the niujiaotang cd rich zinc deposit duyun guizhou province southwest china ore genesis and mechanisms of cadmium concentration the niujiaotang zinc deposit in southeastern guizhou china is a mississippi valley type zn deposit within early cambrian carbonate rocks sphalerite is enriched in cadmium average 1 4 wt cd which occurs mostly as isomorphous impurities in the sphalerite lattice discrete cadmium minerals greenockite and otavite are rare and are found almost exclusively in the oxidation zone of the deposit probably formed as secondary minerals during weathering leaching processes geochemical data show that the sulfides are enriched in heavy sulfur with 34s ranging from 10 0 to 32 8 mean 22 5 the consistent pb isotopic compositions in different sulfide minerals are similar to that of cambrian strata the ore lead probably came from u and th rich upper crustal rocks such as the lower cambrian wuxun formation the ore fluid is of low temperature 101 c to 142 c type with a na ca mg cl dominant composition and is interpreted as oil field brine the data indicate that the metals were mainly derived from the early cambrian strata qingxudong and wuxun formations whereas sulfur is sourced from sulfate in cambrian strata or oil field brines of the majiang petroleum paleoreservoir the genetic model for the deposit invokes an early cambrian shallow sea environment on the yangtze platform zinc and cd in seawater were concentrated in abundant algae via unknown biological mechanisms resulting in large amounts of zn and cd rich algal ooliths during the ordovician concurrent with destruction of the majiang petroleum paleoreservoir oil field brines migrated from the center of the basin to the margin leaching metals from the cambrian strata in the niujiaotang area preexisting zn and cd particularly in the qingxudong and wuxun formation were further mobilized by hot brines rising along the zaolou fault system forming stratiform and generally conformable zn cd orebodies in reactive carbonate lithologies

petrologic and stable isotope constraints on the source and evolution of skarn forming fluids at pine creek california a detailed geochemical analysis of the well exposed multistage pine creek california tungsten deposit has constrained the roles of magmatic metamorphic and meteoric fluids in the development of the zoned scheelite ore and calc silicate skarn metamorphic conditions of p 2 kb t 600 degrees c and x sub co 2 0 25 are recorded in the metasedimentary rocks of the septum metamorphism yielded calcite diopside graphite and calcite dolomite periclase brucite forsterite assemblages in the variably dolomitic siliceous carbonate rocks of the pine creek septurn following intrusion of the morgan creek quartz monzonite metasomatism developed the zoned endoskarn exoskarn at the contact the massive garnet andradite sub 20 50 pyroxene hedenbergite sub 20 80 or quartz skarn assemblage carries most of the scheelite calculated t x sub co 2 equilibria corrected for solid solution effects restrict skarn formation to x sub co 2 0 2 at t 500 degrees or 50 degrees c calculated f sub o 2 f sub f 2 f sub s 2 diagrams indicate f sub o 2 values near quartz fayalite magnetite and f sub s 2 values well below pyrrhotite pyrite during skarn formation slightly falling temperatures were accompanied by increased f sub s 2 and hydration of the skarn assemblages stable isotope data for calcites show a mixing trend from unaltered marble delta 18 o 24ppm delta 13 c 3 5 degrees to values in equilibrium with magmatic fluids delta 18 o 8ppm delta 13 c 6ppm biotite quartz thermometry for the quartz monzonite yields temperatures of 410 degrees to 460 degrees c well below the solidus and suggesting later reequilibration pyroxene quartz analyses suggest that these minerals equilibrated at temperatures of 460 degrees to 560 degrees c the delta 18 o values measured for garnets suggest disequilibrium for the garnet quartz and garnet pyroxene pairs d h results for epidote and biotite suggest that meteoric water influx became more important with time sulfur isotope data are consistent with a reduced magmatic source

mica liquid trace elements partitioning and the granite pegmatite connection the st sylvestre complex western french massif central abstract we constrain the genetic relation between granite and pegmatite parental melts from the variscan saint sylvestre leucogranite complex and associated pegmatite bodies massif central france through compositions of micas using mica trace element concentrations and available partition coefficients for li rb ba cs and f we calculated the trace element contents of granitic and pegmatitic melts at equilibrium with micas biotite in leucogranites and pegmatites ranges mostly from fe biotite to siderophyllite more evolved facies contain protolithionite and zinnwaldite and lepidolite occurs in the most fractionated pegmatite white micas have homogeneous compositions from muscovite to li phengite in granites biotite and muscovite trace element distributions are clustered in pegmatites mica trace element contents globally follow differentiation from the least to the most evolved body the reconstructed pegmatitic and granitic melts present strong compositional similarities such as enrichments in li rb cs and depletion in ba that suggest a similar origin however micas are shown to have selectively equilibrated with the last melt or fluid in contact and so trace element concentrations of early magmatic liquids are rarely preserved inversion of the mica data constrains the composition of parental melts and the trace element evolutions during crystallization leucogranite and pegmatite melts show mutual incompatible element evolutions inconsistent with a parent daughter genetic relation the data and the modelling suggest that they represent non cogenetic melts generated by discrete episodes of partial melting differentiation is thus inherited from source processes rather than being the consequence of fractional crystallization of a common parental melt magma we suggest that both the leucogranites and the pegmatites originate from partial melting of a heterogeneous source rather than pegmatites being the product of granite crystallization

u pb zircon and monazite dating of a mafic ultramafic complex and its country rocks u pb data on zircons from the largest mafic ultramafic body 6 2 km of the french central massif sauviat sur vige yield the following age results primary magmatic crystallization of the gabbroic and peridotitic protoliths took place in the cambro ordovician 496 25 17 m y variable transformation under eclogite facies conditions was hercynian 320 29 36 m y the same age pattern derived by u pb monazite analyses was found also for the immediate country rocks i e kyanite bearing coarse grained metagranites occurring to the w and n of the sauviat massif due to the fact that there is no regional hercynian high grade metamorphism in this part of the french central massif e g duthou 1977 bernard griffiths 1975 both mafic ultramafic complex as well as immediate felsic country rocks must have been emplaced tectonically into pre hercynian acadian caledonian crustal rocks the cause for such a hercynian tectonism is thought to be due to continent continent collision of the spanish with the armorican plate

rbsr and 40ar39ar laser probe dating of high pressure phengites from the sesia zone western alps underscoring of excess argon and new age constraints on the high pressure metamorphism abstract the combined use of the rb sr and 40ar 39ar laser probe methods allows confirmation of the existence of excess argon in phengites from eclogites from the sesia zone western alps despite systematic 40ar 39ar plateau ages 40ar 39ar phengite ages from two different areas show integrated age spreads between 65 4 0 3 ma and 109 5 0 4 ma the youngest 40ar 39ar plateau age 65 9 0 4 ma is concordant with two rb sr phengite whole rock isochron ages from the same outcrop which display a mean age at 64 2 2 5 ma and older than a rb sr biotite whole rock isochron age at 53 0 1 0 ma this age concordance being incompatible with the concept of isotopic closure temperature is probably fortuitous and probably results from an excess argon contamination however the concordance of the rb sr phengite isotopic closure temperature with the temperature reached during the eclogite facies event suggests that the age at 64 2 2 5 ma could be a crystallization age probably contemporaneous with the eclogite facies event in the sesia zone when compared to recent age data from the piemont zone and the dora maira massif the entire data set which needs to be confirmed by extra measurements support diachronous high pressure hp events in the western alps

from magmatic to hydrothermal sn li nb ta w mineralization the argemela area central portugal abstract the argemela mineralized area ama central portugal exhibits a nearly continuous magmatic to hydrothermal ore forming sequence typical of variscan granite related rare metal deposits disseminated and vein type mineralization are distributed in two systems the argemela mine am and the cabeco da argemela ca disseminated mineralization includes montebrasite cassiterite and columbite tantalite dispersed in rare metal granites vein type mineralization occurs both in granites and country rocks the ca system exposes three generations of intragranitic veins with montebrasite in the two earliest and montebrasite wolframite cassiterite and columbite tantalite in the latest country rock veins include a swarm of cassiterite and montebrasite veins am and rare isolated wolframite veins ca disseminated cassiterites are enriched in ta2o5 and nb2o5 compared to vein cassiterites disseminated columbite tantalites evolve toward mn and ta rich chemistries that contrast with the more fe and nb rich compositions in intragranitic veins in the ca system both intragranitic and country rock veins crystallize early mn rich wolframites followed by late more fe rich compositions associated with nb ta poor cassiterite the later replaced by stannite field relations structural mineralogical and geochemical data suggest that the disseminated and vein type mineralization are expressions of a continuous metallogenic evolution initiated at the magmatic stage pursued during the magmatic hydrothermal transition and ended with hydrothermal circulations in country rock the two mineralized systems share similarities but they also show major differences structural analysis demonstrates that the intragranitic veins in the ca and the country rock veins in the am were emplaced under the same tectonic regime therefore the two systems are variants of the same local metallogenic evolution sn and w show markedly contrasted behaviors both have a magmatic source but only sn reaches concentrations leading to saturation of the melt with cassiterite w is deposited later and preferentially to sn in intragranitic veins at the hydrothermal stage cassiterite and wolframite deposition are disconnected and overall wolframite occurs only in minor amounts the metallogenic model proposed for the ama emphasizes magmatic rare metal concentration processes exsolution of magmatic fluids and selective deposition of metals during the magmatic hydrothermal and later hydrothermal stages the sn li nb ta dominated and w poor signature of the ama makes it similar to lct pegmatites of the central iberian zone but is distinctive from the world class w sn cu panasqueira deposit nearby

petrogenesis of syn orogenic rare metal pegmatites in the chinese altai evidences from geology mineralogy zircon u pb age and hf isotope abstract recently an increasing number of anatetic pegmatites have been confirmed in contrast to the granitic pegmatite the anatetic pegmatite has no relation to granite it is therefore necessary to reacquaint the origin and classification of pegmatitic rocks as one of the largest pegmatite provinces the chinese altai provides a natural laboratory for pegmatite study to date the type age source and tectonic setting of the syn orogenic pegmatites in the chinese altai remain unclear in this work the cms chemical composition mineral assemblage structural geology classification is primarily applied on the pegmatitic rocks in the chinese altai five types pegmatitic rocks including metapegmatite pegmatoid pegmatite pseudopegmatite and granite pegmatite are identified by their cms features in addition the geology mineralogy zircon u pb ages and hf isotopes of six pegmatites are presented in this work the pegmatites numbered jmhb02 qbl02 tlt01 aljk01 amlg01 and tmlt01 are dated at 394 8 4 0 ma 402 6 5 5 ma 385 9 3 5 ma 368 0 4 0 ma 358 3 4 6 ma and 333 0 6 0 ma respectively indicating that they were formed during the syn orogenic stage of the altai orogenic process they are exposed in the qiongkuer domain in the southern chinese altai juxtaposed with the late and or post orogenic pegmatites in the same pegmatite field no parental granite can be confirmed for all these pegmatites comparatively the synorogenic pegmatites are scarce and have lower mineralization grades than the late and postorogenic pegmatites zircon hf isotopic compositions of the syn orogenic pegmatites have positive hf t values of 2 06 8 28 and old t dm2 ages of 1 234 860 ma and are consistent with those of most of the syn orogenic i and s granites which indicates that the pegmatites and granites have a common immature crustal source from the early paleozoic metasedimentary rocks compared to the post orogenic pegmatites in the central altai domain the syn orogenic pegmatites in this work have larger positive hf t values and younger t dm2 ages indicating that more juvenile components were involved in their magma sources combining this study with previous works we suggest that the anatexis of metasediments induced by decompression and injection of heat energy and mantle derived materials under a forearc extensional setting led to the formation of the syn orogenic pegmatites in the chinese altai

a simple photogrammetric method to improve quantitative image analysis in geoscience research sedimentary field surveying uses 2d outcrop images to rapidly and easily obtain quantitative information although photogrammetry is capable of a complete and accurate 3d analysis this technique requires great technical expertise as well as intricate and expensive software and hardware rectified digital images instead represent a cost effective and easier method to obtain accurate 2d images for outcrop modeling and mapping the present methodology describes a simple technique to improve quantitative image analysis for outcrop analysis based on widespread conventional pc software and inexpensive hardware the proposed methodology treats outcrop digital images with a software developed by us in order to solve for colinearity equations which calculate coordinates of terrain points thus the geometric correction on the terrain point coordinates to obtain the rectified images can be done with some commercial and widely distributed g i s remote sensing or photogrammetric programs the final rectified image represents a metrically consistent perpendicular view of the outcrop that favors the quantitative image analysis this technique can also be used to improve the photo mosaicking of different perspective corrected images of the same outcrop avoiding the usual misfit of the image frames

