climate instability during the last interglacial period recorded in the grip ice core isotope and chemical analyses of the grip ice core from summit central greenland reveal that climate in greenland during the last interglacial period was characterized by a series of severe cold periods which began extremely rapidly and lasted from decades to centuries as the last interglacial seems to have been slightly warmer than the present one its unstable climate raises questions about the effects of future global warming

formation of willemite in hydrothermal environments willemite zinc silicate is the main zinc mineral in some carbonate hosted ore deposits e g franklin new jersey vazante brazil beltana south australia kabwe zambia recent interest in these unconventional zinc deposits has increased because of high zinc grades that exceed 40 wt percent relatively low environmental impact of ore processing owing to the lack of acid generating sulfides in the waste and advances in ore processing technologies in the past most metallogenic studies proposed formation of willemite deposits by supergene or hypogene alteration of preexisting sulfide deposits however recent data on the vazante beltana and kabwe deposits indicate willemite crystallization at temperatures in excess of 150 c raising the possibility of primary precipitation from hydrothermal fluids we use numerical geochemical modeling to examine the formation of willemite under hydrothermal conditions activity activity diagrams reveal that in the presence of dissolved sulfur and quartz willemite instead of sphalerite will precipitate under oxidizing e g hematite stable sulfate predominant and alkaline ph higher than k feldspar muscovite quartz conditions willemite also becomes more stable relative to sphalerite at high temperature and willemite can coexist with magnetite at 300 c the stabilities and solubilities of sphalerite willemite smithsonite hydrozincite and zincite were calculated for wide ranges of temperature 25 300 c chloride concentration dissolved sulfur and carbon concentrations ph quartz saturation and oxidation potential plots of the solubility of the different minerals as a function of two variables e g temperature and redox state ph and redox state allow us to predict the effects of changing chemical conditions which in turn permits an estimate of the efficiency of particular precipitation processes cooling is an effective process for precipitating sphalerite but not willemite whereas ph increase e g by acidic fluids reacting with carbonates is effective for precipitating willemite but not sphalerite dynamic geochemical models that simulate physicochemical processes are used to understand the formation of the beltana willemite deposit in the adelaide geosyncline of south australia this small high grade deposit 850 000 t at 36 zn is hosted in dolomite of the cambrian ajax limestone next to a tectonic contact with the diapiric halite bearing clastic sediments of the callanna group the orebody is associated with hematite alteration and is characterized by the total absence of sulfides willemite is the only zinc ore mineral and the arsenate hedyphane ca 2 pb 3 aso 4 3 cl is the main lead mineral the model results show that willemite will precipitate in response to water rock interaction and fluid mixing processes at temperatures above 120 c the presence of arsenate in the hydrothermal fluid is likely to have been important at beltana in arsenate absent models sulfate is reduced to sulfide by the precipitation of ferrous iron as hematite resulting in the precipitation of sphalerite and galena in contrast in models including arsenate the reduction of sulfate to sulfide is inhibited and willemite is predicted to precipitate

methodology of the last climatic cycle reconstruction in france from pollen data abstract a numerical method has been devised to reconstruct the climate of the last climatic cycle from pollen data reliable annual reconstructions have been published recently for the two french long sequences of la grande pile and les echets covering the last 140 millennia this paper gives more details on the method used it proposes new reconstructions based on an extended modern pollen data base these reconstructions are consistent with the previous ones especially detailed information is obtained on the seasonal characteristics of climate computations of gcm on the climate variations from 125 kyr to 115 kyr b p are confirmed for western europe the insolation variations have induced at that time a decrease of seasonality accompanied by an increased cyclonic activity in winter this has been the beginning of the first ice growth after the last interglacial other glacier growth phases are detected at the end of substage 5c and particularly at the end of substage 5a

on diagenesis dolomitisation and mineralisation in the irish zn pb orefield marine calcite cementation and lithification of carboniferous carbonate sediments hosting zn pb mineralisation in the irish orefield occurred at or near the seafloor a relatively early fine grained grey replacive dolomite preferentially developed in micrite is widely developed in the waulsortian limestone formation the main host to mineralisation and is pervasive in the southeastern midlands in proximity to the leinster massif this dolomite formed after the first four main stages of calcite cementation but probably also developed within tens of metres of the seafloor as evidenced by incorporation of clasts of dolomite in intraformational sedimentary breccias later coarse grained white dolomite preferentially replaced coarser components of the waulsortian limestone and infilled residual vuggy porosity whilst some of this coarse dolomite may be related to the fine replacive dolomite event a common spatial association with fault zones coupled with primary fluid inclusion data suggest that a significant proportion of this phase precipitated during the onset of fault controlled subsidence and widespread hydrothermal circulation within the irish midlands area fluids up to 250 c and 10 15 wt nacl equivalent sourced from a lower palaeozoic basement equilibrated fluid reservoir infiltrated the carbonate sequence via faults and fractures the more localised development of dolomite cemented breccias white matrix breccias that are frequently associated spatially with mineralisation was a consequence of the increased focusing of these hydrothermal fluids ore formation was broadly synchronous with development of the white dolomite breccias but only happened where mixing occurred between the hydrothermal ore fluids and localised near surface reservoirs of low temperature h2s rich brine in the waulsortian this process led to the precipitation of a distinctive black dolomite that forms a broad halo to massive sulphides although ore stage sulphides postdate significant diagenesis of the host rocks and often display epigenetic textures the fact that much of the cementation occurred soon after carbonate deposition means that mineralisation does not have to have formed after significant burial in fact the occurrence of clasts of hydrothermal dolomite and sulphides in intraformational debris flow breccias is only consistent with mineralising processes occurring in the near seafloor environment relatively soon after host rock deposition the regional development of a distinctive pink dolomite associated with faults and fractures was a post ore event and is considered to mark a regional brine migration linked to the onset of the variscan orogeny the development of this new tectonic and flow regime may have been responsible for the cessation of economic mineralisation in ireland

