fluid inclusions in salton sea scientific drilling project core preliminary results fluid inclusions 191 in calcite quartz k feldspar and epidote from l mm veinlets in cores and well cuttings from 604 2560 m homogenize from 217 to 500 c and vary widely in salinity suggesting a complex history of fluids surrounding these samples no daughter minerals were seen and no clathrates were recognized on freezing vapor rich inclusions under pressure presumably containing co2 and or ch4 were found from a wide range of depths suggesting that effervescence has occurred low salinity fluids 1 2 4 0 wt nacl eq were present as deep as 1939 m the data can be explained by a combination of processes such as thermal metamorphism of evaporites and other sediments and mixing of water from metamorphic dehydration reactions with partly evaporated colorado river water

active circulation of saline ground waters in carbonate platforms evidence from the great bahama bank measurements of salinity temperature and ground water discharges within the great bahama bank provide evidence of active circulation of near normal ocean water beneath north andros island elevated salinity waters 38 42 derived by density refulx from the great bahama bank flow eastward beneath the island and mix with normal salinity cold ground waters 19 20 c from deeper than 250 m in the adjacent oceans east to west flow of water from the straits of florida may be driven by head differences generated by the florida current which impinges on the western margins of the platform alternatively density differences between the reflux and tongue of the ocean sea waters may cause more local circulation on the eastern flanks these flows have important diagenetic implications particularly in the explanation of pervasive secondary dolomitization widely reported at shallow depths in the bahamas

chemistry and environments of dolomitization a reappraisal abstract dolomitization of calcium carbonate can best be expressed by mass transfer reactions that allow for volume gain preservation or loss during the replacement process experimental data as well as textures and porosities of natural dolomites indicate that these reactions must include co 3 2 and or hco 3 supplied by the solution to the reaction site since dolomite formation is thermodynamically favoured in solutions of a low ca 2 mg 2 ratios b low ca 2 co 3 2 or ca 2 hco 3 ratios and c high temperatures the thermodynamic stability for the system calcite dolomite water is best represented in a diagram with these three parameters as axes kinetic considerations favour dolomitization under the same conditions and additionally at low as well as at high salinities if thermodynamic and kinetic considerations are combined the following conditions and environments are considered chemically conducive to dolomitization 1 environments of any salinity above thermodynamic and kinetic saturation with respect to dolomite i e freshwater seawater mixing zones normal saline to hypersaline subtidal environments hypersaline supratidal environments schizohaline environments 2 alkaline environments i e those under the influence of bacterial reduction and or fermentation processes or with high input of alkaline continental groundwaters and 3 many environments with temperatures greater than about 50 c subsurface and hydrothermal environments whether or not massive replacive dolostones are formed in these environments depends on a sufficient supply of magnesium and thus on hydrologic parameters most massive dolostones particularly those consisting of shallowing upward cycles and capped by regional unconformities have been interpreted to be formed according to either the freshwater seawater mixing model or the sabkha with reflux model however close examination of natural mixing zones and exposed evaporitic environments reveals that the amounts of dolomite formed are small and texturally different from the massive replacive dolostones commonly inferred to have been formed in these environments many shallowing upward sequences are devoid of dolomite it is therefore suggested that massive replacive dolomitization during exposure is rare if not impossible rather only small quantities of dolomite cement or replacement are formed which may act as nuclei for later subsurface dolomitization alternatively large scale dolomitization may take place in shallow subtidal environments of moderate to strong hypersalinity the integration of stratigraphic petrographic geochemical and hydrological parameters suggests that the only environments capable of forming massive replacive dolostones on a large scale are shallow hypersaline subtidal environments and certain subsurface environments

a brine interface in the salton sea geothermal system california fluid geochemical and isotopic characteristics data from 71 geothermal production intervals in 48 wells from the salton sea geothermal system ssgs indicate that fluids in that system cluster into two distinct populations in terms of their salinity and their stable isotopic compositions the distinctive hot hypersaline brine typically 20 wt total dissolved solids for which the ssgs is known is overlain by a cooler 260 c fluid with distinctly lower salinity typically 10 wt total dissolved solids hypersaline brines have high and rather consistent 18o shifts produced by water rock interaction and have a very narrow range in d values low tds fluids on the other hand show a wide range in both d and 18o production of both types of fluid from closely spaced geothermal wells in many regions of the ssgs indicates that a relatively sharp salinity interface exists over much of the field the fluid interface typically cross cuts sedimentary bedding but is consistently found where reservoir temperatures are approximately 260 c at these temperatures hypersaline brines have densities of approximately 1 0 gm cm3 while the low tds fluids have densities as low as 0 85 gm cm3 this stable density stratified interface acts as a barrier to convective heat and mass transfer in the ssgs isolating the hypersaline reservoir from overlying dilute fluids a lithologic cap implied by previous ssgs models is unnecessary in such a stratified system since heat and mass transfer across the interface must occur by slow conductive diffusional and interface mixing processes regardless of local permeability

the heating duration and provenance age of rocks in the salton sea geothermal field southern california abstract 40ar 39ar step heating of single k feldspar crystals 500 g in a furnace with a low argon blank and accurate temperature control provide constraints on the thermal history of the salton sea geothermal field southern california estimates of 40ar loss from detrital k feldspars coupled with kinetic information gleaned from laboratory degassing of 39ar allow possible thermal histories to be evaluated samples from core and cuttings of sandstones from the state 2 14 landers 1 dearborn farms 1 magmamax 2 elmore 3 and sinclair 3 wells have been analyzed for this purpose no recent 40ar loss is recognized in any of the 17 crystals measured even for samples currently at temperatures greater than 275 c this observation suggests that the present temperature distributions in these wells have not been maintained for greater than 1000 years however aggregates 100 mg of two k feldspar samples do reveal recent 40ar loss from the state 2 14 well suggesting a time scale of 1000 to 5000 years for near peak thermal conditions these estimates of heating duration are broadly consistent with earlier results from thermal modelling but do not support interpretations that paleotemperatures were once hotter than present provenance ages span the tertiary through middle proterozoic but cluster at ages of 25 70 and 1250 ma these age peaks are very common k feldspar k ar ages in detachment terranes of southeastern california and southwestern arizona both drained by the colorado river this coincidence supports earlier suggestions that the sands of the salton trough were derived mainly from the colorado river unrealistically low apparent activation energies

saline brines and metallogenesis in a modern sediment filled rift the salton sea geothermal system california u s a the salton sea geothermal system ssgs is the site of active hydrothermal metamorphism and metallogenesis in the delta of the colorado river which partially fills the salton trough rift zone at the head of the gulf of california growth of the delta across the rift has isolated the northern part of the salton trough since the pleistocene forming the evaporative salton sea basin whose sediments host the ssgs more than 70 commercial geothermal wells including a 3 2 km deep research borehole drilled as part of the salton sea scientific drilling project sssdp are yielding a wealth of new data from this system within the ssgs active greenschist facies metamorphism is occuring at temperatures 365 c at only 2 3 km depth by reaction of nacakfemncl brines with the deltaic and lacustrine pilo pleistocene sediments two kinds of base metal ore mineralization occur at depth stratabound diagenetic fe sulfides and epigenetic vertical veins containing fezncupb sulfides and fex oxides the vein mineralization occurs as two types an older reduced carbonate sulfide assemblage and a modern oxidized silicate hematite sulfide assemblage the earlier assemblage formed at temperatures up to 100 c higher than the ambient temperatures measured in the wells today implying that cooler oxidized fluids have displaced hot reduced fluids a sharp interface between shallow fluids containing 12 wt tds and deep hypersaline brines containing 15 27 wt tds exists in the ssgs the deeper hypersaline brines are rich in base metals fe 1500 ppm mn 1000 ppm zn 500 ppm whereas the overlying lower salinity fluids contain less than 100 ppm each of fe mn and zn the modern silicate hematite sulfide vein assemblage is precipitating where the two fluids appear to be mixing the two fluid types also have distincty different 18o and d relations the shallow lower salinity fluids have only partially exchanged oxygen with deltaic and lacustrine host rocls the deep hypersaline brines have exchanged oxygen extensively at 250 c with the deltaic sediments d values of the hypersaline brines are typically lighter than either the lower salinity fluids or the modern evaporating groundwaters occupying the salton trough implying a different source for the original fluids the high salinities of the geothermal brines are derived from a combination of evaporation of fossil lakewaters groundwater dissolution of shallow lacustrine evaporites and subsurface hydrothermal metamorphism of buried lacustrine evaporites episodic filling and desiccation of the closed salton sea basin has allowed cold saline brines to form and percolate down into the sedimentary section in some wells anyhydrite meta evaporites and interbedded solution collapse breccias occur at 1 km depth the anyhydrite contains fluid inclusions that are saturated in halite at their homogenization temperatures of 300 c recording the hydrothermal dissolution of bedded salt based on sr and pb isotopic data and whole rock chemical data the source of the metals in the hypersaline brines is from leaching of the host sediments however the origin of reduced s for ore mineralization remains an enigma 34s values for vein sulfides cluster uniformly around zero per mil implying that an isotopically homogeneous source of reduced sulfide exists in the brines it is possible that the vein sulfides receive a constribution from a magmatic s source movement and mixing of brines of different chemistry and oxidation states play a major role in ore genesis our model envisages an early stage in which a deep brine pool accumulated at depth in the sedimentary section by partial section by partial evaporation of basin and dissolution of bedded salts intrusion of rift related basaltic magma into the base of the sedimentary caused heatingand fracturing of the sediments resulting in precipitation of the carbonate sulfide veins during pore fluid expulsion heating also caused a diapir of the hypersaline brine to rise and displace colder less saline shallower pore fluids this brine intrusion was accompanied by pervasive and extensive mineralization as this diapir cooled it began to move downward drawing in shallow more oxidized fluids and causing the formation of the modern silicate hematite sulfide vein ore zone

segmentation and coalescence of cenozoic carbonate platforms northwestern great bahama bank seismic profiles over the northwestern great bahama bank reveal that it is formed by the coalescence of three smaller platforms and significant lateral progradation this conclusion is contrary to previous models which assumed that the bank is the result of continuous accumulation on one huge buildup in late cretaceous to early tertiary time a north south trending depression the straits of andros separated an eastern platform andros bank from a western platform bimini bank initially the straits of andros had dimensions similar to the modem tongue of the ocean but was progressively filled from east to west in early to middle tertiary time a second depression the bimini embayment formed within bimini bank by folding it had a maximum depth of about 470 m this depression was also filled from east to west in addition prograding systems built the western margin of bimini bank more than 25 km westward into the straits of florida

