the fazenda nova gold deposit goias magmatic arc late neoproterozoic intrusion related auriferous mineralization controlled by intracontinental strike slip faulting abstract the fazenda nova gold deposit with an inferred resource of 650 koz at 4 0 g t au cut off grade of 2 0 g t au consists of stockwork au as mineralization hosted in late neoproterozoic dikes in the goias magmatic arc central brazil the dikes which host the mineralization consist of a set of post collisional ilmenite bearing gabbros dolerites trachyandesite porphyry trachyandesite and trachyte porphyry known as the bacil ndia intrusions u pb dating indicates at least two pulses of dike intrusions the first pulse consists of small volume dikes with ages varying from 593 5 to 590 5 ma and the second pulse comprises a large volume of dikes with ages from 574 7 to 572 5 ma three veining or hydrothermal alteration stages are identified in the fazenda nova deposit the first is the early or main stage which comprises the pervasive biotitization of mafic minerals and a stockwork of quartz associated with a disseminated sulfide assemblage of arsenopyrite pyrrhotite stibnite in addition to hydrothermal siderite scheelite rutile and apatite the most common sulfide in this stage is a fine grained acicular arsenopyrite hosting inclusions of gold galena and pyrite grains the temperature estimated for acicular arsenopyrite pyrrhotite and gold is 340 c based on the arsenopyrite geothermometry the intermediate stage is represented by a stockwork of calcite quartz veins associated with hydrothermal sericite chlorite tourmaline epidote and titanite and a sulfide assemblage of pyrrhotite arsenopyrite pyrite the arsenopyrite at this stage displays a tabular texture with no inclusions of sulfides or gold the crystallization temperature of the tabular arsenopyrite pyrite was estimated to be 305 c the c o isotope data from the calcite veins of the second stage provided 13c values ranging from 9 68 to 11 57 and 18o values range from 12 87 to 13 84 these values are equivalent to those of hydrothermal mantle magmatic carbonates the late stage is represented by veining and brecciation infilled by monotonous calcite ankerite with cockade and colloform vein textures and no gold mineralization the dikes and gold mineralization present a strong relationship with the crustal scale strike slip bacil ndia fault this is a second order crustal scale fault that branches out of the major crustal scale moipora novo brazil shear zone mnbsz along a nnw direction the bacil ndia fault played an important role in channeling melts and mineralizing hydrothermal fluids to the upper crust the major activity of this fault occurred at 572 ma with the emplacement of the second pulse of dikes and most likely the development of hydrothermal stages in the fazenda nova deposit the collectively evidence allows fazenda nova to be classified as an epizonal reduced intrusion related deposit following that i the mineralization developed in a post collisional tectonic setting above previously metasomatized subcontinental lithospheric mantle ii the dikes crystallized from metaluminous magmas with a primary oxidation state which formed the ilmenite series intrusions iii the ore deposition had relatively high temperatures 340 300 c and vein textures formed only at shallow depths

effect of partial melting on rock deformation experimental and natural evidences on rocks of granitic compositions abstract deformation tests at constant strain rate 10 5 s 1 at 210 250 mpa confining pressure and at 560 900 c are performed on an aplite with varying water contents the mechanical behaviour is correlated with the microstructure after a stage of plastification depending on the temperature and on the water content the rock strength falls dramatically under such confining pressures before melting and before straining the pores of the wet rocks burst as the thermal expansion of water develops a local high pore pressure which causes cracking and fracturing in our opinion this pore bursting will increase the effects of thermal cracking due to the differential expansion of the minerals the rocks exhibit a uniform crack network with increasing temperature melting occurs in the cataclased rock the crack development and melting depend on the initial water content the loss of cohesion obtained either by cracking or by cracking and melting provokes a dramatic weakening axial shortening induces cracking parallel to 1 and melt transfer into extensional regions deformation is concentrated in dilatant shear bands at the highest temperature 900 c a melt film network is rapidly established and induces a very low flow stress melt effects in naturally deformed gneisses suggest that biotite breakdown furnishes a silicate fluid which induces hydraulic fracture and melt dispersion

mineralogical and stable isotope studies of gold arsenic mineralisation in the sams creek peralkaline porphyritic granite south island new zealand at sams creek a gold bearing peralkaline granite porphyry dyke which has a 7 km strike length and is up to 60 m in thickness intrudes camptonite lamprophyre dykes and lower greenschist facies metapelites and quartzites of the late ordovician wangapeka formation the lamprophyre dykes occur as thin 3 m slivers along the contacts of the granite dyke 18omagma values 5 to 8 vsmow of the a type granite suggest derivation from a primitive source with an insignificant mature crustal contribution hydrothermal gold sulphide mineralisation is confined to the granite and adjacent lamprophyre metapelite country rocks have only weak hydrothermal alteration three stages of hydrothermal alteration have been identified in the granite stage i alteration high fo2 consisting of magnetite siderite biotite stage ii consisting of thin quartz pyrite veinlets and stage iii low fo2 consisting of sulphides quartz and siderite veins and pervasive silicification the lamprophyre is altered to an ankerite chlorite sericite assemblage stage iii sulphide veins are composed of arsenopyrite pyrite galena sphalerite gold chalcopyrite pyrrhotite rutile graphite three phases of deformation have affected the area and the mineralised veins and the granite and lamprophyre dykes have been deformed by two phases of folding the youngest of which is early cretaceous locally preserved early formed fluid inclusions are either carbonic showing two or three phases at room temperature liquid co2 ch4 liquid h2o co2 vapour or two phase liquid rich aqueous inclusions some of which contain clathrates salinities of the aqueous inclusions are in the range of 1 4 to 7 6 wt nacl equiv final homogenisation temperatures th of the carbonic inclusions indicate minimum trapping temperatures of 320 to 355 c which are not too different from vein formation temperatures of 340 380 c estimated from quartz albite stable isotope thermometry 18o values of stage ii and iii vein quartz range from 12 and 17 and have a bimodal distribution 14 5 and 16 with stage ii vein quartz accounting for the lower values siderite in stage iii veins have 18o 12 to 16 and 13c values 5 relative to vpdb unlike those from wangapeka formation metasediments 13cbulk carbon values of 24 to 19 and underlying arthur marble marine carbonates 18o 25 and 13c 0 calculated 18owater 8 to 11 at 340 c and delta 13 text c rm co 2 5 values from vein quartz and siderite are consistent with a magmatic hydrothermal source but a metamorphic hydrothermal origin cannot be excluded 34s values of sulphides range from 5 to 10 relative to cdt and also have a bimodal distribution modes at 6 and 9 correlated with stage ii and stage iii mineralisation respectively the 34s values of pyrite from the arthur marble marine carbonates range from 3 to 13 and wangapeka formation range from 4 to 9 5 indicate that they are potential sources of sulphur for sulphides in the sams creek veins another possible source of the sulphur is the lithospheric mantle which has positive values up to 14 ages of the granite lamprophyre alteration mineralisation and deformation in the region are not well constrained which makes it difficult to identify sources of mineralisation with respect to timing our mineralogical and stable isotope data does not exclude a metamorphic source but we consider that the source of the mineralisation can best be explained by a magmatic hydrothermal source assuming that the hydrothermal fluids were sourced from crystallisation of the sams creek granite or an underlying magma chamber then the sams creek gold deposit appears to be a hybrid between those described as reduced granite au bi deposits and alkaline intrusive hosted au mo cu deposits

flow laws for rocks containing two non linear viscous phases a phenomenological approach abstract mylonitic rocks and rock analogue materials reveal two basic types of structure 1 a load bearing framework lbf of strong phase contains isolated pockets of weak phase 2 interconnected layers of weak phase iwl separate boudins and clasts of strong phase aggregates with the lbf microstructure are characterized by nearly uniform strain rate stress is concentrated in the load bearing framework in aggregates with an iwl microstructure strain rate and sometimes also stress are higher in the interconnected weak phase than in the boudins and clasts of strong phase the degree of stress and strain partitioning depends strongly on the viscous strength contrast and on the relative amounts of the constituent mineral phases based on these observations the rheology of two phase rock is modelled with separate functions for lbf and iwl microstructures a new flow law is derived for rock with iwl structure in which two phases undergo dislocation creep the flow law expresses composite creep strength in terms of temperature bulk strain rate and the volume proportions and creep parameters of the minerals in the rock strain rate and stress are averaged in the constituent phases and slip along phase boundaries maintains strain compatibility within the aggregate composite strengths predicted with the iwl flow law fall well within the uniform stress and uniform strain rate bounds and are generally consistent with the viscous strengths of experimentally deformed bimineralic aggregates a hypothesis of viscous strain energy minimization is used to determine the relative stability of the lbf and iwl microstructures during steady state creep the iwl microstructure is predicted to be stable over a broad range of two phase compositions and mineral strength contrasts whereas the lbf microstructure is stable only in rocks with low volume proportions of weak phase and low to moderate mineral strength contrasts the iwl flow law indicates that rheological stratification in the lithosphere depends strongly on rock composition especially in rocks with low volume proportions of a weak phase and high mineral strength contrasts

fluid inclusion and stable isotope c o and s constraints on the genesis of the high grade langdu cu skarn deposit in yunnan sw china abstract the langdu cu skarn deposit is the highest grade cu deposit average 6 49 cu in the zhongdian polymetallic district in sw china skarns and ore bodies occur primarily in the contact zone of upper triassic qugasi formation fm and late triassic ca 217 ma intrusions disseminated massive stockwork and vein like sulfide ores occur in the skarn quartz calcite veins porphyry and marble skarn minerals include pyroxene garnet actinolite and epidote four types six subtypes of fluid inclusions fis are identified in the sulfide bearing quartz veins type i liquid rich two phase fis have varying vapor liquid ratios and yielded homogenization temperatures of 110 338 c and salinities of 1 7 29 5 wt nacl eqv type ii vapor rich fis contain ch4 n2 bubbles and homogenized to vapor at 262 425 c type iii fis are single phase at room temperature 20 c a vapor phase is present at below 120 c and the fis homogenized at 117 to 114 c type iv daughter mineral bearing fis homogenized by halite disappearance at 295 392 c and have salinities of 37 8 46 6 wt nacl eqv such characteristics suggest that fis were trapped by fluid immiscibility and phase separation the 34scdt values of chalcopyrite and pyrrhotite are of 5 3 to 0 7 on the 34s histogram all langdu data show a cluster of 32s rich 34scdt 5 3 to 4 6 and a cluster of 34s rich 34scdt 1 1 to 0 7 type i calcite in skarn and type ii calcite in quartz sulfide vein have 13cpdb and 18osmow values of 8 4 to 7 8 6 3 to 6 1 and 10 1 to 12 3 and 12 0 to 13 0 respectively type iii calcite from calcite sulfide veins has 13cpdb 5 6 to 0 2 and 18osmow 12 5 to 16 3 biotite garnet and pyroxene syn ore quartz and chlorite yielded 18osmow values of 6 9 4 1 5 13 2 15 3 and 6 8 respectively sulfur carbon and oxygen isotopic features of these hydrothermal minerals and the fis microthermometric data suggest that the ore forming fluids were magmatic sourced and contaminated by meteoric formation water during ascent and or fluid wallrock interactions low 34s 5 3 to 4 6 in sulfides and the ch4 in the fis are interpreted to reflect the reaction between the magmatic fluids and the carbonaceous slate in qugasi fm the methane likely expanded the vapor liquid immiscibility field and reduced the sulfates so42 to sulfides s2 and extensive water rock reaction and fluid mixing may have occurred leading to the high grade cu mineralization at langdu

constraining the relative strengths of high grade metamorphic rocks using foliation refraction angles an example from the northern new england appalachians foliation refraction angles are used to estimate effective viscosity contrasts between metapelitic and metapsammitic layers in amphibolite facies metaturbidites in the presidential range of the white mountains in eastern new hampshire an early formed foliation developed during km scale nappe folding consistently displays larger bedding foliation angles in metapelitic units than metapsammitic units suggesting that the metapelitic units had higher effective viscosities during this deformation we collected bedding foliation angle measurements from a combination of outcrops and locally derived boulders to test the hypothesis that foliation refraction angles could be used to estimate the effective viscosity ratio between different rock types at high metamorphic grades our results show that the metapelitic layers were between 2 and 3 times more viscous than the metaspsammitic layers which we attribute to the presence of large up to 15 cm long abundant up to 30 vol effectively rigid andalusite porphyroblasts in the metapelitic layers we present a methodology for using foliation refraction angles to determine the effective viscosity ratios of different rock types address the practical limitations we encountered and suggest this method is an easy way of estimating the strength contrast between different rock types finally we hypothesize that metamorphism resulting in porphyroblast growth can strengthen large crustal volumes during orogenesis

