geology u pb geochronology and stable isotope geochemistry of the tunca semi massive sulfide mineralization black sea region ne turkey implications for ore genesis abstract upper cretaceous volcano sedimentary sequences of the eastern pontide orogenic belt ne turkey are host to significant vms mineralization including near tunca the initial stages of felsic volcanism within the mineralized area are marked by the eruption of dacitic lavas and breccias of the kizilkaya formation this was accompanied by the emplacement of domelike hematitic dacites autobrecciated and volcaniclastic rocks both in situ and resedimented were likely generated from extrusive portions of these dacite bodies basaltic volcanism is marked by the eruption of the lava flows and pillow lavas of the caglayan formation hiatuses in basaltic activity are marked by thin horizons of volcaniclastics and mudstones the uppermost felsic volcanic units were accompanied by resedimentation of autoclastic facies from previous volcanism and represent the latest phase of upper cretaceous volcanism in the area the semi massive sulfide mineralization is associated with a late stage of the initial felsic volcanism u pb la icp ms zircon dating of a dacitic tuff breccia yielded an age of 88 1 1 2 ma coniacian upper cretaceous which is interpreted to be the age of the sulfide occurrences a concentric zoned alteration pattern is observed in the footwall rocks the alteration pattern is considered to have formed by lateral migration of hydrothermal fluids which had ascended along the discharge conduit fluid inclusion data indicate precipitation or mobilization processes within a relatively narrow temperature range of 152 255 c avg 200 c the low salinity fluids in the fluid inclusions less than 5 9 wt nacl equivalent are consistent with typical modified seawater dominant hydrothermal vent fluids sulfur isotope analysis of the tunca sulfides yields a narrow range of 1 5 4 1 per mil these 34 s values are also typical of many vms deposits most of the recorded 18 o values 7 1 to 14 0 per mil are greater than 9 per mil the most intensely hydrothermally altered rocks tend to have lower 18 o values relative to the less altered rocks collectively the geologic relationships mineralization style and the lack of seafloor ore facies suggest that mineralization is principally of sub seafloor origin the most geologically reasonable interpretation of the genesis of the tunca mineralization is the continuous interaction between the host rocks and seawater derived fluids without significant involvement of a magmatic fluid

evidence for syngenetic precious metal enrichment in an appalachian volcanogenic massive sulfide system the 1806 zone ming mine newfoundland canada the ming deposit baie verte newfoundland canada is an early ordovician bimodal mafic cu au zn ag volcanogenic massive sulfide vms deposit the deposit consists of a number of ore lenses that are stacked in the uppermost part of the rambler rhyolite one of the uppermost lenses the 1806 zone is enriched in au and ag the deposit has been affected by silurian devonian greenschist to amphibolite grade metamorphism and polyphase deformation and this has led to debates as to whether the au ag enrichment in the deposit is syngenetic or a product of a later metamorphic and structural overprint the 1806 zone consists predominantly of discordant sulfide stringer stratabound semimassive to massive sulfides and a weakly mineralized silicified cap zone the ore is largely hosted within the footwall rambler rhyolite that is strongly altered to quartz sericite green mica with sporadic chlorite biotite base and precious metal zoning is developed from the down plunge portion cu au to the up plunge portion cu zn au ag pb of the 1806 zone the ore mineralogy and mineral chemistry are complex and are interpreted to be of an intermediate sulfidation type pyrite and chalcopyrite are the dominant sulfide species with minor to accessory sphalerite galena pyrrhotite and arsenopyrite sulfosalts including ag bearing tennantite tetrahedrite stannite boulangerite and loellingite and precious metal rich phases are common throughout the deposit precious metals occur as 1 mercurian electrum 7 93 20 57 wt hg and 2 various ag phases miargyrite pyrargyrite mercurian stephanite unnamed agcufes phase ag hg au alloys that are more abundant in the up plunge portion of the 1806 zone tellurides and bismuthides are present in trace amounts oxides cassiterite and magnetite are present exclusively in the down plunge portion of the 1806 zone the deposit geometry metals distribution complex ore assemblages abundant sulfosalts and the significant concentration of the epithermal suite of elements e g au ag as hg sb bi in the ore strongly contrast with those of orogenic au deposits and support a syngenetic or synvolcanic origin for precious metals in the ming vms deposit including a possible magmatic input into the 1806 zone ore forming system evidence for a late syndeformation precious metal emplacement via orogenic overprinting is lacking at the 1806 zone and all data point to a syngenetic origin for precious metal enrichment indicating that ordovician appalachian vms deposits are favorable targets for au nevertheless silurian devonian metamorphism and deformation have resulted in textural recrystallization and local remobilization of precious metals and the main fabrics largely control the current geometry of the host succession and of the ming orebody

the development of volcanic hosted massive sulfide and barite gold orebodies on wetar island indonesia wetar island is composed of neogene volcanic rocks and minor oceanic sediments and forms part of the inner banda arc the island preserves precious metal rich volcanogenic massive sulfide and barite deposits which produced approximately 17 metric tonnes of gold the polymetallic massive sulfides are dominantly pyrite locally arsenian with minor chalcopyrite which are cut by late fractures infilled with covellite chalcocite tennantite tetrahedrite enargite bornite and fe poor sphalerite barite orebodies are developed on the flanks and locally overly the massive sulfides these orebodies comprise friable barite and minor sulfides cemented by a series of complex arsenates oxides hydroxides and sulfate with gold present as 10 m free grains linear and pipe like structures comprising barite and iron oxides beneath the barite deposits are interpreted as feeder structures to the barite mineralization hydrothermal alteration around the orebodies is zoned and dominated by illite kaolinite smectite assemblages however local alunite and pyrophyllite are indicative of late acidic oxidizing hydrothermal fluids proximal to mineralization altered footwall volcanic rocks give an illite k ar age of 4 7 0 16 ma and a 40ar 39ar age of 4 93 0 21 ma fluid inclusion data suggest that hydrothermal fluid temperatures were around 250 270 c showed no evidence of boiling with a mean salinity of 3 2 wt equivalent nacl the 34s composition of sulfides ranges between 3 3 and 11 7 and suggests a significant contribution of sulfur from the underlying volcanic edifice the 34s barite data vary between 22 4 and 31 0 close to miocene seawater sulfate whole rock 87sr 86sr analyses of unaltered volcanic rocks 0 70748 0 71106 reflect contributions from subducted continental material in their source region the 87sr 86sr barite data 0 7076 0 7088 indicate a dominant miocene seawater component to the hydrothermal system the mineral deposits formed on the flanks of a volcanic edifice at depths of 2 km spectacular sulfide mounds showing talus textures are localized onto faults which provided the main pathways for high temperature hydrothermal fluids and the development of associated stockworks the orebodies were covered and preserved by post mineralization chert gypsum globigerina bearing limestone lahars subaqueous debris flows and pyroclastics rocks

metallogenic evolution of convergent margins selected ore deposit models abstract metallogenic patterns in convergent margin arcs are related to thickness of the arc crust and its petrologic evolution which are themselves linked by a positive correlation between average silica content of arc rocks and average thickness of arc crust in general tin mercury antimony and fluorine are most abundant in areas of thickest crust although the composition of ore deposits is strongly controlled by deep crustal and mantle features the actual form and type of many ore deposits is controlled by upper crustal features particularly the relative abundance of carbonate and silicate country rock for instance epithermal precious metal deposits are commonly associated with subaerial silicic volcanism which generally forms in the later stages of arc evolution deposits of this type will be absent if arc volcanism is terminated prematurely or removed if subduction of thickened or hot oceanic crust causes extreme uplift and consequent erosion of the upper parts of the arc similarly chimney manto limestone replacement lead zinc silver deposits are common only in terranes with extensive thick limestone units which are commonly formed in shallow shelf environments several important types of convergent margin deposits remain poorly understood acid sulfate deposits in primitive oceanic volcanic arcs are associated with bimodal volcanic suites that appear to have formed at an early stage in the evolution of the arc alkaline porphyry copper deposits are closely associated with calc alkaline porphyry copper deposits commonly in the same volcanic arc although they appear to have formed farther back from the trench or during periods of oblique subduction or arc reversal massive iron oxide deposits and related base and precious metal mineralization are closely associated with silicic magmas that form during later stages of arc evolution in addition to tectonic and petrologic factors metallogenic patterns probably reflect global forcing mechanisms the most important of which are global seawater anoxia and plate reorganizations global anoxia produces deep water environments that favor sedex mineralization and can promote subsurface ore deposition where anoxic bottom waters recharge underlying aquifers plate reorganizations can lead to ore deposition by providing extensional environments in the upper crust and possibly by facilitating partial melting of oceanic crust that has been modified by earlier subducting slabs

