copper mobility in the eastern creek volcanics mount isa australia evidence from laser ablation icp ms of iron titanium oxides the palaeoproterozoic eastern creek volcanics are a series of copper rich tholeiitic basalts which occur adjacent to the giant sediment hosted mount isa copper deposit in queensland australia the volcanic rocks are often cited as the source of metals for the deposit new laser ablation icp ms analyses of iron titanium oxides from the basalts provide evidence for the local mobilisation of copper during regional greenschist facies metamorphism this interpretation is based on the observation that copper bearing magmatic titanomagnetite was destabilised during greenschist facies metamorphism and the new magnetite which crystallised was copper poor petrological observations regional geochemical signatures and geochemical modelling suggest that the mobilised copper was concentrated in syn metamorphic epidote rich alteration zones creating a pre concentration of copper before the main mineralisation event at mount isa geochemical modelling demonstrates this process is enhanced by the addition of co2 from adjacent carbonate rich sediments during metamorphic devolatilisation regional geochemical data illustrate elevated copper concentrations in epidote rich zones high cao but where these zones are overprinted by potassic alteration high k2o copper is depleted a two stage model is proposed whereby after metamorphic copper enrichment in epidote titanite alteration zones an oxidised potassium rich fluid leached copper from the epidote altered metabasalts and deposited it in the overlying sedimentary rocks to form the mount isa copper deposit this ore forming fluid is expressed regionally as potassium feldspar rich veins and locally as biotite rich alteration which formed around major fluid conduits between the metabasalt metal source rocks and the overlying deposit host sequence this model is consistent with the remobilisation of copper from mafic source rocks as has been found at other world class copper deposits

on the variation of so2 emission from volcanoes introduction these only since 1979 and 1980 respectively however as part of an on going research proin 1973 stoiber and jepsen used remote sengram the soz flux from several volcanoes in sing techniques to measure the s02 flux from nicaragua guatemala and sicily have been several volcanoes in central america using measured on a fairly regular basis the object these data they estimated that the global conof this paper will be to examine the results from tribution of so2 from volcanoes was on the order some of those volcanoes and interpret how they of 7 10 g metric tons per year this initial estimight effect flux estimate and prediction mate neglected sulfur in any form other than techniques gaseous so2 and did not include the contribution from erupting volcanoes this number was instrumentation and techniques subsequently modified by stoiber et al 1983b and stoiber et al 1987 this volume to around the instrument used to remotely measure the 1 87 107 tons per year and includes sulfate soz flux is a correlation spectrometer cosaerosols and emission from erupting volcanoes pec developed by barringer resarch in another important application of remote toronto canada primarily for environmental sensing of s02 from volcanoes is forecasting of monitoring of soz the theory of the cospec eruptions this has proven feasible at mt etna operation has been well documented and inter malinconico 1979 and mount st helens ested readers are referred to the following casadevall et al 1981 papers newcomb and millan 1970 moffat for both of these applications it is important and millan 1971 millan et al 1976 millan to understand the longand short term pat 1978 1980 and millan and hoff 1978 terns of so2 emission from volcanoes continthe cospec provides straightforward uous monitoring of all volcanoes is at best techniques for remotely monitoring the soz flux impractical only kilauea and mount st from volcanoes the instrument is compact helens have nearly continuous records and durable and can be used in a variety of modes

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

assessing the origin of old apparent ages derived by pb stepwise leaching of vein hosted epidote from mount isa northwest queensland australia epidote metasomatism affected large areas of tholeiitic metabasalts of the 1 780 ma eastern creek volcanics in the western fold belt of the proterozoic mount isa inlier hydrothermal epidote generally occurs in quartz veins parallel to or boudinaged within the dominant s2 fabrics which formed during the regional metamorphic peak at 1 570 ma associated with the isan orogeny previously published stable isotopic and halogen data suggest that the fluids responsible for epidote formation are metamorphic in origin with an evaporitic component application of the pb stepwise leaching technique to the epidote does not separate radiogenic pb4 and common pb2 generating little spread in 206pb 204pb between 16 0 and 30 5 the causes for this relatively low range are twofold there is little radiogenic pb in the epidotes the most radiogenic steps account for 1 of pb released and both pb2 and uranogenic pb4 substitute into the same site in the epidote crystal lattice consequently age regressions using the pb stepwise leaching data give ages between 150 and 1 500 myrs older than the host rocks and over 450 myrs older than the thermal metamorphic peak these old ages are attributed to chemical inheritance from the host metabasalts via radiogenic pb release by breakdown of phases such as zircon monazite titanomagnetite and ilmenite during metamorphism this idea is supported by trace element data and chrondrite normalized rare earth element patterns that are similar to both the metabasalts and epidotes except for a variable eu anomaly in the latter relatively high fo2 during vein formation fe3 dominates in the epidote crystal lattice would allow the incorporation of th4 and exclusion of u6 and would explain elevated th u ratios up to 12 in epidote compared with the host metabasalts non incorporation of u would explain the relatively low u pb ratios and non radiogenic character of the epidote this process may provide a source of metal for the small u deposits around mount isa and may also suggest a relationship between u mineralization and regional cu mobilization during the isan orogeny our work suggests that non conventional geochronometers should be used only if additional geological information and geochemical data e g mineral chemistry trace elements are available to evaluate any resulting age calculations

rift flank segmentation basin initiation and propagation a neotectonic example from lake baikal new surficial data field landsat tm and topography define morpho tectonic domains and rift flank segmentation in the ol9khon region of the central baikal rift deformation drainage and depositional patterns indicate a change in the locus of active extension that may relate to a recent

