preface archaean geology of the eastern goldfields of western australia regional overview abstract the eastern goldfields region forms part of the yilgarn craton which after the superior province of canada is one of the largest fragments of archaean granite greenstone terrain exposed at the surface of the earth this craton has much in common with the more intensely studied superior province but there are also significant differences between these two cratons both cratons were assembled from numerous smaller crustal fragments in a relatively brief period during the late archaean the superior province was assembled between 2725 and 2695 ma card 1990 and the yilgarn craton was assembled between c 2670 and 2650 ma myers 1993 both cratons contain fragments of older continental crust and volcanic arcs that formed just prior to collision and cratonisation the superior province contains extensive belts of metasedimentary rocks interpreted as accretionary prisms card 1990 whereas metasedimentary rocks form only a minor component of the yilgarn craton and most appear to postdate accretion both cratons contain significant concentrations of gold and support thriving gold mining industries the most prominent difference between the cratons lies in the nature of the granitoid rocks the superior province is dominated by tonalite and granodiorite whereas the yilgarn craton largely consists of granite this applies to granitoid rocks of all ages in each craton granite studies form a major part of this special volume of precambrian research

geochemistry of peruvian near surface sediments sixteen short sediment cores were recovered from the upper edge ueo within wo and below bo the oxygen minimum zone omz off peru during cruise 147 of r v sonne solids were analyzed for major trace elements total organic carbon total inorganic carbon total sulfur the stable sulfur isotope composition 34 s of pyrite and sulfate reduction rates srr pore waters were analyzed for dissolved sulfate sulfide and 34 s of sulfate in all cores highest srr were observed in the top 5 cm where pore water sulfate concentrations varied little due to resupply of sulfate by sulfide oxidation and or diffusion of sulfate from bottom water 34 s of dissolved sulfate showed only minor downcore increases strong 32 s enrichments in sedimentary pyrite to 48 vs v cdt are due to processes in the oxidative part of the sulfur cycle in addition to sulfate reduction manganese and co are significantly depleted in peruvian upwelling sediments most likely due to mobilization from particles settling through the omz whereas release of both elements from reducing sediments only seems to occur in near coastal sites cadmium mo and re are exceptionally enriched in wo sediments 600 m water depth high re and moderate cd and mo enrichments are seen in bo sediments 600 m water depth re mo ratios indicate anoxic and suboxic conditions for wo and bo sediments respectively cadmium and mo downcore profiles suggest considerable contribution to ueo wo sediments by a biodetrital phase whereas re presumably accumulates via diffusion across the sediment water interface to precipitation depth uranium is distinctly enriched in wo sediments due to sulfidic conditions and in some bo sediments due to phosphorites silver transfer to suboxic bo sediments is likely governed by diatomaceous matter input whereas in anoxic wo sediments ag is presumably trapped due to sulfide precipitation cadmium cu zn ni cr ag and t1 predominantly accumulate via biogenic pre concentration in plankton remains rhenium sb as v u and mo are enriched in accordance with seawater te availability lead and bi enrichment in ueo surface sediments is likely contributed by anthropogenic activity mining accumulation rates of toc cd mo u and v from peruvian and namibian sediments exceed those from the oman margin and gulf of california due to enhanced preservation off peru and namibia copyright 2004 elsevier ltd

oxidized and reduced mineral assemblages in greenstone belt rocks of the st ives gold camp western australia vectors to high grade ore bodies in archaean gold deposits hydrothermal sulfide oxide gold mineral assemblages in gold deposits in the archaean st ives gold camp in western australia indicate extremely variable redox conditions during hydrothermal alteration and gold mineralization in space and time reduced alteration assemblages pyrrhotite pyrite occur in deposits in the southwest of the camp e g argo junction deposits and moderately to strongly oxidized assemblages magnetite pyrite hematite pyrite occur in deposits in the central corridor in the northeast e g north orchin revenge deposits reduced mineral assemblages flank the central corridor of oxidized deposits and locally cut across it along e w trending faults oxidized mineral assemblages in the central corridor are focused on gravity lows which are interpreted to reflect abundant felsic porphyritic intrusions at about 1 000 m below present surface hydrothermal magnetite predates and is synchronous with early phases of gold associated albite carbonate pyrite biotite chlorite hydrothermal alteration later stage gold associated pyrite is in equilibrium with hematite the spatial distribution and temporal sequence of iron sulfides and oxides with gold indicate the presence of at least two spatially restricted but broadly synchronous hydrothermal fluids with contrasting redox states sulfur isotope constraints support the argument that the different mineral assemblages reflect differences in redox conditions the 34s values for pyrite for the st ives gold camp range between 8 4 and 5 1 with the negative values occurring in oxidized magnetite rich domains and slightly negative or positive values occurring in reduced pyrrhotitic domains preliminary spatial and paragenetic analysis of the distribution of iron sulfides and oxides in the st ives camp suggests that gold grades are highest where the redox state of the hydrothermal alteration assemblages switches from relatively reduced pyrrhotite pyrite to relatively oxidized magnetite pyrite and hematite pyrite both in space and time gold deposition is inferred to have occurred where fluids of contrasting redox state mixed

late archaean deep marine volcaniclastic sedimentation in an arc related basin the kalgoorlie sequence of the eastern goldfields superterrane yilgarn craton western australia abstract the kalgoorlie sequence 2658 ma is a 3000 m thick succession of volcaniclastic sedimentary rocks and subordinate rhyolite dacite andesite and basalt spatially and temporally associated with subvolcanic sills and dykes of rhyolite and dacite gabbro sills and i type granitoids although basalts are tholeiitic the felsic rocks have ttd affinity the sequence comprises mostly deep marine channel levee turbidite complexes and hemipelagites resedimented syneruptive volcaniclastic deposits are associated with coherent lavas but almost all of the sedimentary fill was recycled from prior cycle volcanic and volcaniclastic sources provenance was mainly dacitic rhyolitic with minor andesitic basaltic and polymictic compositions volcano bound and fault bound architectures and two styles of basin filling are identified progradational or aggradational transverse fans and slope aprons and retrogradational longitudinal fans of a linear trough retrogradation in longitudinal systems is consistent with basin lengthening and deepening six depositional sequences are grouped in four unconformity bounded tectonic stages that record uplift subsidence cycles coeval mafic and felsic magmatism indicate that the tectonic stages represent extensional tectonic settings whereas volcanogenic provenance spatially linked volcanism and plutonic magmatism and deep marine environments support an intra arc site cyclic uplift subsidence and basin lengthening are more consistent with strike slip controls on basin formation than with orthogonal extension the tectonic setting is interpreted to have been a pull apart basin associated with strike slip faulting and amalgamating arc related terranes similar to the modern philippines archipelago the strong similarity between archaean volcaniclastic and modern arc related volcaniclastic environments implies comparable tectonic settings and therefore comparable controlling tectonic factors

robust rock properties through pp ps processing and interpretation marcellus shale the estimation of reservoir properties from seismic amplitude is generally an underdetermined problem which can result in ambiguous estimation of the sought for properties although seismic reflectivity is dependent upon formation p and s wave velocities and density it is common practice to estimate only two attributes from prestack p wave data because of data limitations at far offsets and instabilities of inversion on the other hand the number of petrophysical properties that determine reservoir storability and production are four in the simplest of cases porosity lithology pore fluids type and amount and permeability ambiguity and risk reduction require the inclusion of additional robust information into the estimation of reservoir properties this is particularly true in formations where reservoir storability and production depend on additional petrophysical properties as would be the case of unconventional plays where brittleness and vertical fracture density and orientation are fact

burial of organic carbon and pyrite sulfur in sediments over phanerozoic time a new theory in present day marine sediments almost all of which are deposited in normal oxygenated seawater rates of burial of organic carbon c and pyrite sulfur s correlate positively and bear a constant ratio to one another c s 3 on a weight basis by contrast calculations based on the isotopic model of garrels and lerman 1981 indicate that at various times during the phanerozoic the worldwide burial ratio must have been considerably different than the present day value this ratio change is caused by the requirement that increases in the worldwide mass of organic carbon must be accompanied by equivalent decreases in the mass of sedimentary pyrite sulfur in order to maintain a roughly constant level of o2 in the atmosphere such apparently contradictory behavior can be explained if the locus of major organic carbon burial has shifted over time from normal marine environments as at present to non marine freshwater or to euxinic environments in the geologic past a shift to predominantly freshwater burial can help explain predicted high c s ratios in permo carboniferous sediments and a shift to euxinic environments can help explain predicted low c s ratios during the early paleozoic it is demonstrated that the three environments today exhibit distinguishably different average c s ratios

the largest au deposits in the st ives goldfield yilgarn craton western australia may be located in a major neoarchean volcano sedimentary depo centre the largest neoarchean gold deposits in the world class st ives goldfield western australia occur in an area known as the argo junction region e g junction argo and athena why this region is so well endowed with large deposits compared with other parts of the st ives goldfield is currently unclear because gold deposits at st ives are hosted by a variety of lithologic units and were formed during at least three different deformational events this paper presents an investigation into the stratigraphic architecture and evolution of the argo junction region to assess its implications for gold metallogenesis the results show that the region s stratigraphy may be subdivided into five regionally correlatable packages mafic lavas of the paringa basalt contemporaneously resedimented feldspar rich pyroclastic debris of the early black flag group coarse polymictic volcanic debris of the late black flag group thick piles of mafic lavas and sub volcanic sills of the athena basalt and condenser dolerite and the voluminous quartz rich sedimentary successions of the early merougil group in the argo junction region these units have an interpreted maximum thickness of at least 7 130 m and thus represent an unusually thick accumulation of the neoarchean volcano sedimentary successions it is postulated that major basin forming structures that were active during deposition and emplacement of the voluminous successions later acted as important conduits during mineralisation therefore a correlation exists between the location of the largest gold deposits in the st ives goldfield and the thickest parts of the stratigraphy recognition of this association has important implications for camp scale exploration

magmato hydrothermal space a new metric for geochemical characterisation of metallic ore deposits abstract magmato hydrothermal space is a new mathematical construct where 24 ore and pathfinder elements are used to quantify ore element signatures quantification allows relationships between ore deposit samples to be mapped the broad scale view of magmato hydrothermal space reveals three important trends 1 zn pb sediment hosted mineralisation to igneous associated cu au mineralisation 2 cu au mineralisation to au only mineralisation and 3 ultramafic associated magmatic ni cu pge mineralisation through cu au mineralisation to granitoid associated mo w and sn mineralisation the view provided by magmato hydrothermal space reveals that there is a spectrum of ore element signatures that mirrors the spectrum of ore deposit classes described in the literature geochemical variations within individual ore deposit classes are examined for orogenic au vhms epithermal and sediment hosted cu samples sub groups within each of these classes are compared on element enrichment diagrams and described in the context of magmato hydrothermal space orogenic au samples are divided into two sub groups of as sb rich mineralisation and four sub groups of relatively as sb poor mineralisation the as sb poor sub groups include a te cu ag rich sub group that overlaps with the porphyry cu class and a te w bi mo rich class dominated by granitoid hosted deposits the vhms class ranges from a cu rich sub group that overlaps with porphyry cu and iocg classes through a cu zn sub group to two zn cu sub groups one of which overlaps with the shms class the epithermal class is divided into zn rich cu rich and base metal poor sub groups

