stress slip and earthquakes in models of complex single fault systems incorporating brittle and creep deformations numerical simulations of slip evolution along a cellular vertical strike slip fault in an elastic half space are performed for several models representing discrete fault systems embedded in three dimensional elastic continua the geometry and imposed boundary conditions correspond approximately to the central san andreas fault the simulations incorporate brittle and creep deformations in series the net fault zone deformation rate is the sum of creep rate and frictional slip rate brittle fault properties are given by various distributions of earthquake stress drops on failing segments numerical cells the assumed distributions represent two idealized situations corresponding to different extreme states along an evolutionary path of a fault 1 a strongly disordered state characterized by a wide range of size scales representing immature fault zones and extended spatial domains and 2 a relatively regular state having a narrow range of size scales representing mature highly slipped faults the assumed creep properties are identical in all cases these are prescribed in terms of coefficients characterizing a power law dependency of creep slip rate on stress the combined brittle creep process and employed parameters lead to an overall pine tree stress depth profile with a brittle ductile transition depth of about 12 5 km and variable stress along strike profiles with brittle creep transition around 65 km nw of the 1857 rupture the spatial patterns of simulated hypocenters are statistically similar to observed data the results indicate that the range of size scales characterizing strong fault zone heterogeneities has important manifestations on the seismic response of a fault system a narrow range of size scales leads to frequency size statistics of earthquakes resembling the characteristic earthquake distribution and quasi periodic temporal distribution of large events as in the seismic gap hypothesis on the other hand a wide range of size scales leads to gutenberg richter power law frequency size statistics and random or clustered temporal distribution of large events the simulations demonstrate that treatment of the various observed forms of frequency size and temporal statistics of earthquakes can be unified through the concept of range of size scales characterizing fault zone heterogeneities this has a clear physical interpretation in terms of structural properties of a given fault zone or broad lithospheric domain and is supported by observed earthquake and fault data in some simulated cases the frequency size statistics of small earthquakes fall sharply below the self similar gutenberg richter line the results indicate that small earthquakes prepare the fault for the occurrence of a large event by smoothing during gradual tectonic loading the long wavelength components of stress on the fault this is done through short wavelength stress roughening associated with the numerous ruptures of the small events the above pattern of smoothing roughening of long short wavelengths of stress on the fault is reversed during large scale ruptures of the big events

iron oxide copper gold iocg deposits through earth history implications for origin lithospheric setting and distinction from other epigenetic iron oxide deposits the iron oxide copper gold iocg group of deposits initially defined following discovery of the giant olympic dam cu u au deposit has progressively become too embracing when associated deposits and potential end members or analogs are included the broader group includes several low ti iron oxide associated deposits that include iron oxide p rich iron oxide f and ree rich fe or cu au skarn high grade iron oxide hosted au cu carbonatite hosted cu ree and f rich and iocg sensu stricto deposits consideration of this broad group as a whole obscures the critical features of the iocg sensu stricto deposits such as their temporal distribution and tectonic environment thus leading to difficulties in developing a robust exploration model the iocg sensu stricto deposits are magmatic hydrothermal deposits that contain economic cu and au grades are structurally controlled commonly contain significant volumes of breccia are commonly associated with presulfide sodic or sodic calcic alteration have alteration and or brecciation zones on a large commonly regional scale relative to economic mineralization have abundant low ti iron oxides and or iron silicates intimately associated with but generally paragenetically older than fe cu sulfides have lree enrichment and low s sulfides lack of abundant pyrite lack widespread quartz veins or silicification and show a clear temporal but not close spatial relationship to major magmatic intrusions these intrusions where identified are commonly alkaline to subalkaline mixed mafic even ultramafic to felsic in composition with evidence for mantle derivation of at least the mafic end members of the suite the giant size of many of the deposits and surrounding alteration zones the highly saline ore fluids and the available stable and radiogenic isotope data indicate release of deep volatile rich magmatic fluids through devolatization of causative mantle derived magmas and variable degrees of mixing of these magmatic fluids with other crustal fluids along regional scale fluid flow paths precambrian deposits are the dominant members of the iocg group in terms of both copper and gold resources the 12 iocg deposits with 100 tonnes t resources are located in intracratonic settings within about 100 km of the margins of archean or paleoproterozoic cratons or other lithospheric boundaries and formed 100 to 200 m y after supercontinent assembly their tectonic setting at formation was most likely anorogenic with magmatism and associated hydrothermal activity driven by mantle underplating and or plumes limited amounts of partial melting of volatile rich and possibly metal enriched metasomatized early precambrian subcontinental lithospheric mantle sclm fertilized during earlier subduction probably produced basic to ultrabasic magmas that melted overlying continental crust and mixed with resultant felsic melts with devolatilization and some penecontemporaneous incorporation of other lower to middle crustal fluids to produce the iocg deposits preservation of near surface deposits such as olympic dam is probably due to their formation above buoyant and refractory sclm which resisted delamination and associated uplift most precambrian iron oxide p rich or magnetite apatite kiruna type deposits have a different temporal distribution apparently forming in convergent margin settings prior to or following supercontinent assembly it is only in the phanerozoic that iocg and magnetite apatite deposits are roughly penecontemporaneous in convergent margin settings the phanerozoic iocg deposits such as candelaria chile occur in anomalous extensional to transtensional zones in the coastal cordillera which are also the sites of mantle derived mafic to felsic intrusions that are anomalous in an andean context this implies that special conditions possibly detached slabs of metasomatized sclm may be required in convergent margin settings to generate world class iocg deposits it is likely that formation of giant iocg deposits was mainly a precambrian phenomenon related to the extensive mantle underplating that impacted on buoyant metasomatized sclm generally smaller and rarer phanerozoic iocg deposits formed in tectonic settings where conditions similar to those in the precambrian were replicated

a three dimensional fluid controlled earthquake model behavior and implications we describe the behavior of a three dimensional fluid controlled fault model that couples the dominant mechanical effects of fluid within a cellular fault zone with shear stress accumulation from constant plate motion applied at the downward continuation of the fault improvements from a previous model include long term plate motion loading and porosity creation through dilatant slip which allow the model to evolve to its steady state dynamic equilibrium the examined results include slip and slip deficit accumulation pore pressure buildup and release stress states the emergence of seismic scaling relationships and frequency size statistics of model earthquakes we find that asperities develop naturally within the model reflecting the disorganization of the evolving stress state in mohr space the dynamical interaction of shear stress and effective normal stress perturbs the initial uniform stress state to a complex state that produces transient asperity development along the fault these mohr space asperities spontaneously evolve disintegrate reemerge and migrate along the fault plane the general model behavior is independent of the state of the fluid pressure in four examined cases which span the range of possible fault zone overpressures the equilibrium condition is that which occupies all of the available mohr space maximum slip deficits along the fault depend on the degree of fault weakness ranging from about 3 m for a weak fault to over 30 m for a strong fault after 4000 years of model evolution for events that breach the surface the seismic moment scales with the cube of the source dimension mo l3 which reflects the slipped area times the depth extent of the rupture this scaling crosses isolines of stress drop for confined events mo l2 along isolines of stress drop but no general scaling emerges clusters emerge between stress drop versus seismic moment and stress drop versus source dimension with large events converging to average stress drops of about 8 mpa for a weak fault and about 20 mpa for a strong fault

dynamics of izmit earthquake postseismic deformation and loading of the duzce earthquake hypocenter we have developed dynamic finite element models of izmit earthquake postseismic deformation to evaluate whether this deformation is better explained by afterslip via either velocity strengthening frictional slip or linear viscous creep or by distributed linear viscoelastic relaxation of the lower crust we find that velocity strengthening frictional afterslip driven by coseismic shear stress loading can reproduce time dependent global positioning system data better than either linear viscous creep on a vertical shear zone below the rupture or lower crustal viscoelastic relaxation our best frictional afterslip model fits the main features of postseismic slip inversions in particular high slip patches at and below the hypocenter and on the western karadere segment and limited afterslip west of the hersek delta burgmann et al 2002 the model requires a weakly velocity strengthening fault that is either low effective normal stress in the slipping regions or a smaller value for the parameter describing rate dependence of friction a b than is indicated by laboratory experiments our best afterslip model suggests that the coulomb stress at the duzce hypocenter increased by 0 14 mpa 1 4 bars during the izmit earthquake assuming right lateral slip on a surface dipping 50 to the north and by another 0 1 mpa during the 87 days between the izmit and duzce earthquakes in the marmara sea region within about 160 km of the izmit earthquake rupture this model indicates that the coulomb stresses increased by 15 25 of the coseismic amount during the first 300 days after the earthquake three hundred days after the earthquake postseismic contributions to coulomb stressing rate on the maramara region faults had fallen to values equal to or less than the inferred secular stress accumulation rate our estimates of postseismic coulomb stress are highly model dependent in the marmara region the linear viscous shear zone and viscoelastic lower crust models predict greater postseismic coulomb stresses than the frictional afterslip model near field stress and fault zone rheology estimates are sensitive to the earth9s elastic structure when a layered elastic structure is incorporated in our model it yields a coulomb stress of 0 24 mpa at the duzce hypocenter significantly more than the 0 14 mpa estimated from the uniform elastic model because of the higher near field coseismic stresses the layered elastic model requires a higher value of velocity strengthening parameter a b a b times effective normal stress r to produce comparable postseismic slip a b is estimated at 0 4 and 0 2 mpa respectively for the layered and uniform elastic models these results highlight the importance of understanding the earth9s elastic structure and the mechanism for postseismic deformation if we wish to accurately model coseismic and postseismic crustal stresses

shrimp u pb geochronology of detrital zircons from the treasure lake group new evidence for paleoproterozoic collisional tectonics in the southern hottah terrane northwestern canadian shield the recently defined treasure lake group is a platformal sedimentary sequence more than 3 km thick that was deformed and metamorphosed prior to formation of the great bear continental arc during 1875 1845 ma the group is exposed as remnants in the southern part of the arc its basement is not exposed but is interpreted to comprise the paleoproterozoic hottah terrane which is exposed at hottah lake in the northern part of the arc studies in that region show that the hottah terrane collided with the archean slave craton to the east ca 1883 ma following reversal of subduction polarity from west to east continued eastward subduction under the collision zone formed the great bear arc the present study was undertaken to determine the age and provenance of the treasure lake group thirty nine detrital zircon grains from a sample of a quartz arenite unit in the middle part of the treasure lake group were analyzed by shrimp sensitive high resolution ion microprobe they show an age range from ca 2729 to

