petrology geochemistry and na metasomatism of triassic jurassic non marine clastic sediments in the newark hartford and deerfield rift basins northeastern usa many sandstones and associated siltstones and shales of the triassic jurassic newark supergroup in the newark hartford and deerfield rift basins are immature feldspathic alluvial and lacustrine closed basin deposits they were derived from felsic igneous and metamorphic continental blocks on the flanks of their depositional basins the provenance was dominantly calc alkaline basement rocks similar to the sierra nevada of california the physical and chemical environments of erosion and deposition were probably similar to those in moist pleistocene holocene alluvial basins and playas in the great basin of california and nevada new petrographic and geochemical data for 113 samples indicate that albite rich sandstones and shales of the stockton lockatong and passaic formations in the newark basin and in the portland and east berlin formations of the hartford basin and analcime rich shales up to 52 normative analcime of the lockatong formation have unusually high na2o contents 4 7 textural evidence indicates both sodium replacement in feldspars and formation of authigenic albite and analcime in these rocks by comparison with modem sediments derived from similar provenance using mixing models it is estimated that up to 4 na2o was diagenetically added to the detrital clastics trace metals co cr cu ni zn abundances were apparently enhanced by deposition from brines in the lockatong formation analcime deposition occurred in shales with al2o3 na2o sio2 0 46 this ratio is found only in sediments to which na has been added from salts brines source rocks and the weathering products derived from them may be considered as a closed system in which sodium was retained partly in solids as feldspar and part went into solution during weathering the dilute solution migrated into the depositional basin where evaporation yielded concentrated brine during semi arid to arid climate in the drier parts of 21 000 yr milankovitch cycles reaction of brine during diagenesis at near surface and or shallow depths 2 km with ca plagioclase k feldspar and layer silicates at estimated temperatures 50 c to 100 c and ph 7 9 yielded thermodynamically stable authigenic albite and analcime in the sodium metasomatised clastic sediments low activity of k precluded development of authigenic k feldspar however k from solution incorporated into muds and silts as smectites were transformed to illite some lockatong beds contain anomalously high p2o5 ce la y th and u in authigenic apatite or monazite suggestive of mobilization of these elements in diagenesis

contrasting low and high ca granites in the archean barberton mountain land southern africa abstract two distinct groups of granites can be recognized among the late plutons of the barberton mountain land discrimination between the two groups is provided by geochemical and mineralogical parameters which are considered to reflect the source material from which the magmas were derived a high ca suite has geochemical parameters consistent with derivation from an igneous source i type and an accessory mineral assemblage comprising zircon apatite allanite and sphene a low ca suite contains distinctly different accessory minerals comprising zircon apatite zirconosilicates ca th phosphates very rare xenotime and early monazite and has a chemical signature consistent with derivation from a metasedimentary precursor s type major and trace element trends on harker diagrams support the existence of two distinct granite suites the i type suite exhibits inverse relationships between sio2 and cao tio2 fe2o3 mgo and p2o5 while al2o3 k2o and na2o remain relatively constant these characteristics are consistent with magma formation from an intermediate parent composition and in equilibrium with a hornblendite restite the s type granites formed by vapour absent melting of siliceous metapelite and exhibit inverse correlations between sio2 and al2o3 na2o and k2o with the other major elements remaining relatively unchanged this interpretation of restite unmixing is not unambiguous since rb sr trends could be viewed as reflecting crystal fractionation and partial melting processes high field strength elements tend to reflect the character of accessory mineral phases and are not amenable to modelling in terms of crystal melt equilibria the recognition of possible s and i type suites among the late granite plutons of the barberton mountain land supports recent models which propose plate tectonic analogues in the region the two granite suites are distributed along two subparallel linear arrays which suggests scenarios invoking subduction or hot spot related magmatism with the i types forming in the time span 2740 2690 ma along a western array the putative s types occurring entirely along an easterly belt were emplaced at 3074 and 2820 ma the presence of s type plutons as old as 3074 ma implies that sediments such as the moodies and fig tree groups or their equivalents had been buried to depths of at least 15 km and that by this time at least in the barberton area granites had evolved beyond the ttg mode that typifies many early archaean shield areas

precise u pb titanite age constraints on the emplacement of the bushveld complex south africa the c 2 06 ga bushveld complex intrudes the transvaal supergroup south africa calcsilicate xenoliths within the mafic phase of the bushveld complex rustenburg layered suite or rls preserve calcsilicate xenoliths with high temperature c 1200 c mineralogies that were later metasomatized by hydrous retrograde c 600 700 c fluids whose timing has been unconstrained new u pb isotope data from newly grown titanite within a completely retrogressed xenolith indicates that retrogression occurred at 2058 9 0 8 ma this suggests that retrogression was due to hydrothermal circulation associated with the cooling rls and not slightly later bushveld related granites the new data place a tight constraint on the minimum age of emplacement of the rls

