mechanisms of flexural flow folding of competent single layers as evidenced by folded fibrous dolomite veins abstract flexural flow is thought unlikely to occur in naturally deformed competent isotropic single layers in this study we discuss a particular case of folded bedding parallel fibrous dolomite veins in shale in which the internal strain pattern and microstructural deformation features provide new insights in the mechanisms enabling flexural flow folding strain in the pre folding veins is accommodated by two main mechanisms intracrystalline deformation by bending and intergranular deformation with bookshelf rotation of dolomite fibres the initially orthogonal dolomite fibres allowed a reconstruction of the strain distribution across the folded veins this analysis shows that the planar mechanical anisotropy created by the fibres causes the veins to approximate flexural flow during folding synkinematic veins overgrow the pre folding fibrous dolomite veins microstructures and dolomite growth morphologies reflect growth during progressive fold evolution with evidence for flexural slip at fold lock up homogeneous flattening as evidenced by disjunctive axial planar cleavage subsequently modified these folds from class 1b to 1c folds our study shows that the internal vein fabric has a first order influence on folding kinematics moreover the fibrous dolomite veins show high viscosity contrasts with the shale matrix essential in creating transient permeability for subsequent mineralising stages in the later synkinematic veins during progressive folding

pb isotope fingerprinting of mesothermal gold deposits from central victoria australia implications for ore genesis new pb isotope data from three major mesothermal lode gold deposits ballarat west tarnagulla maldon in central victoria support a model whereby the metals derived from a large reservoir with a long residence time in the crust below the palaeozoic lachlan fold belt the pb isotopic ratios of least radiogenic samples from these deposits are in close agreement with published pb signatures for turbidite hosted gold deposits and for devonian granites elsewhere in the lachlan fold belt despite their spatial distribution and variations in the geological setting the pb signatures point to the extraction and transport of metals from a crustal source area by long lasting large scale hydrothermal systems resulting in the prominent homogenisation of pb isotopic ratios the enduring interaction between large hydrothermal systems and an extensive crustal source reservoir were a vital pre requisite in the formation of the victorian gold province in this regard the prospectivity of victoria is analogous to world class ore provinces elsewhere such as the archaean yilgarn block in western australia

regional syndeformational copper mineralization in the western mount isa block australia the recently proposed syndeformational model for copper mineralization at mount isa has tremendous implications for metallogenesis and exploration and hence a regional assessment of its validity was made detailed studies on the structural setting timing and microstructural development of four brecciated sediment hosted copper deposits situated northwest of mount isa plus the assessment of many small prospects in the same area reveal that all primary copper mineralization is strikingly similar and involves syndeformational syn d 3 veining and alteration variation in and between deposits can be attributed to their host rock lithologies and local structural relationships silica dolomite replacement as recognized at mount isa mine is associated with all copper mineralization in dolomitic host rock sulfide chlorite replacement in arenaceous host rock for example at the mammoth deposit occurs concomitantly with silica dolomite alteration in other deposits that comprise both host rock types the matching alteration history of the deposits indicates a common origin for the mineralization and together with a large number of similar small sized copper occurrences suggests that a major regional copper mineralizing event occurred during the third deformation phase throughout the western mount isa block local structural relationships common to several deposits such as the presence and degree of bedding discordance across low angle d 1 faults together with later d 2 folding may constitute suitable structural traps for subsequent d 3 mineralization therefore the structural geometry together with the alteration features is essential in exploration and assessment of prospects

fluidized breccias a record of brittle transitions during ductile deformation abstract unusual breccias of caledonian age are described in relation to the tectonic and metamorphic history of their greywacke sandstone and siltstone parent rocks the variety of field and textural relationships displayed by the breccias indicate a combination of dilational and non dilational components in a fluidized system of breccia development the velocity of the escaping fluid phase and the viscosity of the fluidized suspension are strongly influenced by competency controls due to their finer grained nature the pelite based breccias allow a greater mobility of the fluid phase and locally record a turbulent expanded bed stage of the fluidized system however the sandstone based breccias lacking the intricate flow patterns retain a replacive non dilational fracture network the breccias occur in a zone of intense d2 deformation the age relationships of the breccias indicate a repeated pattern of brecciation with syntectonic temporal and partly genetic affinities to the s 2 cleavage development the syntectonic dilational elements involving boudinage and hydraulic fracture are coupled with intense pressure solution and conjugate cleavage development metamorphism to lower greenschist facies is synchronous with deformation and brecciation a focussing of the metamorphic fluid phase within the breccia zones is indicated contributing the non dilational components of the brecciation process a simple shear model of the d2 deformation within this zone is proposed the orientation of the breccia zones suggests that their localization is determined by tensional components within the overall d2 stress field the cyclical pattern of the brecciation during the d2 deformation is considered to represent rapid brittle transitions during the ductile deformation stratigraphie controls on the generation of the increased fluid pressures are identified the presence of an impermeable barrier facilitating the necessary conditions for the excess fluid pressures to accumulate is indicated it is suggested that the brecciation process is fundamentally controlled by a seismic pumping phenomenon following periodic shear failure on a major basement fault at depth

the multiple inverse method applied to meso scale faults in mid quaternary fore arc sediments near the triple trench junction off central japan the multiple inverse method for fault slip data is applied to meso scale faults observed in mid quaternary fore arc sediments near the triple trench junction oa the boso peninsula japan data from the otadai umegase and kokumoto formations were processed and three stresses were obtained as significant solutions vertical axial compression and triaxial stresses with the s3 axis in wnw ese and nne ssw directions the triaxial stresses were determined from the otadai and umegase data however the wnw ese tensile stress is not detected from the youngest kokumoto suggesting that the stress is older than the formation the area was subject to a wnw ese tensile stress 01 2 1 0 ma but tensile direction changed to a ne sw trend thereafter the succession was simultaneous with a tectonic event in the landward slope of the sagami trough suggesting that the subduction of the philippine sea plate aaected the stresses in the overriding plate the older tensile stress was probably a manifestation of the gravitational collapse of the hayama mineoka ridge which was growing parallel to the trough the inferred stress history is concordant with the variation of plate convergence at the trough 2000 elsevier science ltd all rights reserved