kiruna type deposits their origin and relationship to intermediate subvolcanic plutons in the great bear magmatic zone northwest canada magnetite apatite actinolite rocks similar to those at kiruna sweden and the st francois mountains missouri are associated with seven epizonal plutons that intrude andesitic stratovolcanoes of a broadly folded early proterozoic continental magmatic arc located in the northwestern canadian shield the plutons mainly medium grained quartz monzonite monzodiorite diorite iugs sheets and laccoliths 5 to 25 km in diameter are spatially temporally and compositionally related to host andesitic stratovolcanoes they were emplaced at 2 to 3 km depth are compositionally heterogeneous metasomatically altered and had plagioclase as the liquidus phase in addition they have kilometer wide alteration halos comprising an inner zone of nearly complete wall rock albitization an intermediate zone of magnetite apatite actinolite veins pods breccias and wall rock replacement and an outer zone of disseminated sulfides one pluton balachey has a halo that is more or less completely outside the body but another rainy lake has a halo that is partly superimposed over it separation of iron phosphate and silicate melts by liquid immiscibility is not supported by 1 the gradual replacement of plagioclase by chessboard albite toward the roof of the rainy lake pluton 2 the chemical trends in the rainy lake intrusion 3 the mineralogical zoning low temperature mineralogy and replacement textures of the magnetite apatite actinolite and 4 the relationship of the magnetite apatite actinolite veins and bodies to the plutons however all available data are compatible with a deuteric origin it is concluded that high temperature low initial water content and shallow level emplacement by intermediate plutons are necessary for the development of kiruna type deposits

geophysical and petrophysical study of an iron oxide copper gold deposit in northern sweden abstract a geophysical petrophysical study has been performed in an area wsw of the city of kiruna northern sweden the sub regional tectonic setting is dominated by two important shear zones which define the boundary of a granitic body many cu fe occurrences are located in proximity of faults related to these major deformation zones particular attention has been given to the tjarrojakka iron oxide copper gold iocg deposit here the bedrock is characterised by intermediate to mafic meta volcanics metamorphosed intermediate to mafic dykes and gabbroic dioritic intrusions of svecofennian ages 1 96 1 75 ga the major cu and fe occurrences are hosted by the meta andesites the aim of the study is to put the deposits into a tectonic framework and test existing hypotheses for their occurrences glacial deposits cover almost the entire area leading to a scarcity of outcrops and inferring that geophysical data are fundamental for geological understanding in addition to this petrophysical analysis is vital for the interpretation of geophysical data gravity airborne magnetics and radiometrics very low frequency and for the definition of geophysical signatures of the deposits the anisotropy of magnetic susceptibility ams was also studied for the tectonic analysis more than 150 oriented samples were collected in a number of outcrops along a profile intersecting the major structures in the tjarrojakka area from the airborne magnetic data two major linear features are interpreted as deformation zones the strike of these deformation zones is approximately nw se and e w respectively the same trends have been defined from other geophysical data such as airborne vlf and ground gravity data a third important structural trend striking sw ne has been defined by k th data and ground magnetic data very good agreement has been found between geophysical lineaments and ams directions magnetic foliations determined by ams measurements confirm the existence of three major trends in the study area sw ne e w and nw se the major fe orebody shows approximately a sw ne strike direction as defined from ground magnetic data this is parallel to the strike of magnetic foliation determined in outcrops 1 km nw of the deposit the epigenetic nature of the cu and fe occurrences in tjarrojakka and their spatial relationship with deformation zones suggest a connection between the formation of the deposits and a tectonic event a later tectonic episode resulted in e w trending deformation in the central area affecting the orebodies themselves other probable compressive deformations have been indicated from petrophysical and geophysical analyses thermomagnetic measurements indicate that fe oxides ti magnetite are common in the area while fe sulphides are almost absent multi domain magnetite has been identified as the most common fe oxide in different rock types while an unstable magnetic mineral has been detected in metamorphosed volcanics a good spatial correlation has been observed between cu deposits and high k th values from radiometric data values that are expressions of potassic alteration

effects of normal stress variations on frictional stability of a fluid infiltrated fault this paper extends the spring slider model of segall and rice 1995 for dilating fluid infiltrated faults to include geometry induced normal stress variations because of the coupling between normal and shear stresses on a dipping plane the shear stress drop accompanying slip is associated with a decrease in normal stress for reverse faulting and with an increase for normal faulting this coupling stabilizes frictional sliding on normal faults and destabilizes it on reverse faults when a fluid phase is present these normal stress variations during slip events also trigger through poroelastic coupling compensating pore pressure changes stabilizing pressure decreases for reverse geometry and destabilizing increases for normal geometry fault stability is quantitatively studied using two parameters the critical spring stiffness k cr separating the stable and unstable slip domains and when numerical simulations exhibit stick slip cycles the stress drop during the slip phases as expected geometric coupling is found to reduce k cr and at for normal faults in comparison with segall and rice s 1995 strike slip geometry and to increase these parameters for reverse faults poroelastic coupling shows opposite consequences the combined effect of shear induced dilatancy and poroelastic coupling is stabilizing for reverse geometry but can be either stabilizing or destabilizing for normal geometry depending on the dilatancy magnitude nevertheless the stability differences between the types of faults are primarily controlled by the geometric coupling between shear and normal stresses for all the parameters we investigated normal faults exhibit lower stress drops and critical stiffnesses than reverse faults finally extrema appearing in the graphs featuring at or k cr versus the diffusion rate demonstrate that the influence of the fluid effects is not maximum in the undrained case surprisingly their influence is maximum at a point intermediate between the drained and undrained limits when the diffusion time is roughly equal to the frictional relaxation time associated with stick phases this phenomenon is presumably caused by subtle second order mechanisms involving the instantaneous pore pressure changes induced by the poroelastic coupling in response to instantaneous friction changes and the interseismic pressure recovery process

tectonomagmatic evolution of the southern great bear magmatic zone northwest territories canada implications for the genesis of iron oxide alkali altered hydrothermal systems the great bear magmatic zone in northwestern canada is a paleoproterozoic volcano plutonic belt of high k calc alkaline to shoshonitic affinity interpreted as a continental arc that formed between 1 87 and 1 85 ga following the short lived calderian orogeny tectonomagmatic evolution of this magmatic zone favored the formation of multiple iron oxide and alkali alteration systems within a time frame of 10 m y as constrained geochronologically within error between 1875 and 1865 ma this illustrates a temporal and genetic relationship between shoshonitic to high k calc alkaline continental arc magmatism and the formation of iron oxide rich deposits and alkali alteration associated with iron oxide copper gold iocg mineralization early rhyolitic magmatism formed the basal sequence of the lou assemblage under a compressive or transpressive tectonic regime between 1872 and 1867 ma an apparent higher magmatic production is linked to a marked change in composition of the volcanic and plutonic rocks from rhyolite to intermediate felsic and locally mafic compositional homogeneity of the intrusive and volcanic rocks of the mazenod and bea assemblages termination of ductile to brittle ductile deformation along the main deformation zones and transition to widespread brittle fracturing and breccia formation are interpreted to reflect a change in the regional stress regime from compressional transpressional to extensional transtensional this change in stress regimes is associated with iron oxide rich mineralization that initially formed the nico au bi co deposit followed by iocg mineralization in the nico and sue dianne deposits as well as albitite hosted u mineralization in the southern breccia

complexity of seismicity due to highly rate dependent friction we study a simple antiplane fault of finite length embedded in a homogeneous isotropic elastic solid in order to understand the origin of seismic source heterogeneity in the presence of nonlinear rate dependent friction all the mechanical properties of the medium and friction are assumed to be homogeneous starting from a heterogeneous initial stress distribution we apply a slowly increasing uniform stress load far from the fault and we simulate the seismicity for more than 20 000 events in some cases the style of seismicity produced by this model is determined by a control parameter which measures the degree of rate dependence of friction for classical friction models with rate independent friction no complexity appears and seismicity is perfectly periodic for weakly rate dependent friction seismicity becomes slightly nonperiodic but most events are still characteristic earthquakes when friction is highly rate dependent seismicity is completely irregular and ruptures of all sizes occur inside the fault highly rate dependent friction destabilizes the healing process producing premature healing of slip and partial stress drop premature healing causes rupture to take the form of narrow propagating slip episodes the so called heaton s 1990 pulses partial stress drop produces large variations in the state of stress which in turn produce earthquakes of different sizes we make the conjecture that all models in which static stress drop is only a fraction of the dynamic stress drop produce stress heterogeneity

fault model for preseismic deformation at parkfield california we construct an earthquake instability model to estimate precursory faulting and ground deformation before the next moderate m 5 5 6 earthquake on the san andreas fault near parkfield california the quasi static model simulates fault slip fault shear stress and ground deformation for all stages of repeated earthquake cycles unstable slip which is the analog of a moderate parkfield mainshock is caused by overall failure of a postulated strong area or patch of the fault zone the brittle patch and surrounding weaker viscous fault are defined by the sign of a spatially varying coefficient of a slip rate dependent fault law stable failure of the patch leading up to mainshock failure causes slip rate to increase at depth amplitudes and timescales of the resulting preseismic deformation anomalies are found for the current earthquake cycle at parkfield starting just after the 1966 mainshock in two ways in the first way we vary parameter values in a sequence of simulations so as to find the simulation giving best agreement with locations moments and recurrence times of past moderate earthquakes and also with fault creep and trilateration line length changes since the 1966 mainshock in the second way we assign values of most parameters from laboratory friction experiments and a temperature depth profile inferred from heat flow data overall agreement between model results and observations is comparable for the two methods in all simulations preseismic deformation anomalies arise from slip rate increase near the model earthquake focus and start several years or less before instability preseismic anomalies in surface fault creep and line length are always too small to detect in current measurements however for some simulations constrained mainly by field data the predicted anomalies in borehole dilatation are large enough to detect

quantitative measure of the variation in fault rheology due to fluid rock interactions we analyze friction data from two published suites of laboratory tests on granite in order to explore and quantify the effects of temperature t and pore water pressure pp on the sliding behavior of faults rate stepping sliding tests were performed on laboratory faults in granite containing gouge granite powder both dry at 23 to 845 c lockner et al 1986 and wet pp 100 mpa at 23 to 600 c blanpied et al 1991 1995 imposed slip velocities v ranged from 0 01 to 5 5 m s and effective normal stresses were near 400 mpa for dried granite at all temperatures and wet granite below 300 c the coefficient of friction shows low sensitivity to v t and pp for wet granite above 350 drops rapidly with increasing t and shows a strong positive rate dependence and protracted strength transients following steps in v presumably reflecting the activity of a water aided deformation process by inverting strength data from velocity stepping tests we determined values for parameters in three formulations of a rate and state dependent constitutive law one or two state variables were used to represent slip history effects each velocity step yielded an independent set of values for the nominal friction level five constitutive parameters transient parameters a b1 and b2 and characteristic displacements dc1 and dc2 and the velocity dependence of steady state friction ss ln v a b1 b2 below 250 data from dry and most wet tests are adequately modeled by using the slip law ruina 1983 and one state variable a 0 003 to 0 018 b 0 001 to 0 018 dc 1 to 20 m dried tests above 250 can also be fitted with one state variable in contrast wet tests above 350 require higher direct rate dependence a 0 03 to 0 12 plus a second state variable with large negative amplitude b2 0 03 to 0 14 and large characteristic displacement dc2 300 to 4000 m thus the parameters a b1 and b2 for wet granite show a pronounced change in their temperature dependence in the range 270 to 350 c which may reflect a change in underlying deformation mechanism we quantify the trends in parameter values from 25 to 600 c by piecewise linear regressions which provide a straightforward means to incorporate the full constitutive response of granite into numerical models of fault slip the modeling results suggest that the succeptibility for unstable stick slip sliding is maximized between 90 and 360 c in agreement with laboratory observations and consistent with the depth range of earthquakes on mature faults in the continental crust