new faults v fault reactivation implications for fault cohesion fluid flow and copper mineralization mount gordon fault zone mount isa district australia fluid flow leading to mineralization can occur both on newly formed faults and on faults that are reactivated subsequent to their initial formation conventional models of fault reactivation propose that under high pore fluid pressures misorientated faults may reactivate due to low fault cohesion timing and orientation data for a mineralized palaeo to mesoproterozoic terrain mount gordon fault zone mgfz indicate that multiple successive new orientations of predominantly strike slip faults developed between 1590 and c 1500 ma requiring that during the younger deformations some earlier formed faults were too cohesive and or had insufficient pore fluid pressures or other potential fault weakening effects to induce reshear low pore fluid pressures were probably not to blame for failed reactivation on all older faults because some young faults did form or reactivate due to high pore fluid pressures as evidenced by jigsaw fit dilatant breccias hypogene copper mineralization in veins and breccia infill and subvertical tensile quartz veins aligned subparallel to 1 the assumption that old faults consistently have little or no cohesion appears to be incorrect in this terrain many older faults display prominent quartz veins along their length which may explain this conclusion furthermore faults with high cohesion may have acted as barriers and compartments so that intersections between them and newly formed faults host mineralization not because of reactivation but because of interaction between new faults and cohesive materials defined either by fault precipitates or rock juxtaposition together these results and observations provide new simple tools to stimulate copper exploration within the region and in fault hosted terrains

sequence stratigraphy in active extensional basins implications for the interpretation of ancient basin fills abstract systematic variations in sequences their component units and their stacking patterns exist within rift basins these variations can be related to displacement gradients associated with the large scale 12 50 km segmentation of normal fault zones which control accommodation sediment supply and basin physiography sediment supply is also strongly influenced by the nature of pre rift drainage networks and variations in bedrock lithology high rates of hangingwall subsidence close to the centre of normal fault segments may cancel out the effects of glacio eustatic sea level fall so that accommodation development is normally characterized by the continual addition of new space the resulting sequences lack type 1 sequence boundaries and lowstand systems tracts and stack into aggradational sequence sets the adjacent footwall is subject to uplift which may lead to subaerial exposure and incision generating a composite type 1 sequence boundary away from the fault zone and near segment boundaries slip rates are much lower and hence relative sea level change is dominated by eustasy here falls in relative sea level are important resulting in type 1 sequences sequence stacking patterns reflect not only eustasy and local fault controlled subsidence but also the interaction of these with adjacent uplifting footwalls and or regional uplift tectonic influence on sequence development will be more pronounced during greenhouse times than icehouse times

geology and alteration geochemistry of the porphyry cu au deposit at bajo de la alumbrera argentina this paper describes the igneous geology alteration mineralogy and geochemistry as well as the ore mineralization history of bajo de la alumbrera a world class porphyry copper gold deposit in northwestern argentina the deposit occurs in the k rich calc alkaline farallon negro volcanic complex located 200 km east of the main andean porphyry copper belt of chile the deposit consists of a composite stock of dacitic porphyry intrusions and extends into the surrounding andesitic volcanic rocks two of the earliest intrusions now occupying the center of the intrusive complex show intense hydrothermal alteration and mineralization these two porphyries host the highest ore grades and were subsequently intruded by several weakly mineralized or barren porphyries intense quartz magnetite k feldspar alteration pervades the first mineralized porphyry whereas similar but lower intensity alteration affected the second laterally this alteration grades into potassic alteration secondary k feldspar and biotite which affects all porphyries except the latest dikes andesitic wall rocks within a hundred meters of the intrusions are dominated by dark hydrothermal biotite an outer halo of propylitic alteration epidote chlorite albite calcite extends up to 1 km into the andesites feldspar destructive alteration sericite pyrite clay minerals gypsum overprints the potassic and propylitic alteration in the volcanic rocks and all porphyries the alteration is locally controlled by faults and late fractures and developed most pervasively upon the outer part of the potassic zone and toward the transition to propylitic alteration in the upper part of the deposit mass balance calculations show small positive changes in net volume for the potassic alteration and minor volume decreases for the chlorite epidote and feldspar destructive alteration the intensely quartz magnetite veined sample of quartz magnetite alteration indicates a total volume increase of 350 percent copper and gold are intimately associated at all scales they occur as composite gold sulfide grains chalcopyrite rarely bornite show a close correlation of ore grades in sample assays and have a closely overlapping distribution on the mine scale a well developed ore shell coincides with potassic alteration and partly with the distribution of intense quartz magnetite veins even though the bulk of the ore minerals now occupy texturally late positions in vugs and fractures a barren zone with intense quartz magnetite potassic alteration and veins but little or no sulfides is interpreted as the focus of upflow of high temperature ore fluids prior to ore mineral saturation and coprecipitation of copper sulfides and gold in the ore shell

rhenium osmium systematics of the mount isa copper orebody and the eastern creek volcanics queensland australia implications for ore genesis the syn tectonic breccia hosted mount isa cu deposit in northwest queensland is the largest sediment hosted cu deposit in australia whole rock samples of chalcopyrite rich cu ore form an isochron with a re os age of 1 372 41 ma this age is more than 100 ma younger than the previously accepted age of cu ore formation an ar ar mineral age for biotite separated from the host rocks within the alteration envelope to the cu orebody this discrepancy cannot be unequivocally resolved due to a lack of other absolute geochronological constraints for cu mineralisation or the deformation event associated with cu emplacement the 1 372 41 ma date may reflect a the time of cu deposition b the time of a hydrothermal event that reset the re os signature of the cu ore or c mixing of the re os isotope systematics between the host rocks and cu bearing fluids however a range of published ar ar and rb sr dates for potassic alteration associated with cu mineralisation also records an event between 1 350 and 1 400 ma and these are consistent with the 1 372 ma re os age the 1 8 ga eastern creek volcanics are a series of tholeiitic basalts with a primary magmatic cu enrichment which occur adjacent to the mount isa cu deposit the whole rock os isotopic signature of the eastern creek volcanics ranges from mantle like values for the upper pickwick member to more radiogenic crustal values for the lower cromwell member the re os isotope signature of the cu ores overlaps with those calculated for the two volcanic members at 1 372 ma hence the os isotope data are supportive of the concept that the os in the cu ores was sourced from the eastern creek volcanics by inference it is therefore postulated that the eastern creek volcanics are the source of cu in the mount isa deposit as both os and cu are readily transported by oxidised hydrothermal fluids such as those that are thought to have formed the cu orebody the pickwick member yields a re os isochron age of 1 833 51 ma which is within error of previously reported age constraints the initial 187os 188os isotopic ratio of 0 114 0 067 os 0 7 is slightly subchondritic and together with other trace element geochemical constraints is consistent with a subcontinental lithospheric mantle source the pickwick member records a minimum age of ca 1 95 ga for melt depletion in the subcontinental lithospheric mantle beneath the mount isa inlier prior to the extraction of the magmas which formed the eastern creek volcanics this corresponds with the end of subduction related magmatism along the eastern margin of the northern australian craton which included the 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

