mafic magmas as sources for gold in middle miocene epithermal deposits of the northern great basin united states evidence from pb isotope compositions of native gold we measured the pb isotope compositions of traces of lead alloyed by gold electrum and naumannite and gangue vein minerals in bonanza epithermal gold deposits of the northern great basin and compared them to possible sources of lead in the region gangue minerals have pb isotope compositions consistent with hydrothermal leaching of lead from local host rocks lead isotope ratios of gold ores however have signatures that are distinct from the gangue minerals and common host rocks in the area such as metasediments and miocene rhyolites the gold ores are isotopically similar to local mafic rocks of the middle miocene bimodal volcanic assemblage indicating that the latter are the likely source of the precious metals the local mafic rocks are also similar to the middle miocene columbia river basalts suggesting that both were derived from a mantle source with similar pb isotope composition our preliminary results indicate that the precious metals were derived from mafic magmas likely related to northern nevada rifting processes and or to the arrival of the columbia river basalt plume and the emergence of the yellowstone hot spot exsolving volatiles from the mafic magmas most likely transported gold upward to the shallow meteoric water dominated epithermal systems where the precious metals precipitated together with quartz and adularia to form bonanza ores that typify these deposits

geochronology geochemistry and fluid characterization of the late miocene buritica gold deposit antioquia department colombia buritica is an intermediate sulfidation epithermal gold deposit located 75 km north of medellin colombia it is hosted by the late miocene buritica andesite porphyry a shallow level pluton dated at 7 41 0 40 ma 2 mswd 2 30 40 ar 39 ar on hornblende which intrudes volcanic and sedimentary rocks of the cretaceous barroso formation and the buritica stock gold mineralization is associated with proximal sericite adularia and distal epidote dominant propylitic alteration and is hosted in two different sets of veins striking and dipping 072 87 s and 105 87 s hydrothermal muscovite from altered host rock yielded a weighted average 40 ar 39 ar age of 7 74 0 08 ma for two samples which is within error of the age of the buritica andesite porphyry sulfur isotope oxygen isotope and fluid inclusion data indicate that a relatively hot and saline fluid 310 c up to 8 wt nacl equiv of probable magmatic origin was dominant during stage 1 of the vein paragenesis and mixed with cooler and less saline groundwater during stages 2 and 3 fluid inclusion assemblages indicate that boiling was likely the main control on gold precipitation with cooling and dilution during fluid mixing as a possible secondary control

application of high resolution x ray computed tomography to mineral deposit origin evaluation and processing abstract high resolution x ray computed tomography ct is the industrial equivalent of medical cat scanning and provides a mechanism for non destructive studies of the three dimensional nature of geological materials hrxct produces two dimensional images slices that reveal the interior of an object as if it had been sliced open along the image plane for viewing a ct image is generated by differences in x ray attenuation that arise from differences in density and composition within the object by acquiring a contiguous set of slices volumetric data for all or part of a sample can be obtained allowing three dimensional inspection and measurement of features of interest ct is particularly effective in the study of metallic ores that commonly contain minerals spanning the range of densities of natural materials available software can produce grain shape size and orientation data from the scanned volume which can be particularly useful for oriented samples ct is particularly useful for studies of gold and other precious metal bearing minerals that typically have significant contrast even with common metallic mineral phases ct precisely defines the in situ location of mineral grains of interest within a sample which then can be studied in conventional petrographic sections and other forms of data collected e g isotope or trace element geochemistry another area of application is fluid inclusions which can be discerned even in opaque phases and fluid and vapor volumes measured for sufficiently large occurrences we summarize ct principles and review available instrumentation as well as scanning and data reduction protocols that provide unique three dimensional information for diverse ore types and applications in mineral deposits geology ct applications for economic geology and other fields will continue to expand as instruments continue to evolve and as scanning protocols and applications are extended for more precise quantification of three dimensional relationships particularly for fine grained particles and small fluid inclusions in larger volumes and for separation of minerals with limited contrast

new insights into the tectonic evolution of the andaman basin northeast indian ocean multibeam swath bathymetry data acquired over an area of about 30 000 km2 together with magnetic and single channel seismic data have been analyzed to understand the tectonic evolution of the andaman basin northeast indian ocean swath bathymetry data revealed several morphotectonic features that divide the basin into a complex western part comprising arc parallel seamount chains and n s trending fault systems and a relatively smooth eastern part a sw ne trending spreading ridge bisects the basin an offset of about 11 8 km at 94 21 e longitude divides the spreading ridge into sw and ne segments high resolution bathymetry and magnetic data enabled us to identify two finer scale segments over the sw part of the ridge that display distinct topographic fabric compared to the ne segment thin sediment cover and presence of axial seamounts characterize sw segments of the ridge whereas the ne segment has thick sediment cover and well maintained rift valley kinematics of basin evolution derived from the identification of the seafloor spreading magnetic anomalies suggest very slow initial opening rates of about 1 6 cm yr and an increase in the spreading rate to about 3 8 cm yr from anomaly 2 to present although this interpretation relies only on magnetic anomalies in the two westernmost segments it is suggested that the true seafloor spreading started about 4 ma in the andaman backarc basin rather than the 11 ma postulated previously we infer based on the integrated analysis of swath bathymetry magnetic and seismological data that the basin has evolved as a consequence of extrusive tectonics that prompted extension and rifting along the plane joining the sagaing and semengko fault systems extension by seafloor spreading during the past 4 myr resulted in the formation of the deep andaman backarc basin this phase has also experienced westward propagation of the spreading center

the miwah high sulphidation epithermal au ag deposit aceh indonesia geology and spatial relationships of gold with associated metals and structures abstract geological mineralogical and whole rock geochemical studies of the plio pleistocene miwah high sulphidation au ag deposit in aceh indonesia revealed evidence of a complex and dynamic interplay between structural development and mineralisation the mineralisation occurs in a zone of predominantly phreatic breccias measuring 1 1 km long by 400 m wide and 200 350 m deep within a fault bounded calc alkaline andesite ridge within this zone low grade disseminated au ag has superimposed narrow high grade au ag zones hydrothermal alteration and mineralisation in three mineralised centres the miwah bluff in the west and the west block m and east block m in the east are controlled by steeply dipping 60 and northerly trending pre to syn mineral feeders this alteration displays an outward sequence of vuggy silica i alunite silica advanced argillic and kaolinite illite argillic a distal chlorite smectite propylitic zone may precede or constitute the end product of the above alteration late silica flooding converted parts of the vuggy silica i and the alunite quartz zone into variably sulphide rich massive silica and vuggy silica ii containing pyrophyllite alunite respectively penecontemporaneous with the massive silicification main stage mineralisation initially deposited intergrowths of pyrite enargite quartz followed by scattered enargite and pyrite and finally rapidly precipitated ag bearing native gold dendrites in vugs with lesser amounts of sulphides the crack seal texture of the host rocks paragenetic variation in precious and base metal mineral species and selective dissolution fracturing and shearing of most of the metallic and gangue minerals are interpreted to reflect transient pulses of fluid flows and mineralisation within zones of structural deformation high grade au zones and associated metals formed clusters along the feeders whereas low grade disseminations of the same metals were spread across the deposit these metals are zoned along the feeders from apical au ag to deep cu as sb te and sn are variably distributed laterally the zonation is from au ag in the feeders through cu as sb bi te and sn and finally to pb or zn in the periphery while these bulk rock metal abundances indicate the efficacy of certain physico chemical conditions of the fluids to accumulate the metals their ratios point to an upward paleo flow path of the fluids along the northerly striking faults and related fractures combining all of the above data we propose that miwah developed in fault fracture networks beneath paleo solfataras in a volcanic crater where the ascending acidic magmatic gas or gas fluids vapours transformed the phreatic breccia host rocks first into vuggy silica and advanced argillic rocks which were subsequently resilicified and mineralised into au and base metal rich massive silica during the depressurisation and condensation of the gas fluid phases mineralisation processes terminated with limited ag enrichment by late stage fluids in the upper segment of the deposit and in deep fractures

seismic images of the sliver strike slip fault and back thrust in the andaman nicobar region 1 the sliver strike slip great sumatra fault gsf traverses mainland sumatra from the sunda strait in the southeast to banda aceh in the northwest and defines the present day plate boundary between the sunda plate in the north and the burmese sliver plate in the south it has been well studied on mainland sumatra but poorly north of banda aceh in the andaman sea here we present deep seismic reflection images along the northward extension of the gsf over 700 km until it joins the andaman sea spreading centre and we interpret these images in the light of earthquake gravity and bathymetry data we find that the gsf has two strands between banda aceh and nicobar island a transpression in the south and a deep narrow active rift system in the north dotted with volcanoes in the center suggesting that the volcanic arc is coincident with rifting farther north of nicobar island an active strike slip fault the andaman nicobar fault cuts through a rifted deep basin until its intersection with the andaman sea spreading centre the volcanic arc lies just east of the rift basin the western margin of this basin seems to be a rifted continental margin tilted westward and flooring the andaman nicobar fore arc basin the andaman nicobar fore arc basin is bounded in the west by back thrusts similar to the west andaman and mentawai faults the cluster of seismicity after the 2004 great andaman sumatra earthquake just north of nicobar island coincides with the intersection of two strike slip fault systems

