multi stage precipitation and redistribution of gold and its collection by lead bismuth and lead immiscible liquids in a reduced intrusion related gold system rirgs dublin gulch western canada abstract dublin gulch reduced intrusion related gold system rirgs located in the selwyn basin area of western canada represents one of the best examples of rirgs mineralization globally and hence can be studied to unravel genesis and evolution of these types of deposits based on textural relationships mineralogy and trace element mineral chemistry three auriferous vein stages were identified the paragenetic sequence for the auriferous vein stages are 1 eagle style es quartz albite low sulfide and sulfosalt content 30 vol as fe w pb bi au ag veins and 3 potato hills style 2 phs 2 high sulfide and sulfosalt content 30 vol fe pb sb zn cu au ag veins in the es and phs 1 veins au is present both as native gold au ag and as invisible gold in arsenopyrite whilst in the phs 2 veins au is present as invisible gold in pyrite native gold micrograins individual grains 1 100 s m in size are observed associated with pb minerals in anhedral to globular cosalite pb2bi2s5 in es veins or galena pbs in the phs 1 veins native gold is also observed as micrograins along arsenopyrite margins and in quartz fractures we suggest a variation on the hydrothermal bi melt collector model to explain the au pb bi association the au ag pb and bi are interpreted to have been locally remobilized from arsenopyrite which shows textures and trace element distribution patterns consistent with fluid and deformation assisted recrystallization we suggest pb and bi were mobilized either as immiscible nanodroplets that coalesced to form larger pb bi liquid accumulations or into the hydrothermal fluid and subsequently exsolved to form immiscible pb bi liquids we propose that remobilized au and ag were collected by these pb bi immiscible liquids subsequent retrograde alteration sulfidation is interpreted to have converted the au ag pb bi alloys to native gold and cosalite es and native gold and galena mineral assemblages phs 1 the similarity of the au ag ratios in native gold and arsenopyrite supports a local source for the native gold micrograins temperatures required to attain liquid pb0 5bi0 5 145 2 c and pb 327 5 c are consistent with arsenopyrite geothermometry es 345 405 c phs 1 380 c these suggested new variations pb bi and pb on the hydrothermal melt bi collector model are important given the common association of native gold with pb bi sulfosalts in many gold deposits and the relatively low temperatures required to maintain these liquids and collect au from the hydrothermal fluid

mobilization of gold as a polymetallic melt during pelite anatexis at the challenger deposit south australia a metamorphosed archean gold deposit the challenger gold deposit in south australia is hosted by pelitic migmatites that underwent peak metamorphism at 7 5 1 5 kbars and at least 800 c nd model ages suggest a protolith age of 2900 ma zircon u pb and garnet sm nd dating indicates that peak metamorphism occurred at 2440 ma and that garnet was present during this event at this time in situ vapor absent melting affected the pelitic host rock indicating that peak metamorphism occurred without introduction of a foreign fluid or melt thus evidence indicating the presence of ore minerals during peak metamorphism implies a prepeak mineralization event this evidence includes the occurrence of invisible gold in lollingite but not in adjacent arsenopyrite and the presence of spherical gold sulfide inclusions in peak metamorphic garnet and other silicates textures developed between pyrrhotite lollingite arsenopyrite and gold the lack of silicate alteration minerals supports this conclusion experimental evidence is presented to support our interpretation that a gold rich polymetallic melt was mobilized into leucosomes synchronously with silicate melt during peak metamorphism visible gold is restricted to migmatitic leucosomes and to a lesser extent melanosomes large inclusions of gold with coexisting arsenopyrite pyrrhotite and bismuth are hosted within peak metamorphic silicate minerals and at grain boundaries trails of micrometric spherical inclusions of predominantly gold and bismuth propagate along annealed fractures from these larger inclusions we show that these gold sulfide inclusions represent the crystallized products of an immiscible polymetallic melt metamorphism of the challenger deposit resulted in partial melting of the pelitic host rock and formation of this gold rich melt these two melts were synchronously redistributed to accumulate as leucosomes during development of a stromatic migmatite formation of a polymetallic melt thereby enabled extensive mobilization of gold producing leucosomes enriched in gold this occurred because the immiscible metallic melt was physically entrained rather than chemically dissolved within the silicate melt gold rich shoots at challenger parallel the plunge of ptygmatically folded leucosomes that are shown to be parasitic to a larger scale fold geometry which appears to be structurally related to the ore shoots it is interpreted that concurrent migration of polymetallic and silicate melt allowed concentration of gold into a series of dilational structures which developed within the larger scale fold geometry challenger represents a new deposit type other deposits around the world such as renco and griffins find are hosted in granulite facies rocks but challenger is the first reported example of leucosome hosted gold in migmatites

fluid inclusion characteristics of intrusion related gold mineralization tombstone tungsten magmatic belt yukon territory canada intrusion hosted low sulfide sheeted vein systems are common within many plutons and stocks of the middle cretaceous tombstone tungsten magmatic belt yukon territory and host significant gold mineralization fluid inclusion characteristics of five such systems namely emerald lake dublin gulch scheelite dome mike lake and mactung constrain the vein forming fluid composition formation temperatures and pressures hydrothermal fluid processes and potential fluid sources the veins contain a wide range of fluid inclusion types ubiquitous type 1a and 1b inclusions are low salinity 1a xnacl 1 kbar at emerald lake and dublin gulch and 2 kbar at mactung and scheelite dome that potentially control fluid composition and evolution a variety of fluid processes may have been responsible for gold precipitation including immiscibility and or release of an evolving magmatic fluid

absolute timing of sulfide and gold mineralization a comparison of re os molybdenite and ar ar mica methods from the tintina gold belt alaska new re os molybdenite dates from two lode gold deposits of the tintina gold belt alaska provide direct timing constraints for sulfide and gold mineralization at fort knox the re os molybdenite date is identical to the u pb zircon age for the host intrusion supporting an intrusive related origin for the deposit however 40ar 39ar dates from hydrothermal and igneous mica are considerably younger at the pogo deposit re os molybdenite dates are also much older than 40ar 39ar dates from hydrothermal mica but dissimilar to the age of local granites these age relationships indicate that the re os molybdenite method records the timing of sulfide and gold mineralization whereas much younger 40ar 39ar dates are affected by post ore thermal events slow cooling and or systemic analytical effects the results of this study complement a growing body of evidence to indicate that the re os chronometer in molybdenite can be an accurate and robust tool for establishing timing relations in ore systems

