glaucophane schists and ophiolites of the northern california coast ranges isotopic ages and their tectonic implications the geology of a strip of 15 quadrangles across the franciscan complex in the coast ranges of northern california is the basis for interpretation of the significance of potassium argon and fossil ages of glaucophane schists ophiolitic materials and associated sedimentary rocks whole rock and mineral separates of franciscan rocks were analyzed and 69 potassium argon ages were obtained white micas and actinolites from blueschist blocks in melange give apparent ages clustering about 142 and 153 m y but they probably represent a continuum blue amphibole ages from blocks range from 98 to 151 m y the younger ages are from the eastern portion of the central melange belt published u pb ages from the ophiolite belt give crystallization ages from 153 to 165 m y k ar ages extend from 143 to 166 m y whole rock ages from the south fork mountain schist range from 113 to 158 m y but cluster around 124 m y the association of older k ar ages with more coarsely crystalline schist suggests that metamorphism occurred prior to 124 m y ago fossil evidence indicates that the basal sediments of the great valley sequence were deposited upon the disrupted coast range ophiolite within 5 to 10 m y of the oldest crystallization ages for ophiolite fossils within melange units broken formations and unsubducted trench slope deposits provide evidence for melange development by the early cretaceous with youngest melange units generally lying to the west available chronologic evidence is compatible with a tectonic model involving melange formation in a restricted flow channel beneath the accretionary wedge of the subduction complex upflow of melange returned blocks of coarsely crystalline metamorphic rocks that were accreted during the initial stages of subduction when the hanging wall was still hot the viability of this and other proposed tectonic models can be assessed by continued careful field documentation of the nature of contacts between key lithotectonic units and by detailed application of temperature sensitive geochronologic tools

a lowstand epikarstic intertidal flat from the middle silurian of gotland sweden this paper describes the sedimentology morphology and diagenesis of recently discovered middle silurian low relief micro epikarst from the klintehamn area on western gotland sweden the epikarst represents the most distal portion of an unconformity that truncates stratigraphic units across a major part of the baltic palaeo basin it formed in the intertidal area during the late stage of platform development and following a short period of regression and siliciclastic influx to the basin at the closing of the cyrtograptus lundgreni chron an overlying oncolite shows signs of repeated exposure to meteoric and marine waters supporting the interpretation of a karstic flat setting four microfacies with different stable isotopic signatures are recognised across the unconformity dolomitic siltstone and altered siltstone in the topmost frojel formation below the contact and ferruginous crinoid algal packstone and ferruginous oncolite grainstone above the contact in the basal lag of the transgressive halla formation endolithic structures of inferred fungal algal origin occur frequently in the two transgressive microfacies the set of observations pointing towards subaerial exposure and karstic processes include a an understanding of the regional geology b erosional relief with undercutting and an associated conglomerate c vertical and subvertical conduits karren indicating gravitationally controlled waters d widening of fissures and subsequent polyphase fillings splitkarren e eluviated internal sediment e g crystal silt f blackening g repeated etching of carbonate allochems h a fitted grain fabric i circum granular cracking j pendant nonluminescent cements k abundant intercrystalline and intergranular secondary porosity and 1 anomalous stable isotopes a synsedimentary iron crust at the contact was not related to karstification but formed a few centimetres below the sediment surface during the ensuing transgression c 2002 elsevier science b v all rights reserved

sedimentology and diagenesis of upper mississippian to lower permian strata talbot lake area jasper national park alberta in the talbot lake area the upper mississippian carbonate sequence comprises in ascending order the turner valley and mount head formations the mount head formation is informally divided into lower and upper members the rocks of the upper mississippian originated by sedimentation associated with successive shallowing upward sequences on the basis of the fossils lithologies textural characters and sedimentary structures the turner valley formation apparently resulted from sedimentation on open marine shoals and in a lagoon the lower member of the mount head formation from sedimentation in shallow subtidal to lower supratidal environments and the upper member of the mount head formation from deposition in upper intertidal to supratidal environments the paraconformity between the lower and upper members of the mount head formation records a short erosional break a disconformity between the upper mississippian carbonates and the upper permian ishbel group displays paleotopographic relief and pale

stratigraphy of upper paleozoic volcanic rocks and regional unconformities in part of the northern sierra terrane california upper paleozoic volcanic rocks in the southern part of the taylorsville strike belt are unconformably truncated at the north fork of the american river by a lens of upper triassic conodont bearing limestone and lower and middle jurassic graywacke slate flysch deposits of the sailor canyon formation the volcanic sequence consists of submarine felsic pyroclastic debris flows and tuffaceous slate of the late devonian sierra buttes formation andesitic turbidites of the taylor formation a lower tuff siltstone member and an upper chert member of the peale formation and fine grained tuff of the reeve formation newly discovered radiolarians date the chert member of the peale as latest late mississippian and or early pennsylvanian shelly fossil fragments found 50 m above the chert member date the lower part of the reeve in this area as late early permian these rocks formed in the distal part of the submarine volcanic apron presumably in an island arc setting and were deposited unconformably on quartz rich clastic rocks and tectonically interlayered chert of the pre late devonian shoo fly complex the peale marks a period of quiescence in the late paleozoic volcanism that culminated in deep water chert deposition during latest mississippian to the middle pennsylvanian the peale reeve contact is a regional hiatus probably nondepositional in the south and becoming erosional to the north that spans most or all of the late pennsylvanian and early permian post late early permian to pre late triassic deformation produced northeast trending folds and a southeast trending fault in the late paleozoic volcanic rocks that are truncated nearly orthogonally by the late triassic unconformity and northwest trending folds in the jurassic sailor canyon formation

regional metamorphic controls on alteration associated with gold mineralization in the eastern goldfields province western australia implications for the timing and origin of archean lode gold deposits alteration assemblages associated with archean epigenetic gold mineralization in the menzies kambalda area of the yilgarn craton western australia vary systematically with regional metamorphic grade up to and including upper amphibolite facies rocks k metasomatism is recorded by the presence of muscovite biotite and k feldspar at progressively higher metamorphic grades high metamorphic grades are commonly developed in broad thermal aureoles around fate syntectonic granitic intrusions metamorphic recrystallization of alteration assemblages is common in the thermal aureoles these relations suggest broad contemporaneity among granitoid intrusion regional metamorphism and mineralization during the final stages of the tectonic evolution of the granite greenstone terrain late syntectonic granitoids acted as centers of heat and fluid flux in large scale synmetamorphic hydrothermal systems that deposited gold possibly from modified mantle derived fluids

early palaeozoic cooling events peri gondwana and beyond the short lived end ordovician hirnantian glaciation allied to marine mass extinction is variously considered as a short lived event or as the peak of long drawn out climatic cooling through at least late ordovician early silurian times evidence from early palaeozoic facies faunas and stable isotope excursions used to interpret climatic cooling events ranges farther from late mid cambrian to late silurian times glacigenic sediments structures and geomorphology provide direct evidence of glacial episodes cool water carbonate deposition which is particularly widespread during the late ordovician boda event in high latitude peri gondwana gondwana and beyond into mid low palaeolatitudes is interpreted as indicating global cooling not warming as has been proposed such carbonates also characterize mid latitude continents widely at horizons earlier in the ordovician and more locally in the mid silurian in high latitude gondwana cool water carbonate mounds have distinctive facies controlled mound faunas across palaeocontinents other facies evidence for palaeoclimates includes black shale deposition including deglacial organic rich hot shales which indicate transgression in epeiric seas and sea level curves interpreted from facies and faunal successions correlation is shown between facies evidence and positive c isotope excursions from which cyclicities are apparent the possible interface of orbitally controlled rhythms is considered against evolving palaeobiogeography and changes in global sea level and in pco2 facies and faunal evidence from peri gondwanan terranes armorica central europe alborz is assessed with that from gondwana mostly north africa south america and correlatives in avalonia baltica and laurentia to establish a wider picture of early palaeozoic cooling events

paleokarst in an upper devonian reef complex of the canning basin western australia abstract evidence for subaerial exposure of an early famennian platform is recorded within the fore reef slope succession in the napier range reef complex of the northern canning basin this observation supports recent sequence stratigraphic interpretations of fluctuations in relative sea level throughout reef evolution in this basin and contrasts with previous outcrop based interpretations that there were no sea level falls except at the frasnian famennian boundary remnants of the paleokarst surface are preserved in allochthonous reef flat blocks that are temporally and spatially related to lenticular siliciclastic units subaerial exposure of the platform margin during a third order sea level fall in the early famennian led to the formation of karst cavities on the reef flat and collapse of parts of the reef margin during this time siliciclastic sands were fed to the slope where they were deposited as a series of aprons along the margin in the early stages of the ensuing transgression renewed marine conditions enabled cyanobacteria to colonize the cavities as sea level continued to rise siliciclastic sands were reworked across the platform filling the encrusted cavities the reef flat blocks were dislodged during the late transgression n response to instability that may have been caused by triggers such as rising pore fluid pressures and or tectonic activity they were emplaced on the siliciclastic slope aprons towards the end of sand deposition as sea level on the platform continued to rise and carbonate production resumed

