high precision isotopic characterization of usgs reference materials by tims and mc icp ms the pacific centre for isotopic and geochemical research pcigr at the university of british columbia has undertaken a systematic analysis of the isotopic sr nd and pb compositions and concentrations of a broad compositional range of u s geological survey usgs reference materials including basalt bcr 1 2 bhvo 1 2 andesite agv 1 2 rhyolite rgm 1 2 syenite stm 1 2 granodiorite gsp 2 and granite g 2 3 usgs rock reference materials are geochemically well characterized but there is neither a systematic methodology nor a database for radiogenic isotopic compositions even for the widely used bcr 1 this investigation represents the first comprehensive systematic analysis of the isotopic composition and concentration of usgs reference materials and provides an important database for the isotopic community in addition the range of equipment at the pcigr including a nu instruments plasma mc icp ms a thermo finnigan triton tims and a thermo finnigan element2 hr icp ms permits an assessment and comparison of the precision and accuracy of isotopic analyses determined by both the tims and mc icp ms methods e g nd isotopic compositions for each of the reference materials 5 to 10 complete replicate analyses provide coherent isotopic results all with external precision below 30 ppm 2 sd for sr and nd isotopic compositions 27 and 24 ppm for tims and mc icp ms respectively our results also show that the first and second generation usgs reference materials have homogeneous sr and nd isotopic compositions nd isotopic compositions by mc icp ms and tims agree to within 15 ppm for all reference materials interlaboratory mc icp ms comparisons show excellent agreement for pb isotopic compositions however the reproducibility is not as good as for sr and nd a careful sequential leaching experiment of three first and second generation reference materials bcr bhvo agv indicates that the heterogeneity in pb isotopic compositions and concentrations could be directly related to contamination by the steel mortar pestle used to process the materials contamination also accounts for the high concentrations of certain other trace elements e g li mo cd sn sb w in various usgs reference materials

mineralogy and geochemistry of gold bearing arsenian pyrite from the shuiyindong carlin type gold deposit guizhou china implications for gold depositional processes arsenian pyrite in the shuiyindong carlin type gold deposit in guizhou china is the major host for gold with 300 to 4 000 ppm au and 0 65 to 14 1 wt as electron miroprobe data show a negative correlation of as and s in arsenian pyrite which is consistent with the substitution of as for s in the pyrite structure the relatively homogeneous distribution of gold in arsenian pyrite and a positive correlation of as and au with au as ratios below the solubility limit of gold in arsenian pyrite suggest that invisible gold is likely present as au1 in a structurally bound au complex in arsenian pyrite geochemical modeling using the laser ablation inductively coupled plasma mass spectrometry la icp ms analysis of fluid inclusions for the major ore forming stage shows that the dominant au species were au hs 2 77 and auhs aq 0 23 gold hydroxyl and gold chloride complexes were negligible the ore fluid was undersaturated with respect to native au with a saturation index of 3 8 the predominant as species was h3aso30 aq pyrite in the shuiyindong deposit shows chemical zonation with rims richer in as and au than cores reflecting the chemical evolution of the ore bearing fluids the early ore fluids had relatively high activities of as and au to deposit unzoned and zoned arsenian pyrite that host most gold in the deposit the ore fluids then became depleted in au and as and formed as poor pyrite overgrowth rims on gold bearing arsenian pyrite arsenopyrite overgrowth aggregates on arsenian pyrite indicate a late fluid with relatively high activity of as the lack of evidence of boiling and the low iron content of fluid inclusions in quartz suggest that iron in arsenian pyrite was most likely derived from dissolution of ferroan minerals in the host rocks with sulfidation of the dissolved iron by h2s rich ore fluids being the most important mechanism of gold deposition in the shuiyindong carlin type deposit

gold bearing arsenian pyrite and marcasite and arsenopyrite from carlin trend gold deposits and laboratory synthesis invisible gold in natural and synthetic arsenian pyrite and marcasite correlates with anomalous as content and fe deficiency and high contents of invisible gold in most natural and all synthetic arsenopyrite correlate with excess as and fe deficiency as rich fe deficient arsenopyrite synthesized hydrothermally contains up to 3 0 wt au uniformly distributed in growth zones of light backscattered electron contrast at the deep star gold deposit carlin trend nevada the sulfide compositions apparently span the full range of metastability from fesz to near feass 40 at s arsenian pyrite contains up to 0 37 wt au but arsenopyrite has excess s and is relatively au poor observed minimum fe contents are 29 1 at in arsenian pyrite and marcasite from the deep star deposit and 31 3 at in synthetic arsenopyrite we suggest that invisible gold in arsenian pyrite and marcasite and arsenopyrite from sediment hosted gold deposits represents au removed from ore fluids by chemisorption at as rich fe deficient surface sites and incorporated into the solids in metastable solid solution however the oxidation state of invisible gold aua au1 remains uncertain because the chemisorption process is intrinsically nonsystematic in terms of crystal chemical parameters and does not result in definitive atomic substitution trends

geochemistry and textures of gold bearing arsenian pyrite twin creeks nevada implications for deposition of gold in carlin type deposits this study relates morphology size texture and chemistry of arsenian pyrite in the twin creeks carlin type gold deposits to the chemical position and amount of gold that it contains arsenian pyrite is confirmed as the most important gold hosting mineral and four types of gold bearing arsenian pyrite are recognized among these four types the lowest amount of gold and arsenic 1 wt as 17 60 ppm au is contained in arsenian pyrite with relatively large grain sizes 10 30 mu m it formed at relatively high temperatures 250 degrees c and k super h super ratios adularia stable probably relatively slowly the largest amount of gold and arsenic 1 05 2 43 wt as 595 1 465 ppm au is contained in arsenian pyrite with small grain sizes up to 2 mu m that formed at low temperatures 120 degrees 200 degrees c and moderate k super h super ratios illite stable probably by relatively rapid deposition xanes exafs and sims analyses show that gold in arsenian pyrite is both structurally bound au i and in submicroscopic inclusions of free gold au highest amounts of structurally bound gold were observed in the relatively coarse grained arsenian pyrite noted above whereas most inclusions of free gold were observed in fine grained arsenian pyrite in general there is a good correlation between gold and arsenic in arsenian pyrite although au as ratios vary from one pyrite to another it is suggested that gold in arsenian pyrite was deposited by direct deposition of native gold and by adsorption of au i from hydrothermal solutions adsorption of gold by arsenian pyrite can take place from solutions that are not saturated with respect to native gold relative volumes of gold bearing pyrite from different paragenetic stages at twin creeks suggest that as much as 50 percent of the total gold in the deposit could have formed from solutions that were not saturated with respect to native gold because both au and au i are found together in arsenian pyrite it is likely that they were deposited by the same process consideration of possible depositional reactions suggests that a decrease in the activity of reduced sulfur is most likely to cause gold deposition

crustal origin for the parana rhyolites discussion of description and petrogenesis of the parana rhyolites southern brazil by garland et al 1995 our criticisms of the garland et al 1995 paper are introduction three fold and concern 1 the interpretation of oxygen the south atlantic ocean divides what was once a single isotope data on the rhyolites 2 radiogenic isotope large igneous province into the larger parana flood basalt fingerprinting of potential source material 3 the major province of south america and the smaller etendeka element composition of melts produced from potential flood basalt province of namibia the term flood basalt sources in the lower crust province is in this case something of a misnomer because a substantial thickness of silicic volcanic rocks is preserved on both sides of the atlantic the parana rhyolites can be divided into two main types the chapeco high ti oxygen isotopes type which have a high concentration of high field one of the problems of using oxygen isotopes to solve strength elements and low sr isotope ratios sr sr at problems of magma genesis is that oxygen isotope ratios 130 ma 0 705 0 708 and the palmas low ti type are changed during low temperature alteration processes which has lower concentrations of high field strength it is well known that large shifts in do value can occur elements and elevated sr isotope ratios sr sr at 130 in glassy volcanic rocks such as the parana rhyolites e g ma 0 714 0 727 mantovani et al 1985 bellieni et al taylor 1968 cerling et al 1985 these problems can 1986 peate et al 1992 garland et al 1995 suggested be circumvented by analysing fresh phenocrysts such as that the former were produced by partial melting of pyroxene which are unaffected by the alteration process high ti parana basalts whereas the more voluminous harris et al 1990 used the pyroxene do values to palmas rhyolites were produced by prolonged asestimate the magma do values of the palmas low ti similation accompanying crystal fractionation of low ti type rhyolite of the parana to be between 10 1 and parana basalt magmas previous workers have suggested 10 7 with the equivalent low ti magma type in the that the palmas type rhyolites are derived from melting etendeka having do values between 9 7 and 11 2 of pre mesozoic middle to lower crust piccirillo et al these high values led harris et al 1990 to suggest an 1988 harris et al 1990 and we believe that garland et origin by crustal melting al 1995 have overlooked several important lines of garland et al 1995 acknowledged the high do evidence in rejecting the crustal melting hypothesis in values and went on to state that the high do values this comment we will draw on data from the etendeka were used by harris et al 1990 to invoke an entirely province of namibia which was once part of the same crustal origin but it has been argued elsewhere that post eruptive alteration can lead to an increase in do continental flood basalt province