identifying mineral prospectivity using seismic and potential field data in the hongniangyu district inner mongolia china abstract since the discovery of several large porphyry mo deposits 0 1 mt mo in the jining region of inner mongolia china during the past decades the area has been receiving wide interest for mining activities however mineral exploration in this area is challenging because most of it is thickly covered by cenozoic sediments geophysical surveys especially the seismic method are becoming increasingly important for investigating deep seated ore controlling structures e g porphyry granites faults and even orebodies supporting the discovery of buried ore deposits recently potential field data and seismic reflection data have been acquired for the hongniangyu area south of jining region in this study we mainly use seismic reflection and potential field data to obtain subsurface geological model in support of mineral exploration targeting gravity and magnetic data were firstly used for spatial analysis of mineral system by mapping the distribution of the faults concealed granites and the sedimentary basin through wavelet analysis tilt angle and euler deconvolution to address the common problem of low signal to noise ratio of seismic reflection data obtained in hard rock terranes we applied a sophisticated workflow including refraction static correction noise filtering surface consistent deconvolution and velocity analysis the obtained post stack migrated seismic profile clearly imaged the geometry of the concealed graben located at the north end of the datong basin and the south east dipping kouquan normal fault controls the depression of the graben and offsets cenozoic sediments by several hectometers up to 500 m joint modeling of seismic gravity and magnetic data further provided evidence of the three buried belts of granites intriguingly one of the concealed granitic belts located in the north of hongniangyu area was found to have both significant chargeability and mo geochemical anomalies indicating a high potential of hosting porphyry type mo ore deposits as expected mo and w mineralized cores were discovered in this target area but limited to only one deep drilling hole the presented results provide the geological framework of hongniangyu district and guidelines for delineating new potential mo mineralization in the jining region

a special issue devoted to proterozoic iron oxide apatite ree and iron oxide copper gold and affiliated deposits of southeast missouri usa and the great bear magmatic zone northwest territories canada preface this special issue is focused on the geology and mineral deposits in the mesoproterozoic st francois mountains terrane of southeast missouri united states and in the paleoproterozoic great bear magmatic zone of northwest territories canada contributions in this special issue emphasize iron oxide apatite ioa rare earth element ree and iron oxide copper gold iocg and affiliated deposits e g hitzman 2000 barton 2014 the volume includes results of studies by the u s geological survey usgs and geological survey of canada gsc derived from research done for the usgs mineral resources program and from gsc research completed under auspices of the geomapping for energy and minerals and targeted geoscience initiative programs these studies took advantage of extensive historic drilling at and near ioa and iocg deposits in missouri and of exceptional field exposures in the great bear magmatic zone collectively results of this cumulative body of work provide new insights into the geological geochemical geophysical and temporal characteristics of ioa ree iocg and affiliated deposits leading to enhanced understanding of diverse magmatic hydrothermal systems and refining of genetic models and exploration strategies for such deposits within these terranes and elsewhere in the world both north american terranes contain iron oxide alkali alteration zones that formed during predominantly i type calc alkaline including s type and shoshonitic in the great bear zone volcano plutonic magmatism later magmatism in both terranes was mainly of a type affinity locally the ioa deposits are spatially associated with high ree concentrations the iocg and affiliated deposits include co and birich variants skarns polymetallic veins and albitite hosted u deposits are also present within the footprints of the regional iron oxide alkali alteration systems mineralization occurred contemporaneously with mafic to felsic volcanism and local volcanogenic sedimentation including within or marginal to large ash flow calderas proterozoic rocks in both the st francois mountains terrane and the great

an earthquake instability model based on faults containing high fluid pressure compartments it has been proposed that large strike slip faults such as the san andreas contain water in seal bounded compartments arguments based on heat flow and stress orientation suggest that in most of the compartments the water pressure is so high that the average shear strength of the fault is less than 20 mpa we propose a variation of this basic model in which most of the shear stress on the fault is supported by a small number of compartments where the pore pressure is relatively low as a result the fault gouge in these compartments is compacted and lithified and has a high undisturbed strength when one of these locked regions fails the system made up of the neighboring high and low pressure compartments can become unstable material in the high fluid pressure compartments is initially underconsolidated since the low effective confining pressure has retarded compaction as these compartments are deformed fluid pressure remains nearly unchanged so that they offer little resistance to shear the low pore pressure compartments however are overconsolidated and dilate as they are sheared decompression of the pore fluid in these compartments lowers fluid pressure increasing effective normal stress and shear strength while this effect tends to stabilize the fault it can be shown that this dilatancy hardening can be more than offset by displacement weakening of the fault i e the drop from peak to residual strength if the surrounding rock mass is sufficiently compliant to produce an instability slip will propagate along the fault until the shear fracture runs into a low stress region frictional heating and the accompanying increase in fluid pressure that are suggested to occur during shearing of the fault zone will act as additional destabilizers however significant heating occurs only after a finite amount of slip and therefore is more likely to contribute to the energetics of rupture propagation than to the initiation of the instability

early proterozoic geomagnetic field in western laurentia implications for paleolatitudes local rotations and stratigraphy abstract the east west trending post orogenic subvertical cleaver dykes from the great bear magmatic arc are dated by the u pb baddeleyite method to be 1740 5 4 ma they have a mean direction of magnetization of d i 136 5 and 57 4 k 64 95 4 5 based on samples from 17 diabase dykes a mean location 65 7 n 118 03 w and a corresponding paleopole at 19 4 n 83 3 w a95 6 1 studies of baked contacts country rocks distant from dykes and later intrusions that cut them show that the dykes became magnetized soon after intrusion the paleopole is not significantly different from that obtained from the 1759 1 ma post orogenic jan lake granite from the hudsonian orogen in southeast saskatchewan these paleopoles fall among a cluster previously determined from post orogenic overprints in a wide variety of rocks and geological settings across laurentia their mean is 20 8 n 94 5 w a95 5 2 collectively they record a widespread remagnetization episode following the hudsonian orogeny during which the polarity of the field remained constant this single polarity interval the cleaver superchron has a mean age of approximately 1747 ma and its duration is at least 13 million years but is probably much longer with further refinement it could become a global statigraphical marker there are 18 paleopoles from red beds and igneous rocks which record the paleofield in western laurentia mainly over an extended interval we estimate to be at least 100 million years between about 1960 and 1830 ma during the hudsonian orogeny over this interval western laurentia remained at a low and essentially constant latitude reversals of the geomagnetic field occurred opposed magnetizations being of approximately equal frequency there are numerous magnetochrons which could in future assist in regional stratigraphic correlations on the basis of six studies we estimate that the reference paleopole for this interval is at 1 1 s 71 7 w a95 12 1 n 6 paleopoles from the other 12 studies are spread over a 100 arc indicating large variable rotations about local vertical axes which probably resulted from transcurrent faulting caused by indentation of the bordering archean slave and superior cratons during the hudsonian orogeny these reference paleopoles show that during the hudsonian orogeny western laurentia was in paleolatitudes of 15 30 during post hudsonian time ca 1750 ma paleolatitudes were about 20 higher paleowind directions inferred by others indicate that laurentia during these intervals was in the northern hemisphere

gold refining by bismuth melts in the iron oxide dominated nico au co bi cu w deposit nwt canada the nico au co bi cu w deposit is located in the great bear magmatic zone nwt canada where numerous polymetallic iron oxide dominated mineralized systems have been recognized petrographic electron microprobe analysis empa and laser ablation inductively coupled plasma mass spectrometry la icp ms studies of host rock alteration and ore mineralogy together with sulfarsenide geothermometry have been carried out to constrain the nature of alteration and or mineralization assemblages in this deposit metasedimentary rocks of the treasure lake group host nico and are pervasively altered to an assemblage of ferrohornblende i actinolite i biotite i magnetite i orthoclase which is cut by barren veins composed of quartz ferrohornblende orthoclase calcite set 1 these alteration events are overprinted by metasomatic prograde and retrograde mineralized assemblages and both brittle and ductile deformation accompanied the metasomatism the prograde assemblage 400 c consists of cobaltite co rich loellingite and co rich arsenopyrite stage i magnetite ii ferrohornblende ii actinolite ii biotite ii pyrite and minor scheelite and orthoclase the earliest retrograde mineralization consists of arsenopyrite stages ii and iii which contains variable amounts of co together with native bi bismuthinite and au with lesser magnetite marcasite pyrite hastingsite and minor quartz the preservation of solidified native bi droplets suggests a temperature range of 270 to 400 c for precipitation of this assemblage the final stage of retrograde mineralization consists of a chalcopyrite bismuthinite hematite chlorite assemblage together with hastingsite emplectite which formed at temperatures of less than 270 c textural and trace element evidence indicates that the au and bi present within arsenides and sulfarsenides in the nico system resulted from the initial partitioning of structurally bound au and or invisible nanometer sized particles of au and bi into the prograde sulfarsenide and arsenide phases which contain up to 81 ppm au the au and bi were remobilized following retrograde alteration of those minerals to arsenopyrite ii molten bi droplets are interpreted to have scavenged au insitu resulting in the formation of the bi au inclusions observed in arsenopyrite ii the second mechanism of gold refining is explained by the occurrence of contemporaneous bi te melt and hydrothermal fluids that also could have fractionated gold during transport in solution and deposited it in fractures interstitially to earlier mineral grains and as disseminations within ca fe amphibole magnetite biotite altered rocks overall the gold upgrading at nico is consistent with the liquid bismuth collector model suggesting that this process was an important control on gold concentration in this and potentially other au bi te fe as s rich iron oxide copper gold iocg deposits

earthquakes slow down for safety in december 1992 part of the san andreas fault ruptured no damage was caused because instead of lasting for a few seconds this earthquake sequence took a week so why are other earthquakes fast

shear heating of a fluid saturated slip weakening dilatant fault zone 2 quasi drained regime 1 this paper analyzes slip on a fluid infiltrated dilatant fault for imposed tectonic strain rates much slower than the rate of fluid exchange between the gouge zone and the surroundings and for exchange of heat slower than of fluid typical of interseismic or most laboratory loading conditions the limiting solution corresponding to the infinitely rapid drainage rate developed in the companion paper by garagash and rudnicki 2003 yielded unbounded slip acceleration for sufficiently large slip weakening of the fault analysis for rapid but finite drainage rates quasi drained condition reveals two types of possible instability the first is inertial instability a seismic event characterized by an unbounded slip rate and attributed to the destabilizing effects of shear heating and fault slip weakening the second corresponds to loss of uniqueness and arises from the small pressure increase caused by shear heating for rapid but finite rates of drainage this instability emerges for even small amounts of thermomechanical coupling and a large range of dilatancy and slip weakening its resolution probably requires a more elaborate frictional description e g rate and state but within the slip weakening framework here leads to either slip arrest or inertial instability the response is always unstable in one of these two ways for sufficiently large thermal coupling or initial stress regardless of the amount of slip weakening and dilatancy the counterintuitive stabilizing effect of increased slip weakening or decreased dilatancy similar to the effect in the companion paper also occurs for nearly drained slip

