magnetostratigraphy of the lebo and tongue river members of the fort union formation paleocene in the northeastern powder river basin montana we analyzed paleomagnetic samples and documented the stratigraphy from two sections near miles city montana to determine the geomagnetic polarity stratigraphy and to constrain the age and duration of the lebo and tongue river members of the fort union formation in the northeastern powder river basin the resulting polarity sequence can be correlated to subchrons c29n c26r of the geomag netic polarity time scale by interpolating measured sediment accumulation rates from the base of c28r to the top of c27n and then extrapolating to the top of the tongue river member and the bottom of the lebo member we developed two age models to estimate the durations of the lebo and tongue river members based on the first model which uses different sedimentation rates for the lebo and tongue river members we estimate the duration of deposition of the lebo to be between 1 30 and 1 74 million years and of the tongue river to be between 1 42 and 1 61 million years using the second model which uses the same sedimentation rate for the lebo and tongue river members we estimate the duration of deposition of the lebo to be between 1 33 and 1 76 million years and of the tongue river to be between 1 00 and 1 25 million years our results indicate a decrease in sediment accumulation rates in c27r which is likely the result of a 0 26 to 0 62 million year long depositional hiatus in the middle of c27r represented by the lebo tongue river contact this unconfor mity occurs 2 million years earlier than previously suggested and is likely contempo raneous with unconformities in the williston basin and in southwestern alberta suggesting that it may be regionally significant

effect of low temperature treatments on pseudo thellier paleointensity determination 1 in paleointensity determination on multidomain md magnetite demagnetization is always easier than restoring remanence in companion acquisition steps as a result paleointensity results follow a convex down curve on the arai plot in an attempt to overcome this problem the effect of systematic low temperature demagnetization ltd on pseudo thellier results was examined ltd was applied not only to the initial anhysteretic remanent magnetization arm simulating natural remanent magnetization or nrm but also after each partial anhysteretic remanent magnetization parm acquisition step three practical benefits of systematic ltd were discovered 1 ltd treated pseudo thellier results always yielded more accurate intensity values regardless of grain size and sample lithology 2 ltd improved the behavior of md magnetite in paleointensity determination although not as much as for single domain and pseudosingle domain grains 3 an unanticipated bonus was a more even distribution of data points on the arai plot resulting from the fact that ltd causes a shift of points toward the upper end of the arai curves constructing a three dimensional map of the distributions and ltd was successful in quantitatively explaining behavior on the arai plots

palaeomagnetism of red beds from the kimberley group western australia implications for the palaeogeography of the 1 8 ga king leopold glaciation abstract the 1 8 ga king leopold glaciation in northwestern australia is recorded in the kimberley basin by subglacial erosional forms cut in an unconformity surface at the top of the speewah group and buried by glaciofluvial deposits at the base of the king leopold sandstone the lowest formation of the kimberley group the glaciofluvial deposits are conformably overlain by texturally mature locally glauconitic quartzarenite of shallow marine origin implying that glaciation occurred near sea level palaeomagnetic data for five sites 29 specimens in the hematite bearing elgee siltstone of the upper kimberley group in the northeastern kimberley basin gave a dip corrected mean direction of declination d 95 4 inclination i 13 8 95 9 7 indicating a palaeolatitude 7 0 5 3 5 0 results for red beds from five sites 28 specimens in the conformably overlying pentecost sandstone gave a dip corrected mean direction of d 94 1 i 13 0 95 11 7 6 6 6 4 5 9 combining our data for the elgee siltstone and pentecost sandstone with results for the elgee siltstone in the southeastern kimberley basin li z x 2000 palaeomagnetic evidence for unification of the north and west australian cratons by ca 1 7 ga new results from the kimberley basin of northwestern australia geophysical journal international 142 173 180 yielded a dip corrected mean direction for 17 sites of d 93 5 i 15 1 95 4 4 palaeolatitude 7 7 2 3 and an overall pole position at latitude p 5 4 s longitude p 211 8 e with confidence semi axes dp 2 3 and dm 4 5 a positive fold test at 99 confidence was obtained for the combined data for the 17 sites the basinwide concordance of results for the elgee siltstone and the positive fold test argue for an early magnetisation acquired prior to late palaeoproterozoic initial folding of the kimberley group and close to the time of deposition our results imply a low palaeolatitude for the kimberley group and the king leopold glaciation palaeomagnetic data for the 1 822 ga plum tree creek volcanics in the pine creek orogen to the east of the kimberley basin are consistent with northwestern australia being in low palaeolatitudes during the late palaeoproterozoic hence the enigma of glaciation near sea level in low palaeolatitudes which marks the neoproterozoic and early palaeoproterozoic applies also to the late palaeoproterozoic red beds from the lansdowne arkose of the speewah group also were studied but only directions ascribable to tertiary regolith weathering processes were found similar findings for the late palaeoproterozoic mcarthur basin in northern australia suggest some units in that basin also record tertiary regolith processes

multicomponent magnetizations from the mississippian mauch chunk formation of the central appalachians and their tectonic implications previous paleomagnetic study of mississippian red beds of the mauch chunk formation from the central appalachians reported shallow inclination directions a positive fold test was obtained after thermal demagnetization typically to only 550 c the near equatorial position for north america indicated by these results has been used to support the idea of a circa 15 latitudinal tectonic offset of the acadia displaced terrain in the northern appalachians the present study of 153 samples from 29 sites occupied in the mauch chunk formation of eastern and southern pennsylvania reveals multicomponent magnetizations a characteristic component declination d 161 0 inclination i 27 8 alpha95 a95 7 9 with unblocking temperatures usually concentrated above 650 c and with dual polarity 10 sites normal 13 sites reversed is isolated in 23 sites and passes a fold test at the 99 confidence level it is interpreted as an early acquired hematitic detrital or chemical magnetization of pre alleghanian orogeny and pre kiaman reverse polarity interval age a southern hemisphere paleolatitude of 15 for the sampling area is indicated another component has generally lower unblocking temperatures which however can extend up to 650 c or more and therefore is also likely to be carried by hematite this secondary magnetization d 170 6 i 5 3 a95 5 0 isolated in 25 sites is of uniform reverse polarity and application of fold tests suggests that it is of synfolding origin a near equatorial paleolatitude 3 n at the time of acquisition is obtained consistent with permo carboniferous kiaman directions from north america incomplete removal of the secondary component can explain the previous results from the mauch chunk comparison of the present paleomagnetic results with those from similar age rocks in the canadian maritimes does not support a latitudinal offset of acadia with respect to interior north america in the early carboniferous in fact according to these and other paleomagnetic data thought to be representative of this age the atlantic bordering continents already were near to a pangea configuration the alleghanian and particularly the hercynian orogenies therefore may not be associated with the closure of a large latitudinal ocean later in the carboniferous and permian alleghanian deformation however may have resulted in the partial bending of the pennsylvanian salient on the basis of apparently systematic deviations in paleomagnetic declinations

paleomagnetism of 1 09 ga lake shore traps keweenaw peninsula michigan new results and implications we report paleomagnetic data from a new section of the 1 09 ga lake shore traps exposed on silver island 10 flows and on the adjacent mainland two flows along the northwestern coastline of the

on the resolution of multivectorial remanences abstract low temperature demagnetization ltd promotes single domain sd like alternating field af and thermal demagnetization properties of anhysteretic remanent magnetization arm and thermoremanent magnetization trm by eliminating low coercivity or low unblocking temperature fractions of remanence we have investigated the effectiveness of ltd in improving the independence of partial arms parms independence of parms and of partial trms ptrms is crucial because multivectorial directional analyses and paleointensity determinations are otherwise compromised in normalized vector plots of af demagnetization of orthogonal parms ltd treated samples showed universal improvement in the degree of independence the greatest improvement was for synthetic sd and pseudo single domain magnetites and natural lake sediments on the other hand independence barely improved for synthetic multidomain magnetite granites and gabbros in general ltd provides a better resolution of superimposed vectors in multicomponent remanence the direction of the original arm corresponding to primary nrm in nature is perfectly recovered however the parm the overprint is not perfectly resolved especially for overprinting coercivities

magnetostratigraphy of chinese loess paleosol sequences abstract as one of the longest and most continuously deposited terrestrial sedimentary archives in the world chinese loess paleosol sequences record paleoclimatic and paleomagnetic variations at a range of time scales magnetostratigraphic studies provide a first order chronological framework for chinese loess sequences in this review we highlight recent developments in loess magnetostratigraphy including pedostratigraphy based on magnetic susceptibility variations we highlight progress in understanding the mechanisms by which the natural remanent magnetization nrm is acquired and discuss in detail the fidelity of paleomagnetic recording in loess records including the recording of magnetic polarity reversals excursions and relative paleointensity variations finally we discuss future prospects for studies of loess nrm

epigenetic nature of the bif hosted gold mineralization at ajjanahalli southern india evidence from ore petrography and fluid inclusion studies abstract gold mineralization at ajjanahalli is bif hosted and structurally controlled ore petrographic studies revealed that gold was introduced as a consequence of focussed fluid flow and sulfidation of an oxide facies bif horizon ubiquitous syn kinematic quartz veins within the bif as well as in the associated metasedimentary units lay testimony to the high fluid flux during deformation fluid inclusion studies involving microthermometry and laser raman microspectroscopy on inclusions from quartz veins from different litho units were carried out the primary fluid preserved as aqueous carbonic inclusions within quartz veins in carbonaceous phyllites is found to be h2o co2 ch4 nacl type near isothermal decompression led to splitting of this fluid at greenschist facies conditions 0 8 kb 263 c to 1 15 kb 309 c and resulted in a low saline aqueous and a carbonic ch4 fluid the auriferous aqueous part reacted with the oxidic bif leading to epigenetic replacement of oxides to pyrite and concomitant precipitation of gold

prefolding and overprint magnetic signatures in precambrian 2 9 2 7 ga igneous rocks from the pilbara craton and hamersley basin nw australia a total of 446 oriented samples have been collected from an archaean intrusion and the fortescue group volcanics and intrusions in the pilbara and hamersley regions of northwestern australia the paleomagnetism and rock magnetism of these samples have been investigated using susceptibility and hysteresis properties and both alternating field af and thermal demagnetization the most consistent paleomagnetic results were achieved by first applying af demagnetization and then stepwise thermal demagnetization evidence of the effects of lightning are widespread which accounts for the efficacy of this particular demagnetization strategy application of the fold test has resulted in placing significant constraints on the acquisition ages of some of the magnetizations and their corresponding pole positions a prefolding pole position from the millindinna complex is latitude 11 9 s longitude 161 3 e dp 6 8 dm 8 4 and is correlated with a radiometric age of 2 86 0 02 ga folding in the millindinna complex occurred prior to the deposition of the fortescue group new geochronological data indicate that the extrusion of the flood basalts commenced about 2 8 ga this provides a minimum age for the magnetization of the millindinna complex deformation responsible for the formation of the mobile belt separating the pilbara and yilgarn blocks during the ophthalmian orogeny has affected the fortescue group and produced a moderate degree of folding in the southern part of the hamersley basin in the region of millindinna complex outcrop the fortescue group is generally flat lying prefolding pole positions from the fortescue group are for the mount roe basalt latitude 52 4 s longitude 178 0 e dp 6 4 dm 9 1 and for the mount jope volcanics latitude 40 5 s longitude 128 7 e dp 19 9 dm 20 8 the overprint magnetization in the mount jope volcanics yields a pole position latitude 35 0 s longitude 211 5 e dp 2 8 dm 5 5 and is interpreted as having been acquired during folding relating to the formation of the ophthalmian fold belt