metal fluxes during magmatic degassing in the oceanic crust sulfide mineralisation at odp site 786b izu bonin forearc volcanogenic massive sulfide deposits are enriched in metals that are either derived from hydrothermal alteration of the basement rocks or supplied by exsolution of metal rich volatiles during magmatic differentiation the extent to which each process contributes to metal enrichment in these deposits varies between different tectonic settings ocean drilling program hole 786b recovered 800 m of upper oceanic crust from a supra subduction zone setting and includes a 30 m thick mineralised zone in situ s isotopic compositions of pyrite decrease from 5 9 2 9 in the upper mineralised zone down to 3 3 2 1 in the extensively altered central mineralisation zone potentially indicating strong magmatic fluid input in this area whole rock data and in situ trace element analyses in sulfide minerals show enrichment of ag as au bi mo s se sb and te in the mineralised zone evaluation of metal behaviour during magmatic differentiation and primary metal fertility of basement rocks suggests that degassing melt is the main source for the high au se and s enrichment observed in the mineralised zone magmatic volatile exsolution occurred late during the magmatic differentiation 2 wt mgo concomitant with oxide crystallisation and metal depletion in the melt comparison of ocean drilling program hole 786b with volcanogenic massive sulfide deposits hosted by boninitic volcanic successions such as in the semail ophiolite the newfoundland appalachians and the flin flon belt suggests that magmatic fluid exsolution could be a common mechanism for au enrichment in bimodal mafic volcanogenic massive sulfide deposits

regional setting and geochronology of the late cretaceous banatitic magmatic and metallogenetic belt the 1 500 km long banatitic magmatic and metallogenetic belt bmmb of romania serbia and bulgaria is a complex calc alkaline magmatic arc of late cretaceous age it hosts a variety of magmatic hydro thermal cu au mo zn pb and fe deposits including europe s only world class porphyry copper deposits regional metallogeny can be linked to subduction of the vardar ocean during the late cretaceous as part of the closure of the neotethys ocean that had separated europe and africa in the mesozoic porphyry cu au mo and intimately associated epithermal massive sul phides dominate in the central segments of the belt in southernmost banat romania serbia and north west bulgaria these districts are the economically most im portant today including major active cu au mines at moldova noua in romania majdanpek veliki krivelj and bor in serbia and elatsite assarel and chelopech in bulgaria more numerous and mostly mined in the past are fe cu and zn pb skarns which occur mainly at the two ends of the belt in eastern bulgaria and in romania this paper summarises some of the deposit characteristics within the geodynamic framework of terminal vardar subduction heterogeneous terranes of the belt including the apuseni mountains at the western end are aligned parallel to the vardar front following continental collision of the dacia and tisza blocks all available geochronological data numerous k ar and some u pb and re os ages are compiled and are complemented by a new high precision re os date for the dognecea skarn deposit south west romania 76 6 0 3 ma these data indicate that magmatism extended over at least 25 million years from about 90 to 65 ma in each segment of the belt within apuseni mountains and banat where magma emplacement was related to syn collisional extension in the orogenic belt of carpathians ore formation seems to be restricted in time and maybe constrained by a shared tectonic event

determination of alteration facies using the normative mineral alteration index selbaie cu zn deposit northern abitibi greenstone belt canada the selbaie cu zn deposit is located in the northeastern portion of the northern archean abitibi volcanic belt it is one of the most important base metal deposits in canada it includes stratiform pyrite levels in the volcanites and shallow water sediments stockworks in a rhyodacitic breccia and a copper rich vein in a caldera context the study of the regional alteration distribution is based on a database of more than 2300 chemical sample analyses using the normat normative mineral alteration index this method allows independent analysis of the numerous lithologic variations in the selbaie district five facies are distinguished in a vast sodium depleted zone from the core to the periphery of the hydrothermal system chlorite pyrophyllite in the mineralized zones chlorite sericite sericite sericite orthoclase and orthoclase at the periphery the carbonates also show zoning with ankerite dolomite at the periphery and siderite magnesite at the core with normat it is possible to highlight overp

the origin and nebular history of the metal phase of ordinary chondrites abstract we present new inaa results for bulk metal from h and ll chondrites both have characteristic elemental patterns similar to l chondrite metal in particular increasing abundances of w and ga from unequilibrated to equilibrated chondrites but a reverse variation of v and cr abundances these characteristics indicate that metal in ordinary chondrites formed by melting and reduction of highly oxidized material the similarities of melting features and the complementary nature of compositions between metal and chondrules suggest that these two components were derived from a common precursor similar to ci or cm material in redox state and compositionally related to the matrix of highly unequilibrated ordinary chondrites co ni abundance ratios are similarly low for bulk metal in the least metamorphosed ordinary chondrites of all three chemical groups this suggests that metal in ordinary chondrites initially had the same composition and formed under the same melting conditions the chondrites that accreted earliest have preserved their melting characteristics while those that accreted later reequilibrated with the ambient gas at different temperatures the differences in redox state between equilibrated and unequilibrated chondrites show that formation of chondritic metal and chondrules by melting occurred during the accretion of ordinary chondrite parent bodies the initial metal composition established during the melting stage 13 wt ni co ni 0 031 is inferred from the metal in highly unequilibrated chondrites the accretion temperatures of about 600 k for ordinary chondrites are calculated from the reequilibration reaction h chondrites were accreted at the highest temperature and are most reduced among the three ordinary chondrite groups ll chondrites were accreted at the lowest temperature and are most oxidized compositionally the metal component that would account for the fe si fractionation among h l and ll chondrites is different from the metal formed by melting indicating that the different fe s ratios of h l and ll chondrites were established before their accretion

cu and mo re enrichment during ductile deformation a case study of the yuleken porphyry cu deposit eastern junggar nw china abstract porphyry cu deposits can be strongly deformed during subsequent tectonic evolution to date research pertaining to the deformation and the enrichment of mineralization in porphyry deposits has suffered from a paucity of proof here we describe the diagnostic petrologic and geochemical signatures of the yuleken porphyry cu deposit from stages of metallogenic overprinting the rocks studied in this work are from deformation metallogenic structures that have 1 a distinguished metallogenic and tectonic evolution 2 a three layer spatial deformation metallogenic structure 3 distinguishably different fluid components within protomylonite mylonite and ultramylonite zones and 4 variational geochemical characteristics of coexisting chalcopyrite and magnetite integrated sulfide and oxide mineral investigations pertaining to the mineral deformation and the trace element variation of the minerals chalcopyrite and magnetite in the yuleken porphyry cu deposit indicated that cu and mo can be enriched during ductile deformation through physical analogous geochemical characteristics of chalcopyrite from different stages and chemical migrations the elemental variation of magnetite magnetites from deformed stages have higher contents of ti v co mn and ga than those from other stages and the variation of the fluid components within the protomylonite mylonite and ultramylonite zones reveal that the migration and precipitation of cu mo and fe were influenced by oxygen fugacity fluid components and temperature variation in conclusion the arc related porphyry deposits were able to become involved in the ductile deformation zone inducing areas of cu and mo enrichment in theory units subjected to ductile deformation may have great potential for porphyry cu mo deposit formation under the influence of magmatism

