geology and geochemistry of wall rock alteration at the carlin gold deposit nevada the carlin disseminated gold deposit occurs in an autochthonous sequence of paleozoic sedimentary rocks exposed in a structural window in the roberts mountains thrust in north central nevada the upper 175 m of the silurian roberts mountains formation hosts the majority of ore at carlin and is characterized by laminated fine grained calcareous and or dolomitic argillaceous siltstone with local coarser grained siltstones and 50 cm thick lenticular interbeds of sand and granule sized calcareous bioclastic debris or fossil hash detailed studies of drill core and exposures in the east pit of the carlin mine show that alteration and mineralization are zoned away from crosscutting fault conduits and these more permeable bioclastic beds indicating that these two features were major inflow zones for hydrothermal fluid in unoxidized rocks unaltered calcareous siltstone 1 containing quartz dolomite calcite illite k feldspar and pyrite is progressively converted to assemblages of 2 quartz dolomite calcite illite pyrite 3 quartz dolomite illite k mica pyrite 4 quartz illite k mica pyrite and 5 quartz kaolinite dickite pyrite adjacent to inflow zones where jasperoids are developed gold most consistently enriches the zone of calcite and dolomite removal 3 and 4 above though it occurs in all zones locally in high concentrations this zoned alteration was accomplished by a co 2 rich acidic fluid this acidic alteration enhanced the passage of fluids by extensive carbonate removal to form zones of higher permeability oxidation is wholly a supergene effect related to deep weathering because the oxidation is superimposed on both mineralized and altered rocks with only minor effect on the major element chemistry it has produced low temperature goethitic fe oxides rather than higher temperature hematite and is not spatially related to au distribution at the mine or on a district scale because of extensive carbonate removal leading to local volume reduction through collapse and or compaction geochemical effects are examined using ratios to relatively immobile elements such as al and ti extensive depletion of ca mg aud co 2 and introduction of si au and s have occurred potassium is depleted in the conversion of illite to dickite kaolinite in proximal silicified inflow zones and fe enriches some pyritized rock carbonate removal and silicification are two separate processes both of which are spatially associated with mineralization mineralized decarbonated rocks and barren footwall rocks commonly are not silicified and intensely silicified proximal alteration zones are generally low grade

the thin hot plume beneath iceland summary we present the results of a seismological investigation of the frequency dependent amplitude variations across iceland using data from the hotspot array currently deployed there the array is composed of 30 broad band passcal instruments we use the parameter t de ned in the usual manner from spectral ratios halderman davis1991 to compare observed s wave amplitude variations with those predicted due toboth anelastic attenuation and diiraction eiects four teleseismic events at a range of azimuths are used to measure t a 2 d vertical cylindrical plume model with a gaussian shaped velocity anomaly is used to model the variations that part of t caused by attenuation was estimated by tracing a ray through iasp91 then superimposing our plume model velocity anomaly and calculating the path integral of 1 oq that part of t caused by diiraction was estimated using a 2 d nite diierence code to generate synthetic seismograms the same spectral ratio technique used for the datawas then used to extract a predictedt thet variations caused by anelastic attenuation are unable to account for the variations we observe but those caused by diiraction do we calculate the t variations caused by diiraction for diierent plume models and obtain our best t plume which exhibits good agreement between the observed and measured t the best t plume model has a maximum s velocity anomaly of 12 per cent and falls to 1 e of its maximum at 100 km from the plume centre this is narrower than previous estimates from seismic tomography which are broadened and damped by the methods of tomography this velocity model would suggest greater ray theoretical traveltime delays than observed however we nd that for such a plume wave front healing eiects at frequencies of 0 03 0 175 hz the frequency range used to pick s wave arrivals causes a 40 per cent reduction in traveltime delay reducing the ray theoretical delay to that observed

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

carlin gold deposits nevada origin in a deep zone of mixing between normally pressured and overpressured fluids gold mineralization at carlin is clearly younger than hydrocarbon maturation pre cretaceous and felsic dike intrusion cretaceous and older than deep oxidation late tertiary within the episode of gold mineralization the main gold ore mgo stage and late gold ore ego stage are distinguished paragenetically with a variety of vein and mineralization types in each mgo stage fluids contained 5 to 10 mole percent co 2 appreciable h 2 s and 3 or 1 wt percent nacl equiv at least portions of mgo stage mineralization were characterized by two phase boiling co 2 exsolution at 215 degrees or 30 degrees c and 800 or 400 bars in contrast ego stage fluids were gas poor with salinities 18 o sub h 2 o values of 5 to 9 per mil whereas ego stage fluids resembled unevolved meteoric water with delta 18 o sub h 2 o values 18 o values shifted from near whole rock values of 12 or 3 per mil to around 0 or 1 per mil as ego stage fluids flooded the system jasperoids also record a large range 9 22 in delta 18 o sub h 2 o values these data indicate the involvement of two very different fluids in ore deposition because mgo and ego stage features are closely associated spatially with each other and with au as sb hg and other ore elements both fluids are believed to have both been present during most stages of ore deposition at pressures of 80 to 85 percent lithostatic depths of 3 8 or 1 9 km are required to accommodate the 800 or 400 bars of pressure recorded in mgo stage fluid inclusions carlin therefore is not an epithermal or hot spring deposit carbon dioxide in gas rich mgo stage fluids may have originated either directly from buried intrusions or their contact aureoles or from low grade regional metamorphism at depth the water may have been originally meteoric and au may be magmatic or derived from leaching of deep metamorphic or sedimentary rocks ore deposition appears to have occurred in zones of throttling at a pressure seal between normally pressured and overpressured regimes where fluids experienced a change from near lithostatic to hydrostatic conditions such pressure seals are common in deep sedimentary basins and may be a key to highly localized gold deposition mixing of two fluids and interaction with host rocks along thin permeable bioclastic horizons are believed to have been the major factors in depositing ore