fault valve behaviour in optimally oriented shear zones an example at the revenge gold mine kambalda western australia abstract quartz vein systems developed in and adjacent to shear zones host major gold deposits in the kambalda region of the norseman wiluna greenstone belt at the revenge mine two groups of mineralised reverse shear zones formed as conjugate near optimally oriented sets during ese subhorizontal shortening adjacent to a major transpressional shear system the shear zones developed at temperatures of about 400 c in a transitional brittle ductile regime deformation was associated with high fluid fluxes and involved fault valve behaviour at transiently near lithostatic fluid pressures during progressive evolution of the shear system early brittle and ductile deformation was overprinted by predominantly brittle deformation brittle shear failure was associated with fault dilation and the formation of fault fill veins particularly at fault bends and jogs a transition from predominantly brittle shear failure to combined shear along faults and extension failure adjacent to faults occurred late during shear zone evolution and is interpreted as a response to a progressive decrease in maximum shear stress and a decrease in effective stresses the formation of subhorizontal stylolites locally subvertical extension veins and minor normal faults in association with thrust faulting indicates episodic or transient reorientation of the near field maximum principal stress from a subhorizontal to a near vertical attitude during some fault valve cycles local stress re orientation is interpreted as resulting from near total shear stress release and overshoot during some rupture events previously described fault valve systems have formed predominantly in severely misoriented faults the shear systems at revenge mine indicate that fault valve action and associated fluctuations in shear stress and fluid pressure can influence the mechanical behaviour of optimally oriented faults

variations in pyrite texture sulfur isotope composition and iron systematics in the black sea evidence for late pleistocene to holocene excursions of the o2 h2s redox transition time dependent variations in the physicochemical properties of pyrite were investigated in four sediment cores from the black sea in the laminated deep basin sediments of unit i and unit ii 86 of pyrite particles are present as fine grained framboidal aggregates in these sediments the dominance of pyrite framboids with a narrow size distribution and maximum size 18 m is evidence of syngenetic water column pyrite formation subjacent to the o2 h2s boundary sediments from the basin margin collected below the impingement of the o2 h2s boundary contain an increased proportion of fine grained euhedral grains of pyrite relative to framboidal aggregates suggesting increased additions of diagenetic pyrite below the sediment water interface the more efficient reduction and sulfidation of iron in the water column of the central region of the basin implies a more reactive source of iron there compared to the basin margins and is tied to increasing pyrite as framboids and greater degrees of pyritization dop in the basin center relative to the basin margins an evaluation of fe s c relations indicates that pyrite formation was limited by sulfate availability during deposition of the oldest lacustrine sediments of unit iii c limited for upper unit iii due to the input of sulfate rich mediterranean seawater and fe limited in the laminated organic carbon rich sediments of units i and ii these changes in pyrite limiting factors occurred within 4 m of compacted burial representing 15 ka sediment fabric and framboid size distributions at a shallow basin margin site suggest that during two extended periods of unit ii deposition the o2 h2s transition retreated to depths below 200 m however during these same intervals the 34s of pyrite remains uniform compared to overlying and underlying laminated sediments implying that this chemical signature may not be uniquely tied to the position of the o2 h2s boundary relative to the sediment water interface but rather is likely related to biogeochemical processes

archean gold bearing quartz veins at the sigma mine abitibi greenstone belt quebec part i geologic relations and formation of the vein system the sigma deposit consists of an archean gold bearing vein system in metavolcanic rocks intruded by an irregular body of porphyritic diorite both affected by regional deformation and in turn cut by younger undeformed feldspar porphyry dikes the veins cut all rock types and are composed chiefly of quartz and tourmaline with minor amounts of carbonates pyrite chlorite scheelite and free gold they were emplaced during or after a greenschist facies metamorphism postdating the intrusion of feldspar porphyry dikes the veins were not significantly affected by any subsequent metamorphism or tectonism two principal types of veins occur in the mine subvertical veins and subhorizontal veins they are contemporaneous and cogenetic subvertical veins occur within steeply dipping east west trending ductile shear zones along which reverse subvertical displacements have taken place the veins occupy openings created by overriding of undulations and irregularities of foliation and slip planes during progressive shearing movements within the shears subhorizontal veins occupy extensional fractures which developed between ductile shear zones preferentially in the more competent host rocks these veins were formed by one or several episodes of open space filling geologic relations indicate that the mineralized veins were emplaced progressively in a dynamic tectonic environment during and shortly after the formation of ductile shear zones a structural analysis has shown that the ductile shear zones and the two vein types can be related to a single strain ellipsoid of the following orientation the z axis trending north south and horizontal the y axis plunging 10 degrees west and the x axis plunging 80 degrees east the deformation responsible for the generation of ductile shear zones and emplacement of the veins is ascribed to a late north south trending compression postdating regional deformation

earthquake faulting as a structural process abstract structural geology is concerned with the history of movement in the earth s crust and the processes by which displacements occur in the upper one third to one half of deforming continental crust displacement is accommodated largely by seismic slip increments on existing faults it follows that earthquakes and related processes are an integral part of structural geology traditionally structural geologists have been preoccupied with the complexity of the finite deformation within fault zones and with the stress states prevailing at the initiation of faults in intact crust future structural work should be directed more towards understanding the dynamic character of fault reactivation during incremental slip and related effects questions of interest include rheological and geometrical controls on the initiation perturbation and termination of ruptures directivity effects associated with rupture propagation the recognition of structures resulting from repeated stress cycling within seismogenic crust and identification of structural features diagnostic of shear stress levels during faulting structures arising from the inter relationships between slip episodes and induced fluid flow are of special importance because these dynamic fault processes appear influential in the development of much fault hosted mineralization mesothermal gold quartz lodes hosted in high angle reverse shear zones of mixed brittle ductile character form illustrative examples of structures that arguably can only be interpreted by seismo structural analysis embodying the concepts listed above

a least squares approach to mass transport calculations using the isocon method a statistically rigorous approach based on the isocon method grant 1986 to evaluate mass transport is presented chemical analyses of multiple samples of unaltered parent and altered rock are used to calculate the average oxide or element concentration and its standard deviation for each rock population in a typical application the uncertainties in element concentration associated with the lack of homogeneity in each population outweigh the analytical uncertainty an efficient algorithm is presented to select the immobile elements by identifying the maximum number of elements that are within their uncertainties compatible with the same isocon the actual isocon is constructed by forcing a line through the origin and optimizing its slope using a weighted least squares procedure for the selected immobile elements thus individual uncertainties for each immobile element are taken into account as an illustration the procedure is applied to a published set of data on potassic and sericitic alterations of quartz monzonite of the bingham porphyry copper system the surprising result indicates that the apparent increase of potassic alteration phases is a consequence of acid leaching of calcium and sodium no mass transport of potassium is indicated silica water and sulfur were added during alteration the best fit isocon requires a small mass increase during each alteration process which however is within the uncertainty obtained for the slope of the isocon a software package is available from the authors in the form of a fortran 77 source code

gold quartz veins in metamorphic terranes and their bearing on the role of fluids in faulting gold quartz vein fields in metamorphic terranes such as greenstone belts provide evidence for the involvement of large volumes of fluids during faulting and may be products of seismic processes near the base of the seismogenic regime in the val d or district of the abitibi greenstone belt canada quartz tourmaline carbonate veins form a vein field 30 15 km in the hanging wall of a crustal scale fault zone which was the main channelway for upward migration of the deeply generated fluids the veins occur in small high angle reverse faults and in adjacent horizontal extensional fractures extending up to 75 m in intact rocks they have formed incrementally during active reverse faulting in response to crustal shortening at depths corresponding to those at the base of the seismogenic zone in actively deforming crust detailed structural and fluid inclusion studies provide evidence for generally lithostatic but fluctuating fluid pressures p of the order of 200 mpa and for cyclic stress reversals during vein formation and provide good support for the fault valve model a comparison of vein characteristics with standard earthquake rupture parameters suggests that each slip increment along veins in reverse faults was accompanied by a small earthquake 4 m 3 or less the large vein field thus represents both the extent of fluid dispersion in the hanging wall of a crustal scale channelway and the distribution of small earthquakes integrated over the lifetime of the hydrothermal system it is proposed that such small earthquakes along veins in reverse faults are related to large earthquakes m 6 nucleating near the base of the seismogenic regime along the nearby crustal scale fault either as aftershocks or as a precursory smarm

reflection transmission coefficients and azimuthal anisotropy in marine seismic studies summary introduce two methods for inferring the direction of vertical fractures from marine seismic data first we derive the necessary reflection and transmission coefficients for an interface between a liquid and an azimuthally anisotropic solid next we show that multicomponent ocean bottom seismometer data from surface airgun sources along two perpendicular shot lines can be rotated into the principal directions of azimuthal anisotropy to determine the orientation of vertical fractures finally we show that p wave amplitude versus offset avo depends on the orientation of the shot line with respect to the vertical fractures thus p wave avo can also be used to determine fracture orientation

load strengthening versus load weakening faulting increases in shear stress along a fault during loading to failure cannot generally occur without changes in the normal stress across the fault n the fault loading parameter n n pf distinguishes situations of load strengthening n 0 where the frictional shear strength of faults increases as tectonic shear stress rises from load weakening environments n t6 0 where it decreases compressional faulting in tectonic regimes with v 3 is always load strengthening unless fluid pressure is rapidly increasing extensional faulting in regimes where v 1 is load weakening unless fluid pressure is dropping rapidly strike slip faulting in terrains where v 2 can be either load weakening or load strengthening the particular case where n 0 so that frictional shear strength stays constant during fault loading is a very special situation corresponding to direct shear load strengthening strike slip faulting appears to correlate with tectonic transpression and load weakening with transtension differing loading characteristics of faults in different tectonic regimes must induce varying patterns of cyclic fluid redistribution accompanying the seismic cycle with implications for earthquake recurrence and precursory groundwater phenomena

