alteration mineralogy and stable isotope geochemistry of paleoproterozoic basement hosted unconformity type uranium deposits in the athabasca basin canada unconformity type uranium deposits are characterized by mineralization developed along the contact between younger sandstone cover and underlying crystalline basement rocks mineralization may extend up to 400 m into the underlying basement rocks whereas sandstone hosted unconformity type deposits have been well studied deposits hosted primarily in the basement have not this study examines the deposits at rabbit lake dawn lake and mcarthur river in the athabasca basin of canada which are hosted by the metamorphic archean and early paleoproterozoic rocks forming the basement to younger late paleoproterozoic sandstones alteration is similar in the three deposits and is characterized by three distinct paragenetic stages 1 preore alteration involving illitization of plagioclase and amphibole followed by chloritization of biotite and illite which formed at ca 230 c 2 ore stage alteration characterized by uraninite and coarse grained illite which formed at ca 240 c 3 postore alteration comprising spherulitic dravite vein chlorite quartz calcite and fe cu co and pb sulfides which formed at ca 135 c fluid circulation associated with emplacement of later mackenzie dikes initiated partial recrystallization of uraninite a later stage of alteration includes kaolinite and iron hydroxide precipitation formed at much lower temperatures of ca 50 c stable isotope compositions of the alteration minerals in conjunction with their paragenesis indicate that oxidized basinal fluids d 43 to 21 18o 3 8 were derived primarily from evolved seawater and leached uranium from the overlying sandstones of the athabasca formation and transported it into the basement via infiltration along fracture zones associated with reverse faults graphitic units in the basement and preore alteration served as both physical fractured zones and chemical reductants traps for the uranium mineralization the basinal fluids were responsible for the preore illite chlorite synore uraninite illite and the early postore alteration events this differs from many other sandstone hosted deposits where both oxidized basinal and reduced basement derived fluids were responsible for uranium precipitation

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 validity of pearce element ratio analysis in petrology an example from the uwekahuna laccolith hawaii chemical variation diagrams formed from ratios with a common denominator exhibit induced correlations which can obscure the effects of true compositional variations nevertheless important conclusions have been obtained using pearce element ratio and isochron diagrams even though these diagrams are formed from axes ratios with a common denominator in this paper we consider two approaches to mitigate the effects of induced correlation and in doing so demonstrate the validity of geochemical analysis using diagrams formed from ratios with a common denominator first we propagate analytical error onto pearce element ratio diagrams to distinguish chemical variations associated with analytical error from chemical variations related to geological processes second we consider an alternative diagram formed from ratios with different denominators which does not exhibit any induced correlation we demonstrate both theoretically and by example that results produced using this diagram are identical to those using diagrams with a common denominator these two lines of evidence confirm that pearce element ratio and isochron diagrams are useful and valid tools in the analysis of geochemical variations

the stable isotope signature of kilometre scale fracturedominated metamorphic fluid pathways mary kathleen australia abstract large calcite veins and pods in the proterozoic corella formation of the mount isa inlier provide evidence for kilometre scale fluid transport during amphibolite facies metamorphism these 10 to 100 m scale podiform veins and their surrounding alteration zones have similar oxygen and carbon isotopic ratios throughout the 200 10 km mary kathleen fold belt despite the isotopic heterogeneity of the surrounding wallrocks the fluids that formed the pods and veins were not in isotopic equilibrium with the immediately adjacent rocks the pods have 13ccalcite values of 2 to 7 and 18ocalcite values of 10 5 to 12 5 away from the pods metadolerite wallrocks have 18owhole rock values of 3 5 to 7 and unaltered banded calc silicate and marble wallrocks have 13ccalcite of 1 6 to 0 6 and 18ocalcite of 18 to 21 in the alteration zones adjacent to the pods the 18o values of both metadolerite and calc silicate rocks approach those of the pods large calcite pods hosted entirely in calc silicates show little difference in isotopic composition from pods hosted entirely in metadolerite thus 100 to 500 m scale isotopic exchange with the surrounding metadolerites and calc silicates does not explain the observation that the 18o values of the pods are intermediate between these two rock types pods hosted in felsic metavolcanics and metasiltstones are also isotopically indistinguishable from those hosted in the dominant metadolerites and calc silicates these data suggest the veins are the product of infiltration of isotopically homogeneous fluids that were not derived from within the corella formation at the presently exposed crustal level although some of the spread in the data may be due to a relatively small contribution from devolatilization reactions in the calc silicates or thermal fluctuations attending deformation and metamorphism the overall l shaped trend of the data on plots of 13c vs 18o is most consistent with mixing of large volumes of externally derived fluids with small volumes of locally derived fluid produced by devolatilization of calc silicate rocks localization of the vein systems in dilatant sites around metadolerite calc silicate boundaries indicates a strong structural control on fluid flow and the stable isotope data suggest fluid migration must have occurred at scales greater than at least 1 km the ultimate source for the external fluid is uncertain but is probably fluid released from crystallizing melts derived from the lower crust or upper mantle intrusion of magmas below the exposed crustal level would also explain the high geothermal gradient calculated for the regional metamorphism

skuppesavon northern sweden a uranium mineralisation associated with alkali metasomatism skuppesavon represents a uranium mineralisation emplaced within a sequence of metavolcanics dominated by rocks of rhyolitic to trachytic composition uraniferous solutions have penetrated along zones previously opened by na ca rich mineralising fluids which caused albitisation of potassium feldspar the partial to complete removal of quartz and the crystallisation of new mineral phases uranium which has been transported mostly as uranyl carbonate complexes has precipitated as uraninite which occurs as impregnations and only very rarely forms fracture infillings the introduction of ca and ti bearing solutions coeval and subsequent to uraninite precipitation has resulted in the association of uraninite and sphene the hydrothermal fluids responsible for the observed mineral parageneses have been activated during metamorphism during the passage of these fluids through the volcano sedimentary country rocks elements such as ca na u and ti were mobilised concentrated and transported access through the rocks was facilitated by structural weaknesses and rock permeability variations

