did the massive magnetite lava flows of el laco chile form by magmatic or hydrothermal processes new constraints from magnetite composition by la icp ms the el laco magnetite deposits consist of more than 98 magnetite but show field textures remarkably similar to mafic lava flows therefore it has long been suggested that they represent a rare example of an effusive fe oxide liquid field and petrographic evidence however suggest that the magnetite deposits represent replacement of andesite flows and that the textures are pseudomorphs we determined the trace element content of magnetite by laser ablation inductively coupled plasma mass spectrometry la icp ms from various settings at el laco and compared them with magnetite from both igneous and hydrothermal environments this new technique allows us to place constraints on the conditions under which magnetite in these supposed magnetite lava flows formed the trace element content of magnetite from the massive magnetite samples is different to any known magmatic magnetite including primary magnetite phenocrysts from the unaltered andesite host rocks at el laco instead the el laco magnetite is most similar in composition to hydrothermal magnetite from high temperature environments 500 c such as iron oxide copper gold iocg and porphyry cu deposits the magnetite trace elements from massive magnetite are characterised by 1 depletion in elements considered relatively immobile in hydrothermal fluids e g ti al cr zr hf and sc 2 enrichment in elements that are highly incompatible with magmatic magnetite rare earth elements ree si ca na and p and normally present in very low abundance in magmatic magnetite 3 high ni cr ratios which are typical of magnetite from hydrothermal environments and 4 oscillatory zoning of si ca mg ree and most high field strength elements and zoning truncations indicating dissolution similar to that formed in hydrothermal fe skarn deposits in addition secondary magnetite in altered brecciated host rock forming disseminations and veins has the same composition as magnetite from the massive lenses euhedral magnetite lining both open spaced veins in the brecciated host rock and along the walls of large hollow chimneys in the massive magnetite lenses also displays oscillatory zoning and most likely formed by fluctuating composition and or physio chemical conditions of the fluid thus the chemical fingerprint of magnetite from the supposed el laco magnetite lava flows supports the hydrothermal model of metasomatic replacement of andesite lava flows by dissolution and precipitation of magnetite from high temperature fluids rather than a magmatic origin from an effusive fe oxide liquid

late mesozoic bimodal volcanic rocks in the jinniu basin middle lower yangtze river belt yrb east china age petrogenesis and tectonic implications abstract late mesozoic intrusive and volcanic rocks are widespread in the southeast hubei province middle lower yangtze river belt yrb east china detailed in situ zircon u pb and hf isotope elemental and sr nd pb isotopic data are presented in this paper for late mesozoic volcanic rocks from the jinniu basin yrb aiming to constrain their age petrogenesis and tectonic implications the jinniu volcanic rocks show a bimodal distribution in composition with dominant rhyolite and dacite and subordinate basalt and basaltic andesite new shrimp and la icpms zircon u pb ages indicate that the volcanic rocks of three formations in the jinniu basin were erupted at quite a short age range of about 5 ma during the early cretaceous 130 125 ma the mafic rocks are moderately enriched in large ion lithophile elements lile e g ba th u and pb and light rare earth elements lree and are characterized by negative nb ta and ti anomalies and relatively high tio2 0 72 2 06 and nb 9 20 26 5 ppm contents these analyses indicate that the geochemical characteristics of the mafic rocks in the jinniu basin are similar to worldwide phanerozoic nb enriched basalt and andesites neba new in situ zircon u pb ages and field geological relationships demonstrate that neba in the southeast hubei province are not spatially or temporally associated with high silica adakitic rocks but were most likely derived from an enriched lithospheric mantle with assimilation of minor crustal materials and then fractional crystallization during the evolution of the magma overall the felsic rocks in the jinniu basin have geochemical characteristics and sr nd pb signatures and in situ zircon hf isotopic compositions similar to those of the mafic rocks compared with the mafic rocks the felsic rocks are characterized by enriched and variable concentrations of lile and ree e g ba 33 3 1372 ppm y 11 4 33 6 ppm ybc 5 07 18 7 and negative eu anomalies eu 0 22 0 98 as well as a wide range of radiogenic nd pb isotopic values with end t 10 2 to 2 4 206pb 204pb i 17 659 18 705 207pb 204pb i 15 478 15 663 and 208pb 204pb i 37 654 38 935 and in situ zircon hf compositions of ehf t 12 7 to 1 8 these features indicate that the genesis of felsic magma in the jinniu basin is consistent with extensive fractional crystallization and large amounts of crustal contamination from an evolved mafic magma sio2 55 the bimodal volcanic rocks in this study provide convincing evidence that early cretaceous volcanic rocks in the yrb developed in a back arc extensional tectonic regime

u pb and th pb dating of apatite by la icpms abstract apatite is a common u and th bearing accessory mineral in igneous and metamorphic rocks and a minor but widespread detrital component in clastic sedimentary rocks u pb and th pb dating of apatite has potential application in sedimentary provenance studies as it likely represents first cycle detritus compared to the polycyclic behavior of zircon however low u th and radiogenic pb concentrations elevated common pb and the lack of a u th pb apatite standard remain significant challenges in dating apatite by la icpms and consequently in developing the chronometer as a provenance tool this study has determined u pb and th pb ages for seven well known apatite occurrences durango emerald lake kovdor mineville mud tank otter lake and slyudyanka by la icpms analytical procedures involved rastering a 10 m spot over a 40 40 m square to a depth of 10 m using a geolas 193 nm arf excimer laser coupled to a thermo elementxr single collector icpms these raster conditions minimized laser induced inter element fractionation which was corrected for using the back calculated intercept of the time resolved signal a tl u bi np tracer solution was aspirated with the sample into the plasma to correct for instrument mass bias external standards plesovice and 91500 zircon nist srm 610 and 612 silicate glasses and stdp5 phosphate glass along with kovdor apatite were analyzed to monitor u pb th pb u th and pb pb ratios common pb correction employed the 207pb method and also a 208pb correction method for samples with low th u the 207pb and 208pb corrections employed either the initial pb isotopic composition or the stacey and kramers model and propagated conservative uncertainties in the initial pb isotopic composition common pb correction using the stacey and kramers 1975 model employed an initial pb isotopic composition calculated from either the estimated u pb age of the sample or an iterative approach the age difference between these two methods is typically less than 2 suggesting that the iterative approach works well for samples where there are no constraints on the initial pb composition such as a detrital sample no 204pb correction was undertaken because of low 204pb counts on single collector instruments and 204pb interference by 204hg in the argon gas supply age calculations employed between 11 and 33 analyses per sample and used a weighted average of the common pb corrected ages a tera wasserburg concordia intercept age and a tera wasserburg concordia intercept age anchored through common pb the samples in general yield ages consistent at the 2 level with independent estimates of the u pb apatite age which demonstrates the suitability of the analytical protocol employed weighted mean age uncertainties are as low as 1 2 for u and or th rich palaeozoic neoproterozoic samples the uncertainty on the youngest sample the cenozoic 31 44 ma durango apatite ranges from 3 7 7 6 according to the common pb correction method employed the accurate and relatively precise common pb corrected ages demonstrate the u pb and th pb apatite chronometers are suitable as sedimentary provenance tools the kovdor carbonatite apatite is recommended as a potential u pb and th pb apatite standard as it yields precise and reproducible 207pb corrected 232th 208pb and common pb anchored tera wasserburg concordia intercept ages

earthquake nests as natural laboratories for the study of intermediate depth earthquake mechanics article the physical mechanism of intermediate depth earthquakes is still under debate in contrast to conditions in the crust and shallow lithosphere at temperatures and pressures corresponding to depths 50 km rocks ought to yield by creep or flow rather than brittle failure some physical process has to enable brittle or brittle like failure for intermediate depth earthquakes the two leading candidates for that are dehydration embrittlement and ther mal shear runaway given their great depth intermediate depth earthquake processes can t be observed direct ly insteadwe must relyona combination ofseismologyand thestudy of laboratory analogs tounderstand them earthquake nests are regions of highly concentrated seismicity that are isolated from nearby activity in this paper we focus on three intermediate depth earthquake nests vrancea hindu kush and bucaramanga and what they reveal about the mechanics of intermediate depth earthquakes we review published studies of tectonic setting focal mechanisms precise earthquake locations and earthquake source physics at these locations with an emphasis on the bucaramanga nest all three nests are associated with subducting litho sphere and at least two of the nests have consistently larger stress drops compared to shallow seismicity in contrast the bucaramanga nest has a larger b value larger variability of focal mechanisms and shows no evidence of aftershock sequences unlike the other two we also report for the first time finding a signifi cant number of repeating earthquakes some with reverse polarity

fluid inclusion evidence for immiscibility in magmatic differentiation abstract after a brief review of recent work on melt inclusions in diamond and in normal magmatic environments the nature of the fluid inclusion evidence for various stages of magmatic immiscibility is summarized during magmatic differentiation by crystal fractionation from originally low volatile content silicate melts yielding normal plutonic rocks through hydrous silicate melts yielding pegmatites to late stage water rich fluids yielding quartz veins and ore deposits the composition of the residual liquid changes drastically in some geologic environments this change in composition of the residual liquid may be continuous with no phase change in the fluid part of the system under most natural conditions however it is far more likely to have one or more stages in the evolution during which globules of a separate new immiscible fluid phase exsolve the term immiscibility is used here in its more general sense to refer to the existence at equilibrium of two or more non crystalline polycomponent solutions fluids differing in properties and generally in composition i believe that the great bulk of the magmatic inclusions that are being studied today have come about through the intervention of one or more such stages of immiscible fluid separation i e magmatic differentiation by fluid immiscibility examples include silicate melt dense co2 fluid silicate melt sulfide melt silicate melt silicate melt silicate melt low density vapor i e vesiculation and as presented in more detail in this paper various stages of silicate melt hydrous saline melt aqueous fluid co2 fluid immiscibility generally only parts of these stages are recorded by the trapping of fluid inclusions and the interpretation of the inclusion record is commonly ambiguous and difficult particularly that stage between hydrous silicate melts and the hydrous saline melts and aqueous solutions responsible for many ore deposits the ambiguity results mainly from problems of inclusion origin and nonrepresentative trapping within any given individual inclusion thus the probability of trapping of only one vs various amounts of two immiscible fluids or of trapping solid phases along with the fluid present severe difficulties in addition there is the almost universal superposition of various stages of trapping of fluids within any given sample these problems can only be approached by very careful petrography and analysis of optimum material combined with field data and experimental p v t x studies of pertinent systems completely unequivocal statements of the course of evolution of the fluids in such natural systems may never be possible

cretaceous ridge subduction along the lower yangtze river belt eastern china the lower yangtze river belt is one of the most important metallogenic belts in china the mechanisms responsible for ore genesis and the formation of related cretaceous igneous rocks such as adakite a type granitoid and nb enriched basalt remain controversial mesozoic granitoids in the lower yangtze river belt were mostly formed in the early cretaceous 140 125 ma and three granitoid belts the inner the south and the north have been defined according to petrological and geochemical characteristics previously based mainly on negative end and high initial sr isotope values the adakitic rocks were generally attributed to partial melting of thickened or delaminated lower crust both of which require crustal thickening mesozoic crustal thickening however is not supported by the development of extensional basins in the region from the late jurassic to cretaceous eastern china was closely associated with subduction of the pacific plate in the south and the izanagi plate in the north the midocean ridge mor between these two plates was drifting toward and likely subducting under the lower yangtze river belt a ridge subduction model can therefore explain the distribution of different magmatic rocks and ore deposits in the belt partial melting of subducting young hot oceanic slabs close to the ridge formed adakitic rocks the negative end values of adakitic rocks can be plausibly interpreted by mixing between adakitic magmas and enriched components in the lithospheric mantle and or crustal materials through afc process a slab window opened during ridge subduction as indicated by a type granitoids in the center of the inner belt nb enriched basalt found in the belt was likely formed by partial melting of a mantle wedge metasomatized by fluids released from the subducting slab at shallow depths

mineralogy chemistry and fluid aided evolution of the pea ridge fe oxide y ree deposit southeast missouri usa the kiruna type pea ridge iron oxide apatite ioa deposit is hosted by a sequence of 1 47 ga rhyolite tuffs of the st francois mountains southeast missouri usa it consists of a series of altered zones composed mainly of amphibole magnetite hematite and quartz together with the presence of several rare earth element y ree rich breccia pipes in many cases the fluorapatite within these zones is rich in inclusions of monazite iron oxide and quartz inclusions plus minor xenotime monazite and minor xenotime are also found intergrown as inclusions in the fluorapatite as well as in surrounding recrystallized magnetite and hematite in the magnetite ore monazite and xenotime typically occur as inclusions within both oxides monazite ce and xenotime y are both relatively poor 2 wt in tho2 and uo2 no significant compositional differences exist in the y ree chemistry between monazite and xenotime inclusions in fluorapatite compared to grains intergrown with magnetite and hematite suggesting that these two ree rich minerals are cogenetic monazite xenotime geothermometry and geochronology of monazite inclusions in fluorapatite provide evidence that formation remobilization of the y ree phosphates took place at ca 50 to 400 c approximately 5 to 10 m y after emplacement of the main iron oxide phosphate orebody evidence from field relationships and fluid inclusion chemistry together with the massive recrystallization and remobilization of fluorapatite monazite xenotime and iron oxides at pea ridge suggest a subvolcanic origin coupled with a strong metasomatic reworking of the ioa deposit