inversion of multicomponent multiazimuth walkaway vsp data for the stiffness tensor vertical seismic profiling vsp an established technique can be used for estimating in situ anisotropy that might provide valuable information for characterization of reservoir lithology fractures and fluids the p wave slowness components conventionally measured in multiazimuth walkaway vsp surveys allow one to reconstruct some portion of the corresponding slowness surface a major limitation of this technique is that the p wave slowness surface alone does not constrain a number of stiffness coefficients that may be crucial for inferring certain rock properties those stiffnesses can be obtained only by combining the measurements of p waves with those of s or ps modes here we extend the idea of horne and leaney who proved the feasibility of joint inversion of the slowness and polarization vectors of p and sv waves for parameters of transversely isotropic media with a vertical symmetry axis vti symmetry we show that there is no need to assume a priori vti symmetry or any other specific typ

prestack waveform inversion of three dimensional seismic data an example from the rock springs uplift wyoming usa abstractapplying seismic inversion to estimate subsurface elastic earth properties for reservoir characterization is a challenge in exploration seismology in recent years waveform based seismic inversions have gained popularity but due to high computational costs their applications are limited and amplitude variation with offset angle inversion is still the current state of the art we have developed a genetic algorithm based prestack seismic waveform inversion methodology by parallelizing at multiple levels and assuming a locally 1d structure such that forward computation of wave equation synthetics is computationally efficient this method is capable of inverting 3d prestack seismic data on parallel computers applying this inversion to a real prestack seismic data volume from the rock springs uplift rsu located in wyoming usa we determined that our method is capable of inverting the data in a reasonable runtime and producing much higher quality results than amplitude variation with offset ang

petrophysical imaging using avo seismic lithology is the process by which rock properties such as lithology porosity and pore fluid content are determined by analysis of seismic and other data i emphasize other data because the seismic method is limited in what can be achieved in a vacuum but can be remarkably robust when combined with other information and constrained by geologically reasonable assumptions i define petrophysical imaging as a seismic lithology method which does not attempt to recover absolute rock properties but rather involves the construction of an image of subsurface rock property variation for example for direct hydrocarbon indication absolute rock properties need not be determined only the deviation from background nonpay behavior is required

brittle ductile deformation in the glarus thrust lochseiten lk calc mylonite the glarus thrust accommodated at least 30 km of northward displacement strongly localized within a 1 m layer of lochseiten lk calc mylonite this layer displays veins in various states of plastic deformation and a wildly refolded foliated gouge texture lattice and shape preferred orientations are observed within the fine grained recrystallized matrix these features indicate the alternate activity of brittle and ductile deformation mechanisms in contrast to the classical view that grain boundary sliding superplasticity is the dominant deformation mechanism it is advocated that fluids derived from the footwall and expelled along the thrust are responsible for hydrofracturing and cataclastic deformation in periods between fracture events deformation was ductile in this new interpretation a substantial amount of the total thrust displacement was accommodated by numerous short lived and strongly localized fracture events at the base of the verrucano thrust sheet rather than a permanently weak decollement lithology

identifying a reference frame for calculating mass change during weathering a review and case study utilizing the c program assessing element immobility and critical ratio methodology abstract the first step in calculating mass changes that occur during the transformation of plutonic rock to saprock involves the determination of a reference frame i e an immobile element whose mass was conserved during the alteration process procedures for identifying the statistical properties of immobile elements have been available since 1990 but have not generally been applied we therefore developed the c program assessing element immobility aei that allows for rapid assessment of those elements possessing the statistical properties of immobility aei utilizes algorithms that test for equal distributions and unchanging means of ratios subcompositional independence and subcompositional invariance following discussion of the general mathematical and statistical procedures implemented in aei we utilize it in a case study involving three sites in an 30 m thick section of regolith zr i is located near the base of the regolith while zr ii and zr iii are located at depths of 20 m and 5 m respectively from each site previous workers chemically analyzed a number of samples from saprock and adjacent unweathered corestones in addition xrd work showed that from zr i to zr ii to zr iii biotite was altered to vermiculite vermiculite kaolinite and vermiculite mixed layer biotite vermiculite kaolinite respectively at zr iii the most dominant clay mineral is kaolinite aei performs a series of tests to assess whether or not a given set of elements possess the statistical properties for immobility it identified the following elements as a potential reference frame al na fe mg ti and p at zr i si al fe mn mg ti and p at zr ii and mg and mn at zr iii however critical ratio methodology indicated that the precision as measured by the smallest 95 confidence interval is greatest for al2o3corestone al2o3saprock and na2ocorestone na2osaprock at zr i for sio2corestone sio2saprock at zr ii and for mgocorestone mgosaprock at zr iii mass balance results using al2o3 and na2o at zr i were nearly identical and as a result only the results derived from al2o3 are reported here using al2o3 sio2 and mgo as reference frame elements for zr i zr ii and zr iii respectively revealed the following statistically significant losses of k mass 14 9 8 at zr i 28 11 9 at zr ii and 33 8 7 at zr iii in addition at zr iii the statistically significant loss of 36 13 11 p mass is attributed to the dissolution of apatite and the small nominal loss of 6 5 5 ca mass may reflect incongruent leaching of plagioclase and or apatite in contrast the small additions of 5 5 4 al 5 2 2 fe and 4 2 2 ti mass at zr iii are likely the result of the translocation of kaolinite and kaolinitized biotite particles into the crack system by downward percolating fluids and the exchange and adsorption of al fe and ti ions onto the illuviated clay lining as it evolved over time overall the interplay between eluviation and illuviation at zr iii translates into a statistically significant 5 3 3 increase in bulk mass

mineral and elemental indicators of post glacial changes in sediment delivery and deposition under a western boundary upwelling system cabo frio southeastern brazil abstract minerals and elements are important proxies that can provide information about variations in the delivery and deposition of coastal ocean sediments associated with past climate changes in this study postglacial changes in the accumulation of sediments on the upper shelf of southeastern brazil are linked to the evolution of regional paleoceanographic and continental paleoclimatic conditions during the last 14 4 kyr cal bp mineralogical and major and trace element analyses of a 14 c dated sediment core identify three main lithostratigraphic units of this core that reveal a succession of changes in sediment delivery and accumulation as postglacial sea level rose and holocene climate on land evolved the depositional setting has transitioned in response to the large sea level rise prior to 9 9 kyr cal bp from being a shallow water high energy environment to a deeper water one this location has since then been a lower energy environment that has persisted into modern times due to the high marine productivity associated with the cabo frio upwelling system in an oxygenated water column the sediments have shown a complex and dynamic redox condition making variations in redox sensitive trace elements indicative of changes in primary production and organic matter diagenesis at this location after sea level stabilized 9 kyr cal bp variations in fluxes of al fe ca and minerals were small indicating that climate driven changes had only secondary controls on delivery of detrital sediment components to the cabo frio shelf modern coastal upwelling conditions and the onset of mid shelf organic rich sediment deposition were established after 9 9 cal kyr bp

prediction of some weathering trends of plutonic and volcanic rocks based on thermodynamic and kinetic considerations abstract the exposed crust consists mainly of plagioclase 35 quartz 20 k feldspar 11 volcanic glass 12 biotite 8 and muscovite 5 quartz is a resistate thus feldspars and glass represent approximately 75 percent of the labile minerals the weathering characteristics of these constituents are summarized in the context of thermodynamic mass balance and kinetic considerations experimentally determined release rate constants were used to predict the proportions of ca na and k released by feldspars of plutonic rocks granites to gabbros to weathering solutions the chemical weathering trends of the weathered residues calculated from the kinetic data conform closely to the initial trends observed in some recent weathering profiles demonstrating the accuracy of the predictions since the weathering of feldspars is controlled by processes that should not change through geological time the relative release rates of ca na and k from the feldspars of granitic rocks can be calculated for future and past episodes of continental weathering experimentally determined release rate constants are not available for a wide range of volcanic glass compositions but the limited data indicate that compositional trends are predictable in weathering profiles developed on volcanic rocks the kinetic data available for rhyolitic glasses accurately predict the initial weathering trends observed in a recent rhyolite weathering profile

constitutive mass balance relations between chemical composition volume density porosity and strain in metasomatic hydrochemical systems results on weathering and pedogenesis relations characterizing the chemical physical and mechanical changes resulting from metasomatic hydrochemical processes are developed using mass balance models which formally link chemical composition to bulk density mineral density volumetric properties porosity and amount of deformation strain rigorous analysis of aqueous solute transport effects is then made possible in a variety of porous media flow environments including chemical weathering pedogenesis soil formation diagenesis ore deposition and enrichment and metamorphism application of these linear constitutive relations to chemical weathering profiles shows that immobile and locally mobile chemical elements with masses conserved on the scale of soil profiles can be accurately identified from analysis of appropriate data arrays and then used as natural geochemical tracers to infer the nature and extent of hydrochemical weathering processes and volume changes during pedogenesis assumptions commonly made in the past about the supposed immobility of certain elements e g ti and zr become unnecessary quantitative differentiation between the effects of residual and supergene fractionation is then easily made these methods are applied to ni rich laterites developed by weathering of ultramafic rocks showing that during ordinary residual enrichment ni is concentrated by as much as 4 protolith peridotite concentrations this occurs simply by silicate mineral dissolution and removal of chemical elements other than ni e g mg with a corresponding reduction in saprolite density and increase in bulk porosity without significant deformation in contrast laterites with mineable concentrations of ni which are similarly undeformed such as the nickel mountain mine in riddle oregon have experienced in addition to residual enrichment strong supergene enrichment by fractionation of ore elements between a leached zone from which ni is extracted and a complementary enriched zone positioned farther along the direction of ground water flow soil forming processes in podzol chronosequences developed on sandy beach terraces of the mendocino coast of california involved soil column collapse of 60 percent by dissolution of silicate minerals in the albic horizon of al and fe leaching and 70 percent dilation expansion in the overlying organic rich layer by root growth the amount of erosion based upon paleosurface reconstructions using the excess mass of fe al pb ga and cu in the zone of supergene enrichment spodic horizon below the ground water table indicates that subsurface erosion by dissolutional collapse is three times that of surficial erosion finally using published chemical data for ti zr and cr on major bauxite deposits in australia where erosion rates are thought to be low we infer that there may have been major amounts of dissolutional collapse to explain the upwards increase of detrital zircon and rutile in weathering profiles

sequential backus averaging upscaling well logs to seismic wavelengths the measure of geologic knowledge spans a tremendously wide geometric scale in the vertical dimension alone the range is from microscopic clay particle morphology and pore throat geometry to basinal megatrends many kilometers thick of a more limited scope and inherent to building synthetic seismograms is the process of relating logging tool resolution measurements to seismic wavelengths a tens of centimeters to tens of meters problem this upscaling can be achieved by sequential backus averaging