dolomitization and isotope stratigraphy of the vazante formation sao francisco basin brazil the vazante formation consists of approximately 1700 m of mainly microbial mats and stromatolitic reefal lenses that were deposited on shallow marine platform and have been entirely dolomitized samples representing different dolomite generations were taken from three boreholes covering the entire spectrum of the vazante carbonates dolomites can be classified based on petrography and geochemistry into four generations ranging in crystal size between 4 m almost micritic and 3 mm and occurring as both replacements and cements the sr ca molar ratios calculated for the dolomitizing fluid 0 0006 0 0138 suggest a contribution from a non marine possibly meteoric water component the 18 oa nd 13 c values vary from 0 1 to 14 3 pdb and 0 2 to 2 3 pdb respectively fluid inclusion study suggests that dolomitization must have commenced at a temperature lower than about 50 c a mixing zone model of dolomitization for dolomites i iii satisfies the constraints from elemental chemistry 18 o 87 sr 86 sr and fluid inclusions the petrographic and chemical criteria of the latest generation dolomite iv reflect conditions of deep burial environment at temperatures above 120 130 c the 13 c values show only small variations at the base of the formation followed by a major negative plunge 4 at the top this plunge correlated with the sturtian glacial phase in contrast to c isotopes only few samples may have retained their near primary 87 sr 86 sr values of 0 70614 0 70734 the lowest 87 sr 86 sr value 0 70614 from fibrous cement in the upper part of the formation correlates with the negative 13 c shift and is also consistent with the sr isotope signature proposed for the glacial sturtian seawater the 34 s values obtained from sulfates trapped in carbonates range between 10 8 and 16 9 with a jump to 21 3 in the overlying formation all isotope signals are within the range suggested for the early neoproterozoic seawater but the precambrian baseline is poorly known and additional work is required to confirm this tentative assignment 2001 elsevier science b v all rights reserved

basement controls on sediment hosted zn pb deposits a pb isotope study of carboniferous mineralization in central ireland lead isotope ratios of galenas from carbonate hosted zn pb or ba or cd or ag deposits in central ireland are homogeneous within individual deposits but vary between deposits the variation correlates directly with geographic position defining isoplumbs and not with style of mineralization age of host rock or structural or physico chemical factors the least radiogenic pb occurs in the northwest and the most radiogenic in the southeast this pattern elegantly reflects irish pre caledonian basement geology lewisian gneisses in the northwest and late proterozoic upper crustal rocks in the southeast that the northwestern end member of the irish pb mixing trend is ultimately lewisian is demonstrated by the concordance of the irish pb isotope trend with the trends defined in proterozoic to tertiary rocks and ores of scotland which contain pb of demonstrably lewisian derivation although geologic relations and pb isotope ratios unambiguously implicate basement in the ore forming process we are unable to discriminate between lewisian gneisses themselves and detritus derived from lewisian rocks in the caledonian metamorphic complex as the direct source of pb comparison of the irish deposits with mcarthur river and mississippi valley deposits leads us to conclude that style of mineralization syngenetic syndiagenetic epigenetic is not a fundamental aspect of sediment hosted zn pb deposits but is a reflection of local stress regimes and physico chemical conditions the fundamental link between the types of sediment hosted zn pb deposits may be the onset of a condition such as extension or compression that provides a mechanism for driving fluids in extensional regimes seismic pumping processes may channel fluids toward the surface or into fractures or favorable pore spaces resulting in a wide variety of styles of mineralization the lack of dilatency in sediments in compressional regimes however may inhibit surface exhalation and favors epigenetic styles of mineralization

mineralogical and geochemical characterization of the riopar non sulfide zn fe pb deposits prebetic zone se spain abstract the present paper reports the first detailed petrological and geochemical study of non sulfide zn fe pb deposits in the riopar area prebetic zone of the mesozoic betic basin se spain constraining the origin and evolution of ore forming fluids in riopar both sulfide and non sulfide zn fe pb calamine ores are hosted in hydrothermally dolomitized lower cretaceous limestones the hypogene sulfides comprise sphalerite marcasite and minor galena calamine ores consist of zn carbonates smithsonite and scarce hydrozincite associated with abundant fe hydr oxides goethite and hematite and minor pb carbonates cerussite three smithsonite types have been recognized i sm i consists of brown anhedral microcrystalline aggregates as encrustations replacing sphalerite ii sm ii refers to brownish subhedral aggregates of rugged appearance related with fe oxi hydroxides in the surface crystals which replace extensively sphalerite and iii sm iii smithsonite appears as coarse grayish botryoidal aggregates in microkarstic cavities and porosity hydrozincite is scarce and appears as milky white botryoidal encrustations in cavities replacing smithsonite also two types of cerussite have been identified i cer i cerussite consists of fine crystals replacing galena along cleavage planes and crystal surfaces and ii cer ii conforms fine botryoidal crystals found infill porosity calcite and thin gypsum encrustations were also recognized the field and petrographic observations of the riopar non sulfide zn fe pb revealed two successive stages of supergene ore formation under meteoric fluid processes i gossan and red calamine formation in the uppermost parts of the ore with deposition of fe hydr oxides and zn and pb carbonates sm i sm ii and cer i occurring as direct replacements of zn pb sulfides and ii gray calamine ore formation with deposition of sm iii cer ii and hydrozincite infilling microkarst cavities and porosity the stable isotope variation of riopar smithsonite is very similar to those obtained in other calamine ore deposits around the world their c o isotope data 18 o 27 8 to 29 6 v smow 13 c 6 3 to 0 4 v pdb puts constrains on i the oxidizing fluid type which was of meteoric origin with temperatures of 12 to 19 c suggesting a supergene weathering process for the calamine ore formation under a temperate climate and ii the carbon source that resulted from mixing between two co 2 components derived from the dissolution of host dolomite 13 c enriched source and vegetation decomposition 13 c depleted component

