evolution of the south central segment of the archean abitibi belt quebec part iii plutonic and metamorphic evolution and geotectonic model textural criteria permit distinction between the pre kenoran and kenoran phases of plutonism and metamorphism the pre kenoran plutons and pre kenoran metamorphic phases are directly related to the volcanic evolution synvolcanic tonalite trondhjemite plutons and swarms of mafic and felsic dykes core central volcanic complexes the volcanic rocks underwent three types of pre kenoran metamorphism namely a pervasive alteration a thermal contact metamorphism that affected narrow aureoles around synvolcanic plutons and a high intensity hydrothermal alteration that affected cross cutting pipes in central volcanic complexes synkinematic kenoran metamorphism resulted in the growth of minerals chlorite actinolite etc parallel to schistosities synkinematic metamorphic grade ranges form the pumpellyite prehnite facies to the amphibolite facies late to post kinematic metamorphic phases resulted in the growth of minerals across schistosities syn to post kinematic plutons are not voluminous in the part o

silicification and metal leaching in semiconformable alteration beneath the chisel lake massive sulfide deposit snow lake manitoba regionally extensive semiconformable zones of silicified fe mg metasomatized and epidotized dominantly mafic volcaniclastic strata and lava flows are exposed 1 to 2 km stratigraphically beneath the chisel lake zn cu massive sulfide deposit snow lake district manitoba the alteration zones occur within a lower proterozoic medium grade regional metamorphic terrane of felsic and mafic volcanics and marine sediments and lie between subvolcanic tonalite sills and the massive sulfide deposit semiconformable alteration occurs at two main stratigraphic positions the lower zone of silicification in pillowed and massive mafic lavas is inferred to have occurred close to the sea floor and probably was not related directly to formation of the chisel lake massive sulfide deposit the stratigraphically higher zone which may be spatially and temporally associated with massive sulfide deposition occurs in a approximately 300 m thick heterolithic mafic volcanic breccia and wacke unit this volcaniclastic hosted alteration is zoned laterally from dominantly silicification and epidotization to mainly fe mg metasomatized garnet chlorite or biotite or staurolite rocks nearer the chisel lake sulfide deposit a discordant footwall fe mg alteration zone directly beneath the sulfide deposit extends toward and may meet the semiconformable fe mg metasomatized zone silicification contributed to partial to complete replacement of volcanic clasts beds in the volcaniclastic unit rocks adjacent to some felsic dikes and pillow interiors by quartz and sodic plagioclase mass balance calculations for silicified rocks and equivalent least altered parts of volcaniclastic beds dikes and pillows indicate that sio 2 increased by up to 50 percent of its initial value and na 2 o by up to 30 percent up to 80 percent of the feo mgo cao and zn was removed during silicification elemental fluxes during fe mg metasomatism are generally opposite those characterizing silicification and are of comparable magnitude epidotization resulted in depletion of na total fe but increased the fe super 2 fe super 3 ratio mg mn k zn and ba and enrichment in ca and sr relative to least altered rocks almost constant interelement ratios of ti zr and al in altered and less altered rocks indicate that these elements were essentially immobile during metasomatism and subsequent medium grade regional metamorphisrm limited data suggest that the heavy ree were also immobile during silicification the subconcordant silicification in the mafic volcaniclastic unit is interpreted to have formed at subsea floor depths of 1 to 2 km where a felsic dike swarm and subvolcanic tonalite sills heated si rich evolved seawater above the temperature of the silica solubility maximum approximately 340 degrees 450 degrees c at pressures below 900 bars causing silica deposition and metal leaching a portion of the fe mg and possibly zn that was leached from the subconcordant silicified zone may have been transported laterally away from this environment thereby producing the semiconformable fe mg metasomatized zones cross stratal structures similar to the hydrothermally altered synvolcanic faults that are known to cut the semiconformable alteration may represent fluid flow paths from a subconcordant metal reservoir in the volcaniclastic unit to the sea floor where massive sulfides were deposited semiconformable pervasively silicified zones particularly those associated with fe mg metasomatized and or epidotized rocks are significantly larger exploration targets than areas of proximal alteration and indicate large scale hydrothermal mass transfer zonation of silicification to fe mg metasomatism laterally within the alteration may provide a vector toward the discordant fe mg enriched alteration zones that commonly underlie volcanic associated massive sulfide deposits

low grade metamorphism of mafic rocks through most of this past century metamorphic petrologists in the united states have paid their greatest attention to high grade rocks especially those which constitute the core zones of exhumed mountain belts the pioneering studies of the 50 s through the 80 s those which applied the principles of thermodynamics to metamorphic rocks focused almost exclusively on high temperature systems for which equilibrium processes could be demonstrated by the 1980 s metamorphic petrologists had developed the methodologies for deciphering the thermal and baric histories of mountain belts through the study of high grade rocks of course low grade metamorphic rocks here defined as those which form at pressures and temperatures up to and including the greenschist facies had been well known and described as well initially through the efforts of alpine and circum pacific geologists who recognized that they constituted an integral and contiguous portion of mountain belts and that they underlay large portions of accreted terranes many of oceanic origins but until the mid 80 s much of the effort in studying low grade rocks for a comprehensive review of the literature to that point see frey 1987 had been concentrated on mudstones volcanoclastic rocks and associated lithologies common to continental mountain belts and arcs in the mid 80 s results of the deep sea drilling project dsdp rather dramatically mitigated a shift in the study of low grade metamorphic rocks

thermobarometry in a subgreenschist to greenschist transition in metabasites of the abitibi greenstone belt superior province canada mineral equilibria in the system cao mgo al2o3 sio2 h2o provide a basis for mapping of four reaction isograds and one bathograd in the low pressure transition from subgreenschist to greenschist facies most of the matachewan area of the abitibi greenstone belt is in the lower pressure bathozone as indicated by the widespread occurrences of the subassemblage prh chl the higher pressure bathozone is indicated by two occurrences of pmp act ep qtz but in these samples the bathograd is displaced to anomalously low pressure by the high fe content of the coexisting minerals this illustrates the need to analyse coexisting minerals calculate activities of end member species and compute p t curves for individual samples before interpreting the isograd bathograd pattern petrographic and microprobe analysis indicates that great care must be taken in the selection of equilibrium assemblages pyroxene phenocrysts in one sample are replaced by the assemblage pmp act ep chl qtz whereas prh act ep chl qtz occurs in the groundmass compositional variation may be more cryptic as in a sample of metabasaltic hyaloclastite that contains two spatially distinct univariant assemblages prh pmp ep chl qtz and prh act ep chl qtz within the devitrified matrix whereas chlorite compositions are similar in both assemblages prehnite and epidote in the latter assemblage are significantly richer in fe and poorer in al accordingly the rock is interpreted to contain two distinct univariant assemblages rather than one invariant assemblage prh pmp act ep chl qtz the displaced univariant curves for this sample intersect at 2 2 kbar and 250 c taking account of all thermobarometric implications the low grade limit of the greenschist facies is at 250 270 c and 2 2 5 kbar corresponding to depths of 7 8 km comparison of apparent p t conditions on both sides of the larder lake cadillac break a regional co2 metasomatized fault zone that is spatially associated with many archaean gold deposits provides an upper limit of not more than c 1 km for post metamorphic south side up dip slip displacement

paludal environments of the mary lee coal zone pottsville formation alabama stacked clastic swamps and peat mires abstract the blue creek and mary lee coals in the lower part of the mary lee coal zone upper pottsville formation northwestern alabama record deposition in peat accumulating environments or mires intervening strata contain a series of stacked clastic swamps forested wetlands which grew on a classtic sediment substrate rather than a peat soil these clastic swamp horizons are identified on the basis of 1 autochthonous erect lycophyte and calamites trunks 2 mud cast prostrate hypoautochthonous lycophyte and calamites trunks 3 compression impression assemblages of autochthonous to hypoautochthonous pteridosperm and lycophyte foliage branches and reproductive structures 4 autochthonous axes and appendages of stigmaria and pinnularia and 5 siderite nodules associated with rooting structures and other plant material sedimentological and biostratinomic features of the mary lee coal to blue creek coal interval are used as the basis for the interpretation that sediment loading and compaction of buried peat controlled the alternation between peat mires and clastic swamps and the stacking of the clastic swamps

quartz cemented breccias from the mid atlantic ridge samples of a high salinity hydrothermal upflow zone textural mineralogical chemical and fluid inclusion studies on a suite of quartz rich sulfide bearing greenstone breccias indicate that high salinity high fluid flow systems were recently active on the mid atlantic ridge south of the kane fracture zone these systems involved fluids with temperatures close to 300 c and salinities almost 3 times that of seawater rounded pebblelike fragments enclosed within a matrix of euhedral quartz crystals appear to have been cemented by quartz deposition from a moving fluid which attained velocities in excess of 1 m s homogenization temperatures and freezing point depression effects in fluid inclusions from texturally early and late quartz crystals indicate temporal evolution of fluid temperatures and nacl equivalent salinities from 290 c and 10 wt to 200 c and 4 7 wt during alteration and cementation of the breccias highly variable cobalt concentrations in the evolving hydrothermal solutions are indicated by fine scale oscillatory zoning in euhedral pyrite within the breccias a continuous supply of the high salinity fluids may be generated either by supercritical two phase separation at depths well below the level of inclusion entrapment or by retrograde dissolution of hydroxy chloride phases in the deeper portions of the hydrothermal system either model requires the initial temperature of the deeply circulating fluids to have been distinctly higher than 415 c and the fluid to have been cooled by conduction or by mixing with lower temperature seawater prior to entrapment within the inclusions

oxygen isotope zonation and alteration in the northern noranda district quebec evidence for hydrothermal fluid flow volcanic rocks of the new vauze norbec area central noranda volcanic complex consist of andesites and rhyolites that have been chemically and isotopically altered by synvolcanic semiconformable alteration processes including spilitization chloritization silicification and epidote quartz alteration andesites have whole rock delta 18 o values of 1 9 to 9 7 per mil those with megascopic mineralogical evidence of hydrothermal alteration have ones of 5 1 to 9 7 per mil rhyolites also show a broad range in delta 18 o values 4 4 to 13 7 per mil all of the volcanic rocks are either enriched or depleted in 18 o with respect to fresh rhyolite 8ppm or fresh andesite 6ppm a marked upward stratigraphic increase in delta 18 o values and the strata bound occurrence and regional zoning of alteration minerals provide evidence of district scale hydrothermal alteration these alteration patterns formed under the conditions of an increasing thermal gradient and a downward decrease in water rock ratio controlled by the cooling of the flavrian lake synvolcanic intrusion

geological processes that control lateral and vertical variability in coal seam moisture contents latrobe valley gippsland basin australia abstract a study throughout the latrobe valley coal measures of coal moisture distribution using downhole bore data and 3d digital models of large bore data sets indicates lateral and vertical moisture variability is controlled by a number of factors these include burial type of overburden age of the seam marine influence coal lithotype and lateral compression on folds and monoclines the same rules appear to apply to the adjacent alberton coal measures in the south gippsland basin suggesting these factors may be widespread in other brown coal basins where moisture contents are still equilibrating through differential compaction in gippsland average moisture down seam decreases by 1 2 per seam and at a rate of approximately 1 every 20 m weight average moisture content for each seam decreases on average at a rate of 0 5 every 1 million years a stepwise decrease in moisture content between the 100 m thick major seams of 1 7 reflects large intervals of time are represented by the relatively thinner 1 10 m thick interseam sediments compression by monoclinal folding can decrease moisture contents up to 7 for any given depth moisture content appears higher on anticlines due to a lowered intensity of compression than in adjacent synclines it is considered likely that coals folded over anticlines are more fractured thereby containing greater amounts of free water use of the lower moisture coals and monoclinal coals may be preferable for future power station developments despite an increase in overburden coal ratios there would be significant savings in a lowered coal volume to calorific value and a consequent reduction in greenhouse gas emission