groundwater flow beneath a hypersaline pond cluett key florida bay florida copyright c 1997 elsevier science b v all rights reserved florida bay carbonate mud islands are known to be sites of holocene diagenesis including dolomitization and the hydrology is an essential component of this process on cluett key a typical mud island in western florida bay groundwater salinities range to 145 beneath a shallow ephemeral pond which occupies 70 of the island a sharp hydrostratigraphic boundary separates low permeability unconsolidated carbonate mud from underlying highly permeable pleistocene limestone we report the results of a 2 year project aimed at determining the rates patterns and driving force of groundwater flow beneath the island water level measurements are used to demonstrate the presence of a large hydraulic head drive caused by the difference in elevations of the pond and florida bay and enhanced by the higher density of groundwaters in the carbonate mud compared with groundwater in the limestone the hydraulic head drive is essentially vertical because florida bay water levels are transmitted with little attenuation to the limestone underlying the island distributions of groundwater density and pore pressures are consistent with vertical groundwater flow based on an estimated vertical hydraulic conductivity of approximately 5 10 3 m day 1 vertical interstitial velocities are on the order of 25 cm year 1 with a residence time in the carbonate mud of approximately 15 years this velocity is very similar to that calculated independently from tritium concentrations in pore waters both horizontal and vertical density gradients exist in the carbonate mud these density variations induce circulations owing to vorticity and may lead to the formation of instability plumes reflux but dynamical scaling suggests that these motions are much slower than those induced by the dominant hydraulic drive buoyancy effects may however be dominant on other lower islands in florida bay where the hydraulic head drive is much smaller than on cluett key diffusion may blur sharp gradients in salt ca 2 or mg 2 but is not an important vehicle for wholesale movement of these species through the island cluett key shares some similarities with holocene carbonate atoll islands the two layer hydrostratigraphy and transmission of tidal signals under the island in contrast to atoll islands however groundwater flow on cluett key is governed by transmission of the mean sea level to the base of the holocene basal tidal fluctuations induce little mixing in the low permeability holocene sediments in addition the groundwater on cluett key is dominantly oceanic not meteoric and is consequently denser than underlying groundwater 1997 elsevier science b v

zinc copper and lead in mid ocean ridge basalts and the source rock control on zn pb in ocean ridge hydrothermal deposits the contents of zn cu and pb in mid ocean ridge basalts morb and the morb source rock control on zn pb in ocean ridge hydrothermal deposits are examined the values of zn cu and pb for submarine mid ocean ridge basalts morb are respectively in ppm average morb 75 75 and 0 7 west valley juan de fuca ridge jfr 87 64 and 0 5 southern jfr 120 and 0 5 and 21 n east pacific rise epr 73 78 and 0 5 values of zn pb range from about 100 240 and cu pb from 100 156 in this study zn is found to correlate positively with tio2 feo mean square of weighted deviates mswd of 1 6 for jfr basalt and inversely with mg number mswd of 3 5 therefore contrary to statements in the literature that zn should be compatible in morb zn is a mildly incompatible element and must be enriched in the glass phase relative to olivine as zn does not fit into the other major phenocryst phase plagioclase in the source of morb zn likely is most enriched in oxides spinel magnetite and titanomagnetite copper generally does not correlate well with other elements in most morb data examined when differentiation is dominated by olivine cu has a tendency to behave incompatibly e g at mg numbers 70 but overall cu shows some tendency towards being a compatible element particularly along the mid atlantic ridge a behavior presumably due to separation of sulfides in which cu but not zn is markedly enriched copper thus may be in dispersed sulfides in the source of morb ocean ridges provide important data on source rock controls for sulfide deposits because in sediment starved ridges much is known about the possible source rocks and mineralization is presently occurring in contrast to zn pb 5 in continental hot cl rich brines zn pb in the hottest sediment starved ridge black smoker hydrothermal fluids at 21 n epr is about 110 similar to local morb 145 but cu pb is closer to 30 possibly due to subsurface deposition of cu at the jfr the best value of zn pb in the hydrothermal fluids is about 175 again similar to local morb 240 but cu is very low in the fluids that are at temperatures less than 300 c the large morb like zn pb in the hottest black smoker fluids suggests a source rock control for the metals that prohibits significant galena in the black smoker deposits of sediment starved ridges in contrast exhalative deposits on sediment swamped ridges have significant galena its presence is suggestive of pb derivation from sediments an origin supported by pb isotope studies of lehuray and colleagues in 1988

putting transport into water rock interaction models new quantitative models for water rock interaction are needed that explicitly couple fluid flow and chemical reactions one such model is based on the finite difference method and is applicable to both one and two dimensional reactive flow in applying the model we find that ignoring the coupling between hydrodynamic dispersion and chemical reactions leads to incorrect predictions of the rates of reactions along fluid flow paths and in cases of which reactions will occur

genesis of native copper lodes in the keweenaw district northern michigan a hybrid evolved meteoric and metamophogenic model the keweenaw peninsula of northern michigan is well known for early diagenetic sulfide dominant sediment hosted stratiform copper mineralization hosted mainly by basal siltstone shale units of the nonesuch formation in the white pine district and later native copper lodes hosted mainly by flow top breccias and amygdaloidal portions of flood basalts and interbedded conglomerates of the stratigraphically lower portage lake volcanics the native copper mineralization is generally attributed to downdip burial metamorphism of the host strata an alternative model is proposed here involving a complementary influx of evolved meteoric water elevated in salinity and progressively equilibrating with its aquifers similar to the fluid which formed the white pine mineralization this model has several advantages over the metamorphogenic model alone it would provide the salinity by dissolution of evaporites and moderately oxidizing conditions as a result of hematitization i e reddening of coarse continental rift sediments necessary for copper transport and the evolved meteoric water would have been an important cupriferous brine owing to the leaching of trace amounts of copper from the voluminous aquifers further reduction of the evolved fluid would also explain the updip deposition of native copper in association with dehematitized pumpellyite and chlorite dominant i e reduced hydrothermal mineral assemblages at the same time this influx of meteoric water from highlands adjacent to the keweenaw rift basin would have provided a mechanism to move the deep seated hot dense ore brine upward along the aquifers of the portage lake volcanics on its ascent from deep portions of the rift basin the gravity driven evolved meteoric brine would have mixed with the fluid generated by burial metamorphism to form a hybrid evolved meteoric metamorphogenic oreforming hydrothermal brine

potential salinity driven free convection in a shale rich sedimentary basin example from the gulf of mexico basin in south texas we investigate the plausibility of salinity driven free thermohaline convection in sedimentary rocks of the south texas part of the gulf of mexico basin using salinity data rayleigh number calculations and numerical models previous studies speculated that free convection could account for high fluxes evidently required for diagenesis in the basin but low permeability shales are calculated to be a barrier to free convection in the study area salinity inversions occur either above or within the transition zone from hydropressures to overpressures the positioning of brines over less saline fluids provides a significant buoyancy force rayleigh number calculations and numerical simulations suggest that homogeneous shaly systems are unstable near the high end of the expected ranges of shale permeability 10 15 10 16 m2 numerical simulations show that the influx of brine into the permeable layers and permeability heterogeneity in the shales are both conducive to free convection simulations indicate that salinity driven free convection can occur at lower permeabilities 10 16 10 18 m2 that may approximate the permeabilities of shales in the zone of extreme overpressures over geologic time simple rayleigh numbers are inadequate to predict the occurrence of free convection in heterogeneous systems salinity driven free convection at depths in some large sedimentary basins such as the gulf of mexico may be more common than expected

numerical models of fluid flow in carbonate platforms implications for dolomitization abstract the role of fluid flow in dolomitization is evaluated through hydrologic simulations designed to capture and test numerically the essential elements of some common dolomitization models a two dimensional finite difference code bethke 1985 was used to simulate single phase flow of variable salinity fluids driven by compaction convection and topography in carbonate platforms and adjacent basinal sediments a simple hydrostratigraphic model consisting of a platform reef complex surrounded by basin filling shale was constructed to examine the magnitude and direction of compaction flow during the initial phase of subsidence most pore fluids in the shale migrate to the surface at velocities less than 0 1 mm yr and basin derived fluids are focused into the reef from only a limited distance of 10 km relatively porous and permeable basinal limestones and sandstones may act as conduits for updip fluid flow into the platform reef complex magnesium mass balance constraints coupled with hydrologic simulations suggest that burial compaction is not an efficient mechanism for regional dolomitization two layer hydrostratigraphic models involving variable salinity fluids show that evaporation of seawater results in downward and seaward fluid migration the magnitude of flow is a function of fluid density even fluids at salinities marginally elevated above normal seawater values are capable of penetration several hundreds of meters into the underlying sediments albeit at rates of less than 1 cm yr paleohydrologic simulations coupled with thermodynamic arguments support that reflux of surface derived waters evaporated past gypsum saturation is a viable dolomitization mechanism calculations utilizing rates of mg mass flux show that dolomitization of large carbonate platforms 10 km3 by fluids below halite saturation requires millions of years of reflux driven flow p thermal convection of seawater through carbonate platforms has been simulated by varying the basal heat flow the thermal pump produces a convection cell in which lateral and vertical flow achieve rates of 75 0 and 13 0 mm yr respectively the potential for dolomitization is high because a large reservoir of mg seawater is tapped elevated temperatures drive fluids further into the stability field for dolomite and kinetic inhibition of dolomite nucleation and growth is decreased simulations of high frequency glacio eustatic sea level oscillations suggest that a dynamic hydrologic regime may be established a sea level fall of 2 m lasting for 10 000 yr can drive lateral flow at rates up to 0 15 m yr dolomitization is possible because marine waters migrate through the carbonate platform in response to gradients of hydraulic head created by eustatic sea level fluctuations

reply sulphur isotope composition of the brunswick no 12 massive sulphide deposit bathurst mining camp new brunswick implications for ambient environment sulphur source and ore genesis the brunswick no 12 massive sulphide deposit occurs within a middle ordovician bimodal volcanic and sedimentary sequence that is thought to have formed in a continental back arc rift covered with a thick succession of carbonaceous hemipelagic and turbiditic sedimentary rocks the deposit consists of three en echelon lenses that are zoned from vent complex to bedded ore and bedded pyrite facies the bedded ore facies has the lowest average 34s values 14 2 but are only slightly less positive than laminated pyrite in footwall sedimentary rocks 34smean 15 1 34s values for the bedded sulphides show an upward increase from 14 2 in bedded ore to 16 5 in bedded pyrite average 34s values for vent complex 15 8 and underlying stringer sulphides 16 1 are consistently more positive than those for bedded ore in carbonaceous shales and siltstone of the patrick brook formation that underlie the deposit 34s values that range between 13 8 and 25 6 and the similarity of these values to those of the bru

