origin of iron rich mississippi valley type deposits the abundance of iron in mississippi valley type lead zinc deposits and districts varies greatly some deposits contain large amounts and others are almost free of iron iron in mississippi valley type deposits is largely paragenetically early pyrite or marcasite that was replaced by sphalerite and galena often in the central part of the deposit or district sedimentary exhalative and irish type base metal deposits which also form from basinal brines have similar variations in iron content calculated metal contents of brines in equilibrium with galena sphalerite and pyrite show that iron is significantly more abundant than lead and zinc in high temperature 200 c relatively acid brines with low sulfur contents whereas zinc dominates under most other conditions including brines with high temperatures and high sulfur contents these results suggest that iron rich mississippi valley type deposits form from brines expelled from the deepest hottest parts of sedimentary basins

synorogenic hydrothermal origin for giant hamersley iron oxide ore bodies geologic mapping basin analysis and calculated fluid compositions indicate that giant orebodies of microplaty hematite and possibly martite goethite in the hamersley province of western australia were formed by heated fluids driven by early paleoproterozoic orogenesis detrital grains of microplaty hematite in the mcgrath trough a foreland basin in front of the northward advancing ophthalmian fold belt constrain the age of the earliest microplaty hematite ore formation to 200 c and locally up to 400 c were involved regional circulation of hydrothermal fluids including heated surface water through reduced banded iron formations occurred during or soon after the ophthalmian orogeny we speculate that martite goethite orebodies previously considered mesozoic cenozoic could also be related to heated paleoproterozoic meteoric fluids migrating northward away from the ophthalmian fold belt

assembling and reactivating the proterozoic capricorn orogen lithotectonic elements orogenies and significance abstract the capricorn orogen was initiated during palaeoproterozoic suturing events that brought together the archaean yilgarn and pilbara cratons to form the west australian craton the orogen comprises palaeoproterozoic plutonic and medium to high grade metamorphic rocks of the gascoyne complex a series of palaeoproterozoic volcano sedimentary and sedimentary basins including the ashburton blair yerrida bryah padbury and earaheedy basins and the deformed margins of the pilbara and yilgarn cratons major pulses of deformation and metamorphism took place during the ca 2200 ma ophthalmian orogeny the 2000 1960 ma glenburgh orogeny the 1830 1780 ma capricorn orogeny and a more localized unnamed event at the end of the palaeoproterozoic 1670 1620 ma the orogen has been the site of repeated intracratonic reactivation with renewed basin formation magmatism and orogeny during the mesoproterozoic and the neoproterozoic the exhumed palaeoproterozoic components of the orogen are unconformably overlain by the intracratonic mesoproterozoic edmund and collier basins bangemall supergroup which have been deformed during the neoproterozoic edmundian orogeny the ophthalmian and glenburgh orogenies affected the northern and southern margins of the orogen respectively the former resulted in the development of a foreland basin along the pilbara craton margin whereas the latter is associated with accretion of an allochthonous element the glenburgh terrane of the gascoyne complex to the northwestern margin of the yilgarn craton effects of the capricorn orogeny extended across the entire orogen and provide a younger limit for amalgamation of the yilgarn and pilbara cratons a number of basins developed along both the northern and southern margin of the orogen during this event and were filled with sediment eroded from the yilgarn craton the gascoyne complex and a presently unexposed early palaeoproterozoic terrane the capricorn orogeny corresponds with a major phase of continental collision both in australia and world wide which appears to mark a major phase of supercontinent assembly mineralization within the orogen includes a wide variety of deposit types that can be related to collisional settings including the world class iron orebodies of the hamersley basin

geotectonic signature and hydrothermal alteration of metabasalts under and overlying the giant serra norte iron deposits carajas mineral province abstract archean metabasalts enclose the serra norte iron deposits in the carajas mineral province brazil they experienced seafloor hydrothermal alteration regional metamorphism under low greenschist facies conditions and hematite associated hydrothermal alteration characterizing i least altered ii chlorite and iii hematite chlorite basalts in the n4 and n5 deposits their petrographic and geochemical changes were caused by both chlorite and hematite hydrothermal alteration nevertheless part of the original igneous textures and some of their primary minerals plagioclase and clinopyroxene are commonly preserved geochemical investigation of the least altered basalt indicates a calc alkaline affinity trace element behavior e g nb and th is consistent with an arc related setting and continental crustal contamination where hydrothermal alteration affects basalts the most distal intermediate and proximal alteration zones are distinguished based on the amount of chlorite hematite less magnetite chlorite composition in i ii and iii marks these three mafic lithotypes with variations in the fet fe mg ratios that clearly reflect buffered through unbuffered conditions mineralogical and textural features such as ruptured amygdales indicate fluid interaction with hydrothermal minerals forming veins and breccias caused by hydraulic fracturing both chlorite and hematite chlorite basalts experienced renewed isotopic equilibrium due to hydrothermal alteration showing a decrease in 18o 7 4 to 1 9 in comparison to the least altered basalt reflecting an increase in fluid rock ratio on most discrimination diagrams and morb normalized spidergram the investigated rocks are comparable to a volcanic arc environment associated with subduction zone these and other evidence such as a a seafloor associated volcanogenic sequence b arc type sedimentation of the grao para group indicated by hyaloclastic tufts breccias and volcaniclastic rocks c jaspilites in the serra norte and d a large subduction signature expressed by enrichment in th all suggest that the grao para basalts overflowed on the continental crust in a back arc environment with the primary magma having experienced significant upper crustal contamination the arc association evolution and the variability of lithological sedimentological igneous volcanic structural metamorphic and other evolutionary aspects of the itacaiunas supergoup is suggestive of a greenstone belt type setting an arc associated evolution and the variability of lithological sedimentological igneous volcanic structural metamorphic and other aspects of the itacaiunas supergoup supports a greenstone belt type setting

regional trends in water chemistry smackover formation southwest arkansas geochemical and physical controls we present results on regional geochemical analyses of formation waters from jurassic carbonate strata of the upper smackover formation from the southwest arkansas shelf gulf coast basin these formation waters have been noted in earlier studies for their exceptional salinities and abnormally br rich composition our detailed and regionally extensive database shows that chemical heterogeneities occur on a relatively small scale and have spatial relations to faults which may act as pathways for fluid migration from deeper and hotter portions of the stratigraphic section one of the most pronounced heterogeneities in water chemistry is an h 2 s rich anomaly in the center of the shelf enrichments in h 2 s are accompanied by large increases in alkali elements li k and b and b which could be produced by higher temperature diagenesis of clay minerals and feldspars in deeper seated siliciclastic strata bromide enrichments in all smackover formation waters are apparent from relations between br and cl contents as well as br and divalent cation carpenter function values as suggested by earlier studies of these waters evaporatively concentrated seawater expelled from associated thick monomineralic evaporite sections may be an important source of br cl and na whereas ca mg and sr concentrations have been modified by carbonate mineral recrystallization and dolomitization mineral equilibria exert significant controls on concentrations of ca mg sr and so 4 in these brines br enrichment likely was produced by halite recrystallization in fluids already enriched in br from seawater evaporation elevated cl br ratios occur in regional association with lower salinity water near t e south arkansas fault system consistent with halite dissolution during meteoric water recharge thus despite relatively uniform reservoir lithology and simple structural setting processes on small spatial scales have resulted in significant heterogeneity in water chemistry within a small area of the gulf coast sedimentary basin although movement of fluids along these fault systems is not demonstrated in our study the regional trends in geochemistry are strongly suggestive of their important role in controlling chemical heterogeneities in smackover formation waters

effect of fractures on seismic amplitudes from explosions improved understanding of the seismic radiation generated by explosions in low coupling damaged fractured media is extremely important for nuclear monitoring as source coupling affects both detection and yield estimation some empirical evidence for seismic amplitude reductions have been noted for nuclear and chemical explosions detonated in fractured media e g sokolova 2008 in order to define the physical mechanism responsible for the amplitude reduction and quantify the degree of the amplitude reduction in fractured rocks we conducted phase i of a multi phase explosion experiment in central new hampshire the experiment involved conducting explosions of various yields including a 46 3 kg explosion in the damage fracture zone of a 231 8 kg explosion and a 46 8 kg shot in nearby undamaged rock our analysis confirms a seismic amplitude reduction in damaged rock by a factor of 2 3 the amplitude differences are frequency dependent with the explosion in the undamaged rock having a higher corner frequency than the explosion in the damaged zone the overshoot parameter for the virgin undamaged rock shots is higher than that for the damaged rock shot we found that the corner frequency correlates with the overshoot parameter and only weakly correlates with the yield additional experiments will be conducted in the near future to further quantify seismic wave characteristics as a function of the depth of burial type of explosives and other factors online material movies of explosions