3 07 2 97 ga greenstone belt formation in the northeastern kaapvaal craton implications for the origin of the witwatersrand basin

metallogeny of precious and base metal mineralization in the murchison greenstone belt south africa indications from u pb and pb pb geochronology the 3 09 to 2 97 ga murchison greenstone belt is an important metallotect in the northern kaapvaal craton south africa hosting several precious and base metal deposits central to the metallotect is the antimony line striking ene for over 35 km which hosts a series of structurally controlled sb au deposits to the north of the antimony line hosted within felsic volcanic rocks is the copper zinc line where a series of small ca 2 97 ga cu zn volcanogenic massive sulfide vms type deposits occur new data are provided for the malati pump gold mine located at the eastern end of the antimony line crystallizations of a granodiorite in the malati pump mine and of the baderoukwe granodiorite are dated at 2 964 7 and 2 970 7 ma respectively zircon u pb while pyrite associated with gold mineralization yielded a pb pb age of 2 967 48 ma therefore granodiorite emplacement sulfide mineral deposition and gold mineralization all happened at ca 2 97 ga it is thus suggested that the major styles of orogenic au sb and the cu zn vms mineralization in the murchison greenstone belt are contemporaneous and that the formation of meso to epithermal au sb mineralization at fairly shallow levels was accompanied by submarine extrusion of felsic volcanic rocks to form associated cu zn vms mineralization

geochemical and paleomagnetic evidence for the occurrence of missing sapropels in eastern mediterranean sediments the cyclic occurrence of sapropels appears to be associated with maxima in the 65 n summer insolation target curve studies of the most recent sapropel have revealed extensive oxidation of organic matter or even the complete removal of this unit geochemical and magnetic signals allow for the detection of such a missing sapropel in older sediments these signals are altered by ongoing diagenetic processes an important parameter to reveal missing sapropels is ba elevated concentrations of ba are always found in visible sapropels and even after oxidation of part of the sapropel the initial ba profile remains visible we use this tool to trace down missing sapropels in kc01b a 37 m kullenberg core from the calabrian ridge eastern mediterranean at least 8 of 11 sampled intervals around insolation maxima contain evidence of a completely oxidized sapropel ba trace metal distributions and magnetic parameters are similar to those observed around visible sapropels

porphyry intrusions and albitites in the bardoc kalgoorlie area western australia and their role in archean epigenetic gold mineralization minor intrusions in the menzies kambalda greenstone belt of the archean eastern goldfields province western australia range from quartz feldspar porphyry to plagioclase hornblende porphyry the porphyries display enrichment of mobile and incompatible elements k to zr and depletion of relatively compatible elements with negative nb p and ti anomalies on mid ocean ridge basalt normalized spidergrams the composition and timing of emplacement of the porphyries are consistent with a genetic relationship with spatially related granitoids porphyries occur in 30 of gold mines in the menzies kambalda belt the association appears to be largely structural since both the intrusions and the mineralizing fluids exploit zones of weaknesses such as lithological contacts and shear zones porphyries have been modified to varying degrees by hydrothermal alteration especially pervasive albitization textural evidence indicates that secondary albite and associated sodic amphibole formed late in the deformatio

characterization of multiple fluid granite interaction processes in the episyenites of avila bejar central iberian massif spain episyenites are small granite hosted pipe like masses characterized by hydrothermal alteration and the major loss of silica compared with the host granites such bodies are fairly common in the hercynian granites of western europe this study deals with late hercynian amphibole bearing and biotite granodiorites monzogranites from the central iberian massif using petrological geochemical major oxides trace elements and ree and isotopic o h data to constrain the paragenesis of alteration the apparent changes in bulk composition and the sources and conditions of fluids involved in fluid rock interactions episyenitization leads to reduction of quartz content albitized plagioclases and chloritized ferromagnesian phases and the process results in increasingly ordered alkali feldspars there are noticeable variations in whole rock na20 k2o mgo cao rb and sr contents with sio2 in some cases behaving differently depending on whether the episyenite is hosted in amphibole granite or biotite granite the lree y zr and nb have also been modified in those syenites associated with amphibole bearing granites this being attributed to destabilization of accessory minerals and recrystallization of new stable mineral assemblages the process is complex and two generations of chlorite early high fe and late low fe are observed whole rocks and mineral separates were studied for o and h isotopes the data rule out meteoric fluids as the cause of syenitization and the later resiliciftcation of some and suggest that at least two different fluids were involved in generating the syenites the first is interpreted to be a high temperature 300 450 c late magmatic deuteric fluid as recorded by feldspar o isotope data later a different fluid with 18o 1 4 and d 13 isotopically indistinguishable from seawater on a deglaciated earth produced the chlorite secondary quartz and minor epidote such a fluid was only available for a short period since feldspars did not re equilibrate the fault systems occupied by the episyenites provided suitable pathways for fluids of both shallow and deep origins over a long term during the carboniferous