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

trace elements in magnetite from massive iron oxide apatite deposits indicate a combined formation by igneous and magmatic hydrothermal processes iron oxide apatite ioa deposits are an important source of iron and other elements e g ree p u ag and co vital to modern society however their formation including the namesake kiruna type ioa deposit sweden remains controversial working hypotheses include a purely magmatic origin involving separation of an fe p rich volatile rich oxide melt from a si rich silicate melt and precipitation of magnetite from an aqueous ore fluid which is either of magmatic hydrothermal or non magmatic surface or metamorphic origin in this study we focus on the geochemistry of magnetite from the cretaceous kiruna type los colorados ioa deposit 350 mt fe located in the northern chilean iron belt los colorados has experienced minimal hydrothermal alteration that commonly obscures primary features in ioa deposits laser ablation inductively coupled plasma mass spectroscopy la icp ms transects and electron probe micro analyzer epma wavelength dispersive x ray wdx spectrometry mapping demonstrate distinct chemical zoning in magnetite grains wherein cores are enriched in ti al mn and mg the concentrations of these trace elements in magnetite cores are consistent with igneous magnetite crystallized from a silicate melt whereas magnetite rims show a pronounced depletion in these elements consistent with magnetite grown from an fe rich magmatic hydrothermal aqueous fluid further magnetite grains contain polycrystalline inclusions that re homogenize at magmatic temperatures 850 c smaller inclusions 500 ppm concentrations

zircon u pb and phlogopite 40 ar 39 ar age of the chengchao and jinshandian skarn fe deposits southeast hubei province middle lower yangtze river valley metallogenic belt china the chengchao and jinshandian deposits in the southeast hubei province are the two largest skarn fe deposits in the middle lower yangtze river valley metallogenic belt mlyrvmb china they are characterized by nw striking orebodies that are developed along the contacts between the late mesozoic granitoid and triassic carbonate and clastic rocks new sensitive high resolution ion microprobe and laser ablation inductively coupled plasma mass spectrometry zircon u pb dating of the mineralization related quartz diorite and granite at chengchao yield ages of 129 2 and 127 2 ma respectively and those at jinshandian of 127 2 and 133 1 ma respectively these results are interpreted as the crystallization age of these intrusions hydrothermal phlogopite samples from the skarn ores at chengchao and jinshandian have the plateau 40ar 39ar ages of 132 6 1 4 and 131 6 1 2 ma respectively these results confirm that both intrusions and associated skarn fe mineralization were formed contemporaneously in the middle early cretaceous time new zircon u pb and phlogopite 40ar 39ar ages in this study when combined with available precise geochronological data demonstrate that there were two discontinuous igneous events corresponding to two episodes of skarn fe bearing mineralization in the southeast hubei province 1 140 136 ma diorites and quartz diorites and 141 137 ma skarn cu fe or fe cu deposits and 2 133 127 ma quartz diorites and granites and 133 132 ma skarn fe deposits this scenario is similar to that proposed for the entire mlyrvmb the intrusions related to skarn fe deposits show obviously petrological and geochemical differences from those related to skarn cu fe or fe cu deposits the former are quartz diorite and diorite in petrology and have similar adakitic geochemical signatures and in equilibrium with a garnet rich residue whereas the latter are petrologically granite and quartz diorite that are distinguishable from adakitic rocks and in equilibrium with a plagioclase residue these features indicated that two episodes of magmatism and the formation of skarn fe bearing deposits in the southeast hubei province mlyrvmb might be associated lithosphere thinning induced by asthenosphere upwelling during the late mesozoic

iron rich melts magmatic magnetite and superheated hydrothermal systems the el laco deposit chile we propose an integrated model that explains the magmatic and hydrothermal features of the unique el laco iron deposit that is located in a pliocene pleistocene volcano of the chilean andes sub volcanic crystallization of an iron rich melt as massive magnetite promoted the exsolution of a small volume of a hydrosaline melt and of large amounts of vapor that led to the formation of an alkali calcic hydrothermal assemblage replacing the host andesite this assemblage is capped and overprinted by a large zone of acid sulfate steam heated alteration forming as a whole a protracted shallow level magmatic hydrothermal system oxygen isotopic data for the massive magnetite 18o 4 3 5 0 and the alkali calcic altered rock diopside 18o 7 2 8 7 magnetite 18o 4 4 6 7 suggest that these rocks are genetically related to the host andesite 18o 7 4 9 6 the estimated temperature of the mineral assemblage 900 c may be the highest recorded in hydrothermal systems is attributed to exsolution of fluids from the crystallizing iron rich melts and is considered unlikely to reflect the convection of surficial fluids

kiyoo wadati and early research on deep focus earthquakes introduction to special section on deep and intermediate focus earthquakes this special section of the journal of geophysical research is dedicated to kiyoo wadati this paper evaluates his contribution to our knowledge of geophysics and especially deep earthquake phenomena in at least three areas wadati wrote several papers before 1936 which profoundly influenced modern geophysics first his work provided the first convincing evidence that deep earthquakes existed second he was a leader in the construction of travel time tables and the determination of mantle velocity structure finally he published the first accurate description of the inclined planar zone of deep earthquakes which extends from trenches beneath volcanic island arcs wadati s work strongly influenced research of many other scientists both before and after world war ii particularly jeffreys gutenberg and benioff several of the questions raised by the research of wadati and others before 1940 are still unanswered today

a method of studying variations in the clustering property of earthquakes application to the analysis of global seismicity we propose a formal algorithm for identification of aftershocks following a main event our approach is based on the analysis of the local statistics the number of events following the initial earthquake displayed as a function of distance and time by comparing this distribution with the random catalog we establish for several magnitude ranges spatiotemporal thresholds for detection of aftershocks high confidence thresholds correspond to the limit at which the number of events derived from the actual catalog corrected for the background seismicity exceeds by a factor of 10 the expected level of background seismicity marginal confidence thresholds correspond to the spatiotemporal boundary along which these two values are approximately equal after several iterations the final thresholds are derived the swarming parameter is defined as the number of aftershocks with a cut off lower magnitude range following an initial earthquake of certain magnitude if a correction for the recurrence law is applied we find that the swarming parameter is magnitude independent this allows us to apply this technique to the analysis of global seismicity using a catalog derived from the isc bulletin for the years from 1964 to 1975 we determine for shallow earthquakes the average values of the swarming parameter in 9 9 cells a global map produced in this manner shows a very wide range of variations in the swarming property undoubtedly reflecting variations in the local geological conditions triple junctions are among the areas with the highest swarming values at the present time there is an insufficient amount of data to study regional variations in the swarming property of deep earthquakes considering the clustering as a function of depth alone we find detectable swarming for two depth ranges from 100 to 250 km and from 450 to 650 km not a single aftershock has been detected among nearly 200 earthquakes withmb 5 in a depth range from 250 to 450 km the average swarming parameter for earthquakes deeper than 100 km is 10 times smaller than that for the shallow earthquakes

aftershock sequences of intermediate depth earthquakes in the pamir hindu kush seismic zone we examine the space time seismicity patterns associated with the 40 largest m 5 6 intermediate depth earthquakes that occurred in the pamir hindu kush zone between 1962 and 1984 to determine if any of these events were followed by aftershock sequences to address this issue we examined data from the soviet central asia regional earthquake catalog with two complementary techniques 1 a subjective technique based on inspection of epicentral maps and 2 a quantitative method which assumed that the background seismicity was a poisson process and which detected aftershocks as significant departures as quantified by the poisson model from background rates of the 40 events we examined only three were identified by both methods as having clearly defined aftershock sequences these three events are also remarkably similar in several other ways 1 they occurred at almost the same point in space 2 they have indistinguishable focal mechanisms 3 they have similar magnitudes and 4 they define two nearly equal time intervals 9 37 and 9 41 years no other events in the pamir hindu kush zone exhibit comparable aftershock behavior six other events show rate anomalies but the subjective method rated them negative or at least ambiguous four of these six we argue are caused by coincidental or man made rate increases at the time of the mainshock the remaining two are noteworthy because they are the two largest events to have occurred beneath the central pamir each of these events shows a small short burst of seismicity around the time period of the largest event in the sequence we suggest two alternative hypotheses to explain our observations 1 more events may have aftershock sequences but the aftershocks are too small to be detected or 2 the three events that had clear aftershocks are unique and trigger a burst of aftershocks only because they have a special relationship to the the most active nest of activity in the hindu kush zone

evidence of wadati benioff zone triggering following the mw 7 9 little sitkin alaska intermediate depth earthquake of 23 june 2014 the 23 june 2014 mw 7 9 earthquake that struck the rat islands region of alaska was a rare intermediate depth event followed by a vigorous aftershock sequence the earthquake s location within a regional network has allowed us to study it in rich detail double difference relocations of aftershocks and static stress modeling reveal some intriguing features most aftershocks are not located on the main shock rupture plane but are concentrated in the adjacent wadati benioff zone in areas of increased stress further a secondary plane of seismicity aligns well with a moderately dipping nodal plane reported by the global centroid moment tensor project and the separation of these locations and the main shock hypocenter from the slab indicates that the main shock ruptured into the oceanic mantle

aftershocks and temporal clustering of deep earthquakes this paper evaluates how the focal depth the geographic location and magnitude affect the incidence of aftershocks and the occurrence of temporal clustering of nearby events for selected deep earthquakes h 70 km in the catalog of the international seismological centre for comparison we also evaluate shallow h 70 km and deep earthquakes for which locations and moment tensors are reported by the harvard group the primary conclusions are as follows 1 the incidence of aftershocks depends strongly on focal depth with aftershocks being less common for events between 100 and 450 km depth than for shallower or deeper events events with focal depth shallower than 40 km have a higher incidence of aftershocks and more pronounced temporal clustering than do deeper events 2 for the data available at present the incidence of aftershocks and amount of temporal clustering for deep events are independent of geographic region except for dependence caused by regional differences in the focal depths of events 3 deep earthquakes with large magnitudes are more likely to have aftershocks than are smaller magnitude events in accordance with models where the number of aftershocks is a poisson random variable that is a function of magnitude however several large events are anomalous in that they have many more aftershocks or many fewer aftershocks in the catalog than predicted by the model

giant kiruna type deposits form by efficient flotation of magmatic magnetite suspensions kiruna type iron oxide apatite ioa deposits are an important source of fe ore and two radically different processes are being actively investigated for their origin one hypothesis invokes direct crystallization of immiscible fe rich melt that separated from a parent silicate magma while the other hypothesis invokes deposition of fe oxides from hydrothermal fluids of either magmatic or crustal origin here we present a new model based on fe and o stable isotopes and trace and major element geochemistry data of magnetite from the 350 mt fe los colorados ioa deposit in the chilean iron belt that merges these divergent processes into a single sequence of events that explains all characteristic features of these curious deposits we propose that concentration of magnetite takes place by the preferred wetting of magnetite followed by buoyant segregation of these earlyformed magmatic magnetite bubble pairs which become a rising magnetite suspension that deposits massive magnetite in regionalscale transcurrent faults our data demonstrate an unambiguous magmatic origin consistent with the namesake ioa analogue in the kiruna district sweden further our model explains the observed coexisting purely magmatic and hydrothermal magmatic features and allows a genetic connection between kiruna type ioa and iron oxide copper gold deposits contributing to a global understanding valuable to exploration efforts

the southeast missouri usa proterozoic iron metallogenic province types of deposits and genetic relationships to magnetite apatite and iron oxide copper gold deposits abstract the southeast missouri usa iron metallogenic province within the mesoproterozoic st francois mountains terrane includes eight major iron deposits and approximately 30 minor deposits three of the major deposits have been mined pilot knob magnetite iron mountain and pea ridge these deposits have similarities to kiruna type iron deposits and likely formed either by crystallization of iron oxide magma and or by hydrothermal replacement of volcanic rocks iron oxides within all district deposits are dominantly magnetite and hematite but the gangue minerals are diverse at pilot knob magnetite the dominant gangue mineral assemblages are albitic plagioclase k feldspar quartz chlorite at iron mountain andradite actinolite apatite quartz and at pea ridge apatite quartz in the main part of the deposit plus ree bearing breccia pipes discussed below pilot knob magnetite and pea ridge have associated carbonates which have been suggested to indicate carbonatitic affinities for these deposits the boss bixby deposit has been described as an iron oxide copper gold iocg deposit and contains the only significant copper resource known in the district 40 mt of 0 8 cu as yet undeveloped the district also has steeply dipping iron oxide vein deposits that represent part of the plumbing system where iron bearing hydrothermal fluids moved upward towards the surface from the iron orebodies developing at depth some of these hydrothermal fluids were exhaled into caldera lakes and formed small deposits composed of laminated oolitic sedimentary hematite

