timing of geothermal activity in an active island arc volcanic setting first 40ar 39ar dating from bouillante geothermal field guadeloupe french west indies mineral separates of adularia have been extracted from three samples of highly silicified hydrothermal breccias newly discovered in the active bouillante geothermal field guadeloupe archipelago and investigated by 40ar 39ar geochronology in order to constrain the timing of geothermal activity in this part of the active lesser antilles island arc the inverse isochron diagram indicates an age of 248 50 ka 2 for all adularia from one breccia sample n 8 with an initial 40ar 36ar ratio of atmospheric composition 309 12 2 attesting that this age is valid this age is concordant with the weighted mean age of 290 40 ka for the same sample adularia from other samples yields concordant ages the obtained 40ar 39ar ages can be related either to the magmatic activity of the bouillante volcanic chain c 850 250 ka ago or to the initiation of the volcanic activity of the active grande decouverte soufriere system 200 ka ago present day our results demonstrate that the bouillante hydrothermal event is coeval with change in the volcanic pulses previously recognized in the magmatic history of the studied area the possible duration calculated for this hydrothermal activity requires at least two superposed volcanic pulses to be developed

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

mid miocene propagation of the yellowstone mantle plume head beneath the columbia river basalt source region columbia river flood basalt volcanism is the product of mid miocene distortion of the yellowstone mantle plume head against the precambrian margin of north america the overall migration pattern of columbia river basalt vents reflects northward propagation of the plume head beneath the west southwest migrating north american plate thermal uplift mimics the vent migration over the 1 5 m y of grande ronde basalt extrusion the projected crest of the plume head migrated 105 km 7 cm yr from the chief joseph dike swarm east oortheast into the clearwater embayment where uplift occurred over the same period at a rate of 0 67 mm yr chemical variations reflect progressive melting of the plume and mantle lithosphere followed by the mixing of these components with an enriched source from the overriding craton rapid thickening of the plume head against the cratonic margin led to decompressional melting and magma rise beneath a thin oceanic lithosphere of accreted terranes

paleohydrology of buckskin mountain national district humboldt county nevada precious metal veins of the national district humboldt county nevada occur in strongly altered miocene volcanic rocks hydrothermal assemblages in buckskin mountain display a marked zonation from the paleosurface downward that correlates with the elevation and physical properties of enclosing rhyolites light stable isotope analyses indicate that meteoric water from two distinct sources comprised the ore fluid the delta d values in fluid inclusions from deep level quartz vein segments 1 250 ft 379 m below the palcosurface range from 115 to 129 per mil fluid inclusion waters from near surface quartz vein segments 0 to 1 250 ft 0 379 m below the paleosurface have delta d 86 to 125 per mil and are generally more depleted than local miocene ground water whose delta d is calculated at 85 per mil the delta d range in inclusion waters from near surface quartz veins has resulted from mixing of the fluids present in deep quartz veins with local ground water and condensed steam derived from boiling of both fluids the fluids present in deep quartz veins probably originated as precipitation on distal highlands with elevations approaching 6 000 ft 1 818 m oxygen isotope data from vein and wall rock minerals record increasing depletion as veins are approached indicating that quartz veins were the major conduits of fluid flow the degree of oxygen exchange between hydrothermal fluid and wall rocks water rock mass ratios and the abundance of hydrothermal phases increase toward the paleosurface the progressive influx of local ground water with increasing elevation is also indicated by chloride dilution the estimated duration of the buckskin mountain hydrothermal system 50 000 to 200 000 yrs is limited by the volume of high temperature alteration assemblages and the heat budget of a proposed intrusion thermal constraints and isotopic data provide an internally consistent range in lifetimes which is several times shorter than that proposed for some active hydrothermal systems these long lived modern systems apparently require multiple thermal and or intrusive events based on geologic isotopic and floral data the palcogeography of ancestral buckskin mountain was dominated by forested volcanic terrane having considerable relief an intermontane basin remnants of which comprise the summit of buckskin mountain today contained scattered thermal pools and patches of steaming ground the basin covered several square miles and stood at 3 000 ft above sea level while adjacent uplands were several thousand feet higher in elevation floras included subalpine spruce and fir mixed conifer hardwood slope forests and basin floor hydrophytes willow and marsh grasses buckskin mountain during the miocene closely resembled present landforms in yellowstone national park wyoming

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

origin of the columbia plateau and snake river plain deflection of the yellowstone plume previous models relating the snake river plain and columbia river basalt northwest united states to the yellowstone plume have not taken into account the effects of the subducted farallon and juan de fuca plates not only did subduction shield the surface from mantle plume activity but it deflected the plume in the direction of plate motion a variety of geophysical and geologic evidence suggests that the plume penetrated the subducted slab at 17 5 ma causing eruption of the columbia river basalt the plate boundary then migrated over the plume and the yellowstone plume has recovered to a more nearly vertical conduit forming the snake river plain and yellowstone plateau

morphology and controls on the position of a gravel sand transition fraser river british columbia alluvial river channels often exhibit a relatively abrupt transition from gravel to sand bedded conditions the phenomenon is well documented but few prior studies have analyzed the spatial variability through reaches where transitions occur the downstream fining pattern observed in the fraser river is cited as a classic example of an abrupt gravel sand transition in a large alluvial channel however important questions regarding the exact location of the transition its sedimentology and morphology and what controls its location remain unanswered here we present observations of the downstream change in bed material grain size river bed topography and channel hydraulics through the reach within which the transition occurs these observations indicate that the gravel sand transition is characterized by a terminating gravel wedge but there are patches of gravel downstream of the wedge forming a diffuse extension we show that there is a dramatic decrease in shear stress at the downstream end of the wedge and a consequent cessation of general gravel mobility we argue that the patches of gravel observed beyond the wedge are the result of enhanced mobility of fine gravel over a sand bed we also find that sand in suspension declines rapidly at the downstream end of the wedge suggesting that sand is delivered to the bed completing the sedimentary conditions for a gravel sand transition we propose that the break in river slope associated with the transition is a consequential feature of the transition

