discovery of new hydrothermal vent sites in bransfield strait antarctica we carried out a search for hydrothermal vents in the central basin of bransfield strait antarctica the zaps zero angle photon spectrometer chemical sensor and instrument package oregon state university ofos ocean floor observation system camera sled and tvg tv grab geomar were used to explore the water column and underlying seafloor these operations were supplemented with a series of dredges hydrothermal plumes over hook ridge at the eastern end of the basin are confined to the e ridge crest and se flank the plumes are complex and sometimes contain two turbidity maxima one widespread feature centered at 1150 m and a smaller more localized but broad maximum at 600 800 m we traced the source of the shallower plume to a sunken crater near the ridge crest using sensors on the zaps instrument package subsequently two tv grabs from the crater brought back hot soupy sediment 42 49 c overlain by hard siliceous crusts and underlain by a thick layer of volcanic ash we also recovered chimney fragments whose texture and mineralogy indicate venting temperatures in excess of 250 c native sulfur and fe sulfides occur in fractures and porous layers in sediment from throughout the area pore water data from the crater site are consistent with venting into a thin sediment layer and indicate phase separation of fluids beneath hook ridge the source of the deeper plumes at hook ridge has yet to be located we also explored a series of three parallel volcanic ridges west of hook ridge called three sisters we detected water column anomalies indicative of venting with the zaps package and recovered hydrothermal barites and sulfides from middle sister we spent considerable time photographing middle sister and hook ridge but did not identify classic vent fauna at either location we either missed small areas with our photography or typical mor vent fauna are absent at these sites

u pb trace element and lu hf properties of unique dissolution reprecipitation zircon from uhp eclogite in sw sulu terrane eastern china abstract a unique dissolution reprecipitation zircon has been identified for the first time in a qtz and phe bearing eclogite from the drill hole ccsd mh southwestern sulu uhp terrane by using laser raman and cathodoluminescence cl imaging combined with trace element analyses and u pb and lu hf isotope data relic inherited domains of partially dissolved zircon show magmatic features and preserve mineral inclusions of cpx pl qtz ap whereas newly formed zircons crystallized around or along cracks of the inherited magmatic domains and contain mineral inclusions of coe grt omp phe ap typical for uhp metamorphism in addition primary fluid inclusions were identified in the coesite bearing zircon domains characterized by a single h2o phase or co2 h2o phase indicating the presence of fluids during uhp conditions strongly dissolved inherited magmatic zircons yielded shrimp 206pb 238u ages of 782 588 ma with an upper intercept age of 780 14 ma new recrystallized or metamorphic zircons yielded consistent triassic uhp ages around 238 225 ma with a weighted mean of 229 2 2 3 ma zircon trace element and hf isotope compositions and mineral inclusions classify the protolith of the sulu eclogite as an intermediate to mafic neoproterozoic igneous rock which was subducted to mantle depth and experienced uhp metamorphism in middle triassic times the juvenile ehf 780 ma value of 7 4 2 9 in inherited magmatic zircon points to a significant input from the depleted mantle to the dabie sulu uhp terrane during middle neoproterozoic new uhp zircon domains have low lu hf th u and significantly higher 176hf 177hf ratios than the inherited magmatic zircons consistent with formation during recrystallization and fractionation of the lu hf system during uhp metamorphism

continental growth at convergent margins facing large ocean basins a case study from mesozoic convergent margin basins of baja california mexico abstract mesozoic rocks of the baja california peninsula form one of the most areally extensive best exposed longest lived 160 my least tectonized and least metamorphosed convergent margin basin complexes in the world this convergent margin shows an evolutionary trend that may be typical of arc systems facing large ocean basins a progression from highly extensional phase 1 through mildly extensional phase 2 to compressional phase 3 strain regimes this trend is largely due to the progressively decreasing age of lithosphere that is subducted which causes a gradual decrease in slab dip angle and concomitant increase in coupling between lower and upper plates as well as progressive inboard migration of the arc axis this paper emphasizes the usefulness of sedimentary and volcanic basin analysis for reconstructing the tectonic evolution of a convergent continental margin phase 1 consists of late triassic to late jurassic oceanic intra arc to backarc basins that were isolated from continental sediment sources new progressively widening basins were created by arc rifting and sea floor spreading and these were largely filled with progradational backarc arc apron deposits that record the growth of adjacent volcanoes up to and above sea level inboard migration of the backarc spreading center ultimately results in renewed arc rifting producing an influx of silicic pyroclastics to the backarc basin rifting succeeds in conversion of the active backarc basin into a remnant backarc basin which is blanketed by epiclastic sands phase 1 oceanic arc backarc terranes were amalgamated by late jurassic sinistral strike slip faults they form the forearc substrate for phase 2 indicating inboard migration of the arc axis due to decrease in slab dip phase 2 consists of early cretaceous extensional fringing arc basins adjacent to a continent phase 2 forearc basins consist of grabens that stepped downward toward the trench filled with coarse grained slope apron deposits phase 2 intra arc basins show a cycle of 1 arc extension characterized by intermediate to silicic explosive and effusive volcanism culminating in caldera forming silicic ignimbrite eruptions followed by 2 arc rifting characterized by widespread dike swarms and extensive mafic lavas and hyaloclastites this extensional rifting cycle was followed by mid cretaceous backarc basin closure and thrusting of the fringing arc beneath the edge of the continent caused by a decrease in slab dip as well as a possible increase in convergence rate phase 2 fringing arc terranes form the substrate for phase 3 which consists of a late cretaceous high standing compressional continental arc that migrated inboard with time strongly coupled subduction resulted in accretion of blueschist metamorphic rocks with development of a broad residual forearc basin behind the growing accretionary wedge and development of extensional forearc trench slope basins atop the gravitationally collapsing accretionary wedge inboard of this ongoing phase 3 strongly coupled subduction together with oblique convergence resulted in development of forearc strike slip basins upon arc basement the modern earth is strongly biased toward long lived arc trench systems which are compressional therefore evolutionary models for convergent margins must be constructed from well preserved ancient examples like baja california this convergent margin is typical of many others where the early to middle stages of convergence phases 1 and 2 create nonsubductable arc ophiolite terranes and their basin fills in the upper plate these become accreted to the continental margin in the late stage of convergence phase 3 resulting in significant continental growth

submarine silicic caldera at the front of the izu bonin arc japan voluminous seafloor eruptions of rhyolite pumice myojin knoll caldera a submarine rhyolitic center 400 km south of tokyo is one of nine silicic calderas along the northern 600 km of the izu bonin ogasawara arc and the first anywhere to receive detailed submersible based study the caldera slightly smaller than the crater lake structure in oregon is 6 7 km in diameter its inner walls are 500 900 m high and it has a remarkably flat floor at 1400 m below sea level mbsl the caldera collapse volume is 18 km 3 suggesting that more than 40 km 3 of pumiceous tephra may have been erupted at the time the caldera formed precaldera seafloor eruptions built a broad volcanic edifice consisting of overlapping composite volcanoes made of rhyolitic lavas shallow intrusions and a variety of volcaniclastic deposits including thick accumulations of rhyolitic pumice erupted at 900 500 mbsl the caldera forming eruption produced a 150 200 m deposit of nonwelded fines depleted pumice that resembles a colossal layer of popcorn at the top of the caldera wall freshly erupted pumice behaved as sinkers or floaters depending on the environment in which it cooled the pumice clasts deposited proximally and exposed in the caldera wall were likely quenched in eruption columns that remained below sea level this pumice ingested seawater and sank as gases filling its vesicles cooled particularly as steam in its vesicles condensed to liquid water some eruption columns may have broken through the sea surface and entered the air especially during vigorous phases of the caldera forming eruption these pumices had the opportunity to ingest air as they cooled becoming floaters as they fell back to the sea these could have been carried distally on the sea surface by the combined effects of ocean currents and wind the age of the caldera is unknown but it may be as young as several thousand years its magmatic system at depth retains sufficient heat to sustain an actively growing intracaldera kuroko type polymetallic sulfide deposit rich in gold and silver and topped by chimneys emitting fluids as hot as 278 c sufficient time has elapsed however for a 250 m high postcaldera dome to grow on the caldera floor and for the caldera rim to be deeply scalloped by slumping

a kuroko type polymetallic sulfide deposit in a submarine silicic caldera manned submersible studies have delineated a large and actively growing kuroko type volcanogenic massive sulfide deposit 400 kilometers south of tokyo in myojin knoll submarine caldera the sulfide body is located on the caldera floor at a depth of 1210 to 1360 meters has an area of 400 by 400 by 30 meters and is notably rich in gold and silver the discovery of a large kuroko type polymetallic sulfide deposit in this arc front caldera raises the possibility that the numerous unexplored submarine silicic calderas elsewhere might have similar deposits

volcanic and hydrothermal processes in submarine calderas the kulo lasi example sw pacific abstract the study area is located at the transition between the northern end of the tonga trench and the north fiji fracture zone where tectonic movements are reputed to be the fastest in the world to the southeast of futuna island a broad area of volcanism occurs within a region characterized by a change in the tectonic fabric between a ne sw oriented volcanic graben and the n s oriented alofi ridge in 2010 the active volcano kulo lasi which represents the most recent volcanic episode in the futuna area was discovered in the center of this extensive volcanic zone kulo lasi is a 20 km diameter shield volcano that rises 400 m above the seafloor it is composed of basaltic to trachy andesitic lava with no obvious geochemical affinity with the tonga subduction that occurs 500 km to the east the central caldera is 5 km in diameter and 300 m deep and is located at a water depth of 1500 m diving operations with the submersible nautile and high resolution auv mapping have revealed the presence of numerous active and inactive hydrothermal fields on the floor and the walls of the caldera four tectono volcanic stages can be distinguished at kulo lasi caldera in stage 1 the shield volcano is built annular reverse faults develop at the summit and control circulation of water rock dominated hydrothermal fluids and high temperature alteration of rocks along the nascent normal faults mixing of hydrothermal fluids with seawater is favored along normal superficial faults leading to the formation of low temperature fe mn mineralization at the summit of the volcano during stage 2 the caldera collapse gradually revealing outcrops of the altered and mineralized zones formed during stage 1 as the magma chamber cools and collapses less heat is available as a result medium to low temperature