rheological transitions in progressive melting of rock and their geological constraints from the fuhu metatexite diatexite profile in guangdong province se china abstract integration of the published experimental data of partially melted amphibolite orthogneiss granite and aplite reveals the existence of three rapid strength drops of the rock assemblage at melt fraction around 7 21 and 41 respectively the first and the last drops are equivalent to the well known melt connectivity transition mct and solid to liquid transition slt the drop at melt fraction around 21 is newly identified and termed as the framework melting transition fmt the fmt is defined as the transition from eutectic quartz feldspars melting to refractory biotite melting the three rheological transitions are well constrained by the geological data from the fuhu profile in guangdong province se china this profile is a 15 m high cliff rocks exposed on the profile are zoned as mottled stripped narrow banded and wide banded migmatites and diatexitic granite from the top of the profile downwards the leucosome proportions of the zones measured on the outcrops combined with the experimental data of the partially melted rocks have significant implications for understanding the role of rheological transitions during crustal melting the mct which corresponds to the boundary between the mottled and the narrow banded migmatites suggests the segregation of melt within the partially melted system along with the compaction of matrix thus the mct is deemed to be the transition of the partially melted system from matrix supported melt stored in pore space to framework supported melt stored in foliation bedding space of protolith the fmt corresponding to the boundary between the narrow and the wide banded migmatites denotes the inception of melting of the solid framework mechanically the fmt can be regarded as the transition of the framework from compaction to fusion and geochemically the transition from eutectic melting of quartz feldspars to that of refractory biotite the increasing melt due to framework melting after the fmt is basically limited within the solid framework until the melt fraction exceeding the slt the slt is geologically constrained by the in situ metatexite diatexite transition that is also termed as magma interface mi the slt mi represents the upper limit of a crustal magma layer because of the downward increasing temperature and melt fraction there should be no consecutive layer of felsic rock existed in the space between the slt mi and the mafic lower crust except the isolated rock blocks within the magma layer

in situ major and trace element compositions of apatite from the yangla skarn cu deposit southwest china implications for petrogenesis and mineralization abstract the yangla copper deposit is a rare triassic intrusion related cu skarn system in the central part of the jinshajiang tectonic belt central sanjiang orogenic belt southwest china major and trace element compositions of apatites from ores fertile calc alkaline intrusions and its mafic microgranular enclaves and barren calc alkaline intrusions from the yangla skarn copper deposit were measured by electron probe microanalysis and laser ablation inductively coupled plasma mass spectrometry results indicate that apatites grown within ores fertile intrusions and its enclaves show similar features but are different from that grown within barren intrusions apatite from fertile intrusions has higher ca and correspondingly lower total trace element contents which partition onto the ca sites relative to that from barren intrusions apatites from the barren intrusions have high f and low cl contents and correspondingly high f cl ratios which may have resulted from the assimilation of sediment apatites from fertile intrusions show relatively high cl and low f contents which may reflect crystallization from magmas with high water contents redox sensitive elements in apatite can be used as indicators of the oxidation state of magmas apatites from the barren intrusion have lower v and as contents higher mn fe and u contents and more pronounced negative eu anomalies than those from fertile intrusion indicating that the barren magma was more reduced consistent with its lower whole rock fe2o3 feo and zircon ce4 ce3 ratios these findings demonstrate that apatites are effective in distinguishing fertile from barren intrusions and can be used as a robust tool in mineral exploration

deformation of partially molten granite a review and comparison of experimental and natural case studies experimental and natural investigations of partially molten granite are compared and reviewed experiments suggest that deformation of partially molten granite with low viscosity melt 104 pa s exhibits a rheological critical melt percentage rcmp in case of high viscosity melts 108 pa s however the relationship between melt fraction and log viscosity of the partially molten granite may be linear considerations about viscosity rheological thresholds and segregation of natural melts suggest that low viscosity melt experiments simulate natural conditions more realistically therefore an rcmp is to be expected under natural conditions both diffusion creep and dislocation creep may occur under natural conditions whereas cataclastic flow is only observed under experimental conditions a melt induced transition from dislocation creep to diffusion creep occurs under experimental and natural conditions melt topology is controlled by the magnitude of differential stress under experimental conditions if differential stress is higher than 100 150 mpa melt pockets are elongate and oriented at a low angle to the maximum compressive stress in contrast in nature melt pockets tend to be oriented subparallel to the foliation plane i e presumably at a high angle to the maximum compressive stress

geology fluid inclusions and isotope geochemistry of the herenping gold deposit in the southern margin of the yangtze craton china a sediment hosted reduced intrusion related gold deposit abstract the herenping gold deposit is a typical gold deposit in the xuefengshan arcuate tectonic zone in the southern margin of the yangtze craton and the major feature of the gold mineralisation in the zone is the varying degrees of tungsten and antimony mineralisation in the ore deposits the herenping gold deposit occurs in the reddish purple sandy slate in the middle section of the madiyi formation of the neoproterozic banxi group and is controlled by the fault fold zone associated with the liulincha anticlinorium and f8 fault zone however its sources of mineralisation and relationships between hydrothermal fluids and magmatic rocks are unclear which similar to those of many gold deposits in metasedimentary terranes in the worldwide the mineralisation can be divided into four stages an albite quartz stage i an albite quartz pyrite gold stage ii a quartz gold polymetallic sulphide stage iii and a quartz carbonate stage iv the gold mineralisation occurred in albite quartz veins in stage ii and in sulphide rich quartz veins in stage iii in the deposit a study of the fluid inclusions reveals that the homogenisation temperature and salinity were slightly higher in stage ii th 186 348 c w 5 2 17 43 wt nacl equiv than in stage iii th 145 249 c w 2 9 15 57 wt nacl equiv indicating that the temperature and salinity of the ore forming fluid are gradually decreasing the h o d 62 3 to 53 0 18o 4 2 11 8 isotope characteristics reveals that the ore forming fluid is a magmatic hydrothermal source the sulphur 34s 4 8 2 0 isotope characteristics and the alteration mineral assemblage sericite chlorite pyrite indicates that the ore forming fluids was reduced the lead 206pb 204pb 17 419 17 436 207pb 204pb 15 519 15 535 and 208pb 204pb 37 86 37 90 isotope characteristics reflect the ore forming materials were likely derived from a deep magmatic source in addition the geochemical characteristics of albitite veins show that they are silicon rich 64 02 66 26 wt and significantly reduced notably the herenping gold deposit is located in the western portion of the tungsten tin metallogenic provinces in south china is host in epimetamorphic detrital rocks and was formed at least two stages of mineralisation typical gold bearing albite quartz veins developed during the primary stage these characteristics are consistent with those of reduced intrusion related deposits such as the muruntau kumtor and telfer deposits thus the herenping gold deposit may be a sediment hosted intrusion related gold deposit based on the phase transition pattern from sulphide rich quartz veins to albite quartz veins to albitite veins with increasing depth in the deposit we propose that reduced intrusions deep in the herenping gold deposit could be discovered in the future which implies the possible existence of a reduced intrusion related gold system in the xuefengshan arcuate tectonic zone

fluid inclusions and h o s pb isotope systematics of the baishan porphyry mo deposit in eastern tianshan china abstract the baishan porphyry mo deposit formed in the middle triassic in eastern tianshan xinjiang northwestern china mo mineralization is associated with the baishan monzogranite and granite porphyry stocks mainly presenting as various types of hydrothermal veinlets in alerted wall rocks with potassic phyllic propylitic and fluorite alteration the ore forming process can be divided into four stages stage i k feldspar quartz pyrite veinlets stage ii quartz molybdenite pyrite veinlets stage iii quartz polymetallic sulfide veinlets and stage iv barren quartz calcite veins four types of fluid inclusions fis can be distinguished in the baishan deposit namely liquid rich two phase l type vapor rich two phase v type solid bearing multi phase s type and mono phase vapor m type inclusions but only the stage i quartz contains all types of fis the stages ii and iii quartz have three types of fis with exception of m type in stage iv quartz minerals only the l type inclusions can be observed the fis in quartz of stages i ii iii and iv are mainly homogenized at temperatures of 271 468 c 239 349 c 201 331 c and 134 201 c with salinities of 2 2 11 6 wt nacl equiv 1 1 10 2 wt nacl equiv 0 5 8 9 wt nacl equiv and 0 2 5 7 wt nacl equiv respectively the ore forming fluids of the baishan deposit are characterized by high temperature moderate salinity and relatively reduced condition belonging to a h 2 o nacl ch 4 co 2 system hydrogen and oxygen isotopic compositions of quartz indicate that the ore forming fluids were gradually evolved from magmatic to meteoric in origin sulfur and lead isotopes suggest that the ore forming materials came predominantly from a deep seated magma source from the lower continental crust the mo mineralization in the baishan deposit is estimated to have occurred at a depth of no less than 4 7 km and the decrease in temperature and remarkable transition of the redox condition from alkalinity to acidity of ore forming fluids were critical for the formation of the baishan mo deposit

experimental deformation of partially molten westerly granite under fluid absent conditions with implications for the extraction of granitic magmas the mechanical behavior of partially molten westerly granite was investigated in the temperature range 800 1100 c 250 mpa confining pressure by means of constant strain rate creep and stress relaxation tests the only water in the samples came from the breakdown of hydrous phases biotite minor chlorite and muscovite and alteration products of feldspars thus the amount of melt was controlled by the test temperature and ranged from 3 at 800 c to 50 at 1100 c over that temperature range strength decreased from 500 mpa to less than 1 mpa and a preliminary constitutive flow law for the partially molten rock was obtained to allow extrapolation to low strain rates the comparative viscosity of the melt alone was estimated at 950 and 1000 c from the distance it could be made to penetrate into a porous sand under a known pressure gradient under all conditions deformation of the matrix of solid grains was by brittle fracture only samples containing up to 10 vol melt failed with the formation of a shear fault zone at higher melt fractions melt filled pores collapsed by shear enhanced compaction squeezing the melt into axial cracks above 40 vol melt unfractured solid grains were carried about passively in the flowing liquid there was no evidence of a rheologically critical melt percentage in this system by analogy with the uniaxial compaction of water saturated soils a simple model is erected to describe a two stage process for the extraction of granitic melts from their protoliths with the aid of nonhydrostatic stress shear enhanced compaction is inferred to drive melt into a network of melt filled veins whereupon porous flow through the high permeability vein network allows rapid drainage of melt to higher crustal levels

country rock contamination of magmas associated with the baogutu porphyry cu deposit xinjiang china abstract the baogutu porphyry cu deposit located in the west junggar region xinjiang china has the reduced mineralization features baogutu is associated with a late carboniferous intrusive complex that was emplaced into lower carboniferous volcano sedimentary sequences the complex consists of a main stage diorite stock and late stage diorite porphyry dikes the former hosts the most of the cu mineralization the main stage stock has a wide range of composition from minor gabbro through dominant diorite to tonalite porphyry these are all inhomogeneous over a few meters with no distinct boundary between the gabbro diorite and tonalite porphyry some felsic microgranular enclaves and heterogeneous phenocrysts occur in these rocks the latter is a single mineral with a range of compositions the elemental and sr nd compositions of the tonalite porphyry have a transitional character from the gabbro and diorite to the country rocks these findings suggest possible country rock contamination mineral composition data suggest that the primary magma at baogutu is oxidized i type magma the gabbro and diorite have high positive e nd t 3 3 to 6 0 and low initial 87 sr 87 sr ratios 0 7035 0 7038 and are characterized by moderately fractionated rare earth element ree patterns pronounced nb depletion and weak negative eu anomalies these observations suggest that the baogutu magmas were derived from partial melting of a metasomatized mantle wedge and underwent significant country rock contamination after emplacement the contamination process involved the felsic components and reduced materials e g carbonaceous sediment which caused the chemical variation of the baogutu rocks and the reduced mineralization features in the baogutu deposit thus contamination played an important role in the formation of the host rocks and associated mineralization at baogutu

