lateral extrusion of lower crust from under high topography in the isostatic limit where there is isostasy the rocks of the lower continental crust are subject to an effective lateral pressure gradient equal to the gradient of topographic load whether compensation is in crustal roots or in the mantle the result is a poiseuille flow planar channel flow in the weak lower crust which removes crust from under mountains and smooths and levels the topography assuming cubic power law creep the flux of crust is proportional to the third power of the topographic gradient to the usual arrhenius term to the tenth power of the absolute temperature of the lower crust and to the negative fifth power of the geothermal gradient the result is that any initial condition with an isolated high tends eventually toward a state where the topography is pancake shaped with a flat central plateau and steeper flanks spreading outward when adjacent pancakes merge most of the relief is eliminated flow may either roughen or smooth the moho depending on whether there are lateral density contrasts in the mantle lithosphere or not although it is difficult to find analytic solutions for this evolution it is easily simulated numerically using various published flow laws for plausible lower crustal rocks lateral extrusion is shown to be insignificant under the oceans marginally significant under shields and platforms important under elevated plains and dominant beneath high plateaus and or hot delaminated regions in particular the basin and range province of the western united states cannot maintain short wavelength 100 km moho relief of more than 1 km for times greater than 10 20 m y at most if the tibetan plateau of china has been delaminated as recently suggested then it may flatten even faster reducing 100 km wavelength topographic features to 200 m relief in no more than 0 03 0 13 m y and leaving only features with wavelengths over 400 km at the present therefore the lower crust of tibet may resemble a hydraulic reservoir as previously suggested because the flatness of the moho is self maintained in these regions information on past tectonics is continually lost and the present moho shape cannot be used to balance cross sections for times in the past the flatness of the moho today is not a constraint on the mechanisms of extension or shortening in these regions nor is it evidence for underplating by intrusions it also follows that very intense and inhomogeneous dilational strains may have occurred locally as in the metamorphic core complexes of the basin and range province

generation and evolution of siliceous high magnesium basaltic magmas in the formation of the permian huangshandong intrusion xinjiang nw china abstract numerous permian orthopyroxene rich mafic ultramafic intrusions in the southern margin of the central asian orogenic belt caob have been emplaced in a post orogenic environment the huangshandong mafic ultramafic intrusion is the largest in the eastern tianshan orogenic belt of the caob and consists of a layered unit intruded by a massive unit the layered unit consists of lherzolite troctolite olivine gabbro hornblende gabbro gabbrodiorite and diorite whereas the massive unit is made of gabbronorite and olivine gabbronorite olivine from the layered unit has fo values of 75 to 84 and contains low nio and mno rocks from this unit have initial 87sr 86sr ratios 0 7031 0 7043 and end values 3 4 7 1 while plagioclase has initial 87sr 86sr ratios ranging from 0 70010 to 0 71569 olivine from the massive unit has relatively low fo values of 59 to 70 and high nio and mno rocks from this unit have relatively low and uniform initial 87sr 86sr ratios 0 7030 and high end values 6 0 7 5 and contain plagioclase with relatively low initial 87sr 86sr ratios 0 70042 to 0 70520 olivines from both units have low cao and cr2o3 both units formed from siliceous high magnesium basaltic magmas were derived from a hydrous depleted mantle source magma forming the layered unit experienced earlier removal of sulfides before emplacement with the earlier sulfide saturation caused by the addition of crustal material the magma that formed the massive unit has undergone fractional crystallization of olivine cr spinel siliceous high magnesium basaltic magmatism produced by melting of a hydrous depleted mantle source may be an important feature of orogenic belts

structural characteristic and origin of intra continental fold belt in the eastern sichuan basin south china block abstract the fold and thrust belt in the eastern sichuan basin is represented by a series of subparallel chevron anticlines under the orogenic tectonic setting within the south china block in meso cenozoic period and the influence of the multi layer detachment fault the deformation of the thrust belt exhibits remarkably layered and large scale intracontinental thrusting structural characteristics in this paper we focus on the structural geometry and deformational mechanisms using the latest two dimensional 2d and three dimensional 3d seismic reflection data in combination with well and outcrop data the multi layer detachment faults especially the upper gypsum bearing detachment in the middle triassic jialingjiang formation and lower detachment with gypsum or shale in the lower middle cambrian system directly control the deformational styles of the study area interpretation of seismic sections indicates that the fold and thrust belt has various deformational styles during folding including fault propagation fold fault bend fold and detachment fold with box fold or pop up structural geometry regional location and structural boundaries play significant roles in controlling the deformational styles and distinct differences exist among the different anticlines the huayingshan anticline located at the front of the thrust belt shows intense structural deformation with northwestward thrusting direction and a relatively weak opposite southeastward thrusting in addition the anticlines exhibit structural differences along strike and the fold and thrust belt in the northern segment is influenced by the north china block

regional structural controls of gold mineralisation bendigo and castlemaine goldfields central victoria australia the bendigo and castlemaine goldfields are classic examples of structurally controlled orogenic gold deposits in the bendigo zone of central victoria se australia detailed mapping and biostratigraphic interpretation has led to a better understanding of the regional structural controls of this type of gold quartz mineralisation mineralised quartz veins are hosted by the castlemaine group an early to middle ordovician turbidite succession at least 3 000 m thick gold deposits are controlled by low displacement faults that are clustered into several belts the goldfields indicating a regional structural control the timing of mineralisation overlapped with that of the major period of deformation including folding cleavage development and regional faulting the bendigo and castlemaine goldfields are located in an area termed the whitelaw thrust sheet bounded by two unmineralised high displacement regional scale faults mapping has revealed an interrelationship between the regional scale faults regional structural style and goldfield location the goldfields lie immediately west of the boundary between the upper and lower portions of the thrust sheet and are characterised by symmetric folds with sub horizontal to synclinal enveloping surfaces relatively low co axial strains and moderate cleavage development the non gold bearing areas immediately east of each goldfield correspond with the lower part of the whitelaw thrust sheet and are characterised by higher non coaxial strains stronger cleavage and folds with wide west dipping limbs giving rise to easterly vergent sections and steeply west dipping enveloping surfaces that mineralisation was an integral part of the thin skinned style of deformation in the central bendigo zone is indicated by timing relationships and the interrelationship between local scale mineralised structures and regional scale features such as large displacement unmineralised faults regional variations in fold style and overall thrust sheet geometry the work supports previous models that suggest mineralised fluids were focussed along a linked system of deep seated faults the primary conduits may have been major regional scale intrazone faults which are inferred to sole into detachments near the base of the castlemaine group it is proposed that these structures linked with minor intrazone faults and then with networks of low displacement mineralised faults that were strongly controlled by folds the location of minor intrazone faults was probably controlled by internal thrust sheet geometry the distribution of gold deposits and of gold production suggests that maximum fluid flow was concentrated along the eastern margins of networks of low displacement faults

petrogenetic character and provenance of metabasalts in the aspiring and torlesse terranes south island new zealand implications for the gold endowment of the otago schist we report quantitative x ray diffraction whole rock geochemical and sm nd isotope data for metabasalts from the aspiring and torlesse terranes in the south island of new zealand these rocks underlie the mesozoic metasedimentary otago schist which is anomalously enriched in gold and host to at least one world class orogenic gold deposit at macraes 125 t au geochemical and nd isotopic similarities between the samples point to a common history of the two terranes rather than the amalgamation of one or two allochthonous plate fragments furthermore geochemical and nd data suggest the metabasalts within both terranes formed in an oceanic but essentially non subduction related setting the origin of the aspiring and torlesse basalts can be linked to the formation of an oceanic plateau that had resulted from a permian mantle plume initiation event proximal to a mid oceanic rise or triple junction given the intrinsically gold enriched nature of certain oceanic character mafic rocks the anomalous gold endowment of the otago schist may have been enhanced via the accretion and subduction of a gold enriched oceanic plateau fragment the metabasalts are generally enriched in gold up to 13 ppb compared to their enclosing metasedimentary rocks typically ca 1 ppb with sulphide rich metabasaltic rocks having up to 550 ppb au however the relatively small volume of metabasalts in the otago schist precludes these rocks as the principal source for otago schist orogenic gold with a primarily metasedimentary source of the gold potentially having a limiting effect on the overall endowment of the otago schist this approach that employs petrogenetic fingerprinting of potentially fertile source rocks for the assessment of gold endowment might prove useful in the conceptual exploration targeting of relatively immature and poorly exposed terrains

synchronous fluvial response to surface uplift in the eastern tibetan plateau implications for crustal dynamics synchronous response of river erosion to surface uplift is often assumed by studies using thermochronometer constrained erosion histories to estimate changes in landscape and tectonics we have tested this assumption by studying river incision histories at two sites heishui and lianghekou located in the westernmost inland part of the deeply incised eastern tibetan plateau apatite fission track age elevation plots for the two sites show clear late miocene inflection points marking the time when apparent erosion rates increased fourfold to eightfold to 300 800 m myr this timing is further refined as 10 12 ma by thermal history modeling and overlaps within analytical uncertainties with that recorded from the eastern tibetan plateau margin 150 200 km to the east demonstrating that river incision propagated rapidly from the margin to the interior in less than 2 ma such synchronous regional river incision is consistent with regional surface uplift generated by crustal extrusion along a detachment at depth