sanjiang tethyan metallogenesis in s w china tectonic setting metallogenic epochs and deposit types abstract tectonically the sanjiang tethyan metallogenic domain stmd is located within the eastern himalayan tibetan orogen in the sanjiang tethys southwestern china although this metallogenic domain was initiated in the early palaeozoic extensive metallogenesis occurred in the late palaeozoic late triassic and himalayan tertiary epochs corresponding tectonic settings and environments in the domain are an arc basin system related to the subduction of the palaeo tethyan oceanic slabs a post collision crustal extension setting caused by the lithospheric delamination or slab breakoff underneath the sanjiang tethys during the late triassic large scale strike slip faulting and thrusting systems due to the indo asian continent collision since the palaeocene in this metallogenic domain important gold copper base metals rare metals and tin ore belts incorporating a large number of giant deposits were developed the main types of deposits include 1 porphyry copper deposits controlled by a large scale strike slip fault system 2 vhms deposits mainly occurring in intra arc rift basins and post collision crustal extensional basins 3 shear zone type gold deposits in the ophiolitic melange zone along the thrusting shearing system 4 hydrothermal silver polymetallic deposits in the triassic intra continental rift basins and tertiary strike slip pull apart basins and 5 himalayan granite related greisen type tin and rare metallic deposits within the metallogenic epochs of the late palaeozoic to cenozoic the styles and types of the ore deposits changed from vhms types in the late palaeozoic through exhalative sedimentary type deposits in the late triassic to porphyry type copper deposits shear zone type gold deposits hydrothermal vein type silver polymetallic deposits greisen type tin and rare metal deposits in the cenozoic correspondingly ore forming metals also changed from a pb zn cu ag association through ag cu pb zn fe ag pb and ag au hg associations to ag cu pb zn cu mo au sn and li rb cs nb zr hf y ce sc associations

quenching of steam charged pumice implications for submarine pyroclastic volcanism abstract huge quantities of silicic pumice have been deposited in intra oceanic convergent margin settings throughout earth s history the association of submarine silicic calderas with thick proximal accumulations of pumice lapilli suggests that these pyroclasts were deposited as a direct result of submarine eruptions yet when first erupted these highly vesicular gas filled clasts had densities significantly less than seawater experiments carried out 1 atm on heated pumice samples whose vesicles were charged with steam the dominant component of magmatic volatiles show that buoyancy of freshly erupted submarine pumice is transient upon quenching the phase change of steam to liquid water creates strong negative pore pressures within the pumice vesicles that accelerate the absorption of surrounding water generating high density pumice and promoting rapid clast sinking variations in the physical properties of steam with temperature and pressure have important implications for submarine pyroclastic eruptions firstly highly vesicular pumice can be deposited on the seafloor at temperatures elevated significantly above ambient if they are erupted at sufficient depths to remain wholly submarine 200 m and either the fluid in which they cool contains heated water and or they only absorb sufficient water to sink secondly the rapid increase in density of the eruption column caused by condensation and the transition from buoyant gas filled to denser water saturated pumice lapilli together with turbulent mixing with the surrounding seawater favour collapse and transport of pyroclasts in water supported gravity currents finally this mixing of the ejecta with seawater and the ease of water ingestion into permeable pumice clasts suggest that water supported transport mechanisms can operate as primary dispersal processes in explosive submarine eruptions

submarine gold mineralization near lihir island new ireland fore arc papua new guinea gold rich siliceous veins with disseminated polymetallic sulfides and pyritic stockwork mineralization have been recovered from the top of conical seamount a shallow 1 050 m water depth submarine volcano located about 10 km south of lihir island papua new guinea grab samples from the summit of conical seamount contain the highest concentration of gold yet reported from the modern sea floor max 230 ppm au avg 26 ppm n 40 the gold occurs in sulfide rich veins of black amorphous silica hosted by intensely altered high k calc alkaline basalts sulfides in the veins consist of sphalerite galena pyrite chalcopyrite marcasite and a variety of cu pb as sb sulfosalts the gold occurs as native gold and electrum in the amorphous silica and as inclusions in the sulfides the highest gold concentrations are associated with high ag as sb and hg zoned alteration adjacent to the veins consists of illite smectite amorphous silica k feldspar secondary plagioclase minor chlorite and trace carbonate the association of gold with illite smectite amorphous silica and k feldspar indicates deposition from near neutral ph hydrothermal fluids however the auriferous polymetallic sulfide veins and the associated alteration are overprinted on stockwork pyrite mineralization that is associated with earlier acid alteration containing alunite aluminum phosphate sulfates kaolinite and other clay minerals the platy habit of the alunite in this assemblage the presence of alumium posphate sulfate minerals and the sulfur isotope ratios of the crystalline pyrite 8 6 to 0 2 34 s n 28 and alunite 7 5 and 6 4 34 s are consistent with a contribution of magmatic volatiles in the earliest stages of the hydrothermal system framboidal pyrite within and at the margins of the mineralized zone has 34 s values suggesting involvement of biogenic activity 11 6 to 13 9 34 s the gold rich veins at conical seamount are distinct from sea floor massive sulfide deposits and represent a new style of mineralization on the modern sea floor the mineralogy alteration geochemistry and texture of the veins resemble those of some subaerial epithermal gold deposits and indicate that features long considered to define a subaerial setting can also form in a submarine environment the proximity of conical seamount to the giant ladolam epithermal gold deposit on nearby lihir island also raises the possibility that both subaerial and submarine gold mineralization in the region may be related to the same district scale magmatic events

formation of volcanogenic massive sulfide deposits the kuroko perspective abstract the main objective of this paper is to identify the geochemical hydrological igneous and tectonic processes that led to the variations in the physical size geometry and chemical mineralogy metal ratios and zoning characteristics of volcanogenic massive sulfide deposits with respect to space from a scale of mining district size area to a global scale and time from a all volcanogenic massive sulfide deposits vmsds appear to have formed in extensional tectonic settings such as at mid ocean spreading centers backarc spreading centers and intracontinental rifts and failed rifts all vmsds appear to have formed in submarine depressions by seawater that became ore forming fluids through interactions with the heated upper crustal rocks submarine depressions especially those created by submarine caldera formation and or by large scale tectonic activities e g rifting become most favorable sites for the formation of large vmsds because of hydrological physical and chemical reasons the fundamental processes leading to the formation of vmsds include the following six processes 1 1 intrusion of a heat source typically a 10 3 km size pluton into an oceanic crust or a submarine continental crust causes deep convective circulation of seawater around the pluton the radius of a circulation cell is typically 5 km the temperature of fluids that discharge on the seafloor increases with time from the ambient temperature to a typical maximum of 350 c and then decreases gradually to the ambient temperatures in a time scale of 100 to 10 000 years the majority of sulfide and sulfate mineralization occurs during the waxing stage of hydrothermal activity 2 2 reactions between low temperature t 2 4 as disseminated gypsum and anhydrite in the country rocks 3 3 reactions of the modified seawater with higher temperature rocks at depths during the waxing stage cause the transformation of the seawater to metal and h 2 s rich ore forming fluids the metals and sulfide sulfur are leached from the county rocks the previously formed gypsum and anhydrite are reduced by fe 2 bearing minerals and organic matter providing additional h 2 s the mass of high temperature rocks that provide the metals and reduced sulfur is typically 10 11 tons 40 km 3 in volume the roles of magmatic fluids or gases are minor in most massive sulfide systems except for so 2 to produce acid type alteration in some systems 4 4 reactions between the ore forming fluids and cooler rocks in the discharge zone cause alteration of rocks and precipitation of some ore minerals in the stockwork ores 5 5 mixing of the ore forming fluids with local seawater within unconsolidated sediments and or on the seafloor causes precipitation of primitive ores with the black ore mineralogy sphalerite galena pyrite barite anhydrite 6 6 reactions between the primitive ores with later and hotter hydrothermal fluids cause transformation of primitive ores to matured ores that are enriched in chalcopyrite and pyrite variations in the mineralogical and elemental characteristics the geometry and the size of submarine hydrothermal deposits are controlled by the following four parameters 1 a the chemical and physical characteristics of seawater composition temperature density which depend largely on the geographical settings e g equatorial evaporating basins 2 b the chemical and physical characteristics of the plumbing system lithology fractures 3 c the thermal structure of the plumbing system which is determined largely by the ambient geothermal gradient and the size and temperature of the intrusive and 4 d the physical characteristics of the seafloor depth basin topography for example the submarine hydrothermal deposits developed in basaltic plumbing systems are generally poor in pb and ba compared to those developed in felsic plumbing systems the lower temperature systems are generally poorer in sulfides but richer in iron oxides and sulfates the higher temperature and larger hydrothermal systems tend to produce chalcopyrite and pyrite rich ores contrasts in the metal ratios between the noranda type archean vmsds and the younger vmsds reflect the differences in the geothermal gradient of the plumbing systems the submarine hydrothermal deposits developed in the near equatorial regions tend to form large continuous bedded type ores because of the likeliness of creating large stratified basins the basic processes of submarine hydrothermal mineralization have remained essentially the same throughout the geologic history from at least 3 5 billion year ago to the present

