the fazenda nova gold deposit goias magmatic arc late neoproterozoic intrusion related auriferous mineralization controlled by intracontinental strike slip faulting abstract the fazenda nova gold deposit with an inferred resource of 650 koz at 4 0 g t au cut off grade of 2 0 g t au consists of stockwork au as mineralization hosted in late neoproterozoic dikes in the goias magmatic arc central brazil the dikes which host the mineralization consist of a set of post collisional ilmenite bearing gabbros dolerites trachyandesite porphyry trachyandesite and trachyte porphyry known as the bacil ndia intrusions u pb dating indicates at least two pulses of dike intrusions the first pulse consists of small volume dikes with ages varying from 593 5 to 590 5 ma and the second pulse comprises a large volume of dikes with ages from 574 7 to 572 5 ma three veining or hydrothermal alteration stages are identified in the fazenda nova deposit the first is the early or main stage which comprises the pervasive biotitization of mafic minerals and a stockwork of quartz associated with a disseminated sulfide assemblage of arsenopyrite pyrrhotite stibnite in addition to hydrothermal siderite scheelite rutile and apatite the most common sulfide in this stage is a fine grained acicular arsenopyrite hosting inclusions of gold galena and pyrite grains the temperature estimated for acicular arsenopyrite pyrrhotite and gold is 340 c based on the arsenopyrite geothermometry the intermediate stage is represented by a stockwork of calcite quartz veins associated with hydrothermal sericite chlorite tourmaline epidote and titanite and a sulfide assemblage of pyrrhotite arsenopyrite pyrite the arsenopyrite at this stage displays a tabular texture with no inclusions of sulfides or gold the crystallization temperature of the tabular arsenopyrite pyrite was estimated to be 305 c the c o isotope data from the calcite veins of the second stage provided 13c values ranging from 9 68 to 11 57 and 18o values range from 12 87 to 13 84 these values are equivalent to those of hydrothermal mantle magmatic carbonates the late stage is represented by veining and brecciation infilled by monotonous calcite ankerite with cockade and colloform vein textures and no gold mineralization the dikes and gold mineralization present a strong relationship with the crustal scale strike slip bacil ndia fault this is a second order crustal scale fault that branches out of the major crustal scale moipora novo brazil shear zone mnbsz along a nnw direction the bacil ndia fault played an important role in channeling melts and mineralizing hydrothermal fluids to the upper crust the major activity of this fault occurred at 572 ma with the emplacement of the second pulse of dikes and most likely the development of hydrothermal stages in the fazenda nova deposit the collectively evidence allows fazenda nova to be classified as an epizonal reduced intrusion related deposit following that i the mineralization developed in a post collisional tectonic setting above previously metasomatized subcontinental lithospheric mantle ii the dikes crystallized from metaluminous magmas with a primary oxidation state which formed the ilmenite series intrusions iii the ore deposition had relatively high temperatures 340 300 c and vein textures formed only at shallow depths

effect of hydrostatic pressure on viscous remanent magnetization in magnetite bearing specimens rocks at depth in the crust acquire a viscous i e time dependent magnetization under the pressure temperature conditions at which they reside there have been numerous studies on the effect of temperature on viscous magnetization but little work has been performed on the effect of hydrostatic pressure we have studied viscous remanent magnetization at 22 c in a 0 1 mt field at 0 1 and 100 mpa for natural and synthetic multidomain magnetite the viscous remanent magnetization was found to increase nearly linearly with log time at both pressures there was little change in the rate of viscous acquisition between 0 1 and 100 mpa over the multidomain grain size range studied thus for rocks buried at depth in the earth the enhancement of magnetic viscosity by thermal fluctuations will dominate over effects due to hydrostatic pressure

geology and magmatic hydrothermal evolution of the giant pebble porphyry copper gold molybdenum deposit southwest alaska the pebble deposit is located 320 km southwest of anchorage alaska it is one of the largest porphyry deposits known with a total resource of 10 78 billion tons bt it comprises the east and west zones which are approximately equal in size with slightly lower grade mineralization in the center of the deposit where the peripheries of the two zones merge the west zone was discovered by cominco america in 1989 and the east zone was discovered by northern dynasty minerals in 2005 the oldest rock in the pebble district is the jurassic cretaceous kahiltna flysch unit which contains basinal turbidites interbedded basalt flows and associated gabbro intrusions these rocks were intruded between 99 and 96 ma by coeval granodiorite and diorite sills followed shortly thereafter by alkalic monzonite intrusions and related breccias subalkalic hornblende granodiorite porphyry plutons of the kaskanak batholith were emplaced at 90 ma similar smaller granodiorite plutons were emplaced around the margins of the batholith and are related to cu au mo mineralization re os dates on molybdenite are between 89 7 and 90 4 ma a late cretaceous volcanic and sedimentary cover sequence completely conceals the east zone whereas the west zone is overlain only by glacial sediments and is exposed in one small outcrop eocene volcanic rocks and subvolcanic intrusions occur east and southeast of the pebble deposit and unconsolidated glacial sediments are widespread the east and west zones represent two coeval hydrothermal centers within a single system the west zone extends from surface to 500 m depth and is centered on four small granodiorite plutons emplaced into flysch diorite and granodiorite sills and alkalic intrusions and breccias the much higher grade east zone extends to at least 1 700 m depth and is hosted by a larger granodiorite pluton and adjacent granodiorite sills and flysch the granodiorite plutons merge with depth on the east side of the deposit high grade mineralization has been dropped 600 to 900 m into the ne trending east graben where the deposit remains undelineated to the east and to depth variations in hypogene grade and metal ratios reflect multiple stages of metal introduction and redistribution hornfels related to the kaskanak batholith formed prior to hydrothermal activity at pebble and is most intensely developed in flysch disseminated and vein hosted cu au mo mineralization dominated by chalcopyrite and locally accompanied by bornite formed with potassic alteration in the shallow part of the east zone and with approximately coeval sodic potassic alteration in the west zone and at depth in the east zone illite kaolinite alteration overprinted potassic and sodic potassic alteration throughout the deposit and variably redistributed copper and gold high grade copper gold mineralization is related to advanced argillic alteration controlled by a synhydrothermal brittle ductile fault zone which overprinted potassic sodic potassic and illite kaolinite alteration in the east zone advanced argillic alteration comprises a core of pyrophyllite alteration associated with chalcopyrite bounded to the west by an upward flaring zone of sericite alteration which contains hypogene bornite digenite covellite and trace enargite and tennantite late quartz veins introduced additional molybdenum in several parts of the deposit grade destructive quartz sericite pyrite alteration forms a halo to the entire deposit and yields outward to propylitic alteration a quartz illite pyrite cap is preserved in the weakly mineralized center of the deposit mineralization at pebble is predominately hypogene a thin incompletely developed zone of supergene mineralization occurs in the west zone and is overlain by a thin leached capping there is no evidence for paleosupergene mineralization or leaching below the cover sequence in the east zone molybdenite contains high concentrations of rhenium throughout the deposit elevated palladium concentrations are associated with pyrophyllite alteration in the east zone the pebble deposit occurs in one of a number of large deep seated magnetic anomalies which are located at the intersection of crustal scale structures both parallel and at high angles to a mid cretaceous magmatic arc this setting is similar to fertile porphyry environments in northern chile and suggests that southwestern alaska is highly prospective for porphyry exploration the large size and high hypogene grades of the pebble deposit may reflect a combination of multiple stages of metal introduction with vertically restricted lateral fluid flow induced by hornfels aquitards in flysch

