heavy mineral sands in the eucla basin southern australia deposition and province scale prospectivity the marine eucla basin in southern australia is emerging as a major new heavy minerals province in australia beach placers are associated with a series of partially buried cenozoic coastal barrier sands formed along an arcuate 2 000 km long basin margin the trace of which is up to 320 km inland of the present coastline the presence of high grade deposits with dominant zircon over ilmenite and lesser amounts of rutile and leucoxene was established with the discovery of the jacinth and ambrosia heavy mineral deposits in late 2004 an additional 10 heavy mineral prospects were subsequently identified and are at various stages of evaluation the eucla basin and its adjacent paleovalley system have a large areal extent that contains a complicated succession of marine and nonmarine strata spanning a wide range of depositional environments four distinct constructional phases for the development of shorelines can be recognized and correlated with major third order sea level events established by others from the marine depositional record as occurring during the middle eocene 42 5 ma late middle eocene 39 36 ma late eocene 36 34 ma and miocene pliocene 15 2 6 ma prevailing westerly winds built extensive dune systems by longshore drift sediment movement was from west to east detrital zircon rocks from the ooldea and barton barriers show a distribution of zircon age that is consistent with the proterozoic musgrave province to the north of the basin as the dominant primary source area of the heavy minerals with a contribution from the albany fraser orogen to the west the likelihood is that these heavy minerals have been recycled via sedimentary basins that flank the musgrave province and include the neoproterozoic to cambrian officer basin and permian to mesozoic deposits of the bight basin our current depositional model is summarized as follows 1 initial rapid transgression and deposition of a shallow marine sand sheet subsequently overlain by shallow marine limestone during middle eocene 2 a major eocene transgression and deposition of a shelf barrier and lagoonal shoreface marine complex during the late middle eocene 3 further transgression and highstand deposition during the late eocene 4 renewed transgression of barrier lagoonal and possibly flooding deltaic sand blanket in the southeastern coastal plain with neotectonic uplift tilting in the western eucla margin during mio pliocene time each stage of reworking increased the potential for heavy mineral concentration in placer deposits

limits on the age of the deccan traps of india from paleomagnetic and plate reconstruction data and their uncertainties revised limits on the age of the deccan traps of india are provided by recent improvements to reconstructions of india relative to surrounding plates since late cretaceous time realistic estimates of the uncertainties in the plate reconstructions and recent paleomagnetic data from the deccans traps and from north america we estimate the age of the deccan traps by rotating its paleomagnetic pole into the north american reference frame different assumed ages for the deccan traps result in very different positions for its paleomagnetic pole relative to north america comparing these positions with the north american apparent polar wander path and accounting for uncertainties in both the paleomagnetic poles and in the reconstructions we place limits on the age of the deccan traps of 62 72 ma significantly younger than found in several prior studies the 62 72 ma limits are consistent with ages obtained independently from radiometric dating and paleontology and are consistent with the hypothesis that the deccan traps were extruded rapidly during reversed polarity chron 29r which contains the cretaceous tertiary boundary

reference frames for plate tectonics and uncertainties abstract paleomagnetic hotspot and mean lithosphere frameworks are compared for the cenozoic era errors in the relative motions of plate pairs accumulate to give uncertainties in reference frames uncertainties in the poles of the mean lithosphere framework are estimated to be less than 1 and for the hotspot framework 1 1 assuming a 50 km error in location of individual hotspots the a 95 s for paleomagnetic data range from 1 to 5 with these uncertainties the differences between the paleomagnetic and mean lithosphere coordinate systems are not found to be statistically significant and the hotspot frame barely differs from the mean lithosphere frame during the early tertiary on the other hand the hotspot and paleomagnetic axes diverge in opposing directions in the mean lithosphere framework and can be distinguished this difference can be attributed to the pacific hotspots having more than 10 motion relative to the mean lithosphere whereas the african set of hotspots does not statistically differ from the mean lithosphere frame one possible explanation is a shift of the pacific hotspots relative to those in the atlantic

textural control on gold distribution in as free pyrite from the dongping huangtuliang and hougou gold deposits north china craton hebei province china article i nfo pyrite from the dongping huangtuliang and hougou gold deposits hebei province china was investigated using a combination of ore microscopy including back scattered imaging and in situ laser ablation inductively coupled plasma mass spectroscopy la icp ms a range of elements including au te ag pb bi cu co ni and as was analyzed with the aim to constrain the textural controls and influence of superimposed deformation on the distribution of invisible gold in as free pyrite thedatasetshowsexcellentcorrespondence betweenmeasuredhighgoldvaluesandthreekeytexturalcriteria i areasofclusteredtellurideinclusions especiallyindongping ii microshearingandfracturing brecciation dongping and huangtuliang and iii pyrite recrystallization huangtuliang invisible gold is only at trace levels in the hougou and gold bearing telluride minerals are not observed this is considered to result from larger scale remobilization of gold during advanced brecciation and recrystallization of pyrite pyrite grains containing clustered inclusions have by far the highest gold concentrations up to 1 wt with consistent values of thousands of ppm au interpretation of textures and la icp ms data infers that in these areas the distribution of telluride inclusions extends pervasively from micron to nanoscale i e as fields of nanoparticles local employment of coupled dissolution reprecipitation reaction between pyrite and fluids driving the k metasomatic alteration dated at 150 ma is invoked to explain such patterns syn deformational grain scale mobilization of gold and other elements took place leading to a portion of the gold being recrystallized within fractures and microshears the deformational event leads to gold enrichment in the huangtuliang pyrite where brittle fracturing brecciation has been followed by a resorption and recrystallization in the latter case foam textured pyrites again without as can contain up to hundreds of ppm gold theinterpreted sequenceof pyritepost depositional eventsis inaccordancewith the protracted magmaticevolution of the area starting with variscan alkaline intrusions 390 ma and reactivated during the yanshanian orogeny 150 ma thestudyshows i thatas freepyritecanreadilycontainsignificantamountsofinvisiblegold bothasnanoparticles

styles of lode gold mineralization contributing to the placers of the indian river and black hills creek yukon territory canada as deduced from microchemical characterization of placer gold grains between 1978 and 2009 approximately 430 000 oz of placer gold were obtained from the indian river and black hills creek which equates to roughly 20 of the production for the entire yukon territory during that period the area is unglaciated exposure is poor and there are few known lode gold occurrences present the technique of microchemical characterization of placer gold grains has been applied to illuminate the style s of source mineralization and their relationship to placer gold from the klondike gold district immediately to the north a total of 2 613 placer gold grains from 22 localities were characterised in terms of the au ag cu and hg content of their alloy and associated suite of opaque mineral inclusions a combination of alloy and inclusion mineralogy was used to define gold signatures which augmented the previous classification of orogenic gold in the klondike gold type 3b 8 25 ag is the main component of the placers in lower dominion creek but is augmented and eventually replaced by type 3a gold 10 40 ag in placers in the main indian river valley probably through erosion of gold bearing veins in the valley floor type 4 gold exhibits highly variable ag which may contain hg to a maximum of 11 wt this gold type also hosts a distinctive inclusion assemblage of complex polymetallic sulphides tellurides sulfotellurides and sulfosalts and has previously been ascribed to local low sulfidation epithermal mineralization placer gold in drainages radiating from eureka dome exhibits various proportions of types 3 and 4 gold depending on location but type 3 gold forms the major component in black hills creek and northerly flowing tributaries of the indian river with the exception of eureka and montana creeks type 5 gold is found only in placers in the middle and lower indian river it is distinguished by slightly elevated 0 05 0 17 cu in the gold alloy together with low 5 9 ag contents inclusions of bi minerals cr bearing magnetite and molybdenite within type 5 gold suggest derivation from an intrusion related source candidates for such a source include undiscovered lode occurrences associated with cretaceous age intrusions to the south of the indian river or deformed cu au mo porphyry occurrences which are known to be present in the same area this analysis of placer gold has indicated that the contribution of low sulfidation epithermal gold from eureka creek to the larger placers of the indian river is minor consequently the placer gold inventory of the indian river is primarily orogenic in origin similarly the characterization of placer gold in blackhills creek strongly suggests an orogenic source this study has demonstrated for the first time that orogenic lode gold mineralization extends a considerable distance to the south of the southern klondike goldfield this information contributes to the regional models of gold mineralization in an area which is currently the focus of intensive exploration