drilling shallow water massive sulfides at the palinuro volcanic complex aeolian island arc italy a subseafloor replacement style barite and sulfide occurrence was drilled in shallow waters at the palinuro volcanic complex the northernmost aeolian arc volcano in the tyrrhenian sea italy using a lander type drilling device 11 successful drill holes yielded a total of 13 5 m of core from a sediment filled depression located at a water depth of 630 to 650 m the longest continuous drill core recovered consists of 4 84 m of massive to semimassive barite and sulfides with abundant late native sulfur overprint seafloor observations suggest that the hydrothermal system associated with the formation of the subseafloor barite and sulfide ore zone is still active although black smoker activity does not occur on the seafloor the recovered drill core shows that the subseafloor deposit is zoned with depth the top of the mineralized zone is comprised of a variably silicified vuggy barite sulfide facies that shows notable polymetallic metal enrichment while the deeper portion of the mineralized zone is dominated by massive pyrite having distinctly lower base and precious metal grades metal zonation of the barite and sulfide deposit is related to the evolution of the hydrothermal fluids in space and time the barite cap and the massive pyrite present in the deeper portion of the mineralized zone appear to have formed early in the paragenesis during the main stage of the mineralization the barite cap was brecciated and cemented by a polymetallic assemblage of barite and pyrite with minor chalcopyrite and tetrahedrite trace famatinite and rare cinnabar lower temperature precipitates formed during the main stage of mineralization include sphalerite galena pyrite opal a and barite which are associated with traces of pb sb as sulfosalts such as bournonite seligmannite or semseyite a distinct mineral assemblage of fine grained anhedral enargite hypogene covellite chalcopyrite and galena is commonly associated with colloform sphalerite galena and pyrite as a late phase of this main stage colloform pyrite and marcasite are the last sulfides formed in the paragenetic sequence the deposit is interpreted to have formed from fluids having an intermediate sulfidation state although excursions to high and very high sulfidation states are indicated by the presence of abundant enargite and hypogene covellite laser ablation and conventional sulfur isotope analyses show that pyrite formed close to the seafloor within the zone of polymetallic metal enrichment has a variable sulfur isotope composition 34s 39 to 3 whereas a more narrow range is observed in the massive pyrite at depth 34s 10 to 0 similar variations were also documented for the late native sulfur overprint overall the negative sulfur isotope ratios at depth the intermediate to very high sulfidation conditions during mineralization and the abundance of native sulfur suggest contributions of magmatic volatiles to the mineralizing fluids from a degassing magma chamber at depth biological processes are interpreted to have played a major role during late stages of ore formation the combination of a subseafloor replacement deposit with a massive to semimassive barite cap rock overlying massive pyrite the intermediate to high sulfidation characteristics and the strong biological influence on the late stages of mineralization are distinct from other modern seafloor massive sulfide deposits and represents a style of submarine mineralization not previously recognized in a modern volcanic arc environment the barite and sulfide occurrence at palinuro shares many characteristics with porphyry related base metal veins and intermediate sulfidation epithermal deposits suggesting that metallogenic processes associated with arc related magmatic hydrothermal systems are not restricted to the subaerial environment

seismological evidence for variable growth of crust along the izu intraoceanic arc 1 the processes that create continental crust in an intraoceanic arc setting are a matter of debate to address this issue we conducted an active source wide angle seismic study to examine along arc structural variations of the izu intraoceanic arc the data used were acquired over a 550 km long profile along the volcanic front from sagami bay to tori shima the obtained structural model showed the existence of felsic to intermediate composition middle crust with a p wave velocity vp of 6 0 6 5 km s 1 in its upper part and 6 5 6 8 km s 1 in its lower part the thickness of the middle crust varied markedly from 3 to 13 km the underlying lower crust also consisted of two layers vp of 6 8 7 2 km s 1 in the upper part and vp of 7 2 7 6 km s 1 in the lower part the upper of these layers was interpreted to consist of plutonic gabbro and the lower layer was interpreted to be mafic to ultramafic cumulates average crustal velocities calculated from our model showed remarkable lateral variation which correlated well with arc volcanism low average crustal seismic velocities 6 7 km s 1 due to thick middle crust were obtained beneath basaltic volcanoes e g o shima miyake jima hachijo jima aoga shima while higher average velocities 7 1 km s 1 were obtained beneath rhyolitic volcanoes e g nii jima kurose south hachijo caldera myoji knoll and south sumisu caldera we concluded from these observations that continental crust grows predominantly beneath the basaltic volcanoes of the izu arc and that rhyolitic volcanism may be indicative of a more juvenile stage of crustal evolution or remelting of preexisting continental crust or both

subaqueous calderas in the archean abitibi greenstone belt an overview and new ideas abstract the 300 700 km abitibi greenstone belt in canada contains numerous world class archean volcanogenic massive sulfide vms deposits yet documentation of archean subaqueous calderas hosting such deposits is lacking the modern sunrise deposit in the myojin knoll caldera of the izu bonin arc shows that submarine calderas are first order sites for vms the hunter mine and normetal calderas as well as the blake river megacaldera complex are reviewed with respect to geometry regional geology physical volcanology dyke emplacement and hydrothermal carbonate alteration these subaqueous calderas are placed into the geodynamic context of the abitibi greenstone belt the abitibi belt displays a complex history of oceanic arc formation evolution collision and fragmentation with numerous types of volcanic edifices and sedimentary basins controlled by oblique subduction and mantle plumes analogue models yielding new insights into the sequence of caldera forming events and their synvolcanic fault patterns are applied to explain archean examples underpressure experiments develop a consistent sequence of fault patterns that compare favourably with natural caldera events during subsidence two sets of faults propagate from the magma chamber margin 1 early reverse outward dipping faults and 2 normal inward dipping faults the largest massive sulfide deposits form preferentially at the caldera margin based on experimental studies and detailed facies mapping the hunter mine caldera is of the segmented piecemeal variety and the normetal caldera is akin to a piston structure the blake river megacaldera contains 1 the e w trending misema 2 the nw trending new senator and 3 the ne trending noranda calderas with the latter two being nested graben type calderas consistent with oblique archean subduction the poorly documented hydrothermal carbonate alteration is discussed and a new exploration model for calderas is presented the alteration study was evaluated at three levels microprobe analysis geochemical staining of drill core and whole rock analyses there were two principal stages of alteration related to vms formation an early silicification phase developed either at depth and or at the seawater interface whereby silicification was bedding parallel in fine grained volcaniclastic deposits near the water surface a pervasive and extensive semi conformable carbonate alteration zone with a discordant focused root zone along faults evolved subsequently three distinct carbonate pairings were observed 1 proximal siderite sideroplesite fe ankerite next to the vms deposit 2 an intermediate ankerite fe dolomite zone and 3 a distal calcite dolomite zone the carbonate phase is associated with the mineralizing event transitions between carbonate species are subtle and changes are indicated by mineral assemblage overlap the silica carbonate hydrothermal alteration pattern is a replacement product and is far more widespread than has been documented this alteration can be easily confused with primary carbonates and chemical precipitation deposits e g banded iron formation if detailed volcano sedimentary facies mapping with alteration has not been conducted and if outcrop is insufficient

the role of granites in volcanic hosted massive sulphide ore forming systems an assessment of magmatic hydrothermal contributions assessment of geological geochemical and isotopic data indicates that a significant subgroup of volcanic hosted massive sulphide vhms deposits has a major or dominant magmatic hydrothermal source of ore fluids and metals this group which is typically characterised by high cu and au grades includes deposits such as those in the neoarchean doyon bousquet laronde and cambrian mount lyell districts these deposits are distinguished by aluminous advanced argillic alteration assemblages or metamorphosed equivalents intimately associated with ore zones in many of these deposits 34ssulphide is low with a major population below 3 34ssulphate differs from coexisting seawater and 34ssulphate sulphide 20 30 these characteristics are interpreted as the consequence of disproportionation of magmatic so2 as magmatic hydrothermal fluids ascended and cooled and as a definitive evidence for a significant magmatic hydrothermal contribution other characteristics that we consider diagnostic of significant magmatic hydrothermal input into vhms ore fluids include uniformly high 3 times modern seawater values salinities or very 18o enriched 18o 5 ore fluids we do not consider other criteria e g variable salinity moderately high 18ofluid 2 5 34ssulphide near 0 metal assemblages or a spatial association with porphyry cu or other clearly magmatic hydrothermal deposits that have been used previously to advocate significant magmatic hydrothermal contributions to be diagnostic as they can be produced by non magmatic processes known to occur in vhms mineral systems however in general a small magmatic hydrothermal contribution cannot be excluded in most vhms systems considered conclusive data that imply minimal magmatic hydrothermal contributions are only available in the paleoarchean panorama district where coeval seawater dominated and magmatic hydrothermal systems appear to have been physically separated this district which is characterised by chloritic and sericitic alteration assemblages and lacks aluminous advanced argillic alteration assemblages is typical of many vhms deposits around the world suggesting that for garden variety vhms deposits a significant magmatic hydrothermal contribution is not required other than deposits associated with advanced argillic alteration assemblages the only deposit for which we ascribe a major magmatic hydrothermal contribution is the devonian neves corvo deposit this deposit differs from other deposits in the iberian pyrite belt and around the world in being extremely sn rich with the sn closely associated with cu and in having formed from high 18o rich fluids 18ofluid 8 5 we consider these characteristics particularly the last as diagnostic of a significant magmatic hydrothermal contribution our analysis indicates that two subgroups of vhms deposits have a major magmatic hydrothermal contribution cu au rich deposits with aluminous alteration assemblages and reduced very sn rich deposits in which sn was introduced in a high temperature ore assemblage comparison with normal vhms deposits suggests that these subgroups of vhms deposits may form in specialised tectonic environments the cu au rich deposits appear to form adjacent to magmatic arcs an environment conducive to the generation of hydrous oxidised melts by melting metasomatised mantle in the wedge above the subducting slab this contrasts with the back arc setting of normal vhms deposits in which relatively dry granites in this contribution we use the term granite sensu latto formed by decompression melting drive seawater dominated hydrothermal circulation the tectonic setting of highly sn rich vhms deposits such as neves corvo is less clear however thick continental crust below the ore hosting basin may be critical as it is in other sn deposits

