structural history and granite emplacement in the rottenstone domain during closure of the trans hudson orogen davin lake northern saskatchewan the rottenstone domain at davin lake northern saskatchewan exhibits structural and granite emplacement evidence for crustal thickening and possible himalayan style extrusion overprinted by transpressional strain increasing toward the contact with the wathaman batholith three discrete rottenstone subdomains parallel the regional strike of the trans hudson orogen i the southeast rottenstone subdomain sersd with gently northwest dipping migmatitic straight gneiss s1 and white granitoid rocks with pinch and swell structures parallel to the straight gneissosity ii the central rottenstone subdomain crsd which partly preserves the same nw dipping fabric s1 but is overprinted at its margins by tight upright f2 folds and includes a stockwork of pink monzogranitic aplites and pegmatites and iii the northwest rottenstone subdomain nwrsd in which the f2 folds are generally tighter and penetrative and its network of narrow white granitoid rocks is deformed and transposed by the f2 folds but in

h2o fluid saturated melting of subducted continental crust facilitates exhumation of ultrahigh pressure rocks in continental subduction zones we present two dimensional numerical models of plate subduction and collision inspired by the scandinavian caledonian orogeny to investigate the possible impact of continental crust partial melting on the exhumation of ultra high pressure metamorphic rocks three possible reactions were tested low temperature solidus representing h2o fluid saturated partial melting and two end member reaction curves for dehydration melting thermo mechanical effects of partial melting were implemented as 1 a viscosity decrease as a determined rheologically critical melt percentage was reached here 0 1 2 a change in effective heat capacity and adiabatic heating cooling accounting for a latent heat term in the heat equation among the 3 tested reactions only h2o fluid saturated partial melting drastically modifies the collision dynamics from the non melting reference model holding all other parameters constant a substantially low general viscosity truncation here 1017 pa s is needed to properly resolve the effect of partial melting on deep collision processes low temperature melting indeed induces the development of a low viscosity buoyant plume prior to slab detachment where migmatites exhume from uhp conditions at rates and with pressure temperature paths similar to the natural values acknowledged for the norwegian caledonides high temperature melting has no drastic influence on early collision dynamics while positive buoyancy remains the first order driver for the exhumation of buried continental rocks exhumation initiates in these cases with eduction subsequent to slab detachment melting and formation of a migmatite plume can later occur along decompression path while continental crust undergoes thermal reequilibration at temperatures above 900 c some of the partially molten material can also relaminate in the overriding plate rather than exhume within the collision zone even if minor in terms of amount of magma produced h2o fluid saturated partial melting at uhp conditions could therefore have a dramatic rheological effect and actually limits continental rocks subduction and facilitates their exhumation

propagation tectonics and multiple accretionary processes of the qinling orogen abstract the qinling orogen was built through collision between the north china and south china blocks previous detailed geological geochemical and geochronological investigations revealed that the mountain range can be divided into four tectonic units with distinct tectono lithostratigraphy which are from north to south the southern sector of the north china block north qinling belt south qinling belt and northern sector of the south china block separated by the kuanping shangdan and mianlue sutures according to the petrology geochemistry and geochronology of ophiolitic melanges and related magmatic rocks as well as the features of sedimentary units we think that the north china block the north qinling belt and the south china block were originally independent continental units while the south qinling belt had been the northern part of the south china block these units experienced three episodes of accretionary tectonic processes and amalgamation from south to north the neoproterozoic accretion took place along the luonan luanchuan fault and kuanping ophiolitic melange belt as a result of southward subduction and subsequent collision between the north qinling and north china blocks during ca 1 0 0 8 ga related to the formation of the supercontinent of rodinia the paleozoic accretion occurred along the shangdan suture resulted from northward subduction of oceanic lithosphere in the early paleozoic and subsequent continental subduction in the late paleozoic late triassic accretion took place along the mianlue suture between the south qinling and south china blocks due to northward subduction of the mianlue oceanic lithosphere during the permian early triassic and subsequent collision in the late triassic after the late triassic collision along the mianlue suture the whole qinling mountain range entered the phase of intense intracontinental deformation

paragenesis and geochemistry of ore minerals in the epizonal gold deposits of the yangshan gold belt west qinling china six epizonal gold deposits in the 30 km long yangshan gold belt gansu province are estimated to contain more than 300 t of gold at an average grade of 4 76 g t and thus define one of china s largest gold resources detailed paragenetic studies have recognized five stages of sulfide mineral precipitation in the deposits of the belt syngenetic diagenetic pyrite py0 has a framboidal or colloform texture and is disseminated in the metasedimentary host rocks early hydrothermal pyrite py1 in quartz veins is disseminated in metasedimentary rocks and dikes and also occurs as semi massive pyrite aggregates or bedding parallel pyrite bands in phyllite the main ore stage pyrite py2 commonly overgrows py1 and is typically associated with main ore stage arsenopyrite apy2 late ore stage pyrite py3 arsenopyrite apy3 and stibnite occur in quartz calcite veins or are disseminated in country rocks post ore stage pyrite py4 occurs in quartz calcite veins that cut all earlier formed mineralization electron probe microanalyses and laser ablation inductively coupled plasma mass spectrometry analyses reveal that different generations of sulfides have characteristic of major and trace element patterns which can be used as a proxy for the distinct hydrothermal events syngenetic diagenetic pyrite has high concentrations of as au bi co cu mn ni pb sb and zn the py0 also retains a sedimentary co ni ratio which is distinct from hydrothermal ore related pyrite early hydrothermal py1 has high contents of ag as au bi cu fe sb and v and it reflects elevated levels of these elements in the earliest mineralizing metamorphic fluids the main ore stage py2 has a very high content of as median value of 2 96 wt and au median value of 47 5 ppm and slightly elevated cu but relatively low values for other trace elements arsenic in the main ore stage py2 occurs in solid solution late ore stage py3 formed coevally with stibnite contains relatively high as median value of 1 44 wt au fe mn mo sb and zn and low bi co ni and pb the main ore stage apy2 compared to late ore stage arsenopyrite is relatively enriched in as whereas the later apy3 has high concentrations of s fe and sb which is consistent with element patterns in associated main and late ore stage pyrite generations compared with pyrite from other stages the post ore stage py4 has relatively low concentrations of fe and s whereas as remains elevated 2 05 3 20 wt which could be interpreted by the substitution of as for s in the pyrite structure these results suggest that syngenetic diagenetic pyrite is the main metal source for the yangshan gold deposits where such pyrite was metamorphosed at depth below presently exposed levels the ore forming elements were concentrated into the hydrothermal fluids during metamorphic devolatilization and subsequently during extensive fluid rock interaction at shallower levels these elements were precipitated via widespread sulfidation during the main ore stage

exhumation driven devolatilization as a fluid source for orogenic gold mineralization at the damang deposit ghana gold mineralization at the damang deposit is unique among currently known orogenic gold deposits in ghana comprising gold hosted within metasediments of the tarkwaian system and contained in a subhorizontal extensional quartz vein array that formed during regional compression the damang region has an extended paragenesis involving numerous structural metamorphic igneous and metasomatic events orogenic gold mineralization occurred late in the geologic paragenesis at damang postdating regional metamorphism and an earlier episode of hydrothermal alteration locally termed pink hematite alteration associated with the intrusion of mafic sills and dikes this earlier pink hematite alteration event involves extensive silicification that changed the rheology of the altered rocks and promoted later fracturing following peak regional metamorphism at around 2005 ma the damang region underwent a short period of rapid exhumation as constrained through numerical thermal modeling of existing pressure temperature time data this exhumation triggered the generation of a subhorizontal fracture array that was fed by fluids released through decompression driven metamorphic devolatilization the interaction between these fluids and the host rock resulted in precipitation of gold in association with sulfide carbonate potassic alteration halos around quartz veins such postpeak metamorphic exhumation driven devolatilization is unlikely to be a singular occurrence and represents a potentially important source of fluid for orogenic gold deposits elsewhere in ghana west africa and globally

textural control on gold distribution in as free pyrite from the dongping huangtuliang and hougou gold deposits north china craton hebei province china article i nfo pyrite from the dongping huangtuliang and hougou gold deposits hebei province china was investigated using a combination of ore microscopy including back scattered imaging and in situ laser ablation inductively coupled plasma mass spectroscopy la icp ms a range of elements including au te ag pb bi cu co ni and as was analyzed with the aim to constrain the textural controls and influence of superimposed deformation on the distribution of invisible gold in as free pyrite thedatasetshowsexcellentcorrespondence betweenmeasuredhighgoldvaluesandthreekeytexturalcriteria i areasofclusteredtellurideinclusions especiallyindongping ii microshearingandfracturing brecciation dongping and huangtuliang and iii pyrite recrystallization huangtuliang invisible gold is only at trace levels in the hougou and gold bearing telluride minerals are not observed this is considered to result from larger scale remobilization of gold during advanced brecciation and recrystallization of pyrite pyrite grains containing clustered inclusions have by far the highest gold concentrations up to 1 wt with consistent values of thousands of ppm au interpretation of textures and la icp ms data infers that in these areas the distribution of telluride inclusions extends pervasively from micron to nanoscale i e as fields of nanoparticles local employment of coupled dissolution reprecipitation reaction between pyrite and fluids driving the k metasomatic alteration dated at 150 ma is invoked to explain such patterns syn deformational grain scale mobilization of gold and other elements took place leading to a portion of the gold being recrystallized within fractures and microshears the deformational event leads to gold enrichment in the huangtuliang pyrite where brittle fracturing brecciation has been followed by a resorption and recrystallization in the latter case foam textured pyrites again without as can contain up to hundreds of ppm gold theinterpreted sequenceof pyritepost depositional eventsis inaccordancewith the protracted magmaticevolution of the area starting with variscan alkaline intrusions 390 ma and reactivated during the yanshanian orogeny 150 ma thestudyshows i thatas freepyritecanreadilycontainsignificantamountsofinvisiblegold bothasnanoparticles

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

source requirements for the golden mile kalgoorlie significance to the metamorphic replacement model for archean gold deposits the golden mile at kalgoorlie represents a giant archean hydrothermal gold system localized by ductile shear zones and hosted mainly by a differentiated tholeiitic sill chlorite carbonate and pyrite alteration zones cover the whole mineralized area 1 km 3 km and calculations suggest that for the golden mile production around 1200 t au the amounts of components added to these alteration zones are 340 mt co2 20 mt k and 5 mt s if one adopts a metamorphic replacement model for gold mineralization in which all ore components derive from devolatilization of greenstones at amphibolite facies or above these data suggest that a source area involving a 5 km thick greenstone slab of area 8 km 8 km could produce the necessary co2 k s h2o and au this is considered a reasonable volume of greenstone belt and under such a model the minimum spacing of large gold deposits would be approximately 20 km along strike neither special au enriched source rocks nor unreasonably large volumes of greenstone b

archaean cratonization and deformation in the northern superior province canada an evaluation of plate tectonic versus vertical tectonic models abstract the archaean minto block northeastern superior province is dominated by tonalite trondhjemite enderbite pyroxene tonalite granodiorite and granite with subordinate mafic rocks and supracrustal belts the plutons have been interpreted as the batholithic roots of andean type plate margins and intra oceanic arcs existing horizontal tectonic models propose that penetrative recrystallization and transposition of older fabrics during terrane assembly at 2 77 and 2 69 ga produced a n nw tectonic grain in the douglas harbour domain northeastern minto block tonalite and trondhjemite dominate the faribault thury complex 2 87 2 73 ga and enderbite constitutes 50 100 km scale ovoid massifs troie and qimussinguat complexes 2 74 2 73 ga magmatic muscovite and epidote in tonalite trondhjemite have corroded edges against quartz plagioclase suggesting resorption during ascent of crystal charged magma foliation maps and air photo interpretation show the common development of 2 10 km scale ovoid structures throughout the douglas harbour domain outcrop and thin section scale structures imply that many plutons experienced a phase of syn magmatic deformation typically followed by high temperature sub magmatic overprints thermobarometric data for plutons indicate near solidus recrystallization at 4 6 kbar pressures the common preservation of syn magmatic fabrics in plutons of different ages seems incompatible with the origin of these fabrics through superimposed regional orogenesis the broad uniformity of intrusion ages and lithologies throughout the minto block and the rarity of shallowly dipping planar fabrics also seem inconsistent with accretion of disparate older terranes each of which should preserve distinct histories a possible alternative explanation for these features is provided by vertical tectonic models whereby buoyant felsic magmas ascended as crystal slurries while dense supracrustal rocks and solidified felsic intrusions emplaced into them subsided as cold fingers 10 20 km scale instabilities shear between upwelling and downwelling limbs would have concentrated in the weak intrusions generating steeply plunging syn magmatic fabrics and producing ductile overprints in solidified rocks

relationships between gold and pyrite at the xincheng gold deposit jiaodong peninsula china implications for gold source and deposition in a brittle epizonal environment the xincheng gold deposit hosted by the early cretaceous 132 to 123 ma guojialing type granitoids in northwest jiaodong peninsula southeast north china craton formed about 2 billion years later than regional metamorphism of the archean jiaodong basement rocks the xincheng deposit comprises mineralized zones with three types of hydrothermal pyrite associated with gold tellurides and a variety of sulfides py1 as disseminated euhedral to subhedral grains in altered granitoids around quartz veins py2 as subhedral grains with brittle cataclastic textures and fractures in quartz pyrite veins and py3 as subhedral partially corroded crystals in sulfide rich veins or veinlets all three generations of pyrite are unzoned and have low trace element contents including very low lattice bound gold contents py1 0 180 ppm py2 0 053 ppm py3 0 060 ppm given that there is 10 to 15 pyrite in the ore zone at xincheng its very low gold content indicates that it contributes 0 2 of gold to the 7 75 g t gold in the orebody instead over 99 of the gold is present as discrete electrum and or gold total range 0 02 59 silver grains which are largely sited in fractures at all scales in pyrite other ore minerals and quartz importantly visible gold in py3 is also sited on solution corroded pyrite grains the pyrite textural and geochemical data indicate that it is impossible to derive the high gold grade orebodies through local remobilization of originally lattice bound gold in pyrite instead the gold is interpreted to have been deposited through sulfidation reactions and phase separation of a h2o co2 ore fluid during progressive brittle cataclastic deformation associated with seismic activity and regional sinistral transtensional shear movement this concomitant fluid infiltration and deformation caused episodic deposition and fracturing and corrosion of earlier formed pyrite and deposited visible gold in dilational cracks the coupled development of the transtensional rather than normal transpressional setting and precipitation of gold within dilational veins and wall rock alteration facilitated the deposition of visible gold and an exceptionally high gold tenor all deposit characteristics indicate that the xincheng gold deposit is a member of the epizonal orogenic deposit class

