intercalibration of astronomical and radioisotopic time the 40ar 39ar radioisotopic dating technique is one of the most precise and versatile methods available for dating events in earth s history but the accuracy of this method is limited by the accuracy with which the ages of neutron fluence monitors dating standards are known calibrating the ages of standards by conventional means has proved difficult and contentious the emerging astronomically calibrated geomagnetic polarity time scale apts offers a means to calibrate the ages of 40ar 39ar dating standards that is independent of absolute isotopic abundance measurements seven published 40ar 39ar dates for polarity transitions nominally ranging from 0 78 to 3 40 ma are based on the fish canyon sanidine standard and can be compared with apts predictions solving the 40ar 39ar age equation for the age of the fish canyon sanidine that produces coincidence with the apts age for each of these seven reversals yields mutually indistinguishable estimates ranging from 27 78 to 28 09 ma with an inverse variance weighted mean of 27 95 0 18 ma normalized residuals are minimized at an age of 27 92 ma indicating the robustness of the solution

late cenozoic evolution of epithermal gold metallogenic provinces in kyushu japan kyushu island japan is located at the junction of the southwest japan arc and the ryukyu arc there are two major late cenozoic epithermal gold silver provinces in kyushu which are termed the northern and southern provinces the provinces are characterized by 1 pliocene volcanism dominated by calc alkaline andesite followed by quaternary volcanism including extrusion of both calc alkaline and tholeiitic magmas 2 formation of extensional grabens 3 pliocene to pleistocene mineralization which was dominated by abundant low sulfidation ls epithermal deposits with a few high sulfidation hs examples the two epithermal gold silver provinces have evolved differently since about 5 ma the northern province has exhibited diminished hydrothermal activity from the pliocene to pleistocene whereas the southern province has witnessed increased hydrothermal activity mainly in easterly and northerly directions changes of tectonic setting from the pliocene to pleistocene account for the variable trends in epithermal gold deposit formation westward oblique subduction of the philippine sea plate beneath the southwest japan arc caused development of the hohi graben and arc related volcanism at about 6 ma this was associated with widespread ls mineralization in and surrounding the hohi graben as is represented by the bajo and taio deposits the subduction of the relatively buoyant kyushu palau ridge during the early pliocene strengthened the coupling between the slab and overriding ryukyu arc leading to polygenetic andesite volcanism with associated hs kasuga iwato and akeshi and ls kushikino mineral deposits forming in the southern province a change of the subduction direction of the philippine sea plate from west to north northwest in the early pliocene increased the orthogonal convergence rate between the southwest japan arc and the philippine sea plate resulting in a decrease of volcanic and hydrothermal activity in the hohi graben of the northern province the more northerly subduction of the philippine sea plate shifted the locus of the kyushu palau ridge subduction northward resulting in underplating of the older 85 60 ma negatively buoyant amami basin oceanic slab in the southern province rather than continued subduction of the young 27 15 ma buoyant shikoku basin slab this replacement caused steepening of the slab angle and slab rollback in the southern province which was associated with regional extension an eastward shift of the ryukyu volcanic front and development of the kagoshima and shimabara grabens as well as the okinawa trough rhyolite and basalt volcanism in addition to andesite volcanism have occurred since 2 ma in the area of the ryukyu back arc coincident ls mineralization at hishikari and ohkuchi was affiliated with the rhyolite volcanism another change of the subduction direction of the philippine sea plate to the northwest occurred at 2 1 ma the forearc sliver of the southwest japan arc shifted westward in association with right lateral strike slip faulting along the median tectonic line due to the increase of the westward convergence rate this shift resulted in shortening and cessation of graben development in the hohi area restricting the subsequent volcanism and related hydrothermal activity to the central part of the graben

apoyo caldera nicaragua a major quaternary silicic eruptive center apoyo caldera near granada nicaragua was formed by two phases of collapse following explosive eruptions of dacite pumice about 23 000 yr b p the caldera sits atop an older volcanic center consisting of lava flows domes and ignimbrite ash flow tuff the earliest lavas erupted were compositionally homogeneous basalt flows which were later intruded by small andesite and dacite flows along a well defined set of n s trending regional faults collapse of the roof of the magma chamber occurred along near vertical ring faults during two widely separated eruptions field evidence suggests that the climactic eruption sequence opened with a powerful plinian blast followed by eruption column collapse which generated a complex sequence of pyroclastic surge and ignimbrite deposits and initiated caldera collapse a period of quiescence was marked by the eruption of scoria bearing tuff from the nearby masaya caldera and the development of a soil horizon violent plinian eruptions then resumed from a vent located within the caldera a second phase of caldera collapse followed accompanied by the effusion of late stage andesitic lavas indicating the presence of an underlying zoned magma chamber detailed isopach and isopleth maps of the plinian deposits indicate moderate to great column heights and muzzle velocities compared to other eruptions of similar volume mapping of the apoyo airfall and ignimbrite deposits gives a volume of 17 2 km3 within the 1 mm isopach crystal concentration studies show that the true erupted volume was 30 5 km3 10 7 km3 dense rock equivalent approximately the volume necessary to fill the caldera a vent area located in the northeast quadrant of the present caldera lake is deduced for all the silicic pyroclastic eruptions this vent area is controlled by n s trending precaldera faults related to left lateral motion along the adjacent volcanic segment break fractional crystallization of calc alkaline basaltic magma was the primary differentiation process which led to the intermediate to silicic products erupted at apoyo prior to caldera collapse highly atypical tholeiitic magmas resembling low k high ca oceanic ridge basalts were erupted along tension faults peripheral to the magma chamber the injection of tholeiitic magmas may have contributed to the paroxysmal caldera forming eruptions