the characterization of viscous remanent magnetization in large and small magnetite particles previous work on the properties of viscous remanent magnetization vrm have shown that it can play an important role in some paleomagnetic investigations the results of this study on the properties of vrm of very fine single domain sd and coarse multidomain md magnetite particles ranging in size from 0 02 to 60 m indicate that the md samples are very sensitive to the previous magnetic history of the samples the af demagnetization causes a dramatic decrease in the ability of md samples to acquire vrm compared to the same sample with an undemagnetized thermal remanent magnetization trm thermal demagnetization of the same md samples causes an increase in the ability to acquire vrm components submicron sd samples show vrm acquisition and decay curves that are independent of previous magnetic treatment both md and sd samples are extremely sensitive to the length of time of zero and weak field storage that occurs prior to the application of the external vrm field a model using the total energy of the system is used to describe the above results

domain size closure domains and the importance of magnetostriction in magnetite abstract previous micromagnetic calculations invoking magnetostatic exchange and crystalline anisotropy energies predict about an order of magnitude more domains in magnetite than have experimentally been observed domains observed on magnetite by the magneto optic kerr effect are of similar size as previously observed ones by the bitter technique simple but quite accurate domain calculations show that for rectangular parallelepipeds of magnetite it is the closure four domain configuration that should be of lowest energy for all multidomain grain sizes unless magnetostrictive energies are considered for the observed domain pattern it is shown that the domains themselves must carry significant magnetoelastic energies

magnetic properties and opaque mineralogy of drilled submarine intrusive rocks summary measurements of natural remanent magnetization nrm curie temperature ferrimagnetic and paramagnetic susceptibility saturation induced and remanent magnetizations coercive forces alternating field properties and viscous magnetization are reported for 50 submarine intrusive rocks drilled during legs 30 37 and 45 of dsdp the collection includes doleritic sill rocks fresh and serpentinized cumulate gabbros and serpentinized cumulate peridotites and serpentinized lherzolites believed to have originated in layers 2b 3b and 4 respectively magnetite with a curie temperature between 520 and 580 c is the principal magnetic mineral in all samples there is no indication of maghemitization or of metamorphism to greenschist facies or above the magnetite in the doleritic and cumulate gabbros is a product of deuteric alteration of titanomagnetite and pyroxene the stable nrm is a primary trm the magnetite in the serpentinized rocks is a secondary product of serpentinization the stable nrm is a crm in most rock types the magnetite is of single domain or pseudo single domain size and soft components of nrm are small the magnetite grain size in some of the doleritic gabbros is much coarser these rocks acquire large viscous magnetizations which however are readily removed by alternating field cleaning the cleaned nrms of legs 30 and 45 rocks have approximately dipole inclinations but the nrms of most of the mutually intruded leg 37 units have been dispersed by tectonic rotations the doleritic gabbros and the serpentinized peridotites have stable directionally coherent nrms 10 3emu cm 3 in intensity their counterparts in layers 2b and 3b 4 are likely contributors to oceanic magnetic anomalies layer 3b cumulate gabbros and tectonic serpentinites in the middle and upper crust are less likely anomaly sources

a fluid inclusion and stable isotope study at the loulo mining district mali west africa implications for multifluid sources in the generation of orogenic gold deposits the world class loulo mining district 15 5 moz resource in the birimian terrane of western mali contains a range of mineralogically diverse styles of orogenic gold mineralization the district is distinguished by as rich orebodies as at yalea as well as tourmaline bearing fe rich orebodies as at gara new fluid inclusion and stable isotope data presented here constrain the nature of the hydrothermal fluids responsible for these different types of mineralization and point towards the role of multifluid sources metamorphic and magmatic in the formation of orogenic gold deposits microthermometric and laser raman studies from yalea main and two other similar orebodies loulo 3 and baboto reveal the dominance of coexisting co 2 n 2 ch 4 type 1 and h 2 o nacl type 2 fluid inclusions these inclusions show evidence of fluid unmixing from reduced quartz faylite magnetite qfm buffered low salinity 10 wt nacl equiv co 2 rich h 2 o nacl n 2 ch 4 primary ore fluids the combination of microthermometric data and geothermometry based on ore and alteration assemblages indicate trapping temperatures and pressures of 270 to 340 c at 1 4 to 1 8 kbar the p t x state of the yalea style mineralizing fluids along with the 18 o fluid compositions of 8 8 to 10 7 is consistent with the derivation of auriferous fluids during greenschist facies regional metamorphism of the host terrane similar fluid compositions were previously reported elsewhere in the birimian crust of west africa and in other orogenic gold districts worldwide the precipitation of gold from the h 2 s rich metamorphic fluid is primarily linked to phase separation of the ore fluid which is controlled by p t fluctuations and or fluid rock interaction with carbonaceous host sedimentary rocks confirmed by low 13 c values of 21 7 to 15 8 fluid inclusion investigations from gara and a similar style orebody yalea north indicate the presence of coexisting co 2 n 2 ch 4 type 1 and mixed salinity 5 21 wt nacl equiv co 2 rich h 2 o nacl inclusions type 3 inclusion assemblages also contain common oxidized hematite magnetite hm buffered high temperature 400 c hypersaline 35 50 wt total dissolved solids metalliferous na fe ca cu ni w pb zn multiphase h 2 o rich co 2 nacl fecl 2 inclusions type 4 this inclusion type has not been previously reported in other birimian terranes the composition of the brines along with carbon isotope data 13 c of 14 4 to 4 5 suggests a magmatic input to the gara style hydrothermal system the coexistence in the fluid inclusion assemblages of the magmatic brines with yalea style co 2 rich metamorphic fluids and the positive correlation between salinity and homogenization temperatures suggest mineralization was locally controlled by fluid mixing the interaction of these two chemically contrasting fluids explains the distinctive petrographic characteristics of the gara style orebodies this includes the growth of widespread multi stage fe 3 rich tourmaline b rich granite source and sodic alteration and ore assemblages consisting of abundant nickeloan pyrite ree phosphates ni co pb zn minor trace sulfides and scheelite gold deposition in the gara style hydrothermal system is related to physical and chemical changes of the two fluids during mixing e g decreases in f o2 and t in the brines and retrograde boiling of the co 2 component in the metamorphic fluids a salting out effect

evidence of fluid inclusions for two stages of fluid boiling in the formation of the giant shapinggou porphyry mo deposit dabie orogen central china abstract the shapinggou porphyry mo deposit one of the largest mo deposits in asia is located in the dabie orogen central china hydrothermal alteration and mineralization at shapinggou can be divided into four stages i e stage 1 ore barren quartz veins with intense silicification followed by stage 2 quartz molybdenite veins associated with potassic alteration stage 3 quartz polymetallic sulfide veins related to phyllic alteration and stage 4 ore barren quartz calcite pyrite veins with weak propylitization hydrothermal quartz mainly contains three types of fluid inclusions namely two phase liquid rich type i two or three phase gas rich co 2 bearing type ii and halite bearing type iii inclusions the last two types of fluid inclusions are absent in stages 1 and 4 type i inclusions in the silicic zone stage 1 display homogenization temperatures of 340 to 550 c with salinities of 7 9 16 9 wt nacl equivalent type ii and coexisting type iii inclusions in the potassic zone stage 2 which hosts the main mo orebodies have homogenization temperatures of 240 440 c and 240 450 c with salinities of 34 1 50 9 and 0 1 7 4 wt nacl equivalent respectively type ii and coexisting type iii inclusions in the phyllic zone stage 3 display homogenization temperatures of 250 345 c and 220 315 c with salinities of 0 2 6 5 and 32 9 39 3 wt nacl equivalent respectively type i inclusions in the propylitization zone stage 4 display homogenization temperatures of 170 to 330 c with salinities lower than 6 5 wt nacl equivalent the abundant co 2 rich and coexisting halite bearing fluid inclusion assemblages in the potassic and phyllic zones highlight the significance of intensive fluid boiling of a nacl co 2 h 2 o system in deep environments up to 2 3 kbar for giant porphyry mo mineralization hydrogen and oxygen isotopic compositions indicate that ore fluids were gradually evolved from magmatic to meteoric in origin sulfur and lead isotopes suggest that the ore forming materials at shapinggou are magmatic in origin re os dating of molybdenite gives a well defined 187 re 187 os isochron with an age of 112 7 1 8 ma suggesting a post collisional setting

