the essence of time fertile skarn formation in the variscan orogenic belt abstract extending from northern america to central china the variscan belt is a paleozoic orogen exceptionally well endowed in magmatic hydrothermal ore deposits including skarn deposits yet the genesis of fertile skarns and their distinction from barren equivalents in orogenic zones is only poorly constrained here we present innovative u pb laser ablation inductively coupled plasma mass spectrometry geochronology of garnet from different skarns in the erzgebirge a classic metallogenic province in central europe garnet ages obtained not only constrain the timing of fertile skarn formation and associated sn w fe zn cu and in mineralization but also clearly distinguish these from barren skarn bodies we show that barren skarns formed during times of peak regional metamorphism at 340 ma whereas mineralized skarns are temporally associated with late orogenic magmatism at 325 313 ma as well as post orogenic magmatism at 308 295 ma the recognition of discrete mineralization events associated with the largest and economically most important skarn deposits provides valuable insight into the punctuated evolution of magmatic hydrothermal systems in ancient collisional orogens on a regional scale this has important implications to direct future mineral exploration

sulfur in serpentinized oceanic peridotites serpentinization processes and microbial sulfate reduction the mineralogy contents and isotopic compositions of sulfur in oceanic serpentinites reflect variations in temperatures and fluid fluxes serpentinization of 1 ma peridotites at hess deep occurred at high temperatures 200 400 c and low water rock ratios oxidation of ferrous iron to magnetite maintained low o2 and produced a reduced low sulfur assemblage including nife alloy small amounts of sulfate reduction by thermophilic microbes occurred as the system cooled producing low 34s sulfide 1 5 to 23 7 in contrast serpentinization of iberian margin peridotites occurred at low temperatures 20 200 c and high water rock ratios complete serpentinization and consumption of ferrous iron allowed evolution to higher o2 microbial reduction of seawater sulfate resulted in addition of low 34s sulfide 15 to 43 and formation of higher sulfur assemblages that include valleriite and pyrite the high so4 total s ratio of hess deep serpentinites 0 89 results in an increase of total sulfur and high 34s of total sulfur mean 8 in contrast iberian margin serpentinites gained large amounts of 34s poor sulfide mean total s 3800 ppm and the high sulfide total s ratio 0 61 results in a net decrease in 34s of total sulfur mean 5 thus serpentinization is a net sink for seawater sulfur but the amount fixed and its isotopic composition vary significantly serpentinization may result in uptake of 0 4 14 1012 g s yr 1 from the oceans comparable to isotopic exchange in mafic rocks of seafloor hydrothermal systems and approaching global fluxes of riverine sulfate input and sedimentary sulfide output

stable isotope compositions of serpentinite seamounts in the mariana forearc serpentinization processes fluid sources and sulfur metasomatism the mariana and izu bonin arcs in the western pacific are characterized by serpentinite seamounts in the forearc that provide unique windows into the mantle wedge we present stable isotope o h s and c data for serpentinites from conical seamount in the mariana forearc and s isotope data for torishima seamount in the izu bonin forearc in order to understand the compositions of fluids and temperatures of serpentinization in the mantle wedge and to investigate the transport of sulfur from the slab to the mantle wedge six serpentine mineral separates have a restricted range of 18o 6 5 8 5 antigorite separates have d values of 29 5 to 45 5 that reflect serpentinization within the mantle wedge whereas chrysotile has low d values 51 8 to 84 0 as the result of re equilibration with fluids at low temperatures fractionation of oxygen isotopes between serpentine and magnetite indicate serpentinization temperatures of 300 375 c two late cross fiber chrysotile veins have higher 18o values of 8 9 to 10 8 and formed at lower temperatures as low as 100 c aqueous fluids in equilibrium with serpentine at 300 375 c had 18o 6 5 9 and d 4 to 26 consistent with sediment dehydration reactions at temperatures 200 c in the subducting slab rather than a basaltic slab source three aragonite veins in metabasalt and siltstone clasts within the serpentinite flows have 18o 16 7 24 5 consistent with the serpentinizing fluids at temperatures 250 c 13c values of 0 1 2 5 suggest a source in subducting carbonate sediments the 34s values of sulfide in serpentinites on conical seamount 6 7 to 9 8 result from metasomatism through variable reduction of aqueous sulfate 34s 14 derived from slab sediments despite sulfur metasomatism serpentinites have low sulfur contents generally 164 ppm that reflect the highly depleted nature of the mantle wedge the serpentinites are mostly enriched in 34s median 34ssulfide 4 5 consistent with a 34s enriched mantle wedge as inferred from arc lavas

different isotope and chemical patterns of pyrite oxidation related to lag and exponential growth phases of acidithiobacillus ferrooxidans reveal a microbial growth strategy abstract the solution chemistry during the initial slow increase of dissolved iron and sulfate and main stage rapid increase of dissolved iron and sulfate of pyrite leaching by acidithiobacillus ferrooxidans af at a starting ph of 2 05 shows significant differences during the initial stage ferrous iron fe 2 is the dominant iron species in solution and the molar ratio of produced sulfate so 4 2 and total iron fe tot is 1 1 thus does not reflect the stoichiometry of pyrite fes 2 during the main stage ferric iron fe 3 is the dominant iron species in solution and the so 4 2 fe tot ratio is with 1 9 close to the stoichiometry of fes 2 another difference between initial and main stage is an initial trend to slightly higher ph values followed by a drop during the main stage to ph 1 84 these observations raise the question if there are different modes of bioleaching of pyrite and if there are what those modes imply in terms of leaching mechanisms different oxygen and sulfur isotope trends of sulfate during the initial and main stages of pyrite oxidation confirm that there are two pyrite bioleaching modes the biochemical reactions during initial stage are best explained by the net reaction fes 2 3o 2 fe 2 so 4 2 so 2 g the degassing of sulfur dioxide so 2 acts as sink for sulfur depleted in 34 s compared to pyrite and is the cause of the so 4 2 fe tot ratio of 1 1 and the near constant ph during the exponential phase pyrite sulfur is almost quantitatively converted to sulfate according to the net reaction fes 2 15 4o 2 1 2h 2 o fe 3 2so 4 2 h we hypothesize that the transition between the modes of bioleaching of pyrite is due to the impact of the accumulation of ferrous iron which induces changes in the metabolic activity of af and may act as an inhibitor for the oxidation of sulfur species this transition defines a fundamental change in the growth strategy of af a mode where bacteria gain energy by oxidation of elemental sulfur to sulfite but show little growth is switched into a mode where bacteria gain a smaller amount of energy by the oxidation of ferrous iron but induce much faster pyrite leaching rates due to the production of ferric iron

