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

late paleoproterozoic extension and a paleostress field reconstruction of the north china craton abstract mafic dyke swarms and aulacogens are major anorogenic extensional events in the late paleoproterozoic north china craton ncc the n nnw mafic dyke swarms are widespread in the ncc whose ages span between 1 83 and 1 77 ga the similar ages and orientations of 1 8 ga dyke swarms in the ncc demonstrate that the amalgamated ncc experienced widespread extension at this time based on the width statistics of dyke swarms on ten survey lines an average crustal extension ratio of 0 35 was found for the ncc the small magnitude of overall extension suggests that the mafic dyke swarms were emplaced into the elastic fractures and indicates that the ncc had become a brittle plate prior to the emplacement of the mafic dyke swarms precisely dated mafic dyke swarms when used as paleostress indicators can be employed in the paleostress field reconstruction of precambrian cratons two dimensional finite element modeling 2 d fem of the ncc in which the various blocks were assigned densities and elastic constants shows that north south compression favors dyke intrusion along generally n nw lines and that deviations in dyke trends can be explained by the effects of boundary constraints and the physical properties of the crust the best fitting model can be considered a plausible representation of the tectonic force acting on the ncc that produces the intraplate stress field that is most consistent with the observed orientation of dyke swarms the results of modeling of the late paleoproterozoic stress field suggest a common tectonic setting for the emplacement of mafic dyke swarms in the central orogenic zone western and east blocks of the ncc the results also show that the north south tectonic forces play an important role in determining the paleostress field in the ncc the widespread extension of the ncc resulted from the north south tectonic forces which may be related to the break up of the late paleoproterozoic supercontinent the paleostress field modeling provides a possible approach to consider the supercontinent paleostress reconstruction and to reveal the mechanisms of the supercontinent break up

the calamine nonsulfide zn pb deposits of belgium petrographical mineralogical and geochemical characterization abstract the belgian nonsulfide zn deposits represent the historical basis for the zinc mining and smelting industry in europe the zinc ores were called calamines and consist of a mixture of zn carbonates smithsonite hydrozincite and zn silicates hemimorphite willemite and zn clays with a variable content of pb minerals the calamines are considered to be oxidation products of primary sulfide ores the most important was the la calamine orebody more than 600 000 tons of zn metal that was continuously exploited for several centuries nonsulfide ores were also exploited at schmalgraf engis dickenbusch fossey rocheux oneux welkenraedt and in other smaller deposits all no longer accessible for study where the calamines extended from the surface to an average depth of 40 to 50 m the primary zn pb sulfide mineralization consists of post variscan hydrothermal veins and replacement bodies mostly occurring in the dinantian visean limestones the sulfides intersect the paleozoic rocks and are truncated and unconformably covered by late cretaceous sediments santonian aachen fm the same setting has been observed for the nonsulfide ores suggesting an early to mid cretaceous weathering consistent with other geological and geochronological data on paleoweathering in europe the extensive occurrence of willemite that appears to be the first deposited nonsulfide mineral is a striking particularity of the belgian calamines most fluid inclusions in willemite 80 and all inclusions in smithsonite are monophase aqueous ice melting temperatures suggest salinities between 0 and 5 wt nacl equiv homogenization temperatures of rare two phase inclusions liquid vapor in willemite display a large variation between 80 and 190 an interval matching the temperature ranges of other willemite ores throughout the world so far considered of hydrothermal origin the stable isotope variation of belgian smithsonites and cerussites is very similar to the range measured in other supergene nonsulfide deposits similar to those of sw sardinia belgian smithsonites exhibit a limited range of 18o values from 27 1 to 30 6 averaging 28 4 0 8 v smow this limited range points to a relatively uniform isotope composition of the oxidizing waters and constant temperatures of smithsonite crystallization carbon isotope values even from the same mine show a considerable range from 11 6 to 1 6 this extensive range indicates at least two carbon sources 13c depleted carbon derived from the organic matter in the soils and 13c enriched carbon originating from marine carbonate host rocks cerussites have oxygen isotope values ranging from 16 8 to 19 3 averaging 17 8 1 0 and carbon isotope values of 18 4 to 14 7 the metallic deposits of eastern belgium have undergone a polyphase history which started with a possibly jurassic sulfide mineralization followed by oxidation under high silica activities and consequently by supergene weathering owing to structure and mineralogical association we do not consider the high homogenization temperatures as reliable minimum temperature estimates of willemite formation however it is possible that the willemite concentrations could have been derived from low temperature localized hydrothermal processes caused by deeply reaching oxidizing fluids smithsonite and hemimorphite often developed at the expenses of willemite have all the characteristics of supergene oxidation products

sulphur isotope characteristics of recrystallisation remobilisation and reaction processes a case study from the ramsbeck pb zn deposit germany syntectonic hydrothermal pb zn vein systems of the ramsbeck deposit germany have been extensively overprinted by late stage fluids responsible for fissure vein mineralisation this has caused remobilisation of vein components notably of sphalerite and galena as well as the formation of various sb sulphosalt minerals including boulangerite semseyite tetrahedrite and bournonite a detailed sulphur isotope study of sphalerite galena and sulphosalts related to different stages of recrystallisation remobilisation and reaction has been carried out using an insitu laser combustion technique primary sphalerite galena ores from flat lying thrust zones are characterised by homogeneous positive 34s values in the range 6 5 to 7 7 in contrast recrystallised and remobilised sphalerite and galena from both overprinted thrust zones and extensional fissure veins show 34s values in the range 1 1 to 2 9 and 0 7 to 8 7 respectively sulphosalt minerals show a similar variation in their sulphur isotope compositions which range between 1 7 and 7 7 fissure vein hosted sphalerite and sulphosalts display significantly more negative 34s values than the primary pb zn ores sphalerite which recrystallised in situ and boulangerite which formed by reactive replacement of primary galena show slightly more positive 34s values than fissure vein hosted minerals the compositional range of remobilised mineral phases can be best explained by varying contributions variable mixing of sulphur derived from 1 the primary sphalerite galena ores of the flat thrust systems and 2 the hydrothermal fluid responsible for the fissure vein mineralisation equilibrium fractionation temperatures calculated for sphalerite galena pairs contrast strongly with fluid inclusion data isotopic fractionation related to the remobilisation has to be interpreted in terms of kinetic processes

willemite zn2sio4 as a possible rb sr geochronometer for dating nonsulfide zn pb mineralization examples from the otavi mountainland namibia abstract the zinc silicate willemite zn 2 sio 4 is the main carrier of zinc in a number of high grade carbonate hosted nonsulfide deposits located in the southern hemisphere that have been recently reinterpreted to be of hypogene hydrothermal origin the timing of willemite mineralization in these deposits is only poorly constrained in this pilot study willemite has been evaluated as a potential rb sr geochronometer that can be used to directly date nonsulfide ore deposits we have analyzed samples of economic stage willemite from the berg aukas and abenab west deposits otavi mountainland namibia which are hosted by neoproterozoic dolomitized and tectonized carbonate rocks rb sr elemental concentration levels and ranges of 87 rb 86 sr and 87 sr 86 sr ratios in these willemite samples are comparable to those observed for the zn sulfide sphalerite which has been already successfully used for direct rb sr dating of carbonate hosted mvt deposits this reflects similar rb sr partitioning mechanisms into zn ore minerals that precipitate either as sulfides from reduced or as silicates from oxidized hydrothermal fluids respectively the rb sr results on willemite presented here reflect mostly low but variable 87 rb 86 sr ratios sufficient to generate radiogenic 87 sr over time this clearly shows that willemite is suitable as an rb sr geochronometer and should be further evaluated for direct rb sr dating of nonsulfide zn deposits isochron regressions combining samples from berg aukas and abenab west willemites yield rb sr ages in the range of ca 500 to 560 ma these values are geochronologically doubtful at first sight due to high excess data scatter mswd 2000 to 3000 massive fine grained willemite samples from abenab west alone n 3 give a still statistically unacceptable mswd 60 isochron regression corresponding to an rb sr age of 574 84 ma more reliable ages are obtained from two statistically adequate small scale rb sr isochron regressions for samples of coarse grained well crystallized berg aukas willemite which yield 499 63 ma mswd 2 6 and 493 2 ma mswd 1 2 respectively regardless of the quality of isochron regressions in terms of mswd the rb sr ages obtained in this study are fully compatible with other available geochronological data that reflect distinct tectonothermal events in the study area therefore on a regional scale they appear to be at least geologically reasonable which has yet to be verified by further studies comparable mineralization ages in the range of ca 490 to 550 ma are also reflected by the few other known economic hypogene willemite deposits in the southern hemisphere for which direct geochronological information is mostly lacking this may point to a global period of hydrothermal activity in the cambrian to lower ordovician under conditions that favored the formation of hypogene nonsulfide zn mineralization