neoproterozoic paleozoic and mesozoic granitoid magmatism in the qinling orogen china constraints on orogenic process abstract the qinling orogen is one of the main orogenic belts in asia and is characterized by multi stage orogenic processes and the development of voluminous magmatic intrusions the results of zircon u pb dating indicate that granitoid magmatism in the qinling orogen mainly occurred in four distinct periods the neoproterozoic 979 711 ma paleozoic 507 400 ma and early 252 185 ma and late 158 100 ma mesozoic the neoproterozoic granitic magmatism in the qinling orogen is represented by strongly deformed s type granites emplaced at 979 911 ma weakly deformed i type granites at 894 815 ma and a type granites at 759 711 ma they can be interpreted as the products of respectively syn collisional post collisional and extensional setting in response to the assembly and breakup of the rodinia supercontinent the paleozoic magmatism can be temporally classified into three stages of 507 470 ma 460 422 ma and 415 400 ma they were genetically related to the subduction of the shangdan ocean and subsequent collision of the southern north china block and the south qinling belt the 507 470 ma magmatism is spatially and temporally related to ultrahigh pressure metamorphism in the studied area the 460 422 ma magmatism with an extensive development in the north qinling belt is characterized by i type granitoids and originated from the lower crust with the involvement of mantle derived magma in a collisional setting the magmatism with the formation age of 415 400 ma only occurred in the middle part of the north qinling belt and is dominated by i type granitoid intrusions and probably formed in the late stage of a collisional setting early mesozoic magmatism in the study area occurred between 252 and 185 ma with the cluster in 225 200 ma it took place predominantly in the western part of the south qinling belt the 250 240 ma i type granitoids are of small volume and show high sr y ratios and may have been formed in a continental arc setting related to subduction of the mianlue ocean between the south qinling belt and the south china block voluminous late stage 225 185 ma magmatism evolved from early i type to later i a type granitoids associated with contemporaneous lamprophyres representative of a transition from syn to post collisional setting in response to the collision between the north china and the south china blocks late mesozoic 158 100 ma granitoids located in the southern margin of the north china block and the eastern part of the north qinling belt are characterized by i type i to a type and a type granitoids that were emplaced in a post orogenic or intraplate setting the first three of the four periods of magmatism were associated with three important orogenic processes and the last one with intracontinental process these suggest that the tectonic evolution of the qinling orogen is very complicated

rheology of the plate interface dissolution precipitation creep in high pressure metamorphic rocks abstract subduction zone models invoke deformation to be concentrated along the plate interface in a region of particularly low temperature geophysical observations do not provide constraints on temperature stress and deformation patterns with desired resolution in contrast the record of high pressure metamorphic rocks exhumed from subduction zones provides details on p t history deformation mechanisms and stress state albeit not readily correlated with the former dynamic situation on larger scale here we review available information on dissolution precipitation creep dpc in high pressure metamorphic rocks which if representative for subduction zones in general can pose constraints on conditions rheology and flow patterns along the plate interface the key observations and conclusions are that 1 deformation is typically highly inhomogeneous and localized into shear zones 2 stresses are generally too low to drive crystal plastic deformation 3 microfabrics suggest dissolution precipitation creep to be the predominant deformation mechanism 4 an aqueous fluid at quasi lithostatic pressure is available throughout allowing for tensile fracturing and crack healing or sealing 5 low stress combined with high strain rates required for localized deformation at typical subduction rates implies low viscosity and 6 contribution of shear heating to the thermal budget of subduction zones should be moderate the dominant deformation mechanism dpc is reviewed in some detail including experimental and theoretical approaches various examples of dpc in high pressure metamorphic rocks are illustrated emphasizing the role of interphase boundaries as sites of dissolution rheology governed by dpc is proposed to control interplate coupling and development of a subduction channel with return flow being a likely candidate for rapid exhumation of high pressure metamorphic rocks

a biogeochemical influence on the secular distribution of orogenic gold orogenic gold deposits are unevenly distributed in geologic time the richest period of orogenic gold deposit formation was the phanerozoic which followed a 1 b y period of relatively meager deposit generation during the proterozoic this proterozoic low gold period developed despite continent building episodes which should have been appropriate for orogenic gold genesis here a brief review of the changes in global geochemistry brought on by evolving interactions between plate tectonics and the biosphere is conducted before the consequences for gold uptake into sedimentary pyrite are discussed it is suggested that the extensive oxygenation of the deep oceans during the second great oxidation event over the period 635 to 510 ma produced conditions where gold was soluble in the deep oceans from that point on regions where bacterial sulfate reduction drove formation of sedimentary pyrite also destabilized this soluble gold promoting its uptake into pyrite and allowing formation of metallogenically enhanced sedimentary sequences these sediments would have become ideal source rocks for gold deposits during subsequent collisional tectonics and metamorphism this biogenic influence on source region enhancement may explain the association during the phanerozoic eon between some of the world s most gold productive regions and sedimentary sequences that contain pyritic carbonaceous rocks

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

deformation and recrystallization of plagioclase along a temperature gradient an example from the bergell tonalite abstract syn to post mylonitic tilting of the bergell tonalite allows investigation of the deformation and recrystallization of plagioclase grains along a temperature gradient from the lower to the upper amphibolite facies at the lowest temperatures recrystallization occurs by nucleation and growth of new grains having a different composition from the old grains in contrast at the highest temperatures recrystallization occurs by subgrain rotation associated with grain boundary migration the temperature increase is inferred to induce this transition in two ways first at higher temperatures the anorthite content of the new grains is higher and thus the comositional difference between old and new grains is lower hence the chemical driving potential is small compared with recrystallization at lower temperatures second higher temperatures facilitate climb of dislocations and thus subgrain formation as well as grain boundary mobility the temperature increase is also associated with a transition in the dominant deformation mechanism intracrystalline plasticity dominates deformation at lower temperatures whereas at higher temperatures deformation occurs primarily by diffusion accommodated grain boundary sliding as indicated by the weakening of the indicatrix preferred orientation and the formation of a mixed plagioclase biotite matrix with increasing deformation therefore the strength of the mid to lower crust may be overestimated by dislocation creep flow laws for plagioclase lower crust rheology for polyphase rocks is better approximated by constitutive relationships involving diffusion accommodated grain boundary sliding

microstructures of melt bearing regional metamorphic rocks the most reliable microstructural criterion for the former presence of felsic melt in regional migmatites is a three mineral quartz k feldspar and sodic plagioclase aggregate in veinlets several other criteria are potentially reliable namely 1 euhedral crystals of feldspar precipitated from liquid or peritectic minerals e g garnet cordierite orthopyroxene k feldspar lining felsic protoleucosomes 2 inclusionfree euhedral overgrowths of feldspar precipitated from liquid or peritectic minerals e g garnet cordierite orthopyroxene k feldspar on residual grains of the same minerals with abundant inclusions in the mesosome 3 aligned euhedral feldspar crystals 4 simple twinning in k feldspar 5 dihedral angles of 60 subtended where a grain of feldspar and or quartz inferred to have pseudomorphed former melt meets two grains of other minerals 6 cuspate volumes of quartz k feldspar or sodic plagioclase especially where surrounded by grains inferred to have been residual during melting 7 veinlets of inferred former melt now mineral pseudomorphs consisting of one of quartz k feldspar or sodic plagioclase preferably though less commonly involving two or three of these minerals along grain boundaries or along inferred former intragranular fractures 8 biotite pseudomorphed by feldspar 9 veinlets of plagioclase that is more sodic than plagioclase grains in the adjacent rock 10 plagioclase with oscillatory zoning 11 microgranophyric intergrowths of quartz and alkali feldspar in patches or veinlets between primary grains 12 symplectic replacement aggregates that can be explained by reactions between peritectic grains and cooling melt and 13 melanosome patches and layers from which leucosome has been extracted however all these criteria must be interpreted with care some other proposed criteria are questionable for example 1 random mineral distributions 2 grain size increase 3 interstitial grains 4 corroded relics of inferred reactant mineral grains surrounded by areas of quartz k feldspar or sodic plagioclase 5 projections into a mineral grain 6 lobes of myrmekite and 7 plagioclase rims with a constant sodic composition occurring on plagioclase cores that are more calcic and or of variable composition

flash vaporization during earthquakes evidenced by gold deposits fluids flowing through cavities in earth s crust can deposit gold thermo mechanical modelling of a fluid filled cavity that expands suddenly during an earthquake shows that the drop in pressure would cause the fluid to vaporize and deposit the gold almost instantaneously

multiple magmatic fabrics in plutons an overlooked tool for exploring interactions between magmatic processes and regional deformation this paper elaborates on the concept of multiple magmatic fabrics in plutons after a general overview of various types of multiple fabrics that may develop in magmatic rocks two case examples of porphyritic granite and melasyenite plutons in the bohemian massif are examined in the jizera granite complex variations in k feldspar phenocryst shape fabric revealed by image analysis of a 200 m long section of an underground tunnel are in contrast with homogeneously oriented magnetic ams fabric carried by coaxial contributions of biotite magnetite and maghemite in the kni eci stolec melasyenite pluton emplacement related margin parallel feldspar foliation was overprinted by flat lying foliation the latter is interpreted to record regional tectonic strain at the two stations examined in detail the crystallographic preferred orientation of biotite and amphibole in the inter phenocryst matrix measured using electron back scatter diffraction ebsd differed from both feldspar fabric and also from the ams principal directions multiple magmatic fabrics in these two plutons are interpreted in terms of fabric superposition where late weak strain is superposed onto a high strength phenocryst framework but is accommodated preferentially by small mineral grains biotite magnetite in the melt bearing matrix this mechanism may explain the discrepancy between mesoscopic feldspar fabric and ams we conclude that multiple magmatic fabrics in plutons may thus result from accumulated strain caused by different processes during final crystallization and as such may serve as a sensitive indicator of the evolving interactions between magmatic and tectonic processes in the earth s crust

timing of collision of the north and south china blocks controversy and reconciliation the qinling orogen was formed by the joining of the north and south china blocks but the timing of their integration has been debated for more than a decade the controversies obviously stem from different approaches to reconstruction of the integration history two contrasting lines of evidence yield two different ages for collision of the north and south china blocks middle paleozoic and late triassic the shangdan suture within the qinling was regarded in previous studies as the trace along which the north and south china blocks collided our studies however demonstrate that there are two sutures within the qinling the well documented shangdan suture and the newly discovered mianlue suture we show in this paper that the late proterozoic to early mesozoic evolution of the qinling involved interactions between the north china block the north and south qinling orogens and the south china block the middle paleozoic collision along the shangdan suture as constrained by some evidence accreted only the south qinling orogen to the southern part i e the north qinling of the north china block contemporaneous rifting of the south china block and subsequent drifting separated the south qinling from the south china block during the middle to late paleozoic the separation of the south from the north china blocks is supported by other evidence in particular geomagnetic data evidently it was the late triassic collision of the south china block with the south qinling orogen along the mianlue suture that led to final integration of the north and south china blocks

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

mineralogy of the sulphide deposits at sulitjelma northern norway abstract the pyritic vms deposits at sulitjelma northern norway have been regionally metamorphosed at amphibolite facies during the caledonide orogen the deposits contain a wide diversity of sulphide sulphosalt and related minerals which are systematically discussed with reference to their occurrence paragenesis and mineral chemistry most of these minerals are however rare to absent within the massive deposits which are dominantly composed of pyrite with subordinate chalcopyrite sphalerite and pyrrhotite galena arsenopyrite cubanite molybdenite stannite and tetrahedrite occur in minor amounts syn metamorphic mobilisation has however resulted in the redistribution of a range of elements including cu pb sb as ni and au these elements have been reconcentrated into mobilised segregations where they have crystallised under retrograde conditions into a range of discrete phases ore mobilisation which took place along tectonically controlled pathways is however of a highly localised nature with a considerable range in mineral textures and parageneses observed in different samples typical associations within these mobilisates include a range of cu pb sb and cu pb as sulphosalts including bournonite meneghinite seligmannite and jordanite breithauptite nisbite costibite arsenopyrite and gudmundite co existing galena is characterised by elevated concentrations of ag bi and se argentian tetrahedrite 9 17 wt ag the chief ag carrier in the deposits as well as other ag bearing phases including pyrargyrite hessite and empressite are strongly enriched in the mobilisates gold as electrum 50 75 wt au and more rarely as aurostibite is also relatively enriched in the mobilisate samples over those from more massive parts of the deposits a further range of minerals including native antimony native arsenic allargentum dyscrasite argentite mckinstryite jalpaite and native silver occur as low temperature breakdown products of earlier formed minerals preserved textures do not contradict a model involving more than a one phase of mobilisation during the metamorphic evolution

a holistic model for the origin of orogenic gold deposits and its implications for exploration the term orogenic gold deposits has been widely accepted but there has been continuing debate on their genesis early syn sedimentary or syn volcanic models and hydrothermal meteoric fluid models are now invalid magmatic hydrothermal models fail because of the lack of consistent spatially associated granitic intrusions and inconsistent temporal relationships the most plausible models involve metamorphic fluids but the source of these fluids is equivocal intra basin sources within deeper segments of the hosting supracrustal successions the underlying continental crust subducted oceanic lithosphere with its overlying sediment wedge and metasomatized lithosphere are all potential sources several features of precambrian orogenic gold deposits are inconsistent with derivation from a continental metamorphic fluid source these include the presence of hypozonal deposits in amphibolite facies domains their anomalous multiple sulfur isotopic compositions and problems of derivation of gold related elements from devolatilization of dominant basalts in the sequences the phanerozoic deposits are largely described as hosted in greenschist facies domains consistent with supracrustal devolatilization models a notable exception is the jiaodong gold deposits of china where ca 120 ma gold deposits are hosted in precambrian crust that was metamorphosed over 2000 million years prior to gold mineralization other deposits in china are comparable to those in the massif central and elsewhere in france in that they are hosted in amphibolite facies domains or clearly post date regional metamorphic events imposed on hosting supracrustal sequences if all orogenic gold deposits have a common genesis the only realistic source of fluid and gold is from devolatilization of a subducted oceanic slab with its overlying gold bearing sulfide rich sedimentary package or the associated metasomatized mantle wedge with co2 released during decarbonation and s and ore related elements released from transformation of pyrite to pyrrhotite at about 500 c although this model satisfies all geological geochronological isotopic and geochemical constraints and is consistent with limited computer based modeling of fluid release from subduction zones the precise mechanisms of fluid flux are model driven and remain uncertain from an exploration viewpoint the model re emphasizes the ubiquitous occurrence of orogenic gold deposits in subduction related orogenic belts and importance of continental scale lithosphere tapping fault and shear zones to focus large volumes of auriferous fluid it confirms the importance of the consistent spacing between world class deposits broadly equivalent to the depth of the moho as derived from empirical observations