deep seismic expression of a foreland basin taranaki basin new zealand a deep seismic reflection profile shot across the south taranaki basin new zealand indicates up to 10 km of crustal thickening beneath the taranaki boundary fault at the eastern margin of the basin the seismic data also show a broad flexure of the entire crust the locus of the flexure producing load appearing to be in the vicinity of the taranaki boundary fault such crustal thickening and flexure suggest a compressional foreland basin style of late cenozoic development rather than the rift graben origin previously assumed this change in interpretation for the south taranaki basin has implications for evaluating the thermal history of the basin and its possibilities for hydrocarbon prospects the study therefore demonstrates the value of deep seismic exploration of a hydrocarbon bearing basin in its early stage of exploration

hysteresis in suspended sediment to turbidity relations due to changing particle size distributions 1 turbidity t is the most ubiquitous of surrogate technologies used to estimate suspended sediment concentration ssc the effects of sediment size on turbidity are well documented however effects from changes in particle size distributions psd are rarely evaluated hysteresis in relations of ssc to turbidity ssc t for single stormflow events was observed and quantified for a dataset of 195 concurrent measurements of ssc turbidity discharge velocity and volumetric psd collected during five stormflows in 2009 2010 on yellow river at gees mill road in metropolitan atlanta georgia regressions of ssc normalized turbidity t ssc on concurrently measured psd percentiles show an inverse exponential influence of particle size on turbidity that is not constant across the size range of the psd the majority of the influence of psd on t ssc is from particles of fine silt and smaller sizes finer than 16 m this study shows that small changes in the often assumed stability of the psd are significant to ssc t relations changes of only 5 m in the fine silt and smaller size fractions of suspended sediment psd can produce hysteresis in the ssc t rating that can increase error and produce bias observed ssc t hysteresis may be an indicator of changes in sediment properties during stormflows and of potential changes in sediment sources trends in the psd time series indicate that sediment transport is capacity limited for sand sized sediment in the channel and supply limited for fine silt and smaller sediment from the hillslope

multiple episodes of tectono thermal disturbances in the huayangchuan u nb pb polymetallic deposit in the xiaoqinling region central china and their significances on metallogeny abstract the huayangchuan u nb pb polymetallic deposit is hosted in a suite of neoarchean to paleoproterozoic gneisses in the xiaoqinling region central china the deposit is composed mainly of discrete small scale carbonatite veins emplaced largely along shallow lying brittle fractures various regimes of deformation from ductile flow to brittle fracturing are documented in the deposit and its adjacent areas these include an earliest phase of contracting deformation d1 manifested as well developed boudinages abundant relict rootless folds and penetrative compositional bands i e gneissosity generally trending e w a subsequent phase of disperse dextral ductile shear d2 along the e w trending gneissosity and a following phase of general ne sw compression d3 resulting in local contraction deformation bands and abundant sinistral ductile shear along the e w trending gneissosity all of which are affected by an overwhelming nne ssw compression d4 with hemi ductile to brittle deformation behaviors the deformation after d4 is generally brittle and may arise from an earlier regime of dominant compression nw se d5 and a later regime of extensive extension d6 multiple thermal events are superimposed on gneisses as revealed by 40ar 39ar dating on biotite collected from various lithologies in the deposit and its adjacent areas which yields ages ranging from late triassic 238 203 ma through earliest cretaceous 139 134 ma and middle cretaceous 117 92 ma to latest cretaceous 72 ma and even early cenozoic 53 ma this sequence of thermal disturbances unlikely predates the d4 deformation given that it is the last ductile deformation experienced by the precambrian rocks ore bearing carbonatite veins have their shallowly lying attitudes consilient with the d4 and d5 deformation and ages compatible with the first two thermal overprints accordingly a two stage mineralization model is proposed for the huayangchuan polymetallic deposit with an early phase of ore veining in the late triassic to latest jurassic during the nearly n s collision between the south and north china blocks and a late phase of ore veining or more possibly reworking in the early cretaceous related to the yanshanian orogeny due to a generally nw directed subduction of the paleo pacific oceanic plate beneath the eastern asia continent

geochronology of the midas low sulfidation epithermal gold silver deposit elko county nevada the midas deposit is the largest known high grade au ag vein deposit located along the northern nevada rift it belongs to a suite of middle miocene low sulfidation epithermal systems associated with magmatism and faulting along the rift interpretation of new 40ar 39ar dates for volcanic rocks and hydrothermal minerals related to gold mineralization and additional isotopic dates throughout the midas region constrain the timing of volcanic tectonic and hydrothermal activity the midas hydrothermal system developed following a change from mafic dominated bimodal volcanism and basin formation to felsic volcanism folding and faulting at about 15 6 ma from 15 6 to 15 2 ma sediments and tuffs were deposited on a relatively impermeable rhyolite flow during this period faulting and tilting of the volcanic edifice created pathways for hydrothermal fluids that flowed to the surface forming sinters and hydrothermal breccias approximately 200 kyr after the change in volcano tectonic regime oblique slip faulting took place along zones of preexisting weakness creating dilational zones and additional channelways for mineralizing fluids at 15 4 ma high grade veins formed in fault zones throughout the region depositing at least 4 million ounces moz of gold and 40 moz of silver the timing of high grade au ag mineralization is identical to the age of rhyolite intrusions whose source magma chamber likely provided the heat necessary to drive the hydrothermal system the age of an unaltered tuff that unconformably overlies opalized sediments establishes that tilting of the units and the hydrothermal system had ceased by 15 2 ma the temporal and spatial coincidence of rhyolite volcanism faulting and high grade mineralization may reflect the importance of contributions from deeper fluid reservoirs containing magmatic components or highly exchanged meteoric waters

