multiple sulfur isotopes monitor fluid evolution of an archean orogenic gold deposit abstract the evolution of a gold bearing hydrothermal fluid from its source to the locus of gold deposition is complex as it experiences rapid changes in thermochemical conditions during ascent through the crust although it is well established that orogenic gold deposits are generated during time periods of abundant crustal growth and or reworking the source of fluid and the thermochemical processes that control gold precipitation remain poorly understood in situ analyses of multiple sulfur isotopes offer a new window into the relationship between source reservoirs of au bearing fluids and the thermochemical processes that occur along their pathway to the final site of mineralisation whereas 34s is able to track changes in the evolution of the thermodynamic conditions of ore forming fluids 33s is virtually indelible and can uniquely fingerprint an archean sedimentary reservoir that has undergone mass independent fractionation of sulfur mif s we combine these two tracers 34s and 33s to characterise a gold bearing laminated quartz breccia ore zone and its sulfide bearing alteration halo in the 6 moz au structurally controlled archean waroonga deposit located in the eastern goldfields superterrane of the yilgarn craton western australia over 250 analyses of gold associated sulfides yield a 34s shift from what is interpreted as an early pre mineralisation phase with chalcopyrite pyrrhotite 34s 0 7 to 2 9 and arsenopyrite cores 34s 0 5 to a syn mineralisation stage reflected in au bearing arsenopyrite rims 34s 7 6 to 1 5 this shift coincides with an unchanging 33s value 33s 0 3 both temporally throughout the au hosting hydrothermal sulfide paragenesis and spatially across the au ore zone these results indicate that sulfur is at least partially recycled from mif s bearing archean sediments further the invariant nature of the observed mif s signature demonstrates that sulfur is derived from a homogeneous mif s bearing fluid reservoir at depth rather than being locally sourced at the site of au precipitation finally by constraining the mif s bearing sulfur source to a fixed reservoir we are able to display the thermochemical evolution of the ore fluid in 34s space and capture the abrupt change in oxidation state that causes au precipitation our results highlight the importance of constraining multiple sulfur isotopes in space and time in order to elucidate the source and evolution of any given au bearing fluid

genetic implications of pyrite chemistry from the palaeoproterozoic olary domain and overlying neoproterozoic adelaidean sequences northeastern south australia abstract sulphide mineralisation associated with rocks from the palaeoproterozoic olary domain od and overlying neoproterozoic adelaidean sequences has undergone a complex history of metamorphism and remobilisation in this study new trace element and sulphur isotopic analyses of pyrites from a large number of deposits and paragenetic generations are combined with an existing data set to build up a sequence of mineralising events linked to the tectonometamorphic evolution of the region the typically high co ni ratios 10 indicate that early strata bound pyrite precipitated from a volcanic related fluid which had fluctuating activities of the two metals during the early stages of the evolution of the willyama basin this period of mineralisation was followed by a diagenetic concentration of sulphide mineralisation at the horizon known as the bimba formation which occurred as a result of the differing redox conditions between the upper and lower sequences in the willyama supergroup during the mesoproterozoic 1600 to 1500 ma olarian orogeny metamorphic remobilisation of strata bound pyrite resulted in an epigenetic signature the trace element concentrations of this generation were controlled primarily by the proximity of mineralisation to the mafic intrusive bodies found throughout the terrane further reworking of existing sulphides during the delamerian orogeny and associated granitoid intrusive rocks led to the formation of a new generation of epigenetic pyrite that occurs in quartz veins in the adelaidean sequences and veins that crosscut olarian fabrics in the olary domain 34s results range from 16 to 11 but most data fall between 2 and 4 this association is suggestive of an initial uniform deep seated crustal reservoir of sulphur which has been repeatedly tapped throughout the metallogenic history of the region the isotopic outliers can be explained by the input of biogenic sulphur or sulphur derived from oxidised possibly evaporitic sediments respectively previous workers have invoked the kupferschiefer and the zambian copperbelt as analogues to mineralisation processes in the olary domain this study shows that 34s and trace element data are suggestive of some affinities with the aforementioned analogues but a more likely link can be made between epigenetic remobilisation in the olary region and the iron oxide copper gold iocg style of mineralisation found at the nearby olympic dam deposit

geology geochronology geochemistry and ore genesis of the wangu gold deposit in northeastern hunan province jiangnan orogen south china abstract the wangu gold deposit in northeastern hunan south china is one of many structurally controlled gold deposits in the jiangnan orogen the host rocks slates of the lengjiaxi group are of neoproterozoic age but the area is characterized by a number of late jurassic cretaceous granites and ne trending faults the timing of mineralization tectonic setting and ore genesis of this deposit and many similar deposits in the jiangnan orogen are not well understood the orebodies in the wangu deposit include quartz veins and altered slates and breccias and are controlled by wnw trending faults the principal ore minerals are arsenopyrite and pyrite and the major gangue minerals are quartz and calcite alteration is developed around the auriferous veins including silicification pyritic arsenopyritic and carbonate alterations field work and thin section observations indicate that the hydrothermal processes related to the wangu gold mineralization can be divided into five stages 1 quartz 2 scheelite quartz 3 arsenopyrite pyrite quartz 4 poly sulfides quartz and 5 quartz calcite the lianyunshan s type granite which is in an emplacement contact with the ne trending changsha pingjiang fracture zone has a zircon la icpms u pb age of 142 2 ma the dayan gold occurrence in the changsha pingjiang fracture zone which shares similar mineral assemblages with the wangu deposit is crosscut by a silicified rock that contains muscovite with a ca 130 ma 40ar 39ar age the gold mineralization age of the wangu deposit is thus confined between 142 ma and 130 ma this age of mineralization suggests that the deposit was formed simultaneously with or subsequently to the development of ne trending extensional faults the emplacement of late jurassic cretaceous granites and the formation of cretaceous basins filled with red bed clastic rocks in northeastern hunan which forms part of the basin and range like province in south china empa analysis shows that the average as content in arsenopyrite is 28 7 atom and the mineralization temperature of the arsenopyrite pyrite quartz stage is estimated to be 245 20 c from arsenopyrite thermometry the high but variable au as molar ratios 0 02 of pyrite suggest that there are nanoparticles of native au in the sulfides an integration of s pb h o he ar isotope systematics suggests that the ore fluids are mainly metamorphic fluids originated from host rocks possibly driven by hydraulic potential gradient created by reactivation of the wnw trending faults initially formed in paleozoic with possible involvement of magmatic and mantle components channeled through regional fault networks the wangu gold deposit shares many geological and geochemical similarities as well as differences with typical orogenic epithermal and carlin type gold deposits and may be better classified as an intracontinental reactivation type as proposed for many other gold deposits in the jiangnan orogen

oxidized and reduced mineral assemblages in greenstone belt rocks of the st ives gold camp western australia vectors to high grade ore bodies in archaean gold deposits hydrothermal sulfide oxide gold mineral assemblages in gold deposits in the archaean st ives gold camp in western australia indicate extremely variable redox conditions during hydrothermal alteration and gold mineralization in space and time reduced alteration assemblages pyrrhotite pyrite occur in deposits in the southwest of the camp e g argo junction deposits and moderately to strongly oxidized assemblages magnetite pyrite hematite pyrite occur in deposits in the central corridor in the northeast e g north orchin revenge deposits reduced mineral assemblages flank the central corridor of oxidized deposits and locally cut across it along e w trending faults oxidized mineral assemblages in the central corridor are focused on gravity lows which are interpreted to reflect abundant felsic porphyritic intrusions at about 1 000 m below present surface hydrothermal magnetite predates and is synchronous with early phases of gold associated albite carbonate pyrite biotite chlorite hydrothermal alteration later stage gold associated pyrite is in equilibrium with hematite the spatial distribution and temporal sequence of iron sulfides and oxides with gold indicate the presence of at least two spatially restricted but broadly synchronous hydrothermal fluids with contrasting redox states sulfur isotope constraints support the argument that the different mineral assemblages reflect differences in redox conditions the 34s values for pyrite for the st ives gold camp range between 8 4 and 5 1 with the negative values occurring in oxidized magnetite rich domains and slightly negative or positive values occurring in reduced pyrrhotitic domains preliminary spatial and paragenetic analysis of the distribution of iron sulfides and oxides in the st ives camp suggests that gold grades are highest where the redox state of the hydrothermal alteration assemblages switches from relatively reduced pyrrhotite pyrite to relatively oxidized magnetite pyrite and hematite pyrite both in space and time gold deposition is inferred to have occurred where fluids of contrasting redox state mixed

characteristics and origin of the zhulazhaga gold deposit in inner mongolia china abstract the zhulazhaga gold deposit in the northern margin of the north china craton ncc is a large tonnage hydrothermal deposit with a reserve of about 50 t at about 4 g t au the deposit is characterized by an epigenetic style of mineralization in metasedimentary rocks dominated by slates low grade metamorphic clastic rocks and minor volcanic rocks gold mineralization is dominantly controlled by structures manifested by stratabound orebodies located immediately to the east and west of a major n s trending fault and associated extensional fractures and ne trending fault zones the mineralization is also preferentially located in the fourth sub member of first member of the agulugou formation there are many auriferous quartz veins parallel to the bedding of the host rocks which contain the orebodies and high grade ore preferentially occurs near to these quartz veins the structures provided pathways for transportation of gold bearing fluids within the formation that appears to form a particularly favorable host rock and depositional setting hydrogen and oxygen isotope data indicate that ore forming fluids were dominated by magmatic water mixed with minor meteoric water sulfur and lead isotope data indicate that some sulfur and lead came from the magmatic fluid the metallogenic age 280 ma of this deposit and the crystallization age 280 290 ma of the granite porphyry in the district indicate a link between the hercynian magmatism and gold mineralization the hercynian tectono magmatism and associated hydrothermal activity remobilized gold and drove ore forming fluids to the favorable depositional environment the northern margin of the ncc is prospective for gold mineralization hosted in metasedimentary rocks which may be one of the most important deposit types for future gold exploration in northern china

the low δ18o late stage ferrodiorite magmas in the skaergaard intrusion result of liquid immiscibility thermal metamorphism or meteoric water incorporation into magma abstractwe report new laser fluorination oxygen isotope analyses of selected samples throughout the skaergaard intrusion in east greenland particularly relying on 1 mg separates of the refractory alteration resistant minerals zircon sphene olivine and ferroamphibole we also reexamine published oxygen isotope data on bulk mineral separates of plagioclase and clinopyroxene our results show that the latest stage strongly differentiated magmas represented by 3 to 6 km3 of ferrodiorites around the sandwich horizon sh where the upper and lower solidification fronts met became depleted in 18o by about 1 5 2 relative to the original skaergaard magma and the normal mantle derived mid ocean ridge basalt earlier studies did not recognize these low 18o ferrodiorite magmas 18o 3 4 because after the intrusion solidified much of the intrusion and its overlying roof rocks were heavily overprinted by low 18o meteoric hydrothermal fluids we consider three possible ways of producing these low