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

extent of power law scaling for natural fractures in rock new data sets from natural faults and extension fractures exhibit simple power law scaling across 3 4 4 9 orders of magnitude regardless of rock type or movement mode the data show no evidence of natural gaps or scaling changes each data set consists of independent measurements made at different observational scales a power law regression to the subset of smaller fractures in each case provides an extrapolation that accurately predicts associated larger fractures consequently data representing a limited range of fracture sizes may be used to characterize a much broader spectrum of fracture sizes

systematics of chlorite alteration at the phelps dodge massive sulfide deposit matagami quebec hydrothermal alteration at the phelps dodge archcan volcanogenic massive sulfide deposit has converted rhyodacite and rhyolite footwall rocks into the following mineral assemblages quartz chlorite albite epidote or sericite quartz chlorite or sericite chlorite or sericite and chlorite talc or stilpnomelane alteration at constant al was initiated by hydration addition of fe mg silicification and leaching of ca na which produced al saturated chlorites with a wide range of fe fe mg ratios within the assemblage quartz chlorite albite epidote or sericite with continued addition of fe and less mg combined with extensive leaching of quartz the chlorite became al undersaturated quartz chlorite talc assemblage and ultimately formed a rock composed essentially of chlorite the chlorite talc or stilpnomelane assemblage is stable in late crosscutting veinlets and in the ore zone chlorites in the si al mg fe system form a rectangular solid solution plane with limited substitution of al for si and extensive substitution of fe for mg in si al space the solid solution plane is outlined by al saturation presence of other al rich mineral and al undersaturation presence of al poor or al free mineral boundaries al iv substitution for si in the chlorites increases from 2 3 to 3 01 formula units based on 8 si al iv with fe fe mg whereas al iv occupancy decreases from 2 88 to 2 65 the valence charge balance is compensated by octahedral site vacancies or by the conversion of fe super 2 to fe super 3 the al iv content of chlorite also increases with temperature chlorites on the al saturated boundary of the solid solution field form a potentially useful geothermometer t degrees c 106 al iv 18 al iv is corrected for changes with the fe fe mg ratio as al iv corrected al iv 0 7 fe fe mg the fe fe mg ratios of bulk rock and contained chlorite correlate well birefringence colors of the chlorite change from green to brown violet and berlin blue as fe fe mg changes from 0 18 to 0 64 the fe fe mg variation apparently depends largely on the proportions of mg rich seawater and fe rich hydrothermal fluid in the altering brine

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

fluid inclusion evidence of petroleum migration through a buried metamorphic dome in the pannonian basin hungary some of the petroleum currently produced in the pannonian basin in hungary stems from buried metamorphic basement domes that form structural highs between neogene sedimentary sub basins in the bekes sedimentary sub basin the source rocks of the petroleum are known to be of middle to late miocene age whereas at least part of the petroleum in the adjacent szeghalom basement dome appears to be of lower miocene age or even older virtually nothing is known about the compositions distribution sources and migration history of this older petroleum although such variables are crucial to future exploration strategies this study addresses the first two of these gaps in knowledge by providing a basic characterization of the older petroleum trapped as fluid inclusions in fracture hosted quartz crystals within the szeghalom dome szd fluid inclusion samples from 9 wells at depths of 1900 to 2200 m were analysed by visible and cie calibrated uv microscopy microthermometry laser raman and 1 h nmr spectroscopy two major types of early miocene oils were found distributed over 5 distinct temporal generations an earlier immature brown oil succeeded by a light gas condensate the early petroleum was undersaturated with respect to methane gas and it migrated through the fracture network at temperatures between about 160 to 130 c entraining blebs of bitumen and flushed by weakly saline water the later gas condensate migration consisted of a heterogeneous mixture of volatile oil methane rich gas weakly saline water at slightly lower temperatures between 135 and 120 c the temperature evolution was caused by cooling of the metamorphic wall rocks during basement uplift culminating in middle miocene exhumation the various hydrocarbons are unevenly distributed across the szeghalom dome showing that pre middle miocene fluid migration through the fracture network was compartmentalized at the km scale 2007 elsevier b v all rights reserved

subsurface fractures and their relationship to stress history in east texas basin sandstone abstract subsurface fractures were sampled with more than 400 m of core from the lower cretaceous travis peak formation a tabular sandstone and shale unit approximately 600 m thick that was deposited in the gradually subsiding gulf coast basin depth to the top of the formation in east texas now ranges between 1800 and 2900 m fractures in sandstone exhibit features that are due to intermittent fracturing and sealing crack seal which suggest that the fractures are the result of episodes of fluid pressure in excess of hydrostatic correlation of the vein mineral precipitation sequence with the diagenetic history and the results of previous 18 o studies of quartz cement indicates that fractures propagated at depths of between 900 and 1500 m during the migration of quartz precipitating fluids fracture permeability provides a mechanism for the passage of large fluid volumes recorded by the precipitation of extensive quartz cement in the formation fractures stopped propagating before maximum burial depth was achieved during approximately 1500 m of subsequent burial fractures either remained partly open or were sealed by postkinematic ankerite and clay minerals

chloritization of the hydrothermally altered bedrock at the igarape bahia gold deposit carajas brazil the igarape bahia gold deposit has developed from weathering of a near vertical hydrothermal cu au mineralization zone the unweathered bedrock composed of chlorite schists is mainly metamorphosed basalts pyroclastic and clastic sedimentary rocks and iron formation contents and fe fe mg ratios of chlorites increase from distal country rock towards the mineralization zone which can be attributed to different water rock ratios and locations in a hydrothermal system in the hydrothermal system high salinity fluids convected through basin floor rocks stripping metals from the recharge zones with precipitation in discharge zones the chlorite with lower fe fe mg ratios indicates alteration by relatively unreacted mg rich fluids occurring within recharge zones by contrast the chlorite with higher fe fe mg ratios in the mineralization zone formed from solutions rich in fe mn au cu h2s and sio2 within a discharge zone the iron formation could also be formed within the discharge zone or on the basin floor from the fe rich fluids the distal country rock with less chlorite content is a hydrothermal product at low water rock ratios whereas the proximal country rock and the host rock with more chlorite content formed at high water rock ratio conditions the al iv contents of chlorites indicate that the formation temperatures of these rocks range from 204 to 266 c with temperatures slightly increasing from distal country rock towards the mineralization zone

development of crystal morphology during unitaxial growth in a progressively widening vein ii numerical simulations of the evolution of antitaxial fibrous veins the development of fibrous morphology and capability of fibres for tracking the opening trajectory were investigated using numerical simulations of a natural antitaxial fibrous vein starting from a non unique best case variation of fracture opening velocity grain size wall roughness growth anisotropy and crystal growth velocity shows that these parameters differ in importance for crystal morphology and tracking capability fibrous veins can be simulated using crack seal opening of the fracture grain boundaries track the opening trajectory if the wall roughness is high opening increments are small and crystals touch the wall before the next crack increment starts