geology petrography and mineral chemistry of the vazante non sulfide and ambrosia and fagundes sulfide rich carbonate hosted zn pb deposits minas gerais brazil the vazante paracatu region represents the most important zn district known in brazil and includes the vazante hypogene non sulfide zn deposit composed mainly of willemite zn2sio4 and sphalerite rich carbonate hosted zn pb deposits fagundes is a stratabound deposit characterized by strong silicification dolomitization and a fe rich carbonate alteration halo the main ore is represented by rhythmically banded colloform and zoned pyrite sphalerite and galena related to wall rock dissolution and sulfide infilling which probably occurs late during the burial diagenesis later veins and breccia ore types reflect epigenetic mobilization related to brittle ductile structures the ambrosia mineralization is mainly fault controlled and related to brecciated dolomite which is tectonically imbricated with black shales and slates typical features include host rock recrystallization minor silicification baroque dolomite and ankerite formation pyrite marcasite sphalerite and minor galena occur in brecciated comb veins and veinlets which overprint stylolites and tectonic fractures suggesting an epigenetic origin for the ore the vazante deposit differs from all other deposits of the district due to the presence of willemitic ore which is composed of willemite dolomite siderite quartz hematite zn rich chlorite barite franklinite and zincite the willemitic ore occurs tectonically imbricate to small sulfide ore bodies which comprises sphalerite and galena metabasites and hydrothermalized dolomites within the vazante shear zone the relationships between willemite formation and the development of mylonitic structures suggest that willemitic mineralization and deformation are synchronous episodes closely related to the vazante shear zone low zn cd ratios in sphalerite from vazante 64 to 98 and fagundes colloform 96 to 244 and zoned sphalerite 89 to 305 could reflect the regional role of mineralizing fluids with similar low zn cd ratios and low contents of reduced sulfur sred high ge up to 2200 ppm and the low fe cu mn and ag contents in late diagenetic fagundes sphalerite might suggest that this metal bearing fluid could be derived from the underlying basin fill which comprise clastic sediments and organic matter rich pelitic sequences systematic relationships among sphalerite composition and paragenetic evolution of the fagundes and ambrosia deposits suggest that progressive fluid mixing processes were important for the genesis of the sulfide rich deposits in the district these mixing processes possibly involved the hot metal bearing fluid with low contents of reduced sulfur and moderate temperature highly saline fluids which could represent an important sulfur supply the predominance of the highly saline brines in later epigenetic mineralization episodes might be related to episodic migration of hydrothermal fluids mainly derived from reduced shale sequences during the brasiliano compressive events the small variation in chemical and sulfur isotopic composition of the vazante sphalerite could imply that the high temperature metal bearing fluid with low zn cd ratios could represent a minor reservoir of reduced sulfur which permitted only subordinate sphalerite precipitation in the vazante deposit the lack of reduced shale sequences above the vazante deposit could represent a limiting factor for h2s supply additionally overall mixture between this hot sulfur deficient metal bearing fluid and meteoric fluids channeled to the vazante shear zone favor the high fo2 s2 conditions responsible for the stability of the vazante willemitic assemblage

