preface archaean geology of the eastern goldfields of western australia regional overview abstract the eastern goldfields region forms part of the yilgarn craton which after the superior province of canada is one of the largest fragments of archaean granite greenstone terrain exposed at the surface of the earth this craton has much in common with the more intensely studied superior province but there are also significant differences between these two cratons both cratons were assembled from numerous smaller crustal fragments in a relatively brief period during the late archaean the superior province was assembled between 2725 and 2695 ma card 1990 and the yilgarn craton was assembled between c 2670 and 2650 ma myers 1993 both cratons contain fragments of older continental crust and volcanic arcs that formed just prior to collision and cratonisation the superior province contains extensive belts of metasedimentary rocks interpreted as accretionary prisms card 1990 whereas metasedimentary rocks form only a minor component of the yilgarn craton and most appear to postdate accretion both cratons contain significant concentrations of gold and support thriving gold mining industries the most prominent difference between the cratons lies in the nature of the granitoid rocks the superior province is dominated by tonalite and granodiorite whereas the yilgarn craton largely consists of granite this applies to granitoid rocks of all ages in each craton granite studies form a major part of this special volume of precambrian research

metamorphic records for subduction erosion and subsequent underplating processes revealed by garnet staurolite muscovite schists in central qiangtang tibet subduction erosion is confirmed as a crucial geodynamic process of crustal recycling based on geological geochemical and geophysical observations at modern convergent plate margins so far not a single metamorphic record has been used for constraining a general tectonic evolution for subduction erosion here we first revealed metamorphic records for a subduction erosion process based on our study of the late paleozoic garnet staurolite muscovite schists in the central qiangtang block tibet provenance analyses suggest that the protoliths of garnet staurolite muscovite schists have the northern qiangtang affinity and were deposited in an active continental margin setting mineral inclusion data show that the early metamorphic stage m1 recorded blueschist facies pressure temperature p t conditions of 0 8 1 1 gpa and 402 441 c indicating that a part of the material from the overriding plate had been abraded into the subduction channel and undergone high pressure low temperature metamorphism the peak metamorphic stage m2 recorded amphibolite facies p t conditions of 0 3 0 5 gpa and 470 520 c the 40ar 39ar cooling ages 263 259 ma yielded from muscovite suggest the amphibolite facies metamorphism 263 ma occurred at oceanic subduction stage the distinctly staged metamorphism defines a clockwise and warming decompression p t t path which reveals an underplating process following the early subduction erosion during the tectonic process the eroded low density material escaped from the cold subduction channel and rise upward into the warm middle lower crust of the upper plate undergoing amphibolite facies metamorphism our new results revealed a complete evolutional process from the early subduction erosion to the subsequent underplating during the northward subduction of the paleo tethys ocean

geology and geochemistry of archaean granites in the kalgoorlie region of the eastern goldfields western australia a syn collisional tectonic setting abstract granites in the kalgoorlie region of the archaean yilgarn craton are predominantly biotite granodiorite and monzogranite with some hornblende bearing varieties granites are divided on the basis of field relationships petrography and chemical composition into five supersuites an older supersuite containing four suites pre dates regional folding and four supersuites post date regional folding the potassic composition of the mainly silica rich granites 68 sio2 suggest that the most appropriate source for the majority of supersuites is continental crust with a composition in the range tonalite to granodiorite most supersuites evolved by fractional crystallization mainly of biotite and plagioclase some suites contain synplutonic dykes and microgranite xenoliths that crystallized from enriched mantle derived magmas there was only limited and local chemical interaction between granite and mantle derived magmas geometric and temporal relationships between regional structures and granites indicate that the granites were emplaced during a major orogenic event however s type granites which are widespread in some modern syn collision settings are absent in the kalgoorlie region the predominance of high fo2 i type granites in the study area probably reflects the absence of large graphite bearing metasedimentary accumulations in the lower crustal source regions the distribution of suites and supersuites implies a common source region beneath the kalgoorlie and gindalbie granite greenstone terranes suggesting that they shared a common geological history dating back to late greenstone deposition

late triassic paleolatitude of the qiangtang block implications for the closure of the paleo tethys ocean abstract to better constrain the late triassic paleolatitude of the qiangtang block and the closure of the paleo tethys ocean a combined paleomagnetic and zircon u pb geochronological study has been conducted on the upper triassic jiapila formation volcanic rocks on the northern edge of the qiangtang block of central tibet 34 1 n 92 4 e these rocks are dated to 204 213 ma progressive thermal or alternating field demagnetization successfully isolated stable characteristic remanent magnetizations chrm that pass both the fold and reversal tests consistent with a primary magnetization these are the first volcanic based paleomagnetic results from pre cretaceous rocks of the qiangtang block that appear to average secular variation well enough to yield a reliable paleolatitude estimate based on our new paleomagnetic data from upper triassic lavas we conclude that the late triassic pole of the qiangtang block was located at 64 0 n 174 7 e with a 95 6 6 n 29 we compile published paleomagnetic data from the qiangtang block to calculate a late triassic latitude for the qiangtang block at 31 7 3 0 n the central paleo tethys ocean basin was located between the north china ncb and tarim blocks to the north and the qiangtang block to the south during late paleozoic early mesozoic a comparison of published early triassic paleopole from the qiangtang block with the coeval paleopoles from the ncb and tarim indicates that the paleo tethys ocean could not have closed during the early triassic and that its width was approximately 32 38 latitude 3500 4200 km however the comparison of our new combined late triassic paleomagnetic result with the late triassic poles of the ncb and tarim as well as numerous geological observations indicates that the closure of the paleo tethys ocean at the longitude of the qiangtang block most likely occurred during the late triassic

regional metamorphic controls on alteration associated with gold mineralization in the eastern goldfields province western australia implications for the timing and origin of archean lode gold deposits alteration assemblages associated with archean epigenetic gold mineralization in the menzies kambalda area of the yilgarn craton western australia vary systematically with regional metamorphic grade up to and including upper amphibolite facies rocks k metasomatism is recorded by the presence of muscovite biotite and k feldspar at progressively higher metamorphic grades high metamorphic grades are commonly developed in broad thermal aureoles around fate syntectonic granitic intrusions metamorphic recrystallization of alteration assemblages is common in the thermal aureoles these relations suggest broad contemporaneity among granitoid intrusion regional metamorphism and mineralization during the final stages of the tectonic evolution of the granite greenstone terrain late syntectonic granitoids acted as centers of heat and fluid flux in large scale synmetamorphic hydrothermal systems that deposited gold possibly from modified mantle derived fluids

tectonic erosion of the peruvian forearc lima basin by subduction and nazca ridge collision 1 subsidence of lima basin part of the peruvian forearc is controlled by tectonic erosion by the subducting nazca plate multichannel seismic reflection data coupled with age and paleowater depth constraints derived from ocean drilling program odp coring now allow the rates of erosion to be reconstructed through time in trenchward locations the forearc has experienced limited recent relative uplift 700 850 m likely due to preferential basal erosion under the center of lima basin long term subsidence driven by basal tectonic erosion dominates and is fastest closest to the trench since 47 ma eocene up to 148 km of the plate margin have been lost at an average rate of up to 3 1 km myr 1 appoximately 110 km of that total appears to be lost since 11 ma implying much faster average rates of trench retreat 10 km myr 1 since collision of the nazca ridge with the lima basin at 11 ma although there is no clear subsidence event at odp site 679 during the time at which nazca ridge was subducting beneath this part of the forearc 4 11 ma the more trenchward odp sites 682 and 688 show significant deepening after 11 ma indicating that subduction of the ridge accelerates tectonic erosion long term rates of crustal erosion in the region of lima basin are greater than estimates of regional arc magmatic productivity implying that such margins are net sinks of continental crust

geochemistry and tectonic setting of two contrasting archaean felsic volcanic associations in the eastern goldfields western australia abstract archaean felsic volcanic rocks from two areas in the eastern yilgarn craton of western australia comprise dacite and rhyolite with subordinate basalt and andesite one suite from one of the areas black flag group is dominated by subaqueous dacite lava lobes and their reworked products the other suite from the other area melita and jeedamya volcanics consists of abundant rhyolitic and rhyodacitic pyroclastic deposits with less abundant dacitic lava flows both suites are intercalated with volumetrically subordinate basic and intermediate volcanic rocks black flag group dacites and rhyolites have steep ree patterns la yb cn about 40 and no eu anomaly high sr and al2o3 low nb and y hence high sr y melita and jeedamya dacites and rhyolites have flatter ree patterns la yb cn between 2 and 9 negative eu anomalies eu eu 0 4 0 6 and higher zr nb y and lower sr hence lower sr y on chemical grounds mafic rocks in both areas are genetically unrelated to felsic volcanic rocks black flag group andesites dacites and rhyolites are probably unrelated by fractional crystallization but resulted from 22 28 partial melting of amphibolite leaving a residual mineralogy of amphibole plagioclase garnet ilmenite clinopyroxene alternatively andesites may have been derived by partial melting of a metasomatised mantle source jeedamya rhyodacite could have been produced by 35 vapour absent melting of tonalite leaving a residual mineralogy of plagioclase orthopyroxene amphibole ilmenite the source would have had a slightly lree enriched pattern with a positive eu anomaly residual garnet in the source of black flag group rocks requires melting at depths greater than 35 km which is below the base of the present tectonically thinned crust the most likely source is subducted oceanic crust with black flag group rocks erupted in a back arc basin melita and jeedamya felsic volcanic rocks resulted from melting of thicker continental crust adjacent to the back arc

petrology and geochemistry of syn to post collisional metaluminous a type granites a major and trace element and nd sr pb o isotope study from the proterozoic damara belt namibia abstract leucocratic a type granites of the proterozoic damara orogen namibia are a distinctive groups of rocks based on their mineralogical chemical and isotopic characteristics a type granites can be distinguished from the leucocratic s type granites on the basis of fe rich amphibole and biotite and higher abundances of zr nb y and lree but lower values of 87 sr 86 sr 500 ma and 18 o in the a type granites in comparison with a type granites described in the literature the damaran a type granites are enriched in cao al2o3 mgo feo ba sr and eu but depleted in ga and zn low k rb rb sr rb ba and sr ba ratios indicate that these melts are not fractionation products of calc alkaline source rocks and therefore these chemical criteria are interpreted as a feature of the source rocks i e undepleted metatonalites the a type granites have high calculated zircon and monazite saturation temperatures mostly in excess of 800 c and it is concluded that these granites represent high temperature partial melts the relationship between major and trace element data and isotopic composition demonstrates that the a type granites originate from several sources and have undergone different processes the lack of evidence for shallow crustal contamination e g lack of country rock xenoliths the low viscosities of the melts as inferred from their high calculated temperature and their high nd concentrations relative to the country rocks suggest that the isotopic characteristics were acquired from an enriched mantle initial sr isotopic compositions vary over a narrow range 0 70486 0 70928 but the initial nd isotopic compositions vary considerably with initial end values between 0 03 and 6 35 this wide range in the nd isotope values is probably the result of interaction of mantle derived magmas with crustal rocks a type granites at baukwab have negatively correlated initial end values and 207 pb 204 pb and 206 pb 204 pb ratios indicating interaction of mantle derived magmas low initial end low 207 pb 204 pb and 206 pb 204 pb with continental crust high initial end high 207 pb 204 pb and 206 pb 204 pb a type granites from albrechtstal and oetmoed have positively correlated pb isotope ratios and show also positive correlations between nd isotopes and pb isotopes suggesting interaction of at least two different source materials the relationship between nd isotopes and nd concentrations precludes simple assimilation instead mixing of different sources seems to be more realistic for the a type granites from oetmoed oxygen isotopes are negatively correlated with sr isotopes indicating that the crustal end member has high 18 o values and rather low initial 87 sr 86 sr ratios whereas a hypothetical mantle component has lower 18 o values but higher initial 87 sr 86 sr ratios the chemical and isotope data suggest that the damaran a type granites were generated by a variety of processes including partial melting of mantle derived tonalitic sources limited crystal fractionation and interaction of these magmas with crustal material these data indicate that the damaran a type suites mark episodes of continental growth during which mantle material although limited in extent is added to the crust