metallogenic efficiency from deposit to region a case study in western zhejiang province southeastern china abstract metallogenic ore efficiency can be defined as the ratio of metal reserves to its total supply and we expanded a typical efficiency calculation model available at the single mineral scale to the regional scale a new cell based model was developed by analogy and by taking some other influences into account i a thicker sedimentary cover ii regional ore and rock controlling structures and their intersections iii the crustal heterogeneity unrelated and related to ores and iv metallic geochemical anomalies finally a regional contour map of weighted efficiency which is essential to metallogenic prediction was obtained it shows that the efficiency anomalies can provide a much better target area for ore positioning than do metallic content anomalies and greater ore efficiency may indicate the likelihood of occurrence of mineral deposits that are larger in size reserves taking northwestern zhejiang province as a case study in which the geochemistry of mesozoic porphyries plausibly show considerable mineralized potential we found that due to the lower magmatic temperature delayed exsolution of the hydrothermal solution and very thick sedimentary cover the cu polymetal ore efficiency associated with granitic plutons in this area is generally low the relatively higher ore productivity efficiency appears mainly in and around small rock bodies rock margins and the contacting zone between different lithologies most ore spots no matter their size fall into areas with nonzero efficiency values there seems to be no interdependence between most ore spots of smaller size and ore efficiency whereas efficiency is essential to regional small and medium scaled metallogensis

an experimental deformation study of partially molten amphibolite application to low melt fraction segregation an amphibolite has been experimentally deformed under subsolidus and partially molten conditions to evaluate the influence of melt on the mechanical behavior of a natural mafic rock and to assess low melt fraction segregation processes experiments were performed under fluid absent conditions at 1 8 gpa between temperatures of 650 c and 1000 c these conditions are similar to those of thickened lower continental crust or intermediate depths in subducting oceanic lithosphere at 850 c melt is granitoid in composition and its viscosity is that of wet granite 103 105 pa s the results can be summarized as follows 1 under subsolidus conditions tests at 650 c and 750 c the amphibolite is macroscopically ductile and deformation is homogeneously distributed throughout the sample 2 at near solidus conditions 800 c 0 5 vol melt fractures 1 10 m in width displace hornblende and plagioclase grains and melt formed in situ is found in some of these cracks the formation of a ductile shear zone in one sample is attributed to the presence of very fine grained reaction products from combined dehydration hydration reactions that involve plagioclase the dehydration reaction products have apparently changed the deformation mechanism and even overrode the melt embrittlement process trapping melt in pockets of lower strain with higher melt fractions 10 15 vol broad melt bearing shear zones form and grains within these zones are brittlely deformed 3 at 20 vol melt 1000 c additional weakening occurs but fractures are not observed and the overall deformation is by viscous flow the results show that at low melt fractions 15 vol fluid absent melting reactions can induce fracture in previously ductile rocks this suggests that the percentage at which melt may escape by fracture is lower than the theoretical critical melt fraction or cmf of 26 40 vol in general estimating the fraction of melt at the onset of segregation cannot be predicted by the cmf melt segregation models have to be adapted to variations in such major factors as pressure temperature type of melting reaction rate of melting and strain rate such models will greatly assist our understanding of continental growth and evolution during orogenesis

strain partitioning during partial melting and crystallizing felsic magmas abstract the rheology of partly molten felsic systems changes dramatically during the onset of melting and through crystallization solid to liquid and liquid to solid transitions are not rheologically identical contradicting the concept of a unique rheological critical melt percentage we use four thresholds separating the successive stages according to the melt fraction for the melting transition 1 the liquid percolation threshold connects the melt pockets at about 8 of melt thus allowing local magma displacement at the metric scale and 2 for greater melt percentages 20 25 a melt escape threshold corresponds to the onset of transport over large distances of the melt and part of the residual solid phase leading ultimately to granitic bodies for the solidification transition 3 the first formation of a rigid framework of crystals which can accumulate stress although the melt can still flow corresponds to the rigid percolation threshold 55 solids and 4 the system becomes totally locked at about 72 75 solid which corresponds to the particle locking threshold the present paper focuses on strain partitioning induced by the coexistence of two contrasting rheological phases between these thresholds on a variety of scales non coaxial deformation is preferentially accommodated by the lower viscosity melt which increases the vorticity in the melt phase and reinforces asymmetries in the flow pattern strain partitioning also increases the scale length of the weakest phase during melting and magma ascent whereas strain partitioning decreases the scale length during crystallizing strain partitioning also affects the migration of melt by raising the particle locking threshold and lowering the melt escape threshold for three dimensional deformations such as transpression the direction of melt segregation e g horizontal may differ from the direction of melt migration e g vertical

geochronology of early mesozoic diabase units in southwestern china metallogenic and tectonic implications two phases of diabase sill forming magmatism are recorded within the badu anticline where magmas were emplaced into upper palaeozoic carbonates and clastic rocks of the youjiang fold and thrust belt in the sw south china block china zircons from these diabase units yield weighted mean u pb ages of 249 2 2 0 ma and 187 1 3 3 ma and magmatic oxygen fugacity values from 20 to 6 average of 12 equating to fmq 5 and 20 to 10 average of 15 equating to fmq 2 respectively these data indicate that the sills were emplaced during early triassic and early jurassic times the discovery of c 250 ma mafic magmatism in this area was probably related to post flood basalt extension associated with the emeishan mantle plume or rollback of the subducting palaeo tethys slab the c 190 ma diabase sills indicate that the southwestern south china block records early jurassic mafic magmatism and lithospheric extension that was likely associated with a transition from post collisional to within plate tectonic regimes the emplacement of diabase intrusions at depth may have driven hydrothermal systems enabling the mobilization of elements from sedimentary rocks and causing the formation of a giant epigenetic metallogenic domain the results indicate that high oxygen fugacity materials within basement rocks caused crustal contamination of the magmas contributing to the wide range of oxygen fugacity conditions recorded by the au bearing badu diabase in addition data from inherited xenocrystic zircons within the badu diabase and detrital zircons from basement rocks suggest that the neoproterozoic jiangshao suture extends to the south of the badu anticline

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

the formation mechanism of reduced porphyry mo deposits in the west junggar region xinjiang the suyunhe example abstract some porphyry mo deposits with relatively reduced ore assemblages characterized by high reduced gases e g ch4 and a lack of magnetite and hematite are generally considered as the reduced porphyry mo deposits nevertheless the role of oxygen fugacity fo2 in controlling mo enrichment and precipitation of molybdenite in such reduced porphyry mo deposits is poorly understood the suyunhe reduced porphyry mo deposit with a mo metal reserve of 0 57mt at an average grade of 0 05 0 09 located in the west junggar terrane in xinjiang is associated with early permian granitic rocks which emplaced into the volcano sedimentary sequences of the middle devonian barluk formation in this study we report that the variation of magmatic fo2 the possible mechanism of mo enrichment and the contributions of the reduced gases or matter to molybdenite precipitation results indicate that the primitive magma is still oxidized with fo2 nno ni nio oxygen buffer but it is then reduced due to contamination by the reduced sequences furthermore the decreasing magmatic fo2 does not significantly prevent the enrichment of mo metals during magmatic evolution by contrast the evolved parental magma characterized by highly differentiated and fluorite rich is responsible for improving mo concentration of melts most of reduced gases are derived from decomposition of organic matter occurring in the reduced volcanic sedimentary rocks of barluk formation more importantly these reduced gases e g ch4 play a role in molybdenite precipitation which may effectively improve the efficiency of metal precipitation

melt segregation in the continental crust before melt will segregate from its restite and source there must be both a driving force and space into which the melt can move the continental crust is anisotropic and in most cases is subject to deviatoric stresses during melting under these conditions melt segregation begins when an interconnected network of melt forms and the source becomes permeable melt is driven by pressure gradients from its restite toward dilatant sites set up in the heterogeneously deforming source the source is continually drained of melt and so need never contain more melt than that required to maintain permeability 26 where the solid matrix breaks down and diapirism can occur

the origin of ch4 rich fluids in reduced porphyry skarn cu mo au systems abstract although most of the methane on earth is of biogenic origin significant abiogenic methane has been identified in a variety of geological environments moreover the presence of methane has been reported recently in fluid inclusions from a number of reduced porphyry deposits the genesis of this methane however remains poorly understood here we present results of a study of magma exsolved ch4 rich aqueous fluid inclusions in the seleteguole reduced porphyry skarn cu mo au deposit nw china four types of fluid inclusions namely intermediate density two phase aqueous inclusions low density vapor inclusions brine inclusions and aqueous liquid inclusions have been distinguished on the basis of the phases present at room temperature the intermediate density inclusions were trapped as a single phase and contain 3 8 10 4 mol kg ch4 corresponding to an oxygen fugacity between fmq 1 and fmq 0 5 this attests to the remarkably reduced nature of the fluid the 13c value of the ch4 hosted fluid inclusions ranges from 29 4 to 19 1 pdb which distinguishes this methane from biogenic methane indeed the high homogenization temperature of the ch4 rich inclusions 400 c implies that the ch4 was exsolved directly from the magma given the environment in which the magma was likely generated i e at or immediately above a subducting plate we propose that the methane was produced by reactions involving organic matter bearing carbonate rocks in the subduction zone although cu mo and au in most porphyry systems are interpreted to have been transported under oxidizing conditions our results indicate that reduced aqueous liquids and vapors are also capable of transporting appreciable concentrations of these metals

methane origin and oxygen fugacity evolution of the baogutu reduced porphyry cu deposit in the west junggar terrain china most porphyry copper deposits worldwide contain magnetite hematite and anhydrite in equilibrium with hypogene copper iron sulfides chalcopyrite bornite and have fluid inclusions with co2 ch4 that are indicative of high fo2 in contrast the baogutu porphyry cu deposit in the west junggar terrain xinjiang china lacks hematite and anhydrite contains abundant pyrrhotite and ilmenite in equilibrium with copper iron sulfides chalcopyrite and has fluid inclusions with ch4 co2 that are indicative of low fo2 the mineralized intrusive phases at baogutu include the main stage diorite stock and minor late stage diorite porphyry dikes the main stage stock underwent fractional crystallization and country rock assimilation contamination and consists of dominant diorite and minor gabbro and tonalite porphyry the country rocks contain organic carbons 0 21 0 79 wt the 13cvpdb values of the whole rocks 23 1 to 25 8 in the wall rocks suggest a sedimentary organic carbon source the 13cvpdb values of ch4 28 2 to 36 0 and co2 6 8 to 20 0 in fluid inclusions require an organic source of external carbon and equilibration of their 13cco2 ch4 values 8 2 25 0 at elevated temperatures 294 830 c suggesting a significant contribution of thermogenic ch4 mineral composition data on the main stage intrusions such as clinopyroxene hornblende biotite magnetite ilmenite sphene apatite and pyrrhotite suggest that the primary magma at baogutu was oxidized and became reduced after emplacement by contamination with country rocks mineral compositions and fluid inclusion gas compositions suggest that the redox state of the system evolved from logfo2 fmq 1 in the magma stage to logfo2 fmq in the hydrothermal stage though oxidized magma was emplaced initially assimilation contamination of carbonaceous country rocks decreased its fo2 such that exsolved fluids contained abundant ch4 and deposited a reduced assemblage of minerals

structural controls on the archean troilus gold copper deposit quebec canada the troilus gold copper deposit is a low grade high tonnage resource hosted in archean calc alkaline intrusive rocks in the superior province of quebec it is best known for being one of the few archean analogues of a porphyry cu au deposit an interpretation with significant implications for both mineral exploration and theories of archean plate tectonics recent research presented here has called into question the prevalent genetic model and proposes instead a two stage history of mineralization controlled by contrasting structural regimes developed as part of an orogen disseminated gold copper mineralization and associated potassic alteration occur in a deformation corridor between two felsic dikes emplaced prior to the peak of regional metamorphism postpeak metamorphic high strain sericitic zones with silicified envelopes host quartz veins and a second phase of gold the present size grade and disposition of the orebody is the result of the superposition of these two events an interpretation that impacts on the genetic model as well as exploration exploitation and grade control