9 processes of tectonism magmatism and mineralization lessons from europe abstract metallogenic provinces in europe range in age from the archaean to the neogene deposit types include porphyry copper and epithermal cu au volcanic hosted massive sulphide vms orogenic gold fe oxide cu au anorthosite fe ti oxide and sediment hosted base metal deposits most of them formed during short lived magmatic events in a wide range of tectonic settings many can be related to specific tectonic processes such as subduction hinge retreat accretion of island arcs continental collision lithosphere delamination or slab tear in contrast most sediment hosted deposits in europe evolved in extensional continental settings over significant periods of time in europe as elsewhere ore formation is an integral part of the geodynamic evolution of the earth s crust and mantle many tectonic settings create conditions conducive to the generation of water rich magma but the generation of ore deposits appears to be restricted to locations and short periods of change in temperature and stress imposed by transitory plate motions crustal influence is evident in the strong structural controls on the location and morphology of many ore deposits in europe crustal scale fault fracture systems many involving strike slip elements have provided the fabric for major plumbing systems rapid uplift as in metamorphic core complexes and hydraulic fracturing can generate or focus magmatic hydrothermal fluid flow that may be active for time spans significantly less than a million years once a hydrologically stable flow is established ore formation is strongly dependent on the steep temperature and pressure gradients experienced by the fluid particularly within the upper crust in europe significant fracture porosity deep in the crystalline basement 1 is not only important for magmatic hydrothermal systems but allows brines to circulate down through sedimentary basins and then episodically upward expelled seismically to produce sediment hosted base metal deposits and kupferschiefer copper deposits emerging research stimulated by geode can improve the predicting power of numerical simulations of ore forming processes and help discover the presence of orebodies beneath barren overburden

geochronological constraints on tourmaline formation in the western fold belt of the mount isa inlier australia evidence for large scale metamorphism at 1 57 ga boron metasomatism during two deformational events resulted in tourmaline formation in parts of late palaeoproterozoic eastern creek volcanics of the western fold belt of the mount isa inlier despite low u pb ratios 1 as measured by laser ablation icp ms when the step leaching process was applied to the tourmaline it process extracted pb fractions with 206 pb 204 pb up to 76 illustrating the method s ability to efficiently separate radiogenic and common pb that are hosted in different crystal lattice sites tourmaline from two syn d2 metamorphic hydrothermal veins at anderson s lode and eldorado yield similar pb step leaching ages of 1573 12 and 1577 48 ma respectively tourmaline from a post d2 but pre d3a vein at king was dated at 1528 51 ma although there is evidence to suggest some contribution from u rich inclusions i e zircon and monazite to some leach steps the isochrons are thought to provide robust ages for hydrothermal tourmaline formation during two discrete thermal events this is supported by estimates of pb closure temperatures in tourmaline derived from ionic porosity models the 1575 ma ages for d2 also reflect the main metamorphic event in the western fold belt but are 40 myr older than accepted age estimates for this episode the revised timing for peak metamorphism tectonism in the western fold belt corresponds with the age of metamorphism in the eastern fold belt suggesting broadly coeval tectonic histories across the entire inlier from at least 1600 ma

forced regressions in a large wave and storm dominated anoxic lake rhaetian sinemurian kap stewart formation east greenland during rhaetian sinemurian time a large wave and storm dominated lake was situated in the jameson land basin east greenland lake deposits consist of alternating black unfossiliferous mudstones and sheet sandstones anoxic conditions dominated at the lake bottom during deposition of the muds and the water column was probably stratified the sandstones were deposited by progradation of wave and storm dominated deltas in a water depth of less than 15 m sequence stratigraphic interpretation suggests that the mudstones were deposited in periods of rising and very high stands of lake level whereas progradation of the deltaic sheet sandstones took place during forced regressions caused by significant falls the lake thus underwent a large number of fairly high amplitude changes in level probably caused by climatic fluctuations the high order cycles can be grouped into several long period cycles that show the same number of major fluctuations as published eustatic sea level curves this similarity suggests a causal link between eustasy and long period variations in the lake the kap stewart formation represents one of the few ancient examples of a large wave and storm dominated lake and it is probably the first documented case of abundant well developed lacustrine forced regressions

depositional and tectonic framework of the rift basins of lake baikal from multichannel seismic data recent multichannel seismic reflection data from lake baikal located in a large active continental rift in central asia image three major stratigraphic units totaling 3 5 to 7 5 km thick in four subbasins a major change in rift deposition and faulting between the oldest and middle rift units probably corresponds to the change from slow to fast rifting in early pliocene time inferred from on land studies a minor modification of fault patterns characterizes the youngest unit a brief comparison of the basins of lake baikal with those of the east african rift system highlights differences in structural style that can be explained by differences in age and evolution of the surrounding basement rocks

biogenic silica accumulation and paleoproductivity in the northern basin of lake baikal during the holocene the effects of the holocene climatic amelioration productivity and preservation on biogenic silica accumulation in the northern basin of lake baikal are evaluated by analyzing biogenic silica organic carbon nitrogen and the stable carbon isotopic composition of organic matter in a 4 m piston core and a companion box core in the early holocene 11 000 8500 yr b p biogenic silica accumulation was low because of low productivity strong oligotrophy and the influx of nutrient limited alpine glacial meltwater in the middle holocene productivity increased significantly during 8500 7000 yr b p but little biogenic silica accumulated in sediments because the dominant primary producer was not diatom flora and because of the relatively higher productivity caused by enhanced terrestrial nutrient influx the highest accumulation rates of biogenic silica were those between 7000 and 4500 yr b p when aquatic productivity increased limnological stratification occurred a mature diatom flora developed in the late holocene the past 4500 yr biogenic silica accumulation decreased because of enhanced recycling and regeneration of silica and a low sedimentation rate

