salt precipitation in magmatic hydrothermal systems associated with upper crustal plutons magmatic hydrothermal systems associated with upper crustal plutons strongly influence volcanic and geothermal processes and form important mineral deposits fluids released from plutons are commonly saline and undergo phase separation into high salinity brines and low salinity vapors upon ascent while brine vapor immiscibility has been extensively studied precipitation of solid salt during phase separation in magmatic hydrothermal systems has generally been considered a rare phenomenon here we show that most porphyry deposits exhibit fluid inclusion evidence best interpreted by solid salt precipitation from ore forming solutions this interpretation naturally links thermodynamics numerical simulations and independent estimates of porphyry ore formation depths salt precipitation imposes major changes on the permeability of the system moreover salt precipitation has implications for ore formation along the liquid vapor halite curve the recognition of salt saturated systems is challenging but very relevant for understanding the evolution of magmatic hydrothermal systems

the significance of clathrates in fluid inclusions and the evidence for overpressuring in the broadlands ohaaki geothermal system new zealand the broadlands ohaaki geothermal system is host to an epithermal environment where gold silver transport and deposition involves deeply derived fluids containing up to 3 3 wt percent co 2 and 0 1 wt percent cl earlier fluid inclusion investigations show that most microthermometric data reflect boiling and mixing in the upper 2 km of the system under modern p t x fluid conditions here we report the microthermometric results for a single quartz crystal from 1 258 m depth well br 25 in the upflow zone of the geothermal system that reveal the presence of clathrates upon freezing due to anomalous concentrations of co 2 7 9 to 14 3 wt these inclusions occupy the core of the quartz crystal and have homogenization temperatures from 291 to 365 c but the anomalous concentrations of co 2 are likely artifacts of deep boiling and heterogeneous trapping of the resulting coexisting gas and liquid phases only a few fluid inclusions t h 300 t m of 1 4 to 1 6 c which lack clathrate reveal more realistic conditions of early quartz precipitation from a modestly overpressured fluid 140 bars relative to the prevailing hydrodynamic boiling conditions 110 bars microthermometric data t h 300 t m of 0 2 to 0 8 c for fluid inclusions in a later formed overgrowth of the quartz crystal match the modern p t x conditions at 1 258 m depth in the well the overall results of the study show that the clathrates are artifacts of two phase trapping of steam and liquid and that the deep liquid became overpressured locally probably due to mineral deposition and sealing of a permeable channel

fault trends on the seaward slope of the aleutian trench implications for a laterally changing stress field tied to a westward increase in oblique convergence 1 normal faults along the seaward trench slope sts commonly strike parallel to the trench in response to bending of the oceanic plate into the subduction zone this is not the circumstance for the aleutian trench where the direction of convergence gradually changes westward from normal to transform motion gloria side scan sonar images document that the aleutian sts is dominated by faults striking oblique to the trench west of 179 e and east of 172 w these images also show a pattern of east west trending seafloor faults that are aligned parallel to the spreading fabric defined by magnetic anomalies the stress strain field along the sts is divided into two domains west and east respectively of 179 e over the western domain sts faults and nodal planes of earthquakes are oriented oblique 9 46 to the trench axis and 69 90 to the magnetic fabric west of 179 e sts fault strikes change by 36 from the e w trend of sts where the trench parallel slip gets larger than its orthogonal component of convergence this rotation indicates that horizontal stresses along the western domain of the sts are deflected by the increasing obliquity in convergence an analytical model supports the idea that strikes of sts faults result from a superposition of stresses associated with the dextral shear couple of the oblique convergence and stresses caused by plate bending for the eastern domain most nodal planes of earthquakes strike parallel to the outer rise indicating bending as the prevailing mechanism causing normal faulting east of 172 w sts faults strike parallel to the magnetic fabric but oblique 10 26 to the axis of the trench on the basis of a coulomb failure criterion the trench oblique strikes probably result from reactivation of crustal faults generated by spreading

geometry of the subducted plate near kodiak island and lower cook inlet alaska determined from relocated earthquake hypocenters abstract using observations from 28 seismograph stations in southern alaska we have used the jhd method to relocate 341 well recorded events with focal depths beneath about 40 km which occurred between 56 4 n and 60 5 n and between 151 6 w and 156 5 w these hypocenters show that the wadati benioff zone has a dip of about 45 however its strike changes by about 15 beneath lower cook inlet at about 59 n near where the volcanic line also bends northward near this bend there is also a distinctly higher level of seismic activity than in the area to the southwest of 59 n a few events are situated about 20 km beneath the main wadati benioff zone in cook inlet and a focal mechanism we have determined for one of these differs from other mechanisms for events in this area focal mechanisms available from previous work show that at intermediate focal depths the area between 59 n and 60 5 n is controlled by horizontal north south compression and as such differs from the region to the southwest relying primarily on the observed change in strike and apparent thickness of the wadati benioff zone we interpret these data to suggest that the subducted lithophere separates into two distinct segments as it subducts beneath cook inlet because the direction of subduction is not perpendicular to the volcanic arc the events situated beneath the wadati benioff zone in cook inlet may be part of the southwestern plate segment although the geometry and available focal mechanisms for events beneath cook inlet are similar to what would be expected in a double wadati benioff zone we suggest instead that these observations result because we are at the boundary between two adjacent segments of the subducting plate

a stable isotope and fluid inclusion study of the qaleh zaricu au ag deposit khorasan province iran abstract the qaleh zari copper deposit located in south khorasan in the central lut region of iran is a polymetallic vein deposit with major amounts of cu au ag and minor amounts of pb zn and bi mineralization occurs in a series of nw se trending fault planes and breccia zones in paleogene andesitic to basaltic volcanic rocks argillization sericitization and propylitization characterize alteration halos bordering mineral veins the main ore minerals are chalcopyrite pyrite galena and sphalerite with quartz calcite and minor chlorite as the main gangue phases microthermometric measurements of fluid inclusions in cogenetic quartz indicate homogenization temperatures between 160 and 300 c and salinities from 1 to 4 wt nacl equiv boiling occurred in the mineralising fluids at 160 1000 m below the paleo water table at pressures of approximately 15 80 bar at various stages in the formation of the ore body the wide range of pressures and temperatures reflects the multi stage nature of the mineralization at qaleh zari the 18 o values in quartz relative to smow and 34 s values in chalcopyrite and galena relative to cdt range from 6 5 to 7 5 and 0 0 1 5 mean 7 0 respectively at 300 c calculated fluid 18 o values are close to 0 these data suggest a magmatic origin for sulfur and a surficial origin for the mineralizing fluid mineralization at qaleh zari is interpreted as epithermal and low sulfidation in style and was probably related to a deep seated magmatic system ore deposition was the result of boiling cooling and pressure reduction

geology fluid inclusion and stable isotope study of the yueyang ag au cu deposit zijinshan orefield fujian province china abstract the large yueyang ag au cu deposit is commonly regarded as a low sulfidation epithermal deposit in the zijinshan orefield fujian province southeastern china the ag ag cu orebodies hosted in the zijinshan granitic batholith are mainly stratoid and lens in shape and controlled by a series of nw trending listric faults with shallow dip angles four mineralization stages are recognized on the basis of mineral assemblage ore fabrics and crosscutting relationships of the ore veins namely pre ore pyrite sericite quartz chlorite main cu chalcopyrite pyrite sericite quartz bornite main ag au ag and au minerals pyrite quartz adularia calcite apatite chalcopyrite galena sphalerite and post ore quartz chalcedony calcite stages fluid inclusions fis in the deposit include aqueous liquid rich wl aqueous vapor rich wv and minor carbonic c and daughter mineral bearing s type ones wl subtype is the main inclusion type in the yueyang deposit accounting for more than 90 in proportion in each stage minor wv subtype inclusions occur in both the main cu and ag stages while the c type and s type ones are only observed in the main cu stage fluid inclusion and h o isotope study indicated that the ore forming fluid of the main cu stage is primarily magmatic vapor which further underwent fluid boiling and mixing with meteoric water while the ore forming fluid of the main ag stage is meteoric water dominated and the precipitation of silver and gold was mainly resulted from fluid boiling and the precipitation of other sulfides on the basis of the aforementioned geological fluid inclusion and stable isotope studies we proposed a two stage model for the yueyang deposit including a magmatic vapor related porphyry type cu mineralization and meteoric water related low sulfidation epithermal ag au cu mineralization although the porphyry cu mineralization is very limited in scale the mineralization and exhumation depths of the yueyang deposit are estimated to be 448 527 m and 18 97 m respectively by comparison with the exhumation depths of other deposits in the zijinshan orefield it is suggested that more epithermal deposits could be found in the southwest of the orefield due to less uplift and exhumation therein

