late cenozoic bending of the bolivian andes new paleomagnetic and kinematic constraints 1 new paleomagnetic measurements of essentially undeformed late cenozoic volcanic rocks in the bolivian andes south america constrain rigid body rotations about vertical axes during the last 13 myr on both limbs of the bolivian orocline in the eastern cordillera thermal and alternating field demagnetization was carried out on samples from 52 sites in three major volcanic complexes 1 13 2 ma los frailes ignimbritic volcanics outcropping at 19 5 s 2 9 5 ma morococala ignimbritic volcanics outcropping at 18 s and 3 13 5 ma shoshonitic to acidic lavas and intrusives outcropping at 17 5 s well defined magnetic components were isolated which are interpreted to represent the earth s magnetic field at the time of volcanic activity the mean magnetic vector for site groupings suggests regional tectonic rotations about vertical axes with respect to stable south america of 10 8 clockwise for the 13 2 ma los frailes volcanic complex 8 9 clockwise for only the miocene 13 5 ma los frailes volcanics and 1 18 anticlockwise for the combined 13 5 ma morococala and eucalyptus volcanic complexes these data are consistent with observed shortening gradients on the eastern margin of the bolivian andes in the sub andean zone suggesting rotation and shortening are synchronous a joint inversion for both tectonic rotation and the amount of shortening assuming a linear variation in the amount of rotation along the length of the southern limb of the bolivian orocline shows that rotation of the eastern cordillera in the last 10 15 myr relative to stable south america varies from zero at the oroclinal hinge at 18 s to a maximum of 13 5 clockwise at 22 s decreasing to zero south of 23 s with an average rotation in the range 5 to 10 clockwise concomitant with this shortening in the sub andean zone decreases from a maximum of 86 km at the oroclinal hinge to 47 km at 22 s and then 33 km at 23 s the lack of evidence for large scale internal faulting in the eastern cordillera since 9 ma suggests either bending of the eastern cordillera was accommodated by more distributed small scale faulting or that relative rotation here mainly occurred prior to 9 ma the small average rotation of the eastern cordillera on the southern limb of the orocline is consistent with only very small rotation 10 of the forearc region in northern chile during the last 20 myr rotation of the northern limb is less well constrained but it is also likely to be small 5 and anticlockwise

gold mineralization in proterozoic black shales example from the haoyaoerhudong gold deposit northern margin of the north china craton abstract the haoyaoerhudong gold deposit in the northern margin of the north china craton ncc is a large tonnage lower grade deposit with a reserve of about 148 tons of gold gold mineralization is characterized by pyrite and pyrrhotite films and thin veins on the schistosity plane of the proterozoic black shales the orebodies strictly controlled by a near ew trending shear zone are stratabound within carbonaceous phyllite and andalusite garnet schist of the bilute formation hydrogen and oxygen isotopic data show that the ore forming fluid was derived from a magmatic source and mixed with meteoric water sulfur and carbon isotope data indicate that most of the sulfur and carbon came from the black shale strata well defined biotite ar ar plateau age and inverse isochron age show that the deposit formed at ca 270ma which suggests a probable link between hercynian magmatism and gold mineralization studies on regional geology ore geology isotope geochemistry and ore forming age substantiate a complex evolutionary history of the deposit the proterozoic black shales rich in gold sulfur and organic matter which were deposited in the proterozoic continental margin rifts comprised the source bed for gold mineralization ew ene structures products of palaeozoic orogenic process provided pathways and mineralization space for ore forming fluids hercynian tectono magmatism and subsequent hydrothermal events remobilized gold and drove the ore forming fluids to dilatational fracture zones related to postcollisional magmatic hydrothermal events the haoyaoerhudong gold deposit is considered as a special type of orogenic gold deposit formed in the compression extension transition stage

physical models of magmatic intrusion during thrusting to investigate how magma rises through the brittle upper crust in a context of compressional tectonics we have performed experiments on scaled physical models powdered silica having a cohesion of 300 pa and an angle of internal friction of 38 was used to represent brittle crust a vegetable oil with a newtonian viscosity of 10 2 paae sa t 50c was used to represent magma a moving piston shortened the models in a box while oil was injected steadily at the base on cooling to room temperature the oil solidified the resulting intrusions were thin sills dykes and laccoliths their shapes and emplacement modes depended on the ratio r between rates of shortening and injection from shapes and orientations of intrusions we infer that hydraulic fracturing was one mechanism of emplacement unconsolidated intrusions strongly influenced thrust formation on the basis of our experiments we suggest that magmas in orogenic belts can rise along thrust faults

k ar dating of miocene magmatism and related epithermal mineralization of the northeastern altiplano of bolivia this paper describes the age and geology of 13 centers of miocene magmatism with related epithermal precious and base metal mineralization on the eastern side of the northern altiplano of bolivia part of the central andean polymetallic belt twenty eight new k ar ages are reported together with a compilation of 12 from the literature the data show four magmatic episodes the two middle miocene episodes 17 15 ma 13 5 12 ma are characterized by clusters of small volcanic and hypabyssal porphyries which host the most important epithermal mineralization in contrast large volcanic centers formed during the two late miocene episodes 10 8 ma 6 5 ma no significance is attached to the secondary age groups the middle and late miocene magmatic mineralization episodes correlate with those of the southern part of the tin belt of the eastern cordillera and formed during the main arc broadening of the central andes