a special issue devoted to gold deposits in northern nevada part 2 carlin type deposits this is the second of two special issues of economic geology devoted to gold deposits in northern nevada readers interested in a general overview of these deposits their economic significance their context within the tectonic evolution of the region and synoptic references on each gold deposit type are directed to the preface of the first special issue john et al 2003 volume 98 issue 2 contains five papers that address regional aspects important to the genesis of gold deposits in northern nevada and five papers that present detailed studies of epithermal deposits and districts all of the regional papers are pertinent to carlin type gold deposits because they address the age of mineralization arehart et al 2003 origin and evolutionary history of the northwest striking mineral belts that localize many deposits grauch et al 2003 nature of the middle and lower crust below these mineral belts howard 2003 district to deposit scale stream sediment and lithogeochemical anomalies theodore et al 2003 and stratigraphy and structure of a district located along a northeast striking lineament peters et al 2003 the nine papers in this second special issue focus on an array of problems pertinent to genetic and exploration models for carlin type deposits in northern nevada fig 1 these investigations sort out and characterize the sequence of deformational igneous and hydrothermal events in mining districts constrain the age of mineralization map paleothermal gradients identify structures and lithologies that are preferentially mineralized ascertain processes of ore formation determine sources of ore fluid components and define fluid flow paths a common theme among these papers is inheritance whereby older features in the mineral belts influence ore formation in subsequent carlin type hydrothermal systems three types of inheritance are inferred by one or more of these investigations 1 structural inheritance where older faults are reactivated during subsequent contractional and or

seismic evidence for a fossil mantle plume beneath south america and implications for plate driving forces a teleseismic travel time study reveals the presence of a fossil plume in the deep upper mantle beneath brazil which has apparently remained geographically fixed with respect to the overly ing continent despite thousands of kilometres of plate motion this result implies that the upper mantle and lithosphere beneath south america have remained coupled since the breakup of gondwanaland and may provide an answer to the long standing question of what forces drive continental plates

ramparts and walls building techniques of kites in the negev highland abstract remarkably the largest archaeological game traps ever recorded in the near east are found in arid landscapes both on plains and in hilly terrains this paper describes the construction methods used on steep slopes in the negev highland apparently in some cases a massive rampart rather than a free standing wall was built around the trap s head the details reflect careful planning and heavy duty work including leveling and then using more than 100 t of stone for the rampart construction new 14 c data date the sayarim site to the early bronze age minimum age and the pitam site to pre late bronze age

geochemical studies of abyssal lavas recovered by dsrv alvin from eastern galapagos rift inca transform and ecuador rift 2 phase chemistry and crystallization history a diverse suite of lavas recovered by dsrv alvin from the eastern galapagos rift and inca transform includes mid ocean ridge tholeiitic basalts morb iron and titanium enriched basalts feti basalts and abyssal andesites rock types transitional in character ferrobasalts and basaltic andesites were also recovered the most mafic glassy basalts contain plagioclase augite and olivine as near liquidus phases whereas in more fractionated basalts pigeonite replaces olivine and iron titanium oxides crystallize plagioclase crystallizes after pyroxenes and iron titanium oxides in andesites possibly due to increased water contents or cooling rates apatite phenocrysts are present in some andesitic glasses ovoid sulfide globules are also common in many lavas compositional variations of phenocrysts in glassy lavas reflect changes in magma chemistry temperature of crystallization and cooling rate the overall chemical variations parallel the chemical evolution of the lava suite and are similar to those in other fractionated tholeiitic complexes elemental partitioning between plagioclase pyroxene and olivine glass pairs suggests that equilibration occurred at low pressure in a rather restricted temperature range various geothermometers indicate that the most primitive morb began to crystallize between 1150 and 1200 c with fo2 10 7 atm coexisting iron titanium oxides in more evolved lavas yield temperatures 1025 c to as low as 910 c withfo2 from 10 8 to 10 12 atm ph 2 o could have been as high as 1 kbar during andesite crystallization compositions of the lavas from the galapagos rift follow the experimentally determined 1 atm qfm liquid line of descent least squares calculations for the major elements indicate that the entire suite of lavas can be produced by fractional crystallization of successive residual liquids from a morb parent magma feti basalts represent 30 65 cumulative weight percent crystallization of plagioclase augite and olivine an additional 30 50 fractionation of pyroxenes plagioclase titanomagnetite and possible apatite is required to generate andesite from feti basalt liquids the presence of partially resorbed mafic xenocrysts in some andesites feti basalt inclusions in these xenocrysts high silica glass inclusions in basaltic andesites and the transitional chemistry of basaltic andesites are evidence that some magma mixing occurred during crystal fractionation the diversity of lava types recovered at single dive sites suggests that low pressure fractional crystallization is a very efficient process beneath the eastern galapagos rift and that isolated magma bodies must be present at shallow levels beneath the accretionary locus voluminous feti basalts erupted at the rift transform intersection are genetically related to the rift lavas but their restricted chemistry reflects different thermal and tectonic controls on their petrogenesis