lower cretaceous tholeiitic dyke swarms from the ponta grossa arch southeast brazil petrology sr nd isotopes and genetic relationships with the parana flood volcanics the lower cretaceous dykes of the ponta grossa arch the most important dyke swarms in brazil are associated with the flood basalts and rare acid flows of the northern parana basin the ponta grossa pg dykes are formed by two pyroxene tholeiites and rare acid rocks the basaltic dykes may be distinguished into two main groups a dominant high tio2 2 wt hti group and a subordinate low tio2 2 wt lti group characterized for similar mgo content by high and low incompatible element contents respectively most pg dykes do not show chemical and isotope evidence supporting important crustal contamination pg dykes with 87sr86sr 0 0 7060 plot in the mantle array sr 17 and nd 3 and can be considered virtually uncontaminated intra dyke chemical variations suggest that the normal or reverse differentiation trends may be related to pre emplacement low pressure differentiation processes the important chemical differences between incompatible element poor and incompatible element rich basaltic dykes cannot be accounted for in terms of fractional crystallization but are compatible with different melting degrees of a garnet peridotite mantle source e g 9 and 20 melting for the hti and lti tholeiites respectively field distribution of the magmatism chemistry and sr nd isotope compositions support that the pg dykes may be related to the flood volcanics of northern parana and not to those of the central and southern parana paleomagnetic data indicate that pg dykes are younger than the parana volcanics all the results suggest that pg dykes were probably feeders of the stratoid volcanics erupted in northern parana towards the continental margin and later eroded chemistry and isotopic data reveal that asthenospheric mantle components were not significant factors in the genesis of pg dykes dyke emplacement occurred during early phases of rifting and or flexuring cutting flood volcanic suites

sediment hosted gold deposits in guizhou china products of wall rock sulfidation by deep crustal fluids sediment hosted gold deposits in guizhou china are hosted in late paleozoic and early mesozoic sedimentary rocks along the southwest margin of the precambrian yangtze craton they have characteristics similar to carlin type gold deposits in nevada and are notably enriched in as sb hg and tl the shuiyindong and yata deposits consist of disseminated strata bound sulfides in permian bioclastic limestone and fault controlled mineralization in middle triassic calcareous clastic rocks respectively mineralization in both deposits consists of barren milky quartz veins disseminated gold bearing arsenian pyrite and arsenopyrite stibnite realgar and orpiment the barren milky quartz veins occur in the ore controlling structures with an envelope of gold mineralization in the host rock consisting of disseminated gold bearing arsenian pyrite and arsenopyrite and replacement style quartz veinlets later drusy quartz stibnite realgar and orpiment fill fractures and vugs on the periphery of gold mineralization petrography microthermometry laser raman spectroscopy and laser ablation inductively coupled plasma mass spectrometric la icp ms analyses of fluid inclusions are used to characterize the chemical evolution of ore fluids at shuiyindong and yata early barren milky quartz veins trapped aqueous fluid with moderate salinity 6 0 wt nacl equiv and minor co2 2 4 mol at a temperature of 230 30 c this fluid contains measurable b na k cs sr as sb and au 3 8 0 5 5 7 2 3 g g determined by la icp ms and is interpreted to approximate the mineralizing fluid responsible for precipitation of early barren quartz veins low salinity 0 9 2 3 wt nacl equiv co2 bearing 6 3 8 4 mol aqueous fluids are recorded in replacement style quartz veinlets they formed gold bearing arsenian pyrite and arsenopyrite and quartz veinlets at a temperature of 210 20 c and pressures of 450 to 1 150 bars corresponding to depths between 1 7 and 4 3 km under lithostatic conditions fluid inclusions in late stibnite orpiment realgar quartz veins contain high co2 58 64 mol and n2 19 2 23 7 mol with trace ch4 up to 1 6 mol the increasing content of co2 and decrease in the concentrations of au and as in aqueous carbonic inclusions are interpreted to result from carbonate dissolution at the level of the deposits during gold mineralization covariance of au as sb and sr concentrations in the fluids is interpreted to reflect deposition of as and sb sulfides and gold concurrent with carbonate crystallization iron is below detection limit 400 g g in all fluid inclusion types suggesting that the ore fluids were fe poor but possibly sulfur rich possibly explaining their exceptionally high gold contents iron in sulfide minerals was probably derived from dissolution of ferroan minerals in the host rocks sulfidized by h2s rich fluids to precipitate arsenian pyrite arsenopyrite and gold pressure fluctuation induced by faulting resulted in local fluid immiscibility and led to late deposition of realgar orpiment stibnite and calcite hydrogen and oxygen isotope compositions of ore fluids d h2o 35 to 68 18o h2o 4 16 5 indicate a metamorphic origin possibly related to crustal thickening and prograde metamorphism during the late yanshanian orogeny

decoupled geochemical behavior of as and cu in hydrothermal systems cu rich and as rich growth zones in pyrite provide new insights into the composition of late stage magmatic fluids and their host hydrothermal ore deposits the pyrite is from the pueblo viejo dominican republic and yanacocha peru high sulfidation gold silver deposits which are thought to form from hydrothermal systems that interacted with magmatic vapor plumes electron microprobe analysis secondary ion mass spectrometry and elemental maps show that pyrite the most common sulfide mineral in both deposits contains three different types of growth zones 1 as rich zones that are enriched in au ag sb te and pb 2 cu rich zones with significantly lower concentrations of these elements and 3 barren pyrite zones with no other elements these zones are interpreted to result from mixing between the pyrite forming fluid and vapors that invaded the main hydrothermal system episodically comparison to experimental studies of elemental partitioning and analyses of fumaroles and fluid inclusions from magmatic hydrothermal systems suggests that the as rich vapor formed at high and possibly magmatic temperatures whereas the cu rich vapor formed at lower temperatures possibly during migration of the original magmatic vapor the presence of finely spaced multiple growth zones in pyrite suggests that the composition of at least high sulfidation hydrothermal systems can be affected intermittently and repetitively by vapors probably from underlying magmas