evolution of magmatic vapor to gold rich epithermal liquid the porphyry to epithermal transition at nevados de famatina northwest argentina fluid inclusions in the subeconomic porphyry cu mo au system at nevados de famatina and closely associated high sulfidation epithermal cu au ag as sb te veins at la mejicana northwest argentina were studied to reconstruct the evolution of hydrothermal fluids from their deep magmatic source to the shallow epithermal environment field geology vein petrography fluid inclusion microthermometry and single inclusion microanalysis by laser ablation inductively coupled plasma mass spectrometry la icp ms were combined to determine the evolution of pressure temperature and ore metal concentrations in the fluids cathodoluminescence imaging complements transmitted light petrography to constrain the successive stages of quartz formation and the entrapment sequence of the fluid inclusion populations aqueous liquid inclusions of 5 wt percent nacl equiv salinity trapped in quartz sericite pyrite qsp veins between 360 and 325 c contain unusually high concentrations of cu 400 5 000 ppm as 200 ppm sb and te both up to 100 ppm and au several ppm along with other ore forming elements these veins are paragenetically transitional between subeconomic porphyry copper mineralization exposed in the valley floor and high sulfidation epithermal veins with high au grades which are preserved along a high ridge adjacent to the porphyry stock low density vapor and hypersaline liquid i e brine inclusions were trapped in early formed quartz of the porphyry stockwork veins between 450 and 600 c the brine contains lower cu and au concentrations than coexisting vapor inclusions and texturally even earlier inclusions of intermediate density that are trapped in phenocrysts and some stockwork veins the latter may be equivalent to a parental fluid the low and intermediate density fluids compositionally overlap in salinity with the aqueous liquids recorded by the later transitional quartz sericite pyrite veins fluid inclusions and geologic time relations indicate that the transitional quartz sericite pyrite veins were the channelways for metal rich aqueous liquids that generated the high sulfidation epithermal cu au deposit these mineralizing liquids are interpreted to have evolved by continuous density increase contraction from a low salinity s rich magmatic fluid of low to intermediate density this vaporlike fluid was produced by exsolution from a magma that existed at greater depth during the late stages of cooling of the magmatic hydrothermal complex and probably underwent minor brine separation prior to reaching the present level of exposure epithermal mineral precipitation occurred upon dilution of the low salinity magmatic fluid with meteoric water which entered the hydrothermal system as it was cooled and successively eroded during continued magmatic fluid ascent

boiling colloid nucleation and aggregation and the genesis of bonanza au ag ores of the sleeper deposit nevada a deep parent composition for bonanza oreforming fluids at the sleeper deposit was calculated by the computer program solveq using fluid inclusion microthermometric and gas data and by assuming equilibrium with the following minerals present in vein samples below the bonanza zones gold chalcedony adularia pyrite chalcopyrite and acanthite the calculated dissolved gold content of 295 ppb is approximately 2 orders of magnitude higher than that assumed for typical geothermal systems thus a gold enriched fluid appears to have been a principal factor in the genesis of bonanza au ag ores at the sleeper deposit geochemical modelling of possible ore forming processes using the computer program chiller with the reconstructed ore forming solution as a starting composition indicates that boiling most closely reproduces observed minerals and their relative abundances in bonanza ores the constraint imposed by the association of amorphous silica with gold precludes all of the mixing scenarios modelled such as mixing with cold and steam heated groundwaters acid sulfate co2 rich modelling indicates that boiling of a gold rich deep solution leads to rapid gold precipitation and that the amount of gold precipitated initially is large relative to other minerals these factors apparently led to nucleation of colloidal gold particles instead of in situ gold deposition or coprecipitation with other phases gold colloids apparently were entrained in the upward flowing hydrothermal solutions and grew as they travelled upon reaching a critical size 10 100 nm they were deposited due to orthokinetic aggregation at an elevation and temperature at which amorphous silica was nucleating and aggregating

occurrences of dendritic gold at the mclaughlin mine hot spring gold deposit two styles of gold dendrites are variably developed at the mclaughlin mine the most abundant occurrence is hosted by amber coloured hydrocarbon rich opal silica likely precipitated from a boiling hydrothermal fluid and complexed with immiscible hydrocarbons forming an amorphous hydrocarbon silica phase this phase likely scavenged particulate gold by electrostatic attraction to the hydrocarbon silica phase the dendritic nature of the gold is secondary and is the result of dewatering of the amorphous hydrocarbon silica phase and crystallization of gold into syneresis fractures the second style of dendritic gold is hosted within vein swarms that focused large volumes of fluid flow the dendrites occur along with hydrocarbon rich silica at the upper contact of the vein margins which isolated the dendrites allowing sufficient time for them to grow in a manner similar to the amber coloured opal the dendrites may have formed by scavenging particulate gold by electrostatic attraction to the hydrocarbon silica phase

precipitation of gold in a low sulfidation epithermal gold deposit insights from submillimeter scale oxygen isotope analysis of vein quartz a discussion sir hayashi et al s 2001 study of vein quartz from the hishikari mine has provided a good test of the model of gold deposition by boiling that we put forward in a paper bogie and lawless 1999 on the significance of the mineralogical and textural variation in low sulfidation epithermal gold deposits our model is based on the textural evidence for the silica supersaturation at elevated temperatures in epithermal gold deposits that is observed within bands of rhythmically banded gold ore this is exactly the material that is shown in figure 5 of hayashi et al 2001 although the authors describe the band of recrystallized chalcedony that typically starts off each rhythmic cycle as bladed quartz other workers e g faure et al 1998 describe this material at hishikari as chalcedony we and earlier workers fournier 1985 saunders 1994 interpret recystallized chalcedony and in some cases quartz that retains ghosts of colloform textures as having initially been amorphous silica that has then recrystallized to chalcedony or quartz as the case may be this point is supported by the work of faure et al 1998 who found excess water in chalcedonic quartz at hishikari that they interpret to be the result of its initially depositing as a hydrous gel in

how sediment promotes narrow rifting application to the gulf of california 1 if the gravitational stress changes related to crustal thinning are large enough to affect the mode of extension then sedimentation should also affect that mode we propose that the weight of sediments reduces the difference in crustal buoyancy forces caused by local crustal thinning allowing a rift to extend more easily in a narrow rift mode we examine the effect of deposition of large amounts of nonlocally derived sediment on extensional style using two dimensional regional scale numerical experiments of extending thick continental crust with varied initial thermal conditions geometries rheological parameters and sedimentation properties depending on initial model conditions the onset of rifting in thick continental crust occurs in the narrow rifting wide rifting or core complex mode with continued extension all cases eventually transition to a narrow rifting mode for a system in wide rift mode moderate to fast sedimentation shortens the time needed to transition to a narrow rift in the gulf of california changes in extensional style correlate with sediment thickness with an earlier transition to narrow rifting in the north versus the south we compare our model results to the extensional history of the gulf of california where sediments may have caused the northern and north central domain of the gulf delfin tiburon and guaymas segments to transition to narrow rifting before the south central domains alarcon segment

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

orogeny and orography the effects of erosion on the structure of mountain belts a numerical model of the coupled processes of tectonic deformation and surface erosion in convergent orogens is developed to investigate the nature of the interaction between these processes crustal deformation is calculated by a two dimensional finite element model of deformation in response to subduction and accretion of continental crust erosion operates on the uplifted surface of this model through fluvial incision which is taken to be proportional to stream power the relative importance of the tectonic and erosion processes is given by a dimensionless erosion number relating convergence velocity rock erodibility and precipitation rate this number determines the time required for a system to reach steady state and the final topographic shape and size of a mountain belt fundamental characteristics of the model orogens include asymmetric topography with shallower slopes facing the subducting plate and an asymmetric pattern of exhumation with the deepest levels of exhumation opposite to subduction these characteristics are modified when the regional climate exhibits a dominant wind direction and orographically enhanced precipitation on one side of the mountain belt the two possible cases are dominant wind in the direction of motion of the subducting plate and dominant wind direction in the opposite direction of the subducting plate velocity models of the former case predict a broad zone of exhumation with maximum exhumation in the orogen interior models of the latter case predict a focused zone of exhumation at the margin of the orogen and at high erosion number a reversal in the topographic asymmetry natural examples of these two cases are presented the southern alps of new zealand exhibits the climate and exhumation asymmetry characteristic of wind in the direction opposite to motion of the subducting plate the asymmetry of topography suggests that erosion is not efficient enough to have reversed the topographic asymmetry the contrasting example of dominant wind in the direction of subduction motion is provided by the olympic mountains of washington state in this case exhumation of deep levels of the cascadia accretionary wedge shows a broad domal pattern consistent with the observed orographic precipitation