a shear zone related greenstone belt hosted gold mineralization in the archean of west greenland a petrographic and combined pb pb and rb sr geochronological study abstract widespread gold showings have been found in an archean greenstone belt on storo in godthabsfjord west greenland the storo greenstone belt sgb comprises metasedimentary and metavolcanic rocks with minor magnetite rich bands gold mineralization is hosted by amphibolites mica schists and garnetite rocks gold occurs in various mineral associations but its presence in lollingite arsenopyrite pyrrhotite parageneses is prevalent to a lesser extent native gold occurs within seriticized plagioclase in sheeted quartz veins and as inclusions in garnet based on petrographic observations and comparison with similar ore mineral textures from other deposits we propose the following model for the deposition of the gold bearing lollingite arsenopyrite pyrrhotite assemblage on storo in the first stage arsenopyrite likely formed with gold in solid solution in the second stage gold bearing arsenopyrite was replaced by lollingite and pyrrhotite this took place during regional prograde metamorphism gold accumulated within lollingite as discrete grains during subsequent retrograde metamorphic conditions lollingite and pyrrhotite reacted and formed arsenopyrite and gold was subsequently exsolved and preferably deposited as discrete grains or films along arsenopyrite lollingite grain boundaries during a later event a garnet overgrowth partly affected the gold bearing lollingite arsenopyrite pyrrhotite assemblage the isotopic composition of pb released from these garnets by pb stepwise leaching experiments pbsl and the bulk pb isotopic compositions of arsenopyrite and pyrrhotite indicate a common source of pb in the fluid from which first the sulfides and subsequently garnet were formed bulk pb analyses and pbsl experiments of arsenopyrite and pbsl experiments of garnet yield ages of 2863 24 ma 2 mswd 0 2 for arsenopyrite and 2748 62 ma 2 mswd 220 for garnet the 2863 24 ma age is thus a minimum age for the mineralization rb sr isotopic analyses of cogenetic biotite from within strongly mineralized zones reveal middle proterozoic ca 1 65 ga mineral ages well in accordance with a regional reheating event known to have affected the area and are thus interpreted as cooling ages thereof this regional reheating event has apparently not affected the pb pb isotopic system of arsenopyrite implying that the blocking temperature of the u pb system in arsenopyrite is likely to be above 300 c the late archean ages for arsenopyrite and garnet are consistent with the regional amphibolite facies metamorphism related to major terrane assemblies in the study area occurring at that time the ore textures preserved today are interpreted as those formed under this regional metamorphic event

the structure and geological development of the erickson gold mine cassiar district british columbia with implications for the origin of mother lode type gold deposits the erickson gold mine is a typical gold quartz vein deposit the veins are hosted by a thrust imbricated gently dipping synformal allochthon of low grade metamorphic devonian to upper triassic basalts argillites and peridotites of oceancrustal origin belonging to the sylvester group part of the slide mountain assemblage the sylvester allochthon lies concordantly on devonian miogeoclinal sedimentary rocks of the north american continental margin and was emplaced in the middle jurassic as a result of the collision of the quesnel arc with north america the veins in the mine are hosted mainly by a moderately dipping system of shear zones with approximately orthorhombic symmetry indicating a triaxial bulk inhomogeneous strain pattern superimposed on the earlier formed gently dipping thrusts steeply dipping extension veinlets rotation of schistosity and downdip slickenlines indicate the maximum shortening axis was subvertical the veins display complex superimposed ribbon and breccia textures in

lower silurian stromatolites in shallow marine environments of the south china block guizhou province china and their palaeoenvironmental significance this study was supported by national natural science foundation of china granted no 41072002 41372022 xdb10010503 and 41521061

mesozoic transpression transtension subduction and metallogenesis in northern and central california middle paleozoic to middle jurassic terrane assemblies in the klamaths and sierran foothills consist of mafic ultramafic complexes fine grained terrigenous strata derived from previously accreted continental margin belts sutured oceanic terranes reflect c 230 myr of margin parallel slip involving chiefly transtension and transpression quartzofeldspathic clastic rocks and blueschists eclogites are very rare little devolatilization occurred at magmagenic depths hence coeval hydrothermal ore deposits and granitoids are uncommon in contrast nearly head on cretaceous subduction of the farallon plate generated the massive klamath sierra nevada volcanic plutonic arc reflecting dewatering of the eastward descending oceanic lithosphere in the magmagenic zone immature great valley forearc and franciscan trench deposits shed from the arc record c 70 myr of rapid crustal growth au bearing solutions rising from magmagenic depths exsolved from plutons and expelled from heated wall rocks were mobilized attending arc construction precipitation of gold bearing quartz veins occurred where h2o co2 bearing fluids encountered major geochemical discontinuities in the wall rocks

an integrated chemical and stable isotope model of the origin of midocean ridge hot spring systems chemical and isotopic changes accompanying seawater basalt interaction in axial midocean ridge hydrothermal systems are modeled with the aid of chemical equilibria and mass transfer computer programs incorporating provision for addition and subtraction of a wide range of reactant and product minerals as well as cation and oxygen and hydrogen isotopic exchange equilibria the models involve stepwise introduction of fresh basalt into progressively modified seawater at discrete temperature intervals from 100 to 350 c with an overall water rock ratio of about 0 5 being constrained by an assumed 18 o h 2o at 350 c of 2 0 per mil h craig personal communication 1984 this is a realistic model because 1 the grade of hydrothermal metamorphism increases sharply downward in the oceanic crust 2 the water rock ratio is high 50 at low temperatures and low 0 5 at high temperatures and 3 it allows for back reaction of earlier formed minerals during the course of reaction progress the results closely match the major element chemistry von damm et al 1985 and isotopic compositions craig et al 1980 of the hydrothermal solutions presently emanating from vents at 21 n on the east pacific rise the calculated solution chemistry for example correctly predicts complete loss of mg and so 4 and substantial increases in si and fe however discrepancies exist in the predicted ph 5 5 versus 3 5 measured and state of saturation of the solution with respect to greenschist facies minerals the calculated d h 2o is 2 6 per mil in excellent agreement with analytical determinations the calculated chemical mineralogic and isotopic changes in the rocks are also in good accord with observations on altered basalts dredged from midocean ridges humphris and thompson 1978 stakes and o neil 1982 as well as with data from ophiolites gregory and taylor 1981 predicted alteration products include anhydrite and clay minerals at low temperatures and a typical albite epidote chlorite tremolite greenschist assemblage at 350 c the models demand that the major portion of the water rock interaction occur at temperatures of 300 350 c interaction at temperatures below approximately 250 c results in negative 18 o h 2o shifts contrary to the observed positive 18 o values of the fluids exiting at midocean ridge vents hydrogen isotope fractionation curves by suzuoki and epstein 1976 lambert and epstein 1980 and liu and epstein 1984 among others are compatible with the model and require d h 2o to increase at all temperatures as a result of seawater basalt interaction

results from the eismint model intercomparison the effects of thermomechanical coupling this paper discusses results from the second phase of the european ice sheet modelling initiative eismint it reports the intercompartison of ten operational ice sheet models and uses a series of experiments to examine the implications of thermomechanical coupling for model behaviour a schematic circular ice sheet is used in the work which investigates both steady states and the response to stepped changes in climate the major finding is that radial symmetry implied in the experimental design can under certain circumstances break down with the formation of distinct regularly spaced spokes of cold ice which extended from the interior of the ice sheet outward to the surrounding zone of basal melt these features also manifest themselves in the thickness and velocity distributions predicted by the models they appear to be a common feature to all of the models which took part in the intercomparison and may stem from interactions between ice temperature flow and surface form the exact nature of these features varies between models and their existence appears to be controlled by the overall thermal regimne of the ice sheet a second result is that there is considerable agreement between the models in their predictions of global scale response to imposed climate change

why are ancient rocky shores so uncommon the geologic history of rocky shore biotas has been neglected by paleontologists under present conditions of high continentality rocky shores account for more than 33 of the world s coastline they are generally located on shores facing narrow continental shelves where the pattern of global plate tectonics exerts much influence over their regional concentration the highest frequency of rocky shores occurs on islands associated with hot spots island arcs and convergent plate margins associated with subduction rocky shores situated in these settings are subject to recycling so their remains are rare in the stratigraphic record some passive continental margins including those along newly rifted oceans and those with low rates of sedimentation exhibit frequencies of rocky shores as high or higher than those found on many convergent plate margins these have a good chance of being retained in the stratigraphic record for geologically significant periods of time epicontinental flooding reduces the

rb sr sm nd and pb isotope systematics of pyrite implications for the age and genesis of lode gold deposits lode gold deposits commonly consist of pyrite and lesser amounts of lead and zinc sulfides accompanied by quartz and calcite their exact origin remains controversial mainly because of the scarcity of reliable geochronological data here we present direct rb sr dating of pyrite from a lode gold deposit and propose the method as a useful geochronological technique for gold mineralization for which age data are poor a positive correlation between present day 87 sr 86 sr and 87 rb 86 sr ratios of pyrite fes 2 subsamples from the linglong gold mine of china corresponds to an age of 122 123 ma which dates the age of gold mineralization the sr nd and pb isotopic compositions of pyrites and the associated rocks suggest that the hydrothermal fluids responsible for the pyrite and gold mineralization were probably derived from a mixed source i e degassing of mafic magmas and meteoric water that had leached the country rocks

gangue mineral 87 sr 86 sr ratios and the origin of mississippi valley type mineralization the 87 sr 86 sr ratio measurements in mississippi valley type gangue minerals and host marine carbonates ranging in age from upper cambrian to mississippian show that in northwestern ohio where the mineralization is sparse the minerals precipitated from fluids in local isotopic equilibrium with the host carbonates in which the minerals were emplaced in contrast most gangue minerals from districts of major mineralization in illinois kentucky virginia and tennessee contain strontium with higher 87 sr 86 sr ratios than the carbonate hosts and appear to have crystallized from fluids containing strontium from silicate minerals in the sedimentary succession no evidence has been found for a magmatie component in the ore fluid strontium data from elmwood tennessee show that the early ore fluids like those of northwestern ohio were in isotopic equilibrium with the host carbonates but that they were mixed with and displaced by introduced fluids containing more radiogenic strontium these findings are consistent with the two fluid mixing model for the mississippi valley mineralization where a fluid indigenous to the host carbonates and containing reduced sulfur was mixed with introduced fluids analogous to metal bearing and low sulfide oil field brines with an extensive history of chemical exchange with clays and other silicate minerals in the sedimentary succession