eocene onset and late miocene acceleration of cenozoic intracontinental extension in the north qinling range weihe graben insights from apatite fission track thermochronology abstract intracontinental extension in central china is accommodated by cenozoic grabens flanking the ordos block such as the weihe graben which accommodate 5 6 mm yr of extension to define the onset and evolution of major cenozoic extension we examine the cooling history of exhumed footwall rocks in the north qinling range adjacent to the weihe graben we present results from 47 new apatite fission track aft samples primarily from two vertical transects of 1 5 2 5 km relief located on the northern flank of the north qinling footwall aft ages that get progressively older with increasing distance from the active range bounding normal fault suggest that cenozoic uplift and southward tilting was a response to range parallel extension correlations between aft ages and both elevation and track lengths combined with thermal modeling of representative samples reveal that the north qinling experienced two major stages of cenozoic exhumation 1 relatively slow exhumation in response to a small magnitude of extension that initiated at 50 ma 2 relatively rapid exhumation in response to a large magnitude of accelerated extension that initiated at 10 ma in addition small changes in cooling rate at 35 ma and 25 ma may reflect minor changes in faulting rate we interpret initial extensional at 50 ma to be a far field effect of initial india asia continental collision in contrast accelerated extension after 10 ma along the qinling is likely linked to the upward and outward growth of the tibetan plateau alternatively lower crustal flow beneath the qinling may have progressively accelerated exhumation in late miocene time

mesozoic large scale lateral extrusion rotation and uplift of the tongbai dabie shan belt in east china the tectonic convergence between the south china block and qinling belt that occurred in mesozoic time was not homogeneous along strike it was mainly concentrated in the shengnongjia and hannan domes which formed a pair of indentors along which the south china block penetrated the qinling belt as a consequence the tongbai dabie shan belt the eastern part of the qinling orogenic belt may have undergone large scale eastward extrusion clockwise rotation and uplift as indicated by its boundary deformation features it was originally located beneath the narrowest part of the qinling belt where it underwent ultrahigh pressure metamorphism in the early triassic

helium chronometry of apatite and titanite using nd yag laser heating we developed a technique for extracting 4he from apatite and titanite using laser heating that permits measurement of u and th and hence u th he ages on a single sample aliquot unlike direct laser heating quantitative he extraction and u th retention can be achieved when samples are indirectly laser heated to temperatures below their melting point resulting he ages are consistent with those obtained from conventional furnace heating of larger samples the major advantages of this technique include faster analysis and far lower more reproducible he blanks that permit single crystal analyses single crystal dating opens a variety of new avenues for u th he chronometry

geochemistry of komatiites from kambalda western australia i chalcophile element depletion a consequence of sulfide liquid separation from komatiitic magmas the type examples of archean volcanic peridotite associated ni sulfide deposits occur at or toward the base of the ultramafic lava pile at kambalda western australia spinilex textured komatiites considered to represent silicate liquids derived from the original magma are significantly depleted in chalcophile elements throughout the entire ultramafic sequence relative to unmineralized and poorly mineralized komatiites elsewhere and to a theoretically modeled sulfide undersaturated komatiite system this depletion is not attributed to postformational modification of the komatiites by sea floor alteration or metamorphism although very low sulfur contents in the same rocks probably result from such processes the depletion is best interpreted as the result of scavenging of metals by sulfide liquid in equilibrium with the silicate magma implying that all the silicate magma now represented by the ultramafic sequence equilibrated with sulfide liquids at the magmatic stage these results provide convincing evidence against formation of the sulfide ores by in situ sulfide separation from the thick komatiitic peridotitc host the magnitude of the depletion in the spinilex textured komatiite divisions of the peridotitc hosts is much less than that expected for such an origin but for the entire sequence it is greater than that expected from metal partitioning relations and mass balance calculations for the volume and composition of the ultramafic sequence and the volume of associated sulfides this suggests that a proportion of the sulfide liquid that formed from the ultramafic magma s was lost from the magmatic system prior to extrusion such loss is a natural consequence of models for ascent and eruption of sulfur rich komatiitic magmas that involve prior concentration of sulfides plus olivine in high level magma chambers or separation and segregation of sulfide liquid during ascent from the upper mantle

two phase growth of high topography in eastern tibet during the cenozoic high topography in eastern tibet is thought to have formed in response to weak lower crust flowing towards the plateau margin thermochronologic analyses of rocks exposed at the eastern plateau margin record periods of mountain growth early in the indo asian collision implying that crustal flow alone could not have created the high topography

sources of metals in the porgera gold deposit papua new guinea evidence from alteration isotope and noble metal geochemistry the porgera gold deposit is spatially and temporally associated with the late miocene mafic alkalic epizonal porgera intrusive complex pic located in the highlands of papua new guinea png the highlands region marks the site of a tertiary age continent island arc collision zone located on the northeastern edge of the australasian craton the pic was emplaced within continental crust near the lagaip fault zone which represents an oligocene suture between the craton and volcano sedimentary rocks of the sepik terrane magmatism at porgera probably occurred in response to the late miocene elimination of an oceanic microplate and subsequent early pliocene collision between the craton margin and an arc system located on the bismarck sea plate gold mineralization occurred within 1 ma of the time of magmatism metasomatism accompanying early disseminated au mineralization in igneous host rocks resulted in additions of k rb mn s and co2 and depletions of fe mg ca na ba and sr rare earth and high field strength elements remained largely immobile pervasive development of illite k feldspar quartz carbonate alteration assemblages suggests alteration by mildly acidic 200 to 350 c fluids at high water rock ratios strontium and lead isotopic compositions of minerals from early base metal sulphide veins associated with k metasomatism and later quartz roscoelite veins carrying abundant free gold and tellurides are remarkably uniform e g 87sr86sr 0 70745 0 00044 n 10 207pb204pb 15 603 0 004 n 15 these compositions fall between those of unaltered igneous and sedimentary host rocks and specifically sedimentary rocks from the jurassic om formation which underlies the deposit igneous rocks 87sr86sr 0 7035 207pb204pb 15 560 om formation 87sr86sr t 0 7153 207pb204pb 15 636 it is therefore suggested that the hydrothermal fluids acquired their sr and pb isotopic signatures by interaction with or direct derivation from a plutonic root of the pic and host sedimentary rocks of the om formation it is likely that au was also derived from one or both of these two sources concentrations of au in unaltered igneous and sedimentary rocks from porgera 10 ppb au do not indicate that either lithology represents a significantly enriched protore although au and platinum group element pge abundances in the igneous rocks suggest a mild primary magmatic enrichment of au relative to the pge average au pt pd mantlenormalized 14 0 6 5 n 8 evidence that the porgera magmas were rich in volatiles permits speculation that au may have been concentrated in a magmatic fluid phase but alternative possibilities such as derivation of au by hydrothermal leaching of solidified igneous materials or sedimentary rocks cannot be excluded at this time

the origin and evolution of mineralizing fluids in a sediment hosted orogenic gold deposit ballarat east southeastern australia the hydrothermal fluids responsible for gold mineralization at the ballarat east gold deposit the second largest orogenic gold deposit in the western lachlan orogen are thought to have links to a variety of processes including metamorphism sedimentation and or magmatism in the current study noble gases and halogens have been used as fluid tracers to reevaluate the origin and evolution of the gold related fluids at the ballarat east deposit gold bearing quartz and carbonate veins from the ballarat east contain low salinity 4 wt nacl equiv aqueous h2o and mixed h2o co2 fluid inclusions these fluid inclusions have variable molar br cl values of between 1 2 10 3 and 2 9 10 3 and i cl values between 150 10 6 and 500 10 6 and br is strongly correlated with i defining a mixing line with a br i ratio of 5 6 the fluid inclusions have 40ar 36ar ratios ranging from 322 close to the atmospheric 40ar 36ar ratio of 296 up to a maximum of 4503 40ar is strongly correlated with cl and defines a mixing line with a 40are cl ratio of 4 6 10 4 40are denotes excess 40ar the fluid inclusions contain 5 1 to 32 ppm 40are by mass and exhibit minimum 36ar concentrations ranging from 3 1 to 11 ppb which exceed air saturated water asw levels by several parts per billion asw 1 3 2 7 ppb fluid inclusion 84kr 36ar and 130xe 36ar values are uniformly enriched in kr and xe relative to air but exhibit limited variation these data provide strong evidence for the involvement of two noble gas and halogen reservoirs this data is compatible with a deeply sourced fluid possibly originating by devolatilization of altered volcanic rocks e g basalts that acquired additional noble gases and organic br plus i by interaction with sedimentary rocks including organic rich shales that are found beneath and surrounding the deposit the data are also consistent with mixing deeply sourced metamorphic fluids with sedimentary formation waters however both interpretations favor the involvement of metamorphic fluids and sedimentary components and highlight the significance of fluid rock interaction as controls on fluid compositions in victorian deposits the data are compatible with genetic models for orogenic gold in which gold mineralization was initiated by metamorphic devolatilization in the lower crust and was linked to lachlan orogenesis at ca 440 ma

mesozoic cenozoic evolution of the danba dome songpan garze east tibet as inferred from la icpms u pb and fission track data abstract the eastern margin of the tibetan plateau is a key region to understand the formation and tectonic evolution of the plateau most studies concentrated on the longmen shan range that corresponds to the tectonically very active boundary between the tibetan plateau and the sichuan basin this work addresses the deformation and exhumation pattern in the poorly understood metamorphic danba dome situated at the conjunction between the southern longmen shan range and the major strike slip xian shui he fault the early mesozoic and cenozoic evolution of the dome are then compared to the regional evolution of eastern tibet the about 830 ma zircon u pb ages obtained on the gezong and gongcai gneiss as well as u pb ages on inherited zircons in the early mesozoic granites confirm the occurrence of neo proterozoic magmatic episodes in the danba basement at 800 900 ma and 650 750 ma following the closure of the palaeotethys ocean granitoids were emplaced in the danba metamorphic basement and triassic cover from about 230 ma to about 170 ma the western part of the lian he kau granitic massif emplaced between 210 and 200 ma within the triassic cover sequence of the dome was only slightly affected by the early mesozoic deformation however in the eastern deeper level compartment of the massif ductile deformation occurred between 200 and 190 ma contemporaneous with the onset of the barrovian type metamorphism dated at 200 180 ma that deformation is compatible with the early mesozoic deformation observed in the longmen shan range and with the development of the large scale decollement level in the songpan garze area apatite fission track data then indicate that following the oligocene middle eocene onset of eastward motion of the songpan garze terrane the danba dome developed as a crustal scale antiform during the late miocene early pliocene 10 4 ma the exhumation was controlled by the lian he kau fault a major transpressive structure parallel to the xian shui he fault and the associated manai thrust faults regionally while the oligocene middle miocene exhumation phase seems to have been of regional extent the late miocene to present day phase is localised along the longmen shan range thrusts as well as along the back thrusts formed by the lian he kau manai and associated faults