structural setting and geochemistry of archaean i type granites in the bardoc coolgardie area of the norseman wiluna belt western australia abstract regional structures in the bardoc coolgardie area of the archaean norseman wiluna greenstone belt in western australia developed during early large scale thrusting followed by a transpressional regime characterized by enewsw shortening and large scale sinistral strike slip faults structural relationships between granites and greenstone successions indicate emplacement of granitic rocks throughout this deformation history emplacement mechanisms include solid or liquid state diapirism forceful dilation of country rocks and passive stoping i type granites are common and include early and late tectonic intrusions the widespread occurrence of i type granodiorite and monzogranite probably derived by partial melting of lower continental crust or subducted oceanic crust suggests a continental margin setting for the belt alkalic quartz diorite occurs as rounded enclaves in granodiorite and is enriched in ba rb sr ce la p and f which suggests a mantle derived volatile component these quartz diorite melts may have formed by partial melting of metasomatized mantle or alternatively mantle degassing may have lowered solidus temperatures in the source region and thus induced melting emplacement of hot volatilerich quartz diorite melts into the lower crust may have lowered solidus temperatures in the source rocks for granodiorite and monzogranite recent models for archaean gold mineralization suggest that auriferous fluids contained co 2 derived from lower crustal or mantle sources in the bardoc coolgardie area co 2 may have been flushed out during partial melting of the lower continental crust however the origin of the gold in the mineralizing fluids requires some other explanation because gold mineralization is generally a late stage event and took place over a considerably shorter time interval than the formation and emplacement of i type granites

an early thrust duplex in the kalgoorlie kambalda greenstone belt eastern goldfields province western australia abstract extensive regional mapping of the stratigraphic sequence throughout the kalgoorlie kambalda greenstone belt in western australia has outlined the geometry of early subhorizontal thrust faults these thrusts stacked and repeated the stratigraphy to form a major thrust duplex south of kambalda the roof thrust to the duplex becomes a major thrust flat to the north at kalgoorlie later transpressional deformation f2 upright folds d3 sinistral shear zones and folding has rotated and displaced the d1 thrusts both sub horizontal deformation and subsequent transpression indicate that compressional tectonics rather than vertical tectonics involving solid state granitoid diapirs were crucial in the development of this granite greenstone terrain

early paleozoic s type granites as the basement of southern qiantang terrane tibet abstract identifying basement rocks is critical to understanding the continent formation and evolution during orogenesis despite two decades of research the age and composition of the basement rocks of the southern qiangtang terrane microcontinent sqt central tibet are still unclear in this study zircon u pb ages and hf o isotopes as well as whole rock nd isotopes are presented for the oldest known igneous rocks in the sqt precise sims zircon u pb dating of these rocks reveals that these early paleozoic granites were crystallized at ca 480 465 ma they exhibit a wide range of isotopic values with unradiogenic whole rock end t and zircon ehf t values ranging between 8 3 7 7 and 8 2 2 3 respectively and by high 18ozircon values of 8 1 11 4 these results reveal that the early paleozoic granites were crystallized from magmas that were almost entirely sediment derived when viewed in concert with regional data and constraints on the tectonic evolution of northern gondwana the sources for the early paleozoic granites are herein proposed to have been deposited in an extensional basin implying that these early paleozoic granites were generated in an extensional setting within an accretionary orogen thus the early paleozoic granites are considered to represent the basement of the sqt after a long period of attachment to northern gondwana the sqt became a small microcontinent when it was rifted and detached from northern gondwana during early permian after this time it drifted northwards to asia and collided with the northern qiangtang terrane to form a unified qiangtang terrane in the late triassic thus the sqt records a complete wilson cycle from construction rifting and departure drifting to collision

tectono stratigraphy of late archaean greenstone terranes in the southern eastern goldfields western australia abstract late archaean greenstone terranes in the southern part of the eastern goldfields are regionally extensive faultbounded entities defined on the basis of small but distinct differences in stratigraphy and structure the greenstones comprise volcano sedimentary successions that were all deposited at the same time c 2720 2675 ma on sialic crust hence the terranes are interpreted to represent contemporaneous probably adjacent basins these basins were affected by the same history of granite plutonism and regional deformation c 2675 2620 ma early subhorizontal deformation is poorly understood but includes at least locally significant thrust stacking d1 with a south to north movement direction this was followed by regional east west extension affecting all supracrustal successions above a province wide detachment now lying at depths between 4 and 7 km this detachment may have evolved from the greenstone basement contact but only after extensive reworking clastic basins developed along or above major extensional faults but no volcanism is associated with this extension subsequent east west d2 shortening resulted in regional scale thrusting folding and associated late stage transcurrent fault movements the d2 event is contemporaneous with a major thermal event represented by voluminous granite emplacement c 2660 ma and associated peak regional metamorphism in the greenstone succession predominantly monzo granite derived from older sialic crust was emplaced throughout the period of basin formation and deformation early synvolcanic granite c 2719 2710 ma is a minor component late synvolcanic granite c 2685 2675 ma is widespread and syn d2 granite c 2660 ma is abundant there are a few small late tectonic intrusions c 2640 ma to 2600 ma the lateral lithostratigraphic persistence of the contemporaneous basins the syn d2 low pressure regional metamorphism and the province wide monzo granite magmatism suggest that the greenstone basins possibly developed in a tectonic setting similar to that of failed rifts above aborted spreading centres the basins closed in response to far field compressional stresses

kinematic history of the boulder lefroy shear zone system and controls on associated gold mineralization yilgarn craton western australia the boulder lefroy shear zone developed in the ca 2 7 to 2 65 ga archean greenstone sequence of the kalgoorlie terrane in the yilgarn craton is remarkable for the number and importance of gold deposits it hosts these include from south to north the following gold districts with tons of contained au st ives 253 t hampton boulder jubilee 123 t golden mile 1 821 t plus mount charlotte 219 t and paddington broad arrow 112 t detailed structural studies reveal that the shear zone system developed in two main stages the first related to a phase of north northwest south southeast trending folding and thrusting regional d 2 and the second related to sinistral shearing regional d 3 that reactivated and linked the preexisting north north west south southeast trending network of thrust planes sinistral movement resulted in only limited displacement less than 12 km and evolved from a ductile shearing in broad zone toward semibrittle shearing along narrow planes regional d 4 the structures suggest a temporal evolution from crustal thickening during d 2 through lateral escape during d 3 to the development of brittle structures during d 4 the fimiston and oroya lodes at the golden mile in kalgoorlie are also exceptional in their gold endowment and associated alteration and were most likely developed during d 2 all other deposits along the shear zone system were formed within 25 m y of the fimiston and oroya lodes late in the tectonic history of the terrane either during d 3 or d 4 intense mineralization within and around the shear zone resulted from multiscale focusing of mineralizing fluids at a regional scale fluids were focused into the shear zone system and its large anticlines which expose deeper mafic and ultramafic rocks at a local scale fluids were further focused into structural pathways resulting from strain induced permeability in rocks with contrasting rheology

eclogites from central qiangtang northern tibet china and tectonic implications abstract eclogites in central qiangtang northern tibet china are characterized by the garnet omphacite rutile phengite quartz assemblage hornblende plagioclase and sphene occur as retrograde amphibolite facies phases in the eclogites pressure temperature estimates for the primary eclogite assemblage are consistent with minimum pressures of 20 25 kbar at 482 to 625 c the retrograde amphibolite facies assemblage suggests conditions of 10 12 kbar and 460 600 c consequently the eclogites from central qiangtang have undergone unambiguous ultrahigh pressure uhp metamorphism the presence of uhp metamorphic rocks implies a likely deep seated continental subduction along the shuanghu suture within the qiangtang terrain the denudation of these uhp metamorphic rocks correlates well with the main interval that the songpan ganzi turbidites accumulated and could have provided a significant source for these sedimentary rocks

the hydrothermal fluid of archaean lode gold deposits at different metamorphic grades compositional constraints from ore and wallrock alteration assemblages a low salinity mixed aqueous carbonic fluid is common to all archaean lode gold deposits throughout the range of mineralising conditions from sub greenschist to lower granulite facies temperatures alteration assemblages and fluid inclusion data give constraints on the fluid composition fluid xco2 is 0 1 0 3 in typical greenschist facies mesothermal deposits at higher temperatures the assemblages are consistent with formation from a fluid of similar composition but slightly higher or lower xco2 cannot be ruled out and fluid inclusion data indicate that ch4 may be an important component in ore fluids at these temperatures fluid ph is neutral or weakly alkaline at all conditions a range of relative oxidation states of four orders of magnitude fo2 is indicated at any temperature with deposits more oxidising relative to qfm at lower temperature sulphur contents of the fluids vary from 10 to 10 3 5m s with a trend towards lower sulphur contents at lower temperatures the relative concentrations of major cations in solution are similar at all conditions with na k ca although ca may be less abundant at low temperatures the broad similarities in ore fluid composition at all temperatures give support to crustal continuum models in which archaean lode gold mineralisation involved either a single fluid moving through the middle and upper crust or derivation of ore fluids by similar processes at different crustal levels many of the compositional differences between high and lowtemperature ore fluids may be attributed to evolution of deep sourced hydrothermal solutions as they rise along structurally controlled conduits the constancy of major ore fluid component concentration e g co2 cl k suggests fluid buffering and high fluid rock ratios along fluid pathways fluid buffered conditions can also explain the ore fluid fo2 temperature relations with equilibria between oxidised and reduced aqueous carbon or sulphur species controlling the oxidation state in contrast the concentrations of components present in lesser abundance in archaean gold ore fluids e g s ca h were probably controlled either by saturation of one or more mineral phases brought on by decreasing temperature or were rock buffered through fluid rock reactions extrapolation to high temperatures of the k na and ca contents of the gold bearing fluids indicates that their composition is consistent with derivation from or final equilibration with rocks of intermediate granitic composition thus giving support to isotopic and geological arguments for ore fluid source regions external to the greenstone belts the fluid oxidation states are characteristic of a wide range of potential source rocks including mantle derived igneous rocks calc alkaline granitoids and magmas and seaflooraltered metabasalts strongly oxidised magmatic sources or unusually oxidising source processes e g co2 streaming during granulitisation of the lower crust are therefore not required in the genesis of arch an lode gold deposits

a metamorphosed stratabound epigenetic origin for a gruneritic archean gold deposit barberton south africa the lily mine located some 24 km northeast of barberton in south africa occurs stratabound within carbonaceous fe and mn rich metapelites containing abundant grunerite it is interpreted to be a metamorphosed epigenetic archean gold deposit the host rocks form part of a graywacke shale sequence close to a major regional structure known as the lily fault they are intensely deformed close to the fault with strain partitioned into finely laminated carbonaceous phyllosilicate rich units the bulk geochemistry of the host rocks is dominated by pelitic ferruginous and quartz rich fractions mixed in varying proportions mineralization is concentrated in rocks composed largely of the pelitic and ferruginous fractions that have prograde metamorphic almandine biotite grunerite magnetite and fe carbonates magnetite rich rocks were produced by progressive metamorphism of greenalite and or siderite and do not reflect oxide facies iron formation much of the quartz rich fraction occurs in deformed and metamorphosed veins associated with the mineralization gold is concentrated in the main reef zone immediately adjacent to the lily fault native gold occurs with arsenopyrite pyrrhotite minor chalcopyrite sphalerite galena and stibnite in association with abundant deformed and metamorphosed veins composed of quartz albite grunerite ankerite sulfides in graphite depleted biotite grunerite tourmaline schists and massive grunerite ankerite rocks ultramafic schists in the footwall of the lily fault exhibit intense k co2 alteration phlogopite dolomite calcite and quartz veining but are essentially barren of gold primary gold concentration probably occurred by destabilization of bisulfide complexes through reactions with reduced iron silicates and carbonaceous matter during channelized fluid influx along the lily fault subsequent metamorphism occurred at t 500 550 c and p 3 5 kbar transitional greenschist amphibolite possibly in association with the emplacement of the nelspruit batholith at 3105 ma