rb sr k ar and stable isotope evidence for the ages and sources of fluid components of gold bearing quartz veins in the northern sierra nevada foothills metamorphic belt california gold bearing quartz veins occur in and near major fault zones in deformed oceanic and island arc rocks west of the main outcrop of the sierra nevada composite batholith veins typically occupy minor reverse faults that crosscut blueschist to amphibolite grade metamorphic rocks whose metamorphic ages range from early paleozoic to jurassic vein micas and carbonate quartz mica assemblages that formed by hydrothermal metasomatism of ultramafic wall rocks in the alleghany grass valley washington and mother lode districts yield concordant k ar and rb sr ages the dated veins are significantly younger than prograde metamorphism penetrative deformation and accretion of their host rocks to the continental margin new and previously published mineralization ages from 13 localities in the sierra foothills range from about 140 to 110 m y ago with mean and median between 120 and 115 m y the age relations suggest that mineralizing fluids were set in motion by deep magmatic activity related to the resumption of east dipping subduction along the western margin of north america following the late jurassic nevadan collision event co 2 bearing fluids responsible for metasomatism and much of the vein mica carbonate albite and quartz deposition in several northern mines were isotopically heavy delta 18 o asymp 8 14ppm delta d between about 10 and 50ppm and do not resemble seawater magmatic or meteoric waters metasomatic and vein filling mica dolomite magnesite and quartz in altered ultramafic rocks generally formed from fluids with similar sr and o isotope ratios at a given locality consistent quartz mica delta 18 o fractionations delta 18 o sub q m 4 5 4 9ppm from various localities imply uniform equilibration temperatures probably between 300 degrees and 350 degrees c on a local mine scale fluids responsible for both carbonate alteration of mafic and ultramafic wall rocks and albitic alteration of felsic and pelitic rocks had similar sr isotope ratios samples from three veins in the central alleghany district fit a 115 7 or 3 m y rb sr isochron with a 87 sr 86 sr i value of approximately 0 7119 inferred 87 sr 86 sr ratios of metasomatic fluids from mines in different parts of the foothills region vary considerably 0 704 0 718 suggesting that sr was derived from sources ranging from western assemblage mesozoic ophiolitic or arc volcanic rocks to early paleozoic continent derived clastic rocks of the shoo fly complex systematic geographic variations in both sr and o isotopes can be rationalized by assuming extensive fluid interaction with rocks similar to the ones that are exposed within a few kilometers of the veins but the ultimate sources of the fluids and of au and other constituents may be independent of these isotopically lighter meteoric fluids deposited some late quartz overgrowths and occupied secondary fluid inclusions in earlier vein quartz

metamorphic origin of ore forming fluids for orogenic gold bearingquartz vein systems in the north american cordillera constraints froma reconnaissance study of δ15n δd and δ18o the western north american cordillera hosts a large number of gold bearing quartz vein systems from the mother lode of southern california through counterparts in british columbia and southeastern alaska to the klondike district in central yukon these vein systems are structurally controlled by major fault zones which are often reactivated terrane bounding sutures that formed in orogens built during accretion and subduction of terranes along the continental margin of north america mineralization ages span mid jurassic to early tertiary and encompass much of the evolution of the cordilleran orogen nitrogen contents and 15n values of hydrothermal micas from veins are between 130 and 3 500 ppm and 1 7 to 5 5 per mil respectively these values are consistent with fluids derived from metamorphic dehydration reactions within the phanerozoic accretion subduction complexes which have 15n values of 1 to 6 per mil the 18o values of gold bearing vein quartz from different locations in the cordillera are between 14 6 and 22 2 per mil but are uniform for individual vein systems the d values of hydrothermal micas are between 110 and 60 per mil ore fluids have calculated 18o values of 8 to 16 per mil and d values of 65 to 10 per mil at an estimated temperature of 300 c d values of ore fluids do not show any latitudinal control these results indicate a deep crustal source for the ore forming fluids most likely of metamorphic origin low dh2o values of 120 to 130 per mil for a hydrous muscovite from the sheba vein in the klondike district reflect secondary exchange between recrystallizing mica and meteoric waters collectively the n h and o isotope compositions of ore related hydrothermal minerals indicate that the formation of these gold bearing veins involved dilute aqueous carbonic and nitrogen bearing fluids that were generated from metamorphic dehydration reactions at deep crustal levels these data are not consistent with either mantle derived fluids or granitoid related magmatic fluids nor do they support a model involving deeply circulated meteoric water

a lithologic tectonic framework for the metallogenic provinces of california the lithologic tectonic framework of california developed principally during mesozoic time when various terranes of oceanic crust and island arc crust were accreted to older sialic crust resulting in westward growth of the continent emplacement of great batholithic masses of granitoid rocks cutting all these crustal types also took place during the mesozoic period the discrete tectonostratigraphic terranes that resulted from these events and subsequent tertiary and quaternary volcanic events are characterized by specific types of metallic mineral deposits or in some terranes by the virtual absence of deposits lead silver zinc replacement type deposits are common in the paleozoic carbonate terrane in the eastern part of the state and occur sporadically elsewhere in the miogeoclinal and cratonal terranes but are absent from the oceanic and island arc terranes the vast majority of contact metasomatic tungsten deposits including all the large ones are in pendants of miogeoclinal rocks in the sierra nevada batholith but the important atolia deposits reside in granitoid rocks that invade oceanic terrane molybdenum distribution closely follows that of tungsten all the large contact metasomatic iron deposits in california are in craton and miogeoclinal terranes but sparse small deposits of this type also occur in island arc terranes of the northern sierra nevada and eastern klamath mountains lode gold deposits although widely scattered show a marked preference for oceanic and island arc terranes that have been invaded by granitoid plutons all the major deposits including late tertiary bonanza deposits such as bodie are in such terranes it appears that magmatic processes were responsible for mobilizing and transporting the gold but the metal was perhaps derived from the eugeosynclinal rocks notably the mafic volcanics most mercury deposits are found in the coast ranges where they commonly occur in silica carbonate rock an alteration product of serpentinite the deposits appear to be spatially related to the coast range thrust and the source of the mercury may have been sedimentary rocks of the underlying franciscan assemblage epigenetic mineralization occurred at several different times during the mesozoic and again during miocene and pliocene time the timing of mineralization events and the distribution of various deposit types indicate that no broad scale zoning of epigenetic deposits exists around the sierra nevada batholith syngenetic deposits are represented mainly by massive sulfides chert associated manganese and chromite the massive sulfide deposits with one exception are restricted to island arc terranes and nearly all of these deposits are in silicic volcanic rocks they are interpreted to be syngenetic with the enclosing rocks although some redistribution of metals may have occurred after the original deposition the deposits occur in volcanic sequences of at least five different ages ranging from early devonian to late jurassic or early cretaceous and along with their enclosing rocks were probably formed at some distance from their present sites chert associated manganese deposits occur mainly in exotic blocks of oceanic crust in melange and probably formed in fairly deep ocean environments chromite is confined to ultramafic rock much of which occupies suture zones separating various accreted terranes

significance of microbialites calcimicrobes and calcareous algae in reefal framework formation from the silurian of gotland sweden abstract the structure of stromatoporoid tabulate coral reefs from the mid to upper silurian of gotland includes complex microbialite and algal framework fabric non skeletal microbialites show variable fabrics structures and morphologies ranging from largely non laminated peloidal irregular masses to well laminated stromatolite crusts indicated by convex upwardly oriented overgrowth structures trichome like arrangement of peloids and intergrowth with skeletal calcimicrobes and metazoans non skeletal microbialites contribute to framework formation during different stages in the hogklint reef development as well as in the marginal or flanking beds of stromatoporoid coral patch reefs in the tofta formation in addition to non skeletal microbialites calcimicrobes mainly rothpletzella and coralline algae graticula gotlandica substantially contribute to framework formation in reefs from the hogklint and the lower hamra formation depending on the occurrence pattern and abundance of microbialites calcimicrobes and calcareous algae four framework types can be differentiated based on the environmental setting type 1 a tabular stromatoporoid and non skeletal microbialite framework characteristic for lower and intermediate parts of the hogklint reefs developed in slightly deeper lower energy fully marine conditions type 2 a graticulacean microbial calcimicrobial framework typically developed in the upper and top parts of the hogklint reefs marking shallow moderate to highly agitated water type 3 a stromatolite coenites framework being part of the reef development in the tofta formation stromatolites formed in sheltered partly restricted parts of stromatoporoid coral patch reef complexes type 4 a stromatoporoid calcimicrobial framework forming relatively small reef bodies in the lower hamra formation situated in a highly agitated very shallow open marine environment these examples indicate that microbial crusts calcimicrobes and graticulacean algae were more important contributors to framework formation in silurian metazoan reefs than previously thought

