helium and carbon fluxes in lake nyos cameroon constraint on next gas burst on 21 august 1986 a lethal gas burst issued from lake nyos in cameroon western africa killing at least 1700 people although the consensus is that the victims died of asphyxiation by co2 of magmatic origin the frequency of such catastrophic degassing events is still unknown the co2 flux at the bottom of lake nyos is estimated based on the measurement of3he4he and4he20ne ratios and the chemical composition of gases exsolved from the lake water the calculated co2 flux at the bottom of lake nyos is 4 4 2 3 1013cm3stp year combined with the estimated release in the august 1986 event of 8 2 1014 cm3 stp the co2 flux suggests that the gas burst may happen about once in18 10 years although the significant uncertainty should be taken into account for the frequency resulting from assumptions such as steady state fluxes in the lake and small fractionization of the c he ratio during the degassing event several yearly measurements of hypolimnetic fluxes and seasonal measurements of epilimnetic fluxes are needed to constrain better the recurrence interval in addition the results of a regional3he4he survey of carbonated mineral springs in northwestern cameroon are discussed in the context of the regional geotectonics

the 1 85 ga mo mineralization in the xiong er terrane china implications for metallogeny associated with assembly of the columbia supercontinent abstract the east qinling region in china hosting several tens of mesozoic magmatic hydrothermal mo deposits is one of the largest molybdenum belts in the world the recently discovered longmendian mo deposit luoning county henan province occurs within the xiong er terrane of the east qinling mo belt here the mo mineralization is confined to altered amphibolite which is replaced by microcline actinolite tremolite epidote and carbonate molybdenite is also observed in microcline dominated felsic differentiates but occurs within the altered amphibolite enclaves here we report re os isotope analyses of five molybdenite and four pyrite samples from longmendian which yield re os isochron ages of 1853 36 ma and 1855 29 ma respectively these ages are in good agreement with the metamorphic age of the taihua supergroup 1 87 1 84 ga the re os data presented in this study indicate that the period of 1 85 ga was a time of widespread melting generating leucocratic microcline dominated granitic melts the timing also coincides with the collision of the eastern and western blocks of the north china craton and its final cratonization at 1 85 ga coinciding with the incorporation of the north china craton within the paleoproterozoic supercontinent columbia

origin of the lethal gas burst from lake monoun cameroun abstract on 15 august 1984 a lethal gas burst issued from a submerged 96 m deep crater in lake monoun in cameroun western africa killing 37 people the event was associated with a landslide from the eastern crater rim which slumped into deep water waters below 50 m are anoxic dominated by high fe 2 600 mg l and hco 3 1900 mg l anoxic and supersaturated with siderite which is a major component of the crater floor sediments the unusually high fe 2 levels are attributed to reduction of laterite derived ferric iron gradually brought into the lake as loess and in river input sulfur compounds are below detection limits in both water and gas gases effervescing from depressurized deep waters are dominantly co 2 with minor ch 4 having 13 c of 7 18 and 54 8 per mil respectively bacterial decomposition of organic matter may account for the methane but 14 c of lake water indicates that only 10 of the carbon is modern giving an apparent age of 18 000 years the dominant source of carbon is therefore attributed to long term emission of co 2 as volcanic exhalation from vents within the crater which led to gradual build up of hco 3 in the lake the density stratification of the lake may have been upset by an earthquake and underwater landslide on 15 august which triggered overturn of the lake and caused nucleation of co 2 in the deep water the resultant ebullition of co 2 from deep lake waters led to a gas burst at the surface and locally generated a water wave up to 5 m high people travelling through the gas cloud were asphyxiated presumably from co 2 and suffered skin discoloration from unidentified components

tectonic evolution of a composite collision orogen an overview on the qinling tongbai hong an dabie sulu orogenic belt in central china abstract the formation of collisional orogens is a prominent feature in convergent plate margins it is generally a complex process involving multistage tectonism of compression and extension due to continental subduction and collision the paleozoic convergence between the south china block scb and the north china block ncb is associated with a series of tectonic processes such as oceanic subduction terrane accretion and continental collision resulting in the qinling tongbai hong an dabie sulu orogenic belt while the arc continent collision orogeny is significant during the paleozoic in the qinling tongbai hong an orogens of central china the continent continent collision orogeny is prominent during the early mesozoic in the dabie sulu orogens of east central china this article presents an overview of regional geology geochronology and geochemistry for the composite orogenic belt the qinling tongbai hong an orogens exhibit the early paleozoic hp uhp metamorphism the carboniferous hp metamorphism and the paleozoic arc type magmatism but the three tectonothermal events are absent in the dabie sulu orogens the triassic uhp metamorphism is prominent in the dabie sulu orogens but it is absent in the qinling tongbai orogens the hong an orogen records both the hp and uhp metamorphism of triassic age and collided continental margins contain both the juvenile and ancient crustal rocks so do in the qinling and tongbai orogens in contrast only ancient crustal rocks were involved in the uhp metamorphism in the dabie sulu orogenic belt without involvement of the juvenile arc crust on the other hand the deformed and low grade metamorphosed accretionary wedge was developed on the passive continental margin during subduction in the late permian to early triassic along the northern margin of the dabie sulu orogenic belt and it was developed on the passive oceanic margin during subduction in the early paleozoic along the northern margin of the qinling orogen three episodes of arc continent collision are suggested to occur during the paleozoic continental convergence between the scb and ncb the first episode of arc continent collision is caused by northward subduction of the north qinling unit beneath the erlangping unit resulting in uhp metamorphism at ca 480 490 ma and the accretion of the north qinling unit to the ncb the second episode of arc continent collision is caused by northward subduction of the prototethyan oceanic crust beneath an andes type continental arc leading to granulite facies metamorphism at ca 420 430 ma and the accretion of the shangdan arc terrane to the ncb and reworking of the north qinling erlangping and kuanping units the third episode of arc continent collision is caused by northward subduction of the paleotethyan oceanic crust resulting in the hp eclogite facies metamorphism at ca 310 ma in the hong an orogen and low p metamorphism in the qinling tongbai orogens as well as crustal accretion to the ncb the closure of backarc basins is also associated with the arc continent collision processes with the possible cause for granulite facies metamorphism the massive continental subduction of the scb beneath the ncb took place in the triassic with the final continent continent collision and uhp metamorphism at ca 225 240 ma therefore the qinling tongbai hong an dabie sulu orogenic belt records the development of plate tectonics from oceanic subduction and arc type magmatism to arc continent and continent continent collision

sm nd and sr isotope systematics of scheelite from the giant au w deposit muruntau uzbekistan implications for the age and sources of au mineralization the application of the sm nd isotope system of scheelite to dating of low sulfide quartz vein hosted au mineralization is still under discussion in the present work new sm nd and rb sr data for scheelite from the giant muruntau myutenbai au deposit kyzylkum western uzbekistan are discussed based on the geological relationship mineralogical properties and trace element characteristics two types of scheelite can be distinguished within the deposit the first one is represented by early bluish luminescent and weakly coloured scheelite generation 1 found within strongly deformed flat quartz veins the apparent isochron defined by this scheelite 351 22 ma is interpreted as a mixing line typically brownish to orange and yellowish luminescent scheelite from steeply dipping veins generation 2 defines a sm nd isochron age of 279 18 ma end 9 5 0 3 mswd 1 5 no evidence for mixing or disturbance by late alteration were found for these scheelites this sm nd isochron age agrees with the rb sr and k ar age range for wall rock alteration in this deposit reported previously the age of 280 ma is interpreted to date the high grade ore formation in the muruntau deposit there are currently no reliable age data available on the magmatic events in the muruntau region probably there is some overlap in time of the hercynian gold deposition with the intrusion of lamprophyric dykes the nd and sr isotopic signatures of scheelite define the wall rocks mainly metasiltstones and metasandstones as the most probable sources for these elements in scheelite

a 1 78 ga large igneous province in the north china craton the xiong er volcanic province and the north china dyke swarm abstract the 1 78 ga xiong er volcanic province xvp and coeval north china giant mafic dyke swarm ncds are the most important magmatic events occurring after the amalgamation of the north china craton ncc the xvp consists of 3 7 km of extrusive volcanics and some feeder dykes sills located along the southern margin of the ncc and extending over an area 0 06 m km2 compositions vary from basalt to rhyolite but are predominantly intermediate in terms of silica content there are also minor sedimentary intercalations and pyroclastic units the sedimentary interlayers indicate an environment changing from continental facies to oceanic facies up section the xvp is characterized by fractional crystallization from an em i type mantle source and both continental arc andean type and rift environments have been proposed the ncds is widespread in the central ncc with an outcrop area 0 1 m km2 and are exposed at variable depths up to 20 km deepest in the north dyke compositions vary from basalt to andesite and dacite but are dominantly mafic and comprise two series of magmatism previous studies revealed that the ncds recorded assimilation and fractional crystallization of an em i type magma source with a minor dm contribution in the younger magmas both syn collisional and intra continental anorogenic environments have been proposed spatial and petrogenic correlations suggest a cogenetic relationship between the ncds and xvp and considered together they define a large igneous province lip of 0 1 m km2 in area and 0 1 m km3 in volume which is also notable for its continuous compositional range from mafic to felsic with no gap at intermediate compositions the petrology is explained by a common magma source that undergoes a silica poor and iron enriched fractionation trend at depth followed by a silica rich and iron poor fractionation trend in shallow level magma conduits dykes and surface lavas a mantle plume is favored as the cause of this 1 78 ga north china lip

zircon u pb dating and sr nd pb hf isotopes of the ore associated porphyry at the giant donggebi mo deposit eastern tianshan nw china abstract the giant donggebi porphyry mo deposit is located in the jueluotage metallogenic belt of eastern tianshan xinjiang northwest china mo mineralization mainly occurs as numerous veinlets in the altered sandstone wall rocks with the development of potassic phyllic argillic and propylitic alteration assemblages outward from a buried porphyritic granite stock zircon crystals from the buried porphyritic granite yield a weighted average 206 pb 238 u age of 236 2 2 ma mswd 1 2 1 n 17 slightly older than the molybdenite re os isotope ages of 231 234 ma suggesting that the donggebi porphyry mo deposit was formed in the triassic post collisional tectonism subsequent to termination of the paleo asia ocean the samples from porphyritic granite show high contents of sio 2 k 2 o and al 2 o 3 low contents of tio 2 mgo and cao and peraluminous high k calc alkaline to shoshonite affinity with obvious lree enrichment and negative eu anomalies they have high initial 87 sr 86 sr ratios of 0 70618 to 0 70821 e nd t values of 0 60 to 1 62 and t dm2 nd ages of 0 88 to 0 96 ga their 206 pb 204 pb t 207 pb 204 pb t and 208 pb 204 pb t values are 17 122 18 577 15 493 15 574 and 37 887 38 006 respectively zircons from the porphyritic granite yield ehf t values of 1 58 to 4 82 and t dm2 hf ages of 0 96 1 36 ga these geochemical and isotopic data imply that the donggebi porphyritic granite originated mainly from partial melting of lower continental crust derived from a depleted mantle the donggebi mo deposit is unique based upon the crustal source for the causative porphyry the distal position of mineralization and the previously revealed co 2 rich ore forming fluids and thus belongs to the collisional or dabie type porphyry deposits as exemplified by the qiane chong and yaochong deposits in dabie shan the donggou deposit in qinling orogen china

evidence of fluid inclusions for two stages of fluid boiling in the formation of the giant shapinggou porphyry mo deposit dabie orogen central china abstract the shapinggou porphyry mo deposit one of the largest mo deposits in asia is located in the dabie orogen central china hydrothermal alteration and mineralization at shapinggou can be divided into four stages i e stage 1 ore barren quartz veins with intense silicification followed by stage 2 quartz molybdenite veins associated with potassic alteration stage 3 quartz polymetallic sulfide veins related to phyllic alteration and stage 4 ore barren quartz calcite pyrite veins with weak propylitization hydrothermal quartz mainly contains three types of fluid inclusions namely two phase liquid rich type i two or three phase gas rich co 2 bearing type ii and halite bearing type iii inclusions the last two types of fluid inclusions are absent in stages 1 and 4 type i inclusions in the silicic zone stage 1 display homogenization temperatures of 340 to 550 c with salinities of 7 9 16 9 wt nacl equivalent type ii and coexisting type iii inclusions in the potassic zone stage 2 which hosts the main mo orebodies have homogenization temperatures of 240 440 c and 240 450 c with salinities of 34 1 50 9 and 0 1 7 4 wt nacl equivalent respectively type ii and coexisting type iii inclusions in the phyllic zone stage 3 display homogenization temperatures of 250 345 c and 220 315 c with salinities of 0 2 6 5 and 32 9 39 3 wt nacl equivalent respectively type i inclusions in the propylitization zone stage 4 display homogenization temperatures of 170 to 330 c with salinities lower than 6 5 wt nacl equivalent the abundant co 2 rich and coexisting halite bearing fluid inclusion assemblages in the potassic and phyllic zones highlight the significance of intensive fluid boiling of a nacl co 2 h 2 o system in deep environments up to 2 3 kbar for giant porphyry mo mineralization hydrogen and oxygen isotopic compositions indicate that ore fluids were gradually evolved from magmatic to meteoric in origin sulfur and lead isotopes suggest that the ore forming materials at shapinggou are magmatic in origin re os dating of molybdenite gives a well defined 187 re 187 os isochron with an age of 112 7 1 8 ma suggesting a post collisional setting