emplacement of a large igneous province as a possible cause of banded iron formation 2 45 billion years ago latest archaean and earliest palaeoproterozoic times from 2 6 to 2 2 billion years ago have generally been viewed as a largely quiescent period of earth history the geological record indicates the very slow deposition of pelagic and chemical sediments1 2 and bears only a limited record of magmatic and tectonic activity3 5 such quiescence is consistent with the contention that the earth s main banded iron formations bifs finely laminated chemical sedimentary rocks rich in iron oxide formed slowly as oxygen abundances in the oceans gradually increased thus reducing the capacity of sea water to retain dissolved iron6 10 here we show that a large igneous province comprising 30 000km3 of dolerite basalt and rhyolite accompanied deposition of a hamersley province bif 2 449 3 million years ago this observation indicates that hamersley bifs formed during a major tectono magmatic event and were deposited very much faster than previously thought at similar rates to or faster than modern pelagic sediments thus the largest palaeoproterozoic bifs rather than simply reflecting a gradual increase in the oxygen content of the oceans during a period of tectonic quiescence are more likely to have formed as a result of an increased supply of suboxic iron and silica rich sea water upwelling onto continental shelves during a pulse or pulses of increased submarine magmatic and hydrothermal activity

structural control and hydrothermal evolution model of unusual high grade metasandstone hosted iron deposits mesoproterozoic eastern chapada diamantina brazil abstract the ibicoara high grade hypogene hematite deposits are located in the sao francisco craton in the chapada diamantina fold thrust belt iron mineralization host rocks are quartz and arkose sandstones from the tombador formation which present planar stratification regionally three deformational phases are identified the first d1 phase developed kink with subordinate chevron folds shear zones related to flexural slip with s c structures stretch lines lx1 slickenlines and slickensides the second phase d2 is represented by dextral and sinistral reverse high angle shear zones with nnw sse and nw se orientation trends respectively high angle tension fractures with an ene wsw orientation during the third deformation phase d3 dextral and sinistral shear zones with ene wsw and ne sw orientations are developed the richest hematite domains are massive having fe2o3total contents to 98 these irregular massive domains are distributed along the primary sedimentary foliation plane parallel stratification and tangential cross bedding of d2 shear zones or tension fractures generated during d2 the irregular massive hematite domains grade laterally into platy and stringer domains in addition there are vein stockwork and breccia mineralization styles that crosscut irregular massive domains or host rock chemical analyses of hematite in irregular massive domains show that fe2o3 correlates negatively with sio2 zr th sr nb hf and ba on the other hand it correlates positively with zn v u cu co and with the sum of rare earth elements ree when normalized to the post archean australian shale paas the distribution of ree generally show negative eu anomalies and a weak enrichment of light rare earth elements lree compared to the heavy rare earth elements hree the paragenetic succession shows the existence of three groups of hematite groups 1 and 2 are mainly related to the irregular massive domains controlled by rock stratification and the s0 s2 foliation in shear zones the distinction between them is given by the finer grain size of the first generation of hematite and the presence of breccia domain in the second group the third group is related to massive vein domains for each group the hematite grain size increases successively and there is a relative increase in the hree in relation to the lree furthermore similarities between the chemical signatures of the first groups 1 and 2 hematite generation with the signatures of the host rock reveal an important influence of the chemical composition of the original rock this is not the case for the late hematite of massive domains as well as vein style hematite suggesting high fluid rock conditions the hematite hydrothermal alteration at ibicoara has structural control with dilational structures associated with d2 shear zones the host lithotype was also a contributing factor due to its porosity and permeability and the presence of sedimentary structures that favored flow of the hydrothermal fluid from shear zones replacement and direct precipitation took place and are responsible for the transfer of solutions and formation of hematite rich areas the hydrothermal fluid probably leached components from the espinhaco supergroup metasedimentary units and dikes that are regionally present and carried these during the deformational phases that inverted the paramirim aulacogen the espinhaco supergroup is characterized by a sequence of siliciclastic rocks predominantly metasandstones and felsic metavolcanic rocks sterile in iron which were deposited between the statherian and stenian periods in the chapada diamantina area the espinhaco supergroup is comprised from base to top by the serra da gameleira formation and the rio dos remedios paraguacu and chapada diamantina groups the chapada diamantina group contains the tombador formation that hosts the high grade iron deposits investigated in the present study in light of the tectonic context and existence of shear related gold and baryte hydrothermal deposits nucleated during the paramirim aulacogen inversion the chapada diamantina thrust and fold belt is considered a potential regional exploration target

a synvolcanic origin for magnetite rich orebodies hosted by bif in the weld range district western australia abstract iron ore deposits hosted by archean banded iron formation bif in the weld range greenstone belt are representative of most of the documented iron ore deposits in the yilgarn craton they include near surface supergene goethite hematite orebodies that overlie and partly modify deeper occurrences of hypogene magnetite and specular hematite ores the cenozoic goethite hematite rich orebodies in these deposits are unequivocally the product of meteoric fluid alteration affecting bif in the near surface supergene environment however the deeper and likely older magnetite and specular hematite rich orebodies have a more contentious origin this study is the first to present a fluid alteration model for hypogene iron mineralization in the yilgarn craton that uses fluid inclusion and mineral chemistry data to constrain the physical chemical characteristics and source of hypogene fluids responsible for mineralization high grade 57 wt fe magnetite rich iron ore at the beebyn deposit defines a discontinuous series of 440 c and co2 rich paired o and c stable isotope data for stage 1 ferroan dolomite suggest that these fluids had a magmatic source while stage 1 magnetite chemistry e g enrichments in mg mn ca and na indicates chemical exchange took place between the fluids and mafic igneous rocks prior to crystallization of magnetite the presence of monophase carbonic fluid inclusions in stage 1 ferroan dolomite suggest that phase separation of a bicarbonate rich aqueous fluid took place in deeper parts of the hydrothermal system which led to the separation of the resultant volatile rich and brine phases during transport of the stage 1 fluid to shallower crustal levels cooling of the hydrothermal system during the stage 2 fluid event involved i an early brine 275 327 c 36 40 equiv wt nacl with cl br and c and o isotopes values that overlap the ranges for magmatic fluids with minor involvement of archean seawater and extensive chemical exchange with country rocks followed by ii pulses of moderate and lower temperature stage 2 brines 125 260 c 2 24 equiv wt cacl2 with cl br o and c isotope na br and ca ca na signatures that suggest cooling of magmatic derived fluids that mixed with archean seawater and reacted with mafic igneous country rocks in areas more distal to fluid pathways the last stage of formation of magnetite rich ore at the beebyn deposit involved the flow of stage 3 fluids through the existing fault network that controlled earlier fluids stage 3 fluids are lower temperature 98 175 c low to high salinity brines with cl br values that overlap reported ranges for 3 2 ga vent fluids and seawater thus they are likely the product of heated archean seawater that was chemically modified through interaction with mafic country rocks magnetite talc veins at the madoonga deposit have fluid halogen ratios for fluid inclusions hosted by magnetite that are compatible with a range of possible sources including low grade metamorphic fluids geothermal brines or oil field formation waters although their common spatial association with semi massive sulfides suggests their likely precipitation from brines derived from heated archean seawater more locally developed specular hematite quartz veins that cut folded magnetite rich ores at the beebyn and madoonga deposits are the product of iron redistribution in bif by heated meteoric fluids or seawater with precipitation of specular hematite as a consequence of oxidation and cooling of the fluid

cavity decoupling of small explosions in limestone abstract a series of small chemical explosions were conducted in may 2012 at kirtland air force base new mexico as a part of the humble redwood iii experiments two charges of 111 kg of ammonium nitrate and fuel oil were detonated at 7 m depth in limestone the second explosion was detonated in a small asymmetric cavity produced by the first explosion at the same working point as the first explosion light detection and ranging lidar was used to image the cavity and to determine an equivalent spherical radius of 0 82 m the seismic amplitudes for the cavity shot were reduced by a factor of 3 4 compared to the confined shot in limestone in this article we quantify the generation of the small cavity and the seismic decoupling it produced

seismic coupling of chemical explosions in intact and fractured granite in barre vermont abstract in this article we investigate seismic amplitude reduction for the chemical explosions conducted in low coupling fractured rocks the explosions used in this study were detonated in the fracture zones left by earlier explosions repeat shots in granitic rocks in barre vermont the seismic amplitudes from the repeat shots were compared with the amplitudes from the same yield explosion conducted in the intact rock in the same area the results of the experiments show seismic amplitude reduction for the repeat shots by a factor of 1 5 2 in the entire frequency range the amplitude reduction observed for the fractured rock explosions can be explained by compaction due to pore collapse other inelastic losses and by reduced elastic moduli in a localized damage zone around the charge we used the mueller and murphy 1971 model to calculate spectra of the first and repeat explosions to account for the reduction of the elastic moduli in the damage zone we propose a hybrid medium model with the intact medium outside the elastic radius and the medium with reduced elastic moduli inside the elastic radius the contribution from the pore collapse and other inelastic losses can be modeled by changing the compaction parameter d both models higher compaction and the hybrid elastic moduli predict reduced amplitudes at all frequencies and qualitatively agree with the observations because of the complexities in the explosion processes and the uncertainties in the damage zone configuration the proposed model may not exactly reproduce the explosion source however using the models with a combination of the parameters can provide bounds on the resulting spectral ratios