magmatic hydrothermal fluids at the sedimentary rock hosted intrusion related telfer gold copper deposit paterson orogen western australia pressure temperature composition constraints on the ore forming fluids the neoproterozoic telfer deposit one of australia s largest gold copper deposits is located in the paterson orogen several highly differentiated calc alkaline to alkali calcic peraluminous granites intruded the metasedimentary rocks near 5 20 km telfer contemporaneously with structurally controlled gold copper mineralization fluid inclusion assemblages with different fluid inclusion types were identified in samples from a range of different vein types these inclusion types range from three phase aqueous laq vaq shalite high salinity 50 wt nacl equiv high temperature 460 c inclusions to two phase aqueous or two phase aqueous carbonic low to moderate salinity 2 22 wt nacl equiv moderate to high temperature 480 c fluid inclusions fluid inclusion trapping mechanisms and interpreted precipitation mechanisms for gold and copper include 1 adiabatic cooling between 450 and 200 c in all veins and 2 locally fluid phase separation at about 300 c the trapping pressure of fluid inclusion assemblages trapped during phase immiscibility was calculated to be approximately 1 5 kbar for fluid inclusion assemblages that lack evidence for phase immiscibility a pressure at the temperature of final homogenization of up to 3 kbar was calculated this high pressure value is interpreted to be related to local fluid overpressure as a consequence of fault zone movement in faults and fractures that localized gold at telfer phase immiscibility and gold precipitation were induced during sharp pressure decrease accompanying fault zone movement in situ laser inductively coupled plasma mass spectrometry icp ms analyses of fluid inclusions revealed high trace element contents in all fluid inclusion assemblages manganese fe ratios of 0 24 is observed given the high temperatures and salinities of up to 480 c and 42 wt nacl equiv au and cu were likely transported as chloride complexes this interpretation is supported by the observation that the highest base metal contents occur in the highest salinity fluid inclusion potassium ca ratios of 1 in most assemblages the high homogenization temperatures 480 c in many fluid inclusion assemblages and the high trace element contents e g fe mg k na in most of the fluid inclusion assemblages are compatible with involvement of a magmatic hydrothermal fluid during gold copper mineralization this fluid was probably derived from the coeval granites in the telfer area and thus telfer is interpreted to be a distal intrusion related metasedimentary rock hosted gold copper deposit type

shear criteria in granite and migmatite deformed in the magmatic and solid states microstructural criteria for the determination of the sense of shear in rocks homogeneously deformed in the partially melted state are similar to those which apply to solid state deformation sense of shear determination is either direct deduced from the sense of rotation of markers or indirect involving the obliquity between the shear and foliation planes or between the successive foliations imprinted at different stages of progressive deformation this study is a by product of the detailed structural and microstructural investigation of a high grade metamorphic rock pile variscan vosges massif france which underwent subhorizontal shearing during partial melting and further solidification depending on the rock chemistry on the position in the pile and the relative timing of progressive deformation layered migmatites and homogeneous granites were variously deformed in the partially melted and solid states the sense of shear obtained from these rock types using the criteria presented here consistently gives a top to sw direction

magmatic oxidation state of the baogutu porphyry copper deposit in the west junggar of china implication for ore formation abstract the ore forming processes in the baogutu porphyry copper deposit are spatially and temporally related to the late carboniferous intrusions in the west junggar of china the mineralized intrusions consisting of granodiorite quartz diorite and diorite are characterized by moderate zircon ce4 ce3 ratios 15 9 163 and eu eu ratios 0 25 0 57 eu eu eun smn gdn 1 2 estimated magmatic oxidation state based on amphibole crystallized in the deep magma chamber logfo2 fmq 1 7 is lower comparing with that based on amphibole crystallized in the shallow magma chamber logfo2 fmq 3 5 the baogutu ore forming magmas were moderately oxidized with an average logfo2 value of fmq 2 5 based on amphibole oxygen barometer or fmq 2 7 based on magnetite ilmenite oxygen barometer these data indicate that the baogutu ore forming magmas were moderately oxidized

rheology of crystal bearing silicate melts an experimental study at high viscosities the viscosity of partially crystallized mg3al2si3o12 melts has been measured under uniaxial compression in the interval 1010 1013 poise as a function of the volume fraction of crystals these inclusions are well rounded spherulites of aluminous enstatite having the same composition as the melt and whose growth rate is negligible at the temperature of the measurements the viscosity increases by less than 1 order of magnitude for crystal fractions of 40 vol and remains newtonian up to the maximum stress exerted namely 1 kbar the einstein roscoe equation 0 1 m n provides very good fits to the measurements only if either the m or n parameter is allowed to depend on temperature for modeling of magmatic processes however the widely recommended constant values m 0 6 and n 2 5 should be adequate the rheology changes abruptly when the clustered spherulites begin to oppose shear deformation at a crystal fraction of about 40 vol the viscosity becomes non newtonian with yield strengths of a few tens of bars at temperatures at which the viscosity of the melt is higher than 1010 poise as long as the crystal fraction remains lower than 70 vol the deformation proceeds in an irregular manner with a nonuniform distribution of crystals and melt the deformation becomes again regular at low stresses with lower melt fractions but samples undergo extensive fracturation along the direction of uniaxial stress similar rheology changes have been observed during the isothermal crystallization of li2si2o5 melts which produces small ellipsoidal inclusions these results suggest that the influence of solid suspensions on the rheology of magmas is primarily determined by the crystal fraction even though additional measurements would be useful to determine the possible influence of other factors such as the size distribution or the shape of the inclusions

the effects of porphyroblast growth on the effective viscosity of metapelitic rocks implications for the strength of the middle crust numerical models are used to examine the effects of porphyroblast growth on the rheology of compositionally layered rocks metapelites and metapsammites and by extension the middle crust during prograde metamorphism as porphyroblast abundance increases during prograde metamorphism metapelitic layers will strengthen relative to porphyroblast free metapelitic units and potentially relative to quartzofeldspathic metapsammitic units as metapelitic layers become stronger the integrated strength of compositionally layered successions increases potentially causing large volumes of mid crustal rock to strengthen altering the strain rate distribution in the middle crust and affecting the geodynamic evolution of an orogenic belt the growth of effectively rigid porphyroblasts creates strength heterogeneities in the layer undergoing porphyroblast growth which leads to complex strain rate distributions within the layer at the orogen scale the strengthening of large crustal volumes on the order of thousands of cubic kilometres changes the strain rate distribution which may change exhumation rates of high grade metamorphic rocks the geothermal structure and the topography of the orogen the presence of a strong zone in the middle crust causes strain rate partitioning around the zone suppressed uplift rates within and above the zone and leads to the development of a basin on the surface

c o h n salt fluids associated with contact metamorphism mcgerrigle mountains quebec a raman spectroscopic study abstract contact metamorphism and metasomatism of a sequence of cambro ordovician pelitic calcareous and psammitic metasediments in the thermal aureole of the devonian mcgerrigle pluton has resulted in the generation of a wide variety of fluid compositions which can be described by the system c o h n salt microthermometric and raman spectroscopic data on fluid inclusions trapped in quartzsulfide veins adjacent to the intrusive contact indicate that three fluid types existed in the aureole 1 h 2 o 2 h 2 o co 2 and 3 ch 2 n 2 co 2 the aqueous fluids have salinities which range from 0 to 50 eq wt nacl cacl 2 this range in fluid compositions resulted from the progressive devolatilization of a variety of lithologies and the release of waters from the intrusion the range in salinity of the aqueous fluids is attributed to variable contributions of high salinity waters from the intrusions and lower salinity dehydration and formation waters some of the co 2 may be orthomagmatic however most of it probably came from decarbonation of calcareous and possibly carbonaceous lithologies ch 4 and n 2 were generated in pelitic units during thermal metamorphism ch 4 from reactions involving bitumen and graphite and n 2 from nh 2 in silicate minerals and possibly from bitumen the restriction of high mole fractions of ch 4 and n 2 to water free fluid inclusions can be explained by fluid immiscibility within the fracture system the partitioning of co 2 between water poor and water rich fluids was less extreme and in some cases co 2 and h 2 o may have been completely miscible

how do metals escape from magmas to form porphyry type ore deposits abstract many precious and base metals cu au ag mo w sn in porphyry type deposits are extracted from a granitic to granodioritic silicate melt and transported by magmatic hydrothermal fluids to the site of ore deposition metals abundance increases from some ppb in the melt to in ore bodies assuming a unique magmatic source for metals it implies an exceptional combination of enrichment process acting over up to 4 orders of magnitude previous models of magma differentiation and hydrothermal circulations omitted to consider the role of metal segregation during magma chamber evolution models for the formation and evolution of intrusions have recently shifted from the so called melting storage assimilation homogenization mash paradigm basically steady state to the mantle melting segregation ascent emplacement m m sae paradigm dynamic and discontinuous in time successive magma injections of variable compositions progressively build the magma chamber this is supported by field relations e g cross cutting dykes and stocks textures e g partially resorbed enclaves or mineral fabrics geochemistry e g hybridization signatures and age dating aswell as istopic studies as melt crystallizes and forms a mush 50 crystals volatiles also exsolve forming a magmatic volatile phase mvp while melt motion slows down metals partition between the three phases melt crystals and mvp they generally prefer the mvp owing to more favorable partition coefficients this is indicated by metal content in coeval melt and fluid inclusions with homogenization temperatures above 600 c or in volcanic fumaroles here by simulating the physical interactions between the three phases we suggest a model of fluid sparging during which metals segregate towards the mvp by diffusion and are further transported by advection as metal complexes the model also provides estimates of metal enrichment an undimensional peclet number rules the competition between diffusion and advection basically scaling with respect to the inverse of the product of melt viscosity by metal diffusivity d the threshold value of the peclet number between diffusion and advection is roughly 10 9 fast diffusive metals au cu ag w readily diffuse from silicate melt and towards bubbles of the mvp to escape the mush through tubular structures the mvp must overcome a critical gas saturation level about 20 vol usually reached after several magma injections advection then takes over and transports the metal enriched mvp towards the top of the magma chamber this leads to nearly coeval 1 separation between a high salinity liquid phase and a low salinity vapor phase 2 fluid rock interactions resulting in potassic advanced argillic and phyllic alterations and 3 metal deposition the metal enrichment scales as the ratio between partition coefficient i e related to the gradient in chemical potential diffusivity i e related to the gradient of concentration and melt viscosity i e related to the gradient of momentum the rates at which all such gradients relax determe metals enrichment inducing chemical and physical instabilities leading to a cyclic process the whole cycle also encompasses the case of a partial non completed full enrichment yielding to barren intrusions a tentative model generalizes the sparging fluid model to other metal deposits linked to an intrusion such generalization should be interpreted as predicting metal enrichment by 3 4 orders of magnitude rather than predicting an exact value

reduced fluids in porphyry copper gold systems reflect the occurrence of the wall rock thermogenic process an example from the no 1 deposit in the xiongcun district tibet china abstract high fo2 conditions characterize the majority of global porphyry copper deposits and contain highly oxidized minerals such as magnetite and anhydrite in contrast the no 1 porphyry cu au deposit in the xiongcun district tibet china has abundant pyrrhotite reduced fluids ch4 co2 and a relative lack of highly oxidized minerals which are indicative of low fo2 conditions scanning electron microscopy fluid inclusion c h o he ar isotopes and whole rock organic carbon contents and isotope analysis were used to constrain the evolution of ore forming fluid the origin of ch4 and metal deposition mechanisms for the no 1 deposit the he ar isotopic compositions 3he 4he 0 11 0 96 ra 40ar 36ar 418 7 2920 2 suggest that the ore forming fluids predominantly derived from crust source with minor mantle input the h o isotopic analysis results 18oh2o 1 8 to 5 2 d 106 to 89 9 indicate that the ore forming fluids were derived from a magmatic source that mixed with some meteoric waters the element compositions of zircons and fluid melt inclusions from the mineralized middle jurassic quartz diorite porphyry reveal that the primary magma was characterized by high log fo2 nno conditions the quartz diorite porphyry intruded into the carbon bearing wall rocks produced abundant ch4 by thermal decomposition of organic matter 13cch4 26 3 to 28 5 which changed the redox state of the porphyry copper system from oxidized to reduced condition ore elements were deposited via fluid boiling as a consequence of the rapid decrease in temperature and pressure