fluid density distribution in a high temperature stratified thermohaline system implications for saline hydrothermal circulation density distribution within the salton sea geothermal system of fluids ranging from 20 c to 325 c has been computed using chemical and thermal data from geothermal production well tests and curve fit models of nacak chloride solution properties density corrections can easily be made to 0 01 g cm3 for solute effects of each of the dominant chloride salts as well as pressure above vapor saturation field data on dissolved co2 is too sparse to routinely compute a correction however this moderately but variably abundant component can be shown to produce only minor 0 01 g cm3 errors in density estimates fluid density within the shallow cool 20 wt the density of brine climbs rapidly to near 1 0 g cm3 as the salinity increase across this interface overwhelms the thermal effect on fluid density this steep density gradient precludes all but diffusional conductive or perhaps double diffusive convective mass and heat transfer in this transitional regime measurement uncertainties for reservoir depths temperatures and salinities commonly exceed errors of our density model limiting the accuracy of details in our modeled density distribution gross scale relationships of this salinity stratified geothermal system are however apparent permitting rational discussion of extrapolations to conditions beneath the presently explored reservoir and inference of dynamics during the systems evolution

reservoir scale heterogeneity in depositional packages and diagenetic patterns on a reef rimmed platform upper miocene mallorca spain predictive models for interwell scale variations in heterogeneous carbonate rock are best made from outcrop studies of well exposed limestone and dolomite such as the upper miocene reef complex that crops out in the sea cliffs of mallorca spain the sea cliff sections reveal highly complex lithofacies stacking patterns that could lead to ambiguous lateral correlations of coeval units the stratigraphic complexity and distribution of primary and secondary porosity are the result of a sea level driven hierarchical stacking of different magnitudes of accretional units thickest sections of porous and permeable rocks are in the aggradational portions of the reef upper slope and outer lagoon units the relative volume of the various accretional units and heterogeneity in the lithofacies architecture were dependent on the amount of carbonate production which was related to 1 accommodation changes controlled by sea level fluctuations and 2 depositional profile moldic porosity mostly from the dissolution of aragonitic constituents is the predominant porosity type and its heterogeneous distribution is related to the lateral and vertical distribution of lithofacies the secondary porosity in much of this reef complex was produced mainly during early dolomitization dolomite patterns are complicated apparently mainly related to shallow flooding of the platform during third or fourth order sea level highs and to geographic location of permeable pathways for brine reflux probably primarily through fourth order aggradational reef units many of these stratigraphic complexities and diagenetic patterns are below the resolution of seismic and well analyses thus models based on outcrop data such as this can enhance reservoir development in certain shallow water carbonate rocks

chemical and isotopic 87sr 86sr δ18o δd constraints to the formation processes of red sea brines abstract about twenty deeps filled with hot brines and or metalliferous sediments are located along the red sea axis these brines present a well suited framework to study the hydrothermal activity in such a young ocean the present study outlines the results of a geochemical approach combining major trace element and isotopic oxygen hydrogen strontium analyses of brines in six of the deeps to evaluate different processes of brine formation and to compare the evolution of each deep important heterogeneities in temperature salinity hydrographic structure and chemistry are recorded each brine having its own characteristics the intensity of hydrothermal circulation varies among the deeps and ranges from being strong atlantis ii and nereus to weak port soudan and even to negligible valdivia and suakin and it varies along the entire red sea axis these observations do not favour a unique formational model for all of the brines for example the brines of the suakin deep appear to have been formed by an old sea water which dissolved evaporite beds without significant fluid circulation and hydrothermal input while others such as atlantis ii or nereus deeps appear to be dominated by hydrothermal influences a striking feature is the absence of a relationship between the position of the deeps along the axis and their evolutionary maturity

density driven groundwater flow in closed desert basins field investigations and numerical experiments abstract with a numerical model duffy and al hassan demonstrated that free convection of brine could occur under the playa lake in a closed basin since then continued effort has been made to further our understanding of the role of free convection in closed basin hydrology in this paper we present the results of 5 years of field investigation in pilot valley nevada of a literature search for field studies in closed basins around the world and of numerical simulations to examine the effect of heterogeneity on free convection and the effect of climate changes observations along the divide playa transect revealed evidence of forced convective flow and the existence of a hinge line the salinity profile and the inferred flux suggest that diffusion is the dominant solute transport mechanism under the field sites but the stable isotopes o h suggest that free convection of playa brine also occurs we propose a model that explains these observations first free convection could occur farther out on the playa where the density of surface brines is higher second once free convective flow penetrates the surface sediments it could migrate laterally in the more permeable layers of alluvium third free convection could occur during periods of time when brines pond on the surface for extended periods the search for field studies of groundwater processes in other closed basins leads to a few cases where free convection has occurred on a basin scale based on this survey we discuss the geologic and climatic conditions that favor free convection numerical simulations are carried out to examine the effect of basin scale heterogeneity in hydraulic conductivity the results indicate that heterogeneity changes the pattern of flow by creating multiple convection cells finally we address how a closed basin may respond to climate change climate changes alter the salinity of surface brines and the freshwater recharge both acting as boundary conditions for subsurface processes using dimensional analysis and numerical simulations we evaluate the effect of a warming and drying trend on free convection and how the geologic features of a closed basin affect the response 1997 elsevier science b v

hydrologic framework of a sabkha along arabian gulf during the last 4 000 to 5 000 years sedimentary offlap and a relative fall in sea level have resulted in the development of broad gently seaward sloping about 1 3 000 planar areas or sabkhas along much of the south shore of the arabian persian gulf the depth of the water table below the sabkha surface increases with distance from the sea until it reaches 1 0 to 1 5 m after which it remains constant when the water table depth is in the range of 1 0 to 1 5 m the sabkha surface has reached a state of deflational equilibrium the water table beneath the sabkha always slopes seaward indicating that the direction of ground water flow is toward the sea the subsurface flow system beneath the sabkha is part of a regional seaward flowing groundwater regime in the seaward part of the sabkha a downward flow component is superimposed on the general seaward ground water flow pattern as a result of the infiltration and percolation of water from shallow bodies of seawater that are propelled inland over the sabkha surface by strong onshore winds farther inland where marine flooding does not occur the vertical flow component is directed upward in response to evaporative losses at the sabkha surface the net rate of evaporation from the central parts of the sabkha is on the order of 6 cm factor of 2 of ground water per year a regional ground water flow system which causes the water table level to rise as the sabkha progrades appears to be a requirement for the development and the preservation of broad sabkhas fluctuations in sea level during sabkha development influence the rate of progradation and the thickness of sedimentary facies the refluxing of sabkha brines beneath the lagoons in the abu dhabi area indicates a mechanism for preserving anhydrite during marine transgressions thus permitting the development of stacked sabkha successions which are common in the rock record

outcrop analog for cyclic shelf reservoirs san andres formation of permian basin stratigraphic framework permeability distribution geostatistics and fluid flow modeling vertical and horizontal transects were sampled from core and outcrop of the san andres formation at lawyer canyon guadalupe mountains new mexico to assess permeability variation in a geologic framework of upward shallowing carbonate cycles and to show the potential effect these variations have on viscous dominated flow behavior in analogous reservoirs these cycles occur in a ramp crest facies tract are 3 13 m 10 45 ft thick and contain both vertical and lateral variation of lithofacies thicker cycles consist of a basal dolomudstone which is overlain by burrowed dolopackstone and capped by bar flank ooid peloid dolograinstone and bar crest ooid dolograinstones in vertical transects permeability is extremely variable about the mean yet upward increasing trends coinciding with the succession of lithofacies typify a given cycle semi variance analysis shows permeability to be uncorrelated vertically at distances greater than 5 5 m 18 ft which is the average cycle thickness suggesting that the cycles may equate to a fluid flow unit in a reservoir semi variance analysis of measurements collected along a horizontal transect within bar crest dolograinstones of a single cycle shows permeability is uncorrelated at distances greater than 3 6 m 12 ft this correlation distance appears to be controlled by alternating porous and tightly cemented zones that formed during dolomitization vertical and lateral variogram models were fit to the spatial parameters to generate a variety of conditionally simulated permeability fields fluid flow simulations show viscous dominated flow behavior is compartmentalized by both the individual cycles and groups of cycles the basal dolomudstones are potential baffles to flow crossover between cycles but poorly developed cycles i e those that are mud rich and lack well developed bar flank and bar crest facies result in the greatest compartmentalization of fluid flow within a succession of cycles

9 processes of tectonism magmatism and mineralization lessons from europe abstract metallogenic provinces in europe range in age from the archaean to the neogene deposit types include porphyry copper and epithermal cu au volcanic hosted massive sulphide vms orogenic gold fe oxide cu au anorthosite fe ti oxide and sediment hosted base metal deposits most of them formed during short lived magmatic events in a wide range of tectonic settings many can be related to specific tectonic processes such as subduction hinge retreat accretion of island arcs continental collision lithosphere delamination or slab tear in contrast most sediment hosted deposits in europe evolved in extensional continental settings over significant periods of time in europe as elsewhere ore formation is an integral part of the geodynamic evolution of the earth s crust and mantle many tectonic settings create conditions conducive to the generation of water rich magma but the generation of ore deposits appears to be restricted to locations and short periods of change in temperature and stress imposed by transitory plate motions crustal influence is evident in the strong structural controls on the location and morphology of many ore deposits in europe crustal scale fault fracture systems many involving strike slip elements have provided the fabric for major plumbing systems rapid uplift as in metamorphic core complexes and hydraulic fracturing can generate or focus magmatic hydrothermal fluid flow that may be active for time spans significantly less than a million years once a hydrologically stable flow is established ore formation is strongly dependent on the steep temperature and pressure gradients experienced by the fluid particularly within the upper crust in europe significant fracture porosity deep in the crystalline basement 1 is not only important for magmatic hydrothermal systems but allows brines to circulate down through sedimentary basins and then episodically upward expelled seismically to produce sediment hosted base metal deposits and kupferschiefer copper deposits emerging research stimulated by geode can improve the predicting power of numerical simulations of ore forming processes and help discover the presence of orebodies beneath barren overburden