lithologically controlled invisible gold yukon canada the newly discovered cretaceous coffee orogenic gold deposit 4 moz resource consists of an extensive oxidised zone developed on primary sulphidic rock the primary mineralised rock is characterised by invisible gold in arsenian pyrite that has replaced biotite in selected host rocks the deposit has a cryptic surface expression and is an example of an extremely subtle exploration target hydrothermal emplacement was controlled by extensional fractures with breccias but most mineralisation was focused on biotite bearing granitic gneiss metasedimentary gneisses and younger biotite granite fine grained 0 1 mm arsenian pyrite replaced biotite along mineral cleavage planes and followed biotite rich metamorphic and post metamorphic structural fabrics arsenian pyrite also formed overgrowths on earlier coarse grained up to 2 mm barren hydrothermal pyrite arsenian pyrite is concentrically zoned on the 1 10 m scale with respect to as sb and au contents and typically contains 5 wt as 500 mg kg sb and 500 mg kg au in solid solution biotite replacement was accompanied by sericitisation silicification and ankerite impregnation hydrothermal alteration involved dilution and localised depletion of k na and al in silicified host rocks but most ca mg and fe concentrations remained broadly constant magnesium rich ultramafic host rocks were only weakly mineralised with auriferous arsenian pyrite and have fuchsite and magnesite alteration near surface oxidation has liberated nanoparticulate and microparticulate supergene gold which remains essentially invisible varying degrees of oxidation extend as deep as 250 m below the present subdued topographic surface well beyond the present vadose zone and this deep oxidation may have occurred during post mineralisation uplift and erosion in the cretaceous oxidation has leached some as from the surficial mineralised rocks decreasing the geochemical signal which is also obscured by the localised presence of high background as up to 100 mg kg in metasedimentary quartzites in the region antimony provides more reliable soil anomalies than as but most sb anomalies are 100 mg kg the persistence of invisible gold into the extensive supergene zone with little gold particle size enhancement has ensured that no placer deposits have formed in nearby streams further restricting the surface footprint and au dispersal halo of this subtle exploration target

evidence for relative motions between the indian and australian plates during the last 20 m y from plate tectonic reconstructions implications for the deformation of the indo australian plate we use plate tectonic reconstructions to establish whether motions between india and australia occurred since chron 18 43 ma we test the africa antarctica australia india plate circuit closure at chrons 5 10 ma 6 21 ma and 13 36 ma using a compilation of magnetic anomalies and fracture zone traces from the southeast southwest central indian and the carlsberg ridges additional reconstructions at chrons 23 55 ma and 26 61 ma are used to estimate the overall motion between india and australia relative motions between the indian and australian plates are estimated using the plate circuit india africa australia a new statistical approach based on spherical regression analyses is used to assess the uncertainty of the best fitting finite rotations from the uncertainties in the data the uncertainty in a rotation is described by a covariance matrix directly related to the geometry of the reconstructed plate boundary to the distribution and estimated errors of the data points along it our parameterization of the rotations allows for simple combination of the rotation uncertainties along a plate circuit path results for chron 5 are remarkably consistent with present day kinematics in the indian ocean except that the arabian and indian plates are found to be separate plates comparisons of the motions between the indian and african plates across the carlsberg ridge with that between the australian and african plates across the central indian ridge evidence a significant counterclockwise rotation of the australian plate relative to the indian plate about a pole located in the central indian basin the determinations are consistent for chrons 26 13 6 and 5 determination at chron 23 is different but questionable due to the small number of available data we propose two alternative solutions that both predict convergence within the wharton and central indian basins and extension in the vicinity of the chagos laccadive ridge the first solution assumes that all the deformation in the equatorial indian ocean started 7 ma ago as found by ocean drilling program leg 116 hence all the determinations at different times represent the total motion between india and australia the averaged india australia euler vector chrons 5 6 13 and 26 11 1 s 78 0 3 54 lies within the central indian basin and yields a n s contraction of 46 52 km at 85 and 80 63 km at 90 however the difference of the india australia euler vectors at chrons 5 and 6 suggests that the india australia convergence started between 10 and 21 ma following the continent continent collision of india with asia in the early miocene the second averaged solution chrons 6 13 and 26 5 2 s 74 3 e 5 93 predicts a total n s contraction of 123 73 km at 85 e and 178 91 km at 90 e both models are compatible with the deformation pattern observed in the equatorial indian ocean

fine resolution multiscale mapping of clay minerals in australian soils measured with near infrared spectra 1 clay minerals are the most reactive inorganic components of soils they help to determine soil properties and largely govern their behaviors and functions clay minerals also play important roles in biogeochemical cycling and interact with the environment to affect geomorphic processes such as weathering erosion and deposition this paper provides new spatially explicit clay mineralogy information for australia that will help to improve our understanding of soils and their role in the functioning of landscapes and ecosystems i measured the abundances of kaolinite illite and smectite in australian soils using near infrared nir spectroscopy using a model tree algorithm i built rule based models for each mineral at two depths 0 20 cm 60 80 cm as a function of predictors that represent the soil forming factors climate parent material relief vegetation and time their processes and the scales at which they vary the results show that climate parent material and soil type exert the largest influence on the abundance and spatial distribution of the clay minerals relief and vegetation have more local effects i digitally mapped each mineral on a 3 arc second grid the maps show the relative abundances and distributions of kaolinite illite and smectite in australian soils kaolinite occurs in a range of climates but dominates in deeply weathered soils in soils of higher landscapes and in regions with more rain illite is present in varied landscapes and may be representative of colder more arid climates but may also be present in warmer and wetter soil environments smectite is often an authigenic mineral formed from the weathering of basalt but it also occurs on sediments and calcareous substrates it occurs predominantly in drier climates and in landscapes with low relief these new clay mineral maps fill a significant gap in the availability of soil mineralogical information they provide data to for example assist with research into soil fertility and food production carbon sequestration land degradation dust and climate modeling and paleoclimatic change

hotspots mantle plumes flood basalts and true polar wander persistent long lived stationary sites of excessive mantle melting are called hotspots hotspots leave volcanic trails on lithospheric plates passing across them the global constellation of fixed hotspots thus forms a convenient frame of reference for plate motions through the orientations and age distributions of volcanic trails left by these melting anomalies hotspots appear to be maintained by whole mantle convection in the form of upward flow through narrow plumes evidence suggests that plumes are deflected little by horizontal flow of the upper mantle mantle plumes are largely thermal features and arise from a thermal boundary layer most likely the mantle layer just above the core mantle boundary experiments and theory show that gravitational instability drives flow beginning with the formation of diapirs such a diapir will grow as it rises fed by flow through the trailing conduit and entrainment of surrounding mantle the structure thus develops a large spherical plume head and a long narrow tail on arrival at the base of the lithosphere the plume head flattens and melts by decompression producing enormous quantities of magma which erupt in a short period these are flood basalt events that have occurred on continents and in ocean basins and that signal the beginning of major hotspot tracks the plume supported hotspot reference frame is fixed in the steady state convective flow of the mantle and is independent of the core generated axial dipole paleomagnetic reference frame comparison of plate motions measured in the two frames reveals small but systematic differences that indicate whole mantle motion relative to the earth s spin axis this is termed true polar wander and has amounted to some 12 since early tertiary time the direction and magnitude of true polar wander have varied sporadically through the mesozoic probably in response to major changes in plate motions particularly subduction zone location that change the planet s moments of inertia