shallow water eruption fed mafic pyroclastic deposits along a paleoproterozoic coastline kangerluluk volcano sedimentary sequence southeast greenland abstract the 200 300 m thick volcano sedimentary sequence at kangerluluk is part of the psammite zone one of four major zones which constitute the 1 8 ga ketilidian orogen in south greenland three lithofacies are emphasized in the study 1 the conglomerate sandstone 2 the volcanic and 3 the pyroclastic lithofacies the 2 40 m thick conglomerate sandstone lithofacies represents a subaerial to subaqueous fan delta deposit matrix and clast supported conglomerates are interpreted as debris flow and longitudinal gravel bar deposits erosive based conglomerate channel fills attest to stream incision trough crossbedded sandstone interpreted as lunate megaripples planar bedded sandstone indicative of upper flow regime bar top sands and small scale trough crossbeds reflecting ripples follow up section form collectively with the conglomerate 0 40 2 50 m thick fining upward sequences the sandstone dominated unit up section from the conglomerates and composed of planar and low angle crossbeds minor ripples and graded beds as well as mudstone is indicative of a lower shoreface deposit below normal wave base the clastic sedimentary rocks are suggestive of a fan delta setting the 100 200 m thick volcanic lithofacies composed of pillowed and pillow brecciated lava flows is consistent with shallow water deposition interstratification of lava flows with both conglomerate sandstone and pyroclastic lithofacies intrusion of dykes into volcaniclastic rocks and peperite formation accentuate contemporaneity between volcanism and sedimentation and is a common feature of island arcs the 1 50 m thick pyroclastic lithofacies with sharp depositional contacts to the overlying volcanic and underlying conglomerate sandstone lithofacies was emplaced in a subaqueous setting the lithofacies is divided into a planar to crossbedded tuff lapilli tuff and a bedded lapilli tuff breccia whereby both deposits are inferred to result from shallow water surtseyan type eruptions the 5 15 m thick bedded lapilli tuff breccia with abundant bomb sag structures and graded beds is considered a result of subaqueous eruptions strong enough to form an insulating steam cupola characterized by ballistically emplaced bombs that rapidly collapsed allowing for transport via mass flow processes the deposits are considered proximal to the vent the 2 50 cm thick planar to crossbedded tuff lapilli tuff featuring abundant euhedral and broken crystals of feldspar 2cm and minor pyroxene 1 cm are massive graded crossbedded and stratified the planar but laterally discontinuous beds characterized by abundant low angle scours are interpreted as low to high concentration sediment gravity flows produced directly from subaqueous tephra jets that collapsed due to massive water ingestion local breccia size pyroclasts disrupting beds are interpreted as bomb sags the mafic eruption fed surtseyan type deposits postulated to be a subaqueous counterpart of cold subaerial base surges originate from subaqueous tuff cones formed along a rugged volcanic dominated shoreline featuring high energy fan deltas

the lightning creek sill complex cloncurry district northwest queensland a source of fluids for fe oxide cu au mineralization and sodic calcic alteration the lightning creek cu au prospect is hosted by a cogenetic suite of plutonic i type granitoids the dominant rock type is a porphyritic quartz monzodiorite that is intruded by more fractionated rocks including monzogranite and alkali feldspar granite a series of flat lying sills are interpreted to be late stage differentiates based on their timing mineralogy and chemistry in parts of the prospect there is pervasive sodic calcic alteration pyroxene after amphibole albite after k feldspar and oligoclase of the plutonic rocks this alteration predates sill emplacement and is unrelated to veining or fracturing of any kind the presence of small amounts of carbonate in the altered rocks suggests that the fluids were co2 bearing quartz and feldspar separates from these altered rocks have oxygen isotope compositions similar to those from fresh quartz monzodiorite suggesting that the fluids were hot and of magmatic composition sodium and ca were added and k fe cl and cu were stripped during what is interpreted as an autometasomatic event the sills display considerable textural and mineralogical complexity and evolved from equigranular quartzofeldspathic rocks aplites with magmatic chemistry to unusual fe rich rocks albite magnetite quartz that exhibit a range of bizarre spherulitic textures some of the albite and magnetite in the sills is secondary albite forms pseudomorphs after k feldspar na fe ca alteration along sill margins and within sills at the contacts between different textural zones halos of disseminated magnetite clinopyroxene fe ca na alteration are developed adjacent to early magnetite veins fluid inclusion studies indicate that these rocks crystallized at temperatures in excess of 500 c and at pressures in excess of 1 5 kbar the range of spherulitic textures is taken to indicate crystallization under hydrous conditions with the episodic release of a fluid phase this magmatic fluid phase was dominated by h2o co2 and chlorine and underwent phase separation into a co2 rich vapor and a hypersaline brine 33 55 wt nacl equiv the hypersaline fluid was enriched in fe 10 wt and cu 1 wt pixe analysis in addition to na k and ca where this fluid was retained within fe rich portions of the sills it caused ca fe na alteration pyroxene albite magnetite growth at the expense of quartz where the fluid was expelled from the sills it produced quartz magnetite clinopyroxene albite veins broadly coeval with the early magnetite veins although rich in cu these granitoid derived magmatic fluids did not generate significant cu au mineralization perhaps because of the high temperatures involved and or a lack of reduced sulfur in the fluids or host rock however the amount of iron present is estimated from the aeromagnetic anomaly to be in excess of 2 000 million tonnes mt a later generation of calcite chlorite pyrite chalcopyrite veins contain traces of cu au mineralization fluid inclusion and stable isotope work indicate that these veins probably crystallized from cooler 200 c more dilute 15 28 wt nacl equiv fluids perhaps generated by the admixture of a meteoric component the conclusions reached in this study have implications for understanding the genesis of fe oxide cu au deposits and related sodic calcic alteration the study indicates the potential for co2 rich granitoid magmas to evolve hypersaline fe and cu rich fluids capable of causing intense magnetite veining and cu au mineralization autometasomatic sodic calcic alteration of the granitoids may be an important precursor to mineralization contributing fe k cu and cl to the magmatic fluids

some guidelines in reconstructing ancient sea conditions from wave ripplemarks abstract the collection of substantial field data on wave ripple mark spacing wavelength steepness grain size and symmetry index from the miocene burdigalian sandstones of western switzerland has allowed apparently close reconstruction of sea conditions results indicate however that ancient wave reconstructions must be restricted at present to data from relatively steep ripple marks vfi less than 7 5 i e vortex ripples data from the flatter ripple marks post vortex types formed at high oscillatory shear stresses do not provide reasonable estimates of wave conditions penecontemporaneous overwashing and or post depositional compaction ripple formation near the destruction condition rather than the threshold of sediment movement orbital diameters far exceeding ripple spacing and superimposed unidirectional currents are all factors which seriously affect wave reconstructions evaluation of ancient wave conditions provides insight into geologically important phenomena such as local water depth and basin size

