high grade magnetite mineralization at 1 86 ga in neoarchean banded iron formations gongchangling china in situ u pb geochronology of metamorphic hydrothermal zircon and monazite

composition and origin of intermediate solid solutions in the system thorite xenotime zircon coffinite abstract a comprehensive survey of the accessory mineral assemblages in variscan granites of the german erzgebirge and pan african granites from jordan revealed the occurrence of intermediate solid solutions of the tetragonal thorite xenotime zircon coffinite mineral group with partially novel compositions these solid solutions preferentially formed in evolved and metasomatically altered p poor leucogranites of either i or a type affinity thorite from the erzgebirge contained up to 18 8 wt y2o3 16 1 wt zro2 and 23 3 wt uo2 xenotime and zircon have incorporated th in abundances up to 36 3 wt and 41 8 wt tho2 respectively extended compositional gradation with only minor gaps is confined to hydrated members of this mineral group and is observed to exist between thorite and xenotime thorite and coffinite and y hree bearing thorite and zircon complex hydrous solid solutions containing elevated abundances of three or more of the endmembers are subordinate previously reported intermediate solid solutions between anhydrous zircon and xenotime and anhydrous zircon and thorite are not observed and are in conflict with experimental work demonstrating very limited miscibility between anhydrous species of endmember composition the majority of hydrous intermediate solid solutions in the th y zr u system are likely thermodynamically unstable instead they are probably metastable responses to unusual physico chemical conditions involving various parameters and conditions the relative importance of which is incompletely known leaching and dissolution of preexisting accessory phases during interaction with f bearing hydrous fluids enriched in th y hree zr and or u and common deposition of the various elements at disequilibrium supersaturation seems to play a key role but other processes may be of similar importance experimental work involving hydrous conditions and complex systems composed of more than two endmembers are needed to shed light into the stability relations of the chemically uncommon compositions treated in this study

residence of ree y th and u in granites and crustal protoliths implications for the chemistry of crustal melts a systematic study with laser ablation icp ms scanning electron microscopy and electron microprobe revealed that 70 95 wt of ree except eu y th and u in granite rocks and crustal protoliths reside within reeythu rich accessories whose nature composition and associations change with the rock aluminosity the accessory assemblage of peraluminous granites migmatites and high grade rocks is composed of monazite xenotime in low ca varieties apatite zircon thorthosilicate uraninite and betafite pyrochlore metaluminous granites have allanite sphene apatite zircon monazite and thorthosilicaie peralkaline granites have aeschinite fergusonite samarskite bastnaesite fluocerite allanite sphene zircon monazite xenotime and th orthosilicate granulite grade garnets are enriched in nd and sm by no less than one order of magnitude with respect to amphibolite grade garnets granulitegrade feldspars are also enriched in lree with respect to amphibolite grade feldspars accessories cause non henrian behaviour of ree y th and u during melt solid partitioning because elevated fractions of monazite xenotime and zircon in common migmatites are included within major minerals their behaviour during anatexis is controlled by that of their host settling curves calculated for a convecting magma show that accessories are too small to settle appreciably being separated from the melt as inclusions within larger minerals biotite has the greatest tendency to include accessories thereby indirectly controlling the geochemistry of ree y th and u we conclude that ree y th and u are unsuitable for petrogenetical modelling of granitoids through equilibrium based trace element fractionation equations

modeling the role of sodic alteration in the genesis of iron oxide copper gold deposits eastern mount isa block australia liberation of iron and potassium by widespread postmetamorphic albitization of country rocks was one of the likely contributing processes in the formation of both barren and mineralized magnetite chalcopyrite biotite gold hematite clinopyroxene actinolite apatite ironstones in the cloncurry district of the proterozoic mount isa block whole rock geochemical data indicate nearly immobile al ga ti zr during transformation of a variety of least altered rocks toward albitite the data indicate that the addition of na from a brine to the rock accompanied the loss of fe k ba rb ca sr co v mn pb and zn from those altered rocks and enrichment in the brine but that cu was not systematically stripped from a variety of wall rocks during albitization conversely the formation of metasomatic ironstones the immediate hosts to some cu au ores involves addition of most of the same elements that were lost during albitization the correlation between intensity of alteration its distribution and timing e g in breccias cored by ca 1530 1500 ma granitoids and convergence of all rock types toward magmatic stable isotope values provides strong evidence for a substantial component of igneous derived fluid simulations of the albitization process were carried out isothermally at 550 c and 350 mpa and polythermally from these conditions down to 400 c and 200 mpa using the gibbs minimization method with hch software and the unitherm database both simple fluid rock mixing models and more complicated reactor style algorithms used a range of input fluids from fluids equilibrated fluid with two feldspar granite through to na k ratios consistent with fluid inclusion results and geochemical data for initially unaltered wall rocks the apparent paradox of widespread albitization resulting from fluid released by two feldspar granites can be explained by relatively small shifts away from the k feldspar albite equilibrium curve and even isothermal model fluids derived from two feldspar granites produce albitites in calc silicate rocks outboard of granite proximal k feldspar clinopyroxene skarns matching field patterns those models with fluid na k ratios similar to those of fluid inclusions produced the most realistic alteration assemblages dominated by albite for both isothermal conditions and decreasing temperature which approximate those observed in the field pixe and microthermometric data on fluid inclusions from quartz in two feldspar quartz monzonite and pegmatite at the top of the mount angelay pluton indicate bulk na k molar ratios in the fluid of between 10 and 20 considerably higher than our thermodynamically calculated values for fluid in equilibrium with two feldspar granite of around 3 at 550 c 350 mpa such shifts may have been attained by admixture of magmatic hydrothermal fluid with small amounts of nacl brines trapped along grain boundaries in scapolite bearing calc silicate wall rocks by fluid immiscibility due to high initial co2 contents in the felsic intrusions by contributions from mafic magmas or from dissolution of salt rich layers into the intrusions prior to crystallization and fluid release with increasing amounts of fluid rock interaction in the models the fluids were enriched in k fe and ca approaching compositions observed in fluid inclusions in the ore deposits these fluids reacted with pelitic rocks which are common ore hosts would produce magnetite clinopyroxene biotite actinolite alteration at high temperature similar to the proximal alteration around ore deposits we infer that precipitation of sulfides in the cu au deposits was the result of mixing of cu bearing brine of ultimately magmatic origin but modified extensively via albitization with sulfur bearing fluids or reaction of the brine with sulfur bearing rocks when cu was absent from the initial magmatic fluid barren ironstones may have been the result