identifying types of remanent magnetization in igneous contact zones abstract the magnetization of host rock adjacent to an igneous intrusion e g dyke will be completely reset near the contact partially reset in a zone of hybrid directions farther away and unaffected at even greater distances if the intrusion related direction and the host direction are known the hybrid zone magnetization can be analysed in terms of ptrm vrm and crm overprints of the host magnetization using orthogonal component plots of thermal demagnetization results straight line segments representing the host and the intrusion induced magnetization show a sharp intersection in cases of 1 a pure ptrm overprint 2 a vptrm overprint of one magnetic carrier 3 two magnetic minerals with a vptrm partial overprint of the high blocking temperature mineral and a complete vptrm overprint of the low blocking temperature mineral and 4 a crm overprint where its blocking temperature range is entirely distinct from that of the host component a gradual transition between both components during thermal demagnetization is expected for a two magnetic minerals both of which have a vptrm partial overprint b a vptrm partial overprint of a continuous range of magnetic mineral compositions and c a crm overprint in which the blocking temperature range partially overlaps that of the host component swings from one direction to the other and back are observed if the blocking temperature of one of the components completely overlaps that of the other one of them being due to crm

magnetostratigraphy of the ludlow member of the fort union formation lower paleocene in the williston basin north dakota to determine the geomagnetic polarity stratigraphy and the duration and age of the ludlow member of the fort union formation lower paleocene we constructed a 325 m composite lithostratigraphic section of the upper cretaceous hell creek formation and the lower paleocene ludlow and tongue river members of the fort union formation in the little missouri river valley of north dakota usa we analyzed paleomagnetic samples from nine of the logged sections the principal magnetic carrier in the ludlow member sediments is likely titanomaghemite as indicated by predominantly irreversible thermomagnetic curves measured from sandstone siltstone and carbonaceous shale samples the analyzed paleomagnetic samples document a series of polarity zones that can be correlated from c29n to c27r on the geomagnetic polarity time scale gpts we infer that the magnetization of the samples is primary because the characteristic directions are consistent with those of the paleocene of north america and the reversal stratigraphy from this section corresponds to the gpts with reasonable sediment accumulation rates by extrapolating the measured sediment accumulation rate from the cretaceous tertiary k t boundary to the top of c28n and then to the top of the ludlow member we estimate the duration of the member to range from 2 31 to 2 61 m y this is the fi rst estimate for the duration and age of the ludlow member and it can be used as an important tool for interpreting rates of biotic recovery after the k t extinction

geochemistry and petrogenesis of amphibolites kolar schist belt south india evidence for komatiitic magma derived by low percentages of melting of the mantle the kolar schist belt of the dharwar craton of south india is an archean greenstone belt dominated by metavolcanic rocks the mafic metavolcanic rocks occur as komatiitic and tholeiitic amphibolites the komatiitic amphibolites occur along the margins of the n s trending synformal belt they are much less abundant than the tholeiitic amphibolites and have 14 to 21 3 wt per cent mgo the komatiitic amphibolites from the west central part of the belt have two distinctive ree patterns 1 those enriched in the middle to light ree but depleted in ce relative to nd and 2 those with patterns that are convex up i e depleted in both light and heavy ree although more depleted in the light ree associated tholeiites have light ree depleted to flat ree patterns komatiitic and tholeiitic amphibolites from the eastern part of the belt have enriched light ree patterns the tholeiitic amphibolites from the kolar schist belt are similar to the th i and th ii types of archean tholeiites of condie 1981 the komatiitic amphibolites are similar to komatiites and komatiitic basalts of barberton mountainland but have higher feo and tio 2 abundances and lower yb gd ratios the petrogenetic interpretations for these rocks are based primarily on a modification of the mgo feo diagram of hanson langmuir 1978 and modelling of zr ni and ree all of the rocks have undergone some fractionation while the modelling does not give accurate temperatures pressures compositions and extents of melting of the mantle sources for the various amphibolites it does present an approach which can be used for estimating these parameters for example the komatiitic amphibolites appear to be derived from melts generated by 10 to 25 per cent melting of the mantle over a range of depths and temperatures greater than 80 km and 1575 c the variation in the p t conditions of magma generation is possibly due to adiabatic melting in mantle diapirs with a range of feo mgo ratios if the tholeiitic amphibolites are derived from similar mantle sources it is not clear that they are their parent melts may have been generated by similar extents of melting but at depths of less than 80 km the komatiitic amphibolites from the west central part of the belt were generated from light ree depleted mantle whereas those from the eastern part of the belt appear to have been generated from light ree enriched mantle the sources for the komatiitic amphibolites in both areas were significantly enriched in feo relative to pyrolite thus a light ree depleted and a light ree enriched source appear to have provided mafic volcanics with similar major element chemistry to this belt during its evolution

linear and nonlinear thellier paleointensity behavior of natural minerals 1 we tested single domain sd and multidomain md theories of the acquisition and thermal demagnetization of thermoremanent magnetization trm and partial trm ptrm in thellier paleointensity determination using as samples plagioclase and mafic minerals extracted from matachewan diabase dikes susceptibility hysteresis and coercivity data indicate that the plagioclase contains mainly sd grains of magnetite while the mafic minerals clinopyroxene biotite and actinolite contain mainly md magnetite the extracted minerals were used in simulated thellier experiments with laboratory trm as a proxy for natural remanent magnetization nrm the plagioclase behaved ideally as predicted by neel sd theory giving a linear nrm ptrm arai plot with positive ptrm checks the mafic minerals behaved nonideally with strongly curved arai plots sagging below the ideal sd line in accord with observations for md magnetites and with predictions that in heating to moderate temperatures t more trm is lost than can be regained as ptrm causing convex down arai plots in experiments with ptrm parallel to nrm all ptrm tail checks were zero but ptrm checks increasingly deviated from the arai curve toward the sd line as t increased however in repeat thellier experiments ptrms at all t agreed with the original ptrms the negative ptrm checks were therefore not due to physicochemical alteration but rather to departure from initial states with different magnetic histories than those of the original ptrms the inability of ptrm checks to reliably distinguish between md behavior and irreversible alteration could lead to overestimates of paleointensity from straight line fits to the low t unaltered part of the arai curve or underestimates from fits to high t data alternating field cleaning to 15 mt before each thellier step made the arai plot only marginally less curved low temperature demagnetization ltd zero field cycling through the 120 k verwey transition hardly changed the curvature of the arai plot for ptrm parallel to nrm for ptrm perpendicular to nrm ltd led to an inflected arai curve straddling the ideal sd line data of either form would not give reliable paleointensity values no presently available pretreatment renders the paleointensity data of md grains usable sd and nearly sd grains which behave ideally in the thellier experiment remain the only useful recorders of absolute paleointensity

deciphering deformation in ultramafic intrusions via magnetic fabric ams and palaeomagnetic studies savannah ni pge camp nw australia abstract knowledge of the internal architecture of mafic ultramafic intrusions is critical to efficient targeting of magmatic ni pge mineralisation however the visual homogeneity of such rocks in drill core coupled with poor outcrop in many areas can make their structure difficult to decipher using standard techniques in this study we use measurements of anisotropy of magnetic susceptibility ams and remanent magnetisation to examine the structural history of four adjacent temporally equivalent ultramafic intrusions from the east kimberley province nw australia whilst temporally equivalent the intrusions have contrasting shape including bladed dyke sill and stock geometries and are hosted in rocks of contrasting rheology including mafic granulite and meta sedimentary migmatite early palaeomagnetic signatures present at savannah and savannah north originally had shallow nne to ene oriented inclinations consistent with acquisition at low palaeolatitudes at ca 1800 ma however these primary magnetisations were subsequently rotated during the ca 1810 ma mount stafford event and or ca 1400 1200 ma yampi orogeny demonstrating that significant deformation occurred post emplacement magmatic fabrics observed in ams data for savannah savannah north and dave hill all show evidence of ne or sw rotation during the same events however whilst there are similarities in the implied shortening directions across all four intrusions they each preserve a unique deformation style which is attributed to the unique shape of each intrusion coupled with the contrasting rheology of their respective host rocks each intrusion preserves a common dominant palaeomagnetic signature which is consistent with a high grade metamorphic overprint at ca 1200 ma during the latter stages of the yampi orogeny the lack of subsequent rotation of this dominant palaeomagnetic signature demonstrates that the majority of deformation pre dates the yampi event this study demonstrates that even where tectonic conditions are similar that intrusion shape and host rock rheology exert strong control on their deformation style

geomagnetic paleointensity at 2 41 ga as recorded by the widgiemooltha dike swarm western australia abstract absolute geomagnetic paleointensity measurements were conducted on samples from six mafic dikes of the 2 41 ga widgiemooltha swarm western australia rock magnetic analyses indicate that the paleointensity signal is carried by nearly stoichiometric pseudosingle domain magnetite and or low ti titanomagnetite paleointensity values were determined using the thellier double heating method supplemented by low temperature demagnetizations the ltd thellier method in order to reduce the effect of magnetic remanence carried by large pseudosingle domain and multidomain magnetite grains thirty one samples from five dikes yielded successful paleointensity determinations with the mean value of 41 2 3 8 t which corresponds to a virtual dipole moment of 6 65 0 98 a m 2 the mean and range of paleofield strength values are similar to those of the recent earth s magnetic field and are consistent with a compositionally driven geodynamo established by the earliest paleoproterozoic era the existence of a stable dipolar geomagnetic field during the proterozoic indicated by paleointensity and paleodirectional data from this and prior studies seems incompatible with a young age of the inner core suggested by recent models of the earth s thermal evolution acquisition of reliable paleointensity estimates using the novel approaches such as the ltd thellier method is crucial in constraining the development of more realistic earth like models of long term geodynamo behavior

measuring heterogeneous remanence in paleomagnetism 1 remanence directions of from the same block sample may be inconsistent or unrepresentative due to orientation and location heterogeneity of their remanence bearing minerals rbm magnetization heterogeneity is usually undetectable at the specimen level but we replicated its effects by measuring 8 small specimens with stable magnetizations 8 or 5 2 cm3 these were assembled into a single large multi specimen inside 125 cm3 containers that were measured in a molspin bigspin magnetometer large specimen remanence directions deflect towards the direction of any strongly magnetized sub specimen differences between the large specimen remanence and that for the group of individually measured sub specimens worsened when one sub specimen was mis oriented these discrepancies were cancelled or reduced using larger numbers of specimen orientations in the magnetometer conventional schemes with 4 or 6 different measurement orientations may fail to suppress heterogeneity effects whereas our 12 orientation protocol may succeed for most specimens acceptable remanence homogeneity is present where similar remanence directions are recorded from 4 6 and 12 different spin orientations

direction and intensity of the geomagnetic field in the middle devonian and lower ordovician southern mugodjary ophiolites urals abstract a considerable quantity of petromagnetic and palaeomagnetic measurements in the field has allowed us to obtain reliable palaeomagnetic results for such difficult and complex objects as ophiolites eifelian baked sediments diabases dyke contacts and gabbros and tremadocian baked sediments were studied continuous thermocleaning conglomerate and fold tests were applied the palaeointensity was investigated by means of several palaeomagnetic methods wilson burakov van zijl shaw and bagina petrova the mean intensities of the geomagnetic fields are 7 and 16 5 t with mean palaeomagnetic pole positions of 9 n 134 e and 6 s 83 e respectively these data demonstrate that southern urals mugodgary was located near the northern margin of the east european plate during ordovician devonian time the allochthonous crustal oceanic plate was thrust upon the edge of the plate and deformed as late as the permian changes in the polarity of the geomagnetic field are recorded in the succession of sheeted dykes a direct corroboration of the vine mathews hypothesis