carbonate sulfide equilibria and stratabound disseminated epigenetic gold mineralization a proposal based on examples from alleghany california u s a abstract stratabound disseminated pyritic au ore bodies were produced by reactions between wall rocks and through flowing fluids in mesozoic epigenetic au quartz vein systems in the sierra nevada metamorphic belt equilibrium relations among fe bearing carbonate and sulfide minerals were critical in determining which rock types were likely to host disseminated mineralization along portions of discordant veins the compositions of metasomatic carbonates in hydrothermally altered wall rocks at alleghany california u s a were larely predetermined by the relative proportions of fe mg and ca in the unaltered wall rocks thus coexisting solid solutions in the magnesite siderite and dolomite ankerite series from a variety of different wall rocks yield an empirical phase diagram for a large part of the ca co 3 mgco 3 feco 3 system at the temperature of metasomatism 325 50 c because fe mg silicates were unstable in alteration zones adjacent to the veins wall rock fe was partitioned between carbonates and sulfides pyritization and disseminated au mineralization occur in a variety of igneous and metasedimentary wall rocks in which the initial molar fe fe mg 0 5 in altered wall rocks with initial molar fe fe mg 0 5 fe was incorporated almost entirely within mg rich carbonates x feco 3 0 6 in magnesite siderite solutions it is proposed that the co 2 rich vein fluid responsible for the alteration and mineralization was partially buffered with respect to h 2 s co 2 h 2 ratios by equilibrium between pyrite and mg 0 4 fe 0 6 co 3 graphite as it traversed and altered intermediate volcanic and sedimentary rocks this fluid then locally reacted with lower fe fe mg rocks to form fe bearing dolomite magnesite assemblages and reacted with higher fe fe mg rocks to form ankerite pyrite assemblages gold precipitated from saturated solutions of bisulfide complexes partly in response to fluid desulfidation and reduction caused by the pyritization reactions in terranes dominated by intermediate metavolcanic and metasedimentary rocks favorable host rocks for this type of mineralization need not have high fe contents but do require high fe fe mg ratios they may include felsic volcanic and plutonic rocks fe rich tholeiitic differentiates banded fe formations and a variety of siliceous and argillaceous sedimentary rocks rocks which tend not to be heavily sulfidized because they have low initial fe fe mg ratios include ultramafic and mafic igneous rocks and some argillaceous sedimentary rocks exploration guidelines based on these principles may be useful elsewhere in the sierra nevada and in other comparable heterogeneous metamorphic terranes if modified to reflect the dominant buffering rock types in a given fluid flow path carbonate sulfide equilibria are capable of approximately buffering the carbonate sulfide ratios of co 2 rich vein fluids f co 2 10 2 8 at 325 c 200mpa or 2000 bar the alleghany fluid f co 2 10 3 2 or 10 mol co 2 had a molar co 2 h 2 s ratio of approximately 10 3 assuming graphite saturation at lower co 2 fugacities fe bearing silicates entered the buffering assemblages carbonatization reactions could potentially de sulfidize some wall rocks releasing s and associated metals to the fluid this would be most likely to occur in pyrite bearing mafic and ultramafic rocks and some argillites

timing of pre antler deformation in the shoo fly complex sierra nevada california the shoo fly complex between quartz hill and fall creek mountain contains paleozoic folds and probable thrust faults that predate the intrusion of the bowman lake batholith u pb analyses from six zircon fractions show that the youngest phase of the batholith is 409 16 m y old previous suggestions that paleozoic deformation in the shoo fly complex is an early expression of the late devonian early mississippian antler orogeny are now viewed as less likely because the antler orogeny postdated deformation of the shoo fly by at least 20 m y

palaeokarst tidal erosion surfaces and stromatolites in the silurian eke formation of gotland sweden irregular scalloped erosion surfaces in the shelf carbonate sequences of the lower eke formation ludlow series upper silurian in the east of gotland sweden comprise series of shallow hollows separated by sharp crested ridges and cavities with sculptured undercut walls cut into lithified sediment these represent analogues of the solution basins formed in modern coastal and subaerial karst terrains discrete erosional cavities merge on enlargement by breaching of the intervening walls to leave remnant tapering ridges sets of the ridges and basins seen in surface view show an average width of basins of 1 2 m with relief of 40 50 cm and pronounced n s axes for the ridges this alignment may reflect the local drainage direction the lowermost erosion surface passes laterally into a planar mineralized horizon at the top of the underlying hemse group that was resistant to and forms the base level of erosion because of restricted exposure of higher eke formation sediments the upper limit of erosion remains unknown there is no evidence of caliche or subaerial diagenetic textures but solution vugs are common in the eroded limestones marine hard bottom biota attached to some surfaces and transition from scalloped to planar surfaces indicate erosion in tidal zones but subaerial karstic erosion is also inferred the resubmerged karst eroded topography is overlain by shallow marine carbonates including small organic buildups finely layered stromatolitic mats developed over the initial infill in subtidal environments and grew to form domed mounds within the erosional cavities they abut sharply against bounding side walls and overhangs some emergence is evident from desiccation features in the upper parts of mounds biostratigraphical evidence dating the events from initial uplift and karstic erosion to covering of the drowned relief topography places the whole sequence within upper ludlow times

gold quartz veins and auriferous granite at the oriental mine alleghany district california in the vicinity of serpentinite wall rocks the three fissure veins at the oriental mine alleghany district california contain small high grade ore shoots carrying hundreds of troy ounces of gold per ton in addition minable zones of albitized wall rock granite ore contain an average of 0 2 ounces of gold per ton in the veins most native gold is associated with arsenopyrite but in the granite it occurs as microscopic inclusions within the pyrite various geothermometers and paragenetic relations suggest that vein gold deposition occurred during the end stages of mineralization at temperatures between approximately 200 degrees and 300 degrees c at pressures between 670 and 2 500 bars and at depths exceeding 2 5 km during the early stages of mineralization when most hydrothermal alteration and gold deposition in granite probably occurred temperatures were above 340 degrees c and possibly as high as 491 degrees c muscovite paragonite calcite dolomite dawsonite and albite fluid inclusion daughter minerals have been identified in situ by refractive index measurements and by x ray powder diffraction the near neutral to alkaline character of the mineralizing fluids implied by the presence of 0 34 m dissolved dawsonite and the low equivalent nacl contents of fluid inclusions suggest that gold may have been complexed with sulfur or arsenic rather than occurring as chloride complexes wall rock alteration effects include envelopes of intense carbonatization and desilication surrounding the principal gold quartz veins but also occurring along splits and fractures emanating from the main vein structures for example in granite the main alteration effects dissolution of original quartz and formation of albite occur along nearly flat lying fractures chiefly in the footwall of the oriental vein chemical analyses suggest that desilication of granite and removal of silica during ankeritization of mafic minerals in the other rock types liberated sufficient amounts of silica to form the vein quartz at the oriental mine beyond the fringes of the main alteration envelopes is an outer essentially pervasive alteration zone characterized by sericitization and chloritization which grades into greenschist facies country rocks and may predate mineralization gold in high grade ore shoots may have been precipitated by reactions involving reducing fluids derived from the wall rock serpentinite near which all high grade vein ores occur limited fluid inclusion data for ore shoot quartz however seem to be consistent with the alternative possibility of temperature controls for ore deposition

a stepped karst unconformity as an early silurian rocky shoreline in guizhou province south china abstract where succeeded by marine strata karst unconformities signify a former rocky coastline such relationships may help sort out relative sea level changes and aspects of local geography controlling facies distribution an exceptional example of an early silurian karst shore is well exposed near the village of wudang in central guizhou province near the capital city of guiyang in south china here the lower silurian kaochaitien formation oversteps 63 m of paleotopographic relief in limestones belonging to the llanvirn guniutan formation and caradoc to early ashgill huanghuachong formation ordovician the corresponding rise in sea level took place coeval with tectonic uplift as confirmed by a regionally diachronous relationship in the ordovician silurian boundary across a 250 km track from central to northern guizhou province the change in sea level also fits with a global rise of sea level in late aeronian later llandovery early silurian time borings of the ichnofossil trypanites are reported from the karst surface of the huanghuachong formation and silurian strata fill sink holes in this unit over 5 m deep the silurian karst shoreline near wudang is integrated with other regional data to construct a paleogeographic map covering the northern half of guizhou province

uranium lead isotopic ages from the sierra nevada batholith california this study provides new information on the timing and distribution of mesozoic magmatic events in the sierra nevada batholithic complex chiefly between 36 and 37 n latitude u pb ages have been determined for 133 zircon and 7 sphene separates from 82 samples of granitoid rocks granitoid rocks in this area range in age from 217 to 80 m y triassic intrusions are restricted to the east side of the batholith jurassic plutons occur south of the triassic plutons east of the sierra nevada as isolated masses within the cretaceous batholith and in the western foothills of the range cretaceous plutons form a continuous belt along the axis of the batholith and occur as isolated masses east of the sierra nevada no granitic intrusions were emplaced for 37 m y east of the sierra nevada following the end of jurassic plutonism however following emplacement of the eastern jurassic granitoids regional extension produced a fracture system at least 350 km long into which the dominantly mafic calc alkalic independence dike swarm was intruded 148 m y ago the dike fractures probably represents a period of regional crustal extension caused by a redistribution of the regional stress pattern accompanying the nevadan orogeny intrusion of cretaceous granitic plutons began in large volume about 120 m y ago in the western sierra nevada and migrated steadily eastward for 40 m y at a rate of 2 7 mm y this slow and constant migration indicates remarkably uniform conditions of subduction with perhaps downward migration of parent magma generation or a slight flattening of the subduction zone such steady conditions could be necessary for the production of large batholithic complexes such as the sierra nevada the abrupt termination of plutonism 80 m y ago may have resulted from an increased rate of convergence of the american and eastern pacific plates and dramatic flattening of the subduction zone u pb ages of the giant forest alaskite sequence in sequoia national park are all in the range 99 3 m y indicating a relatively short period of emplacement and cooling for this nested group of plutons u pb ages of a mafic inclusion and its host granodiorite indicate that both were derived from a common source or that the mafic inclusion was totally equilibrated with the granodioritic magma comparison of isotopic ages determined by different methods such as zircon u pb sphene u pb hornblende k ar and biotite k ar suggests that zircon u pb ages generally approximate the emplacement age of a pluton however some plutons probably contain inherited or entrained old zircons and the zircons of some samples are disturbed by younger thermal and metamorphic events the ages reported here are consistent with u pb age determinations previously made on granitic rocks to the north stern et al 1981 the age distribution of granitic belts determined here is in general agreement with those established by k ar dating evernden and kistler 1970 but does not differentiate the five epochs of plutonism determined in their study