alteration associated with gold deposition at the getchell carlin type gold deposit north central nevada wall rock alteration at the getchell underground deposit was examined to determine the effects of au bear ing fluids on host lithologies and the relationship between k bearing alteration minerals and au deposition the major minor and trace element geochemistry of highly altered and mineralized to unmineralized rocks from the getchell deposit was quantified for more than 50 samples collected along 13 transects through calcareous siltstone and carbonaceous limestone and along one transect through a rhyodacite dike each transect in sedimentary rocks was collected along a single homogeneous bed that could be followed from high grade ore to moderately altered rock or waste rock analyses were obtained for 39 elements 10 oxides and loss on ignition using multiple techniques petrographic studies were integrated with geochemistry and x ray diffraction and electron microbeam analyses to identify ore and alteration minerals and to correlate mineralogy with geochemical fluxes geochemical analyses indicate that ti al zr and th behave as immobile elements immobility isocon diagrams show that significant hg sb se te tl and cs are typically added to the wall rocks with au fe is im mobile in some transects added to wall rocks in others and removed from rhyodacite host rocks minor s and w are usually added to the rock whereas minor cu and mo are variably added or removed sio 2 appears to be either added during alteration or remain constant elements that are consistently removed during alteration include ca mn sr and sc and minor mg ba and k na was significantly removed from the rhyodacite au correlates positively with ag hg sb se si te and tl and negatively with ca mn and sc element fluxes correspond with observations of mineral abundances and show that sulfidation and decar bonitization are spatially associated with gold mineralization au hg sb tl cu and possibly te and w were incorporated in trace element rich pyrite as host rock fe was sulfidized fe was added to mineralized rocks in some transects indicating that pyritization occurred in addition to sulfidation cs is a component of galkhaite a trace mineral that is relatively common in some parts of the getchell system decarbonitization removed as much as 95 wt percent calcite from mineralized rocks in some transects and is responsible for loss of ca mn sr and sc although sio 2 as and s are significantly added to carlin type systems they do not appear as major added components in the isocon diagrams this is because the halos for these components are much larger than the areas examined by the sample transects x ray diffraction analyses identified minerals in ore and waste samples and quantified clay minerals including illite and montmorillonite kaolinite was not identified or is present in low abundances in both mineralized and unmineralized rocks montmorillonite does not correlate with gold and textures are consistent with pore filling deposition of montmorillonite following au deposition illite abundance exhibits a positive correlation with au r 2 0 80 intergrowths of illite with ore stage pyrite support a genetic relationship between the two minerals and indicate that illite formed as part of the au related hydrothermal event mineral abundances and textures suggest that illite formed by alteration of k feldspar during au deposition results show that dating of k bearing phases in sedimentary rocks other than illite would provide ages unrelated to gold deposition

formation timescales of large martian valley networks abstract assessing timescales for formation of the ancient martian valley networks is key to interpreting the early climate on mars we determined the likely formation times for seven of the largest ancient valley networks on mars using the darcy weisbach equation for average flow velocity three different sediment transport models and a range of input parameters to encompass possible formation conditions with runoff rates similar to intense storms in arid regions on earth the minimum formation timescales of these martian valley networks range from 10 5 to 10 7 yr depending on the specific valley network shorter formation timescales require hurricane scale flows that if minimized with assumptions of continuous formation unlike even terrestrial rates could complete large valley network formation in as little as 200 to 5000 yr though this is not the preferred interpretation while these results do not support impact induced climate change as the sole mechanism for creating precipitation sufficient to incise the large valley networks neither do they extend the amount of time required to form the networks beyond their range in age consistent with hypotheses that valley incision was constrained to a relatively short period of martian history near the noachian hesperian boundary approximately 3 6 to 3 8 ga

visible gold in arsenian pyrite at the shuiyindong carlin type gold deposit guizhou china implications for the environment and processes of ore formation abstract carlin type gold deposits are best known for the scarcity of visible gold in their ores it has long been recognized that the majority of gold is invisible such that it cannot be resolved by conventional microscopy and resides in arsenian pyrite shuiyindong differs in that sub m to m sized native gold is present in arsenian pyrite veinlets and disseminations it is also the largest 55 tonnes and highest grade 7 to 18 ppm stratabound carlin type gold deposit in guizhou china and has produced 5 tonnes of gold from sulfide refractory ores extracted by underground mining methods in this study an electron microprobe analyzer empa was used to map the spatial distribution of invisible gold and sub m to m size visible gold particles in arsenian pyrite in high grade ore samples from the shuiyindong the samples studied are hosted in permian bioclastic ferroan limestone of the longtan formation and exhibit evidence of decarbonation silicification and sulfidation arsenian pyrite with detectable au 400 to 3800 ppm is disseminated in altered limestone and was deposited in two stages separated by an episode of corrosion in a veinlet the results show that there are two populations of native gold in arsenian pyrite one is comprised of sub m size gold particles 0 1 to 0 2 m that are occasionally present in the gold bearing arsenian pyrite disseminated in the host rocks this arsenian pyrite is interpreted to have been formed by sulfidation of ferroan calcite and dolomite another is comprised of coarser 1 to 6 m native gold grains present in the arsenian pyrite veinlet either on the first stage where it has been corroded or on the second stage the lack of fluid inclusion or other evidence of boiling and the low iron content of fluid inclusions in quartz suggest the veinlet formed by sulfidation of another fluid containing fe the fe bearing fluid may be a depleted ore fluid that gained fe by dissolution of ferroan limestone after h 2 s had been consumed the association of the largest visible gold grains with an episode of corrosion suggests that fluids episodically became undersaturated with arsenian pyrite while remaining saturated with gold e g ph decrease or an increase in the oxidation state this may have resulted from incursion of relatively acidic or oxidized fluids that were able to dissolve arsenian pyrite and remain saturated with gold in this case sulfidation of iron from the host rock was the most important depositional mechanism for au bearing arsenian pyrite with or without grains of native gold

the etendeka igneous province magma types and their stratigraphic distribution with implications for the evolution of the parana etendeka flood basalt province detailed geochemical and field data for the volcanic sequence and intrusions of the etendeka igneous province are used to construct a stratigraphic framework for petrogenetic interpretation of the evolution of the etendeka parana continental flood volcanic event geochemical and petrographic characterization of over 1 000 analyzed samples allows 8 mafic and 17 silicic magma types to be recognized both silicic and mafic types can be grouped into high ti and low ti suites on the basis of elevated ti relative to other elements the mafic magmas are khumib high ti tafelberg kuidas horingbaai huab tafelkop albin and esmeralda all low ti amongst the silicic types the goboboseb springbok wereldsend grootberg and beacon low ti quartz latites and the nil desperandum high ti latite have been described previously in addition the hoas low ti nadas sechomib and hoarusib all high ti latites and the fria low ti sarusas ventura khoraseb naude and elliott all high ti quartz latites are described for the first time here there is a marked provinciality in the distribution of the high and low ti suites with the former concentrated in the northern etendeka region and the latter dominant in the southern etendeka stratigraphic distribution of magma types allows two new formations to be defined in the northern etendeka the khumib formation of basaltic flows and the skeleton coast formation dominated by silicic sheets the geochemical provinciality hampers precise correlations between northern and southern etendeka available evidence suggests that the lower part of the awahab formation in the southern etendeka is coeval with the lower part of the khumib formation and that the silicic units in the upper part of the tafelberg formation probably correlate with the skeleton coast formation the paucity of khumib dykes in relation to tafelberg dykes and their field relationships with regard to the volcanic sequence in the northern etendeka suggests that the main khumib eruptive centers lay further north consistent with southward thinning of the khumib basalts in the southern etendeka the doros complex is the eruptive center of the plume derived tafelkop basalts which probably built a shield volcano within the regional flood lava field this work indicates that the etendeka has a greater variety of both silicic and low ti mafic magma types than are known from the se parana detailed comparisons reveal that all the important silicic types in the parana have geochemical equivalents in the etendeka new correlations are santa maria fria ourinhos khoraseb guarapuava sarusas thus extending the previously recognized correlations between southern etendeka quartz latites and the caixas do sul and anita garibaldi rhyolites these correlations emphasize that very large volume silicic systems develop with pronounced lithospheric rifting in continental flood basalt provinces

tectonic significance of the florianopolis dyke swarm parana etendeka magmatic province a reappraisal based on precise u pb dating abstract the first precise id tims u pb baddeleyite zircon dating of dykes representative of the main activity in the florianopolis dyke swarm parana etendeka magmatic province south brazil reveals a restricted age range 134 7 0 3 to 133 9 0 7 ma this is at least 5 ma older than indicated by previous 40 ar 39 ar stepwise heating dating and precludes a direct relationship with syn rifting magmatism connected with opening the south atlantic ocean instead as indicated by their age and geochemical signature these dykes are feeders to the nearby urubici lavas and coeval khumib lavas in etendeka these new geochronological results also cast doubt on a large 10 ma time span for the emplacement of the florianopolis dyke swarm implied by 40 ar 39 ar dates but the determination of its upper age limit still requires accurate dating of younger low volume fine grained dykes with distinct low ti chemistry crystals suitable to u pb dating were not obtained thus far from these dykes