the ca 1740 1710 ma leichhardt event inversion of a continental rift and revision of the tectonic evolution of the north australian craton abstract interpretation and modelling of high resolution regional geophysical data of the central leichhardt river fault trough in the mount isa inlier are used to determine the timing of a major basin inversion event following the development of the ca 1780 1740 ma leichhardt superbasin inversion of the leichhardt superbasin formed the regional north south trending leichhardt anticline during east west shortening the limbs of the anticline are overprinted by several east west trending wedge shaped non magnetic sub basins filled with ca 1710 ma calvert and isa superbasin successions these relationships suggest inversion of the leichhardt superbasin occurred between ca 1740 and 1710 ma the event is also known to have affected the northern and eastern north australian craton the scale of the inversion suggests it was a significant event that we have defined as the leichhardt event this event requires a major tectonic driver to the east of the north australian craton possibly the accretion of a micro continental ribbon to the east of the mount isa inlier the results of this study have implication for paleogeographic reconstruction of the australian continent during the formation of nuna because eastern north australian craton faced an ocean at ca 1740 1720 ma the results also challenge the significance and intensity of crustal shortening associated with the ca 1600 1500 ma isan orogeny throughout the western mount isa inlier

uranium deposits of the mount isa region and their relationship to deformation metamorphism and copper deposition the mount isa region contains over 20 uranium deposits few of which contain economic concentrations of uranium they are thought to be significant in the exploration for new copper deposits based on the model that the uranium deposits are located in the pathways of oxidized hydrothermal fluids that formed the postmetamorphic mount isa copper orebodies there are two uranium deposit types both hosted in volcanosedimentary rocks we suggest that they represent one mineralization event but different metamorphic histories examples in the eastern low grade metamorphic zone are dominated by a brannerite anatase apatite assemblage and are low in uranium the examples in the western high grade metamorphic zone are characterized by uraninite and titanite with higher uranium concentrations the chemical association of u ti and ca is consistent for all deposits however phosphate is only important at low metamorphic grade the most likely mechanism of formation in the lower metamorphic grade anderson s lode example is the complete replacement of chlorite in the sedimentary host rock matrix by the mineralizing assemblage in this model chlorite acted as a reductant allowing the deposition of uranium from an oxidized fluid this mechanism of uranium deposition is similar to that proposed for unconformity type uranium deposits we conclude that these uranium deposits are metamorphosed equivalents of proterozoic unconformity type uranium deposits found elsewhere in northern australia at anderson s lode brannerite and apatite were concentrated in veinlets by pressure dissolution after the deposition of the original assemblage these pressure dissolution seams are cut by premetamorphic quartz chlorite veins examples at higher metamorphic grade have textures that suggest uranium deposition was pre to synmetamorphic at the latest this precludes any genetic or temporal connection between the uranium deposits and the postmetamorphic mount isa copper deposit

