the iberian type of volcano sedimentary massive sulphide deposits the iberian pyrite belt located in the sw iberian peninsula contains many paleozoic giant and supergiant massive sulphide deposits including the largest individual massive sulphide bodies on earth total ore reserves exceed 1500 mt distributed in eight supergiant deposits 100 mt and a number of other smaller deposits commonly with associated stockwork mineralizations and footwall alteration haloes massive sulphide bodies largely consist of pyrite with subordinated sphalerite galena and chalcopyrite and many other minor phases although substantial differences occur between individual deposits both in mineral abundance and spatial distribution these deposits are considered to be volcanogenic roughly similar to volcanic hosted massive sulphides vhms however our major conclusion is that the iberian type of massive sulphides must be considered as a vhms sub type transitional to shms

controls on the siting and style of volcanogenic massive sulphide deposits abstract volcanogenic massive sulphide vms deposits form in subaqueous environments from circulating hydrothermal fluids heated by volcanic activity these deposits form as sulphide mounds stratiform exhalative and or replacive bodies and commonly have stockwork vein mineralization in their immediate footwall these various styles are essentially facies of mineralization each one being the product of a particular set of conditions that control the ore forming processes and the consequent geometry and architecture style of the deposits these controls include the physical and or chemical nature of the host rocks the temperature and composition of the hydrothermal fluids and the redox state of the depositional environment the style of exhalative deposits is controlled by the salinity of the vented fluids and the redox state at the seafloor hydrothermal fluids with salinities less than twice that of seawater that vented into open oxic oceanic environments typically formed small mound and chimney complexes unless they were rapidly covered by sediments or volcanic rocks the massive sulphides were rapidly oxidized and partly dissolved by seawater in contrast stratiform sheet like deposits are typically formed in anoxic bottom waters anoxic marine conditions were periodically of global extent particularly prior to 2 4 ga or of a regional nature local anoxic conditions can also be self induced by the exhalation of saline and reducing hydrothermal fluids that ponded in bathymetric depressions such as second or third order basins to form a brine pool these exhalative systems may have been initiated as chimney vent complexes and subsequently overlain by stratiform sulphides formed under the self induced anoxic conditions deposits formed in anoxic environments can be significantly larger than those in oxic settings and this is attributed to several factors that include longer lived hydrothermal circulation more efficient sulphide precipitation and reduced or inhibited oxidation thereof replacement of volcanic and sedimentary strata by sulphide typically occurs within the feeder zones beneath the exhalative mineralization however successions with abundant porous permeable and or reactive rocks such as glassy and or pumiceous volcaniclastic rocks and in some cases limestone favoured the development of large replacive deposits that may have had little surficial expression on the sea floor vms deposits at spreading centres within oceanic crust formed almost exclusively as mounds most of them have not been preserved likely due to oxidation of the sulphides in the prevailing oxic environment and or destruction of oceanic crust during subsequent subduction intra continental rifts arc rifts and back arc rifts commonly have more complexity in their structure and facies architecture and environments and can host all styles of vms mineralization in these settings early extension favoured the formation of restricted basins with ideal conditions for the onset of hydrothermal activity and development of anoxic bottom waters whereas in mature rifts the conditions were less conducive for the formation of regionally extensive anoxic environments formation of replacive deposits was permissible in all settings with porous or reactive subsea floor strata replacive mineralization is the most likely to be preserved in the geological record due to the sulphides being physically shielded from oxidative weathering and mechanical erosion at the seafloor the various styles of vms mineralization can rarely be distinguished using a single criterion in most cases multiple criteria are required mound style mineralization is distinguished by a mound or lens shaped morphology b presence of chimney fragments c presence of abundant sedimented sulphide breccias d location on a stratigraphic boundary ore horizon and e association with a thin horizon or thicker stratigraphic interval of fine grained clastic rocks e g shale mudstone that accumulated at slow sedimentation rates stratiform exhalative mineralization is distinguished by a sheet like morphology prior to deformation b presence of fine grained clastic host rocks that accumulated at relatively slow rates e g mudstone c presence of local or extensive planar stratification replacive mineralization is characterized by a irregular geometry and distribution of sulphide bodies b gradation from massive sulphides to semi massive sulphides and disseminated mineralization with relict textures of the host rock and c originally pumiceous glassy or reactive host rocks emplaced at high depositional rates mass flow deposits lavas carbonate altered mass flow deposits or limestone one deposit or district may comprise two or more of these main styles of mineralization in many cases the main styles of vms mineralization present in a particular region can be predicted from examination of the facies architecture and depositional environments of the host succession recognition of the style s of mineralization that occur in a particular basin or mineral belt enables exploration models to be improved and should influence the strategy of exploration for vms deposits

stratigraphy of the tectonically imbricated lithological succession of the neves corvo mine area iberian pyrite belt portugal biostratigraphic research based on palynomorphs and ammonoids of the tectonically imbricated lithological succession of the neves corvo mine in the portuguese part of the iberian pyrite belt has yielded ages for all formerly recognised lithostratigraphic units these can be assembled in three main lithological sequences 1 detrital sandy shale substrate phyllite quartzite formation of late famennian age 2 volcano sedimentary complex divided into a lower and an upper suite in which one basic three dolerite sills and four felsic volcanic units and a mineralised package of massive sulphides are identified with ages which range from the late famennian to the late visean 3 flysch succession mertola formation composed of shale and greywacke dated as late visean to early serpukhovian precise biostratigraphic dating of the sedimentrary host rocks of massive sulphide mineralisation constrains the age of the latter to the late strunian 354 8 354 0 ma three stratigraphic hiatuses corresponding to early middle strunian tournaisian and early visean respectively and a south westward progressive unconformity were also recognised sequences 1 and 2 are related to extensional episodes while sequence 3 marks the beginning of compressive tectonic inversion which gave rise to south westward flysch progradation in close relation to a foreland basin development our results lead to the reinterpretation of the tectonic structure of the neves corvo mine with implications for the interpretation of the regional basin dynamics and metal exploration

geological fate of seafloor massive sulphides at the tag hydrothermal field mid atlantic ridge deep sea mineral deposits potentially represent vast metal resources that could make a major contribution to future global raw material supply increasing demand for these metals many of which are

mineralogy and geochemistry of epigenetic features in metalliferous sediment atlantis ii deep red sea finely laminated and undisturbed except in the area of the southwest basin in this part bedding resedimentation and brecciation are common in addition some sediment cores contain vein minerals precipitated in open fissures which crosscut unlithified metalliferous sediments these veins are the conduits by which new hot brine vents into the deep vein mineralogy is dominated by anhydrite talc smectite pyrite sphalerite and chalcopyrite vertical zonation due to cooling and reaction of the incoming brine as it rises through the metalliferous sediment near the southwest basin west basin transition metalliferous sediment has been locally recrystallized to a hematite magnetite pyroxene assemblage probably due to the intrusion of basalt into the metalliferous sediments modified journal abstract