a proposed new type of arsenian pyrite composition nanostructure and geological significance this report describes a new form of arsenian pyrite called as3 pyrite in which as substitutes for fe fe as s2 in contrast to the more common form of arsenian pyrite as1 pyrite in which as1 substitutes for s fe as s 2 as3 pyrite has been observed as colloformic overgrowths on as free pyrite in a hydrothermal gold deposit at yanacocha peru xps analyses of the as3 pyrite confirm that as is present largely as as3 empa analyses show that as3 pyrite incorporates up to 3 05 at of as and 0 53 at 0 1 at 0 27 at 0 22 at 0 08 at and 0 04 at of pb au cu zn ni and co respectively incorporation of as3 in the pyrite could be written like as3 y au 1 y 2 fe2 where au and vacancy help to maintain the excess charge hrtem observations reveal a sharp boundary between as free pyrite and the first overgrowth of as3 pyrite 20 40 nm thick and co linear lattice fringes indicating epitaxial growth of as3 pyrite on as free pyrite overgrowths of as3 pyrite onto as free pyrite can be divided into three groups on the basis of crystal size 8 20 nm 100 300 nm and 400 900 nm and the smaller the crystal size the higher the concentration of toxic arsenic and trace metals the yanacocha deposit in which as3 pyrite was found formed under relatively oxidizing conditions in which the dominant form of dissolved as in the stability field of pyrite is as3 in contrast reducing conditions are typical of most environments that host as1 pyrite as3 pyrite will likely be found in other oxidizing hydrothermal and diagenetic environments including high sulfidation epithermal deposits and shallow groundwater systems where probably kinetically controlled formation of nanoscale crystals such as observed here would be a major control on incorporation and release of as3 and toxic heavy metals in oxidizing natural systems

the bero volcanic complex extension of the parana etendeka igneous province into sw angola abstract an extension of the etendeka parana igneous province into sw angola occurs as minor basalt lavas intrusive gabbro sheets minor mafic dykes and thick sheets and lava flows with minor pyroclastics of quartz latite composition this suite crops out along the eastern margin of the cretaceous namibe basin in sw angola the quartz latites from one locality have been referred to informally as the giraul volcanics but the name giraul has previously been used for cretaceous conglomerates we propose the name bero volcanic complex for this suite of intrusive and extrusive rocks on the basis that the full compositional range of this diverse suite crops out along the rio bero major and trace element compositions and sr nd and pb isotopic compositions indicate that the basalts and gabbros are equivalent to the high ti khumib urubici and pitanga types from the etendeka and parana the basalts underlie the quartz latites which are cut by mafic dykes some of which are compositionally equivalent to the paranapanema lavas in the parana five different geochemical types of high ti quartz latite are recognised amongst the silicic volcanics 3 of which have very close geochemical affinities to the ventura sarusas and khoraseb types of the northern etendeka their relative stratigraphic position in the bero volcanic sequence is the same as in the etendeka sequence and extend significantly the area over which these types were erupted the two remaining types chinguau and high nb are not known from either the etendeka or the parana provinces

early mars volcanic sulfur storage in the upper cryosphere and formation of transient so2 rich atmospheres during the hesperian in a previous paper chassefiere et al 2013 we have shown that most volcanic sulfur released to the early mars atmosphere could have been trapped in the upper cryosphere under the form of co2 so2 clathrates huge amounts of sulfur up to the equivalent of an 1 bar atmosphere of so2 would have been stored in the noachian upper cryosphere then massively released to the atmosphere during the hesperian due to rapidly decreasing co2 pressure it could have resulted in the formation of the large sulfate deposits observed mainly in hesperian terrains whereas no or little sulfates are found at the noachian in the present paper we first clarify some aspects of our previous work we discuss the possibility of a smaller cooling effect of sulfur particles or even of a net warming effect we point out the fact that co2 so2 clathrates formed through a progressive enrichment of a pre existing reservoir of co2 clathrates and discuss processes potentially involved in the slow formation of a so2 rich upper cryosphere we show that episodes of sudden destabilization at the hesperian may generate 1000 ppmv of so2 in the atmosphere and contribute to maintaining the surface temperature above the water freezing point

early mars serpentinization derived ch4 reservoirs h2 induced warming and paleopressure evolution ch4 has been observed on mars both by remote sensing and in situ during the past 15 yr it could have been produced by early mars serpentinization processes that could also explain the observed martian remanent magnetic field assuming a cold early mars a cryosphere could trap such ch4 as clathrates in stable form at depth the maximum storage capacity of such a clathrate cryosphere has been recently estimated to be 2 x 10 19 to 2 x 10 20 moles of methane we estimate how large amounts of serpentinization derived ch4 stored in the cryosphere have been released into the atmosphere during the noachian and the early hesperian due to rapid clathrate dissociation and photochemical conversion of ch4 to h 2 these episodes of massive ch4 release may have resulted in transient h 2 rich atmospheres at typical levels of 10 20 in a background 1 2 bar co2 atmosphere the collision induced heating effect of h 2 present in such an atmosphere has been shown to raise the surface temperature above the water freezing point we show how local and rapid destabilization of the cryosphere can be induced by large events such as the hellas basin or tharsis bulge formation and lead to such releases our results show that the early mars cryosphere had a sufficient ch4 storage capacity to have maintained h 2 rich transient atmospheres during a total time period up to several million years or tens of million years having potentially contributed to the formation of valley networks during the noachian early hesperian

origin of high grade gold ore source of ore fluid components and genesis of the meikle and neighboring carlin type deposits northern carlin trend nevada the meikle mine exploits one of the world s highest grade carlin type gold deposits with reserves of ca 220 t gold at an average grade of 24 7 g t locally gold grades exceed 400 g t several geologic events converged at meikle to create these spectacular gold grades prior to mineralization a devonian hydrothermal system altered the bootstrap limestone to fe rich dolomite subsequently the rocks were brecciated by faulting and late jurassic intrusive activity the resulting permeability focused flow of late eocene carlin type ore fluids and allowed them to react with the fe rich dolomite fluid inclusion data and mineral assemblages indicate that these fluids were hot ca 220 c of moderate salinity 2 s rich gold rich pyrite formed by dissolution of dolomite and sulfidation of its contained fe where dissolution and replacement were complete ore stage pyrite and other insoluble minerals were all that remained locally these minerals accumulated as internal sediments in dissolution cavities to form ore with gold grades 400 g t petrographic observations geochemical data and stable isotope results from the meikle mine and other deposits at the goldstrike mine place important constraints on genetic models for meikle and other carlin type gold deposits on the northern carlin trend the ore fluids were meteoric water d 135 18 o 5 that interacted with sedimentary rocks at a water rock ratio of ca 1 and temperatures of ca 220 c the absence of significant silicification suggests that there was little cooling of the ore fluids during mineralization these two observations strongly suggest that ore fluids were not derived from deep sources but instead flowed parallel to isotherms the gold was transported by h 2 s 34 s 9 which was derived from paleozoic sedimentary rocks the presence of auriferous sedimentary exhalative mineralization in the local stratigraphic sequence raises the possibility that preexisting concentrations of gold contributed to the carlin type deposits taken together our observations suggest that meteoric water evolved to become an ore fluid by shallow circulation through previously gold and sulfur enriched rocks carlin type gold deposits formed where these fluids encountered permeable reactive fe rich rocks

the oxygen isotope composition of karoo and etendeka picrites high δ18o mantle or crustal contamination oxygen isotope compositions of karoo and etendeka large igneous province lip picrites and picrite basalts are presented to constrain the effects of crustal contamination versus mantle source variation olivine and orthopyroxene phenocrysts from lavas and dykes mg 64 80 from the tuli and mwenezi nuanetsi regions of the ca 180 ma karoo lip have 18o values that range from 6 0 to 6 7 they appear to have crystallized from magmas having 18o values about 1 1 5 higher than expected in an entirely mantle derived magma olivines from picrite and picrite basalt dykes from the ca 135 ma etendeka lip of namibia and karoo age picrite dykes from dronning maud land antarctica do not have such elevated 18o values a range of 18o values from 4 9 to 6 0 and good correlations between 18o value and sr nd and pb isotope ratios for the etendeka picrites are consistent with previously proposed models of crustal contamination explanations for the high 18o values in tuli mwenezi picrites are limited to 1 alteration 2 crustal contamination and 3 derivation from mantle with an abnormally high 18o previously a variety of models that range from crustal contamination to derivation from the enriched mantle lithosphere have been suggested to explain high concentrations of incompatible elements such as k and average end and esr values of 8 and 16 in mwenezi nuanetsi picrites however the primitive character of the magmas mg 73 combined with the lack of correlation between 18o values and radiogenic isotopic compositions mgo content or mg is inconsistent with crustal contamination thus an 18o enriched mantle source having high incompatible trace element concentration and enriched radiogenic isotope composition is indicated high 18o values are accompanied by negative nb and ta anomalies consistent with the involvement of the mantle lithosphere whereas the high 18o themselves are consistent with an eclogitic source magma 18o values about 1 higher than expected for mantle derived magma are also a feature of the bushveld mafic and ultramafic magmas and the possibility exists that a long lived 18o enriched mantle source has existed beneath southern africa a mixed eclogite peridotite source could have developed by emplacement of oceanic lithosphere into the cratonic keel during archaean subduction