structural and mechanical controls on intrusion related deposits of the tombstone gold belt yukon canada with comparisons to other vein hosted ore deposit types abstract intrusion related gold deposits at the clear creek scheelite dome and dublin gulch properties of the tombstone gold belt tgb yukon territory have dominantly e striking steeply dipping auriferous quartz extension veins within intrusions in adjacent metasedimentary rocks gold is hosted in subvertical nw to nnw striking sinistral faults as veins and breccias in e striking extension veins and locally in e to ene striking fault veins these structural relationships indicate low magnitude broadly e w directed shortening and n s extension during stock emplacement and gold mineralisation at 92 ma the lack of any deviation or deflection of the extension vein orientations in the country rocks with respect to their orientation within the stocks indicates consistent stress trajectories in both rock types these tgb deposits formed at 5 8 km depth where mean and differential stresses may be greater in magnitude than in shallower porphyry environments many porphyry systems feature magmatic related stresses that dominated the local stress field with more variable vein orientations the result conversely orogenic gold systems usually exhibit strong dimensionality in vein orientations fault hosted mineralisation in metasedimentary rocks of the tgb deposits in this study is comparable in geometry but generally smaller in size than in many orogenic gold systems intrusion related systems of the tgb exhibit intermediate structural styles of mineralisation that provide a useful bridge in understanding the diversity of mechanically controlled structural styles in otherwise mostly unrelated gold deposit types

cation ratio and accelerator radiocarbon dating of rock varnish on mojave artifacts and landforms the first accelerator radiocarbon dates of rock varnishes are reported along with potassium argon ages of lava flows and conventional radiocarbon dates of pluvial lake shorelines in an empirical calibration of rock varnish k ca 2 ti 4 ratios with age in the mojave desert eastern california this calibration was used to determine the cation ratio dates of 167 artifacts although cation ratio dating is an experimental method some dates suggest human occupation of the mojave desert in the late pleistocene

textural control on gold distribution in as free pyrite from the dongping huangtuliang and hougou gold deposits north china craton hebei province china article i nfo pyrite from the dongping huangtuliang and hougou gold deposits hebei province china was investigated using a combination of ore microscopy including back scattered imaging and in situ laser ablation inductively coupled plasma mass spectroscopy la icp ms a range of elements including au te ag pb bi cu co ni and as was analyzed with the aim to constrain the textural controls and influence of superimposed deformation on the distribution of invisible gold in as free pyrite thedatasetshowsexcellentcorrespondence betweenmeasuredhighgoldvaluesandthreekeytexturalcriteria i areasofclusteredtellurideinclusions especiallyindongping ii microshearingandfracturing brecciation dongping and huangtuliang and iii pyrite recrystallization huangtuliang invisible gold is only at trace levels in the hougou and gold bearing telluride minerals are not observed this is considered to result from larger scale remobilization of gold during advanced brecciation and recrystallization of pyrite pyrite grains containing clustered inclusions have by far the highest gold concentrations up to 1 wt with consistent values of thousands of ppm au interpretation of textures and la icp ms data infers that in these areas the distribution of telluride inclusions extends pervasively from micron to nanoscale i e as fields of nanoparticles local employment of coupled dissolution reprecipitation reaction between pyrite and fluids driving the k metasomatic alteration dated at 150 ma is invoked to explain such patterns syn deformational grain scale mobilization of gold and other elements took place leading to a portion of the gold being recrystallized within fractures and microshears the deformational event leads to gold enrichment in the huangtuliang pyrite where brittle fracturing brecciation has been followed by a resorption and recrystallization in the latter case foam textured pyrites again without as can contain up to hundreds of ppm gold theinterpreted sequenceof pyritepost depositional eventsis inaccordancewith the protracted magmaticevolution of the area starting with variscan alkaline intrusions 390 ma and reactivated during the yanshanian orogeny 150 ma thestudyshows i thatas freepyritecanreadilycontainsignificantamountsofinvisiblegold bothasnanoparticles

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

tracing the transition of gold from source to sponge to sink the late timing of gold is a characteristic feature of orogenic gold deposits for many of these deposits gold and other metals were liberated from the host greenstone belt and or early sulfides concomitant with the syndeformational and fluid assisted metamorphic transition of pyrite to pyrrhotite and remobilized into late high grade ore shoots however this metamorphic driven gold upgrading model is usually inferred at least in part from mineral textures which can be equivocal in the absence of geochemical and or isotopic constraints herein we report new bulk mineral separate and in situ pb isotope results for minerals in key textures in the paleoproterozoic meliadine gold district arsenopyrite and pyrite porphyroblasts occur in paleoproterozoic quartz ankerite veins and are enveloped by remobilized pyrrhotite chalcopyrite and galena crystals that accompany gold in late microtextural sites a texture that forms the basis for the gold upgrading model early sieve textured arsenopyrite and pyrite are gold rich and tend to yield the least radiogenic pb isotope compositions high 207 pb 206 pb and 208 pb 206 pb ratios suggesting these relict domains escaped complete recrystallization during later reworking late inclusion free arsenopyrite and pyrite overgrowths scatter to more radiogenic pb isotope compositions low 207 pb 206 pb and 208 pb 206 pb ratios and are typically gold poor which we attribute to reworking and precipitation from a younger and more radiogenic fluid prior to and or during the precipitation of free gold in variably plunging high grade ore shoots at 1 90 to 1 85 ga remobilized sulfides are the youngest sulfide phases but locally yield low 206 pb 204 pb and 207 pb 204 pb ratios and thus spurious old model ages which overlap with the least radiogenic sieve textured arsenopyrite pyrite domains the isotopic signature of these remobilized sulfides was likely inherited from pb liberated during reworking of early arsenopyrite and pyrite and a second crustal pb source that was introduced during late remobilization our results highlight the importance of element recycling from early sulfides and or the neoarchean deposit host rocks during significantly younger paleoproterozoic reworking and gold remobilization the in situ pb isotope results of early and remobilized sulfides document the transition of gold from source to sponge to sink which is likely a feature that occurred during the formation of other orogenic style gold deposits

re os and u pb geochronology of the clear creek dublin gulch and mactung deposits tombstone gold belt yukon canada absolute timing relationships between plutonism and mineralization new re os molybdenite and u pb zircon dates from deposits of the tombstone gold belt provide direct timing constraints for sulphide mineralization and crystallization of spatially associated plutons at clear creek re os molybdenite dates 93 6 0 3 to 92 4 0 4 ma are in excellent agreement with the host intrusion u pb zircon age 92 3 0 3 ma consequently we propose the nominally younger existing 40 ar 39 ar hydrothermal mica ages 91 7 0 4 and 90 0 0 3 ma for clear creek are a result of slow cooling at dublin gulch the re os molybdenite date 93 2 0 3 ma for a late stage vein is nominally younger than the host intrusion u pb zircon age 94 0 0 3 ma in agreement with the deposits paragenesis the u pb zircon date 92 1 0 2 ma for the mactung pluton which lies adjacent to the w skarn has a younger age than the re os molybdenite dates 97 5 0 5 ma fluid inclusion geochemical data for ore fluid indicate a magmatic origin for the w skarn however the u pb and re os age data suggest that the exposed mactung stock is not the source of ore fluid for the w skarn and that the progenitor pluton for ore fluid is unknown the robust age constraints obtained for mineralization and magmatism in this study are in accord with the hypothesis that these deposits are intrusion related ore systems the results are also in agreement with previous studies demonstrating that the re os molybdenite chronometer can provide accurate and precise dates for the timing of metallic mineralization with rigorous sampling protocol