local and regional mass transfer during thrusting veining and boudinage in the genesis of the giant shale hosted paracatu gold deposit minas gerais brazil multiscale structural and geochemical studies have been applied to understand the genesis of the paracatu deposit in brazil a shallow dipping bulk tonnage vein style low grade au ag pb zn orebody hosted in 1000 ma black phyllites of the paracatu formation associated with intense shearing accompanying thrusting during the 680 ma brasiliano orogeny massive to laminated quartz sulfide carbonate veins and associated alteration and au mineralization formed early in the deformation history and these veins were boudinaged during subsequent progressive increase in shear strain geochemical profiles across the ore allow recognition of a relatively homogeneous protolith at 100 m scales with respect to ti al zr v and rare earth elements including a footwall with protolith attributes similar to those of the ore oxygen hydrogen and sulfur isotopes do not reveal a distinctive external fluid source rather they reflect fluid rock equilibration with the host phyllites during greenschist facies regional metamorphism detailed geochemical sampling around a prominent population of smaller boudinaged veins shows that si ca and sr were removed and al k and ti were residually concentrated during formation of synboudinage dark selvages this process of mass transfer occurred at scales more local than the processes responsible for ore genesis despite the local processes of chemical equilibration and redistribution many of the main ore components appear externally derived at scales broader than the orebody gold as ag sb bi and to some extent pb and zn show a strong spatial and statistical correlation with boudinaged quartz sulfide carbonate veins and these are surrounded by a broader 10 50 m vertical scale halo of enriched k ba and volatiles loss on ignition with depleted na and sr the majority of gold is present as inclusions in pyrite and arsenopyrite within and immediately surrounding the veins and gold bearing pyrite shows mineral chemistry consistent with a hydrothermal origin the main population of large veins locally preserve preboudinage complex internal vein textures including sulfide rich wall rock laminae similar to laminated fault fill veins in many orogenic style vein deposits these larger veins contain most of the gold in the paracatu resource metals were most likely sourced distal to the paracatu formation and were precipitated in sheeted shear or tensile veins during the early stages of the brasiliano thrusting the oblate strain recorded by the boudinaged mineralized veins is anomalous and distinctive to paracatu at km scales even though the thrusting is regional although the orebody may have been physically dislocated by thrusting from the place where the early veins formed the anomalous strains recorded suggest a perturbation in the footwall to the regional thrust or perhaps a spatially restricted competency contrast within the stratigraphy which contributed to the localization of the present mineralization during the thrusting

abrupt increase in greenland snow accumulation at the end of the younger dryas event the warming at the end of the last glaciation was characterized by a series of abrupt returns to glacial climate the best known of which is the younger dryas event1 despite much study of the causes of this event and the mechanisms by which it ended many questions remain unresolved1 oxygen isotope data from greenland ice cores2 4 suggest that the younger dryas ended abruptly over a period of about 50 years dust concentrations2 4 in these cores show an even more rapid transition 20 years this extremely short timescale places severe constraints on the mechanisms underlying the transition but dust concentrations can reflect subtle changes in atmospheric circulation which need not be associated with a large change in climate here we present results from a new greenland ice core gisp2 showing that snow accumulation doubled rapidly from the younger dryas event to the subsequent preboreal interval possibly in one to three years we also find that the accumulation rate change from the oldest dryas to the bo11ing allerod warm period was large and abrupt the extreme rapidity of these changes in a variable that directly represents regional climate implies that the events at the end of the last glaciation may have been responses to some kind of threshold or trigger in the north atlantic climate system

geochemistry and spatial distribution of heavy metals in oxisols in a mineralized region of the brazilian central plateau abstract the majority of the soils of the central plateau of brazil are product of long time development over stable surfaces usually associated with erosion and re deposition cycles in this context to successfully study the geochemistry and spatial distribution of heavy metals it is essential to understand the geology as well as the geomorphology of the landscape and the properties of the metals related to their mobility the main objective of this work was to evaluate contents of naturally occurring heavy metals co cu mn ni pb and zn in some highly weathered tropical soils as related to their geochemistry geology and geomorphology additionally it was aimed to interpret the spatial distribution patterns of these metals and of major elements al fe ti and mg evaluating the association and taking the influence of the geology and geomorphology into consideration in geo referenced sites ranging from 480 to 1040 m in altitude samples were collected at two depths 0 20 and 60 80 cm ground sieved and prepared for analysis heavy metals and major element contents were determined by extraction with aqua regia descriptive statistics pearson correlation coefficients principal components analysis pca and spatial variability analyses were conducted and soil metal concentrations for the whole area were predicted by kriging interpolation a close association between pb zn and mn was observed using pca and pearson s correlation highest concentrations of pb zn and mn were determined in soils between 650 and 550 m altitude surrounding the dolomite massif hills and ambrosia and fagundes mineral deposits copper and fe were found to be geochemically associated with highest concentrations observed in soils formed from carbonaceous phyllite with quartzite layers cobalt and ni concentrations were associated with areas characterized geomorphologically as alluvial colluvial deposits at the lowest altitudes within the region although there is a marked dispersive action of tropical weathering on metal distribution in the region cu pb and zn were found spatially associated with their geological source this probably happens because of greater affinity of those metals to fe and mn oxides formed predominantly under tropical conditions and present in higher concentrations near to the sources of these heavy metals on the other hand both the low affinity of ni and co for these oxide minerals and the region s geomorphology allowed for their migration towards the drainage network to the regions of sediment accumulation at lower altitudes during the landscape s evolution

characterization and origin of low t willemite zn2sio4 mineralization the case of the bou arhous deposit high atlas morocco willemite zn2sio4 usually reported in hypogene non sulfide deposits is described as the main ore mineral in the carbonate hosted bou arhous zinc deposit this deposit is located in the high atlas intracontinental range that formed during the tertiary based on a set of microscopic observations it was possible to establish that willemite replaces primary sphalerite on the basis of cathodoluminescence imaging three successive generations of willemite are distinguished with evidence of dissolution reprecipitation processes willemite is also variably enriched in ge up to 1000 ppm while ge contents lower than 100 ppm are reported in the primary sulfide minerals depending on the willemite generation this substitution was positively or negatively correlated to the zn pb substitution according to the nature of zoning sector versus oscillatory the incorporation of ge was either controlled by crystallographic factors or by the nature of the mineralizing fluids willemite is associated with other oxidation related mineral species like cerussite pbco3 but is not in isotopic equilibrium and therefore not considered to be cogenetic oxygen isotope compositions support the formation of willemite at temperatures below 130 c from mixed meteoric and deeper hydrothermal fluids the formation of the high atlas belt during the tertiary has contributed to the exhumation of the sulfide minerals and the development of vertical conduits for percolation of meteoric water and ascending hydrothermal fluids in addition to a local contribution of silicate minerals of the host limestone hydrothermal fluids probably transported si and ge that are incorporated in willemite