geochemistry and textures of gold bearing arsenian pyrite twin creeks nevada implications for deposition of gold in carlin type deposits this study relates morphology size texture and chemistry of arsenian pyrite in the twin creeks carlin type gold deposits to the chemical position and amount of gold that it contains arsenian pyrite is confirmed as the most important gold hosting mineral and four types of gold bearing arsenian pyrite are recognized among these four types the lowest amount of gold and arsenic 1 wt as 17 60 ppm au is contained in arsenian pyrite with relatively large grain sizes 10 30 mu m it formed at relatively high temperatures 250 degrees c and k super h super ratios adularia stable probably relatively slowly the largest amount of gold and arsenic 1 05 2 43 wt as 595 1 465 ppm au is contained in arsenian pyrite with small grain sizes up to 2 mu m that formed at low temperatures 120 degrees 200 degrees c and moderate k super h super ratios illite stable probably by relatively rapid deposition xanes exafs and sims analyses show that gold in arsenian pyrite is both structurally bound au i and in submicroscopic inclusions of free gold au highest amounts of structurally bound gold were observed in the relatively coarse grained arsenian pyrite noted above whereas most inclusions of free gold were observed in fine grained arsenian pyrite in general there is a good correlation between gold and arsenic in arsenian pyrite although au as ratios vary from one pyrite to another it is suggested that gold in arsenian pyrite was deposited by direct deposition of native gold and by adsorption of au i from hydrothermal solutions adsorption of gold by arsenian pyrite can take place from solutions that are not saturated with respect to native gold relative volumes of gold bearing pyrite from different paragenetic stages at twin creeks suggest that as much as 50 percent of the total gold in the deposit could have formed from solutions that were not saturated with respect to native gold because both au and au i are found together in arsenian pyrite it is likely that they were deposited by the same process consideration of possible depositional reactions suggests that a decrease in the activity of reduced sulfur is most likely to cause gold deposition

textures and in situ chemical and isotopic analyses of pyrite huijiabao trend youjiang basin china implications for paragenesis and source of sulfur many carlin like au deposits occur within the late paleozoic and triassic youjiang basin of southwest china the huijiabao trend in guizhou province contains over 300 metric tons t 10 6 moz of au at an average grade of 7 to 18 g t in a narrow corridor that is about 20 km long and 5 km wide petrographic and sem studies of pyrite in barren host rocks and high grade orebodies led to the recognition of four stages of pyrite py1 consists of fine grained framboidal crystals in black mudstone py2 is comprised of coarser grained euhedralsubhedral clusters that are spatially related to organic matter py3 is coarse grained euhedral and occurs as overgrowths on py1 and py2 py3 s porous texture inclusion of randomly oriented detrital minerals and association with quartz recrystallization suggest it was deformed during late triassic orogenesis with py1 and py2 py4 generally occurs as rims on py1 to py3 and is intergrown with arsenopyrite sensitive high resolution ion microprobe shrimp 34s analyses of each pyrite type and arsenopyrite show that py1 is related to py2 and that py3 is related to py4 and arsenopyrite the s isotope compositions of py1 7 5 to 5 9 and py2 5 3 to 7 9 are bimodal which suggests that h2s was generated by biogenic sulfate reduction in open marine and sulfate limited systems during sedimentation and or diagenesis the compositions of py3 2 6 to 1 5 py4 1 2 to 1 5 and arsenopyrite 0 8 to 0 9 are homogeneous and have an intermediate range of values near 0 that suggest that h2s was derived either from average pyrite 0 2 in sedimentary rocks or from a concealed magmatic source laser ablation inductively coupled plasma mass spectrometer la icp ms trace element analyses as ni co cu ag se v support different origins and show that py3 and py4 are ore related the lower w co w ni and w s w se ratios of py1 and py2 are consistent with formation during sedimentation or diagenesis whereas the higher ratios of py3 py4 and arsenopyrite are consistent with a hydrothermal origin the lower concentrations of au in py1 0 23 2 5 ppm and py2 0 06 12 ppm show that little au was added during sedimentation or diagenesis the higher concentrations of au in hydrothermal py3 1 1 110 ppm and py4 0 34 810 ppm indicate that most of the au was introduced during subsequent hydrothermal fluid flow the low au contents of arsenopyrite 0 09 0 52 ppm suggests they formed from au depleted fluids the au as ratios of py1 and py2 are typical of diagenetic pyrite whereas py3 and py4 have ratios that approach those of ore stage pyrite in nevada carlin type deposits the fracturing of py3 and its cementation by py4 suggests that ore fluid movement was associated with deformation published isochron ages on arsenopyrite re os 200 ma and late calcite realgar veinlets sm nd 135 ma in the huijiabao trend are older than mafic dikes 84 ma exposed 20 km to the east if the 200 and 135 ma ages are valid h2s and au may be derived from a sedimentary source because igneous intrusions of this age have not been found if these ages are not valid and the gold deposits are actually late cretaceous in age then h2s and au may be derived from a magmatic source additional geochronology and isotopic tracer studies are needed to resolve this uncertainty

disequilibrium partial melting and rheomorphic layer formation in the contact aureole of the basistoppen sill east greenland the tholeiitic basistoppen sill was intruded into the upper part of the skaergaard complex shortly after the skaergaard magma had solidified heat from the cooling basistoppen magma caused disequilibrium partial melting in the adjacent skaergaard ferrogabbros olivine ferrobustamite and magnetite were selectively melted and removed from the rock as an iron rich melagabbro magma plagioclase acted as a refractory phase during partial melting and was left behind as an anorthositic gabbro restite modal and grain size layering formed rheomorphically in the previously solidified host rocks as a result of partial melting and recrystallization the rheomorphic layers are distinct from those found elsewhere in the intrusion

metallogeny in response to lithospheric thinning and craton destruction geochemistry and u pb zircon chronology of the yixingzhai gold deposit central north china craton article magmatism and associated metallogeny provide important constraints on geodynamic processes the hengshan terrain of northern taihang mountains located within the central north china craton is well known for several mesozoic gold deposits here we investigate the yixingzhai quartz vein hosted gold deposit associated with the sunzhuang quartz monzodiorite intrusive complex we present u pb data on zircon grains from the monzodiorite which constrain the timing of emplacement of the pluton as ca 134 ma coeval with the formation of yixingzhai quartz vein gold deposit at ca 131 ma the magmatism and metallogenesis during early creta ceous in the hengshan terrain are part of a major regional magmatic and metallogenic event in the taihang mountains and elsewhere in the north china craton at ca 130 ma ago we characterize the zircon grains from the sunzhuang quartz monzodiorite through morphology internal structures and chemistry and also present data from analyses of the chemistry co and ni and s pb d o isotopes of the minerals from the yixingzhai gold deposit and sunzhuang quartz monzodiorite our results reveal a lower crustal signature with mantle input for the source magma of the sunzhuang pluton the magma was enriched in water and alkalis and shows a wide range of crystallization temperature 850 550 c and mainly 650 to 700 c under high oxygen fugacity the ore minerals and the mineralizing fluids in the yixingzhai gold deposit were derived from magma generated in the lower crust with additional input of mantle components our study links the gold metallogenesis with magmatism in the hengshan terrain based on the new results reported in this study and in conjunction with our interpretation of the ultra broadband high precision magnetotelluric sounding profile from the region we suggest that the lithosphere beneath the hengshan terrain was strongly thinned and decoupled between the crust and mantle in the early cretaceous and that the partly destructed lithosphere was largely preserved through the cenozoic to present the metallogenesis in the hengshan terrain was closely related to the lithosphere thinning and crust mantle decoupling in the north china craton

trace element and sulfur isotopic evidence for redox changes during formation of the wallaby gold deposit western australia abstract wallaby is a major gold deposit of the st ives gold field of western australia with an estimated resource of 8 million ounces of gold it has a well established paragenesis across five vein sets that displays macroscopic evidence of changing redox through time from hematite to magnetite micro analysis of pyrite from each vein generation shows a progressive and gradual change in redox conditions the sulfur isotope composition has a 34 s range of 7 7 to 9 8 using 3 m spots on the sensitive high resolution iron micro probe stable isotope shrimp si negative values indicative of an oxidized sulfur signature are found in the earliest generation of pyrite which coexists with hematite that also contains high concentrations of as ni zn ag sb cu and pb conversely positive values representative of reduced sulfur are found in later generations of pyrite with lower concentrations of as ni sb cu zn and pb these later pyrite crystals display higher as ni as sb and as bi and lower cu te these geochemical trends are the result of redox controlled transport and partitioning into pyrite of minor and trace elements now within the pyrite structure previous studies suggested a single orogenic event formed the wallaby deposit this is not supported by the present study trace element ratios such as as ni clearly delineate the high au generations and could be used as an exploration tool it is suggested that pyrite from the wallaby gold deposit formed via desulfidation and a gradual change in redox conditions within an evolving fluid and did not result from mixing of two separate fluids as previously advocated utilizing pyrite to link the entire fluid history of wallaby demonstrates the general use of pyrite as a valuable mineral tracer in gold bearing fluid systems

uplift of the western altiplano plateau evidence from the precordillera between 20 and 21 s northern chile 1 we present a quantitative reconstruction of uplift of the western flank of the altiplano plateau central andes one of the largest monoclines on the earth on the basis of an analysis of tectonic structures syntectonic deposits and geophysical data uplift occurred on a west vergent slowly propagating system of high angle reverse faults merging into a joint detachment that ramps down to midcrustal levels below the plateau edge the upper ramps determine local fold geometries while the lower ramp controls large scale surface tilting and uplift at 20 s this fault system was active between 30 ma and 5 10 ma with maximum shortening rates of 0 22 mm yr between 17 and 10 ma it generated some 2600 m of surface uplift with only minor shortening of 3000 m its activity was largely synchronous to eruption of large volume ignimbrites from a midcrustal source geophysical data indicate that the fault system localized deformation at the boundary between a cool strong forearc crust and a presumably fluid rich and or partially molten zone underneath the plateau area the systematic relation between crustal melting and shortening with uplift at the western plateau margin can be traced along most of the plateau flank with a stepwise decrease in age of deformation and magmatism toward the south indicating discontinuous addition of plateau segments crustal thickening to as much as 70 km from westward underthrusting in the back arc parts of the plateau isostatically compensated the tectonic surface uplift and monocline formation with respect to a stable forearc which only reacted with minor tilting