the massive dolomitization of platformal and basinal sequences proposed models from the paleocene northeast sirte basin libya abstract the paleocene carbonate succession in the northeast sirte basin is composed of two shallowing upward ramp cycles where each cycle is under and overlain by deeper water pelagic facies a significant proportion of each of these two cycles is dolomitized petrographic study supported by geochemical data stoichiometry stable isotopes trace elements and fluid inclusions and integrated with broader tectono sedimentary information has provided the basis for interpreting these paleocene dolomites the use of this integrated approach in the study of dolomites suggests that despite the much publicized uncertainties in interpreting geochemical analyses of ancient dolomites the results of the paleocene dolomites show that the geochemical characteristics are generally consistent with regional stratigraphic distribution and petrographic observations four distinct types of dolomite have been recognized in this part of the sirte basin based on the stratigraphic position and petrographic criteria two of these types have a platformal setting and the other two are basinal the platform varieties consist of dolomicrites and pervasive stratal dolomites the dolomicrites interpreted to be of syn sedimentary origin were probably a product of reflux of seawater with elevated salinity as suggested by palaeoenvironmental analysis and supported by geochemical evidence the average s 80 value is 0 1 pdb the average sr content is 639 ppm the pervasive dolomites were formed during the progradation of the platform sequences and probably stabilized and augmented during shallow burial a meteoric marine mixing zone is thought to have been the most likely process for the formation of these dolomites this interpretation is supported by geochemical evidence the average 18o is 2 4 pdb the average sr content is 72 ppm combined with a favourable stratigraphic position the most characteristic feature related to both mixing zone and reflux dolomitization is the basinward movement of the dolomitizing fluids which suggests that the formation of these platform dolomites was related to a lowstand system tract the two basinal varieties comprise thick over 300 m basinal dolomudstones and fracture filling sparry dolomites the stratigraphic position of the finely crystalline basinal dolomudstones within very thick shale successions as a result of being very close to the depocentre of the sirte basin combined with geochemical evidence the average 18o is 6 4 pdb suggest that the dolomitizing fluids were basin derived with mg2 released from dewatering through compaction of basinal shales the occurrence of this type of dolomite provides one of the rare examples of large scale dolomitization of thick basinal sequences late diagenetic fracture filling dolomites exhibit a structural control on their distribution geochemical evidence including fluid inclusion analysis and the lightest oxygen isotopic signature of 7 3 pdb suggests that highly saline formation brines were the solutions responsible for their formation

the significance of viscosity in density dependent flow of groundwater abstract many modeling studies of variable density groundwater flow have been performed in the last few decades in most of these studies fluid density is considered to vary with concentration while the variation of viscosity with concentration is neglected the consequences of this negligence is not completely known the present study uses a numerical simulation approach to investigate the density viscosity concentration relationship during groundwater flow and solute transport through a density stratified system fluid density is assumed to increase with depth from freshwater at the surface through brackish and saline waters to brines at the bottom half of the system the system mimics field observations at the atikokan research area ara in northwestern ontario canada hypothetical unit basin models consisting of recharge midline and discharge area regimes are employed simulations with the density concentration equation of state and a constant freshwater viscosity in the density stratified system causes groundwater to sink against the buoyancy forces of the system more water is recharged into the system than necessitated by the buoyancy the configurations and lengths of travel paths and travel time of conservative contaminants are inaccurately predicted accounting for concentration in the viscosity equation causes groundwater floating in agreement with the expected buoyancy physics of the system overestimation of concentration dependent density causes sinking whereas overestimation of viscosity results in overfloating and underestimation of groundwater recharge even in density stratified fluids with salinity of seawater recharge and through flow of water may be slightly overestimated if a concentration dependent depsity is used along with a constant freshwater viscosity the concentration dependence of both density and viscosity are to be analyzed carefully during groundwater flow and solute transport simulations in systems with considerable fluid density variations

characteristics and origin of the oak dam east breccia hosted iron oxide cu u au deposit olympic dam region gawler craton south australia oak dam east is a large 560 mt of 41 56 wt fe cu au u bearing massive fe oxide deposit in the northeastern gawler craton south australia it is one of several unmined deposits in the same province as the giant olympic dam deposit and it lies beneath more than 500 m of younger cover rocks the oak dam east breccia complex formed in a near surface environment and was characterized by an early history of skarnlike albite calc silicate magnetite formation from hypersaline 400 500 c fluids large scale brecciation was an early feature and the resulting complex was progressively filled in and replaced by 1 hematite and goethite hematite stage i 2 magnetite apatite quartz actinolite magnetite stage 3 hematite goethite quartz chalcedony pyrite chlorite monazite hematite stage ii and 4 chalcopyrite pitchblende illite hydromus covite florencite carrolite cu u au mineralization stage hematite stage ii assemblages are volumetrically most abundant and were formed from fluids of 1 to10 wt percent nacl equiv that were boiling at 170 to 190 c and had 18o values more negative than 3 7 to 1 7 per mil interpreted as meteoric water the upper 200 m of the breccia complex contains spectacular colloform hematite void fillings interpreted to have replaced hydrothermal goethite the cu u mineralized zones resemble olympic dam ore in terms of geometries being grossly subhorizontal 10 to 70 m thick and crosscutting with respect to the host breccias the mineralization is zoned vertically from pyrite 34s 6 to 0 at depth to shallower chalcopyrite 34s 14 to 7 high grade pitchblende occurs centrally within the chalcopyrite zone e g 10 m of 0 46 cu and 3 700 ppm u3o8 straddling a boundary between k mica illite alteration and underlying fe mg chlorite alteration the deposit is assigned to the 1570 to 1595 ma iron oxide cu au u event in the eastern gawler craton on the basis of its similarity to other cu au u deposits in the province an la icpms pb isotope age for monazite in hematite stage ii was calculated to be 1455 20 ma and possibly as young as 1370 54 ma to explain all isotopic data this is an age of isotopic resetting our preferred model is that cu u mineralization developed within a stratified hydrothermal brine complex analogous to the salton sea geothermal system copper fe and s in the deep brine was sourced from an evolving deep reservoir with local compositional controls on sulfur isotopes including cooling boiling and replacement of previous pyrite chalcopyrite u veins and disseminations formed as a result of mixing with overlying steam heated u rich saline ground water mixing induced cooling and dilution only occurred episodically following hydrofracturing events that ephemerally linked the brine layers within the substantially cemented upper complex by contrast high grade uo2 formation occurred as deep upflow waned via reaction of the upper brine with hematite stage ii fe mg chlorite along an interface that descended with time this resulted in an increase of u grades through time caused by leaching and reprecipitation

mixed convection processes below a saline disposal basin abstract saline groundwater and irrigation drainage are commonly diverted and stored in both natural and artificial depressions throughout the murray darling basin of australia the disposal basin brines that are formed are often denser than ambient groundwater under certain conditions these dense brines may become unstable causing them to mix with groundwater over distances several orders of magnitude greater than due to diffusion alone a model is developed to study the mixed convection processes below a saline disposal basin located between a recharge and discharge zone numerical simulations are performed in cross section using the 2 d density dependent model sutra saturated unsaturated transport it is shown that the salt front movement is related to both the ability of the dense saline brines to mix convectively with underlying groundwaters rayleigh convection and the strength of the regional advective velocity both homogeneous and heterogeneous aquifer systems are studied and the effects of anisotropy are considered our numerical results suggest that the behaviour of a dense brine plume overlying less dense groundwater in a homogeneous porous medium depends on the magnitude of at least two non dimensional numbers a rayleigh number and modified peclet number defined in terms of basin scale hydrogeologic parameters including dispersion it is shown that the onset of gravitational instabilities and the formation of free convective cells begins when the magnitude of a non dimensional parameter combining the rayleigh and modified peclet number exceeds a certain critical value 1997 elsevier science b v all rights reserved

carbonate ramp depositional systems abstract the classification tectonic settings stratigraphy and early diagenesis of carbonate ramp systems are reviewed carbonate ramps are common in all geological periods but were dominant at times when reef constructing organisms were absent or inhibited ramps can be subdivided into inner mid and outer ramp environments the mid ramp zone extends from fair weather wave base to normal storm wave base although the water depths which these boundaries represent vary an additional outer slope environment occurs on distally steepened ramps as with siliciclastic shelves a range of wave storm and tide dominated ramps can be recognised and this forms the most convenient basis for ramp classification the carbonate productivity profile of ramps differs from that of rimmed shelves with the inner ramp showing lower production rates than comparable shallow water facies on rimmed shelves the zone of greatest organic carbonate sediment production appears to have shifted from the mid ramp to the inner ramp since the late jurassic carbonate ramps occur in most types of sedimentary basin but are best developed where subsidence is flexural and gradients are slight over large areas as in foreland and cratonic interior basins and along passive margins the featureless depositional profiles of many ramps means that sequence geometries are best observed on regional seismic lines some show low angle sigmoidal or shingled clinoforms suggesting that ramps may seldom be homoclinal but possess subtle slope geometries which reflect depositional environments because of their low angle slopes ramps respond differently to rimmed shelves during relative sea level changes although results seem to be strongly dependent on the rate of relative sea level change during a minor fall shallow ramp facies belts will simply shift basinwards in a forced regression in contrast the whole surface of a steep sloped flat topped rimmed shelf may be exposed so that sediment production ceases or is drastically reduced during a major fall shallow ramp bounded basins may empty completely conversely ramps also flood gradually whereas rimmed shelves do so more rapidly homoclinal ramps develop no resedimented lowstand deposits rimmed shelves and distally steepened ramps in contrast may develop lowstand talus or turbiditic wedges distally steepened ramps may behave more like homoclinal ramps during minor base level falls and like rimmed shelves during major base level falls many ramps consist of layered successions of several ramp sequences stacked one upon the other ramp stacks of this sort may show gross vertical accretion but individual ramp sequences seldom appear to develop in a keep up style apart from minor organic buildups as with many rimmed shelves steepening of the outer ramp margin due to tectonism slope inheritance or differential sedimentation may promote the development of a distally steepened ramp or rimmed shelf a wide variety of organisms have constructed buildups in mid and outer ramp environments isolated buildups may seed early in ramp development accrete to wave base or sea level and continue growth by stacking through successive ramp sequences so that depositional and diagenetic features within them are in concert with those of the shallow ramp the location of isolated buildups on ramps is governed by tectonism halokinesis antecedent topography or by the subtle slope geometry of the previous ramp sequence diagenesis on ramps shows some major variations compared with diagenesis on steep sloped flat topped carbonate platforms ramp bounded basins may form prolific petroleum sourcing and reservoiring systems and offer a range of subtle stratigraphic play types and lateral facies variations which determine reservoir quality and distribution isolated buildups in the mid and outer ramp environments represent one of the commonest petroleum reservoirs in ramp systems and tend to have their foundations in transgressive systems tracts grainstone and packstone reservoirs are widespread and range from shoreline carbonate sandbodies to major detached shoal complexes or shoals over offshore highs grainstone sandbodies occur in both highstand systems tracts and in prograding lowstand wedges

boron isotope evidence for the involvement of non marine evaporites in the origin of the broken hill ore deposits identifying the palaeogeographic setting and mode of origin of stratabound ore deposits can be difficult in high grade metamorphic terranes where the effects of metamorphism may obscure the nature of the protoliths here we report boron isotope data for tourmalines from the early proterozoic broken hill block in australia which hosts giant lead zinc silver sulphide deposits with one exception the 11b 10b ratios are lower than those for all other tourmalines from massive sulphide deposits and tour malinites elsewhere in the world we propose that these low ratios reflect leaching of boron from non marine evaporitic borates by convecting hydrothermal fluids associated with early proterozoic continental rifting a possible modern analogue is the salton sea geothermal field in california