formation of gold deposits review and evaluation of the continuum model abstract the continuum model has been one of the more widely advocated genetic models for gold only deposits such as those found in archaean greenstone belts it postulates that hydrothermal gold deposits were formed throughout a 20 25 km vertical crustal profile from temperatures above 700 c to below 180 c and that the deposition occurred synchronous with the peak of metamorphism the continuum model is reviewed at this stage because we believe that it does not successfully account for many aspects of gold deposit formation the most obvious shortcoming is in considering ore deposits found in rocks of uppermost amphibolite and granulite facies domains where temperatures were appropriate for partial melting the main chemical condition that favours melting is access to h2o from either aqueous fluid or the breakdown of hydrous minerals a major gold forming hydrothermal event at the peak of high grade metamorphism as implied by the continuum model is incompatible with partial melting of wallrocks instead of forming a hydrothermal gold deposit an aqueous fluid introduced during partial melting would be consumed to produce further melt five gold deposits are documented from high metamorphic grade domains within four separate archaean cratons one is the type example used in the continuum model the other four have been significant producers partial melting has been recorded in the wallrocks adjacent to gold mineralisation at big bell hemlo challenger and renco gold deposits in the sulphide rich ore itself at challenger and hemlo deposits and as cross cutting dykes and migmatites at griffins find for these five deposits and indeed in general the continuum model does not easily account for aspects of the fluid source fluid composition gold transport and metal deposition the evidence that has been used to support deposit formation at the peak of high grade metamorphism as required by the continuum model is generally compatible with one or more alternative genetic models textural and structural siting of the gold high variance mineral assemblages proportion of various gold host rocks and relative element enrichments at high metamorphic grade can be accounted for by gold formation at greenschist facies conditions followed by progress of the deposit to high metamorphic grade metamorphic model moreover the type examples in the continuum model of deposits in sub greenschist facies domains appear to be incorrectly ascribed and are likely to be in greenschist facies domains the continuum model does not adequately explain archaean gold deposits

cambrian to triassic geodynamic evolution of central qiangtang tibet abstract central qiangtang is a tectonic melange between the northern qiangtang terrane nqt and the southern qiangtang terrane sqt and it preserves a complete cambrian to triassic geological record related to tethyan evolution e g high pressure metamorphic rocks and ophiolites thus this region is the key to understanding the geodynamic processes that occurred during the dispersion of gondwana and the accretion of laurussia given previous studies on sedimentary paleomagnetic and seismic records this paper presents an integrated interpretation of ophiolites magmatism metamorphism and tectonism and reconstructs the cambrian triassic tectonic evolution of central qiangtang as described below 1 following the neoproterozoic early cambrian pan african orogeny and associated postcollisional extension the southward subduction of the proto tethys oceanic plate produced andean type magmatism along the northern margin of gondwana a back arc basin between the nqt and sqt further opened during the late cambrian early ordovician accompanying with trench retreat and overriding plate extension resulting from slab rollback 2 with the middle ordovician late silurian successive incorporation of micro continental terranes e g qaidam kunlun and tarim terranes into the northern margin of gondwana the back arc basin closed through subduction polarity reversal from southward to northward 3 during the late devonian early carboniferous the southward subduction of an oceanic plate to the north of the nqt separated the nqt and sqt through back arc extension consequently the nqt drifted northward rapidly and the back arc basin gradually evolved into a mature ocean basin i e longmu co shuanghu paleo tethys ocean lspto 4 subsequently a late carboniferous early permian continent continent collision between the nqt and the south china block scb resulted in the northward subduction of the lspto and the ongoing subduction probably induced the opening of a back arc basin between the nqt and the scb during the late permian 5 finally diachronous collision between the nqt and sqt closed the lspto during the early middle triassic and the termination of the collisional orogenic event was evidenced by strong late triassic magmatism related to postcollisional extension in the qiangtang block in this paper we propose that the opening and closure of back arc basins controlled the cambrian triassic tectonic evolution of central qiangtang which provides significant insights into the dispersion of gondwana and the accretion of laurussia

magma sources for mesozoic anorogenic granites of the white mountain magma series new england usa the magma sources for granitic intrusions related to the mesozoic white mountain magma series in northern new england usa are addressed relying principally upon nd isotopes many of these anorogenic complexes lack significant volumes of exposed mafic lithologies and have been suspected of representing crustal melts sm nd and rb sr isotope systematics are used to evaluate magma sources for 18 felsic plutons with ages ranging from about 120 to 230 ma the possibility of crustal sources is further examined with analyses of representative older crust including paleozoic granitoids which serve as probes of the lower crust in the region multiple samples from two representative intrusions are used to address intrapluton initial isotopic heterogeneities and document significant yet restricted variations 1 in end overall mesozoic granite plutons range in end t from 4 2 to 2 3 with most 2 to 0 and in initial 87sr 86sr from 0 7031 to 0 709 the isotopic variations are roughly inversely correlated but are not obviously related to geologic geographic or age differences older igneous and metamorphic crust of the region has much lower nd isotope ratios with the most radiogenic paleozoic granitoid at end 180 ma of 2 8 these data suggest mid proterozoic separation of the crust in central northern new england moreover the bulk of the mesozoic granites cannot be explained as crustal melts but must have large mantle components the ranges of nd and sr isotopes are attributed to incorporation of crust by magmas derived from midly depleted mantle sources crustal input may reflect either magma mixing of crustal and mantle melts or crustal assimilation which is the favored interpretation the results indicate production of anorogenic granites from mantle derived mafic magmas

lithospheric electrical structure in the central tibetan plateau and its tectonic significance abstract the himalaya tibetan plateau that is situated in the southwest china is an ideal natural field laboratory for continental dynamics in this study through the careful processing and analyzing of the magnetotellurics data the reliable 2 d electrical model was obtained to examine the relationship between the lithospheric electrical structure and dynamics in the central himalaya tibetan plateau six conductors mainly result from the partial melting and aqueous fluids and three resistors are related to different tectonic evolution in the model combined with other observations we summarize that the upwelling hot material partial melting aqueous fluids subduction erosion and crushed zone contribute to the error between the electrical location and the geographic location of the indus yarlung zangbo suture the subducted indian plate is shown by the different angles and different locations of its frontier in different areas the electrical structure beneath the lhasa terrane corresponds well with the geological features of an old locally reworked crust dominantly in the central lhasa terrane with two young juvenile crustal blocks surrounding the central terrane and it also shows the relationship among the dynamics magmatism and mineralization in the gangdise metallogenic belt meanwhile we indicate that the subducted polarity of the bangong nujiang tethyan ocean lithosphere was double sided and the smooth subduction of the longmu tso shuanghu tethyan ocean lithosphere may cut through the moho surface approximately 20 km above all these results can provide helpful constrains to understand the evolution mechanism of the himalaya tibetan plateau

triassic subduction of the paleo tethys in northern tibet china evidence from the geochemical and isotopic characteristics of eclogites and blueschists of the qiangtang block abstract a 500 km long high pressure metamorphic belt occurs in the qiangtang block of northern tibet the occurrence of high pressure metamorphic rocks eclogite and blueschist has a significant bearing on the paleo tethys subduction and plate suturing processes a few studies have been conducted on petrology mineralogy and geochronology of high pressure rocks in this paper we present the results of a geochemical and isotopic investigation on eclogite and blueschist from this belt and discuss the probable geodynamic implications geochemical analyses show that the two rock types eclogite and blueschist have grossly similar bulk compositions but they can be subdivided into two groups based on their tio 2 contents the low tio 2 2 2 75 wt rocks are more akin to ocean island basalts oib the positive e nd t values ranging from 0 4 to 6 3 for all samples suggest that the basaltic protoliths of the eclogites and blueschists were derived from a long term depleted mantle source however their geochemical characteristics require that the source be metasomatized and enriched in large ion lithophile and light rare earth elements shortly before the formation of these basaltic protoliths like for most oib and e morb of the world the recognition of oceanic basalts suggests that a paleozoic ocean should be present in the central qiangtang block this ocean was hypothesized to be the western extension of the changning menglian paleo tethys ocean in the eastern margin of the tibetan plateau the eclogite and blueschist were formed by northward subduction of this paleozoic ocean in the triassic

three dimensional velocity structure of the northern hikurangi margin raukumara new zealand implications for the growth of continental crust by subduction erosion and tectonic underplating traveltimes between shots from nine marine seismic reflection lines and nine onshore recorders were used to construct a 3 d p wave velocity model of the northern hikurangi subduction margin new zealand from north to south between raukumara basin and raukumara peninsula the moho of the overriding plate increases in depth from 17 to similar to 35 km low seismic p wave velocities of 3 5 5 0 km s are localized within a similar to 10 km thick prism in the lower crust of the overriding plate immediately updip of the intersection between the subduction thrust and moho and beneath the topographic crest of east cape ridge and the raukumara range southward this region of low seismic velocities and surface uplift increases in distance from the trench as the thickness of the crust in the overriding plate increases we interpret this low velocity volume to be underplated sedimentary rocks and crustal materials that were tectonically eroded by subduction beneath the trench slope the buoyancy and low strength of these subducted materials are proposed to assist the escape from a subduction channel near the base of the crust and drive local rock uplift in the middle crust our observations of very low velocity suggest high fluid filled porosities of 12 18 and the implied buoyancy anomaly may enhance underplating at greater depths the process is driven by the contrast between upper crustal quartz feldspar mineralogy and the denser diabase or olivine rich lithologies of the lower crust and mantle we estimate a rate of lower crustal underplating at the northern hikurangi margin of 20 7 km 3 ma 1 km 1 since 22 ma we suggest that underplating provides an efficient means of accreting subducted sediment and tectonically eroded material to the lower crust and that the flux of forearc crustal rocks into the mantle at subduction zones may be systematically overestimated

silver isotopes as a source and transport tracer for gold a reconnaissance study at the sheba and new consort gold mines in the barberton greenstone belt kaapvaal craton south africa

the nature and dimensions of regional and local gold related hydrothermal alteration in tholeiitic metabasalts in the norseman goldfields the missing link in a crustal continuum of gold deposits the archaean lode gold deposits at norseman western australia consist of auriferous quartz veins in dextral reverse ductile brittle shear zones within tholeiitic metabasalts of upper greenschist to amphibolite facies metamorphic grade three types of deposits northern central southern are delineated on the basis of their spatial distribution veining style alteration mineraloty and metamorphic grade of host rocks northern deposits hosted in upper greenschist to lower amphibolite facies rocks comprise massive to laminated quartz veins with selvedges of quartz chlorite calcite biotite plagioclase assemblages central deposits hosted in lower amphibolite facies rocks consist of laminated to massive quartz veins with selvedges of quartz actinolite biotite plagioclasecalcite assemblages southern deposits hosted in middleamphibolite facies metabasalts consist of banded quartz diopside calcite microcline zoisite veins all deposits exhibit variable ductile deformation of veins and contiguous alteration haloes consistent with a syn deformational genesis at high temperatures from northern to southern deposits the alteration assemblages are indicative of higher temperatures of formation and there are progressively greater degrees of dynamically recovered textures in alteration and gangue minerals these observations imply that a thermal variation of gold related hydrothermal alteration exists within the norseman terrane over a distance of 40 km with tnorthern tcentral tsouthern this thermal zonation is corroborated by t xco 2 phase relations between vein selvedge assemblages which signify formation temperatures of approximately 420 475 c 470 495 c and 500 c for northern central and southern deposits respectively the sum of structural petrographic and mineral chemistry data indicates that the alteration assemblages formed in high temperature open hydrothermal systems and have not been subsequently metamorphosed the thermal differences between the deposit groups may reflect 1 a temperature gradient at relatively constant p corresponding to the proximity of the deposits to regional granitoid complexes or 2 formation of the deposits at progressively deeper crustal levels from north to south in either case the deposits represent a continuum of gold deposition from upper greenschist to amphibolite facies now exposed in an oblique section through the archaean crust at norseman

