawater are markedly higher than those of volcanic rocks and riverine waters we conducted high resolution zn concentration and zn isotope analyses on carbonate rocks across the permian triassic boundary ptb in the meishan section of south china an abrupt increase of zn concentration and a concomitant 0 5 decrease in 66zn occur 35 k y before the mass extinction and carbon isotope 13c minima mass balance calculation demonstrates that a 0 5 negative shift in 66zn within thousands of years requires rapid and massive input of isotopically light zn from volcanic ashes hydrothermal inputs and or extremely fast weathering of large igneous provinces a positive 66zn shift of as much as 1 0 following the mass extinction demonstrates that primary productivity recovered and reached a maximum in fewer than 360 k y our finding provides insights into the marine zn cycling across the ptb and clarifies the temporal relationship and duration of events including intensive volcanism carbon isotope excursion mass extinction and widespread ocean anoxia

metal content and p t evolution of co2 bearing ore forming fluids of the haftcheshmeh cu mo porphyry deposit nw iran abstract the haftcheshmeh cu mo porphyry deposit is located in the nw part of the arasbaran metallogenic magmatic zone nw iran ore mineralisation is generally hosted within the gabbro diorite and granodiorite porphyry stocks presenting various types of hydrothermal quartz veins and veinlets associated with potassic to sericitic alteration stage i corresponds to barren quartz k feldspar biotite anhydrite veins a veins with minor metal sulphides pyrite chalcopyrite surrounded by k feldspar and biotite halos related to potassic alteration stage ii is described with quartz molybdenite chalcopyrite pyrite bornite magnetite hematite biotite veins predominantly occurring in the potassic alteration halos b veins stage iii shows pyrite quartz chalcopyrite molybdenite galena sphalerite associated with sericitic alteration halos d veins the coexistence of halite saturated aqueous rich and low salinity vapour rich fluid inclusions in the h2o nacl co2 system is evidence of fluid boiling in each alteration mineralising stage this process was subsequently followed by meteoric water input diluting the co2 bearing ore forming fluid la icpms data of fluid inclusions associated with the successive stages reveal elevated na k fe mn cu and mo content the metal content of fluid inclusions of stages ii and iii is in large part due to accidentally trapped minerals molybdenite and chalcopyrite started precipitating between stages i and ii from a metal poor fluid the long duration 1 ma of the magmatic hydrothermal activity and associated co2 fluxes and repeated fluid boiling events lead to ore deposition at temperatures of 450 to 250 c and at depths between 3 0 and 1 0 km from near lithostatic to near hydrostatic conditions

resources of gold in phanerozoic epithermal deposits we have estimated earth s endowment of gold in phanerozoic epithermal deposits using a tectonic diffusion model which simulates the emplacement of deposits at a shallow crustal depth and their subsequent vertical tectonic migration in the crust the calculation was calibrated by least squares comparison of a calculated age frequency distribution to the age frequency distribution for 448 epithermal deposits of phanerozoic age results indicate that 17 percent of the epithermal deposits that formed through phanerozoic time remain in the crust today whereas 83 percent have been removed by erosion assuming a similar age distribution for all 1 181 epithermal deposits in our compilation indicates that 307 000 deposits formed throughout phanerozoic time that 63 000 of these remain in the crust and that 244 000 have been eroded grade and tonnage data of gold in 757 epithermal deposits in the compilation have an arithmetic average of 34 7 t and yield an estimate of 2 2 106 t of gold for epithermal deposits remaining in the crust using the model based vertical distribution of deposits remaining in the crust we estimate that epithermal deposits in the upper kilometer of the united states contain 2 9 104 t of gold adjusting for areas of outcropping volcanic rocks in each continent increases our estimate to 4 0 104 t these estimates are similar to the 1 7 104 t gold resource estimated by the u s geological survey using completely different methods several comparisons based on these results provide insights into the efficiency of processes that form epithermal deposits and the outlook for sustainability for instance all of the epithermal deposits that formed through phanerozoic time represent only about 0 03 percent of the gold in the crust only about 0 007 percent of crustal gold remains in epithermal deposits the rest has been eroded and recycled this is consistent with comparisons based on the flux of gold in the taupo volcanic zone which indicate that epithermal deposits trap only 0 1 to 0 2 percent of the gold that moves through convergent margin hydrothermal systems epithermal gold deposits that remain in the crust can supply current production from them for 1 400 to 5 000 years depending on success of exploration and depth of mining finally we are consuming gold from epithermal deposits about 17 000 times faster than earth is replenishing the supply