from intrusion related to orogenic mineralization the wasamac deposit abitibi greenstone belt canada abstract the wasamac deposit is an example of archean greenstone hosted gold deposit located in the abitibi belt 15 km southwest of rouyn noranda the deposit is hosted by a second order ductile shear zone of the cadillac larder lake fault zone cllfz known as the francoeur wasa shear zone fwsz it regionally sits at the boundary between the orogenic gold district of noranda and the kirkland lake gold district dominated by intrusion related gold systems this specific location in between two different gold mineralization environments sets the wasamac deposit apart as a prime candidate for investigating hydrothermal processes along the cllfz within the wasamac deposit gold distribution is constrained to the altered mylonitized portion of the fwsz lode systems are absent hydrothermal alteration and associated disseminated mineralization occurs as a replacement of the blake river group metavolcanic units the hydrothermal signature displays two distinct alkaline alteration assemblages potassic and albitic each associated with specific gold characteristics 1 potassic alteration is characterized by the crystallization of microcline carbonates and quartz within this assemblage gold is associated with porous pyrite enriched in te ag au mo pb bi w deposited under oxidizing conditions such characteristics are widely described in the kirkland lake area and are found in examples of syenite related mineralization such as the beattie and malartic gold deposits 2 the albitic alteration assemblage composed of albite sericite and carbonates reflects more reduced hydrothermal conditions with mineralization characterized by free native gold this hydrothermal event is coeval with the brecciation of early gold rich pyrite reflecting a structural overprint that controlled late stage gold characteristics these alteration and structural features are common in orogenic gold deposits both worldwide and regionally particularly at the neighbouring kerr addison and francoeur deposits and in lode gold systems such as in the sigma lamaque deposit the gold mineralization at wasamac has similar characteristics to both intrusion related gold systems and structurally controlled orogenic gold deposits hydrothermal and structural crosscutting relationships at wasamac indicate that a structurally controlled hydrothermal event overprinted earlier potassic magmatic hydrothermal alteration this observation supports a multistage process of gold concentration during which new gold characteristics metal anomalies fluid conditions and alteration assemblages replaced earlier stages of gold enrichment in places completely obliterating previous signatures we propose that the wasamac deposit was originally related to an alkaline intrusion buried at depth beneath the francoeur wasa shear zone

gas migration pathways and slope failures in the danube fan black sea abstract a large geophysical dataset including bathymetry and 2d and 3d p cable seismic data revealed evidence of numerous gas flares near the s2 canyon in the danube fan northwestern black sea this dataset allows us to investigate potential relationships between gas migration pathways gas vents observed at the seafloor and submarine slope failures vertical gas migration structures as revealed in the seismics appear to be concentrated near submarine slope failure structures where these seismically defined features extend upwards to the seafloor they correlate with the location of gas flares however not all these structures reach the seafloor in some cases because they are capped by overlying sediments a strong correlation is inferred between gas migration pathways heterogeneous mass transport deposits and contacts between adjacent units of contrasting lithology although missing age constrains prevent a final judgement we discuss the potential relationship between submarine slope failures and gas migration in order to determine if gas migration is a precursor to failure or if the presence of slope failures and associated mass transport deposits facilitates the migration of gas our observations indicate that lithological heterogeneity mass transport deposits and minor sediment deformation control gas migration pathways and the formation of gas chimney like features gas migration is focused and gradual resulting in gas flares where the chimney like features extend to the seafloor with no evidence of erosive features such as pockmarks

geochemistry of shale and sedimentary pyrite as a proxy for gold fertility in the selwyn basin area yukon selwyn basin area strata contain sedimentary pyrite with au above background levels when analyzed by laser ablation inductively coupled mass spectrometry hyland group rocks contain framboidal pyrite contents of 670 ppb au 1223 ppm as and 5 3 ppm te the mean of all types of sedimentary pyrite in the hyland group is 391 ppb au 1489 ppm as and 3 8 ppm te these levels are similar to sedimentary pyrite in host lithologies from major orogenic gold districts in new zealand and australia comparison of whole rock and pyrite data show that rocks deposited in continental slope settings with significant terrigenous input contain pyrite that is consistently enriched in au as te co and cu although data are limited whole rock samples of stratigraphic units containing au rich pyrite also contain high au indicating that most of the au is within sedimentary pyrite based on geologic characteristics and comparison of pyrite chemistry data with whole rock chemistry selwyn basin area strata have the necessary ingredients to form orogenic gold deposits au enriched source rocks metamorphic conditions permissive of forming a metamorphic ore fluid and abundant structural preparation for channeling fluids and depositing ore

cation ratio dating a new rock varnish age determination technique abstract rock varnish coats many surfaces of geomorphic and archaeologic interest in arid lands all varnish dating techniques are limited by the time lag between the exposure of a surface to subaerial processes and the onset of varnishing they are valid only where manganese is not remobilized after deposition for example in most arid environments the premise of a new age determination method cation ratio dating is that the ratio of the more mobile cations e g k and ca to titanium in varnish decreases with time although there are many inherent assumptions and potential limitations cation ratio dating has been verified on relative age sequences from a death valley debris cone negev desert talus flatirons and prehistoric lake levels at searles lake in california varnish cation ratios have been calibrated to independently dated surfaces in the coso volcanic field and vicinity in california tentative absolute dates have been assigned to geomorphic surfaces in the coso area cation ratios have been used to distinguish relative ages of archaeologic artifacts in southwestern north america and to demonstrate that varnish at the south stoddard locality mojave desert did not form in 25 yr