a miocene tungsten mineralization and its implications in the western bangong nujiang metallogenic belt constraints from u pb ar ar and re os geochronology of the jiaoxi tungsten deposit tibet china abstract the jiaoxi quartz vein type wolframite deposit is the first tungsten mineralization discovered in the western part of the bangong nujiang metallogenic belt which is well known for the porphyry epithermal skarn cu au mineralization the jiaoxi deposit is associated with a porphyritic monzogranite that intrudes sandstone and shale in the shiquanhe ophiolite melange the ages of the magmatism and mineralization in the deposit are here constrained by the dates obtained from the porphyritic monzogranite hydrothermal muscovite and molybdenite laser ablation inductively coupled plasma mass spectrometry la icp ms zircon u pb dating of two porphyritic monzogranite samples yields weighted mean 206pb 238u coincident age of 14 1 ma two samples of hydrothermal muscovite from wolframite bearing quartz veins yield plateau 40ar 39ar ages of 13 4 0 1 and 13 3 0 1 ma respectively re os model ages of five molybdenite samples range from 11 4 0 2 to 11 8 0 2 ma with a weighted mean model age of 11 6 0 2 ma these dates indicate that the tungsten mineralization at jiaoxi was formed during the middle miocene this miocene tungsten mineralization is significantly different from the early and late cretaceous cu au mineralization in the western part of the bangong nujiang metallogenic belt our dating results show that there is a middle miocene tungsten mineralization in the western part of the bangong nujiang metallogenic belt the tectonic setting for the tungsten mineralization is here interpreted to be the regional extension associated with the india asia continental collision the discovery of the jiaoxi deposit has important significance on regional tungsten prospecting

evolution of the mo rich scheelite skarn mineralization at kozbudaklar western anatolia turkey evidence from mineral chemistry and fluid inclusions abstract the kozbudaklar scheelite skarn deposit in the tavsanli zone located approximately 22 km southeast of bursa is hosted by the triassic calcic inonu marble and eocene topuk pluton at least four stages have been recognized through skarn evolution scheelite skarn distributed close to the topuk pluton occurred during the early stage 1 and late stage 2 prograde substages the early prograde endo and exoskarn are composed of hedenbergite hd 96 joh 4 plagioclase an 55 64 and hedenbergite hd 61 94 joh 4 7 accompanied by calcic garnet grs 38 94 sps 1 5 alm 0 and scheelite pow 1 6 the second stage represents a relatively oxidized mineralogy dominated by diopside hd 16 48 joh 0 9 subcalcic garnet grs 24 92 sps 0 11 alm 0 31 and scheelite pow 7 32 the stage 3 and 4 mineral assemblages are characterized by few hydrous minerals in the retrograde stage and intense fracturing fluid inclusions from skarn rocks are indicative of multiple fluid events 1 low moderate salinity 5 16 wt nacl equiv inclusions homogenized dominantly by a high temperature 308 c to 600 c liquid phase in stage 1 fluid inclusions in an early garnet homogenized over a similar temperature range 440 c and 459 c into both liquid and vapor phases eutectic temperatures ranging from 61 7 c to 35 0 c that indicate the presence of h 2 o nacl mgcl 2 cacl 2 co 2 solutions 2 coexisting daughter mineral bearing high salinity 29 5 70 wt nacl equiv and vapor rich moderate salinity 11 5 16 7 wt nacl equiv inclusions that homogenized in the liquid phase by the disappearance of the vapor phase at a similar temperature range 308 c to 600 c in stage 2 eutectic temperatures range from 67 9 c to 51 8 c that shows the presence of h 2 o nacl co 2 ch 4 n 2 solutions 3 low moderate salinity 12 5 7 6 wt nacl equiv and temperature 320 c to 215 c inclusions homogenized by the liquid phase in stage 3 eutectic temperatures range from 59 5 c to 44 2 c indicating the presence of h 2 o nacl mgcl 2 cacl 2 co 2 solutions 4 inclusions of low salinity 9 9 0 9 wt nacl equiv and homogenization temperature 183 c to 101 c in stage 4 these data show that the kozbudaklar skarn deposit was formed in a magmatic hydrothermal system in this model carbonaceous fluids may have been exsolved from the plutonic rock during its emplacement and crystallization fluid inclusion data indicate that fluid boiling and immiscibility occurred at temperatures between 440 c and 459 c and pressures ranging from 50 mpa to 60 mpa based on hydrostatic considerations early scheelite was precipitated with relatively reduced mineral compositions as a result of depressurization mo rich scheelite with oxidized minerals formed via high salinity and vapor rich inclusions the second scheelite mineralization occurred in a normal hydrothermal system by an infiltration mechanism at pressures between approximately 40 and 1 5 mpa at shallow depths

the zn pb ag skarns of zacatepec northeastern oaxaca mexico a study of mineral assemblages and ore forming fluids abstract the zn pb ag skarns of zacatepec are located in the eastern sierra madre del sur southern mexico they are hosted by the cuicateco terrane a composite unit made up of deformed jurassic and cretaceous volcanic and sedimentary rocks deposited in an arc environment that also hosts several precious and base metal epithermal deposits and cu mo porphyries the mineralization occurs as irregular masses along the contact between cretaceous limestones and tertiary porphyritic dacites over widths of few decimeters an epidote rich ep44 97czo01 55pie00 03 endoskarn is developed for the most part the mineralization consists in skarns of calcic garnet ad62 22gr78 38 with disseminated hematite calcic garnets show chemical oscillatory zonation skarns rich in calcic clinopyroxenes di28 54hd22 55jo07 32 locally occur farther from the porphyry contact as a result of a retrograde event the above assemblages are locally silicified and ore minerals sphalerite galena pyrite chalcopyrite and minor scheelite and chamosite deposited the iron contents in sphalerite range between 5 0 and 9 4 mol fes an assemblage of sulfosalts consisting of vikingite ag5pb8bi13s30 and an ag bi s phase probably of the aikinite group occurs locally magnesian skarns develop and are composed of almost pure diopside di98 100hd00 01jo00 clinochlore chrysotile and vesuvianite notoriously rich in ti 6 48 to 7 93 wt tio2 the magnesian skarns yield the highest metal grades up to 595 g t of ag 9 94 pb and 3 27 zn after weathering of the ores botryoidal crusts of supergene minerals as smithsonite rosasite cu zn 2 co3 oh 2 and hemimorphite developed microthermometric studies of fluid inclusions were carried out in calcite calcic garnet quartz and sphalerite homogenization to the liquid phase th occurred between 160 c and 470 c salinities were calculated assuming an h2o nacl system and range from 10 5 to 19 8 wt nacl eq the temperature of formation of the prograde skarn calculated from microthermometric data with an assumed pressure of 500 bar ranges between 470 c and 510 c a slight increase in o2 caused the formation of garnet after clinopyroxene the formation of the retrograde assemblage implies a decrease in temperature and initially in salinity coupled with a great increase in o2 these changes can be explained by mixing with cooler oxidizing and dilute meteoric water subsequently a progressive increase in fluid salinity as th dropped from 300 c suggests that boiling took place triggering sulfide precipitation

a study of sulfur isotopes in determining the genesis of goynuk and celaldagi desandre pb zn deposits eastern yahyali kayseri central turkey abstract pb zn deposits are widespread and common in various part of the taurus belt most of the deposits are related to faults in limestones and dolomitic limestones goynuk and celaldagi desandre deposits are typical of those found in the yahyali district of the aladag region located northwest of the eastern taurus the concordant nature of these deposits with wall rocks differentiates them from other deposits in the same region the following typical important features characterize the concordant and fault related deposits 1 the absence of any rocks indicating magmatic activity 2 no evidence of thermochemical changes along the ore body wall rocks contact 3 the presence of a simple mineral assemblage in the paragenesis 4 the presence of marcasite reflecting the low temperature occurrence 5 considerable carbonatization and sulfatization and 6 the higher abundance of pb zn cu ni and co in wall host rocks compared with those in the unaltered limestones the 34 s values for galena are within the range of 0 5 to 0 9 n 10 mean 0 17 0 49 for goynuk and 8 4 to 10 9 n 10 mean 9 28 0 75 for celaldagi desandre the origin of sulfur in celaldagi desandre is probably seawater whereas goynuk reflects the complete reduction of seawater sulfate within a closed basen the concordant pb zn deposits in the aladag region are carbonate hosted deposits these were probably associated with rifting processes in the eastern mediterranean prior to the early middle triassic the fault related deposits formed from leaching transportation and re deposition of the concordant deposits by seawater within fault zones

the dewulu reduced au cu skarn deposit in the xiahe hezuo district west qinling orogen china implications for an intrusion related gold system abstract the triassic west qinling orogen which formed by the continental collision between the south china block and north china craton following the subduction and closure of the paleo tethyan ocean hosts numerous gold deposits that have commonly considered as products of metamorphic dehydration during the oceanic subduction and subsequent continental collision however whether or not there are gold deposits that are genetically related to regional magmatism remains poorly understood here we present mineralogical compositional and geochronological data of the dewulu au cu skarn deposit in the xiahe hezuo district to understand the ore genesis and its genetic link to many coeval sediment hosted disseminated and magmatic hosted vein gold deposits in the same district which collectively reveal a possible intrusion related gold system the dewulu au cu skarn deposit in the eastern zone of the xiahe hezuo district is associated with the early triassic i type ilmenite series dewulu quartz diorite pluton that intrudes permian marine clastic and carbonate rocks ore related hydrothermal biotite separates yield a well defined 40ar 39ar plateau age of 239 9 1 4 ma 2 which agrees with a previous zircon u pb age 238 6 1 5 ma at 2 of the dewulu quartz diorite and thus demonstrates a temporal and likely genetic link between the two both prograde and retrograde skarn assemblages are well developed in the dewulu au cu deposit prograde garnet and pyroxene are compositionally dominated by grossularite and hedenbergite respectively indicating a reduced skarn system au cu mineralization is largely represented by the arsenopyrite loellingite chalcopyrite pyrrhotite bornite assemblage and is associated with retrograde skarns gold is mostly hosted in arsenopyrite loellingite and chalcopyrite and has close textural relations with native bismuth or bismuthides mineral stability relationships and fluid inclusion microthermometric data confirm that the prograde skarns formed at a low oxygen fugacity high temperature 630 c and a depth range of 2 9 to 6 5 km arsenopyrite and chlorite geothermometers indicate that the retrograde skarns and late quartz sulfide calcite assemblages formed at 550 to 345 c and

crustal thinning recorded by the shape of the namurian westphalian leucogranite in the variscan belt of the northwest massif central france in the western part of the variscan belt of the french massif central the gueret massif is an extensional allochthon emplaced to the southeast during namurian westphalian time the internal structure of the coeval leucogranites surrounding the western edge of the gueret massif consists of the plateau d9aigurande brame st sylvestre and st goussaud massifs these plutons are asymmetric diapirs rooted in the marche bussiere madeleine and arrenes faults respectively the plutons indicate diverging extravasation from the gueret massif whatever their shapes the plutons have a northwest trending stretching lineation that is also marked in the granite host rocks by retrograde metamorphic minerals the same extension direction is also inferred from namurian westphalian leucogranites and dikes intruding the gueret massif extension related granitoids present typical structural patterns such as asymmetric shapes extravasation from the collapsing area and persistence of a stretching lineation parallel to the regional extension direction these features may be used to diagnose crustal thinning

ore deposits of the french massif central insight into the metallogenesis of the variscan collision belt the french massif central fmc represents the whole west european variscan wev belt in terms of both the geodynamic evolution and the metallic content thus a study of the metallogenic evolution of the fmc may elucidate the conditions that allow the mineralisation of a collision belt since recent collision belts e g the himalayas or the alps show that mineralisation does not necessarily result from the collision process the palaeozoic history of the fmc is divided into three geodynamic stages unevenly involved from the metallogenic view point the eo variscan stage cambrian to silurian was not important the meso variscan stage devonian early carboniferous was of limited importance and most of the mineralisations formed during the neo variscan stage late carboniferous early permian in addition some more mineralisation was produced during the mesozoic because of the thermal reactivation linked with the alpine orogenies

chemistry origin and evolution of mineralized granites in the lachlan fold belt australia the metallogeny of i and s type granites granites crop out over 20 percent of the total exposed area of the lachlan fold belt and comprise subequal proportions of i and s types despite the volume of granite present economic mineralization is sporadic major sn mineralization is related to s and i type granites that are both reduced and have undergone extended feldspar dominated crystal fractionation mo showings are common but as for cu large deposits are absent scheelite skarn mineralization is represented by the major king island deposits however granite related w mineralization of a similar scale is not found elsewhere within the belt the i type granites of the belt represent the products of partial melting of older tonalitic granodioritic source rocks in the deep crust which were themselves ultimately derived through fusion of mantle materials m types this evolution is also mirrored in the nature of related mineralization which changes from cu dominated at the chemically primitive end of this series through to lithophile dominated in the more evolved i types most of the lachlan fold belt i types have not undergone significant crystal fractionation and are neither strongly oxidized nor reduced two prominent mineralized i type suites the boggy plain supersuite of the central lachlan fold belt and the grassy suite of king island are anomalously mineralized relative to other lachlan i types both are chemically distinct from other i types in the belt by being enriched in the incompatible elements and ni and cr mafic s types are universally restite rich and unmineralized s type magmas are in general more evolved chemically than 1 types are all relatively reduced and when highly fractionated are related to sn or w mineralization the ore element ratios sn w cu mo etc of granite related mineralization in the lachlan fold belt is a straightforward function of the relative oxidation state and degree of fractionation within the associated granite