mineral deposits in the rotokawa geothermal pipelines new zealand abstract at the point of flashing at 224 c and 25 bar absolute black silica scales enriched in cu 5 6 ag 4 0 te 3 0 zn 1 5 pb 0 6 and au 0 2 form in the wellhead of production well rk 9 silver pb cu and au are deposited as tellurides cu pb and zn as sulfides and au as a native element these elements decrease rapidly downstream of the wellhead to other parts of the surface geothermal pipeline mercury b as se sb and reduced s are partially transported in the gas phase and largely deposited at a distance from the wellhead total s and b occur primarily in the steam line where b is stabilised by silica deposited from carry over brine and s by sulfide mineralisation arsenic and se are mainly found in scales from the steam and mixing reinjection lines hg in the condensate and mixing reinjection lines and sb in the mixing reinjection line where 80 105 c low salinity condensate mixes with 140 170 c brine a decrease in the solubility of minerals in solution is the primary cause for deposition of opal a and increased concentrations of base and precious metal concentrations in scales scale deposition is thus induced by flashing at the wellhead of the production wells attached to the ormat binary plant phase separation dilution cooling and mixing of cooled brine and condensate the reaction of steel with circulating solutions and gases results in a corrosion mineral assemblage reflecting local oxidation potentials and variations in h2s partial pressures along the pipeline corrosion of steel introduces and augments trace elements such as p cr mn ni and mo in the pipeline solutions selective incorporation or sorption of elements onto opal a and opal ct or corrosion products such as goethite and pyrite may increase concentrations of li b cl and p in opal cr mn ni cu zn and mo in goethite and cu zn as hg sb and se in pyrite wherever water circulates in the pipeline system silica precipitates initially from solution as a gel which later solidifies to opal a and subsequently crystallises to opal ct then quartz aggregates the transformation of opal a to more ordered and stable silica polymorphs in the pipelines is a kinetically induced dissolution reprecipitation crystal growth process that is accelerated at temperatures 200 224 c

no evidence of extraterrestrial noble metal and helium anomalies at marinoan glacial termination abstract high concentrations of extraterrestrial iridium have been reported in terminal sturtian and marinoan glacial marine sediments and are used to argue for long likely 3 12 myr durations of these cryogenian glaciations reanalysis of the marinoan sedimentary rocks used in the original study supplemented by sedimentary rocks from additional terminal marinoan sections however does not confirm the initial report new platinum group element concentrations and 187os 188os and 3he 4he signatures are consistent with crustal origin and minimal extraterrestrial contributions the discrepancy is likely caused by different sample masses used in the two studies with this study being based on much larger samples that better capture the stochastic distribution of extraterrestrial particles in marine sediments strong enrichment of redox sensitive elements particularly rhenium up section in the basal postglacial cap carbonates may indicate a return to more fully oxygenated seawater in the aftermath of the marinoan snowball earth sections dominated by hydrogenous osmium indicate increasing submarine hydrothermal sources and or continental inputs that are increasingly dominated by young mantle derived rocks after deglaciation sedimentation rate estimates for the basal cap carbonates yield surprisingly slow rates of a few centimeters per thousand years this study highlights the importance of using sedimentary rock samples that represent sufficiently large area time products to properly sample extraterrestrial particles representatively and demonstrates the value of using multiple tracers of extraterrestrial matter