oxide and sulphide minerals in highly oxidized rb depleted archaean granulites of the shevaroy hills massif south india oxidation states and the role of metamorphic fluids the shevaroy hills of northern tamil nadu southern india expose the highest grade granulites of a prograde amphibolite facies to granulite facies deep crustal section of late archaean age these highly oxidized quartzofeldspathic garnet charnockites generally show minor high tio2 biotite and amphibole as the only hydrous minerals and are greatly depleted in the incompatible elements rb and th peak metamorphic temperatures garnet orthopyroxene and pressures garnet orthopyroxene plagioclase quartz are near 750 degrees c and 8 kbar respectively pervasive veinlets of k feldspar exist throughout dominant plagioclase in each sample and show clean contact with orthopyroxene they are suggested to have been produced by a low h2o activity migrating fluid phase under granulite facies conditions most likely a concentrated chloride carbonate brine with high alkali mobility accompanied by an immiscible co2 rich fluid silicate oxide and sulphide mineral assemblages record high oxygen fugacity pyroxenes in the felsic rocks have high mg mg fe 0 5 0 7 the major oxide mineral is ilmenite with up to 60 mole per cent exsolved hematite utilizing three independent oxygen barometers ferrosilite magnetite quartz ferrosilite hematite quartz and magnetite hematite in conjunction with garnet orthopyroxene exchange temperatures samples with x hm ilm 0 1 yield a consistent oxygen fugacity about two log units above fayalite stability less oxidized samples x hm ilm 0 4 pyrite is the dominant sulphide and pyrrhotite is absent pyrite grains in these samples have marginal alteration to magnetite along the rims signifying a high temperature oxidation event moderately oxidized samples 0 1 x hm ilm 0 4 have abundant co existing pyrrhotite pyrite and magnetite the most reduced granulite samples have pyrrhotite as the dominant sulphide with little or no pyrite and no coexisting magnetite chalcopyrite is a common accessory mineral of pyrite and pyrrhotite in all the samples textures in some samples suggest that it formed as an exsolution product from pyrrhotite extensive vein networks of magnetite and pyrite associated principally with the pyroxene and amphibole give evidence for a pervasive highly oxidizing fluid phase thermodynamic analysis of the assemblage pyrrhotite pyrite and magnetite yields consistent high oxidation states at 700 800 degrees c and 8 kbar the oxygen fugacity in our most oxidized pyrrhotite bearing sample is 10 12 65 bar at 770 degrees c there are strong indications that the shevaroy hills granulites recrystallized in the presence of an alkali rich low h2o activity fluid probably a concentrated brine it cannot be demonstrated at present whether the high oxidation states were set by initially oxidized protoliths or effected by the postulated fluids the high correspondence of maximally rb depleted samples with the highest recorded oxidation states suggests that the rb depletion event coincided with the oxidation event probably during breakdown of biotite to orthopyroxene k feldspar we speculate that these alterations were effected by exhalations from deep seated alkali basalts which provided both heat and high oxygen fugacity low a h2o fluids it will be of interest to determine whether greatly rb depleted granulites in other precambrian terranes show similar highly oxidizing signatures

inferred glacio eustatic control for high frequency depositional sequences of the plio pleistocene kazusa group a forearc basin fill in boso peninsula japan abstract the plio pleistocene forearc basin fill of the kazusa group as much as 3000 m thick is represented by a series of marine environments ranging from deep sea basin plain to shallow sea it developed during about 2 4 to 0 45 ma and is well exposed in the boso peninsula japan chronostratigraphic resolution of the kazusa group is partly within about 0 05 m y based on the combination of fission track dating of volcanic ash layers and bio and magneto stratigraphy many volcanic ash layers are intercalated in the kazusa group and provide distinct chronostratigraphic surfaces within the heterogeneous lithologies detailed outcrop studies on depositional facies discontinuities and lateral terminations of volcanic ash layers allow high resolution sequence stratigraphic interpretation of the kazusa group seventeen depositional sequences were identified in the kazusa group each depositional sequence was tentatively correlated with the stable oxygen isotope sea level index integrated with the age framework of the kazusa group this correlation indicates that frequencies of the depositional sequences can be estimated to be in the range of 0 2 0 02 m y and are within fourth and fifth order stratigraphic cyclicities depositional sequences constitute third order transgressive and highstand sequence sets and the kazusa group is interpreted to represent a third order composite sequence with a duration of about 1 95 m y development of the third order composite sequence was also controlled by an overall rise and fall of glacio eustatic sea levels overprinted by tectonic events related to fluctuations in oceanic plate motion which occurred at about 2 6 ma and 0 5 ma respectively

shear sense a new approach that resolves conflicts between criteria in metamorphic rocks investigation of microstructural relationships in major movement zones in metamorphic rocks where the sense of displacement is known from regional geological relationships indicates numerous problems with current concepts of shear sense criteria and their application the direction of apparent shearing commonly conflicts from one criterion to another e g from the symmetry of quartz c axis orientation diagrams to the asymmetry of extensional crenulation cleavages this implies that interpretations of shear sense along foliations from some mesoscale and microscale criteria have been erroneous a new approach to interpreting shear sense involving the use of strain fields resolves conflicts in mesoscopic and microscopic criteria and provides a method for determining coherent shear sense histories extending back before the last shearing event for any foliated metamorphic rock it also provides a powerful tool for determining the structural metamorphic path that a rock has followed within an orogen for determination of the shear sense on the last foliation developed in a rock this approach uses geometries developed around competent heterogeneities such as quartz pebbles pegmatite pods veins porphyroclasts porphyroblasts and breccia clasts a shear sense history is derived by applying this approach to earlier foliations preserved within the heterogeneities and their strain shadows

multicomponent magnetization in western dolpo tethyan himalaya nepal tectonic implications abstract a palaeomagnetic study has been carried out in the tethyan himalaya th the northern margin of greater india twenty six palaeomagnetic sites have been sampled in triassic low grade metasediments of western dolpo two remanent components have been identified a pyrrhotite component characterized by unblocking temperatures of 270 335 c yields an in situ mean direction of d 191 7 i 30 9 k 29 5 95 5 7 n 23 sites the component fails the fold test at the 99 confidence level kin situ kbed 6 9 and is therefore of postfolding origin for reason of the low metamorphic grade this pyrrhotite magnetization is believed to be of thermo chemical origin geochronological data and inclination matching indicate an acquisition age around 35 ma the second remanence component has higher unblocking temperatures 400 c and up to 500 580 c range and resides in magnetite a positive fold test and comparison with expected triassic palaeomagnetic directions suggest a primary origin the postfolding character of the pyrrhotite component and its interpreted age of remanence acquisition implies that the main himalayan folding is older than 35 ma in the western dolpo area this study also suggests that the second metamorphic event neo himalayan was more significant in the dolpo area than the first eo himalayan one a clockwise rotation of 10 15 is inferred from the pyrrhotite component which is compatible with oroclinal bending and or rotational underthrusting models this rotation is also supported by the magnetite component indicating that no rotation of the tethyan himalaya relative to india took place before 35 ma

archean gold bearing quartz veins at the sigma mine abitibi greenstone belt quebec part i geologic relations and formation of the vein system the sigma deposit consists of an archean gold bearing vein system in metavolcanic rocks intruded by an irregular body of porphyritic diorite both affected by regional deformation and in turn cut by younger undeformed feldspar porphyry dikes the veins cut all rock types and are composed chiefly of quartz and tourmaline with minor amounts of carbonates pyrite chlorite scheelite and free gold they were emplaced during or after a greenschist facies metamorphism postdating the intrusion of feldspar porphyry dikes the veins were not significantly affected by any subsequent metamorphism or tectonism two principal types of veins occur in the mine subvertical veins and subhorizontal veins they are contemporaneous and cogenetic subvertical veins occur within steeply dipping east west trending ductile shear zones along which reverse subvertical displacements have taken place the veins occupy openings created by overriding of undulations and irregularities of foliation and slip planes during progressive shearing movements within the shears subhorizontal veins occupy extensional fractures which developed between ductile shear zones preferentially in the more competent host rocks these veins were formed by one or several episodes of open space filling geologic relations indicate that the mineralized veins were emplaced progressively in a dynamic tectonic environment during and shortly after the formation of ductile shear zones a structural analysis has shown that the ductile shear zones and the two vein types can be related to a single strain ellipsoid of the following orientation the z axis trending north south and horizontal the y axis plunging 10 degrees west and the x axis plunging 80 degrees east the deformation responsible for the generation of ductile shear zones and emplacement of the veins is ascribed to a late north south trending compression postdating regional deformation