formation of fe si mn oxyhydroxides at the pacmanus hydrothermal field eastern manus basin mineralogical and geochemical evidence samples of fe si mn oxyhydroxides were collected from the pacmanus hydrothermal field which lies in a young back arc setting in the eastern manus basin the purpose of the study was to understand the origin and characteristics of fe si mn oxyhydroxides associated with massive sulfides in a back arc basin the pacmanus fe si mn oxyhydroxides are composed of fe oxyhydroxides and mn oxyhydroxides with opal a and nontronite they have very low concentrations of trace elements except for ba mo v and u and rare earth elements and they show ree distribution patterns with positive eu anomalies and slight enrichments of lrees the fe si mn oxyhydroxides appear to be precipitated mainly from hydrothermal fluid with limited seawater contamination and scavenged trace metals are predominantly from the ambient seawater the differences in the ree distribution patterns between the fe oxyhydroxide fraction and mn oxyhydroxide fraction originate from diagenetic processes there are diverse filamentous microtextures resembling unique microbial populations suggesting microbially mediated mineralization during the precipitation of the fe si mn oxyhydroxides

the geochemistry of hydrothermal alteration at the salgadinho copper deposit portugal the recently discovered salgadinho copper deposit 7 km nne of s luis portugal is located in the southernmost linear belt of outcropping low metamorphic grade deformed palaeozoic rocks famennian in the sw part of the iberian pyrite belt the stratabound replacement pyrite chalcopyrite mineralisation is present in variably altered felsic pyroclastic rocks which are overlain by pyritic graphitic shales and tuffs which have undergone alteration in the lowermost 5m the altered pyroclastic rocks are characterised by pale green celadonitic fluoro muscovite and in the most intense zone of alteration quartz ankerite and ore minerals are present pyrite chalcopyrite sphalerite tetrahedrite galena bournonite no exhalative cu zn or zn pb ore has been discovered associated with the submarine volcanic stratabound cu rich alteration zone alteration of feldspars and groundmass has involved a net loss from the system and gains in fe2 f h2o mg fe3 ca mn p ti s cu zn pb as sb ag and au at the expense of si k al and na the great enrichment of f in the altered rocks the association of zonal alteration facies with coarse grained pyroclastic masses and the intimate association of pale green celadonitic fluoro muscovite with mineralisation at salgadinho and most other deposits of the iberian pyrite belt represents a powerful exploration guide for submarine exhalative ores

manganiferous cherts of the franciscan assemblage i general geology ancient and modern analogues and implications for hydrothermal convection at oceanic spreading centers there are several hundred ophiolitic manganiferous chert deposits primarily of late jurassic to early cretaceous age known within the franciscan assemblage of california the sequences typically consist of one to three massive manganiferous chert lenses containing 30 to 50 percent mn and averaging 1 m in thickness by 15 m subcircular diameter these are separated by an average 2 to 10 m of thin bedded radiolarian cherts and overlie basalts or greenstones both their geology and chemistry indicate that the ore lenses are hydrothermal and may have formed on the flanks of a mid ocean ridge or within a back arc basin it is proposed that the sequences developed as a result of sea floor spreading over a series of deep hydrothermal seawater convection cells paralleling a spreading center and spaced roughly 5 to 10 km apart chemical profiles of mn fe si al cu ni zn co ba ti the rare earth elements and 87 sr 86 sr have been determined through two complete sections these profiles indicate hydrothermal input of mn si cu ni zn and ba and detrital or hydrogenous input of al and co they illustrate the use of ti as a measure of relative detrital sedimentation rates fe is strongly fractionated from mn within the ores fe mn 0 1 and fe mn ratios decrease upward throughout each section suggesting preferential deposition of fe within the sediment and of mn at the seawater interface rare earth element distributions reflect the interaction of sea water and underlying basalts sr isotopic ratios of the ores and basalts demonstrate both strong and moderate seawater influences respectively fluid inclusion analyses on veins of undetermined age show seawater salinity temperatures of roughly 200 degrees c and tentative entrapment pressures corresponding to 1 700 m water depth early and intermediate veins were injected into unconsolidated siliceous sediment producing a characteristic bleached and pseudobrecciated texture an analogy is drawn with the present day field of hydrothermal mounds near the galapagos rift and with ophiolitic complexes of the northern apennines and other localities

nature age and tectonic setting of granitoid hosted orogenic gold deposits of the jiaodong peninsula eastern north china craton china the jiaodong gold province along the southeastern margin of the north china craton has combined production and resources of 900 tonnes of gold more than 95 of the gold in this province is hosted in mesozoic granitoids which intruded late archean metamorphic basement rocks episodically at ca 160 150 and 130 126 ma the deposits of the jiaodong peninsula form one of the largest provinces of granitoid hosted lode gold deposits in the world the jiaodong gold deposits occur as massive auriferous quartz veins hosted in subsidiary second or third order faults and as disseminated and stockwork style mineralization hosted in alteration zones along major regional scale faults mineralization took place during brittle reactivation d3 of pre existing d2 ductile shear zones wall rock alteration is dominated by k feldspar and sericite with also sulfide minerals and extensive silicification closest to the orebodies the alteration was accompanied by the introduction of volatiles fe mg k rb ag as au bi and sb enrichment of base metals in the gold lodes is generally low although locally significant gold deposition took place at temperatures of 250 to 350 c for most gold deposits and the ore fluids are co2 bearing and have calculated salinities of 6 13 wt nacl equiv deposits from the zhao ye xixia and muping rushan belts have similar if not identical structural controls alteration patterns ore mineral assemblages and stable isotope signatures lead isotope data indicate that the ore stage leads were buffered by their host rocks the granitoid hosted gold deposits in the zhao ye belt are most radiogenic the greenstone hosted deposits in the xixia belt is the least radiogenic and the granitoid hosted deposits in the muping rushan belt have intermediate compositions gold mineralization in the zhao ye xixia and muping rushan belts in the jiaodong gold province appears to be broadly contemporaneously at ca 130 120 ma as constrained by shrimp u pb zircon dating on host rocks and a post gold dike as well as by ar ar rb sr and k ar dating of hydrothermal alteration minerals and ores this early cretaceous age is about 100 million years younger than the peak of ultra high pressure metamorphism in the qinling dabie sulu orogen between the north china and yangtze cratons and about 500 km south of the gold province however it overlaps with the extension and exhumation stage of the collisional orogeny and the subduction of the pacific plate beneath the eurasian continental crust whereas the majority of the jiaodong gold deposits occur on the margin of the north china craton some also occur in the suture zone between the two cratons they are significantly landward relative to the subduction zone of the pacific plate compared with typical orogenic lode gold deposits globally lode gold deposits of ca 130 120 ma occurred on both sides of the tan lu fault zone available data favor a temporal link of gold mineralization with pacific plate subduction