re os dating of molybdenite from the pueblo viejo au ag cu and douvray cu au districts hispaniola re os dating of three samples of hydrothermal molybdenite collected from the pueblo viejo au ag cu district 5 5 moz gold past production 16 2 moz proven plus probable gold ore reserves yields ages of 112 1 0 4 112 0 0 4 and 111 5 0 4 ma these re os dates confirm an early cretaceous age for mineralization coeval with tholeiitic intraoceanic island arc volcanism and in particular with an episode of felsic magmatism and extension across the early cretaceous arc 110 118 ma re os dating of a sample of hydrothermal molybdenite collected from the douvray porphyry cu mo deposit with inferred resources of 1 257 mlb cu 0 276 moz au 6 84 moz ag and 4 367 tons mo yields an age of 93 3 0 3 ma coeval with calc alkaline magmatism along the greater antilles island arc

sulphur and lead isotope geochemistry of sulphide minerals from the zn pb cu ag au lemarchant volcanogenic massive sulphide vms deposit newfoundland canada abstract the lemarchant zn pb cu ag au volcanogenic massive sulphide vms deposit in the newfoundland appalachians canada is an important example of a precious metal bearing vms deposit the deposit contains vms and sulphosalt rich mineral assemblages which are paragenetically distinct initial zn pb sulphosalt mineralization was deposited from low temperature 300 c less oxidized hydrothermal fluids with lower sulphur activity in situ microanalyses of pyrite chalcopyrite and galena yield a wide range of 34s values 6 4 to 15 1 sulphides from paragenetically early lower temperature zn pb sulphosalt assemblages have the lowest average 34s values 4 6 whereas the highest 34s values occur in the late stage high temperature cu rich sulphide assemblages variability in 34s occurs in all styles of mineralization and is consistent with s derived from thermochemical reduction of seawater sulphate igneous leaching and potentially disproportionated magmatic so2 although it is difficult to distinguish explicitly between leached igneous and magmatic sources of sulphur mineralogical and geochemical data from lemarchant suggest a possible contribution of magmatic so2 during the deposition of epithermal style mineralization at the earlier stages of deposit formation lead isotope compositions of galena show little variability with style of mineralization suggesting similar pb sources throughout ore deposition most pb at lemarchant is derived from underlying neoproterozoic sandy brook group volcanic rocks with potential input of juvenile pb from the lake ambrose basaltic rocks of the tally pond group and or from more juvenile neoproterozoic basement rocks

metal sources in the middle valley massive sulphide deposit northern juan de fuca ridge pb isotope constraints abstract massive sulphides drilled from the bent hill deposit during ocean drilling program leg 139 at middle valley northern juan de fuca ridge have 206 pb 204 pb 18 43 18 86 207 pb 204 pb 15 43 15 63 208 pb 204 pb 38 08 38 60 these span 35 of the total pb isotope range measured in hydrothermal sulphides from the east pacific ocean 206 pb 204 pb ratios overlap the range of local basalts but 208 pb 204 pb and 207 pb 204 pb require a contribution from a more radiogenic source that is similar to the overlying sedimentary sequence three samples have lower 207 pb 204 pb ratios than the mean basalt value which may reflect the preservation of heterogeneities in the local basalts the pb isotopic compositions of primary pyrrhotites are indistinguishable from the secondary pyrite dominated assemblage which recrystallised from low temperature hydrothermal fluids this may be explained by the importance of the sulphosalts as a host for pb in the deposit clastic sulphides from sediments at bent hill are at the radiogenic end of the massive sulphide range suggesting that surficial sulphides may not provide an unbiased sample of the past hydrothermal circulation responsible for massive sulphide deposition if the pb content of sediment is 100 times greater than the basalts the sulphide pb isotopes imply that the hydrothermal fluids have extracted pb from 60 to 900 times more basalt than sediment lead in the massive sulphide deposit has been extracted from 2 10 8 3 10 8 m 3 of oceanic crust demonstrating the local nature of hydrothermal circulation at mid ocean ridges the 3 he 4 he of hydrothermal fluids in the sulphides 6 2 10 6 7 8 10 6 show only half the variation between the sediment and basalt end members displayed by pb isotopes this is consistent with volatile acquisition from magma degassing while the pb is leached during hydrothermal fluid interaction with crystallised oceanic crust

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

geology alteration and origin of archean au ag cu mineralization associated with the synvolcanic chibougamau pluton the brosman prospect abitibi greenstone belt canada the au ag cu brosman prospect is located in the line lake area on the northern limb of the chibougamau anticline in the northeast part of the archean abitibi greenstone belt this prospect comprises a broad n s subvertical mineralized envelope formed in a synvolcanic fault related environment this envelope is at least 300 m long up to 30 m wide and is known to a depth of 950 m where it remains open it exhibits two contrasting mineralized zones that are representative of the varying styles of mineralization in the area the auriferous zone consists of both ubiquitous pyrite disseminations and au bearing silicate sulfide veins in strongly fractured intrusive effusive and volcaniclastic rocks showing propylitic and chloritic alteration assemblages the polymetallic zone on the other hand is characterized by discrete cu ag au rich magnetite sulfide veins and lesser pyrite disseminations within distal propylitic and proximal potassic to sodic alteration zones the polymetallic zone is hosted by a chaotic assemblage of gabbro and tuff bodies interpreted as a talus megabreccia several lines of evidence including the spatial distribution pattern and nature of the alteration and mineralization assemblages metal associations mass balance calculations pyrite composition and crosscutting relationships are indicative of a preregional deformation synvolcanic origin for the hydrothermal activity crosscutting relationships alteration characteristics as well as whole rock and la icp ms pyrite analyses reveal two stages of hydrothermal alteration and mineralization stage i consists of both ubiquitous disseminated pyrite found in the polymetallic and auriferous zones and contemporaneous magnetite sulfide veins restricted to the polymetallic zone stage ii is restricted to the auriferous zone and corresponds to au bearing silicate sulfide veins with characteristic vein halo chloritic alteration overprinting stage i alteration and mineralization this hydrothermal activity is interpreted to have occurred concurrently with the development of the fertile deeper magmatic hydrothermal system related to the chibougamau pluton stage i hydrothermal activity related to the early build up of the volcanic sequence formed a continuum of mineralization from discrete massive magnetite sulfide veins to ubiquitous pyrite disseminations in a submarine talus megabreccia generated along synvolcanic faults afterward a late au rich contribution to the hydrothermal fluids possibly related to intermediate to felsic magmatism the 2707 6 1 4 ma line lake stock produced stage ii silicate sulfide veins overprinting mineralization and alteration in favorably oriented and sustainably permeable fracture zones of the auriferous zone overall this study suggests that the brosman prospect is a rare example of archean polymetallic mineralization produced by a shallow subaqueous epithermal style subsystem developed in a broader fertile magmatic hydrothermal arc setting

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

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

the gold content of volcanogenic massive sulfide deposits volcanogenic massive sulfide deposits contain variable amounts of gold both in terms of average grade and total gold content with some vms deposits hosting world class gold mines with more than 100 t au previous studies have identified gold rich vms as having an average gold grade expressed in g t exceeding the total abundance of base metals expressed in wt however statistically meaningful criteria for the identification of truly anomalous deposits have not been established this paper presents a more extensive analysis of gold grades and tonnages of 513 vms deposits worldwide revealing a number of important features in the distribution of the data a large proportion of deposits are characterized by a relatively low gold grade 2 g t with a gradual decrease in frequency towards maximum gold grades defining a log normal distribution in the analysis presented in this paper the geometric mean and geometric standard deviation appear to be the simplest metric for identifying subclasses of vms deposits based on gold grade especially when comparing deposits within individual belts and districts the geometric mean gold grade of 513 vms deposits worldwide is 0 76 g t the geometric standard deviation is 2 70 g t au in this analysis deposits with more than 3 46 g t au geometric mean plus one geometric standard deviation are considered auriferous the geometric mean gold content is 4 7 t au with a geometric standard deviation of 26 3 t au deposits containing 31 t au or more geometric mean plus one geometric standard deviation are also considered to be anomalous in terms of gold content irrespective of the gold grade deposits with more than 3 46 g t au and 31 t au are considered gold rich vms a large proportion of the total gold hosted in vms worldwide is found in a relatively small number of such deposits the identification of these truly anomalous systems helps shed light on the geological parameters that control unusual enrichment of gold in vms at the district scale the gold rich deposits occupy a stratigraphic position and volcanic setting that commonly differs from other deposits of the district possibly due to a step change in the geodynamic and magmatic evolution of local volcanic complexes the gold rich vms are commonly associated with transitional to calc alkaline intermediate to felsic volcanic rocks which may reflect a particularly fertile geodynamic setting and or timing e g early arc rifting or rifting front at the deposit scale uncommon alteration assemblages e g advanced argillic aluminous strongly siliceous or potassium feldspar alteration and trace element signatures may be recognized e g au ag as sb bi hg te suggesting a direct magmatic input in some systems