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

origins of nd sr pb isotopic variations in single scheelite grains from archaean gold deposits western australia accessory gangue scheelite cawo4 from the archaean mt charlotte lode au deposit can be divided into two types with different rare earth element ree signatures in some scheelite grains specific ree signatures are reflected by different cathodoluminescence colours which can be used to map their often complex oscillatory intergrowths domains with specific ree contents from two grains were sampled for sm nd rb sr and pb isotopic analyses using a micro drilling technique type i scheelite is strongly enriched in middle ree mree and eu anomalies are either absent or slightly positive four fragments collected from type i regions of two crystals have initial 87sr 86sr and end values ranging from 0 70141 to 0 70163 and 2 5 to 3 5 respectively and pb isotope ratios reflecting the composition of greenstone sequence this may indicate that nd and pb have their source either locally or regionally in the greenstones basic greenstone lithologies have 87sr 86sr 0 7015 and the radiogenic sr signatures indicate that part of the sr originated from felsic lithologies located either within or beneath the host greenstone pile alternatively the sr signature may have evolved from preferential leaching of a rb rich mineral during hydrothermal alteration of the greenstone the ree patterns of type ii scheelite are either flat or mree depleted and have strong positive eu anomalies three fragments collected from type ii regions of the same two crystals have initial 87sr 86sr ratios and end values between 0 70130 and 0 70146 and 1 1 to 2 6 respectively and pb isotope signatures that are once again similar to that of the greenstone this implies that 87sr 86sr ratios in type ii fluids were closer to those of the host dolerite 0 7008 0 7013 due to more extensive fluid interaction with the dolerite a positive correlation between na and ree suggests that ree3 are accommodated by the coupled substitution ree3 na 2 ca2 into both type i and type ii scheelite this is consistent with a fractional crystallisation model to explain the change in ree patterns from type i to type ii but not with a model involving different coupled substitutions and fluids from different origins we propose that the complex ree and isotopic signatures of scheelite at mt charlotte are related to small m to medium km scale processes involving mixing between fresh batches of hydrothermal fluid with fluids that had already been involved in extensive wall rock alteration the very high end values measured in some scheelites have been previously used to link gold mineralisation with komatiites containing unusually high sm nd ratios however tiny 20 m grains of secondary hydroxyl bastnasite were found within micro fractures of one scheelite grain containing an extremely high end signature the hydroxyl bastnasite probably formed during recent ree redistribution within the scheelite as a result of meteoric fluid circulation the scale of this cryptic low temperature alteration is sufficient to explain the anomalously high endi values observed in scheelite from western australia

the role of komatiitic and picritic magmatism and s saturation in the formation of ore deposits abstract the introduction of high temperature high magnesium komatiitic and picritic magmas into the earth s upper crust has given rise either directly or indirectly to many of the world s major ore deposits there is a direct association between komatiitic magmatism and kambalda style ni cu pge deposits the link between magma type and mineralization is not as clear cut in the case of the noril sk cu ni pge deposits but nevertheless there is an association between picritic magmatism and ore deposit development komatiitic and or picritic magmatism may also have contributed to the formation of many types of hydrothermal ore deposits recent nd isotope data generated by kent et al 1994 provide strong evidence for the hypothesis proposed by keays in 1983 that the au in archaean au deposits was derived either directly or indirectly from komatiites through the action of metamorphic fluids the importance of komatiitic and picritic magmas in ore forming processes arises from the fact that they are among the few mafic ultramafic magma types that are s undersaturated at the time of magma formation s saturation of magmas leads to marked depletion in the chalcophile metals for example czamanske and moore 1977 estimated that a third of the original cu and ni contents of morb were lost from the magmas due to extraction into magmatic sulphides the depletion in au and the pge is much greater due to their much larger partition coefficients keays and scott 1976 komatiitic and picritic magmas are s undersaturated because they are high temperature magmas produced by large degrees of partial melting of upper mantle sourc regions that were already depleted in s through earlier partial melting events most mafic magmas produced by less than 25 partial melting are s saturated and hence depleted in the ore forming chalcophile elements in contrast komatiitic and picritic magmas have their full complement of chalcophile metals when they do become s saturated they form sulphides that are strongly enriched in ni cu au pge and other strongly chalcophile metals these may accumulate to form ore deposits directly e g magmatic ni cu pge sulphides or be dispersed in komatiitic picritic rocks or their fractionation products making these source rocks for hydrothermal au or vms deposits

systematic variations in magmatic sulphide chemistry from mid ocean ridges back arc basins and island arcs abstract immiscible sulphide liquids preserved as magmatic sulphide globules are hosted in igneous rocks of highly variable composition formed during magmatic processes at different tectonic settings here we report on compositional in situ data of magmatic sulphides from mid ocean ridge back arc and island arc magmatic systems iron ni rich monosulphide solid solutions mss that mainly consist of pyrrhotite and pentlandite are the dominant phases in magmatic sulphide globules in subduction related magmas mss are most abundant in relatively evolved melts and characterised by low ni and high fe contents as well as low ni cu ratios 1 thus mss chemistry varies systematically in lavas from the different tectonic settings in terms of their fe ni distribution and ni cu ratio the observed mineralogical and chemical variations between mss from the different settings reflect early s saturation accompanied by olivine fractionation along mid ocean ridges compared to later stage s saturation in arc systems associated with fe ti oxide fractionation these systematics are probably related to the relatively oxidised character of subduction zone magmas opposed to more reduced mid ocean ridge melts large ni cu variations in mss from the same locality and in individual samples suggest that s saturation is a multistage or continuous process leading to inclusions of magmatic sulphides that represent different fractionation stages in the ascending magma

mesozoic basin development and tectonic evolution of the dabieshan orogenic belt central china 1 the dabieshan orogenic belt is a zone of long lived shortening with late stage extension that formed during mesozoic time in central china regional basin analysis coupled with structural cross sections provides a means of reconstructing the paleogeography of the region from triassic continental collision through early tertiary extension the overall distribution of different basin types in the orogenic belt indicates that there was a prolonged period of shortening throughout the mesozoic that became less intense over time meanwhile extension became more common from jurassic into tertiary time either compression or extension was isochronous but limited to different geographic regions and or crustal levels or alternated repeatedly orogen wide through time nonetheless the orogen underwent gradual transition from overall shortening and crustal thickening to dominantly extension and rift basin formation although these events overlapped in time from the jurassic through early cretaceous penecontemporaneous compression and extension across the orogen may reflect long term 100 m y shortening and crustal thickening leading to gravitational spreading of the resultant thick crustal welt we suspect that this prolonged history of shortening across two nearby 100 km apart suture zones played some role in the exhumation of ultrahigh pressure rocks from beneath the dabieshan

siderophile element abundances in the upper mantle evidence for a sulfide signature and equilibrium with the core abstract measurements of the intrinsic oxygen fugacities of samples from the earth s upper mantle have shown that the characteristic oxidation state ranges from values close to fe ni metal saturation to more oxidized values tectonic processes can account for the progressive and variable oxidation of a mantle that was originally in equilibrium with metal so that the oxygen fugacity data give strong support to the possibility of protocore protomantle equilibrium the data for apparently overabundant siderophile elements in the mantle are reviewed and 2 groups group a ge au pd pt ru rh ir re os group b sn sb ag ga cu as co ni w mo are recognized group a abundances result from an overprinting by a late accreting chondritic component of a mantle that had suffered major extraction of the indigenous complement by core forming metal chou 1978 group b however are 10 more abundant than group a and may be distinguished as a group by their greater chalcophile tendencies this is highlighted for example by the large cl normalized sn ge ratio of the mantle which is unexpected as a result of metal silicate fractionation alone but can be accounted for if some sn together with sulfur was retained in the mantle during core separation industrial experience in the equilibrium of molten fe and silicate slags shows that sulfur is not overwhelmingly partitioned into metal so that the concept of partial 20 of original accreting component retention of sulfur and group b elements in the mantle has experimental and theoretical support following core separation progressive loss of sulfide from the mantle is indicated by the anomalously high ni at a given mg number in archaean basalts and siliceous mg poor sediments it is also a feature of oceanic and continental basalts that some u pb fractionation must have occurred in the mantle source regions following core separation and this is most satisfactorily achieved if pb is partially removed from u in a sulfide phase