uranium deposits associated with na metasomatism from central ukraine a review of some of the major deposits and genetic constraints abstract the paper reviews geological structural mineralogical fluid inclusion geochemical and stable isotope characteristics of u deposits related to na metasomatism in the central ukrainian uranium province cuup the province contains numerous u deposits hosted in gneisses migmatites and granites of the ingul paleoproterozoic megablock and in iron formations along its margin with the fore mid dnieper mesoarchean megablock the richest u deposits are distributed along the nearly e w striking subotin mashorin tectonic zone but they are tightly controlled by n s and more rarely sw ne striking crustal scale shear zones na metasomatism and u mineralisation were controlled by protracted tectonic activity along these shears zones from ductile to brittle regimes of deformation a metasomatic zoning is developed from outermost unaltered lithologies inwards to dequartzified rocks episyenites aegirine albitites na metasomatites andradite epidote ca amphibole calcite rich rocks ca mg metasomatites and ultimately to biotite albitites k metasomatites main u ore deposition predominantly consisting of brannerite and uraninite with lesser amounts of u ferropseudobrookite is associated with the ca mg and k metasomatised zones superimposed on earlier aegirine albitites na metasomatites were formed at temperatures exceeding 500 c possibly triggered by the onset of a plume in the mantle associated with the intrusion of the highly potassic korsun novomyrgorod rapakivi granitic pluton at about 1750 1800 ma as constrained by u pb geochonologic data on the granites and uranium mineralisation these intrusions have been partly reworked tectonically with recrystallisation under lower p t conditions the mineral paragenesis of ca mg and k metasomatites crystallised during the cooling of the system down to 370 340 c ca mg and k metasomatisms resulted in further dequartzification of the rocks and their enrichment mainly in ca mg k u zr fe 2 and hf the hydrothermal fluids were oxidised moderately saline and highly alkaline the high alkalinity of the fluids explains the dissolution of quartz the deposition and stability of albite the high zr hf ratios up to 224 in the metasomatites and the high solubility of u and zr u deposition was controlled by mixing with a reduced fluid which explains the increase of whole rock fe 2 concentration during uranium deposition the last widespread hydrothermal event corresponds to a chlorite epidote paragenesis superimposed on all previous mineral associations at temperatures down to less than 100 c h o and c isotopic ratios in minerals and fluid inclusions extract from albitites suggest a surface formation water origin for the hydrothermal fluids producing na metasomatism and a mixed metamorphic surficial formation water origin for the fluids producing ca mg and k metasomatism and the main u mineralisation

albitization in the antimony line murchison greenstone belt kaapvaal craton a geochemical and geochronological investigation abstract the 3 09 2 97 ga murchison greenstone belt is one of several archean volcano sedimentary belts within the kaapvaal craton in southern africa among the diverse ore deposits found within the belt a set of sb au deposits are located in a major quartz carbonate altered brittle ductile structure known as the antimony line the antimony line is thus clearly related to hydrothermal fluid circulation in this study we focus on albitites that run along the antimony line petrological and geochemical investigations indicate that albitization developed at the expense of a granodioritic protoliths under high fluid rock ratios and also that sb enrichment was concomitant with albitization oxygen isotopes on albitites point to a crustal origin for the hydrothermal fluid responsible for the albitization process geochronology on zircon and hydrothermal monazite identifies a 2 97 2 92 ga magmatic crystallization event disconnected from a ca 2 8 ga hydrothermal alteration and a potentially younger event around 2 0 ga these data highlight a likely magmatic related primary enrichment at 2 97 2 92 ga followed by a secondary metamorphic related mobilization event around 2 8 ga leading to a sb enrichment in the albitites and a last episode of alteration around 2 0 ga

regional scale cretaceous albitization in the pyrenees evidence from in situ u th pb dating of monazite titanite and zircon abstract albitization is a common metasomatic process active in various geodynamic contexts in the northern pyrenees there are several occurrences of albitites but until now only one occurrence has been dated 117 ma ar ar dating this paper presents new u th pb ages for several albitite occurrences throughout the pyrenees to test whether they are contemporaneous and if so to specify the regional extent of the albitization event ages obtained from large euhedral titanite and monazite grains from distinct albitites are 110 8 and 98 2 ma respectively the zircon u th pb isotopic system did not record this cretaceous metasomatic event even when grains were selected in metasomatically zr enriched rocks or in hydrothermal structures millimetre sized veins cross cutting granitoids we argue that the total time span of 20 ma recorded by albitites corresponds to a long lived hydrothermal system that was active during the rotation of iberia around europe along the north pyrenean fault because albitization and talc mineralization have the same spatial and temporal distribution in the pyrenees we argue that these two metasomatic phenomena are two independent records of this single regional scale long lived hydrothermal system

role de la deformation du granite dans la genese des episyenites feldspathiques des massifs de lovios geres galice et des zaer maroc central l etude de la deformation du granite dans les zones encaissant les bandes d episyenites feldspathiques de deux massifs hercyniens a permis de degager un modele tectonique rendant compte des caracteristiques structurales de celles ci les bandes d episyenites feldspathiques se forment lors des phases ultimes de la mise en place des intrusions granitiques sous l effet d un champ de contrainte local susceptible d engendrer des surpressions fluides aboutissant a une fracturation de type hydraulique la genese des mineralisations filoniennes intragranitiques a tungstene etain associees spatialement a ces episyenites doit etre rapportee a des evenements tectoniques et a des circulations fluides posterieures