world class base and precious metal deposits a quantitative analysis over 62 percent of the 193 000 metric tons of gold discovered to date is located in four countries and more than 68 percent occurs in four types of mineral deposits about 55 percent of the 1 740 000 metric tons of silver found is in four countries and 45 percent is in four types of deposits fifty six percent of the 1 52 billion metric tons of discovered copper is from four countries and four types of deposits contain 88 percent of the total over 50 percent of both the 713 000 000 metric tons of zinc and 349 000 000 metric tons of lead discovered to date come from four countries and 70 percent of both metals occur in four types of deposits all discovered gold would fit in a cube with a height of 22 m silver in a 55 m cube copper in a 550 m cube zinc in a 460 m cube and lead in a 310 m cube at least 74 percent of gold silver zinc and lead is in deposits having average grades above the respective median grades and 44 percent of copper is in deposits with average grades above the median grade of all deposits lower grade deposits contain less total metal than higher grade deposits tonnage of mineralized rock is an even better predictor of contained metal with over 96 percent of each metal s total residing in deposits having greater than median size and between 47 and 79 percent of metal contained in the largest 10 percent of deposits world class deposits defined as the upper 10 percent of deposits in terms of contained metal account for over 86 percent of all gold 79 percent of silver 84 percent of copper 71 percent of zinc and 73 percent of lead these giant deposits contain at least 100 metric tons 3 2 moz gold 2 400 metric tons 77 moz silver 2 million metric tons copper 1 7 million metric tons zinc or 1 million metric tons lead mineral deposits occur rarely in the earth s crust and large ones are especially uncommon this analysis shows that only the unusually large deposits can significantly affect supply

boron and calcium isotope composition in neoproterozoic carbonate rocks from namibia evidence for extreme environmental change the level and evolution of atmospheric carbon dioxide throughout earth s history are key issues for palaeoclimate reconstructions especially during times of extreme climate change such as those that marked the neoproterozoic the carbon isotope ratios of marine carbonates are crucial in the correlation and identification of neoproterozoic glacial deposits and they are also used as a record for biogeochemical cycling and potential proxy for atmospheric pco2 likewise the boron and calcium isotope compositions of marine carbonates are potential proxies for palaeo seawater ph and the ratio of calcium fluxes into and out of seawater respectively and together they may be used to estimate atmospheric carbon dioxide here we use b and ca isotopes to estimate palaeoenvironmental conditions in the aftermath of a major neoproterozoic glaciation in namibia the validity of the b and ca isotope variation in the ancient marine carbonates is evaluated using the oxygen isotope composition of the carbonates and its correlation to the carbon isotope variation a negative 2 7 to 6 2 11b excursion occurs in the postglacial carbonates and is interpreted to reflect a temporary decrease in seawater ph associated variations in 44ca values ranging between 0 35 and 1 14 are linearly coupled with the carbon isotope ratios and imply enhanced postglacial weathering rates the reconstructed seawater ph and weathering profiles indicates that high atmospheric co2 concentrations were likely during the melt back of neoproterozoic glaciations and precipitation of cap carbonates however the b isotope trend suggests that these concentrations rapidly ameliorated and they do not co vary with 13c thus models attempting to link long lived negative 13c excursions to elevated pco2 need to be reconsidered

gold in magmatic hydrothermal solutions and the rapid formation of a giant ore deposit the ladolam hydrothermal system on lihir island papua new guinea hosts one of the youngest and largest gold deposits in the world several deep more than 1 kilometer geothermal wells were drilled beneath the ore bodies to extract water at 275 c and to facilitate open pit mining using a titanium down hole sampler we determined that the deep geothermal brine of magmatic origin contains 15 parts per billion gold at the current gold flux of 24 kilograms per year this deposit could have formed within 55 000 years the combination of sustained metal flux and efficient metal precipitation led to the formation of a giant hydrothermal gold deposit in a short period