magnetic overprinting in southeastern australia and the thermal history of its rifted margin the magnetizations of many rock units in southeastern australia have been severely overprinted interpretation of early work was hindered by this and one of the aims of the current research is to ascertain the nature of the overprint mechanism its geological significance and its regional extent results from three igneous bodies from the sydney basin which have been studied previously in a reconnaissance fashion have provided a new insight into the reason for such widespread magnetic overprinting the hornsby breccia hb and milton monzonite mm yield similar magnetic signatures both containing discrete high and low temperature magnetizations with directions of dec 72 inc 83 95 4 and dec 13 inc 76 95 2 respectively for hb and dec 60 inc 78 95 4 and dec 348 inc 79 95 6 respectively for mm the corresponding pole positions are 29 s 166 e a95 7 and 59 s 139 e a95 4 for hb and 22 s 171 e a95 7 and 55 s 158 e a95 11 for mm the paleomagnetic direction and pole from the third body studied the mogo hill basalt mh are dec 241 inc 81 95 5 and 41 s 130 e a95 9 this pole position and the two low temperature overprint pole positions from hb and mm are consistent with ages ranging from 100 m y to approximately 70 m y when compared to the late mesozoic apparent polar wander path we relate the overprinting to uplift erosion and rapid supracrustal cooling of the southeastern margin of australia associated with initial phases of marginal rift development prior to seafloor spreading in the tasman sea the temperatures of rocks now exposed may have reached 200 c or higher before the rapid cooling blocked and stabilized the overprint magnetizations

palaeomagnetism of the dundas fossey trough tasmania oroclinal rotation and late cretaceous overprinting abstract the dundas fossey trough a curved belt of early palaeozoic rocks in western and northern tasmania is hypothesised to have developed as an orocline driven by asthenospheric flow behind a rapidly retreating subduction zone on the east gondwana margin rocks of the wurawina supergroup from the trough were sampled for a palaeomagnetic test of this hypothesis rock magnetic analysis identifies both magnetite and hematite remanence carriers hematite carries a normal polarity overprint which was acquired at relatively low temperature thermal demagnetisation over the magnetite unblocking range reveals a linear trend that defines component a which displays both normal and reversed polarity component a passes a modified version of the fold test indicating that it is pre deformational the declination of component a correlates linearly with regional strike thus passing the orocline test a palaeomagnetic pole for the cambrian owen group after correction for oroclinal rotation is displaced from the gondwana polar wander path suggesting that the western limb of the trough may have been rotated anticlockwise by about 40 prior to the development of the orocline presumably during known early to middle ordovician deformation adjustment of the palaeomagnetic declination data to account for this earlier phase of rotation improves the fit parameters of the orocline test the palaeomagnetic results are consistent with the orocline model for the dundas fossey trough and support the inference that the trough extends beneath later cover in southeastern tasmania overprint component b yields a mean pole in broad agreement with other overprints previously observed both in tasmania and the eastern seaboard of the australian mainland and like them appears to have been acquired during the late cretaceous normal polarity superchron we speculate that magnetic overprinting in tasmania is related to enhanced oxidation by meteoric water driven by regional uplift

uniformity in sulfur isotope composition in the orogenic gold deposits from the dharwar craton southern india the sulfur isotope composition of sulfides mainly pyrite and arsenopyrite from gold deposits prospects of the dharwar craton such as hutti hira buddini uti kolar chigargunta ajjanahalli and jonnagiri has a narrow range 34s 1 1 to 7 1 such craton scale uniformity of the above gold camps is noteworthy in spite of the wide diversity in host rock compositions and their metamorphic conditions and suggests a magmatic or average crustal source of sulfur for all deposits studied in addition our study points towards gold precipitation from reduced ore fluids with near homogeneous sulfur isotope compositions

alteration geochemistry and fluid inclusion characteristics of the greenstone hosted gold deposit of hutti eastern dharwar craton india gold mineralization of the hutti mine southern india is situated in closely spaced laminated quartz veins and associated alteration haloes along steeply dipping shear zones within a sequence of rather uniform amphibolites intense shearing has resulted in large scale mylonitization of the wall rocks anastomosing shear zones with intervening lensoid bodies of unsheared amphibolites are characteristic features of the deposit the general pattern of symmetrical alteration comprises a distal zone of chlorite rich rock with a proximal biotite rich zone adjacent to laminated quartz veins arsenopyrite thermometry yielded a temperature range of 350 477 c for the biotite alteration zone which preceded the formation of the laminated quartz veins mass balance calculations on the alteration zones indicate a gradual mass and volume loss during alteration the alteration is accompanied by intense potash metasomatism and addition of sulfur which resulted in the formation of arsenopyrite pyrrhotite and pyrite results of fluid inclusion studies suggest that low salinity 3 9 13 5 wt nacl equivalent h2o co2 rich fluids were responsible for gold rich laminated quartz vein formation in the hutti deposit these fluids constituted a later counterpart of the protracted fluid activity that first formed the biotite alteration zone the estimated p t values range from 1 0 to 1 7 kbar at 280 320 c these data along with the alteration assemblages and the characteristic gold sulfide association both in the altered wall rock and laminated quartz veins suggest that gold transported as reduced bisulfide complexes was deposited in response to sulfidation reactions in the wall rocks comparison of p t conditions of formation of gold quartz veins at hutti with two other large gold deposits in the eastern dharwar craton namely kolar 1 8 kbar 280 c and western ramagiri 1 45 1 7 kbar 240 270 c indicates broadly similar lode gold forming conditions in the dharwar craton

paleomagnetism of the newcastle range northern queensland eastern gondwana in the late paleozoic 1 the newcastle range is an extensive 2500 km2 and well exposed caldera system erupted on the trailing edge of eastern gondwana between 325 and 295 ma paleomagnetic samples were collected from ignimbrites and associated microgranitoid intrusions from the central northern and southern calderas from which three components of magnetization are recognized component 1 is considered to be a viscous magnetization acquired during the brunhes chron a presumed permian component c2 is found in seven paleomagnetic sites with a mean pole at 30 9 s 139 7 e k 13 9 a95 16 8 asd 21 7 agreeing with previously reported permian data from australia carboniferous units have a well defined characteristic component c3 distinguished by dual polarity predominantly reversed and moderate to steep inclination directions paleomagnetic polarities in the newcastle range volcanics are formation dependent and new constraints on the timing of carboniferous volcanism 325 317 ma are consistent with recent reanalysis of the base of the permo carboniferous reversed superchron pcrs a mean paleomagnetic pole calculated from 15 vgps lies at 63 4 s 125 e k 26 22 a95 7 6 asd 15 8 suggesting that australia remained at midlatitudes into the middle carboniferous this paleomagnetic pole is consistent with similarly aged poles from western gondwana the conformity of which indicates contributions from nondipole components of the earth s paleofield were probably not significant in the time immediately preceding the pcrs

paleomagnetism of the 765 ma luakela volcanics in northwest zambia and implications for neoproterozoic positions of the congo craton the 765 ma luakela volcanics of northwest zambia are a northeast trending belt of basaltic to andesitic flows within an essentially undeformed and unmetamorphosed siliciclastic succession that unconformably overlies neoarchean and paleoproterozoic rocks of the congo craton margin paleomagnetic analysis of 65 samples from nine sites isolated three magnetization components component a carried mainly by single domain magnetite is directed very shallowly to the southeast and northwest and is interpreted as a primary thermoremanent magnetization component b carried mainly by hematite and oriented mainly shallowly upwards to the southwest is interpreted to be a secondary overprint possibly acquired during latest neoproterozoic pan african deformation in the lufilian arc a low stability component c probably carried by multi domain magnetite or hematite or both is directed very steeply downward and is similar to late paleozoic results from the region paleopoles lva and lvb corresponding to components a and b are similar to previous results from mafic rocks of similar age in tanzania lva 40 2 s 122 0 e coincides with a reliable pole for the 748 ma mbozi syenite gabbro complex confirming the position of the congo craton at 765 to 750 ma the secondary lvb pole 38 3 n 90 9 e of uncertain age but younger than 765 ma coincides with the pole for the ca 795 ma gagwe lavas casting doubt on the assumed primary nature of the gagwe remanence our new data imply that the congo craton rotated ca 90 clockwise between 750 ma and the time of b component acquisition contrary to previous scenarios in which congo rotated counter clockwise between ca 800 and 750 ma

fluid inclusion characteristics of the uti gold deposit hutti maski greenstone belt southern india abstract the uti gold deposit occurs within amphibolites of the late archaean hutti maski greenstone belt of southern india gold mineralization is associated with intense silicification biotite actinolite and sulfide rich alteration zones in between small anastomozing shear zones characteristic biotite k feldspar alteration and arsenopyrite thermometry constrain the mineralizing event to above 400 c which is concordant with evidence of dynamic recrystallization in mineralized quartz veins shearing concomitant alteration and au mineralization took place at post peak metamorphic conditions a post mineralization sub greenschist facies assemblage of prehnite pumpellyite and hydrogarnet later imprinted the rocks fluid inclusion microthermometric studies on mineralized and barren quartz veins reveal similar fluid chemistry p t and surprisingly a complete absence of carbonic inclusions fluid inclusion textures and the shape of th histograms point towards moderate reequilibration during a phase of isothermal decompression the microthermometric data coupled with stability relations of the hydrous ca al silicates indicate post mineralization isobaric cooling followed by a near isothermal decompression the observed temperature salinity variation is schematically explained by simple cooling of a low salinity heated meteoric fluid followed by isothermal mixing with a high salinity granitic fluid however fluid mixing was later than and unrelated to mineralization on the other hand formation of auriferous sulfide bearing quartz veins at uti was essentially due to remobilization and attendant fluid rock interaction involving suitable host rocks that contained primary gold

hysteresis and susceptibility characteristics of magnetite and titanomagnetites interpretation of results from basaltic rocks abstract the hysteresis and susceptibility characteristics down to liquid nitrogen temperatures have been investigated in over one thousand basalt samples of tertiary age from the society islands norfolk island reunion mauritius france and australia the variation in these properties with temperature is similar to that observed by radhakrishnamurty synthetic samples covering grain sizes from single to multidomain grains of pure magnetite titanomagnetite and cation deficient titanomagnetites of varying composition have been investigated to determine their low temperature characteristics comparison of the results from these synthetic samples with those of natural basalts shows that the composition and domain state predominating in any given sample of basalt cannot be determined uniquely from low temperature characteristics alone however optical electron microprobe and af demagnetization characteristics can be combined with the low temperature properties to determine the predominant grain size and magnetite composition the results show that the simple classification scheme put forward by radhakrishnamurty is questionable samples showing increase in susceptibility and no change in coercivity down to liquid nitrogen temperature are not of the cation deficient type these properties appear to be those associated with titanomagnetites containing exsolved ilmenite lamellae formed during initial cooling this unique characteristic may be useful for the quick selection of samples most suitable for palaeointensity studies

reversal asymmetry in mesoproterozoic overprinting of the 1 88 ga gunflint formation ontario canada non dipole effects or apparent polar wander abstract eighty two palaeomagnetic samples of calcareous and jaspilitic grainstones iron formation or taconite and chert carbonate were collected from the 1 88 ga gunflint formation at 22 sites in the thunder bay area ontario twenty clasts of gunflint taconite also were sampled from the basal conglomerate of the overlying mesoproterozoic sibley group anisotropy of magnetic susceptibility measurements indicate the gunflint formation in the sampling area has not experienced regional dynamic metamorphism analyses by variable field translation balance and x ray diffraction show that the predominant magnetic mineral is hematite but a small amount of magnetite also is present in some samples altogether 213 gunflint specimens and 59 sibley conglomerate specimens were subjected to stepwise thermal demagnetisation and 74 gunflint specimens to stepwise alternating frequency demagnetisation the following components were isolated for the taconites gunflint magnetite normal declination d 293 4 inclination i 30 8 95 7 2 n 21 reverse d 86 7 i 54 6 95 5 8 n 29 gunflint hematite normal d 243 6 i 23 6 95 6 0 n 11 reverse d 70 3 i 51 4 95 3 2 n 79 sibley clasts magnetite normal d 282 7 i 33 4 95 7 6 n 20 sibley clasts hematite normal d 254 5 i 56 2 95 8 4 n 13 reverse d 110 6 i 55 7 95 8 3 n 11 none of these sets passed the reversal test with the normal component generally being the shallower fold tests were negative or inconclusive and the conglomerate test also was negative chert carbonate at one other site appears to have acquired a remanence carried by magnetite d 97 3 i 78 2 95 6 3 n 18 prior to folding related to keweenawan 1 1 ga logan diabase sill emplacement most of the components we identified match components for keweenawan sills volcanic rocks intrusions and baked contact rocks in the thunder bay area indicating that keweenawan magmatism caused widespread chemical remagnetisation of the proterozoic country rock in our sampling area although others have argued that asymmetry was a feature of the keweenawan geomagnetic field the declinations of our gunflint and sibley hematite and magnetite components are different suggesting that the components were acquired at significantly different times we conclude that the reversal asymmetry shown by our gunflint and sibley data may best be ascribed to apparent polar wander during keweenawan times