characterisation and 3d modelling of a nuggety vein hosted gold ore body sunrise dam western australia abstract in high nugget gold ore bodies samples taken from drill core for gold assay are typically too small to allow for the extreme spatial variability in grade and are a poor representation of the underlying distribution of mineralisation the gq north lode at the sunrise dam gold mine in western australia is a good example of an orebody which has a very strong nugget effect coefficient of variation 20 and that has proved very problematic to model gold is hosted in vein stockworks and shear zones and although there is a clear spatial relationship between mineralisation and alteration high vein density and well developed foliations the relationship is best defined statistically because the association between high gold grades and various combinations of these features is non trivial we present a method for automating the inclusion of geological data proxies for gold mineralisation into the prediction of mineralised rocks using conditional probability the method uses the gold assays and the logged geological data to calculate the probability that rocks with particular geological features will be mineralised the ore body can then be modelled automatically using interpolation software with isosurfaces indicating the regions with highest probability of gold mineralisation a good understanding of the geological features associated with mineralisation and consistent geological logging are important prerequisites for successful conditional probability modelling of drill hole data

geochemical characteristics of chilean upwelling sediments at 36 s abstract we examined the cycling of c s fe and the trace elements te ag cd mo mn re and se in chilean upwelling sediments from water depths of 127 2744 m at 36 s the sediments are clay rich with moderate total organic carbon toc and low biogenic caco3 contents for the shallow site within oxygen minimum zone omz which shows some thioploca biomass highest observed toc contents correspond to highest productivity induced toc flux and or enhanced preservation under o2 deficient conditions pyrite formation may be restricted by the concurrent presence of elemental s in thioploca and or the availability of reactive iron in the other sediments 370 m water depth pyrite formation is strongly restricted due to the low availability of metabolizable toc for sulfate reduction calculations of the degree of bacterially mediated remineralization suggest that more nitrogen than toc was remineralized by sulfate reduction than expected from redfield c n relationship manganese is significantly depleted due to reductive dissolution of mn hydr oxides within all sediments mo is only significantly enriched vs the lithogenic background in the shallow core implying anoxic conditions contents of re and mo as well as re mo ratios indicate suboxic conditions for the other cores cd and re contents decrease with increasing water depth reflecting decreasing reducing conditions in contrast authigenic ag contents increase with water depth suggesting enrichment controlled by seawater availability and or opaline input and not redox conditions elevated se contents in all cores may be either due to bio detrital input or diffusion from seawater accumulation rates of toc ts and te off chile are in the same order as those from the gulf of california and lower than those off peru distinct seasonality in upwelling and dilution of chilean sediments with terrigenous material lead to rather low metabolizable toc contents and lower te trapping capacity

crustal scale shear zones and their significance to archaean gold mineralization in western australia many large archaean epigenetic gold deposits show a broad spatial relationship to regional lineaments in greenstone belts although in detail they are sited in subsidiary brittle ductile fault structures fluids originating from a deep source follow a complex path and re equilibrate with different lithologies and with metamorphic fluid during migration to higher crustal levels temperature and pressure conditions at or below the amphibolite greenschist metamorphic boundary where most gold deposits are located favour the establishment of brittle ductile and brittle subsidiary structures the preferred structural setting of gold deposits physical gradients between the regional ductile structures and more brittle subsidiary structures ensure transient strongly localized fluid flow into the latter where lower temperatures and suitable host rocks with high fe fe mg ratios favour gold deposition the multi source origin and continuous re equilibration of the fluid with crustal rock which includes granitoid and greenstone belt lithologies of different ages is reflected in the diverse isotopic and geochemical signature of the gold deposits

assessing metasomatic mass and volume changes using the bootstrap with application to deep crustal hydrothermal alteration of marble a statistically rigorous approach for determining likely errors on estimates of mass and volume change in metasomatic systems is presented and then used to assess mass transfer resulting from hydrothermal alteration of marble during regional metamorphism analysis of metasomatic effects using standard statistical methods designed for unconstrained univariant data often fails to provide useful results for several important reasons 1 the concentration of any constituent in a composition is constrained to be between 0 and 100 wt percent 2 the sum of the concentrations of all constituents must be 100 wt percent the closure constraint compositions are multivariate and provide information only about the relative proportions of constituents 3 the maximum possible mass or volume loss is 100 percent 100 corresponds to complete mass or volume loss 4 rock bulk density must be greater than 0 g cm super 3 5 the underlying probability distributions for mass and volume changes are commonly non normal to address these issues we use statistical procedures recently developed to treat the special properties of compositional data including closure and the bootstrap method to compute accurate confidence intervals for assessing how far in error best estimates of mass and volume change are likely to be the bootstrap deals effectively with non normality and constraints 1 3 and 4 we apply our approach to gain a better understanding of synmetamorphic acadian orogeny hydrothermal alteration of upper greenschist facies and amphibolite facies marble beds of the wepawaug schist connecticut the marbles lost significant amounts of volatiles dominantly co 2 and h 2 o si ti k rb sr and ba best estimates of total mass and volume change are 27 and 32 percent respectively the bulk of the lost mass was volatiles approximately 140 g kg super 1 sio 2 approximately 60 g kg super 1 and k 2 o approximately 15 g kg super 1 numbers are g lost per kg of parent rock volatile loss was probably regional in scope loss of si ti k rb sr and ba occurred over minimum length scales on the order of typical hand sample dimensions maximum length scales remain to be determined the fortran 77 computer code that performs the calculations described herein is available from the senior author

pyrite framboids and their development a new conceptual mechanism it has been suggested that the two morphologies of sedimentary pyrite framboids and euhedra may reflect two distinct pathways of pyrite formation framboids form indirectly via iron monosulphides whereas euhedra form from direct precipitation from solution a third pathway which is bridging these two forms is proposed here namely the continuous growth from a monosulphide globule through framboids to a euhedral single crystal it is also suggested that framboids probably occur over a range of three orders of magnitude from the least complex microframboids through framboids to polyframboids

the archean amphibolite facies coolgardie goldfield yilgarn craton western australia nature controls and gold field scale patterns of hydrothermal wall rock alteration the late archean coolgardie goldfield at the western margin of the norseman wiluna belt yilgarn craton comprises an arcuate belt of deformed mafic ultramafic and sedimentary rocks which is bounded to the west by the syntectonic calooli monzogranite greenstones at coolgardie preserve a broad metamorphic gradient with peak metamorphic temperature varying from 480 50 c at the center of the gold field to 545 50 c adjacent to the western granitoid greenstone contact at an approximate pressure of 3 to 4 kbar gold field scale variations in the gangue and ore mineralogy of zoned wall rock alteration assemblages around lodes the ore geochemistry and the isotope chemistry of vein minerals at coolgardie are correlated with plan view distance from the calooli monzogranite evidence supporting synpeak metamorphic gold mineralization in the coolgardie goldfield includes the equilibrium textural relationships between gold sulfides and high temperature silicate gangue the occurrence of undeformed auriferous quartz veins enveloped by garnet hornblende plagioclase calcite alteration which crosscut peak metamorphic fabrics and the siting of variably deformed gold ores in synpeak metamorphic structures conditions of gold mineralization at deposits less than 1 to 2 km from the calooli monzogranite are determined from geothermometry and barometry to be 510 50 c to 590 25 c at 3 to 4 kbar whereas those at greater distances from the monzogranite are 490 to 525 50 c at 3 to 4 kbar alteration assemblages in mafic host rocks can be divided into garnet bearing garnet hornblende plagioclase calcite and garnet absent biotite amphibole plagioclase calcite the presence or absence of garnet is mainly controlled by the mg number of the host mafic rocks ore in deposits with garnet bearing alteration is enriched in ag na pb s and w but only weakly enriched or depleted in k 2 o and other large ion lithophile elements co 2 as mo sb and te whereas deposits with biotite amphibole plagioclase calcite alteration are strongly enriched in ag as s sb w co 2 and large ion lithophile elements sulfide oxide assemblages are regionally zoned from pyrite ilmenite in deposits in granitoids and adjacent to the granitoid greenstone contact through pyrrhotite ilmenite pyrite in garnet bearing alteration 1 to 2 km from the contact to arsenopyrite pyrrhotite ilmenite assemblages in biotite bearing alteration 2 km from the greenstone granitoid contact this variation is potentially related to gradients in fluid f o2 away from the granitoids isotopic compositions of oxygen in quartz 18 o 10 8 12 4 scheelite 18 o 4 0 4 1 and oxygen and carbon in calcite 18 o 8 9 13 2 13 c 0 5 to 5 3 are generally more positive in deposits with garnet bearing alteration than in those with biotite bearing alteration 18 o quartz 6 6 11 8 18 o scheelite 2 3 4 6 18 o calcite 8 7 11 4 13 c calcite 4 3 to 8 4 whereas both alteration styles have dd biotite and dd amphibole values in the range 65 to 86 per mil these differences are interpreted to reflect interaction of isotopically heavy ore fluids with relatively depleted greenstone host rocks during fluid migration through structurally controlled conduits the gold field scale variations in alteration mineralogy and ore chemistry are considered to be related not to initial ore fluid composition but to temperature to host rock composition and to changes in fluid composition resulting from reaction with greenstone belt rocks the correlation between the calculated temperature of alteration and distance from the western granitoid greenstone contact suggests that the calooli monzogranite played some genetic role in determining the nature of hydrothermal alteration across coolgardie

quartz vein morphology and implications for formation depth and classification of archaean gold vein deposits abstract a simplified scheme to categorise quartz vein morphology from gold deposits is based on the growth direction of quartz or pseudomorphed chalcedony in the veins the categories are scale independent and are 1 face control 2 displacement control 3 parallel control 4 radiating 5 non directional control 6 replacement and 7 modified quartz vein textures do not indicate any particular type of mineralisation for the depth continuum of archaean lode gold deposits of the yilgarn craton all seven categories of texture are variously present however crustal depth is a major control parallel and radiating textures are characteristic of gold deposits formed at the near surface displacement and non directional control are typical of deposits at midcrustal greenschist and amphibolite conditions and face control shows a range of conditions from the near surface to midcrustal replacement and modified textures embrace a wide variety of alteration and deformation conditions and can occur in gold deposits at all crustal levels

