cyclic fluid infiltration in structurally controlled ag pb cu occurrences schladming eastern alps abstract polysulphide occurrences in the schladming crystalline complex eastern alps are associated with the intersection of moderately n dipping shear zones type i structures and nw se and ne sw trending steep faults type ii structures geochemical and mineralogical profiling across the type i structures shows progressive mineral alteration feldspar breakdown silicification sericitisation from the margin to the center these observations are best interpreted by an external acidic fluid infiltrating these structures such alteration halos in deep seated structures are excellent leaching sites and transport channels for available metals however precipitation of ore took place at the contact with carbonates where metal bearing complexes became unstable as a result of fluid neutralisation the origin of this carbonate is linked with type ii structures showing distinct carbonatisation but no wall rock alteration it is therefore suggested that the ore occurrences in type i and type ii structures were formed during the cyclic infiltration of an acidic and a co 2 bearing fluid fluid inclusions from quartz carbonate ore veins are consistent with this interpretation pt estimates and the orientation of shear zones and faults are consistent with the geodynamic setting of cretaceous cooling and exhumation of the austroalpine realm east of the tauern window the exhumation of the schladming crystalline complex is therefore interpreted as the setting for a cyclic process of increased fluid pressure vein opening fluid infiltration ore precipitation and vein closure

fluid flow and volume loss during mylonitization an origin for phyllonite in an overthrust setting north carolina u s a abstract in the western blue ridge province of the southern appalachians north carolina granite gneisses are converted to phyllitic mylonites phyllonites along the base of the rector branch thrust sheet the mylonites show evidence for fluid infiltration and large volume losses with decreasing distance from the thrust and local shear zones feldspars undergo grain size reduction by cracking followed by chemical breakdown to muscovite sericite and zoisite the following reaction was important during mylonitization 3 k feldspar 2 h muscovite 2 k 6 sio 2 quartz displays evidence for dynamic recrystallization whereas feldspar deforms brittely which together with the secondary mineral assemblages suggest that deformation occurred at temperatures of between 300 c and 500 c in the presence of a fluid the mylonites are enriched by a factor of at least 2 5 in elements such as tio 2 p 2 o 5 zr y v la ce and depleted in sio 2 in comparison to their protolith gneisses the immobile element enrichments are attributed to enrichment in residual phases such as ilmenite zircon apatite and epidote in the mylonites and are interpreted as due to minimum volume losses of 60 mass balance calculations based on major element chemistry indicate the mylonites lost large amounts of sio 2 and smaller amounts of alkalies and were infiltrated by fluid rock weight ratios of between 10 2 and 10 3 phyllonitization as a process involves several deformation mechanisms including cataclasis and solution of feldspar as well as dislocation creep of quartz and micas reaction enhanced softening was apparently important in strain localization

palinspastic reconstruction and topographic evolution of the eastern alps during late tertiary tectonic extrusion this paper presents a new palinspastic restoration of the eastern alps for neogene time and an attempt to reconstruct the neogene palaeogeology palaeotopography and palaeohydrography in connection with the structural evolution the eastern alps underwent radical horizontal displacement during the neogene due to large strike slip systems and formation of structural windows our palinspastic reconstruction considers a the rearrangement of tectonic units dismembered during tectonic extrusion b the tectonic denudation driven by displacement of the crystalline blocks c geochronological arguments and d the sedimentary record of the syn extrusion basins the rearrangement of tectonic blocks results in a remarkably good fit of highly dismembered zones both in crystalline and sedimentary areas and shows the pre miocene unstretched pattern of the eastern alps reduced to 65 of its present day e w elongation using this structural frame and considering the sedimentary record a set of palaeogeologic and palaeotopographic sketch maps with the palaeo river systems is presented for three time slices pre syn and post extension situation in late oligocene and early miocene times the western eastern alps were already mountainous whereas the eastern part of the orogen formed lowlands or hilly areas enhanced block movement in the course of the extrusion process around the early middle miocene boundary led to the formation of intramontane sedimentary basins and a fault induced reorientation of the drainage pattern which forms the basis of the modern river system in the area east of the tauern window this region where pre miocene land surfaces are preserved probably became a mountainous area not before late miocene time and never reached the elevations of the areas further west

distinction between tectonic mixing and mass transfer processes in a ductile shear zone abstract a petrological geochemical and microtextural study of an upper amphibolite facies shear zone developed in interlayered mafic and felsic layers permits the relative roles of tectonic mixing and fluid assisted mass transfer processes to be determined geochemical evidence indicates that the chemical changes in the deformed rocks result from mixing of mafic and felsic layers together with fluid assisted mass transfer within the shear zone during mylonitization most major elements and some trace elements lree rb sr ba cu ni exhibited mobile behavior the hrees ti v sc co and fe on the other hand were immobile based on mass conservation of these elements a two component mixing model using mafic and felsic rocks from outside the shear zone as end members explains the major and trace element data the chemical composition of the felsic mylonite is modeled by mixing 12 5 mafic rock and 88 5 felsic rock whereas the mafic mylonite is modeled by mixing 55 4 mafic rock and 45 4 felsic rock a closed system mixing model yields a good fit for immobile elements such as hrees ti v and sc but significant chemical differences between the calculated data and observed data are explained by fluid assisted mass transfer of mobile elements under open system conditions a 24 volume loss in the felsic mylonite and 13 volume loss in mafic mylonite are derived from the mass balance assuming the hree ti v and sc were immobile