petrographic and geochemical evidence for hydrothermal evolution of the north deposit mt tom price western australia high grade iron mineralisation 65 fe in the north deposit occurs as an e w trending synclinal sheet within banded iron formation bif of the early proterozoic dales gorge member and consists of martite microplaty hematite ore three hypogene alteration zones between unmineralised bif and high grade iron ore are observed 1 distal magnetite siderite iron silicate 2 intermediate hematite ankerite magnetite and 3 proximal martite microplaty hematite apatite alteration zones fluid inclusions trapped in ankerite within ankerite hematite veins in the hematite ankerite magnetite alteration zone revealed mostly h2o cacl2 pseudosecondary and secondary inclusions with salinities of 23 9 1 5 1 n 38 and 24 4 1 5 1 n 66 eq wt cacl2 respectively pseudosecondary inclusions homogenised at 253 59 9 c 1 n 34 and secondary inclusions at 117 10 0 c 1 n 66 the decrepitation of pseudosecondary inclusions above 350 c suggests that their trapping temperatures are likely to be higher i e 400 c hypogene siderite and ankerite from magnetite siderite iron silicate and hematite ankerite magnetite alteration zones have similar oxygen isotope compositions but increasingly enriched carbon isotopes from magnetite siderite iron silicate alteration 8 8 0 7 1 n 17 to hematite ankerite magnetite alteration zones 4 9 2 2 1 n 17 when compared to the dolomite in the wittenoom formation 0 9 0 7 1 n 15 that underlies the deposit a two stage hydrothermal supergene model is proposed for the formation of the north deposit early 1a hypogene alteration involved the upward movement of hydrothermal cacl2 rich brines 150 250 c likely from the carbonate rich wittenoom formation 13c signature of 0 9 0 7 1 n 15 within large scale folds of the dales gorge member fluid rock reactions transformed unmineralised bif to magnetite siderite iron silicate bif with subsequent desilicification of the chert bands stage 1b hypogene alteration is characterised by an increase in temperature possibly to 400 c depleted 13c signature of 4 9 2 2 1 n 17 and the formation of hematite ankerite magnetite alteration and finally the crystallisation of microplaty hematite late stage 1c hypogene alteration involved the interaction of low temperature 120 c basinal brines with the hematite ankerite magnetite hydrothermal assemblage leaving a porous martite microplaty hematite apatite mineral assemblage stage 2 supergene enrichment in the tertiary resulted in the removal of residual ankerite and apatite and the weathering of the shale bands to clay

the role of deformation in the formation of banded iron formation hosted high grade iron ore deposits hamersley province australia abstract the hamersley province western australia hosts some of the world s largest iron ore deposits but despite decades of research their genesis is still extensively debated many iron ore deposits are hosted in complexly deformed archean to paleoproterozoic banded iron formations comprising thin chert and iron oxide bands interlayered with silicate rich shales and carbonates current iron ore genesis models have identified a strong structural control on ore formation linked to extensive hypogene and supergene fluid circulation along fault structures these fluid pathways facilitate the removal of vast amounts of gangue minerals leading to enrichment of the iron oxide residue to iron ore however the evolution of the associated structures has not yet been considered as a key element in ore genesis here we show through multiscale structural analyses that deformation not only forms suitable fluid channels but that folding and shearing also result in significant synkinematic removal of gangue minerals our multidisciplinary investigation of the structural evolution of the mount tom price deposit combines microtectonic field geology and 3d implicit modelling techniques to establish a link between deformation structures at various scales microscale shear bands and outcrop scale asymmetric parasitic folds share striking similarities in their evolution and their controlling mechanisms both features record substantial non coaxial deformation accompanied by volume changes due to stress induced silica remobilisation the closely spaced layering of rheologically different lithologies within hamersley province strata plays a crucial role in complex multilayer deformation which resulted in extensive strain partitioning our study suggests that deformation was of major significance in the upgrading of banded iron formation to iron ore and was active from the early stages of banded iron formation during diagenesis deformation structures also established a micro to deposit scale lateral and vertical fluid network which enabled infiltration by hypogene and supergene fluids during or after deformation these new insights have important implications for iron ore genesis models structural applications in the mine environment and for understanding complex multilayer deformation with volume loss

dating hypogene iron mineralization events in archean bif at weld range western australia insights into the tectonomagmatic history of the northern margin of the yilgarn craton this study is the first to constrain the absolute timing of hypogene iron mineralization in archean bif located in the yilgarn craton in situ shrimp u th pb geochronology on xenotime and monazite grains has been used to constrain the age of hypogene magnetite replacement ores at the beebyn deposit and hypogene magnetite vein ores at the madoonga deposit in the weld range study area the beebyn magnetite replacement ores c 2627 ma are younger than the published maximum depositional age of the bif hosts of the wilgie mia formation 2792 9 ma and partially overlaps crystallization ages of granitic rocks 2757 2606 ma that intrude supracrustal rocks throughout the study area these plutons and their probable subvolcanic expressions are considered to be likely sources of energy and fluids responsible for magnetite replacement ores in contrast madoonga magnetite veins record multiple dates the first at 2857 41 ma followed by events at c 2775 ma through 1812 ma the two oldest monazite dates of 2857 41 and 2832 51 ma are interpreted to be mineralization ages for magnetite veins possibly related to subseafloor volcanism and base metal vms systems whereas the younger dates i e 2775 1812 ma probably represent multiple episodes of phosphate mineral precipitation related to reactivation of structures and overprinting by discrete pulses of hydrothermal fluids the madoonga bifs are genetically distinct from the beebyn bifs and are the oldest dated bifs at weld range being older than c 2857 ma but younger than the c 2970 ma felsic volcanic rocks in the stratigraphic footwall to the madoonga bifs hydrothermal events at the madoonga deposit 2215 2120 ma coincide with published dates for rifting mafic ultramafic magmatism and basin development along the northern margin of the yilgarn craton 2215 2145 ma whereas the younger phosphate mineral dates correspond with the glenburgh 2002 1947 ma and capricorn orogenies 1817 1772 ma tectonic activity along the northern margin of the yilgarn craton coincides with reported ages for hematite mineralization in bif hosted iron ore deposits in the hamersley basin and pilbara craton suggesting the far reaching effects of tectonism along paleocratonic boundaries as drivers for iron mineralization in bif

carbonate alteration of the upper mount mcrae shale beneath the martite microplaty hematite ore deposit at mount whaleback western australia the formation of large martite microplaty hematite ore deposits in northwest australia remains a contentious topic in part because important evidence supporting a unifying genetic model has not been observed at all deposits carbonate replacement of silica has been found along normal faults below ore at the mount tom price and giles mini deposits which suggests an early hypogene process during ore formation however such rocks have not been identified at the largest martite microplaty hematite deposit mount whaleback in this study samples of the mount mcrae shale are examined for their chemistry mineralogy and petrography these samples were collected from several key locations including an area that immediately underlies ore along the mount whaleback fault at mount whaleback compared to unaltered black mount mcrae shale from wittenoom gorge in the north and altered black and red mount mcrae shale at mount whaleback reddish green mount mcrae shale along the mount whaleback fault is greatly enriched in mgo and cao and depleted in sio2 this chemistry arises from significant amounts of fine to medium grained ferroan dolomite and ankerite and cross cutting chlorite and carbonate veins the composition is distinct from that produced during regional metamorphism and most likely represents hydrothermal alteration after metamorphism the lack of carbonate rich silica poor rocks in the overlying dales gorge member at mount whaleback is consistent with pervasive oxidation of most rocks in the region during or after ore genesis a process that removed carbonates although several questions remain unanswered these results support models that invoke an early hypogene stage during the formation of the martite microplaty hematite deposits in the hamersley province

giant iron ore deposits of the hamersley province related to the breakup of paleoproterozoic australia new insights from in situ shrimp dating of baddeleyite from mafic intrusions banded iron formations of the ca 2770 2405 ma hamersley province of western australia were locally upgraded to high grade hematite ores during the early paleoproterozoic by a combination of hypogene and supergene processes after the initial rise of atmospheric oxygen ore genesis was associated with the stratigraphic break between the lower and upper wyloo groups of the ashburton province and has been variously linked to the ophthalmian orogeny late orogenic extensional collapse and anorogenic continental extension small spot in situ pb pb dating of baddeleyite by sensitive high resolution ion microprobe shrimp has resolved the ages of two key suites of mafic intrusions constraining for the first time the tectonic evolution of the ashburton province and the age and setting of iron ore formation mafic sills dated as ca 2208 ma were folded during the ophthalmian orogeny and then cut by the unconformity at the base of the lower wyloo group a mafic dike swarm that intrudes the lower wyloo group and has a close genetic relationship to iron ore is ca 2008 ma slightly younger than a new syneruptive 2031 6 ma zircon age for the lower wyloo group these new ages constrain the ophthalmian orogeny to the period between ca 2208 and 2031 ma before lower wyloo group extension sedimentation and flood basalt volcanism the ca 2008 ma dikes pre s ent a new maximum age for iron ore genesis and deposition of the upper wyloo group thereby linking ore genesis to a ca 2050 2000 ma period of continental extension similarly recorded by paleoproterozoic terrains worldwide well after the initial oxidation of the atmosphere by ca 2320 ma