redox controlled generation of the giant porphyry cu au deposit at pulang southwest china some porphyry cu au deposits with relatively reduced ore assemblages characterized by high hydrothermal pyrrhotite contents and a lack of primary hematite and magnetite are generally considered to be associated with reduced i type granitoids however the role of magmatic oxygen fugacity fo2 in controlling cu au mineralization in such reduced porphyry deposits is poorly understood the giant late triassic ca 216 ma pulang porphyry cu au deposit of southwest china shows typical reduced ore assemblages this study reported the systematical variation of upper crustal magmatic fo2 of pulang deposit based on detailed investigations of mineral crystallization sequences and compositional features of the mineralization related porphyries early p1 and late p2 porphyry results indicate that magma of the mineralization related porphyries experienced complex fo2 fluctuations during its upper crustal evolution the early primary magma had very high initial fo2 with fmq 3 0 at depths of 12 km fmq is the deviation of logfo2 from the fayalite magnetite quartz fmq buffer the fo2 of evolved parental magma subsequently decreased with fmq 1 9 due to injection of relatively reduced dioritic magmas fmq 1 4 to 2 3 from a deeper chamber 17 21 km depth into the primary magma chamber at 10 12 km depth magma mixing had largely ceased at 6 10 km depth the parental magma then ponded within the reduced tumugou formation at a depth of 3 7 km where magmatic fo2 decreased to a moderately oxidized state fmq 1 6 and finally to a moderately reduced state reflected by log fe2o3 feo ratios of 0 5 for p2 porphyry results of this study of magmatic fo2 indicate that porphyry magmas associated with reduced pulang ore assemblages were initially generated as highly oxidized magma which was subsequently reduced through magma mixing and contamination by reduced sedimentary rocks of the tumugou formation the sharp fo2 decrease at very shallow depth prevented the early loss of cu and au because the magma remained oxidized until it was emplaced at 3 7 km depth moderately reduced magmas may thus have a genetic association with porphyry cu au mineralization

manaslu leucogranite a collision signature of the himalaya a model for its genesis and emplacement the manaslu pluton is one of 10 leucogranites that formed in the overthrusted higher himalaya after the indo eurasian collision field and analytical data indicate that the underlying migmatites of the tibetan slab may be where the leucogranitic melts were generated the himalayan crustal thrusting of a hot slab over a rather cold volcano sedimentary pile le fort 1975a provides the necessary release of fluids these fluids cross the main central thrust mct induce the partial anatexis of the overheated tibetan slab and produce a leucogranitic magma the emplacement of the magma at first is located along the main disharmonic plane above the mct between the infrastructure and the superstructure there it generates a convective hydrothermal system extending very far laterally according to the stratification of permeabilities progressive emplacement of the granite proceeds as the convected fluids including the fluids released by the saturated magma dissolve the mainly calcareous host rocks of tibetan sedimentaries a frozen image of this caving out progression is given by the extensive network of granitic dikes outside the pluton the two quite independant fluid cycles of the generation and of the emplacement were triggered or guided by the tectonics due to collision this two fold model dominated by fluid activity may be of importance for other leucogranites and granites

mineralogical evidence for crystallization conditions and petrogenesis of ilmenite series i type granitoids at the baogutu reduced porphyry cu deposit western junggar nw china mossbauer spectroscopy epm and la mc icpms analyses abstract primary ore forming minerals retain geochemical signatures of magmatic crystallization information and can reveal the petrochemical conditions prevalent at the time of their formation the baogutu deposit is a typical reduced porphyry cu deposit amphibole and biotite fe 3 fe ratios minerals feldspar biotite amphibole zircon and apatite in situ elemental and apatite nd isotopic compositions were determined by mossbauer spectroscopy electron probe microanalysis and laser ablation multiple collection inductively coupled plasma mass spectrometry respectively to investigate the magma oxidation state petrogenesis source features and to constrain the carbon species at magmatic stages for the intrusive phases the results show that the primary plagioclase and amphibole in the mineralized diorite to granodiorite porphyry and post ore hornblende diorite porphyry are distinct an 26 55 versus an 60 69 mg hornblende versus tschermakite in particular the amphibole shows distinct major and trace element compositions with light rare earth element enrichments and negative eu anomalies in mg hornblende and light rare earth element depletions and no eu anomalies in tschermakite all the analyzed biotites are primary igneous phases with a biotite phenocryst profile showing significant variations of zn cr sc and sr from core to rim these results may indicate the occurrence of mixing between two distinct magmas during mineral formation titanium in zircon and si in amphibole thermometries indicate that magma crystallized at 900 c and continued to 650 c in situ apatite nd isotope end t 5 6 7 6 t dm2 620 460 ma indicate absence of significant reduced sedimentary contamination and the source of juvenile lower crust slightly decreasing fe 3 fe ratios from biotite and amphibole to whole rock indicate decreasing oxygen fugacity during magma crystallization recalculated biotite compositions according to fe 3 fe ratios indicate f o 2 values of less than ni nio buffer nno which show slightly lower values than that estimated according to zircon melt distribution coefficients ce anomalies nno 0 6 these values are consistent with the features of reduced porphyry cu deposits crystallization of other mineral phases significantly affects the reliability of oxybarometer of zircon melt distribution coefficients eu anomalies and mn contents in apatite this oxidation state suggests that only co 2 was present at the magmatic stage and implies that ch 4 formed during co 2 reduction occurring later hydrothermal alteration the alteration of primary amphibole to actinolite released ti al fe mn na and k to the fluid with later precipitation of titanite albite and minor ilmenite and magnetite during actinolite alteration

metal fractionation between magmatic brine and vapor determined by microanalysis of fluid inclusions the major and trace element compositions of individual fluid inclusions from a range of magmatic hydrothermal ore deposits were analyzed by laser ablation inductively coupled plasma mass spectrometry to explore the behavior of ore forming components during fluid phase separation boiling in high temperature saline fluid systems data from 13 samples showing unambiguous evidence for coeval trapping of a liquid brine and a coexisting vapor phase identify two groups of elements with drastically different geochemical behavior na k fe mn zn rb cs ag sn pb and tl preferentially partition into the brine probably as cl complexes whereas cu as au probably as hs complexes and b selectively partition into the vapor fluid phase separation is probably a major previously underestimated process in the chemical differentiation that contributes to the extreme range of selective element enrichments in magmatic hydrothermal systems from deep plutons through porphyry style and greisen deposits to epithermal mineralization and volcanic fumaroles

a model for the rheology of particle bearing suspensions and partially molten rocks an edited version of this paper was published by agu copyright 2009 american geophysical union

the batalha au granite system tapajos gold province amazonian craton brazil hydrothermal alteration and regional implications abstract the tapajos gold province tgp is part of the 2 10 1 87 ga ventuari tapajos or tapajos parima tectonic province of the central amazonian craton the ventuari tapajos province resulted from an ocean continent orogeny and includes two calc alkaline magmatic arcs that are represented by the cuiu cuiu complex 2 1 1 90 ga and parauari intrusive suite 1 92 1 88 ga the parauari magmatic event was accompanied by sedimentation and intrusion of post tectonic 1 88 ga calc alkaline granites and was followed by subalkaline and alkaline a type magmatism related to the 1 87 ga uatuma event the 1880 ma batalha granite consists of metaluminous to peraluminous calc alkaline to sub alkaline hornblende biotite syeno to monzo granites and shows a late to post collisional affinity intense pervasive and fissural post magmatic hydrothermal alteration linked to gold mineralization affected the batalha granite the early alteration stage involved na metasomatism represented by crystallization of interstitial albite and replacement of plagioclase and k feldspar by albite precipitation of albite caused na impoverishment and k enrichment in the hydrothermal fluid leading to microcline and biotite crystallization and pervasive replacement of older feldspars by k feldspar pervasive and fissural propylitization overprints magmatic and early hydrothermal mineral associations and is characterized mainly by epidote clinozoisite chlorite albite carbonate and sulphide fissural to pervasive sericitization was the last hydrothermal stage and led to crystallization of sericite quartz and sulphide gold mineralization was related to the last two alteration stages most importantly sericitization the hydrothermal alterations were related to exsolved magmatic aqueous fluids 500 290 c and 2 6 1 2 kbar the batalha granite shares certain characteristics with both 1 porphyry related systems biotite halogen content large alteration haloes and base metals associated with gold and 2 intrusion related gold systems reduced environment depth of emplacement and gold mineralization associated with sericitic alteration classification of the batalha gold mineralization is thus not straightforward however calc alkaline to sub alkaline subvolcanic granites similar to batalha i e with identical hydrothermal alteration patterns are associated with paleoproterozoic quartz alunite high sulphidation volcanic hosted epithermal gold mineralization in the tgp implying coeval evolution of the hydrothermal systems thus the shallower and more oxidized batalha granite may have generated a porphyry cu au type deposit this has new implications for gold exploration in the tgp apart from the known gold deposits within shear zones there is high potential for low grade large volume gold and base metal deposits

metamorphism transient mid crustal rheology strain localization and the exhumation of high grade metamorphic rocks 1 we present a series of three dimensional numerical models investigating the effects of metamorphic strengthening and weakening on the geodynamic evolution of convergent orogens that are constrained by observations from an exposed mid crustal section in the new england appalachians the natural mid crustal section records evidence for spatially and temporally variable mid crustal strength as a function of metamorphic grade during prograde polymetamorphism our models address changes in strain rate partitioning and topographic uplift as a function of strengthening weakening in the middle crust as well as the resultant changes in deformation kinematics and potential exhumation patterns of high grade metamorphic rock results suggest that strengthening leads to strain rate partitioning around the zone and suppressed topographic uplift rates whereas weakening leads to strain rate partitioning into the zone and enhanced topographic uplift rates deformation kinematics recorded in the orogen are also affected by strengthening weakening with complete reversals in shear sense occurring as a function of strengthening weakening without changes in plate boundary kinematics

crustal anatexis and melt migrations in the higo metamorphic terrane west central kyushu kumamoto japan abstract this paper deals with the anatectic migmatites and leucogranites developed in a high t low p metamorphic terrane kyushu japan the migmatites in this terrane are divided into metatexites and diatexites and they occur at and above the garnet cordierite grade layers lenses and pods of leucogranite a few centimeters up to 6 m in thickness are wide spread in the metamorphic terrane and are most abundant not in the highest grade zone but in intermediate grade zones in which migmatites do not develop the migmatite leucosomes are typically depleted in orthoclase component while many leucogranites contain abundant orthoclase it is suggested that the leucogranites are crystal cumulates precipitated from peraluminous melts that had been generated and ascended from deeper levels in the crust and that the zone of metatectic migmatites represents both the source region and passage region for the anatectic melts it is thus clear that the anatectic melts had segregated and ascended on the order of 5 km in the crust of this region diatexites on the other hand represent solid melt mixtures at more advanced stages of partial melting dominated by biotite breakdown implications of the observations made in the higo terrane are discussed in reference to an evolutionary scheme for crustal anatexis and granite magma genesis

experimental determination of au solubility in rhyolite melt and magnetite constraints on magmatic au budgets the solubility of au metal in rhyolite melt and coexisting magnetite has been determined at 800 c 140 mpa and f 0 2 nno in a au metal saturated sulfur free vapor brine silicate melt system whole crystals of magnetite suspended within quenched rhyolite glass were analyzed by la icp ms these data yield a solubility of au in magnetite on the order of 2 g g the solubility of au metal in rhyolite melt hosting the magnetite crystals is on the order of 500 ng g these data indicate a partition coefficient for au between magnetite and melt d m t m e l l au 4 using reasonable estimates of the mass fraction of magnetite that crystallizes in crustal magmatic systems we modeled the proportion of au sequestered by magnetite during fractional crystallization we considered fractionation in two steps the idealized derivation of a rhyolite by fractionation of basalt at depth and the closed system crystallization of the rhyolite in a magma chamber below the site of ore deposition magnetite sequesters 14 54 au from basaltic parent melt that produces a rhyolite melt via crystal fractionation at modal abundances of 1 5 less then 4 au is sequestered from a crystallizing rhyolite melt because of low modal abundances of magnetite 2 our experimental and model results suggest that early crystallization of magnetite can play a mitigating role in the ability of a calc alkaline magmatic system to yield a au rich ore fluid