fault valve behaviour in optimally oriented shear zones an example at the revenge gold mine kambalda western australia abstract quartz vein systems developed in and adjacent to shear zones host major gold deposits in the kambalda region of the norseman wiluna greenstone belt at the revenge mine two groups of mineralised reverse shear zones formed as conjugate near optimally oriented sets during ese subhorizontal shortening adjacent to a major transpressional shear system the shear zones developed at temperatures of about 400 c in a transitional brittle ductile regime deformation was associated with high fluid fluxes and involved fault valve behaviour at transiently near lithostatic fluid pressures during progressive evolution of the shear system early brittle and ductile deformation was overprinted by predominantly brittle deformation brittle shear failure was associated with fault dilation and the formation of fault fill veins particularly at fault bends and jogs a transition from predominantly brittle shear failure to combined shear along faults and extension failure adjacent to faults occurred late during shear zone evolution and is interpreted as a response to a progressive decrease in maximum shear stress and a decrease in effective stresses the formation of subhorizontal stylolites locally subvertical extension veins and minor normal faults in association with thrust faulting indicates episodic or transient reorientation of the near field maximum principal stress from a subhorizontal to a near vertical attitude during some fault valve cycles local stress re orientation is interpreted as resulting from near total shear stress release and overshoot during some rupture events previously described fault valve systems have formed predominantly in severely misoriented faults the shear systems at revenge mine indicate that fault valve action and associated fluctuations in shear stress and fluid pressure can influence the mechanical behaviour of optimally oriented faults

orogen parallel strike slip faults bordering metamorphic core complexes the salzach enns fault zone in the eastern alps austria abstract the approximately ene trending salzach enns fault eastern alps contributed to late stage exhumation of the tauern metamorphic core complex in response to oblique convergence of european and adriatic plates six stages of kinematic evolution with early ductile and later brittle deformational structures were recognized as follows 1 initial ductile deformation is represented by a combination of sinistral shear dominated noncoaxial strain and coaxial strain 2 early brittle sinistral strike slip is combined with a pure shear deformation and represents roughly n s contraction it is followed by 3 sinistral strike slip and 4 ca n s extension due to normal slip striations 5 subsequent n s compression resulted in counterclockwise rotation of the maximum principal stress from ne to nw and led to final dextral strike slip displacements along the salzach enns fault the exhumation of the tauern window was accommodated by dip slip on the fault plane at shallow levels and coaxial flattening at deeper structural levels brittle sinistral faults at shallow structural levels produced a larger amount of strike slip displacement than ductile shear zones at the deeper structural levels which is explained as a scissors like movement and which resulted in the eastward tilting of the tauern window

hydrologic segmentation of high temperature shear zones structural geochemical and isotopic evidence from auriferous mylonites of the renco mine zimbabwe abstract combined structural mineralogical and geochemical observations in auriferous mylonites of the renco mine hosted by late archaean high grade metamorphic granitoids in southern zimbabwe are used to describe the spatially heterogeneous fluid flow and metasomatism that occurred synchronous with deformation at mid to upper amphibolite facies metamorphic conditions significantly the narrow on average 1m wide mylonitic shear zones are internally zoned reflecting a pronounced hydrologic segmentation during deformation shear zones typically consist of two distinct domains i anastomosing quartz feldspar biotite hornblende mylonites and or quartz mylonites and ii tabular shaped pods referred to as lithons that are enveloped by mylonites and that exhibit evidence of transient episodes of brittle fracturing and ductile creep whole rock geochemistry and mass balance calculations indicate dramatic element volume up to 100 and associated mass gains for the brittle ductile lithons that are mineralogically reflected in a volumetrically abundant sulphide mineralisation and the formation of a pervasively developed silicate alteration paragenesis in contrast mylonites have experienced only minor element and volumetric changes and minor alteration 18 o values for whole rocks and quartz are enriched in lithons compared to wall rocks and enveloping mylonites which implies the influence of externally derived fluids these results indicate a strongly domainal fluid flow and mass transfer fluid advection was dominated by microscopic and macroscopic fracture permeabilities related to periods of coseismic dilatancy in lithons the patchy but partly interconnected distribution of lithons enveloped by mylonites indicates that deformation in the high temperature shear zones was characterised by transient periods of seismic slip rather than continuous aseismic creep based on the well preserved internal mineralogical and textural development of the renco shear zones a more general model applicable for fluid advection coupled with the rheological behaviour of mid crustal shear zones is presented

a least squares approach to mass transport calculations using the isocon method a statistically rigorous approach based on the isocon method grant 1986 to evaluate mass transport is presented chemical analyses of multiple samples of unaltered parent and altered rock are used to calculate the average oxide or element concentration and its standard deviation for each rock population in a typical application the uncertainties in element concentration associated with the lack of homogeneity in each population outweigh the analytical uncertainty an efficient algorithm is presented to select the immobile elements by identifying the maximum number of elements that are within their uncertainties compatible with the same isocon the actual isocon is constructed by forcing a line through the origin and optimizing its slope using a weighted least squares procedure for the selected immobile elements thus individual uncertainties for each immobile element are taken into account as an illustration the procedure is applied to a published set of data on potassic and sericitic alterations of quartz monzonite of the bingham porphyry copper system the surprising result indicates that the apparent increase of potassic alteration phases is a consequence of acid leaching of calcium and sodium no mass transport of potassium is indicated silica water and sulfur were added during alteration the best fit isocon requires a small mass increase during each alteration process which however is within the uncertainty obtained for the slope of the isocon a software package is available from the authors in the form of a fortran 77 source code

