major types and time space distribution of mesozoic ore deposits in south china and their geodynamic settings the ore deposits of the mesozoic age in south china can be divided into three groups each with different metal associations and spatial distributions and each related to major magmatic events the first event occurred in the late triassic 230 210 ma the second in the mid late jurassic 170 150 ma and the third in the early mid cretaceous 120 80 ma the late triassic magmatic event and associated mineralization is characterized by peraluminous granite related w sn nb ta mineral deposits the triassic ore deposits are considerably disturbed or overprinted by the later jurassic and cretaceous tectono thermal episodes the mid late jurassic magmatic and mineralization events consist of 170 160 ma porphyry skarn cu and pb zn ag vein deposits associated with i type granites and 160 150 ma metaluminous granite related polymetallic w sn deposits the late jurassic metaluminous granite related w sn deposits occur in a ne trending cluster in the interior of south china such as in the nanling area in the early mid cretaceous from about 120 to 80 ma but peaking at 100 90 ma subvolcanic related fe deposits developed and i type calc alkaline granitic intrusions formed porphyry cu mo and porphyry epithermal cu au ag mineral systems whereas s type peraluminous and or metaluminous granitic intrusions formed polymetallic sn deposits these cretaceous mineral deposits cluster in distinct areas and are controlled by pull apart basins along the south china continental margin based on mineral assemblage age and space time distribution of these mineral systems integrated with regional geological data and field observations we suggest that the three magmatic mineralization episodes are the result of distinct geodynamic regimes the triassic peraluminous granites and associated w sn nb ta mineralization formed during post collisional processes involving the south china block the north china craton and the indo china block mostly along the dabie sulu and songma sutures jurassic events were initially related to the shallow oblique subduction of the izanagi plate beneath the eurasian continent at about 175 ma but i type granitoids with porphyry cu and vein type pb zn ag deposits only began to form as a result of the breakup of the subducted plate at 170 160 ma along the nne trending qinzhou hangzhou belt also referred to as qin hang or shi hang belt which is the neoproterozoic suture that amalgamates the yangtze craton and cathaysia block a large subduction slab window is assumed to have formed in the nanling and adjacent areas in the interior of south china triggering the uprise of asthenospheric mantle into the upper crust and leading to the emplacement of metaluminous granitic magma and associated polymetallic w sn mineralization a relatively tectonically quiet period followed between 150 and 135 ma in south china from 135 ma onward the angle of convergence of the izanagi plate changed from oblique to parallel to the coastline resulting in continental extensional tectonics and reactivation of regional scale ne trending faults such as the tan lu fault this widespread extension also promoted the development of ne trending pull apart basins and metamorphic core complexes accompanied by volcanism and the formation of epithermal cu au deposits granite related polymetallic sn w deposits and hydrothermal u deposits between 120 and 80 ma with a peak activity at 100 90 ma

geochemical identification of episodes of gold mineralisation in the barberton greenstone belt south africa abstract the morphology and mineral chemistry of gold and associated sulphides at sheba fairview and new consort gold mines in the barberton greenstone belt bgb identify two main types of mineralization the first type occurs associated with sulphides mainly pyrite either as inclusions 10 30 m or as sub microscopic gold the second gold type consists of large gold grains 100 m within the silicates mostly quartz la icp ms studies reveal that some gold and associated sulphide grains contain high values of cl br na and i the elemental relationships reflect the different chemistry and precipitation processes of possible source fluids and identify several episodes of mineralisation in the study area one of them formed due to a boiling process in a supercritical hydrothermal environment this paper reports on the compositional characteristics of these gold grains the significance of the halogen contents and the implications for possible sources of the gold and associated sulphides

geochronology and geochemistry of the granitoids and ore forming age in the xiaoyao tungsten polymetallic skarn deposit in the jiangnan massif tungsten belt china implications for their petrogenesis geodynamic setting and mineralization abstract the xiaoyao tungsten polymetallic skarn deposit in the eastern jiangnan massif of yangtze block is located at the contact between a granodiorite pluton and sinian cambrian limestone the intrusions in the tungsten rich district comprise the xiaoyao and other granodiorite plutons and granite porphyry dikes the age determinations by la icp ms u pb dating of zircons indicate that the granodiorite formed at 149 4 1 1 ma whereas the granite porphyry was emplaced at 133 2 0 7 ma re os dating of molybdenite from the skarn orebodies yielded a weighted average age of 148 7 2 3 ma n 5 these ages indicate that the tungsten mineralization is temporally related to the granodiorite the granodiorites are metaluminous a cnk 0 86 0 98 and in the high k calc alkaline series they contain hornblende and have a negative correlation between p2o5 and sio2 indicating that they are typical i type intrusions the granite porphyries exhibit high alkali contents na2o k2o 7 97 9 53 elevated feot feot mgo ratios 0 83 0 94 high concentrations of zr nb ce and y and high zr saturation temperatures average of 812 c thus they are geochemically similar to a type intrusions the initial 87sr 86sr and end t values range respectively from 0 7074 to 0 7083 and from 7 9 to 1 3 for the granodiorite and from 0 7008 to 0 7083 and from 6 3 to 4 7 for the granite porphyry in addition two stage nd model ages t2dm of 1 0 1 6 ga for the granodiorite and 1 3 1 4 ga for the granite porphyry indicate that the proterozoic crustal rocks of the shangxi group could have contributed to the xiaoyao magmas the rhenium contents of the molybdenite grains vary from 32 to 136 ppm suggesting that the molybdenum was derived mainly from a mixture of mantle and crustal sources based on the new geochemical data and regional geological investigations we propose that the late jurassic mineralization related i type granodiorite was derived from the neoproterozoic shangxi metamorphic rocks with some additional input of mantle material the magmatism was triggered by asthenospheric upwelling induced by slab tearing during oblique subduction of the paleo pacific plate beneath the south china block

geology geochronology and hf isotope geochemistry of the longtougang skarn and hydrothermal vein cu zn deposit north wuyi area southeastern china abstract the recently discovered longtougang skarn and hydrothermal vein cu zn deposit is located in the north wuyi area southeastern china the intrusions in the ore district comprise several small porphyritic biotite monzonite porphyritic monzonite and porphyritic granite plutons and dikes the mineralization is zoned from a lower zone of cu rich veins and cu zn skarns to an upper zone of banded zn pb mineralization in massive epidote altered rocks the deposit is associated with skarn potassic epidote greisen siliceous and carbonate alteration molybdenite from the cu rich veins yielded a re os isochron age of 153 6 3 9 ma which is consistent with u pb zircon ages of 154 0 1 3 ma for porphyritic monzonite 154 0 0 8 ma for porphyritic biotite monzonite and 152 0 0 8 ma for porphyritic granite geological observations suggest that the cu mineralization is genetically related to the porphyritic biotite monzonite and porphyritic monzonite all the zircons from intrusive rocks in the ore district are characterized by ehf t values between 13 41 and 4 38 and hf model ages tdm2 between 2054 and 1482 ma reflecting magmas derived mainly from a proterozoic crustal source molybdenite grains from the deposit have re values of 14 6 27 7 ppm indicative of a mixed mantle crust source the porphyry skarn abundant cu and hydrothermal vein type pb zn ag deposits in the north wuyi area are related to the late jurassic porphyritic granites and early cretaceous volcanism respectively the late jurassic mineralization related granites were derived from the crustal anatexis with some mantle input which was triggered by asthenospheric upwelling induced by slab tearing during oblique subduction of the paleo pacific plate beneath the south china block and the early cretaceous mineralization related granitoids mainly from crust material formed within a series of nne trending basins during margin parallel movement of the plate