rare earth elements and the petrogenesis of aubrites ree measurements in the bishopville aubrite confirm that oldhamite cas is the dominant ree carrier while most other phases including the major silicates are extremely ree depleted in oldhamite a variety of ree patterns are observed which could not have been produced during crystallization from a single fractionating magma although the aubrites have clearly undergone igneous processing oldhamite which is highly refractory may be a relict phase in this case whole rock ree data cannot provide information on the igneous evolution of these meteorites

geological features and genesis of gold deposits hosted by low grade metamorphic rocks in central western inner mongolia the central western inner mongolia area located in the western section of the northern margin of the north china craton is one of the most important gold metallogenic provinces in north china the gold deposits and prospects hosted by low grade metamorphic rocks are widely distributed in two parallel ew trending protrozoic rift belts i e the bayan obo belt in the north and the chaiertaishan belt in the south up till now twenty three ore deposits and prospects have been discovered of which changshanhao zhulazaga and saiyinwusu are the largest ones gold mineralization occurs in proterozoic carbonaceous sandstone siltstone carbonaceous phyllite phyllite slate and schist in the stratoid layer stratiform stockwork and veinlike forms all these gold deposits are characterized by shallow burial large size simple composition and easy mining systematic geological investigations indicate that wall rocks of these deposits and prospects vary from place to place but most of them show an intimate spatial temporal relation with the caledonian ductile brittle fracture zone and the hercynian intrusive dyke swarm geometry of individual ore bodies is mainly controlled by interstratified fracture zones all the geological features of the gold deposits hosted by low grade metamorphic rocks in central western inner mongolia are similar to features of the muruntau gold deposit in uzbekistan the relationship between the regional crustal evolution and gold metallogeny can be summarized as follows middle proterozoic stage tectono magmatism caused by the intense rifting occurring within the early precambrian continent or along its margin resulted in the spatial preparation of gold and other ore forming materials and formed a number of gold source beds and protores along certain strata early paleozoic stage a number of gold prospects were formed by regional metamorphism and its related hydrothermal event caused by regional tectonic movement and magmatic intrusion with middle to large sized gold deposits of this stage also identified late paleozoic stage gold deposits with economic significance are considered to have been caused by the re working of the early formed deep rooted faults and its related hercynian mafic to felsic magmatism the extensional faulting and high alkaline magmatism might have played the key role during the formation of the ore deposits geological and geochemical features of the gold deposit hosted by the low grade metamorphic rocks occurring in central western inner mongolia have aroused much attention among geologists both in china and abroad an integrated analysis of the geological setting and geological and geochemical features of these deposits and their related intrusive rocks will greatly upgrade the understanding of the ore forming processes of the gold deposits and prospects hosted by low grade metamorphic rocks in central western inner mongolia meanwhile the genetic model and mineral exploration criteria of these deposits and prospects can also be used during the comprehensive evaluation of the concealed gold deposits hosted by low grade metamorphic rocks in central western inner mongolia and its neighboring areas

the permian triassic granitoids in bayan obo north china craton a geochemical and geochronological study abstract granitoids near the bayan obo giant rare earth element ree deposit at the north margin of the north china craton ncc the world s largest light ree lree deposit have been taken by some authors as the key factors that controlled the mineralization in contrast others proposed that the ree deposit has been partially destructed by these granitoids here we report systematic studies on geochronology and geochemical characteristics of granitoids of different distances from the orebodies to investigate the genesis and their relationship to the giant bayan obo deposit granitoids studied here including granites and quartz monzonites are peraluminous with a cnk 0 99 1 11 lree enriched and heavy ree hree depleted with variable ree concentrations total ree 54 330 ppm and large negative eu anomaly eu 0 19 0 70 the ree patterns are distinct from those of ore bearing dolomites some samples have slightly higher lree concentrations which may have been contaminated by the orebodies during intrusion trace elements of the granitoids are characterized by positive pb anomaly strong negative ti anomaly and nb ta and sr anomalies the granites exhibit negative ba anomaly the granitoids plot within the post collision granite field in the pearce diagram which is consistent with the tectonic regime the quartz monzonites and one granite have a type granite characteristics and belong to the a 2 subgroup zircons in these granitoids have high th u values which are typical for magmatic zircons high precision u pb dating for these zircons by secondary ion mass spectrometry sims and laser ablation inductively coupled plasma mass spectrometry la icp ms yields permian triassic 206 pb 238 u ages ranging from 243 2 to 293 8 ma the formation of the granitoids is 55 ma later than the latest ore forming age the zircons have low la concentrations 0 02 12 ppm high sm la n 0 8 685 and ce ce 1 4 80 the ti in zircon temperature of the granitoids ranges from 590 to 770 c all these evidences suggest that the granitoids have no contribution to the formation of the bayan obo deposit granitoids that are close to the orebodies had limited interaction with it and gained some lree enriched characteristics during magmatisms nevertheless their effects to the orebodies are subtle all the granitoids formed in a post collisional tectonic regime at convergent margins which is consistent with plate subduction during the closure of the palaeo asian ocean which started in the neoproterozoic and lasted until the carboniferous permian