on the rheologically critical melt fraction abstract with increasing melt fraction the strength of partially molten granite decreases from a value characteristic of solid competent rock to a value nearly equal to that of the melt previously published mechanical and microstructural data indicate that deformation in partially molten rocks often involves brittle processes thus the pressure in the melt is expected to be important in determining strength when the volume changes during deformation strength and fluid flow will be coupled by such parameters as permeability k storage capacity per unit volume or storativity s melt compressibility f grain size d fluid viscosity and strain rate experiments on brittle rock at low temperatures show that the strain rate at which the internal fluid pressure can be maintained constant is approximately proportional to k s an evaluation of published experimental data suggests that this relation also holds for partially molten granites indicating that the strength of these rocks depends on their transport properties since the permeability is related to below a critical melt fraction rcmf the melt pressure will change if deformation is not iso volumetric by assuming a power law relationship between k and we estimate that rcmf f d m 1 n 1 where m and n relate permeability to grain size and respectively

nonlinear acoustic scattering from a gassy poroelastic seabed a model for difference frequency backscatter from trapped bubbles in sandy sediments was developed a nonlinear volume scattering coefficient was computed via a technique similar to that of ostrovsky and sutin nonlinear sound scattering from subsurface bubble layers in natural physical sources of underwater sound edited by b r kerman kluwer dordrecht 1993 pp 363 373 which treats the case of bubbles surrounded by water biot s poroelastic theory is incorporated to model the acoustics of the sediment biot fast and slow waves are included by modeling the pore fluid as a superposition of two acoustic fluids with effective densities that differ from the pore fluid s actual density and account for its confinement within sediment pores the principle of acoustic reciprocity is employed to develop an expression for the backscattering strength model behavior is consistent with expectations based on the known behavior of bubbles in simpler fluid media

zoning of mineralization in hypogene porphyry copper deposits insight from comb microfractures within quartz chalcopyrite veins in the hongshan porphyry cu deposit western yunnan sw china abstract the origin of zonal mineralization in porphyry copper deposit is important for understanding the mineralization processes we propose a new modified orthomagmatic genetic model for mineralization zoning in hypogene porphyry copper deposits this new model is based on the features and formation mechanism of comb microfractures in quartz chalcopyrite veins within pyrite vein from the hongshan porphyry copper deposit in zhongdian county western yunnan province sw china the main evidence for this model is volume expansion related to crystallization of chalcopyrite magnetite and k metasomatism in the deposit comb microfractures are well developed in quartz chalcopyrite veins and are present as comb quartz veinlets consisting of a zone of central longitudinal quartz overprinted by laterally grown quartz combs chalcopyrite fragments lie perpendicular to the central quartz veinlet and were dismembered by the quartz combs the combed microfractures are typical tensional hydrofractures the formation of the comb microfractures is related to volume expansion that was induced by crystallization of chalcopyrite from a chalcopyrite melt that resulted in the subsequent increase of volumetric pressure in the confined residual silica melt the formation mechanism of the comb microfractures including volume expansion induced by crystallization increases volumetric pressure hydrofracturing and fluid expulsion and was the most likely process for zoning of minerals in hypogene porphyry copper deposits fabrics in the veins and veinlets are consistent with overpressuring and injection and are common structures that are directly related to volumetric pressure and crystallization of chalcopyrite and magnetite and k metasomatism in hypogene porphyry copper deposits the volume expansion ratio of chalcopyrite mineral to melt and that of magnetite mineral to melt are approximately 19 vol and 20 vol respectively the volume expansion rate of a monomolecular lattice is 8 vol for orthoclase replacing plagioclase we suggest that zoning of mineralization in hypogene porphyry copper deposits is mainly related to hydrofracturing migration and distribution of ore forming fluids including 1 the enclosure and formation of a mush core 2 concentration of ore forming fluids in the orthoclase shell 3 k metasomatism and pressure building 4 hydrofracturing and migration of ore forming fluids and 5 prior crystallization of magnetite and chalcopyrite that will expel residual fluids upwards and outwards due to the change in volumetric pressure

age and petrogenesis of the andagul granodiorite and its implications on gold mineralization of the kassan region western kyrgyzstan tian shan abstract a number of significant gold deposits in the kassan region in west kyrgyzstan are associated with the andagul granitic intrusion these gold deposits share similarities with the class of intrusion related gold deposits implying that the gold mineralization may be genetically related to the intrusion in order to evaluate this potential relationship it is important to study the petrogenesis of the intrusion this paper examines the sources and physiochemical conditions of magma generation and crystallization of the andagul intrusion and their implications for gold mineralization through an integrated study involving zircon geochronology mineral chemistry whole rock chemistry and isotopic geochemistry the andagul intrusion consists mainly of granodiorite which is characterized by metaluminous and median k calc alkaline compositions high lree and lile low hfse right inclined ree pattern with a flat hree pattern and insignificant eu anomalies eu eu n 0 79 0 85 these geochemical characteristics together with relatively low positive end t values 2 1 4 3 moderately low 87sr 86sr i ratios 0 7049 0 7059 relatively low initial pb isotopic ratios 206pb 204pb t 17 76 18 51 207pb 204pb t 15 56 15 62 and 208pb 204pb t 37 89 38 33 juvenile ehf t 2 29 4 80 but relatively old model ages t2dm hf 1026 1175 ma t2dm nd 728 907 ma indicate that the magma was mainly sourced from partial melting of juvenile basaltic lower crust with the involvement of a small proportion ca 5 10 of precambrian basement rocks the high zr saturation temperatures 807 c to 825 c relatively high mg values 43 52 and presence of mafic enclaves and development of acicular apatite and oscillatory plagioclase and titanite suggest that the magmatism was related to injection of hot mantle material the magma generation and intrusion took place during 299 310 ma based on la icpms zircon u pb dating suggesting a post collisional setting remelting of sulfide accumulate residual with high au cu ratios in the juvenile lower crust formed in the pre collisional plate subduction related magmatic arc stage is proposed to account for the au mineralization in the kassan region the high water contents 4 0 5 2 wt h2o and weakly oxidized or slightly reduced redox state fmq 0 33 0 49 deduced from mineral composition and zircon ce anomaly are favorable for the transport and enrichment of gold in the magmatic fluids these geochemical characteristics suggest that the andagul granodiorite and related granitic rocks in the kassan region have the potential to provide ore forming materials and fluids for gold mineralization

importance of melt fraction and source rock composition in crustal genesis the example of two granitic suites of northern portugal in the montalegre area northern portugal two varieties of syn to late tectonic hercynian aluminous granitoids are distinguished 1 1 muscovite rich two mica granites forming many successively emplaced bodies with little chemical variation the chemical characteristics of these units suggest partial melting of various crustal rocks under distinct p t and water saturation conditions there has been no homogenization between the different melt batches the rocks contain very few restitic minerals segregation processes may have operated either in the source rock or in an early magmatic stage 2 2 a cordierite biotite and muscovite bearing granitic body its mineralogical composition is extremely heterogeneous and it contains many xenoliths the genesis of this granite results essentially from partial melting of peraluminous orthogneisses the melt fraction was higher than the rheological critical melt percentage rcmp during anatexis the rigid framework of the source rock broke down leading to migration of both melt and residual minerals towards upper crustal levels the heterogeneity of the resulting granite is mostly related to an incomplete separation of residual crystals and melt the existence of biotite and cordierite rich samples is the result of accumulation of residual minerals during the magma emplacement this study emphasizes the role of the source rock composition which controls the degree of partial melting at given p t and water saturation conditions during crustal melting the strong geochemical differences between the two kinds of granitoids are related to the near absence of residual minerals in the muscovite rich granites and to the presence of residual minerals in the cordierite bearing granite it is suggested that the muscovite rich granites are mostly low melt fraction rcmp the presence of many residual crystals alkali feldspars and plagioclases as well as dark minerals in the cordierite granite implies a high viscosity of the melts and therefore water undersaturated melting conditions

mineralogy of the aktogai giant porphyry cu deposit in kazakhstan insights into the fluid composition and oxygen fugacity evolution abstract it has been proposed that most giant porphyry cu deposits are closely associated with oxidized magmas most recently many methods including zircon trace elements mn content in apatite and magnetite ilmenite mineral pair were used to reveal the primary magmatic oxygen fugacity of the porphyry cu deposits hydrothermal alterations are products of water rock reactions and can form various hydrothermal minerals which can be used for exploration the aktogai group including aktogai aidarly and kyzilkia contains more than 12 5 mt cu 0 4 wt and 80 t au 0 007 g t 0 4 g t chlorites show significant variations in the aktogai deposit which are responsible for degrees of tschermak and di trioctahedral substitution chlorites are divided into type i and ii based on fe fe mg ratios of chlorites which are predominately controlled by hydrothermal fluid composition and oxygen fugacity the chlorite i have higher fe fe mg ratios than chlorite ii the formation temperatures of chlorite are 248 322 c based on empirical geothermometry t5 sericites belong to muscovite and phengitic muscovite and their compositions are controlled by tschermak and fe 3 vi al 3 vi substitution water rock reaction leaches out some elements into hydrothermal fluids therein si 4 and k precipitate as quartz and sericite at all alteration stages but fe 2 and ca 2 precipitate as specific minerals such as chalcopyrite and calcite at main mineralization or late stage the chlorite i form under a fe rich condition with high water rock ratio while chlorite ii form under a low water rock ratio and fe poor condition which may be responsible for the sulfides and epidote precipitation according to zircon trace elements mn content in apatite magnetite ilmenite mineral pair and fe fe mg and fe 3 tfe ratios of chlorites the oxygen fugacity changes from high at magmatic and early hydrothermal stages through low at main mineralization stage to slightly high at late or post mineralization stage our results combined with previous works suggest that fe contents and fe fe mg ratios of chlorites are good indicators for exploration

baogutu an example of reduced porphyry cu deposit in western junggar abstract classic oxidized porphyry copper deposits opcd are characterized by high tonnage and elevated f o 2 and contain highly oxidized minerals including primary anhydrite and hematite in contrast the baogutu deposit contains abundant hypogene pyrrhotite and methane rich fluid inclusions characteristic of lower tonnage reduced porphyry copper deposits rpcd mineral paragenesis and mineral chemistry studies reveal two mineralization periods magmatic and hydrothermal with the hydrothermal period further subdivided into three paragenetic stages including ca na alteration potassic alteration and propylitization locally overlapped by phyllic alteration several independent geothermometers indicate that the temperatures of the magma potassic and propylitization alteration are estimated to be 600 900 c 200 400 c and 200 300 c respectively multiple indicators including mineral assemblages apatite so 3 content whole rock fe 2 o 3 feo ratios and fluid compositions indicate the f o 2 of the magma and hydrothermal fluid to be nno and nno nno 2 respectively hydrothermal fluids associated with primary biotite yield log f h 2 o f hcl fluids values of 4 8 6 2 for diorite and 4 1 4 5 for granodiorite porphyry the log f s2 of the magmatic and potassic alteration are estimated to be 0 7 to 3 0 and 5 5 to 11 0 respectively based on pyrrhotite and sulfide assemblages mineral assemblage and hessite composition suggest the log f te2 of the potassic alteration is 8 5 to 14 5 the low tonnage baogutu deposit displays significantly lower f o 2 than opcds nno 2 and on this basis could be classified as a rpcd other physicochemical conditions including t f s2 log f h2o f hcl fluid do not show obvious differences to those of opcds we deduce that the low f o 2 of baogutu metallogenic granitoids and aqueous fluids likely produced the unusual pyrrhotite arsenopyrite mineral assemblage and the nacl h 2 o ch 4 co 2 ore forming fluids and small tonnage typical of rpcds previous obtained fluid inclusion h o isotope data and sulfide s pb isotope data suggest the methane rich ore forming fluids were derived from deep mantle source with little contamination from sedimentary components however detailed studies are needed to clarify the origin of the ch 4