temperature dependence of magnetic domains in magnetite crystals synthetically grown and natural multidomain magnetite crystals d 30 m have strongly temperature dependent magnetic domain structures the unblocking of domain walls at moderate temperatures t 120 c makes these magnetite grains unsuitable as paleomagnetic recorders approximately 15 of the smaller hydrothermally grown magnetite crystals d 10 m show stable domain patterns when heated to 350 c these magnetites with temperature independent domain structures can possibly account for paleo magnetic remanences with high blocking temperatures

magnetic properties opaque mineralogy and magnetic anisotropies of serpentinized peridotites from odp hole 670a near the mid atlantic ridge abstract magnetic mineralogy and magnetic properties of 16 samples of serpentinized peridotites harzburgites from odp hole 670a are studied the degree of serpentinization was determined by a new method which combines saturation magnetization and whole rock iron content a unique curie temperature t c of 580 c corresponding to pure magnetite was observed microprobe analyses whole rock metal content and curie temperatures indicate the absence of awaruite ni 3 fe or native metals hysteresis parameters reveal the contribution of pseudo single domain psd grains in samples with a low degree of serpentinization and large multi domain md grains in highly serpentinized samples with j rs j s 0 08 natural remanent magnetization nrm is mainly unstable and possibly composite particularly in the highly serpentinized samples the intensity j a of anhysteretic remanent magnetization arm in the geomagnetic field is much higher than j n intensity of nrm with j a j n averaging 2 4 the koenigsberger ratio q is low and susceptibility k depends on degree of serpentinization ranging from 1 2 to 51 5 10 6 m 3 kg 1 nrm inclinations are scattered with some tendency to lie near the horizontal plane which is close to the foliation plane anisotropies of susceptibility of arm and of saturation remanent magnetization srm were studied and compared with spinel foliation determined for three less serpentinized samples in these samples magnetic anisotropies seem to be mimetic of high temperature deformation before serpentinization when the degree of serpentinization is high later magnetic fabrics probably related to magnetite veins dominate previous anisotropies the temperature dependence of susceptibility and of viscous remanent magnetization vrm were studied the thermal variation of susceptibility shows a shallow hopkinson effect at about 300 400 c time dependence of induced and remanent magnetization increases with temperature reaching maximum values at about 300 400 c based on these observations induced magnetization and vrm of lower crustal and upper mantle rocks and their contribution to marine magnetic anomalies are discussed

magnetization of lower crust and interpretation of regional magnetic anomalies example from lofoten and vesteralen norway the interpretation of regional magnetic anomalies commonly observed over continental crust in both satellite and near surface magnetic data has remained enigmatic in its requirement for laterally extensive thick highly magnetic and deep seated sources presented here are the results of a detailed field analytical and experimental study of the magnetic mineralogy and magnetic properties of rocks from lofoten and vesteralen norway a province of deep seated origin with associated regional magnetic and gravity anomalies these rocks have a bulk magnetic character of surprising uniformity with susceptibilities hysteresis and curie temperatures defined solely by coarse grained magnetite stable remanence is observed in samples from sw lofoten but is not of significant magnitude the thermal enhancement of magnetic susceptibility the hopkinson effect is not important rocks from the deepest part of the section have the largest average susceptibilities 6 10 2 si and magnitudes of natural remanent magnetization nrm 5 10 4 a m2 kg these values of susceptibility together with the oft observed viscous character of the nrm suggest that these granulite facies lithologies particularly those of sw lofoten are perfect examples of lower crustal sources of typical regional magnetic anomalies furthermore these intermediate pressure granulites define a unique magnetic petrology especially for mafic compositions at higher grades of metamorphism high pressure granulites and eclogites or lower grades greenschist and amphibolite facies such magnetite bearing assemblages will be unstable and their high or low grade equivalents will be only weakly magnetic this evidence and these conclusions may be used to constrain models of lower crustal sources of regional magnetic anomalies over stable cratons it portrays the lower crust as a highly magnetic unit defined by metamorphic grade with a curie isotherm of 575 c magnetite and magnetization parallel to the ambient geomagnetic field induced and viscous components

stretched exponential relaxation of viscous remanence and magnetic dating of erratic boulders viscous remanence continuously increases with the duration of reorientation of rocks and the remanence gets partially overprinted in rocks parallel to the earth s magnetic field this overprinted viscous remanence is unblocked at a certain temperature that enables the estimation of the time required for the rock to acquire the magnetism by assuming the exponential law of neel s single domain theory however previous results of dating the rocks by the exponential law have shown older ages than radiometric or cosmogenic exposure ages neel s exponential decay law is applicable to a system whose magnetic grains have an identical relaxation time however in real systems the expected behavior is not usually observed because relaxation times vary for individual grains moreover the variation of viscous remanence with the logarithmic law for a distribution of relaxation times is predicted to be concave downwards here we found that the stretched exponential law exp t 1 n with 0 n 1 explains previously published data on viscous decay the observed stretched exponential relaxation can be interpreted in terms of the global relaxation of a system containing many relaxing species each of which decays exponentially in time with a specific fixed relaxation rate using this law we derived an extended time temperature relationship of magnetite involving the neel s exponential decay law with n 0 and a system containing many relaxing times with variable n the extended time temperature relationship shows that the age of a coral tsunami boulder with high anomalous unblocking temperatures can be fitted with an assigned radiometric age

remagnetization in lava flows recording pretransitional directions basaltic lava flows associated with the last reversal in three distinct volcanic sequences of the island of la palma are characterized by overlapping components with very high unblocking temperatures a medium high temperature mht component with normal polarity is resistant up to 500 550 c while in most cases the high temperature ht primary reverse component cannot be isolated below 520 c the same situation with opposite polarities is observed also in a lava flow associated with the onset of the upper reunion reversal in the gamarri sequence in ethiopia the presence of both polarities within all these single flows cannot be reconciled with the hypothesis that a full reversal was systematically recorded during their cooling the direction of mht is either close or similar to the direction of the ht component of the overlying flows which indicates that remagnetization occurred shortly after emplacement rock magnetic studies microscopic observations microprobe analyses and remagnetization experiments have been carried out to investigate the origin of mht the rock magnetic parameters do not exhibit specific anomalies the unblocking temperatures are mostly higher than the simulations involving reheating by the overlying flow combined with thermoviscous overprinting caused by slow cooling the most plausible scenario is that baking by the overlying flows was accompanied by low temperature oxidation of titanomagnetite to cation deficient titanomagnetite this description also includes the presence of cracks fractures and other heterogeneities of the lava which induce variability between parallel profiles similarities with characteristics observed at steens mountain could suggest that the hypothesis of rapid geomagnetic changes recorded by a single lava flow should be considered with caution

the moho as a magnetic boundary revisited we have re examined the question of whether it is generally true that in continental regions a relatively magnetic crust overlies a relatively non magnetic mantle whereas the original conclusion was based on limited data the present study is based on about 400 globally distributed xenolith samples while it is now clear that the moho represents a maximum velocity gradient in a complex transition from crust to mantle and while crustal magnetization is highly variable the original concept of a magnetic crust overlying a non magnetic mantle is strongly supported by the new data a large upper mantle xenolith suite of ultramafic composition is characterized by non magnetic chrome spinels and magnesian ilmenites such rocks are magnetic only where demonstrably altered the most significantly magnetic rocks within the crust are mafic types prograded to granulite grade or mafic melts of basaltic composition which crystallized deep in the crust at high pressure and temperature prograded rocks have characteristic magnetite curie points and are strongly magnetic especially the more mafic compositions while the mafic rocks formed by crystallization at depth have curie points between 400 570 c and are strongly magnetic induced magnetizations are commonly several a m and can readily account for long wavelength magnetic anomalies measured by satellite and aircraft thus there is no need to search for exotic sources for missing crustal magnetization no metal or other peculiar magnetic species have been observed by us in any of our unaltered samples mineral assemblages including magnetic components are consistent with oxidizing conditions close to the fmq buffer