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

the carbonate hosted willemite prospects of the zambezi metamorphic belt zambia zambian willemite zn2sio4 deposits occur in the metasedimentary carbonate rocks of the proterozoic katangan supergroup the most important orebodies are located around kabwe and contain both sulphides and willemite in dolomites of low metamorphic grade the star zinc and excelsior prospects lusaka area discovered in the early 1920s occur in the metamorphic lithotypes of the late proterozoic zambezi supracrustal sequence which were deposited in a transtensional basin formed during the oblique collision of the kalahari and congo cratons the deposits are hosted by the limestone and dolomitic marbles of the cheta and lusaka formations structural analysis indicates that several fracture sets host the deposits which may be genetically related to the pan african mwembeshi dislocation zone a major geotectonic boundary between the lufilian arc and the zambezi belt in both prospects willemite replaces the marbles and is found along joints and fissures with open space filling textures and locally may develop colloform and vuggy fabrics as well silver as well as traces of germanium and cadmium have been detected within the willemite ore and lead or zinc sulphides are scarce or absent calcite locally replaces willemite willemite is associated with specular hematite and franklinite and post dates the zn spinel gahnite in the paragenesis genthelvite zn4be3 sio4 3s occurs as a minor phase in irregular aggregates the willemites from the lusaka area though mn poor show green cathodoluminescence colours and bright green fluorescence in short wave uv as the high temperature willemites in usa thermometric analyses of primary fluid inclusions in willemite yield homogenization temperatures that range from 160 c to 240 c and salinities of 8 16 wt equiv nacl the homogenization temperatures suggest a hypogene hydrothermal origin for the willemite concentrations the geochemistry of fluid inclusion leachates suggests that the hydrothermal fluids were brines derived from highly evaporated seawater precise age constraints are currently lacking for the lusaka area deposits though the deposits are not deformed indicating that they post date the lufilian orogeny 520 ma the possibility of precursor ores exists the gahnite franklinite willemite deposits could have been derived from a metamorphosed primary sulphide or even nonsulphide concentration that has subsequently been completely destroyed however there is no real evidence of such a primary source for the willemite mineral association the lusaka zinc ores may have been produced by an extensive hydrothermal system with fluids discharging along basinal fracture zones controlled by the pre pan african rifting stage a paragenesis similar to that of the lusaka prospects has been proposed to be a vector towards massive sulphide ores in several parts of the world therefore it is possible that these small willemite showings in zambia may be part of a much bigger and still unexplored zinc province

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

reequilibration of fluid inclusions in low temperature calcium carbonate cement calcium carbonate cements precipitated in low temperature near surface vadose environments contain fluid inclusions of variable vapor to liquid ratios that yield variable homogenization temperatures cements precipitated in low temperature phreatic environments contain one phase all liquid fluid inclusions neomorphism of unstable calcium carbonate phases may cause reequilibration of fluid inclusions stable calcium carbonate cements of low temperature origin which have been deeply buried contain fluid inclusions of variable homogenization temperature and variable salt composition most inclusion fluids are not representative of the fluids present during cement growth and are more indicative of burial pore fluids therefore low temperature fluid inclusions probably reequilibrate with burial fluids during progressive burial reequilibration is likely caused by high internal pressures in inclusions which result in hydrofracturing the resulting fluid inclusions population could contain a nearly complete record of burial fluids in which a particular rock has been bathed

mineralogy paragenesis and mineral zoning of the west fork mine virburnum trend southeast missouri asarco9s west fork mine the newest mine in the viburnum trend began production in 1985 and reached full production in 1988 although significant mineralogical and paragenetic studies have been performed on the other ore deposits of the viburnum trend no attempt has been made to correlate these studies with mineral zoning patterns the distinct mineral zoning present at the west fork mine provides a unique opportunity to relate mineral paragenetic sequence to mineral zoning the present study concentrated on the main ore horizon m bed which is composed of four north south trending linear metal zones which together consist of an inner zone of colloform iron and zinc sulfides bordered outward by zones of disseminated iron and zinc sulfides and fringed by outermost zones of galena that also overprint the inner zone principal metallic sulfides in the ores at the west fork mine are pbs galena zns sphalerite and wurtzite and fes 2 pyrite and marcasite paragenetic sequence and metal zoning show that the ore forming fluids migrated initially through the inner zone where zns was deposited and subsequently traveled outward depositing fes 2 followed by pbs the presence of rapidly deposited colloform zns and fes 2 and bladed galena in the central fe zn zone indicates that ore fluids which deposited the sulfides in the central zone were more saturated with respect to metal sulfides outer layers of the colloform zns bodies in the fe zn zone are characterized by an abundant complex intergrowth of sphalerite and wurtzite schalenblende x ray analysis indicates that the wurtzite within the schalenblende inverted to sphalerite thereby producing pseudomorphous sphalerite after wurtzite because wurtzite is the sulfur deficient form of zns it can be postulated that sulfur fugacities were low at the end of the main stage of zinc sulfide deposition subsequent increase in sulfur fugacity during the main stage of iron deposition caused inversion of wurtzite to sphalerite interlayered pyrite and marcasite are evidence that the ore fluids during the main stage of deposition were oscillating above and below a ph of 5 minerals subsequently deposited in the outer zones were precipitated more slowly from fluids that were relatively depleted in metals the distinct differences in ore character between the metal zones may well be a result of multiple fluids separate fluids may have been channelized each fluid representing a zone or may have mixed through time the results of this study are not compatible with ore deposition from a single fluid source

bedding parallel fibrous veins beef and cone in cone worldwide occurrence and possible significance in terms of fluid overpressure hydrocarbon generation and mineralization abstract bedding parallel fibrous veins are common worldwide in sedimentary basins especially within strata of low permeability the term beef refers to bedding parallel veins of fibrous minerals where the fibres are mutually parallel and have formed quasi vertically more complex on a smaller scale are cone in cone structures yet these are also common within bedding parallel veins for both beef and cone in cone we have compiled a worldwide catalogue 157 localities typically the veins consist mainly of white gangue minerals for example calcite gypsum or quartz but may also contain accessory minerals of economic interest for example bitumen sulphides emerald pitchblende or gold fluid inclusions may contain oil or gas calcite beef 110 localities is common in organic rich shale of marine carbonate origin especially of 1 cambrian ordovician 2 devonian carboniferous 3 early jurassic or 4 cretaceous to palaeogene ages gypsum beef 30 localities is common in evaporitic or lacustrine strata of continental origin especially of triassic or neogene ages quartz beef 17 localities is common within meta turbidite sequences especially of ordovician or proterozoic ages because these modal ages seem to reflect climatic controls we infer that the fibre forming mineral species have not travelled far vertically the same conclusion holds for accessory minerals typical temperatures of formation are 1 up to 60 c for gypsum beef 2 70 c to 120 c for calcite beef and 3 200 c to 350 c for quartz beef hydrocarbon bearing calcite beef may be a good indicator of a petroleum system in which oil or gas migrate together with aqueous solutions we argue that beef and cone in cone layers result from tensile fracturing and vertical dilation coeval with fibre growth possible causes are either 1 force of crystallization or 2 seepage forces due to fluid overpressure for layers that form at depths of several km fluid overpressure is the more likely cause