abundance of co 2 rich fluid inclusions in a sedimentary basin hosted cu deposit at jinman yunnan china implications for mineralization environment and classification of the deposit the jinman cu deposit is hosted in sandstones and slates of the jurassic huakaizuo formation in the mesozoic to cenozoic lanping basin in western yunnan china despite the fact that cu mineralization occurs mainly in quartz carbonate veins controlled by faults and fractures the jinman deposit was classified as a sediment hosted stratiform cu deposit mainly because it is hosted in a sedimentary basin characterized by abundant red beds with many stratiform cu deposits a detailed petrographic and microthermometric study of fluid inclusions from the jinman deposit reveals the presence of abundant co2 rich fluid inclusions together with aqueous inclusions the co2 rich inclusions have co2 melting temperatures mainly from 58 0 c to 56 6 c homogenization temperatures of the carbonic phase mostly into the liquid phase mainly between 22 c and 30 c clathrate melting temperatures from 1 8 c to 9 2 c with corresponding salinities from 1 6 to 13 4 wt nacl equivalent and total homogenization temperatures from 226 c to 330 c the aqueous inclusions have first melting temperatures from 60 c to 52 c ice melting temperatures from 41 4 c to 2 3 c with salinities from 3 9 to 29 0 wt nacl equivalent and total homogenization temperatures mainly from 140 c to 250 c these fluid inclusion characteristics are comparable to those of orogenic or magmatic mineralization systems and are uncharacteristic of basinal mineralization systems suggesting that it is inappropriate to classify the jinman deposit as a sediment hosted stratiform cu deposit the results of this study together with geochemical data reported previously suggest that the jinman deposit formed in a hydrothermal system that involved both extra basinal deeply sourced co2 rich fluid and basinal aqueous fluid

linking of regional and local hydrothermal systems in the mid crust by shearing and faulting abstract the concentration of mass during regional metamorphism and metallogenesis requires interactions between regional and local hydrothermal systems that are scale and time dependent during regional low to medium temperature metamorphism in which fluid flow is driven by broad thermal or mechanical gradients fluid flow rates are on average 8 m s and are limited by restricted permeabilities and gentle gradients in hydraulic head in contrast coseismic fluid flow rates in major fault zones may reach 10 2 m s as a consequence there may be a limited capacity for individual faults to substantially control fluid flow in the region because of the large contrast in flow rates and permeabilities between faults and low permeability rocks to produce a major epigenetic ore deposit in overpressured crust fluid flow systems with such diverse flow rates must interact in such a way that metal dissolution and transport from regional fluid flow systems culminate in focused fluid flow with metal deposition this is best achieved through two steps one in which km scale hydrothermal systems channel into 100 m scale ductile shear zones largely by reaction and deformation enhanced permeability in the shear zone and a second one in which rapid fluid pressure changes accompanying brittle faulting or intense shearing within the broader shear zones causes metal deposition and ore genesis

hematite breccia hosted iron oxide copper gold deposits require magmatic fluid components exposed to atmospheric oxidation evidence from prominent hill gawler craton south australia

fluid and mass transfer during metabasalt alteration and copper mineralization at mount isa australia metabasalts surrounding the metasediment hosted base metal deposit at mount isa have been modified by metamorphism polyphase deformation and several stages of hydrothermal alteration this study aims to understand the large scale hydrothermal processes that determined the chemical composition of syn to postmetamorphic ore brines prior to being focused into chemicaly anomalous metasediments that acted as a chemical trap for localized copper precipitation field mapping of the metabasalts provided the framework to determine the relative timing of district scale alteration events which were then studied by mineralogic observations and bulk mass balance measurements microthermometry raman microspeetrometry and br cl analyses of fluid inclusions and h o c and s stable isotope measurements on vein minerals were used to characterize the hydrothermal fluids associated with the alteration events all fluids are moderately to highly saline variably ca enriched with high br cl compared to modern seawater and isotopiealy most akin to modern evolved basin or low grade metamorphic brines the timing and fluid tracer observations in conjunction with thermodynamic mass transfer modeling permit the following correlation between district scale alteration processes and the mine scale hydrothermal events associated with copper deposition at mount isa district scale epidote sphene alteration of metabasalts temporally overlapped with the peak of greenschist facies metamorphism and deformation and is widely distributed in flow top breccias and as halos around synmetamorphic quartz calcite veins it is associated with calcic brines with 12 to 38 wt percent cacl 2 nacl equiv and thermodynamic modeling suggests that the observed mineralogy and bulk mass changes can be explained by focusing of metabasalt equilibrated fluids and by reaction with similar rocks at somewhat higher temperature similar timing and fluid characteristics are associated with dolomitic alteration at the mount isa mine where mixing of metabasalt derived calcic brine with metamorphic co 2 from the metasediments led to massive dolomite precipitation kilometer scale fracture zones associated with carbonate fe oxide alteration calcite albite magnetite or hematite or biotite or chlorite cut all penetrative foliations in the metabasalts east of mount isa they are locally associated with uranium mineralization and are characterized by medium salinity nacl rich fluid inclusions 8 93 wt nacl equiv carbonate fe oxide alteration is interpreted to mark channelways of infiltration by oxidized sulfate bearing brines from outside the metabasalt sequence possibly from an overlying evaporitic cover sequence fluid characteristics evidence for extreme copper mobility and timing criteria of alteration relative to deformation suggest a temporal and possibly genetic correlation with the main copper and silica deposition stage at the mount isa mine mg chlorite rutile alteration is restricted to metabasalts immediately below the copper deposit and the vicinity of the mount isa fault zone where higher grade amphibolite facies metabasalts were upthrust against the greenschist facies metabasalts and metasediments now hosting the copper ore bodies mg chlorite rutile alteration of metabasalts and copper mineralization in the adjacent reduced sediments can both be explained by fluid focusing along an actively extending contact zone between the oxidized metabasalts and the reducedmetasediments the brines focused into this contact zone were initially oxidized as a result of infiltration through the carbonate fe oxide fracture zones and less altered metabasalts and became reduced by reaction with the metasediments the unusual geochemical and isotopic low 13 c and high 34 s composition of mg chlorite rutile alteration can be explained as a result of back flow of reduced fluids from the metasediments into the metabasalts although mg chlorite rutile alteration of metabasalts probably liberated some of the copper now present in the orebodies this alteration may be a relatively localized effect associated with the site of ore deposition rather than an essential sourcing mechanism for the copper the regional feature of greatest significance to ore formation at mount isa and to exploration for similar high grade copper deposits elsewhere is not the somewhat elevated initial copper content of the metabasalts nor their locally significant copper depletions but their moderately high oxidation state evident from the ubiquitous occurrence of fe rich epidote and the large scale fe oxide altered fracture zones oxidation of the basalts or any other silicate rocks favored high copper solubility along extensive transport paths and provided the essential chemical contrast to the depositional environment which was anomalously reduced and sulfide rich as a result of an earlier process of sedimentary to diagenetic pb zn fe s mineralization