the longer it has been since the last earthquake the longer the expected time till the next we adopt a iognormal distribution for earthquake interval times and we use a locally determined rather than a generic coefficient of variation to estimate the probability of occurrence of characteristic earthquakes we extend previous methods in two ways first we account for the aseismic period since the last event the seismic drought in updating the parameter estimates second in calculating the earthquake probability we allow for uncertainties in the mean recurrence time and its variance by averaging over their likelihood both extensions can strongly influence the calculated earthquake probabilities especially for long droughts in regions with few documented earthquakes as time passes the recurrence time and variance estimates increase if no additional events occur leading eventually to an affirmative answer to the question in the title the earthquake risk estimate begins to drop when the drought exceeds the estimated recurrence time for the parkfield area of california the probability of a magnitude 6 event in the next 5 years is about 34 per cent much lower than previous estimates furthermore the estimated 5 year probability will decrease with every uneventful year after 1988 for the coachella valley segment of the san andreas fault the uncertainties are large and we estimate the probability of a large event in the next 30 years to be 9 per cent again much smaller than previous estimates on the mojave pallett creek segment the catalog includes 10 events and the present drought is just approaching the recurrence interval so the estimated risk is revised very little by our methods

trace elements in magnetite as petrogenetic indicators we have characterized the distribution of 25 trace elements in magnetite mg al si p ca sc ti v cr mn co ni cu zn ga ge y zr nb mo sn hf ta w and pb using laser ablation icp ms and electron microprobe from a variety of magmatic and hydrothermal ore forming environments and compared them with data from the literature we propose a new multielement diagram normalized to bulk continental crust designed to emphasize the partitioning behavior of trace elements between magnetite the melt fluid and co crystallizing phases the normalized pattern of magnetite reflects the composition of the melt fluid which in both magmatic and hydrothermal systems varies with temperature thus it is possible to distinguish magnetite formed at different degrees of crystal fractionation in both silicate and sulfide melts the crystallization of ilmenite or sulfide before magnetite is recorded as a marked depletion in ti or cu respectively the chemical signature of hydrothermal magnetite is distinct being depleted in elements that are relatively immobile during alteration and commonly enriched in elements that are highly incompatible into magnetite e g si and ca magnetite formed from low temperature fluids has the lowest overall abundance of trace elements due to their lower solubility chemical zonation of magnetite is rare but occurs in some hydrothermal deposits where laser mapping reveals oscillatory zoning which records the changing conditions and composition of the fluid during magnetite growth this new way of plotting all 25 trace elements on 1 diagram normalized to bulk continental crust and elements in order of compatibility into magnetite provides a tool to help understand the processes that control partitioning of a full suit of trace elements in magnetite and aid discrimination of magnetite formed in different environments it has applications in both petrogenetic and provenance studies such as in the exploration of ore deposits and in sedimentology

the marcona magnetite deposit ica south central peru a product of hydrous iron oxide rich melts marcona the preeminent andean magnetite deposit 1 9 gt 55 4 fe and 0 12 cu is located in the iron oxide copper gold iocg subprovince of littoral south central peru fe oxide and cu zn pb sulfide mineralization was controlled by northeast striking faults transecting a middle jurassic aalenian to oxfordian andesitic shallow marine arc and a succession of contiguous plate boundary parallel late jurassic to mid cretaceous volcanosedimentary basins at marcona hydrothermal activity was initiated in the earliest middle jurassic 161 177 ma by high temperature mg fe metasomatism represented by cummingtonite and phlogopite magnetite assemblages subsequently during the terminal eruptions 156 162 ma of the arc widespread albite marialite alteration na cl metasomatism was followed by the emplacement of an en echelon swarm of massive magnetite ore bodies with subordinate overprinted magnetite sulfide assemblages hosted largely by paleozoic metasilici clastics the magnetite orebodies exhibit abrupt smoothly curving contacts dike like to tubular apophyses and intricate amoeboid interfingering with dacite porphyry intrusions there is no convincing megascopic or microscopic evidence for large scale fe metasomatism associated with the main sulfide poor mineralization the largest 400 mt minas 2 3 4 orebody is interpreted as a bimodal magnetite dacite intrusion comprising commingled immiscible melts generated through the dissolution of metasedimentary quartz in parental andesitic magma oxygen and sulfur stable isotope geothermometry indicates that the evolution at ca 159 ma from magnetite biotite calcic amphibole phlogopite fluorapatite to magnetite phlogopite calcic amphi bole pyrrhotite pyrite assemblages coincided with quenching from above 800 c to below 450 c and the concomitant exsolution of dilute aqueous brines subsequently chalcopyrite pyrite calcite pyrrhotite sphalerite galena assemblages in part metasomatic were deposited from lower temperature 360 c brines the cu poor marcona kiruna type magnetite and cu rich iocg deposits in the district therefore although spatially contiguous represent contrasting ore deposit types the former are interpreted as the product of fe oxide melt coexisting with dacite magma within an andesitic arc which failed during the closure of a back arc basin the weak associated magmatic hydrothermal cu sulfide mineralization at marcona was generated through melt vesiculation and contrasts with the considerably higher grade cu and ag rich orebodies of the major cu rich iocg deposits in the central andes e g la candelaria punta del cobre mantoverde raul condestable and mina justa which were the products of cool oxidized hydrothermal fluids plausibly expelled from the adjacent basins during tectonic inversion

a deep earthquake aftershock sequence and implications for the rupture mechanism of deep earthquakes a distinguishing characteristic of deep earthquakes has been the absence of observable aftershock sequences1 2 here we report the first extensive deep earthquake aftershock sequence to be observed it was recorded by an array of eight broadband seismographs following the 9 march 1994 deep tonga earthquake the aftershocks show a power law decay with time following the main shock as is typical of shallow events most of the well located aftershocks are concentrated along a steeply dipping plane consistent with one of the nodal planes of the main shock mechanism and the mechanisms of three large aftershocks assuming these aftershocks denote the main shock rupture area they define a 50 65 km fault plane extending across the entire width of the active seismic zone and into the surrounding aseismic region the width of the aftershock zone is wider than the expected width of the metastable olivine wedge demonstrating that either the width of the metastable olivine material exceeds previous estimates or the aftershocks are not confined in such a wedge

seismic domains within the gissar kokshal seismic zone soviet central asia by evaluating seismological data from the belt of shallow intraplate seismicity known as the gissar kokshal seismic zone in soviet central asia in conjunction with recent data on the shallow crustal structure and longer term geologic deformation we show that certain size classes of earthquakes are characteristic of adjacent and in some places overlapping geologic tectonic settings we call these settings seismic domains they are both geologically and seismologically distinct i e they contain different rocks and structures and have different frequency magnitude relationships for example in terms of number of events gissar kokshal seismicity is dominated by the microearthquakes that have occurred within the sedimentary rocks of the vakhsh fold thrust belt these are all shallow and have occurred mostly within the frontal portions of the thrust sheets more rarely moderate size m 5 5 earthquakes have ruptured the thrust ramps on the other hand the major strain release in this region has accompanied the large 5 5 m 8 1 shallow earthquakes on basement faults in the southern gissar range adjacent to the fold thrust belt within these ranges there is relatively little microearthquake activity that dichotomy indicates that a significantly different seismic hazard is associated with each of these areas but the seismological distinction is obscured along the front of the fold thrust belt where the domain of small shocks in the sedimentary rocks overlaps the domain of large earthquakes on basement faults to explain the localization of the large basement events along the southern flank of the fold thrust belt adjacent to the fold thrust belt we hypothesize a mechanism of thrust sheet loading thrust loads are greatest in this region since thrust sheets have overridden large basement block uplifts beneath the illiac and peter the first range faults

a wide azimuth streamer acquisition pilot project in the gulf of mexico a 3d wide azimuth waz marine streamer survey was designed to image deep objectives below a complex salt structure in a deepwater area of the gulf of mexico numerous technical and logistical challenges were considered in developing the design for this survey the final candidate designs were tested with synthetic data modeling and imaging the final selected wide azimuth design was used for a survey acquired over the friesian prospect in the gulf of mexico we present some of the modeling studies supporting the wide azimuth survey design as well as acquisition and processing results from the friesian waz pilot project

aftershock occurrence rate decay properties for intermediate and deep earthquake sequences we perform a detailed analysis of seven aftershock sequences with intermediate or deep focus mainshocks occurring in the aleutian islands japan the pamir hindu kush phk tonga and bolivia for each sequence we determine the modified omori law mol p value and goodness of fit for all sequences we are able to find a time interval where the sequence conforms to mol decay with either c 0 or c variable the two sequences with deep focus mainshocks have slightly larger p values than the intermediate depth sequences despite the rarity of well defined intermediate or deep aftershock sequences neither the observed p values nor the overall sequence decay behavior significantly differ from that observed for shallow sequences

sulfide saturation and magma emplacement in the formation of the permian huangshandong ni cu sulfide deposit xinjiang northwestern china the eastern tianshan orogenic belt northwestern china hosts 120 mafic ultramafic intrusions and at least six of them host significant ni cu sulfide mineralization of which the permian huangshandong intrusion is the largest the mafic ultramafic rocks of the huangshandong intrusion comprise lherzolite gabbronorite gabbro and diorite individual orebodies are composed of pyrrhotite pentlandite and chalcopyrite as disseminated to net textured or massive to semimassive sulfide mineralization oxide bearing sulfide mineralization contains 1 to 5 magmatic magnetite that has ilmenite and spinel exsolution lamellae silicate rocks and sulfide mineralization have variable whole rock platinum group elements pge 0 38 362 ppb cu 42 97 000 ppm and ni 26 33 000 ppm concentrations the mineralization is broken out into two types termed oxide rich and oxide poor the oxide rich sulfide mineralization has high pge concentrations with low 187re 188os ratios 38 9 151 whereas oxide poor sulfide mineralization has low pge concentrations with high 187re 188os 159 781 both oxide rich and poor sulfide mineralization have highly variable os values 49 189 and 30 278 respectively oxide rich sulfide mineralization contains magnetite with variable amounts of mgo tio2 al2o3 feototal cr v zn and sn indicating that they crystallized from different stages during magma differentiation we propose that the host intrusion and associated sulfide mineralization were derived from high mg basaltic magmas from a mantle source that was previously modified by subducted oceanic slab oxide poor sulfide mineralization of the huangshandong intrusion formed from a magma that underwent 0 03 sulfide removal before emplacement and was thus pge depleted oxide rich sulfide mineralization formed from magma that had much lower amounts of sulfide 0 003 removed in the early stage before entering the huangshandong magma chamber an early sulfide saturation event was likely associated with fractionation of silicate minerals in a deep staging magma chamber the second sulfide saturation event that formed the sulfide mineralization was probably triggered by selective crustal contamination and fractionation in the high level chamber where mixing of magmas and accumulation of immiscible sulfide occurred

geology and geochronology of magnetite apatite deposits in the ning wu volcanic basin eastern china the ning wu volcanic basin is an important part of the middle lower yangtze river valley metallogenic belt in eastern china where magnetite apatite deposits are common these magnetite apatite deposits can be classified into two types the first type includes the washan taocun heshangqiao and dongshan deposits which are hosted in diorite porphyry and adjacent volcanic rocks the second type includes gushan baixiangshan and hemushan deposits developed at the contact between gabbro diorite and sedimentary rocks we used 40ar 39ar dating on phlogopite and u pb la icp ms dating of zircon in ore related diorite porphyry and gabbro diorite to determine the timing of iron mineralization in the basin the ages of the magnetite apatite deposits such as washan taocun heshangqiao gushan baixiangshan hemushan deposits are from 130 ma to 131 ma thus the magnetite apatite deposits in the ning wu volcanic basin all formed at about 130 ma later than the skarn and porphyry cu au deposits in the adjacent fault uplift areas such as edongnan jiu rui anqing guichi and tongling clusters that are between 146 ma and 135 ma based on available petrological and geochemical data the magnetite apatite deposits in the ning wu volcanic basin are interpreted to have formed under regional lithospheric extension and thinning and ore forming elements were derived from basaltic melts sourced from enriched lithospheric mantle and the crust

