paleolatitudes determined from paleomagnetic data from vertical cores most of the paleomagnetic data available from oceanic plates consist of measurements of inclination but not declination from azimuthally unoriented vertical cores similarly the basic paleomagnetic data to be analyzed from crustal blocks that have rotated by varying amounts about vertical axes are a set of paleoinclinations a new method is proposed for determining the paleocolatitude of a sampling site and the associated confidence limits paleomagnetic inclinations are first converted to paleocolatitudes and averaged to produce a mean colatitude after showing that this mean is biased we derive a paleocolatitude correction to remove the bias using a model for geomagnetic secular variation based on the latitude dependence of secular variation during the past 5 million years after testing the validity of the model by applying it to paleomagnetic data from quaternary lavas for which both inclination and declination are known we apply the method to inclination only paleomagnetic data from suiko seamount the most complete data set from a drilling site the method is then applied to paleomagnetic data from all deep sea drilling project sites for the pacific plate to obtain for each site an unbiased mean paleocolatitude and its confidence limits these paleocolatitudes and confidence limits are then used as input for a least squares analysis to obtain early and late cretaceous poles for the pacific plate and 95 confidence limits for these poles it is shown that the scatter of individual site paleocolatitudes about the best fit pole is consistent with the confidence limits at each site as determined by the new method of analysis

fission track evidence of the cenozoic thermal history of the idaho batholith abstract apatite and zircon fission track data are combined with published k ar biotite dates from the idaho batholith to outline the cenozoic thermal history of central idaho apatite apparent ages reveal that parts of the batholith have been at temperatures below 100 c for over 50 ma suggesting that much of the batholith has remained at shallow crustal levels throughout cenozoic time eocene plutonism and attendant hydrothermal circulation resulted in thermal resetting of apatite and zircon dates from near the plutons concordant zircon and biotite dates indicate that hydrothermal systems associated with eocene plutonism operated at shallow crustal levels so that cooling occurred rapidly as geothermal activity waned the calculated depth to the 105 c apatite annealing isotherm suggests that as much as 3 km of erosion has occurred in deep canyons during the last 10 ma presumably associated with final erosion of the batholith following the establishment of drainage at low elevation along the snake river plain

classification of dolomite rock textures abstract dolomite rock textures can be classified according to crystal size distribution and crystal boundary shape the classification scheme presented here is largely descriptive but carries genetic implications because size distribution is controlled by both nucleation and growth kinetics and crystal boundary shape is controlled by growth kinetics size distributions are classified as unimodal or polymodal crystal boundary shapes are classified as planar or nonplanar if the evidence permits a complete classification includes a description of recognizable allochems matrix and void filling allochems and preexisting cements may be unreplaced partially replaced replaced mimically or replaced nonmimically allochems may be dissolved leaving molds matrix can be unreplace partially replaced or replaced by a unimodal or polymodal size dolomite unimodal size distributions generally indicated a single nucleation event on a unimodal substrate polymodal sizes can be formed by multiple nucleation events on a unimodal or polymodal substrate or differential nucleation on an originally polymodal substrate planar crystal boundaries develop when crystals undergo faceted growth and nonplanar boundaries develop when crystals undergo nonfaceted growth nonplanar boundaries are characteristic of growth at elevated temperature 50 c and or high supersaturation both planar and nonplanar dolomite can form as a cement replacement of caco3 or neomorphism of a precursor dolomite

geomagnetic fluctuations during a polarity transition the extensive roza member of the columbia river basalt group washington state has intermediate paleomagnetic directions bracketed by underlying normal and overlying reverse polarity flows a consistent paleomagnetic direction was measured at 11 widely distributed outcrops the average direction has a declination of 189 and an inclination of 5 with greater variation in the inclination rietman 1966 in this study the roza member was sampled in two pasco basin drillcores where it is a single cooling unit and its thickness exceeds 50 m excellent core recovery allowed uniform and dense sampling of the drillcores during its protracted cooling the roza flow in the drillcores recorded part of a 15 5 ma geomagnetic polarity transition the inclination has symmetric quasicyclic intraflow variation while the declination is nearly constant consistent with the results from the outcrops thermal models of the cooling flow provide the timing for remanence acquisition the inclination is inferred to have progressed from 0 to 15 and back to 3 over a period of 15 to 60 years at rates of 1 6 to 0 5 yr because the geomagnetic intensity was probably weak during the transition these apparently high rates of change are not significantly different from present day secular variation these results agree with the hypothesis that normal secular variation persists through geomagnetic transitions the iow amplitude quasicyclical fluctuations of the field over tens of years recorded by roza suggest that the geomagnetic field reverses in discrete steps and that more than 15 60 years were required to complete this reversal

diagenesis of devonian reefal carbonates in the oscar range canning basin western australia abstract the oscar range part of the extensive devonian reef complex of the northern canning basin is a large atoll 80 10 km in which frasnian and famennian late devonian reefs grew around an emergent island of precambrian metasedimentary rocks many sediments in the oscar range had high depositional porosities but this porosity was largely occluded by diagenetic alteration diagenesis included marine cementation dolomitization pressure solution burial cementation and karstification field mapping petrography cathodoluminescence and carbon and oxygen isotope analyses have been used to document six major diagenetic episodes that affected oscar range sediments earliest cementation occurred in the marine environment where reef margin and associated subfacies developed abundant non luminescing radiaxial fibrous and microcrystalline calcite cements based on convergent covariant isotopic trends marine cements had a characteristic late devonian oxygen signature of 4 5 0 5 pdb and a carbon signature of 2 0 0 5 pdb in well cemented parts of the reef complex synsedimentary neptunian fissures formed and were filled with cement and sediment cements in these fissures have isotopic ratios that are essentially identical to the values determined for marine cements thereby suggesting that the fissures acted as conduits f r seawater flow and cementation early in the history of the reef complexes marine diagenesis was followed by marine burial and possible meteoric phreatic cementation and dolomitization marine cements are overgrown by non luminescing scalenohedral cements that grade into banded bright and non luminescing blocky calcite cements that are common on the platform but are missing in many reef margin and marginal slope deposits banded luminescence may represent fluctuating reducing and oxidizing conditions in pore waters across the intermittently emergent platform the reef complexes however were generally drowned reefs with little chance for prolonged meteoric exposure this sedimentologic evidence suggests that banded luminescing cements may have formed in the marine burial environment the next cement generation consists of non luminescing non ferroan bloc y calcites that occur throughout the reef complex stratigraphic relationships combined with isotopic results 18o 6 0 to 9 0 13c 2 0 suggest that this spar is a burial cement resulting from late devonian through early carboniferous burial the reef complex was exhumed and eroded in the late carboniferous dedolomite dolo moldic porosity pisolitic calcrete crusts iron oxide stained blocky cements and deep 200 m sinkholes formed at the unconformity surface non ferroan irregularly banded moderate to bright luminescing blocky calcite precipitated throughout the reef complex unconformity related cements calcrete crusts and dedolomites have distinctive isotopic signatures that show little variation in oxygen 7 0 to 9 0 but have a broad range of carbon values 1 5 to 8 0 highly negative carbon values are probably related to the interaction of meteoric fluids with isotopically light soil gas at the er sion surface in post carboniferous time most of the remaining porosity was filled with non ferroan bright luminescing blocky calcite that formed by pressure solution reprecipitation during burial isotopic analyses 18o 8 0 to 12 0 13c 1 5 support a burial origin for this generation of cements these cements fill primary porosity tension gashes along stylolites and hairline fractures along which hematite is leached a final generation of ferroan homogeneous dull to non luminescing blocky calcite cement formed in a late burial event that according to isotopic evidence 18o 10 5 to 15 0 13c 2 0 to 9 0 was probably related to movement of basinal brines through the reef complexes