magmatic arc asymmetry and distribution of anomalous plutonic belts in the batholiths of california effects of assimilation crustal thickness and depth of crystallization in order to better understand geologic fac tors controlling pronounced regional variations in whole rock chemistry mineralogy and mineral chemistry in the batholiths of california we calculate the magmatic intensive variables f hf f h 2 o f hf f hcl and f o 2 regional scale west to east increases in f oh in mafic silicates corresponding to the systematic i wc to i mc and i sc progression reflect orders of magnitude increase in f hf f h 2 o attending pluton crystallization low f hf f h 2 o of formation of western i wc types is consistent with their derivation from low fluorine source rocks in subducted oceanic slabs or the upper mantle in contrast higher f hf f h 2 o of crystallization of i mc and i sc types to the east implies the involvement of 1 progressively greater amounts of continental crustal source material such as biotite bearing metamorphic rocks their unweathered sedimentary derivatives retaining f rich mafic minerals or their fusion products and or 2 source materials which become more f rich toward the continental interior the regional distribution of i mc and i sc types suggests that the precambrian craton of western north america or derivative sediments may extend farther north in california and be morphologically more complex than previously thought from seemingly out of place occurrences of i wc plutons on the eastern slopes of the sierra nevada batholith we infer the existence of regions where precambrian basement was thin or absent in mesozoic time which prevented extensive cratonal contamination of subducted slab or upper mantle derived magmas new methods for estimating t f o 2 relations in the magmas demonstrate that i scr granites crystallize at oxygen fugacities as much as five orders of magnitude lower than those of i wc i mc and i sc types under conditions at or below the maximum stability limit of graphite in equilibrium with a c o h s gas phase the local scale formation of i scr granites in plutonic belts within specific wall rock terranes containing highly reducing sediments or metasediments may occur by contamination of i types with graphitic pelite or in some cases by the direct fusion of this reducing pelitic wall rock the spatial distribution of i scr granite provinces therefore is controlled simply by wall rock lithology amphibole geobarometry demonstrates a general west to east decrease in crystallization pressure across the sierra nevada batholith in contrast the peninsular ranges batholith displays a west to east crystallization pressure increase the bulk of the california batholiths crystallized at pressures less than 4 5 kb and depths less than about 15 19 km in the southern sierra nevada batholith the san gabriels and the eastern peninsular ranges however plutons crystallized at pressures exceeding 6 kb at deep crustal levels 23 km reconstruction of the pre erosion top of the batholith shows that in an east west cross section the central sierra nevada batholith was a horizontal tabular body with an aspect ratio of at least five to one

imaging a low viscosity zone beneath the kutcharo caldera eastern hokkaido japan using geodetic data abstract geodetically observed volcano deformation has been analysed in terms of either elastic or viscoelastic crustal rheology for which the presence of magma has usually been considered as a deformation source from a rheological viewpoint however a magma and the peripheral rocks it influences are presumably low viscosity materials which allows detection of the presence of magma as a zone that has rheologically less strength in this study using a 3 dimensional finite element model that introduces a rectangular low viscosity zone lvz into relatively high viscosity crust the response of the linear maxwell viscoelastic crust to the emplacement of a spheroidal sill is adopted for interferometric synthetic aperture radar insar data of the kutcharo caldera eastern hokkaido japan to image a spatial crustal viscosity variation the observed temporal displacement at the surface point where the line of sight los displacement is largest in the deformation field requires the lvz to have a spatially uniform viscosity of 4 5 10 17 pa s the lvz is also required to be present immediately below a 5 km elastic layer down to mid crustal depths or deeper with a horizontal extent of at least 10 km models with acceptable minimum sizes of lvz are found to satisfy the overall temporal displacement field with root mean square misfits of less than 2 cm the geodetic data reasonably accept the presence of an lvz beneath the kutcharo caldera that is consistent with the magnetotelluric image of the accumulated magma providing the mechanical aspect of the low resistivity domain the lvz model behaviour suggests an important implication that the accumulation of magma in the middle to lower crust is detectable using geodetic data after magma emplacement into the upper crust where the deformation rate represents the degree of magmatic influence on the crustal viscosity

trace element compositions and br cl ratios of fluid inclusions in the tsushima granite japan significance for formation of granite derived fluids abstract fluid inclusions in quartz samples from a miarolitic cavity two quartz veins and a hydrothermal ore vein in the tsushima granite japan were analyzed by particle induced x ray emission to examine the chemistry and process of formation of hydrothermal fluids in an island arc granite most of the inclusions were polyphase or vapor and there were smaller numbers of two phase aqueous inclusions the inclusions contained cl k ca ti mn fe cu zn ge br rb sr ba and pb for each inclusion there was a strong positive correlation between cl content and contents of other elements identified concentration ranges for most elements other than rb and ge in polyphase inclusions from the miarolitic cavity were comparable to those from cavities in alkaline granites those from the ore vein were comparable to large scale continental hydrothermal ore deposits the lower rb and higher ge contents in the polyphase inclusions of the tsushima granite may be characteristic of hydrothermal fluids from calc alkaline granites in an island arc setting br cl ratios by weight for the vapor and two phase inclusions were 0 0013 0 0030 and differed among the three geological settings br cl ratios of polyphase inclusions increased with increasing cl content in single crystal and polycrystalline quartz and high values of more than 0 0100 were found the high br cl ratios and the differences among the geological settings sampled may be due to pressure dependences of partitioning of cl and br between fluid and magma during fluid segregation and between liquid and vapor during boiling using a simple model based on these dependences we calculated br cl ratios greater than 0 01 in brine generated at pressures

melt segregation in migmatites anatectic stromatic migmatites have a symmetrical layered structure with a low ratio of thickness to length and a periodicity features that have not been explained satisfactorily but which are related to physical processes of melt segregation we evaluate the compaction model for segregation as it applies to migmatites and develop models for melt segregation based upon convection driven by volume change and upon advection down pressure gradients that result from applied differential stress acting on an anisotropic multilayer protolith compaction by gravity driven two phase flow results in asymmetric separation yields calculated segregation times that are slow or geologically unreasonable and yields relative volumes that are not consistent with the abundance of leucosome seen in natural migmatites although calculated segregation times for very wet granite might allow segregation by compaction some driving force in addition to gravity is needed to cause widespread melt segregation in the crust for segregation at low volume fraction of melt we develop a two dimensional two phase matrix and fluid viscous flow cell model length much greater than thickness with phase changes which predicts either expansion or contraction convection depending on whether volume change vr is positive or negative convection leads to melt segregation to form a stromatic structure in a geologically reasonable timescale at moderate volume fraction of melt segregation may occur by filter pressing in compositionally layered rocks in response to applied differential stress melt migration is by porous medium flow driven by differences in mean normal stress between the layers as a consequence of differences in rheology and shear enhanced matrix collapse calculated segregation rates are fast and the model yields adequate volume of leucosome the positive vr for volatile phase absent melting reactions under crustal pressures promotes melt escape unless extensional deformation facilitates melt accumulation if the rate of melt production exceeds the rate of melt escape then the increase in melt pressure may lead to melt enhanced embrittlement and fracture and melt may migrate out of the system in contrast water rich volatile phase present melt producing reactions have a negative vr which promotes melt retention after segregation unless deformation generates melt escape pathways evolution of stromatic structure through anatectic erosion of mesosome by increasing the melt fraction in situ may lead to breakdown of the solid matrix and will lead to instability due to buoyancy and magma may become mobile and entrain residual material restite

experimental detection of a slow acoustic wave in sediment at shallow grazing angles following recent experimental results at sea n p chotiros proc oceans 89 that suggest the existence of a previously undetected type of acoustic wave in sandy sediments an experiment was designed to detect and measure the speed of acoustic waves in an isolated environment the experiment was conducted in a laboratory tank containing 1 m of unwashed river sediment under a 3 m water column observations were made of the travel time and attenuation of a pulse from an acoustic source located above the water sediment interface to a set of probes below the interface it was observed that at normal incidence the pulse traveled at about 1750 m s while at shallow grazing angles the pulse traveled through the sediment at close to 1200 m s an interesting possible explanation exists in the biot model m a biot j acoust soc am 28 168 191 1956 which predicts a slow acoustic wave in porous materials work funded by onr under noarl management

some comments on the rheologically critical melt percentage abstract the concept of rheologically critical melt percentage rcmp originally proposed by arzi tectonophysics 44 1978 173 184 for partially molten granitic rocks is re examined it is shown that there is no experimental support to show the presence of rcmp the published experimental data suggest that the effective viscosity of partially molten granitic rocks is reduced rapidly and continuously with increasing melt fraction it is also shown that the experimental data may be modeled by means of the upper bound behavior the voigt bound of two phase material by assuming a melt localization which implies that there is no partitioning of strain between the solid and the melt

geology and gold molybdenum porphyry mineralisation of the archean taschereau launay plutons abitibi quebec abstract porphyry gold deposits constitute a new challenge for mineral exploration in archean greenstone belts yet the relationships between intrusive rocks and mineralisation remain poorly understood the messegay gold occurrence is situated in the taschereau launay plutonic complex of the northern volcanic zone of the abitibi greenstone belt the taschereau intrusion 2718 2 ma represents a complex network of sills composed of tonalite with minor amounts of diorite monzonite and gabbro the launay granite which intrudes the taschereau pluton is late orogenic and its emplacement was controlled by an early reverse ductile n s shear zone intrusive rocks have been subjected to four episodes of alteration epidote chlorite carbonate magnetite hematite microcline albite pyrite and sericite quartz shear zone hosted and metasomatic au mo mineralisation is mainly associated with albite pyrite assemblages but also occurs within zones of intense magnetite hematite microcline and sericite quartz alteration the messegay gold occurrence is interpreted as a new type of disseminated gold molybdenum porphyry deposit associated with a zone of magmatic differentiation in a magnetite series pluton

geochronological isotopic and mineral geochemical constraints on the genesis of the diyanqinamu mo deposit inner mongolia china abstract the diyanqinamu mo deposit located in central inner mongolia in china is a newly discovered mo deposit due to the lack of detailed deposit scale studies the metallogenesis of the deposit has been a matter of debate in this study petrography zircon u pb geochronology sr nd and pb isotopes and apatite mineral geochemistry have been used to constrain the genesis of the diyanqinamu mo deposit petrographic evidence indicates that the granites in the diyanqinamu deposit have experienced fluid exsolution as demonstrated by the corroded quartz in the porphyritic granites skeletal quartz in the aplitic granites and the unidirectional solidification texture ust in the apex of the aplitic granites sr and nd isotopic data of the purple fluorite from the fluorite molybdenite veins showing the 87sr 86sr i 0 7046 0 7052 avg 0 7048 and end t values 2 4 3 8 avg 3 3 are similar to those of the porphyritic and aplitic granites this similarity implies that the mo ore forming fluids may have been derived from the granitic magmas the hypothesis is further supported by the fluorine content 3 79 5 48 wt avg 4 53 wt in apatite from the porphyritic granites and the widely distributed fluorite in the hydrothermal alteration mineral assemblages which indicate that the porphyritic granites contain high f content and the hydrothermal fluids are also f rich petrographic observation also shows that magnetite occurs commonly in the hydrothermal mineral assemblages indicating that the hydrothermal fluids are characterized by high oxygen fugacity fo2 it is important to note that the intruding granites are essentially barren and the mo is mainly hosted in the andesite and volcaniclastic rocks in comparison pb isotopic compositions of molybdenite are similar to those of both the andesite and k feldspar from the porphyritic granites which suggests that both the intruding granites and ore hosting andesite have contributed to the ore forming materials this hypothesis is further supported by the abnormally high mo for the andesite our new evidence suggests that the diyanqinamu mo deposit may have formed in a post collisional extensional environment ore forming materials may have been derived mainly from the granites and minimally from the host rocks the ore fluids are characterized by high fo2 and being f rich and the trigger for the mo mineralization has been attributed to the reduction of fo2 and or temperature for the ore fluids

controls on low pressure anatexis low pressure anatexis whereby rocks melt in place after passing through the andalusite stability field develops under more restricted conditions than does low pressure metamorphism our thermal modelling and review of published work indicate that the following mechanisms operating alone may induce anatexis in typical pelitic rocks without inducing wholesale melting in the lower crust i magmatic advection by pervasive flow ii crustal scale detachment faulting and iii the presence of a high heat producing layer of these only magmatic advection by pervasive flow and crustal scale detachment faulting have been shown quantitatively to provide sufficient heat to cause widespread melting combinations of the above mechanisms with pluton scale magmatic advection shear heating removal of the lithospheric mantle or with each other provide additional means of developing suitable high temperatures at shallow crustal levels to generate low pressure anatexis