geology of the ivanhoe hg au district northern nevada influence of miocene volcanism lakes and active faulting on epithermal mineralization the mercury gold deposits of the ivanhoe mining district in northern nevada formed when middle miocene rhyolitic volcanism and high angle faulting disrupted a shallow lacustrine environment sinter and replacement mercury deposits formed at and near the paleosurface and disseminated gold deposits and high grade gold silver veins formed beneath the hot spring deposits the lacustrine environment provided abundant meteoric water the rhyolites heated the water and the faults flow units and lakebeds provided fluid pathways for the hydrothermal fluids a shallow lake began to develop in the ivanhoe area about 16 5 ma the lake progressively expanded and covered the entire area with fine grained lacustrine sediments lacustrine sedimentation continued to at least 14 4 ma and periodic fluctuations in the size and extent of the lake may have been responses to both climate and nearby volcanism the eruption of rhyolite and andesite flows and domes periodically disrupted the lacustrine environment and produced interfingered flows and lake sediments the major pulse of rhyolitic volcanism took place between 15 16 0 05 and 14 92 0 05 ma high angle faulting began in the basement about 15 2 ma penetrated to and disrupted the paleosurface after 15 10 0 06 ma and largely ceased by 14 92 0 05 ma ground motion related to both faulting and volcanism created debris flows and soft sediment deformation in the lakebeds mercury gold mineralization was coeval with rhyolite volcanism and high angle faulting and it took place about 15 2 to 14 9 ma at and near the paleosurface hydrothermal fluids migrated through tuffaceous sediments above relatively impermeable volcanic and paleozoic units creating chalcedonic cinnabar bearing replacement bodies and sinters disseminated gold was deposited in sedimentary and volcanic rocks beneath the mercury deposits although the hydrologic path between the two ore types is unclear higher grade gold silver deposits formed in massive rhyolites and paleozoic quartzites at deeper levels and these mineralized zones possibly represent the feeder zones for the higher level deposits fluctuations in the ground water table locally produced hydrothermal oxidation of the near surface mercury and disseminated gold deposits the locus of mineralization shifted with time moving south and east from its inception point in the west central part of the district thus although mineralization in the district took place during a span of 300 000 years the duration of mineralization at any one place probably was much shorter the low sulfidation deposits of the ivanhoe district formed at the same time and under similar conditions as those in the nearby midas district 15 km to the northwest which includes the large high grade ken snyder gold silver epithermal vein deposit the exposures in the ivanhoe district are interpreted to represent the near surface example of the paleosurface that originally was present above the midas mineralizing system the resulting combined ivanhoe midas model provides an exploration guide for epithermal deposits in similar geologic environments in northern nevada

age of the emeishan flood magmatism and relations to permian triassic boundary events the permian triassic p t mass extinction the greatest biological mortality event in the earth s history was probably caused by dramatic and global forcing mechanisms such as the siberian flood volcanism here we present the first set of high precision 40 ar 39 ar dating results of volcanic and intrusive rocks from the emeishan traps south china which define a main stage of the flood magmatism at v251 253 ma and a subordinate precursory activity at v255 ma this time span is generally coeval with or slightly older than the age of the p t boundary estimated by the ash beds in the meishan stratotype section and the main eruption of the siberian traps our data reinforces the notion that the eruption of the emeishan traps rather than eruption of the siberian traps accounted for the formation of the p t boundary ash beds in south china the emeishan flood magmatism which occurred in the continental margin comprising thick marine limestone formations moreover may have triggered rapid release of large volumes of methane and carbon dioxide that could have been responsible for the global n 13 c excursion and associated environmental crisis leading to the mass extinction at the p t boundary fl 2002 elsevier science b v all rights reserved

lithospheric topography tilted plumes and the track of the snake river yellowstone hot spot 1 the trace of the snake river yellowstone hot spot is the world s best example of a mantle plume that has been overridden by continental lithosphere the standard model calls for the plume head to rise under northern nevada and be forced northward to form basalts of the columbia plateau subsequent movement of north america to the southwest over the plume tail created a hot spot trace on the surface we present a new conceptual model for the origin of this feature that resolves inconsistencies in the current standard model and explains the recent documentation of a thermal anomaly in the mantle below yellowstone today that plunges 65 wnw our model implies that the plume tail was forced beneath thinned cratonic lithosphere to the se along with part of the plume head and has remained in this orientation for the last 12 ma we infer that almost all of the volcanism in se oregon and sw idaho prior to 12 ma results from overriding the southern extension of the plume head not the plume tail and that a distinct plume tail hot spot track was not established until formation of the bruneau jarbidge eruptive center around 12 ma the plume tail track may also be controlled by a preexisting structural boundary in lithosphere that is thinner than adjacent lithosphere this model demonstrates the potential importance of lithospheric topography on controlling the surface manifestation of plume volcanism and the complexity that may arise when lithospheric thickness is nonuniform