in situ elemental and isotopic analysis of fluorapatite from the taocun magnetite apatite deposit eastern china constraints on fluid metasomatism metasomatic alteration of fluorapatite has been reported in several iron oxide apatite ioa deposits but its effect on elemental and isotopic variations has not been well understood in this study we present integrated elemental u pb sr and o isotopic microanalytical data on fresh and altered domains in fluorapatite from the taocun ioa deposit eastern china to evaluate the timing and nature of the metasomatism and its effects on the ore forming event orebodies of the taocun deposit are spatially associated with a subvolcanic intermediate intrusion which displays zonal alteration patterns with albite in the center and increasing actinolite chlorite epidote and carbonate toward the margin both disseminated and vein type ores are present in the taocun deposit and fluorapatite commonly occurs with magnetite and actinolite in most ores fluorapatite grains from the both types of ores have been variably metasomatized through a coupled dissolution reprecipitation mechanism many trace elements including na cl s si mg sr u th and rees y were variably leached from the fluorapatite grains during this process and the sr and o isotopic signatures of the grains were also modified the altered fluorapatite grains domains have in situ 87 sr 86 sr ratios 0 70829 0 70971 slightly higher than those of the fresh fluorapatite 0 70777 0 70868 and 18 o values 3 0 to 3 4 variably lower than the primary domains 5 3 to 7 5 the sr and o isotopes of the primary fluorapatite are consistent with or slightly higher than those of the ore hosting intrusion implying that the early stage ore forming fluids were magmatic in origin but underwent weak interaction with the country rocks u pb dating of the fresh and altered domains of the fluorapatite yielded indistinguishable ages of 131 ma which are the same as the age of the ore hosting intrusion in combination with fluid inclusion data we propose that the metasomatism of fluorapatite was induced by hydrothermal fluids at a late stage of the ore forming event the shifts to higher 87 sr 86 sr ratios and lower 18 o values in the altered fluorapatite indicate that the alteration was induced by fluids with more radioactive sr and lighter o isotope signatures the metasomatic fluids were likely dominated by meteoric waters that were mixed with the earlier magmatic fluids and interacted with sedimentary rocks our study highlights that elemental and isotopic compositions of fluorapatite can be significantly modified by hydrothermal fluids during ore forming events thus instead of traditional bulk rock analysis in situ microanalysis is important to provide accurate constraints on the magmatic and or hydrothermal evolution of complex ore forming systems

an enhanced image of the pamir hindu kush seismic zone from relocated earthquake hypocentres summary we determine the shape of the seismic zone in the pamir hindu kush region de ned by 30 42 0 n 68 78 0 e by obtaining improved hypocentral locations with 90 per cent con dence limits of less than 30 km the depth error bar for most of the earthquakes of about 6000 shallow and intermediate depth earthquakes available s and depth phase arrival times are also used together with the p wave arrival times in the joint hypocentre determination technique to obtain the best possible hypocentral locations the study region is divided into three depth ranges 0 60 60 160 and 160 km the 0 60 km depth zone is then subdivided laterally into 19 blocks with the deeper regions divided into two blocks each the improved delineation of the seismic zone obtained by using the relocated hypocentres implies that the intermediate depth seismicity in the pamir hindu kush region is most simply explained by a single s shaped seismic zone 700 km long and no more than 30 km wide and with most activity concentrated at 100 300 km depth the main features observed are 1 the eastward steepening of the north dipping hindu kush seismic zone through to its overturning at its eastern end beneath the pamirs where it dips to the southeast 2 the curvature and forking of the subducting slab at depths greater than 200 km within the eastern part of the hindu kush seismic zone 3 the very abrupt cut oi in intermediate depth seismicity at 90 110 km depth with no extension to shallower depths under the pamirs and with a persistent gap between the intermediate and shallow seismicity in the northern pamirs and 4 the unusual horizontal t axes for intermediate depth earthquakes of the pamir seismic zone which align with its curvature this study shows that the seismic zone under the hindu kush has stress axes which follow the classical pattern for subducting slabs controlled by gravity whereas the pamir region has horizontal t axes that follow the trend of the contorted seismic zone this suggests that the pamir seismic zone is a slab deformed due to ow in the upper mantle

identification of aftershocks of deep earthquakes by a new ratios method we describe a new statistical method for identifying pairs or groups of related events in sequences which resemble but are not identical to a poisson process for a particular earthquake we form the ratio r nb na of the relative origin times of the na th subsequent event and the nb th previous event we then find the probability that such a ratio could occur if the sequence were a poisson process presumably sequences where the ratio is too small to be probable contain related events and the subsequent events are aftershocks since the method requires knowledge of the origin times of only a few preceding and subsequent events it is more powerful than methods which require knowledge of the mean activity rate of the poisson process using this ratios method we searched the isc catalog for aftershocks of earthquakes with focal depths exceeding 70 km from the world as a whole events with at least one aftershock can be found in all depth ranges including 250 km to 450 km however the incidence of aftershocks is significantly lower for events between 100 and 450 km than for shallower or deeper events

spatial and temporal patterns of earthquake distribution in soviet central asia application of pair analysis statistics we have examined the soviet regional earthquake catalog for central asia for the period 1962 to 1983 to characterize the spatial and temporal structure of the earthquake process in this highly active continental collision zone three areas of apparently homogeneous earthquake distribution were identified as characteristic of distinct seismic regimes 1 a segment of the range bounding gissar kokshal fault zone 2 the interior of the crystalline massif of the tien shan mountain range and 3 the tadjik depression fold thrust belt the pair analysis technique applied in our study compares the spatial and temporal distribution of pairs of earthquakes with those determined by uniformly distributed events in the same area and time interval we have found the earthquake distribution in this region to be dominated by significant nonrandomness in both space and time the spatial nonrandomness is manifested in two major ways spatial clustering appears to characterize the overall structure of the earthquake process in the region as it is observed regardless of the scale considered and even upon removal of foreshocks aftershocks and swarms from the catalog anomalies at larger distances could also be distinguished suggesting the existence of a regular spatial alternation of subareas of higher and lower levels of seismic activity the nonrandomness in temporal distribution of earthquakes is also characterized by clustering at short time intervals in contrast with the spatial patterns earthquakes appear to be randomly distributed in time after removal of foreshocks aftershocks and swarms

unbending and horizontal fracture of the subducting pacific plate as evidenced by the 1993 kushiro oki and the 1981 and 1987 intermediate depth earthquakes in hokkaido abstract a large number of aftershocks of the 1993 kushiro oki earthquake m jma 7 8 1987 hidaka earthquake m jma 7 0 and 1981 western hidaka earthquake m jma 7 1 have been observed by the seismic network of hokkaido university these three earthquakes are the largest events that have occurred in the depth range from 100 km to 200 km under the pacific side of hokkaido for 109 years they migrated from west to east in the last 12 years their focal mechanism solutions represent typical down dip extension in the lower zone of the double seismic zone in the descending pacific plate we investigated aftershock distributions of the three intermediate depth earthquakes to study physical properties of the descending slab aftershocks of the kushiro oki earthquake spread horizontally over an area of 50 km 40 km a vertical cross section of this aftershock distribution shows that it extends from the lower seismic zone almost to the upper seismic zone it shows a slightly concave shape of about 35 km in length aftershocks of the hidaka and western hidaka earthquakes also extend horizontally over areas of 10 km 10 km and 8 km 12 km respectively these main shocks occurred in the lower seismic zone the horizontal aftershock distributions of all three large earthquakes indicate that the horizontal nodal planes of the focal mechanisms rather than the vertical nodal planes are the fault planes such a characteristic suggests that the descending lithosphere under hokkaido has been unbent macroscopically over the last 12 years unbending and the long horizontal faults of the intermediate depth earthquakes may be caused by mantle flow in the asthenosphere beneath the pacific plate we suggest that the descending slab of about 100 km thickness constitutes two layers a seismic brittle upper layer and an aseismic ductile lower layer small ruptures may tend to occur easily along both boundary surfaces of the upper layer under the action of forces especially unbending and form the double seismic zone

man made changes of seismicity rates seismicity catalogs contain important information about processes which occur in seismically active regions of the earth many authors have examined these catalogs for patterns and variations in patterns which might reflect changes in these processes we have found that these catalogs include a complex mixture of real and man made changes one must identify and account for the man made changes before the real ones can be identified and understood many man made changes in seismicity catalogs are manifested as changes in seismicity rates and can therefore be identified through careful examination of these rates obvious effects include increases or decreases in the detection and reporting of smaller events which accompany the installation or closure of seismic stations these types of changes can be recognized by examining the distribution of seismicity rate changes in the magnitude domain they are characterized by increases or decreases in the number of small events in the catalog at times when the number of larger events remains constant systematic changes in the magnitudes assigned to events can also be identified by examining seismicity rates because they cause apparent changes in rates of data sets with magnitude cutoffs the sign of the apparent rate change depends on the sign of the magnitude change and the type of cutoff used the effects of detection changes can be easily remedied by using a magnitude cutoff which eliminates the smaller events from consideration the techniques we have developed allow one to determine the optimum cutoff magnitude corrections are necessary for remedying the effects of systematic changes in magnitude estimates these corrections can be determined through modeling of observed rate changes caused by these shifts man made changes are present in all seismicity catalogs whether local regional or teleseismic they must be accounted for if these catalogs are to provide meaningful information on real process changes in the earth

dissolution reprecipitation process of magnetite from the chengchao iron deposit insights into ore genesis and implication for in situ chemical analysis of magnetite abstract magnetite formed in different environments commonly has distinct assemblages and concentrations of trace elements that can potentially be used as a genetic indicator of this mineral and associated ore deposits in this paper we present textural and compositional data of magnetite from the chengchao iron deposit daye district china to provide a better understanding in the formation mechanism and genesis of the deposit and shed light on analytical protocols for in situ chemical analysis of hydrothermal magnetite magnetite grains from the ore related granitoid pluton mineralized endoskarn magnetite dominated exoskarn and vein type iron ores hosted in marine carbonate intruded by the pluton were examined using scanning electron microscopy and analyzed for major and trace elements using electron microprobe back scattered electron images reveal that primary magnetite from the mineralized skarns and vein type ores were all partly reequilibrated with late stage hydrothermal fluids forming secondary magnetite domains that are featured by abundant porosity and have sharp contact with the primary magnetite these textures are interpreted as resulting from a dissolution reprecipitation process of magnetite which however are mostly obscure under optically primary magnetite grains from the mineralized endoskarn and vein type ores contain high sio 2 0 92 3 21 wt al 2 o 3 0 51 2 83 wt and low mgo 0 15 0 67 wt whereas varieties from the exoskarn ores have high mgo 2 76 3 07 wt and low sio 2 0 03 0 23 wt and al 2 o 3 0 54 1 05 wt this compositional contrast indicates that trace element geochemical composition of magnetite is largely controlled by the compositions of magmatic fluids and host rocks of the ores that have reacted with the fluids compared to its precursor mineral secondary magnetite is significantly depleted in most trace elements with sio 2 deceasing from 1 87 to 0 47 wt on average and al 2 o 3 from 0 89 to 0 08 wt in mineralized endoskarn and vein type ores and mgo from 2 87 to 0 60 wt in exoskarn ores on the contrary average content of iron is notably increased from 69 2 wt to 71 9 wt in the secondary magnetite grains the results suggest that the dissolution reprecipitation process has been important in significantly removing trace elements from early stage magnetite to form high grade high quality iron ores in hydrothermal environments the textural and compositional data confirm that the chengchao iron deposit is of hydrothermal origin rather than being crystallized from immiscible iron oxide melts as previously suggested this study also highlights the importance of textural characterization using various imaging techniques before in situ chemical analysis of magnetite as is the case for texturally complicated u th bearing accessory minerals that have been widely used for u pb geochronology study