the geology of the nifty copper deposit throssell group western australia implications for ore genesis the nifty syndeformational sediment replacement cu deposit is located approximately 450 km southeast of port hedland western australia in subgreenschist grade rocks of the neoproterozoic paterson orogen the resource of 99 million metric tons mt 1 7 percent cu is hosted in a syncline within carbonaceous and dolomitic shales of the upper broadhurst formation the cu mineralization occurred during neoproterozoic dy2 deformation and formed at the center of a zoned hydrothermal alteration system interbedded dolomitic and carbonaceous shale of the upper and lower units of the nifty member was altered s in two stages early siliceous s alteration with minor chlorite pyrite hematite sericite and stilpnomelane which was replaced by syntectonic black quartz in the core of the deposit on the fringes of the alteration framboidal pyrite in the carbonaceous shale was replaced by quartz chalcopyrite euhedral pyrite sphalerite and galena inward is a sequence of interlayered hydrothermal quartzdolomite and unaltered shale beds the hydrothermal quartz dolomite first appears as spots and 2 to 4 mm thick veins with light gray diffuse margins in areas of intense spotty alteration dolomitic alteration forms bedding parallel zones farther inward and proximal to the highest ore grade is silicified dolomitic shale which grades into fine grained quartz and chalcopyrite with minor fluorapatite and carbonaceous material synmineralization fluid inclusions are small 10 microns liquid vapor inclusions with homogenization temperatures between 200 degrees to 370 degrees c median 270 degrees c and salinity of 8 to 27 wt percent nacl equiv median 15 wt nacl equiv sulfur isotope data from framboidal pyrite range between delta 34 scdt of 27 and 16 per mil chalcopyrite delta 34 scdt ranges between 6 and 6 per mil and delta 34 scdt for euhedral synmineralization pyrite ranges between 12 to 12 per mil carbon and oxygen isotope results from synmineralization vein stage carbonates and dolomitic alteration overlap with delta 13 cvpdb values from 2 to 10 per mil and delta 18o vsmow values from 14 to 21 per mil it is likely that decarbonation of host rock caused the large shift in delta 13 cvpdb values the formation of the nifty cu deposit is linked to deformation tectonic loading associated with the miles orogeny driving fluids from deep in the yeneena basin hydrothermal fluids were focused along dy2 thrust faults during tightening of the nifty syncline thrust faulting penetrated carbonaceous and dolomitic shales allowing hydrothermal fluids to react with the nifty member early fluid pulses were weakly oxidized and contained low concentrations of base metals these fluids altered the host rock and precipitated green quartz the fluid evolved to a hotter more saline reduced and moderately acidic fluid with significant cu concentration these fluids reacted with the dolomite dominated nifty member forming the dolomitic alteration and siliceous replacement of carbonaceous shale and dolomitic mudstone chalcopyrite precipitation was controlled mainly by changes in ph accompanied by a slight decrease in temperature the mineralization and alteration styles at nifty are similar to the copper orebodies at mount isa

the structure and development of the baikal rift depression abstract the baikal rift depression located in southeastern siberia is one of the best investigated intra continental rift structures in the world in addition to being the site of comprehensive studies by russian geo scientists for decades the baikal depression is now the subject of new international investigations using advanced technical means and technologies the present paper summarizes the principal results concerning the morphostructure of the baikal rift the stratigraphy of the cenozoic deposits and the stages of the baikal depression development based on cross sections which exhibit the impact of both tectonic and climatic processes the development of the baikal depression occurred through two stages i the pre crypto rift stage of the late cretaceous eocene and ii active rifting of the oligocene recent the second stage is divided into several steps and substeps separated by tectonic phases formation of the baikal rift zone appears to be related to the indo eurasian plate collision related to a model of passive rifting the uplifted shoulders of the depression have a complex morphostructure overlain by cenozoic deposits which are thin and restricted in area the near marginal zones of the rift are largely defined by the system of longitudinal blocks of different width surfaces of the blocks are inclined either to the uplifted shoulder of the rift bound by listric faults typical for western edges of the depression or to the axis of the depression cenozoic deposits on the blocks consist of accumulative plains composed primarily of thick lacustrine sequences

comparative sequence stratigraphy of low latitude versus high latitude lacustrine rift basins seismic data examples from the east african and baikal rifts abstract lakes baikal malawi and tanganyika are the world s three largest rift valley lakes and are the classic modern examples of lacustrine rift basins all the rift lakes are segmented into half graben basins and seismic reflection datasets reveal how this segmentation controls the filling of the rift basins through time in the early stages of rifting basins are fed primarily by flexural margin and axial margin drainage systems at the climax of syn rift sedimentation however when the basins are deeply subsided almost all the margins are walled off by rift shoulder uplifts and sediment flux into the basins is concentrated at accommodation zone and axial margin river deltas flexural margin unconformities are commonplace in the tropical lakes but less so in high latitude lake baikal lake levels are extremely dynamic in the tropical lakes and in low latitude systems in general because of the predominance of evaporation in the hydrologic cycle in those systems evaporation is minimized in relation to inflow in the high latitude lake baikal and in most high latitude systems and consequently major sequence boundaries tend to be tectonically controlled in that type of system the acoustic stratigraphies of the tropical lakes are dominated by high frequency and high amplitude lake level shifts whereas in high latitude lake baikal stratigraphic cycles are dominated by tectonism and sediment supply variations