gold mineralization in as rich mesothermal gold ores of the bogosu prestea mining district of the ashanti gold belt ghana remobilization of invisible gold the bogosu prestea mining district of southwestern ghana is a 33 km section of the early proterozoic ashanti gold belt greenschist facies carbonaceous and carbonate bearing turbidites and greywackes and mafic dikes host numerous economic mesothermal gold deposits structurally higher ores in the bogosu concession have brittle deformation and consist of disseminated sulphide lodes in tectonically disrupted sedimentary rocks and carbonate altered mafic dikes most gold occurs as micrometre size particles in arsenian pyrite and as invisible gold in arsenian pyrite and arsenopyrite the structurally deeper ores of the adjoining prestea concession are associated with brittle ductile deformation and consist of extensive crack seal quartz veins and graphitic shear zones only minor amounts of invisible gold were detected in these deeper lodes gold occurs dominantly as abundant microscopic and larger particles in sulphide arsenide minerals and in gangue the gold distribution patterns revealed by sims microprobe analysis and ion maps emp and colour staining suggest that most of the primary gold in the bogosu prestea system precipitated in solid solution with sulphide arsenide minerals however post depositional concentration and redistribution occurred in increasing degree with 1 increase in metamorphic hydrothermal gradients in the gold system depth 2 decrease in the refractory properties of the host mineral and 3 increase in the amount of post depositional host mineral recrystallization and deformation gold evolved from primary solid solution within sulphide arsenide minerals to colloidal and micrometre size particles concentrated in voids fractures and internal grain boundaries and finally to microscopic and larger particles at sulphide arsenide grain margins and in the gangue assemblage the general conclusions presented here are applicable to as rich gold deposits of all ages worldwide the presence of gold in late fractures is insufficient evidence for late stage introduction of gold in mesothermal gold systems

petrogenetic significance of au bi te s associations the example of maldon central victorian gold province australia abstract mineralization at maldon victorian goldfields australia is part of the western lachlan foldbelt and is hosted by deformed lower ordovician metasediments intruded by the late devonian harcourt granite gold bi te s associations in the deposit include four gold bearing minerals maldonite native gold jonassonite and aurostibite and a range of sulphotellurides from the tetradymite group all of which are paragenetically tied to native bismuth the simpler phase associations involving bismuth maldonite and one or the other bi sulpho tellurides resemble the equivalents of eutectics in the system au bi te and hint at crystallisation from melts the complex associations include symplectites of gold and bismuth resulting from replacement of maldonite and are dominated by co existing sulphur bearing species joseite a joseite b an unnamed phase with bulk composition approximating to bi 3 te s 2 and jonassonite the complexity of such assemblages is also underpinned by chemical structural inhomogeneity within the unnamed phase i e nanoscale domains of bi 14 te s 9 and bi 5 te s 3 as documented from electron diffractions obtained from a fib tem foil although such lattice scale intergrowths reveal polysomatic disorder that can be associated with replacement of one telluride by another the structural modules in the overall stacking sequence indicate formation under local equilibrium conditions the mineralogical complexity of all au bi te s associations can be modelled in terms of interaction between simpler assemblages and sulphur bearing fluids three stages all involving gold minerals are recognised 1 bismuth maldonite hedleyite 2 bi sulphotellurides jonassonite and decomposition of maldonite gold bismuth and 3 bismuthinite gold from decomposition of maldonite or jonassonite the last stage is coincident with chloritisation and occurs during or following granite emplacement thermodynamic modelling of maldonite replacement by bismuthinite and gold indicates conditions to overlap with the pyrite hematite buffer at 258 c if neutral fluids are involved the general replacement model for the deposit shows that local scale reworking of maldonite and resultant gold remobilisation contributed to the apparent abundance of native gold this interpretation is consistent with the protracted geological history at maldon where granite intrusion post dates an orogenic event initial sulphur poor assemblages crystallised from melts predating granite emplacement the latest sulphidation event is attributed to retrograde fluids in the contact aureole whereas the earlier one could also have been produced during the multi stage deformation although observed assemblages and textures are the product of a complex sequence of overprinting there is no need to invoke input of gold from more than a single fluid generation

radiocarbon and cation radio ages for rock varnish on tioga and tahoe marainal boulders of pine creek eastern sierra nevada california and their paleoclimatic implications abstract accelerator mass spectrometry 14 c analyses of organic matter extracted from rock varnishes on morainal boulders yield limiting minimum ages for three crests of the tioga glaciation at pine creek in the eastern sierra nevada varnish started to form on boulders of the outermost tioga moraine before 19 000 yr b p and varnish originated on the innermost tioga moraine before 13 200 yr b p comparisons with lake level paleohydrological paleoecological colluvial and rock varnish micromorphological data indicate that central eastern california and western nevada experienced a moisture effective period during the late pleistocene but after the tioga maximum and perhaps as tioga glaciers receded from the mouth of pine creek canyon varnishes on tahoeage morainal boulders at pine creek have cation ratio ages of about 143 000 156 000 yr b p suggesting that the tahoe glaciation should not be correlated with oxygen isotope stage 4 in the early wisconsin but rather with stage 6 varnishes on morainal boulders of an older glaciation at pine creek are dated by cation ratio at about 182 000 187 000 yr b p

gold refining by bismuth melts in the iron oxide dominated nico au co bi cu w deposit nwt canada the nico au co bi cu w deposit is located in the great bear magmatic zone nwt canada where numerous polymetallic iron oxide dominated mineralized systems have been recognized petrographic electron microprobe analysis empa and laser ablation inductively coupled plasma mass spectrometry la icp ms studies of host rock alteration and ore mineralogy together with sulfarsenide geothermometry have been carried out to constrain the nature of alteration and or mineralization assemblages in this deposit metasedimentary rocks of the treasure lake group host nico and are pervasively altered to an assemblage of ferrohornblende i actinolite i biotite i magnetite i orthoclase which is cut by barren veins composed of quartz ferrohornblende orthoclase calcite set 1 these alteration events are overprinted by metasomatic prograde and retrograde mineralized assemblages and both brittle and ductile deformation accompanied the metasomatism the prograde assemblage 400 c consists of cobaltite co rich loellingite and co rich arsenopyrite stage i magnetite ii ferrohornblende ii actinolite ii biotite ii pyrite and minor scheelite and orthoclase the earliest retrograde mineralization consists of arsenopyrite stages ii and iii which contains variable amounts of co together with native bi bismuthinite and au with lesser magnetite marcasite pyrite hastingsite and minor quartz the preservation of solidified native bi droplets suggests a temperature range of 270 to 400 c for precipitation of this assemblage the final stage of retrograde mineralization consists of a chalcopyrite bismuthinite hematite chlorite assemblage together with hastingsite emplectite which formed at temperatures of less than 270 c textural and trace element evidence indicates that the au and bi present within arsenides and sulfarsenides in the nico system resulted from the initial partitioning of structurally bound au and or invisible nanometer sized particles of au and bi into the prograde sulfarsenide and arsenide phases which contain up to 81 ppm au the au and bi were remobilized following retrograde alteration of those minerals to arsenopyrite ii molten bi droplets are interpreted to have scavenged au insitu resulting in the formation of the bi au inclusions observed in arsenopyrite ii the second mechanism of gold refining is explained by the occurrence of contemporaneous bi te melt and hydrothermal fluids that also could have fractionated gold during transport in solution and deposited it in fractures interstitially to earlier mineral grains and as disseminations within ca fe amphibole magnetite biotite altered rocks overall the gold upgrading at nico is consistent with the liquid bismuth collector model suggesting that this process was an important control on gold concentration in this and potentially other au bi te fe as s rich iron oxide copper gold iocg deposits

