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

comparative stratigraphy and subsidence history of mesozoic rift basins of north atlantic mesozoic rift basins around the north atlantic all possess many of the following upper jurassic to lower cretaceous oxfordian to albian stratigraphic elements 1 an early oxfordian hiatus 2 backfill deposits in upper jurassic paleovalleys 3 multiple tithonian transgressive regressive sequences 4 early berriasian regressive sequences 5 a sub valanginian unconformity overlain by coarse siliciclastic sandstones and conglomerates 6 valanginian to hauterivian fining upward sequences 7 condensed barremian sequences and 8 prominent aptian unconformities more local features include abrupt kimmeridgian deepening lusitanian basin portugal albian deepening cantabrian and aquitaine basins northern spain and southern france and late albian early cenom nian source rejuvenation cantabrian basin northern iberia decompaction and backstripping of sediments from well and outcrop sections in six basins also reveal broad similarities in trends of rates of apparent basement subsidence relatively high rates 50 100 m m y characterize onset of rift phases in the triassic and the kimmeridgian locally in the lusitanian basin of portugal kimmeridgian rates reach about 250 m m y in the cantabrian and aquitaine basins a third phase of rapid subsidence characterizes the aptian albian interval after the main phase of continental breakup around iberia this phase may be due to transtensional stresses as iberia continued to rotate relative to europe tensional intraplate stresses or gravity driven partial collapse of the new continental marg n the north atlantic mesozoic basins share many features because they had a common origin controlled by intracontinental rifting and subsequent separation of europe and north america pulses of extension generally are reflected in the stratigraphy of all of the basins superimposed on this record are the effects of eustatic sea level variations the main difference between the canadian jeanne d arc basin and iberian basins is the significantly greater early and middle jurassic basement subsidence rate in the former about 40 m m y vs 0 10 m m y this difference may result from prolonged extension on the canadian side perhaps due to northward transfer of shear stress from left lateral transform motion between the grand banks and northwestern africa

magmatism in the iberian pyrite belt petrological constraints on a metallogenic model igneous formations associated with massive sulphide deposits in the iberian pyrite belt ipb are essentially composed of basic lavas and dolerites and dacitic to rhyolitic volcanites intermediate lavas are subordinate the basic rocks show variable geochemical characteristics lavas and dolerites comparable to recent within plate alkaline basalts seem restricted to the western and southern parts of the ipb whereas basic rocks comparable to continental tholeiites or arc related basalts occur across the whole belt the felsic rocks are classified as calc alkaline and belong to the low al2o3 and high yb type at given sio2 al2o3 and tio2 contents they show variable zr nb and hree contents heavy rare earth element fractionation decreases from the dacites to the rhyolites gd yb n 1 whereas the negative eu anomaly becomes more pronounced the characteristics of the rhyolites are typical of sulphide fertile volcanic packages trace element modelling suggests that the felsic rocks evolved from a dacitic parent magma through fractional crystallization of hornblende and plagioclase partial melting of an amphibolite protolith which appears as the most probable model for the origin of this dacitic magma requires a high t p gradient in the crust the occurrence of alkaline basalts and continental tholeiites is consistent with formation of the ipb in a tensional tectonic setting however the associated island arc tholeiites suggest a location in a domain of plate convergence emplacement in a fore arc basin over a recently accreted crustal segment is envisaged as a possible hypothesis to account for the geological and petrological constraints a high geothermal gradient and eruption in a submarine tensional basin could have been two key ingredients for the development of massive sulphide deposits within the ipb

fluid inclusions in quartz from oceanic hydrothermal specimens and the bingham utah porphyry cu deposit a study with pixe and sxrf abstract individual natural fluid inclusions in quartz were selected for non destructive microprobe analysis by synchrotron x ray fluorescence sxrf and proton induced x ray emission pixe in order to compare and contrast the compositions of porphyry type ore forming brines and two types of ocean crustal brines the inclusions contained brines with high salt concentrations typically 20 40 wt total and one or more daughter crystals the x ray and proton beams produced detectable characteristic x rays for many elements and allowed the quantification of concentrations for many elements with atomic number z 14 fluid inclusions associated with the core of the bingham ut porphyry cu mo deposit contain nacl brines that also contain element typical range in ppm k 12 000 100 000 ca 17 000 mn 8000 fe 120 000 cu 8000 zn 5000 br 2000 these values are in good agreement with published estimates and results from sxrf and pixe experiments are generally consistent the bingham fluid compositions closely resemble brines from the porphyry mo cu deposit at questa nm inclusions from two suites of oceanic plutonic rock samples gave pixe results consistent with published sxrf analyses from other inclusions in the same samples new inclusion analyses from the mathematician ridge locality indicate the nacl brines also contain on average element ppm k 3500 ca 35 000 mn 4900 cu 400 zn 200 br 1000 new inclusion analyses from the oceanographer transform locality indicate nacl brines that also contain on average k 37 000 ca 35 000 mn 10 000 cu 400 zn 2200 br 1000 these values are calculated on the assumption that fe in the inclusions is 50 000 ppm compared to continental ore forming brines the oceanic brines have higher ca and lower k and cu concentrations

the genesis of mid ocean ridge basalt the tholeiitic volcanics erupted at mid ocean ridges mid ocean ridge basalts or morb constitute the dominant volcanic lithology on earth analyses of tachylites from atlantic pacific and indian ocean spreading centres range widely in 100 mg mg fe2 ratios m and m varies from 70 to 30 glasses with m 55 65 are the most common variants and only a small percentage of glass analyses has m approaching 70 the latter defines the m value of basaltic melts in equilibrium with residual upper mantle source peridotites with m 88 the frequency histogram of the m values of average compositions of morb glasses at 88 ocean floor localities is similar in analysis distribution to the frequency histograms depicting variation in the m values of glasses from the various spreading centres m values and nickel contents of morb and the nature and compositions of the near liquidus phases crystallized experimentally from morb melts at elevated pressures have been applied to identify primary unfractionated melts erupted in a mid ocean ridge environment however ni abundances and high pressure phase relationships are not necessarily unique or definitive parameters of primary melts the latter are generally linked genetically with mg rich lherzolitic source rocks of pyrolite type m 90 the spectrum of m values displayed by morb glasses with a definite bias towards relatively fe rich compositions average m of approximately 600 morb glasses is 58 6 suggests that the melts may have evolved either via ferromagnesian fractionation of relatively mg rich parental melts m 70 80 or by partial melting of a heterogeneous upper mantle with variable m values or as a result of magma mixing of already fractionated melts and primitive magma batches for a number of reasons fractonation models based on the extraction of olivine or one or more of olivine plagioclase and clinopyroxene either from picritic melts m 75 or primitive basaltic melts with m 70 are questionable as prime controls of morb chemistry these include 1 the extreme rarity of quenched picritic or mg basaltic melts in ocean ridge environments 2 the lack of adequate evidence of the appropriate of necessity voluminous complementary cumulates dunites allivalites troctolites anorthosites demanded by olivine plagioclase or olivine plagioclase fractionation models and 3 the aberrent frequencies of glass m values whereby the assumed derivatives m 55 65 are much more abundant and presumably much more voluminous than the alleged parents or transitional derivatives 65 m 75 the nature of the trends of na2o cao and al2o3 in galapagos spreading centre tachylites of extended composition m 65 30 indicates the gabbroic fractionation is also unlikely to exert important controls on morb chemistry as their m values increase mid ocean ridge basalts increase in al ca ni co cr and decrease in ti mn na k and p except for al and ca these trends are similar to those displayed by upper mantle peridotites increasing in m i e becoming more refractory following one or more partial melting episodes it is suggested that at least a majority of mid ocean ridge basalts is intrinsically primary and generated by variable degrees of partial melting of heterogeneous lherzolitic upper mantle 80 m 90 with variable abundances of elements such as ti al ca and na and also depleted in large ion lithophile lil elements negative europium anomalies in the rare earth patterns of some oceanridge basalts ferrobasalts with low m are ascribed mainly to the persistence of residual plagioclase in relatively fe rich plagioclase lherzolite source rocks following low degrees of partial melting the partial melting events leading to the generation of mid ocean ridge basalts took place over a relatively modes pressure range approximately 8 15 kb which encompassed the transition of plagioclase lherzolite to spinel lherzolite this proposal appears consistent with the nature and occurrence of megacrysts xenocrysts of tschermakitic cr diopside ca43mg52fe5 olivine mg 89 91 plagioclase an92 85 and spinel fe2al60cr38 in some morb the megacryst compositions suggest that these phases represent disaggregated plagioclase peridotite or spinel lherzolite acquired by melts during their passage through the oceanic upper mantle

dating the oldest terrestrial rocks fact and fiction abstract this paper is a review of well documented geochronological and related isotopic evidence for the age and total crustal residence time of the oldest known terrestrial rocks mainly using rb 1bsr sm 1bnd and u 1bpb decay schemes the oldest known stable sialic crust of true continental character predominantly composed of calc alkaline orthogneisses was produced during major mantle differentiation some 3 7 3 5 ga ago and is represented on several continents enclaves of older supracrustal rocks as well as a major remnant of a volcano sedimentary sequence of greenstone belt affinity dated at 3 8 ga are preserved within the surrounding gneisses in southern west greenland recent ionmicroprobe evidence for 4 1 ga old detrital zircon grains in a younger early archaean metasedimentary sequence in western australia is also discussed several cases are reviewed where claims for early archaean 3 4 ga dates are believed to be erroneous and to result from oversimplistic and incorrect interpretation of isotopic data some of the geological and geochemical mechanisms which have given rise to claims for spuriously old sm 1bnd u 1bpb and pb pb dates are critically discussed to supplement published data we present several new sets of whole rock sm 1bnd and pb pb isotopic data from west greenland labrador zimbabwe and eastern india relevant to the overall topic of this review

phase separation and fluid mixing in subseafloor back arc hydrothermal systems a microthermometric and oxygen isotope study of fluid inclusions in the barite sulfide chimneys of the lau basin fluid inclusions in barite and sulfide in chimneys both active and inactive from three hydrothermal sites of the back arc lau basin were studied with microthermometric and isotopic methods to determine the chemistry and evolution of hydrothermal aqueous fluids strontium isotope compositions of sulfides from the lau basin reflect the presence of anhydrite and barite inclusions the 87sr 86sr ratios of these two sulfate minerals vary from 0 7045 to 0 7078 and are interpreted as the result of mixing between various proportions of the hydrothermal end member and pure seawater the microthermometric study of fluid inclusions reveals that mixing with seawater involved different kinds of aqueous fluid end members a high temperature mg depleted end member of high salinity 5 5 wt eq nacl was found at the vai lili site a uncommon low temperature mg rich end member was also identified at the hine hina site in association with barite deposition at the vai lili site very low salinity fluids were produced in addition to a very saline brine 30 wt nacl that was trapped inside anhydrite precipitated from an active vent at a temperature of 342 c oxygen isotope ratios of water inclusions range from 2 to 4 4 for chalcopyrite barite and sphalerite minerals the 18o enrichment and the high salinities of many fluids from the lau basin are accounted for by the specificity of the back arc setting this non mid ocean ridge setting is characterized by the shallow depth of the hydrothermal systems that allows frequent unmixing of high temperature liquids the abundance of silicic magmas also provided magmatic fluids including brines that mixed with seawater derived aqueous fluids