tectonic framework and evolution of the gawler craton southern australia the gawler craton is the major crustal province in the southern australian proterozoic and is pivotal in models seeking to describe the evolution of proterozoic australia the craton is host to the olympic dam iron oxide cu au u ree deposit as well as a number of other iron oxide copper gold iocg au and iron ore deposits the evolution of the gawler craton is dominated by two major phases of tectonic activity late archean and late paleoproterozoic early mesoproterozoic in total spanning ca 1 billion years the late archean 2560 2500 ma basin development was coeval with arclike felsic magmatism and mafic ultramafic magmas including komatiites collisional deformation between 2480 and 2420 ma led to ca 400 m y of tectonic quiescence conceivably within an early paleoproterozoic continental interior the second major phase of tectonic activity was in the middle to late paleoproterozoic and early mesoproterozoic 2000 1450 ma paleoproterozoic tectonism was initially dominated by rift related events which produced a series of basins over the interval ca 1900 to 1730 ma some of which contain significant iron ore reserves transient contractional deformation at ca 1850 ma led to batholith scale granitic magmatism in part derived from melting of the late archean basement major basin development was finally terminated by the 1730 to 1690 ma kimban orogeny the effects of which are widespread and include the formation of crustal scale shear systems granitic magmatism and low to high grade metamorphism the kimban orogeny was followed by a renewed period of extension between ca 1680 and 1640 ma leading to local magmatism and sedimentation which appears in part to have been coeval with the high grade ca 1650 ma ooldean event in the western gawler craton at ca 1620 to 1615 ma the generation of the arc related st peter suite in the southern gawler craton implies that craton components were located at an active plate margin the arc environment was superseded by a transition to a continental interior regime which coincided with the voluminous gawler range volcanics 1595 1590 ma and hiltaba suite granitoids 1595 1575 ma the hiltaba suite is dominated by high t fractionated felsic rocks with coeval mafic magmatism confirming a mantle involvement emplacement of the hiltaba suite coincided with the major mineralizing interval in the gawler craton regionally two distinct mineral systems have been recognized the olympic iocg province in the eastern part of the craton and the gold dominated systems within the central gawler gold province the spatial distribution of iocg versus au dominated mineral systems appears to reflect regional variations in crustal composition and hiltaba suite petrogeneses hiltaba aged granites in the olympic iocg province are isotopically more evolved richer in u and th and oxidized compared to similar aged granites in the au dominated province modern day heat flow in the iocg province is significantly higher 90 10 mwm 2 compared to the au dominated province 54 5 mwm 2 suggesting there are important lithospheric compositional differences between the two metallogenic provinces which may reflect an older phase of craton assembly widespread northwest southeast contractional deformation coeval with the emplacement of the hiltaba suite is expressed by the formation and or reactivation of shear zones ranging up to crustal scale within this regime northwest trending structures such as those in the vicinity of the olympic dam deposit are likely to have accommodated dilation associated with strike slip movements intersection between these structures and northeast trending contractional faults may have formed suitable structural traps for 1590 to 1580 ma mineralization the timing of syn hiltaba deformation overlaps with transpression and medium to high grade metamorphism associated with the ca 1570 to 1540 ma kararan orogeny which dominates the geophysical architecture of the central northern part of the craton the youngest phase of deformation in the craton is expressed by reactivation of shear zones between ca 1470 and 1450 ma and regional cooling

testing models for post glacial cap dolostone deposition nuccaleena formation south australia sedimentologically and geochemically distinctive carbonate sequences consistently drape the glacial deposits associated with the younger cryogenian ice age the presence of ice rafted debris in the basal dolostone implies that at least the lower portion of the cap sequence records deglaciation an isochronous model proposes that cap dolostones were deposited synchronously around the world regardless of water depth whilst a diachronous model proposes that deposition tracked glacioeustatic flooding during deglaciation the nuccaleena formation of the adelaide rift complex arc south australia exhibits many of the unique sedimentary features observed in other younger cryogenian cap dolostones around the world some bedforms are the product of wind driven oscillatory flow thus constraining post glacial dolostone deposition to 400 m these sedimentary features are absent in the deepest basinal facies in the northern arc suggesting that this region was below storm wave base even at the glacial sea level lowstand in the north flinders ranges there is a distinct relationship between lateral facies variability in the pre syn and post glacial sediments and the axes of 50 km scale structural folds the northern fold limbs are characterised by basinal facies whilst the southern limbs are associated with upper slope facies we interpret the abrupt facies transitions to reflect lower slope to upper slope shelf breaks across a series of linked south facing half grabens the majority of cap dolostone carbon isotope records show monotonic declines in 13c of 2 with starting points between 0 5 and 3 5 an isochronous model implies a 3 0 lateral gradient from platform to lower slope that varies dramatically on a short spatial scale and non systematically with palaeobathymetry if pre and syn glacial facies are used to infer palaeobathymetry and cap dolostones are deposited diachronously as sea level rises during deglaciation then 13c values becomes progressively lighter with time implying that cap dolostone deposition tracked the glacioeustatic sea level rise over a series of half grabens that deepened to the north the carbon isotope dataset cannot rule out uniquely isochronous or diachronous models given the high frequency spatial variability of 13c values temperature cannot be the dominant control on the isotopic variability of the cap dolostone