seismicity of the forearc marginal wedge accretionary prism three different types of seismic data have been examined for seismic events occurring within the zone called the accreted wedge or forearc marginal wedge that underlies the inner trench wall of some arcs these types of data are 1 teleseismically recorded earthquakes that have been reported in the literature as occurring in major arc trench regions these events fail to demonstrate that earthquakes occur within the accreted wedge because the uncertainty of focal depth usually exceeds the depth dimension of the accreted wedge these data include many tsunamigenic earthquakes 2 local earthquakes located by combined ocean bottom seismograph and land networks in the arc trench region in the new hebrides and the central and eastern aleutian trench none of the more reliable of these hypocenters lies within the accreted wedge 3 s p intervals measured at stations on islands located on the outer ridge or at ocean bottom seismograph stations on the forearc marginal wedge these data do not show the existence of events occurring within the accreted wedge e g from 18 ocean bottom seismograph stations with a cumulative operation time of about 1 year the smallest s p time is about 2 5 s for events in the new hebrides and about 4 s for events in the adak and kodiak regions we found no s p time smaller than 2 s from 6 years of seismograms recorded at middleton island alaska and no s p time smaller than 4 s from 25 years of seismograms recorded on barbados all of the events could have occurred outside the forearc marginal wedge although we have located no events with hypocenters which are situated unambiguously within the accreted wedge we have located many events which occurred in the vicinity of the accreted wedge some occur in the upper portion of the benioff zone shallower than about 40 km and others occur in the leading edge of the overriding plate these locations reported in this paper which are within or close to the seismic networks comprise some of the most reliably located shallow events ever reported in convergent margins of island arcs the absence of seismic activity within the accreted wedge in any island arc that we studied suggests that the deformation evident in this region occurs aseismically

the effect of oceanic crustal structure on phase changes and subduction abstract recent numerical models for the thermal regime of subduction zones are in disagreement with observations of depth to formation of eclogite which require higher temperatures than those found in the models this discrepancy may be reconciled if more accurate geometric depictions of the subducted slab are incorporated in the modelling studies the transition from 15 dip along the thrust plane to 30 along the benioff zone starts at depths of about 40 km dehydration reactions and the gabbro eclogite phase change are presumed to increase the average density of the lithosphere and to cause the steepening of the subducted lithosphere the benioff zone seismicity which starts at about 40 km depth may itself be a result of stresses caused by the phase changes since the gabbro eclogite transformation occurs in gabbroic sections of the crust thickened crustal sections beneath oceanic plateaus will affect subduction first the width of the thrust zone is reduced due to the increased dip angle because of early transformation of lower and warmer crustal sections second incomplete conversion of the entire crustal thickness results in an average density intermediate between unsubducted oceanic lithosphere and fully transformed normal lithosphere the first effect leads to decreased thrust mechanism seismicity because the coupling between the plates is reduced the second effect leads to flat lying subcontinental subduction or arcward advance of the trench in suboceanic subduction

mineralization and hydrothermal history of the tiwi geothermal system philippines tiwi is one of several large geothermal fields in the philippines in 1992 the drilling of matalibong 25 provided nearly 1 650 m of continuous core from the geothermal reservoir a maximum temperature of 275 c at 1 829 m and intense hydrothermal alteration were encountered in this well six stages of alteration and vein mineralization have been documented in the cored portion of the well the earliest stage which reflects the initial development of the system is represented by the deposition of chalcedony and clays stage 1 the transition to a high temperature environment is marked by the appearance of sericite stage 2 deposited by steam heated waters stage 3 reflects episodic cycles of fluid upwelling and boiling followed by the incursion of cooler fluids veins deposited by boiling fluids are filled with quartz adularia epidote pyrite and base metal sulfides whereas heating of the recharging fluids led to the deposition of calcite and or anhydrite maximum temperatures of fluid inclusions trapped in two quartz crystals deposited by the upwelling fluids range from 325 to 332 c temperatures varied widely during the recharge phase of these cycles fluid inclusions trapped in calcite and anhydrite suggest that mineral deposition occurred at temperatures ranging from 333 c to less than 270 c fluid inclusion salinities indicate that seawater dominated below a depth of 1 600 m whereas fresh waters dominated at shallower depths gaseous species trapped in these inclusions were released by crushing or thermal decrepitation and analyzed with a quadrupole mass spectrometer co2 ch4 and n2 ar ratios indicate that the gaseous species trapped in calcite and anhydrite were derived primarily from meteoric and crustal sources in contrast gaseous species in quartz from matalibong 25 and advanced argillic assemblages from other tiwi wells have magmatic and crustal origins stage 4 represents a second major episode of sericite deposition subsequent mineralization consisted of wairakite epidote followed by actinolite and then calcite stage 5 the presence of actinolite implies that temperatures exceeded 300 c at the end of this stage thermochemical modeling indicates that the modern stage 6 fluids are again in equilibrium with sericite 40ar 39ar spectrum dating of stage 3 adularia from three depths has been combined with the mineral parageneses and fluid inclusion homogenization temperatures to constrain the thermal history of the geothermal system taken together these data record the deposition of adularia at 330 c between 314 and 279 ka minor cooling followed by reheating to produce stage 5 actinolite at 200 to 220 ka incursion of marginal waters and cooling to 235 c by 190 ka a long period of quiescence to 50 ka and finally development of the modern thermal regime at 10 to 50 ka in response to a recent subvolcanic intrusion

stable isotope study of the mineralization and alteration in the madjarovo pb zn district south east bulgaria the madjarovo ore district is centred on the exposed section of a lower oligocene volcano and consists of radially disposed pb zn precious metal veins and attendant intermediate sulfidation wallrock alteration earlier high sulfidation and potassic porphyry style alterations are found in the centre of the district spatially associated with monzonitic intrusions the total duration of all mineralization and alteration was ca 300 ka stable isotope analyses s o h have been carried out on a suite of sulfides sulfates and silicates from the mineralization high and intermediate sulfidation alterations and a suite of basement rocks these data range between the following limits delta 34 s sulfides 1 2 operatorname to 6 8 text delta 34 s sulfates 21 4 text to 24 9 delta 18 o sulfates 5 8 operatorname to 11 6 delta d sulfates 35 1 operatorname to 67 1 delta 18 o silicates 5 2 text to 13 5 delta d operatorname silicates 36 3 to 77 0 we also analysed d of fluid inclusions in quartz and barite for which we obtained respectively the ranges of 43 6 to 78 6 and 58 4 to 67 1 the data show that high sulfidation alteration was dominated by magmatic fluids with minor meteoric water whereas the fluids responsible for the intermediate sulfidation alteration were essentially magmatic the fluids responsible for the intermediate sulfidation pb zn mineralization were mixed magmatic meteoric and certainly contained a significant meteoric component sulphur is likely derived from basement and or igneous sources the evolution of alteration and mineralization styles from potassic porphyry copper style to high sulfidation to intermediate sulfidation can be understood in terms of changing ore fluid composition resulting from an increasing permeability of the system and an increasingly remote source of magmatic fluid with time these changes link directly to the geological evolution of this volcanic centre

saline fluid inclusions in sphalerite from the broadlands ohaaki geothermal system a coincidental trapping of fluids being boiled toward dryness report of a fluid inclusion study of sphalerites from the broadlands ohaaki geothermal system the findings indicate that brine filled fluid inclusions do exist in sphalerite from one location but that their origin is a consequence of local boiling toward dryness rather than introduction of a deep brine

fracture analysis hydrothermal mineralization and fluid pathways in the neogene geitafell central volcano insights for the krafla active geothermal system iceland abstract studies on hydrothermal fluids circulation and tectonic structures improve the geothermal exploration and exploitation in iceland in this paper we present the results of an integrated study structural kinematic fluid inclusions analyses carried out on a fossil and exhumed geothermal system at geitafell se iceland considered an analogue of the active krafla geothermal system ne iceland our work is addressed to the relationships between the main structures and the paleo geothermal fluids circulation at the boundary between the magma chamber with magma of gabbroic composition and host rocks with the aim to get information on those factors having controlled the hydrothermal fluids flow its storage and the rocks fluid interaction at 1 2 km crustal depth the structural study is mostly based on defining the architecture of mineralized brittle shear zones where by measuring length and width of mineralized fractures permeability was computed in 1 3 10 14 m2 as regards the hydrothermal paleo fluids fluid inclusions in hydrothermal quartz and calcite usually associated with epidote and chlorite i e the typical alteration minerals found in icelandic geothermal fields recorded a fluid circulation with temperature ranging from about 255 c to 320 c and boiling at the highest temperature transient pressure drop likely related to fracture opening and propagation during tectonic activity triggered local boiling process testified by fluid inclusions in a calcite sample during a late evolution the system progressively cooled down to 60 90 c paleo geothermal fluids were characterized by low salinity meteoric water although higher salinity fluids up to 10 6 wt nacl equiv entered in the system and mixed with meteoric fluid in the end based on results from fluid inclusions derived fluid density permeability values and modeling of viscosity hydraulic conductivity between 2 8 10 8 to 1 8 10 7 m s was also computed within the fluids channelling brittle shear zones we finally encourage the key study of fossil geothermal systems as a tool for getting parameters commonly obtained after drilling in active systems