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

middle cambrian to permian subduction related accretionary orogenesis of northern xinjiang nw china implications for the tectonic evolution of central asia abstract middle cambrian to permian subduction related records are widely distributed in northern xinjiang which can be grouped into the chinese altay east junggar eastern tien shan west junggar yili and tarim domains by integrating paleogeographic and geological data we suppose that the chinese altay east junggar eastern tien shan domain was more closely located to siberia while the west junggar and yili domains occupied an intermediate position near the kazakhstan block in the early paleozoic paleoasian ocean distribution of andean type magmatic arcs island arcs accretionary wedges ophiolitic slices and or microcontinents shows an archipelago paleogeography forming a huge accretionary active margin sequences the tarim domain was on the opposite side of the early paleozoic paleoasian ocean remaining passive margin these tectonic units drifted northwards and approached the southern active margin of the siberian craton in the late paleozoic leading to termination of the paleoasian ocean and formation of a complicated orogenic collage between siberian craton and the tarim block between the end permian and triassic these multiple accretion processes significantly contributed to the lateral growth of central asia

carbonate hosted nonsulfide pb zn deposits of the quesnel lake district british columbia canada carbonate hosted pb and zn mineralization in the quesnel lake district flipper creek dolomite flats main gunn and que deposits forms an 8 km long belt in metasedimentary and metavolcanic rocks of the cariboo terrane a displaced piece of the ancestral north american margin the mineralization is strata bound dolostone hosted and consists of pb and zn bearing sulfide and nonsulfide minerals the supergene nonsulfides mimic the morphology of preexisting sulfides occurring as disseminations veins and fracture fillings and irregular replacement zones in the neoproterozoic to upper cambrian dolostones of the cunningham formation the main sulfides are galena sphalerite and minor amounts of pyrite which have been partially or completely transformed into nonsulfide minerals by supergene processes the main nonsulfide minerals are smithsonite znco 3 and hemimorphite zn 4 si 2 o 7 oh 2 h 2 o with variable amounts of cerussite pbco 3 anglesite pbso 4 hydrozincite zn 5 co 3 2 oh 6 and iron oxides the 18 o vsmow 13 c vpdb and 87 sr 86 sr values of the nonmineralized and mineralized dolostones are indistinguishable and consistent with the composition of carbonates that underwent postdepositional recrystallization and isotopic reequilibration during deep burial fluid rock interaction the 18 o vsmow and 13 c vpdb values range from 17 4 to 20 and 0 8 to 1 4 respectively and 87 sr 86 sr values are above 0 71048 carbon and oxygen ratios of smithsonite show a limited range of values which are slightly lower than those of the host dolostones with 18 o vsmow values of 16 9 to 17 8 and 13 c vpdb values of 1 6 to 0 9 the carbon isotope values suggest that the host dolostones are the carbon source for smithsonite the 18 o smithsonite values are lower than typical values for nonsulfides formed under warm humid and arid to hyperarid climates elsewhere in the world the precipitation temperatures derived from the isotope data for the quesnel lake smithsonite vary between 9 and 33 c this temperature range is favorable for supergene smithsonite formation for most deposits of the quesnel lake district examined in this study close spatial relationships including textures and morphological similarities between sulfide and nonsulfide zones coexisting sulfide nonsulfide assemblages and stable oxygen and carbon isotope data suggest that the oxidation of sulfides occurred after the laramide orogeny

germanium geochemistry and mineralogy abstract germanium is enriched in the following geologic environments 1 1 iron meteorites and terrestrial iron nickel 2 2 sulfide ore deposits particularly those hosted by sedimentary rocks 3 3 iron oxide deposits 4 4 oxidized zones of ge bearing sulfide deposits 5 5 pegmatites greisens and skarns and 6 6 coal and lignitized wood in silicate melts ge is highly siderophile in the presence of native iron nickel otherwise it is highly lithophile among silicate minerals ge is concentrated in those having less polymerized silicate tetrahedra such as olivine and topaz in deposits formed from hydrothermal solutions ge tends to be enriched mostly in either sulfides or in fluorine bearing phases it is thus concentrated both in some hydrothermal sulfide deposits and in pegmatites greisens and skarns in sulfide deposits that formed from solutions having low to moderate sulfur activity ge is concentrated in sphalerite in amounts up to 3000 ppm sulfide deposits that formed from solutions having higher sulfur activity allowed ge to either form its own sulfides particularly with cu or to substitute for as sn or other metals in sulfosalts the ge in hydrothermal fluids probably derives from enrichment during the fractional crystallization of igneous fluids or is due to the incorporation of ge from the country rocks particularly from those containing organic material germanium bonds to lignin derivative organic compounds that are found in peat and lignite accounting for its common concentration in coals and related organic material germanium is precipitated from water together with iron hydroxide accounting for its concentration in some sedimentary and supergene iron oxide deposits it also is able to substitute for fe in magnetite in a variety of geologic environments in the oxidized zone of ge bearing sulfide deposits ge is concentrated in oxides hydroxides and hydroxy sulfates sometimes forming its own minerals it is particularly enriched in some iron and manganese bearing oxides and hydroxides including goethite up to 5300 ppm and hematite up to 7000 ppm

fracturing and fluid flow in a sub decollement sandstone or a leak in the basement crack seal texture within fracture cements in the triassic el alamar formation ne mexico shows that the fractures opened during precipitation of quartz cements later overlapping calcite cements further occluded pore space previous workers defined four systematic fracture sets a oldest to d youngest with relative timing constrained by crosscutting relationships quartz fluid inclusion homogenization temperatures are higher within set b 148 20 c than in set c 105 12 c these data and previous burial history modelling are consistent with set c forming during exhumation fluid inclusions in set c quartz have higher salinity than those in set b 22 9 v 14 2 wt nacl equivalent respectively and set c quartz cement is more enriched in 18 o 20 2 v 18 7 vsmow under most assumptions about the true temperature during fracture opening the burial duration the amount of cement precipitated and fluid flow patterns it appears that the fracture fluid became depleted in 18 o and enriched in 13 c this isotopic evolution combined with increasing salinity suggests that throughout fracture opening there was a gravity driven influx of fluid from upsection jurassic evaporites which form a regional decollement fracture opening amid downward fluid motion suggests that fracturing was driven by external stresses such as tectonic stretching or unloading rather than increases in fluid pressure

new insights into the genesis of willemite zn2sio4 from zinc nonsulfide deposits through trace elements and oxygen isotope geochemistry abstract willemite is a common mineral mainly occurring in the so called hypogene nonsulfide zn pb mineralizations deposited from what have been interpreted as high temperature hydrothermal or metamorphic fluids considering that a comprehensive compilation of geochemical data on willemite is currently lacking the aim of this study is to provide evidence for specific geochemical signatures of willemites with contrasting genesis for this work we analyzed a range of specimens taken from the mineralogy collection of the natural history museum in london the specimens are from several deposit types 1 stratiform hypogene deposits franklin and sterling hill usa 2 structurally controlled hypogene deposits star zinc excelsior surprise lusaka area zambia kabwe zambia berg aukas and tsumeb namibia 3 supposedly supergene deposits mumbwa area zambia and altenberg vieille montagne belgium textures and structures of the willemite bearing specimens were initially investigated by scanning electron microscope energy dispersive analysis sem eds trace element concentrations were later measured in situ on selected willemite crystals by laser ablation la icp ms statistical analyses principal component analysis pca multivariate analysis of variance post hoc tukey hsd test conducted on the data obtained by la icp ms showed that statistically significant differences exist between willemites derived from the various considered deposits the high t stratiform willemite samples contain significant amounts of mn and mg and result statistically different from the all the other analyzed specimens structurally controlled hypogene willemites characterized by lower mn and mg concentrations and by higher pb cu as b ge contents are statistically inhomogeneous both at the district and at the deposit scale high t willemite from the lusaka area contains more mn than the low t hydrothermal deposits that show instead higher concentrations of other elements e g pb cu ge among the willemite generations associated with the supposedly supergene mineralizations only the mumbwa willemites form a cluster that is significantly displaced from the other deposits the 18o isotopic compositions match the mentioned trace element distribution willemites with similar origin are characterized by similar oxygen isotopic ratios on the basis of the present work willemite types can be the distinguished as follows high t willemites are characterized by high mn mg concentrations between 100 ppm to unit wt and by low and positive 18o compositions around 3 to 5 v smow low t hydrothermal willemites show mn mg concentrations below 50 ppm associated with high pb cu as ge concentrations above 100 ppm they also have high and positive 18o compositions between 9 and 15 v smow willemites derived from either cold or warm meteoric fluids have possibly low concentrations of the above elements and negative 18o compositions 3 and 9 v smow the supposedly supergene altenberg vieille montaigne willemite having low mn mg and high pb ge as concentrations and showing 18o compositions between 11 and 12 5 should have formed from low t hydrothermal fluids