bi melt formation and gold scavenging from hydrothermal fluids an experimental study abstract the role of polymetallic melts in scavenging ore components has recently been highlighted in the context of fluid poor metamorphosed ore deposits in contrast the role of polymetallic melts in systems dominated by hydrothermal fluids remains poorly understood using a simple au bi model system we explored experimentally whether such polymetallic melts can precipitate directly from a hydrothermal fluid and investigated the ability of these melts to scavenge au from the solution the experiments were conducted in custom built flow through reactors designed to reproduce a hydrothermal system where melt components are dissolved at one stage along the flow path e g bi was dissolved by placing bi minerals along the fluid path whereas melt precipitation was caused further along the flow path by fluid rock interaction bi rich melts were readily obtained by reaction with pyrrhotite graphite or amorphous fes when au was added to the system bi au melts with compositions consistent with the au bi phase diagram were obtained in the case of fluid reaction with pyrrhotite epitaxial replacement of pyrrhotite by magnetite was observed with textures consistent with an interface coupled dissolution reprecipitation reaction icdrr in this case the metallic melt precipitated as blebs that were localized at the replacement front or within the porous magnetite direct fractionation of bi au melts from a hydrothermal fluid or precipitation of a bi melt followed by partitioning of au from ambient fluid offer new pathways to the enrichment of minor ore components such as au without requiring fluid saturation with respect to a au mineral this mechanism can explain the strong geochemical affinity recognized between au and low melting point chalcophile elements such as bi in many gold deposits examples of deposits where such a model may be applicable include orogenic gold deposits and gold skarns contrary to models involving migration of polymetallic melts to explain element remobilization only small quantities ppm of polymetallic melts are required to affect the au endowment of a deposit via interaction with a hydrothermal fluid the experiments also show that micro environments can play a critical role in controlling melt occurrences for example reaction fronts developing via icdr reactions can promote melt formation as observed during the replacement of pyrrhotite by magnetite the associated transient porosity creates space for the melt and promotes melt fluid exchanges whereas the reaction front provides local geochemical conditions favorable to melt precipitation e g reduced low ah2s aq and catalytic surface

the role of fluids in syntectonic mass transport and the localization of metamorphic vein type ore deposists abstract in low to medium grade metamorphic environments the mobilization of species is closely associated with the operation of solution precipitation creep processes during foliation development such deformation processes involve dissolution of material from source sites which are usually discrete dissolution surfaces having orientations controlled by the applied stress and displacement fields mass transfer occurs via solute diffusion in an essentially static fluid network or by transport of dissolved species in a migrating fluid precipitation of material removed from dissolution sites occurs in a variety of sink sites including pore surfaces grain scale microcracks and veins sink sites may be close to or distantly removed from source sites the transport of material during deformation involving solution precipitation creep is largely controlled by the mobility of metamorphic fluids in regions of high fluid mobility significant mass transfer may occur over large distances fluid flow mass transfer paths and the development of sink sites in such cases are constrained by the structurally controlled development of high fluid pressure domains and the consequent mechanical enhancement of permeability by hydraulic fracture processes and the development of grain scale dilatancy these processes may result in focussed fluid flow and precipitation of mobilized species in restricted regions a proportion of syntectonically mobilized material is precipitated to form vein type deposits in hydraulic fracture arrays which develop in high fluid pressure environments the microstructures developed in minerals deposited in these sites reflect the interaction of a range of nucleation and growth processes which usually involve multiple generations of fracture growth and mineral crystallization the formation of syntectonic gold quartz vein deposits in central victoria australia illustrates the importance of structural controls on both the large scale migration of metamorphic fluids and the development of sites suitable for the precipitation of metamorphically mobilized species

architecture and geodynamic evolution of the st ives goldfield eastern yilgarn craton western australia abstract a new structural evolution consisting of both extensional and contractional events has been defined for the st ives goldfield in the south central kalgoorlie terrane of the eastern yilgarn craton in western australia these events shaped the development of the fault architecture which controlled the location of the regional anticlines the magmatic centres and the deposition of the archaean greenstone successions the fundamental grain of the st ives goldfield is north northwest trending this trend is marked by faults which developed during d1 extension which was oriented east northeast west southwest across these faults we map major stratigraphic changes in the thickness and composition of units especially of the previously undivided black flag group volcaniclastic rocks the centre of the st ives goldfield is dominated by the kambalda anticline this north northwest trending regional fold was likely established early during the d1 extensional history and was fully established during subsequent east northeast oriented d2 contraction the regional anticline is an important architectural element because 1 magmatism and gold mineralising fluids were focussed into this domed region and 2 deformation was partitioned across the limbs and crest of this structure the d3 event involved regional uplift and extension resulting in the formation of late basins merougil conglomerate locally and the emplacement of granitoids sourced from a metasomatised mantle wedge mafic type porphyries the most significant gold event in terms of endowment occurred during d4b sinistral strike slip shearing and associated thrusting e g tramways and republican thrusts these thrusts were previously interpreted as the first contractional structures to deform the area d1 but are here reinterpreted as relatively late d4b in this d4b event the north northwest trending faults underwent sinistral strike slip shearing and were linked across the kambalda anticline by accommodation structures represented by generally east to east northeast trending thrusts reactivation of d1 transfer structures may have influenced the location of these later accommodation structures late stage mineralisation during d5 was the result of dextral strike slip brittle shearing

element mobility and spatial zonation associated with the archean hamlet orogenic au deposit western australia implications for fluid pathways in shear zones mass balance equations can be used to understand metasomatism here we use 2726 multi element chemical analyses to quantify the effect of metasomatism associated with au mineralisation at the hamlet gold mine hamlet yilgarn craton western australia principal components analysis is first used to identify covarying elements and six groups of elements are identified 1 relatively immobile elements with zr and hf providing an appropriate reference frame for mass balance calculations 2 elements associated with mafic rocks such as ca co fe ge mg mn and zn 3 elements related to potassic alteration ba cs k rb and tl 4 elements related to sodic alteration be na and sr 5 chalcophile elements including bi weakly associated with au and 6 au te and s representing the main au mineralising event a statistically rigorous approach is then used to quantify element addition and depletion and provides 95 confidence limits for changes in mass to achieve this the basaltic host rocks are divided into four subunits based on cr content sample subsets are then created to select altered rocks and least altered protolith rocks using discrimination diagrams pairings of least altered altered samples from each basalt subunit are used to construct matrices via bootstrapping mass balance results from these matrices indicate similar element mobility associated with au mineralisation across the four basalt subunits with up to 24 mass loss key potassic and sodic group elements are enriched in the au mineralised zones at the expense of elements with a mafic association spatial patterns of mass change are linear and repetitive within the steeply dipping hamlet shear zone interpreted to represent a mesh like geometry of metasomatism within the plane of the shear zone localised enrichment of k na and bi are related to au distribution while ca depletion zones reflect the movement of reactive fluid along the shear plane

early diagenesis and sulphur speciation in sediments of the oman margin northwestern arabian sea the pore waters from two boxcores from the oman margin from sites niop484 and niop487 respectively in and below the oxygen minimum zone in the water column were analysed for concentrations of iodide sulphide thiosulphate phosphate silica ammonia nitrate nitrite dissolved iron dissolved manganese and dissolved sulphur in addition the solid phase of samples from the same cores was analysed for organic carbon pyritic sulphur acid volatile sulphide elemental sulphur organic polysulphides humic sulphur reactive iron phases and reactive manganese phases pore water profiles show that post oxic diagenesis dominates in these holocene sediments only minor amounts of reduced sulphur species were found in pore waters despite the low oxygen concentration of the bottom water and the relatively high organic matter content of the sediments significant amounts of solid phase reduced sulphur were only found in the lower half of the boxcore at station niop484 consequently sulphate reduction is not as significant in these sediments relative to sediments from other upwelling areas and thus more sedimentary organic matter is preserved

application of multi element relationships in stream sediments to mineral exploration a case study of walawe ganga basin sri lanka abstract this study on the application of multi element relationships in stream sediments to mineral exploration in the walawe ganga basin presents one of the first of its kind in sri lanka in order to determine the nature of these sediments multi element analysis of selected grain size some elements notably zr hf th u ce and la show very high enrichment factors e g 54 for zr as compared to the upper crustal abundance these are presumably associated with heavy minerals such as zircon rutile and monazite found in abundance in the stream sediments of the walawe ganga basin principal component analysis pca of the geochemical data show that multi element relationships could be effectively used to delineate target areas for mineral exploration with the use of pca this study reveals that areas with associations of calc silicate marble and charnockitic rocks are probable source regions for mineral occurrences particularly in the axial regions of anticlines

discriminating induced seismicity from natural earthquakes using moment tensors and source spectra earthquake source mechanisms and spectra can provide important clues to aid in discriminating between natural and induced events in this study we calculate moment tensors and stress drop values for eight recent induced earthquakes in the western canadian sedimentary basin with magnitudes between 3 2 and 4 4 as well as a nearby magnitude 5 3 event that is interpreted as a natural earthquake we calculate full moment tensor solutions by performing a waveform fitting procedure based on a 1 d transversely isotropic velocity model in addition to a dominant double couple dc signature that is common to nearly all events most induced events exhibit significant non double couple components a parameter sensitivity analysis indicates that spurious non dc components are negligible if the signal to noise ratio snr exceeds 10 and if the 1 d model differs from the true velocity structure by less than 5 estimated focal depths of induced events are significantly shallower than the typical range of focal depths for intraplate earthquakes in the canadian shield stress drops of the eight induced events were estimated using a generalized spectral fitting method and fall within the typical range of 2 to 90 mpa for tectonic earthquakes elastic moduli changes due to the brittle damage production at the source presence of multiple intersecting fractures dilatant jogs created at the overlapping areas of multiple fractures or non planar pre existing faults may explain the non dc components for induced events

degree of pyritization of iron as a paleoenvironmental indicator of bottom water oxygenation abstract a new method is presented for recognizing the degree of bottom water oxygenation in organic carbon bearing rock the method uses the parameter degree of pyritization dop which is the ratio of pyritic fe to pyritic fe acid soluble fe sediments with ages ranging from devonian to cretaceous have been classified on paleoecological and sedimentological criteria as having been deposited in aerobic conditions deposited in fully oxygenated bottom waters restricted marine conditions deposited in waters with low oxygen concentrations or inhospitable bottom conditions where little or no oxygen is present and h2s may be continually or intermittently present aerobic dop

antitaxial crack seal vein microstructures and their relationship to displacement paths abstract the microstructures developed in an example of a syntectonic crack seal vein indicate that antitaxial fibres do not necessarily track the incremental opening history during crack seal vein growth this observation serves as a caution against the uncritical use of fibrous microstructures to make inferences about displacement histories during fibre formation it is demonstrated that crack seal processes can lead to the development of both irregular and laminated vein microstructures as well as fibrous microstructures the types of microstructures which develop in crack seal veins and the extent to which they reflect the opening paths of veins depend on a range of factors the most important of these include the nucleation and growth kinetics of phases precipitated from fluids in opening fractures and the location of sites of material accretion during successive crack seal increments

rapid computation of multioffset vertical seismic profile synthetic seismograms for layered media by rearranging the formulas for the responses of buried sources and receivers in the kennett reflectivity algorithm we have obtained a new algorithm that is very efficient for computing multioffset vsp synthetic seismograms the rearrangement of the response formulas is quite general inasmuch as it applies to both isotropic and anisotropic kennett codes our new algorithm and the original single receiver algorithm can both be made to run much faster on vector computers by taking the layer loop out of the p loop but leaving it inside the frequency loop the resulting vector codes can still compute the response of media with frequency dependent velocities