scrubbing process and chemical equilibria controlling the composition of light hydrocarbons in natural gas discharges an example from the geothermal fields of el salvador 1 the compositional features of fluids from both fumarolic discharges and productive geothermal wells of ahuachapan chipilapa berlin chinameca and san vicente geothermal systems el salvador are described and discussed in order to investigate the complex geochemical interactions involving geothermal fluids within the shallowest part of the hydrothermal circulation pathways our results highlight that secondary processes are able to strongly affect and modify the chemical characteristics of geothermal gases once they discharge to the surface as natural manifestations mainly in relation to the chemical physical properties of each gas species the effects of both gas dissolution in shallow aquifers and gas water rock chemical interactions on gas discharge composition make it difficult to get a correct evaluation of the thermodynamic conditions that characterize the geothermal reservoirs by applying the common geoindicators based on the chemical equilibria of the h2o co2 h2 ch4 co system differently the composition of the c1 c2 c3 alkanes and the c3 and c4 alkane alkene pair established within the geothermal reservoirs under the control of chemical reactions remains stable in samples collected from discharging gas vents these results suggest that the relative abundances of hydrocarbons characterized by similar structure and molecular size seem to be mainly regulated by the diffusion velocity of gases through the liquid dominated system therefore the chemical features of the light organic gas fraction of naturally discharging fluids can be successfully utilized for the evaluation of geothermal reservoir temperatures and redox conditions providing useful indications in terms of geothermal exploration and exploitation on this basis the distribution speciation and relative abundances of light hydrocarbons can also be considered highly promising in geochemical monitoring of active volcanic systems

a chlorine disinfectant for excess argon released from k felsspar during step heating abstract the release of excess 40 ar 40 ar e from virtually all alkali feldspars at low temperatures 40 ar 39 ar step heating experiments obscures potentially valuable age and thermal history information we report a method that takes advantage of the differential release of 40 ar e between contiguous isothermal heating steps due to the decrepitation of fluid inclusions and which permits derivation of a correction for the contaminating argon differences in age and cl k between successive temperature pairs commonly yield a single well correlated array on a 40 ar k versus cl k plot that identifies the cl correlated composition of 40 ar e 40 ar e cl affecting that sample eighteen of the twenty k feldspar samples we have measured yield a single 40 ar e cl component that varies from 1 1 10 3 to 4 2 10 6 making possible recovery of otherwise unobtainable thermochronological information the upper bound on the 40 ar e cl distribution of 10 3 may reflect ar saturation of pore fluids or the maximum duration between deuteric alteration and cooling below ar closure in k feldspar

minimum volume of a tephra fallout deposit estimated from a single isopach a simple method is presented to calculate the minimum volume of an exponentially thinning fall deposit for which only one isopach is available this method exploits the fact that the product of the area enclosed within an isopach a and its thickness t reaches a maximum value ta max for a certain isopach it is shown that the minimum volume is vmin 3 69ta which is equal to the actual volume when ta ta max calculation of vmin for 327 isopachs from 74 fallout deposits shows that it is often just a bit lower than the volume calculated from all the available isopachs it is therefore proposed that vmin provides a useful minimum volume estimate for fallout deposits for which only one isopach is available

the chemistry of geothermal waters in iceland iii chemical geothermometry in geothermal investigations abstract new data from geothermal wells in iceland have permitted empirical calibration of the chalcedony and na k geothermometers in the range of 25 180 c and 25 250 c respectively the temperature functions are t c 1112 4 91 log sio 2 273 15 t c 933 0 993 log na k 273 15 concentrations are expressed in ppm these temperature functions correspond well with the chalcedony solubility data of fournier 1973 and the thermodynamic data for low albite microcline solution equilibria of heloeson 1969 a new co2 geothermometer is proposed which is considered to be useful in estimating underground temperatures in fumarolic geothermal fields its application involves analysis of co2 concentrations in the fumarole steam the temperature function which applies in the range 180 300 c is logco2 37 43 73192 t 11829 103 t2 0 18923t 86 187 logt where t is in k and co2 in moles per kg of steam