a new sulphur isotopic study of some iberian pyrite belt deposits evidence of a textural control on sulphur isotope composition the sulphide deposits of the iberian pyrite belt ipb represent an ore province of global importance our study presents 113 new sulphur isotope analyses from deposits selected to represent the textural spectrum of ores measured 34s values range from 26 to 10 mostly for massive and stockwork ores in agreement with data previously published in situ laser 34s analyses reveals a close correlation of 34s with texture primary diagenetic textures are dominated by relatively low 34s 8 to 2 whereas stockwork feeder textures are dominated by higher 34s 3 to 5 intermediate textures mainly coarse textures in stratiform zones have intermediate 34s although they are mostly dominated by the high 34s component rare barite has a homogeneous 34s around 18 which is consistent with direct derivation from lower carboniferous seawater sulphate a dual source of sulphide sulphur in the ipb deposits has been considered a hydrothermal source derived from reduction of coeval seawater sulphate in the convective systems is represented by sulphide in the feeder zones here variations in 34s are caused by variations in the extent of the sulphate reduction which governs the so4 h2s ratio the second end member was derived from the bacterial reduction of coeval seawater sulphate at or near the surface as reflected in the primary textures a distinct geographical variation in 34s and texture from sw more bacteriogenic and primary textures to ne more hydrothermal textures and 34s which reflects a variation in the relative input of each source was likely controlled by local geological environments given that the sulphur isotope characteristics of the ipb deposits are unlike most vms and kuroko deposits and noting the dominance of a mixed reduced sedimentary and volcanic environment we suggest that the ipb could represent an ore style which is intermediate between volcanic and sedimentary hosted massive sulphide types

variscan tectonics in the iberian pyrite belt south portuguese zone this paper aims to discuss the structural evolution of the iberian pyrite belt during the variscan orogeny it provides new structural data maps and cross sections from the eastern part of the iberian pyrite belt regional geology of the south portuguese zone and lithostratigraphy of the iberian pyrite belt are first briefly summarised three roughly homoaxial deformation phases are distinguished and are mainly characterised by south verging multi order folds axial planar cleavages and thrusts three structural units are distinguished the la puebla de guzman and valverde del camino antiforms are rooted units related to the propagation of southward directed thrust systems that may branch onto the lower decollement level of the south portuguese zone el cerro de andevalo is a structurally higher unit mainly composed of allochthonous d1 thrust nappes no evidence of sinistral transpression has been found in the transected cleavage and the strike of s3 with respect to s2 better evidence of transpression is the moderately to steeply westerly plunging folds that show s type asymmetry in down plunge view variscan deformation in the iberian pyrite belt is defined as the combination of a dominant southwards shear and a sinistral e shear caused by oblique continental collision between the south portuguese plate and the iberian massif

shale basins sulfur deficient ore brines and the formation of exhalative base metal deposits abstract the massive sulfide deposits of the iberian pyrite belt are interbedded with felsic volcanic rocks and shale and underlain by several thousand meters of siliciclastic sedimentary rocks known as the pq group isotope geochemistry and regional geology are both consistent with equilibration of the ore forming fluids with the pq group prior to ore deposition near the former seafloor the average cu zn pb ratio of the pq group rocks ca 26 55 19 is similar to the weighted average of all the massive sulfide orebodies combined ca 25 52 23 the genetic relationship between massive sulfide deposits and a siliciclastic sedimentary metal source is explained here by a thermodynamic model proposing that mildly reducing redox conditions imposed by equilibration with the sedimentary rocks are most critical for the formation of an effective ore forming fluid relatively metal rich but organic poor pyrite bearing shale undergoing dewatering of saline pore fluids is an effective source for the generation of sulfur deficient but relatively iron and base metal rich brines thus we propose that the giant deposits of the iberian pyrite belt owe their existence not to exceptionally metal enriched e g magmatic fluids but to the existence of a fairly ordinary but large metal source in reactive siliciclastic sediments combined with an underlying igneous heat source and a particularly efficient mechanism of sulfide precipitation by mixing with h 2 s rich fluids at or near the seafloor essentially similar mineral equilibria are imposed when saline fluids are buffered by typical continental basement rocks leaching of retrograde minerals and possibly residual salts from their magmatic or metamorphic prehistory is expected to generate similar variably metal rich but relatively sulfide deficient fluids thus the existence of mildly reducing rocks can be the dominant chemical control in the source of fluids generating many volcanogenic irish type or sedex deposits many of which are known to precipitate their metal load in response to biogenic sulfide addition at the ore deposition site

source and evolution of ore forming hydrothermal fluids in the northern iberian pyrite belt massive sulphide deposits sw spain evidence from fluid inclusions and stable isotopes a fluid inclusion and stable isotopic study has been undertaken on some massive sulphide deposits aguas tenidas este concepcion san miguel san telmo and cueva de la mora located in the northern iberian pyrite belt the isotopic analyses were mainly performed on quartz chlorite carbonate and whole rock samples from the stockworks and altered footwall zones of the deposits and also on some fluid inclusion waters homogenization temperatures of fluid inclusions in quartz mostly range from 120 to 280 c salinity of most fluid inclusions ranges from 2 to 14 wt nacl equiv a few cases with th 80 110 c and salinity of 16 24 wt nacl equiv have been also recognized in addition fluid inclusions from the soloviejo mn fe jaspers 160 190 c and 6 wt nacl equiv and some late to post hercynian quartz veins 130 270 c and 4 wt nacl equiv were also studied isotopic results indicate that fluids in equilibrium with measured quartz 18ofluid 2 to 4 chlorites 18ofluid 8 14 dfluid 45 to 27 whole rocks 18ofluid 4 7 dfluid 15 to 10 and carbonates 18oankerite 14 5 16 13cfluid 11 to 5 evolved isotopically during the lifetime of the hydrothermal systems following a waxing waning cycle at different temperatures and water rock ratios the results fluid inclusions 18o d and 13c values point to a highly evolved seawater along with a variable but significant contribution of other fluid reservoirs such as magmatic and or deep metamorphic waters as the most probable sources for the ore forming fluids these fluids interacted with the underlying volcanic and sedimentary rocks during convective circulation through the upper crust

the geology and gravity field in the central core of the vredefort structure the centre of the vredefort structure in south africa comprises an uplifted granitic basement with an associated 300 g u gravity high ringed by upturned sedimentary strata detailed geological mapping of the basement core has delineated a three fold concentric zoning of outer granite gneisses ogg steynskraal formation sf and inlandsee leucogranofels ilg an increase in metamorphic grade occurs toward the centre of the structure from greenschist relics on the basement perimeter to upper amphibolite and granulite facies in the sf and granulite facies in the ilg a marked lithological change occurs within the sf in which perimeter greenschists and inner amphibolites give way to paragneisses in the ilg paragneiss and mafic granulite relics predominate these divisions are consistent with the standing on edge model for the basement 1 three regional metamorphic episodes and two thermal events have been recognised m1 and m3 both attained granulite facies at low medium load pressures in the ilg m1 granulite facies assemblages extended into the ogg but the m3 facies was restricted to the ilg m2 was a k metasomatism event limited to the ogg an extensive thermal episode m4a is provisionally correlated with the collar metamorphism pyroxene hornfels facies t 800 c was attained in the centre as opposed to conditions of p 4 5 kbar and t 650 c for alkali granite stocks in the sedimentary collar 2 this event was thus pre or syn uplift but not post uplift a high level high temperature p 3 kbar and t 800 900 c event m4b occurred at a late stage in the overall sequence a suite of amphibole orthopyroxene rocks of basaltic komatiite type cao al2o3 1 are chemically related to younger high mg tholeiites these tholeiites are correlated with similar rocks in the sedimentary collar and were intruded syn to post m4a in several episodes of decreasing basicity shock metamorphic features granitic breccias and pseudotachylite are linked to the late stage history of the structure and a case is presented for a dynamic metamorphic front advancing outward and upward from the centre in response to unroofing of the basement the central gravity high3 is delineated by 1239 new gravity stations at 1 km and 0 5 km intervals a low frequency oval shaped anomaly with a long axis trending nw se has eccentrically positioned on it a plateau extending southwards as a spur a dyke is responsible for the spur but the source for the plateau is from a recent borehole thought to be largely due to sheared altered harzburgite modelling of the low frequency gravity field component was based on the mapped divisions of the ogg sf and ilg and a density contrast of 200 kg m3 was determined between the ogg and the sf and ilg lithologies a purely crustal solution for the excess mass distribution in the basement is conclusively demonstrated

gold enrichment in active geothermal systems by accumulating colloidal suspensions the origins of high grade hydrothermal ore deposits are debated but active geothermal systems provide important clues to their formation the highest concentrations of gold are found in geothermal systems with direct links to island arc magmatism yet similar concentrations have also been found in the absence of any input from arc magmas for example in the reykjanes geothermal field iceland here we analyse brine samples taken from deep wells at reykjanes and find that gold concentrations in the reservoir zone have increased over the past seven years from an average of 3 ppb to 14 ppb the metal concentrations greatly exceed the maximum solubility of gold in the reservoir under saturated conditions and are now nearly two orders of magnitude higher than in mid ocean ridge black smoker fluids the direct analogues of reykjanes deep liquids we suggest that ongoing extraction of brine the resulting pressure drop and increased boiling have caused gold to drop out of solution and become trapped in the reservoir as a colloidal suspension this process may explain how the stock of metal in the reservoirs of fossil geothermal systems could have increased over time and thus become available for the formation of gold rich ore deposits