chemical zoning and homogenization of pasamonte type pyroxene and their bearing on thermal metamorphism of a howardite parent body we studied thermal metamorphism in the surface layer of a eucrite parent body by examining pyroxene zoning ca mg fe zoning of pyroxenes in the pasamonte eucrite can be classified into three types type 1 with a core composition of ca5mg62fe32 and rim of ca41mg20fe4o type 2 with core of ca5mg62fe32 and rim of ca25mg34fe41 and type 3 with core of ca5mg66fe29 and rim of ca7mg32fe61 type 1 typically occurs adjacent to a mesostasis type 2 typically occurs adjacent to a surrounding plagioclase and type 3 is common in pyroxenes in the matrix pasamonte is less metamorphosed than previously believed because of the wide range of composition of zoned pyroxenes on the basis of ca mg fe variation data we have calculated the degree of homogenization of zoned pyroxenes by numerically solving the diffusion equation a pasamonte type pyroxene may yield a juvinas type pyroxene by thermal metamorphism the homogenization of juvinas pyroxene could not have taken place during initial cooling and may be related to reheating events although our calculation rests upon some assumptions

sodium calcium metasomatism chemical temporal and spatial relationships at the yerington nevada porphyry copper deposit tilting associated with extensional tectonics has exposed 1 800 m of continuous palcovertical relief through the jurassic age yerington porphyry copper deposit the rarely observed root zones of a porphyry system are characterized at yerington by sodium calcium metasomatism the principal hydrothermal reactions involve the replacement of primary k feldspar by oligoclase and of primary biotite by actinolite copper has been removed sodic calcic alteration developed at deep intrusive contacts nearly simultaneously with potassic alteration and copper mineralization at shallow intrusive contacts the reactions accompanying potassic alteration are the reverse of those accompanying sodic calcic alteration plagioclase reacted to form k feldspar hornblende reacted to form biotite the potassic and sodic calcic alteration couplet principally accompanied each of two separate intrusive events 1 luhr hill granite stock and associated porphyry dikes and 2 walker river granite porphyry dikes the younger walker river intrusion was emplaced at slightly deeper levels than the older luhr hill complex vertically stacked and partially superimposed alteration and mineralization patterns resulted from differences in depth and in timing of emplacement at lower temperatures late sodic alteration was superimposed on early sodic calcic alteration at deeper levels in the system whereas late quartz sericite pyrite alteration was superimposed on potassic alteration at higher levels sodic alteration is characterized by the replacement of k feldspar by albite and of biotite by chlorite copper deposited during potassic alteration was remobilized the transition to lower temperature alteration is associated only with the walker river intrusion the hydrothermal system associated with the emplacement of the luhr hill complex had not evolved to lower temperature assemblages prior to emplacement of the walker river dikes phase equilibria and geologic reconstruction of depth of emplacement suggest that temperatures during early sodic calcic alteration were limited to the range of 360 degrees to 480 degrees c at pressures between 300 and 800 bars late sodic alteration took place at similar pressures but at temperatures less than 360 degrees c nearly simultaneous development of potassic and sodic calcic alteration resulted from the flow of fluids away from intrusions at shallow levels potassic alteration and from the flow of fluids toward intrusions at deeper levels sodic calcic alteration because k na and na ca activity ratios of a fluid in equilibrium with granitic rocks decrease with decreasing temperature cooling of high temperature fluids can generate potassic alteration and heating of fluids to high temperatures can generate sodic or sodic calcic alteration

retrograde fluid infiltration in the high grade modum complex south norway evidence for age source and ree mobility the high grade metamorphic basement of the modum complex south norway exhibits retrogradation and alteration due to late stage fluid infiltration extensive alteration zones of albite and calcite rich veining occur especiallywithin and around numerous metagabbros the gabbros intruded at 1224 15ma are now partly altered to amphibolites due to the subsequent high grade metamorphism two generations of albite rich rocks have been recognized 1 a fine grained foliated type 2 a coarse grained crosscutting type both types show a typical greenschist facies mineralassemblage albite actinolite chlorite talc the calcite veins dykesrepresent a younger generation of veins than both albite rich types u pb data for spheneof type 1 yielded an age of 1080 3 ma determining a point on the retrogradep t t path of the modum complex increasing albitisation of themetagabbros leads to a decrease in 143nd 144nd oand an increase in 87sr 86 o albite and calcite rich samples show negative nd and positive sr suggesting that fluids which interacted with the metagabbros originated from a crustal reservoir the nd and sr isotopic data show disequilibrium at the microscale as well as at the macroscale negative sm nd model ages of the albite rich rocks demonstrate that rare earth elements rees were mobile and fractionated during albitisation

