sedimentary rhythm of mn carbonate laminae induced by east asian summer monsoon variability and human activity in lake ohnuma southwest hokkaido northern japan abstract past and ongoing precipitations of mn carbonate rhodochrosite have occurred in the freshwater lake ohnuma which is situated about 8 km south of the hokkaido komagatake volcano southwest hokkaido northern japan lake sediments from a surface and a longer 4 m core contain the sedimentary rhythms of rhodochrosite laminae and volcanic deposits which were identified by nondestructive micro xrf fluorescence analyses the three tephra ash layers from the hokkaido komagatake volcano characterized by peaks of al si k ca and ti correspond to regionally recognized tephras 1640 1929 and 1996 ce based on ashfall isopachs and 137cs 210pb and 14c ages determined from the two cores the sedimentation rate showed a drastic change around 1929 ce 0 3 cm yr for 1640 1929 ce and 2 7 cm yr for 1929 2012 ce the mn content record of lake ohnuma sediments composed of rhodochrosite laminae showed a periodic variation with a decadal centennial scale over the interval 1640 2012 ce sediment petrography and hydrochemical measurements suggest that the metastable phase of rhodochrosite was deposited during summer after being precipitated followed by the crystal growth of rhodochrosite within the anoxic sediments on the basis of these results high mn content is interpreted as an indicator of strengthened summer stratification associated with reduced rainfall runoff the ti si ratios are representative of the relative abundance of detrital inputs from the watershed the combined signals of elevated mn content and ti si ratio for 1935 2012 ce indicate eutrophication and hypoxia in the lake and enhanced land erosion in the watershed associated with the post 1970s intensive land use such as livestock farming for 1640 1935 ce the lake ohnuma record shows an antiphase relationship between the mn content and ti si ratio which implies that the detrital inputs reflected the strength of the east asian summer monsoon easm and the reduced rainfall runoff resulted in weaker mixing followed by depleted o2 in the water column the antiphase variation in mn content and ti si ratio suggest weak easm during the little ice age 1850 ce followed by strong easm between 1860 and 1929 ce

regional geologic and petrologic framework for iron oxide apatite rare earth element and iron oxide copper gold deposits of the mesoproterozoic st francois mountains terrane southeast missouri usa this paper provides an overview on the genesis of mesoproterozoic igneous rocks and associated iron oxide apatite ioa rare earth element iron oxide copper gold iocg and iron rich sedimentary deposits in the st francois mountains terrane of southeast missouri usa the st francois mountains terrane lies along the southeastern margin of laurentia as part of the eastern granite rhyolite province the province formed during two major pulses of igneous activity 1 an older early mesoproterozoic ca 1 50 1 44 ga episode of volcanism and granite plutonism and 2 a younger middle mesoproterozoic ca 1 33 1 30 ga episode of bimodal gabbro and granite plutonism the volcanic rocks are predominantly high silica rhyolite pyroclastic flows volcanogenic breccias and associated volcanogenic sediments with lesser amounts of basaltic to andesitic volcanic and associated subvolcanic intrusive rocks the iron oxide deposits are all hosted in the early mesoproterozoic volcanic and volcaniclastic sequences previous studies have characterized the st francois mountains terrane as a classic a type within plate granitic terrane however our new whole rock geochemical data indicate that the felsic volcanic rocks are effusive derivatives from multicomponent source types having compositional similarities to a type within plate granites as well as to s and i type granites generated in an arc setting in addition the volcanic hosted ioa and iocg deposits occur within bimodal volcanic sequences some of which have volcanic arc geochemical affinities suggesting an extensional tectonic setting during volcanism prior to emplacement of the ore forming systems the missouri iron orebodies are magmatic related hydrothermal deposits that when considered in aggregate display a vertical zonation from high temperature magmatic hydrothermal ioa deposits emplaced at moderate depths 1 2 km to magnetite dominant ioa veins and iocg deposits emplaced at shallow subvolcanic depths the shallowest parts of these systems include near surface iron oxide only replacement deposits surficial epithermal sediment hosted replacement deposits synsedimentary ironstone deposits and mn rich exhalite deposits alteration associated with the ioa and iocg mineralizing systems of the host volcanic rocks dominantly produced potassic with lesser amounts of calcic and sodic rich mineral assemblages no deposits are known to be hosted in granite implying that the mineralizing systems were operative during a relatively short postvolcanic period yet prior to intrusion of the granitoids companion studies in this special issue on mineral chemistry stable isotopes and iron isotopes suggest that the magnetite within the ioa deposits formed from high temperature fluids of magmatic or magmatic hydrothermal origin however the data do not discriminate between a magmatic hydrothermal source fluid exsolved from an fe rich immiscible liquid or an fe rich silicate magma mineral chemical fluid inclusion and stable isotope data from these new studies record the effects of metasomatic fluids that interacted with crustal reservoirs such as volcanic rocks or seawater

understanding past climatic and hydrological variability in the mediterranean from lake prespa sediment isotope and geochemical record over the last glacial cycle here we present stable isotope and geochemical data from lake prespa macedonia albania border over the last glacial cycle marine isotope stages 5 1 and discuss past lake hydrology and climate tic oxygen and carbon isotopes as well as responses to climate of terrestrial and aquatic vegetation toc rock eval pyrolysis carbon isotopes pollen the lake prespa sediments broadly fall into 5 zones based on their sedimentology geochemistry palynology and the existing chronology the glacial sediments suggest low supply of carbon to the lake but high summer productivity intermittent siderite layers suggest that although the lake was likely to have mixed regularly leading to enhanced oxidation of organic matter there must have been within sediment reducing conditions and methanogenesis mis 5 and 1 sediments suggest much more productivity higher rates of organic material preservation possibly due to more limited mixing with longer periods of oxygen depleted bottom waters we also calculated lakewater 18o from siderite authigenic glacial and calcite endogenic holocene and show much lower lakewater 18o values in the glacial when compared to the holocene suggesting the lake was less evaporative in the glacial probably as a consequence of cooler summers and longer winter ice cover in the holocene the oxygen isotope data suggests general humidity with just 2 marked arid phases features observed in other eastern and central mediterranean lakes