anatectic migmatites from the roof of an ocean ridge magma chamber a well preserved contact aureole at the base of the sheeted dyke is likely to be centred at or very close to the sheeted complex in the troodos ophiolite cyprus records the partial melting dyke gabbro boundary this region of the crust is a of the roof of a mid ocean ridge magma chamber hydrothermally dynamic environment that is subjected to rapid changes altered dykes within the lowermost 10 30 m of the sheeted dyke in rheology and thermal structure it is here that melt is complex were recrystallized to pyroxene and hornblende hornfels episodically extracted to build the upper crust e g deand closest to the underlying gabbros 30 at water undersaturated conditions and temperatures if not all of the lower crust e g phipps morgan chen 875 c geological considerations indicate that the duration of 1993 quick denlinger 1993 boudier et al 1996 melting events was probably on the time scale of eruptive cycles i e and magmatic heat is rapidly transferred across a thin years to centuries this resulted in disequilibrium partial melting 100 m conductive boundary layer e g lowell and highly imperfect melt segregation within the contact aureole burnell 1991 and advectively transported out of the we hypothesize that partial melting is a common process within crust by hydrothermal fluids e g cann et al 1985 the roof zones of axial magma chambers at fast spreading ridges geophysical surveys along the east pacific rise epr and that a component of geophysically imaged axial magma chambers provide indirect evidence that the depth and internal may be partially melted upper crust rather than partially crystallized properties e g melt volumes proportion of melt and magma field relations suggest that where the highest degrees of crystals of amcs are not static and change on the time melting occurred a mixture of anhydrous hornfels and hydrous scale of eruptive cycles years to centuries wilcock leucocratic melt disaggregrated and was assimilated into the magma delaney 1996 singh et al 1998 chamber it is well known from ophiolite studies that the roofs of amcs can be marked by polyphase magmatism and intense interactions between hydrothermal fluids and rocks particularly within the uppermost few hundred

geological and isotopic constraints on ore genesis huangjindong gold deposit jiangnan orogen southern china abstract the huangjindong gold deposit with gold resources of 80 t is hosted in neoproterozoic slates along the chang ping fault in the jiangnan orogen southern china the orebodies are dominated by auriferous arsenopyrite pyrite quartz scheelite veins quartz rich breccias that are cemented by late arsenopyrite pyrite muscovite and quartz auriferous stibnite quartz veins and disseminated gold in arsenopyrite pyrite sericite quartz altered rocks neither significant alteration zoning nor ore element zoning are recognized within the orebodies although the orebodies at shallower crustal levels are slightly more stibnite rich at the regional scale the first order control on the distribution of gold deposits is the nne to ne trending crustal scale sinistral strike slip chang ping fault as in other gold provinces globally this controlling fault is not gold mineralized the gold occurrs in wnw to e w trending zones of thrusts and fracture arrays associated with tight locked up asymmetrical folds the broadly e w trending orebodies formed independently within approximately ne trending fault bounded corridors defined by the first order chang ping and second order ni wan and keng shang chang long faults hydrothermal activity was initiated at the ductile brittle transition but evolved to brittle dominated deformation all within a transpressional environment oxygen and hydrogen isotope compositions of fluids d 44 4 to 34 2 18o 8 6 to 9 5 calculated from gold related hydrothermal quartz and hydrothermal sericite respectively are consistent with a metamorphic source sulfur isotope compositions of the gold bearing arsenopyrite and pyrite median values between 8 0 and 6 0 are generally distinct from 34s of the regional neoproterozoic slates in the jiangnan orogen but overlap the compositions of pyrites in global sediment hosted orogenic gold deposits lead isotope compositions of the gold bearing arsenopyrite and pyrite 206pb 204pb 17 049 to 18 417 207pb 204pb 15 263 to 15 712 208pb 204pb 37 597 to 39 220 suggest that the lead was sourced from metamorphic rocks involved in the orogenic events therefore both the stable and radiogenic isotopic data indicate that ore forming fluids and metals were derived from a metamorphic source although the precise nature of that source is unclear on the basis of these geological and isotopic data huangjindong is interpreted as an orogenic gold deposit the gold ore bodies show classic structural controls being adjacent at the province scale to the crustal scale chang ping fault but located in thrusts and associated tight anticlinal fold hinges that lie in corridors between markedly oblique relatively brittle fault arrays that parallel the chang ping fault

fluid present disequilibrium melting in neoarchean arc related migmatites of daeijak island western gyeonggi massif korea the melting process of meta igneous rocks was investigated via field petrographic and geochemical analyses of the neoarchean 2 51 ga migmatite complex in daeijak island western gyeonggi massif this complex consists primarily of garnet free amphibolites and tonalitic migmatites both of which contain hornblende plagioclase and quartz as major constituents neosomes and leucosomes in the migmatite have dioritic tonalitic and tonalitic trondhjemitic compositions respectively compositions of hornblende xfe 0 39 0 42 and plagioclase an24 27 vary little between the neosomes and leucosomes the amphibolites show distinct depletions in nb ta zr and ti relative to large ion lithophile elements suggesting an arc related origin for their basaltic protolith leucosomes have lower contents of k2o mgo feo tio2 zr rb and rare earth elements ree than amphibolites and neosomes but are higher in sio2 na2o and sr contents leucosomes and neosomes have positive eu eu n 1 32 7 26 and negative 0 71 0 97 eu anomalies respectively which are attributed to the variable degree of plagioclase fractionation during the partial melting the p t condition for the migmatite formation was estimated to be 700 730 c and 4 7 5 5 kbar primarily based on the hornblende plagioclase thermobarometry and phase equilibria various lines of textural evidence such as the channel flow of melt along migmatitic layers and the segregation of melt into shear bands or boudin necks suggest a syn deformation crystallization of melt chemical disequilibrium in migmatites is documented not only by petrographic and geochemical data but also by the ree modeling between melt product and source rock disequilibrium process is most likely attributed to the rapidity of melt extraction or migration compared to chemical diffusion rate in summary the fluid present disequilibrium melting of dioritic tonalitic protoliths has produced tonalitic trondhjemitic leucosomes in a dynamic arc environment prevalent in the neoarchean crust of the korean peninsula as well as the shandong peninsula north china craton

mineralogical and isotopic studies of base metal sulfides from the jiawula ag pb zn deposit inner mongolia ne china abstract the jiawula ag pb zn deposit is located in the northern part of the da hinggan mountains metallogenic belt in the eastern section of the central asian orogenic belt sphalerite galena pyrite chalcopyrite and arsenopyrite are the major sulfide minerals occurring in this deposit here we report results from electron probe micro analysis epma thermoelectricity and sulfur isotope studies of the constituent silver minerals and sulfide phases of the jiawula deposit petrographic observations and epma study reveal abundant silver mineralization in the ore especially within sphalerite and galena discrete grains of silver minerals including argentite pyrargyrite and canfieldite and isomorphism in silver bearing sulfides are identified silver and tellurium contents in galena are relatively high and show interrelationship significant substitution of s by te in the galena lattice facilitates silver entering the galena structure which might have been promoted by relatively low lg fs2 the thermoelectric coefficient of pyrite shows a marked gradient from n type to p type from pre to post metallogenic stages pyrites in jiawula are enriched in co and as and in the absence of ni displaying features typical of epithermal deposits the isotopic data present a close relationship between the sulfur source and magmatism

fluid chemical evolution as a factor in controlling the distribution of gold at the archean golden crown lode gold deposit murchison province western australia the golden crown lode gold deposit is one of a number of northwest southeast to north south trending quartz veins hosted by a steeply dipping 500 m thick differentiated dolerite sill the great fingall sill and adjacent graphitic shale in the day dawn district of the murchison province in the archean yilgarn craton western australia quartz veins of the golden crown deposit are shear zone hosted crosscut the dominant host rock stratigraphy at a high angle and are laminated partly boudinaged and generally bifurcate at their extremities veins are dominated by dark gray or milky white quartz with subordinate calcite and ankerite and in order of abundance arsenopyrite pyrrhotite pyrite galena chalcopyrite and minor sphalerite the vein sulfides coexist with native gold and are most abundant at vein margins textural evidence indicates that precipitation of the majority of these minerals overlapped and that most phases can be considered to have been in equilibrium during vein genesis mineral chemistry fluid inclusions alteration and ore assemblages show that there are significant systematic gradients in intensive variables within the golden crown deposit these are recorded over the approximately 450 m of vertical exposure of the vein system and over up to 30 m into the wall rocks based on proximal alteration assemblages and zoning patterns zoned wall rock alteration haloes are divisible into four sequences three of which are closely related to gold mineralization specific alteration sequences occur within different parts of the vein system biotite alteration predominates in the deeper portions whereas muscovite alteration characterizes the upper and southern portions of the system higher grade gold zones within the golden crown deposit are limited to the northern end of the system in the deeper portion of the veins and expand along strike to the south in the upper portions fluid inclusions interpreted to be trapped during gold mineralization are low salinity 10 wt nacl equiv h2o co2 ch4 inclusions with variable carbonic to aqueous contents that are interpreted to be due to fluid immiscibility inferred methane concentrations in the carbonic phase vary significantly from 19 7 mole percent in the vicinity of the shale contact to 5 2 mole percent in the shallower southern end of the deposit carbon and oxygen isotope compositions of carbonates 13ccalcite 10 5 to 1 9 13cankerite 5 3 to 4 1 18ocalcite 7 0 18 3 and 18oankerite 7 7 9 2 and oxygen isotope compositions of vein quartz 18oquartz 10 2 12 5 are well within the range of known archean orogenic lode gold deposits combining the results of all geothermometers and geobarometers a temperature of 300 to 400 c and pressure of 2 5 0 5 kbars are determined with temperature variations over the exposed quartz vein of less than 40 c the as content at of arsenopyrite and au ag ratios of native gold show significant compositional variation which appears to be controlled by parameters such as a h2s aq and f o2 not temperature gradients on the basis of the variations in fluid composition determined in the reefs the two major processes responsible for high grade gold mineralization are interpreted to have been phase separation in the deeper portions of the ore forming system approximately below 400 m level and above these levels mixing between two modified ore fluids one reduced fluid that had reacted with graphitic shale and a relatively oxidized fluid that had partly equilibrated with dolerite