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

paleomagnetic study of french guyana early jurassic dolerites hypothesis of a multistage magmatic event a detailed paleomagnetic and anisotropy of magnetic susceptibility ams study was carried out on 34 sites of early jurassic dolerite dykes from french guyana which formed during the initial opening of the central atlantic ocean four types of ams fabrics are recognized i normal fabric 21 dykes defined by clustering of k1 k2 axes on the dyke plane whereas the k3 axis is nearly perpendicular to it this fabric is interpreted as due to magma flow the subhorizontal inclination of the k1 axis permitted to suggest that the french guyana dykes could be fed by horizontal magma fluxes from a distant magma source ii reversal fabric 8 dykes is characterized by the k2 k3 plane close to the dyke plane and the k1 perpendicular to dyke orientation such fabric was attributed to the local shearing stress iii intermediate fabric 1 dyke is defined by k1 k3 axes close to the dyke plane and k2 axis is perpendicular to this plane it was interpreted as due to vertical compaction of a static magma column iiii other fabric 4 dykes does not show any preferential orientation scanning electronic microscope and susceptibility versus temperature experiments show that minerals of the titanomagnetite family are main magnetic remanence carriers two magnetic components were isolated ages of magnetic remanences are estimated at 198 3 2 0 ma to 192 3 1 5 ma their virtual geomagnetic poles are calculated pole a v 73 2 n p 15 3 e k 288 8 a95 3 4 n 8 and pole b v 81 6 n p 89 1 e k 69 8 a95 4 2 n 18 these two groups probably correspond to two distinct magmatic events which occurred in a short period this hypothesis is consistent with published 40 ar 39 ar radiometric ages though with mini plateau spectra these paleomagnetic results suggest the presence of magmatic pulses led to the construction of the central atlantic magmatic province in french guyana during the early jurassic fl 2000 elsevier science b v all rights reserved

cu rich scales in the reykjanes geothermal system iceland seawater dominated fluids discharge from the subaerial reykjanes geothermal system iceland there is a sharp pressure decrease in surface pipes at an orifice throttle point and cu rich scales deposit at this orifice that consist largely of bornite and digenite along with sphalerite and other sulfides the bornite and digenite form complex intergrowths with sphalerite and galena accompanied by high concentrations of gold and silver up to 590 ppm and 2 3 wt respectively the precipitates form in response to rapid boiling flashing of the hydrothermal fluids due to the sharp decrease in pressure 37 22 bars at the orifice downstream of the well head the boiling and concomitant separation of the vapor phase results in a temperature decrease from 252 to 220 c across this throttle over distances of centimeters there is a nearly quantitative deposition of metals from solution compared to concentrations in reservoir liquids resulting in formation of the metastable cu fes solid solution rich in silver and gold plus sphalerite and galena a high degree of supersaturation of metals in the quenched liquid is indicated by the very fine grain size of the sulfides and dendritic textures postdepositional cooling likely caused exsolution of bornite and digenite from the high temperature solid solution in addition silver was expelled from the cu fe s solid solution and remobilized into late fractures in the scales whereas the gold appears to have remained in solid solution or as submicroscopic inclusions in the bornite and digenite the observed mineral assemblage at reykjanes contrasts with that of sea floor black smokers despite similar fluid compositions the sea floor hydrothermal vents characteristically have cu rich linings composed predominantly of chalcopyrite which is precipitated by a combination of conductive cooling of high temperature 350 c hydrothermal fluids and mixing with seawater the difference in sulfide mineralogy between the reykjanes orifice scales with cu sulfides dominated by bornite and digenite and many sea floor vents may provide an indication of the subsea floor mineralization where fluids boil before discharging at sea floor vents

neo tethyan magmatism and metallogeny in myanmar an andean analogue abstract plate margin tectonism magmatic and hydrothermal processes and the genesis of mineral deposits are intricately linked in myanmar two near parallel magmatic belts that together contain a significant proportion of that country s mineral wealth have contrasting metallogenic endowments the mogok mandalay mergui belt hosts crustal melt s type granites with significant tin tungsten mineralization while the wuntho popa arc comprises i type granites and granodiorites with copper gold mineralization the spatial juxtaposition of the two belts and their distinct but consistent metallogenic endowment bears strong similarities to the metallogenic belts of the south american cordillera recent u pb age dating has shown the potential for the two belts to be near contemporary from the late cretaceous to eocene here we investigate whether an andean type setting during subduction of neo tethys could explain the observed magmatism and mineralization within these two belts

effects of magmatic volatile influx in mafic vms hydrothermal systems evidence from the troodos ophiolite cyprus abstract the troodos ophiolite cyprus is the principal on land analogue for mafic hosted volcanogenic massive sulfide vms deposits this study for the first time presents sulfur isotope 34s data on a regional scale from vms deposits and other mineralised zones across the troodos ophiolite in combination 34s se s ratios and trace element chemistry e g se cu and au of different hydrothermal sulfides are used to assess variations in magmatic volatile influx and the source of metals and sulfur in ancient hydrothermal systems sulfur isotope analyses n 180 across 19 mineral deposits indicate a variable source of sulfur in the troodos vms hydrothermal system this in turn allows a variable source of metals to be inferred pyrite 34s range from 5 5 to 13 2 with an average of 4 6 n 160 for all deposits investigated these 34s variations cannot only be explained by variable proportions of thermochemical seawater sulfate reduction 34s 18 to 19 and leaching of primary magmatic sulfur from igneous rocks 34s 0 1 consequently two processes are proposed explaining the trace metal and 34s variation across the troodos ore forming systems including i a variable source of metals in the sheeted dyke complex and ii the addition of a magmatic volatile rich phase to the vms hydrothermal systems two distinct lava units exist in the troodos stratigraphy namely the upper and lower pillow lavas upl and lpl the more primitive upl are enriched in au se and cu relative to as sb and zn that are concentrated in the lpl some vms deposits pre date the formation of the upl e g agrokipia a suggesting se cu and au depleted source rocks hence the stratigraphic position of vms deposits and the ratio of upl lpl affinity elements e g as zn sb vs cu se au imply a systematic relationship between trace element distribution and stratigraphic depth this relates to the relative proportion of upl and lpl affinity lavas within the metal source region 34s values 106 ratios 500 in pyrite suggest the contribution of a magmatic volatile component e g apliki and skouriotissa we demonstrate that the source of metals and sulfur in the troodos vms hydrothermal system is affected by regional scale processes related to i variable source lithologies and ii the contribution of a magmatic volatile phase to some troodos vms hydrothermal systems