relative motion of the nazca farallon and south american plates since late cretaceous time by combining reconstructions of the south american and african plates the african and antarctic plates the antarctic and pacific plates and the pacific and nazca plates we calculated the relative positions and history of convergence of the nazca and south american plates despite variations in convergence rates along the andes periods of rapid convergence averaging more than 100 mm a between the times of anomalies 21 49 5 ma and 18 42 ma and since anomaly 7 26 ma coincide with two phases of relatively intense tectonic activity in the peruvian andes known as the late eocene incaic and mio pliocene quechua phases the periods of relatively slow convergence 50 to 55 30 mm a at the latitude of peru and less farther south between the times of anomalies 30 31 68 5 ma and 21 and between those of anomalies 13 36 ma and 7 correlate with periods during which tectonic activity was relatively quiescent thus these reconstructions provide quantitative evidence for a correlation of the intensity of tectonic activity in the overriding plate at subduction zones with variations in the convergence rate

cenozoic evolution of neotethys and implications for the causes of plate motions africa north america eurasia plate circuit rotations combined with red sea rotations and new estimates of crustal shortening in iran define the cenozoic history of the neotethyan ocean between arabia and eurasia the new constraints indicate that arabia eurasia convergence has been fairly constant at 2 to 3 cm yr since 56 ma with slowing of africa eurasia motion to 1 cm yr near 25 ma coeval with the opening of the red sea ocean closure occurred no later than 10 ma and could have occurred prior to this time only if a large amount of continental lithosphere was subducted suggesting that slowing of africa significantly predated the arabia eurasia collision these kinematics imply that africa s disconnection with the negative buoyancy of the downgoing slab of lithosphere beneath southern eurasia slowed its motion the slow steady rate of northward subduction since 56 ma contrasts with strongly variable rates of magma production in the urumieh dokhtar arc implying magma production rate in continental arcs is not linked to subduction rate

uncertainties in plate reconstructions relative to the hotspots pacific hotspot rotations and uncertainties for the past 68 million years summary a method previously used for estimating the uncertainty in rotation of a plate relative to the hotspots is shown to be insufficient because of the neglect of the uncertainty in one of the three degrees of freedom of a rotation two new methods for estimating best fitting rotations and associated uncertainties for the rotation of a plate relative to the hotspots are presented both methods require a priori estimates of the uncertainties in the locations of individual hotspots and their ancient tracks for a particular age the use of a priori uncertainties permits the hypothesis of hotspot fixity to be tested the first method the two hotspot method leads to simple geometrical interpretations of best fitting rotations and of the eigenvectors and eigenvalues of the appropriate covariance matrix this simplicity comes at the cost of using only two hotspots and their associated tracks with assumed equal circular uncertainties the second method the n hotspot method allows use of an arbitrary number of hotspots 2 with unequal elliptical uncertainties this generality and power comes at the cost of losing the simple geometrical interpretation of the first method elliptical uncertainties are generally more appropriate because hotspot tracks are less well known along strike than across strike because of gaps and uncertainties in ages along the tracks when both methods are applied to the estimate of the pacific hotspot rotation since the age of the hawaiian emperor elbow using the tracks of both the hawaiian and louisville hotspots they produce similar rotations and similar uncertainties the n hotspot method is further applied to estimate pacific hotspot motion at six additional ages from 10 9 ma to 67 7 ma corresponding to the ages of key magnetic anomalies used in global plate reconstructions the goodness of the resultant fit shows that the assigned uncertainties and assumption of hotspot fixity are mutually consistent the pre elbow i e pre 47 9 ma or emperor pole of rotation is shown to differ significantly from the post elbow i e post 47 9 ma or hawaiian pole of rotation moreover rotation over the past 47 9 ma is shown to have occurred in at least three statistically distinct stages from 47 9 to 20 1 ma from 20 1 ma to 6 ma and from 6 ma to the present the change in pole position at 20 1 ma is accompanied by a highly significant 50 per cent increase in the rate of rotation of the pacific plate relative to the hotspots these encouraging results suggest that the new methods can be used to improve and to quantify plate motions relative to the hotspots for a wide variety of applications

plate reconstruction uncertainties using empirical probability density functions we present a method of determining plate reconstruction uncertainties using empirical probability density functions pdfs we convert estimated positioning errors of fracture zone and magnetic anomaly crossings into data pdfs which describe the uncertainty in the location of each fracture zone or magnetic anomaly crossing an empirical pdf is formed for the rotation between a single plate pair based on the ability to align the rotated data along a series of ridge and transform segments monte carlo techniques are then used to sample the pdfs for rotations between single plate pairs to form a pdf for a sequence of rotations this is accomplished with a frequency of occurrence approach for a gridded representation of possible combined rotations we also develop pdfs for the reconstructed positions of points on the rotated plates from the pdf for the rotation confidence regions for any of the pdfs are formed by contouring the cumulative probabilities of grid points within the pdf we provide comparisons with other previously published studies for the anomaly 13 single plate pair reconstruction between the pacific and antarctic plates as well as for the anomaly 6 19 8 ma combined reconstruction between the pacific and north american plates results for the anomaly 13 reconstruction are generally consistent with previous studies but smaller confidence regions are attributed to the more robust nature of the empirical pdf approach that we have used the 95 confidence region for the anomaly 6 reconstruction is less than half that of the worst case scenario the benefit of our monte carlo technique is that it adequately samples the pdfs for all of the rotations thus accurately including the effects of coupling uncertainties between pole location and amount of rotation

a method for bounding uncertainties in combined plate reconstructions we present a method for calculating uncertainties in plate reconstructions that does not describe the uncertainty in terms of uncertainties in pole positions and rotation angles if a fit of magnetic anomalies of the same age and fracture zones that were active as transform faults at that time can be found such a reconstruction can be perturbed and degraded by small rotations about each of three orthogonal axes partial uncertainty poles if the uncertainty in the reconstruction is a consequence of independent small but acceptable rotations about these axes then the uncertainties in reconstructed points will be elliptical in shape the dimensions and orientation of such ellipses will depend upon the magnitudes of the perturbing rotations and upon the relative geometry of the partial uncertainty poles and the points in question in a sequence of rotations each rotation will contribute an elliptical region of uncertainty for each reconstructed point and these ellipses can be combined as independent statistical quantities to obtain a confidence ellipse for the sequence of rotations as a test we calculated uncertainties for three points on the pacific plate with respect to north america at the time of anomaly 6 20 ma the computed uncertainties are similar in shape to those that we previously obtained for a sequence of marginally acceptable rotations but the major axes of the ellipses presented here are about 25 shorter

gold mineralization in as rich mesothermal gold ores of the bogosu prestea mining district of the ashanti gold belt ghana remobilization of invisible gold the bogosu prestea mining district of southwestern ghana is a 33 km section of the early proterozoic ashanti gold belt greenschist facies carbonaceous and carbonate bearing turbidites and greywackes and mafic dikes host numerous economic mesothermal gold deposits structurally higher ores in the bogosu concession have brittle deformation and consist of disseminated sulphide lodes in tectonically disrupted sedimentary rocks and carbonate altered mafic dikes most gold occurs as micrometre size particles in arsenian pyrite and as invisible gold in arsenian pyrite and arsenopyrite the structurally deeper ores of the adjoining prestea concession are associated with brittle ductile deformation and consist of extensive crack seal quartz veins and graphitic shear zones only minor amounts of invisible gold were detected in these deeper lodes gold occurs dominantly as abundant microscopic and larger particles in sulphide arsenide minerals and in gangue the gold distribution patterns revealed by sims microprobe analysis and ion maps emp and colour staining suggest that most of the primary gold in the bogosu prestea system precipitated in solid solution with sulphide arsenide minerals however post depositional concentration and redistribution occurred in increasing degree with 1 increase in metamorphic hydrothermal gradients in the gold system depth 2 decrease in the refractory properties of the host mineral and 3 increase in the amount of post depositional host mineral recrystallization and deformation gold evolved from primary solid solution within sulphide arsenide minerals to colloidal and micrometre size particles concentrated in voids fractures and internal grain boundaries and finally to microscopic and larger particles at sulphide arsenide grain margins and in the gangue assemblage the general conclusions presented here are applicable to as rich gold deposits of all ages worldwide the presence of gold in late fractures is insufficient evidence for late stage introduction of gold in mesothermal gold systems