distribution of variscan i and s type granites in the eastern alps a possible clue to unravel pre alpine basement structures abstract meta granitoid rocks with variscan intrusion ages have been investigated in the three large austroalpine basement massifs of the schladminger tauern the seckau bosenstein tauern and the semmering raabalpen the first two massifs are dominated by calc alkaline i type granitoids mainly granodiorites tonalites and are thus similar to most other intra alpine variscan granitoid massifs while the semmering raabalpen area is one of the rare examples of an intra alpine s type batholith this confirms previous concepts according to which the semmering raabalpen area is part of a separate variscan terrane within the austroalpine basement it is suggested that this basement terrane already had some affinities to the thick mature crust that is typical for the internal moldanubian zone of the variscides the widespread occurrence of carboniferous i type plutons suggests the presence of a less evolved or newly underplated variscan lower crust in most other parts of the eastern alps this kind of crust was probably typical for the former southern flank of the variscan orogen

late cretaceous exhumation of the metamorphic gleinalm dome eastern alps kinematics cooling history and sedimentary response in a sinistral wrench corridor abstract the metamorphic gleinalm dome eastern alps was uplifted and exhumed within a releasing structure in a sinistral wrench corridor during the late cretaceous the dome is confined by a system of ductile shear zones including low angle normal faults and steep sinistral tear faults which define a large releasing structure with the metamorphic dome in its center the fabrics developed within all ductile shear zones record processes which were operating during decreasing temperatures from initial epidote amphibolite upper greenschist facies conditions with crystal plastic fabrics in quartz to temperatures below ca 300 c with predominantly cataclastic fabrics a cooling path based on 40ar 39ar amphibole 95 4 1 2 ma and muscovite ages 87 6 0 6 84 3 0 7 ma together with sphene zircon and apatite fission track data indicate cooling through ca 500 c at ca 94 ma to below ca 250 200 c at 65 ma subsidence of the adjacent late cretaceous kainach gosau basin occurred synchronously with cooling and uplift of the gleinalm dome internal depositional patterns record rapid subsidence at the time of cooling with internal synsedimentary block rotation above an intra crustal ductile normal fault the sinistral wrench corridor of the eastern alps developed by sinistral displacement of the austroalpine units against a relatively stable europe during the late cretaceous

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

chemical changes associated with deformation of granites under greenschist facies conditions the example of the zawidow granodiorite se lusatian granodiorite complex poland abstract small scale subvertical retrograde shear zones are developed in the zawidow granodiorite located in the eastern part of the lusatian massif southwest poland heterogeneous deformation under conditions of greenschist facies metamorphism has produced mylonite zones that show a broad range of microstructural development three fabric types in the granodiorite deformation have been distinguished a weakly deformed protolith b moderately deformed in lens shaped domains c strongly foliated in fabric types b and c whole rock analyses show a decrease in cao mno cr sr pb whereas na2o and sio2 increase slightly overall al2o3 tio2 and zr remain essentially constant this pattern of element variation is consistent with the observed feldspar breakdown and increase of sericite and also with the transformation of biotite into chlorite mass balance calculations based on major and trace element geochemistry indicate constant volume or less than 10 volume loss from this study it is apparent that a low grade metamorphic deformation event does not have to have a profound influence on whole rock geochemical composition

geochemical characteristics across the oxic anoxic interface rote faule front within the kupferschiefer of the lubin sieroszowice mining district sw poland abstract samples of profiles from lower zechstein in the lubin sieroszowice mining district sw poland were collected within and at different distance to the oxidized facies rote faule developed within the zechstein strata around the zary pericline the samples include kupferschiefer underlying weissliegendes sandstone and overlying zechstein limestone from rote faule zones adjacent cu mineralized strata and drill cores from the pb zn mineralized zone more distant to the rote faule front the kupferschiefer samples show differences in ore paragenesis and composition of organic matter which are accompanied by variations in base metal and trace element contents and isotope ratios of organic carbon and carbonates the results from rock eval analyses provide evidence for an oxidative alteration of organic matter in kupferschiefer samples within and adjacent to the rote faule zones related to base metal mineralization the extent of this alteration is reflected by differences in the temperature of maximum pyrolysis yield t max hydrogen and oxygen indices hi oi in contrast to kupferschiefer from pb zn mineralized areas organic matter is characterized by lower hydrogen equivalents and by higher t max values the changes in the isotopic composition of organic carbon and carbonates reflect the alteration of organic matter kerogen within or close to rote faule zones is enriched in 13 c caused by the preferential release of isotopically light organic compounds through progressive degradation of organic matter the opposite tendency towards lower 13 c values of calcite provides evidence for organic matter remineralization isotopic exchange reactions of carbonate with the isotopically distinct oxidizing brines at temperatures between 100 and 120 c caused decreasing 18 o values of calcite the regional zonation in the kupferschiefer with respect to sulfide paragenesis and base metal composition in the mines is explained by metal precipitation from oxidizing brines through decreasing eh conditions during the infiltration of the brines into the reduced kupferschiefer relationships between selected trace elements organic carbon and sulfur contents and hi values are consistent with that model of ore formation