venting of acid sulfate fluids in a high sulfidation setting at nw rota 1 submarine volcano on the mariana arc a comprehensive survey of hydrothermal plumes and their geochemistry revealed 16 hydrothermally active volcanoes along 1 200 km of the mariana arc from 13 5 n to 22 5 n of these 16 one volcano nw rota 1 is discharging sulfuric acid rich fluids producing some of the largest chemical anomalies ever observed in non buoyant hydrothermal plumes two types of venting were observed one of focused flow rich in al s si co2 fe mn and 3he and a second of diffuse flow rich in fe mn co2 and 3he but without al s and si the plume waters from the acid rich focused flow showed decreases in ph up to 0 73 ph units compared to ambient seawater and an estimated 80 percent of this change was brought about by volcanic and or hydrothermal so2 and possibly hcl with the remaining 20 percent by co2 conservative estimates suggest that the ph of the venting fluids was 1 0 the chemistry of this plume was also greatly different from that observed in any other hydrothermal setting an acid sulfate or high sulfidation fluid source was indicated from plume water samples rich in sulfuric acid native s al sulfate phosphate minerals and weathered silicate and amorphous silica phases the alunite group minerals and acidic fluids indicate that the most important hydrothermal reactions taking place within the hydrothermal system were resulting in advanced argillic alteration of the host volcanic rocks gas chemistry of the fluids revealed an he isotope signature with r ra 8 35 and 3he co2 3 25 109 these values are similar to those found along the global midocean ridge mor suggesting that the source of the 3he and co2 found here is predominantly from the upper mantle consistent with the position of nw rota 1 closest to the back arc in a chain of cross arc volcanoes the chemical signatures of the plumes arising from the diffuse flow remain greatly enriched in fe mn 3he and co2 but compared to the focused flow are low in mineral acidity h al and s the source of this diffuse flow is likely a fluid similar to that producing the acid rich plume but with a lower water to rock ratio and a longer reaction time resulting in the consumption of the mineral acidity this results in an increase in ph and the deposition of the alunite phosphate sulfate and other minerals close to the seafloor by comparison it is likely that the low ph focused flow ejects the vast majority of its mineral load into the water column which is not conducive to the formation of a mineral deposit at the sea floor and potentially would produce a barren prospect

the spectrum of ore deposit types volcanic environments alteration halos and related exploration vectors in submarine volcanic successions some examples from australia variations in shape metal content alteration mineralogy and volcanic host rocks of the ore deposits in the two major volcanic hosted massive sulfide vhms districts of eastern australia the cambrian mount read volcanics and the cambro ordovician mount windsor subprovince strongly reflect their volcanic environment conditions of ore formation and hydrothermal alteration processes lens and sheet style polymetallic zinc rich deposits such as rosebery hellyer que river and thalanga are considered to have formed in moderate to relatively deep water environments 500 1 000 m these deposits probably formed either on the sea floor e g hellyer que river or by replacement of porous volcaniclastic units directly below the sea floor e g rosebery the footwall alteration associated with these polymetallic vhms deposits was controlled by host rock permeability and porosity which are in turn related to volcanic facies type degree of fracturing and synvolcanic structural architecture focusing of hydrothermal fluids along synvolcanic structures has resulted in well zoned chlorite sericite footwall alteration pipes within footwall lavas at hellyer on the other hand diffuse fluid flow through very thick pumice breccia at rosebery and hercules has resulted in strata bound sericite dominated footwall alteration zones parallel to the paleosea floor and the ore lenses massive and disseminated pyritic cu au deposits such as those in the mount lyell field and at highway reward formed by subsea floor replacement and are associated with only minor zinc lead massive sulfide ore these deposits formed from higher temperature fluids 300 degrees c in which copper transport is enhanced and are commonly located in felsic volcanic centers dominated by shallow porphyritic intrusions e g highway reward the cu au ore lenses may be strata bound e g mount lyell or crosscutting pipes e g highway reward depending on the structure and permeability characteristics of the felsic volcanic host rocks the presence of high sulfidation alteration minerals e g pyrophyllite zunyite in some of the cu au deposits e g mount lyell field indicates that fluids were relatively acidic and suggests the possibility of magmatic fluid input into the hydrothermal system alteration zonation associated with the cu au vhms deposits is more symmetrical than that of the zn rich deposits with sericite rich alteration extending into the hanging wall in keeping with the subsurface replacement origin of these deposits synvolcanic gold rich deposits with high gold base metal ratios are less common than the cu au and zn rich vhms ore types the gold rich ores e g henty south hercules are strata bound in nature have low sulfide contents and are associated with central zones of intense silicification surrounded by envelopes of sericite pyrite and carbonate alteration volcanological and geochemical studies at henty indicate the gold rich ore formed by the replacement of particular volcanic units deposited in a relatively shallow water environment dominated by volcaniclastic facies lavas and limestones this spectrum of cu au zn rich and au only deposits in the mount read volcanics and the mount windsor subprovince is interpreted to represent a continuum from classic sea floor vhms ores toward those with features more akin to porphyry cu au and epithermal au ag deposits this spectrum relates to the interplay between factors in the submarine volcanic environment and the character of the hydrothermal fluid as follows 1 proportions of volcaniclastic lava and subvolcanic intrusive facies 2 depth of seawater 3 permeability and porosity of volcanic host rocks 4 balance between magmatic components and seawater components in the ore fluid and 5 temperature and acidity of the ore fluid mineralogical lithogeochemical and isotopic studies have revealed a range of alteration vectors useful in exploration for both the zn rich and cu au vhms deposits carbonate and white mica compositional variations are highlighted as important mineralogical vectors thallium and antimony halos may be useful trace element vectors and oxygen and sulfur provide important isotope vectors toward the center of the hydrothermal system

divining gold in seafloor polymetallic massive sulfide systems hydrothermal fluids on the modern seafloor are important carriers of base and precious metals in a wide range of volcanic and tectonic settings the concentrations and distribution especially of gold and silver in associated seafloor massive sulfide sms deposits are strongly influenced by variable source rocks fluid chemistry and precipitation mechanisms compositional data of 130 sms deposits around the world show a large range of gold and silver grades in part reflecting strong buffering of the hydrothermal fluids by their host rocks geochemical reaction path modeling shows that in most cases the investigated hydrothermal fluids are undersaturated with gold and silver and solubilities can be orders of magnitude higher than the au and ag concentrations measured in the corresponding fluids precipitation of gold during conductive cooling of mid ocean ridge black smoker mor fluids occurs at low temperatures but can be very rapid with 90 of the gold deposited in the first 25 c of cooling below 150 c the result is a zn au polymetallic assemblage with au and ag deposited at the same time together with pb and sulfosalts in ultramafic dominated um systems the strongly reduced hydrothermal fluids promote the deposition of gold at higher temperatures and explain the correlation between gold and copper in these deposits in this case the lower stability of the auhs complex at low o2 buffered by fayalite magnetite and quartz results in gold deposition at 250 c with early bornite and chalcopyrite and before sphalerite and silver producing a high temperature cu au assemblage in sediment hosted sed systems the much higher ph stabilizes au hs 2 and keeps gold in solution to very low temperatures after the precipitation of chalcopyrite sphalerite and galena resulting in au poor polymetallic sulfides and very late stage deposition of gold commonly with amorphous silica in arc related arc systems gold deposition occurs at somewhat higher temperatures than in the mor case in part because the fluids start with higher gold concentrations this can be explained by probable direct magmatic contributions and the high o2 of the fluids which promotes the solubility of gold at the source during cooling gold precipitates at about 160 c with sphalerite tennantite silver and galena resulting in an au rich polymetallic sulfide assemblage the mixing of hydrothermal fluids with seawater generally causes oxidation and eventually a decrease in the ph at a mixing ratio of 1 1 causing an initial increase in the solubility of gold and silver this can delay gold deposition from aqueous species to very low temperatures these complex systematics make prediction of au and ag grades difficult however important new data are coming to light on the actual concentrations of the precious metals in hydrothermal fluids in particular the input of magmatic volatiles and leaching of pre existing gold can lead to significant increases in the au and ag concentrations of the venting fluids and earlier deposition in several cases it appears that at least part of the gold load is present as nanoparticles in suspension allowing bulk gold concentrations that may be far in excess of liquid saturation boiling at the seafloor is now widely observed even at great water depths close to the critical point of seawater model calculations of phase separation during boiling show the competing effects on gold solubility of h2 h2s and co2 partitioning into the vapor which can result in highly variable gold to base metal ratios in the deposits flashing of the vent fluids into steam at high temperatures is also commonly observed and can lead to spectacular au grades with a strong cu au association in the deepest and hottest vents