evolution of regoliths and landscapes in deeply weathered terrain implications for geochemical exploration abstract thick commonly lateritic regoliths are widespread in inter tropical regions of the world and present particular challenges in exploration these are best tackled through a sound understanding of the evolution of the landscapes in which they occur the regoliths formed under humid warm to tropical conditions and although they may have been modified by later climatic changes i e to more humid or more arid conditions many chemical and mineralogical characteristics are retained these include the geochemical expressions of concealed mineralization erosional and depositional processes control the preservation and occurrence of specific regolith units that may be used as sample media and in turn target size element associations and contrast thereby influencing sampling procedures analysis and data interpretation these parameters are best summarized in terms geochemical dispersion models based on the degree of preservation of the pre existing lateritic regolith regolith landform mapping permits an assessment of the terrain in terms of such models in relict regolith landform regimes in which the lateritic regolith is largely preserved broad multi element anomalies in the upper ferruginous horizons lateritic residuum can be detected using sample intervals of 1 km or more in contrast in erosional regimes where this material is absent anomalies in upper saprolite and the soil and lag derived from it are more restricted in area and closer sampling intervals 200 40 m or less may be necessary lag and soil are generally ineffective in depositional areas except where the sediments are very thin e g

revised history of early tertiary plate motion in the south west pacific reanalysis of early tertiary magnetic anomalies on the pacific plate south of the campbell plateau indicates that in early tertiary time there was a previously unrecognized triple junction of the pacific the antarctic and a third plate now beneath the bellingshausen sea corresponding revised parameters for pacific antarctica motion do not require early tertiary displacements within antarctica or new zealand but suggest a change in direction of the pacific plate over the hotspots of approximately the same sense and timing as the hawaiian emperor bend

hydrothermal transport and depositional processes in archean lode gold systems a review abstract compelling evidence now exists for the formation of archean lode gold deposits over a substantial range in metamorphic conditions and crustal depth the commonality of ore fluid isotopic and geochemical characteristics the extreme fluid rock ratios within actual ore zones and the observed depth continuity within individual orebodies all suggest that these deposits formed within vertically extensive crustal scale hydrothermal systems calculations based on new thermodynamic data and estimated ore fluid conditions suggest that gold would have been transported as au hs 2 over most of this range gold transport in the ore fluids that formed some diopside and diopside k feldspar class deposits t 550 c would have been as aucl2 whereas the au hs complex may have been significant in some of the lower temperature 270 c less alkaline ore fluids the high solubility of gold in lode gold ore fluids at amphibolite facies conditions 10 s to 1000 s ppb suggests that fluids may have been undersaturated in gold within their source regions promoting the ore fluid s capacity to retain gold in solution over the large transport distances involved ore depositional mechanisms were also likely to vary between crustal levels fluid rock interactions and wallrock sulphidation in particular appear to be the most important type of precipitation mechanism however with decreasing depth of formation the scope of other depositional mechanisms notably phase separation and fluid mixing becomes greater

errors in plate rotations as described by covariance matrices and their combination in reconstructions we use covariance matrices to describe the errors in plate rotations and to combine the contrbutions of individual plate pairs for plate reconstructions first for the rotation of a pair of plates the covariance matrix which describes the errors in the rotation is defined then we show how individual covariance matrices can be combined in a circuit of successive plate rotations to assess the errors in a plate reconstruction using the covariance matrix for accumulated errors from a sequence of rotations we can determine the uncertainties in the position of selected points on one plate in the plate circuit to another that is assumed to be fixed as an example the position of the pacific plate relative to north america is reconstructed at chron 6 20 ma this choice is made to allow a direct comparison with published error assessments using the accumulated covariance matrix we find similar uncertainties in the positions of their selected points on the pacific plate as a further test this reconstruction is repeated using seafloor spreading data that directly relate antarctica to africa bypassing the link of india this eliminates the largest source of error in the plate circuit that due to the short ridge segment between africa and india this revised shortened circuit results in generally smaller errors in the positions of points on the pacific plate the covariance matrices of the two alternative circuits are used to construct the optimal estimate and its associated error the optimal estimate and its error region intersect the two alternative reconstructions and their error regions

motion between the indian capricorn and somalian plates since 20 ma implications for the timing and magnitude of distributed lithospheric deformation in the equatorial indian ocean summary approximately 2200 magnetic anomaly crossings and 800 fracture zone crossings flanking the carlsberg ridge and central indian ridge are used to estimate the rotations of the indian and capricorn plates relative to the somalian plate for 20 distinct points in time since 20 ma the data are further used to place limits on the locations of the northern edge of the rigid capricorn plate and of the southern edge of the rigid indian plate along the central indian ridge data south of and including fracture zone n the fracture zone immediately south of the vema fracture zone which intersects the central indian ridge near 10 s are well fit assuming rigid capricorn and somalian plates while data north of fracture zone n are not in agreement with prior results data north of fracture zone h which intersects the central indian ridge near 3 2 s are well fit assuming rigid indian and somalian plates while data south of and including fracture zone h are not resulting in a smaller rigid indian plate and a wider diffuse oceanic plate boundary than found before the data are consistent with capricorn somalia motion about a fixed pole since 8 ma but require rotation about a pole 15 farther away from the central indian ridge from 20 to 8 ma the post 8 ma pole also indicates capricorn somalia displacement directions that are 7 clockwise of those indicated by the pre 8 ma stage pole in contrast india somalia anomaly and fracture crossings are well fit by a single fixed pole of rotation for the past 20 ma india somalia motion has changed little during the past 20 myr nonetheless astronomically calibrated ages for reversals younger than 12 9 ma allow resolution of the following small but significant changes in spreading rate india somalia spreading slowed from 31 to 28 mm yr 1 near 7 9 ma and later sped up to 31 mm yr 1 near 3 6 ma capricorn somalia spreading slowed from 40 to 36 mm yr 1 near 11 0 ma later sped up to 38 mm yr 1 near 5 1 ma and further sped up to 40 mm yr 1 near 2 6 ma the motion between the indian and capricorn plates is estimated by differencing india somalia and capricorn somalia rotations which differ significantly for all 20 pairs of reconstructions india has rotated relative to the capricorn plate since at least 20 ma if about a pole located near 4 s 75 e the rate of rotation was slow 0 11 0 01 myr 1 95 per cent confidence limits from 20 to 8 ma but increased to 0 28 0 01 myr 1 95 per cent confidence limits at 8 ma the onset of more rapid rotation coincides within uncertainty with the inferred onset at 7 8 ma of widespread thrust faulting in the central indian basin and with the hypothesized attainment of maximum elevation and initiation of collapse of the tibetan plateau at 8 ma the plate kinematic data are consistent with steady india capricorn motion since 8 ma and provide no evidence for previously hypothesized episodic motions during that interval the convergence since 8 ma between the indian and capricorn plates significantly exceeds by 13 to 20 km the convergence estimated from three north south marine seismic profiles in the central indian basin where and how the additional convergence was accommodated is unclear

pacific plate reconstructions and uncertainties abstract the pacific plate is positioned into the hotspot reference frame first directly relative to the pacific hotspots and then indirectly relative to the african hotspots by using seafloor spreading data the reconstructions are repeated for 10 15 37 48 and 61 ma the errors associated with each link for both approaches are expressed by covariance matrices which can be combined to yield the overall error in the case of the relative plate motion circuit assuming the hotspots are fixed and that the reconstructions record the true relative positions of the plates the reconstructions should be equivalent within their uncertainties in fact however there is agreement within the uncertainties only for 10 ma with the misfit increasing with age for the earlier reconstructions these misfits can he accounted for by three alternative explanations large hotspot location uncertainties a shift in time of the pacific group of hotspots relative to the african group or an additional plate boundary in the relative plate motion circuit which we assume to be in antarctica by examining each of these possibilities in turn we conclude that the uncertainties in individual hotspot locations required to bring the reconstructions into agreement are unreasonably large and that the data available at present do not allow us to distinguish which of the remaining two possibilities may be the primary source of reconstruction errors