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

the flow of surface derived fluids through alice springs age middle crustal ductile shear zones reynolds range central australia the south east reynolds range central australia is cut by steep north west trending alice springs age c 334 ma shear zones that are up to hundreds of metres wide and several kilometres long with reverse senses of movement amphibolite facies 550 600 c 500 600 mpa shear zones cut metapelites while greenschist facies shear zones 420 535 c 400 650 mpa cut metagranites the sheared rocks commonly underwent metasomatism implying that the shear zones were the pathways of significant fluid flow altered granites within greenschist facies shear zones have gained si and k but lost ca and na relative to their unsheared counterparts suggesting that the fluid flowed down temperature and hence probably upward through the shear zones time integrated fluid fluxes calculated from silica addition are up to 2 1 1010 mol m 2 c 4 2 105 m3 m 2 similar time integrated fluid fluxes are also estimated from changes in k and na the sheared granitic rocks locally have 18o values as low as 0 which is much lower than the 18o values of the adjacent unsheared granites 7 to 9 implying that the fluid which flowed through these shear zones was derived from the surface for the estimated time integrated fluid fluxes the fluids would be able to retain their isotopic signature for many tens to hundreds of kilometres the flow of surface derived fluids into the ductile middle crust with subsequent expulsion upwards through the shear zones may have been driven by seismic activity accompanying the alice springs deformation

element mobility and volumetric strain in brittle and brittle viscous shear zones of the superdeep well ktb germany abstract mass balance studies of brittle and brittle viscous shear zones of the superdeep well ktb germany show that element mobility and associated volumetric strain is markedly different in metabasites and paragneisses shear zones in metabasites show thickening and volume increase due to mineralization of prehnite epidote calcite within open fractures and pore space gains in al 2 o 3 cao and sio 2 are compatible with these observations shear zones in paragneisses show either constant volume or volume loss volume loss of the paragneiss shear zones can be explained by pressure solution of quartz and by retrograde mica forming reactions associated with significant gains in potassium group elements k rb ba and losses in sio 2 the differences in volumetric strain between paragneiss and metabasite shear zones can be explained by the different deformation mechanisms and rheology of paragneiss and metabasite at t ca 250 350 c the rigid metabasites supported long lived open fractures and pores where new minerals could precipitate from a fluid phase the paragneisses on the other hand were weak because of increasing amounts of crystal plasticity and pressure solution of quartz which is typical for the brittle viscous transition of quartz bearing rocks open fractures and pores were rapidly closed hampering the deposition of new minerals graphite enrichment has been found in both paragneiss and metabasite shear zones there is clear evidence that graphite enrichment results from fluid phases which carried c into the shear zones from external sources suggesting that c was largely mobile during the shearing processes

benthic foraminiferal biogeography in nw european fjords a baseline for assessing future change the seaboard extending from northern svalbard to scotland is the only region of the world where fjords have been comprehensively studied for their live stained benthic foraminiferal faunas these modern faunas provide essential baseline data for the interpretation of the postglacial and continuing environmental changes in those fjords and this is the first biogeographic synthesis the data come from the surface sediment assemblages mainly sampled in the 1990 s from all the available literature due to limited information of shallow water assemblages in the north only the species occurrences in deeper water from below the halocline are considered amongst these only common species species occurring in more than one fjord are included there is a clear pattern of distribution with five groups of taxa 5 widespread species found throughout the region 53 species reaching their northern limit 13 species reaching their southern limit 11 deep sea species 1 recently introduced species although there is an abrupt change in temperature from tanafjorden in northern norway to hornsund in southern svalbard the faunal change from n to s is progressive throughout the investigated region the area of overlap of the northern and southern species corresponds with the previously recognised boundary between the barents sea province and the norwegian coast province based on shelf and upper slope invertebrate macrofaunal benthos and plankton temperature is the main abiotic control on the distributions for the fjords which have shallow sills separating them from the open shelf it is likely that most of the foraminiferal colonisers of the deeper fjord basins are sourced from the shelf or slope via propagules one species has recently been introduced from further south into the southern region probably through the discharge of ballast water from ships the biodiversity of the pristine svalbard fjords extends below what is considered to reflect acceptable ecological status for mainland norway illustrating the need to introduce new methods to determine possible deviations from the reference conditions as defined in the eu s water framework directive wfd 2000 60 ec altogether 347 species have so far been recorded in norwegian waters 214 in fjords 60 above and 180 below the halocline of which 26 occur both above and below the halocline and 266 on the shelf and slope 133 of which also occur in fjords