linearity spectrum analysis of multi component magnetizations and its application to some igneous rocks from south eastern australia summary a method that enables the objective resolution of almost parallel multi component magnetizations is described and demonstrated a feature distinguishing this method from others is its simultaneous analysis of demagnetization data from a group of specimens rather than the analysis of data from one specimen at a time the only prerequisite is that the specimens are derived from a homogeneous source thus for a formation carrying a simple single component magnetization all specimens from the formation may be simultaneously reduced for a more complicated two component magnetization it is shown that only specimens from a particular site can be considered homogeneous and for a complex three component system each sample often requires undivided attention thus the workload is proportionally increased to achieve analyses of comparable reliability from data of variable quality new pole positions from mesozoic intrusions of the sydney basin nsw are from the marsden park breccia pipe 48 s 127 e a95 6 the st marys breccia pipe 46 s 150 e a95 8 the prospect dolerite 60 s 142 e a95 13 and 53 s 180 e a95 6 and from the dundas breccia pipe 58 s 162 e a95 36 and 31 s 195 e a95 16 the last two formations possess multi component magnetizations these pole positions are consistent with previous results from south eastern australia

geological and palaeomagnetic significance of the kulgera dyke swarm musgrave block nt australia summary the kulgera dyke swarm consists of olivine tholeiites which have intruded late proterozoic transitional granulite gneisses and granites of the eastern musgrave block in central australia preliminary rb sr results suggest that the dolerites were emplaced at 1054 f 14ma in addition a rb sr age of 1060 f 10ma on a biotite from a pegmatite indicates thermal resetting of the country rock minerals during dyke emplacement palaeomagnetic investigations of the dykes yield a primary thermoremanent magnetization direction corresponding to a palaeomagnetic pole at 17 s 266 e ag5 12 in addition to this primary magnetization an overprint component was present in many of the samples providing a palaeomagnetic pole at 30 s 138 e ags 24 which is similar to previous results from other central australian rocks affected by the alice springs orogeny the results extend the area of influence of the carboniferous alice springs orogeny southward into the musgrave block further the results provide no evidence for an earlier late proterozoic petermann orogeny affecting the musgrave block in the kulgera region however the possibility that a petermann orogeny thermal overprint has been erased by the alice springs orogeny cannot be dismissed

remagnetization 1100ma of voosges lake tuff 1542ma and other mid proterozoic rocks nipigon graben northern ontario abstract recently discovered horizons of non deformed felsic welded crystal tuff in the proterozoic rift system of northern ontario yield a geologically stable paleomagnetic vector using alternating field af thermal td and low temperature demagnetization ltd techniques a single stable characteristic remanence chrm vector is found in all specimens declination inclination 115 73 5 95 2 1 n 58 it is overprinted by a bruhnes epoch vrm the tuffs paleopole position coincides with that of ubiquitous sub horizontal diabase sheets which intruded at 1110 ma during late proterozoic keewanawan volcanism the latter produced numerous volcanic and intrusive rocks along the margins of the lake superior paleo rift system during the interval 1087 1120 ma but the sills are most extensive although the voosges lake tuff shares the same paleopole location as the diabase sheets 1109 ma the nearby seagull pluton 1112 ma and the keweenawan volcanic rocks its primary zircons yielded a mesoproterozoic crystallization age of 1542 3 ma this is similar to the nearby english bay felsic intrusions dated at 1537 2 10 ma davis d w sutcliffe r h 1985 u pb ages from the nipigon plate sic and northern lake superior geol soc am bull 96 1572 1579 whereas the paleomagnetic magnetic signal appears primary in the 1542 ma voosges tuff it was completely reset by the late proterozoic keewanawan paleofield 1100 ma the tuff units are separated from the diabase sills by stratigraphic thicknesses of 50 m and show no traditional petrographic evidence of annealing or thermal metamorphism since it is improbable that the 1542 ma tuff returned to the same paleolatitude at 1109 ma we postulate that metasomatic processes caused a 1109 ma remagnetization a similar event could explain unusual chrms reported for certain other pre keweenawan proterozoic sites with adjacent keewanawan diabase sheets for example some paleopoles reported for the gunflint formation 1878 ma and sibley formation 1339 ma perhaps 1537 ma by association with the english bay intrusions might have been acquired due to secondary remagnetization

paleomagnetic results indicate pre permian counter clockwise rotation of the southern tamworth belt southern new england orogen australia 1 visean ignimbrites from three fault bounded blocks within the southern tamworth belt southern new england orogen australia retain a prefolding high temperature component of remanence carried mainly by magnetite paleomagnetic data indicate that the southern tamworth belt occupied low to moderate paleolatitudes during the early carboniferous normal and reversed polarities with low to moderate inclinations suggest that the steep reverse polarity latest carboniferous to earliest permian overprint is absent from the high temperature component data set the mean characteristic remanence direction of three early to middle visean ignimbrite flows 32 sites from the rouchel block is declination dec 154 2 and inclination inc 38 8 95 12 5 k 98 8 which corresponds to a paleo south pole at latitude 64 9 s longitude 258 1 e dp 5 5 dm 9 2 where dp is the angular length of the semiaxis of the ellipse of confidence of the calculated paleopole that lies along the pole to site great circle and dm is the angular length of the semiaxis perpendicular to dp six middle to late visean ignimbrite flows 29 sites from the gresford block retain a mean remanence of dec 159 6 and inc 54 1 95 13 7 k 24 7 corresponding to a paleo south pole at latitude 72 8 s longitude 228 4 e dp 13 4 dm 19 2 despite widespread overprinting throughout much of the myall block reliable information was obtained from three late visean ignimbrite flows five sites with a mean direction of dec 118 1 and inc 42 5 95 12 0 k 41 6 from which a paleo south pole of latitude 35 7 s longitude 233 4 e dp 9 1 dm 14 7 is calculated the calculated paleo south poles do not fall on the early carboniferous segment of the published apparent polar wander path for australia and suggest that the southern tamworth belt has undergone significant counter clockwise rotation relative to cratonic australia inferred rotations for the rouchel gresford and myall blocks are approximately 80 80 and 120 respectively stratigraphic and geological evidence suggest rotation took place between the mid namurian and latest carboniferous and is thought to have resulted from sinistral strike slip during compression at the eastern margin of australia

an improved methodology for thellier type paleointensity determination in igneous rocks and its usefulness for verifying primary thermoremanence we propose a new methodology for thellier type paleointensity experiments in standard paleointensity experiments the blocking temperatures of most stable remanences are often not reached before thermal alteration of magnetic mineralogy begins and subsequent data have to be discarded we emphasize that when alteration begins at low temperature and continues throughout the experiment which then fails in standard analyses the resulting unblocking temperature tub spectrum of the alteration product does not necessarily overlap with the whole tub spectrum of natural remanent magnetization nrm because tub depends on physical characteristics of the new grains not the temperatures at which they were formed nrm may survive in an uncontaminated higher tub window if alteration remanence only has low tub then partial thermoremanent magnetization ptrm checks at temperature step tj 1 can be used to correct for alteration that has occurred at tj and furthermore thermal demagnetization of a full trm acquired in the laboratory at the end of the progressive thellier thellier experiment should reveal the true uncontaminated tub spectrum of the nrm in higher temperature intervals we propose a new experimental sequence which allows monitoring and correction of alteration and provides two semi independent estimates of palaeointensity in the circumstances described above we illustrate the issues involved through new experimental work on igneous rocks from a paleozoic charnockitic syenite where we unravel trm from multidomain isothermal remanence despite massive alteration during laboratory heating finally we point out the value of these experiments for verifying primary trm in igneous rocks indicating properties that distinguish trm from other remanence

fluid evolution in the kolar gold field evidence from fluid inclusion studies gold mineralization in the kolar schist belt of the dharwar craton occurs dominantly in the form of a sulfide poor au quartz lode the champion lode exposed in the mysore and other mines and sulfide rich auriferous lodes from the nundydroog mine fluid inclusion microthermometric experiments were conducted on primary inclusions in quartz intimately associated with au mineralization homogenization studies on aqueous biphase l v aqueous polyphase l v halite and aqueous carbonic lco2 vco2 laq inclusions from the champion lode furnish a temperature range of 120 to 420 c freezing of aqueous biphase inclusions and dissolution of halite in the aqueous polyphase inclusions provide salinity of 5 to 50 wt nacl equivalent fluid inclusion thermobarometry from the total homogenization of aqueous carbonic inclusions and from intersecting isochores of coeval pure carbonic lco2 vco2 and pure aqueous inclusions constrain the p t path of evolution of the fluid in the champion lode gold precipitation was likely to have been brought about in response to a sharp fall in pressure with attendant unmixing of liquid co2 from the parent h2o co2 fluid of possible metamorphic origin this would imply transportation of gold by some pressure sensitive complex such as the au carbonyl fluid characteristics are different in the sulfide rich auriferous lodes as indicated by the virtual absence of the co2 bearing and the halite bearing inclusions the fluid evolution path as evident from the crude positive colinearity of temperature and salinity is due to mixing of a low 200 c temperature low saline 7 wt nacl equivalent fluid with a high temperature 400 c high saline 50 wt nacl equivalent fluid the lack of co2 and association of au with sulfides indicate a different mode of gold transport as chloride or bisulfide complexing deposition of which was possibly brought about by fluid mixing

metamorphism in archaean greenstone belts calculated fluid compositions and implications for gold mineralization an inescapable consequence of the metamorphism of greenstone belt sequences is the release of a large volume of metamorphic fluid of low salinity with chemical characteristics controlled by the mineral assemblages involved in the devolatilization reactions for mafic and ultramafic sequences the composition of fluids released at upper greenschist to lower amphibolite facies conditions for the necessary relatively hot geotherm corresponds to those inferred for greenstone gold deposits xco2 0 2 0 3 this result follows from the calculation of mineral equilibria in the model system cao mgo feo al2o3 sio2 h2o co2 using a new expanded internally consistent dataset greenstone metamorphism cannot have involved much crustal over thickening because very shallow levels of greenstone belts are preserved such orogeny can be accounted for if compressive deformation of the crust is accompanied by thinning of the mantle lithosphere in this case the observed metamorphism which was contemporaneous with deformation is of the low p high t type for this type of metamorphism the metamorphic peak should have occurred earlier at deeper levels in the crust i e the piezothermal array should be of the deeper earlier type however at shallow crustal levels the piezothermal array is likely to have been of deeper later type as a consequence of erosion thus while the lower crust reached maximum temperatures and partially melted to produce the observed granites mid crustal levels were releasing fluids prograde into shallow crustal levels that were already retrograde we propose that these fluids are responsible for the gold mineralization thus the contemporaneity of igneous activity and gold mineralization is a natural consequence of the thermal evolution and does not mean that the mineralization has to be a consequence of igneous processes upward migration of metamorphic fluid via appropriate structurally controlled pathways will bring the fluid into contact with mineral assemblages that have equilibrated with a fluid with significantly lower xco2 these assemblages are therefore grossly out of equilibrium with the fluid in the case of infiltrated metabasic rocks intense carbonation and sulphidation is predicted if as seems reasonable gold is mobilized by the fluid generated by devolatilization then the combination of processes proposed most of which are an inevitable consequence of the metamorphism leads to the formation of greenstone gold deposits predominantly from metamorphic fluids