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

the long canyon deposit anatomy of a new off trend sedimentary rock hosted gold discovery in northeastern nevada the long canyon deposit is a new sedimentary rock hosted gold discovery located in northeastern nevada over 150 km east of the well explored carlin trend unlike most examples of this type the deposit is hosted in a sequence of cambro ordovician carbonate rocks deposited in a periodically emergent platform to shelf edge environment the strata were strongly deformed and weakly metamorphosed during a mid mesozoic orogenic event that segmented and extended a brittle 80 m thick dolomite layer into a series of elongate n to ne trending boudin blocks while the enclosing limestone underwent ductile deformation this unusual structural architecture focused subsequent contractional deformation as well as tertiary extensional brittle deformation forming pathways for meteoric and hydrothermal fluids which formed and enlarged dissolution cavities and dissolution breccias along boudin margins boudin necks and normal faults gold occurs primarily in zones of polyphase dissolution breccias that are localized in minor faults and fold hinges in the structurally complex areas along and adjacent to dolomite block margins two general phases of brecciation are present including calcareous breccias and later fe as oxide rich breccias which carry the highest gold grades breccias are accompanied by variable pervasive silicification alteration assemblages include zoned dickite white mica hematite in the cores of mineralized zones with peripheral kaolinite sericite and goethite hematite and ultimately more distal illite smectite local preservation of fine grained sulfides encapsulated in calcite and silica suggests that the iron oxide assemblages may represent oxidation of fine grained pyrite marcasite arsenopyrite assemblages as currently defined the long canyon deposit extends for 3 km in a northeasterly direction and consists of several subparallel zones of mineralization focused on boudin block margins boudin necks and incipient boudins it contains a combined measured indicated and inferred estimated resource comprising 2 2 million ounces of gold at an average grade of 2 3 g t gold metallurgical work indicates that the deposit is highly amenable to cyanidation by heap leaching or milling the long canyon deposit is open to further exploration and resource additions and is shallowly buried relatively high grade and thoroughly oxidized the stratigraphic position of the long canyon deposit on the paleozoic shelf and its location well east of the majority of identified major trends indicate significant potential for new discoveries in this underexplored area of eastern nevada

phyllosilicates in hydrothermally altered basalts from dsdp hole 504b leg 83 a tem and aem study phyllosilicates occurring as replacements of olivine clinopyroxene and interstitial materials and as veins or fracture fillings in hydrothermally altered basalts from dsdp hole 504b leg 83 have been studied using transmission and analytical electron microscopy the parageneses of phyllosilicates generally change systematically with depth and with the degree of alteration which in turn is related to permeability of basalts saponite and some mixed layer chlorite smectite are the dominant phyllosilicates at the top of the transition zone chlorite corrensite and mixed layer chlorite corrensite occur mainly in the lower transition zone and upper levels of the sheeted dike zone chlorite talc and mixed layer talc chlorite are the major phyllosilicates in the sheeted dike zone although replacement of talc or ohvine by saponite is observed the phyllosilicates consist of parallel or subparallel discrete packets of coherent layers with packet thicknesses generally ranging from 100 a to a few hundred a the packets of saponite layers are much smaller or less well defined than those of chlorite corrensite and talc indicating poorer crystal linity of saponite by contrast chlorite and talc from the lower transition zone and the sheeted dike zone occur in packets up to thousands of a thick the si si al ratio of these trioctahedral phyllosilicates increases and fe fe mg decreases in the order chlorite corrensite saponite and talc these relations reflect optimal solid solution consistent with minimum misfit of articulated octahedral and tetrahedral sheets variations in composition of hydrothermal fluids and precursor minerals especially in si si al and fe fe mg ratios are thus important factors in controlling the parageneses of phyllosilicates the phyllosilicates are generally well crystallized discrete phases rather than mixed layered phases where they have been affected by relatively high fluid rock ratios as in high permeability basalts in veins or areas adjacent to veins intense alteration in basalts with high permeability indicating high fluid rock ratios is characterized by pervasive albitization and zeolitization minimal alteration in the basalts without significant albitization and zeolitization is characterized by the occurrence of saponite mixed layer chlorite smectite in the low temperature alteration zone and mixed layer chlorite corrensite or mixed layer talc chlorite in the high temperature alteration zone textural non equilibrium for phyllosilicates is represented by mixed layering and poorly defined packets of partially incoherent layers the approach to textural equilibrium was controlled largely by the availability of fluid or permeability

overlapping cretaceous and eocene alteration twin creeks carlin type deposit nevada we report here new ar ar dates for adularia illite and muscovite from the twin creeks carlin type deposit all illite samples were obtained from altered intrusive and volcanic rocks that are interlayered with mineralized sedimentary rocks thus largely avoiding complications caused by detrital illite and muscovite in sedimentary rocks interpretation of the illite age spectra was based in part on newly developed models for the behavior of argon in fine grained illite the results indicate that all adularia in the megapit area formed at about 42 ma eocene whereas most illite in the megapit area and muscovite in alteration zones beneath chimney creek formed at about 109 to 103 ma cretaceous additional events involving formation of illite at 200 ma and muscovite at about 310 ma were also observed but were not widespread enough to be considered significant our eocene date for adularia is in complete agreement with previous measurements on adularia from twin creeks but our cretaceous dates are considerably older than dates determined in previous studies of the nearby getchell district particularly the osgood stock which has a date of 98 to 92 ma our results suggest that the osgood stock is part of a more protracted cretaceous igneous hydrothermal event that took place between about 109 and 83 ma the relationship of adularia and illite to gold mineralization is not completely clear making it impossible to use our data to assign a unique age to the twin creeks deposit we favor an eocene age for some gold mineralization because the adularia that we dated is intimately intergrown with auriferous arsenian pyrite typical of carlin type mineralization however adularia has a very restricted distribution in the deposit and temperatures at which it formed were probably not great enough to reset cretaceous and older ages of illite illite is much more widespread and its association with gold is supported by correlation between gold content and k al ratios of mineralized material thus cretaceous illitization was at least an important ground preparing event at twin creeks and it is not possible to show from our data that gold ore or protore were not deposited during this event