precious metals in the jimberlana intrusion western australia implications for the genesis of platiniferous ores in layered intrusions a neutron activation study of the distribution of ir pd and an in the jimberlana intrusion has shown that pd and au partition strongly into the sulfides the pd content of sulfides is governed to a large degree by the r factor a measure of the amount of silicate liquid in equilibrium with the sulfide melt the pd content of the sulfide horizons from the center of the jimberlana intrusion is high but appreciably less than that of the merensky reef and the pt horizon of the stillwater complex the comparatively low pd content of the jimberlana sulfides is believed to be due to the sulfides having reached equilibrium with a smaller volume of silicate liquid than the pt rich sulfides of the bushveld and stillwater complexes at the margin of jimberlana the sulfide content of the rocks may be as high as 10 percent these sulfides have precipitated rapidly resulting in a very low r factor and consequently they have anomalously low precious metal contents variations in the ir content of rocks are independent of the presence of sulfides in the jimberlana intrusion this may have resulted from the f sub o 2 of the magma being less than that of the ir 2 s 3 ir buffer allowing pd but not ir to enter the sulfide melt

an introduction to remobilization information from ore body geometry and experimental considerations abstract mobilization involves the translocation and concentration of originally dispersed metallic constituents remobilization is the translocation resulting in modified concentration and distribution of pre existing metalliferous concentrations mobilization and remobilization proceed by chemical liquid state mechanical solid state and mixed processes the paradox of remobilization is that the more complete the process the more difficult becomes substantiation regional metamorphic remobilization is commonly invoked to explain the morphology of supposedly once stratiform massive sulphides and their feeder and alteration systems analysis of the principal dimensions of stratiform massive sulphides in non metamorphic and regionally metamorphosed deposits shows that whereas comparative morphology may be a guide to remobilization it does not constitute proof similarly feeder and alteration morphology suggests that geometric analysis may be of value in establishing mechanical remobilization but the more complex alteration distributions which exist could well lead to misinterpretation in highly deformed terranes ore composition internal structure and zonation influence the ore s capacity to be remobilized mechanical remobilization requires rheological divergence between sulphides and their host rocks experimental deformation of dry sulphides shows that most common sulphides pass through the brittle ductile transition before entering the field of regional metamorphism but that the strength order of sulphides silicates and carbonates remains dynamic until all are undergoing steady state flow within this context experimental strength orders support galena weakest experimental considerations indicate that during diagenesis and early prograde metamorphism the brittle ductile transition will result in widely distributed micro and mesoscale mechanical remobilization nevertheless there is usually insufficient gross behavioural contrast to engender discordant macroscale remobilization possible exceptions particularly in relation to thrusting are outlined but we conclude that macroscale mechanical remobilization during the brittle ductile transition is an uncommon occurrence experimental data were also examined relative to subsequent prograde to peak metamorphism and strength contrasts during decaying and retrogressive metamorphism because the largest strength difference between sulphides and the host sequence is in the lower part of the 350 650 c metamorphic range we contend that macroscale mechanical remobilization is more probable during prograde metamorphism in the 350 500 c interval we similarly believe that macroscale mechanical remobilization is more likely during deformation accompanying retrogressive cooling in the range 500 350 c and we note that it may also result from deformation exploiting the brittle ductile contrasts between silicate and sulphide rocks during the water deficient non retrogressive metamorphic decay cycle chemical mobilization and remobilization require the dissolution of base metals by metamorphic fluids the compositions of hydrothermal low grade metamorphic fluids and their precipitates in active geothermal systems show that the fluids can dissolve transport and form concentrations of base and precious metals similarly fluid inclusion compositions from low to medium grade metamorphic rocks show that such fluids can dissolve a few ppm of base metals in both cases low salinities and low dissolved base metal contents raise doubts over whether such metamorphic fluids can form ore grade base metal concentrations we submit that they can metamorphogenesis is favoured by high initial base metal concentrations preferential dissolution of sulphides over silicates and fluid dominated metamorphism as found in a multiple pass fluid system preferential dissolution of sulphides and the need for high water rock ratios are supported by theoretical considerations experimental work on mafic felsic and sedimentary source rocks and field based studies in the context of mobilization the work strongly implies that prograde metamorphic devolatilization and a mobile fluid phase have substantial ore forming capability retrograde revolatilization does not remobilization is more complex although favoured by high initial metal concentrations dissolution is inhibited by the low permeability of homogeneous low silicate massive sulphides and the consequent peripheral fluid channeling it is conversely enhanced by fine grainsize in discordant fluid focussing retro grade shear zones in all cases for remobilization to form a new ore body both fluid flow and the precipitation process must be focussed and efficient

fission track dating of phosphate minerals and the thermochronology of apatite several phosphate minerals have been investigated for their usefulness in geochronology and thermochronology by the fission track method of these apatite ca5 po4 3 f cl oh has proved pre eminently suitable for this purpose for reasons discussed below apatite was one of the first minerals amongst many others to be investigated for fission track dating by fleischer and price 1964 and has subsequently become by far the most important of all the minerals used for dating by this method the usefulness of apatites for fission track dating arises from their near universal tendency to concentrate uranium within their structure at the time of crystallization and their widespread occurrence in all of the major rock groups price and walker 1963 first recognized the possibility that the accumulation of radiation damage tracks in natural minerals from the spontaneous nuclear fission of 238u within their lattices could be used for geological dating they also demonstrated that a simple chemical etching procedure served to enlarge these fission tracks to optical dimensions so that they could be observed and measured under an ordinary optical microscope price and walker 1962b 1963 this simple procedure quickly opened the way for a wide range of nuclear particle track studies in natural minerals and glasses e g fleischer et al 1975 the most important of which was fission track dating fleischer et al 1964 also showed that spontaneous or fossil fission tracks provided an explanation for various anomalous etch pits in apatite which had puzzled crystallographers over many years the essence of a fission track age determination involves measuring the number of tracks that have accumulated over the lifetime of the mineral along with an estimate of the amount of uranium that is present knowing the rate of spontaneous fission decay a geological age can be calculated detailed

constructing the longmen shan eastern tibetan plateau margin insights from low temperature thermochronology 1 contrasting models of upper crustal shortening versus lower crustal flow have been proposed to explain the formation of thickened crust in the longmen shan lms eastern tibetan plateau tp margin these models require different structural kinematics along the lms whose structural geometry is defined by three parallel nw dipping fault zones from foreland southeast to hinterland northwest they are the guanxian anxian fault yingxiu beichuan fault ybf and wenchuan maowen fault wmf newly derived and previously published low temperature thermochronology data from the lms were synthesized to constrain the spatial exhumation and test previous models the results show that 1 exhumation increases abruptly across the range bounding ybf suggesting the fault being the main thrust boundary between the lms and the sichuan basin to the east 2 younger late cenozoic cooling ages are found on the hinterland wmf where a dichotomy of ages on the hanging wall versus footwall suggests late cenozoic thrust activity and 3 toward the hinterland to the west exhumation rates decrease twofold over a distance of 30 40 km this exhumation pattern indicates a westward decrease of tectonic uplift providing the regional topography approached a steady state whereby exhumation is in balance with tectonic uplift the observed exhumation estimates support an upper crustal configuration where thrusts in the lms merge gradually into a gentle detachment seated at a depth of 20 30 km results of this study support a revised upper crustal thrusting model

reconstruction of the deformed collision zone between india and asia by backward motion of lithospheric blocks 1 on the basis of a synthesis of tectonic data available on the india asia collision we present a first attempt to reconstruct the evolution of the collision zone assuming that the deformation of the lithosphere is localized along narrow shear zones and that the interiors of mantle blocks in between remain relatively undeformed we define block contours from the fault pattern and move back the blocks along their boundary faults along convergent or extensional boundaries the crust is assumed to shorten or stretch coherently step by step we go backward in time to finally reach the collision onset for each time step we find a solution compatible with the data set available and the position of the adjacent blocks for each block the search for compatibility at the scale of the entire collision zone allows for solving the kinematics of regions with fewer data and suggests plausible scenarios for regions where data is lacking for each step we calculate large scale displacement maps and determine euler poles for each block for the most recent time step the map proposed is compared to gps motions the deformation budget implies that extrusion absorbed 30 of the convergence between india and siberia during the entire collision span but varied with time accounting for as little as 3 or as much as 60 of this convergence at different epochs

archaean basic volcanism in the eastern goldfields province yilgarn block western australia abstract petrographic petrological and geochemical studies have demonstrated the presence of three distinctive basic volcanic suites in the eastern goldfields province yilgarn block western australia these are termed the high magnesian series basalts hmsb the low magnesian series basalts lmsb and the siliceous high magnesian series basalts shmsb the hmsb and shmsb constitute differentiation series which contain both high mgo 9 5 14 wt and low mgo the hmsb and lmsb were derived from source mantle regions which were variably depleted in the incompatible elements archaean komatiites were derived from similarly depleted source regions and it is argued that the main petrogenetic difference between these three volcanic suites was the degree of partial melting from which they were derived the depleted nature of the source regions may have been induced by earlier small degree source mantle regions of the shmsb were variably enriched in the incompatible elements and water and parental magmas of the shmsb were derived from moderately hydrous conditions of partial melting the relative proportions of each basalt suite varies considerably between the layered successions examined for example the basic volcanics overlying the komatiites at kambalda are shmsb while the footwall volcanics consist predominantly of hmsb with subordinate lmsb however the norseman succession where no ultramafic volcanics are known to occur is comprised mainly of lmsb with a smaller proportion of hmsb