numerical analysis of seawater circulation in carbonate platforms ii the dynamic interaction between geothermal and brine reflux circulation density driven seawater circulation may occur in carbonate platforms due to geothermal heating and or reflux of water of elevated salinity in geothermal circulation lateral contrasts in temperature between seawater and platform groundwa ters warmed by the geothermal heat flux result in upward convective flow with colder seawater drawn into the platform at depth with reflux circulation platform top waters concentrated by evaporation flow downward displacing less dense underlying ground waters we have used a variable density groundwater flow model to examine the pattern magnitude and interaction of these two different circulation mechanisms for mesosaline platform top waters 50 and brines concentrated up to saturation with respect to gypsum 150 and halite 246 geothermal circulation most active around the platform margin becomes restricted and eventually shut off by reflux of brines from the platform interior towards the margin the persistence of geothermal circulation is dependent on the rate of brine reflux which is proportional to the concentration of platform top brines and also critically dependent on the magnitude and distribution of permeability low permeability evaporites can severely restrict reflux whereas high permeability units in hydraulic continuity enhance brine transport reduction in permeability with depth and anisotropy of permeability kv kh focuses flow laterally in the shallow subsurface 1 km resulting in a horizontally elongated brine plume aquifer porosity and dispersivity are relatively minor controls on reflux platform brines can entrain surficial seawater when brine generating conditions cease but the platform top remains submerged a variant of reflux we term latent reflux brines concentrated up to gypsum saturation have relatively long residence times of at least 100 times the duration of the reflux event they thus represent a long term control on post reflux groundwater circulation and conse quently on the rates and spatial patterns of shallow burial diagenesis such as dolomiti zation

hydrothermal fluid migration and brine pool formation in the red sea the atlantis ii deep numerical heat and fluid flow simulations of the atlantis ii deep in the red sea were conducted to investigate the development migration and discharge of hydrothermal fluids into a submarine depression and determine the conditions necessary to form a brine pool high salinity fluids are predicted to form by leaching miocene evaporates migrate and convect within young oceanic crust and discharge onto the seafloor predicted fluid discharge temperatures t max 301 c discharge fluid velocities v max 0 09 m s and salinities s max 21 wt increase over time and reach values comparable to modern seafloor observations established convection patterns and discharge behavior are robust and are not greatly affected by geometry of rock property changes modeling results were used to calculate the minimum conditions for hydrothermal fluids from a developing hydrothermal system to mix with seawater reverse buoyancy and begin to form a brine pool in a submarine depression under conditions encountered on the seafloor t 25 300 c s 5 25 wt fluid mixtures predicted to pond on the seafloor range from late in the mixing process 99 at low temperatures t 26 c to much earlier 36 mixing at higher temperatures t 94 c a model of brine pool evolution is proposed that describes the processes and conditions necessary to initiate brine pool formation and compares formation conditions with accumulated ore material in the atlantis ii deep and other locations

stability of hydrous melt at the base of the earth s upper mantle seismological observations have revealed the existence of low velocity and high attenuation zones above the discontinuity at 410 km depth at the base of the earth s upper mantle1 2 it has been suggested that a small amount of melt could be responsible for such anomalies3 4 5 the density of silicate melt under dry conditions has been measured at high pressure4 6 7 8 9 10 11 12 13 and found to be denser than the surrounding solid thereby allowing the melt to remain at depth but no experimental investigation of the density of hydrous melt has yet been carried out here we present data constraining the density of hydrous basaltic melt under pressure to examine the stability of melt above the 410 km discontinuity we infer that hydrous magma formed by partial melting above the 410 km discontinuity may indeed be gravitationally stable thereby supporting the idea that low velocity or high attentuation regions just above the mantle transition zone may result from the presence of melt

lithostratigraphy of the state 2 14 borehole salton sea scientific drilling project sedimentary rocks from the california state 2 14 research borehole of the salton sea scientific drilling project located in the imperial valley of southern california were deposited in the continental basin of the salton trough lacustrine shale and siltstone are the dominant lithologies sandstones were deposited in lake margin meander channel fill and lacustrine delta environments framework modal clast compositions of the sandstones indicate a dominant colorado river provenance rare pebbly mudstones were deposited as storm induced debris flows primary depositional structures in the sediments are commonly disrupted by seismic disturbances nonmarine gastropods and ostrocods are relatively uncommon and occurrences of stratabound sulfides suggest periods of brackish water conditions bedded anhydrite and centimeter scale mudcracks record intervals of dessication the state 2 14 sedimentary rocks are interpreted as the basinward facies of the borrego and brawley formations which range from pliocene to pleistocene in age

metamorphosed plio pleistocene evaporites and the origins of hypersaline brines in the salton sea geothermal system california fluid inclusion evidence abstract the salton sea geothermal system ssgs occurs in plio pleistocene deltaic lacustrine evaporite sediments deposited in the salton trough an active continental rift zone temperatures up to 365 c and hypersaline brines with up to 26 wt tds are encountered at 1 3 km depth in the sediments which are undergoing active greenschist facies hydrothermal metamorphism previous models for the origins of the na ca k cl brines have assumed that the high salinities were derived mainly from the downward percolation of cold dense brines formed by low temperature dissolution of shallow non marine evaporites new drillcores from the central part of the geothermal field contain metamorphosed bedded evaporites at 1 km depth consisting largely of hornfelsic anhydrite interbedded with anhydrite cemented solution collapse shale breccias fluid inclusions trapped within the bedded and breccia cementing anhydrite homogenize at 300 c identical to the measured downhole temperature and contain saline na ca k cl brines some of the inclusions contain up to 50 vol halite sylvite and carbonate crystals at room temperature and some halite crystals persist to above 300 c upon laboratory heating the data are consistent with the trapping of halite saturated na ca k cl fluids during hydrothermal metamorphism of the evaporites and accompanying solution collapse of interbedded shales we conclude that many of the salt crystals in inclusions are the residuum of bedded evaporitic salt that was dissolved during metamorphism by heated connate fluids therefore the high salinities of the salton sea geothermal brines are derived in part from the in situ hydrothermal metamorphism and dissolution of halides and caso 4 from relatively deeply buried lacustrine evaporites this fact places important constraints on modeling fluid flow in the ssgs as brines need not have migrated over great distances the brines have been further modified to their present complex na ca k fe mn cl compositions by on going sediment metamorphism and water rock interaction

hydrothermal alteration and mineralization in the neves corvo volcanic hosted massive sulfide deposit portugal i geology mineralogy and geochemistry the neves corvo volcanic hosted massive sulfide deposit is one of the largest 300 mt and richest known deposits of the iberian pyrite belt the deposit is unique because of its extremely high cu and sn grades 45 mt at 6 cu plus 4 5 mt at 12 cu and 2 2 sn more than 300 000 metric tons t of tin metal is contained in several types of tin mineralization that grade from 500 ppm to 60 percent sn the deposit consists of five orebodies including corvo the focus of the present study the volcanic volcaniclastic and sedimentary lithofacies that directly host the neves corvo mineralization consist of a rhyolitic dome cryptodome hyaloclastite complex facies interpretation indicates that volcanic activity occurred at moderate water depth below storm wave base in variably subsiding basins of different orders several syn and postore modification events included gravity driven mass transport processes and subsequent low angle thrusting and asymmetric detachment folding which shaped the present deposit the ore geology and geochemistry and the textural mineralogical and geochemical aspects of the ore forming process in the corvo orebody collectively suggest that ore formation resulted from a long lived hydrothermal system including two main mineralizing events an early stage of stringer and massive cassiterite deposition and a subsequent episode of massive sulfide ore generation two spatially independent stockworks fed these genetically related mineralizing systems the early deposition of stringer and massive cassiterite ores took place almost exclusively in the tin corridor a structural alignment bounded by synvolcanic faults textural analysis and geochemical data both indicate minimal fluid rock interaction during ascent of the tin bearing fluid the massive and semimassive cassiterite ores at corvo most likely formed by direct venting into seawater and or emplacement in water laden unlithified sediments and early hydrothermal products that mantled the footwall volcanic sequence stringer and massive cassiterite precipitation is thought to have been a product of a short lived episode in the overall ore forming system the vein filling mineral assemblage in the cassiterite stockwork quartz cassiterite pyrite donbassite kaolinite chamosite coupled with whole rock and mineral geochemical data indicate that the tin bearing fluid had a low ph and relatively high temperature 380 c the massive sulfide related hydrothermal alteration in corvo is essentially strata bound the permeability contrasts in the uppermost portion of the footwall succession played an important role in controlling the fluid flow the hydrothermal alteration is zoned and consists of an inner chlorite donbassite quartz sulfide sericite core that grades outward into successive enveloping k sericite quartz sulfide and na sericite quartz sulfide halos textural evidence from the mine to the microscopic scale and mass balance considerations indicate that extensive silicate replacement in the coherent volcanic rocks of the footwall sequence and disseminated replacement mineralization in the uppermost volcaniclastic and or sedimentary units were major mechanisms of massive sulfide deposition at corvo alteration mineralogy and geochemistry indicate that the neves corvo ore fluids were hotter and more acidic than typical ore forming fluids of the iberian pyrite belt the geochemical characteristics of the tin and copper mineralization also differ from the ores of typical deposits of the iberian pyrite belt and imply the involvement of additional metal sources possibly magmatic in the ore forming system copper enrichment due to mechanical and fluid assisted tectonometamorphic remobilization processes accounts for the formation of extremely high grade ore shoots in the deposit

dolomitization of the capitan formation forereef facies permian west texas and new mexico seepage reflux revisited interpretation of seepage reflux dolomitization is commonly restricted to intervals containing evaporites even though several workers have modelled reflux of mesosaline brines this study looked at the partially dolomitized forereef facies of the capitan formation to test the extent of reflux dolomitization and evaluate the possible role of the near backreef mesosaline carbonate lagoon as an alternative source of dolomitizing fluids the capitan formation forereef facies ranges from 10 to 90 dolomite most of the dolomite is fabric preserving and formed during early burial after marine cementation before and or during evaporite cementation and before stylolitization within the forereef facies dolomite follows depositional units with debris flow and grain flow deposits the most dolomitized and turbidity current deposits the least the amount of dolomite increases with stratigraphic age and decreases downslope within the reef facies dolomite is restricted to haloes around fractures and primary cavities except where the reef facies lacks marine cements and in contrast is completely dolomitized this dolomite distribution supports dolomitization by sinking fluids oxygen isotopic values for fabric preserving dolomite 1 8 o 0 9 1 0 n 101 support dolomitization by sea water to isotopically enriched sea water these values are closer to the near backreef dolomite 1 8 o 2 1 0 7 n 48 than the hypersaline backreef dolomite 1 8 o 3 6 0 9 n 11 therefore the fabric preserving dolomite is consistent with dolomitization during seepage reflux of mainly mesosaline brines derived from the near backreef carbonate lagoon the occurrence of mesosaline brine reflux in the capitan formation has important implications for dolomitization in forereef facies and elsewhere first any area with a restricted carbonate lagoon may be dolomitized by refluxing brines even if there are no evaporite facies present secondly such brines may travel significant distances vertically provided permeable pathways such as fractures are present therefore the absence of immediately overlying evaporite or restricted facies is not sufficient cause to eliminate reflux dolomitization from consideration