geologic setting and genesis of the mule canyon low sulfidation epithermal gold silver deposit north central nevada the mule canyon mine exploited shallow low sulfidation epithermal au ag deposits that lie near the west side of the northern nevada rift in northern lander county nevada mule canyon consists of six small deposits that contained premining reserves of about 8 2 mt at an average grade of 3 81 g au tonne it is an uncommon mafic end member of low sulfidation au ag deposits associated with tholeiitic bimodal basalt rhyolite magmatism the ore is hosted by a basalt andesite eruptive center that formed between about 16 4 to 15 8 ma during early mafic eruptions related to regionally extensive bimodal magmatism hydrothermal alteration and au ag ores formed at about 15 6 ma and were tightly controlled by north northwest to north striking high angle fault and breccia zones developed during rifting emplacement of mafic dikes and eruption of mafic lava flows hydrothermal alteration assemblages are zoned outward from fluid conduits in the sequence silica adularia adularia smectite smectite intermediate argillic and smectite carbonate propylitic all alteration types contain abundant pyrite and or marcasite arsenopyrite field relations indicate that silica adularia alteration is superimposed on argillic and propylitic alteration little or no steam heated acid sulfate alteration is present probably the result of a near surface water table during hydrothermal alteration and ore deposition two distinct ore types are present at mule canyon early replacement and later open space filling replacement ores consist of disseminated and vesicle filling pyrite marcasite and arsenopyrite in argillically altered or weakly silicified rocks ore minerals consist of au bearing arsenopyrite and arsenian pyrite overgrowths on earlier formed pyrite and marcasite open space filling ores include narrow stockwork quartz adularia veins banded and crustiform opaline and chalcedonic silica adularia veins silica adularia cemented breccias and sparse carbonate pyrite and or marcasite veins ore minerals consist mostly of electrum and ag sulfide and selenide minerals with minor to major amounts of pyrite marcasite and arsenopyrite and local stibnite both types of ores have similar geochemical signatures characterized by high au ag as sb and se contents locally high hg mo tl and w contents and low cu pb and zn contents stable isotope data indicate that ore fluids consisted dominantly of meteoric water that evolved by deep circulation through paleozoic sedimentary rocks at low water rock ratios about 1 and high temperatures 200 c calculated isotopic compositions of ore fluids are 18oh2o 3 to 7 per mil dh2o 107 to 124 per mil 13cco2 0 to 6 per mil and 34sh2s 3 to 8 per mil the ore fluids obtained much of their h2s and co2 and probably scavenged ore metals and trace elements from the paleozoic sedimentary rocks some h2s and co2 may have been derived from degassing miocene magmas mule canyon formed at shallow depths probably about 100 m below the paleosurface ore fluids were dilute nearly neutral in ph reduced h2s rich and co2 bearing peak temperatures in ore zones reached 230 to 265 c at nearly lithostatic pressures when some crystalline quartz adularia precipitated but most ore formed at temperatures 200 c at near hydrostatic pressures and was accompanied by precipitation of opaline and chalcedonic silica adularia calcite and dolomite deposition of gold in as rich overgrowths on pyrite and or marcasite in disseminated ores occurred owing to decreasing h2s in the ore fluids resulting from sulfidation reactions later electrum and ag selenide precipitation in open spaces occurred owing to boiling loss of h2s to the vapor phase and cooling mule canyon is similar to most other low sulfidation au ag deposits associated with miocene tholeiitic bimodal basalt rhyolite magmatism in the great basin such as sleeper midas and buckhorn major differences at mule canyon are the high fe sulfide contents of some ores and altered wall rocks the paucity of silicification and the presence of only narrow and discontinuous gold bearing siliceous veins these differences primarily are due to the mafic composition of the host rocks mule canyon differs from low sulfidation deposits associated with subduction related calc alkaline porphyry intrusions such as the comstock lode in that a much smaller proportion of water salts and metals was derived from crystallizing magmas

distribution and geochronology of oregon plateau u s a flood basalt volcanism the steens basalt revisited the timing and petrogenesis of mid miocene flood basalt volcanism in the northwest united states has been extensively addressed yet the chemical characteristics and temporal details of the steens basalt exposed on the oregon plateau are poorly defined steens basalt volcanism has generally been accepted to have occurred at 16 6 ma coeval and or just prior to the onset of columbia river basalt group volcanism to the north new major and trace element analyses and nine 40 ar 39 ar ages ranging from 15 51 0 28 to 16 58 0 18 ma were obtained on oregon plateau flood basalt lava flows from stratigraphic sections in close proximity to steens mountain additionally new 40 ar 39 ar ages were obtained on the uppermost and thirty first lava flow down from the top of the 1 km section of steens basalt exposed at steens mountain and yield eruption ages of 16 59 0 10 and 16 55 0 10 ma respectively field relations between these basalt sections suggest that multiple eruptive centers were present in the vicinity of steens mountain the chemical and chronologic data presented in this study illustrate that flood basalts with the steens basalt chemical signature erupted across the southern oregon plateau over a much greater timespan than what is typically quoted for the steens mountain type section these data suggest that the main volume of steens basalt volcanism erupted over at least an 1 m y duration from eruptive loci in the vicinity of steens mountain while likely much less voluminous volcanism and lithospheric input of mafic magma appears to have occurred for n2 ma across the oregon plateau these new geochemical and geochronological constraints verify a common temporal link between steens malheur gorge region and columbia river basalt group crbg volcanism this direct temporal link requires that petrogenetic and tectonic models of mid miocene northwestern u s flood basalt volcanism recognize that the northern columbia plateau and southern oregon plateau portions of this mid miocene basalt province were erupting simultaneously during portions of the regional event 2007 elsevier b v all rights reserved

calculation of in situ acoustic sediment attenuation using off the shelf horizontal adcps in low concentration settings the use of off the shelf acoustic doppler velocity profilers adcps to estimate suspended sediment concentration and grain size in rivers requires robust methods to estimate sound attenuation by suspended sediment theoretical estimates of sediment attenuation require a priori knowledge of the concentration and grain size distribution gsd making the method impractical to apply in routine monitoring programs in situ methods use acoustic backscatter profile slope to estimate sediment attenuation and are a more attractive option however the performance of in situ sediment attenuation methods has not been extensively compared to theoretical methods we used three collocated horizontally mounted adcps in the fraser river at mission british columbia and 298 observations of concentration and gsd along the acoustic beams to calculate theoretical and in situ sediment attenuation conversion of acoustic intensity from counts to decibels is influenced by the instrument noise floor which affects the backscatter profile shape and therefore in situ attenuation we develop a method that converts counts to decibels to maximize profile length which is useful in rivers where cross channel acoustic profile penetration is a fraction of total channel width nevertheless the agreement between theoretical and in situ attenuation is poor at low concentrations because cross stream gradients in concentration sediment size or gsd can develop which affect the backscatter profiles we establish threshold concentrations below which in situ attenuation is unreliable in fraser river our results call for careful examination of cross stream changes in suspended sediment characteristics and acoustic profiles across a range of flows before in situ attenuation methods are applied in river monitoring programs

geochronology of the sleeper deposit humboldt county nevada epithermal gold silver mineralization following emplacement of a silicic flow dome complex the high grade gold silver deposits at the sleeper mine are low sulfidation quartz adularia type epithermal deposits formed during the final stages of igneous hydrothermal activity of a small middle miocene silicic flow dome complex in north central nevada potassiumargon and 40 ar 39 ar ages of alkali feldspar indicate that the rhyolitic flow dome complex was emplaced before about 16 5 ma hydrothermal alteration and mineralization followed lasting from about 16 2 to 14 ma based on ages of adularia associated with electrum there were multiple pulses of alteration and mineralization but all occurred within a period of less than 2 m y later supergene alteration formed opal and alunite about 5 4 ma but produced no au or ag mineralization other than some remobilization to produce locally rich pockets of secondary au and ag enrichment and is unrelated to the older magmatic hydrothermal system the sleeper deposit in the northern part of the great basin is genetically related to bimodal volcanism that followed a long period of arc related andesitic volcanism in the same general region