relative timing of deformation and two stage gold mineralization at the hutti mine dharwar craton india gold mineralization at hutti is confined to a series of nine parallel n s to nnw sse trending steeply dipping shear zones the host rocks are amphibolites and meta rhyolites metamorphosed at peak conditions of 660 40 c and 4 1 kbar they are weakly foliated s1 and contain barren quartz extension veins the auriferous shear zones reefs are typically characterized by four alteration assemblages and laminated quartz veins which in places occupy the entire reef width of 2 10 m and contain the bulk of gold mineralization a 1 5 m wide distal chlorite sericite biotite calcite plagioclase alteration zone can be distinguished from a 3 5 m wide proximal biotite plagioclase quartz muscovite calcite alteration zone gold is both spatially and temporally associated with disseminated arsenopyrite and pyrite mineralization an inner chlorite k feldspar quartz calcite scheelite tourmaline sphene epidote sericite alteration halo which rims the laminated quartz veins is characterized by a pyrrhotite chalcopyrite sphalerite ilmenite rutile and gold paragenesis the distal chlorite sericite and proximal biotite plagioclase alteration assemblages are developed in microlithons of the s2 s3 crenulation cleavage and are replaced along s3 by the inner chlorite k feldspar alteration indicating a two stage evolution for gold mineralization ductile d2 shearing alteration and gold mineralization formed the reefs during retrograde evolution and fluid infiltration under upper greenschist to lower amphibolite facies conditions 560 60 c 2 1 kbar the reefs were reactivated in the d3 dextral strike slip to oblique slip environment by fault valve behavior at lower greenschist facies conditions ca 300 350 c which formed the auriferous laminated quartz veins later d4 crosscutting veins and d5 faults overprint the gold mineralization the alteration mineralogy and the structural control of the deposit clearly points to an orogenic style of gold mineralization which took place either during isobaric cooling or at different levels of the archean crust from overlaps in the tectono metamorphic history it is concluded that gold mineralization occurred during two tectonic events affecting the eastern dharwar craton in south india between ca 2550 2530 ma 1 the assemblage of various terranes of the eastern block and 2 a tectono magmatic event which caused late to posttectonic plutonism and a thermal perturbation it differs however from the pre peak metamorphic gold mineralization at kolar and the single stage mineralization at ramagiri notably greenschist facies gold mineralization occurred at hutti 35 90 million years later than in the western dharwar craton

low temperature demagnetization of saturation remanence in magnetite bearing dolerites of high coercivity we studied 16 magnetite bearing dolerite dyke samples of high coercive force hc ranging from 11 to 50 mt that had been used successfully in precambrian palaeomagnetic studies each dolerite was given a saturation remanent magnetization whose change was measured as the sample was cooled to 77 k in zero field and warmed back to room temperature only the three dolerites of highest hc 40 mt show little change on cooling suggesting that their magnetite is mostly in elongated single domain grains the rest of the dolerites are likely to be dominated by pseudosingle domain magnetite cooling to 135 k causes their remanence to decrease by 37 per cent on average in rough proportion to the decrease in saturation magnetostriction as expected if internal stresses oppose domain wall motion cooling from 135 k to 77 k causes remanence to decrease further by 26 per cent on average probably mostly because of domain reorganization forced by magnetite s verwey crystallographic transition warming back to room temperature causes some of the remanence loss to be recovered perhaps because internal stresses act as a bridge between different easy axes below the verwey temperature 122 k and above the isotropic point 135 k this recoverable low temperature demagnetization averages 23 6 per cent of the initial saturation remanence while the permanent demagnetization averages 40 9 per cent recoverable low temperature demagnetization is even larger for natural remanence averaging 46 9 per cent for the six dolerites measured while the corresponding permanent demagnetization averages 13 6 per cent large recoverable low temperature demagnetization seems to be characteristic of pseudosingle domain magnetite in which high internal stresses block domain wall motion and may be common in mafic igneous rocks like our dolerite samples whose magnetite is intergrown with ilmenite lamellae measuring natural remanence of such rocks before after and while at 77 k should help separate remanence carried by multidomain magnetite mostly permanently demagnetized by single domain magnetite mostly unchanged and by pseudo single domain magnetite mostly responsible for recoverable demagnetization

climatic regimes of tropical convection and rainfall abstract annual distribution and phase propagation of tropical convection are delineated using harmonic and amplitude phase characteristics analysis of climatological pentad mean outgoing longwave radiation and monthly frequencies of highly reflective cloud an annual eastward propagation of peak rainy season along the equator from the central indian ocean 60 e to arafura sea 130 e is revealed this indicates a transition from the withdrawal of the indian summer monsoon to the onset of the australian summer monsoon significant bimodal variations are found around major summer monsoon regions these variations originate from the interference of two adjacent regimes the convergence zones over the eastern north pacific the south pacific and the southwest indian ocean are identified as a marine monsoon regime that is characterized by a unimodal variation with a concentrated summer rainfall associated with the development of surface westerlies equatorward of a monsoon trough conversely the central nor

palaeomagnetism of iron formation from the late palaeoproterozoic frere formation earaheedy basin western australia palaeogeographic and tectonic implications abstract oriented cores of unweathered frere formation 1 9 1 8 ga including shaly and peloidal iron formation were obtained from two holes drilled to 116 m depth on opposite limbs of an anticline in the southern earaheedy basin at the northeastern margin of the yilgarn craton anisotropy of magnetic susceptibility measurements indicated that the frere formation in this area was unaffected by regional metamorphism the predominant magnetic mineral in the iron formation is hematite but a small amount of magnetite also is present in some samples stepwise thermal demagnetisation was conducted for 263 specimens many specimens acquired spurious components of magnetisation after treatment to 300 500 c ascribable to the oxidation of siderite to magnetite on heating however other specimens displayed a stable high temperature 680 c dual polarity component a carried by hematite that gave a bedding dip corrected direction of d 314 3 i 24 8 k 38 5 95 2 6 n 78 a fold test was positive at 99 level of confidence indicating acquisition of remanence prior to stanley folding in the earaheedy basin that occurred at 1 78 ga near the end of capricorn orogenesis the presence of polarity reversals in the stratigraphic section from one drill hole and within some specimens argues for the identification of a dipole field axis and the sufficient averaging of secular variation to define a palaeopole for the frere formation located at 45 2 s 220 0 e dp 1 3 dm 2 4 component a is ascribed to a chemical remanent magnetisation crm acquired soon after deposition and indicates a palaeolatitude of 13 0 1 5 for the frere formation the frere pole plots adjacent to but is still significantly different from late to post capricorn orogeny overprint poles from the hamersley province of the southern pilbara however vertical axis relative anticlockwise rotation of 10 15 for the frere pole brings it close to these pilbara overprint poles the data imply that the earaheedy basin and possibly the yilgarn craton underwent clockwise rotation of 10 15 relative to the pilbara craton during stanley folding this finding accords with evidence for tectonic settings marked by sinistral strike slip deformation adjacent to the yilgarn pilbara cratonic boundary during deposition and subsequent folding in the earaheedy basin the data also imply that the west australian and north australian cratons were in close proximity by 1 8 ga

palaeomagnetic cleaning strategies abstract cleaning is a term used in palaeomagnetism to refer to laboratory methods designed to demagnetize preferentially the less stable components of magnetization although the diversity of methods suggests that there is a wide choice available this is illusory except in simple cases the wrong cleaning strategy may yield misleading results cleaning methods include thermal magnetic alternating field or a f low temperature chemical shock and microwave cleaning a cleaning strategy involves one or more cleaning methods applied in a specific sequence the design of a strategy is made easier with the a priori knowledge of magnetic mineralogy and granulometry or in other words rock magnetic properties although not a substitute for other rock magnetic experiments the variation of low field susceptibility with temperature k t not only provides information on magnetic mineralogy and granulometry but it also draws attention to chemical alteration that may occur during thermal cleaning apart from the rapid though reversible decrease of k associated with curie temperatures other diagnostic k t features include the low temperature about 140 c peak typical of multidomain md magnetite the high temperature hopkinson peak typical of fine grained magnetite and other magnetic minerals and the classic lepidocrocite maghemite haematite profile which is irreversible examples are given where the remanence caused by lightning can be effectively eliminated only by applying a f or low temperature lt pre treatment such remanence may be an isothermal remanent magnetization irm or a combination of irm and anhysteretic remanent magnetization arm as the unblocking temperature of relatively low coercivity md grains may extend to high temperature it is often desirable to suppress md remanence an example is given of palaeointensity determination with the without lt pre treatment the nature of the lt transition is briefly addressed in the light of a rare double low temperature peak which may reflect both the isotropic point and the verwey transition

hydrothermal gold mineralization at the hira buddini gold mine india constraints on fluid evolution and fluid sources from boron isotopic compositions of tourmaline we determined the boron isotope and chemical compositions of tourmaline from the hira buddini gold deposit within the archean hutti maski greenstone belt in southern india to investigate the evolution of the hydrothermal system and to constrain its fluid sources tourmaline is a minor but widespread constituent in the inner and distal alteration zones of metabasaltic and metadacite host rocks associated with the hydrothermal gold mineralization the hira buddini tourmaline belongs to the dravite schorl series with variations in al fe fe mg ca ti and cr contents that can be related to their host lithology the total range of 11b values determined is extreme from 13 3 to 9 0 but 95 of the values are between 4 and 9 the boron isotope compositions of metabasalt hosted tourmaline show a bimodal distribution with peak 11b values at about 2 and 6 the wide range and bimodal distribution of boron isotope ratios in tourmaline require an origin from at least two isotopically distinct fluid sources which entered the hydrothermal system separately and were subsequently mixed the estimated 11b values of the hydrothermal fluids based on the peak tourmaline compositions and a mineralization temperature of 550 c are around 1 and 10 the isotopically lighter of the two fluids is consistent with boron released by metamorphic devolatilization reactions from the greenstone lithologies whereas the 11b rich fluid is attributed to degassing of i type granitic magmas that intruded the greenstone sequence providing heat and fluids to the hydrothermal system

a cautionary tale for palaeomagnetists a spurious apparent single component remanence due to overlap of blocking temperature spectra of two components the magnetic properties of the upper cretaceous red limestones from the montsec thrust sheet in the pyrenees reveal an unusual arrangement of the unblocking temperatures and coercivities of the magnetic mineral phases which leads to an apparently single component remanence being isolated on thermal demagnetization that this is an artefact of the almost complete overlap of the blocking temperature spectra of the two real components can be deduced from alternating field and thermal demagnetization of a composite isothermal remanent magnetization irm when two irm components are imparted to the sample at right angles to each other and each affecting grains with different coercivities thermal demagnetizations of the composite irm does not isolate the components but alternating field demagnetization does illustrating the superiority of the latter treatment in this case

development of a low temperature insert for the measurement of remanent magnetization direction using superconducting quantum interference device rock magnetometers data on the directional changes of a full magnetization vector during cycling to cryogenic temperatures can provide important insights into the low temperature magnetic properties of natural and synthetic materials these data also provide an empirical basis for the application of low temperature treatments in paleomagnetism for example the removal of viscous magnetization in magnetite bearing rocks however existing instruments only allow continuous measurement of magnetization along a single axis hampering experimental and theoretical advances in rock magnetism and the implementation of low temperature techniques into regular paleomagnetic practices here we describe development of a novel low temperature insert designed in collaboration with william s goree inc which allows measurement of directional behavior of a full magnetization vector during zero field low temperature cycling pilot experiments on well controlled polycrystalline samples of pseudo single domain psd and multidomain magnetite as well as on a natural sample containing psd magnetite indicate that the orientation of a saturation isothermal remanent magnetization sirm imparted at room temperature remains constant during low temperature cycling to 20 k this observation lends additional support to low temperature cycling as a cleaning technique in paleomagnetism the sirm imparted in an individual crystal of magnetite showed systematic albeit small changes upon both cooling and warming through the verwey temperature which may reflect switching between the easy magnetization directions however the switching effect may be significantly attenuated by crystallographic twinning in magnetite below the transition overall our results demonstrate the potential of the directional low temperature magnetometry for the advancement of our understanding of the properties of natural and synthetic materials