a multidisciplinary study of invisible gold in arsenopyrite from four mesothermal gold deposits in siberia russian federation characterization of gold in four siberian mesothermal gold deposits was done through a multidisciplinary investigation including ore microscopy inaa sims epma and 197 au moessbauer spectroscopy the deposits are olympiada and veduga in the enisei mountain range and nezhdaninskoye and sentachan in the verkhoyansk kolyma area in which vein and disseminated orebodies are hosted by sedimentary sequences metamorphosed to amphibolite olympiada and greenschist facies ore minerals include arsenopyrite and pyrrhotite or pyrite as major constituents with minor scheelite and sphalerite chalcopyrite and native gold as minor constituents with or aurostibite galena gudmundite tetrahedrite stibnite and berthierite some of the gold mineralization is metallurgically refractory in all four deposits largely due to the presence of invisible gold in arsenopyrite and to a lesser extent in pyrite several generations of arsenopyrite are recognized in the auriferous zones of the veduga nezhdaninskoye and sentachan deposits textural observations indicate late deposition of native gold with respect to arsenopyrite no evidence was found of arsenopyrite recrystallization and the crystallization of native gold is considered to result from an evolution of fluids rather than redeposition of gold wide variations in the gold contents of arsenopyrite occur within deposits within samples and within individual grains inna analyses of individual arsenopyrite grains 30 70 mu m in size show that the gold concentration ranges from 197 au moessbauer spectra of arsenopyrite concentrates from the veduga nezhdaninskoye and sentachan deposits exhibit mainly a component with isomer shifts between 3 3 and 3 6 mm s which is interpreted as strong evidence of the presence of chemically bound gold in the arsenopyrite crystal structure the spectra of samples from the veduga deposit also contain minor amounts of native gold for which negative isomer shifts near 1 2 mm s are expected metallic gold dominates in the 197 au moessbauer spectra of samples from the olympiada deposit but varying amounts of chemically bound gold are also visible in the spectra some attributed to aurostibite it is concluded that the ores from these deposits are refractory partly because the gold occurs in arsenopyrite as chemically bound gold as well as submicroscopic metallic particles probably as small as 2 nm this study also shows that au bearing arsenopyrite is usually depleted in as with respect to its stoichiometric composition but correlation of au enrichment to fe deficiency was not found it is inferred from 197 au moessbauer spectroscopy that substitution of fe super 3 for au super 3 is most likely the temporal change in different modes of gold occurrence i e as native gold or invisibly in sulfides in the mineralized zones and veins is interpreted to result from an evolution of fluids rather than to be a recrystallization phenomenon dissemination of au bearing sulfides within the altered halos infers that a pervasive fluid rock interaction was an effective mechanism for deposition of minerals and gold absorption of gold complexes onto sulfide surfaces is also suggested to have played an important role in the deposition of native gold during the latest episodes in the formation of the deposits

multi stage enrichment processes for large gold bearing ore deposits abstract a review of previous studies of the world s large hydrothermal gold deposits indicates that the largest deposits tend to show complicated parageneses where multiple gold enrichment events and processes have been involved in the deposit generation these observations suggest that multistage processes may even be a requirement for the formation of large deposits in some deposits e g witwatersrand boddington cadia sukhoi log or carlin the different enrichment processes occur millions of years apart in others such as many large porphyry deposits the different stages are much closer in time in many deposits particularly sedimentary hosted deposits early diffuse enrichment occurs within a particular province that is then upgraded by more focused processes e g sukhoi log kalgoorlie the presence of this early diffuse enrichment could explain the tendency for gold deposits to cluster into camps this model has important implications as the presence or absence of multiple gold events could be used to discriminate at the exploration and feasibility stages between small deposits with single stage ore genesis and more complicated deposits with multistage enrichment and the potential for larger gold endowment

bi melt formation and gold scavenging from hydrothermal fluids an experimental study abstract the role of polymetallic melts in scavenging ore components has recently been highlighted in the context of fluid poor metamorphosed ore deposits in contrast the role of polymetallic melts in systems dominated by hydrothermal fluids remains poorly understood using a simple au bi model system we explored experimentally whether such polymetallic melts can precipitate directly from a hydrothermal fluid and investigated the ability of these melts to scavenge au from the solution the experiments were conducted in custom built flow through reactors designed to reproduce a hydrothermal system where melt components are dissolved at one stage along the flow path e g bi was dissolved by placing bi minerals along the fluid path whereas melt precipitation was caused further along the flow path by fluid rock interaction bi rich melts were readily obtained by reaction with pyrrhotite graphite or amorphous fes when au was added to the system bi au melts with compositions consistent with the au bi phase diagram were obtained in the case of fluid reaction with pyrrhotite epitaxial replacement of pyrrhotite by magnetite was observed with textures consistent with an interface coupled dissolution reprecipitation reaction icdrr in this case the metallic melt precipitated as blebs that were localized at the replacement front or within the porous magnetite direct fractionation of bi au melts from a hydrothermal fluid or precipitation of a bi melt followed by partitioning of au from ambient fluid offer new pathways to the enrichment of minor ore components such as au without requiring fluid saturation with respect to a au mineral this mechanism can explain the strong geochemical affinity recognized between au and low melting point chalcophile elements such as bi in many gold deposits examples of deposits where such a model may be applicable include orogenic gold deposits and gold skarns contrary to models involving migration of polymetallic melts to explain element remobilization only small quantities ppm of polymetallic melts are required to affect the au endowment of a deposit via interaction with a hydrothermal fluid the experiments also show that micro environments can play a critical role in controlling melt occurrences for example reaction fronts developing via icdr reactions can promote melt formation as observed during the replacement of pyrrhotite by magnetite the associated transient porosity creates space for the melt and promotes melt fluid exchanges whereas the reaction front provides local geochemical conditions favorable to melt precipitation e g reduced low ah2s aq and catalytic surface