the role of evaporites in the genesis of base metal sulphide mineralisation in the northern platform of the pan african damara belt namibia geochemical and fluid inclusion evidence from carbonate wall rock alteration the otavi mountain land is a base metal sulphide ore province in northern namibia where deposits are hosted by platform carbonates of the otavi group in a foreland fold and thrust belt on the northern edge of the pan african damara belt deposits have been classified as the berg aukas or tsumeb types based on differences in ore association stratigraphic position and geochemistry of ores and gangue carbonates mineralisation at these deposits is accompanied by carbonate alteration in the form of dolomite and calcite veins carbonate recrystallisation calcitisation and carbonate silicification based on cathodoluminescence imaging trace and rare earth element ree o and c isotope and fluid inclusion data a series of carbonate generations constituting wall rock alteration around the tsumeb and kombat tsumeb type and berg aukas berg aukas type deposits was established similar data obtained on the recently discovered khusib springs deposit indicate a strong affinity to tsumeb type deposits tsumeb type deposits are distinguished from berg aukas type deposits by having trace element and ree concentrations that are significantly higher in the alteration products compared to the carbonate host rocks only around tsumeb type deposits a relative enrichment in light ree is noted for the hydrothermal carbonate generations that are cogenetic with the main stage of mineralisation microthermometric results from fluid inclusions in carbonate alteration phases and associated quartz indicate relatively high salinity 17 23 wt nacl equivalent for the main mineralising and subsequent sulphide remobilisation stages at the deposits investigated estimated mineralisation temperatures are significantly higher for tsumeb type deposits 370 405 c with early sulphide remobilisation in tsumeb at 275 c whereas they are lower at berg aukas up to 255 c fluid inclusion leachate analysis suggests that most of the observed salinity can be ascribed to dissolved predominantly ca and mg carbonates and chlorides with subordinate nacl na cl br leachate systematics indicate a derivation of the fluid salinity from the interaction with evaporitic rocks en route tsumeb type mineralisation is interpreted to be derived from fluids expelled during pan african orogeny in the more intensely deformed internal zones of the damara belt further south when the high salinity fluids reached the carbonate platform after having scavenged high concentrations of base metals base metal sulphide precipitation occurred in zones of high porosity provided by karst features in the carbonate sequence results obtained for the berg aukas type deposits emphasise their derivation from basinal brines similar to mississippi valley type deposits and confirm that mineralisation of the berg aukas and tsumeb types are both spatially and temporally distinct

an eburnean base metal source for sediment hosted zinc lead deposits in neoproterozoic units of namibia lead isotopic and geochemical evidence the lead isotopic composition of galena from the neoproterozoic sediment hosted zn pb sulphide deposit at rosh pinah and the oxidised zn deposit at skorpion in the pan african gariep belt southwestern namibia as well as that of galena from minor occurrences in the wider rosh pinah ore province was investigated and is compared with that of other major sediment hosted base metal ore deposits hosted by neoproterozoic strata in southwestern africa and brazil the isotope data were supplemented by a geochemical provenance study of the argillitic host rock to the rosh pinah deposit and its stratigraphic equivalents the pre orogenic rosh pinah deposit and the inferred progenitor for the secondary skorpion deposits have very similar pb isotopic compositions in contrast syn orogenic deposits e g tsumeb type show a larger proportion of radiogenic pb from the country rocks that were infiltrated by orogenic ore fluids

sediment hosted lead zinc deposits of the neoproterozoic bambui group and correlative sequences sao francisco craton brazil a review and a possible metallogenic evolution model abstract the proterozoic sediment hosted zn pb sulfide and non sulfide deposits of the sao francisco craton brazil are partially syn diagenetic and epigenetic and were probably formed during extensional events the majority of the deposits occur within shallow water dolomites the pb isotopic data of sulfides are relatively homogeneous for individual deposits and plot above the upper crust evolution curve of the plumbotectonic model some of the deposits are characterized by highly radiogenic lead 206 pb 204 pb 21 originating from the highly radioactive crust of the sao francisco craton pb and s isotopic data suggest the sources of metal and sulfur for the deposits to be the basement rocks and seawater sulfates in the sediments respectively the relatively high temperatures of formation 100 to 250 c and moderate salinity 3 to 20 nacl equiv of the primary fluid inclusions in the sphalerite crystals suggest the participation of basinal mineralizing fluids in ore formation the steep paleo geothermal gradient generated by the radioactively enriched basement rocks probably assisted in heating up the circulating mineralizing fluids