cenozoic eucla basin and associated palaeovalleys southern australia climatic and tectonic influences on landscape evolution sedimentation and heavy mineral accumulation abstract the eucla basin including the vast nullarbor plain lies on the margins of the yilgarn musgrave and gawler cratons in southern australia and owes its distinctive landscape to a unique set of interactions between eustatic climatic and tectonic processes over the last 50 ma understanding of the history of the basin and the palaeovalleys that drained from the surrounding cratons are important because they contain major mineral deposits and the sediments derived from them contain remobilised gold uranium and heavy minerals in particular a remarkably preserved palaeoshoreline sequence along the north eastern margin of the eucla basin is highly prospective for heavy mineral placer deposits the record of marine marginal marine estuarine fluvial and lacustrine environments as constrained mainly by an extensive borehole dataset reflects major depositional events during the palaeocene early eocene middle late eocene oligocene early miocene middle miocene early pliocene and pliocene quaternary these events reflect the key role of eustatic sea level variation which during highstands inundated the craton margins flooding palaeovalleys to up to 400 km inboard of the present coastline however a systematic eastward migration of the depocentre across the eucla basin during the neogene together with apparent flow reversals in a number of palaeovalley systems draining the gawler craton suggest that the eucla basin has also been subject to differential vertical movements expressed as a west side up east side down tilting of 100 200 m this differential movement forms part of a broader north down southwest up dynamic topographic tilting of the australian continent associated with relatively fast 6 7 cm yr northward plate motion since fast spreading commenced in the southern ocean at 43 ma we suggest that the evolving dynamic topography field has played a key role in facilitating development of placer deposits largely through multistage eastward reworking of near shore sequences during highstand transgressive cycles on a progressively tilting platform under the influence of persistent westerly weather systems

the nature of gold bearing palaeochannel sediments in the gidji area north of kalgoorlie western australia abstract remnants of an extensive palaeodrainage network are preserved in the kalgoorlie region of western australia as buried palaeochannels at gidji a major palaeochannel up to 60 m deep is infilled with sand and clay rich facies and overlain by younger alluvial colluvial sands and gravels aeolian sands and silts playa deposits red soils and lag materials the palaeochannel sediments consist predominantly of low crystallinity kaolinite quartz and minor smectite they contain very little muscovite illite or chlorite and can be readily distinguished from underlying in situ saprolite by their chemical characteristics particularly k al and mg al ratios low rare earth element contents and low k rb ratios these sediments were largely deposited during the mid late eocene as an homogenised mixture derived from a low relief and highly weathered pre eocene landsurface they have undergone post depositional weathering resulting in mobilisation and redeposition of iron as oxides oxyhydroxides the stratigraphy of the palaeochannel sediments indicates a change from dominantly fluvial to fluvio lacustrine conditions during deposition in areas of gold mineralisation the presence of palaeochannels complicates geochemical exploration areas of thick palaeochannel clays mask bedrock anomalies placer and supergene concentrations of gold associated with the palaeochannels can be confused with bedrock anomalies unless a multi element approach is taken the gidji palaeochannel environment contains a combination of bedrock and placer supergene gold anomalies primary bedrock anomalies generally have as sb and w as associated pathfinder elements some pathfinder elements e g as have been concentrated in ferruginous pisoliths and gravels or mechanically dispersed in near surface alluvial colluvial materials giving rise to false anomalies

a sequential data assimilation approach for the joint reconstruction of mantle convection and surface tectonics with the progress of mantle convection modelling over the last decade it now becomes possible to solve for the dynamics of the interior flow and the surface tectonics to first order we show here that tectonic data like surface kinematics and seafloor age distribution and mantle convection models with plate like behaviour can in principle be combined to reconstruct mantle convection we present a sequential data assimilation method based on suboptimal schemes derived from the kalman filter where surface velocities and seafloor age maps are not used as boundary conditions for the flow but as data to assimilate two stages a forecast followed by an analysis are repeated sequentially to take into account data observed at different times whenever observations are available an analysis infers the most probable state of the mantle at this time considering a prior guess supplied by the forecast and the new observations at hand using the classical best linear unbiased estimate between two observation times the evolution of the mantle is governed by the forward model of mantle convection this method is applied to synthetic 2 d spherical annulus mantle cases to evaluate its efficiency we compare the reference evolutions to the estimations obtained by data assimilation two parameters control the behaviour of the scheme the time between two analyses and the amplitude of noise in the synthetic observations our technique proves to be efficient in retrieving temperature field evolutions provided the time between two analyses is 10 myr if the amplitude of the a priori error on the observations is large 30 per cent our method provides a better estimate of surface tectonics than the observations taking advantage of the information within the physics of convection

revised and synthetic apparent polar wander paths of the african eurasian north american and indian plates and true polar wander since 200 ma we have reviewed paleomagnetic data available for the eurasian african north american and indian plates over the last 200 ma selection criteria are those generally accepted with an emphasis on evidence for lack of remagnetization accurate dating and proper structural analysis this results in 23 35 51 and 2 poles for eurasia africa north america and india respectively we believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander apw paths than averaging of larger data sets involving less stringent selection criteria we propose and describe revised apw paths but more importantly we next use relative motion models to transfer all data in a common reference frame we find good agreement between transferred data when they are averaged separately for each plate in independent 20 ma windows this is a check on consistency of paleomagnetic data kinematic models and of the geocentric dipole hypothesis transferred data from all plates are averaged in 20 ma windows to generate synthetic apw paths for all plates studied these synthetic paths are in agreement with the original revised apw paths that use only data from a single plate moreover both geographic and time resolution are improved spatial confidence intervals are of the order of 5 the synthetic paths display interesting features such as a previously ill recognized apw loop for eurasia paleomagnetic and hotspot apw are next compared and a determination of true polar wander tpw is derived we find significant tpw amounting to over 20 in the last 200 ma tpw appears to be episodic with a standstill between 180 and 110 ma there is general agreement with a previous study of livermore et al 1984 the tpw standstill appears to correlate with a time of decreasing reversal frequency ending with the cretaceous long normal superchron other periods of fast tpw would seem to correspond to increasing reversal frequency however it is suggested that the major tpw hairpin at 50 ma might correspond to the collision of india with eurasia tentative correlations with core or mantle indicators can be understood as a result of couplings between the core mantle and lithosphere we believe primarily related to episodic evolution of the d layer

the origin and evolution of the klondike goldfields yukon canada abstract the world famous klondike goldfields are located in the unglaciated part of west central yukon canada since their discovery over 100 years ago they have produced an estimated 311 tonnes of gold primarily from bench and creek placers that are fluvial in origin and range from pliocene to holocene in age historically the placers are classified into three levels of gravel with four main units these include the high level white channel gravel pliocene presently the most important gold bearing unit which sits nonconformably on an erosional bedrock surface i e the white channel strath and is overlain and interbedded with the glaciofluvial klondike gravel pliocene the intermediate level gravel pleistocene the least important economically and the low level gravel pleistocene holocene historically the most important gold bearing unit but it has been mined three or four times now the goldfields originated from the weathering and erosion of early cretaceous discordant mesothermal quartz veins and the light grey color of the matrix of the white channel gravel is due mainly to weathering and diagenetic alteration by groundwater flow the concentration of placer gold is related to a hierarchy of physical scales at the lithofacies scale metres bed roughness determined sites of gold deposition at the element scale tens of metres gravel bars were preferentially enriched in gold at the reach scale hundreds of metres stream gradient was an important factor at the system scale hundreds of km braided river environments transported large amounts of gold and at the sequence scale thousands of km economic placers formed initially in the high level white channel gravel and later in the intermediate level and low level gravel the white channel strath is interpreted as an erosional tectonic terrace that formed during isostatic uplift and under conditions of dynamic equilibrium the high level white channel gravel and klondike gravel are interpreted as a depositional climatic terrace that formed during a reversal in the tectonically induced downcutting which is attributed to the initial and most extensive of the pre reid glaciations 3 ma in the yukon the intermediate level gravel is interpreted as minor erosional complex response terraces that formed during static equilibrium when there were pauses in valley floor degradation which are attributed to the subsequent and less extensive pre reid glaciations the low level gravel formed also during valley floor degradation and may represent a return to dynamic equilibrium conditions hence the dominant forcing mechanisms controlling the evolution of the goldfields were isostatically compensated exhumation and climatic change related to the repeated glaciation of the yukon in addition the lowering of baselevel from high level to intermediate level and finally to low level gravel was accompanied by a decrease in accommodation space as indicated by a decrease in gravel thickness which resulted in an increase in the concentration of the placer gold