genesis and exploration implications of epithermal gold mineralization and porphyry style alteration at the endeavour 41 prospect cowal district new south wales australia endeavour 41 is a deep level structurally controlled epithermal gold deposit hosted by early ordovician subaqueous volcanosedimentary rocks calc alkalic to shoshonitic mafic to intermediate sills dikes and stocks intruded the volcanosedimentary units in the middle ordovician the most felsic intrusions together with pyroxene bearing dikes are temporally related to gold mineralization postmineralization intrusions are exclusively of mafic character endeavour 41 evolved from early high temperature porphyry style veins and alteration to lower temperature epithermal style gold mineralization early magnetite and garnet bearing veins stage 1 and 2 respectively associated with actinolite magnetite and biotite bearing alteration assemblages have been cut by gold bearing veins and associated alteration assemblages there were two main epithermal style gold mineralizing events 1 quartz pyrite calcite adularia chlorite veins stage 3 and 2 carbonate base metal sulfide veins calcite ankerite quartz pyrite sphalerite galena chalcopyrite ag tellurides arsenopyrite apatite hematite illite muscovite and chlorite stage 4 gold occurs principally as a refractory phase in pyrite it also occurs as grains of au ag tellurides and as inclusions of free gold in pyrite sphalerite and chalcopyrite hydrothermal alteration associated with gold mineralized veins produced early epidote and k feldspar epidote bearing alteration halos and later stage illite muscovite k feldspar and calcite rich alteration halos the highest gold grades are associated with muscovite and illite alteration stable isotope analyses and fluid inclusion data provide evidence of a magmatic hydrothermal component to the mineralizing fluids fluid inclusion data suggest that gold precipitated from boiling saline waters 9 0 wt nacl at temperatures of about 310 c stage 3 veins are estimated to have formed approximately 1 km below the paleosurface at hydrostatic pressure 90 bars stage 4 illite formed at temperatures below 280 c stage 3 calcite has 13 c calcite and 18 o calcite values that range from 5 2 to 4 6 and from 11 6 to 12 1 respectively calculated fluids for these mineral values at 300 c 13 c fluid 3 18 o fluid 6 are consistent with a magmatic hydrothermal source of carbon and oxygen during stage 3 a component of meteoric waters is inferred for stage 4 because 13 c carbonate and 18 o carbonate values range from 6 9 to 0 5 and from 10 9 to 30 1 respectively corresponding to 13 c fluid and 18 o fluid values of 5 and 2 at 200 to 250 c the 34 s sulfide values for early vein stages range between 4 9 and 0 5 stage 3 has 34 s sulfide values ranging from 5 2 to 0 8 with the most 34 s enriched values deposited away from the mineralized center stage 4 sulfides have isotopic compositions from 7 5 to 2 5 the negative isotopic values are consistent with oxidized sulfate predominant magmatic hydrothermal fluids sulfur isotopic zonation patterns show that the most negative 34 s values correlate with gold enriched domains and also with areas that contain high temperature porphyry style alteration facies the negative sulfur isotope values define zones of upflow for the mineralizing magmatic hydrothermal fluids the paragenetic history of endeavour 41 records a transition from deep level to shallow level magmatic hydrothermal activity this transition implies erosion and unroofing of the system synchronous with mineralization high temperature assemblages e g actinolite magnetite biotite and k feldspar epidote indicate that epithermal mineralization occurred proximal to a magmatic hydrothermal center and that there is potential for the discovery of porphyry copper gold mineralization below the current level of diamond drilling

epithermal gold veins in a caldera setting banska hodrusa slovakia the central zone of the large miocene stiavnica stratovolcano in the western carpathians hosts epithermal au mineralization of intermediate sulfidation type located at deep levels of the historic rozalia base metal mine at banska hodrusa the au mineralization occurs as subhorizontal veins at the base of pre caldera andesites close to the roof of a subvolcanic granodiorite intrusion the veins are dismembered by a set of quartz diorite porphyry sills and displaced by the younger steeply dipping rozalia base metal vein and parallel structures the base metal vein structures are related to resurgent horst uplift in the caldera center the au mineralization formed during two stages based on fluid inclusion evidence both stages formed from fluids of low salinity 0 3 wt nacl eq which underwent extensive boiling at moderate temperatures 280 330 c variable pressure conditions 39 95 bars neglecting the effect of co2 indicate continual opening of the system and a transition from suprahydrostatic towards hydrodynamic conditions at shallow depths 550 m the fluid inclusions of the rozalia base metal vein show homogenization temperature peaks at 285 and 187 c and salinities between 1 and 4 wt nacl eq precipitation of au is considered to be the result of prolonged boiling of fluids and associated decrease in au solubility oxygen and hydrogen isotope data for quartz and carbonate from the au veins show a relatively homogeneous fluid composition 2 7 to 1 1 18o 78 to 62 d the combined 18ofluid and dfluid values suggest a mixed character of fluids falling between the fields of typical magmatic and meteoric water influenced by 18ofluid shift due to fluid rock isotopic exchange end stages of open system boiling and fractionation could have been reached at least locally significantly lower isotopic composition of meteoric fluids associated with au mineralization compared to those associated with the intrusion related mineralizations could have resulted from changing paleoclimate and or analytical problems of extraction of water from fluid inclusions the proposed genetic model for the au deposit at rozalia mine highlights the importance of hydrothermal activity during the early stage of caldera collapse caldera subsidence established new convective fluid flow paths along marginal caldera faults which acted as infiltration zones major metal precipitation occurred within subhorizontal structures that formed as the result of a collapse related stress field a shallow differentiated magma chamber at the base of the volcano was the likely source of heat and magmatic components for the mineralizing fluids

fluid inclusion gas compositions from an active magmatic hydrothermal system a case study of the geysers geothermal field usa abstract hydrothermal alteration and the active vapor dominated geothermal system at the geysers ca are related to a composite hypabyssal granitic pluton emplaced beneath the field 1 1 to 1 2 million years ago deep drill holes provide a complete transect across the thermal system and samples of the modern day steam the hydrothermal system was liquid dominated prior to formation of the modern vapor dominated regime at 0 25 to 0 28 ma maximum temperatures and salinities ranged from 440 c and 44 wt nacl equivalent in the biotite hornfels adjacent to the pluton to 305 c and 5 wt nacl equivalent at distances of 1730 m from the intrusive contact the major minor and noble gas compositions of fluid inclusions in the hydrothermally altered rocks were integrated with microthermometric and mineralogic data to determine their sources and the effects of mixing and boiling major and minor gaseous species were released from the inclusions by crushing or thermal decrepitation noble gases were released by crushing the samples were analyzed by mass spectrometry the analyses document the presence of magmatic crustal and meteoric components in the trapped fluids hydrothermal fluids present during the liquid dominated phase of the system contained gaseous species derived mainly from crustal and magmatic sources at the geysers n 2 ar ratios greater than 525 and 3 he 4 he ratios of 6 10 7 ra are diagnostic of a magmatic component crustal gas has co 2 ch 4 ratios less than 4 n 2 ar ratios between 45 and 525 and low 3 he 4 he ratios 0 5 ra meteoric fluids have co 2 ch 4 ratios greater than 4 and n 2 ar ratios between 38 air saturated water and 84 air however n 2 ar ratios between 15 and 110 can result from boiling ratios less than 15 reflect the incorporation of n 2 into nh 3 bearing clay minerals in the central geysers the incursion of meteoric fluids occurred during the transition from the liquid to vapor dominated regime variations in the relative ch 4 co 2 and h 2 contents of the gas analyses demonstrate that boiling took place under open system conditions the gas data indicate that the inclusions have remained closed to the diffusion of he and h 2 since their formation

hydrothermal fluids in epithermal and porphyry au deposits in the central slovakia volcanic field the neogene central slovakia volcanic field in the carpathian arc contains various au deposits hosted by central zones of large andesite stratovolcanoes fluids involved in mineralization have been studied at three different types of deposit mostly by fluid inclusion and stable isotope techniques the rozalia mine in the stiavnica stratovolcano hosts intermediate sulphidation style au ag epithermal mineralization in subhorizontal veins related to hydrothermal activity during an early stage of caldera collapse associated fluids of low salinity underwent extensive boiling at 280 330 c on transition from suprahydrostatic towards hydrodynamic conditions at shallow depths c 550 m from fluids of mixed magmatic and meteoric origin the kremnica ore field hosts a large system of low sulphidation style au ag veins contemporaneous with rhyolite magmatism and situated on resurgent horst faults fluids were of low salinity predominantly of meteoric origin and showed gradual decrease in temperature along the system c 270 140 c related to a decrease in erosion level from c 500 to c 50 m the biely vrch au porphyry deposit in the javorie stratovolcano is associated with quartz stockwork in diorite porphyry intrusion the major type of ore bearing fluid was high temperature magmatic vapour 720 380 c accompanied by fe rich salt melt gold precipitated in a high temperature but low pressure subvolcanic environment