cold spots during the cenozoic evolution of the eastern alps thermochronological interpretation of apatite fission track data abstract the tectonical and morphological evolution of the eastern alps during the cenozoic is discussed in the light of sedimentary record and thermochronological data of crystalline basement rocks the regional distribution of apatite fission track data in the central eastern alps reveals the differential cenozoic exhumation in contrast to the penninic of the tauern window which has been strongly uplifted and eroded since the early miocene the amount of neogene denudation was much smaller in most areas east of the hohe tauern a sudden change in the regional cooling behaviour is observed at the katschberg fault system which must have been active in post oligocene times within the austroalpine only the schladming tauern have experienced a rather strong denudation the seckau core complex east of the pols line and the area of the northern koralpe cooled to below 100 c at the end of the cretaceous and in the early eocene respectively and have never been heated again during the cenozoic these parts of the austroalpine nappe pile were always near to the surface and therefore may be summarized under the term cold spots a major post cretaceous thickening of the austroalpine can be excluded because it would have produced more uplift and erosion any remnants of miocene or older landforms are certainly not preserved in the area of the tauern window the uppermost planation surfaces in the crystalline basement at the western border of the styrian basin koralpe probably developed in the late miocene

linking of regional and local hydrothermal systems in the mid crust by shearing and faulting abstract the concentration of mass during regional metamorphism and metallogenesis requires interactions between regional and local hydrothermal systems that are scale and time dependent during regional low to medium temperature metamorphism in which fluid flow is driven by broad thermal or mechanical gradients fluid flow rates are on average 8 m s and are limited by restricted permeabilities and gentle gradients in hydraulic head in contrast coseismic fluid flow rates in major fault zones may reach 10 2 m s as a consequence there may be a limited capacity for individual faults to substantially control fluid flow in the region because of the large contrast in flow rates and permeabilities between faults and low permeability rocks to produce a major epigenetic ore deposit in overpressured crust fluid flow systems with such diverse flow rates must interact in such a way that metal dissolution and transport from regional fluid flow systems culminate in focused fluid flow with metal deposition this is best achieved through two steps one in which km scale hydrothermal systems channel into 100 m scale ductile shear zones largely by reaction and deformation enhanced permeability in the shear zone and a second one in which rapid fluid pressure changes accompanying brittle faulting or intense shearing within the broader shear zones causes metal deposition and ore genesis

gold mineralization without quartz veins in a ductile brittle shear zone macraes mine otago schist new zealand greenschist facies schist which hosts the macraes mine in east otago new zealand has been pervasively altered by post metamorphic lower greenschist facies fluids over a 120 m thick section perpendicular to foliation metamorphic titanite has been replaced by rutile and epidote has been replaced by a variety of metamorphic minerals including siderite chlorite muscovite and calcite the early stages of this alteration occurred during development of a ductile cleavage associated with kilometre scale recumbent folding the cleavage was widely overprinted by a subparallel set of spaced mm scale microshears which are locally enriched in rutile and hydrothermal graphite strain was then concentrated into narrow m scale zones where more intensely deformed portions of the rock are crossed and highly disrupted by closely spaced 100 m scale microshears the highly strained rocks show a combination of mylonitic and cataclastic microstructures including crystal plastic grain size reduction and recrystallization of micas to form a new foliation muscovite has grown at the expense of albite in the mylonitic cataclasites hydrothermal alteration was accompanied by addition of pyrite arsenopyrite and gold without development of quartz veins gold precipitated with sulphides during reduction of the fluid by hydrothermal graphite the whole altered rock sequence was later cut sporadically by mesothermal quartz veins which contain gold scheelite rutile pyrite and arsenopyrite this deposit displays a continuum of post metamorphic processes and hydrothermal fluid flow which occurred during uplift of the schist belt

breakdown of feldspar volume gain and lateral mass transfer during mylonitization of granitoid in a low metamorphic grade shear zone abstract in a low metamorphic grade shear zone from quadrilatero ferrifero southeastern brazil granitoid rocks have been transformed into phyllonites and mylonites via activation of crystal plastic processes in quartz and fluid assisted reaction softening in feldspars quartz deformed by basal a and prism a slip with concurrent subgrain rotation recrystallization breakdown of matrix plagioclase an 22 35 via mica producing softening reactions required cationic exchanges between ca na and k forming replacement perthites in the k feldspar megacrysts which occupy around 60 of the original rock volume subsequent breakdown of the perthitic plagioclase an 12 15 led to disruption of the megacrysts enabling strain accommodation that localized the mylonite and phyllonite zones domainal volume and mass balance calculations based on major and minor elements indicate only small positive changes in mass and volume around 15 in the whole system however accentuated differences ranging from mass and volume losses of 25 and 35 at the margins to gains of 85 and 95 in the centre respectively exist between the individual longitudinal subzones occupied by the different tectonite types this scenario indicates that deformation proceeded as a nearly isochemical process although with lateral mobility of major elements principally si al and k within the shear zone these results are different from those previously reported for other granitic mylonites most of them indicating volume losses in compressional tectonic settings it is suggested that the extensional tectonic framework attributed to the moeda bonfim shear zone where mass volume losses are in principle not required enhanced the access of fluid and consequent transport of components into the shear zone principally fe si and k producing the small gains of mass and volume observed