the mobility of zirconium and other immobile elements during hydrothermal alteration abstract development of zircon and other zr phases in hydrothermal deposits indicates that zr can be highly mobile in these systems mobility is most common in but not restricted to f rich hydrothermal systems related to alkalic f rich igneous suites these suites can range from peralkaline through metaluminous to peraluminous a few examples are neither alkalic nor f rich three locations in the trans pecos magmatic province texas u s a demonstrate this hydrothermal zr mobility all three igneous systems are alkalic and f rich but vary in alkali al ratios peralkaline rhyolites and trachytes in the christmas mountains contain as much as 2100 ppm zr mostly in aegirine or arfvedsonite zircon is rare or absent fluorspar replacement deposits in limestone at contacts with the rhyolites contain as much as 38 000 ppm zr occurring as small disseminated zircons the deposits also are enriched in a variety of incompatible elements including be rare earth elements ree y nb mo hf pb th and u the sierra blanca intrusions a series of mildly peraluminous f rich rhyolite laccoliths contain as much as 1000 ppm zr mostly as zircon hydrothermal zircon occurs as overgrowths on magmatic grains as veinlets connected to overgrowths and in fluorspar replacement bodies in adjacent limestone the highest zr concentrations in fluorspar are 200 ppm metaluminous quartz monzonite from the infiernito caldera contains 400 600 ppm zr mostly as zircon euhedral zircon in quartz fluorite veins in the quartz monzonite indicates mobility of zr zirconium concentrations in the veins are unknown but the paucity of zircon suggests little zr enrichment relative to the host zircon and more rarely zirconolite occur in skarn in the ertsberg district of irian jaya indonesia unlike in texas related igneous rocks are metaluminous and the hydrothermal system was f poor worldwide hydrothermal zircon other zr phases and ti and al bearing phases occur in skarn epithermal precious metal veins volcanogenic massive sulfide deposits and mylonites we propose that differences in zr mineralogy of igneous source rocks is an important factor in determining the availability of zr to hydrothermal fluids although zr concentrations in the sierra blanca and christmas mountains rhyolites are similar zr enrichment in fluorspar was much greater in the christmas mountains we suggest that hydrothermal solutions could easily break down aegirine and arfvedsonite to release zr but that zircon was only moderately attacked therefore far more zr was available for transport and subsequent deposition in the christmas mountains than at sierra blanca availability of other trace elements probably is also governed by their mineral host although zr mobility is most common in f rich hydrothermal systems related to alkalic and f rich igneous systems mobility at ertsberg may have been promoted by sulfate complexing

zircon solubility in alkaline aqueous fluids at upper crustal conditions field evidence and thermodynamic data at ambient conditions suggest that complexing of zr with hydroxyl ions at high ph enhances the solubility of zircon we tested this hypothesis by measuring the solubilities of the assemblages zircon zrsio 4 baddeleyite z b and zircon quartz z q in neutral to alkaline fluids at 0 2 gpa and 450 750 c in neutral to alkaline fluids 0 0 1 1m naoh zircon dissolved incongruently in quartz undersaturated fluids to form baddeleyite at 450 c various na zr silicates precipitated from fluids saturated and undersaturated in quartz we observed no significant dependence of solubility on temperature the measured solubilities of zircon in pure h 2o at 600 and 750 c are only slightly higher than the procedural blank of 3 3 10 7 m zr however the measured solubility of zircon increases with increasing aqueous silica suggesting that zirconium complexes with silica in the fluid at 600 c linear regression of experimental results yielded ln zr 0 25 2 9 ln si where terms in parentheses represent molal concentrations zircon solubility also seems to increase with increasing hydroxyl concentration in the fluid enhanced solubility in silica rich alkaline fluids may cause dissolution reprecipitation of zircon and resetting of its isotopic clocks suggesting that some zircon u pb ages may correspond to fluid events

fluorine and chlorine behaviour during progressive dehydration melting consequences for granite geochemistry and metallogeny dehydration melting of biotite is the main control on crustal differentiation in the mid to lower continental crust because this reaction produces the most voluminous and most mobile granitic melts biotite breaks down over a broad temperature interval so the partitioning behaviour of elements between biotite and melt is likely to vary it has been hypothesized that fluorine may stabilize biotite to higher melting temperatures because biotite is typically f rich in ultra high temperature uht metamorphic rocks if true f would be an important influence on crustal differentiation because not only would it broaden the temperature range of melting but also elevated f concentration decreases melt viscosity furthermore ligand partitioning between biotite and melt may be an important influence on the metallogeny of magmas this study used electron microprobe analysis of biotite in rocks from the ballachulish and rogaland metamorphic aureoles to investigate the concentration of f and cl in biotite heated to 600 1 000 c results show a broad increase in biotite f content up to 5 04 f with temperature until 850 920 c beyond which f content decreases 2 5 f chlorine concentrations in biotite are consistently lower 1 cl and show a progressive decrease after the onset of partial melting it was found that mg content and the processes that control mg distribution are most strongly correlated with f and cl concentration in biotite calculations based on these results indicate that f enriched biotite could be a significant source of f for continental crust derived melts generation of a hot f rich melt at uht conditions could be important for transporting lower crustal metals to the upper crust