shallow submarine epithermal pb zn cu au ag te mineralization on western milos island aegean volcanic arc greece mineralogical geological and geochemical constraints abstract milos island contains several epithermal deposits e g profitis ilias chondro vouno pb zn ag au te cu triades galana agathia kondaros pb zn ag bi w mo cu au and katsimoutis kondaros vani pb zn ag mn of late pliocene to early pleistocene age these deposits are hosted in calc alkaline volcanic rocks emplaced as a result of three successive magma pulses in an emergent volcanic edifice submarine rhyolitic to rhyodacitic cryptodomes at ca 2 7 ma profitis ilias chondro vouno submarine to subaerial andesite to dacite domes at ca 2 2 to 1 5 ma triades galana kondaros katsimouti vani hydrothermal alteration of the volcanic rocks includes advanced argillic both hypogene and steam heated argillic phyllic adularia sericite and propylitic types in the northern sector triades galana agathia kondaros initial magma degassing derived from andesitic dacitic intrusives along ne sw to e w trending faults resulted in the development of pre ore hypogene advanced argillic alteration dickite alunite diaspore pyrophyllite halite and pyrite in a submarine environment mineralogical data indicate common features among the profitis ilias chondro vouno kondaros katsimoutis vani and triades galana mineralized centers all of which are characterized by the presence of galena fe poor sphalerite and chalcopyrite as well as abundant barite adularia sericite and to a lesser extent calcite which are typical of intermediate sulfidation epithermal type deposits locally at triades galana and kondaros agathia high sulfidation conditions prevailed as suggested by the presence of coexisting enargite and covellite the high silver and gold content of the western milos deposits is derived from ag bearing sulfosalts polybasite pearceite pyrargyrite freibergite and tellurides gold at profitis ilias both as native gold and silver gold tellurides is present in base metal precipitates within multicomponent blebs which recrystallized to form hessite petzite altaite coloradoite and native gold mineralogical evidence e g microchimney structures copper sulfides widespread occurrence of barite aragonite suggests that precious metal mineralization in western milos mineralization formed in a submarine setting we present information on the surface distribution of au ag cu pb zn as sb hg mo bi w and cd at western milos gold is enriched at profitis ilias chondro vouno deposits and to a lesser extent at triades galana arsenic is absent from the southern sector but shows elevated concentrations together with molybdenum bismuth and tungsten at the northern sector triades galana vani deposits the differences in precious and base metal abundances may be related to the depths at which the deposits are exposed and or different sources of magma the metal signatures of the triades galana and agathia kondaros katsimouti vani mo bi w as hg ag au occurrences compared to profitis ilias te au ag reflect different sources of magma dacite rhyodacite for profitis ilias andesite dacite for triades galana and dacite for kondaros katsimoutis the enrichment of te mo w and bi in the deposits is a strong indication of a direct magmatic contribution of these metals at western milos precious and base metal vein mineralization was deposited during episodic injection of magmatic volatiles and dilution of the hydrothermal fluids by seawater the mineralization represents seafloor sub seafloor precipitation of sulfides that formed in stockwork zones base and precious metal mineralization formed from intermediate to high sulfidation state fluids and mostly under boiling conditions as indicated by the widespread occurrence of adularia associated with metallic mineralization we speculate that the widespread occurrence of boiling and the shallow depth of the precious and base metal emplacement prevented the formation of seafloor massive sulfides

mineralogy and mineral chemistry of the metamorphosed and precious metal bearing ming deposit canada abstract the ming deposit in the northwestern canadian appalachians is a metamorphosed bimodal mafic precious metal au ag bearing and cu rich volcanogenic massive sulfide vms deposit consisting of several spatially proximal lenses in a sericite to chlorite altered rhyodacitic footwall the various lenses 1807 zone 1806 zone ming south up plunge ming south down plunge and the lower footwall zone have variations in cu au ag and zn grades that reflect varying physico chemical conditions of ore formation this study describes the complex ore mineralogy of the orebodies and constrains the genesis of the deposit using field methods mineralogy whole rock sulfide geochemistry and micro analytical methods the orebodies are predominantly pyrite chalcopyrite with lesser sphalerite pyrrhotite and trace galena and arsenopyrite with the exception of the lower footwall zone which consists of a high temperature 320 c chalcopyrite pyrrhotite pyrite cubanite assemblage the other orebodies 1806 1807 ming south up plunge and down plunge contain trace amounts of tellurides hessite altaite tsumoite unnamed bismuthtelluride sulfosalts ag poor and ag rich tennantite tetrahedrite meneghinite agsb phases stannite and precious metal phases electrum aghg au alloys the 1807 zone is enriched in te bi and se whereas the 1806 zone is telluride free and contains as sb hg au and ag mineral chemistry of sphalerite shows strong variations in fe content 1 12 11 04 wt intermediate fe 4 33 6 33 wt fe and fe rich 7 327 11 04 wt fe sphalerite are common in all orebodies whereas fe poor sphalerite 1 12 3 57 wt fe occurs exclusively in the 1807 and 1806 zones tennantite tetrahedrite is typically ag poor 0 25 2 19 wt in the 1807 zone but is significantly enriched in ag up to 29 3 wt in the 1806 zone galena in the 1807 zone and ming south orebodies is commonly myrmekitically intergrown with tellurides and has high concentrations of te bi se and ag in contrast galena in the 1806 zone is less enriched in te bi and se but high in ag variations in mineralogy epithermal elements as bi hg sb se sn te and precious metal ag au content and mineral chemistry between the different orebodies indicate that they were formed from predominantly reduced acidic hydrothermal fluids with varying te2 s2 se2 s2 and mbi msb ratios as temperatures steadily decreased from 300 c to

mobility of au and related elements during the hydrothermal alteration of the oceanic crust implications for the sources of metals in vms deposits volcanogenic massive sulphide vms deposits are commonly enriched in cu zn and pb and can also be variably enriched in au as sb se and te the behaviour of these elements during hydrothermal alteration of the oceanic crust is not well known ocean drilling program odp hole 1256d penetrates a complete in situ section of the upper oceanic crust providing a unique sample suite to investigate the behaviour of metals during hydrothermal alteration a representative suite of samples was analysed for au as sb se and te using low detection limit methods and a mass balance of metal mobility has been carried out through comparison with a fresh mid oceanic ridge basalt morb glass database the mass balance shows that au as se sb s cu zn and pb are depleted in the sheeted dyke and plutonic complexes by 46 12 27 5 2 5 0 5 27 6 8 4 0 7 9 6 1 6 7 9 0 5 and 44 6 respectively arsenic and sb are enriched in the volcanic section due to seawater derived fluid circulation calculations suggest that large quantities of metal are mobilised from the oceanic crust but only a small proportion is eventually trapped as vms mineralisation the quantity of au mobilised and the ratio of au to base metals are similar to those of mafic vms and ten times enrichment of au would be needed to form a au rich vms the cu rich affinity of mafic vms deposits could be explained by base metal fractionation both in the upper sheeted dykes and during vms deposit formation

gold in turkey a missing link in tethyan metallogeny abstract the gold metallogeny of turkey constitutes a sector of the tethyan eurasian metallogenic belt temb within the alpine himalayan orogenic system that formed from jurassic cretaceous to the present this orogenic system produced many different types of deposits related to subduction collision post collision and rifting processes gold deposits as well as other mineral deposits of turkey are mainly concentrated in late mesozoic and tertiary rocks evaluation of the gold metallogeny of turkey is based on a gis database compilation of known gold deposits and prospects currently available data show that turkey has a gold endowment including reserves and resources of approximately 31 5 m oz 979 tonnes in 51 deposits 21 of which contain more than 0 2 m oz gold the other 30 deposits contain a total of approximately 1 m oz 31 tonnes gold resources two recent discoveries kisladag and copler currently contain total resources of 17 6 m oz au 549 tonnes more than 50 of the total turkish gold endowment turkey possesses a wide spectrum of gold deposits related to mesozoic and cenozoic volcanoplutonic arcs however porphyry gold copper epithermal gold including both high and low sulfidation styles and gold rich volcanic associated massive sulfide including both kuroko and cyprus types are the most economically important to date orogenic gold including listwanite hosted placer gold and skarn hosted gold are relatively less important or abundant deposit types other potential gold systems for exploration include carlin type gold detachment fault related gold iron oxide copper gold and gold in carbonate replacement and manganese deposits