the enigmatic yinshan fold and thrust belt of northern china new views on its intraplate contractional styles the east to east northeast trending yinshan belt lies within north china extending westward at least 1100 km from china s eastern coast to inner mongolia this intraplate jurassic cretaceous belt underwent contractional and normal faulting folding and contemporaneous terrestrial sedimentation and magmatism current views on its contractional deformational style favor relatively limited thick skinned faulting of archean basement and cover units these views are challenged however by recent discoveries in the eastern part of the belt of south directed ductile nappe formation and large displacement 40 45 km thin skinned northward thrust faulting both involving archean and younger rock units collision of the siberian and north china plates upon closure of a jurassic and early cretaceous mongolo okhotsk ocean more than 800 1100 km to the north may have been responsible for yinshan north south contraction some patterns of contraction e g jurassic cretaceous ductile nappe formation appear to have been influenced by a superposed magmatic regime related to westward subduction of a pacific basin plate beneath the north china plate

geochronology isotopic chemistry and gold mineralization of the black slate hosted haoyaoerhudong gold deposit northern north china craton abstract the haoyaoerhudong gold deposit is the largest gold deposit in the north margin of the north china craton gold province and contains over 7 moz of gold at an average grade of 0 62 g t the deposit is hosted in the carbonaceous and pyritic slate phyllite and schist and is controlled by a tight syncline and shear zones the high grade orebodies contains abundant pyrite veins and pyrite quartz veins three stages of pyrite have been identified including the diagenetic disseminated pyrite pyrite veins caused by peak matamorphism and pyrite quartz veins forming during post peak metamorphism native gold has been observed in pyrite veins and pyrite quartz veins the 40ar 39ar plateau age of a biotite separate from a tails of boudinaged pyrite quartz veins is 260 1 2 9 ma combined with previously published 40ar 39ar mica age data and low closure temperature of mica these results suggest that pyrite veins and pyrite quartz veins were formed during the peak to post peak metamorphism during 285 260 ma respectively the lower limit of the formation age of sedimentary disseminated pyrites has constrained to 1670 1560 ma by the intruded mafic ultramafic dikes disseminated pyrite separates have 34s values ranging from 39 40 to 17 85 206pb 204pb of 19 144 21 892 207pb 204pb of 15 681 15 864 and 208pb 204pb of 37 502 38 925 suggesting they were formed from seawater sulfate and has experienced strong sulfur isotopic fractionation in contrast hydrothermal pyrites from pyrite veins and pyrite quartz veins have 34s values ranging from 6 8 to 16 47 206pb 204pb of 18 566 18 922 207pb 204pb of 15 645 15 684 and 208pb 204pb of 38 924 38 983 which may reflect dissolution reprecipitation of disseminated sulfides from the pre existing organic rich sediments the mineral paragenetic geometric and cross cutting relationships of pyrite veins and pyrite quartz veins at haoyaoerhudong suggest that gold was most likely introduced into pyrite accompanying sedimentation of the organic rich shales and then became enriched during diagenesis subsequently the hydrothermal fluids following metamorphism and shear zone activity make dissolution of the gold in the diagenetic pyrite and precipitated in the intersection of shear zone and tight syncline

tectonic constraints on the timing relationships of metamorphism fluid production and gold bearing quartz vein emplacement abstract there is an ongoing discussion about the genetic significance of the postmetamorphic emplacement of many gold mineralizations as a contribution to this discussion this paper reviews the timing relationships between metamorphic devolatilization and metamorphism at different crustal levels for different tectonic processes for example denudation accompanying or following thickening causes metamorphism to occur later at depth than at shallow levels thus many internally heated terrains e g barrovian terrains are of deep later type that is peak metamorphism and devolatilization at deeper levels occurs at later times metamorphic processes of the deep later type may produce and release metamorphic fluids during prograde metamorphism at depth and precipitate them in the form of postmetamorphic quartz veins at shallow levels magmatic heating from below on the other hand is a deep earlier process causing inverse timing relationships in many metamorphic terrains deep later and deep earlier processes may compete for example if denudation at the surface accompanies heating from below if denudation and heating from below are of similar duration deep later characteristics prevail and a strongly bi modal age distribution of peak metamorphism in different crustal levels may arise the upper crust will experience metamorphism early but largely contemporaneously and the lower crust will experience metamorphism much later this process is particularly favourable for substantial simultaneous devolatilization of large sections of the lower crust to postdate metamorphism at shallow levels thus such processes may result in late infiltration events in cooling rocks of low metamorphic grade the common observation of postmetamorphic fluid infiltration and mineralization may therefore indicate but does not require that the mineralising fluids are related to the same metamorphic event

re os system of black schist from the mesoproterozoic bayan obo group central inner mongolia china and its geological implications abstract the mesoproterozoic bayan obo group in central inner mongolia china represents a sedimentary sequence deposited in the bayan obo rift basin new re os dates are reported here for the black shale schist from the group re os geochronology of the black schist provides an age of 1447 42 ma which is consistent with the previous work from the literature and provides a direct and new constraint for the age of the group the dating indicates that the re os isotopes for the black schist have remained undisturbed during lower amphibolite facies metamorphism the ca 250 ma age for gold bearing sulfides also indicates that the re os isotopes for the schist have not been affected by hydrothermal overprinting of reduced fluid the initial 187 os 188 os value of 0 93 0 14 for the schist is interpreted to represent the os isotopic composition of the seawater during deposition of the group which indicates that the os composition of mesoprotozoic seawater fluctuated during the mesoproterozoic and was temporary very radiogenic similar to present day seawater the re os data suggests that rifting of the bayan obo basin peaked during ca 1447 ma and the bilute formation in the bayan obo group may have been deposited in the bayan obo sea in a restricted or semi restricted depositional environment