high spatial resolution u pb geochronology and pb isotope geochemistry of magnetite apatite ore from the pea ridge iron oxide apatite deposit st francois mountains southeast missouri usa the pea ridge iron oxide apatite ioa deposit is one of the major rhyolite hosted magnetite deposits of the st francois mountains terrane which is located within the mesoproterozoic 1 5 1 3 ga granite rhyolite province in the u s midcontinent precise and accurate determination of the timing and duration of oreforming processes in this deposit is crucial for understanding its origin and placing it within a deposit scale and regional geologic context apatite and monazite well established u pb mineral geochronometers are abundant in the pea ridge orebody however the potential presence of multiple generations of dateable minerals processes of dissolution reprecipitation and occurrence of micrometer sized intergrowths and inclusions complicate measurements and interpretations of the geochronological results here we employ a combination of several techniques including id tims and high spatial resolution geochronology of apatite and monazite using la sc icpms and shrimp and pb isotope geochemistry of pyrite and magnetite to obtain the first direct age constraints on the formation and alteration history of the pea ridge ioa deposit the oldest apatite tims 207 pb 206 pb dates are 1471 1 and 1468 1 ma slightly younger than but within error of the 1474 to 1473 ma u pb zircon ages of the host rhyolites dating of apatite and monazite inclusions within apatite provides evidence for at least one younger metasomatic event at 1 44 ga and possibly multiple superimposed metasomatic events between 1 47 and 1 44 ga lead isotop analyses of pyrite show extremely radiogenic 206 pb 204 pb ratios up to 80 unsupported by in situ u decay this excess radiogenic pb in pyrite may have been derived from the spatially associated apatite as apatite recrystallized several tens of million years after its formation the low initial 206 pb 204 pb ratio of 16 5 and 207 pb 204 pb ratio of 15 4 for individual magnetite grains indicate closed u pb system behavior in this mineral and are consistent with derivation of the pb from a mantle like source

compressive strength and failure modes of lithophysae rich topopah spring tuff specimens and analog models containing cavities the presence of lithophysae in some units of topopah spring tuff at yucca mountain nevada the u s high level nuclear waste repository have a detrimental effect on the engineering properties of the rock mass and its performance the lithophysae were formed by pockets of gas trapped within the falling volcanic ash that formed the tuff units the porosity associated with the lithophysae is termed macroporosity because of the large pore size as compared with traditional rock pores in this paper lithophysae rich tuff and analog models both cylindrical and cubic made of plaster of paris containing artificially created cavities were tested to assess the effect of macroporosity on both the uniaxial compressive strength and failure modes of the specimens as expected compressive strength decreases with increasing porosity due to lithophysae in tuff and cavities in plaster analog specimens failure modes of cylindrical specimens were also investigated the failure modes observed were grouped into four distinct categories spalling axial splitting shear failure and web failure the failure modes transition from spalling through web failure as the percentage of macroporosity within the specimen increased

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

mineral thermometry and fluid inclusion studies of the pea ridge iron oxide apatite rare earth element deposit mesoproterozoic st francois mountains terrane southeast missouri usa mineral thermometry and fluid inclusion studies were conducted on variably altered and mineralized samples from the mesoproterozoic pea ridge iron oxide apatite ioa rare earth element ree deposit in order to constrain p t conditions fluid chemistry and the source of salt and volatiles during early magnetite and later ree mineralization scanning electron microscopy sem cathodoluminescence and sem backscatter electron images show that quartz and rutile precipitated before during and after magnetite and ree mineral growth ti in quartz and zr in rutile equilibration temperatures range from 350 to 750 c in the amphibole magnetite hematite and silicified zones where t increased during magnetite and quartz growth and dropped precipitously after fracturing and brecciation late drusy quartz cements within a ree rich breccia pipe record the lowest t 315 400 c liquid vapor rich and hypersaline hematite calcite fluid inclusions are common and liquid co 2 is present locally salinities define three populations saline 10 27 wt nacl equiv hypersaline 34 60 wt nacl equiv and dilute 0 10 wt nacl equiv the wide range of eutectic melting temperatures 67 to 19 c suggests that saline inclusions trapped variable proportions of a cacl mgcl fecl bearing fluid end member and an nacl kcl fluid end member homogenization temperatures and pressures of these saline inclusions suggest they were trapped when fluids unmixed into brine and vapor at t the na cl na k and cl br ratios of fluid inclusion extracts provide evidence for mixtures of magmatic hydrothermal fluids and evaporated seawater extracts from magnetite hematite and pyrite plot in the magmatic hydrothermal field indicating that fe was derived from a magmatic source their enrichments in mg and ca are consistent with a mafic magmatic source the positive correlation between na mg and na ca ratios may be due to halite saturation or albitization of igneous rocks extracts from barite in the ree rich breccia pipes are enriched in na and br and plot near the seawater evaporation trend he is highly enriched relative to ne and ar in fluid inclusion extracts which precludes air as a source of he although the he is mostly of crustal origin pyrite with a 3 he 4 he r r a of 0 1 contains up to 12 mantle he many extracts have low 20 ne 22 ne ratios due to nucleogenic production of 22 ne in high f o minerals such as fluorapatite or f biotite the arrays of data for 3 he 4 he r r a and 22 ne 20 ne suggest that volatiles were derived from two sources a moderate f mafic magma containing mantle he and a high f silicic magma with crustal he together with other evidence cited in this report these data 1 support a magmatic hydrothermal origin for the mesoproterozoic magnetite apatite deposit with ore fluids derived from a concealed mafic to intermediate composition intrusion 2 suggest that the ree minerals in breccia pipes were either derived from apatite or precipitated in response to decompression and cooling during breccia pipe formation 3 provide evidence for the influx of basinal brine magmatic fluids from granitic intrusions and meteoric water after breccia pipe formation and 4 show that pea ridge was relatively unaffected by the late paleozoic mississippi valley type mvt pb zn system in overlying cambrian sedimentary rocks

