a tale of shales the relative roles of production decomposition and dilution in the accumulation of organic rich strata middle upper devonian appalachian basin abstract a new consensus on the processes responsible for organic carbon burial in ancient epeiric seas has emerged more firmly grounded in the uniformitarian framework of modern oceanography and biogeochemistry this consensus recognizes the interdependent roles of sedimentation primary production and microbial metabolism in favor of earlier end member models e g production vs preservation in this study one of the classic stratigraphic sequences upon which the preservation end member was based is re interpreted in light of this new consensus the study employs an extensive new sedimentological biogeochemical database from cores drilled in western new york the database spans over 500 m and 15 my of devonian deposition in the appalachian basin and provides a framework for comparative study of organic matter burial the major conclusions are 1 few organic rich units were deposited under pervasive anoxic sulfidic water columns 2 establishment and breakdown of seasonal thermoclines on annual or longer timescales were the predominant mode of stratification and 3 under such conditions remineralization of bio limiting nutrients may have played a key role in organic matter burial by creating a eutrophication pump this pump may have augmented an already rising nutrient inventory such that productivity levels exceeded the threshold required for development of suboxic to anoxic conditions in sediments and episodically in bottom waters a final conclusion asserts that the master variable for organic matter accumulation was relative sea level change which exerted influence on clastic dilution preservation and production processes sea level rise events led to sediment starvation and organic carbon concentration in distal basin sediments as well as to decreased effectiveness of seasonal mixing and thus longer build up intervals for remineralized nutrients episodic mixing of nutrient enriched bottom waters led to enhanced production ultimately increased clastic sediment delivery and water column mixing during relative sea level fall diluted surface sediment organic content such that respiratory demand could be met by increased oxygen supply thus terminating deposition of strata enriched in organic carbon

burial of organic carbon and pyrite sulfur in sediments over phanerozoic time a new theory in present day marine sediments almost all of which are deposited in normal oxygenated seawater rates of burial of organic carbon c and pyrite sulfur s correlate positively and bear a constant ratio to one another c s 3 on a weight basis by contrast calculations based on the isotopic model of garrels and lerman 1981 indicate that at various times during the phanerozoic the worldwide burial ratio must have been considerably different than the present day value this ratio change is caused by the requirement that increases in the worldwide mass of organic carbon must be accompanied by equivalent decreases in the mass of sedimentary pyrite sulfur in order to maintain a roughly constant level of o2 in the atmosphere such apparently contradictory behavior can be explained if the locus of major organic carbon burial has shifted over time from normal marine environments as at present to non marine freshwater or to euxinic environments in the geologic past a shift to predominantly freshwater burial can help explain predicted high c s ratios in permo carboniferous sediments and a shift to euxinic environments can help explain predicted low c s ratios during the early paleozoic it is demonstrated that the three environments today exhibit distinguishably different average c s ratios

remnant colloform pyrite at the haile gold deposit south carolina a textural key to genesis auriferous iron sulfide bearing deposits of the carolina slate belt have distinctive mineralogical and textural features traits that provide a basis to construct models of ore deposition our identification of paragenetically early types of pyrite especially remnant colloform crustiform and layered growth textures of pyrite containing electrum and pyrrhotite establishes unequivocally that gold mineralization was coeval with deposition of host rocks and not solely related to paleozoic tectonic events ore horizons at the haile deposit south carolina contain many remnants of early pyrite 1 fine grained cubic pyrite disseminated along bedding 2 fine grained spongy rounded masses of pyrite that may envelop or drape over pyrite cubes 3 fragments of botryoidally and crustiform layered pyrite and 4 pyritic infilling of vesicles and pumice detailed mineral chemistry by petrography microprobe sem and eds analysis of replaced pumice and colloform structures containing both arsenic compositional banding and electrum points to coeval deposition of gold and the volcanic host rocks and thus confirms a syngenetic origin for the gold deposits early pyrite textures are present in other major deposits of the carolina slate belt such as ridgeway and barite hill and these provide strong evidence for models whereby the sulfide ores formed prior to tectonism the role of paleozoic metamorphism was to remobilize and concentrate gold and other minerals in structurally prepared sites recognizing the significance of paragenetically early pyrite and gold textures can play an important role in distinguishing sulfide ores that form in volcanic and sedimentary environments from those formed solely by metamorphic processes exploration strategies applied to the carolina slate belt and correlative rocks in the eastern united states in the avalonian basement will benefit from using syngenetic models for gold mineralization

the porosities of ordinary chondrites models and interpretation densities and porosities for 285 ordinary chondrites have been assembled and analyzed measured chondrite porosities are bimodal finds have an average porosity of 30 we conclude that mild degrees of weathering fill pore spaces lowering grain densities and porosities without significantly changing the bulk size or mass of the sample by assuming an original pristine grain density as a function of the meteorite s mineralogy determined by its class we can derive model pristine porosities these model porosities cluster around an average value of 10 for all classes of ordinary chondrites ordinary chondrites do not show any correlation of porosity model or measured with petrographic grade or sample size over a range from 0 2 g to 2 kg however we do see a correlation between shock state and porosity shock blackened meteorites are less porous than other meteorites furthermore less severely shocked meteorites show a much broader range of porosities with the maximum porosity seen among meteorites of a given shock class falling linearly as a function of that shock class this is consistent with the idea that shock compresses and closes pore space analysis of meteorite porosity provides a lower bound to the fine scale porosity of asteroids our densities even with 10 primordial porosity are significantly higher than inferred densities of possible asteroid parent bodies these asteroids are probably loose piles of rubble