geodynamics of the eastern yilgarn craton abstract over the last decade there have been significant advances in our understanding of the stratigraphy magmatism deformation metamorphism and timing of mineralisation in the eastern yilgarn craton eyc of western australia the integration of these disciplines has enabled a holistic review of the tectonic history of the eyc which favours a paraautochthonous tectonic model a significant advance has been the recognition of a 2810 ma rifting event off the eastern margin of the youanmi terrane which set up the north northwest trending architecture of the eyc rifting was followed by a 100 myr long phase of tectonic activity initiated by the establishment of a convergent margin at 2715 ma characterised by a west dipping subduction zone to the east of the eyc subduction was associated with the deposition of 2715 2670 ma volcanic stratigraphy and the emplacement of voluminous tonalite trondhjemite granodiorite magmatism which resulted in magmatic thickening of the crust volcanism was terminated by a 5 myr pulse of east northeast contraction followed by mid orogenic extension possibly linked to lithospheric and lower crustal delamination the lack of ultra high pressure metamorphism and the presence of high geothermal gradients preclude this contractional event from recording continent continent collision mid orogenic extension at 2665 ma resulted in the introduction of metasomatised mantle melts mafic granites and syenites deposition of siliciclastic basins which record metamorphic anticlockwise ptt paths and the start of significant economic gold mineralisation in the eyc high stretching factors and or delamination associated with this extension event resulted in significant heat input into the base of the crust which eventually led to the emplacement of low ca crustal melt granites and cratonisation of the eyc major gold mineralisation postdates mid orogenic extension and was associated with renewed compression transpression 2650 ma and the development of steep predominantly north northwest striking sinistral and later north to north northeast striking dextral strike slip faults during low ca granite emplacement

mass change calculations in altered rock series a technique is presented whereby precursors of altered rocks in a fractionated rock series can be identified and changes in chemical components mass and volume accurately calculated the procedure utilizes elements with high degrees of immobility in hydrothermal and associated interactive water rock systems igneous rock series and sedimentary and metamorphic rocks with primary continuity of chemical composition are amenable to this treatment in the procedure for igneous rocks best fit fractionation lines are established for an immobile igneous compatible incompatible element pair the latter element functioning as the fractionation monitor alteration paths are linear and pass through the origin intersections of the fractionation and alteration lines yield the immobile element concentrations in the precursors to the altered rocks mobile components are determined from fractionation lines and monitor values total mass changes are proportional to the displacement of altered samples from the fractionation line masses of other elements in an altered sample reconstructed values are recalculated to the concentration of the monitor element in the precursor chemical changes are the differences between the calculated precursor compositions and the reconstructed altered rock compositions errors are mainly a function of the positioning of the fractionation lines

elastic wave propagation in media with parallel fractures and aligned cracks a model of parallel slip interfaces simulates the behaviour of a fracture system composed of large closely spaced aligned joints the model admits any fracture system anisotropy triclinic the most general monoclinic orthorhombic or transversely isotropic and this is specified by the form of the 3 3 fracture system compliance matrix the fracture system may be embedded in an anisotropic elastic background with no restrictions on the type of anisotropy to compute the long wavelength equivalent moduli of the fractured medium requires at most the inversion of two 3 3 matrices when the fractures are assumed on average to have rotational symmetry transversely isotropic fracture system behaviour and the background is assumed isotropic the resulting equivalent medium is transversely isotropic and the effect of the additional compliance of the fracture system may be specified by two parameters in addition to the two isotropic parameters of the isotropic background dilute systems of flat aligned microcracks in an isotropic background yield an equivalent medium of the same form as that of the isotropic medium with large joints i e there are two additional parameters due to the presence of the microcracks which play roles in the stress strain relations of the equivalent medium identical to those played by the parameters due to the presence of large joints thus knowledge of the total of four parameters describing the anisotropy of such a fractured medium tells nothing of the size or concentration of the aligned fractures but does contain information as to the overall excess compliance due to the fracture system and its orientation as the aligned microcracks which were assumed to be ellipsoidal with very small aspect ratio are allowed to become non fiat i e have a growing aspect ratio the moduli of the equivalent medium begin to diverge from the standard form of the moduli for flat cracks the divergence is faster for higher crack densities but only becomes significant for microcracks of aspect ratios approaching 0 3

residual migration velocity analysis in the plane wave domain over the last few years migration velocity analysis methods have been developed for 2 d and 3 d models by extending the assumptions and approximations used for rms velocity models computational requirements for these analyses have increased dramatically because top down layer stripping migration is needed to derive interval velocities directly instead of using rms velocities and then converting into interval velocities we establish exact equations for 1 d and 2 d residual velocity analysis in the depth plane wave domain and use these in an iterative and interactive migration velocity analysis program the new method updates interval velocities directly in a top down residual difference correction for all layers after prestack depth migration instead of top down layer stripping migration followed by residual analysis this makes the new method a suitable tool for migration velocity analysis especially for 3 d surveys we test the method on synthetic and field data the field data results show that a reasonable velocity model is obtained and most common image gathers are correctly imaged using no more than four iterations

anisotropic 3d full waveform inversion abstractwe have developed and implemented a robust and practical scheme for anisotropic 3d acoustic full waveform inversion fwi we demonstrate this scheme on a field data set applying it to a 4c ocean bottom survey over the tommeliten alpha field in the north sea this shallow water data set provides good azimuthal coverage to offsets of 7 km with reduced coverage to a maximum offset of about 11 km the reservoir lies at the crest of a high velocity antiformal chalk section overlain by about 3000 m of clastics within which a low velocity gas cloud produces a seismic obscured area we inverted only the hydrophone data and we retained free surface multiples and ghosts within the field data we invert in six narrow frequency bands in the range 3 to 6 5 hz at each iteration we selected only a subset of sources using a different subset at each iteration this strategy is more efficient than inverting all the data every iteration our starting velocity model was obtained using standard psdm model bui

sulfate reduction and formation of iron sulfide minerals in nearshore sediments from qi ao island pearl river estuary southern china abstract sulfate reduction usually leads to the formation of iron sulfide minerals in marine sediments and strongly influences the global coupled carbon iron sulfur biogeochemical cycles in marine sediments here we report sulfate reduction and pyrite formation from the coastal sediments near qi ao island pearl river estuary using a combination of sedimentary pore water and solid sediment geochemical analyses strong enrichments in reactive iron 276 8 358 2 mol g were observed especially at the sites located near the river outlets high contents of reactive iron and low values of the degree of iron pyritization dop indicate that the limiting effect of reactive iron on pyrite formation can be excluded at these sites in the upper sedimentary layer conservative mixing of river water and marine water reinforces the fact that sulfate reduction is not appreciable the limited sulfate supply and low sulfate reduction rates lead to not enough dissolved sulfide available for formation of iron sulfide minerals however below this dilution mixing layer pore water geochemical profiles indicate that a fraction of sulfate consumption is mediated by upward diffusing methane an additional sulfide is produced from anaerobic oxidation of methane aom and later reacts with reactive iron resulting in enhancement of pyrite formation in the sediments by contrast it appears that sulfate concentration is a primary factor controlling for pyrite formation in these estuarine environments our study highlights the need in future work for an integrated analysis of the hydrologically driven change in the bottom seawater sulfate concentrations to better understand the regulation factors of global sulfur reservoirs in coastal sediments of estuarine environments as sulfur burial is an important factor in determining the past atmospheric oxygen level

trace element content of pyrite from the kapai slate st ives gold district western australia the kapai slate is a continuous pyrite rich carbonaceous shale horizon within the st ives au district that is spatially related to high grade au mineralization in situ laser ablation inductively coupled mass spectrometry la icpms trace element analyses in situ sensitive high resolution ion microprobe stable isotope shrimp si s isotope analyses and optical microscopy pyrite texture analyses were used to examine the different pyrite types in the kapai slate and au deposits these data were also used to confirm that the trace element signature of sedimentary pyrite can be preserved in rocks that underwent upper to mid greenschist facies metamorphism and significant hydrothermal overprint the data were further utilized to gain a more detailed understanding of the ocean conditions during deposition of the kapai slate and determine whether some of the au and s in the st ives district could have been sourced from the kapai slate seven different types of pyrite were identified fine grained sedimentary pyrite py 1 nodular sedimentary pyrite py 2 remobilized sedimentary pyrite py 3 coarse grained inclusions poor late pyrite py 4 inclusion rich magnetite series pyrite py 5 ore stage pyrite py 6 and pyrite associated with the mafic units py 7 each type of pyrite was found to have distinctive trace element compositions and s isotope signatures the results of the la icpms analyses provide evidence for early trace element enrichment in the kapai slate sedimentary pyrite median values of 158 ppm ni 387 ppm co 82 ppm cu 727 ppm as 1 91 ppm mo 13 ppm se 0 25 ppm au 7 72 ppm te and 3 36 ppm ag for py 1 and 223 ppm ni 158 ppm co 99 ppm cu 856 ppm as 1 27 ppm mo 10 2 ppm se 0 57 ppm au 10 09 ppm te and 6 62 ppm ag for py 2 concentrations of ni and co are low relative to other late archean sedimentary pyrite median of 813 and 465 ppm respectively and mo levels are near that of the euxinic shales of the similar aged jeerinah formation in the hamersley basin western australia these data suggest that the kapai slate was deposited in an anoxic to euxinic basin with relatively low biological productivity the 33 s and 34 s signatures of the sedimentary pyrite suggest two different sources of s positive 34 s and negative 33 s signatures indicate bacterial reduction of so 4 2 from seawater whereas positive 34 s and positive 33 s signatures indicate an elemental s 8 source indicating the pyrite formed later during diagenesis this s isotope signature is consistent with a transition between a near sediment environment to a more distal environment source analyses of the ore phase pyrite yield weakly positive 33 s values this suggests there was a minor contribution of sedimentary s to the more significant oxidized ore forming fluids which is consistent with a small contribution of au from a sedimentary source approximations of the degree of sedimentary pyrite destruction in the pyrrhotite pyrite dominated zones and pyrrhotite magnetite pyrite zones of the northern part of the st ives district were used to calculate the amount of au released from the early sedimentary pyrite the calculation suggests that a minor though possibly locally significant amount of au could have been sourced from the kapai slate