integrated model for meguma group lode gold deposits nova scotia canada meguma group lode gold deposits represent concentrations of a variety of auriferous quartz veins in lower paleozoic metaturbidites of the meguma terrane of southern nova scotia our studies incorporating extensive field regional and detailed mapping and laboratory petrography fluid inclusions geochemistry isotopes studies of 30 of these deposits in the eastern meguma terrane suggest that mineralization occurred at ca 370 ma 40 ar 39 ar dating of hydrothermal vein minerals during widespread incursion of mafic and felsic magmatism into the crust concurrent with subvertical shear zone development thus postdating regional acadian deformation by some 30 40 m y isotopic data s c o sr indicate that vein forming fluids were not solely magmatic and were in part exotic to the meguma group thus earlier magmatic and metamorphogenic lateral secretion models that considered either late stage magmatic fluids or the enveloping meguma group metasedimentary rocks as the sole source of vein components are invalidated

alpine metamorphism of calcareous metasediments in the western hohe tauern tyrol mineral equilibria in cohs fluids during tertiary regional metamorphism in the western hohe tauern reaching maximum p t conditions around 6 kb 550 c in calcareous metasediments reaction of pyrite to pyrrhotite is suggested by regional distribution and textural relations in rocks without graphite pyrite is common at all metamorphic grades in graphite bearing rocks however the dominant fe sulfide is pyrite at lower grade and pyrrhotite at higher grade furthermore in graphite bearing high grade rocks pyrite is restricted to assemblages with mg rich silicates several factors control pyrite pyrrhotite relations increase of temperature is most effective by increase of pyrrhotite vs pyrite stability field shift of silicate sulfide reactions toward the stability field of pyrrhotite creation of sulfur free fluids from devolatilization reactions and increase in the proportions of sulfur bearing fluid species presence of graphite also favours progress of pyrite to pyrrhotite reaction as shown by different f s 2 stabilities and changes in the amount of minerals and fluid during metamorphic heating of graphite bearing and graphite free assemblages an opposite effect is shown by assemblages with low fe contents in fe mg silicates due to the enlarged stability field of such minerals with increasing mg and f content another inhibition of pyrite to pyrrhotite reaction is suggested to be due to relatively high sulfur contents of h2o rich infiltrating fluids

depletion and enrichment zones in the bendigo gold field a possible source of gold and implications for exploration a regional lithogeochemical survey in the bendigo region of central victoria australia covering an area of 3 750 km2 and including the giant bendigo gold field and other large regional gold fields such as maldon castlemaine and fosterville was completed in 2004 by ionex pty ltd a total of 142 rock samples were collected and analyzed for gold and associated elements extensive areas having an anomalously low gold content termed depletion zones were detected in the vicinity of known gold fields and constitute a large proportion of the surveyed area the results show that the gold content in the depletion zones is 70 percent below regional background levels the bendigo gold field itself is located within a positive gold anomaly enrichment zone with an average value of 14 ppb in all rock types and an extent of approx 100 km2 to the north is a depletion zone on the order of 700 to 800 km2 with an average gold content of less than 0 5 ppb in all rock types in total the gold enrichment and depletion zones occupy an area of more than 1 000 km2 against a regional background of 0 7 to 4 7 ppb au avg 1 29 ppb if this depletion zone extends to a depth of 2 km the volume of depleted rock would be at least 1 400 km3 and the extent of apparent depletion would be 3 000 to 4 000 tonnes t of gold the quantum of apparent gold depletion at bendigo is on the same order of magnitude as the gold enrichment in the bendigo gold field and associated dispersion haloes 3 000 t au substantial depletion zones are also present adjacent to the other important gold deposits within the survey area the presence of extensive gold depletion zones adjacent to the major gold deposits of central victoria raises the possibility that these rocks were a source of gold in the deposits these findings provide possible new criteria for the appraisal of prospective regions

analysis of fault slip inversions do they constrain stress or strain rate fault slip data commonly are used to infer the orientations and relative magnitudes of either the principal stresses or the principal strain rates which are not necessarily parallel or equal at the local scale of an individual fault the shear plane and slip direction define the orientations of the local principal strain rate axes but not in general the local principal stress axes at a large scale the orientations of p and t axes maxima for sets of fault slip data do not provide accurate inversion solutions for either strain rate or stress the quantitative inversion of such fault slip data however provides direct constraints on the orientations and relative magnitudes of the global principal strain rates to interpret the inversion solution as constraining the global principal stresses requires that 1 the fault slip pattern must have a characteristic symmetry no lower than orthorhombic 2 the material must be mechanically isotropic and 3 there must be a linear constitutive relationship between the global stress and the global strain rate isotropic linear elastic constitutive equations are appropriate to describe the local deformation surrounding an individual slip discontinuity fault slip inversions however constrain the characteristics of a large scale cataclastic flow which is described by constitutive equations that are probably but to an unknown degree anisotropic and nonlinear such material behavior would not strictly satisfy the requirements for the stress interpretation thus at the present state of knowledge fault slip inversion solutions are most reliably interpreted as constraining the principal strain rates

formation of massive sulfide lenses by replacement of folds the hercules pb zn mine tasmania the hercules pb zn deposit consists of a number of isolated ellipsoidal massive sulfide pods hosted in a tuffaceous slate unit of middle cambrian age these pods are aligned roughly parallel to the main cleavage and have completely discordant to moderately folded bedding mineralization is localized in the short limb zones of parasitic f 2 folds from the micro to the macroscopic scale antithetic shearing induced along the bedding during d 3 caused localized dilation and fracturing of some of the short limb zones this resulted in alteration and replacement late in d 3 such that sa was overprinted by the mineralization the following paragenetic replacement sequence exists pyrite i quartz chlorite pyrite ii sphalerite galena chalcopyrite pb isotope age data are consistent with the derivation of the deposit from dispersed primary sulfides in the mount read volcanics by synmetamorphic mobilization into structural traps at higher levels in the crust

foliation boudinage control on the formation of the rosebery pbzn orebody tasmania abstract the rosebery pb zn ag au massive sulphide deposit occurs within the middle cambrian mount read volcanics of western tasmania it is hosted by a tuffaceous slate lens that is surrounded by felsic volcanics the volcanic belt and its ordovician cover were deformed and metamorphosed during an early to middle devonian orogeny tabberabberan orogeny during this orogeny d 3 e w shortening was partially accommodated by reverse bedding parallel shearing within the slate unit and immediately underlying volcanic rocks this caused reactivation and intensification of an early foliation s 2 and the development of asymmetric chocolate tablet foliation boudinage structures with boudin necks parallel to the x and y directions the mineralization occurs along extensional shear planes that are associated with foliation boudinage mineralogical zoning in the orebody can be directly related to dilational bends in shears associated with foliation boudin necks it is proposed that these structures gaped during the d 3 deformation allowing infiltration of metalliferous hydrothermal fluids along shear plane and cleavage directions into the fractured wall rock progressive metasomatic replacement of the wall rock resulted in the formation of a sulphide sulphate silicate orebody the orebody is completely discordant to folds in the host rock this observation confirms a structurally controlled origin and refutes earlier interpretations of the deposit as a tightly folded chemical sediment isotope data indicate that the orebody was probaby derived from volcanogenic sulphides within the mount read volcanics the rosebery orebody is best regarded as a metamorphogenic deposit that formed by dissolution upward transport concentration and re precipitation of primary sulphides the proposed macroscopic model complements separate microstructural evidence by the author indicating a syn d 3 timing of mineralization