rheology of granitic magmas during ascent and emplacement flow abstract considerable progress has been made over the past decade in under standing the static rheological properties of granitic magmas in the continental crust changes in h2o content co2 content and oxidation state of the interstitial melt phase have been identified as important compositional factors governing the rheodynamic behavior of the solid fluid mixture although the strengths of granitic magmas over the crystallization interval are still poorly constrained theoretical investigations suggest that during magma ascent yield strengths of the order of 9 kpa are required to com pletely retard the upward flow in meter wide conduits in low bagnold number magma suspensions with moderate crystal contents solidosities 0 1 0 3 viscous fluc tuations may lead to flow differentiation by shear enhanced diffusion ams and mi crostructural studies support the idea that granite plutons are intruded as crystal poor liquids 50 with fabric and foliation development restricted to the final stages of emplacement if so then these fabrics contain no information on the ascent verti cal transport history of the magma deformation of a magmatic mush during pluton emplacement can enhance significantly the pressure gradient in the melt resulting in a range of local macroscopic flow structures including layering crystal alignment and other mechanical instabilities such as shear zones as the suspension viscosity varies with stress rate it is not clear how the timing of proposed rheological transitions formulated from simple equations for static magma suspensions applies to mixtures undergoing shear new theories of magmas as multiphase flows are required if the full complexity of granitic magma rheology is to be resolved

geology distribution and classification of gold deposits in the western qinling belt central china gold deposits of the western qinling belt occur within the western part of the qinling dabie sulu orogen which is located between the precambrian north china and yangtze cratons and east of the songpan ganzi basin the early paleozoic to early mesozoic orogen can be divided into northern central and southern zones separated by the shangdan and lixian shanyang thrust fault systems the northern zone consists of an early paleozoic arc accreted to the north china craton by ca 450 ma the central zone which contains numerous orogenic gold deposits is dominated by clastic rocks formed in a late paleozoic basin between the converging cratonic blocks the southern zone is characterized by the easternmost exposure of triassic sedimentary rocks of the songpan ganzi basin these early to late triassic turbidities in part calcareous of the immense songpan ganzi basin also border the western qinling belt to the west carlin like gold deposits are abundant 1 along a westward extension of the southern zone defined by a window of early paleozoic clastic rocks extending into the basin and 2 within the easternmost margin of the basinal rocks to the south of the extension and in adjacent cover rocks of the yangtze craton triassic and early jurassic synkinematic granitoids are widespread across the western qinling belt as well as in the songpan ganzi basin orogenic lode gold deposits along brittle ductile shear zones occur within greenschist facies highly deformed devonian and younger clastic rocks of the central zone mainly coarse grained gold along with pyrite pyrrhotite arsenopyrite and minor base metal sulfides occur in networks of quartz veinlets brecciated wall rock and are disseminated in altered wall rock isotopic dates suggest that the deposits formed during the late triassic to middle jurassic as the leading edge of the yangtze craton was thrust beneath rocks of the western qinling belt many gold bearing placers are distributed along the river systems that flow south from the lode bearing central zone carlin like gold deposits have only been identified during the last decade in the southern zone of the western qinling and in the northeastern corner of the songpan ganzi basin the deposits mainly contain micron diameter gold in arsenical pyrite are characterized by the common occurrence of cinnabar stibnite realgar and orpiment exhibit strong silicification carbonatization pyritization and decalcification dissolution textures and are structurally controlled the lack of reactive host lithologies may have prevented development of large 100 tonnes of gold stratigraphically controlled orebodies which are typical of the carlin deposits in the western usa these deposits are hosted by triassic turbidities and shallow water carbonates and an early paleozoic inlier in the songpan ganzi basin that extends in an east west belt for about 300 km rather than true carlin deposits these carlin like deposits may be some type of shallow crustal i e epithermal hybrid with features intermediate to nevada style carlin deposits and the orogenic gold deposits to the immediate north these carlin like deposits also overlap in age with the early mesozoic orogenic gold deposits and therefore also formed during the final stages of collision between the cratons and intermediate basin closure

plio pleistocene transition in the continental record from lake baikal diatom biostratigraphy and age model abstract lake baikal basin in the continental interior asia preserved a unique continuous sedimentary archive of the late cenozoic large scale regional environmental changes of the past were recorded in the composition of fossil diatom flora here the lake baikal planktonic diatom biostratigraphy is extended over the plio pleistocene transition it includes 11 new local diatom assemblage zones ldaz for the time interval 1 25 3 6 ma the comparison of the parallel bdp 96 1 and bdp 98 drill core records allows constructing the complete diatom biostratigraphic record at the time of the inception of the northern hemisphere glaciations diatom biostratigraphic zones around matuyama gauss paleomagnetic reversal are correlated with individual stages of marine oxygen isotope stratigraphy contrary to previous assumptions the period 2 8 2 6 ma in lake baikal record was characterized not by a prolonged minimum in diatom abundance but by a distinct maximum in the abundance of small celled diatom taxa tertiarius baicalensis and stephanodiscus binderanus et var hyalinus which proliferated in the lake during the interval of relatively warm and humid climate the lake baikal diatom biostratigraphic change at around 1 8 ma occurs only at a species level of the genus cyclotella by contrast around 2 8 2 5 ma spectacular appearances and extinctions occurred at a high taxonomic level of genera including the genera of tertiarius thalassiobeckia and stepahanopsis making the biostratigraphic changes around matuyama gauss reversal more prominent at least three distinct biostratigraphic boundaries are observed at 2 69 ma 2 65 ma and at 2 46 ma none of these boundaries however coincides with the paleomagnetic reversal at 2 58 ma thus the new well resolved lake baikal biostratigraphic record points to the difficulty of assigning a single golden spike biostratigraphic boundary around matuyama gauss paleomagnetic reversal in continental asia

fluid immiscibility and gold deposition in the xincheng deposit jiaodong peninsula china a fluid inclusion study abstract the xincheng gold deposit located in west jiaodong peninsula in southeast north china craton is a representative mesothermal lode deposit hosted in late mesozoic granitoids in jiaodong gold mineralization occurs as disseminated and stockwork style ores within the hydrothermal breccias and cataclastic zones controlled by the jiaojia fault whereas echelon tensile auriferous veins hosted in the ne and nne trending subsidiary faults cutting the granitoids occur subordinately according to crosscutting relationships and mineral paragenesis four paragenetic stages were identified which are pyrite quartz sericite stage 1 quartz pyrite stage 2 quartz polysulfide stage 3 and quartz carbonate stage 4 gold was deposited during the quartz pyrite and quartz polysulfide stages on the basis of microthermometry and raman spectroscopy on fluid inclusions contained within the quartz veins from stages 2 and 3 three types of fluid inclusions were recognized 1 type 1 h 2 o co 2 inclusions that show high temperatures ca 260 c low salinities 2 4 8 9 wt equiv nacl and variable xco 2 0 03 to 0 20 2 type 2 aqueous inclusions with medium temperatures ca 220 c and low to moderate salinities 3 1 13 3 wt equiv nacl 3 type 3 pure co 2 inclusions with a carbonic phase density of 0 712 0 03 g cm 3 types 1 and 2 inclusions appear in the same growth phase of the quartz grains from the breccias and tensile auriferous veins these coexisting inclusions are likely formed by fluid immiscibility due to unmixing from a single homogeneous h 2 o co 2 parent fluid at trapping p t conditions of 221 to 304 c average 261 19 c and 780 to 2080 bar the fluid immiscibility is interpreted to be initiated by fluid pressure decrease at ca 300 c the ore fluid p t x conditions of the xincheng gold deposit are the same as those for mesothermal deposits gold was most probably transported as a au hs 2 complex at xincheng fluid immiscibility over the temperature interval of 221 304 c resulted in significant h 2 s loss from the hydrothermal solution thereby reducing au hs 2 solubility with concomitant deposition of gold the mineralizing process of the granitoid hosted xincheng lode gold deposit is likely related to the fluid immiscibility

a proposed new type of arsenian pyrite composition nanostructure and geological significance this report describes a new form of arsenian pyrite called as3 pyrite in which as substitutes for fe fe as s2 in contrast to the more common form of arsenian pyrite as1 pyrite in which as1 substitutes for s fe as s 2 as3 pyrite has been observed as colloformic overgrowths on as free pyrite in a hydrothermal gold deposit at yanacocha peru xps analyses of the as3 pyrite confirm that as is present largely as as3 empa analyses show that as3 pyrite incorporates up to 3 05 at of as and 0 53 at 0 1 at 0 27 at 0 22 at 0 08 at and 0 04 at of pb au cu zn ni and co respectively incorporation of as3 in the pyrite could be written like as3 y au 1 y 2 fe2 where au and vacancy help to maintain the excess charge hrtem observations reveal a sharp boundary between as free pyrite and the first overgrowth of as3 pyrite 20 40 nm thick and co linear lattice fringes indicating epitaxial growth of as3 pyrite on as free pyrite overgrowths of as3 pyrite onto as free pyrite can be divided into three groups on the basis of crystal size 8 20 nm 100 300 nm and 400 900 nm and the smaller the crystal size the higher the concentration of toxic arsenic and trace metals the yanacocha deposit in which as3 pyrite was found formed under relatively oxidizing conditions in which the dominant form of dissolved as in the stability field of pyrite is as3 in contrast reducing conditions are typical of most environments that host as1 pyrite as3 pyrite will likely be found in other oxidizing hydrothermal and diagenetic environments including high sulfidation epithermal deposits and shallow groundwater systems where probably kinetically controlled formation of nanoscale crystals such as observed here would be a major control on incorporation and release of as3 and toxic heavy metals in oxidizing natural systems

mobility of gold during metamorphism of the dalradian in scotland abstract mobility of au and related metals during metamorphism has been suggested to be the source of metals enriched in orogenic au deposits this study investigates the mobility of au as and sb during metamorphism of the dalradian metasedimentary rocks of scotland the metamorphic processes in the dalradian of scotland are extremely well studied and the terrane is an ideal area to investigate mobility of these metals our results show that of the 25 major and trace elements analysed only au as sb s and volatile contents as shown by loss on ignition loi values show systematic variation with the metamorphic grade of the samples average au concentrations decrease from 1 1 0 55 ppb and 0 72 0 34 ppb in chlorite and biotite zone rocks down to 0 4 0 22 ppb and 0 34 0 13 ppb in kyanite and sillimanite zone rocks average as concentrations decrease from 4 8 ppm range 0 5 to 17 8 ppm and 1 96 1 9 ppm in chlorite and biotite zone rocks down to 0 24 0 15 ppm and 0 2 0 12 ppm in kyanite and sillimanite zone rocks average sb concentrations decrease from 0 18 0 15 ppm and 0 11 0 10 ppm in chlorite and biotite zone rocks down to 0 04 0 02 ppm in both kyanite and sillimanite zone rocks sulphur and loi concentrations also show significant decreases mass balance calculations indicate that compared to chlorite and biotite zone samples sillimanite zone samples have an average mass loss of 62 14 94 4 and 74 14 for au as and sb respectively every 1 km3 of chlorite biotite zone mixed psammitic pelitic protolith rock that is metamorphosed to sillimanite zone conditions would release 1 5 t au 8613 t as 270 t sb and 1 02 mt s the mobility of these elements is strongly controlled by the paragenesis of sulphide minerals pyrite sphalerite galena and cobaltite as well as gersdorffite decrease in abundance with increasing metamorphic grade in the dalradian metasedimentary rocks a critical aspect of the sulphide paragenesis is the transition of pyrite to pyrrhotite this transition is complete by mid greenschist facies in the loch lomond samples but is more gradual at glen esk occurring between biotite and sillimanite zones the au as and sb content of the sulphide assemblage also decreases with increasing metamorphic grade and we suggest that this is a controlling factor on the mobility of these metals from the dalradian metasedimentary rocks during metamorphism chlorite may be an important host mineral for as in the greenschist facies rocks breakdown of chlorite indirectly drives the mobility of au as and sb as this produces the bulk of metamorphic fluid that drives transition between pyrite and pyrrhotite we suggest that there is potential for significant undiscovered mineralisation in the central and sw highlands of scotland however as the total mass of gold mobilised is lower than observed in other metasedimentary terranes such as the otago and alpine schist s new zealand very efficient fluid focussing and trapping mechanisms would be required to form large deposits in the dalradian of scotland

tectonic and district to deposit scale structural controls on the ge erke orogenic gold deposit within the dashui zhongqu district west qinling belt china abstract ge erke 100 t gold is one of six mesozoic gold deposits within the dashui zhongqu district that collectively have a non jorc resource of about 150 tonnes gold out of a total of 1100 tonnes gold for the host west qinling belt the deposits in the dashui zhonggu district are deeply weathered to depths of up to 400 m with secondary fe oxides having replaced pyrite and other sulfides this makes traditional ore deposit studies involving mineralogical geochemical and isotopic analysis difficult thus their association has been controversial with correlations to deposits that include carlin and intrusion related types postulated however these associations can be discounted in favour of their classification as epizonal orogenic gold deposits at ge erke gold mineralization was focused in the hangingwall of a broadly south dipping southern margin of a granite pluton in a wide variety of fault trends this complexity was caused by heterogeneous stresses developed around the rigid pluton by the broadly ne sw directed regional principal maximum stress responsible for the sinistral movement along the crustal scale faults which control the regional alignment of the west qinling gold deposits at the orebody and ore shoot scale gold mineralization is controlled by rheology contrasts particularly the fault contacts between crystalline limestone units and granite dykes which are themselves in a variety of orientations because of inhomogeneous strain around the parent granite pluton in this region the secondary faults that subparallel the mianlue suture and associated crustal scale faults are poorly gold mineralized in the absence of granite plutons during exploration such buried plutons and the orientations of their margins can best be detected by gravity surveys although magnetic surveys may also be useful