alteration and mineralization of an oceanic forearc and the ophiolite ocean crust analogy mineralogical chemical and isotopic o c s and sr analyses were performed on minerals and bulk rocks from a forearc basement section to understand alteration processes and compare with mid ocean ridges mor and ophiolites ocean drilling program hole 786b in the izu bonin forearc penetrates 103 m of sediment and 725 m into volcanic flows breccias and basal dikes the rocks comprise boninites and andesites to rhyolites most of the section was affected by low temperature 100 c seawater alteration with temperatures increasing downward the rocks are partly 5 25 altered to smectite fe oxyhydroxide calcite and phillipsite and exhibit gains of k rb and p loss of ca variable changes in si na mg fe sr and y and elevated 18o and 87sr 86sr higher temperatures 150 c in the basal dikes below 750 m led to more intense alteration and formation of chlorite smectite corrensite albite k feldspar and quartz chlorite a 5 m thick hydrothermally altered and pyritized zone at 815 m in the basal dikes reacted with mixtures of seawater and hydrothermal fluids to mg chlorite albite and pyrite and gained mg and s and lost si and ca focused flow of hydrothermal fluids produced serialization halos na k sericite quartz pyrophyllite k feldspar and pyrite along quartz veins at temperatures of 200 250 c high 87sr 86sr ratios of chloritized 0 7055 and sericitized 0 7065 rocks indicate involvement of seawater via mixing with hydrothermal fluids low 34s of sulfide 2 to 5 5 and sulfate 12 5 are consistent with input of magmatic so2 into hydrothermal fluids and disproportionation to sulfide and sulfate alteration processes were generally similar to those at mors but the arc section is more intensively altered in part because of the presence of abundant glassy rocks and mafic phases the increase in alteration grade below 750 m and the mineralization in the basal dikes are analogous to changes that occur near the base of the volcanic section in mor and the troodos ophiolite

gabbroic xenoliths and host ferrobasalt from the southern juan de fuca ridge rare isotropic gabbroic xenoliths occur in sheet and lobate flow fragments of nearly aphyric ferrobasalt collected along a 12 km section of the southern juan de fuca ridge xenoliths comprise 1 of the dredge contents and range in size from 1 cm3 glomerocryst to 240 cm3 the xenoliths have ophitic to intersertal texture with 5 50 interstitial glass of ferrobasaltic composition more evolved than the host lava on the basis of texture and mineralogy the xenoliths have been subdivided into three types type i plagioclase olivine glass type ii plagioclase augite glass olivine and type iii plagioclase augite olivine glass pigeonite partially inverted fe ti oxides mineral and glass inclusion compositions suggest a sequence of evolution for the three xenolith types in which type i is the least evolved and type iii is the most evolved application of a graphical pyroxene geothermometer to augite in xenolith types ii and iii yields crystallization temperatures of 1100 1200 c and to host lamellae pairs in inverted pigeonite yields subsolidus equilibrium temperatures of 1100 1150 c coexisting titanomagnetite ilmenite pairs in type iii xenoliths yield temperature estimates of 1000 1070 c and log f02 9 7 to 10 8 we infer that the xenoliths represent the partially crystalline mush boundary zone of a magma chamber based on the abundance of interstitial glass zonation of mineral grains in the most crystalline samples and coherence of chemical trends between interstitial glass glass inclusions and mineral phases the evolved composition of the xenoliths provides evidence for the presence of melts more fractionated than the host ferrobasalt in the magma chamber the erupted ferrobasalt is a hybrid lava formed by mixing these highly evolved melts with more primitive melts

p and s wave scattering from mantle plumes we discuss a new method for seismically imaging mantle plumes by modeling the plume as a vertical elastic cylinder and analyzing the effects of the cylinder on an incoming teleseismic plane wave the incident plane wave decomposes into cylindrical waves for which the scattering coefficients can be computed by imposing continuity of stress and displacement at the cylinder boundary we extended this development to cylinders with gradational boundaries by numerically integrating the elasto dynamic equations synthetic seismograms calculated using this technique for a cylinder with a range of parameters appropriate for a mid pacific mantle plume epicentral distance 50 90 t 200 400 k boundary width 0 60 km have the following characteristics 1 a delay of the p wave arrival for an observational point directly behind the plume 2 the focusing of energy into the region directly behind the plume where the focal length is 2 times the plume diameter and 3 the presence of secondary phases and especially the excitation of a transverse component whose amplitude is up to 25 of the radial amplitude most of the phases arise from p to s conversions and reflections at the boundary of the cylinder increasing the epicentral distance reduces the focal length and strengthens the focusing effect for receivers near the plume increasing the thermal anomaly reduces both the focal length and the amplitude of the secondary phases using gradational boundaries does not affect the scattering of low frequency waves but it does cause the reduction or disappearance of the secondary arrivals at high frequency because of the reduced efficiency of the conversions synthetic seismograms for an incident s wave arrival show a time delay more than twice as long as for the p wave and stronger focusing but only weak secondary phases considerable coupling takes place between sv and sh on the basis of these findings we outline an observational program to image the hawaiian plume stations should preferably be installed close to but not directly on top of the most likely position of the plume axis