some aspects of magnetic viscosity in subaerial and submarine volcanic rocks summary the magnetic viscosity of 334 upper tertiary and pre bruhnes quaternary volcanic rocks from the massif central france and the steens mountain oregon and of 40 basaltic cores from dsdp leg 37 has been investigated thellier s viscosity index follows a log normal distribution with mean values equal to 6 and 3 5 per cent for subaerial and submarine rocks respectively for subaerial rocks the average intensity of the viscous remanent magnetization vrm acquired in the earth s field since the beginning of the bruhnes polarity epoch t 0 7 myr is estimated to be equal to one quarter of the average intensity of the primary remanence alternating field demagnetization of vrms acquired in low fields for acquisition times t ranging from 2 day to 32 month indicates the resistance to alternating fields is quite different from sample to sample and increases linearly with log t neel s diagnostic parameter of domain structure of the grains involved in magnetic viscosity shows that hard vrm is carried by single domain grains and soft vrm carried by multidomain particles single domain particles carrying hard vrm in subaerial volcanic rocks are almost equant magnetite intergrowths with size near the superparamagnetism threshold resulting from high temperature oxidation of titanomagnetite soft vrm is carried by low curie point homogeneous titanomagnetite unlike his single domain theory neel s multidomain theory of magnetic viscosity does not account quantitatively for the resistance of vrm to alternating fields

a model of ocean basin crustal magnetization appropriate for satellite elevation anomalies a model of ocean basin crustal magnetization measured at satellite altitudes is developed which will serve both as background to which anomalous magnetizations can be contrasted and as a beginning point for studies of tectonic modification of normal ocean crust the model is based on published data concerned with the petrology and magnetization of the ocean crust and consists of viscous magnetization and induced magnetization estimated for individual crustal layers thermal remanent magnetization and chemical remanent magnetization are excluded from the model because seafloor spreading anomalies are too short in wavelength to be resolved at satellite altitudes the exception to this generalization is found at the oceanic magnetic quiet zones where thermal remanent magnetization and chemical remanent magnetization must be considered along with viscous magnetization and induced magnetization

redox controlled generation of the giant porphyry cu au deposit at pulang southwest china some porphyry cu au deposits with relatively reduced ore assemblages characterized by high hydrothermal pyrrhotite contents and a lack of primary hematite and magnetite are generally considered to be associated with reduced i type granitoids however the role of magmatic oxygen fugacity fo2 in controlling cu au mineralization in such reduced porphyry deposits is poorly understood the giant late triassic ca 216 ma pulang porphyry cu au deposit of southwest china shows typical reduced ore assemblages this study reported the systematical variation of upper crustal magmatic fo2 of pulang deposit based on detailed investigations of mineral crystallization sequences and compositional features of the mineralization related porphyries early p1 and late p2 porphyry results indicate that magma of the mineralization related porphyries experienced complex fo2 fluctuations during its upper crustal evolution the early primary magma had very high initial fo2 with fmq 3 0 at depths of 12 km fmq is the deviation of logfo2 from the fayalite magnetite quartz fmq buffer the fo2 of evolved parental magma subsequently decreased with fmq 1 9 due to injection of relatively reduced dioritic magmas fmq 1 4 to 2 3 from a deeper chamber 17 21 km depth into the primary magma chamber at 10 12 km depth magma mixing had largely ceased at 6 10 km depth the parental magma then ponded within the reduced tumugou formation at a depth of 3 7 km where magmatic fo2 decreased to a moderately oxidized state fmq 1 6 and finally to a moderately reduced state reflected by log fe2o3 feo ratios of 0 5 for p2 porphyry results of this study of magmatic fo2 indicate that porphyry magmas associated with reduced pulang ore assemblages were initially generated as highly oxidized magma which was subsequently reduced through magma mixing and contamination by reduced sedimentary rocks of the tumugou formation the sharp fo2 decrease at very shallow depth prevented the early loss of cu and au because the magma remained oxidized until it was emplaced at 3 7 km depth moderately reduced magmas may thus have a genetic association with porphyry cu au mineralization

high temperature viscous magnetization of oceanic deep crustal and mantle rocks as a partial source for magsat magnetic anomalies the magsat data have yielded a global cartography of long wavelength anomalies of lithospheric origin the magnetizations needed to model these anomalies are much higher than those measured on samples by laboratory experiments an enhancement of both induced and viscous magnetization at high temperature has often been put forward to account for the missing magnetization experimental measures of viscous magnetization mv at high temperature are presented for oceanic deep crustal and mantle rocks at room temperature mv hereafter defined in experiments as the magnetization acquired after 400 s in a weak field is negligible in comparison with the room temperature induced magnetization mk0 at higher temperatures between 200 c and 450 c mv represents about 20 of mk0 and is further enhanced near 500 c ranging from 25 to 130 of mk0 an extrapolation of these results to geological times shows that the viscous magnetization acquired since the last reversal of the earth magnetic field may be of the order of magnitude of the induced magnetization mk0 above 200 c and possibly predominant near 500 c however a magnetic uppermost mantle is still needed to explain the magnetizations of oceanic lithosphere derived from magsat data

acquisition of viscous remanent magnetization abstract viscous remanent magnetization vrm has been frequently suggested as a possible source of magnetic anomalies in ocean crust and as an important contributor of natural remanent magnetization in some volcanic and sedimentary rocks most references to vrm assume a linear dependence with log t yet long term experimental data on vrm while scarce often disagree with this simple notion here we investigate the angular dependence grain size dependence initial state dependence and time t dependence of vrm acquisition we observe a non linear log t dependence of vrm acquisition over all time intervals 104 s for all the samples regardless of various experimental conditions echoing earlier studies we also find that vrm acquisition is strongly dependent on the initial magnetic state of the samples samples in a thermally demagnetized initial state were most susceptible to vrm acquisition when vrm is produced parallel to the initial thermal remanence vrm acquisition is negligible vrm acquisition is more prevalent for magnetites in single domain and multidomain md grains than in the pseudo single domain range in md magnetites frequent removal and restoration of samples for the purpose of zero field remanence measurement apparently increases the efficiency of vrm acquisition

magnetic properties of exposed deep crustal rocks from the superior province of manitoba abstract a traverse across precambrian rocks of the superior province in central manitoba provided 56 samples for magnetic study from the pikwitonei granulite domain and the adjacent cross lake subprovince these rocks represent a cross section of the continental crust presently exposed at the surface thirty one samples are dominantly paramagnetic and three contain pyrrhotite twenty two samples from silicic plutonic rocks which make up about 80 of the section by volume contain nearly pure magnetite and have susceptibilities that are almost constant between room temperature and 540 c averaging about 1 10 2 si units the mafic supracrustal rocks with much higher iron content are about an order of magnitude less magnetic because they contain little or no magnetite susceptibility is generally independent of metamorphic grade or apparent depth in the crust the discrepancy between induced magnetizations of measured lower crustal rocks and the source requirements of long wavelength magnetic anomalies may be due to a high temperature viscous magnetization in the deep crust