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

modeling of gold scavenging by bismuth melts coexisting with hydrothermal fluids the effect of gold scavenging by bismuth melts is investigated using equilibrium thermodynamic modeling of an aqueous solution mineral melt system the calculations for the au bi na cl s h o system performed at temperatures between 300 and 450 c demonstrate that au concentrations in the melt are several orders of magnitude higher than in the coexisting fluid indicating the possible formation of economic gold deposits from undersaturated aqueous fluids in which mineralization would not be expected in the absence of a bismuth melt the model applies to any deposit where a bi melt is stable and coexists with a hydrothermal fluid examples of such deposits are known from skarn intrusion related orogenic and volcanogenic massive sulfide vms gold systems in sulfur poor systems the partitioning curves presented here can be used directly to correlate the gold concentration in the fluid and the au grade in the ore e g escanaba trough vms deposit these results also illustrate important principles generally applicable to understanding magmatic hydrothermal and metamorphic deposits that may have contained significant volumes of more complex polymetallic melts

arsenopyrite pyrite association in an orogenic gold ore tracing mineralization history from textures and trace elements laser ablation inductively coupled plasma mass spectrometry la icp ms spot analysis and element mapping coupled with high resolution focused ion beam scanning electron microscopy fib sem imaging has been performed on au hosting sulfides from a proterozoic orogenic gold deposit tanami gold province north central australia gold distribution patterns in the arsenopyrite pyrite assemblage which probably crystallized toward the end of the au forming event suggest a process of initial scavenging of au into arsenopyrite and subsequent remobilization of that au following brittle fracture gold expelled from the sulfide lattice during the remobilization event is reconcentrated around the margins of the same arsenopyrite grains and within swarms of crosscutting microfractures the distribution of pb bi and ag closely mimic au indicating that these elements were also reconcentrated preservation of oscillatory zonation patterns for co ni sb se and te in arsenopyrite imply however that no significant degree of sulfide recrystallization took place residual concentrations of invisible gold in grain centers are 5 ppm in arsenopyrite and 1 ppm in coexisting pyrite the presence of submicron sized pores is suggestive of a fluid driven process rather than solid state diffusion micron scale 2 10 m remobilized gold is accompanied by fine particles 200 nm 2 m and nanoparticles 200 nm of gold the extensive variation of au concentrations within single arsenopyrite grains underlines the significance of textures when using trace element data to understand how gold ores evolved

bimodal distribution of gold in pyrite and arsenopyrite examples from the archean boorara and bardoc shear systems yilgarn craton western australia this study investigates the microstructures geochemistry and hydrothermal evolution of gold bearing pyrite and arsenopyrite from six orogenic gold deposits in the archean eastern goldfields province western australia scanning electron microscope sem electron microprobe emp and laser ablation inductively coupled plasma mass spectroscopy la icp ms analyses show that the gold bearing minerals possess a number of similar textural features including the occurrence of invisible gold within initial phases of growth and later stage visible gold associated with alteration rims the alteration rims are characterized by a higher than average atomic mass mainly owing to arsenic enrichment and are preferentially located along fractures and grain boundaries in the pyrite and arsenopyrite these observations suggest that visible gold formation is associated with hydrothermal alteration of preexisting pyrite and arsenopyrite textural observations and la icp ms data suggest that some invisible gold was remobilized from early formed pyrite and arsenopyrite to form visible gold during development of these alteration rims gold may also have been added by hydrothermal fluids during a later stage of mineralization in situ geochemistry and phase relationships of alteration rims are used to further constrain the hydrothermal process responsible for formation of alteration rims and visible gold in fractures based on sulfide stability relations our data indicate that development of arsenopyrite alteration rims associated with late stage visible gold formation was related to an increase in temperature maximum increase from 310 to 415 c and up to of sic orders of magnitude increase in sulfur fugacity whereas changes in oxygen fugacity were less important la icp ms analyses show that the relative and absolute variations in selected trace element au ag sb bi ba te pb co and mo concentrations can also be used to distinguish between unaltered and altered pyrite and arsenopyrite in general trace elements within pyrite and arsenopyrite have a relatively uniform distribution whereas later stage alteration rims have more variable trace element distributions although the observed textures are typical of prograde metamorphic coronae we suggest that they are the consequence of variations in fluid conditions and chemistry and that mineralization occurred in response to syn and or postpeak metamorphic fluid infiltration

the last pluvial climatic episodes in the deserts of southwestern north america long term variations in the effective moisture of deserts are linked both to glacial interglacial climatic cycles and through the milankovitch theory to orbitally induced changes in solar radiation intensity1 4 for southwestern north america comparison of palaeobotanical data with model predictions of palaeoclimate reveals two distinct pluvial regimes during the late wisconsin and early holocene 23 8 kyr bp southward displacement of the aleutian low pressure centre and strengthened westerlies from 40 to 30 n were important features of the full glacial 18 kyr bp pluvial maximum increased cyclonic storms and weakened monsoons led to a winter precipitation regime a feature seen both in the fossil record5 6 and climate models7 8 dominantly zonal flow during this period contrasts with the latest wisconsin and early holocene 12 8 kyr bp when meridional circulation transported maritime tropical air into the desert interior we now show that plant macrofossil assemblages from packrat neotoma spp middens9 in the mojave desert provide evidence for increased temperatures and summer rainfall during this period10 12 the anomalously late persistence to 8 kyr bp of woodland in low latitude low elevation deserts of the south west13 is attributed to enhanced monsoonal circulation3

the dublin gulch intrusion hosted gold deposit tombstone plutonic suite yukon territory canada the dublin gulch intrusion is a member of the tombstone plutonic suite a linear belt of middle cretaceous intrusions that extend across the yukon territory like many of the intrusions in this suite the dublin gulch intrusion is associated with several different zones of gold and tungsten mineralization within and immediately adjacent to the intrusion the eagle zone 50 3 mt 0 93 g t gold located in the southwestern part of the dublin gulch intrusion hosts the most significant concentration of gold in the area the gold occurs in a broadly east west striking steeply south dipping series of sheeted veins the veins consist of early quartz scheelite pyrrhotite pyrite arsenopyrite and are associated with k feldspar albite alteration envelopes these grade out to and are overprinted by sericite carbonate chlorite alteration the same assemblage also occurs in veinlets that refracture sheeted quartz veins and contain the majority of the gold the gold occurs with molybdenite lead bismuth antimony sulfosalts galena and bismuthinite gold correlates strongly with bismuth r 2 0 9 a relationship common to several intrusion related gold deposits but has a poor correlation with all other elements tungsten and molybdenum have a weak inter element correlation r 2 0 55 and paragenetically pre date the majority of gold precipitation lead zinc copper silver antimony and arsenic have moderate to strong inter element correlations 0 58 to 0 93 the change from tungsten bearing mineralization through to gold bismuth rich ores with elevated syn to post ore lead zinc copper silver antimony and arsenic can be grossly correlated with a change in hydrothermal fluid composition early scheelite bearing quartz contains primary co2 rich fluid inclusions which are post dated by secondary inclusions with higher salinities up to 15 wt nacl equiv and less co2 these latter inclusions are interpreted to coincide with the later gold bismuth and base metal mineralization the favored genetic model is one in which early co2 rich fluids exsolved from a magma with an initially high co2 content but progressively became more saline and h2o rich as the system evolved