undepleted mantle composition beneath hawaii compositions of undepleted mauna loa mantle have been calculated assuming derivative residuum relations between a tholeiitic picrite from the 1852 eruption of mauna loa and mantle residua whose compositions are delineated by experimental data and the compositions of the most refractory picrite phases namely mg rich olivine and cr rich spinel the preferred calculated composition based on 40 picrite 60 residuum composed of olivine orthopyroxene and cr spinel has a lower 100 mg mg fe ratio 84 1 than the ratios of pyrolite like undepleted mantle compositions 88 90 and compared with many such compositions it also has significantly higher abundances of tio2 al2o3 cao na2o k2o and p2o5 the higher abundances of incompatibles have implications for the volumes and character of potential derivative melts a calculated mode which assumes that a titaniferous pargasitic amphibole is a component of the undepleted lherzolite indicates that amphibole could be a significant primary phase at temperatures slightly above the lherzolite solidus amphibole would be eliminated on incongruent melting and its breakdown products mainly clinopyroxene plus minor olivine would become part of a re equilibrated anhydrous peridotitic mineral assemblage the lower 100 mg mg fe ratio of the calculated composition would permit the generation of primary basaltic melts with relatively low 100 mg mg fe2 ratios at lower 40 degrees of melting such lower ratios are usually attributed to olivine fractionation of more mg rich primary melts from pyrolite like mantle sources

evolution and source of ore fluids in the stringer system hellyer vhms deposit tasmania australia evidence from fluid inclusion microthermometry and geochemistry the hellyer deposit is a classic large tonnage high grade mound style volcanic hosted massive sulphide vhms deposit in the cambrian mt read volcanic belt of western tasmania in the footwall directly underlying the deposit there is an extensively altered pipe which cootains a well developed and preserved stringer zone the vein paragenesis at hellyer indicates that premineralization stage i veins consist entirely of quartz and occur throughout the alteration pipe the synmineralization stage 2 veins are the most abundant veins in the stringer zone and consist of three sub stages stage 2a veins of crustiform quartz pyrite and carbonate with minor amounts of chalcopyrite sphalerite and galena stage 2b veins with abundant base metal sulphides minor quartz carbonate and barite gangue and stage 2c veins of coarsely crystalline barite with variable amounts of pyrite sphalerite galena and carbonate stages 3 6 veins are postmineralization veins and are related to the devonian tabberabberan orogeny textural petrographic and microthermometric investigations of fluid inclusions in the hellyer stringer system indicate that type i primary liquid vapour inclusions occur along growth planes of crustiform quartz crystals or within colour banding of zoned sphalerite these inclusions are 10 15 m in size and yielded homnogenisation temperatures of 170 220 c in early 2a veins 165 322 c in main stage 2b veins and 190 256 c io late stage 2c veins these data suggest a waxing and waning thermal history however the average salinity remained between 8 11 naci equiv wt in all stage 2 veins chalcopyrite bearing primary fluid inclusions have been recognised in the base metal rich stage 28 veins no evidence for presence of co2 e g formation of clathrates was recorded by rnicrothermometry however laser raman spectroscopic lrs analysis indicates the presence of co2 1 mole in the stage 2b veins and no detectable co2 in 2a and 2c vein stages semi quantitative sem wds microprobe analyses of fluid inclusion decrepitates indicate that the hellyer ore fluid was enriched in potassium and calcium but depleted in magnesium relative to seawater pixe microanalysis of fluid inclusions in quartz indicates that the stage 2b ore fluids have a significantly higher base metal concentration compared to the stage 2a veins the postmineralization stage 4 veins have a variable but lower base metal content in this study there was no fluid inclusion evidence of boiling cation composition higher salinities relative to seawater and the presence of co2 suggest that recycled seawater alone cannot be the sole source of the ore fluids this interpretation is in agreement with previous isotopic studies in the hellyer stringer system although direct input of bulk ore constituents from a magma chamber cannot be demonstrated from the present fluid inclusion data such a contribution of ore fluids from a magmatic source cannot be ruled out the possible input from the magmatic source may have occurred during the base metal rich stage 2b vein formation characterised by the intensifying temperature of deposition higher base metals and co2 contents

petrology and geochemistry of the nyamuragira volcano zaire abstract nyamuragira volcano in the western rift of the east africa rift valley is characterized by violent volcanic eruptions the most recent of which occurred in 1967 1971 1976 1977 1980 and 1981 1982 its extrusives consist of a highly potassic suite of olivine basanite tephritic phonolites among which phonolitic tephrite is the commonest the petrographic features of the whole suite such as phenocryst and groundmass mineral assemblages and modal variations are rather simple major elements vary smoothly from olivine basanite to phonolitic interstitial glass without any abrupt change with increasing k 2 o in addition the typical incompatible trace elements ba ce f nb rb and sr increase continuously with the increase of k 2 o these suggest that chemical variations were controlled mainly by the effective fractionation of olivine and clinopyroxene associated with plagioclase and titanomagnetite furthermore in the k 2 o incompatible trace element diagrams trend lines extrapolated to zero converge on the coordinate axes of the vertical and horizontal lines this suggests that these incompatible elements were preferentially moved toward the primitive basaltic magma during the partial melting of metasomatized upper mantle peridotites furthermore these elements are concentrated almost completely in residual liquids during fractional crystallization

hydrothermal alteration of felsic volcanic rocks associated with massive sulphide deposition in the northern iberian pyrite belt sw spain abstract massive sulphide deposits of the northern iberian pyrite belt ipb are mainly hosted by felsic volcanic rocks of rhyolitic to dacitic composition beneath most of the massive ores of this area e g concepcion san miguel aguas tenidas este or san telmo deposits there is usually a wide hydrothermal alteration halo associated with stockwork type mineralization within these alteration envelopes there are two principal rock types 1 chlorite rich rocks linked to the inner and more intensely altered zones and dominantly comprising chlorite pyrite quartz sericite carbonate rutile zircon chalcopyrite and 2 sericite rich rocks more common in the peripheral zones and showing a dominant paragenesis of sericite quartz pyrite chlorite carbonate rutile zircon sphalerite mass balance calculations comparing altered and least altered felsic volcanic rocks suggest that sericitization was accompanied by moderate enrichment in mg fe and h 2 o with depletion in si na and k and a slight net mass loss of about 3 chloritization shows an overall pattern which is similar to that of the sericitic alteration but with large gains in fe mg and h 2 o and minor enrichment in si s and mn and a significant loss of na and k and a minor loss of ca and rb however chloritization has involved a much larger net mass change mass gain of about 28 only a few elements such as nb y zr ti p and lree appear to have remained inert during hydrothermal alteration whilst ti and al have undergone very minor mobilization the results point to the severity of the physico chemical conditions that prevailed during the waxing stage of the ore forming hydrothermal systems further mineralogical and geochemical studies of the altered footwall rocks in the studied deposits indicate that hydrothermal ore bearing fluids reacted with host rocks in a multi stage process which produced a succession of mineralogical and chemical changes as the temperature increased

correlation of upper jurassic lower cretaceous continental sequences from the southern biscay margin northern spain biostratigraphical data indicate that fresh water carbonate deposits from the southern part of the the cantabrian basin northern spain are of earliest cretaceous berriasian age charophyte assemblages enable correlation with fresh water carbonate rocks of the western cameros basin to the south the charophytes also support correlation with berriasian marginal marine carbonate sequences in the jura mountains southern england and northern germany indicating that these continental carbonate rocks represent fresh water equivalents of the 9purbeckian9 facies the biostratigraphical correlation is enhanced by the parallel facies evolution observed in the upper jurassic berriasian of the cameros and cantabrian basins interpretation of these successions as non marine depositional sequences permits member scale correlation between the two basins further strong facies similarities in the overlying alluvial clastic rocks of the two areas suggest a revised correlation of the overlying valanginian albian deposits the two basins were connected at least intermittently during the late jurassic early cretaceous only during periods of higher sea level did facies differences develop as a result of greater marine influence in the north at other times vast alluvial lake and marsh systems extended over the region leading to the deposition of genetically related alluvial and lacustrine or palustrine deposits in these linked half graben systems in cantabria and the cameros basin strong lateral variations in facies thickness and stratigraphy reflect erosion at the onset of late jurassic rifting differential subsidence in the early cretaceous and erosion beneath aptian albian unconformities improved understanding of the successions described here may assist interpretation of similar upper jurassic lower cretaceous sequences on the northern margin of the biscay rift such as those in the wessex and north celtic sea basins

petrogenetic implications of tholeiitic basalt glasses from the east pacific rise and the galapagos spreading center basalt glasses dredged from the east pacific rise and the galapagos spreading center show a wide range of composition with fm values feototal feototal mgo from 0 49 to 0 72 analyses for major elements of 40 unaltered glasses and of 17 of these glasses for trace elements demonstrate an analogous line of differentiation over a considerable distance in the east pacific using data from the experiments of peridotite partial melting and their implication for primary melts the composition of the initial basalt magma in equilibrium with the residual phase is extrapolated to be fm 0 40 0 48 the importance of apatite introduced from the parental peridotite for the incompatible elements ree f and p2o5 within various basalt differentiation series is demonstrated

fluids from the neves corvo massive sulphide ores iberian pyrite belt portugal abstract the neves corvo cu sn zn mine is probably the richest vms deposit in the world the research was conducted in order to know if pristine volcanogenic fluids are present in the massive sulphide ores and proceeds to their characterization and interpretation the samples were collected inside thick massive sulphide bodies that are undoubtedly of primary origin petrographic relations between quartz and sulphide phases mainly pyrite chalcopyrite suggest that the quartz and the sulphides were formed simultaneously three types of fluid inclusions were observed 1 low salinity aqueous fluid inclusions either isolated in clusters or along trails 2 low salinity h2o co2 ch4 nacl fluid inclusions in intergranular arrays or intracrystalline trails some in growth zones some of which form clathrates on freezing and 3 only in one sample high salinity fluid inclusions with halite without any regular distribution in the crystals the aqueous low salinity fluid is the most abundant consisting of modified seawater resulted from convection through the basement homogenization temperatures lie between 140 and 370 c this wide range of th is typical of vms deposits from ipb and reflects the evolution of the hydrothermal system with time salinity ranges from 3 to 6 wt equiv nacl except for a small group of the high salinity fluid inclusions which have around 30 wt eq nacl these high salinity fluids are interpreted as brines resulting from sporadic deep subsea floor boiling that could originate either from seawater or from a magmatic fluid the carbon dioxide and methane could have been derived from reactions with organic matter enclosed in the metasediments underlying the ores however a magmatic origin cannot be precluded