petrology and oxygen isotope geochemistry of a fossil seawater hydrothermal system within the solea graben northern troodos ophiolite cyprus hydrothermal mineral zonations and o isotope patterns of the northern troodos complex do not parallel the ophiolite pseudostratigraphy but reflect the convective geometry of an upper cretaceous seawater hydrothermal system large areas of the sheeted intrusive complex sic including the subaxial region of the solea graben are composed of 18o rich subgreenschist mineral assemblages and may represent regions of diffuse seawater recharge other areas of the sic are recrystallized to distinctive epidosite rocks granular high variance assemblages of epidote quartz chlorite that are depleted in 18o al2o3 na2o k2o zr cu and zn and are enriched in cao and sr compared with other mafic volcanic and dike rocks of the solea graben epidosite alteration occurred at temperatures of 310 370 c and involved fluids with 18o values and salinities similar to those of upper cretaceous seawater the epidosite zones are discordant with earlier mineral o isotope zonations and with the axis of spreading in the solea graben suggesting a postspreading off axis origin the seawater hydrothermal system responsible for solea graben massive sulfide deposits was probably driven by hypabyssal intrusions not exposed emplaced in a terminal failed spreading episode the geometries of o isotope surfaces within the solea graben imply that the epidosites formed in fossil upflow and deep recharge conduits depletions in base metals show that epidosite alteration liberated cu and zn to mineralizing fluids within the fossil upflow zone

regional alteration systems associated with volcanogenic massive sulfide mineralization at panorama pilbara western australia outstanding exposure across the panorama district of the archean pilbara block of western australia reveals a cross section through a ca 3 24 ga massive sulfide bearing volcanic pile and an underlying coeval subvolcanic intrusion in an area of low metamorphic grade and very low strain this has provided a rare opportunity to map a complete regional scale hydrothermal alteration system associated with volcanogenic massive sulfide mineralization and to assess the role of a subvolcanic intrusion in driving such a system four major alteration facies can be defined in the panorama district these are feldspar bearing background alteration and feldspar sericite quartz alteration and completely feldspar destructive sericite quartz and chlorite quartz alteration these alteration facies affect all of the volcanic pile and the upper parts of the underlying subvolcanic intrusion regional semiconformable alteration zones which are predominantly feldspar bearing are crosscut by broad corridors of transgressive feldspar destructive alteration a convective hydrothermal model is invoked to explain this distribution of alteration facies where semiconformable alteration zones represent seawater recharge and transgressive feldspar destructive alteration zones mark the path of evolved seawater discharging back to the sea floor the interpreted discharge sites are coincident with base metal sulfide deposits at the top of the volcanic pile geologic relationships and the distribution of alteration zones clearly implicate the strelley granite as the subvolcanic intrusion which drove the hydrothermal alteration systems granophyric textures and miarolitic cavities in the upper parts of the intrusion suggest that a magmatic fluid phase was exsolved early in its crystallization history granite hosted copper zinc tin veins near the top of the intrusion are interpreted to be a product of this magmatic fluid this raises the possibility of a substantial contribution of magmatically derived metal to the overlying volcanogenic massive sulfide hydrothermal alteration systems

development of a brine dominated hydrothermal system at temperatures of 400 500 c in the upper level plutonic sequence troodos ophiolite cyprus abstract microthermometric analyses of quartz hosted fluid inclusions in fresh and epidotized plagiogranites from the troodos ophiolite cyprus indicate that brine rich fluids 46 56 wt nacl equivalent at temperatures 400 500 c were pervasive throughout many of the plagiogranite bodies high salinity fe bearing inclusions along healed microfractures homogenize by halite dissolution at temperatures of 400 500 c liquid dominated low salinity 2 7 wt nacl secondary fluid inclusions are ubiquitous in all plagiogranite and gabbro samples studied homogenization of the low salinity inclusions occurs at uncorrected temperatures of 200 400 c the quartz hosted high salinity fluids represent either phase separation of hydrothermal seawater or an exsolved magmatic aqueous phase at uncorrected temperatures of 400 500 c migration and segregation of brine and vapor phases into fractures near the margins of the crystallizing plagiogranite bodies resulted in preferential entrapment of the fe rich brines in the deep seated high temperature portions of the hydrothermal system subsequent fracturing of the upper plutonic rocks at temperatures of 200 400 c allowed penetration by seawater which pervasively altered the plagiogranites hydration reactions involving formation of secondary minerals and or mixing with phase separated fluids resulted in fluid salinities two times that of seawater

chemostratigraphic division of the blake river group rouyn noranda area abitibi quebec the effusive rocks of the blake river group in the abitibi volcanic belt rouyn noranda region belong to a bimodal sequence in which andesites and rhyolites clearly dominate the identification of calc alkaline and tholeiitic affinities is made upon examination of the major trace and rare earth element ree content thus the andesites 58 normalized sio2 value without volatiles of the calc alkaline units have average k 4800 ppm ba 160 ppm and rb 13 ppm values that are greater than the average k 1800 ppm ba 130 ppm and rb 3 ppm values for andesites 57 sio2 that belong to associated tholeiitic units furthermore tholeiitic andesites have distinctive average values of ti 9600 ppm and y 40 ppm that are higher than the average values of ti 6700 ppm and y 25 ppm of calc alkaline andesites an effective discrimination between the calc alkaline and tholeiitic affinities is obtained using the zr y and ti zr ratios which are respectively less than 4 and greater than 70 in andes

discussion on the concepts in paleoenvironmental reconstruction from coal macerals and petrographic indices abstract organic facies analyses quantify the coal constituents and plot various associations to discriminate the paleoenvironment for coal bearing successions this allows the relation of coal composition to mire ecosystems or environments coal petrographic models are used extensively to reconstruct the nature of ancient peat forming environments many authors proposed relations between specific maceral assemblages and or micro lithotypes and peat forming environments the key controlling factors which affect peat environment include hydrogeology redox ph vegetation type clastic influx sedimentation and peat accumulation rate etc recent advancements in coal maceral study and organic petrology reveal the pros and cons of the available indices and models the main reasons for the failure of the petrographic models are oversimplification of the effects of humification on tissue preservation vs destruction the use of post diagenetic processes e g geochemical gelification in determining depositional environments changes in petrographic composition related to floral evolution geological age rank increase and compaction lack of distinction between different inertinite maceral in some models here the widely used petrographic indices and models are reviewed based on the observations of several workers and the applicability and concepts of paleo environmental reconstruction are discussed a multi disciplinary approach including petrography palynology chemistry etc has been recommended which is more logical and scientific than the exclusive use of petrographic composition for paleoenvironmental interpretation

metabasalts axial hot springs and the structure of hydrothermal systems at mid ocean ridges knowledge of the chemical transfer and mineralogical transformations that occur when sea water reacts with basalt at elevated temperatures and pressures can be used along with geological and geophysical data to deduce the typical structure and evolutionary sequence for hydrothermal systems within the oceanic crust along the axis of a mid ocean ridge studies of metabasalt and metadiabase dredged from fault scarps along the axial valley of the mid atlantic ridge reveal consistent relationships among the bulk chemistry of the altered rocks and their secondary mineralogy mineral abundances and mineral compositions especially for chlorite these relationships can be interpreted in terms of the distribution of alteration with respect to time temperature and water rock ratio in young crust assemblages of chl ab ep act chl ab ep act qtz chl ab qtz and chl qtz are produced at successively higher effective sea water to rock ratios within the temperature range of the greenschist facies 250 to 450 c toward higher ratios chlorites tend to become more mg rich pillow basalts of layer 2 are commonly altered under these conditions by sea water on the descending rather than the ascending limb of a convection system this type of alteration which reflects moderately high water rock ratios may characteristically occur above a still partially molten magma chamber as the magma chamber solidifies and is then penetrated more pervasive alteration of the deeper part of layer 2 and of layer 3 occurs at lower water rock ratios producing solutions such as those recovered from springs on the galapagos rift and the east pacific rise at 21 n localized upwelling limbs of the convection system can produce veins filled with quartz sulfides and fe rich chlorite

fluid overpressure along an oligocene out of sequence thrust in the shimanto belt sw japan abstract out of sequence thrusts osts exposed in ancient accretionary prisms are considered as fossil analogs of present day megasplay faults in subduction margins and can provide direct information about the conditions of deformation during thrust activity in modern as well as in ancient accretionary prisms first order megasplay faults or osts truncate or merge with faults of lesser importance called second order osts structural analysis of the makinokuchi fault a branch of an oligocene to lower miocene second order ost in the tertiary shimanto belt of central kyushu sw japan brings information about the conditions of deformation at the time of thrusting the studied exposure shows that the fault footwall and to a much lesser extent the fault hanging wall consist of quartz cemented syntectonic dilatant hydraulic breccias testifying to pore fluid pressures larger than the least principal stress component the footwall sandstones are crossed by several centimeters thick quartz veins that merge with the footwall breccias the continuity between the veins and the breccias suggest that the veins acted as conduits which likely collected fluids from the footwall side sandstones upward and toward the fault fluid inclusions indicate that the quartz cementing the breccias and that filling the feeder veins crystallized from similar fluids and under similar pressure and temperature conditions 245 285 c and 5 8 km depth these similarities suggest that the fluids responsible for syn tectonic hydraulic brecciation were collected from the footwall through the conduits the fluid inclusion trapping temperatures are close to the temperatures expected to be reached along the seismogenic zone our analysis shows that fluid overpressures can play a key role in the growth and activity of second order osts in accretionary prisms and suggests that fluids collected along second order osts or splay faults may flow upward along first order osts or megasplay faults

massive sulfide deposits of the noranda area quebec ii the aldermac mine the original aldermac mine near noranda contained several cu zn massive sulfide lenses hosted by felsic to mafic volcanic rocks of the late archean blake river group the original nos 3 6 orebodies which consisted of massive pyrite with lesser magnetite pyrrhotite chalcopyrite and sphalerite contained 1 87 mt of cu zn ore that averaged 1 47 cu zn was not recovered the orebodies occurred within felsic breccias and tuffs up to 100 m thick that are stratigraphically overlain by an extensive dome of mainly massive rhyolite and rhyodacite up to 250 m thick and at least 550 m across most of the volcanic rocks that laterally flank and overlie the felsic dome are dacitic to andesitic flows breccia and tuff with minor rhyolites and associated subvolcanic sills of quartz feldspar porphyry and gabbro the new massive sulfide deposit discovered in 1988 lies 150 200 m east of the mined out orebodies at a similar stratigraphic level within altered felsic breccia and tuff the sulfides are mainly in

supercritical two phase separation of hydrothermal fluids in the troodos ophiolite the hot 350 c black smoker springs of the ocean floor are the vents of hydrothermal systems in which cold seawater descends below the ocean floor is heated reacts with the host rock and is transformed into metal bearing brine the salinity of these brines varies between vents from 0 7 to 2 0 times seawater salinity1 4 this variation has been ascribed to crustal hydration5 crystallization of chloride bearing minerals6 or two phase separation of seawater into components of differing salinity4 7 12 here we present fluid inclusion evidence for two phase separation in the ancient troodos sulphide forming systems of cyprus and discuss with the nacl h2o system as a model how this process could explain observed salinity variations in hydrothermal fluids

composite synvolcanic intrusions associated with precambrian vms related hydrothermal systems large subvolcanic intrusions are recognized within most precambrian vms camps of these 80 are quartz diorite tonalite trondhjemite composite intrusions the vms camps spatially associated with composite intrusions account for 90 of the aggregate sulfide tonnage of all the precambrian intrusion related vms camps these low alumina low k and high na composite intrusions contain early phases of quartz diorite and tonalite followed by more voluminous trondhjemite they have a high proportion of high silica 74 sio2 trondhjemite which is compositionally similar to the vms hosting rhyolites within the volcanic host rock successions the quartz diorite and possibly tonalite phases follow tholeiitic fractionation trends whereas the trondhjemites fall within the composition field for primitive crustal melts these transitional m i type primitive intrusive suites are associated with extensional regimes within oceanic arc environments subvolcanic composite intrusions related to the archean sturgeon lake and noranda and paleoproterozoic snow lake vms camps range in volume from 300 to 1 000 km3 three have a sill morphology with strike lengths between 15 and 22 km and an average thickness between 1 500 and 2 000 m the fourth has a gross stock like shape the vms deposits are principally restricted to the volcanic strata above the strike length of the intrusions as are areally extensive thin exhalite units the composite intrusions contain numerous internal phases which are commonly clustered within certain parts of the composite intrusion these clusters underlie eruptive centers surrounded by areas of hydrothermal alteration and which contain most of the vms deposits early quartz diorite and tonalite phases appear to have intruded in rapid succession evidence includes gradational contacts magma mixing and disequilibrium textures they appear to have been emplaced as sill dike swarms these early phases are present as pendants and xenoliths within later trondhjemite phases the trondhjemite phases contain numerous internal contacts indicating emplacement as composite sills common structural features of the composite intrusions include early xenolith phases abundant small comagmatic dikes fractures and veins and in places columnar jointing internal phases may differ greatly in texture from fine to coarse grained aphyric and granophyric through seriate to porphyritic mineralogical and isotopic evidence indicates that early phases of each composite intrusion are affected by pervasive to fracture controlled high temperature 350 450 c alteration reflecting seawater rock interaction trondhjemite phases contain hydrothermal magmatic alteration assemblages within miarolitic cavities hydrothermal breccias and veins this hydrothermal magmatic alteration may in part be inherited from previously altered wall rocks two of the four intrusions are host to cu mo rich intrusive breccias and porphyry type mineralization which formed as much as 14 ma after the main subvolcanic magmatic activity the recognition of these precambrian subvolcanic composite intrusions is important for greenfields vms exploration as they define the location of thermal corridors within extensional oceanic arc regimes which have the greatest potential for significant vms mineralization the vms mineralization may occur for 2 000 m above the intrusions in some cases vms mineralization has been truncated or enveloped by late trondhjemite phases of the composite intrusions evidence that much of the trondhjemitic magmatism postdates the principal vms activity is a critical factor when developing heat and fluid flow models for these subseafloor magmatic hydrothermal systems