magnetic properties of granitic rocks from the southern half of the idaho batholith influences of hydrothermal alteration and implications for aeromagnetic interpretation the measured magnetic susceptibilities of rocks from more than 600 outcrops in the southern idaho batholith have a good spatial correspondence with the observed intensities of magnetic anomalies koenigsberger ratios are low 0 3 in 75 of cored samples indicating that remanent magnetizations are unimportant relative to induced magnetizations which are related to susceptibilities in aeromagnetic anomaly interpretations in this region extremely low susceptibilities 25 10 5 within the eastern and western parts of the batholith are the sources of positive aeromagnetic anomalies the associated lithologies include mesozoic tonalite and granodiorite as well as a diverse tertiary group of epizonal stocks and batholiths intense meteoric hydrothermal activity associated with the emplacement of these tertiary plutons caused a reduction in in some rocks although coarse igneous magnetite is surprisingly resistant to hydrothermal corrosion and oxidation however under conditions of lower water rock ratios a significant increase in commonly occurred in the leucocratic plutons generally reflecting growth of hydrothermal magnetite because the measured susceptibilities do not necessarily represent more than the uppermost part of the 14 to 20 km thick crust that gives rise to magnetic anomalies an apparent discordance between susceptibility measurements and susceptibilities inferred from aeromagnetic data can occur such discordance patterns occur near the margins of the batholith where granitic rocks are thin and also in hydrothermally altered regions where subsurface tertiary plutons are present magnetic models suggest that the idaho batholith thickens to 20 km in its east central part which is underlain by enormous tertiary intrusive bodies

paleohydrogeology of the canadian rockies and origins of brines pb zn deposits and dolomitization in the western canada sedimentary basin results of stable isotopic and fluid inclusion studies of cambrian carbonate units in the southern canadian rockies indicate that during the late devonian or early mississippian a warm brine migrated from miogeoclinal shales in the western part of the western canada sedimentary basin into the eastern carbonate sequences this fluid had a temperature of 150 25 c salinity values of 20 to 25 equivalent wt nacl and isotopic values of 18 o 7 to 0 13 c 1 2 d 70 19 and 87 sr 86 sr 0 710 the fluids formed large bodies of coarse sparry dolomite as well as pb zn magnesite and talc mineralizations in the cambrian units it is probable that this event was also responsible for the development of pb zn mineralization specifically pine point in the devonian units as well as a component of the present day basinal brines in the basin

construction of the silurian and devonian seawater 87sr 86sr curve the estimated path of seawater 87sr 86sr during the silurian and devonian is based on 86 analyses 84 whole rocks presented here and in other published results the seawater 87sr 86sr rose steadily throughout the silurian from a poorly constrained value near 0 70793 sw 114 at the ordovician silurian boundary the 87sr 86sr value at the silurian devonian boundary is near 0 70871 sw 36 based on an earliest devonian limestone suite from new york the 87sr 86sr falls through the early devonian to reach a low near 0 70772 sw 135 in the eifelian and givetian and rises to near 0 70803 sw 104 in famennian time the seawater 87sr 86sr at the devonian mississippian boundary is estimated to be near 0 70805 sw 102 the middle paleozoic sample suite is judged less suitable for defining seawater 87sr 86sr than those from other paleozoic periods the middle paleozoic seawater 87sr 86sr variation indicates domination of continental sr contributions during the silurian and later devonian the high ratio continental sr contribution is controlled by the climate and the area age and composition of exposed crystalline rocks the dominance of the oceanic low ratio sr during the early and part of middle devonian is coincident with a reported increase in continental volcanism the estimated rise and fall of sealevel leading to flooding or exposure of the continents does not appear to control seawater 87sr 86sr during the middle paleozoic

multistage dolomitization in rainbow buildups middle devonian keg river formation alberta canada abstract more than 80 partially to completely dolomitized buildups of middle devonian keg river formation occur in the rainbow sub basin of northwestern alberta in six buildups a b e f g and tehze four petrographic types of dolomites are identified gray fine crystalline dolomite floating dolomite rhombs patches matrix dolomite and saddle dolomite the petrographic and geochemical data indicate that these dolomites probably formed in three principle stages of dolomitization during progressive burial gray fine crystalline dolomite less than 1 of total dolomite by volume is interpreted as penecontemporaneously dolomitized lime muds by either normal marine waters or evaporitic brines during early exposure the following evidence suggests that this dolomite probably formed early 1 it usually occurs in fractures and breccias presumably related to the early exposure of the buildups 2 it is absent in rugs and molds that formed in the subsurface environment and 3 locally clasts of gray fine crystalline dolomite are embedded in marine limestones rhomb patch dolomite is the major type of dolomite in dolomitic limestones and is widespread throughout the limestone buildups matrix dolomite is fabric destructive and is most abundant in the dolostone buildups both these dolomites probably formed during shallow to intermediate burial because they 1 postdate stylolites 2 are widespread throughout the buildups and crosscut various facies and 3 have light oxygen isotope compositions and low sr but high fe and mn concentrations dolomitizing fluids for dolomite rhombs patches and matrix dolomites were possibly derived from 1 mechanical compaction 2 dehydration of the adjacent muskeg gypsum at relatively shallow burial depths and 3 intermediate depth basinal fluids conducted updip along porous and permeable conduits during d vonian and carly carboniferous times compaction fluids were probably responsible for the early dolomite rhombs and patches however the main phase of matrix dolomitization took place later probably during late mississippian to jurassic time when the east side of the basin was intermittently tilted and uplifted causing updip flow of intermediate depth basinal brines saddle dolomite occurs as late stage vug and fracture fillings it is most depleted in 18o but most enriched in mn and fe suggesting precipitation during deeper burial from warm basinal fluids these fluids were expelled during upper cretaceous sedimentary and tectonic loading of the western side of the basin and then moved up the east side of the basin along existing platform fault and fracture conduits the source of the mg is uncertain some or most of the mg may have resulted from chemical compaction of the earlier matrix dolomite