40ar 39ar polyorogenic mineral age record in the northern mauritanide orogen west africa abstract northern segments of the mauritanide orogen in the akjoujt region are dominated by suprastructural nappe complexes these structurally overlie other tectonic elements including 1 autochthonous parautochthonous foreland units composed of late proterozoic late paleozoic frasnian sedimentary cover sequences and unconformably underlying crystalline basement late archean middle proterozoic of the west african shield 2 parautochthonous successions largely represented by low grade metasedimentary rocks and 3 allochthonous infrastructural complexes with characteristics similar to those of late proterozoic early paleozoic pan african tectonic elements exposed in central and southern parts of the mauritanide orogen amphibole from various structural levels of the ijibiten and akjoujt suprastructural nappe complexes generally display internally discordant saddle shaped 40 ar 39 ar release spectra suggestive of extraneous excess argon contamination one sample records a well defined intermediate temperature plateau of 2420 ma muscovite within lower and intermediate structural levels of the complexes displays variably discordant 40 ar 39 ar age spectra which suggest that variable thermal rejuvenation of older up to 2600 ma mineral systems occurred during a ca 310 300 ma metamorphism uppermost structural levels of the complexes are represented in part by penetratively mylonitic metagranite muscovite within these rocks displays internally concordant 40 ar 39 ar age spectra which define plateau ages of 300 310 ma there is no indication in either amphibole or muscovite results of any late proterozoic early paleozoic pan african thermal record together with field relationships the 40 ar 39 ar results suggest that structural units of the suprastructural nappe complexes are in part composed of variably overprinted basement rocks these structurally overlie allochthonous pan african sequences and therefore must have been derived west of and now largely conceal the pan african orogen the complexes were transported eastward to higher structural levels during late paleozoic orogenesis the extent of late paleozoic thrusting within the akjoujt region contrasts with more limited transport distances documented southward in the orogen this contrast probably reflects a kinematic influence of the reguibat promentory of the west african shield which during the late paleozoic collision of gondwana and laurentia effected extensive compression in proximal sequences

excess argon in k ar and ar ar geochronology the k ar and ar ar dating techniques occasionally produce anomalously old ages attributed to excess argon and such data is often rejected as not offering any insight into the age thermal history or geochemistry of the rock however improvements in the quantification of argon geochemistry now provide a framework to model excess argon in both open and closed systems solubility data for argon in hydrous fluids melts and emerging data for minerals can be used to understand the behaviour of excess argon and provide valuable insights into the environment in which the samples cooled to their argon retention or closure temperature treating excess argon as a trace element also throws light on its behaviour in minerals above the closure temperature in deeply buried dry systems such as eclogites blueschists granulites and even in the lithospheric mantle extremely low partition coefficients between k feldspar and hydrous fluid phases predict lower excess argon susceptibility than micas and this is observed in fluid poor systems variation of partition coefficients can lead to excess argon in fluids being introduced into minerals or removed from minerals as grain boundary fluids change during flow through a rock however excess argon can also be introduced or removed from minerals by varying temperature without the need for fluid flow high mineral melt and mineral fluid partition coefficients are also the reason why excess argon is often concentrated in inclusions within minerals partition coefficients between minerals and hydrous fluids as low as 10 6 lead fluid inclusions to dominate the radiogenic argon budget particularly in low potassium minerals melt inclusions are less dominant but become critical in dating younger samples

the plinian eruptions of 1912 at novarupta katmai national park alaska the three day eruption at novarupta in 1912 consisted of three discrete episodes episode i began with plinian dispersal of rhyolitic fallout layer a and contemporaneous emplacement of rhyolitic ignimbrites and associated proximal veneers the plinian column was sustained throughout most of the interval of ash flow generation in spite of progressive increases in the proportions of dacitic and andesitic ejecta at the expense of rhyolite accordingly plinian layer b which fell in unbroken continuity with purely rhyolitic layer a is zoned from 99 to 15 rhyolite and accumulated synchronously with emplacement of the correspondingly zoned ash flow sequence in mageik creek and the valley of ten thousand smokes vtts only the andesiterichest flow units that cap the flow sequence lack a widespread fallout equivalent indicating that ignimbrite emplacement barely outlasted the plinian phase on near vent ridges the passing ash flows left proximal ignimbrite veneers that share the compositional zonation of their valley filling equivalents but exhibit evidence for turbulent deposition and recurrent scour episode ii began after a break of a few hours and was dominated by plinian dispersal of dacitic layers c and d punctuated by minor proximal intraplinian flows and surges after another break dacitic layers f and g resulted from a third plinian episode iii intercalated with these proximally are thin intraplinian ignimbrites and several andesite rich fall flow layers both cd and fg were ejected from an inner vent 400 m wide nested within that of episode i into which the rhyolitic lava dome novarupta was still later extruded two finer grained ash layers settled from composite regional dust clouds layer e which accumulated during the d f hiatus includes a contribution from small contemporaneous ash flows and layer h settled after the main eruption was over both are distinct layers in and near the vtts but distally they merge with cd and fg respectively they are largely dacitic but include rhyolitic shards that erupted during episode i and were kept aloft by atmospheric turbulence published models yield column heights of 23 26 km for a 22 25 km for cd and 17 23 km for fg and peak mass eruption rates of 0 7 1x108 0 6 2x108 and 0 2 0 4x108 kg s 1 respectively fallout volumes adjusted to reflect calculated redistribution of rhyolitic glass shards are 8 8 km3 4 8 km3 and 3 4 km3 for episodes i ii and iii microprobe analyses of glass show that as much as 0 4 km3 of rhyolitic glass shards from eruptive episode i fell with cde and 1 1 km3 with fgh most of the rhyolitic ash in the dacitic fallout layers fell far downwind se of the vent near the rhyolite dominated ignimbrite however nearly all of layers e and h are dacitic showing that the downwind rhyolitic ash is of co plinian rather than co ignimbrite origin