initial gold contents of silicic volcanic rocks bearing on the behavior of gold in magmatic systems many fresh silicic volcanic rocks have markedly lower initial gold contents than previously recognized of 129 carefully selected glassy silicic volcanic rocks analyzed 113 contain f o 2 more readily accommodate gold the mean of 23 relatively silicic intermediate rocks is 0 54 ppb au with tholeiitic andesites icelandites generally higher in gold than calc alkalic types there is little evidence that particular geologic regions are intrinsically richer in gold than others bulk composition melt structure and the amount and timing of separation of vapor mineral and sulfide and or metal melt phases would appear to largely determine the gold contents of silicic magmas the various lines of evidence for the removal of gold from magma by the separation of vapor crystalline and immiscible melt phases indicate that fresh volcanic rocks provide only minimum limits on magmatic gold concentrations in most cases magmatic values will be much higher elevated gold contents 0 3 to 1 0 ppm of some porphyry deposits suggest the existence of salic magmas containing gold in concentrations appreciably greater than the 1 2 ppb concentrations of basalts

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

gold metallogeny associated with craton destruction a geophysical perspective from the north china craton abstract currently ranking as the largest producer of gold in the world china s gold reserves are spread over 200 major gold deposits and several minor deposits a large part of these belong to the late mesozoic gold deposits in the north china craton ncc that occur along craton margins as well as within the cratonic interior in reactivated paleo sutures and show a close spatio temporal relationship with zones of lithospheric thinning and craton destruction here we integrate and evaluate geophysical information from the ncc through an analysis of receiver function and tomography that suggest mantle upwelling accompanied by lower crustal or lithospheric delamination our results identify that the major gold belts in the ncc are largely located above zones of mantle upwelling and craton destruction the faults and paleo sutures provided the pathways for migration of ore bearing fluids with the granitoids offering favorable conditions for gold deposition

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

large scale fractures related to inception of the yellowstone hotspot during middle miocene time western north america was subject to flood basalt volcanism dike swarm injection and broad scale fracturing and folding of the crust we propose a simple model to account for these events and for a regional pattern of geologic and geophysical features aeromagnetic maps reveal some of the most important elements of this pattern which are several narrow arcuate anomalies here referred to as the northern nevada rifts these rifts extend hundreds of kilometers across nevada and are likely caused by highly magnetic middle miocene mafic dikes with the aid of filtering techniques the anomalies can be traced into oregon together with other geologic features such as fold axes dike swarms and faults they produce a spoke like pattern fanning over 2208 of arc that converges toward a point near the oregon idaho border lat 448n a possible cause for this pattern is a point source of stress at the base of the crust related to the formation of the yellowstone hotspot the spoke like pattern however does not persist at large distances from the emerging hotspot several hundred kilometers to the south the northern nevada rifts deviate significantly 308 from a radial trend we show that a simple model imposing a point source of stress at the base of the crust and a regional stress field aligned with the presumed middle miocene stress direction fits the observed fracture pattern it thus accounts for both the radial pattern present near the nascent hotspot and the far field pattern due to regional stresses

isotope systematics in vein gold from brusson val d ayas nw italy 1 pb pb evidence for a piemonte metaophiolite au source the monte rosa gold district nw italian alps comprises hundreds of late alpine epigenetic gold bearing quartz veins we report a lead isotope study on an important one of these veins fenilia brusson val d ayas which reveals a complex isotope geochemistry of petrographically cogenetic vein minerals petrographic observations allow to distinguish a quartz dominated main stage and a quartz poor final stage mineral assemblage most of the free gold crystallized in the beginning of the final stage whereas sulphides were deposited during its waning three distinct lead isotope fields result for the paragenetic sequence of vein minerals the variable lead isotope signature 15 517 15 706 as well as from the uniform radiogenic lead 15 649 207pb 204pb 15 691 characteristic of the waning final stage we favour a model where the deeply circulating final stage fluid the ultimate source of which remains obscure acquired lead and thus possibly its gold in piemonte metaophiolitic rocks beneath the monte rosa nappe during ascent the primitive gold bearing fluid was variably contaminated by an evolved crustal lead 15 706 207pb 204pb 15 743 compatible with derivation from monte rosa nappe rocks during deposition of final stage minerals the pb isotope composition of the fluid changed to a constant signature becoming again dominated by a radiogenic lead source thus within an assemblage defined as cogenetic on petrographic grounds the pb isotopic signature is not necessarily uniform and source modelling based on lead isotopes of associated minerals may be inappropriate it remains open at present whether the inferred piemonte metaophiolite gold source is confined to the fenilia vein in val d ayas only or whether it might be more widespread in the monte rosa gold district