crustal architecture beneath the southern midcontinent usa and controls on mesoproterozoic iron oxide mineralization from 3d geophysical models abstract several types of critical mineral bearing ore deposits in the southern midcontinent region of the u s are hosted in mesoproterozoic igneous rocks largely concealed beneath paleozoic cover discerning the architecture of igneous intrusions and volcanic centers in the crust is fundamental to understanding the geologic evolution of this vast region and its mineral resources to advance the understanding of the geologic framework beneath the southeast missouri iron metallogenic province we invert continental scale magnetic and gravity anomaly data to three dimensional 3d physical property models the regional models image altered and mineralized igneous rocks near the proterozoic basement surface and underlying intrusive complexes that extend down to the moho at shallow crustal levels our models confirm that iron oxide apatite rare earth element ioa ree deposits and iron oxide copper gold cobalt iocg deposits occur within or near the edges of large low density low susceptibility early mesoproterozoic ca 1 4 ga silicic calderas and ca 1 3 ga granitic plutons previous isotopic and geochemical studies conclude that the iron deposits and their volcanic host rocks originated from mantle derived and crustal melts that erupted during regional extension extension was associated with thermal event s that produced the large scale silicic magmatism related to the ca 1 45 ga eastern granite rhyolite province egrp and the 1 35 ga southern granite rhyolite province sgrp we postulate that early in the evolution of the egrp several trans crustal magmatic plumbing systems developed that are evident in the 3d models the southeast missouri metallogenic province is underlain by one such magmatic system that is expressed as a northwest trending 50 km wide by 200 km long elongate track of high susceptibility at deep crustal levels the high susceptibility corridor splays upward through the crust to the proterozoic surface where the iron deposits are the epigenetic manifestation of this magmatic event our findings confirm that the iron deposits with no distinct connection at the surface are connected to one large magmatic system at depth we propose that other similar susceptibility tracks which are present along the top of the mantle mark additional feeder zones that allowed magma to ascend to the main eruptive centers that produced the granite rhyolite provinces the early mesoproterozoic extensional tectonic framework established crustal scale pathways that controlled the distribution of subsequent magmatic activity including the ca 1 4 ga calderas and underlying intrusions ca 1 3 ga silicic plutons and phanerozoic alkaline intrusions if these interpretations are correct our study has identified large areas that are prospective for critical mineral bearing ore deposits and importantly suggests that the mesoproterozoic architecture may have influenced subsequent magmatism and hydrothermal activity in the southern midcontinent of the u s

dynamic simulations of slip on a smooth fault in an elastic solid we report on numerical simulations of slip evolution along a two dimensional slip varies only with depth vertical strike slip fault in an elastic half space using a framework incorporating full inertial elastodynamics the model is a follow up on earlier quasi static and quasi dynamic simulations of deformations along smooth fault systems in elastic continua the fault is driven below a crustal depth of 24 km by a constant plate velocity of 35 mm yr deformation at each fault location in the crustal zone is the sum of slip rate contributions from rate and state dependent friction and power law creep where both processes have temperature dependent and hence depth varying coefficients and both take place locally under the same stress the simulations employ two versions of rate and state dependent friction a slip law which requires nonzero velocity for state evolution and an ageing slowness law which incorporates state evolution and restrengthening in stationary contact the assumed constitutive laws and distribution of frictional parameters are compatible with laboratory experiments the elastodynamic calculations are based on spectral representations of variables and a new algorithm providing a unified computational framework for calculations of long deformational histories containing short periods of rapid instabilities the simulations show dynamic rupture propagation and wave phenomena not accounted for in the previous quasi static and quasi dynamic works however the results are qualitatively similar to those obtained by corresponding quasi static and quasi dynamic calculations slip histories along a smooth fault simulated here with full elastodynamics for various constitutive laws and model parameters consist mostly of quasi periodic large events this finding indicates that inertial dynamics does not provide a generic mechanism for generating spatio temporal complexities of slip on the other hand calculations done for cases representing approximately strongly disordered systems do show rich slip histories with a range of event sizes this result is compatible with our previous conclusions that the origin of observed broad distributions of earthquake sizes is strong fault zone heterogeneities the fully dynamic calculations illustrate the evolution of nucleation phases of instabilities associated with accelerating and expanding creep final slip values of model earthquakes in full elastodynamic calculations are larger than those of corresponding quasi static and quasi dynamic events the dynamic overshoot in simulations with the slip version of friction is larger than in those employing the ageing slowness law

crustal earthquake instability in relation to the depth variation of frictional slip properties recent stability studies using constitutive relations of the type found by dieterich ruina and others to describe frictional slip of rocks in the laboratory have provided a new explanation of the depth cutoff of shallow crustal earthquakes the class of friction laws discussed has the property that the sliding stress depends on normal stress temperature slip rate and slip history for sliding at a fixed slip rate v and fixed environment e g normal stress temperature etc the shear strength evolves toward a steady state value ss v stability analysis show that for d ss v dv 0 velocity strengthening implies stable steady state sliding at least for a broad class of constitutive laws more fully described in the paper experiments by dieterich and by tullis and weeks on westerly granite with mature sliding surfaces indicate that d ss v dv is negative at room temperature whereas higher temperature experiments by stesky show that d ss v dv becomes positive above approximately 300 c therefore in the case of the earth where temperature increases with depth the above observations seem to suggest that the depth cutoff of crustal earthquake activity can be understood in terms of the variation of the frictional response with depth from a regime with d ss v dv 0 this is not inconsistent with but rather refines the suggestion by sibson and others that the depth cutoff is due to a transition from brittle friction to ductile flow further our results show definitively that the shallow depth confinement of seismicity is compatible with a model in which deformation is localized to a fault zone extending well below the seismogenic depth the depth confinement does not require a model showing transition to a zone of broadly distributed creep flow beneath the seismogenic zone following mavko a two dimensional quasi static strike slip fault model is analyzed but using different numerical procedures and a depth variation of frictional properties based on the laboratory data mentioned above and the lachenbruch sass depth variation of temperature for the san andreas fault the resulting predictions of such features as the confinement of crustal earthquakes to shallow depths the development of locked patches the recurrence time for the seismic cycle the seismic stress drop and displacement etc are generally in agreement with the observed characteristics of large scale strike slip earthquakes along the san andreas fault calculations based on the laboratory data predict for example that stick slip nucleates around 5 7 km that large seismic motion occurs around the nucleation depth and above but diminishes gradually to zero at 13 15 km depths that rapid postseismic creep occurs over another 3 4 km depth and that at greater depths steady slip consistent with the average plate velocity is only modestly perturbed by the earthquakes occurring above the most uncertain parameter is the slip weakening distance l for evolution of fault surface state we illustrate how features of the predicted earthquake cycles vary with l over the range for which calculations are feasible

magmatic evolution of the southern great bear continental arc northwestern canadian shield geochronological constraints the calc alkaline great bear continental arc in the wopmay orogen developed after a collision ca 1890 ma of the archean slave craton with the paleoproterozoic hottah terrane to the west u pb zircon dating of three volcanic and six intrusive rocks from the southern part of the arc shows four stages of development i intrusion of a few small sodic leucogranite plutons at 1873 2 ma into a previously folded metasedimentary sequence ii abundant calc alkaline felsic volcanism and subvolcanic intrusions during the period 1870 1866 ma bracketed by five ages iii intrusion of large calc alkaline granitic plutons including the marian river batholith dated by zircon and titanite from two samples at 1866 2 3 ma and iv emplacement of the potassic faber lake rapakivi granite at 1856 2 3 ma the arc was developed on the hottah terrane due to easterly subduction of an oceanic plate under the amalgamated slave craton hottah terrane the oldest exposed rocks in the southern part of the arc are remnants

physical properties of rocks from borehole tj71305 and geophysical outline of the tjarrojakka cu prospect northern sweden abstract in the last decades fe oxide cu au iocg deposits attracted the interest of exploration geologists and geophysicists general geophysical descriptions of iocg deposits have been published in the recent past but there is still a lack of detailed geophysical investigations we present a petrophysical study of rock samples from exploration borehole tj71305 which intersects the cu prospect in the tjarrojakka iocg mineralised area northern sweden furthermore geophysical data are compiled and analysed to tentatively define a geophysical signature at local scale for this type of deposit the study area is dominated by intrusive and volcanic rocks of middle proterozoic age the latter hosting the cu and fe occurrences the fe occurrences are clearly defined from both aeromagnetic and ground magnetic data and are also indicated by gravity geoelectric and electromagnetic data enrichment in ferromagnetic minerals in the area is suggested by the high values of magnetic susceptibility commonly obtained for different rock types magnetite and ti magnetite are the dominant magnetic minerals haematite with variable contents of ti was detected and it is probably a result of oxidation of magnetite in alteration zones in tjarrojakka a clear spatial relationship is noted between cu occurrences and high k th ratios this ratio is calculated from airborne radiometric data and is an expression of enrichment in potassium due to alteration the cu prospect is also indicated by high gravity and magnetic anomalies by clear positive anomalies in induced polarisation data and by negative anomalies for the imaginary part of ground electromagnetic slingram data however high density high susceptibility low resistivity ti magnetite is associated with the cu prospect and this may lead to misinterpretation of potential field electromagnetic and geoelectric data

stability regimes of a dilatant fluid infiltrated fault plane in a three dimensional elastic solid 1 we investigate in a systematic parameter space study dilatant effects on slip evolution of a fluid infiltrated fault in the continuum limit the fault is governed by rate and state dependent friction and an empirical law for porosity evolution we focus on the response of systems as a function of fluid related parameters such as the degree of overpressurization dilatancy and diffusivity this study emphasizes the exploration of the parameter space for homogeneous along strike properties to investigate the evolution of spatiotemporal slip depending on hydromechanical processes three types of responses emerge first system wide unstable stick slip develops for drained conditions and for undrained conditions if mechanisms leading to an increase in pore space are less effective the critical stiffness depends on hydraulic diffusivity and dilatancy which is shown to correspond with interevent times of simulated stick slip events during instabilities the evolution of hydraulic variables differ significantly between drained and undrained conditions second stable creep is a result of dilatant processes third systems situated in transitional stability regimes develop nonuniform slip pattern in space and time revealing a possible explanation for rupture termination and observed stable afterslip although these patterns are produced by models located in transition zones of the parameter space the occurrence of heterogeneous slip evolution is persistent for an extensive range of parameter values since transition zones contain an broad range of plausible conditions in the crust they do not represent extreme cases

magnetic and gravity gradiometry framework for mesoproterozoic iron oxide apatite and iron oxide copper gold deposits southeast missouri high resolution airborne magnetic and gravity gradiometry data provide the geophysical framework for evaluating the exploration potential of hidden iron oxide deposits in mesoproterozoic basement rocks of southeast missouri the data are used to calculate mineral prospectivity for iron oxide apatite ioa rare earth element ree and iron oxide copper gold iocg deposits results delineate the geophysical footprints of all known iron oxide deposits and reveal several previously unrecognized prospective areas the airborne data are also inverted to three dimensional density and magnetic susceptibility models over four concealed deposits at pea ridge ioa ree boss iocg kratz spring ioa and bourbon iocg the pea ridge susceptibility model shows a magnetic source that is vertically extensive and traceable to a depth of greater than 2 km a smaller density source located within the shallow precambrian basement is partly coincident with the magnetic source at pea ridge in contrast the boss models show a large 625 m wide vertically extensive and coincident dense and magnetic stock with shallower adjacent lobes that extend more than 2 600 m across the shallow precambrian paleosurface the kratz spring deposit appears to be a smaller volume of iron oxides and is characterized by lower density and less magnetic rock compared to the other iron deposits a prospective area identified south of the kratz spring deposit shows the largest volume of coincident dense and nonmagnetic rock in the subsurface and is interpreted as prospective for a hematite dominant lithology that extends from the top of the precambrian to depths exceeding 2 km the bourbon deposit displays a large bowl shaped volume of coincident high density and high magnetic susceptibility rock and a geometry that suggests the iron mineralization is vertically restricted to the upper parts of the precambrian basement in order to underpin the evaluation of the prospectivity and three dimensional models an extensive statistical summary of density and apparent magnetic susceptibility measurements is presented that includes data on several hundred samples taken from the deposits altered wall rocks and unaltered country rocks