sources of mineralising fluids for the olympic dam deposit south australia smnd isotopic constraints abstract the olympic dam cu u au ag deposit south australia is hosted by the olympic dam breccia complex a high level hematite rich hydrothermal breccia system the deposit is characterised by extreme enrichments of ree which are a product of the ore forming process therefore making the sm nd isotopic system an ideal tool for clarifying the complex relationships between the different ore and breccia types pyrite chalcopyrite and bornite chalcocite rich hematitic ores share an initial end signature of 2 5 which suggests that these ore types are cogenetic these ores together with sulphide poor hematitic breccias yield a 14 point sm nd isochron age of 1572 99 ma the host roxby downs granite has an initial nd of 5 and therefore cannot alone have been the source of the mineralising fluids for the hematitic ores in contrast to hematitic rocks volumetrically minor magnetite rich assemblages have the same initial nd signature as the host granite suggesting that they are cogenetic assuming that both hematite and magnetite were precipitated from solution their contrasting isotopic signatures require that ore deposition involved two distinct fluids the isotopic data do not allow the distinction between a model involving concurrent activity of such fluids in a mixing regime and a model involving temporally separate overprinting of magnetite by hematite cu sulphides the ore signature of nd 2 5 cannot have been derived solely from pre existing crustal rocks of the gawler craton it must have involved a contribution from a mantle derived source rock or magma the most likely source of this nd component are the mantle derived rocks produced during the gawler range volcanics event e g altered alkaline mafic ultramafic dykes within the deposit which have initial isotopic signatures as high as nd 4 assuming end member compositions equivalent to these dykes and the roxby downs granite the ore signature implies that 30 of the nd in the ores was derived from the mafic ultramafic source if nd was leached from source rocks rather than derived from magmatic volatiles the inferred rock fractions contributing ree to the deposit are 13 mafic ultramafic rocks and 87 roxby downs granite or its volcanic equivalent massbalance calculations indicate that 13 rock fraction of mafic ultramafics can account for 50 of the cu contained in the deposit the involvement of mafic ultramafic rocks is therefore inferred to have been crucial to the genesis of the cu enriched ores at olympic dam

u pb and 40ar 39ar geochronology and isotopic constraints on the genesis of copper gold bearing iron oxide deposits in the hasancelebi district eastern turkey the hasancelebi deposit in eastern turkey with proven reserves of 95 million metric tons is a copper gold bearing iron oxide deposit it is chiefly hosted by rocks that underwent widespread sodic calcic and potassic styles of alteration in the hasancelebi district pervasive na ca and k fe alteration types are overprinted by sericitization and by late alteration that occur in multiple overprinting systems the age of hydrothermal alteration and mineralization ca between 74 68 ma overlaps with the age of alkaline magmatism in the hasancelebi district crystallization and cooling of alkaline magmatism is associated with hydrothermal features that spanned the duration of the district s igneous history the 40ar 39ar and u pb geochronology of magmatism indicate that the hydrothermal system was synchronous with diabase and syenite and or microsyenite porphyry intrusions and consisted of several discrete phases of na ca and k fe alteration the oldest alteration phase 1 formed at ca 74 4 to 74 3 ma it is spatially and temporally associated with the emplacement of diabase dikes and contains scapolite and phlogopite the next younger alteration phase 2 formed at about 71 3 ma and is spatially and temporally associated with syenite porphyry and microsyenite porphyry intrusions it consists of scapolite garnet pyroxene and actinolite that are superimposed on the first alteration phase the next alteration assemblage phase 3 formed at about 68 6 ma and consists of phlogopite 2 and magnetite mineralization still later alteration phase 4 overprints phases 1 2 and 3 and consists of sericite quartz chalcopyrite hematite calcite fluorite and barite oxygen 18o and hydrogen d isotope analyses were carried out on phlogopite sericite barite calcite fluorite and quartz representing the mineral assemblage formed during phases 1 and 4 of the hydrothermal alteration the calculated 18o h2o and d h2o composition of the fluids that formed preore k fe alteration and postore alteration of phase 4 range between 15 1 to 8 5 and 124 to 85 6 per mil respectively and partially overlap the range for traditional magmatic waters oxygen and h isotope compositions from alteration minerals highlight two major points na ca and k fe alteration assemblages are likely derived from high temperature primary magmatic fluids and late sericitization is likely derived from fluids exsolved from cooling alkaline plutons during hydrothermal fracturing and consequent degassing at moderate temperatures the initial ndi values of the alteration zones range from 2 02 to 2 08 close to the calculated nd value of associated syenite porphyry at the time of alteration nd 68 64ma 1 9 nd sr isotope compositions combined with 40ar 39ar and u pb geochronology of the associated alteration suggest that the fluids were sourced from coeval diabase dikes and syenite porphyry and or microsyenite porphyry intrusions the rare earth element characteristics 40ar 39ar and u pb geochronology of alteration and fresh magmatic rocks 18o d nd and sr isotope systematics together suggest that the fluids responsible for the high to moderate temperature assemblages are largely derived from the coeval alkaline igneous rocks diabase and microsyenite porphyry dikes that intrudes the sedimentary volcanosedimentary basin

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

the biogeochemistry of ferruginous lakes and past ferruginous oceans abstract anoxic and iron rich ferruginous conditions prevailed in the ocean under the low oxygen atmosphere that occurred through most of the archean eon while euxinic conditions i e anoxic and hydrogen sulfide rich waters became more common in the proterozoic ferruginous conditions persisted in deep waters ferruginous ocean regions would have been a major biosphere and earth surface reservoir through which elements passed through as part of their global biogeochemical cycles understanding key biological events such as the rise of oxygen in the atmosphere or even the transitions from ferruginous to euxinic or oxic conditions requires understanding the biogeochemical processes occurring within ferruginous oceans and their indicators in the rock record important analogs for transitions between ferruginous and oxic or euxinic conditions are paleoferruginous lakes their sediments commonly host siderite and ca carbonates which are important precambrian records of the carbon cycling lakes that were ferruginous in the past or euxinic lakes with cryptic iron cycling may also help understand transitions between ferruginous and euxinic conditions in shallow and mid depth oceanic waters during the proterozoic modern ferruginous meromictic lakes which host diverse anaerobic microbial communities are increasingly utilized as biogeochemical analogues for ancient ferruginous oceans such lakes are believed to be rare but regional and geological factors indicate they may be more common than previously thought while physical mixing processes in lakes and oceans are notably different many chemical and biological processes are similar the diversity of sizes stratifications and water chemistries in ferruginous lakes thus can be leveraged to explore biogeochemical controls in a range of marine systems near shore off shore silled basins or those dominated by terrestrial or hydrothermal element sources ferruginous systems both extant and extinct lacustrine and marine host a continuum of biogeochemical processes that highlight the important role of iron in the evolution of earth s surface environment