genesis and magmatic hydrothermal evolution of the yangla skarn cu deposit southwest china the yangla skarn cu deposit 150 mt at 1 03 cu is located in the central segment of the jinshajiang metallogenic belt within the sanjiang three rivers region southwest china skarn orebodies are mainly developed between different units of devonian carbonate and quartz sandstone rocks with stratiform like shapes or within the contact zone between granitoids and marbles re os dating of molybdenite intergrowth with chalcopyrite yielded a well constrained 187re 187os isochron age of 232 0 1 5 ma with a weighted average age of 231 8 1 3 ma both coeval with the related intrusions 233 1 1 4 and 231 0 1 6 ma at 2 by zircon u pb dating from our previously published work field and textural relationships indicate three hydrothermal stages characterized by assemblages of prograde skarn pre ore stage retrograde skarn and cu fe mo bi sulfides main ore stage and pb zn sulfides associated with calcite and quartz late ore stage as well as one supergene stage marked by secondary cu mineralization malachite and azurite skarns contain garnets with andraditic compositions ad96 gr2 3py0 1 and clinopyroxene two series hd6di94 and hd86di13jo1 with low mn fe ratios 0 1 typical of cu skarn deposits worldwide three stages of fluid evolution were observed by a detailed fluid inclusion study 1 early fluids were trapped under two phase conditions as evidenced by the coexistence of brine homogenization temperatures 560 600 c average salinity 49 4 1 7 wt nacl equiv n 33 and vapor rich inclusions in pre ore stage andradite and diopside trapped at 600 bars or a depth of approximately 2 km assuming lithostatic pressure conditions 2 main ore stage fluid inclusions in quartz were also trapped under two phase conditions boiling as identified by the coexistence of vapor and liquid rich fluid inclusions liquid rich inclusions homogenized between 312 and 389 c average 350 24 7 c n 20 with salinities of 2 4 to 5 6 wt nacl equiv average 4 2 0 9 wt nacl equiv n 13 and a depth of 2 km 200 bars hydrostatic pressure conditions 3 late ore stage fluids are represented by inclusions in calcite characterized by homogenization temperatures ranging from 220 to 290 c average 249 27 c n 14 and salinities between 2 1 and 8 0 wt nacl equiv average 4 6 1 5 wt nacl equiv n 11 sulfur isotope compositions of sulfide minerals pyrite chalcopyrite pyrrhotite and molybdenite from the main and late ore stage have a narrow range of 34s values from 1 9 to 2 6 consistent with a magmatic origin calcite in the late ore stage has 13c values ranging from 3 2 to 5 9 and 18o from 7 2 to 18 0 distinct from the host rock marble compositions 13c 1 2 4 3 18o 10 8 23 9 when corrected for temperature 250 c estimated from fluid inclusion analysis in calcites these calcite data correspond to ore fluid 13cfluid values of 2 0 to 4 6 and 18ofluid values of 0 5 to 11 2 clustering at 10 which are consistent with a magmatic origin lead isotope compositions of sulfides 206pb 204pb 18 273 18 369 207pb 204pb 15 627 15 677 and 208pb 204pb 38 445 39 611 are similar to those of the granitic intrusions and sedimentary wall rocks but distinct from those of basalts 206pb 204pb 18 282 19 133 207pb 204pb 15 564 15 665 and 208pb 204pb 38 367 38 942 in the mining area taken together these geologic geochemical and isotopic data confirm that yangla is a typical cu skarn deposit

phase relations in the fe zn s system to 5 kbars and temperatures between 325 degrees and 150 degrees c sphalerite pyrite and pyrrhotite were recrystallized together in a saturated ammonium iodide solution at 150 degrees to 325 degrees c and 0 4 to 5 1 kbars the data were obtained from ten sets of experiments comprising gold capsules stacked consecutively within the bores of cold seal pressure vessels which were heated for periods of between 300 and 750 days five capsules contained magnetite as an additional reactant a p t composition map has been constructed for sphalerite buffered by pyrite and hexagonal pyrrhotite the upper p t range for the present study includes the lower temperature boundary for the sphalerite geobarometer which has been located near 285 degrees c at 0 5 kbars and 300 degrees c at 5 kbars the provisional lower p t range for the present data extends from approximately 180 degrees c at 0 5 kbars to 190 degrees c at 5 kbars within the investigated p t interval isobaric composition curves for sphalerite show a progressive decrease in iron content with increasing pressure and decreasing temperature hexagonal pyrrhotite was the only iron monosulfide identified in the experimental reaction products and was found to increase in iron content with increasing pressure its occurrence in these long term experiments supports earlier predictions that monoclinic pyrrhotite may be a metastable phase down to low temperatures hexagonal pyrrhotite may stably invert to monoclinic pyrrhotite at temperatures below 150 degrees c under isotropic stress conditions however at higher temperatures monoclinic pyrrhotite appears to be favored by anisotropic stress conditions which produce lattice strain under these conditions it may form and persist metastably as the preferred phase below the sphalerite geobarometer boundary the presence of magnetite does not affect the compositions of sphalerite or pyrrhotite under the conditions of study the present calibration may be applied to hydrothermal vein deposits provided codeposition of phases can be proven however it is expected to have limited application for massive sulfide ores that have prograded or retrograded to temperatures around or below 300 degrees c this is because the calibration requires buffering by pyrite and hexagonal pyrrhotite whereas most lower temperature assemblages appear to have been buffered by pyrite and monoclinic pyrrhotite over a range of temperatures pressures and anisotropic stress conditions this latter buffer has restricted sphalerite compositions between around 11 and 12 mole percent fes

fluid inclusion and stable isotope constraints c o h on the dagbasi fe cu zn skarn mineralization trabzon ne turkey abstract the dagbasi fe cu zn skarn mineralization developed along the contact between the block and lens shaped limestones of the lower cretaceous berdiga formation and the upper cretaceous dagbasi granitoid the exoskarn type mineralization is characterized by prograde stage garnet and pyroxene while the retrograde stage is characterized by epidote tremolite actinolite and chlorite quartz and calcites were observed in both stages of the skarn development the ore minerals mainly consist of magnetite and hematite with a lesser amount of pyrrhotite pyrite chalcopyrite sphalerite and minor galena the homogenization temperatures th and salinity values of the prograde stage halite bearing fluid inclusions are in the range of 412 514 c and 48 8 61 8 wt nacl equ respectively the second stage liquid and vapor rich fluid inclusion assemblage reveals that boiling at temperatures of 353 458 c took place after the formation of halite bearing fluid inclusions final stage liquid rich fluid inclusions were characterized by low th 160 and 327 c and salinity values 0 5 and 6 2 wt nacl equ the decreasing salinity trend of the fluid inclusions versus th indicated that meteoric water was involved in the hydrothermal solutions eutectic temperatures te of the prograde stage fluid inclusions were found to be cacl2 dominated while retrograde stage inclusions contained different salt combinations rather than a specific salt type the minimum trapping pressures of the early stage brine fluid inclusions were calculated to be between 710 and 884 bar while later stage inclusions had much lower trapping pressures between 195 and 445 bar the 18o isotopes of prograde stage quartz garnet and pyroxenes are close to the composition of the hydrothermal solutions of magmatic sources moreover retrograde stage quartz epidote tremolite actinolite and calcite minerals and their equilibrated solutions were found to be highly depleted by 18o isotopes therefore the fluid inclusion and stable isotope constraints suggest that the hydrothermal solutions of magmatic origin were responsible for the prograde skarn stage while a mixture of magmatic and meteoric solutions were responsible for the ore formation in a shallow skarn environment

skarn gold mineralization at the geodo mine south korea the geodo bi te bearing skarn deposit is located in the taebaeksan mining district south korea both endoskarn and exoskarn are associated with the cretaceous eopyeong granitoids which consist of magnetite bearing oxidized fe 2 o 3 feo 0 4 quartz monzodiorite to monzodiorite gold in the geodo mine occurs in a retrograde alteration stage within a garnet rich proximal zone skarn minerals associated with gold ore include fe rich garnet ad 57 95 gr 1 41 and a wide range of clinopyroxene hd 20 100 di 0 79 compositions broadly similar to those of typical gold bearing skarn deposits the paragenetic stages of the bi and te bearing skarn in the geodo mine consist of early calc silicate alteration followed by three mineralization stages fe oxide cu rich and au rich the fe oxide and cu rich stages were deposited at higher temperatures than the au rich stage based on mineral assemblage stabilities gold in au rich ores from the geodo mine occurs as native gold ranging from 5 to 400 m in diameter and is closely related to the bi and te bearing minerals bismuthinite and tetradymite which at geodo tend to be strongly enriched in bi and depleted in s compared to ideal compositions this association of gold with bismuthinite and tetradymite suggests that gold was deposited at relatively low temperatures consistent with fluid inclusion studies on calcite that indicate that the gold was deposited from fluids ranging from 181 to 281 c and 8 0 to 10 9 wt nacl equiv and with chlorite geothermometry indicating a range of 198 to 211 c the presence of significant amounts of bi and te bearing minerals associated with au suggests that liquid bismuth may have acted as a collector increasing the concentration of gold

composition and evolution of ore fluids in a magmatic hydrothermal skarn deposit the chemistry of brine vapor and low salinity fluid inclusions measured by proton induced x ray emission from the bismark skarn deposit mexico is consistent with an evolving magmatic hydrothermal system with no evidence for external fluid inputs the results support a model that invokes early phase separation of magmatic fluids into brine and vapor two phase field at high temperature and lithostatic pressure followed by the entrapment of a low salinity magmatic fluid one phase field at lower temperature and hydrostatic pressure the early brine and vapor inclusions contain high pb and zn concentrations and low cu however the vapor contains significantly more cu than the brine and was likely transported as a sulfur complex the fluid phase changes observed and behavior of cu are comparable to those of porphyry cu systems the later low salinity fluid at bismark represents a distinct pulse of magmatic fluid with a high k ca ratio in which base metals including cu were transported by chloride complexes paragenetic relationships and variations in the relative concentrations of cu pb and zn suggest that this fluid was primarily responsible for ore deposition the relative cu pb and zn concentrations in fluid inclusions at bismark are consistent with those measured from base metal ores in high temperature carbonate replacement deposits throughout mexico and suggest that the bulk metal budget of these deposits is primarily controlled by magmatic hydrothermal fluids deficient in cu

metallogenic model of the trepca pb zn ag skarn deposit kosovo evidence from fluid inclusions rare earth elements and stable isotope data the trepca pb zn ag skarn deposit 29 mt of ore at 3 45 pb 2 30 zn and 80 g t ag is located in the kopaonik block of the western vardar zone kosovo the mineralization hosted by recrystallized limestone of upper triassic age was structurally and lithologically controlled ore deposition is spatially and temporally related with the postcollisional magmatism of oligocene age 23 26 ma the deposit was formed during two distinct mineralization stages an early prograde closed system and a later retrograde open system stage the prograde mineralization consisting mainly of pyroxenes hd54 100jo0 45di0 45 resulted from the interac tion of magmatic fluids associated with oligocene 23 26 ma postcollisional magmatism whereas there is no direct contact between magmatic rocks and the mineralization the deposit is classified as a distal pb zn ag skarn abundant pyroxene reflects low oxygen fugacity 10 31 bar and anhydrous environment fluid inclu sion data and mineral assemblage limit the prograde stage within a temperature range between 390 and 475 c formation pressure is estimated below 900 bars isotopic composition of aqueous fluid inclusions hosted by hedenbergite d 108 to 130 18o 7 5 8 0 mn enriched mineralogy and high ree content of the host carbonates at the contact with the skarn mineralization suggest that a magmatic fluid was modified during its infiltration through the country rocks the retrograde mineral assemblage comprises ilvaite magnetite arsenopyrite pyrrhotite marcasite pyrite quartz and various carbonates increases in oxygen and sulfur fugacities as well as a hydrous character of min eralization require an open system model the opening of the system is related to phreatomagmatic explosion and formation of the breccia arsenopyrite geothermometer limits the retrograde stage within the temperature range between 350 and 380 c and sulfur fugacity between 10 8 8 and 10 7 2 bars the principal ore minerals galena sphalerite pyrite and minor chalcopyrite were deposited from a mod erately saline ca na chloride fluid at around 350 c according to the isotopic composition of fluid inclusions hosted by sphalerite d 55 to 74 18o 9 6 to 13 6 the fluid responsible for ore deposition was dominantly meteoric in origin the 34s values of the sulfides spanning between 5 5 and 10 point to a magmatic origin of sulfur ore deposition appears to have been largely contemporaneous with the retrograde stage of the skarn development postore stage accompanied the precipitation of significant amount of carbonates including the travertine deposits at the deposit surface mineralogical composition of travertine varies from calcite to siderite and all carbonates contain significant amounts of mn decreased formation temperature and depletion in the ree content point to an influence of ph neutralized cold ground water and dying magmatic system