dissolution solution transfer diffusion versus fluid flow and volume loss during deformation metamorphism dissolution and solution transfer during deformation metamorphism are controlled by the partitioning of deformation into progressive shearing and shortening components progressive shearing is readily accommodated by slip on the planar crystal structure of phyllosilicates and graphite without accumulating dislocation density gradients across grain boundaries progressive shortening is accommodated by the cores of most other minerals including sulphides these minerals develop strain and hence dislocation density gradients on their rims due to progressive shearing along grain boundaries these gradients are particularly large when the mineral abuts phyllosilicate or graphite the resulting chemical potential gradients between the core and rim drive dissolution causing removal of the highly strained grain margins removal of dissolved material by solution transfer is aided by the geometry of shearing of phyllosilicates and graphite around other grains in an active anastomosing foliation interlayers and interfaces on boundaries lying at a low angle to the direction of shearing and oriented relative to the sense of shear such that they can open gape by small amounts water present in these interlayer spaces becomes destructured considerably enhancing diffusion rates along the foliation penetrative volume loss especially in deforming metamorphosing pelitic rocks is large at all metamorphic grades increasing and becoming more penetrative with depth to at least the transition into granulite and eclogite facies transference of material by fluid flow from deep to high levels in the earth s crust is precluded because thousands to tens of thousands of rock volumes of fluid are required necessitating continual recirculation of fluid from shallow to deep crustal levels in one large or several small sets of cells unless some extremely large scale form of fluid channelling is possible reassessment of diffusion mechanisms and hence rates during deformation and pervasive foliation generation in large volumes of rock where fluid channeling cannot provide enough fluid indicates that diffusion can proceed with sufficient rapidity that massive recirculation of fluid is no longer required the amount of fluid can be reduced sufficiently to allow large volume losses by a one way flow of fluid to the earth s surface in deforming metamorphosing environments where the fluid pressure equals or exceeds the hydrostatic pressure deformation partitioning controlled dissolution progressively changes the bulk chemistry of a rock containing phyllosilicates or graphite during deformation metamorphism because matrix minerals other than phyllosilicates and graphite are preferentially removed the large size of porphyroblasts if present tends to preserve them from dissolution hence the bulk chemistry operative during subsequent porphyroblast growth can have changed considerably from that operative when the first porphyroblasts grew in rocks in which bedding is still well preserved

timing of gold mineralisation in western and central victoria australia new constraints from shrimp ii analysis of zircon grains from felsic intrusive rocks abstract age constraints provided by felsic intrusive rocks indicate that gold mineralisation in western and central victoria australia occurred over a prolonged period of time ranging from prior to the late silurian to the late devonian an early phase of gold mineralisation associated with regional deformation pre dated the emplacement of late silurian felsic porphyry dykes at 413 3 ma in the central lode system of the magdala deposit stawell gold mineralisation associated with the final stages of the main regional deformation also pre dated the intrusion of early ca 400 ma to late devonian ca 370 ma plutons at stawell wonga deposit mt bute tarnagulla tallangalook maldon and heathcote in both the stawell pyrenees and ballarat metallogenic provinces consistent with preliminary dating studies of hydrothermal micas from major gold deposits later phases of gold mineralisation throughout western and central victoria were either temporally associated with or post dated the intrusion of late silurian to late devonian felsic dykes examples of this style of mineralisation include the wonga deposit at stawell mafeking fosterville maryborough minor occurrences near linton and the woods point region preliminary constraints from ballarat west indicate that late devonian dykes intruded at ca 386 ma were pervasively altered by interaction with an auriferous fluid similar to that responsible for gold deposition within the guiding star lode but that this was probably related to remobilisation during emplacement of the dykes into a pre existing mineralised structure with the exception of the woods point region and ballarat west these later phases often have a magmatic affiliation indicated by an association with cu mo bi sb and w these complex timing relationships reflect the diachronous nature of deformation metamorphism and magmatism in the western lachlan fold belt of southeast australia and suggest at least two discrete periods of gold mineralisation in the region

how to resolve the controls on mesothermal vein systems in a goldfield characterized by sparse kinematic information and fault reactivation a structural and graphical approach current mine exposures of auriferous veins in the charters towers goldfield ctgf are characterized by a scarcity of kinematic indicators furthermore the dip direction of exposed and historically mined orebodies largely coincides with the orientation of pre existing discontinuities consequently the reactivation of earlier structures during gold deposition is strongly indicated both factors complicate the structural analysis of the auriferous vein systems as slip on earlier faults likely occurred before new rock failure and evolution of fault networks reflecting the stress field present at the time of gold deposition despite access to drillcore open pits and underground workings extracted data were limited to ten occurrences of wear groove striations and three reverse separations of dykes and a pegmatite cut by sulphide bearing quartz veins because of the scarcity of kinematic information the structural analysis focused on the geometry of veins and ore shoots also applying basic graphical and kinematic methods

foliation development and refraction in metamorphic rocks reactivation of earlier foliations and decrenulation due to shifting patterns of deformation partitioning reactivation of early foliations accounts for much of the progressive strain at more advanced stages of deformation its role has generally been insufficiently emphasized because evidence is best preserved where por phyroblasts which contain inclusion trails are present reactivation occurs when progressive shearing operating in a synthetic anastomosing fashion parallel to the axial planes of folds changes to a combination of coarse and fine scale zones of progressive shearing some of which operate antithetically relative to the bulk shear on a fold limb reactivation of earlier foliations occurs in these latter zones reactivation decrenulates pre existing or just formed crenulations generating shearing along the decrenulated or rotated pre existing foliation planes partitioning of deformation within these foliation planes such that phyl losilicates and or graphite take up progressive shearing strain and other minerals accom modate progressive shortening strain causes dissolution of these other minerals this results in concentration of the phyllosilicates in a simi lar but more penetrative manner to the for mation of a differentiated crenulation cleavage except that the foliation can form or intensify on a fold limb at a considerable angle to the axial plane of synchronous macroscopic folds reactivation can generate bedding parallel schistosity in multideformed and meta morphosed terrains without associated folds heterogeneous reactivation of bedding gen erates rootless intrafolial folds with sigmoidal axial planes from formerly through going struc tures reactivation causes rotation or refrac tion of axial plane foliations forming in the same deformation event causing reactivation in those beds or zones in which an earlier foli ation has been reactivated and results in destruction of the originally axial plane foli ation at high strains reactivation also provides a simple explanation for the apparently wrong sense but normally observed rotation of gar net porphyroblasts whereby the external foli ation has undergone rotation due to antithetic shear on the reactivated foliation alter natively the rotation of the external foliation can be due to its reactivation in a subsequent deformation event porphyroblasts with inclusion trails com monly preserve evidence of reactivation of earl ier foliations and therefore can be used to ident ify the presence of a deformation that has not been recognized by normal geometric methods because of penetrative reactivation reac tivation often reverses the asymmetry between pre existing foliations and bedding on one limb of a later fold leading to problems in the geo metric analysis of an area when the location of early fold hinges is essential the stretching lineation in a reactivated foliation can be rad ically reoriented potentially causing major errors in determining movement directions in mylonitic schistosities in folded thrusts geometric relationships which result from reactivation of foliations around porphyroblasts can be used to aid determination of the timing of the growth of porphyroblasts relative to deformation events other aspects of reac tivation however can lead to complications in timing of porphyroblast growth if the presence of this phenomenon is not recognized for example d grown porphyroblasts may be dis solved against reactivated s and hence appear to have grown syn d