linking the late cretaceous to paleogene pacific plate and the atlantic bordering continents using plate circuits and paleomagnetic data 1 late cretaceous to paleogene paleomagnetic data from the pacific plate the emperor seamounts can be compared with data from the atlantic bordering continents through the use of plate circuit reconstructions here we summarize the uncertainties in all data sets and present formal tests we report agreement between late cretaceous pacific paleomagnetic data and predictions based on the estimates of non pacific pole positions from synthetic apparent polar wander paths this congruency points to the veracity of the plate circuits and the accuracy of the paleomagnetic estimates in contrast to the agreement seen for the late cretaceous small discrepancies are observed in the comparisons of the pacific paleogene data and predictions from synthetic apparent polar wander paths such a disparity in a younger time interval is unexpected given the agreement of the late cretaceous data the possibility that minor temporally variable nondipole field components contribute to the discrepancy cannot be completely discounted however an alternative and more straightforward explanation is suggested by further comparisons of the mean non pacific paleomagnetic data and the highest quality poles that contribute to the means in particular we note that 1 the pacific paleogene data are in full agreement with coeval poles from north america meeting strict reliability criteria and 2 the non pacific paleogene poles of synthetic apparent polar wander paths are dominated by results from the north atlantic igneous province naip but taken as a whole the naip data fail a paleomagnetic reversal test hence minor discrepancies between paleocene paleomagnetic data from the pacific and atlantic hemispheres may point to limitations of the latter which incorporate a relatively large number of older lower quality data these findings call for renewed data collections utilizing comprehensive rock magnetic and paleomagnetic demagnetization procedures to improve resolution of paleocene non pacific data the uncertainties of the pacific paleolatitude data and the non pacific reference poles are larger than the differences related to the use of the two alternative plate circuits through east to west antarctica and through australia to the lord howe rise that link the atlantic and pacific hemispheres while no preference can thus be given to either plate circuit their overall consistency with paleomagnetic data suggests that they can be used to investigate long term motion of the pacific plate since late cretaceous times

plates and their motions the last reviews which covered research in plate motions were those of herron and pitman 1975 on marine magnetic anomalies and dewey 1975 on plate tectonics both reporting on research for the period 1971 1974 in the last 12 years there has been considerable refinement of relative motion models as well as further development of global models also major effort has gone into defining reference frames for the study of plate motions and discriminating between them more recently the role of errors in individual plate rotations and the associated uncertainties in reconstructions have received attention in this report i concentrate on work published in the last 4 years 1983 1986 citing a minimum of publications from previous years however some papers in press for early 1987 have been included this report first reviews work on relative plate motions for the present and past and reconstructions and then models for these motions global ones for driving forces and local ones determining subduction history and back arc spreading the problem of reference frames both hotspot and paleomagnetic is discussed next i end with a review of work on the errors in plate motions

lode and placer gold composition in the klondike district yukon territory canada implications for the nature and genesis of klondike placer and lode gold deposits we have determined the major and trace element composition of approximately 2 700 gold particles from 21 gold bearing mesothermal quartz veins and 35 placer gold samples from the klondike district in western yukon territory measured au ag cu and hg contents were used to define a characteristic geochemical signature or fingerprint for each of the vein samples these signatures were then compared with the various compositional populations that we have distinguished within each of the placer samples preliminary conclusions derived from the study include 1 placer gold in both recent stream deposits and in the pliocene to pleistocene white channel gravels is detrital in origin 2 the placer gold is mainly if not entirely derived from mesothermal quartz veins 3 all lode sources for the placer gold have not yet been located and 4 gold composition data can be used to identify the lode signature for placer gold and if it has not been removed by erosion help locate and link placer gold to its specific lode source

oxygen isotope dating of the australian regolith australia has had a long and complex history of intense surficial weathering and this has led to the extensive development of laterites bauxites and deeply weathered profiles of a variety of ages across the continent1 2 a detailed knowledge of the age s of such weathering phenomena is of great importance for the reconstruction of australia s post palaeozoic geomorphic and palaeoclimatic history however in many instances absolute geo chronological information is not obtainable using conventional techniques here we describe and evaluate a new method that can provide an estimate of the age of intensely weathered profiles based on the oxygen isotope composition of minerals developed by deep weathering lateritization processes this method allows the definition of four ages of weathering in eastern australia from the late palaeozoic to the present and suggests that some profiles previously thought to be late cretaceous or tertiary in age may be of pre late mesozoic age the oxygen isotope data indicate that the minerals used in this study have retained their original isotopic composition for periods of up to 250 million years despite the continued presence of meteoric waters of progressively higher 18o content during this interval

uncertainties and implications of the late cretaceous and tertiary position of north america relative to the farallon kula and pacific plates we present updated global plate reconstructions and calculated uncertainties of the pacific kula and farallon vancouver plates relative to north america for selected times since 68 ma improved magnetic data from the indian ocean decrease the uncertainties in the global plate circuit approach these uncertainties are now considerably smaller than those inherent in equivalent reconstructions based on the assumption of fixed hotspots major differences between these results and those of others are due to our use of more detailed africa north america reconstructions separate vancouver and farallon plate reconstructions and the assumption of a rigid antarctica plate during cenozoic time the uncertainties in the relative positions of the pacific and north america plates since the time of anomaly 7 26 ma range up to 100 km in position or from 1 to 3 m y in time if the mendocino triple junction initiated at about 28 5 ma its position would have been at 31 3 n 130 km relative to fixed north america unacceptable overlap of oceanic crust of the pacific plate with continental crust of western north america in the anomaly 10 30 ma reconstruction is a minimum of 340 200 km along an azimuth of n60 e and may be accounted for by basin and range extension pacific north america displacement in the past 20 ma is found to be considerably less than that calculated by fixed hotspot reconstructions farallon vancouver north america convergence velocity decreased greatly between the times of anomalies 24 and 21 56 to 50 ma prior to the 43 ma age of the hawaiian emperor bend and the often quoted 40 ma end of the laramide orogeny a change in direction of farallon north america convergence occurred sometime between 50 and 42 ma and also may not correlate with the time of the hawaiian emperor bend the lack of data from subducted parts of the farallon and kula plates permits many possibilities regarding the position of the kula farallon ridge the age of subducted crust or the position of oceanic plateaus during the laramide orogeny leaving open the question of the relationship between plate tectonic scenarios and tectonic style during laramide time displacements of points on the various oceanic plates along the west coast of an arbitrarily fixed north america during the interval between anomalies 30 31 and 18 68 to 42 ma are found to be pacific plate 1700 200 km northward farallon plate 3200 400 km northeastward vancouver plate 3000 400 km northeastward kula plate if attached to the pacific plate after a24 time 2500 400 km northward