age and source constraints for the giant muruntau gold deposit uzbekistan from coupled re os he isotopes in arsenopyrite the muruntau gold deposit uzbekistan is one of the largest gold deposits known worldwide but its origin remains controversial we used re os arsenopyrite geochronology to precisely determine the age of main stage gold mineralization at muruntau to be 287 5 1 7 ma which overlaps the emplacement of proximal post tectonic granitoid magmatism additionally we suggest that arsenopyrite growth may have occurred over an interval of at least 2 m y os initial ratios derived from arsenopyrite were coupled with he isotopic data from fluid inclusions within arsenopyrite to constrain the source of ore metals and fluids muruntau arsenopyrite yields relatively unradiogenic initial os 0 37 0 27 and elevated 3he 4he ratios 0 23 0 33 r a relative to purely crustal os he reservoirs these data suggest the presence of a mantle derived component in the ore system that was probably introduced during the generation of the granitoid magmas these new timing and source constraints provide important new insight into the generation of this giant gold deposit and they necessitate reexamination of genetic models for muruntau and potentially other giant orogenic gold deposits worldwide

h o s and pb isotope geochemistry of the awanda gold deposit in southern tianshan central asian orogenic belt implications for fluid regime and metallogeny abstract the awanda gold deposit is located within the southern tianshan accretionary complex a late paleozoic orogenic belt in the northern margin of tarim block in nw china the gold mineralization here occurs in primarily within shear zones and secondary s faults with quartz mica schist as the principal host rock the sulfide mineral assemblage is dominated by arsenopyrite and pyrrhotite with minor pyrite and gudmundite alteration includes silicification sericitization and rare carbonatization the measured 18 o values for the gold bearing quartz range from 15 9 to 20 3 and the estimated 18 o fluid values range from 7 8 to 12 2 the d values for fluid inclusions from quartz range from 116 3 to 87 6 the 34 s values of gold associated sulfide minerals range from 3 6 to 6 2 and the calculated 34 s h2s values of h 2 s for the hydrothermal fluids range from 4 8 to 5 8 with an average of 0 4 lead isotopic compositions of sulfides from gold bearing ores show 206 pb 204 pb ratios ranging from 17 4625 to 18 5356 207 pb 204 pb ratios from 15 4943 to 15 6468 and 208 pb 204 pb ratios from 37 8825 to 38 8238 respectively collectively the isotope h o s pb characteristics suggest that the ore forming fluids were of metamorphic origin derived through dehydration of carbonaceous black shale with the sulfur sourced from crustal components and the lead from a mixture of host rocks and ore forming metamorphic fluids this deposit can be classified as orogenic gold deposit and share many major characteristics with those of muruntau type gold deposits during late paleozoic the southern tianshan accretionary complex was intensely deformed into an orogenic belt by folding and thrusting generating regional thrust faults at different scales the ore controlling subsidiary faults also formed subsequently with the ore forming fluids derived from dehydration of deeply buried carbonaceous black shale and the ore fluids channeled into shear zones leading to gold precipitation

permian magmatic sequences of the bilihe gold deposit in central inner mongolia china petrogenesis and tectonic significance abstract the bilihe gold deposit is located in the eastern section of the ondor sum yanji suture at the southern margin of the xing an mongolian orogenic belt xmob and the northern margin of the north china craton ncc central inner mongolia the magmatic rocks in the ore district are generally high k calc alkaline enriched in lree zr and hf and depleted in hree nb ta and p the magmatic evolution sequences are norite gabbro granodiorite porphyry granite or norite gabbro andesite dacite porphyry granodiorite which show a trend of decreasing tio2 feo mgo cao and p2o5 with increasing sio2 in the bilihe ore district hydrothermal processes were coeval with granitic magmatism for a period of 17 myr 272 255 ma the ages of the granite granodiorite porphyry granodiorite and dacite porphyry are 271 5 264 1 ma 269 8 255 8 ma 268 3 ma and 268 6 259 4 ma respectively the magmatic rocks contain magmatic hydrothermal and magmatic hydrothermal zircons the magmatic zircons have ce 4 la 2 5 the hydrothermal zircons have ce 3 ppm and smn lan

isotopic geochemistry of the sawayaerdun orogenic type gold deposit tianshan northwest china implications for ore genesis and mineral exploration the sawayaerdun gold deposit is hosted by carbonaceous metasediments and is considered to be the largest muruntau type gold deposit in the chinese tianshan metallogenic belt gold mineralization at sawayaerdun occurs in quartz veins associated with three major hydrothermal events an early barren quartz vein stage middle stage mineralized quartz veins with pyrite and late carbonate quartz veins the isotopic compositions of quartz and sulfides from the sawayaerdun gold deposit show some variation but are generally comparable to those of other orogenic type gold deposits fluids trapped in early stage quartz have a 18o range of 13 6 to 15 4 d of 48 to 75 13c of 0 5 to 4 2 and 30si of 0 2 to 0 in contrast isotopic compositions of fluids trapped in middle stage quartz have 18o values of 6 7 to 14 7 d of 56 to 110 13c of 0 4 to 10 1 and 30si of 0 3 to 0 diagenetic and hydrothermal pyrite have similar sulfur 1 8 to 0 9 and pb isotopic values that are associated with host rock compositions the early stage 18o and 13c rich fluids are probably derived from metamorphic decarbonation of the sedimentary host rock at depth leading to the precipitation of early barren quartz veins in the middle stage a decrease in the regional pressure and temperature regime could have resulted in the incorporation of external fluids into the ore forming system these external fluids with isotopic signatures similar to that of the host rock and generally rich in 34s and radiogenic pb mixed with original ore forming fluids to generate extensive metal precipitation late stage fluids trapped by calcite veins show isotopic compositions similar to meteoric water indicating the cessation of hydrothermal fluid circulation at sawayaerdun occurred at this time the metallogenetic model illustrated by stable and pb isotopes is also consistent with fluid inclusion studies in sawayaerdun the development of mineralization at sawayaerdun is strongly linked to fluid mixing as witnessed by the isotopic signatures of fluids from identified ore bearing zones the isotopic compositions of other anomalous zones at sawayaerdun are similar to those of the mineralized zones suggesting a high potential for further exploration 2012 elsevier b v