deep structure of an arc continent collision earthquake relocation and inversion for upper mantle p and s wave velocities beneath papua new guinea we examine mechanisms of lithospheric growth and modes accommodating subcrustal convergence following the cessation of subduction by analyzing the seismicity and upper mantle velocities in the new guinea arc continent collision earthquakes in the papua new guinea png region from 1967 to 1984 have been relocated in a joint inversion with seismic velocities using p and s wave arrival times recorded by seismographs in png a total of 957 well recorded earthquakes were chosen for use in three dimensional velocity inversions from an initial catalog of 12 960 events based on the stability of initial hypocenter relocations constant velocity blocks were defined as irregular polyhedra to give fine detail in heavily sampled areas without introducing a large number of poorly controlled parameters and to exploit a priori knowledge of the geometry of velocity anomalies spherical geometry was used to accurately calculate long ray paths up to 1500 km in length most of the 700 best located hypocenters from the png data set are in the upper mantle beneath the finisterre huon fh ranges the newly accreted island arc terrane beneath the papuan peninsula and in the new britain seismic zone an oceanic subduction system these hypocenters show a well defined seismic zone dipping vertically or steeply to the north beneath the northern fh ranges from 125 to 250 km depth continuous along strike with the new britain seismic zone to the east which shows earthquakes to 600 km depth the steeply dipping intermediate depth seismicity flattens near 100 km depth beneath the fh ranges and forms a subhorizontal seismic zone beneath the fh ranges by contrast seismicity continues upward to the surface beneath new britain where the arc has not yet collided with new guinea the 100 km deep flat zone does not continue south of the fh ranges but appears to be truncated below the eastern and southern boundaries of the island arc terrane and no clear evidence for a south dipping seismic zone could be found the relationship of the flat part of the slab 100 km beneath the fh ranges and surface faulting is difficult to discern the apparent surface suture bounding the south side of the fh ranges is 100 km directly above the southern termination of intermediate depth seismicity nowhere in png could clear evidence for arc polarity reversal be found from seismicity all of the well located subcrustal earthquakes beneath the papuan peninsula lie in a narrow horizontal line between 125 and 175 km depth that follows the center of the peninsula and are distinctly separated from the fh seismic zone to the north these events do not require a southward subducting plate beneath the papuan peninsula an alternate and reasonable explanation for these events is that they are a result of unstable thickening of the lithosphere information on lateral heterogeneity in the mantle is limited because large variations in crustal velocities dominate the observed pattern of travel time residuals allowing for only crustal heterogeneity reduces travel time residual variances by 40 from a simple one dimensional structure while allowing for additional mantle heterogeneity results in 10 further reduction for the various parameterization s attempted here inversions with several different block geometries show low velocities at 35 to 171 km km depth beneath the bismarck sea back arc basin and high velocities just south of the intermediate depth fh seismic zone the pattern of lateral heterogeneity supports the inference from seismicity of north dipping slabs beneath both northeast new guinea and new britain

hydrothermal breccias and veins at the kelian gold mine kalimantan indonesia genesis of a large epithermal gold deposit mineralized hydrothermal breccias and veins formed during and after the waning stages of maar diatreme related volcanic activity at kelian kalimantan indonesia subsurface phreatic explosions occurred around the margins of the diatreme breccia complex focusing high temperature fluid flow and generating several large mineralized hydrothermal breccia bodies tectonic phreatomagmatic and hydraulic processes also contributed to hydrothermal brecciation explosive phreatic brecciation was followed by in situ hydraulic brecciation and then by minor veining as the system returned to steady state geothermal conditions brecciation and mineralization mostly occurred 400 to 700 m below the paleowater table the kelian deposit contained more than 240 t au prior to mining precious metals occur with sulfide minerals as disseminations in sheeted and conjugate veins and as breccia cement there is a progression from pyrite dominated stage 1 to base metal sulfide dominated stages 2 and 3 to sulfosalt dominated hydrothermal breccias and veins stage 4 in terms of gangue minerals the system evolved from illite quartz to adularia and or quartz and or illite dominated and then carbonate dominated boiling produced abundant bladed carbonate during stages 3 and 4 overall carbonate and base metal sulfide minerals are abundant whereas quartz is comparatively minor free gold is most abundant in stages 3 and 4 but also occurs in stages 1 and 2 native gold occurs principally as inclusions in pyrite sphalerite galena arsenopyrite quartz bladed carbonate and sulfosalts hydrothermal alteration assemblages are zoned about contacts faults breccias and veins and consist of secondary quartz illite pyrite chlorite and various carbonate minerals kelian preserves evidence of a magmatic component to the mineralizing fluids fluid inclusion analyses revealed the presence of saline fluid inclusions in particular from stage 3 isothermal mixing of low salinity 0 to 2 wt nacl equiv with moderate salinity fluids 10 to 25 wt nacl equiv rather than boiling resulted in the range of salinity and homogenization temperatures of fluid inclusions in sphalerite carbonate and quartz deposited during stage 3 salinities of 4 to 6 wt percent nacl equiv for inclusions in adularia and quartz from stages 2 and 4 and in rhodochrosite and proustite pyrargyrite from stage 4 may also reflect a component of mixing of low salinity water with a moderately saline fluid sulfur isotope values from kelian sulfides 34s 1 4 to 5 5 per mil are consistent with a magmatic sulfur source both boiling and fluid mixing contributed to high grade au mineralization at kelian rapid formation hundreds of years or less is inferred for the individual hydrothermal breccia bodies 2008 society of economic geologists inc

inversion of the assimilation and fractional crystallization afc equations characterization of contaminants from isotope and trace element relationships in volcanic suites the assimilation and fractional crystallization afc equations of depaolo 1981 are inverted with the aim of characterizing the chemistry of contaminants involved in magma evolution from observed isotope and trace element relationships equations are derived for contours of the ratio of the rate of assimilation to the rate of fractional crystallization in the isotope or trace element region where the contaminant might be located even for volcanic suites in which afc is thought to have been the dominant process the characterization of the contaminant is poorly constrained although afc is likely to be a normal process it leaves a recognizable isotopic signature in volcanics only when magmas assimilate material derived from continental crust in subduction related volcanics this can be the case where the volcanic arc lies on or near continental crust

self ordering and complexity in epizonal mineral deposits epizonal base and precious metal deposits makeup a range of familiar deposit styles including porphyry copper gold epithermal veins and stockworks carbonate replacement deposits and polymetallic volcanic rock hosted vhms deposits they occur along convergent plate margins and are invariably associated directly with active faults and volcanism they are complex in form variable in their characteristics at all scales and highly localized in the earth s crust more than a century of detailed research has provided an extensive base of obser vational data characterizing these deposits from their regional setting to the fluid and isotope chemistry of mineral deposition this has led to a broad understanding of the large scale hydrothermal systems within which they form low salinity vapor released by magma crystallization and dispersed into vigorously convecting ground water systems is recognized as a principal source of metals and the gases that control redox conditions within systems the temperature and pressure of the ambient fluid anywhere within these systems is close to its vapor liquid phase boundary and mineral deposition is a consequence of short timescale perturbations generated by localized release of crustal stress however a review of occurrence data raises questions about ore formation that are not addressed by traditional genetic models for example what are the origins of banding in epithermal veins and what controls the frequency of oscillatory lamina tion what controls where the phenomenon of mineralization occurs and why are some porphyry deposits for example so much larger than others the distinctive self organized characteristics of epizonal deposits are shown to be the result of repetitive coupling of fracture dilation consequent on brittle failure phase separation boiling and heat transfer between fluid and host rock process cou pling substantially increases solute concentrations and triggers fast far from equilib rium depositional processes since these coupled processes lead to localized transient changes in fluid characteristics paragenetic isotope and fluid inclusion data relate to conditions at the site of deposition and only indirectly to the characteristics of the larger scale hydrothermal system and its longer term behavior the metal concentra 1 the us government has the right to retain a nonexclusive royalty free license in and to any copyright covering this paper

inversion of source parameters for subcrustal earthquakes in the hellenic arc summary a one station method was developed to invert simultaneously for seismic moment focal depth and the orientations of the nodal planes this method was tested on seismograms recorded at the broadband grefenberg array grf for earthquakes in the central to eastern hellenic arc and southern turkey the complete p and s body wave responses including all near source surface reflections were synthesized using the reflectivity method the minimum misfit between the observed and theoretical seismograms in the wwssn lp band was determined by cross correlation searching the whole parameter space of strike dip and rake of a pair of orthogonal nodal planes and adjusting the source depth and seismic moment the initial searching step of 20 was reduced to 5 in the vicinity of the minimum misfit position for a final search the best fitting solution was then compared to the harvard hrvd moment tensor solution best double couple which was derived using long period data and to the p wave first motion polarities reported by the isc most solutions based on grf data agree well with those found by hrvd but in some cases some features of the observed grf signals cannot be matched by the hrvd solution or solutions similar to it the nine earthquakes we analysed in detail had magnitudes between 5 2 and 5 8 our depth determinations ranged from 35 to 155 km the deepest events 80 155 km were located near rhodes and to the east of it one earthquake located in southern turkey was confirmed to have a focus at about 125 km depth the focal mechanisms of the intermediate depth earthquakes in the eastern hellenic arc show p axes approximately parallel to the strike of the deep slab