a fluid inclusion and stable isotope study of synmetamorphic copper ore formation at mount isa australia earlier structural studies indicate that the dolomitic and siliceous breccias and contained copper mineralization at mount isa queensland australia formed during regional deformation and greenschist facies metamorphism of their host mid proterozoic metasediments the metasomatic breccia body locally called silica dolomite is zoned with an outer halo of sparry dolomitic alteration replacing finely laminated dolomitic pyritic metasediments the core of the dolomitic breccia next to the underlying altered greenstones is overprinted by siliceous breccia containing the bulk of the chalcopyrite ore fluid inclusions in quartz and dolomite were studied by microthermometry micro raman spectrometry and semiquantitative electron microprobe analysis of inclusion salts two types of aqueous inclusions are restricted to dolomitic breccia cacl 2 rich group 1 with a cation ratio na ca k mg nearly equal 30 10 3 1 and salinity approximately 25 wt similar to recent salton sea geothermal brines and low salinity with 10 to 20 mole percent co 2 co 2 rich group 2 these two aqueous inclusion types are intimately associated in the same crystals and have probably both been trapped during dolomitic alteration siliceous alteration superimposed on dolomitic breccia contains nacl rich fluid inclusions group 3 na ca k mg nearly equal 300 10 30 1 minor ch 4 microstructures suggest that earlier higher salinity 10 20 wt and later lower salinity 4 9 wt variants of this nacl rich fluid bracket the main stage of chalcopyrite introduction there are no low density vapor inclusions indicative of any stage of fluid boiling rare high density ch 4 or co 2 rich carbonic inclusions are associated with nacl rich aqueous inclusions in a few samples secondary inclusions of a highly calcic low temperature brine group 4 occur along late healed cracks and postdate all major stages of mineral precipitation the c o and h isotope compositions of dolomites quartz and hydrous silicates and of fluid inclusion extracts from samples of the siliceous alteration zone were imposed by the externally derived weakly ch 4 co 2 bearing nacl rich fluid at high fluid rock ratios and over a small range of temperatures this fluid may have originated as a highly evolved basin brine or an evaporite derived metamorphic fluid the early cacl 2 rich fluid may have had an origin similar to the nacl rich ore fluid but its different delta d and the mass balance constraints arising from its chemical composition suggest that it has not been derived from the nacl rich fluid by progressive rock interaction within the zoned alteration system similarly an independent source is likely for the co 2 rich fluid the absolute pressure temperature ranges for the two alteration processes seem to be slightly different but it is uncertain whether the quartz chalcopyrite mineralization occurred at slightly lower temperature or higher pressure compared with the dolomitic alteration the latter is confined to 270 degrees to 350 degrees c and 700 to 1 500 bars by isochore and mineral stability data the following working hypothesis for the syntectonic and synmetamorphic fluid rock interaction and copper ore formation at mount isa is proposed in an initial stage dolomitic alteration occurred by chemical interaction between the urquhart shale and moderate amounts of two fluids of different chemical and hydrological origin represented by the cacl 2 rich and the co 2 rich fluid inclusions fluid mixing may have been aided by extension on the limb of a regional fold which temporarily lowered fluid pressures thus leading to hydraulic brecciation and fluid influx from two fluid regimes above and below the co 2 rich fluid probably represents the local metamorphic fluid of the urquhart shale whereas the cacl 2 rich brine was introduced from the underlying altered greenstones the initial stage of brecciation and dolomitic alteration was followed by the main stage of silicification and copper introduction by the much more copious nacl rich fluid probably at near lithostatic fluid pressure quartz and chalcopyrite precipitation occurred by a combination of slight cooling a ph increase and possibly an increase in sulfur activity as the initially reduced acid and possibly s deficient nacl rich fluids reacted with the previously dolomitized pyritic and dolomitic urquhart shale

theoretical wave modeling of large wave ripples in volcaniclastic sediments ordovician llewelyn volcanic group north wales abstract volcaniclastic sedimentary rocks of ordovician age underlying the garth tuff member of the capel curig volcanic formation north wales were deposited on a shallow marine shelf deposition at the capel curig anticline 2 km southwest of capel curig was on a high energy proximal offshore to foreshore analysis of large wave ripples and associated sedimentary structures suggests a shallow marine depositional environment above storm wave base and in water depths of 10 30 m this estimate of water depth from wave calculations is similar to that estimated by facies analysis

u pb rb sr and sm nd chronology of granitic basement hydrothermal albitites and uranium mineralization lagoa real south bahia brazil the granites orthogneisses hydrothermal albitities and rocks which have suffered uranium mineralization from the lagoa real district south state of bahia brazil have been investigated by u pb rb sr and sm nd techniques u pb values on zircons from the granitic protolith give an age of 1725 ma u pb on u mineralization dates the primary mineralization at 1395 ma and indicates a reworking at 480 ma which may represent the age of the thrusting of the lagoa real complex over espinhaco metasediments during the brazilian orogeny these two dates are given by rb sr on albitites but from sparse information and are not supported by unequivocal arguments the uranium deposition and sodium metasomatism however cannot be linked either with the thermal history of the granite or with brazilian thrusting sm nd gives scattered results which are suggestive of autochtonous reworkings of rees these results lead to the following suggestions 1 magmatic activity of subalkaline affinity existed within the sao francisco craton at about 1 7 ga 2 such subalkaline plutons are likely sources for u mineralization 3 at about 1 4 ga an unknown event caused hydrothermal activity leading to u deposition at lagoa real 4 brazilian overthrusting at about 480 ma did not play any genetic role in the hydrothermal activity

downstream changes in sand composition roundness and gravel size in a short headed high gradient stream northwestern italy abstract we assessed changes in grain composition and roundness of the 1 2 and 2 3 size fractions of sand along a 16 km length of the graveglia river liguria italy to its mouth at the tyrrhenian sea and then for 1 km along the beach rocks exposed in the drainage area include ophiolites ribbon chert limestone shale sandstone and quaternary sands the erosion of shale and limestone bedrock yields large amounts of sand but sand grains of these rock types decrease rapidly in abundance downstream chiefly through dilution rather than abrasion carbonate rock fragments that reach the beach survive wave abrasion but most shale grains do not predictably monocrystalline and polycry talline quartz and chert are the most durable grain types but serpentine basalt and unidentified rock fragments also show significant resistance to both stream and wave abrasion polycrystalline quartz feldspar basalt serpentine sandstone and shale grains are more abundant in the coarser sand size fraction monocrystalline quartz carbonate rock fragments and unidentified rock fragments are more abundant in the finer sand size fraction and chert and plutonic rock fragments have no size preference a comparison of the outcrop areas of the different rock types with the corresponding percentages of different grain types present in each of three drainage areas indicates that shale which breaks down readily to silt and clay size particles and chert which does not readily break down into sand size fragments are underrepresented in the sands limestone and serpentine however are overrepresented in comparison to outcrop area in certain drainage areas the overrepresentation of limestone reflects in part a sampling bias sand was sampled adjacent to limestone outcrops but the overrepresentation of serpentine reflects its resistance to weathering compared to other aluminosilicates and its durability shale limestone and serpentine increase in roundness downstream and along the beach to a maximum limiting value of 5 rho units medium grained shale clasts attain high rounding in only a few kilometers of transport downstream decrease in gravel size follows an exponential formula that can be expressed in linear form using phi size size 0 1167 distance km 10 29