stable isotope o h s c and n systematics of quartz vein systems in the turbidite hosted central and north deborah gold deposits of the bendigo gold field central victoria australia constraints on the origin of ore forming fluids the central and north deborah gold deposits are located in one of 15 mining centers of the bendigo gold field central victoria australia a major world class gold province developed in a turbidite terrane gold mineralization occurs primarily within domes and reverse fault systems in multiply deformed lower ordovician metaturbidites deformation and thrusting occurred between 450 and 420 ma coevally with regional metamorphism at zeolite to greenschist facies in the central and north deborah deposits six stages of quartz veining each with characteristic geometry and mineral paragenesis developed sequentially stages 1 2 and 4 are gold bearing whereas stages 3 5 and 6 are barren gold related quartz veins were deposited at temperatures of 350 25 c based on oxygen isotope geothermometry of silicate phases quartz muscovite and sulfur isotope compositions of coexisting sulfide mineral pairs sphalerite galena this result is consistent with temperatures inferred from fluid inclusions the 18o values of gold bearing vein quartz and coexisting hydrothermal muscovite are uniform within and between the six vein stages ranging from 15 9 to 17 2 per mil and 11 6 to 12 8 per mil respectively the d values of hydrothermal muscovites are also uniform at 68 to 55 per mil ore fluids have calculated 18o values of 8 to 11 per mil and d values of 37 to 17 per mil pyrite of synsedimentary diagenetic origin in slate and sandstone displays sulfur isotope compositions of 23 5 to 8 0 per mil and 0 1 to 11 9 per mil respectively sulfides from gold bearing quartz veins have a range of 7 4 to 8 1 per mil median ca 2 5 the relatively uniform vein pyrites indicate a large uniform sulfur source most likely from desulfidation reactions of sulfide minerals interaction of the ore forming fluid with depleted sulfides at the site of deposition may have shifted the sulfur isotope signatures of source sulfides to more negative values carbon isotope compositions of carbonates range from 5 1 to 9 0 per mil median ca 6 5 nitrogen concentrations and nitrogen isotope compositions of hydrothermal muscovite from quartz veins are 652 to 895 ppm and 2 84 to 4 49 per mil respectively collectively the data define a band in n content vegh rsus 15n coordinate space from higher n and lower 15n in the phanerozoic metasedimenary hosted central and north deborah gold deposits to lower n and higher 15n in predominantly meta igneous hosted neoarchean gold provinces collectively the h o s c and n isotope compositions of ore related hydrothermal minerals indicate that the dilute aqueous carbonic and n bearing ore forming fluids were generated from metamorphic dehydration reactions at deeper crustal levels and were variably influenced by exchange with wall rocks during transport to the site of deposition this genetic model for the central and north deborah gold deposits is consistent with other turbidite hosted lode gold deposits and counterparts in volcanic plutonic terranes in contrast these data are not consistent with either mantle derived fluids or magmatic fluids nor do they support a deeply circulated meteoric surface water model for vein formation

tectonics of the northern canadian cordillera imaged using modern magnetotelluric analysis abstract magnetotelluric data from lithoprobe snorcle corridor 2 in western canada crossing the rocky mountain trench tintina fault between 56 and 60 n have been analyzed using modern mt methods to provide new insights into the tectonics of the north american cordillera and ancestral north america resistivity images of the crust were obtained using two independent sets of unconstrained 2 d inversions and constrained 2 d inversions incorporating seismic constraints structure to the west of the tintina fault includes uniform moderate mid to lower crustal resistivity in a seismically defined westward tapering wedge of ancestral north american rocks to the east between the tintina fault and the liard line a transfer zone in asymmetric neoproterozoic rifting the wedge includes a major synformal conductor that parallels seismic reflections the large lateral changes in resistivity can be explained by transition between different blocks of asymmetric rifting the synformal conductor is attributed to graphitic rocks from the aida formation of the mesoproterozoic muskwa assemblage that were re distributed in a sag basin on the lower plate margin in the blackwater block spatially complex upper crustal resistivity in the cordillera can be interpreted in terms of lithological variations and alteration processes an east dipping conductor in stikinia is attributed to east dipping jurassic and relatively high conductivity beneath the eskay rift may be explained by crustal fertilization a conductor in the cache creek terrane is attributed to listwanite alteration of ultramafic bodies on the king salmon fault and high conductivity in the slide mountain terrane within the sylvester allochthon to localized carbonaceous alteration a series of conductors above north american basement is attributed to deformation and metamorphic processes associated with mesozoic thrust faulting to the west of the tintina fault the source material for these conductors is likely from the stikinia terrane and to the east it is likely from the muskwa assemblage

uniformitarianism as a guide to rocky shore ecosystems in the geological record literature on pleistocene and neogene rocky shores is reviewed from a uniformitarian perspective to assess the fidelity of physical and biological information entrained in the geological record through present day processes coverage by latitude spans tropical to subarctic shores variations in paleotopography are illustrated by unconformities between sedimentary accumulations and rocks of igneous or non igneous origin former rocky shores occur across a range of geographic scales that include i spot localities ii embayments with continuous transition from exposed outer to protected inner shores and iii entire islands with windward and leeward shores differences in rate and mag nitude of sea level change affect rocky shore physiography terrace deposits characterize the pleistocene but ramp de posits typify older neogene shores on average species with hard parts account for about 50 of the modern rocky intertidal fauna on a regional basis regardless of latitude biodiversity is highest in mid latitudes fixed organisms with encrusting wedging or boring habits corals barnacles oysters other bivalves and vermetid gastropods show high potential for fossilization in growth position mobile organisms that cling to rocks gastropods crabs and echinoids suffer post mortem transport but may be immured within neptunian dikes or interstices among cobbles and boulders at best localized fossil deposits from the pleistocene reflect 30 of the biodiversity of skeletonized organisms tabulated from modern rocky shores on a regional basis uniformitarianism provides little insight into the diversity of soft bodied organisms excluding stromatolites that joined the ecosystem in precambrian times and the extent to which soft bodied organisms dominated shore life through later times

crustal evolution of the eastern canadian cordillera the cordillera is not like the himalayas the himalayas are supported by the indian continent that was thrust under the asian continent the eastern cordillera is supported not by imbricated continents but by imbricated continental margin of the ancient western north america that ancient margin was compressed and imbricated as it was confined between the north american craton and the minicontinent of stikinia in the eastern canadian cordillera tectonic thickening is in the collision zone and not on either side of it as for example in the tibetan plateau where the underthrust indian continent has duplicated the thickness of the crust in the eastern canadian cordillera jurassic and early cretaceous thrusts and faults are directed away from the collision zone toward the converging blocks the oppositely directed thrust faults outline wedge shaped sheets of rock sedimentary and volcanic rocks cover the crystalline rocks on which they were deposited basement and the underlying upper mantle may have been detached from each other during the compression the basement therefore would have imbricated internally between the cover and the mantle as a duplex structure confinement of the ancient margin s basement between the colliding blocks requires that the volume of basement under the margin be the same as that presently between the colliding blocks the possibility of such an equality has been tested and confirmed in cross section the cross section of the ancient western margin from that test has many features comparable to cross sections recently drawn through the eastern continental margin of canada

petrography and environmental controls on the formation of phanerozoic marine carbonate hardgrounds early marine seafloor lithification of carbonate sediments leads to the formation of hardgrounds and is known from rocks as old as the proterozoic hardground surfaces however are more commonly recorded in the phanerozoic sedimentological archive while ecological studies of hardground biota abound the environmental and physico chemical parameters leading to the development of seafloor lithification remain in many cases poorly understood and documented this paper reviews published evidence on the petrography mineralogy and geochemistry of phanerozoic carbonate hardgrounds within a process oriented context of their environmental controls hardgrounds typically develop in warm and shallow agitated tropical waters that are saturated with respect to caco3 but are also reported from hemi pelagic to bathyal realms and from cool water temperate settings a range of early marine cement types are documented from present day hardgrounds whereas preserved cement fabrics and related mineralogies are less diverse in early phanerozoic hardgrounds carbonate hardgrounds are widespread during calcite sea periods e g ordovician and cretaceous as opposed to some preservation bias ancient aragonite seas e g permian and triassic obviously the relation between seawater chemistry and hardground abundance and attributes is not an easy one here the concept of aragonite versus calcite seas serves as a first order template for the sake of a structured discussion whereas modern aragonite mode oceans document a significant diversity in spatial and bathymetric seawater properties this spatio temporal complexity is perhaps reflected in the scarce record of calcite sea hardgrounds from the devonian or paleogene rock record contrasted by the abundance in aragonite holocene and recent seas holocene hardgrounds allow for the direct assessment of rates of and reasons for early subaquatic lithification and in most cases escaped subsequent non marine diagenetic overprint conversely studies of ancient hardgrounds are often biased by early biological and mechanical erosion the degree of diagenetic overprint and tectonic to orogenic processes the formation distribution and physical properties of hardgrounds depend on the global climatic context greenhouse icehouse modes on sea level changes on ocean stratification on the spatial extent of epicontinental seas and carbonate depositional environments and on the spatially different mineralogies and rates of carbonate production the fabrics and mineralogies of early hardgrounds cement seem to substantially hinge on variations in atmospheric co2 thus the application of lessons from modern to ancient hardground case examples is not straightforward a holistic model explaining the full variability of controls on submarine hardground formation and their cement petrography throughout the phanerozoic is as yet lacking a more critical view on secular changes in hardground formation patterns is clearly needed future work should rely on the combination of sedimentological stratigraphic palaeoecological petrographic and geochemical approaches specific emphasis should be given to time intervals triassic carboniferous cambrian etc where the bulk of reported case studies is at best scarce