the role of thrusting in the structural development of the mount isa mine and its relevance to exploration in the surrounding region resolution and prediction of the structure underground at mount isa from the surface geology is now possible using evidence for north to south movement on large scale thrusts during the first deformation at 1610 ma this evidence for the direction of movement during thrusting is preserved 1 macroscopically within three successive thrust imbricates as the north south alignment of respective truncations of older stratigraphic units by the angular unconformity at the base of the mount isa group 2 macroscopically and mesoscopically as a zone of vertical north south striking foliation with a horizontal stretching lineation that formed on a sinistral transform fault during dalong the wonga duchess belt 3 mesoscop ically as a regionally distributed north south oriented stretching lineation li that is locally well preserved on s0 surfaces in the hinges of folds with north south axial planes that formed during the second regional deformation and 4 microscopically as a horizontal north south aligned mineral elongation lineation defined by inclusion trails preserved in porphyroblasts that grew early during the second deformation adjacent to the wonga duchess belt the swing in strike of a large scale 70o west dipping lateral ramp on the western margin of the large scale kokkalukkanurker duplex from north south parallel to the direction of movement during thrusting to north northwest south southeast caused a portion of the meerenurker roof thrust lying on this lateral ramp to be overturned for at least 50 km near mount isa this overturning resulted from tilting the 70o west dipping lateral ramp through another 70 o as it rode obliquely north south over the north northwest south southeast striking portion of the same lateral ramp on the next imbricate to the south this generated the gross structural relationships seen in three dimensions at mount isa whereby the greenstone basement that truncates mount isa group rocks below the mine overlies them to the west the second deformation involved intense east west shortening and formed macroscopic folds that conveniently expose varying levels of the thrust pile generated during the first deformation in higher grade rocks west of mount isa and to either side of the wonga belt axial plane schistosity development and or reactivation of bedding during this deformation was quite intense and tended to obliterate and or rotate foliations and lineations formed during the first deformation only in the hinges of the folds formed during the second deformation where the deformation was less intense or silicification had occurred are earlier lineations preserved relatively unaffected by this event the geology of the mount isa valley and the rocks to the west of the mine is controlled by a series of north south striking high angle reverse faults that slice through the meerenurker thrust as it crosses the above mentioned lateral ramp on the western edge of the large scale duplex two of the faults mount gordon and mount isa dip steeply west with similar dis placements of 4 to 6 km west side up the other two faults holly and tastoka dip steeply east with the latter fault having a more significant displacement of 2 5 to 3 km east side up that greatly affects the geology on the eastern side of the mount isa valley the effects of these faults on the geometry produced by thrusting during the first deformation and folding during the second allow accurate prediction of the third dimension anywhere in the mount isa valley and the region immediately to the west consequently they can be used to predict the location of structurally controlled sites of potential copper mineralization similar to those found in the mount isa mine where mineralization formed during the third deformation

fluid flow and alkali transport within a metamorphic terrane western north carolina abstract this paper presents preliminary results from study of a metasedimentary terrane near the great smoky mountains of north carolina grade of metamorphism ranges from garnet to staurolite kyanite zone the stratigraphic section is dominated by metasandstone but contains thick pelitic horizons over the central portion anakeesta formation of the studied interval metasandstone beds contain numerous isolated calcite concretions these react to form leucocratic calc silicate rock at variable grade of metamorphism metasandstones themselves are unreactive study of calc silicate reaction permits the following statements regarding fluid flow and alkali transport within the studied terrane within unreacted concretions the typical mineral assemblage is quartz oligoclase muscovite biotite calcite reaction between micas and calcite proceeded through several stages forming as prograde minerals andesine bytownite clinozoisite zoisite garnet hornblende and chlorite reaction occurred in the presence of a fluid phase containing a h2o co2 ratio of 9 and resulted in massive expulsion of na2o k2o and co2 since postulated reactions would have produced on average a fluid phase whose h2o co2 ratio is only one flow of water rich fluid through the concretions accompanied reaction rough estimate suggests a volumetric ratio of external fluid to rock of at least 1 1 the metamorphic grade at which concretions undergo reaction is controlled by stratigraphic position of the host metasandstone relative to metashale within the anakeesta formation and overlying strata extensive reaction occurs near the pelitic staurolite and kyanite isograds within massive metasandstones of the underlying strata reaction is delayed to the middle staurolite kyanite zone 5 km upgrade some reaction occurs within the upper garnet zone but this is restricted to certain thin metasandstone beds interlayered with metashale study of dehydration reactions within metashale demonstrates that calc silicate reaction was coupled to production of h2o rich fluids within pelitic strata transport of h2o rich fluids beyond outcrop scale was by advection diffusion was limited to outcrop distance on most outcrops all isolated concretions show the same degree of reaction but at larger scale separate metasandstone packets bounded by metashale show differing degrees of reaction also all rocks underwent some degree of penetrative deformation during time of reaction the suggestion is made that advective flow was in fact channelized into fractures within metasandstone and that diffusion between fractures promoted reaction within outcrop size volumes of rock overall direction of fluid transport was in part upsection and in part channelized within metasandstone strata downsection flow was limited for the most part alkalis liberated by calc silicate reaction appear to have left the terrane metasandstones do not appear to constitute a sink for alkalis and no sink of any sort was found for sodium potassium metasomatism of pelitic rock did occur and is marked by conversion of chlorite to biotite but such k deposition is limited to sections within which metashale and calc silicate rock are within outcrop distance no sink is evident for most liberated potassium it is within conjecture that the liberated elements caused alkali metasomatism elsewhere within the metamorphic belt but such an event has yet to be demonstrated

structural evolution of the orogenic gold deposits in central victoria australia the role of regional stress change and the tectonic regime abstract this review synthesizes the main characteristics of the orogenic gold deposits in western and central victoria australia and presents new insights into the relationships between orogen scale deformation and the 12 hard rock gold deposits that have each produced 30 t au we examine five of these large deposits stawell ballarat bendigo fosterville and costerfield from the start of the delamerian orogeny at ca 515 ma to the end of the tabberabberan orogeny at ca 370 ma the eastern continental margin of gondwanan australia experienced both accretionary growth and the development of the lachlan orocline central western victoria lies on the western edge of this oroclinal structure and is partly underlain by a proterozoic continental fragment the selwyn block all of the 30 t orogenic gold deposits in the region lie either on or to the west of the selwyn block from the delamerian orogeny onwards the deformation front progressively moved eastwards and the principal stress direction rotated clockwise from ne sw through e w and ultimately to n s so that the initial faulting and folding created pathways and traps for the subsequent mineralizing fluids through the brittle to brittle ductile crust thus the 455 440 ma benambran orogeny was the major mineralizing event at stawell and resulted in some early mineralization in the bendigo zone the 420 410 ma bindian orogeny gave rise to significant additional mineralization at ballarat bendigo and fosterville while the 385 370 ma tabberabberan orogeny drove mineralization at costerfield and a second bonanza event at fosterville the largest gold deposits formed where the fluid pathways either remained open or reopened in this rotating stress field allowing multiple pulses of gold rich fluids

microstructural evidence for an epigenetic origin of a proterozoic zinc lead silver deposit dugald river mount isa inlier australia the zn pb ag deposit contained in the metasediments of middle proterozoic age at dugald river forms one of a number of significant zones of sulphide mineralization within the eastern fold belt of the mount isa inlier it is characterized by its high zn pb grade high zn zn pb ratio and strong structural controls with the present resource standing at 38 million tons averaging 13 zn 2 1 pb and 42 g t ag microstructural timing relationships and a variety of microscopic ore textures indicate that the deposit resulted from replacement and partial infill of carbonaceous and pyritic host rocks by hydrothermal ore forming fluids during the d4 deformation event this genesis is contrary to earlier syngenetic based interpretations but accords with the discordant nature and structural controls on emplacement of the mineralization key timing criteria include 1 truncation of s2 and or s3 by the late ore minerals 2 replacement textures in undeformed mineral paragenesis 3 slightly preferred dimensional orientation and undulating extinction of quartz and muscovite that are intergrown but which crystallized earlier than sulphide minerals in veins the presence of these microstructural relationships throughout the deposit and the complete absence of any syngenetic stratiform precursor to zinc lead silver mineralization indicates that the zinc lead and silver at dugald river were epigenetically introduced rather than just having undergone syngenetic deposition during sedimentation and remobilization during deformation the regional distribution of the mineralization in a specific stratigraphic zone is most likely due to the partitioning of deformation between different rock types this caused the weaker lithology to accommodate significantly higher strain than adjacent more competent units resulting in fracturing and localization of the syntectonic mineralization