gold deposits of the northern margin of the north china craton multiple late paleozoic mesozoic mineralizing events the northern margin of the north china craton is well endowed with lode gold deposits hosting a resource of approximately 900 tonnes t of gold the 1 500 km long region is characterized by east trending blocks of metamorphosed archean and proterozoic strata that were episodically uplifted during variscan indosinian and yanshanian deformational and magmatic events at least 12 gold deposits from the daqinshan yan liao includes the zhangjiakou yanshan and chifeng gold districts and changbaishan gold provinces contain resources of 20 100 t au each most deposits are hosted in uplifted blocks of precambrian metamorphic rocks although felsic paleozoic and mesozoic plutons are typically proximal and host 30 of the deposits the lodes are characterized by sulfide poor quartz veins in brittle structures with low base metal values and high au ag ratios although phyllic alteration is most common intensive alkali feldspar metasomatism characterizes the wulashan dongping and zhongshangou deposits but is apparently coeval with variscan alkalic magmatism only at wulashan stepwise 40ar 39ar geochronology on 16 samples from gangue and alteration phases combined with unpublished shrimp u pb dates on associated granitoids suggest that gold mineralizing events occurred during variscan indosinian and yanshanian orogenies at circa 350 250 200 180 150 and 129 ma however widespread permo triassic 250 ma and early jurassic 180 ma thermal events caused variable resetting of most of the white mica and k feldspar argon spectra as well as previously reported k ar determinations compiled and new stable isotope and fluid inclusion data show that most 18o values for ore stage veins range from 8 to 14 indicating a fluid in equilibrium with the precambrian metamorphic basement rocks d values from fluid inclusions range widely from 64 to 154 which is indicative of a local meteoric component in some veins and highly variable 34s data 7 to 17 even within individual deposits indicate various local country rock sources for sulfur fluid inclusions from all districts show variable homogenization temperatures between 240 and 400 c and are consistently characterized by low salinity h2o co2 ch4 n2 solutions although the data are largely consistent with that from orogenic gold veins intrusion related veins and epithermal veins are also recognized the multiple episodes of mineralization are coincident with episodic tectonic reactivations and associated magmatism along the northern margin of the north china craton electronic supplementary material to this paper can be obtained by using the springer link server located at http dx doi org 10 1007 s00126 001 0239 2

pyrite re os and zircon u pb dating of the tugurige gold deposit in the western part of the xing an mongolia orogenic belt china and its geological significance abstract the tugurige gold deposit in the northeastern margin of the north china craton ncc and western part of the xing an mongolia orogenic belt xmob is a large tonnage quartz vein type deposit with a resource of 24 t at about 4 g t au gold mineralization is hosted by porphyritic syenogranite and controlled by nw trending faults our study provides the first precise age for the tugurige gold mineralization and host porphyritic syenogranite the u pb age of zircons from the syenogranite is 276 2 ma and re os dating of six pyrite separates from the deposit yields an isochron age of 268 15 ma the results show that the ages of syenogranite and gold mineralization are the same within error furthermore geological and geochronological evidence indicate that the syenogranite and gold mineralization have very close genetic relationships and that they are products of permian magmatism related to post collisional extension of the central asian orogenic belt caob the initial os isotopic compositions 187 os 188 os 1 26 0 69 of pyrite indicate that the source of os and by inference other ore related materials are derived from the crust the late paleozoic is an important period in which economic gold mineralization was deposited in the caob

geology and structural evolution of the muruntau gold deposit kyzylkum desert uzbekistan abstract the muruntau gold deposit in the kyzylkum desert of uzbekistan is the largest single deposit 1100 tonnes of gold of the class of low sulfide syndeformation synigenous gold deposits formed in the brittle ductile transition zone of the crust within transpressional shear zones hosted by the cambrian to ordovician besopan suite the ores were deposited in pre existing thrust fault and metamorphism related permeabilities and in synmineralization dilational zones created in a large fault related fold the besopan suite is a 5 000 m thick sequence of turbiditic siltstones shales and sandstones the ore is primarily localized at the base of the besopan 3 unit which is a 2 000 m thick series of carbonaceous shales siltstones sandstones and cherts initial gold deposition took place within the sangruntau tamdytau shear zone which was developed along the stratigraphic contact between the besopan 3 and besopan 4 units during the mineralization process folding of the besopan suite and a left step adjustment in the sangruntau tamdytau shear zone were caused by two concurrent events 1 the activation of the left lateral muruntau daugyztau shear zone that developed at nearly a 90 angle to the preceding shear zone and 2 the intrusion of granitoid plutons these structural events also resulted in the refocusing of hydrothermal fluid flow into new zones of permeability