the bronzewing lode gold deposit western australia p t x evidence for fluid immiscibility caused by cyclic decompression in gold bearing quartz veins abstract the bronzewing lode gold deposit is located in the yandal greenstone belt in the yilgarn craton of western australia gold mineralization is hosted in tholeiitic basalt that is metamorphosed to greenschist facies individual ore bodies are controlled by a complex gold bearing quartz vein system that comprises shear and extension veins formed during a progressive d2 deformation event gold is localized in quartz along fractures and deformed grain boundaries and is interpreted to have formed late in the formation of the veins detailed petrography and microthermometry on primary pseudosecondary and secondary fluid inclusions trapped in gold bearing shear and extension veins revealed five types of fluid inclusions 1 co2 ch4 h2o nacl inclusions of variable salinity 0 1 to 17 5 eq wt nacl containing between 10 and 99 mol co2 and molar volumes that range from 22 to 76 cm3 2 h2o nacl inclusions of variable salinity 0 4 to 22 1 eq wt nacl 3 co2 ch4 inclusions with up to 58 mol ch4 and molar volumes between 54 and 73 cm3 4 ch4 h2o inclusions with ch4 ranging from 80 to 90 mol and 5 ch4 inclusions with low molar volumes of 19 to 23 cm3 types 1 2 and 3 constitute a fluid inclusion assemblage that occurs consistently in primary pseudosecondary and secondary trails and clusters within the gold bearing quartz vein system these co existing fluids are interpreted to have formed by fluid immiscibility of a low salinity homogeneous parent fluid at about 300 c and 1500 bars locally type 2 fluid inclusions exhibit total homogenization temperatures that are on average 100 c less than the co genetically trapped type 1 aqueous carbonic inclusions this discrepancy is interpreted to have involved co2 effervescence in response to fluid pressure fluctuations types 4 and 5 fluid inclusions are rare and are only present locally in secondary trails and clusters in extension veins significant pressure fluctuations but relatively constant homogenization temperatures for types 1 2 and 3 fluid inclusions suggest that cyclic decompression of the hydrothermal fluids due to seismic activity along the shear zones that host the gold bearing veins triggered fluid immiscibility the process of fluid immiscibility and subsequent lowering of gold solubility is interpreted to be the most efficient precipitation mechanism for gold in the d2 shear zone hosted vein system at bronzewing

a new estimate of the conditions for transition from basal a to prism c slip in naturally deformed quartz abstract quartz c axis fabrics of naturally deformed quartz in metacherts from the ryoke metamorphic belt in the yanai district southwestern japan suggest that the prism c prism a and basal a slip systems were activated during the plastic deformation under high temperature metamorphic conditions with increasing temperature quartz c axis fabric patterns change from girdle type through x maximum with a faint crossed girdle to x maximum the dominant orientation of subgrain boundaries in the quartz also changes from prism to basal with increasing temperature closely correlated with the change in quartz c axis fabric the fabric transition is also accompanied by an abrupt increase of recrystallized quartz grain size moreover observations with transmission electron microscopy tem clearly identify the dominant activation of c slip through the presence of basal subgrain boundaries and free dislocations with the 0001 burgers vector in the quartz from the metachert samples exhibiting x maximum c axis fabric based on these observations it is concluded that the mechanism switch from basal a to prism c slip systems occurred with increasing temperature in the metacherts the transition temperature is petrologically estimated to be ca 550 600 c at the natural strain rate

nucleation of ductile shear zones in a granodiorite under greenschist facies conditions neouvielle massif pyrenees france abstract the problem of ductile shear zone nucleation under greenschist facies conditions is approached from the example of small scale shear zones developed in the neouvielle granodioritic pluton pyrenees france by means of field and microstructural observations and chemical analyses these shear zones are not related to pre existing fractures and exhibit networks of numerous conjugate fairly parallel and regularly spaced centimetre scale brittle ductile shear zones involving diffuse localization mechanisms although the mode of deformation depends on the minerals present deformation in the shear zones is basically controlled by hydration processes hydration and consequent fluid controlled alteration and deformation are related to fluid migration towards developing cracks all these cracks are extensional and formed on the scale of at most a few grains and frequently single grains a comparison of chemical compositions of undeformed and sheared granodiorite shows that the shear zones can be interpreted as isochemical and isovolumetric systems we propose a sequence of mechanisms by which the nucleation of a small scale brittle ductile shear zones spreading out within a granodioritic rock may occur in these mechanisms shear zone nucleation occurs independently of pre existing fractures and results from the heterogeneous character of the polymineralic rock in the studied granodiorite the mineral heterogeneities favour focusing of locally derived fluids by processes involving grain scale hydraulic fracturing fluid focusing generates instabilities by local softening and subsequently shear zone nucleation

fluid channelling during ductile shearing transformation of granodiorite into aluminous schist in the tauern window eastern alps ductile shearing in the core of the tauern window austria transformed metagranodiorite into si undersaturated garnet chlorite staurolite schist at a depth of c 35 40 km during the alpine orogeny four distinct zones have been recognized extending from the wallrock into the centre of the shear zone zone i unaltered metagranodiorite with subordinate amphibolite zone ii biotite white mica garnet schist zone iii biotite phengite schist zone iv quartz absent garnet chlorite staurolite schist with garnets up to 10 cm across whole rock analyses show a dramatic decrease in sio2 from 65 wt in zone i to 35 wt in zone iv ca na and sr also decrease across the shear zone whereas al ti fe mg p cr ni zn and rb all increase towards zone iv mass balance calculations indicate that shearing was accompanied by up to 60 volume loss near the centre of the shear zone comparison of the tauern window samples with other shear zones in granitic hosts indicates that silica loss accompanied by gains in mg fe and ti is typical for volume loss shear zones but is distinctly different from the element behaviour exhibited in shear zones that are thought to represent approximately isovolumetric behaviour in the samples studied here volume loss appears to have resulted from channellized fluid flow during shearing producing time integrated fluid fluxes of 108 cm3 cm 2 in zone iv this large volume of fluid may have originated in part from dehydration of flysch carried beneath the metagranodiorites during eocene movement on the north penninic subduction zone development of an inverted thermal gradient during subduction would have allowed the fluid to scavenge large amounts of silica from the shear zone during ascent and heating