visible gold in arsenian pyrite at the shuiyindong carlin type gold deposit guizhou china implications for the environment and processes of ore formation abstract carlin type gold deposits are best known for the scarcity of visible gold in their ores it has long been recognized that the majority of gold is invisible such that it cannot be resolved by conventional microscopy and resides in arsenian pyrite shuiyindong differs in that sub m to m sized native gold is present in arsenian pyrite veinlets and disseminations it is also the largest 55 tonnes and highest grade 7 to 18 ppm stratabound carlin type gold deposit in guizhou china and has produced 5 tonnes of gold from sulfide refractory ores extracted by underground mining methods in this study an electron microprobe analyzer empa was used to map the spatial distribution of invisible gold and sub m to m size visible gold particles in arsenian pyrite in high grade ore samples from the shuiyindong the samples studied are hosted in permian bioclastic ferroan limestone of the longtan formation and exhibit evidence of decarbonation silicification and sulfidation arsenian pyrite with detectable au 400 to 3800 ppm is disseminated in altered limestone and was deposited in two stages separated by an episode of corrosion in a veinlet the results show that there are two populations of native gold in arsenian pyrite one is comprised of sub m size gold particles 0 1 to 0 2 m that are occasionally present in the gold bearing arsenian pyrite disseminated in the host rocks this arsenian pyrite is interpreted to have been formed by sulfidation of ferroan calcite and dolomite another is comprised of coarser 1 to 6 m native gold grains present in the arsenian pyrite veinlet either on the first stage where it has been corroded or on the second stage the lack of fluid inclusion or other evidence of boiling and the low iron content of fluid inclusions in quartz suggest the veinlet formed by sulfidation of another fluid containing fe the fe bearing fluid may be a depleted ore fluid that gained fe by dissolution of ferroan limestone after h 2 s had been consumed the association of the largest visible gold grains with an episode of corrosion suggests that fluids episodically became undersaturated with arsenian pyrite while remaining saturated with gold e g ph decrease or an increase in the oxidation state this may have resulted from incursion of relatively acidic or oxidized fluids that were able to dissolve arsenian pyrite and remain saturated with gold in this case sulfidation of iron from the host rock was the most important depositional mechanism for au bearing arsenian pyrite with or without grains of native gold

geochemical and isotopic composition of auriferous pyrite from the yongxin gold deposit central asian orogenic belt implication for ore genesis abstract the yongxin gold deposit is a large fracture controlled ore deposit in the duobaoshan gold copper metallogenic belt within the eastern domain of the central asian orogenic belt caob silicification sericitization and potassic alteration are the main hydrothermal alteration types which display successive zoning surrounding the ore bodies pyrite galena sphalerite and chalcopyrite are the major metallic minerals in the deposit and gold is mainly distributed in pyrite here we investigate the trace element composition and isotope pb and s characteristics of pyrite from this deposit the auriferous pyrite is mainly allotriomorphic and homogeneous in chemistry with a homogenous distribution of as ni mo and fe from the core to rim of grains these features are consistent with the 207pb 206pb 206pb 204pb and 208pb 204pb 206pb 204pb of the grains that suggest a single source with no evidence for involvement of late fluids the 34s values range from 3 5 to 5 4 with an average of 4 48 suggesting a mantle derived magmatic provenance for the sulfur the high co content and co ni ratios low ni and mo ni ratios suggest a mafic ultramafic source for the yongxin gold deposit the co ni as relation indicates a magmatic hydrothermal affinity with a classification as epithermal gold deposit high arsenic and gold contents on the rims of pyrite grains indicate that au and as were simultaneously precipitated in anoxic low temperature and near surface conditions

formation of gold deposits a metamorphic devolatilization model a metamorphic devolatilization model can explain the enrichment segregation timing distribution and character of many goldfields such as those found in archean greenstone belts slate belts and other gold only provinces in this genetic model hydrated and carbonated greenschist facies rocks particularly metabasic rocks are devolatilized primarily across the greenschist amphibolite facies boundary in an orogenic setting devolatilization operates on the scale of individual mineral grains extracting not just h2o and co2 but also s and in turn au elevated gold in solution is achieved by complexing with reduced s and by h2co3 weak acid buffering near the optimal fluid ph for gold solubility the buffering is more important than being at the point of maximum gold solubility low salinity ensures low base metal concentrations in the auriferous metamorphic fluid migration of this fluid upwards is via shear zones and or into hydraulic fracture zones in rocks of low tensile strength the geometry of the shear zones dictates the kilometre scale fluid migration paths and the degree of fluid focusing into small enough volumes to form economic accumulations of gold deposition of gold from solution necessitates breakdown of the gold thiosulphide complex and is especially facilitated by fluid reduction in contact with reduced carbon bearing host rocks and or by sulphidation of wallrocks to generate iron bearing sulphide and precipitated gold as such black slate carbon seams banded iron formation tholeiitic basalt magnetite bearing diorite and differentiated tholeiitic dolerite sills are some of the important hosts to major goldfields gold deposition is accompanied by carbonation sulphidation and muscovite biotite alteration where the host rock is of suitable bulk composition the correlation of major gold deposits with rock type even when the gold is primarily in veins argues for rock dominated depositional systems not fluid dominated ones as a consequence a general role in gold deposition for fluid mixing temperature decrease and or fluid pressure decrease and boiling is unlikely although such effects may be involved locally several geological features that are recorded at gold only deposits today reflect subsequent modifications superimposed upon the products of this generic metamorphic devolatilization process overprinting by higher grade metamorphism and deformation and or palaeo weathering may provide many of the most obvious features of goldfields including their mineralogy geochemistry geometry small scale timing features geophysical response and even mesoscopic gold distribution

seismic imaging of a megathrust splay fault in the north chilean subduction zone central andes abstract prominent trench parallel fault systems in the arc and fore arc of the chilean subduction zone can be traced for several thousand kilometers in north south direction these fault systems possibly crosscut the entire crust above the subduction megathrust and are expected to have a close relationship to transient processes of the subduction earthquake cycles with the motivation to image and characterize the structural inventory and the processes that occur in the vicinity of these large scale fault zones we re processed the ancorp 96 controlled source seismic data set to provide images of the faults at depth and to allow linking geological information at the surface to subsurface structures the correlation of the imaging results with observed hypocenter locations around these fault systems reveals the origin and the nature of the seismicity bound to these fault systems active and passive seismic data together yield a picture of a megathrust splay fault beneath the longitudinal valley at mid crustal level which can be observed from the top of the subduction plate interface and which seems to be connected to the precordilleran fault system pfs known at the surface this result supports a previously proposed tectonic model where a megathrust splay fault defines the western altiplano as a crustal scale fault bend fold furthermore we clearly imaged two branches of the uyuni kenayani fault ukf in a depth range between 0 and 20 km in summary imaging of these faults is important for a profound understanding of the tectonic evaluation and characterization of the subduction zone environment for which the results of this study provide a reliable basis

geochemistry of dendritic anorthosites and associated pegmatites in the skaergaard intrusion east greenland evidence for metasomatism by a chlorine rich fluid abstract in the upper zone uza of the skaergaard intrusion dendritic anorthosites and pegmatites are found in a zone of deformed gabbro along the flank of a trough banded structure apatite biotite some having over 2 wt c1 and amphibole have higher chlorine contents than those found in the surrounding rocks and indicate the presence of a high temperature aqueous fluid having a high hcl hf fugacity ratio the fluid must have been derived locally by exsolution of magmatic volatiles because hydrous minerals in nearby hydrothermal veins have low chlorine contents and the structures are far from the margin of the intrusion apatite and quartz in the pegmatites have abundant fluid inclusions containing cubic halide crystals giving evidence for a separate aqueous fluid coexisting with an evolved residual melt the pegmatites are also characterized by mafic and felsic clots with compositions similar to immiscible silicate liquids dendritic anorthosites have equal or higher abundances of zr hf ta and rare earth elements than the surrounding gabbros yet they contain little pyroxene or fe ti oxides the unusual trace element enrichment patterns and the associated rare assemblage of zirconolite zircon baddeleyite and allanite suggest that the chlorine rich fluid was enriched in trace elements derived from the residual melt and the reacted mafic minerals deformation of the magma at a crystallinity below the critical melt fraction resulted in localized exsolution and channeling of a magmatic aqueous fluid and an evolved residual melt dissolution of pyroxene olivine and fe ti oxides allowed for further deformation and expulsion of the mafic melt leaving a plagioclase rich residue these structures are the product of enhanced reaction locally of an aqueous fluid which may have moved upward as a nearly planar reactive front throughout the lower crystallization zone in addition to the transport of trace elements migration of a high temperature fluid through the zone of crystallization could aid in the compaction of igneous cumulates and the development of layering this study presents the first evidence for a high temperature chlorine rich aqueous fluid in the layered series of the skaergaard intrusion and illustrates the important effect of such fluids on the physical and chemical evolution of layered intrusions

partial melting at the earth s surface implications for assimilation rates and mechanisms in subvolcanic intrusions abstract the relatively recent routine availability of isotopic and elemental analytical results from large sample suites of igneous rocks has revolutionized the study of magmatic processes large uncertainties remain however with regard to the highly variable rates of magma differentiation and ascent through the crust although the repose interval prior to an eruption may be equivalent to magma residence time at a certain level in the crust this cannot be assumed a priori even where this happens to be the case uncertainties in determining such intervals for prehistoric eruptive sequences may be large with respect to process rates consequently independent insights from geologically well constrained situations must be sought this paper first presents documentation of an example wherein fragments of dacite incorporated by a mafic lava flow on the surface were melted to the point that they viscously deformed in response to stresses in this flow the transformation from the solid state to a largely molten state is inferred to have occurred within a few weeks these results are then discussed in the context of examples from the literature that support rapid melting and digestion of xenoliths in magma it is suggested that the rate of crustal assimilation may be sufficiently rapid that it can operate within the durations of extended eruptive episodes years decades which are generally much shorter than typical repose periods at arc volcanoes decades millennia