nonuniform and unsteady sliding of a plate boundary in a great earthquake cycle a numerical simulation using a laboratory derived friction law a numerical study is conducted to simulate complicated sliding behavior and earthquake activity on a subducting plate boundary a 2 d model of a uniform elastic half space with a semi infinite thrust fault is set up and the frictional stress prescribed by a rate and state dependent friction law is assumed to act on the plate boundary fault spatial nonuniformity of friction parameters representing rate dependence of friction and of slip dependence of friction are introduced in the model to obtain complicated sliding behavior in the numerical simulation analogs of great earthquakes that break the entire seismogenic plate boundary repeatedly occur at a constant time interval smaller events of seismic or aseismic sliding occur during a great earthquake cycle regions of rate strengthening of friction and of a large characteristic distance in slip dependence of friction behave as barriers or asperities rupture propagation is often arrested in such a region and a great earthquake occurs later when the region is broken the variety of earthquake activity observed in many regions along real plate boundaries may be explained by similar nonuniformity in friction parameters conversely the friction parameters on plate boundaries might be estimated from comparison of theoretical simulations with observations of earthquake activity simulation results indicate that spatiotemporal variation in stress due to aseismic sliding may play an important part in generating earthquakes

space time distribution of afterslip following the 2003 tokachi oki earthquake implications for variations in fault zone frictional properties 1 gps time series following the september 25 2003 ut tokachi oki earthquake m w 8 0 reveal significant deformation we use the network inversion filter to invert for the spatial and temporal evolution of afterslip in the 30 days following the earthquake afterslip is concentrated adjacent to the coseismic rupture zone between the 2003 rupture and the inferred source regions of the 1968 tokachi oki and the 1973 nemuro oki earthquakes the inversion shows a rapid decay of slip rate in the 2003 rupture zone with slower deceleration in the surrounding regions the stress velocity paths for the afterslip regions approximately follow d d in v 0 6 mpa suggestive of steady state velocity strengthening friction the complementary spatial pattern of coseismic rupture and afterslip may indicate along strike spatial variations of frictional properties although other interpretations can not be ruled out

the rock physical property database of british columbia and the distinct petrophysical signature of the chilcotin basalts rock physical properties provide the link between geophysical surveys and their geological interpretation the british columbia rock physical properties database compiled by the geological survey of canada now comprises 3876 values of density 930 values of electric resistivity 12 356 values of magnetic susceptibility and 2576 values of magnetic remanence and koenigsberger ratio the measurements are linked to sample locations lithologies and geological formation or unit maps histograms and biplots are used to demonstrate useful links between lithology and physical properties and serve as a background for future rock physical properties studies as expected density and resistivity are controlled mostly by porosity and mineralogy magnetic susceptibility has a bimodal distribution maxima at 4 10 4 and 2 10 2 si controlled by magnetite concentration magnetic remanence is shown to be more important than usually considered in magnetic survey analysis with koenigsberger ratios greater than u

slip complexity in earthquake fault models we summarize studies of earthquake fault models that give rise to slip complexities like those in natural earthquakes for models of smooth faults between elastically deformable continua it is critical that the friction laws involve a characteristic distance for slip weakening or evolution of surface state that results in a finite nucleation size or coherent slip patch size h models of smooth faults using numerical cell size properly small compared to h show periodic response or complex and apparently chaotic histories of large events but have not been found to show small event complexity like the self similar power law gutenberg richter frequency size statistics this conclusion is supported in the present paper by fully inertial elastodynamic modeling of earthquake sequences in contrast some models of locally heterogeneous faults with quasi independent fault segments represented approximately by simulations with cell size larger than h so that the model becomes inherently discrete do show small event complexity of the gutenberg richter type models based on classical friction laws without a weakening length scale or for which the numerical procedure imposes an abrupt strength drop at the onset of slip have h 0 and hence always fall into the inherently discrete class we suggest that the small event complexity that some such models show will not survive regularization of the constitutive description by inclusion of an appropriate length scale leading to a finite h and a corresponding reduction of numerical grid size

earthquakes as a coupled shear stress high pore pressure dynamical system the migration coalescence and localization of slip seismicity and zones of high pore pressure are modeled using a porosity reduction mechanism to drive pore pressure within a fault zone in excess of hydrostatic increased pore pressure in discrete cells creates zones of low effective stress which induces slip that may propagate to surrounding cells depending on the local state of stress at slip stress is transferred using the solution for a rectangular dislocation in an elastic half space and pore pressures are redistributed by conserving fluid mass using simple assumptions about fault rheology and permeability it is shown that the interaction between shear stress and effective stress evolves to a state of earthquake clustering with repeated events locked zones and large variations in fault strength the model evolves from a uniform shear stress state on a strong fault to a heterogeneous shear stress state on a weak fault

prominent hill a hematite dominated iron oxide copper gold system the discovery of iron oxide copper gold iocg mineralization at prominent hill in the mount woods inlier south australia by minotaur resources ltd in november 2001 was based on an empirical exploration model that focused specifically on the hematite end member of the iocg ore deposit model type copper and gold bearing hematite rich breccias formed by repetitive hydrothermal brecciation milling and explosive venting within a volcanic setting high pressure spalling chemical corrosion of breccia clasts and compositional layering within the breccias are distinctive features iron oxide sericite silica alteration is pervasive within and marginal to the main breccias and is surrounded by a wider zone of less intense alteration iron copper gold barium fluorine uranium cerium and lanthanum are strongly correlated and associated with a relatively low temperature hydrothermal system mineralization was synchronous with volcanism and sedimentation within a narrow east west trending graben that developed at 1585 ma

formation of albitite hosted uranium within iocg systems the southern breccia great bear magmatic zone northwest territories canada uranium and polymetallic u mineralization hosted within brecciated albitites occurs one kilometer south of the magnetite rich au co bi cu nico deposit in the southern great bear magmatic zone gbmz canada concentrations up to 1 wt u are distributed throughout a 3 by 0 5 km albitization corridor defined as the southern breccia zone two distinct u mineralization events are observed primary uraninite precipitated with or without pyrite chalcopyrite molybdenite within magnetite ilmenite biotite k feldspar altered breccias during high temperature potassic iron alteration subsequently pitchblende precipitated in earthy hematite specular hematite chlorite veins associated with a low temperature iron magnesium alteration the uraninite bearing mineralization postdates sodic albite and more localized high temperature potassic iron biotite magnetite k feldspar alteration yet predates potassic k feldspar boron tourmaline and potassic iron magnesium hematite k feldspar chlorite alteration the southern breccia zone shares attributes of the valhalla australia and lagoa real brazil albitite hosted u deposits but contains greater iron oxide contents and lower contents of riebeckite and carbonates potassium ni and th are also enriched whereas zr and sr are depleted with respect to the aforementioned albitite hosted u deposits field relationships geochemical signatures and available u pb dates on pre syn and post mineralization intrusions place the development of the southern breccia and the nico deposit as part of a single iron oxide alkali altered ioaa system in addition this case example illustrates that albitite hosted u deposits can form in albitization zones that predate base and precious metal ore zones in a single ioaa system and become traps for u and multiple metals once the tectonic regime favors fluid mixing and oxidation reduction reactions

short lived 1 9 ga continental margin and its destruction wopmay orogen northwest canada a 1 9 ga continental margin in the northwest of the canadian shield evolved in less than 15 m y from a zone of crustal stretching through a phase of passive margin subsidence to attempted subduction beneath an exotic microcontinent reversal of subduction polarity and generation of a major episutural magmatic arc occurred within 15 m y following microcontinental accretion terminal collision occurred 90 m y at most after initial rifting certain differences in detail between this collided margin and those of phanerozoic age may be related to subduction of newly rifted lithosphere rapid evolution of the orogen the contracted passive margin phase in particular is consistent with but does not prove more rapid recycling of oceanic plates at that time

the cloncurry lineament geophysical and geological evidence for a deep crustal structure in the eastern succession of the mount isa inlier the cloncurry lineament is a major feature evident from wavelet processed magnetic and gravity potential field data over the mount isa inlier eastern succession it is laterally continuous over 200 km of strike length and is inferred from the gravity data to extend to at least 30 km depth it delineates the contact between two major paleoproterozoic sedimentary sequences and is spatially associated with a number of mineral deposits including cannington ag pb zn ernest henry cu au and many smaller cu au deposits mapping and magnetic forward modelling along several profiles perpendicular to the cloncurry lineament suggests that it corresponds to the eastern margin of a 5 10 km wide deformation zone within the calc silicate dominated doherty formation the western boundary of this deformation zone is a distinct narrow fault zone the cloncurry fault the deformation zone consists of multiple generations of shearing and faulting accompanied by metasomatic alteration large variations in the magnetic response within the deformation zone can be modelled as sub vertical brecciation zones breccia types range from highly magnetic actinolite albite magnetite breccias through to late stage non magnetic quartz k feldspar hematite breccias the long lived history of multiple reactivation strike continuity width and vertical extent of the deformation zone suggest that it is a major crustal structure the structure may have originated during basin formation and later acted as a fluid pathway for alteration and base metal mineralization

imaging of firn isochrones across an antarctic ice rise and implications for patterns of snow accumulation rate it has been an underlying assumption in many studies that near surface layers imaged by ground penetrating radar gpr can be interpreted as depositional markers or isochrones it has been shown that gpr layers can be approximately reproduced from the measured electrical properties of ice but these material layers are generally narrower and more closely spaced than can be resolved by typical gpr systems operating in the range 50 400 mhz thus gpr layers should be interpreted as interference patterns produced from closely spaced and potentially discontinuous material layers and should not be assumed to be interpretable as precise markers of isochrones we present 100 mhz gpr data from lyddan ice rise antarctica in which near surface 50 m deep layers are clearly imaged the growth of the undulations in these layers with depth is approximately linear implying that rather than resulting from a pattern of vertical strain rate they do correspond to some pattern of snowfall variation furthermore comparison of the gpr layers with snow stake measurements suggests that around 80 of the rms variability in mean annual accumulation is present in the gpr layers the observations suggest that at least in this case the gpr layers do approximate isochrones and that patterns of snow accumulation over lyddan ice rise are dominated by extremely persistent spatial variations with only a small residual spatial variability if this condition is shown to be widely applicable it may reduce the period required for measurements of surface elevation change to be taken as significant indications of mass imbalance