porphyroblast inclusion trails the key to orogenesis detailed microstructural analysis of inclusion trails in hundreds of garnet porphyroblasts from rocks where spiral shaped inclusion trails are common indicates that spiral shaped trails did not form by rotation of the growing porphyroblasts relative to geographic coordinates they formed instead by progressive growth by porphyroblasts over several sets of near orthogonal foliations that successively overprint one another the orientations of these near orthogonal foliations are alternately near vertical and near horizontal in all porphyroblasts examined this provides very strong evidence for lack of porphyroblast rotation the deformation path recorded by these porphyroblasts indicates that the process of orogenesis involves a multiply repeated two stage cycle of 1 crustal shortening and thickening with the development of a near vertical foliation with a steep stretching lineation followed by 2 gravitational instability and collapse of this uplifted pile with the development of a near horizontal foliation gravitational spreading near coaxial vertical shortening and consequent thrusting on the orogen margins correlation of inclusion trail overprinting relationships and asymmetry in porphyroblasts with foliation overprinting relationships observed in the field allows determination of where the rocks studied lie and have moved within an orogen this information combined with information about chemical zoning in porphyroblasts provides details about the structural metamorphic p t t paths the rocks have followed the ductile deformation environment in which a porphyroblast can rotate relative to geographic coordinates during orogenesis is spatially restricted in continental crust to vertical ductile tear transcurrent faults across which there is no component of bulk shortening or transpression

controls on syntectonic replacement mineralization in parasitic antiforms haile gold mine carolina slate belt strata bound au ag mo mineralization in the haile gold mine area is hosted by several ellipsoidal pyrite quartz replacement bodies within a laminated phyllite unit that is bounded on both sides by cambrian volcaniclastic schists the orebodies occur within two east northeast trends corresponding to the highly strained limbs of an upright macroscopic f 2 synform and show opposing en echelon distributions along each trend although the orebodies are associated with strata bound and locally stratiform massive pyrite mineralization and have been heterogeneously deformed detailed textural and structural studies strongly suggest that both the silicic orebodies and massive pyrite lenses formed synchronously with early to late d 2 deformation by syntectonic replacement of parasitic doubly plunging antiformal fold hinges ore textures include massive to laminated silicified phyllites in low strain domains silicic mosaic breccias and lensoidal silicic augen within highly strained pyrite sericite schists many fragmental textures previously interpreted as primary in origin resulted from brittle ductile deformation associated with heterogeneous syntectonic alteration a four stage syn d 2 alteration paragenesis involved i fine grained quartz pyrite or k feldspar or molybdenite replacement along bedding and s 2 in some anticlinal fold hinges ii extensive development of steeply dipping thin quartz veins quartz replacement and local mosaic slab breccias along s 2 iii local replacement of quartz and pyrite by intense sericitization within high strain shear zones developed marginal to and crosscutting silicic zones and iv locally extensive medium to coarse grained pyrite or molybdenite replacement along sericitic high strain zones stages ii and iii are separated by a generation of barren massive quartz veins lowgrade gold occurs with stage i silicification but high grade gold is locally associated with stage iv pyritization some remobilization of gold occurred with pyrite dissolution during stage iii sericitization controls on fluid rock interaction at haile reflect shifting patterns of deformation partitioning at different scales where replacement dominantly occurred in progressive shortening domains and selective dissolution occurred in progressive shearing zones the mineralization events were predated by syntectonic feldspar porphyry dikes and postdated by late retrogressed lamprophyric dikes and unmetamorphosed diabase dikes mineralization resulted from hydrothermal alteration by predominantly metamorphic fluids with a possible magmatic contribution syn to postpeak greenschist facies metamorphism during the later stages of taconian orogenesis

syndeformational carbonate replacement model for the copper mineralization at mount isa northwest queensland a microstructural study the mount isa base metal deposits show a definite spatial separation between stratiform ag pb zn lenses generally regarded as syngenetic and cu ores within a recrystallized and brecciated host lithology the silica dolomite microstructural evidence indicates that the cu orebodies and the silica dolomite formed during early syndeformational syn d 3 replacement processes in a dolomitic shale sequence three stages of alteration are distinguished 1 dolomite recrystallization silicification 2 dolomitization and 3 chalcopyrite or fe sulfide or quartz or chlorite deposition structural timing constraints suggest a post d 2 to pre s 3 interval for the first stage whereas the last two stages occurred during early d 3 the formation of the silica dolomite with the cu orebodies is separated in time from the adjacent syngenetic stratiform ag pb zn ores by a period including at least two regional deformation events

the geology and gold deposits of the victorian gold province abstract the palaeozoic succession of victoria represents a major world gold province with a total production of 2500 t of gold i e 78 million oz on a global scale central victoria represents one of the most gold mineralized areas outside the witwatersrand of south africa and remains the prime example of a slate belt gold province also known as turbidite hosted or shale greywacke gold province production came from quartz veins 1000 t modern placers 1200 t and palaeoplacers 300 t up to 7000 mines produced gold from quartz veins however the 168 mines of this type which produced over 1 t au contributed 68 of the primary gold production twelve goldfields have each produced 30 t au approximately one million ounces from all sources with bendigo 697 t ballarat 408 t and castlemaine 173 t being the largest these twelve goldfields have contributed 71 of the combined primary and secondary production gold mining commenced in 1851 declined dramatically around 1914 and has not increased significantly since the palaeozoic succession of victoria is part of an extensive fold belt along much of eastern australia i e the tasman orogenic belt in victoria the succession is dominated by cambrian to early devonian clastic metasedimentary rocks that have undergone major upright deformation and low grade metamorphism culminating in the middle devonian tabberabberan orogeny north south inliers of cambrian metavolcanic and metasedimentary rocks associated with regional fault zones divide the province into several geologically distinct zones in central victoria there are particularly well developed acid volcanic complexes of late devonian age and synchronous peraluminous granites which represent high temperature crustal melting tertiary to quaternary basalts have covered some of the palaeozoic gold deposits and have also covered several rich gold placer deposits of cainozoic age larger goldfields including bendigo and ballarat occur in ordovician slate and greywacke sequences but a wide range of metamorphosed sedimentary and igneous rock types are host to goldfields most if not all primary deposits demonstrate structural control although the controlling structural features differ between deposits wallrock alteration is strongly influenced by host rock in metasedimentary rocks it is limited and subtle but it is more pervasive in mafic and felsic igneous rocks carbonates muscovite and pyrite are the most widespread alteration minerals in these rocks representing co 2 k and s addition enrichment of as within gold deposits is common with sb important in some small deposits whereas base metals are only abundant locally and bi w mo and te show strong spatial association with granites and are rarely associated with economic gold deposits granites are an important component of the palaeozoic succession in victoria no large deposits are within granites but a few goldfields including maldon 65 t au are within contact aureoles of either s or i type granites of differing degrees of fractionation and silica content a single stage but protracted and possibly diachronous period of gold introduction near the time of the middle devonian tabberabberan deformation can explain geological relationships at many gold deposits the possibility that this event was slightly older i e silurian or early devonian in the west of the state cannot be precluded from the available evidence the mineralizing event appears linked to the same thermal event as diorite lamprophyre dykes and graite intrusions acidic and basaltic volcanism deformation and regional metamorphism the limited amount of fluid chemistry data from victorian gold deposits suggests low salinity fluids which contained co 2 and variable methane with homogenization temperatures from 150 to 300 c the sulphur isotopic compositions of pyrite are mostly 1 to 5 with some values slightly above 10 mineralizing temperatures are inferred to be near 300 c and pressure around 1 2 kbar sulphide rich gold deposits in the east of victoria many of which contain significant base metals contrast with the rest of the province which contains mostly gold deposits with lower sulphide content and this eastern area should possibly be viewed as separate from the victorian gold province per se the benefits to victorian gold mining coming from the 1980 s global gold boom have been insignificant compared to the historical importance of the victorian gold province there seems little justification for assuming that all the large deposits have been discovered and there is reason to believe new deposit styles may emerge to complement the limited current exploration activity which is focussed on the search for extensions and repeats of known deposit types a major advance in scientific understanding of these oft neglected deposits could kindle a major exploration and production revival