remobilisation of base metals and gold by variscan metamorphic fluids in the south iberian pyrite belt evidence from the tharsis vms deposit the tharsis massive sulphide deposit one of the major vms type deposits in the iberian pyrite belt ipb was severely deformed by the variscan tectono thermal events the question of whether or not these events affected the metal distribution in the deposit has been addressed by simultaneously studying the mineral parageneses tharsis stockwork and the fluid circulation at local and regional scales the results are 1 the early paragenesis in the stockwork q1 quartz pyrite chlorite phengitefcobaltitefankerite was strongly overprinted by a late post kinematic mineral deposition including new quartz veins q3 quartz and base metal sulphides chalcopyrite sphalerite bi and te minerals pyrite and galena and gold 2 at a regional scale fluids accompanying the peak metamorphism conditions ca 300 mpa ca 300 jc were of c o h n nacl type co2 dominated with ch4 and n2 and are considered to be metamorphic on the basis of microthermometry and geochemistry the late to post kinematic evolution retrograde stage was characterised by a pressure drop down to 40 mpa lithostatic to hydrostatic transition and a heat input leading to temperatures z430 jc then decreasing to temperature around 170 jc fluids of the retrograde type exhibit both dilution of the c o h n nacl fluid by a low salinity meteoric water and progressive loss of volatile components 3 fluids of the retrograde type pervasively percolated through the tharsis stockwork and were responsible for the strong mineral overprint on the early deformed paragenesis all the measurable fluid inclusions f i record these late fluids there are primary fluid inclusions in q1 but they are systematically imploded due to the external overpressure generated by the variscan tectonic events although base metal distribution in the stockwork is basically the result of the

integration of subsidence and sequence stratigraphic analyses in the cretaceous carbonate platforms of the prebetic jumilla yecla region spain abstract an integrated analysis of subsidence and sequence stratigraphy of the cretaceous successions of the jumilla yecla region betics se spain is supported by abundant stratigraphical sedimentological and palaeontological data with the aim to document and explain the accommodation changes that controlled the evolution and architecture of the carbonate platforms generated during that time on the southern continental margin of iberia the cretaceous shallow marine carbonates and clastics that extensively crop out in the jumilla yecla region are divided into 11 sequence sets major stratigraphic units bounded by tectonically induced unconformities which can be subdivided into several third order depositional sequences and their constituent system tracts all these genetic units build up a regional chronostratigraphic framework which is herein used to support subsidence calculations from the results of the subsidence analysis seven intervals with characteristic tectonosedimentary patterns were distinguished for cretaceous time from these intervals the first three respectively early tithonian early late berriasian late berriasian late hauterivian and latest hauterivian to earliest late albian were controlled by extensional tectonics strong enough to mask during most part of the time the long term thermal subsidence inherent to the continental margin this tectonism was related to the iberia africa divergence and the opening of the north atlantic later the fourth interval late albian mid cenomanian and the sixth interval late coniacian to late santonian were characterised regionally by overall homogeneous subsidence patterns controlled by thermal subsidence sediment loading and a relative tectonic quiescence finally the fifth and the seventh intervals respectively latest cenomanian early coniacian and campanian maastrichtian were characterised by strong tectonic movements and complex subsidence patterns which were related to changes in intraplate stresses related to the onset of the convergence between africa and iberia and with the evolution of the bay of biscay

radioelement concentrations in the deep profile through precambrian basement of the vredefort structure the vredefort basement granite in south africa forms the core of an undomed and overturned sequence of precambrian strata slawson 1976 proposed that the granite core has also been overturned exposing to view a thick section through the archean crust incrasing in depth toward the center of the dome profiles across the vredefort basement provide an opportunity for studying the distribution of u th and k with depth in the upper and middle crust detailed radioelement concentration profiles across the granitic basement showed that 1 the central part of the basement core is depleted in th and u relative to the perimeter and the concentration of th and u falls off regularly from the basement margin inward 2 the distribution of the radioelements and the distribution of heat production with depth over the outer 8 km are consistent with an exponential depth function 3 the felsic rocks of the central basement are characterized by high k u and th u ratios and the ratios k u k th and th u increase from the margin inward and 4 mafic granulites which occur in the central regions of the dome have normal k u k th and th u ratios high grade metamorphic environment resulted in the depletion of bothmore u and th relative to k in the greater proportion of the felsic rocks from the central regions of the vredefort basement depletion of u was far greater than that of th the normal th u k u and k th ratios in mafic rocks in close proximity to the felsic rocks suggest that mineral assemblages have a strong control on the behavior of the radioelements during regional metamorphism in the vredefort basement less

carbonic fluid overprints in volcanogenic massive sulfide deposits examples from the kelan volcanosedimentary basin altaides china the tiemurte and the dadonggou zn pb cu volcanogenic massive sulfide vms deposits situated in the devonian kelan volcanosedmentary basin of the south margin of the chinese altaides were metamorphosed and overprinted by metamorphic sulfide quartz veins during early carboniferous to early permian two mineralizing periods of ore mineral growth can be identified 1 disseminated banded and massive sulfide ores related to a primary depositional sea floor volcanic hydrothermal activity and 2 the foliated sulfide quartz veins q1 related to synorogenic metamorphism and late chalcopyrite bearing quartz veins q2 cutting the schist related to a younger metamorphic overprinting event carbonic co 2 ch 4 n 2 fluid inclusions are ubiquitous in q1 and q2 veins a few carbonic fluid inclusions may be primary and some may be pseudosecondary whereas the vast number of carbonic fluid inclusions are secondary representing later events a microthermometry study shows that primary carbonic fluid inclusions in q1 and q2 have tm co2 ranging from 64 5 to about 59 4 c with th co2 13 4 to about 18 6 c the secondary carbonic fluid inclusions exhibit two behaviors when cooling and heating the tm co2 of the first group l co2 ranges from 63 3 to 57 7 c and that of the second group l co2 ch4 n2 ranges from 83 4 to 61 c the second group of carbonic fluids has much higher ch 4 and or n 2 proportions than the first group the trapping temperatures for the carbonic inclusions have been estimated to be 243 1 to 412 1 c for tiemurte and 216 to 430 c for dadonggou on the basis of some l co2 l h2o inclusions associated with carbonic inclusions and the trapping pressures have been estimated to be 120 to 340 mpa which are consistent with deformation p t conditions of quartz and slightly less than the p t conditions of the biotite and garnet metamorphic zones these abundant carbonic inclusions at the tiemurte and the dadonggou deposits were not a part of a volcanogenic ore producing system but represent a much younger event possibly having originated from a synorogenic metamorphism which may have contributed to orogenic gold

a pan african alkaline pluton intruding the saramuj conglomerate south west jordan the geological setting petrography and bulk mineral chemistry of a monzodiorite and a presumably consanguineous megaporphyry with large up to 25 cm labradorite megacrysts both intruding the upper proterozoic saramuj conglomerate in south west jordan south eastern shore of the dead sea were examined the crystallization temperatures of the monzodiorite and the megaporphyry as determined from pyroxene thermometry and supported by contact metamorphic mineralogy are about 700 and 900 c respectively the intrusion depth of the monzodiorite is about 3 4 km the monzodiorite was emplaced in the saramuj conglomerate at about 595 2 ma ago according to rb sr and u pb age determinations

estimation of the degree of partial melting by na2o k2o and al2o3sio2 of basic magmas abstract equations for estimating the degree of partial melting f of basic magmas in terms of na2o k2o and al 2 o 3 sio 2 and based on previously published batch equilibrium melting and experimental and petrochemical data are proposed as follows for f 0 10 0 50 f a 1 0 957 na 2 o k 2 o s na 2 o k 2 o m 0 0149 for f f b 1 0 694 na 2 o k 2 o s na 2 o k 2 o m 0 0516 for f 0 15 0 35 f c 0 9348 al 2 o 3 sio 2 s al 2 o sio 2 m 0 1532 0 108 al 2 o 3 sio 2 s al 2 o sio 2 m 0 597 for f f d 1 2437 al 2 o 3 sio 2 s al 2 o sio 2 m 0 2470 3 308 al 2 o 3 sio 2 s al 2 o sio 2 m 0 237 the subscripts s and m denote source and supposedly primary magma or melt respectively a correct estimate or choice of source composition and composition of primary magma for the calculation is important if both compositions can be reasonably assigned or estimated then the degree of partial melting can be calculated from one of these equations with errors generally

geometry and genesis of feeder zones of massive sulphide deposits constraints from the rio tinto ore deposit spain vein related data have been collected around the giant rio tinto orebody in southern spain within the root zones of the massive sulphide deposits here we report the main results of this study concerning the geometry of the stockwork and the conditions of formation although field and thin section studies have shown that a wide range of vein configurations exist from micro cracks fluid inclusion planes to large paleo flow channels two groups seem to dominate the first corresponds to small constricted micro cracks and capillary flow channels now mainly filled with quartz whereas the veins of the second group have large widths are continuous over several meters and are filled with quartz and sulphides most are tension veins and only very few quartz tends to post date the quartz dominated veins quartz pyrite the vein thickness and spacing distribution is modal rather than logarithmic and their densities are not fractal but are characterized by a poisson distribution from the immediate sub surface zone to more than 100 m below the base of the massive sulphide deposits most hydrothermal quartz sulphide stockwork veins are sub parallel to the base of the massive sulphide deposit the assumption that the base of this deposit corresponds to a paleo horizontal plane implies that most veins were sub horizontal this is particularly evident for small veins but the larger ones can be strongly oblique to the base of the deposit the hydrothermal fluids that generated the massive sulphide deposits and underlying stockworks were very saline and probably underwent sub or super critical phase separation in the root zones of the system this phase separation was the probable mechanism producing the periodic over pressures of at least 20 mpa that were necessary to generate the sub horizontal veins of the stockworks

fluid inclusion evidence for subsurface phase separation and variable fluid mixing regimes beneath the deep sea pacmanus hydrothermal field manus basin back arc rift papua new guinea 1 altered volcanic rocks were cored from over 350 m below the seafloor at the papua new guinea australia canada manus basin hydrothermal field pacmanus deep sea hydrothermal field in the eastern manus back arc basin fluid inclusions in anhydrite veins reveal phase separation and fluid mixing beneath the seafloor the anhydrite precipitated from high temperature fluids 150 385 c at roman ruins a site of active high temperature venting 220 276 c measured by submersible the fluid inclusion thermal depth profile is uniform and high temperature 242 368 c at snowcap a site of warm water effusion 6 65 c the fluid inclusions indicate high temperatures at depth 270 385 c but both low and high temperatures in the shallower section this indicates a flow regime dominated by vertical advection and shallow entrainment and mixing with cool seawater inclusions at snowcap exhibit extreme salinity variations due to phase separation at temperatures above 350 c fluids contain na cl fe zn mg and ba and a minor gas component such as co2 or ch4 most inclusions at roman ruins exhibit salinities that fall within the range of those observed at modern active vent sites along the mid ocean ridge system fluid inclusion temperatures support a hypothesis developed previously from sr isotopic analysis that the subseafloor at snowcap is characterized by mixing between deep sourced hot hydrothermal fluids and cold seawater like fluid both heating of seawater and cooling of upwelling hydrothermal fluids can be recognized by combining isotopic and fluid inclusion data in contrast to snowcap the regime at roman ruins is less varied with uniformly high temperature upwelling fluids that have hydrothermally dominated sr isotopic ratios