prolonged magmatism and time constraints for sediment deposition in the early archean barberton greenstone belt evidence from the upper onverwacht and fig tree groups the single zircon evaporation shrimp ion microprobe and conventional dissolution techniques were used to determine 207pb 206pb and u pb ages on samples from the upper onverwacht and fig tree groups of the early archean barberton greenstone belt south africa zircons from dacitic rocks of the upper hooggenoeg formation yield ages of 3445 3452 ma a tuff in the basal kromberg formation has a mean age of 3416 5 ma a tuffaceous band 5 cm thick in the uppermost kromberg formation contains igneous zircons with a mean age of 3334 3 ma the 1700 m section of kromberg formation between these two samples is composed of basaltic lavas minor komatiites and cherty metasediments the overlying mendon formation is composed of interbedded komatiitic lavas and metasediments with a minimum thickness of 600 m a cherty stromatolitic metasediment 300 m above the base contains several thin ash layers with a mean zircon age of 3298 3 ma the basal fig tree group has units as old as 3259 3 ma and upper units in the fig tree are as young as 3225 3 ma xenocrystic zircons in the upper onverwacht and overlying fig tree group samples suggest that successive igneous units inherited zircons from underlying units and that over several hundred million years episodes of intermediate to felsic igneous activity took place at 20 40 ma intervals structural repetition by isoclinal folding and thrust faulting are important components of late greenstone belt evolution but should not obscure the importance of the prolonged interval of magmatic evolution represented by the thick pile of volcanic rocks observed in the barberton greenstone belt

simultaneous pure and simple shear the unifying deformation matrix abstract any simultaneous combination of finite simple shear and finite pure shear is a linear transformation which can be expressed as a single transformation matrix for two dimensions the matrix is upper triangular with an off diagonal term called the effective shear strain is a simple function of the pure and simple shear components for three dimensions a simultaneous combination of thrusting in the x direction thrusting in the y direction and a wrench in the x direction in addition to 3 orthogonal components of coaxial strain can also be represented by a 3 3 upper triangular matrix here three off diagonal terms xy x y and y z occur xy is a simple function of the horizontal coaxial strain values and thrusting in the x direction yz depends on the coaxial strain components in the y and z directions and the thrusting in the y direction while xz is related to all six strain components the matrix also allows for volume change either homogeneously or preferentially in a single direction a method of decomposing the deformation matrix into a series of incremental deformation matrices where each incremental deformation records the same kinematic vorticity number as the finite deformation is shown the orientation and magnitude of the finite strain ellipsoid ellipse is easily and accurately found at any increment during the deformation

timing of deposition and deformation of the moodies group barberton greenstone belt south africa very high resolution of archaean surface processes abstract thin and largely discontinuous volcanic tuff beds in the alluvial fluvial and marginal marine siliciclastic strata of the middle archaean moodies group barberton greenstone belt bgb contain rare to abundant zircon dating of magmatic zircon grains from five tuff beds and one cross cutting dike yield ages which span the entire spectrum of bgb history from ca 3550 ma to 3215 ma and show variable degrees of post depositional alteration until ca 2500 ma the youngest pristine zircon grains in all tuff samples analyzed by ca id tims chemical abrasion ion dissolution thermal ion mass spectrometry yield upper intercept and concordia ages between 3227 6 0 3 and 3216 0 3 ma and those grains analyzed by la sf icp ms laser ablation sector field inductively coupled plasma mass spectrometry and shrimp sensitive high resolution ion microprobe yield ages between 3228 9 and 3214 9 ma most of these ages are either identical within error or significantly older than a ca id tims age of 3223 4 0 3 ma obtained from zircon grains of a tuff horizon of the schoongezicht formation at the top of the underlying fig tree group we interpret this age to mark the time of volcanic eruption whereas the significantly older and overlapping ages of the stratigraphically higher moodies tuff zircon grains point either to a high degree of magmatic inheritance and or a detrital contribution from surrounding sedimentary rocks during or after ash tuff deposition the minimum age for deposition of moodies group sediments and deformation is provided by a la icp ms zircon u pb age of 3219 9 ma obtained from an undeformed felsic dike which crosscuts folded moodies strata this age overlaps with a precise ca id tims u pb zircon age of 3216 0 0 3 ma obtained from a tuff horizon of the upper moodies group overall the new age data collectively imply that the up to 3 7 km thick sandstone dominated moodies group was deposited and deformed within

petrogenesis and geochronology of cretaceous adakitic i and a type granitoids in the ne yangtze block constraints on the eastern subsurface boundary between the north and south china blocks abstract a careful literature investigation for the north china and yangtze precambrian basements shows a distinct overlap of the nd pb hf isotopes between these two cratonic basements the isotopes of early cretaceous adakitic granitoids from chuzhou east of the tanlu fault zone in the northeastern yangtze block fall into the overlap fields these isotopes therefore cannot be used to distinguish the derivation of the chuzhou adakitic rocks from the north china or yangtze basements moreover high resolution deep seismic probing across the dabie sulu uhp terranes and east central china all indicates the yangtze craton was thrust beneath the north china craton the chuzhou adakitic rocks were most likely derived from the archean to paleoproterozoic lower crust of the yangtze craton the north china and yangtze cratons are most probably separated by the dabie tanlu sulu imjingang belts and a subsurface north china yangtze boundary located near nanjing is unsuitable

stress analysis post orogenic extension and 3 01 ga gold mineralisation in the barberton greenstone belt south africa gold deposits in the barberton greenstone belt bgb have formed along a common network of brittle ductile shear zones that developed during extension and emplacement of felsic porphyry dykes at 3015 ma at a deposit scale mineralised shears occur in en echelon arrays distributed as riedel p and anti riedel shears with envelopes that connect groups of mine workings along nnw ene ne and ese trends the dip of the mineralised shear envelopes is generally moderate to steep 50 80 the existence of the directional alignment of gold deposits at the scale of the greenstone belt has been demonstrated with fry analysis stress inversion for mineralised shear zones from workings near 28 mines using a bingham axial distribution and an optimized right dihedron approach provide near identical results with 1 vertical and 3 horizontal nw the stress regime is generally radial extensional or pure extensional at the scale of the greenstone belt the mineralised shear envelopes comprise two sets of near orthogonal trends positioned symmetrically with respect the measured far field stress 1 a nw and ne trending pair with the latter accommodating pure normal slip and the former both dextral or sinistral strike slip and 2 a nnw and ese trending pair accommodating dextral transtensional and sinistral transtensional movements respectively the fact that all mineralised shears developed systematically in response to a similar far field stress suggests that mineralisation formed during a single tectonic stage pb pb ages of 3017 1 17 8 ma and 3008 8 16 ma from a syn mineralisation porphyry dyke near golden quarry indicates gold deposition at 3015 ma i e 85 ma after the craton had stabilised older porphyry dykes intruded near sheba and consort mines have been dated at 3100 ma and 3260 ma respectively and provide a minimum and maximum age constraint on accretionary deformation in the area we propose that the main phase of gold enrichment in the bgb occurred after tectonic and thermal stabilisation of the crust at a time the craton was acting as a stable terrane mineralised brittle ductile shear zones allowed ingress of deeply sourced fluids during cratonic extension mine camps display a cluster spacing of 12 km which suggests a regular distribution of controlling shears possibly as a function of the elastic rigidity of the lithosphere rather than linking mineralisation to orogenic processes during greenstone belt accretion we link gold mineralisation to the opening of fault bounded intracratonic basins including the pongola and witwatersrand basins and concomitant igneous activity