the remarkable re os chronometer in molybdenite how and why it works the re os rhenium osmium chronometer applied to molybdenite mos2 is now demonstrated to be remarkably robust surviving intense deformation and high grade thermal metamorphism successful dating of molybdenite is dependent on proper preparation of the mineral separate and analysis of a critical quantity of molybdenite unique to each sample such that recognized spatial decoupling of 187re parent and 187os daughter within individual molybdenite crystals is overcome highly precise accurate and reproducible age results are derived through isotope dilution and negative thermal ion mass spectrometry id ntims spatial decoupling of parent daughter precludes use of the laser ablation icp ms microanalytical technique for re os dating of molybdenite the use of a reference or control sample is necessary to establish laboratory credibility and for interlaboratory comparisons the rb sr k ar and 40ar 39ar chronometers are susceptible to chemical and thermal disturbance particularly in terranes that have experienced subsequent episodes of hydrothermal magmatic activity and therefore should not be used as a basis for establishing accuracy in re os dating of molybdenite as has been done in the past re os ages for molybdenite are almost always in agreement with observed geological relationships and when available with zircon and titanite u pb ages for terranes experiencing multiple episodes of metamorphism and deformation molybdenite is not complicated by overgrowths as is common for some minerals used in u pb dating e g zircon monazite xenotime nor are re and os mobilized beyond the margins of individual crystals during solid state recrystallization moreover inheritance of older molybdenite cores incorporation of common os and radiogenic os loss are exceedingly rare whereas inheritance common pb and pb loss are common complications in u pb dating techniques therefore molybdenite ages may serve as point in time markers for age comparisons

geology and he ar s pb isotope constraints on the genesis of the sidaogou gold deposit in liaodong peninsula northeastern north china craton abstract the large sidaogou gold deposit in the liaodong peninsula is a typical alteration type gold system hosted in paleoproterozoic metamorphic rocks of gaixian formation ore paragenesis indicates sidaogou mineralization occurred in four stages 1 early ore stage s1 quartz albite 2 main ore stage s2 quartz sericite pyrite gold 3 secondary ore stage s3 quartz polymetallic sulfide scheelite biotite gold and 4 late ore stage s4 calcite ankerite quartz sulfide associated hydrothermal alterations in wall rocks are dominated by albitization s1 silicification s1 to s4 sericitization s2 sulfidation s2 to s3 and carbonatization s4 with minor biotitization s3 developed locally gold mineralization was primarily occurred in s2 stage with intensive silicification sericitization and sulfidation in metamorphic wall rocks and occurred to a lesser extent in s3 stage with weak alteration possibly due to fairly weak activity and small fluxes of the ore fluids at gold precipitation stages s2 to s3 three generations of ore related pyrite py1 py2 and py3 are identified nanosims in situ s isotope analyses show that both py1 and py2 from the s2 stage have relatively broad ranges of 34s values with significant intra grain variations py1 7 7 16 5 py2 3 7 14 8 varying between those of wall rocks 11 6 20 3 and typical magmatic sources as well as the regional contemporary magmatic hydrothermal gold system cmhgs this can be interpreted by a mixed process formed by magmatic hydrothermal fluids leaching different proportion of sulfur from wall rocks during extensive fluid rock interaction py3 in the s3 stage has relatively narrow range of 34s values py3 0 6 3 7 similar to those of adjacent cmhgs indicating a magmatic origin without notable wall rock contamination due to weak fluid rock interaction the s2 stage gold bearing pyrite py1 dominated shows 206pb 204pb ratios from 17 882 to 18 938 207pb 204pb ratios from 15 592 to 15 825 and 208pb 204pb ratios from 37 811 to 39 003 which are between those of nearby early cretaceous sanguliu granitic pluton and wall rocks 206pb 204pb i 17 958 20 085 207pb 204pb i 15 664 15 976 and 208pb 204pb i 37 474 40 021 implying that the lead was originated from mixture sources ore fluids from the s2 stage pyrite py1 dominated have 3he 4he ratios range from 0 21 0 01 to 0 57 0 03 ra average 0 34 ra indicating a crustal source with minor mantle contributions moreover the 3he 4he ratios are similar to those of the adjacent cmhgs demonstrating a similar magmatic origin for the ore fluids the 40ar 36ar ratios range from 547 to 11994 indicating that the ore fluids contained 48 to 98 crust derived radiogenic 40ar which most likely inherited from wall rocks combining the geologic evidences and isotopic results we propose that the sidaogou deposit was formed by magmatic hydrothermal fluids exsolved from sanguliu pluton or its deep magma during fluid rock interaction sulfur lead and 40ar were partly scavenged from the wall rocks and then sequestered by ore related pyrite nevertheless gold was mainly of magmatic origin our study reinforces the previous conclusion that s pb isotope compositions of ore minerals could be strongly modified by wall rocks during fluid rock interaction

fluid inclusion geochemistry and 40ar 39ar geochronology constraints on the genesis of the jianchaling au deposit china abstract the giant jianchaling gold deposit is located in the shaanxi province china the mineralization is hosted by wnw trending faults in the mianxian lueyang yangpingguan mly area the mineralization can be divided into three stages based on mineralogical assemblages and crosscutting relationships of mineralized quartz veins these stages from early to late are characterized by the mineral assemblage of 1 quartz coarse grained pyrite pyrrhotite pentlandite dolomite 2 quartz pyrite gold sphalerite galena carbonate arsenopyrite fuchsite and 3 dolomite calcite quartz fine grained pyrite realgar orpiment three types of fluid inclusions have been recognized in this study based on petrographic and microthermometric measurements including pure co 2 and or ch 4 pc type nacl h 2 o w type and nacl co 2 h 2 o c type fluid inclusions these fluid inclusion types are present in quartz from the stage 1 and 2 assemblages whereas the stage 3 quartz only contains w type fluid inclusions the stage 2 assemblage is associated with the mineralization at the jianchaling deposit fluid inclusions of stage 1 quartz homogenize mainly between 250 and 360 c with salinities up to 15 6 wt nacl equiv whereas the stage 3 dolomite with homogenization temperatures of 160 220 c and salinities of 1 1 7 4 wt nacl equiv this indicates that the ore fluid system evolved from co 2 rich probably metamorphic hydrothermal to co 2 poor meteoric fluid all three types of fluid inclusions can be observed in the stage 2 quartz suggesting that this heterogeneous association was trapped from a boiling fluid system these inclusions homogenized at temperatures of 200 250 c and salinities of 1 2 12 4 wt nacl equiv the estimated trapping pressures of the fluid inclusions are between 117 and 354 mpa in stage 1 suggesting an alternating lithostatic hydrostatic fluid system which was controlled by a fault valve at the depth of 12 km two fuchsite samples collected from the stage 2 polymetallic quartz veins yielded well defined 40 ar 39 ar isotopic plateau ages of 197 2 and 194 2 ma and 39 ar 36 ar 40 ar 36 ar normal isochrones of 198 2 and 199 2 ma this indicates that the mineralization at jianchaling is early jurassic ca 198 ma in age we propose that jianchaling is an orogenic gold deposit and formed during continental collision related to the northward subduction of the mian lue oceanic plate during the early jurassic we also conclude that the beginning of the continental collision between the yangtze and the north china cratons took place around 200 ma

rb sr sm nd and pb isotope systematics of pyrite implications for the age and genesis of lode gold deposits lode gold deposits commonly consist of pyrite and lesser amounts of lead and zinc sulfides accompanied by quartz and calcite their exact origin remains controversial mainly because of the scarcity of reliable geochronological data here we present direct rb sr dating of pyrite from a lode gold deposit and propose the method as a useful geochronological technique for gold mineralization for which age data are poor a positive correlation between present day 87 sr 86 sr and 87 rb 86 sr ratios of pyrite fes 2 subsamples from the linglong gold mine of china corresponds to an age of 122 123 ma which dates the age of gold mineralization the sr nd and pb isotopic compositions of pyrites and the associated rocks suggest that the hydrothermal fluids responsible for the pyrite and gold mineralization were probably derived from a mixed source i e degassing of mafic magmas and meteoric water that had leached the country rocks

la icp ms zircon u pb dating trace element and hf isotope geochemistry of the heyu granite batholith eastern qinling central china implications for mesozoic tectono magmatic evolution article i nfo mesozoic granitoids are widespread in the southern margin of the north china craton and of particular geo logical interests because of their indication for lithospheric evolution and close association with porphyry mo mineralization in this contribution in situ la icp ms u pb hf isotope and trace element analyses of zircon crystals have been combined to constrain the emplacement time and petrogenesis of the heyu granitic bath olith which hosts the yuchiling porphyry mo system henan province china as inferred from field investiga tions the concentrically zoned heyu batholith mainly consists of four texturally distinguishable phases and displays a pattern of monotonically decreasing u pb age from 143 0 1 6 ma for phase 1 through 138 4 1 5 ma for phase 2 through to ca 135 ma for phase 3 and to 133 6 1 3 ma for the mo mineralized yuchiling granite porphyry phase 4 the ehf t values 27 7 to 3 4 and the two stage hf model ages 1403 to 2924 ma of zircon grains together with the ages of local strata suggest that the magmas forming the heyu batholith were dominantly sourced from the crust with input of a mantle component the higher ce ce ratios of the zircon crystals in the mineralized yuchiling porphyry compared to those in the mo barren phases suggests that fluids exsolved from oxidic magmas might be more favorable for mo enrichment a regional synthesis suggests that intensive lithospheric thinning in the southern margin of the north china craton should not be earlier than 130 ma

rotational reflectance properties of arkoma basin dispersed vitrinite insights for understanding reflectance populations in high thermal maturity regions abstract analysis and interpretation of dispersed vitrinite reflectance data in regions of high thermal maturity 2 vitrinite reflectance have been equivocal partly because of an increase in width and complexity of reflectance histograms with increasing mean reflectance such complexity is illustrated by random reflectance rran data from the arkoma basin that display a linear increase in standard deviation of rran with an increase in mean rran from 1 to 5 evaluating how much of the dispersion in these data is the result of vitrinite anisotropy and how much is the result of mixing of kerogen populations by sedimentary processes and or sampling procedures has been problematic automated collection of reflectance data during polarizer rotation provides preliminary data for solution of this problem rotational reflectance data collected from a subset of arkoma basin samples reveal positive linear relationships among maximum r max random rran rotational rrot and minimum r min reflectance as well as a systematic increase in bireflectance r max r min with increasing reflectance r max and rrot display lower standard deviations and narrower more nearly unimodal histograms than rran and r min suggesting that r max and rrot are superior less ambiguous indices of thermal maturity these data patterns are inferred to be mostly an indication of increasing vitrinite anisotropy with increasing thermal maturity suggesting that the linear covariance observed between mean rran and standard deviation in dispersed organic data sets from regions of high thermal maturity may be explained mostly as the result of increasing vitrinite anisotropy with increasing thermal maturity

discovery of sapphirine bearing mg al granulites in the north china craton implications for paleoproterozoic ultrahigh temperature metamorphism abstract we report here for the first time the occurrence of mg al granulites within the khondalite belt of the north china craton and provide robust evidence for extreme crustal metamorphism at ultrahigh temperature uht conditions in this region the uht indicator assemblages in these rocks include equilibrium sapphirine quartz high alumina orthopyroxene sillimanite quartz and high temperature perthites the surrounding khondalites also carry spinel quartz equilibrium assemblage the sapphirines occurring in various microstructural associations are markedly magnesian x mg 0 69 0 77 with a composition varying from 7 9 3 to 2 2 1 end members spinel is a solid solution of mg spinel and hercynite x mg 0 41 0 54 with zno content varying from 0 48 to 1 79 wt garnet shows a compositional range between pyrope and almandine x mg 0 39 0 48 with minor grossular 2 6 3 8 mol and spessartine 0 5 1 8 mol the orthopyroxenes display marked compositional variations in terms of al 2 o 3 content with the core of the medium grained orthopyroxene adjacent to sillimanite in the matrix showing the highest al 2 o 3 content 8 4 wt the history of prograde to peak metamorphism of these rocks is indicated by inclusions of sapphirine spinel quartz and sillimanite within garnet sapphirine formed during the peak stage through the reaction spl qtz sil spr occurrence of sapphirine and quartz inclusions trapped in sillimanite suggests the reaction spr qtz grt opx sil a series of retrograde reactions are observed in these rocks as corona and symplectite textures around early coarse grained minerals such as opx sil qtz crd grt sil qtz crd grt qtz crd opx grt o 2 crd qtz fe 2 o 3 in mag or ilm and finally opx kfs h 2 o bt qtz whereas the stability of sapphirine quartz and orthopyroxene sillimanite quartz in the matrix assemblage suggests t 1000 c and p 10 kbar peak metamorphism conventional geothermobarometric estimates confirm the ultrahigh temperature nature and also trace the retrograde conditions 930 970 c at p 10 kbar al in opx 900 1000 c perthite 930 990 c grt opx and 910 940 c sapphirine spinel electron microprobe dating of monazites in textural association with the uht assemblages yield a sharply defined isochron age of 1917 48 ma which correlates with the 1927 11 ma obtained in a previous study from monazites in the surrounding khondalites carrying spinel quartz assemblage an evaluation of the assemblages and textures using appropriate petrogenetic grids indicate that following peak ultrahigh temperature conditions the rocks underwent initial isobaric cooling and subsequent isothermal decompression and we interpret these trajectories to be part of an overall anti clockwise p t evolution the study area located within the northern margin of the north china craton defines a collisional orogenic belt where continental collision and deep subduction was possibly followed by extensional collapse of the orogen evidence for the extensional phase is provided by the several mafic magmatic intrusives of paleoproterozoic age occurring in the region which suggest a possible link between the thermal anomaly associated with extreme crustal metamorphism in this region with mantle derived magmas the uht metamorphism in north china craton coincided with the assembly of the paleoproterozoic supercontinent columbia