tin bearing sodic episyenites associated with the proterozoic a type agua boa granite pitinga mine amazonian craton brazil this paper reports the first occurrence of tin mineralized episyenites in the amazonian craton the agua boa and madeira plutons in the pitinga region are stanniferous proterozoic a type granites of the rapakivi series featuring metasomatic episyenitization the biotite granite facies of the agua boa pluton is metasomatically altered to sodic episyenite and minor potassic episyenite and micaceous episyenite the sodic episyenites formed by a albitization of k feldspar b vug formation by dissolution of magmatic quartz c vug filling by albite chlorite lithian muscovite cassiterite fluorite k feldspar and d deposition of late quartz cassiterite in remaining cavities in the potassic episyenites vugs produced by quartz dissolution are filled by secondary k feldspar which also replaces magmatic phases micaceous episyenites formed by zinnwaldite replacement of secondary albite in the sodic episyenites in contrast to the sodic episyenites the potassic and micaceous episyenites contain only traces of cassiterite relative to unaltered biotite granite the sodic episyenites are strongly depleted in sio2 k2o lree and sr moderately depleted in hree and enriched in na2o al2o3 rb and sn the potassic episyenites resemble the sodic episyenite in sio2 and al2o3but exhibit higher values of k2o ba y and rb their na2o ree and sn concentrations match those of the biotite granite host metasomatic episyenitization is attributed to 1 a high temperature gradient and fluid rock ratio during subsolidus cooling of the agua boa pluton 2 quartz dissolution in the biotite granite facies by reaction with an high temperature silica undersaturated fluid of probable magmatic origin 3 hydrothermal cavity filling by a low temperature silica saturated fluid of meteoric or mixed magmatic meteoric character significant tin values in metasomatic sodic episyenites near the pitinga mine suggest that exploration of similar rocks in other a type granites may be worthwhile

cretaceous albitization and dequartzification of hercynian peraluminous granite in the salvezines massif french pyrenees abstract the salvezines massif in northern french pyrenees has undergone extensive albitization the massif consists of gneisses and a leucogranite hosted in paleozoic sediments schists and carbonates the leucogranite was emplaced at the end of the hercynian orogeny the granite underwent monazite fractionation during magmatic stages which induced lowering of ree and th contents in the most evolved rocks hydrothermal alteration during late magmatic stages is also identified through the development of a tetrad effect in the ree patterns of the granite and through the fractionation of ratios like zr hf out of the charac charge radius controlled range the granite underwent partial to complete sub solidus albitization progressive metasomatic overprint induced metasomatic replacement of feldspars into albite and dissolution of quartz episyenitization together with new albite formation hydrothermal muscovite with a sheaf like structure sometimes crystallised in albitites inherited muscovites from magmatic stage and newly formed muscovites from albitites have distinct chemical compositions with the latter being much more phengitic the initial peraluminous chemistry of the leucogranite was lost during albitization and composition evolved towards a pure albite one a cnk 1 albitization induced gains in na and al and losses in si and k leaching of ree and u is also identified during albitization as well as the non charac behaviour of some elements 18o values of quartz and muscovite in albitized granites are in the magmatic range about 12 and 9 5 respectively feldspars acquired high 18o values during albitization up to 15 muscovite grains from albitites have higher 18o values than inherited ones and tend towards isotopic equilibrium with albite the albitizing fluids have interacted with high 18o rocks probably local carbonates prior to alter granite muscovites inherited from magmatic stages display very characteristic disturbed 39ar 40ar age spectra with saddle shapes when a muscovite associated with albitization provided a plateau age at 117 5 ma for this alteration event albitization in the salvezines massif just preceded the regional talc chlorite hydrothermal mineralization both events might then represent two outward signs of a huge hydrothermal system at the time of the rotation of the iberian plate around europe the north pyrenean metamorphism is identified by 39ar 40ar analyses at ca or younger than 100 ma

the significance of rb sr ages of biotite and phlogopite for the thermal history of the central and southern marginal zones of the limpopo belt of southern africa and the adjacent portions of the kaapvaal craton abstract generally rb sr biotite and phlogopite ages range from 2050 ma to 1950 ma between the murchison thabazimbi and triangle tuli sabi shear zones of southern africa in the rocks comprising the central and southern marginal zones of the limpopo belt and adjacent portions of the kaapvaal craton in some shear zones however similar ages ocurr as young as 800 ma in contrast within the kaapvaal craton south of the murchison thabazimbi shear zone rb sr biotite ages range between 3150 ma and 2100 ma and show no regular distribution in all cases the ages are interpreted to indicate the time elapsed since the biotite last cooled below its closure temperature approximately 320 c the regular pattern of 2050 ma to 1950 ma rb sr biotite ages north of the murchison thabazimbi shear zone suggests that the present erosional surface approximates a paleo isothermal surface this surface was exposed by uplift and erosion which was completed by 1900 ma to 1800 ma ago when the sedimentary and volcanic rocks of the soutpansberg group were deposited the regional uplift was accommodated by motion on the murchison thabazimbi and triangle tuli sabi shear zones and probably was related to tectonism associated with the emplacement of the anorogenic bushveld and phalaborwa complexes other anorogenic granitic magmatism accompanied uplift the depth represented by the paleo isothermal surface and the magnitude of the uplift that exhumed it cannot be constrained precisely it is suggested however that the majority of the 10 to 15 km by which crust in this area is thinner than that of the adjacent cratons is the manifestation of this uplift