the waihi epithermal gold silver base metal sulfide quartz vein system new zealand temperature and salinity controls on electrum and sulfide deposition the large andesite hosted quartz vein system at waihi contains a major epithermal au ag deposit total production of 6 4 moz au and 39 9 moz ag ag au 6 2 it is notable for a large vertical extent up to 575 m of economic au ag ore a pervasive adularia quartz illite chlorite pyrite alteration zone bounds the larger quartz veins and is associated with veinlets of quartz calcite pyrite adularia illite this zone is enveloped by an illite quartz adularia chlorite calcite pyrite alteration zone which grades out through a transition zone characterized by interlayered illite smectite into a smectite chlorite calcite alteration zone in the least altered andesites the formation of the adularia quartz chlorite calcite pyrite and some illite in these alteration assemblages was associated with an initial stage of vein filling by platy calcite and quartz this was followed by a main stage of quartz and sulfide vein filling with crustiform quartz at deeper levels and colloform quartz plus minor adularia at shallow levels deposition of main stage quartz and sulfides was accompanied by illite overprinting adularia in the wall rock and filling fractures in the quartz calcite pyrite adularia veinlets sulfide bearing bands associated with the main stage quartz show a zonation with decreasing depth from a coarse grained sphalerite galena pyrite chlorite assemblage at deep levels to a finer grained sphalerite galena pyrite chalcopyrite acanthite electrum assemblage at medium to shallow levels fluid inclusion studies indicate that the main stage quartz was deposited from fluids which ranged in temperature and composition from up to 295 c and 1 7 wt percent nacl equiv at depth to 190 c and 0 2 wt percent nacl equiv at shallow levels these are true salinities because the fluids extracted from the fluid inclusions have low co 2 contents 0 05 0 24 wt quartz within electrum poor zn pb fe sulfide bands was deposited at higher temperatures 295 242 c and salinities than quartz in electrum bearing zn pb fe cu ag sulfide bands 260 190 c temperature versus depth profiles for fluid inclusions with evidence of boiling indicate that the paleowater table was located about 160 m above the present erosion level during deposition of the main stage quartz minor amethyst as the final stage of vein filling was locally preceded by inesite and was not associated with sulfides and electrum amethyst was deposited at 253 to 170 c from fluids containing up to 4 5 wt percent nacl equiv these higher salinities are attributed to concentration of salts in the residual liquid by localized open system boiling chloride depth chloride enthalpy and 18 o water enthalpy plots indicate mixing of deep fluids up to 1 7 wt nacl for the main stage quartz with very low salinity steam heated ground water oxygen isotope 18 o values for vein quartz and amethyst range from 6 1 to 11 6 per mil and for vein calcite from 2 29 to 9 4 per mil calculated 18 o water values define a trend from 2 9 per mil at shallow levels to 0 6 per mil at depth extrapolation of the dilution trend on a 18 o water enthalpy plot indicates that the deep fluid 18 o water 0 6 was mixed with heated 160 180 c ground water of local meteoric water composition 5 5 dilution of a deep chloride fluid 1 2 wt nacl equiv carrying zinc and lead as chloride complexes with steam heated ground water is inferred to have been responsible for deposition of the electrum poor zn pb fe sulfide assemblage from 300 to 240 c at a paleodepth below about 500 m deposition of this assemblage may also have been promoted by decrease of sulfur in solution through loss of h 2 s gas by boiling during episodic fracturing the decrease in salinity following dilution favored the transport of gold silver and copper as hydrosulfide complexes and led to the deposition of the electrum bearing zn pb fe cu ag sulfide assemblage over a temperature range of 260 to less than 190 c at paleodepths of 500 to less than 250 m in this assemblage the decrease of sulfur in solution as a result of deposition of the associated chalcopyrite pyrite and galena is a likely mechanism for electrum deposition

synthetic fluid inclusions as recorders of microfracture healing and overgrowth formation rates aqueous fluid inclusions have been synthesized within fluorite microfractures t 200 c p p sat t 30 days quartz microfractures and quartz overgrowths t 400 to 300 c p 400 bars t 20 days the experiments were designed to determine the time necessary to form the inclusions within a minute in a fluorite host and within a day in a quartz host the results permit estimation of the time needed to heal microfractures or create overgrowths which is accomplished by measuring the melting temperature t m of the inclusions in fluorite and the homogenization temperature t h of the inclusions in quartz for the fluorite experiments the results show that fluorite nh4cl solution equilibrium took 82 hours moreover the healing process appears to be an irregular process along the microfracture fluorite experiments mimic boiling processes occurring in natural hydrothermal or epithermal systems and show how a boiling fluid may be progressively salted during vaporization and trapped as fluid inclusions for the experiments involving quartz the quartz h2o nacl solution equilibrium was reached on the first day for synthetic quartz and on the sixth day for natural quartz this difference is linked to the hydration state of the two types of quartz the decrease in t h of synthetic fluid inclusions from the core overgrowth boundary to the external part of the quartz overgrowth shows that the growth of quartz is a progressive and protracted process which involves a silica oversaturated solution

do fluid inclusions preserve δ18o values of hydrothermal fluids in epithermal systems over geological time evidence from paleo and modern geothermal systems milos island aegean sea stable isotope compositions of quartz d 18 oquartz and fluid inclusion waters d 18 ofi and ddfi were analysed from profitis ilias a low sulphidation epithermal gold mineralisation deposit on milos island greece to establish if d 18 ofi preserve a record of paleogeothermal processes previous studies show that mineralisation at profitis ilias resulted from extreme boiling and vaporisation with a zone located at approximately 430 m above sea level asl representing the transition between liquid and vapor dominated systems miner depos 36 2001 32 the deposit is also closely associated with an active geothermal system whose waters have a well characterised stable isotope geochemistry pflumio c boulegue j liakopoulos a briqueu l 1991 source transport and deposition of metals balkema rotterdam the samples were collected over an elevation interval of 440 m 210 650 m asl to give information on the liquid and vapor dominated sections of the paleosystem the data show systematic variations with sample elevation samples from the highest elevations ca 650 m asl have the lightest d 18 ofi 7 3x and ddfi 68 0x whilst the deepest ca 210 m asl are isotopically heavier d 18 ofi 0 3x ddfi 19 0x relative changes in d 18 ofi closely parallel those in ddfi d 18 oquartz shows an opposite trend from the lightest values 13 9x at the lowest elevations to the heaviest 15 1x at the highest elevations d 18 ofi shows correlations with other parameters for example variable fluid inclusion homogenisation temperatures in the vapor dominated part of the system correlate with a rapid shift in ddfi 33 3xto 50 5x and d 18 ofi 4 1xto 6 2x gold contents also increase in the same zone up to 50 ppm au comparable correlations in d 18 oquartz or d 18 ocalculated estimated geothermal fluid from fluid inclusion homogenisation data are absent d 18 ocalculated are always 5 10xheavier than d 18 ofi comparison with the present day geothermal field shows that ddfi and d 18 ofi are similar isotope data for the modern system and fluid inclusion waters fall on linear trends subparalleling the meteoric water line and project towards seawater values numerical modelling favours kinetically controlled fractionation to explain differences in d 18 ocalculated and d 18 ofluid rather than diffusive posttrapping reequilibration the evidence suggests that in low temperature epithermal systems d 18 ofi may represent

anatomy of a megathrust the 2010 m8 8 maule chile earthquake rupture zone imaged using seismic tomography abstract knowledge of seismic velocities in the seismogenic part of subduction zones can reveal how material properties may influence large ruptures observations of aftershocks that followed the 2010 m w 8 8 maule chile earthquake provide an exceptional dataset to examine the physical properties of a megathrust rupture zone we manually analysed aftershocks from onshore seismic stations and ocean bottom seismometers to derive a 3 d velocity model of the rupture zone using local earthquake tomography from the trench to the magmatic arc our velocity model illuminates the main features within the subduction zone we interpret an east dipping high p wave velocity anomaly 6 9 km s as the subducting oceanic crust and a low p wave velocity we assess the role played by physical heterogeneity in governing megathrust behaviour greatest slip during the maule earthquake occurred in areas of moderate p wave velocity 6 5 7 5 km s where the interface is structurally more uniform at shallow depths high fluid pressure likely influenced the up dip limit of seismic activity the high velocity bodies lie above portions of the plate interface where there was reduced coseismic slip and minimal postseismic activity the northern velocity anomaly may have acted as a structural discontinuity within the forearc influencing the pronounced crustal seismicity in the pichilemu region our work provides evidence for how the ancient geological structure of the forearc may influence the seismic behaviour of subduction megathrusts