timing and development of the heise volcanic field snake river plain idaho western usa the snake river plain srp developed over the last 16 ma as a bimodal volcanic province in response to the southwest movement of the north american plate over a fixed melting anomaly volcanism along the srp is dominatedby eruptions of explosive high silica rhyolites and represents some of the largest eruptions known basaltic eruptions represent the final stages of volcanism forming a thin cap above voluminous rhyolitic deposits volcanism progressed generally from west to east along the plain episodically in successive volcanic fields comprised of nested caldera complexes with major caldera forming eruptions within a particular field separated by ca 0 5 1 ma similar to and in continuation with the present day yellowstone plateau volcanic field passage of the north american plate over the melting anomaly at a particular point in time and space was accompanied by uplift regional tectonism massive explosive eruptions and caldera subsidence and followed by basaltic volcanism and general subsidence the heise volcanic field in the eastern srp idaho represents an adjacent and slightly older field immediately to the southwest of the yellowstone plateau volcanic field five large volume 0 5 km 3 rhyolitic ignimbrites constitute a time stratigraphic framework of late miocene to early pliocene volcanism for the study region field relations and high precision 4 0 ar 3 9 ar age determinations establish that four of these regional ignimbrites were erupted from the heise volcanic field and form the framework of the heise group these are the blacktail creek tuff 6 62 0 03 ma walcott tuff 6 27 0 04 ma conant creek tuff 5 51 0 13 ma and kilgore tuff 4 45 0 05 ma all errors reported at 2a the fifth widespread ignimbrite in the region is the arbon valley tuff member of the star light formation 10 21 0 03 ma which erupted from a caldera source outside of the heise volcanic field these results establish the conant creek tuff as a distinct and widespread ignimbrite in the heise volcanic field eliminating former confusion resulting from previous discordant k ar and fission track dates new 4 0 ar 3 9 ar determinations when combined with geochemical lithologic geophysical and field data define the volcanic and tectonic history of the heise volcanic field and surrounding areas volcanic units erupted from the heise volcanic field also provide temporal control for tectonic events associated with late cenozoic extension in the snake range and with uplift of the teton range wyoming in the snake range movement of large 0 10 km 3 slide blocks of mississippian limestone exposed 50 km to the east of the heise field occurred between 6 3 and 5 5 ma and may have been catastrophically triggered by the caldera eruption of the 5 51 0 13 ma conant creek tuff this slide block movement of 300 vertical meters indicates that the snake range had significant relief by at least 5 5 ma in jackson hole the distribution of outflow facies of the 4 45 0 05 ma kilgore tuff related to eruption from the kilgore caldera in the heise volcanic field on the eastern srp indicates that the northern teton range was not a significant topographic feature at this time

how fast does gold trickle out of volcanoes the enormous hydrothermal gold ore deposit on lihir island papua guinea may have formed in less than 55 000 years

origins of calcite in a boiling geothermal system the formation of hydrothermal calcite relates to the movement of carbon dioxide in a geothermal system as governed by boiling dilution and condensation in this paper we show how these processes control the occurrence distribution and stable isotope composition of calcite based on a study at broadlands ohaaki the two principal calcite occurrences in the broadlands ohaaki geothermal system are 1 as replacement of rock forming minerals and volcanic glass and 2 as platy crystals infilling voids both are stable over a broad temperature range from 160 degrees c to 300 degrees c replacement calcite is widespread and forms through hydrolysis reactions involving calcium alumino silicates and sub boiling liquids that contain 0 3 to 0 75 m co 2 platy calcite in contrast forms over a restricted vertical interval of a few hundred meters within the upflow zone it precipitates from boiling fluids through exsolution of carbon dioxide as indicated by coeval liquid rich and vapor rich fluid inclusions and its formation in the two phase zone fluid inclusion data help to define the boiling paths of fluids from which platy calcite formed homogenization temperatures range from 160 degrees c to 310 degrees c and are consistent within the present geothermal regime ice melting temperatures range from 0 0 to 1 0 degrees c and indicate the presence of up to 0 5 m dissolved carbon dioxide model boiling curves calculated to match these data show how the concentration of dissolved carbon dioxide in the preboiled fluid dictates the depth of first boiling most fluid inclusion data lie along a model boiling path characteristic of the centre of the upflow zone in which the rising fluid initially containing 0 75 m co 2 begins to boil at approximately 320 degrees c and approximately 2000 m depth data from well br 18 instead matches a curve in which the rising fluid initially containing 0 53 m co 2 begins boiling at approximately 245 degrees c and approximately 900 m depth the shallowing of the depth of first boiling likely results from dilution of dissolved carbon dioxide in the parent chloride water as it rises and mixes with marginal waters calcite precipitates from both shallow formed steam headed groundwater and deeply derived chloride water and these waters are isotopically distinct at broadlands ohaaki the delta 18 0 values of calcite at 200 degrees c range from 0 5 to 7 5 per mil whereas delta 18 0 values of calcite at 200 degrees c range from 4 to 10 per mil taking appropriate temperature dependent fractionation factors into account these data indicate equilibration with chloride water delta 18 0 h2o 4 5 per mil and steam heated groundwater delta 18 o h2o 7 0 per mil respectively oxygen isotopes of hydrothermal calcites in the nearby wairakei and waiotapu geothermal systems show similar patterns consistent with the occurrence of both chloride and steam heated waters there calcite formation is explained by a model that describes the distribution of two phase conditions and aqueous carbon dioxide concentrations in a column of hydrothermal fluid rising through a rock matrix of isotropic permeability in this ideal situation platy calcite forms along the inner margin of the two phase zone having the shape of an inverted cone whereas replacement calcite mostly forms in the surrounding one phase liquid only zone the sparse occurrence of calcite at less than or equal to 800 m depth in the central upflow of the ohaaki sector at broadlands ohaaki is compatible with this model and appears related to the exsolution of dissolved carbon dioxide through boiling deeper in the system

geochronology of age progressive volcanism of the oregon high lava plains implications for the plume interpretation of yellowstone 1 the high lava plains province hlp is a late cenozoic bimodal volcanic field at the northern margin of the basin and range province in southeastern oregon that hosts a westward younging trend of silicic volcanism that crudely mirrors northeastward migration of silicic volcanism along the yellowstone snake river plain ysrp trend we present 40ar 39ar ages for 19 rhyolite domes 5 rhyolite ash flow tuffs and 34 basaltic lavas from the hlp the previously identified trend of westward migration of hlp rhyolites is confirmed the rate of propagation is 33 km m y from 10 to 5 ma slowing to 13 km m y after 5 ma the duration of silicic volcanism at any locus is 2 m y three older hlp dacite domes yielded ages of 15 5 ma basalts are not age progressive we identify several episodes of increased basaltic activity at 7 5 7 8 5 3 5 9 and 2 3 ma with the younger episode likely continuing into the recent the hlp and ysrp trends emerged from the axis of middle miocene basaltic volcanism of the columbia river and steens basalts we propose a model in which 1 miocene flood basalts and widespread silicic rocks are the result of emplacement of a plume head near the craton margin enhanced by flow up a topographic gradient along the base of the lithosphere at the craton margin 2 the hlp trend is the result of westward flow originating at the craton margin and 3 the ysrp trend is the trace of the motion of the north american plate over the tail of the plume