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

crustal scale structures in the proterozoic mount isa inlier of north australia their seismic response and influence on mineralisation abstract the proterozoic mount isa inlier in northern australia is prospective for base metals and gold it contains a western fold belt including the leichhardt river fault trough and an eastern fold belt separated by older basement rocks of the kalkadoon leichhardt belt sediments and volcanics in both fold belts were deposited in rifts which were subsequently shortened by up to 50 mineralisation appears to be partitioned large tonnage lead and zinc deposits are more prevalent in the leichhardt river fault trough and most gold and copper occurrences are in the eastern fold belt cross sections of the inlier derived from coincident seismic reflection and refraction profiling are dominated by the youngest tectonic events the refraction data imply a west dipping lens of high velocity intermediate to mafic rock in the middle to upper crust in the east of the inlier it is collinear with another lens in the lower crust in the west of the inlier the lenses form a belt of high velocity rock cutting the crust from top to bottom and from east to west the reflection data reveal different styles of compression related structures in the east and west of the inlier thin skinned tectonics dominate in the eastern fold belt the sediments and volcanics are thrust to the west along a number of shallowly east dipping upper crustal detachments the detachments in turn are cut by steeply east dipping reverse faults which link into the zone of high velocity rocks defined by the refraction data in contrast faults in the western fold belt are steep and penetrate to mid crustal levels and probably also link into the belt of high velocity material deeper in the crust the partitioning into different tectonic styles occurs across the kalkadoon leichhardt belt which appears to have acted as a buttress during the crustal shortening published mineralisation models attribute the lead zinc mineralisation to circulating fluids at shallow crustal levels within the leichhardt river fault trough high reflectivity of faults with a close spatial association with mineralisation is attributed to alteration along the fault caused by migrating fluids copper gold mineralisation in the eastern fold belt is scattered but known major deposits lie along trend from a thrust fault shown in the seismic data to be highly reflective this fault links via the upper crustal detachments and the high velocity lenses into the middle to lower crust and is seen as a likely control on fluid migration pathways from lower crustal levels into the supracrustal eastern fold belt the partitioning of the tectonic styles seen in the seismic data and the mode of linking of faults into the middle to lower crust are seen as primary factors in the partitioning of mineralisation in the region

the proterozoic mount isa fault zone northeastern australia is it really a ca 1 9 ga terrane bounding suture abstract in marked contrast to palaeoproterozoic laurentia the location of sutures and boundaries of discrete crustal fragments amalgamated during palaeoproterozoic formation of the north australian craton remain highly speculative interpretations of suture locations have relied heavily on the analysis of regional geophysical datasets because of sparse exposure of rocks of the appropriate age the mount isa fault zone has been interpreted as one such palaeoproterozoic terrane bounding suture furthermore the coincidence of this fault zone with major shale hosted massive sulphide pb zn ag orebodies has led to speculations that trans lithospheric faults may be an important ingredient for the development of this deposit type this study has integrated geophysical and geochemical data to test the statute of the mount isa fault as a terrane bounding suture forward modelling of gravity data shows that basement rocks on either side of the mount isa fault have similar densities these interpretations are consistent with geochemical observations and sm nd data that suggest that basement lithologies on either side of the mount isa fault are geochemically and isotopically indistinguishable from each other and that the mount isa fault is unlikely to represent a suture zone that separates different palaeoproterozoic terranes our data indicate that the crustal blocks on both sides of the mount isa fault zone must have been in within close proximity of each other since the palaeoproterozoic and that the western fold belt was part of the ancestral north australian craton well before the 1 89 1 87 ga barramundi orogeny it appears that deep crustal variations in density may be related to the boundary between a shallowly west dipping high density mafic to ultramafic plate and low density basement rocks this interpretation in turn impacts on crustal scale models for the development of shale hosted massive sulphide pb zn mineralisation which do not require trans lithospheric faults to tap deep seated metal reservoirs and or mantle plumbing systems the approach applied herein demonstrates the value of multi disciplinary investigations to the critical assessment of long lived proterozoic fault systems which in the absence of methodical analysis are commonly assumed to represent terrane bounding sutures

mapping white mica alteration associated with the jiama porphyry skarn cu deposit central tibet using field swir spectrometry abstract field measurement of the short wave infrared swir reflectance from drill hole samples proved to be an effective method to map variations in white mica composition tschermak substitution associated with hydrothermal alteration zonation of the jiama porphyry skarn cu deposit tibetan plateau china this swir mineral spectral information provides geological insights into a the possible nature of the related magmatic intrusions b the physicochemistry of the associated hydrothermal fluids c the interactions between these fluids and country rocks and d potential vectors to economic mineralization the results suggest that the al oh band wavelength of white mica can used as an indicator in the subsequent steps of prospecting

continental climate response to orbital forcing from biogenic silica records in lake baikal changes in insolation caused by periodic changes in the earth s orbital parameters provide the primary forcing for global ice ages1 6 but it is not clear to what extent the climates in continental interiors are controlled directly by regional variations in insolation and to what extent they are driven instead by the highly nonlinear response of the oceans and ice sheets here we investigate this question using the record of biogenic silica in lake baikal as a proxy for climate change in this high latitude mid continental region we find a good correlation between this record and that of marine oxygen isotopes4 over the past 250 kyr the baikal record exhibits both a strongly nonlinear component manifested in a 100 kyr periodicity and weaker direct insolation components manifested in the 41 kyr obliquity and 23 and 19 kyr precession orbital cycles these results show that even though extreme continental climates such as this are influenced directly by insolation variations they are dominated by the nonlinear rhythm of the oceans and ice sheets