fe oxides in jasperoids from two gold districts in nevada characterization geochemistry and u th he dating abstract this study examines the whole rock geochemistry fe oxide texture and crystal morphology and fe oxide u th he date variation of jasperoids as well as the relationships between these characteristics in order to evaluate jasperoids as an indicator for gold mineralization jasperoid samples with a range of textures appearances and expected gold concentrations were collected from two gold districts in nevada the northern carlin trend gold quarry mine and the battle mountain district marigold mine battle mountain prospect in our jasperoid samples whole rock concentrations of au are positively correlated with as ag pb sb u w and tl the following fe oxide textures are associated with anomalous gold concentrations disseminated submicron crystals feathery and acicular crystal morphologies concentric zonation from hematite to goethite and botryoidal textures calculated closure temperatures for the u th he system in observed jasperoid fe oxide morphologies within our dataset range from 26 to 136 c indicating variable susceptibility to he diffusion at near surface conditions fe oxide u th he dates range from 29 4 0 5 ma to 0 11 0 01 ma displaying intra and intersample variation we attribute the date variation to multiple factors including he implantation parent isotope exchange and he loss through closure temperature sensitivity and alpha ejection the intra and intersample variation within the fe oxide u th he dates limits our ability to make geologic interpretations our case study identifies important limitations of the u th he method in jasperoids and provides insight on how he date variation might be avoided in future studies

porphyry epithermal transition maricunga belt northern chile the refugio aldebaran and la pepa districts in the maricunga belt of northern chile contain advanced argillic alteration zones that locally host high sulfidation epithermal gold deposits in proximity to porphyry gold copper deposits the spatial association suggests a genetic link mineralized zones are characterized by four main vein types that formed at different times and have specific zonal relationships a veinlets are the earliest and deepest vein type they are restricted to potassic alteration zones in intrusive rocks a veinlets contain variable amounts of quartz magnetite biotite and chalcopyrite and locally have k feldspar halos they have nonmatching irregular vein walls and lack internal symmetry hypersaline liquid rich inclusions coexisting with vapor rich inclusions in a veinlets indicate temperatures as high as nearly 700 c and pressures between 200 and 400 bars assuming a lithostatic load depths of 0 8 to 1 6 km are inferred zones of abundant a veinlets contain mostly 1 ppm gold and 0 1 to 0 4 percent hypogene copper banded quartz veinlets occur mostly above a veinlets and cut a veinlets where they overlap dark gray bands the color resulting from a high density of vapor rich fluid inclusions and micron sized grains of magnetite commonly occur as symmetric pairs near the vein walls vein walls are parallel and slightly wavy vuggy vein centers are common and alteration envelopes are absent data from rare liquid rich inclusions in banded quartz veinlets indicate temperatures 350 c at pressures between 20 and 150 bars assuming a hydrostatic load depths of 0 2 to 1 5 km are inferred zones of abundant banded quartz veinlets generally contain 0 5 to 2 ppm gold and 0 1 percent hypogene copper d veins are pyrite veins with quartz sericite pyrite halos they are widespread and crosscut a veinlets and banded quartz veinlets the brittle nature of d veins and limited fluid inclusion data suggest temperatures 400 c d veins serve as important time lines they are nowhere truncated or crosscut by intrusions a veinlets or banded quartz veinlets quartz alunite replacement veins referred to as ledges in this paper are typical of the high sulfidation epithermal environment they are mostly limited to overlying volcanic rocks they contain local core zones of vuggy residual quartz that can contain enargite or at higher elevations barite of the three districts studied only la pepa has mineable quartz alunite ledges which contain an average gold grade of about 20 ppm a spectrum of porphyry style deposits exists cerro casale at aldebaran shares many characteristics of porphyry copper deposits worldwide whereas verde at refugio is a true porphyry gold deposit potassic alteration zones and a veinlets are strongly developed at cerro casale whereas they are absent at verde banded quartz veinlets predominate at verde whereas they occur only at the upper levels of cerro casale the pancho deposit at refugio and the cavancha deposit at la pepa are telescoped systems in which banded quartz veinlets overprint potassic alteration zones and a veinlets a veinlets and banded quartz veinlets cut and are cut by intrusions indicating multiple cycles of intrusion potassic alteration a veinlets banded quartz veinlets during formation of porphyry style mineralization banded quartz veinlets are thought to have formed by flashing of magmatic fluids during episodic transitions from lithostatic to hydrostatic pressure loss of sulfur to the vapor phase during flashing inhibited formation of copper sulfides in banded quartz veinlets and therefore resulted in high gold copper ratios where rising magmatic vapors condensed into overlying meteoric water along faults barren quartz alunite ledges formed this conclusion is supported by equivalent 40ar 39ar dates on hydrothermal biotite associated with porphyry style ore and alunite from barren ledges at aldebaran 40ar 39ar dates at la pepa indicate alunite formed at least 140 000 years to as long as 900 000 years after hydrothermal biotite within the high sulfidation epithermal environment the development of ore depends on the ability of late moderate salinity magmatic fluids to reach the surface without condensing a brine upon ascent cooling and boiling of the moderate salinity fluid below its critical temperature results in the formation of sericite at depth and alunite near the surface that is essentially synchronous with high sulfidation ore formation the timing of the switch from lithostatic pressures to brittle hydrostatic conditions relative to the life of the hydrothermal system might determine how much porphyry style ore forms relative to high sulfidation epithermal ore

is spreading prolonged episodic or incipient in the andaman sea evidence from deepwater sedimentation abstract the central andaman basin cab is generally accepted to be a site of continuous sea floor spreading since the early pliocene 4 0 ma the adjacent alcock and sewell rises and part of the east andaman basin have been interpreted as probable miocene oceanic crust published seismic lines across the eastern half of the spreading centre show that 100 s m thickness of sediment are present right up to the central trough the central trough margins are faulted uplifted and tilted away from the central trough the youngest sediment is ponded and onlaps the tilted central trough margin while older faulted sediment lies within the trough such a configuration is incompatible with continuous spreading instead either spreading in the central basin was episodic probably comprising a late miocene early pliocene phase of spreading followed by extension accommodated in the alcock and sewell rise area by faulting and dike intrusion and then a recent quaternary return to spreading in the central trough or the central trough marks an incipient spreading centre in hyper thinned continental or possibly island arc crust to resolve these possibilities regional satellite gravity data was inverted to determine crustal type and thickness the results indicate the cab is oceanic crust however the adjacent regions of the alcock and sewell rises and the east andaman basin are extended continental crust these regions were able to undergo extension before seafloor spreading and when seafloor spreading ceased unpublished seismic reflection data across the east andaman basin supports the presence of continental crust under the basin that thins drastically westwards towards the spreading centre episodic seafloor spreading fits with gps data onshore that indicate the differential motion of india with respect to se asia is accommodated on widely distributed structures that lie between the trench and the sagaing fault

factors controlling normal fault offset in an ideal brittle layer we study the physical processes controlling the development and evolution of normal faults by analyzing numerical experiments of extension of an ideal two dimensional elastic plastic brittle layer floating on an inviscid fluid the yield stress of the layer is the sum of the layer cohesion and its frictional stress faults are initiated by a small plastic flaw in the layer we get finite fault offset when we make fault cohesion decrease with strain even in this highly idealized system we vary six physical parameters the initial cohesion of the layer the thickness of the layer the rate of cohesion reduction with plastic strain the friction coefficient the flaw size and the fault width we obtain two main types of faulting behavior 1 multiple major faults with small offset and 2 single major fault that can develop very large offset we show that only two parameters control these different types of faulting patterns 1 the brittle layer thickness for a given cohesion and 2 the rate of cohesion reduction with strain for a large brittle layer thickness 22 km with 44 mpa of cohesion extension always leads to multiple faults distributed over the width of the layer for a smaller brittle layer thickness the fault pattern is dependent on the rate of fault weakening a very slow rate of weakening leads to a very large offset fault and a fast rate of weakening leads to an asymmetric graben and eventually to a very large offset fault when the offset is very large the model produces major features of the pattern of topography and faulting seen in some metamorphic core complexes

postmagmatic magnetite apatite assemblage in mafic intrusions a case study of dolerite at olympic dam south australia an assemblage of magnetite and apatite is common worldwide in different ore deposit types including disparate members of the iron oxide copper gold iocg clan the kiruna type iron oxide apatite deposits a subtype of the iocg family are recognized as economic targets as well a wide range of competing genetic models exists for magnetite apatite deposits including magmatic magmatic hydrothermal hydrothermal metasomatic and sedimentary exhalative the sources and mechanisms of transport and deposition of fe and p remain highly debatable this study reports petrographic and geochemical features of the magnetite apatite rich vein assemblages in the dolerite dykes of the gairdner dyke swarm 0 82 ga that intruded the roxby downs granite 0 59 ga the host of the supergiant olympic dam iocg deposit these symmetrical only few mm narrow veins are prevalent in such dykes and comprise besides usually colloform magnetite and prismatic apatite also further minerals e g calcite quartz the genetic relationships between the veins and host dolerite are implied based on alteration in the immediate vicinity 4 mm of the veins in particular ti magnetite ilmenite is partially to completely transformed to titanite and magmatic apatite disappears we conclude that the mafic dykes were a local source of fe and p re concentrated in the magnetite apatite veins uranium pb ages for vein apatite and titanite associated with the vein in this case study suggest that alteration of the dolerite and healing of the fractures occurred shortly after dyke emplacement we propose that in this particular case the origin of the magnetite apatite assemblage is clearly related to hydrothermal alteration of the host mafic magmatic rocks

nanogeoscience in ore systems research principles methods and applications introduction and preface to the special issue martin reich robert m hough artur deditius satoshi utsunomiya cristiana l ciobanu and nigel j cook