comparative mineralogy and geochemistry of gold bearing sulfide deposits on the mid ocean ridges a comparative study of the mineralogy and geochemistry of sulfide deposits on mid ocean ridges in the northeast pacific and the mid atlantic reveals common characteristics associated with primary gold enrichment average gold contents of 0 8 to 5 ppm au occur in sulfides from southern explorer ridge and axial seamount northeast pacific and from the tag hydrothermal field and snakepit vent field mid atlantic ridge the enrichment of gold in these deposits is consistently related to a phase of late stage low temperature 1 ppm au occur exclusively in pyritic assemblages and commonly with abundant fe poor sphalerite and a suite of complex pb sb as sulfosalts amorphous silica and locally barite or carbonate are important constituents of the gold rich precipitates but do not contain gold themselves high temperature 350 c black smoker assemblages consisting dominantly of pyrite chalcopyrite pyrrhotite isocubanite and abundant anhydrite are uniformly gold poor 0 2 ppm au to the extent that individual sulfides can be mechanically separated chemical analyses by neutron activation indicate that gold is most abundant in sphalerite up to 5 7 ppm au but also occurs in pyrite and marcasite samples of sphalerite with abundant inclusions of fine grained sulfosalts locally contain up to 18 ppm au suggesting that sulfosalts may be repositories for gold no free gold has been observed at 4000 magnification of polished specimens indicating that the gold is present only as submicroscopic inclusions or as a chemical constituent within the sulfides samples from gold rich deposits in the northeast pacific and mid atlantic are compared with similar but relatively gold poor sulfides from the galapagos rift and 13 n on the east pacific rise epr and with barren sulfides from 11 n epr 21 n epr the endeavour ridge and the southern juan de fuca ridge trace element analyses of more than 170 samples show that gold enrichment in almost all of the deposits is associated with high concentrations of ag as sb pb and zn and locally with high cd hg tl and ga in contrast gold is typically depleted in samples with high co se and mo the close association of au with ag as sb and pb may reflect the common behavior of these metals as aqueous sulfur complexes e g au hs 2 at low temperatures similar mineralogical and geochemical associations are observed in sulfide deposits from modern back arc settings and in the ancient geologic record

geomorphological variations at hydrothermal sites in the southern mariana trough relationship between hydrothermal activity and topographic characteristics abstract this study presents the first detailed geomorphological characterization of field scale geological features associated with hydrothermal systems in the southern mariana trough using near bottom swath mapping data collected by the autonomous underwater vehicle auv urashima during cruise yk09 08 and dive observation data acquired by the submersible shinkai6500 during cruise yk10 11 the motivation of this study is to examine the relationship between geomorphological characteristics and hydrothermal activity and to examine the nature of tectonic and volcanic controls on the hydrothermal system in this area two of the hydrothermal sites in the study area near 12 57 n 143 37 e are located on the active backarc spreading axis the snail and yamanaka sites one is located at the eastern foot of the axial high the archean site and two are located on an off axis knoll about 5 km from the spreading axis the pika and urashima sites the on axis area is divided into tectonically dominant and volcanically dominant zones volcanically dominant zones are characterized by mounds height 5 30 m diameter 250 320 m cut by fissures the snail and yamanaka sites are located adjacent to these fissures and are possibly represented local activity associated with a 4th order segment scale diking event on the basis of comparisons with previously studied cases on the east pacific rise with similar on axis geological characteristics in contrast to the on axis sites the off axis sites show no evidence of faulting the archean site at the foot of the axial high is characterized by a single mound height 50 100 diameter 250 300 m pronounced off axis lava flows and the presence of high amplitude rugged seafloor features the site is located at the top of the mound numerous ridge lines height mainly 2 6 m extend radially from the top of the mound and several chimney like structures up to approximately 6 m high occur on the top and slopes of the mound the pika site is located on the western peak of an off axis knoll and the newly discovered urashima site is located at the northern foot of the western peak of the same knoll the western peak is characterized by bumpy seabed textures formed by numerous smaller scale mounds and ridge lines however the eastern peak has a very smooth top and slope and shows no signs of hydrothermal activity numerous mounds heights 5 75 m diameters 50 350 m are developed on the comparatively gentle slope of the knoll in contrast to the numerous ridge lines height mainly 1 6 m developed on the relatively steep slopes of the knoll on the basis of the associated geomorphological features the three off axis sites archean pika and urashima were identified as localities created by relatively long term large scale hydrothermal activity as compared with sites in the on axis area the sustained activity at off axis sites appears closely related to an off axis upwelling magma system the three off axis hydrothermal sites are composed mainly of breccia assemblages that probably originated from hydrothermal activity with black smoker venting these areas are characterized by numerous ridge lines conical mounds and bumpy seabed texture whereas the on axis sites are characterized by the absence of ridge lines and the presence of white smoker and shimmering observed on dome shaped pillow mounds hence the distribution of ridge lines mound morphology and bumpy seabed texture is likely to correlate with hydrothermal activity

sulfur isotope evidence for magmatic contributions to submarine and subaerial gold mineralization conical seamount and the ladolam gold deposit papua new guinea conical seamount is a submarine alkali basalt volcano situated 10 km south of lihir island in the tabar feni island chain papua new guinea polymetallic zn pb ag au as sb veins pyritic stockwork and hydrothermally altered rocks discovered on the summit of conical seamount represent a unique example of submarine gold mineralization with mineralogical chemical and textural characteristics common to some subaerial epithermal systems the giant ladolam epithermal gold deposit 42 moz au is situated in the crater of an alkali stratovolcano on lihir island portions of the epithermal mineralization and alteration assemblages at ladolam are similar to those observed on conical seamount conical seamount has the lightest hydrothermal i e nondiagenetic sulfide d34s values 17 5 to 6 1 measured to date from modern sea floor hydrothermal systems a previous model for the conical seamount hydrothermal system suggested a contribution of magmatic volatiles in the earliest stages of mineralization stage 1 followed by a combination of sulfur leached from the underlying volcanic rocks and reduced seawater sulfate during the main base and precious metal precipitating event stage 2 new sulfur isotope data allow a reinterpretation of this model that suggests a greater input of magmatic volatiles plus boiling throughout both stage 1 and 2 mineralization sulfides from the ladolam gold deposit have a range of sulfur isotope values 12 9 to 3 6 similar to that of conical seamount these sulfur isotope data have previously been interpreted as the result of magmatic volatiles escaping from a crystallizing magma and mixing with dilute meteoric ground water anhydrite associated with the gold mineralization has two populations of d34s values 8 14 and 20 22 which indicate the interaction of magmatic volatiles with seawater that has previously been discounted the range of sulfide and sulfate d34s values at ladolam is reinterpreted to be due to a complex interaction of magmatic hydrothermal fluids meteoric ground water and seawater native sulfur in thermal areas in luise caldera and pyrite in offshore hydrothermal vents in luise harbor have similar d34s values to the ladolam epithermal mineralization indicating that fluids similar to the young ladolam ore forming event are still reaching the floor of luise caldera sulfur isotope data from conical seamount and the ladolam deposit suggest that magmatic volatiles have contributed a significant amount of sulfur to both these gold rich hydrothermal systems

caldera forming processes and the origin of submarine volcanogenic massive sulfide deposits certain volcanogenic massive sulfide vms ore deposits form in submarine calderas this association is well known but the link between caldera formation and the origins of the deposits remains poorly understood here we show that the size and location of a vms deposit within a submarine caldera may be determined by how and when the caldera formed these spatial temporal conditions control development of the hydrothermal system associated with the vms deposit we propose that caldera opening along outward dipping faults promotes magma degassing seawater influx and high temperature leaching resulting in a metal rich hydrothermal fluid these outward dipping faults are considered to provide critical pathways for ore forming fluids responsible for some caldera hosted vms deposits and may also be fundamentally important for the formation of many other caldera hosted ore deposit types

submarine hydrothermal mineralization in the okinawa trough sw of japan an overview abstract the okinawa trough is a heavily sedimented rifted back arc basin formed in an intracontinental rift zone submarine hydrothermal activity is located within the six back arc rifts located in the middle and southern okinawa trough and its distribution is controlled principally by tectonic factors subduction of the daito and gagua ridges beneath the ryukyu arc has resulted in fracturing of the brittle lithosphere beneath the okinawa trough hydrothermal activity is strongest in the volcanic arc rift migration phenomenon vamp area plus the jade site and southernmost part of the okinawa trough spot area which form the prolongation of these two ridges these areas are characterized by extremely high heat flow locally submarine hydrothermal fluids from the okinawa trough tend to be strongly influenced by interaction of the hydrothermal fluids with organic matter in the sediment resulting in high alkalinity and nh 4 concentrations of the fluids the fluids also contain high concentrations of co 2 of magmatic origin submarine hydrothermal mineralization in the trough is diverse the clam site consists principally of carbonate chimneys interaction of the hydrothermal fluid with organic matter in the sediment is particularly strong at this site this is most probably a sediment hosted deposit in which sulphide minerals have deposited within the sediment column leaving spent ore fluids to emerge at the seafloor the jade site consists of active and inactive sulphide sulphate chimneys and mounds the zn pb rich sulphides at this site contain the highest concentrations of pb ag and au so far recorded in submarine hydrothermal sulphide deposits at minami ensei knoll and hatoma knoll active and inactive chimneys consist principally of anhydrite and barite as a result of phase separation of the hydrothermal fluids beneath the seafloor an intense black smoker has recently been discovered at yonaguni knoll in the spot area if it is confirmed that sulphide mineralization is dominant at this site this could be a highly prospective area the most prospective areas for economic grade minerals in the okinawa trough appear to be the jade site and the spot area