origin of the atypical puy les vignes w breccia pipe massif central france constrained by trace element and boron isotopic composition of tourmaline abstract the puy les vignes w deposit is an atypical wolframite bearing hydrothermal breccia pipe hosted in migmatitic biotite sillimanite gneisses in the northwestern french massif central the deposit is characterized by volumetrically important tourmaline alteration allowing to investigate the fluid evolution of the ore forming hydrothermal system four generations of hydrothermal tourmaline tur 1 tur 4 formed during pre syn and post mineralization stages were identified based on detailed petrographic observations and were analyzed in situ for their chemical and boron isotopic compositions at the grain scale tourmaline commonly shows oscillatory zoning and dissolution textures resulting from a multi stage crystallization in a fluid dominated system the different generations of hydrothermal tourmaline have dravite schorl compositions and show similar major and trace element contents falling into the field of metamorphic rocks high concentrations of v cr sr and low concentrations of li sn in tourmaline suggest a metamorphic dominated origin of these elements the boron isotopic compositions of tourmaline range between 13 3 and 7 8 and cannot unambiguously distinguish between a magmatic and a metamorphic fluid origin our data indicate that the chemical and boron isotopic composition of tourmaline was dominantly controlled by high temperature fluid rock interactions between the metamorphic basement and boron rich reduced and low salinity hydrothermal fluids based on these results we propose a fluid evolution scenario for the puy les vignes ore forming hydrothermal system release of magmatic hydrothermal fluids from an unexposed peraluminous leucogranite at ca 324 ma is proposed as the main mechanism responsible for early greisenization and formation of disseminated tur 1 this episode was followed by massive tourmalinization and hydraulic brecciation of the overlying gneisses producing a tourmaline rich tur 2 crackle breccia by interactions with boron rich hydrothermal fluids fluid assisted reopening and collapse of the former tourmalinite at ca 318 ma yielded a matrix supported quartz tourmaline breccia pipe and formation of wolframite bearing quartz veins accompanied by deposition of tur 3 finally post ore hydraulic fracturing at ca 300 ma led to formation of tourmaline rich microbreccias tur 4 possibly from metamorphic fluids

neo tethyan magmatism and metallogeny in myanmar an andean analogue abstract plate margin tectonism magmatic and hydrothermal processes and the genesis of mineral deposits are intricately linked in myanmar two near parallel magmatic belts that together contain a significant proportion of that country s mineral wealth have contrasting metallogenic endowments the mogok mandalay mergui belt hosts crustal melt s type granites with significant tin tungsten mineralization while the wuntho popa arc comprises i type granites and granodiorites with copper gold mineralization the spatial juxtaposition of the two belts and their distinct but consistent metallogenic endowment bears strong similarities to the metallogenic belts of the south american cordillera recent u pb age dating has shown the potential for the two belts to be near contemporary from the late cretaceous to eocene here we investigate whether an andean type setting during subduction of neo tethys could explain the observed magmatism and mineralization within these two belts

fluid inclusion rare earth element geochemistry and isotopic characteristics of the eastern ore zone of the baiyangping polymetallic ore district northwestern yunnan province china abstract the baiyangping cu ag polymetallic ore district is located in the northern part of the lanping simao foreland fold belt which lies between the jinshajiang ailaoshan and lancangjiang faults in western yunnan province china the source of ore forming fluids and materials within the eastern ore zone were investigated using fluid inclusion rare earth element ree and isotopic c o and s analyses undertaken on sulfides gangue minerals wall rocks and ores formed during the hydrothermal stage of mineralization these analyses indicate 1 the presence of five types of fluid inclusion which contain various combinations of liquid l and vapor v phases at room temperature a h 2 o l b h 2 o l h 2 o v c h 2 o v d c m h n v and e h 2 o l co 2 l sometimes with co 2 v these inclusions have salinities of 1 4 19 9 wt nacl equivalents with two modes at approximately 5 10 and 16 21 wt nacl equivalent and homogenization temperatures between 101 c and 295 c five components were identified in fluid inclusions using raman microspectrometry h 2 o dolomite calcite ch 4 and n 2 2 calcite dolomitized limestone and dolomite contain total ree concentrations of 3 10 38 93 ppm whereas wall rocks and ores contain ree concentrations of 1 21 196 ppm dolomitized limestone dolomite wall rock and ore samples have similar chondrite normalized ree patterns with ores in the huachangshan xiaquwu and dongzhiyan ore blocks having large negative ce and eu anomalies which may be indicative of a change in redox conditions during fluid ascent migration and or cooling 3 34 s values for sphalerite galena pyrite and tetrahedrite sulfide samples range from 7 3 to 2 1 a wide range that indicates multiple sulfur sources the basin contains numerous sources of s and deriving s from a mixture of these sources could have yielded these near zero values either by mixing of s from different sources or by changes in the geological conditions of seawater sulfate reduction to sulfur 4 the c o isotopic analyses yield 13 c values from ca zero to 10 and a wider range of 18 o values from ca 6 to 24 suggestive of mixing between mantle derived magma and marine carbonate sources during the evolution of ore forming fluids although potential contributions from organic carbon and basinal brine sources should also be considered these data indicate that ore forming fluids were derived from a mixture of organism basinal brine and mantle derived magma sources and as such the eastern ore zone of the baiyangping polymetallic ore deposit should be classified as a lanping type ore deposit

reservoir analog studies using multimodel photogrammetry a new tool for the petroleum industry an advanced photogrammetric method called multimodel stereo restitution is a potential new tool for the petroleum industry when outcrop investigations are necessary such as in reservoir analog studies the method is based on very simple field photography techniques allowing the geologist to use his own standard small frame camera e g 24 36 or 60 60 mm formats it can be applied to geological studies of virtually any scale and outcrop mapping is significantly improved in detail accuracy and volume the method is especially useful when investigating poorly accessible exposures on steep mountain faces and canyon walls the use of multimodel photogrammetry is illustrated by a study of upper cretaceous deltaic sediments from the atane formation of west greenland detailed vertical cross sections of all outcrops in the area are measured and in addition horizontal maps of individual sand bodies are compiled on this basis data such as width thickness ratios sinuosity and the shapes of sand bodies and paleochannels crucial in reservoir analog studies are quantified and collected further potential applications of the method in petroleum explorations are discussed true scale mapping of lithologies in large continuous exposures can be used in understanding basin evolution and in seismic modeling close range applications can be used when modeling fault geometries and in studies of individual bed forms clay laminae cemented horizons and diagenetic fronts

some new data on the genesis of the linghou cu pb zn polymetallic deposit based on the study of fluid inclusions and c h o s pb isotopes abstract the linghou deposit located near hangzhou city of zhejiang province eastern china is a medium sized polymetallic sulfide deposit associated with granitic intrusion this deposit is structurally and lithologically controlled and commonly characterized by ore veins or irregular ore lenses in this deposit two mineralization events were identified of which the former produced the cu au ag orebodies while the latter formed pb zn cu orebodies silicification and calc silicate skarn type phyllic and carbonate alternation are four principal types of hydrothermal alteration the early cu au ag and late pb zn cu mineralizations are characterized by quartz sericite pyrite chalcopyrite bornite au ag minerals magnetite molybdenite and calcite dolomite sphalerite pyrite chalcopyrite galena respectively calcite clusters and calcite quartz vein are formed during the late hydrothermal stage the nacl h 2 o co 2 system fluid coexisting with nacl h 2 o system fluid and showing the similar homogenization temperatures 385 c and 356 c respectively and different salinities 16 89 21 68 wt nacl eqv and 7 70 15 53 wt nacl eqv suggests that fluid immiscibility occurred during the cu au ag mineralization stage and might have given rise to the ore metal precipitation the ore forming fluid of the pb zn cu mineralization mainly belongs to the nacl h 2 o co 2 system of high temperature 401 c and mid high salinity 10 79 wt nacl eqv fluids trapped in the quartz chalcopyrite vein cu au ag ores pb zn cu ores and calcite clusters yielded 18 o h2o and d values varying from 5 54 to 13 11 and from 71 8 to 105 1 respectively indicating that magmatic fluids may have played an important role in two mineralization events the 13 c pdb values of the calcite change from 2 78 to 4 63 indicating that the co 3 2 or co 2 in the ore forming fluid of the pb zn cu mineralization was mainly sourced from the magmatic system although dissolution of minor marine carbonate may have also occurred during the ore forming processes the sulfide minerals have homogeneous lead isotopic compositions with 206 pb 204 pb ranging from 17 958 to 18 587 207 pb 204 pb ranging from 15 549 to 15 701 and 208 pb 204 pb ranging from 37 976 to 39 052 indicating that metallic elements of the linghou deposit came from a mixed source involving mantle and crustal components based on geological evidence fluid inclusions and h o c s pb isotopic data the linghou polymetallic deposit is interpreted as a high temperature skarn carbonate replacement type two types of mineralization are both related to the magmatic hydrothermal system with the cu au ag mineralization having a close relationship with granodiorite

comparative behavior of sr nd and hf isotopic systems during fluid related deformation at middle crust levels abstract we have carried out a comparative rb sr sm nd and lu hf isotopic study of a progressively deformed hercynian leucogranite from the french massif central belonging to the la marche ductile shear zone in order to investigate the respective perturbation of these geochronometers with fluid induced deformation the one meter wide outcrop presents a strongly deformed and mylonitized zone at the center and an asymmetric deformation pattern with a higher deformation gradient on the northern side of the zone ten samples have been carefully collected every 10 cm north and south away from the strongest deformed mylonitic zone they have been analyzed for a complete major trace element data set oxygen isotopes rb sr sm nd and lu hf isotopic systematics we show that most of major and trace elements except sio2 alkaline elements k2o rb and some metal transition elements cu are progressively depleted with increasing deformation this depletion includes ree y but also hfs elements ti hf zr nb which are commonly considered as immobile elements during upper level processes variations in elemental ratios with deformation e g decrease in lree mree hree nd hf th sr increase in rb sr u th and constant sr nd lead to propose the following order of element mobility u th sr nd hf hree we conclude in agreement with previous tectonic and metallogenic studies that trace element patterns across the shear zone result from circulation of oxidizing f rich hydrothermal fluids associated with deformation a temperature of the fluid of 470 480 c can be deduced from the 18o equilibrium between quartz muscovite pairs elemental fractionation induces perturbation of the rb sr geochronometer the well defined 87rb 86sr 87sr 86sr correlation gives an apparent age of 294 19 ma slightly younger than the 323 4 ma age of leucogranites in this area this apparent age is interpreted as dating event of intense deformation and fluid circulation associated with mass transfer and exhumation of the ductile crust shortly after the leucogranite emplacement sm nd and lu hf isochron type diagrams do not define any correlation because of the low fractionated sm nd and lu hf ratios isotopic data demonstrate that only the lu hf geochronometer system is not affected by fluid circulation and gives reliable tdm age 1 29 0 03 ga and ehf signatures by contrast the sm nd geochronometer system gives erroneous old tdm ages of 2 84 4 ga there is no positive end ehf correlation because of decreasing end values with deformation at constant ehf values however end ehf values remain in the broad end ehf terrestrial array which strongly indicates that fluid induced fractionation can contribute to the width of the terrestrial array the strong ehf negative values of the leucogranite are similar to metasedimentary granulitic xenoliths from the french massif central and confirm the generation of the leucogranite by several episodes of reworking of the lower crust

trace element geochemistry of sphalerite in contrasting hydrothermal fluid systems of the freiberg district germany insights from la icp ms analysis near infrared light microthermometry of sphalerite hosted fluid inclusions and sulfur isotope geochemistry the historic silver mining district of freiberg germany comprises hydrothermal vein style mineralization of permian and cretaceous age we compare sphalerite compositions with associated ore forming fluids and constrain the behavior of critical metals such as in ge and ga in contrasting hydrothermal environments fluid inclusion studies reveal that the permian veins formed due to boiling and cooling of a low salinity 0 to 6 eq w nacl magmatic hydrothermal fluid at 350 to 230 c in contrast cretaceous veins formed by mixing of highly saline 17 to 24 eq w nacl cacl2 and variable na na ca ratios brines at low temperatures 120 c sulfides of the permian ore stage have a narrow range of 34svcdt from 2 3 to 0 9 while the sulfides of the cretaceous stage have a large scatter and significantly more negative 34svcdt values 30 9 to 5 5 supporting the different nature of the hydrothermal systems contrasting fluid systems and ore forming mechanisms correspond to markedly different trace element systematics in sphalerite permian sphalerite is significantly enriched in in up to 2500 g g in relative to two sphalerite generations of cretaceous veins the latter have higher ge up to 2700 g g ge and ga up to 1000 g g ga concentrations the observed trace element systematics of different sphalerite generations imply that in is enriched in high temperature low to intermediate salinity fluids with a significant magmatic hydrothermal fluid component while ge and ga are more concentrated in low temperature high salinity crustal fluids with no obvious magmatic hydrothermal affiliation