juvenile crust formation in the northeastern kaapvaal craton at 2 97 ga implications for archean terrane accretion and the source of the pietersburg gold abstract combined u pb and lu hf isotope data of detrital and magmatic zircon grains provide evidence that the murchison greenstone belt ne kaapvaal craton south africa hosts an important archean suture zone which primarily formed by a collision 2 97 ga ago between a back arc system to the north and an evolved terrane the proto kaapvaal craton to the south this model is supported by the observations that igneous rocks of the northern terrane comprising mafic rocks and tonalites of the rooiwater complex and felsic volcanics of the rubbervale formation were both emplaced at about 2 97 ga and show highly superchondritic hft of 4 4 to 5 1 in contrast u pb hf isotope data of detrital zircon grains from quartzitic schists of the murchison unit and the la france formation both part of the southern terrane provide evidence for a southern provenance which was affected by granitoid intrusions accompanied by crust re working at 3 53 3 42 ga hft 1 8 to 4 8 3 30 3 20 ga hft 1 8 to 6 3 and 3 13 3 05 ga hft 1 3 to 5 6 pointing to a connection with the barberton greenstone belt and swaziland a spatial separation of the two terranes prior to 2 97 ga is furthermore supported by the facts that the youngest detrital zircon grains of the southern terrane having ages between 2 99 and 2 97 ga show subchondritic hft between 6 5 and 1 5 in contrast to the highly superchondritic hft obtained from the contemporaneous magmatic rocks of the northern terrane the juvenile character of the rooiwater and rubbervale magmatic rocks along with the occurrence of vms deposits makes them a likely candidate for the source of the gold pyrite bearing conglomerates exposed in the adjacent pietersburg greenstone belt

albite rich silica depleted metasomatic rocks at emuford northeast queensland mineralogical geochemical and fluid inclusion constraints on hydrothermal evolution and tin mineralization late paleozoic fluorine rich granites and their host metasediments in the emuford district of the herberton tin field contain small zones of intense albitization that host cassiterite mineralization sodium metasomatism was accompanied by hydrothermal leaching of granite quartz to form vuggy albite rocks episyenites vugs created by quartz dissolution were filled in by hydrothermal minerals including albite k feldspar and or muscovite and cassiterite late phases are dominated by quartz and fluorite swarms of parallel veinlets crosscut the albitized zones these veinlets were formed by repeated fracturing in response to local fluid over pressures they contain infill mineralogy similar to the vugs and probably formed at least partly contemporaneously with vug infill due to the overprinting at various scales of alteration vug infill and vein formation stages chemical changes associated with episyenite formation are only qualitatively constrained however the mineralogical and textural features of the alteration zones suggest that most major and trace elements had substantial mobility during their formation major chemical changes associated with episyenite formation include leaching of sio 2 k 2 o rb and ba with introduction of na 2 o and minor sr during vug and vein filling stages there was an introduction of sio 2 al 2 o 3 na 2 o k 2 o cao f and sn the erratic behavior of ree is probably related to the growth of fluorite and minor accessory phases zircon monazite xenotime with preferential enrichment of heavy ree in rocks with the highest fluorine concentrations fluid inclusion study indicates that vug and vein filling stages of the hydrothermal evolution involved two types of fluids a high salinity fluid 30 50 equiv wt nacl of probable magmatic origin and homogenization temperatures up to 428 degrees c was associated with vug filling in the granite hosted deposits a lower salinity fluid 5 15 equiv wt nacl containing variable amounts of co 2 and having homogenization temperatures between 160 degrees and 420 degrees c was associated with veinlet formation in the granite hosted deposits and with vein and cavity filling in the metasediment hosted deposit this lower salinity fluid may represent a condensed vapor phase derived from boiling of the magmatic fluid or may be a mixture of magmatic and meteoric components the close association of albitization and quartz dissolution suggests a genetic link between the two processes it is proposed that exchange of na for k in the fluorine rich hydrothermal fluids caused a substantial increase in quartz solubility and led to quartz dissolution in the albitized rocks

mineralogical and geochemical characteristics of hydrothermal alteration and episyenitization in the konigshain granites northern bohemian massif germany the late hercynian granites of konigshain underwent multistage hydrothermal processes extensive high temperature late magmatic alteration is for example indicated by low zr hf and an ree pattern displaying the tetrad effect intensive post magmatic alteration of the granite occurred along brittle structures at least two main stages of post magmatic hydrothermal alteration are involved the first high temperature stage which is characterized by albitization and or quartz leaching episyenitization resulted from fluid rock interaction with late magmatic fluids that very probably mixed with external low salinity fluids quartz dissolution was triggered by vapour condensation and or the cooling of these fluids below 450 c along brittle structures the high porosity resulting from quartz leaching during stage 1 assisted subsequent circulation of low temperature fluids at stage 2 the latter is characterized by the chloritization and illitization of episyenites almost all major and trace elements were enriched or depleted during one of the main alteration stages however zr hf th and ti were immobile during post magmatic alteration the significant depletion of lree and the enrichment of hree in albitized samples is controlled by the dissolution of monazite and the new formation of hree rich polycrase y or aeschynite y during post magmatic stage 1 negative ce anomalies of episyenites are associated with illitization and suggest oxidizing conditions during stage 2