kilometre scale thermohaline overturn of pore waters in the louisiana gulf coast convincing evidence exists for the large scale convection of sea water through the upper oceanic crust even at considerable distances from mid ocean ridges1 2 there has been considerable interest in verifying the existence of convective overturn of pore fluid in large sedimentary basins a mass transport process which could profoundly influence the transport of heat hydrocarbons metals and diagenetically reactive dissolved constituents3 4 the existence of fluid convection in sediments of the united states gulf coast has been inferred on the basis of the large volumes of fluid necessary to account for the observed diagenetic alteration5 and the existence of thermal anomalies that cannot be explained by simple conductive transport of heat3 6 theoretical rayleigh darcy calculations6 7 support the notion that thermally driven convection is possible here i present geochemical and physical evidence for the existence of density inversions in gulf coast pore fluids sufficient to drive large scale convective fluid flow at rates conceivably as high as metres per year the density inversions are caused in part by the dissolution of salt diapirs and the formation of dense saline brines at shallow depths

ore facies mineral chemical and fluid inclusion characteristics of the hossein abad and western haft savaran sediment hosted zn pb deposits arak mining district iran abstract the arak basin in the northern part of the malayer esfahan metallogenic belt memb of iran hosts 9 zn pb sulfide deposits within a jurassic sedimentary succession of black fine grained sandstone siltstone and shale this succession was deposited in an extensional back arc environment between the sanandaj sirjan zone ssz and the central iranian microcontinent two of the 9 deposits western haft savaran and hossein abad are the focus of study here three ore facies can be distinguished based on the morphology of the orebodies and the sulfide mineralogy and textures a stockwork b massive and c bedded mineralization the stockwork and massive mineralization are interpreted to have formed in the subsurface whereas bedded mineralization was sedimented on the seafloor the fe contents of sphalerites from massive and stockwork mineralization from the two deposits together with the presence of pyrrhotite inclusions indicate that sphalerite precipitated from relatively low f s 2 11 fluid homogenization temperatures of fluid inclusions in sphalerite quartz and siderite in massive mineralization range from 105 to 309 c and average 208 c with salinities between 4 03 and 11 1 wt nacl eq and averaging 5 77 wt nacl eq these widely ranging salinities and homogenization temperatures reflect mixing of shallow connate water with venting metalliferous fluid that is of deep basinal origin and deposition of mineralization predominantly below the seafloor collectively the host rock types ore facies and textures deposition in an reduced ambient environment fluid inclusion temperatures and salinities and mineralizing fluid behaviour at the seafloor are all compatible with a vent proximal sedimentary exhalative sedex origin for the western haft savaran and hossein abad deposits

active ore formation at a brine interface in metamorphosed deltaic lacustrine sediments the salton sea geothermal system california the salton sea geothermal system is an area of active hydrothermal metamorphism of pliocene to pleistocene deltaic lacustrine and evaporitic sediments deposited in a modern continental rift zone temperatures up to 365 degrees c hypersaline metalliferous brines with up to 26 wt percent total dissolved solids and active base metal ore formation accompany low pressure greenschist and amphibolite facies metamorphism at depths less than 3 2 km drill cores recovered by the salton sea scientific drilling project shed new light on the nature and origin of ore formation in the eastern portion of the salton sea geothermal system base metal ore mineralization occurs in vertical fractures that comprise the major form of fluid permeability at 1 to 3 km in the modern geothermal system type 1 veins are dominated by carbonate contain pyrrhotite and other sulfides are completely sealed and occur in wall rock showing minimal alteration fluid inclusion data indicate that they were deposited under a thermal gradient much higher than that now found in the eastern part of the system type 2 veins are dominated by silicates and hematite are permeable and occur in wall rocks showing pervasive chloritization and epidotization fluid inclusion data indicate that they are forming now from the geothermal brines under the modern thermal gradient fluid inclusions record an apparent steep progressive salinity increase in the vein systems in the upper 2 km of the system however fluid production data from salton sea geothermal wells indicate that reduced metalliferous hypersaline brine averaging 23 wt percent total dissolved solids is overlain by more oxidized metal poor fluid averaging 5 wt percent total dissolved solids a sharp interface exists between the fluids at 1 to 2 km paralleling the 250 degrees c isothermal surface a model is developed for type 2 vein ore formation involving mixing at the interface between the two fluids magmatic intrusion induces fracturing heating and the expulsion of reduced connate fluids to cause early type 1 vein mineralization following the rising thermal front deep hypersaline brine rises diapirically and mixes with overlying fluids in vertical fractures transecting the ascending interface forming type 2 veins the hypersaline brine rises to a level of density temperature stabilization causing pervasive greenschist metamorphism of the host sediments the diapir then begins to cool and descend shallow oxidized brine is drawn in as the hypersaline brine descends promoting late stage hematite in type 2 veins the eastern salton sea geothermal system presently appears to be in this latter mode of cooling and retrograde brine descent ore formation in the system is caused by the coincidence of transaxial entry of a major river into an active rift zone deposition of metal bearing deltaic sediments to form a closed basin sedimentary environment episodic lacustrine evaporite formation and injection of heat and elements by rift related magmatic intrusions at depth the presence of an upwelling brine diapir in the system may be analogous to conditions immediately prior to the exhalation of metalliferous fluids that formed stratiform fe zn pb cu sulfide and oxide deposits in paleorift zone environments

distribution of porosity and permeability in platform dolomites insight from the permian of west texas saller and henderson 1998 presented an article concerning porosity and permeability variations in two west texas oil fields north riley and south cowden fields these fields occur within dolomitized permian platform carbonates near the basinward shelf edge of the central basin platform a paleogeographic element that influenced permian deposition in west texas their carefully documented study outlines several anomalous aspects concerning porosity distribution they noted in particular that porosity distribution is not strictly facies dependent but that porosity tends to increase in a basinward direction they suggested that porosity developed largely during dolomitization by the action of hypersaline evaporative fluids that flowed downdip in a basinward direction during the permian these brines were generated in restricted shelf interior lagoons and moved downdip parallel with bedding in a shallow reflux type of hydrologic system driven by subsurface fluid density differences in the permian clear fork and grayburg formations as shown here in figure 1 see figure 18 in saller and henderson 1998 figure 1 part a shows the scenario proposed by saller and henderson 1998 for porosity creation dolomitization and dolomite cementation of the permian clear fork and grayburg formations of west texas they suggested that macrodissolution of calcium carbonate occurred contemporaneously with dolomitization and dolomite cementation this process of brine backreaction however would cause a net increase of porosity in the shelf interior the opposite of what is observed part b shows a two stage scenario proposed here to circumvent the problem of limestone macrodissolution by shelf interior brines that must have been initially in approximate bulk equilibrium with calcium carbonate the first stage records dolomitization by replacement of limestone with little macrodissolution in stage two evaporative shelf interior brines may have evolved in equilibrium with dolomite only and became undersaturated with calcite and aragonite reflux migration of these brines downdip may

copper arsenic decoupling in an active geothermal system a link between pyrite and fluid composition abstract over the past few decades several studies have reported that pyrite hosts appreciable amounts of trace elements which commonly occur forming complex zoning patterns within a single mineral grain these chemical zonations in pyrite have been recognized in a variety of hydrothermal ore deposit types e g porphyry cu mo au epithermal au deposits iron oxide copper gold carlin type and archean lode au deposits among others showing in some cases marked oscillatory alternation of metals and metalloids in pyrite growth zones e g of cu rich as au ag depleted zones and as au ag rich cu depleted zones this decoupled geochemical behavior of cu and as has been interpreted as a result of chemical changes in ore forming fluids although direct evidence connecting fluctuations in hydrothermal fluid composition with metal partitioning into pyrite growth zones is still lacking in this study we report a comprehensive trace element database of pyrite from the tolhuaca geothermal system tgs in southern chile a young and active hydrothermal system where fewer pyrite growth rims and mineralization events are present and the reservoir fluid i e ore forming fluid is accessible we combined the high spatial resolution and x ray mapping capabilities of electron microprobe analysis empa with low detection limits and depth profiling capacity of secondary ion mass spectrometry sims in a suite of pyrite samples retrieved from a 1 km drill hole that crosses the argillic 20 450 m and propylitic 650 1000 m alteration zones of the geothermal system we show that the concentrations of precious metals e g au ag metalloids e g as sb se te and base and heavy metals e g cu co ni pb in pyrite at the tgs are significant among the elements analyzed as and cu are the most abundant with concentrations that vary from sub ppm levels to a few wt i e up to 5 wt as 1 5 wt cu detailed wavelength dispersive spectrometry wds x ray maps and sims depth vs isotope concentration profiles reveal that pyrites from the tgs are characterized by chemical zoning where the studied elements occur in different mineralogical forms arsenic and co occur as structurally bound elements in pyrite cu and au in pyrite can occur as both solid solution and submicron sized particles of chalcopyrite and native au or au tellurides respectively pyrites from the deeper propylitic zone do not show significant zonation and high cu co as concentrations correlate with each other in contrast well developed zonations were detected in pyrite from the shallow argillic alteration zone where cu co rich as depleted cores alternate with cu co depleted as rich rims these microanalytical data were contrasted with chemical data of fluid inclusions in quartz and calcite veins high cu as ratios and borehole fluid low cu as ratios reported at the tgs showing a clear correspondence between cu and as concentrations in pyrite forming fluids and chemical zonation in pyrite these observations provide direct evidence supporting the selective partitioning of metals into pyrite as a result of changes in ore forming fluid composition most likely due to separation of a single phase fluid into a low density vapor and a denser brine capable of fractionating cu and as

source of solutes to the coastal sabkha of abu dhabi an ascending brine model is proposed to address the observed isotope geochemistry solute composition and solute and water fluxes in the coastal sabkha of the emirate of abu dhabi mass balance measurements document that 95 of the solutes are derived from ascending continental brines minor amounts are derived from rainfall and from groundwater entering from up gradient areas nearly 100 of the annual water loss is from evaporation and not lateral discharge direct rainfall on the sabkha and subsequent recharge to the underlying aquifer account for 90 of the annual water input to the system the remaining 10 comes from both lateral and ascending groundwater flow thus the water and solutes in this system are from different sources solute concentrations of conservative i e nonreactive elements in the coastal sabkha covered aquifer are consistent with the fluid pore volumes of ascending brine calculated from hydrologic properties calcium to sulfate ratios and sulfur isotopes are consistent with this source of solute from the underlying tertiary formations recharging rainwater dissolves halite and other soluble minerals on the surface causing the solution to become more dense and sink to the bottom of the aquifer where it vertically mixes with less dense ascending brines solutes are returned to the surface by capillary forces and recycled or lost from the system by eolian or fluvial processes thus the system becomes vertically mixed consistent with the presence of tritium throughout the aquifer but there is essentially no horizontal mixing of seawater with groundwater the observed seawater solutes in the supratidal zone come from interstitial seawater trapped by the rapid progradation of the sediments into the arabian gulf and are not refluxed or laterally mixed the ascending brine model contrasts significantly with both the seawater flooding and evaporative pumping models previously proposed as a source of solutes to the coastal sabkha of the emirate of abu dhabi use of these earlier models leads to incorrect conclusions and raises serious questions about their applicability in the evaluation of sabkhat in the geologic record