sources of iron for pyrite formation in marine sediments more than two hundred aerobic continental margin aerobic deep sea dysaerobic and anaerobic euxinic sediments have been examined for their variations in different operationally defined iron fractions each of which represents a different reactivity towards dissolved sulfide aerobic continental margin deep sea and dysaerobic sediments contain similar contents of highly reactive iron dithionite soluble iron plus pyrite iron poorly reactive iron iron soluble in hcl less that soluble in dithionite and unreactive iron total iron less that soluble in hcl by contrast non turbidite euxinic samples from the black sea as well as euxinic samples from the cariaco basin and framvaren are enriched in highly reactive iron these sediments contain a small lithogenous fraction and a large biogenous organic c rich fraction which decays by sulphate reduction in an iron rich water column to form pyrite rich sediment other anaerobic euxinic samples from the black sea orca basin and kau bay contain lower concentrations of biogenous sediment and are not therefore enriched in highly reactive iron degrees of pyritization dop for all the aerobic dysaerobic and anaerobic euxinix samples except those low in biogenous material are consistent with analogous ancient sediments and indicate that most pyrite formation occurs form the highly reactive iron fraction

metamorphic sulfide silicate equilibria in the massive sulfide deposits at ducktown tennessee examination of the distribution and composition of the ferromagnesian silicates in the country rocks wall rocks and ore zone at ducktown tennessee indicates that high values for f sub s 2 and f sub o 2 during metamorphism have produced distinct transitions in the mineralogy and chemistry of the ferromagnesian silicates in the wall rocks and ore zone these transitions include from country rocks to ore zone 1 appearance of primary chlorite 2 disappearance of staurolite and 3 disappearance of garnet the compositions of the ferromagnesian silicates country rocks to ore zone vary as follows x bt fe 0 62 to 0 19 x gt fe 0 85 to 0 37 x stt fe 0 84 to 0 42 and x chl fe 0 51 to 0 14 these differences parallel the transitions in oxide sulfidegraphite mineralogy described in nesbitt and kelly 1980 in the wall rocks at ducktown application of experimental phase equilibria to the assemblages indicates that the compositions of the ferromagnesian silicates were controlled by complex reactions involving muscovite pyrrhotite quartz and the ferromagnesian silicates good agreement is observed between f sub s 2 f sub o 2 conditions inferred from oxide sulfide graphite assemblages and ferromagnesian silicate assemblages the observation of variations in the mineralogy and chemistry of the ferromagnesian silicates around metamorphosed ore deposits can double or triple the target area in addition an improved understanding of the variations in f sub s 2 and f sub o 2 and the resultant reactions among the ferromagnesian silicate in the wall rocks of metamorphosed ore deposits aids in the interpretation of the origin of such deposits

origin of spilitic rocks from the southern slope of the greater caucasus a suite of 39 spilitic basalts and andesites of bajocian age was analyzed for the major elements and li rb sr ba sc v cr co ni and seven ree hf ta and th were analyzed in 20 samples sr and o isotope ratios have been determined for selected whole rock samples the occurrence of biotite and actinolite indicates maximum temperatures of hydrothermal reactions slightly above 400 c prehnite pumpellyite and epidote formed at lower temperatures chlorite and sodic plagioclase are abundant products of hydrothermal seawater alteration a comparison between weakly and intensively altered basalts demonstrates that the largest proportion of al v cr co ni and the ree was immobile during spilitization enrichment in 18o is correlated with the gain of h2o co2 na2o and li which exceeds pre spilitic levels initial 87sr86sr ratios for 10 whole rock samples range from 0 7040 to 0 7049 relics of fresh clinopyroxene phenocrysts from the spilitic basalts and andesites have initial 87sr86sr ratios between 0 7037 and 0 7044 and 18o values between 4 5 and 5 6 smow whole rock samples have higher 87sr86srinit ratios and higher 18o values when compared to their clinopyroxene phenocrysts which is explained as a result of alteration by seawater average 18o values of 11 smow along with the enrichment of li suggest that the major alteration occurred at temperatures below 200 c the spilitic basalts have higher average sc v cr co and ni concentrations relative to the spilitic andesites the abundances of these elements in the spilitic andesites are quite similar to those in typical orogenic andesites the correlation of cr and ni in both the spilitic basalts and the spilitic andesites is explained by various degrees of fractionation of olivine and clinopyroxene although ta and th may have been mobilized to minor extents the majority of the analyzed samples has hf ta th relations which are typical of subduction related volcanic rocks assuming an olivine tholeiite as the parental melt from which the basaltic and andesitic magmas were derived the average la and ce concentrations in these rocks require a metasomatically enriched peridotite source with 5 times chondritic la and ce abundances the coexistence of andesites and basalts at the southern slope of the greater caucasus along with the occurrence of ophiolites in the lesser caucasus suggests that both could be related to a middle jurassic subduction zone in this region