u pb geochronology of the eastern abitibi subprovince part 2 noranda kirkland lake area u pb zircon ages for 15 volcanic and plutonic units in the noranda and kirkland lake areas help constrain the history of volcanism plutonism sedimentation and deformation in the south central part of the abitibi belt volcanism occurred over an interval of at least 50 ma beginning with the deposition of the volcanic and volcaniclastic units within the pacaud structural complex at 2747 ma following a period of apparent quiescence magmatism resumed at 2730 2725 ma with the eruption of volcanic rocks in the normetal and lac abitibi area from 2715 until about 2698 ma volcanism occurred sporadically throughout much of the area culminating in the eruption of the blake river group from 2703 to 2698 ma several large intrusive bodies yield ages that indicate that they are plutonic equivalents of the blake river group plutons that are considered to have been emplaced during the kenoran orogeny give ages that are only slightly younger than the youngest volcanic units of the blake river group emphasizing

two phase separation of fossil hydrothermal fluids in the mid indian ridge ophiolites abstract rocks of the ophiolite suite were dredged from the axis of the mid indian ridge at 1 n three stages of their hydrothermal alteration were identified 1 actinolite epidote albite 420 450 c 2 epidote chlorite quartz and 3 chlorite smectite 190 c epidote chlorite quartz veins cutting basaltic breccias include an opaque amorphous na bearing silica rock that grades into cryptocrystalline quartz sometimes it contains rare transparent microcrystalline inclusions which may be inferred from their high na cl and si contents to be quartz intergrown with halite the specific bulk composition structure and texture of the matter allow the interpretation that it is a lithified fossil colloid precipitated from a highly saline hydrothermal solution as a result of phase separation in the na si cl h 2 o system at elevated pt conditions the phase separation of hydrothermal fluids is proposed as a mechanism generating supersaturated salt silicate solutions with simultaneous precipitation of ore components the latter is confirmed by the negligible fe and mn concentrations in both the colloidal matrix and the microcrystalline inclusions in the presence of disseminated ore mineralization in the examined basalt breccias

fluid rock interactions in the lower oceanic crust thermodynamic models of hydrothermal alteration rocks from the lower oceanic crust show both petrologic and isotopic evidence for extensive alteration by circulating hydrothermal fluids derived from seawater deeply circulating fluids may have a substantial but as yet largely undetermined impact on the thermal regime of spreading centers magmatic processes at mid ocean ridges and elemental fluxes at ridge hydrothermal systems in the present study the consequences of water rock interactions during the penetration of seawater into hot lower oceanic crust were investigated using thermodynamic reaction path models alteration assemblages predicted for reaction of an evolving seawater with olivine gabbro gabbronorite and troctolite during heating from 300 to 900 c closely resemble alteration assemblages observed in rock samples from the lower oceanic crust these assemblages include sodic plagioclase actinolite diopsidic clinopyroxene epidote chlorite prehnite analcime quartz at lower temperatures 300 500 c and plagioclase hornblende diopside chlorite trace magnetite at intermediate temperatures 500 700 c the relative abundances of these alteration products show much greater variation with temperature than with water rock w r ratio indicating that variations in the amounts of hydrated minerals such as amphiboles may not be an accurate indication of the w r ratio experienced by rocks in the lower crust since the same amount of fluid may produce widely differing amounts of amphibole at slightly different temperatures depending on the temperature of interaction some rocks in the lower oceanic crust may have experienced much higher w r ratios than has previously been proposed based on the amount of amphibole present at higher temperatures 750 900 c the models predict that very little alteration occurs before the fluid equilibrates with the rock producing only trace amounts of amphibole as an alteration phase with the remainder of the rock retaining its igneous mineral composition essentially unchanged this result suggests that extensive penetration of rocks by aqueous fluids may occur in the lower ocean crust at temperatures 700 c and leave very little conspicuous petrologic evidence for their presence such cryptic alteration is consistent with isotopic evidence for hydrothermal alteration of oxygen and sr isotopes by seawater in lower ocean crust rock samples which otherwise retain their igneous compositions and textures taken together the model results allow for much more extensive circulation of fluids in the lower oceanic crust and at much higher temperatures than has been proposed by previous studies based on petrologic interpretations

depositional record of an avulsive fluvial system controlled by peat compaction neogene most basin czech republic this paper illustrates the response of a fluvial depositional system to the interplay between peat compaction and clastic sediment supply at a range of spatial and temporal scales as documented by extensive exposures in an open cast mine in the most basin part of the oligo miocene oh e rift eger graben basin system in the czech republic the most basin is characterized by the occurrence of a number of phenomena resulting from syn and post depositional interactions between clastic sedimentary systems and the underlying accumulation of organic material that was the precursor of the main lignite seam of up to 45 m thickness the studied strata are interpreted as deposits of an avulsive mixed load fluvial system the large scale depositional architecture documents an existence of at least five stratal packages up to 1500 m wide and up to several tens of metres thick representing a record of long term evolution of a clastic floodplain bordered by accumulating peat within each of the packages several small scale channel belts were documented individual packages are separated by carbonaceous mudstones indicating a period of reduced clastic input and interpreted as due to avulsion of the fluvial channels out of the floodplain limit two main mutually linked processes controlled the evolution of the studied fluvial system i syndepositional compaction of the underlying peat and ii avulsions of the channels away from the original floodplain resulting in formation of a new floodplain the processes which caused the channels of the hrabak fluvial system to reach the avulsion threshold were i decrease of rate of creation of accommodation leading to increased sinuosity and thus to a decreased channel slope and ii cross floodplain tilting of the channel belt caused by differential compaction of underlying organic rich substratum

conditions and timing of metamorphism in the southern abitibi greenstone belt quebec regional metamorphism ranging in grade from the subgreenschist facies to the greenschist amphibolite facies transition affects all archean supracrustal rocks 2677 ma in the rouyn noranda area contact metamorphic minerals associated with the posttectonic preissac lacorne batholith 2643 ma show no evidence of a regional retrograde event accordingly the age of regional metamorphism can be bracketed between 2677 and 2643 ma three reaction isograds were mapped in subgreenschist facies metabasites dividing the low grade rocks into three metamorphic zones the pumpellyite actinolite zone the prehnite pumpellyite zone and the prehnite epidote zone in addition the pumpellyite actinolite epidote quartz bathograd corresponding to a pressure of approximately 200 mpa occurs on both sides of the porcupine destor fault low pressure regional metamorphism is also indicated both by the occurrence of an actinolite oligoclase zone and the persistence of pre regional metamorphic andalusite the coincidence

oxygen isotope alteration in the noranda mining district abitibi greenstone belt quebec a regional whole rock oxygen isotope survey carried out in the noranda district shows how oxygen isotope alteration can be used to guide exploration for massive sulfide deposits the survey is based on 588 outcrop whole rock oxygen isotope determinations from 478 sites in a approximately 35 by 50 km area between the destor porcupine and cadillac larder lake breaks from east of the horne mine to west of the magusi river mine the mean isotopic composition of all samples is 7 4 or 2 per mil smow nested analysis of variance of 478 isotopic determinations from 371 of these sites shows that 74 percent of the total variance in whole rock delta 18 o r r whole rock is associated with outcrop location 21 percent with outcrop sampling and 5 percent with sample analysis an analysis of 259 determinations indicates that rock type is statistically no more important than outcrop sampling the statistical analysis indicates that isotopic variations greater than or 1 per mil can be used to map the hydrothermal circulation responsible for massive sulfide mineralization specific traverses visually confirm the lack of 18 o dependence on rock type and the utility of a 2 per mil interpretive band the large scale pattern of isotopic alteration in the noranda district is dominated by a nearly continuous 2 to 3 km wide annulus at the margins of the flavrian pluton of delta 18 o r 18 o r 18 o r depletion is surrounded up section by a approximately 10 km wide band of patchy but intense 18 o enrichment 8 5 14ppm the combined pattern of concentric light and heavy alteration has a diameter of approximately 30 km the clericy intrusion shows a similar pattern of isotope alteration on a smaller 10 km diam scale the lake dufault pluton seems to have been isotopically altered by continuing convection driven by the flavrian hydrothermal system modeling suggests that the 18 o depletion is produced by water circulating toward and along the edges of an intrusion and that the surrounding heavy alteration is produced either by seawater recharge or limited hydrothermal discharge the large scale and coherency of the alteration pattern centered on the flavrian pluton is remarkable and indicates that this pluton drove a major long lived approximately 10 m y hydrothermal system in which convection penetrated to an 8 kin depth the intensity and coherency of the 18 o depletion correlates with the tonnage of associated massive sulfide mineralization oxygen isotope depletion may in general mark the margins of plutons that have driven sufficient hydrothermal circulation to produce economic massive sulfide deposits up section it may be possible to screen plutons for up section massive sulfide exploration potential by collecting and isotopically analyzing approximately 50 samples from five traverses across a pluton9s margins additional samples may provide details light fingers that guide exploration to particular deposits

the iceland research drilling project synthesis of results and implications for the nature of icelandic and oceanic crust study of a 3 km thick section of tertiary basalts in eastern iceland has resulted in the formulation of a model for crustal construction based on accumulation of lava flows on the flanks of large central volcanoes and has led to the development of alteration and physical property profiles in a crustal segment formed at the mid atlantic ridge deep drilling at reydarfjordur revealed the existence of subaerial lava flows and volcaniclastic rocks to a crustal depth of 3 5 km three major stratigraphic units composed largely of basalts are recognized each characterized by distinctive major and trace element patterns each of these units is believed to be the product of a separate episode of mantle melting and fractionation and probably reflects accumulation on the flanks of a central volcano extensive fractionation in shallow magma chambers led to intermediate and silicic lavas which mark the cores of the central volcanic complexes dike density does not increase downward below about 1 5 km crustal depth and the sampled dikes appear to have been intruded laterally along rifts on the flanks of the central volcanoes secondary mineral assemblages are dominated by smectite and zeolites near the top of the section and by epidote and quartz near the base in general the temperature of alteration increases downward but the distribution of individual minerals is irregular locally earlier secondary mineral assemblages are overprinted by contact metamorphism along dike margins the upper surface of greenschist facies metamorphism crops out at the surface in thingmuli volcano but it lies at an estimated crustal depth of about 4 km in the reydarfjordur borehole located only 3 km east of the edge of thingmuli thus the metamorphic zones clearly crosscut the lava stratigraphy physical property profiles correlate well with variations in secondary mineralogy and the top of layer 3 velocities corresponds closely with the projected upper boundary of amphibolite facies rocks at about 6 km crustal depth magnetic susceptibility decreases with depth and is extrapolated to zero at about 4 3 km crustal depth close to the estimated upper limit of greenschist facies metabasalts a much greater crustal thickness leading to the emplacement of high level magma chambers within the extrusive part of the crust is thought to account for many of the differences in crustal structure and evolution between iceland and the rest of the mid atlantic ridge