compositional polarity of triassic granitoids in the qinling orogen china implication for termination of the northernmost paleo tethys abstract the qinling orogen was formed from the closure of the northernmost paleo tethys sea and the tectonic suturing of the yangtze and north china cratons the timing of this collision and the tectonic framework are debated the widely developed triassic granitoids in the western qinling orogen offer a key to understand the tectonic evolution of this region here we compile the geological geochemical and geochronological data of the triassic granitoids from the qingling orogen and conclude that the granitoids north of the mian lue suture were emplaced in an active continental margin related to the northward subduction of the mian lue oceanic plate during 248 200 ma the granitoids can be classified into i and s types with the former constituting the major variety northward from the mian lue fault an s type granite belt appears followed by and locally overlapping with an i type granite belt the i type granite belt can be subdivided into four sub belts from south to north with increasing contents of k2o k2o na2o sio2 th and u and the ratios of k2o na2o rb sr and 87sr 86sr i but decreasing contents of na2o al2o3 mg and ehf t except for the northernmost belt which occurs in the huaxiong block of the north china craton this geochemical polarity of i type granitoids and the zoned distribution of s and i types granitoids cannot be explained through continental collision orogeny including syn to post collision but can be well interpreted by considering the progressive subduction of the northernmost branch of the paleo tethys as represented by the mian lue ocean thus the termination of the northernmost paleo tethys and the onset of the continental collision between the yangtze and the north china plates are considered to have occurred at about 200 ma during the transition from triassic to jurassic our model is also supported by the available data from other studies and provides a revised framework for the timing and tectonics of assembly of the yangtze and the north china cratons

introduction to tectonics of china abstract the continental crust of china is a mosaic of cratonic blocks and orogenic belts containing small cratons and terranes with various tectonic settings they have diverse origins and complex histories of amalgamation and often suffered repeated reworking after multiple episodes of amalgamation in the last three decades extensive geological geochemical and geophysical investigations have been carried out on these cratonic blocks and intervening orogenic belts producing an abundant amount of new data and competing interpretations this provides important insights into understanding the formation and evolution of the chinese continents the papers assembled in this volume present a timely and comprehensive overview on major advancements and controversial issues related to the formation and evolution of continental crust in china complex tectonic histories were experienced not only by the large scale cratonic blocks and orogenic belts but also by small scale terranes and orogens between and inside these blocks nevertheless our understanding of lithotectonic units and geological processes has been greatly advanced by recent studies of zirconology and geochemistry for various rock types from major petrotectonic units in china it has been further advanced from integrated interpretations of geochemical and petrological data for petrogenesis of magmatic rocks an overview of these observations and interpretations provides new insights into understanding the continental plate tectonics and the chemical geodynamics of subduction zones

crustal growth in the north china craton at 2 5ga evidence from in situ zircon u pb ages hf isotopes and whole rock geochemistry of the dengfeng complex abstract the dengfeng complex is located on the southern margin of the north china craton ncc and belongs to the southern portion of the trans north china orogen this terrane is important to understand the formation and evolution of ncc during late neoarchean 2 5 ga the dengfeng complex is well exposed in the junzhao region and comprises two distinct lithologic units supracrustal assemblage and plutonic rocks la icpms magmatic zircon u pb age data shows that the rocks formed within the range of 2547 2504 ma the available hf isotope data indicate that the majority of ca 2 5 ga zircons from the dengfeng complex have high e hf t values close to the initial hf isotope ratios of the contemporaneous depleted mantle these data indicate that the rocks in the dengfeng represent juvenile crust the ttg gneisses in the dengfeng complex display low mg 41 48 mgo 59 69 mgo 3 5 6 6 wt ni 82 130 ppm and cr 148 237 ppm abundances elevated sr 1759 1927 ppm and ba 1742 2289 ppm concentrations and high lree la n 38 487 such geochemical features are similar to archean sanukitoids a two stage model is applied here to explain the genesis of metadiorites of dengfeng complex 1 firstly the mantle is metasomatized either by melts or by aqueous fluids from a subducted slab 2 subsequently sanukitoid magmas were produced by partial melting of the hybridized mantle furthermore the amphibolites of supracrustal rocks have a mixture of morb and arc like geochemical affinities suggesting the development of a back arc in the southern ncc at ca 2 5 ga the contemporary late neoarchean ttgs sanukitoids and morb back arc association may represent a late neoarchean tectonic melange implying a neoarchean subduction accretion process which would suggest that modern style plate tectonics processes was probably initiated in the southern ncc by 2 5 ga

geology fluid inclusion geochemistry and 40ar 39ar geochronology of the wulasigou cu deposit and their implications for ore genesis altay xinjiang china abstract the wulasigou cu deposit occurs as veins controlled by a nw trending structure in a devonian volcano sedimentary basin of the altay orogenic belt xinjiang china igneous and sedimentary rocks exposed in the area have undergone greenschist facies metamorphism the ore forming process can be divided into early middle and late stages represented by respectively pyrite quartz polymetallic sulfide quartz and carbonate quartz veins veinlets and or replacement bodies the early veins were deformed and brecciated during a compressional or transpressional event the middle stage veinlets filled fractures in the early stage vein and alteration assemblages and are undeformed suggesting a tensional shear setting the late stage veinlets are mainly open space fissure fillings that cut veins and replacement bodies formed in the earlier stages four types of fluid inclusions fis including aqueous w type mixed carbonic aqueous m type purely carbonic c type and daughter mineral bearing s type have been identified in copper related quartz and calcite from the wulasigou deposit the early stage quartz contains m and w type primary fis that completely homogenized at temperatures of 322 412 c with low salinities of 0 9 6 5 wt nacl equiv in contrast the late stage quartz or calcite contains only the w type fis with homogenization temperatures of 101 234 c and salinities of 0 9 2 9 wt nacl equiv this indicates that the metallogenic system evolved from co 2 rich metamorphic to co 2 poor through input of meteoric fluids all four types of fis can only be observed in the middle stage minerals where they show evidence of vein formation during an episode of fluid immiscibility these fis homogenized at temperatures ranging mainly from 230 to 347 c with salinities clustering 2 7 10 2 wt nacl equiv for the w m and c types and 34 7 38 2 wt nacl equiv for the s type respectively the metal precipitation resulted from a decrease in copper solubility during the fluid immiscibility episode the estimated trapping pressures for the middle stage fluids are 1 55 3 55 kbar suggesting an alternating lithostatic hydrostatic fluid system controlled by fault valve activity at a depth of 13 15 5 km muscovite separates from the middle stage polymetallic quartz veinlets yield a well defined 40 ar 39 ar isotopic plateau age of 219 41 2 10 ma and an 39 ar 36 ar 40 ar 36 ar isochron age of 219 73 2 17 ma this age postdates the final paleo asia ocean closure at ca 250 ma by about 30 ma and indicates that the cu mineralization at wulasigou has occurred in the triassic continental collision setting hence the wulasigou cu deposit may be the first example of orogenic lode cu deposits formed in accretionary orogeny or continental collision

isotope geochemistry of the tieluping silver lead deposit henan china a case study of orogenic silver dominated deposits and related tectonic setting the tieluping silver deposit which is sited along ne trending faults within the high grade metamorphic basement of the xiong er terrane in the qinling orogenic belt is part of an important recently discovered mesozoic orogenic type ag pb belt ore formation includes three stages an early barren quartz pyrite stage e an intermediate polymetallic sulfide ore stage m and a late barren carbonate stage l carbon sulfur and lead isotope systematics indicate that the e stage fluids are deeply sourced the l stage fluids are shallow sourced meteoric water whereas the m stage fluids are a mix of deep sourced and shallow sourced fluids sulfur and lead isotope data show that the ore forming fluids must have originated from a source with elevated radiogenic lead and low 34s values that differs significantly from exposed geologic units in the xiong er terrane the lower crust and the mantle this supports the view that the carbonate shale chert sequences of the guandaokou and luanchuan groups south of the machaoying fault might be the favorable sources although little is known about their isotopic compositions a tectonic model that combines collisional orogeny metallogeny and hydrothermal fluid flow is proposed to explain the formation of the tieluping silver deposit during the mesozoic collision between the north china craton and south china block early mid triassic indosinian orogeny crustal slabs containing the carbonate shale chert sequences of the guandaokou and luanchuan groups locally rich in organic matter carbonaceous shale were thrust northwards beneath the xiong er terrane along the machaoying fault metamorphic devolatilisation of this underthrust slab probably provided the ore forming fluids to develop the ag pb ore belt which includes the tieluping silver deposit fluids and magmas were emplaced during extensional stages related to the jurassic cretaceous yanshanian orogeny

nd and sr isotope study of hydrothermal scheelite and host rocks at omai guiana shield implications for ore fluid source and flow path during the formation of orogenic gold deposits omai is a high tonnage low grade world class gold deposit located in the paleoproterozoic guiana shield it is the second most important gold deposit in the guiana shield after las cristinas venezuela and one of the largest in south america 4 0 million oz sm nd and sr isotope data are presented for host rocks and for scheelite from auriferous quartz carbonate scheelite sulfide telluride veins from the omai deposit gold bearing veins are hosted by the paleoproterozoic barama mazaruni supergroup a greenstone belt sequence consisting of mafic volcanic rocks interbedded with sedimentary rocks that are intruded by quartz feldspar porphyry and rhyolite dikes this lithologic sequence was folded and metamorphosed to lower greenschist facies during the paleoproterozoic trans amazonian orogeny the volcano sedimentary unit was intruded by a post tectonic quartz monzodiorite diorite hornblendite stock initial nd isotope ratios for the omai volcanic rocks range from nd 2 1 to 4 2 these values suggest that this part of the guiana shield was a site of new crust formation during the paleoproterozoic and was not contaminated by older archean reworked continental crust initial nd isotope ratios for the omai stock range between 0 5 and 2 3 which suggest limited contamination with previously formed continental crust although the nd isotopic ratios of gold related scheelites overlap with those of the host rocks particularly the tholeiitic basalts at the interpreted time of vein emplacement the lack of both isotopic mixing and significant nd movement during the hydrothermal process suggest that the nd isotope composition can be used to determine the isotopic characteristics of the ore fluid source area at omai the ore fluid is largely derived from a radiogenic nd source represented by mantle or lower crustal reservoirs strontium isotope ratios for the scheelites cluster tightly between 0 7019 and 0 7021 the sr isotope data suggest that unlike nd sr was significantly mobile during the hydrothermal process the fluids responsible for the omai deposit may have picked up sr along the flow path the constant low sr isotope values of scheelites probably reflect the key role that the local tholeiitic basalts played as the main source of sr in the fluids whereas nd isotopes identify the fluid source area the sr isotopes map the fluid flow paths

metallogeny during continental outgrowth in the columbia supercontinent isotopic characterization of the zhaiwa mo cu system in the north china craton abstract the eastern qinling molybdenum belt eqmb in central china represents the largest mo province in the world located at the southern margin of the north china craton this mo province is largely associated with mesozoic granitoids here we report re os isochron age of 1761 33 ma 2 mswd 2 9 from molybdenite in the recently discovered zhaiwa porphyry mo cu system within the eqmb this age coincides with the peak eruption period 1 78 1 75 ga of the xiong er group volcanic rocks which unconformably overlie the taihua supergroup and are interpreted to have developed on a continental arc the re concentrations 1 to 43 ppm of the zhaiwa molybdenites are similar to those of typical porphyry cu mo au systems developed on continental arcs the 34 s values of the zhaiwa ores 2 7 7 3 average 5 2 suggest that the sulfur was mainly sourced from a magmatic system the sulfides from the ores yield sr i average of 0 70533 similar to that of the xiong er group 0 70547 albeit higher than that of the kuanping group 0 70334 and lower than the taihua supergroup 0 70763 suggesting a mixed source the average e nd 1 76 ga of the sulfides from the ores 0 9 falls between the averages of the taihua supergroup ca 6 8 and the kuanping group ca 7 4 the pb isotope ratios of the ores are similar to but slightly higher than those of the taihua supergroup reflecting preferential leaching of radiogenic pb from the taihua supergroup by magmatic fluids integrated data and interpretations constrain the zhaiwa mo cu deposit to be a porphyry mineral system developed within a ca 1 76 ga continental arc related to oceanic plate subduction beneath the southern margin of the north china craton a correlation between tectonic settings and metallogenic types clearly indicate that the xiong er group was formed in a volcanic arc confirming paleo mesoproterozoic continental outgrowth along the margins of the columbia supercontinent the isotopic characterization of the zhaiwa deposit provides new insights into the enigma of the widespread distribution of large and super large mo deposits of eqmb along the southern margin of the north china craton