tiny particles building huge ore deposits particle based crystallisation in banded iron formation hosted iron ore deposits hamersley province australia abstract the world s major source of iron ore is hosted in precambrian banded iron formations these chemical meta sedimentary rocks are composed of alternating laminae of iron oxide minerals and chert despite the economic significance of high grade iron ore deposits controversy persists after decades of research on how banded iron formations became upgraded to form iron ore the fundamental requirement for iron ore formation is the removal of vast amounts of chert coupled with an increased concentration of iron oxide minerals here we assess the fate of colloidal hematite inclusions encapsulated in chert after quartz dissolution and examine their role in the formation of hematite ore we have analysed hematite ores from the hamersley province australia using a combination of petrography high resolution electron microscopy and x ray diffraction these techniques reveal the presence of abundant nano and microscale hematite particles that we suggest form the building blocks of larger hematite crystals within the iron ore our textural observations indicate that hematite colloids that were previously encapsulated inside the microcrystalline quartz grains in chert layers are released during quartz dissolution and subsequently reassemble in a self similar fashion to from new microplaty hematite crystals via non classical crystallisation pathways progressive growth and fusion leads to the transformation of hematite microplates into hematite bands which resemble pre existing iron oxide laminae of banded iron formations in contrast to previous models we observe the direct transformation of banded iron formations to microplaty hematite and have not found evidence that significant amounts of hematite formed through metamorphism of goethite or that intermediate carbonate minerals were involved during the upgrading of banded iron formations to pure hematite ore given the strong evidence for hypogene fluids found in many deposits we have also assessed the role that such warm highly saline fluids may have played during the evolution of iron ore using insights from crystal chemistry we conclude that fluid infiltration impacts many aspects of iron ore formation by controlling hematite colloid liberation and aggregation and finally controlling the transformation of the colloids into macroscopic crystals of hematite via non classical crystallisation mechanisms our study underlines the significance of hypogene fluids in the upgrading of banded iron formation to iron ore however we suggest that their influence was mainly passive as hematite was not precipitated directly from these fluids

radial gravitational gliding on passive margins abstract gravitational gliding of uppermost sediments down a passive margin is possible if there is a basal layer of evaporite or other soft material to allow detachment in examples from the gulf of mexico and the brazilian margin gliding has produced three main structural domains an uppermost domain of downdip extension an intermediate domain of rigid gliding and a lowermost domain of downdip contraction domain boundaries are established by changes in slope in this paper we examine three kinds of gravitational gliding depending on the paths followed by material particles in ideal parallel gliding particle paths are parallel straight lines trending downslope this should occur where the margin is perfectly straight in ideal radial gliding particle paths are radii of a circle and the margin is shaped like a circular cone natural margins will not have ideal shapes but divergent gliding will tend to occur off coastal salients convergent gliding off coastal re entrants a simple kinematic model based on ductile behaviour illustrates some essential features of radial gliding changes in radius during divergent gliding produce strike parallel extension during convergent gliding they produce strike parallel contraction vertical strains also differ divergent gliding produces an uppermost domain of strong vertical thinning balanced by extensions in all horizontal directions similarly convergent gliding produces a lowermost domain of strong vertical thickening balanced by contractions in all horizontal directions these deformed states cannot be restored by simple techniques based on section balancing we have done three experiments using analogue materials sand to model the brittle behaviour of sediments silicone putty to model the ductile behaviour of basal layers of evaporite the experiments were properly scaled to account for gravitational forces experiment i reproduced convergent gliding above a basement with a conical upper surface strike parallel contraction was responsible for radial folds on the lower slopes the lowermost domain had a complex fold pattern indicating contraction in all horizontal directions experiment 2 reproduced divergent gliding above a basement with a pyramidal upper surface strike parallel extension produced radial rifts on the upper slopes the uppermost domain had a complex polygonal pattern of normal faults indicating extension in all horizontal directions experiment 3 also reproduced divergent gliding but due to non uniform sediment thickness we have examined the many seismic profiles both downdip and alongstrike available for the brazilian margin alongstrike profiles show structures which we attribute to strike parallel strains around the santos re entrant syn sedimentary growthfolds indicate strike parallel contraction increasing downslope this argues for a mechanism of convergent gliding in contrast around the campos salient normal growth faults and associated salt rollers indicate strike parallel extension decreasing downslope this argues for a mechanism of divergent gliding the pattern of oil fields in the campos area including the giant marlim field seems to be closely controlled by divergent gravitational gliding and associated processes radial gliding should occur on other passive margins as well especially where the coastline is sinuous structures around the western gulf of of mexico seem to indicate convergent gliding at the scale of the entire gulf structures at a smaller scale around the mississippi delta seem to indicate divergent gliding

geochemistry of metal rich brines from central mississippi salt dome basin u s a abstract oil field brines are the most favored ore forming solutions for the sediment hosted mississippi valley type ore deposits detailed inorganic and organic chemical and isotope analyses of water and gas samples from six oil fields in central mississippi one of the very few areas with high metal brines were conducted to study the inorganic and organic complexes responsible for the high concentrations of these metals the samples were obtained from production zones consisting of sandstone and limestone that range in depth from 1900 to 4000 m 70 120 c and in age from late cretaceous to late jurassic results show that the waters are dominantly bittern brines related to the louann salt the brines have extremely high salinities that range from 160 000 to 320 000 mg l total dissolved solids and are na ca cl type waters with very high concentrations of ca up to 48 000 mg l and other alkaline earth metals but with low concentrations of aliphatic acid anions the concentrations of metals in many water samples are very high reaching values of 70 mg l for pb 245 mg l for zn 465 mg l for fe and 210 mg l for mn the samples with high metal contents have extremely low concentrations 2 s samples obtained from the smackover formation limestone have low metal contents that are more typical of oil field waters but have very high concentrations up to 85 mg l of h 2 s computations with the geochemical code solmineq 87 give the following results 1 both pb and zn are present predominantly as aqueous chloride complexes mainly as pbcl 4 2 and zncl 4 2 respectively 2 the concentrations of metals complexed with short chained aliphatic acid anions and reduced s species are minor 3 organic acid anions are important in controlling the concentrations of metals because they affect the ph and buffer capacity of the waters at subsurface conditions and 4 galena and sphalerite solubilities control the concentrations of pb and zn in these waters

structural evolution of the mount wall region in the hamersley province western australia and its control on hydrothermal alteration and formation of high grade iron deposits abstract the discovery of two relatively small but high grade iron ore deposits near mt wall an intensely faulted part of the southwestern hamersley province provides unique insights into the structural control on ore formation in this region the deposits have many geological features typical of the high grade microplaty hematite group which also contains the much larger mt tom price paraburdoo and mt whaleback deposits the deposits are structurally controlled along early normal faults and contain abundant microplaty hematite and martite and are largely confined to the dales gorge member of the brockman iron formation in addition to the microplaty hematite martite ore there are martite goethite ores and rare magnetite goethite or magnetite hematite ores below the modern weathering surface hydrothermally altered zones in wallrock bif from the lower dales gorge member contain magnetite hematite and carbonate talc bearing mineral assemblages a staged ore genesis model involving early extension and fluid circulation along normal faults hypogene silica leaching and carbonate alteration followed by deep meteoric oxidation with microplaty hematite formation and finally weathering can explain most features of the mt wall deposits the role of deformation was to provide pathways for mineralising fluids and initiate the seed points for the mineralised systems high grade iron in the wellthandalthaluna deposit is situated between the nw to nnw trending boolgeeda creek fault and a synthetic joining splay the northern fault both are high angle normal faults and formed during early extension in this part of the province faults are characterised by localised small scale deformation and brecciation deep carbonate alteration and oxidation recent weathering has penetrated deeply into the fault zones converting the carbonate rich assemblages into goethite mineralisation in the arochar deposit is situated in the overlap or relay zone between two segments of the mt wall fault zone a moderately to steeply southerly dipping normal fault system which at arochar is intruded by dolerite dykes at both locations the ore controlling faults are offset by later nw trending dextral and normal faults fault relay zones or fault splay zones were likely zones of increased permeability and fluid flow during fault development or reactivation and may also have been important in initiating mineralisation in larger deposits such as mt tom price and mt whaleback however structural controls on the largest iron ore deposits are often obscured due to the intensity and scale of ore development whereas they are better preserved in the smaller deposits recognition that carbonate bearing protores at mt wall survived for nearly two billion years until intense recent weathering converted them to martite goethite or magnetite goethite ores may imply that more of the giant hematite goethite deposits of the hamersley province had hydrothermal precursors and were not formed by supergene processes alone