timing of gold mineralization at red lake northwestern ontario canada new constraints from u pb geochronology at the goldcorp high grade zone red lake mine and the madsen mine the goldcorp high grade zone at the red lake mine has reserves proven and probable established at 1 775 million tons mt at an average grade of 80 6 g t au new u pb geochronologic data combined with detailed mapping and crosscutting relationships provide timing constraints and new insights into the formation of the exceptionally rich goldcorp high grade zone the results show that the main stage of the high grade mineralization in the high grade zone formed before 2712 ma and that a second stage of gold mineralization much smaller in terms of total gold content but spectacular in terms of grade formed after 2702 ma this second stage is attributed to gold remobilization caused by the enhanced thermal gradient and deformation associated with the emplacement of the cat island pluton 2699 ma approximately 7 km east of the deposit or by the ca 2 63 to 2 66 ga postorogenic regional thermal event indicated by hornblende muscovite and biotite ar ar cooling ages from the uchi subprovince it is proposed that the main stage of high grade mineralization formed between ca 2723 to 2712 ma possibly synchronous with emplacement of the dome and mckenzie island stocks the abino granodiorite and hammell lake batholith as well as with penetrative d2 main stage regional deformation lamprophyre dikes spatially associated with the deposit postdate main stage mineralization by at least 10 m y a postore granodiorite dike from the madsen mine the second largest deposit in the district was dated at 2698 1 ma this age is similar to a postalteration and late deformation diorite dike at the creek zone showing near the former starratt olsen mine which was dated at 2696 2 ma the crosscutting relationships confirm that the main stage of mineralization at the madsen mine is pre 2698 ma a folded and metamorphosed polymictic conglomerate located stratigraphically above the campbell red lake deposit and deposited after ca 2747 but before 2712 ma is correlated with the newly recognized huston assemblage crosscutting and overprinting relationships between the conglomerate and the alteration indicate protracted multistage aluminous and iron carbonate quartz veining and or alteration event s before and after deposition of the conglomerate the age of the conglomerate confirms that the campbell red lake deposit is proximal to a folded regional unconformity consistent with the empirical relationship between large gold deposits and unconformities elsewhere in archean greenstone belts this paleosurface represents a key first order exploration target and it is proposed that areas of high potential for gold exploration in red lake occur in the balmer assemblage rocks within 500 m to 1 km of the unconformity

gold telluride nanoparticles revealed in arsenic free pyrite pyrite the most abundant sulfide on earth and a common component of gold deposits can be a significant host for refractory gold this is the first documentation of pore attached composite au telluride nanoparticles in arsenic free pyrite trace elements mapping in pyrite from an intrusion hosted au deposit with orogenic overprint dongping china shows trails of tellurides overlapping co ni zonation intragranular microfracturing anomalous anisotropy and high porosity are all features consistent with devolatilization attributable to the orogenic event the pyrite hosted nanoparticles are likely the frozen solid expression of te rich au ag pb bearing vapors discharged at this stage nanoparticle formation as presented here provides the smallest scale tool to fingerprint au trapping during crustal metamorphism

a revised chronology for the last pleistocene lake cycle in the central lahontan basin radiocarbon dates of plant materials from packrat middens in caves below the elevation of the last high stand of pleistocene lake lahontan in conjunction with radiocarbon dates of ancient archaeological materials provide evidence that the last high stand terminated before 12 070 yr b p this new information suggests that the last major fluctuation in the level of lake lahontan was approximately synchronous with those of lakes bonneville and russell and that these three lakes probably responded contemporaneously to the same regional paleoclimatic changes

insights into the liquid bismuth collector model through analysis of the bi au stormont skarn prospect northwest tasmania the liquid bismuth collector model has previously been proposed to explain the common association between gold and bismuth seen in many ore deposits and involves the scavenging of gold from hydrothermal fluid by liquid bismuth here textural relationships and temperature and chemical conditions of mineralization at the stormont bi au skarn prospect in northwestern tasmania are analyzed to investigate whether the liquid bismuth collector model can explain gold accumulation at this prospect the calc silicate skarn assemblage containing grandite hedenbergite actinolite magnetite epidote quartz and calcite suggests that initial metasomatism involved temperatures 400 to 500 c visible gold is associated with native bismuth bismuthinite maldonite galena and an unknown bi te s mineral native bismuth is directly associated with grandite which formed during initial stages of metasomatism this implies that bismuth would have precipitated from a hydrothermal fluid as a liquid heating experiments showed that bismuth alloy inclusions in garnet melt between temperatures of 250 and 288 c the mineralization setting at stormont and textural relationships suggest that gold was scavenged from hydrothermal fluids by liquid bismuth rhythmically zoned grandite crystals provide evidence for fluctuating hydrothermal fluid conditions which may have contributed to dissolution and reprecipitation of gold thus facilitating an ore zone refining process within the prospect this refining process is made possible by the tendency of liquid bismuth to scavenge gold from significantly under saturated fluids infiltrating gold undersaturated fluids can dissolve gold not in contact with bismuth which can then be preferentially redeposited upon encountering a bismuth droplet enhancing the correlation between the two elements a long lived and focused fluid flow system would maximize the efficiency of this ore zone refining process which is likely to be relevant to gold deposition at a range of deposit types

melt recharge f o2 t conditions and metal fertility of felsic magmas zircon trace element chemistry of cu au porphyries in the sanjiang orogenic belt southwest china the magmatic hydrothermal pulang cu deposit triassic and the beiya au cu deposits eocene are located in the sanjiang copper porphyry belt southwest china zircon chemistry was used to constrain the magmatic evolution and oxidation state of the porphyries the results show that porphyries of the beiya district formed from an early oxidized melt and a later relatively reduced and more evolved magma whereas pulang experienced a normal cu porphyry evolutionary trend the pulang porphyries crystallized from more oxidized magma fmq 2 9 4 6 average 4 0 1 0 n 3 with an average temperature of 709 6 c compared to the beiya porphyries fmq 0 6 3 5 average 1 9 1 3 n 5 with a mean magmatic temperature of 780 22 c these data combined with data from other cu and au rich porphyries in the sanjiang belt i e machangjing cu yao an au are consistent with previous experimental work showing that elevated cu and au solubilities in magma require oxidizing conditions a compilation of existing geochemical data for magmatic zircons from fertile and barren porphyry systems worldwide establishes an optimal diagnostic interval on ceiv ceiii tti in zircon and eu eu n plots for generating magmatic hydrothermal cu au deposits

geomorphic and paleoclimatic implications of latest pleistocene radiocarbon dates from colluvium mantled hollows california radiocarbon analyses of charcoal from basal colluvium in 11 california hollows show a clustering of dates between 9000 and 15 000 b p an indication that changes in the storage and discharge of colluvium from hillslopes accompanied the pleistocene holocene transition hollows are sites of topographically induced convergence and deposition of colluvial debris and evacuation of this debris was apparently more thorough and possibly more frequent in the latest pleistocene perhaps due to a combination of changes in vegetation and rainfall characteristics one hypothesis is that greater storm intensities occurred in the latest pleistocene and induced a higher frequency of landslides in hollows and a regional extension of channel heads upslope relative to the holocene during the last pleistocene california hollows apparently contained smaller volumes of colluvium in storage the increased storage of debris during the holocene may have resulted in a diminished supply of sediment to stream channels