petrogenesis of the red mountain pluton laramie anorthosite complex wyoming implications for the origin of a type granite abstract the red mountain pluton the youngest intrusion in the laramie anorthosite complex is one of five monzonitic to granitic a type plutons intruded at 1 43 ga in the laramie mountains of southeastern wyoming the red mountain pluton ranges in composition from fayalite monzonite clinopyroxene quartz monzonite and biotite hornblende quartz syenite to granite the red mountain pluton is petrologically and geochemically distinct from the northern and southern sherman batholith the sybille intrusion and the maloin ranch pluton it has higher feo t feo t mgo and higher k 2 o is more strongly enriched in rees and at any given silica content it has lower abundances of tio 2 feo mgo cao and p 2 o 5 the fayalite monzonite in the red mountain pluton crystallized at temperatures around 1000 c and oxygen fugacities 1 2 log units below that of the fmq buffer similar to the crystallization conditions of the sybille and maloin ranch plutons the biotite hornblende quartz syenite crystallized at oxygen fugacities near or slightly below those of fmq most of the units in the pluton carry mantle like nd sr and pb isotopic compositions similar to the least contaminated anorthositic and ferrodioritic rocks of the lac indicating that the pluton evolved mainly via differentiation substantial crustal assimilation occurred only in the late dikes and the granite we suggest that all of the 1 43 ga a type intrusions in southeastern wyoming are ultimately derived from tholeiites extreme differentiation or partial melting produced monzonitic magmas that have assimilated varying amounts of siliceous crust the differences between the red mountain pluton and other a type granitoids in southeastern wyoming are related to the extent of differentiation prior to crustal assimilation the compositional ranges of a type granitoids in southeastern wyoming suggest that the geochemical spectrum of a type granites worldwide similarly are produced from evolved mantle derived melt that has assimilated varying proportions of crustal material

fluid wall rock interaction in an archean hydrothermal gold deposit a thermodynamic model for the hunt mine kambalda gold mineralization at the hunt mine kambalda is situated in a 20 m thick steeply dipping shear zone within a sequence of rather uniform metabasalts zonal alteration of the host rocks occurred around fractures which acted as conduits during gold mineralization limited major element mobility ensured assemblages of low variance over most of the alteration zone providing a suitable environment for the application of thermodynamics in conjunction with detailed petrographic geochemical electron microprobe analysis of mineral compositions and fluid inclusion studies microthermometry equilibrium equations involving fluid components and alteration minerals are used to determine conditions during the alteration event and in particular activity gradients of components profiles across the schist zone show increasing f sub h 2 s around auriferous quartz veins log f sub h 2 s 0 6 but constant f sub o 2 log f sub o 2 29 7 these results are near the magnetite pyrrhotite pyrite triple point for the inferred temperature of 350 degrees c and explain the pyrite pyrrhotite magnetite zonation sequence outward from veins the profile of x sub co 2 is very close to the value of 0 25 determined independently from fluid inclusions the k super h super profile shows increases adjacent to auriferous veins corresponding to the zone of biotite stability using a value for fluid salinity of 2 equiv wt percent nacl ph was calculated to be around 6 9 the solubility of gold in solution was estimated for the calculated conditions for the complex au hs super 2 the maximum solubility is around 0 5 mg kg super 1 i e 0 5 ppm for au cl super 2 it is close to 10 super 6 mg kg super 1 indicating the efficacy of sulfide transport of gold compared to chloride transport under these conditions the calculated decrease in f sub h 2 s away from the auriferous quartz veins results in an 80 percent or greater decrease in gold solubility suggesting hat the sulfidation of fe rich wall rocks is a geologically reasonable and very efficient mechanism for precipitating gold from solution at elevated temperatures

transcurrent shearing granite sheeting and the incremental construction of the tabular 3 1 ga mpuluzi batholith barberton granite greenstone terrane south africa structural petrographic and geochronological studies show that the tabular 3 1 ga mpuluzi batholith in the barberton granite gneiss terrane in south africa was emplaced via a combination of external and internal processes external structural controls are indicated by systematic variations in intrusive relationships and strain along the margins of the mpuluzi batholith and are consistent with an emplacement of the granite in a dilational jog within a ne ene trending system of dextral transcurrent synmagmatic shear zones internally the mpuluzi batholith is essentially made up of granite sheets the structurally higher parts of the granite are made up of shallowly dipping sheets that are underlain by an anastomosing network of steeply dipping variably deformed dykes and sheets these granite sheets at lower structural levels intruded either into the actively deforming shear zones or into extensional sectors between and along the bounding shear zones multiple intrusive relationships and geochronological evidence suggests that granite sheeting and the assembly of the pluton occurred over a period of 3 13 ma the spatial and temporal relationship between deformation and magma emplacement reflects episodes of incremental dilation related to deformation along the bounding shear zones and granite sheeting the transition to the mainly subhorizontal granite sheets at higher structural levels of the tabular mpuluzi batholith indicates the intrusion of the granites during subhorizontal regional shortening where the reorientation of the minimum normal stress to vertical attitudes at the shallow levels of emplacement allowed for vertical dilation and subhorizontal emplacement of the granite sheets

generation of metaluminous a type granites by low pressure melting of calc alkaline granitoids melting experiments on two calc alkaline rocks a tonalite and a granodiorite demonstrate that shallow dehydration melting of hornblende and biotite bearing granitoids generates metaluminous a type granitic melts profuse crystallization of calcic plagioclase orthopyroxene during low pressure p 4 kbar incongruent melting explains all of the major and trace element characteristics of a type granites e g their low al ca mg sr and eu contents and high ga al and k na ratios as well as their common association with anorthositic and mangeritic rocks the residual assemblage produced during melting of the same rocks in the deep crust p 8 kbar is dominated by clinopyroxene and as a consequence most of the chemical features diagnostic of a type granites are lost the high temperatures 900 c and low h 2 o contents 4 wt that are characteristic of a type magmas are adequately explained by the h 2 o poor nature of the granitoid source materials and require neither a granulitic nor a f rich source previous attempts to explain the origin of a type granites have focused on the nature of the source material these experiments show that pressure of melting is at least as important as source composition in engendering a type melts

are discontinuous chondrite normalized ree patterns in pegmatitic granite systems the results of monazite fractionation abstract all 14 stable rare earth elements rees in biotite muscovite granite and tourmaline rich granite of the harney peak granite black hills south dakota usa have been analyzed using inductively coupled plasma mass spectrometry chrondrite normalized ree patterns of the tourmaline rich granites are discontinuous between nd and sm the discontinuity was modeled successfully by the fractional crystallization of monazite from a biotite muscovite granite initial composition this explanation may also apply to the development of such discontinuous patterns in other highly evolved rocks

mesozoic large scale magmatism and mineralization in south china preface abstract south china is famous for its polymetallic deposits with more than 50 of the world s w and sb reserves and 20 of the sn reserves as well as abundant cu mo pb zn au ag ta nb hree and u this special issue reports recent progress on the tectonic evolution magmatism and ore deposits of south china which leads to a more comprehensive understanding of the mesozoic large scale magmatism and mineralization events 1 south china has been located in backarc settings during multiple subductions since the paleozoic which may have contributed to pre enrichments of the ore forming materials 2 the pacific plate subduction was a principle factor that controlled the late mesozoic large scale magmatism and mineralization in south china through flat subduction ridge subduction slab tearing and slab rollbacks