differential exhumation at eastern margin of the tibetan plateau from apatite fission track thermochronology abstract new apatite fission track aft ages from mesozoic sediments in the sichuan basin combined with previous fission track data demonstrate differential uplift and exhumation across the basin particularly significant change in exhumation at least 2000 m was found across the huaying mts modeled temperature time histories and the boomerang plot of aft dataset across the basin suggest rapid cooling and exhumation events during 120 80 ma and at 20 10 ma they reflect the start of the basin scale differential uplift and exhumation which effected the eastern growth of tibetan plateau in particular nested old age center separated by huaying mts was found in the center to northwest part of the sichuan basin a simplified one dimensional steady state solution model was developed to calculate the mean exhumation rate which is 0 05 0 2 mm yr in most parts of the basin it suggests a slow exhumation across much of the basin the regional pattern of aft age length and erosion rate supports a progressive change from the nested old age center towards the southwest this pattern supports the idea of a prolonged steady state uplift and exhumation process across the basin controlled by cratonic basin structure the eastern growth of the tibetan plateau has exerted a significant effect on the rapid exhumation of the southwestern part of the sichuan basin but not on all of the basin during the late cenozoic

contrasting tectonic settings and sulfur contents of magmas associated with cretaceous porphyry cu mo au and intrusion related iron oxide cu au deposits in northern chile porphyry cu mo au and iron oxide copper gold iocg deposits share many similarities e g fe cu and au contents but also have important differences e g the predominance of sulfide minerals in porphyry deposits and iron oxides in iocg deposits genetic comparisons are complicated by the broad definition of iocg deposits here we restrict our study to iocg deposits that are related to igneous intrusive systems in the mesozoic coastal cordillera of northern chile both porphyry and iocg deposits occur in close spatial and temporal proximity offering the chance to examine what controls their different modes of formation from detailed examination of the timing geochemistry and tectonic setting of associated igneous rocks based on new and published data we find that rocks associated with mid cretaceous iocg deposits 125 110 ma are largely indistinguishable from those associated with slightly earlier 125 ma and later 110 ma porphyry cu mo au deposits magmas related to iocg deposits were formed during a brief period of back arc transtension in the mid cretaceous and are on average somewhat more mafic dioritic locally alkaline and isotopically primitive compared to granodioritic magmas associated with porphyry deposits formed during normal contractional arc tectonics in the later cretaceous however these compositional ranges overlap and the differences are not clear enough to be diagnostic we measured the so3 content of igneous apatite from selected samples of these rocks to test the hypothesis that the difference in sulfur content of the ore deposits was due to differences in sulfur content of the associated magmas early igneous apatite crystals occurring as inclusions in silicate phenocrysts from the carmen de andacollo porphyry cu au deposit re os molybdenite ages of 103 9 0 5 ma 103 6 0 5 ma are significantly richer in s 0 25 0 17 wt so3 n 69 than similar apatite crystals from two iocg deposits candelaria casualidad and a sample of regional mid cretaceous igneous rock from near productora 0 04 0 02 wt so3 n 76 using published partition coefficients for s between apatite and oxidized silicate melt we semi quantitatively estimate corresponding magmatic sulfur contents of 0 02 wt s in the carmen de andacollo magmas versus 0 001 to 0 005 wt s in the iocg associated magmas this is an order of magnitude difference and the opposite of what would be expected if the difference were due to bulk magma composition sulfur solubility is generally higher in mafic magmas whereas here the s content is higher in the more felsic porphyries we conclude that the porphyry forming magmas indeed had higher s contents than the iocg related magmas and suggest that these differences reflect different petrogenetic processes during normal subduction magmas derived from the metasomatized mantle wedge are hydrous moderately oxidized and s rich and have the potential to generate s rich porphyry type deposits in contrast in back arc extensional settings upwelling asthenospheric melts carry a weaker subduction signature including lower s contents interaction of these s poor magmas with previously subduction modified upper plate lithosphere is more likely to give rise to s poor iocg deposits

sea floor massive sulfide deposits from 21 degrees n east pacific rise juan de fuca ridge and galapagos rift bulk chemical composition and economic implications

origin and tectonic significance of a mesozoic multi layer over thrust system within the yangtze block south china abstract in the yangtze block south china a well developed mesozoic thrust system extends through the xuefeng and wuling mountains in the southeast to the sichuan basin in the northwest the system comprises both thin and thick skinned thrust units separated by a boundary detachment fault the dayin fault to the northwest the thin skinned belt is characterized by either chevron anticlines and box synclines to the northwest or chevron synclines to the southeast the former structural style displays narrow exposures for the cores of anticlines and wider exposures for the cores of synclines thrust detachments occur along silurian fs and lower cambrian fc strata and are dominantly associated with the anticlines to the southeast this style of deformation passes gradually into one characterized by chevron synclines with associated principal detachment faults along silurian fs cambrian fc and lower sinian fz strata there are however numerous secondary back thrusts therefore the thin skinned belt is like the valley and ridge province of the north american applachian mountains the thick skinned belt structurally overlies the thin skinned belt and is characterized by a number of klippen including the xuefeng and wuling nappes it is thus comparable to the blue ridge province of appalachia the structural pattern of this thrust system in south china can be explained by a model involving detachment faulting along various stratigraphic layers at different stages of its evolution the system was developed through a northwest stepwise progression of deformation with the earliest delamination along lower sinian strata fz analyses of balanced geological cross sections yield about 18 1 21 total 88 km shortening for the thin skinned unit and at least this amount of shortening for the thick skinned unit the compressional deformation from southeast to northwest during late jurassic to cretaceous time occurred after the westward progressive collision of the yangtze block with the north china block and suggests that the orogenic event was intracontinental in nature

textural evidence for remobilization in metamorphic environments remobilization of stratabound massive sulphide deposits involves both dislocation flow mechanical remobilization and fluid state processes chemical remobilization textural support for such remobilization is presented at micro meso and macroscales microscale evidence establishes the degree the amount of a deposit affected of remobilization likewise mesoscale textures establish the degree and may provide mesoscale models of macroscale remobilization processes but still do not establish the extent the distance over which translocation may have occurred of remobilization where macrotexture indicates a departure from an original conformable relationship gross remobilization is suggested and may be the consequence of mechanical and or chemical processes the case for gross mechanical and or chemical remobilization is often difficult to sustain because alternative hypotheses e g hydrothermal replacement may be compatible with the observed relationships remobilization of a conformable ore layer may involve internal redistribution during which sulphides have been modified by dislocation flow and or fluid state processes within the layer constraints or external where the layer constraints have been substantially transgressed during dislocation flow and or fluid phase remobilization if mesoscale textures indicate remobilization of the external type then macroscale gross remobilization remains a possibility subject to substantiation

sediment and shear hosted gold mineralization of the tartoq group supracrustals southwest greenland abstract gold mineralization occurs in the archaean tartoq group supracrustals which are dominated by greenschist amphibolite facies metamorphosed mafic rocks although deformed recognizable features of the stratigraphy pillowed lavas intercalated sills and interflow sediments suggest shallow to deep sea floor deposition large intensely carbonatized structural zones cut through the greenstones subparallel to regional layering and postdating major folding they are interpreted as major shear zones and are characterized by internal ductile deformation due to high fluid pressures with brittle deformation of peripheral zones erratic high grade gold mineralization is associated with quartz pyrite lenses and veins that occur subparallel to layering within the shear zones moderate but more consistent values are found in more extensive pyrite arsenopyrite quartz layers also subparallel to layering these lenses and layers occur adjacent to or within the major carbonatized shear zones microscopically gold is found as small 10 m grains in both pyrite and arsenopyrite low but consistently anomalous grades of gold are also found in strongly contorted banded muscovite rich quart zpyrite rocks these are thought tobe thin chemically deposited interflow sediments within the greenstone pile analogous to oxide facies banded iron formation which is found nearby metamorphism has obscured critical textures but the gold was either chemically deposited along with exhalative silica and sulphides or may have been introduced later during deformation and alteration of primary bif the tartoq group rocks are similar in most respects to those of other archaean greenstone belts being distinguished only by their excellent outcrop exposure current models for the origin of archaean gold deposits are adequate to explain the tartoq gold mineralization a multistage model of gold concentration is proposed gold is initially leached from mafic volcanics and possibly underlying gneisses and redeposited at anomalous levels in chemical sediments these may be further enriched by sulphidation reactions high grade gold mineralization is typically epigenetic in style where very large volumes of fluid are channeled through shear zones undergoing brittleductile deformation