quartz latite rheoignimbrite flows of the etendeka formation north western namibia the etendeka formation of north western namibia consists of a sequence of interbedded quartz latites and tholeiitic basalts and forms part of the karoo igneous province in southern africa the age of the etendeka formation is approximately 130 135 ma the quartz latites make up a significant proportion of the stratigraphic succession 25 of the total stratigraphic thickness and form as much as 60 of the outcrop area in the southern etendeka apart from some systematic differences between pitchstones and devitrified quartz latite largely explained by alteration processes individual quartz latite units exhibit remarkably uniform compositions with no significant vertical or lateral variation geochemistry can be used as a primary criterion for the correlation of major quartz latite units over much of the southern etendeka area enabling the reconstruction of the etendeka formation stratigraphy in this region individual quartz latite units occur as voluminous 400 2600 km3 widespread up to 8800 km2 sheet like deposits typically between 40 and 300 m thick each unit consists of basal main and upper zones the main zone generally constitutes over 70 of the thickness of the unit and typically consists of texturally featureless devitrified quartz latite in contrast the basal and upper zones of the flow are characterised by flow banding pitchstone lenses and breccia with rare occurrences of pyroclastic textures the quartz latites are sparsely porphyritic 10 phenocrysts with glassy or devitrified groundmass textures the phenocrysts consist of plagioclase pyroxene titanomagnetite and rare ilmenite pyroxene geothermometry indicates high 1000 1100 c temperatures of crystallisation which coupled with the absence or primary hydrous phases indicates that the quartz latites were relatively hot h2o undersaturated magmas the quartz latites display features common to both rhyolite lavas and ignimbrites and are clearly the products of an unusual eruption style the local preservation of pyroclastic textures and the broad areal extent of these units lead to the conclusion that the quartz latites are high temperature rheomorphic ignimbrites i e rheoignimbrites a combination of high eruption temperature and relatively low viscosity helps to explain the often completely welded and homogeneous textures observed in most quartz latite outcrops in the etendeka area

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

replacement of pyrrhotite by pyrite and marcasite under hydrothermal conditions up to 220 c an experimental study of reaction textures and mechanisms the transformation of pyrrhotite to fe disulfide pyrite and or marcasite under hydrothermal conditions was studied experimentally by probing the effects of temperature up to 220 c vapor saturated pressures s ii concentrations ph and availability of oxygen on reaction progress and on the resulting textures the pyrrhotite to fe disulfide reaction proceeded by a dissolution reprecipitation mechanism under all conditions marcasite and pyrite formed under both oxic and anaerobic conditions which is inconsistent with the traditionally assumed polysulfide route for fes2 formation oxidants required for polysulfide formation the nature of the products was controlled by the level of supersaturation of the solution with respect to fe disulfide minerals marcasite formed preferentially at low ph or s ii deficient solutions saturation index 1000 different textures were obtained for the replacement of pyrrhotite by either pyrite or marcasite pyrite formation proceeded by direct replacement of pyrrhotite and simultaneously by overgrowth from solution the pyrite crystals were 10 m in size and randomly oriented in comparison marcasite crystals were 1 m in size and no significant overgrowth was observed at ph21 c 3 the marcasite nanocrystals showed the well known crystallographic relationship with pyrrhotite but at ph21 c 3 96 the marcasite crystallites were randomly oriented these experimental results confirm that the preservation of the crystallographic orientation is not a distinguishing feature between dissolution reprecipitation and solid state reactions the different textures among pyrite and marcasite reflect the dominance of crystal growth pyrite vs nucleation marcasite as a precipitation mechanism

linking high grade gold mineralization to earthquake induced fault valve processes in the porgera gold deposit papua new guinea processes responsible for high grade gold enrichment are of significant economic interest and have long been elusive structural and geophysical models postulate that earthquake induced fault valve processes are frequently responsible for gold mineralization in structurally controlled hydrothermal systems although a comprehensive geochemical record of these processes has been lacking the porgera gold deposit papua new guinea provides an opportunity to examine the complex processes surrounding high grade au mineralization in structurally controlled hydrothermal systems new high resolution trace element and sulfur isotope data obtained in situ by laser ablation inductively coupled plasma mass spectrometry and sensitive high resolution ion microprobe stable isotope analysis of pyrite across a well constrained paragenetic sequence reveal a temporal evolution in fluid chemistry the data uncover a stratigraphy of repeated high au negative 34 s and low au positive 34 s zones within individual pyrite crystals present in the highest grade gold event stage ii results provide the first clear geochemical documentation of rapid complete switching in ore forming processes preserved in individual pyrite crystals throughout an evolving hydrothermal system it is postulated that geochemical variations are attributable to rapid au deposition during pressure release due to fault failure followed by switching to more reduced rock buffered metal poor fluids from adjacent sediments as the rupture seals the cyclicity of this process is reflected in the pyrite zonation seen at the porgera deposit this study demonstrates the usefulness of applying contemporary analytical techniques to pyrite in hydrothermal systems to gain new insights into ore genesis and fluid pathways revealing that deposition of exceptionally high grade ore at the porgera deposit was the result of multiple events in a fluctuating ore forming environment

new bulk sulfur measurements of martian meteorites and modeling the fate of sulfur during melting and crystallization implications for sulfur transfer from martian mantle to crust atmosphere system abstract sulfur storage and transport between different reservoirs such as core mantle crust and atmosphere of mars are tied to igneous processes martian meteorites carry a record of mantle melting and subsequent differentiation history of martian magmas investigation of s geochemistry of martian meteorites can thus provide an understanding of how s is transferred from the martian interior to the exosphere in this study we measured bulk s concentration of 7 martian meteorites and modeled the behavior of s during both isobaric crystallization of primary martian magmas and isentropic partial melting of martian mantle comparisons between measured data and modeled results suggest that 1 sulfides may become exhausted at the source during decompression melting of the mantle and mantle derived basalts may only become sulfide saturated after cooling and crystallization at shallow depths and 2 in addition to degassing induced s loss mixing between these differentiated sulfide saturated basaltic melts and cumulus minerals with without cumulate sulfides could also be responsible for the bulk sulfur contents in some martian meteorites in this case a significant quantity of s could remain in martian crust as cumulate sulfides or in trapped interstitial liquid varying from 2 to 95 percent by weight our modeling also suggests that generation of sulfide undersaturated parental magmas requires that the mantle source of martian meteorites contain

an experimental study of the mechanism of the replacement of magnetite by pyrite up to 300 c abstract we present the results of an experimental study into the sulfidation of magnetite to form pyrite marcasite under hydrothermal conditions 90 300 c vapor saturated pressures a process associated with gold deposition in a number of ore deposits the formation of pyrite marcasite was studied as a function of reaction time temperature ph sulfide concentration solid weight to fluid volume ratio and geometric surface area of magnetite in polytetrafluoroethylene lined autoclaves ptfe and a titanium and stainless steel flow through cell marcasite was formed only at ph 21 c 21 c 1 11 while pyrite predominated at ph 21 c 2 and formed even under basic conditions up to ph 21 c 12 13 marcasite formation was favored at higher temperatures fine grained pyrrhotite formed in the initial stage of the reaction together with pyrite in some experiments with large surface area of magnetite grain size 120 c 0 96 4 42 and that rate of the transformation increased from 120 to 190 c scanning electron microscope sem imaging revealed that micro pores 0 1 5 m scale existed at the reaction front between the parent magnetite and the product pyrite and that the pyrite and or marcasite were euhedral at ph 21 c 155 c 4 42 the experimental results are in good agreement with sulfide phase assemblage and textures reported from sulfidized banded iron formations pyrite marcasite and pyrrhotite have been found to exist or co exist in different sulfidized banded iron formations and the microtextures show no evidence of sub m scale pseudomorphism of magnetite by pyrite