structure and metamorphism of the archean murchison belt kaapvaal craton south africa critical examination of five popular prejudices applied to greenstone belts in general and the murchison belt in particular show that 1 the term murchison schist belt is preferred to that of murchison greenstone belt which differs significantly from the barberton greenstone belt 2 the murchison belt is tectonized such that no simple stratigraphy may be erected 3 the murchison belt lacks a synclinal keel 4 heterogeneous oblique to dip slip shear characterizes much of the murchison belt but strike slip deformation occurred in the letaba shear zone along the northeastern margin of the belt and 5 contact metamorphism was a limited phenomenon that was not responsible for the highest grade rocks of the murchison belt the belt comprises a number of domains each with its own characteristic lithologies structural style and metamorphic history bounded by tectonic breaks which have juxtaposed rocks from different crustal levels

ore forming fluids in archean gold bearing quartz veins at the sigma mine abitibi greenstone belt quebec canada archean gold bearing quartz veins at the sigma mine occur in deformed metavolcanics intruded by two generations of preore porphyries the veins were emplaced during a late compressional tectonic event and they consist primarily of quartz tourmaline carbonates pyrite and scheelite vein minerals have been deformed and are also cut by abundant healed fractures outlined by fluid inclusions in the veins gold typically occurs in late fractures and in recrystallized quartz microthermometric sem eda leachate analysis and gas chromatographic techniques were used to study fluid inclusions in quartz most fluid inclusions at sigma occur in late fractures and are secondary the fluid inclusions are commonly found in fractures parallel to or less commonly within gold bearing fractures and are genetically related to the gold three types of fluid inclusions are identified 1 high salinity na ca cl aqueous inclusions 25 34 wt salt with small amounts of co 2 some containing halite calcite and nahcolite as daughter minerals 2 co 2 rich inclusions containing small amounts of h 2 o and ch 4 and 3 low salinity h 2 o co 2 inclusions 10 equiv wt nacl with co 2 contents which are variable but commonly in the range of 15 to 30 mole percent co 2 leachate analyses show that in addition to na and ca the aqueous fluids contain minor amounts of k fe mg and li homogenization temperatures of h 2 o co 2 inclusions with constant phase ratios range from 285 degrees to 395 degrees c those of aqueous inclusions range from 60 degrees to 295 degrees c the aqueous and co 2 rich inclusions are interpreted to have trapped two coexisting fluids resulting from the unmixing of a homogeneous h 2 o co 2 parent fluid itself occasionally trapped in late fractures the ore bearing fluid is interpreted to be a low salinity 10 equiv wt nacl fluid containing 15 to 30 mole percent co 2 at minimum temperatures in the range of 285 degrees to 395 degrees c the relatively low homogenization temperatures of aqueous inclusions are interpreted to be the result of co 2 effervescence from a co 2 saturated aqueous fluid due to pressure fluctuations at the time of entrapment it is also proposed that the onset of gold deposition is related to the onset of fluid unmixing

isotopic peculiarities of an archaean pegmatite union mine mica south africa abstract an isotopic investigation rb sr u pb pb pb and sm nd of the archaean union mine pegmatite northern province south africa is reported the intrusion of this pegmatite devoid of zircon and monazite has been dated at 2912 2 6 ma by pb pb step leaching of magmatic garnet this age is supported by an identical two garnet apatite sm nd isochron of 2917 27 ma a rb sr k feldspar garnet albite isochron of 2023 11 ma corresponds to a later hydro thermal overprint isotope resetting during this overprint was incomplete and revealed that various parent daughter isotope pairs within minerals of the union mine pegmatite behaved in exceedingly different manners under the same physico chemical conditions sr isotope resetting was incomplete and resulted in open system behaviour of the pegmatite k feldspar almost quantitatively lost its radiogenic sr which had accumulated between the time of intrusion and the overprint sr loss from muscovite was variable and incomplete as indicated by a wide range in ages from 2088 74 ma to 2744 13 ma only insignificant portions of this sr were incorporated into adjacent albite as revealed by a micro drilled rb sr profile through a muscovite albite grain boundary the 87 sr 86 sr initial ratio is 0 8301 11 for the proterozoic isochron which is far less radiogenic than the calculated value of 2 24 for a closed system it is therefore concluded that sr was lost from the system and that the rb sr ratio of the whole rock was changed apatite on the other hand seems to have behaved as a closed system and retains a low initial sr ratio of 0 7125 u pb analyses of apatite reveal that this mineral has behaved as an almost closed system for pb diffusion as well but was open for u significant loss during the overprint this has resulted in high reverse discordance of apatite u pb analysis and in a geologically meaningless secondary pb pb isochron the data demonstrate that both preferential loss of daughter sr over parent rb and parent u over daughter pb isotopes can occur in some minerals of one and the same rock this process altered the parent daughter ratios to such an extent that in the case of apatite a secondary pb pb isochron with an erroneous age of 4119 66 ma was established it appears likely that preferential loss of daughter or parent isotopes is the reason for spurious secondary whole rock isochrons reported and reviewed by moorbath and taylor 1986 and moorbath et al 1986 such losses far exceeding predictions based on volume diffusion experiments may occur by means of fast transport diffusion which in part may depend on the micro textures of the host minerals geochemical studies of early archaean rocks require substantial back correction for radiogenic decay and the example of the union mine pegmatite shows that changes of parent daughter isotope ratios may occur long after rock formation however such changes may go unnoticed in high precision geochronology i e garnet dating in this study but will lead to erroneous correction for radiogenic decay and hence to erroneous conclusions regarding the origin of the studied samples