geochemical and geologic study of the volcano sedimentary sulfide orebody of la zarza province of huelva spain the la zarza orebody containing some 100 000 000 metric tons of ore is one of the three most important ones in the huelva province spain it overlies the second phase of acid volcanic activity within a volcanic sedimentary succession of visean age the stratigraphically lower part of the orebody consists of a finely bedded polymetallic ore overlain by a massive pyrite facies 90 of the orebody which is in turn covered by a more or less discontinuous resedimented pyritic capping the distribution of the metals within the orebody differs from the classical pattern in la zarza the pb zn contents are greater in the lower parts of the deposit than in the upper parts while cu is more or less independent of other metals and shows a typical basal concentration in a breccia type ore and stockwork restricted to a small area the foot and hanging walls are made up of acid epiclastic rocks and pelites and are practically without lavas however it is possible to recognize volcanic eruption centers some kilometers to the east of the mine below the sulfide orebody an intensively chloritized zone some hundred meters in diameter most probably represents a hydrothermally altered pipe overall the wall rocks are characterized by their enrichment in mg and impoverishment in na and si within the chloritized pipe an overall lowering of alkali values is accompanied by a strong increase in mg as analyzed by spectrometry and atomic absorption the distribution of trace elements in wall rocks and ore cu pb zn v co cr and mn tends to show the lack of consanguinity between ore and wall rocks on the one hand and the reasonably clear association between the ore and a late volcanic hydrothermalism on the other furthermore the high co ni ratios correspond to a hot sulfide deposition there is no distinct dispersion halo except for the presence of some disseminated sulfides around and very close to the ore the distribution of the elements and their correlations differ betweeen the ore and wall rocks although it lies on a volcanic sedimentary footwall the la zarza orebody is thus rooted in a hydrothermally altered pipe nevertheless the migration of magnesium and other metals is unconnected and there is evidence of a certain independence among the different metals above all in the case of copper

flow rates in the axial hot springs of the east pacific rise 21 n implications for the heat budget and the formation of massive sulfide deposits abstract the first in situ measurements of flow rates in submarine hot springs temperatures between 275 and 350 c were made in the hot springs of the east pacific rise at 21 n during november 1981 the flow rates ranged between 0 7 and 2 4 m s we estimate that the total rates at which heat and mass are transported from the three hot spring areas obs national geographic and southwest vents are ca 2 2 108 w and 150 kg of fluid s respectively the very high rate of heat loss precludes the existence of hot springs as steady state features of the east pacific rise at 21 n we estimate that hot springs are active for a maximum of ca 40 000 years chemical and or mechanical clogging of the vents can drastically reduce the lifetime of the hot springs using a simple model of plume development 1 we calculated that less than 3 of the sulfide particles that are entrained in the hydrothermal plumes at 21 n settle from the plumes before dispersal by a lateral submarine current at a height of ca 250 m above the seafloor 2 the time that is required to form the sulfide mounds that surround the hot springs by the accumulation of sulfide particles that settle from the plumes is ca 70 85 years the current rate at which sulfide particles settle from the hydrothermal plumes and accumulate near the hot springs appears to be too small to lead to the formation of large massive sulfide deposits such as the mavrouni deposit on cyprus 3 within the time limits that are suggested by our thermal energy balance calculations

bismuth and cobalt minerals as indicators of stringer zones to massive sulphide deposits iberian pyrite belt the stringer zones and commonly the interaction zone at the base of the massive sulphide mounds in the iberian pyrite belt contain bismuth and cobalt minerals that are not found in the overlying massive sulphides these are fairly rare cobalt sulphoarsenides cobaltite alloclasite galucodot that were formed at the beginning of the massive sulphide genesis and fairly common bismuth sulphides bismuthinite hammarite wittichenite cosalite kobellite joseite etc including species rare at world scale nuffieldite giessenite jaskolskiite that were deposited from last stage high temperature 300 c copper bearing fluids containing bi te se the last stage fluids precipitated chalcopyrite containing cu bi te se sulphosalts at the base of the sulphide mound to form a high cupriferous zone their interaction with the massive sulphides is reflected by the formation of an exchange zone a few metres thick showing chalcopyrite disease textures at the base of the mound this zone forms the upper limit of potentially economic copper enrichment and of bismuth minerals gold is undoubtedly in part if not totally related to this last phase the bismuth concentrations being equivalent in the massive sulphides and the stringers the presence of bismuth minerals in the stringer zones results from high temperature conditions combined with a rarity of galena which impedes absorption of available bi the distribution of these bismuth minerals provides a basic mineralogical zoning in the stringer zone with a deep low as2 zone containing native bismuth and tellurides and a shallow higher as2 zone in contact with the massive ore sensu stricto and containing complex bismuth sulphides these results make it possible to distinguish between sulphide veinlets belonging to stockwork zones of massive orebodies and veinlets of an ambiguous nature and provide mineralogical criteria for the proximity of copper rich zones they enrich the very complex mineralogy of the iberian pyrite belt

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

alteration mineralogy lithochemistry and stable isotope geochemistry of the murgul artvin ne turkey volcanic hosted massive sulfide deposit implications for the alteration age and ore forming fluids abstract the murgul artvin ne turkey massive sulfide deposit is hosted dominantly by late cretaceous calc alkaline to transitional felsic volcanics the footwall rocks are represented by dacitic flows and pyroclastics whereas the hanging wall rocks consist of epiclastic rocks chemical exhalative rocks gypsum bearing vitric tuff purple vitric tuff and dacitic flows multi element variation diagrams of the hanging wall and footwall rocks exhibit similar patterns with considerable enrichment in k rb and ba and depletion in nb sr ti and p the chondrite normalized rare earth element rees patterns of all the rocks are characterized by pronounced positive negative eu anomalies as a result of different degrees of hydrothermal alteration and the semi protected effects of plagioclase fractionation mineralogical results suggest illite illite smectite chlorite kaolinite and chlorite in the footwall rocks and illite smectite kaolinite and chlorite illite in the hanging wall rocks overall the alteration pattern is represented by silica sericite chlorite and chlorite carbonate epidote sericite and quartz albite zones increments of ishikawa alteration indexes resulting from gains in k 2 o and losses in na 2 o and the chlorite carbonate pyrite index towards to the center of the stringer zone indicate the inner parts of the alteration zones calculations of the changes in the chemical mass imply a general volume increase in the footwall rocks abnormal volume increases are explained by silica and iron enrichments and a total depletion of alkalis in silica zone relative k increments are linked to the sericitization of plagioclase and glass shards and the formation of illite smectite in the sericite zone in addition fe enrichment is always met by pyrite formation accompanied by quartz and chlorite illite is favored over chlorite smectite and kaolinite in the central part of the ore body due to the increase in the al k na ca ratio although the rees were enriched in the silicification zone light rees show depletion in the silicification zone and enrichment in the other zones in contrast to the heavy rees behavior hydrothermal alteration within the hanging wall rocks apart from the gypsum bearing vitric tuffs is primarily controlled by chloritization with proportional fe and mg enrichments and sericitization the 18 o and d values of clay minerals systematically change with increasing formation temperature from 6 6 to 8 7 and 42 to 50 for illites and 8 6 and 52 for chlorite respectively the o and h stable isotopic data imply that hydrothermal alteration processes occurred at 253 332 c for illites and 136 c for chlorite with a temperature decrease outward from the center of the deposit the positive 34 s values 20 3 to 20 4 for gypsum suggest contributions from seawater sulfate reduced by fe oxide hydroxide phases within altered volcanic units thus the hydrothermal alteration possibly formed via a dissolution precipitation mechanism that operated under acidic conditions the k ar dating 73 62 ma of the illites indicates an illitization process from the maastrichtian to early danian period