timing and mechanism of gold mineralization at the wang ershan gold deposit jiaodong peninsula eastern china abstract the wang ershan gold deposit located in the southern jiaojia goldfield is currently the largest gold deposit hosted within the subsidiary faults in jiaodong peninsula with a gold reserve of 60 t gold at a grade of 4 07 g t au it is hosted in the late jurassic linglong biotite granites and controlled by the second order n to nne trending wang ershan fault and its subsidiary faults which is broadly parallel to the first order jiaojia fault in the goldfield gold mineralization occurs as both disseminated and stockwork style and quartz sulfide vein style ores mainly within altered cataclasites and breccias and sericite quartz and potassic alteration zones respectively mineralization stages can be divided into 1 the pyrite quartz sericite stage 2 the quartz pyrite stage 3 the quartz sulfide stage and 4 the quartz carbonate stage two sericite samples associated with the main ore stage pyrites from pyritic phyllic ores of the deposit with weighted mean plateau 40 ar 39 ar age of 120 7 0 6 ma and 119 2 0 5 ma respectively were selected for 40 ar 39 ar geochronology on the basis of petrography and microthermometry three types of primary fluid inclusions related to the ore forming event were identified type 1 h 2 o co 2 nacl type 2 aqueous and type 3 co 2 fluid inclusions in decreasing abundance stage 1 quartz contains all three primary fluid inclusions while stages 2 and 3 quartz contain both type 1 and 2 inclusions and stage 4 quartz contains only type 2 inclusions the contemporaneous trapping similar salinities and total homogenization temperature ranges and different homogenization phases of type 1 and type 2 inclusions indicate that fluid immiscibility did take place in stages 1 2 and 3 ores with p t conditions of 190 to 85 mpa and 334 to 300 c for stage 1 and 200 to 40 mpa and 288 to 230 c for stages 2 and 3 combined with the h o c s pb isotopic compositions ore forming fluids may have a metamorphic dominant mixed source which could be associated with the dehydration and decarbonisation of a subducting paleo pacific plate and characterized by medium high temperature 285 350 c co 2 bearing 8 mol with minor ch 4 1 4 in carbonic phase and low salinity 3 38 8 45 eq wt nacl during mineralization the fluid finally evolved into a medium low temperature nacl h 2 o system au hs 2 was the most probable gold transporting complex at wang ershan due to the low temperature 157 350 c and near neutral to weakly acidic ore fluids the reaction between gold bearing fluids and iron bearing wall rocks and fluid immiscibility processes caused via fluid pressure cycling during seismic movement along fault zones that host lode gold orebodies which led to breakdown of au hs 2 are interpreted as the two main precipitation mechanisms of gold deposition in general the wang ershan deposit and other deposits in the jiaojia camp have concordant structural system and wall rock alteration assemblages nature of orebodies and gold occurrence conditions as well as the similar geochronology ore forming fluids system and stable isotope compositions thus gold mineralization in the jiaojia goldfield was a large scale unified event with consistent timing origin process and mechanism

origin and evolution of ore fluid and gold deposition processes at the giant taishang gold deposit jiaodong peninsula eastern china abstract the early cretaceous taishang gold deposit is the largest deposit in the linglong goldfield jiaodong peninsula with a pre mining endowment of 326 t of gold it is hosted in the 165 150 ma linglong biotite granite and controlled by the nee to ne trending potouqing fault near the northern end of the regional zhaoping fault system hydrothermal alteration is well developed in the footwall of the potouqing fault and is characterized by a narrow zone of sericitization followed by a wider zone of silicification and an even wider zone of potassic alteration the main orebodies no i orebody comprise disseminated and stockwork veinlet systems although about 5 of the resources are in larger quartz veins no ii orebody four stages have been identified for both styles of mineralization on the basis of cross cutting relationships and mineralogical and textural characteristics pyrite quartz sericite stage 1 quartz pyrite stage 2 quartz pyrite base metal sulfide stage 3 and quartz carbonate stage 4 gold was mainly deposited in stages 2 and 3 with minor amounts in stage 1 petrographical microthermometric and laser raman spectroscopic studies on fluid inclusion assemblages in quartz and calcite from the four stages reveal three types of primary fluid inclusions type 1 h 2 o rich aqueous carbonic type 2 co 2 rich aqueous carbonic and rare type 3 carbonic inclusions stage 1 quartz primary inclusions are only type 1 inclusions with an estimated composition of 88 h 2 o 10 co 2 4 5 wt nacl equiv and 0 5 ch 4 with trapping at 336 c and 1 7 kbars the gold rich stages 2 and 3 from both orebodies typically contain primary fluid inclusion assemblages with both type 1 and 2 inclusions which show similar phase transition temperatures and were trapped between 246 and 294 c the stage 4 quartz and calcite contain only primary type 1 inclusions which are estimated to have a composition of 93 h 2 o 6 0 co 2 3 6 wt nacl equiv and trace amount of ch 4 and were trapped at temperatures of 236 c the 34 s values of hydrothermal pyrite from the four stages have a narrow range from 4 5 to 8 0 and are within the ranges for whole rock sulfides from the archean jiaodong group and magmatic pyrite from mesozoic granitoid and intermediate basic dikes the 18 o values of hydrothermal quartz range mainly from 10 9 to 12 5 and remain constant for all four stages calculated fluid 18 o values are 1 3 10 0 the d water values calculated from hydrothermal sericite range from 60 to 45 considering the fluid inclusion compositions 18 o and d compositions of ore forming fluids and regional geological events the most likely ultimate potential fluid and metal reservoirs would be the paleo pacific oceanic slab and its overlying sediments which were thrust below the high grade metamorphic rocks of the jiaodong peninsula the initial ore forming fluids were medium high temperature co 2 rich and low salinity h 2 o co 2 nacl ch 4 homogeneous fluids over the duration of the hydrothermal system the fluid remained fairly consistent in p t x although becoming slightly more water rich during final post ore activity fluid immiscibility occurred during stages 2 and 3 ore deposition at pressures that fluctuated strongly from 1700 to 580 bars during hydrofracturing the hs 2 ion was the most probable gold transporting complex at taishang wall rock sulfidation and episodic pressure drops with associated fluid unmixing and other chemical changes were the two main mechanisms of ore deposition

multistage sedimentary and metamorphic origin of pyrite and gold in the giant sukhoi log deposit lena gold province russia gold mineralization at sukhoi log in eastern siberia is hosted in a deformed neoproterozoic organic bearing and pyritic black shale and siltstone sequence that is folded into a tight overturned anticline the deposit contains about 30 million ounces of gold at an average grade of 2 0 g t au and is one of the largest known undeveloped gold resources the high grade gold zone forms a gently dipping tabular body in the core of the anticline the best gold grades occur in narrow bedding parallel pyrite quartz veinlets that have been folded during the main deformation event lower grade gold is associated with disseminated pyrite developed in and around the high grade core of the deposit detailed paragenetic studies of the mineralization and host rocks have defined six stages of pyrite development in the carbonaceous sediments the two earliest forms of pyrite termed py1 and py2 are commonly developed in stratiform layers of micron sized crystals framboids and fine euhedra which are interpreted as synsedimentary to early diagenetic in origin coarser grained bedding parallel aggregates of inclusion rich pyrite termed py3 contain inclusions of arsenopyrite native gold and gold tellurides and are interpreted to form during late diagenesis and earliest deformation coarse euhedral pyrite py4 overgrows the earlier pyrite py1 py2 and py3 and the slaty cleavage developed in the host rocks indicating a syndeformation timing late stage inclusion free pyrite py5 overgrows and replaces earlier sulfides and is considered to be syn to late deformation laser ablation inductively coupled plasma mass spectrometry la icpms analyses of the various pyrite types indicate that the synsedimentary py1 contains the highest levels of invisible gold varying from 0 4 to 12 1 ppm with a mean of 3 22 ppm au and 1 900 ppm as py1 is also enriched in a suite of trace elements mo sb ni co se te ag cu pb zn mn ba cr u v which are similar to those concentrated by organic processes in euxinic sedimentary environments later generations of pyrite from py2 to py5 including pyrite in bedding parallel pyrite quartz veinlets contain progressively lower contents of invisible gold and most other trace elements however this metamorphic and postmetamorphic pyrite contains microinclusions of free gold arsenopyrite pyrrhotite sphalerite and chalcopyrite the paragenetic textural and chemical relationships at sukhoi log suggest that gold was clearly initially introduced prior to cleavage development accompanying sedimentation of the organic rich shales and fixed during diagenesis within the structure of diagenetic arsenian pyrite subsequently accompanying deformation gold was liberated from recrystallized diagenetic pyrite to become concentrated as free gold and gold tellurides within metamorphic pyrite and folded bedding parallel pyrite quartz veinlets two key processes are considered vital to the formation of the sukhoi log deposit original synsedimentary and early diagenetic concentration of gold dissolved within arsenian pyrite in organic rich black shales and metamorphic processes that liberated gold from the early forms of arsenian pyrite to be concentrated as free gold and gold tellurides within late diagenetic and metamorphic pyrite and associated pyrite quartz veinlets in the core of an overturned anticline these ore forming processes are unlikely to be unique to sukhoi log other black shale and turbidite hosted deposits that occur in rifted continental margin environments which have undergone collision and basin inversion may form by similar processes

extreme variation of sulfur isotopic compositions in pyrite from the qiuling sediment hosted gold deposit west qinling orogen central china an in situ sims study with implications for the source of sulfur high spatial resolution textural scanning electron microscope sem chemical electron microprobe emp and laser ablation inductively coupled plasma mass spectrometry la icp ms and sulfur isotopic secondary ion mass spectrometry sims analyses of pyrite from the qiuling sediment hosted gold deposit 232 4 ma in the west qinling orogen central china were conducted to distinguish pyrite types and gain insights into the source and evolution of sulfur in hydrothermal fluids the results reveal an enormous variation 27 1 to 69 6 in sulfur isotopic composition of pyrite deposited during three paragenetic stages pre ore framboidal pyrite which is characterized by low concentrations of as au cu co and ni has negative 34s values of 27 1 to 7 6 that are interpreted in terms of bacterial reduction of marine sulfate during sedimentation and diagenesis of the paleozoic carbonate and clastic sequences the predominant lithologies in the deposit area and the most important hosts of many sediment hosted gold deposits throughout the west qinling orogen the ore stage hydrothermal pyrite contains high concentrations of au as cu sb tl and bi and has a relatively narrow range of positive 34s values ranging from 8 1 to 15 2 the sulfur isotope data are comparable to those of ore pyrite from many triassic orogenic gold deposits and paleozoic sedimentary exhalative sedex pb zn deposits in the west qinling orogen both being hosted mainly in the devonian sequence this similarity indicates that sulfur responsible for the auriferous pyrite at qiuling was largely derived from the metamorphic devolatization of paleozoic marine sedimentary rocks post ore stage pyrite which is significantly enriched in co and ni but depleted in au and as has unusually high 34s values ranging from 37 4 to 69 6 that are interpreted to result from thermochemical reduction of evaporite sulfates in underlying cambrian sedimentary rocks with very high 34s values the variations in au content and sulfur isotopic compositions across a single ore stage pyrite grain may reflect displacement of indigenous groundwater with low 34s values by auriferous metamorphic fluids with high 34s values the very low grade metamorphism of the host rocks and the metamorphic derivation of sulfur for the ore pyrite indicate that the qiuling sediment hosted gold deposit is an epizonal manifestation of an orogenic gold system in the west qinling orogen

la icp ms trace element analysis of pyrite from the chang an gold deposit sanjiang region china implication for ore forming process abstract the chang an gold deposit yunnan province is one of five large gold deposits in the ailaoshan gold belt that is the most important ore belt of the sanjiang tethyan metallogenic domain geochemical study of pyrite in the deposit was conducted using laser ablation inductively coupled plasma mass spectroscopy three types of hydrothermal pyrite were identified in the ores and wall rocks i e the coarse euhedral crystals in syenite py1 the coarse grains disseminated in altered sandstones or sandstone ores py2 and the fine grained euhedral pyrite in sandstone ores py3 their trace elements exhibit different concentrations associations and rim core zoning implying different geneses and crystallization processes the cores of py1 were formed by magmatic fluids and have the lowest concentrations of au as cu and zn the cores of py2 were formed by metamorphic fluids and have relatively high au ag ni pb and cu concentrations the py3 grains and the growth rims of py1 and py2 show consistently high contents of au as pb and co suggesting that all of them were rapidly deposited from a mixing fluid system that was probably composed of fluids of metamorphic and magmatic origins this interpretation is supported by the observation that the high grade ores generally contain lots of py2 and py3 hence we consider that the deposit was formed during india asia collision slightly postdate the 33 35 ma tectono magmatism

arsenopyrite pyrite association in an orogenic gold ore tracing mineralization history from textures and trace elements laser ablation inductively coupled plasma mass spectrometry la icp ms spot analysis and element mapping coupled with high resolution focused ion beam scanning electron microscopy fib sem imaging has been performed on au hosting sulfides from a proterozoic orogenic gold deposit tanami gold province north central australia gold distribution patterns in the arsenopyrite pyrite assemblage which probably crystallized toward the end of the au forming event suggest a process of initial scavenging of au into arsenopyrite and subsequent remobilization of that au following brittle fracture gold expelled from the sulfide lattice during the remobilization event is reconcentrated around the margins of the same arsenopyrite grains and within swarms of crosscutting microfractures the distribution of pb bi and ag closely mimic au indicating that these elements were also reconcentrated preservation of oscillatory zonation patterns for co ni sb se and te in arsenopyrite imply however that no significant degree of sulfide recrystallization took place residual concentrations of invisible gold in grain centers are 5 ppm in arsenopyrite and 1 ppm in coexisting pyrite the presence of submicron sized pores is suggestive of a fluid driven process rather than solid state diffusion micron scale 2 10 m remobilized gold is accompanied by fine particles 200 nm 2 m and nanoparticles 200 nm of gold the extensive variation of au concentrations within single arsenopyrite grains underlines the significance of textures when using trace element data to understand how gold ores evolved

evidence for strain rate variation and an elevated transient geothermal gradient during shear zone evolution in the cordillera blanca peru abstract the cordillera blanca shear zone cbsz occupies the footwall of the cordillera blanca detachment fault an active low moderate angle detachment fault extending for 200 km along the western margin of the cordillera blanca in central peru recrystallized quartz paleopiezometry two feldspar thermometry on asymmetric strain induced myrmekite and structural analysis from the cbsz provide evidence for shear zone evolution within a transient elevated geotherm during syn convergent extension shear zone width varies by a factor of six along strike with widths of up to 400 m within the central segment of the shear zone microstructurally derived strain rates calculated from quartz paleopiezometry and two feldspar thermometry results are on the order of 10 14 to 10 12 s 1 and are broadly consistent with macrostructurally derived strain rates of 10 14 to 10 13 s 1 calculated from shear zone thickness and previously determined exhumation rates within the context of existing thermochronological data constraints on batholith emplacement and previously determined offset along the cordillera blanca detachment our data suggest that ductile deformation in the cbsz occurred within a transient elevated geothermal gradient of at least 50 c km and potentially as high as 100 c km which deviated from a 30 c km gradient for an unspecified duration of time we discuss the evolution of the cordillera blanca where observed deformation fabrics are related to strain localization along an exhuming fault as well as thermal weakening from the interplay between faulting and magmatism

melt assisted interior to margin switch from dislocation to diffusion creep in coarse grained plagioclase evidence from a deformed anorthosite pluton abstract at bolangir eastern india massive anorthosites in the pluton interior grade into anorthosites with outward dipping margin parallel foliation neighboring the pluton margin sheets and veins of ferrodiorites residual melts of anorthosite crystallization concordant and discordant to the margin parallel foliation suggest pluton deformation at near magmatic conditions t 950 c and p 6 12 kbar in the pluton interior the larger than centimeter sized magmatic plagioclase grains are replaced by aggregates of smaller 100 600 m dynamically recrystallized internally strained grains with un equilibrated boundaries neighboring the pluton margin poly sized 200 2500 m plagioclase grains in anorthosites are of two types strain free rectangular shaped plagioclase grains with high energy an richer margins indenting neighboring plagioclase grains formed by diffusion creep whereas unstrained end to end touching euhedral plagioclase grains showing tilling represent magmatic flow textures the pluton interior to margin switch in plagioclase deformation from grain boundary migration accommodated dislocation creep to grain boundary diffusion creep is attributed to the increasing melt fraction melt crystal ratio during syn deformation pluton emplacement plagioclase grains in the bolangir pluton are significantly coarser compared to plagioclase aggregates