repeated hydrothermal quartz crystallization and cataclasis in the bavarian pfahl shear zone germany abstract field and microstructural data of the pfahl shear zone in north eastern bavaria germany reveal the intimate spatial temporal connection between fragmentation fluid influx and quartz crystallization these processes and their interaction led to complex structured quartz units i a dense network of early quartz veins ii two domains of fine grained reddish to grayish quartz masses iii an extended central zone of massive white quartz and iv late cross cutting closely spaced parallel fractures and partly open quartz veins the fine grained quartz domains result from repeated and coeval cataclasis fluidization and quartz precipitation material transport in these domains is at least partly governed by the flow of mobile fluid quartz particle suspensions the complex internal meso to microstructures of the massive white quartz are generated by repeated processes of fragmentation and grain growth in general the brittle part of the pfahl shear zone represents a key example of cyclic dissolution precipitation and fragmentation on large scale

fluid flow alteration and polysulphide mineralisation associated with a low angle reverse shear zone in the lower palaeozoic of the anglo brabant fold belt belgium abstract in the lower palaeozoic rocks of the brabant massif belgium a recently discovered polysulphide mineralisation is related to a low angle reverse shear zone this shear zone has been attributed to the main early devonian deformation event data from boreholes and outcrops allow a detailed investigation of the alteration pattern and palaeofluid flow along this shear zone macroscopic observations of the mineralogy and quantitative changes in the phyllosilicate mineralogy indicate that this shear zone is characterised by an envelope of intense sericitisation and silicification in addition chloritisation is associated with this alteration the alteration zone may reach a thickness of 250 m ore mineralisation occurred synkinematically and is spatially related to the shear zone the mineralisation consists of pyrite marcasite arsenopyrite pyrrhotite chalcopyrite sphalerite galena stibnite and smaller amounts of tetrahedrite and other sulphosalts it is concentrated in quartz sulphide veins or occurs diffusely in the host rock the mineralising fluids have a low salinity h 2 o co 2 ch 4 nacl kcl composition and a minimum temperature of 250 320 c the 18 o values of quartz vary between 12 3 and 14 5 smow and d compositions of the fluid inclusions in the quartz crystals range from 65 to 35 v smow the d and the calculated 18 o values of the mineralising fluids fall in the range typical for metamorphic fluids and partly overlap with that for primary magmatic fluids the 34 s values between 4 7 and 10 6 cdt fall outside the interval typical for i type magmas important migration of likely metamorphic fluids causing a widespread alteration and a polysulphide mineralisation along a low angle shear zone has thus been identified for the first time in the caledonian anglo brabant fold belt

application of high resolution sequence stratigraphy to northwest hutton field northern north sea implications for management of a mature brent group field the middle jurassic brent group reservoirs in northwest hutton comprise shallow marine marginal marine and coastal plain reservoir sandstones of both channelized and sheet geometries the connectivity and production characteristics of these sandstones can be predicted through the application of high resolution sequence stratigraphic correlation techniques the broom formation overlies a basal brent sequence boundary and is a lowstand to transgressive systems tract tidal estuarine complex the overlying rannoch formation comprises shoreface sands that represent a highstand systems tract the overlying etive formation is a multilayer fluvial to estuarine channelized system of near fieldwide development interpreted as incised valley fills the superjacent ness formation is genetically related to the etive and is the main oil bearing interval over much of the field the lower ness comprises mouth bar parasequences and is interpreted to be a transgressive systems tract to the etive lowstand the upper ness is a nonmarine delta plain succession interpreted as a highstand systems tract upper ness channel sands typically correlate between several wells and are oriented west east an interval of widespread stacked channels indicates a change in drainage orientation and is related to a high frequency relative sea level fall the uppermost reservoir unit is the tarbert formation related to a further major sequence boundary original repeat formation tester data support a stratigraphically well connected reservoir model compartmentalization exists and is due to stratigraphic layering laterally persistent mudstones creating low vertical permeability and variable dimensions of channels and valley fills

numerical constraints on rates of metamorphism fluid production and fluid flux during regional metamorphism one dimensional numerical models of regional metamorphism were constructed in order to constrain rates of metamorphism rates of fluid production and fluid flux as a function of time and position in thickened continental crust four end member numerical experiments result from combinations of sawtooth single thrust versus homogeneous crustal thickening geometries and continuous versus discontinuous metamorphic reactions that release up to 5 wt volatiles from the lower half of the thickened crust metamorphic reactions consume substantial amounts of heat and retard the thermal evolution of thickened continental crust by several million years peak rates of metamorphism defined as the rate of advance of the 400 c isotherm range from 1 4 to 5 7 mm yr 1 in the saw tooth models and from 0 5 to 0 6 mm yr 1 in the homogeneous models maximum rates of fluid production range from 90 mg fluid m 3 yr 1 in the sawtooth discontinuous reaction model to 6 mg fluid m 3 yr 1 in the homogeneous continuous reaction model fluid production in the sawtooth models produce a maximum fluid flux of 0 2 kg fluid m 2 yr 1 out of the 40 km thick lower plate the maximum fluid flux in the homogeneous models are lower 2 yr 1 fluid fluxes resulting from all models require significant focusing into channels spaced greater than 10 km apart in order to perturb the overlying thermal structure of the continental crust in these numerical experiments most fluid flow infiltrates overlying rocks at temperatures substantially below peak metamorphic temperatures suggesting that strongly focused or multi pass fluid circulation systems may be required to explain reported high fluid rock ratios