growth and mass wasting of volcanic centers in the northern south sandwich arc south atlantic revealed by new multibeam mapping new multibeam swath bathymetric sonar data acquired using an em120 system on the rrs james clark ross supplemented by sub bottom profiling reveals the underwater morphology of a not vert similar 12 000 km2 area in the northern part of the mainly submarine south sandwich volcanic arc the new data extend between 55 45 s and 57 20 s and include protector shoal and the areas around zavodovski visokoi and the candlemas islands groups each of these areas is a discrete volcanic center the entirely submarine protector shoal area close to the northern limit of the arc forms a 55 km long east west trending seamount chain that is at least partly of silicic composition the seamounts are comparable to small subaerial stratovolcanoes in size with volumes up to 83 km3 indicating that they are the product of multiple eruptions over extended periods zavodovski visokoi and the candlemas island group are the summits of three 3 3 5 km high volcanic edifices the bathymetric data show evidence for relationships between constructional volcanic features including migrating volcanic centers structurally controlled constructional ridges satellite lava flows and domes and mass wasting of the edifices mass wasting takes place mainly by strong erosion at sea level and dispersal of this material along chutes probably as turbidity currents and other mass flows that deposit in extensive sediment wave fields large scale mass wasting structures include movement of unconsolidated debris in slides slumps and debris avalanches volcanism is migrating westward relative to the underlying plate and major volcanoes are asymmetrical being steep with abundant recent volcanism on their western flanks and gently sloping with extinct eroded volcanic sequences to their east this is consistent with the calculated rate of subduction erosion of the fore arc

effects of glacier retreat on the outbursts of goesvatnet southwest spitsbergen svalbard effects of the retreat of gasbreen south spitsbergen svalbard on the evolution of the ice dammed lake goesvatnet are shown for the period 1899 1991 the retreat and lowering of the damming ice masses have changed not only the sotred lake volume the lake geometry and the elevation slope and length of the subglacial outlet tunnel but also the frequency and magnitude of ourburst floods of goesvatnet for the estimation of peak discharges of outburst floods we computed an unbiased regression equation related to the progressive enlargement of subglacial tunnels using lake volume data and peak discharge data from the literature the derived equation is very similar to the original form of the clague mathews formula and answers the question why this formula has worked well in many cases peak discharges of goesvatnet in various years were estimated by means of the derived equation effects of the changed lake geometry as well as the changed length and slope of the subglacial outlet tunnel on the discharge during outbursts will be discussed by means of the nye clarke model observation of an out burst of goesvatnet in summer 1991 indicates that the outbursts may have been triggered by pressure decrease in the subglacial outlet tunnel increased discharge whereas flotation of the ice dam can be excluded

geological setting and mineralogical and geochemical investigations on sulfide deposits near 13 n on the east pacific rise geological mineralogical and geochemical investigations of hydrothermal deposits on one of the most studied segments of the east pacific rise near 13 n show that sulfides occur in four main tectonic settings these are 1 in the middle part of the central graben where active zones are located 2 at the top of the central graben where a second line of deposits is observed 3 on the marginal high which has sulfides both on its western tectonic side and at its top where a large deposit is located and 4 on the flank of the southeastern seamount which is the site of the most important deposit of the area the nature of the distribution of the samples through the area studied shows that hydrothermal activity has existed for more than 20 000 years mineral assemblages are dependent on the sample location the sulfide samples exhibit a wide range of mineralogical assemblages varying from high temperature hydrothermal cu fe rich chimneys black smokers which are characteristic structures of young edifices to mature cu fe rich massive sulfides similar to those found in the core of massive sulfide lenses mined on shore with respect to these observations a classification into nine types of sulfide assemblages is proposed hydrothermal activity at the axis is now increasing in intensity increase in temperature decrease in f s2 and f o2 the relative paucity of zn rich massive sulfides on the off axis structures is likely to be a consequence of the depletion of zn in the hydrothermal fluids r mode factor analysis applied to a population of nineteen representative bulk chemical analyses shows fractionation of the reduced sulfur fluid during the discharge into three main groups cu fe co se ca sr zn cd ag as pb and si ba displaying the principal types of sulfide samples collected in the 13 n area microprobe investigations conducted on sulfide minerals define the distribution of trace metal elements such as ag co se cd and as during the process of hydrothermal maturation of the deposits

high resolution magnetic signature of active hydrothermal systems in the back arc spreading region of the southern mariana trough high resolution vector magnetic measurements were performed on five hydrothermal vent fields of the back arc spreading region of the southern mariana trough using shinkai 6500 a deep sea manned submersible a new 3 d forward scheme was applied that exploits the surrounding bathymetry and varying altitudes of the submersible to estimate absolute crustal magnetization the results revealed that magnetic anomaly derived absolute magnetizations show a reasonable correlation with natural remanent magnetizations of rock samples collected from the seafloor of the same region the distribution of magnetic anomaly derived absolute magnetization suggests that all five andesite hosted hydrothermal fields are associated with a lack of magnetization as is generally observed at basalt hosted hydrothermal sites furthermore both the pika and urashima sites were found to have their own distinct low magnetization zones which could not be distinguished in magnetic anomaly data collected at higher altitudes by autonomous underwater vehicle due to their limited extension the spatial extent of the resulting low magnetization is approximately 10 times wider at off axis sites than at on axis sites possibly reflecting larger accumulations of nonmagnetic sulfides stockwork zones and or alteration zones at the off axis sites

carbon isotopic distribution of methane in deep sea hydrothermal plume myojin knoll caldera izu bonin arc implications for microbial methane oxidation in the oceans and applications to heat flux estimation the concentration and stable carbon isotopic composition 13c of methane have been measured for both seafloor hydrothermal venting fluids and effluent plume waters supplied from the vents at the myojin knoll caldera izu bonin arc in the western north pacific the hydrothermal end member concentrations and 13c of methane show near homogeneity among vents 41 2 mol kg and 16 3 0 8 pdb respectively while those in the effluent plume are stratified in the caldera and vertically exhibit a large variation 2 1 11 nmol kg and 29 0 to 11 3 pdb respectively comparison of concentration and 13c data between vent fluids and plume waters suggest that the methane is not conserved but microbially oxidized along with the plume gradual decrease of diffusive methane flux in proportion to distance from the vent fields supports the occurrence of significant microbial oxidation within the plume assuming steady state emission of methane from the vents 1 kinetic isotope effect due to the microbial oxidation k12 k13 2 methane flux from the vents fvents 3 heat flux from the vents q and 4 average turnover time of hydrothermal methane tall are estimated to be k12 k13 1 005 0 001 fvents 90 340 mol d q 30 110 mw and tall 60 240 d respectively the estimated turnover times however are not uniform within the water column around the vents a turnover time of less than 50 d for methane is one of the shortest values in the pelagic ocean while those at the distant points correspond to values more typical for deep ocean water

the changing vision of marine minerals abstract non fuel marine minerals are reviewed from the perspective of resources and their value as active analogs that can advance understanding of types of ancient ore deposits that formed in marine settings the theory of plate tectonics is the largest influence in expanding our vision of marine minerals and in developing our understanding of geologic controls of mineralization in space and time prior to the advent of plate tectonics we viewed the ocean basins as passive sinks that served as containers for particulate and dissolved material eroded from land this view adequately explained marine placer deposits heavy minerals and gems aggregates sand and gravel and precipitates phosphorites and manganese nodules although numerous sites of placer mineral deposits are known on continental shelves worldwide current activity pertains to diamond mining off southwestern africa tin mining off southeastern asia and intermittent gold mining off northwestern north america which are all surpassed economically by worldwide recovery of marine sand and gravel in turn dwarfed by offshore oil and gas with the advent of plate tectonics plate boundaries in ocean basins are recognized as active sources of mineralization in the form of hydrothermal massive sulfide deposits and proximal lower temperature deposits hosted in oceanic crust mafic at ocean ridges and felsic at volcanic island arcs and of magmatic ni cu sulfide chromite and pge deposits inferred to be present in the oceanic upper mantle lower crust based on their occurrence in ophiolites some 300 sites of hydrothermal active and relict mineralization most of them minor are known at this early stage of seafloor exploration on ocean ridges in fore arc volcanoes at back arc spreading axes and in arc rifts deposits formed at spreading axes and transported off axis by spreading are present in oceanic lithosphere but are virtually unknown the tag trans atlantic geotraverse hydrothermal field in the axial valley of the mid atlantic ridge latitude 26 n is considered to exemplify a major volcanogenic massive sulfide vms deposit forming at a spreading axis the most prospective of these occurrences lie within the 200 nautical mile 370 km wide exclusive economic zone eez of the nations of the volcanic island arcs of the western pacific where metal content of massive sulfides ag au ba cu pb sb zn exceeds that at ocean ridges plate tectonics early provided a framework for mineralization on the scale of global plate boundaries and is providing guidance to gradually converge on sites of mineralization through regional scales of plate reorganization with the potential to elucidate the occurrence of individual deposits e g eocene carlin type gold deposits investigation of the spectrum of marine minerals as active analogs of types of ancient mineral deposits is contributing to this convergence consideration of questions posed by brian skinner 1997 of what we do and do not know about ancient hydrothermal mineral deposits demonstrates the ongoing advances in understanding driven by investigation of marine minerals