petrogenesis of the proterozoic athabasca basin northern saskatchewan canada and its relation to diagenesis hydrothermal uranium mineralization and paleohydrogeology abstract in the athabasca basin proterozoic sandstones unconformably overlie archean and aphebian rocks multiple fluid events involving isotopically and chemically distinct fluids which have migrated laterally for considerable distances and along fault zones have produced a paragenetically identifiable assemblage of clay silicate and oxide minerals in the basin and basement rocks altered sandstones and the underlying metasedimentary rocks proximal to fault zones host high grade u ni co unconformity type uranium ore deposits and contain an alteration assemblage of highly crystalline 2m 3t illite 1m kaolinite clinochlore and sudoite hematite dravite and euhedral quartz in the sandstones 18o values of coeval quartz hematite quartz illite quartz dravite illite hematite and kaolinite hematite fluid inclusion temperatures in diagenetic quartz and d values of 1m kaolinite 2m 3t illite dravite and in fluid inclusions in the quartz indicate formation from a basinal fluid that had d values of 60 10 and which underwent salinity temperature and 18o increases from 5 to 34 wt nacl 120 to 240 c and 5 to 3 respectively as a result of increasing burial depth and sustained water rock interactions with the basinal sediments over substantial time periods 18 o 16 o and d h ratios of sudoite and clinochlore in meta sedimentary basement rocks within the alteration haloes indicate formation from a basement derived fluid with 18o and d values of 4 4 and 30 10 respectively that was isotopically distinct from the basinal fluids 18o and d values of late kaolinites in reactivated fault zones indicate formation mainly from low temperature 50 25 c fluids having 18o and d values of 16 and 130 respectively similar to relatively modern meteoric waters in the area some kaolinites have 18o and d values which suggest they formed from meteoric waters at mid latitudes and have experienced varying degrees of oxygen and hydrogen isotope exchange with relatively modern meteoric waters in the re activated faults illite sudoite and dravite have d values that vary by 100 without concurrent changes to their 18o values indicating preferential hydrogen isotope exchange with relatively modern meteoric waters at low temperatures and under conditions of high integrated water rock ratios rb sr isochron and model ages of 1477 57 and 952 ma respectively for the crystalline 2m 3t illites indicate the timing of the high temperature basinal fluid events associated with fluid mixing rb sr and k ar ages on illites with the lowest d values are as young as 300 ma and indicate late stage resetting of their radiogenic isotope systematics as a result of substantial interactions with relatively modern meteoric waters the evolution of sr isotopic compositions of fluids in the basin and basement rocks as reflected by the 87 sr 86 sr ratios of chlorite illite and dravite indicates that mixing of two isotopically distinct fluids in the vicinity of faults was the process by which uranium was precipitated similar to that suggested by oxygen and hydrogen isotope systematics in the clays and silicate minerals nd ratios in the high temperature diagenetic and hydrothermal clay and silicate minerals and the cogenetic uranium ores suggest they all have formed from similar fluids which have interacted substantially with the basin sediments in general the isotopic chemical microthermometric and petrologic data indicate that the athabasca basin has had a protracted fluid history comparable to that documented for more modern sedimentary basins the data clearly show that the major sandstone aquifers have been affected by widespread lateral flow of diagenetic fluids over distances of hundreds of kilometers and also that these fluid migration paths have been modified by cross formational fluid flow near active fault zones

drilling induced isothermal remanent magnetization the recovery of core samples is important in petroleum exploration mineral exploration and scientific drilling projects and often complete orientation of the samples azimuth and plunge is desirable recovered cores are usually not azimuthally oriented because of the costs associated with deployment and operation of downhole orientation tools inexpensive paleomagnetic orientation methods have been used with considerable success in the borehole environment van der voo and watts 1978 kodama 1984 bleakly et al 1985a b evans and mailol 1986 layer et al 1988 mcwilliams and pinto 1988 in some cases the technique has been hampered by secondary magnetizations associated with the drillstring and or coring tool magnetizations which have partially or completely overprinted the primary and secondary magnetizations used for orientation

oxygen 18 oxygen 16 and d h studies of plutonic granitic and metamorphic rocks across the cordilleran batholiths of southern british columbia hydrogen and oxygen isotope ratios of 500 samples mainly from granitic plutons were measured along a 700 km e w traverse across the accreted terranes of southern british columbia latitudes 49 52 n despite the geological complexity and range of intrusive ages late triassic to tertiary and although there are steps in the isotopic values at some geologic boundaries e g across the strait of georgia two clear patterns emerge 1 the 18 o 16 o and d h ratios of the waters involved in hydrothermal interactions with the granitic rocks show a regular eastward trend of depletion in d and 18 o enormous areas were affected by the hydrothermal processes but the most intense alteration is localized along major north trending lineaments e g okanagan lake 2 independent of the hydrothermal effects the primary 18 o values of the granitic rocks also change systematically eastward from 7 0 to 8 5 in vancouver island reaching a minimum of 5 5 to 7 0 in the western and central coast plutonic complex then increasing progressively from the eastern coast batholith to the okanagan batholith and attaining a maximum of 10 0 to 12 0 in the nelson batholith two groups of samples are unique in their high d values the first group is represented by two geographically isolated batholiths guichon and thuya that were not affected by the tertiary meteoric hydrothermal systems and that have therefore preserved their early jurassic to triassic k ar ages the second group is represented by the jurassic plutons of vancouver island there the hydrothermal fluids were both d rich and 18 o rich 18 o 0 as evidenced by the fact that feldspars in the altered granites are enriched in 18 o relative to coexisting quartz both anomalies can be explained if these terranes were located closer to the equator and or in a maritime environment at the time of intrusive and hydrothermal activity in agreement with available paleomagnetic data excluding these anomalous areas two distinct ages of meteoric hydrothermal activity can be identified cretaceous in the west and early to mid tertiary in the east the isotopic trends in the rocks are similar to the present day patterns of meteoric waters in the region with one primary difference the paleowaters are enriched in d by about 20 compatible with a northward translation of these terranes a climatic change or both the similarities of the patterns suggest a topography similar to that of the present day a mountain chain along the coast during the early tertiary the whole rock 18 o values of the granitic rocks determined by extrapolation of the trends of 18 o quartz versus 18 o feldspar to correct for hydrothermal alteration effects suggest that the main part of the coast plutonic complex formed from magmas similar to those in oceanic island arcs derived from basaltic crust or from an upper mantle source toward the east an increasingly larger component of sedimentary rocks must be present in the source regions of the granitic magmas it may be significant that right at the edge of the north american craton there is a drop in the 18 o of the plutons from 11 down to 9 5 suggesting involvement of old granitic crust instead of sedimentary rocks these new data from southern british columbia confirm and amplify previous studies that have documented 1 the existence of widespread and pervasive hydrothermal effects throughout the north american cordillera and 2 regular west to east asymmetries in the amounts of different kinds of parent rocks in the source regions from which these mesozoic and cenozoic granitic magmas were derived