trace element signature of pyrite from the los colorados iron oxide apatite ioa deposit chile a missing link between andean ioa and iron oxide copper gold systems although studies have proposed that iron oxide apatite ioa deposits may represent the deeper roots of some andean iron oxide copper gold iocg systems their genetic links remain obscure and controversial a key question when considering an integrated genetic model is whether a magmatic hydrothermal fluid that precipitates massive magnetite will continue transporting significant amounts of dissolved fe cu and au after ioa precipitation here we provide new geochemical data for accessory pyrite from the los colorados ioa deposit in the chilean iron belt that confirm the role of this sulfide as a relevant repository for economic metals in ioa deposits pyrite occurs at los colorados as disseminated grains and as veinlets associated with magnetite and actinolite that postdate the main igneous magnetite stage electron probe microanalysis epma data for pyrite show anomalously high co and ni concentrations up 3 9 and 1 5 wt respectively and relatively high as contents 100s of ppm to a maximum of 2 000 ppm when combined with results from secondary ion mass spectrometry sims spot analyses pyrite data show significant amounts of cu that range from sub ppm values 100 ppb up to 1 000s of ppm plus nonnegligible concentrations of zn pb cd sb se and te up to 100 ppm the highest contents of cu measured wt level most likely record the presence of cu bearing submicron sized mineral inclusions contents of au and ag are up to 1 and 10 ppm respectively with maximum concentrations that can rise up to 800 ppm au and 300 ppm ag due to the presence of submicron sized inclusions the high co ni ratios of pyrite from los colorados are consistent with a magmatic hydrothermal origin associated with a greater mafic affinity compared to pyrite from porphyry cu deposits furthermore the geochemical signature of los colorados pyrite shares important similarities of composition and microtexture with the few published data for pyrite from iocg deposits e g ernest henry australia and manto verde chile these findings combined with recent geochemical and isotopic studies that support an igneous origin for the dike shaped magnetite orebodies at los colorados point to a magmatic source of mafic to intermediate composition for the contained metals and support the hypothesis that ioa systems can source fe cu au rich fluids based on experimental studies these ioa derived fluids may continue transporting significant amounts of metals to form iocg mineralization at shallower levels in the crust

recurring large deep earthquakes in hindu kush driven by a sinking slab hindu kush subduction zone produces large intermediate depth earthquakes within a small volume every 10 15 years here we study the last three m 7 events within the cluster and find complex and diverse rupture processes however their main subevents appear to recur on the same fault patch dipping 70 to the south this recurrence requires an average of 9 6 cm yr slip rate on the patch much higher than the 1 cm yr surface convergence rate measured geodetically the high slip rate is likely caused by significant slab internal deformation such as localized slab stretching necking we infer that the hindu kush subducted slab below 210 km is sinking through the mantle at a vertical rate of 10 cm yr

spatial patterns of aftershocks of deep focus earthquakes this paper presents precise relative relocations for aftershocks of 59 intermediate and deep earthquakes using p pp and pkp arrival times read by us or reported in the literature these aftershocks included 36 rupture subevents occurring within 1 min of the initial event and 71 true aftershocks occurring later and identified statistically as being related to the initial event we compare the relative locations with the orientations of the wadati benioff zones and with the available focal mechanisms using statistical methods not previously applied to earthquake data surprisingly neither true aftershocks nor rupture subevents cluster along the nodal planes of intermediate or deep earthquake focal mechanisms aftershocks more than 20 km from the initial events occur preferentially in the plane of the wadati benioff zone while those lying closer have isotropically distributed directions rupture subevents occur after the travel time of the s wave from the initial event larger magnitude earthquakes generally possess rupture subevents lying farther from the initial event whereas true aftershocks can occur 50 km or more from initial events having magnitudes as small as 5 3 except following a few unusually large earthquakes with focal depths of about 100 km we find no rupture subevents or true aftershocks more than about 80 km from the initial hypocenter the existence of many aftershocks far from nodal planes does not favor models in which deep earthquake failure is simply slip along a planar fault rather the aftershocks may occur in response to a general redistribution of stress caused either by the occurrence of the initial event or possibly by nearby continuing phase transitions alternatively the failure zones of deep earthquakes may be surfaces which are frequently curved by 40 or more from the orientation of the initial nodal plane perhaps due to inhomogeneous stress distributions

erosional processes as a control on the structural evolution of an actively deforming fold and thrust belt an example from the pamir tien shan region central asia the foreland of the pamir mountains displays an unusual orogenic topography in which an active north vergent thin skinned fold and thrust belt shows no topographic foredeep in advance of the thrust sheets instead the thrusts are impinging on a topographic barrier formed by the southern edge of the tien shan the deformation front of this thrust belt is coincident with a deeply incised river valley occupied by a large river the surkhob we suggest that this river played a key role in the evolution of this orogenic system by acting as a lateral conveyor belt for the transport of mass delivered to the deformation front by thrusting on the basis of reconnaissance geologic mapping it is clear that the present system developed in the late miocene or pliocene when the tien shan were uplifted within an existing foreland basin as the tien shan highland developed a longitudinal river within the foreland basin was trapped between the north directed thrusts and the tien shan highland with continued deformation the river was squeezed into a narrow canyon and since that time the system has been in an unusual equilibrium where thrusting acts to push the deformation front northward toward the tien shan but fluvial erosion and transport act to keep it in a constant position geologic relationships indicate that the vakhsh thrust the frontal thrust of the north directed system is now emerging on an erosion surface beneath a thin quaternary cover throughout the canyon suggesting that the river has been unable to keep pace with the tectonic mass flux and the thrust front as well as the river have been pushed northward across the beveled edge of what was originally the southern edge of the tien shan we suggest that the original deformation front of the tien shan now lies 10 km south of the present day topographic front of the tien shan beneath the north vergent vakhsh thrust as indicated by 1 a near vertical zone of intense basement seismicity the petrovsky seismic zone 2 an en echelon array of backthrusts along the south flank of the peter the first range and 3 footwall cutoff relationships of the vakhsh thrust system order of magnitude mass balance considerations based on sediment budgets support this conclusion but suggest that the mass imbalance may be small thus large volumes of material may have passed through the thrust fron and been carried out of the system by erosion a relationship that may account for the narrow 30 50 km wide band of deformed sedimentary rock between the pamir salient and the tien shan the present near equilibrium is maintained by long term tectonic transport rates and fluvial transport rates the latter closely linked to climate particularly precipitation thus although there is apparently a mass pile up within this system other systems of this type might show grossly different structural and stratigraphic relationships depending on the amount of rainfall and corresponding variations in rates of fluvial sediment transport similar settings can be recognized in the active thick skinned thrust systems of the tien shan and ancient systems like the laramide belt of western north america

active tectonics of the pamirs and karakorum the source parameters of moderate to large sized earthquakes and the spatial distribution of earthquake hypocenters are used to investigate the active tectonics and three dimensional configuration of the subducted lithosphere in the pamir karakorum intracontinental convergence zone the continental collision has resulted in intense deformation in the upper crust the deformation is distributed over a broad area and is absorbed by thrust and strike slip faulting along the northern margin of the pamirs where the deformation due to continental collision is most intense the northern pamir thrust and several major fault zones mark the present boundary most of the earthquakes that occurred in these fault zones are shallow crustal events with focal mechanism solutions correlated with the tectonic features of the region along the pamir front thrust faulting dominates while on the western and eastern edges of the pamirs the style of deformation is characterized by oblique thrusting with a component of strike slip motion thrust type events beneath the tadjik depression indicate that both the sedimentary rocks and the basement are involved in shortening right lateral strike slip motion is observed on the eastern edge of the pamirs where they border the tarim basin and in the talas fergana kun lun fault zone a southward dipping seismic zone beneath the pamirs and karakorum indicates that the asian lithosphere has been subducted along the pamir front to a depth of at least 200 km this interpretation is consistent with published geological and geophysical data focal mechanism solutions of some intermediate depth events beneath the pamirs show strike slip faulting with approximately n s horizontally oriented p axes indicating that nearly horizontal compression at the intermediate depth is the predominant mode of deformation a 90 km deep event beneath the karakorum is interpreted as occurring in the subducted asian lithosphere the fact that the p axis of this event is oriented parallel to the descending plate suggests downdip compression within the asian lithosphere the overall tectonics of the region is interpreted as a consequence of the underthrusting asian lithosphere being impinged upon by the shallow northward underthrusting indian lithosphere

abundant aftershock sequence of the 2015 mw7 5 hindu kush intermediate depth earthquake the 2015 m w7 5 hindu kush earthquake occurred at a depth of 213 km beneath the hindu kush region of afghanistan while many early aftershocks were missing from the global earthquake catalogues this sequence was recorded continuously by eight broad band stations within 500 km here we use a waveform matching technique to systematically detect earthquakes around the main shock more than 3000 events are detected within 35 d after the main shock as compared with 42 listed in the advanced national seismic system catalogue or 196 in the international seismological centre catalogue the aftershock sequence generally follows the omori s law with a decay constant p 0 92 we also apply the recently developed double pair double difference technique to relocate all detected aftershocks most of them are located to the west of the hypocentre of the main shock consistent with the westward propagation of the main shock rupture the aftershocks outline a nearly vertical southward dipping plane which matches well with one of the nodal planes of the main shock we conclude that the aftershock sequence of this intermediate depth earthquake shares many similarities with those for shallow earthquakes and infer that there are some common mechanisms responsible for shallow and intermediate depth earthquakes

routine low damage apatite u pb dating using laser ablation multicollector icpms 1 apatite is a common u bearing accessory mineral with a u pb closure temperature of 500 c making u pb dating of apatite a potentially valuable thermochronometer however its low u concentration and tendency to incorporate common lead has limited widespread application to destructive isotope dilution methods we overcome previous limitations by using a nu plasma multicollector icpms with an attached short pulse excimer laser and by identifying two new matrix matched reference apatites to correct for elemental fractionation gem quality 485 myr old apatite from madagascar which we independently characterized by id tims analysis and 523 5 ma apatite from the mcclure mountain syenite source of the40ar 39ar reference mmhb common pb is corrected using measured 204pb isobarically corrected for hg interference and a five step iterative process using stacey and kramers common pb model we accurately reproduce ages of numerous independently characterized apatites regularly achieving precision of 2 2 by pooling as few as five 30 m spot analyses data quality in apatite with low u concentrations low 206pb 204pb values 30 and young ages 75 ma is compromised by the goal of avoiding significant grain damage such limitations can be overcome by using spot sizes 65 m or greater but at the expense of substantial grain damage for single detrital apatite grains with ages of 500 ma precision of 4 2 was achieved by pooling 2 to 3 spots per grain the accuracy of our detrital results is supported by a good age match with similar closure temperature 40ar 39ar detrital hornblende ages from the same sediment

aftershock sequences of moderate sized intermediate and deep earthquakes in the tonga subduction zone intermediate and deep focus earthquakes are generally thought to produce few aftershocks the two year operation of a network of broadband seismographs in the tonga fiji region provides an opportunity to monitor the aftershock activity of moderate sized earthquakes in addition to the march 9 1994 mw 7 6 event that shows 144 aftershocks we found that all nine intermediate and deep earthquakes with mw 6 0 showed some aftershock activity suggesting that aftershock sequences are ubiquitous on average deep tonga earthquakes show an order of magnitude fewer aftershocks above a given magnitude than shallow california earthquakes of similar moment the largest aftershocks of each sequence have an mb that is 2 0 units smaller on average than the mw of the mainshock whereas the california earthquakes show a mean difference of 1 1 since b values of the deeper sequences appear to be similar to shallow sequences the difference in aftershock productivity results from the larger size difference between the mainshock and the larger aftershocks the most prolific sequences show an activity level not far below shallow earthquakes of the same moment this is in contrast with observations of little aftershock activity for deep earthquakes in south america japan and kuriles and suggests that the tonga subduction zone is unusually prolific in aftershock production perhaps due to cold slab temperatures

formation of anhydrous and hydrous skarn in cu au ore deposits by magmatic fluids most skarn ore deposits are characterized by two distinctly different alteration styles an early prograde stage with anhydrous minerals such as garnet and pyroxene forms from relatively high temperature hypersaline liquid a later retrograde stage with hydrous minerals such as epidote amphibole and chlorite plus sulfide ore minerals forms from lower temperature lower salinity fluids these two alteration stages commonly have been thought to reflect a dominance of magmatic and meteoric water respectively with relevance to the source of ore metals we report data from two different skarn systems one being part of the world s largest cu au resource stable isotope compositions of anhydrous and hydrous alteration minerals from both deposits indicate a magmatic source for both the prograde and retrograde stages 18o averages 5 0 per mil for garnet range 3 4 7 2 6 5 per mil for pyroxene 4 3 8 2 and 7 1 per mil for amphibole 4 3 8 7 the d values of late amphibole are more complex with magmatic values 77 to 78 for one deposit and both magmatic and lighter values for another deposit that could be explained either by magmatic degassing or by limited mixing with meteoric water we conclude that the differences in fluid composition prograde versus retrograde stages resulted from a magmatic fluid that intersected its solvus during the early stage creating vapor and hypersaline liquid whereas in the later stage this magmatic fluid did not intersect its solvus because it followed a different cooling path this late low salinity liquid only boiled once its vapor pressure curve was reached causing sulfide ore to precipitate during the retrograde stage