geochemistry and paleotectonic setting of felsic volcanic rocks in the finlayson lake volcanic hosted massive sulfide district yukon canada the finlayson lake volcanic hosted massive sulfide vhms district represents one of canada s most recent vhms discovery regions with 34 million metric tons mt of massive sulfide mineralization found since the mid 1990s felsic volcanic rocks are associated with three units the fire lake unit the kudz ze kayah unit and the wolverine succession significant accumulations of polymetallic felsic vhms deposits kudz ze kayah gp4f and wolverine have only been discovered in the kudz ze kayah unit and wolverine succession to date in the hanging wall of the money creek thrust felsic volcanic and high level intrusive rocks in the fire lake unit have calc alkalic and tholeiitic affinities with low high field strength element hfse contents and intermediate zr sc 9 4 43 4 and zr tio2 254 864 ratios these rocks are interlayered with mafic rocks with arc geochemical signatures to date devoid of significant vhms mineralization and represent bimodal magmatism within an evolving devonian mississippian continental arc system the kudz ze kayah unit in the footwall of the money creek thrust stratigraphically overlies the fire lake unit and consists of felsic volcanic rocks with high hfse contents within plate a type signatures and high zr sc 15 3 190 3 and zr tio2 630 2 185 ratios the kudz ze kayah unit felsic rocks are crosscut and overlain by alkalic mafic rocks are associated with abundant carbonaceous sedimentary rocks and represent magmatism within a devonian mississippian ensialic back arc rift basin environment the wolverine succession unconformably overlies the kudz ze kayah unit felsic rocks below the wolverine deposit have geochemical attributes similar to the kudz ze kayah unit with high hfse contents within plate a type signatures and high zr sc 29 9 84 2 and zr tio2 391 1 220 ratios in contrast aphyric rhyolite flows in the hanging wall of the deposit have much lower hfse contents and the lowest zr sc 3 5 27 7 and zr tio2 181 591 ratios in the district all the felsic rocks of the wolverine succession are interlayered with abundant carbonaceous sedimentary rocks and are overlain by midocean ridge basalt morb like basaltic rocks the wolverine succession is interpreted to have formed within an early mississippian ensialic back arc basin environment that eventually evolved to sea floor spreading the variation in the hfse budgets of the felsic rocks of the finlayson lake district likely reflects variations in the source and or temperature of crustal melting in particular felsic rocks of the fire lake unit have higher nb ta and lower ti sc ratios than other volcanic rocks in the district suggesting possible derivation from mafic crustal sources and or lower crustal fusion temperatures the kudz ze kayah unit and footwall rocks to the wolverine deposit are inferred to have formed from high temperature partial melting of continental crust the hanging wall aphyric rhyolites from the wolverine deposit may have formed from either lower temperature continental crustal melting or may have been derived from the mixing of hfse depleted n morb mafic magmas and evolved continental crust polymetallic felsic volcanic associated vhms deposits within the finlayson lake district are preferentially associated with hfse enriched felsic rocks with high zr sc 15 3 190 3 and zr tio2 391 2 105 ratios the hfse and rare earth element ree systematics of vhms associated felsic rocks of the finlayson lake district are different from prospective felsic rocks from archean vhms environments in the superior province and are displaced toward higher zr y and la ybn ratios their hfse and ree systematics are similar to many phanerozoic vhms environments in particular those at least partially to fully underlain by evolved continental crust the geochemical differences between the felsic rocks of the finlayson lake district and those from archean vhms environments most likely reflect differences in the substrates from which the felsic rocks were derived e g evolved versus juvenile

geochemistry of the host rocks of the volcanogenic massive sulfide deposit at que river tasmania que river is a moderate sized high grade polymetallic volcanogenic massive sulfide deposit located in the cambrian mount read volcanics of western tasmania several subvertical lenses of massive sulfide occur in a sequence dominated by variably altered intermediate to silicic volcaniclastic rocks the sequence was deformed and regionally metamorphosed to prehnite pumpellyite facies during the devonian approximately 240 host rock samples have been analyzed in an attempt to determine their primary geochemical affinities and to evaluate the geochemical effects of alteration on binary variation diagrams elements such as zr nb y ti and al define linear fields that characteristically project toward the origin the linear behavior of these elements on such plots is in marked contrast to that of almost all other elements which show very complex distribution patterns the linear patterns are interpreted to reflect immobility of the components mentioned with movement up and down the linear arrays being controlled by concentration or dilution due to subtraction or addition of other components thus primary ratios between immobile components have been preserved and can therefore be used to infer the primary geochemical affinities of the altered rocks the intersection point of the linear trends defined by the que river rocks with a low pressure fractional crystallization trend defined by cenozoic orogenic lavas probably provides the best estimate of the initial composition of the rocks prior to alteration in a plot of zr tio 2 vs nb y the que river rocks define a restricted area dominantly in the fields defined by modern andesite and dacite rhyodacite basalts and rhyolites are apparently absent in terms of their primary geochemical affinities the host rocks of the mineralization are probably best described as a high k calc alkaline suite similar in many ways to modern continental margin andesitic rocks however the que river rocks are by comparison abnormally depleted in ti and enriched in transition metals such as ni cr and v the geochemistry of alteration is complex and not readily amenable to quantitative evaluation elements such as fe mg and mn have been added to the system whereas ca and na have been removed both si and k have been partly remobilized during alteration among the major and minor elements mn shows the greatest relative enrichment over its inferred primary abundance however the mn anomalies are sporadic and do not define a neat halo although most of the observed geochemical alteration appears to have accompanied mineralization the andesites have been modified from their inferred primary compositions probably during the regional metamorphism hydrothermal alteration is not symmetrical about the ore lenses although the presently observed displacement between the massive sulfide lenses and the alteration halo is probably due largely to the devonian deformation it is possible that there is also a primary component the apparent immobility of al implies that the abundant phyllosilicates whose development is clearly related to mineralization do not represent simple chemical precipitates or sediments rather their development would appear to be controlled by the local availability of immobile al