geochemistry and mineralization age of magnesian skarn type iron deposits of the janggun mine republic of korea the janggun iron deposits republic of korea occur as lens shaped magnesian skarn magnetite and base metal sulfide orebodies developed in the cambrian janggun limestone formation mineralization stage of the deposits can be divided into two separate events the skarn stage 107 ma consists of magnetite pyrrhotite base metal sulfides carbonates and magnesian skarn minerals the hydrothermal stage 70 ma consists of base metal sulfides native bismuth bismuthinite tetrahedrite boulangerite bournonite and stannite mineral assemblages chemical compositions and thermodynamic considerations indicate that formation temperatures log fs2 and log fo2 values of ore fluids from the skarn stage were 433 to 345 c 8 1 to 9 7 bar and 29 4 to 31 6 bar and the hydrothermal stage was 245 to 315 c 10 4 to 13 2 bar and 33 6 to 35 4 bar respectively thermochemical considerations indicate that the xco2 during magnesian skarnization ranged from 0 06 to 0 09 and the activity of h presumably decreased when the fluids equilibrated with host dolomitic limestone which resulted in a ph change from about 6 1 to 7 8 and decreases in fo2 and fs2 the 34s values of ore sulfides have a wide range from 3 2 to 11 6 cdt calculated 34sh2 s values of ore fluids are 2 9 to 5 4 skarn stage and 8 7 to 13 5 hydrothermal stage these are interpreted to represent an initial deep seated igneous source of sulfur which gave way to influence of oxidized sedimentary sulfur to hydrothermal stage the 13c values of carbonates in ores range from 4 6 to 2 5 pdb it is likely that carbon in the ore fluids was a mixture of deep seated magmatic carbon and dissolved carbon of dolomitic limestone the 18oh2 o and d values smow of water in the ore fluids were 14 7 to 1 8 and 85 to 73 during the skarn stage and 11 1 to 0 2 and 87 to 80 in the hydrothermal stage

evaluation of the volatilization and infiltration effects on the stable isotopic and mineralogical variations in the carbonate rocks adjacent to the cretaceous muamsa granite south korea the muamsa granite in the hwanggangri mineralized zone south korea intruded the cambro ordovician samtaesan formation which dominantly consists of calcitic marble and dolomitic marble with minor intercalations of pelitic rock and amphibolite intrusion of the granite caused an isotopic depletion of 13c and 18o in the carbonate rocks calcitic carbonates show systematic depletion from unrecrystallized limestone through calcitic marble to skarn it was initiated with volatilization accompanying calcsilicate forming reactions and then with increased permeability h2o rich siliceous fluids of igneous origin actively infiltrated the system promoting fluid rock interaction for the skarn fluid infiltration yielded fluid rock ratios as high as 4 2 in open system and caused increased 18o depletion rates compared to the volatilization stage more than 70 of the isotopic depletion in the calcitic marble is attributed to the infiltration process with such strong infiltration mineral assemblages such as phlogopite in the absence of k feldspar phlogopite diopside and wollastonite are commonly observed in the marble the dolomitic marble was comparatively less depleted in heavy isotopes compared to the calcitic marble and formed under a wide range of xco2 and low fluid rock ratios 1 0 active decarbonation in dolomitic marble was restricted by silica poor dolomite protolith which caused limited fluid infiltration and resulted in high xco2 mineral assemblages represented by dolomite calcite diopside forsterite and more 13c depleted nature compared to dolomite calcite tremolite forsterite assemblage

dating of archean auriferous quartz vein deposits in the abitibi greenstone belt canada 40 ar 39 ar evidence for a 70 to 100 m y time gap between plutonism metamorphism and mineralization the sigma mine at val d9or quebec in the abitibi belt is typical of vein type gold deposits in archcan greenstone belts although the relative timing of gold mineralization with respect to magmatism metamorphism deformation and hydrothermal events in the mine is well documented precise isotopic age constraints have until recently been lacking this paper presents the results of a 40 ar 39 ar dating study of the sigma mine and of the davidson tisdale deposit in the abitibi belt in the sigma mine dates on metamorphic amphibole indicate that the regional metamorphism which predates the vein formation occurred at ca 2 68 ga muscovite from the gold bearing quartz veins yields a plateau date of 2579 or 9 ma 2sigma whereas biotite from the veins shows disturbed saddle shaped age spectra the mild disturbance of the biotite is attributed to later diabase dike emplacement and a 40 ar 39 ar date on a dike indicates that this event is younger than 2 3 ga thermal effects of the grenville orogeny 1 2 1 0 ga may also be present a muscovite separate from a gold bearing quartz vein at the davidson tisdale deposit yields a 2607 or 10 ma 2sigma plateau date although the amphibole and muscovite dates at the sigma mine bracket the gold mineralization event at between ca 2 68 and 2 58 ga a number of tectonothermal histories are possible for this time interval it is concluded based on all evidence that the muscovite plateau dates are the time of vein and gold formation which thus occurred in the sigma mine ca 100 m y and in the davidson tisdale deposit ca 70 m y after syn to post tectonic magmatism and regional metamorphism

the timing of w sn rare metals mineral deposit formation in the western variscan chain in their orogenic setting the case of the limousin area massif central france new 40ar 39ar dating performed on rare metal granites and w sn deposits in the northern limousin has provided evidence of two metallogenic episodes an early namurian episode c 325 ma was contemporaneous with the emplacement of the large peraluminous leucogranite bodies which are associated with small w sn deposits but also with some larger deposits at puy les vignes 323 4 0 9 ma and moulin barret 323 7 0 8 ma formed at a shallower level above cryptic granite plutons these new data indicate that the metallogenic potential of the namurian leucogranites might have been underestimated most other w sn deposits in the northern limousin area are attributed to a mid westphalian episode c 310 ma and are contemporaneous with the emplacement of all the rare metal granites both episodes were related to leucogranite emplacement and associated fluid circulations but in two different geodynamic contexts the early namurian episode may be related to syncollisional extension of the variscan belt whereas the mid westphalian one occurs during generalized extension and rapid exhumation of the belt associated with the granulite facies metamorphism of the lower lithosphere probably related to the delamination of the lower lithosphere thus w sn and rare metals ta nb be li deposits are clearly temporally and probably genetically related to leucogranitic magmatism

cooling pattern and mineralization history of the saint sylvestre and western marche leucogranite pluton french massif central i 40ar 39ar isotopic constraints abstract the saint sylvestre western marche leucogranite complex lc is a composite pluton emplaced 324 m y ago in the variscan continental crust of the northwestern limousin french massif central petrographic structural and new isotopic data are used to constrain the cooling pattern of the pluton in the temperature range for ar retention in muscovite part i and the genetic relationships with the subsequent uranium mineralization postdating the time of intrusion by 40 50 m y part ii a suite of thirty seven muscovite and nine biotite concentrates selected over the entire plutonic complex were analyzed by the 40 ar 39 ar technique muscovite is characterized by well defined plateau ages ranging between 314 and 301 ma while biotite gives less well behaved age spectra with apparent ages varying between 320 and 300 ma the muscovite age variations are systematic and clearly associated with regional trends at the scale of the pluton field petrographic gravimetric and structural relationships indicate that the age pattern is controlled by the intrusion shape and the regional post cooling fault network cutting through the laccolith when restored to their approximate pre faulting geometry the age variations are shown to correlate positively with sample elevation with a top to base difference as large as 10 14 m y over a mean intrusion thickness of 3 km this pattern is interpreted to reflect cooling driven by erosion and exhumation of the thickened variscan crust during the westphalian and is confirmed by two dimensional thermal modelling part ii the data show that 40 ar 39 ar mica ages can postdate the time of crystallization of the host pluton by up to 23 ma depending on the level sampled at the surface they warn against interpreting k ar mica ages from similar low relief igneous settings as closely postdating intrusive events without a precise knowledge of the thermal structure cooling mechanism and post cooling faulting of the pluton in turn provided that the pre faulting geometry of the intrusion is properly assessed the vertical cooling pattern can reveal fine scale variations in the thermal structure of old eroded intrusions that may be extremely useful for recovering quantitative information on the mechanisms and rates of cooling in ancient orogens

how long can a hydrothermal system be sustained by a single intrusive event published numerical calculations show that the convective systems which produce near surface geothermal systems or ore deposits are likely to cool their intrusion heat source in at most a few tens of thousands of years even if the intrusion is large generally hydrothermal circulation near surface geothermal activity and magmatic intrusion should be closely tied in time and space long lived approximately 1 m y hydrothermal activity normally suggests multiple pulses of intrusion with associated pulses of hydrothermal circulation however if the permeability of the intruded environment is just above that which allows convection and the volume of the intrusion is very large calculations presented here show that a single episode of intrusion can sustain hydrothermal circulation and near surface geothermal activity for approximately 800 000 yr for a deep sill heat source these unusually long lived hydrothermal systems can vent through a few very widely separated discharge sites and have the potential to produce isolated massive sulfide deposits of unusually large size

40 ma of hydrothermal w mineralization during the variscan orogenic evolution of the french massif central revealed by u pb dating of wolframite we present u pb thermal ionization mass spectrometer tims ages of wolframite from several granite related hydrothermal w sn deposits in the french massif central fmc located in the internal zone of the variscan belt the studied wolframite samples are characterized by variable u and pb contents typically 10 ppm and show significant variations in their radiogenic pb isotopic compositions the obtained u pb ages define three distinct geochronological groups related to three contrasting geodynamic settings i visean to namurian mineralization 333 327 ma coeval with syn orogenic compression and emplacement of large peraluminous leucogranites ca 335 325 ma ii namurian to westphalian mineralization 317 315 ma synchronous with the onset of late orogenic extension and emplacement of syn tectonic granites ca 315 310 ma and iii stephanian to permian mineralization 298 274 ma formed during post orogenic extension contemporaneous with the permian volcanism in the entire variscan belt the youngest ages 276 274 ma likely reflect the reopening of the u pb isotopic system after wolframite crystallization and may correspond to late hydrothermal alteration e g ferberitization our results demonstrate that w sn mineralization in the fmc formed during at least three distinct hydrothermal events in different tectono metamorphic settings over a time range of 40 ma

comment on formation and evolution processes of the salanfe w au as skarns aiguilles rouges massif western swiss alps by chiaradia m mineralium deposita 38 154 168

the beauvoir topaz lepidolite albite granite massif central france the disseminated magmatic sn li ta nb be mineralization the beauvoir topaz lepidolite albite granite french massif central is the latest intrusion in a variscan peraluminous granitic complex composed of three units successively emplaced the concealed la bosse granite the colettes two mica granite and the beauvoir granite the 900 m deep drill hole of the geologie profonde de la france deep drilling program of the continental crust has provided a continuously cored section of the beauvoir granite the exposed section of the beauvoir granite is presently mined for kaolin in the upper 200 m it also represents a huge subeconomic disseminated li sn ta be deposit sharing many characteristics with rare metal bearing pegmatites compared to similar li f rich igneous bodies the beauvoir granite is strongly enriched in sn 200 1 400 ppm ta 20 400 ppm and be 20 300 ppm li is located in lepidolite and amblygonite ta and nb mainly in columbo tantalite and uranium rich microlite and sn in cassiterite be is located in lepidolite crystals and in unidentified mineral s the major element composition of the beauvoir granite is similar to that of macusani volcanic glasses of peru and ongonite subvolcanic rocks of mongolia the main structural textural mineralogical and geochemical characteristics of the beauvoir granite can be explained by extreme fractionation of f and li rich magmas and associated fluid phases most trace elements show a strong upward enrichment the high f sub o 2 low solidus temperature and high fluorine content of the beauvoir magma have considerably reduced sn and w fractionation in the expelled magmatic fluid despite its high chlorine content 25 30 wt nacl equiv however f and li enriched geochemical halos have developed in the enclosing mica schist in response to the boiling of the magmatic fluid the relatively deep emplacement of the granite 3 km has limited the development of hydraulic fracturing and subsequent hydrothermal circulation and mineralization thus despite the very high specialization of the beauvoir granite no significant vein type mineralization is directly related to it the la bosse quartz ferberite stockwork was developed before the emplacement of the colettes and beauvoir granites and is related to the earlier concealed la bosse granite