oxygen hydrogen sulfur and carbon isotopes in the pea ridge magnetite apatite deposit southeast missouri and sulfur isotope comparisons to other iron deposits in the region oxygen hydrogen sulfur and carbon isotopes have been analyzed in the pea ridge magnetite apatite deposit the largest historic producer among the known iron deposits in the southeast missouri portion of the 1 5 to 1 3 ga eastern granite rhyolite province the data were collected to investigate the sources of ore fluids conditions of ore formation and provenance of sulfur and to improve the general understanding of the copper gold and rare earth element potential of iron deposits regionally the 18 o values of pea ridge magnetite are 1 9 to 4 0 consistent with a model in which some magnetite crystallized from a melt and other magnetite perhaps the majority precipitated from an aqueous fluid of magmatic origin the 18 o values of quartz apatite actinolite k feldspar sulfates and calcite are significantly higher enough so as to indicate growth or equilibration under cooler conditions than magnetite and or in the presence of a fluid that was not entirely magmatic a variety of observations including stable isotope observations implicate a second fluid that may ultimately have been meteoric in origin and may have been modified by isotopic exchange with rocks or by evaporation during storage in lakes sulfur isotope analyses of sulfides from pea ridge and seven other mineral deposits in the region reveal two distinct populations that average 3 and 13 two sulfur sources are implied one was probably igneous melts or rocks belonging to the mafic to intermediate composition volcanic suite that is present at or near most of the iron deposits the other was either melts or volcanic rocks that had degassed very extensively or else volcanic lakes that had trapped rising magmatic gases the higher 34 s values correspond to deposits or prospects where copper is noteworthy the central dome portion of the boss deposit the bourbon deposit and the vilander prospective area the correspondence suggests that 1 sulfur either limited the deposition of copper or was cotransported with copper and 2 sulfur isotope analysis may be useful in evaluating southeast missouri iron deposits for copper and possibly for gold

iron and oxygen isotope signatures of the pea ridge and pilot knob magnetite apatite deposits southeast missouri usa new o and fe stable isotope ratios are reported for magnetite samples from high grade massive magnetite of the mesoproterozoic pea ridge and pilot knob magnetite apatite ore deposits and these results are compared with data for other iron oxide apatite deposits to shed light on the origin of the southeast missouri deposits the 18 o values of magnetite from pea ridge n 12 and pilot knob n 3 range from 1 0 to 7 0 and 3 3 to 6 7 respectively the 56 fe values of magnetite from pea ridge n 10 and pilot knob n 6 are 0 03 to 0 35 and 0 06 to 0 27 respectively these 18 o and the 56 fe values suggest that magnetite crystallized from a silicate melt typical igneous 56 fe ranges 0 06 0 49 and grew in equilibrium with a magmatic hydrothermal aqueous fluid we propose that the 18 o and 56 fe data for the pea ridge and pilot knob magnetite apatite deposits are consistent with the flotation model recently proposed by knipping et al 2015a which invokes flotation of a magmatic magnetite fluid suspension and offers a plausible explanation for the igneous i e up to 15 9 wt tio 2 in magnetite and hydrothermal features of the deposits

occurrence and origin of marialitic scapolite in the humboldt lopolith n w nevada the occurrence and origin of marialitic scapolite in the humboldt lopolith was investigated in the field and in the laboratory using petrographic and experimental techniques scapolite occurs in three modes as a pervasive replacement of plagioclase and other minerals in gabbro diorite and extrusive rocks as a poikiloblastic mineral in scapolitite dikes and as a fracture filling mineral with analcime albite and sphene in scapolite veins additional secondary minerals associated with scapolite include epidote prehnite hornblende and diopside salite clinopyroxene relations with these minerals suggest that most marialitic scapolite grew at temperatures around 400 c scapolite composition varies from eqan12 to eqan37 containing from 72 to 96 atomic cl in the r position experiments on systems of similar compositions indicate that nacl h2o fluid having more than 40 mol nacl is needed to stabilize the scapolite

a late holocene palaeoenvironmental record from lake tizong northern cameroon using diatom and carbon stable isotope analyses abstract a late holocene record based on diatom and stable carbon isotopes from lake tizong northern cameroon provides a history of environmental changes over the last 4100 years several coarser sediment layers among which the two younger ones are of pyroclastic origin interrupt the fine clayey sediment of the core the detailed chronology of the core supported by 24 radiocarbon 14c dates and proxies data results revealed an erosive phase registered in the sedimentary column from 2200 to 1500 cal bp the diatom ecological groups suggest that between 4100 and 2800 cal yrs bp the lake level was much higher than after corresponding to a relatively greater precipitation minus evaporation p e ratio as well as increased runoff in the lake catchment these conditions were favourable to the development of c3 plants in the lake catchment as indicated by lower 13c values and higher c n ratios than after this hydrological phase is also characterized by eutrophic turbid and probably circum neutral to alkaline waters after this episode higher 13c values between 2800 and 2500 cal bp suggest increased water use efficiency of terrestrial plants and or potentially more c4 plant debris input into the lake an indication of savannas patches developing due probably to changes in the rainfall distribution marked lake level declines are recorded at 2500 2200 2100 and at 1400 1000 cal yrs bp these low stands are characterized by higher inputs of windblown diatoms up to 4 2 than before which confirms that the ne trade winds were strengthening this corresponds primarily to a reduction in the p e ratio but probably also to greater inter annual or seasonal variability when drier periods or seasons became more prolonged and intense than previously consequently savannas were maintained as suggested by relatively higher than before 13c values as well as independently supported by regional pollen data after 1000 cal bp the lake level rose towards sub modern conditions with a deep neutral and eutrophic water column carbon stable isotopes suggest a reduction of organic matter input while savannas were maintained despite the return to more humid conditions the trends of climatic changes observed in the lake tizong reveal the variability in timing magnitude and regional extent of known climatic events