geochronology and geochemistry of the giant qian echong mo deposit dabie shan eastern china implications for ore genesis and tectonic setting abstract the qian echong mo deposit is a giant porphyry deposit in the dabie shan of eastern china molybdenum mo mineralization mainly occurs as numerous veinlets in altered schists with the development of potassic phyllic argillic and propylitic alteration assemblages a buried granite porphyry stock and locally exposed dikes are spatially and genetically related to the mo mineralization zircon crystals from exposed quartz porphyry rhyolitic porphyry and granite porphyry dikes as well as a buried granite porphyry dike yield weighted average 206 pb 238 u ages of 128 9 1 1 ma mswd 0 30 2 n 13 127 42 0 94 ma mswd 0 58 2 n 17 127 44 0 98 ma mswd 1 6 2 n 16 and 126 6 1 4 ma mswd 0 67 2 n 15 respectively the buried granite porphyry stock yields a zircon u pb weighted average age of 124 7 1 6 ma mswd 1 5 2 n 16 which is slightly younger than the exposed dikes but within error of the age of the buried granite porphyry dike five molybdenite samples from the ores yield re os isotope ages of 123 31 1 02 to 128 49 1 40 ma which are consistent with the u pb ages for the igneous rocks these age data constrain magmatic hydrothermal activity at the qian echong mo deposit to a period of about six million years from 129 to 123 ma this period overlaps the recognized regional mo mineralization event in the dabie shan of 142 111 ma which took place during the post collisional tectonism subsequent to amalgamation of the yangtze and north china blocks the qian echong granite porphyry stock and dikes have high contents of sio 2 k 2 o and al 2 o 3 and low contents of tio 2 mgo and cao showing a peraluminous high k calc alkaline to shoshonite affinity with obvious lree enrichment and a negative eu anomaly the rocks have a high initial 87 sr 86 sr of 0 70729 to 0 71788 and highly negative e nd t values of 16 2 to 26 1 with t dm2 nd ages of 2 24 to 3 03 ga their 206 pb 204 pb t 207 pb 204 pb t and 208 pb 204 pb t values range from 16 017 to 16 701 15 252 to 15 368 and 37 095 to 37 578 respectively this sr nd pb isotopic signature indicates that the causative granitic intrusions for the qian echong deposit were mainly melts with components of both the northern dabie complex and the taihua and xiong er groups that are part of the precambrian basement of the north china block the melts were generated during the tectonic extrusion of the dabie shan and their emplacement onto the basement of the north china block the qian echong mo deposit is unique based upon the crustal source for the causative porphyry localization of most mineralization in the surrounding schist country rocks and the co 2 rich ore forming fluid thus we propose the qian echong deposit to represent a new sub type of porphyry mo deposit herein termed collisional or dabie type porphyry deposit

triassic mineralization with cretaceous overprint in the dahu au mo deposit xiaoqinling gold province constraints from shrimp monazite u th pb geochronology abstract the xiaoqinling terrane in central china is the second largest gold producing area in china previous application of age constraints has yielded ages for the gold mineralization ranging from paleoproterozoic through triassic indosinian to jurassic and cretaceous yanshanian shrimp u th pb dating of hydrothermal monazite in a mo mineralized quartz vein from the dahu au mo deposit in the northern xiaoqinling terrane identifies two mineralization episodes hydrothermal monazite intergrown with molybdenite yields an initial precipitation age of 216 5 ma which is identical to but more precise than the molybdenite re os isochron age of 218 41 ma a younger generation of monazite is distinguished by data falling on a mixing array it is inferred that the younger monazite was formed at the expense of the older ones during the yanshanian probably during a single event that occurred at 125 ma combining these results with available geochronological data and geological evidence we conclude that molybdenum mineralization accompanied by monazite deposition took place in the indosinian followed by a yanshanian tectono hydrothermal event that resulted in large scale gold mineralization in the xiaoqinling terrane our observation demonstrates that monazite is sensitive to hydrothermal overprint and thus is useful for dating fluid infiltration event

pb sr nd isotope constraints on the fluid source of the dahu au mo deposit in qinling orogen central china and implication for triassic tectonic setting abstract the dahu au mo deposit is a structure controlled lode system occurring in the northern xiaoqinling terrane huaxiong block qinling orogen this paper reports a new sr nd pb isotope dataset obtained for ore sulfides and the hostrocks within the taihua supergroup in an attempt to constrain the source of the ore forming fluids from a new dimension 16 sulfide samples yield i sr ratios of 0 70470 0 71312 with an average of 0 70854 e nd t values between 13 5 and 18 1 with average of 15 1 and 206 pb 204 pb i 207 pb 204 pb i and 208 pb 204 pb i ratios of 17 033 17 285 15 358 15 438 and 37 307 37 582 with averages of 17 162 15 405 and 37 440 respectively 5 gneiss samples from the taihua supergroup yield i sr ratios of 0 70947 0 73201 averaging 0 72294 e nd t values of 20 0 to 31 1 averaging 25 1 and 206 pb 204 pb i 207 pb 204 pb i 208 pb 204 pb i ratios of 17 127 18 392 15 416 15 604 and 37 498 37 814 with averages of 17 547 15 470 and 37 616 respectively these data show that the ore sulfides have less radiogenic sr nd pb isotope systematics than the hostrocks and suggest that the ore forming fluids which interacted with the wallrocks to form ores must be sourced from a depleted mantle or a depleted subducted oceanic slab in combination with the spatial scenario and geochemical signatures of the triassic magmatites and mineral systems we argue that in the triassic the mianlue ocean was not completely closed and that the northward oceanic plate subduction still survived along the mian lue suture which caused the late triassic magmatism and associated mineralization in qinling orogen including the au mo dahu deposit

re os and sr nd pb isotope constraints on source of fluids in the zhifang mo deposit qinling orogen china abstract the zhifang mo deposit is located in the northeastern qinling orogen along the southern margin of the north china craton the deposit represents a quartz vein system hosted in the mesoproterozoic xiong er group volcanic rocks we identify three hydrothermal stages early middle and late characterized by veinlets of quartz pyrite quartz molybdenite pyrite chalcopyrite galena sphalerite and quartz carbonate assemblages respectively five molybdenite samples from the zhifang deposit yield re os ages ranging from 241 2 1 6 ma to 247 4 2 5 ma with an isochron age of 246 0 5 2 ma 2 mswd 7 4 and a weighted mean age of 243 8 2 8 ma 2 mswd 5 5 the re os age shows that the mo mineralization occurred during the indosinian orogeny and suggests that the mineralization is unrelated to the yanshanian magmatism or the paleo mesoproterozoic volcanic hydrothermal event this study also reports a new sr nd pb isotope dataset from ore sulfides in an attempt to constrain the source of the ore forming fluids ten sulfide samples from middle stage of the zhifang mo deposit yield i sr t ratios of 0 710286 0 711943 with an average of 0 711004 e nd t values between 19 5 and 14 8 with an average of 16 7 and 206 pb 204 pb i 207 pb 204 pb i and 208 pb 204 pb i ratios of 17 126 17 535 15 374 15 466 and 37 485 37 848 with averages of 17 380 15 410 and 37 631 respectively one pyrite from the early stage yield i sr t of 0 722711 0 722855 with an average of 0 722783 which is higher than those of the middle stage sulfides and suggests equilibration with wallrocks the e nd t values are in the range of 17 3 to 16 6 with a mean at 17 0 and 206 pb 204 pb i 207 pb 204 pb i and 208 pb 204 pb i ratios are 17 386 15 405 and 37 622 respectively the ore sulfides show higher pb isotope ratios higher e nd t and lower i sr t values than the host rocks the results suggest that the ore forming fluids had lower i sr t and higher e nd t values than the ore sulfides and were possibly sourced from the dengfeng complex the southward subduction of the north china craton beneath the huaxiong block during the triassic was possibly responsible for the formation of the waifangshan orogenic mo system

assembling north china craton within the columbia supercontinent the role of double sided subduction abstract an evaluation of recent s wave receiver functions s wave velocities and two versions of p wave tomographic images along various transects in the north china craton provides some clues on the subduction collision history of the different crustal blocks and their final amalgamation within the paleoproterozoic columbia supercontinent interpretation of a n s seismic section of the craton suggests thick slab debris sinking to various depths in the mantle the w e seismic corridors show the preservation of a thick 200 km lithospheric root tectosphere beneath the ordos block and its variable and extensive erosion towards the yanliao block eastern block this zone is characterized by layers with marked velocity contrast and suggests repeated stacking of the remnants of underplated and accreted paleoproterozoic oceanic lithosphere the present day lithosphere asthenosphere boundary beneath this region probably marks the erosional plane along which decratonization occurred through subduction erosion from the east and thermal and material erosion by upwelling asthenosphere from below resulting in the partial destruction of the tectosphere and its thinning towards the east within the asthenosphere below the yanliao block younger and thinner slabs predominate in the absence of any prominent thick high velocity layers these younger slabs define a westward polarity and constitute a mega scale duplex formed by underplating through phanerozoic subduction process particularly the pacific plate subduction from the east the lithologic associations within the inner mongolia suture zone dividing the yinshan block to the north and ordos block to the south correspond to an accreted ocean plate stratigraphic sequence with the tonalite trondhjemite granodiorite ttg gneisses charnockites and calc alkaline granites representing a continental arc built up through subduction from the north the seismic transects bring out a contrasting polarity in the subduction regime with an oblique east to southward subduction of the yinshan block and a westward subduction of the yanliao block here i propose a double sided subduction history for the ncc similar to the ongoing subduction process in the western pacific such double sided subduction is considered to promote rapid amalgamation of continental fragments within supercontinents and the subduction polarities and mantle dynamics of ncc are therefore considered to be critical in evaluating the final assembly of the paleoproterozoic supercontinent columbia

an inference on the process of gas outburst from lake nyos cameroon abstract a large volume of lethal gas was released from lake nyos in the northwest province of cameroon on august 21 1986 some 50 days after the event a hydro physical and chemical survey of lake nyos was carried out by four japanese scientists including the author vertical density distributions of lake water showed remarkably stable stratification through the whole water column chemical analyses showed high concentrations of dissolved co2 and fe which contribute to stable stratification assuming an initial saturation of dissolved co2 in lake water released gas volume was estimated to be 0 68 km3 as a difference between saturated and remaining gas contents in the present lake temperature drops due to quasiadiabatic expansion of co2 gas exsolved from various water layers were estimated for every 10 m layer thickness assuming a constant temperature distribution and a complete thermal exchange between expanding gas and lake water the calculated temperature profile agreed well with the profile measured by ctd conductivity temperature depth profiler the height of the gas cloud over the lake and the flow velocity of draining gas from the outlet of the rim were evaluated using a simple geometrical flux model assuming a sinusoidal variation of ejection with equal volume mixing with air and head loss coefficient of cd 0 3 the calculated maximum gas height of 110 m and the corresponding drain velocity of 19 m s 1 were obtained

ore forming conditions and genesis of the huogeqi cu pb zn fe deposit in the northern margin of the north china craton evidence from ore petrologic characteristics abstract the huogeqi cu pb zn fe deposit is located in the langshan district in the western segment of the northern margin of the north china craton the deposit is hosted by upper greenschist lower amphibolite facies 550 650 c rocks of the langshan group most orebodies are hosted in a shear zone that developed in parallel with the sedimentary bedding of the langshan group fe was precipitated coeval with the deposition of the host rocks i e a syngenetic origin whereas cu pb and zn were precipitated in hydrothermal systems postdating formation of the host rocks i e an epigenetic origin the hydrothermal mineralization process can be subdivided into two stages 1 a main stage with predominant cu pb zn mineralization stage where ore forming structures are characterized by brittle ductile and brittle shear deformation the main stage sulfide veins cut the mylonite fabrics of the host rocks mineral paragenesis and compositions indicate that main stage cu mineralization took place at 330 440 c and 3 4 3 9 kbar this pressure temperature p t condition is consistent with that of the brittle ductile transition zone but lower than that of the peak metamorphism of the host rocks compared with cu mineralization the main stage pb zn mineralization took place at relatively lower temperature and pressure the syngenetically formed fe orebodies and iron formations in the huogeqi deposit were favorable sites for epigenetic cu precipitation the h 2 s rich cu fluids would have reacted with fe in the host rocks when flowing through fe orebodies and iron formations such a reaction would have led to a reduction in h 2 s of the ore forming fluids and consequently cu precipitation 2 the late stage cu mineralization is of less economic importance characterized by open space filling textures and a low temperature mineral assemblage that constrains the ore forming temperature as lower than 330 c this late stage mineralization took place in an epithermal like hydrothermal system at a shallower crustal depth

geology geochemistry and ore genesis of the wenyu gold deposit xiaoqinling gold field qinling orogen southern margin of north china craton abstract the wenyu giant gold deposit is hosted in the precambrian taihua supergroup metamorphic rocks within the xiaoqinling terrane qinling orogen on the southern margin of the north china craton the mineralization can be divided into three stages quartz pyrite veins early quartz sulfide veins middle main and carbonate quartz veinlets late with gold being mainly introduced in main stage quartz formed in two earlier stages contains three compositional types of fluid inclusions i e pure co2 co2 h2o and nacl h2o but the late stage minerals only contain the nacl h2o inclusions the inclusions in quartz formed in the early main and late stages yield total homogenization temperatures of 262 417 c 236 407 c and 114 239 c respectively with salinities no higher than 13 wt nacl equiv trapping pressures estimated from co2 h2o inclusions are 139 399 mpa and 111 316 mpa in the early and main stages corresponding to mineralization depths of 14 km and 11 km respectively fluid boiling and mixing caused rapid precipitation of sulfides and native au through boiling and inflow of meteoric water the ore forming fluid system evolved from co2 rich to co2 poor in composition and from metamorphic to meteoric as indicated by decreasing 18owater values from early to late the carbon sulfur and lead isotope compositions suggest the hostrocks within the taihua supergroup to be a significant source of ore metals integrating the data obtained from the studies including regional geology ore geology fluid inclusion and c h o s pb isotope geochemistry we conclude that the wenyu gold deposit was an orogenic type system formed in the tectonic transition from compression to extension during the jurassic early cretaceous continental collision between the north china and yangtze cratons