the relative roles of folding and faulting in controlling gold mineralization along the deborah anticline bendigo victoria australia gold mineralization along the deborah anticline bendigo australia occurs in quartz veins that are categorized into two main groups those associated with folds and those associated with faults the relative chronology of the structural development and quartz vein emplacement is characterized by the initial development of the fold and the subsequent formation of bedding and cleavage parallel veins saddle reefs may have developed at this stage reverse faults and their associated veins formed after folds had locked up major gold hosting structures include saddle reefs reverse faults and composite fold fault structures where reverse faults have disrupted saddle reefs some reverse faults are interpreted to have propagated along preexisting structures such as bedding parallel veins although gold is hosted in fold related structures the majority is found in veins associated with later faults folds and fold related veins strongly controlled the location of these later formed reverse faults and their associated veins a fault based fracture system was superimposed on preexisting dilational sites such as bedding parallel veins and saddle reefs and it is possible that this allowed for the introduction of a larger volume of fluids during faulting than was possible during folding

the role of fluids in syntectonic mass transport and the localization of metamorphic vein type ore deposists abstract in low to medium grade metamorphic environments the mobilization of species is closely associated with the operation of solution precipitation creep processes during foliation development such deformation processes involve dissolution of material from source sites which are usually discrete dissolution surfaces having orientations controlled by the applied stress and displacement fields mass transfer occurs via solute diffusion in an essentially static fluid network or by transport of dissolved species in a migrating fluid precipitation of material removed from dissolution sites occurs in a variety of sink sites including pore surfaces grain scale microcracks and veins sink sites may be close to or distantly removed from source sites the transport of material during deformation involving solution precipitation creep is largely controlled by the mobility of metamorphic fluids in regions of high fluid mobility significant mass transfer may occur over large distances fluid flow mass transfer paths and the development of sink sites in such cases are constrained by the structurally controlled development of high fluid pressure domains and the consequent mechanical enhancement of permeability by hydraulic fracture processes and the development of grain scale dilatancy these processes may result in focussed fluid flow and precipitation of mobilized species in restricted regions a proportion of syntectonically mobilized material is precipitated to form vein type deposits in hydraulic fracture arrays which develop in high fluid pressure environments the microstructures developed in minerals deposited in these sites reflect the interaction of a range of nucleation and growth processes which usually involve multiple generations of fracture growth and mineral crystallization the formation of syntectonic gold quartz vein deposits in central victoria australia illustrates the importance of structural controls on both the large scale migration of metamorphic fluids and the development of sites suitable for the precipitation of metamorphically mobilized species

fault fracture density and mineralization a contouring method for targeting in gold exploration abstract a widely observed correlation between high fracture density and mineralization throughout terranes and geological time indicates a fundamental underlying ore forming process in archaean greenstone hosted deposits high density fracturing was accompanied by enhanced fluid flow during fault fracture network development producing regional scale fluid pressure gradients that focussed hydrothermal fluids into preferentially fractured areas fracture density is both increased and decreased during faulting and fault healing and fracture density accumulates over time in zones of high palaeo fluid flow localised zones where the density of fracturing is increased by deformation become permeability nodes for migrating hydrothermal fluids leading to large zones of alteration and gold precipitation the ora banda mining centre in western australia contains significant gold deposits that appear to demonstrate a close association between high density fracturing and gold precipitation fracture density in the ora banda mines was enhanced by fault fault intersections fault contact intersections and changes in fault geometry the mine scale relationships between fracture density and gold mineralization are repeated at smaller and larger scales hence these relationships may be used in targeting for gold exploration contouring the density of fracturing in a region provides a semi quantitative way to rank areas for exploration and uses data from mapping drilling and high quality geophysical data as a basis for analyses fracture density contouring is complementary to other prospectivity analysis methods

utilization of pre existing competent and barren quartz veins as hosts to later orogenic gold ores at huangjindong gold deposit jiangnan orogen southern china most if not all orogenic gold deposits form during the late stages of deformation in some cases there is barren pre mineralization hydrothermal alteration which may be part of a single progressive hydrothermal event or a temporally distinct event relative to deposition of gold at the huangjindong orogenic gold deposit in the jiangnan orogen of southern china there are two phases of vein generation the earlier of which is largely barren structural analysis petrography mineralogy and in situ laser ablation inductively coupled plasma mass spectrometry la icp ms gold mapping of different generations of gold bearing sulfides reveal the relationship between different phases of hydrothermal alteration and quartz veins pre existing barren and brittle quartz veins which are controlled by tight folds and thrust faults provide favorable competent hosts for later overprinting by gold bearing ore bearing fluids that infiltrated during ongoing reverse sinistral fault movement these overpressured fluids caused hydrofracturing to complete brecciation of segments of the earlier quartz veins and associated pre existing breccias with deposition of auriferous quartz sulfide veins and breccia cements or matrices within them although fluid inclusion data are non definitive fluid pressure fluctuations are interpreted to have resulted in phase separation within the ore fluid which was primarily responsible for destabilization of gold complexes and deposition of native gold with other invisible gold bearing ore related sulfides formed by sulfidation reactions in hydrofractures and breccia matrices thus competent barren quartz veins in fold thrust belts may locally host superimposed gold mineralization and provide favorable targets for gold resources

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

esr dating of fossil shells in the middle to upper pleistocene strata in japan abstract the accuracy of the esr ages of fossil shells from the boso peninsula ooiso hills and the miyazaki plain in japan in the middle to upper pleistocene were examined the measurement test with a thermoluminescent dosimeter at the sampling point suggests that the dose rate of cosmic rays must be added to the calculated dose rate from chemical analysis the results show that the esr ages are in good agreement with the stratigraphically estimated ages however according to the chemical analysis of the mollusc shell itself several shells give an abnormally high concentration of uranium this may indicate that the shell has absorbed uranium from outside especially in sandy sediments