metallogenesis at the neves corvo vhms deposit portugal a contribution from the study of fluid inclusions abstract neves corvo one of europe s most outstanding currently producing mines was the focus of two recent studies on fluid inclusions the present contribution provides new data and is intended to give an overview of the fluid inclusion research carried out on the deposit the study focuses on samples from all stratigraphic levels above the ores in the copper and tin ores and in the main thrust in massive copper ore pyrite plus chalcopyrite occur along growth zones in quartz suggesting that quartz and sulfides are coeval in these samples the most frequent fluid is a low salinity h 2 o nacl fluid but an aqueous carbonic fluid of low density also occurs especially in intercrystalline domains homogenization temperatures show two distinct modes one at 160 to 170 c and the other between 220 and 280 c in one sample a high salinity h 2 o nacl fluid with 30 wt nacl equiv was observed this fluid is interpreted to represent the product of phase separation at depth and it could have been derived from either seawater or a magmatic fluid however the most likely cause for ore deposition was temperature decrease it seems however that three separate events are registered low medium and high temperature in the evolution of the volcanogenic hydrothermal system in all but the massive ore and the main thrust the dominant fluids are low salinity aqueous carbonic in character frequently showing liquid co 2 at room temperature pressure oscillations frequently found in these fluids are interpreted as due to variations in the pressure regime from lithostatic to hydrostatic at the same depth co 2 ch 4 are thought to have resulted from reactions with organic matter dispersed in the metasediments which are very common at the stockwork and rubane ores and below the orebodies the highest p t conditions during metamorphism were estimated around 320 mpa and 350 c the rubane ores are interpreted as metamorphogenic ores as they were formed during orogeny also the fissural copper ore is better described as a metamorphosed stockwork

fluid inclusion and stable isotopic constraints on fluid sources and evolution of the luojiahe cu deposit in the southern margin of the north china craton abstract the luojiahe cu deposit in the zhongtiaoshan region is located in the southern margin of the north china craton the orebodies are hosted in the mafic volcanic sedimentary sequences of the metamorphosed greenschist facies neoarchean songjiashan group the luojiahe cu mineralization can be divided into the primary volcanogenic massive sulfide vms mineralization stage stage i banded or stockwork ores and the subsequent metamorphic remobilization stage stage ii coarse vein ores three types of quartz selected for fluid inclusion fi studies were collected from the stage i banded q1 and stockwork q2 ores and stage ii coarse vein q3 ores four types of fis were identified 1 liquid rich fis l type 2 pure vapor and vapor rich fis v type 3 daughter mineral bearing fis s type and 4 ch4 h2o fis c type systematical microthermometric and h o isotopic studies show that the stage i ore forming fluids consist predominantly of high salinity evolved seawater 125 220 c 23 9 27 9 wt nacl equiv and some magmatic hydrothermal fluids 249 339 c 34 5 42 2 wt nacl equiv the two fluid end members are represented by the l type fis in q1 and the s and v type fis in q2 the temperature and salinity variation trends of the l type fis in q1 indicate a mixing process between the hot evolved seawater and cold seawater at stage i furthermore the v and s type fi coexistence in q2 and their microthermometric data suggest that fluid unmixing has occurred in original magmatic fluids at stage i in contrast the stage ii ore forming fluids consist of ch4 rich metamorphic fluids 192 350 c 10 6 43 2 wt nacl equiv carbon isotopic analysis of the stage ii calcite 4 58 to 10 83 and graphite 32 01 to 39 16 in the ore hosting chlorite schist indicates that the metamorphic ore forming fluids had exchanged carbon isotope with graphite the generation of ch4 may have resulted from the interaction between h2o released by metamorphic devolatilization and graphite the continuous consumption of h2o in the hydrothermal fluid system may have increased the fluid salinity and triggered fluid unmixing in the ch4 nacl h2o system in addition the vms metallogenic environment is generally favorable for microbial communities it is considered that the graphite at luojiahe may have been derived from sedimentary organic matter formed in seafloor hydrothermal vent systems as also supported by carbon isotopic data we propose that at stage i the main mineralization may have been resulted from 1 fluid mixing of hot evolved seawater and cold seawater in the near surface environment and 2 fluid unmixing caused by the percolation of magmatic fluids into syn volcanic faults forming the stockwork ores at stage ii the interaction between h2o and graphite may have resulted in the reduction of ore forming fluids and cu precipitation and fluid unmixing in the ch4 nacl h2o system may have further promoted the cu mineralization

geochronology of the deep profile through archean basement at vredefort with implications for early crustal evolution the presence of archean granite forming the core of an updomed and overturned sequence of strata at vredefort south africa has been known for over 70 years recent geophysical geochemical and geological evidence has given rise to the proposal that the basement granite core has also been overturned presenting a section of the earth s granitic crust to view and that radial traverses in the vredefort basement penetrate through a 15 km thick section of the archean crust when moving from the contact of collar strata to the center of the dome the upper levels of the vredefort crust are dominated by a relatively homogeneous outer granite gneiss ogg displaying marked gradients in large ion lithophile elements these rocks grade inward into the deeper levels which occur in granulite facies metamorphism the deeper levels of the vredefort crust exposed in the central parts of the dome consist predominantly of felsic inlandsee leucogranofels ilg which occurs interleaved with inclusions of mafic metavolcanic and metasedimentary rocks termed steynskraal metamorphic zone smz a comprehensive isotopic study using four decay schemes 87rb 232th 238u and 235u on this crustal section is now reported the results permit unique and direct tests on the theories of archean crust forming processes in the upper vredefort crust whole rock isochron ages of 3050 m y are obtained from the four decay schemes and almost all evidence of an extended crustal history prior to 3050 m y has been obliterated establishment of strong vertical gradients in rb sr ratio shortly before setting of rb sr whole rock systems has wide implications for the interpretation of sr initial ratios although the vredefort upper levels of the crust has a low 87sr 86sr initial ratio of 0 7019 0 0002 it probably participated in a crustal prehistory for at least 500 m y in the deeper levels of the vredefort basement the geochronology is much more complex rb sr and th pb isochrons of 3500 m y are recorded in the mafic granulite relicts a companion paper welke and nicolaysen this issue provides evidence for an early crust forming event in this sector 3800 m y ago from 3500 m y onward these deeper crustal levels did not undergo addition of new archean crust forming material on a major scale nevertheless a most important younger archean event affected these levels 2800 m y ago cessation of high grade metamorphism is recorded by setting of the rb sr isotopic system in the felsic rocks and zircon u pb ages an episode of strong uranium depletion affected both the felsic and mafic rocks of this sector 2800 m y ago the inlandsee leucogranofels is evidently a rock unit with a long and complex history beginning with the emplacement as a crustal unit at least 3800 m y ago since then it has been subject to reworking and resetting of isotopic systems which terminated 2800 m y ago in the deeper crust there is scant evidence of a 3050 m y event similarly there is no record of a 2800 m y old imprint in the upper vredefort crust thus the last major events that affected the geochemical constitution of the vredefort upper and deeper crustal sectors differed in time and nature nevertheless they brought about isotopic setting in a downward moving sequence while the overlying crust remained cooler rigid and unaffected these events probably reflect significant discontinuities in the cooling of the archean crust

basement components of the xiangshan yuhuashan area south china defining the boundary between the yangtze and cathaysia blocks abstract the exposed metamorphic rocks and xenolithic metamorphic rocks in the xiangshan yuhuashan xsyhs area in central jiangxi province south china were studied in order to reveal the compositions of the basement rocks and their tectonic implications bulk geochemistry and zircon u pb hf isotopic compositions suggest that the basement metamorphic rocks in the xsyhs area are mainly composed of neoproterozoic after 822 683 ma meta sedimentary rocks the exposed meta sedimentary rocks show higher k 2 o na 2 o and al 2 o 3 na 2 o cao and lower eu eu than those occurring as xenoliths in the yuhuashan volcanic intrusive complex indicating relatively strong weathering and leaching in the provenance area geochemical data suggest that the detritus of the exposed metasedimentary rocks was derived mainly from felsic to intermediate igneous rocks similar to neoproterozoic sedimentary rocks in the southern yangtze block whereas the xenolithic metasedimentary rocks contain more recycled material similar to those in the cathaysia block these xenoliths from the mid lower crust also have significantly high u contents consequently they probably are an important source for uranium in the xsyhs volcanic rocks u pb ages of detrital zircons show that the protoliths of the xenoliths are mainly composed of neoproterozoic neoarchean and paleoproterozoic detritus similar to the components of the wuyi terrane in the eastern cathaysia block in contrast the protoliths of the exposed metasedimentary rocks consist mainly of neoproterozoic detritus with minor neoarchean and paleoproterozoic fragments similar to those in the southern yangtze block both geochemistry and zircon u pb hf isotope compositions suggest that the deep basement metamorphic rocks in the xsyhs area belong to the wuyi terrane whereas the surface metamorphic rocks are derived from the southern yangtze block therefore the real boundary between the yangtze and cathaysia blocks lies north of the study area the exposed yangtze related metamorphic rocks were overthrust southward from the southern yangtze block and onto the cathaysia basement during the indosinian to yanshanian orogeny

the vredefort structure an introduction and a guide to recent literature abstract the 50 km wide vredefort structure appears to present the eroded roots of an area affected by rapid uplift and violent rock deformation widely regarded as shock deformation on an immense scale the origin of the structure is disputed external impact and internal agencies have been suggested this note introduces the vredefort structure and its geological environment emphasizing information published in the decade prior to the 1987 international workshop on cryptoexplosions and catastrophes in the geological record with a special focus on the vredefort structure this typical cryptoexplosion structure has a central and polygonal basement uplift containing violently deformed and brecciated rocks with the northern half surrounded by steeply dipping and heavily faulted collar strata and then by a rim synclinorium of less deformed strata the gains made in research on all three sectors are noted traverses through the basement from collar to centre effectively penetrate through a 15 km thick section of crust the archaean history of the crustal section has been documented the thermal and fluid inclusion evolution near the time of uplift has been studied and a deep seated co 2 rich gas explosion has been proposed as an alternative to the impact hypothesis for the shock deformation meticulous study of the effects of thermal metamorphism on the shock deformations in the collar strata led to a bold proposal more than one shock event in the evolution of vredefort analysis of the character of the faulting of the collar supports an earlier proposal regarding inward movement of blocks of collar strata renewed research on shatter cone orientations in the collar strata and two detailed studies of the structural history within the rim synclinorium are noted through the intensive mapping geophysical exploration and drilling which have been completed within the witwatersrand and transvaal basins close spatial and temporal connections between vredefort and the bushveld complex have been documented in both cases basin centres were intruded by horizontal mafic eruptives and basin subsidence took place after these eruptives had consolidated and cooled the vredefort structure has long been discussed as a possible deeply eroded remnant of a major cratering event or events the connections of this structure with the bushveld complex carry important implications for such discussion