post orogenic 1500 ma thermal history of the proterozoic eastern fold belt mount isa inlier australia the paucity of sedimentary rocks younger than 1500 ma within the mt isa inlier renders it difficult to determine its post orogenic exhumation and tectonic history except by inference from the stratigraphy of the surrounding areas we present hornblende white mica biotite and alkali feldspar 40ar 39ar data from the eastern fold belt of the inlier which reveal a previously unrecognised complex post orogenic non linear cooling history plateau and total fusion 40ar 39ar ages range between 1500 and 1130 ma and record increases in regional cooling rates of up to 18 c my during 1460 1410 and 1280 1050 ma forward modelling of the alkali feldspar 40ar 39ar arrhenius parameters reveals subsequent increases in cooling rates during 740 640 and 600 500 ma the cooling episodes were driven by both erosional exhumation at maximum rates of 0 9 km my and thermal relaxation following magmatic events early mesoproterozoic cooling is synchronous with exhumation and shearing during the anmatjira event in the arunta block and late mesoproterozoic cooling occurred at the same time as magmatic events in the arunta and musgrave blocks as well as grenville aged orogenesis in the albany fraser belt more localised cooling during the middle neoproterozoic may be a far field response to the separation of australia and laurentia whereas latest neoproterozoic cambrian cooling and exhumation was probably driven by collisional tectonics associated with the petermann ranges and delamerian orogenic events major changes in the stress field of the australian plate that are synchronous with the assembly and break up of rodinia and gondwana resulted in the repeated brittle reactivation of the eastern fold belt probably via the displacement of long lived basement faults

low latitude meteoric fluid flow along the cloncurry fault cloncurry district nw queensland australia geodynamic and metallogenic implications hydrothermal alteration within the mount isa block mib is typically largely to have ceased at the end of the isan orogeny ca 1 50 ga however late large brittle quartz rich vein systems including those of the cloncurry fault cut mesoproterozoic tectonic fabrics intrusions and products of regional na ca hydrothermal alteration associated features display affinities to iron oxide cu au ore deposits including specular hematite bearing veins and cu au u lree ba mineralization stable isotope data suggest late veins in the vicinity of the cloncurry fault eastern fold belt efb formed from fluids with 18oh2o and dh2o 250 c of 4 to 3 and 22 to 4 respectively the high dh2o moderate 18oh2o and cacl2 nacl rich character of the saline fluids indicates they were mixtures of low latitude meteoric fluids and evaporite derived basinal brines which with time became progressively dominated by meteoric water deposited barite and cooled the low latitude meteoric signature demonstrates that the fault fracture systems were active when in equatorial regions paleomagnetic data for the mib and the australian protocontinent together with structural and thermal considerations and geochronological arguments suggest the veins probably formed sometime after 1 10 ga and potentially as late as during the alice springs orogeny ca 0 34 ga the late quartz veining was unrelated to two phases of regional na ca alteration and economically significant fe oxide cu au mineralization at 1 60 1 58 and 1 53 1 50 ga

geochemistry of post 1540 ma granites in the cloncurry district northwest queensland proterozoic fe oxide cu au co mineralization in the cloncuriy district is spatially and temporally related to intrusion of voluminous postpeak metamorphic granites that form most of the williams and naraku batholiths detailed mapping and geochemical study of these granites in the southern part of the district have identified two groups of intrusions that are referred to here as the cloncurry supersuite and eureka supersuite the cloncurry supersuite is composed mainly of monzogranite and syenogranite which consist of quartz plagioclase alkali feldspar hornblende biotite with accessory magnetite titanite apatite and zircon the timing of intrusion of the wiley igneous complex is interpreted to be pre or syn d 3 based on overprinting relationships while hornblende geobarometry suggests an emplacement depth of approximately 14 km the granites are subalkaline high k metaluminous rocks and range from 59 to 74 wt percent sio 2 increasing sio 2 is accompanied by increasing k 2 o rb and th and decreasing mgo fe 2 o 3 feo tio 2 cao p 2 o 5 sr eu zr cu zn ni v and sc this is consistent with fractionation of observed phases including plagioclase amphibole biotite magnetite titanite and apatite the eureka supersuite ranges from diorite to syenogranite in composition more mafic varieties contain pyroxene and hornblende while more felsic varieties contain biotite as the only ferromagnesian silicate sodiccalcic alteration events are closely linked in space and time to intrusion of eureka supersuite magmas the timing of intrusion is interpreted to be pre and post d 3 based on overprinting relationships between different plutons and mylonitic fabrics hornblende geobarometry suggests an emplacement depth of approximately 10 km for the mount angelay igneous complex eureka supersuite granites in the mount angelay igneous complex range from 51 to 76 wt percent sio 2 the granites are alkaline and k 2 o rich leading to their classification as a shoshonite series with increasing sio 2 content tio 2 al 2 o 3 fe 2 o 3 t mno mgo cao p 2 o 5 sr eu cu zn ni v and sc decrease indicating fractionation of observed minerals including apatite magnetite pyroxene hornblende plagioclase and biotite some samples are characterized by very high abundances of k 2 o p 2 o 5 sr ba and light rare earth elements lree compared to other eureka supersuite samples this may reflect their derivation from a source that was variably enriched in incompatible elements compared with many other shoshonitic rocks high k eureka supersuite samples are strongly depleted in sr and to a lesser extent eu and enriched in y and heavy rare earth elements hree this suggests that garnet was not a stable phase in the source region that the source contained plagioclase as a stable phase and or the granites evolved by extensive plagioclase fractionation partial melting in the lower crust to generate post 1540 ma granites was most likely caused by intrusion of basic magma into the crust eureka supersuite magmas were probably derived from partial melting of variably incompatible element enriched gabbroic rocks while cloncurry supersuite magmas were most likely derived from partial melting of more felsic material of dioritic to tonalitic composition

reconstruction of ancient sea conditions with an example from the swiss molasse ancient sea conditions can be estimated from the grain size spacing and steepness of preserved ripple marks the element of greatest uncertainty in such reconstructions is the relationship between near bed orbital diameter of water particles and the ripple spacing this relationship is simple for vortex ripples of high steepness but is problematical for the low steepness forms known as post vortex rolling grain or anorbital ripples the existence field for wave ripples is between the threshold velocity for sediment movement and the onset of sheet flow most low steepness forms occurring close to the bed planation threshold a range of maximum period of formative waves can be obtained using combinations of orbital diameter and orbital velocity assuming linear wave theory to be a reasonable approximation probable wave heights wave lengths and water depths can be investigated using the transformation of wave parameters in shallowing waters and the constraints on wave dimensions provided by the wave breaking condition given reasonable estimates of wave height crude estimates of wave power allow a comparison of ancient wave influenced sequences with modern counterparts wave ripple marks preserved in the upper marine molasse of western switzerland have been investigated results which are in agreement with regional geology suggest deposition in a seaway of approximately 100 km width where moderate period waves t 3 6 s were generated the depositional facies belts were adjusted to the prevailing waves tides and fluvial outflows