carbon sequestration on mars on earth carbon sequestration in geologic units plays an important role in the carbon cycle scrubbing co 2 from the atmosphere for long term storage while carbonate is identified in low abundances within the dust and soils of mars at 1 wt in select meteorites and in limited outcrops no massive carbonate rock reservoir on mars has been identified to date here we investigate the largest exposed carbonate bearing rock unit the nili fossae plains combining spectral thermophysical and morphological analyses to evaluate the timing and carbon sequestration potential of rocks on mars we find that the olivine enriched 20 25 basalts have been altered by low temperature in situ carbonation processes to at most 20 fe mg carbonate thus limiting carbon sequestration in the nili fossae region to 0 25 12 mbar of co 2 during the late noachian early hesperian before or concurrent with valley network formation while this is large compared to modern day co 2 reservoirs the lack of additional comparably sized post late noachian carbonate bearing deposits on mars indicates ineffective carbon sequestration in rock units over the past 3 7 b y this implies a thin atmosphere 500 mbar during valley network formation extensive post noachian atmospheric loss to space or diffuse deep sequestration by a yet to be understood process in stark contrast to earth s biologically mediated crust atmosphere carbon reservoir ratio of 10 4 10 5 mars ratio is a mere 10 10 3 even if buried pre noachian crust holds multiple bars

invisible gold in arsenian pyrite and arsenopyrite from a multistage archaean gold deposit sunrise dam eastern goldfields province western australia the sunrise dam gold mine 11 1 moz au is the largest deposit in the archaean laverton greenstone belt eastern goldfields province yilgarn craton western australia the deposit is characterized by multiple events of fluid flow leading to repeated alteration and mineralization next to a major crustal scale structure the au content of arsenian pyrite and arsenopyrite from four mineralizing stages d1 d3 d4a and d4b and from different structural and lithostratigraphic environments was measured using in situ laser ablation inductively coupled plasma mass spectrometry pyrite contains up to 3 067 ppm au n 224 whereas arsenopyrite contains up to 5 767 ppm n 19 gold in arsenopyrite d4a stage was coprecipitated and remained as invisible gold nanoparticles and or lattice bound during subsequent deformation events in contrast gold in pyrite is present not only as invisible gold but also as micrometer size inclusions of native gold electrum and au ag tellurides pristine d1 and d3 arsenian pyrite contains relatively low au concentrations 26 ppm the highest au concentrations occur in d4a arsenian rich pyrite that has recrystallized from d3 pyrite textures show that this recrystallization proceeded via a coupled dissolution reprecipitation process and this process may have contributed to upgrading au grades during d4a in contrast au in d4b pyrite shows grain scale redistribution of invisible gold resulting in the formation of micrometer scale inclusions of au minerals the speciation of au at sunrise dam and the exceptional size of the deposit at province scale result from multiple fluid flow and multiple au precipitating mechanisms within a single plumbing system

evaluation of the role of sulfidation in deposition of gold screamer section of the betze post carlin type deposit nevada this study was undertaken to evaluate the relation between sulfidation and gold deposition in the screamer section of the betze post carlin type deposit we also attempted to determine the source of sulfur in the deposit and the possibility that more than one gold depositing event contributed metal to the screamer system gold ore at screamer forms a generally stratiform body hosted largely by the wispy member of the devonian popovich formation and lacks obvious alteration of wall rock gangue minerals gold at screamer is hosted by arsenian pyrite that forms disseminated grains and overgrowths on diagenetic pyrite a strong correlation is observed between the gold content of rock samples and their proportion of ore related arsenian pyrite as determined by point counts isocon plots show that mineralization at screamer involved addition of both sulfur and iron along with gold arsenic antimony tungsten and local silica barium and phosphorus fe al vs s al plots show that most ore samples at screamer do not contain enough sulfur to account for all of their iron as pyrite petrographic examination shows that the iron occurs in pyrite ferroan dolomite and iron bearing micas in order of decreasing abundance the fe al vs s al plots also show that screamer samples with high gold contents contain more pyrite than samples with low gold contents and that samples inside the ore zone have more pyrite than those in the surrounding area these relations are interpreted to indicate that screamer has undergone at least two pyrite forming events the first event which probably took place during diagenesis involved incomplete sulfidation that left some iron in carbonate and silicate minerals the second event which probably took place during gold mineralization sulfidized most remaining iron and added pyrite in the screamer ore zone the 34 s values of chemically extracted sulfur from disseminated pyrite at screamer range from 13 8 to 16 5 per mil and 34 s values for hand picked separates of pyrite and other sulfides in veins cutting these rocks range from 21 2 to 11 7 per mil a significant fraction of samples with high gold contents and of samples in the ore zone regardless of gold content have 34 s values between 1 and 5 per mil these data suggest that early diagenetic sulfur with a wide range of 34 s values was overprinted by sulfur with isotopic compositions in the 1 to 5 per mil range that was associated with gold mineralization the 34 s values for gold related sulfur at screamer are lower than those reported for bulk mineral separates from most other carlin type deposits and from sims analyses of sulfides from the proximal post part of the betze post system and could be magmatic limited evidence can be found for multiple gold forming events in the screamer zone tungsten which might have been introduced by the jurassic age goldstrike stock is widespread in the deposit and correlates closely with gold a few samples with high tungsten gold ratios found along fault zones might be part of an earlier phase of mineralization related to the goldstrike stock a 34 s value for pyrite in one of these samples is similar to the high values reported for sulfides in auriferous polymetallic mineralization in the post section of the deposit but this mineralization is not reported to contain tungsten other veins containing sphalerite have lower 34 s values and lack consistent gold values whereas there is no correlation between gold values and the degree to which the host rocks have undergone sulfidation there is a strong and highly significant correlation between gold values and the abundance of ore related pyrite this shows that simple sulfidation of immediately adjacent wall rock cannot account alone for gold deposition at screamer and it might not be the only ore depositing process in some other carlin deposits a more general extended sulfidation process could be important however one likely process involves mixing of an invading mineralizing fluid containing sulfur and gold with a wall rock fluid containing iron derived from adjacent or distal wall rocks recognition of the source s for this iron and fluid flow pathways responsible for introducing it to the ore zone could provide useful guidance in exploration for the next carlin trend

gold and trace element zonation in pyrite using a laser imaging technique implications for the timing of gold in orogenic and carlin style sediment hosted deposits laser ablation icp ms imaging of gold and other trace elements in pyrite from four different sedimenthosted gold arsenic deposits has revealed two distinct episodes of gold enrichment in each deposit an early synsedimentary stage where invisible gold is concentrated in arsenian diagenetic pyrite along with other trace elements in particular as ni pb zn ag mo te v and se and a later hydrothermal stage where gold forms as either free gold grains in cracks in overgrowth metamorphic and or hydrothermal pyrite or as narrow goldarsenic rims on the outermost parts of the overgrowth hydrothermal pyrite compared to the diagenetic pyrites the hydrothermal pyrites are commonly depleted in ni v zn pb and ag with cyclic zones of co ni and as concentration the outermost hydrothermal pyrite rims are either as au rich as in moderate to highgrade deposits such as carlin and bendigo or co ni rich and as au poor as in moderate to low grade deposits such as sukhoi log and spanish mountain the early enrichment of gold in arsenic bearing syngenetic to diagenetic pyrite within black shale facies of sedimentary basins is proposed as a critical requirement for the later development of carlin style and orogenic gold deposits in sedimentary environments the best grade sediment hosted deposits appear to have the gold climax event toward the final stages of deformation related hydrothermal pyrite growth and fluid flow