sulfur isotopes in sediment hosted orogenic gold deposits evidence for an early timing and a seawater sulfur source we report sulfur isotopic compositions of sulfides of various paragenetic stages in the giant sukhoi log sediment hosted orogenic au deposit in russia the overall mean value and the significant variability in early pyrite indicate that the sulfur was from the reduction of seawater sulfate the later generations of sulfide have 34s values in successively smaller ranges coincident with the mode that is around the median value of the whole data set together with textural evidence sulfide trace element data and gold occurrence the data demonstrate that metamorphism has gradually homogenized the early sulfur accompanied by the segregation of quartz and the release of au from the lattice of early pyrite and its reprecipitation as inclusions in later pyrite the s isotopic compositions of sulfides in sukhoi log and many other major orogenic au deposits hosted in sedimentary rocks of various ages show a pattern generally parallel to the seawater sulfate curve through geologic time indicating that the sulfur in most sediment hosted orogenic au deposits was probably also originally from the reduction of seawater sulfate we conclude that sulfidation and gold mineralization in many sediment hosted orogenic gold deposits was early during basin evolution when seawater was the principal active fluid rather than later during or after basin inversion as proposed in current models

roof assimilation at fast spreading ridges an investigation combining geophysical geochemical and field evidence 1 the role of assimilation at the roof of the axial magma chamber amc at fast spreading mid ocean ridges is investigated using field geophysical and geochemical data field observations from ophiolites indicate that roof assimilation is a common process but provide little constraint on the amount of material assimilated arguments based on geophysical data that include the subsidence of the layer 2a 2b boundary across the width of the amc and the depth of large axial summit troughs require at least 50 100 m of assimilation however significant variations in the depth of the amc on the east pacific rise epr over short along axis distances suggest up to 500 m fluctuations in the depth of the amc may be common on short timescales geochemical modeling of the over enrichment of chlorine in epr basalts suggests that 20 of the oceanic crust may go through a cycle of crystallization alteration then assimilation this is far more than can be accounted for by roof subsidence and suggests that fluctuations in the level of the roof must account for the majority of the assimilation assimilation most likely occurs on a number of different timescales ranging from decadal due to perturbations in the thermal regime related to diking to longer period fluctuations controlled by magma supply from the mantle large scale roof assimilation has implications for the accretion of the lower crust due to heat losses associated with assimilation and for the interpretation of the geochemistry of the oceanic crust

isotope geochemistry and genesis of the liyuan gold deposit shanxi north china abstract the liyuan lode gold deposit is located in the shanxi province central north china craton gold orebodies are mainly hosted in archean metamorphic rocks and structurally controlled by the nne trending faults gold occurs in disseminated and auriferous quartz sulfide veins and veinlets within hydrothermally altered rocks mineralization can be divided into three intervals 1 quartz pyrite stage 2 quartz polymetallic sulfides stage and 3 quartz carbonate stage gold formed mainly in the middle stage sericite sample associated with the middle stage pyrite from phyllic alteration zones yields an well defined 40ar 39ar plateau age of 133 3 1 2 ma which is remarkably consistent with zircon u pb age 133 4 1 1 ma of the quartz porphyry dikes in the liyuan mine indicating a close relationship between gold mineralization and granitic magmatism in the area the sulfur isotopic compositions of pyrite py1 2 from the early stage have a narrow range from 0 3 to 4 1 indicating a deep seated magmatic source however the sulfides py3 5 sphalerite galena from the middle stage have lower 34sv cdt values of 7 2 to 3 0 which is thought to be a result of fluid oxidation during gold mineralization the pb isotopic compositions of sulfides from liyuan ores have 206pb 204pb 207pb 204pb and 208pb 204pb ratios of 16 671 16 860 15 236 15 255 and 41 452 44 159 respectively indicting a high thorium lower continental crust source of ore forming materials the 13cvpdb and 18ovsmow values of late stage calcite ranging from 7 1 to 5 8 and 12 2 to 13 7 respectively illustrating a deep seated magmatic source of carbon bearing portion given the absence of granite bodies at the present mining levels we speculate that the ore forming fluids may have been exsolved from a concealed granite pluton at greater depth based on the regional tectono magmatic events ore geology geochronologic and isotopic data obtained in this study together with the previous published fluid inclusion data we suggest that the liyuan deposit is an orogenic gold deposit that is probably related to the early cretaceous granite magmatism in the interior of the north china craton

coupled caal nasi diffusion in plagioclase feldspar experiments and applications to cooling rate speedometry abstract the rate of caal nasi interdiffusion in plagioclase feldspar was determined under 1 atm anhydrous conditions over the temperature range 1400 to 1000 c in calcic plagioclase an80 81 by homogenizing coherent exsolution lamellae the dependence of the average interdiffusion coefficient on temperature is given by the expression d 10 99 cm 2 sec exp 123 4 kcal mol rt t in k this value is for diffusion perpendicular to the 03 1 interface of the lamellae caal nasi interdiffusion is 4 to 5 orders of magnitude slower than oxygen diffusion in the temperature range 1400 to 1200 c and possibly 10 orders of magnitude slower at subsolidus temperatures the large differences in diffusion rates explain the apparent contradiction posed by the plagioclases of large layered intrusions e g the skaergaard which retain delicate ca na compositional zoning profiles on the micron scale but have undergone complete oxygen isotopic exchange with heated meteoric groundwater from the surrounding wall rocks caal nasi diffusion is slow the closure temperature is high within the solidus liquidus interval and ca na zoning is preserved oxygen diffusion is faster the closure temperature is lower 350 400 c and the feldspars exchange oxygen with the low temperature hydrothermal fluids the complex micron scale oscillatory zones in plagioclase can also be used as cooling rate speedometers for volcanic and plutonic plagioclase cooling histories typical of large mafic intrusions e g the stillwater are slow begin at high initial temperatures 1200 c and result in homogenization of oscillatory zones on the scale of 10 microns the oscillatory zones found in the plagioclase of granodioritic plutons are preserved because cooling is initiated at a lower temperature 1000 c limiting diffusion to submicron length scales despite the slow cooling rate of the intrusion

the mongol okhotsk belt in mongolia an appraisal of the geodynamic development by the study of sandstone provenance and detrital zircons abstract the mongol okhotsk belt formed in a late stage of jurassic orogeny in the composite central asian orogenic belt the present paper investigates the late palaeozoic mesozoic sandstones associated with the belt in mongolia aimed at reconstructing the time and mode of ocean formation subduction and collision we apply provenance analysis including 1 heavy mineral and sandstone framework grain analysis 2 u pb laser ablation icp ms dating of detrital zircons for identifying contemporaneous volcanic arc activity and the recycling of older crustal material and 3 on dated zircon grains we analyze trace element contents and hf isotopic ratios in order to characterize the rock types in which they crystallized and the magma sources respectively the investigated samples are derived from 1 the adaatsag and doschgol terranes which represent the suture zone 2 the hangai hentei belt to the northwest and 3 the ereendavaa terrane and the middle gobi volcanic belt to the southeast of the suture the latter two are concurrent with the northern and southern margins of the mongol okhotsk ocean tectono stratigraphic arguments suggest that the mongol okhotsk ocean opened during the silurian within the early palaeozoic collage in the suture zone permian syn sedimentary zircon trace element contents confirm mafic rock sources and the mantle involvement in the magmatism epsilon hf t from 13 up to 20 n and s directed bi vergent subduction developed as revealed by contemporaneous zircons 1 along the northern margin hangai hentei from silurian early carboniferous subduction and accretion prevailed epsilon hf t from 3 up to 12 in associated zircons which was re initiated during the permian and 2 the contemporaneous silurian devonian southern margin ereendavaa middle gobi still represented an extensional continental margin showing reworking of neoproterozoic early palaeozoic zircons from the basement it turned into an active continental margin with starting arc magmatism in the carboniferous zircon mean epsilon hf t 5 75

chemistry morphology and origin of magmatic reaction chromite stringers associated with anorthosite in the upper critical zone at winnaarshoek eastern limb of the bushveld complex

apatite dissolution into peraluminous haplogranitic melts an experimental study of solubilities and mechanisms apatite ap dissolution and solubility in the peraluminous haplogranite system were investigated in three sets of experiments at 750 c and 200 mpa ph2o as functions of ap grain size abundance melt ap ratio and melt asi alumina saturation index mol al2o3 na2o k2o cao between an asi of 1 1 1 metaluminous to mildly peraluminous the solubility of ap as measured by p2o5 in the melt is low 0 1 wt p2o5 which agrees with previous work in equilibrium runs with more strongly peraluminous compositions however ap solubility increases linearly with asi to p2o5 0 63 wt in melt at asi 1 3 and can be denned by a simple equation p2o5 3 4 3 1 asi r 0 833 which agrees within error with previous calculations reported in the literature in experiments where ap grains are sparce and widely separated i e simulating ap distribution in an ap bearing peraluminous protolith undergoing anatexis the dissolution of ap involves transient disequilibrium that promotes higher solubility of p2o5 up to nearly six times the equilibrium values adjacent to ap at a given bulk asi of melt and this increases the al si of melt in the ap dissolution aureole the approach to equilibrium between ap and melt is hastened by increasing the ap melt ratio within the local melt pool e g the capsule or by decreasing ap grain size for a similar melt ap ratio diffusivity calculations using inverse error function methods for the normal diffusion of p and ca yield dp 10 11 cm2 s and dca 10 10 cm2 s confirming prior results that diffusion of p away from ap surfaces is the rate limiting factor in ap dissolution these results combined with previous work at higher temperatures but on similar hydrous melts yield an activation energy e for p diffusion of 16 5 kcal mol and a frequency factor d0 of 1 4 10 3 cm2 s phosphorus diffusion through the melt is accompanied by uphill diffusion of al resulting in an increase of up to 2 wt al2o3 in melt near the ap within the ap dissolution aureole melt contains one additional al cation for every p cation i e al p 1 1 which is suggestive of an associated alpo4 complex the exchange reaction in melt is represented by alpsi 1 in which 2 to4 clusters are created in the place of 1 this exchange should be accompanied by discernable changes in melt properties e g density molar volume viscosity the results have applications to equilibrium and local disequilibrium effects during anatexis of apbearing metapelites and to the saturation limits of ap during crystallization of peraluminous magmas the dissolution of flux bearing minerals such as ap e g those containing p b or f may influence the stability and solubility of other trace element bearing minerals during anatexis of metapelites texture e g grain size and distribution and spacial relationship to other minerals plays an important role in reaction kinetics and at least temporarily in mineral solubilities the disequilibrium behavior of accessory minerals has critical consequences for ree modelling of granites