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

formation of wulong mafic dikes via mixing of enriched lithospheric mantle derived and crustal magmas eastern north china craton abstractzircon u pb age and geochemical data are reported for the wulong dike swarms from the liaodong peninsula ne china the studied dikes include diorite lamprophyre and dolerite together with previous dating the diverse rocks have a peak age of ca 120 125 ma they are marked by inversely zoned phenocrysts with mg and cr rich rims and they exhibit mantle like signatures i e high mg cr ni associated with prominent subduction type element enrichments the early diorites have relatively radiogenic sr 0 7143 and less radiogenic nd to 11 0 isotopes coupled with high and variable mgo cr ni contents indicating that they were mainly derived from ancient lower crust with subordinate contributions of enriched lithospheric mantle the transitional lamprophyres are characterized by high rb sr ratios 0 22 0 54 but low initial 87sr 86sr ratios 0 7069 0 7093 moderate negative end t values 14 4 to 12 6 and low promantle elements such as mg cr ni reflecting that juvenile lower crust

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

native gold and native copper grains enclosed by olivine phenocrysts in a picrite lava of the emeishan large igneous province sw china a native gold bleb found in an olivine phenocryst in a picrite lava from the emeishan large igneous province elip may be the first documented case of the transport of gold as a distinct precious metal phase in a mantle derived magma four picrite layers have been recognized in the lower part of the volcanic succession in the lijiang area in the western part of the elip the native gold bleb was found enclosed in an olivine phenocryst in the second picritic layer of a basalt picrite succession in the elip the gold bleb is spheroidal about 30 m in diameter consists of pure gold and does not contain any other elements in addition native copper grains were also discovered in the serpentinized olivine phenocrysts and native zinc and moissanite sic were separated from an 20 kg sample of picrite the paragenesis of these minerals suggests that the primary magmas were s unsaturated the native gold and native copper grains are considered to be xenocrysts from the mantle transported to shallow depths by a rising plume and then captured by the picritic melts the discovery of native gold and native copper grains provides direct evidence that the gold in the hydrothermal gold deposits and the native copper deposits in the elip lavas ultimately may be derived from a mantle plume

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

colloidal transport of gold and silica in epithermal precious metal systems evidence from the sleeper deposit nevada bonanza vein gold ore from the sleeper deposit nevada was deposited at a depth of less than 500 m and exhibits a characteristic texture of alternating colloform bands of fine grained quartz and bands of opaline silica containing abundant electrum textural evidence indicates that the quartz bands recrystallized from a gel like amorphous precursor whereas the gold rich bands were deposited originally as amorphous precipitate formed by the coagulation of colloidal silica and gold particles gold and silica precipitated initially as colloidal particles at deeper levels in the hydrothermal system and were mechanically transported upward by the hydraulic action of the ore forming fluids coagulation of the colloidal particles may have been caused by solution cooling or boiling and catalyzed by hydrated polycationic al 3 aqueous species transport of gold as colloidal particles in epithermal systems indicates that at least in some instances higher gold content is possible than in true solution a gold enriched ore fluid appears necessary to explain the origin of the bonanza grade ore at the sleeper mine and the colloidal gold transport process locally might have played a role in the genesis of the more common disseminated epithermal precious metal deposits

relationship of epithermal gold deposits to large scale fractures in northern nevada geophysical maps of northern nevada reveal at least three and possibly six large scale arcuate features one of which corresponds to the northern nevada rift that possibly extends more than 1 000 km from the oregonidaho border to southern nevada these features may reflect deep discontinuities within the earth s crust possibly related to the impact of the yellowstone hot spot because mid miocene epithermal gold deposits have been shown to correlate with the northern nevada rift we investigate the association of other epithermal gold deposits to other similar arcuate features in northern nevada mid miocene and younger epithermal goldsilver deposits also occur along two prominent aeromagnetic anomalies west of the northern nevada rift here we speculate that mid miocene deposits formed along deep fractures in association with mid miocene riftrelated magmatism and that younger deposits preferentially followed these preexisting features statistical analysis of the proximity of epithermal gold deposits to these features suggests that epithermal gold deposits in northern nevada are spatially associated with large scale crustal features interpreted from geophysical data

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

native gold in a hawaiian alkalic magma native gold found in fresh basanite glass from the early submarine phase of kilauea volcano hawaii may be the first documented case of the transport of gold as a distinct precious metal phase in a mantle derived magma the gold bearing glass is a grain in bedded volcanic glass sandstone japan marine science and technology center jamstec sample s508 r3 collected by the submersible shinkai 6500 at 3879 m depth off kilauea south flank extensive outcrops there expose debris flow breccias and sandstones containing submarine erupted alkalic rock fragments and glasses from early kilauea precipitation of an immiscible gold liquid resulted from resorption of magmatic sulfides during crystallization differentiation with consequent liberation of sulfide hosted gold elevated whole rock gold concentrations to 36 ppb for fresh lavas and clasts from early kilauea further show that some magmas erupted at the beginning stages of hawaiian shield volcanoes were distinctly gold rich most likely owing to limited residual sulfide in their mantle source alkalic magmas at other ocean islands may also be gold rich and oceanic hot spot provinces may contain underappreciated gold resources