the coupled geochemistry of au and as in pyrite from hydrothermal ore deposits the ubiquity of au bearing arsenian pyrite in hydrothermal ore deposits suggests that the coupled geochemical behaviour of au and as in this sulfide occurs under a wide range of physico chemical conditions despite significant advances in the last 20years fundamental factors controlling au and as ratios in pyrite from ore deposits remain poorly known here we explore these constraints using new and previously published empa la icp ms sims and pixe analyses of as and au in pyrite from carlin type au epithermal au porphyry cu cu au and orogenic au deposits volcanogenic massive sulfide vhms witwatersrand au iron oxide copper gold iocg and coal deposits pyrite included in the data compilation formed under temperatures from 30 to 600 c and in a wide variety of geological environments the pyrite au as data form a wedge shaped zone in compositional space and the fact that most data points plot below the solid solubility limit defined by reich et al 2005 indicate that au1 is the dominant form of au in arsenian pyrite and that au bearing ore fluids that deposit this sulfide are mostly undersaturated with respect to native au the analytical data also show that the solid solubility limit of au in arsenian pyrite defined by an au as ratio of 0 02 is independent of the geochemical environment of pyrite formation and rather depends on the crystal chemical properties of pyrite and post depositional alteration compilation of au as concentrations and formation temperatures for pyrite indicates that au and as solubility in pyrite is retrograde au and as contents decrease as a function of increasing temperature from 200 to 500 c based on these results two major au as trends for au bearing arsenian pyrite from ore deposits are defined one trend is formed by pyrites from carlin type and orogenic au deposits where compositions are largely controlled by fluid rock interactions and or can be highly perturbed by changes in temperature and alteration by hydrothermal fluids the second trend consists of pyrites from porphyry cu and epithermal au deposits which are characterised by compositions that preserve the au as signature of mineralizing magmatic hydrothermal fluids confirming the role of this sulfide in controlling metal ratios in ore systems

pyrite and pyrrhotite textures and composition in sediments laminated quartz veins and reefs at bendigo gold mine australia insights for ore genesis the various types textures and compositional zoning of pyrite in the gold bearing saddle reefs quartz veins and surrounding sedimentary rocks provide new information on the potential source and timing of gold and arsenic and related fluid processes responsible for mineralization at bendigo nodular diagenetic pyrite in the black shale tops to sandstone turbidites is enriched in invisible gold and arsenic with mean values of 0 61 ppm au 1 300 ppm as and au ag 1 based on la icpms analyses other elements enriched in the diagenetic pyrite within the organic rich shales are mn zn mo cu v ba ag cd tl co ni bi pb and te in contrast euhedral and growth zoned hydrothermal pyrite in the turbidites and bedding parallel laminated quartz veins contains lower contents of most trace elements but has higher contents of invisible au and as especially on the outermost rim of the pyrite the gold rich pyrite rims generally become thicker a few to hundreds of microns in proximity to the gold bearing saddle reefs pyrite in the reef commonly has the highest levels of invisible au and as and the lowest levels of other trace elements it is characterized by au ag 1 and au pb 0 01 in the deepest stratigraphic levels below known productive gold reefs diagenetic pyrite in the most carbonaceous shales has been replaced by pyrrhotite during metamorphism la icpms analyses reveal that the disseminated pyrrhotite contains similar levels of ni and co to the diagenetic pyrite but is strongly depleted in as and au the spatial relationships between organic rich shales folded bedding parallel laminated quartz veins and gold arsenic bearing saddle reefs combined with the consistent trends in the trace element composition of pyrite hosted by these three geological elements is interpreted to indicate that the black shales were an initial source of au and as and the laminated quartz veins acted as the initial pathways for hydrothermal fluid flow carrying au and as from the source shales to the saddle reefs maximum gold and arsenic input into the reefs principally as free gold plus arsenopyrite occurred late during deformation toward the end of the hydrothermal cycle and is expressed by the au as rich rims to hydrothermal pyrite in the sedimentary host rocks laminated quartz veins and reefs this corresponds with final fold lockup and the development of through going fault arrays linking adjacent anticlines the source of au and as for this final and most economically important fluid flow event is considered to be from carbonaceous shales deeper in the basin where original gold bearing diagenetic arsenian pyrite reacts with organic matter and is converted to pyrrhotite with release of au as and s to the metamorphic fluid

trace elements in fluid inclusions of sediment hosted gold deposits indicate a magmatic hydrothermal origin of the carlin ore trend the carlin type deposits in nevada western usa constitute the world9s second largest gold ore province these structurally and stratigraphically controlled sediment hosted ore bodies are characterized by carbonate dissolution attending hydrothermal precipitation of gold rich arsenian pyrite the origin of the mineralizing fluids and the source of the gold remain debated conceptual models favoring either sedimentary metamorphic or magmatic fluid sources are based on isotopic tracers giving ambiguous results here we use the trace element compositions of fluid inclusions to separate geochemical signals of the large scale fluid source from effects of deposit scale fluid interaction with the sedimentary host rocks specifically we compare the ratios of rb k b as sr and ba between clearly magmatic hydrothermal cu au ores at copper canyon in the battle mountain eureka trend with the gold quarry and chukar footwall deposits on the carlin trend that contain high grade gold in similar sedimentary host rocks results indicate that both ore districts can be related to upper crustal hydrous magmatic intrusions but are now exposed at different levels of erosion and formed at different distances from their magmatic fluid source fluid compositions are best explained by separation of a deep magmatic fluid into rb k enriched brine and b as au enriched vapor followed by cooling and contraction of the magmatic vapor phase to an epithermal liquid which reacted with sr ba bearing sedimentary rocks during ascent and eventual precipitation of au rich arsenian pyrite

partitioning of arsenic between hydrothermal fluid and pyrite during experimental siderite replacement abstract arsenian pyrite is the major host mineral of gold in carlin type gold deposit ctgd and forms during the interaction of h2s bearing hydrothermal fluids with fe bearing carbonates e g siderite ankerite fe rich dolomite in the wall rock experimental replacement of siderite at hydrothermal conditions i e 200 c psat was used to study the parameters that control the formation of ctgd pyrite i e ph time ah2s and the partitioning of as between the hydrothermal fluid and newly formed pyrite hydrothermal pyrite formed via replacement of siderite in experiments mainly occurs as two types 1 porous massive pyrite at alkaline ph conditions or 2 clusters of dense euhedral to anhedral pyrite grains at slightly acidic conditions ph concentration of as in newly formed pyrite depends strongly on the as concentration of the hydrothermal fluid used in the experiment calculated partition coefficients vary between 300 and 1700 with lowest d values at high as concentrations of the fluid in the studied system as 1 substitutes for s 2 as indicated by the correlation of high as with low s concentrations experimental findings agree well with previous studies in natural ctgd on as substitution and the partitioning of as between ore stage pyrite and hydrothermal fluids trapped as fluid inclusions the newly constrained partition coefficients for as help to explain the large heterogeneity in natural ctgd pyrite compositions which is caused by changes in the fluid composition due to the strong partitioning of as into the forming pyrite

low palaeopressure of the martian atmosphere estimated from the size distribution of ancient craters the decay of the martian atmosphere which is dominated by carbon dioxide is a component of the long term environmental change on mars from a climate that once allowed rivers to flow to the cold and dry conditions of today the minimum size of craters serves as a proxy for palaeopressure of planetary atmospheres because thinner atmospheres permit smaller objects to reach the surface at high velocities and form craters the aeolis dorsa region near gale crater on mars contains a high density of preserved ancient craters interbedded with river deposits and thus can provide constraints on atmospheric density at the time of fluvial activity here we use high resolution images and digital terrain models from the mars reconnaissance orbiter to identify ancient craters in deposits in aeolis dorsa that date to about 3 6 gyr ago and compare their size distribution with models of atmospheric filtering of impactors we obtain an upper limit of 0 9 0 1 bar for the martian atmospheric palaeopressure rising to 1 9 0 2 bar if rimmed circular mesas interpreted to be erosionally resistant fills or floors of impact craters are excluded we assume target properties appropriate for desert alluvium if sediment had rock mass strength similar to bedrock at the time of impact the paleopressure upper limit increases by a factor of up to two if mars did not have a stable multibar atmosphere at the time that the rivers were flowing as suggested by our results then a warm and wet co 2 h 2o greenhouse is ruled out and long term average temperatures were most likely below freezing