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

timing of deformation in the norseman wiluna belt yilgarn craton western australia establishing relative and absolute time frameworks for the sedimentary magmatic tectonic and gold mineralisation events in the norseman wiluna belt of the archean yilgarn craton of western australia has long been the main aim of research efforts recently published constraints on the timing of sedimentation and absolute granite ages have emphasized the shortcomings of the established rationale used for interpreting the timing of deformation events in this paper the assumptions underlying this rationale are scrutinized and it is shown that they are the source of significant misinterpretations a revised time chart for the deformation events of the belt is established the first shortening phase to affect the belt d1 was preceded by an extensional event d1e and accompanied by a change from volcanic dominated to plutonic dominated magmatism at approximately 2685 2675 ma later extension d2e controlled deposition of the ca 2655 ma kurrawang sequence and was followed by d2 a major shortening event which folded this sequence d2 must therefore have started after 2655 ma at least 20 ma later than previously thought and after the voluminous 2670 2655 ma high ca granite intrusion younger transcurrent deformation d3 d4 waned at around 2630 ma suggesting that the crustal shortening deformation cycle d2 d4 lasted approximately 20 30 ma contemporaneous with low volume 2650 2630 ma low ca granites and alkaline intrusions time constraints on gold deposits suggest a late mineralisation event between 2640 2630 ma thus d2 d4 deformation cycle and late felsic magmatism define a 20 30 ma long tectonothermal event which culminated with gold mineralisation the finding that d2 folding took place after voluminous high ca granite intrusion led to research into the role of competent bodies during folding by means of numerical models results suggest that buoyancy driven doming of pre tectonic competent bodies trigger growth of antiforms whereas non buoyant competent granite bodies trigger growth of synforms the conspicuous presence of pre folding granites in the cores of anticlines may be a result from active buoyancy doming during folding 2002 elsevier science b v all rights reserved

crustal material recycling induced by subduction erosion and subduction channel exhumation a case study of central tibet western china based on p t t paths of the eclogite bearing baqing metamorphic complex

recalibration of the garnet muscovite gm geothermometer and the garnet muscovite plagioclase quartz gmpq geobarometer for metapelitic assemblages the garnet muscovite gm geothermometer and the garnet muscovite plagioclase quartz gmpq geobarometer have been simultaneously calibrated under conditions of t 1 4 450 760 c and p 1 4 0 8 11 1 kbar using a large number of metapelitic samples in the compositional ranges x grt fe 1 4 0 53 0 81 x grt mg 1 4 0 05 0 24 x grt ca 1 4 0 03 0 23 in garnet x pl ca 1 4 0 17 0 74 in plagioclase and fe 1 4 0 04 0 16 mg 1 4 0 04 0 13 al 1 4 1 74 1 96 in muscovite on the basis of 11 oxygens the resulting gm thermometer yielded similar temperature estimates mostly within 50 c to that of the garnet biotite thermometer and successfully discerned the expected systematic temperature change of prograde sequences thermal contact zones and an inverted metamorphic zone the resulting gmpq barometer yielded similar pressure estimates mostly within 1 0 kbar to the garnet aluminum silicate plagioclase quartz gasp barometer and placed the aluminosilicate bearing samples in the appropriate aluminosilicate stability fields application of the gmpq barometer to thermal contact aureoles or rocks within limited geographical areas confirmed the expected constant pressures that should exist in these settings the random errors of the gm thermometer and the gmpq barometer are estimated to be 16 c and 1 5 kbar respectively when biotite or aluminosilicate is absent in metapelites metamorphic p t conditions may be determined by simultaneously applying the gm thermometer and the gmpq barometer

high grade metamorphic processes which influence archaean gold deposits with particular reference to big bell australia mineralogical and geochemical evidence indicates that partial melting and desulphidation have occurred in the big bell gold deposit decarbonation may also have occurred to account for the lack of a carbonate alteration halo this is compatible with the present data but difficult to test the big bell deposit consists of auriferous sulphide bearing lode schists with muscovite and k feldspar and surrounding biotite schists all derived by intense premetamorphic alteration of rocks of mafic composition assemblages which include cordierite sillimanite k feldspar garnet biotite quartz suggest peak metamorphic conditions of 4 5 kbar and 650 700 c based on phase relations geobaro meters and garnet biotite fe mg exchange partitioning partial melting occurred at peak metamorphism particularly in the altered mafic rocks in and around the deposit and its occurrence may have been essential to the preservation of the deposit melting greatly limited the importance of devolatilization reactions resulting in negligible aqueous fluids and no means of removing appreciable gold minor gold loss may have accompanied desulphidation a diversity of complex metamorphic assemblages occurs around the mine compared to the assemblages developed regionally variable bulk rock composition influences this contrast but there is no evidence of higher metamorphic grades at the mine nor that this might have been the prime control on the different assemblages in this narrow belt it is suggested that the big bell and hemlo deposits are the higher metamorphic grade equivalents of the more abundant greenschist facies gold deposits within archaean greenstone belts this interpretation is favoured by the host rock setting and geochemical characteristics of big bell alternative models that suggest that this class of deposit is a new type must account for the absence of high grade equivalents of the greenschist facies deposits and also the lack of low grade equivalents of the big bell hemlo type archaean gold deposits in high grade metamorphic terrains have undergone a series of processes that are not recorded in the more typical gold deposits of the greenschist facies

2d and 3d numerical models on compositionally buoyant diapirs in the mantle wedge we present 2d and 3d numerical model calculations that focus on the physics of compositionally buoyant diapirs rising within a mantle wedge corner flow compositional buoyancy is assumed to arise from slab dehydration during which water rich volatiles enter the mantle wedge and form a wet less dense boundary layer on top of the slab slab dehydration is prescribed to occur in the 80 180 km deep slab interval and the water transport is treated as a diffusion like process in this study the mantle s rheology is modeled as being isoviscous for the benefit of easier to interpret feedbacks between water migration and buoyant viscous flow of the mantle we use a simple subduction geometry that does not change during the numerical calculation in a large set of 2d calculations we have identified that five different flow regimes can form in which the position number and formation time of the diapirs vary as a function of four parameters subduction angle subduction rate water diffusivity mobility and mantle viscosity using the same numerical method and numerical resolution we also conducted a suite of 3d calculations for 16 selected parameter combinations comparing the 2d and 3d results for the same model parameters reveals that the 2d models can only give limited insights into the inherently 3d problem of mantle wedge diapirism while often correctly predicting the position and onset time of the first diapir s the 2d models fail to capture the dynamics of diapir ascent as well as the formation of secondary diapirs that result from boundary layer perturbations caused by previous diapirs of greatest importance for physically correct results is the numerical resolution in the region where diapirs nucleate which must be high enough to accurately capture the growth of the thin wet boundary layer on top of the slab and subsequently the formation morphology and ascent of diapirs here 2d models can be very useful to quantify the required resolution which we find for a 1019 pa s mantle wedge to be about 1 km node spacing for quadratic order velocity elements

the subduction plate interface rock record and mechanical coupling from long to short timescales abstract short and long term processes at or close to the subduction plate interface e g mineral transformations fluid release seismicity and more generally deformation might be more closely related than previously thought increasing evidence from the fossil rock record suggests that some episodes of their long geological evolution match or are close to timescales of the seismic cycle this contribution uses rocks recovered episodically from subduction zones together with insights from thermomechanical modelling to provide a new dynamic vision of the nature structure and properties of the plate interface and to bridge the gap between the mechanical behavior of active subduction zones e g coupling inferred from geophysical monitoring and fossil ones e g coupling required to detach and recover subducted slab fragments based on critical observations and an exhaustive compilation of worldwide subducted oceanic units for which the presence near the plate interface rock types pressure temperature t p gradients thickness and timing of detachment can be assessed the present study demonstrates how long term mechanical coupling exerts a key control on detachment from the slab and potential rock recovery critical assessment of rock t p characteristics indicates that these fragments can indeed be used as natural probes and provide reliable information on subduction interface dynamics down to 2 8 gpa rock clusters are identified at depths of 30 55 60 and 80 km with some differences between rock types data also reveal a first order evolution with subduction cooling in the first 5 myr which is interpreted as reflecting a systematic trend from strong to weak mechanical coupling after which subduction is lubricated and mostly inhibits rock recovery this contribution places bounds on the plate interface constitution regular thickness

hydrothermal transport and depositional processes in archean lode gold systems a review abstract compelling evidence now exists for the formation of archean lode gold deposits over a substantial range in metamorphic conditions and crustal depth the commonality of ore fluid isotopic and geochemical characteristics the extreme fluid rock ratios within actual ore zones and the observed depth continuity within individual orebodies all suggest that these deposits formed within vertically extensive crustal scale hydrothermal systems calculations based on new thermodynamic data and estimated ore fluid conditions suggest that gold would have been transported as au hs 2 over most of this range gold transport in the ore fluids that formed some diopside and diopside k feldspar class deposits t 550 c would have been as aucl2 whereas the au hs complex may have been significant in some of the lower temperature 270 c less alkaline ore fluids the high solubility of gold in lode gold ore fluids at amphibolite facies conditions 10 s to 1000 s ppb suggests that fluids may have been undersaturated in gold within their source regions promoting the ore fluid s capacity to retain gold in solution over the large transport distances involved ore depositional mechanisms were also likely to vary between crustal levels fluid rock interactions and wallrock sulphidation in particular appear to be the most important type of precipitation mechanism however with decreasing depth of formation the scope of other depositional mechanisms notably phase separation and fluid mixing becomes greater

characterisation and 3d modelling of a nuggety vein hosted gold ore body sunrise dam western australia abstract in high nugget gold ore bodies samples taken from drill core for gold assay are typically too small to allow for the extreme spatial variability in grade and are a poor representation of the underlying distribution of mineralisation the gq north lode at the sunrise dam gold mine in western australia is a good example of an orebody which has a very strong nugget effect coefficient of variation 20 and that has proved very problematic to model gold is hosted in vein stockworks and shear zones and although there is a clear spatial relationship between mineralisation and alteration high vein density and well developed foliations the relationship is best defined statistically because the association between high gold grades and various combinations of these features is non trivial we present a method for automating the inclusion of geological data proxies for gold mineralisation into the prediction of mineralised rocks using conditional probability the method uses the gold assays and the logged geological data to calculate the probability that rocks with particular geological features will be mineralised the ore body can then be modelled automatically using interpolation software with isosurfaces indicating the regions with highest probability of gold mineralisation a good understanding of the geological features associated with mineralisation and consistent geological logging are important prerequisites for successful conditional probability modelling of drill hole data

tectonic evolution of the early mesozoic blueschist bearing qiangtang metamorphic belt central tibet this is the published version copyright 2003 american geophysical union all rights reserved

application of proton microprobe data to trace element partitioning in volcanic rocks abstract in situ proton microprobe analyses are used to calculate partition coefficients d for mn fe cu zr ga rb sr y zr ba and pb and more restricted data for ni ge nb and as in pyroxenes olivines biotites hornblendes feldspars and fe ti oxides from 32 volcanic rocks all with glassy matrices these range from leucitite and picritic basalt to high silica and peralkaline rhyolites data for pigeonite aenigmatite allanite and leucite are included where these occur as additional phases coefficients have also been rayleigh fractionation corrected this correction being significant for those elements with high coefficients in specific mineral phases and where pronounced crystallisation has occurred partition coefficients are presented as plots vs nbo t ratios of coexisting glasses melts negative correlations are observed based on log transformed data for mn zn fe and y pyroxenes mn zn and fe olivines mn and zn fe ti oxides nb y sr and zr hornblendes sr plagioclase and ba cu zn and fe feldspars these element partition coefficients are not correlated with bulk mineral compositions zr and ga are not correlated with either glass or mineral compositions except zr in hornblende the correlations support the role of melt polymerisation in controlling element partitioning at least for those elements listed although possible effects of temperature cannot be uniquely isolated in the present data set the coefficients presented here are generally consistent with published values for equivalent rock compositions one difference being that some of the higher published coefficients e g sr in plagioclase are not matched in the present data set from a practical viewpoint the relative regularity of the coefficient data presented does facilitate choice of relevant coefficient values that are required for petrogenetic modelling procedures

the coolgardie goldfied western australia district scale controls on an archaean gold camp in an amphibolite facies terrane archaean lode gold deposits in the coolgardie goldfield western australia occur in mafic and ultramafic rocks which have been metamorphosed to the amphibolite facies mineralisation was broadly synchronous with peak metamorphism the main phase of granitoid emplacement and regional deformation several different structural styles are represented by the deposits of the coolgardie goldfield mineralisation occurs along sheared felsic porphyry ultramafic rock contacts in gabbro hosted quartz vein sets in fault bounded quartzvein sets and in laminated quartz reefs sited in brittleductile shear zones the structures hosting mineralisation formed in response to a progressive deformation event related to granitoid emplacement in an east west compressional far field stress regime but with local heterogeneous stress orientations this occurred after an earlier period of thrust stacking with probable north south directed tectonic transport two contrasting styles of goldrelated wallrock alteration are associated with the auriferous lodes of the coolgardie goldfield a high temperature assemblage formation temperature 500 c characterised by the proximal alteration assemblage garnet hornblende plagioclase pyrrhotite contrasts with a medium temperature assemblage formation temperature 500 c consisting of calcic amphibole biotite plagioclase calcite arsenopyrite pyrrhotite the distribution of the two styles of gold related alteration is controlled by distance from voluminous syntectonic granitoids located to the west of the coolgardie goldfield with the high temperature style of alteration more proximal to the granitoid greenstone contact than the medium temperature style the occurrence of gold deposits that formed under amphibolite facies conditions throughout the coolgardie goldfield supports a crustal continuum model for archaean lode gold deposits which proposes that gold is deposited in metamorphic environments that range from the sub greenschist to granulite facies in addition the data from coolgardie suggest that syntectonic synmineralisation granitic plutons may play a significant role in controlling the style of gold associated wallrock alteration at deep crustal levels