bedded jaspers of the ordovician lokken ophiolite norway seafloor deposition and diagenetic maturation of hydrothermal plume derived silica iron gels sedimentary beds of jasper red hematitic chert in the ordovician lokken ophiolite of norway are closely associated with volcanogenic massive sulphide vms deposits the jaspers occur in the immediate hangingwall and laterally peripheral to the large lokken 25 30 mt and small hoydal 0 1 mt vms deposits and are exposed discontinuously for several kilometres along strike massive or laminated types predominate jasper sulphide debris flow deposits are also abundant near vms deposits the jaspers contain hematite rich laminae showing soft sediment deformation structures and microtextural evidence that record the presence of a colloidal precursor and an origin as gels early textures include 1 straight or curved chains of hematitic filaments 3 10 m in diameter and 20 100 m long 2 branching networks of 15 25 m thick tubular structures surrounded by cryptocrystalline hematite and filled with quartz and euhedral hematite 3 small up to 10 m spherules composed of cryptocrystalline hematite and silica and 4 up to 50 m silica spherules with hematitic cores the small filaments seem to have been deposited in varying proportions in the primary laminae possibly together with hematitic and siliceous microspheroids diagenetic changes are represented by polygonal syneresis cracks and the presence of cryptocrystalline originally opaline silica chalcedony quartz carbonate and cryptocrystalline hematite and or goethite forming botryoidal masses and spheroids 10 m to 5 mm in diameter coarser euhedral grains of quartz carbonate and hematite are integral parts of these textures bleached silica rich jaspers preserve only small relics of fine grained hematite rich domains and locally contain sparse pockets composed of coarse euhedral hematite epidote

hydrothermal alteration and mineralization in the neves corvo volcanic hosted massive sulfide deposit portugal i geology mineralogy and geochemistry the neves corvo volcanic hosted massive sulfide deposit is one of the largest 300 mt and richest known deposits of the iberian pyrite belt the deposit is unique because of its extremely high cu and sn grades 45 mt at 6 cu plus 4 5 mt at 12 cu and 2 2 sn more than 300 000 metric tons t of tin metal is contained in several types of tin mineralization that grade from 500 ppm to 60 percent sn the deposit consists of five orebodies including corvo the focus of the present study the volcanic volcaniclastic and sedimentary lithofacies that directly host the neves corvo mineralization consist of a rhyolitic dome cryptodome hyaloclastite complex facies interpretation indicates that volcanic activity occurred at moderate water depth below storm wave base in variably subsiding basins of different orders several syn and postore modification events included gravity driven mass transport processes and subsequent low angle thrusting and asymmetric detachment folding which shaped the present deposit the ore geology and geochemistry and the textural mineralogical and geochemical aspects of the ore forming process in the corvo orebody collectively suggest that ore formation resulted from a long lived hydrothermal system including two main mineralizing events an early stage of stringer and massive cassiterite deposition and a subsequent episode of massive sulfide ore generation two spatially independent stockworks fed these genetically related mineralizing systems the early deposition of stringer and massive cassiterite ores took place almost exclusively in the tin corridor a structural alignment bounded by synvolcanic faults textural analysis and geochemical data both indicate minimal fluid rock interaction during ascent of the tin bearing fluid the massive and semimassive cassiterite ores at corvo most likely formed by direct venting into seawater and or emplacement in water laden unlithified sediments and early hydrothermal products that mantled the footwall volcanic sequence stringer and massive cassiterite precipitation is thought to have been a product of a short lived episode in the overall ore forming system the vein filling mineral assemblage in the cassiterite stockwork quartz cassiterite pyrite donbassite kaolinite chamosite coupled with whole rock and mineral geochemical data indicate that the tin bearing fluid had a low ph and relatively high temperature 380 c the massive sulfide related hydrothermal alteration in corvo is essentially strata bound the permeability contrasts in the uppermost portion of the footwall succession played an important role in controlling the fluid flow the hydrothermal alteration is zoned and consists of an inner chlorite donbassite quartz sulfide sericite core that grades outward into successive enveloping k sericite quartz sulfide and na sericite quartz sulfide halos textural evidence from the mine to the microscopic scale and mass balance considerations indicate that extensive silicate replacement in the coherent volcanic rocks of the footwall sequence and disseminated replacement mineralization in the uppermost volcaniclastic and or sedimentary units were major mechanisms of massive sulfide deposition at corvo alteration mineralogy and geochemistry indicate that the neves corvo ore fluids were hotter and more acidic than typical ore forming fluids of the iberian pyrite belt the geochemical characteristics of the tin and copper mineralization also differ from the ores of typical deposits of the iberian pyrite belt and imply the involvement of additional metal sources possibly magmatic in the ore forming system copper enrichment due to mechanical and fluid assisted tectonometamorphic remobilization processes accounts for the formation of extremely high grade ore shoots in the deposit

the mattagami lake mine archean zn cu sulfide deposit quebec hydrothermal coprecipitation of talc and sulfides in a sea floor brine pool evidence from geochemistry 18 o 16 o and mineral chemistry discussion

fluids and thrusting abstract recent studies of the structure of the continental crust by cocorp and birps have shown that major thrust structures in the lower crust are ubiquitous plate tectonic processes of continental crust destruction or construction at sites where subduction or collision occurs the himalayas or the andes involve thrusting of continental materials or ocean crust beneath continents and local overthrusting may accompany transcurrent motion in all such cases massive fluid flow must occur as first stressed by m k hubbert and w w rubey thrusting mechanisms require that fluid pressures are close to or greater than lithostatic pressures when thrusting occurs compaction of the underplate must occur rapidly expelling pore water crustal thickening leads to gradual heating of the underplate with prograde metamorphism near the top and even melting at the base if the overthrust plate is thick and hot at the base retrograde metamorphism will occur at the base of the overthrust plate and rising fluids will encounter inverted thermal gradients in such a region veins need not occur but leaching phenomena may dominate if an underthrust plate is hydrated and or contains sedimentary aquifers large fluid volumes are expelled through shear zones into the overthrust plate the scale of fluid release processes can be large thus in thin skinned tectonics where the overthrust plate is 10 15 km thick induced fluid release can easily reach 4 109 g m 2 of thrust surface the flow of such fluids will be controlled by the thrust surface and lithology both of which will influence hydraulic fracture mechanisms and spacing of fractures the chemistry of thrust derived fluids may be highly variable depending on lithology and the time constants of thrusting vertical thermal and redox environments will be similarly dependent in the subduction process impressive quantities of fluids must pass back up the thrust surface and such fluids have recently been directly observed during drilling dewatering of spilites serpentinites and sediment layers of the underthrust oceanic lithosphere must produce massive fluid flow back to the surface but some of these fluids may form hydrated minerals in the overlying mantle and at great depth flux mantle melting extreme metasomatism in blueschist belts must result from such fluids a case of particular interest involves the cessation of subduction when a high level slab reaches thermal equilibrium in general flow regimes in the thrust and fault zones associated with collisions follow a sequence from conditions of high t p with locally derived fluids at low water rock ratios during initiation of the structures to high fluxes of reduced metamorphic fluids along conduits as the structures propagate and intersect hydrothermal reservoirs later in the tectonic evolution and at shallower crustal levels there may be incursion of oxidizing near surface fluid reservoirs into the faults these fluids may have extremely low 18o values where mountain ranges form on rebound faults and high altitude depleted fluids penetrate down the rebound structure extensive mineralization may be associated with such thrust derived fluids examples which will be discussed include the mother lode au deposits of california u s a and the large u deposits of lagoa real brazil