geochemical constraints on cu fe and fe skarn deposits in the edong district middle lower yangtze river metallogenic belt china abstract copper and iron skarn deposits are economically important types of skarn deposits throughout the world especially in china but the differences between cu and fe skarn deposits are poorly constrained the edong ore district in southeastern hubei province middle lower yangtze river metallogenic belt china contains numerous fe and cu fe skarn deposits in this contribution variations in skarn mineralogy mineralization related intrusions and sulfur isotope values between these cu fe and fe skarn deposits are discussed the garnets and pyroxenes of the cu fe and fe skarn deposits in the edong ore district share similar compositions i e dominantly andradite ad 29 100 gr 0 68 and diopside di 54 100 hd 0 38 respectively this feature indicates that the mineral compositions of skarn silicate mineral assemblages were not the critical controlling factors for variations between the cu fe and fe skarn deposits intrusions associated with skarn fe deposits in the edong ore district differ from those cu fe skarn deposits in petrology geochemistry and sr nd isotope intrusions associated with fe deposits have large variations in their la yb n ratios 3 84 24 6 and eu anomalies eu 0 32 1 65 and have relatively low sr y ratios 4 2 44 0 and high yb contents 1 20 11 8 ppm as well as radiogenic sr nd isotopes end t 12 5 to 9 2 and 87 sr 86 sr i 0 7067 to 0 7086 in contrast intrusions associated with cu fe deposits are characterized by relatively high sr y 35 0 81 3 and la yb n 15 0 31 6 ratios low yb contents 1 00 1 62 ppm without obvious eu anomalies eu 0 67 0 97 as well as 87 sr 86 sr i 0 7055 to 0 7068 and end t 7 9 to 3 4 geochemical evidence indicates a greater contribution from the crust in intrusions associated with fe skarn deposits than in intrusions associated with cu fe skarn deposits in the edong ore district the sulfides and sulfates in the cu fe skarn deposits have sulfur isotope signatures that differ from those of fe skarn deposits the cu fe skarn deposits have a narrow range of 34 s values from 6 2 to 8 7 in sulfides and 13 2 to 15 2 in anhydrite while the fe skarn deposits have a wide range of 34 s values from 10 3 to 20 0 in pyrite and 18 9 to 30 8 in anhydrite sulfur isotope data for anhydrite and sedimentary country rocks suggest that the formation of skarns in the edong district involved the interaction between magmatic fluids and variable amounts of evaporites in host rocks

the lower changjiang yangzi yangtze river metallogenic belt east central china intrusion and wall rock hosted cu fe au mo zn pb ag deposits abstract the lower valley of changjiang from wuhan of the hubei province in the west to zhenjiang of the jiangsu province in the east contains more than 200 polymetallic cu fe au mo zn pb ag deposits and is one of the most important metallogenic belts in china this metallogenic belt situated at the northern margin of the yangzi craton and bordered by the dabieshan ultrahigh pressure metamorphic belt to the north consists mainly of cambrian triassic marine clastic sedimentary rocks and carbonate and evaporite rocks which overlay a precambrian basement and are intruded by yanshanian 205 to 64 ma granitoid intrusions and subvolcanic complexes repeated tectonism from late proterozoic to triassic resulted in extensively developed networks of faults and folds involving the cambrian triassic sedimentary strata and the precambrian basement the yanshanian granitoid intrusions and subvolcanic complexes in the lower changjiang metallogenic belt are characterized by whole rock 18 o of 8 to 10 initial 87 sr 86 sr of 0 704 to 0 708 and ndt from 10 to 17 and have been interpreted to have originated from mixing between juvenile mantle and old crustal materials also the yanshanian granitoids exhibit eastward younging and increase in alkalinity i e from older calc alkaline in the west to younger subalkaline alkaline in the east which are related to oblique collision between the yangzi and sino korean cratons and tectonic evolution from early compressional to late extensional or rifting regimes most polymetallic deposits in the lower changjiang metallogenic belt are clustered in seven districts where the yanshanian magmatism is particularly extensive from west to east edong jiurui anqing guichi luzhong tongling ningwu and ningzhen mineralization is characterized by the occurrence of three distinct types of orebodies in individual deposits orebodies in yanshanian granitoid intrusions skarn orebodies at the contact zones between the yanshanian intrusions and late paleozoic early mesozoic sedimentary rocks and stratabound massive sulfide orebodies in the late paleozoic early mesozoic sedimentary strata the most important host sedimentary strata are the middle carboniferous huanglong formation lower permian and lower middle triassic carbonate and evaporite rocks the intrusion hosted and skarn orebodies exhibit well developed zonation in alteration assemblages metal contents and isotopic compositions within individual deposits and apparently formed from hydrothermal activities related to the yanshanian magmatism the stratabound massive sulfide orebodies in the late paleozoic early mesozoic sedimentary strata have long been suggested to have formed from sedimentary or volcano sedimentary exhalative processes in shallow marine environments however extensive research over the last 40 years failed to produce unequivocal evidence for syngenetic mineralization on the basis of geological relationships and isotope geochemical characteristics we propose a carbonate hosted replacement deposit model for the genesis of these stratabound massive sulfide orebodies and associated skarn orebodies this model suggests that epigenetic mineralization resulted from interactions between magmatic fluids evolved from the yanshanian intrusions with carbonate and evaporite wall rocks mineralization was an integral but distal part of the larger hydrothermal systems that formed the proximal skarn orebodies at the contact zones and the intrusion hosted orebodies the stratabound massive sulfide deposits of the lower changjiang metallogenic belt share many features with the well studied high temperature carbonate hosted replacement deposits of northern mexico and western united states particularly with respect to association with small shallow granitoid complexes structural and stratigraphic controls on mineralization alteration assemblages geometry of orebodies metal association metal zonation and isotopic systematics

longevity of magmatic hydrothermal systems in the daye cu fe au district eastern china with implications for mineral exploration the daye district represents one of the largest concentrations of cu fe au skarn deposits in china with additional porphyry cu mo deposits these deposits are closely associated with late mesozoic intrusions of intermediate to felsic composition previous studies have yielded important age constraints for several deposits timing and history of metallogenesis for the entire district however remain not well understood in this paper we present twenty five new ages re os 40ar 39ar and u pb to better understand the temporal framework of the district wide polymetallic mineralization these ages when combined with existing data indicate a prolonged history of mineralization lasting 25 million years from late jurassic to early cretaceous 157 132 ma and that most ore deposits of the district are results of multistage hydrothermal activity discrete episodes of ore formation coincide well with emplacement ages of granitoid intrusions within the district as revealed by recent zircon u pb dating this age consistency substantiates a genetic relationship between ore formation and intrusive magmatism prolonged and episodic magmatic and hydrothermal activities must have played significant roles in the formation of polymetallic ore deposits in the district by repeatedly supplying heat fluids and metals previous studies have demonstrated that the daye district was dominated by lithospheric extension during late jurassic to early cretaceous when extensive magmatism and mineralization took place under the extensional setting the area could have experienced significant uplift or extensional exhumation that would have brought early stage intrusions and associated ore deposits to shallower levels with ore bodies formed by subsequent magmatic hydrothermal events tending to be localized at greater depth this in turn indicates potentials to find new ore bodies at depth of known deposits especially for those formed by episodic magmatic hydrothermal processes this view is partly confirmed by the recent success in deep exploration beneath the tieshan fe cu and tonglushan cu au fe deposits

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

the role of evaporites in the formation of magnetite apatite deposits along the middle and lower yangtze river china evidence from la icp ms analysis of fluid inclusions abstract numerous magnetite apatite deposits occur in the ningwu and luzong sedimentary basins along the middle and lower yangtze river china these deposits are located in the contact zone of gabbro dioritic porphyries with surrounding volcanic or sedimentary rocks and are characterized by massive vein and disseminated magnetite apatite anhydrite mineralization associated with voluminous sodic calcic alteration petrologic and microthermometric studies on multiphase inclusions in pre to syn mineralization pyroxene and garnet from the deposits at meishan ningwu basin luohe and nihe both in luzong basin demonstrate that they represent extremely saline brines 90 wt nacl equiv that were trapped at temperatures of about 780 c laser ablation icp ms analyses and raman spectroscopic studies on the natural fluid inclusions and synthetic fluid inclusions manufactured at similar p t conditions reveal that the brines are composed mainly of na 13 24 wt k 7 11 wt ca 7 wt fe 2 wt cl 19 47 wt and variable amounts of so 4 3 39 wt their cl br na k and na b ratios are markedly different from those of seawater evaporation brines and lie between those of magmatic fluids and sedimentary halite suggesting a significant contribution from halite bearing evaporites high s b and ca na ratios in the fluid inclusions and heavy sulfur isotopic signatures of syn to post mineralization anhydrite 34 s anh 15 2 to 16 9 and pyrite 34 s py 4 6 to 12 1 further suggest a significant contribution from sedimentary anhydrite these interpretations are in line with the presence of evaporite sequences in the lower parts of the sedimentary basins the combined evidence thus suggests that the magnetite apatite deposits along the middle and lower yangtze river formed by fluids that exsolved from magmas that assimilated substantial amounts of triassic evaporites during their ascent due to their fe oxide dominated mineralogy their association with large scale sodic calcic alteration and their spatial and temporal associations with subvolcanic intrusions we interpret them as a special type of iocg deposits that is characterized by unusually high contents of na ca cl and so 4 in the ore forming fluids evaporite assimilation apparently led to the production of large amounts of high salinity brine and thus to an enhanced capacity to extract iron from the gabbro dioritic intrusions and to concentrate it in the form of ore bodies hence we believe that evaporite bearing sedimentary basins are more prospective for magnetite apatite deposits than evaporite free basins

the pilot knob magnetite deposit in the proterozoic st francois mountains terrane southeast missouri usa a magmatic and hydrothermal replacement iron deposit abstract the pilot knob magnetite deposit is located in southeast missouri within the 1380 1480 ma st francois mountains terrane rhyolitic trachytic volcanic rocks the deposit is tabular dips about 45 and is sill like in nature being approximately parallel to the bedding in the host tuffs the deposit was uncovered by erosion and exposed to weathering in the late proterozoic and is overlain in angular unconformity by the cambrian lamotte sandstone this proterozoic weathering cycle apparently had little effect on the deposit with only the updip edge being converted to hematite textural and mineralogical features of the deposit suggest a combined magmatic and hydrothermal replacement origin the magnetite rich ores that make up the bulk of the deposit are interpreted as having crystallized from an iron rich magma and a surrounding envelope of lower to moderate grade ores where magnetite has clearly replaced the tuffaceous host rocks are interpreted as hydrothermal in origin after the development of the higher grade magnetite ores and the envelope of lower to moderate grade ores late hydrothermal minerals were deposited as cross cutting veins and breccia fill the two most abundant minerals in the higher grade portions of the deposit are magnetite and albitic plagioclase and petrologically the higher grade ores could be described as a magnetite sodic syenite the most abundant gangue mineral within the lower grade impregnated envelope of ores around the higher grade ores is k feldspar apparently relict from the rhyolites trachytes thin lenses within the higher grade ores contain calcite as a matrix mineral to the magnetite and are considered to indicate carbonatitic affinity rare earth elements are elevated in one of five whole rock chemical analyses of the ores and the ree bearing mineral ferriallanite has been identified minor portions of the deposit below the proterozoic weathering cap consist of high grade hematite ores having equilibrium textures at depth the deposit is intruded by the shepherd mountain gabbro a 120 m thick near horizontal dike which resulted in minor contact metamorphism of the ore

metallogeny of copper and iron deposits in the eastern yangtse craton east central china abstract the eastern yangtse craton of central to eastern china is an important fe cu metallogenic province based on studies of the regional geology and tectonic evolution this metallogenic belt is controlled by faults and aulacogens in the continental plate in the early yanshan epoch jurassic the dominant west northwest and east west lithospheric faults controlled the distribution of cu mo and au mineralization whereas in the late yanshan epoch cretaceous the north northeast and northeast lithospheric faults controlled cu fe mineralization in a mineral district the distribution of ore fields is under the dual control of basement fault s and near surface fault s based on the relation of ring and lineament faults to intrusions a three level model of vertical zonation is proposed systematic petrochemical studies of mesozoic igneous rocks and mineralization identify three subseries 1 a calc alkaline intermediate to silicic subseries rich in potassium 2 a calc alkaline intermediate to felsic subseries rich in sodium and 3 a calc alkaline intermediate to mafic subseries rich in sodium genetic types of ore deposits are diverse including fe and cu skarns cu porphyries fe ore magma injections ningwu luzong fe deposits from late orthomagmatic to pegmatitic pneumato hydrothermal and to mesothermal epithermal associated with subvolcanic rocks and remobilized sedimentary fe and or cu deposits the genetic types of ore deposits form two metallogenic series 1 a sedimentary metallogenic series active mainly in the paleozoic and 2 a magmatic hydrothermal metallogenic series consisting of three subseries active predominantly in the jurassic and cretaceous with the latter being more important in this region the superposition and compounding of these two metallogenic series contribute to many styles of mineralization in this mineral province