relationships between volcano gravitational spreading and magma intrusion volcano spreading with its characteristic sector grabens is caused by outward flow of weak substrata due to gravitational loading this process is now known to affect many present day edifices a volcano intrusive complex can form an important component of an edifice and may induce deformation while it develops such intrusions are clearly observed in ancient eroded volcanoes like the scottish palaeocene centres or in geophysical studies such as in la reunion or inferred from large calderas such as in hawaii the canaries or galapagos volcanoes volcano gravitational spreading and intrusive complex emplacement may act simultaneously within an edifice we explore the coupling and interactions between these two processes we use scaled analogue models where an intrusive complex made of golden syrup is emplaced within a granular model volcano based on a substratum of a ductile silicone layer overlain by a brittle granular layer we model specifically the large intrusive complex growth and do not model small scale and short lived events such as dyke intrusion that develop above the intrusive complex the models show that the intrusive complex develops in continual competition between upward bulging and lateral gravity spreading the brittle substratum strongly controls the deformation style the intrusion shape and also controls the balance between intrusive complex spreading and ductile layer related gravitational spreading in the models intrusive complex emplacement and spreading produce similar structures to those formed during volcano gravitational spreading alone i e grabens folds en echelon fractures therefore simple analysis of fault geometry and fault kinetic indicators is not sufficient to distinguish gravitational from intrusive complex spreading except when the intrusive complex is eccentric from the volcano centre however the displacement fields obtained for 1 a solely gravitational spreading volcano and for 2 a gravitational spreading volcano with a growing and spreading intrusive complex are very different consequently deformation fields like those obtained from geodetic monitoring can give a strong indication of the presence of a spreading intrusive complex we compare the models with field observations and geophysical evidence on active volcanoes such as la reunion island indian ocean ometepe island nicaragua and eroded volcanic remnants such as ardnamurchan scotland and suggest that a combination between gravitational and intrusive complex spreading has been active

new constraints on fluid sources in orogenic gold deposits victoria australia fluid inclusion microthermometry raman spectroscopy and noble gas plus halogen geochemistry complemented by published stable isotope data have been used to assess the origin of gold rich fluids in the lachlan fold belt of central victoria south eastern australia victorian gold deposits vary from large turbidite hosted orogenic lode and disseminated stockwork gold only deposits formed close to the metamorphic peak to smaller polymetallic gold deposits temporally associated with later post orogenic granite intrusions despite the differences in relative timing metal association and the size of these deposits fluid inclusion microthermometry indicates that all deposits are genetically associated with similar low salinity aqueous co2 bearing fluids the majority of these fluid inclusions also have similar 40ar 36ar values of less than 1500 and 36ar concentrations of 2 6 58 ppb by mass that are equal to or much greater than air saturation levels 1 3 2 7 ppb limited amounts of nitrogen rich fluids are present at a local scale and have the highest measured 40ar 36ar values of up to 5 700 suggesting an external or distinct source compared to the aqueous fluids the predominance of low salinity aqueous carbonic fluids with low 40ar 36ar values in both orogenic and intrusion related gold deposits is attributed to fluid production from common basement volcano sedimentary sequences and fluid interaction with sedimentary cover rocks turbidites aqueous fluid inclusions in the stawell magdala deposit of western victoria including those associated with n2 preserve mantle like br cl and i cl values in contrast fluid inclusions in deposits in the eastern structural zones which contain more abundant shales have elevated molar i cl ratios with maximum values of 5 170 10 6 in the melbourne zone br i ratios in this zone range from 0 5 to 3 0 that are characteristic of fluid interaction with organic rich sediments the maximum i cl and characteristic br i ratios provide evidence for organic br and i released during metamorphism of the shales therefore the regional data provide strong evidence for the involvement of sedimentary components in gold mineralisation but are consistent with deeper metamorphic fluid sources from basement volcano sedimentary rocks the overlying sediments are probably involved in gold mineralisation via fluid rock interaction

gold distribution in as deficient pyrite and telluride mineralogy of the yangzhaiyu gold deposit xiaoqinling district southern north china craton the mesozoic yangzhaiyu lode gold deposit is situated in the southern edge of the north china craton gold mineralization is hosted in archean amphibolite facies metamorphic rocks and consists mainly of auriferous quartz veins pyrite is the predominant sulfide mineral with minor amounts of chalcopyrite sphalerite and galena based on morphology and paragenesis there are three generations of pyrite termed as first generation g1 second generation g2 and third generation g3 they have distinct contents occurrences and distribution patterns of gold the coarse grained euhedral g1 pyrite contains negligible to low levels of gold whereas both invisible and visible gold are present in the fine to medium grained g2 pyrite that is characterized by abundance of microfractures and porosities forming a foam like texture laser ablation inductively coupled plasma mass spectrometry la icp ms depth profiles indicate that invisible gold occurs either as solid solution or as nanoparticles of gold bearing tellurides in the g2 pyrite visible gold is widespread and present as irregular grains and stringers of native gold mostly along grain boundaries or filling microfractures of pyrite likely resulting from remobilization of invisible gold once locked in the g2 pyrite the g3 pyrite invariably intergrown with chalcopyrite sphalerite and galena contains the highest levels of invisible gold there is a positive correlation between au ag and te indicating that gold occurs as submicroscopic au bearing telluride inclusions in the host minerals whenever gold either invisible or visible is present as is always below or only marginally higher than the detection limit of la icp ms this indicates that as played an insignificant role in gold mineralization tellurides are widespread in the auriferous quartz veins consisting mainly of petzite calaverite hessite altaite and tellurobismuthite native gold commonly occurs as intergrowths with tellurides textural evidence indicates a precipitation sequence in a temporal order of calcaverite petzite altaite tellurobismuthite and hessite little amount of sulfide phases has been found in association with the tellurides indicating that tellurides were deposited under low s fugacity fs2 and or high te fugacity fte2 conditions the textural relationships when combined with fluid inclusion microthermometric data of auriferous quartz veins and tellurides thermodynamic data permit estimation for logfte2 during telluride formation which are 6 8 to 10 8 at 300 c and 9 6 to 17 6 at 250 c available geochronological and geochemical data suggest that te was most likely derived from the late mesozoic magmatic rocks widespread in the xiaoqinling district and other parts of the southern north china craton which were emplaced broadly contemporaneous with gold mineralization at yangzhaiyu this study highlights the role of te and tellurides as important gold scavengers in as deficient ore fluids

origin of the jiaodong type xinli gold deposit jiaodong peninsula china constraints from fluid inclusion and c d o s sr isotope compositions abstract the xinli gold deposit within the sanshandao cangshang gold belt in the jiaodong gold province is a typical jiaojia type ore deposit with high grade quartz sulfide vein veinlet stockworks that cut mesozoic granodiorite four ore stages are distinguished in the following sequences i quartz k feldspar sericite pyrite ii quartz pyrite iii quartz polymetallic sulfide and iv quartz calcite pyrite fluid inclusions in quartz and calcite in the ores contain c o h fluids dominantly of six types with varied liquid vapor ratio ore fluid in the main ore stage is characterized by medium low temperature low salinity reducing condition and nacl h 2 o co 2 ch 4 in composition the 18 o water smow of quartz in ore stages ii and iii range from 2 82 to 5 34 and the d smow are between 69 6 and 88 3 both suggest that the ore fluid was dominantly magmatic water the 13 c pdb of the calcite are from 6 4 to 2 4 indicating mantle contribution to the ore fluid the pyrite 34 s cdt from four ore stages are basically similar varying from 9 42 to 11 62 which implies significant involvement of crustal materials the initial 87 sr 86 sr ratios in the calcite ranging from 0 710657 to 0 711542 also denote a crustal contribution the fluid composition and systematic h o c s sr isotopic data are explained by a model of slab subduction induced gold mineralization in the model the ore fluid and metals were considered to associate with the dehydration and desulfidation of the subducting paleo pacific slab and the subsequent devolatilization of enriched mantle wedge the h 2 o in the fluid might have been released from the devolatilization of enriched mantle wedge the co 2 could be from mantle wedge or subcontinental lithospheric mantle and the sulfur and strontium would be initially released from the subducted seafloor sediments this process was further supported by the geochemical features of intermediate basic dykes contemporaneous to the gold ore deposits the deposit features and ore forming process of the xinli gold deposit hardly concur those of the orogenic gold deposit and the pluton related gold deposit the study supports the argument of that the gold deposits in the jiaodong belong to a unique gold type related to a complex slab subduction related process the geodynamic engine of the jiaodong type gold deposit is specific i e the slab subduction coupling with the consequent lithospheric thinning and structure adjustment

the identification of the guqiong orogenic ag au polymetallic deposit in southern tibet evidences from mineralogy geochronology and fluid evolution abstract guqiong is an ag au polymetallic deposit located in the eastern part of the au sb ore belt along the indus yarlung zangbo suture in the north himalaya in this study we have identified three major alteration mineralization stages for guqiong based on the mineral assemblages and their paragenetic relationships namely the i pre ore alteration forming mainly ore barren thick quartz veins and vuggy quartz ii main ore stage that comprises quartz veins with sulfide mineralization including ag au bearing minerals in this stage sericite occurs closely associated with sulfides iii the post ore stage characterized by the formation of calcite and quartz with trace sulfides fluid inclusion assemblage fia microthermometry was conducted on barren and mineralized quartz and sphalerite three types of inclusions are identified based on their petrographic features and laser raman spectra i e the pure carbonic pc aqueous carbonic three phase c1 and two phase c2 and aqueous w types the fias in stage i ore barren quartz are mainly pc type and c1 type which are characterized by being high temperatures 260 to 300 c low salinities 0 8 to 3 5 wt nacl eqv and with abundant co2 sphalerite of main ore stage contains mainly w type with limited c2 type of low temperatures 158 to 180 c and medium salinities 5 6 to 6 7 wt nacl eqv meanwhile the fia in mineralized quartz is featured by the general coexistence of c2 type and w type with local pc type and c1 type inclusions the fluid system may have been a mixture of the two aforementioned fluids in barren quartz and sphalerite as indicated by two significantly different fias c type and w type with contrasting homogenization temperatures 260 to 290 c and 180 to 220 c respectively but comparable salinities 0 2 to 8 2 and 0 2 to 8 1 wt nacl eqv respectively the fluid d 18o and 13c values of the main ore stage quartz are 172 to 114 8 8 to 13 1 and 13 4 to 7 7 respectively the negative 13c values are interpreted as devolatilization products of biogenic carbon with a metamorphic origin while the d and 18o values likely reflect meteoric water involvement into metamorphic fluids we propose that fluid mixing had occurred and caused sulfide precipitation at the main ore stage additionally the estimated fluid trapping depth of 5 to 6 km obtained using 3 d phase diagram is consistent with the formation depth of many orogenic type deposits worldwide 40ar 39ar dating on sericite yielded a well defined plateau age of 42 03 0 42 ma with normal and inverse isochron ages of 42 8 1 6 ma and 42 9 1 9 ma respectively these ages bracket the ag au mineralization at guqiong place it in the main stage india asia collision and further support its orogenic type origin

a carbonaceous sedimentary source rock model for carlin type and orogenic gold deposits this paper presents evidence and arguments that carbonaceous sedimentary rocks were a source for au and as in sediment hosted orogenic and carlin type gold deposits and develops a corresponding genetic model in this two stage basin scale model gold and arsenic are introduced early into black shale and turbidite basins during sedimentation and diagenesis stage 1 and concentrated to ore grades by later hydrothermal structural or magmatic processes stage 2 in reduced continental margin basin settings organic matter sedimented under anoxic to euxinic conditions immobilizes and concentrates gold arsenic and a range of trace elements particularly v ni se ag zn mo cu u present in marine bottom waters into fine grained black mudstone and siltstone of slope and basin facies during early diagenesis gold and certain other trace elements ni se te ag mo cu pge are preferentially partitioned into arsenian pyrite that grows in the muds these processes produce regionally extensive black shale and turbidite sequences enriched in syngenetic gold and arsenic commonly from 5 to 100 ppb au and 10 to 200 ppm as rare organic and sulfide rich metalliferous black shales may contain up to 1 to 2 ppm au and over 1 000 ppm as present as refractory gold in arsenian pyrite and nanoparticles of free gold during late diagenesis and early metamorphism stage 2 the diagenetic arsenian pyrite is recrystallized to form coarser grained pyrite generations and the organic matter is cooked to bitumen under higher grade metamorphism lower greenschist facies and above arsenian pyrite in carbonaceous shales is converted to pyrrhotite these processes release gold arsenic sulfur and other elements sb te cu zn mo bi tl and pb from the source rocks to become concentrated by hydrothermal processes locally to produce gold ores in structural sites such as fold hinge zones shear or breccia zones within or above the black shale sequence la icp ms analyses of diagenetic pyrite in carbonaceous sediments both associated and not associated with gold deposits suggests that invisible gold contents of greater than 250 ppb in diagenetic pyrite are indicative of carbonaceous shale source rocks with the potential to produce economic gold deposits application of this sedimentary source rock model enables a systematic exploration approach for sediment hosted gold deposits based on the distribution composition and structure of carbonaceous shale sequences and their contained diagenetic pyrite

strain distribution across a partially molten middle crust insights from the ams mapping of the carlos chagas anatexite aracuai belt east brazil abstract the easternmost part of the neoproterozoic aracuai belt comprises an anatectic domain that involves anatexites the carlos chagas unit leucogranites and migmatitic granulites that display a well developed fabric microstructural observations support that the deformation occurred in the magmatic to submagmatic state structural mapping integrating field and anisotropy of magnetic susceptibility ams revealed a complex 3d structure the northern domain displays gently dipping foliations bearing a nw trending lineation southward the lineation trend progressively rotates to ew then sw and the foliation is gently folded the eastern domain displays e w and ne sw trending foliations with moderate to steeply dips bearing a dominantly ns trending lineation magnetic mineralogy investigation suggests biotite as the main carrier of the magnetic susceptibility in the anatexites and ferromagnetic minerals in the granulites crystallographic preferred orientation cpo measurements using the electron backscatter diffraction ebsd technique suggest that the magnetic fabric comes from the crystalline anisotropy of biotite and feldspar grains especially the delineation of several structural domains with contrasted flow fabric suggests a 3d flow field involving westward thrusting orthogonal to the belt northwestward orogen oblique escape tectonics and ns orogen parallel flow this complex deformation pattern may be due to interplay of collision driven and gravity driven deformations