detrital zircon record of neogene exhumation of the central alaska range a far field upper plate response to flat slab subduction far field deformation in the upper plate of many ancient and modern convergent margins is commonly attributed to flat slab subduction processes recent studies have suggested that the high topography of the central alaska range alaska usa for example is the far field deformational response to neogene shallow subduction processes in southern alaska in this study we analyze six stratigraphically controlled detrital zircon samples from miocene pliocene sandstone of the tanana foreland basin to evaluate the temporal and spatial links between exhumation of the central alaska range and shallow subduction the tanana basin located north of the central alaska range contains 2 km of neogene strata lower miocene strata from the basin have detrital zircon ages with dominant precambrian populations these grain ages are attributed to sediment derivation from the yukon composite terrane source area located at the northern edge and north of the alaska range during the early middle miocene the provenance of the tanana basin switched to a distinctly mesozoic source signal these mesozoic age grains are attributed to sediment derived from source areas within and south of the alaska range the early middle miocene change in provenance from predominantly precambrian to mesozoic sediment sources in the context of previously published basin analysis studies is interpreted to represent the onset of neogene uplift of the central alaska range the timing of exhumation of the central alaska range is important not only for understanding the tectonics of the highest mountain range in north america but additionally because recent modeling studies predict that this range is one of the first expected products of neogene flat slab subduction along the southern alaska convergent plate boundary results of our study provide a needed chronologic framework to fully evaluate the temporal and spatial upper plate responses to shallow subduction processes

petrology and u pb geochronology of footwall porphyritic rhyolites from the wolverine volcanogenic massive sulfide deposit yukon canada implications for the genesis of massive sulfide deposits in continental margin environments porphyritic rhyolite sills form an important component of the footwall of the wolverine volcanogenic massive sulfide vms deposit yukon canada and occur proximal to mineralization in the immediate deposit area wolverine lynx zone and at similar stratigraphic levels along strike fisher puck and sable zones porphyritic rhyolites are of two types an older quartz feldspar porphyritic qfp rhyolite suite and a younger feldspar porphyritic fp suite both the qfp and fp suites of intrusions are semiconcordant suggesting a silllike morphology and are altered and crosscut by veinlet mineralization suggesting that they are pre to synmineralization the margins of qfp suite of intrusions contain minor xenoliths of surrounding shales and poorly developed chilled margins suggesting emplacement into partially consolidated sedimentary rocks whereas the fp suite of intrusions shows well developed chilled margins indicative of emplacement into fully solidified sedimentary rock these features suggest that the qfp suite of intrusions represents an older phase of rhyolitic magmatism whereas the fp suite represents a younger event this is supported by u pb zircon ages which indicate a 352 4 1 5 ma emplacement age for the fp suite and a 347 to 346 ma emplacement for the fp suite two ages at 347 8 1 3 and 346 0 2 2 ma both suites of porphyries have inherited proterozoic zircon and have ratios of la smucn 1 and nb thucn 1 ucn upper continental crust normalized indicating derivation from and or extensive interaction with ancient upper continental crustal materials the fp suite however has elevated high field strength element hfse and rare earth element ree contents high zircon saturation temperatures and higher nb ta ratios and lower ti sc ratios than the qfp suite these features are interpreted to reflect that the fp suite of magmas was hotter 900 c melts with a greater mantle component in their genesis both suites however are interpreted to have formed due to basaltic upwelling crustal melting and crust mantle mixing during ensialic back arc basin activity the presence of mantle heat within the wolverine basin from 352 to 347 to 346 ma a minimum of 5 m y suggests that sustained mantle heat flow was critical to the genesis of the wolverine porphyries it is also suggested that this sustained mantle heat was responsible for the wolverine hydrothermal system and that upwelling mantle may be essential in providing the heat to drive hydrothermal systems even in continental margin type vms environments e g bathurst iberian pyrite belt the wolverine porphyries are among the most hfse and ree enriched felsic rocks associated with vms mineralization globally the high hfse and ree concentrations in these rocks are interpreted to be due to

restraining bend tectonics in the santa cruz mountains california imaged using 10be concentrations in river sands reverse faults frequently generate large and destructive earthquakes yet their seismic hazard remains diffi cult to assess with traditional paleoseismic tools because their surface expressions are often complex and subtle this contribution assesses the utility of millennial scale denudation rates derived from in situ cosmogenic 10be for revealing the spatial patterns and magnitudes of rock uplift produced by slip along reverse faults we present seventeen basin averaged denudation rates from rivers draining the santa cruz mountains along the san andreas fault california usa which closely reproduce known uplift rate patterns associated with a restraining bend along the fault an additional component of vertical deformation appears to be superposed on the uplift due to the restraining bend this may result from regional transpression further irregularities in the fault trace or interactions with neighboring faults our results indicate that 10be derived denudation rates can reveal patterns of rock uplift adjacent to reverse faults over length scales relevant for characterizing their seismic hazard potential

the behaviour of so called immobile elements in hydrothermally altered rocks associated with volcanogenic submarine exhalative ore deposits evidence is available that some elements notably zr tio2 y sc ce and nb are largely immobile during the alteration of volcanic rocks owing to metamorphism hydrothermal events and weathering e g floyd and winchester 1978 however it is shown by reference to analyses of rocks from the environment of five volcanogenic massive sulphide bodies that while zr tio2 and ce are mostly immobile even during intense hydrothermal alteration y and particularly sc and nb may be extremely mobile when elements are removed by solution in a hydrothermal fluid it seems that reaction rates are such that these elements are almost totally removed from the rock therefore of the so called immobile trace elements only zr and tio2 may be used with any reliability to identify the degree of magmatic differentiation in an hydrothermally altered rock however if an element has been mobile it is usually readily identified as having moved