epithermal au ag and related deposits of the hauraki goldfield coromandel volcanic zone new zealand epithermal au ag vein deposits of the hauraki goldfield in the coromandel volcanic zone new zealand produced 320 000 kg au and 1 5 million kg ag between 1862 and 2006 from a total land area of approximately 2 900 km2 approximately 69 percent of the production was from the world class martha deposit at waihi some 97 percent of total past gold production was from deposits hosted in miocene andesite and dacite of the coromandel group although this unit represents only 61 percent of the rocks exposed in the goldfield the deposits consist of quartz calcite vein systems 0 5 to 4 5 km long within alteration zones up to 20 km2 in size they were mined over vertical intervals of 170 to 330 m but up to 575 m at martha and 700 m at karangahake electrum and acanthite are the main ore minerals although pyrite sphalerite galena chalcopyrite sulfosalt minerals and te and se bearing minerals are present in some deposits stibnite and cinnabar occur locally as late minerals and some deposits are overprinted by late massive calcite fluid inclusion ice melting temperature tm measurements indicate that fluids associated with base metal and precious metal deposition had apparent salinities of between 0 5 and 2 wt percent nacl equiv with a maximum of 6 1 wt percent nacl equiv when fitted to hydrostatic boiling point with depth curves the homogenization temperature th data define paleodepths spanning a vertical interval from near the water table to a depth of approximately 1 300 m the presence of vapor rich fluid inclusions and occurrences of lattice textures and vein adularia indicate that periods of boiling coincided with deposition of some epithermal ores sulfur and carbon isotope values of vein minerals are consistent with a magmatic origin for sulfur and carbon oxygen isotope values of vein quartz and calcite are consistent with derivation from evolved meteoric water with possibly a small magmatic water component in some samples the epithermal deposits can be broadly subdivided into northern southern and eastern groups based on their host rocks age variability of vein strike au ag ratios occurrence or relative abundance of key minerals such as adularia calcite and arsenopyrite the grain size of vein quartz and differences in vein textures early ca 14 10 ma steeply dipping planar veins occur in the oldest andesite and dacite in the northern portion of the goldfield but some of these are bonanza grade veins that contained significant gold resources larger veins typically with well developed crustiform textures formed in andesite and dacite in the southern gold field at 7 to 6 ma and these contributed most of the au and ag that has been recovered from the goldfield in contrast rhyolite in the eastern goldfield hosts smaller less well developed veins plus stockworks that have produced the least amount of au and ag epithermal mineral deposits in the hauraki goldfield evolved through time reflecting changes in volcanic activity and more speculatively tectonic and structural history and setting

structural evolution of the golden cross epithermal au ag deposit new zealand golden cross located in the hauraki goldfield of new zealand is a classic example of a fault fracture hosted epithermal vein system gold silver mineralization was mined in two areas the open pit stock work veins and the underground empire vein system which are collectively referred to as the empire zone open pit stockwork veins are either parallel or subperpendicular to northeast striking moderately southeast dipping bedding bedding perpendicular veins are mostly pure extensional structures whereas bedding parallel veins open in an extensional regime with a minor component of shear vein frequency across strike is typically 15 m and most veins are 0 01 to 0 2 m thick the underground empire vein system consists of the steeply west dipping empire vein and a complex array of footwall veins the 2 to 10 m thick empire vein strikes northeast dips steeply 65 85 northwest has a mineralized strike length of about 500 m and a vertical extent of 250 m it is localized in a deflection of the empire fault where the dip of the fault decreases the footwall veins are low angle gently west dipping veins up to 100 m long hosted in extensional shear and pure extension fractures they splay off the empire vein at an average angle of 30 up to seven vein types record the multiple fault fracture openings that provided highly permeable pathways for hydrothermal fluids in the empire zone the empire zone was initially characterized by a pervasive network of thin barren sulfide veins followed by two to five stages of electrum bearing quartz veins the most voluminous of these stages were quartz adularia veins with well developed colloform and crustiform bands and breccia textures the final stage was massive coarsely crystalline calcite veins similarities in vein type orientation and crosscutting relationships between the empire vein system and stockwork veins suggest that they formed under similar stress regimes and that mineralization in the two areas overlapped in time at least in part host rocks at golden cross dip 50 to the southeast if the vein system and bedding are rotated so that bedding is restored to horizontal the empire fault and empire vein reflect a steeply southeast dipping normal fault after rotation the former footwall veins form a network of subvertical veins in the hanging wall of the empire fault they form an en echelonlike array similar to horse tail extensional fracture networks that commonly develop in normal faults in the near surface therefore we infer that before tilting empire zone fractures and veins formed in an extensional regime with 1 subvertical 2 subhorizontal oriented northeast south west and 3 subhorizontal oriented northwest southeast consistent with the interpreted rift tectonics of the coromandel volcanic zone during the late miocene time of mineralization

biogenicity of gold and silver bearing siliceous sinters forming in hot 75 c anaerobic spring waters of champagne pool waiotapu north island new zealand champagne pool a large hot spring at waiotapu in north island new zealand is rimmed by a subaerial sinter dam and a shallow subaqueous shelf that is composed of orange sinter rich in metallic sulphides orange siliceous flocs also rich in sulphides are in constant circulation in the spring pool and form loose sediment on the shelf the orange sinters and flocs are rich in as sb tl and hg and have high concentrations of au 100 ppm and ag 330 ppm most metallic sulphides are amorphous and disseminated throughout the sinter instead of forming distinct mineral phases the shelf sinters are domal and resemble stromatolites the neutral chloride waters ph 5 5 temperature 75 c however are virtually anaerobic examination of the sinters by scanning electron microscopy confirms that they are laminated and contain an abundant low diversity assemblage of filamentous bacilliform and coccoid microbes the flocs are similarly composed of very small silicified filaments silicification involved replacement of the cell walls and extensive encrustation by opal a based on size and morphology these microbes are probably anaerobic bacteria or archaea by providing substrates for nucleation of the silica the microbes are indirectly contributing to the formation of the gold bearing sinters

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

magmatic and tectonic extension at mid ocean ridges 1 controls on fault characteristics we use 2 d numerical models to explore the thermal and mechanical effects of magma intrusion on fault initiation and growth at slow and intermediate spreading ridges magma intrusion is simulated by widening a vertical column of model elements located within the lithosphere at a rate equal to a fraction m of the total spreading rate i e m 1 for fully magmatic spreading heat is added in proportion to the rate of intrusion to simulate the thermal effects of magma crystallization and the injection of hot magma into the crust we examine a range of intrusion rates and axial thermal structures by varying m spreading rate and the efficiency of crustal cooling by conduction and hydrothermal circulation fault development proceeds in a sequential manner with deformation focused on a single active normal fault whose location alternates between the two sides of the ridge axis fault spacing and heave are primarily sensitive to m and secondarily sensitive to axial lithosphere thickness and the rate that the lithosphere thickens with distance from the axis contrary to what is often cited in the literature but consistent with prior results of mechanical modeling we find that thicker axial lithosphere tends to reduce fault spacing and heave in addition fault spacing and heave are predicted to increase with decreasing rates of off axis lithospheric thickening the combination of low m particularly when m approaches 0 5 as well as a reduced rate of off axis lithospheric thickening produces long lived large offset faults similar to oceanic core complexes such long lived faults produce a highly asymmetric axial thermal structure with thinner lithosphere on the side with the active fault this across axis variation in thermal structure may tend to stabilize the active fault for longer periods of time and could concentrate hydrothermal circulation in the footwall of oceanic core complexes

segmentation and morphology of the central indian ridge between 3 s and 11 s indian ocean abstract the segmentation pattern of 750 km long central indian ridge cir between 3 s and 11 s latitudes in the indian ocean has been studied using multibeam bathymetry and magnetic data twelve ridge segments were identified that are separated by well defined transform faults and non transform discontinuities magnetic model studies qualify the ridge as a slow spreading ridge with average full spreading rates varying from 26 to 38 mm yr the disposition of the magnetic anomalies suggests that the plate opening direction has not changed during the last 0 4 ma period along axis variations in the magnetic anomalies presence of axial volcanic ridges on the inner valley floor variations in the depth and geometry of the rift valley suggest distinct variati ons in the accretionary processes along the ridge based on these characteristics and the segmentation pattern we suggest that ten ridge segments are undergoing less magmatic phase of extension while two segments have shown characteristics of magmatic accretion the linear segments with narrow and shallow rift valley floor and near symmetric magnetic anomalies are identified as segments with magmatic accretion the influence of diffuse plate boundary zone on the young seafloor fabric generated by the cir has been examined the seafloor topography different from the normal ridge parallel fabric observed at few places over the ne flank of the cir is suggested to be the consequence of gradual and progressive influence of the distributed diffuse plate boundary between the indian capricorn plates on the newly generated oceanic lithosphere further we documented distinct ridge transform intersection rti highs three of these rti highs are identified as oceanic core complexes megamullion structures megamullion structures are found to be associated with less magmatic sections increased seismicity has been observed at the less magmatic segment ends suggesting the predominance of tectonic extension prevalent at the sparsely magmatic sections