geology geochemistry and origin of volcanic rock hosted uranium deposits in northwestern nevada and southeastern oregon usa abstract northwestern nevada and southeastern oregon have the largest uranium u deposits in tertiary volcanic rocks in the us most deposits are in or adjacent to calderas or rhyolite lava dome fields and are hydrothermal almost all are associated with rhyolites that have high primary u concentrations 9 to 20 ppm but the rhyolites range from peralkaline to peraluminous caldera related occurrences are hosted by peralkaline rocks in the mcdermitt and virgin valley calderas at mcdermitt major deposits are along or just inboard of the southwestern kings river and northern ring fractures bretz opalite of the caldera complex the kings river deposits are strongly enriched in trace elements associated with epithermal precious metal deposits e g as au ag mo and sb much of the u occurs as hydrothermal uraniferous zircon and whole rock samples contain as much as 5 zr gangue minerals include quartz adularia fluorite and numerous sulfides 40 ar 39 ar dating of adjacent volcanic rocks and adularia indicates that mineralization was contemporaneous with igneous activity the bretz opalite deposits which include the large aurora resource 7 5 106 kg u3o8 are less enriched in epithermal trace elements and contain uraninite coffinite chalcedony opal and numerous sulfides at virgin valley deposits are mostly in tuffaceous sediments adjacent to ring fracture rhyolites trace element enrichment is minor u is hexavalent in all identified u minerals and associated phases include opal organic material and pyrite deposits in the lakeview u district oregon are hosted by weakly peraluminous rhyolite domes and tuffs and are highly enriched in epithermal trace elements the mineral assemblage includes coffinite opal and numerous sulfides minor u occurrences at buff peak nevada the newly recognized westernmost topaz peraluminous rhyolite in the us are also trace element enriched gangue minerals include quartz adularia and sulfides on the basis of their geologic setting geochemistry age and mineralogy the mcdermitt lakeview and buff peak deposits are hydrothermal sparse published fluid inclusion data indicate temperatures between 200 c and 330 c virgin valley deposits most likely are hydrothermal minor deposits in outflow ash flow tuffs are distant from any intrusive heat source and probably formed from low temperature groundwater

paleomagnetism and geochemistry of 1144 ma lamprophyre dikes northwestern ontario implications for the north american polar wander and plate velocities

metamorphic history of the hemlo gold deposit from al2sio5 mineral assemblages with implications for the timing of mineralization textural relations between al2sio5 phases and deformation fabrics provide constraints on the metamorphic history of the hemlo gold deposit kyanite in the deposit is most common within and on the margins of boudinaged quartz realgar veins and less commonly as rotated porphyroblasts within the matrix of schistose rocks kyanite predates the main d2 schistosity sillimanite postdates kyanite occurring irregularly as discrete knots and foliae that run parallel to but sometimes cut across the principal d2 foliation indicating that sillimanite postdates the d2 foliation we regard kyanite to be part of the peak metamorphic assemblage with sillimanite representing a partial later overprint most likely related to fluid infiltration rare andalusite occurs in two associations as late stage clean idioblastic crystals and as large fractured grains locally overprinted by sillimanite in boudinaged quartz realgar veins we suggest two possible origins for this second form of andalusite one involving generally late growth the second involving early growth prior to the development of peak metamorphic kyanite although not unambiguous we prefer the second scenario p t conditions from petrogenetic grid constraints and new geothermobarometric estimates indicate 6 7 gpa 600 650 c for the peak kyanite grade metamorphism ca 2677 ma and 4 5 gpa 600 c for the later sillimanite overprint ca 2672 ma we see these two events as part of an evolving p t path in a single metamorphic event in our early andalusite scenario the andalusite may have formed from pyrophyllite breakdown at 2 4 gpa 450 c possibly associated with emplacement of the regional suite of granodiorite plutons ca 2686 ma late andalusite formed sporadically on the retrograde path the occurrence of deformed andalusite and aligned kyanite in and on the margins of boudinaged auriferous realgar stibnite quartz veins provides evidence in support of a premetamorphic mineralization event

proterozoic paleomagnetism in the nipigon embayment of northern ontario pillar lake lava waweig troctolite and gunflint formation tuffs abstract by using unusual combinations of demagnetization techniques proterozoic paleomagnetic vectors and paleopoles are provided for two recently discovered post tectonic proterozoic units near armstrong northern ontario and also for well dated gunflint formation which by previous techniques yielded problematical paleomagnetic data the first paleomagnetic data are provided also for the seagull pluton and inspiration sills characteristic remanent magnetizations chrm for the pillar lake lavas indicate a keweenawan age more specifically 1000 1040 ma by comparison with the well established apwp for the late proterozoic superior craton four combinations of demagnetization techniques yield declinations in the range 108 133 and inclinations in the range 65 to 70 n 100 which define paleopoles near 200 w 48 n corresponding to a location on the keweenawan apwp near 1040 ma in the underlying basement a recently discovered proterozoic igneous complex the waweig troctolitic complex yields new paleomagnetic data with declination and inclination 42 54 n 14 defining a paleopole at 238 w 09 n its ages may be 1400 1600 or 2000 ma by comparison with the presently available ambiguous and sparsely populated apwp the first paleomagnetic results for the seagull pluton u pb age 1113 ma yield a mean declination of 87 4 75 7 n 32 corresponding to a keweenawan paleopole near 233 42 n consistent with other paleopoles near 1200 ma tuffs of the oft studied but problematical gunflint formation u pb age1878 ma yielded stable and presumably primary vectors using several different demagnetization techniques on the same specimens their mean primary declination and inclination 303 48 8 n 17 yields a paleopole now located near 178 w 42 n comparable with the published locations of paleopoles of 2000 ma of broader interest we recognized that low temperature demagnetization preceding conventional demagnetization techniques enhanced the isolation of characteristic vectors combining the conventional techniques thermal and af demagnetization also improved the resolution of characteristic vectors not achieved by other means low grade metamorphism affected the non tectonized proterozoic cover to the canadian shield due to burial or hydrothermal effects obfuscating or erasing primary vectors in some lithologies and especially at certain sites

pre and post folding magnetizations from the early devonian snowy river volcanics and buchan caves limestone victoria summary multicomponent magnetizations of the snowy river volcanics have been resolved into pre folding dec 340 1 inc 59 2 and post folding dec 10 5 inc 72 3 elements the former is attributed to initial cooling of the volcanics while the latter is probably related to the waning stages of the deformational episode which was presumably accompanied by cooling we argue that the pole position from the pre folding magnetizations lat 74 3 s long 222 7 e represents the australian palaeopole for the early devonian a reassessment of other australian pole positions determined from rocks of about this age suggests that remagnetization during the carboniferous may be common interpretation of the pole path prior to the devonian is equivocal and awaits further study the post folding magnetizations from the volcanics are similar in stability and direction to those from the buchan caves limestone dec 27 8 inc 75 6 which appears to be completely remagnetized the age of this remagnetizing event appears to be mid carboniferous if our interpretation of the pole path for this time is correct it is noted that a much unappreciated attribute of the fold test lies with identifying not just the relative age of magnetization but also the palaeohorizontal only when the palaeohorizontal is established can the calculation of a palaeopole be justified

influence of structural setting on sulphur isotopes in archean orogenic gold deposits eastern goldfields province yilgarn western australia the published mean 34s values of ore related pyrites from orogenic gold deposits of the eastern goldfields province yilgarn craton lie between 4 and 4 as for orogenic gold deposits worldwide most deposits have positive means and a restricted range of 34s values but some have negative means and wider ranges of 34s values wall rock carbonation and back mixing of similar source fluids with different fluid pathways can explain some of the more negative 34s signatures however structural setting appears to be the most important factor controlling ore fluid oxidation state and hence the distribution of 34s values in gold related pyrites shear hosted deposits appear to have experienced fluid dominated processes such as phase separation whereas stockwork vein hosted or disseminated deposits formed under conditions of greater rock buffering at victory defiance in particular negative 34s values are more common in gently dipping dilational structures compared to more compressional steeply dipping structures it appears most likely that fluid pressure fluctuations during fault valve cycles establish different fluid flow regimes in structures with different orientations rapid fluid pressure fluctuations in dilational structures during seismic activity can cause partitioning of reduced gas phases from the ore fluid during extreme phase separation and hence are an effective method of ore fluid oxidation leading to large local fluctuations in oxidation state it is thus not necessary to invoke mixing with oxidised magmatic fluids to explain 34s signatures indicative of oxidation in any case available robust geochronology in the eastern goldfields province does not support the direct involvement of oxidised magmatic fluids from adjacent granitic intrusions in orogenic gold genesis thus negative mean 34s values and large variations in 34s values of ore related pyrites in world class orogenic gold deposits are interpreted to result from multiple mechanisms of gold precipitation from a single ubiquitous ore fluid in varying structural settings rather than from the involvement of oxidised ore fluids from a different source such signatures are indicative but not diagnostic of anomalously large orogenic gold systems

geology and alteration in the golden mile kalgoorlie the giant kalgoorlie gold deposit 1 200 metric tons au recovered is surrounded by a wide wall rock alteration halo 1 x 3 km this alteration primarily represents the selectively pervasive formation of carbonate chlorite and localized intense pyrite and muscovite alteration which all formed synchronously with gold mineralization the carbonate alteration halo is interpreted as epigenetic because of evidence that carbonate chlorite assemblages overprint regional metamorphic fabrics furthermore the geometry of the alteration zone boundaries is controlled by ductile shear zones formed during regional deformation and the geochemistry of all the alteration zones suggests that one fluid was responsible for all assemblages there is no evidence of intense premetamorphic carbonation in the kalgoorlie area kalgoorlie is situated in the highly auriferous eastern goldfields province of the yilgarn block within a narrow corridor rift zone that contains several large gold deposits regional metamorphism has produced upper greenschist facies assemblages but has retained original rock textures at kalgoorlie the most important gold host is the golden mile dolerite 80 of gold produced a differentiated tholeiitic sill intruded between a lower mafic ultramafic pile including the paringa basalt the next most important gold host and overlying felsic rocks and metasediments the kalgoorlie syncline is the dominant tectonic structure in the main gold producing area of the golden mile adjacent to this infold of metasediments the golden mile dolerite and paringa basalt are the site of a number of ductile shear zones which formed synchronously with the syncline these zones and the many smaller shear zones between them have provided fluid channelways throughout the golden mile and are the site of most economic gold mineralization three alteration zones have been mapped in the golden mile dolerite on the basis of the mineralogic siting of fe i e the chlorite carbonate and pyrite zones the additional actinolite zone representing the unaltered regional metamorphic mafic assemblage has been converted to chlorite zone assemblages throughout the mined areas of the golden mile the carbonate zone bleached dolerite can be traced for 2 km through the main mineralized parts of the golden mile and to the full depth of lake view mine the boundary between the chlorite and carbonate alteration zones is locally sharp and controlled by ductile shear zones but is generally strata bound and preferentially developed in the more fe rich parts of the dolerite the pyrite zone which includes all economically gold mineralized areas is the site of the strongest muscovite alteration and is most intense and continuous in ductile shear zones the chlorite zone chlorite calcite or ankerite forms from mild carbonation of actinolite zone assemblages the carbonate zone ankerite siderite is a more local expression of this carbonation even after extensive carbonate alteration relative major element abundances are little changed and the main added components are limited to co 2 s k and rb the pyrite zone i e the gold lodes has ankerite muscovite quartz albite pyrite and minor siderite bearing assemblages it represents significant addition of sulfur the three alteration zones not only reflect differing degrees of alteration intensity but are also strongly influenced by host rock fe mg and this ratio largely determines the stability of chlorite favored by low fe mg and thus the distribution of the carbonate zone favored by high fe mg progressive deformation within the shear zones reworked existing mineralization kalgoorlie fits a metamorphic replacement model of gold formation by auriferous metamorphic fluids interacting with fe rich host rocks its great size is a function of a very well developed fluid channelway system via ductile shear zones and later fault zones and the favorable chemical nature of the host rocks especially the golden mile dolerite the full width of carbonation chlorite and carbonate zones reflects overlapping alteration envelopes around the closely spaced fluid pathways the processes responsible for the formation of most of the gold at kalgoorlie were essentially similar to those at several other archean gold deposits but on a larger scale