the behavior of metals pb zn as mo cu during crystallization and degassing of rhyolites from the okataina volcanic center taupo volcanic zone new zealand tthe taupo volcanic zone tvz new zealand is a region of voluminous and frequent rhyolitic volcanism and widespread geothermal activity additionally the hydrothermal systems of the tvz contain relatively high concentrations of base and precious metals here we present an extensive dataset of major element volatile and trace element including pb zn as mo cu abundances in melt inclusions pumice glasses and minerals from eight eruptions within the okataina volcanic center ovc of the tvz to investigate the behavior of metals during melt evolution the high sio2 rhyolites of the ovc contain high concentrations of volatiles 6 wt h2o 0 25 wt cl and underwent significant degassing prior to and during eruption the ovc melts contain moderate concentrations of metals 11 24 ppm pb 20 50 ppm zn 2 7 ppm as 2 5 ppm mo 5 ppm cu ferromagnesian minerals amphibole biotite and orthopyroxene in the ovc pumice have high concentrations of zn 1500 ppm and plagioclase and biotite contain moderate amounts of pb 11 ppm the melt inclusion and pumice glass trace element data reveal complex histories of magma mixing and mingling prior to eruption however discrete melt batches are easily identified based on trace element geochemistry variations in incompatible trace elements within these melt batches suggest that the ovc rhyolites underwent at least 20 25 fractional crystallization during quartz crystallization and melt inclusion entrapment at pressures of 100 200 mpa and little to no crystallization 5 during ascent and eruption comparison of melt inclusion metal and incompatible element e g u concentrations reveals that melt pb mo and as increase whereas melt zn decreases during fractional crystallization at depth 100 200 mpa these observations can be explained by minor partitioning of the metals pb mo and as into the fractionating minerals and stronger partitioning of zn into the ferromagnesian phases supported by calculated metal d values and analyzed metal concentrations in ovc minerals interestingly throughout both deep vapor saturated crystallization and during extensive degassing during magma ascent and eruption as recorded by pumice glasses the metals analyzed here do not appear to partition appreciably into the vapor we propose that the lack of volatility of the metals analyzed in this study can be attributed to a combination of several factors first vapor melt partitioning requires the presence of ligands commonly cl s and oh with which the metals may complex given the low cl h2o ratios in the ovc melts and the extensive degassing of h2o compared with cl it seems likely that the rhyolites would have exsolved h2o rich vapor with insufficient cl to transport metals in particular pb and zn into the vapor phase either at depth or during magma ascent second the overall small volumes of vapor present during crystallization at pressures of 100 200 mpa would have impeded significant vapor melt partitioning of the metals finally the estimated very rapid ascent of the ovc melts from depths of 4 8 km suggests that there was insufficient time at low pressure for diffusion of metals out of the melt these results imply that there may be an indirect connection between the rhyolites and the metals of the hydrothermal systems of the tvz as the metals and other species such as cl remain in the rhyolitic magmas upon eruption they are available in the large volumes of rhyolite emplaced in the upper crust of the tvz for leaching by heated meteoric waters

160 m y record of marine sedimentary phosphorus burial coupling of climate and continental weathering under greenhouse and icehouse conditions a compilation of all meaningful phosphate data published in the proceedings of the deep sea drilling project and the ocean drilling program is used here for the extraction of a 160 m y marine burial record of bulk phosphorus all sediment types 5648 measurements and a 100 m y marine burial record of biogenic phosphorus pelagic biogenic sediment types 1754 data these records serve as estimates for total and dissolved bioavailable phosphorus flux rates which in turn are highly dependent on total and chemical continental weathering rates respectively the similarity of both records indicates that in the past 100 m y changes in total weathering rates may have been tracked by changes in chemical weathering rates prior to 32 ma phosphorus burial and long term sea level change are positively correlated from 32 ma to the present this correlation is inverse this points to a fundamental change in feedback mechanisms between continental weathering phosphorus and climate which was probably linked to the onset of major glaciation