hydrothermal alteration and hydrologic evolution of the golden cross epithermal au ag deposit new zealand golden cross located in the coromandel peninsula new zealand is a classic example of a volcanic rock hosted low sulfidation epithermal gold silver deposit gold and silver ore is confined to the empire vein system and shallow level stockwork the veins are hosted by miocene to early pliocene andesites and dacites of the waipupu formation and the waiharakeke dacite that are unconformably overlain by the postore whakamoehau andesite hydrothermal alteration minerals display distinct spatial and temporal zonation around veins as defined along three cross sections 1 000 m long 450 m deep located 200 m apart which transect the empire vein system along these sections replacement quartz chlorite and pyrite are ubiquitous with the abundance of quartz veinlets increasing toward major veins replacement adularia envelops the empire vein system and shallow stockwork in each section it is coextensive with and is variably replaced by illite that progressively grades upward and laterally into a zone of interstratified illite smectite that mantles the deposit replacement calcite and minor siderite formed contemporaneously with and also overprint the above minerals whereas late barren calcite veins crosscut mineralized quartz veins kaolinite pyrite veinlets together with rare very local alunite formed during late stage hydrothermal activity clay mineral zonation is well developed illite occurs at depth and close to the veins grading outward and upward into illite smectite with minor smectite occurring 600 m east of the empire vein system this overall zonation reflects paleothermal gradients of 150 c on the periphery to 220 c near the veins consistent with the observed t h range of 150 to 240 c for fluid inclusions in quartz platy calcite and late barren calcite veins final ice melting temperatures for inclusions mostly range from 0 0 to 1 4 c corresponding to apparent salinities of less than 2 4 wt percent nacl equiv ice melting temperatures combined with vapor bubble expansion on crushing indicate the presence of dissolved co 2 in some platy calcite and late stage barren calcite the co 2 content is estimated to range from 0 35 to 3 5 wt percent with the lower limit set by fluid inclusion vapor expansion during crushing and the upper limit by the absence of any observable clathrates depth estimates based on inclusions in platy calcite suggest that the shallow level stockwork zone formed about 100 m below the paleowater table under hydrostatic conditions veins and alteration minerals at golden cross formed in the shallow part 400 m of a hydrothermal system analogous to geothermal systems in the nearby taupo volcanic zone the assemblage of quartz adularia chlorite illite calcite and pyrite reflects the upflow of near neutral ph to weakly alkaline chloride waters contemporaneously steam heated co 2 rich waters formed on the margins and reacted with the wall rock to form the assemblage of calcite siderite smectite illite smectite and kaolinite eventual thermal collapse and invasion of these peripheral co 2 rich waters into the former upflow zone produced late stage barren calcite veins and an overprint of clay carbonate alteration minerals at the same time localized steam heated acid sulfate waters generated above the water table descended to form the late overprint of kaolinite pyrite and alunite

accretion and erosion in subduction zones the role of fluids convergence occurs today along about 60 000 km of plate boundaries parsons 1981 in this paper we consider frontal subduction which occurs along about 40 000 km of convergent plate boundaries using the kine matic solution of de mets et al 1990 and including 0 5 km 2 yr1 of marginal basin openings otsuki 1989 we find that 2 95 km 2 per year disappear in frontal subduction at an average rate of 7 3 cm yr the upper overriding plate is significantly modified not only through the addition of a large amount of magmatic material markhinin 1968 but also through tectonic processes which result either in outgrowth of the margin through addit ion of sediments silver 1969 or in recession through frontal and subcrustal erosion which appears to be linked to sediment subduction coats 1962 gilluly 1963 the various phenomena observed must in some way be related to variations in mechanical coupling between the two plates uyeda kanamori 1979 had proposed that the transition from erosion to accretion results from increasing mechanical coupling the marianas trench was presented as typical of an crosional margin and the chile trench of an accreting margin ruff kanamori 1980 and kana mori 1986 showed that the magnitude of the subduction earthquakes increases with the convergence velocity and the inverse of the age of the subducted lithosphere this correlation was qualitatively explained by the control of age on the d ip of the platc and of convergence velocity on the

geologic evolution of the caylloma epithermal vein district southern peru the silver and base metal rich caylloma epithermal district in the tertiary volcanic belt of southern peru has been worked intermittently since the incaic period and has produced over 100 million ounces moz of ag intermediate sulfidation mineralization is present in veins hosted by miocene andesitic volcanic and volcaniclastic rocks with minor ore in underlying folded jurassic sedimentary basement new 40 ar 39 ar dates give a host rock age of 20 30 0 11 ma andesitic volcanic matrix a hydrothermal alteration age of 18 35 0 17 ma adularia in vein wall rock and postmineralization ages of 11 8 0 8 and 12 25 0 07 ma sanidine and biotite from a rhyolite dome gangue minerals include quartz calcite rhodonite rhodochrosite pyrite and minor adularia illite barite and helvite ore minerals include sphalerite galena chalcopyrite and tetrahedrite hydrothermal alteration is pervasive in lava flows but weak and localized near veins in volcaniclastic rocks hydrothermal alteration types include silicification quartz adularia and quartz illite and propylitization chlorite calcite illite banded veins show four stages of mineral precipitation 1 early sugary quartz chalcedony pyrite 2 manganese minerals 3 quartz sulfides and 4 late calcite quartz cyclic bands in the manganese stage early sulfides coarse to medium grained quartz late rhodonite calcite chalcedony are a few millimeters to 5 cm thick and form ore bands up to 1 m thick veins occupy dextral normal faults 020 050 45 70 se and extension fractures 060 090 70 90 se and are 1 to 25 m wide and as much as several kilometers long the veins display complex and multi episodic filling with textures characteristic of open space precipitation such as crustiform banding symmetric banding vugs breccias and cockade and comb textures we present a structural model in which the principal veins formed in subparallel northeast striking dextral normal faults and related extension fractures within a northwest striking structural corridor bounded by sinistral regional faults second and third order veins formed by bookshelf sliding related to movement on higher order dominantly strike slip faults slickenlines have low rakes 20 50 sw and fault kinematic analysis suggests the extension axis was oriented 4 330 and the shortening axis 55 234 in agreement with the strain field commonly proposed for the early middle miocene of southern peru ore grade is discontinuous with high grade ore zones having strike lengths of tens to hundreds of meters and extending about 300 m downdip ore shoots in fault hosted veins are narrow subvertical zones a few tens of meters wide measured in the plane of the vein separated by lower grade ore zones ore shoots in extension fractures are shorter downdip and horizontally more continuous some ore shoots occur as half cymoids that form along the main vein only in the hanging wall of synmineral transverse faults mineral composition and metal content are zoned horizontally and vertically silver base metals and calcite and manganese minerals increase to the northeast and downward and sulfide content increases gradually with depth shallower levels of the hydrothermal system are exposed to the west and southwest in the district evidence from sulfur isotopes and high salinity fluid inclusions is consistent with a model in which magmatic fluids played a role in the hydrothermal system introduced through channels of enhanced structural permeability the cyclic nature of mineral precipitation with abrupt changes in mineralogy textures and fluid composition leads us to postulate that cyclic injections of magma derived hydrothermal fluids occurred economic mineralization is estimated to have formed at a depth of about 650 m below the paleowater table with a temperature of formation of 270 c

the p velocity within the tonga benioff zone determined from traced rays and observations p waves with travel time residuals between 0 s and 12 s are observed at regional stations in samoa afi and raoul island rao for 39 earthquakes in tonga with focal depths between 70 km and 300 km these anomalously large residuals apparently are produced because seismic phases travel along the strike of the tonga benioff zone within the high velocity subducted lithosphere for up to 1200 km before arriving at afi and rao to eliminate erroneous residuals caused by poor event locations we selected 11 stations and reread the available p times at these stations for the 39 events these arrivals and pp p intervals were used to relocate the events using a variant of the joint hypocenter determination method then the pattern of residuals at afi and rao not used in the relocation was compared to the pattern of residuals expected for various models of the subducted lithosphere as determined by ray tracing the observed pattern of residuals at afi is consistent with the ray traced models if some of the first arrivals are produced by rays traveling directly along the strike of the subducting lithosphere and if others are produced by rays which reflect once off the upper surface of the subducting lithosphere before arriving at afi the observed residuals can be explained by a model where the p velocity in the subducted lithosphere is 8 higher than the velocity in the herrin model the residuals are fit even better by a layered slab model in which the seismic velocity is about 6 higher than the herrin velocity at the upper surface of the subducted lithosphere and about 9 higher at the bottom of the slab these velocity contrasts could be produced if the temperature in the slab was 700 c cooler than the surrounding mantle and if there were no partially melted material within the slab the existence of these anomalously large residuals suggests that the high velocity region in the upper 300 km of the mantle beneath tonga must be fairly continuous over distances of 1000 km and more

subduction zone calibration and teleseismic relocation of thrust zone events in the central aleutian islands abstract teleseismically determined epicenters for thrust zone earthquakes in the central aleutian islands are systematically mislocated by about 40 km to the north relative to local network solutions we attribute the mislocation to travel time anomalies produced by the higher p wave velocity in the subducting slab these travel time anomalies can be estimated by seismic ray tracing through a thermally modeled velocity structure for the slab observed teleseismic station residuals and slab travel time anomalies computed from local network hypocenters plotted as a function of event distance to the center of curvature of the island arc show good agreement this distance versus residual relationship can be used to validate the local network hypocenters and to constrain the length of subducting lithosphere computed slab source corrections when incorporated in a standard location program reduce teleseismic mislocations to 20 km the addition of empirical station corrections reduces the mislocation to about 10 to 15 km depth and origin time estimates can be further improved through use of better station corrections a local station within 2 and depth phases