fluid rock interaction history during stabilization of early dolomites upper knox group lower ordovician u s appalachians abstract the lower ordovician upper knox group is characterized by shallowing upward peritidal cycles that are extensively dolomitized to form massive dolomite most 85 replacive dolomite is stratigraphically controlled composed of very fine to medium crystalline planar to nonplanar early dolomite composed of zones 1 2 and 3 dolomites remaining dolomite occurs as medium to coarsely crystalline nonplanar late replacive dolomite zones 2 and 3 late dolomites that selectively replaces limestone associated with stylolites and fractures or forms massive dolomite fronts complexly zoned zone 2 to 6 saddle dolomite cements make up less than 5 of all dolomite and primarily fill solution voids and fractures early dolomites are near stoichiometric and relative to early ordovician marin carbonates are similar to or significantly depleted in 18o and sr slightly to significantly enriched in mn and fe and have similar 87sr 86sr values relative to early dolomites late zones 2 and 3 dolomites are depleted in 18o and st enriched in mn and fe and have similar 87sr 86sr values early dolomites are interpreted to have formed from modified seawater during seaward progradation of tidal flats within each high frequency fifth and fourth order cycle duration late dolomites based on their texture and geochemistry formed as a result of recrystallization of early dolomites as direct replacement of limestone at elevated temperatures and as cement in fractures and secondary solution voids covariant trends between textures and geochemical compositions for early dolomite indicate that presentday compositions of knox early dolomite record a history of progressive diagenetic modification stabilization or recrystallization involving varying degrees of changes of textural and geochemical compositions during multiple episodes of dolomitization early dolomites wit the most extensively altered textures are more stoichiometric more depleted in 18o and sr and more enriched in mn and fe relative to texturally less altered early dolomite initial stabilization may have occurred syndepositionally in modified seawater continued stabilization of early dolomite likely occurred in fresh or mixed waters of a middle ordovician meteoric aquifer associated with knox unconformity development final stabilization occurred by deep burial brines at elevated temperatures as indicated by 1 extensive replacement and overgrowth of early dolomite by late dolomites 2 a progressive increase in crystal size and abundance of nonplanar crystal boundaries with increase in replacement by late dolomites and 3 the similarity of the geochemical compositions of texturally most extensively altered early dolomites and late dolomites

paleomagnetic constraints on the tectonic history of the foreland belt southern canadian cordillera preliminary results in the southern canadian cordillera the paleomagnetic memory of paleozoic carbonate strata in the front ranges and inner foothills of the foreland thrust and fold belt retains no record of their known deposition at low latitudes instead each folded structure exhibits a similar but asynchronous sequence of events including an eastwardly progressing predeformational chemical remagnetization during the cordilleran orogeny the remagnetization of a western front ranges structure occurs during a period of normal polarity before 130 ma the paleomagnetic pole requires that the subsequent deformation of the western front ranges is jurassic or younger the remagnetization of a front ranges structure in the lewis thrust sheet occurs during a period of normal polarity after 130 ma but before deformation which from other evidence occurred around 75 ma the predeformational remagnetization of an inner foothills structure occurs during a reversed magnetic period that we interpret to be after 75 ma an early cretaceous sill in the lewis thrust sheet was remagnetized during a reverse polarity chron prior to the end of lewis thrust deformation when about 70 of the present dip of the sill was acquired remagnetization consistently predates deformation whereas it occurs later at more easterly localities there are also similarities in character and style of the remagnetizations among localities when coupled with the eastward progression of the deformation our observations suggest that an important and pervasive but hitherto unrecognized and unappreciated orogenic chemical process affected paleozoic carbonate strata in the van of the deforming cordilleran tectonic wedge

a stable isotope study of retrograde alteration in sw connemara ireland dalradian metamorphic rocks lower ordovician meta igneous rocks mgs and caledonian granites of the connemara complex in sw connemara all show intense retrograde alteration alteration primarily involves sericitization and saussuritization of plagioclase the alteration of biotite and hornblende to chlorite and the formation of secondary epidote the alteration is associated with sealed microcracks in all rocks and planes of secondary fluid inclusions in quartz where it occurs and was the result of a phase of fluid influx into these rocks in hand specimen k feldspar becomes progressively reddened with increasing alteration mineralogical alteration in the mgs and caledonian granites took place at temperatures 275 15 c and in the mgs pfluid is estimated to be 1 5 kbar during alteration the d values of alteration phases are 18 to 29 fluid inclusions 47 to 61 chlorites and 11 to 31 epidotes chlorite 18o values are 0 2 to 4 3 while 18o values for quartz k feldspar pairs show both positively sloped mgs and highly unusual negatively sloped caledonian granites arrays diverging from the normal magmatic field on a plot the stable isotope data show that the fluid that caused retrogression continued to be present in most rocks until temperatures fell to 200 140 c the retrograde fluid had d 20 to 30 in all lithologies but the fluid 18o varied both spatially and temporally within the range 4 to 7 the fo2 of the fluid that deposited the epidotes in the mgs varied with its 18o value with the most 18o depleted fluid being the most oxidizing the d values together with low 0 18o values for the retrograde fluid in some lithologies indicate that this fluid was of meteoric origin this meteoric fluid was probably responsible for the alteration in all lithologies during a single phase of fluid infiltration the variation in retrograde fluid 18o values is attributed to the effects of variable oxygen isotope shifting of this meteoric fluid by fluid rock interaction infiltration of meteoric fluid into this area was most likely accomplished by convection of pore fluids around the heat anomaly of the galway granite soon after intrusion at 400 ma however convective circulation of meteoric water and mineralogical alteration could possible have occurred considerably later

strontium and oxygen isotopic variations in mesozoic and tertiary plutons of central idaho regional variations in initial 87sr 86sr ratios r i of mesozoic plutons in central idaho locate the edge of precambrian continental crust at the boundary between the late paleozoic mesozoic accreted terranes and precambrian sialic crust in western idaho the r i values increase abruptly but continuously from less than 0 704 in the accreted terranes to greater than 0 708 across a narrow 5 to 15 km zone characterized by elongate lens shaped highly deformed plutons and schistose metasedimentary and metavolcanic units the chemical and petrologic character of the plutons changes concomitantly from ocean arc type diorite tonalite trondhjemite units to a weakly peraluminous calcic to calcalkalic tonalite granodiorite granite suite the idaho batholith plutons in both suites yield late cretaceous ages but permian through early cretaceous bodies are confined to the accreted terranes and early tertiary intrusions are restricted to areas underlain by precambrian crust the two major terranes were juxtaposed between 75 and 130 m y ago probably between 80 and 95 m y oxygen and strontium isotopic ratios and rb and sr concentrations of the plutonic rocks document a significant upper crustal contribution to the magmas that intrude precambrian crust magmas intruding the arc terranes were derived from the upper mantle subducted oceanic lithosphere and may have been modified by anatexis of earlier island arc volcanic and sedimentary units plutons near the edge of precambrian sialic crust represent simple mixtures of the precambrian wall rocks with melts derived from the upper mantle or subducted oceanic lithosphere with r i of 0 7035 rb sr varies linearly with r i producing pseudoisochrons with apparent ages close to the age of the wall rocks measured 18o values of the wall rocks are less than those required for the assimilated end member by sr o covariation in the plutons however indicating that wall rock 18o was reduced significantly by exchange with circulating fluids metasedimentary rocks of the belt supergroup are similarly affected near the batholith documenting a systematic depletion in 18o as much as 50 km from the margin of the batholith plutons of the bitterroot lobe of the idaho batholith are remote from the accreted terranes and represent mixtures of precambrian wall rocks with melts dominated by continental lower crust r i 0 708 rather than mantle pseudoisochrons resulting from these data are actually mixing lines that yield apparent ages less than the true age of the wall rocks and meaningless ri assimilation fractional crystallization models permit only insignificant amounts of crystal fractionation during anatexis and mixing for the majority of plutons of the region