a geochemical process model for the mount isa copper orebodies in this paper we present new data and review existing data on the distribution lithochemistry and paragenesis of copper ore at mount isa and provide the first description of a body of anhydrite dolomite barite and hematite recently intersected at the northern end of the copper system it has been previously proposed that the extraordinary spatial coincidence of giant copper and lead zinc deposits at mount isa is due to the emplacement of copper into host rocks made chemically favorable for copper mineralization by enrichment in dolomite and or pyrite and mechanically favorable by the presence of preexisting layers of ductile lead and zinc sulfide ore several recent studies however suggest that copper and much if not all of the lead and zinc were deposited contemporaneously we provide evidence that high grade 2 5 copper is associated with fractured and silicified rocks early breccia cementing quartz is commonly euhedral and contains numerous growth zones defined by disordered carbon and brine inclusions containing ch4 this quartz is inferred to have been deposited from a cooling brine basement brine that equilibrated with rocks beneath the level of the deposit consistent with earlier studies we have noted that there is often late dolomite dolomite ii infilling of vuggy porosity defined by this euhedral quartz chalcopyrite however postdates both phases and there is textural evidence of both dolomite and quartz dissolution during chalcopyrite formation the nature of the copper bearing fluid basinal brine was deduced from the intimate association of chalcopyrite and anhydrite and from previous fluid inclusion studies this brine was oxidized and sulfur rich but undersaturated in quartz consistent with this fluid being introduced from above as proposed in several previous studies new reaction path modeling suggests that reduction of the oxidized brine by the carbonaceous and urquhart shale was the most likely depositional process many other rock types in the region also had the capacity to reduce oxidized ore forming brines thus ore can be anticipated in these other units provided that sufficient porosity and permeability existed during ore formation the modeling further suggests that the presence of dolomite is not a prerequisite to economic copper grades although carbonate dissolution possibly contributed with fracturing to enhanced porosity and permeability furthermore sulfide rich rocks were no more effective at precipitating copper than sulfide absent rocks consistent with several lines of evidence demonstrating that pyrite or other sulfide minerals were not consumed during ore formation mixing of the basinal and basement brines could also have caused precipitation of copper but the predicted gangue assemblages differ from those observed in nature various radiometric ages are consistent with formation of the mount isa copper deposits after initiation of the unconformable south nicholson basin although we recognize that more precise and accurate dating of the ore and fluid flow events are required basal red bed sediments of the south nicholson basin would have provided suitable aquifers for the oxidized copper rich basinal brines we speculate that the driving force for brine flow was the development of topography to the east of mount isa

chemical mass transfer modelling of ore forming hydrothermal systems current practise and problems abstract chemical mass transfer modelling is a numerical approach to predicting the progress of multicomponent fluid rock reactions in natural hydrothermal systems based on thermodynamic data for minerals and fluid species quantitative simulations of hydrothermal wall rock alteration and ore mineral deposition can be used to develop and test geological models for ore formation which are a basis for mineral exploration and resource assessment in practice the application of thermodynamic mass transfer modelling to specific ore forming systems faces two main problems even for the limiting assumption of local equilibration between fluids and minerals the first problem relates to the geometry of fluid rock interaction where the scheme of the model must be adjusted to the geological question of interest one dimensional reactor models defining reaction progress as a function of integrated fluid flux along a path through a chemically reactive rock do not necessarily provide a more realistic description of chemical mass transfer than simple titration models without temporal and spatial dimension because in most ore depositing systems the fluids are partially channelized by structures like veins or faults the efficiency of metal extraction at the ore deposition site is commonly determined by a sensitive balance between fluid focusing on one hand and wall rock reaction promoting ore mineral precipitation on the other an example of gold and scheelite mineralization associated with mesothermal quartz veins illustrates how ore deposition and alteration zoning parallel to the main fluid flow direction in partially channelized systems can be described by a two dimensional multibox model the second problem relates to the extent and quality of thermodynamic data explicit consideration of fe bearing silicate solid solutions is essential for predicting realistic mass balance in complex fluid silicate sulphide reactions associated with hydrothermal ore formation although a high level of internal consistency of multicomponent data sets has been achieved by simultaneous fitting of multiple thermodynamic and equilibrium measurements significant inconsistencies persist among the most comprehensive data compilations published for the three main groups of species relevant to hydrothermal ore deposits aqueous species in high temperature aqueous brines silicate oxide carbonate minerals including fe mg silicates and sulphur bearing minerals including sulphides and sulphates discrepancies between thermodynamic predictions and observed brine composition and mineral assemblages in the salton sea geothermal system suggest that the redox scale for rock forming silicates may be inconsistent with that of sulphides and aqueous s c o h species this analysis is preliminary but illustrates the importance of field testing of thermodynamic data particularly for hydrothermal conditions between 150 and 450 c which lie between the high temperatures of reliable equilibrium experiments among rock forming minerals 500 c and the low temperatures of many experiments defining the thermodynamics of aqueous species mostly

accurate and precise quantification of major and trace element compositions of calcic sodic fluid inclusions by combined microthermometry and la icpms analysis abstract determination of absolute trace element concentrations in fluid inclusions using laser ablation la icpms requires an internal standard i e the concentration of one element must be independently known from independent observation microthermometric determination of the last melting temperature of ice hydrohalite or halite is routinely used to calculate apparent salinities in wt nacl equivalent using phase relations in the binary h 2 o nacl system to estimate na concentration calculating the concentrations of all other elements requires an empirical correction if additional salt concentrations are of similar magnitude as that of nacl if cacl 2 is the main additional salt component as in many low temperature basin and basement brines absolute na concentrations wt nacl abs can be obtained by observing two melting temperatures hydrohalite and either ice or halite uniquely defining the major element composition of the fluid in the ternary model system h 2 o nacl cacl 2 and allowing na to be used as internal standard for quantifying all minor and trace elements test results for a range of compositions show that calcium concentration can be determined more precisely by microthermometry than by la icpms analysis but that both methods agree within error the combined approach of microthermometry and la icpms analysis described here permits reliable quantification of major ca na as well as trace element concentrations in sodic calcic brine inclusions even in ca rich host minerals such as fluorite or ca bearing carbonates