geochemistry of metals and metalloids in siliceous sinter deposits implications for elemental partitioning into silica phases abstract sinter deposits are formed by precipitation of silica from hydrothermal fluids that have reached the surface environment they are commonly found around hot springs and represent surface expressions of underlying geothermal systems and or low sulfidation epithermal gold silver hydrothermal deposits several studies have reported ppm to weight percent concentrations of metals e g au ag cu and metalloids e g as sb b in sinters capping geothermal systems and epithermal gold silver deposits however the relation between the maturity of the siliceous sinter and its metal enrichment remains unknown here we use geochemical and mineralogical data that link the silica crystallinity degree with trace metal and metalloid contents in sinter in this paper we provide in situ trace element data in metal rich silica sinter samples from the puchuldiza geothermal field in the altiplano of northern chile that record the complete diagenetic sequence from non crystalline opal a to microcrystalline quartz combined sem xrd and la icp ms data show that the concentration of metals and metalloids in sinters from puchuldiza display a strong correspondence with silica crystallinity while arsenic and boron are predominantly enriched in the more amorphous silica phases opal a ct gold and silver show higher concentrations in the more crystalline phases opal c quartz silica structural morphological and geochemical transformations from its initial precipitation to its final maturation after diagenesis are responsible for this differential enrichment during the initial stages gold and silver are incorporated into silica microspheres as cationic species and or metal nanoparticles or colloids while arsenic and boron incorporation is controlled by as bearing accessory minerals and fe oxyhydroxides as diagenesis progresses and the crystallinity of silica increases diffusion driven processes such as ostwald ripening may progressively enrich gold and silver in the sinter while metalloids are depleted owing to the low retention of arsenic by silica these findings indicate that the diagenetic transitions of silica defined by significant structural changes that involve generation of surface defects and the creation of reactive sites may play an important role in elemental uptake by silica in near surface environments

new isotopic evidence bearing on bonanza au ag epithermal ore forming processes new cu s and pb isotope data provide evidence for a magmatic source of metal loid s and sulfur in epithermal au ag deposits even though their ore forming solutions are composed primarily of heated meteoric ground waters the apparent isotopic discrepancy between ore metals and ore forming solutions and even between the ore and associated gangue minerals indicates two different sources of epithermal ore forming constituents 1 a shallow geothermal system that not only provides the bulk of water for the ore forming solutions but also major chemical constituents leached from host rocks silica aluminum potassium sodium calcium to make gangue minerals and 2 metals and metalloids as te sb etc and sulfur se derived from deeper magma bodies isotopic data are consistent with either vapor phase transport of metal loids and sulfur and their subsequent absorption by shallow geothermal waters or formation of metallic au ag cu phases nanoparticles at depth from magmatic fluids prior to encountering the geothermal system the latter is most consistent with ore textures that indicate physical transport and aggregation of nanoparticles were significant ore forming processes the recognition that epithermal au ag ores form in tectonic settings that produce magmas capable of releasing metal rich fluids necessary to form these deposits can refine exploration strategies that previously often have focused on locating fossil geothermal systems

genesis of the jianzhupo sb pb zn ag deposit and formation of an ore shoot in the wuxu ore field guangxi south china abstract the jianzhupo sb pb zn ag deposit in the southeastern dachang sn polymetallic ore belt guangxi region south china is the largest deposit in the wuxu district and is hosted by devonian limestone and mudstone the deposit mainly comprises a stage i jamesonite sphalerite ore shoot and stage ii jamesonite sphalerite calcite quartz jscq veins we use homogenization temperatures salinities and the h o isotopic compositions of fluid inclusions from the jianzhupo deposit to constrain the genesis of the deposit and determine factors that controlled the formation of the ore shoot fluid inclusions from both stages of mineralization contain large volumes of co2 and minor bitumen c6h6 and ch4 fluid inclusions from the ore shoot yield homogenization temperatures of 120 c 290 c mainly 250 c 270 c and salinities of 3 4 13 6 wt nacl equiv fluid inclusions from the stage ii mineralization yield homogenization temperatures of 150 c 350 c mainly 240 c 310 c and salinities of 1 4 12 0 wt nacl equiv ore forming fluids of the jianzhupo deposit yield 18oh2o and dv smow values of 4 9 9 9 and 78 to 111 respectively and plot close to the composition of magmatic water on the dv smow vs 18oh2o diagram co2 in the ore forming fluids was likely generated through deep skarnization beneath the wuxu ore field rather than from metamorphism thermal degradation or the oxidization of organic matter h o isotopic compositions of ore forming fluids that contain large volumes of co2 with minor bitumen and c6h6 indicate that they were derived from magmatic water that was mixed with basin brines ore forming fluids of the stage i ore shoot yield lower 18oh2o values 4 9 6 7 than those associated with the stage ii mineralization 7 5 9 9 moreover ore forming fluids of the ore shoot and the jscq veins yield strongly and weakly positive correlations between homogenization temperature and salinity respectively this difference suggests that rapid and large scale mixing between magmatic water and basin brines was the key factor that controlled the formation of the ore shoot

modes of extensional faulting controlled by surface processes we investigate the feedbacks between surface processes and tectonics in an extensional setting by coupling a 2 d geodynamical model with a landscape evolution law focusing on the evolution of a single normal fault we show that surface processes significantly enhance the amount of horizontal extension a fault can accommodate before being abandoned in favor of a new fault in simulations with very slow erosion rates a 15 km thick brittle layer extends via a succession of crosscutting short lived faults heave 10 km using simple scaling arguments we quantify the effect of surface mass removal on the force balance acting on a growing normal fault this leads us to propose that the major range bounding normal faults observed in many continental rifts owe their large offsets to erosional and depositional processes

comparison of seafloor tectonic fabric at intermediate fast and super fast spreading ridges influence of spreading rate plate motions and ridge segmentation on fault patterns we have conducted a comparative study of the tectonic morphology of young seafloor using seamarc ii side scan sonar surveys of the intermediate spreading ecuador rift the fast spreading east pacific rise epr 8 30 10 n and the super fast spreading epr 18 19 s we find that characteristics of fault populations are not only a function of spreading rate but also vary along axis within individual ridge segments i e with proximity to large and short offset discontinuities we also find that fault azimuths can be used to examine plate kinematics on a finer scale than can be obtained using magnetic data alone most of the variation in fault populations with spreading rate can be explained by an inverse relationship between spreading rate and thickness of the brittle layer for example regions of super fast spreading are characterized by the largest numbers of short faults the smallest average fault spacing and throw and the highest fault density in addition clusters of short closely spaced antithetic faults subsidiary to long master inward dipping faults are common within the super fast spreading area presumably the result of a thinner weaker brittle layer faults facing away from the ridge axis occur in increasing numbers with increasing spreading rate such that few outward facing faults are found at slow to intermediate rates and approximately equal numbers of inward and outward facing faults are observed at the fastest rates rapid thickening of the brittle layer with distance from the ridge may account for the predominance of inward facing faults at slower spreading rates outward facing faults at all spreading rates have shorter mean lengths and lower vertical offsets these differences may reflect the shorter time outward facing faults are active owing to increasing strength of the lithosphere with distance from the ridge fault lengths and spacings in all areas approximate exponential distributions the extensional strain represented by fault populations is calculated from the displacement and length distributions of faults and strain estimates of 4 are obtained for each area assuming that fault spacing reflects fracture depth extent where faults initiate we infer a brittle layer thickness of 1 km when faulting begins fault populations are examined for ridge segment scale variations in amagmatic extension we see evidence for greater amagmatic extension associated with long term reduced magma supply along the eastern third of the ecuador rift evidence for local increased brittle extension is also found within 15 km of transform faults discordant zones left by overlapping spreading centers oscs are characterized by low fault abundances at oscs discrete events of ridge tip propagation may accommodate extension taken up elsewhere along the ridge by normal faulting fault azimuths do appear to be useful indicators of plate motion within the epr 8 30 10 n area fault trends record a recent change in pacific cocos plate motion 3 6 at 1 m y consistent with magnetic anomaly and fault lineation data from elsewhere along the northern epr within the ecuador rift fault azimuths scatter within 3 of predicted trends and are consistent with constant spreading about one pole for the past 1 5 m y

colloidal gold in sulphur and citrate bearing hydrothermal fluids an experimental study abstract the stability of colloidal gold gold nanoparticles in sulphur bearing and citrate bearing solutions was investigated at temperatures up to 225 c using synchrotron x ray near edge spectroscopy xanes and up to 350 c with a visual check of colour changes the citrate based colloidal gold solutions with or without colloidal silica in the solution are only stable up to 225 c in contrast the gold colloids in na2s solutions are not stable upon heating to 150 c but stable up to 300 c when 0 5 1 5 wt of colloidal silica is present in the solution the colloidal gold particles started to aggregate and deposit from the solution with the aggregation and growth of silica particles at 350 c the concentrations of gold as colloids in the solutions are up to 0 5 mmol 95 ppm more than three orders of magnitude higher than gold solubility as aqueous complexes under the same condition calculated based on available thermodynamic data these results provide the first evidence that high concentrations of colloidal gold are stable in sulphur bearing fluids at elevated temperatures 300 c this implies that the formation of gold colloids is an effective way to concentrate gold in hydrothermal sulphur bearing fluids to form high grade gold ores