hydrogen isotope systematics of submarine basalts the d h ratios and water contents in fresh submarine basalts from the mid atlantic ridge the east pacific rise and hawaii indicate that the primary d h ratios of many submarine lavas have been altered by processes including 1 outgassing 2 addition of seawater at magmatic temperature and 3 low temperature hydration of glass decreases in d and h2o from exteriors to interiors of pillows are explained by outgassing of water whereas inverse relations between d and h2o in basalts from the galapagos rise and the famous area are attributed to outgassing of ch4 and h2 a good correlation between d values and h2o is observed in a suite of submarine tholeiites dredged from the kilauea east rift zone where seawater added directly to the magma affected only the isotopic compositions of hydrogen and argon analyses of some glassy rims indicate that the outer millimeter of the glass can undergo lowtemperature hydration by hydroxyl groups having d values as low as 100 d values vary with h2o contents of subaerial transitional basalts from molokai hawaii and subaerial alkali basalts from the society islands indicating that the primary d values were similar to those of submarine lavas extrapolations to possible unaltered d values and h2o contents indicate that the primary d values of most thoteiite and alkali basalts are near 80 5 the weight percentages of water are variable 0 15 0 35 for mor tholeiites about 0 25 for hawaiian tholeiites and up to 1 1 for alkali basalts the primary d values of 80 for most basalts are comparable to those measured for deep seated phlogopites these results indicate that hydrogen in marked contrast to other elements such as sr nd pb and o has a uniform isotopic composition in the mantle this uniformity is best explained by the presence of a homogeneous reservoir of hydrogen that has existed in the mantle since the very early history of the earth

cyclic deposits and hummocky cross stratification of probable storm origin in upper cretaceous rocks of the cape sebastian area southwestern oregon abstract cyclic deposits containing hummocky cross stratification occur in the upper part of the cape sebastian sandstone of bourgeois 1980 a shallow marine transgressive sandstone of late cetaceous age on the southern oregon coast the cycles average 1 6 m in thickness and consist where complete of a lower hummocky cross stratified sandstone a middle planar and ripple bedded sandstone with a shale bed in its middle part and an upper bioturbated sandstone noteworthy features of the hummocky cross stratification include the presence of depositional domes in addition to scoured depressions the absence of significant bedform migration and the presence of a small proportion of dip angles greater than the angle of repose 34 in addition to the large proportion of low 15 deg dip angles the lower stratified fining upward part of the cycle up to the top of the shale bed is interpreted as having accumulated under conditions of initially great but gradually decreasing current velocity and deposition rate the currents probably had a strong oscillatory component and the depositional event is inferred to have been a storm the part of the planar and ripple bedded sandstone above the shale bed was probably deposited during relatively fair weather after the storm but before re establishment of a normal benthic fauna the bioturbated sandstone is interpreted to have been deposited during fair weather or during minor storms separated by long intervals of fair weather

an 40 ar 39 ar investigation of the contact effects of a dyke intrusion kapuskasing structural zone ontario an 40ar 39ar study on biotite and hornblende from baked tonalite gneiss adjacent to a 14 m wide kapuskasing dyke in the chapleau block of the kapuskasing structural zone employed resistance furnace step heating of bulk mineral separates single grain laser step heating and laser spot dating of individual grains in thick section all three methods yielded concordant results for the biotite indicating that it has been uniformly overprinted at ca 2 05 ga this date is probably a close estimate of the age of the dyke intrusion the argon data from the hornblende are more difficult to interpret all three methods suggest that the hornblende was partially overprinted and then affected by a later influx of excess argon from the external environment laser spot dates record an apparent age gradient extending radially outward from 2 25 ga in the grain cores to 2 4 2 7 ga in the grain rims corresponding 37ar 39ar values show no systematic variation across the hornblende although 38ar 39ar ratios display a similar pattern to the age distribution with lower values in the cores than in the rims the bulk separate and single grain step heating spectra mimic a volume diffusion ar loss profile that would appear to be inconsistent with the laser spot dates three possible interpretations to explain this apparent discrepancy may involve either experimental difficulties an effective radius of diffusion which is much smaller than the actual grain radius or an effective radius of diffusion that is of the order of the grain radius it is clear that for the 40ar 39ar dating of samples that have experienced complex geologic histories it is important to utilize both resistance furnace and laser techniques

uranium enrichment in archaean crustal basement associated with overthrusting uranium deposits cumulatively in the 100 000 tonne u3o8 range occur within ductile shear zones transecting archaean basement gneisses at lagoa real north east of caetite state of bahia brazil1 3 fig 1 the gneisses dated at 2 600 3 000 myr are at amphibolite to granulite facies of meta morphism there is a general concensus that such rocks form near the base of the crust and typically exhibit pronounced depletion of u relative to the upper crust5 6 to the east and west of lagoa real the archaean gneisses overlie proterozoic meta sediments and volcanics of the espinhaco supergroup along well documented thrust surfaces7 8 limited u pb dating of the deposits indicates that u mineralization occured at 820 myr ref 9 involving removal of si and k coupled with na addition in oxidizing conditions v pb and light rare earth elements lree were introduced along with uranium from hypersaline co2 hydrocarbon rich fluids with 18o 4 at temperatures of 540 c depths of 15 km and pfluid pload we show here that the geological constraints are consistent with release through hydrofracturing of 1 000 km3 of over pressurized meteoric water recharged formation brines in the proterozoic sediments up through the hot overriding gneisses

ore lead isotope pattern from the iglesiente sulcis area sw sardinia and the problem of remobilization of metals in sw sardinia syngenetic to syndiagenetic pb zn ores occur in cambrian carbonates along the unconformity between the cambrian and ordovician in permo triassic karsts and in vein type deposits related to late hercynian granites which also contact metamorphosed some cambrian deposits in all types of deposits the leadisotope ratios show similarly high 238u 204pb and high w 232th 204pb values indicating a crustal origin for the lead most of the cambrian ores contain isotopically similar or identical leads whereas in the younger deposits the isotope ratios vary and suggest that especially the lead of permo triassic ores may consist to a large extent of remobilized cambrian possibly also ordovician ore lead plus a hercynian component

petrologic hypothesis testing with pearce element ratio diagrams derivation of diagram axes in petrology pearce element ratio per diagrams have been used i to determine whether members of a rock suite are co genetic ii to identify the minerals involved in differentiation processes and iii to evaluate the extent to which those mineral are involved the axis coefficients of each diagram are chosen such that sorting of minerals or combinations of minerals will generate unique and predictable trends unfortunately selection of the optimal combination of axis coefficients is a difficult task especially if the system being investigated has a large number of phases or complicated solid solution minerals our work has established a formal set of rules and matrix operations which facilitate the determination of per diagram axes coefficients this methodology can be used to determine the unit molar vector displacement caused by the addition or subtraction of a specific mineral given a set of axis coefficients it can also be used to create per diagrams on which minerals have predetermined vector displacements by designating all vector displacements to be parallel axis coefficients for assemblage test diagrams can be determined to test the following hypothesis the observed chemical variation is due to the addition or removal of a specific set of minerals alternatively by designating all vector displacements to be mutually perpendicular phase discrimination diagrams can be created which test whether the observed chemical variations require a specific phase to be involved in differentiation phase discrimination diagrams also provide a means to estimate the extent of that involvement this methodology facilitates construction of powerful yet simple per diagrams which provide an effective means of testing alternative differentiation hypotheses