the bif hosted high grade iron ore deposits in the archean koolyanobbing greenstone belt western australia structural control on synorogenic and weathering related magnetite hematite and goethite rich iron ore the koolyanobbing banded iron formation bif hosted iron ore deposits total premining resources 150 million metric tons mt indicated reserves 32 mt are located in the mesoarchean lower succession bif of the koolyanobbing greenstone belt younami terrane yilgarn craton in western australia in the koolyanobbing greenstone belt a deformation sequence that broadly correlates with the proposed deformation history of most greenstones belts within the southern cross domain includes d 1 structures mainly small scale f 1a and f 1b folds formed in a north south to northwest southeast compressional regime a ductile to brittle deformation sequence d 2 to d 4 generated during east west compression and a late stage brittle segmentation of bif and reactivation of faults attributed to d 5 the formation of the seven known medium 45 58 wt fe to high grade 58 68 wt fe magnetite martite specularite and goethite bearing orebodies can be subdivided into four archean stages and one weathering related upgrade from the permian and or mesozoic to recent times the archean ore forming stages comprise 1 early fe mg ca metasomatism causing local ferroan carbonate and ferroan talc alteration of the metamorphosed quartz magnetite bif protolith 2 sequential syn d 2a coaxial to syn d 4 transpressional tight folding driven removal of carbonate quartz and minor ferroan talc by solution and mechanical transfer producing residual enrichment of medium to high grade magnetite ore 3 magnetite mineralization in syn d 2b and syn d 4 breccias and fractures forming medium grade ore zones or overprint magnetite in bif and first stage magnetite ore and 4 mineralization of hydrothermal specularite and locally associated ferroan dolomite quartz alteration and local oxidation of magnetite in and near brittle d 4 faults fractures and reactivated f 1 and f 2a fold cores modern weathering related leaching of carbonate and minor quartz pseudomorphic goethite replacement of existing iron oxides and gangue and coeval or subsequent to oxidation in the vadose zone formed goethite martite ore with local relics of specularite or magnetite and or kenomagnetite the intensity and localization of this supergene modification is in most deposits at koolyanobbing controlled by existing hypogene magnetite specularite rich medium to high grade ore zones and or carbonate altered bif at depth the existence of high grade ore below the weathering horizons suggests the possibility of further concealed magnetite and or specularite rich orebodies within the deposits and region

genesis of high grade hematite orebodies of the hamersley province western australia a reply sir we are very thankful for the constructive comments provided by kneeshaw and kepert unfortunately the competitive ownership structure of the iron ore mining industry in the hamersley province limits our personal knowledge of the mount whaleback deposit to occasional mine visits and to the available literature published and unpublished we do however have access to the data on the smaller giles mini and lido deposits 50 and 10 km west of mount whaleback respectively and to extensive exploration mapping in the ophthalmia ranges to the north direct proof for rotation of the orebody as it is preserved at paraburdoo in the approximately 50 dip of the overlying proterozoic hematite conglomerates of the mount mcgrath formation is not preserved at mount whaleback however we believe that there is sufficient information available in the literature to think that a rotational model for a large part of the deposit is perfectly plausible first and above all as stated in taylor et al 2001 f2 folds are more upright with axial planes dipping about 50 s in the southern and western part of the deposit away from the whaleback fault e g as shown on section no 315120e fig 4 in powell et al 1999 interestingly the ore controlling faults are also more upright in this part of the deposit which suggests an intimate link between the dip of these fault planes and dip of the axial planes of f2 folds secondly

chlorine isotopes in fluid inclusions determination of the origins of salinity in magmatic fluids abstract thermal ionisation mass spectrometry tims of the cs 2 cl ion has been used to determine the 37 cl value of paleofluids trapped in fluid inclusions modifications to the existing published method have lowered the precision by a factor of 2 0 09 1 thereby allowing real differences to be measured with greater certainty the method has been applied to determine 37 cl of two sets of magmatic fluids trapped in fluid inclusions samples were studied from the capitan pluton in new mexico and the sw england batholith both of which have high temperature fluids whose d and 18 o signatures are distinctly magmatic relative to standard mean ocean chloride smoc the 37 cl values of the capitan samples cluster around 0 whereas those from sw england cluster around 1 8 the br cl ratios of the capitan fluid inclusions are low indicative of halite dissolution and are comparable to local high salinity waters derived from leaching permian evaporites the br cl ratios of the sw england fluid inclusions are much higher and within the range of values normally considered to be magmatic we conclude that the 37 cl value of the capitan magmatic fluids confirms the evidence from br cl ratios that cl was derived from an evaporite source whereas the 37 cl value of the sw england fluids appears to be more representative of a deep magmatic source

iron deposits hosted by banded iron formations in the yilgarn craton products of sequential iron enrichment by magmatic marine and meteoric fluids abstract mesoarchean algoma type bif in the yilgarn craton are locally enriched by secondary processes to iron concentrations approaching 72 wt fe while their near surface goethite hematite ores are unequivocally the product of supergene fluid alteration their deeper magnetite and crystalline hematite martite ores have a contentious origin this study shows that magnetite ores formed after diagenesis and peak greenschist facies metamorphism of bif coinciding with regional deformation events the ores formed by a multi stage process involving initial replacement of primary quartz bands in bif by hypogene carbonate minerals followed by the dissolution of the carbonate and concentration of magnetite at the beebyn deposit the earliest fluids were hot 440 c co2 vapor rich and were likely derived from felsic magmatic fluids in contrast the fluids responsible for carbonate alteration at the matthew ridge koolyanobbing and windarling deposits were cooler 366 to 125 c and aqueous rich with salinities from 0 3 to 43 9 wt nacl equiv their cl br and c o isotopic ratios suggest they are the product of mixing between magmatic fluids and heated seawater in all deposits later cooling 178 to 98 c of these fluids led to dissolution of carbonate minerals and concentration of magnetite dated monazite and xenotime from magnetite ores demonstrates that fluid alteration of bif began in the mesoarchean and continued episodically through to the paleoproterozoic later specular hematite rich veins that cut and modify magnetite ores crystallized from aqueous rich fluids that are more oxidized with salinities from 0 to 28 7 wt nacl equiv and crystallization temperatures of 243 to 85 c their cl br and c o isotopic fluid ratios indicate that they formed from mixing between heated seawater and meteoric waters the bif hosted iron deposits in the yilgarn craton show remarkable similarities with the well studied iron deposits in the carajas mineral province and hamersley basin they all show the same transition through time from magnetite to hematite rich ores subsequent episodic tectonically driven fluid alteration events led to further iron enrichment in these deposits the transition in ore types most likely corresponds to a common guiding process namely the uplift of crustal rocks containing bif resulting in the progressive shallowing of fluid alteration pathways and the greater influence of cooler more oxidized hydrothermal fluids unlike iron deposits hosted by algoma type bif in the yilgarn and carajas iron deposits hosted by superior type bif in the hamersley basin display a greater influence of oxidized marine and meteoric waters without involvement of magmatic fluids

hydrothermal origin for the 2 billion year old mount tom price giant iron ore deposit hamersley province western australia giant iron ore deposits such as those in the hamersley province of northwestern australia may contain more than a billion tonnes of almost pure iron oxides and are the world s major source of iron it is generally accepted that these deposits result from supergene oxidation of host banded iron formation bif accompanied by leaching of silicate and carbonate minerals new textural evidence however shows that formation of iron ore at one of those deposits mount tom price involved initial high temperature crystallisation of magnetite siderite iron silicate assemblages this was followed by development of hematite and ferroan dolomite bearing assemblages with subsequent oxidation of magnetite leaching of carbonates and silicates and crystallisation of further hematite preliminary fluid inclusion studies indicate both low and high salinity aqueous fluids as well as complex salt rich inclusions with the range of fluid types most likely reflecting interaction of hydrothermal brines with descending meteoric fluids initial hematite crystallisation occurred at about 250 c and high fluid pressures and continued as temperatures decreased although the largely hydrothermal origin for mineralisation at mount tom price is in conflict with previously proposed supergene models it remains consistent with interpretations that the biosphere contained significant oxygen at the time of mineralisation

a pre 2 2 ga age for giant hematite ores of the hamersley province australia one current model for the origin of giant hematite ores of the hamersley province requires a two step process formation of martite and goethite by supergene enrichment followed by burial metamorphism to convert goethite to microplaty hematite our new structural stratigraphic and geoehronological data suggest that at least some microplaty hematite ores were present before metamorphism owing to burial under the lower wyloo group a shrimp u pb zircon age from volcaniclastic breccia within the lower wyloo group suggests that hematite detritus in that group was derived from hematite ores older than 2209 or 15 ma the action of pre 2 2 ga oxidizing fluids is implied although their origin supergene versus hypogene remains controversial

fluid and gas migration in the north german basin fluid inclusion and stable isotope constraints fluid inclusions have been studied in minerals infilling fissures quartz calcite fluorite anhydrite hosted by carboniferous and permian strata from wells in the central and eastern part of the north german basin in order to decipher the fluid and gas migration related to basin tectonics the microthermometric data and the results of laser raman spectroscopy reveal compelling evidence for multiple events of fluid migration the fluid systems evolved from a h2o nacl kcl type during early stage of basin subsidence to a h2o nacl cacl2 type during further burial locally fluid inclusions are enriched in k cs li b rb and other cations indicating intensive fluid rock interaction of the saline brines with lower permian volcanic rocks or sediments fluid migration through carboniferous sediments was often accompanied by the migration of gases aqueous fluid inclusions in quartz from fissures in carboniferous sedimentary rocks are commonly associated with co genetically trapped ch4 co2 inclusions p t conditions estimated via isochore construction yield pressure conditions between 620 and 1 650 bar and temperatures between 170 and 300 c during fluid entrapment the migration of ch4 rich gases within the carboniferous rocks can be related to the main stage of basin subsidence and stages of basin uplift a different situation is recorded in fluid inclusions in fissure minerals hosted by permian sandstones and carbonates aqueous fluid inclusions in calcite quartz fluorite and anhydrite are always h2o nacl cacl2 rich and show homogenization temperatures between 120 and 180 c co genetically trapped gas inclusions are generally less frequent when present they show variable n2 ch4 compositions but contain no co2 p t reconstructions indicate low pressure conditions during fluid entrapment always below 500 bar the entrapment of n2 ch4 inclusions seems to be related to phases of tectonic uplift during the upper cretaceous a potential source for nitrogen in the inclusions and reservoirs is corg rich carboniferous shales with high nitrogen content intensive interaction of brines with carboniferous or even older shales is proposed from fluid inclusion data enrichment in li ba pb zn mg and sulfur isotopic compositions of abundant anhydrite from fissures the mainly light 34s values of the fissure anhydrites suggest that sulfate is either derived through oxidation and re deposition of biogenic sulfur or through mixing of so42 rich formation waters with variable amounts of dissolved biogenic sulfide an igneous source for nitrogen seems to be unlikely since these rocks have low total nitrogen content and furthermore even extremely altered volcanic rocks from the study area do not show a decrease in total nitrogen content