the late cretaceous donlin creek gold deposit southwestern alaska controls on epizonal ore formation the donlin creek gold deposit southwestern alaska has an indicated and inferred resource of approximately 25 million ounces moz au at a cutoff grade of 1 5 g t the ca 70 ma deposit is hosted in the late cretaceous kuskokwim flysch basin which developed in the back part of the arc region of an active continental margin on previously accreted oceanic terranes and continental fragments a hypabyssal mainly rhyolitic to rhyodacitic and commonly porphyritic 8 3 km dike complex part of a regional ca 77 to 58 ma magmatic arc formed a structurally competent host for the mineralization this deposit is subdivided into about one dozen distinct prospects most of which consist of dense quartz carbonate veinlet networks that fill north northeast striking extensional fractures in the northeast trending igneous rocks the sulfide mineral assemblage is dominated by arsenopyrite pyrite and typically younger stibnite gold is refractory within the arsenopyrite sericitization carbonatization and sulfidation were the main alteration processes fluid inclusion studies of the quartz that hosts the resource indicate dominantly aqueous ore fluids with also about 3 to 7 mol percent co2 ch4 and a few tenths to a few mole percent nacl kcl the gold bearing fluids were mainly homogeneously trapped at approximately 275 to 300 c and at depths of 1 to 2 km some of the younger stibnite may have been deposited by late stage aqueous fluids at lower temperature measured 18o values for the gold bearing quartz range between 11 and 25 per mil the estimated 18o fluid values range from 7 to12 per mil suggesting a mainly crustally derived fluid a broad range of measured d values for hydrothermal micas between 150 and 80 per mil is suggestive of a contribution from devolatilization of organic matter and or minor amounts of mixing with meteoric fluids gold associated hydrothermal sulfide minerals are characterized by 34s values mainly between 16 and 10 per mil with the sulfur derived from diagenetic pyrite and organic matter within the flysch basin a smaller group of 34s measurements which shows values as depleted as 27 per mil suggests a different local sulfur reservoir in the basin for the later hydrothermal episode dominated by stibnite initial nd of 8 7 to 3 1 and 87sr 86sr measurements of 0 706 to 0 709 for the ore hosting dikes also indicate a crustal reservoir for some of the late cretaceous magmatism overlapping lead isotope data for these intrusive rocks and for sulfide minerals suggest a crustal contribution for the lead in both copper and gold bearing stockwork veinlets in hornfels occur at dome a prospect located at the northern end of the donlin creek deposit these stockworks are cut by the younger auriferous gold veins that define the main donlin creek gold mineralization highly saline gas rich heterogeneously trapped fluids deposited the stockworks at temperatures approximately 100 c hotter than those of the main gold forming event at donlin creek the genetic relationship of the dome prospect to the main donlin creek gold resource is equivocal the epizonal donlin creek deposit shows affinities to the gold systems interpreted by various workers as orogenic or intrusion related it shows important differences from typical epithermal and carlin like deposits the ore forming fluids were derived by either broad scale metamorphic devolatilization above rising mantle melts or exsolution from a magma that was dominated by a significant flysch melt component

is the palea kavala bi te pb sb au district northeastern greece an intrusion related system abstract intrusion related gold systems are generally characterized by a au bi te sn w metal assemblage genetically linked to the emplacement of granitoids the palea kavala ore system greece consists of 150 minor fe mn pb zn ag fe mn au fe as au fe cu au and bi te au occurrences that occur primarily in quartz calcite sulfide veins hypogene mineralization or as supergene bodies in overlapping zones centered on the 21 22 ma granodioritic kavala pluton which intrudes metamorphic rocks of the paleozoic rhodope metamorphic core complex the pluton consists mostly of granodiorite with lesser amounts of diorite tonalite and monzodiorite which was emplaced along the regional e w trending kavala komotini fault the recently discovered 4 km long e w trending so called kavala vein is a sheeted quartz vein system of bi te pb sb au mineralization that crosscuts the kavala pluton and the schists and gneisses of the rhodope massif the kavala vein system is comprised of quartz with lesser amounts of k feldspar plagioclase and muscovite quartz sericite pyrite alteration is pervasive but minor kaolinite is also present pyrite 5 of vein volume contains inclusions of tetradymite some gold bearing bismuthinite and cosalite sulfur isotope values n 27 of pyrite from the kavala and chalkero veins as well as pyrite and galena from garizo hill fe mn pb vein range from 1 9 to 1 0 with one outlier of 4 6 and suggest a magmatic sulfur source homogenization temperatures th of type i two phase aqueous liquid vapor and type ii three phase h2o co2 rich fluid inclusions that homogenize into the liquid phase in quartz from the kavala and chalkero veins range from 216 0 to 420 0 c n 216 and 255 7 to 414 0 c n 112 respectively the th of type iii two phase aqueous liquid vapor which homogenize into the vapor phase ranges from 210 4 to 323 4 c n 28 the salinities of type i and type ii inclusions range from 15 9 to 22 6 wt nacl equiv and 5 5 to 11 2 wt nacl equiv respectively eutectic temperatures of 58 5 to 44 3 c for type i inclusions suggest the presence of appreciable cacl2 in addition to nacl clathrate melting temperatures for type ii inclusions of 56 7 c indicate that co2 is the major component of the gaseous phase however up to 6 ch4 is present in some inclusions the presence of a zoned metallogenetic district centered on bi te pb sb au mineralization within the kavala pluton the presence of both magnetite and ilmenite in the kavala pluton and the two high temperature high salinity immiscible carbonic and aqueous fluids associated with the kavala and chalkero veins are consistent with them being part of an intrusion related gold system that formed along the ilmenite magnetite buffer

mineral potential mapping in an accreted island arc setting using aeromagnetic data an example from southwest alaska the distribution of volcanogenic massive sulfide vms porphyry epithermal alaska type ultramafic mafic complexes intrusion related au and granitoid sn w ore deposits in southwest alaska supports current metallogenic models linking the formation of these deposit types to the emplacement of different suites of igneous rocks during the evolution of this convergent plate margin regional scale aeromagnetic data provide a continuous set of observations over the deposits and show contrasting patterns over the igneous rock suites hosting the various deposit types combined with surface geologic data and regional metallogenic constraints aeromagnetic data filtered to enhance the anomalous magnetic field and map magnetic domains were used to produce a mineral potential map across this accreted island arc setting the reduced to pole upward continuation and total horizontal gradient transform maps show anomalies that could represent porphyry epithermal deposits within the intraoceanic and continental arc terranes the tilt derivative transform highlights lineaments within the back arc that may represent zones with potential for vms deposits the truncations of tilt derivative lineaments outline a major magnetic domain boundary between the back arc and craton margin which is prospective for granitoid sn w deposits annular tilt derivative highs outline granitoids that could be associated with intrusion related au deposits within the craton margin shallow magnetite rich alaska type ultramafic mafic complexes are mapped by their short wavelength high amplitude anomalies successful mineral potential mapping across southwestern alaska as performed in the present study suggests that filtered aeromagnetic data can be effectively used in mineral exploration in convergent continental margin settings

behavior of oceanic crustal magnetization at high temperatures viscous magnetization and the marine magnetic anomaly source layer although the source layer for marine magnetic anomalies has been assumed to be the extrusive basalts of up permost ocean crust recent studies indicate that lower crustal rocks may also contribute because the temperature at which magnetization of crustal rocks achieves long term stability is crucial to any source layer contribution we undertook high temperature vrm viscous remanent magnetization experi ments on samples of basalt dike and gabbroic sections sam ples were heated at temperature intervals up to to while a magnetic field was applied for periods between 6 hours and 28 days results show that the dike and gabbro samples achieve maximum vrm acquisition near 250oc well below the tc of 580oc the basalt sample shows a peak at 68oc also well below t results of this pilot study indicate that the critical isotherm for stable magnetization acquisition is de fined by the vrm behavior of the specific crustal section