shear zones fault networks and archean gold conjugate ductile shear zones developed mostly on granite greenstone margins are a dominant structural feature of the yilgarn craton these greenstone belt scale shear zones lack both gold deposits and gold related alteration previous interpretations of the shear zones as part of a linked system as craton scale terrane boundaries or as crustal scale pathways for gold mineralizing fluids all appear equivocal instead gold mineralizing fluids in the yilgarn craton illustrated here by the gold rich yandal belt were focused by a network of middle to upper crustal brittle faults in the late archean the faults are 040 to 060 trending dextral and 090 to 120 trending sinistral the maximum compression direction is about 075 gold mineralization occurs in laminated and other schistosity parallel quartz veins where brittle faults refract along earlier subsidiary within greenstone ductile shear zones and in quartz veins within late archean faults

paleogeographic maps of the continents bordering the indian ocean since the early jurassic paleomagnetic data from the african plate from the early jurassic onward are critically re evaluated and updated resulting in a set of 30 paleopoles from which an apparent polar wander path is derived using 20 ma intervals this path is then used in conjunction with kinematic models of the indian and atlantic oceans to place other continents bordering the indian ocean eurasia india islands and plateaus madagascar seychelles mascarene and blocks involved in the collision of india with eurasia south tibet indochina in an absolute paleogeographic frame fixed with respect to africa paleogeographic maps are presented for the early jurassic pre breakup the jurassic cretaceous boundary anomaly ml5 the early cretaceous m2 the late cretaceous a 34 the cretaceous tertiary boundary a 29 the early eocene a 24 the middle eocene a 21 and the oligocene miocene boundary a 6 observed and predicted paleomagnetic directions for landmasses other than africa i e for western europe china korea india and australia which are not used in deriving the maps are found to be in very satisfactory agreement the quality of reconstructions as measured by rms discrepancies between observed and predicted paleolatitudes is of the order of 5 the main features of the paleogeographic maps are discussed with the aim of relating oceanic and on land geodynamic events the constraints provided by tibetan data on the india asia collision and the 20 ma reorganization of geology and plate boundaries in a band extending from the south china sea through tibet to the red sea system are discussed in more detail we emphasize the episodic nature of the breakup of gondwanaland and formation of the indian ocean and subsequent interaction with eurasia in which six major discrete events are identified separated by 30 ma phases of rather uniform plate motion

acceleration and deceleration of india asia convergence since the cretaceous roles of mantle plumes and continental collision 1 a strong 50 35 ma decrease in india asia convergence is generally ascribed to continent continent collision however a convergence rate increase of similar magnitude occurred between 65 50 ma an earlier increase occurred at 90 ma both episodes of accelerated convergence followed upon arrival of a mantle plume below and emplacement of a large igneous province lip on the indian plate we here first confirm these convergence rate trends reassessing the indo atlantic plate circuits then using two different numerical models we assess whether plume head arrival and its lateral asthenospheric flow may explain the plate velocity increases and whether decreased plume flux and increasing continent plume distance may explain deceleration even without continental collision the results show that plume head arrival can indeed lead to absolute indian plate motion accelerations on the order of several cm yr followed by decelerations on timescales similar to the reconstructed fluctuations the 90 ma increase could potentially be explained as response to the morondova mantle plume alone the 65 50 ma convergence rate increase however is larger than can be explained by plume head spreading alone we concur with previous hypotheses that plume induced weakening of the indian continental lithosphere asthenosphere coupling and an increased slab pull and ridge push efficiency are the most likely explanations for the large convergence rate increase the post 50 ma decrease is best explained by orogeny related increased trench resistivity decreased slab pull due to continental subduction and possibly restrengthening of lithosphere asthenosphere coupling upon plume demise

the formation of fluvio lacustrine ferruginous pisoliths in the extensive palaeochannels of the yilgarn craton western australia abstract iron pisoliths and nodules are common within the fluvio lacustrine kaolinitic clays of tertiary paleochannels developed in the yilgarn craton of southern western australia deposition within these channels began in the late eocene with sediments comprised a fining upward sequence from sands at the base to lacustrine clay deposits above iron pisoliths are found randomly distributed in the clays in deposits more than 300 km apart the pisoliths are composed of goethite and hematite and minor quartz and clays they are 2 to 30 mm in diameter and formed of an inner nucleus surrounded by up to 200 iron oxide laminations there is no evidence for transportation of pisoliths into the channels but the internal fragmented nature of some of the pisoliths suggests some movement has occurred and multiple generations of pisoliths have developed the interface between river water and sediment was a redox boundary between reduced slightly acidic sediments and oxidised river water pisoliths formed on or just beneath the surface of the sediment by redox reactions likely mediated by microorganisms pisoliths have undergone cycles of exposure growth and movement linked to the height and course of the palaeoriver system

some geometrical aspects of uncertainties in combined plate reconstructions the uncertainties in both the position of a pole of rotation and the rotation angle describing a plate reconstruction depend not only on the number and quality of data used but also on two geometrical factors the length of plate boundary represented by the data and the distance from the best fit pole to the center of the data region we describe a simple geometrical method that can be used to calculate minimum uncertainties in reconstructions based on positions of magnetic anomalies and fracture zones with finite uncertainties uncertainties in pole positions and angles corresponding to 10 km uncertainty in individual data points can when combined yield uncertainties in reconstructed positions greatly in excess of 10 km per rotation for example an uncertainty of 10 km in anomaly 6 reconstructions in the south pacific southeast indian northwest indian and north atlantic oceans would result in uncertainty of up to 190 km in the reconstructed position of the pacific plate with respect to north america at 19 8 m y b p existing data show that the realistic uncertainties for this case are twice as large as the calculated minimum uncertainties

multistage gold mineralization at the lapa mine abitibi subprovince insights into auriferous hydrothermal and metasomatic processes in the cadillac larder lake fault zone the lapa gold deposit contains reserves of 2 4 mt at 6 5 g t au and is one of the few deposits located directly within the cadillac larder lake fault zone cllfz a first order crustal scale fault that separates the archean abitibi subprovince from the pontiac subprovince to the south gold mineralization is predominantly hosted in highly strained and altered upper greenschist lower amphibolite facies mafic to ultramafic rocks of the piche group auriferous ore zones consist of finely disseminated auriferous arsenopyrite pyrrhotite pyrite and native gold disseminated in biotite and carbonate altered wall rocks native gold which is also present in quartz dolomite calcite veinlets is locally associated with sb bearing minerals especially at depth 1 km from surface where the deposit is characterized by a au sb as association at vertical depth greater than 1 km gold is associated with arsenopyrite and pyrrhotite au as association the mineralogy and paragenesis of the lapa deposit metamorphosed ore and alteration assemblages record the superposition of three metamorphic episodes m1 m2 and m3 and three gold mineralizing events spatial association between biotitized wall rocks and auriferous arsenopyrite indicates that arsenopyrite precipitation is concomitant with potassic alteration the predominant au as association recognized across the deposit is related to gold in solid solution in arsenopyrite as part of a pre m2 low grade auriferous hydrothermal event however the occurrence of hornblende oligoclase porphyroblasts overprinting the biotite alteration and the presence of porous clusters and porphyroblasts of arsenopyrite with native gold and pyrrhotite indicate an auriferous metasomatic event associated with peak m2 prograde metamorphism late retrograde metamorphism m3 overprints the hornblende oligoclase m2 assemblage within the host rocks proximal to ore by an actinolite albite assemblage by precipitation of free gold and sb sulfosalts at lower p t the complex relationships between ore structural features and metamorphic assemblages at lapa are related to the tectonometamorphic evolution of the cadillac larder lake fault zone at different times and crustal levels and varying heat and fluid flow regimes the lapa deposit demonstrates that early low grade gold mineralization within the cadillac larder lake fault zone has benefited from late gold enrichment s during prograde and retrograde metamorphism suggesting that multi stage processes may be important to form gold rich orogenic deposits in first order crustal scale structures