the setting style and role of magmatism in the formation of volcanogenic massive sulfide deposits throughout earth s history all volcanogenic massive sulfide vms hosting environments are associated with specific assemblages of mafic and felsic rocks with distinct petrochemistry petrochemical assemblages indicative of formation at anomalously high temperatures within extensional geodynamic environments in mafic dominated juvenile ophiolitic vms environments there is a preferential association with mafic rocks with boninite and low ti tholeiite mid ocean ridge basalt morb and or back arc basin basalt affinities representing forearc rifting or back arc initiation mid ocean ridges or back arc basin spreading or back arc basins respectively felsic rocks in juvenile oceanic arc environments in archean terrains are high field strength element hfse and rare earth element ree enriched in post archean juvenile oceanic arc terrains felsic rocks are commonly hfse and ree depleted and have boninite like to tholeiitic signatures in vms environments that are associated with continental crust i e continental arc and back arc and dominated by felsic volcanic and or sedimentary rocks evolved environments felsic rocks are the dominant hosts to mineralization and are generally hfse and ree enriched with calc alkalic a type and or peralkalic affinities representing continental arc rifts continental back arcs and continental back arcs to continental rifts respectively coeval mafic rocks in evolved environments have alkalic within plate ocean island basalt like and morb signatures that represent arc to back arc rift versus back arc spreading respectively the high temperature magmatic activity in vms environments is directly related to the upwelling of mafic magma beneath rifts in extensional geodynamic environments e g mid ocean ridges back arc basins and intra arc rifts underplated basaltic magma provides the heat required to drive hydrothermal circulation extensional geodynamic activity also provides accommodation space at the base of the lithosphere that allows for the underplated basalt to drive hydrothermal circulation and induce crustal melting the latter leading to the formation of vms associated rhyolites in felsic dominated and bimodal vms environments rifts also provide extensional faults and the permeability and porosity required for recharge and discharge of vms related hydrothermal fluids rifts are also critical in creating environments conducive to preservation of vms mineralization either through shielding massive sulfides from seafloor weathering and mass wasting or by creating environments conducive to the precipitation of subseafloor replacement style mineralization in sedimented rifts subvolcanic intrusions are also products of the elevated heat flow regime common to vms forming environments shallow level intrusive complexes i e within 1 3 km of the seafloor may not be the main drivers of vms related hydrothermal circulation but are likely the manifestation of deeper seated mantle derived heat i e 3 10 km depth that drives hydrothermal circulation these shallower intrusive complexes are commonly long lived i e millions of years and reflect a sustained thermally anomalous geodynamic environment such a thermally anomalous environment has the potential to drive significant hydrothermal circulation and therefore multi phase long lived subvolcanic intrusive complexes are excellent indicators of a potentially fertile vms environment the absence of intrusive complexes however does not indicate an area of low potential as they may have been moved or removed due to post vms tectonic activity in some cases shallow level intrusive systems contribute metals to the vms hydrothermal system

trace element budgets in the ohio sunbury shales of kentucky constraints on ocean circulation and primary productivity in the devonian mississippian appalachian basin abstract the hydrography of the appalachian basin in late devonian early mississippian time is modeled based on the geochemistry of black shales and constrained by others paleogeographic reconstructions the model supports a robust exchange of basin bottom water with the open ocean with residence times of less than forty years during deposition of the cleveland shale member of the ohio shale this is counter to previous interpretations of these carbon rich units having accumulated under a stratified and stagnant water column i e with a strongly restricted bottom bottom water circulation a robust circulation of bottom waters is further consistent with the palaeoclimatology whereby eastern trade winds drove upwelling and arid conditions limited terrestrial inputs of siliciclastic sediment fresh waters and riverine nutrients the model suggests that primary productivity was high 2 g c m 2 d 1 although no higher than in select locations in the ocean today the flux of organic carbon settling through the water column and its deposition on the sea floor was similar to fluxes found in modern marine environments calculations based on the average accumulation rate of the marine fraction of ni suggest the flux of organic carbon settling out of the water column was approximately 9 of primary productivity versus an accumulation rate burial of organic carbon of 0 5 of primary productivity trace element ratios of v mo and cr mo in the marine sediment fraction indicate that bottom waters shifted from predominantly anoxic sulfate reducing during deposition of the huron shale member of the ohio shale to predominantly suboxic nitrate reducing during deposition of the cleveland shale member and the sunbury shale but with anoxic conditions occurring intermittently throughout this period

geomorphic limits to climate induced increases in topographic relief recognition of the potential for strong dynamic coupling between atmospheric and tectonic processes has sparked intense cross disciplinary investigation and debate on the question of whether tectonics have driven long term climate change or vice versa it has been proposed that climate change might have driven the uplift of mountain summits through an isostatic response to valley incision because isostasy acts to compensate mean elevations the debate hinges on the question of whether climate change can significantly increase topographic relief or more precisely increase the volume of missing mass between summits and ridges here we show that in tectonically active mountain ranges geomorphic constraints allow only a relatively small increase in topographic relief in response to climate change thus although climate change may cause significant increases in denudation rates potentially establishing an important feedback between surficial and crustal processes neither fluvial nor glacial erosion is likely to induce significant isostatic peak uplift