geochemistry of late metamorphic hydrothermal alteration and graphitisation of host rock macraes gold mine otago schist new zealand abstract the macraes gold deposit in the otago schist new zealand formed during late metamorphic fluid flow through a lower greenschist facies shear zone mineralisation occurred near to the brittle ductile transition at about 300 c large volumes of host rock in a shear zone up to 120 m thick have been hydrothermally altered by this fluid activity most alteration is not structurally controlled apart from proximity to the shear zone ductile and brittle microshears traverse the most mineralised rocks and some structural control of fluid flow occurred as well fluid flow was slow similar to that in metamorphic rocks mm year and diffusion through interconnected fluid was a significant chemical process localised extensional hydrofractures m scale are filled with mineralised quartz most alteration of the host rocks was isochemical with respect to the lithophile elements and mineralised rocks have been variably enriched in as au sb w mo and bi but not co or cd addition of sulphur has occurred to both host rocks and mineralised rocks up to 1 wt above a background of 0 1 wt host rock sulphur is mainly pyritic and is not structurally controlled mineralised rocks have pyrite and arsenopyrite along microshears pyrite chalcopyrite sphalerite and galena have formed from sulphidation of silicates with no addition of metals graphite has been added to mineralised rocks along microshears up to 3 wt locally above a background of 0 1 wt noncarbonate carbon graphite deposition may have occurred as a result of mixing of two fluids water methane and water carbon dioxide graphitisation and sulphidation reactions released low d water which accumulated in the slow moving mineralising fluid distinction between this low d reaction water and meteoric water incursion is difficult

mesozoic metallogeny in east china and corresponding geodynamic settings an introduction to the special issue abstract the giant east china mesozoic metallogenic province hosts some of the world s largest resources of tungsten tin molybdenum antimony and bismuth ores of gold silver mercury lead zinc copper uranium and iron are also of major importance the province and its constituent metallogenic belts or regions south china middle lower yangtze river valley east qinling dabie interior of north china craton yan liao and north east china are the products of several pulses of igneous activity and mineralisation between 240 and 80 ma each successive stage has produced a distinctive suite of deposits that can be readily related to the geodynamic evolution of the region during the mesozoic this geodynamic evolution is linked to a complex series of tectonic events involving far field subduction plate collisions crustal thickening post collision collapse and rifting

a review of the relationships between granitoid types their origins and their geodynamic environments abstract granitoids are divided into several types according to their mineral assemblages their field and petrographical features and their chemical and isotopic characteristics this typology complements most of the recent classifications because it is not based solely on chemical and isotopic criteria but also on the field petrographical and mineralogical criteria it thus has the advantage of distinguishing the various granitoid types in the field in most cases the proposed classification shares many similarities with the twenty most used genetic classifications of granitoids both types of peraluminous granitoids are of crustal origin the tholeiitic alkaline and peralkaline granitoids are of mantle origin and both types of calc alkaline granitoids are of mixed origin and involve both crustal and mantle materials each granitoid type is generated and emplaced in a very specific tectonic setting each stage of the wilson cycle is characterised by typical associations of granitoids well typed and precisely dated granitoids can then complement structural approaches and indicate on the geodynamic environment with reference to some case studies the use of granitoids as tracers of the geodynamic evolution is also proposed and discussed

molybdenite re os and albite 40ar 39ar dating of cu au mo and magnetite porphyry systems in the yangtze river valley and metallogenic implications abstract the area of the middle lower yangtze river valley eastern china extending from wuhan hubei province to western zhenjiang jiangsu province hosts an important belt of cu au mo and fe deposits there are two styles of mineralization i e skarn porphyry stratabound cu au mo fe deposits and magnetite porphyry deposits in several nne trending cretaceous fault bound volcanic basins the origin of both deposit systems is much debated we dated 11 molybdenite samples from five skarn porphyry cu au mo deposits and 5 molybdenite samples from the datuanshan stratabound cu au mo deposit by icp ms re os isotope analysis nine samples from the same set were additionally analyzed by ntims on re os results from the two methods are almost identical the re os model ages of 16 molybdenite samples range from 134 7 2 3 to 143 7 1 6 ma 2 the model ages of the five samples from the datuanshan stratabound deposit vary from 138 0 3 2 to 140 8 2 0 ma with a mean of 139 3 2 6 ma their isochron age is 139 1 2 7 ma with an initial os ratio of 0 7 8 1 mswd 0 29 these data indicate that the porphyry skarn systems and the stratabound deposits have the same age and suggest an origin within the same metallogenic system albite 40 ar 39 ar dating of the magnetite porphyry deposits indicates that they formed at 123 to 125 ma i e 10 20 ma later both mineralization styles characterize transitional geodynamic regimes i e the period around 140 ma when the main ns trending compressional regime changed to an ew trending lithospheric extensional regime and the period of 125 115 ma of dramatic ew trending lithospheric extension

fluid inclusion analysis of silicified palaeoarchaean oceanic crust a record of archaean seawater abstract in recent years the role of archaean seawater and hydrothermal fluid in the extensive silicification of palaeoarchaean volcano sedimentary successions has been a matter of considerable debate in an attempt to constrain the conditions of silica precipitation and the sources and chemical composition of the interacting fluids we used fluid inclusion microthermometry bulk crush leach and oxygen isotope analyses of chert and quartz veins in silicified komatiites and sediments from the barberton greenstone belt south africa chert vein margins consist of microcrystalline quartz and carbonaceous matter whereas the vein centres are often filled with macrocrystalline quartz that contains abundant fluid inclusions oxygen isotope ratios of vein chert and macrocrystalline quartz vary from 18 to 21 with the macrocrystalline quartz having slightly higher 18o values 0 7 0 3 the data are consistent with silica precipitation during low temperature 100 c hydrothermal processes on the archaean seafloor macrocrystalline quartz contains homogeneous 2 phase l v inclusions at room temperature with a relatively constant vapour fraction the inclusions have a salinity of 3 11 wt nacl equiv and homogenisation temperatures th of 150 200 c whereas some of the inclusions form intragranular fluid inclusion clusters that appear to be primary other inclusions form transgranular fluid inclusion trails and are clearly secondary both types of inclusions share the same microthermometric characteristics indicating that fluid entrapment occurred during a later metamorphic event and not near the seafloor the event likely coincided with regional deformation and metamorphism at 3 23 ga chlorite thermometry from vein host rocks reveals peak conditions of 257 31 c in conjunction with microthermometry the data correspond to a crustal depth of 3 6 km a secondary origin of fluid inclusions is also consistent with the cl br na cl and na k ratios of the macrocrystalline veins which are similar to those found in metamorphic quartz veins the fluid inclusions thus do not provide information on the conditions and temperatures during chert formation while the fluids potentially carry the geochemical signature of modified archaean seawater i e hydrothermal fluid characterised by low cl br and na k ratios and low mg contents the fluid composition was likely modified during regional metamorphism