wave formed structures and paleoenvironmental reconstruction clifton h e and dingier j r 1984 wave formed structures and paleoenvironmental reconstruction in b greenwood and r a davis jr editors hydrodynamics and sedimentation in wave dominated coastal environments mar geol 60 165 198 wave formed sedimentary structures can be powerful interpretive tools because they reflect not only the velocity and direction of the oscillatory currents but also the length of the horizontal component of orbital motion and the presence of velocity asymmetry within the flow several of these aspects can be related through standard wave theories to combinations of wave dimensions and water depth that have definable natural limits for a particular grain size threshold of particle movement and that of conversion from a rippled to fiat bed indicate flow velocity limits the ratio of ripple spacing to grain size provides an estimate of the length of the near bottom orbital motion the degree of velocity asymmetry is related to the asymmetry of the bedforms though it presently cannot be estimated with confidence a plot of water depth versus wave height h h diagram provides a convenient approach for showing the combination of wave parameters and water depths capable of generating any particular structure in sand of a given grain size natural limits on wave height and inferences or assumptions regarding either water depth or wave period based on geologic evidence allow refinement of the paleoenvironmental reconstruction the assumptions and the degree of approximation involved in the different techniques impose significant constraints inferences based on wave formed structures are most reliable when they are drawn in the context of other evidence such as the association of sedimentary features or progradational sequences

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

a shallow marine volcaniclastic facies model an example from sedimentary rocks bounding the subaqueously welded ordovician garth tuff north wales u k abstract volcaniclastic sedimentary rocks bounding the ordovician garth tuff in north wales were deposited in a shallow marine setting adjacent to a magmatic arc the volcaniclastic sedimentary rocks are primarily of angular to euhedral quartz angular euhedral feldspar and volcanic rock fragments these grains are of a pyroclastic origin but have been reworked to various degrees sedimentary rock fragments rounded quartz muscovite biotite iron rich chlorite and various clay minerals occur in lesser amounts the sedimentary rocks can be divided into a proximal offshore to foreshore facies association poffa and an offshore to lower shoreface facies association olsfa the sand dominated poffa exposed near capel curig is characterized by large wave ripples hummocky cross stratification co sets of trough cross beds reactivation surfaces plane beds and graded turbidite layers in suspension deposited mudstone water depths calculated from bedding plane exposure of large wave ripples and determined by the presence of hummocky cross stratification and by facies association vary from a few meters to around 30 m 45 50 m theoretical maximum in a 45 m thick stratigraphic section beneath the tuff eight kilometers to the southeast the garth tuff is bounded by a thick sequence of suspension deposited laminated mudstone with thin graded beds of siltstone and fine grained sandstone this facies association olsfa represents the introduction of material by distal turbidites and by pelagic sedimentation the olsfa was deposited in water depths well below storm wave base possibly in excess of 200 m no evidence of storm waves or surface generated currents occur until over 40 m above the tuff this study documents marine sedimentary rocks deposited in water depths ranging from foreshore to offshore as the bounding facies of the garth tuff it is thus reasonable to conclude that the garth tuff was emplaced and welded in water depths greater than the thickness of the tuff in the area of this study

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

chronology of magmatism skarn formation and uranium mineralization mary kathleen queensland australia uranium mineralization is hosted in a metasedimentary sequence intruded by felsic and mafic bodies in which the zircon u pb systems are 1 730 to 1 740 m y old felsic dikes characterized by exceptionally high uranium contents crystallized at essentially the same time 1 737 or 15 m y primary rb sr systematics in these igneous rocks are profoundly disturbed u pb discordia establish age of uranium mineralization as 1 550 or 15 m y derivation of uranium protore in the host rocks prior to regional metamorphism and 1 550 m y uraninite formation was probably by ca 1 740 m y hydrothermal activity modified journal abstract

magmatic hydrothermal albite actinolite apatite rich rocks from the cloncurry district nw queensland australia abstract albite quartz actinolite apatite rich rocks with accessory titanite form a carapace that caps a small dome like intrusion of roxmere pluton and provide evidence of the accumulation and release of magmatic fluids that may have contributed to regionally extensive na ca alteration in the proterozoic cloncurry district australia the roxmere pluton was emplaced after the peak amphibolite facies metamorphism and into psammitic metasedimentary rocks that are stratigraphically younger and overlie the calc silicate rich mary kathleen group na ca hydrothermal assemblages closely spatially related to the roxmere pluton suggest that na ca alteration in this case is inconsistent with excepted up temperature metamorphic fluid circulation models the carapace rocks comprise two variants aplite and pegmatite both of which were affected by alteration and brecciation aplitic material predominates and the carapace has a brecciated top and a distinct textural zonation from an aplite rich pegmatite poor base to a relatively aplite poor pegmatite rich top this zonation is interpreted to represent the progressive accumulation of volatile phases toward the top of the carapace which culminated in fluid overpressuring and brecciation of the granitoid stock the convoluted and ptygmatic habit of the aplitic and pegmatitic material within the carapace is similar in nature to the comb quartz unidirectional solidification textures that are developed in the roof zones of some porphyry stocks these complex textures are interpreted to have formed from a single phase of fluid saturation the roxmere pluton or a deeper equivalent has a na mg and ca rich composition and a clinopyroxene amphibole rich mineralogy that is consistent with the formation of the contained albite actinolite apatite rich phases however the source of the rocks within the carapace is equivocal given that their contact relationships are obscured the 18 o composition of minerals within the carapace has the following ranges albite 7 1 8 7 actinolite 8 0 8 5 and quartz 8 9 11 4 albite and quartz 18 o equilibria suggest that these minerals in the pegmatitic and aplitic components formed at temperatures of 510 c to 540 c the calculated 18 o composition of a fluid in equilibrium with these minerals 6 0 to 8 0 at temperatures between 450 c and 550 c is consistent with a magmatic origin these estimated temperatures and 18 o fluid composition are comparable to estimates for regional na ca alteration in contrast d analysis of actinolite 136 to 150 within the carapace demonstrates that these fluids have a significantly lower d signature compared to na ca alteration assemblages associated with regional alteration 70 to 90 the low values in the carapace could either be interpreted as being produced from meteoric fluids or from degassed magmatic fluids although a meteoric fluid model is deemed improbable as meteoric fluids in the district have a documented d composition of ca 0 0 as a consequence an open system degassed magmatic hydrothermal fluid origin is considered to be the most plausible explanation for the low isotopic signatures and is also consistent with the complex textural relationships described within the carapace