mineralogical evidence for h2 degassing during serpentinization at 300 c 300 bar abstract hydrogen is produced in large amounts during hydrothermal alteration of peridotite in low spreading rate mid ocean ridges this production is directly linked to reducing conditions in hydrothermal fluids induced by the oxidation of fe2 in primary minerals olivine and pyroxene to fe3 in secondary minerals magnetite and serpentine a better knowledge of iron speciation in serpentine is therefore crucial to the quantification of hydrogen production during the serpentinization process for the first time we have determined the amount of ferric iron in altered peridotite as a function of alteration time we investigated experimentally the alteration of powdered lherzolite in pure water at 300 c 300 bar for each experimental run 0 7 18 34 and 70 days h2 degassing was measured using in situ gas chromatography and the experimental products were analyzed using xrd raman and x ray absorption spectroscopy at the iron k edge in parallel aqueous solutions were analyzed by icp aes our results show quasi complete serpentinization at 70 days with replacement of primary olivine and pyroxene by secondary lizardite and magnetite the fe3 fetotal ratio is linearly dependent on the hydrogen production and ranges from 0 to 0 66 at the end of the experiment our results reveal strong variations in fe3 in serpentine for different alteration times from 0 to 100 of ferric iron including up to 12 of tetrahedral iron hydrogen was produced in three main stages 1 a first stage during which the h2 production rate reaches a maximum at 18 days and is controlled by the crystallization of magnetite 2 an intermediate stage during which serpentine incorporates ferric iron and thus plays a major role up to 50 in the hydrogen formation and 3 a final stage during which magnetite amount increases from 2 to 5 of the mineral assemblage the last alteration stage is accompanied by a slight increase of the fe3 fetotal ratio while the rate of hydrogen production decreases at the end of the experiment consequently variations of the ferric iron contents in natural oceanic peridotites may constitute a good indicator of the hydrogen potential of various ultramafic hydrothermal fields

high radiogenic heat producing granites and metamorphism an example from the western mount isa inlier australia the origins of metamorphism 600 c and 3 4 kbar in the western mount isa inlier australia remain controversial for a number of reasons 1 no synmetamorphic intrusive bodies can be recognized 2 high temperatures appear to be sustained for periods 100 m y and 3 metamorphism follows an extended phase of thermal subsidence we show that the burial of granite batholiths enriched in radiogenic elements beneath the thick insulating sedimentary succession of the mount isa group deposited in response to repeated rift sag cycles was capable of generating steep upper crustal thermal gradients immediately prior to the isan orogeny these gradients are appropriate to peak metamorphic conditions such that the ensuing isan orogeny required no significant additional heat input this result is significant in that it may provide a mechanism for understanding the origins of high temperature metamorphism in other terranes where the involvement of transient heating is not obvious

proterozoic low pressure high temperature metamorphism and an anticlockwise p t t path for the hazeldene area mount isa inlier queensland australia in the hazeldene area situated in the mount isa inlier queensland the metamorphic grade changes from chlorite zone through biotite and cordierite zones to sillimanite k feldspar zone microstructural studies of rocks near the sillimanite isograd demonstrate that cordierite grew early during the development of a steep foliation s2 was replaced by biotite andalusite and sillimanite at the metamorphic peak late in s2 and in turn by kyanite chlorite adjacent to localized small post d2 shear zones although the anticlockwise p t t path is well defined the precise p t conditions are uncertain because of problems with experimental and thermodynamic data the best estimate for the metamorphic peak for rocks close to the sillimanite isograd is around 600 c at 4 kbar the metamorphism has been dated at 1544 ma and was synchronous with a major crustal shortening event because proposed extensional events occurred more than 60 ma earlier their contribution to the peak metamorphic thermal perturbation would have been insignificant the syn metamorphic mica creek pegmatites the abundance of high heat producing elements in the nearby pre d2 sybella granite and advective heat by fluids which caused considerable metasomatism in the hazeldene area may have each contributed to the thermal budget however the metamorphic thermal gradient may be 80 c km 1 or higher strongly suggesting a local magmatic control as none are known in the area such syn metamorphic plutons would have to lie beneath the exposed high grade rocks

formation of h2 ch4 and n species during low temperature experimental alteration of ultramafic rocks abstract hydrogen methane and ammonium are important chemical species for chemolithotrophic microorganisms that sustain subsurface endolithic communities it is well known that production of h 2 through serpentinization of the primary fe ii containing minerals olivine and pyroxene in ultramafic rocks and reduction of co 2 to ch 4 are processes that occur at higher temperatures knowledge is however limited about these reactions under low temperature conditions and how they are affected by fe ii containing secondary minerals such as brucite and serpentine which are commonly found in these environments in this experimental study we explored the formation of h 2 ch 4 and nh 4 during low temperature 25 c reactions between deionised water and 1 unaltered 2 medium altered and 3 highly altered dunites over a period of 99 days hydrogen was detected in all three experiments and the concentration increased over time the o 2 concentration decreased over time but o 2 was still present at significant levels when h 2 started to form this suggests that the h 2 was formed locally at the surface of mineral grains rather than in the hypoxic solution the highly altered dunite gave the highest and the unaltered dunite the lowest h 2 concentrations and while the o 2 concentration decreased fast with the medium altered dunite significant amounts of o 2 still remained with the unaltered dunite at the end of the experiments both the medium and highly altered dunite gave o 2 concentrations close to or below the detection limit during the last part of the experiment factors that likely affected the h 2 production are fe content mineralogy dissolution rates and surface areas of the minerals as well as the presence of catalytic minerals such as chromite and magnetite methane was also detected and increased with time in all experiments the results indicate that ch 4 formation also occurred locally on the surface of minerals detection of both nh 4 and no 3 in the aqueous solutions indicates that previously absorbed n species were leached out during the experiment increasing nh 4 and decreasing no 3 concentrations over time suggest no 3 reduction likely catalysed by mineral surfaces the results imply that low temperature water rock reactions in moderately to highly altered ultramafic rocks provide reduced chemical species that can be utilized as electron donors in energy yielding metabolic processes by subsurface microorganisms such reactions could play an important role in sustaining subsurface microbial communities particularly in ophiolites and near seafloor parts of the ultramafic oceanic lithosphere at slow spreading mid ocean ridges and possibly also in other rocks and sediments the results suggest that brucite and possibly also serpentine may be more important sources of fe ii and thus h 2 formation than olivine in low temperature ultramafic systems formation of h 2 in reducing microenvironments on mineral surfaces surrounded by oxic conditions increases the extent of h 2 formation and the subsurface habitat for aerobic h 2 oxidizing microorganisms