the carbonate hosted mvt morro agudo zn pb deposit central brazil abstract morro agudo is the largest known mississippi valley type deposit mvt in brazil with original estimated reserves of 18 3 mt 5 08 zn and 1 75 pb of which 3 92 mt 4 zn and 1 45 pb remained in 2017 in spite of having been mined for more than 30 years a comprehensive study of morro agudo metallogenic features was still lacking in the international literature this deposit is hosted within the 250 km long vazante sequence a n s trending mesoproterozoic neoproterozoic carbonate dominant sedimentary sequence thrusted over neoproterozoic rocks during the brasiliano orogeny sulfide bodies in morro agudo are restricted to the morro do calcario formation as a 1300 m long ore envelope averaging 9 m thick comprising stacked up stratabound orebodies that thin out down dip and are limited to the ne by the main fault five orebodies are described a breccia hosted orebody ghi b sphalerite rich doloarenite orebody jk c galena rich doloarenite orebody l d lower replaced stratiform orebody m e upper replaced stratiform orebody n comparison of these orebodies reveals mineral zonation patterns and variations in zn pb grade consistent with sulfide textural relationships these relationships indicate five mineralization styles that provide spatial and timing constraints for morro agudo a stratiform sulfides b massive ore c laminated ore d breccia hosted ore e late stage fracture filling ore carbon and oxygen stable isotope data for each mineralization style shows a pattern of mixing of signatures of mineralizing fluids and barren host rocks and supports our interpretation that these mineralization styles occurred in more than one stage of ore formation the ore was later offset by a set of strike slip normal faults associated with compression related to the brasiliano orogeny of which the main fault falha principal is the most important because it limits the deposit to the ne previous interpretation of upper orebodies as part of a sedex system is revisited in the present work in our model data used to support a syngenetic origin for morro agudo such as synsedimentary structures pb isotopes in sulfides and fluid inclusion data indicate a younger barren generation of sulfides deposited long before zn pb mineralization during sedimentation of the morro do calcario formation in the mesoproterozoic these early sulfides were preserved in cherts and as massive pyrite beds that were later overprinted by sphalerite and galena overprinting thus mixed meso and neoproterozoic pb isotopic sources and led to a long standing argument over whether the morro agudo ores were syngenetic or epigenetic this interpretation is also in agreement with the current classification of the vazante sequence as containing imbricated neoproterozoic and mesoproterozoic sedimentary rocks based on our work of sulfide textural relationships stable isotopes whole rock geochemistry and previously published data we argue that morro agudo ores formed by at least two stages of percolation of mildly hot hydrothermal fluid in the neoproterozoic the primary reason for sulfide precipitation is interpreted to be mixing of mildly hot metal bearing basinal brines and seawater connate waters

the calamines and the others the great family of supergene nonsulfide zinc ores abstract nonsulfides is a term which comprises a series of oxidized zn pb ore minerals it has also been used to define a special deposit type mainly considered as derived from the weathering of zn pb sulfide concentrations however nonsulfide zinc deposits have been distinguished between supergene and hypogene according to their mineralogy geological characteristics and genetic setting the supergene deposits formed by weathering and oxidation at ambient temperatures whereas the hypogene ones are considered hydrothermal or associated with metamorphic processes on primary sulfide ores in this review paper a comparison between a number of several nonsulfide deposits has been carried out typical calamines peculiar calamines and others the whole group comprises deposits of typical supergene origin mixed supergene hypogene mineralizations and oxidized concentrations characterized by different metals only locally associated with zinc the zn pb nonsulfide concentrations hosted in carbonate rocks which are mainly attributed to wall rock replacement and direct replacement supergene processes are the typical calamines liege district belgium iglesias district italy silesia cracow district poland peculiar calamine deposits are those mineralizations that have been generally considered as supergene but which are instead genetically related at least partly to hypogene processes e g angouran iran jabali yemen though mineralogically and texturally similar to supergene nonsulfide deposits the others are prevailingly supergene nonsulfide zinc deposits not hosted in carbonate rocks skorpion namibia yanque peru or characterized by other metals as main commodities like lead magellan australia silver sierra mojada mexico wonawinta australia or vanadium otavi mountainland namibia minerals of current economic importance in most calamine deposits are smithsonite hydrozincite and cerussite this mineralogical association is generally simple but when the calamines are dolomite hosted one of the consequences of the wall rock replacement process is the generation of a series of economically useless zn and mg bearing mixed carbonate phases secondary deposits hosted in silicatic sedimentary or volcanic rocks mainly contain hemimorphite and or sauconite lead ag and v rich nonsulfide ores are characterized by a more complex mineralogical association mixed pb carbonates pb sulfates pb phosphates pb arsenates various ag sulfosalts and zn pb cu vanadates carbon and oxygen stable isotope studies allow distinguishing between supergene and hypogene nonsulfide deposits evaluating the effects of oxidative heating and even gaining indirect paleoclimatic information the oxygen isotope variation of the individual carbonate minerals within a deposit is relatively small indicating constant formation temperatures and a single meteoric fluid source carbon isotope values are highly variable thus suggesting several isotopically distinct carbon sources periods of paleoclimatic switch overs from seasonally humid arid to hyperarid have been considered as the most favorable conditions for the formation and preservation of supergene nonsulfide deposits however while several recent nonsulfide deposits throughout the world are positioned between 15 and 45 n latitude thus pointing to a warm and humid weathering climate others have been deposited in sub arctic regions the economic value of the nonsulfide zn pb ag v ores is highly variable because more than in the case of metallic sulfide deposits it resides not only on the geological setting but also on their mineralogy that can directly influence processing and metallurgy

the hakkari nonsulfide zn pb deposit in the context of other nonsulfide zn pb deposits in the tethyan metallogenic belt of turkey abstract the hakkari nonsulfide zinc deposit is situated close to the southeastern border of turkey here both sulfide and nonsulfide zn pb ores are hosted in carbonate rocks of the jurassic cudi group with features typical of carbonate hosted supergene nonsulfide zinc mineralization the regional strike extent of the mineralized district is at least 60 km the age of the supergene deposit has not been determined but it is probable that the main weathering happened during upper tertiary possibly between upper miocene and lower pliocene the hakkari mineralization can be compared to other carbonate hosted zn pb deposits in turkey and an interpretation made of its geological setting the zinc mineral association at hakkari typically comprises smithsonite and hemimorphite which apparently replace both sulfide minerals and carbonate host rock two generations of smithsonite are present the first is relatively massive the second occurs as concretions in cavities as a final filling of remnant porosity some zinc is also hosted within fe mn hydr oxides lead is present in cerussite but also as partially oxidized galena lead can also occur in mn hydr oxides max 30 pbo the features of the supergene mineralization suggest that the hakkari deposit belongs both to the direct replacement and the wall rock replacement types of nonsulfide ores mineralization varies in style from tabular bodies of variable thickness

carbonate hosted nonsulphide zn pb mineralization of southern british columbia canada many carbonate hosted sulphide deposits in the salmo district of southern british columbia have near surface zn and pb bearing iron oxide rich gossans the gossans formed when carbonate hosted base metal sulphides were subjected to intense supergene weathering processes and metals were liberated by the oxidation of sulphide minerals two types of supergene carbonate hosted nonsulphide deposits direct replacement red ore and wallrock replacement white ore are present in the salmo district the direct replacement deposits formed by the oxidation of primary sulphides the base metals passed into solution and were redistributed and trapped within the space occupied by the oxidized portion of the sulphide protore depending on the extent of replacement of the sulphides by zn pb and fe bearing oxides silicates carbonates and phosphates the resulting ore can be called mixed sulphides and nonsulphides or simply nonsulphide the wallrock replacement deposits formed when base metals liberated by the oxidation of sulphides were transported by circulating supergene solutions down and or away from the sulphides to form wallrock replacement deposits the direct replacement nonsulphide zones of the salmo district overlay the sulphide bodies in which they replaced the sulphides and carbonates forming large irregular replacement masses encrustations and open space fillings they consist predominantly of hematite goethite hemimorphite zn4si2o7 oh 2 h2o minor hydrozincite zn5 co3 2 oh 6 cerussite pbco3 and traces of willemite zn2sio4 the wallrock replacement zones consist mainly of hemimorphite with local occurrences of iron oxides hopeite zn3 po4 2 4h2o and tarbuttite zn2 po4 oh no remnants of sulphides were observed in the replacement zones the salmo nonsulphide deposits were formed by prolonged weathering of mississippi valley type mvt mineralization that underwent dissolution and oxidation of the pyrite sphalerite and galena protore the weathering also leached out highly mobile zn less mobile pb and left behind the iron oxides precipitating zn and pb silicates within the protore or at a distance from the protore