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

microseismicity and focal mechanisms of the intermediate depth bucaramanga nest colombia the bucaramanga nest of intermediate depth seismicity in northern colombia was investigated from microearthquakes m sub b less than or equal to4 3 recorded from june 20 to july 6 1979 by a local array of 12 digital three component seismographs of 161 events studied 142 are from the nest and 19 define a thin wadati benioff w b zone mainly from 110 to 190 km depth the w b zone part of the bucaramanga segment strikes n10 sup 0 e with a dip increasing wtih depth from 30 sup 0 e to near vertical beneath the nest the 89 best located events from within the nest define a volume of about 8 km nw by 4 km ne by 4 km depth centered at 161 km depth with relative location accuracies of 0 5 to 1 0 km double couple fault plane solutions of 59 events were determined from an objective procedure using p wave polarities and s p amplitude ratios there is no dominant trend in focal mechanisms nor any reasonable correlation with regional stress patterns or previously determined mechanisms from teleseismic data the subduction and assimilation of a linear feature such as an island arc undergoing a phase transformation may explain the existence of the bucaramanga nest however the extraordinarilymore high rate of seismicity suggests that the nest may be moving rate several orders of magnitude faster than the rate of relative plate motion alone the intense seismicity and variation in focal mechanisms suggest that the nest represents some aspect of stress release associated with the ascent of fluids perhaps brine or magma that will ultimately result in the initiation of volcanic activity over this recently emplaced w b zone copyright american geophysical union 1987 less

immiscible iron and silica rich melts and magnetite geochemistry at the el laco volcano northern chile evidence for a magmatic origin for the magnetite deposits abstract the pliocene el laco volcano northern chile hosts a large iron oxide deposit that occurs as stratiform bodies with underlying feeder zones dykes hosted by hydrothermally altered andesite lava flows phenocrysts in the andesite hosting the mineralization have abundant melt inclusions displaying strong evidence for melt immiscibility most of the pyroxene and the plagioclase phenocrysts contain abundant fe rich melt inclusions with low ti magnetite pyroxene apatite and minor anhydrite the glassy groundmass of these melt inclusions is chemically similar to sub alkaline rhyolitic rocks e g 70 sio 2 si al k na rich large resorbed plagioclase phenocrysts also host another group of melt inclusions with a similar silica rich groundmass but hosting evenly distributed immiscible and complex fe rich spheroidal globules that are enriched in ti p mg ca and s these immiscible globules and the ti poor magnetite daughter crystals in the melt inclusions in pyroxene phenocrysts are thought to be the parental iron oxide melt liquid after which their coalescence leads to the formation of magnetite melts from which the magnetite that formed the el laco iron oxide deposits crystallized and were emplaced the mineralogy and chemistry of these melt inclusions tracks the chemical evolution of the fe ox melts and supports a magmatic origin for the el laco magnetite this is mainly recognized by the continuum of compositions between volcanic separate immiscible fe oxide melts and magnetite ores the trace element composition of the fe rich melt inclusions and of the magnetite in the host andesite and the ore bodies reveals a systematic depletion in most of the elements that are compatible in magnetite the ti content of the magnetite also records the separation and crystallization of the iron rich melt typically the magnetite in the orebodies and the melt inclusions is ti poor compared with titanomagnetite found in the andesite groundmass and that hosted by the hydrothermally altered andesite that is interpreted as being attributed to its crystallization under oxidizing and sulfur rich conditions the cause of this unusual chemical behaviour is probably due to the mechanism s governing the crystallization of the iron rich melt including the temperature changes in chemistry prior to eruption and an increase in the redox conditions of the silicate melt magma oxidation facilitates the prevalence of ferric iron and formation of ti poor magnetite preventing the incorporation of ti in the exsolving iron rich melt the likely reason for the formation of these large ti poor magnetite deposits is likely the contamination of the parental andesitic magma by oxidized crustal rocks and concomitant crystallization of magnetite under oxidizing conditions something that had a significant impact on the elemental distribution

a genetic link between iron oxide apatite and iron skarn mineralization in the jinniu volcanic basin daye district eastern china evidence from magnetite geochemistry and multi mineral u pb geochronology various styles of ore deposits may form from a single magmatic hydrothermal system identification of a possible genetic link between different ore types in a region is not only of critical importance for a better understanding of the magmatic hydrothermal processes but can also help in successful mineral exploration both iron oxide apatite ioa and iron skarn deposits are closely associated with intrusive rocks of intermediate to felsic in composition but whether these two ore types can form from the same magmatic intrusion remains poorly understood in this study we present a comparative study between a newly identified subsurface ioa ore body located at the apex of a diorite porphyry and the iron skarn ore bodies located immediately above it in the jinniu volcanic basin of the daye district middle lower yangtze river metal logenetic belt mlyrmb eastern china in order to highlight a genetic link between these two styles of mineralization the ioa ores are dominated by ti rich magnetite with variable amounts of fluorapatite diopside and actinolite this mineralogical assemblage is distinctly different from the iron skarn ores which consist mainly of ti depleted magnetite and subordinate pre ore garnet and diopside and post ore quartz chlorite calcite and pyrite in addition magnetite from the ioa ores is characterized by well developed ilmenite lamellae and has high concentrations of ni v co and ga consistent with high temperature crystallization whereas magnetite grains from the iron skarn ores usually exhibit oscillatory growth zones and contain much lower ni v co and ga indicating their formation under relatively low temperatures titanite and fluorapatite from the ioa ores have u pb ages of 132 5 2 4 ma to 128 4 3 0 ma which match a titanite u pb age for the associated iron skarn ores 1323 2 0 ma and are consistent with zircon u pb ages for the ore hosting diorite porphyry 130 4 0 7 ma to 130 3 0 5 ma this age consistency supports a possible genetic link among the diorite porphyry ioa ores and iron skarn ores we propose that the ioa and skarn ores are the products of two consecutive mineralization stages of the same magmatic hydrothermal system involving a high temperature hypersaline fluid coexisting with the diorite porphyry magma during emplacement and a subsequent low temperature diluted hydrothermal fluid other ioa and iron skarn deposits of similar ages 130 ma are found in a series of volcanic basins in the mlyrmb which forms one of the world s largest ioa metallogenic belts the close association of the two ore styles identified at daye provides a useful exploration guide for ioa and iron skarn deposits both on a local and regional scale

a tectono genetic model for porphyry skarn stratabound cu au mo fe and magnetite apatite deposits along the middle lower yangtze river valley eastern china abstract the middle lower yangtze river valley metallogenic belt yrb situated along the northern margin of the yangtze craton is characterized by porphyry skarn stratabound cu au mo fe deposits in the areas of uplift and magnetite apatite deposits in cretaceous fault basins following detailed field investigations and a review of published data we recognize two episodes of magmatism and mineralization in the yrb 1 156 137 ma high k calc alkaline granitoids associated with 148 135 ma porphyry skarn stratabound cu au mo fe deposits and 2 135 123 ma shoshonitic series associated with 134 9 122 9 ma magnetite apatite deposits a type granitoids and associated alkaline volcanic have a small age range from 126 5 to 124 8 ma and are temporally spatially and genetically associated with the second episode the geodynamic history of the yrb did not experience the paleozoic to mesozoic lithospheric thickening that took place in the north china craton this process is inferred to be linked to partial melting of the delaminated lower crust at high pressures resulting in the development of c type adakitic rocks the petrochemical and sr nd isotopic data show that both the shoshonitic series and a type granitoids are quite different from adakites with only some of the k calc alkaline granitoids having adakitic signatures previous ore genesis models were established based on an assumed relationship with adakites and a continuous tectono thermal evolution from 150 to 100 ma all data obtained for the middle lower yangtze river region consistently show that the tan lu regional strike slip fault zone initiated at 233 6 to 225 6 ma from the collision between the north china and yangtze cratons and was reactivated at ca 160 ma the tan lu fault was caused by the oblique subduction of the izanagi plate which along the yrb the low angle subducted slab and the overlying crust was disrupted or broken due to the disharmonious movement of the two blocks the high k calc alkaline granitoids magmas were derived from melting of the subducted slab with some input of crustal material these magmas were emplaced at the intersections between ne and ew trending faults and formed porphyry skarn stratabound cu au mo fe deposits between 156 and 137 ma after 135 ma the subducted plate changed its direction of motion to northeast now running parallel to the eurasian continental margin and leading to large scale continental extension the shoshonitic series and subsequent a type granitoids magmatism and the development of magnetite apatite ores in the yrb took place in both fault basins and ne trending rifts between 135 and 124 ma

second order moment of central california seismicity 1969 1982 the second order moment cross correlation function of earthquakes in the u s geological survey central california catalog between 1969 and 1982 was calculated with respect to a magnitude threshold m 4 0 over interevent distances up to 80 km and interevent times up to 320 days the statistical procedure results in a representation of the spatial temporal structure of the catalog associated with m 4 0 earthquakes and is capable of revealing patterns too weak to be detected in the space time distribution of seismicity for individual earthquake sequences a method is introduced for identifying aftershocks based on a physical two parameter model of the earthquake interaction process the results show that the aftershock process dominates the second order moment and may even obscure the statistical expression of a precursory process a concentration of foreshocks within 15 km and 3 days of m 4 0 main shocks exhibits an apparent migration toward the main shock loci with velocity 2 6 5 3 km d this concentration may be related to an observed tendency for m 4 0 events to cluster auto correlate over this interevent range with the identified aftershocks removed the residual catalog is poissonion in space and time when two m 4 0 earthquakes occur within 80 km and 40 days of each other aftershock productivity appears to be relatively enhanced in the earlier sequence this suggests that aftershock populations are not solely dependent on their main shocks and that unusually productive aftershock sequences may be predictors of future moderate earthquakes

aftershocks of the march 9 1994 tonga earthquake the strongest known deep aftershock sequence the march 9 1994 mw 7 6 depth 564 km deep tonga earthquake produced an exceptionally strong aftershock sequence for a deep earthquake using eight temporary broadband stations deployed on the fiji tonga and niue islands we identify 144 aftershocks with mb ranging from 3 2 to 6 0 during the first 41 days following the mainshock the number and seismic moment of the march 9 aftershocks lie within the range of aftershock productivities of typical shallow earthquakes of similar moment the aftershocks show a power law decay with time following the mainshock with an exponent p value of 1 0 similar to shallow aftershock sequences fifty aftershocks can be accurately located using a hypocentroidal decomposition method these aftershocks align along a near vertical ene striking plane similar to one of the mainshock nodal planes sixteen of the 18 well located aftershocks during the first 2 5 hours are located to the nne of the mainshock epicenter consistent with nne rupture propagation derived from waveform analysis later aftershocks are located south of the epicenter suggesting aftershock expansion beyond the region of substantial moment release in the mainshock several of the aftershocks are located outside of the normal bounds of the seismically active slab consistent with evidence that the mainshock rupture terminated outside the active slab five possible foreshocks occurred along the preferred fault plane within the 22 days prior to the mainshock at least three triggered aftershocks occurred at distances of 50 to 110 km within the first six hours following the mainshock triggered aftershocks in adjacent slab material may be common for large deep earthquakes while there is some variation in the focal mechanisms of the aftershocks most resemble the mainshock focal mechanism all the mechanisms show compressional axes dipping to the west consistent with background seismicity in this region overall most of the aftershock characteristics of the 1994 tonga event are similar to those found for shallow earthquakes suggesting similarities between shallow and deep earthquakes in the mechanical processes that produce aftershocks

seismotectonics of intermediate depth earthquakes from properties of aftershock sequences abstract aftershock sequences following two intermediate depth earthquakes have been analyzed to determine their spatial and temporal distributions the kanaga pass earthquake m b 6 0 occurred at 105 km depth in the central aleutian islands data were provided by the central aleutians seismic network the iwaizumi town earthquake m b 6 4 occurred at 75 km depth under northern honshu japan data were provided by the network of tohoku university for both events the hypocenter distribution of the aftershocks agreed well with focal mechanism solutions derived by others with down dip compression for both both events were followed by numerous aftershocks that decayed with time initially according to the modified omori relation with p value close to 1 in both cases activity at a higher rate than that expected from the omori relation began after several months without a strong aftershock to trigger it the kanaga pass activity is modeled by two omori sequences the second beginning 64 days after the mainshock the p values are 0 92 and 1 04 respectively the corresponding b values are 1 01 and 1 03 the iwaizumi town sequence is modeled by a single omori sequence p 0 96 plus a constant rate of activity 0 25 events day for 63 days beginning 120 5 days after the mainshock the b value of the main sequence was 0 91 0 14 up to the start of the linear surge for which the b value was 1 23 0 47 no strong aftershock or event in the vicinity of the aftershock zone occurred to trigger the secondary activity in either case the cause of the secondary activity is not known the two sequences occurred in subduction zones with similar properties and in both cases previous seismicity shows a deficiency of strong earthquakes in the main thrust zone above the aftershock source volumes a strong creep event or silent earthquake in a weakly coupled interplate contact zone is a plausible explanation for the secondary sequences the unusual setting of the iwaizumi event at the bottom of the upper plane of the double planed wadati benioff zone under honshu may play a role in that case