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

detection and location of earthquakes in the central aleutian subduction zone using island and ocean bottom seismograph stations a network of eight university of texas ocean bottom seismographs obs operated for 6 weeks in 1978 about 50 km offshore of adak island alaska and nearby islands in 1979 a similar network of nine instruments was deployed for 7 weeks farther offshore within and up to 100 km seaward of the aleutian trench for shallow earthquakes on the outer trench slope for shallow earthquakes in the thrust zone and for intermediate depth events we have analyzed the obs and island based network data and evaluated the island network s capabilities for earthquake detection and location and for focal mechanism determination our three major conclusions are presented the first concerns shallow earthquakes on the outer trench slope in 1979 about 30 earthquakes occurred within the aleutian trench and up to 60 km seaward of the trench axis the island network located none of these events and detected p phases for only three of them ray tracing shows that the islands lie in a geometric shadow zone for events on the outer trench slope the best located events are shallower than 20 km and exhibit first motions consistent with normal faulting several authors have suggested that these events are caused by bending of the oceanic lithosphere at the outer rise prior to subduction if so then the event locations reported here show that the bending stresses exceed the strength of lithosphere only in a narrow zone extending about 10 km landward and 60 km seaward of the trench axis the second conclusion concerns shallow earthquakes in the thrust zone epicenters determined by island stations alone are virtually identical to epicenters determined using data from both island and obs stations the locations are similar whether they are determined using flat layered velocity models or a more realistic model and a ray tracing scheme nearly all the earthquakes detected by the obs network are also recorded by island stations three composite focal mechanism solutions that use data from both island and obs stations have p axes parallel to the direction of plate convergence the thrust zone events are separated from events on the outer trench slope by a gap in seismicity about 50 km wide situated on the inner trench slope the third conclusion concerns earthquakes deeper than 70 km epicenters determined using island network stations alone lie 10 to 80 km south of those determined using obs and island stations with the differences between epicenters depending both on event depth and on the velocity model used ray tracing with a realistic velocity model shows that for events at 200 km depth locations are about 80 km north of locations determined from island data alone these results do not support the observed increase in the dip of the benioff zone beneath 100 km depth that is suggested by locations determined from island network data alone using a flat layered velocity model

the evolution of volcano hosted geothermal systems based on deep wells from karaha telaga bodas indonesia temperature and pressure surveys fluid samples and petrologic analyses of rock samples from deep drill holes at the karaha telaga bodas geothermal field on the volcanic ridge extending northward from galunggung volcano west java have provided a unique opportunity to characterize the evolution of an active volcano hosted geothermal system wells up to 3 km in depth have encountered temperatures as high as 353 c and a weakly altered granodiorite that intruded to within 2 to 3 km of the surface the intrusion is shallowest beneath the southern end of the field where an acid lake overlies a nearly vertical low resistivity structure 10 ohm m defined by magnetotelluric measurements this structure is interpreted to represent a vapor dominated chimney that provides a pathway to the surface for magmatic gases four distinct hydrothermal mineral assemblages document the evolution of the geothermal system and the transition from liquid to vapor dominated conditions the earliest assemblage represents the initial liquid dominated system generated during emplacement of the granodiorite between 5910 76 and 4200 150 y bp tourmaline biotite actinolite epidote and clay minerals were deposited contemporaneously at progressively greater distances from the intrusive contact assemblage 1 at 4200 150 y bp flank collapse and the formation of the volcano s crater kawah galunggung resulted in catastrophic decompression and boiling of the hydrothermal fluids this event initiated development of the modern vapor dominated regime chalcedony and then quartz were deposited as the early low salinity liquids boiled assemblage 2 both vapor and liquid rich fluid inclusions were trapped in the quartz crystals liquid rich fluid inclusions from the southern part of the field record salinities ranging from 0 to 26 weight percent nacl cacl 2 equivalent and locally contain fluorite daughter crystals we suggest based on temperature salinity relationships and evidence of boiling that these fluids were progressively concentrated as steam was lost from the system however mixing with fluids derived from the underlying intrusion or generated during the formation of acid so 4 water on the vapor dominated chimney margins could have contributed to the observed salinities as pressures declined co 2 and so 4 rich steam heated water drained downward depositing anhydrite and calcite assemblage 3 in the fractures limiting further recharge fluid inclusions with salinities up to 31 weight percent nacl equivalent were trapped in these minerals as the descending water vaporized the final assemblage is represented by precipitates of nacl kcl and fecl x deposited on rock surfaces in portions of the vapor dominated zone that boiled dry vapor dominated conditions extend over a distance of at least 10 km and to depths below sea level deep wells drilled into the underlying liquid dominated reservoir in the northern and central part of the volcanic ridge produce low salinity fluids representing recent recharge of meteoric and steam heated water the evolution of volcanic hosted vapor dominated geothermal systems can be described by a five stage model stage 1 involves the formation of an over pressured liquid dominated geothermal system soon after magmatic intrusion in stages 2 and 3 pressures progressively decrease and a curtain of steam heated water surrounding a magmatic vapor dominated chimney at 350 c and 14 2 mpa develops the relatively low pressure near the base of the chimney causes liquid inflow adjacent to the intrusion and the development of a secondary marginal vapor dominated zone in stage 4 the magmatic vapor discharge from the intrusion becomes small vapor pressure declines and the secondary vapor dominated zone expands above the intrusion in stage 5 the vapor dominated zone floods because heat from the intrusion is insufficient to boil all liquid inflow a more common liquid dominated volcanic hosted system then develops

fluid evolution in the baia mare epithermal gold polymetallic district inner carpathians romania the baia mare district romania represents a complex segment of the neogene quaternary subduction related volcanic chain of the carpathians in this province the calc alkaline volcanism and associated mineralization are closely related to a major strike slip fault and an underlying pluton the ore deposits of the district have many features typical of low sulfidation or adularia sericite epithermal deposits the combination of fluid inclusion microthermometry raman spectroscopy bulk crush leach analysis laser ablation optical emission spectrometry la oes of individual fluid inclusions and stable isotope data has been used to characterize the ore forming solutions involved with gold polymetallic mineralization in three of the largest deposits ore fluids at baia sprie cavnic and sasar deposits had temperatures of 320 to 150 c and salinities in the range 0 21 wt nacl equiv during phreatomagmatic activity fracturing and periods of vein dilation the pressure fluctuations may have caused boiling the hydrothermal fluids were na k li ca chloride solutions with lesser amounts of so4 and f the na k ratios and the stable isotope compositions are consistent with equilibration of the fluids with country rocks at progressively lower temperatures the halogen ratios br cl 103 in the range 0 13 1 21 together with sulfur isotopic systematics 34s 0 3 5 2 per mil are consistent with a dominantly magmatic origin of the solutes and probably of the ore fluids themselves

seismicity focal mechanisms and tectonics in the last four years there has been a tremendous increase in the detail of tectonic processes reported in the typical seismotectonic study new data from many local and regional networks of seismographs established during the previous four year period plus the now routine application of improved hypocentral location procedures long period body wave modelling and moment tensor inversion using digitally recorded data have increased resolution of both the hypocentral location and the nature of the faulting process consequently research emphasis has been on local tectonics or those aspects of regional and global tectonics requiring high resolution i briefly describe below some of the highlights of these studies

migration of geothermal fluids in extensional terrains the ore deposits of the boccheggiano montieri area southern tuscany italy an integrated study based on fluid inclusion 18o composition and structural analyses was carried out on a pliocene fossil hydrothermal system located to the south of the present active larderello geothermal field in the boccheggiano montieri area the study area is typified by mineralized cataclastic levels related to late oligocene early miocene thrust surfaces and to the following two generations of normal faults of miocene and pliocene ages respectively within the damage zone of the pliocene boccheggiano fault the mineralization is mainly made up of quartz and pyrite quartz pb zn sulfides or quartz pb zn sulfides fluorite carbonates assemblages occur instead in the older cataclastic levels two generations of liquid rich fluid inclusions were recognized in quartz and fluorite the first one with homogenization temperatures ranging between 172 and 331 c and salinity between 0 0 and 8 8 wt naclequiv records the early stage of hydrothermal activity the second generation of fluid inclusions documents a later stage with homogenization temperature from 124 to 288 c and salinity from 0 2 to 1 9 wt naclequiv fluid inclusions analyses also indicate that mixing of fluid with distinct salinities and or temperatures was a widespread process during the early stage and that fluid temperatures decreased moving from the boccheggiano fault toward the more distal and older cataclastic levels the 18o values of water in equilibrium with hydrothermal quartz which range from 5 7 to 0 1 are related to the circulation of meteoric water mixed with saline water that leached the evaporite level and enriched in 18o through water rock interaction and or with magmatically derived fluids results indicate that the damage zone of the pliocene boccheggiano fault represented the main channel for the flow of meteoric water which was heated at depth then mixed with high salinity fluids and finally ascend to infiltrate along the older cataclastic levels our results based on fluid inclusions oxygen isotopic compositions and structural analyses indicate that a single fluid flow path run through the damage zone of the boccheggiano fault and the older cataclasites which were thus hydraulically connected

spatial distribution and mechanisms of earthquakes in the southern new hebrides arc from a temporary land and ocean bottom seismic network and from worldwide observations in 1977 a wide aperture seismic network of land and ocean bottom stations obs was operated for 6 weeks in the southern new hebrides island arc data on the spatial distribution and mechanisms of small events recorded by this local network were integrated with worldwide observations of moderate to large size new hebrides earthquakes of the past 17 years to study the structure and deformational processes in the new hebrides subduction zone surprisingly small differences less than about 10 km were found between the locations of shallow and intermediate depth earthquakes as located by the temporary local network and by the international seismological centre using worldwide stations analysis of the p wave travel time data from the obs station closest to the trench shows limited evidence for a high velocity zone about 5 higher than the surrounding mantle associated with the descending plate the thrust zone between the descending and upper plates is defined mainly by the spatial distribution and focal mechanisms of moderate to large size events and their aftershocks but it is not very well defined by the spatial distribution of small size events located in this study shallow seismic activity located beneath the trench and beneath the interplate thrust zone indicates a lower limit of about 40 km for the thickness of the seismically active part of the descending plate beneath the thrust zone focal mechanisms of moderate to large size earthquakes located within the upper plate and geological observations on the islands suggest that the upper plate in the region of erromango and tanna is divided into a series of blocks that are differentially uplifted along mainly nw se striking faults during the obs experiment several shallow events were well located beneath the coriolis trough a riftlike feature located to the east of the volcanic arc well determined depths of these events are between 11 and 22 km the well located intermediate depth events define a 20 km thick benioff zone that has a dip of about 70 the spatial distribution of events within the descending slab is very strongly clustered in a persistent pattern that is seen in both the short term sample of small events determined by the local network and the long term sample of locations based on worldwide data several focal mechanisms of moderate to large size intermediate depth events show components of lateral extension along the strike of the arc and some show components of lateral compression both of which could be interpreted as the result of a lateral bending of the descending plate in the part of the southern new hebrides arc where the trench begins to curve around to the east