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

mineralogy geochemistry and age constraints on the zn pb skarn deposit of maria cristina quebrada galena northern chile the maria cristina zn pb skarn deposit hosted by carbonate rocks of the lower cretaceous chanarcillo group in northern chile is described to provide a basis of comparison to adjacent 2 km barite deposits and regional pb zn and ba deposits with mississippi valley type affinities strongly retrograded garnet andradite to ad30gr70 diopsidic pyroxene and epidote skarn and barite bearing massive sulfides sphalerite pyrite galena marcasite magnetite occur at the contact of potassically altered diorite porphyry of mid cretaceous age the skarn mineral compositions similar to those of cu skarns may reflect emplacement in a high f o2 context sulfur isotope values of sulfides 8 8 to 7 1 and barite 14 0 26 1 indicate disequilibrium conditions and different sulfur sources including magmatic sulfur lead isotope ratios of galena and strontium isotope ratios of barite indicate a similar metal reservoir for the different ore deposit types constituted by the intercalated lower cretaceous volcanic rocks or mid cretaceous intrusive rocks 40ar 39ar dating of hydrothermal k feldspar suggests an age between 94 and 90 ma for the mineralization

geochemical constraints on the genesis of hercynian two mica leucogranites from the massif central france abstract late hercynian two mica leucogranites dated at 340 280 3a old occur across the entire massif central france and form dykes and stocks up to 130 km in length they are peraluminous molecular al 2 o 3 na 2 o k 2 o cao asi 1 16 1 25 and have low initial nd 8 2 to 6 1 indicating derivation from the metasedimentary lower crust as sampled in crustal xenoliths nd 340 8 6 to 4 3 nd 280 9 0 to 4 8 their high sio 2 70 78 wt p 2 o 5 up to 0 5 rb and ga and low ba sr and ree are mainly a result of partial melting processes rather than fractionation partial melting was initiated by underplating of the lower crust by mantle derived basic magmas between 360 and 270 ma and was intensified by granulite facies metamorphism between 300 and 280 ma from this model there is little similarity between the genesis of two mica leucogranite in the massif central and that in the himalaya with which it has previously been compared

yttrium and rare earth elements in fluids from various deep sea hydrothermal systems rare earth element ree and yttrium y concentrations were measured in fluids collected from deep sea hydrothermal systems including the mid atlantic ridge mar i e menez gwen lucky strike tag and snakepit the east pacific rise epr i e 13 n and 17 19 s and the lau vai lili and manus vienna woods pacmanus desmos back arc basins bab in the south west pacific in most fluids y is trivalent and behaves like ho chondrite normalized y ree y reen concentrations of fluids from mar epr and two bab sites i e vai lili and vienna woods showed common patterns with lree enrichment and positive eu anomalies ree analysis of plagioclase collected at lucky strike strengthens the idea that fluid ree contents are controlled by plagioclase phenocrysts other processes however such as ree complexation by ligands cl f so42 secondary phase precipitation and phase separation modify ree distributions in deep sea hydrothermal fluids ree speciation calculations suggest that aqueous ree are mainly complexed by cl ions in hot acidic fluids from deep sea hydrothermal systems ree concentrations in the fluid phases are therefore influenced by temperature ph and duration of rock fluid interaction unusual y reen patterns found in the pacmanus fluids are characterized by depleted lree and a positive eu anomaly the demos fluid sample shows a flat y reen pattern which increases regularly from lree to hree with no eu anomaly these manus basin fluids also have an unusual major element chemistry with relatively high mg so4 h2s and f contents which may be due to the incorporation of magmatic fluids into heated seawater during hydrothermal circulation ree distribution in pacmanus fluids may stem from a subseafloor barite precipitation and the ree in demos fluids are likely influenced by the presence of sulfate ions

tracing metal sources in peribatholitic hydrothermal w deposits based on the chemical composition of wolframite the example of the variscan french massif central abstract we present a complete dataset for major minor and trace elements in wolframite fe mn wo4 based on the example of several peribatholitic hydrothermal w deposits located in the variscan french massif central fmc the wolframite samples were characterized down to the micrometer scale by combining scanning electron microscopy observations and in situ chemical analysis by electron probe microanalyzer and laser ablation inductively coupled plasma mass spectrometry most samples are fe dominated wolframites and are characterized by variable fe fe mn values for the studied deposits from 0 36 to 0 86 without significant alteration features wolframites present intracrystalline variations for concentrations of several minor and trace elements principally for nb ta and sn which reflect growth zoning the minor and trace element compositions of the different wolframite samples show at first order similar enrichments in ta nb sn zn and in as well as depletions in lree li th ti zr hf pb v co and mg compared to the upper continental crust second order compositional variations are observed between the studied deposits which can be separated in four main regional geochemical groups this work highlights that the chemical composition of wolframite is controlled by both the crystallochemical parameters and the composition of the primary hydrothermal fluid the regional variations observed in the geochemical signatures of wolframite reflect local variations in the composition of the hydrothermal fluid i e variable metal sources therefore this suggests that the minor and trace element contents in wolframite likely represent direct markers of the source of the hydrothermal fluid and ultimately of the metals the comparison between the trace element compositions of wolframite with the compositions of regional granitic rocks and worldwide shales suggests that evolved peraluminous granites represent the main source of metals in peribatholitic hydrothermal w deposits from the fmc whereas metasedimentary rocks have a limited to no influence on the metal budget

sphalerite geochemistry of the zn pb orebodies in the taebaeksan metallogenic province korea abstract the taebaeksan region located in the northeastern part of south korea is an important metallogenic province which hosts numerous zn pb w mo fe cu au ag mineral systems the zn pb orebodies in the region inclusive of skarn carbonate replacement and vein breccia ore types are hosted on the early paleozoic carbonate sequences and they are associated with late cretaceous to early paleogene magmatic hydrothermal activities we collected zn pb orebodies from 15 areas including non metal deposits such as high ca calcite marble as well as exploration sites in the taebaeksan metallogenic province thereafter we studied the geochemistry of the sphalerite present in them to understand the regional zn pb mineralization process and to establish an exploration strategy the sphalerites collected from the region contained detectable amounts of fe mn cd co cu ga ge mo ag in sn sb w tl pb and bi which were identified using microbeam techniques like epma and la icp ms sphalerites from economically significant zn pb deposits characteristically have high mn and low cd content while w mo bearing deposits have remarkably high cd content correlation trends between trace elements demonstrate the coupled substitution mechanisms such as 3 zn2 2 cu or ag sn4 and 2 zn2 cu in3 occurring in the sphalerite high temperature ores such as skarn and vein breccia are relatively enriched with co and in while ga ag and sn tend to be concentrated in the relatively low temperature ores such as carbonate replacement type ores lead isotope compositions of the sphalerites show a well defined positive linear trend with those of the local cretaceous intrusions and the sedimentary sequence of the taebaeksan region major economic zn pb deposits show relatively more signatures of non radiogenic pb isotope closer to the intrusions as compared to the minor and sub economic zn pb orebodies the pb isotope signature suggests that cretaceous magmatic hydrothermal fluids with a minimal degree of fluid rock interactions are required for economic zn pb mineralization in the taebaeksan metallogenic province

tungsten mineralization in the tien shan gold belt geology petrology fluid inclusion and stable isotope study of the ingichke reduced tungsten skarn deposit western uzbekistan abstract the ingichke reduced w skarn deposit 100kt wo 3 with the average grade of 0 58 wo 3 is the largest one among w deposits known in the tien shan gold belt it is associated with an early permian multiphase granitoid pluton that comprises ilmenite series to ilmenite titanite bearing medium to high k metaluminous to peraluminous i type granitoid monzodiorite granodiorite to granite leucogranite rocks the pluton is related to crustal accretionary type reservoirs of granitoid magma likely formed in response to an ascent of asthenospheric material at the post collisional stage it is part of the continuum of plutonic suites related to reduced w au polymetallic deposits with further links to reduced intrusion related au deposits the deposit comprises abundant pyroxene mainly hedenbergitic skarn and a minor garnet skarn mostly grossular rich garnet with elevated almandine and spessartine contents hydrosilicate propylitic alteration amphibole chlorite calcite quartz with abundant pyrrhotite and minor chalcopyrite partially overprints skarn and is in turn overprinted by zones of phyllic alteration quartz sericite fe carbonate locally with albite scheelite and sulfides bi and au minerals prograde and retrograde skarns were formed from aqueous dominantly magnesian sodic chloride moderately saline 10 14 to 4 7 wt nacl eq hot fluid with its progressing enrichment in ca hydrosilicate propylitic and phyllic alteration assemblages were formed from boiling low salinity carbonic aqueous magnesium sodic calcic fluids at moderate 330 355 c to lower 250 265 c temperatures respectively with separation of carbonic with varying co 2 ch 4 ratio and aqueous chloride fluids trapping pressures ranged from 2000 to 1500 bars indicating a lithostatic depth of some 6 to 8 km i e a lower intermediate level in an integrated model of reduced w skarn deposits the stable isotope data indicate an influence of a sedimentary limestone source of oxygen and carbon at the retrograde skarn stage 18 o h2o 8 1 to 9 0 13 c co2 2 5 to 0 3 and a dominantly magmatic mantle to crustal derived source of water 18 o h2o 7 0 0 5 at the hydrosilicate propylitic and phyllic alteration stages and carbon 13 c co2 7 8 to 4 7 at the hydrosilicate propylitic stage at the phyllic alteration stage a strong enrichment in the light carbon isotope 13 c co2 10 1 to 9 4 indicates a possible influence of a reduced organic carbon source a magmatic source of sulfur is envisioned for the hydrosilicate propylitic 34 s 2 0 0 5 and phyllic 34 s 1 0 0 3 alteration stages

the mineralogy of copper bearing skarn to the east of the sungun chay river east azarbaidjan iran abstract a calcic copper bearing skarn zone in east azarbaidjan nw of iran is located to the east of the sungun chay river skarn type metasomatic alteration and mineralization occurs along the contact between upper cretaceous impure carbonates and an oligo miocene cu bearing granitoid stock both endoskarn and exoskarn are developed along the contact exoskarn is the principal skarn zone enclosed by a marmorized and skarnoid hornfelsic zone the skarnification process occurred two stages 1 prograde and 2 retrograde the prograde stage is temporally and spatially divided into two sub stages a metamorphic bimetasomatic sub stage i and b prograde metasomatic sub stage ii sub stage i began immediately after the intrusion of the pluton into the enclosing impure carbonates sub stage ii commenced with segregation and evolution of a fluid phase in the pluton and its invasion into fractures and micro fractures of the marmorized and skarnoid hornfelsic rocks developed during sub stage i the introduction of considerable amounts of fe si and mg led to the development of substantial amounts of medium to coarse grained anhydrous calc silicates from texture and mineralogy the retrograde metasomatic stage can be divided into two discrete sub stages a early sub stage iii and b late sub stage iv during sub stage iii the previously formed skarn zones were affected by intense multiple hydro fracturing phases in the cu bearing stock in addition to fe si and mg substantial amounts of cu pb zn along with volatile components such as h 2 s and co 2 were added to the skarn system consequently considerable amounts of hydrous calc silicates epidote tremolite actinolite sulfides pyrite chalcopyrite galena sphalerite bornite oxides magnetite hematite and carbonates calcite ankerite replaced the anhydrous calc silicates sub stage iv was concurrent with the incursion of relatively low temperature more highly oxidizing fluids into skarn system bringing about partial alteration of the early formed calc silicates and developing a series of very fine grained aggregates of chlorite clay hematite and calcite

carbon oxygen isotopic geochemistry of the yangla cu skarn deposit sw china implications for the source and evolution of hydrothermal fluids abstract the yangla cu deposit is the largest cu skarn deposit in the jinshajiang tectonic belt based on the detailed observation of crosscutting relationships three mineralization stages i e pre ore ore and supergene have been identified in the yangla deposit the pre ore stage is dominated by prograde skarn the ore stage is characterized by the precipitation of hydrous silicate minerals fe oxides fe cu mo sulfides quartz and calcite whose mineral assemblages were formed in the early and late sub ore stages the early sub ore stage is marked by retrograde alteration with the deposition of hydrous silicate minerals e g actinolite epidote and chlorite fe oxides abundant fe cu mo sulfides quartz and minor calcite whilst the late sub ore stage associated with silicic and carbonate alteration is represented by widespread thick quartz or calcite veins with disseminated pyrite chalcopyrite galena and sphalerite we present new carbon oxygen c o isotopic compositions of the ore hosting marble and hydrothermal calcite of this deposit the hydrothermal calcite in the yangla deposit was precipitated from both the early and late sub ore stages calcite i from the early sub ore stage is anhedral and occurs as spot in the skarn or locally replaces the skarn minerals calcite ii from the late sub ore stage is distinguished by being coarse grained subhedral to euhedral and its occurrence in thick veins calcite i contains lower 13 c pdb 7 0 to 5 0 and 18 o smow 7 2 to 12 7 than calcite ii 13 c pdb 4 5 to 2 3 18 o smow 10 7 to 19 4 in the 13 c pdb vs 18 o smow diagram the calcite i and calcite ii data fall close to the igneous carbonatite field and between the fields of igneous carbonatite and marine carbonates respectively this suggests a dominantly magmatic origin for the early sub ore fluids and there might have been increasing carbonate wall rock involvement towards the late sub ore stage the ore hosting marble 13 c pdb 4 8 to 0 3 18 o smow 10 2 to 23 9 also shows a positive 13 c pdb vs 18 o smow correlation which is interpreted to reflect the decreasing alteration intensity during the interactions between the hydrothermal fluids and ore hosting carbonates simulated calculation suggests that both the calcite i and calcite ii precipitated at 350 c to 250 c and 250 c to 150 c respectively we proposed that co 2 degassing and water rock interactions were likely the two major processes that precipitated the calcite and led to the observed c o isotopic features of the yangla cu deposit