geology geochronology and sulphur isotope geochemistry of the black schist hosted haoyaoerhudong gold deposit of inner mongolia china implications for ore genesis abstract the haoyaoerhudong gold deposit is located in the urad middle banner area of inner mongolia in china and is hosted by black schist of the bilute formation in the mesoproterozoic bayan obo group the gold mineralization is within a 4500 m long and 100 m wide zone associated with sulphide quartz veins following the schistosity and crosscutting fractures in the host rocks locally as disseminated sulphide in altered black schist the sulphide mineral assemblage is pyrrhotite pyrite arsenopyrite chalcopyrite sphalerite and is associated with a weak hydrothermal alteration characterized by a silica halo with variable amounts of biotite sericite and carbonate the bilute formation was intruded by multi stage dykes these include metamorphosed porphyritic diorite that pre dates mineralization unmetamorphosed amphibole olivine gabbro that pre dates mineralization and tourmaline bearing pegmatite that post dates mineralization in order to constrain the timing of the gold mineralization and host rocks zircons from the porphyritic diorite amphibole olivine gabbro and tourmaline bearing pegmatite were extracted for u pb dating gold bearing pyrite samples were analysed for 34 s to help determine the possible source of s in the mineral zones a weighted mean 206 pb 238 u age date of 288 3 ma mswd 2 4 was obtained for the porphyritic diorite a weighted mean 206 pb 238 u age date of 282 1 ma mswd 0 58 was obtained from the amphibole olivine gabbro and a weighted mean 206 pb 238 u age date of 182 1 ma mswd 1 04 was obtained from the tourmaline bearing pegmatite these dates constrain the timing of gold mineralization to between 282 1 and 182 1 ma and are consistent with the hydrothermal alteration ages of ca 267 246 ma reported from the literature these new zircon dates indicate that the regional metamorphism of the bayan obo group ceased by the time the haoyaoerhudong gold mineralization was deposited and the genesis of mineralization is not related to the regional metamorphic event the gold bearing pyrites gave positive 34 s v cdt values between 6 8 and 13 4 and are different from the 34 s widely distributed values for the typical black rocks indicating that the sulphur is not derived from the black bilute formation but sourced from some other relatively uniform reservoir widespread contemporaneous granitic magmatism is developed in the haoyaoerhudong area and we suggest that the sulphur forming the gold bearing sulphides is probably sourced from a carbonate contaminated granitic magma reservoir the new zircon dates for the dykes and the 34 s data for gold bearing sulphides indicate that the gold mineralization did not form from metamorphosed black schist in the bayan obo group but is associated with magmatic hydrothermal activity

late cenozoic uplift of the eastern cordillera bolivian andes abstract this paper analyses late cenozoic uplift in the bolivian andes using the morphology of well preserved regional paleosurfaces in the eastern cordillera that define three axially draining braided river catchments that formed between 12 and 9 ma rock uplift since the formation of the paleodrainage systems which has been quantified using four different methods is 1705 695 m with a mean erosion of 230 90 m as a consequence of entrenchment of the drainage systems the lack of faulting or tilting in the regions immediately farther west strongly suggest that rock uplift of the paleodrainage systems also extends to the western margin of the eastern cordillera balanced structural cross sections require crustal deformation at depth beneath the eastern cordillera in order to accommodate underthrusting of the brazilian shield beneath the thin skinned fold and thrust belt on the eastern margin of the bolivian andes in this case the observed uplift of the eastern cordillera is easily explained by sliding up a ramp in the major decollement dipping in the range 4 16 w as a consequence of the observed 60 110 km of shortening farther east in the subandean zone contemporaneous uplift of the western margin of the eastern cordillera is easily explained in terms of crustal thickening as a result of ductile squeezing in the lower crust accommodating at depth the subandean shortening it remains unclear how this uplift relates to that of the regions farther west in the altiplano and volcanic arc except that uplift in the eastern cordillera coincides with a phase of intense shortening in the northern altiplano commencing at 9 5 ma and continuing to 2 7 ma and possibly younger

the giant muruntau gold deposit geologic geochronologic and fluid inclusion constraints on ore genesis we reinterpret the regional geologic setting of the giant muruntau gold deposit and report new 40 ar 39 ar isotope age determinations and a laser raman microprobe analysis of fluid inclusions new 40 ar 39 ar isotope age determinations of hydrothermal sericite selvages to gold stage quartz veins are in excellent agreement with older rb sr dates on auriferous quartz veins they suggest triassic sericite formation at 245 and 220 ma some 30 m y later than subjacent felsic intrusions as defined by rb sr data these dates call into question the role of magmatic fluid metal and heat input indeed the rb sr pluton ages may be a reflection of hydrothermal activity rather than cooling from magmatic conditions the muruntau deposit is situated adjacent to a major rift which has been periodically active from the devonian to the tertiary and has accumulated a substantial thickness of chemical volcaniclastic and clastic sediments including red beds and evaporites we infer that the muruntau deposit is so large because of the presence of a major source of reduced sulfur in these nearby evaporitic sediments via thermochemical sulfate reduction some support for the role of reduced sulfur is provided by the presence of detectable h2s in fluid inclusions from late stage auriferous quartz veins reduced sulfur rich fluids could have been focused into the depositional site along major northeast trending structures which have influenced rift basin architecture as recently as the jurassic oxidation could therefore be an important ore precipitating mechanism a change from early ch4 to later syngold co2dominant fluid inclusions may be an indication of this process