divining gold in seafloor polymetallic massive sulfide systems hydrothermal fluids on the modern seafloor are important carriers of base and precious metals in a wide range of volcanic and tectonic settings the concentrations and distribution especially of gold and silver in associated seafloor massive sulfide sms deposits are strongly influenced by variable source rocks fluid chemistry and precipitation mechanisms compositional data of 130 sms deposits around the world show a large range of gold and silver grades in part reflecting strong buffering of the hydrothermal fluids by their host rocks geochemical reaction path modeling shows that in most cases the investigated hydrothermal fluids are undersaturated with gold and silver and solubilities can be orders of magnitude higher than the au and ag concentrations measured in the corresponding fluids precipitation of gold during conductive cooling of mid ocean ridge black smoker mor fluids occurs at low temperatures but can be very rapid with 90 of the gold deposited in the first 25 c of cooling below 150 c the result is a zn au polymetallic assemblage with au and ag deposited at the same time together with pb and sulfosalts in ultramafic dominated um systems the strongly reduced hydrothermal fluids promote the deposition of gold at higher temperatures and explain the correlation between gold and copper in these deposits in this case the lower stability of the auhs complex at low o2 buffered by fayalite magnetite and quartz results in gold deposition at 250 c with early bornite and chalcopyrite and before sphalerite and silver producing a high temperature cu au assemblage in sediment hosted sed systems the much higher ph stabilizes au hs 2 and keeps gold in solution to very low temperatures after the precipitation of chalcopyrite sphalerite and galena resulting in au poor polymetallic sulfides and very late stage deposition of gold commonly with amorphous silica in arc related arc systems gold deposition occurs at somewhat higher temperatures than in the mor case in part because the fluids start with higher gold concentrations this can be explained by probable direct magmatic contributions and the high o2 of the fluids which promotes the solubility of gold at the source during cooling gold precipitates at about 160 c with sphalerite tennantite silver and galena resulting in an au rich polymetallic sulfide assemblage the mixing of hydrothermal fluids with seawater generally causes oxidation and eventually a decrease in the ph at a mixing ratio of 1 1 causing an initial increase in the solubility of gold and silver this can delay gold deposition from aqueous species to very low temperatures these complex systematics make prediction of au and ag grades difficult however important new data are coming to light on the actual concentrations of the precious metals in hydrothermal fluids in particular the input of magmatic volatiles and leaching of pre existing gold can lead to significant increases in the au and ag concentrations of the venting fluids and earlier deposition in several cases it appears that at least part of the gold load is present as nanoparticles in suspension allowing bulk gold concentrations that may be far in excess of liquid saturation boiling at the seafloor is now widely observed even at great water depths close to the critical point of seawater model calculations of phase separation during boiling show the competing effects on gold solubility of h2 h2s and co2 partitioning into the vapor which can result in highly variable gold to base metal ratios in the deposits flashing of the vent fluids into steam at high temperatures is also commonly observed and can lead to spectacular au grades with a strong cu au association in the deepest and hottest vents

mineral precipitation in the quartz reefs of the bendigo gold deposit victoria australia gold precipitation at bendigo postdates cleavage development and was initiated during folding increased during a reverse faulting stage in bedding parallel quartz veins and culminated in quartz reefs associated with a strike slip faulting event some gold was remobilized along carbonate rich cataclasites associated with later brittle faults understanding the paragenesis of the sulfide and gangue minerals associated with the auriferous quartz reefs is critical to unraveling the cycle of ore genesis from source to sink in this study a petrographic approach closely tied to macroscopic structural observations along with stable isotope and fluid inclusion analyses places constraints on the relative timing of gold mineralization with respect to deformation early sulfide assemblages in bedding and cleavage parallel veins are dominated by pyrite pyrrhotite siderite and are free of visible gold later assemblages in reactivated bedding parallel veins and other fold related veins are characterized by the presence of arsenopyrite ankerite gold fault related veins and the massive quartz reefs are rich in sphalerite and galena with associated gold but lack pyrrhotite small amounts of late stage antimony bearing minerals occur in many vein types and postdate the precipitation of gold decreasing temperature and increasing sulfur activity controlled successively younger sulfide assemblages in the quartz veins stable oxygen isotope data show that quartz from all vein types 18 o 15 9 19 0 homogenized with the host rocks whereas carbon 13 c 14 0 to 4 1 and oxygen 18 o 4 5 24 0 in carbonate have a wider range in values that is interpreted to be a function of decreasing temperature this corresponds with early siderite pyrrhotite and anhedral pyrite and with later ankerite and ferroan dolomite associated with arsenopyrite galena and euhedral pyrite that are crosscut by calcite veins fluid inclusions in quartz veins are predominantly composed of water with carbon dioxide with smaller proportions of nitrogen and methane particularly in the later strike slip faults the sporadic occurrence of methane suggests that there was an open fluid system with incoming fluids from an external source mixing with those in the host rocks although several recent studies have argued that synsedimentary preenrichment may be a significant factor determining the size and distribution of gold in the bendigo orogenic gold deposits we believe there is little direct or deposit scale evidence for a relationship between original metal content in the host rock metasedimentary rocks and the distribution of the gold mineralization in specific structural sites it is suggested that deep seated faults act as conduits for fluid flow and the source for the gold at bendigo the gold is externally derived from deeply sourced auriferous metamorphic fluids that have been focused into discrete structural sites

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

continuous deformation of the tibetan plateau from global positioning system data global positioning system velocities from 553 control points within the tibetan plateau and on its margins show that the present day tectonics in the plateau is best described as deformation of a continuous medium at least when averaged over distances of 100 km deformation occurs throughout the plateau interior by ese wnw extension and slightly slower nne ssw shortening relative to eurasia material within the plateau interior moves roughly eastward with speeds that increase toward the east and then flows southward around the eastern end of the himalaya crustal thickening on the northeast ern and eastern margins of the plateau occurs over a zone 400 km wide and cannot be the result of elastic strain on a single major thrust fault shortening there accommodates much of india s penetration into eurasia a description in terms of movements of rigid blocks with elastic strain associated with slip on faults between them cannot match the velocity field

spatial variation in meso cenozoic exhumation history of the longmen shan thrust belt eastern tibetan plateau and the adjacent western sichuan basin constraints from fission track thermochronology abstract the ne trending longmen shan thrust belt marking the eastern margin of the tibetan plateau adjacent to the sichuan basin is characterized by the paradox of high topographic gradients but low convergence rates and thus critical for understanding of geodynamic processes involved in the eastward growth of the plateau many low temperature thermochronological studies focused on the central and southern portions of the longmen shan and western sichuan basin to investigate the cenozoic exhumation history of the eastern tibetan plateau but little work has been done on their northern portions furthermore the spatial variation in both cenozoic and mesozoic thermal histories of this region has been poorly documented in this paper zircon zft and apatite aft fission track analyses for samples collected from three transects and three boreholes covering the northern central and southern longmen shan and the western sichuan basin provide new data demonstrating several cooling events in this region since the indosinian deformation they occurred during the late triassic 200 ma at the end of early cretaceous 100 ma during the early cenozoic 60 40 ma the early miocene 20 25 ma and the late miocene 9 14 ma respectively on regional scale the longmen shan experienced a slow cooling between the mesozoic and early cenozoic followed by a rapid cooling during the late cenozoic when the rapid regional exhumation began at 10 ma with a sharp increase in exhumation rate from 0 2 mm yr up to 0 9 mm yr locally comparison of zft and aft ages shows that during the mesozoic the northern segment of the longmen shan belt cooled more rapidly than the central and southern segments while younger aft ages indicate more rapid exhumation in central and southern segments of the belt during the late cenozoic similar trend is demonstrated within the western sichuan basin where the onset of exhumation changes from 45 ma in the northern portion to 20 25 ma in the central and southern portions of the basin the thermochronological data reveal that the yingxiu beichuan and guanxian anxian faults have accommodated significant differential exhumation by thrusting activity and associated denudation with a greater amount of differential exhumation from north to south we propose that the major cause for the rise in topography and rapid exhumation in the longmen shan thrust belt during the late cenozoic is the thrusting activities integrated with the regional uplift of the eastern tibetan plateau a process unrelated to the channel flow within the lower crust

focused modern denudation of the longmen shan margin eastern tibetan plateau we use river sediment load data to map the pattern of modern denudation across the longmen shan margin of the tibetan plateau suspended sediment load with corrections of bed load and solute load contributions is used to calculate watershed averaged denudation rates decadal erosion is spatially heterogeneous and seasonally modulated by monsoon flows which account for 80 90 of the sediment load enhanced denudation occurs in a 50 km wide band on the hanging wall of the longmen shan and huya fault zones reaching 0 5 0 8 mm yr these rates are similar to kyr scale rates deduced from cosmogenic 10be and to myr scale rates from low temperature thermochronology the sediment flux derived erosion rates decrease with increasing distance plateauward to less than 0 05 mm yr at a distance 200 km northwest of the foot of the longmen shan the gradient in precipitation across this margin alone cannot explain the one order of magnitude spatial difference in erosion rather the river sediment load data delineates a zone of relatively rapid denudation around active faults that carry the longmen shan in their hanging wall from the similarity of denudation rates measured over myr kyr and decadal time scales we propose that erosion of the longmen shan margin has approached a flux steady state the erosional efflux is balanced by advection of rock toward the longmen margin above the 20 nw dipping ramp of the margin bounding fault our results suggest that high amounts of landslide material mobilized by earthquakes such as the mw 7 9 2008 wenchuan event are gradually removed by rivers smoothing sediment flux over time our results also suggest that caution should be exercised when interpreting young cooling ages as evidence of the initiation of plateau uplift advection of an already high plateau into the belt of higher erosion rate at the longmen shan could also give rise to an abrupt cooling history

constraints on the collision and the pre collision tectonic configuration between india and asia from detrital geochronology thermochronology and geochemistry studies in the lower indus basin pakistan abstract knowledge of the timing of india asia collision is a fundamental prerequisite for understanding the evolution of the himalayan tibetan orogen and its role in global climate oceanic chemistry and ecological evolution despite much active research the basic pre collision tectonic configuration and the timing of terminal india asia suturing remain debated for example debates regarding when and how the intervening kohistan ladakh arc was sutured with india and asia still remain elusive some models propose the arc collided with asia at about 100 ma with india asia collision at ca 55 ma whilst a newer model proposed the arc s collision with india at 50 ma and subsequently with asia at 40 ma another example is the recent proposition that an oceanic greater india basin separated the tethyan himalaya microcontinent from the remaining indian plate until 20 25 ma with the consumption of this oceanic basin marking the final collision at this time these controversies relate to whether the commonly documented 50 ma contact represents the terminal india asia suturing or the amalgamation between various arcs or microcontinents with india or asia here we present an integrated provenance study of geochronology thermochronology and geochemistry on the late cretaceous pleistocene sediments from the lower indus basin on the indian plate the detrital zircon u pb and fission track data show a reversal in sediment source from a pure indian signature to increasing inputs from the suture zone and the asian plate between the middle paleocene and early oligocene the nd and sr isotopes narrow down this change to 50 ma by revealing input of asian detritus and the establishment of a nd sr isotopic pattern similar to the present day indus fan by 50 ma with no significant variations up section contrary to what might be expected if later major collisions had occurred our isotopic data indicate that greater india was occupied by a fluvial deltaic system analogous to the present day indus and named as the paleo indus which has been transporting asian detritus southward across the suture zone and kohistan ladakh arc since 50 ma suggesting no other ocean basins intervened between india and asia after this time in this region our data require that in the west the india asia collision were accomplished by 50 ma