fluid evolution in the southern part of the proterozoic vazante group brazil implications for exploration of sedimentary hosted base metal deposits abstract the proterozoic vazante group carbonate rocks were submitted to multiple stages of fluid circulation from diagenesis to orogenesis as documented by detailed mineralogical fluid inclusion and isotopic studies of the upper morro do pinheiro and lower pamplona members from the serra do poco verde formation in the southern part of the paleo basin these units are the main hosts for the hypogene structurally controlled zinc silicate deposits in the vazante zinc district including the vazante mine which is considered to be the largest willemitic zn 2 sio 4 deposit in the world with estimated total resources of 40 60 mt at 20 zn five hydrothermal alteration types were identified in the southern extension of the vazante group i early stage alteration comprises dolomite substitution bands and nodules associated with moderate salinity h 2 o nacl cacl 2 fluids with temperatures around 90 c interpreted as late diagenetic the calculated c and o isotopic compositions of the fluids suggest meteoric and or connate origin and interaction with organic carbon ii pre ore stage alteration is evidenced by dog tooth dolomite and quartz with minor fe oxi hydroxides which fills dissolution voids formed by h 2 o nacl cacl 2 mgcl 2 fluids at temperatures around 100 to 150 c the c isotopic data also indicate interaction with organic carbon the two first stages are poor in ore related elements iii ore stage encompasses four phases of mineral precipitation the first is distal and characterized by red stained dolostones due to disseminated hematite and red dolomite the second is the main phase of the ore stage composed of massive red dolomite massive hematite and willemite with enrichment mainly in fe 2 o 3 sio 2 ag as cd cu ni pb se and zn the third phase comprises white dolomite hematite and traces of willemite with enrichment in mno cd ni and pb the calculated c and o isotopic compositions of the fluids at 180 c associated with pervasive alteration yielded values heavier than the diagenetic stage and lower than the host rock the fourth phase corresponds to zn chlorite and quartz which are associated with h 2 o nacl fluids of variable salinities and distinct temperatures 90 140 c and 170 190 c iv pyrite bearing vein characterized by pyrite sphalerite white dolomite fine hematite and late quartz with c and o isotopic and fluid inclusion compositions that are similar to the three phases of dolomite of the ore stage these data suggest a co genetic relationship of these two alteration styles v the late sulfide stage characterized by rare galena bearing stringers with sphalerite chalcocite greenockite covellite and white dolomite that cross cut the main ore stage phases the ore related fluid compositions associated with the silicate zinc mineralization in the southern part of the vazante group are also similar to the fluids reported in previous work for the sulfide zinc lead deposits in the northern part indicating favorable conditions for metal transport during the brasiliano orogeny sulfide ore would have precipitated in zones where sulfur was available and silicate zinc in structures where mineralizing fluids interacted with evolved meteoric water this finding implies that carbonate sequences in other districts with sulfide lead zinc deposits may also host hypogene silicate zinc deposits

geology of the beltana willemite deposit flinders ranges south australia beltana is a high grade hypogene willemite deposit hosted in lower cambrian carbonate rocks in the arrowie basin northern flinders ranges south australia it is situated adjacent to a major growth fault on the basin margin ooid grainstone units of the woodendinna dolomite and units of archaeocyathid rich wilkawillina limestone are the main host lithologies lead minerals in subeconomic quantities are also present in karstic collapse breccias surrounding the willemite orebodies mineralization is structurally controlled and associated with brecciation and extensive hematite rich hydrothermal zincian dolomitization ore minerals include willemite and coronadite with lesser mimetite hedyphane and smithsonite late stage gangue minerals include manganocalcite dolomite and minor quartz the texture of willemite is heterogeneous resulting from various depositional mechanisms such as partial to massive replacement of the carbonate host rock internal sedimentation fracture fill brecciation and vein fill on the periphery of the deposit smithsonite formed by weathering of willemite beltana is centered on a karstic collapse breccia that extends at least 100 m vertically formed in part through corrosion by acidic ore solutions the geochemical signature of the orebody includes high levels of zn pb cd as and mn notably silver is absent from the deposit and sulfur concentrations are low 20 ppm fluid inclusion studies yield a low minimum temperature range of ore deposition between 50 and 170 c k ar dating of coronadite associated with the willemite orebody indicates an age of formation of 435 5 ma premining resources of willemite ore were 850 000 t at 36 percent zn and an associated body of subeconomic lead contained more than 800 000 t at 8 9 percent pb 3 9 percent zn and 1 percent as the deposit has some similarities with mississippi valley type deposits but differs in ore and alteration mineral assemblages

glaciation and sea level change for ireland and the irish sea since late devensian midlandian time the sea level change around the coast of ireland and the irish sea for the past 20 000 years is primarily the combined result of the glacio isostatic adjustment of the crust to the removal of ice over the british isles and the total eustatic change from the global ice sheets however the isostatic effects due to the removal of ice from northern europe and north america and the addition of meltwater into the oceans also make a significant contribution predictions of sea level change based on glacio hydro isostatic models are compared with observations to constrain the ice volume over ireland at the time of the last glacial maximum and the maximum ice height appears to have been of the order of 600 m the models predict well the spatial variability in sea level change observed across the region for holocene and lateglacial time with levels above present being predicted only for northeastern ireland and north of about morecambe bay the models do not support suggestions that lateglacial levels along the east coast of ireland or the coast of wales were 50 150 m above their present levels consistent models that would produce such large lateglacial highstands are incompatible with all other sea level and glacial evidence for the british isles palaeobathymetry and palaeoshoreline reconstructions for the irish sea indicates that a tenuous landbridge between britain and ireland developed only across the celtic sea between about 18000 and 14000 years bp