constraints on depositional conditions and ore fluid source for orogenic gold districts in the west qinling orogen china implications from sulfide assemblages and their trace element geochemistry abstract yangshan mian lue ning and manaoke represent three large orogenic gold districts in the triassic west qinling orogen the third largest gold province in china 1100 t gold resource although their component gold deposits have similar geological characteristics representative deposits from these three gold districts have contrasting ore mineral assemblages and gold speciation suggestive of diversity in gold source and or ore forming depositional conditions or processes decrease in p t conditions of ore formation and metamorphic grades of host rocks in deposits from the mian lue ning through yangshan to manaoke districts is matched by variation in their mineral paragenesis from pyrite chalcopyrite sphalerite galena native gold through pyrite arsenopyrite stibnite to scheelite pyrite arsenopyrite stibnite realgar invisible gold in the deposits of the yangshan and manaoke gold districts contrasts with native gold in the mian lue ning gold district concentrations of ag as au bi cu pb sb tl in hydrothermal pyrite show a gradual systematic increase from mian lue ning through yangshan to manaoke pyrite in the huachanggou gold deposit from the mian lue ning district has high co and low as and au concentrations in contrast with pyrite which is zoned with respect to as and au in gold deposits from the other two districts these variations between gold deposits in the different districts could be due to variable local ore fluid sources derived from the different host rock sequences during metamorphic devolatilization as also allowed by mass balance calculations however previously unmetamorphosed voluminous source rocks that have experienced amphibolite facies metamorphism with liberation of gold and other metals during conversion of gold bearing pyrite into pyrrhotite are limited it appears more likely that an external deeply sourced fluid most logically from devolatilization of the pyrite bearing sediment wedge above the triassic subduction zone associated with the formation of the west qinling orogen was responsible for gold mineralization in this model variation between districts was due to fluid interactions with contrasting host rock sequences under contrasting p t conditions

remobilization of metasomatized mantle lithosphere a new model for the jiaodong gold province eastern china the 120 ma jiaodong province in the north china craton composed of abundant mesozoic granites that intrude precambrian metamorphic basement is bordered by the sulu orogenic belt which formed through the collision between the north china and yangtze cratons in the triassic insights on the geodynamic setting and source of gold and fluids in the jiaodong gold province are based on a detailed study of the sanshandao deposit the 13cpdb and 18o values of hydrothermal calcite from this deposit range from 4 3 to 6 5 and 11 4 to 15 1 respectively which are compatible with a mantle source pre ore mafic dykes in the deposit show typical arc like geochemical features suggesting that the mantle source was metasomatized prior to the basic magmatism the 34s values of gold related pyrite from sanshandao and other large deposits in the province range from 10 9 to 11 5 and are higher than those of the country rocks but consistent with the signature of orogenic gold deposits sourced from a neoproterozoic sedimentary reservoir these high positive 34s values are interpreted to be inherited from auriferous neoproterozoic sedimentary rocks of the northern yangtze craton which were subducted into the mantle lithosphere beneath the north china craton during the formation of eclogite in the adjacent sulu orogenic belt in the triassic the syn ore lower cretaceous mafic dykes display oceanic island basalt like geochemical features suggesting that asthenosphere upwelling triggered the release of gold and sulfur from an enriched and fertilized mantle lithosphere contributing to the auriferous fluid which formed the widespread gold mineralization in the jiaodong province at ca 120 ma

cretaceous cenozoic tectonic history of the jiaojia fault and gold mineralization in the jiaodong peninsula china constraints from zircon u pb illite k ar and apatite fission track thermochronometry the jiaojia fault jjf in the jiaodong area of eastern china is an important nne trending structure that is subsidiary to the regional tancheng lujiang tan lu fault zone and hosts 1200 t of gold reserves contained in disseminated and stockwork ore dominantly in the footwall of the fault we present new zircon u pb apatite fission track and illite k ar data along the jjf and have delineated its tectonic history focusing on its formation and reactivation zircon u pb dating shows that the shangzhuang granite is a composite body with ages between 132 1 and 127 1 ma illite k ar ages for the fault s gouge range from 83 2 to 68 2 ma and the measured apatite fission track ages for ores are between 55 and 21 ma previous zircon u pb geochronology and structural studies suggest that the jjf was originally activated in the jurassic during 160 150 ma as a sinistral fault the jjf was a normal fault in the early cretaceous due to nw se orientated tension and ne sw compression which lasted from 135 to 120 ma this was followed by sinistral strike slip faulting due to nw se compression and ne sw tension during 120 110 ma and it changed to normal displacement at ca 110 ma our apatite fission track data analysis and thermal modeling of representative samples suggest that there was a subsequent dextral reactivation of the fault at ca 55 ma previous age data of ca 130 110 ma for gold mineralization along the jjf coincides with the early cretaceous magmatism and is coeval with the transition from normal faulting to sinistral strike slip faulting of the jjf in early cretaceous which is interpreted to be due to changing direction of the subducting pacific plate

structural and geochronological studies on the liba goldfield of the west qinling orogen central china the liba goldfield located to the northeast of the zhongchuan granite in the west qinling orogen wqo of mainland china contains the largest known gold resource of 2 8 moz in the zhongchuan area devonian metasedimentary rocks host the structurally controlled gold mineralization which is associated with silica sericite chlorite carbonate alteration two major styles of mineralization occurred at the goldfield which are disseminated sediment hosted and quartz vein hosted types pyrite arsenopyrite and arsenian pyrite are major gold carriers and gold also occurs as native gold grains and electrum spatially associated with the sulfides numerous felsic intermediate dykes have a similar structural control as the mineralization and their contacts with host rocks are recognized as favorable zones for mineralization detailed fieldwork in conjunction with geochronological studies has helped to define the deformation history and gold metallogenesis of the goldfield three major phases of deformation have been recognized in the zhongchuan area the first deformation d1 event was compressional in broadly a n s orientation the second d2 event was also compressional and orientated in a ne sw direction and the third d3 event was post mineralization and was associated with the emplacement of barren calcite and anhydrite veins compression related to d2 is the key process that controlled the distribution of igneous dykes and gold mineralization in the liba goldfield both igneous and hydrothermal fluids preferentially focused along dilational jogs under local trans extension which took place during the late stage of d2 precise dating with high resolution ion microprobe shrimp u pb on zircon and 40ar 39ar on muscovite biotite hornblende and plagioclase of crosscutting pre mineralization granitic porphyry and diorite dykes have constrained the mineralization age to after ca 227 ma 40ar 39ar analysis of minerals formed in hydrothermal alteration zones associated with gold mineralization indicates that there was a widespread ca 216 ma hydrothermal event that affected almost all lithologies in the area this detailed investigation is the first study to tightly constrain the timing of gold mineralization in the wqo the broadly overlapping timing and similar structural control of the mineralization and igneous dykes show a promising correlation which could be potentially used to map this late triassic gold mineralization event in the wqo

tethys tectonic evolution and its bearing on the distribution of important mineral deposits in the sanjiang region sw china abstract the sanjiang region in se tibet plateau and nw yunnan is known to have formed by amalgamation of gondwana derived continental blocks and arc terranes as a result of oceanic subduction followed by continental collision from paleozoic to mesozoic in this paper we provide a synthesis of tectonic evolution magmatism and metallogeny in the region based on data from literatures early paleozoic ophiolites 473 439 ma in the changning menglian belt indicate the existence of a proto tethys ocean in this region two episodes of subduction related magmatism in the early paleozoic one occurred in the baoshan and tengchong blocks at 502 455 ma and the other occurred in the simao block at 421 401 ma are regarded as evidence for two different events of subduction of the proto tethys ocean at different locations the proto tethys was succeeded in early devonian by the paleo tethys which comprised the main ocean and three branches ailaoshan jinshajiang and garze litang the changning menglian main ocean existed from middle devonian to middle triassic the remnants of the oceanic crust are preserved in a few places in the longmu tso shuanghu suture as well as in the changning menglian ophiolite belt the eastward subduction of the main oceanic plate from early permian to early triassic formed a prominent arc terrane stretching 1500 km from yunnan to eastern tibet from the waning stage of subduction to post subduction numerous s type granite plutons with ages varying between 230 and 219 ma such as the lincang batholith in yunnan were emplaced at or close to the suture this event produced several hydrothermal w sn deposits in the region the tectonic evolution and associated magmatism of the jinshajiang and ailaoshan branch oceans are generally comparable to those of the main ocean however the branch oceans were subducted westward instead the garze litang branch ocean also underwent westward subduction from middle devonian to late triassic arc related high sr y porphyry intrusions and associated porphyry skarn cu mo au deposits are common in the jinshajiang ailaoshan region especially in the yidun arc which formed prior to jurassic the vms deposits in the sanjiang region formed in diverse tectonic settings including middle silurian back arc basins carboniferous oceanic islands paleozoic subduction zones and triassic post subduction rifting environments the mesozoic and early cenozoic evolution of the baoshan and tengchong blocks was largely influenced by eastward oceanic subduction of the meso and neo tethys from late permian to middle cretaceous and from late cretaceous to 50 ma respectively abundant early cretaceous granitoids and associated skarn type pb zn and sn fe deposits in the baoshan and tengchong blocks were produced in the background of the shan boundary oceanic slab subduction to the west and the break off of the nujiang bitu oceanic slab to the north the subduction of the neo tethys oceanic plate beneath the tengchong block from late cretaceous to paleogene formed abundant s type granitoids and many skarn type and greisen type sn w deposits granitoids formed at 105 to 81 ma and contemporaneous hydrothermal w mo ag and au deposits which temporally coincided with the subduction of the neo tethys are common in the yidun arc terrane

p t fluid deformation regime of the ediacaran serra do cavalo magro orogenic gold deposit ribeira belt brazil abstract the serra do cavalo magro orogenic gold deposit is located in the southern ribeira belt brazil gold quartz veins are hosted in calymmian 1500 1450 ma metasedimentary and metabasic rocks and ediacaran 610 600 ma granitic rocks geochronological data available in previous literature and structural petrological relationships with host rocks and shear zones indicate that the gold mineralization was formed within 580 540 ma during the late stage evolution of the brasiliano pan african orogeny related with gondwana supercontinent assembly petrological modeling indicates peak metamorphic conditions from 560 c and 7 kbar chlorite biotite phyllite to 625 c and 6 8 kbar garnet biotite phyllite recorded in immediate host rocks gold mineralization occurs in quartz shear and extensional veins structurally controlled by ne trending second and third order sinistral transcurrent shear zones the nnw trending subvertical and subhorizontal extensional veins are oriented 55 85 in relation to the fault planes indicating the shear zones were severely misoriented to frictional reactivation deformation within the shear zones was accommodated by bulging recrystallization of quartz aggregates while feldspar aggregates from the granitic protolith underwent cataclastic flow or were replaced by sericite epidote producing phyllonites gold precipitated during stages of vein deformation within microfractures in shear and extensional veins with quartz previously subjected to bulging recrystallization metamorphic ore bearing fluids are recorded in distinct assemblages of low to moderate salinities 1 18 wt nacleq h2o nacl cacl2 and h2o co2 n2 nacl cacl2 fluid inclusions and co2 n2 inclusions microthermometric data indicate entrapment conditions of 250 350 c and 0 3 2 3 kbar recording hydrostatic to supralithostatic fluid pressures in post peak metamorphic stages sharp fluid pressure fluctuations 0 4 1 6 kbar resulted from earthquake rupture events and the fault valve behavior induced circulation of fluids from distinct sources with limited partial fluid mixing favoring the gold transportation and precipitation

geological and geochemical characteristics and formation mechanisms of the zhaishang carlin like type gold deposit western qinling mountains china abstract located in the western part of the min li metallogenic belt within the western qinling mountains the zhaishang gold deposit is a giant carlin like disseminated gold deposit discovered recently the ore deposit is present both in rocks of low grade metamorphic middle devonian and lower permian clastic formation which is composed of quartz sandstone siltstone calcareous slate and argillaceous limestone gold mineralization is strictly controlled by a fault zone minerals in ores are quite complex and consist of sulfides sulfosalt oxides sulfate carbonate tungstate telluride native metals and polymetallic alloys the diversity of mineral in the ores and the existence of microscopic visible native gold constitute the outstanding features of the gold deposit we believe that the mineral source of the ore deposit has a close connection with the host rocks because all samples show the light of the ree distribution patterns the s c pb h and o isotopic compositions show that the ore forming elements were mainly derived from the country rocks the underlying rocks and partially deep sources data obtained suggest that the ore forming fluids were derived predominantly from an active meteoric groundwater system mineralization and related alteration have features of a low water rock ratio ore forming temperatures are estimated to have been in the range from 120 to 240 c the dissolution of ferruginous limestone in the host rocks and the sulfidation of the dissolved iron by h2s introduced by ore fluids constituted not only the most important depositional mechanisms for the existence of microscopic visible gold grains but also favorable conditions for gold enrichment in the zhaishang gold deposit

new advances in trace element geochemistry of ore minerals and accessory phases the trace element composition of ore minerals is of fundamental importance to the understanding of mineralizing processes in situ analyses of ore minerals by empa sims and la icp ms have become essential approaches to many geochemical studies progress in micro beam techniques allows the examination at any location in the sample with unprecedented detail

non linear feedback loops in the rheology of cooling crystallizing felsic magma and heating melting felsic rock at least six major parameters control the rheology of partially molten systems melt content rate of melt production reaction to strain of the solid component reaction to strain of the molten component temperature and chemical composition of the source rocks we examine their interactions to understand the rheology of partially molten rocks and partly crystallised magmas this paper especially focuses on the rheology in the transitional domains between two pairs of thresholds that bracket a transitional regime between a solid state and a fluid behaviour during melting and crystallisation respectively we review related information and point out non linear effects that develop during heating of melting rocks and cooling of crystallising magmas owing to the non linear interactions positive or negative feedback loops accelerate or damp the system melt in migmatite experiences shear softening which along with strain partitioning facilitates melt segregation conversely the augmenting number of rigid crystals during cooling increases the suspension viscosity shear hardening which soon inhibits magma movement those effects reinforce the asymmetry between solid to melt and melt to solid transitions they severely contradict the concept of one rheological critical melt percentage valid for both melting and crystallisation transitions fabric acquisition competes with nucleation and crystal growth thus leading to hysteresis of the stress stain rate curves implications to field observations include horizontal magma segregation magma extraction and successive magma intrusions

structural criteria for identifying granitic cumulates abstractcumulates are igneous rocks that reflect relative concentration of crystals and or loss of melt and that therefore did not crystallize entirely from a magma of their current whole rock composition however this definition does not help to identify cumulates and so the problem is to determine whether the current whole rock composition reflects cumulate processes which is where structural criteria can be useful identification of cumulates in granitoids can be difficult especially in the absence of major whole rock compositional differences between cumulate and host structural and structural mineralogical criteria diagnostic of or at least consistent with granitic cumulates include 1 locally high concentrations of particular minerals relative to concentrations in the bulk of the intrusion 2 abundant euhedral crystals in contact at least with regard to cores that touched before any overgrowths and consequent molding of minerals about each other occurred 3 abundant antecrysts or mantle