nonredox transformations of magnetite hematite in hydrothermal systems the transformation of magnetite to hematite or hematite to magnetite in nature has generally been considered a redox reaction and linked to a specific redox state of fluid however a nonredox reaction fe2o3 hm fe2 h2o fe3o4 mt 2h may have been the principal mechanism for the transformations of iron oxides in nature especially in hydrothermal environments for example the transformation of goethite and or hematite primary precipitates to magnetite in banded iron formations bifs probably occurred through nonredox reactions with fe2 bearing hydrothermal fluids during the accumulation of a bif sequence rather than through redox reactions involving organic matter during and or after the bif deposition the proposed mechanisms for the transformation of magnetite to hematite provides new exploration strategies for hematite rich secondary ores extending the target for orebodies to much deeper zones below the paleosurface another important implication of the proposed mechanism is that the presence or absence of magnetite and or hematite in geologic formations may or may not provide meaningful information on the redox state of fluid

the northern margin of the capricorn orogen western australia an example of an early proterozoic collision zone abstract the early proterozoic 2200 1600 ma capricorn orogen is a major zone of deformation metamorphism and granite emplacement between the archaean yilgarn and pilbara cratons of northwest australia capricorn orogen structures are recognized along the southern margin of the pilbara craton deforming the archaean granite greenstone terrain of the sylvania inlier together with rocks of the hamersley ashburton and blair basins in the sylvania inlier deformation is characterized by large scale shear zones interpreted as nne directed thrusts in the hamersley basin orogenic deformation forms the ophthalmia fold belt and can be divided into two groups of structures in the southwest large scale dome and basin folds pre ashburton basin are present in the southeast folds and faults are part of a younger syn ashburton basin n directed foreland fold and thrust belt directly linked to deformation in the sylvania inlier the deformed ashburton basin and adjacent parts of the hamersley basin form the ashburton fold belt two periods of deformation are recognized early recumbent folding d1a post ashburton basin pre blair basin and later dextral wrench faulting and associated folding d2a syn or post blair basin capricorn orogen structures are interpreted as the result of an oblique continent continent collision between the pilbara and yilgarn cratons along the southern pilbara margin collision first occurred in the southeast and migrated northwest with the ashburton basin evolving as a foreland basin late stage dextral wrench faulting was the result of westward extrusion of material caught between the opposing craton margins

a new fluid flow model for the genesis of banded iron formation hosted martite goethite mineralization with special reference to the north and south flank deposits of the hamersley province western australia

regional phase excitation and propagation in the lop nor region of central asia and implications for p lg discriminants ital pn and ital lg spectra for lop nor explosions and nearby earthquakes were analyzed for source and propagation effects using a modified inversion method of ital xie 1993 spectral analyses were performed on waveform data from incorporated research institution for seismology chinese digital seismic network kazakhstan and krgyzstan networks ital pn amplitude variations of a factor of 20 are observed across the small aperture lt 200 km kyrgyzstan network and are probably related to three dimensional moho structure close to the network comparisons of ital pn amplitudes and spectral ratios provide evidence for nonisotropic radiation pattern effects in the case of earthquakes and overshoot in explosion time histories complex source effects are born out in our inversion results 1 logarithm of seismic moment m sub 0 from ital pn and ital lg spectra scale linearly with body wave magnitude m sub b for explosions 5 0 le m sub b le 6 0 and for earthquakes 4 0 le m sub b le 5 5 2 pn m sub 0 estimates are similar to lg m sub 0 for earthquakes but are higher than lg m sub 0 for explosions 3 corner frequency f sub c scale as quarter root of m sub 0 for explosions and cube root for earthquakes 4 at the same m sub 0 level pn f sub c aremore 4 endash 5 times higher than lg f sub c for earthquakes and explosions and 5 ital pn source spectra are characterized by significant overshoot for explosions our results suggest that the cause of maximum separation between earthquake and explosion populations on plots of p lg spectral ratios which typically occurs for frequencies above explosion pn f sub c between 2 and 4 hz for explosions in this study may be related to overshoot in the reduced velocity potential copyright 1999 american geophysical union less

salt flow and diapirism related to extension at crustal scale abstract the analysis of the structure and history of salt diapirs in large ancient evaporitic basins such as the southeastern north sea basin suggests that diapirism is controlled by crustal scale extension laboratory experiments on small scale models designed to simulate configurations comparable to the southeastern north sea basin show that diapirs which grow in a sedimentary cover during basement extension can develop close to basement normal faults or at a significant distance from them if the displacement on the basement faults is transfered horizontally along the salt layer for given values of salt viscosity strain rate and friction coefficient in brittle rocks the pattern of salt diapirism in a particular area depends on the ratio of cover thickness to salt thickness as well as the absolute thicknesses of cover and salt layers the experiments give criteria for identifying different types of diapirs and provide models of horizontal flow in a salt layer undergoing extension and diapir development some dynamic implications of the experiments are applied to the southeastern north sea basin

geochemistry of evaporite bearing series a tentative guide for the identification of metaevaporites literature on metasedimentary rocks indicates that former evaporites have been assumed where brines are present as fluid inclusions or minerals such as scapolite tourmaline and lazurite which are considered as evidence of equilibria with brines very few works take into account the chemical composition of rocks because chemical data on evaporite bearing sedimentary series are scarce

na cl br systematics of mineralizing brines in mississippi valley type deposits new and published analyses of fluid inclusion leachates from mississippi valley type mvt deposits can be divided into two groups on the basis of na br and cl br ratios mvt leachates from the illinois kentucky and cincinnati arch districts and from cubic galena in the viburnum trend have na br and cl br ratios that extend to values significantly above that of seawater which are characteristic of evaporite dissolution brines mvt leachates from polaris and octahedral galena in the viburnum trend have na br and cl br ratios that plot below seawater and along the compositional trend formed by evaporation solubility volume constraints require that brines formed by seawater evaporation had high dissolved metal contents preliminary correlation of leachate compositions allows delineation of two brine provinces in the midcontinent united states 1 an early high br province found only in southeastern missouri and 2 a later low br province of probable permian age that extended from the cincinnati arch to the tri state district these observations along with na cl br data for modern brines from the illinois basin argue against models for single stage midcontinent mvt brine flow based on recharge from late paleozoic arkoma foreland basins to the south

oxygen isotope compositions of iron oxides from high grade bif hosted iron ore deposits of the central hamersley province western australia constraints on the evolution of hydrothermal fluids the hamersley province of northwest western australia is one of the world s premier iron ore regions with high grade martite microplaty hematite iron ore deposits mostly hosted within banded iron formation bif sequences of the brockman iron formations of the hamersley group these high grade iron ores contain between 60 and 68 wt percent fe and formed by the multistage interaction of hydrothermal fluids with the host bif formation the oxygen isotope compositions of magnetite and hematite from bif hydrothermal alteration assemblages and high grade iron ore were analyzed from the mount tom price paraburdoo and channar iron ore deposits the 18o values of magnetite and hematite from hydrothermal alteration assemblages and high grade iron ore range from 9 0 to 2 9 per mil a depletion of 5 to 15 per mil relative to the host bif the 18o values are spatially controlled by faults within the deposits a response to higher fluid flux and larger influence on the isotopic compositions by the hydrothermal fluids the oxygen isotope composition of hydrothermal fluids 18ofluid indicates that the decrease in the 18o content of iron oxides was due to the interaction of both basinal brines and meteoric fluids with the original bif late stage talc bearing ore at the mount tom price deposit formed in the presence of a pulse of 18o enriched basinal brine indicating that hydrothermal fluids may have repeatedly interacted with the bifs during the paleoproterozoic