early mesozoic mo mineralization in the qinling orogen an overview abstract as one of the most important mo provinces in the world the qinling orogen is host to mainly late mesozoic late jurassic early cretaceous porphyry mo systems recently additional early mesozoic mainly triassic plus minor early jurassic mo deposits have been identified which occur as fault controlled quartz veins or as carbonatite veins and as such they are fundamentally different from the commonly observed porphyry systems furthermore early mesozoic porphyry systems are also present but they are different from those of late mesozoic age thus early mesozoic mo systems not only offer a new and comprehensive view of the mo metallogenesis but also provide unique insight into the tectonic evolution of the qinling orogen taking into consideration the role of mineral deposits as ideal probes of the geodynamic evolution the known early mesozoic mo deposits in qinling can be classified into three types based on their geological and geochemical features porphyry carbonatite and orogenic systems porphyry mo systems including porphyry porphyry skarn and intrusion related veins are spatially and genetically associated with granitic porphyries they display typical porphyry type alterations and are formed by magmatic water carbonatites are considered as mantle derived igneous rocks and mo mineralization is associated with magmatic fluids orogenic mo systems occur as fault controlled quartz lodes and the ore forming fluids are medium high temperature co2 rich metamorphic fluids spatially early mesozoic porphyry mo systems are concentrated in the western and middle part of the qinling orogen whereas carbonatite veins and orogenic mo systems are located in the northeastern part temporally orogenic mo deposits were the earliest mainly between 220 and 250 ma followed by carbonatite veins mainly between 205 and 225 ma and porphyry mo systems mainly between 190 and 205 ma with some exceptions there is generally a trend of decreasing deposit ages toward the western part of the orogen detailed comparison reveals that the early mesozoic porphyry mo systems in the qinling orogen are different from the late mesozoic ones in terms of their economic importance spatial distribution ore causative intrusions hydrothermal alteration and the ore forming fluids to explain the above geological and geochemical features the triassic qinling is considered as an active continental margin caused by the northward subduction of the mian lue oceanic plate a transition from continental arc to back arc basin is suggested for the formation of triassic orogenic and carbonatite vein mo deposits whereas continental arc is host to the early mesozoic porphyry mo systems

petrogenesis and tectonic setting of volcanic rocks in the xiaoshan and waifangshan areas along the southern margin of the north china craton constraints from bulk rock geochemistry and sr nd isotopic composition abstract as part of the xiong er volcanic belt along the southern margin of the north china craton volcanic rocks in the xiaoshan and waifangshan areas have a compositional range from the basaltic andesite andesite dacite to rhyolite which display consistent variation trends in terms of their major and trace elements and sr nd isotopic compositions the variable yb contents with nearly constant la yb and tb yb ratios of volcanic rocks in two areas suggest that the fractional crystallization may have played an important role in the differentiation from the basaltic andesite through andesite and dacite to rhyolite the volcanic rocks in these two areas are characterized by the lile and lree enrichments and negative hfse anomalies implying hydrous melting of a mantle wedge in a subduction zone variable sr nd ratios of the basaltic andesite and andesite are interpreted as a result of the fluid addition from a subducting slab non radiogenic nd isotopic compositions as well as high zr y and nb y ratios suggest that the volcanic rocks in these areas were derived from an enriched mantle source on the other hand the volcanic rocks of the basaltic andesite and andesite possess markedly higher fe ti and hfse concentrations than those of typical intra oceanic arcs implying that the mantle source from which the volcanic rocks were derived was metasomatised by siliceous melts during the archean to paleoproterozoic subduction collision in the trans north china orogen these data suggest that in the paleo mesoproterozoic the southern margin of the north china craton was most likely an andean type continental arc in which slab dehydration not only induced the melting of a pre existing metasomatised mantle source but also released lile enriched fluids into the mantle source masking the inherent hfse enriched characteristics of the volcanic rocks along the southern margin of the craton the results of this study indicate that the north china craton like many other continental components e g north america greenland baltica amazonia australia etc of the supercontinent columbia nuna also underwent a subduction related outgrowth along its southern margin during the paleo mesoproterozoic time

shrimp u pb zircon geochronology of palaeoproterozoic metasedimentary rocks in the north china craton evidence for a major late palaeoproterozoic tectonothermal event abstract although metasedimentary successions are widely distributed across the north china craton those of palaeoproterozoic age are restricted to three orogenic belts namely the trans north china orogen the khondalite belt and the jiao liao ji belt prior to this work some of those successions were considered to be archaean in age the palaeoproterozoic metasedimentary rocks contain detrital igneous zircon grains with ages ranging from 3 0 to 2 1 ga with the majority recording an age between 2 3 and 2 0 ga this latter group define a series of magmatic events that occurred between formation of the archaean basement and deposition of the metasedimentary rock protoliths the metasedimentary rocks occur as two associations older volcano sedimentary units that formed between 2 37 and 2 0 ma and younger sedimentary units that formed mainly between 2 0 and 1 88 ga all these rock associations were then subjected to metamorphism during the late palaeoproterozoic 1 88 and 1 85 ga tectonothermal event named the luliang movement in early chinese literature which was a continent continent collision that led to the amalgamation of the north china craton

geology geochronology and isotopic geochemistry of the xiaoliugou w mo ore field in the qilian orogen nw china case study of a skarn system formed during continental collision this study is financially supported by the national natural science foundation of china 41502068

the evolution of thermal structure and water chemistry in lake nyos abstract we collected a time series of physical and chemical data to gain a better understanding of the dynamics of lake nyos measurements of water and gas chemistry and temperature made during january march and may 1987 are compared to data taken in september 1986 just after the initial co 2 gas release there is no pattern of change in overall heat content of the lake although heat input to bottom waters 185 208 m has occurred at a rate of 1600 mw m 2 this increase in heat content translates to a change from 23 38 to 24 12 c at 200 m and can be explained by geothermal heat flow and addition of thermal spring water concentrations of ca 2 mg 2 na k fe 2 and alkalinity have increased only in bottom waters in situ lake processes such as sulfate and iron reduction are unable to account for the changes in alkalinity observed chemical changes are consistent with a scenario where slightly thermal soda water is being input to the bottom of the lake measurements of p co 2 at depth ranged from 18 to 28 of saturation and exhibited horizontal variability overall recharge of co 2 in bottom waters is negligible mainly because of increasing ion concentrations in bottom water total stability of the water column increased 33 from 48 800 j m 2 in september 1986 to 64 700 j m 2 in may 1987 as long as co 2 concentrations remain the same this level of stability is higher than could be disrupted by common limnologic or meteorologic processes there is thermal and chemical evidence that a buildup of dissolved iron and co 2 in bottom waters must have preceded the august 1986 gas release in addition a survey of all crater lakes in cameroon indicates that only lakes nyos and monoun contain high concentrations of dissolved iron and co 2 thus there is a low probability of any other cameroonian lake releasing a substantial volume of co 2

tectonics and surface effects of the supercontinent columbia abstract assembly of the supercontinent columbia at about 1 85 1 90 ga coincided with several events that affected the entire earth the oldest worldwide network of orogenic belts formed at the same time although some granite granodiorite gg suites had formed earlier the gg suites became common in the 1 8 1 9 ga orogenic belts these suites succeeded the older tonalite trondhjemite granodiorite ttg suites which were not produced after 1 8 ga changes on the earth s surface at 1 8 1 9 ga include rapid increase in the concentration of oxygen in the atmosphere and oceans and probably the evolution of eukaryotes all of these surface changes occurred as columbia accreted and the assembly of columbia may have contributed to the drastic changes in the earth s surface environment as well as to the evolution of primitive life forms

gas buildup in lake nyos cameroon the recharge process and its consequences the gases dissolved in lake nyos cameroon were quantified recently december 1989 and september 1990 by two independent techniques in situ measurements using a newly designed probe and laboratory analyses of samples collected in pre evacuated stainless steel cylinders the highest concentrations of co2 and ch4 were 0 30 mol kg and 1 7 mmol kg respectively measured in cylinders collected 1 m above lake bottom probe measurements of in situ gas pressure at three different stations showed that horizontal variations in total dissolved gas were negligible total dissolved gas pressure near the lake bottom is 1 06 mpa 10 5 atm 50 as high as the hydrostatic pressure of 2 1 mpa 21 atm comparing the co2 profile constructed from the 1990 data to one obtained in may 1987 shows that co2 concentrations have increased at depths to below 150 m based on these profiles the average rate of co2 input to bottom waters was 2 6 108 mol a increased deep water temperatures require an average heat flow of 0 32 mw into the hypolimnion over the same time period the transport rates of co2 heat and major ions into the hypolimnion suggest that a low temperature reservoir of free co2 exists a short distance below lake bottom and that convective cycling of lake water through the sediments is involved in transporting the co2 into the lake from the underlying diatreme increased ch4 concentrations at all depths below the oxycline and a high14c content 41 modern in the ch4 4 m above lake bottom show that much of the ch4 is biologically produced within the lake the ch4 production rate may vary with time but if the co2 recharge rate remains constant co2 saturation of the entire hypolimnion below 50 m depth would require 140a given present day concentrations

fluid inclusion study of the nannihu giant porphyry mo w deposit henan province china implications for the nature of porphyry ore fluid systems formed in a continental collision setting abstract the nannihu mo w deposit located in the henan province of china is a giant porphyry deposit formed in the qinling orogen during the jurassic continental collision between the north china and yangtze cratons mo w mineralization is associated with the nannihu porphyritic monzogranite occurring as numerous stockwork veinlets in the altered wallrocks and the causative porphyry both the porphyry stock and wallrocks underwent intense hydrothermal alteration ranging outwardly from potassic to phyllic alteration and to carbonation zones with increasing distance from the intrusion the hydrothermal ore forming process can be divided into four stages characterized by veinlets from early to late 1 quartz potassic feldspar molybdenite pyrite 2 quartz molybdenite veins containing minor pyrite and chalcopyrite 3 quartz polymetal sulfide and 4 quartz carbonate fluorite most of the ores are formed during stages 2 and 3 four types of fluid inclusions fis are distinguished in this study based on petrographic and microthermometric criteria i e pure co 2 nacl h 2 o co 2 h 2 o and daughter mineral bearing fluid inclusions all of the four types of fis can be observed in the hydrothermal quartz formed in stages 1 2 and 3 while the stage 4 minerals contain only nacl h 2 o fluid inclusions fluid inclusions of stage 1 are mainly homogenized between 350 and 460 c with salinities ranging from 5 7 to 17 9 wt nacl equiv the stage 2 fis yield homogeneous temperatures of 300 380 c and salinities of 3 5 16 9 wt nacl equiv fis of stage 3 are homogenized between 250 and 370 c with salinities of 1 8 14 3 and 28 6 39 1 wt nacl equiv fis of stage 4 are homogenized at temperatures of 115 to 265 c yielding salinities of 0 5 to 1 2 wt nacl equiv daughter minerals such as halite chalcopyrite and some unknown minerals are frequently present in stage 3 fis which probably represent a reducing and oversaturated fluid system resulting from fluid boiling the daughter mineral bearing fis coexist with vapor and liquid rich nacl h 2 o fis that have contrasting salinities these fis are homogenized in divergent ways at similar temperatures suggesting that fluid boiling took place in stages 2 and 3 the estimated pressures range from 70 to 270 mpa in stage 1 through 30 150 mpa in stage 2 to 30 85 mpa in stage 3 corresponding to a depth of no less than 3 km in other words the ore forming fluids are characterized by high temperature high salinity high f o 2 and high co 2 content and fluid boiling resulted in co 2 release f o 2 decrease and ore mineral precipitation we suggest that the co 2 rich fluid is a distinctive feature of porphyry systems developed in continental collision setting in transitional compressional to extensional regime contrasting to the co 2 poor nacl h 2 o fluids observed in volcanic arcs

u pb dating of baddeleyite and zircon from the shizhaigou diorite in the southern margin of north china craton constrains on the timing and tectonic setting of the paleoproterozoic xiong er group abstract the xiong er volcanism marks an important thermal event in the southern margin of the north china craton during the paleoproterozoic precise u pb ages of zircon and baddeleyite from the shizhaigou diorite which intruded the majiahe formation of the upper part of the xiong er group were determined in this study by multi collector sims the mean 207 pb 206 pb ages are 1789 3 5 ma for baddeleyite and 1780 11 ma for zircon from shizhaigou diorite respectively the major and trace elements and sr nd isotopic compositions indicate an em i mantle type signature and crustal contamination features combining with previous chronological data it is proposed that the age of the xiong er volcanism is between 1770 ma and 1800 ma with a peak at 1780 ma our result supports the view that the xiong er volcanic rocks formed in an extensional rift environment

genesis of the dadonggou pb zn deposit in kelan basin altay nw china constraints from zircon u pb and biotite 40ar 39ar geochronological data abstract the genesis of polymetallic deposits in southern altay nw china has been disputed between a syngenetic seafloor hydrothermal process and an epigenetic orogenic type mineralization the dadonggou pb zn deposit occurs as nw trending veins in the devonian kangbutiebao formation volcanic sedimentary sequence in the kelan basin southern altay a set of integrated zircon u pb and biotite 40ar 39ar geochronological data were applied to constrain the forming ages of the ores and their country rocks three samples of host volcanic rocks yielded weighted mean 206pb 238u ages of 397 1 4 5 ma 391 7 3 6 ma and 391 1 4 2 ma respectively indicating that the kangbutiebao formation was deposited in a devonian back arc basin two biotite samples separated from the pb zn containing quartz veins yielded 40ar 39ar plateau ages of 205 9 2 1 ma and 204 3 2 2 ma respectively which represent the age of the pb zn mineralization that is attributed to the closure of the kelan back arc basin and the late triassic orogeny combining the available geological and geochronological data this contribution outlines the successive evolution from the development of a devonian back arc basin to the late triassic post subduction orogeny and proposes that the dadonggou pb zn deposit is an epigenetic orogenic type deposit placed in the late triassic orogeny