a marine biogeochemical perspective on black shale deposition abstract deposition of marine black shales has commonly been interpreted as having involved a high level of marine phytoplankton production that promoted high settling rates of organic matter through the water column and high burial fluxes on the seafloor or anoxic sulfidic water column conditions that led to high levels of preservation of deposited organic matter or a combination of the two processes here we review the hydrography and the budgets of trace metals and phytoplankton nutrients in two modern marine basins that have permanently anoxic bottom waters this information is then used to hindcast the hydrography and biogeochemical conditions of deposition of a black shale of late jurassic age the kimmeridge clay formation yorkshire england from its trace metal and organic carbon content comparison of the modern and jurassic sediment compositions reveals that the rate of photic zone primary productivity in the kimmeridge sea based on the accumulation rate of the marine fraction of ni was as high as 840 g organic carbon m 2 yr 1 this high level was possibly tied to the maximum rise of sea level during the late jurassic that flooded this and other continents sufficiently to allow major open ocean boundary currents to penetrate into epeiric seas sites of intense upwelling of nutrient enriched seawater would have been transferred from the continental margins their present location onto the continents this global flooding event was likely responsible for deposition of organic matter enriched sediments in other marine basins of this age several of which today host major petroleum source rocks bottom water redox conditions in the kimmeridge sea deduced from the v mo ratio in the marine fraction of the kimmeridge clay formation varied from oxic to anoxic but were predominantly suboxic or denitrifying a high settling flux of organic matter a result of the high primary productivity supported a high rate of bacterial respiration that led to the depletion of o2 in the bottom water a high rate of burial of labile organic matter albeit a low percentage of primary productivity in turn promoted anoxic conditions in the sediment pore waters that enhanced retention of trace metals deposited from the water column

carbon sulfur iron systematics of the uppermost deep water sediments of the black sea box cores recovered during leg 4 of the 1988 r v knorr black sea oceanographic expedition from deep water regions of the basin were dominated by coccolith rich microlaminated unit 1 sediment and muddy gray turbidite layers both organic carbon oc and pyrite sulfur values for unit 1 display narrow ranges with mean concentrations of 5 3 1 1 1 wt and 1 3 0 3 wt respectively unit 1 is not enriched in pyrite s relative to sediments deposited under oxygenated bottom waters normal marine sediments with comparable oc concentrations carbon sulfur relationships evaluated on a calcium carbonate free basis to avoid spurious correlations resulting from dilution effects demonstrate that oc and pyrite s are essentially decoupled these observations combined with the persistence of elevated pore water sulfide to depth and a strong correlation between pyrite s and the detrital fe component argue strongly for limitation of pyrite formation in unit 1 by the availability of reactive fe unit 1 fe limitation is further indicated by degree of pyritization dop studies a measure of the extent to which the original potentially reactive fe has been transformed to pyrite these studies show sulfidation of reactive fe ranging from 57 to 78 with dop values independent of oc concentration unit 1 dop profiles suggest that the majority of the pyrite is formed in the sulfidic water column and or very close to the sediment water interface pyrite s concentrations of unit 1 when compared with the particulate reduced sulfur fluxes measured in time series sediment traps are compatible with predominantly water column pyrite formation because of the limitations in the supply of reactive fe associated with the comparatively high supply of oc the microlaminated sediment is characterized by c s ratios greater than those typical of holocene oxically deposited sediments the turbidite muds of the deep basin display high reduced s values relative to holocene normal marine sediments in samples with low oc concentrations low c s ratios this reflects pyrite formation under anoxic sulfidic bottom water conditions in a probable upper slope source region as well as during transport and final deposition intermediate dop values for the turbidites in part a product of their rapid rate of deposition reveal that fe limitation is ultimately not a factor and that further pyrite should form during burial however the very rapid rate at which unit 1 pyrite forms suggests fundamental differences between the overall reactivities of the fe phases associated with the microlaminated and turbiditic sediments the signature of water column anoxia with regard to sedimentary pyrite formation is clearly indicated by the high dop values of unit 1 and the comparatively high levels of s associated with the low concentrations of oc of the turbidite muds this agrees with similar conclusions based on studies of ancient sedimentary rocks

peralkaline and calc alkaline hosted volcanogenic massive sulfide deposits of the bonnifield district east central alaska volcanogenic massive sulfide vms zn pb cu ag au deposits of the bonnifield mining district formed during late devonian early mississippian magmatism along the western edge of laurentia the largest deposits dry creek and wtf have a combined resource of 5 7 million tonnes at 10 zn 4 pb 0 3 cu 300 grams per tonne g t ag and 1 6 g t au these polymetallic deposits are hosted in high field strength element hfse and rare earth element ree rich peralkaline pantelleritic metarhyolite and interlayered pyritic argillite and mudstone of the mystic creek member of the totatlanika schist formation mystic creek metarhyolite and alkali basalt chute creek member constitute a bimodal pair that formed in an extensional environment a synvolcanic peralkaline quartz porphyry containing veins of fluorite sphalerite pyrite and quartz intrudes the central footwall at dry creek the anderson mountain deposit located 32 km to the southwest occurs within calc alkaline felsic to intermediate composition metavolcanic rocks and associated graphitic argillite of the wood river assemblage felsic metavolcanic rocks there have only slightly elevated hfses and rees the association of abundant graphitic and siliceous argillite with the felsic volcanic rocks together with low cu contents in the bonnifield deposits suggests classification as a siliciclastic felsic type of vms deposit bonnifield massive sulfides and host rocks were metamorphosed and deformed under greenschist facies conditions in the mesozoic primary depositional textures generally uncommon consist of framboids framboidal aggregates and spongy masses of pyrite sphalerite the predominant base metal sulfide encloses early pyrite framboids galena and chalcopyrite accompanied early pyrite formation but primarily formed late in the paragenetic sequence silver rich tetrahedrite is a minor late phase at the dry creek deposit gold and ag are present in low to moderate amounts in pyrite from all of the deposits electrum inclusions occur in dry creek sphalerite contents and ratios of trace elements in graphitic argillite that serve as proxies for the redox state of the bottom waters in the basin indicate that dry creek mineralization took place in suboxic to periodically anoxic bottom waters trace element data show higher contents of tl mn as in pyrite from the anderson mountain deposit compared to the dry creek or wtf deposits and thus suggest that anderson mountain may have formed at lower temperatures or under slightly more oxidizing conditions no exact modern analogue for the tectonic setting of the bonnifield vms deposits is known although the back arc regions of the okinawa trough and woodlark basin satisfy the requirement for a submarine extensional setting adjacent to a continental margin limited occurrences of peralkaline volcanic rocks occur in these two potential analogues but the peralkalinity of those rocks is much less than that of the mystic creek member metarhyolites in the bonnifield district the highly elevated trace element e g zr nb contents of mystic creek metarhyolites suggest that a better analogue may be a submarine rifted continental margin the calc alkaline composition of the host rocks to the anderson mountain deposit suggests that mineralization there formed in a continental margin arc outboard of the extended continental margin setting of the peralkaline hosted dry creek and wtf deposits