early yellowstone hotspot magmatism and gold metallogeny abstract high grade epithermal gold deposits in the northern great basin have long been associated with regional miocene basaltic to rhyolitic volcanism previous models for the low sulfidation epithermal gold ores in this region have generally portrayed the bimodal magmas as a source of heat to drive large scale convection of meteoritic water that leached gold from crustal sources and deposited it in hydrothermal vein systems or required that the gold evolve from fractionated silicic magmas new data of the present study indicate a more direct genetic link to the plume related basaltic magmas of the region laser 40ar 39ar incremental heating plateau ages for single crystals of adularia from several of these low sulfidation epithermal gold deposits range from 16 6 ma to 15 5 ma adularia from the jumbo deposit yields three concordant plateau ages with a combined statistical result of 16 54 0 04 ma 95 confidence level mswd 0 23 plateau ages for adularia from other deposits in the region and from gold bearing veins in the owyhee mountains of southwestern idaho yield similar ages up to 16 5 ma however some veins are as young as ca 15 5 ma and the grain to grain ages for a given sample can vary by up to ca 0 5 ma observed variations in age among the adularia crystals of a given rock sample indicate varying amounts of extraneous argon and also loss of radiogenic 40ar among the population of grains for a particular sample the single crystal results are interpreted to indicate a 16 5 15 5 ma interval for formation of gold bearing adularia veins in the region the initiation and duration of this gold forming event appears contemporaneous within uncertainties with the basaltic volcanism at the steens mountain section and an ensuing one million year episode of basaltic volcanism from multiple centers in the region brueseke et al 2007 trace amounts of lead are alloyed with gold in the deposits studied the isotopic compositions of this lead are not compatible with regional crustal units that host the gold ores or the silicic igneous lithologies of the region but have the same lead isotopic composition as basalts of the earliest yellowstone plume represented by the earliest lavas of the columbia river basalt province the steens basalts and stonyford volcanic complex hanan et al 2008 we propose that the gold studied and its traces of alloyed lead were derived together from the mantle released from basaltic magma chambers of the province and carried by low density fluids into shallow geothermal systems during the earliest stages of yellowstone hotspot magmatism

quantitative fault studies on the east pacific rise a comparison of sonar imaging techniques a synthesis is presented of published statistics on the lengths scarp heights and spacing of normal faults on the flanks of the east pacific rise epr between 19 s and 21 n nine data sets are included which were collected at different times employing one of four different sonar mapping systems the scripps deep tow instrument seamarc i seamarc ii and gloria with widely ranging data collection capabilities the epr in this region is morphologically quite similar and in some cases the data sets were obtained from the same area of the seafloor it is assumed therefore that data collection procedure and system resolution have had a greater influence on the fault statistics observed than varying tectonic processes we try to account for these sampling effects such that quantitative comparisons of fault development at midocean ridges may then be made we discuss the problem of identifying faults in sonar images and the errors associated with measurements of the relevant parameters we also investigate the statistical properties of the fault populations and derive expressions for correcting observed values of the population means at finite resolution to obtain the corresponding values at infinite resolution the theoretical predictions compare well with the results of a decimation procedure applied to real data sets to simulate diminishing resolution we also discuss the effects that surveying geometry can have on the interpretation of fault lengths the data synthesis is augmented by an analysis of the correlation between scarp height and fault length for areas near 3 5 s and 12 n on the epr the correlation which we find to be linear is used to estimate 1 the minimum operational fault target size for each of the four mapping systems and 2 the topography due to faulting in an area where only fault lengths are known

clay minerals of late pleistocene sites jonzac and les cottes sw france applications of x ray diffraction analyses to local paleoclimatic and paleoenvironmental reconstructions abstract the crystal chemical properties of clay minerals are closely dependent on the physicochemical conditions prevailing at the time when they formed in soils and weathered zones the degree of alteration of primary minerals and the composition of the secondary products are clearly linked to climate and plant cover in particular changes are rapid under temperate conditions which favor the development of thick soils but slow in cold contexts favoring thin and fragile soils in this paper the mineral composition of two pleistocene clay rich deposits is investigated in order to contribute to reconstructing paleoenvironmental changes jonzac charentes sw france and les cottes vienne sw france are two rock shelters with long archaeological sequences which include faunal remains and were recently radiometrically dated the decomposition of xrd patterns at each site allows identification of the principal mineral phases and semi quantitative estimation of their relative proportions the variation of these proportions is analyzed through indexes based on the relative intensities of the decomposed bands the clay fraction of both sites mostly results from the transformation of the minerals forming the sediments in the surrounding area of the rock shelter terrace including contributions from the hillside mineralogical clay data are compared with the faunal spectrum identified at both sites in order to discuss paleoenvironmental implications at jonzac during the quina mousterian period dated to mis4 the environment was cold and dry the dominant mineralogical phases are inherited from the rocks weak pedogenesis this is consistent with the faunal analysis data typical of the mis4 during the mousterian of acheulean tradition period dated to mis3 the mineralogical composition of the clay deposits changed significantly suggesting a less wet environment the aurignacian deposits dated to the end of mis3 early mis2 are rich in mixed layer minerals mlms which are produced by the hydrolysis processes in soils the climatic conditions probably became milder however because no faunal remains were found this evolution is uncertain the les cottes deposits dated between mis3 and early mis2 are mineralogically typical of a cold environment where soil reactions are not very active however some weak variations of the indexes suggest temporary temperate conditions during the les cottes interstadial and in the aurignacian the parallelism between mineral signature and paleontological evidence in recording relatively rapid variations between contrasted climates suggests that clay mineral assemblages from rock shelter deposits can be used to assess paleoclimatic and paleoenvironmental dynamics at a local scale

tectonics and history of the andaman sea region abstract the andaman sea is an active backarc basin lying above and behind the sunda subduction zone where convergence between the overriding southeast asian plate and the subducting australian plate is highly oblique the effect of the oblique convergence has been formation of a sliver plate between the subduction zone and a complex right lateral fault system the late paleocene collision of greater india and asia with approximately normal convergence started clockwise rotation and bending of the northern and western sunda arc the initial sliver fault which probably started in the eocene extended through the outer arc ridge offshore from sumatra through the present region of the andaman sea into the sagaing fault with more oblique convergence due to the rotation the rate of strike slip motion increased and a series of extensional basins opened obliquely by the combination of backarc extension and the strike slip motion these basins in sequence are the mergui basin starting at 32 ma the conjoined alcock and sewell rises starting at 23 ma east basin separating the rises from the foot of the continental slope starting at 15 ma and finally at 4 ma the present plate edge was formed alcock and sewell rises were separated by formation of the central andaman basin and the faulting moved onshore from the mentawai fault to the sumatra fault system bisecting sumatra

whole rock geochemical techniques for evaluating hydrothermal alteration mass changes and compositional gradients associated with epithermal au ag mineralization volcanic rocks hosting quartz calcite adularia illite epithermal deposits undergo elemental mass changes associated with k metasomatism k h metasomatism and h metasomatism that are developed progressively upward and outward from the site of mineralization as hydrothermal fluid ascends boils and cools resultant hydrothermal mineral zonation shows increasingly k rich mineral assemblages with proximity to deposits thus patterns of k enrichment provide a vector toward those deposits we describe whole rock geochemical techniques for identifying these patterns and for evaluating attendant hydrothermal mineral zonation whole rock geochemical anomalies are evaluated by calculating mass changes associated with hydrothermal alteration using a modified version of gresens 1967 equation to compare the composition of altered rocks to fresh rock equivalents hydrothermally altered rocks most affected by k metasomatism will be characterized by the largest k gains and generally na and ca losses mass changes associated with k metasomatism are also evaluated graphically using plots of molar 2ca na k al versus molar k al since molar values are used to construct the plot compositions of altered rocks can be compared to the compositions of primary and secondary k na ca and al bearing minerals that are located in the same compositional space allowing identification of important hydrothermal minerals e g adularia illite smectite and alteration processes that are reflected in trends from fresh rock compositions toward the compositions of hydrothermal minerals the intensity of k metasomatism encompassing both k gains and na and ca losses can be represented by the slope of the line between an altered rock composition and the origin i e the molar k 2ca na k value determinations of mass changes in altered rocks surrounding selected epithermal deposits demonstrate the predominance of k metasomatism proximal to and commonly increasing in intensity toward mineralized veins comparisons of k mass changes to trace element concentrations indicate that the area affected by k metasomatism is more extensive 100s to 1 000s of meters than that containing anomalous concentrations of precious metals base metals and pathfinder elements 10s to 100s of meters therefore whole rock geochemical techniques potentially extend the area over which geochemical targeting may be effective data from this study show that intensity of k metasomatism molar k 2ca na k values and concentrations of precious metals and pathfinder elements increase toward ore and are greatest proximal to ore so that pathfinder element and whole rock geochemical anomalies are complementary