evidence for rapid environmental changes in low latitudes during the late silurian lau event the burgen 1 drillcore gotland sweden erosional outliers are an important component of the ludlow late silurian stratigraphy of gotland sweden however due to the poor degree of exposure outcrop studies have not revealed a detailed section from any of these outliers the first complete stratigraphy of such an erosional remnant the burgen outlier is presented here based on the recently recovered burgen 1 drillcore stratigraphical units encountered are from oldest to youngest the uppermost hemse group 35 m eke formation 0 4 m burgsvik formation 3 4 m and the basal hamra formation 12 in preserved based on carbon stable isotopes and lithological correlation to nearby outcrops that are zoned by conodonts it can be concluded that the most profound late ludlow sedimentary changes took place shortly after the onset of the globally recognized lau event delta 13 c values reach 8 7 parts per thousand in the lower hamra formation sedimentary changes include development of discontinuity surfaces widespread occurrence of an intraclastic conglomerate increased microbial activity rothpletzella and wetheredella and influx of clastics to the basin these changes are linked to substantial base level changes during the event a general comparison with the prague basin suggests contemporaneous sedimentary changes on baltica and on cratonic elements from peri gondwana during the event

geology mineral equilibria and isotopic studies of the mcdame tungsten skarn prospect north central british columbia the mcdame prospect occurs in hadrynian to ordovician metasediments near cretaceous felsic stocks three prograde skarn and ore facies and one retrograde facies are lithologically and structurally controlled lithostatic pressure during formation of mcdame skarns was on the order of 1 500 bars temperature t and co 2 mole fraction x conditions of the metasomatic fluid are estimated for the different skarn facies by assuming an idealized iron free system sulfur isotope data from mcdame porphyry skarn and hornfels form six discrete groups skarn average delta 34 s 7 7 between ordinary magmatic sulfur delta 34 s 0 and cambrian sedimentary sulfate delta 34 s 30 rubidium strontium isotopes from kuhn stock quartz feldspar porphyry define a 69 or 2 m y isochron with a high initial ratio of 0 712 indicating that the kuhn stock had a sialic crustal component lead isotope data from vein and skarn deposits near cassiar cluster around the upper crustal shale curve for the canadian cordillera indicating an upper crustal source for the lead thus anatexis and assimilation of continental crust produced granitic melts rich in lithophile elements the kuhn stock is an oxidized granitoid of sedimentary origin geochemically specialized in k 2 o k rb u and u th and anomalous in w and mo differentiation processes have concentrated these lithophile elements in magmatic fields that produced the mcdame tungsten skarn deposit modified journal abstract

differential preservation of primary isotopic signatures in silurian brachiopods from northern europe abstract isotopic and geochemical compositions of silurian calcitic brachiopod shells carbonate host rocks and calcite cements from two northern european sites with different diagenetic histories are compared gotland sweden oslo graben norway the sequence exposed on gotland is characterized by repeated synsedimentary subaerial exposure and meteoric influence but only insignificant thermal alteration during shallow burial diagenesis in contrast silurian sediments exposed in the oslo area show only few signs of early exposure but were severely affected by thermal alteration during burial of the succession the preservation potential of brachiopod shells in each basin is different most nonluminescent brachiopods from gotland show well preserved microstructures and trace element contents very similar to those of modern brachiopod shells tightly clustered isotope values of individual brachiopod populations do not suggest significant meteoric alteration of primary marine isotope signatures in contrast there is strong evidence that all brachiopod shells from the oslo area contain diagenetically altered 18o values alteration of norwegian brachiopods is not always reflected in their trace element contents and cathodoluminescence characteristics but under sem the shells often display corrosion and recrystallization scattered oxygen isotope values in nonluminescent brachiopods 18o 18 to 5 5o oo together with low 18o values the case study from the oslo graben demonstrates that high fluid rock ratios and thermal overprinting during burial diagenesis may result in pervasive diagenetic alteration of 18o values in nonluminescent brachiopod shells this alteration can be detected by combined petrographic and geochemical investigations on brachiopod shells diagenetic cements and carbonate host rocks brachiopods from deeply buried and hydrothermally altered sites should not be used for deciphering marine 18o signals however the data presented here argue against a general rejection of lower paleozoic brachiopods as indicators of marine isotope signatures

palaeoecology and evolution of marine hard substrate communities abstract marine organisms have occupied hard substrates since the archaean shells rocks wood and sedimentary hardgrounds offer relatively stable habitats compared to unconsolidated sediments but the plants and animals which inhabit them must develop means to gain and defend this premium attachment space hard substrate communities are formed by organisms with a variety of strategies for adhering to and or excavating the substrates they inhabit while mobile grazers organically attached and even soft bodied organisms may leave evidence of their former presence in ancient hard substrate communities a superior fossil record is left by sessile encrusters with mineralised skeletons and by borers which leave trace fossils furthermore encrusters and borers are preserved in situ retaining their spatial relationships to one another and to the substrate spatial competition ecological succession oriented growth and differential utilisation of exposed vs hidden substrate surfaces can all be observed or inferred hard substrate communities are thus excellent systems with which to study community evolution over hundreds of millions of years here we review the research on modern and ancient hard substrate communities and point to some changes that have affected them over geological time scales such changes include a general increase in bioerosion of hard substrates particularly carbonate surfaces through the phanerozoic this is at least in part analogous to the infaunalisation trends seen in soft substrate communities encrusting forms show an increase in skeletalisation from the palaeozoic into the mesozoic and cenozoic which may be a response to increasing levels of predation hard substrate communities considering borers and encrusters together show a rough increase in tiering through the phanerozoic which again parallels trends seen in soft substrate communities this extensive review of the literature on living and fossil hard substrate organisms shows that many opportunities remain for large scale studies of trends through time at the community and clade levels palaeontologists will especially benefit by closer integration of their work with that of neontologists particularly in aspects of ecology such as larval recruitment competition and succession

assessing mechanisms of environmental change palynological signals across the late ludlow silurian positive isotope excursion δ13c δ18o on gotland sweden abstract the silurian is characterised by strong environmental changes as indicated by several pronounced positive 13c and 18o excursions the mechanisms responsible for these isotopic shifts are a matter of much debate the purpose of the present study is a quantitative high resolution evaluation of the palynomorph distribution across the late ludlow ludfordian isotope excursion on gotland marine and terrestrial palynomorphs have been chosen as palaeoenvironmental indicators because they are widely distributed and exceptionally well preserved moreover the combined analysis of marine and terrestrial palynomorphs allows the reconstruction of contemporaneous palaeoenvironmental change in the marine and terrestrial realms to correlate our results with the carbon isotope stratigraphy we have sampled only localities with published isotope data from diagenetically unaltered brachiopod shells our results show that fluctuations in the composition of the acritarch assemblages are closely correlated with the stable isotope development low abundances of acritarchs occur in times of high stable isotope values and vice versa indicating that an increase in marine productivity cannot have been the reason for the positive 13c excursion the results are in good agreement with climatic models for the silurian assuming alternating humid and arid climatic conditions in low latitudes times of high stable isotope values correspond to arid climatic conditions in low latitudes with low input of terrestrial nutrients resulting in impoverished acritarch and conodont communities whereas times of low stable isotope values correlate with humid climate high nutrient input and abundant and diverse acritarch communities the major change in acritarch communities took place during the increase of the stable isotope values after hemipelagic planktonic or nektonic organisms graptolites conodonts have been affected this indicates that environmental changes connected with the stable isotope excursion first affected deeper water settings and later the photic zone the absolute abundance of terrestrial spores closely mirrors the marine 13c and 18o development showing that both the marine and the terrestrial realm were synchronously affected by the climatic changes the high abundance of spores during the isotope excursion might be explained by increased aeolian input

ordovician intracratonic sediments from the lac st jean and chicoutimi areas quebec eastern canada the transgressive sequence of the ordovician intracratonic basin of the present day lac st jean and chicoutimi areas is characterized by a variety of sediments that were deposited in a number of en

δ13c records across the late silurian lau event new data from middle palaeo latitudes of northern peri gondwana prague basin czech republic abstract during the late silurian the prague basin was located in middle southern latitudes in contrast to palaeocontinents positioned in tropical and subtropical latitudes like baltica no reefs are developed which is in accordance with the predicted cooler water the prague basin represents a relatively restricted and shallow rift basin with a complex tectonic history sections in different palaeoenvironments have been studied to document the most prominent silurian stable carbon isotope excursion recorded during the late silurian ludfordian lau event from this part of peri gondwana deeper water deposits of the kopanina formation investigated in the present study were deposited on the slope to basin transition near the kosov volcanic centre in the western part of prague basin the sediments are developed as an alternation of dark partly laminated limestones and marls with an increase of the limestone marl ratio in the upper part of the succession a pronounced positive carbon isotope excursion starts in the neocullograptus kozlowskii graptolite and in the upper polygnathoides siluricus conodont zone the maximum of the shift is observed in the lower part of an interval characterised by the ananaspis fecunda cyrthia postera community the maximum values scatter around 8 which represent the highest values reported hitherto from the prague basin in low latitudes often a decrease of 13c values towards deeper water settings is reported in contrast in the present study the 13c values of about 8 are much higher than those recorded from the contemporaneous shallow water sections studied in the classical muslovka and po ary quarries the most reasonable explanation is the presence of stratigraphical gaps in the shallow parts of the basin as indicated by karstification these gaps were caused by a sea level drop another effect of this sea level fall was a strongly reduced sedimentation of the cephalopod limestone facies around volcanic and tectonic elevations

