fluid inclusion and stable isotope constraints on the genesis of the world class zhuxi w cu skarn deposit in south china abstract scheelite mineralization accompanied by traces of chalcopyrite occurs in skarn systems in the giant zhuxi deposit in the taqian area northeastern jiangxi province south china three paragenetic stages of skarn with ore deposition have been recognized the prograde skarn retrograde skarn and hydrothermal quartz sulfide stages microthermometer data showed that the prograde skarn were formed at relatively high temperatures 450 500 c and pressures 0 97 1 38 kbars and low oxidation to weak reduction conditions the raman microprobe data demonstrated that the fluid of the stage was h2o nacl kcl cacl2 ch4 c2h4 system the retrograde skarn alteration formed at 1 24 kbars and relatively medium temperatures 280 320 c and the ore forming fluid was h2o nacl kcl cacl2 system during the hydrothermal quartz sulfide stage the fluids were methane bearing aqueous low salinity 0 9 8 0 wt nacleqv and low homogeneous temperature 160 240 c correspondingly the minimum fluid pressures were evaluated between 75 and 252 bars the h and o isotopic values from the prograde and retrograde stages implied that the mineralizing fluids were derived principally from magmatic water the mineralizing fluids in the quartz sulfide hydrothermal stages were mingled with magmatic water with some meteoric water as well as minor contributions of wall rocks the sulfur and lead isotopes indicated that ore forming materials including sulfur lead copper zinc and iron in the zhuxi deposit were derived from local magmatic and sedimentary sources geological fluid inclusion and isotopic data supported that the zhuxi deposit is a typical skarn type w cu deposit related to ilmenite series granitic rocks and formed in a compressional tectonic environment

fluid inclusion evidence for immiscibility in magmatic differentiation abstract after a brief review of recent work on melt inclusions in diamond and in normal magmatic environments the nature of the fluid inclusion evidence for various stages of magmatic immiscibility is summarized during magmatic differentiation by crystal fractionation from originally low volatile content silicate melts yielding normal plutonic rocks through hydrous silicate melts yielding pegmatites to late stage water rich fluids yielding quartz veins and ore deposits the composition of the residual liquid changes drastically in some geologic environments this change in composition of the residual liquid may be continuous with no phase change in the fluid part of the system under most natural conditions however it is far more likely to have one or more stages in the evolution during which globules of a separate new immiscible fluid phase exsolve the term immiscibility is used here in its more general sense to refer to the existence at equilibrium of two or more non crystalline polycomponent solutions fluids differing in properties and generally in composition i believe that the great bulk of the magmatic inclusions that are being studied today have come about through the intervention of one or more such stages of immiscible fluid separation i e magmatic differentiation by fluid immiscibility examples include silicate melt dense co2 fluid silicate melt sulfide melt silicate melt silicate melt silicate melt low density vapor i e vesiculation and as presented in more detail in this paper various stages of silicate melt hydrous saline melt aqueous fluid co2 fluid immiscibility generally only parts of these stages are recorded by the trapping of fluid inclusions and the interpretation of the inclusion record is commonly ambiguous and difficult particularly that stage between hydrous silicate melts and the hydrous saline melts and aqueous solutions responsible for many ore deposits the ambiguity results mainly from problems of inclusion origin and nonrepresentative trapping within any given individual inclusion thus the probability of trapping of only one vs various amounts of two immiscible fluids or of trapping solid phases along with the fluid present severe difficulties in addition there is the almost universal superposition of various stages of trapping of fluids within any given sample these problems can only be approached by very careful petrography and analysis of optimum material combined with field data and experimental p v t x studies of pertinent systems completely unequivocal statements of the course of evolution of the fluids in such natural systems may never be possible