mineralogy and geochemistry of the sodium metasomatism related uranium occurrence of aricheng south guyana the aricheng south uranium occurrence is associated with na metasomatism that affected the granitoids of the kurupung batholith in western guyana the mineral paragenesis indicates that late magmatic albitization was followed by chlorite alteration of biotite a minor amount of uraninite occurs in fractures in the newly formed albite crystals often in company of calcite the main mineralization stage occurred later than albitization and chloritization and is represented by brannerite disseminated in a groundmass of fine grained hydrothermal zircon whole rock geochemistry supports the temporal dissociation of albitization from the main ore stage brannerite zircon and uraninite are often partially altered to secondary brannerite zircon and coffinite respectively stable oxygen chlorite calcite and hydrogen chlorite isotope compositions suggest that a highly evolved meteoric fluid or at least one corresponding to a very high rock fluid ratio 18o of approx 3 4 to 4 and d of approx 80 may have caused the pre ore alteration assemblage the fluids in equilibrium with main ore stage zircon have 18o of approx 6 8 and appear to be of magmatic origin the aricheng occurrence geochemically mineralogically thermally and paragenetically resembles the valhalla u deposit in northern australia despite differences between the deposits host lithologies whereas the lagoa real and espinharas u deposits in brazil have host rock lithology that resembles that of aricheng

ages of altered granites adjoining the witwatersrand basin with implications for the origin of gold and uranium one of the main problems in understanding the origin of the witwatersrand basin mineralization concerns the source of the prodigious concentrations of gold and uranium in the basin another problem is the age and nature of the rocks in the source area recent data1 2 indicate that the source area was largely 3 200 2 900 myr old becoming progressively younger as the basin evolved granites adjoining the basin are commonly overprinted by pervasive and vein related hydrothermal alteration which resulted in an enrichment of both gold and uranium3 4 here we report uranium lead data demonstrating that these hydrothermally altered granites were intruded sporadically both before and during the 360 myr period5 constraining witwatersrand sediment deposition and that hydrothermal alteration was associated with granitoid intrusion and subsequent cooling thus fertilization of the crust occurred during basin evolution a feature that supports the idea19 that gold and uranium mineralization occurred simultaneously with sediment deposition

u th pb geochronology of zircon and monazite from syenite and pincinite xenoliths in pliocene alkali basalts of the intra carpathian back arc basin abstract phreatomagmatic eruptions in the lucenec basin in slovakia ejected fragments of igneous rocks of unknown age and ranging in composition from gabbro to syenite and orthopyroxene granodiorite pincinite two kinds of syenites with different fe3 contents composed of anorthoclase and interstitial metaluminous trachyte to alkali rhyolite glass represent the final products of alkali basalt differentiation at depths of 10 and 15 18 km pincinite composed of plagioclase quartz orthopyroxene ilmenite and interstitial peraluminous dacite glass is a quenched supersolidus anatectic melt formed at the depth of 17 20 km by high temperature dehydration melting of biotite bearing gneissic protolith zircon from pincinite and syenite and monazite from syenite were dated using excimer laser ablation icp ms the obtained ages cluster at 5 8 0 1 and 5 4 0 1 ma thus corroborating the genetic link between the two rock types and constraining the residence time of at least 300 100 ka of the parental basalt in the deep crustal magma storage zones the inferred ages overlap the upper miocene pliocene boundary and are considerably younger than spatially associated basaltic lava flows dated using the k ar method consequently the fluviolacustrine poltar formation overlying the maar structure must have sedimented during the pliocene and not during the upper miocene pontian as previously assumed

structures related to the antimony line murchison schist belt kaapvaal craton south africa abstract sporadic epigenetic antimony gold mineralisation along the ene trending antimony line of the murchison schist belt is structurally controlled and spatially related to carbonated rock types with quartz carbonate veins in tension fractures the antimony line is a discontinuous zone of brittle ductile deformation and is locally oblique to lithological layering within a broad ductile shear zone this tectonically early d 1 shear zone has an oblique to dip slip reverse north side up movement sense with downward facing fold structures in the hangingwall later d 2 fold structures deformed the antimony line and its wallrocks into steep structures the antimony line is intruded by granitoids which post date d 1 and are not directly related to mineralisation the source of which may have been metamorphic fluids