paleostress and fluid pressure regimes inferred from the orientations of hishikari low sulfidation epithermal gold veins in southern japan abstract the orientation distribution of dilational fractures is affected by the state of stress around the fractures and by the pressure of the fluid that opened the fractures thus the distribution can be inverted to determine not only the stress but also the pressure condition at the time of vein formation however epithermal ore veins that we observe today are the results of a great number of intermittent upwelling of overpressured fluids with different pressures here we define driving pressure index dpi as the representative non dimensionalized fluid pressure for the fluids we collected the orientations of 1000 ore veins in the hishikari gold mine which were deposited at around 1 ma in southern kyushu japan it was found that the majority of the veins were deposited under an extensional stress with a nw se trending 3 axis and a northeasterly inclined 1 axis with relatively high stress ratio the representative driving pressure ratio was 0 2 data sets obtained at different depths in the mine indicated a positive correlation of representative driving pressure ratios with the depths the correlation suggests repeated formation and break of pressure seals during the mineralization our compilation of the pliocene quaternary stress regimes in southern kyushu including the result of the present study suggests that epithermal gold mineralization was associated with distributed extensional deformations in southern kyushu and strain localization into an intra arc rift seems to have terminated the mineralization

seismic petrologic and geodetic analyses of the 1999 dome forming eruption of guagua pichincha volcano ecuador abstract guagua pichincha located 14 km west of quito ecuador is a stratovolcano bisected by a horseshoe shaped caldera in 1999 after some months of phreatic activity guagua pichincha entered into an eruptive period characterized by the extrusion of several dacitic domes vulcanian eruptions and pyroclastic flows we estimated the three dimensional 3 d p wave velocity structure beneath guagua pichincha using a tomographic inversion method based on finite difference calculations of first arrival times hypocenters of volcano tectonic vt earthquakes and long period lp events were relocated using the 3 d p wave velocity model a low velocity anomaly exists beneath the caldera and may represent an active volcanic conduit petrologic analysis of eruptive products indicates a magma storage region beneath the caldera having a vertical extent of 7 8 km with the upper boundary at about sea level this zone coincides with the source region of deeper vt earthquakes indicating that a primary magma body exists in this region lp swarms occurred in a cyclic pattern synchronous with ground deformation during magma extrusions the correlation between seismicity and ground deformation suggests that both respond to pressure changes caused by the cyclic eruptive behavior of lava domes

thermochronology of economic mineral deposits dating the stages of mineralization at panasqueira portugal by high precision 40 39 ar age spectrum techniques on muscovite 40 ar 39 ar age spectrum dates for 13 muscovites have been used to reconstruct the thermal history thermochronology of the panasqueira portugal tin tungsten deposit a deposit spatially associated with a belt of hercynian plutons muscovite samples with an age difference as small as 2 2 m y 0 7 of the age are statistically distinct statistics are even better for comparison of multiple samples from separate events that is a difference of 0 9 m y 0 3 can be resolved in this approximately 300 m y old deposit the major tin and tungsten ore forming stages which are the oxide silicate stage the main sulfide stage and greisenization occurred between 296 3 or 0 8 1 sigma and 291 6 or 0 8 m y 1 sigma the first substage of the oxide silicate stage was a short lived thermal pulse at 296 3 or 0 6 m y the fluids responsible may have emanated from the known granite cupola the main sulfide stage was active at 294 5 or 0 9 m y as a slightly longer lived pulse with oldest evidence for this stage 295 8 or 0 6 m y coming from areas farthest away from the known cupola and youngest evidence 293 5 or 0 8 m y closest to the cupola a second substage of the oxide silicate stage occurred as a short lived thermal pulse at 292 9 or 0 7 m y synchronous with greisenization of the cupola and alteration of the silica cap at 292 1 or 0 4 m y the duration of activity of the oxide silicate stage the main sulfide stage greisenization and alteration of the silica cap based on the ages of all 13 muscovites was greater than 4 2 or 0 5 m y 1 sigma minor argon loss from all dated muscovites occurred during later reheating probably during the longer lived pyrrhotite alteration stage a single center the known cupola had a prolonged role and was the source for main sulfide stage oxide silicate stage ii greisenization and alteration of the silica cap and possibly oxide silicate stage i and the pyrrhotite alteration stage however a separate source for these latter two stages cannot be ruled out this study is an example of a new and powerful application of 40 ar 39 ar age spectrum dating of muscovite because of the high precision demonstrated in this study it is now possible to establish time constraints necessary for solving some of the long standing problems in economic geology beyond this the unique geologic situation of panasqueira has allowed us to quantify the thermal characteristics of muscovite published fluid inclusion data have been used to estimate a muscovite argon closure temperature of approximately 325 degrees c during rapid cooling or short reheating and a temperature of approximately 270 degrees c during slow cooling or extended reheating argon loss patterns displayed by all dated muscovites resulted from reheating after original closure the mechanism for this argon loss appears to have been argon transport by volume diffusion thus 40 ar 39 ar age spectrum dating of muscovite can be used to evaluate thermal conditions controlling argon diffusion as well as age duration and number of episodes of mineralization