stabilization of early formed dolomite a tale of divergence from two mississippian dolomites abstract several large hydrocarbon accumulations in alberta canada are hosted in dolomitized successions of stacked thin sabkha capped cycles of visean age porosity is micro intercrystalline and occurs in dolomitized restricted subtidal and intertidal muds that have their fine primary fabric preserved two such fields are here considered that illustrate divergent and contrasting modes of dolomite stabilization despite initial similarities in facies and textures the dolomite in the upper debolt formation of the dunvegan field nw alberta forms planar e or microsucrosic fabrics with crystals in the 1 20 m range the dolomite is non ferroan ca rich average of 58 mol caco 3 and poorly ordered its stable isotopic signatures range from 0 12 to 3 4 vpdb for 18 o mean 1 3 and 0 9 to 4 3 vpdb for 13 c mean 2 6 the average radiogenic 87 sr 86 sr ratio for this dolomite is 0 7077 both sets of values are consistent with dolomite precipitation from mississippian marine or modified marine evaporated seawater these parameters are strongly reminiscent of holocene protodolomites and hence suggestive of a sabkha dolomitization process shallow seepage reflux or evaporitive pumping this dolomite with its high associated porosity average of 15 and up to 38 relatively unaltered mineralogical and chemical signatures both preserved despite 4 km of burial depth suggests a very unique set of relatively non reactive physico chemical conditions during burial likely a closed system in contrast the dolomite from the mount head formation of the shell waterton field sw alberta has undergone measurable neomorphic alteration in several stages in deeper burial environments open system such alteration has affected its crystal size range 18 o ranges between 1 5 in the least altered dolomite and 13 2 vpdb 13 c ranges from 3 9 to 1 5 vpdb and 87 sr 86 sr ranges from 0 7078 to 0 7090 the dolomite has retained a degree of non stoichiometry with an average ca content of 55 mol caco 3 the mount head dolomites clearly indicate ongoing reactivity between the rocks and basinal fluids during burial and are in this respect representative of the norm for dolostones in the western canada sedimentary basin

large scale fluid flow in the middle devonian presqu ile barrier western canada sedimentary basin saddle dolomite cements are regionally extensive in the middle devonian presqu9ile or keg river barrier and are spatially associated with mississippi valley type mineralization at pine point northwest territories from northeastern british columbia to pine point over a lateral distance of 400 km these dolomite cements show general trends of decreasing 87 sr 86 sr ratios 0 7106 to 0 7081 and homogenization temperatures 178 to 92 c with some increase in 18 o values 16 to 7 pdb these regional trends suggest that hotter and more radiogenic basinal fluids moved eastward updip along the presqu9ile barrier and mixed with cooler ambient formation waters these movements of basinal fluids were probably related to tectonic thrusting and compression sedimentary loading and tectonic uplift on the western margin of the western canada sedimentary basin which began to form during late jurassic to early cretaceous time and climaxed during the late cretaceous to paleocene the middle devonian presqu9ile barrier appears to have acted as a deeply buried regional conduit system that played an important role in focusing and channeling these basinal fluids these regional fluid flows appear to have been responsible for extensive burial dolomitization secondary migration of hydrocarbons and local mississippi valley type mineralization in the western canada sedimentary basin

insights into hydrological regime of lake vostok from differential behavior of deuterium and oxygen 18 in accreted ice 1 we use isotopic data deuterium and oxygen 18 of the recently recovered deepest vostok ice core section down to 3650 m depth to study processes leading to the formation of lake ice and the hydrological regime of subglacial lake vostok the significant variability of the lake ice isotopic content implies fluctuations in physical conditions of ice formation mainly volume and or growth rate of frazil ice crystals as well as variations of the isotope composition of the freezing water the latter implies a poor mixing of the source waters glacier melt and hydrothermal water with the water of the main lake body poor mixing within the lake may have important consequences for the lake s chemical and gas balance and particularly for its microbiological content a poorly mixed lake may provide ecological niches where microbial life can hide from high oxygen concentrations likely typical for the lake we also show that the isotopic content of the main lake s input meltwater significantly differs from that of the output lake ice which can be explained by the contribution of an additional hydrothermal source this latter conclusion is supported by the observed noncovariant behavior of deuterium and oxygen 18 isotopes in the lake ice

palaeomagnetism of granitic intrusives from the precambrian basement under eastern kansas orienting drill cores using secondary magnetization components summary the precambrian basement under east central kansas was drilled at two circular aeromagnetic positives one at osawattamie and one at big springs the core retrieved from these sites is a coarse to medium grained granite which has been dated by u pb to be 1350 ma old the palaeomagnetism of these azimuthally unoriented cores was studied to see if a technique which uses low coercivity low temperature magnetization components to orient the cores would allow an independent confirmation of the core s mid proterozoic age orthogonal projection plots of the alternating field af and thermal demagnetization data show that the magnetization of these cores is relatively simple having only two components a low temperature low coercivity magnetization with steep positive inclinations and a shallow negative inclination characteristic magnetization for the osawattamie core or a positive moderate inclination characteristic magnetization for the big springs core if the declination of the low temperature low coercivity component is aligned parallel to the present field declination the characteristic directions may be azimuthally oriented this allows the calculation of palaeomagnetic poles for the big springs core lat 4 5 s long 29 9 e and the osawattamie core lat 20 2 n long 39 3 e which are consistent with irving s apparent polar wander path for laurentia at about 1300 1400 ma comparison of anhysteretic remanent magnetization arm viscous remanent magnetization vrm and isothermal remanent magnetization af demagnetization curves with a natural remanent magnetization nrm demagnetization curve suggests that the osawattamie core probably acquired a piezoremanent magnetization prm parallel to the core axis during drilling