anomaly threshold estimation and data normalization using eda statistics application to lithogeochemical exploration in lower cretaceous zn pb carbonate hosted deposits northern spain abstract a lithogeochemical survey of lower cretaceous sediments 1293 samples in the basque cantabrian basin northern spain was carried out in order to estimate the mean contents of major oxides and ba co cu ni pb rb s sr v zn and zr in different lithological subsets and to identify anomalies related with zn pb carbonate hosted mineralization after a detailed evaluation of different anomaly recognition methods using both real and synthetic data the cut off values used in the exploratory data analysis eda were selected as thresholds a prior classification of the rocks based on chemical mineralogical and petrographical data guarantees appropriate homogeneous with respect to major oxides rock groups during the evaluation of the data a data standardization method was developed and applied to the samples collected from the carranza sector n 330 zinc and pb values for each sample were normalized to the particular threshold and interquartile range of their corresponding rock type these normalized values were plotted in geochemical maps the results reveal the presence of zn up to 0 6 and pb max 1380 ppm anomalies concentrated in carbonate facies mainly in limestones dolostones and dolomitic limestones magnesium rich carbonates host the most significant anomalies in the carranza area the great majority of outliers occur around known zn pb mineral occurrences and are concentrated at the edge of the carbonate platform in some cases fault control of the anomalies is indicated

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

magmatic composition and tectonic setting of altered volcanic rocks of the fennell formation british columbia the upper paleozoic fennell formation in south central british columbia consists of basalts and associated marine sediments metamorphosed at low greenschist facies conditions although the microphe

a reappraisal of the structure of the spanish segment of the iberian pyrite belt the major structural features of the iberian pyrite belt are described in terms of geometry deformation mechanisms scale timing kinematics and the mutual relationships among the various architectural elements the result of such an analysis allows this zone to be considered as a s verging thin skinned fold and thrust belt propagating southwards over a mid crustal basal detachment this was the response in the footwall of the suture to the major phase of hercynian oblique collision between the south portuguese plate and the ossa morena zone of the iberian autochthon this thin skinned event inverted a previous extensional structure acquired during the initial stages of the collisional process and intimately linked to the formation of the ore deposits that make this region a world class metallogenic province

fluid inclusion petrography abstract a procedure of fluid inclusion studies is proposed with emphasis on the criteria of selecting fluid inclusions for detailed microthermometry and spectroscopic analysis an overview of descriptive and genetic classifications of fluid inclusions in single crystals and in massive rocks is given with the intention of further differentiating the commonly used terms primary and secondary fluid inclusions some principles of fluid inclusion modification are explained cathodoluminescence cl studies of quartz with the optical high power cl microscope and the electron microprobe provided with a cl detector are an important help in fluid petrography cl textures are subdivided in primary growth textures and a wide variety of secondary microtextures which are in part induced by fluid inclusions the latter is grouped in textures indicative of local lower crystal order increasing defect structures and microtextures indicative of local quartz healing reduction of the defect structures microtextures showing the genetic relationship between fluid inclusions and the host mineral provide information about the possible post entrapment changes of fluid inclusions and therewith testify their geological relevance

two stage terrane assembly in western gondwana insights from structural geology and geophysical data of central borborema province ne brazil abstract combined geophysical and structural data from the transversal subprovince of the borborema province ne brazil highlight the internal structure of and interrelationships between the constituent terranes radiometric and magnetic maps show distinctive signatures for the archean paleoproterozoic alto moxoto and meso to neoproterozoic alto pajeu and pernambuco alagoas terranes mapped radiometric and first and second order magnetic lineaments associated with euler deconvolution enable correlation between geophysical data and major structures in addition to early related deformation of the alto moxoto terrane combined analysis of late transposition foliations lineations and kinematic criteria reveal a complex structural evolution marked by two distinct assembly stages the first phase is characterized by thrust tectonics with top to the south vergence resulting in the juxtaposition of the allochtonous alto pajeu terrane with the structurally underlying alto moxoto terrane the terrane boundary is delineated by the serra de jabitaca shear zone which is associated with low dipping collisional granitic sheets and ca 1 0 ga mafic ultramafic rocks interpreted as obtudcted ophiolite remnants later strike slip movements strongly folded and obliterated thrust related markers and the continental scale e w pernambuco lineament is interpreted as the result of lateral assembly between the composite alto pajeu alto moxoto terranes and the pernambuco alagoas terrane during the metamorphic peak of the brasiliano orogeny evolution of the borborema province reflects accretionary processes during the assembly of western gondwana

geochemistry across an exposed section of archaean crust at vredefort south africa with implications for mid crustal discontinuities abstract the central region of the vredefort structure consists of a semi circular multi layered sequence of crystalline rocks which are nearly vertical in attitude and which increase in metamorphic grade towards the core of the structure together with the overlying precambrian strata this sequence provides a cross section through almost the entire crustal section of the kaapvaal craton 36 km the upper part of the vredefort crystalline crust consists of a 3 0 ga sequence of differentiated felsic rocks in amphibolite facies the lower crust consists of a complex and heterogeneous both chemically and isotopically high grade metamorphic terrain of charnockites granulites mafic and felsic and supracrustal rocks the upper crust is separated from the lower crust by the vredefort discontinuity a brittle ductile shear zone characterised by a high concentration of pseudotachylite and brecciated rock petrographic chemical and isotopic evidence suggest that the upper and lower crust have undergone very different styles of evolution this is indicative of different geological environments prior to their present juxtaposition we speculate that the upper and the lower parts of the vredefort crystalline crust were juxtaposed during intracratonic thrusting 2 8 ga ago and that the entire 36 km section of crust including the vredefort discontinuity were rotated and uplifted into their present vertical position during the 2 0 ga vredefort catastrophe

continental sedimentation in an evolving rift basin the lower cretaceous of the western cameros basin northern spain abstract the lower cretaceous of the western cameros basin comprises up to 2 5 km of clastic and carbonate continental sediments deposited in a variety of fluvial and lacustrine settings kimmeridgian berriasian red beds and lacustrine limestones are overlain by the valanginian aptian pedroso group which is dominated by sheet flood sandstones and distal alluvial mudstones with vertebrate footprints mudcracks and intercalated carbonate paleosols lacustrine limestones are also present commonly containing spongiostromate oncoids the upper part of the sequence the salas group and utrillas formation aptian lower cenomanian comprises quartzose pebbly sands and conglomerates with rare muddy paleosols these sediments are interpreted as braided stream deposits deposition of this thick continental sequence may be correlated with the extensional history of the cameros basin and of the evolving bay of biscay rift to the north clastic supply was dominantly controlled by tectonic factors facies evolution may also have reflected climatic influence

geologic map of the basque cantabrian basin and a new tectonic interpretation of the basque arc a new printable 1 200 000 bedrock geological map of the onshore basque cantabrian basin is presented aimed to contribute to future geologic developments in the central segment of the pyrenean cantabrian alpine orogenic system it is accompanied in separate appendixes by a historic report on the precedent geological maps and by a compilation above 350 bibliographic citations of maps and academic reports usually overlooked or ignored that are central to this contribution structural scrutiny of the map permits to propose a new tectonic interpretation of the basque arc implementing previously published partial reconstructions it is presented as a printable 1 400 000 tectonic map the basque arc consists of various thrust slices that can expose at the surface basement rocks palaeozoic to lower triassic and their sedimentary cover uppermost triassic to tertiary from which they are detached by intervening upper triassic evaporites and associated rocks the slice bounding thrusts are in most cases reactivated normal faults active during meso cenozoic sedimentation that can be readily related to basement discontinuities generated during the hercynian orogeny

fluid inclusions of the massive sulfide deposits in the skellefte district sweden abstract the massive sulfide deposits of the skellefte district are located in northern sweden the ores are hosted by precambrian submarine volcanics and sedimentary rocks both rocks and ores have been deformed and metamorphosed mainly to greenschist facies aqueous two phase liquid vapor primary inclusions in sphalerite quartz and calcite from six mines ravliden kristineberg ostra hogkulla bjurtrask renstrom and langdal indicate that the sulfide ores were formed under non boiling conditions during two major stages of hydrothermal activity at temperatures of 210 and 295 c respectively the ore forming fluids were ca na cl bearing hydrothermal solutions with a salt content of 4 2 eq wt nacl later recrystallization of the ores occurred at lower temperatures involving solutions with higher salinities

a new interpretive procedure for whole rock u pb systems applied to the vredefort crustal profile granulite grade precambrian gneisses have usually undergone at least one period of strong u depletion whole rock u pb isotope studies can determine the time s of the severe depletion and this work attempts to place such studies on a more rigorous footing two stage u pb systems can be described in terms of one major episodic differentiation into rocks with varying u pb ratios while three stage systems can be described by two such distinct episodes most of the precambrian granulites that have been isotopically analyzed have histories too complex to be described as two stage systems however it is demonstrated here that u pb data on whole rock suites can yield the complete u pb chemical history of a three stage system in terms of u pb ratios for a suite of granulites present day 207pb 204pb and 206pb 204pb ratios and element concentration data allow these ratios to be calculated at a number of specific past times and plotted as an array the degree of scatter in each of these past arrays is graphed as a function of time the point of least scatter denotes the age of the end of stage 2 in the history of the system the array slope and the dating of the end of stage 2 also permit the beginning of stage 2 to be calculated all other parameters in the system u and pb concentrations pb isotopic ratios can now be determined for each individual rock throughout its history the new interpretive method also distinguishes sensitively among various kinds of uranium fractionation which may have operated during the differentiation episodes it is applied here to uranium depleted granulites in the deeper part of the vredefort crustal profile the times of the two fractionating episodes are calculated at 3860 and 2760 m y respectively the vredefort 3070 m y event when geochemical systems in the upper half of the crystalline basement became permanently closed evidently had little significance for the lower half of the crystalline basement some fundamental features of archaean basement evolution whose detailed significance is still poorly understood were elucidated for the first time by this u pb isotope study on whole rock samples rocks which suffered little uranium depletion at the end of stage 1 suffered least at the end of stage 2 rocks which were most affected at the end of stage 1 suffered the strongest depletion at the end of stage 2 evidently repeated high grade metamorphism purges certain granulites of their u content but leaves others relatively unscathed