partition coefficients of hf zr and ree between zircon apatite and liquid concentration ratios of hf zr and ree between zircon apatite and liquid were determined for three igneous compositions two andesites and a diorite the concentration ratios of these elements between zircon and corresponding liquid can approximate the partition coefficient although the concentration ratios between apatite and andesite groundmass can be considered as partition coefficients those for the apatite in the diorite may deviate from the partition coefficients the hree partition coefficients between zircon and liquid are very large 100 for er to 500 for lu and the hf partition coefficient is even larger the ree partition coefficients between apatite and liquid are convex upward and large d 10 100 whereas the hf and zr partition coefficients are less than 1 the large differences between partition coefficients of lu and hf for zircon liquid and for apatite liquid are confirmed these partition coefficients are useful for petrogenetic models involving zircon and apatite

iib trioctahedral chlorite from the barberton greenstone belt crystal structure and rock composition constraints with implications to geothermometry iib trioctahedral chlorite in the barberton greenstone belt bgb metavolcanic rocks was formed during pervasive greenschist metamorphism the chem ical composition of the chlorite is highly variable with the fe fe mg ratio ranging from 0 12 to 0 8 among 53 samples the chemical variation of the chlorite results from the chemical diversity of the host rock especially the mgo content of the rock but major details of the variation pattern of the chlorite are due to the crystal structure of the chlorite all major cation abundances in the chlorite are strongly correlated with each other sil icon increases with mg and decreases with fe while aliv and alvi decrease with mg and increase with fe2 a complex exchange vector explains over 90 of the chlorite compositional variation mg4sife2 3alvi 1 aliv 1 which has 3 parts fe mg substitution coupled with one part tschermakite substitution this ratio is required to maintain the charge and site balances and the dimensional fit between the tetrahedral and octahedral sheets the subtle change in al substitution in chlorite implies that alvi is preferentially ordered in the m 4 site and about 84 of the alvi present is in the m 4 sites when they are nearly filled with alvi based on 47 analyzed chlorite bearing rock samples chlorite chl composition is strongly correlated with the mgo content of the host rock calculated correlation coefficients are 0 91 for sio2chl mgorock 0 87 for al2o3chl mgorock 0 89 for mgochl mgorock and 0 85 for feochl mgorock only weak correlations have been found between chlorite oxides and other oxides of rock between same oxides in chlorite and rock sio2 0 67 al2o3 0 59 feo 0 41 however mgochl is saturated at about 36 wt in rocks that have mgo above 22 wt the mgochl is about 5 wt when the host rock approaches 0 wt of mgo this implies that mg substituting into the chlorite is approximately limited to 1 5 9 2 mg atoms per formula unit and 1 0 3 2 aliv chlorite geothermometers can not be applied to all bgb samples however the empirical chlorite geothermometer based on aliv of chlorite may be applicable to chlorites formed under metamorphic conditions because it can predict the chemical composition of the chlorite from basaltic and dacitic samples in this study an estimated temperature of about 320 c for the greenschist metamorphism of the greenstone belt through this geothermometer is consistent with that obtained by other geothermometers

reassessment of archean crustal development in the barberton mountain land south africa based on u pb dating this paper presents u pb ages for 23 samples from the barberton mountain land an archean granite greenstone terrain in the kaapvaal craton of south africa a reexamination of the geology of this area in the light of previous u pb dating and the new ages indicates that much of its deformation can probably be accounted for by a single short lived compressional event within the time span 3230 3215 ma which was synchronous with deposition of the moodies group and the upper part of the fig tree group sediments this event may have been responsible for thrusting of much older sequences in the onverwacht group including slivers of 3 54 ga old gneiss circa 3 47 ga old komatiitic and tholeiitic sequences and circa 3 45 ga old felsic sequences key constraints on the timing of deformation are previously determined 3227 3225 ma ages for detrital zircons in thrusted units of the fig tree group a 3229 4 3 ma age for a deformed porphyry which is pre or synthrusting a previously determined 3227 3 ma age for an undeformed porphyry in an inferred thrust fault 3227 1 ma ages for both the kaap valley pluton and an ignimbrite in the stolzburg syncline both of which predate regional upright folding and a 3216 2 ma age for the dalmein pluton which appears to cut regional upright ne sw trending folds in the kromberg syncline part of the final phase of deformation associated with regional compression areally extensive granitoid sheets and syenogranitic plutons were subsequently emplaced into the barberton region at about 3105 ma the linear configuration of the syenogranitic plutons which are also aligned with the gabbroic usushwana complex as well as the geometry of the granitoid sheets suggests that the circa 3105 ma magmatism was caused by incipient rifting around a nw se trending crustal fracture this episode was probably coeval with a late regional transtensional phase of deformation recorded in the greenstone belt some of the above interpretations are in conflict with previous models for early development of the barberton greenstone belt a 3352 6 ma age from a metagabbro which is comagmatic with a proposed ultramafic sheeted dike complex is over 100 m y younger than the probable age of volcanic rocks which host the dikes this conflicts with the suggestion that much of the onverwacht group comprises an ophiolitic section obducted in an intraoceanic setting at circa 3 45 ga that partially melted to form trondhjemitic magmas which intruded along active thrust faults present data suggest that despite its antiquity and the diversity of ages and lithologies found within it the barberton greenstone belt can generally be understood in terms of actualistic tectonic processes at convergent margins

new u pb and 40ar 39ar ages from the northern margin of the barberton greenstone belt south africa implications for the formation of mesoarchaean gold deposits gold mineralization in the mesoarchaean new consort gold mine in the barberton greenstone belt south africa is situated in the immediate hanging wall of an exhumed composite granitoid gneiss dome known as the stentor pluton the granitoid greenstone contact is characterized by a contact parallel high strain zone that separates the mid crustal gneisses from the greenschist facies greenstone belt in contrast to the lower greenschist facies deposits in the central parts of the greenstone belt the mineralized shear zones at new consort are hosted by distinctly higher grade metamorphic rocks ranging from upper greenschist to upper amphibolite facies conditions a distinct change of ore and alteration assemblages with increasing metamorphic grade points to a temporal and spatial relationship between gold mineralization and the juxtaposition of the stentor gneiss dome against the overlying greenstone belt examination of the structural and petrologic evolution of the mineralized high strain zone combined with new u pb and ar ar data and previously published ages indicates a multistage tectono metamorphic evolution early high t medium p metamorphism ca 600 700 c and 5 1 kbar in the structurally deeper parts of the mine is interpreted to be related to an early period of orogen parallel ne sw extension and associated detachment shearing during the main d2 accretionary event in the centre of the greenstone belt between ca 3260 and 3225 ma recent work has shown that these early high t fabrics were reactivated at ca 3105ma during the subsequent d3 tectonism and associated emplacement of large potassic granite batholiths the gold mineralization was coeval with ne sw extensional exhumation and solid state doming of the stentor gneiss dome during the subsequent d4 deformation this late stage deformation and the gold mineralization postdate the emplacement of the d3 related potassic granites by more than 50 million years as indicated by 1 a u pb age of 3046 27ma for a retrograde titanite generation in a d2 related amphibolite sample 113 and 2 a 3027 7 5ma age for hydrothermal titanite from the so called footwall lens mineralization which temperature has been estimated at 590 40 c this high t event is also recorded by a gcorrected h 3040 4ma hornblende ar ar age from the same sample suggesting that cooling of the stentor gneiss dome shortly followed the exhumation a considerably younger ar ar age 2966 4 ma has been obtained for an amphibolite from the contact parallel high strain zone pointing to non uniform cooling following the mineralization the gold mineralization was largely contemporaneous with the emplacement of nnw trending syn kinematic granitic pegmatites and strain localization along the pegmatite swarm at ca 3030 3040 ma the mineralization is also considerably younger than the gold mineralization in the greenschist facies gold deposits in the barberton greenstone belt which has previously been dated at ca 3080 ma in conjunction with previously published ages the data indicate the au mineralization in the barberton greenstone belt to have occurred during at least two mineralization episodes at ca 3080 and 3040 ma the main phase of au mineralization was introduced during the end of an either protracted or episodic structural and thermal evolution of the granitoid greenstone terrain that may have lasted some 200 ma