aseismic slip transients emerge spontaneously in three dimensional rate and state modeling of subduction earthquake sequences 1 to investigate the possible physical mechanisms of recently observed aseismic slip events in the cascadia japan and mexico subduction zones we apply a dieterich ruina rate and state friction law to a three dimensional model of a shallow subduction fault that is loaded by imposed steady plate slip rate far downdip along the thrust interface friction properties are temperature and hence depth dependent such that sliding is stable at depths below about 30 km the system is perturbed into a significantly nonuniform slip mode along strike by introducing small along strike variations in the constitutive parameters a and a b in addition to large heterogeneous earthquake slip at seismogenic depths and associated postseismic transients we found that slip events which have clearly aseismic slip rates emerge spontaneously around the downdip end of the seismogenic zone both transients which start well after a seismic event and those which are triggered by other transients are observed from the simulations the slip velocity depth range and sometimes along strike migration speed of simulated transients are similar to the observations from natural subduction zones unstable stable transitional friction properties near the downdip end of the seismogenic zone are suggested to be an ingredient allowing such transients simulated transients can weaken the locking intensity of the updip seismogenic zone while enhancing that of the transition zone spatial temporal correlation of aseismic transients and nearby seismicity in the guerrero mexico area suggests that transients may signal a period of increased probability for nucleating their high frequency counterparts as damaging subduction thrust events

time dependent distributed afterslip on and deep below the izmit earthquake rupture surface deformation transients measured with the global positioning system during the 87 days between the 17 august 1999 izmit earthquake and the 12 november 1999 duzce earthquake indicate rapidly decaying aseismic fault slip on and well below the coseismic rupture elastic model inversions for time dependent distributed fault slip using a network inversion filter approach show that afterslip was highest between and below the regions of maximum coseismic slip and propa gated downward to or even below the base of the crust maximum afterslip rates decayed from greater than 2 m yr immediately after the i zmit earthquake to about 1 2 m yr just prior to the duzce earthquake maximum afterslip occurred below the eastern karadere rupture segment and near the i zmit hypocenter afterslip in the upper 16 km decayed more rapidly than that below the seismogenic zone these observations are consistent with a phase of rapid aseismic fault slip concentrated near the base of the seismogenic zone continued loading from the rapid deep afterslip along the eastern rupture zone is a plausible mechanism that helped trigger the nearby mw 7 2 12 november duzce earthquake

petrophysical properties density and magnetization of rocks from the northern part of the baltic shield abstract density magnetic susceptibility and remanent magnetization of more than 30 000 samples of crystalline rocks from the northern part of the baltic shield have been measured by the national geological surveys of the nordic countries these data were compiled into a database as part of a project involving finland norway and sweden the nordkalott project in addition more than 20 000 in situ susceptibility measurements are available for a large part of the baltic shield from all these measurements a number of general conclusions can be drawn regarding the distribution of magnetic minerals in the different lithologies 1 1 the precambrian metasupracrustal rocks show a rather typical bimodal distribution in susceptibility the spatial distribution of these susceptibility modes gives rise to the banded patterns observed in high resolution low altitude aeromagnetic measurements 2 2 the amount of remanence in the various rock types is generally low as indicated by q values konigsberger ratios of around 0 2 on average thus magnetization is parallel to the earth s magnetic field simplifying the interpretation ol the total field magnetic survey results 3 3 plutonic rocks as a whole have the same bimodal susceptibility distribution but individual plutons have usually unimodal susceptibility distributions and thereby create characteristic local anomaly patterns 4 4 metamorphic processes significantly affect the distribution of magnetite well documented examples include the amphibolite granulite facies transition tectonophysics vol 192 the serpentinization of ultramafic rocks pure appl geophys vol 114 and oxidation in fracture zones geoexploration 15 the metamorphic development of magnetite appears to reflect in many cases the variation in primary composition 5 5 the range in density variation and magnetic susceptibility in crystalline rocks is now well defined in local structures it is however always necessary to determine the actual petrophysical contrasts in order to minimize the ambiguity in quantitative interpretation of potential field anomalies

alteration facies linkages among iron oxide copper gold iron oxide apatite and affiliated deposits in the great bear magmatic zone northwest territories canada high temperature metasomatism driven by ascent of voluminous saline fluid columns in the upper crust plays a major role in the genesis of iron oxide alkali alteration ore systems but fundamental questions remain on genetic linkages among iron oxide copper gold iocg iron oxide apatite ioa albitite hosted uranium and skarn deposits that they produce excellent surface exposures of such systems in the great bear magmatic zone of northernwestern canada record the depth to paleosurface prograde evolution of iron oxide alkali alteration facies and mineralization across the belt albitite corridors that are tens of kilometers in length record the earliest reactions between highly saline fluids and host rocks along fault zones and subvolcanic intrusions pervasive albitization partitioned metals from the host rocks into the ascending fluid column leaving behind structurally weakened corridors of porous albitite these corridors were cut replaced and overprinted by amphibole and magnetite bearing calcic iron alteration assemblages in extreme cases the discharge of calcium iron and specialized metals formed iron oxide apatite deposits vanadium rare earth elements while recharging the outgoing fluids in sodium potassium and base and precious metals as temperatures declined and fluid chemistry evolved through fluid rock reactions the formation of potassic iron alteration assemblages breccias and sulfides resulted in magnetite and hematite group iocg mineralization within carbonate units skarns formed prior to are replaced by and evolved to calcic iron alteration facies skarns can locally host base metal mineralization tectonically uplifted albitite breccias replaced by potassic iron alteration assemblages became a preferential host for uranium mineralization the results of this study also illustrate that permutations and cyclical build up of alteration products can arise from a combination of faulting differential uplift and renewed magmatism framed within an alteration facies deposit model alteration zones and mineral occurrences play a pivotal role in predicting the mineral potential of iron oxide and alkali altered systems at district to deposit scales

trace element geochemistry of magnetite and its relationship to cu bi co au ag u w mineralization in the great bear magmatic zone nwt canada the paleoproterozoic great bear magmatic zone is the focus of ongoing exploration for iron oxide copper gold iocg deposits and also hosts iron oxide apatite occurrences examples of iocg deposits in the great bear magmatic zone include sue dianne and nico and other smaller prospects including damp fab and nori ra the past producing terra mine property hosts significant iocg like alteration that contains dome shaped iron oxide apatite bodies petrographic study has identified multiple generations of magnetite at nico fab and nori ra and for the most part a single generation of magnetite at sue dianne damp and terra laser ablation inductively coupled plasma mass spectrometry la icp ms documents important geochemical differences in v ni cr and co concentrations within the magnetite variations of trace elements in magnetite from the great bear magmatic zone could be a result of 1 host rock fluid equilibration during regional metamorphism 2 postmetamorphic hydrothermal metasomatism of treasure lake group metasedimentary rocks 3 preferential solubility of co over ni within the fe rich fluids 4 changes in oxygen fugacity f o2 and 5 partitioning of elements into coprecipitating sulfides regionally the cr co ratio is higher in barren and pre ore magnetite compared to magnetite coprecipitated with ore minerals and or present in ore rich veins and breccias locally at the nori ra prospect the v ni ratio in magnetite differentiates between barren and ore related magnetite and at damp and sue dianne the co ni ratio is extremely high and clearly different from that of other great bear magmatic zone magnetite samples these results provide the first database for geochemically characterized magnetite from different stages of iocg alteration and illustrate the potential use of magnetite as an indicator mineral in the exploration for iocg deposits

uranium in iron oxide copper gold iocg systems the olympic dam iron oxide copper gold iocg deposit is currently the world s largest producer of uranium other iocg deposits generally have anomalous though uneconomic uranium grades the fluids that generate iocg deposits are thought to be saline and highly oxidized and would be capable of leaching and transporting significant uranium the reasons for uranium enrichment in certain iocg deposits remain speculative but could include the composition of mineralization related magmas of the wall rocks altered by hydrothermal fluids or of the fluids mixing at the site of mineralization there is sparse data available on uranium abundance in iocg ores there is even less reliable data on the uranium content of unaltered wall rocks for the deposits however the data that is available suggest that the uranium grade of iocg deposits may be primarily related to the uranium content of their unaltered host rocks many iocg deposits appear to have approximately 10 to 40 times enrichment in the ore compared to unaltered host rocks this suggests that exploration for uranium rich iocg deposits should focus on areas with anomalously high uranium contents in host rock sequences

fault stability inferred from granite sliding experiments at hydrothermal conditions seismicity on crustal faults is concentrated in the depth interval 1 3 to 12 15km tse and rice 1986 suggested that the lower bound on seismicity is due to a switch with increasing temperature from velocity weakening destabilizing to velocity strengthening stabilizing friction they inferred this transition from friction data for dry granite however pore fluids exist at elevated temperatures throughout the crust and may strongly influence strength and sliding behavior we present new data from sliding experiments on granite at elevated t 23 to 600 plus elevated ph2o 100 mpa our results show velocity strengthening at room temperature but velocity weakening from 100 to 350 c except at 250 from 350 to 600 there are systematic trends from velocity weakening to strong velocity strengthening and from high to low friction neither trend was seen in tests on dry granite the velocity dependence data imply the potential for unstable slip in the interval 100 to 350 using a geotherm to map temperature to depth this interval closely matches the observed earthquake distribution

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

seismicity on a fault controlled by rate and state dependent friction with spatial variations of the critical slip distance 1 we perform systematic simulations of slip using a quasi dynamic continuum model of a two dimensional 2 d strike slip fault governed by rate and state dependent friction the depth dependence of the a b and l frictional parameters are treated in an innovative way that is consistent with available laboratory data and multidisciplinary field observations various realizations of heterogeneous l distributions are used to study effects of structural variations of fault zones on spatiotemporal evolution of slip we demonstrate that such realizations can produce within the continuum class of models realistic features of seismicity and slip distributions on a fault we explore effects of three types of variable l distributions 1 a depth dependent l profile accounting for the variable width of fault zones with depth 2 uncorrelated 2 d random distributions of l with different degrees of heterogeneity and 3 a hybrid distribution combining the depth dependent l profile with the 2 d random l distributions the first type of l distribution with relatively small l over the depth range corresponding to the seismogenic zone and larger l elsewhere generates stick slip events in the seismogenic zone and ongoing creep above and below that region the 2 d heterogeneous parameterizations generate frequency size statistics with event sizes spanning 4 orders of magnitude our results indicate that different degrees of heterogeneity of l distributions control 1 the number of simulated events and 2 the overall stress level and fluctuations other observable trends are 3 the dependency of hypocenter location on l and 4 different nucleation phases for small and large events in heterogeneous distributions

on the relationship between alteration facies and metal endowment of iron oxide alkali altered systems southern great bear magmatic zone canada the great bear magmatic zone in northern canada hosts large iron oxide and alkali altered systems that developed within a 10 m y period during the paleoproterozoic in the eastern treasure lake and duke sectors and at the nico deposit early albitization associated with magmatism was followed by tungsten mineralization that was spatially associated with extensive skarn and fe skarn alteration subsequent fluid rock reactions led to substantial iron enrichment that culminated in iron oxide apatite mineralization at the transition from compressional transpressional to extensional transtensional stress regimes changes in magma composition increased the metal budget of exsolved magmatic hydrothermal fluids and played a key role in the formation of au co bi mineralization at the nico deposit and its satellite showings in the duke zone this process happened as the systems regionally transitioned to magnetite bearing potassic iron alteration the continued evolution of magmatism and fluid chemistry then contributed to the formation of iron oxide copper gold iocg and variant mineralized zones in the sue dianne and nico deposits and other showings and albite hosted uranium mineralized zones elsewhere the great bear magmatic zone illustrates that the evolution of magmatically derived fluids during a transition between different stress regimes can play a significant role in metal endowment and the ability of metasomatic systems to form skarn iron oxide apatite iocg and albitite hosted uranium mineralization