traveltime dispersion in an isotropic elastic mantle strong lower mantle signal in differential frequency residuals we study wavefield effects of direct p and s waves in elastic and isotropic 3 d seismic structures derived from the temperature field of a high resolution mantle circulation model more specifically we quantify the dispersion of traveltime residuals caused by diffraction in structures with dynamically constrained length scales and magnitudes of the lateral variations in seismic velocities and density 3 d global wave propagation is simulated using a spectral element method intrinsic attenuation i e dissipation of seismic energy is deliberately neglected so that any variation of traveltimes with frequency can be attributed to structural effects traveltime residuals are measured at 15 22 5 34 and 51 s dominant periods by cross correlation of 3 d and 1 d synthetic waveforms additional simulations are performed for a model in which 3 d structure is removed in the upper 800 km to isolate the dispersion signal of the lower mantle we find that the structural length scales inherent to a vigorously convecting mantle give rise to significant diffraction induced body wave traveltime dispersion for both p and s waves the difference between long period and short period residuals for a given source receiver pair can reach up to several seconds for the period bands considered here in general these differential frequency residuals tend to increase in magnitude with increasing short period delay furthermore the long period signal typically is smaller in magnitude than the short period one that is wave front healing is efficient independent of the sign of the residuals unlike the single frequency residuals the differential frequency residuals are sur prisingly similar between the lower mantle and the whole mantle model for corresponding source receiver pairs the similarity is more pronounced in case of s waves and varies between different combinations of period bands the traveltime delay acquired in the upper mantle seems to cancel in these differential signals depending on the associated wavelengths and the length scales of structure at shallow depth differential frequency residuals may thus prove useful to precondition tomographic inversions for the lower mantle structure such as to reduce the influence of the upper mantle for certain length scales overall standard deviations of the diffraction induced traveltime dispersion between the longest 51 s and the shortest 15 s period considered here are 0 6 and 1 0 s for p and s waves respectively for comparison the corresponding standard deviations of the 15 s residuals are 1 0 s and 2 8 s in the lower mantle model standard deviations are 0 3 and 0 6 s respectively which gives an average lower mantle contribution to the total dispersion of 50 per cent for p waves and 60 per cent for s waves

corrensite and mixed layer chlorite corrensite in metabasalt from northern taiwan tem aem empa xrd and optical studies many chloritic minerals in low grade metamorphic or hydrothermally altered mafic rocks exhibit abnormal optical properties expand slightly upon glycolation expandable chlorite and or have excess alvi relative to aliv as well as significant ca k and na contents chloritic minerals with these properties fill vesicles and interstitial void space in low grade metabasalt from northern taiwan and have been studied with a combination of tem aem empa xrd and optical microscopy the chloritic minerals include corrensite which is an ordered 1 1 mixed layer chlorite smectite and expandable chlorite which is shown to be a mixed layer chlorite corrensite corrensite and some mixed layer chlorite corrensite occur as rims of vesicles and other cavities while later formed mixed layer chlorite corrensite occupies the vesicle cores the tem observations show that the mixed layer chlorite corrensite has ca 20 and the corrensite has ca 50 expandable smectite like layers consistent with xrd observations and with their abnormal optical properties the aem analyses show that high si and ca contents high alvi aliv and low fevi fe mg vi ratios of chlorites are correlated with interstratification of corrensite or smectite like layers in chlorite the aem analyses obtained from 200 500 a thick packets of nearly pure corrensite or chlorite layers always show that corrensite has low aliv siiv and low fevi fe mg vi while chlorite has high aliv siiv and high fevi fe mg vi this implies that the trioctahedral smectite like component of corrensite has significantly lower aliv siiv and fevi fe mg vi the ratios of fevi fe mg vi and aliv siiv thus decrease in the order chlorite corrensite smectite the proportions of corrensite or smectite like layers relative to chlorite layers in low grade rocks are inferred to be controlled principally by fe mg ratio in the fluid or the bulk rock and by temperature compositional variations of chlorites in low grade rocks which appear to correlate with temperature or metamorphic grade more likely reflect variable proportions of mixed layered components the assemblages of trioctahedral phyllosilicates tend to occur as intergrown discrete phases such as chlorite corrensite corrensite smectite or chlorite corrensite smectite a model for the corrensite crystal structure suggests that corrensite should be treated as a unique phase rather than as a 1 1 ordered mixed layer chlorite smectite

source of iron for sulfidation and gold deposition twin creeks carlin type deposit nevada this study was undertaken to determine the source of iron in comus formation sedimentary rocks that were sulfidized during deposition of gold in the megapit area of the twin creeks carlin type deposit sedimentary rocks in and near the megapit contain ferroan dolomite largely as overgrowths on iron poor dolomite iron to form these overgrowths appears to have been released from mafic volcanic rocks that are interlayered with the sedimentary rocks these igneous rocks have undergone two stages of hydrothermal alteration the first stage involved formation of albite and iron rich chlorite possibly caused by interaction with seawater the second stage involved destruction of the iron rich chlorite by illite or sericite which released iron to form ferroan dolomite in the sedimentary rocks comparisons show that transfer of iron from the igneous rocks to the sedimentary rocks can account for the present distributions of iron in these rocks relative to basalts comus formation igneous rocks are enriched in iron and potassium these results suggest that ferroan dolomite in sedimentary rocks is not solely a product of diagenetic processes and can form when iron is released from adjacent iron bearing igneous rocks recognition of this additional mechanism for formation of ferroan dolomite expands the range of geologic settings that can be favorable for formation of gold deposits formed by sulfidation

new constraints on the properties of the yellowstone mantle plume from p and s wave attenuation tomography 1 we estimate attenuation t for teleseismic p and s arrivals to seismometers in the yellowstone intermountain seismic array tomographically invert these data for upper mantle qp 1 and qs 1 structure and with the aid of the upper mantle velocity model of waite et al 2006 interpret the results for mantle temperature partial melt and water content because attenuation analysis is susceptible to contamination by noise we employ time and frequency domain analyses and a careful assessment of the uncertainty associated with each estimate by using the misfit between actual and predicted traces or spectra the greatest noise source is signal generated noise which affects s wave results more than p wave results s waves also appear more susceptible to the effects of plume focusing we find the upper mantle to be highly attenuative but that above 200 250 km the low velocity plume is progressively less attenuative than the adjacent mantle with decreasing depth we conclude that water dissolved in mantle minerals causes the upper mantle to be highly attenuative that the yellowstone plume is only 50 c warmer than the surrounding mantle below the north america lithosphere and that melting within this plume begins at depths of 200 250 km we attribute the lower attenuation in the partially molten plume to the dehydration of the solid matrix as water partitions into the melt the source of the upper mantle hydration is attributed to subduction including water flux from a hydrated transition zone and laramide age shallow subduction and direct hydration of north america lithosphere