regional folding low angle thrusting and permeability networks structural controls of gold mineralization in the hope reef at fairview mine barberton greenstone belt south africa abstract the hope reef forms part of a number of high grade mineralized but hitherto undocumented auriferous shear zones at fairview mine in the barberton greenstone belt in south africa collectively referred to as the hanging wall reefs the reefs are hosted by steeply dipping greywacke units of the fig tree group on the western limb of the regional scale ulundi syncline the currently mined shallow hope reef represents a top to the northwest low angle thrust that formed as a secondary accommodation structure during continued nw subhorizontal shortening after the lock up of the ulundi syncline northwest shortening strains correspond to the main phase of upright regional folding d2 3 in the greenstone belt the formation of shear and extensional vein sets documents fault valve behaviour along the low angle thrust driven by close to lithostatic fluid pressures and under very low differential stresses the hanging wall reefs at fairview mine underline the formation of mineralized reefs in the barberton gold district with vastly different orientations structural and lithological controls either in sequence or broadly contemporaneous but during the late stage shortening of the greenstone belt the absolute timing of this deformation and associated hydrothermal events remain unclear

zircon u pb geochronology and hf isotopic compositions of the mesozoic granites in southern anhui province china abstract mesozoic magmatism is extensively developed in eastern china along the eastern asian continental margin but the geochronological framework is not well understood for each area in this study seventy eight samples were collected from the mesozoic igneous rocks in south anhui province east central china in order to precisely constrain their formation ages and contributions to crustal growth zircon u pb age determination using ion probe and laser ablation techniques indicate that magmatism in the area can be divided into two separate stages at 150 136 and 136 120 ma respectively the first stage of granitic rocks not documented in other areas of eastern china is mostly composed of hornblende bearing i type granodiorite and monzogranite and is considered to be formed by slab tearing during pacific subduction the second stage of magmatism coeval to that in other areas of eastern china is characterized by the occurrence of a type granite and syenite and their volcanic counterparts although i type granitic rocks are also developed locally these rocks are mostly distributed around and within the fault controlled volcanic basins and some are closely related to detachment shearing indicating their formation in an extensional setting zircon hf isotopic analyses indicate that archean material might be present in the area but that the main episode of crustal growth took place during the mesoproterozoic with very little juvenile material in the mesozoic

a tectono genetic model for porphyry skarn stratabound cu au mo fe and magnetite apatite deposits along the middle lower yangtze river valley eastern china abstract the middle lower yangtze river valley metallogenic belt yrb situated along the northern margin of the yangtze craton is characterized by porphyry skarn stratabound cu au mo fe deposits in the areas of uplift and magnetite apatite deposits in cretaceous fault basins following detailed field investigations and a review of published data we recognize two episodes of magmatism and mineralization in the yrb 1 156 137 ma high k calc alkaline granitoids associated with 148 135 ma porphyry skarn stratabound cu au mo fe deposits and 2 135 123 ma shoshonitic series associated with 134 9 122 9 ma magnetite apatite deposits a type granitoids and associated alkaline volcanic have a small age range from 126 5 to 124 8 ma and are temporally spatially and genetically associated with the second episode the geodynamic history of the yrb did not experience the paleozoic to mesozoic lithospheric thickening that took place in the north china craton this process is inferred to be linked to partial melting of the delaminated lower crust at high pressures resulting in the development of c type adakitic rocks the petrochemical and sr nd isotopic data show that both the shoshonitic series and a type granitoids are quite different from adakites with only some of the k calc alkaline granitoids having adakitic signatures previous ore genesis models were established based on an assumed relationship with adakites and a continuous tectono thermal evolution from 150 to 100 ma all data obtained for the middle lower yangtze river region consistently show that the tan lu regional strike slip fault zone initiated at 233 6 to 225 6 ma from the collision between the north china and yangtze cratons and was reactivated at ca 160 ma the tan lu fault was caused by the oblique subduction of the izanagi plate which along the yrb the low angle subducted slab and the overlying crust was disrupted or broken due to the disharmonious movement of the two blocks the high k calc alkaline granitoids magmas were derived from melting of the subducted slab with some input of crustal material these magmas were emplaced at the intersections between ne and ew trending faults and formed porphyry skarn stratabound cu au mo fe deposits between 156 and 137 ma after 135 ma the subducted plate changed its direction of motion to northeast now running parallel to the eurasian continental margin and leading to large scale continental extension the shoshonitic series and subsequent a type granitoids magmatism and the development of magnetite apatite ores in the yrb took place in both fault basins and ne trending rifts between 135 and 124 ma

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

the chemical composition of rivers and snow affected by the 2014 2015 barðarbunga eruption iceland abstract the 2014 15 bar arbunga volcanic eruption was the largest in iceland for more than 200 years this eruption released into the atmosphere on average 60 000 tonnes day of so2 30 000 tonnes day of co2 and 500 tonnes day of hcl affecting the chemical composition of rain snow and surface water the interaction of these volcanic gases with natural waters decreases fluid ph and accelerates rock dissolution this leads to the enhanced release of elements including toxic metals such as aluminium to these waters river monitoring including spot and continuous osmotic sampling shows that although the water conductivity was relatively stable during the volcanic unrest the dissolution of volcanic gases increased the so4 f and cl concentrations of local surface waters by up to two orders of magnitude decreasing the carbon alkalinity in addition the concentration of sio2 ca mg na and trace metals rose considerably due to the water molten lava and hot solid lava interaction the presence of pristine lava and acidic gases increased the average chemical denudation rate calculated based on na flux within jokulsa a fjollum catchment by a factor of two compared to the background flux melted snow samples collected at the eruption site were characterised by a strong dependence of the ph on so4 f and cl and metal concentrations indicating that volcanic gases and aerosols acidified the snow protons balanced about half of the negatively charged anions the rest was balanced by water soluble salts and aerosols containing a variety of metals including al fe na ca and mg the concentrations of f al fe mn cd cu and pb in the snowmelt water surpassed drinking and surface water standards snowmelt river water mixing calculations indicate that low alkalinity surface waters such as numerous salmon rivers in east iceland will be more affected by polluted snowmelt waters than high alkalinity spring and glacier fed rivers

tectonic history of the barberton greenstone belt south africa 490 million years of archean crustal evolution rocks of the 3570 3080 ma barberton greenstone belt record a geological history of at least 490 million years m y and were affected by regional thermal perturbations for over 1500 m y small tectonic wedges of 3570 3511 ma schistose tonalite are the oldest rocks yet recognized but little is known about their tectonothermal history the earliest well recognized tectonothermal events 3490 3450 ma represent mid ocean ridge like processes these were followed by two periods of arc related and trench related processes separated by 160 m y the first 3445 3416 ma recording an intraoceanic suprasubduction like environment and the second 3260 3225 ma intra arc and interarc like processes culminating in arc amalgamation accretion like convergent processes dominated between 3230 ma and 3080 ma overlapping with the assembly of the kaapvaal craton at 3080 ma the barberton greenstone belt experienced a sudden shift from dominantly convergent to dominantly transtensional tectonism and was mineralized with mesothermal gold along a regional shear system soon afterward transtension became distributed across the entire kaapvaal craton heralding the onset of formation of the witwatersrand basin by 3070 ma