structural control hydrothermal alteration zonation and fluid chemistry of the concealed high grade 4ee iron orebody at the paraburdoo 4e deposit hamersley province western australia high grade iron ore of the 4ee orebody of the 4e deposit 200 mt at 63 5 wt fe occurs as a southerly dipping sheet within banded iron formation bif of the paleoproterozoic dales gorge and joffre members of the brockman iron formation structural reconstruction of the 4e deposit shows that reactivation of the 18e fault and development of the nw striking steeply sw dipping 4e and 4ee normal faults resulted in preservation of the 4ee orebody below the 4e deposit and 400 m below the modern topographic surface three hypogene alteration zones between low grade bif and high grade iron ore are observed 1 distal magnetite quartz dolomite stilpnomelane hematite pyrite 2 intermediate magnetite dolomite hematitechlorite quartz stilpnomelane and 3 proximal hematite dolomite chlorite pyrite magnetite hydrothermal alteration is temporally and spatially constrained by nw trending dolerite dikes that intruded the 4e and 4ee faults prior to hypogene alteration six vein types v1 v6 are recognized at the 4e deposit the veins both cut and parallel the primary bif layers and were emplaced contemporaneously with the hydrothermal alteration zones that record the transformation of low grade bif to high grade iron ore our integrated structural hydrothermal alteration and fluid flow model proposes that during early stage 1a hypogene fluid flow in the 4e orebody occurred during a period of continental extension and enhanced heat flow within sedimentary basins to the south of the paraburdoo range heated basinal brines were focused by the nw striking steeply sw dipping 4e and 4ee normal faults and reacted with bif of the dales gorge and joffre members the warm to hot 160 255 c ca rich 26 6 31 9 equiv wt cacl2 basinal brine interacted with magnetite chert layers transforming them into magnetite quartz dolomite stilpnomelane hematite pyrite bif the iron rich brine up to 2 8 wt fe likely originated from evaporated seawater that had lost mg and na and gained li and ca through fluid rock reactions with volcaniclastic rocks and carbonate successions within the wittenoom formation the first incursion of deeply circulating low salinity 5 8 9 5 wt nacl equiv heated 106 201 c modified meteoric water is recorded in late stage 1a minerals this modified meteoric water had lost some of its na through wall rock interaction with plagioclase possibly by interaction with dolerite of the weeli wooli formation that directly overlies the joffre and dales gorge members stage 1b involved continuing reactions between the hydrothermal fluids and the magnetite quartz dolomite stilpnomelane hematite pyrite bif and produced both the intermediate magnetite dolomite hematite chlorite pyrite and the proximal hematite dolomite magnetite stilpnomelane alteration assemblages microplaty 10 80 m platy 100 250 m and anhedral hematite increasingly replace magnetite in the intermediate alteration zone forming the proximal alteration zones that consist of microplaty platy anhedral hematite and magnetite the intermediate and proximal alteration zones represent the mixing of a hot 250 400 c high salinity ca rich 30 40 wt cacl2 equiv sr rich basinal brine with low temperature and low salinity 5 wt nacl equiv modified meteoric water that was heated 100 200 c during its descent into the upper crust heterogeneous mixing of the two end member fluids resulted in the trapping of primary fluid inclusion assemblages containing a wide range of trapping temperatures up to 200 c and salinities up to 25 wt nacl equiv stage 1c of the hypogene hydrothermal fluid is characterized by low temperature 110 c low salinity 5 wt nacl meteoric water that interacted with the proximal hematite dolomite magnetite stilpnomelane altered bif leaving a porous hematite apatite high grade ore supergene alteration affected the orebody since the cretaceous and produced a hematite goethite alteration assemblage resulting in destruction of the hypogene alteration zones that are only preserved below the depth of modern weathering discovery of the concealed 4ee orebody of the 4e deposit demonstrates that structural geology plays a critical role in the exploration for high grade iron orebodies structural reconstruction should be considered a critical exploration activity in structurally complex terranes where concealed orebodies may exist

synthesis of salt tectonics in the southern north sea uk abstract a structural model encompassing the southern north sea basin west of the central graben has been developed this model consists of a rift system affecting the post salt section around the basin margin and a large area of detached compressional buckle folds within the basin this pattern is initially a response to gravity sliding of the post salt section on the salt within the basin during the late triassic to late jurassic a close relationship between the location and trend of the peripheral graben system and basement structures in the pre salt is noted pre jurassic extension across the peripheral graben systems was balanced by the sum of fault heaves at the pre salt rotliegend level and shortening across salt cored buckle folds in the post salt section salt pillows and swells passively infilled the cores of these gravity induced buckle folds cretaceous and tertiary inversion involved basin tilt and renewed movement on basin bounding basement faults notably reverse movements did not propagate from basement structures up into the peripheral graben systems the post salt sedimentary section experienced gravity spreading in response to inversion related uplift resulting in syn inversion extensional faulting in the sole pit high where the mesozoic section was thickest this extension combined with a loss of fault heave in the pre salt section was balanced by amplification of salt cored buckle folds in the centre of the basin in the context of the model described here salt pillows represent passive infill of thin skinned compressional buckle folds which later amplified during thick skinned basement shortening crestal collapse of such folds occurs via normal faulting accompanied by reactive diapirism such reactive diapirs establish conduits through which salt may leak leading to pillow deflation and ultimately conduit preservation as a salt wall flanked by rim synclines in areas where the buckle folds were emergent the salt structures described here are related to cover folds and faults which in turn reflect episodes of basin extension tilting and inversion hence individual salt structures can be said to be only remotely connected with regional intraplate stresses

prolonged history of episodic fluid flow in giant hematite ore bodies evidence from in situ u pb geochronology of hydrothermal xenotime abstract absolute ages for hydrothermal mineralization and fluid flow are critical for understanding the geological processes that concentrate metals in the earth s crust yet many ore deposits remain undated because suitable mineral chronometers have not been found the origin of giant hematite ore deposits which are hosted in precambrian banded iron formations bifs remains contentious several models have been formulated based on different sources and timing for the mineralizing fluids supergene metamorphic syn orogenic late orogenic extensional collapse and syn extensional precise geochronology of the ore offers a means of discriminating between these models in this study two u pb chronometers xenotime and monazite have been identified in high grade hematite ore bodies from the mount tom price mine in the hamersley province northwestern australia both phosphate minerals occur as inclusions within the hematite ore and as coarser crystals intergrown with martite hematite pseudomorph after magnetite and microplaty hematite indicating that the xenotime and monazite precipitated during mineralization in situ u pb dating by ion microprobe indicates that both phosphate minerals grew during multiple discrete events our results suggest that ore genesis may have commenced as early as 2 15 ga with subsequent hydrothermal remobilization and or mineralization at 2 05 ga 1 84 ga 1 67 ga 1 59 ga 1 54 ga 1 48 ga and 0 85 ga the location of the ore bodies along ancient fault systems and the coincidence of at least some of the u pb phosphate dates with episodes of tectonothermal activity in the adjacent proterozoic capricorn orogen implies that fluids were channelled through major structures in the southern pilbara craton during discrete phases of tectonic compression and extension our results show that the hematite ore bodies formed at sites of repeated focussed hydrothermal fluid flow in contrast to the aforementioned models our results imply that iron ore formation was probably a long lived multi stage process spanning more than one billion years

constraints on the origins of fluids forming irish zn pb ba deposits evidence from the composition of fluid inclusions the numerous zn pb deposits in the irish midlands together with the quantity and grade of the ore make this a world class base metal province despite significant exploration and research there is still disagreement on the origin of the mineralizing fluids in this study we have measured the composition of fluid inclusions using a crush leach technique these data constrain the possible sources of the fluids both during and after mineralization chloride and br concentrations indicate that the main ore fluid at tynagh and silvermines was seawater that had evaporated until the salinity was between 12 and 18 wt percent however na k and li data show that water rock interactions have resulted in the depletion of na and the enrichment of k and li relative to the concentrations expected for evaporated seawater the fluids that produced postore dolomitization were also derived from seawater that had evaporated to higher salinities and had precipitated halite the concentrations of na k and li are what would be expected for seawater at this degree of evaporation and show that these fluids did not interact with the same lithologies as the ore fluids the major change to their fluid composition was exchange of mg for ca during dolomitization we suggest that the ore fluids could only have reached their high salinity by evaporation of seawater on the extensive shallow water shelf regions that existed over much of the irish midlands

structural and hydrothermal evolution of the shear zone related iron oxide enrichment in metavolcano sedimentary rocks of the intracontinental aracuai orogen the case of the espirito santo iron deposit abstract the rio sao joao metavolcano sedimentary sequence is located in the eastern border of the northern espinhaco fold thrust belt in the intracontinental aracuai west congo orogen mafic schist quartz amphibole and carbonate itabirites as well as a carbonate silicate rock occur in the study area these contain mineral assemblages compatible with the amphibolite facies metamorphic conditions magnetite and hematite rich itabirites form a series of iron deposits in the region near igapora caetite bahia state brazil the espirito santo iron deposit has compact hematite layers truncating quartz itabirite with magnetite varying from 10 to 55 vol of the rock volume siliciclastic and mafic rocks also host iron rich domains variably enriched in magnetite and hematite the deposit is hosted in compressional duplexes with structural top towards the sw these structures have an sn 1 sn foliation represented by compositional banding and parallel schistosity that is observed at all scales other common features are s c c boudins pinch and swell structures as well as sheath syn tectonic folds and a mineral stretching lineation lxn hydrothermal alteration was contemporaneous to the development of duplex and the sn 1 sn foliation a distal alteration zone is dominated by a potassic muscovite and or biotite chlorite assemblage and the intermediate zone by carbonate development the proximal alteration zone constitutes the iron oxide magnetite and hematite domains sulfide minerals as well as quartz veins silicification in association with other phases also formed hydrothermal magnetite is associated with c structures which grows either including silicates and skeletal carbonates or forming corrosion borders in these minerals lamellar hematite forms polycrystalline aggregates characterizing the s c structure in the sn 1 sn foliation as well as the axial planar foliation sn in intrafolial folds and the mineral stretching line lxn data obtained by the la icpms technique show that in general in quartz rich and amphibole itabirites hydrothermal magnetite is richer in ree than an early stage magnetite hydrothermal magnetite ree pattern is close to that of the country rock of the mineralization the formation of hematite and magnetite rich domains is related to the percolation of hydrothermal fluids that leached all components of the original silicates quartz and the carbonates of all host rocks and precipitated new hydrothermal phases taking advantage of the ediacaran shear structures