ore genesis and fluid evolution of the kaladawan south zn pb cu ore field eastern altyn mountains nw china evidence from fluid inclusions h o isotopes and geochronology abstract the newly discovered kaladawan south zn pb cu orefield in eastern altyn mountains is composed of five zn pb cu deposits with the largest one containing 414 4 thousand tonnes kt zn 265 2 kt pb and 149 2 kt cu stratiform lenticular orebodies are hosted by the ordovician volcanic sedimentary sequences subjected to greenschist facies metamorphism two major ore forming stages with significant cross cutting relationship are identified at kaladawan south stage i of the earlier massive banded disseminated barite rich sulfide ores and stage ii of the later polymetallic sulfide quartz veins with economically less importance fluid inclusions fis locked in quartz of stage i massive sulfide ores are homogenized completely at 226 354 c with calculated seawater like salinities of 2 7 4 2 wt nacl equiv these characteristics of ore forming fluids coupled with ore textures of typical vms deposits are interpreted to be originated from the modified seawater by sea floor volcanic hydrothermal activities in compared the stage ii fis yielded lower homogenization temperatures ranging 251 c 357 c with higher salinities of 0 8 14 4 wt nacl equiv laser raman analyses identified large amounts of co 2 ch 4 n 2 in the hydrothermal fluids of stage ii the 18 o h2o values 0 5 5 7 and d h2o values of 68 1 to 32 3 of stage ii quartz are plotted in the range of metamorphic fluids in the d o isotopic diagram which indicating that the co 2 rich and mesothermal fluids are metamorphic derived the phlogopite paragenetic in the stage ii polymetallic vein yielded a 40 ar 39 ar plateau age of 435 4 3 9 ma similar to the age of the metamorphic muscovite 451 0 4 1 ma from the ore hosting schist based on the ore deposit geology mineral assemblages and fluid geochemistry the kaladawan south zn pb cu mineralization is attributed to be an ordovician vms type deposit overprinted by the silurian metamorphic fluids

microthermometric evidence for the formation of permian vhms deposits in tasik chini area central belt of peninsular malaysia abstract the vhms deposits of tasik chini area are located in the pahang state of central belt peninsular malaysia the deposits occur in a package of permian volcanics within mixed volcano sedimentary sequence and have been exploited from the bukit botol and bukit ketaya deposits they have many similarities with kuroko style massive sulfide deposits the host rocks at both deposits are felsic volcanic rocks of rhyolitic to rhyodacitic composition mineralization occurs as distinct ore zonation forming a stringer to massive sulfide zone at the footwall followed by barite lenses and exhalite layers fe mn si ore at the stratigraphic top the sulfide phases are characterized by pyrite as the major sulfide mineral with subordinate chalcopyrite sphalerite and rare galena traces of gold silver and sn bearing minerals occur in the massive sulfide and barite ores primary fluid inclusions within quartz and barite occur as two types 1 type i two phase liquid vapour inclusions without daughter minerals and 2 type ii three phase liquid co2 bearing carbonic inclusions these three phase inclusions include liquid h2o liquid co2 and vapour co2 with variable co2 content and often decrepitate before complete homogenization microthermometric measurements of the type i inclusions show that the temperature of homogenization th varies between 180 c and 310 c with salinities ranging from 1 0 to 15 0 wt nacl equivalent in the quartz samples homogenization temperatures th of 190 c to 300 c and salinities between 1 0 and 13 0 wt nacl equivalent are recorded from the barite samples type ii inclusions have an initial melting temperature range of 57 1 c to 57 5 c clathrate melting temperature tmclathrate of 7 4 c to 8 0 c and co2 homogenization temperature thco2 between 30 1 c and 30 3 c and salinity ranging from 4 0 to 5 1 wt nacl equivalent densities pressure and depth of ore forming fluids range from 0 711 to 0 970 g cm3 12 to 93 bars and 1500 m depth of seawater laser raman spectroscopic analysis shows the presence of co2 in the carbonic inclusions the fluid inclusion results suggest that the main ore fluid during the formation of the tasik chini vhms deposit is a seawater dominated fluid but contribution from a modified seawater or possibly magmatic origin is also indicated by higher salinities relative to seawater 3 2 wt nacl and the presence of co2 taking into account the regional geodynamic evolution it is also concluded that the formation of vhms deposits in the tasik chini area occurred in a back arc geotectonic setting

structural evolution of the southernmost segment of the west european variscides the south portuguese zone sw iberia abstract the south portuguese zone spz represents the southernmost unit of the iberian massif it is mainly composed of three structural domains from north to south the beja acebuches ophiolitic complex baoc the pulo do lobo antiform pla and the iberian pyrite belt ipb this study proposes a structural analysis of the spanish part of the spz that allows us to point out two main kinds of deformation one accommodated by early top to the south and following sinistral strike slip tectonics in the northern part of the spz and the other by top to the south thrusting in a wide southern branch this transition underlining the strain partitioning is analysed by lattice preferred orientation of quartz using the texture goniometry method it shows that the deformation is accommodated in the pla at low to middle temperature by basal and prismatic a slip quartz textures suggest increasing thermal conditions of deformation from thrust to strike slip tectonics our work within the ipb allows us to present a sequence of deformation showing a primary south verging ductile thrusting and coeval crustal thickening in response to the thin skinned tectonics the progressive deformation generated backthrusts while it turns shallower southward these features are summarised in an interpretative cross section of the spz that underlines the main structural style of deformation the fore mentioned southward propagating thin skinned thrusts

microthermometry and chemical composition of fluid inclusions from the mt chalmers volcanic hosted massive sulfide deposits central queensland australia implications for ore genesis mt chalmers is a mound style volcanic hosted massive sulfide vhms deposit in central queensland australia the ore lenses are hosted by rhyolitic volcanics and sedimentary rocks of early permian age the two ore lenses main lode and west lode consist of cu zn pb massive sulfide underlain by cu rich stringer mineralization textural and petrographic investigations of fluid inclusions indicate that primary type i inclusions up to 25 microns are found in quartz from the stringer mineralized zone and microthermometric studies of these inclusions yielded homogenization temperatures of 160 285 degrees c and salinities of 5 10 5 nacl equiv wt laser raman spectroscopic lrs analysis indicates the presence of co2 0 1 1 mol in the mt chalmers vhms systems semiquantitative sem wds microprobe analyses of fluid inclusion decrepitates indicate that the mt chalmers ore fluids were enriched in potassium and calcium but depleted in magnesium relative to seawater pixe microanalysis of fluid inclusions in quartz from the stringer zone also indicates a significant base metal concentration in these fluids cation composition and higher salinities relative to seawater suggest that recycled seawater cannot be the sole source of the ore fluids high base metal content and the presence of co2 in the fluid inclusions imply that magmatic input of ore metals copper in particular accompanying seawater leaching of the footwall volcanic pile is a distinct possibility in terms of fluid composition the k ca fe variation of the mt chalmers ore fluid is comparable with those of typical epithermal deposits e g thames new zealand and porphyry copper deposits bingham ut the cu zn fe 10 plot of the mt chalmers ore fluids indicates that there is significant copper in the system comparable to copper enrichment in a porphyry copper system the mt chalmers ore fluids also show similar copper content with the cu rich end member ore fluid composition of the mineralized stage 2b veins of the hellyer vhms deposit whereas the stage 2a veins of hellyer are more enriched in lead and zinc overall the ore fluids have a variable chemistry with a continuum of compositional data from vhms to epithermal porphyry style ore fluids shallow water emplacement 300 m for the vhms mineralization has been postulated for the mt chalmers deposit based on the presence of trace fossils in the footwall and hanging wall sedimentary rocks and volcanic facies studies however fluid inclusion studies do not rule out a moderate to deeper submarine environment as there is no fluid inclusion evidence of boiling at mt chalmers boiling of ore fluids would probably have occurred as in most epithermal systems if the ore fluid exhaled onto the seafloor under a shallow environment

major element constraints on melting differentiation and mixing of magmas from the galapagos 95 5 w propagating rift system projections in the pseudo quaternary system diopside plagioclase olivine quartz di pl ol q are used in this paper as a base on which to plot additional compositional petrologic or tectonic variables used in this manner the projections provide a sensitive means of displaying and evaluating the chemical variability of mid ocean ridge basalt morb suites such as that of the galapagos 95 5 w propagating rift system and allow mantle controls on the chemical variability of morb to be distinguished from the predominant effects of shallow level crystal fractionation and mixing primitive lavas mg 63 from the 95 5 w region form a broadly linear array within the di pl ol q tetrahedron parallel to the di ol join in terms of two recent high pressure studies of morb petrogenesis stolper 1980 takahashi and kushiro 1983 the primitive liquids in this array are not primary but have evolved by less than 10 olivine fractionation from an array of primary liquids which separated from residual mantle over a range of pressures between 10 20 kb with appropriate assumptions as to residual lithology and mg of residual olivine the nature and origin of 95 5 w primary magmas can be deduced 1 lavas of the normal rift are derived from a broad range of primary magmas that separated from compositionally uniform in terms of major elements mantle over a pressure range of 10 20 kb 2 beneath the newly forming propagating rift melting is initiated perhaps by pressure release at shallow depth 9 11 kb degree of melting and source composition appear similar to those for the normal rift within a few km of the propagating rift tip this shallow source is joined by a second deeper 11 12 kb more iron rich source both sources persist for at least 100 km along the propagating rift but the deeper source appears to become dominant farther behind the tip maximum pressure for melt separation anywhere beneath the propagating rift is less than 15 kb in addition to these source effects the projections allow a more complete description of the shallow level fractionation and mixing effects along the propagating rift primitive lavas of restricted composition erupted near the rift tip give way within 2 3 km to an increasingly diverse suite of lavas characterized by variable extents of fractionation and by numerous single mixing events consistent with magmatic evolution in small only occasionally interconnected magma bodies beyond 15 km the diversity gradually declines toward a restricted population of cpx saturated lavas which characterizes the propagating rift beyond 75 km and is consistent with the presence of a large well mixed magma chamber in this region