finite element models of stress orientations in well developed strike slip fault zones implications for the distribution of lower crustal strain finite element models are used to examine the effects of strike slip earthquakes on stresses in an elastic layer overlying a finite width viscoelastic shear zone in the lower crust the overall dimensions of the model are 300 km wide 400 km long and 50 km deep three geometries for the lower crustal shear zone are considered 1 a viscoelastic half space approximation with a shear zone that extends to the model boundaries 300 km in width 2 a wide shear zone model 70 km in width and 3 a narrow shear zone model 10 km in width earthquakes are simulated with a fault plane that slips without friction at the desired time step and is centered above the shear zone extending to a depth of 15 km and running the length of the mesh far field plate velocity boundary conditions are enforced at the model edges so that stress on the fault evolves naturally a coulomb type failure criterion based on the average shear stress on the fault is set such that the earthquake cycle is 250 years the models are run until a limit cycle is reached and transient stresses have decayed away we focus on the maximum changes in the stress field during the earthquake cycle by examining stresses before and immediately after each earthquake in addition to comparisons of the separate components of the stress tensor we present results in the form of maximum compressive stress orientations plunge angles of the principal stress axes and beach ball diagrams that facilitate visualizing the full tensor stresses are concentrated in the upper crust where it overlies the finite width viscoelastic shear zone which causes the plunge angles of the principal stress axes to rotate from andersonian orientations of 90 and 0 to angles that approach 45 in the lower crust our results suggest that an examination of stress orientations in the upper and middle crust from borehole breakouts and focal mechanisms may provide insight as to the distribution of strain in the lower crust and may eventually allow us to distinguish between localized and distributed deformation models for the lower crust in active strike slip zones

a model for complex slip behavior on a large asperity at subduction zones 1 3 d numerical simulations for rupture process of a large asperity on a subduction fault are carried out in order to examine whether the whole area of an asperity always breaks at once or not during an earthquake our quasi static simulation in an elastic half space includes a rate and state dependent friction we examine the effect of lateral dimension of an asperity which extends the whole seismogenic width in dip direction on the slip behavior in cases of the asperity with comparable dimension to the seismogenic width the whole area of the asperity can break at once and its seismic cycle shows a regular periodic behavior however in cases with much larger lateral dimension the asperity does not break its whole area at once in addition its slip behavior becomes complex within a certain range this suggests that the slip complexity in earthquake fault may be an intrinsic property in the system

lithology and hydrothermal alteration control the distribution of copper grade in the prominent hill iron oxide copper gold deposit gawler craton south australia the prominent hill iron oxide copper gold iocg deposit located in the gawler craton of south australia contains ca 278 million metric tons mt of ore at 0 98 cu 0 75 g t au and 2 5 g t ag in contrast to the predominantly granite hosted olympic dam iocg deposit prominent hill is mainly within unmetamorphosed sedimentary rocks comprising coarse clastic to laminated argillaceous lithologies with some volcaniclastic components and variable carbonate including local massive dolomite essentially unmetamorphosed sedimentary rocks and structurally underlying mafic to intermediate composition lavas inferred to be members of the lower gawler range volcanics host the economically mineralized hematite breccias the volcanic sedimentary package was downfaulted and tilted along a major east west fault north of which similar but regionally low grade metamorphosed rocks were affected by subeconomic skarn mineralization and on a more regional scale of the mount woods domain intruded by granitic and gabbroic bodies hydrothermal alteration and mineralization at prominent hill involved pervasive and texturally destructive replacement of formerly calcareous dolomitic and siliciclastic breccia components hydrothermal alteration minerals comprise hematite magnetite siderite ankerite quartz sericite chlorite kaolinite fluorapatite fluorite barite ree u minerals including monazite uraninite and coffinite together with cu sulfides including chalcopyrite bornite and chalcocite in the highest grade ore brecciation and replacement caused mechanical mixing as well as chemical alteration of primary lithologies such that sedimentary contacts became obscured mass balance calculations identify al ti si and zr as least mobile components during hematite chlorite sericite to weak hematite quartz alteration because zr was not regularly assayed in drill cores we use concentration ratios of ti al and si from the deposit scale assay database to delineate the distribution of lithochemical units prior to hydrothermal alteration and cu mineralization the resulting lithochemical model based on one horizontal and five vertical cross sections is used as a basis for mapping alteration patterns calculated from molar fe si fe si al k na and k al ratios these chemical patterns in conjunction with mineral stoichiometry indicate that the spatial distribution of hematite chlorite variably phengitic sericite and or illite kaolinite quartz bearing alteration is superimposed on the pattern of interpreted lithologic contacts the alteration patterns confirm visual logging results showing that hematite enrichment correlates only partially with the distribution of cu grades of 0 25 wt a subvertical body of complete replacement by hematite and quartz with consistent but subeconomic gold enrichment forms a cu barren core in the central and eastern parts of the deposit zones of increasing k al and k na ratios extend upward and westward from this cu barren core transgressively overprinting lithologic contacts the degree of hematite quartz replacement can be measured by a hematite quartz alteration index here termed the hmsi value fe si fe si al which inversely correlates with the normal probability for cu grade areas of highest cu grade 1 wt spatially correlate with irregular zones having intermediate molar alteration indices 0 34 k al 0 40 20 k na 36 and hmsi 0 98 hematite breccias and cu ore deposition developed after tilting of the host sequence into its present steep orientation as indicated by geopetal structures within the breccia matrix thus the economic mineralization occurred late in the deformational history of the region and after extrusion of the lower gawler range volcanics the formation of the prominent hill orebodies occurred during or after upthrusting of deeper seated rocks containing subeconomic cu in skarns north of the fault faulting as well as ore formation may be related to orogenic processes in the central and northern part of the mount woods domain iron oxide introduction was decoupled from and at least partly preceded hydrothermal deposition of high grade cu geochemical and petrographic data indicate that economic cu mineralization occurred together with mildly acidic hematite chlorite sericite siderite alteration of originally carbonate illite and feldspar bearing sedimentary rocks the presence of copper enrichment with an intermediate degree of cation leaching from the host rocks indicates that ph neutralization of initially highly acidic metal transporting fluids was an essential factor causing cu sulfide deposition distinct ranges in na k al ratios and low hmsi values offer potential as exploration indicators pointing toward higher ore grades these results from prominent hill are consistent with recently published mineralogical studies at the giant olympic dam deposit indicating similar ore depositional controls despite lithologically different host rocks

atmospheric sources for audio magnetotelluric amt sounding the energy sources for natural source magnetotelluric mt frequencies 1 hz are electromagnetic em waves caused by distant lightning storms and which propagate within the earth ionosphere waveguide the properties of this waveguide display diurnal seasonal and 11 year solar cycle fluctuations and these temporal fluctuations cause significant signal amplitude attenuation variations especially at frequencies in the 1 to 5 khz so called audiomagnetotelluric amt dead band in the northern hemisphere these variations increase in amplitude during the nighttime and the summer months and they correspondingly decrease during the daytime and the winter months thus one problem associated with applying the amt method for shallow 3 km exploration can be the lack of signal in certain frequency bands during the desired acquisition interval in this paper we analyze the time variations of high frequency em fields to assess the limitations of the efficient applicability of the amt method we demonstrate th

slip patterns and earthquake populations along different classes of faults in elastic solids numerical simulations of slip instabilities on a vertical strike slip fault in an elastic half space are performed for various models belonging to two different categories the first category consists of inherently discrete cellular fault models such are used to represent fault systems made of segments modeled by numerical cells that can fail independently of one another their quasi independence is assumed to provide an approximate representation of strong fault heterogeneity due to geometric or material property disorder that can arrest ruptures at segment boundaries the second category consists of models having a well defined continuum limit these involve a fault governed by rate and state dependent friction and are used to evaluate what types of property heterogeneity could lead to the quasi independent behavior of neighboring fault segments assumed in the first category the cases examined include models of a cellular fault subjected to various complex spatial distributions of static to kinetic strength drops and models incorporating rate and state dependent friction subjected to various spatial distributions of effective stress normal stress minus pore pressure the results indicate that gradual effective stress variations do not provide a sufficient mechanism for the generation of observed seismic response strong and abrupt fault heterogeneity as envisioned in the inherently discrete category is required for the generation of complex slip patterns and a wide spectrum of event sizes strong fault heterogeneity also facilitates the generation of rough rupture fronts capable of radiating high frequency seismic waves the large earthquakes in both categories of models occur on a quasi periodic basis the degree of periodicity increases with event size and decreases with model complexity however in all discrete segmented cases the models generate nonrepeating sequences of earthquakes and the nature of the large quasi periodic events is highly variable the results indicate that expectations for regular sequences of earthquakes and or simple repetitive precursory slip patterns are unrealistic the frequency size fs statistics of the small failure episodes simulated by the cellular fault models are approximately self similar with b 1 2 and ba 1 where b and ba are b values based on magnitude and rupture area respectively for failure episodes larger than a critical size however the simulated statistics are strongly enhanced with respect to self similar distributions defined by the small events this is due to the fact that the stress concentrated at the edge of a rupture expanding in an elastic solid grows with the rupture size when the fault properties e g geometric irregularities are characterized by a narrow range of size scales the scaling of stress concentrations with the size of the failure zone creates a critical rupture area terminating the self similar earthquake statistics in such systems events reaching the critical size become on the average unstoppable and they continue to grow to a size limited by a characteristic model dimension when however the system is characterized by a broad spectrum of size scales the above phenomena are suppressed and the range of apparent self similar fs statistics is broad and characterized by average b and ba values of about 1 the simulations indicate that power law extrapolations of low magnitude seismicity will often underestimate the rate of occurrence of moderate and large earthquakes the models establish connections between features of fs statistics of earthquakes range of self similar regimes local maxima and structural properties of faults dominant size scales of heterogeneities dimensions of coherent brittle zones the results suggest that observed fs statistics can be used to obtain information on crustal thickness and fault zone structure

metasomatic alteration control of petrophysical properties in the great bear magmatic zone northwest territories canada the great bear magmatic zone in the northwest territories of canada contains large iron oxide alkali alteration systems noted for having high potential for iron oxide apatite iron oxide copper gold and affiliated ore deposits physical properties density magnetic and electrical were measured on 824 rocks samples selected to represent the range of metasomatic alteration types in the region mineralogical and geochemical classification of the prograde iron oxide and alkali alteration facies reveals large variations in physical properties through the evolution of the metasomatic systems in particular deep and early sodic alteration produced rocks having low densities and low magnetic susceptibilities during calcium and iron precipitation rocks gain extremely high density and susceptibility due to crystallization of amphibole and especially magnetite subsequent hightemperature potassic and iron altered rocks are marked by cocrystallization of magnetite with k feldspar or biotite and as the transition from magnetite to hematite takes place k feldspar crystallizes instead of iron oxides leading ultimately to potassic felsites having low densities and susceptibilities subsequent cooler shallower and more oxidized potassic iron alteration produced high densities but moderate susceptibilities owing to crystallization of hematite understanding these major variations in physical properties of rocks enables detailed geophysical mapping of alteration zones which builds and improves the regional context for integrated mineral exploration vectors to potential ore deposits