the proterozoic albitite hosted valhalla uranium deposit queensland australia a description of the alteration assemblage associated with uranium mineralisation in diamond drill hole v39 the valhalla uranium deposit located 40 km north of mount isa queensland australia is an albitite hosted mesoproterozoic u deposit similar to albitite hosted uranium deposits in the ukraine sweden brazil and guyana uranium mineralisation is hosted by a thick package of interbedded fine grained sandstones arkoses and gritty siltstones that are bound by metabasalts belonging to the ca 1 780 ma eastern creek volcanics in the western succession of the mount isa basin alteration associated with u mineralisation can be divided into an early main and late stage the early stage is dominated by laminated and intensely altered rock comprising albite reibeckite calcite titano magnetite brannerite the main stage of mineralisation is dominated by brecciated and intensely altered rocks that comprise laminated and intensely altered rock cemented by brannerite apatite uranoan zircon uraninite anatase albite reibeckite calcite and hematite the late stage of mineralisation comprises uraninite red hematite dolomite calcite chlorite quartz and pb fe cu sulfides brannerite has u pb and pb pb ages that indicate formation between 1 555 and 1 510 ma with significant pb loss evident at ca 1 200 ma coincident with the assemblage of rodinia the oldest ages of the brannerite overlap with 40ar 39ar ages of 1 533 9 ma and 1 551 7 ma from early and main stage reibeckite and are interpreted to represent the timing of formation of the deposit these ages coincide with the timing of peak metamorphism in the mount isa area during the isan orogeny lithogeochemical assessment of whole rock data that includes mineralised and unmineralised samples from the greater mount isa district reveals that mineralisation involved the removal of k ba and si and the addition of na ca u v zr p sr f and y u th ratios indicate that the ore forming fluid was oxidised whereas the crystal chemistry of apatite and reibeckite within the ore zone suggests that f and text po 3 4 were important ore transporting complexes 18o values of co existing calcite and reibeckite indicate that mineralisation occurred between 340 and 380 c and involved a fluid having 18ofluid values between 6 5 and 8 6 reibeckite d values reveal that the ore fluid had a dfluid value between 98 and 54 the mineral assemblages associated with early and main stages of alteration plus 18ofluid and dfluid values and timing of the u mineralisation are all very similar to those associated with na ca alteration in the eastern succession of the mount isa basin where a magmatic fluid is favoured for this style of alteration however isotopic data from valhalla is also consistent with that from the nearby mount isa cu deposit where a basinal brine is proposed for the transport of metals to the deposit based on the evidence to hand the source fluids could have been derived from either or both the metasediments that underlie the eastern creek volcanics or magmatism that is manifest in the mount isa area as small pegmatite dykes that intruded during the isan orogeny

isotopic and structural responses of granite to successive deformation and metamorphism within the sybella granite west of mount isa in northern australia differential response of u pb and rb sr isotopic systems to deformation metamorphism enables ages of emplacement and three discrete deformation metamorphic events to be determined based on zircon u pb systems two of the plutons were emplaced at 1671 pm 8 ma and 1668 pm 26 21 ma and the third at 1610 pm 10 ma a separate microgranite body gives aberrantly old u pb ages of up to 1780 ma due to partial inheritance of older zircon the three discrete deformation metamorphic events d 1 d 2 and d 3 occurred at 1610 pm 13 ma 1544 pm 12 ma and 1510 pm 13 ma respectively based on whole rock rb sr isotopic systems the youngest body was emplaced syntectonically and has possible significance to the origin of the mount isa ag pb zn orebodies homogenization of rb and sr isotopes via extensive syntectonic fluid migration associated with deformation partitioning and resultant solution transfer appears to have been

reduction of co2 during serpentinization of olivine at 300 c and 500 bar co 2 reduction processes occurring during experimental serpentinization of olivine at 300 c and 500 bar confirm that ultramafic rocks can play an important role in the generation of abiogenic hydrocarbon gas data reveal that conversion of fe ii in olivine to fe iii in magnetite during serpentinization leads to production of h 2 and conversion of dissolved co 2 to reduced c species including methane ethane propane and an amorphous carbonaceous phase hydrocarbon gases generated in the process fit a schulz flory distribution consistent with catalysis by mineral reactants or products magnetite is inferred to be the catalyst for methanization during serpentinization because it has been previously shown to accelerate fischer tropsch synthesis of methane in industrial applications involving mixtures of h 2 and co 2 the carbonaceous phase was predominantly aliphatic but had a significant aromatic component although this phase should ultimately be converted to hydrocarbon gases and graphite if full thermodynamic equilibrium were established its formation in these experiments indicates that the pathway for reduction of co 2 during serpentinization processes is complex and involves a series of metastable intermediates