the transition from thick skinned to thin skinned tectonics in the basque cantabrian pyrenees the burgalesa platform and surroundings interpretation of seismic data in the margins of the burgalesa platform in the basque cantabrian pyrenees has allowed proposition of a new structural model that combines different modes of deformation during oblique tectonic inversion conditioned by the distribution of triassic salts deformation was decoupled by the presence of the salt horizon between basement involved thrusts inverting formerly triassic and late jurassic early cretaceous extensional faults and a detached thrust system involving the upper triassic to neogene sedimentary package structural units experiencing different styles of deformation are not only stacked vertically above and below the salt but most importantly they change from one to the other along strike across the transversal edges of the triassic salts the burgalesa platform detached thrust system was confined between the basement involved structures of the cantabrian mountains westward and the nw tip of the iberian basement involved structures san pedro southward this together with the obliquity between the pyrenean shortening direction and the strike of the previous extensional faults mostly during the late stages of deformation determined the strike slip reactivation of the basement involved inverted faults and the lateral extrusion of the burgalesa platform detached mesozoic successions above the salt towards the se to form a prominent thrust salient oblique to the main pyrenean trend the proposed model combines thick skinned with thin skinned structural styles during oblique tectonic inversion and is consistent with the surface data including the fracture system the available subsurface data and the mechanical stratigraphy

late hercynian polymetallic vein type base metal mineralization in the iberian pyrite belt fluid inclusion and stable isotope geochemistry s o h cl late variscan vein type mineralization in the iberian pyrite belt related to the rejuvenation of pre existing fractures during late variscan extensional tectonism comprises pyrite chalcopyrite quartz galena sphalerite quartz stibnite arsenopyrite quartz pyrite quartz cassiterite scheelite fluorite galena sphalerite chalcopyrite and quartz manganese oxide mineral assemblages studies of fluid inclusions in quartz stibnite and barite as well as the sulfur isotopic compositions of stibnite galena and barite from three occurrences in the central part of the iberian pyrite belt reveal compelling evidence for there having been different sources of sulfur and depositional conditions quartz stibnite mineralization formed at temperatures of about 200 c from fluids which had undergone two phase separation during ascent antimony and sulfide are most probably derived by alteration of a deeper lying volcanic hosted massive sulfide mineralization as indicated by 34s signatures from 1 45 to 2 74 sub critical phase separation of the fluid caused extreme fractionation of chlorine isotopes 37cl between 1 8 and 3 2 which correlates with a fractionation of the cl br ratios the source of another high salinity fluid trapped in inclusions in late stage quartz from quartz stibnite veins remains unclear by contrast quartz galena veins derived sulfide and metals by alteration of a sedimentary source most likely shale hosted massive sulfides the 34s values in galena from the two study sites vary between 15 42 and 19 04 barite which is associated with galena has significantly different 34s values 0 2 to 6 44 and is assumed to have formed by mixing of the ascending fluids with meteoric water

salinity of oceanic hydrothermal fluids a fluid inclusion study abstract an extensive microthermometric study of quartz epidote plagioclase anhydrite and sphalerite hosted fluid inclusions from ophiolitic semail oman and trinity california ophiolites and oceanic east pacific rise hydrothermal vents gorringe bank odp leg 111 hole 504b crust has been carried out in order to constrain a model accounting for wide salinity variations measured in the oceanic hydrothermal fluids recorded salinities in fluid inclusions vary between 0 3 and 52 wt nacl eq however more than 60 of the mean standard deviation salinities of the samples are within the range 3 2 0 3wt nacl eq microthermometric error and the mean salinity of all fluid inclusions without the brines is 4 0 wt nacl eq with a standard deviation of 1 6 wt nacl eq whereas most samples display slightly higher salinities than seawater several samples exhibit very high salinities more than two times that of seawater these high salinities are restricted to the plagiogranites semail and trinity ophiolites which mark the top of the fossil magma chamber in the transition zone between the plutonic sequence and the sheeted dyke complex the fluid inclusion population studied in the plagiogranites is characterized by the occurrence of four major fluid inclusion families 1 low to medium salinity liquid vapor fluid inclusions which homogenize into the liquid phase 2 low salinity liquid vapor fluid inclusions with pseudocritical homogenization 3 low to medium salinity liquid vapor fluid inclusions which homogenize into the vapor phase and 4 high salinity liquid vapor halite fluid inclusions which homogenize into the liquid phase by halite dissolution and exhibit salinities as high as 52 wt nacl eq these fluid inclusion families are interpreted as resulting from phase separation occurring in hydrothermal or magmatic fluids within the transition zone between the hydrothermal system and the magma chamber at temperatures higher than 500 c very low salinities less than half that of seawater have been found only in one dioritic sample from the gorringe bank mineralogical and petrological data from the semail ophiolite shows the absence of cl rich minerals furthermore hydration of the crust although very important cannot account for the large salinity variations thus the large salinity variations measured are interpreted as resulting from phase separation and generation of brine rich fluids at the top of the magma chamber this phase separation and subsequent phase segregation and brine accumulation may result in a double diffusive convection of the fluids in the hydrothermal system

fluid inclusion studies on a multiply deformed metamorphosed volcanic associated massive sulfide deposit joma mine norway meso and microscale structural analysis and the spatial distribution morphology and microthermometry of fluid inclusions reveal part of the fluid evolution of a multiply deformed metamorphosed volcanic associated massive sulfide deposit in the scandinavian caledonides norway the ore and host rocks have suffered greenschist facies metamorphism and four deformation events metamorphism peaked during d 2 to d 3 and waned to subgreenschist facies by d 4 of the four distinct fluid inclusion types from d 2 to d 4 structural settings primary inclusions occurring in crack seal growth zones type i in d 2 vein quartz are the only preserved inclusions related to d 1 to d 3 ore remobilization and vein growth all subsequent inclusions are secondary within healed microfracture space type ii secondary inclusions exist in vein quartz regardless of structural setting their formation cannot therefore predate d 4 both type i and type ii inclusions underwent a pervasively destructive decrepitation event syn to more probably post d 4 the leakage and or decrepitation is attributed to internal overpressures accompanying rapid decompressive regional uplift after the d 4 event two populations of postdecrepitation inclusions types iii and iv have consistent vapor to liquid ratios and yield low temperatures t h 137 degrees 297 degrees c and low salinities 0 6 7 wt nacl equiv they possibly record flushing by meteoric water late in the uplift history

sedimentary succession and palaeoenvironments within a fault controlled basin the wealden of the santander area northern spain abstract the so called wealden sediments are no less than 2000 m thick in santander and north burgos northern spain the succession rests on an extensive scoured and channeled surface over marine carbonates of early callovian age and ends with the incoming of the urgonian marine strata and equivalents lower aptian two major sedimentary cycles megacylothems and the beginning of a third are recognized following a widespread break in the succession each megacyclothem opens with a coarse grained clastic interval of fluviatile origin and then grades up into finer grained clastics and carbonates recording fluviatile lacustrine and shallow marine conditions the succession accumulated in an elongated fault bounded basin trending e w detritus came from source areas lying to the south and west of the depositional area which was at times connected with the sea to the east and possibly to the north the development of the basin was controlled by the reactivation of late hercynian faults acting contemporaneously with sedimentation the frequency of faulting varied considerably periods of relative calm alternating with periods of strong activity these determined the characteristics of the successive megacyclothems

along strike variation of the sheeted dike complex in the oman ophiolite insights into subaxial ridge segment structures and the magma plumbing system 1 along strike variations of the 60 km long sheeted dike complex in the northern oman ophiolite were studied in order to understand the shallow magma plumbing system beneath the fossil fast spreading ridge the presence of numerous dikes intruding into the layered gabbro defines the northern end of the paleoridge segment at wadi fizh the postulated segment center is located at wadi thuqbah which has a thicker mono transition zone than elsewhere along the fossil segment aphyric dikes predominate in the sheeted dikes of which 99 are simple while multiple and composite dikes are few the thickness of 1511 dikes ranges from 13 m with an average thickness of 71 3 cm restored dike trends in the 30 km long northern half of the dike complex display the ns trending north domain and nne ns trending south domain bordered at the south of wadi bani umar al gharbi in the domain boundary dikes gradually change strikes or are mutually intrusive dike thickens northward with the largest peak along wadi fizh at the northern end of the paleoridge segment and a small peak at wadi bani umar al gharbi most dikes have a bulk mg of 55 66 which overlaps the majority of morb less common highly evolved dikes with mg 34 40 characterize the north domain thicker dikes 3 m tend to have high mg while thinner dikes 2 m are highly variable in mg the regional variations of the dike trends and the whole rock compositions can be explained by the secular variation in the structures of the paleoridge segment comparable to a third order segment of the present mid ocean ridge system spanning a few tens of thousands of years initially the segment long melt lens developed along the paleoridge axis which fed long and thick dikes with high mg with decreasing supply of magma the melt lens split up into the north and south smaller lenses bordered by a deval that fed thinner dikes intruding into the former thick dikes cut off of the northern melt lens from the magma source changed the melt composition to highly evolved low mg magmas which subsequently intruded as short evolved dikes meanwhile the main melt lens in the segment center continued feeding the high mg dikes which were maintained by the larger melt lens size and intermittent magma supply from deep magma chambers

magmatic evolution and tectonic setting of the iberian pyrite belt volcanism rocks has been carried out to determine the primary magmatic aynity tectonics iberian pyrite belt and tectonic setting of the iberian pyrite belt volcanism most of the basaltic rocks are continental tholeiites but a few samples show an alkaline aynity the origin of the basaltic rocks and their diversity of compositions are explained by a single mixing model between e and n morb mid ocean ridge basalt and as introduction similation of crustal material calc alkaline intermediate and silicic volcanic hosted massive sulphide deposits are mainly rocks include basaltic andesites andesites dacites and rhyolites associated world wide with calc alkaline submarine volvolumetrically dacites and rhyolites are the most abundant inter canism petrological and geochemical data together with mediate and silicic rocks are not related by fractional crystallization stratigraphic and structural studies have been of major nor is there a relationship between the basaltic and calc alkaline importance in constraining geological models of the volrocks by the same process we suggest that in the iberian pyrite canism associated with massive sulphide deposits inbelt silicic calc alkaline magmas were generated on a large scale tegrated studies in areas such as the kuroko province in by the invasion of continental crust by mafic magmas generated in japan ohmoto 1983 the mount read volcanics in the underlying upper mantle the diversity of compositions shown tasmania crawford et al 1992 and the mount windsor by dacites and rhyolites can mainly be explained either by diverences volcanics in northwestern australia stolz 1995 have in the composition of the source rocks or by diverent degrees of revealed that this volcanism may be developed during partial melting of upper crust rocks andesites however formed by diverent stages of the subduction process always being mixing between basaltic magmas and upper crust material the new located on the overriding plate geochemical data agree with previously published tectonostratigraphic this paper documents the petrology and geochemistry data which suggest that the iberian pyrite belt volcanism formed of the volcanism of the iberian pyrite belt an early on the south portuguese plate owing to strike slip tectonics this carboniferous metallogenic province that extends from portugal to spain in southwest iberia and constitutes local extensional tectonic setting was related to transtension as a