assembly of proterozoic australia implications of a revised pole for the 1070 ma alcurra dyke swarm central australia summary the pole position for the late mesoproterozoic alcurra formerly kulgera dyke swarm ads musgrave block central australia has been revised by adding new data from two more dykes a palaeomagnetic fold test is positive at the 99 percent confidence level and the pole position is latitude 2 8 s longitude 80 4 e dp 7 2 dm 10 7 new 40ar 39ar determinations on single biotite grains from country rock gneiss 20 cm from the contact with an alcurra dyke yield ages from 1068 2 ma to 1085 2 ma 2 in addition 40ar 39ar determinations on two samples of aggregates of very fine grained biotite from the mesostasis of a stuart dyke sd arunta inlier northern territory yield ages of 1059 2 and 1066 3 ma 2 these new ages are in agreement with those of the 1070 ma warakurna large igneous province west central australia indicating that both the ads and sd belong to that suite palaeomagnetic poles determined by others for the ads and sd are indistinguishable from each other but cannot be presently combined as a single pole because the published information on the sd is incomplete an analysis of the palaeogeography implied by the dispersed palaeomagnetic poles from members of the extended warakurna suite shows that the present relationships between the major precambrian cratons west north and south australian which constitute australia west of the tasman line are inconsistent with the inferred relationship at 1070 ma nor is it apparent that internal deformations within the separate cratons are the cause of the dispersion our findings imply that these major australian cratons must have finally assembled sometime after 1070 ma

paleomagnetism of the 1 88 ga sokoman formation in the schefferville knob lake area quebec canada and implications for the genesis of iron oxide deposits in the central new quebec orogen abstract thirty nine oriented block samples of iron formation were collected at 13 sites including opposite limbs of major folds from the 1 88 ga sokoman formation knob lake group in the schefferville knob lake area of the central new quebec orogen northern quebec the samples assayed up to 80 24 fe2o3t 54 08 fe implying fe enrichment of the iron formation up to ore grade anisotropy of magnetic susceptibility measurements on 245 standard specimens indicate a well preserved bedding parallel fabric in the iron formation suggesting minimal alteration of the magnetic mineralogy since deposition and or a mimetic secondary magnetic mineralogy the iron formation has not been internally deformed since the magnetic mineralogy was established analyses by variable field translation balance and x ray diffraction showed that the predominant magnetic mineral is hematite but a small amount of magnetite also is present in most samples following low temperature pre treatment as appropriate stepwise thermal and alternating field demagnetization of 218 specimens revealed a low temperature post folding component maximum tub 400 c d 27 1 i 20 1 95 10 9 from seven sites pole position of 40 6 s 257 0 e and components carried by magnetite maximum tub 580 c d 35 8 i 3 9 95 9 1 from 10 sites pole position of 29 6 s 250 9 e and hematite maximum tub 680 c d 40 0 i 1 6 95 18 6 from seven sites pole position of 26 8 s 247 0 e the components carried by magnetite and hematite are pre syn and post folding depending on the sampling site indicating that the magnetization was acquired continuously with deformation in the new quebec orogen at 1 84 1 83 ga no evidence was found for acquisition of magnetization during the mesozoic when many of the iron oxide orebodies in the schefferville knob lake area are thought to have formed our findings imply that an episode of fe enrichment of iron formation in the sokoman formation involved the circulation of hydrothermal fluids related to late paleoproterozoic orogenesis such orogenic circulation of fluids may have contributed to the development of hematitic orebodies in the central new quebec orogen

mineralogy geochemistry and fluid inclusions of the qinglong sb au deposit youjiang basin guizhou sw china abstract the qinglong deposit is a newly characterized large sb au deposit in the youjiang basin guizhou sw china four mineralization stages are identified i e pre ore early ore late ore and post ore in this study based on crosscutting relationships our study shows that the qinglong sb mineralization early ore stage was dominated by quartz stibnite fluorite whilst the sb au mineralization late ore stage comprises quartz stibnite pyrite electron probe microanalysis epma indicates that the au is present as au1 in the arsenian pyrite besides our new fluid inclusion data from fluorite stibnite and quartz show that the sb au rich hydrothermal fluids were derived from the mixing fluids consisting of h2o nacl hydrocarbon and minor co2 in the early ore stage the ore forming fluids are characterized by medium to low temperatures 161 294 c mean 220 c medium to low salinities 0 35 13 18 wt nacl equiv low ph and the presence of hydrocarbons and co2 in comparison the late ore stage ore forming fluids are featured by low temperatures 113 255 c mean 175 c low salinities 0 18 6 30 wt nacl equiv and nearly neutral ph the 34s s values 2 8 to 2 3 of the ore forming fluids suggest that the sulfur at qinglong was dominantly magmatic derived from the early ore to late ore stage the ore forming fluid temperature and oxygen fugacity fo2 decrease but the ph increases from acidic to neutral such physicochemical transition may have significantly decreased the sb solubility from 10 s to 100 s of ppm to 0 001 ppm therefore we propose that fluid mixing was the main process that substantially reduced the fluid temperature and oxygen fugacity which effectively lowered the sb solubility and led to abundant stibnite precipitation the stibnite precipitation had likely decreased the h2s concentrations of the ore forming fluids thereby destabilizing the auhs0 complexes in the fluids and caused local au precipitation

regional sulfate hematite sulfide zoning in the auriferous mariana anticline quadrilatero ferrifero of minas gerais brazil the distribution of mineral deposits characterised as barite deposits hematite rich auriferous deposits and auriferous tourmaline sulfide deposits displays a regional sulfate hematite sulfide zoning along the thrust delineated limbs of the mariana anticline in the south eastern part of the quadrilatero ferrifero of minas gerais brazil cross cut relationships of barite veins and sulfide lodes indicate that sulfidation occurred in a late tectonic context which is here attributed to the collapse of the 0 6 ga brasiliano thrust front reconnaissance s isotopic data from barite and pyrite antonio pereira barite deposit and its adjacent gold deposit respectively and arsenopyrite passagem de mariana gold deposit suggest a new interpretation for the hydrothermal fluid overprint in the mariana anticline the antonio pereira barite has 33s values that are near zero constraining the sulfate source to rocks younger than 2 45 ga the barite 34s values are between 19 6 and 20 8 the passagem arsenopyrite and tourmaline have co ni ratios that define a positive linear trend with the antonio pereira pyrite the latter has homogenous 34s values between 8 8 and 8 9 which are compatible with thermochemical reduction of aqueous sulfate with the s isotopic composition of the antonio pereira barite

the dharwar craton southern india interpreted as the result of late archaean oblique convergence abstract the dharwar craton comprises two distinct parts separated by a steep n s sinistral shear zone in the western part a pre 2900 ma complex of orthogneisses granodiorites and narrow tracts of supracrustal rocks sargur group forms the basement to volcanic and sedimentary basins of the dharwar supergroup ca 2800 2550 ma late archaean deformation is characterised by ne sw crustal shortening and steep n s sinistral shear zones the eastern part is underlain by parallel steep n s or nw se linear belts of calc alkaline anatectic and juvenile granites and granodiorites dharwar batholith ca 2750 2510 ma with intervening volcanic and sedimentary schist belts ca 2800 2550 ma the plutonic belts are 15 25 km wide up to 150 km long and bounded by steep nw se high strain zones up to 2 km wide with sinistral shear sense except one which is dextral magmatic state fabrics and structures in the plutonic rocks are parallel to solid state sinistral shear fabrics in the high strain zones but diffuse magmatic banding is commonly oblique to these zones and coincides with the plane of instantaneous shortening in sinistral shear magmatic state structures swarms of vertical nw se dykes of granite and the vertical wedge shape of the linear belts are consistent with emplacement of the batholith during sinistral shear when magma pressure exceeded regional horizontal compressive stress upright folds and schistosity steep reverse faults and effects of regional ht lp metamorphism show that deformation was partitioned into ne sw shortening in the schist belts during emplacement of the plutonic belts in the sinistral shear regime the western part of the craton is interpreted as the foreland to an accretionary arc represented by the batholith and schist belts intra arc basins in the east ne sw shortening and sinistral transcurrent displacements in the foreland and arc are consistent with arc normal and arc parallel displacements during oblique convergence analogous to mesozoic cenozoic convergent settings

paleomagnetism and u pb geochronology of the clarence head dykes arctic canada orthogonal emplacement of mafic dykes in a large igneous province the north south trending clarence head dyke swarm located on devon and ellesmere islands in the canadian high arctic has a trend orthogonal to that of the neoproterozoic franklin swarm that surrounds it the clarence head dykes are dated by the u pb method on baddeleyite to between 716 1 and 713 1 ma ages apparently younger than but within the published age range of the franklin dykes alpha recoil in baddeleyite is considered as a possible explanation for the difference in ages but a comparison of the u pb ages of grains of equal size from both swarms suggests that recoil distances in baddeleyite are lower than those in zircon and that the clarence head dykes are indeed a distinctly younger event within the period of franklin magmatism the clarence head dykes represent a large swarm tangential to and cogenetic with a giant radiating dyke swarm 800 km from the indicated source the preferred mechanism for the emplacement of the clarence head dykes is the exploitation of concentric zones of e

late proterozoic reconstructions of north west scotland and central canada magnetic fabrics paleomagnetism and tectonics red beds dominate the stoer group and the unconformably overlying torridon group having accumulated between 1400 and 1000 ma in a rift of the hebridean foreland the sequences were weakly strained and shortened e w and each underwent successive diagenetic changes which altered their magnetic properties and caused their different characteristic post depositional magnetizations anisotropy of magnetic susceptibility ams reveals an n s vertical tectonic foliation superimposed on the bedding planar ams sub fabric the tectonic ams sub fabric was isolated by comparing normalized and non normalized mean tensors of multiple specimens the n s vertical ams tectonic foliation postdates warps of bedding thus the high susceptibility minerals including those carrying palaeomagnetic signals re oriented or recrystallized after deposition and diagenesis thus characteristic remanence vectors chrm were acquired long after deposition first in the stoer group then in the torridon chrms were isolated for 143 stoer specimens and 94 torridon specimens using two cycles of low temperature demagnetization followed by at least 12 steps of thermal demagnetization for the stoer group two structurally integral groups of sites yield mean magnetizations of 317 0 43 1 95 11 3 n 49 and 309 9 52 1 95 10 6 n 23 some stoer specimens bear a torridon age overprint 133 3 525 95 9 6 n 19 the younger torridon group yields 138 4 52 2 95 7 4 n 42 134 9 45 5 95 16 0 n 15 and 134 4 55 1 95 11 3 n 19 from three structurally integral clusters of sites these late chemical magnetizations postdate bedding warps faults and tectonic ams fabrics they do not warrant local rigid body tilt corrections however we restored the chrm directions for the regional post cambrian eastwards tilt which affected the entire foreland from the lewisian basement to the overlying proterozoic strata paleopoles calculated from the strata s restored vectors were rotated into laurentian coordinates for comparison with canadian proterozoic palaeopoles the stoer group was magnetized in a normal polarity geomagnetic field whereas the torridon group was magnetized in a reversed polarity epoch however both sequences include at least one polarity transition indicating long duration chemical magnetizations successfully averaging secular variation at a minimum red bed magnetization processes exceeded 0 1 ma and may have encompassed several million years during stoer group magnetization the hebridean craton was at paleolatitudes of 26 44 n when the torridon group magnetized it was at 23 27 s accepting published ages this defines a minimum southward latitudinal displacement rate of 27 40 km ma restored stoer group paleopoles lie in the present day north central pacific ocean 190e 25n corresponding to the laurentian apwp dated at between 1090 and 1109 ma torridon group paleopoles lie in the south central pacific 210e 25s near the poorly documented 900 ma southernmost part of the laurentian apwp