relating sulfide mineral zonation and trace element chemistry to subsurface processes in the reykjanes geothermal system iceland abstract the nature and distribution of sulfide minerals and their trace element chemistry in the seawater dominated reykjanes geothermal system was determined through the study of cuttings and core from wells that intersect different regions of the hydrothermal cell from the near surface to depths of 3000 m the observed sulfide mineral zonation and trace element enrichment correlate well with the present day thermal structure of the system isocubanite and pyrrhotite are confined to the deep low permeability regions whereas an assemblage of chalcopyrite and pyrite predominates in the main convective upflow path the presence of marcasite in the uppermost regions of the system reflects weakly acidic conditions ph 2 the presence of chalcopyrite disease in sphalerite may be an indication that the system is experiencing a heating trend following the logic of zone refining in volcanogenic massive sulfide systems sulfide sulfur at all analyzed depths in the reykjanes geothermal system was derived from a mixture of basaltic and reduced seawater sources petrographic evidence suggests that seawater derived hydrothermal fluids have altered primary igneous sulfides in the host rocks a process that has been proposed as a major control of aqueous sulfide production in mid ocean ridge environments calculations show that igneous sulfides in the host basalts likely account for less than 5 of the total available ore metal budget in the system however their contribution to fluid metal budgets is probably significant because of their relatively high solubility the processes documented by this study are likely analogous to those operating in the feeder and deep reaction zones of mid ocean ridge seafloor hydrothermal systems the results show that sulfide mineral zonation and trace element chemistry vary as a function of physicochemical parameters that are relevant to the characterization and exploration of geothermal energy resources

free thermohaline convection in sediments surrounding a salt column complex groundwater convection patterns are possible near salt domes because groundwater is subject to both lateral heat and salinity gradients in order to assess the mechanisms responsible for driving convection near salt domes we use dimensional analysis and numerical simulations to investigate coupled heat and salt transport in homogeneous sediments surrounding a cylindrical salt column the dimensional analysis does not require the boussinesq assumption the coupled heat solute and groundwater transport equations are controlled by three dimensionless parameters the rayleigh number the lewis number and the buoyancy ratio the buoyancy ratio is the ratio of salinity to temperature effects on groundwater density and it directly affects the groundwater flow equation a finite difference numerical multigridding algorithm is used to iteratively solve the coupled transport equations the multigridding technique was about 3 times faster than a point wise successive overrelaxation solution boundary conditions for the numerical simulations were adjusted to represent different contrasts in the thermal gradient between the salt and the overlying sediments the contrast in thermal gradient is parameterized by the thermal conductivity ratio and is responsible for isotherms being elevated near the salt the analysis suggests that a wide range of convective flow patterns are possible with flow occurring either up or down along the salt flank the sense of convection is dependent mainly on the value of the buoyancy ratio and how sharply isotherms are pulled up near the salt these factors in turn depend on the regional salinity variation the time since diapirism and the thermal conductivity of water saturated sediments while this analysis provides useful insight into the mechanisms driving free convection near salt domes the assumptions about medium and fluid properties may limit the applicability of dimensional analysis in simulating flow in specific geologic settings

spatial patterns of diagenesis during geothermal circulation in carbonate platforms geothermal convection of seawater deep in carbonate platforms could provide the necessary supply of magnesium for dolomitization at temperatures high enough to overcome kinetic limitations we used reactive transport simulations to predict the rates and spatial patterns of dolomitization during geothermal convection in a platform that was 40 km across and 2 km thick in the simulations porosity and permeability decrease with depth to account for sediment compaction dolomitization of a platform consisting of medium grained 0 0 5 mm sediments occurred in a broad band ranging from 2 5 km depth near the margin to 1 5 km depth near the platform center the area of dolomitization is deep enough that temperatures exceed 50 c but not so deep that low permeabilities restrict mass transport complete dolomitization in the center of this zone is estimated to require at least 60 my incorporation of permeability contrasts permeable beds and reactive beds focused dolomitization strongly and reduced the estimated time required for dolomitization by as much as 50 percent dolomitization created magnesium depleted calcium rich fluids in less than 10 ky and results support a link between dolomitization and anhydrite precipitation where adequate sulfate is available

geochim cosmochim acta m a a schoonen and h l barnes erratum to schoonen and barnes 1991 mechanisms of pyrite and marcasite formation from solution iii hydrothermal processes 55 3491 3504

meta evaporite sequence exhalative chemical sediments and associated rocks in the proterozoic willyama supergroup south australia implications for metallogenesis abstract the willyama supergroup comprises proterozoic meta sedimentary and meta igneous rocks in western new south wales and eastern south australia and contains the giant broken hill pb zn ag deposit in addition to numerous small stratiform stratabound and vein mineral deposits new lithological geochemical and fluid inclusion data from the olary block west of broken hill suggest the presence of former evaporite rocks now metamorphosed to a variety of chemically and mineralogically unusual lithologies these evaporite rocks overlie relatively oxidised strata and are in turn overlain by reduced graphite bearing pelites rocks of sodic composition abound represented by albite quartz rich rocks calcite actinolite epidote garnet clinopyroxene inferred to represent metamorphosed analcime rich strata of felsic volcaniclastic provenance with variable proportions of carbonates for example calcite dolomite trona and shortite these rocks are interspersed with potassic rocks possibly also with an evaporitic origin in addition to normal clastic meta sediments the presence of halogen rich minerals in a distinctive mineral assemblage hypersaline fluid inclusions pseudomorphic voids and minerals after gypsum and carbonate in addition to tourmalines with isotopically light boron isotopes is interpreted as evidence for a non marine evaporite parent rock for portions of the olary block types of mineral occurrences in the olary block form a continuum between chemical sediments formed by the exhalations of hydrothermal fluids to intrasediment deposits inferred to have been formed due to the chemical changes imposed on these and or diagenetic fluids during their passage through the sedimentary pile disseminated stratabound pyrite including cobaltian types and chalcopyrite extend along strike for many kilometres and are interpreted to have formed by a diagenetic mechanism the presence of massive sulphide bearing strata iron formations barite tourmalinite and manganiferous rocks are typical of chemical sediments derived from hot spring sources the olary block meta evaporites have similar whole rock compositions to tertiary analcime rich rocks of the green river formation wyoming these new data imply that portions of the adjacent broken hill block may also comprise meta evaporites for example the quartz albite rich strata of the thackaringa group and the calcsilicates of the lower broken hill group and that the disseminated cobaltian pyrite mineralisation in the thackaringa group may not be of exhalative origin

skeletal aragonite dissolution from hypersaline seawater a hypothesis abstract hypersaline seawater has often been invoked as a mechanism to explain the pervasive dolomitization of ancient platform carbonates but its potential in causing skeletal aragonite dissolution of these carbonates has rarely been investigated previous experimental and theoretical studies have demonstrated that hypersaline seawater is undersaturated with respect to aragonite when evaporation reaches a certain degree it is contended here that similar undersaturation could also have occurred in ancient evaporitic seas geological evidence from the miocene carbonates of se spain gulf of suez red sea and iraq suggests that this may have been the case despite differences in their geological settings these carbonates have common diagenetic features including 1 widespread dissolution of skeletal aragonite with little or no calcite cementation and 2 pervasive dolomitization dolomite occurs as both a replacement of mg calcite and mouldic pore filling cement the association of the dolomites with evaporites their relatively heavy oxygen isotopic values and widespread distribution suggest a hypersaline seawater origin of these dolomites petrographic data indicate that skeletal aragonite was dissolved during dolomitization because the aragonite fossil moulds contain dolomite cements but no pre dolomitization calcite cements implying that the dolomitizing fluids hypersaline seawater were probably undersaturated with respect to aragonite the dissolved calcium may have been subsequently incorporated into the dolomite this may help to explain the lack of calcite cementation despite the extensive skeletal aragonite dissolution

boron isotope systematics of tourmaline formation in the sullivan pb zn ag deposit british columbia canada abstract we report here the results of 54 boron isotope analyses of tourmaline associated with the giant sullivan pb zn ag deposit in southeastern british columbia canada the 11 b values range from 11 1 to 2 9 which is almost as great as the range found worldwide in tourmalines from 33 massive sulfide deposits and tourmalinites in dominantly clastic metasedimentary terranes the major control on the overall 11 b values of the sullivan tourmalinites is the boron source potential controls over the large range of the data also include 1 differences in formation temperatures of the tourmalinites 2 different stages of tourmaline formation 3 variations in the proportions of dissolved boron incorporated into the tourmaline rayleigh fractionation 4 seawater entrainment and 5 post depositional metamorphism the boron isotope data at sullivan are consistent with boron derivation from leaching of footwall clastic sediments however the great abundance of tourmaline in the sullivan deposit suggests that the local clastic sediments were not the sole source of boron and we argue that non marine evaporites buried deep below the orebody are the most viable source of this additional boron it is likely that some of the variation in tourmaline 11 b values reflect mixing of boron from these two sources comparison of the potential effects of these controls with geologic and other geochemical evidence suggests that major causes for the wide range of 11 b values measured at sullivan are seawater entrainment and rayleigh fractionation although in places post depositional alteration and thermal metamorphism were important in determining 11 b values of some of the recrystallized tourmalinites

fate of reflux brines in carbonate platforms active reflux may occur during periods of platform top brine generation but the role and fate of these brines after reflux events are uncertain we have used a numerical flow model to investigate and quantify the response of reflux brines to changes in platform top salinity simulations suggest that reflux brines originally concentrated to gypsum saturation 150 have a relatively long platform residence time on the order of 100 times the duration of the reflux event when brine generating conditions cease brines will continue to sink through the platform entraining seawater a variant of reflux circulation we term latent reflux mesosaline brines intercepted by drilling of carbonate margins by the ocean drilling program may have originated from pleistocene reflux event s on the adjacent platform top and be currently moving by latent reflux latent reflux circulation could deliver a significant quantity of dissolved reactants to platform carbonates including mg for dolomitization