polygenetic polyphase karsted hardground omission surfaces in lower silurian neritic limestones anticosti island eastern canada abstract exquisitely preserved and well exposed rocky paleoshoreline omission surfaces in lower silurian chicotte formation limestones on anticosti island quebec are interpreted to be the product of combined marine and meteoric diagenesis the different omission features include 1 planar erosional bedding tops 2 scalloped erosional surfaces 3 knobs ridges and swales at bedding contacts and 4 paleoscarps an interpretation is proposed that relates specific styles of omission surface to different diagenetic depositional processes that acted in separate terrestrial peritidal shallow neritic zones such processes were linked to fluctuations in relative sea level with specific zones of diagenesis such as 1 karst corrosion 2 peritidal shallow subtidal erosion 3 subtidal seawater flushing and cementation and 4 shallow subtidal deposition most surfaces are interpreted to have been the result of initial extensive shallow water synsedimentary lithification forming hardgrounds that were as sea level fell planed by inner neritic abrasion successively altered by exposure and subaerial corrosion only to be buried by sediments as sea level rose again this succession was repeated several times resulting in a suite of recurring polyphase omission surfaces through many meters of stratigraphic section synsedimentary cloudy marine cements are well preserved and are thus interpreted to have been calcitic originally aragonite components are rare and thought to have been dissolved just below the silurian seafloor large mollusks that survived such seafloor removal were nonetheless leached and the resultant megamolds were filled with synsedimentary calcite cement these silurian inner neritic strandline omission surfaces are temporally unique they are part of a suite of marine omission surfaces that are mostly found in early to middle paleozoic neritic carbonate sedimentary rocks these karsted hardgrounds formed during a calcite sea time of elevated marine carbonate saturation and extensive precipitation of marine cement the contemporaneous greenhouse atmosphere was supercharged with co2 leading to profound surface karst under strongly acid rain younger peritidal omission surfaces although potentially formed during aragonite sea or calcite sea times would have been subject to very different terrestrial diagenetic process with lower atmospheric pco2 values but increasingly complex biogenic soils producing dissimilar alteration features

stable isotope systematics in mesozoic granites of central and northern california and southwestern oregon 18o d and h2o contents were measured for whole rock specimens of granitoid rocks from 131 localitics in california and southwestern oregon with 41 new determinations in the klamath mountains and sierra nevada initial strontium isotope ratios are known for 104 of these samples large variations in 18o 5 5 to 12 4 d 130 to 31 water contents 0 14 to 2 23 weight percent and initial strontium isotope ratios 0 7028 to 0 7095 suggest a variety of source materials and identify rocks modified by secondary processes regular patterns of variation in each isotopic ratio exist over large geographical regions but correlations between the ratios are generally absent except in restricted areas for example the regular decrease in d values from west to east in the sierra nevada batholith is not correlative with a quite complex pattern of 18o values implying that different processes were responsible for the isotopic variations in these two elements in marked contrast to a good correlation between 87sr 86sr o and 18o observed in the peninsular ranges batholith to the south such correlations are lacking except in a few areas d values on the other hand correlate well with rock types chemistry and 87sr 86sr o except in the coast ranges where few of the isotopic signatures are primary the uniformly low d values of samples from the mojave desert indicate that meteoric water contributed much of the hydrogen to the rocks in that area even so the 18o values and 18o fractionations between quartz and feldspar are normal in these same rocks

a three dimensional ice sheet flow solution an accurate three dimensional reduced model shallow ice approximation flow with velocity depending on all three spatial coordinates is constructed for the commonly adopted isotropic viscous law with temperature dependent rate factor the solution is for steady flow with a prescribed temperature distribution but can be extended to flow with a coupled energy balance and to unsteady flow the accuracy hinges on the reduction to a two point ordinary differential equation problem for the surface profile on an unknown span for which established accurate numerical methods are available this is achieved by setting one horizontal velocity component in elliptic cylindrical coordinates to zero but the other two components depend on all three spatial variables while not of direct physical interest such an exact solution is valuable as a test solution for the large scale numerical codes commonly used in ice sheet modelling which have not yet been subjected to such a comparison

palaeokarst evidence for widespread regression and subaerial exposure in the middle katian upper ordovician of baltoscandia significance for global climate we report on widespread and contemporary palaeokarst in the middle katian upper ordovician of sweden estonia and latvia the first major palaeokarst horizon to be reported from the ordovician of baltoscandia the solution features occurwithin a fewmetres of thick zone of limestone immediately belowthe widespread fjacka shale and are related to an unconformity with preserved palaeorelief in the slandrom limestone in sweden and the time equivalent saunja formation in estonia and latvia facies evidence for a karst origin comes fromseveral outcrops and core sections and includes 1 frequent karren likemorphologies interpreted as swiss cheese karst sensu baceta et al 2001 2 local occurrences of solution collapse breccia 3 presence of bladed pseudospar crystals in solution cavities and 4 carbon isotope values indicating meteoric influence to the succession these findings are herein put in context with previously reported large scale erosional channels that may cut down several tens of metres below the fjacka shale in the subsurface baltic sea area and with regional anomalous thickness variations in the slandrom limestone and saunja formation all together forming strong support for regional exposure of the baltoscandian continent in the middle katian high resolution stable isotopic data show that the regression and lowstand of sea level overlap with thewaynesville carbon isotope excursion it resulted in basin wide cessation of carbonate production near the amorphognathus superbus and a ordovicicus conodont zonal boundary the contemporary development of palaeokarst in different confacies belts of the basin suggests that this was an extraordinary sea level lowstand herein interpreted as reflecting a middle katian glaciation less

formation and significance of a middle silurian ravinement surface on gotland sweden a laterally extensive and conspicuously smooth erosional surface is exposed near the wenlock ludlow boundary on east central gotland sweden it occurs in the reef complex area of a carbonate platform has an undulating topography and separates truncated bioherms and biostromes from overlying allochthonous high energy deposits on a basin regional scale the surface is associated to a shift from a prograding to a retrograding platform and to a substantial hiatus in basin marginal areas estonia the significance of the surface is further indicated by a clear cut truncation of the reef complex including m sized stromatoporoids along a distance of at least 20 km b an erosional relief exceeding 2 08 in c a conspicuously smooth nature in both palaeolows and palaeohighs d present although scarce subaerial diagenetic indications e g shallow karst features at the unconformity surface and pendant meniscus cement in the lowermost part of overlying strata e a locally occurring basal conglomeratic lag in overlying strata and 0 peritidal indications and as evident from at least one quarry onlapping geometry in overlying strata the unconformity has implications for the analysis of the middle silurian baltic basin evolution as well as for the interpretation of erosional surfaces on carbonate platforms in general based on the above characteristics the formation of the unconformity is attributed to a relative sea level fall causing subaerial exposure followed by transgressive abrasion in a rocky shore environment the unconformity hence constitutes a ravinement surface which based on the associated basin regional sedimentary changes is interpreted as coinciding with a regional exposure surface sequence boundary it thus increases our understanding of the hitherto poorly understood palaeogeographic evolution of the middle silurian baltic basin further the transgressive erosion was significant as well as recurrent as indicated by the clear cut truncation of large stromatoporoids at the unconformity surface and by truncated marine fossil fragments in upper edges of karst infills the unconformity therefore constitutes a good example of the capability of transgressive erosion in creating stratigraphic incompleteness and hence in removing subaerial indications on previously exposed carbonate platforms c 2004 elsevier b v all rights reserved less

geochemistry of fluid inclusions from the mcintyre hollinger gold deposit timmins ontario canada type 1 h 2 o co 2 type 2 h 2 o co 2 type 3 ch 4 co 2 h 2 o and type 4 ch 4 fluid inclusions type 1 type 3 and some type 2 inclusions are primary or pseudosecondary in origin whereas type 4 and most type 2 inclusions are secondary the primary or pseudosecondary type 1 and 2 inclusions homogenized at temperatures of 220 degrees to 385 degrees c secondary type 2 inclusions homogenized at 160 degrees to 215 degrees c crushing tests confirm the presence of co 2 in samples with type 1 and 2 inclusions visually estimated x co2 values in type 1 inclusions range from 3 to 24 mole percent with bulk molar volumes of 19 to 28 cm 3 mole mineralizing fluids were at equilibrium with graphite at temperatures of 380 degrees to 480 degrees c and pressures of 1 to 3 kb calculated oxygen fugacities lie near the quartz fayalite magnetite buffer the mineralizing fluids differ compositionally from fluids in other types of ore deposits but resemble fluids derived from metamorphic rocks and deep level felsic intrusions modified journal abstract

carbon and oxygen isotopic composition of silurian brachiopods gotland sweden palaeoceanographic implications abstract one hundred and seventy two non luminescent brachiopod shells from the silurian sequence of gotland were investigated for their carbon and oxygen isotopic composition diagenetic alteration of the shell calcite was excluded by means of petrographic trace element and cathodoluminescence analysis oxygen and carbon isotopic signals vary considerably within the uppermost llandovery to the ludlow average 18o and 13c values of well defined stratigraphic units range from 2 4 to 5 9 and 1 to 8 respectively we observed parallel trends in the carbon and oxygen isotope curves with the most prominent shifts occurring in the lower wenlock upper wenlock and middle ludlow average 18o values 2 4 to 2 9 recorded in the middle ludlow are among the highest 18o values reported for the early palaeozoic the recorded 18o variations cannot solely be explained by changes in sea surface temperature and salinity the variations in the isotopic signals correlate negatively with changes in sea level heavy carbon and oxygen signals coincide with sea level lowstands whereas light signals are recorded during sea level highstands sea level highstands were probably characterized by the formation of warm saline waters on subtropical epicontinental shelves inducing oxygen deficiency in the basins alumshale facies the low 18o values are attributed to 18o depletion in surface waters owing to the storage of 18o enriched saline waters in the basins low 13c values are explained by a low surface water productivity since nutrients were not supplied to the photic zone but rather stored in the deeper ocean during sea level lowstands colder waters that formed in higher latitudes may have represented the main intermediate water source a better ventilation of the basins is documented by the deposition of well aerated sediments whereas the high 18o values are explained by the change in oceanic circulation the high 13c values are attributed to an enhanced primary productivity and possibly to an enhanced burial of corg the model discussed here may provide an attractive scenario for accounting for the recorded short term variations in the isotopic signals without the necessity of introducing an isotopic correction factor for silurian sea water