sulfide remobilization during low temperature alteration of seafloor basalt abstract the behavior of sulfide minerals during the physical and chemical changes accompanying seafloor alteration was studied in three basalt flows from the bottom of d s d p hole 418a leg 53 the rocks are mildly altered and contain primary authigenic and vein sulfide minerals sulfide habit mineralogy and trace element content are inter related and are correlated with the extent and type of silicate and oxide alteration incipient alteration at 90 100 c was accompanied by low temperature reequilibration of pyrrhotite and locally by the oxidation of pyrrhotite to pyrite plus magnetite the dominant stage of alteration at 90 c is characterized by dissolution and local redistribution of pyrite and chalcopyrite whose precipitation appears to be controlled by the water rock ratio and the extent to which the water has been modified by reaction with the basalt chalcopyrite was concentrated relative to pyrite by slight changes in fluid composition caused by reaction with other minerals concurrent precipitation of smectite causes a net increase in rock volume tending to restrict seawater access calculations of rock cooling rate through time suggest that the most prolonged hydrothermal circulation occurs at low temperatures giving rise to pervasive low temperature alteration assemblages

carbon geochemistry of serpentinites in the lost city hydrothermal system 30 n mar abstract the carbon geochemistry of serpentinized peridotites and gabbroic rocks recovered at the lost city hydrothermal field lchf and drilled at iodp hole 1309d at the central dome of the atlantis massif mid atlantic ridge 30 n was examined to characterize carbon sources and speciation in oceanic basement rocks affected by long lived hydrothermal alteration our study presents new data on the geochemistry of organic carbon in the oceanic lithosphere and provides constraints on the fate of dissolved organic carbon in seawater during serpentinization the basement rocks of the atlantis massif are characterized by total carbon tc contents of 59 ppm to 1 6 wt and 13ctc values ranging from 28 7 to 2 3 in contrast total organic carbon toc concentrations and isotopic compositions are relatively constant 13ctoc 28 9 to 21 5 and variations in 13ctc reflect mixing of organic carbon with carbonates of marine origin saturated hydrocarbons extracted from serpentinites beneath the lchf consist of n alkanes ranging from c15 to c30 longer chain hydrocarbons up to c40 are observed in olivine rich samples from the central dome iodp hole 1309d occurrences of isoprenoids pristane phytane and squalane polycyclic compounds hopanes and steranes and higher relative abundances of n c16 to n c20 alkanes in the serpentinites of the southern wall suggest a marine organic input the vent fluids are characterized by high concentrations of methane and hydrogen with a putative abiotic origin of hydrocarbons however evidence for an inorganic source of n alkanes in the basement rocks remains equivocal we propose that high seawater fluxes in the southern part of the atlantis massif likely favor the transport and incorporation of marine dissolved organic carbon and overprints possible abiotic geochemical signatures the presence of pristane phytane and squalane biomarkers in olivine rich samples associated with local faults at the central dome implies fracture controlled seawater circulation deep into the gabbroic core of the massif thus our study indicates that hydrocarbons account for an important proportion of the total carbon stored in the atlantis massif basement and suggests that serpentinites may represent an important as yet unidentified reservoir for dissolved organic carbon doc from seawater

formation processes of framboidal pyrite abstract pyrite framboid formation may be the result of four consecutive processes 1 nucleation and growth of initial iron monosulfide microcrystals 2 reaction of the microcrystals to greigite fe 3 s 4 3 aggregation of uniformly sized greigite microcrystals i e framboid growth and 4 replacement of greigite framboids by pyrite the uniform morphology uniform size range and ordering of the microcrystals in individual framboids as well as the range of observed framboid structures from irregular aggregates to densely packed spherical aggregates and polyframboids are consequences of these processes using dlvo theory derjaguin landau verwey and overbeek we have evaluated the stability of colloidal iron monosulfide suspensions with ionic strengths typical of marine and lacustrine waters in addition to van der waals attractive and double layer repulsive forces a term is included to account for the ferrimagnetic properties of greigite numerical models predict that magnetically saturated greigite particles 0 1 m in diameter will rapidly aggregate in either marine or fresh water the aggregation model is in agreement with the sequence of greigite formation followed by pyrite framboid formation established in a previous experimental study sweeney and kaplan 1973 and is consistent with the occurrence of framboids composed of other magnetic minerals e g greigite magnetite and magnesioferrite based on the temperature dependent magnetic properties of greigite and aging experiments in hydrothermal solutions this mechanism for framboid formation via precursor greigite could operate to temperatures of 200 c consistent with the occasional occurrence of pyrite framboids in the paragenesis of metalliferous ore deposits