faulting within the eastern baikal rift as characterized by earthquake studies earthquake relocations focal mechanism determinations waveform modeling and stress analyses using focal mechanism information are used to examine the faulting processes within the western baikal rift 95 to 108 5 results of the study show that high angle dip 70 faulting is most likely to occur in the more youthful parts of the rift and is most likely to be associated with m 4 5 events like the eastern rift nearly all earthquakes occurred on faults dipping 30 suggesting that in no part of the rift is significant deformation occurring on low angle structures focal mechanism information does not indicate there is a sharp boundary to the western rift as observed in the eastern rift less normal faulting rakes 60 to 120 occurs in the western rift than the eastern rift waveform modeling of the two largest earthquakes 1950 mondy and 1959 srednebaikal in the western rift gives focal depths of 10 to 15 km the 1959 event appears to have occurred along the eastern boundary fault on lake baikal just to the north of proval bay a feature created a m 7 1 earthquake in 1862 the 1950 event may have occurred along the southern boundary fault of the tunka depression stress orientation information indicates a near horizontal nw se to wnw ese striking 3 in the eastern part of the western rift

silicate replacive high sulfidation massive sulfide orebodies in a porphyry cu au system bor serbia the cu au deposit of bor serbia represents a continuum of mineralization styles from porphyry style ore occurring in quartz magnetite chalcopyrite veins and chalcopyrite disseminations to high sulfidation epithermal cu au ores in pyrite chalcopyrite and anhydrite sulfide veins decisive for the great economic importance of bor is the presence of exceptionally rich high sulfidation massive sulfide orebodies composed of pyrite covellite chalcocite digenite and minor anhydrite and enargite they form irregular bodies measuring 0 5 10 million tons of ore grading up to 7 cu hosted by andesites and surrounded by intense argillic alteration this study focuses on a small but rich underground orebody mined out recently where limited drillcore is preserved for quantitative geochemical study this paper documents the vein relationships within the deep porphyry style orebody of borska reka the transitional porphyry epithermal veins and the overlying and laterally surrounding epithermal massive sulfides of the bor deposit geological observations indicate that the formation of massive sulfide orebodies concludes the ore formation mass balance calculations recast into geologically realistic bulk fluid rock reactions confirm textural evidence that near isovolumetric replacement of andesite host rocks is the dominant formation mechanism of massive sulfide orebodies at bor whereby all lithophile elements including si are dissolved and only ti stays relatively immobile while net volume changes for individual mineralization styles within the massive sulfide orebody vary from 16 volume loss to 127 volume gain overall volume change for the whole massive sulfide orebody was probably slightly negative brecciation is important only as means of creating channelways for reactive fluid that turns the andesite protolith into massive sulfide whereas net breccia infill occurred only locally

noble gas and halogen constraints on mineralizing fluids of metamorphic versus surficial origin mt isa australia abstract the origin of hydrothermal fluids responsible for silica dolomite alteration associated with cu ore in the giant mt isa deposit of queensland north east australia has been investigated main stage quartz and dolomite veins and late to post ore quartz from the buck quartz fault in the orebody footwall plus the regional mt isa and railway faults contain two phase liquid vapour fluid inclusions with variable salinity fluid inclusion molar br cl values are mostly in the range 1 5 12 10 3 and molar i cl values are in the range 9 186 10 6 greater than seawater br cl and i cl values can characterise either sedimentary formation waters with a bittern brine origin by the sub aerial evaporation of surface water beyond the point of halite saturation or some metamorphic dehydration fluids with a deep crustal source we propose that these alternative origins can be distinguished in our samples based on the relationship between 40 ar 36 ar and 36 ar concentration metamorphic fluids formed by dehydration of crystalline rock are predicted to have low 36 ar concentrations with the salinity 40 ar 36 ar and 40 ar e cl values determined by the whole rock concentration of cl 36 ar oh k and age the main stage quartz and buck quartz fault samples host fluid inclusions with 36 ar 40 ar 36 ar in the range 10 000 28 000 and high 40 ar e cl values of 3 8 10 4 these values are consistent with metamorphic fluids sourced by devolatilisation of either the underlying eastern creek volcanics or the barramundi basement in contrast surface derived fluids acquire elevated 40 ar 36 ar values 296 through fluid rock interaction which leads to a simultaneous increase in 36 ar concentration the main stage ca rich dolomite fluid inclusions have high salinities close to the point of halite saturation 26 wt 1 7 5 5 ppb 36 ar 40 ar 36 ar in the range 1600 8400 and 40 ar e cl of 0 2 1 10 4 these values are most similar to those expected in an evolved bittern brine however the 36 ar concentration and 40 ar 36 ar value exhibit a weakly negative correlation which is the opposite to that expected in surface derived fluids only fluid inclusions in samples from the railway fault and some fluid inclusions in samples from the buck quartz fault have 36 ar concentrations of greater than air saturated water asw 1 3 2 7 ppb 36 ar up to 27 ppb and 40 ar 36 ar values of 1000 2500 that are expected for fluids with an exclusively sedimentary formation water origin fluid inclusions in the mt isa fault samples have the highest 36 ar concentrations of up to 100 ppb some of the lowest 40 ar 36 ar values and the highest br cl value of up to 30 10 3 these features distinguish it from all other sample groups in the mine area silica dolomite alteration and cu mineralization are interpreted to have resulted from mixing of the deeply derived metamorphic fluid best preserved in quartz fluid inclusions and a surface derived bittern brine partially preserved within the main stage dolomite fluid inclusions inflow of a second surficial fluid along regional faults and into the mine through the late to post d 3 buck quartz fault may have terminated mineralization the involvement of metamorphic fluids is compatible with the syn d 3 1530 ma timing of mineralization during the final stages of the isan orogeny