fluid inclusion studies a thematic session on fluid inclusions was held on 15 16 december 1986 at the university of southampton as part of the annual meeting of the mineral deposits studies group and was supported by the applied mineralogy group of the mineralogical society four of the papers are published in this volume referees d h m alderton a v bromley r p foster a p gize r a ixer n j jackson d a c manning and j j wilkinson and abstracts are presented for five other contributions providing as always an annual forum for the reporting and discussion of mineral deposits research the 1986 meeting focused in part on fluid inclusion studies in metallogenesis opening the meeting keynote speaker roedder roedder and howard referred to a recent study of the taolin zn pb fluorite deposit people9s republic of china in which homogenization temperatures of 120 200 c and salinity determinations of 0 14 wt nacl equiv were more indicative of low temperature epithermal mineralization than a mississippi valley type deposit some of the data particularly the homogenization data differed significantly from the results of an earlier study and were used to emphasize the problems that can be encountered in fluid inclusion studies particular reference was made to the stretching or decrepitation of earlier lower th inclusions by later hotter fluids later hotter fluids in the more typical mw type environment of the illinois fluorite district petroleum rich and aqueous inclusions were the subject of investigation by gize and richardson who observed that the petroleum phase was a strongly

the archean amphibolite facies coolgardie goldfield yilgarn craton western australia nature controls and gold field scale patterns of hydrothermal wall rock alteration the late archean coolgardie goldfield at the western margin of the norseman wiluna belt yilgarn craton comprises an arcuate belt of deformed mafic ultramafic and sedimentary rocks which is bounded to the west by the syntectonic calooli monzogranite greenstones at coolgardie preserve a broad metamorphic gradient with peak metamorphic temperature varying from 480 50 c at the center of the gold field to 545 50 c adjacent to the western granitoid greenstone contact at an approximate pressure of 3 to 4 kbar gold field scale variations in the gangue and ore mineralogy of zoned wall rock alteration assemblages around lodes the ore geochemistry and the isotope chemistry of vein minerals at coolgardie are correlated with plan view distance from the calooli monzogranite evidence supporting synpeak metamorphic gold mineralization in the coolgardie goldfield includes the equilibrium textural relationships between gold sulfides and high temperature silicate gangue the occurrence of undeformed auriferous quartz veins enveloped by garnet hornblende plagioclase calcite alteration which crosscut peak metamorphic fabrics and the siting of variably deformed gold ores in synpeak metamorphic structures conditions of gold mineralization at deposits less than 1 to 2 km from the calooli monzogranite are determined from geothermometry and barometry to be 510 50 c to 590 25 c at 3 to 4 kbar whereas those at greater distances from the monzogranite are 490 to 525 50 c at 3 to 4 kbar alteration assemblages in mafic host rocks can be divided into garnet bearing garnet hornblende plagioclase calcite and garnet absent biotite amphibole plagioclase calcite the presence or absence of garnet is mainly controlled by the mg number of the host mafic rocks ore in deposits with garnet bearing alteration is enriched in ag na pb s and w but only weakly enriched or depleted in k 2 o and other large ion lithophile elements co 2 as mo sb and te whereas deposits with biotite amphibole plagioclase calcite alteration are strongly enriched in ag as s sb w co 2 and large ion lithophile elements sulfide oxide assemblages are regionally zoned from pyrite ilmenite in deposits in granitoids and adjacent to the granitoid greenstone contact through pyrrhotite ilmenite pyrite in garnet bearing alteration 1 to 2 km from the contact to arsenopyrite pyrrhotite ilmenite assemblages in biotite bearing alteration 2 km from the greenstone granitoid contact this variation is potentially related to gradients in fluid f o2 away from the granitoids isotopic compositions of oxygen in quartz 18 o 10 8 12 4 scheelite 18 o 4 0 4 1 and oxygen and carbon in calcite 18 o 8 9 13 2 13 c 0 5 to 5 3 are generally more positive in deposits with garnet bearing alteration than in those with biotite bearing alteration 18 o quartz 6 6 11 8 18 o scheelite 2 3 4 6 18 o calcite 8 7 11 4 13 c calcite 4 3 to 8 4 whereas both alteration styles have dd biotite and dd amphibole values in the range 65 to 86 per mil these differences are interpreted to reflect interaction of isotopically heavy ore fluids with relatively depleted greenstone host rocks during fluid migration through structurally controlled conduits the gold field scale variations in alteration mineralogy and ore chemistry are considered to be related not to initial ore fluid composition but to temperature to host rock composition and to changes in fluid composition resulting from reaction with greenstone belt rocks the correlation between the calculated temperature of alteration and distance from the western granitoid greenstone contact suggests that the calooli monzogranite played some genetic role in determining the nature of hydrothermal alteration across coolgardie

reaction mechanism for the replacement of calcite by dolomite and siderite implications for geochemistry microstructure and porosity evolution during hydrothermal mineralisation carbonate reactions are common in mineral deposits due to co2 rich mineralising fluids this study presents the first in depth integrated analysis of microstructure and microchemistry of fluid mediated carbonate reaction textures at hydrothermal conditions in doing so we describe the mechanisms by which carbonate phases replace one another and the implications for the evolution of geochemistry rock microstructures and porosity the sample from the 1 95 moz junction gold deposit western australia contains calcite derived from carbonation of a metamorphic amphibole plagioclase assemblage that has further altered to siderite and dolomite the calcite is porous and contains iron rich calcite blebs interpreted to have resulted from fluid mediated replacement of compositionally heterogeneous amphiboles the siderite is polycrystalline but nucleates topotactically on the calcite as a result the boundaries between adjacent grains are low angle boundaries 10 which are geometrically similar to those formed by crystal plastic deformation and recovery growth zoning within individual siderite grains shows that the low angle boundaries are growth features and not due to deformation low angle boundaries develop due to the propagation of defects at grain faces and zone boundaries and by impingement of grains that nucleated with small misorientations relative to each other during grain growth the cores of siderite grains are aligned with the twin planes in the parent calcite crystal showing that the reactant fe entered the crystal along the twin boundaries dolomite grains many of which appear to in fill space generated by the siderite replacement also show alignment of cores along the calcite twin planes suggesting that they did not grow into space but replaced the calcite where dolomite is seen directly replacing calcite it nucleates on the fe rich calcite due to the increased compatibility of the fe bearing calcite lattice relative to the pure calcite both reactions are interpreted as fluid mediated replacement reactions which use the crystallography and elemental chemistry of the calcite experiments of fluid mediated replacement reactions show that they proceed much faster than diffusion based reactions this is important when considering the rates of reactions relative to fluid flow in mineralising systems

metamorphism in archaean greenstone belts calculated fluid compositions and implications for gold mineralization an inescapable consequence of the metamorphism of greenstone belt sequences is the release of a large volume of metamorphic fluid of low salinity with chemical characteristics controlled by the mineral assemblages involved in the devolatilization reactions for mafic and ultramafic sequences the composition of fluids released at upper greenschist to lower amphibolite facies conditions for the necessary relatively hot geotherm corresponds to those inferred for greenstone gold deposits xco2 0 2 0 3 this result follows from the calculation of mineral equilibria in the model system cao mgo feo al2o3 sio2 h2o co2 using a new expanded internally consistent dataset greenstone metamorphism cannot have involved much crustal over thickening because very shallow levels of greenstone belts are preserved such orogeny can be accounted for if compressive deformation of the crust is accompanied by thinning of the mantle lithosphere in this case the observed metamorphism which was contemporaneous with deformation is of the low p high t type for this type of metamorphism the metamorphic peak should have occurred earlier at deeper levels in the crust i e the piezothermal array should be of the deeper earlier type however at shallow crustal levels the piezothermal array is likely to have been of deeper later type as a consequence of erosion thus while the lower crust reached maximum temperatures and partially melted to produce the observed granites mid crustal levels were releasing fluids prograde into shallow crustal levels that were already retrograde we propose that these fluids are responsible for the gold mineralization thus the contemporaneity of igneous activity and gold mineralization is a natural consequence of the thermal evolution and does not mean that the mineralization has to be a consequence of igneous processes upward migration of metamorphic fluid via appropriate structurally controlled pathways will bring the fluid into contact with mineral assemblages that have equilibrated with a fluid with significantly lower xco2 these assemblages are therefore grossly out of equilibrium with the fluid in the case of infiltrated metabasic rocks intense carbonation and sulphidation is predicted if as seems reasonable gold is mobilized by the fluid generated by devolatilization then the combination of processes proposed most of which are an inevitable consequence of the metamorphism leads to the formation of greenstone gold deposits predominantly from metamorphic fluids

optimally oriented fault valve thrusts evidence for aftershock related fluid pressure pulses a thrust vein network from the triumph gold deposit western australia is explained in terms of an extremely high rate of fluid pressure increase prior to failure relative to the rate of stress increase the thrust fault is a small displacement fault characterized by a thick fault parallel shear vein plus multiple low angle extension veins with orientations that demonstrate the thrust was optimally oriented relative to the locally imposed crustal stresses large extension veins have irregular margins are dominantly composed of coarse milky quartz with no obvious laminations or solid inclusion trails and are regularly spaced along the thrust 1 2 m the fault vein geometries indicate the triumph thrust is a rare candidate for fault valve failure of an optimally oriented thrust and it is possible the structure formed in a small number of failure events during load weakening of the thrust an analysis using the coulomb criterion shows that load weakening of a thrust occurs when fluid pressure increases relative to tectonic stress by a factor dependent on the orientation of the thrust thrust and reverse faults in dry crust load strengthen prior to failure but the poroelastic behavior of sealed fluid saturated crust is enough to induce load weakening in compressive environments thus poroelastic load weakening is expected to be an important failure mechanism in hydrothermal environments however in the case of the triumph thrust dilatant shear failure necessitates a fluid pressure increase which is an order of magnitude larger still the observations and results are consistent with a pulse of high fluid pressure migrating up through fault or fracture networks that have elevated permeability relative to the wall rock under conditions of transiently low differential stress fluid pressure differences resulted between the fault and wall rock leading to extension fracture and fault failure such conditions may occur when adjacent large earthquakes induce damage breach overpressured fluid reservoirs and generate fluid pressure driven aftershocks

the tectonic setting and evolution of the 2 7 ga kalgoorlie kurnalpi rift a world class archean gold province the yilgarn craton results from three major mantle input events at ca 3 0 2 9 2 8 and 2 7 ga that have interacted with 3 0 ga continental crust zircon geochronology and sm nd isotopic data subdivide the craton into an older yilgarn proto craton and the younger more primitive eastern goldfields superterrane egst formation of the kalgoorlie kurnalpi rift kkr within the egst was associated with the 2 7 ga event which exploited weakened crust at the eastern margin of the yilgarn proto craton where thick sequences of komatiite and basalt were erupted between ca 2710 and 2690 ma in the kalgoorlie terrane calc alkaline volcanism in the kurnalpi terrane began at ca 2730 ma and continued to ca 2690 ma overlapping rifting and plume related volcanism in the kalgoorlie terrane deposition of siliciclastic sedimentary rocks within basins at ca 2660 resulted from an intra orogenic extensional event and coincided with the transition from high ca to low ca granite magmatism and peak emplacement of intrusions with a metasomatised mantle source component most aspects of the kkr are satisfied by broadly coincident plume related magmatism in the kalgoorlie terrane and westward subduction to the east of the burtville terrane geochemical characteristics of 2730 2700 ma calc alkaline volcanism and 2685 2630 ma low sio2 and alkali rich intrusions support models for a continental margin subduction zone setting world class gold deposits formed in reactivated margins of the kkr which became flux zones for mantle derived magmas hydrothermal fluids and heat during 2675 2620 ma orogenesis the orogenic gold mineralisation can be subdivided into proximal intrusion related and distal source deposits

triassic continental subduction in central tibet and mediterranean style closure of the paleo tethys ocean the qiangtang metamorphic belt qmb in central tibet is one of the largest and most recently documented high pressure hp to near ultrahigh pressure near uhp belts on earth lu hf ages of eclogite and blueschist facies rocks within the qmb are 244 223 ma indistinguishable from the age of uhp metamorphism in the qinling dabie orogen results of a u pb detrital zircon study suggest that protoliths of the qmb include upper paleozoic qiangtang continental margin strata and sandstones that were derived from a paleozoic arc terrane that developed within the paleo tethys ocean to the north we attribute qmb hp metamorphism to continental collision between the qiangtang terrane and a paleo tethys arc terrane this collision and the coeval south china north china collision may have slowed convergence between laurasia and gondwana derived terranes and initiated mediterranean style rollback and backarc basin development within much of the remnant paleo tethys ocean realm