elevated temperature viscous remanent magnetization of natural and synthetic multidomain magnetite abstract the time temperature relationship of magnetization is a subject of much interest and debate by paleomagnetists rock magnetists and magnetic anomaly modellers we have investigated this relationship by studying the viscous remanent magnetization vrm of coarse grained multidomain md magnetite our experiments covered the temperature range from 22 to 400 c times from minutes to months and included both australian granulite samples and multidomain magnetite samples synthesized by the glass ceramic method vrm acquisition was found to generally increase with temperature but not always at the rate predicted from classical thermal fluctuation theories thermal cycling between room temperature at which the measurements were made and the vrm acquisition temperature sharply decreased the temperature dependence of the vrm room temperature vrm acquisition accelerates with time when plotted on a semilog plot whereas at elevated temperature the curves are quasilinear against log time for both the natural and synthetic samples this change in behavior may suggest a variation in the vrm acquisition mechanism as a function of temperature for md magnetite the granulites have a nearly linear increase in vrm acquisition rate with temperature whereas the glass ceramics display little change in the acquisition rate between 22 and 200 c but increase by nearly a factor of 3 by 400 c the increase in vrm of the glass ceramics between 200 and 400 c is in general qualitative agreement with thermal fluctuation theory there was no systematic change in the rate of vrm acquisition with grain size for the multidomain magnetites used in this study elevated temperature e g 400 c vrm acquisition by the deep crustal granulites if extrapolated over the brunhes chron would produce a magnetization of several a m which if true is of the order required by models for the source of long wavelength magnetic anomalies

magnetic viscosity of some doleritic basalts in relation to the interpretation of the oceanic magnetic anomalies the growth of induced magnetization vim with time t in a 5 oe acquisition field followed by subsequent decrease in zero field of the viscous remanent magnetization vrm with time t was investigated at room temperature for 5 oceanic doleritic basalts and at temperature up to 105 c for 1 dike sample both vim acquisition and vrm decay are enhanced by increasing temperature and time however the growth and decay do not uniformly obey a log t law on an experimental time of 2 hours the results show that far from the ridge axes at the bottom of oceanic pillow layer 2a and in the upper part of the dike layer 2b the induced magnetization might largely dominate the remanent one while only a small fraction of the primary trm corresponding to the highest blocking temperatures tb would be preserved the partial trm corresponding to the lowest tb having been replaced by a viscous magnetization consequently the doleritic basalts would rarely if ever contribute to oceanic magnetic anomalies

the sushibar an automated system for paleomagnetic investigations 1 we present a new automated system based on a three axis superconducting magnetometer and a custom made coil designed to experiment on cylindrical specimens used in typical paleomagnetic investigations the system which resembles a sushi bar facilitates stepwise alternating field demagnetization of up to 99 samples per loaded track it also enables researchers to explore magnetic properties using an anhysteretic remanent magnetization arm in any coercivity window up to peak alternating fields of 95 mt with direct current bias fields up to 0 17 mt for example partial arm parm spectra characterize magnetic grain size distributions in rocks yet rarely are parm spectra measured because the complete curve for one sample takes at least two hours to acquire manually the sushibar achieves 99 such curves in slightly less than 100 hours using the sushibar we measured the parm sprectra as well as the viscosity and anisotropy of arm in three discrete switching field windows of continental sediments from the xishuigou section western china the average grain size remains constant along the 2200 m thick section yet magnetic viscosity varies systematically from bottom to top of the section samples with high magnetic viscosities also have higher proportions of non viscous material on average principal anisotropy axis directions from the lowest switching fields correlate well with principal axis directions from anisotropy of magnetic susceptibility principal axis directions defined at higher switching fields systematically deviate from those at lower switching fields perhaps defining the fabric of the remanence carrying grains

geological analysis of aeromagnetic data from southwestern alaska implications for exploration in the area of the pebble porphyry cu au mo deposit aeromagnetic data are used to better understand the geology and mineral resources near the late cretaceous pebble porphyry cu au mo deposit in southwestern alaska the reduced to pole rtp transformation of regional scale aeromagnetic data shows that the pebble deposit is within a cluster of magnetic anomaly highs similar to pebble the iliamna kijik and neacola porphyry copper occurrences are in magnetic highs that trend northeast along the crustal scale lake clark fault a high amplitude short to moderate wavelength anomaly is centered over the kemuk occurrence an alaska type ultramafic complex similar anomalies are found west and north of kemuk a moderate amplitude moderate wavelength magnetic low surrounded by a moderate amplitude short wavelength magnetic high is associated with the gold bearing shotgun intrusive complex the rtp transformation of the district scale aeromagnetic data acquired over pebble permits differentiation of a variety of jurassic to tertiary magmatic rock suites jurassic cretaceous basalt and gabbro units and late cretaceous biotite pyroxenite and granodiorite rocks produce magnetic highs tertiary basalt units also produce magnetic highs but appear to be volumetrically minor eocene monzonite units have associated magnetic lows the rtp data do not suggest a magnetite rich hydrothermal system at the pebble deposit the 10 km upward continuation transformation of the regional scale data shows a linear northeast trend of magnetic anomaly highs these anomalies are spatially correlated with late cretaceous igneous rocks and in the pebble district are centered over the granodiorite rocks genetically related to porphyry copper systems the spacing of these anomalies is similar to patterns shown by the numerous porphyry copper deposits in northern chile these anomalies are interpreted to reflect a late cretaceous magmatic arc that is favorable for additional discoveries of late cretaceous porphyry copper systems in southwestern alaska

a fluid inclusion and stable isotope study of the moonta copper gold deposits south australia evidence for fluid immiscibility in a magmatic hydrothermal system abstract the copper gold ores of the moonta mineral field in the eastern part of the archaean mesoproterozoic gawler craton south australia are vein deposits occupying fractures and shear zones within the palaeoproterozoic moonta porphyry the veins show a spatial relationship with mesoproterozoic granites equivalents of the hiltaba suite granites that are widespread in the gawler craton the ores were deposited in four hydrothermal stages separated by episodes of fracturing the first two stages were dominated by fe oxides the third by fe sulfides and the final stage by a cu fe co au zn pb s assemblage three types of fluid inclusions are present in quartz associated with the oxide and sulfide minerals type 1 fluid inclusions are two phase vapour rich with homogenization temperatures up to 473 c and low to moderate salinity 2 to 22 wt nacl equivalent type 2 are two phase liquid rich with lower homogenization temperatures up to 269 c and low to moderate salinities 0 7 to 22 wt nacl equivalent type 3 are liquid rich solid s bearing with homogenization temperatures up to 467 c and high salinities 28 to 55 wt nacl equivalent the coexistence of types 1 and 3 suggests that these fluid inclusions resulted either from trapping of boiling fluids or represent two immiscible fluids most likely derived from a magma the fluid history reflected in fluid inclusion characteristics was complex involving variable amounts of boiling cooling and mixing in the vein system salinity homogenization temperature relationships indicate mixing of the hypersaline fluid and the vapour rich fluid with a surface derived fluid of low temperature and low to moderate salinity d h 2 o values 22 to 31 and 18 o h 2 o values 2 9 to 5 1 calculated from analyses of chlorite and 18 o h 2 o values calculated from analyses of quartz 0 6 to 8 8 on the basis of mean to maximum fluid inclusion homogenization temperatures are intermediate between magmatic water and surface derived fluids i e seawater or meteoric water the narrow range of 18 o values of quartz 10 8 to 12 7 may be explained by mixing of a magmatic fluid and either seawater or a low latitude moderately evolved meteoric water with a coupled decrease in fluid temperature and 18 o content similar ranges for 34 s values of ore sulfides 2 3 to 6 4 and disseminated sulfides in the moonta porphyry and interdigitated doora schist 1 5 to 4 6 suggest assimilation of crustal sulfur by the granitic magma or ore fluids the combined fluid inclusion and stable isotope data support previous proposals based on field and mineralogical evidence for a genetic relationship between the moonta ores and the mesoproterozoic granitoids

organic and black carbon 13c and 14c through the santa monica basin sediment oxic anoxic transition black carbon bc is a significant percentage of sedimentary organic carbon soc at abyssal ocean sites but its presence in shelf sediments is not well studied approximately 1600ad santa monica basin bottom waters shifted from oxic to very low oxygen dysoxic deposition conditions under oxic deposition conditions bc was 11 4 of soc whereas after the overlying water became dysoxic and sediments became anoxic bc was 5 2 1 2 of soc this shift may reflect the preferential remineralization of non black soc under oxic conditions there is an offset between bc and soc 14c ages which changes with oxidation conditions suggesting that bc storage is related to oxygen exposure and confirming a previously published report of the vulnerability of bc to sedimentary oxidation middelburg 1999 terrestrial carbon is 17 5 of total soc in this core s anoxic region and 31 11 of this terrestrial carbon is bc