multistage processes linked to tectonic transition in the genesis of orogenic gold deposit a case study from the shanggong lode deposit east qinling china abstract the shanggong lode deposit represents one of the largest mesozoic orogenic gold deposits in the east qinling metallogenic belt of central china here we investigate the texture trace element and sulfur isotopic composition of pyrite from this deposit to gain insights in the ore genesis three types of pyrite are recognized by its texture py1 is represented by coarse grained cubic and pentagonal dodecahedron belonging to the stage i quartz pyrite vein py2 comprises fine grained disseminated pentagonal dodecahedron belonging to stage ii quartz sulfides vein py2 can be further divided into core and rim porous cores py2c carrying abundant visible mineral inclusions and significantly higher arsenic concentration and clean rims py2r with low arsenic systematic la icpms trace elements and sims sulfur isotopic analyses reveal two distinct stages of mineralization with diverse source materials 1 the early stage is represented by py1 which contains 0 65 70 ppm au average 11 6 ppm with relatively low and variable concentrations of cu ag te pb zn and bi the 34 s values of py1 range from 2 7 to 7 2 suggesting that the sulfur source for the early stage fluid was mainly derived from the taihua and xiong er groups and that the depleted 34 s values reflect changes of ph f o2 and fluid rock interaction 2 the main stage is represented by py2 which contains au in the range of 0 14 189 ppm average 39 7 ppm the py2 carries significantly higher contents of compatible elements ni co v and cr and ore metals e g au ag pb zn as te than those in py1 with the ore metals occurring as invisible lattice bound or nanosized particles and visible micron sized inclusions the 34 s values of py2 show marked depletion with a range of 16 3 to 11 7 attributing to the fractionation between barite and sulfide minerals together with previous studies and trace element characters in py2 we identify a contribution of au from mantle source into the ore forming fluid system during the main stage we propose that the two stages of gold mineralization in the shanggong deposit in response to the collisional assembly of qinling orogenic belt preserve evidence for diverse metal sources with the early stage derived from partial melting of the lower crust taihua and xiong er groups in the compressional stage and the main stage characterized by mantle contribution in a post collisional extensional setting our findings may have important implications for the exploration of regional triassic gold and molybdenum deposits

genesis of the 1 76 ga zhaiwa mo cu and its link with the xiong er volcanics in the north china craton implications for accretionary growth along the margin of the columbia supercontinent abstract the east qinling molybdenum belt hosting numerous mesozoic magmatic hydrothermal mo deposits is one of the largest molybdenum provinces in the world the recently discovered zhaiwa mo cu deposit luoning county henan province occurs within the xiong er terrane of the east qinling orogenic belt the mo cu mineralization is associated with widespread stockworks or veinlets occurring within altered amphibolite here we investigate the characteristics of the ore mineralization through systematic fluid inclusion petrography microthermometry and laser raman spectroscopy the typical inclusions in the veins and stockworks are aqueous daughter mineral bearing and a subordinate co2 rich type the homogenization temperatures of the aqueous inclusions in quartz molybdenite veins are up to 370 c which together with some daughter mineral bearing inclusions homogenized by dissolution of halite at 500 c indicates that the ore forming fluids were derived from a hypothermal fluid system the daughter mineral bearing inclusions assign high salinities for the ore forming fluids correlating the ore deposit to a magmatic hydrothermal system we also present h o isotope systematics which support this interpretation the recently reported re os molybdenite age of 1 76 0 03 ga from this deposit is coeval to the xiong er group andesite dominated volcanic sequence 1 8 1 45 ga based on which we propose that the zhaiwa deposit represents the upper domain of a porphyry system developed within a subduction related setting with accretion to the southern margin of the north china craton our study provides new constraints on the outgrowth of the columbia supercontinent during paleoproterozoic

the xiong er volcanic belt at the southern margin of the north china craton petrographic and geochemical evidence for its outboard position in the paleo mesoproterozoic columbia supercontinent abstract the xiong er volcanic belt covering an area of more than 60 000 km 2 along the southern margin of the north china craton has long been considered an intra continental rift zone and recently interpreted as part of a large igneous province formed by a mantle plume that led to the breakup of the paleo mesoproterozoic supercontinent columbia however such interpretations cannot be accommodated by lithology mineralogy geochemistry and geochronology of the volcanic rocks in the belt lithologically the xiong er volcanic belt is dominated by basaltic andesite and andesite with minor dacite and rhyolite different from rock associations related to continental rifts or mantle plumes which are generally bimodal and dominated by mafic components however they are remarkably similar to those rock associations in modern continental margin arcs in some of the basaltic andesites and andesites amphibole is a common phenocryst phase suggesting the involvement of h 2 o rich fluids in the petrogenesis of the xiong er volcanic rocks geochemically the xiong er volcanic rocks fall in the calc alkaline series and in most tectono magmatic discrimination diagrams the majority of the xiong er volcanic rocks show affinities to magmatic arcs in the primitive mantle normalized trace element diagrams the xiong er volcanic rocks show enrichments in the lile and lree and negative nb ta ti anomalies similar to arc related volcanic rocks produced by the hydrous melting of metasomatized mantle wedge nd isotope compositions of the xiong er volcanic rocks suggest that 5 15 older crust has been transferred into the upper lithospheric mantle by subduction related recycling during archean to paleoproterozoic time available shrimp and la icp ms u pb zircon age data indicate that the xiong er volcanic rocks erupted intermittently over a protracted interval from 1 78 ga through 1 76 1 75 ga and 1 65 ga to 1 45 ga though the major phase of the volcanism occurred at 1 78 1 75 ga such multiple and intermittent volcanism is inconsistent with a mantle plume driven rifting event but is not uncommon in ancient and existing continental margin arcs taken together the xiong er volcanic belt was most likely a paleo mesoproterozoic continental magmatic arc that formed at the southern margin of the north china craton similar paleo mesoproterozoic continental magmatic arcs were also present at the southern and southeastern margins of laurentia the southern margin of baltica the northwestern margin of amonzonia and the southern and eastern margins of the north australia craton which are considered to represent subduction related episodic outbuilding on the continental margins of the paleo mesoproterozoic supercontinent columbia therefore in any configuration of the supercontinent columbia the southern margin of the north china craton could not have been connected to any other continental block as proposed in a recent configuration but must have faced an open ocean whose lithosphere was subducted beneath the southern margin of the north china craton

shrimp and la icp ms zircon geochronology of the xiong er volcanic rocks implications for the paleo mesoproterozoic evolution of the southern margin of the north china craton the paleo mesoproterozoic xiong er volcanic rocks along the southern margin of the north china craton are lithologically and geochemically similar to those formed in subduction related continental margin volcanic arcs the volcanic rocks are primarily composed of basaltic andesites and andesites with minor dacites and dacitic rhyolites traditionally the xiong er volcanic rocks have been divided from lower to upper into the xushan jidanping and majiahe formations but the ages of volcanic rocks in these formations have not been well constrained which has hindered further understanding the tectonic significance of the xiong er volcanic belt at the southern margin of the north china craton shrimp and la icp ms u pb zircon analyses combined with cathodeluminescence cl images have enabled resolution of xenocrystic and magmatic zircons that can be directed toward determination of the ages of the xiong er volcanic rocks shrimp and la icp ms u pb analyses on magmatic zircons from two basaltic andesite samples one dacite sample and one rhyolite sample of the xushan formation known as the lowest sequence of the xiong er volcanic rocks indicate that the volcanic eruption of this formation occurred at 1 78 ga but most xenocrystic inherited zircons in these samples yielded 207pb 206pb ages ranging from 2 55 ga to 1 91 ga of three samples collected from the jidanping formation two rhyolite samples 05xe015 and 05xe100 yielded weighted mean 207pb 206pb ages of 1778 5 5 ma and 1751 14 ma respectively similar to the ages of the volcanic rocks in the xushan formation whereas one dacite sample 05xe066 gave a weighted mean 207 pb

genetic links between porphyry mo and peripheral quartz vein mo cu mineralization in the baituyingzi district eastern inner mongolia ne china abstract although there are important implications for mineral exploration the genetic links between porphyry mo and peripheral mo cu vein deposits remain unclear to constrain their possible relationships this paper studied hydrothermal veins ar ar geochronology and o h s pb isotopes from three mo cu deposits in the baituyingzi district ne china mineralization in the baituyingzi district occurs as quartz stockworks e g the baituyingzi porphyry mo deposit surrounding the baituyingzi monzogranite porphyry and 0 1 1 5 m wide quartz veins e g the kulitu and baimashigou mo cu deposits along regional faults apart from the quartz unidirectional solidification textures stage 1 observed at baituyingzi the sequence of hydrothermal activity at each deposit was similar although with minor differences and includes stage 2 sulfide barren and cu bearing quartz veins related to potassic alteration e g k feldspar biotite hematite stage 3 mo bearing quartz veins with no obvious alteration stage 4 pyrite rich quartz veins related to quartz muscovite illite alteration and stage 5 calcite quartz veins representing late hydrothermal activity 40ar 39ar plateau ages of muscovite from stage 4 demonstrated that hydrothermal activities in baituyingzi 241 4 1 4 ma and kulitu 239 4 1 2 ma were contemporaneous consistent with earlier re os ages indicating that the three deposits were co genetic in the early triassic moreover similar fluid inclusion assemblages homogenization temperature ranges up to 500 c and o h isotopic ratios 18ofluid 4 14 to 7 89 and d 97 to 90 indicative of magmatic origin of stage 2 veins and 34s values 0 3 and radiogenic lead isotope ratios of sulfide samples were identified from each deposit further confirming that the three deposits belong to individual mineralized centers integrated with previous data we consider that 1 the baituyingzi hydrothermal system formed during the intrusion of the 248 ma baituyingzi monzogranite porphyry and 2 the different stress regimes e g regional structures and local fluid pressures wall rock compositions and fluid characteristics e g salinity of initial fluid branches resulted in the formation of three hydrothermal deposits with different vein attitudes and cu mo ratios thus our study reveals that the mo cu vein deposits can constitute individual mineralization centers of a porphyry system as a result of different fluid branches that evolved along individual t p wall rock routes this finding is a novel supplement to the magmatic hydrothermal mineralization model and can provide guidance for the exploration of similar mo cu vein deposits

re os sm nd u pb and stepwise lead leaching isotope systematics in shear zone hosted gold mineralization genetic tracing and age constraints of crustal hydrothermal activity abstract a combined re os sm nd u pb and stepwise pb leaching pbsl isotope study of hydrothermal mo w bearing minerals and base metal sulfides from two adjacent shear zone hosted gold deposits ran kimberley in the harare shamva greenstone belt zimbabwe constrain the timing of the mineralizing events to two periods during an initial late archean event 2 60 ga a first molybdenite scheelite bearing paragenesis was deposited in both shear zone systems followed by a local reactivation of the shear systems during an early proterozoic 1 96 ga tectono thermal overprint during which base metal sulfides and most of the gold was re deposited while pbsl has revealed an open system behavior of the u pb systematics in molybdenite and wolframite from the ran mine initial archean re os ages are still preserved implying that this system in these minerals was more resistant to the overprint a similar retentivity could be shown for the sm nd system in scheelite and powellite associated with the above ore minerals re os isotopic data from the proterozoic mineralization in the kimberley mine point to a recent gain of re most pronouncedly affecting fe rich sulfides such as pyrrhotite a significant re loss in powellitic scheelite an alteration phase of molybdenite bearing scheelite coupled with a marked loss of u in w mo ore minerals complements the observation of a major re uptake in fe sulfides during oxidizing conditions in a weathering environment pyrrhotite under these conditions behaves as an efficient re sink lead isotope signatures from pbsl residues of molybdenite powellite and quartz indicate a continental crustal source and or contamination for the mineralizing fluid by interaction of the fluids with older sedimentary material as represented by the direct host country rocks our investigation reveals the potential of the re os isotopic system applied to crustal hydrothermal ore minerals for genetic tracing and dating purposes the simplified chemical separation of re and os from geological material used in this study together with improvements of chemical yields will enable high precision data to be collected rapidly on crustal material with low os concentrations in the future

isotope and fluid inclusion geochemistry and genesis of the qiangma gold deposit xiaoqinling gold field qinling orogen china abstract the qiangma gold deposit is hosted in the 1 9 ga taihua supergroup metamorphic rocks in the xiaoqinling terrane qinling orogen on the southern margin of the north china craton the mineralization can be divided as follows quartz pyrite veins early quartz polymetallic sulfide veinlets middle and carbonate quartz veinlets late stages with gold being mainly introduced in the middle stage three types of fluid inclusions were identified based on petrography and laser raman spectroscopy i e pure carbonic carbonic aqueous co 2 h 2 o and aqueous inclusions the early stage quartz contains pure carbonic and co 2 h 2 o inclusions with salinities up to 12 7 wt nacl equiv bulk densities of 0 67 to 0 86 g cm 3 and homogenization temperatures of 280 365 c the early stage is related to h 2 o co 2 n 2 ch 4 fluids with isotopic signatures consistent with a metamorphic origin 18 o water 3 1 to 5 2 d 37 to 73 the middle stage quartz contains all three types of fluid inclusions of which the co 2 h 2 o and aqueous inclusions yield homogenization temperatures of 249 346 c and 230 345 c respectively the co 2 h 2 o inclusions have salinities up to 10 9 wt nacl equiv and bulk densities of 0 70 to 0 98 g cm 3 with vapor bubbles composed of co 2 and n 2 the isotopic ratios 18 o water 2 2 to 3 6 d 47 to 79 suggest that the middle stage fluids were mixed by metamorphic and meteoric fluids in the late stage quartz only the aqueous inclusions are observed which have low salinities 0 9 9 9 wt nacl equiv and low homogenization temperatures 145 223 c the isotopic composition 18 o water 1 9 to 0 5 d 55 to 66 indicates the late stage fluids were mainly meteoric water trapping pressures estimated from co 2 h 2 o inclusions are 100 285 mpa for the middle stage suggesting that gold mineralization mainly occurred at depths of 10 km fluid boiling and mixing caused rapid precipitation of sulfides and native au through boiling and inflow of meteoric water the ore forming fluid system evolved from co 2 rich to co 2 poor in composition and from metamorphic to meteoric as indicated by decreasing 18 o water values from early to late the carbon sulfur and lead isotope compositions suggest the hostrocks within the taihua supergroup to be a significant source of ore metals integrating the data obtained from the studies including regional geology ore geology and fluid inclusion and c h o s pb isotope geochemistry we conclude that the qiangma gold deposit was an orogenic type system formed in the tectonic transition from compression to extension during the jurassic early cretaceous continental collision between the north china and yangtze cratons