serpentinization and heat generation constraints from lost city and rainbow hydrothermal systems abstract the discovery of ultramafic hosted hydrothermal systems at rainbow 36 n mar and lost city a vent site approximately 15 km west of the mar at 30 n provides unique perspectives on chemical and heat generating processes associated with serpentinization at a range of chemical and physical conditions heat balance calculations together with constraints imposed by geochemical modeling indicate that significant changes in temperature are not likely to occur at either vent system as a result of the exothermic nature of olivine hydrolysis at rainbow the relatively high temperatures in subseafloor reaction zones in excess of 400 c which must be linked to magmatic processes inhibit olivine hydrolysis effectively precluding mineralization induced heating effects geochemical modeling of the lost city vent fluids indicates temperatures in excess of those measured 40 75 c the relatively high subseafloor temperatures 200 50 c requires conductive cooling of the fluids on ascent to the seafloor a scenario in keeping with the mineralization of chimney structures actually observed although the intermediate temperatures predicted for subseafloor reaction zones at lost city could be expected to enhance olivine to serpentine conversion dissolved cl k cl and na cl ratios of the lost city vent fluids are virtually unchanged from seawater values and indicate little hydration of olivine which is a necessary condition for exothermic heat generation by serpentinization apparently the fluid rock mass ratio is too high or fluid residence times too low for this to occur to any significant extent thus in spite of the off axis location of the lost city vents and apparent lack of a localized heat source mineralization reactions likely play an insignificant role in accounting for hydrothermal circulation it is more likely that tectonic processes associated with the slow spreading mar permit access of seawater to relatively deep and still hot lithospheric units and or near axis magmatic heat sources before venting additional chemical and physical temperature flow rate data for lost city and similar hydrothermal systems are needed to test key elements of the proposed model

zircon u pb and phlogopite 40 ar 39 ar age of the chengchao and jinshandian skarn fe deposits southeast hubei province middle lower yangtze river valley metallogenic belt china the chengchao and jinshandian deposits in the southeast hubei province are the two largest skarn fe deposits in the middle lower yangtze river valley metallogenic belt mlyrvmb china they are characterized by nw striking orebodies that are developed along the contacts between the late mesozoic granitoid and triassic carbonate and clastic rocks new sensitive high resolution ion microprobe and laser ablation inductively coupled plasma mass spectrometry zircon u pb dating of the mineralization related quartz diorite and granite at chengchao yield ages of 129 2 and 127 2 ma respectively and those at jinshandian of 127 2 and 133 1 ma respectively these results are interpreted as the crystallization age of these intrusions hydrothermal phlogopite samples from the skarn ores at chengchao and jinshandian have the plateau 40ar 39ar ages of 132 6 1 4 and 131 6 1 2 ma respectively these results confirm that both intrusions and associated skarn fe mineralization were formed contemporaneously in the middle early cretaceous time new zircon u pb and phlogopite 40ar 39ar ages in this study when combined with available precise geochronological data demonstrate that there were two discontinuous igneous events corresponding to two episodes of skarn fe bearing mineralization in the southeast hubei province 1 140 136 ma diorites and quartz diorites and 141 137 ma skarn cu fe or fe cu deposits and 2 133 127 ma quartz diorites and granites and 133 132 ma skarn fe deposits this scenario is similar to that proposed for the entire mlyrvmb the intrusions related to skarn fe deposits show obviously petrological and geochemical differences from those related to skarn cu fe or fe cu deposits the former are quartz diorite and diorite in petrology and have similar adakitic geochemical signatures and in equilibrium with a garnet rich residue whereas the latter are petrologically granite and quartz diorite that are distinguishable from adakitic rocks and in equilibrium with a plagioclase residue these features indicated that two episodes of magmatism and the formation of skarn fe bearing deposits in the southeast hubei province mlyrvmb might be associated lithosphere thinning induced by asthenosphere upwelling during the late mesozoic

sulfur mineralogy and geochemistry of serpentinites and gabbros of the atlantis massif iodp site u1309 abstract in situ uplifted portions of oceanic crust at the central dome of the atlantis massif mid atlantic ridge 30 n were drilled during expeditions 304 and 305 of the integrated ocean drilling program iodp and a 1 4 km section of predominantly gabbroic rocks with minor intercalated ultramafic rocks were recovered here we characterize variations in sulfur mineralogy and geochemistry of selected samples of serpentinized peridotites olivine rich troctolites and diverse gabbroic rocks recovered from hole 1309d these data are used to constrain alteration processes and redox conditions and are compared with the basement rocks of the southern wall of the atlantis massif which hosts the lost city hydrothermal field 5 km to the south the oceanic crust at the central dome is characterized by ni rich sulfides reflecting reducing conditions and limited seawater circulation during uplift and exhumation seawater interaction in gabbroic dominated domains was limited as indicated by homogeneous mantle like sulfur contents and isotope compositions of gabbroic rocks and olivine rich troctolites local variations from mantle compositions are related to magmatic variability or to interaction with seawater derived fluids channeled along fault zones the concomitant occurrence of mackinawite in olivine rich troctolites and an anhydrite vein in a gabbro provide temperature constraints of 150 200 c for late circulating fluids along local brittle faults below 700 m depth in contrast the ultramafic lithologies at the central dome represent domains with higher seawater fluxes and higher degrees of alteration and show distinct changes in sulfur geochemistry the serpentinites in the upper part of the hole are characterized by high total sulfide contents high 34ssulfide values and low 34ssulfate values which reflect a multistage history primarily controlled by seawater gabbro interaction and subsequent serpentinization the basement rocks at the central dome record lower oxygen fugacities and more limited fluid fluxes compared with the serpentinites and gabbros of the lost city hydrothermal system our studies are consistent with previous results and indicate that sulfur speciation and sulfur isotope compositions of altered oceanic mantle sequences commonly evolve over time heterogeneities in sulfur geochemistry reflect the fact that serpentinites are highly sensitive to local variations in fluid fluxes temperature oxygen and sulfur fugacities and microbial activity