timing of gold and arsenic sulfide mineral deposition at the getchell carlin type gold deposit north central nevada pregold mineralization at the getchell carlin type gold deposit includes quartz and base metal vein mineralization associated with intrusion of a cretaceous granodiorite stock the veins contain minor pyrite and trace chalcopyrite arsenopyrite galena and sphalerite the pyrite is moderately coarse and in thin section has high relief is well polished and is fractured and locally cemented by the gold ore assemblage white micas are associated with veins near the granodiorite intrusion gold was not observed or detected by fire assay analyses of samples or electron microprobe analyses of pyrites microprobe analyses show that pregold pyrites have near stoichiometric compositions variable low arsenic is present in pyrite in samples overprinted by gold mineralization secondary ion mass spectrometry sims analyses detected trace gold in the coarse near stoichiometric pyrite in overprinted samples the pregold vein assemblage was fractured and cemented by gold ore stage mineralization the gold ore stage assemblage consists of gold and arsenic enriched pyrite and marcasite encompassed by jasperoid and drusy quartz and local late fluorite orpiment and galkhaite the consistent spatial association of jasperoid and ore pyrite reflects their near contemporaneous formation the ore pyrite occurs as either fine irregularly shaped grains or rims on earlier gold free pyrite in thin section the pyrite is visibly distinct and has a low polishing relief and a poor polish sims and electron microprobe analyses show that ore pyrites commonly contain 8 to 11 wt percent arsenic and as much as 2 400 ppm gold near the end of the ore stage fluorite orpiment and galkhaite precipitated locally in open space created by faulting or limestone dissolution late ore stage mineralization consists dominantly of open space filling realgar and calcite with minor quartz stibnite and framboidal pyrite realgar conforms to euhedral crystal faces of ore stage quartz fluorite and galkhaite calcite filled most remaining open space and conforms to euhedral crystal faces of quartz fluorite and realgar significant textural observations argue against an earlier interpretation that the gold mineralization and arsenic minerals formed as two discrete events separated by 40 m y in a few areas where calcite and realgar are in contact with ore stage quartz containing gold bearing pyrite realgar and calcite enclose gold bearing pyrite grains massive realgar and calcite distal from these contacts do not contain the gold bearing pyrite the presence of ore pyrite in realgar calcite and jasperoid requires close timing of these minerals and shows that there is no major time break separating ore stage and late ore stage mineral deposition textures indicate instead that the ore stage and late ore stage formed as part of a single evolving hydrothermal system the consistent successive overgrowth of younger minerals on perfectly preserved euhedral faces of older minerals supports the continuous evolution of the gold ore stage into the late ore stage these results are consistent with fluid inclusions that indicate that paragenetically successive minerals precipitated from an aqueous ore fluid with consistent salinity and gas contents but at declining temperatures results show that within error the 34 39 and 42 ma ages determined for fluorite and galkhaite at getchell and adularia at the nearby twin creeks mine respectively most closely approximate the timing of gold deposition at getchell

reactive iron in marine sediments abstract a combined field laboratory study has been undertaken to explore the mineralogy concentrations and reactivity towards sulfide of iron in marine sediments also considered is the importance of bacterial fe liberation in regulating pore water chemistry two contrasting marine environments are included one is the relatively fe poor foam site and the other is the fe rich sediment of the subaqueous mississippi delta results show that oxide minerals are the most important fe phases in early diagenetic pyrite formation however viewed separately lepidocrocite and ferrihydrite are more reactive towards sulfide than goethite and hematite when fe oxides are present in relatively high concentrations dissolved sulfide is nearly absent from sediment pore waters with concomitant high concentrations of dissolved fe even in the presence of active sulfide production by sulfate reduction a combination of experimental results and diagenetic modelling shows that in some sediments pore water fe finds it origin in the bacterial reduction of iron oxides this seems the case even though greater amounts of fe are reduced by reaction of sulfide with iron oxides it appears that distinct microenvironments may exist in marine sediments where in one microenvironment sulfide reacts with fe oxides locally precipitating fe sulfide minerals in another fe reduced and solubilized by microorganisms migrates freely into solution

rayleigh wave dispersion along the hawaiian swell a test of lithospheric thinning by thermal rejuvenation at a hotspot summary in this paper we present a seismological test of the thermal rejuvenation model for the formation of the hawaiian swell the archetype of a midplate hotspot swell twostation measurements of rayleigh wave group and phase velocities between midway atoll and o ahu provide the basis for the test comparison of the observed dispersion curves with age dependent regionalized dispersion curves suggests that the swell has the seismic velocity structure of 50 110 myr old lithosphere rather than that of 20 50 myr old lithosphere as suggested by the rejuvenation model joint inversion of the group and phase velocity curves for isotropic velocity models yields an estimate of 100 km for the seismic lithosphere s thickness along the swell this value is difficult to reconcile with the 40 50 km thickness demanded by the rejuvenation model preliminary measurements of the coefficients of anelastic attenuation are also presented although the accuracy of these values is difficult to assess they too appear to be consistent with thick lithosphere