reduced porphyry copper gold deposits a new variation on an old theme it is a widely accepted fact that porphyry cu au deposits form from highly oxidized ore fluids despite this assertion several otherwise typical porphyry cu au deposits exhibit clear evidence of having formed from relatively reduced ore fluids these reduced porphyry cu au deposits lack primary hematite magnetite and sulfate minerals i e anhydrite but contain abundant hypogene pyrrhotite commonly have carbonic rich ore fluids with substantial ch 4 and are associated with ilmenite bearing reduced i type granitoids based on a synthesis of theoretical experimental and field data a variation on the classic porphyry cu model is proposed to explain the formation of reduced porphyry cu au deposits and their relatively cu poor but au rich nature the proposed reduced porphyry cu au model does not contradict the current understanding of porphyry cu au formation rather it adds yet another variation on the theme of porphyry cu au genesis

magmatic process recorded in plagioclase at the baogutu reduced porphyry cu deposit western junggar nw china abstract despite the fact that porphyry cu deposits contain large amounts of cu in one or more small stocks few studies have discussed evidence for significant magma recharge in porphyry cu deposits a systematic elemental and sr isotopic study of plagioclase crystals from mineralized diorite and granodiorite porphyry constrains the processes of crystallization and magma recharge at the baogutu reduced porphyry copper deposit western junggar nw china large compositional changes in an 12 24 mol are observed along with strong positive correlations between an and feo significant resorption textures are also preserved in plagioclase crystals as well as repeated oscillatory zoning in an and feo and complex sr isotope variations three types of crystals with different core to rim sr isotope variations are recognized type i crystals have core to rim increases in 87sr 86sr i that could be explained by diffusion for example sr isotope variations recorded in bck2 1 2 crystal could be generated by diffusion with simulated maximum crystal residence times of 100 500 years with the proximate value of 300 years type ii crystals with different variation trends in 87sr 86sr i on opposite sides of the core could be produced either by multi stage crystallization or by analytical uncertainty whereas type iii crystals with complex core to rim variations in 87sr 86sr i may record repeated magma recharge events all these results suggest repeated recharging of the magma chamber by hotter more mafic and less radiogenic sr isotope melts therefore more mafic melt injection recorded in plagioclase profiles may provide significant metal contributions to the magma chamber which ultimately results in cu mineralization at the baogutu reduced porphyry copper deposit

cretaceous reduced granitoids in the goodpaster mining district east central alaska the 5 moz 1 oz troy 31 103 g gold veins of the liese zone and nearby prospects in the goodpaster mining district of east central alaska are spatially and temporally associated with late early to late cretaceous reduced granitoids that are divided into a granite suite tonalite suite and diorite suite in decreasing age synkinematic to postkinematic biotite hornblende granite granodiorite pegmatite and two mica granite with accessory garnet compose the granite suite 109 107 ma the tonalite suite 107 103 ma forms small to large bodies of postkinematic hornblende biotite granodiorite to tonalite with rare granite intruding the granite and parts of the tonalite suites are aplite and pegmatite that grade through sugary pegmatoidal or aplitic quartz veins and finally into quartz veins along strike the diorite suite 95 4 93 7 ma consists of small stocks of diorite to tonalite that intrude the older suites and are inferred to in trude the shallowly dipping auriferous quartz veins limited data indicate granitoid emplacement at 5 9 km depths consis tent with formation of the auriferous quartz veins based upon published fluid inclusion data the weakly peraluminous granite and tonalite suites are distinguished by variable amounts of monazite and zircon whereas the diorite suite is metaluminous contains 5 magnetite and lacks monazite all suites are subalkalic calc alkaline and have low mag netic susceptibilities high large ion lithophile element high field strength element lile hfse and depleted nb and ti the granite suite has higher 206 pb 204 pb values 19 4 19 6 than the diorite 19 1 overall the granite and tonalite suites likely represent melts generated late during crustal thickening that intruded along shallowly dipping faults during exhuma tion whereas the diorite suite represents postdeformation melts that underwent less interaction with the old silicic crust

deformational behaviour of migmatites and problems of structural analysis in migmatite terrains an understanding of the response of migmatites to deformation is crucial to an interpretation of their structures and in anatectic and intrusive migmatite terrains due consideration must be given to the modification of deformation processes imposed by melts in partially molten systems containing more than 30 melt the classical theories of fold formation are inapplicable the folds produced are of the viscous fold type their geometry is unpredictable and may mimic structures due to refolding this is particularly true at very high degrees of melting where flow of melt carrying rafts of solids begins failure to distinguish viscous folds from the products of refolding will lead to errors in interpreting the structural and metamorphic history of an area and to consequent misinterpretation of the true structural relations between geometrically complex high grade and more regularly deformed low grade areas

a reinterpretation of the role of granitoids in the genesis of neoproterozoic gold mineralization in the telfer dome western australia the telfer gold mine in the paterson province of western australia is one of australia s largest gold deposits with more than 4 million oz m oz or 125 t of gold produced since mining commenced in 1977 hosted by low grade dominantly marine metasedimentary sequences of the upper yeneena group approximately 1 000 750 ma the neoproterozoic mineralization 700 600 ma is localized within the northwest trending telfer dome which consists of two en echelon asymmetric doubly plunging anticlines called the main dome and the west dome within these two domes gold and minor copper occur as a series of vertically stacked strata bound to stratiform gold copper sulfide quartz reefs up to several meters in thickness linked by zones of intense stockwork and sheeted veins carbon oxygen boron lead and sulfur isotope data including shrimp microanalysis from ore sulfides and alteration minerals carbonates and tourmalines in conjunction with tourmaline and pyrite compositional data indicate that the ore fluid solutes were derived chiefly from sedimentary host rocks and not neoproterozoic granitoids a model for telfer mineralization is proposed such that granitoids act primarily as sources of heat that drive thermal convection cells in which heated saline formational contact metamorphic fluids scavenge gold copper and sulfur from the surrounding sedimentary wall rocks at depth this style of ore fluid generation shares some similarities with a convective porphyry copper gold model although the lack of hornfelses proximal granitoids and types of alteration characteristic of porphyry copper deposits implies that the telfer dome deposits did not form in the near porphyry environment these data do suggest however highly channelized fluid flow to the domal sites of mineralization after an initial period of extensive deep fluid circulation and leaching of sulfur and metals from the sedimentary wall rocks adjacent to the pluton s a regional scale deep structure capable of focusing these deeper fluids to higher stratigraphic levels is the north northwest trending telfer lineament upon which the telfer dome is situated periodic reactivation of this structure during the the paterson orogeny would allow for episodes of widespread fluid advection at depth under essentially lithostatic fluid pressures followed by rapid ingress of fluids into the structure triggered by a decrease in fluid pressure during reactivation

experimental deformation of partially melted granitic aggregates abstract the effects of varying amounts of partial melt on the deformation of granitic aggregates have been tested experimentally at conditions 900 c 1500 mpa 10 4 to 10 6 s where melt free samples deform by dislocation creep with microstructures approximately equivalent to those of upper greenschist facies experiments were performed on samples of various grain sizes including an aplite 150 m and sintered aggregates of quartz albitemicrocline 10 50 and 2 10 m water was added to the samples to obtain various amounts of melt 1 15 in the aplite 1 5 in the sintered aggregates optical and tem observations of the melt distribution in hydrostatically annealed samples show that the melt in the sintered aggregates is homogeneously distributed along an interconnected network of triple junction channels while the melt in the aplites is inhomogeneously distributed the effect of partial melt on deformation depends an melt amount and distribution grain size and strain rate for samples deformed with 1 melt all grain sizes exhibit microstructures indicative of dislocation creep for samples deformed with 3 5 melt the 150 m and 10 50 m grain size samples also exhibit dislocation creep microstructures but the 2 10 m grain size samples exhibit abundant tem scale evidence of dissolution precipitation and little evidence of dislocation activity suggesting a switch in deformation mechanism to predominantly melt enhanced diffusion creep at natural strain rates melt enhanced diffusion creep would predominate at larger grain sizes although probably not for most coarse grained granites the effects of melt percentage and strain rate have been studied for the 150 m aplites for samples with 5 and 10 melt deformation at 10 6 s squeezes excess melt out of the central compressed region allowing predominantly dislocation creep conversely deformation at 10 5 s produces considerable cataclasis presumably because the excess melt cannot flow laterally fast enough and a high pore fluid pressure results for samples with 15 melt deformation at both strain rates produces cataclasis presumably because the inhomogeneous melt distribution resulted in regions of decoupled grains which would produce high stress concentrations at point contacts at natural strain rates there should be little or no cataclasis if an equilibrium melt texture exists and if the melt can flow as fast as the imposed strain rate however if the melt is confined and cannot migrate a high pore fluid pressure should promote brittle deformation

redox state of the baogutu reduced porphyry cu deposit in the central asian orogenic belt abstract reduced porphyry cu deposits are scarce and their origins remain enigmatic the late carboniferous baogutu reduced porphyry cu deposit in the western junggar terrane nw china provides an opportunity to address this issue in this study we conducted systematic analyses in terms of petrography mineral chemistry major elements trace elements and fe isotopes to investigate the redox state of its primary magma and further constrain the origin of baogutu copper deposit based on the petrological observation and mineral chemistry two magmatic stages have been recognized during the formation of ore forming diorite the early stage is characterized by the crystallization of magnetite and high mg mg mg fe2 hornblende under oxidized condition nno 2 36 calculated by the equation given by scaillet and evans 1999 the late stage is characterized by the crystallization of ilmenite and low mg biotite under reduced condition nno in this study we first report the fe isotopic compositions of reduced porphyry copper deposit and discuss its application for the origin of ore deposit the 56fe values of less altered diorites and granodiorites 0 083 and 0 054 fall into the field of arc magmas consistent with their major and trace elements results the 56fe values of sulfides chalcopyrite and pyrite show a large span 0 228 to 0 614 and the early formed disseminated sulfides are enriched in light fe isotopes relative to the late formed vein hosted sulfides which indicates that hydrothermal fluid evolves toward heavy fe isotope enrichment the average 56fe of all hydrothermal sulfides chalcopyrite and pyrite is much higher than that of baogutu silicate rocks which demonstrates that the hydrothermal fluid are fe isotopically heavy and it well explains the positive correlations between 56fe and indices of hydrothermal alteration for altered plutonic rocks and wall rocks in summary major elements trace elements and fe isotopes of the baogutu porphyries show the affinity of arc magmas same as the typical oxidized porphyry cu deposits both petrological observations and fe isotope features of the baogutu porphyry cu deposit indicate that the primary magma of baogutu porphyries is oxidized and its reduced features are secondary

in situ la mc icp ms trace element and nd isotopic compositions and genesis of polygenetic titanite from the baogutu reduced porphyry cu deposit western junggar nw china abstract titanite sphene catisio 5 is sensitive to changes in temperature oxygen and water fugacity and fluid composition in order to understand formation processes and the nature of hydrothermal fluids various types of titanite from cu ores at the baogutu reduced porphyry cu deposit were chosen for detailed study magmatic titanite is associated with biotite plagioclase and k feldspar whereas hydrothermal titanite occurs with k feldspar chlorite actinolite and calcite the formation of hydrothermal titanite was related to hydration of igneous minerals under high f h 2 o whereas the widespread replacement of ilmenite by titanite without magnetite indicates a relatively low oxygen fugacity magmatic titanite has low al high fe y sn zr nb and ree contents relative to hydrothermal titanite on the basis of the zr in titanite and al in chlorite geothermometers formation temperatures for magmatic and hydrothermal titanite are estimated to be 687 739 c and 250 670 c respectively the gradual decrease in ree y and sn contents from magmatic to late hydrothermal titanite was probably caused by precipitation of ree bearing minerals magmatic and hydrothermal titanites have similar chondrite normalized ree patterns with negative eu anomalies and relatively flat hree randomly selected titanites have nd isotopic compositions similar to the host rocks thus both magmatic and hydrothermal titanite are believed to have been predominantly derived from a mantle source