experimental modeling of role of gravity and lateral shortening in zagros mountain belt dynamically scaled analogs of the geologic structures of the zagros mountain belt are used to argue that different parts of the zagros mountains of iran record different combinations of the effects of a gravity driven overturn and a southwest northeast lateral shortening superimposed on the zagros overturn partially scaled material models have been used to simulate the zagros geodynamics which involve layer parallel compression of a 6 to 7 km thick phanerozoic carbonate cover containing a pattern of preshortening diapirs the folds in the zagros form rapidly 1 5 mm yr in a 20 to 30 km wide zone reactive some of the preshortening diapirs and generate new synshortening listric diapirs a third set of postshortening diapirs rises from the hormuz decollement behind the fold thrust front model buckle folds superimposed on diapirs or pillows tend to avoid and curve around preshortening diapirs which flatten in the synclines model profiles show that lateral shortening induces residual salt at depth to flow toward and rise through the anticlinal cores as synshortening or postshortening diapirs i suggest that any salt pillows in currently diapir free zones of the zagros fold thrust belt may surface as diapirs through the anticlines in the future in the absence of well data seismic and field observations i have used only three map pattern criteria to recognize the timing of diapirs these are locally obscured by extrusive salt sheets preshortening diapirs are generally small and elongate parallel to fold axes and are restricted to synclines synshortening diapirs are elongate perpendicular to the fold axes and are restricted to anticlines postshortening diapirs can be the same size as synshortening diapirs or larger and can be circular they are more common in the anticlines

nature and evolution of fluids associated with specularite bearing fe and au pge jacutinga mineralization during the brasiliano orogeny in the eastern sao francisco craton minas gerais brazil abstract hydrothermal fluids related to the development of the aracuai orogen during the brasiliano cycle have affected paleoproterozoic banded iron formation bearing sequences along the eastern border of the sao francisco craton controversy surround the origin of specularite in high grade schistose iron ore bodies in the serpentina range serpentina type ore versus specularite rich s free au pge quartz veins jacutinga type that occur at the eastern border of the quadrilatero ferrifero mining district chemical and thermobarometric characteristics of fluid inclusions from serpentina type quartz specularite veins show that metamorphic aqueous saline fluids of low to moderate temperature i e salinity between 6 3 and 13 7 wt nacl eqv and homogenization temperatures from 91 to 228 c were responsible for silica leaching and fe enrichment along shear zones in the statherian serra da serpentina group these serpentina type fluids present a distinct pattern with na ca k the fluids are depleted in li na ba ca k cu and mg and enriched in as and sr in comparison to the jacutinga type fluids in contrast other two distinct fluids and chemical processes were involved to precipitate fe oxides with au pge in jacutinga type veins the first type is an aqueous carbonic fluid from the catas altas and itabira deposits these jacutinga type fluids present a pattern with na k ca mg and k li ba sr the fluids are interpreted to be of brasiliano age and are typically associated with anatectic pegmatites from santa maria de itabira the high salinity of these brines 23 2 25 3 wt nacl eqv from the itabira ore likely originated from their percolation through evaporitic layers from salt concentration during deformation or from fluid rock interactions mixing between high temperature aqueous carbonic pegmatite forming fluids and low temperature aqueous metamorphic brines could have induced an abrupt change in the physical chemical conditions of the system which led to au and pge precipitation in the itabira jacutinga type deposit a second jacutinga type fluid is observed at the gongo soco and barao de cocais deposits this fluid is interpreted as an aqueous saline metamorphic fluid with the pattern na ca k mg low to moderate salinity 4 3 16 4 wt nacl eqv and temperatures of 77 to 305 c it is relatively enriched in b depleted in li ba na cu zn and k and follows the trend k li ba in both types of fluids responsible for jacutinga type mineralization gold was transported as chlorine complexes under acidic and oxidizing conditions the metal precipitation may have occurred due to a rapid depressurization during failure of shear zones operating in the upper levels of the crust or variations in the ph and redox conditions during the interaction with the host dolomitic itabirite

fluid immiscibility in natural processes use and misuse of fluid inclusion data abstract many occurrences of fluid immiscibility in natural geologic systems have been reported recently most often from fluid inclusion studies however the interpretation of fluid inclusion data in terms of immiscibility sometimes suffers from ambiguity of the vocabulary insufficient knowledge of the immiscibility constraints and insufficient knowledge of the topology in the tpx space of natural fluid systems for such reasons some authors have been misled to erroneous interpretations the term chemical immiscibility is unambiguously redefined as a multiphase multicomponent equilibrium the consequences of this definition are directly derived from the phase rule and concern the possible equations that relate the various parameters temperature pressure volumes compositions to each other these equations already put constraints on the topology of the phase equilibria in fluid systems a particular expression of the phase rule is proposed which takes into account the multiphase constant bulk volume constant bulk composition constraints in fluid inclusions the consequences of such an expression are of major importance in fluid inclusion studies the phase relations of some simple systems that approximate quite efficiently the natural complex fluids are then detailed h2o nacl co2 ch4 h2o co2 h2o co2 nacl the effects of these topologies and of the supplementary constraints constant bulk composition and constant bulk volume assumed for fluid inclusions isopleth isochoric systems are discussed

macflincor and its application to fluids in archean lode gold deposits abstract macflincor was developed to process laboratory data gathered on fluid inclusions and to calculate isochores for geologically important fluids composed of h2o c02 ch4 nacl and n2 within the program interactive diagrams are available to describe those chemical systems which cannot be adequately constrained numerically the user can choose from various published equations of state describing fluid behavior and can easily compare the results obtained by using different equations the program once the basic fluid parameters have been established allows the data to be used to determine the p t trapping conditions to evaluate possible pressure corrections and to compare different depths of formation based on the pressure calculations and given geothermal gradients for illustrative purposes macflincor has been utilized here to constrain different physical chemical parameters of fluid inclusions trapped in ore related quartz veins and breccias of archean lode gold deposits in western australia inferred pressures from individual deposits show a wide range of values which may be due to post entrapment modifications and or observational uncertainties including estimation of liquid vapor ratios

seismic source studies for chemical explosions in granite abstract nuclear explosion monitoring is commonly performed at regional or teleseismic distances and consequently the spectra of the recorded seismic waves are significantly affected by the propagation path study of the explosion source signature requires separation of the source and the propagation effects in this article we analyze the data from two chemical explosions 54 kg and 123 kg of ammonium nitrate and fuel oil conducted in central new hampshire we use small calibration explosions 0 454 kg composition b boosters to remove the propagation effects and to extract the source time functions stfs for the larger explosions using the empirical green s function method comparison of the deconvolved signals with the mueller murphy model mueller and murphy 1971 shows that the observed stfs have significantly longer duration than the characteristic time scale predicted by the model source function period increase can be attributed to differences between nuclear and chemical sources alternatively it can be explained by nonlinear attenuation effects near the source of shallow explosions

fluid inclusion evidence for p v t x evolution of hydrothermal solutions in late alpine gold quartz veins at brusson val d ayas northwest italian alps

genesis of high grade hematite orebodies of the hamersley province western australia a discussion sir taylor et al 2001 have presented a hypogene supergene model for the genesis of the premium martite microplaty hematite ores derived from banded iron formation bif to replace the supergene metamorphic model morris 1980 1985 1998 harmsworth et al 1990 the new model is based on hydrothermal features in two of ten tom price hematite deposits recorded previously as carbonate metasomatism of bif ewers and morris 1981 and as localized postore metasomatism harmsworth et al 1990 morris 1998 from new data taylor et al 2001 proposed that hypogene leaching of free silica with addition of further siderite in bif followed by compaction produced a general hematite ore precursor during deep weathering episodes or even within tertiary weathering profiles oxidation of magnetite to martite and siderite to microplaty hematite and ankerite followed by leaching of residual gangue enriched this modified bif to martite microplaty hematite ore in contrast in the supergene metamorphic model martite microplaty hematite ore genesis is linked to metamorphism of precursor ores of the phanerozoic supergene martite goethite type these comprise about 90 percent of the hamersley bif hosted resource and deposits range to over a billion tonnes harmsworth et al 1990 and 250 m deep with high phosphorus levels 0 07 0 17 p although stratigraphically thinned by leaching 35 they maintain primary bif banding and texture with gangue minerals altered to phosphorus bearing goethite pseudomorphs their martite microplaty hematite analogues 0 06 p are attributed to proterozoic supergene enrichment of bif with burial metamorphism partly converting the goethite pseudomorphs to secondary largely microplaty hematite yet preserving the primary banding some 20 known deposits of this type range from small pockets of enrichment to the massive mount whaleback 1700 mt later re exposures of the microplaty hematite deposits produced further stratigraphic thinning total 45 50 with loss of phosphorus by leaching of remnant goethite however significant