beef veins and thrust detachments in early cretaceous source rocks foothills of magallanes austral basin southern chile and argentina structural evidence for fluid overpressure during hydrocarbon maturation abstract we describe 1 bedding parallel veins of fibrous calcite beef and 2 thrust detachments which we believe provide good evidence for fluid overpressure in source rocks for petroleum our examples are from the surface or subsurface of the magallanes austral basin which lies at the southern tip of south america there the best source rocks for petroleum are of early cretaceous age in the central parts of the basin these source rocks have become overmature but at the eastern edge onshore and offshore they are today either immature or in the oil window in tierra del fuego the foothills of the andes consist mainly of sedimentary rocks which have undergone thin skinned thrusting in the vicuna area chile early cretaceous source rocks have reached the surface above thrust detachments which are visible on seismic data and well data at the surface we have found calcite beef containing hydrocarbons solid and or fluid in the rio jackson and vicuna formations which have reached the wet gas window in the rio gallegos area argentina the source rocks have not reached the surface but seismic and well data provide good evidence for thin skinned thrusting above flat lying detachments in early cretaceous source rock where it is in the early oil window in contrast there is little or no deformation where the source rock is still immature thus the deformation front coincides with the maturity front next to the central parts of the basin where the source rocks have reached the surface within the andes proper they have undergone low grade metamorphism within these source rocks we have found beef veins but of quartz not calcite to the east within the foreland basin seismic and well data provide evidence for a few compressional structures including thin skinned detachments in the deeply buried source rock finally in the northern part of the basin santa cruz province argentina where it is shallower the source rocks have reached the surface in the foothills above a series of back thrusts at lago san martin the source rocks have reached the oil window and they again contain calcite beef in conclusion where we have examined early cretaceous source rocks at the surface they contain either calcite beef if they have reached the late oil window or wet gas window or quartz beef if they are overmature independent evidence for overpressure in the form of source rock detachments comes from subsurface data especially at the southern end of the basin where the source rocks are not overmature and deformation is relatively intense thus we argue that hydrocarbon generation has led to overpressure as a result of chemical compaction and load transfer or volume changes or both

crystallization pressure versus crack seal as the mechanism for banded veins banded fibrous veins are often pointed to as evidence for episodic crack opening driven by oscillations in fluid pressure or bulk strain advances in understanding the geochemistry of precipitation data on veins and experiments suggest that pressure due to growing crystals may be an alternate explanation for many observations on these types of veins we propose that some veins originate at sites of precipitation and then propagate due to the pressure exerted by the crystal growth as materials precipitate the vein walls are pushed apart the resulting veins have shapes typical for mode i cracks except that mechanically crystallization pressure replaces the role of internal fluid pressure in their propagation a nonzero remote differential stress serves to align veins mechanical and geochemical considerations suggest that this process will be most important in fine grained rocks such as greenschist grade pelites where diffusion from sites of dissolution rather than advection is the dominant mass transport process veins may owe their orientation to tectonism but their initiation and growth are due to processes that supersaturate the pore fluid veins formed by this mechanism involve cracking to the extent that precipitation forces an original flaw to extend during precipitation cyclic quartz mica bands may indicate geochemical self organization at the vein wall driven by pressure solution enhanced supersaturation in the pore fluid and nonlinear precipitation kinetics at the vein wall

behavior of critical metals in metamorphosed pb zn ore deposits example from the pyrenean axial zone rare metals ge ga in cd are key resources for the development of green technologies and are commonly found as trace elements in base metal mineral deposits many of these deposits are in orogenic belts and the impact of recrystallization on rare metal content and distribution in sphalerite needs to be evaluated based on laser ablation inductively coupled plasma mass spectrometry la icp ms analyses and micro imaging techniques such as laser induced breakdown spectroscopy libs and electron backscattered diffraction ebsd we investigate the minor and trace element composition related to sphalerite texture for three types of mineralization from the pyrenean axial zone paz vein mineralization type 2b appears significantly enriched in ge and ga compared to disseminated and stratabound mineralizations type 1 and type 2a respectively in vein mineralization the partial recrystallization induced by deformation led to the remobilization of ge ga and cu from the sphalerite crystal lattice into accessory minerals we propose that the association of intragranular diffusion and fluid rock reaction were likely responsible for the formation of patchy oscillatory zoning in sphalerite and the crystallization of ge rich accessory minerals chemical and textural heterogeneity is common in sphalerite from various world class deposits and a full understanding of these heterogeneities is now crucial to assess the rare metal potential and associated extraction processes of deformed base metal ores

on the geological availability of germanium based on a detailed statistical analysis of chemical data published in the scientific literature estimates were made of the minimum amounts of recoverable ge contained within sulphidic zinc ores and coals given current processing technologies it is expected that at least 119 kt 7 kt in zinc ores and 112 kt in coal of recoverable germanium exist within proven reserves at present stage of knowledge at grades in excess of 100 ppm in sphalerite and 200 ppm in coal while at least 440 kt 50 kt in zinc ores and 390 kt in coal should become recoverable in the future being associated to coal reserves at 8 200 ppm ge and zinc resources containing in excess of 100 ppm ge in sphalerite mississippi valley type mvt deposits are expected to be the most important hosts of germanium rich sphalerite while both brown and hard coals are expected to be equally important as hosts of germanium the approach taken in this publication shows that reliable minimum estimates for the availability of by product metals lacking suitable reserve resource data may be attained by using robust statistical methods and geochemical data published in the scientific literature

the petroleum generation potential and effective oil window of humic coals related to coal composition and age abstract a worldwide data set of more than 500 humic coals from the major coal forming geological periods has been used to analyse the evolution in the remaining hydrogen index hi and total quality index qi generation potentials with increasing thermal maturity and the effective oil window oil expulsion window all samples describe hi and qi bands that are broad at low maturities and that gradually narrow with increasing maturity the oil generation potential is completely exhausted at a vitrinite reflectance of 2 0 2 2 ro or tmax of 500 510 c the initial large variation in the generation potential is related to the original depositional conditions particularly the degree of marine influence and the formation of hydrogen enriched vitrinite as suggested by increased sulphur and hydrogen contents during initial thermal maturation the hi increases to a maximum value himax similarly qi increases to a maximum value qimax this increase in hi and qi is related to the formation of an additional generation potential in the coal structure the decline in qi with further maturation is indicating onset of initial oil expulsion which precedes efficient expulsion liquid petroleum generation from humic coals is thus a complex three phase process i onset of petroleum generation ii petroleum build up in the coal and iii initial oil expulsion followed by efficient oil expulsion corresponding to the effective oil window efficient oil expulsion is indicated by a decline in the bitumen index bi when plotted against vitrinite reflectance or tmax this means that in humic coals the vitrinite reflectance or tmax values at which onset of petroleum generation occurs cannot be used to establish the start of the effective oil window the start of the effective oil window occurs within the vitrinite reflectance range 0 85 1 05 ro or tmax range 440 455 c and the oil window extends to 1 5 2 0 ro or 470 510 c for general use an effective oil window is proposed to occur from 0 85 to 1 7 ro or from 440 to 490 c specific ranges for himax and the effective oil window can be defined for cenozoic jurassic permian and carboniferous coals cenozoic coals reach the highest himax values 220 370 mg hc g toc and for the most oil prone cenozoic coals the effective oil window may possibly range from 0 65 to 2 0 ro or 430 to 510 c in contrast the most oil prone jurassic permian and carboniferous coals reach the expulsion threshold at a vitrinite reflectance of 0 85 0 9 ro or tmax of 440 445 c