4 transpressional tectonics lower crust decoupling and intrusion of deep mafic sills a model for the unusual metallogenesis of sw iberia abstract sw iberia is interpreted as an accretionary magmatic belt resulting from the collision between the south portuguese zone and the autochthonous iberian terrane in variscan times 350 to 330 ma in the south portuguese zone pull apart basins were filled with a thick sequence of siliciclastic sediments and bimodal volcanic rocks that host the giant massive sulphides of the iberian pyrite belt massive sulphides precipitated in highly efficient geochemical traps where metal rich but sulphur depleted fluids of dominant basinal derivation mixed with sulphide rich modified seawater massive sulphides formed either in porous reactive volcanic rocks by sub seafloor replacement or in dark shale by replacement of mud or by exhalation within confined basins with high biogenic activity crustal thinning and magma intrusion were responsible for thermal maturation and dehydration of sedimentary rocks while magmatic fluids probably had a minor influence on the observed geochemical signatures the ossa morena zone was a coeval calc alkaline magmatic arc it was the site for unusual mineralization particularly magmatic ni cu and hydrothermal fe oxide cu au ores iocg most magmatism and mineralization took place at local extensional zones along first order strike slip faults and thrusts the source of magmas and iocg and ni cu deposits probably lay in a large mafic ultramafic layered complex intruded along a detachment at the boundary between the upper and lower crust here juvenile melts extensively interacted with low grade metamorphic rocks inducing widespread anatexis magma contamination and further exsolution of hydrothermal fluids hypersaline fluids 18 o fluid 5 4 to 12 were focused upward into thrusts and faults leading to early magnetite mineralization associated with a high temperature 500 c albite actinolite salite alteration and subsequent copper gold bearing vein mineralization at somewhat lower temperatures assimilation of sediments by magmas led in turn to the formation of immiscible sulphide and silicate melts that accumulated in the footwall of the layered igneous complex further injection of both basic and sulphide rich magmas into the upper crust led to the formation of ni cu rich breccia pipes younger 330 to 280 ma peraluminous granitoids probably reflect the slow ascent of relatively dry and viscous magmas formed by contact anatexis these granitoids have w sn and pb zn related mineralization that also shows geochemical evidence of major mantle crust interaction late epithermal hg cu sb and pb zn ag mineralization was driven by convective hydrothermal cells resulting from the high geothermal gradients that were set up in the zone by intrusion of the layered igneous complex in all cases most of the sulphur seems to have been derived from leaching of the host sedimentary rocks 34 s 7 to 20 with only limited mixing with sulphur of magmatic derivation the metallogenic characteristics of the two terranes are quite different in the ossa morena zone juvenile magmatism played a major role as the source of metals and controlled the styles of mineralization in the south portuguese zone magmas only acted as heat sources but seem to have had no major influence as sources of metals and fluids which are dominated by crustal signatures most of the magmatic and tectonic features related to the variscan subduction and collision seem to be masked by those resulting from transpressional deformation and deep mafic intrusion which led to the development of a metallogenic belt with little resemblance to other accretionary magmatic arcs

hydrothermal precipitates associated with bimodal volcanism in the central bransfield strait antarctica polymetallic sulfide sulfate mineralization enriched in pb ag as sb hg occurs in the bransfield strait a late tertiary quaternary marginal basin close to the antarctic peninsula the mineralization is associated with bimodal volcanism and pelagic and volcaniclastic sediment in rifted continental crust hydrothermal precipitates have been recovered from two shallow 1 050 1 000 m water depth submarine volcanoes hook ridge and three sisters in the central bransfield strait mineralization at hook ridge consists of polymetallic sulfides massive barite and pyrite and marcasite crusts in semilithified pelagic and volcaniclastic sediment native sulfur commonly infills void space and cements the volcaniclastic sediment the polymetallic sulfides are dominated by sphalerite with minor galena enargite tetrahedrite tennantite pyrite chalcopyrite and traces of orpiment cemented by barite and opal a the presence of enargite at hook ridge the abundance of native sulfur and the low fe content of sphalerite indicate a high sulfur activity of the hydrothermal fluids responsible for mineralization the sulfur isotopic composition of hook ridge precipitates documents the complexity of the sulfur sources in this hydrothermal system with variable influence of biological activity and possibly magmatic contributions homogenization temperatures and salinities of fluid inclusions in barite and opal a suggest that boiling may have affected the hydrothermal fluids during their ascent the discovery of massive barite silica precipitates at another shallow marine volcano three sisters volcano attests to the potential for hydrothermal mineralization at other volcanic edifices in the area the characteristics of the mineralization in the bransfield strait with rifting of continental crust the presence of bimodal volcanism including highly evolved felsic volcanic rocks the association with sediments and the pb ag as sb hg enrichment are similar to the setting of massive sulfide deposits in the okinawa trough and distinct from those of sediment dominated hydrothermal systems such as escanaba trough middle valley and guaymas basin the geological setting of the bransfield strait is also broadly similar to that of some of the largest volcanogenic massive sulfide deposits in the ancient record such as the iberian pyrite belt

delimiting geochemical anomalies in the exploration of covered deposits with multifractal methods and using stream sediment data from the iberian pyrite belt southwest iberia abstract the iberian pyrite belt ipb is a world class province of upper paleozoic age for volcanic and shale hosted massive sulfides that includes 5 of the 10 supergiant 150 mt deposits of their kind in the world the volcano sedimentary sequences hosting the ores are locally exposed commonly covered by flysch type successions of variable thickness the baixo alentejo flysch group bafg as a result of primary sedimentary features and superimposed effects due to variscan folding and tectonic stacking in addition and when affected by late developed geomorphic depressions the exhumed paleozoic basement is also capped by meso cenozoic sediments the total thickness of cover sequences on top of the litho stratigraphic sequence of interest can reach 100 s of meters creating problems for mineral exploration a large stream sediment multi element geochemical database for the ipb was done in the 90 s during a wide ranging regional survey this database was reassessed by means of multifractal methods in view of their faculty to enhance the detection of faint geochemical anomalies and provide geochemical indicators to deep buried ore systems several anomalies were detected namely for cu zn and co along nw se to wnw ese trends the main anomalous geochemical trends identified include zones that i extend along the mira formation of bafg opening for the first time two non traditional areas for further exploration studies ii run along the boundary between the mira and mertola formations of bafg probably related to different geochemical signatures and late developed fault controlled epigenetic ore systems and iii overlap windows of volcanic sedimentary sequences within the cenozoic detrital cover and the site below which the deep buried deposit of lagoa salgada is located

dsdp hole 504b the first reference section over 1 km through layer 2 of the oceanic crust hole 504b provides a reference section for correlation of in situ petrological and geophysical studies with regional crustal models based on dredged samples and remote surveys and at least for the medium spreading rate costa rica rift hole 504b confirms that the main features of the upper part of an idealized ophiolite sequence occurs in the oceanic crust