geology mineral chemistry and formation conditions of calc silicate minerals of astamal fe lree distal skarn deposit eastern azarbaijan province nw iran the astamal fe lree skarn deposit is the largest iron skarn in nw iran it is a unique case as a distal skarn deposit which crops out approximately 600 m from the associated oligo miocene granodioritic pluton this deposit formed in the south southwest of the pluton where fractures and faults within the upper cretaceous volcano sedimentary host rocks acted as conduits for the mineralizing fluids the deposit contains three iron ore bodies southern northern and eastern the main mineral assemblage within the ore zones is characterized by magnetite pyrrhotite and pyrite with lesser quantities of chalcopyrite hematite goethite and limonite the skarn minerals predominantly comprise garnet epidote actinolite calcite quartz clinopyroxene and chlorite in order of abundance retrograde alteration is strongly developed in the skarn zone where most of the garnet has been pervasively altered to secondary minerals e g epidote calcite and quartz both in the rims and the cores whereas the majority of the clinopyroxene has been replaced by a hydrous retrograde mineral assemblage e g tremolite actinolite and chlorite garnets with andradite grossular compositions are the dominant mineral in the skarn zone which are generally isotropic with a narrow compositional range along the growth lines adr94 3 64 5grs21 9 2 7alm11 1 0 2 these garnets are fe rich and have high fe fe al ratios between 0 96 and 0 78 cu and ni are enriched in the garnets this suggests that these elements were enriched in the hydrothermal fluids from which the garnet precipitated this is supported by the presence of chalcopyrite and ni bearing massive magnetite in the study area which also suggests that cu and ni were enriched in the late stage ore bearing hydrothermal fluids clinopyroxene with a hedenbergitic composition is generally homogenous and has particularly high fe fe mg ratios between 0 99 and 0 86 and is poor in tio2 mno and cr2o3 high zn concentrations were also detected in the clinopyroxenes up to 1044 ppm despite an absence of significant zn mineralization such as sphalerite in the district therefore it is believed that the proportion of zn in the hydrothermal fluids decreased significantly from the time of clinopyroxene formation to the period of the sulfide deposition phase allanite and lree bearing epidotes are the main lree bearing minerals in this deposit the epidote is also fe rich with high fe fe al ratios between 0 32 and 0 44 due to a lack of replacement texture between both garnet and clinopyroxene and garnet and actinolite which is formed by alteration of clinopyroxene it is believed that these two minerals have grown simultaneously and are coexisting minerals in the astamal skarn these minerals can be stable and coexisting at temperatures between 490 and 560 c and log o2 16 to 31

lower crustal rejuvenation and growth during post thickening collapse insights from a crustal cross section through a variscan metamorphic core complex the variscan lower crust of europe acquired its main features seismic layering intrusion of mantle derived magmas granulite grade metamorphism when the upper crust underwent post thickening extension through gravitational collapse we suggest that post thickening collapse is a geodynamic process that contributes to the growth and differentiation of continental lithosphere in collapsed mountain belts

the genesis of lct type granitic pegmatites as illustrated by lithium isotopes in micas abstract isotopic compositions in the monts d ambazac pegmatite field french massif central exhibit a narrow range of mica 7li values ranging from 3 6 to 3 4 the value obtained in biotite from the host saint sylvestre granite falls within this range 7li 1 5 lithium concentrations are consistent with the degree of magmatic evolution of each pegmatite type from 630 ppm in type ii up to 13 500 ppm in the more evolved type vi pegmatite although the rare element contents e g li cs and ta of the micas are consistent with pegmatite differentiation 7li are firstly independent of the degree of magmatic differentiation independent of pegmatite type and secondly independent of the content of li and other flux elements such as be and cs muscovite sampled in pegmatite v from the chabannes locality is the only pegmatite to exhibit a 7li variation from an intermediate unit 1 7 to an internal pegmatitic unit 3 4 the nature of this 7li variation suggests that there was extensive fractional crystallization during the pegmatite s consolidation the independence of 7li evolution from the degree of magmatic evolution and the presence of distinct major rare element bearing phases throughout the pegmatite field tend to confirm that the 7li values recorded in mica are inherited from crustal source rocks common to the granite and pegmatite forming melts we propose that the distinct pegmatite subtypes beryl columbite vs lepidolite petalite subtypes observed throughout the monts d ambazac pegmatite field reflect the diverse contributions of crustal protoliths the lack of evidence of surrounding alteration combined with the absence of increasing li content within the host granite tends to confirm that the 7li values obtained within this pegmatite field are primary and that no li diffusional process and or mixing driven li isotope fractionation has overprinted these isotopic compositions in light of these results the process of partial melting of protoliths enriched in rare element bearing phases e g mica and garnet seems to be more responsible for li isotope fractionation than li diffusion or fractional crystallization at the temperature of pegmatite consolidation finally we discuss the use of li isotopic compositions to identify the most highly evolved pegmatitic systems

mineralogy and metasomatic evolution of distal strata bound scheelite skarns in the riba de alva mine northeastern portugal the metasediment hosted distal scheelite skarns from the riba de alva mine barca de alva escalhao region northeastern portugal consist of three main levels of strata bound boudin shaped 0 5 to 4 m thick bodies having a total resource of 380 000 tons t with an average grade of 0 51 percent wo3 the boudinage of the cambrian metasedimentary rocks of the slate greywacke complex occurred early during the hercynian orogeny prior to the granitic complex intrusion and the skarn forming process the granitic complex corresponds to reduced highly evolved and metallogenically specialized s type leucogranites comparable to those commonly associated with mo poor w f skarns four different skarn types were recognized pyroxene skarn with fe diopside anorthite amphibole epidote skarns with actinolite clinozoisite bytownite phlogopite ore skarn with clinozoisite scheelite fluorite calcite labradorite apatite fe axinite armenite scapolite and late stage veins with quartz or fluorite or calcite clinozoisite andesine or albite adularia clinopyroxene ranges from di80hd20 to di60hd40 amphibole compositions range from actinolite to mg hornblende and are f rich 0 5 1 9 wt clinozoisite often zoned with fe poor cores shows little compositional variation in the skarn types having a low pistacitic component 5 13 ps plagioclase compositions grade from anorthite in pyroxene skarn through andesine and albite in late stage veins phlogopite as amphibole is f rich 2 45 4 70 wt an extremely pure mo free scheelite the only ore mineral present is restricted to the ore skarn type the riba de alva skarns were formed after intrusion of the hercynian granitic complex by selective infiltrative metasomatic alteration of argillaceous dolomitic limestone an early stage of prograde alteration led to the formation of the pyroxene skarn through reaction with an si rich and fe bearing fluid scheelite mineralization occurred during retrograde alteration which was mainly fracture controlled and is characterized by a pervasive infiltration metasomatism with minor diffusion in the skarn walls this alteration started with hydration reactions to form amphibole epidote skarns probably under lower xco2 conditions ore skarn formation with scheelite deposition took place later during this retrograde stage mainly along veins within the strata bound boudins as the result of a decrease in temperature and or significant changes in the activity of some fluid components such as w f na mn p and b and particularly an increase in aca monomineralic late stage veins represent the waning stage of the skarn forming process

microfracturing and fluid mixing in granites w sn ore deposition at vaulry nw french massif central abstract the vaulry w sn mineralisation located at the eastern boundary of the blond rare metal leucogranite is contained in a set of subvertical quartz veins locally with muscovite and minor quartz selvages the sequence of deposition was 1 milky quartz predominantly as fracture filling generally affected by subsequent ductile deformation 2 hyaline quartz wolframite cassiterite 3 minor sulphides other sets of quartz veinlets although generally barren are observed in the blond massif fluid migration at the microscopic scale within the granite and in the vicinity of quartz fractures was constrained by studying the geometry of fluid inclusion planes and fluid inclusion chemistry in and outside the mineralised area three major sets of subvertical fluid inclusion planes are recognised a n050 060 e set mostly developed in the veins and in the immediate vicinity a n110 130 e set regionally developed in the granite and a n140 160 e set of local extent as a whole the density of fip decreases from the mineralised zones toward the barren part of the pluton except for the n140 160 e set these are locally abundant around quartz veinlets with similar orientations that form a broad n s band near the blond locality mineralising fluids observed as primary inclusions in cassiterite and in undeformed hyaline quartz are mostly aqueous with moderate salinity and a minor volatile component at variance with many other w sn deposits in the variscan belt ore deposition occurred around 315 c at an estimated depth of 5 5 km under hydrostatic to slightly suprahydrostatic pressures it resulted from fluid mixing in the central part of a large hydrothermal system between two end members i a hot 425 430 c moderately saline fluid that contained a diluted volatile component and although na dominated minor amounts of li and ca the estimated 18o indicates that this fluid was completely equilibrated with the tectono magmatic pile pseudo metamorphic fluid ii a cold 230 c low salinity fluid evolved meteoric water that mixed with and eventually overprinted the early moderately saline fluid responsible for granite muscovitization at 425 430 c later a second hydrothermal system was initiated by the percolation of heated meteoric water with very low salinity the system was by then at least 3 5 km deep and the fluid was heated to 300 c these characteristics are reminiscent of the ca 305 ma episyenitic hydrothermal system known elsewhere in the n limousin

genetic environment of w skarn and pb zn vein mineralization associated with the imog granite in the taebaeksan mineralized district south korea abstract w skarn and pb zn vein mineralization were developed in the imog and seojin deposits in association with the imog granite that intruded the cambro ordovician carbonate rocks in the taebaeksan mineralized district south korea garnet pyroxene ratios in the skarn of both deposits decreased as away from the granite in the imog deposit garnet compositions vary from andradite to grossular series with increasing distance from the igneous contact however andradite is dominant in the seojin deposit in both the deposits diopside series remained dominant with a slight increase in johannsenite composition in late stage the skarn ore minerals mainly comprise scheelite and pyrrhotite whereas the vein ores comprise galena sphalerite and arsenopyrite in sphalerite a decrease in the cds contents and an increase in the mns contents from w skarn to pb zn vein ore can be attributed to leaching from sedimentary rocks and high mobility in magmatic hydrothermal fluids respectively the 34sh2s values of sulfide minerals were 3 3 4 9 for w skarn and 4 1 6 7 for pb zn vein ore in the imog deposit mostly belonging to magmatic origin but a slight increase is observed in the vein stage due to the interaction with carbonate host rock the isotopic depletion of 18o and 13c values from host rock to skarn and pb zn vein can be mostly attributed to the infiltration of external fluids involving magmatic components the effects of organic carbon in the host rock and input of meteoric water resulted in distinctly low 13c and 18o values in the skarn and vein samples respectively the sulfur isotope geothermometer and mineral stability in combination with fluid inclusion data constrained the upper limit of skarn formation to 520 c and 3 kbar and a temperature range of 0 5 kbar for w and pb zn mineralization in the imog deposit but pressure would be higher in the seojin deposit the skarn was formed by the intrusion of oxidized magma and scheelite mineralization occurred with decreasing oxygen fugacity the pb zn vein ores were precipitated with decreasing sulfur fugacity and increasing ph under the influence of meteoric water and carbonate host rock

metallogenesis in the french part of the variscan orogen part i u preconcentrations in pre variscan and variscan formations a comparison with sn w and au abstract in the french part of the variscan orogen the pre variscan metamorphic rocks and the variscan granites resulting from pure crustal recycling are enriched in uranium and tin data on tungsten and gold are too scarce for a reasonable estimation of the average concentrations of these elements to be obtained the spatial distribution of uranium preconcentrations is transverse to the main variscan structural domains and the distribution of the sn w mineralizations prompts the same conclusions u sn w and au metal enrichment in crustal domains in europe seems to result from late proterozoic to early palaeozoic basic and acid magmatism for uranium at least two main stages of partial melting early palaeozoic and late carboniferous lead to enrichment of the metal in peraluminous leucogranites these leucogranites represent the main source of late variscan uranium and tin within the peraluminous granitic complexes the main metal source is related to the late intrusion of small granitic cupolas strongly enriched in metals in the case of uranium the efficiency of the hydrothermal remobilization depends on the proportion of metal located in uraninite which is easily leachable by hydrothermal solutions the emplacement of these cupolas is structurally controlled in well defined areas by large shear zones these shear zones are continuously active during the successive emplacement of the granitic magmas most of the hydrothermal uranium mineralization is related to the permian reactivation of the shear zones in a brittle stage these zones then channel the hydrothermal fluids involved in the last metal concentration step leading to the formation of the orebodies the efficiency of metal remobilization is dependent on a close spatial relationship between magmatic structures enriched in uranium and brittle structures channeling the hydrothermal fluids

carbon and oxygen isotope constraints on fluid sources and fluid wallrock interaction in regional alteration and iron oxide copper gold mineralisation eastern mt isa block australia the source of metasomatic fluids in iron oxide copper gold districts is contentious with models for magmatic and other fluid sources having been proposed for this study 18o and 13c ratios were measured from carbonate mineral separates in the proterozoic eastern mt isa block of northwest queensland australia isotopic analyses are supported by petrography mineral chemistry and cathodoluminescence imagery marine meta carbonate rocks ca 20 5 18o and 0 5 13c calcite and graphitic meta sedimentary rocks ca 14 18o and 18 13c calcite are the main supracrustal reservoirs of carbon and oxygen in the district the isotopic ratios for calcite from the cores of na ca alteration systems strongly cluster around 11 18o and 7 13c with shifts towards higher 18o values and higher and lower 13c values reflecting interaction with different hostrocks na ca rich assemblages are out of isotopic equilibrium with their metamorphic hostrocks and isotopic values are consistent with fluids derived from or equilibrated with igneous rocks however igneous rocks in the eastern mt isa block contain negligible carbon and are incapable of buffering the 13c signatures of co2 rich metasomatic fluids associated with na ca alteration in contrast plutons in the eastern mt isa block have been documented as having exsolved saline co2 rich fluids and represent the most probable fluid source for na ca alteration intrusion proximal skarn like cu au orebodies that lack significant k and fe enrichment e g mt elliott display isotopic ratios that cluster around values of 11 18o and 7 13c calcite indicating an isotopically similar fluid source as for na ca alteration and that significant fluid wallrock interaction was not required in the genesis of these deposits in contrast k and fe rich intrusion distal deposits e g ernest henry record significant shifts in 18o and 13c towards values characteristic of the broader hostrocks to the deposits reflecting fluid wallrock equilibration before mineralisation low temperature low salinity low 18o 10 calcite and co2 poor fluids are documented in retrograde metasomatic assemblages but these fluids are paragenetically late and have not contributed significantly to the mass budgets of cu au mineralisation