underground fumaroles excess heat effects in vein formation we examine the heat extracted from a fractured hydrothermal system following the sudden dilation of fractures in the host rock this dilation increases the permeability of the system and veins form along irregular fractures where the opening is localized for simplicity we will consider one such vein in this paper fluid in the system is assumed to be initially close to vapor saturation and in thermal equilibrium with the surrounding host rock the rapid opening of the vein leads to approximately isenthalpic vapor separation and causes a pressure drop to propagate through the system as the two phase fluid in the fracture system is constrained to the vapor pressure curve the depressurization is accompanied by a rapid cooling of the fluid fluid flow is also induced from a hydrothermal reservoir toward the dilated discharge vein the temperature difference between the host rock and the two phase fluid in the fracture system drives heat from the rock into the fluid the excess heat 1 1 derived from the host rock in this manner evaporates additional liquid in the two phase flow which develops substantially larger vapor fractions than those due only to the rapid initial adiabatic depressurization of the fluid such additional vapor separation therefore leads to high solute concentrations in the liquid phase and potentially results in faster deposition of minerals in the fracture system and local veins a model is developed here to describe the minimum excess heat transfer i e the worst case scenario into the two phase saturated liquid vapor mixture as it is drawn horizontally from the hydrothermal reservoir toward the discharge vein excess heat effects are shown to be effective within tens to hundreds of meters of actively forming veins and a principal cause of oscillatory banding and lamination fabrics this transient heat extraction process applies to a wide range of shallow crustal deposit styles associated with brittle and brittle ductile deformation these deposit styles range from epithermal bonanza veins sea floor massive sulfide deposits mesothermal veins in shear zones to the higher temperature veins and stockworks of hypothermal and porphyry mineralization 1 fn 1

silica and gold textures in bonanza ores of the sleeper deposit humboldt county nevada evidence for colloids and implications for epithermal ore forming processes

an early permian epithermal gold system in the tulasu basin in north xinjiang nw china constraints from in situ oxygen sulfur isotopes and geochronology abstract the axi and jingxi yelmand gold deposits being the largest gold deposits in the chinese north tianshan nw china are located ca l0 km apart in the tulasu basin and are hosted by the late devonian early carboniferous dahalajunshan formation in situ la icp ms titanium analyses on quartz from the axi and jingxi yelmand deposits are broadly identical accordingly the calculated ore forming temperatures by ti in quartz thermometer give average temperatures of 279 c and 294 c respectively results of in situ sims analyses of oxygen and sulfur isotopes on quartz and pyrite from these two deposits are similar temperature corrected fluids of the axi deposit have 18 o values of 2 6 8 1 and 34 s values of 0 8 2 4 whereas the fluids of the jingxi yelmand deposit have 18 o of 6 4 8 9 and 34 s of 0 4 to 4 0 the oxygen and sulfur isotopes from the two deposits indicate a magmatic origin la icp ms zircon u pb ages of aqialehe formation sandstone provided a lower limit for the mineralization timing of the axi deposit 288 ma in situ sims u pb analyses on entrapped zircon 297 ma and newly recognized 284 5 ma columnar rhyolite implies that the jingxi yelmand deposit formed in the early permian based on the magmatic affinity of the ore fluids similar age and ore formation temperatures we propose that the axi and jingxi yelmand deposits comprise an epithermal gold system which was driven by the same permian magma in the tulasu basin the ore geological features together with our new results indicate that the axi and jingxi yelmand deposits are intermediate and high sulfidation type epithermal deposits respectively

high temperature gold copper extraction with chloride flux in lava tubes of tolbachik volcano kamchatka subvolcanic environments in supra subduction zones are renowned for hosting epithermal deposits that often contain electrum and native gold including bonanza examples this study examined mineral assemblages and processes occurring in shallow crust volcanic settings using recent eruption 2012 2013 of the basaltic tolbachik volcano in the kamchatka arc the tolbachik eruptive system is characterized by an extensive system of lava tubes after cessation of magma input the tubes maintained the flow of hot oxidized gases that episodically interacted with the lava surfaces and sulphate chloride precipitates from volcanic gases on these surfaces the gas rock interaction had strong pyrometamorphic effects that resulted in the formation of molten salt oxidized tenorite hematite cu rich magnesioferrite and skarn like silicate mineral assemblages by analogy with experimental studies we propose that a combination of these processes was responsible for extraction of metals from the basaltic wall rocks and deposition of cu fe and cu fe oxides and native gold

scanning electron microscope imaging of gold electrum nanoparticles in middle miocene bonanza epithermal ores from northern nevada usa scanning electron microscope sem imaging of high grade gold ores from several middle miocene bonanza epithermal au ag deposits from northern nv shows that a significant amount of gold occurs as electrum nanoparticles and larger microparticles the latter of which apparently formed by continued growth of nanoparticle precursors the particles occur as discrete grains disseminated in gangue silicate minerals or as self organized aggregates termed fractal dendrites common nanoparticle shapes observed include spheres polyhedral crystals dodecahedra octahedra and also triangular and hexagonal plates although gold nanoparticles locally occur in other types of hydrothermal gold deposits it appears that evidence for nanoparticles is best preserved in epithermal ores due to the ubiquitous co deposition of silica nanoparticles as opal in these ore forming systems it is possible that nanoparticle formation and aggregation could be intermediary steps in the formation of larger gold crystals under disequilibrium ore forming conditions where and when the nanoparticles formed in the evolving ore forming system are not well constrained and warrants more study two possibilities include i e 1 extreme boiling flashing at shallow depth led to the in situ formation and aggregation of nanoparticles or 2 electrum nanoparticles formed from a deep magmatic fluid and were physically transported up to the epithermal setting or perhaps both processes operated in concert

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

discussion is spreading prolonged episodic or incipient in the andaman sea evidence from deepwater sedimentation by c k morley and a alvey 2015 abstract in 2005 i published a paper on the tectonics and geological history of the andaman sea in which i stated again the idea that the andaman sea is experiencing sea floor spreading in a basin i called the central andaman basin fig 1 the sea floor in the basin shows signs of what my colleagues and i had previously postulated to be either episodic spreading or episodic deposition of sediment in the first publication in which we had described this region in 1974 we had favored the idea that the spreading and not deposition had varied episodically in my 2005 paper i changed my interpretation to episodically changing rates of introduction and deposition of sediment caused by glacial vs non glacial fluctuations of sea level the recent paper by morley and alvey was mainly a discussion of my 2005 paper favoring the idea of an episodically varying rate of spreading in their analysis however they state some other interpretations and differences in opinion this short communication is mainly a rebuttal to their discussion

the fluid inclusion and mineralogic record of the transition from liquid to vapor dominated conditions in the geysers geothermal system california the geysers an active vapor dominated geothermal system related to a hypabyssal granitic pluton was liquid dominated from its inception at 1 2 to 0 3 ma when the modern vapor dominated regime formed as the hydrothermal liquids boiled off a vapor dominated heat pipe developed in which the flow of steam was balanced by downward percolation of condensate a record of the transition from liquid to vapor dominated conditions is contained within late stage quartz and calcite crystals that are common in the upper part of the steam reservoir and overlying cap rock samples from three localities were examined in detail quartz from these veins is characterized by corroded crystal faces solid inclusions of contemporaneous calcite that are generally etched coeval high and low salinity fluid inclusions primary low salinity liquid rich inclusions and abundant vapor rich inclusions primary vapor rich inclusions up to 2 mm across were trapped mainly on rhombic and prism faces of skeletal quartz crystals during periods of rapid growth these inclusions nucleate crystals of co2 that disappear by sublimation rather than melting at temperatures ranging from 92 9 to 56 7 c liquid rich fluid inclusions from veins near the top of the modern steam reservoir yield homogenization temperatures ranging from 304 to 188 c these temperatures commonly exceed boiling point to depth relationships based on the paleoground surface suggesting that pressures were initially greater than hydrostatic temperature salinity relationships indicate that the early shallow liquids had salinities of 1 5 to 2 wt percent nacl equiv and that boiling began by 300 c as the fluids boiled off salinities of the residual liquid increased to 3 to 4 wt percent nacl equiv trapping of low salinity fluids 0 0 0 5 wt nacl equiv interpreted as condensate in primary liquid rich inclusions suggest vapor dominated conditions had begun to develop by the time temperatures had dropped to 290 c the primary origin of these inclusion fluids indicates that they were capable of depositing quartz and calcite during the evolution of the vapor dominated system the hydrothermal fluids were at different times but at the same location undersaturated in equilibrium or oversaturated with respect to quartz and calcite the hydrothermal fluids initially deposited quartz and calcite as they boiled off dissolution of these minerals occurred as the condensate which was initially undersaturated with respect to quartz and calcite percolated down subsequent deposition of these minerals occurred as the condensate or mixtures of condensate and boiled liquid that leaked back into the fractures vaporized during periods of intermittent pressure reduction the top of the vapor dominated system migrated downward with time as the overlying fractures were progressively sealed off today pressures within the steam reservoir are vaporstatic and temperatures range from 240 to at least 342 c