explanation for many of the unusual features of the massive sulfide deposits of the iberian pyrite belt newly published fluid inclusion data from quartz in stockwork veins beneath seven massive sulfide lenses in the iberian pyrite belt suggest that the lenses were formed from fluids that on reaching the sea reversed buoyancy and ponded in basins sulfides quenched in the resulting brine pool would have settled to form a sulfide mud this process provides a relatively efficient trapping mechanism for metal in the fluids and effectively excludes ambient seawater accounting for the deposits tending to have the characteristics of large size sheet like form absence of relict chimney structures and a mineral content characterized by pyrite arsenopyrite and absence or scarcity of barite marcasite and fe oxides if total s was less than total metals in the stockwork fluids some or all of the more soluble zn and pb could have been swept from the basin at the overflow accounting for the variable but generally low zn and pb contents of the ores the lack of sedimentary source for the high salinities implicates magmatic intrusions possibly similar to those related to sn w mineralization

brine pool deposition for the zn pb cu massive sulphide deposits of the bathurst mining camp new brunswick canada i comparisons with the iberian pyrite belt abstract the ordovician zn pb cu massive sulphide ore deposits of the bathurst mining camp share many features with those of the devonian carboniferous iberian pyrite belt particularly the tendency to large size tonnage and metal content shape as far as can be determined after allowing for deformation metal content particularly fe cu pb zn and sn mineral assemblages pyrite arsenopyrite pyrrhotite and lack or rarity of sulphates sulphide textures particularly framboidal pyrite lack of chimney structures and rubble mounds irregular metal or mineral zoning and the low degree of zone refining compared to hokuroku ores the major differences between the provinces are the lack of vent complexes and the presence of sn cu ores in the iberian pyrite belt there are also similarities in the geological setting of the two camps both lie within continental terranes undergoing arc continent and continent continent collision and in each case massive sulphide mineralisation followed ophiolite obduction the ore deposits are associated with bimodal volcanic rocks derived from morb and continental crust and marine shales and mineralisation was locally accompanied or followed by deposition of iron formations fluid inclusion data from veins in stockworks from at least six of the iberian massive sulphide deposits point to sulphide deposition having taken place in basins containing mostly spent saline ore forming fluids brine pools and it is suggested that most of the major features of the bathurst deposits can be explained by similar processes the proposed model is largely independent of ocean sulphate and o 2 content whereas low values of each are requisites for the current spreading plume model of sulphide deposition in the bathurst camp

iron isotope behavior during fluid rock interaction in k feldspar alteration zone a model for pyrite in gold deposits from the jiaodong peninsula east china abstract mechanisms for fe isotope fractionation in hydrothermal mineral deposits and in zones of associated k feldspar alteration remain poorly constrained we have analyzed a suite of bulk samples consisting of granite displaying k feldspar alteration precambrian metamorphic rocks and pyrite from gold deposits of the jiaodong peninsula east china by multi collector inductively coupled plasma mass spectrometry pyrites from disseminated j type ores show a 56fe variation from 0 01 to 0 64 overlapping with the signature of the host granites 0 08 to 0 39 in contrast pyrites from quartz veins l type ores show a wide range of fe isotopic composition from 0 78 to 0 79 negative values are never seen in the j type pyrites the fe isotope signature of the host granite with k feldspar alteration is significantly heavier than that of the bulk silicate earth the fe isotopic compositions of precambrian metamorphic rocks across the district display a narrow range between 0 16 and 0 19 which is similar to most terrestrial rocks concentrations of major and trace elements in bulk samples were also determined so as to evaluate any correlation between fe isotope composition and degree of alteration we note that during progressive k feldspar alteration to rocks containing 70 wt sio2 75 ppm rb and this work demonstrates that the fe isotope composition of pyrite displays a significant response to the process of pyrite precipitation in hydrothermal systems and that systematic fractionation of iron isotopes occurs during fluid rock reaction in the k feldspar alteration zone of the linglong granite the implications of the results are that processes of mineralization and associated fluid rock interaction which are ubiquitously observed in porphyry style cu au mo and other hydrothermal deposits may be readily traceable using fe isotopes

hydrothermal alteration of a 1 km section through the upper oceanic crust deep sea drilling project hole 504b mineralogy chemistry and evolution of seawater basalt interactions deep sea drilling project hole 504b penetrates 1076 m into oceanic layer 2 and is the first hole to pass through the transition from pillow basalts altered at low temperatures into hydrothermally metamorphosed sheeted dikes alteration of the crust at site 504 occurred in four stages related to the movement of the crust away from the spreading axis 1 dikes reacted with seawater 200 300 c in the upwelling zone of an axial convection cell at the spreading axis resulting in the formation of greenschist facies parageneses in veins and host rocks mixing of the upwelling hydrothermal fluids with seawater circulating in the overlying more permeable pillow section occurred in the upper part of the lithologic transition zone causing a steep temperature gradient at the base of the pillow section secondary minerals formed in the lower pillow section from the resultant reducing mixed fluids at temperatures of around 100 c at the same time the initial effects of seafloor weathering began in the upper 320 m of the pillow section at low temperatures 50 c and under conditions of open circulation of oxidizing seawater 2 following refracturing of the dikes a second stage of axial upwelling occurred hydrothermal fluids 200 380 c were probably similar to those presently sampled from spreading ridges on the seafloor mixing of these fluids with seawater in the lithologic transition zone caused deposition of quartz epidote and sulfides in veins and formation of a sulfide rich stockworklike zone within this transition zone alteration of the pillow section proceeded under the prior conditions with the effects of seafloor weathering extending progressively downward into the crust 3 seawater recharge penetrated to depths of at least 1075 5 m subbasement and deposited anhydrite locally in veins 4 off axis alteration of the dikes was characterized by formation of zeolites calcite and prehnite in veins and host rocks from more highly evolved and lower temperature 100 250 c fluids alteration in the upper pillow section evolved from seafloor weathering conditions to more reducing and rock dominated as cracks in the basalt were sealed with secondary minerals and the basement was covered with a layer of sediment zeolites and calcite were the last phases to form throughout the pillow section sealing of the crust to convective cooling also allowed conductive reheating of the crust from below calculations of seawater crustal chemical fluxes from whole rock data are complicated by the low recovery and heterogeneity of alteration effects but indicate that basalt seawater interactions are a sink for seawater mg and k and a source for si and ca

the mattagami lake mine archean zn cu sulfide deposit quebec hydrothermal coprecipitation of talc and sulfides in a sea floor brine pool evidence from geochemistry 18 o 16 o and mineral chemistry orebody 2 is 21 million metric tons grading 10 5 percent zn 0 17 percent cu and 0 014 oz au plus 1 15 oz ag per short ton quartz in the orebody has a delta 18 o of 10 to 11 9 per mil or approximately 2 per mil heavier than footwall quartz signifying precipitation temperatures 250 degrees to 280 degrees c this is corroborated by filling temperatures of primary fluid inclusions within gangue quartz t f 240 degrees 270 degrees c both isotopic and fluid inclusion temperatures signify a broad thermal zonation from approximately 270 degrees c at the base to approximately 240 degrees c at the top of the orebody minimum water depths of 300 m are indicated from nonboiling hydrothermal fluids of 3 to 5 weight percent nacl equivalent at approximately 250 degrees c a approximately 50 degrees c cooling of fluids during passage from the footwall to a suggested sea floor brine pool above the ore zone may have been induced by turbulent mixing of approximately 20 percent pristine archean marine bottom water at approximately 70 degrees c downward into the brine pool chert in the overlying key tuffite has delta 18 o of 10 to 12 per mil probably representing equilibrium precipitation from brine pool fluids at 200 degrees to 250 degrees c spreading laterally across the sea floor in a gravitationally stable boundary layer modified journal abstract