redistribution of germanium during dynamic recrystallization of sphalerite rare metals are essential to the development of the green technologies that are at the core of low carbon societies in nature these metals are frequently present in trace amounts scattered in base metal ore deposits but the physico chemical processes that are responsible for their concentration into strategic minerals are still poorly understood based on laser induced breakdown spectroscopy libs coupled with electron backscattered diffraction ebsd analysis this study shows that plastic deformation and subsequent syntectonic recrystallization of sphalerite zinc sulfide zns led to the spatial redistribution of germanium ge from a background level of a few hundreds of parts per million in undeformed primary sphalerite to tens of weight percent in neocrystallized ge minerals during dynamic recrystallization ge is likely released from the crystal lattice of parent sphalerite and subsequently concentrated in ge minerals leaving behind a ge depleted recrystallized sphalerite matrix identifying how rare metals concentrate through deformation and syntectonic recrystallization at the mineral scale is essential to understand the spatial redistribution and localization at the deposit scale this study highlights the importance of coupling in situ chemical mapping analysis with macro and microstructural characterization when targeting rare metals in deformed ore

subsurface fracture analysis and determination of in situ stress direction using fmi logs an example from the santonian carbonates ilam formation in the abadan plain iran abstract the relationship between the present day stress field and natural fractures can have significant implications for subsurface fluid flow in particular fractures that are aligned in orientations favourable for reactivation by either shear or tensile failure in the in situ stress field often exhibit higher hydraulic conductivities the ilam formation of southwestern iran is an important hydrocarbon reservoir containing numerous natural fractures however little is known about the state of stress in this region or any of iran s petroleum provinces we conducted analysis of the present day maximum horizontal stress orientation and the density orientation and hydraulic conductivity of natural fractures in the ilam carbonates using high resolution formation micro imager resistivity logs in two wells a total of 51 breakouts with an overall length of 215 m were observed in the two wells indicating a maximum horizontal stress orientation of 68 n 7 6 in well a and 58 n 6 3 in well b furthermore the wellbore derived stress orientations determined herein are consistent with those inferred from nearby earthquake focal mechanism solutions indicating that stresses in the sedimentary cover are linked to the resistance forces generated by arabia eurasia collision furthermore the correlation between stress orientations estimated from earthquake focal mechanism solutions and breakouts indicates that focal mechanism solution data which is often considered to be unreliable for stress field analysis near transform margins may provide reliable information on the stress orientation near continental collision zones the image log data also reveals three sets of open and presumably hydraulically conductive fractures with strikes of i 160 170 n ii 110 140 n and iii 070 080 n fracture set iii is consistent with being formed and open in the present day stress field however fracture sets i and ii strike at a high angle to the present day maximum horizontal stress and are interpreted herein to be the result of either pre or syn folding related forces the observation that different sets of open fractures in the field can be either sensitive or insensitive to the present day stress is critical for improving hydrocarbon recovery

oxygen isotope fractionation in zinc oxides and implications for zinc mineralization in the sterling hill deposit usa oxygen isotope fractionation in the zinc oxides has been calculated by means of the modified increment method the results suggest that zincite is slightly enriched in 18o relative to the franklinite of the spinel type structure but considerably depleted in 18o relative to the franklinite of the inverse spinel type structure the zinc oxides are significantly depleted in 18o relative to water under hydrothermal and metamorphic conditions the oxygen isotope analyses of mineral pairs including the zinc oxides and the common gangue minerals such as calcite and quartz can constitute a sensitive isotope geothermometer application of oxygen isotope geothermometry to natural assemblages is attempted for the calcite zinc ore mineral pairs from the sterling hill deposit in usa the results indicate that the temperature of the zinc mineralization may be in the range from 410 to 630 c and thus lower than the metamorphic temperatures of granulite facies a metamorphic fluid could have been involved in the formation of the zinc ore minerals franklinite would structurally be an inverse spinel in the infancy of its formation and thus could have originally evolved from zn2 substitution to fe2 of magnetite at the high temperatures

a chlorite solid solution geothermometer the los azufres mexico geothermal system chlorite constitutes a major hydrothermal alteration product of metamorphism of andesites in the active geothermal system of los azufres mexico electron microprobe analyses performed on a set of crystals from each sample show wide variations in composition correlation coefficients among chemical constituents were calculated it is shown that the tetrahedral charge is positively correlated with the octahedral vacancy and negatively with the iron content and there is almost no correlation with the octahedral aluminium and magnesium content a procedure is proposed to select end members and substitution vectors and to give a general formula for these chlorites

petedunnite caznsi 2 o 6 a new zinc clinopyroxene from franklin new jersey and phase equilibria for zincian pyroxenes

correlation of homogenization temperatures of accessory minerals from sphalerite bearing deposits and ga ge model temperatures abstract the ga ge ratio in sphalerite can be related to the temperature of the initial fluids from which the sphalerite grew after mixing with various amounts of fluids from different origins the extent of mixing of the two fluids can be qualitatively derived from the pressure corrected homogenization temperature which represents the temperature after mixing of the fluids and the ga ge temperatures of sphalerite which indicates the temperature of the hot metal bearing initial fluids different temperature information strongly indicate that mixing of fluids did occur prior to sulphide formation

palaeoseismic events recorded in archaean gold quartz vein networks val d or abitibi quebec canada archaean gold quartz vein deposits are commonly hosted in high angle reverse shear zones and are interpreted to have formed in a regime of horizontal compression and high fluid pressure environment this paper presents the results of a combined structural and fluid inclusion study on three gold quartz vein deposits of the val d or area abitibi quebec consisting of subhorizontal extensional veins and e w steeply dipping shear veins crack seal structures tourmaline fibres stretched quartz crystals and open space filling textures indicate that the subhorizontal veins formed by hydraulic fracturing under supralithostatic fluid pressure co2 rich and h2o nacl fluid inclusions interpreted as two coexisting immiscible fluids occur typically in microcracks of different orientations interpreted to have formed in the 1 2 plane horizontal co2 rich fluid inclusion planes are contemporaneous with the opening of these veins 3 vertical vertical h2o nacl fluid inclusion planes as well as some microstructures such as deformed minerals indicate that the same extensional veins have experienced episodic vertical shortening 3 horizontal alternating with the opening events deformation and slip opening also occurred in shear veins in which preferred orientation of fluid inclusion planes is not clear except that the h2o nacl fluid inclusion planes tend to be oriented at high angles to the slip direction the successive opening and collapse events in subhorizontal extensional veins are correlated with deformation and slip opening events in shear veins respectively and are attributed to cyclic fluid pressure fluctuations in the system they are thus consistent with the fault valve model sudden drop in fluid pressure from supralithostatic to lower values induces fluid unmixing and occurs immediately post failure following seismic rupturing along the shear zone sealing of the shear veins allows the fluid pressure to build up again and the subhorizontal extensional veins to form the intermittent change in orientation of 3 in horizontal extensional veins is ascribed to a total shear stress release along the neighbouring shear veins after rupture the complex internal structures of the gold quartz veins provide an excellent record of archaean seismic activity along high angle reverse faults