reflection image spectroscopy across the andean subduction zone abstract this paper presents new insights into the south american subduction zone from reprocessed seismic images we applied a 3d kirchhoff prestack depth migration scheme to data sets containing different narrow frequency ranges in order to extract additional details from seismic reflection images this approach accounts for the effects of scattering on the seismic image especially for structures below a heterogeneous overburden the reflection image in such environments will differ significantly when focusing on different frequency ranges due to the frequency dependence of scattering that is likely to be present the narrow frequency range images uncover reflectors in one frequency range that are masked in another range furthermore the images enable for instance the characterization of the medium in terms of scatterer concentration and thus improve the structural interpretation the analysis of these images might help to distinguish between small scale structures in the high frequency band and large scale structures in the low frequency band we call this imaging approach reflection image spectroscopy ris we applied the ris approach to the ancorp 96 data set an onshore deep seismic reflection profile across the south american central andes the narrow frequency range images revealed additional details that we interpret as features directly linked to fluid migration processes in the subduction zone furthermore structural details of the oceanic crust and the overlying mantle and crust are revealed from the narrow frequency range images we interpret that the top of the so called nazca reflector at 70 km depth marks the upper limit of the hydrated mantle wedge whereas the bottom of the reflector represents the top of the subducted oceanic crust the compilation of our results with local earthquake data confirms this interpretation similar features observed in another deep seismic profile precorp 95 support this interpretation too furthermore the ris images show a highly reflective heterogeneous zone between the nazca reflector at 70 km depth and a prominent mid crustal bright spot quebrada blanca bright spot at about 30 km depth we associate this zone with a complex network of ascending fluids or partial melts initiated by ascending fluids released from the subducted oceanic plate this observation links the quebrada bright spot directly to the dehydrating oceanic plate

the granite hosted gold deposit of moulin de cheni saint yrieix district massif central france petrographic structural fluid inclusion and oxygen isotope constraints the moulin de cheni orogenic gold deposit is the only granite hosted deposit of the saint yrieix district french massif central it occurs in 338 1 5 ma old peraluminous leucogranites and is characterized by intense microfracturing and bleaching of the granite in relation to pervasive sulfide crystallization formation of quartz veins and gold deposition occurred in two successive stages an early mesozonal stage of quartz sulfide fe as s deposition usually devoid of gold and a late epizonal stage of base metal and gold deposition both stages postdate peak metamorphism and granite intrusion the genesis of the deposit is the result of four successive fluid events 1 percolation of aqueous carbonic metamorphic fluids under an assumed lithostatic regime of 400 450 c at a maximum depth of 13 km 2 formation of the main quartz lodes with coeval k alteration and introduction of as and s from aqueous carbonic fluids percolating along regional faults arsenopyrite and pyrite deposition was linked to the alteration of fe silicates into k feldspar and phengite at near constant iron content in the bulk granite temperature was similar to that of the preceding stage but pressure decreased to 100 50 mpa suggesting rapid uplift of the basement up to 7 5 km depth 3 the resulting extensional tectonic leads to the deposition of gold boulangerite galena and sphalerite in brecciated arsenopyrite and pyrite from aqueous fluids during a mixing process temperature and salinity decrease from 280 to 140 c and 8 1 wt eq nacl to 1 6 wt eq nacl respectively 4 sealing of the late fault system by barren comb quartz which precipitated from dilute meteoric aqueous fluids 1 6 wt eq nacl to 0 9 wt eq nacl under hydrostatic conditions at 200 150 c

investigating sulfur pathways through the lithosphere by tracing mass independent fractionation of sulfur to the lady bountiful orogenic gold deposit yilgarn craton abstract as sulfur is the primary complexing ligand for gold transport in aqueous fluids recent work has demonstrated that the archean systems can preserve mass independent fractionation of sulfur mif s to yield near zero values 33s1 0 0 5 in such cases interpretation of the geological significance of such isotopic signatures may be compromised as it is known that mass dependent fractionation of sulfur mdf s can yield small 33s values herein as 33sb often owing to geobiological reactions involving sulfur in this study we reassess the range of possible values of 33sb from the traditionally accepted value of 0 0 2 to demonstrate that this range is not constant but rather directly linked to the degree of 34s fractionation we present a multiple sulfur isotope dataset from the neoarchean lady bountiful orogenic gold deposit from the yilgarn craton western australia this deposit contains pyrite with an isotopic composition of 34s 2 1 0 8 and 33s 0 14 0 06 2sd on the mean the analysis of these results as well as of a data compilation from other 2 65 ga orogenic gold deposits of the yilgarn craton indicates that 33s1 values are too large to be the result of purely mdf s processes in addition when compared to a compilation of 3400 33s0 measurements of archean sedimentary rocks which shows highly variable positive and negative signals the remarkably constant positive mif s1 signature of gold deposits from the yilgarn craton with 33s1 values between 0 and 0 6 is evidence that the source of the auriferous fluids equilibrated at depth by mixing with a non mif s0 bearing sulfur reservoir by enhancing the application of mif s1 33s1 as a powerful and indelible tracer to understand the source and emplacement pathways of gold bearing fluids through the lithosphere the outcomes from this study demonstrate the recorded 33s1 signature in the auriferous fluids reflects the presence of sulfur that was at least partially sourced from an archean sediment reservoir

seismic imaging of a convergent continental margin and plateau in the central andes andean continental research project 1996 ancorp 96 1 a 400 km long seismic reflection profile andean continental research project 1996 ancorp 96 and integrated geophysical experiments wide angle seismology passive seismology gravity and magnetotelluric depth sounding across the central andes 21 s observed subduction of the nazca plate under the south american continent an east dipping reflector nazca reflector is linked to the down going oceanic crust and shows increasing downdip intensity before gradual breakdown below 80 km we interpret parts of the nazca reflector as a fluid trap located at the front of recent hydration and shearing of the mantle the fluids being supplied by dehydration of the oceanic plate patches of bright quebrada blanca bright spot to more diffuse reflectivity underlie the plateau domain at 15 30 km depth this reflectivity is associated with a low velocity zone p to s wave conversions the upper limits of high conductivity and high vp vs ratios and to the occurrence of neogene volcanic rocks at surface we interpret this feature as evidence of widespread partial melting of the plateau crust causing decoupling of the upper and lower crust during neogene shortening and plateau growth the imaging properties of the continental moho beneath the andes indicate a broad transitional character of the crust mantle boundary owing to active processes like hydration of mantle rocks in the cooler parts of the plate margin system magmatic underplating and intraplating under and into the lowermost crust mechanical instability at moho etc hence all first order features appear to be related to fluid assisted processes in a subduction setting

trace element characteristics of different pyrite types in mesoarchaean to palaeoproterozoic placer deposits a comparative in situ la icp ms trace element study on pyrite from three different variably auriferous archaean to palaeoproterozoic palaeoplacer deposits in the ouro fino syncline quadrilatero ferrifero brazil the elliot lake area north of lake huron se canada and several deposits within the witwatersrand basin south africa revealed systematic differences between morphologically different pyrite types and between the various palaeoplacer deposits especially the ni and au concentrations as well as co ni and mo ni ratios were found to be systematically different in detrital compact detrital porous and post sedimentary hydrothermal pyrite grains from different source areas high co ni ratios and low au concentrations are typical of post sedimentary pyrite which is hydrothermal in origin in contrast relatively low co ni ratios and high au contents characterise detrital porous banded and concentric pyrite grains au 1 ppm which are syn sedimentary in origin in the elliot lake area and the witwatersrand basin detrital compact rounded pyrite is characterised by high co ni ratios which is in agreement with derivation from a hydrothermal source low au concentrations in this pyrite type support the contention of the gold and the pyrite in these deposits coming from different source rocks in contrast derivation from an originally diagenetic pyrite is suggested for the detrital compact pyrite in the ouro fino syncline because of low to intermediate co ni ratios high au contents may indicate a genetic relationship between pyrite and gold there systematic differences exist between the three areas with respect to au ni co mo and cu distributions in detrital pyrite which reflects differences in the provenance a predominantly mafic ultramafic source is indicated for the ouro fino a felsic source for the elliot lake and a mixed felsic mafic provenance for the witwatersrand pyrite populations independently of pyrite type the higher au endowment of the studied witwatersrand and ouro fino conglomerates are also reflected by an overall higher au concentration in the respective pyrite grains compared to the relatively au poor samples from elliot lake in general a strong positive correlation between au and pb levels in the various pyrite grains is noted analogous to pb which is well known for not being easily accommodated in the pyrite crystal lattice but occurring as discrete pbs phases au is considered to be present mainly in the form of discrete au phases in minute pores and interstices of the pyrite grains rather than within the pyrite lattice

influence of structural setting on sulphur isotopes in archean orogenic gold deposits eastern goldfields province yilgarn western australia the published mean 34s values of ore related pyrites from orogenic gold deposits of the eastern goldfields province yilgarn craton lie between 4 and 4 as for orogenic gold deposits worldwide most deposits have positive means and a restricted range of 34s values but some have negative means and wider ranges of 34s values wall rock carbonation and back mixing of similar source fluids with different fluid pathways can explain some of the more negative 34s signatures however structural setting appears to be the most important factor controlling ore fluid oxidation state and hence the distribution of 34s values in gold related pyrites shear hosted deposits appear to have experienced fluid dominated processes such as phase separation whereas stockwork vein hosted or disseminated deposits formed under conditions of greater rock buffering at victory defiance in particular negative 34s values are more common in gently dipping dilational structures compared to more compressional steeply dipping structures it appears most likely that fluid pressure fluctuations during fault valve cycles establish different fluid flow regimes in structures with different orientations rapid fluid pressure fluctuations in dilational structures during seismic activity can cause partitioning of reduced gas phases from the ore fluid during extreme phase separation and hence are an effective method of ore fluid oxidation leading to large local fluctuations in oxidation state it is thus not necessary to invoke mixing with oxidised magmatic fluids to explain 34s signatures indicative of oxidation in any case available robust geochronology in the eastern goldfields province does not support the direct involvement of oxidised magmatic fluids from adjacent granitic intrusions in orogenic gold genesis thus negative mean 34s values and large variations in 34s values of ore related pyrites in world class orogenic gold deposits are interpreted to result from multiple mechanisms of gold precipitation from a single ubiquitous ore fluid in varying structural settings rather than from the involvement of oxidised ore fluids from a different source such signatures are indicative but not diagnostic of anomalously large orogenic gold systems