geochronology and geochemistry of granitoids related to the giant dahutang tungsten deposit middle yangtze river region china implications for petrogenesis geodynamic setting and mineralization abstract porphyry and skarn deposits in the middle yangtze valley within the northern yangtze craton have a combined tungsten resource of 3 million tonnes mt and represent one of the most important tungsten regions in the world the dahutang porphyry tungsten deposit with reserves of 1 mt is one of the largest deposits uranium pb analyses for the ore related granitoids yield ages of 147 4 0 58 ma 148 3 1 9 ma for porphyritic biotite granite 144 7 0 47 ma 146 1 0 64 ma for fine grained granite and 143 0 0 76 ma 143 1 1 2 ma for granite porphyry a progressive youngling of ages that is consistent with field observations geochemical data show that the three types of granite are characterized by enrichments in rb pb and u and depletion in ba nb p and ti with asi molar al 2 o 3 cao na 2 o k 2 o 1 1 that is characteristic of a peraluminous melt the p 2 o 5 contents of the granites are 0 13 0 37 and have a positive correlation with sio 2 and they are thus s type intrusions they exhibit initial 87 sr 86 sr of 0 721 to 0 731 and e nd t of 5 06 to 7 99 for porphyritic biotite granite 0 7196 to 0 7289 and 6 29 to 6 74 for fine grained granite and 0 7153 to 0 7365 and 5 09 to 7 64 for granite porphyry chondrite normalized rare earth element ree patterns for the granites are characterized by enrichment in the light ree and a strong negative eu anomaly indicating that they were derived from the proterozoic pelitic and psammitic basement strata and experienced strong fractional crystallization of plagioclase our ca 150 140 ma age for the dahutang s type magmatism and w mineralization is identical to that of the i type magmatism related to cu au mo fe bearing porphyry and skarn deposits along the middle to lower yangtze river valley we propose that the latest jurassic to earliest cretaceous granitoids and ores formed during a tearing of the subducting izanagi slab which caused the upwelling of asthenosphere and resulting mantle crust interaction the s type granitoids and related w ore systems resulted from the re melting of the proterozoic crust whereas the i type granitic rocks and related ores are attributed to the partial melting of the subducted slab

geochemistry and nd sr isotopic studies of late mesozoic granitoids in the southeastern hubei province middle lower yangtze river belt eastern china petrogenesis and tectonic setting abstract a geochemical and isotopic study was carried out for the mesozoic yangxin tieshan and echeng granitoid batholiths in the southeastern hubei province eastern china in order to constrain their petrogenesis and tectonic setting these granitoids dominantly consist of quartz diorite monzonite and granite they are characterized by sio 2 and na 2 o compositions of between 54 6 and 76 6 wt and 2 9 to 5 6 wt respectively enrichment in light rare earth elements lree and large ion lithophile elements lile and relative depletion in y concentrations ranging from 5 17 to 29 3 ppm and yb 0 34 2 83 ppm with the majority of the granitoids being geochemically similar to high sio 2 adakites hsa their initial nd e nd 12 5 to 6 1 and sr 87 sr 86 sr i 0 7054 0 7085 isotopic compositions however distinguish them from adakites produced by partial melting of subducted slab and those produced by partial melting of the lower crust of the yangtze craton in the late mesozoic the granitoid batholiths in the southeastern hubei province exhibit very low mgo ranging from 0 09 to 2 19 wt with an average of 0 96 wt and large variations in negative to positive eu anomalies eu eu 0 22 1 4 especially the tieshan granites and yangxin granite porphyry eu eu 0 22 0 73 geochemical and nd sr isotopic data demonstrate that these granitoids originated as partial melts of an enriched mantle source that experienced significant contamination of lower crust materials and fractional crystallization during magma ascent late mesozoic granitoids in the southeastern hubei province of the middle lower yangtze river belt were dominantly emplaced in an extensional tectonic regime in response to basaltic underplating which was followed by lithospheric thinning during the early cretaceous

timing of skarn deposit formation of the tonglushan ore district southeastern hubei province middle lower yangtze river valley metallogenic belt and its implications abstract the tonglushan ore district in the middle lower yangtze river valley metallogenic belt includes the tonglushan cu fe the jiguanzui au cu and the taohuazui au cu skarn deposits they are characterized by ne striking ore bodies and hosted at the contact of triassic carbonate rocks and late mesozoic granitoid deposits new sensitive high resolution ion microprobe shrimp and laser ablation inductively coupled plasma mass spectrometry la icp ms zircon u pb molybdenite re os and phlogopite 40 ar 39 ar ages indicate that these skarn deposits formed between 140 3 1 1 and 137 3 2 4 ma these dates are identical to the zircon u pb ages for host quartz diorites ranging from 140 2 to 139 1 ma these results confirm that both skarn mineralization and related intrusions were initiated during the early cretaceous the high rhenium contents 261 4 1152 g g of molybdenites indicate that a metasomatic mantle fluid was involved in the ore forming process of these skarn ore systems this conclusion is consistent with previously published constraints from sulfur deuterium and oxygen isotope compositions and the geochemical signatures and sr nd isotopic data of the mineralization hosting intrusions geological and geochronological evidence demonstrates that there were two igneous events in the tonglushan ore district the first resulted in the emplacement of quartz diorite during the early cretaceous 140 2 to 139 1 ma and the second is characterized by the eruption of volcanic rocks during the mid early cretaceous 130 2 to 124 2 ma the former is spatially temporally and genetically associated with skarn gold bearing mineralization 140 3 1 1 to 137 3 2 4 ma the recognition of these two igneous events invalidates previous models that proposed continuous magmatism and associated mineral deposits in the middle lower yangtze river valley metallogenic belt

a type granites in northeastern china age and geochemical constraints on their petrogenesis abstract a type granites are widely distributed in northeastern china ne china they were emplaced during three major episodes the permian late triassic to early jurassic and early cretaceous and evolved in different tectonic regimes according to their mineralogical and geochemical characteristics two subgroups of a type granites aluminous and peralkaline can be recognized the peralkaline subgroup contains alkali mafic minerals such as riebeckite arfvedsonite and sodic pyroxene while the aluminous subgroup contains annite and fe rich calcic or sodic calcic amphibole with respect to the aluminous subgroup the peralkaline granites contain higher rb ga and total rare earth elements ree but lower mgo cao al 2 o 3 ba and sr based on the discrimination criteria of eby geology 20 1992 641 the permian and late triassic to early jurassic a type granites belong to the a 2 post orogenic type whereas the early cretaceous granites are of the a 1 anorogenic type nd isotopic compositions of these a type granites indicate their derivation from a dominantly juvenile crustal source their origin is thought to have involved partial melting of an underplated lower crustal source because the generation of a type granites requires high melting temperature we propose models involving slab break off lithospheric delamination and extension the permian a type granites have the same age range as those in eastern junggar southern mongolia and central inner mongolia they occur along a major suture and form a narrow belt between the north china and siberian cratons we suggest that their formation was associated with post collisional slab break off the late triassic to early jurassic a type granites are likely to be the product of lithospheric delamination after the final collision of the major crustal blocks in the late paleozoic to early triassic the early cretaceous a type granites have an anorogenic affinity and were possibly associated with rifting in eastern china at this time associated with the onset of paleo pacific subduction consequently we conclude that the a type granites in ne china were generated at three different times involving multiple processes operative in different tectonic environments