oxygen isotope map of the giant metamorphic hydrothermal system around the northern part of the idaho batholith u s a abstract determinations of 18o values from 100 outcrops of belt supergroup wallace fm metasedimentary rocks in the idaho panhandle reveal a regular regional pattern that was produced by pervasive fluid infiltration and isotopic exchange low grade argillites at large distances 60 km from the idaho batholith have high 18o values 15 compatible with their probable primary values pelitic rocks with anomalously low 18o values of 8 7 to 12 7 occur in the following zones 1 in a 5000 km2 zone of schist and gneiss peripheral to the idaho batholith generally coincident with high grade sillimanite bearing assemblages 2 in high grade metasedimentary roof pendants within the idaho batholith 3 peripheral to small cretaceous stocks and 4 within and near the scapolite bearing zone south west of st regis on 18o 18o plots data from coexisting minerals define trends with unit slopes indicating that the reductions in 18o occurred under high grade metamorphic conditions this metamorphism culminated in the emplacement of the idaho batholith probably as a consequence of profound crustal thickening associated with the cretaceous accretion of the wallowa seven devils arc terranes with north america the huge low 18o region is bounded by a steep 18o gradient 0 1 0 5 km that occurs in low grade rocks along and near the lewis and clark line well below the biotite isograd this boundary zone may be analogous to but is not nearly as sharp as those of meteoric hydrothermal systems in many regions the important ore deposits of the coeur d alene district are located in this peripheral zone suggesting that the metamorphic hydrothermal system may have been intimately involved in their formation in addition the metamorphic hydrotermal system in idaho is larger deeper and higher in temperature than typical meteoric hydrothermal systems and it involved fluids with much higher 18o values that were probably dominantly derived from formation waters accordingly this system produced rocks with 18o values similar to those of the idaho batholith and mineral assemblages that approach isotopic equilibrium under high temperature conditions

late laramide dolomite recrystallization of the husky rainbow a hydrocarbon devonian reservoir northwestern alberta canada paleomagnetic and geochemical evidence the rainbow field is in reefal carbonates of the middle devonian keg river formation in the western canada sedimentary basin alternating field and thermal step demagnetization was done on specimens from unoriented core from a vertical an inclined and a horizontal well core in the dolomite reservoir although they had no viscous remanent magnetization component to use for orientation most specimens had a well defined characteristic remanent magnetization chrm that resides in single to pseudosingle domain magnetite by rotating the mean chrm direction around its core axis a small circle can be generated for each core and the small circles intersect in the true chrm direction of d 168 i 73 5o 95 5 8 k 32 2 its paleopole of 164 e 83 n a95 10 defines a tertiary age with one sigma limits of middle eocene to middle miocene age petrographic examination defines four generations of dolomite matrix dolomite has 60 100 m diameter crystals that were later recrystallized to 200 400 m

mesozoic alteration of permian volcanic rocks thuringer wald germany ar sr and o isotope evidence abstract besides older granites and gneisses the thuringer wald in central europe consists predominantly of altered permian volcanic and pyroclastic rocks the alteration history of these volcanic rocks was investigated by k ar 39 ar k 40 ar rb sr 18 o d and x ray studies on feldspar and biotite separates alteration is accompanied by increases in the triclinicity and 18 o values 14 to 19 of alkali feldspars an increase in the 18 o values of low potassium biotites 10 and in sericitized plagioclases 12 to 19 and also the generation of apparent mesozoic isotopic ages in contrast volcanic biotites with primary magmatic 18 o signatures still yield early permian ages and biotites of the granitic basement give carboniferous ages the observed isotope changes of the volcanic feldspars indicate episodic fluid exchange with meteoric and formation brines 18 o 5 at low temperatures 200 to 100 c during tectonic fracturing of the basement as a result sanidines were replaced by porous microclines which were subject to isotope exchange and ar loss feldspars and biotites of the granitic wall rocks seem to be less affected by alteration processes which may be due to their lower permeabilities plateau shaped 40 ar 39 ar age spectra of volcanic alkali feldspar indicate complete resetting of the k ar system whereas staircase spectra point to partial retention or repeated ar loss because the feldspars represent mixtures of microcline and sanidine remnants the apparent ages 260 to 160 ma represent maximum ages for the time of replacement rb sr feldspar model ages 180 ma or 230 to 170 ma respectively support the k ar data the minimal 40 ar 39 ar spectra ages are concordant with both k ar and 40 ar 39 ar sericite model ages 160 to 150 ma of sericitized plagioclases the model ages favour jurassic episodic fluid reactions with alkali feldspars and plagioclases in the thuringer wald

revised and synthetic apparent polar wander paths of the african eurasian north american and indian plates and true polar wander since 200 ma we have reviewed paleomagnetic data available for the eurasian african north american and indian plates over the last 200 ma selection criteria are those generally accepted with an emphasis on evidence for lack of remagnetization accurate dating and proper structural analysis this results in 23 35 51 and 2 poles for eurasia africa north america and india respectively we believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander apw paths than averaging of larger data sets involving less stringent selection criteria we propose and describe revised apw paths but more importantly we next use relative motion models to transfer all data in a common reference frame we find good agreement between transferred data when they are averaged separately for each plate in independent 20 ma windows this is a check on consistency of paleomagnetic data kinematic models and of the geocentric dipole hypothesis transferred data from all plates are averaged in 20 ma windows to generate synthetic apw paths for all plates studied these synthetic paths are in agreement with the original revised apw paths that use only data from a single plate moreover both geographic and time resolution are improved spatial confidence intervals are of the order of 5 the synthetic paths display interesting features such as a previously ill recognized apw loop for eurasia paleomagnetic and hotspot apw are next compared and a determination of true polar wander tpw is derived we find significant tpw amounting to over 20 in the last 200 ma tpw appears to be episodic with a standstill between 180 and 110 ma there is general agreement with a previous study of livermore et al 1984 the tpw standstill appears to correlate with a time of decreasing reversal frequency ending with the cretaceous long normal superchron other periods of fast tpw would seem to correspond to increasing reversal frequency however it is suggested that the major tpw hairpin at 50 ma might correspond to the collision of india with eurasia tentative correlations with core or mantle indicators can be understood as a result of couplings between the core mantle and lithosphere we believe primarily related to episodic evolution of the d layer

changes in the hoar isotope composition of clays during retrograde alteration abstract k ar ages of illite alteration associated with middle proterozoic athabasca unconformity type u deposits in saskatchewan range from 414 to 1493 ma the k ar ages correlate with water contents and d values such that illites with young k ar ages have d values as low as 169 and water contents as high as 7 7 wt whereas illites with older ages have d values near 70 and water contents near 4 wt water extracted at 400 c from illites with low d values and high water contents has low d and 18 o values similar to those of modern meteoric water suggesting that some of the illites associated with the original deposition of the ore underwent varying degrees of retrograde alteration the alteration is initiated by hydration of sites in the interlayer region of the illite which results in the partial resetting of the k ar ages and introduction of excess structural water in the form of interlamellar water the interlamellar water is enriched in 18 o by about 7 per mil relative to the water that physically surrounded the clay particle further alteration decreases the d value and increases the 18 o value of the illite by isotopic exchange between the mineral and the interlamellar water although the chemical compositions and xrd patterns of the altered illites indicate that no detectable smectite component is present in the samples the isotopic results suggest that the altered illites may be an early precursor in the formation of mixed layer illite smectite by retrograde alteration of pure illite the wide variation of d values of chlorite and kaolinite from these u deposits is analogous to that of the illite suggesting that retrograde alteration of clays by meteoric water can be substantial the general association of altered clays with areas containing the highest concentrations of u is probably related to localized permeability within the ore zone