fluid inclusion characteristics and geological significance of the xi ao copper tin polymetallic deposit in gejiu yunnan province abstract the newly discovered xi ao cu sn polymetallic deposit is located in the inner alteration zone of the laochang kafang granite edge in gejiu yunnan province the mineralization process of this deposit can be divided into four stages cassiterite and chalcopyrite mainly formed at stages ii and iii respectively fluid inclusions at different stages were studied by petrography microthermometry and laser raman spectrometry five fluid inclusion types are observed microthermometry results revealed that the ore forming fluid evolved from high temperature with low to intermediate salinity to low temperature with low salinity at stage i the fluid was under near critical conditions and then separated into coexisting liquid and vapor phases at stage ii finally the fluid was boiling and immiscible at stage iii a meteoric fluid mixing process was also observed at this stage laser raman spectra identified ch4 at the skarn and mineralization stages combined with other geological features the deposit was concluded to have formed under a relatively reduced environment the co2 component abruptly increased at stage iii and probably originated from the exterior fluorine may play an important role in mineralization cassiterite and chalcopyrite were mainly deposited at 340 440 and 300 400 c under pressures of 18 50 and 12 36 mpa respectively boiling and immiscibility are probably the main reason for mineral deposition the deposit of cassiterite and chalcopyrite showed similar salinities but higher temperatures than other mineralization types of deposits in the gejiu area this finding indicated that the study deposit probably represents the high temperature part of the gejiu mineralization system thereby benefiting mineral explorations in the gejiu area

the xinqiao cu s fe au deposit in the tongling mineral district china synorogenic remobilization of a stratiform sulfide deposit abstract the cu s fe au deposit at xinqiao is located in the tongling mineral district anhui province people s republic of china it is a large scale polymetallic deposit with drilling indicated reserves of 0 5 million tonnes of cu averaging 0 71 75 5 million tonnes of s averaging 29 3 and 24 9 million tonnes of fe averaging 46 as well as 11 2 tonnes of au averaging at 4 7 g tonne plus 248 4 tonnes of ag averaging at 248 4 g tonne there are two major mineralization types in the xinqiao deposit one is the economically significant stratiform orebody hosted in the lower carboniferous gaolishan siltstone c1g and overlying middle carboniferous huanglong limestone c2h while the other is the less important skarn type mineralization restricted to the contact between the yanshanian diorite 110 168 ma and the lower permian qixia marble p1q the ore in the xinqiao deposit is dominated by massive pyrite with minor amount of chalcopyrite magnetite pyrrhotite galena sphalerite quartz and dolomite four stages of deformation have been recognized to affect the sedimentary rocks in the xinqiao area the first two d1 and d2 produced mainly ne sw and nne ssw trending open folds whereas d3 is characterized by normal faults striking nne ssw and d4 is dominated by post mineralization strike slip faults segmenting the orebody the structural information obtained by this study not only revealed the transgressive nature of the stratiform orebody but also identified a strong structural control on the formation of the deposit it is concluded that the xinqiao polymetallic deposit was formed through remobilization of a carboniferous sulfide source bed by hydrothermal solutions related to late yanshanian plutonism 110 ma during which d3 normal faulting played an important role in localization of the mineralization such a two stage ore forming model is consistent with the most recent genetic interpretations of the deposit

assessment of the controls on 234u 238u activity ratios recorded in detrital lacustrine sediments abstract understanding landscape evolution on geological time scales has become increasingly important in the light of recent climate warming and intensified land use uranium isotopes 234u 238u activity ratios can be used to reconstruct catchment wide erosion from fine grained detrital matter as the lighter nuclide 234u is lost from grains loss of 234u is mainly attributed to recoil of 234th during decay of 238u however mobilisation of 234u is also controlled by preferential oxidation and leaching of 234u from detrital grains further considerations are required concerning the sediment mineralogy and uranium bound to authigenic or endogenic matter here we extend the discussion about the controls on previously published 234u 238u activity ratios and uranium concentrations of detrital matter along a 5 43 m long late glacial to holocene sedimentary record from lake ohrid north macedonia albania lake ohrid is chosen to study how erosion responds to anthropogenic disturbances and climate variability which are common through the late pleistocene and holocene in the mediterranean region we compared uranium concentration and isotope data to redox sensitive mineralogical and biogeochemical proxies mineralogical and biogeochemical data show no control on uranium concentration and 234u 238u activity ratios rock magnetic redox proxies indicate uranium mobilisation from detrital matter in more oxic environments no correlation between 234u 238u activity ratios and rock magnetic data is observed which implies that the redox environment has no control on the isotope signal monte carlo simulations reveal that post depositional preferential mobilisation of 234u has only negligible impact on the calculated sediment residence times our model implies that pre depositional leaching can results in shorter sediment residence times but low 234u 238u activity ratios during cold and dry intervals imply that recoil is likely the main processes explaining loss of 234u from the detrital grain this is explained by the time scales studied herein in 10 000 years during which preferential leaching is seen to be not significant