regulation of bacterial sulfate reduction and hydrogen sulfide fluxes in the central namibian coastal upwelling zone abstract the coastal upwelling system off central namibia is one of the most productive regions of the oceans and is characterized by frequently occurring shelf anoxia with severe effects for the benthic life and fisheries we present data on water column dissolved oxygen sulfide nitrate and nitrite pore water profiles for dissolved sulfide and sulfate 35 s sulfate reduction rates as well as bacterial counts of large sulfur bacteria from 20 stations across the continental shelf and slope the stations covered two transects and included the inner shelf with its anoxic and extremely oxygen depleted bottom waters the oxygen minimum zone on the continental slope and the lower continental slope below the oxygen minimum zone high concentrations of dissolved sulfide up to 22 mm in the near surface sediments of the inner shelf result from extremely high rates of bacterial sulfate reduction and the low capacity to oxidize and trap sulfide the inner shelf break marks the seaward border of sulfidic bottom waters and separates two different regimes of bacterial sulfate reduction in the sulfidic bottom waters on the shelf up to 55 of sulfide oxidation is mediated by the large nitrate storing sulfur bacteria thiomargarita spp the filamentous relatives beggiatoa spp occupy low o 2 bottom waters on the outer shelf sulfide oxidation on the slope is apparently not mediated by the large sulfur bacteria the data demonstrate the importance of large sulfur bacteria which live close to the sediment water interface and reduce the hydrogen sulfide flux to the water column modeling of pore water sulfide concentration profiles indicates that sulfide produced by bacterial sulfate reduction in the uppermost 16 cm of sediment is sufficient to account for the total flux of hydrogen sulfide to the water column however the total pool of hydrogen sulfide in the water column is too large to be explained by steady state diffusion across the sediment water interface episodic advection of hydrogen sulfide possibly triggered by methane eruptions may contribute to hydrogen sulfide in the water column

an accurate ray based offset to angle transform from normal moveout uncorrected multicomponent data in a transversely isotropic medium with vertical symmetry axis a new ray based approach for converting multicomponent normal moveout uncorrected offset domain prestack seismic data into angles is presented from any two way zero offset time sample and a given angle of incidence the exact ray and a few neighboring rays that contribute to the given angle of incidence are traced through the medium to their corresponding source and receiver locations to compute their raypath lengths traveltimes and offsets a polynomial then is fitted locally between these computed offsets and their corresponding traveltimes next the traveltimes for the actual offsets on data that lie within these traced rays are computed from this local polynomial fit and their corresponding time samples are partially stacked and moved to their two way zero offset times repetition of this procedure for all zero offset time samples and for all desired angles produces an angle gather the procedure allows computation of both primary p p and mode converted p sv angle gathers also by calculating the exact raypath length an accurate geometric spreading compensation can be computed and applied to these gathers by using the methodology for a horizontally stratified transversely isotopic medium with a vertical symmetry axis a vti medium it is possible to extract a much more accurate angle domain response from the ray based method than is possible from a traditional nmo based method accuracy out to large angles for both primary and mode converted reflection suggests that this ray based transform potentially could be used in a multicomponent prestack waveform inversion scheme

1d elastic full waveform inversion and uncertainty estimation by means of a hybrid genetic algorithm gibbs sampler approach abstract stochastic optimization methods such as genetic algorithms search for the global minimum of the misfit function within a given parameter range and do not require any calculation of the gradients of the misfit surfaces more importantly these methods collect a series of models and associated likelihoods that can be used to estimate the posterior probability distribution however because genetic algorithms are not a markov chain monte carlo method the direct use of the genetic algorithm sampled models and their associated likelihoods produce a biased estimation of the posterior probability distribution in contrast markov chain monte carlo methods such as the metropolis hastings and gibbs sampler provide accurate posterior probability distributions but at considerable computational cost in this paper we use a hybrid method that combines the speed of a genetic algorithm to find an optimal solution and the accuracy of a gibbs sampler to obtain a reliable estimation of the posterior probability distributions first we test this method on an analytical function and show that the genetic algorithm method cannot recover the true probability distributions and that it tends to underestimate the true uncertainties conversely combining the genetic algorithm optimization with a gibbs sampler step enables us to recover the true posterior probability distributions then we demonstrate the applicability of this hybrid method by performing one dimensional elastic full waveform inversions on synthetic and field data we also discuss how an appropriate genetic algorithm implementation is essential to attenuate the genetic drift effect and to maximize the exploration of the model space in fact a wide and efficient exploration of the model space is important not only to avoid entrapment in local minima during the genetic algorithm optimization but also to ensure a reliable estimation of the posterior probability distributions in the subsequent gibbs sampler step

pyrite formation in the water column and sediments of a meromictic lake we compared concentrations of organic carbon partitioning of iron and sulfur stable isotopic ratios of major sulfur species and pyrite morphologies in the water column and sediments of oxic and euxinic portions of a sulfate rich meromictic lake green lake near fayetteville new york in order to examine the effects of euxinic conditions on pyrite formation and sedimentary fe s c relations about 15 of the pyrite in sediments of the euxinic basin of green lake forms in the water column the bulk of pyrite in both euxinic and oxic sediments forms in the upper 10 cm of the sediment and is limited by availability of reactive iron oxic and euxinic sediments in green lake cannot be clearly distinguished on the basis of concentrations of organic carbon or pyrite degree of pyritization or sulfur isotopic ratios of pyrite but can be distinguished by using size distributions of pyrite framboids therefore fe s c relations in ancient shales must be used with caution when attempting to reconstruct paleoenvironmental redox conditions

early diagenesis of sulfur in a tropical upwelling system cabo frio southeastern brazil the early diagenesis of sulfur was assessed in four short sediment cores on the continental shelf off southeastern brazil that were deposited under the influence of an upwelling tropical system this tropical upwelling area allows a direct focus on the coupled roles of hydrodynamic and bioturbation driven influences on sulfate reduction sulfide re oxidation and corresponding stable sulfur isotope signal formation under the depositional conditions of cabo frio the degree of reactive iron pyritization was limited by both dissolved sulfide availability and pyrite oxidation events textural analyses of pyrite framboids provide evidence of re oxidation processes reflecting dynamic redox conditions in the sediments the isotope composition of pore water sulfate remained close to the modern seawater value but very light stable sulfur isotope ratios 34 s 32 s of chromium reducible sulfur essentially pyrite are found that reflect intense bioturbation induced sulfur re cycling the sulfur isotope signatures developing in these tropical upwelling sites are similar to those of modern euxinic systems although they are caused by a superimposition of sulfate reduction and an intense oxidative sulfur cycle

fluid infiltration into fault zones chemical isotopic and mechanical effects fluid infiltration into fault zones and their deeper level counterparts brittle ductile shear zones is examined in diverse tectonic environments in the 2 7 ga abitibi greenstone belt major tectonic discontinuities with lateral extents of hundreds of kilometres initiated as listric normal faults accommodating rift extension and acted as sites for komatiite extrusion and locally intense metasomatism during reverse motion on the structures accommodating shortening of the belt these transcrustal faults were utilised as a conduit for the ascent of trondhjemitic magmas from the base of the crust and of alkaline magmas from the asthenosphere and for the discharge of thousands of cubic kilometres of hydrothermal fluids such fluids were characterised by 18o 6 2 d 50 20 13c 4 4 and temperatures of 270 to 450 c probably derived from devolatilisation of crustal rocks undergoing prograde metamorphism hydrothermal fluids were more radiogenic 87sr 86sr 0 7010 to 0 7040 and possessed higher than did contemporaneous mantle komatiites or tholeiites and thus carried a contribution from older sialic basement a provinciality of87sr 86sr and 13c is evident signifying that fault plumbing sampled lower crust which was heterogeneous at the scale of tens of kilometres mineralised faults possess enrichments of large ion lithophile lil lil elements including k rb ba cs b and co2 and rare elements such as au ag as sb se te bi and w fluids were characterised by xco2 0 1 neutral to slightly acidic ph low salinity 3 wt k na 0 1 they carried minor ch4 co and n2 and they underwent transient effervescence of co2 during decompression clastic sediments occupy graben developed at fault flexures the40ar 39ar release spectra indicate that fault rocks experienced episodic disturbance on time scales of hundreds of millions of years

the role of early formed structures on the development of the world class st ives goldfield yilgarn wa a revised structural interpretation for the victory to kambalda area of the world class st ives goldfield in the archean yilgarn craton has mapped out the distribution of wnw trending faults within this area of the field these previously cryptic wnw trending structures had been identified in gravity data and also by isopach thickness variations the wnw trending faults acted as transfers syn gold mineralization although only discrete segments of these faults were active during the main stage of gold mineralization where mineralized the faults transferred strain from a complex combination of block on block movement associated with thrusting and strike slip movement on nw and n trending faults along some segments n trending mineralized faults terminate against the wnw trending faults many of the wnw trending faults correlate with major strike changes on regional and camp scale faults and they are domain boundaries for the critical n trending fault segments that host high grade gold within contractional jogs the wnw trending faults also show evidence for an older deformation history prior to main stage gold which may extend back to early basin development associated with ultramafic and mafic volcanism they are inferred to have been a series of early wnw trending normal faults and breached relay ramps associated with oblique rifting along an older nnw trending basement boundary

palaeoseismic events recorded in archaean gold quartz vein networks val d or abitibi quebec canada archaean gold quartz vein deposits are commonly hosted in high angle reverse shear zones and are interpreted to have formed in a regime of horizontal compression and high fluid pressure environment this paper presents the results of a combined structural and fluid inclusion study on three gold quartz vein deposits of the val d or area abitibi quebec consisting of subhorizontal extensional veins and e w steeply dipping shear veins crack seal structures tourmaline fibres stretched quartz crystals and open space filling textures indicate that the subhorizontal veins formed by hydraulic fracturing under supralithostatic fluid pressure co2 rich and h2o nacl fluid inclusions interpreted as two coexisting immiscible fluids occur typically in microcracks of different orientations interpreted to have formed in the 1 2 plane horizontal co2 rich fluid inclusion planes are contemporaneous with the opening of these veins 3 vertical vertical h2o nacl fluid inclusion planes as well as some microstructures such as deformed minerals indicate that the same extensional veins have experienced episodic vertical shortening 3 horizontal alternating with the opening events deformation and slip opening also occurred in shear veins in which preferred orientation of fluid inclusion planes is not clear except that the h2o nacl fluid inclusion planes tend to be oriented at high angles to the slip direction the successive opening and collapse events in subhorizontal extensional veins are correlated with deformation and slip opening events in shear veins respectively and are attributed to cyclic fluid pressure fluctuations in the system they are thus consistent with the fault valve model sudden drop in fluid pressure from supralithostatic to lower values induces fluid unmixing and occurs immediately post failure following seismic rupturing along the shear zone sealing of the shear veins allows the fluid pressure to build up again and the subhorizontal extensional veins to form the intermittent change in orientation of 3 in horizontal extensional veins is ascribed to a total shear stress release along the neighbouring shear veins after rupture the complex internal structures of the gold quartz veins provide an excellent record of archaean seismic activity along high angle reverse faults