subglacial hydrology for an ice sheet resting on a deformable aquifer glissements et deformation de la calotte de glace en relation avec le materiau de l aquifere et la densite des chenaux de drainage

dolomite occurrence evolution and economically important associations abstract dolomite is not a simple mineral it can form as a primary precipitate a diagenetic replacement or as a hydrothermal metamorphic phase all that it requires is permeability a mechanism that facilitates fluid flow and a sufficient supply of magnesium dolomite can form in lakes on or beneath the shallow seafloor in zones of brine reflux and in early to late burial settings it may form from seawater from continental waters from the mixing of basinal brines the mixing of hypersaline brine with seawater or the mixing of seawater with meteoric water or via the cooling of basinal brines bacterial metabolism may aid the process of precipitation in settings where sulfate reducing species flourish and microbial action may control primary precipitation in some hypersaline anoxic lake settings dolomite is a metastable mineral early formed crystals can be replaced by later more stable phases with such replacements repeated a number of times during burial and metamorphism each new phase is formed by the partial or complete dissolution of an earlier dolomite this continual re equilibration during burial detracts from the ability of trace elements to indicate depositional conditions and resets the oxygen isotope signature of the dolomite at progressively higher temperatures because subsurface dolomite evolves via dissolution and reprecipitation a bed of dolomite can retain or create porosity and permeability to much greater burial depths and into higher temperature realms than a limestone counterpart dolomitization also creates new crystals with new rhomb growth following the dissolution of less stable precursors repetition of this process without complete pore cementation can generate intercrystalline porosity a number of times in the rock s burial history intercrystalline porosity is a highly interconnected style of porosity that gives dolomite reservoirs their good fluid storage capacity and efficient drainage the fact that many dolomite reservoirs formed via brine reflux means that they sit beneath an evaporite seal in both platform and basinwide evaporite settings the same association of evaporites sulfate source and entrained hydrocarbons means that burial conditions are also suitable for thermochemical sulfate reduction and the precipitation of base metals this tends to occur at higher temperatures 60 c 80 c and so the resulting dolomites tend to be ferroan and consist of saddle shaped crystals

calibration of the c 14 timescale over the past 30 000 years using mass spectrometric u th ages from barbados corals uranium thorium ages obtained by mass spectrometry from corals raised off the island of barbados confirm the high precision of this technique over at least the past 30 000 years comparison of the u th ages with 14c ages obtained on the holocene samples shows that the u th ages are accurate because they accord with the dendrochronological calibration before 9 000 yr bp the 14c ages are systematically younger than the u th ages with a maximum difference of 3 500 yr at 20 000 yrbp the u th technique thus provides a way of calibrating the radiocarbon timescale beyond the range of dendrochronological calibration

highly 13c enriched carbonate and organic matter in the neoproterozoic sediments of the bambuigroup brazil abstract paleontological and radiometric dating methods rb sr k ar and pb pb suggest neoproterozoic ages for sedimentary carbonate rocks of the bambuigroup in the southern part of the sa o francisco basin of central brazil the 13c values for carbonates fall within two ranges 2 to 2 and 10 to 16 for both groups coexisting organic carbon is depleted in 13c by 28 2 relative to the carbonates similar to numerous neoproterozoic examples worldwide secular variations in 13c in the bambuigroup show a significant positive excursion beginning with values near 0 in carbonates immediately above a diamictite and rising to values as high as 16 upsection the sr isotope data for carbonates from the bambuigroup agree with the age inferred from the secular variation in oceanic 13c values the striking feature in this study is that both the carbonate and total organic carbon are considerably more enriched in 13c than previously reported for neoproterozoic carbonates elsewhere it is not clear if these unusual 13c values are caused by primary or secondary processes

airborne radar evidence for tributary flow switching in institute ice stream west antarctica implications for ice sheet configuration and dynamics despite the importance of ice streaming to the evaluation of west antarctic ice sheet wais stability we know little about mid to long term dynamic changes within the institute ice stream iis catchment here we use airborne radio echo sounding to investigate the subglacial topography internal stratigraphy and holocene flow regime of the upper iis catchment near the ellsworth mountains internal layer buckling within three discrete topographically confined tributaries through ellsworth independence and horseshoe valley troughs provides evidence for former enhanced ice sheet flow we suggest that enhanced ice flow through independence and ellsworth troughs during the mid to late holocene was the source of ice streaming over the region now occupied by the slow flowing bungenstock ice rise although buckled layers also exist within the slow flowing ice of horseshoe valley trough a thicker sequence of surface conformable layers in the upper ice column suggests slowdown more than 4000 years ago so we do not attribute enhanced flow switch off here to the late holocene ice flow reorganization intensely buckled englacial layers within horseshoe valley and independence troughs cannot be accounted for under present day flow speeds the dynamic nature of ice flow in iis and its tributaries suggests that recent ice stream switching and mass changes in the siple coast and amundsen sea sectors are not unique to these sectors and that they may have been regular during the holocene and may characterize the decline of the wais