the hekla eruption 1980 1981 the sixteenth eruption of hekla since 1104 began on august 17th 1980 after the shortest repose period on record only ten years the eruption started with a plinian phase and simultaneously lava issued at high rate from a fissure that runs along the hekla volcanic ridge the production rate declined rapidly after the first day and the eruption stopped on august 20th a total of 120 million m3 of lava and about 60 million m3 of airborne tephra were produced during this phase of the activity in the following seven months steam emissions were observed on the volcano activity was renewed on april 9th 1981 and during the following week additional 30 million m3 of lava flowed from a summit crater and crater rows on the north slope

adularia in epithermal veins queensland morphology structural state and origin four types of adularia i e sub rhombic rhombic tabular and pseudo acicular are recognised from examination of samples from ten epithermal vein deposits and prospects in queensland based on morphology of the individual crystals further investigation of the structural state of adularia reveals that each group has some specific features in terms of the degree of al si disordering which can be related to various crystallisation conditions and the thermal history sub rhombic adularia is commonly 2 4 mm in size and subhedral with more or less rhombic outlines and has a relatively ordered al si distribution 2t1 0 84 reflecting slow crystallisation conditions adularia of this type in association with coarse grained quartz is predominant at deep levels of epithermal systems where boiling is initiated in an environment of low permeability and the fluid is slightly supersaturated with respect to adularia and quartz tabular adularia characterised by its lath shape and disordered structure 2t1 values ranging from 0 64 to 0 74 is likely to have formed when the fluid moves up to a more permeable environment and starts boiling violently relatively high temperatures and rapidly changing conditions account for its special morphology and disordered structure rhombic adularia showing very small crystal size 0 2 mm with euhedral rhombic form has an intermediate degree of al si ordering pseudo acicular adularia is interpreted as pseudomorphs after carbonate and its high al si ordered structure is attributed to the presence of a carbonate precursor these two types of adularia commonly occur within crustiform and colloform bands in association with high grade ore and chalcedony or fine grained quartz which often displays various recrystallisation textures it is most likely that adularias of these two types are formed when extensive boiling is protracted microprobe analyses indicate the composition of all adularia types close to pure kalsi3o8 sericite and or carbonate altered adularias consistently display more ordered structures suggesting that the post crystallisation thermal regime affects the structural state of altered adularia even at temperatures as low as in epithermal environments

plinian airfall deposits of basaltic composition two widespread scoria fall deposits locally known as the fontana lapilli and san judas formation are demonstrated to have issued from masaya caldera complex nicaragua and to be the first documented plinian airfall deposit of basaltic composition data from maps of median grain size average maximum pyroclast diameter and average maximum diameter of lithic fragments provide conclusive evidence of the plinian nature of both deposits the volume within the 1 mm isopach of the fontana lapilli is 12 km3 3 4 km3 dense rock equivalent and within the san judas formation it is 1 2 km3 0 35 km3 dre theoretical eruption parameters calculated for the fontana lapilli and san judas formation respectively include muzzle velocity 170 m s and 150 m s column height 50 km and 18 km volume eruption rate 2 105 m3 s and 1 104 m3 s and duration 2 h and 10 h these are all similar to the calculated parameters for silicic plinian eruptions basaltic magmas are generally considered incapable of erupting in the manner characteristic of plinian eruptions recognition of them argues that factors other than high viscosity may be of fundamental importance