a fault constitutive relation accounting for thermal pressurization of pore fluid 1 the heat generated in a slip zone during an earthquake can raise fluid pressure and thereby reduce frictional resistance to slip the amount of fluid pressure rise depends on the associated fluid flow the heat generated at a given time produces fluid pressure that decreases inversely with the square root of hydraulic diffusivity times the elapsed time if the slip velocity function is crack like there is a prompt fluid pressure rise at the onset of slip followed by a slower increase the stress drop associated with the prompt fluid pressure rise increases with rupture propagation distance the threshold propagation distance at which thermally induced stress drop starts to dominate over frictionally induced stress drop is proportional to hydraulic diffusivity if hydraulic diffusivity is 0 02 m2 s estimated from borehole samples of fault zone material the threshold propagation distance is 300 m the stress wave in an earthquake will induce an unknown amount of dilatancy and will increase hydraulic diffusivity both of which will lessen the fluid pressure effect nevertheless if hydraulic diffusivity is no more than two orders of magnitude larger than the laboratory value then stress drop is complete in large earthquakes

magnetic effects of hydrothermal alteration in porphyry copper and iron oxide copper gold systems a review abstract magnetic anomaly patterns can be used as a tool for mapping lithology metamorphic zones and hydrothermal alteration systems as well as identifying structures that may control passage of magmas or hydrothermal fluids associated with mineralisation reliable geological interpretation of mineralised systems requires an understanding of the magmatic metamorphic and hydrothermal processes that create alter and destroy magnetic minerals in rocks predictive magnetic exploration models for porphyry copper and iron oxide copper gold iocg deposits can be derived from standard geological models by integrating magnetic petrological principles with petrophysical data deposit descriptions and modelling of observed magnetic signatures of these deposits even within a particular geological province the magnetic signatures of similar deposits may differ substantially due to differences in the local geological setting searching for look alike signatures of a known deposit is likely to be unrewarding unless pertinent geological factors are taken into account these factors include the tectonic setting and magma type composition and disposition of host rocks depth of emplacement and post emplacement erosion level depth of burial beneath younger cover post emplacement faulting and tilting remanence effects contingent on ages of intrusion and alteration and metamorphism because the effects of these factors on magnetic signatures are reasonably well understood theoretical magnetic signatures appropriate for the local geological environment can qualitatively guide exploration and make semiquantitative predictions of anomaly amplitudes and patterns the predictive models also allow detectability of deposit signatures to be assessed for example when deposits are buried beneath a considerable thickness of nonmagnetic overburden are covered by highly magnetic heterogeneous volcanic rocks or there is a strong regional magnetic gradient this paper reviews the effects of hydrothermal alteration on magnetic properties and magnetic signatures of porphyry copper and iron oxide copper gold systems and presents examples of predictive magnetic exploration models and their predicted signatures in various geological circumstances

geophysical signature of the nico au co bi cu deposit and its iron oxide alkali alteration system northwest territories canada models of the three dimensional physical property variation of the nico au co bi cu deposit northwest territories and the southern breccia albitite hosted uranium occurrences and their iron oxide alkali alteration envelopes were derived from inversion of high resolution aeromagnetic gravity and magnetotelluric mt data at deposit to regional scales in turn integration of the geophysical results with physical property measurements and geologic observations leads to a new understanding of the geometry of the deposit adjacent mineralized and altered zones and potential cogenetic links within the host metasomatic system nico which is a variant of the magnetite group iron oxide copper gold iocg class of deposit is spatially associated with a discrete zone of lower electrical resistivity occurring within a broader ne dipping zone of higher density the high density zone overlaps a ne dipping zone of higher magnetic susceptibility and is truncated to the southwest by a nw striking geophysical discontinuity interpreted as a major fault zone this inferred fault divides the magnetite altered metasedimentary rocks hosting the nico deposit from the albite altered rocks within the southern breccia corridor to the southwest that host the uranium mineralization having been active during development of the metasomatic system this fault influenced the formation of these distinct but complementary deposit types

magnetotelluric evidence for a deep crustal mineralizing system beneath the olympic dam iron oxide copper gold deposit southern australia the iron oxide copper gold olympic dam deposit situated along the margin of the proterozoic gawler craton south australia is the world s largest uranium deposit and sixth largest copper deposit it also contains significant reserves of gold silver and rare earth elements gaining a better understanding of the mechanisms for genesis of the economic liberalization is fundamental for defining exploration models in similar crustal settings to delineate crustal structures that may constrain mineral system fluid pathways coincident deep crustal seismic and magnetotelluric mt transects were obtained along a 220 km section that crosses olympic dam and the major crustal boundaries in this paper we present results from 58 long period 10 104 s mt sites with site spacing of 5 10 km a two dimensional inversion of mt data from 33 sites to a depth of 100 km shows four notable features 1 sedimentary cover sequences with low resistivity 1000 m archean crustal core from a more conductive crust and mantle to the north typically 500 m 3 to the north of olympic dam the upper middle crust to 20 km is quite resistive 1000 m but the lower crust is much more conductive 100 m and 4 beneath olympic dam we image a low resistivity region 100 m throughout the crust coincident with a seismically transparent region we argue that the cause of the low resistivity and low reflectivity region beneath olympic dam may be due to the upward movement of co2 bearing volatiles near the time of deposit formation that precipitated conductive graphite liberalization along grain boundaries simultaneously annihilating acoustic impedance boundaries the source of the volatiles may be from the mantle degassing or retrograde metamorphism of the lower crust associated with proterozoic crustal deformation

predicting rock type and detecting hydrothermal alteration using machine learning and petrophysical properties of the canadian malartic ore and host rocks pontiac subprovince quebec canada abstract the canadian malartic deposit is a world class intrusion related archean gold deposit hosted in the pontiac subprovince superior province in quebec canada laboratory petrophysical properties measurements were performed on 824 rock samples collected from the various rock types observed within the ore body and peripheral host rocks the various rock types present in the malartic district mainly meta sedimentary rocks felsic intermediate intrusive rocks and mafic dykes have contrasting grain densities and magnetic susceptibilities using support vector machines it is shown that these two physical properties can be used to predict the rock type of a sample with an average precision and recall rate of 89 within the meta sedimentary rocks class variations in magnetic susceptibility are due to the changes in mineralogy associated with hydrothermal alteration these are caused by the destruction of iron bearing silicate minerals and magnetite in unaltered rocks 10 4 to 10 3 si to form pyrite carbonates k feldspar and fe depleted hydrothermal biotite in altered rocks 10 5 to 10 4 si within the felsic intermediate intrusive rocks grain densities below 2 7 g cm3 and magnetic susceptibilities in the 10 6 to 10 4 si range yield the highest probabilities that a rock has been submitted to carbonate and pyrite alteration however magnetic susceptibility and grain density of these intrusive rocks are also dependent on their fe2o3 al2o3 and tio2 al2o3 ratios which are not related to the hydrothermal alteration footprint but are rather due to distinct protolith compositions within the mafic dykes grain density is the best indicator of hydrothermal alteration unaltered mafic dykes 2 95 to 3 10 g cm3 are mostly composed of amphibole whereas altered mafic dykes 2 70 to 2 95 g cm3 have reduced amphibole contents and higher abundances of carbonates pyrite quartz and biotite alteration the support vector machine classifier is extended to predict if meta sedimentary rocks felsic intermediate intrusive rocks and mafic dykes have undergone hydrothermal alteration with average f1 scores of 73 69 and 93 respectively in altered meta sedimentary rocks the integration of grain density and magnetic susceptibility allows the identification of altered but unmineralized samples the classifier is further extended to predict if the gold content of meta sedimentary rocks is above or below threshold values of 0 01 0 1 and 1 ppm with average f1 scores of 83 80 and 76 respectively using conceptual models of the rock physical properties at the malartic district scale it is shown that ground magnetic surveys are the most promising geophysical tool for early stage greenfield exploration of this type of deposit however depending on the scale at which the surveys are conducted magnetic susceptibility contrasts between the various investigated rock types can overshadow the specific signatures of hydrothermally altered rocks this in part explains why past airborne geophysical exploration campaigns for this type of deposit in the malartic district were inconclusive finally the machine learning process used in this case study can be applied in advanced exploration stages during which drilling and either subsequent laboratory petrophysical analyses of core samples or downhole geophysical surveys produce large amounts of data that can be used to train prediction models

effects of slip slip rate and shear heating on the friction of granite the stability of fault slip is sensitive to the way in which frictional strength responds to changes in slip rate and in particular to the effective velocity dependence of steady state friction ss ln v this quantity can vary substantially with displacement temperature and slip rate to investigate the physical basis for this behavior and the possible influence of shear heating we slid initially bare granite surfaces in unconfined rotary shear to displacements of hundreds of millimeters at normal stresses n of 10 and 25 mpa and at room temperature we imposed step changes in slip rate within the range 10 2 to 103 5 m s and also monitored frictional heating with thermistors embedded in the granite the transient response of to slip rate steps was fit to a rate and state dependent friction law using two state variables to estimate the values of several parameters in the constitutive law the first 20 mm of slip shows rising friction and falling ss ln v further slip shows roughly constant friction ss ln v and parameter values suggesting that a steady state condition is reached on the fault surface at v 10 m s ss ln v 0 004 0 001 at higher rates the response is sensitive to normal stress at n 25 mpa granite shows a transition to effective velocity strengthening ss ln v 0 008 0 004 at the highest slip rates tested at 10 mpa granite shows a less dramatic change to ss ln v 0 at the highest rates the maximum temperature measured in the granite is 60 c at 25 mpa and 103 5 m s temperatures are in general agreement with a numerical model of heat conduction which assumes spatially homogeneous frictional heating over the sliding surface the simplest interpretation of our measurements of ss ln v is that the granite is inherently velocity weakening ss ln v 0 mimics velocity strengthening these results have implications for the frictional behavior of faults during earthquakes high slip rates may cause a switch to effective velocity strengthening which could limit peak coseismic slip rate and stress drop for fluid saturated faults strengthening by this mechanism may be partly or fully offset by weakening due to thermal pressurization of a poorly drained pore fluid

on the physical mechanism of silent slip events along the deeper part of the seismogenic zone 1 to investigate the mechanism of recently observed silent slip events we simulated earthquake preparation processes using the dieterich ruina rate and state dependent friction law to ensure realistic modeling of the unstable stable transition we considered small cut off velocity to an evolution effect in the friction law for the transition zone when the cut off velocity to the evolution effect is significantly smaller than that of a direct effect steady state friction behaves as velocity weakening at low slip velocity and velocity strengthening at high slip velocity this frictional behavior was experimentally and theoretically confirmed for the unstable stable transition zone the results of our numerical simulations show that silent slips of which velocity is higher than the velocity of relative plate motion eventually propagates horizontally along the unstable stable transition over a period of several years silent slip events can be interpreted as being caused by the transitional behavior of the fault constitutive law