mesozoic volcanogenic massive sulfide vms deposits in mexico abstract volcanogenic massive sulfide vms deposits are the most conspicuous type of deposits that formed during the mesozoic in mexico many mexican vms deposits display classical kuroko type mineral zonation and structure and some of them as cuale and la minita formed in shallow submarine environments the most prospective time window for the formation of vms deposits in mexico comprises the late jurassic and the early cretaceous vms stopped forming during the progressive continentalization of mexico since its metallotectonic processes dominated by extensive tectonics changed giving way to compression during the late early cretaceous new vms deposits did not form until after the opening of the gulf of california mesozoic vms deposits in mexico occur in submarine volcano sedimentary sequences that deposited essentially in association with back arc basins now found roughly along the boundaries between tectonostratigraphic terranes or within juvenile and slightly evolved arcs at the internal parts of the terranes while few others occur on the epicontinental seafloor and hinterlands of eastern mexico vms deposits are especially abundant in the guerrero composite terrane and are also present in the alisitos and parral terranes however new evidence referred in this paper indicate the western continental edge of the cratonic block of oaxaquia as a promising new prospective region for vms deposits interestingly no vms deposits are found in the northern part of the guerrero composite terrane despite the occurrence of marine volcano sedimentary sequences similar to those in the south such absence can be related to differential extensional unroofing much larger in the southern part of the guerrero composite terrane than in the northern part many vms deposits occur along or close to terrane boundaries especially around the guerrero composite terrane this distribution reflects the association between vms deposits and back arc basins which represented the frontal part of terranes or sub terranes that were ultimately accreted to the oaxaquia cratonic block as a consequence vms deposits usually display strong deformation and thrusting and their mineral and compositional zonation can be found overturned due to such common association between these deposits and terrane boundaries that reactivated during the cenozoic vms deposits are especially susceptible to overprinting by later metallogenic processes unrelated to vms producing environments this susceptibility may be held accountable for the complex mineral associations found in the francisco i madero deposit the inclusion fluids in the cuale la minita el rubi tizapa and campo morado deposits have salinities that range from 2 5 to 20 0 wt nacl equiv and temperatures of homogenization from 110 to 420 c although a particular fluid inclusion assemblage with daughter halite and sylvite from a stockwork at the tizapa deposit ranged salinities from 39 7 to 64 7 wt nacl and from 35 9 to 43 5 wt kcl and temperatures of homogenization from 440 to 550 c the general characteristics listed above may account for mineralizing fluids from magmatic marine and modified marine sources the entrainment of cool and oxidizing seawater within upwelling fluids increased as the paleohydrothermal systems waned the waning stages are normally represented by barite rich mineralizations and in the case of the la minita deposit this notoriously outlasted by mn oxide mineralization that reflects even more oxidizing conditions a similar evolution is deduced in cuale which is the only known mesozoic vms deposit in mexico that formed in a shallow submarine environment together with la minita the shallower the formation of vms deposits is the more quickly mineralizing fluids evolve into highly oxidized end members once the paleohydrothermal systems cooled down the extremely high salinity and hot inclusion fluids in tizapa despite the unavailability of further geochemical data are likely to represent magmatic brines that did not undergo significant mixing with seawater as for the temporal evolution of the studied deposits they generally exhibit 1 an increasing magmatic contribution from early to middle stages for both fluids as mineralizing fluids grew hotter and more saline and sulfur and 2 the waning of hydrothermal activity towards the last stages on mineralization which is characterized by the prevalence of seawater as fluids grew cooler and more diluted and oxidizing thus reflecting quite a typical evolution for most vms deposits elsewhere new 34s values for the same vms deposits reflect multiple sources of sulfur in the mineralizing fluids magmatic sedimentary metasedimentary and marine different degrees of dominance of either source for sulfur in these deposits and in their stages of mineralization have been inferred in a way that also responds to a schematic evolution from magmatic to seawater dominated paleohydrothermal systems

resources of the sea floor in his perspective rona reviews recent advances in knowledge of sea floor resources knowledge is expanding from those minerals eroded from land metals gemstones sand and gravel to those minerals concentrated by plate tectonic processes in ocean basins exploration of the oceans is at an early stage and development of marine mining is subject to economic and environmental considerations

geochemical characteristics of late carboniferous mineralization in the east tianshan a case study of the meiling deposit in the kalatage area abstract the meiling deposit in the kalatage area of the east tianshan which is the most recently identified late carboniferous hydrothermal vein deposit is located in a series of basic intermediate acidic volcanic pyroclastic rocks and contains ore bodies composed of single quartz sulfide veins and quartz sulfide vein groups in a stockwork en echelon arrangement with cu as the primary ore forming element and zn au and ag as secondary ore forming elements this study examines the geochemical characteristics of sub volcanic rocks and analyses fluid inclusions and stable isotopes h o and s to determine the metallogenic characteristics of the meiling deposit and discusses the pattern and geological significance of the mineralization in the kalatage area the quartz porphyry dacite porphyry and granite porphyry from the meiling deposit which are closely related to the mineralization are calc alkaline magmatic rocks that formed in an island arc setting most of the primary quartz fluid inclusions are liquid rich two phase fis with homogenization temperatures mainly of 154 251 c and salinities of 3 1 5 6 nacl equivalent and the sphalerite fluid inclusions are liquid rich two phase fis characterized by omogenization temperatures mainly of 156 263 c and salinities of 2 4 5 3 nacl equivalent which are low temperature and low salinity h and o isotope analysis show that in the polymetallic sulfide quartz stage the d values ranged from 79 3 to 54 0 the 18oh2o values ranged from 8 99 to 2 57 and the primary ore forming fluids were mixtures of a dominant meteoric water phase with minor amounts of magmatic fluid s isotope analysis shows that in the polymetallic sulfide quartz stage the 34s values of the main sulfides were distributed between 1 6 and 5 2 similar to the range of a homogenous sulfur source these results together with the s isotopic characteristics of the wall rocks from the meiling deposit demonstrate that the ore forming metals might have originated from the volcanic subvolcanic wall rocks these results show that the metallogenic characteristics of the meiling deposit are consistent with those of low temperature epithermal deposits and clearly demonstrate that the meiling deposit is a low temperature epithermal deposit that formed in an island arc setting the kalatage area provides a natural laboratory for studying the characteristics of geological mineralization during the arc basin system transformation process

post collisional crustal extension setting and vhms mineralization in the jinshajiang orogenic belt southwestern china abstract the jinshajiang orogenic belt job of southwestern china located along the eastern margin of the himalayan tibetan orogen includes a collage of continental blocks joined by paleozoic ophiolitic sutures and permian volcanic arcs three major tectonic stages are recognized based on the volcanic sedimentary sequence and geochemistry of volcanic rocks in the belt westward subduction of the paleozoic jinshajiang oceanic plate at the end of permian resulted in the formation of the chubarong dongzhulin intra oceanic arc and jamda weixi volcanic arc on the eastern margin of the changdu continental block collision between the volcanic arcs and the yangtze continent block during early middle triassic caused the closing of the jinshajiang oceanic basin and the eruption of high si and al potassic rhyolitic rocks along the permian volcanic arc slab breakoff or mountain root delamination under this orogenic belt led to post collisional crustal extension at the end of the triassic forming a series of rift basins on this continental margin arc significant potential for vhms deposits occurs in the submarine volcanic districts of the job mesozoic vhms deposits occur in the post collisional extension environment and cluster in the late triassic rift basins

native gold and gold rich sulfide deposits in a submarine basaltic caldera higashi aogashima hydrothermal field izu ogasawara frontal arc japan sulfide deposits with extremely high au concentrations up to 275 ppm avg 102 ppm n 15 high au ag ratios 0 24 n 15 and low cu cu zn ratios 0 03 n 15 were discovered in 2015 in active hydrothermal fields at a water depth of 760 m in a basalt dominated submarine caldera in the izu ogasawara frontal arc japan native gold grains occur in massive sulfide fragments concretions and metalliferous sediments from a sulfide mound 40 m across and 20 m high with up to 30 m high black smoker chimneys tiny native gold grains up to 14 m in diameter are mainly present in sulfide fallouts from chimney orifices and plumes larger native gold grains up to 150 m long occur mostly as discrete particles and or with amorphous silica and sulfides the larger gold grains are interpreted to represent direct precipitation from au bearing hydrothermal fluids circulating in and or beneath the unconsolidated sulfide mound deposits sulfur isotope compositions from a limited number of sulfide separates n 4 range from 4 3 to 5 8 34s similar to the quaternary volcanic rocks of the arc barite separates have values of 22 2 and 23 1 close to modern seawater values and indicate probable seawater sulfate origin the cu zn and pb concentrations in bulk samples of sulfide rich rocks are similar to those of volcanogenic massive sulfides formed in continental crustal environments the gold is interpreted to have formed by low temperature hydrothermal activity perhaps genetically different from systems with documented magmatic contributions or from seafloor hydrothermal systems in other island arc settings its presence suggests that basalt dominated submarine calderas situated on relatively thick continental crust in an intraoceanic arc setting such as the higashi aogashima knoll caldera may be perspective for gold mineralization

submarine hydrothermal activity and mineralization on the kurile and western aleutian island arcs n w pacific the kurile arc consists of at least 100 submarine volcanoes and 5 submarine caldera located mainly in the rear arc the arc is seismically very active particularly in the south and is characterized by extensive volcanism and hydrothermal activity at least one subaerial volcano on the arc medvezhy located on iturup island has an extremely shallow magma chamber and is intensely active three types of submarine hydrothermal deposit were recovered from this area hydrothermal manganese crusts nontronite and hydrothermal manganese crusts overlain by hydrogenous manganese oxides in addition submarine hydrothermal fe oxyhydroxides enriched in p were sampled from the submerged caldera of kraternaya bight in the central part of the arc these deposits appear to be analagous to deposits from santorini caldera on the aegean arc piip submarine volcano located in the western aleutian arc is characterized by two shallow summit craters both intermediate temperature anhydrite gypsum barite amorphous silica pyrite calcite and aragonite and low temperature nontronite fe oxyhydroxides hisingerite and ferromanganese crusts hydrothermal minerals were recovered from this seamount together with bacterial mats and giant clams the maximum measured temperature of the hydrothermal fluids was 133 c although the actual temperature may have reached 250 c both the kurile and aleutian arcs have the potential to host major submarine hydrothermal systems detailed exploration of these poorly studied areas using modern geophysical geochemical and sampling techniques is therefore strongly recommended