the cu stockwork and massive sulfide ore of the feitais volcanic hosted massive sulfide deposit aljustrel iberian pyrite belt portugal a mineralogical fluid inclusion and isotopic investigation the variscan feitais volcanic hosted massive sulfide deposit in the aljustrel district of the iberian pyrite belt consists of 55 million metric tons of zn pb cu massive sulfide overlying a cu rich stockwork the massive ore is overlain by up to 30 m of feldspar phyric rhyolitic volcaniclastic rock and locally by a jasper and or chert layer up to 15 m thick the massive sulfide orebody consists dominantly of pyrite sphalerite galena chalcopyrite tetrahedrite tennantite arsenopyrite and bournonite together with minor quartz chlorite sericite carbonate and barite the orebody is up to 100 m thick and is underlain by a tabular alteration zone of chloritedominated locally silicified felsic volcanic rock the upper 30 to 60 m of which contains chalcopyrite quartzchlorite sericite carbonate bearing stockwork vein let s that prior to deformation were at a shallow angle to the base of the massive orebody chloritized footwall rocks extend up to 20 m below the cu stockwork zone and are underlain by up to 50 m of quartz sericite pyrite altered rhyolitic rock the stockwork veins also contain pyrite tetrahedrite tennantite sphalerite and arsenopyrite pyrite both in stockwork and massive ore locally displays partly recrystallized framboidal reniform and cellular textures two generations of quartz q1 and q2 and carbonate in the stockwork veins contain primary in growth zones and pseudosecondary fluid inclusions with homogenization temperatures of 270 to 315 c and salinities of 2 2 to 8 1 wt percent nacl equiv the 34s cdt values of massive and stockwork ores range from 15 4 to 4 7 mean 2 8 and 11 2 to 11 9 mean 0 4 per mil respectively the lowest values from colloformtextured pyrite with no evidence of oxidation of sulfide sulfur during mineralization the most negative values indicate an origin by biogenic reduction of seawater sulfate the 13c pdb values for carbonates 7 5 to 13 7 and 9 3 to 14 3 per mil in massive and stockwork ore respectively indicate an origin mostly by oxidation of methane derived from organic matter in underlying sedimentary rocks and possibly a contribution of magmatic carbon there are no significant lateral or vertical variations in s isotope values in sulfides or c o isotope values in carbonates either in massive or stockwork ore the 18o smow values for quartz in stockwork and massive sulfide are 11 6 to 13 9 and 16 7 to 17 9 per mil respectively coexisting and texturally contemporaneous carbonate and quartz in stockwork veins are not in isotopic equilibrium indicating that the c o isotope values may have been reset the 18o values of fluid calculated to be in equilibrium with quartz at fluid inclusion homogenization temperatures are 4 2 to 5 2 per mil barite from the hanging wall and massive ore yields 34s values 21 9 27 9 equal to or slightly higher than those of coeval seawater 87sr 86sr ratios 0 708438 0 709063 are slightly more radiogenic than those of coeval seawater 0 7080 0 7085 and much more radiogenic than those of coeval volcanic rocks 0 703304 0 706642 probably representing mixtures between seawater sr and radiogenic sr in fluids sourced in the crustal pile deposition of the massive sulfide on the sea floor is suggested by its stratiform nature the stronger alteration of footwall relative to hanging wall rocks the stockwork system terminating sharply at the base of the massive sulfide the presence of sedimentary like textures in the massive sulfide the absence of replacement fronts and the presence of framboidal and other sea floor depositional textures indicative of fluid quenching the sheetlike form lack of rubble mounds and chimneys scarcity of barite reduced mineral assemblage and metal zoning distinguish feitais from kuroko type deposits it shares most of the characteristics of those iberian pyrite belt deposits for which a brine pool origin has been proposed based on fluid inclusion data

active hydrothermal vents and sulfide deposits on the southern juan de fuca ridge massive sulfide deposits rich in zinc and silver were recovered from the juan de fuca ridge 500 km west of oregon in september 1981 the samples recovered are composed largely of zinc sulfide with lesser amounts of iron lead and copper sulfide most of the deposits occur at a series of hydrothermal vents within a relatively continuous depression in the center of a smooth 1 km wide valley along the ridge axis the depression appears to be formed by collapse of a lava lake possibly modified by extensional faulting the axial valley floor outside the depression is underlain by fresh glassy ferrobasalt sheet and lobate flows two types of sulfide mineral deposits were dredged from one of the hydrothermal vents 1 angular slabs of dark gray zinc sulfide and 2 subrounded fragments of porous light gray zinc sulfide the samples contain fresh sphalerite zoned wurtzite pyrite and minor amounts of marcasite galena and chalcopyrite cubanite the spreading rate of the juan de fuca ridge and the composition of the sulfide samples are generally similar to the east pacific rise lat 21 n site however the texture and geologic setting of the sulfide deposits are significantly different

fe zn cu pb mineralization in the salton sea geothermal system imperial valley california the salton sea geothermal system is an area of active metamorphism in a plio pleistocene deltaic sedimentary sequence within it hypersaline brines containing up to 250 000 ppm total dissolved solids and temperatures up to 365 degrees c are causing incipient sulfide and oxide mineralization we have made a detailed analysis of ore mineralization in two boreholes drilled to depths of up to 2 500 m in this system major ore minerals observed are in order of decreasing abundance pyrite hematite sphalerite chalcopyrite pyrrhotite marcasite and galena the associated nonore minerals are mainly calcite quartz epidote anhydrite adularia and chlorite ore mineralization can be divided into three main types diagenetic sulfide mineralization occurring at depths less than 760 m and temperatures less than 250 degrees c consists of fine grained stratiform iron sulfides occurring as cement in sandstone and as disseminations and bands in shale metamorphic sulfide mineralization occurring at depths greater than 760 m and temperatures greater than 250 degrees c includes the progressive development of porphyroblastic pyrite with increasing depth and temperature its replacement by other fe cu zn pb sulfides at depths greater than 1 220 m and its decomposition into skeletal aggregates at depths greater than 1 525 m concurrent with hornfelsic recrystallization of the host sediments vein and related pore filling ore mineralization is of two contrasting types hematite silicate sulfide or sulfate and sulfide carbonate or silicate the first type occurs in open porous veins in single vertical zones at least 220 m thick in each borehole the second type occurs in veins that are generally sealed and occur in relatively thin scattered vertical intervals in both boreholes thermodynamic analysis indicates that the modern reservoir brines are near equilibrium with the hematite silicate sulfide or sulfate vein assemblage at 300 degrees c with ph 5 4 and log a sub o 2 30 reactions between hematite pyrite and iron bearing silicates chlorite epidote probably control the redox state of the present brines calculated metal chloride complex solubilities for fe zn pb and cu agree well with brine analyses hematite pyrite and chalcopyrite are supersaturated galena and sphalerite are undersaturated in the modern brines the phase relations of the presently sealed sulfide carbonate or silicate veins require that earlier fluids if formed at the same temperature and ph had to be more reduced and sulfur rich than the present slightly oxidized sulfur poor brines contrasting fluid redox states may be caused by a late lateral or fault controlled influx of more oxidized surficial waters lack of acid alteration implies boiling has not occurred to produce the oxidized fluids early formed diagenetic iron sulfides are commonly replaced by later cu pb zn sulfides and iron oxides and enveloped and resorbed by veins acting as nuclei and sources of s and fe for later mineralization precipitation probably occurs when seismic or hydraulic fracturing allows the metal rich sulfur poor brines to interact with earlier pyrite sulfur in the host rocks thus the geothermal system as presently explored is best classified as an incipient stratabound sulfide deposit that is being overprinted by hydrothermal and metamorphic processes