petrogenetic evolution of felsic volcanic sequences associated with phanerozoic volcanic hosted massive sulphide systems the role of extensional geodynamics abstract felsic volcanic hosted massive sulphide deposits form in many different geotectonic environments usually associated with episodes of extension however empirical geochemical links between felsic volcanism geotectonic environment and extensional geodynamics have not been adequately documented so as to distill the fundamental relationships between these features and formation of volcanic hosted massive sulphide vhms mineralization which is important considering most vhms deposits are hosted in felsic volcanic bearing sequences using felsic geochemical data from six different vhms mineralized phanerozoic geotectonic settings it seems that the felsic geochemical data are consistent with the overall geotectonic interpretations for these areas which in turn helps in stratigraphic and geotectonic interpretations for detailed vhms exploration in extensional geodynamic settings bimodal volcanism is common compared to the continuum of compositions observed under compressional to neutral high stress regimes in calc alkaline arc building environments the empirical geochemical metallogenic vhms relationships evident in the archean felsic volcanic rocks in particular the low zr y n yb n 2 o conditions in different source areas and geotectonic environments they seem to be the predominant controls on the compositional evolution of felsic magmas in extensional settings because of the limited degree of assimilation and fractional crystallization afc affecting their compositions with rapid intrusion into the uppermost crust to positions were they achieve neutral buoyancy in the volcanic to subvolcanic environment within plate felsic or a type anorogenic type and anomalous ocean ridge or type felsic magmas that have tholeiitic affinities relatively low alkali elements are common within parts of the volcanic sequences hosting massive sulphide deposits reflecting in part a relationship to extension the high contents of high field strength elements zr ga lree hree y nb etc and other lines of evidence indicate they are relatively high temperature relatively reduced magmas probably derived from previously dehydrated granulitic or melted oceanic simatic or continental sialic crust low p granulites or amphibolites but also in some cases possibly mantle rocks such that they are significantly water undersaturated during melting i e a h 2 o 1 reduced viscosities typical of high temperature melts enable rapid intrusion to higher levels diking vs diapirism higher rates of heat advection from mantle mafic magmas into the thickened oceanic and or continental crust during lithospheric extension are needed to facilitate melting of these rock to form a type and or or type partial melts tholeiitic felsic this usually results in bimodal magmatism as magma mixing is not favored in extensional settings especially at high magma production intrusion and extrusion rates i e high heat flow this would inhibit ferromagnesian phase in particular hornblende and magnetite fractionation especially at low f o 2 in the felsic magmas that would deplete the melt in y hree as well as ore related elements particularly zn which is 2 to 3 times higher in a type and or type rocks compared to other magma types therefore the empirical felsic igneous geochemical metallogenic relationships observed by others low zr y n yb n

u pb zircon and geochemical evidence for bimodal mid paleozoic magmatism and syngenetic base metal mineralization in the yukon tanana terrane alaska new shrimp sensitive high resolution ion microprobe u pb zircon ages and trace element geochemical data for mafic and felsic metaigneous rocks of the pericratonic yukon tanana terrane in east central alaska help define the tectonic setting of mid paleo zoic magmatism and syngenetic hydrothermal zn pb ag mineralization along the ancient pacific margin of north america we compare data from similar greenschist facies sequences of bimodal volcanic and subvolcanic rocks associated with carbonaceous and siliciclastic marine sedimentary rocks in the wood river area of the alaska range and the salcha river area of the yukon tanana upland and from amphibolite facies augen gneiss and mafic gneiss amphibolite in the goodpaster river area of the upland allowing for analytical uncertainties igneous crystallization age ranges of 376 353 ma 378 346 ma and 374 358 ma are indicated by 13 new shrimp u pb dates for the wood river salcha river and goodpaster river areas respectively bimodal magmatism is indicated by late devonian crystallization ages for both augen gneiss 371 3 and 362 4 ma and associated orthoamphibolite 369 3 ma in the upland and by stratigraphic interleaving of mafic and felsic rocks in the alaska range metabasites in all three study areas have elevated hfse high field strength element and ree rare earth element contents indicative of generation in a within plate extensional tectonic setting within plate trace element signatures also are indicated for peralkaline metarhyolites that host the largest volcanogenic massive sulfide deposits of the bonnifield district in the wood river area and for metarhyolite tuff interlayered with the carbonaceous nasina assemblage which hosts sedimentary exhalative sulfide occurrences in the salcha river area most of the other felsic metaigneous samples from the alaska range and the yukon tanana upland have geochemical signatures that are similar to those of both average upper continental crust and continental margin arc rocks generated in thick continental crust given the absence in our study areas of intermediate composition magmatic products generally found in most arcs and the presence of bimodal magmatism the alkalic within plate chemistry of the mafic rocks and some of the felsic rocks and the widespread occurrence of interlayered carbonaceous sedimentary rocks indicative of deposition within a restricted marine basin or submerged continental margin we consider it most likely that this prolonged late devonian to early mississippian magmatic episode resulted from attenuation of the ancient continental margin of western north america rather than development of an arc as proposed by many others