the origin of magnetic anomalies in lower crustal rocks labrador mid to lower crustal granulites near wilson lake in central labrador exhibit pronounced aeromagnetic anomalies areas characterized by high and low field strength correlate with the wilson lake allochthon and the surrounding autochthon respectively the induced magnetization in both units has mean values of 0 34 a m allochthon and 0 33 a m autochthon however the mean values of remanent magnetizations are 9 2 a m allochthon and 3 3 a m autochthon in directions contributing to the present geomagnetic field therefore the remanent magnetization plays the main role in generating the observed magnetic anomaly thermal demagnetizations indicate that titanhematite is stable to 585 600 c the absence of significant verwey transitions indicates that titanhematite is the predominant carrier in some rocks and a significant carrier along with magnetite in others any interpretation of the magnetic anomalies above these rocks must consider magnetic remanence as the principle anomaly source

emplacement depth and carbon dioxide rich fluid inclusions in intrusion related gold deposits phanerozoic intrusion related gold deposits have many consistent characteristics such as a spatial temporal and geochemical association with moderately reduced predominantly ilmenite series i type intrusions the deposits exhibit a range of characteristics that vary over a wide range of emplacement depths 7 km deposits in shallow crustal settings 5 km have characteristics of mesothermal environments and are hosted by plutons containing sheeted veins greissen and disseminated gold fluid characteristics also vary with depth deposits in shallow environments contain high temperature 350 c immiscible brine 30 wt nacl equiv and low salinity 5 wt nacl equiv vapor that commonly contains carbon dioxide deposits in deeper environments contain abundant low salinity carbon dioxide rich aqueous fluids 10 wt nacl equiv that in some deposits are postdated by moderate to high salinity brines 10 40 wt nacl equiv these contrasting fluid types are interpreted to be magmatic in origin and are the result of the complex interplay between exsolution of different volatiles carbon dioxide water and chlorine from felsic magmas emplaced at different crustal levels experimental studies have shown that carbon dioxide will exsolve from felsic magmas at much higher pressures than water and chlorine due to its lower solubility in the melt thus deposits formed from felsic magmas in deeper environments will contain abundant carbon dioxide rich fluids subsequent fluids released from the magma are likely to be water rich and more saline felsic magmas in shallow environments will contain lower concentrations of carbon dioxide that will partition into the vapor phase during phase separation between brine and vapor

magnetic mineralogy in an archean crustal cross section implications for crustal magnetization a traverse across precambrian rocks from the kapuskasing uplift in ontario provided 31 samples from a crustal cross section that is now exposed at the surface about half the samples are dominantly paramagnetic magnetite accompanied in a few samples by minor amounts of pyrrhotite is the only significant magnetic mineral in the other half magnetic susceptibilities of these samples are constant between room temperature and about 540 c above which temperature they drop rapidly to zero magnetic susceptibility does not depend on rock type apparent original depth in the crust or degree of metamorphism the average induced magnetization of the most magnetic units in this section is less than 1 a m which is much lower than the source requirements for the deep crust deduced from aeromagnetic and satellite magnetometer surveys

thermal demagnetization of partial thermoremanent magnetization three well characterized glass ceramic samples containing magnetite crystals of different domain states have been given partial thermoremanent magnetizations ptrms in the temperature interval 400 350 c for varying lengths of time 10 min to 5 days and in magnetic fields from 0 5 to 5 oe 0 05 0 5 mt the thermal demagnetization behavior of the ptrms changes with domain state in the sense that with decreasing coercivity increasing grain size demagnetization curves become less steep and may not completely demagnetize until heated to the curie temperature numerical calculations based on neel s single domain sd theory invoking field dependence and experimentally determined as well as theoretical volume distributions based on power laws have been performed to predict viscous magnetization acquisition and demagnetization temperatures we observe that experimental results do not match any of the theoretical predictions there is no discernible field dependence of demagnetization curves for the sd and pseudo single domain samples and even for the soft multidomain md sample in the field range investigated an important consequence for thermally derived paleointensity data is that they may not be entirely reliable especially if the sample remanence is due to md grains

magnetic petrology of deep crustal rocks ivrea zone italy magnetic petrology is an extension of petrology integrating magnetic property studies with conventional petrology for the purpose of understanding the development and modification of the magnetization in rocks the magnetic properties of a suite of samples from the ivrea zone northern italy are described with the aim of developing a magnetic petrology for deep crustal rocks samples studied include a variety of metamorphosed mafic granulites granofelses plagioklasfelse amphibolite pyriclasite and pelitic quartzo feldspathic rocks acid granulites stronalite and kinzigite gneisses schists and representative ultramafic rocks phlogopite peridotite pyroxenite hornblendite ilmenite is the chief oxide mineral in a majority of the samples but in a few mafic rocks magnetite is the principal oxide constituent cr al spinel is common in ultramafic samples and rutile is present in some metapelites sulfides are generally subordinate in abundance to oxide minerals the mafic rocks in the ivrea section have the broadest range of magnetic properties and include the most strongly magnetic samples the ivrea ultramafic rocks are moderately magnetic the magnetite present appears to be secondary and is associated with crustal alterations while pelitic and quartzo feldspathic lithologies are dominantly paramagnetic or only weakly magnetic the main ferromagnetic mineral in all strongly magnetic samples is magnetite that is nearly pure fe3o4 in composition consequently curie temperatures are 565 580 c for these rocks pyrrhotite contributes to the magnetism in several ultramafic and mafic rocks and it is the sole ferromagnetic mineral in the pelitic and quartzo feldspathic samples the results of our study indicate that certain mafic rock types amphibolites mafic granulites pyriclasites are the most likely source of the ivrea zone s regional scale magnetic anomalies if the ivrea zone represents a tectonically exposed cross section of continental crust the geographic distribution of these rock types suggests the presence of multiple and variably thick strongly magnetic layers with magnetite curie temperatures in the deep crust petrographic evidence suggests however that the magnetic mineralogy of the sampled ivrea rocks might have been changed subsequent to peak metamorphic conditions therefore considerable care must be taken in evaluating the magnetic record in these crustal cross sections particularly if they are to be used to model the magnetic characteristics of the lower crust

evidence for magnetostrictive control of intrinsic susceptibility and coercive force of multidomain magnetite in rocks abstract it is shown that intrinsic susceptibility xi susceptibility in the absence of self demagnetizing fields as well as coercive force hc of multidomain magnetite in rocks are likely to be commonly magnetostrictively controlled presumably through internal stresses opposing domain wall motion we use observations of the thermal variation of observed susceptibility x per cm3 of magnetite in the sample and coercive force hc on cooling to 130 k for seven basaltic rocks and for 0 25 0 21 mm magnetite grains dispersed in cement as expected theoretically with magnetostrictively controlled xi a plot of 1 x versus j s 2 for each sample where is the polycrystalline magnetostriction coefficient and js is saturation magnetization gives a reasonably straight line whose 1 x intercept agrees with n average self demagnetizing factor of magnetite grains in the sample given by h c j r n h c j r h c j ri where jr is saturation remanence and ri is jr when n 0 as expected theoretically with magnetostrictively controlled hc a plot of hc versus j s for each sample reveals an approximate proportionality this also implies magnetostrictive control of xi because a plot of 1 x versus h c j s for each sample yields a straight line of 1 x intercept agreeing with n from h c j r n h c j r h c j ri as expected if hc and xi are both magnetorestrictively controlled