a sulfur isotopic profile through the troodos ophiolite cyprus primary composition and the effects of seawater hydrothermal alteration the sulfide mineralogy sulfur contents and sulfur isotopic compositions were determined for a section through the troodos ophiolite as represented by drillcore from holes cy1a cy2a and cy4 plus selected outcrops with the goals of understanding the geochemistry of s during hydrothermal alteration of the crust and obtaining a mass balance for sulfur in altered ophiolitic crust primary 34s values of the ophiolite are 0 1 consistent with a morb or bab like sulfur source the volcanic rocks have low sulfur contents mean 40 ppm and generally negative 34s values to 26 1 these are the results of sulfur loss through oxidation by seawater at low temperatures 100 c and isotopic fractionation during partial oxidation of igneous sulfides the sheeted dike complex is uniformly enriched in 34s 34s 5 4 as the result of mixing of reduced cretaceous seawater sulfate 17 with primary crustal sulfide sulfate was reduced through oxidation of ferrous iron in the dikes and conversion of igneous pyrrhotite to secondary pyrite decreasing water rock ratios with depth in the plutonic section led to a general decrease in 34s values downward to igneous values in the lower gabbros seawater effects are present locally to the base of the section however cooling of diffuse upwelling hydrothermal fluids t 250 c produced s and metal enrichments in the upper dikes 1 26 wt s whereas sulfur was lost from the lower dikes and upper plutonic rocks which contain 200 ppm and 10 1640 ppm s respectively through higher temperature 350 c alteration reactions epidosites at the dikes gabbro transition contain 30 ppm s with 34s 5 2 6 5 these rocks lost sulfur and metals during reaction with 34s enriched deep hydrothermal fluids at high temperatures 350 400 c and high water rock ratios in basal hydrothermal upflow zones the ultimate effect of alteration of ophiolitic crust is redistribution of igneous sulfur within the crust and exchange of crustal sulfur for seawater sulfur with little net change in the sulfur content of the crust overall the bulk altered troodos crust is enriched in 34s having 34s 3 6 fluxes of sulfur between seawater and ophiolitic crust are an order of magnitude smaller than modern riverine input to and sedimentary sulfide output from the oceans the amount of seawater sulfur that reacted with troodos ophiolite crust is approximately twice that for a composite section of oceanic crust consistent with the generally greater intensity of alteration of ophiolitic crust compared to modern oceanic crust

contemporaneous formation of adjacent porphyry and epithermal cu au deposits over 300 ka in northern luzon philippines there is commonly a close spatial relation between porphyry cu au and high sulfidation epithermal cu au deposits throughout the world although a genetic association has not been proven nowhere is this spatial association better seen than in northern luzon philippines where the lepanto epithermal cu au deposit overlies the far southeast fse porphyry cu au deposit both world class orebodies fresh rock and hydrothermal mineral separates yield k ar ages indicating that premineralization and postmineralization volcanism occurred at 2 2 1 8 ma and 1 2 0 9 ma respectively and that the hydrothermal system was active from 1 5 to 1 2 ma k ar ages of alunite from lepanto have the same range as those of hydrothermal biotite and illite from the fse deposit indicating that both epithermal and porphyry mineralization formed from an evolving magmatic hydrothermal system that was active for about 300 ka this temporal relation strengthens the argument for a genetic link between these two styles of ore deposit and has implications for exploration where one style of mineralization is found there is potential for the other nearby

zircon crystallization and the lifetimes of ore forming magmatic hydrothermal systems magmatic hydrothermal copper ore formation involves multiple pulses of subvolcanic porphyry intrusion vein opening and hydrothermal ore deposition it is driven by larger subjacent magma reservoirs acting as the source of fluid and ore forming components high precision u pb ages of individual zircon crystals from porphyries immediately predating and postdating cu au mineralization at bingham canyon utah united states and bajo de la alumbrera northwestern argentina show a significant spread of reliably concordant ages this demonstrates zircon crystal formation over a protracted period of 1 m y which is interpreted to record the lifetime of the magma reservoir from which porphyries and ore fluids were extracted the youngest zircons in all pre ore and post ore intrusions overlap within a much shorter time interval of 0 32 m y at bingham canyon and 0 090 m y at alumbrera these youngest zircons of each intrusion are interpreted to bracket the maximum duration of porphyry emplacement and ore formation to short periods consistent with thermal constraints this study illustrates that age brackets based on individual magmatic zircon grains are geologically more informative than the calculation of means and standard deviations based on apparently normal age distributions in zircon populations

30 000 years of hydrothermal activity at the lost city vent field strontium carbon and oxygen isotope data and radiocarbon ages document at least 30 000 years of hydrothermal activity driven by serpentinization reactions at lost city serpentinization beneath this off axis field is estimated to occur at a minimum rate of 1 2 10 4 cubic kilometers per year the access of seawater to relatively cool fresh peridotite coupled with faulting volumetric expansion and mass wasting processes are crucial to sustain such systems the amount of heat produced by serpentinization of peridotite massifs typical of slow and ultraslow spreading environments has the potential to drive lost city type systems for hundreds of thousands possibly millions of years

garnet u pb dating of the yinan au cu skarn deposit luxi district north china craton implications for district wide coeval au cu and fe skarn mineralization abstract several recent studies have shown that garnet u pb dating may potentially provide robust constraints on the ages of carbonatite alkaline magmatism and skarn ore formation here we present textural trace element and laser ablation inductively coupled plasma mass spectrometry u pb isotope data of garnet from the yinan au cu skarn deposit in luxi district eastern north china craton to place tight constraints on the timing of mineralization three hydrothermal andradite rich garnet samples jl 8 thz 38 and tj 13 collected respectively from jinlong tonghanzhuang and gongquan ore segments of the yinan au cu skarn deposit contain 2 31 10 36 ppm u and are characterized by flat time resolved depth profiles for u indicating that this element is hosted in crystallographic lattice in the garnets the positive correlation between u and lrees concentration suggests that the incorporation of u into the garnet is largely controlled by substitution mechanisms the three garnet samples yield weighted mean 206pb 238u dates of 127 3 ma 127 5 ma and 126 7 ma 2 respectively these ages are reproducible within analytical uncertainties and are consistent with a zircon u pb age of 129 2 ma of the ore related diorite intrusion and thus are interpreted as the age of au cu mineralization at yinan the results presented here when combined with existing isotopic age data demonstrate that au cu skarn mineralization throughout the southeastern luxi district occurred coevally in the early cretaceous and is genetically associated with the tongjing and jingchang granitic dioritic complexes the au cu mineralization and associated magmatism coincide in time with the iron skarn mineralization and associated magmatism 134 126 ma across the luxi district strongly suggesting a causal link between the district wide skarn mineralization and pervasive intermediate to felsic magmatism both are products of destruction of lithospheric mantle beneath the north china craton this study confirms that garnet u pb dating can be a robust geochronometer for garnet bearing hydrothermal deposits and can elucidate associated geological processes