magmatic salt melt and vapor extreme fluids forming porphyry gold deposits in shallow subvolcanic settings the recently discovered biely vrch deposit in the western carpathian magmatic arc is the most extreme example of a porphyry gold deposit being practically free of copper molybdenum or any other sulfide minerals microanalytical data on fluid inclusions in quartz veinlets including a characteristic type of banded veinlets show that this deposit formed from nearly anhydrous fe k na cl salt melts containing 10 ppm au coexisting with hydrous vapor of very low density this exceptional fluid evolution required an fe rich dioritic source magma that was emplaced at shallow subvolcanic depth 3 5 km directly exsolving a hypersaline liquid and magmatic vapor at high temperature 850 c during ascent to the level of the porphyry intrusion 0 5 1 km fluid expansion at high temperature but low pressure led to halite precipitation and further water loss to the vapor generating an increasingly fe k rich salt melt that transported high concentrations of au but negligible cu into the fractured porphyry stock the low sulfur fugacity resulting from fluid expansion suppressed precipitation of sulfide explaining the gold only enrichment in this globally recurring but rare type of gold ore

solubility of cepo4 monazite and ypo4 xenotime in h2o and h2o nacl at 800 c and 1 gpa implications for ree and y transport during high grade metamorphism abstract monazite and xenotime are important accessory minerals in metasediments because they host ree and are useful for geochronology and geothermometry it is therefore essential to understand their behavior during metasomatic processes that attend metamorphism and subduction in order to constrain the solubility of monazite and ypo 4 xenotime at high pressure and temperature we carried out weight loss experiments on synthetic single crystals of cepo 4 monazite and ypo 4 xenotime using hydrothermal piston cylinder methods at 800 c and 1 gpa in h 2 o and h 2 o nacl fluids results indicate that cepo 4 and ypo 4 dissolved congruently and that their solubilities in pure h 2 o are very low 0 04 0 04 and 0 25 millimolal respectively the solubility of cepo 4 rises to an increasing extent with added nacl to 7 94 0 07 millimolal at 50 mol nacl in contrast the solubility of ypo 4 rises to a decreasing degree with increasing nacl to 4 36 0 08 millimolal at 50 mol nacl best fit equations for the solubilities of the two phases are m cepo 4 3 56 10 5 5 82 10 3 x nacl 1 97 10 2 x 2 nacl and m ypo 4 2 48 10 4 1 26 10 2 x nacl 8 89 10 3 x 2 nacl the solubility of ypo 4 is greater than that of cepo 4 at nacl mole fractions x nacl of 0 00 0 27 the solubility enhancement behavior implies that ce dissolves as anhydrous chloride complexes whereas y forms mixed cl oh solutes the results provide a simple mechanism for redistributing ree and y in deep crustal and upper mantle environments the h 2 o ce ratio inferred for subduction zone melts and silicate rich fluids can also be produced by a cepo 4 saturated fluid with x nacl 0 1 in addition neutral ph h 2 o nacl fluids can transport substantial ree and y obviating the need to invoke highly acid solutions in environments where they are unlikely

the 1994 bolivia and tonga events fundamentally different types of deep earthquakes the 1994 bolivia and tonga events show large differences in source and aftershock properties the bolivia earthquake characterized by high stress drop slow rupture propagation and a weak aftershock sequence appears similar to other large deep earthquakes that are isolated from active seismic zones including the 1970 colombia earthquake in contrast the 1994 tonga event showed stress drop rupture propagation velocities and an aftershock sequence similar to that found for shallow earthquakes the aftershock sequence of the 1994 tonga event is many times stronger than has been observed for any other deep earthquake a survey of deep earthquake aftershocks suggests that aftershock occurrence is correlated with the overall subduction zone magnitude frequency relation b value the largest deep events occur in isolated regions lacking smaller earthquakes or in subduction zones characterized by anomalously low b values the 1994 tonga event is an exception and is the largest deep earthquake to occur within an active subduction zone showing a large b value perhaps explaining the strong aftershock sequence the bolivia and tonga events may represent end members of a population of deep earthquakes showing exceptional diversity in source properties or they may represent two fundamentally different types of deep earthquakes perhaps resulting from different physical mechanisms

geological development and phanerozoic crustal accretion in the western segment of the southern tien shan kyrgyzstan uzbekistan and tajikistan abstract the tien shan form a high intracontinental mountain belt lying north of the main india asia collision mountains and consist of re activated paleozoic orogens the western segment of the southern tien shan lies northwest of the pamir and west of the talas fergana fault the stratigraphy lithology igneous and metamorphic petrology and geochemistry of this segment indicate that it was formed by the assembly of lower paleozoic arcs which developed into microcontinents with upper paleozoic mature shelf and slope clastic and carbonate sediments precambrian continental crust is confined to two small blocks along its southern margin the bulk of the southern tien shan consists of vendian to silurian oceanic and slope clastic rocks resting on oceanic lithosphere and overlain by thick passive margin devonian to mid carboniferous mature shelf clastics and carbonates these are unconformably overlain by syn and post orogenic immature clastic sediments derived from mountains on the north formed by closure of a carboniferus southern tajik and a northern vendian to carboniferous turkestan ocean with the southern tien shan microcontinent sandwiched between associated with these collisions are late carboniferous to permian intrusives which form three south to north though overlapping suites a southern calc alkaline granodiorite granite suite an intermediate gabbro monzodiorite granite suite and a northern alkaline monzodiorite granite alaskite suite the gabbro monzodiorite granite suite forms the earliest subduction related magmatism of the southern tien shan rare earth element patterns are consistent with derivation from a primitive or slightly enriched mantle the other suites show more crustal contamination rb and sr vary with depth and degree of partial melting and are consistent with progressive involvement of crustal material in partial melts during collision the gradual change in composition within each complex lasting in some cases from 295 to 250 ma the entire permian may be explained by a consecutive shift in the melting sedimentary cover of the subducting plate from oceanic crust through transitional crust to marginal continental crust like the central asian orogenic belt the main focus of igcp 420 the tien shan represent a net addition of continental crust during the phanerozoic very little of the belt has any precambrian precursor

reequilibration processes in magnetite from iron skarn deposits textural and compositional data for magnetite from nine iron skarn deposits in canada romania and china show that most samples have reequilibrated by dissolution and reprecipitation oxy exsolution and or recrystallization the dissolution and reprecipitation processes are most extensive and are present in most magnetite samples examined whereas the oxy exsolution occurs only in high ti magnetite forming exsolution lamellae of fe ti al oxides electron microprobe analysis indicates that the reequilibration processes have significantly modified the minor and trace element compositions of magnetite notably si mg ca al mn and ti whereas oxy exsolution is effective in decreasing the ti content of high ti magnetite many analyses of magnetite grains from the skarn deposits plot variably in the banded iron formations bif iron oxide copper gold iocg or porphyry cu fields using the ti v versus ca al mn discrimination diagram this pattern suggests that trace element data for magnetite that has unusual composition and or reequilibrated cannot be reliably used as a petrogenetic indicator mixing of externally derived saline fluids with fe rich magmatic hydrothermal solutions an increase in temperature and local decreasing pressure and f o 2 are considered the most important causes for the dissolution and reprecipitation or recrystallization of the magnetite increasing f o 2 and decreasing temperature may facilitate oxy exsolution of fe ti al oxides in high ti magnetite results presented here highlight the importance of detailed textural characterization prior to in situ chemical analysis of magnetite grains so that mineral compositions can be properly evaluated in terms of the genesis and evolution of iron skarn deposits

facies analysis of a 1 9 ga continental margin back arc felsic caldera province with diverse zn pb ag cu au sulfide and fe oxide deposits bergslagen region sweden the bergslagen mining district in south central sweden has been a major metal producer for more than 1 000 years in geologic terms bergslagen is the intensely mineralized part of a large early proterozoic mainly 1 90 1 87 ga felsic magmatic region of mainly medium to high metamorphic grade in the baltic shield the district contains a diverse range of ore deposit types including banded iron formation magnetitecalc silicate skarn manganiferous skarn and carbonate hosted iron ore apatite bearing iron ore stratiform and strata bound zn pb ag cu au sulfide ores and w skarn most ore deposits occur in hydrothermally altered metavolcanic rocks and associated metalimestones and skarns this study is a field oriented regional facies analysis based on recognition of primary volcanic and sedimentary features through the strong overprints of hydrothermal alteration deformation and metamorphism the region is interpreted as an extensional probably back arc active continental margin magmatic region it evolved through stages of intense magmatism thermal doming and crustal extension followed by waning extension waning volcanism thermal subsidence reversal from extension to compressional deformation regional metamorphism and structural inversion the volcanic successions comprise interfingering proximal near vent medial volcano flanks and distal volcano margin facies associations of rhyolitic pyroclastic caldera volcanoes and subordinate dacite rhyolite complexes nonwelded to poorly welded pyroclastic flow and fallout units and their rapidly resedimented subaerial and subaqueous equivalents are the most abundant volcanic facies subvolcanic porphyritic intrusions and cryptodomes are also abundant the pyroclastic debris was shed from mainly shallow water and subaerial proximal areas to shallow and moderately deep water above and below storm wave base distal areas most ore deposits formed during the regional waning volcanic stage and occur in medial to distal facies associations that comprise rhyolitic ash siltstone limestone and vitric crystal sandstone and breccia these mineralized facies associations were deposited in subaqueous environments mainly below the fair weather wave base but locally above the storm wave base water depths are interpreted to have been mainly 10 to 500 m with local deeper areas although the ore deposits occur in medial to distal facies associations many occur above or laterally adjacent to subsided volcanic vent complexes this suggests a spatial temporal and possible genetic relationship between several ore deposit types and the late magmatic stage of these volcanic centers except for the apatite bearing iron deposits it is uncertain if the ores are genetically related to specific magmatic hydrothermal events from specific volcanoes this is possible especially for the base metal sulfide deposits which have intense localized footwall alteration in felsic volcanic rocks however an important role of the volcanic centers may have been to provide strong positive pertubations or magmatic hydrothermal input to the regional convective geothermal system the main zn pb ag cu au ore deposits span a range between two end member types stratiform ash siltstone hosted sea floor deposits that have similarities to some stratiform sediment hosted zn pb ag deposits especially the broken hill type deposits and strata bound volcanic associated limestone skarn subsea floor replacement deposits that more closely resemble felsic volcanic associated massive sulfide deposits facies models are presented for both the banded iron formations are interpreted as chemical suspension deposits rhythmically interbedded with rhyolitic ash siltstone the skarn iron ores are strata bound in limestones and include metamorphosed sedimentary ores and metasomatic replacements apatite bearing iron ores differ in setting from the other ores in being formed within a dacitic intrusive complex that may be the root zone of a dacite rhyolite volcanic center

velocity structure of the pamir hindu kush region possible evidence of subducted crust the arrival times of compressional p and shear s waves from approximately 580 microearthquakes recorded by a temporary array in the pamir hindu kush region in central asia are used to deduce one and three dimensional velocity structures of this region the results for one dimensional structures imply that the moho is at 70 5 depth also there is a velocity reversal near 160 km depth which is inferred to be the beginning of the low velocity zone this reversal continues to depths of approximately 230 km below 230 km velocities are somewhat higher than those of normal mantle at similar depths 9 3 km s versus 8 4 km s for p waves the outstanding feature of the results for three dimensional velocity structures is a broad 40 km centrally located region with 8 to 10 lower velocities than those in the surrounding regions this low velocity region envelopes the seismic zone at depths between 70 and 150 km the region may actually extend beyond these depths but the results for shallower and deeper structure lack sufficient resolution to decide several tests using both hypothetical and real data were performed to estimate the reliability of the three dimensional solutions the results of these tests suggest that the inferred velocities are reasonably accurate representations of the average velocities in the blocks although one must be cautious of the effects of averaging in interpreting the solution the low velocity region is inferred to be a manifestation of substantial quantities of subducted continental crust therefore while subduction has occurred in the pamir hindu kush the results of the three dimensional inversions suggest that continental rather than only oceanic lithosphere has been subducted to depths of at least 150 km appendices are available with the entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j81 013 1 00 payment must accompany order