application of arsenopyrite geothermometry and sphalerite geobarometry to the taebaek pb zn ag deposit at yeonhwa i mine republic of korea the taebaek pb zn ag deposit of the yeonhwa i mine republic of korea occurs in a broadly folded and reverse faulted terrain of paleozoic sedimentary rocks the taebaeksan basin the orebodies consist of several thin tabular orebodies of hydrothermal replacement type where they are hosted by carbonate rocks the pb zn ag mineralization can be divided into four distinct stages based upon the mode of occurrence of ore minerals ore textural relationships and their composition based on temperatures inferred from arsenopyrite compositions by means of electron microprobe and fluid inclusions the estimated temperatures for the stages i ii iii and iv reach 330 to 350 c 270 to 340 c 230 to 250 c and 220 c respectively the sulphur activity atm of ore formation at the taebaek deposit was estimated for each stage as 10 11 to 10 11 5 10 9 5 to 10 13 10 13 5 to 10 15 and 10 15 respectively even though application of sphalerite geobarometry is problematic because of the absence of good mineral assemblages sphalerite coexisting with pyrite but not with pyrrhotite was used to estimate the minimum mineralization pressure about 1 kbar

invisible gold in arsenian pyrite and arsenopyrite from a multistage archaean gold deposit sunrise dam eastern goldfields province western australia the sunrise dam gold mine 11 1 moz au is the largest deposit in the archaean laverton greenstone belt eastern goldfields province yilgarn craton western australia the deposit is characterized by multiple events of fluid flow leading to repeated alteration and mineralization next to a major crustal scale structure the au content of arsenian pyrite and arsenopyrite from four mineralizing stages d1 d3 d4a and d4b and from different structural and lithostratigraphic environments was measured using in situ laser ablation inductively coupled plasma mass spectrometry pyrite contains up to 3 067 ppm au n 224 whereas arsenopyrite contains up to 5 767 ppm n 19 gold in arsenopyrite d4a stage was coprecipitated and remained as invisible gold nanoparticles and or lattice bound during subsequent deformation events in contrast gold in pyrite is present not only as invisible gold but also as micrometer size inclusions of native gold electrum and au ag tellurides pristine d1 and d3 arsenian pyrite contains relatively low au concentrations 26 ppm the highest au concentrations occur in d4a arsenian rich pyrite that has recrystallized from d3 pyrite textures show that this recrystallization proceeded via a coupled dissolution reprecipitation process and this process may have contributed to upgrading au grades during d4a in contrast au in d4b pyrite shows grain scale redistribution of invisible gold resulting in the formation of micrometer scale inclusions of au minerals the speciation of au at sunrise dam and the exceptional size of the deposit at province scale result from multiple fluid flow and multiple au precipitating mechanisms within a single plumbing system

40ar39ar dating of the skaergaard intrusion we report the first 40ar 39ar data for the skaergaard intrusion based on analyses of hornblende and biotite from a transgressive granophyre from upper zone a uza of the layered series hornblende separates heated in a resistance furnace and laser heated single crystals of biotite from the granophyre give plateau ages that are consistent with isochron ages of 55 48 0 30 and 55 40 0 14 ma respectively 2 internal uncertainty with initial 40ar39ar ratios of 292 4 19 0 and 291 1 7 2 if uncertainties in the ages of 40ar 39ar standard fluence monitors are considered external uncertainties 2 errors are 0 76 and 0 72 ma respectively resistance heating analysis of biotite separated from a gabbro from lower zone a lza of the skaergaard layered series indicates that the sample has been extensively contaminated by ar inherited from old crust and does not constrain the intrusion age apart from indicating that it is less than approximately 57 ma old the ar cooling ages are consistent within uncertainty with both pluton cooling models presented by norton and taylor 1 but the data agree best with their model s2 which assumes a smaller permeability 10 13 cm2 combination of the new ages and the norton and taylor cooling models constrains the emplacement and final crystallization of the skaergaard magma chamber to be at 55 75 0 30 ma and 55 65 0 30 ma respectively evidence for late crystallizing quartz and tridymite near the sandwich horizon sh combined with phase equilibria constraints on the near solidus crystallization temperature of the sh 975 25 c require the pressure of sh solidification to be 60 15 mpa this means that the tertiary basalts and sediments overlying the kangerdlugssuaq region were 3 2 0 8 km thick at 55 65 0 30 ma assuming that the 6 7 km thick tertiary section exposed further east along the east greenland coast is representative of that which accumulated above the skaergaard we infer that the skaergaard emplacement occurred during the main stage of the east greenland flood basalt event after approximately half of the tertiary section had erupted the onset of coastal flexure in the kangerdlugssuaq region was after 55 48 0 76 ma

geology mineralogy and stable isotope geochemistry of the dzuunmod area in northern mongolia constraints for gold ore genesis and sources abstract the dzuunmod area located in the north khentii gold belt nkgb of northern mongolia includes lode gold deposits such as gatsuurt sujigtei and boroo with several minor gold deposits and occurrences they show similar hydrothermal alteration assemblages sericitic siliceous and potassic and ore mineral assemblages pyrite and arsenopyrite with minor amount of galena sphalerite and chalcopyrite gold occurs as native form and invisible gold in pyrite and arsenopyrite the major sulfide minerals are separated into earlier non auriferous stage and later auriferous grains containing invisible gold native gold postdates the major sulfide mineralization gold and arsenic content of pyrite grains indicates that gold exists mainly as solid solution form au 1 in the gatsuurt and boroo deposit whereas gold nanoparticle au0 is present in the sujigtei deposit high co ni and mo ni ratios of pyrite grain suggest a post sedimentary or hydrothermal origin and the ore forming fluid was significantly affected by fluid host rock interactions during mineralization processes large variation of 34s values of pyrite and arsenopyrite from 2 6 to 17 2 indicates that sulfur seems to be mainly derived from a source with heterogeneous sulfur isotope composition even though the role of magmatic sulfur as one of possible sulfur sources cannot be ruled out consistent with geological evidence relatively positive 34s values suggest that sulfidation plays an important role for gold and sulfide precipitation the calculated 18o values of hydrothermal fluid from the measured 18o values of quartz samples from 14 7 to 17 7 indicate a metamorphic derivation of ore forming fluid gold mineralization processes in the dzuunmod area seem to occur several times by multiple input of hydrothermal fluid and fluid host rock interactions the gold deposits in the dzuunmod area are considered to be orogenic gold type influenced by fluid host rock interactions in the deposit area

geodynamics of the western part of the mongolia okhotsk collisional belt trans baikal region russia and mongolia abstract after the western edge of the mongolian microcontinent joined the siberian continent in the region of central mongolia in the earliest permian these two continental blocks remained turned at an angle of about 120 with respect to each other and separated on greater extent of their present day boundary by an enormous gulf of the paleopacific called the mongolia okhotsk ocean closure of this ocean at the early middle jurassic boundary led to the complete collision of siberia and mongolia which by then had already become part of the mongolia north china continent this main collisional episode which lasted through the middle and late jurassic involved thrusting folding and magmatism and produced the mongolia okhotsk belt the onon island arc which was located in the mongolia okhotsk ocean was squeezed between the two major continents inasmuch as the third element the island arc was involved in the collision it is reasonable to distinguish two branches of the mongolia okhotsk suture these branches control the spatial distribution of gold mineralization in the trans baikal region on the southeastern periphery of siberia the crust thickened considerably after the collision and a plateau like uplift formed in the early cretaceous when compression ceased the collisional uplift collapsed and the thrusts were transformed into low angle normal faults the motions on which were responsible for the formation of rift basins and exhumation of metamorphic core complexes

coupling of anatectic reactions and dissolution of accessory phases and the sr and nd isotope systematics of anatectic melts from a metasedimentary source advances in field observations and experimental petrology on anatectic products have motivated us to investigate the geochemical consequences of accessory mineral dissolution and nonmodal partial melting processes incorporation of apatite and monazite dissolution into a muscovite dehydration melting model allows us to examine the coupling of the rb sr and sm nd isotope systems in anatectic melts from a muscovite bearing metasedimentary source modeling results show that 1 the sm nd ratios and nd isotopic compositions of the melts depend on the amount of apatite and monazite dissolved into the melt and 2 the relative proportion of micas muscovite and biotite and feldspars plagioclase and k feldspar that enter the melt is a key parameter determining the rb sr and 87 sr 86 sr ratios of the melt furthermore these two factors are not in practice independent in general nonmodal partial melting of a pelitic source results in melts following one of two paths in e nd 87 sr 86 sr ratio space a higher temperature fluid absent path path 1 represents those partial melting reactions in which muscovite biotite dehydration and apatite but not monazite dissolution play a significant role the melt will have elevated rb sr 87 sr 86 sr sm nd and e nd values in contrast a lower temperature fluid fluxed path path 2 represents those partial melting reactions in which muscovite biotite dehydration plays an insignificant role and apatite but not monazite stays in the residue the melt will have lower rb sr 87 sr 86 sr sm nd and e nd values than its source the master variables controlling both accessory phase dissolution and hence the sm nd system and melting reaction and hence the rb sr systematics are temperature and water content the complexity in sr nd isotope systematics in metasediment derived melts as suggested in this study will help us to better understand the petrogenesis for those granitic plutons that have a significant crustal source component

liquid segregation parameters from amphibolite dehydration melting experiments we have experimentally obtained some of the parameters necessary for understanding the segregation of silicate liquid from amphibolite undergoing dehydration melting at 1 gpa and 750 1000 c the solidus for this calcic amphibolite 68 mg hb 32 pl an 90 is 750 c amphibolites begin to melt at relatively high temperatures in the garnet absent field but the solidus appears to backbend at 1 gpa coincident with the garnet in boundary hornblende breakdown due to garnet formation releases h 2o and causes melting thus in the garnet present field 1 gpa the amphibolite dehydration melting solidus may be coincident with the h 2o saturated solidus liquid interconnectivity may be achieved at 900 c and 5 vol liquid based on both physical and chemical data from solid rock runs mass balance calculations from powdered rock runs suggest that small amounts 5 15 vol of hydrous 4 wt h 2o low viscosity 10 3 10 4 pa s heavy rare earth element depleted felsic liquid may be segregated during amphibolite dehydration melting at 875 c the rapid breakdown of coarse grained hb cores may lead to the formation of transient h 2o saturated liquids with even lower viscosities 10 2 pa s although comprising only a small portion of the melting cycle these h 2o rich conditions may enhance the segregation of liquid by reduction of liquid viscosities and by mechanical effects on the restite e g increased deformation and liquid fracturing during anatexis of coarse grained natural rocks transient conditions may control the initial stages of liquid segregation

splay fault triggering by great subduction earthquakes inferred from finite element models we have investigated the influence that megathrust earthquake slip has on the activation of splay faults using a 2 d finite element method fem taking into account the effects of gravity and variations in the frictional strength properties of splay faults we simulated both landward dipping and seaward dipping splay fault geometries and imposed depth variable slip distributions of subduction events our results indicate that the two types of splay fault exhibit a similar behavior with variations in frictional properties along the faults affecting only the seismic magnitude the triggering process is controlled by a critical depth megathrust slip concentrated at depths shallower than the critical depth will favor normal displacement while megathrust slip concentrated at depths deeper than the critical depth is likely to result in reverse motion our results thus provide a useful tool for predicting the activation of secondary faults and may have direct implications for tsunami hazard research