rocky shoreline deposits in the lower silurian upper llandovery telychian chicotte formation anticosti island quebec the upper llandovery telychian chicotte formation is a regionally extensive crinoid rich unit exposed in the south central part of anticosti island in the gulf of st lawrence the chicotte encrinites represent an inner ramp crinoidal sand shoal complex about 80 m thick that prograded over deeper middle to outer ramp facies of the underlying jupiter formation in response to a long term sea level fall the typical depositional unit in the chicotte formation is a metre scale subtidal cycle indicating that higher frequency sea level changes were also present metre scale cycles are typically characterized by coarsening upward locally cross bedded encrinitic material capped by a sharp erosional surface these bounding surfaces change progressively upsection from simple planar to low relief scalloped erosional surfaces to complex polyphase surfaces formed by distinct but superimposed erosional events three dimensional paleomorphological features with local relief up to 50 m associated with erosional sur

early cretaceous gold silver mineralization in the sylvester allochthon near cassiar north central british columbia k ar dates on sericite from several gold silver bearing white quartz veins in the cassiar area indicate that mineralization occurred in the early cretaceous at about 130 ma thus these veins predate the mid cretaceous cassiar batholith and late cretaceous and early tertiary plutons in the immediate area the early cretaceous date probably represents either a thermal precursor to emplacement of the cassiar batholith or a structurally related event associated with allochthonous emplacement of the sylvester group either of these events may have caused circulation of the meteoric fluids responsible for the veins

timing and structural expression of the nevadan orogeny sierra nevada california the nevadan orogeny was a very short lived event in the late jurassic that involved the deformation of a great variety of rock types and paleozoic and mesozoic terranes throughout the extent of the sierra nevada the nevadan structures show great variation in style but relatively constant orientations these relations can be explained by considering the prior histories of the various terranes slaty cleavages and tight folds are the characteristic main phase structures in the western belt of jurassic island arc volcanic rocks and flysch type sedimentary rocks a strip of phyllites and greenschists along the eastern edge of the belt apparently represents similar jurassic rocks that were deformed and metamorphosed at greater depths probably during underthrusting of the western belt beneath the central belt the central belt of paleozoic metasedimentary and metavolcanic rocks shows the most extreme variation in style of main phase structures from weak spaced to crenulation cleavages in the south where polyphase deformed rocks formed a structural basement to slaty and phyllitic cleavages and asymmetric to isoclinal folds in the north where most of the paleozoic basement rocks lack penetrative pre nevadan fabrics eastward directed thrust faulting apparently was important only in the northern part of the range where main phase deformation was most intense the eastern belt of jurassic and triassic magmatic arc volcanic and sedimentary rocks defines the core of a major synclinorium and the rocks contain penetrative slaty cleavages and asymmetric tight to isoclinal folds a late phase of nevadan structures consisting of northeast trending cleavages and minor folds also shows a marked variation in style from relatively intensely developed in the north to very weakly developed in the south the regional extent and geometry of the nevadan structures indicate that the nevadan orogeny involved underthrusting of island arc rocks on the west and significant crustal shortening in the central and eastern belts these features suggest that the nevadan orogeny resulted from the collision of the island arc western belt with an andean type arc eastern belt situated at the western edge of north america

geological and geochemical controls on the silver content fineness of gold in gold silver deposits abstract a compilation of microprobe fineness data and a re evaluation of bullion production fineness data suggest that the gold fineness model of fisher 1945 can be updated and applied to a wide range of gold mineralised environments the major deposit classes are characterised by the overall average or range of deposits averages and total range of gold fineness values as follows archaean 940 780 1000 slate belt 920 800 1000 plutonic 825 650 970 porphyry 700 1000 650 1000 volcanogenic 650 850 520 870 epithermal 440 1000 0 1000 the archaean including witwatersrand slate belt and plutonic classes are characterised by a high and consistent average fineness and a narrow fineness range there is no clear subdivision within these classes according to deposit style or inferred genesis the porphyry volcanogenic and epithermal classes have a variable average deposit fineness and a wide total range of fineness values within these three classes higher fineness is typical of deposits in andesitic rather than rhyolitic volcanic settings deposits with cu au rather than cu mo or pb zn element associations and deposits with telluride or selenide minerals solubility calculations indicate that in all these environments gold and silver form the same aqueous species with agcl 2 dominating for silver and au hs 2 dominating for gold in all the environments the fluids transport an equal amount of gold and silver in the form of their bisulphide complexes therefore the observed gold fineness values can not be explained by the transporting mechanism only low silver in the ores and higher gold fineness values observed in the archaean plutonic and slate belt environments can be attributed to sulphidation as the dominant mechanism of ore deposition sulphidation of wall rocks destabilises only the bisulphide complexes of gold and silver in contrast epithermal volcanogenic and porphyry environments are characterised by a diversity of ore depositing mechanisms cooling boiling mixing which act alone or in combination and destabilise chloro and bisulphide complexes of gold and silver the high silver concentration in the ores low fineness values and large variation in the fineness values in these environments can be attributed to the complexities in the ore depositing mechanisms

exact solutions to the thermomechanically coupled shallow ice approximation effective tools for verification we describe exact solutions to the thermomechanically coupled shallow ice approximation in three spatial dimensions although artificially constructed these solutions are very useful for testing numerical methods in fact they allow us to verify a finite difference scheme that is to show that the results of our numerical scheme converge to the correct continuum values as the grid is refined in three dimensions comparison of numerical results with exact solutions has helped us to precisely quantify and understand some of the numerical errors we are making our verified numerical scheme shows the basal temperature spokes which arose in the eismint european ice sheet modelling initiative ii intercomparison payne and others 2000 a careful analysis describes these warm spokes as numerical errors which occur when the derivative of the strain heating term with respect to the temperature is large on the other hand the appearance of basal temperature spokes in a verified numerical scheme strongly suggests that they are a feature of the eismint ii experiment f continuum problem in fact they are clear evidence of an unstable equilibrium point of the continuum problem this paper is a sequel to bueler and others 2005 which addresses exact solutions and verification in the isothermal case

mid ludfordian coeval carbon isotope natural gamma ray and magnetic susceptibility excursions in the mielnik ig 1 borehole eastern poland dustiness as a possible link between global climate and the silurian carbon isotope record abstract the graptolite bearing upper silurian succession of the mielnik ig 1 borehole eastern poland represents a periplatform setting on a neritic carbonate platform located on the palaeocontinent of baltica and records widely recorded positive mid ludfordian carbon isotope excursion cie the initial increase of 13 c values occurs in the uppermost part of the neocucullograptus kozlowskii zone whereas pronounced increase and maximal values 6 74 are noted in the n kozlowskii pseudomonoclimacis latilobus interzone pristiograptus dubius bloom interval the thickness time relation in the section indicates that the main initial shift in carbon isotope ratio of 2 7 occurred over a very short time estimated to be around 4000 years and is coeval with a rapid initiation of deposition of rock forming silt sized detrital dolomite and quartz the cie decline is interpreted as steeper than in inner shelf sections and is coincident with a flooding event in the ps latilobus slovinograptus balticus zone whereas the end of the excursion is coeval with the next flooding surface marked by the ps latilobus uncinatograptus acer graptolite assemblage combined sgr spectral gamma ray facies and faunal records during the cie suggest prolonged sea level lowstand conditions the abundant detrital material dolomite siltstone facies occurring exclusively in the cie interval is noted in microfacies as well as in the geophysics record as pronounced negative excursions in total natural gamma ray and magnetic susceptibility both resulting from dilution by diamagnetic and low radioactivity minerals the perfectly sorted vertically equally sized silt material contains unique constant and low diversified grain assemblage euhedral dolomite quartz minute ooids suggesting its provenance exclusively from the shallower and or emerged parts of the carbonate platform interior the spot findings of graptolites and nautiloids along with delicate parallel lamination of sediments indicates relatively deep environment with absence of traction processes these features along with the lack of dolomite detritus in accompanying redeposited carbonates suggest its derivation by wind with regard to the earlier noted presence of aeolian detritus in the middle ludfordian we suggest dustiness as a plausible driving force of the cie in the hypothesis proposed bioavailable fe bearing dust inflow in concert with antiestuarine circulation on shelfs caused nutrient n p ratio disturbances resulting in methanogenesis in surface layer of the seas the surface layer methanogenesis should resulting in bathymetric gradient of isotope fractionation what is consistent with the cie record coincidence of sea level lowstand low latitude aridity and gustiness of the global climate are in accordance with the hypothetical short lived glaciation in mid ludfordian time

the structure of shear related vein type gold deposits a review abstract the structure of vein type gold deposits is defined by the shapes and geometrical relationships of mineralized bodies the form of the mineralization making up these bodies and the sequence of vein forming events most mineralized zones occur within or are spatially associated with shear zones especially shear zones in larger systems of intersecting shear zone sets they range in shape from tabular to linear and in form from disseminated to breccia to stockwork or sheeted veinlet zones to single veins there typically is a complex history of mineral deposition which overlaps and is genetically related to the deformation that generated the host structural zone the purpose of this paper is to consider the structural features of vein type deposits in relation to the geometrical properties and evolution of veinhosting fractures particularly shear zones the evidence indicates that veins are localized by tectonically generated dilatancy in an environment of low mean stress caused by high fluid pressure it is proposed that a major cause of tectonic dilation of shear zones is the interference between intersecting shears during bulk inhomogeneous flattening by movement on systems of intersecting shear zone sets

silurian conodont based correlations between gotland sweden and saaremaa estonia correlations between the silurian successions of gotland and the east baltic are based on the narrowest stratigraphic units that can be distinguished in both areas in order further to increase precision true ends of ranges in measured sections have been used instead of pooled formational faunas where outcrops and collections permit some previous correlations are confirmed in other cases there are considerable changes both in levels of correlation and in precision the major differences are found mostly when one or more of the major gaps have remained unrecognized and stratigraphic units therefore have been extended and translated upwards or downwards at least some of these gaps decrease in magnitude southwards in estonia gaps at these levels are also noted on gotland although they are small and probably do not extend across the island the patterns through time of deposition during secundo episodes and gaps during primo episodes agree very well with what can be predicted from a model of oceanic and climatic cyclicity