medium temperature fumarolic gas sampling in order to sample medium temperature fumaroles gases are collected by means of iron or glass tubings and conveyed to the condensation system through glass dewar tubes one type of condenser was designed for condensate measurement and gas sampling for radon activity determination the other with naoh absorbing solution for the s cl ratio measurement and the sampling of rare gases with this method it is possible to obtain precise gas vapour measurements as well as air free gas samples

the kago low sulfidation gold and silver deposit a peripheral mineralisation to the nansatsu high sulfidation system southern kyushu japan abstract the kago deposit is a small deposit located at the southern tip of the satsuma peninsula of southern kyushu japan it lies proximal to the well known nansatsu type mineralisation province dominated by high sulfidation type epithermal deposits the deposit was heavily mined in the 18 th century largely for its relatively higher gold compared to that of surrounding and regional deposits the kago deposit is a typical low sulfidation deposit characterised by adularia quartz veins composed of electrum ag tetrahedrite polybasite chalcopyrite and pyrite based on mine records the grade ranged from 4 1 to 13 3 g t au and 2 6 6 6 g t ag alteration grades from low to high temperature argillic into a propylitic zone at the extremes of the vein exposures carbonate is absent fluid inclusion microthermometry reveals a typical temperature range of 220 240 c with salinity of 0 7 2 6 nacl eq wt electrum from high grade ore ranges from 66 to 69 wt au 40ar 39ar age dating of adularia bearing colloform crustiform and brecciated veins suggests a mineralisation event from 4 23 to 4 0 ma 18o of veined and silicified quartz ranges from 4 0 to 18 4 18oh2o of fluids in equilibrium with quartz in the dominant range of measured fluid inclusion temperatures ranges from 6 5 to 0 2 34s of pyrite has a narrow range from 1 8 to 2 7 the deposit lies at the northern extent of the classic nansatsu high sulfidation epithermal area in which a number of silicified bodies punctuate the region in a roughly semi circular shape the kago deposit lies within the principle mineralisation age range of the high sulfidation deposits which range from 5 5 to 3 7 ma the structural displacement of the kago deposit from the nansatsu mineralisation and the differing host rocks has greatly influenced alteration ore and rock water interaction of the ore depositing fluids here we seek to establish the relationship that this extended mineral province has between the differing styles of mineralisation

review of methods of estimating rhyolitic tephra volumes applications to the taupo volcanic zone new zealand there have been at least 35 rhyolitic plinian eruptions in the last 50 kyr from 2 volcanic centres in taupo volcanic zone tvz new zealand to determine the total volume of erupted material calculation techniques are reviewed and further developed all involve extrapolation from isopach maps three methods assume an exponential decay of thickness with distance from source and a further two are based on the area enclosed by each isopach comparison of methods shows those based on area to be more accurate except the log log plot of area and thickness which requires excessive extrapolation a log volume vs thickness plot is considered superior because minimal extrapolation is needed for most tephras volumes calculated by the different methods are in reasonable agreement volumes previously calculated by the crystal concentration method are 2 3 times greater than those from the six methods used here total pumice fragmentation is probably not required for the liberation of crystals and this would decrease calculated volumes during the last 50 kyr magma has been erupted at a constant mean rate in the taupo volcanic zone a simple step function shows a strong relationship between deviations from the mean rate and either the next erupted volume or the time to the next eruption extrapolation of these step functions suggests future eruptions in less than 400 years for okataina and about 8000 years for taupo the volume of rhyolitic material erupted from tvz within the last 50 kyr is estimated at 360 km3 of tephra 350 km3 of ignimbrite and 55 km3 of extrusive equivalent to about 350 km3 of magma at an average rate of eruption of 7 km3 year at least 4200 km3 of rhyolitic magma has been erupted since the inception of tvz about 0 6 m y ago if back are spreading of tvz at 7 mm yr is compensated for by dyke intrusion into the upper 10 km of crust 8400 km3 of magma is required the total volume of magma generated by partial melting is estimated at 12 600 km3 removal of this melt would cause subsidence of 2 1 km the present depth to subvolcanic basement in tvz

precipitation of gold in a low sulfidation epithermal gold deposit insights from a submillimeter scale oxygen isotope analysis of vein quartz sir we welcome the opportunity to respond to bogie and lawless discussion of our paper on the submillimeter scale oxygen isotope variation of vein quartz from the low sulfidation style gold deposit at the hishikari mine the disputed point is whether or not the bladed and fine grained quartz precipitated initially as chalcedony and recrystallized to quartz in a subsequent stage similarity of texture between chalcedony e g typically found in agate and flaky or acicular bladed quartz suggests that there is some truth in bogie and lawless idea the texture of bladed quartz suggests rapid crystallization from solutions supersaturated with respect to quartz however it is difficult to define only from the texture whether