strata bound fe co cu au bi y ree deposits of the idaho cobalt belt multistage hydrothermal mineralization in a magmatic related iron oxide copper gold system mineralogical and geochemical studies of strata bound fe co cu au bi y rare earth element ree deposits of the idaho cobalt belt in east central idaho provide evidence of multistage epigenetic mineralization by magmatic hydrothermal processes in an iron oxide copper gold iocg system deposits of the idaho cobalt belt comprise three types 1 strata bound sulfide lenses in the blackbird district which are cobaltite and less commonly chalcopyrite rich with locally abundant gold native bismuth bismuthinite xenotime allanite monazite and the be rich silicate gadolinite y with sparse uraninite stannite and bi tellurides in a gangue of quartz chlorite biotite muscovite garnet tourmaline chloritoid and or siderite with locally abundant fluorapatite or magnetite 2 discordant tourmalinized breccias in the blackbird district that in places have concentrations of cobaltite chalcopyrite gold and xenotime and 3 strata bound magnetite rich lenses in the iron creek area which contain cobaltiferous pyrite and locally sparse chalcopyrite or xenotime most sulfide rich deposits in the blackbird district are enclosed by strata bound lenses composed mainly of cl rich fe biotite some deposits have quartz rich envelopes whole rock analyses of 48 co and or cu rich samples show high concentrations of au up to 26 8 ppm bi up to 9 16 wt y up to 0 83 wt rees up to 2 56 wt ni up to 6 780 ppm and be up to 1 180 ppm with locally elevated u up to 124 ppm and sn up to 133 ppm zn and pb contents are uniformly low 821 and 61 ppm respectively varimax factor analysis of bulk compositions of these samples reveals geochemically distinct element groupings that reflect statistical associations of monazite allanite and xenotime biotite and gold detrital minerals chalcopyrite and sparse stannite quartz and cobaltite with sparse selenides and tellurides significantly cu is statistically separate from co and as consistent with the general lack of abundant chalcopyrite in cobaltite rich samples paragenetic relations determined by scanning electron microscopy indicate that the earliest y ree be mineralization preceded deposition of co cu au and bi allanite xenotime and gadolinite y commonly occur as intergrowths with and inclusions in cobaltite the opposite texture is rare monazite in contrast forms a poikiloblastic matrix to cobaltite and thin rims on allanite and xenotime reflecting a later metamorphic paragenesis allanite and xenotime also show evidence of late dissolution and reprecipitation forming discordant rims on older anhedral allanite and xenotime and separate euhedral crystals of each mineral textural data suggest extensive deformation of the deposits by folding and shearing and by pervasive recrystallization all during cretaceous metamorphism sensitive high resolution ion microprobe u pb geochronology by aleinikoff et al 2012 supports these paragenetic interpretations documenting contemporaneous mesoproterozoic growth of early xenotime and crystallization of megacrystic a type granite on the northern border of the district these ages are used together with mineralogical and geochemical data from the present study to support an epigenetic iocg model for fe co cu au bi y ree deposits of the idaho cobalt belt a sulfide facies variant of iocg deposits is proposed for the blackbird district in which reducing hydrothermal conditions favored deposition of sulfide minerals over iron oxides this new model includes mesoproterozoic vein mineralization and related fe cl metasomatism that formed the biotite rich lenses a predominantly felsic magmatic source for metals in the deposits given their local abundance of y rees and be and a major sedimentary component in the hydrothermal fluids based on independent sulfur isotope and boron isotope data for sulfides and ore related tourmaline respectively

timing of iron oxide cu au u hydrothermal activity and nd isotope constraints on metal sources in the gawler craton south australia the eastern gawler craton hosts australia s premier uranium bearing iron oxide copper gold iocg belt the 500 km long olympic cu au u province in addition to the olympic dam cu u au and prominent hill cu au deposits numerous barren and weakly mineralized iocg prospects are present in the province new geochronological data for hydrothermal minerals combined with constraints from host rock ages demonstrate that alteration and associated iocg mineralization formed between 1570 and 1600 ma in three districts of the province this iocg hydrothermal activity temporally overlaps with magmatism of the hiltaba suite and gawler range volcanics titanites in paragenetically early magnetite bearing alteration in the olympic dam and prominent hill districts yield u pb ion probe ages of 1576 5 and 1567 10 ma respectively molybdenite in veins crosscutting magnetite biotite and albitic alteration in the moonta wallaroo district have re os ages of 1574 6 and 1599 6 ma respectively these represent minimum ages of the iocg alteration assemblages in this district a muscovite 40ar 39ar age of 1575 11 ma provides a minimum age of paragenetically later hematitic alteration in the olympic dam district neodymium isotope compositions were determined for 44 whole rock samples from barren and weakly mineralized cu au prospects and host rocks in the olympic dam and prominent hill districts the new geochronological framework enables comparison of the nd isotope data across two iocg districts at the time of formation of the olympic dam deposit ca 1590 ma magnetite rich weakly cu mineralized alteration from five prospects yields a relatively narrow range end 1590 values of 5 8 to 4 1 both hematite and magnetite rich alteration yield generally similar end 1590 values that match values from fresh and weakly altered paleoproterozoic metasedimentary and metagranitic rocks 6 6 to 3 5 as well as from most felsic hiltaba suite intrusions and gawler range volcanics in the eastern gawler craton ca 6 to 4 these data are consistent with crustal sources for ree and by implication for associated copper in the barren and weakly mineralized prospects mineralization and alteration in these minor iocg systems can be geochemically discriminated from the giant olympic dam deposit where greater inputs of mantle derived ree and other ore components are evident 2007 society of economic geologists inc