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

seismic evidence for a slab tear at the puerto rico trench 1 the fore arc region of the northeast caribbean plate north of puerto rico and the virgin islands has been the site of numerous seismic swarms since at least 1976 a 6 month deployment of five ocean bottom seismographs recorded two such tightly clustered swarms along with additional events joint analyses of the ocean bottom seismographs and land based seismic data reveal that the swarms are located at depths of 50 150 km focal mechanism solutions found by jointly fitting p wave first motion polarities and s p amplitude ratios indicate that the broadly distributed events outside the swarm generally have strike and dip slip mechanisms at depths of 50 100 km while events at depths of 100 150 km have oblique mechanisms a stress inversion reveals two distinct stress regimes the slab segment east of 65 w longitude is dominated by trench normal tensile stresses at shallower depths 50 100 km and by trench parallel tensile stresses at deeper depths 100 150 km whereas the slab segment west of 65 w longitude has tensile stresses that are consistently trench normal throughout the depth range at which events were observed 50 100 km the simple stress pattern in the western segment implies relatively straightforward subduction of an unimpeded slab while the stress pattern observed in the eastern segment shallow trench normal tension and deeper trench normal compression is consistent with flexure of the slab due to rollback these results support the hypothesis that the subducting north american plate is tearing at or near these swarms the 35 year record of seismic swarms at this location and the recent increase in seismicity suggest that the tear is still propagating

mineralogy of the volcanic influenced great northern coal seam in the sydney basin australia abstract the mineralogy of the individual coal plies and intra seam claystone bands in the great northern seam of the sydney basin has been evaluated using optical and scanning electron microscopy and quantitative x ray diffraction techniques the uppermost two claystone bands are tonsteins consisting mainly of well ordered kaolinite with graupen to vermicular textures idiomorphic crystals of k feldspar within these tonsteins may represent members of the anorthoclase sanidine series or a sodic sanidine and indicate an acid to intermediate volcanic ash input in contrast the lowermost parting was largely derived from epiclastic sediment admixed with minor volcanic material such as high temperature quartz and a different type of k feldspar component the mineral fraction of the coals especially in the middle and upper parts of the seam is dominated by authigenic kaolinite with a very low abundance of quartz apart from the tonsteins authigenic processes therefore appear to be the dominant mechanism of mineral matter formation authigenic k feldspar also occurs in the lower few metres of the seam with a variety of modes of occurrence including cell and cleat infillings cross cutting veins and thin laminae parallel to the organic matter and detrital clay bands a late syngenetic low temperature hydrothermal fluid injection process is suggested for formation of the feldspar veins the origin of the fluid is uncertain but is most likely associated with contemporaneous volcanic activity

the relationship between plate curvature and elastic plate thickness a study of the peru chile trench the age of the nazca plate where it enters the peru and northern chile trenches varies from 30 ma in the north to 45 ma in the south as its dip beneath the south american continent steepens from 13 to 30 if the elastic thickness te of oceanic lithosphere depends only on its age and therefore thermal state we would expect that te determined from fitting the flexure of the lithosphere over the outer rise as revealed in the depth geoid and gravity anomalies would increase from the peru trench in the north to the northern chile trench further south we find that the opposite is true the lithosphere appears suffer outboard of the peru trench than it does offshore chile to explain deflections of the lithosphere seaward of peru the isotherm controlling the elastic ductile transition must be near 800 c providing the thermal structure of the plate is that predicted by the standard thermal plate model because the decrease in plate stiffness to the south is correlated with an increase of plate curvature over the outer rise and outer trench wall we interpret our result in terms of inelastic yielding of the oceanic lithosphere when bent to high strains the magnitude of the reduction in strength however suggests that the standard oceanic yield strength envelope underestimates the amount of failure for lithosphere bent to high strains unless the stress field off northern chile is characterized by 100 mpa level tension the more highly bent segment of subducting lithosphere at this trench also dips at a steeper angle at greater depth beneath the continent but detailed analysis of plate geometry does not support a relationship between slab dip as determined by earthquake hypocenters and elastic behavior over the outer rise

mineralogical and fluid inclusion studies of low sulfidation epithermal veins at osilo sardinia italy several precious metal bearing low sulfidation epithermal veins occur in the rolling topography of the osilo area northern sardinia the sa pala de sa fae and the sa pedra bianca veins were subject to intense diamond drilling exploration in the mid 1990 s the veins extend for 1 3 km dip steeply and range from 1 to 10 m in width high k calc alkaline volcanic deposits containing plagioclase phenocrysts along with lesser pyroxene amphibole magnetite olivine and sanidine form the main host rocks gold grades in drill intersections range from 50 m from the vein the assemblage albite fe chlorite illite pyrite montmorillonite calcite k feldspar prevails regionally and its formation is attributed to minor metasomatism of the country rock involving the addition of water carbon dioxide and hydrogen sulfide at distances 10 m from the vein the assemblage quartz k feldspar pyrite illite dominates forming an alteration envelope that cross cuts regional alteration quartz and k feldspar increase in abundance towards the vein quartz is the main vein mineral and it displays a range of morphologies and textures including crustiform colloform banding quartz pseudomorphs of platy calcite breccias and coarse euhedral crystals electrum and argentite which are the main gold and silver minerals deposited during the early stages of vein mineralization with rhomb shaped crystals of k feldspar adularia pyrite plus lesser marcasite arsenopyrite stibnite and sphalerite are the other sulfide phases in veins kaolinite halloysite jarosite form a late assemblage overprinting earlier hydrothermal alteration it is mostly restricted to shallow depths of a few meters except near veins most of this assemblage likely formed from weathering and oxidation of sulfides microthermometric measurements were made on quartz hosted two phase liquid vapor inclusions containing 75 liquid mean homogenization temperatures 750 measurements range from 220 to 250 c and ice melting temperatures 550 measurements range from 0 0 to 2 3 c the presence of co existing vapor rich and liquid rich inclusions with quartz pseudomorphs of platy calcite indicate that boiling conditions existed slight vapor bubble expansion of a few fluid inclusions subjected to crushing experiments indicates inclusion fluids contained variable but low concentrations of dissolved gas this study shows that gold silver mineralization formed in subvertical channels from ascending solutions at 250 c at around 300 to 450 m below the paleo water table in a typical low sulfidation epithermal environment hydrothermal solutions that produced vein mineralization and related alteration were dilute 4 1 equivalent wt nacl and 4 wt co2 near neutral ph reduced and at times boiling

speculations on the petroleum geology of the accretionary body an example from the central aleutians abstract in the 300 km wide adak amlia sector of the central aleutian trench 36 000 km 3 of offscraped trench fill makes up the wedge shaped mass of the aleutian accretionary body within this wedge seismic reflection profiles reveal an abundance of potential hydrocarbon trapping structures these structures include antiforms thrust and normal faults and stratigraphic pinchouts maximum closure on these features is 2 km in addition the silt and possibly sand size sediment within the offscraped turbidite deposits and the porous diatomaceous pelagic deposits interbedded with and at the base of the wedge may define suitable reservoirs for the entrapment of hydrocarbons potential seals for these reservoirs include diagenetically altered and produced siliceous and carbonate sediment the organic carbon input into the central aleutian trench based on carbon analyses of dsdp legs 18 and 19 core samples suggests that the average organic carbon content within the accretionary body is approximately 0 3 0 6 heat flow across the aleutian terrace indicates that at present the oil generation window lies at a depth of 3 6 5 km at depths of 8 km which corresponds to the maximum depth the offscraped sediment has been seismically resolved beneath the lower trench slope the probable high 170 180 c temperatures prohibit all but gas generation the dewatering of trench sediment and subducted oceanic crust should produce an abundance of fluids circulating within the accretionary body these fluids and gases can conduct hydrocarbons to any of the abundant trapping geometries or be lost from the system through sea floor seepage in the aleutian accretionary body all the conditions necessary for the formation of oil and gas deposits exist the size and ultimate preservation of these deposits however are dependent on the deformational history of the prism both during accretion and after the accretion process has been superceded by subsequent tectonic regimes