pyroclastic geology of the rhyolitic volcano of la primavera mexico explosive eruptions of this large quaternary rhyolitic volcano have produced voluminous pumice fall deposits and ignimbrites a stratigraphie study reveals more than 30 fall deposits totalling more than 90 km3 which have come from many different vents some are of plinian type and one is among the largest known in the world the main ignimbrite rio caliente is of intra plinian type in all 140 km3 of rhyolitic material were erupted during the past 105 years of which 50 km5 are dispersed well outside the volcano the dense rock equivalent volume is 60 km3 of which fall deposits ignimbrites and lava bodies comprise 45 25 and 30 respectively the average output rate of 0 06 km3 per century is nearly an order of magnitude less than for the most productive rhyolitic volcanoes known a circular area 10 km across in the centre of la primavera contains updomed lacustrine ashes and associated sediments including a remarkable giant pumice bed which probably accumulated in a caldera lake la primavera has no record of historical eruptions and hot springs are the only signs of present activity but the updoming is thought to be due to the uprise of a new acid pluton beneath the volcano and future eruptions are probable

bladed quartz and its relationship to gold mineralization in the hishikari low sulfidation epithermal gold deposit japan bladed quartz frequently is observed in low sulfidation epithermal gold deposits bladed calcite also has been documented in boiling zones of some active epithermal systems and boiling in these systems has been directly linked to gold mineralization in the hishikari low sulfidation epithermal gold deposit the distribution and texture of fluid inclusions within bladed quartz reveal a similar relationship to gold precipitation the formation of bladed quartz at hishikari involves several stages 1 deposition of bladed calcite 2 precipitation of fine grained adularia and quartz on the surface of calcite blades 3 dissolution of calcite blades leaving cavities in the interstices between aggregates of adularia and quartz and 4 infilling of the cavities by later quartz i e pseudomorphs of the original bladed calcite bladed quartz is present largely in the deeper part of the vein system beneath the high grade gold ore zone at hishikari this distribution may be explained by the fact that the original bladed calcite formed at depth in the system where boiling and loss of co 2 initially caused the precipitation of the calcite and quartz formed as pseudomorphs of the original calcite blades

40ar 39ar geochronology of the pongkor low sulfidation epithermal gold mineralisation west java indonesia abstract the pongkor gold deposit located approximately 90 km southwest of jakarta is well known as the typical low sulfidation ls epithermal gold deposit and the largest gold vein deposit in indonesia the pongkor gold mineralisation has been the subject of several geological studies determination of the duration of gold mineralisation events however is less well understood the mineralisation events are important for understanding the genesis of ore deposits this study presents 40ar 39ar age results for four adularia samples from ciguha gold bearing quartz vein yielding plateau ages of 1 80 0 03 ma 2 33 0 32 ma and 1 88 0 02 ma plus one weighted mean age of 1 95 0 04 ma adularia from the kubang cicau gold bearing quartz veins yielded a 40ar 39ar plateau age of 2 02 0 03 ma the period of adularia growth in the ciguha vein may have extended from ca 2 07 to 1 77 ma that includes the ages of 1 95 0 04 ma and 1 88 0 02 ma the time interval indicates multiple generations of the adularia the data suggest that mineralisation of ciguha may have continued episodically or continuously the age of adularia from the kubang cicau gold bearing quartz vein 2 02 0 03 ma is within the error of the ciguha vein age and the adularia growth of kubang cicau vein reveals at the beginning of ciguha s adularia growth the data suggest that gold mineralisation in the pongkor deposit for kubang cicau and ciguha veins was initiated at the relatively similar time the 40ar 39ar adularia ages of this study are within error of the previously reported 40ar 39ar results milesi et al 1999 but significantly younger than the previously reported k ar adularia dating result kageyama 1999 in rosana and matsueda 2002

eruption of the nevado del ruiz volcano colombia on 13 november 1985 tephra fall and lahars a small plinian eruption of the nevado del ruiz volcano in colombia ejected 3 5 x 1010 kilograms of mixed dacite and andesite tephra on 13 november 1985 with a maximum column height of 31 kilometers above sea level small pyroclastic flows and surges generated during the initial stage of the eruption caused surface melting of approximately 10 of the volcano9s ice cap leading to meltwater floods the erosive floods incorporated soils and loose sediments from the volcano9s flanks and developed into lahars which claimed at least 25 000 lives

stochastic modeling for the stress inversion of vein orientations paleostress analysis of pliocene epithermal veins in southwestern kyushu japan plio pleistocene epithermal quartz veins in southern kyushu japan include gold deposits the coherent trend of the ore veins suggests tectonic control for their formation however the stress regime during the formation has been controversial to solve this problem we improved existing methods for inferring paleostresses from vein orientations it was assumed that veins making a cluster were formed intermittently from thermal fluids with various pressures the present method determines stress ratio and stress axes with 95 confidence regions the method was applied to mid pliocene quartz veins cropping out at hashima southwestern kyushu we obtained a normal faulting regime of stress with the trend of sigma3 at 167 10 and the stress ratio at 0 20 0 13 0 09 the low stress ratio and the lack of slickenlines and slickenfibers on vein walls suggest that the host rock was subject to a small differential stress i e a weak tectonic stress when the veins were formed