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

fluidization of wet sediments during the emplacement and cooling of various igneous bodies during the emplacement and cooling of igneous bodies fluidization of wet sediment by water vapour may occur by heating or by pressure relief during the opening of fractures momentary fluidization causes sediment reconstitution and local transport continuous fluidization can result in substantial sediment displacement theoretical considerations show that fluidization due to heating is not likely to occur at depths where pressure is much greater than 312 bars studies of the emplacement of sills of andesite ayrshire fluid rhyolite ramsey island and viscous rhyolite moelwyn hills recognize a wide range of phenomena attributable to wet sediment fluidization and in places modify existing interpretations the transgressive bases of the subaqueously welded ash flow tuffs of the capel curig volcanic formation are consistent with emplacement attended by fluidization of the subjacent sediment evidently fluidization is a means by which large volumes of wet sediment can be replaced by igneous material with minimal disturbance of the remaining host also fluidization can result in exceptionally complex contact relationships between sedimentary host and intrusive rocks and can cause fractures formed during cooling to be thoroughly pervaded by sediment fluidization is commonly associated with the formation of peperites

calculation of oxygen isotope fractionation in hydroxyl bearing silicates abstract the modified increment method has been applied to the calculation of oxygen isotope fractionation factors for hydroxyl bearing silicate minerals the order of 18o enrichment obtained in common rock forming minerals is pyrophyllite kaolinite tourmaline talc prehnite topaz illite phengite lepidolite muscovite staurolite epidote glaucophane serpentine chlorite tremolite hornblende phlogopite biotite humite norbergite ilvaite hydroxyl bearing silicates are enriched in 18o relative to hydroxyl groups but depleted in 18o relative to anhydrous counterparts three sets of self consistent fractionation factors between quartz and the hydroxyl bearing silicate minerals between calcite and the silicate minerals and between the silicate minerals and water have been calculated for a temperature range of 0 1200 c the fractionation factors calculated for mineral pairs are applicable to isotopic geothermometry in igneous metamorphic and sedimentary petrology they can be used as a test of isotopic equilibrium or disequilibrium in natural mineral assemblages over all temperature ranges of geological interest the difference in oxygen isotope composition between the hydroxyl bearing mineral and the oh group is quantitatively demonstrated to be temperature dependent and therefore can be used as a single mineral geothermometer

mineralogical and chemical evolution of the ernest henry fe oxide cu au ore system cloncurry district northwest queensland australia the ernest henry cu au deposit was formed within a zoned post peak metamorphic hydrothermal system that overprinted metamorphosed dacite andesite and diorite ca 1740 1660 ma the ernest henry hydrothermal system was formed by two cycles of sodic and potassic alteration where biotite magnetite alteration produced in the first cycle formed ca 1514 24 ma whereas paragenetically later na ca veining formed ca 1529 11 8 ma these new u pbtitanite age dates support textural evidence for incursion of hydrothermal fluids after the metamorphic peak and overlap with earlier estimates for the timing of cu au mineralization ca 1540 1500 ma a distal to proximal potassic alteration zone correlates with a large up to 1 5 km k fe mn ba enriched alteration zone that overprints earlier sodic alteration mass balance analysis indicates that k fe mn ba alteration largely produced during pre ore biotite and magnetite rich alteration is associated with k rb cl ba fe mn and as enrichment and na ca and sr depletion the aforementioned chemical exchange almost precisely counterbalances the mass changes associated with regional na ca alteration this initial transition from sodic to potassic alteration may have been formed during the evolution of a single fluid that evolved via alkali exchange during progressive fluid rock interaction cu au ore dominated by co precipitated magnetite minor specular hematite and chalcopyrite as breccia matrix forms a pipe like body at the core of a proximal alteration zone dominated by k feldspar alteration both the core and k feldspar alteration overprint na ca alteration and biotite magnetite k fe alteration ore was associated with the concentration of a diverse range of elements e g cu au fe mo u sb w sn bi ag f ree k s as co ba and ca mineralization also involved the deposition of significant barite k ba feldspar calcite fluorite and complexly zoned pyrite the complexly zoned pyrite and variable k ba feldspar versus barite associations are interpreted to indicate fluctuating sulphur and or barium supply together with the alteration zonation geochemistry and overprinting criteria these data are interpreted to indicate that cu au mineralization occurred as a result of fluid mixing during dilation and brecciation in the location of the most intense initial potassic alteration a link between early alteration na ca and k fe and the later k feldspathization and the cu au ore is possible however the ore related enrichments in particular elements especially ba mn as mo ag u sb and bi are so extreme compared with earlier alteration that another fluid possibly magmatic in origin contributed the diverse element suite geochemically independently of the earlier stages structural focussing of successive stages produced the distinctive alteration zoning providing a basis both for exploration for similar deposits and for an understanding of ore genesis

hydrodynamics and sedimentation in wave dominated coastal environmentsreconstruction of ancient sea conditions with an example from the swiss molasse ancient sea conditions can be estimated from the grain size spacing and steepness of preserved ripple marks the element of greatest uncertainty in such reconstructions is the relationship between near bed orbital diameter of water particles and the ripple spacing this relationship is simple for vortex ripples of high steepness but is problematical for the low steepness forms known as post vortex rolling grain or anorbital ripples the existence field for wave ripples is between the threshold velocity for sediment movement and the onset of sheet flow most low steepness forms occurring close to the bed planation threshold a range of maximum period of formative waves can be obtained using combinations of orbital diameter and orbital velocity assuming linear wave theory to be a reasonable approximation probable wave heights wave lengths and water depths can be investigated using the transformation of wave parameters in shallowing waters and the constraints on wave dimensions provided by the wave breaking condition given reasonable estimates of wave height crude estimates of wave power allow a comparison of ancient wave influenced sequences with modern counterparts wave ripple marks preserved in the upper marine molasse of western switzerland have been investigated results which are in agreement with regional geology suggest deposition in a seaway of approximately 100 km width where moderate period waves t 3 6 s were generated the depositional facies belts were adjusted to the prevailing waves tides and fluvial outflows

isolated pods of subaqueous welded ash flow tuff a distal facies of the capel curig volcanic formation ordovician north wales acidic welded tuffs of the capel curig volcanic formation were deposited from ash flows that transgressed from a proximal subaerial to a distal submarine environment isolated pods of welded tuff enveloped by marine sediments represent a distal facies of the formation deposited beyond the limits of the continuous ash flow tuff sheets various mechanisms for the separation of the tuff pods from a parent flow are discussed and it is concluded that it is most likely that the formation of the pods was due to disruption of a subaqueous ash flow during transport rather than by downslope collapse at the front of a previously deposited ash flow tuff sheet

hydrothermal mobility of ti zr and ree examples from the bergell and adamello contact aureoles italy contact metamorphic marbles affected by metasomatic fluids at the contact to the bergell and adamello intrusives contain various accessory minerals zirconolite allanite titanite rutile geikielite hoegbomite which provide new evidence for the hydrothermal mobility of ti and zr in both examples ti and zr migrated along with u th y and ree in a metasomatic fluid rich in potassium the composition of the main minerals fluorine rich phlogopite pargasite and titanian clinohumite and the abundance of fluor apatite demonstrate that fluorine and phosphorus were important components of the fluid the textural relationships indicate that the formation of the accessory phases is linked to the crystallization of the hydrous minerals