first direct evidence for natural occurrence of colloidal silica in chalcedony hosted vacuoles and implications for ore forming processes different forms of abundant silica e g quartz and chalcedony are closely associated with many types of ore deposits in igneous metamorphic and sedimentary environments occurrence of quartz and metal bearing minerals together strongly indicates that silica is an important component in hydrothermal fluids transporting and concentrating economic metals to the ore grade however chemical and physical characteristics of such ore forming media remain debated understanding of the environment in which chalcedony forms is largely hampered by the lack of chalcedony hosted fluid inclusions our study reports for the first time fortuitously preserved large sized up to 150 m fluid inclusions in chalcedony from the gonchak deposit of optical calcite in the early triassic basalts belonging to the siberian large igneous province the application of microthermometric methods scanning electron microscopy with cathodoluminescence and laser raman spectroscopy to the fluid inclusions and their host chalcedony recognized the formation of chalcedony from a colloidal suspension the fluid inclusions represent a gel like saline aqueous fluid that is residual after precipitation of spherulitic chalcedony aggregates with numerous h2o bearing and h2o poor layers we propose that the colloidal nature of fluids forming chalcedony lends strong support to the natural existence of experimentally predicted silicothermal fluids such fluids can be instrumental in mobilizing and transporting large quantities of both silica and nano and micro particles of ore minerals followed by efficient separation of the latter from coagulating silica gel into ore rich zones and bodies

a study by electron microscopy of gold and associated minerals from round mountain nevada abstract details regarding the mineralization patterns of gold in epithermal systems are poorly understood a refined understanding of gold microtextures the interface between gold and associated minerals and the role that nanoparticles play in gold mineralization could provide insight into the details of gold growth and may contain indicators of gold ore concentration mechanisms furthermore a refined understanding of the interface also may explain variation in cyanide leaching extraction efficiency and may enable enhancement of recovery methods macrocrystalline gold samples from round mountain nevada were analyzed using field emission scanning electron microscopy and transmission electron microscopy this study suggests that gold nanoparticles are common in round mountain gold and associated mineral phases and may play an important role in the formation of macrocrystalline gold in this deposit microtopographic evidence indicates that the two dimensional nucleation and growth mechanism is dominant and nanotextural evidence suggests that nanoparticulate gold was the first stage of growth in the formation of these macrocrystalline samples that grew rapidly at high degrees of supersaturation results suggest that the interface between gold and quartz and other related minerals is far more complex than previously thought and that textures present at the interface and in the bulk gold can help explain mineralization history and can also have implications for gold recovery efficiency for macrocrystalline bearing ore

the favona epithermal gold silver deposit waihi new zealand the favona deposit is a new epithermal vein discovery 595 000 oz au 2 367 500 oz ag resource located less than 2 km east of the world class martha deposit 6 7 million oz au 42 1 million oz moz ag in the hauraki goldfield of new zealand the concealed favona vein is hosted in late miocene andesitic lava flows and occurs to the east of the nearby historically mined silverton amaranth mascotte and union veins hydrothermal alteration is characterized by a variety of alteration minerals that display distinct zonation along four subsurface cross sections 800 m long 450 m deep that transect the favona cowshed and amaranth veins hydrothermal quartz and pyrite are ubiquitous adularia is widespread and envelopes veins it locally coexists with hydrothermal albite adjacent to the amaranth vein at depth both adularia and albite are variably replaced by illite which together with broadly coextensive chlorite are generally restricted to the foot wall of the favona and cowshed veins although both minerals appear locally at depth in the hanging wall interstratified illite smectite containing 10 to 70 percent smectite and rare discrete smectite are mostly confined to the hanging wall of the favona and cowshed veins late stage kaolinite cristobalite and rare alunite locally overprint the above minerals at shallow levels whereas late stage overprinting calcite occurs at depth near the amaranth cowshed and southern portion of the favona veins aeromagnetic data show that the favona and martha deposits both fit within a broad zone of hydrothermally induced demagnetization that is approximately 3 7 3 0 km alteration is broadly similar at each deposit although albite and calcite are more abundant at martha fluid inclusion data indicate that both the favona and martha deposits formed at similar levels with veins developed over extensive vertical intervals exceeding 400 and 600 m respectively based on these data we propose that the veins comprising the favona and martha deposits formed at similar shallow levels 1 km in a single paleogeothermal system it is unclear if the martha and favona veins systems formed within a single upflow zone a migrating upflow or within two separate upflow zones regardless widespread alteration surrounding the veins includes quartz adularia albite chlorite illite plus pyrite and reflects formation from alkali chloride waters that contained moderate concentrations of aqueous co2 focused and episodic boiling of these chloride waters along extensive open conduits resulted in the formation of gold and silver bearing colloform and crustiform banded quartz veins steam heated waters formed above and on the margins of the paleogeothermal system and during the eventual thermal collapse resulted in overprinting by kaolinite cristobalite and alunite from descending steam heated acid sulfate waters and late calcite from localized steam heated co2 rich waters the discovery of the favona deposit next to the large martha deposit highlights the importance of determining the physical limits of hydrothermal alteration in a system in order to identify the limits of paleogeothermal activity and define the area that may be explored for veins the veins at favona martha and golden cross empire vein combined with a resource at karangahake collectively total more than 3 5 moz au and should encourage the exploration of known epithermal deposits throughout the hauraki goldfield for additional resources

2 hydrothermal ore deposits related to post orogenic extensional magmatism and core complex formation the rhodope massif of bulgaria and greece abstract the rhodope massif in southern bulgaria and northern greece hosts a range of pb zn ag cu mo and au ag deposits in high grade metamorphic continental sedimentary and igneous rocks following a protracted thrusting history as part of the alpine himalayan collision major late orogenic extension led to the formation of metamorphic core complexes block faulting sedimentary basin formation acid to basic magmatism and hydrothermal activity within a relatively short period of time during the early tertiary large vein and carbonate replacement pb zn deposits hosted by high grade metamorphic rocks in the central rhodopean dome e g the madan ore field are spatially associated with low angle detachment faults as well as local silicic dyke swarms and or ignimbrites ore formation is essentially synchronous with post extensional dome uplift and magmatism which has a dominant crustal magma component according to pb and sr isotope data intermediate and high sulphidation pb zn ag au deposits and minor porphyry cu mo mineralization in the eastern rhodopes are predominantly hosted by veins in shoshonitic to high k calc alkaline volcanic rocks of closely similar age base metal poor high grade gold deposits of low sulphidation character occurring in continental sedimentary rocks of synextensional basins e g ada tepe show a close spatial and temporal relation to detachment faulting prior and during metamorphic core complex formation their formation predates local magmatism but may involve fluids from deep mantle magmas the change in geochemical signatures of palaeogene magmatic rocks from predominantly silicic types in the central rhodopes to strongly fractionated shoshonitic bulgaria to calc alkaline and high k calc alkaline greece magmas in the eastern rhodopes coincides with the enrichment in cu and au relative to pb and zn of the associated ore deposits this trend also correlates with a decrease in the radiogenic pb and sr isotope components of the magmatic rocks from west to east reflecting a reduced crustal contamination of mantle magmas which in turn correlates with a decreasing crustal thickness that can be observed today hydrogen and oxygen isotopic compositions of the related hydrothermal systems show a concomitant increase of magmatic relative to meteoric fluids from the pb zn ag deposits of the central rhodopes to the magmatic rock hosted polymetallic gold deposits of the eastern rhodopes

diffusion and fluid interaction in itrongay pegmatite madagascar evidence from in situ 40ar 39ar dating of gem quality alkali feldspar and u pb dating of protogenetic apatite inclusions alkali feldspar 40ar 39ar and apatite upb geochronological studies have typically invoked two mechanisms to account for apparent loss of radiogenic 40ar and 206 208pb some studies have suggested that the radiogenic isotopes were lost by volume diffusion and used these dates to constrain temporal variations of rock temperatures others have argued that the radiogenic isotopes were lost due to interaction with fluids and related these dates to chemical alteration these two end member interpretations have fundamentally different implications for tectonic models derived from geochronological data and therefore it is important to reliably identify the principal mechanism for loss of radiogenic isotopes here we revisit the mechanisms of 40ar loss in the famous gem quality alkali feldspar from the itrongay pegmatite in madagascar previous studies have suggested that volume diffusion is the dominant mechanism of 40ar loss providing key evidence to support the use of 40ar 39ar dating of alkali feldspar for thermochronology we attempted to verify these results by obtaining time temperature paths from petrologically characterised cogenetic feldspar and apatite from the itrongay pegmatite and comparing them with each other however our results suggest that only a minor component in the variability of 40ar 39ar dates of itrongay feldspar is related to the diffusive loss of 40ar and that this loss was not compatible with the majority of previously proposed models which hinders quantitative interpretations the crystal studied here grew in five episodes related to the influx initially of co existing dense sio2 rich solution and co2 dominated fluid the first and supposedly the following two episodes and subsequently of h2o rich fluid supposedly the final two episodes much greater component in the variability of the acquired 40ar 39ar dates is interpreted to reflect the differences in the ages of these growth episodes which we estimate to span from 477 ma to 176 ma the first four episodes apatite inclusions in this crystal are interpreted to be xenocrysts derived from the country rocks of the itrongay pegmatite these yield older upb dates than the estimated age of their host feldspar and have apparently experienced diffusive loss of 206 207pb prior to entrapment our 40ar 39ar results indicate that there is a lack of unambiguous evidence for diffusive loss of 40ar from alkali feldspar that can be readily interpreted for thermochronological purposes however in situ 40ar 39ar dating of alkali feldspar appears to be a promising tool for tracking fluid flow events in the earth s crust whose applicability is not restricted to sedimentary rocks our upb results corroborate previous suggestions that upb dating of apatite can be used for thermochronology