effects of regional uplift deformation and meteoric hydrothermal metamorphism on k ar ages of biotites in the southern half of the idaho batholith k ar ages of biotites from the southern half atlanta lobe of the idaho batholith are younger than the true emplacement ages of the rocks for mesozoic plutons with true ages probably greater than 100 m y the k ar ages range from 95 m y along the east and west margins of the batholith to 37 m y in the east central zone there is a conspicuous age gap between 46 and 53 m y however a north trending group of epizonal eocene plutons in the east central zone has k ar ages of 44 6 m y values which closely approximate the crystallization ages these eocene plutons which were emplaced at shallow levels and underwent rapid cooling are responsible for widespread 10 000 km2 meteoric hydrothermal alteration of the mesozoic country rocks partial chloritization and lowering of the k2o deuterium and 18o contents of the biotites accompanied the hydrothermal activity although the intensity of the hydrothermal effects is geographically correlated with the degree of lowering of the k ar ages the production of anomalously young k ar ages is attributed in part to regional uplift and doming in addition to the hydrothermal metamorphism in any given part of the atlanta lobe the k ar ages of the higher elevation samples become systematically older with increasing elevation this effect is observed in almost all of the biotites both those that have undergone intense hydrothermal alteration and those that show no evidence of any kind of hydrothermal effects the only exceptions are biotites that underwent complete ar loss such samples occur only at low elevations and within the contact metamorphic aureoles of the eocene plutons age versus elevation trend lines are linear with slopes that indicate uniform late cretaceous and early tertiary uplift 0 10 0 14 mm yr and cooling of the terrane the trend lines have almost identical slopes throughout the region but the positions of the trend lines shift systematically upward to higher elevations from west to east indicating that subsequent doming and faulting of the terrane modified the relative elevations of different subregions the zone where this more recent uplift is most prominent almost coincides with the axis of the eocene intrusions furthermore the k ar age versus elevation trend lines are systematically truncated both at low elevations and close to contacts with eocene plutons apparently because of a temperature increase due to widespread hydrothermal heating of the mesozoic rocks by the eocene batholiths in rocks located at present day elevations below about 1000 1500 m in the central part of the atlanta lobe biotites lost practically all their radiogenic ar during the eocene intrusive event

the occurrence and origin of late paleozoic remagnetization in the sedimentary rocks of north america although it has been known for 25 years that some paleozoic sedimentary rock units in europe and north america were remagnetized during pennsylvanian or permian time it is only very recently that the complex and widespread nature of the late paleozoic remagnetization phenomenon has been generally acknowledged it is now recognized that many paleozoic paleomagnetic poles for north america that were once considered reliable are in fact the result of remagnetization and as a consequence the paleomagnetic data base for the paleozoic is undergoing rapid and drastic revision the causes of late paleozoic remagnetization in north america are currently the focus of much interest and active research the remagnetization can reside in either hematite or magnetite and different remagnetization mechanisms have been important in different settings chemical remagnetization processes some related to specific diagenetic events are dominant in hematite bearing sandstones and carbonates in magnetite bearing carbonates both chemical and thermoviscous remagnetization processes appear to have been important but it is difficult to determine which process is the dominant one in some settings some of the observed remagnetizations can be linked to the migration of chemically active and perhaps hot fluids during the mountain building events that affected much of the continent during the late paleozoic paleomagnetic studies promise to be important in assessing the role of orogeny in driving fluid migrations within sedimentary basins and in constraining the age of the migrations and the nature of the fluids

nd isotope mapping of the allochthon boundary thrust on the shores of georgian bay ontario significance for grenvillian crustal structure and evolution nearly 50 new nd isotope analyses are presented for the shawanaga region of georgian bay ontario to study crustal evolution in the grenvillian central gneiss belt depleted mantle t dm nd model ages are used to map a major grenvillian tectonic boundary the allochthon boundary thrust abt which in the shawanaga area separates gneisses with t dm ages above and below 1 65 ga this is lower than the 1 8 ga age cut off observed further north and is attributed to a southward increase in mesoproterozoic magmatic reworking of an original palaeoproterozoic continental margin causing a progressive southward decrease in nd model ages between shawanaga island and franklin island nd isotope mapping yields an abt trajectory that closely matches published geological mapping and passes within 100 m of four retrogressed eclogite bodies typically associated with the thrust boundary this validation of the method gives confidence in the mapped trajectory south of snake island where sparse outcrop inhibits lithological mapping the new results suggest that published 1 7 1 9 ga t dm ages in the lower go home domain of the central gneiss belt further south are also indicative of parautochthonous crust hence we propose that the main ramp of the abt is located in the immediate hangingwall of the go home domain much further south than generally recognized this has important implications for the large scale crustal structure of the sw grenville province suggesting that the abt ramp has a similar curved trajectory to the grenville front and the central metasedimentary belt boundary thrust

paleogeographic evolution of the united states pacific northwest during paleogene time paleogeographic reconstructions for oregon and washington during paleogene time illustrate a major transition from a dominantly compressional prior to middle eocene time to an extensional tectonic regime this transition resulted in the development of three phases of paleogene basin evolution in the united states pacific northwest during the initial phase basins formed along the continental margin during collision of oceanic islands sediments in these basins were derived from nearby orogenic highlands the second phase of basin development began in middle eocene time and consisted of rapid subsidence of individual basins that formed within a broad forearc region nonmarine basins that formed during this phase were caused by extension possibly associated with transcurrent faulting rapid sedimentation in both marine and nonmarine basins during this time consisted dominantly of sandstone derived from cretaceous plutonic sources far to the east the final stage of basin development was the modification

anticorrelated rb sr and k ar age discordances leuchtenberg granite ne bavaria germany the leuchtenberg granite oberpfalz ne bavaria displays a continuous differentiation trend ranging from mildy peraluminous coarse grained porphyritic biotite granites bg to strongly peraluminous medium to fine grained garnet bearing muscovite granites gmg the rb sr and k ar age determinations of whole rock and mineral samples from the granite and associated intermediate rocks redwitzites have revealed two divergent age gradients rb sr wholerock dates decrease and initial 87sr 86sr ratios increase for successively more evolved subsets of the granite all bg samples 87rb 86sr 2 16 yield a date of 326 2 ma with a low initial 87sr 86sr ratio of 0 70778 0 00013 1 while all gmg samples 87rb 86sr 70 to 1000 yield a younger date of 317 2 ma with an enhanced initial 87sr 86sr ratio of 0 7146 0 0039 the k ar measurements on biotites and muscovites give closely concordant dates for the gmg 326 323 ma and the southern lobe of the bg 324 320 ma the northern lobe of the bg including the redwitzites shows a well defined trend of decreasing k ar dates from 320 ma to 300 ma towards the northwest critical consideration of both isotope systems leads to the conclusion that the rb sr system of the gmg was disturbed by a later hydrothermal event the ca 326 ma whole rock rb sr date for the bg is not in conflict with any of the k ar mineral dates and is taken as approaching the crystallization age of the leuchtenberg granite the k ar age progression within the northern lobe of the bg indicates that this part either cooled down over a protracted period of some 20 ma or experienced reheating at ca 300 ma the study highlights the potential of combined rb sr and k ar dating in deciphering detailed chronology on the scale of a single igneous intrusion