the sphene centered ocellar texture an effect of grain supported flow and melt migration in a hyperdense magma mush abstractthe sphene centered ocellar texture consists of leucocratic ocelli with sphene titanite crystals at the center enclosed in a biotite rich matrix this texture has been recognized worldwide in hybrid intermediate rocks on the basis of structural petrological and geochronological data from selected outcrops of the variscan ribadelago pluton nw iberian massif we propose that the ocelli were formed by migration and accumulation of a residual melt through a plagioclase and biotite dominated crystalline framework at the late stage of crystallization the magma acted as a hyperdense suspension and reacted to the pressure gradient caused by the regional stress field entering the domain of grain supported flow microstructures reveal that aligned crystal domains arose in the crystal framework from the shearing and compaction of the crystal mush and behaved as magmatic microshears relative displacement of adjacent crystal clusters along these microshears corresponded to the onset of reynolds di

geologic framework and tectonic evolution of the qinling orogen central china abstract the geologic framework of the qinling orogen was built up through interplay of three blocks the north china block including the north qinling the south qinling and the south china block separated by the shangdan and mianlue sutures the shangdan suture resulted from middle paleozoic collision of the north china block and the south qinling the mianlue suture resulted from late triassic collision of the south qinling and the south china block present upper crust of the qinling is structured dominantly by thrust fold systems the north qinling displays thick skinned deformation with crystalline basement involved whilst the south qinling is characterized by thin skinned thrusts and folds detached above the lower sinian two types of precambrian basement crystalline and transitional are defined according to lithology and metamorphic grade and different in age stratigraphic and sedimentary architecture is characterized by distinct zonation the qinling orogen experienced a prolonged continental divergence and convergence between blocks during the period from late neoproterozoic to early paleozoic times the south qinling was the northern margin of the south china block and the north qinling was the southern margin of the north china block separated by a proto tethyan qinling ocean the north qinling evolved into an active margin when the proto tethyan qinling ocean subducted northward during ordovician time collision of the south and north qinling took place in middle paleozoic along the shangdan suture synchronous with the collision rifting occurred at the southern rim of the south qinling and was followed by the opening of the paleo tethyan qinling ocean during the late paleozoic resulting in the splitting of the south china block from the south qinling collision of the south qinling and the south china block came about in the late triassic along the mianlue suture the late triassic collisional orogeny caused extensive fold and thrust deformation and granitoid intrusions throughout the qinling and led to final amalgamation of the north and south china blocks

structure geochronology and petrogenesis of the late triassic puziba granitoid dikes in the mianlue suture zone qinling orogen china we present structural field observations in situ zircon u pb ages hf isotopic compositions geochemistry and sr nd isotope data from the puziba granitoid intrusions in the qinling orogenic belt china these intrusions occur as ene wsw oriented dike swarms in a highly deformed phyllite matrix melange along several major fault zones within the mianlue suture zone granitoid rocks have a relatively wide range of sio 2 69 77 wt volatile free and total alkali contents na 2 o k 2 o 2 76 4 83 wt and k 2 o na 2 o ratios 1 and they mostly fall in the granite granodiorite fields in the total alkalis versus silica diagram they display relative enrichments of light rare earth elements lrees and relatively flat heavy rare earth elements patterns hrees with weak negative eu anomalies eu 0 56 0 95 on chondrite normalized rare earth element ree diagrams they show enrichment in th and zr and depletion in high field strength elements hfses e g nb p and ti they are enriched in large ion lithophile elements lile and lrees and have low zircon e hf t 2 8 to 4 2 values relatively high initial 87 sr 86 sr ratios 0 7107 0 7188 and variable mg 22 46 their u pb zircon crystallization age is ca 215 ma but they also show inherited zircon age clusters in the paleoproterozoic mesoproterozoic neoproterozoic and paleozoic with two prominent peaks at ca 753 ma and ca 805 ma the high rb and ta elemental abundances smooth chondrite normalized ree patterns and e nd t values 5 to 3 of the puziba dike rocks characterize them as k rich calc alkaline granites their magmas were derived from partial melting of garnet amphibole bearing lower crust that was triggered by slab breakoff induced asthenospheric upwelling in the aftermath of the oblique collision of the north and south china blocks during the late triassic emplacement of the ene wsw oriented puziba dike swarms was associated with a transtensional stress regime along across the collisional mianlue suture zone

boron sulphur and copper isotope systematics in the orogenic gold deposits of the archaean hattu schist belt eastern finland abstract the hattu schist belt is located in the western part of the archaean karelian domain of the fennoscandian shield the orogenic gold deposits with au bi te geochemical signatures are hosted by ne sw n s and nw se oriented shear zones that deform 2 76 2 73 ga volcanic and sedimentary sequences as well as 2 75 2 72 ga tonalite granodiorite intrusions and diverse felsic porphyry dykes mo w mineralization is also present in some tonalite intrusions both separate from and associated with au mineralization somewhat younger unmineralized leucogranite intrusions 2 70 ga also intrude the belt lower amphibolite facies peak metamorphism at 3 5 kbar pressures and at 500 600 c temperatures affected the belt at around 2 70 ga and post date hydrothermal alteration and ore formation in this study we investigated the potential influence of magmatic hydrothermal processes on the formation of orogenic gold deposits on the basis of multiple stable isotope b s cu studies of tourmaline and sulphide minerals by application of in situ sims and la icp ms analytical techniques crystal chemistry of tourmaline from a mo w mineralization hosted by a tonalite intrusion in the hattu schist belt is characterized by fe3 al3 substitution indicating relatively oxidizing conditions of hydrothermal processes the range of 11b data for this kind of tourmaline is from 17 2 to 12 2 the hydrothermal tourmaline from felsic porphyry dyke swith gold mineralization has similar crystal chemistry e g dravite povondraite compositional trend with fe3 al3 substitution and 11b values between 19 0 and 9 6 the uvite foitite compositional trend and 11b values between 24 1 and 13 6 characterize metasomatic hydrothermal tourmaline from the metasediment hosted gold deposits composition of hydrothermal vein filling and disseminated tourmaline from the gold bearing shear zones in metavolcanic rocks is transitional between the felsic intrusion and metasedimentary rock hosted hydrothermal tourmaline but the range of average boron isotope data is essentially identical with that of the metasediment hosted tourmaline rock forming magmatic tourmaline from leucogranite has 11b values between 14 5 and 10 8 and the major element composition is similar to that of the metasediment hosted tourmaline the range of 34svcdt values measured in pyrite chalcopyrite and pyrrhotite is from 9 1 to 8 5 which falls within the typical range of sulphur isotope data for archaean orogenic gold deposits in the hattu schist belt positive 34svcdt values characterize metasediment hosted gold ores with sulphide parageneses dominated by pyrrhotite and arsenopyrite the 34svcdt values are both positive and negative in ore mineral parageneses within felsic intrusive rocks in which variable amounts of pyrrhotite are associated with pyrite purely negative values were only recorded from the pyrite dominated gold mineralization within metavolcanic units therefore the shift of 34svcdt values to the negative values reflects precipitation of sulphide minerals from relatively oxidizing fluids the range of measured 65cunbs978 values from chalcopyrite is from 1 11 to 1 19 positive values are common for mineralization in felsic intrusive rocks and negative values are more typical for deposits confined to metasedimentary rocks positive and negative 65cunbs978 values occur in the ores hosted by metavolcanic rocks there is no correlation between sulphur and copper isotope data obtained in the same chalcopyrite grains evaluation of sulphur and boron isotope data together and comparisons with other archaean orogenic gold provinces supports the hypothesis that the metasedimentary rocks were the major sources of sulphur and boron in the orogenic gold deposits in the hattu schist belt variations in major element and boron isotope compositions in tourmaline as well as in the 34svcdt values in sulphide minerals are attributed to localized involvement of magmatic fluids in the hydrothermal processes the results of copper isotope studies indicate that local sources of copper in orogenic gold deposits may potentially be recognized if the original distinct signatures of the sources have not been homogenized by widespread interaction of fluids with a large variety of rocks and provided that local chemical variations have been too small to trigger changes in the oxidation state of copper during hydrothermal processes

lithospheric controls on the formation of provinces hosting giant orogenic gold deposits ages of giant gold systems 500 t gold cluster within well defined periods of lithospheric growth at continental margins and it is the orogen scale processes during these mainly late archaean palaeoproterozoic and phanerozoic times that ultimately determine gold endowment of a province in an orogen a critical factor for giant orogenic gold provinces appears to be thickness of the subcontinental lithospheric mantle sclm beneath a province at the time of gold mineralisation as giant gold deposits are much more likely to develop in orogens with subducted oceanic or thin continental lithosphere a proxy for the latter is a short pre mineralisation crustal history such that thick sclm was not developed before gold deposition in constrast orogens with protracted pre mineralisation crustal histories are more likely to be characterised by a thick sclm that is difficult to delaminate and hence such provinces will normally be poorly endowed the nature of the lithosphere also influences the intrinsic gold concentrations of potential source rocks with back arc basalts transitional basalts and basanites enriched in gold relative to other rock sequences thus segments of orogens with thin lithosphere may enjoy the conjunction of giant scale fluid flux through gold enriched sequences although the nature of the lithosphere plays the crucial role in dictating which orogenic gold provinces will contain one or more giant deposits the precise siting of those giants depends on the critical conjunction of a number of province scale factors such features control plumbing systems traps and seals in tectonically and lithospherically suitable terranes within orogens

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

a classification scheme for epigenetic archaean lode gold deposits recent research has shown that archaean lode gold deposits occur in environments that range in their metamorphic grade from prehnite pumpellyite to lower granulite facies based on this data a new classification is proposed for these deposits in this classification shallow level gold deposits are classed as epizonal the so called mesothermal deposits are mesozonal and the deeper deposits commonly in mid amphibolite or lower granulite terrains are hypozonal

shearing of magma along a high grade shear zone evolution of microstructures during the transition from magmatic to solid state flow abstract syntectonic plutons may record short lived geological events related to crustal melting and deformation of the continental crust therefore interpretation of microstructure and orientation of fabrics is critical in order to constrain space time temperature deformation relationships during pluton crystallization here we describe the transition from magmatic to solid state deformation in the late variscan diorite granite suite dgs emplaced along the santa lucia shear zone the systematic collection of meso microstructural and quartz axis data allow us to examine the spatial distribution and the mode of superposition of different fabrics we identify three magmatic microfabric types thought to reflect the microstructural evolution at decreasing melt content during pluton crystallization our data suggest that diffusion creep dislocation creep and grain scale fracturing cooperated in accommodating the shearing of the partially molten quartzofeldspathic aggregate we suggest that the switch from upward to horizontal magmatic flow occurred at melt fractions gt 0 40 and that most of the hypersolidus fabrics formed during horizontal flow reflecting the stress field imposed by the shear zone and preserving no memory of the ascent stage

gold and trace element zonation in pyrite using a laser imaging technique implications for the timing of gold in orogenic and carlin style sediment hosted deposits laser ablation icp ms imaging of gold and other trace elements in pyrite from four different sedimenthosted gold arsenic deposits has revealed two distinct episodes of gold enrichment in each deposit an early synsedimentary stage where invisible gold is concentrated in arsenian diagenetic pyrite along with other trace elements in particular as ni pb zn ag mo te v and se and a later hydrothermal stage where gold forms as either free gold grains in cracks in overgrowth metamorphic and or hydrothermal pyrite or as narrow goldarsenic rims on the outermost parts of the overgrowth hydrothermal pyrite compared to the diagenetic pyrites the hydrothermal pyrites are commonly depleted in ni v zn pb and ag with cyclic zones of co ni and as concentration the outermost hydrothermal pyrite rims are either as au rich as in moderate to highgrade deposits such as carlin and bendigo or co ni rich and as au poor as in moderate to low grade deposits such as sukhoi log and spanish mountain the early enrichment of gold in arsenic bearing syngenetic to diagenetic pyrite within black shale facies of sedimentary basins is proposed as a critical requirement for the later development of carlin style and orogenic gold deposits in sedimentary environments the best grade sediment hosted deposits appear to have the gold climax event toward the final stages of deformation related hydrothermal pyrite growth and fluid flow

grain scale processes in actively deforming magma mushes new insights from electron backscatter diffraction ebsd analysis of biotite schlieren in the jizera granite bohemian massif abstract in the porphyritic jizera granite bohemian massif three distinct types of lattice preferred orientations of biotite grains were revealed in schlieren delineated magmatic structures using the electron backscatter diffraction ebsd method 1 biotite basal planes 001 reorient from schlieren subparallel near the schlieren base to schlieren perpendicular in the upper part of the schlieren both orientations share subhorizontal n s to nne ssw trending a axes 2 in some domains the a axes are steep and at a high angle to the schlieren plane while the c axes plunge shallowly and rotate around an ill defined a axis 3 in other domains the ebsd coincides with background magnetic fabric of the host granite revealed using the anisotropy of magnetic susceptibility ams method that is the a axes plunge shallowly to the se or nw while the c axes are subhorizontal and cluster around the ne sw trend these multiple biotite orientations in the schlieren are interpreted to reflect 1 velocity gradient in laminar magma flow along channel like conduits localized within the high strength host phenocryst framework 2 grain scale gravity driven constrictional deformation of the magma mush and 3 overprinting background tectonic deformation transmitted across large parts of the magma chamber prior to its final crystallization the grain scale mechanisms of biotite fabric acquisition in the schlieren presumably involved rotation of biotite crystals during flow with the biotite alignment reflecting the flow geometry and kinematics replaced after flow cessation by melt aided grain boundary sliding of those biotite crystals still enclosed in melt pockets within otherwise static highly crystallized magma mush the latter process was sufficient to reorient biotite grains but not to cause destruction of the schlieren using the jizera granite as a case example we argue that the lattice preferred orientation of mineral grains in mafic schlieren is highly sensitive to reorient in response to processes both associated with the schlieren formation e g localized magma flow and those that occur later and are superimposed onto the effectively solid high strength magma mush