magmatic hydrothermal processes in sangdong w mo deposit korea study of fluid inclusions and 39ar 40ar geochronology abstract the sangdong scheelite molybdenite deposit in northeast south korea consists of strata bound orebodies in intercalated carbonate rich layers in the cambrian myobong slate formation among them the m1 layer hosts the main orebody below which lie layers of f1 f4 host footwall orebodies each layer was first skarnized with the formation of a wollastonite garnet pyroxene assemblage hosting minor disseminated scheelite the central parts of the layers were subsequently crosscut by two series of quartz veining events hosting minor scheelite and major scheelite molybdenite ores respectively the former veins associate amphibole magnetite amphibole alteration whereas the latter veins host quartz biotite muscovite mica alteration deep quartz veins with molybdenite mineralization are hosted in the cambrian jangsan quartzite formation beneath the myobong formation in the sunbawi area which is in close proximity to the sangdong deposit quartz veins with scheelite mineralization are hosted in precambrian metamorphic basement three muscovite 39ar 40ar ages between 86 6 0 2 and 87 2 0 3 ma were obtained from m1 and f2 orebodies from the sangdong deposit and sunbawi quartz veins the upper cretaceous age of the orebodies is concordant with the published ages of the hidden sangdong granite 87 5 4 5 ma this strongly suggests that the intrusion is causative for the sangdong w mo ores and sunbawi veins fluid inclusions in the quartz veins from the m1 and f2 orebodies the deep quartz molybdenite veins and the sunbawi veins are commonly liquid rich aqueous inclusions having bubble sizes of 10 30 vol apparent salinities of 2 8 wt nacl eqv and homogenization temperatures of 180 350 c the densities of the aqueous inclusions are 0 70 0 94 g cm3 no indication of fluid phase separation was observed in the vein to constrain the formation depth in the sangdong deposit fluid isochores are combined with ti in quartz geothermometry which suggests that the m1 and f2 orebodies were formed at depths of 1 3 km and 5 6 km below the paleosurface respectively the similarity of the cs cesium concentrations and rb sr ratios in the fluid inclusions of the respective orebodies indicate an origin from source magmas having similar degrees of fractionation and enrichment of incompatible elements such as w and mo high s concentrations in the fluids and possibly organic c in the sedimentary source likely promoted molybdenite precipitation in the sangdong orebodies whereas the scheelite deposition in the deep quartz molybdenite veins hosted in the quartzite is limited by a lack of ca and fe in the hydrothermal fluids the molybdenite deposition in the sunbawi quartz molybdenite veins hosted in the precambrian metamorphic basement rocks was possibly limited by a lack of reducing agents such as organic c

mineral assemblages of the francisco i madero zn cu pb ag deposit zacatecas mexico implications for ore deposit genesis abstract the francisco i madero deposit central mexico occurs in the mesozoic guerrero terrane which hosts many ore deposits both cretaceous volcanogenic massive sulfides and tertiary epithermal and skarn deposits it is hosted by a 600 m thick calcareous pelitic unit of lower cretaceous age crosscut by porphyritic dikes that strike nw se a thick felsic volcanic tertiary sequence consisting of andesites and rhyolitic ignimbrites unconformably overlies the cretaceous series at the base the mineralization consists of several mantos developed within calcareous beds they are dominantly composed of sphalerite pyrrhotite and pyrite with minor chalcopyrite arsenopyrite and galena at the top of the orebody there are calcic skarns formed through prograde and retrograde stages the resulting mineral assemblages are rich in manganoan hedenbergite hd 75 28 di 40 4 jh 40 20 andraditic garnets adr 100 62 grs 38 0 epidote ep 95 36 czo 60 5 pie 8 0 chamosite calcite and quartz the temperature of ore deposition estimated by chlorite and arsenopyrite geothermometry ranges from 243 to 277 c and from 300 to 340 c respectively the pressure estimated from sphalerite geobarometry averages 2 1 kbar this value corresponds to a moderately deep skarn and agrees with the high cu content of the deposit paragenesis p t conditions and geological characteristics are compatible with a distal dike related zn skarn deposit its style of mineralization is similar to that of many high temperature carbonate replacement skarn deposits in the southern cordillera

geochemical evidence for a multistage magmatic genesis of ta sn li mineralization in the granite at beauvoir french massif central the beauvoir rare metal granite at the southern edge of the echassieres granite massif french massif central is mineralized by disseminated ta sn and li the granite body formed from an independent melt which was emplaced soon after the main granite facies of the massif the geochemistry of the beauvoir granite relates it to the phosphorus rich type of ta bearing granites and geochemical and mineralogical data establish its similarity to pegmatite melts the beauvoir granite results built up from two distinct intrusions both intrusions are cogenetic and were emplaced within a short time interval a geochemical study of a deep drill hole bored down to 900 m under the surface to the floor of the granite sheet with selected key zones observed in greater detail has allowed the recreation step by step of a long process leading to the economic concentration of ta sn and li the comparison of major element trace element and isotopic geochemistry with the petrology makes it possible to unravel the numerous processes contributing to the ore formation local events need to be determined before main events can be identified constraints on the interpretation of the ore genesis are provided by a careful observation of data interpretations of these observations are strongly backed up by experimental data e g keppler 1993 the source rock was most likely enriched in sn and w at about 10 ppm precluding a slightly enriched granitic melt by partial melting it is not known whether the melt was already enriched in ta but this is not a prerequisite subsequently deep seated fractional crystallization produced granitic magma similar in composition ta nearly equal 9 m to the adjacent colettes granite and further fractionation processes led to a melt already strongly enriched in ta ca 40 ppm strongly peraluminous and enriched in the depolymerizing elements f and p the viscosity was therefore very low allowing easy ascent during transport small scale fluctuations in the viscosity were amplified by upward movement leading to an f and p rich melt preceding the magma batch this less viscous melt again underwent crystallization while moving through the upper

a two stage exhumation in western french massif central new geochronological evidences of syn collisional extension abstract systematic 40ar 39ar dating of granitoids and surrounding metamorphic rocks has been performed in the limousin area w of french massif central in order to provide new constrains concerning its late orogenic thermal and tectonic evolution the same 40ar 39ar age of ca 335 338 ma is found on the present day outcrop surface for most of the limousin metamorphic rocks hence indicating that the entire massif was uniformly cooled down at this period these ages are very close to the 40ar 39ar ages proposed in the sioule and brevenne areas in the literature but also to the coupled tectonic and magmatic activity in the south limousin these results help to document the existence of two exhumation stages each one associated with an extensional stage a first exhumation stage has been active at ca 335 340 ma available data indicate that this extension and associated exhumation and cooling occurred over a distance of more than 200 km in the internal zones of the belt whereas compression was still active in the northern and southern parts of the belt a second exhumation stage may be related to the late orogenic extension and collapse of the belt at ca 310 305 ma and is characterized by the uplift of the st sylvestre complex by unroofing of the overlying metamorphic and granitoidic formations

late orogenic carboniferous extensions in the variscan french massif central the variscan french massif central experienced two successive stages of extension from middle carboniferous to early permian in the northern massif central the first stage began in the late visean immediately after nappe stacking and is well recorded by namurian westphalian synkinematic plutonism the middle carboniferous leucogranites widespread in the nw massif central limousin and sioule area were emplaced within a crust extending along a ne sw direction at the same time the hanging wall or gueret extensional allochton moved toward the se several examples of the synextensional plutonism are also recognized in central limousin saint mathieu dome la porcherie and cornil leucogranites these examples illustrate the relationship between granite emplacement and crustal scale deformation characterized by nw se stretching and ne sw shortening in the central and southern massif central cevennes ch taigneraie and margeride areas plutonism is dominantly granodioritic and exhibits the same structural features nw se maximum stretching and overturning to the se middle carboniferous namurian westphalian extension was parallel to the variscan belt both in the massif central and southern armorican area this extensional regime was active from the late visean in the north while compression dominated in the southernmost domains montagne noire and pyrenees the second extensional stage occurred from late carboniferous to early permian this event was responsible for the opening of intramontane coal basins brittle deformation in the upper crust and ductile normal faulting localized on the margin of cordierite granite migmatite domes data from the coal basins show that the half graben is the dominant structural style except for basins located along submeridianal left lateral faults which have pull apart geometries late carboniferous extension occurred along the ne sw direction the ne sw maximum stretching direction can be found in the whole massif central but is more developed in the eastern part the extensional direction is transverse to the general trend of the belt and top to the ne shearing is dominant correlations of these two extension directions with neighboring variscan massifs are discussed

5 late variscan mineralizing systems related to orogenic processes the french massif central abstract the french massif central constitutes an exceptional study area due to the diversity of its metallic deposits its internal position in the variscan belt and the abundance of available geological geophysical and metallogenic data obtained within the geofrance 3d programme the deposits formed towards the end of the orogenic evolution represent the economic products of two distinct mineralizing systems a au sb hydrothermal system and a w sn and rare metals magmatic hydrothermal system which were simultaneously active during a short time span between ca 310 and 300 ma two types of gold deposit can be distinguished on the basis of their depth of emplacement deep seated gold deposits developed under lithostatic to hydrostatic pressure during rapid exhumation and shallow gold deposits emplaced under hydrostatic pressure with no significant uplift deposits of w sn and rare metals were emplaced in the upper crust during final crystallization of specialized magmas after their rapid ascent perhaps enhanced by simultaneous regional uplift the gold bearing systems are associated with a complex network of re activated crustal scale faults initially active during the period between 335 and 315 ma normal motion along the faults coeval with 335 to 315 ma granite migmatite domes played a major role in the 3d distribution of the hydrothermal plumbing system gold and related metals were carried within huge hydrothermal cells which reached ca 100 km by 10 km in area and 30 km in depth in contrast granites rich in magmatophile elements w sn rare metals generated smaller hydrothermal cells 10 km by 10 km in area and

rare earth elements and hydrothermal ore formation processes abstract the geochemical behaviour of ree is influenced by nearly all important hydrothermal ore formation processes including fluid rock interactions fluid precipitations adsorption and scavenging onto particles and changes in fluid temperature pressure ph eh alkalinity and ligand concentration destabilization of ree complexes in response to these physicochemical changes and possible chemical crystallographic controls determine the concentration and distribution of ree within hydrothermal minerals mineraloids and amorphous phases alteration assemblages of intrusive related mineralisations may exhibit a wide range of ree distributions and ree fractionation trends the ree distribution within these hydrothermally altered lithologies will depend on the ree concentration in the rock and the fluid the partitioning behaviour of the ree between the rock phases and the fluid and the types of alteration reactions which take place reactive rocks such as carbonate rich lithologies of skarn deposits may gain or loose significant amounts of ree during fluid rock interactions ree geochemical investigations combined with fluid inclusion and stable isotope studies may point to the origin of alteration products and the source s of ore fluids ancient massive sulphide ores commonly exhibit anomalous eu concentrations similar to many recent submarine hydrothermal precipitates the recognition of hydrothermal fluid seawater sedimentary or volcanically derived ree distributions within ores chemical sediments and hydrothermally altered lithologies may help to constrain genetic modelling of massive sulphide deposits care should be taken with the interpretation of ree distributions from metamorphosed hydrothermal ore deposits becase metamorphic and diagenetic alteration reactions may change the ree distributions mobility of ree may occur during diagenesis of carbonate rich rocks and during the development of shear zones and migmatites in contrast very large fluid rock ratios are necessary to cause significant changes in ree patterns of silicate rich rocks during diagenesis and regional metamorphism

mineralogical chemical and isotopic u pb method studies of hercynian uraniferous mineralizations margnac and fanay mines limousin france abstract mineralogical and chemical studies of pitchblendes pyrites and coffinites from the margnac and fanay mines limousin france indicate that chemical exchanges between uraniferous solutions and vein wall rocks are reflected in the chemical compositions of pitchblendes an age of 276 ma is given for the primary pitchblende pyrite association by the u pb method isotopic analysis of pb in galena crystallized during coffinitization of pitchblende gives an interval of 6 ma between pitchblende precipitation and its remobilization as coffinite analyses by electron microprobe indicate that this hydrothermal transformation affects more or less all the pitchblende spherulites even if the volume of coffinite is low this leads to si enriched pitchblende and explains the systematic age of 270 ma obtained by pb pb and concordia diagrams a mineralogically undetected remobilization stage is also found in some samples at 170 175 ma finally this work specifies the life time 20 30 ma of the geothermal system which is considered to be responsible for the episyenitization of the two mica granite the precipitation of pitchblende and its remobilization as coffinite