cretaceous reduced granitoids in the goodpaster mining district east central alaska the 5 moz 1 oz troy 31 103 g gold veins of the liese zone and nearby prospects in the goodpaster mining district of east central alaska are spatially and temporally associated with late early to late cretaceous reduced granitoids that are divided into a granite suite tonalite suite and diorite suite in decreasing age synkinematic to postkinematic biotite hornblende granite granodiorite pegmatite and two mica granite with accessory garnet compose the granite suite 109 107 ma the tonalite suite 107 103 ma forms small to large bodies of postkinematic hornblende biotite granodiorite to tonalite with rare granite intruding the granite and parts of the tonalite suites are aplite and pegmatite that grade through sugary pegmatoidal or aplitic quartz veins and finally into quartz veins along strike the diorite suite 95 4 93 7 ma consists of small stocks of diorite to tonalite that intrude the older suites and are inferred to in trude the shallowly dipping auriferous quartz veins limited data indicate granitoid emplacement at 5 9 km depths consis tent with formation of the auriferous quartz veins based upon published fluid inclusion data the weakly peraluminous granite and tonalite suites are distinguished by variable amounts of monazite and zircon whereas the diorite suite is metaluminous contains 5 magnetite and lacks monazite all suites are subalkalic calc alkaline and have low mag netic susceptibilities high large ion lithophile element high field strength element lile hfse and depleted nb and ti the granite suite has higher 206 pb 204 pb values 19 4 19 6 than the diorite 19 1 overall the granite and tonalite suites likely represent melts generated late during crustal thickening that intruded along shallowly dipping faults during exhuma tion whereas the diorite suite represents postdeformation melts that underwent less interaction with the old silicic crust

magnetic properties of rocks of the kapuskasing uplift ontario canada and origin of long wavelength magnetic anomalies summary the sources of long wavelength magnetic anomalies lwa in the crust are poorly understood we have measured remanent and induced magnetizations of 210 samples of anorthosite tonalite and mafic gneiss from the kapuskasing uplift an exposed partial crustal crosssection in northwestern ontario canada anorthosites have generally high q ratios of remanent induced magnetization in the range 0 3 60 and their natural remanent magnetization nrm is resistant to both thermal and alternating field af demagnetization however anorthositenrms 0 001 0 3am 1 aretooweaktoexplainlwaamplitudes maficgneisses also have relatively high q ratios peaking in the range 1 10 and the nrm is resistant to thermal and af demagnetization nrm and induced magnetization im intensities are in the ranges 0 01 2 and 0 01 0 6 a m 1 respectively tonalites have a bimodal distribution of magnetization the more strongly magnetic group has both nrm and im intensities in the range 0 1 5 a m 1 and wide ranging q values from 0 1 to 10 approximately some tonalites could be an lwa source although the long term stability of their nrms at high temperature in the crust is questionable because unblocking temperatures are broadly distributed from 100 to 600 c in general q values measured at surface temperatures overestimate remanence at depth in the deep crust im remains more or less constant but remanence decreases both reversibly and irreversibly leading to q ratios of 0 2 0 3 for multidomain grains and 1 3 for single domain grains near their blocking temperatures thermoviscous magnetization over the brunhes chron could add substantially to the effective induced magnetization typically induced thermoviscous magnetization in the direction of the present earth s field will outweigh remanence in the direction of an ancient field as a source of lwa originating in the deepest crust remanence may play a larger role for mid crustal sources where single domain grains are well below their blocking temperatures

thermoremanent magnetization of the oceanic lithosphere inferred from a thermal evolution model implications for the source of marine magnetic anomalies abstract thermoremanent magnetization models of the oceanic crust and the uppermost part of the oceanic mantle are derived on the basis of a typical thermal evolution model of the oceanic lithosphere using different magnetic blocking temperature ranges the magnetization of the basaltic layer 2a has a similar pattern but with a lower intensity to that of the standard sea floor spreading magnetic model however the magnetization patterns of the lower crust and upper mantle are skewed so that in a given polarity period the total magnetization of a lithospheric column which is created early in the period is greater than that of a column which is created later in the period magnetic anomalies of the magnetization models are calculated at sea level and at magsat elevation for slow moderate and fast sea floor spreading velocities although the magnetization models are derived based on purely physical reasoning without using any a priori information about the observed marine and magsat magnetic anomalies they do yield magnetic anomalies which are similar to the observed anomalies the models explain several features of the marine and magsat anomalies the anomalous skewness of marine magnetic anomalies is explained in terms of the magnetization of the oceanic lower crust the magnetization of the oceanic upper mantle may account for the anomalous skewness observed over the cretaceous quiet zones in the atlantic ocean the skewness of the magnetic anomalies calculated at sea level decreases with the increase in the sea floor spreading velocity

crustal magnetization and temperature at depth beneath the yilgarn block western australia inferred from magsat data variations in crustal magnetization along a seismic section across the archean yilgarn block of western australia inferred from magsat data are interpreted as a subtle thermal effect arising from variations in depth to the curie isotherm the isotherm lies deep within the mantle of the eastern part of the province but transects the crust mantle transition and rises well into the crust on the western side the model is consistent with heat flow variations along the section line the mean crustal magnetization implied by the model is approximately 2 a m the temperature variation implied by the model is consistent with the hypothesis that the crust mantle transition seen seismically corresponds to the mafic granulite eclogite phase transition within a zone of igneous crustal underplating

shotgun deposit granite porphyry hosted gold arsenic mineralization in southwestern alaska usa the shotgun deposit 30 tonnes gold inferred resource using 0 5 ppm cutoff consists of stockwork au as mineralization in a late cretaceous 69 7 0 3 ma 40ar 39ar granite porphyry stock located approximately 150 km north of dillingham sw alaska preliminary metallurgical results indicate 90 gold is recoverable by cyanide leaching with average estimated concentrations of 1 4 ppm au 6 1 ppm ag 0 5 as 0 07 cu 40 ppm mo and 8 ppm bi shotgun is a low fo2 porphyry au as deposit that shares characteristics with both intrusion related gold and porphyry copper deposits the bulk of gold occurs in intense quartz stockwork and breccias it is texturally identical to cu mo porphyries but has different mineralization and alteration assemblages shotgun contains arsenopyrite pyrite pyrrhotite chalcopyrite lacks k alteration and instead contains vein and pervasive albite quartz sericite carbonate as temperature decreased early high temperature low fs2 mineralization arsenopyrite loellingite pyrrhotite evolved to later lower temperature moderately high fs2 conditions pyrite bornite chalcopyrite in addition ores at shotgun contain primary native gold native bismuth bi te sulfides cubanite maldonite scheelite and sphalerite as well as supergene covellite chalcocite marcasite and native copper arsenopyrite geothermometry yields deposition temperatures of 470 to 630 c fluid inclusion microthermometry and laser raman spectroscopy indicate that vapor rich inclusions h2o co2 ch4 homogenize to vapor at 360 c and saline aqueous inclusions contain 40 to 60 wt nacl equiv probably nacl kcl and homogenize to liquid at 280 to 600 c coexisting vapor and liquid rich fluid inclusions in quartz suggest boiling occurred stable isotope data include 18o of 16 4 to 17 1 quartz d of 124 to 105 sericite and 34s of 5 5 to 5 0 asp cpy at 380 c fluid compositions of 18o of 10 4 to 11 1 and d of 95 to 74 are compatible with a magmatic fluid source gold correlates fairly well r 0 60 with ag bi mo and te when combined with the evidence for high temperature fluids the fragmental morphology of the intrusion stockwork veining and exsolution of vapor rich and aqueous saline rich fluid inclusions indicate a direct relationship between early mineralization and magmatism the shotgun deposit shares similarities with porphyry cu mo au deposits stockwork breccia and reduced nonporphyry intrusion related au as bi deposits sulfide mineralogy geochemistry and oxidation state they appear to be products of the same style of magmatic hydrothermal systems and differ only in depth and thus volatile release history the shotgun deposit is essentially an intrusion related au as bi deposit emplaced at pressures and temperatures typical of a porphyry system

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