carboniferous arc magmatism in the qiangtang area northern tibet zircon u pb ages geochemical and lu hf isotopic characteristics and tectonic implications abstract the longmu co shuanghu suture zone lssz which lies between the southern qiangtang baoshan block and northern qiangtang qamdo block on the tibetan plateau represents remnants of the paleo tethys ocean previous investigations have focused on the ophiolite high pressure zone and metamorphic belt in this zone whereas few studies have considered the arc magmatism the present study examines a suite of carboniferous arc magmatic rocks in the western region of the lssz that consist of acidic to basic volcanic rocks including rhyolite dacite andesite basaltic andesite basalt and pyroclastics zircon u pb dating of three samples two andesites and one dacite by laser ablation inductively coupled plasma mass spectrometry demonstrates that these rocks were emplaced at ca 351 346 ma the basalts have low cr and ni abundances indicating that they were generated by varying degrees of partial melting of an enriched lithospheric mantle that was metasomatized by subduction derived components zircons in the felsic rocks have positive e hf t values of 0 8 14 8 indicating an origin by varying degrees of partial melting of juvenile basaltic lower crust consisting mainly of underplated magmas with similar compositions to the basalts geochemical data show that the volcanic rocks are enriched in light rare earth elements and large ion lithophile elements e g th u and pb but are strongly depleted in high field strength elements e g nb ta and ti have high al 2 o 3 and low zr concentrations and are calc alkaline these magmas are therefore subduction related volcanic arc rocks indicating that paleo tethys subduction began in the early carboniferous

late archean granitoid hosted lode gold deposits yilgarn craton western australia deposit characteristics crustal architecture and implications for ore genesis granitoid hosted lode gold deposits constitute a sub group of late archean lode gold deposits in the yilgarn craton of western australia they share a number of common characteristics including 1 a strong structural control on a variety of scales 2 in most instances the presence of mineralisation in adjacent supracrustal sequence 3 wallrock alteration and vein assemblages consistent with the addition of sio2 k2o co2 s na2o but dependent on the p t conditions of the host environment and ore fluid 4 a metal association comprising au ag as bi te w mo and low concentrations of cu pb and zn and 5 deposition from a low moderate salinity near neutral h2o co2 ch4 fluid over a temperature and pressure range from 250 to 600 c and 1 to 4 kbar which corresponds to a range of crustal environments from lower greenschist to upper amphibolite facies these characteristics suggest that the granitoid hosted lode gold deposits represent a coherent genetic group that is equivalent to the more abundant greenstone hosted lode gold deposits of the craton in combination lode gold deposits irrespective of their host rocks and host terrane formed over a range of crustal levels and developed late in the tectonic sequence of the craton granitoids which host lode gold mineralisation are older than the inferred age of mineralisation by at least 10 million years and in combination with thermodynamic and isotope studies indicate that the host granitoids are not the dominant source of the ore fluid however the host granitoids do provide a structural and to a lesser extent chemical trap for gold bearing fluids in combination trace element and isotope studies of greenstone and granitoid hosted deposits suggest that the ore fluid originated from a deep mid to lower crustal source and at least interacted with granitic rocks at some stage the ore fluid composition is quite constant over the range of crustal levels from deep to shallow implying that the extent of modification by interaction with fluid conduit rocks is variable but not substantial

seismic structure of the north central chilean convergent margin subduction erosion of a paleomagmatic arc we study the erosive convergent margin of north central chile at similar to 31 degrees s by using high resolution bathymetric wide angle refraction and multichannel seismic reflection data to derive a detailed tomographic 2 d velocity depth model in the overriding plate our velocity model shows that the lowermost crustal velocities beneath the upper continental slope are 6 0 6 5km s which are interpreted as the continental basement composed by characteristic metamorphic and igneous rocks of the coastal cordillera beneath the lower and middle continental slope however the presence of a zone of reduced velocities 3 5 5 0km s is interpreted as the outermost fore arc composed of volcanic rocks hydrofractured as a result of frontal and basal erosion at the landward edge of the outermost fore arc the bathymetric and seismic data provide evidence for the presence of a prominent trenchward dipping normal scarp similar to 1km offset which overlies a strong lateral velocity contrast from similar to 5 0 to similar to 6 0km s this pronounced velocity contrast propagates deep into the continental crust and it resembles a major normal listric fault we interpret this seismic discontinuity as the volcanic continental basement contact of the submerged coastal cordillera characterized by a gravitational collapse of the outermost fore arc subduction erosion has most likely caused large scale crustal thinning and long term subsidence of the outermost fore arc

blueschist bearing metamorphic core complexes in the qiangtang block reveal deep crustal structure of northern tibet a 500 km long belt of metamorphic exposures in the qiangtang block provides an opportunity to study the internal structure of northern tibetan crust metamorphic rocks exposed at two widely separated areas along this belt consist of blueschist bearing melange and are bounded by late triassic early jurassic domal low angle normal faults we propose that this melange was underplated to the qiangtang block and was subsequently exhumed by detachment faulting both the underplating and the exhumation occurred during early mesozoic southward subduction of oceanic lithosphere along the jinsha suture this model predicts that the deeper crust of much of northern tibet consists of accretionary melange in contrast to the continental crystalline crust of southern tibet and may account for north south variations of cenozoic tectonism in tibet

volcanic associated nickel deposits from around the widgiemooltha dome western australia volcanic peridotitc associated nickel sulfide deposits occur in archean komatiitic flow sequences around the widgiemooltha dome in the eastern goldfields region of western australia the deposits are typically small shoots 40 volatile free al 2 o 3 2 content of metamorphic fluids and the timing of fluid access to the rocks along the eastern side of the widgiemooltha dome host rocks have undergone serpentinization and variable but generally extensive prograde talc carbonate alteration e g redross dordie rocks north and widgiemooltha 3 deposits in the north munda deposit they have experienced serpentinization with minor carbonate alteration and in addition prograde metamorphic reconstitution to forsteritc talc rich assemblages dunitic units in the west wannaway and wannaway north deposits escaped prograde serpentinization talc carbonate alteration and significant metamorphic reconstitution but have been retrogressively serpentinized sulfides from the different alteration environments show systematic mineralogical and compositional differences at all deposits peak metamorphic temperatures 550 degrees 600 degrees c resulted in reversion of sulfides to a dominant fe ni co cu bearing monosulfide solid solution accompanied by pentlanditc or fe cu intermediate solid solution in some cases most of the present ore mineralogies and textures relate to exsolution of low temperature sulfides from metamorphic monosulfide solid solution with reequilibration and recrystallization during slow postmetamorphic cooling deformation produced a tectonite fabric in the ores characterized by preferred orientation of sulfide grains and mineral layering breccia ores were formed in response to brittle deformation in host rocks and continued ductile behavior in the sulfides deformation also affected the overall configuration of the ore shoots and led to significant sulfide mobilization and ore shoot distension after allowing for the metamorphic imprint a number of primary features of the ore environment can be isolated and there is strong evidence for the prior existence of magmatic sulfides the association of ores with early formed largely dunitic units similar bulk ni cu and ni co ratios for ores from different deposits precious metal data low s se ratios and distinctive magmatic spinels in the sulfides all point to a magmatic control on the ores strong positive correlation between ni and mgo in host units and similar ni mgo ratios in both mineralized and unmineralized units suggest that sulfide liquids formed and equilibrated with the parent magma at depth before being extruded as part of olivine rich crystal mushes

geodynamics of the eastern yilgarn craton abstract over the last decade there have been significant advances in our understanding of the stratigraphy magmatism deformation metamorphism and timing of mineralisation in the eastern yilgarn craton eyc of western australia the integration of these disciplines has enabled a holistic review of the tectonic history of the eyc which favours a paraautochthonous tectonic model a significant advance has been the recognition of a 2810 ma rifting event off the eastern margin of the youanmi terrane which set up the north northwest trending architecture of the eyc rifting was followed by a 100 myr long phase of tectonic activity initiated by the establishment of a convergent margin at 2715 ma characterised by a west dipping subduction zone to the east of the eyc subduction was associated with the deposition of 2715 2670 ma volcanic stratigraphy and the emplacement of voluminous tonalite trondhjemite granodiorite magmatism which resulted in magmatic thickening of the crust volcanism was terminated by a 5 myr pulse of east northeast contraction followed by mid orogenic extension possibly linked to lithospheric and lower crustal delamination the lack of ultra high pressure metamorphism and the presence of high geothermal gradients preclude this contractional event from recording continent continent collision mid orogenic extension at 2665 ma resulted in the introduction of metasomatised mantle melts mafic granites and syenites deposition of siliciclastic basins which record metamorphic anticlockwise ptt paths and the start of significant economic gold mineralisation in the eyc high stretching factors and or delamination associated with this extension event resulted in significant heat input into the base of the crust which eventually led to the emplacement of low ca crustal melt granites and cratonisation of the eyc major gold mineralisation postdates mid orogenic extension and was associated with renewed compression transpression 2650 ma and the development of steep predominantly north northwest striking sinistral and later north to north northeast striking dextral strike slip faults during low ca granite emplacement

archaean carbon reservoirs and their relevance to the fluid source for gold deposits it is commonly assumed1 2 that seawater derived carbon in altered volcanic rocks is the only major pre metamorphic carbon in carbonate reservoir in archaean greenstone belts thus carbonates from archaean gold deposits with stable isotope ratios too negative median 13c 3 to have been derived from such carbon 13c usually assumed to be near 0 have been interpreted as being derived from a local felsic magmatic source1 barley and groves3 however recognize two carbonate alteration styles that predate regional metamorphism and gold mineralization in the norseman wiluna greenstone belt of western australia evidence is summarized below these are seafloor alteration and fault controlled regional alteration which as shown below have completely different carbon isotope compositions the latter median 13c 4 8 implies a major juvenile carbon co2 flux from the mantle during greenstone belt evolution and is thus of fundamental importance in itself however here we show why the mere existence of two carbon in carbonate reservoirs and variation in carbon isotope ratios of carbonates between individual gold deposits in one area negate some of the fundamental assumptions used to support magmatic fluid models1

the age of the kambalda greenstones resolved by ion microprobe implications for archaean dating methods abstract a chert horizon within the archaean basalt pile at kambalda western australia contains zircons from air fall tuffs erupted as the sediment was laid down ion microprobe analyses of these magmatic zircons constrain the long sought age of the kambalda lavas to 2692 4ma 2 and show that regional metamorphism and deformation of the greenstone pile occurred within 30 ma of deposition rare zircon xenocrysts are evidence of pre greenstone crust as old as 3450 ma exposed remnants of which are yet to be identified subsets of both the magmatic and xenocryst zircons show evidence of hitherto unsuspected low temperature u and or pb mobility in the proterozoic long after stabilisation of the craton the ion microprobe data unravel complexities of isotopic inheritance and alteration illustrating the hazards of using bulk or whole rock samples in archaean isotopic investigations identification of interflow cherts as a precisely dateable rock type opens the way to direct ion probe dating of mafic ultramafic piles and close numerical age control of the complex but ill exposed stratigraphic and structural relations in this richly mineralised greenstone belt

tectonic history of subduction zones inferred from retrograde blueschist p t paths many phanerozoic convergent plate junctions are marked by discontinuous blueschist belts reflecting relatively high pressure p prograde trajectories common blueschist paragneisses such as those of the western alps exhibit widespread overprinting by greenschist and or epidote amphibolite facies assemblages for this type of high p belt retrograde metamorphism involved fairly rapid nearly isothermal decompression some terranes underwent continued heating during early stages of pressure release uplift probably occurred as a consequence of the entrance of an island arc oceanic plateau or segment of continental crust into the subduction zone collision resulting in marked deceleration or cessation of underflow and buoyant approximately adiabatic rise of the stranded recrystallized subduction complex other high p belts such as the franciscan of western california preserve metamorphic aragonite and lack a low p overprint retrogression approximately retraced the prograde p t temperature path or for early formed high grade blocks occurred at even higher p t ratios parts of this type of metamorphic belt evidently migrated slowly back up the subduction zone in response to isostatic forces during continued plate descent and refrigeration upward motion took place as tectonically imbricated slices as laminar return flow in melange zones and perhaps partly a lateral spreading extension of the underplated accretionary prism retrograde p t trajectories of high pmore belts therefore provide important constraints on the tectonic evolution of convergent plate junctions less