hydrothermal alteration of magmatic zircon related to nacl rich brines diffusion reaction and dissolution reprecipitation processes magmatic zircon from altered gabbros adjacent to the 1 07 ga lala fe cu deposit sw china was modified by nacl rich brines related to the fe cu mineralization the modified zircon grains are composed mostly of both inclusion free and porous domains and some grains also have overgrowth rims the inclusion free domains commonly overgrown by the porous domains are roughly homogeneous under bse imaging but display oscillatory or sector zoning under cl imaging in contrast the porous domains are distinctly mosaic like under cl imaging and contain abundant pores and mineral inclusions such as thorite xenotime ree rich phases actinolite albite biotite and calcite the inclusion free domains have concentrations of non formula elements for example al ca and fe much higher than the magmatic zircon and are thus interpreted to be products of interaction between magmatic zircon possibly metamictized and the fluids via a diffusion reaction process however these resultant domains have retained the th and u contents ree patterns u pb ages and hf isotopes of the precursor magmatic zircon on the other hand the porous domains have stoichiometric end member compositions that is lowered th u ree y and p and given that they are porous and inclusion rich we proposed that they have formed from the precursor element rich inclusion free domains or magmatic zircon via a fluid induced dissolution reprecipitation process it is notable that the porous domains retained the 177hf 176hf ratios of the precursors but possess modified and meaningless u pb ages instead a meaningful u pb age 1006 62 ma similar to the timing of the lala deposit is well recorded by the overgrowth rims with distinctly high lrees la 50 700 ppm and elevated hf isotopic ratios our new results reveal that in the presence of the nacl rich brines the nature of the precursor magmatic zircon that is high element budgets and or metamictization plays a key role on hydrothermal alteration of zircon

unraveling the sequence of serpentinization reactions petrography mineral chemistry and petrophysics of serpentinites from mar 15 n odp leg 209 site 1274 1 the results of detailed textural mineral chemical and petrophysical studies shed new light on the poorly constrained fluid rock reaction pathways during retrograde serpentinization at mid ocean ridges uniformly depleted harzburgites and dunites from the mid atlantic ridge at 15 n show variable extents of static serpentinization they reveal a simple sequence of reactions serpentinization of olivine and development of a typical mesh texture with serpentine brucite mesh rims followed by replacement of olivine mesh centers by serpentine and brucite the serpentine mesh rims on relic olivine are devoid of magnetite conversely domains in the rock that are completely serpentinized show abundant magnetite we propose that low fluid flux serpentinization of olivine to serpentine and ferroan brucite is followed by later stages of serpentinization under more open system conditions and formation of magnetite by the breakdown of ferroan brucite modeling of this sequence of reactions can account for covariations in magnetic susceptibility and grain density of the rocks citation bach w h paulick c j garrido b ildefonse w p meurer and s e humphris 2006 unraveling the sequence of serpentinization reactions petrography mineral chemistry and petrophysics of serpentinites from mar 15 n odp leg 209 site 1274 geophys res lett 33 l13306

the solubility of fluorite in h2o and h2o nacl at high pressure and temperature abstract the solubility of fluorite in h2o and h2o nacl has been measured at 600 1000 c 0 5 2 0 gpa in a piston cylinder apparatus fluorite dissolves congruently at all conditions investigated the data indicate that the concentration of fluorite dissolved in h2o is low at 600 c and 0 5 gpa 0 0014 mol kg h2o but increases strongly with temperature t and pressure p to 0 247 mol kg h2o at 1000 c 2 gpa the data were fit with the equation log mcaf2 6 587 2 774p1 2 2 266 10 3 4 699 10 4p t where mcaf2 is molality of caf2 t is in kelvin and p is in gpa fluorite solubility was also measured in h2o nacl fluids at 800 c and 1 0 gpa results indicate a strong increase in dissolved fluorite concentration with increasing nacl content from 0 028 mol kg h2o at xnacl 0 to 1 673 mol kg h2o at xnacl 0 486 where xnacl is the mole fraction of nacl in the fluid the data were fit to mcaf2 0 01035 1 141xnacl 4 694xnacl2 the results indicate that f mobility will be further enhanced by saline brines in high p t igneous and metamorphic environments high f solubility in h2o and h2o nacl in the deep crust and upper mantle can promote mobilization of high field strength elements rare earth elements u and th through transport as f bearing complexes