fluid inclusion and noble gas studies of the dongping gold deposit hebei province china a mantle connection for mineralization the dongping gold deposit 100 t au occurs about 200 km inboard of the northern margin of the north china craton the deposit is mainly hosted by syenite of a middle paleozoic alkalic intrusive complex that was emplaced into late archean basement rocks both groups of rocks are intruded by late jurassic to early cretaceous crustal melt granite dikes and stocks some within a few kilometers of the deposit the gold ores were deposited during this latter magmatic period at about 150 ma a time that was characterized by widespread regional north south compression that formed the east west trending yanshan deformational belt the ores include both the telluride mineral bearing low sulfide quartz veins and the highly k feldspar altered syenite with most of the resource concentrated in two orebodies 1 and 70 fluid inclusion microthermometry indicates heterogeneous trapping of low salinity e g 5 7 wt nacl equiv fluids that varied from a few to 60 mole percent nonaqueous volatile species laser raman spectroscopy confirms that the vapor phase in these inclusions is dominated by co2 but may be comprised of as much as 9 mole percent h2s and 20 mole percent n2 methane concentrations in the vapor phase are consistently 1 mole percent the variable phase ratios are consistent with fluid immiscibility during ore formation fluid inclusion trapping conditions are estimated to be 250 to 375 c and 0 6 to 1 0 kbar helium isotope studies of fluid inclusions in ore stage pyrites indicate 3he 4he ratios of 2 1 to 5 2 ra ra 1 4 10 6 for air for orebody 1 and 0 3 to 0 8 ra for orebody 70 the former data suggest that at least 26 to 65 percent mantle helium occurs in the fluids that deposited the veins in orebody 1 the lower values for orebody 70 which is characterized by a more disseminated style of gold mineralization are interpreted to reflect an increased interaction of ore fluids with surrounding crustal rocks which may have contributed additional 4he to the fluids a mantle source for at least some of the components of the gold forming fluid is consistent with upwelling of hot asthenosphere and erosion of as much as 100 to 150 km of cool archean lithosphere beneath the craton during this time the dongping deposit is located along the 100 km wide north south gravity lineament which marks the western border of the thinned crust as both regional metamorphism of mesoproterozoic and younger cover rocks and widespread granite magmatism also occurred at ca 150 ma it is unclear as to whether one or both of these also contributed fluid and or metals to the hydrothermal system importantly these new data suggest that economically significant gold deposits of similar mineral style and fluid composition which are scattered along the margins of the craton may all be products of a fluid originally partly sourced within the mantle

evolution of ore forming fluids in the sawayaerdun gold deposit in the southwestern chinese tianshan metallogenic belt northwest china the sawayaerdun gold deposit was discovered in the early 1990s and is the largest orogenic gold deposit in the chinese tianshan gold mineralization occurs in quartz veins which are hosted in carbonaceous metasediments and controlled by faults and fractures three major hydrothermal events can be identified at sawayaerdun an early barren quartz vein stage middle stage quartz veins with pyrite and gold mineralization and late carbonate quartz veins fluid inclusion studies of quartz pyrite and carbonate formed during the major hydrothermal stagesshow distinct changes in fluid temperature composition and redox conditions occurred during fluid evolution three types of fluid inclusions have been identified at sawayaerdun h2o nacl l v h2o nacl co2 l v and co2 l v fluids trapped in early barren quartz have high temperatures 280 420 c low salinities 3 wt nacl equiv high cl and are co2 rich middle stage quartz contains fluids with lower temperatures 210 310 c and variable salinities 0 8 17 5 wt nacl equiv they are commonly s rich co2 containing and relatively reduced compared with early stage fluids in contrast hydrothermal pyrite hosted in middle stage quartz veins trapped fluids formed over a narrower temperature range 220 270 c more oxidized high co2 ch4 ratio and have higher ca k and mg than fluids in quartz late stage carbonate is dominated by fluids with low temperatures 200 c and low contents of cl s and k fluid immiscibility probably commenced at the onset of mineralization along with middle stage quartz formation resulting in the generation of coexisting fluid inclusions with distinguishable vapor liquid ratios and salinities the compositional difference between fluids trapped in pyrite and quartz indicate fluid mixing may have occurred during the mineralization stage the oxidized fluids probably a mixture of meteoric water and fluids generated during water host rock interaction emerged into the ore forming system to generate extensive metals precipitation the fluid immiscibility and fluid mixing are most likely caused by pressure decrease during late paleozoic uplift and erosion in the chinese southern tianshan area comparison of fluids between four major mineralization zones at sawayaerdun indicate that zone ii is similar to the most important zone iv and the order of similarity is probably zone ii xi i which could be applied to the future mineral exploration in this area

u pb zircon ages and the evolution of the michipicoten plutonic volcanic terrane of the superior province ontario the archean michipicoten greenstone belt of the superior province in ontario is made up of supracrustal rocks divided into lower middle and upper metavolcanic rocks with associated metasedimentary rocks the belt has been intruded by granitic rocks and is also surrounded by granitic terranes based on u pb zircon geochronology it appears that volcanism in the area extended from at least 2749 to 2696 ma and plutonism and tectonic activity extended from at least 2888 to 2615 ma the various granitic and also one gabbroic plutons both internal and external to the greenstone belt were emplaced concomitantly with the three volcanic cycles as well as before and after the formation of the volcanic rocks zircon ages reported here together with previously published ages show that the area evolved in six major volcanic and plutonic events i 2888 ma plutonism ii 2743 ma volcanism and plutonism iii 2717 ma volcanism and plutonism iv 2696 ma volcanism and plutonism v 2668 ma plutonism and vi