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

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

stable carbon isotope ratios of rock varnish organic matter a new paleoenvironmental indicator stable carbon isotope ratios of organic matter in rock varnishes of holocene age from western north america and the middle east show a strong association with the environment this isotopic variability reflects the abundance of plants with different photosynthetic pathways in adjacent vegetation analyses of different layers of varnish on late pleistocene desert landforms indicate that the carbon isotopic composition of varnish organic matter is a paleoenvironmental indicator

the au bearing vein system of the limarinho deposit northern portugal genetic constraints from bi chalcogenides and bi pb ag sulfosalts fluid inclusions and stable isotopes abstract the limarinho gold deposit northern portugal located in the variscan iberian massif is a system of gold bearing quartz veins that crosscut a peraluminous synkinematic syn d3 315 310 ma variscan granite the mineralized structures are subvertical and trend n30 40e parallel to the penacova regua verin fault the quartz veins record two main stages of mineral deposition i and ii indicating successive periods of vein re opening and are crosscut by stage iii irregular veinlets stage i is characterized by milky quartz mq and arsenopyrite apy 1 and produces a greisen like alteration stage ii comprises clear quartz cq k feldspar chlorite rutile and metallic minerals mainly arsenopyrite apy 2 apy 3 and pyrite with galena chalcopyrite and sphalerite in lesser amounts the stage iii is characterized by chlorite and pyrite the hydrothermal fluids in stages i and ii are low salinity 4 n 2 amount toward the stage ii that indicates mixing with metamorphic fluids temperatures of 460 40 c and pressures from 230 to 350 mpa are inferred for the as fe mineralization of stage i the highest p values indicate lithostatic conditions at a depth of around 13 km the inferred p t conditions for the stage ii range from 340 to 425 c and from 60 to 225 mpa a decompression of more than 250 mpa through stages i to ii without a substantial cooling of around 150 c is observed in accordance with similar variscan belt gold deposits of western europe estimated temperatures for stage iii are from 300 c to lower than 200 c the pressures being around 60 mpa late stage ii and stage iii are related to aqueous fluids with variable salinity as a result of a mixing between two end members a hotter and more saline fluid 6 7 wt nacleq and a cooler and dilute one 18 o values for aqueous carbonic fluids 10 5 to 7 6 do not permit to discriminate between magmatic and metamorphic the 18 o and d values obtained for chlorite 4 0 to 5 0 and 24 2 to 23 1 respectively are more indicative of a meteoric signature the 34 s values of the fluid in equilibrium with arsenopyrite range from 10 0 to 7 8 however the values obtained for pyrite around 2 6 can be only explained by the incorporation of an isotopically heavier sulfur from a different source probably sedimentary a detailed study of the ore has shown two different gold parageneses the first bi te s comprises native au or maldonite along with native bi hedleyite and joseite b the second cu ag pb bi s te comprises electrum cu ag pb bi sulfosalts such as lillianite gustavite s s heyrovskyite vikingite berryite and matildite pb bi sulfotellurides and hessite lamellar exsolution textures observed in the lillianite gustavite s s indicate initial temperatures above 400 c the paragenesis becomes ag enricher as temperature goes down matildite 111 exsolutions indicate a low limit of mineralization slightly above 200 c we propose different mechanisms of gold precipitation 1 a gold scavenged by bi melts in the first case 2 remobilization of this early gold together with new gold deposition due to sulfur extraction and or fluid mixing from the hydrothermal fluid during the stage of prevailing sulfide precipitation we conclude that the existence of at least two gold bearing hydrothermal events may have enhanced the gold grade in this type of gold deposits

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

the hemlo gold deposit ontario an example of melting and mobilization of a precious metal sulfosalt assemblage during amphibolite facies metamorphism and deformation the timing of mineralization at the hemlo gold deposit has long been a controversial issue we present arguments that use partial melting of the ore assemblage to explain the mechanism of ore mobilization and resolve problems with the model of premetamorphic mineralization the results have implications for mine exploitation and exploration strategies we show that the ore mineral assemblage underwent partial melting during middle amphibolite facies metamorphism 600 650 c 6 7 kbars primarily through breakdown of stibnite and arsenopyrite in a high s2 environment concurrent deformation led to segregation of the resulting sb and as rich sulfosalt melt interaction between this melt and a range of unmelted sulfides led to further melting and incorporation of other elements into the melt the gold bearing melt was mobilized from compressional high strain sites into dilational domains such as boudin necks and extensional fractures developed in competent lithologies ore minerals that did not melt significantly pyrite molybdenite pyrrhotite and sphalerite were not extensively mobilized and largely remained within high strain compressional domains this segregation of melt from residue thus resulted in the observed heterogeneous distribution of ore minerals within the deposit crystallization of the sulfosalt melt produced a diverse suite of ore minerals dominated by stibnite and realgar and containing an array of rare sulfosalts native elements intermetallic compounds and tellurides some sulfosalt melt persisted to low temperature 300 c allowing continued small scale localized mobilization during late deformation although gold occurs at moderate concentrations within the compressional high strain domains it is particularly concentrated in the dilational domains a consequence of its mobilization within a sulfosalt melt our model of partial melting of the ore mineral assemblage with consequent mobilization explains how ore minerals that are unstable even at greenschist facies conditions came to be hosted in structures that formed at or near the peak of amphibolite facies metamorphism

k ar dating of the cima volcanic field eastern mojave desert california late cenozoic volcanic history and landscape evolution thirty four k ar ages and supporting paleomagnetic measurements from flows of the cima volcanic field provide a detailed volcanic history and a temporal basis for analysis of evolving landforms the cima field has undergone three periods of volcanic activity spanning late miocene through latest pleistocene time 1 7 6 to 6 5 2 4 5 to 3 6 and 3 1 0 to at least 0 015 m y ago these precisely dated sequences afford a unique opportunity to quantify the long term effects of pedogenesis and erosion on volcanic landforms in an arid environment morphologic and pedologtc data indicate that the most stable geomorphic surfaces in the field occur on flows between 0 25 and 0 75 m y old younger flows are dominated by eolian aggradation and outcrop rubbling and older flows are dominated by surface runoff and fluvial dissection a process response model involving progressive pedogenesis and a subsequent shift from infiltration to surface runoff is proposed to explain this temporal variation in land surface stability