gilt by association origins of pyritic gold ores in the victory mesothermal gold deposit western australia sulfur isotope data and calculated fluid rock reaction paths constrain the origin of pyritic gold ores in the victory mesothermal gold deposit of western australia pyrite 34s values range from 4 4 to 5 1 and are negatively correlated with gold tenor in mafic host rocks indicating a significant increase in fluid oxidation state accompanied mineralization geologic evidence argues strongly that mineralization occurred with wall rock sulfidation but this process alone cannot produce the degree of fluid oxidation necessary to explain the sulfur isotope range in contrast either carbonation of ferric iron bearing minerals or separation of immiscible fluid phases both of which are consistent with the available geologic data can drive fluid oxidation to the point of aqueous sulfate dominance coupled decreases in gold solubility and sulfide sulfur isotope values that are predicted by such a scenario provide an explanation for the trend between 34s and gold tenor observed at victory because both wall rock carbonation and fluid phase separation involve loss of aqueous sulfide but do not require precipitation of iron sulfides these processes may explain the poor correlation between gold tenor and pyrite abundance in many mesothermal deposits and the occurrence of some gold ores in iron poor rocks

supersaturation and crystal growth in the roof zone of the skaergaard magma chamber the coarse grained upper border series rocks of the skaergaard intrusion contain abundant skeletal crystals of magnetite and ilmenite skeletal and hopper crystals of apatite and less abundant sector zoned augite crystals and hopper zircon crystals in addition the melanogranophyres which occur as pods and lenses in the lower part of the upper border series and the upper part of the layered series are characterized by very coarse grained dendritic ferrohedenbergite crystals skeletal hopper and sectorzoned crystals are not present in the layered series gabbros the development of these unusual crystal morphologies in the upper border series requires that the roof zone magma was intermittently supersaturated and indicates that the skaergaard magma chamber was compositionally zoned and that heat loss through the roof maintained a temperature gradient in the magma that was greater than the adiabatic gradient it is suggested that supersaturation developed in the roof zone of the intrusion as a result of convective overturn and magma mixing during the early stages of crystallization and as a result of sudden volatile loss during the later stages of crystallization when the upper border series rocks became rigid enough to fracture

orogenic gold common or evolving fluid and metal sources through time abstract orogenic gold deposits of all ages from paleoarchean to tertiary show consistency in chemical composition they are the products of aqueous carbonic fluids with typically 5 20 mol co 2 although unmixing during extreme pressure fluctuation can lead to entrapment of much more co 2 rich fluid inclusions in some cases ore fluids are typically characterized by significant concentrations of ch 4 and or n 2 common estimates of 0 01 0 36 mol h 2 s a near neutral ph of 5 5 and salinities of 3 7 wt nacl equiv with na k ca mg this fluid composition consistency favors an ore fluid produced from a single source area and rules out mixing of fluids from multiple sources as significant in orogenic gold formation nevertheless there are broad ranges in more robust fluid inclusion trapping temperatures and pressures between deposits that support a model where this specific fluid may deposit ore over a broad window of upper to middle crustal depths much of the reported isotopic and noble gas data is inconsistent between deposits leading to the common equivocal interpretations from studies that have attempted to define fluid and metal source areas for various orogenic gold provinces fluid stable isotope values are commonly characterized by the following ranges 1 18 o for precambrian ores of 6 to 11 and for phanerozoic ores of 7 to 13 2 d and 34 s values that are extremely variable 3 13 c values that range from 11 to 2 and 4 15 n of 10 to 24 for the neoarchean 6 5 to 12 for the paleoproterozoic and 1 5 to 10 for the phanerozoic secular variations in large scale earth processes appear to best explain some of the broad ranges in the o s and n data fluid rock interaction particularly in ore trap areas may cause important local shifts in the o s and c ratios the extreme variations in d mainly reflect measurements of hydrogen isotopes by bulk extraction of waters from numerous fluid inclusion generations many which are not related to ore formation radiogenic isotopes such as those of pb sr nd sm and os measured on hydrothermal minerals are even more difficult to interpret for defining metal source particularly as the low salinity ore fluids transport limited amounts of these elements and significant amounts of these may be locally added to the minerals during alteration reactions at the sites of gold deposition noble gas and halogen data are equally equivocal fluid exsolution from granitoids emplaced into the upper and middle crust metamorphism of the crust or fluids entering trans crustal fault zones from below the crust all remain as permissive scenarios associated with orogenic gold formation as the abundant geochemical data are equivocal however geological and geochronological data weigh heavily against a magmatic hydrothermal model in the upper to middle crust there is no universal temporal association between orogenic gold and magmatism and where there is an overlap in age there is no specific type of magmatism consistently associated with gold formation nor element zonation around any specific pluton a crustal metamorphic model for fluid and metal sources is very consistent with geological geochronological and geochemical data although metamorphism on a regional scale that releases these components into major fault zones can be associated with many processes along active continental margins these can include crustal thickening and radiogenic heating slab rollback and heating during crustal extension or subduction of a spreading ridge heating the base of an accretionary prism in rare examples where phanerozoic orogenic gold deposits are hosted in precambrian high grade metamorphic terranes fluids and metals must however enter a transcrustal fault system from a sub crustal source this could either be a devolatilized subducted relatively flat perhaps stalled slab and its overlying sediment or the corner of the fertilized mantle wedge that releases a fluid during a thermal event

late cenozoic deformation and uplift of the western flank of the altiplano evidence from the depositional tectonic and geomorphologic evolution and shallow seismic activity northern chile at 19 30 s 1 we analyze the west vergent thrust system wts along the western flank of the altiplano in northern chile 18 s 21 s in our study area 19 20 s 19 50 s the wts consists of three thrust propagation monocline folds flexures developing growth strata the relative uplift accommodated by the flexures is rapid between 26 and 8 ma 0 1 mm yr diminishing to 0 02 mm yr after 8 ma approximately 2000 m of relative surface uplift was accommodated by the flexures since the late oligocene sedimentological and geomorphological analysis shows that westward tilting of the forearc occurred after 10 ma coeval with the shifting of deformation from the altiplano to the sub andean zone where the underthrusting of the brazilian craton would have resulted in crustal thickening surface uplift in the orogen and westward ductile subcrustal flow forearc tilting is accommodated by east vergent thrusts ets issued from the benioff zone beneath the central depression emerging into the western cordillera contributing 500 1400 m of surface uplift the wts connects the ets in the brittle ductile crustal transition 25 km depth continuing farther east as the altiplano low velocity zone configuring the western altiplano as a crustal scale fault bend fold forearc tilting would be caused by westward ductile flow in the lower crust pushing the rigid forearc in the ets meanwhile between 19 s and 21 s the wts accommodates dextral strike slip and 3 km of n s shortening occurred in the coastal cordillera transcurrence and strain partitioning are probably the result of slight plate convergence obliquity strong coupling within the interplate zone westward continental concavity and high elevation opposing horizontal contraction

genesis of two different types of gold mineralization in the linglong gold field china constrains from geology fluid inclusions and stable isotope abstract the dongfeng and linglong gold deposits are located in the northwest jiaodong peninsula north china craton the deposits are mainly hosted in the mesozoic granitoids and structurally controlled by the zhaoyuan pingdu fault zone gold mineralization at dongfeng occurs as disseminated ores and sulfide stockworks typically enveloped by broad alteration selvages in contrast mineralization at linglong is characterized by massive auriferous quartz veins with narrow alteration halos three stages of mineralization were identified in both deposits with the early stage represented by quartz pyrite the middle stage by gold quartz pyrite or gold quartz base metal sulfides and the late stage by quartz carbonate pyrite respectively four types of fluid inclusions were distinguished based on petrography microthermometry and laser raman spectroscopy including 1 pure co2 fluid inclusions type i 2 h2o co2 nacl fluid inclusions type ii 3 h2o nacl fluid inclusions type iii and 4 daughter mineral bearing or multiphase fluid inclusions type iv in the dongfeng gold deposit the early and middle stage quartz mainly contains primary type ii fluid inclusions that completely homogenized at temperatures of 276 341 c with salinities of 2 8 11 7 wt nacl equivalent and temperatures of 248 310 c with salinities of 3 3 10 8 wt nacl equivalent respectively a few primary type i fluid inclusions could be observed in the early stage quartz in contrast the late stage quartz contains only the type iii fluid inclusions with homogenization temperatures of 117 219 c and salinities of 0 5 8 5 wt nacl equivalent the estimated pressures for the middle stage fluids are 226 338 mpa suggesting that gold mineralization mainly occurred at paleodepths of deeper than 8 4 12 5 km the mineralization resulted from extensive water rock interaction between the h2o co2 nacl fluids and wallrocks in the first order fault in the linglong gold deposit the early stage quartz mainly contains primary type ii fluid inclusions and a few type i fluid inclusions of which type ii fluid inclusions have salinities of 3 3 7 5 wt nacl equivalent and homogenization temperatures of 271 374 c the middle stage quartz mainly contains all four types of fluid inclusions among which the type ii fluid inclusions yield homogenization temperatures of 251 287 c and salinities of 5 5 10 3 wt nacl equivalent while the type iii fluid inclusions have homogenization temperatures of 244 291 c and salinities of 4 1 13 3 wt nacl equivalent fluid inclusions in the late stage quartz are type iii fluid inclusions with low salinities of 0 3 8 2 wt nacl equivalent and low homogenization temperatures of 103 215 c the trapping pressure estimated for the middle stage fluids is 228 326 mpa suggesting that the gold mineralization mainly occurred at paleodepths of about 8 4 12 1 km precipitation of gold is possibly a consequence of phase separation or boiling of the h2o co2 nacl fluids in response to pressure and temperature fluctuations in the open space of the secondary faults the 34s values of pyrite are similar for the dongfeng and linglong deposits and show a range of 5 8 to 7 0 and 5 9 to 7 4 respectively oxygen and hydrogen stable isotopic analyses for quartz yielded the following results 18o 3 8 to 6 4 and d 90 5 to 82 7 for the dongfeng deposit and 18o 0 0 to 8 9 and d 77 4 to 63 7 for the linglong deposit stable isotope data show that the ore forming fluids of the two gold deposits are of magmatic origin with gradual incorporation of shallower meteoric water during after mineralization