metamorphic evolution of sulphide rich chromitites from the chernichevo ultramafic massif se bulgaria abstract the upper mantle rocks of the metamorphosed ophiolite of chernichevo rhodope metamorphic complex in southern bulgaria host small chromite ores with unusual mineralization of base metal sulphides rich in platinum group elements mineralogical and chemical data indicate that after their formation in the mantle the chernichevo s chromite ores were modified by the intrusion of an alkaline mafic melt which resulted in the precipitation of a suite of metasomatic minerals sulphides calcite apatite and ilmenite accompanied by an increase in feo tio2 ga zn vn mn and especially ti and fe2o3 contents in the chromite the degree of chemical modification and abundance of metasomatic minerals are positively correlated and mark the extent of reaction of the chromitite with the intruding melt sulphide segregation promoted the concentration of high amounts of pges up to 3661 ppb particularly pt and pd yielding chromite ores with a typical flat to positive sloped chondrite normalized pattern subsequently the chromite ores were deformed and metamorphosed together with their host rocks at ultra high pressure uhp 2 5 gpa 1200 c to be later retrograded under eclogite and finally hydrous amphibolite facies conditions giving rise to three microstructural types metamorphism of the most metasomatized i e sulphide rich chromitites at temperatures 700 c within the conditions of uhp and eclogite facies resulted in the formation of 1 non porous recrystallized chromite consisting of a granoblastic microstructure made up of coarse grained blasts and finer grained chromite neoblasts in contrast hydrous metamorphism on the less metasomatized i e sulphide poor chromitite under the conditions of amphibolite facies ca 482 483 c resulted in the formation of 2 partly altered chromite characterized by unaltered cores surrounded by fe2 rich and al depleted porous chromite containing abundant clinochlore and 3 porous chromite corresponding to a chromite that was entirely transformed by the metamorphic alteration to fe2 rich and al depleted porous chromite during metamorphism magmatic ni fe cu sulphides originally formed during the metasomatic event in the mantle were altered resulting in a major leaching of cu rich sulphides leading to significant remobilization of pt and pd

the salton sea scientific drilling project in march 1986 a research borehole called the state 2 14 reached a depth of 3 22 km in the salton sea geothermal system of southern california this was part of the salton sea scientific drilling project sssdp the first major i e multimillion dollar research drilling project in the u s continental scientific drilling program the principal goals of the project were to investigate the physical and chemical processes of a high temperature high salinity magmatically driven hydrothermal system the borehole encountered temperatures of up to 355 c and produced metal rich alkali chloride brines containing 25 wt of total dissolved solids the rocks penetrated exhibit metamorphism and ore genesis in action they show a progressive transition from unconsolidated lacustrine and deltaic sediments to hornfelses with lower amphibolite facies mineralogy accompanied by pervasive veins containing iron copper lead and zinc ore minerals the sssdp included an intensive program of rock and fluid sampling flow testing and downhole logging and scientific measurement the purpose of this paper is to introduce this special section of the journal of geophysical research on the sssdp to describe briefly the background of the project and the drilling and testing of the borehole to summarize the initial scientific results and to discuss how the lessons learned are applicable to future scientific drilling projects

free thermohaline convection beneath allochthonous salt sheets an agent for salt dissolution and fluid flow in gulf coast sediments basinward migration of jurassic salt in the u s gulf of mexico has resulted in the emplacement of large allochthonous salt sheets into shallow miocene to holocene sediments although comparatively little direct information is available on the environment below these salt bodies it is reasonable to suppose that the formation of dense brines by dissolution of the base of these sheets may induce free thermohaline pore fluid convection within the sediments below we derived equations which make it possible to quantitatively estimate rates of dissolution of these subsurface salt structures from these calculations and by geologically realistic numerical simulations it can be shown that free convection beneath allochthonous salt sheets has the potential for being a significant mechanism for both salt dissolution and mass transport even if the underlying sediments have permeabilities as low as 10 17 m2 0 01 md the calculated maximum darcy fluxes and rates of salt dissolution rapidly increase with sediment permeability when the vertical permeability of the underlying sediment is 10 17 m2 0 01 md salt is dissolved from the base of the sheet at an average rate of 3 5 m m y 1 the corresponding fluid velocities are such that over a 10 m y period the integrated fluid flux in the underlying sediments would be 104 m3 m 2 by comparison integrated fluid flux for compactive expulsion is 103 m3 m 2 thus for the offshore sediments of the gulf of mexico thermohaline convection beneath an allochthonous salt sheet is a significant driving mechanism for fluid flow with potentially important implications for heat and mass transport diagenesis and salt tectonics

chemical mass transfer modelling of ore forming hydrothermal systems current practise and problems abstract chemical mass transfer modelling is a numerical approach to predicting the progress of multicomponent fluid rock reactions in natural hydrothermal systems based on thermodynamic data for minerals and fluid species quantitative simulations of hydrothermal wall rock alteration and ore mineral deposition can be used to develop and test geological models for ore formation which are a basis for mineral exploration and resource assessment in practice the application of thermodynamic mass transfer modelling to specific ore forming systems faces two main problems even for the limiting assumption of local equilibration between fluids and minerals the first problem relates to the geometry of fluid rock interaction where the scheme of the model must be adjusted to the geological question of interest one dimensional reactor models defining reaction progress as a function of integrated fluid flux along a path through a chemically reactive rock do not necessarily provide a more realistic description of chemical mass transfer than simple titration models without temporal and spatial dimension because in most ore depositing systems the fluids are partially channelized by structures like veins or faults the efficiency of metal extraction at the ore deposition site is commonly determined by a sensitive balance between fluid focusing on one hand and wall rock reaction promoting ore mineral precipitation on the other an example of gold and scheelite mineralization associated with mesothermal quartz veins illustrates how ore deposition and alteration zoning parallel to the main fluid flow direction in partially channelized systems can be described by a two dimensional multibox model the second problem relates to the extent and quality of thermodynamic data explicit consideration of fe bearing silicate solid solutions is essential for predicting realistic mass balance in complex fluid silicate sulphide reactions associated with hydrothermal ore formation although a high level of internal consistency of multicomponent data sets has been achieved by simultaneous fitting of multiple thermodynamic and equilibrium measurements significant inconsistencies persist among the most comprehensive data compilations published for the three main groups of species relevant to hydrothermal ore deposits aqueous species in high temperature aqueous brines silicate oxide carbonate minerals including fe mg silicates and sulphur bearing minerals including sulphides and sulphates discrepancies between thermodynamic predictions and observed brine composition and mineral assemblages in the salton sea geothermal system suggest that the redox scale for rock forming silicates may be inconsistent with that of sulphides and aqueous s c o h species this analysis is preliminary but illustrates the importance of field testing of thermodynamic data particularly for hydrothermal conditions between 150 and 450 c which lie between the high temperatures of reliable equilibrium experiments among rock forming minerals 500 c and the low temperatures of many experiments defining the thermodynamics of aqueous species mostly

accurate and precise quantification of major and trace element compositions of calcic sodic fluid inclusions by combined microthermometry and la icpms analysis abstract determination of absolute trace element concentrations in fluid inclusions using laser ablation la icpms requires an internal standard i e the concentration of one element must be independently known from independent observation microthermometric determination of the last melting temperature of ice hydrohalite or halite is routinely used to calculate apparent salinities in wt nacl equivalent using phase relations in the binary h 2 o nacl system to estimate na concentration calculating the concentrations of all other elements requires an empirical correction if additional salt concentrations are of similar magnitude as that of nacl if cacl 2 is the main additional salt component as in many low temperature basin and basement brines absolute na concentrations wt nacl abs can be obtained by observing two melting temperatures hydrohalite and either ice or halite uniquely defining the major element composition of the fluid in the ternary model system h 2 o nacl cacl 2 and allowing na to be used as internal standard for quantifying all minor and trace elements test results for a range of compositions show that calcium concentration can be determined more precisely by microthermometry than by la icpms analysis but that both methods agree within error the combined approach of microthermometry and la icpms analysis described here permits reliable quantification of major ca na as well as trace element concentrations in sodic calcic brine inclusions even in ca rich host minerals such as fluorite or ca bearing carbonates

density of basaltic melt at high pressure and stability of the melt at the base of the lower mantle abstract we made density measurements of model morb and picritic melts up to about 15 gpa using the floating diamond method the compression behavior of the melts is fitted by the vinet equation of state and extrapolated to higher pressure a density crossover between the morb melt and the mantle peridotite is suggested at the base of the lower mantle thus the ultra low velocity zone in the d layer may be explained by the existence of gravitationally stabilized basaltic melt formed by melting of the accumulated slab materials this region might become unstable due to progressive melting of slab materials and or supply of volatiles

oceanic magmatism in sedimentary basins of the northern gulf of california rift rift related magmatism in the northernmost gulf of california and the adjacent subaerial salton trough and cerro prieto basins comprises intermediate to rhyolitic surfi cial and buried lava fl ows and domes including their xenolith cargo in addition geothermal drill wells frequently penetrate subsurface gabbroic to granitic sills and dikes which intruded into colorado river delta fl uviatile and lacustrine sediments combined singlecrystal u th pb and u th he zircon ages reveal late pleistocene to holocene eruption ages for three volcanic centers in adjacent rift basins from n to s salton buttes eruption age 2 48 0 47 ka 95 confi dence cerro prieto maximum eruption age 73 7 ka and roca consag eruption age 43 6 ka u th zircon and allanite crystallization ages are close to the eruption ages with the exception of roca consag lava the zircon population of which is dominated by zircon with ca 1 ma crystallization ages a population interpreted to be recycled from an unknown crustal source underlying the wagner basin nd isotopic ratios for subsurface micro gabbros from cerro prieto end 8 9 overlap with values for midoceanic ridge basalts morb from the east pacifi c rise adjacent to the southern gulf of california cerro prieto microgranites and salton sea basaltic xenoliths have similarly elevated end values the lowest end value for late pleistocene holo cene igneous rocks from the northern gulf of california is for cerro prieto dacitic lava end 0 6 this value implies minor 20 assimilation of continental crustal rocks which however is an upper limit because of crystal scale evidence for magma contamination by unconsolidated sediment at the time of eruption zircon crystals in felsic rocks rhyolite lavas intrusive microgranites and granophyre xenoliths have trace element and submantle 18 o compositions that are robust indicators for a mafi c source that has exchanged oxygen by interacting with meteoric hydrothermal fl uids collectively these data imply that oceanic rifting has initiated in the salton trough and cerro prieto basins there morb type magmas formed mafi c intrusions within thick sedimentary basin fi ll where they became exposed to deep reaching hydrothermal fl uids diverse intermediate to high silica rhyolitic magmas that are prevalent at the surface are produced by fractional crystallization of mafi c parental magmas with minor assimilation of sediments or pre rift basement rocks and by partial melting of hydrothermally altered mafi c intrusions

the lavrion pb zn ag rich vein and breccia detachment related deposits greece involvement of evaporated seawater and meteoric fluids during postorogenic exhumation