analysis of sulfides in whole rock matrices by elemental analyzer continuous flow isotope ratio mass spectrometry abstract potential matrix effects in the sulfur isotopic analysis of whole rocks by elemental analyzer continuous flow isotope ratio mass spectrometry ea cfirms have been investigated by measuring mixtures of a well calibrated laboratory sulfide reference mineral and common rock forming minerals mixtures of sulfide and quartz orthoclase plagioclase kaolinite olivine magnetite ilmenite garnet halite fluorite calcite and graphite show consistent s yields and 34s values generally within 0 2 of the measured pure sulfide value however erratic results are obtained for mixtures of reference sulfide and pyroxene amphibole or biotite these minerals may suppress sample peaks cause extreme peak tailing and prohibit isotope ratio measurements pure sulfide minerals introduced after the mixtures also showed suppressed sample peaks suggesting so2 adsorption on mineral and residual ash surfaces removal of the residual ash resulted in normal measurements for reference sulfide minerals whole rocks with s concentrations in excess of 100 ppm can be successfully analyzed using ea cfirms provided that measurements are carefully monitored for evidence of so2 retention during and after combustion

sulfur in peridotites and gabbros at lost city 30 n mar implications for hydrothermal alteration and microbial activity during serpentinization variations in sulfur mineralogy and chemistry of serpentinized peridotites and gabbros beneath the lost city hydrothermal field at the southern face of the atlantis massif mid atlantic ridge 30n were examined to better understand serpentinization and alteration processes and to study fluid fluxes redox conditions and the influence of microbial activity in this active peridotite hosted hydrothermal system the serpentinized peridotites are characterized by low total sulfur contents and high bulk d 34 s values close to seawater composition low concentrations of 34 s enriched sulfide phases and the predominance of sulfate with seawater like d 34 s values indicate oxidation loss of sulfide minerals and incorporation of seawater sulfate into the serpentinites the predominance of pyrite in both serpentinites and gabbros indicates relatively high fo2 conditions during progressive serpentinization and alteration which likely result from high fluid fluxes during hydrothermal circulation and evolution of the lost city system from temperatures of 250 to 150 c sulfate and sulfide minerals in samples from near the base of hydrothermal carbonate towers at lost city show d 34 s values that reflect the influence of microbial activity our study highlights the variations in sulfur chemistry of serpentinized peridotites in different marine environments and the influence of long lived moderate temperature peridotite hosted hydrothermal system and high seawater fluxes on the global sulfur cycle

subsurface processes at the lucky strike hydrothermal field mid atlantic ridge evidence from sulfur selenium and iron isotopes at lucky strike near the azores triple junction the seafloor setting of the hydrothermal field in a caldera system with abundant low permeability layers of cemented breccia provides a unique opportunity to study the influence of subsurface geological conditions on the hydrothermal fluid evolution coupled analyses of s isotopes performed in conjunction with se and fe isotopes have been applied for the first time to the study of seafloor hydrothermal systems these data provide a tool for resolving the different abiotic and potential biotic near surface hydrothermal reactions the 34s between 1 5 and 4 6 and se values between 213 and 1640 ppm of chalcopyrite suggest a high temperature end member hydrothermal fluid with a dual source of sulfur sulfur that was leached from basaltic rocks and sulfur derived from the reduction of seawater sulfate in contrast pyrite and marcasite generally have lower 34s within the range of magmatic values 0 1 and are characterized by low concentrations of se 50 ppm for 82se 76se ratios the 82se values range from basaltic values of near 1 5 to 7 the large range and highly negative values of hydrothermal deposits observed cannot be explained by simple mixing between se leached from igneous rock and se derived from seawater we interpret the se isotope signature to be a result of leaching and mixing of a fractionated se source located beneath hydrothermal chimneys in the hydrothermal fluid at lucky strike we consider two sources for s and se 1 the end member hydrothermal fluid with basaltic se isotopic values 1 5 and typical s isotope hydrothermal values of 1 5 2 a fractionated source hosted in subsurface environment with negative 34s values probably from bacterial reduction of seawater sulfate and negative 82se values possibly derived from inorganic reduction of se oxyanions fluid trapped in the subsurface environment is conductively cooled and has restricted mixing and provide favorable conditions for subsurface microbial activity which is potentially recorded by s isotopes fe isotope systematic reveals that se rich high temperature samples have 57fe values close to basaltic values 0 whereas se depleted samples precipitated at medium to low temperature are systematically lighter 57fe values between 1 to 3 an important implication of our finding is that light fe isotope composition down to 3 2 may be explained entirely by abiotic fractionation in which a reservoir effect during sulfide precipitation was able to produce highly fractionated compositions