toward a self consistent high resolution absolute plate motion model for the pacific the hot spot hypothesis postulates that linear volcanic trails form as lithospheric plates move relative to stationary or slowly moving plumes given geometry and ages from several trails one can reconstruct absolute plate motions apm that provide valuable information about past and present tectonism paleogeography and volcanism most apm models have been designed by fitting small circles to coeval volcanic chain segments and determining stage rotation poles opening angles and time intervals unlike relative plate motion rpm models such apm models suffer from oversimplicity self inconsistencies inadequate fits to data and lack of rigorous uncertainty estimates in addition they work only for fixed hot spots newer methods are now available that overcome many of these limitations we present a technique that provides high resolution apm models derived from stationary or moving hot spots given prescribed paths the simplest model assumes stationary hot spots and an example of such a model is presented observations of geometry and chronology on the pacific plate appear well explained by this type of model because it is a one plate model it does not discriminate between hot spot drift or true polar wander as explanations for inferred paleolatitudes from the emperor chain whether there was significant relative motion within the hot spots under the pacific plate during the last 70 m y is difficult to quantify given the paucity and geological uncertainty of age determinations evidence in support of plume drift appears limited to the period before the 47 ma hawaii emperor bend and apart from the direct paleolatitude determinations may have been somewhat exaggerated

uncertainties in reconstructions of the pacific farallon vancouver and kula plates and constraints on the rigidity of the pacific and farallon and vancouver plates between 72 and 35 ma to study the rigidity of the pacific and farallon plates between 72 and 35 ma we analyzed the magnetic anomalies and fracture zones in the pacific ocean between longitudes 125 w 180 and latitudes 43 s 60 n a set of consistent rotations indicates that the pacific and farallon plates between the pioneer and agassiz fracture zones were rigid plates between 72 and 35 ma thus the location of a missing tertiary boundary within the pacific or antarctic plates is restricted to be south of latitude 43 s the farallon plate broke into two plates at the time of the change in the pacific farallon spreading direction north of the pioneer and mendocino fracture zones sometime between 59 and 49 ma the northern fragment termed the vancouver plate was subducted beneath most of north america we present rotations and uncertainties for the histories of motion of the pacific and kula plates from anomaly 32 to anomaly 25 of the pacific and vancouver plates from anomaly 21 to anomaly 13 and of the pacific and farallon plates from anomaly 32 to anomaly 13

buried inset valleys in the eastern yilgarn craton western australia geomorphology age and allogenic control abstract a contributory network of narrow fluvially incised valleys filled with a distinctive sequence of tertiary continental to shallow marine sediments is widespread throughout the eastern yilgarn craton in southwestern australia previously referred to as deep leads or paleochannels these are herein termed inset valleys to more accurately reflect their geomorphic expression and emphasize their subordinate and entrenched position with the bedrock surface of a much broader and older system of primary valleys owing to minimal denudation and tectonic deformation during the late cenozoic the inset valleys are excellently preserved and can be readily traced in boreholes and open cut mines throughout the eastern yilgarn craton and possibly also adjacent parts of southwestern australia with a combined catchment areal extent of at least documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmary

the uncertainties of finite rotations in plate tectonics incomplete knowledge of the pattern of magnetic lineations and fossil transform faults represented by sea floor spreading data on two plates generated by the same spreading center leads to uncertainties in a reconstruction of the past relative configuration of the plates a reconstruction may be represented by the finite rotation that describes one configuration relative to another in this paper a method of reconstruction is provided that reflects the uncertainties in the data the method minimizes a weighted least squares measure of fit as a function of the rotation parameters for a given rotation the measure of fit represents the sum of squares of the weighted distances of fixed and rotated data points representing corresponding former plate margin segments from a common plate margin the common margin is estimated from the locations of both fixed and rotated data points the estimated margin consists of a set of great circle arcs it is shown how the method of reconstruction may be utilized to obtain an uncertainty region for the pole and angle of rotation that characterize the finite rotation these techniques are then used to study the uncertainties of reconstructions in the south pacific for the times of anomalies 13 and 18 the best fit reconstructions and the uncertainty region for the anomaly 18 pole are in good agreement with previous work however the uncertainties in the anomaly 13 pole were substantially underestimated by previous investigators

constraints on jalisco block motion and tectonics of the guadalajara triple junction from 1998 2001 campaign gps data a gps campaign network in the state of jalisco was occupied for 36 h per station most years between 1995 and 2005 we use data from 1998 2001 to investigate tectonic motion and interseismic deformation in the jalisco area with respect to the north america plate the twelve stations used in this analysis provide coverage of the jalisco block and adjacent north america plate and show a pattern of motion that implies some contribution to jalisco block boundary deformation from both tectonic motion and interseismic deformation due to the offshore 1995 earthquake the consistent direction and magnitude of station motion on the jalisco block with respect to the north america reference frame 2 mm year to the southwest 95 confidence level perhaps can be attributed to tectonic motion however some station velocities within and across the boundaries of the jalisco block are also non zero 95 confidence level and the overall pattern of station velocities indicates both viscoelastic response to the 1995 earthquake and partial coupling of the subduction interface together termed interseismic deformation our results show motion across the northern colima rift the eastern boundary of the jalisco block which is likely to be sinistral oblique extension rather than pure extension we constrain extension across both the colima rift and the northeastern boundary of the jalisco block the tepic zacoalco rift to 8 mm year 95 confidence level slow compared to relative rates of motion at nearby plate boundaries

mineralogy and heavy metal content in sediments from rio gato carelmapu and cucao southern chile in southern chile with a mild and rainy climate high levels of heavy metals have been detected in many gold placer deposits many of the contaminants pose risks to human life and consequently damage the trofic chain in this environment the study zones selected correspond to the handicraft gold bearing sands of rio gato los muermos carelmapu and cucao these are all located in the x region of los lagos in chile the type of methodology used in the characterization of the associated mineralization consists of testing each representative sample with a grain size distribution statistical parameter analysis and a mineralogical analysis using a binocular magnifying glass a petrographic microscope xrd and sem edx the chemical composition was defined by means of x ray fluorescence and micro chemical analysis the results of the study about sediments in southern chile are presented the major concentrations of heavy minerals are generally located in areas of dynamic river energy in the samples more than 70 of the heavy minerals were distributed among grain sizes corresponding to thin sand with good grain selection meshes of 0 25 0 06 mm the main mineral phases present in the analysed samples were gold zircon olivine ilmenite hornblende hypersthene hematite garnet chromite chlorite augite amphibolitic epidote etc the main heavy metals found as a result of the study were mercury lead cadmium chrome tellurium indium zinc cobalt copper platinum gold etc and as well some less common elements like cerium praseodymium gadolinium neodymium samarium and lanthanum this research work is only a starting point for carrying out a risk probability mapping of heavy metals and the mineralogy of the cucao carelmapu and rio gato gold bearing sands

paleomagnetic tests of pacific plate reconstructions and implications for motion between hotspots the plate motion circuit through the south pacific and antarctica is shown to fail paleomagnetic tests of consistency these failures imply that reconstructions of pacific basin plates relative to surrounding plates inferred from this circuit are systematically in error and that estimates of motion between hotspots inferred from this circuit are probably too large therefore the motions between hotspots remain poorly known and may be much smaller than previously estimated

reevaluation of plate motion models based on hotspot tracks in the atlantic and indian oceans abstract plate motion models based on hotspot tracks in the atlantic and indian oceans predict minimal movement less than a few millimeters per year between these hotspots and their counterparts in the pacific ocean for the past 100 m yr whereas plate circuit exercises indicate relative motions of 20 mm yr hotspot based models also suggest that the rajmahal traps india were located 1000 km away from the kerguelen hotspot at 115 ma and the deccan traps india were located a similar distance from the reunion hotspot at 65 ma this is at odds with conclusions derived from paleomagnetism plate circuits and geochemical parameters that suggest a genetic link between flood basalt provinces in india and hotspots in the indian ocean these divergent views may be explained by plume action 1000 km from its center or errors in the hotspot motion models the latter hypothesis is scrutinized in this article by examination of the radiometric ages for hotspot tracks in the atlantic and indian oceans the