tertiary meteoric hydrothermal systems and their relation to ore deposition northwestern united states and southern british columbia tertiary meteoric hydrothermal systems have altered the rocks exposed over more than 5 of the land surface of the northwestern united states and southern british columbia including at least 25 000 km 2 in idaho the systems typically involved convective circulation of fluid derived from ordinary meteoric groundwaters around crystallizing calc alkaline epizonal plutons emplaced into coeval volcanic cover rocks these individual systems had widely ranging lifetimes of 10 3 to 10 6 years and operated locally throughout the cenozoic although the most profound development of such activity occurred during eocene time individual systems varied in size from a few tens of square kilometers yankee fork idaho or less to several thousand square kilometers sawtooth and castro ring zones idaho typically regional propylitization aacompanied the fluid circulation although the higher temperature alteration assemblages were developed locally as were intense alteration effects e g silicification sericitization etc near some veins and in mining districts a significant amount probably 25 50 of the mineral production and potential in the region is closely related to tertiary meteoric hydrothermal systems oxygen and hydrogen isotopic data clearly demonstrate the close geologic association of meteoric hydrothermal systems and mineralization in 1 the paleocene cu zn pb mn main stage mineralization at butte montana 2 numerous eocene epithermal deposits principally valued for au and ag but also including significant deposits of cu pb zn f sb etc as at republic washington and in several mining districts in the idaho batholith and the challis volcanic field 3 several eocene skarn deposits valued for w ima idaho and cu mackay idaho 4 important lead silver vein and replacement deposits of tertiary bluebell british columbia and of probable cretaceous and early tertiary age wood river idaho 5 several potentially economic mo be and u bearing eocene porphyry plutons and 6 miocene epithermal deposits most prominently the au and ag bearing veins at silver city and delamar idaho the hg deposits at the mcdermitt caldera nevada and oregon and at weiser idaho and au deposits in the western cascade range and lake county oregon a close spatial association has been demonstrated between ore deposits and rocks having anomalous 18 o values and low d values the most important deposits are associated with relatively small generally 5 300 km 2 zones of low 18 o values and they are particularly closely linked with zones of very steep 18 o 16 o gradients in the altered rocks these associations hold much promise for the use of 18 o and d contour maps in future exploration efforts

variations in geometric measures of topographic surfaces underlain by fractured granitic plutons silicate melts impose systematic fracture patterns on their hosts and their own subsolidus portions as they crystallize and cool to ambient conditions weathering of these plutonic bodies accentuates the fractures and produces fragmented topographic surfaces whose geometric measures exceed those of a euclidean surface because geometric measures of these surfaces are potentially indicative of the percolation properties of the fracture network contours and vertical sections were digitized from 1 250000 and 1 240000 scale maps and their statistical fractal dimensions d were computed regions underlain by granitic plutons with similar fragmentation patterns were found to haved values ranging from 1 15 to 1 28 within a single pluton this range of values is caused by noise and several geologic factors the values contain noise introduced by the map making and digitizing procedures that amounts tod 1 05 however all values lie well above this threshold and they correlate closely with local differences in weathering smallerd values derive from contours where accumulation of screen or glacial erosion has smoothed the surface whereas largerd values derive from contours along aretes and in unglaciated terrains in each of these regions largerd values occur locally where fractures are most frequent and or continuous larged values were also derived from regions underlain by host rocks that lie in the near field region of the stress field caused by the pluton the fractral values derived are apparently related to the magma hydrothermal event they can potentially be used to derive properties of the percolation networks that were active during the hydrothermal events

effects of normal faulting on fluid flow in an ore producing hydrothermal system comstock lode nevada abstract normal faults in the comstock lode mining district nv usa are delineated by low 18o belts and strong isotopic gradients perpendicular to strike these faults perturb and also offset the regional pattern of 18o contours produced by pervasive hydrothermal alteration of the alta formation and have preserved the down dropped hanging wall rocks while exposing the footwall rocks to greater erosion fluid rock interaction was greatly enhanced within 200 m wide zones centered on the normal faults where permeability was increased and fluids were focused parallel to the fault plane the effects of displacement and permeability enhancement around the faults are investigated using oxygen isotope profiles through the comstock and occidental faults and across steeper secondary normal faults such as the coryell fault that may have formed after the principal faults were rotated to more shallow dips oxygen isotope profiles record both post hydrothermal displacement and syn deformational enhancement of fluid flow around the faults fluid focusing along normal faults perturbs the 18o values by increasing the water rock ratio this isotopic effect can be mathematically described for any distance x from the fault using the formula f a e bx 2 which is analogous to the fundamental solution for diffusion away from a plane source the maximum magnitude of 18o depletion a depends on the effects of permeability temperature mineralogy fluid isotopic composition and integrated fluid flux the constant b is related to the scale of exchange enhancement around the fault the characteristic width w 1 b of the fluid focusing effects is remarkably similar at 100 m for all three faults examined coupling this isotope exchange effect with a step function which accounts for fault displacement provides a good fit to the observed 18o fault profiles the 18o distribution around faults can be used to infer relationships and timing among the processes of structural deformation hydrothermal convection and ore deposition in the comstock district

tectonic inferences from oxygen isotopes in volcano plutonic complexes of the india asia collision zone nw india the india asia collision zone in ladakh nw india is characterized by two major granitic batholiths the northern karakoram granitic complex and the southern ladakh granitic complex are separated by thrust bound belts of ophiolites flysch and calc alkaline volcanics khardung volcanics of late paleozoic to tertiary age oxygen isotope analyses of whole rock and coexisting mineral separates indicate that a the ladakh granitic complex belongs to an i type suite with 18o 5 6 to 6 1 and b the karakoram granitic complex comprises two contrasting suites of granitoids an older predominantly i type with 18o 6 2 to 7 7 and a younger s type with 18o 9 5 to 10 7 the khardung volcanics show wide variation in 18o values ranging from 0 75 to 10 7 indicating isotopic exchange with a meteoric hydrothermal convective system we suggest that the older i type suite of the karakoram batholith represents jurassic to early cretaceous arc magmatism along the bangong nujiang north saltoro suture this implies a continental collision between the lhasa shyok kohistan block and the asian plate prior to the tertiary collision between india and asia india asia collision resulted in the rejuvenation of the north saltoro suture with the emplacement of the younger anatectic granite of the karakoram batholith