multistage gold mineralization events in the archean tati greenstone belt northeast botswana constraints from integrative white mica ar ar garnet u pb and sulfides pb pb geochronology abstract the tati greenstone belt tgb in northeastern botswana hosts numerous au deposits shashe mupane and signal hill associated with sulfides garnet and white mica in this study integrative white mica ar ar garnet u pb and sulfides pb pb dating techniques were combined with whole rock and sulfide pb isotope characteristics to track the source s of gold and constrain the timeframes of gold mineralization events all sulfides and arsenopyrite samples from the tgb yielded overlapping pb pb errorchron ages of 2227 66 ma and 2220 73 ma respectively which coincide with the shashe sulfides pb pb errorchron age of 2250 110 ma these relatively imprecise pb pb dates suggest heterogeneity in the initial pb isotope ratios of sulfides at mupane whereas au mineralization associated hydrothermal almandine garnet yielded overlapping tera wasserburg lower intercept 206pb 238u age and a concordia age of 2119 18 ma mswd 1 03 and of 2105 24 ma mswd of concordance 1 10 respectively sulfides produced an errorchron pb pb age of 2873 140 ma which despite the large error remains geologically meaningful as it coincides within error with the first neoarchean limpopo liberian orogeny 2 70 2 65 ga granitoids intrusion emplacement 2 65 2 73 ga and deposition of banded iron formation 2 73 0 15 ga ore related white mica from signal hill yielded an overlapping ar ar plateau age of 1987 24 ma and a weighted mean ar ar age of 1987 13 ma n 15 mswd 4 3 which coincide within error with the 2 05 1 95 ga second limpopo liberian tectonic cycle herein considered to have possibly triggered the au mineralization in this area the obtained radiometric dates point to at least three well constrained gold mineralization events with two of them possibly triggered by two different regional tectonic events lead isotope compositions of most of the sulfides overlap with those of spatially associated schists and granitoids thus suggesting these units possibly represent pb and by inference au rock sources the genetic model of the tgb au deposits is consistent with many greenstone hosted gold deposits worldwide suggesting our results are not solely of local regional interest but can be used to characterize greenstone hosted gold deposits worldwide

seafloor hydrothermal systems driven by the serpentinization of peridotite 1 heat balance models predict that heat released upon serpentinization of peridotites can result in a wide range of hydrothermal venting temperatures temperatures ranging from a few to a few tens of c are likely temperatures 300 c may be possible in conjunction with significant heat input from below low fluid flow rate 10 kg s a high rate of serpentinization 100 kg s or some combination of these factors the volume of fresh lithosphere accessible to serpentinization may limit the duration of an episode of hydrothermal venting when the serpentinization rate exceeds 10 kg s hydrothermal venting driven by high rates of serpentinization may occur episodically as new reaction surfaces are made available by tectonic activity or crack propagation given the uncertainty of several parameters the current episode of venting at the lost city vent field on the mid atlantic ridge may range from 102 to 104 years

ages and isotope evolution of the agua rica cu au porphyry epithermal deposit nw argentina abstract the link between porphyry epithermal deposits and continental arc magmatism is widely accepted but in flat slab settings the source of metals is still debated the central andes region hosts numerous important mineral deposits within the flat subduction zone including the world class agua rica cu au mo porphyry epithermal deposit which is associated with the farallon negro volcanic complex the suite of mineralization at agua rica includes a porphyry cu phase an associated epithermal assemblage and a supergene enrichment overprint the main mineralization is considered to be related to the trampeadero and seca norte dacite porphyry intrusions for which u pb shrimp zircon ages are 6 09 0 17 and 5 81 0 32 ma to 5 41 0 25 respectively rhenium os dating of pyrite from the trampeadero porphyry yielded an isochron with an age of 5 85 0 42 ma and molybdenite yielded a model age of 5 61 0 03 ma the seca norte body was responsible for the main porphyry style mineralization while the older trampeadero porphyry served as a host rock magmatic hydrothermal activity at agua rica commenced at approximately the same time as it was ending in other parts of the volcanic complex pb isotope signatures of sulfide samples from agua rica indicate a significant addition of crustal material to a mantle source neogene magmatism in the area is likely related to the subduction of the juan fernandez ridge which was subjacent to the deposit by the time of the porphyry intrusions upper crustal mash mixing assimilation storage and homogenization in the melting zone above the slab is inferred as the source of the shallow magma reservoir beneath the farallon negro volcanic complex

two ages of porphyry intrusion resolved for the super giant chuquicamata copper deposit of northern chile by ela icp ms and shrimp zircon u pb ages measured in situ by excimer laser ablation inductively coupled plasma mass spectrometry ela icp ms and verified by sensitive high resolution ion microprobe shrimp on ore bearing felsic porphyries from the chuquicamata porphyry copper molybdenum deposit northern chile identify two discrete igneous events the volumetrically dominant east porphyry has an age of 34 6 0 2 ma whereas the bench and west porphyries yield ages of 33 3 0 3 ma and 33 5 0 2 ma respectively the age of the east porphyry is indistinguishable from a re os age for early molybdenite mineralization 35 ma and the oldest reported 40 ar 39 ar ages for hydrothermal alteration confirming a genetic link with mineralization previous geological studies and 40 ar 39 ar and re os geochronology identify two main hydrothermal events high temperature potassic alteration with chalcopyrite at 33 4 0 3 ma followed by lower temperature quartz sericite alteration with pyrite at 31 1 0 3 ma the ages of the west and bench porphyries match the ages for potassic alteration younger quartz sericite alteration may reflect an additional fourth intrusion concealed at depth the anomalously large size of chuquicamata appears to be due to a protracted igneous history resulting in the superposition of at least two temporally distinct magmatic hydrothermal systems

stable isotope o h s relationships in tertiary basalts and their mantle xenoliths from the northern hessian depression w germany 18o 16o 34s 32s and d h ratios as well as vacuum fusion h2o contents were measured for late tertiary volcanic basaltic rocks ranging in composition from quartz tholeiites and alkali olivine basalts to melilite bearing olivine nephelinites and for peridotite xenoliths from the northern hessian depression of w germany measured oisotope ratios in both basalts and peridotites were corrected for variable degree of post eruption secondary alteration the ranges and means of corrected 18o values smow for the north hessian lavas and peridotites are i 8 tholeiites ca 6 1 to 7 3 x 6 6 ii 21 alkali olivine basalts ca 5 4 to 7 6 x 6 5 iii 19 nepheline basanites limburgites and olivine nephelinites ca 5 3 to 8 0 x 6 6 and iv 23 peridotites 5 1 to 7 0 x 6 0 the 34s values cdt for the tholeiites range from 0 6 to 1 4 x 0 03 and for the alkali basalts range from 0 9 to 8 6 x 2 5 the approximate d value smow of the pristine basalts and peridotites is estimated to have been ca 90