geology of the delamar silver mine idaho a volcanic dome complex and genetically associated hydrothermal system the delamar silver mine located in southwest idaho produced over 17 million ounces of silver and 230 thousand ounces of gold by bulk mining methods between 1977 and late 1987 the mine occurs within the silver city rhyolite an extensive and complex sequence of middle miocene silicic volcanic flows and domes that unconformably overlie a thick pile of miocene alkali olivine basalt the basalt overlies late cretaceous granite which is correlative with the idaho batholith all three major rock types contain epithermal silver gold veins although rhyolites such as those at delamar are the only hosts for low grade large tonnage mineralization the delamar mine occurs within a sequence of six pervasively altered silicic volcanic units from oldest to youngest these units have the following informal designations 1 porphyritic latite 2 quartz latite 3 tuff breccia 4 porphyritic rhyolite 5 banded rhyolite and 6 millsite rhyolite the porphyritic rhyolite unit is spatially temporally and genetically related to silver gold mineralization and is the principal host rock at delamar this unit was emplaced as coalescing and overlapping lava domes and was preceded by cogenetic eruption of tuff breccia in the form of tuff rings the emplacement of tuff rings and lava domes was controlled by structures related to a shallow level magma body and regional basin and range faulting onlap of banded rhyolite preceded hydrothermal activity and served as an important control on localizing ore deposition the mineralogic character of ore and alteration at delamar is similar to adularia sericite type volcanic hosted epithermal deposits but the geologic setting is more like that of acid sulfate deposits

precious metal vein systems in the national district humboldt county nevada the national district 20 miles 33 km southeast of mcdermitt nevada produced 200 000 ounces of gold and 750 000 ounces of silver during the period 1906 1941 precious metal quartz sulfide silicate veins near the townsite of national and on buckskin mountain occur within a 3 000 ft thick 910 m sequence of miocene volcanic and intrusive rocks which vary in composition from basalt to rhyolite veins formed about 15 6 million years ago and associated rhyolites are approximately 18 to 15 m y old the volcanic pile at national is highly faulted and deposits are localized within orthogonal fault sets that coincide with major upper crustal structures in the northern great basin intrusive rhyolite and eruptive centers spatially related to veins at national partly define the goosey lake depression a shallow basin situated on the western edge of the owyhee volcanic plateau wall rock in the district is progressively hydrolyzed and sulfidized to assemblages dominated by chlorite calcite illite montmorillonite quartz pyrite and kaolinite quartz pyrite as veins are approached alteration assemblages exposed by exploration on buckskin mountain include the paleosurface at buckskin planar open space filled veins disperse upward into structurally unconfined flat lying zones of kaolinite quartz quartz alunite and chalcedonic silica sinter quartz altered rhyolite the latter being the paleosurface assemblage metallic minerals in four nested and coalescing vein stages include cinnabar stibnite electrum silver sulfosalts and selenides and base metal sulfides vein silicates are quartz kaolinite illite and at depth muscovite the spatial distribution of hydrothermal sulfides and silicates throughout buckskin mountain is strongly influenced by physical properties of premineralization volcanic rocks porous and permeable rhyolite tuff capping buckskin mountain is pervasively hydrolyzed to laterally extensive silica alunite and kaolinite bearing assemblages which lack tabular zones of open space filling underlying dense and brittle rhyolite flows confine hydrothermal assemblages to a relatively few planar fractures flanked by parallel selvedges of progressively altered wall rock at buckskin mountain two distinct but synchronous thermal events are documented by fluid inclusion studies the earlier event involved temperatures ranging from approximately 295 degrees to 250 degrees c and culminated in rupture of overlying rhyolite producing an inverted conical mass of bilithic breccia fluids associated with the second postbrecciation event boiled at 100 degrees to 255 degrees c from the paleosurface to depths exceeding 2 000 ft 606 m near surface hydrothermal assemblages were deposited from low salinity 1 2 wt nacl solutions containing 2 weight percent dissolved gases precious metal precipitation occurred at a temperature of about 250 degrees c and a pressure of about 40 bars largely from boiling solution periodic restrictions to convective heat loss resulting from silica deposition and episodic fluid circulation cause observed temperature and pressure data to deviate somewhat from an ideal hydrostatic model the vertical temperature gradient is consistent with a water saturated melt of granitic composition about 10 000 ft 3 030 m below the paleosurface during mineralization close spacing of isotherms below 200 degrees c reflects paleosurface proximity cooling by conduction and steam loss as well as accelerated convective cooling of fluid circulating through permeable rhyolite tuff integration of hydrothermal assemblages thermal measurements fluid mineral equilibria and active hot springs data suggests that as temperatures in buckskin mountain decreased from 250 degrees c at depth to 100 degrees c at the surface ph increased from approximately 3 7 to near neutral sigma s increased from 10 super 4 5 m to approximately 10 super 2 6 m and f sub o 2 decreased from 10 super 39 to 10 super 48 the distribution of alteration assemblages precious metal minerals stibnite and cinnabar was apparently controlled by boiling a sulfldation gradient temperature decreases and near surface hydrology

the northern nevada rift regional tectono magmatic relations and middle miocene stress direction as defined by the most recent aeromagnetic surveys the north northwest trending northern nevada rift zone extends for at least 500 km from southern nevada to the oregon nevada border at several places along the rift the magnetic anomaly is clearly related to north northwest trending dikes and flows that based on new radiometric dating erupted between 17 and 14 ma and probably during an even shorter time interval the tectonic significance of the rift is dramatized by its length its coincidence in time and space at its northern terminus with the oldest silicic caldera complex along the yellowstone hot spot trend and its parallelism with the subduction zone along the north american coast prior to the establishment of the san andreas fault the northern nevada rift is also equivalent in age trend and composition to feeder dikes that fed the main eruptive pulse 95 volumetrically off the columbia river flood basalts in northern oregon 15 5 16 5 ma because of these similarities both regions are considered to be part of an enormous lithospheric rift that propagated rapidly south southeast and north northwest respectively from a central mantle plume the site of the initial breaching of the north america plate by this plume is probably the mcdermitt volcanic center at the north end off the rift near the oregon nevada border the present north northwest trend of the rift and its internal elements such as dikes and lava filled grabens record the orientation of the arc normal extensional stress in this back arc region at the time of emplacement paleomagnetic evidence presented by others and interpreted to indicate block rotations at three sample localities is not consistent with either a rotation of dikes within the rift or with a regional rotation of the entire rift the present north northwest trend of the rift reflects the state of stress in the basin and range during middle miocene time and is consistent with stress indicators of similar age throughout the basin and range and rio grande rift provinces