synthesis of fe pumpellyite and its stability relations with epidote hydrothermal synthesis of fe pum pellyites was conducted using high pressure cold seal apparatus and solid oxygen buffering techniques at temperatures between 250 c and 550 c and 2 0 9 1 kbar pfluid fe pumpellyites were synthesized from partially crystalline gel mixtures of compositions 4cao 2 1al2o3 1 5feo 0 3mgo 6sio2 ii and 3cao 1 5 al2o3 2 7feo 0 3mgo 6sio2 iii in the presence of excess h2o at pfluid of 5 9 1 kbar temperatures between 275 c and 325 c and fo2 defined by the qfm and hm buffers for both of these compositions ii and iii the condensed synthetic run products included minor 7a chlorite garnet fe oxide the cell dimensions and aggregate refractive index a 19 13 2 a b 5 940 4 a c 8 847 5 a 97 37 6 and n 1 702 2 of the pum pellyite synthesized from the bulk composition ii mix are compatible with those of natural pumpellyites containing similar total fe contents attempts at synthesizing fe pumpellyites from a mg free bulk composition were not successful these results are consistent with the total absence of natural mg free pumpellyites the higher temperature higher oxygen fugacity assemblages of the equivalent bulk compositions ii and iii consist of epidote minor amounts of chlorite garnet quartz hematite and magnetite the results of these synthesis experiments accord with the mineral parageneses observed in low grade metabasites which imply that fe pumpellyites are replaced by epidote with increasing temperature and or fo2 and that fe3 is preferentially partitioned into epidote with respect to coexisting pum pellyite in addition these synthesis experiments indicate that fe bearing pumpellyites crystallize at and are stable to lower temperatures than more aluminous pumpellyites a result also consistent with natural systems

chemical variations in assemblages including epidote and or chlorite in the fossil hydrothermal system of saint martin lesser antilles abstract epidote and or chlorite are common minerals in the roots of the fossil geothermal system of saint martin lesser antilles they appear in four distinct assemblages 1 epidote actinolite quartz magnetite near the contact between the tuffaceous host rocks andesitic modal composition and the quartz diorite intrusion of philipsburg 2 epidote chlorite quartz in host rocks as far as a lateral distance of about 3 km from the intrusion 3 epidote chlorite haematite quartz locally in iron and manganese rich host rocks 4 chlorite phengite magnetite appearing as late sealing of porosity in fracture controlled quartz veins with strongly phengitized wall rocks all these assemblages constitute a large alteration grading from propylitic alteration to thermal metamorphism actinolite bearing assemblage detailed microprobe studies of epidotes replacing plagioclases and of chlorites replacing glass and mafic minerals reveal notable compositional variations which have been studied with respect to temperature paleogradients estimated from fluid inclusions study distance from the thermal source and fo2 conditions the mean ps pm 100 fe3 mn3 al3 fe3 mn3 of epidotes vary from 21 in the presence of magnetite near the intrusion to 32 in haematite bearing iron and or manganese volcanic and sedimentary formations the intra grain chemical scattering of epidotes increases with increasing distance of the pluton and decreasing temperature of crystallization all the chlorites coexisting with epidote are mg rich xfe

plagioclase and epidote buffering of cation ratios in mid ocean ridge hydrothermal fluids experimental results in and near the supercritical region abstract experiments have been performed with na ca k cl fluids of seawater chlorinity and diabase basalt and plagioclase bearing mineral mixtures at 350 425 c and 250 400 bars to help constrain hydrothermal alteration processes at mid ocean ridges dissolved ca na and ph for all experiments responded systematically to differences in dissolved sio2 concentrations and the compositions of plagioclase reactants diabase alteration at low fluid rock mass ratios 0 5 to 1 produces fluids undersaturated with respect to quartz during hydration of primary olivine and orthopyroxene whereas basalt alteration under similar conditions yields fluids slightly supersaturated with respect to quartz during breakdown of glass to smectite and amphibole fluid chemistry in all experiments appears to approach a partial equilibrium state with the albite and anorthite components in plagioclase and approaches a ph consistent with plagioclase alteration to epidote trace element data from vent fluids specifically b and sr together with major element chemistry ca na sio2 ph provides evidence that the reaction zone for black smoker fluids at mid ocean ridges is composed of only slightly altered diabase and is characterized by small amounts of epidote nearly fresh plagioclase and clinopyroxene and partially to completely hydrated olivine and orthopyroxene fluids reacting with this rock may be undersaturated with respect to quartz so pressure estimates based on the quartz geobarometer should be regarded as minimums using equilibrium between plagioclase the dominant reactant and epidote the dominant reaction product in experiments we estimate that temperatures in reaction zones are in excess of 375 c for most vent systems these temperatures are higher than measured vent temperatures suggesting that hotspring fluids commonly loose heat during ascent to the seafloor

actinolite forming reaction at low pressure and the role of fe 2 mg substitution the 6km thick karmutsen metabasites exposed over much of vancouver island were thermally metamorphosed by intrusions of jurassic granodiorite and granite observation of about 800 thin sections shows that the metabasites provide a complete succession of mineral assemblages ranging from the zeolite to pyroxene hornfels facies around the intrusion the reaction leading to the appearance of actinolite which is the facies boundary between prehnite pumpellyite and prehnite actinolite facies was examined using calcite free karmutsen metabasites collected from the route along the elk river in the prehnite pumpellyite facies xfe3 fe3 fe3 al in prehnite pumpellyite and epidote buffered by the four phase assemblage prehnite pumpellyite epidote chlorite systematically decreases with increasing metamorphic grade such a trend is the reverse of that proposed by cho et al 1986 this may be related to the higher f o 2 in the mt menzies area the actinolite forming reaction depends on the value of xfe3 in pumpellyite if using a low value of fe3 3 89 pr 0 06 0 48 ep 0 26 0 60 chl h2o 2 10 pm 0 08 0 17 act 0 88 qz is delineated the number in parentheses stands for the xfe3 value in ca al silicates on the other hand replacing the xfe3 of 0 08 in pumpellyite with a higher xfe3 value 0 24 changes the reaction to 0 41 pm 0 02 chl 0 42 qz 0 11 pr 0 62 ep 0 10 act h2o the first hydration reaction forms pumpellyite and actinolite on the high temperature side whereas the second dehydration reaction consumes pumpellyite to form prehnite epidote and actinolite the former reaction seems to explain the textural relationship of ca al silicates in the study area however actinolite forming reaction changes to a different reaction depending on the compositions of the participating minerals although in the other area even physical conditions may be similar to those in the study area chemographic analysis of phase relations in the pra facies indicates that the appearance of prehnite depends strongly on the bulk feo mgo ratio this may explain the rarity of prehnite in common metabasites in spite of the expected dominant occurrence in the conventional pseudo quaternary ca al fe3 fm system an increasing feo mgo ratio stabilizes the pr act assemblage and reduces the stability of the pm act one therefore the definition of pumpellyite actinolite facies should include not only pm act but also the absence of pr act assemblages in addition to the possible role of high x co 2 cho and liou 1987 and or high f o 2 to mask the appearance of prehnite the effect of the feo mgo ratio is emphasized

a field chemical and stable isotope study of subseafloor metamorphism of the josephine ophiolite california oregon the josephine ophiolite is a large complete ophiolite generated in a late jurassic back arc basin along western north america the ophiolite underwent subseafloor metamorphism under a steep thermal gradient as indicated by downward changes in mineralogy and 18o values characteristic mineral zonation includes mica orthoclase and hematite in the lower extrusives greenschist facies assemblages in the upper sheeted dikes and amphibolite facies assemblages in the lower sheeted dikes and high level gabbro cumulate gabbros are only incipiently altered but are depleted in 18o indicating high temperature alteration with fluids that evolved at low water w rock r ratios a second off axis circulation system is recorded by metalliferous sediments that occur 8 21 m above the ophiolite alteration in the upper sheeted dike complex and especially the extrusive sequence is heterogeneous at outcrop scale as indicated by large variations in mineralogy chemistry and 18o values within individual pillows and dikes the heterogenity is due to 1 variations in w r ratio and starting material during alteration by downwelling seawater e g pillow cores versus glassy rims and 2 localized discharge mineralization mostly epidosites superimposed on the background recharge alteration epidosites occur largely as dike parallel stringers irregular replacement of pillow and massive lava and replacement of interpillow and pillow breccia matrices the chemistry of most extrusives and sheeted dikes is characterized by loss of ca and gain in na and mg consistent with alteration by downwelling fluids in addition the sheeted dikes are depleted in k2o and the lower sheeted dikes and high level gabbros are depleted in zn and cu the 18o and d values for most samples from the extrusive sequence indicate alteration by seawater at 50 rotation of the entire crustal sequence at the spreading axis suggests that the ophiolite formed at a slow spreading center where magma chambers were episodic

district scale alteration associated with massive sulfide deposits in the mount read volcanics western tasmania the distribution of secondary mineral assemblages in six mineralized areas of the mount read volcanics defines a district wide zonation which is subconformable with massive sulfide host horizons and the contacts of sill like granitoids where present the relationships between alteration patterns and elements of cambrian geology strongly suggest that the alteration is related to cambrian hydrothermal circulation responsible for massive sulfide mineralization there is a later greenschist facies metamorphic overprint the major components of the principal assemblages are quartz albite chlorite epidote quartz albite chlorite calcite quartz sericite chlorite and quartz k feldspar chlorite several complex assemblages involving the above minerals biotite magnetite pyrite and tourmaline are characteristic of granite contacts secondary biotite is also present sporadically in the rosebery area probably related to devonian granitoids at depth there is a districtwide zonation of sodic or calcic versus nonsodic noncalcic assemblages in felsic volcanic rocks in preore volcanics nonsodic noncalcic assemblages occur immediately beneath the host horizon and may persist several hundred meters before passing into a zone including granite contacts where calcium minerals and albite occur sporadically the nonsodic noncalcic assemblages are predominantly sericitic but assemblages with prominent k feldspar occur locally in postore volcanics sericitic non sodic noncalcic assemblages may persist up to 200 m above the ore horizon before passing into relatively fresh volcanics with common albite and calcic greenschist minerals preore assemblages probably largely reflect downward circulation upwelling zones are characterized by intense sericite chlorite or chlorite development with or without k feldspar lateral spreading of solutions immediately beneath the ore horizon is common chlorite compositions preserve local patterns due to cambrian hydrothermal processes but they do not vary systematically with alteration assemblages throughout the province chlorite mg to fe ratios appear to have been preserved through metamorphism a combination of structural and alteration data is useful to mapping in the mount read volcanics this approach indicates that most syngenetic deposits and sedimentary lenses in the rosebery hercules area may lie on a single host horizon and suggests a continuity of geology from mount farrell through red hills to mount sedgwick

the northern samail ophiolite an oxygen isotope microprobe and field study geological petrological and oxygen isotopic data are presented for 228 whole rock and mineral samples collected from a 100 20 km area of the northern samail ophiolite in oman most of these samples are from three detailed profiles through the pillow lavas sheeted dikes and layered gabbros of this laterally heterogeneous fragment of cretaceous oceanic crust down to and across the petrologic moho the profiles encompass a range of petrologic and tectonic styles and each profile exhibits distinctive 18 16 o variations compared to one another and to mid ocean ridge basalts as a result of pervasive seawater hydrothermal interaction that varied in intensity along strike in the ophiolite in general 18 o depletions are observed in the layered gabbros and 18 o enrichments in most of the sheeted dikes and pillow lavas similar to results previously observed in the southern part of the ophiolite ibra area where 18 o depletions within the gabbroic section are quantitatively balanced by 18 o enrichments in the shallower parts of the oceanic crust the wadi hilti profile selected as an example of relatively intact crust differs from ibra in having more uniform and slightly higher 18 o in the gabbros 5 4 to 6 3 as well as in containing more hydrous alteration minerals amphibole epidote chlorite and prehnite the profiles in the wadi kanut shafan and wadi rajmi sections are much more complex and reveal the impact of off axis intrusions and deep crustal shearing plagiogranite wehrlite intrusions in the shafan kanut area superimposed a local hydrothermal aureole on the ophiolite evident in dikes highly depleted in 18 o quartz sulfide veins abundant epidote thullite and chlorite in shallower rocks and low temperature hydrous alteration of deeper gabbroic rocks the latter produced an overall increase in whole rock 18 o 6 2 to 6 9 such late stage intrusions are found throughout the northern half of the samail ophiolite the wadi rajmi area which is a possible fossil transform or propagating rift represents the most complex of the three profiles it also contains the most abundant highly deformed and hydrothermally altered rocks together with the deepest and largest zone of 18 o depletion yet found in any ophiolite locally 18 o 2 0 conduits for large volumes of high temperature hydrothermal fluids were provided by fractures now occupied by low 18 o gabbro pegmatites and low 18 o dikes material balance estimates for the regional samples and from the various transects through the ophiolite give crustal bulk 18 o averages 5 9 to 6 3 that are within sampling error almost identical to the average morb basalt value of about 5 8 if both vertical and lateral crustal heterogeneities are integrated into a three dimensional model this supports and amplifies the conclusion of earlier workers that the 18 o of seawater is buffered and controlled by hydrothermal interaction with oceanic crust as long as the cumulative effects both spatial and temporal of all seafloor magmatic hydrothermal processes are considered the very slight out of balance enrichment of the integrated crustal average 18 o compared to morb may be explained by the ubiquitous mineralogical and isotopic evidence for a late low temperature alteration event in the basal gabbros these effects are prominent in the vicinity of the petrologic moho and may indicate exchange with low temperature aqueous fluids during or after detachment of the obducted slab