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

likely mantle plume activity in the skellefte district northern sweden a reexamination of mafic ultramafic magmatic activity its possible association with vms and gold mineralization abstract most attention has been given to the geology of the extensive vms and subordinate precious metals mineralization in the skellefte district less attention has been given to indications of deep seated origins of felsic and mafic ultramafic volcanic rocks of vms and precious metals mineralizing fluids and the primary origins of these metals a holistic view of the significance of mafic ultramafic volcanic rocks to both the geotectonic evolution of the area and the existence of its important base and precious metals deposits has never been presented these subjects are discussed in this investigation primitive mantle normalized spider diagrams of rare earth elements ree distinguish two groups of mafic ultramafic volcanic rocks each with distinct geochemical characteristics a mid ocean ridge morb type and a geochemically unusual and problematic calc alkaline basalt cab type which is the main subject of this investigation the morb type mafic volcanic rocks are mostly older than the skellefte group felsic volcanic rocks hosting the vms deposits whereas the more primitive cab type mafic ultramafic volcanic rocks are mostly younger a common source for these cab type mafic mgo wt 14 volcanic rocks is suggested by their similar and distinctive geochemical features these are near chondritic al undepleted al 2 o 3 tio 2 ratios moderate to strong high field strength element hfse depletion light rare earth element lree enrichment and moderate heavy rare earth element hree depletion they outcrop throughout an area of at least 100 100 km gold mineralization is spatially associated with ultramafic volcanic rocks zr and hf depletion has been shown to be associated with al depletion in mafic ultramafic volcanic rocks elsewhere and has been attributed to deep seated partial melting in ascending mantle plumes zr and hf depletion in cab type al undepleted mafic ultramafic volcanic rocks is therefore unusual the solution to this dilemma is suggested to be contamination of an al depleted mantle plume by felsic crustal rocks whereby al depleted ultramafic magmas become al undepleted it will be argued that this model has the potential to explain previous observations of deep seated origins the spatial association of ultramafic volcanic rocks with occurrences of gold mineralization and even the primary origin of metals in vms deposits

late cenozoic uplift of southeastern tibet the age of surface uplift in southeastern tibet is currently unknown but the initiation of major river incision can be used as a proxy for the timing of initial uplift the topographically high eastern plateau and gently dipping southeastern plateau margin are mantled by an elevated low relief relict landscape that formed at a time of slow erosion at low elevation and low tectonic uplift rates prior to uplift of the eastern tibetan plateau thermochronology from deep river gorges that are cut into the relict landscape shows slow cooling between ca 100 and ca 10 20 ma and a change to rapid cooling after ca 13 ma with initiation of rapid river incision at 0 25 0 5 mm yr between 9 and 13 ma a rapid increase in mean elevation of eastern tibet beginning at this time supports tectonic climate models that correlate the lateral eastern expansion of high topography in tibet with the late miocene intensification of the indian and east asian monsoons

the geological evolution of the tibetan plateau the geological evolution of the tibetan plateau is best viewed in a context broader than the india eurasia collision zone after collision about 50 million years ago crust was shortened in western and central tibet while large fragments of lithosphere moved from the collision zone toward areas of trench rollback in the western pacific and indonesia cessation of rapid pacific trench migration 15 to 20 million years ago coincided with a slowing of fragment extrusion beyond the plateau and probably contributed to the onset of rapid surface uplift and crustal thickening in eastern tibet the latter appear to result from rapid eastward flow of the deep crust probably within crustal channels imaged seismically beneath eastern tibet these events mark a transition to the modern structural system that currently accommodates deformation within tibet

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

orogenic gold mineral systems of the western lachlan orogen victoria and the hodgkinson province queensland crustal metal sources and cryptic zones of regional fluid flow abstract orogenic gold mineral systems in the western lachlan orogen victoria and the hodgkinson province queensland produced gold provinces characterised by vastly different scales of gold endowment and strongly uneven distribution of gold mineralisation within each province the volume of hydrous pyrite bearing rocks undergoing metamorphic devolatilisation during thermo tectonic events driving orogenic gold mineral systems represents a fundamental first order constraint on the total gold endowment and its broad spatial distribution both between and within the provinces most of the largest gold deposits in both regions occur in linear richly endowed metallogenic zones oblique to the dominant regional structures and related to deep crustal domain boundaries these boundaries with only subtle surface expressions were the major regional structural controls which promoted focused near vertical flow of mineralising metamorphic fluids above the outer margins of cratonic blocks in the lower crust recognised major faults represented only more local scale and often indirect controls on the focused fluid flow particularly effective above the deep cratonic block boundaries overlain by relatively thick crustal source rocks

the jinhe qinghe fault an inactive branch of the xianshuihe xiaojiang fault zone eastern tibet abstract the xianshuihe xiaojiang fault zone xfz forms a prominent linear nw se trending tectonic feature along the southeastern margin of the tibetan plateau the spatial and temporal evolution of the predominantly strike slip movement along the fault remains unclear as estimates for the initiation age of its different segments range from 5 to 13 ma the jinhe qinghe fault jqf the south part of the longmen shan thrust belt provides a key for understanding the tectonic role and deformation history of the xianshuihe xiaojiang fault zone in terms of timing magnitude of displacement and spatial temporal evolution this paper presents research on the structure and chronology of the jqf the results of a geometric and kinematic study of the jinhe qinghe fault show it to be a thrust fault with a left lateral strike slip component the strike slip offset extends at least 13 18 km to southwest based on the shortening of its folds and faulting and based on stream deflection ten apatite fission track apparent ages show a pronounced change in age elevation gradient at 17 2 ma consistent with an abrupt increase of exhumation due to uplift of the hanging wall of the jqf at that time combined with the age of deformed tertiary strata in the footwall of the jqf and the geometric relationships of the jqf with the chenghai fault and the xfz this indicates that the jqf was active between 17 and 5 ma based on the fault chronology geometry and movement we suggest that the fault was a transpressional structure at the south end of the xfz during the period from 17 to 5 ma

principles of mobilization dissolution of metals in mafic and ultramafic rocks the role of immiscible magmatic sulphides in the generation of hydrothermal gold and volcanogenic massive sulphide deposits abstract immiscible magmatic sulphides exert a major control over both the absolute precious and base metal contents of magmas and on the availability of these metals from rocks during ore formation it is concluded that only those mafic magmas that attained s saturation at a late stage are not depleted in the chalcophile elements and are hence suitable source rocks for metals in hydrothermal ore deposits immiscible magmatic sulphides are not only strongly enriched in the ore forming metals but they are also much more reactive than silicates the ore forming metals may be liberated from the sulphides at the sea floor alteration stage and deposited in interflow chemical sediments au deposits or volcanogenic massive sulphide deposits subsequent mobilization remobilization of au from the interflow sediments may concentrate au within the interflow sediments to form economic au deposits the mobilized remobilized au may also be transported to form epigenetic style gold deposits in archaean greenstone belts and in palaeozoic slate belts

the growth of northeastern tibet and its relevance to large scale continental geodynamics a review of recent studies recent studies of the northeastern part of the tibetan plateau have called attention to two emerging views of how the tibetan plateau has grown first deformation in northern tibet began essentially at the time of collision with india not 10 20 myr later as might be expected if the locus of activity migrated northward as india penetrated the rest of eurasia thus the north south dimensions of the tibetan plateau were set mainly by differences in lithospheric strength with strong lithosphere beneath india and the tarim and qaidam basins steadily encroaching on one another as the region between them the present day tibetan plateau deformed and its north south dimension became narrower second abundant evidence calls for acceleration of deformation including the formation of new faults in northeastern tibet since 15 ma and a less precisely dated change in orientation of crustal shortening since 20 ma this reorientation of crustal shortening and roughly concurrent outward growth of high terrain which swings from nne ssw in northern tibet to more ne sw and even ene wsw in the easternmost part of northeastern tibet are likely to be in part a consequence of crustal thickening within the high tibetan plateau reaching a limit and the locus of continued shortening then migrating to the northeastern and eastern flanks these changes in rates and orientation also could result from removal of some or all mantle lithosphere and increased gravitational potential energy per unit area and from a weakening of crustal material so that it could flow in response to pressure gradients set by evolving differences in elevation