os os dating of copper and molybdenum deposits along the middle and lower reaches of the yangtze river china the 186 os 187 os and 188 os 187 os dating techniques were modified and applied to molybdenites from two cu vein deposits and three mo skarn deposits along the middle and lower reaches of the yangtze river china the results show that these deposits formed at 138 to 133 ma identical to the ages of 140 to 130 ma for the host granitoids re os and os os ages of two of the samples agree within analytical uncertainties the close temporal relationship between the granitoid intrusions and the formation of the deposits supports a magmatic model for the origin of these deposits the results show that the novel os os dating technique is capable of resolving geologic problems although the accuracy obtained by the present work 1 2 is lower than that of the re os method

the jc tin skarn deposit southern yukon territory ii a carbon oxygen hydrogen and sulfur stable isotope study the jc tin deposit is a stanniferous skarn hosted within mississippian sediments intruded by the mid cretaceous seagull batholith the stable isotope compositions of silicate carbonate and sulfide minerals have been used to interpret the evolution of fluid sources during sequential stages of skarn formation the delta 13 c and delta 18 o values of remnant calcite from the earliest contact metamorphic stage 0 rocks are the result of decarbonation of the protolith marble coupled with infiltration of variable amounts of meteoric water garnet pyroxene skarn stage i was produced by predominantly magmatic fluids during initial skarn forming metasomatism stage ii amphibole rich skarn was formed by magmatic water mixed with up to 50 percent meteoric water this meteoric water had undergone stable isotope exchange with the jc granite at a low water to rock ratio as it moved upward along the contact of the granite with the surrounding wall rock the magmatic water component of stage ii fluids is inferred to have moved vertically through the solid cap of the granite before mixing with the exchanged meteoric water stage iii epidote quartz calcite skarn formed in the presence of a fluid consisting of magmatic water which had reequilibrated with the granite cap at temperatures between 350 degrees and 500 degrees c and subsequently mixed with up to 45 percent meteoric water the delta 13 c and delta 13 o values of stage iii calcite suggest the presence of an additional fluid component which had equilibrated with the protolith marble at similarly high temperatures these observations reinforce the hypothesis that originally highly saline stage iii fluids were diluted by low salinity meteoric water at the periphery of the skarn replacement zone and suggest that the surrounding marble horizon may have provided a pathway by which such fluids entered the skarn during formation of stage iv biotite quartz fluorite cassiterite skarn the fluid was predominantly magmatic with no detectable involvement of meteoric water this stage represented a renewed influx of magmatic fluid following the increased involvement of meteoric water during stages ii and iii and may have been related to a discrete episode of aqueous fluid separation from the underlying granitic magma the return to a predominantly magmatic fluid at stage iv is concomitant with an increase in both fluid temperature and salinity relative to stage iii fluids during the fracturing and formation of breccia zones which accompanied formation of stage vi pyrite marcasite carbonate skarn meteoric water which had initially equilibrated at low temperature 13 s values for all skarn sulfide minerals 1 0 to 2 5ppm is consistent with a magmatic source for sulfur throughout skarn formation stage iii and iv mineral pairs which remained in isotopic equilibrium after their initial crystallization yield oxygen isotope geothermometric temperatures consistent with those determined by fluid inclusion microthermometry