extreme 18o enriched volcanics and 18o evolved marine water aljustrel iberian pyrite belt transition from high to low rayleigh number convective regimes abstract quartz eye keratophyric tuffs at aljustrel s portugal iberian pyrite belt possess unusually high uniform whole rock values up to 18 1 with a mean of 16 7 0 7 because the quartz eye cores have 18o 12 0 to 13 5 the felsic tuffs may have had an original whole rock oxygen isotope composition of 10 to 11 the deduced enrichments of up to 8 in keratophyric derivatives is attributed to isotope exchange with abundant marine water under temperatures which diminished to 100 c during thermally driven convective cooling the isotopic uniformity requires elevated permeability of 10 8 cm2 such that the second critical rayleigh number was exceeded and drifting non steady state convection cells dominated with relatively smooth thermal structures in mineralised counterparts of the tuffs beneath the feitais estacao zn pb cu massive sulphide orebodies whole rock values are 12 and quartz 13 3 15 4 chlorite 3 2 6 3 fractionations correspond to temperatures of 220 270 c and a calculated fluid 18o of 1 4 to 5 7 this local isotopic overprinting in vent domains of the regionally high 18o was induced by a stable second stage convective regime imposed by drastic reduction of permeability accompanying capping of the geothermal discharge by hydrothermal cherts and or progressive spilitisation of the tuffs at lower water rock coupled with higher temperatures the recirculating marine water underwent variable 18o enrichment up to 5 7 by exchange with the high 18o tuffs an upwards increase in 18o quartz and quartz reflects progressive cooling of the geothermal discharge from 15 4 270 c in the stockwork through 18 3 220 240 c in the orebody to 20 1 110 130 c in overlying hydrothermal cherts probably induced by entrainment of ambient marine bottom water in sub seafloor aquifers

giant pyritic base metal deposits the example of feitais aljustrel portugal detailed study of hanging wall rocks and alteration above the giant feitais orebody of aljustrel iberian pyrite belt portugal shows that cherts that constitute the immediate hanging wall to the sulfide deposit result from hydrothermal alteration of laterally equivalent fe and or mn rich jaspers textures geochemistry and lack of fossil radiolarians indicate that these jaspers are hydrothermal precipitates hydrothermal alteration of the jaspers took place concomitantly with sulfide precipitation through reaction with hot 150 c reduced fluids with replacement of hematite by magnetite pyrite and fe2 phyllosilicates on the basis of the above and considering also the results of previous studies on hydrothermal activity in the aljustrel volcanic rocks and sulfide ores we propose that sulfide precipitation may have taken place under a thin veneer of siliceous sediments present day massive sulfide formation in the guaymas basin and in the salton sea under hundreds of meters of sediments may be approximate analogues in the sense that the presence of a caprock reduces dispersion of black smoke and causes more efficient precipitation

mineralogy and geochemistry of hydrothermal sediments from the serpentinite hosted saldanha hydrothermal field 36 34 n 33 26 w at mar abstract the saldanha hydrothermal field is located at the top of a serpentinized massif mount saldanha ms at a non transform offset nto5 along the mid atlantic ridge mar south of the azores it is one of the rare known sites on a worldwide basis where direct evidence of low temperature 7 9 c hydrothermal activity has been provided by direct observation of hydrothermal fluid venting through small orifices in the ocean floor sedimentary cover this study focuses on the mineralogy and geochemistry of 14 sediment cores collected at ms for comparison four samples collected at the rainbow site nto6 were also studied mount saldanha hydrothermal sediments are highly diluted within a dominant foraminiferal nanofossiliferous ooze with small fragments of underlying rocks the mineral assemblage of the hydrothermal component is characterized by sulphides nontronite smectites poorly crystallized mn oxyhydroxides and amorphous material cu zn and fe sulphides mn mg oxy hydroxides and putative manganobrucite were also identified in one sample collected at an orifice vent in this sample micro chimneys conduits composed of isocubanite and sphalerite were also identified mount saldanha sediments show a clear enrichment in elements such as mn mg fe cu p and v derived from hydrothermal fluids and ni cr and co derived from ultramafic rocks the geochemical data together with the observed mineral assemblage suggest that the hydrothermal fluids are at a higher temperature than those measured at the escape orifices 7 9 c and a strong enrichment in mg mainly at the top of the mount agrees with extensive mixing of the hydrothermal fluid with unmodified seawater nevertheless the mineral assemblage of ms sediments is consistent with the precipitation from hydrothermal fluids at much lower temperatures than at rainbow the presence of serpentinized and steatitized talcshist ultramafic rocks and the occurrence of a strong methane anomaly within the overlying water column collectively suggest that the hydrothermal circulation at ms is driven by exothermic reactions closely associated with the serpentinization process rainbow sediments have a higher concentration in transition metals and consequently an enrichment in sulphides these differences are likely to be a consequence of the higher temperature of hydrothermal fluids reflected in the composition of hydrothermal solutions and of a stronger hydrothermal flux at the rainbow site

the teton fault wyoming topographic signature neotectonics and mechanisms of deformation we integrated geophysical and geological methods to evaluate the structural evolution of the active teton normal fault wyoming and its role in the development of the dramatic topography of teton range and jackson hole the teton fault bounds the precipitous eastern front of the teton range and is marked by large well preserved postglacial fault scarps that extend for 55 km along the range front comparison of variations in surface offsets with the topographic expression of the teton range crest and drainage divide and the overall structure of the range suggests that the effects of postglacial faulting cannot be discriminated from the influence of pre extensional structures and differential erosion on the footwall topography in contrast the effects of multiple scarp forming normal faulting earthquakes are expressed by the anomalous drainage pattern and westward tilt of the hanging wall jackson hole toward the teton fault kinematic boundary element fault models suggest that the westward tilt of the valley floor is the product of 110 125 m of displacement on a 45 75 e dipping teton fault in the past 25 000 75 000 years comparisons with historic normal faulting earthquake displacements imply that this range of displacement corresponds to 10 50 m 7 scarp forming earthquakes a total throw of 2 5 to 3 5 km across the teton fault is suggested by inverse ray tracing and forward gravity models these models also suggest that laramide age structures have been offset across the teton fault and obscure its geophysical signature but also continue to influence the structural and topographic expression of the footwall and hanging wall blocks paleomagnetic analyses of the 2 0 ma huckleberry ridge tuff suggest that the overall westward tilt of the teton range is a result of about 10 of west side down tilt across the teton fault since tuff emplacement this suggests that much if not all of the throw across the teton fault has accumulated in the past 2 m y complex demagnetization and rock magnetic behavior and local emplacement of the huckleberry ridge tuff on preexisting topography preclude determination of the amount or variations in throw along strike of the teton fault from the paleomagnetic data