metamorphism of a late jurassic volcano plutonic arc northern california usa the metamorphic history of the middle to upper jurassic volcanic and hypabyssal rocks exposed in the klamath mountains and sierra nevada of california is related in part to the rifting of a volcano plutonic arc the callovian to kimmeridgian rocks exposed in the region consist of from north west to south east a back arc ophiolite a rifted volcanic arc and a volcanic arc complex all of these units have been metamorphosed and contain various combinations of the phases chlorite amphibole epidote prehnite and pumpellyite projection of coexisting phases onto the composition plane mgo mgo feo and al2o3 fe2o3 0 75 cao na2o through quartz water albite and epidote results in consistent mineralogical compatibilities within each region but crossing tie lines between regions this suggests that the volcanic and hypabyssal rocks from each region have equilibrated under different intensive conditions the back arc ophiolite in the north has suffered subseafloor high t p hydrothermal metamorphism with geothermal gradients on the order of 100 c km 1 the rifted volcanic arc has undergone synchronous burial hydrothermal and contact metamorphism metamorphic field gradients in the region pass through the prehnite pumpellyite and greenschist facies suggesting geothermal gradients on the order of 30 c km 1 the southernmost volcanic arc complexes contain metavolcanics of the pumpellyiteactinolite and greenschist facies suggesting moderate to high p t metamorphism and geothermal gradients on the order of 20 c km 1 the apparent increase in rifting and calculated geothermal gradients from south east to north west suggest that the observed very low and low grade metamorphism may be a response to enhanced thermal gradients during extension of the volcanic arc this correlation between the extent of rifting and metamorphism is consistent with a model of diastathermal metamorphism of a propagating rift along the western margin of north america during the late jurassic the plate tectonic setting may be analogous to the present day andaman sea region

mineralogy and stable isotope geochemistry of hydrothermally altered oceanic rocks mineralogical and isotopic variations observed in altered glassy and crystalline rocksfrom the east pacific rise and the mid atlantic ridge provide information about the temperatures of alteration and seawater rock ratios for various hydrothermal regimes within the oceanic crust a systematic increase in alteration temperature is evident for the glassy rocksin the sequence 1 nontronite and celadonite vesicle fillings 35 c 2 saponite rich pillow breccias 130 170 c 3 calcite rich greenstone breccias and epidote rich greenstone 200 350 c theseresults include the highest temperatures thus far reported for saponite formation the seawater dominated hydrothermal alteration process that formed the saponite rich pillow breccias is characterized by high water rock ratios 0 1 low to moderate temperatures a seawater origin of most of the carbon in vein calcites 13 c 0 and the predominance of fe rich saponite and calcite as secondary phases greenstones chlorite quartz epidote and greenstone breccias chlorite quartz albite calcite are altered in a rock dominated system with lower water rock ratios 50 1 to 13 c 4 the crystalline rocks diabase gabbro and metagabbro are affected to varying degrees by pervasive high temperature seawater interactions that commence soon after solidification producing varying proportions of fine grained secondary minerals including talc smectite chlorite vermiculite actinolite and sodicplagioclase hydrothermal solutions derived from alteration of the crystalline rocks are of the appropriate temperature and isotopic composition to alter the overlying glassy rocks to the observed mineralogies as well as being the source of metal rich deposits associated with the oceanic spreading centers

evolution of the south central segment of the archean abitibi belt quebec part ii tectonic evolution and geomechanical model in this paper we describe the relations between the paleogeographic and tectonic evolution of the southwestern part of the archean abitibi and bellecombe belts volcanism in the abitibi belt created a very thick anisotropic plate composed of competent volcanic rocks and broken by the duparquet destor break the depocenters of the upper division of diverse volcanic rocks subsided about 10 km relative to their surroundings and some central volcanic complexes within this division were consolidated by synvolcanic plutons and their thermal metamorphic aureole the cadillac break a normal fault separated the abitibi and bellecombe belts the latter consisted of comparatively incompetent sedimentary rocks on top of a basement composed of ultramafic mafic flows north south compression of the volcanic terrain during the kenoran orogeny produced a set of flexure folds f1 that curve around the consolidated cores of central volcanic complexes generally in an easterly direction synclinoria nucleated at the dee

archean metavolcanics from the rouyn noranda district abitibi greenstone belt quebec 2 mobility of trace elements and petrogenetic constraints qualitative evaluation of altered mafic pillows from the rouyn noranda district of the abitibi metavolcanic belt reveals that for low grade metamorphism the high field strength elements hfs zr y ti and the rare earth elements ree are immobile however where alteration is extreme the ree elements are leached and redeposited by carbonate rich and k rich metasomatic fluids whilst zr y and zr ti remain unchanged rubidium ba and to a lesser extent sr are mobile at all levels of alteration however provided certain filters are applied the original geochemical values of the lavas for the alkaline elements the hfs elements and the ree can be defined the geochemical variations thus identified for the volcanic series of the rouyn noranda district are ascribed to a combination of contamination and fractional crystallization of a primitive tholeiitic magma

metabasalts from the mid atlantic ridge new insights into hydrothermal systems in slow spreading crust an extensive suite of hydrothermally altered rocks were recovered byalvin and dredging along the mark mid atlantic ridge south of the kane fracture zone 23 24 n where detachment faulting has provided a window into the crustal component of hydrothermal systems rocks of basaltic composition are altered to two assemblages with these characteristics i type i albitic plagioclase an02 10 mixed layer smectite chlorite or chlorite actinolite quartz sphene 20 of the clinopyroxene is altered and cu and zn are leached the geochemical signature of these alteration types reflects the relative proportion and composition of secondary minerals and the degree of alteration of primary phases and does not show simple predictive relationships element mobilities indicate that both alteration types formed at low water rock ratios the mark assemblages are typical of the greenschist and transition to the amphibolite facies and represent two distinct albeit overlapping temperature regimes type i 180 to 300 c and type ii 250 to 450 c by analogy with dsdp odp hole 504b and many ophiolites the mark metabasalts were altered within the downwelling limb of a hydrothermal cell and type i and ii samples formed in the upper and lower portions of the sheeted like complex respectively episodic magmatic and hydrothermal events at slow spreading ridges suggest that these observed mineral assemblages represent the cumulative effects of more than one hydrothermal event groundmass and vein assemblages in the mark metabasalts indicate either that alteration conditions did not change during successive hydrothermal events or that these assemblages record only the highest temperature event lack of retrograde reactions or overprinting of lower temperature assemblages e g zeolites suggests that there is a continuum in alteration conditions while crustal segments remain in the ridge axis environment the type ii samples may be representative of thereaction zone where compositions of hydrothermal fluids actively venting at the seafloor today become fixed this prediction necessitates interaction between hydrothermal fluids and intersertal glass and or mafic phases in addition to plagioclase in order to produce the observed range in vented fluid ph

composition of geothermal minerals from saline and dilute fluids krafla and reykjanes iceland abstract in high temperature geothermal areas t 200 c in iceland the alteration minerals formed show a regular zoning with increasing temperature i e smectite chlorite epidote and actinolite to test the effect of varying composition of the hydrothermal circulation fluid drillhole chips from two contrasting areas were analyzed in detail the reykjanes area is fed with sea water while krafla is fed with meteoric water and has a cl concentration of 28 ppm both areas have tholeiitic basalts lavas and hyaloclastites as original reservoir rocks although both systems have the same sequence of alteration minerals the actual composition of the minerals differs between systems reflecting the different chemical composition of the circulating waters in reykjanes with its brine fluid the composition of the original rock appears not to affect the composition of the alteration minerals while the reverse appears to be the case in krafla

crystal size of epidotes a potentially exploitable geothermometer in geothermal fields crystal size of epidotes crystallized in quartz epidote veins is used as the basis for a new geothermometer from the fossil geothermal field of saint martin lesser antilles the epidote bearing alteration paragenesis is developed as far as 3 km from a quartz diorite pluton at temperatures of 220 350 c the length width ratio of the epidote grains is constant for all the analyzed samples and suggests isotropic growth environments however the length and width of the grains vary exponentially with temperature the obtained results offer new perspectives for simple grain size geothermomeery but must be extended to other geologic environments to clarify the influence of different rock types

alteration in the irdp drill hole compared with other drill holes in iceland the overall alteration pattern in the drill hole at reydarfjordur is very similar to alteration patterns observed in icelandic geothermal areas and in low grade metamorphosed basalts in deep crustal sections elsewhere in iceland however more detail is obtained by the study of the irdp drill core than by study of drill cuttings sampled in previous drill holes in iceland a comparatively high fossil thermal gradient is obtained at reydarfjordur by a combination of mineral stability data and the observed occurence of secondary minerals this high gradient is consistent with the measured dike dilation at the drill site and the location of the drill site adjacent to a central volcano

phase relations of pumpellyite actinolite facies metabasites in the sanbagawa metamorphic belt in central shikoku japan abstract sanbagawa metabasites metamorphosed at conditions near the upper limit of the pumpellytic actinolite facies were examined in terms of phase equilibria in the five component system al2o3 fe2o3 cao mgo feo the fe3 content of epidote measured as x ep fe fe fe al of epidote in the assemblage epidote chlorite actinolite pumpellyite decreases gradually towards the higher grade pumpellyite free areas the progressive change in xfeep can be detected within one metabasite bed 200 meters thick near the upper limit of the pumpellyite actinolite facies the mg fe2 substitution as expressed by variation of fe fe mg in chlorite 0 40 0 55 has little effect on the fe3 al ratios of epidote and pumpellyite in the above mentioned assemblage the lowet xfeep in the pumpellite bearing assemblage is 0 15 and hence the upper limit of the pumpellyite actinolite facies is defined by the appearance of an epidote chlorite actinolite assemblage with x ep fc c 15

a special issue devoted to abitibi ore deposits in a modern context preface the abitibi greenstone belt which is part of the larger abitibi subprovince in the superior province of the canadian shield e g card and ciesielski 1986 is the largest known archcan greenstonegranite terrane 300 km n s x 400 450 km mineral production and reserves are significant particularly in relation to volcanogenic massive sulfide deposits and structurally controlled archcan goldbearing quartz vein systems the term au quartz vein being used in the sense of lindgren 1933 e g the gold quartz veins of ontario p 662669 volcanogenie massive sulfide production and reserves to 1989 1990 total 424 million metric tons

stages in the compaction of peat examples from the stephanian and permian of the massif central france thick coal seams in the intramontane basins of the massif central allow a detailed study of peat coal compaction three stages of compaction can be detected stage 1 occurred during the deposition of the peat coal beds which thicken and dip away from penecontemporaneous sandstones suggest rapid compaction stage 2 compaction of peat related to overburden pressure in some cases resulted in the formation of lakes over peat beds the thickness of the lacustrine deposits may give an indication of the amount of compaction that took place one possibility is that the early compaction was related to the progradation of siliciclastics over the peat stage 3 occurred after the organic sediment achieved some rigidity compaction can be calculated from deformed clastic dykes

distribution of alteration zones in the upper oceanic crust aging of the upper oceanic crust produces alteration zones that have distinct mineralogical and geochemical characteristics five zones are recognized in the troodos ophiolite and in situ oceanic crust 1 a sea floor weathering zone swz 2 a low temperature zone ltz 3 a transition zone tz 4 an upper dike zone udz and 5 a mineralized zone mz the mz may be overlain by the swz ltz or tz but in the known examples it is always underlain by the udz the development of the swz is dependent upon the duration of cold seawater circulation which is dependent upon seafloor topography and the rate and nature of sedimentation the boundary between the low and high temperature zones is largely controlled by permeability contrasts lateral variation in the depth of this boundary indicates that the thermal gradient is not uniform within the crust this along with the evidence for disequilibrium assemblages implies that regional metamorphic zones are not appropriate to describe the distribution of alteration effects