mineralization mechanisms in the shangfanggou giant porphyry skarn mo fe deposit of the east qinling china constraints from h o c s pb isotopes abstract the giant shangfanggou skarn porphyry mo fe deposit one of the largest mo deposits in the east qinling orogen is hosted by the neoproterozoic luanchuan group on the southern margin of the north china craton the mineralization occurs mainly as veinlets and in veinlet disseminated form within altered schists skarns hornfelses and or marbles the hydrothermal alteration and mineralization at shangfanggou took place over four stages stage 1 involved potassic alteration and silicification in the porphyry skarnization and magnetization in carbonate strata and hornfelsization in schists quartz molybdenite veins associated with phyllic alteration were produced in the porphyry during stage 2 along with sericitization chloritization serpentinization and silicification in the skarns hornfelses and or marbles quartz polymetallic sulfide veins related to phyllic alteration were produced during stage 3 and ore barren quartz calcite pyrite veins associated with weak carbonatation were produced during stage 4 c o and d isotope compositions of hydrothermal calcite dolomite magnetite phlogopite and quartz from different stages of mineralization during the formation of the shangfanggou deposit range from 12 9 to 8 8 3 2 to 22 7 and 100 to 80 respectively sulfur isotope compositions of sulfides from the ores of the shanfanggou deposit range from 3 6 to 4 4 206 pb 204 pb 207 pb 204 pb and 208 pb 204 pb ratios of 17 648 to 18 742 apart from one magnetite with a value of 20 78 15 526 to 15 690 and 38 197 to 39 823 in the shanfanggou deposit the evolution of the ore forming fluids in the shangfanggou deposit as revealed by 18 o d and 13 c isotope studies was one of progressive modification of the magmatic fluids through fluid wall rock interactions along conduits during fluid ascent and this signifies the involvement of formation strata water during the earlier stages of mineralization and the predominance of meteoric waters over magmatic fluids in the later stages the carbon sulfur and lead isotope compositions suggest mixing took place between the magmas and the host rocks within the region integrating the data obtained from studies of the regional geology ore geology fluid inclusions and c h o s pb isotope geochemistry we conclude that the shangfanggou mo fe deposit is a porphyry skarn type system that formed during a transition from compressional to extensional tectonics during the jurassic early cretaceous continental collision between the north china craton and yangtze craton the ore materials were sourced from magmatic ore forming fluids that mixed with components derived from carbonaceous wall rocks during their ascent and which were gradually mixed with meteoric waters especially in the later stages

the lake nyos gas disaster chemical and isotopic evidence in waters and dissolved gases from three cameroonian crater lakes nyos monoun and wum abstract to better understand the cause of the nyos gas disaster of august 21 1986 we conducted geochemical and limnological surveys in october 1986 of three lakes nyos monoun and wum which are located in the cameroon volcanic zone that is characterized by a prevalence of young alkaline basalts and basanitoids lake wum was studied as a non active control co2 is dissolved in significant concentrations about 1 5 of saturation in gas active lakes nyos and monoun but is virtually absent in lake wum stable isotopic ratios of total dissolved carbon 13c 3 for nyos and 5 5 for monoun and of helium 5 7 ratm for nyos and 3 6 ratm for monoun indicate a mantle origin of these gases however so42 and cl concentrations are found to be very low concentrations of dissolved chemical species like fe2 mg2 ca2 and hco3 are high in the two gas active lakes whereas they are very low in the gas inactive lake high salinities in the gas active lakes are probably due to dissolution of indigenous mafic rocks and transported lateritic soil in acidic co2 rich warm water the gas active lakes are characterized by increasing temperature and salinity with increasing depth indicating an active influx of heat and dissolved materials at the bottom density estimates show that the lake water is stably stratified in spite of the inverse temperature profile of the lakes on account of dissolved chemical species the concentrations of dissolved carbonate species co2 aq and hco3 are positively correlated with those of ionic dissolved species indicating their common occurrence in the bottom water the august 1986 gas bursts from lake nyos were most likely caused by rapid exsolution of dissolved co2 within the lake an explosive process such as a phreatic eruption or a co2 gas jetting from beneath the bottom is unlikely because of low concentrations of cl and so42 no oxygen isotopic shift low turbidity and no reported perturbation of the bottom sediments exsolution of co2 bubbles could occur if co2 saturated bottom water was displaced upwards by an increased influx of high salinity water from the bottom during the rainy season exsolution of co2 at the upper layers was possibly accelerated by upwelling of a two phase fluid co2 bubbles and solution a mechanism known as a pneumatic lift pump resulting in discharge of a large amount of co2 gas the h2s concentration in the gas cloud must have been kept far below the lethal level because of a high fe2 concentration of the lake water

the shanggong gold deposit eastern qinling orogen china isotope geochemistry and implications for ore genesis abstract the shanggong au deposit in the xiong er terrane east qinling china has resources of about 30 ton au making it one of the largest orogenic mesothermal au deposits hosted in volcanic rocks of the mesoproterozoic xiong er group three stages of hydrothermal activity are recognized early middle and late of which two early and middle were ore producing and characterized by quartz pyrite and polymetallic sulfides respectively the third and late stage is represented by a carbonate quartz assemblage hydrogen oxygen and carbon isotope systematics of the shanggong deposit from a previous work suggest that the early stage fluids were derived from magmatic and or metamorphic devolatilization of sedimentary rocks at depth this is supported by new c s and published sr and pb isotopic data presented in this paper these new data 13c values ranging from 1 5 for early stage ankerite to 2 2 for late stage ankerite negative 34s values for sulfides from the middle stage 19 2 to 6 3 suggest a contribution from organic matter and that the ore fluid evolved from deeply sourced to shallowly sourced with those of the middle stage representing a mixture of these two fluid systems the comparison of the hydrogen oxygen carbon sulfur lead strontium isotope systematics between the shanggong deposit and the main lithologies in the xiong er terrane shows that neither these nor the underlying lower crust and mantle or combinations thereof could be considered as the source of ore fluids for the shanggong au deposit a likely source was a carbonaceous carbonate sandstone shale chert sequence in the underthrusted guandaokou and luanchuan groups exposed south of the xiong er terrane ar ar and rb sr isochron ages for mineral phases of the early middle and late stages together with geological field data constrain the timing of the hydrothermal activity and au metallogenesis at 242 10 167 7 and 112 7 ma respectively this metallogenesis and associated granitic magmatism can be related to the continental collision between the yangtze and north china cratons that resulted in the formation of the qinling orogen led to the different hydrothermal systems that were responsible for the three stages that formed the shanggong au deposit over a period of about 130 myrs

diverse and multistage mo au ag pb zn and cu deposits in the xiong er terrane east qinling from triassic cu mineralization abstract the xiong er terrane is one of the representative regions in east qinling orogenic belt various polymetallic deposits have been discovered in the xiong er terrane such as porphyry mo carbonatite hosted type mo quartz vein type mo breccia pipes type au and orogenic type au ag pb zn deposits indicating that diverse significant mineralization occurred in the xiong er terrane recently numerous cu deposits have been observed in the xiong er terrane xianglugou is a quartz vein type copper mineralization which was discovered in xiayu district southern of the xiong er terrane four chalcopyrite samples from xianglugou yield re os isochron age of 230 31 ma 2 mswd 0 8 this is the first evidence for triassic cu mineralization in the xiong er terrane this not only shows the xiong er terrane contains considerable mo au and ag pb zn reserve but also indicates that there is a great potential of cu mineralization the discoverable cu mineralization can further emphasize the significance of the widespread triassic mineralization and widened exploration ideas and goals in the future in the xiong er terrane the xiong er terrane experienced three polymetallic mineralization events including paleoproterozoic triassic and late jurassic to early cretaceous deposits during the qinling orogen evolution these metallogenic events may be related to multiple tectonic events the paleoproterozoic mineralization was associated with the continental outgrowth occurred along the southern margin of the north china craton before 1760 ma the triassic cu mineralization may be associated with the northward subduction of the north qinling orogen beneath the huaxiong block during the collision between north china craton and yangtze craton late jurassic to early cretaceous mineralization perhaps caused by decompressional extension of the qinling orogenic belts and the north china craton intense destruction and lithospheric thinning which is now generally related to the westward subduction of paleo pacific plate

re os isotopic dating of molybdenites in the xiaoliugou w mo deposit in the northern qilian mountains and its geological significance abstract the xiaoliugou is a large scale w mo deposit newly explored in the northern qilian mountains caledonian fold belt northwestern china it is a vein and skarn type deposit genetically associated with the xiaoliugou caledonian granodiorite stock re os isotopic dating for the molybdenite in the deposit yields an isochron age of 462 13 ma 2 and model ages of 436 to 496 ma these results suggest that w mo mineralization occurred before the collision of plates and mineralized substances mainly originated from the crust

the significance of siderite in the sediments from lake nyos cameroon abstract small amounts 1 3 wt of endogenic siderite feco3 and traces of pyrite fes2 are present in a 1 m long sediment core from lake nyos thermodynamic modeling shows that the deep waters are supersaturated with respect to siderite and pyrite no other carbonates were found to be stable this rare occurrence of siderite is symptomatic of the build up of large concentrations of co2 and fe2 in the lake nyos waters a search for siderite in the deeper sediments could reveal the existence of previous gas burst events that have occurred at lake nyos or in other cameroon lakes

the geology and geochronology of the waifangshan mo quartz vein cluster in eastern qinling china abstract the waifangshan mo quartz vein cluster occurs at the northernmost margin of the qinling orogen the orebodies occur as quartz veins and are controlled by low angle faults we identify three hydrothermal stages characterized by veinlets of quartz pyrite quartz molybdenite pyrite chalcopyrite galena sphalerite and quartz carbonate assemblages respectively the early stage quartz veins were structurally deformed brecciated and filled by non deformed middle or late stage veinlets suggesting that the mineralization is associated with a tectonic transition from compression to extension five molybdenite separates from the zhifang mo deposit yield individual re os ages from 241 2 1 6 to 247 4 2 5 ma with a weighted mean age of 243 8 2 8 ma individual re os isotope ages of four molybdenite samples from the badaogou mo deposit range from 238 9 2 1 to 255 8 2 2 ma with a weighted mean age of 246 10 ma four analyses of molybdenite separates from the xiangchungou mo deposit yield individual re os isotope ages of 243 8 8 8 to 247 7 4 4 ma with a weighted mean age of 246 0 1 1 ma all of these data yield a weighted mean age of 241 9 1 9 ma coeval with the accretionary orogeny we thus conclude that the waifangshan mo quartz veins formed during a transition from continental arc to back arc setting the waifangshan mo quartz veins share similar geological geochemical and geochronological features with the orogenic type systems we propose that the waifangshan mo quartz veins are an orogenic type mo mineralization and possibly represent the deepest member in the crustal continuum model of the orogenic type mineral systems the molybdenite samples from the mo quartz veins in the waifangshan area have low re contents 1 457 to 39 16 ppm suggesting a continental crust dominated source

mesozoic molybdenum deposits in the east qinling dabie orogenic belt characteristics and tectonic settings abstract the east qinling dabie orogenic belt accommodates the largest mo ore district in the world it contains 8 43 mt of proven mo metal reserves which accounts for 66 of the total proven chinese mo reserves the mo ore district includes 24 deposits and 12 occurrences with four major types of mo mineral systems i e porphyry porphyry skarn skarn and hydrothermal veins the latter can be further subdivided into quartz vein and carbonatite vein types although mo mineralization in the belt began in the paleoproterozoic 1680 24 to 2044 14 ma all economically significant deposits were formed during the mesozoic re os dating of molybdenite has shown that there are three episodes of mo mineralization i e late triassic 233 221 ma late jurassic to early cretaceous 148 138 ma and early to middle cretaceous 131 112 ma late triassic mo deposits developed as molybdenite quartz veins and carbonatite vein types stable isotope systematics c o s and high contents of re and sr indicate that the carbonatite mo veins are mantle derived porphyry and porphyry skarn mo mineral deposits were formed in the late jurassic to early cretaceous and early to middle cretaceous the late jurassic to early cretaceous granite porphyries that are associated with the mo deposits usually occupy less than 1 5 km2 at the surface and are situated in the east qinling area far west of china s continental margin on the other hand the early to middle cretaceous batholiths and granite porphyries with associated mo deposits are located in the dabieshan area and eastern part of the east qinling area the late jurassic to early creataceous granitoids and related mo deposits possibly formed in a back arc extensional setting of the eurasian continental margin which was probably triggered by the oblique subduction of the izanagi plate the early to middle cretaceous batholiths and granite porphyries are linked to the tectonic regime of lithospheric thinning asthenospheric upwelling and partial melting of the crust induced by a change in izanagi plate motion parallel to the continent margin in the east qinling dabie belt there are vein type pb zn ag deposits surrounding porphyry and or porphyry skarn mo w deposits forming well defined ore clusters the same spatial arrangement i e porphyry mo stockworks and outlying pb zn ag ore veins is also observed at the deposit scale thus mo porphyry stockworks and distal polymetallic veins belong to the same ore system and may reflect an outward temperature decrease from the highly fractionated granite plutons both porphyry stockworks and polymetallic veins can be used as vectors for further prospecting