pyrite pyrrhotite stability in a metamorphic aureole implications for orogenic gold genesis it is generally thought that orogenic gold deposit formation is associated in many cases with metamorphic fluids these fluids are mainly liberated by chlorite breakdown in the upper greenschist facies and there is some debate as to whether the pyrite to pyrrhotite desulfidation reaction precedes or coincides with this fluid release since pyrite is an important metal host in metamorphic rocks its breakdown to pyrrhotite may allow complexation of au with liberated sulfur thus the question of the temperature pressure conditions required for pyrite desulfidation is important because it determines which tectonic processes and which parts of the crust are relevant to gold deposit genesis here we investigate metal redistribution during pyrite breakdown using laser ablation inductively coupled plasma mass spectrometry and bulk rock analysis on greenschist to amphibolite facies rocks of the ballachulish metamorphic aureole scotland results suggest that the pyrite to pyrrhotite transition field spans a broader temperature range than indicated by recent modeling and that pyrite breaks down simultaneously with chlorite gold as bi sb mn and w were likely removed from the rock in chlorite derived fluid nickel co ga ge mo tl v and cr are retained in the rock by uptake in pyrrhotite copper is retained by growth of chalcopyrite as temperature increases whereas pb and zn are retained by progressive uptake in nonsulfide minerals these elements are not depleted during amphibolite facies metamorphism the tendency of s to react with silicate hosted fe and be retained in the rock implies that fe poor pyritic sedimentary rocks may produce more s and metal rich fluids in source regions

fluid immiscibility in metamorphic rocks evidence of fluid immiscibility in metamorphic rocks comes in the best of all worlds from fluid inclusion observations in fact fluid immiscibility should be anticipated over the full range of metamorphic conditions provided that appropriate bulk fluid compositions are present in a review paper entitled metamorphic fluids the evidence from fluid inclusions crawford and hollister 1986 summarized the role of fluid immiscibility in metamorphism was only recognized after serious study of fluid inclusions in metamorphic rocks was underway in the late 1970s and has yet to attract the attention of more than a handful of petrologists studying metamorphic rocks since then tremendous progress has been made a huge amount of fluid inclusion studies have convincingly shown that immiscible fluids can be present in rocks of all bulk compositions and at all p t conditions thermodynamic databases of minerals solid solutions and fluid mixtures along with the development of gibbs free energy minimization programs now allow for detailed prediction of the evolution of phase assemblages and mineral compositions in p t space significant progress has also been made in understanding reaction kinetics diffusional and convective mass transport and the interpretation of fluid rock interaction during metamorphism given the vast fluid inclusion evidence of immiscibility it would appear however that phase petrology and mass transport in any specific rock where fluids are involved are not correctly interpreted if possible or proven immiscibility is not considered relatively few studies directly link mineral reactions with immiscible fluids therefore this review aims at working out the effects of fluid immiscibility on the progress of metamorphic reactions and fluid transport over a wide range of conditions questions arise directly from the fluid inclusion record in metamorphic rocks this chapter however is organized the other way round at first it summarizes phase relations of fluid systems pertinent to

direct evidence for the source of uranium in the baiyanghe deposit from accessory mineral alteration in the yangzhuang granite porphyry xinjiang province northwest china

towards quantitative evaluation of gas injection using time lapse seismic data of particular concern in the monitoring of gas injection for the purposes of storage disposal or improved oil recovery is the exact spatial distribution of the gas volumes in the subsurface in principle this requirement is addressed by the use of 4d seismic data although it is recognized that the seismic response still largely provides a qualitative estimate of moved subsurface fluids exact quantitative evaluation of fluid distributions and associated saturations remains a challenge to be solved here an attempt has been made to produce mapped quantitative estimates of the gas volume injected into a clastic reservoir despite good results using three accurately repeated seismic surveys time delay and amplitude attributes reveal fine scale differences though large scale agreement in the estimated fluid movement these differences indicate disparities in the nature of the two attributes themselves which can be explained by several possible causes of most impact are the effects of processing and migration wave interference effects and noise from non repeatability of the seismic surveys this subject highlights the need for a more careful consideration in 4d acquisition amplitude processing and use of true amplitude preserving attributes in quantitative interpretation

release of uranium from highly radiogenic zircon through metamictization the source of orogenic uranium ores although circumstantial evidence from ore deposit mineralogy and geochemistry can imply potential sources for fluids and metals rarely is direct evidence for metal leaching from source rocks seen in the vicinity of deposits here we investigate the source of metals for a series of fault zone and shear zone hosted uranium occurrences in the mount isa inlier australia as well as containing uranium these deposits are enriched in zr and rare earth elements rees requiring that unusual fluids were responsible for addition of these typically immobile elements during the isan orogeny highly saline metamorphic fluids infiltrated the sheared margin of a highly evolved granite intrusion which contains elevated u th f zr and rees synchrotron x ray fluorescence spectroscopy and high resolution electron microprobe mineral mapping show that this radioactive element rich characteristic caused zircon crystals to become highly metamict allowing the elements therein to become mobile thus when orogenic fluids pervasively infiltrated along shear zones 100 m y after granite intrusion their unusually saline character allowed enhanced dissolution of regional carbonates and fluorite from the granite providing the ligands needed for transport of uranium and the normally immobile elements from the metamict zircons