mass volume and chemical changes in the alteration zone at the norbec mine noranda quebec a funnel shaped alteration pipe in archean rhyolite and andesite below massive sulphide zn cu ores at the norbec mine in northwestern quebec was outlined from drill core samples geochemical parameters and normative alteration mineralogy the pipe has a mass of 37 9 million tonnes giving a volume of 13 106 m3 which represents volume increase of 10 relative to the unaltered host volcanic rocks a bulk chemical composition was calculated using weighting procedures for volumes of influence for the samples net mobile mass change or chemical flux for the alteration pipe was 5 8 106 tonnes inclusion of the massive ore lens yields a flux of 9 6 106 tonnes for the whole hydrothermal system the largest additions to the system in millions of tonnes were feo 4 2 sio2 3 8 s 1 8 k2o 0 55 and mgo 0 5 the only depletions were na2o 1 4 and cao 0 44 base and precious metals accounted for 3 4 of the total element flux

hydrothermal alteration of a 1 km section through the upper oceanic crust deep sea drilling project hole 504b mineralogy chemistry and evolution of seawater basalt interactions deep sea drilling project hole 504b penetrates 1076 m into oceanic layer 2 and is the first hole to pass through the transition from pillow basalts altered at low temperatures into hydrothermally metamorphosed sheeted dikes alteration of the crust at site 504 occurred in four stages related to the movement of the crust away from the spreading axis 1 dikes reacted with seawater 200 300 c in the upwelling zone of an axial convection cell at the spreading axis resulting in the formation of greenschist facies parageneses in veins and host rocks mixing of the upwelling hydrothermal fluids with seawater circulating in the overlying more permeable pillow section occurred in the upper part of the lithologic transition zone causing a steep temperature gradient at the base of the pillow section secondary minerals formed in the lower pillow section from the resultant reducing mixed fluids at temperatures of around 100 c at the same time the initial effects of seafloor weathering began in the upper 320 m of the pillow section at low temperatures 50 c and under conditions of open circulation of oxidizing seawater 2 following refracturing of the dikes a second stage of axial upwelling occurred hydrothermal fluids 200 380 c were probably similar to those presently sampled from spreading ridges on the seafloor mixing of these fluids with seawater in the lithologic transition zone caused deposition of quartz epidote and sulfides in veins and formation of a sulfide rich stockworklike zone within this transition zone alteration of the pillow section proceeded under the prior conditions with the effects of seafloor weathering extending progressively downward into the crust 3 seawater recharge penetrated to depths of at least 1075 5 m subbasement and deposited anhydrite locally in veins 4 off axis alteration of the dikes was characterized by formation of zeolites calcite and prehnite in veins and host rocks from more highly evolved and lower temperature 100 250 c fluids alteration in the upper pillow section evolved from seafloor weathering conditions to more reducing and rock dominated as cracks in the basalt were sealed with secondary minerals and the basement was covered with a layer of sediment zeolites and calcite were the last phases to form throughout the pillow section sealing of the crust to convective cooling also allowed conductive reheating of the crust from below calculations of seawater crustal chemical fluxes from whole rock data are complicated by the low recovery and heterogeneity of alteration effects but indicate that basalt seawater interactions are a sink for seawater mg and k and a source for si and ca

compositions and parageneses of secondary minerals in the onikobe geothermal system japan abstract parageneses and chemical compositions of secondary minerals in many core samples of the onikobe geothermal system japan were investigated in detail in order to evaluate the controls of geothermal waters on alterations of andesitic dacitic rocks depth zonations of secondary minerals were delineated for cazeolites as mordenite laumontite yugawaralite wairakite and clay minerals as smectite alkaline smectite chlorite smectite chlorite compositional variations with depth hence temperature are apparent for smectite chlorite and illite chlorite interstratified clay minerals but not for wairakite both prehnite and epidote at depth have fe rich compositions and become more aluminous with increasing depth the ubiquitous occurrence of pyrite and rare magnetite and the absence of hematite suggest that the onikobe system had fs2 fo2 conditions within the pyrite stability field the occurrence of abundant calcite and ca zeolites indicates that thermal waters at depth are neutral to slightly alkaline and xco2 is very low to stabilize ca al silicates this conclusion is consistent with the production of a remarkable amount of alkaline high temperature steam and thermal waters in a newly drilled production well

from peat bog to lignite seam a new method to calculate the consolidation coefficient of lignite seams wielkopolska region in central poland only one quantitative method is described in the literature to estimate the consolidation coefficient of lignite seams more studies concentrate on the compaction of plant tissues which explains why the obtained results are overestimated moreover most studies do commonly not determine the consolidation of the whole peat bog but usually those of some of its elements only we propose a new approach which conceptually is fairly close to the hager et al fortschr geol rheinld westf 29 319 352 1981 method our method has been tested on an example of the first middle polish lignite seam and the second lusatian lignite seam from some wielkopolska lignite deposits in central poland the consolidation coefficients cc range between 2 34 and 2 56 for the second lusatian lignite seam and between 1 80 and 2 14 for the first middle polish lignite seam

hydrothermal alteration processes at midocean ridges experimental and theoretical constraints from ca and sr exchange reactions and sr isotopic ratios diabase and basalt were reacted with na ca k cl fluids of seawater chlorinity at 375 425 c 375 400 bars and fluid rock mass ratios of 0 5 1 0 to assess the role of basalt chemistry and texture on sr and ca mobility during high temperature hydrothermal alteration an additional experiment utilizing an 84sr spiked fluid was performed to help quantify reaction rates of processes affecting sr mobility the experimental results help to constrain reaction processes responsible for the chemistry of hot spring fluids at midocean ridges alteration of basalt and diabase is characterized by formation of tremolite actinolite smectite chlorite and clinozoisitic epidote smectite chlorite respectively diabase alteration produced dissolved sr ca ratios similar to those observed for ridge crest hot spring fluids whereas alteration of a mostly crystalline basalt produced significantly lower ratios this observation supports the premise that the sr ca ratios observed in vent fluids may be produced during deep seated reaction of the hydrothermal fluids with diabase dikes and or gabbro at relatively low fluid rock ratios and suggests that hydrothermal alteration of primary igneous minerals at relatively high temperatures 400 c leads to formation of a mineral with low dsr ca possibly hydrothermal plagioclase results of the 84sr spiked experiment indicated that only 4 of the sr in basalt was mobilized after 800 hours of reaction despite the fact that b and li were nearly quantitatively leached it is thus suggested that b and li are good indicators of the amount of fresh rock encountered by fluids while sr concentration and isotopic data can be used to estimate the degree of alteration that the sr bearing primary minerals have undergone the degree of alteration of subsurface reservoirs may be estimated by dividing the fluid rock ratio obtained for highly mobile elements by the fluid rock ratio obtained from path integrated sr isotope and concentration data based on this method for example it can be estimated that only 5 of the primary sr bearing minerals encountered by fluids in the subsurface at epr 21 n are actually converted to secondary phases

mineralogical and geochemical characteristics of hydrothermal alteration of basalt in the kuroko mine area japan implications for the evolution of a back arc basin hydrothermal system abstract basalt in the furutobe district of the kuroko mine area in japan is characterized by abundant chlorite and epidote fluid inclusion studies indicate that chlorite is formed at lower temperatures 230 250 c than epidote 250 280 c the seawater basalt mass ratio for the early chlorite rich alteration was high max 40 but that for the later alteration was low 0 1 1 8 the cao na2o and sio2 of the bulk rock correlate negatively with mgo while feo and fe correlate positively with mgo these changes in the characteristic features of hydrothermal alteration from early to late are generally similar to those for a mid ocean ridge geothermal system accompanying basalt alteration the mgo feo ratios of chlorite and actinolite and the fe2o3 concentration of epidote from the basalt are greater than those of mid ocean ridge basalt probably owing to the differences in the fe2o3 feo and mgo feo ratios of the parent rocks the lower cao concentration and the higher na2o concentration of the bulk rock compared with altered mid ocean ridge basalt can be interpreted in terms of the difference in original bulk rock compositions the furutobe basalt as well as other submarine back arc basalts contains more vesicles filled with hydrothermal minerals epidote calcite quartz chlorite pyrite than do the mid ocean ridge basalts the abundance of vesicles plays an important role in controlling the secondary mineralogy and geochemistry of hydrothermally altered submarine back arc basin basalts

chemical interaction of aqueous solutions with epidote feldspar mineral assemblages in geologic systems ii equilibrium constraints in metamorphic geothermal processes thermodynamic analysis of the system na sub 2 o k sub 2 o cao feo fe sub 2 o sub 3 al sub 2 o sub 3 sio sub 2 h sub 2 o hcl co sub 2 at pressures and temperatures to 5 kb and 600 sup 0 c affords quantitative description and interpretation of phase relations among epidote garnet plagioclase and alkali feldspar solid solutions in hydrothermal systems comparison of computed and observed compositions of these minerals and coexisting fluids suggests that the calculations afford close approximation of equilibrium and mass transfer in metamorphic geothermal processes the standard molal enthalpies of decomposition reactions for plagioclase are of the order of 4 to 50 kcal mole sup 1 which requires the activity of the anorthite component of plagioclase coexisting with epidote solid solutions to increase with increasing temperature at constant pressure in contrast because the standard molal volumes of these reactions are also negative a sub caal sub 2 si sub 2 o sub 8 decreases with increasing pressure at constant temperature either stoichiometric epidote or clinozoisite is compatible with plagioclase quartz calcite and an aqueous phase but only at x sub co sub 2 less than or equal to 0 2 the complex zoning commonly exhibited by both plagioclase and epidote solid solutions in geologic systems can be attributed to minor isothermal isobaric changes in the composition of coexisting aqueous solutions logarithmic activity and fugacitymore diagrams constructed with provision for solid solution permit documentation of such changes and facilitate thermodynamic interpretation of mineral and fluid compositions in metamorphic geothermal systems less

epidote bearing veins in the state 2 14 drill hole implications for hydrothermal fluid composition epidote bearing veins in state 2 14 drill core from 900 to 2960 m depth were examined using backscattered electron microscopy and electron probe microanalysis to characterize the mineralogy parageneses texture and composition of vein minerals in order of decreasing abundance minerals in epidote bearing veins are pyrite calcite k feldspar quartz anhydrite hematite chlorite fe cu zn sulfides actinolite titanite and allanite the downhole distribution of minerals in epidote bearing veins pyrite and quartz varies as a function of depth and includes 1 calcite above 2000 m 2 k feldspar between 1700 and 2745 m 3 anhydrite between 2195 and 2745 m 4 hematite sulfides above 2773 m and 5 actinolite below 2890 m where present k feldspar was the first mineral to precipitate in veins followed by epidote in all other veins epidote was the earliest vein mineral to form calcite quartz anhydrite hematite and sulfides were paragenetically later compositional zoning common in most vein epidotes is typically symmetric with al rich cores and fe3 rich rims the minimum mole fraction of ca2fe3si3o12 oh xps in vein epidotes decreases systematically with increasing depth from 0 33 at 906 m to 0 21 at 2900 m and the maximum xps at any given depth is greater than 0 33 thermodynamic analyses of phase relations among vein filling minerals and aqueous solutions at depths near 1867 m and 300 c indicate that the modern reservoir fluid in the salton sea geothermal system is in equilibrium with calcite hematite quartz epidote xps 0 33 anhydrite the predicted fugacity of co2 14 bars for the modern salton sea brine is in close agreement with the calculated value of fco2 for the 1867 m production fluid theoretical phase diagrams in the system cao k2o fe2o3 al2o3 sio2 h2o o2 s2 co2 demonstrate that the mineralogies and mineral parageneses recorded hi epidote bearing veins and the observed variations in al fe3 content of vein epidotes may result from only minor changes in the fugacity of co2 o2 and s2 of the geothermal fluid