metallogeny of the north china craton link with secular changes in the evolving earth abstract the north china craton ncc has experienced a complex geological evolution since the early precambrian and carries important records of secular changes in tectonics and metallogeny here we synthesize the salient geological and tectonic features of the evolution and destruction of the ncc vis a vis major metallogenic events and the formation of potential ore deposits we identify a close relationship between the major geological events in the ncc and those reported elsewhere on the globe we trace the records of a regular change in the pattern of metallogeny mineral deposit character spatial distribution and genetic mechanisms which closely match the timing and styles of the major geological and tectonic events in this craton the ncc went through five major tectonic cycles 1 neoarchean crustal growth and stabilization 2 paleoproterozoic rifting subduction accretion collision with imprints of the great oxidation event goe 3 late paleoproterozoic neoproterozoic multi stage rifting 4 paleozoic orogenesis at the margins of the craton and 5 mesozoic extensional tectonics associated with lithospheric thinning and decratonization coinciding with these major geological events are five major metallogenic systems identified as follows 1 an archean bif system 2 paleoproterozoic cu pb zn and mg b systems 3 a mesoproterozoic ree fe pb zn system 4 a paleozoic orogenic cu mo system and 5 mesozoic intracontinental au and ag pb zn and mo systems the ore deposit types in each of these metallogenic systems show distinct characteristics and tectonic affinities from early precambrian through late precambrian to paleozoic and mesozoic the ncc records a transition from primitive to modern style plate tectonics evidence for imbricated oceanic plate stratigraphy in a subduction accretion setting and collisional orogenesis along at least three major zones of ocean closure are documented major transitions in tectonic style and surface environmental changes recorded in other parts of the world are also reflected in the geological history and metallogenic events in the ncc large scale gold deposits formed through intraplate tectonics during the mesozoic provide important insights into mantle dynamics and crust mantle interaction associated with lithospheric thinning and craton destruction the ncc provides one of the best examples for documenting secular changes in the geological history and metallogenic epochs of an evolving earth

composition and evolution of the archaean crust in central henan china the upper archaean in central henan province is composed chiefly of rocks of the dengfeng complex and the taihua complex rocks of the dengfeng complex show essential features of a granite greenstone terrane they consist of greenstone belts and surrounding granitic and migmatite gneiss terranes when the late superposed structures are removed the archaean conspicuously exhibits alternating gneiss domes and synformal basins which are occupied by granitic rocks and greenstone belts respectively the succession of the greenstone belts is divided into three groups 1 1 the lower group consists of ultramafic complexes which are only locally exposed 2 2 the middle group which comprises most of the greenstone belts contains tholeiitic metabasalts and felsic volcanics which are bimodal in character 3 3 the upper sedimentary group consists of molasse and turbidite formations the metamorphic grade in the dengfeng complex is up to the medium pressure almandine amphibolite facies the mafic volcanics are characterized by flat to slightly enriched ree patterns that resemble the th2 type of condie in most other elements the rocks are also similar to the tholeiitic metabasalts in typical greenstone belts throughout the world however they possess certain unique characteristics as for the granitic rocks the early plutons tend to be na rich and are geochemically similar to tonalitic rocks while the later intrusives tend to be k rich the taihua complex is composed primarily of felsic gneisses and metamorphosed supracrustal rocks interbedded with mafic ultramafic rocks the metamorphic grade in the complex ranges from granulite to upper amphibolite facies geochemical characteristics of the rocks are also similar to those of other high grade terranes throughout the world in general an overall structural grain characterizes the taihua complex which constitutes an elongate fold belt with characteristic features of linear mobile belts that are frequently found in high grade terranes the extensive dengfeng granite greenstone terrane and the linear taihua high grade gneissic terrane are bounded by thrusts or contacts of unknown nature and together make up the late archaean crust in central henan they may have developed more or less contemporaneously on a pre existing sialic crust in this region controlled by shallow convection cells in the upper mantle their development may have occurred in two different tectonic settings the rift type and the subsiding type setting respectively

fluid inclusion geochemistry and ore genesis of the longmendian mo deposit in the east qinling orogen implication for migmatitic hydrothermal mo mineralization abstract the 1 85 ga longmendian mo deposit in east qinling is the oldest mo system recognized in china it is unique for three reasons 1 quartzofeldspathic orebodies are away from any intrusion or fault 2 it is closely associated with migmatitic rocks and 3 it has remarkably high re content in molybdenite 504 to 1660 ppm the origin of the deposit is poorly constrained in the longmendian deposit strong mo mineralization is always associated with hydrothermally altered migmatitic amphibolites to probe into ore genesis detailed fluid inclusion studies are carried out on both mineralized migmatitic amphibolites and ore barren rocks four compositional types of fluid inclusions are observed including co 2 ch 4 pc type co 2 h 2 o c type daughter mineral bearing s type and h 2 o nacl w type quartz in mesosome and melanosome of mineralized migmatitic amphibolites contains all of the four types of inclusions measurements of immiscible inclusion assemblages in the ores constrain the mo mineralization temperatures and pressures to be 225 390 c and 114 265 mpa respectively primary fluid inclusions in barren migmatitic gneisses are dominated by s type and minor of c and pc types which are identical to those in the leucosome of migmatitic amphibolites these inclusions yield lower homogenization temperatures than those in the mineralized mesosome and melanosome of migmatitic amphibolites suggesting that the ore causative injected melts should have higher temperatures and originated from depths such features of the ore forming fluids indicate that the longmendian deposit was a migmatitic hydrothermal system caused by high temperature melt injection this interpretation can also be supported by the observations below 1 tourmaline is abundant in melanosome but absent in mesosome 2 the consistent quartzofeldspathic composition of leucosome is independent of mesosome 3 leucosome in migmatitic amphibolite crosscuts each other 4 halite bearing fluid inclusions are prevalent in studied samples which is the feature of granitic rocks instead of the scenarios of in situ migmatites and 5 the trapping temperatures of fluid inclusions in mineralized migmatitic amphibolite are much lower than those required for partial melting but higher than those obtained from barren migmatitic rocks

ore geology and fluid inclusion geochemistry of the tiemurt pb zn cu deposit altay xinjiang china a case study of orogenic type pb zn systems abstract the tiemurt pb zn cu deposit is hosted in a devonian volcanic sedimentary basin of the altay orogenic belt and is thus interpreted to have formed by sea floor hydrothermal exhalation in previous studies our investigation discovered that the deposit is not stratiform or stratabound but structure controlled instead the hydrothermal ore forming process can be divided into the early middle and late stage represented by pyrite quartz polymetallic sulfide quartz and carbonate quartz veinlets respectively the early stage veins and contained minerals are structurally deformed and brecciated suggesting a compressional or transpressional tectonic regime the middle stage veinlets intrude and infill the fissures of the early stage assemblages and show no deformation suggesting a tensional shear setting the late stage veinlets mostly infill open space fissures that crosscut veins and replacements formed in the earlier stages four types of fluid inclusions fis including aqueous type w carbonic aqueous type c pure carbonic type pc and solid bearing type s are identified at the tiemurt deposit the early stage minerals contain the c and w type primary fis that are totally homogenized at temperatures of 330 390 c with low salinities of 0 8 11 9 wt nacl eqv whilst the late stage quartz or calcite contains only the w type fis with homogenization temperatures of 118 205 c and salinities of 1 4 3 4 wt nacl eqv this indicates that the ore fluid system evolved from co2 rich probably metamorphic to co2 poor meteoric fluids and that a significant co2 escape must have occurred all the four types of fis can be only observed in the middle stage minerals and even in a microscopic domain of a crystal representing an association trapped from a boiling fluid system these fis homogenize at temperatures ranging from 270 to 330 c with two salinity clusters of 1 9 14 5 and 37 4 42 4 wt nacl eqv respectively this implies that metal precipitation resulted from fluid boiling co2 escape and transient oversaturation the estimated trapping pressures of fis range from 130 to 380 mpa suggesting an alternating lithostatic hydrostatic fluid system controlled by a fault valve at the depth of 13 km therefore the tiemurt pb zn cu deposit is likely an example of orogenic pb zn cu systems formed in collision orogeny rather than a pre collision vms or sedex system developed in devonian and a new metallogenic model is proposed to interpret the formation of the tiemurt pb zn cu deposit

recent ph and co2 profiles at lakes nyos and monoun cameroon implications for the degassing strategy and its numerical simulation abstract in situ ph profiles are reported for the first time for lakes nyos and monoun the ph profiles were converted to co2 profiles using hco3 profiles calculated from conductivity data recent observations 1993 1996 at lake nyos indicates that co2 still accumulates below 180 m depth at a rate of 125 mmol year 1 at lake monoun the majority of co2 is present below a depth of 60 m only 25 m below the saturation depth consequently a potential danger of gas explosion is high at both lakes and artificial degassing of the lakes should be performed as soon as possible a system for industrial degassing of the lakes is proposed the system based on the self sustained gas lift principle consists of multiple pipes 14 cm in diameter with different intake depths 12 pipes for lake nyos four each at 185 195 and 205 m and three pipes for lake monoun at 70 80 and 90 m the stepped degassing at different depths is intended to keep the maximum stability of the lakes the proposed degassing operation was simulated using the dyresm code for both lakes in 5 years approximately 50 of currently dissolved co2 in lake nyos and 90 in lake monoun will be removed the expected changes in the thermal and chemical structures of the lakes as degassing proceeds will be most easily monitored with a carefully calibrated ctd equipped with a ph sensor the simulation indicates that the discharged degassed water will sink to a level of neutral buoyancy i e to a maximum of 70 m at lake nyos and 35 m at lake monoun there would be no possibility of triggering a gas explosion by this plunge of discharged water because the water present there would have already been replaced by water at lower co2 concentration during the degassing from shallower pipes

dynamics of co2 driven lake eruptions on 21 august 1986 a massive release of carbon dioxide from lake nyos in cameroon killed about 1 700 people a similar event occurred on 15 august 1984 at lake monoun also in cameroon it was suggested1 5 that the co2 released was initially dissolved in the hypolimnion dense lower layer of the lake and was released by eruptive outgassing because of its violence the nyos outburst was at first thought6 to have been volcanic but undisturbed sediments and other evidence indicate that no large volcanic eruption occurred7 9 recent experiments10 11 have shown that decompression of co2 saturated water is able to power explosive eruptions here i analyse the dynamics of co2 driven lake water eruptions by deriving an equation of state for gas liquid mixtures and using it to integrate the bernoulli equation which describes the dynamics of the bubbly flow i find that under certain conditions these eruptions can be violent the lake surface exit velocity of an initially gas saturated water parcel may reach 89m s 1 for lake nyos and 51m s 1 for lake monoun the dynamics are similar to those of water driven volcanic eruptions which are also powered by gas exsolution from a liquid

geology c h o s pb isotope systematics and geochronology of the yindongpo gold deposit tongbai mountains central china implication for ore genesis abstract the yindongpo gold deposit is located in the weishancheng au ag dominated polymetallic ore belt in tongbai mountains central china the ore bodies are stratabound within carbonaceous quartz sericite schists of the neoproterozoic waitoushan group the ore forming process can be divided into three stages represented by early barren quartz veins middle polymetallic sulfide veinlets and late quartz carbonate stockworks with most ore minerals such as pyrite galena native gold and electrum being formed in the middle stage the average 18owater values changed from 9 7 in the early stage through 4 9 in the middle stage to 5 9 in the late stage with the d values ranging between 65 and 84 the 13cco2 values of ore fluids are between 3 7 and 6 7 with an average of 1 1 the h o c isotope systematics indicate that the ore fluids forming the yindongpo gold deposit were probably initially sourced from a process of metamorphic devolatilization and with time gradually mixed with meteoric water the 34s values range from 0 3 to 5 2 with peaks ranging from 1 to 4 fourteen sulfide samples yield 206pb 204pb values of 16 990 17 216 207pb 204pb of 15 419 15 612 and208pb 204pb of 38 251 38 861 both s and pb isotope ratios are similar to those of the main lithologies of the waitoushan group but differ from other lithologic units and granitic batholiths in the tongbai area which suggest that the ore metals and fluids originated from the waitoushan group the available k ar and 40ar 39ar ages indicate that the ore forming process mainly took place in the period of 176 140 ma during the transition from collisional compression to extension and after the closure of the oceanic seaway in the qinling orogen the yindongpo gold deposit is interpreted as a stratabound orogenic style gold system formed during the transition phase from collisional compression to extension the ore metals in the waitoushan group were extracted transported and then accumulated in the carbonaceous sericite schist layer the carbonaceous sericite schist layer especially at the junction of collapsed anticline axis and fault structures became the most favorable locus for the ore bodies

sm nd sr c and o isotope systematics in hydrothermal calcite fluorite veins implications for fluid rock reaction and geochronology article i nfo the 147 sm 143 nd isotope system may be used to determine the age of hydrothermal vein growth for single veins or sets of veins providing that the minerals in these veins formed from a hydrothermal fluid with initially homogeneous 143 nd 144 nd in this study we determined the 147

in situ trace element and sulfur isotope of pyrite constrain ore genesis in the shapoling molybdenum deposit east qinling orogen china abstract the early cretaceous ca 128 ma shapoling vein type molybdenum mo mineralization associated with the huashan intrusion is a newly discovered deposit in the xiong ershan district east qinling china where pyrite is a ubiquitous mineral phase here we present results from an integrated study on the texture in situ trace element chemistry from laser ablation inductively coupled plasma mass spectrometry la icp ms and sulfur isotopic composition using secondary ion mass spectrometry sims with a view to understand the geochemical variation genesis of mo mineralization and ore forming process the distinct textural patterns as revealed by petrographic observations suggest three types of pyrites py1 is composed of coarse grained euhedral pyrite in the wall rock and early barren quartz vein at stage i py2 is represented by subhedral anhedral grains coexisting with molybdenite in quartz veins at stage ii and py3 forms irregular interstitial grains in the quartz k feldspar molybdenite dominated veins at stage iii the mo enrichment mainly occurred in py3 with co ni ratios 1 and 34s values mainly