widespread active detachment faulting and core complex formation near 13 n on the mid atlantic ridge oceanic core complexes are massifs in which lower crustal and upper mantle rocks are exposed at the sea floor1 2 3 they form at mid ocean ridges through slip on detachment faults rooted below the spreading axis2 3 4 5 6 to date most studies of core complexes have been based on isolated inactive massifs that have spread away from ridge axes here we present a survey of the mid atlantic ridge near 13 n containing a segment in which a number of linked detachment faults extend for 75 km along one flank of the spreading axis the detachment faults are apparently all currently active and at various stages of development a field of extinct core complexes extends away from the axis for at least 100 km our observations reveal the topographic characteristics of actively forming core complexes and their evolution from initiation within the axial valley floor to maturity and eventual inactivity within the surrounding region there is a strong correlation between detachment fault morphology at the ridge axis and high rates of hydroacoustically recorded earthquake seismicity preliminary examination of seismicity and seafloor morphology farther north along the mid atlantic ridge suggests that active detachment faulting is occurring in many segments and that detachment faulting is more important in the generation of ocean crust at this slow spreading ridge than previously suspected

geochemistry of high h2 and ch4 vent fluids issuing from ultramafic rocks at the rainbow hydrothermal field 36 14 n mar the flores diving cruise was part of the mast iii amores 1995 1998 program funded by the european union one of the major achievements of the flores cruise was the discovery of the rainbow hydrothermal field hosted in ultramafic rocks south of the amar segment on the mid atlantic ridge mar the rainbow hydrothermal fluids exhibit temperatures of 365 c ph of 2 8 high chlorinity 750 mmol kg and low silica 6 9 mmol kg the uniformity in endmember major minor trace element concentrations and gas contents suggests that all rainbow fluids originate from the same deep source although h2s content is relatively low 1 20 mmol kg all vent fluids show extraordinary high h2 16 mmol kg ch4 2 5 mmol kg and co 5 mol kg endmember concentrations compared to fluids collected from other vent sites along the mar hydrogen represents more than 40 of the total gas volume extracted from the fluids at rainbow h2 production is likely associated with alteration of olivine and orthopyroxene minerals during serpentinization given that exposures of ultramafic rock may be common particularly along slow spreading ridges the production of h2 may have important implications for microbial activity at and beneath the seafloor

initiation of zn pb mineralization in the pingbao pb zn skarn district south china constraints from u pb dating of grossular rich garnet abstract in this study we discuss the application of u pb dating on u rich common pb bearing grossular rich garnet and the issues of common pb correction grossular rich garnet samples were collected from the baoshan and huangshaping areas of the pingbao skarn district hunan south china the baoshan garnet comprises gr64 3 and34 0 to and99 0 gr0 2 gr grossular and andradite with minor spessartine 0 43 1 45 almandine 0 0 61 and pyrope 0 01 0 91 the huangshaping 304 garnet comprises gr41 7 and56 0 to gr70 2 and25 9 with minor spessartine 1 42 2 10 whereas the huangshaping 301 garnet comprises gr62 0 and26 6 spe11 2 to gr74 5 and20 9 pyr2 5 spe2 1 spe spessartine pyr pyrope the baoshan garnet grains are subhedral to euhedral with well developed core rim textures and contain approximately 1 10 ppm u the huangshaping 304 garnet is also euhedral with grains size ranging from several to tens of millimeters the core rim textured garnet grains are compacted together with interstitial quartz and contain about 5 10 ppm u the huangshaping 301 garnet is anhedral to tabular with interstitial quartz and magnetite and contains up to 579 ppm u in situ u pb dating of the baoshan huangshaping 304 and 301 garnet samples yielded consistent anchored y intercept ages at 162 6 2 9 ma n 60 mswd 1 5 160 2 1 5 ma n 59 mswd 1 9 and 160 9 0 3 ma n 60 mswd 4 1 respectively these ages are consistent with the new and published zircon and titanite u pb ages 162 160 ma indicating that the garnet u pb ages are reliable and the technique is robust we suggest that the non u bearing mineral correction has involved in the third party common pb ratio which is believed to be a better correction for the system common pb

precise molybdenite re os and mica ar ar dating of the mesozoic yaogangxian tungsten deposit central nanling district south china the yaogangxian deposit in the central nanling region south china consists of vein type ore bodies hosted in cambrian to jurassic strata and mesozoic granitic intrusions wolframite and molybdenite are the dominant ore minerals intergrown with gangue minerals of quartz feldspar phlogopite and muscovite we have carried out molybdenite re os and phlogopite and muscovite 40ar 39ar dating to better understand the timing and genesis of mineralization re os dating of eight molybdenite samples yielded model ages ranging from 152 0 3 5 to 161 1 4 5 ma with an average of 156 0 ma the re os analyses give a well defined 187re 187os isochron with an age of 154 9 2 6 ma mswd 2 4 hydrothermal phlogopite and muscovite display extremely flat 40ar 39ar age spectra phlogopite yields a 40ar 39ar plateau age of 153 0 1 1 ma whereas muscovite yields a plateau age of 155 1 1 1 ma both 40ar 39ar ages are in good agreement with the re os ages placing the timing of tungsten mineralization at about 154 ma this age is consistent with the field relationships our new data when combined with published geochronological results from other major deposits in this region suggest that large scale w sn mineralization occurred throughout the central nanling region in the late jurassic