carbon and oxygen isotope constraints on fluid sources and fluid wallrock interaction in regional alteration and iron oxide copper gold mineralisation eastern mt isa block australia the source of metasomatic fluids in iron oxide copper gold districts is contentious with models for magmatic and other fluid sources having been proposed for this study 18o and 13c ratios were measured from carbonate mineral separates in the proterozoic eastern mt isa block of northwest queensland australia isotopic analyses are supported by petrography mineral chemistry and cathodoluminescence imagery marine meta carbonate rocks ca 20 5 18o and 0 5 13c calcite and graphitic meta sedimentary rocks ca 14 18o and 18 13c calcite are the main supracrustal reservoirs of carbon and oxygen in the district the isotopic ratios for calcite from the cores of na ca alteration systems strongly cluster around 11 18o and 7 13c with shifts towards higher 18o values and higher and lower 13c values reflecting interaction with different hostrocks na ca rich assemblages are out of isotopic equilibrium with their metamorphic hostrocks and isotopic values are consistent with fluids derived from or equilibrated with igneous rocks however igneous rocks in the eastern mt isa block contain negligible carbon and are incapable of buffering the 13c signatures of co2 rich metasomatic fluids associated with na ca alteration in contrast plutons in the eastern mt isa block have been documented as having exsolved saline co2 rich fluids and represent the most probable fluid source for na ca alteration intrusion proximal skarn like cu au orebodies that lack significant k and fe enrichment e g mt elliott display isotopic ratios that cluster around values of 11 18o and 7 13c calcite indicating an isotopically similar fluid source as for na ca alteration and that significant fluid wallrock interaction was not required in the genesis of these deposits in contrast k and fe rich intrusion distal deposits e g ernest henry record significant shifts in 18o and 13c towards values characteristic of the broader hostrocks to the deposits reflecting fluid wallrock equilibration before mineralisation low temperature low salinity low 18o 10 calcite and co2 poor fluids are documented in retrograde metasomatic assemblages but these fluids are paragenetically late and have not contributed significantly to the mass budgets of cu au mineralisation

a subaqueous welded tuff from the ordovician of county waterford ireland abstract the metal man tuff mmt from the ordovician of county waterford ireland was emplaced and welded in water depths greater than the thickness of the pyroclastic flow the mmt is the basal member of the middle tramore volcanic formation mtvf of the 5 km thick tramore group the mmt consists of a 10 m thick basal graded zone that represents a pyroclastic flow consisting of angular clasts of black mudstone pumice gray flow banded rhyolite and pink massive rhyolite set in a matrix of non deformed ash shards and pumice maximum grain size grades from large cobbles and small boulders to pebbles the basal 10 30 cm is depleted with respect to the largest boulders resulting in an inversely graded basal layer the basal graded zone passes upward into a transition zone with a strong eutaxitic foliation defined by elongated fiamme of mudstone and flattened pumice overlying this is an upper welded zone with a pronounced eutaxitic foliation columnar jointing flattened ash shards and shards deformed around phenocrysts and spheroids the presence of these features indicate that the deposit is welded was hot and was in motion as the shards deformed the mmt represents a pyroclastic flow that was a hot primary product of an eruption rather than re mobilized cold pyroclastic debris the mmt is bounded by suspension deposited fine grained tuff tuffaceous mudstone and terrigenous mudstone deposited below storm wave base many of the mudstone horizons contain brachipod faunas from shelf depth water nowhere in the 5 km thick tramore group is there terrigenous sandstone evidence of unidirectional flowing water nor any indication of shoreline alluvial environments or subaerial exposure it thus seems reasonable to conclude that the mmt was emplaced and welded subaqueously the geochemistry of the mmt is typical of other high silica 70 78 wt sio2 rhyolite from the ordovician of ireland the chemistry of the mmt is consistent from top to bottom allowing it to be distinguished from associated syndepositional intrusive rock and fine grained tuffs

the petrology and geochemistry of alteration assemblages in the eastern creek volcanics as a guide to copper and uranium mobility associated with regional metamorphism and deformation mount isa queensland

volcaniclastic sedimentation in and around an ordovician subaqueous caldera lower rhyolitic tuff formation north wales the lower rhyolitic tuff formation of ordovician age in north wales records the collapse infilling and subsequent resurgence of a volcanic caldera with an original diameter of about 15 km this volcanic center controlled patterns of volcaniclastic sedimentation providing enough topographic relief for both a shallow lagoon depositional basin within the caldera and a source of sediment derived from the rim within the caldera sediment consists of tuffaceous laminated siltstone and immature coarse grained volcaniclastic sandstone containing plane beds ripple cross laminations symmetrical wave ripples and hummocky cross stratification coarse grained matrix supported conglomerate layers and layers of ash flow tuffs are also present these sediments accumulated in shallow water above fair weather wave base conglomerate units represent debris flows from the caldera rim a nearby shoreline and elevated areas associated with resurgent domes sedimentation outside of the caldera consisted of deposition of background suspension and volcanic ash suspension and turbidite deposition on a pyroclastic apron the outer margin of the apron was dominated by fine grained suspension and turbidite deposition whereas the inner margin of the apron contains hummocky cross stratification and other evidence of reworking by episodic storm waves local highs with associated shallow water sedimentation existed outside the caldera even though deposited in high energy marine environments all sedimentary rocks are both texturally and mineralogically very immature this textural immaturity differs from the typical very mature marine sediments and was caused by rapid depositional rates and a local volcanic sediment source

age dating of individual grains of uraninite in rocks from electron microprobe analyses it is shown that an electron microprobe analysis of the u th and pb content of a single uraninite uo2 grain may be used to calculate the age of formation of that grain this is known as a chemical age examples are shown for a range of natural samples with a wide range of ages comparison with ages derived from isotopic measurements shows the results to be valid approximate formulae which have been proposed for the calculation of the chemical age are not sufficiently reliable especially where the th content is high and for older ages a method for calculating the age by iteration is presented this uses the full and exact formula and is shown to produce a reliable age for the uraninite compared to isotopic measurements the method is particularly useful for samples where the minerals and mineral textures record information on more than one event two examples of samples recording information on dual ages are discussed to show how these ages can be related to events in the formation of the host rocks in one example an alluvial heavy mineral concentrate is shown to contain uraninites from two distinct sources with significantly different ages in the other example two phases of u mineralisation are distinguished in the same rock sample one phase relates to the genesis of the sediments from which the rock was formed the second phase records remobilisation of the u during later tectonic movement

thrusting and duplex formation at mount isa queensland australia large scale repetition of mount isa and haslingden group rocks in the anticline north of mount isa is interpreted as the core of a thrust duplex which has been subsequently folded parallel to the direction of thrusting displacement of mount isa group rocks above the duplex is extremely significant to the regional geology and exploration for more mount isa style pb zn and cu orebodies interpretation of the geometry of the thrust belt indicates thrusting on different stratigraphical levels below and above the duplex during its formation horses formed from a heelwards protruding footwall ridge instead of a trough and the complex effects on stratigraphical cross sections through duplexes of sloping lateral footwall ramps on horses