the jc tin skarn deposit southern yukon territory i geology paragenesis and fluid inclusion microthermometry the jc tin deposit is one of several stanniferous skarns associated with the mid cretaceous seagull batholith at jc a ridgelike lobe of the seagull batholith granite has intruded a thick sequence of mississippian quartzite containing a 30 to 35 m carbonate horizon skarn has replaced this carbonate along its elongate contact with the granite ridge for a distance of more than 850 m the paragenetic sequence in the jc skarn is contact metamorphic skarnoid stage 0 grossularite diopside quartz calcite early skarn stage i andradite hedenbergite stage iia fe amphibole magnetite stage iib fe amphibole pyrrhotite or chalcopyrite or sphalerite stage iii epidote quartz calcite or axinite or sn al sphene late skarn stage iv early veins beryl quartz or tourmaline or danalite stage iv main stage biotite quartz fluorite or arsenopyrite or chalcopyrite or cassiterite stage v fe chlorite muscovite calcite stage vi veins breccias calcite pyrite or marcasite the sequential stages of skarn development display a progression from massive skarn replacement of the protolith calcite marble stages i and ii to more diffuse or veinlike replacement of earlier skarn assemblages stages iii to vi many of the early skarn silicates contain appreciable tin especially andradite up to 0 79 wt sno 2 fe amphibole 0 07 0 61 wt sno 2 epidote 0 12 1 56 wt sno 2 and the sn al sphene the replacement of earlier stage iia skarn at stage iv was an important ore forming process precipitating tin as cassiterite in a narrow zone above the skarn granite contact microthermometric measurements of primary fluid inclusions in epidote and axinite yield a temperature range for stage iii skarn formation of 410 degrees to 500 degrees c at an estimated pressure of 750 bars stage iii fluids ranged in salinity from 20 to 27 wt percent 4 5 6 5 molal total cl and had high cacl 2 nacl ratios 11 these fluids are interpreted to be the result of mixing of originally more saline magmatic fluids with meteoric water during the latest stage of early skarn formation low salinity pseudosecondary inclusions in epidote may represent a relatively unmixed sample of this meteoric water component stage iii fluids appear to have become cacl 2 enriched as a consequence of skarn forming reactions primary fluid inclusions in cassiterite imply a temperature range of 430 degrees to 560 degrees c and salinities of 25 to 43 wt percent 6 12 molal total cl for stage iv fluids these fluids contained predominantly cacl 2 nacl h 2 o and a lesser component of kcl with cacl 2 nacl kcl ranging from 0 6 to 7 7 the apparent increase in both fluid temperature and salinity at stage iv is attributed to a renewed episode of fluid release from the underlying granite with concomitant greisen alteration of the upper part of the jc granite ridge a trend toward very high cacl 2 nacl kcl ratios in the stage iv fluids may be attributable to the progress of cacl 2 producing kcl consuming replacement reactions within the skarn however variations in the overall salinity and other compositional characteristics of these fluids may have been largely controlled by the process of aqueous phase separation from the jc granite the high salinity of the stage iv fluids implies derivation from a granitic melt which exsolved a subcritical two phase aqueous fluid the observation of extremely saline two phase fluids in greisens and other types of granite hosted deposits provides more direct evidence of such a process the relatively low salinity observed in some other examples of granite related sn and w mineralization may be a direct consequence of supercritical rather than subcritical aqueous fluid separation from the associated magmas

characteristics and genesis of gangdese porphyry copper deposits in the southern tibetan plateau preliminary geochemical and geochronological results porphyry type cu mo au deposits have been discovered along the gangdese magmatic arc in the southern tibetan plateau extensive field investigations and systematic studies of geochemistry s pb isotopic tracing together with re os and 40ar 39ar isotopic dating indicate that the mineralisation of the copper belt is genetically related to emplacement of late orogenic granitic porphyries during the post collisional crustal relaxation period of the late himalayan epoch these porphyries are petrochemically k enriched and belong to shoshonitic to high k calc alkaline series they display enrichment of large ion lithophile elements lile rb k u th sr pb and depletion of high field strength elements hfse nb ta ti and the heavy rare earth elements hree and y without eu anomalies these characteristics demonstrate that subduction played a dominant role in their petrogenesis and residual garnet was left in the magma sources pb isotope data show a linear correlation in the plumbotectonic framework diagram ranging from orogenic pb in the eastern segment of the copper belt to mantle pb in the western segment these constitute a mixing line of the indian oceanic morb with indian oceanic sediments and suggest that the porphyry magmas were dominantly derived from partial melting of subducted oceanic crusts mixed with a minor quantity of sediments and mantle wedge components the gangdese porphyry copper polymetallic belt has alteration characteristics and zonation typical of porphyry type copper deposits which include potassic alteration k feldspathisation and biotitisation silicification sericitisation and propylitisation mineralisation mainly occurs in strongly altered granitic cataclasite at the exo contact with veinlet disseminated textures the porphyries themselves are weakly mineralised with disseminated pyrite and chalcopyrite the copper deposits contain simple ore mineral associations consisting of chalcopyrite pyrite bornite molybdenite sphalerite and oxidised minerals of malachite covellite and molybdite during supergene oxidation primary ores underwent secondary enrichment to form economic orebodies with cu grade ranging from 1 to 5 ore sulphides of the copper belt display s and pb isotopic compositions identical to the ore bearing porphyries their 34s values vary between 3 8 and 2 4 and are typical of mantle sulphur the 206pb 204pb 207pb 204pb and 208pb 204pb ratios vary in the ranges 18 106 to 18 752 15 501 to 15 638 and 37 394 to 39 058 respectively and yielded radiogenetic lead enriched signatures twelve molybdenite samples from the copper belt yielded isochron ages of 14 76 0 22 ma and 13 99 0 16 ma for the nanmu and chongjiang deposits and model ages of 13 5 to 13 6 ma for the lakang e deposit meanwhile 40ar 39ar isotopic dating of two biotite phenocrysts from the chongjiang and lakang e deposits give plateau ages of 13 5 1 0 ma and 13 42 0 10 ma respectively during the geodynamic evolution of the gangdese collision orogenic belt intrusion of the ore bearing porphyries took place just before the rapid uplift and e w extension of the southern plateau and the ore forming process may have occurred simultaneously with the uplift and extension 14 0 1 ma