hydrothermal alteration and microbial sulfate reduction in peridotite and gabbro exposed by detachment faulting at the mid atlantic ridge 15 20 n odp leg 209 a sulfur and oxygen isotope study whole rock sulfur and oxygen isotope compositions of altered peridotites and gabbros from near the 15 20 n fracture zone on the mid atlantic ridge were analyzed to investigate hydrothermal alteration processes and test for a subsurface biosphere in oceanic basement three processes are identified 1 high temperature hydrothermal alteration 250 350 c at sites 1268 and 1271 is characterized by 18o depletion 2 6 4 4 elevated sulfide s and high 34s up to 2 wt and 4 4 10 8 fluids were derived from high temperature 350 c reaction of seawater with gabbro at depth these cores contain gabbroic rocks suggesting that associated heat may influence serpentinization 2 low temperature 150 c serpentinization at sites 1272 and 1274 is characterized by elevated 18o up to 8 1 high sulfide s up to 3000 ppm and negative 34s to 32 1 that reflect microbial reduction of seawater sulfate these holes penetrate faults at depth suggesting links between faulting and temperatures of serpentinization 3 late low temperature oxidation of sulfide minerals caused loss of sulfur from rocks close to the seafloor sulfate at all sites contains a component of oxidized sulfide minerals low 34s of sulfate may result from kinetic isotope fractionation during oxidation or may indicate readily oxidized low 34s sulfide derived from microbial sulfate reduction results show that peridotite alteration may be commonly affected by fluids heat derived from mafic intrusions and that microbial sulfate reduction is widespread in mantle exposed at the seafloor

o and h stable isotope compositions of oceanic crust and upper mantle rocks exposed in the hess deep near the galapagos triple junction abstract a large variety of rocks consisting of basalts dolerites gabbros and ultramafics are exposed on the intra rift ridge in the hess deep representing a section of the oceanic crust formed at the east pacific ridge 1 we conducted a stable isotope study of o and h to assess the nature of the interaction between seawater and the oceanic crust in this region fresh basalt and dolerite samples have magmatic morb values 18o 6 0 105 d 70 and low h2o content 57 and h2o contents 1 3 most gabbros have 18o 5 6 essentially identical to their initial magmatic values their d values are generally higher than 57 suggesting that they have reacted with some seawater derived fluids as is also suggested by their mineralogy in gabbros displaying little or no visual evidence of alteration of their plagioclases and pyroxenes 18oplagioclase pyroxene is small 0 3 and consistent with magmatic values the calcic an 50 plagioclases have magmatic 18o 5 8 albitized prehnitized plagioclases have variable 18o values 4 8 it also appears that the 18o modified gabbros are those in which plagioclases are transformed into albite and or prehnite which are characteristic of greenschist facies conditions the lack of calcic an 50 plagioclases depleted in 18o and hornblende suggests that most of the gabbros did not react extensively under amphibolite facies conditions with seawater derived fluids t 400 c or that they were subsequently altered to greenschist facies gabbros the serpentinized peridotites were depleted in 18o 3 3 4 9 in one sample the 18oserpentine magnetite 5 7 corresponds to a temperature of about 325 50 c these values are compatible with serpentinization by seawater derived fluids at temperatures between 200 and 350 c another sample has a higher 18o value of 10 corresponding to a serpentinization temperature of less than 50 c the 18o mass balance between seawater and the oceanic crust seems to have attained steady state where enrichment in 18o of metabasalts is balanced by the depletion in 18o of metadolerites the contribution of the gabbros is minor the depletion in 18o of the serpentinized peridotites is not taken into account since we believe that the high temperature serpentinization is characteristic of the hess deep and cannot be extrapolated to the normal oceanic crust

la mc icp ms u pb dating of low u garnets reveals multiple episodes of skarn formation in the volcanic hosted iron mineralization system awulale belt central asia volcanic hosted iron deposits of the eastern awulale metallogenetic belt in central asia possess a reserve of over 1 2 billion tons of iron ores and constitute one of the most important basements for high grade iron resources in china skarns are widespread in these deposits and closely associated with iron mineralization the ages of these skarns are unclear and their genesis remains debated preventing further investigation into their metallogenic processes we focused on garnets in nine ore bearing skarns from three large scale iron deposits chagangnuoer dunde and beizhan in the eastern awulale belt u pb dating was conducted on these garnets using our in house reference material the taochong garnet tc 13 pb pb isochron age 126 2 23 ma initial pb 207 pb 206 ratio 0 845 0 022 laser ablation multicollector inductively coupled plasma mass spectrometry la mc icp ms was employed in the garnet u pb dating and high precision u pb ages 03 1 6 were obtained highlighting the advantages of la mc icp ms in dating low u minerals the garnet u pb ages of the nine skarn samples fall into three groups i e 329 0 5 1 326 3 3 ma two samples 316 3 2 9 311 2 2 4 ma six samples and 295 6 1 0 ma one sample implying three episodes of skarn alteration in the volcanic hosted iron mineralization system the first and second episodes of skarns formed as a result of contact metasomatism between coeval volcanic rocks and limestone and they have economically important iron mineralization the third was likely caused by a local postcollision granitic intrusion but its metallogenic potential deserves further assessment

lost city life a new type of hydrothermal vent field in the atlantic ocean called lost city was discovered in 2000 as boetius 1 describes in her perspective recent detailed mapping of this hydrothermal vent field as well as chemical and microbial analyses kelley et al 2 reveal that an extraordinary array of micro and macro organisms inhabit the giant white carbonate chimneys of lost city 1 http www sciencemag org cgi content full 307 5714 1420 2 http www sciencemag org cgi content short 307 5714 1428

the role of pre impact topography in impact melt emplacement on terrestrial planets abstract on terrestrial planets flow like deposits of impact melt are commonly found exterior to fresh impact craters often these deposits are asymmetric about the crater rim and the direction of flow may relate to either the pre impact topography or the impact azimuth in this work we seek to determine the primary mechanism responsible for impact melt emplacement on two very different terrestrial worlds the moon and venus newly derived stereo topography data allows us to investigate the role of pre impact topography in melt emplacement on venus for the first time we determine that pre impact topography plays an important role in melt emplacement around complex craters on the moon but not on venus this difference may relate to the differences in gravity and hence crater depth and melt volume on the two worlds melt trapped in the relatively deep lunar craters may require additional momentum to be pushed over the crater rim and therefore emerges preferentially from the region of rim crest low this added momentum might come from uplift during the crater modification stage for the shallower craters on venus the downrange momentum imparted in oblique impacts may be sufficient to push the melt up and over the crater rim explaining the correlation between melt direction and impact azimuth understanding the emplacement of impact melt on terrestrial planets of different sizes and geologic histories provides added insight into the impact cratering process