involvement of anomalously as au rich fluids in the mineralization of the heilan gou gold deposit jiaodong china evidence from trace element mapping and in situ sulfur isotope composition abstract the mesozoic 120 ma heilan gou gold deposit located in the jiaodong gold province eastern margin of the north china craton ncc consists of several ultra high grade up to 600 g t auriferous quartz veins in the early cretaceous guojialing granodioritic pluton ore related pyrite crystals from this deposit show complex textures in this study emp nano sims trace element mapping and in situ sulfur isotope analyses were conducted to characterize the textural and compositional features two types of pyrite were identified type i crystals py i hosted in gold quartz pyrite veins are generally homogeneous in chemical composition they contain low concentrations of as based on the distinctive textural and compositional signatures between py iia and py iib it is proposed that anomalously as au rich fluids were injected into hydrothermal systems during the late ore forming stage which may have intensively interacted with certain sedimentary rocks the involvement of these fluids firstly documented in this study can largely explain the occurrence of native gold ore shoots at heilan gou in addition the intimate association between as and au highlights the important roles of as in enrichment of au in the jiaodong gold district

photochemical and climate consequences of sulfur outgassing on early mars abstract ancient mars might have been warm and wet compared to today but climate models have trouble reproducing this warmth partly because of the faintness of the young sun and partly because of inherent limitations to co2 h2o greenhouse warming in particular rayleigh scattering of incoming sunlight by a dense co2 rich atmosphere leads to a high planetary albedo thereby reducing the amount of sunlight absorbed by the planet it has been recently suggested that the presence of 1 100 ppmv so2 in mars early atmosphere might have provided enough additional greenhouse warming to maintain a warm wet early mars we show however that this warming should have been more than offset by cooling from sulfate and sulfur aerosols in early martian atmosphere hence the paradox of mars early climate remains unresolved

nanosims element mapping and sulfur isotope analysis of au bearing pyrite from lannigou carlin type au deposit in sw china new insights into the origin and evolution of au bearing fluids abstract sulfur isotope signatures of au bearing pyrite from lannigou au deposit a typical carlin type au deposit in sw china provide valuable information about the origin of the ore forming minerals analysis by nanosims was used to determine s isotope compositions of au bearing pyrite and to map the grain scale distributions of au cu as and s in pyrite from the deposit based on different textural pattern of pyrites revealed by back scattered electron bse images they are divided into three types py 1 diagenetic pyrite without core rim structure py 2 pyrite with an au free core and a rhythmically zoned au bearing rim py 3 au bearing pyrite with rhythmic zoning across the entire grain the element distributions and s isotope compositions of four paragenetic stages are recognized on the basis of textural observation py 1 grains and the au free homogeneous cores of zoned crystals were formed in stage 1 while the au bearing rims of the zoned crystals with rhythmic zonation of as and cu and to a lesser degree au were formed in two superimposed stages stage 2 formed the inner zone that is enriched in as alone and stage 3 formed the outer zone that is enriched in both au and as other sulfides such as realgar cinnabar and stibnite are formed in the last stage the relationship between au and as distributions in pyrite rim is complicated changing from coupled to decoupled at the nanoscale such complexity is interpreted to reflect fluctuation of fluid composition and temperature with time which in turn affect the modes of occurrence of as and au it is inferred that as mainly occurs in the crystal lattice replacing s whereas au is mainly present as nanoparticles that were trapped in pyrite during crystal growth the au bearing rims of the zoned pyrite crystals are characterized by highly variable 34s values from 1 1 to 18 1 which exceed the values of the triassic calcareous host rocks 10 14 in contrast the 34s values of the au free cores of zoned pyrite crystals vary over a narrower interval and are mainly between 6 and 12 close to the values of pyrite crystals in the sedimentary country rocks our new analyses also reveals that the 34s values of the au bearing fluids generally increase during the formation of the deposit the observed s isotope variations are consistent with mixing between a magmatic related fluid with mantle like 34s value 0 and a sedimentary or deep basin brine fluid with elevated 34s value 18 with an increasing contribution from the latter with time the notably varied values of 34s and the disseminations of au and other trace elements such as as and cu in pyrite crystals indicate that the process responsible for au precipitation in this deposit occurred in an open hydrothermal system

direct analysis of ore precipitating fluids combined ir microscopy and la icp ms study of fluid inclusions in opaque ore minerals laser ablation inductively coupled plasma mass spectrometry la icp ms in combination with near infrared microscopy of fluid inclusions hosted by ore minerals that are opaque to visible light can provide the composition of ore precipitating fluids we applied the two techniques to well constrained fluid inclusion assemblages hosted by pyrite enargite and quartz to trace the source and evolution of the fluids in high sulfidation epithermal veins overprinting a porphyry copper deposit at rosia poieni romania despite some analytical limitations caused by the sulfide host minerals the data demonstrate that fluids trapped in apparently cogenetic quartz and ore minerals are chemically different systematic changes in major and trace element ratios between liquid vapor vapor rich and brine fluid inclusion assemblages in the three minerals record an evolving fluid source at the porphyry to epithermal transition regarding their cs na k ratios most of epithermal quartz hosted fluid inclusion assemblages form a well defined cluster which coincides with the narrow range of the porphyry stage fluids trapped in early quartz of the porphyry stockwork veins their cu na k ratios are 10 to 100 times lower compared to the pyrite hosted inclusions and correspond to the lowest cu na k ratios recorded for the porphyry stage fluids by contrast pyrite hosted vapor rich fluid inclusions have the highest cu na k similar to the highest cu na k ratios measured in the porphyry stage fluid inclusions the results led to the conclusion that the gangue and ore minerals in the high sulfidation epithermal veins at rosia poieni formed by successive pulses of chemically distinct hydrothermal fluids that were successively exsolved from residual melt batches of a progressively crystallizing magma at greater depth these results are consistent with detailed textural observations but petrography alone could not have led to this unambiguous conclusion

copper arsenic decoupling in an active geothermal system a link between pyrite and fluid composition abstract over the past few decades several studies have reported that pyrite hosts appreciable amounts of trace elements which commonly occur forming complex zoning patterns within a single mineral grain these chemical zonations in pyrite have been recognized in a variety of hydrothermal ore deposit types e g porphyry cu mo au epithermal au deposits iron oxide copper gold carlin type and archean lode au deposits among others showing in some cases marked oscillatory alternation of metals and metalloids in pyrite growth zones e g of cu rich as au ag depleted zones and as au ag rich cu depleted zones this decoupled geochemical behavior of cu and as has been interpreted as a result of chemical changes in ore forming fluids although direct evidence connecting fluctuations in hydrothermal fluid composition with metal partitioning into pyrite growth zones is still lacking in this study we report a comprehensive trace element database of pyrite from the tolhuaca geothermal system tgs in southern chile a young and active hydrothermal system where fewer pyrite growth rims and mineralization events are present and the reservoir fluid i e ore forming fluid is accessible we combined the high spatial resolution and x ray mapping capabilities of electron microprobe analysis empa with low detection limits and depth profiling capacity of secondary ion mass spectrometry sims in a suite of pyrite samples retrieved from a 1 km drill hole that crosses the argillic 20 450 m and propylitic 650 1000 m alteration zones of the geothermal system we show that the concentrations of precious metals e g au ag metalloids e g as sb se te and base and heavy metals e g cu co ni pb in pyrite at the tgs are significant among the elements analyzed as and cu are the most abundant with concentrations that vary from sub ppm levels to a few wt i e up to 5 wt as 1 5 wt cu detailed wavelength dispersive spectrometry wds x ray maps and sims depth vs isotope concentration profiles reveal that pyrites from the tgs are characterized by chemical zoning where the studied elements occur in different mineralogical forms arsenic and co occur as structurally bound elements in pyrite cu and au in pyrite can occur as both solid solution and submicron sized particles of chalcopyrite and native au or au tellurides respectively pyrites from the deeper propylitic zone do not show significant zonation and high cu co as concentrations correlate with each other in contrast well developed zonations were detected in pyrite from the shallow argillic alteration zone where cu co rich as depleted cores alternate with cu co depleted as rich rims these microanalytical data were contrasted with chemical data of fluid inclusions in quartz and calcite veins high cu as ratios and borehole fluid low cu as ratios reported at the tgs showing a clear correspondence between cu and as concentrations in pyrite forming fluids and chemical zonation in pyrite these observations provide direct evidence supporting the selective partitioning of metals into pyrite as a result of changes in ore forming fluid composition most likely due to separation of a single phase fluid into a low density vapor and a denser brine capable of fractionating cu and as