origin of rare earth element enriched hematite breccias at the olympic dam cu u au ag deposit roxby downs south australia hematite breccias the host rocks to cu u au ag ore at the olympic dam deposit occur as steeply dipping northwest striking dikelike bodies within fractured granite the breccia complex has a strike length of over 5 km and extends to depths greater than 1 km both the deposit as a whole and the individual breccia bodies are zoned from weakly brecciated sericitized and hematized granite on the margins through heterolithic breccias to hematite quartz microbreccia at the center hematite occurs as euhedral laths and fine grained aggregates in the matrices of all breccia types in veins and fragments as pseudomorphs after feldspar and rarely as pseudomorphs after siderite and magnetite relict magnetite is rare and most iron oxide was deposited as hematite heterolithic breccias include fragments of earlier breccia and veins minor sedimentary rocks including bedded hematite and laminated barite also are present as clast components in the upper portion of the deposit volcaniclastic conglomerate and siltstone occur in a fault bounded block approximately 500 x 500 m in plan view copper iron sulfides occur as interstitial grains most common microveinlets and rare clasts they are most abundant in heterolithic breccias and are zoned laterally from chalcocitebornitc to chalcopyrite pyrite zonal boundaries are subparallel to steep breccia contacts copper iron sulfides commonly are intergrown with fluorite and locally rim corroded quartz grains the breccias are highly enriched in light lree and heavy rare earth elements hree avg approximately 5 000 ppm ree in hematite rich rocks total lree content and la lu ratios are correlated with hematite abundance maximum la values 10 000 x chondrite occur in hematite quartz microbreccias from the geographic center of the deposit five hydrothermal ree phases have been identified bastnaesite florencite monazite xenotime and britholite they occur in all breccia types and in a variety of habits disseminated in quartz sericite matrices of granite breccia in quartz sericite veins interstitial to hematite laths intergrown with barite in laminated sedimentary rocks and as inclusions in hematite and in sulfide grains the hematite breccias formed by progressive hydrothermal brecciation and iron metasomatism of the granite host minor sedimentary components preserved as blocks and fragments within the upper portion of the deposit point to a near surface environment for the breccia complex textural relations indicate that most of the copper was introduced late relative to hematization and brecciation based on spatial relations between barren zones oxidized sulfide grains and high grade chalcocite ore some supergene redistribution of copper probably occurred the abundance of ree in hydrothermal phases ree enrichment of altered relative to unaltered wall rock concentration of ree in the center of the system and variable slopes of chondrite normalized patterns combined with the lack of evidence of intrusion of unusual magmas at the present level of exposure attest to extensive transport and deposition of ree by hydrothermal fluids responsible for hematization and breccia formation this study demonstrates hydrothermal ree mobility on a scale previously undocumented and suggests that ree may be particularly mobile in f rich hydrothermal systems

alteration oxygen isotope and fluid inclusion study of the meishan iron oxide apatite deposit se china the meishan deposit 338 mt at 39 fe comprises massive ores in the main orebody and stockwork and disseminated ores along the main orebody four stages of mineralization and related alteration have been identified the second stage of mineralization which was the main stage of iron mineralization formed stringer disseminated iron ores as well as the main meishan orebody the fourth stage formed small pyrite and or gold orebodies above or alongside the main magnetite orebody stage 2 apatites have homogenization temperatures of 257 485 c and salinities of 7 3 11 wt nacleq calculated 18ofluid values of magnetite and apatite from the disseminated ores vary between 7 7 and 14 9 which is similar to values observed in the massive ores 8 1 12 9 the high 18o fluids at meishan have been interpreted as being of magmatic hydrothermal origin these fluids are indicative of the boiling of ore forming fluids quartz occurring as cavity fillings gives homogenization temperatures from 202 to 344 c with most values lying between 250 and 330 c corresponding salinities are 5 wt nacleq calculated 18ofluid values are 6 4 to 6 8 these values indicate that the lower temperature 250 330 c quartz was deposited from a cooling magmatic hydrothermal fluid stage 3 siderites contain fluid inclusions that homogenized between 190 and 310 c mainly between 210 and 290 c corresponding salinities are 4 8 wt nacleq stage 4 quartz carbonate veinlets contain fluid inclusions that homogenized at moderate to low temperatures 150 230 c and exhibit low salinities 2 10 wt nacl eq 18ofluid values of the mineralizing fluids for the quartz and calcite can be calculated to vary from 0 7 to 5 6 and 6 3 to 10 2 respectively while there is some overlap the 18o values of the fluids are generally lower than those observed in the massive and disseminated magnetite ores d values for the quartz and calcite vary between 154 and 123 and 123 and 111 respectively these values suggest late stage input of a shallow sourced isotopically light meteoric fluid at the temperature of pyrite and gold deposition the fluid inclusion and stable isotope data indicate a cooling magmatic hydrothermal system that progressed from isotopically heavy to isotopically depleted fluids as it cooled such fluid evolution is comparable with those of other kiruna type deposits worldwide

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

arc related sodic hydrothermal alteration in the western united states sodium rich hydrothermal alteration is widely developed in permian to jurassic arc igneous rocks of the western united states but it is rare in younger rocks na rich alteration reflects a paleogeographic control on development two varieties naca and na can be defined by mineralogic and compositional changes both are common in permian to jurassic arc plutonic and volcanic rocks and are accompanied by fe and cu zn mineralization geologic relations and mineralogic stable isotopic and fluid inclusion data indicate that the fluids responsible for sodium rich alteration were isotopically heavy moderately to highly saline fluids of marine formation and or meteoric origin with or without a magmatic component the transition away from widespread na ca rich alteration in the late mesozoic corresponds to and is consistent with the change from an early mesozoic marine and or arid rifted arc setting to a fully emergent continental arc environment in the cretaceous