submarine volcanoes and high temperature hydrothermal venting on the tonga arc southwest pacific submarine hydrothermal vents and associated seafloor mineralization on the tonga arc have been found for the first time in the summit calderas of two shallow water volcanoes greatly extending the known areas and diversity of seafloor hydrothermal activity in the western pacific region the highest temperature vents 245 265 c occur at water depths of 385 540 m near the summit of one volcano at 24 s the vents are spatially related to basaltic dike swarms exposed at a summit cone and in the caldera walls clusters of large to 10 m high barite anhydrite and sulfide chimneys on the summit cone are vigorously discharging clear hydrothermal fluids with temperatures on the seawater boiling curve there is abundant evidence of phase separation which can be seen as flame like jets of steam h2o vapor at the chimney orifices pyrite marcasite sphalerite and chalcopyrite line the interiors of the highest temperature vents similar to black smoker chimneys on the mid ocean ridges

the gondou hydrothermal field in the ryukyu arc a huge hydrothermal system on the flank of a caldera volcano high resolution geophysical mapping was conducted from an autonomous underwater vehicle on the flank of daisan kume knoll in the ryukyu arc southwest of japan 1 m resolution bathymetry identified 264 spires 173 large mounds and 268 small mounds within a depression that is up to 1600 m wide and up to 60 m deep at water depths between 1330 and 1470 m hydrothermal venting is strongly inferred from the observation of plumes in sidescan sonar imagery and positive temperature anomalies over the spires and mounds this field named the gondou field has a giant mound g1 with a diameter of 280 m and a height of 80 m mound g1 has distinctive summit ridges comprised of multiple spires where acoustic plumes with temperature anomalies up to 1 12 c are observed indicative of high temperature venting other than mound g1 a number of active large mounds more than 30 m wide and spires over 10 22 m tall are common and they concentrate in the central and southern areas of the field suggesting that these areas are the center of present hydrothermal activity acoustic plumes imaged by side scan sonar at the gondou field are different in character from bubble plumes imaged in other hydrothermal fields in the ryukyu arc the plumes are diffused and deflected as they rise through the water column and have a shape consistent with black smokers

differential preservation in the geologic record of intraoceanic arc sedimentary and tectonic processes abstract records of ancient intraoceanic arc activity now preserved in continental suture zones are commonly used to reconstruct paleogeography and plate motion and to understand how continental crust is formed recycled and maintained through time however interpreting tectonic and sedimentary records from ancient terranes after arc continent collision is complicated by preferential preservation of evidence for some arc processes and loss of evidence for others in this synthesis we examine what is lost and what is preserved in the translation from modern processes to the ancient record of intraoceanic arcs composition of accreted arc terranes differs as a function of arc continent collision geometry forward facing collision can accrete an oceanic arc on to either a passive or an active continental margin with the arc facing the continent and colliding trench and forearc side first in a backward facing collision involving two subduction zones with similar polarity the arc collides backarc first with an active continental margin the preservation of evidence for contemporary sedimentary and tectonic arc processes in the geologic record depends greatly on how well the various parts of the arc survive collision and orogeny in each case preservation of arc terranes likely is biased towards those that were in a state of tectonic accretion for tens of millions of years before collision rather than tectonic erosion the prevalence of tectonic erosion in modern intraoceanic arcs implies that valuable records of arc processes are commonly destroyed even before the arc collides with a continent arc systems are most likely to undergo tectonic accretion shortly before forward facing collision with a continent and thus most forearc and accretionary prism material in ancient arc terranes likely is temporally biased toward the final stages of arc activity when sediment flux to the trench was greatest and tectonic accretion prevailed collision geometry and tectonic erosion vs accretion are important controls on the ultimate survival of material from the trench forearc arc massif intra arc basins and backarc basins and thus on how well an ancient arc terrane preserves evidence for tectonic processes such as subduction of aseismic ridges and seamounts oblique plate convergence and arc rifting forward facing collision involves substantial recycling melting and fractionation of continent derived material during and after collision and so produces melts rich in silica and incompatible trace elements as a result forward facing collision can drive the composition of accreted arc crust toward that of average continental crust

drill core based facies reconstruction of a deep marine felsic volcano hosting an active hydrothermal system pual ridge papau new guinea odp leg 193 abstract pual ridge is a deep marine felsic volcanic edifice in the eastern manus back arc basin papua new guinea with an estimated volume of 6 to 9 km 3 it is 1 1 5 km wide 20 km long and rises 500 600 m above the surrounding ocean floor the active pacmanus hydrothermal field on the crest of pual ridge at 1640 1690 m below sea level was the target of ocean drilling program leg 193 variably altered dacite lavas have been recovered from the subsurface of a low t discharge site snowcap and a high t black smoker site roman ruins reaching a maximum depth of 380 m below seafloor mbsf volcanic facies interpretation of these cores is difficult due to incomplete recovery and widespread pseudoclastic textures generated by fracturing and multi phase incomplete fluid dacite interaction however distinction of genuine volcaniclastic facies and facies with alteration related clastic appearance is important in order to define paleo seafloor positions within the volcanic stratigraphy that may be prospective for massive sulfide mineralization this has been accomplished using remnant primary characteristics indicative of transportation such as polymictic composition grading or textural evidence for differential movement of individual clasts three phases of volcanic activity can be distinguished and a proximal facies association dominated by coherent facies of dacite lavas exists below snowcap at roman ruins a medial facies association consists of lava flows with about equal proportions of coherent and volcaniclastic facies endogenous growth was an important process during lava flow emplacement and groundmass textures such as perlite flow banding and spherulites indicate that cooling rates were variable locally allowing for high temperature devitrification a tube pumice breccia unit is interpreted as the resedimented facies of a quench fragmented highly vesicular dacite lava carapace sulfide accumulations in the subsurface are restricted to roman ruins suggesting that the abundance of volcaniclastic facies favored ascent of metal bearing fluids due to the inherent high permeability evidence for mineralization at paleo seafloor positions at 110 and 195 mbsf indicate a complex interrelationship between volcanic and hydrothermal activity a facies model has been developed which may help to locate subaqueous felsic lava dominated volcanic centers and associated sulfide deposits in ancient volcano sedimentary successions

gold and native copper in supergene sulphides from the mid atlantic ridge in 1985 black smokers and massive sulphide deposits were discovered in the tag hydrothermal field in the rift valley of the mid atlantic ridge near latitude 26 08 n ref 1 sulphide samples from chimneys and mounds were recovered by dredging at depths between 3 620 and 3 670 m mineralogical and chemical analyses of the samples reveal 1 primary unaltered zn fe cu sulphides containing 0 8 4 0 p p m au 0 02 0 12 oz per ton and 2 secondary cu rich sulphides with native copper formed by supergene alteration of primary cu fe sulphides containing up to 16 4 p p m au 0 48 oz per ton as free native gold this is the first reported occurrence of secondary enrichment of gold and copper in recent submarine sulphides the high gold grades and native copper associated with secondary cu sulphides resemble occurrences in some supergene gossans overlying ancient massive sulphide deposits on land2

petrographic investigations of seafloor sediments from the kita bayonnaise submarine caldera shichito iwojima ridge izu ogasawara arc northwestern pacific abstract seafloor sediment samples were recovered by box and gravity corers from the kita bayonnaise submarine caldera shichito iwojima ridge izu ogasawara arc northwestern pacific the samples were separated into heavy light and claysized fractions for assessment of the hydrothermal contribution to the sediments of the submarine caldera and have been determined by microscopy sem edx and xrd heavy fractions are composed of predominant magnetic minerals dominant silicates and common sulfides and sulfate minor sulfosalt rutile phosphates carbonates and mn oxyhydroxides primary hydrothermal phases are composed of barite pyrite sphalerite chalcopyrite galena fahlore marcasite and rutile secondary mineral associations are usually present around galena and chalcopyrite grains the assemblages of barite pellet type pyrite chalcopyrite sphalerite and sphalerite chalcopyrite pyrite with galena or fahlore suggest that hydrothermal mineralization took place in the caldera the assemblages of epidote carbonate or chlorite in heavy transparent fractions and of sericite quartz feldspar in light fractions are indicative of hydrothermal alteration origin because of only dacite sampled from the caldera some of the smectite chlorite sericite and kutnahorite in clay sized fractions could also support the existence of hydrothermal activity other minerals consisting of hypersthene augite hornblende quartz feldspar and biotite are observed in heavy and light fractions as detritus of dacite in the caldera significant amounts of sulfides and barite mixtures ranging from about 1 0 to 3 5 wt in treated fractions 0 063 0 25 mm in gravity core samples suggest that hydrothermal mineralization took place several times in the caldera many lines of evidence indicate the presence of hydrothermal mineralizations associated with ag and th in the kita bayonnaise submarine caldera the mineralogical analyses are powerful for exploring signs of past or present hydrothermal mineralization in submarine calderas

remelting of an andesitic crust as a possible origin for rhyolitic magma in oceanic arcs an example from the izu bonin arc