origin of high potassium talc alkaline i type granitoids many i type granitoid magmas are generated through partial melting of older metaigneous rocks and the compositions of such melts are broadly calc alkaline and metaluminous these melts are granitic to tonalitic and result from thermal extremes in their lower crustal source regions data on the experimental partial melting of common crustal rocks suggest that high k i type granitoid magmas can be derived only from the partial melting of hydrous calc alkaline to high k calc alkaline mafic to intermediate metamorphic rocks in the crust because of their low k2o contents metabasaltic rocks of all kinds are unsuitable as sources and models that propose mixing of mantle derived basaltic magmas and crustal melts are also inadequate there is no requirement that i type calc alkaline magmatism be related in any way to subduction processes

exsolution of magmatic volatile phases from cl enriched mineralizing granitic magmas and implications for ore metal transport abstract to understand cl dissolution in aluminosilicate liquids and the exsolution of cl rich magmatic volatile phases experiments were conducted to determine the solubility of nacl kcl and h2o in felsic liquids at 0 5 and 2 kbar the cl content of h2o poor nacl saturated and kcl saturated silicate liquids is low i e 1 3 wt to very low i e 0 2 wt and varies with changes in pressure and composition the cl concentration increases with the f concentration and the molar al na ca mg si ratio of the liquid and decreases with increasing activity of h2o in the system exsolution of a volatile phase depends on the partial pressures of all dissolved volatiles and low cl solubilities in nacl saturated and kcl saturated silicate liquids imply that exsolution of a cl bearing volatile phase will occur early in cl bearing granitic magmas i e prior to extensive melt crystallization and or at comparatively low water fugacities the solubility behavior of h2o and cl is very similar to that of co2 and h2o in felsic liquids small quantities of co2 are known to facilitate volatile phase exsolution holloway 1976 and in a similar manner volatile phases may exsolve early in cl enriched granite magmas whereas 5 to 6 wt dissolved h2o is necessary for volatile phase exsolution from a co2 free and cl free haplogranite liquid at 2 kbar and 800 c a cl rich brine will exsolve if the liquid contains only 1 wt h2o and 0 26 wt cl at the same conditions these new solubility data are interpreted in light of h2o f and cl concentrations in felsic liquids determined from silicate melt inclusions to constrain the exsolution of cl bearing magmatic volatile phases from mineralizing granitic magmas felsic magmas genetically associated with cu porphyry and mo porphyry mineralization contain sufficient h2o and cl to become saturated with respect to a hypersaline brine without strong pressure reduction boiling of the volatile phase i e exsolution of immiscible vapor and brine or strong volatile enrichment resulting from extensive crystal fractionation experiments were also conducted to investigate the solubility of mo in highly saline volatile phases coexisting with granitic liquids at 2 and 0 5 kbar the apparent partition coefficient for mo in the volatile phase s relative to silicate liquid d mo is defined as the computed concentration of mo in a volatile phase or phases the measured concentration of mo in granite glass d mo ranges from 8 to 80 as the nacl and kcl content of the volatile phase s increases from 15 to 90 wt because mo does not complex with cl in aqueous fluids it appears that mo solubility may be a strong function of the activity of na and k in alkali chloride rich volatile phases

interpretation of the conolly contour diagram of fault related veins the conolly diagram is a section or plan on which the distances from the vein to an arbitrary reference plane are contoured this graphical technique makes it possible to depict the shape of the vein in a form which resembles topographic contour maps the conolly diagram can be used to analyse the relation between oreshoot development in fault related veins the attitude of the irregular fault plane and slip direction of the fault