gushan magnetite apatite deposit in the ningwu basin lower yangtze river valley se china hydrothermal or kiruna type abstract the gushan deposit is one of the typical magnetite apatite deposits associated with dioritic porphyries in the lower yangtze river valley belt of the eastern yangtze craton the origin of this deposit is still uncertain and remains a controversial issue divergent opinions are centered on whether the iron deposits are magmatic or hydrothermal in origin however our field observations and mineralogical studies combined with previous published petrological and geochemical features strongly suggest that the main ore bodies in the gushan magnetite apatite deposit are magmatic specific evidence includes the existence of gas bubbles tubes and miarolitic and amygdaloidal structures melt flow banding structure and the presence of ore breccia new electron microprobe analyses of the pyroxene phenocrysts of the dioritic porphyry genetically associated with the gushan magnetite apatite deposit show that the fe contents in the evolving magma dramatically decrease and then gradually increase because there is no evidence of mafic magma recharge this scenario decreasing fe could be plausibly interpreted by fe rich melts separated from fe poor silicate melts i e liquid immiscibility was triggered by minor addition of phosphorus by crustal contamination the occurrence of massive iron ore bodies can be satisfactorily explained by the immiscible fe rich melt with enormous volatile contents was driven to the top of the magma chamber due to the low density the hot and volatile rich iron ore magma was injected along fractures and spaces between the dioritic intrusions and wall rocks and led to an explosion near the surface resulting in the immediate fragmentation of the roof of the intrusion and wall rocks forming brecciated ores moreover other types of ores can be considered as a result of post magmatic hydrothermal activities our proposed metallogenic model involving the kiruna type mineralization is consistent with the observed phenomenon in the gushan deposit

carbon and oxygen stable isotopic signatures of methanogenesis temperature and water column stratification in holocene siderite varves abstract manganoan siderite fe mn co 3 occurs in abundance of up to 19 dry weight as the sole endogenic carbonate within a succession of holocene organic rich varved sediments from freshwater otter lake ol michigan radiocarbon dating and varve counts from a 7 m piston core constrain periods of major siderite accumulation to sediments older than 1200 cal yr bp sediment petrography suggests that siderite was a seasonal precipitate confined to the summer layer of the varve couplet bulk sediment chemistry reveals cycles in abundance of manganese and aluminum coincident with centennial scale cycles of siderite accumulation siderite 13 c and 18 o are enriched when the mineral is abundant and depleted when it is least abundant samples with high abundance of siderite precipitated in oxygen isotopic equilibrium with modern lake water but are 13 c enriched relative to modern waters samples from intervals of low siderite abundance are 13 c and 18 o depleted relative to equilibrium with modern lake water these data suggest that abundant siderite precipitation occurred when the ol water column was ferruginous iron meromictic allowing for enhanced ferrous iron concentrations and dissolved inorganic carbon dic enriched in 13 c below the chemocline where methanogenesis in waters and sediments influenced dic composition seasonal siderite precipitation was triggered by water column alkalinity fluctuations driven by summer calcite dissolution manganese substitution in siderite lowered kinetic barriers to low temperature mineral precipitation climate changes and basin filling influenced lake stratification and the rate of siderite precipitation the siderite occurrence reported here displays remarkable similarity to its occurrence in paleogene maar lake deposits suggesting that further studies of holocene lacustrine siderites may provide insight into ancient sedimentary systems and environments

evaporitic source model for igneous related fe oxide ree cu au u mineralization we propose that many igneous related fe oxide rich ree cu au u bearing deposits form by hydrothermal processes involving evaporitic ligand sources either coeval salars or older evaporites these deposits are abundant in both phanerozoic and proterozoic extensional continental and continent margin settings they commonly form in global arid zones but they also occur where magmatism is superimposed upon older evaporites magmatic compositions exert only second order control mainly on alteration mineralogy and on element abundances hot s poor brines generated by interaction with evaporitic materials are consistent with geologic settings and help rationalize the distinctive element enrichments siderophile lithophile and hydrothermal alteration sodic locally alkaline found in these systems this model contrasts with immiscible oxide melt and magmatic hydrothermal origins commonly proposed for these deposits although all three mechanisms can occur 31 refs 3 figs 1 tab

metal source and tectonic setting of iron oxide copper gold iocg deposits evidence from an in situ nd isotope study of titanite from norrbotten sweden titanite occurs as a widespread accessory phase in mineralized zones and alteration associated with iron oxide copper gold iocg and iron oxide apatite ioa deposits of norrbotten county sweden and is a major host of the ree in these deposits in situ analyses of sm nd isotope ratios in titanites previously analysed for u pb geochronology and trace element composition confirms previous interpretations of grain scale isotopic heterogeneity initial nd isotope ratios expressed relative to chur range from 3 to 8 in ioa deposits from 1 to 9 in iocg deposits and from 2 to 4 in the most cu rich deformed iocg deposits of the nautanen deformation zone within individual ioa deposits at valkommen malmberget from 1 4 to 5 7 in grain cores and 7 2 to 8 2 in grain rims rim at gruvberget and from 3 0 to 6 0 in grain cores and from 5 8 to 7 1 in grain rims at luossavaara in iocg deposits at rakkurijarvi iocg dpeosits at nautanen from 1 3 to 2 3 these values are consistent with the derivation of the ree and potentially economically enriched metals from the local volcanic sequence either via granitic melts or directly by leaching by metasomatic fluids the most cu rich deposits reflect the involvement of more basic protoliths the age distribution of these deposits suggest ioa deposit formation during the collisional phase of the svecofennian orogeny 1 9 1 8 ga and iocg mineralization during this phase and during post orogenic collapse 1 8 1 7 ga whilst model ages indicate the ultimate enrichment of the continental crust in these metals during pre collisional extensional and subduction related basic magmatism these processes underscore the importance of continental cycles both in producing the preserved geological record of orogenic ore deposition and in the generation of fertile continental crust from which metals can be mobilised by subsequent events nd varies relative to chur from 3 1 to 4 0 nd varies from 6 1 to 7 1 and in deformed iocg dpeosits at nautanen from 1 3 to 2 3 these values are consistent with the derivation of the ree and potentially economically enriched metals from the local volcanic sequence either via granitic melts or directly by leaching by metasomatic fluids the most cu rich deposits reflect the involvement of more basic protoliths the age distribution of these deposits suggest ioa deposit formation during the collisional phase of the svecofennian orogeny 1 9 1 8 ga and iocg mineralization during this phase and during post orogenic collapse 1 8 1 7 ga whilst model ages indicate the ultimate enrichment of the continental crust in these metals during pre collisional extensional and subduction related basic magmatism these processes underscore the importance of continental cycles both in producing the preserved geological record of orogenic ore deposition and in the generation of fertile continental crust from which metals can be mobilised by subsequent events