calculation of oxygen isotope fractionation in hydroxyl bearing silicates abstract the modified increment method has been applied to the calculation of oxygen isotope fractionation factors for hydroxyl bearing silicate minerals the order of 18o enrichment obtained in common rock forming minerals is pyrophyllite kaolinite tourmaline talc prehnite topaz illite phengite lepidolite muscovite staurolite epidote glaucophane serpentine chlorite tremolite hornblende phlogopite biotite humite norbergite ilvaite hydroxyl bearing silicates are enriched in 18o relative to hydroxyl groups but depleted in 18o relative to anhydrous counterparts three sets of self consistent fractionation factors between quartz and the hydroxyl bearing silicate minerals between calcite and the silicate minerals and between the silicate minerals and water have been calculated for a temperature range of 0 1200 c the fractionation factors calculated for mineral pairs are applicable to isotopic geothermometry in igneous metamorphic and sedimentary petrology they can be used as a test of isotopic equilibrium or disequilibrium in natural mineral assemblages over all temperature ranges of geological interest the difference in oxygen isotope composition between the hydroxyl bearing mineral and the oh group is quantitatively demonstrated to be temperature dependent and therefore can be used as a single mineral geothermometer

description and interpretation of the composition of fluid and alteration mineralogy in the geothermal system at svartsengi iceland abstract the bulk composition and mineralogy of hydrothermally altered tholeiite along with the composition and speciation of fluid have been determined for a well defined alteration zone at 240 c and 110 bars at svartsengi iceland mass balances between the geothermal fluid and altered tholeiite relative to a seawater fresh water mixture and unaltered tholeiite indicate the overall reaction per 1000 cm 3 is 1325 gm plagioclase 1228 gm pyroxene 215 gm oxide minerals break down to form 685 gm chlorite 636 gm albite 441 gm quartz 249 gm epidote 266 gm calcite 201 gm oxide minerals 15 gm pyrite requiring an influx of 123 gm co 2 10 gm h 2 s and 4 gm na 2 o and a release of 57 gm sio 2 35 gm feo 21 gm cao 8 gm mgo and 4 gm k 2 o principal reactions deduced from textural evidence include na ca exchange in plagioclase precipitation of quartz calcite and anhydrite and formation of chlorite and epidote by reactions between groundmass minerals and fluid thermodynamic analyses of authigenic minerals and downhole fluid indicate that the fluid maintains a state close to equilibrium with the secondary mineral phases chlorite epidote albite quartz calcite prehnite anhydrite pyrite and magnetite whereas remnant primary labradorite and augite are out of equilibrium with the fluid water rock ratios for the system are determined under a variety of assumptions however the open nature of the system makes comparisons with experimental and theoretical closed system studies ambiguous

archean mafic metavolcanics from the rouyn noranda district abitibi greenstone belt quebec 1 mobility of the major elements reply in a suite of archean mafic pillows from the rouyn noranda region of quebec s abitibi greenstone belt including both tholeiitic and calc alkaline varieties spanning the prehnite pumpellyite to upper greenschist metamorphic facies three types of alteration can be defined i chlorite epidote actinolite ii chlorite epidote and iii chlorite sericite the number of mineral phases decreases as a result of progressive hydration from type i to type iii alteration albitization resulting from substitution of in calcic plagioclase is highly variable in type i alteration but in types ii and iii the plagioclase is totally albitized and in some cases silicified chloritization is closely linked to increasing hydration and ca leaching with mgo and feo substituting for cao in ferromagnesian minerals calcium was mobilized and carried by solutions as evidenced by the variable concentration of epidote at the margins of pillows this calcium leaching generated an excess of al2o3 with respect to the combine

metal depleted root zones of the troodos ore forming hydrothermal systems cyprus the cupriferous pyrite deposits of cyprus were precipitated from hydrothermal solutions derived by interaction of contemporaneous seawater with hot mafic rock at the ancient troodos spreading centre here we identify the zones in which this interaction took place the zones occur in the lower part of the sheeted dyke complex and within them 30 50 of the rock is made up of epidosite an epidote quartz rock replacing the dykes as sheets and pipes the epidosites contain abundant fluid inclusions which give trapping temperatures of 350 400 c or even higher and contain water normally near seawater in salinity zones of epidosite are elongate parallel to the strike of the sheeted dykes and are up to 1 km wide the rocks throughout these zones are strongly depleted in cu and zn and the metals removed are sufficient to supply the ore deposits in fact several large ore deposits lie along strike from zones of epidosite all of these features support the identification of the epidosites as the hydrothermal reaction zones the location of the epidosite zones immediately above the gabbros of the plutonic complex supports the hypothesis that the heat to drive the ore forming systems came from the underlying magma as is also likely for modern black smoker springs

the scuddles cu zn prospect an archean volcanogenic massive sulfide deposit golden grove district western australia the scuddles volcanogenic massive sulfide prospect together with the nearby gossan hill golden grove prospect constitute the largest known potentially economic cu zn sulfide deposits in the archean yilgarn block of western australia enclosed in a submarine sequence of felsic to intermediate volcanics epiclastics and exhalative chemical sediments the scuddles deposit lies in a regionally extensive mineralized horizon this horizon is up to 80 m thick and is dominated by sulfidic and cherty exhalites and epiclastics a sequence of felsic to intermediate lavas pyroclastics and epiclastics lies above the mineralized horizon with felsic pyroclastics and epiclastics beneath pyroclastic rocks may have been resedimented by mass flow processes the entire sequence has sustained the effects of hydrothermal alteration and regional metamorphism and dips steeply to the west lying on the east limb of a major synclinal structure due to alteration major element values of the rocks are not an accurate guide to their primary composition however abundances of ti zr y nb and v are commonly unaffected and confirm an andesitic to rhyolitic spectrum zoning of ore metals in the scuddles mineralized horizon is typical of that in many precambrian felsic volcanic associated exhalative sulfide deposits the top is defined by a laminated magnetite bearing iron formation enriched in fe mn and base metals which conformably overlies several lenses of massive fe zn sulfides in places containing significant pb ag au and cu grading downward into pyritic sulfides in turn the massive sulfides overlie a zone of epigenetic stockwork stringer and disseminated cu fe sulfides in the middle and lower portions of the mineralized horizon massive to disseminated exhalative sulfides are in places accompanied by quartz chlorite talc and other silicates carbonates and magnetite also interpreted to be at least partly exhalative in origin the lower cu fe sulfide zone is considered to have formed immediately below the sea floor perhaps occupying an extensive feeder system for hot springs the upper zn fe sulfide zone may have formed by precipitation out of hydrothermal plumes as they vented into and mixed with seawater ore mineral deposition was attended by the development of a volumetrically large 10 km long several hundred meters thick strata bound zone of hydrothermal alteration well represented in the footwall and mineralized horizon upper hanging wall rocks are little altered but elsewhere the sequence displays moderate to intense feldspar destructive development of quartz chlorite sericite and locally abundant carbonate talc and sulfides these mineralogical changes are accompanied by pervasive depletion of na ca and sr and locally k the mineralized horizon and much of the footwall is enriched in fe mg mn base and precious metals as s co 2 and h 2 o interpreted to be the result of addition from exhalative sources the strata bound alteration zone represents a volume of hydrothermal flux in which major sites of sulfide deposition at scuddles and gossan hill were focused zones of fe mg base metal enrichment below and adjacent to positions of submarine metal bearing exhalations precipitation of exhalites occurred under relatively quiescent conditions in the mineralized horizon however the stratigraphic sequence indicates that at other times active mass flow deposition was taking place

regional scale hydrothermal alteration in the central blake river group western abitibi subprovince canada implications for vms prospectivity the late archean blake river group is a thick succession of predominantly mafic volcanic rocks within the southern zone of the abitibi greenstone belt it contains a number of silicic volcanic centers of different size including the large noranda volcanic complex which is host to 17 past producing volcanogenic massive sulfide deposits the noranda complex consists of a 7 to 9 km thick succession of bimodal mafic and felsic volcanic rocks erupted during five major cycles of volcanism massive sulfide formation coincided with a period of intense magmatic activity cycle iii and the formation of the noranda cauldron hydrothermal alteration in these rocks is interpreted to reflect large scale hydrothermal fluid flow associated with rapid crustal extension and rifting of the volcanic complex the alteration includes abundant albite chlorite epidote and quartz silicification which exhibit broad stratigraphic and structural control and correlate with previously mapped whole rock oxygen isotope zonation the mine sequence volcanic rocks are characterized by abundant iron rich chlorite fe fe mg 0 5 hydrothermal amphibole ferroactinolite and coarse grained epidote of clinozoisite composition 10 wt fe2o3 lacking the clinozoisite solid solution alteration in the mine sequence volcanic rocks persists along strike well beyond the limits of the main ore deposits as far as several tens of kilometers and can be readily distinguished from greenschist facies metamorphic assemblages at a regional scale the lack of similar alteration in the pre cauldron sequences is consistent with limited 18o depletion and suggests that the early history of the volcanic complex did not support large scale high temperature fluid flow in these rocks comparisons with a much smaller barren volcanic complex in nearby ben nevis township reveal important differences in the alteration mineralogy between volcanoes of different size with implications for area selection during regional scale mineral exploration the ben nevis complex consists of a 3 to 4 km thick succession of mafic intermediate and felsic volcanic rocks centered on a small subvolcanic intrusion alteration of the volcanic rocks comprises mainly low temperature assemblages of prehnite pumpellyite magnesium rich chlorite fe fe mg 10 wt fe2o3 and calcite actinolite magnetite alteration occurs proximal to the intrusive core of the complex but the limited extent of this alteration indicates only local high temperature fluid circulation adjacent to the intrusion a distal zone of carbonate alteration is located 4 6 km from the center of the volcano although iron bearing carbonates are present locally within this zone the absence of siderite argues against a high temperature origin for this alteration these observations do not offer positive encouragement for the existence of a fossil geothermal system of sufficient size or intensity to have produced a large massive sulfide deposit

salinity of oceanic hydrothermal fluids a fluid inclusion study abstract an extensive microthermometric study of quartz epidote plagioclase anhydrite and sphalerite hosted fluid inclusions from ophiolitic semail oman and trinity california ophiolites and oceanic east pacific rise hydrothermal vents gorringe bank odp leg 111 hole 504b crust has been carried out in order to constrain a model accounting for wide salinity variations measured in the oceanic hydrothermal fluids recorded salinities in fluid inclusions vary between 0 3 and 52 wt nacl eq however more than 60 of the mean standard deviation salinities of the samples are within the range 3 2 0 3wt nacl eq microthermometric error and the mean salinity of all fluid inclusions without the brines is 4 0 wt nacl eq with a standard deviation of 1 6 wt nacl eq whereas most samples display slightly higher salinities than seawater several samples exhibit very high salinities more than two times that of seawater these high salinities are restricted to the plagiogranites semail and trinity ophiolites which mark the top of the fossil magma chamber in the transition zone between the plutonic sequence and the sheeted dyke complex the fluid inclusion population studied in the plagiogranites is characterized by the occurrence of four major fluid inclusion families 1 low to medium salinity liquid vapor fluid inclusions which homogenize into the liquid phase 2 low salinity liquid vapor fluid inclusions with pseudocritical homogenization 3 low to medium salinity liquid vapor fluid inclusions which homogenize into the vapor phase and 4 high salinity liquid vapor halite fluid inclusions which homogenize into the liquid phase by halite dissolution and exhibit salinities as high as 52 wt nacl eq these fluid inclusion families are interpreted as resulting from phase separation occurring in hydrothermal or magmatic fluids within the transition zone between the hydrothermal system and the magma chamber at temperatures higher than 500 c very low salinities less than half that of seawater have been found only in one dioritic sample from the gorringe bank mineralogical and petrological data from the semail ophiolite shows the absence of cl rich minerals furthermore hydration of the crust although very important cannot account for the large salinity variations thus the large salinity variations measured are interpreted as resulting from phase separation and generation of brine rich fluids at the top of the magma chamber this phase separation and subsequent phase segregation and brine accumulation may result in a double diffusive convection of the fluids in the hydrothermal system