origin of ore fluids in the muruntau gold system constraints from noble gas carbon isotope and halogen data abstract hydrothermal vein minerals directly associated with native gold mineralization in the muruntau vein system uzbekistan have been studied for noble gas carbon isotope and halogen chemistry of the trapped ore related fluids helium trapped in early arsenopyrite 1 which has preserved the original fluid signature better than associated scheelite and quartz indicates a small input from a mantle source 5 of total he however the overwhelming majority of the he in the fluid 95 is from crustal sources the noble gases ne kr and xe in the sample fluids are dominated by gases of atmospheric origin the carbon isotope 13 c 2 1 to 5 3 and halogen characteristics of the fluids log br cl 2 64 to 3 23 lend further support for the activity of juvenile fluids during the main ore stage the high proportion of crustal components in the ore forming fluids may be explained by intense fluid rock interaction and is also supported by previous nd and sr isotope studies the involvement of a juvenile fluid component during the main stage of hydrothermal activity at muruntau 275 ma can be linked to the emplacement of lamprophyric dikes at muruntau due to apparently overlapping ages for high temperature alteration related ore vein formation and intrusion of the dikes the input of mantle derived fluids possibly related to the hercynian collisional event in the western tien shan stimulated intense fluid rock interaction in the crust in this context the mantle derived fluid should be considered as one possible carrier of metals significant amounts of external meteoric fluids circulating in fracture systems are interpreted to have modified the noble gas signature of fluid in quartz mostly during late low temperature fluid circulation

in situ u pb zircon dating using laser ablation multi ion counting icp ms high resolution in situ u pb zircon geochronology on zoned grains can obtain isotope signatures from multiple growth or thermal events we present a method using laser ablation multicollector inductively coupled plasma mass spectrometry la mc icp ms to overcome complications associated with intricately zoned zircon crystals through in situ sampling of zircon volumes as small as 12 m 25 m and 40 m in diameter by about 10 m in depth high precision u pb age of a series of zircon standard covering a wide age range of 30 to 1 065 ma was acquired using la mc icp ms the precision of measured pb u ratios in homogeneous zircon is about 2 2 resulting in routinely achieved precision of u pb ages obtained by external calibration of 1 2 or better all masses of interest can be simultaneously recorded with a multi ion counting system mic operating in static mode and the short ablation required to achieve such precision results in spatial resolution that is superior to comparable u pb zircon analyses by single collector icp ms the resulting present u pb age for five zircon reference samples and two geological samples show an excellent agreement with the previously reported id tims or shrimp data

magmatism in the huarina belt bolivia and its geotectonic implications abstract the huarina belt is a tectono stratigraphic unit extending along the western side of the eastern cordillera and southern altiplano it includes the well known bolivian tin belt both geophysical and isotopic evidence suggest that the huarina belt is located between three cratonic blocks in a major lithospheric weakness zone that has probably been inherited from the proterozoic most of the phanerozoic backarc magmatism in the bolivian andes occurred along this belt local ordovician basalts and andesites and clasts of devonian granitoids occurring in late devonian early carboniferous sedimentary units are testimonies of a scarce palaeozoic igneous activity during the mesozoic rift related magmatism resulted in peraluminous to metaluminous high k calcalkaline triassic granitoids as well as discrete cretaceous centers of alkaline and carbonatitic rocks in the cenozoic huge high k generally peraluminous ignimbrite sheets along with numerous volcanic and intrusive centers were generated along the belt shoshonitic alkaline and ultrapotassic rocks have been locally described on the basis of both geochemical and geophysical evidence a general evolution model is proposed for the huarina belt during paleozoic and mesozoic times the predominant extensional regime of the continental crust led to lithospheric thinning basin generation and production of within plate magmas during the cenozoic prevailing compressional conditions led to a gradual thickening of the lithosphere inducing delamination of its denser and less viscous root the mantle generated magmas interplayed widely with crustal melts giving rise to the dominantly peraluminous magmas

regional geologic setting and metallogenesis of central inner mongolia china guides for exploration of mesothermal gold deposits abstract the central inner mongolia district forms one of the most important metallogenic provinces in china because the district has been influenced by repeated distinct tectonomagmatic activity the genetic relationships and spatial temporal evolution of ancient tectonism and associated geologic structures and metallogeny are discussed in this paper an integrated genetic model for gold metallogeny is proposed during the late archean to paleozoic repeated north southward accretional tectonics between the paleo mongolia oceanic plate and the north china paleoplate led to the formation of not only the e w trending deep crustal faults but also a series of sedimentary volcanic and metamorphic events numerous stratiform fe cu au pb zn and ree ores formed in association with sedimentation and volcanism within the platform environments at this time during the late paleozoic to early mesozoic the central inner mongolia district was largely influenced by northwestward subduction of the circum pacific plate beneath the eurasia plate a series of ne sw trending faults together with s type granitoid intrusions formed as a result of this subduction mesothermal type gold deposits e g the saiyingwusu laoyanghao shibaqin wulashan and donghuofang deposits formed during the permian to jurassic 158 5 to 275 4 ma preferentially at or near the intersections between ne sw trending and e w trending fault systems at these intersections the areas of not only late paleozoic to early mesozoic granitoids but also late archean to early proterozoic metamorphic rocks especially the gold rich seertengshan wulashan and bayan obo group rocks may be promising exploration targets for mesothermal gold deposits