pre ore burial dolomitization adjacent to the carbonate hosted gays river zn pb deposit nova scotia the gays river formation of the lower windsor group in the vicinity of the gays river zn pb deposit was completely dolomitized via a volume for volume transformation prior to mineralization premineralization porosity was the precursor limestone porosity comprising fenestrae dissolution molds and intercrystalline intraskeletal and growth pores which remained essentially unchanged through dolomitization regionally the dolomites one replacement and one cement overlapped with sediment compaction and dissolution of aragonite the dolomites are characterized by low 18opdb values 14 to 4 average 8 slightly radiogenic 87sr 86sr ratios 0 70778 0 70900 and locally high percentages of feco3 and mnco3 up to 13 5 and 9 5 mol respectively the presented petrographic and geochemical data compiled with existing fluid inclusion microthermometry data suggest dolomitization by hot brines in a burial setting high fe and mn concentrations south of the studied area with progressive depletion toward

geology mineralogy and geochemistry of the vazante northern extension zinc silicate deposit minas gerais brazil thesis master geological sciences geological engineering queen s university 2012 02 29 16 45 36 41

carbon and oxygen isotopic covariations in hydrothermal calcites isotopic covariations of carbon and oxygen in hydrothermal calcites are quantitatively modeled in terms of the following three mixing processes 1 mixing between two different fluids which leads to the precipitation of calcite 2 mixing between fluid and rock a calcite precipitation due to fluid rock interaction b secondary alteration of primary calcite by interaction with a subsequent fluid the models are derived from mass balance equations a distinction among the three mixing processes can be made on a 13c vs 18o diagram which places important constraints on the genesis of hydrothermal mineralization the variables which control the ultimate isotopic composition of hydrothermal calcites include the composition of the initial fluid and the wallrock temperature and dissolved carbon species owing to significant temperature dependent fractionation effects during equilibrium precipitation of calcite from a hydrothermal fluid the mixing processes may be distinguished by telltale patterns of isotopic data in 13c vs 18o space in particular caution must be exercised in postulating the fluid mixing as the cause for mineral deposition this is demonstrated for hydrothermal pb zn deposits in the western harz mountains germany a positive correlation between 13c and 18o values is observed for calcites from the bad grund deposit in the upper harz two sample profiles through calcite veins show similar correlations with the lowest values at the center of the veins and the highest values at the vein margins because the correlation array has a greater slope than for calcite precipitation at equilibrium in a closed system and because fluid mixing may not proceed perpendicular to the vein strike it is assumed that a fluid rock interaction is responsible for the observed correlation and thus for the precipitation of calcite a deep seated fluid is inferred with a 13c value of 7 and a 18o value of 10 as well as h2co3 as the dominant dissolved carbon species precipitation temperatures of the calcites are estimated to be about 280 170 c quite different isotopic distributions are observed for calcites from the st andreasberg deposit in the middle harz an alteration model is suggested based mainly on the isotopic distribution through a calcite vein in addition to a primary fluid which has the same isotopic composition as that in the bad grund deposit and thus seems to be responsible for the precipitation of calcite associated with sulfides an evolved hco 3 dominant subsurface fluid with 13c about 20 15 and 18o 0 is deduced to alter the primary calcite at low temperatures of 70 40 c

sulfide carbonate relationships in the navan tara zn pb deposit ireland samples from the mineralized micrite the bottom part of the 1 5 lens were investigated they contain zn and fe bearing low mg calcite dolomite fedolomite minrecordite cazn co3 2 sphalerite pyrite melnikovite and overgrown quartz zn dolomite consists probably of camg co3 2 and cazn co3 2 domains about 10 35 nm in size zn dolomite is sulfidized replaced by zns which is demonstrated by both reflected light and electron microscope methods cazn co3 2 from the navan ore has cell parameters a0 0 484 0 004 and c0 1 608 0 012 nm and is rhombohedral it forms close tiny intergrowths with calcite base metals at navan were exhaled to form at least partly carbonate minerals which subsequently reverted to sulfides in the diagenetic environment the reversion of base metals carbonates to sulfides is connected with reduction in volume the reduction in volume allowed the ore zone to retain its permeability after lithification of the host rock it allowed lateral flow of hydrothermal solutions which deposited further generations of sulfides and increased the base metal content producing a high grade ore

nonsulfide and sulfide rich zinc mineralizations in the vazante ambrosia and fagundes deposits minas gerais brazil mass balance and stable isotope characteristics of the hydrothermal alterations the vazante group hosts the vazante nonsulfide zinc deposit which comprises high grade zinc silicate ore znsio4 and late diagenetic to epigenetic carbonate hosted sulfide rich zinc deposits e g morro agudo fagundes and ambrosia in the sulfide rich deposits hydrothermal alteration involving silicification and dolomitization was related with ground preparation of favorable zones for fluid migration e g fagundes or with direct interaction with the metalliferous fluid e g ambrosia at vazante hydrothermal alteration resulted in silicification and dolomite siderite jasper hematite and chlorite formation these processes were accompanied by strong relative gains of sio2 fe2o3 t rb sb v u and la which are typically associated with the nonsulfide zinc mineralization all sulfide rich zinc ores in the district display a similar geochemical signature suggesting a common metal source from the underlying sedimentary sequences oxygen and carbon isotope compositions of hydrothermally altered rocks reveal a remarkable alteration halo at the vazante deposit which is not a notable feature in the sulfide rich deposits this pattern could be attributed to fluid mixing processes involving the metalliferous fluid and channelized meteoric water which may control the precipitation of the vazante nonsulfide ore sulfide deposition resulted from fluid rock interaction processes and mixing between the ascending metalliferous fluids and sulfur rich tectonic brines derived from reduced shale units