the lower changjiang yangzi yangtze river metallogenic belt east central china intrusion and wall rock hosted cu fe au mo zn pb ag deposits abstract the lower valley of changjiang from wuhan of the hubei province in the west to zhenjiang of the jiangsu province in the east contains more than 200 polymetallic cu fe au mo zn pb ag deposits and is one of the most important metallogenic belts in china this metallogenic belt situated at the northern margin of the yangzi craton and bordered by the dabieshan ultrahigh pressure metamorphic belt to the north consists mainly of cambrian triassic marine clastic sedimentary rocks and carbonate and evaporite rocks which overlay a precambrian basement and are intruded by yanshanian 205 to 64 ma granitoid intrusions and subvolcanic complexes repeated tectonism from late proterozoic to triassic resulted in extensively developed networks of faults and folds involving the cambrian triassic sedimentary strata and the precambrian basement the yanshanian granitoid intrusions and subvolcanic complexes in the lower changjiang metallogenic belt are characterized by whole rock 18 o of 8 to 10 initial 87 sr 86 sr of 0 704 to 0 708 and ndt from 10 to 17 and have been interpreted to have originated from mixing between juvenile mantle and old crustal materials also the yanshanian granitoids exhibit eastward younging and increase in alkalinity i e from older calc alkaline in the west to younger subalkaline alkaline in the east which are related to oblique collision between the yangzi and sino korean cratons and tectonic evolution from early compressional to late extensional or rifting regimes most polymetallic deposits in the lower changjiang metallogenic belt are clustered in seven districts where the yanshanian magmatism is particularly extensive from west to east edong jiurui anqing guichi luzhong tongling ningwu and ningzhen mineralization is characterized by the occurrence of three distinct types of orebodies in individual deposits orebodies in yanshanian granitoid intrusions skarn orebodies at the contact zones between the yanshanian intrusions and late paleozoic early mesozoic sedimentary rocks and stratabound massive sulfide orebodies in the late paleozoic early mesozoic sedimentary strata the most important host sedimentary strata are the middle carboniferous huanglong formation lower permian and lower middle triassic carbonate and evaporite rocks the intrusion hosted and skarn orebodies exhibit well developed zonation in alteration assemblages metal contents and isotopic compositions within individual deposits and apparently formed from hydrothermal activities related to the yanshanian magmatism the stratabound massive sulfide orebodies in the late paleozoic early mesozoic sedimentary strata have long been suggested to have formed from sedimentary or volcano sedimentary exhalative processes in shallow marine environments however extensive research over the last 40 years failed to produce unequivocal evidence for syngenetic mineralization on the basis of geological relationships and isotope geochemical characteristics we propose a carbonate hosted replacement deposit model for the genesis of these stratabound massive sulfide orebodies and associated skarn orebodies this model suggests that epigenetic mineralization resulted from interactions between magmatic fluids evolved from the yanshanian intrusions with carbonate and evaporite wall rocks mineralization was an integral but distal part of the larger hydrothermal systems that formed the proximal skarn orebodies at the contact zones and the intrusion hosted orebodies the stratabound massive sulfide deposits of the lower changjiang metallogenic belt share many features with the well studied high temperature carbonate hosted replacement deposits of northern mexico and western united states particularly with respect to association with small shallow granitoid complexes structural and stratigraphic controls on mineralization alteration assemblages geometry of orebodies metal association metal zonation and isotopic systematics

geology of gold bearing skarn deposits in the middle and lower yangtze river valley and adjacent regions abstract the middle and lower yangtze river valley and adjacent regions are the most important metallogenic belt of gold and copper bearing skarn deposits in china the total gold reserves in this belt have been estimated at more than 600 t the gold bearing skarns are mainly distributed in the southeastern hubei tongling and northern anhui regions favorable tectonic settings are depressions and fold zones of the platforms i e mobile belts these skarns are hosted by platformal limestone dolomitic limestone and dolomite of the triassic carboniferous permian and middle to lower cambrian formations the related intrusions are yenshanian 180 to 113 ma calc alkaline quartz monzodiorite granodiorite quartz monzonite monzogabbro and their hybabyssal facies the intrusions have high fe2o3 feo 0 5 and intermediate initial 87 sr 86 sr ratios 0 7046 to 0 7087 their ree distribution patterns are lree enriched and exhibit smooth right dipping curves these suggest that the source materials mainly came from upper mantle with contamination by sialic crustal components the auriferous skarns are both calcic and magnesian but calcic skarns are most common the constituent minerals of the calcic skarns are diopside garnet wollastonite vesuvianite and scapolite whereas magnesian skarns are dominated by forsterite spinel diopside phlogopite chondrodite and clinohumite with abundant superimposed serpentine clinochlore and brucite the compositions of coexisting pyroxenes and garnets are diopside and andradite indicating the high oxygen fugacity and low acidity conditions gold is closely associated with cu pb zn sulfides and exists mainly in the form of native gold and electrum arsenides tellurides bismuthides and selenides are present in many ore deposits therefore cu as bi te ag pb zn se and co are the major metals present in the deposits and are important geochemical ore searching indicators in some au fe cu magnesian skarns magnesiomagnetite magnesioferrite and ludwigite are locally abundant the metasomatic zoning in many gold skarn deposits is very distinct consisting of an outward sequence of fe cu cu mo cu au au cu au pb zn the geologic characteristics of au cu skarn deposits that formed in the mobile platformal setting of china have distinct differences compared to au skarns formed in orogenic belts at convergent plate margins in british columbia and the western usa