authigenic minerals from the paola ridge southern tyrrhenian sea evidences of episodic methane seepage abstract paola ridge along the nw calabrian margin southern tyrrhenian sea is one of the few reported deep sea sites of precipitation of authigenic carbonates in the tyrrhenian sea here the changing composition of the seeping fluids and the dynamic nature of the seepage induced the precipitation of pyrite siderite and other carbonate phases the occurrence of this array of authigenic precipitates is thought to be related to fluctuation of the sulfate methane transition zone smtz concretions of authigenic minerals formed in the near sub bottom sediments of the paola ridge were investigated for their geochemical and isotopic composition these concretions were collected in an area characterized by the presence of two alleged mud volcanoes and three mud diapirs the mud diapirs are dotted by pockmarks and dissected by normal faults and are known for having been a site of fluid seepage for at least the past 40 kyrs present day venting activity occurs alongside the two alleged mud volcanoes and is dominated by co2 rich discharging fluids this discover led us to question the hypothesis of the mud volcanoes and investigate the origin of the fluids in each different domed structure of the study area in this study we used stable isotopes carbon and oxygen of carbonates coupled with rare earth element ree composition of different carbonate and non carbonate phases for tracing fluid composition and early diagenesis of authigenic precipitates the analyses on authigenic precipitates were coupled with chemical investigation of venting gas and sea water authigenic calcite aragonite concretions from surficial sediments on diapiric structures have depleted 13c isotopic composition and slightly positive 18o values by contrast siderite concretions generally found within the first 6 m of sediments on the alleged mud volcanoes yielded positive 13c and 18o values the siderite ree pattern shows consistent lree light ree fractionation mree medium ree enrichment and positive gd and la anomalies as shown by the ree distribution the 13c depleted composition and their association with chemosymbiotic fauna calcite aragonite precipitated at time of moderate to high methane flux close to the seafloor under the influence of bottom seawater authigenic siderite on the other hand formed in the subseafloor during periods of lower gas discharges under prolonged anoxic conditions within sediments in equilibrium with 13c rich dissolved inorganic carbon dic and 18o rich water likely related to methanogenesis and intermittent venting of deep sourced co2

neodymium isotopic study of rare earth element sources and mobility in hydrothermal fe oxide fe p ree systems rare earth element ree enriched igneous related hydrothermal fe oxide hosted fe p ree systems from four areas in north america have been analyzed for their neodymium iosotopic composition to constrain ree sources and mobility in these systems the nd isotopic results evidence a common pattern of ree concentration from igneous sources despite large differences in age proterozoic to tertiary tectonic setting subduction vs intraplate and magmatic style mafic vs felsic in the middle proterozoic st francois mountains terrane of southeastern missouri epsilon sub nd for fe p ree apatite monazite xenotime deposits ranges from 3 5 to 5 1 similar to associated felsic to intermediate igneous rocks of the same age epsilon sub nd 2 6 to 6 2 at the mid jurassic humboldt mafic complex in western nevada epsilon sub nd for fe p ree apatite mineralization varies between 1 1 and 2 4 similar to associated mafic igneous rocks minus 1 0 to 3 5 in the nearby cortez mountains in central nevada mid jurassic felsic volcanic and plutonic rocks epsilon sub nd minus 2 0 to minus 4 4 are associated with fe p ree apatite monazite mineralization having similar epsilon sub nd minus 1 7 to minus 2 4 at cerro de mercado durango mexico all assemblages analyzed in this tertiary rhyolite hosted fe oxide deposit have identical isotopic compositions with epsilon sub nd minus 2 5 these data are consistent with coevalmore igneous host rocks being the primary source of ree in all four regions and are inconsistent with a significant contribution of ree from other sources interpretations of the origin of these hydrothermal systems and their concomitant ree mobility must account for nonspecialized igneous sources and varied tectonic settings less

proterozoic iron oxide cu u au ree deposits further evidence of hydrothermal origins the emmie bluff fe oxide deposit south australia is unequivocally of hydrothermal origin and exhibits overprinting relations visible at both the meso and microscopic scale that strongly support a model involving two temporally distinct fluids as previously proposed for the enormous olympic dam deposit the evidence indicates that the first stage of magnetite rich ore formation was associated with high temperature fluids from a major mid proterozoic felsic intrusive event this was followed by infiltration of lower temperature fluids that produced the hematite rich mineralization from authors

siderite concretions indicators of early diagenesis in the gammon shale cretaceous the upper cretaceous gammon shale in the northern great plains contains abundant siderite concretions that formed during the early compactional history of the fine grained marine sediments the relative depth of siderite precipitation can be estimated on the basis of textural mineralogical and isotopic evidence siderite concretions that formed at shallow depths in the sediment column contain a large volume percent carbonate 75 85 preserve uncompacted sediment textures and have oxygen isotope ratios similar to those of carbonates precipitated in equilibrium with seawater delta 18 o nearly equal e 0 to 2 per thousand pdb in contrast lower volume percent carbonate values 55 75 and relative decreases in 18 o 16 o values characterize concretions that formed longer after deposition and or at greater depths in the accumulating sediments both volume percent carbonate and delta 18 o values are strongly correlated with independently determined sediment accumulation rates concretions in rapidly deposited sediments formed at shallow depths 200 m later in the burial history the gammon served as both source rock and reservoir for accumulations of biogenic methane and like the siderite concretions the methane was generated during early diagenesis both siderite precipitation and methane accumulation require interstitial pore waters that have sufficiently high bicarbonate activities and that are depleted in dissolved sulfur and free oxygen because of chemical requirements siderite did not precipitate until after the onset of methane generation therefore if the depth of siderite formation is known the minimum depth for the onset of methane accumulation can be inferred the presence of abundant pyrite in slowly accumulated gammon sediments suggests by analogy to data from recent sediments that the depth of siderite precipitation and methane accumulation was controlled by the sedimentation rate in slowly accumulating sediments most organic matter was probably consumed by aerobic organisms and sulfate reducing anaerobic bacteria whereas in areas of more rapid deposition large quantities of organic matter were available for methane generation