lateral variations in mylonite zone thickness as influenced by fluid rock interactions linville falls fault north carolina abstract over a distance of approximately 20 km along strike the linville falls mylonite varies in thickness from 1 m at linville falls to 60 m at banner elk along strike pressure temperature and displacement variations are minimized allowing this study to focus on the influences of fluid behavior and protolith mineralogy on fault zone development the protolith at linville falls contains mainly k feldspar perthite and quartz while at banner elk the protolith contains plagioclase and quartz at linville falls quartz deformed by dynamic recrystallization feldspar by intragranular fracturing and alteration to quartz and mica and mica by sliding along cleavage planes modal mineralogies change from the protolith to the mylonite with quartz decreasing from 39 to 19 and feldspar from 59 to 1 5 muscovite increases from mass balance calculations based on major and trace element geochemistry indicate approximately 75 volume loss at linville falls and 20 at banner elk fluid rock ratios estimated from the calculated depletions of si are an order of magnitude higher at linville falls than at banner elk fluids infiltrated the fault zone over a thicker zone at banner elk than at linville falls because the plagioclase altered more readily than k feldspar creating new pathways for fluids fluids migrated preferentially through channels along the fault zone creating a three dimensional network of higher fluid flow

calcite twins their geometry appearance and significance as stress strain markers and indicators of tectonic regime a review twin lamellae are a widespread feature in calcites of any type and origin twinning in calcite is an important intracrystalline deformation mechanism with low critical resolved shear stress and virtually no temperature dependence in contrast to most slip systems in rock forming minerals twinning therefore permits intracrystalline deformation at very low temperature and very little confining pressure twinning alone however cannot lead to large strains because only one independent slip system is effectively available twins lead to discontinuous shearing on the grain scale which gives rise to considerable strain incompatibilities at grain boundaries twinning consequently leads to strain hardening detailed observations of the geometry of twin lamellae show that incompatibilities at grain boundaries are eliminated by pressure solution on the grain scale at low temperature and dynamic recovery processes at higher temperatures twins are found in various tectonic environments and record very small strains in rocks deformed under very little cover different twinning strain and twinning stress methods and palaeopiezometers are critically reviewed all methods are limited to strains of less than 15 shortening or extension by twinning twinning nevertheless is an important although not directly measurable contribution to the total strain in highly strained calcite rocks at least up to middle greenschist facies under which conditions it is the most effective contributor to the formation of strong lattice preferred orientations in calcite tectonites the appearance of calcite twin lamellae changes systematically as a function of deformation temperature micro twins and straight narrow 1 5 m but fewer twins are developed above approximately 200 c curved twins twinned twins and completely twinned grains indicate the progressive importance of other slip systems and larger intracrystalline strains are possible at ca 250 c and above ancient straight twin lamellae are modified into irregular geometries by recrystallization and grain boundary migration the appearance of calcite twins is proposed as an approximate but rapid and easy to use geothermometer in low temperature environments

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred

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

origin and incremental evolution of brittle ductile shear zones in granitic rocks natural examples from the southern abitibi belt canada abstract metre scale shear zones developed in the mooshla granitic pluton exhibit a plethora of internal mesoand microstructures revealing their mode of nucleation growth and termination the subvertical e w brittle ductile shear zones are developed in the most isotropic part of the intrusion and consist of phyllonites characterized by mylonitic fabrics and well developed down dip mineral lineations they are exposed on a single large and flat outcrop perpendicular to the mineral lineation these brittle ductile shear zones are in close spatial association with a fracture system along which they have nucleated and propagated joints fractures and brittle faults contain abundant evidence of ductilely deformed and recrystallized minerals suggesting that they underwent ductile shearing after their formation the deformation was accompanied by the influx of a metasomatic fluid which altered original fe mg bearing minerals and plagioclase into quartz epidote chlorite sericite and carbonate that precipitated in dilatant fractures and narrow breccia zones we recognize two different types of lateral terminations i shear zones that terminate into joints and fractures that are parallel or oblique to the shear direction and ii curved segments of shear zones that connected with neoformed splay fractures at high angle to the shear direction in the latter case sharp bending and severe perturbation of the shear zone orientation occur at the shear zone termination where ductile shearing is transfered into an adjacent shear zone via dilatational fractures this demonstrates that shear zones tended to grow towards each other and coalesce to form a well developed anastomosing network we propose that narrow dilatational fractures and joints acted as paleo weakness planes along which fluid rock interaction and resulting reaction softening occurred the deformation scenario involves episodic fluid drawn into dilatant sites responsible for cyclic fluid pressure fluctuation in the system dilatancy related fracturing and reaction softening lead to deformation under brittle to plastic conditions and to shear strain localization