rapid pulses of uplift subsidence and subduction erosion offshore central america implications for building the rock record of convergent margins integrated ocean drilling program iodp expedition 334 to southern costa rica central america documented unprecedented subduction erosion in an area of active seismic slip widespread subduction erosion of the upper plate initiated when the cocos ridge an overthickened aseismic ridge arrived at the middle america trench subduction erosion was coeval with the rapid formation of deposition centers on the forearc of the upper plate the completely recovered shelf sequence constrains a short 2 m y interval of extreme subsidence 1200 m with a rapid pulse occurring during the first 0 3 m y this event removed an estimated 1 2 106 km3 of forearc material at a rate of 1690 km3 m y km of trench during a time of rapid 1035 m m y shelf sediment accumulation at this erosive margin a sediment starved trench persisted in spite of abundant sediment supply because subduction erosion led to the creation of forearc basins similar rapid pulses of subduction erosion may punctuate the evolution of many margins contributing disproportionately to crustal recycling at subduction zones with implications for the evolution of continental crust and mountain belts and recycling of continental material into the mantle

the source of lead in archaean lode gold deposits of the menzies kalgoorlie kambalda region yilgarn block western australia the archaean greenstone terrane between menzies and kambalda exhibits a coherent although deformed stratigraphic sequence intruded by granitoids and bounded by major nnw trending shear and or fault zones the greenstone terrane hosts a large number of lode gold prospects and deposits including the giant kalgoorlie deposits the initial pb isotope compositions of lode gold deposits as determined from ore related galena and pyrite vary systematically in a linear trend on a207pb 204pb versus206pb 204pb diagram which reflects crustal heterogeneity at the time of mineralisation deposits hosted within a 90 km section of the menzies boorara shear zone have a uniform radiogenic initial pb isotope composition irrespective of temperature of mineralisation and proximity to granitoid gneiss in plan view the pb in these deposits is considered to be derived largely from older felsic crust underlying the greenstone belt and was accessed via this major shear zone system deposits in a transect unrelated to a major shear zone show a systematic correlation between initial pb isotope compositions and proximity to granitoid gneiss and or to mineralisation temperature these compositions are less radiogenic than those within the menzies boorara shear zone but trend on a207pb 204pb versus606pb 204pb diagram between this isotope signature and the uniform pb isotope signature which characterises the 100 km greenstone transect from the mt pleasant area through kalgoorlie to kambalda these data are interpreted to reflect pb derivation from discrete crustal segments within and below the greenstones and require that mineralisation was related to crustal scale hydrothermal systems that accessed both sialic mid to lower crust and the greenstone succession

interpretation of post entrapment fluid inclusion re equilibration at the three mile hill marvel loch and griffins find high temperature lode gold deposits yilgarn craton western australia abstract fluid inclusions that are interpreted to be related to vein filling of quartz veins at the three mile hill and marvel loch gold deposits in amphibolite facies rocks and the griffins find deposits in granulite facies rocks in the yilgarn craton of western australia are dominantly low salinity h 2 o co 2 ch 4 mixtures or co 2 ch 4 fluids at each deposit inclusions have a wide range of compositions with respect to both the aqueous carbonic ratio and the co 2 ch 4 ratio of the carbonic phase and most fluids could not have been in equilibrium with the mineral assemblage in the vein and adjacent rock inclusion densities suggest a range of p t conditions of entrapment and are in general not consistent with the conditions of vein formation indicated by vein assemblages diffusional addition of h 2 into inclusions diffusional loss of h 2 o and reduction of inclusion volume are possible during cooling and uplift along the inferred p t path the inclusion populations could have been derived from an originally uniform population of low salinity aqueous dominated h 2 o co 2 inclusions by a combination of these processes inclusion modification as the cause of the complex inclusion populations is supported by relations of molar volume to composition and to an extent by variations in fluid salinity if inclusion re equilibration is the cause of inclusion variability it was of variable intensity within a single vein system and within individual clusters of inclusions in some samples and is suggested to have been a function of the local petrological and textural environment

isotopic study of the manaslu granite himalaya nepal inferences on the age and source of himalayan leucogranites a detailed isotopic study of the manaslu leucogranite was carried out a u pb age of 25 ma and a whole rock rb sr age isochron of 18 ma were obtained suggesting that the magmatic activity lasted at least 7 ma initial sr isotopic ratios are very high 0 740 to 0 760 and initial nd isotopic ratios are low nd in 13 to 16 and they show the existence of large isotopic variations even at the metre scale these are not the result of perturbations by fluids but rather they reflect the initial isotopic heterogeneity of the source material which has not been obliterated by magmatic processes e g fusion mixing by convection these results also support the crustal origin of this leucogranite the tibetan slab paragneisses whose sr and nd isotopic ratios are very similar to those of the granite at an age of 20 ma are the most probable parental material nd model ages for both the leucogranite and the gneisses are in the range 1 5 2 ga a model of formation of the manaslu granite by coalescence of different batches of magma is in agreement with the present data

diversity within a unified model for archaean gold mineralization in the yilgarn craton of western australia an overview of the late orogenic structurally controlled gold deposits abstract the archaean yilgarn craton has produced 3000 tonnes of gold mainly from structurally controlled deposits that formed during the latest stages of an orogenic event that affected the entire craton and culminated in the period 2 66 2 63 ga as a group these late orogenic deposits encompass a wide range of host rocks structural settings and structural styles and alteration types however several consistent features justify grouping the deposits into a single class of mineralization these features include timing relative to orogenesis high gold to base metal ratios an association with potassium metasomatism and carbonation of host rocks low sulfide contents and a low salinity h 2 o co 2 ch 4 ore fluid most deposits formed in domains of low mean stress during east west regional compression and preserve evidence in the form of quartz vein arrays and hydraulic breccias for high fluid pressures and rock dilation the range of ore depositional conditions implied by alteration assemblages at various deposits makes the term mesothermal an inappropriate description for this class of mineralization the term late orogenic structurally controlled gold deposits is proposed as one which more completely encompasses the class of mineralization described in this overview late orogenic structurally controlled gold deposits formed in thermal equilibrium with high grade metamorphic rocks at the margins of the greenstone belts but were retrograde with respect to lower grade host rocks in the cores of greenstone belts these relationships probably reflect diachronous metamorphism and a single gold mineralization event which was contemporaneous with the later high grade metamorphism geochronological data indicate gold deposition at ca 2 63 ga in the western part of the yilgarn craton isotopic data suggest similar depositional ages in the eastern part of the craton eastern goldfields superterrane but this timing is difficult to reconcile with field relationships that suggest broad contemporaneity between regional metamorphism and gold mineralization at ca 2 66 ga the main competing genetic models for late orogenic structurally controlled gold mineralization in the yilgarn craton propose an orthomagmatic deeply sourced or metamorphogenic ore fluid none of these models can account satisfactorily for all the features of the late orogenic structurally controlled gold deposits

interplate earthquakes as a driver of shallow subduction erosion basal erosion is a prevalent process at subduction zones and plays an important role in the mass balance of global plate tectonics in contradiction with the theoretical expectation that basal erosion requires high basal friction and hence compression in the upper plate extensional faulting is commonly observed in submarine wedges that undergo such erosion here we propose a model to explain this apparent paradox in terms of stress fl uctuations during earthquake cycles in this model basal erosion occurs during large earthquakes when the shallow rate strengthening part of the plate interface strengthens and its overlying wedge weakens but extension occurs during interseismic relaxation of wedge stress the mechanics of basal erosion provide important information on the nature of the updip limit of the megathrust seismogenic zone in margins dominated by basal erosion

geochemistry of cretaceous granites from mianning in the panxi region sichuan province southwestern china implications for their generation abstract the cretaceous granites of mianning located in the northern panxi region were emplaced after collision of the tibetan plateau and yangtze block these granites have very high k 2 o na 2 o ga zr nb y ree except eu and very low mgo cao p 2 o 5 and sr contents relative to m i or s type granites based on the chemical discrimination criteria of whalen et al whalen j b currie k l chappell b w 1987 a type granites geochemical characteristics distribution and petrogenesis contributions to mineralogy and petrology 95 407 419 most of them are a type granites moreover the granites plot in the range of post collision granites and belong to the a2 type elevated initial sr isotopic ratios 0 72 suggest their derivation dominantly from a crustal source these features are consistent with granite formation in a post orogenic setting such as after subduction or collision between of the tibetan plateau and yangtze block in addition the granites are characterized by low abundances of ba sr p ti and eu positive correlation between ba and eu anomalies and negative correlation between rb and k rb plots of rb vs sr suggest that fractional crystallization affected the final compositions of these granites after melting from a dominantly crustal source from the late proterozoic to late mesozoic the crustal composition compared to that of the mantle appears to have increased in the panxi region while the mantle component played an important part in the generation of cretaceous granites in southeastern china its influence was relatively minor in the panxi region thus there was a significant difference in mantle evolution between southeastern china and the panxi region which led to different metallogenic processes

provenance and palaeogeographic implications of detrital zircons from the lower carboniferous riwanchaka formation of the central tibetan plateau the longmu co shuanghu suture zone which divides the qiangtang terrane into the northern and southern qiangtang blocks is regarded as a key locality in reconstructing the evolutionary history of the palaeo tethys ocean and the break up of gondwana however although low temperature high pressure metamorphic rocks and ophiolites have been documented within the longmu co shuanghu suture zone it remains unclear whether it is an in situ suture zone and represents the relic of the main palaeo tethys ocean the uncertainty stems mainly from the limited systematic studies of the provenance palaeontological evidence and depositional settings of strata on either side of the longmu co shuanghu suture zone i e northern and southern qiangtang blocks here we report new detrital zircon u pb ages and palaeontological data from lower carboniferous strata riwanchaka formation of the northern qiangtang block central tibet the riwanchaka formation contains warm climate biota with cathaysian affinities provenance analysis reveals that the formation has detrital zircon spectra similar to those from strata of the yangtze plate and it contains a large proportion of zircons with ages 360 ma similar to the timing of synsedimentary magmatic arc activity implying an active continental margin setting associated with northward subduction of the palaeo tethyan oceanic lithosphere conversely the carboniferous permian strata from the southern qiangtang block contain cool water faunas of gondwanan affinity and exhibit minimum zircon crystallization ages that are markedly older than their depositional ages suggesting a passive continental margin setting the differences in provenance palaeontological assemblages and depositional settings of the carboniferous to permian strata either side of the longmu co shuanghu suture zone indicate the existence of an ancient ocean between the northern and southern qiangtang blocks combining the new findings with previous studies on high pressure metamorphic rocks arc magmatism and ophiolites we support the interpretation that the longmu co shuanghu suture zone is an in situ suture zone that represents the main suture of the palaeo tethys ocean

provenance of eclogitic metasediments in the north qilian hp lt metamorphic terrane western china geodynamic implications for early paleozoic subduction erosion abstract in this contribution we present new petrological geochemical zircon u pb and hf isotopic data for eclogitic metasediments from the north qilian orogenic belt in which early paleozoic ophiolitic sequences and hp lt metamorphic rocks have been previously recognized the studied metasediments contain eclogite facies assemblages reflecting p t conditions of 450 560 c and 1 9 2 4 gpa consistent with those of adjacent eclogites ree compositions of the eclogitic metasediments overlap those of average upper continental crust the shrimp and la mc icpms u pb data of zircons from metasediments demonstrate input from sources with major age components about 500 ma and 1600 1900 ma peak at 1800 ma with a smaller population at 660 800 ma peak at 750 ma and minor 1 9 ga zircon ages the youngest detrital zircons suggest a maximum depositional age of ca 475 ma and combined with p t conditions and previously determined metamorphic age suggest a burial rate of 0 6 0 7 cm yr zircon hf isotopic analyses show that 500 ma zircons have positive ehf t mainly between 8 and 16 by contrast meso to paleoproterozoic detrital zircons show a broad spectrum of ehf t mainly between 5 and 10 with t dm hf of 1800 2500 ma these data suggest that eclogitic metasediments are derived from a mixture of proterozoic continent crust and juvenile early paleozoic oceanic and or island arc crust and their protoliths likely were deposited in a terrigenous dominated continental margin basin rather than a pelagic oceanic crust environment the new results are consistent with subduction erosion of the active continental margin during south dipping subduction as these sediments formed in a fore arc environment close to the qilian block to the south were transported in the subduction zone to 60 70 km depth prior to their exhumation