gold mineralization in the guilaizhuang deposit southwestern shandong province china insights from phase relations among sulfides tellurides selenides and oxides abstract the guilaizhuang gold deposit is composed of limestone and breccia type ores in the limestone ores gold is hosted by pyrite as bearing pyrite and arsenopyrite whereas in the breccia ores tellurides are the main gold carriers selenium often isomorphously substitutes for sulfur in sulfides in the limestone ores but occurs as selenides in the breccia ores hematite is the only oxide in the breccia ores associated with barite and other tellurides in this paper the thermodynamic parameters especially the gibbs free energies of formation and reaction of related sulfides tellurides selenides and oxides are computed to explain the distinct mineral styles in the limestone and breccia type ores we construct the phase relations among minerals delineated by phase diagrams at 250 c the temperature for gold precipitation in the guilaizhuang deposit according to the phase relations between sulfides and selenides we propose that gold has been transported as au hs hse 2 and released during rapid formation of pyrite as pyrite and arsenopyrite at log f s 2 g between 12 8 and 11 4 250 c in the breccia ores log f te 2 g and log f se 2 g are constrained within 12 9 to 9 4 and 12 4 to 6 9 250 c respectively based on the mineral assemblages observed in the ores the phase relations between tellurides and selenides indicate that calaverite the only stable gold telluride mineral in nature was not stable in the breccia ores at 250 c during the gold precipitation based on empa analysis the so called calaverite revealed from microscopic observations is in fact au ag telluride probably derived from the decomposition of sylvanite auagte 4 and petzite auag 3 te 2 the only two stable au ag tellurides in nature at high temperature however we do not exclude the possibility that calaverite occurs in this deposit which may have precipitated in early relatively high tellurium concentration stage preceding gold mineralization selenium was not detected in gold bearing tellurides implying that selenide is unrelated with gold precipitation directly in the breccia ores but can buffer the tellurium fugacity and thus influence the gold precipitation indirectly furthermore in log f o 2 g range of 35 4 at 250 c the majority of the tellurides and selenides can be stable in the breccia ores but residual sulfides are proposed to be oxidized into sulfate minerals which could also free some gold into ores although less important relative to that formed from telluride decomposition

40 ar 39 ar dating and mineral paragenesis for carlin type gold deposits along the getchell trend nevada evidence for cretaceous and tertiary gold mineralization the orebodies at the getchell and twin creeks mines were studied through mineral paragenesis geologic relationships and 40 ar 39 ar dating mineral paragenetic relationships are based on observations made during the logging of 18 000 m of drill cuttings and core and crosscutting relationships recognized in the field and main pit at the getchell mine ages for igneous and mineralizing events were determined through 40 ar 39 ar incremental heating analyses of 15 samples of biotite k feldspar sericite and vein adularia a thermal history for the area was also developed using k feldspar multiple diffusion domain results and age determinations on cogenetic minerals which have different argon closure temperatures mineral paragenetic relationships and 40 ar 39 ar dating at the getchell and twin creeks deposits document that gold mineralization occurred in five stages between 95 and 42 ma stage 1 pyrrhotite quartz arsenopyrite chalcopyrite biotite gold mineralization in skarn is associated with 95 ma dacite dikes and granodiorite plugs stage 2 chalcopyrite sphalerite galena pyrite arsenopyrite or gold and silver mineralization in skarn is related to the 92 ma osgood mountains stock secondary k feldspar and sericite in stage 3 quartz pyrite kaolinite gold mineralization formed at 83 ma and are not related to 95 or 92 ma igneous activity stage 4 low grade quartz pyrite gold mineralization formed at about 75 ma in the matrix of a breccia pipe that cuts 83 ma silicic and argillic ores adularia intergrown with stibnite orpiment pyrite quartz gold or realgar mineralization yields precise 40 ar 39 ar plateau ages of 42 11 or 0 43 and 41 90 or 0 25 ma for stage 5 this study documents the temporal complexity of mineralization and igneous activity at the getchell and twin creeks deposits minor gold in skarn type mineralization stages 1 and 2 associated with 95 and 92 ma igneous intrusions preceded and was not genetically related to carlin type gold mineralization stages 3 and 5 the carlin type deposits at getchell and twin creeks therefore represent a composite orebody formed by the overprinting of 83 ma stage 3 quartz pyrite gold by 42 ma stage 5 orpiment stibnite pyrite gold mineralization

three dimensional modeling of plume lithosphere interaction in order to understand better the dynamics of hot spots such as hawaii we present a three dimensional numerical model for the interaction of a thermal plume with a moving lithosphere the model domain is a rectangular box filled with fluid whose viscosity depends upon temperature and pressure the lithosphere is represented by a layer of cold highly viscous fluid moving with an imposed horizontal velocity u in the x direction and a thermal plume is generated by a circular temperature anomaly on the bottom of the box the steady flow is determined numerically using a hybrid spectral finite difference technique the flow is characterized by a stagnation streamline of width y x1 5 that represents the edge of the spreading plume material we illustrate the detailed behavior of the model using the example of the hawaiian plume our best fitting hawaiian model is obtained by adjusting the plume buoyancy flux b until the predicted topography anomaly matches the observed hawaiian swell topography we find b 4100 kg s 1 which implies a plume radius of 90 km for an assumed plume mantle temperature contrast of 300 c the predicted topography is supported primarily by density anomalies beneath the lithosphere and cannot be explained by lithospheric erosion we therefore conclude that the classical lithospheric reheating model is unable to account for hotspot swells the horizontal flux of buoyancy associated with the swell exceeds b by up to 80 suggesting that current estimates of b for mantle plumes are too high empirically derived scaling laws for the width of the stagnation streamline and for the topography anomaly exhibit power law dependencies on b and u that agree well with those predicted by the refracted plume model of olson 1990 the principal weakness of the model is that the predicted geoid topography ratio of 0 010 for the hawaiian swell is about twice the observed value of 0 004 0 006