statistical empirical index of chemical weathering in igneous rocks a new tool for evaluating the degree of weathering abstract chemical weathering indices are useful tools in characterizing weathering profiles and determining the extent of weathering however the predictive performance of the conventional indices is critically dependent on the composition of the unweathered parent rock to overcome this limitation the present paper introduces an alternative statistical empirical index of chemical weathering that is extracted by the principal component analysis pca of a large dataset derived from unweathered igneous rocks and their weathering profiles the pca analysis yields two principal components pc1 and pc2 which capture 39 23 and 35 17 of total variability respectively the extent of weathering is reflected by variation along pc1 primarily due to the loss of na 2 o and cao during weathering in contrast pc2 is the direction along which the projections of unweathered felsic intermediate and mafic igneous rocks appear to be best discriminated therefore pc1 and pc2 represent independent latent variables that correspond to the extent of weathering and the chemistry of the unweathered parent rock subsequently pc1 and pc2 were then mapped onto a ternary diagram mfw diagram the m and f vertices characterize mafic and felsic rock source respectively while the w vertex identifies the degree of weathering of these sources independent of the chemistry of the unweathered parent rock the w index has a number of significant properties that are not found in conventional weathering indices first the w index is sensitive to chemical changes that occur during weathering because it is based on eight major oxides whereas most conventional indices are defined by between two and four oxides second the w index provides robust results even for highly weathered sesquioxide rich samples third the w index is applicable to a wide range of felsic intermediate and mafic igneous rock types finally the mfw diagram is expected to facilitate provenance analysis of sedimentary rocks by identifying their weathering trends and thereby enabling a backward estimate of the composition of the unweathered source rock

structural control of greenstone hosted gold mineralization by transcurrent shearing a new interpretation of the kalgoorlie mining district western australia abstract the kalgoorlie mining district encloses the golden mile and mt charlotte mining areas and has produced 1200 metric tons of gold it is classed as one of the largest archaean gold deposits in the world the district is located in the norseman wiluna greenstone belt of the yilgarn craton western australia this greenstone belt is characterized by nnw trending transcurrent shear zones some of which are traceable over more than 500 km strike length mineralization in the district is controlled by second third and fourth order structures located between principal shear zones of regional extent an early deformation phase of regional folding d1 associated with prograde regional metamorphism of low grade preceded two phases of wrenching the geometry of mineralized shear zones in both mining areas as well as field and microstructural studies allow us to distinguish an early phase of transpressional sinistral shearing d2 from a later phase of transcurrent dextral shearing d3 the earlier sinistral shear system in the golden mile area was in part reactivated but its gross original geometry was preserved these observations are consistent with clay model experiments carried out by the authors hydrothermal carbonate sericite alteration and gold mineralization commenced during the sinistral shear event and continued into the dextral event mineralization within individual shear zones forms tabular orebodies whereas pipe shaped ore shoots occur at the intersection of shear zones the ore shoots contain high grade telluride mineralization locally relative timing relationships show that mineralization postdates the peak of regional metamorphism hydrothermal activity in the district was preceded by the intrusion of calc alkaline porphyry dykes of granodioritic to tonalitic composition and accompanied by the intrusion of lamprophyre dykes

azimuthal anisotropy analysis of p wave seismic data and estimation of the orientation of the in situ stress fields an example from the rock springs uplift wyoming usa abstractestimating the orientation and magnitude of maximum and minimum horizontal in situ stress is important for characterizing naturally fractured unconventional and carbon sequestered reservoirs for naturally fractured reservoirs they are needed to guide directional drilling for unconventional reservoirs they are used for optimal placements of hydraulic fractures and for carbon sequestered reservoirs they are used to avoid fracturing of overlying seal rocks in addition a knowledge of stress fields can be used to induce fractures within the target reservoirs and enhance additional storage for carbon sequestration experiments the orientation and magnitude of in situ stress can be calculated at the well locations for locations away from the wells analysis of the azimuthal dependence of the amplitude variation with angle gradient or azimuthal angle stacks are used to quantify anisotropy which are then related with well data and other geologic information for stress estimation such azimutha

a note on fault reactivation reactivation of existing faults whose normal lies in the 1 3 plane of a stress field with effective principal compressive stresses 1 2 3 is considered for the simplest frictional failure criterion n n p where and n are respectively the shear and normal stresses to the existing fault p is the fluid pressure and is the static friction for a plane oriented at to 1 the stress ratio for reactivation is 1 3 1 cot 1 tan this ratio has a minimum positive value at the optimum angle for reactivation given by 12tan 1 1 but reaches infinity when 2 beyond which 3 0 is a necessary condition for reactivation an important consequence is that for typical rock friction coefficients it is unlikely that normal faults will be reactivated as high angle reverse faults or thrusts as low angle normal faults unless the effective least principal stress is tensile

defining element associations and inferring geological processes from total element concentrations in australian catchment outlet sediments multivariate analysis of continental scale geochemical data abstract in this paper the geochemical composition of surficial regolith is statistically analysed and compared to independent geoscientific datasets to infer processes governing regolith composition surface 0 10 cm depth and sub surface 60 80 cm depth transported sediment samples from the national geochemical survey of australia were analysed for total element content in both coarse censored data

zonation of hydrothermal alteration at the san antonio gold mine bissett manitoba canada the san antonio mine is a mesothermal gold deposit hosted in the leucogabbroic portion of the san antonio sill a differentiated mafic intrusion in the western part of the superior province of the precambrian canadian shield the gold mineralization is associated with a system of quartz carbonate veins within a moderately plunging cylinder of hydrothermally altered rocks alteration zones delimited by mapping alteration isograds encompass individual veins and merge to form the mine scale alteration the alteration zones formed through metasomatism of co 2 k and lesser na as determined by mass balance calculations on whole rock chemical data alteration minerals are dependent on the original rock composition with actinolite and epidote characteristic of unaltered host rock boundaries between individual zones can be mapped as alteration isograds in order of increasing alteration or decreasing distance from a vein or shear zone these isograds in the leucogabbro are 1 chlorite and calcite 2 paragonite and ankerite and 3 ankerite and muscovite and in the melagabbro 1 chlorite and calcite and 2 ankerite and chlorite zone 1 contains evidence of the transition from titanite to rutile and calcite paragonite abundant at the san antonio mine is much less common in other areas of gold alteration thus making the san antonio alteration unique the formation of paragonite is due to co 2 metasomatism in an aluminum rich host rock its coexistence with muscovite and hydrothermal albite is significant as it defines the k super na super ratio of the hydrothermal fluid associated with gold mineralization using k super h super na super h super and co 2 the path of alteration was modeled corresponding to observations of texture and paragenesis thermodynamic calculations also suggest that the isograds represent the degree of carbonate potassic and sodic metasomatism by reaction of the gold bearing hydrothermal fluid with the unaltered metamorphic rocks and display the importance of original rock composition on the mineralogy of alteration zones the conditions of alteration and gold mineralization can be directly determined by understanding the alteration mineralogy and changes at zone boundaries this ore fluid contained subequal amounts of potassium and sodium about 5 percent co 2 and was within the temperature range of 230 degrees to 375 degrees c

bayesian inference gibbs sampler and uncertainty estimation in geophysical inversion the posterior probability density function ppd m d obs of earth model m where d obs are the measured data describes the solution of a geophysical inverse problem when a bayesian inference model is used to describe the problem in many applications the ppd is neither analytically tractable nor easily approximated and simple analytic expressions for the mean and variance of the ppd are not available since the complete description of the ppd is impossible in the highly multi dimensional model space of many geophysical applications several measures such as the highest posterior density regions marginal ppd and several orders of moments are often used to describe the solutions calculation of such quantities requires evaluation of multidimensional integrals a faster alternative to enumeration and blind monte carlo integration is importance sampling which may be useful in several applications thus how to draw samples of m from the ppd becomes an important aspect of geophysical inversion such that importance sampling can be used in the evaluation of these multi dimensional integrals importance sampling can be carried out most efficiently by a gibbs sampler gs we also introduce a method which we called parallel gibbs sampler pgs based on genetic algorithms ga and show numerically that the results from the two samplers are nearly identical we first investigate the performance of enumeration and several sampling based techniques such as a gs pgs and several multiple maximum a posteriori map algorithms for a simple geophysical problem of inversion of resistivity sounding data several non linear optimization methods based on simulated annealing sa ga and some of their variants can be devised which can be made to reach very close to the maximum of the ppd such map estimation algorithms also sample different points in the model space by repeating these map inversions several times it is possible to sample adequately the most significant portion s of the ppd and all these models can be used to construct the marginal ppd mean covariance etc we observe that the gs and pgs results are identical and indistinguishable from the enumeration scheme multiple map algorithms slightly underestimate the posterior variances although the correlation values obtained by all the methods agree very well multiple map estimation required 0 3 of the computational effort of enumeration and 40 of the effort of a gs or pgs for this problem next we apply gs to the inversion of a marine seismic data set to quantify uncertainties in the derived model given the prior distribution determined from several common midpoint gathers

the crustal continuum model for late archaean lode gold deposits of the yilgarn block western australia most archaean gold ores belong to a coherent genetic group of structurally controlled lode deposits that are characteristically enriched in au with variable enrichments in ag as w sb bi te b and pb but rarely cu or zn and are surrounded by wallrock alteration haloes enriched in k lile and co2 with variable na and or ca addition evidence from the yilgarn block of western australia combined with similar evidence from canada and elsewhere indicates that such deposits represent a crustal continuum that formed under a variety of crustal regimes over at least a 15 km crustal profile at pt conditions ranging from 180 c at 1 kb to 700 c at 5 kb individual deposits separated by tens to hundreds of kilometres collectively show transitional variations in structural style of mineralisation vein textures and mineralogy of wallrock alteration that relate to the pt conditions of their formation at varying crustal depths specific transitions within the total spectrum may be shown also by deposits within gold camps although nowhere is the entire continuum of deposits recorded from a single gold camp or even greenstone belt

the structure of shear related vein type gold deposits a review abstract the structure of vein type gold deposits is defined by the shapes and geometrical relationships of mineralized bodies the form of the mineralization making up these bodies and the sequence of vein forming events most mineralized zones occur within or are spatially associated with shear zones especially shear zones in larger systems of intersecting shear zone sets they range in shape from tabular to linear and in form from disseminated to breccia to stockwork or sheeted veinlet zones to single veins there typically is a complex history of mineral deposition which overlaps and is genetically related to the deformation that generated the host structural zone the purpose of this paper is to consider the structural features of vein type deposits in relation to the geometrical properties and evolution of veinhosting fractures particularly shear zones the evidence indicates that veins are localized by tectonically generated dilatancy in an environment of low mean stress caused by high fluid pressure it is proposed that a major cause of tectonic dilation of shear zones is the interference between intersecting shears during bulk inhomogeneous flattening by movement on systems of intersecting shear zone sets