compositional order disorder and stable isotope characteristics of al fe epidote state 2 14 drill hole salton sea geothermal system epidote ca2fe3si3o12 oh ca2al3si3o12 oh is a common hydrothermal mineral in metasediments and veins at depths 900 m in the state 2 14 drill hole of the salton sea geothermal system the mole fraction of ca2fe3si3o12 oh in epidotes xps from this drill hole ranges from 0 11 to 0 42 and complex compositional zoning of octahedral fe3 and al is typical within single grains with increasing depth there is an overall but irregular decrease in the fe3 content of epidotes in metasandstones in most samples vein epidotes are more fe3 rich and exhibit a wider compositional range than metasandstone epidotes from the same depth octahedral fe3 in the m 1 sites in four epidotes evaluated by 57fe mossbauer spectroscopy ranges from 7 5 1 7 to 11 4 1 5 of the total iron the most ordered epidote formed in veins at 2618 m in the biotite zone this epidote has an ordering parameter 1 2xfe3 m 1 of 0 85 0 03 which corresponds to a calculated state of equilibrium order disorder at 390 60 c in close agreement with the probable downhole temperature of 340 c two epidote samples from the chlorite calcite zone 1420 m 265 c 1867 m 300 c are more disordered corresponding to calculated states of equilibrium order disorder of 450 c these data suggest that the analyzed epidotes from the chlorite calcite zone are in a metastable state of substitutional order disorder the measured value of depidote from 1420 m is 96 which is 5 6 lighter than epidotes from 1867 m depidote 90 2227 m depidote 91 and 2618 m depidote 90 when compared to the reservoir fluid from the december 1985 flow test the derived fractionation of hydrogen isotopes between epidote and the geothermal brine is 19 at 1867 m and 340 c this value is in accord with published experimental hydrogen isotope fractionations between iron rich epidote and aqueous electrolyte solutions therefore considering the complexity of the salton sea brine the isotopic temperature is similar to the inferred downhole temperature

a review of the timing of coalification in the light of coal seam erosion clastic dykes and coal clasts abstract coal seam erosion clastic dykes penetrating coal seams and studies of coal clasts occurring in late paleozoic basins are used to demonstrate that coalification to the rank of subbituminous and bituminous coal occurred rapidly at shallow depth in some parts of the variscan orogen implying a precondition of high paleotemperatures for the coal basins the origin of clastic dykes particularly in the bolsovian westphalian c deposits of the plze and kladno rakovnik continental basins czech republic is shown to be a relevant factor in determining the timing of coalification pesek j 1978 erosion and clastic dikes in coal seams of the central bohemian basins and their significance for determination of plant substance coalification folia musei rerum naturalium bohemiae occidentalis geologica 12 1 34 is of the opinion that the 50 100 cm deep fractures filled with superposed clastics originated in fully ranked high volatile bituminous coal exposed during a hiatus between the early westphalian c early bolsovian and late westphalian d late asturian these dykes have either irregular shape or form conic bodies with sharp boundaries their filling does not show any signs of postsedimentary deformation coal clasts occurring in clastic sediments either close to or far above the roof of coal seams are a few millimetres up to several centimetres in size being subangular or angular however there also exist clasts of which one side is angular to subangular whereas the opposite side is markedly sharp in some basins coal clasts are embedded in units of the same age they are mostly of somewhat lower rank by 0 01 0 03 than the nearest coal seam whereas gayer r a pesek j s korova i valterova p 1996 coal clasts in the upper westphalian sequence of the south wales coal basin implication for the timing of maturation and fracture permeability in gayer r harris j eds coalbed methane and coal geology geological society special publication 109 pp 103 130 presumed in the south wales foreland basin a burial level of about 1 km with an elevated geothermal gradient 60 c km enhanced by hot fluids along thrusts resulting in rapid coalification to the bituminous coal rank no deep burial of organic matter could have occurred in the intramontane basins of the bohemian massif coalification of the late paleozoic seams must have proceeded at a very high rate the only logical reason leading to such rapid coalification of biomass could have been high value of geothermal gradient at least in the basins of the bohemian massif

heavy metal and sulfur transport during subcritical and supercritical hydrothermal alteration of basalt influence of fluid pressure and basalt composition and crystallinity crystalline basalt diabase and basalt glass have been reacted with a na ca k cl fluid of seawater ionic strength at 350 425 c 375 400 bars pressure and fluid rock mass ratios of 0 5 1 0 to assess the role of temperature basalt diabase chemistry and texture on heavy metal and sulfur mobility during hydrothermal alteration alteration of basalt diabase is characterized by cation fixation and hydrolysis reactions which show increased reaction progress with increasing temperature at constant pressure correspondingly ph in a series of 400 bar experiments ranges from 4 8 to 2 7 at 350 and 425 c respectively and is typically lower for alteration of a sio2 rich crystalline basalt than for other rock types due in part to relatively high sio2 concentrations in solution high sio2 concentrations stabilize hydrous na and ca rich alteration phases causing ph to decrease according to reactions such as 3 0 caal2si2o8 1 0 ca 2 0 h2o 2 0 ca2al3si3o12 oh 2 0 h phases experimentally produced include mixed layer chlorite smectite ca rich amphibole and clinozoisite clinozoisite was identified as a replacement product of plagioclase from diabase solution interaction experiments in direct response to h production dissolved fe mn and h2s concentrations increase dramatically for early stage reaction h2s typically exceeds fe and mn however at 425 c and after long term reaction at 400 c h2s is lost from solution apparently in response to pyrite replacement of oxide and silicate phases pyrrhotite formed at temperatures 375 c whereas magnetite was identified in all run products except from basalt glass alteration cu and zn concentrations in solution are not simple functions of ph these metals achieve greatest solubility in fluids from experiments at 375 400 c except when basalt glass is used as a reactant the relatively low concentrations of these species in solution during basalt glass reaction may be due to adsorption by fine grained alteration phases

interplay between tectonics and compaction in a rift margin lacustrine delta system miocene of the eger graben czech republic the early miocene bilina palaeodelta consists of fluvio deltaic and lacustrine clastics deposited along the south eastern margin of the extensional most basin part of the eger graben in north bohemia czech republic the bilina succession shows evidence of repeated advances of an axial deltaic system across a thick accumulation of organic material and clay in the hangingwall of an active fault exposures up to ca 4 5 km long in the bilina open cast mine help bridge the gap between seismic scale and typical outcrop scale of observation and thus allow the relationships between small scale and basin scale stratal geometries to be evaluated the bilina palaeodelta deposits include sand dominated fluvial channel fills and heterolithic sheets interpreted as delta plain strata sand dominated mouth bar wedges and heterolithic sheets of prodeltaic deposits passing distally into lacustrine clays the depositional environment is interpreted as a fluvial dominated mixed load lacustrine delta with a high degree of grain size segregation at the feeder channel mouths on the largest temporal and spatial scales variable tectonic subsidence controlled the overall advance and retreat of the delta system the medium term transgressive regressive history was probably driven by episodes of increased subsidence rate however at this temporal scale the architecture of the deltaic sequences deltaic lobes and correlative lacustrine deposits was strongly affected by i compaction of underlying peat and clay which drove lateral offset stacking of medium term sequences and ii growth of a fault propagation fold close to the active bilina fault at the smallest scale the geometries of individual mouth bars and groups of mouth bars short term sequences reflect the interaction among sediment loading compaction and growth faulting that produced high frequency relative lake level fluctuations and created local accommodation at the delta front

the geology of the new insco copper deposit noranda district quebec the new insco copper deposit is an archean volcanogenic massive sulfide orebody situated on the northwestern edge of the noranda mining district in the superior province of the canadian shield the ore occurs near the base of a thin unit of pillowed and brecciated basalt that overlies rhyolitic volcaniclastic rocks both the basalt and rhyolite are heavily chloritized and sericitized and form a zone of alteration that is depleted in sio2 na2o and cao and enriched in feot mgo k2o cu zn and s the units are separated from an underlying massive rhyolite by a diorite intrusion and from an overlying series of pillowed basalts by a fault the orebody is a pyrrhotite rich lens of massive sulfide containing conformable bands of disseminated chalcopyrite and pyrite that is interlayered with lenses of pyrite chalcopyrite bearing siliceous ore the massive ore is interpreted to have been deposited in a distal environment with pyrrhotite as a primary mineral it was recrystallized during regional metamorphis

geochemistry of volcanic rocks associated with cu zn and ni cu deposits in the abitibi subprovince volcanogenic massive sulfide vhms deposits in the abitibi subprovince are preferentially associated with volcanic successions containing 150 m thicknesses of felsic volcanic rocks approximately 50 by area of volcanic terranes and are found within volcanic sequences of at least three distinct affinities group i which is host to greater than half of the volcanogenic massive sulfide deposits by tonnage and which comprises only approximately 10 percent by area of volcanic terranes is composed of bimodal tholeiitic basalt basaltic andesite and high silica rhyolite the basaltic andesites and high silica rhyolites are characterized by high high field strength element and heavy rare earth element ree contents low light to heavy ree ratios most with la n yb n 0 8 3 and strong negative eu anomalies the kamiskotia matagami and chibougamau lower cycle volcanogenic massive sulfide areas all of which are also underlain by large synvolcanic gabbroic complexes are associated with group i volcanic sequences the kidd creek potter normetal and horne deposits are also included in this category group ii which is host to one third of the volcanogenic massive sulfide deposits by tonnage and which is also approximately 10 percent by area of volcanic terranes is composed of bimodal transitional tholeiitic to calc alkalic andesitc and rhyolite characterized by intermediate high field strength element contents and slightly higher ree ratios la n yb n 1 4 noranda camp excluding the horne deposit and val d or camp group iii is host to only one known deposit the selbaie mine which is unusual in that much of its mineralization cuts stratigraphy the selbaie mine sequence contains calc alkalic andesite rhyolite with relatively low high field strength element and ree contents and higher ree ratios la n yb n 3 9 the vast majority of volcanogenic massive sulfide bearing mafic and felsic volcanic rocks in the abitibi subprovince have la n yb n 5 barren volcanic sequences are group iv calc alkalic basaltic andesite to rhyodacite with low high field strength element and relatively high ree ratios la n yb n 8 20 represented by the upper skead group the quebec hunter mine group and the upper cycle chibougamau rocks and group v mafic to felsic alkalic volcanic rocks with high ree ratios la n yb n 12 62 represented by the timiskaming opemisca and ridout series rocks group i is most similar to thickened oceanic rift suites e g galapagos spreading center iceland east rift group ii is similar to suites in rifled island arcs e g hokuroku district japan and group iii is comparable to continental arc suites e g southern volcanic zone central chile groups iv and v are comparable to arc related suites derived from metasomatized mantle with variable amounts of crustal contamination e g setouchi area japan roman province italy known ni cu deposits in volcanic sequences are hosted exclusively in komatiitic flows and hypabyssal sills represented by chill compositions with high mgo contents 20 35 wt anhydrous very low incompatible element contents and depleted signatures la n sm n 0 5 0 8 zr y 2 5 alexo marbridge and shaw dome deposits in comparison to most barren komatiites in the abitibi subprovince their depleted trace element signatures indicate minimal crustal assimilation which would appear contrary to the widely held view that crustal contamination e g addition of silica sulfur oxygen of a mantle derived melt is necessary for sulfur saturation and magmatic sulfide segregation this problem may be reconciled by considering the dynamics of komatiite flows lesher and arndt 1990 where contaminated present address 23 euclid avenue ottawa ontario canada kis 2w2 komatiite liquid segregated and deposited sulfide and was subsequently swept downstream and replenished by uncontaminated magma over the deposit as the flow ebbed and cooled when considered together with geologic features such as evidence for thermal erosion in the footwall langmuir deposits and the presence of sulfidic footwall rocks shaw dome deposits the distinctive depleted geochemical signature of mineralized komatiites is useful in exploration for ni cu deposits

u pb zircon geochronology in the southwestern abitibi greenstone belt superior province the abitibi belt is the largest continuous greenstone belt in the superior province of the canadian shield it comprises several composite komatiitic tholeiitic calc alkalic and sedimentary sequences that are folded transected by major faults and intruded by various generations of plutonic rocks precise u pb geochronology has been carried out in the belt for the past decade to solve chronostratigraphic and metallogenetic problems this paper presents new zircon ages and reassesses previously published ones now refined by the addition of abraded and concordant zircon analyses volcanic and subvolcanic units of the timmins area yield the following ages 2727 1 5 ma for a tuff of the upper deloro formation 2703 1 5 ma for a tuff of the upper tisdale formation and 2698 4 ma for the krist fragmental assigned to the top of the tisdale formation the age of a dunite intrusive into the upper deloro formation is revised at 2707 3 ma whereas the aquarius diorite east of timmins yields a poorly define

geochemistry of the alteration pipe at the amulet upper a deposit noranda quebec the amulet upper a orebody is a stratiform pyritiferous copper zinc deposit situated within a sequence of andesitic pillow lavas underlying the massive sulphides is a cylindrical alteration pipe c