radiation damage in zircon and monazite monazite and zircon respond differently to ion irradiation and to thermal and irradiation enhanced annealing monazite cannot be amorphized by 800 kev kr ions at temperatures greater than 175 c whereas zircon can be amorphized at temperatures up to 740 c the damage process i e elastic interactions leading to amorphization in radioactive minerals metamictization is basically the same as for the ion beam irradiated samples with the exception of the dose rate which is much lower in the case of natural samples the crystalline to metamict transition in natural samples with different degrees of damage from almost fully crystalline to completely metamict is compared to the sequence of microstructures observed for ion beam irradiated monazite and zircon the damage accumulation process representing the competing effects of radiation induced structural disorder and subsequent annealing mechanisms irradiation enhanced and thermal occurs at much higher temperatures for zircon than for monazite the amorphization dose expressed as displacements per atom is considerably higher in the natural samples and the atomic scale process leading to metamictization appears to develop differently ion beam induced amorphization data were used to calculate the decay event dose required for amorphization in terms of a critical radionuclide concentration i e the concentration above which a sample of a given age will become metamict at a specific temperature this equation was applied to estimate the reliability of u pb ages to provide a qualitative estimate of the thermal history of high u natural zircons and to predict whether actinide bearing zircon or monazite nuclear waste forms will become amorphous metamict over long timescales

metallogeny of the zoujiashan uranium deposit in the mesozoic xiangshan volcanic intrusive complex southeast china insights from chemical compositions of hydrothermal apatite and metal elements of individual fluid inclusions abstract the zoujiashan u deposit in the mesozoic xiangshan volcanic intrusive complex is one of the largest volcanic related type u deposits in china chemical composition of hydrothermal apatite and metal contents in individual fluid inclusions of quartz and fluorite are investigated in order to understand the precipitation mechanism of u and to further constrain the origin of u and ore forming fluids of the zoujiashan deposit illitization hematitization and u mineralization successively occurred within rhyolite and porphyritic lava three types of hydrothermal apatite are identified apatite type 1 ap1 and apatite type 2 ap2 are both euhedral subhedral crystals found in hematitization zone while only ap2 shows evident irregular chemical zoning along its rims apatite type 3 ap3 occurs as anhedral aggregates closely associated with u minerals in u ore veins implying synchronous precipitation of ap3 and u ores comparable chemical compositions of ap1 and cores of ap2 including similar high cl content indicate both of them were precipitated from a cl rich fluid that induced hematitization the elevated f contents of ap3 and rims of ap2 implies that a f rich fluid was involved and altered ap2 during the main stage of u deposition ap1 and cores of ap2 have lower mn contents than ap3 and rims of ap2 indicating an oxidized character for the cl rich fluid and a reducing character for the f rich fluid metal contents of fluid inclusions in fluorite and quartz occurring in u ores suggest that two end members of fluids including a u rich and a u poor were involved in the main stage of u mineralization mixing between these two fluids led to the deposition of u and th minerals through oxidation reduction reactions and destabilization of th f complexes respectively the u rich fluid is depleted in th and sr compared with the u poor fluid based on comparison of chemical compositions of apatite and fluid inclusions we suggest that the cl rich fluid precipitating ap1 and ap2 corresponds to the u rich end member fluid while the f rich fluid precipitating ap3 represents a mixture between the two end members the high u 12 7 58 5 ppm in the u rich end member fluid is in agreement with a relatively high oxygen fugacity and a surface derived origin the u poor end member fluid has lower rb cs than the u rich end member and the unaltered host rocks indicating the u poor fluid was derived from a mantle wedge that experienced addition of cs by fluids generated from the devolatilization of the subducted paleo pacific slab

newly discovered uranium mineralization at 2 0 ma in the menggongjie granite hosted uranium deposit south china abstract the southeastern part of the nanling metallogenic province south china contains numerous economically important granite hosted hydrothermal vein type uranium deposits the miao ershan mes uranium ore field is one of the most important uranium sources in china hosts the largest chanziping carbonaceous siliceous pelitic rock type uranium deposit and several representative granite hosted uranium deposits the geology and geochemistry of these deposits have been extensively studied however accurate and precise ages for the uranium mineralization are scarce because uranium minerals in these deposits are usually fine grained and may have formed in several stages thus hindering the understanding of the uranium metallogenesis of this province the menggongjie mgj uranium deposit is one of the largest granite hosted uranium deposits in the mes ore field uranium mineralization in this deposit occurs at the central part of the mes granitic complex accompanied with silicification fluorination k metasomatism and hematitization the ore minerals are dominated by uraninite occurring in quartz or fluorite veinlets along fractures in altered granite in situ sims u pb dating on the uraninite yields the u pb isotopic age of 1 9 0 7 ma which is comparable to the chemical u th pbtol uraninite age of 2 3 0 1 ma such ages agree well with the eruption ages of the extension related quaternary volcanics 2 1 1 2 ma in south china suggesting that the uranium mineralization have formed at an extensional setting possibly related to the quaternary volcanic activities therefore our robust new dating results of the mgj uranium deposit make it the youngest granite hosted uranium deposit reported so far in south china and the mineralization event represents a newly identified mineralization epoch