mineralogical and geochemical controls on the formation of the woods point dike swarm victoria australia evidence from the morning star dike and implications for sourcing of au within orogenic gold systems the woods point dike swarm of eastern victoria australia has produced over 133 tonnes t of au here as in a number of other orogenic au provinces a close spatial relationship is observed between orogenic au mineralization and intrusive rocks this study focuses on the well endowed morning star dike a gabbro to gabbro diorite dike that has produced 28 t of hydrothermal quartz carbonate pyrite vein hosted au and a further 24 t of placer gold derived from primary rocks in the area immediately around morning star the fractionated magma that formed the dikes within the woods point dike swarm was sourced from a staging magma chamber at a depth of 16 to 23 km here mantle derived magmas were driven to s saturation by assimilation of crustal material leading to the formation of cu ni platinum group element pge au bearing sulfides most of which segregated and settled toward the bottom of the staging chamber emplacement of the dike initiated with tapping of the staging chamber and release of the magmas within the chamber forming the woods point dike swarm flow differentiation and the formation of dike bulges enabled the segregation of sulfide bearing mafic units and intermediate sulfide barren units within these dikes this is exemplified by primitive amphibole rich sulfide bearing gabbroic sections and more evolved sulfide barren gabbro diorite portions of the morning star dike we suggest that the source of au within the morning star dike as well as that in other members of the woods point dike swarm was the cu pge au sulfides left at depth within the magma conduit system for example within the thomson river copper mine dike mass balance calculations undertaken suggest that 104 million tonnes mt of magmatic sulfide may exist within the magma conduit system these magmatic sulfides if they are the equivalent of the sulfides preserved in the morning star dike probably contain around 3 ni and 10 cu during postmagmatic metamorphism we infer that hydrothermal fluids flowed along dikes interacting with magmatic sulfides and leaching some of the au which was subsequently deposited in quartz carbonate veins at higher levels in the system

paleoproterozoic turbidite hosted gold deposits of the ashanti gold belt ghana west africa comparative analysis of turbidite hosted gold deposits and an updated genetic model abstract the paleoproterozoic gold deposits of the ashanti gold belt ghana occur mainly as auriferous quartz veins in graphitic shears proximal to contacts between deformed and metamorphosed turbidites and tholeiitic volcanics alternately block faulted tarkwaian sediments they are suggested in this article to belong to that class of gold deposits recognized as turbidite hosted hence evidence from previous investigations of turbidite hosted and turbidite associated gold deposits occurring in circumstances roughly similar to the ashanti deposits are used where relevant to produce an updated genetic model for the latter given the relatively significant amounts of graphite associated with the major ashanti deposits the role of non carbonate carbon is assessed in the light of conclusions reached by work on other turbidite hosted and turbidite associated gold belts conclusions as to the origins of mineralizing fluids and temporary repositories of gold auriferous interflow sediments reached by work on these other belts are also analysed a concept of the tectonic development of the ashanti belt kumasi basin area introduced in this article is deemed to be critical to the origin of the ashanti gold deposits it encompasses the role of non carbonate carbon turbidites and the primary sources and secondary repositories of au and as in the genesis of the major ashanti gold deposits it examines the significance of the intimate relationship between calc alkaline volcanism and the basin sediments the occurrences of felsic metasandstones in transition zone sediments and ttg granitic material and fragments of chemical sediments in turbidites all of which are incompatible with one or the other of existing concepts of tectonic development in the area

out of sequence thrusts out of sequence thrusts oost are those thrusts which do not obey the foreland propagating or in sequence deformation style they include both isolated thrusts which develop hindward of the thrust front and sequences of break back thrusts which propagate from the foreland to the hinterland two end members of a series of oost types are recognized 1 older in sequence thrusts which are reactivated along their entire length and 2 completely new thrusts which propagate through already deformed thrust sheets between the two end members are thrusts composed partially of reactivated in sequence thrust sequences and partially of new entirely out of sequence segments oosts can be initiated for a variety of reasons including 1 keeping the orogenic wedge at critical taper 2 break back sequences from the suture zone in the overriding plate 3 ramping to overcome a sticking point which inhibits in sequence thrust propagation and 4 during simultaneous displacement along two stacked thrusts culminations which bow up segment of the upper thrust may be chopped through to permit continued displacement on the upper thrust many different types of thrust behavior including gravity sliding plucking and derivation of isolated horses from ramps may mimic some of the characteristics of oosts consequently it may be difficult to conclusively prove an oost origin for a complex thrust geometry

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

orogenic disseminated gold in phanerozoic fold belts examples from victoria australia and elsewhere abstract orogenic gold mineralisation in the lachlan orogen of southeastern australia and in other phanerozoic fold belts has generally been assumed to be confined entirely to quartz veins however an improved understanding of the tectonic structural and geochronological framework in which these occurrences form and recent advances towards the extent of wallrock alteration around these veins cast doubt on the validity of this deeply entrenched lode gold paradigm ore grade orogenic disseminated gold mineralisation has been recognised in a number of fold belts that are known to host significant orogenic lode gold deposits lachlan orogen australia buller terrane and otago schist new zealand meguma terrane canada tien shan mountains in uzbekistan and kyrgyzstan yana kolyma province baikal verhoyansk and allakh yun fold belts in north central mongolia and far east russia disseminated mineralisation in these collisional belts is invariably hosted by pervasively altered greenschist to subgreenschist facies rocks these occurrences also share a number of characteristics with some carlin type gold deposits in the great basin of western north america e g getchell regarding structural relationships fluid composition mineral paragenesis and a common association of carbonaceous matter with mineralisation features of alteration physico chemical characteristics of the ore bearing fluids and strong structural control in all of the studied occurrences point to a close genetic association with lode gold mineralisation the two styles are considered to represent end members of a crustal continuum of orogenic gold emplacement in phanerozoic fold belts with disseminated mineralisation more likely to develop at shallower levels and within more permeable units during the waning stages of uplift and exhumation these conditions facilitate ground preparation via brittle fracturing and the development of intricate stockwork systems efficient gold segregation from bisulphide complexes is largely controlled by fluid mixing of an evolved metamorphic fluid with either more oxidised ascending fluids or meteoric fluids and accelerated by the presence of carbonaceous matter at the site of deposition

quantifying micron scale grain detachment during weathering experiments on limestone weathering in carbonate rocks is often assumed to be governed by chemical dissolution nevertheless chemical processes can be coupled to mechanical mechanisms with small grains undergoing partial dissolution along grain boundaries followed by detachment from the rock surface crucially this process can even extend down to the micron scale although chemo mechanical detachment could be critical for the understanding of carbonate weathering at the global scale the role it plays has not been directly quantified to calculate the contribution of grain detachment to surface retreat rates and to determine the impact of the flow regime we carried out a series of flow through weathering experiments on micritic limestone using atomic force microscopy we obtained high resolution in situ data of surface topography for reacting rock surfaces in all the experiments both grain detachment and chemical dissolution were observed under the laminar flow conditions we explored we found no clear correlation between flow rate and the size of detached grains or between the flow rate and the frequency of grain detachment events importantly our results establish that grain detachment contributes significantly to the overall surface retreat on average accelerating mass loss by 38 in addition to speeding up weathering this micron scale mechanism could also influence the evolution of porosity in aquifers and hydrocarbon reservoirs and provide a natural flux of colloids that could transport heavy metals or radionuclides in groundwater

platinum group element segregation and mineralization in a noritic ring complex formed from proterozoic siliceous high magnesium basalt magmas in the vestfold hills antarctica

wallrock alteration associated with turbidite hosted gold deposits examples from the palaeozoic lachlan fold belt in central victoria australia abstract the study of wallrock alteration phenomena around slate belt hosted gold deposits can provide valuable information regarding the physico chemical conditions of the ore bearing fluids and the timing relationships between deformation peak metamorphism granitoid emplacement and mineralisation thus studying the processes involved in the interaction of the hydrothermal fluids with the host rock is critical in order to constrain genetic models and ultimately as a tool for exploration largely due to the characteristics of the host rocks turbidite hosted mesothermal gold deposits were historically believed to lack such diagnostic alteration haloes and their study has long been neglected however recent studies in the palaeozoic lachlan fold belt in southeastern australia and in similar tectonic settings elsewhere have demonstrated that alteration around many of these deposits is more widespread and pervasive than previously thought a review of existing information and new data strongly support the viability of studying hydrothermal alteration as a guide to turbidite hosted mineralisation and as an indicator of the character of the solutions associated with ore deposition characteristically the auriferous reef structures are surrounded by broad bleached zones while disseminations of carbonate spots pyrite and arsenopyrite porphyroblasts are the most obvious features of alteration depending on parameters such as fault geometry fluid pressure and reactivity and permeability of the host rocks these disseminations can occur up to a kilometre away from the auriferous structures sericitisation and carbonatisation are commonly discernible within 3 to 10 m wide haloes wallrock alteration chemistry of turbidite hosted gold deposits in the lachlan fold belt shows consistent enrichments of k2o co2 s and as and the anomalous values of these elements also provide the most useful dispersion haloes for turbidite hosted gold exploration due to both their regularity and extent

sulphide mineral evolution and metal mobility during alteration of the oceanic crust insights from odp hole 1256d fluxes of metals during the hydrothermal alteration of the oceanic crust have far reaching effects including buffering of the compositions of the ocean and lithosphere supporting microbial life an