possible submarine advanced argillic alteration at the basin lake prospect western tasmania australia the basin lake copper gold prospect lies in western tasmania s mount read volcanics and is hosted in a series of calc alkaline andesites quartz feldspar porphyries mudstones carbonates and sandstones between the tyndall group and the central volcanic complex alteration at the basin lake prospect occurs over a strike length of 1 4 km and includes thin strata bound pyrophyllite quartz paragonite kaolinite pyrite alunite alteration zones up to 12 m wide and containing up to 50 wt percent pyrophyllite with local fluorite veining these zones grade out to paragonite muscovite kaolinite quartz pyrite and muscovite carbonate chlorite alteration zones extensive propylitic alteration chlorite carbonate epidote affects most other rocks outside these zones mineralization consists of thin strata bound zones of massive and vein pyrite tennantite and chalcopyrite with trace covellite and galena hosted mainly within an intensely silicified core of the pyrophyllite quartzsericite alteration zone pyrite has 34s values of 1 4 to 6 9 per mil although marginal vein pyrite in the propylitic zone has 34s values around 12 4 per mil large silicified glacial erratic boulders at surface contain massive and vein pyrite enargite and tennantite with minor barite and trace covellite stannoidite and mawsonite pyrite and enargite have 34s values of 1 7 to 6 8 per mil barite has 34s values around 35 2 per mil with 87sr 86sr around 0 7108 the alteration and mineralization at the basin lake prospect is similar to that associated with high sulfidation copper gold systems formed by acidic relatively oxidized fluids a new geochemical vector here termed the advanced argillic alteration index aaai 100 sio2 sio2 10mgo 10cao 10na2o has been devised to help quantify the intensity of alteration the values of the aaai at basin lake are similar to those of several high sulfidation epithermal deposits the low sulfide 34s values are similar to those at other sulfide occurrences in the mount read volcanics that have previously been considered to be barren are lower than those of nearby volcanic hosted massive sulfide deposits and may indicate a magmatic fluid component however the 34s and 87sr 86sr values of basin lake barite at the assumed highest exposed level of the system and higher 34s values in pyrite from marginal veins are similar to those of cambrian volcanic hosted massive sulfide systems indicating the involvement of reduced seawater sulfate at these locations calcite carbon and oxygen isotope values silicate oxygen isotope values and the unusual abundance of carbonate close to advanced argillic alteration indicate fluid mixing and suggest that acidic magmatic fluids were likely neutralized by seawater this occurrence strengthens the case for prospecting the mount read volcanics and other similar submarine volcanic belts for copper gold and gold only deposits that formed by the actions of hyperacid oxidized fluids

reconstruction of an early permian sublacustrine magmatic hydrothermal system mount carlton epithermal au ag cu deposit northeastern australia the mt carlton au ag cu deposit northern bowen basin northeastern australia is an uncommon example of a sublacustrine hydrothermal system containing economic high sulfidation epithermal mineralization the deposit formed in the early permian and comprises vein and hydrothermal breccia hosted au cu mineralization within a massive rhyodacite porphyry v2 open pit and stratabound ag barite mineralization within volcano lacustrine sedimentary rocks a39 open pit these orebodies are all associated with extensive advanced argillic alteration of the volcanic host rocks stable isotope data for disseminated alunite 34s 6 3 29 2 18oso4 0 1 to 9 8 18ooh 15 3 to 3 4 d 102 to 79 and pyrite 34s 8 8 to 2 7 and void filling anhydrite 34s 17 2 19 2 18oso4 1 8 5 7 suggest that early advanced argillic alteration formed within a magmatic hydrothermal system the ascending magmatic vapor 34sss 1 3 was absorbed by meteoric water 50 60 meteoric component producing an acidic ph 1 condensate that formed a silicic quartz alunite quartz dickite kaolinite zoned alteration halo with increasing distance from feeder structures the oxygen and hydrogen isotope compositions of alunite forming fluids at mt carlton are lighter than those documented at similar deposits elsewhere probably due to the high paleolatitude s60 of northeastern australia in the early permian veins of coarse grained banded plumose alunite 34s 0 4 7 0 18oso4 2 3 6 0 18ooh 10 3 to 2 9 d 106 to 93 formed within feeder structures during the final stages of advanced argillic alteration epithermal mineralization was deposited subsequently initially as fracture and fissure filling au cu rich assemblages within feeder structures at depth as the mineralizing fluids discharged into lakes they produced syngenetic ag barite ore isotope data for ore related sulfides and sulfosalts 34s 15 0 to 3 0 and barite 34s 22 3 23 8 18oso4 0 2 to 1 3 and microthermometric data for primary fluid inclusions in barite th 116 233 c 0 0 1 7 wt nacl are consistent with metal deposition at temperatures of 200 40 c for au cu mineralization in v2 pit and 150 30 c ag mineralization in a39 pit from a low salinity sulfur and metal rich magmatic hydrothermal liquid that mixed with vapor heated meteoric water the mineralizing fluids initially had a high sulfidation state producing enargite dominated ore with associated silicification of the early altered wall rock with time the fluids evolved to an intermediate sulfidation state depositing sphalerite and tennantite dominated ore mineral assemblages void filling massive dickite 18o 1 1 to 2 1 d 121 to 103 with pyrite was deposited from an increasingly diluted magmatic hydrothermal liquid 70 meteoric component exsolved from a progressively degassed magma gypsum 34s 11 4 19 2 18oso4 0 5 3 4 occurs in veins within postmineralization faults and fracture networks likely derived from early anhydrite that was dissolved by circulating meteoric water during extensional deformation this process may explain the apparent scarcity of hypogene anhydrite in lithocaps elsewhere while the mt carlton system is similar to those that form subaerial high sulfidation epithermal deposits it also shares several key characteristics with magmatic hydrothermal systems that form base and precious metal mineralization in shallow submarine volcanic arc and back arc settings the lacustrine paleosurface features documented at mt carlton may be useful as exploration indicators for concealed epithermal mineralization in similar extensional terranes elsewhere

the susu knolls hydrothermal field eastern manus basin papua new guinea an active submarine high sulfidation copper gold system susu knolls comprises three steep sided conical volcanic peaks standing on a n nw trending ridge in the eastern manus basin a complex zone of convergence between the major indo australian and pacific plates the knolls consist of three porphyritic andesite to dacite domes each 1 0 to 1 5 km in diameter with crests ranging from 1 150 to 1 520 m below sea level an intense hydrothermal plume with peak transmission anomalies in excess of 40 originates from susu knolls and associated hydrothermal venting with fluid temperatures exceeding 300 c and sulfide mineralization has been detected on the crests of all three edifices the most important of these is the 2 47 million metric ton mt solwara 1 copper gold deposit on suzette knoll slated for mining by nautilus minerals the porphyritic dacite is commonly strongly altered by reaction with acidic fluids fragments of sulfide mineralization collected as part of the seafloor talus surrounding the crest of north su and south su are characterized by an assemblage of pyrite fukuchilite enargite covellite chalcopyrite by contrast there are actively venting chimneys on the crest of suzette that are typically zoned with chalcopyrite pyrite tennantite inner zones and barite dominated outer zones the knolls are covered by black sulfidic sediments that contain up to 2 4 wt cu and 3 2 ppm au primary feldspars have been obliterated by the hydrothermal activity that ranges from incipient to intense and which is characterized by natroalunite alunite north su only cristobalite tridymite and rare quartz and kaolinite most volcanic rocks exhibit a patchy surficial coating of native sulfur which may also fill vesicles no altered rocks were dredged from suzette because of the thick sulfidic sediment cover mass balance calculations for altered and unaltered rock from north and south su show that major and trace lithophile elements are depleted in the altered rocks except for ti al si and zr all near immobile in concert with the destruction of the primary minerals and removal of primary components by acidic fluids sulfur and chalcophile trace elements i e as au ba cd cu mo pb and se typically associated with magmatic cu au mineralization are enriched by orders of magnitude in both leached and mineralized rock sulfide and native sulfur 34s values from all three domes range from 7 4 to 0 4 indicating a magmatic component for the sulfur a small group of ore elements i e ag bi in sb and zn are strongly enriched in mineralized breccias and depleted in the altered dacites suggesting redistribution during alteration the presence of advanced argillic alteration the paragenesis of the fe cu as sulfide assemblage and presence of native sulfur together with the sulfur isotope evidence for magmatic input into the hydrothermal fluid at susu knolls are consistent with a submarine high sulfidation magmatic cu au hydrothermal system furthermore the sulfide assemblage pyrite enargite covellite chalcopyrite observed at north and south su and orders of magnitude enrichment of chalcophile elements in both altered and mineralized volcanic rocks relative to unaltered volcanic rock are also characteristic of subaerial high sulfidation epithermal mineralization the susu knolls hydrothermal field is a good example of a modern high sulfidation cu au submarine hydrothermal system