scheelite mineralization in central east greenland numerous scheelite mineralized areas have been located in the caledonian fold belt of central east greenland 70 74 1 2 n the fold belt comprises archaean and proterozoic metamorphic complexes flanked by late proterozoic and lower palaeozoic sediments and intruded by upper proterozoic and caledonian granitic rocks scheelite mineralization occurs in marble skarns in upper proterozoic metasediments often spatially associated with caledonian or older granitic intrusives skarns and quartz veins in late proterozoic sediments lower ebg quartzite up to 7 km from outcropping caledonian granites veins in fault zones cutting late proterozoic sediments upper ebg limestones in this paper the three types are described with emphasis on structural setting mineralogy and trace element chemistry it is concluded that most scheelite mineralization is associated with caledonian igneous activity either at or near granite contacts or as mineralization distal to granites at depth of the various types described the scheelite veins in late proterozoic limestone are considered to be of the best economic potential

aluminous granite as a source magma of hydrothermal ore deposits an experimental study partition ratios of zinc and lead between synthetic granitic melts and aqueous phase were determined at 800 degrees c and under p sub h 2 o 3 5 or 0 25 kb two starting glasses in the system sio 2 al 2 o 3 cao na 2 o were prepared their bulk chemical compositions are similar i e sio 2 content of 75 65 percent in glass 1 and of 74 68 percent in glass 2 glass 1 is prepared to be slightly alkaline with 1 89 percent normative wollastonite and glass 2 is slightly aluminous with 2 42 percent normative corundum the partition ratio of sodium between aqueous fluid and melt k p na can be expressed by the following equation over the whole range of chloride concentrations in the aqueous solution m v cl mole kg regardless of the normative composition of the melt k p na 0 468 x m v cl however the partition ratio of zinc k p zn for the aluminous melt is more than two orders of magnitude larger than that for the alkaline melt k p zn varies with the square of the chloride concentration in the aqueous phase and is satisfied by the following two equations k p zn 9 40 x m v cl 2 aluminous melt 2 and k p zn 0 0833 x m v cl 2 alkaline melt 1 a similar contrast was also observed in the partition ratios of lead between aqueous phase and melts k p pb 2 26 x m v cl 2 aluminous melt 2 and k p pb 0 0474 x m v cl 2 alkaline melt 1 the log of the partition ratio shows an excellent inversely proportional relation with the ph at 25 degrees c of the aqueous solution which was equilibrated with granitic melts and quenched a slope of 2 is obtained for alkaline melt and is reasonably explained by the equation pbo melt 2h super vapor 2cl super vapor pbcl sub 2 vapor h 2 o the slope for the aluminous melt 1 2 may be explained only by complex reactions involving silicate ligands in the melt the results indicate that only aluminous granite is capable of producing hydrothermal fluids which would contain a significant amount of ore metals in other words aluminous granite can be ore bearing by postmagmatic processes this is likely to happen when a granitie magma which originally was slightly alkaline changed its composition to slightly aluminous during diapiric uplift by assimilation of pelitic sediments if this compositional evolution happened then granitic magma would tend to store up base metals during the early stages of its evolution and release them very efficiently in the later stages to form a hydrothermal ore solution