ore metamorphism and pyrite porphyroblast development at the cherokee mine ducktown tennessee very large pyrite porphyroblasts are a striking feature of the metamorphosed massive pyrrhotite rich sulfide ores at ducktown tennessee the crystals up to 300 mm across are commonly euhedral but exhibit varying degrees of elongation and postgrowth fracturing and granulation the inclusion patterns within the various porphyroblasts indicate growth before during and after the development of schistosity in the silicate gangue the sphalerite inclusions within single porphyroblasts show no systematic compositional variations from core to rim partly because the larger porphyroblasts were formed by the aggregation of numerous growing pyrite grains and partly because fractures and chalcopyrite allowed many of the sphalerite inclusions to reequilibrate at lower temperatures and pressures sphalerite inclusions surrounding single growth centers which were selected to avoid reequilibration problems show that they grew at a nearly constant pressure of 6 8 or 0 8 kb this information is used to relate the growth of the pyrite porphyroblasts to the pressure temperature evolution of the deposit pyrrhotite which forms the matrix for the porphyroblasts ranges compositionally from 47 4 at percent fe in the matrix to 47 7 at percent fe immediately adjacent to the pyrite crystals chalcopyrite commonly occurs as fracture fillings and as pressure shadows adjacent to the pyrite porphyroblasts minor sphalerite galena and magnetite are present gangue minerals are dominantly quartz calcite sheet silicates actinolite and talc

anoxia vs productivity what controls the formation of organic carbon rich sediments and sedimentary rocks discussion the prevailing explanation for the origin of organic rich sediments and rocks invokes deposition under conditions of anoxia however recent research suggests that high primary production and not water column anoxia provides the first order control on the accumulation of organic rich facies in the modern oceans oxygen minima do not appear to have any direct effect on carbon accumulation in continental margin or marginal sea environments sediments accumulating in the modern black sea the type euxinic basin are not particularly enriched in organic matter despite the presence of an anoxic water column although a sapropel containing extremely high carbon concentrations was deposited during the holocene at a time when the basin was oxic results of a recently published co pled ocean atmosphere model indicate that during the cretaceous thermohaline and surface circulation in the oceans was similar to or more intense than modern conditions despite the overall equable climate such conditions confound the idea that circulation in the cretaceous atlantic for example was punctuated by oceanic anoxic events brought about by more sluggish circulation sporadic temporal and spatial increases in primary production reflecting changes in the behavior and or state of the ocean atmosphere system constitute a more tenable explanation for the occurrence of modern and quaternary carbon rich sediments and cretaceous black shales consequently the fundamental control on the accumulation of carbon rich facies in the oceans and marginal seas is not the presence or absence of anoxia

geochemical variation within the northern ryukyu arc magma source compositions and geodynamic implications the major and trace element and pb sr nd isotopic compositions of quaternary mafic lavas from the northern ryukyu arc provide insights into the nature of the mantle wedge and its tectonic evolution beneath the volcanic front in the northern part of the arc the subducted slab of the philippine sea plate bends sharply and steepens at a depth of 80 km lavas from the volcanic front have high abundances of large ion lithophile elements and light rare earth elements relative to the high field strength elements consistent with the result of fluid enrichment processes related to dehydration of the subducting slab new pb isotopic data identify two distinct asthenospheric domains in the mantle wedge beneath the south kyushu and northern ryukyu arc which in a parallel with data from the lau basin appear to reflect mantle with affinities to indian and pacific type mid ocean ridge basalt morb indian ocean morb type mantle contaminated with subducted ryukyu sediments can account for the variation of lavas erupted on south kyushu and probably in the middle okinawa trough in contrast magmas of the northern ryukyu volcanic front appear to be derived from sources of pacific morb type mantle contaminated with a sedimentary component along arc variation in the northern ryukyus reflects increasing involvement of a sedimentary component to the south compositions of alkalic basalts from the south kyushu back arc resemble intraplate type basalts erupted in nw kyushu since 12 ma we propose that the bending of the subducted slab was either caused by or resulted in lateral migration of asthenospheric mantle yielding indian ocean type characteristics from a mantle upwelling zone beneath nw kyushu and the east china sea this model also accounts for 1 extensional counter clockwise crustal rotation 4 2 ma 2 voluminous andesite volcanism 2 ma and 3 the recent distinctive felsic magmatism in the south kyushu region

contemporaneous ascent of within plate type and island arc type magmas in the beppu shimabara graben system kyushu island japan abstract within plate wp type and island arc ia type magmas have erupted in north kyushu and south kyushu respectively during quaternary time the former is related to the eastern end of the east asian extensional province and the latter to the subduction zone of the philippine sea plate the beppu shimabara graben system in the central part of kyushu island is located in the area of overlap between them quaternary volcanic rock samples from the graben system can be chemically classified as wp and ia types by their multi element patterns and positions on a zr nb y discrimination diagram rocks of both type and of similar ages occur throughout the graben system furthermore the rocks of wp type give systematically higher 87sr 86sr ratios 0 7048 0 7049 than those of the associated ia type around 0 7040 the relations between 87sr 86sr and nb y ratios of the former are clearly distinguished from those of the latter these results demonstrate that this graben system is characterized by the contemporaneous ascents of wp and ia type magmas to the same quaternary volcanic areas such rivalrous ascents of wp and ia type magmas seem to be a common feature of magmatism associated with graben in extensional provinces related to subduction zones of island arcs and their boundary areas