the coupled geochemistry of au and as in pyrite from hydrothermal ore deposits the ubiquity of au bearing arsenian pyrite in hydrothermal ore deposits suggests that the coupled geochemical behaviour of au and as in this sulfide occurs under a wide range of physico chemical conditions despite significant advances in the last 20years fundamental factors controlling au and as ratios in pyrite from ore deposits remain poorly known here we explore these constraints using new and previously published empa la icp ms sims and pixe analyses of as and au in pyrite from carlin type au epithermal au porphyry cu cu au and orogenic au deposits volcanogenic massive sulfide vhms witwatersrand au iron oxide copper gold iocg and coal deposits pyrite included in the data compilation formed under temperatures from 30 to 600 c and in a wide variety of geological environments the pyrite au as data form a wedge shaped zone in compositional space and the fact that most data points plot below the solid solubility limit defined by reich et al 2005 indicate that au1 is the dominant form of au in arsenian pyrite and that au bearing ore fluids that deposit this sulfide are mostly undersaturated with respect to native au the analytical data also show that the solid solubility limit of au in arsenian pyrite defined by an au as ratio of 0 02 is independent of the geochemical environment of pyrite formation and rather depends on the crystal chemical properties of pyrite and post depositional alteration compilation of au as concentrations and formation temperatures for pyrite indicates that au and as solubility in pyrite is retrograde au and as contents decrease as a function of increasing temperature from 200 to 500 c based on these results two major au as trends for au bearing arsenian pyrite from ore deposits are defined one trend is formed by pyrites from carlin type and orogenic au deposits where compositions are largely controlled by fluid rock interactions and or can be highly perturbed by changes in temperature and alteration by hydrothermal fluids the second trend consists of pyrites from porphyry cu and epithermal au deposits which are characterised by compositions that preserve the au as signature of mineralizing magmatic hydrothermal fluids confirming the role of this sulfide in controlling metal ratios in ore systems

on thrusting regional unconformities and exhumation of high grade greenstones in neoarchean orogens the case of the waroonga shear zone yilgarn craton abstract during the neoarchean the dominant tectonic style progressively changed from an episodic overturn stagnant lid regime to modern style plate tectonics the neoarchean strengthening of continental lithosphere changed the style of deformation of orogenic belts the case study presented here provides insights into how such transition in tectonic style occurred a matter that is generally controversial we present structural and metamorphic data from the c 2660 ma waroonga shear zone wsz in the neoarchean yilgarn orogen western australia the wsz contains a syntectonic pluton and older high grade greenstones the tectonic fabric in the pluton developed during melt present thrusting followed by syn cooling wrench dominated transpression mafic greenstones preserve three metamorphic assemblages the m1 assemblage grt cpx qtz records peak p t conditions of 12 1 kbar and 800 50 c followed by isothermal decompression to 9 kbar m2 these anhydrous assemblages might predate the wsz greenstones then underwent decompression at c 2660 ma 3 4 kbar 600 650 c defined by the amphibole rich m3 assemblage synkinematic with the tectonic fabric in the wsz we show that shearing along the wsz exhumed these greenstones by at least 10 km inducing major uplift and erosion archean accretionary orogens developed on weak lithosphere where deformation suppressed crustal thickening orogenic relief and synorogenic exhumation of orogenic roots however our study indicates a genetic link between i strain localization along contractional structures inducing large scale uplift ii exhumation of high grade greenstones iii development of inverse metamorphic gradients iv establishment of a regional unconformity with clastic sediments fed by the uplifted terrane v incorporation of portions of the newly formed orogenic basins into the footwall of the wsz these features imply that the yilgarn orogenic lithosphere at c 2660 ma was stiff enough to allow tectonic processes analogous to those that dominate modern style orogenic belts representing an intermediate stage between archean and modern type orogens

pyrite and pyrrhotite textures and composition in sediments laminated quartz veins and reefs at bendigo gold mine australia insights for ore genesis the various types textures and compositional zoning of pyrite in the gold bearing saddle reefs quartz veins and surrounding sedimentary rocks provide new information on the potential source and timing of gold and arsenic and related fluid processes responsible for mineralization at bendigo nodular diagenetic pyrite in the black shale tops to sandstone turbidites is enriched in invisible gold and arsenic with mean values of 0 61 ppm au 1 300 ppm as and au ag 1 based on la icpms analyses other elements enriched in the diagenetic pyrite within the organic rich shales are mn zn mo cu v ba ag cd tl co ni bi pb and te in contrast euhedral and growth zoned hydrothermal pyrite in the turbidites and bedding parallel laminated quartz veins contains lower contents of most trace elements but has higher contents of invisible au and as especially on the outermost rim of the pyrite the gold rich pyrite rims generally become thicker a few to hundreds of microns in proximity to the gold bearing saddle reefs pyrite in the reef commonly has the highest levels of invisible au and as and the lowest levels of other trace elements it is characterized by au ag 1 and au pb 0 01 in the deepest stratigraphic levels below known productive gold reefs diagenetic pyrite in the most carbonaceous shales has been replaced by pyrrhotite during metamorphism la icpms analyses reveal that the disseminated pyrrhotite contains similar levels of ni and co to the diagenetic pyrite but is strongly depleted in as and au the spatial relationships between organic rich shales folded bedding parallel laminated quartz veins and gold arsenic bearing saddle reefs combined with the consistent trends in the trace element composition of pyrite hosted by these three geological elements is interpreted to indicate that the black shales were an initial source of au and as and the laminated quartz veins acted as the initial pathways for hydrothermal fluid flow carrying au and as from the source shales to the saddle reefs maximum gold and arsenic input into the reefs principally as free gold plus arsenopyrite occurred late during deformation toward the end of the hydrothermal cycle and is expressed by the au as rich rims to hydrothermal pyrite in the sedimentary host rocks laminated quartz veins and reefs this corresponds with final fold lockup and the development of through going fault arrays linking adjacent anticlines the source of au and as for this final and most economically important fluid flow event is considered to be from carbonaceous shales deeper in the basin where original gold bearing diagenetic arsenian pyrite reacts with organic matter and is converted to pyrrhotite with release of au as and s to the metamorphic fluid

the competition between rates of deformation and solidification in syn kinematic granitic intrusions resolving the pegmatite paradox abstract while fully crystallized granites rich in feldspar generally serve to strengthen the continental crust their precursor melts are assumed to be important agents of crustal weakening many syn tectonic granitic pegmatites are deformed within shear zones but ubiquitously preserve undeformed primary magmatic textures implying that they were largely molten during shearing yet the shapes of pegmatite bodies indicate that they deformed with a greater competence than their surroundings this co located pair of material behaviours is paradoxical we interpret field relationships in a typical pegmatite shear zone association torrisdale nw scotland and propose a mechanism by which syn tectonic granitic melts may in effect act as competent bodies while not yet fully crystallized competence was rapidly increased by preferential crystallization on intrusion margins that served to encapsulate residual melt inside stiff rinds further crystallization may have been pulsed as the concentrations of crystallization inhibitors fluxes increased in residual fluids postulating the existence of initial stiff rinds also consistent with modern estimates for rates of feldspar crystallization cms yr from undercooled hydrous silicic magma to form pegmatites these greatly outpace strain rate estimates for shear zones thus fully liquid granitic melts may only be present fleetingly and have little opportunity to weaken deforming crust before crystallization begins

fast strain rates during pluton emplacement magmatically folded leucocratic dikes in aureoles of the mount stuart batholith washington and the tuolumne intrusive suite california finite strain analysis and thermal modeling of magmatically folded leucocratic dikes in the mount stuart batholith washington and the tuolumne intrusive suite california yield strain rates in the range of 10 2 to 10 1 3 s 1 compared to published regional strain rates 10 1 3 to 10 1 5 s 1 wall rock strain rates associated with dike fed expansion 10 7 s 1 and rates resulting from numerical modeling of crystal plastic creep in aureoles 10 1 1 s 1 our fast rates are several orders of magnitude higher field and microstructural observations suggest that multiple material transfer processes including rigid rotation ductile flow cracking and potentially melt assisted granular flow operated in the aureoles to accommodate emplacement of these two plutons calculated durations of pluton construction and bulk shortening in the aureoles indicate that aureole deformation requires only slow long term strain rates of 10 1 4 s 1 thus our local fast strain rates indicate that aureoles may be characterized by pulsating high strain rate surges we suggest that magmatically folded dikes in these and other pluton aureoles around the world may be used as evidence for fast host rock strain rates during pluton emplacement

timing of cadomian deformation and magmatism within la hague nw france the la hague peninsula normandy nw france contains the most northerly components of the late neoproterozoic cadomian orogeny u pb dates for single grain and small multiple grain zircon separates from carefully selected plutons provide geochronological constraints on the timing of deformation and magmatism within the region the syntectonic jardeheu quartz diorite and omonville granodiorite were emplaced and contemporaneously deformed during the development of a greenschist facies fabric pervasive across the peninsula u pb zircon dates of 610 1 ma for the jardeheu quartz diorite and 608 2 ma for the omonville granodiorite are similar to a previously reported sensitive high resolution ion microprobe u pb date for metamorphism of the basement and indicate that a regionally significant period of metamorphism and magmatism occurred between 620 and 608 ma an emplacement age of 573 9 1 3 0 9 ma for the syntectonic thiebot granodiorite places limits on the timing of deformation along the thiebot shear zone zircons from the undeformed st martin monzonite give a precise u pb age of 581 3 1 1 ma the new dates point to reactivation of magmatism and localized greenschist facies deformation between 582 and 573 ma

geological and geochemical constraints on the genesis of the huachanggou gold deposit western qinling region central china the huachanggou gold deposit is located in lueyang county shaanxi province central china it is situated in the mian lue sutural zone in the western qinling orogen and controlled by a wnw striking ductile shear zone the ore deposit is developed in spilite limestone and phyllite in the middle lower devonian sanhekou group and the wall rocks show evidence of deformation pyrite is the main gold bearing mineral and the native gold is either visible or microscopic the huachanggou gold deposit is considered to be an orogenic gold deposit controlled by a ductile shear zone related to the indosinian qinling orogenic event the ores show a strong enrichment in au ag and as but exhibit a simple assemblage of au and ag that are strongly correlated together gold was deposited from a moderate to low salinity salt solution co2 hydrothermal fluid at medium temperature variations in s pb c and o isotopic composition indicate that the sulfur was derived from the reduction of seawater sulfate in the wall rocks the lead was derived mainly from the wall rocks the carbon was derived from the dissolution of marine carbonate the variations in h and o isotopic composition indicate that the ore forming fluids were derived originally from metamorphic fluid with meteoric water mixing in later stages au migrated mainly in the form of au hs 2 to form the deposit boiling of the fluid triggered the precipitation of the metal sulfides that contain the gold

metabasalts as sources of metals in orogenic gold deposits although metabasaltic rocks have been suggested to be important source rocks for orogenic gold deposits the mobility of au and related elements as sb se and hg from these rocks during alteration and metamorphism is poorly constrained we investigate the effects of increasing metamorphic grade on the concentrations of au and related elements in a suite of metabasaltic rocks from the otago and alpine schists new zealand the metabasaltic rocks in the otago and alpine schists are of morb and wpb affinity and are interpreted to be fragments accreted from subducting oceanic crust gold concentrations are systematically lower in the higher metamorphic grade rocks average au concentrations vary little between sub greenschist 0 9 0 5 ppb and upper greenschist facies 1 0 0 5 ppb but decrease significantly in amphibolite facies samples 0 21 0 07 ppb the amount of au depleted from metabasaltic rocks during metamorphism is on a similar scale to that removed from metasedimentary rocks in otago arsenic concentrations increase with metamorphic grade with the metabasaltic rocks acting as a sink rather than a source of this element the concentrations of sb and hg decrease between sub greenschist and amphibolite facies but concentration in amphibolite facies rocks are similar to those in unaltered morb protoliths and therefore unaltered oceanic crust cannot be a net source of sb and hg in a metamorphic environment the concentrations of au as sb and hg in oceanic basalts that have become integrated into the metamorphic environment may be heavily influenced by the degree of seafloor alteration that occurred prior to metamorphism we suggest that metasedimentary rocks are much more suitable source rocks for fluids and metals in orogenic gold deposits than metabasaltic rocks as they show mobility during metamorphism of all elements commonly enriched in this style of deposit

synsedimentary diagenetic and metamorphic pyrite pyrrhotite and marcasite at the homestake bif hosted gold deposit south dakota usa insights on au as ore genesis from textural and la icp ms trace element studies the paleoproterozoic homestake deposit northern black hills south dakota is the largest banded iron formation bif hosted gold deposit in the world and one of the largest single gold deposits globally 1 300 t au mined at an average grade of 8 4 g t the origin of the deposit has remained in dispute from its discovery with views broadly falling into two groups 1 the gold and associated elements were externally sourced and deposited in a suitable structural and or chemical trap e g iron formation and 2 gold was indigenous to the host iron formation from which it was remobilized into dilatant structural zones during deformation and metamorphism in recent times most workers have favored the former model appealing to the broad synchroneity of paleoproterozoic metamorphism felsic magmatic activity and gold event timing la icp ms analysis of synsedimentary to early diagenetic sulfides at homestake and surrounding area indicates that the au in the orebodies may have originally had a significant syngenetic component in particular la icp ms mapping of multistage pyrites from carbonaceous shale in the upper poorman formation shows that au and a suite of trace elements co ni cu zn se mo ag sb te au hg tl pb and bi similar to those in diagenetic pyrite from around the globe are contained in anhedral sponge textured cores and nodules surrounded by relatively trace element barren au poor euhedral pyrite rims furthermore interelement ratios e g co ni ni ag of the trace element rich spongy and nodular pyrites are also similar to those of typical diagenetic pyrite la icp ms imaging of pyrrhotite in the same rocks reveals multistage growth in this mineral as well with the earlier generation having higher concentrations of ag sb pb tl and bi than its presumed metamorphic counterpart imaging of marcasite in the shales of the upper poorman formation demonstrates this mineral s high abundance of w and tl relative to all generations of pyrite and pyrrhotite mass balance calculations indicate that the volume of upper poorman formation in the mine area approx 12 km3 could potentially yield 4 500 t of au a value greater than the total mined resource by more than a factor of three these geochemical data and calculations suggest that a significant portion of the gold at homestake may have been sourced from the relatively thick carbonaceous and sulfidic black shale facies in the stratigraphic footwall to the host iron formation in a manner similar to other sediment hosted gold districts

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

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

the crustal continuum model for late archaean lode gold deposits of the yilgarn block western australia most archaean gold ores belong to a coherent genetic group of structurally controlled lode deposits that are characteristically enriched in au with variable enrichments in ag as w sb bi te b and pb but rarely cu or zn and are surrounded by wallrock alteration haloes enriched in k lile and co2 with variable na and or ca addition evidence from the yilgarn block of western australia combined with similar evidence from canada and elsewhere indicates that such deposits represent a crustal continuum that formed under a variety of crustal regimes over at least a 15 km crustal profile at pt conditions ranging from 180 c at 1 kb to 700 c at 5 kb individual deposits separated by tens to hundreds of kilometres collectively show transitional variations in structural style of mineralisation vein textures and mineralogy of wallrock alteration that relate to the pt conditions of their formation at varying crustal depths specific transitions within the total spectrum may be shown also by deposits within gold camps although nowhere is the entire continuum of deposits recorded from a single gold camp or even greenstone belt