enargite luzonite hydrothermal vents in manus back arc basin submarine analogues of high sulfidation epithermal mineralization abstract active and inactive hydrothermal chimneys composed almost entirely of enargite and luzonite rare minerals in seafloor hydrothermal deposits were found at the summits of two submarine volcanoes north su and kaia natai in the manus back arc basin detailed mineralogical and geochemical studies revealed that most probably these deposits precipitated at t 200 330 c and high fs2 the negative 34s values 8 58 to 3 70 of the enargite luzonite are best explained by disproportionation reactions of magmatic so2 and suggest that the high fs2 is likely provided by direct magmatic input of so2 into the hydrothermal system fractionation of cu stable isotopes during the precipitation of enargite luzonite 65cu ranges from 0 20 to 0 35 is inferred to be associated with either rayleigh type fractionation or redox processes cu oxidation to cu2 and the mass balance of dissolved cu and cu2 species in the hydrothermal fluid the trace element composition of enargite and luzonite indicates a temporal fluctuation of the chemistry of the ore forming fluid with an increase of fe ga tl au hg pb and ag and decrease of sb sn te ge and v concentrations with time and points out that this type of deposits is the richest in au average 11 9 ppm and te average 169 ppm among all other types of seafloor metal deposits in addition to the widespread inorganic precipitation of enargite and luzonite in this setting there is evidence that this mineralization may be biogenically mediated on the external surfaces of the active vents fungi like filaments mineralized by luzonite imply that the fungi dikarya subkingdom may be implicated in a mechanism of bio sequestration of as s and cu and provide the initial substrate for luzonite precipitation the studied enargite luzonite deposits have characteristics similar to those of subaerial high sulfidation epithermal mineralization back arc basin setting acid sulfate and boiling ore forming fluids altered advanced argillic stage dacitic host rocks major enargite luzonite and minor pyrite barite and s0 34s au mineralization at depth

petrogenesis of the rambler rhyolite formation controls on the ming vms deposit and geodynamic implications for the taconic seaway newfoundland appalachians canada the ming cu zn ag au volcanogenic massive sulfide vms deposit is hosted by the upper cambrian lower ordovician rambler rhyolite formation which consists of a folded northeast plunging felsic dome complex developed in the uppermost segment of the obducted supra subduction baie verte oceanic tract in the north central newfoundland appalachians the deposit is overlain by interstratified mafic volcanic flows and volcaniclastic rocks of the snooks arm group the upper 1 km of the rambler rhyolite formation consists of a coherent felsic lithofacies at its base overlain by a quartz bearing volcaniclastic dominated lithofacies units 1 2 and 1 3 geochemically these rocks are light rare earth element lree and large ion lithophile element lile enriched la yb pn 4 20 th 2 58 4 05 ppm and high field strength element hfse depleted y 5 64 6 59 ppm rhyodacite with calk alkalic affinities th yb 2 zr y 7 the last pulse of felsic volcanism unit 1 3 is intimately associated with the massive sulfide lenses and has fi and fii type rhyolite signatures high la yb ratios commonly attributed to deep 30 km crustal melting the end t values of the rhyolites range from 2 5 to 1 3 indicating the rocks were influenced by crustal material a 10 percent batch partial melt of a garnet amphibole metamorphosed normal mid oceanic ridge basalt n morb and island arc tholeiite iat crustal material can generate a melt similar to the felsic rocks hosting the ming deposit however the iat reproduces much better the absolute abundances of lile th sr hfse nb zr y and middle rare earth elements mree of the felsic rocks it is interpreted that the felsic melt came from melting of a subducted slab with iat affinity thus explaining the depth of melt generation and fi fii signatures of the felsic rocks moreover the slab derived siliceous melt may also have contributed magmatophile elements for example ag au se te sn sb hg into the hydrothermal system once it had reached near surface depths and may explain the epithermal suite element enrichment in the ming deposit this study shows that strongly fractionated felsic volcanic rocks associated with boninites such as those associated with the ming vms deposit can be important hosts for precious metal enriched vms and may be as prospective as tholeiitic felsic volcanic sequences for example fiii fiv type rhyolite low la yb ratios the base of the snooks arm group is comprised of spatially restricted sulfide bearing mafic breccia overlain by a regionally extensive sedimentary sequence end t 3 1 to 5 5 which is in turn overlain by interstratified high mg basalt end t 1 6 th enriched back arc basin basalt babb enriched mid oceanic ridge basalt e morb end t 6 6 and lree enriched low ti tholeiitic tuffs end t 0 5 four generations of mafic to intermediate dikes and sills cross cut the rambler rhyolite formation and share similar geochemical characteristics to the extrusive rocks of the snooks arm group suggesting comagmatic relationships the combination of variations in nb yb th yb and end t values within and between units suggest melts derived from depleted to enriched mantle material with melts synchronously generated by both slab metasomatized mantle wedge and upwelling back arc asthenosphere

erratum to variations of sulphur isotope signatures in sulphides from the metamorphosed ming cu au volcanogenic massive sulphide deposit newfoundland appalachians canada the ming deposit is an early ordovician bimodal mafic cu au volcanogenic massive sulphide vms deposit in the newfoundland appalachians that was metamorphosed to upper greenschist lower amphibolite facies conditions and deformed in the silurian and devonian the ming deposit consists of several spatially proximal ore bodies of which the 1806 zone 1807 zone ming south up plunge and down plunge and the lower footwall zone are the focus of this paper the ore bodies have similar stratigraphic sequences the ore bodies can be divided into 1 a silicified horizon that caps the massive sulphides 2 semi massive to massive sulphides and 3 sulphide mineralization in a rhyodacitic footwall sulphide mineralization in a rhyodacitic footwall includes a sulphide stringers immediately below the semi massive to massive sulphides and b chalcopyrite pyrrhotite pyrite stringers distally from semi massive to massive sulphides in the lower footwall zone pyrite chalcopyrite pyrrhotite arsenopyrite and galena were analysed by in situ secondary ion mass spectrometry sims for sulphur isotope compositions the isotopic signatures of pyrite chalcopyrite pyrrhotite and arsenopyrite fall within a limited range of 2 8 to 12 0 for semi massive to massive sulphides and sulphide mineralization in the footwall the silicified horizon capping the semi massive to massive sulphides has higher 34s 5 8 19 6 especially for pyrrhotite mean 17 2 2 2 n 8 the sulphur isotope composition of galena is more heterogeneous especially within semi massive to massive sulphides and sulphide stringers ranging from 0 8 to 17 3 mean 6 1 4 3 n 35 and 7 6 to 17 1 mean 13 7 5 3 n 3 respectively geothermometric calculations give insufficient formation and metamorphism temperatures for neighbouring mineral pairs because sulphides were not in isotopic equilibrium while deposited in early ordovician or re equilibrated during silurian devonian metamorphism respectively therefore original isotopic compositions of sulphides at the ming deposit have been preserved modelling of the source of sulphur shows that 1 reduced seawater sulphate and 2 sulphur leached from igneous wall rock and or derived from magmatic fluids are the main sources of sulphur in the ming deposit the influence of igneous sulphur igneous wall rock magmatic fluids increases with temperature and is an important sulphur source for the semi massive to massive sulphides and footwall mineralization in addition to a contribution from thermochemical sulphate reduction tsr of seawater it is difficult to distinguish between sulphur leached from igneous rocks and magmatic fluid related sulphur and it is possible that both sources contributed to the ores at the ming deposit in addition to igneous sulphur the heavy isotopes of the silicified horizon are consistent with the sulphur in this horizon being derived only from thermochemical sulphate reduction of early ordovician seawater sulphate