geochemistry and evolution of ore forming fluids of the yueshan cu au skarn and vein type deposits anhui province south china abstract the yueshan mineral belt is geotectonically located at the centre of the changjiang deep fracture zone or depression of the lower yangtze platform two main types of ore deposits occur in the yueshan orefield cu au fe skarn deposits and cu mo au pb zn hydrothermal vein type deposits almost all deposits of economic interest are concentrated within and around the eastern and northern branches of the yueshan dioritic intrusion in the vicinity of the zongpu and wuhen intrusions there are many cu pb zn au s vein type and a few cu fe au skarn type occurrences fluid inclusion studies show that the ore forming fluids are characterised by a cl s na k chemical association hydrothermal activity associated with the above two deposit types was related to the yueshan intrusion the fluid salinity was high during the mineralisation processes and the fluid also underwent boiling and mixed with meteoric water in comparison the hydrothermal activity related to the zongpu and wuhen intrusions was characterised by low salinity fluids chlorine and sulphur species played an important role in the transport of ore forming components hydrogen and oxygen isotope data also suggest that the ore forming fluids in the yueshan mineral belt consisted of magmatic water mixed in various proportions with meteoric water the enrichment of ore forming components in the magmatic waters resulted from fluid melt partitioning the ore fluids of magmatic origin formed large cu au deposits whereas ore fluids of mixed magmatic meteoric origin formed small to medium sized deposits the sulphur isotopic composition of the skarn and vein type deposits varies from 11 3 to 19 2 and from 4 2 to 10 0 respectively these variations do not appear to have been resulted from changes of physicochemical conditions rather due to compositional variation of sulphur at the source s and by water rock interaction complex water rock interaction between the ore bearing magmatic fluids and sedimentary wall rocks was responsible for sulphur mixing lead and silicon isotopic compositions of the two deposit types and host rocks provide similar indications for the sources and evolution of the ore forming fluids hydrodynamic calculations show that magmatic ore forming fluids were channelled upwards into faults fractures and porous media with velocities of 1 4 m s 9 8 10 1 to 9 8 10 7 m s and 3 6 10 7 to 4 6 10 7 m s respectively a decrease of fluid migration velocity in porous media or tiny fractures in the contact zones between the intrusive rocks and the triassic sedimentary rocks led to the deposition of the ore forming components the major species responsible for cu transport are deduced to have been cucl cucl2 cucl32 and cucloh whereas au was transported as au2 hs 2s2 au hs 2 auhs and auh3sio4 complexes cooling and a decrease in chloride ion concentration caused by fluid boiling and mixing were the principal causes of cu deposition gold deposition was related to decrease of ph total sulphur concentration and fo2 which resulted from fluid boiling and mixing geological and geochemical characteristics of the two deposit types in the yueshan mineral belt suggest that there is a close genetic relationship with the dioritic magmatism geochronological data show that the magmatic activity and the mineralisation took place between 130 and 136 ma and represent a continuous process during the yanshanian time the cooling of the intrusions and the mineralisation event might have lasted about 6 ma the cooling rate of the magmatic intrusions was 80 to 120 c my 1 which permitted sufficient heat supply by magma to the ore forming system

alteration mineralization and genesis of the zoned tongshan skarn type copper deposit anhui china abstract the tongshan skarn type copper deposit is located in the anqing guichi ore cluster of the iron copper metallogenic belt which occurs along the middle lower yangtze river valley china in the study area skarnization and mineralization took place along the contact zone between carbonates and granodiorite porphyries the contact zone shows significant horizontal and vertical variations in alteration and mineralization in the horizontal direction the garnet content is high in the skarns near the intrusive body proximal skarns the diopside content is high farther from the intrusive body distal skarns and hedenbergite is concentrated in the skarns adjacent to the marble zone limestones located far from the marble zone experienced a strong silicification in the vertical direction from higher to lower levels the rocks change from hornfels to calcareous skarn to magnesian skarn mineralogical studies show that the skarns near the intrusion are relatively oxidized and the garnet in the skarns is relatively andradite rich high concentrations of cu are found in the porphyries with quartz veins as well as in the calcic skarns magnesian skarns hornfelses and marbles which are located at distances of 13 10 43 and 25 m from the porphyries respectively high concentrations of zn are found in silicified limestones and skarns located even farther from the porphyries the present findings suggest that the tongshan deposit was subjected to prograde alteration and mineralization followed by retrogression the alteration can be divided into a sequence of stages contact metamorphism prograde metasomatism early retrogression and late retrogression the copper mineralization occurred mainly during the early retrogression and the copper was further enriched in quartz veins within the porphyries during the late stages of magma evolution