contrasting sulfur geochemistry and fe al and mo al ratios across the last oxic to anoxic transition in the cariaco basin venezuela an abrupt transition from oxic to anoxic sulfidic euxinic marine bottom waters occurred in the cariaco basin in response to increasing productivity resulting from the late pleistocene post glacial rise in sea level and corresponding increase in surface water nutrient availability the microlaminated sediments of the euxinic interval which span the last f14 5 ky suggest a predominance of water column syngenetic pyrite formation based on 1 high pyrite sulfur spy concentrations in the surficial sediment layers 2 values for degree of pyritization dop that generally do not increase appreciably with increasing burial 3 ratios of total iron fet to al that are elevated above the continental baseline recorded in the underlying oxic sediments and 4 spy isotope trends that largely mimic the d 34 shsof the modern water column intermediate dop values in the microlaminated deposits and fet al ratios that are slightly above continental levels indicate an iron reservoir controlled by scavenging during syngenetic pyrite formation in combination with intermediate rates of fe bearing siliciclastic accumulation as predicted from the relative rates of siliciclastic delivery fet al and dop data lie between end member values observed in the modern black sea as viewed broadly fet al and dop relationships in euxinic sediments reflect the balance between syngenetic fe scavenging and temporal and spatial gradients in siliciclastic input pyrite concentrations are generally low in the underlying oxic marine deposits because of limitations in the supply of organic carbon corg however the upper 80 cm of the fe rich corg poor bioturbated sediment show evidence for a strong diffusional hsoverprint from the overlying fe limited euxinic marine environment this post glacial transition manifests in pyrite overprints that are strongly 34 s depleted relative to those in restricted presently euxinic marine settings elsewhere in the world such as the black sea where the sedimentary sequence spanning the last glacial interglacial transition begins with a shift from freshwater to corg poor oxic marine deposition and thus dominantly sulfate diffusion trends for mo al ratios in the microlaminated sediments suggest that mo is enriched by roughly two orders of magnitude above the continental levels recorded in the oxic deposits organic matter plays a role by enhancing hsproduction and or by providing a substrate for mo scavenging significant mo enrichment via diffusion into the upper portion of the bioturbated zone was not observed despite hs rich pore waters as recorded in the heavy iron sulfide overprint we have

review of radiometric data from the yukon crystalline terrane alaska and yukon territory the results of more than 20 years of geochronological studies in the yukon crystalline terrane in east central alaska and the western yukon territory suggest at least six igneous and thermal metamorphic events plutonism during mississippian early jurassic mid cretaceous late cretaceous and early tertiary times is indicated evidence also indicates that mississippian early jurassic late early cretaceous and late cretaceous thermal metamorphic events have affected parts of the terrane the western part of the terrane was affected by a significant regional metamorphic event in late early cretaceous time followed by a terrane wide mid cretaceous plutonic event the pattern of k ar ages allows division of the terrane into domains bounded by northeast trending lineaments

extreme sulfur isotope fractionation in the late devonian dry creek volcanogenic massive sulfide deposit central alaska abstract the dry creek zn pb cu ag au volcanogenic massive sulfide vms deposit in east central alaska occurs in a late devonian sequence of peralkaline rhyolite tuff minor graphitic argillite and local peralkaline quartz porphyry rhyolite intrusions principal mineralized facies are semi massive and massive sulfide in variably silicified and graphitic rhyolite tuff massive sulfide in graphitic argillite disseminated sulfides in graphitic and non graphitic rhyolite tuff and vein hosted sulfides in a subvolcanic peralkaline quartz porphyry intrusion in situ analysis of the sulfur isotope composition of sulfide minerals from all facies of the deposit shows a total range in 34s values from 48 0 to 23 1 this remarkable 71 1 variation is more than twice the largest range known for sulfides in nearly all individual vms deposits both modern and ancient which typically is high precision ca id tims u pb geochronology of zircons from a synvolcanic peralkaline quartz porphyry intrusion yields a weighted mean 206pb 238u date of 363 02 0 43 ma 2 total uncertainty this date indicates that the formation of graphitic and sulfidic sediments at dry creek and contemporaneous vms mineralization occurred at least 3 6 m y before and hence are unrelated to widespread black shale deposition during the hangenberg event at ca 359 ma

zircon thermometry of high temperature rhyolites near volcanic associated massive sulfide deposits abitibi subprovince canada high temperature high silica rhyolites are commonly found near cu zn volcanic associated massive sulfide vms deposits in the archean abitibi subprovince they are characterized by anhydrous phenocrysts feldspar quartz in concentrations of generally 73 sio 2 high incompatible element contents rb sr 1 zr y

development of framboidal pyrite during diagenesis low grade regional metamorphism and hydrothermal alteration distribution patterns microstructural relationships and compositional variations for framboidal pyrite in the paleozoic sedimentary host rocks to gold deposits on the northern carlin trend nevada and at fosterville in central victoria collectively demonstrate that it is not all of synsedimentary early diagenetic origin framboidal pyrite also nucleated in hydrothermal veins formed at several kilometers depth and along dissolution surfaces stylolites and crenulation cleavages formed during metamorphism at up to anchizonal grade i e prehnitepumpellyite facies there are no obvious differences in either the size range or internal morphology of framboids formed in these various settings suggesting that a single relatively simple crystallization path may operate over a wide depth range in the upper crust at fosterville anomalously metal rich including au framboidal pyrite is most abundant adjacent to laminated shear veins that developed by slip along bedding surfaces during folding of the sedimentary host rocks microstructural relationships and pb isotope compositions for the framboidal pyrite and other similarly distributed early formed sulfides support development in response to the localized episodic influx of metal enriched h 2s bearing hydrothermal fluids during regional deformation between 440 and 400 ma these findings may have implications for other similarly deformed metasedimentary successions where the discovery of au bearing framboidal pyrite has recently been cited as evidence for significant au enrichment during sedimentation 2009 society of economic geologists inc