paleosecular variation and time averaged field recorded in late pliocene holocene lava flows from mexico 1 this paper presents results from 13 paleomagnetic sites from an area west of mexico city and 7 sites from an area of dispersed monogenetic volcanism in the state of san luis potosi accompanied by seven 40ar 39ar radiometric dates an analysis of secular variation and time averaged paleomagnetic field in the trans mexican volcanic belt tmvb using compiled data both newly obtained and from the literature is presented interpretation can best be constrained after excluding from the data set sites that appear to be tectonically affected the selected data include 187 sites of late pliocene holocene age the mean direction among these sites is dec 358 8 inc 31 6 95 2 0 k 29 this direction does not overlap the expected geocentric axial dipole gad but is consistent with a gad plus a 5 quadrupole the virtual geomagnetic pole scatter of this group of sites 12 7 with lower and upper 95 confidence limits of 11 9 and 14 1 is consistent with the value expected from model g 13 6

no evidence for brunhes age excursions santo antao cape verde abstract we present new 40 ar 39 ar ages and palaeomagnetic directions from two re examined sequences of basaltic lava flows on the island of santo antao cape verde tarrafal and agua nova we find no evidence for the fully reversed brunhes age excursions reported in knudsen et al earth planet sci lett 206 199 214 2003 at tarrafal complex geology including repeated gravitational collapses and or considerable erosion followed by in filling of subsequent valleys by younger lavas coupled with an insufficient number of dated reversely and normally magnetised flows led earlier researchers to suggest a series of brunhes age excursions at agua nova the use of single samples to determine palaeodirections resulted in sampling of anomalous reversed directions creating apparent excursions we report new palaeomagnetic directions from agua nova based on multiple measurements per flow determined by both alternating field and thermal demagnetisation all flows of brunhes age have virtual geomagnetic pole latitudes 60 our findings highlight the need for precise age constraints and a high sampling density of dated flows with multiple directional results per flow when using long lava sequences to reconstruct palaeofield behaviour

determination of domain structure in igneous rocks by alternating field and other methods abstract af alternating field demagnetization arm anhysteretic remanent magnetization and strong field hysteresis properties of a large collection of mostly continental igneous rocks are reported here the collection included rocks whose magnetic carriers were believed from previous work to be of one of three types md multidomain sd psd single domain pseudo single domain or a bimodal mixture of md grains e g discrete opaques and sd psd material e g silicate inclusions two series of subaerial basalts with a full range of deuteric oxidation classes included examples of all three classes of behaviour sd psd rocks have relatively hard inflected af decay curves decay rate initially increasing then decreasing md rocks have soft exponential like decay curves and bimodal rocks have a combination of these characteristics relative hardnesses of normalized decay curves of remanences acquired in weak intermediate and strong fields the lowrie fuller test are also distinctively different for the three classes and the results support the theory developed in an accompanying paper 1 that lowrie fuller characteristics are an expression of the shapes of decay curves the lowrie fuller test although its result can be expressed as a numerical parameter is not capable of fine scale classification of domain structure or grain size the shape of the arm induction curve does have a quasi continuous variation with grain size however the parameter ar j rs initial anhysteretic susceptibility normalized to saturation remanence which is easily measured with standard paleomagnetic instrumentation is potentially useful for magnetic granulometry although ar itself was not diagnostic of grain size

chemical remagnetization and burial diagenesis testing the hypothesis in the pennsylvanian belden formation colorado lower pennsylvanian belden formation carbonate rocks from colorado were subjected to paleomagnetic rock magnetic and geochemical studies to test whether there is a connection between a widespread chemical remanent magnetization crm carried by authigenic magnetite and burial diagenesis thermal demagnetization results indicate the presence of two components of natural remanent magnetization nrm after removal of a low unblocking temperature nrm 250 c remanence that is interpreted to be a modern viscous magnetization an intermediate unblocking temperature 250 400 c remanence component with normal and reversed polarity tertiary directions is interpreted to be a thermoviscous remanent magnetization many limestones also contain a high unblocking temperature 400 570 c remanence component which is interpreted to be a crm fold tests from different parts of the basin indicate that the crm was acquired either before or during laramide folding this crm is interpreted to be carried by authigenic magnetite that formed by replacement of pyrite hysteresis ratios are consistent with those reported for other remagnetized carbonates and indicate that the crm is carried by single domain pseudo single domain magnetite although elevated 87sr 86sr values indicate passage of radiogenic fluids through the limestones the results of contact vein tests do not support the hypothesis that these fluids were responsible for the crm the time of crm acquisition which varies from late paleozoic to cretaceous coincides with the modeled time of organic matter maturation in different parts of the basin this suggests that diagenetic reactions that were triggered by low to moderate burial temperatures may have caused the magnetite authigenesis and probably gave rise to the crm

ltd thellier paleointensity of 1 2 ga nova floresta mafic rocks amazon craton 1 absolute paleointensity at 1 2 ga was obtained on basalts and gabbros from nova floresta formation amazon craton brazil magnetic carriers in these rocks are single domain to multi domain pure to nearly pure magnetite they present a strong decrease in remanence after low temperature demagnetization ltd accompanied by an increase in median destructive fields incorporation of ltd to thellier double heating measurements improved paleointensity estimates and enabled paleofield recovery for otherwise sterile rock samples giving twenty seven successful determinations for six sites 45 success rate site mean paleofield values are within 4 90 t and 15 55 t mean at 8 37 4 07 t these results correspond to a mean vdm of 2 08 0 94 1022 am2 thus increasing the number of low field determinations that dominate the still scarce precambrian database our study exemplifies the need for alternative approaches in the study of ancient rocks

can relative paleointensities be determined from the normalized magnetization of the wind blown loess of china although the wind blown loess and interbedded paleosol sequence in the loess plateau of china has been extensively studied and regarded as one of the most complete terrestrial records of both the geomagnetic field and climatic changes during the quaternary little attention has been paid to it as a source of relative paleointensity in this study we examine the malan loess l1 the last glacial sediments in the lingtai section to determine whether loess is able to reliably record the relative paleointensity toward that end both the conventional normalizing method as well as the pseudo thellier method tauxe et al 1995 were used in conjunction with the examination of the rock magnetic properties and natural remanent magnetization nrm rock magnetic properties of l1 show the uniformity in terms of magnetic mineralogy and grain size suggesting that they may be suitable for relative paleointensity studies normalized remanences derived from l1 show highs between both 72 66 ka and 62 51 ka and lows near 63 ka 42 ka and between 20 and 10 ka comparable results using different normalization parameters suggest that large amplitude variation of normalized magnetization may reflect the intensity variation of the geomagnetic field however comparisons of the normalized magnetization with contemporaneous marine records show some dissimilarities in particular the intensity low at 20 10 ka corresponding to the last glacial maximum is not compatible with the sint 800 composite record on the other hand spectral coherence analyses of middle and high frequency components of the normalized magnetizations suggest some climatic influence thus this apparent discrepancy may be explained by climatic changes in the loess plateau and the resulting effect on the nrm

the role of a transcrustal shear zone in orogenic gold mineralization at the ajjanahalli mine dharwar craton south india the ajjanahalli gold mine is spatially associated with a late archean craton scale shear zone in the eastern chitradurga greenstone belt of the dharwar craton india gold mineralization is hosted by an 100 m wide antiform in a banded iron formation original magnetite and siderite are replaced by a peak metamorphic alteration assemblage of chlorite stilpnomelane minnesotaite sericite ankerite arsenopyrite pyrite pyrrhotite and gold at ca 300 to 350 c elements enriched in the banded iron formation include ca mg c s au as bi cu sb zn pb se ag and te whereas in the wall rocks as cu zn bi ag and au are only slightly enriched strontium correlates with cao mgo co2 and as which indicates cogenetic formation of arsenopyrite and mg ca carbonates the greater extent of alteration in the fe rich banded iron formation layers than in the wall rock reflects the greater reactivity of the banded iron formation layers the ore fluids as interpreted from their isotopic composition 18o 6 5 8 5 initial 87sr 86sr 0 7068 0 7078 formed by metamorphic devolatilization of deeper levels of the chitradurga greenstone belt arsenopyrite chalcopyrite and pyrrhotite have 34s values within a narrow range between 2 1 and 2 7 per mil consistent with a sulfur source in chitradurga greenstone belt lithologies based on spatial and temporal relationships between mineralization local structure development and sinistral strike slip deformation in the shear zone at the eastern contact of the chitradurga greenstone belt we suggest that the ajjanahalli gold mineralization formed by fluid infiltration into a low strain area within the first order structure the ore fluids were transported along this shear zone into relatively shallow crustal levels during lateral terrane accretion and a change from thrust to transcurrent tectonics based on this model of fluid flow exploration should focus on similar low strain areas or potentially connected higher order splays of the first order shear zone

trading partners tectonic ancestry of southern africa and western australia in archean supercratons vaalbara and zimgarn original connections among the world s extant archean cratons are becoming tractable by the use of integrated paleomagnetic and geochronologic studies on paleoproterozoic mafic dyke swarms here we report new high quality paleomagnetic data from the similar to 2 41 ga widgiemooltha dyke swarm of the yilgarn craton in western australia confirming earlier results from that unit in which the primary origin of characteristic remanent magnetization is now confirmed by baked contact tests the corresponding paleomagnetic pole 10 2 degrees s 159 2 degrees e a 95 7 5 degrees in combination with newly available ages on dykes from zimbabwe allow for a direct connection between the zimbabwe and yilgarn cratons at 2 41 ga with implied connections as early as their cratonization intervals at 2 7 2 6 ga the proposed zimgarn supercraton was likely distinct from vaalbara kaapvaal plus pilbara at 2 4 ga but both of those entities independently fragmented at ca 2 1 2 0 ga reassembling into the kalahari and west australian cratons by 1 95 1 8 ga c 2012 elsevier b v all rights reserved

granitoid associated orogenic intrusion related and porphyry style metal deposits in the archean yilgarn craton western australia abstract the yilgarn craton hosts three types of archean granitoid associated metallogenic systems orogenic gold intrusion related and porphyry systems these systems may occur in the same terrane and share many broad characteristics including spatial coincidence with granitoids gangue silicate mineralogy metal associations and local structural controls features of archean orogenic gold systems spatially associated with granitoids include i a broad range of granitoid host compositions ii gold mineralization is mostly late with respect to yilgarn wide granitoid emplacement peak metamorphism and deformation iii orebodies are structurally controlled and iv gold deposition occurs over a large range of temperatures pressures and crustal depths 2 to 15 km from a co 2 rich low to moderate salinity reduced ore fluid that was derived from a metamorphic and or distal magmatic fluid source these systems do not display deposit scale vertical or lateral zonation of metals away from granitoid contacts host granitoid age composition or oxidation state are not important in controlling gold mineralization processes whereas structural setting and fluid flux are paramount archean intrusion related au mo w and mo au systems in the yilgarn craton are commonly small 2 bearing moderate to high salinity fluid during ore deposition archean porphyry cu mo au cu mo and cu au systems are spatially and temporally associated with volumetrically small pervasively altered felsic plutons and dikes surrounded by altered and mineralized supracrustal countryrock lateral zonation of metals away from porphyry centers commonly includes a decrease in cu au and ag au ratios with distance from the source pluton microthermometry studies on fluid inclusions in gold bearing veins suggest that early ore fluids were high temperature and high salinity and that mineralization occurred at 2 o 3 feo are demonstrated in phanerozoic terranes but are presently ill defined in the yilgarn craton crustal depth estimation is important in defining areas likely to have preserve shallow forming archean porphyry systems