mineralogy and composition of kuroko deposits from northeastern honshu and their possible modern analogues from the izu ogasawara bonin arc south of japan implications for mode of formation the kuroko deposits of ne honshu are a key type deposit for the study of volcanogenic massive sulfide deposits however these deposits have not been studied in detail since the early 1980 s and knowledge of their mode of formation is now dated in this study we present the analysis of 12 samples of the kuroko deposits 12 samples of submarine hydrothermal minerals from the sunrise deposit and 6 samples from suiyo seamount both of which are located on the izu ogasawara bonin arc for 27 elements for the kuroko deposit cd sb ag pb hg as zn cu are highly enriched au te bi ba mo are moderately enriched in tl are somewhat enriched and fe is not significantly enriched relative to the average continental crust within each of these deposits a similar pattern of element associations is apparent zn pb with as sb cd ag hg tl and au fe cu ba with as sb ag tl mo te and au si ba with ag and au caso4 the enrichment of the chalcophilic elements in these deposits is consistent with hydrothermal leaching of these elements from the host rocks which are dominantly rhyolite dacite in the case of the kuroko deposits rhyolite in the case of the sunrise deposit and dacite rhyolite in the case of the suiyo seamount deposit however this pattern of element enrichment is also similar to that observed in fumarolic gas condensates from andesitic volcanoes this suggests that there may be a significant magmatic contribution to the composition of the hydrothermal fluids responsible for the formation of the kuroko deposits although it is not yet possible to quantify the relative contributions of these two sources of elements the compositional data show that sunrise and suiyo seamount deposits are much closer compositionally to the kuroko deposits from ne honshu than are the submarine hydrothermal deposits from the jade site in the okinawa trough which contain on average significantly higher concentrations of pb zn sb as and ag than each of these deposits in spite of the greater similarity in tectonic setting of the hokuroku basin in which the kuroko deposits formed to the okinawa trough intracontinental rifted back arc basin compared to myojin knoll and suiyo seamount active arc volcanoes it appears that submarine hydrothermal deposits from myojin knoll and suiyo seamount are closer analogues of the kuroko deposit than are those from the okinawa trough the present data are consistent with the magmatic hydrothermal model for the formation of kuroko type deposits as formulated by urabe and marumo urabe t marumo k 1991 a new model for kuroko type deposits of japan episodes 14 246 251

textural and mineralogical changes associated with the incipient hydrothermal alteration of glassy dacite at the submarine pacmanus hydrothermal system eastern manus basin variably altered dacite from the pacmanus vent field in the eastern manus back arc basin papua new guinea was studied to determine the textural and mineralogical characteristics of hydrothermal alteration taking place in the immediate subsurface of this modern seafloor hydrothermal system detailed textural investigations show that fluid flow through the glassy dacite has been strongly controlled by the primary volcanic textures quench fractures and networks of interconnected perlitic cracks linking vesicles provided pathways for hydrothermal fluids flowing up to the seafloor hydrothermal alteration along these pathways resulted in the formation of pseudoclastic textures textural evidence suggests that alteration of the glassy dacite has not been sustained the samples have been affected by incipient hydrothermal alteration that is typically not preserved in ancient volcanic rock hosted massive sulfide deposits interaction of the glassy dacite with hydrothermal fluids primarily resulted in the conversion of volcanic glass to dioctahedral smectite only minor amounts of trioctahedral smectite were formed destruction of the volcanic glass and the formation of smectite caused pronounced changes in the chemistry of the dacite samples in particular a decrease in the sio2 whole rock content and the na2o k2o ratio the two alkali elements behaved differently during hydrothermal alteration due to preferential incorporation of k into the interlayer position of the newly formed dioctahedral smectite smectite formation occurred under rock dominated conditions although the addition of mg was required to form trioctahedral smectite from the silicic volcanic glass primary plagioclase was resistant to hydrothermal alteration highlighting the fact that the destruction of volcanic glass and feldspar are not necessarily contemporaneous in massive sulfide forming hydrothermal systems incipient alteration of the glassy dacite close to the seafloor occurred at temperatures below 150 c in an environment that allowed the development of steep temperature gradients comparison of the new data to the findings of deep drilling during odp leg 193 suggests that the smectite rich alteration in the immediate subsurface of the pacmanus hydrothermal vent field represents the low temperature equivalent of illite and chlorite rich alteration associations forming in the upflow zones of the hydrothermal fluids in the deeper portion of the volcanic sequence

volcanogenic related origin of sulfide rich quartz veins evidence from o and s isotopes at the geant dormant gold mine abitibi belt canada the geant dormant gold mine is a sulfide rich quartz vein gold deposit hosted by a volcano sedimentary sequence and an associated felsic endogenous dome and dikes the auriferous quartz sulfide veins were preceded by two synvolcanic gold bearing mineralizing events early sulfidic seafloor related and later disseminated pyrite in the felsic dome this deposit differs from classical archean auriferous quartz vein deposits by the low carbonate and high sulfide contents of the veins and by their formation prior to ductile penetrative deformation the 18o values of quartz associated with seafloor related auriferous sulfides average 11 9 0 6 n 3 the seafloor hydrothermal fluids had a 18o value of 3 2 calculated at 250 c the oxygen isotope composition of quartz and chlorite from veins average 12 5 0 3 n 20 and 5 9 1 1 n 4 respectively assuming oxygen isotope equilibrium between quartz and chlorite the veins formed at a temperature of 275 c which is consistent with the calculated temperature of 269 10 c from chlorite chemistry the gold bearing fluids had a 18o value of 4 7 calculated at 275 c the 34s values of sulfides from the three gold events range from 0 6 to 2 8 n 32 and are close to magmatic values sulfur isotope geothermometry constrains the sulfide precipitation in the gold bearing veins at a temperature of 350 c the similarity of the isotope data the calculated 18o of the mineralizing fluids and the likely seawater fluid source suggest that the three mineralizing events are genetically related to a volcanogenic hydrothermal system the high value of the auriferous fluids 18o 4 7 is attributed to a significant magmatic fluid contribution to the evolved seawater dominated convective hydrothermal system the two stage filling of veins at increasing temperature from quartz chlorite 275 c to sulfides 350 c may reflect the progressive maturation of volcanogenic hydrothermal systems these results together with field and geochemical data suggest that formation of gold rich volcanogenic systems require specific conditions that comprise a magmatic fluid contribution and gold from arc related felsic rocks coeval with the mineralizing events this study shows that some auriferous quartz vein orebodies in archean terranes are formed in volcanogenic rather than mesothermal systems

age constraints and characteristics of subaqueous volcanic construction the archean hunter mine group abitibi greenstone belt the archean hunter mine group hmg is a 6 7 km thick south facing volcanic sequence at the southern limit of the northern volcanic zone abitibi greenstone belt the hmg is composed of 1 felsic lava flows and endogenous and exogenous domes with autoclastic breccia deposits 2 pyroclastic fire fountain products and high to low concentration volcaniclastic sediment gravity flows of autoclastic and pyroclastic origin interstratified with iron formation 3 mafic pillowed and brecciated flows 4 a prominent felsic dyke swarm and 5 abundant mafic intrusions the stratigraphy is complex as inferred by detailed volcano sedimentary facies mapping a 5 7 km wide n trending felsic dyke swarm coupled with abundant synvolcanic and synsedimentary faults chaotic breccias and mineralization suggest a central volcanic depression which is consistent with a caldera structure voluminous effusive activity and explosive fire fountaining eruptions contributed to caldera development rifting of this arc edifice is supported by the presence of felsic dykes and mafic tholeiitic sills the lithofacies and overlying komatiites flows are consistent with a deep water marine setting u pb age determinations help constrain the hmg caldera formation to 6 million years the sampled quartz feldspar phyric dykes yielded u pb zircon ages of 2728 3 4 3 4 and 2728 9 0 8 ma and together with a previously published dyke age of 2729 6 1 4 ma from the same dyke swarm a case is made that the swarm evolved over 1 million years 2728 6 2729 6 ma a lobate flow and flow breccia revealed similar ages of 2724 6 4 6 1 7 and 2727 6 4 2 2 0 ma whereas the up to 1 km thick gabbro diorite roquemaure sill precedes both felsic dykes and flows as indicated by the 2731 8 2 2 2 0 ma age the 2706 4 5 7 4 3 ma feldspar quartz phyric dykes occurred late in the history of the hmg and were not associated with subaqueous volcanic construction the 207pb 206pb ages of analyzed zircon fractions have an inherited component within error of the oldest crystallization ages obtained for the hmg complex collectively these age determinations and inheritance as well as stratigraphy support a tripartite division into a 2734 2730 ma lower formational stage a 2732 ma middle formational stage and a 2730 2728 ma upper formational stage the study shows that age constraints without detailed facies mapping may lead to erroneous conclusions on volcanic construction and that felsic volcanic dykes and flows may inherit an earlier component of their own volcanic edifice

timing and characteristics of the archean subaqueous blake river megacaldera complex abitibi greenstone belt canada abstract the archean blake river group of the abitibi greenstone belt represents a megacaldera complex which evolved over 8 11 m y from approximately 2704 to 2696 ma the early misema caldera developed from a series of amalgamated shield volcano complexes identified by the remnants of mafic dyke and sill systems these remnants are found in the jevis clericy montsabrais renault clifford tannahill and colnet regions where summit calderas are delineated by circular ring dyke structures the secondary 330 trending new senator caldera formed within the envelope of the misema caldera and exhibits a box work graben type structure finally the felsic dominated 070 striking noranda caldera well known for its vms endowment represents the final collapse of the megacaldera complex deformation has overprinted the calderas but structural patterns can be used in order to reconstruct the pre existing volcanic architecture structures such as mafic ring dyke complexes and rhyolitic dome flow complexes have nucleated fold geometries and synvolcanic fractures were reactivated within zones of ductile deformation precise u pb geochronological analyses were conducted in selected areas with specific emphasis on the misema and new senator calderas felsic volcanism progressed throughout the evolution of the megacaldera complex and intermediate to felsic units have been dated to limit this evolution the misema caldera formed between 2704 and 2702 ma via the amalgamation of shield volcanoes that were probably active prior to 2704 ma the new senator caldera was generated between 2702 and 2700 ma during paroxysmal felsic volcanism followed by the collapse of the noranda caldera culminating between 2700 2696 ma the misema caldera was generated by a gravitational stress field consistent with the formation of ring and radial dyke architecture whereas the se trending new senator caldera is more compatible with a sw trending principal compression direction related to oblique convergence in the abitibi belt the noranda caldera is interpreted as a ne rift structure that formed in the final stages of megacaldera evolution