trace element zoning of sulfides and quartz at sheba and fairview gold mines clues to mesoarchean mineralisation in the barberton greenstone belt south africa abstract the fairview and sheba mines are two of the major gold mines in the paleoarchean barberton greenstone belt of southern africa at these mines gold is associated with quartz carbonate rutile veins and occurs both as invisible gold finely dispersed in sulfides primarily pyrite and arsenopyrite and as visible electrum grains hosted in pyrite up to approximately 1000 ppm au are contained in pyrite and up to approximately 1700 ppm in arsenopyrite mapping of trace element distribution in sulfide minerals using electron microprobe and proton probe techniques revealed multiple events of ore formation and au mineralisation at fairview mine three stages of pyrite formation were identified the last of which is associated with arsenopyrite electrum and other sulfide minerals sphalerite chalcopyrite galena gersdorffite and sb sulfides at sheba mine pyrite was deposited in two stages and electrum is associated with the second stage at both mines the last stage of sulfide formation is the main stage of au deposition and is associated with mobilisation of au as sb cu zn and ni the host rock composition seems to have affected the composition of pyrite since higher ni and co concentrations up to 1 4 and 1 6 wt respectively have been measured in meta ultra mafic host rocks in comparison with chert and metagreywacke arsenopyrite is chemically zoned and has sb and s rich cores and as and ni rich rims this zoning indicates variations in fluid compositions decreasing sb and increasing ni and crystallisation conditions increasing as content for increasing temperature geothermometric estimates based on the as content of arsenopyrite as 32 at indicate temperatures up to 420 c for the crystal rims petrographic and cathodoluminescence observations of quartz associated with gold mineralisation show only local brittle deformation and no plastic deformation this supports the notion that the ore transporting veins were emplaced late in the deformation history variations of cathodoluminescence of quartz are correlated with changing al contents al 0 16 wt and can be related to fluctuations in the ph of the mineralising fluids

multi stage alteration rheological switches and high grade gold mineralization at sheba mine barberton greenstone belt south africa abstract structural petrographic and geochemical data from sheba gold mine in the barberton greenstone belt of south africa indicate that gold mineralization formed at the end and as the result of a multi stage structural fluid flow and alteration history this multi stage evolution is a prerequisite for the formation of high grade 40 g t au ore bodies the sheba mine is situated in well bedded metaturbidites of the mesoarchaean fig tree group and structurally infolded ultramafic lavas in the first order refolded ulundi syncline in the northern parts of the belt original komatiitic lava flows are pervasively altered to talc carbonate greyschist this regionally widespread alteration relates to an early phase of deformation and associated fluid flow the reaction softening associated with this alteration accentuates rheological anisotropies within the heterogeneous wall rock sequence this results in the localization of strain along the contact between talc carbonate greyschist and overlying massive greywacke and chert units during later stage refolding and associated flexural slip pervasive quartz veining silicification k enrichment and the progressive replacement of the earlier talc carbonate assemblages by a high variance quartz fuchsite green schist alteration testify to the localized fluid flow along this contact the newly formed finely intergrown green schist assemblage leads in turn to a reaction hardening and embrittlement of the former greyschist chert contact high grade gold mineralization is hosted by brittle quartz carbonate vein networks and breccias developed along brittle ductile structures that cross cut the competent green schist at high angles the siting of gold and predominance of free gold suggest that gold precipitation is related to extreme fluid pressure cycling and phase separation of the mineralizing fluid during the formation of the brittle fracture networks the thrust kinematics of the late stage controlling structure studied here indicate that the mineralized structure accommodated regional nw se subhorizontal shortening when further tightening of the ulundi syncline passed the lock up stage could no longer be accommodated by flexural slip the results highlight that the formation of favourable conditions for high grade mineralization such as those at sheba mine may be the result of the interaction of temporally distinct fluid flow and alteration events and lithological heterogeneities during the progressive structural evolution of a region

ore shoot formation in the main reef complex of the fairview mine multiphase gold mineralization during regional folding barberton greenstone belt south africa gold production in fairview mine in the barberton greenstone belt south africa is focused on high grade steeply plunging ore shoots contained within a gently undulating system of spaced low displacement broadly bedding parallel faults and shear zones collectively referred to as the main reef complex this shear zone system is developed in metaturbiditic sedimentary rocks of the mesoarchean fig tree group on the steep western limb of the regional d3 refolded and distinctly arcuate ulundi syncline shear zones are preferentially localized along lithological contacts with pronounced rheological contrasts particularly pronounced along the limbs of tightly infolded ultramafic schist against fig tree group metasedimentary rocks the main ore shoot of the mrc at fairview shows a steep southeasterly plunge that can be traced for 2000 m down plunge parallel to the fold hinge f3b that refolds the ulundi syncline fabrics and structures in the ore shoot record combined top to the nw thrust sense and dextral strike slip kinematics and strongly constrictional strains interpreted to indicate dextral transpression and associated steep extrusion of the rocks during progressive nw se d3 shortening a combination of structural and lithological controls govern ore shoot formation and gold mineralization the main ore shoot corresponds to dilational jog geometry that developed during dextral transpression bedding parallel shearing consistent with the flexural slip refolding f3b of the ulundi syncline high grade pockets correlate with sheared commonly dismembered graphite rich and sulfide mineralized shale units testifying to the preferential fluid focusing and strain localization into incompetent shale units during deformation cross cutting relationships with other auriferous reefs from the fairview mine complex indicate a late timing of the mrc type mineralization underlining the complex fluid focusing and utilization of differently orientated structures with different kinematics during progressive deformation and either episodic or protracted fluid flow events

an atmospheric source of s in mesoarchaean structurally controlled gold mineralisation of the barberton greenstone belt abstract the barberton greenstone belt of southern africa hosts several mesoarchaean gold deposits the ores were mostly formed in greenschist facies conditions and occur as hydrothermal alteration zones around extensional faults that truncate and post date the main compressional structures of the greenstone belt ore deposition was accompanied by the intrusion of porphyries which has led to the hypothesis that gold may have been sourced from magmas because the transport of au in the hydrothermal fluids is widely believed to have involved s complexes tracing the origin of s may place strong constraints on the origin of au we measured multiple s isotopes in sulfide ore from sheba and fairview mines of the barberton greenstone belt to distinguish deep s sources e g magmas from surface s sources i e rocks of the volcano sedimentary succession that contain s processed in the atmosphere preserved as sulfide and sulfate minerals ion probe sims analyses of pyrite from ore zones indicate mass independent fractionation of s isotopes 33s 0 6 to 1 0 and the distribution of the analyses in the 33s 34s space matches the distribution peak of previously published analyses of pyrite from the entire volcano sedimentary succession notwithstanding that the h2o co2 components of the fluids may have been introduced from a deep source external to the greenstone belt rocks the fact that s bears an atmospheric signature suggests the hypothesis that the source of au should also be identified in the supracrustal succession of the greenstone belt our findings differ from conclusions of previous studies of other archaean shear hosted au deposits based on mineralogical and isotopic evidence which suggested a magmatic or mantle source for au and imply that there is no single model that can be applied to this type of mineralisation in the archaean