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

zinc germanium ores of the tres marias mine chihuahua mexico the tres marias carbonate hosted zn ge deposit in chihuahua mexico contains sphalerite with the highest average ge 960 ppm and willemite with the highest reported ge contents of mississippi valley type mvt deposits worldwide this has prompted current exploration efforts to focus on the deposit as a high grade source of germanium the sulfide rich ore type 125 000 t at 20 zn and 250 g t ge contains fe rich botryoidal sphalerite type i associated with solid hydrocarbons this type exhibits distinctive intimately intergrown lamellar texture of high fe sphalerite average 9 9 wt fe and 800 ppm ge and a somewhat less fe rich sphalerite phase average 5 5 wt fe and 470 ppm ge reddish brown banded sphalerite type ii average 5 7 wt fe and 1 320 ppm ge is subordinately followed by galena and pyrite the sulfide poor oxidized zinc ore up to 50 wt zn 250 to 300 ppm ge is a fine grained often friable alteration product of the sulfide ore and associated limestone and breccia host while some areas are dominated by carbonates and sulfates others are enriched in silicates such as hemimorphite and willemite the gangue assemblage includes goethite hematite and amorphous silica or quartz minor wulfenite greenockite cinnabar and descloizite also occur willemite occurs as interstitial replacement of sphalerite and fracture fillings in the oxidized ore and can be unusually rich in pb up to 2 0 wt and ge up to 4 000 ppm oscillatory zonation reflects trace element incorporation into willemite from the oxidation of primary ge bearing sphalerite and galena by siliceous aqueous fluids the tres marias deposit has hybrid characteristics consisting of a primary low temperature mvt ge rich zn pb sulfide ore body overprinted by ge rich hemimorphite willemite and fe oxide mineralization

natural fractures in carbonate rich tight oil reservoirs from the permian lucaogou formation southern junggar basin nw china insights from fluid inclusion microthermometry and isotopic geochemistry abstract natural fractures within tight reservoirs form preferred pathways for hydrocarbon charge and provide space for hydrocarbon preservation however the mechanism that controls the formation of natural fractures in tight reservoirs remains controversial this study investigates natural fractures within carbonate rich tight oil reservoirs from the permian lucaogou formation in the jimsar sag southern junggar basin drill core and image log observations show that fractures in the lucaogou formation can be categorized into two types bedding parallel fractures and inclined fractures the carbon and oxygen isotopic compositions of calcite cements in the bedding parallel fractures range from 0 6 to 6 0 13cpdb and from 15 5 to 8 9 18opdb respectively which are similar to those of the organic rich host rock this suggests that the bedding parallel fractures likely formed in a closed fluid system fluid inclusion analyses of the aqueous inclusions in calcite cements from the bedding parallel fractures record homogenization temperatures of 91 5 c 130 4 c indicating that they formed at the cretaceous to cenozoic during when the lucaogou formation reached peak oil generation stage therefore the bedding parallel fractures were most likely generated by hydrocarbon generation overpressure the presence of hydrocarbon inclusions in bedding parallel fracture cements supports natural fracturing by high pore fluid pressure the 13cpdb and 18opdb of calcite cements within the inclined fractures vary from 3 6 to 9 6 and from 11 0 to 2 9 respectively which are close to those of the carbonate rich parent rock this also supports a closed fluid system scenario the relatively lower fluid inclusion trapping temperatures 70 5 c 109 7 c suggest that they were likely generated by decrease of confining stress during uplift processes therefore both an increase in pore fluid pressure and a decrease in confining stress contribute to the formation of natural fractures in the lucaogou formation tight oil reservoirs

post variscan exhumation of the central anti atlas morocco constrained by zircon and apatite fission track thermochronology the origin of the anti atlas relief is one of the currently debated issues of moroccan geology to constrain the post variscan evolution of the central anti atlas we collected nine samples from the precambrian basement of the bou azzer el graara inlier for zircon and apatite fission track thermochronology zircon ages cluster between 340 20 and 306 20 ma whereas apatite ages range from 171 7 ma to 133 5 ma zircon ages reflect the thermal effect of the variscan orogeny tectonic thickening of the ca 7 km thick paleozoic series likely enhanced by fluid advection apatite ages record a complex mesozoic cenozoic exhumation history track length modelling yields evidence that i the precambrian basement was still buried at ca 5 km depth by permian times ii the central anti atlas was subjected to erosional exhumation during the triassic early cretaceous then buried beneath ca 1 5 km thick cretaceous paleogene deposits iii final exhumation took place during the neogene contemporaneously with that of the high atlas

regional fractures i a mechanism for the formation of regional fractures at depth in flat lying reservoirs 1 regional fractures may form slowly at depth in extension and parallel to regional horizontal tectonic compression such fracture sets can be common in reservoirs despite the absence of flexure of the strata and are commonly important contributors to reservoir permeability such fractures make good targets for deviated wells high formation pore pressure is crucial to creating low effective confining stresses and brittle rock properties in the reservoirs conditions conducive to fracturing despite deep burial but pore pressure does not exceed the least principal stress and does not cause tensile fracturing the mechanism of natural hydraulic fracturing is incompatible with the characteristics of most regional fracture sets rather many regional fractures form duri g far field compression initiate at locally induced tensile stresses caused by flaws in the rock and propagate in the plane of the maximum and intermediate compressive stresses the differential stress required for initiation and propagation of regional fractures is well below that necessary for shear failure in the presence of high pore pressure fractures can be held open at depth by a tectonic basinwide dilatancy of the strata and the open void space commonly becomes mineralized

isotopic footprints of the giant precambrian caixiashan zn pb mineralization system abstract carbonate replacement zn pb deposits are formed through fluid circulation within carbonate rocks and are an important global source of zn and pb the caixiashan carbonate replacement deposit is one of the largest zn pb systems in china the mesoproterozoic syn sedimentary stage i of the caixiashan deposit is characterized by sulfides with negative 34s values 25 3 to 8 6 generated by organic sulfate reduction which co precipitated with the host rocks the neoproterozoic main sulfide precipitation stage ii was caused by thermo sulfide reduction resulting in positive 34s values 7 9 12 3 carbonates from stage i have 13cpdb of 1 2 to 2 8 and 18osmow of 17 1 24 0 similar to those of marine carbonate rocks dolomite and calcite from stage ii have 13cpdb ranging from 5 1 to 0 2 and 18osmow of 14 1 19 7 consistent with dissolution of carbonate rock in contrast dolomite and calcite from stage iii have mantle c o signatures with the 13cpdb ranging from 7 5 to 0 4 and 18osmow from 5 8 to 13 2 c o isotopic signatures suggest that the original carbonates were dissolved by the acid generated from sulfide precipitation consistent with the sulfide precipitation process subsequent precipitation of carbonates created large volumes of dolomite alteration that coated the sulfides and possibly contributed to the preservation of the ore as dolomite would dissolve more slowly than calcite carboniferous magmatism stage iii resulted in a magmatic overprint on the stage ii sulfide the metal sources of these sulfide ore packets are mainly derived from the mesoproterozoic carbonate rocks as indicated by the lead isotopes of sulfides and also c o isotopes of carbonates the deposit is characterized by a complex paragenesis with multiple overprinting sources of hydrothermal fluids and sulfide over 700 m y the syn sedimentary stage 1019 ma and main mineralization stage 859 837 ma at caixiashan are chronologically and tectonically associated with the assembly and break up of the rodinia supercontinent whereas the magmatic hydrothermal modification stage 353 ma is associated with the closure of the paleo asian ocean during the early carboniferous

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

changing fluid conditions during folding an example from the central appalachians abstract fluids inclusion microthermometry and stable isotope analyses of vein minerals in the central appalachian patterson creek anticline record dynamic changes in fluid temperature pressure and chemistry during folding these changes resulted from variations in fracture connectivity and fluid communication between stratigraphically restricted fluid reservoirs minerals in early formed bed parallel and ene nnw striking cross fold veins show that prior to folding fluids in silurian through lower upper devonian rocks were well stratified based temperature salinity and isotopic composition stable isotopes data indicates that the fluid systems were essentially closed with no communication between layers as folding commenced early minerals in ne nw striking veins in the siluro devonian carbonate rocks record 2 km of burial by syn tectonic sedimentation with corresponding increases in fluid temperature and pore fluid pressure an additional 2 km of burial along with the influx of low salinity methane saturated brines into the middle and upper devonian clastic rock sequence is recorded by later quartz in the ne nw striking veins at some point during folding stable isotope data indicate that fluid communication increased along fracture and faults between major stratigraphic fluids reservoirs the culmination of this process was the breaching of the stratigraphic seal at the base of the devonian shales allowing fluid infiltration from the devonian shale sequence into the upper portion of the helderberg group this resulted in the local precipitation of quartz and possibly the enhancement of solution structures