multistage gold mineralization at the lapa mine abitibi subprovince insights into auriferous hydrothermal and metasomatic processes in the cadillac larder lake fault zone the lapa gold deposit contains reserves of 2 4 mt at 6 5 g t au and is one of the few deposits located directly within the cadillac larder lake fault zone cllfz a first order crustal scale fault that separates the archean abitibi subprovince from the pontiac subprovince to the south gold mineralization is predominantly hosted in highly strained and altered upper greenschist lower amphibolite facies mafic to ultramafic rocks of the piche group auriferous ore zones consist of finely disseminated auriferous arsenopyrite pyrrhotite pyrite and native gold disseminated in biotite and carbonate altered wall rocks native gold which is also present in quartz dolomite calcite veinlets is locally associated with sb bearing minerals especially at depth 1 km from surface where the deposit is characterized by a au sb as association at vertical depth greater than 1 km gold is associated with arsenopyrite and pyrrhotite au as association the mineralogy and paragenesis of the lapa deposit metamorphosed ore and alteration assemblages record the superposition of three metamorphic episodes m1 m2 and m3 and three gold mineralizing events spatial association between biotitized wall rocks and auriferous arsenopyrite indicates that arsenopyrite precipitation is concomitant with potassic alteration the predominant au as association recognized across the deposit is related to gold in solid solution in arsenopyrite as part of a pre m2 low grade auriferous hydrothermal event however the occurrence of hornblende oligoclase porphyroblasts overprinting the biotite alteration and the presence of porous clusters and porphyroblasts of arsenopyrite with native gold and pyrrhotite indicate an auriferous metasomatic event associated with peak m2 prograde metamorphism late retrograde metamorphism m3 overprints the hornblende oligoclase m2 assemblage within the host rocks proximal to ore by an actinolite albite assemblage by precipitation of free gold and sb sulfosalts at lower p t the complex relationships between ore structural features and metamorphic assemblages at lapa are related to the tectonometamorphic evolution of the cadillac larder lake fault zone at different times and crustal levels and varying heat and fluid flow regimes the lapa deposit demonstrates that early low grade gold mineralization within the cadillac larder lake fault zone has benefited from late gold enrichment s during prograde and retrograde metamorphism suggesting that multi stage processes may be important to form gold rich orogenic deposits in first order crustal scale structures

evidence for prograde metamorphic evolution of sri lankan pelitic granulites and implications for the development of continental crust petrological evidence for prograde metamorphic evolution is obtained for pelitic granulites of the newly defined highland complex in sri lanka it includes the occurrences of relict kyanite quartz staurolite corundum kyanite hercynite kyanite and sapphirine kyanite spinel within garnet in the sillimanite quartz rich rocks of special interest is the mineralogy of khondalites graphitic garnet sillimanite k feldspar quartz gneisses the matrix of which is free of hydrous minerals indicating that dehydration reactions have been completed in this kind of rocks however relict hydrous minerals such as al ti rich more than 58 wt al2o3 and more than 1 wt tio2 staurolite and extremely ti rich up to 9 8 wt tio2 biotine occur as inclusions within garnet the local relict mineral assemblages within garnet in khondalites indicate the following continuous staurolite breakdown reactions which may have progressed as temperature increased and pressure decreased staurolite quartz kyanite garnet h2o staurolite corundum kyanite garnet h2o staurolite kyanite hercynite garnet h2o staurolite sillimanite hercynite garnet h2o some of these dehydration reactions are inferred to have also taken place in other widespread pelitic granulites of the highland complex moreover garnet in the highland complex pelitic granulites occasionally preserves growth zoning though modified slightly to extensively by post growth volume diffusion based on the evidence a clockwise prograde p t path of evolution is inferredfor meta sedimentary granulites belonging to the highland complex on the other hand no evidence for prograde metamorphism has so far been obtained in lower grade rocks of the wanni complex to the northwest of the highland complex such a difference between these complexes may be the first petrological feature which supports the new geotectonic division of the sri lankan basement rocks based virtually on the distinctly different neodymium model ages it suggests that these two complexes were amalgamated during the very last part of prograde metamorphism of the highland complex rocks that is during decompression and heating probably by the lower crustal lateral flow after tectonic overthickening and delamination the clockwise prograde p t t path followed by the meta sedimentary granulites of the highland complex is in marked contrast with the near isobaric cooling path for meta igneous granulites suggested by some authors it is proposed that both types of p t t paths albeit for different rock types are possible and resulted from diverse deep crustal processes such as underplating of continental crust by sediments and magmas or near convergent plate boundaries

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

calibration of a ti in muscovite geothermometer for ilmenite and al2sio5 bearing metapelites abstract the ti in muscovite geothermometer was empirically calibrated as ln t oc 7 258 0 289 ln ti 0 158 mg fe mg 0 031 ln p kbar using ilmenite and al2sio5 bearing assemblages in metapelites under p t conditions of 450 800 c and 0 1 1 4 gpa the calibration was conducted for muscovites containing ti 0 01 0 07 fe 0 03 0 16 mg 0 01 0 32 and mg fe mg 0 05 0 73 respectively on the basis of 11 oxygen per formula unit such compositional range covers more than 90 natural muscovites and the random error of this thermometer is estimated to be of 65 c the geothermometer was validated against a set of independently determined temperature conditions between different degrees in samples from different prograde inverted and contact metamorphic terranes application of this thermometer beyond the calibration conditions is not encouraged

the origin and pre cenozoic evolution of the tibetan plateau different hypotheses have been proposed for the origin and pre cenozoic evolution of the tibetan plateau as a result of several collision events between a series of gondwana derived terranes e g qiangtang lhasa and india and asian continent since the early paleozoic this paper reviews and reevaluates these hypotheses in light of new data from tibet including 1 the distribution of major tectonic boundaries and suture zones 2 basement rocks and their sedimentary covers 3 magmatic suites and 4 detrital zircon constraints from paleozoic metasedimentary rocks the western qiangtang amdo and tethyan himalaya terranes have the indian gondwana origin whereas the lhasa terrane shows an australian gondwana affinity the cambrian magmatic record in the lhasa terrane resulted from the subduction of the proto tethyan ocean lithosphere beneath the australian gondwana the newly identified late devonian granitoids in the southern margin of the lhasa terrane may represent an extensional magmatic event associated with its rifting which ultimately resulted in the opening of the songdo tethyan ocean the lhasa northern australia collision at similar to 263 ma was likely responsible for the initiation of a southward dipping subduction of the bangong nujiang tethyan oceanic lithosphere the yarlung zangbo tethyan ocean opened as a back arc basin in the late triassic leading to the separation of the lhasa terrane from northern australia the subsequent northward subduction of the yarlung zangbo tethyan ocean lithosphere beneath the lhasa terrane may have been triggered by the qiangtang lhasa collision in the earliest cretaceous the mafic dike swarms ca 284 ma in the western qiangtang originated from the panjal plume activity that resulted in continental rifting and its separation from the northern indian continent the subsequent collision of the western qiangtang with the eastern qiangtang in the middle triassic was followed by slab breakoff that led to the exhumation of the qiangtang metamorphic rocks this collision may have caused the northward subduction initiation of the bangong nujiang ocean lithosphere beneath the western qiangtang collision related coeval igneous rocks occurring on both sides of the suture zone and the within plate basalt affinity of associated mafic lithologies suggest slab breakoff induced magmatism in a continent continent collision zone this zone may be the site of net continental crust growth as exemplified by the tibetan plateau c 2012 international association for gondwana research published by elsevier b v all rights reserved

gold mineralisation throughout about 45 ma of archaean orogenesis protracted flux of gold in the golden mile yilgarn craton western australia the golden mile deposit was discovered in 1893 and represents today the largest archaean orogenic lode gold system in the world 50 m oz produced gold the golden mile deposit comprises three major styles of gold mineralisation fimiston oroya and charlotte styles fimiston style lodes formed at 250 to 350 c and 100 to 200 mpa and are controlled by brittle ductile fault zones their subsidiary fault zone and vein networks including breccias and open cavity infill textures and hydrothermally altered wall rock fimiston lodes were formed late d1 prior to d2 regional upright folding hydrothermal alteration haloes comprise a progression toward the lode of diminishing chlorite an increase in sericite and in fe content of carbonates lodes contain siderite pyrite native gold 17 different telluride minerals au ag tellurides contain 25 of total gold tourmaline haematite sericite and v rich muscovite oroya style lodes formed at similar p t conditions as the fimiston lodes and are controlled by brittle ductile shear zones associated dilational jogs that are particularly well developed at the contact between paringa basalt and black shale interflow sedimentary rocks and altered wall rock the orebodies are characterised by micro breccias and zones of intense shear zone foliation very high gold grades up to 100 000 g t au and the common association of tellurides and vanadian mica green leader oroya lodes crosscut fimiston lodes and are interpreted to have formed slightly later than fimiston lodes as part of one evolving hydrothermal system spanning d1 and d2 deformation ca 2 675 2 660 ma charlotte style lodes exemplified by the mt charlotte deposit are controlled by a sheeted vein stockwork complex of north dipping quartz veins and hydrothermally altered wall rock the mt charlotte orebody formed at 120 to 440 c and 150 to 250 mpa during movement along closely spaced d4 2 625 ma and reactivated d2 faults with the quartz granophyre in the golden mile dolerite exerting a strong lithological control on gold mineralisation veins consist of quartz carbonate minor scheelite and wall rock alteration comprises chlorite destruction and growth of ferroan carbonate sericite pyrite native gold pyrite pyrrhotite is zoned on the scale of vein haloes and of the entire mine giving a vertical temperature gradient of 50 100 c over 1 000 vertical metres the structural hydrothermal model proposed consists of four major stages 1 d1 thrusting and formation of fimiston style lodes 2 d2 reverse faulting and formation of oroya style lodes 3 d3 faulting and dissecting of fimiston and oroya style lodes and 4 d4 faulting and formation of mt charlotte style sheeted quartz vein system the giant accumulation of gold in the golden mile deposit was formed due to protracted gold mineralisation throughout episodes of an archaean orogeny that spanned about 45 ma fluid conduits formed early in the tectonic history and persisted throughout orogenesis with the plumbing system showing a rare high degree of focussing efficiency and duration in addition to the long lasting fluid plumbing system the wide variety of transient structural and geochemical traps multiple fluid sources and precipitation mechanism contributed towards the richest golden mile in the world

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

rb sr k ar and stable isotope evidence for the ages and sources of fluid components of gold bearing quartz veins in the northern sierra nevada foothills metamorphic belt california gold bearing quartz veins occur in and near major fault zones in deformed oceanic and island arc rocks west of the main outcrop of the sierra nevada composite batholith veins typically occupy minor reverse faults that crosscut blueschist to amphibolite grade metamorphic rocks whose metamorphic ages range from early paleozoic to jurassic vein micas and carbonate quartz mica assemblages that formed by hydrothermal metasomatism of ultramafic wall rocks in the alleghany grass valley washington and mother lode districts yield concordant k ar and rb sr ages the dated veins are significantly younger than prograde metamorphism penetrative deformation and accretion of their host rocks to the continental margin new and previously published mineralization ages from 13 localities in the sierra foothills range from about 140 to 110 m y ago with mean and median between 120 and 115 m y the age relations suggest that mineralizing fluids were set in motion by deep magmatic activity related to the resumption of east dipping subduction along the western margin of north america following the late jurassic nevadan collision event co 2 bearing fluids responsible for metasomatism and much of the vein mica carbonate albite and quartz deposition in several northern mines were isotopically heavy delta 18 o asymp 8 14ppm delta d between about 10 and 50ppm and do not resemble seawater magmatic or meteoric waters metasomatic and vein filling mica dolomite magnesite and quartz in altered ultramafic rocks generally formed from fluids with similar sr and o isotope ratios at a given locality consistent quartz mica delta 18 o fractionations delta 18 o sub q m 4 5 4 9ppm from various localities imply uniform equilibration temperatures probably between 300 degrees and 350 degrees c on a local mine scale fluids responsible for both carbonate alteration of mafic and ultramafic wall rocks and albitic alteration of felsic and pelitic rocks had similar sr isotope ratios samples from three veins in the central alleghany district fit a 115 7 or 3 m y rb sr isochron with a 87 sr 86 sr i value of approximately 0 7119 inferred 87 sr 86 sr ratios of metasomatic fluids from mines in different parts of the foothills region vary considerably 0 704 0 718 suggesting that sr was derived from sources ranging from western assemblage mesozoic ophiolitic or arc volcanic rocks to early paleozoic continent derived clastic rocks of the shoo fly complex systematic geographic variations in both sr and o isotopes can be rationalized by assuming extensive fluid interaction with rocks similar to the ones that are exposed within a few kilometers of the veins but the ultimate sources of the fluids and of au and other constituents may be independent of these isotopically lighter meteoric fluids deposited some late quartz overgrowths and occupied secondary fluid inclusions in earlier vein quartz