guagua pichincha volcano ecuador fluid geochemistry in volcanic surveillance abstract the densely populated metropolitan area of quito is located on the slopes of the active guagua pichincha volcano at only 10 km from the crater recently the italian ministry of foreign affairs sponsored a project for the mitigation of volcanic hazard in this area the geochemical study carried out as part of this project was aimed at constructing a geochemical model of the zone for use in volcanic surveillance according to this geochemical model a hydrothermal aquifer t 200 240 c fed both by meteoric waters and by fluids released by a magma body lies at shallow levels beneath guagua pichincha crater the crater fumaroles are essentially fed by steam boiled off from the hydrothermal aquifer the high flow rate fumaroles located in the dome area show significant so2 contents which suggest a relatively high contribution of magmatic fluids in the zone of the aquifer feeding them the absence of so2 in the fumarolic discharges near the southern crater wall indicates instead that the magmatic fluids dissolve entirely into the aquifer here the hot springs located at the western end of the crater represent the lateral discharge of the hydrothermal aquifer on the basis of this model it is likely that an increment in the flux of both the magmatic fluids and the heat from a magma body produces an increase albeit small of the pressure temperature conditions of the hydrothermal system and consequent changes in flow rate and fluid chemistry of the fumarolic vents in particular total sulphur and possibly hydrochloric acid may increase in all the vents and sulphur dioxide may appear in other fumarolic discharges the varying thermodynamic conditions in the hydrothermal aquifer can be evaluated on the basis of the equilibria among carbon species and hydrogen only minor delayed changes are expected in the physical chemical characteristics of the springs located at the western end of the crater

the history of vein formation determined by 40ar 39ar dating of adularia in the hosen 1 vein at the hishikari epithermal gold deposit japan the hosen 1 vein has a symmetrical banded structure within the hishikari epithermal gold deposit in japan the vein can be divided into four macroscopic bands we used 40ar 39ar dating of adularia by incremental step heating to determine the duration of mineralization in the individual bands of the vein and the history of vein formation plateau ages were obtained from all adularia samples and no significant excess 40ar is recognized from inverse isochrons the derived age spectra suggest that thermal overprinting of the epithermal system did not affect the plateau ages although early bands must have been reheated by the thermal pulses associated with later bands the 40ar 39ar ages of adularia indicate that vein formation occurred episodically in distinct periods separated by intervals of about 30 000 to 110 000 yr the hosen 1 vein grew in at least six increments at the 20 m level and at least four increments at the 62 5 m level the total duration of mineralization from the margin to the center of the hosen 1 vein was about 260 000 yr during which time the temperature of the hydrothermal fluid decreased from 190 to 210 c to less than 170 c

timing of magmatic activity and mineralization and evidence of a long lived hydrothermal system in the fresnillo silver district mexico constraints from 40ar 39ar geochronology the ore deposits of fresnillo have produced silver for more than 450 years and contain resources of approximately 1 goz mineralization in fresnillo occurs in epithermal veins hosted by a middle jurassic to late cretaceous volcanosedimentary sequence guerrero terrane late paleocene conglomerates and eocene to oligocene tuffs and flows of trachyte rhyolite and rhyodacite lie unconformably on top of the guerrero terrane steam heated acid sulfate alteration containing alunite veins occurs in eocene tuffs above some of the mineralized veins oligocene rhyolitic domes and a quartz monzonite stock also occur in the area whereas sporadic miocene basaltic flows and dikes are the youngest igneous rocks in the district spatial association of tertiary volcanic rocks intrusions acid sulfate alteration and epithermal veins makes the district ideal for evaluating the temporal relations between magmatism and hydrothermal activity using precise 40ar 39ar geochronology sanidine single crystal laser fusion analyses reveal discrete volcanic eruptions at 44 73 0 04 40 92 0 04 40 83 0 04 31 55 0 03 and 31 04 0 05 ma dating of orthoclase from the fortuna quartz monzonite was problematic due to excess argon contamination but a preferred age of 32 65 0 05 ma is suggested two adularia samples from the jarillas vein give statistically identical ages of 29 75 0 12 and 29 68 0 10 ma adularia from valdecanas vein shows a climbing age spectrum related to hydrothermal alteration however a terminal step with an age of 29 74 0 07 ma suggests that the two veins may be coeval alunite recorded two distinct ages at 30 56 0 04 and 31 03 0 05 ma alunite and adularia ages suggest that acid sulfate alteration and mineralization occurred separately well after emplacement of the dated phase of the fortuna stock and consequently it cannot be the source of heat for the hydrothermal system that formed those finally the 3 m y difference in age between epithermal mineralization and the fortuna stock is indicative of a long lived but episodic hydrothermal system which could have been driven by multiple pulses of small and shallow intrusions