episodic metamorphic reactions during orogenesis the control of deformation partitioning on reaction sites and reaction duration textural unconformities or truncations are common in porphyroblasts with complex inclusion trails they reflect cycles of successive foliations that develop against competent porphyroblasts during orogenesis and are preserved by successive growth increments their truncational character results from shear and dissolution along a particular foliation generating a differentiated crenulation cleavage the increment of porphyroblast growth that follows a textural unconformity may or may not mark a significant compositional change depending on the amount of movement of the rock through p t space between cleavage forming events although historically interpreted to result from a significant metamorphic hiatus most textural unconformities indicate that the reactions involved in the formation of these minerals are episodic during continuous prograde metamorphism starting and stopping as a function of the stage of crenulation of the matrix foliation and the pattern of deformation partitioning such episodic reaction behaviour can only occur for multivariant reactions or successive but different univariant reactions the reason why garnet is the most common porphyroblast to exhibit evidence for episodic reactions is probably the fact that it grows by multivariant reactions over a much wider p t range than most other common porphyroblast phases porphyroblast growth is micrometasomatic it is episodic because a significant reduction of strain occurs within domains of progressive shortening each time continuous progressive shearing domains form on their margins this stops microfracture development across the progressive shortening domains thereby preventing rapid access and interaction of fluid ions and complexes with porphyroblast boundaries shifting patterns of deformation partitioning and resulting small scale juxtaposition of different compositional layers spreads the duration and location of multivariant reactions and causes differential timing of porphyroblast growth along a particular stratigraphic horizon it may also locally preserve metastable metamorphic assemblages in regionally metamorphosing deforming pelites near simultaneous cessation of porphyroblast growth on all rims once continuous differentiated progressive shearing domains have formed nearby precludes fluid recirculation as a significant process for removal of material during cleavage development alternatively diffusion of simple molecules and dissociated ions along actively shearing and micro gaped phyllosilicates with recomplexing in fluid filled microfractures readily explains the control of deformation partitioning on reaction site and reaction duration

hydraulic fracturing stress measurements and in situ stress field in the kanto tokai area japan abstract hydraulic fracturing stress measurements have been made in fifteen boreholes of various depths from 100 to 800 m in the kanto tokai area since 1978 about 90 sets of in situ stresses have been obtained successfully the maximum and minimum horizontal compressive stresses increase linearly with depth in each borehole the difference between the maximum and minimum principal stresses also increases with depth the increase of stress difference with depth is interpreted in terms of large stress relaxation in shallow parts of the boreholes where low confining pressure and many pre existing microcracks are dominant the maximum shear stress at a depth of 400 m ranges from 1 to 8 mpa depending on the site it is not always the case that the regions of small shear stresses are inactive in microseismicity and crustal movement this phenomenon is attributed to the relaxation of shear stress at the measured depths and to the regional variation in increasing rate of shear stress the maximum compressive stress direction is obtained from detection of the fracture azimuth after hydraulic fracturing stress direction measured at each borehole agrees well with that estimated from geologic and seismic methods near the measurement site several stress provinces where the stress directions appear almost uniform are defined in the kanto tokai area the regional distribution of the stress directions is understood in terms of the relative movement of three plates the philippine sea pacific and eurasian plates which are in contact with each other in this area

mount isa silica dolomite and copper orebodies the result of a syntectonic hydrothermal alteration system complex hydrothermal alteration system postdates deposition of the mount isa group and the lead zinc silver orebodies silica dolomite results from differential progressive metasomatic alteration of the entire range of microfacies constituting the lead zinc sequence the main components of this alteration are phyllosilicate development dolomitization and silicification this study concentrates on the latter two components and their role in breccia host formation all stages from incipient bedding and vein controlled alteration to total pervasive brecciation can be related to a deformational history of folding cleavage development and faulting a regional folding event d 2 formed the fold limb which constitutes the ore setting dolomite replacement veins and pseudobreccia complexes developed with varying intensity in favorable zones combinations of bedding and fracture controlled replacement continued until total dolomitization of the former shales occurred all forms and stages of dolomitic pseudobreccias are preserved by pseudomorphic quartz replacement in silicification zones which form the core of the orebody systems the separation in timing and mode of formation of lead zinc and copper orebodies is significant for exploration in the mount isa inlier the lead zinc mineralization appears to accumulate either syngenetically or at a very early stage in diagenesis but the copper has a structural control and a basic igneous association and is estimated to have been introduced at least 150 m y later modified journal abstract

hydrothermal breccias in vein type ore deposits a review of mechanisms morphology and size distribution abstract breccias are among the most widely distributed rock textures found in hydrothermal vein type deposits previous studies have mainly been interested in developing qualitative descriptive approaches leading to a confusing profusion of terms brecciation originates in numerous ways resulting in highly complex classification systems and frequent misinterpretations of facies field observations are difficult to reconcile with physical theories of fragmentation partly due to the fact that few satisfactory quantitative tools have been developed a review of the main brecciation processes occurring in hydrothermal vein type deposits allows for the discrimination between chemical and physical mechanisms including tectonic comminution wear abrasion two types of fluid assisted brecciation hydraulic and critical volume expansion or reduction impact and collapse each of these mechanisms can be distinguished using nonscalar parameters that describe breccia geometry including fragment morphology size distribution of the fragments fabric and dilation ratio the first two parameters are especially important because 1 the morphology of the fragments allows chemical and physical mechanical breccias to be distinguished and 2 the particle size distribution psd is a function of the energy input during breccia formation the slope of the cumulative psd fractal dimension ranges from high values for high energy brecciation processes to low values for low energy processes indicated by an isometric distribution the evolution of a vein system can be divided into three stages propagation wear and dilation these stages are separated by one threshold of mechanical discontinuity and one of hydraulic continuity these two thresholds also mark the transition between different types of brecciation mineralization occurs during all three stages and may display different textures due to pressure variations the use of quantitative parameters in fault related hydrothermal breccias allows a better understanding of the physical parameters related to a vein environment including structural setting and crustal level as well as fluid rock interactions recognition of the different breccia types could also be important during the early stages of mineral exploration

a comparison of orogenic gold mineralisation in central victoria aus western south island nz and nova scotia can implications for variations in the endowment of palaeozoic metamorphic terrains abstract the palaeozoic western lachlan orogen in victoria se australia the buller terrane western south island new zealand and the meguma terrane nova scotia canada are remarkably similar in regard to their geological age and history structural make up and the setting and formation of orogenic lode and disseminated style gold deposits within these fold belts a comprehensive review of the principal characteristics of orogenic gold deposits in each of the three historically important gold provinces illustrates the many similarities but it also reveals several major geological structural tectonic geochemical and geochronological differences that may account for the disparity in contained gold and overall endowment between the three fold belts aspects that stand out as potentially playing a critical role in the generation of a phanerozoic world class orogenic gold province include 1 the presence of a hydrated oceanic character substrate that can provide a fertile source rock for both fluids and metals 2 asthenospheric thermal input to trigger and sustain crustal devolatilisation and melting 3 the existence of a number of near vertical deep seated faults 4 substantial transcurrent movement and 5 evolution of an accretionary subduction system that promotes development of an extensive fore arc system with prolonged fluid generation and circulation quantification of these controlling factors provide a significantly improved understanding of the processes that control the genesis of orogenic gold deposits and also have the potential to enable discrimination between more and less gold favourable orogens and parts of orogens

subducting seamounts and deformation of overriding forearc wedges around japan abstract two subducting seamounts under inner trench slopes have been identified around japan on the basis of magnetic anomalies morphology and geological structure the first one is located under the foot of the inner trench slope at the junction between the japan trench and the kuril trench another one occurs beneath the slope slightly seaward of the tosabae the basement high at the trench slope break along the nankai trough off shikoku the magnetic anomalies of seamount origin are accompanied by the characteristic morphology of a forearc wedge i e a swell landward and a depression seaward the seamounts beneath the inner trench slopes have preserved magnetization showing reasonably consistent directions which suggests that the subducting seamounts have kept roughly their original shapes the morphology of the forearc wedge can be explained by a subducting seamount on the oceanic crust pushing the forearc material forward and upward deformation of the forearc wedge by the subducting seamount extends to the forearc basin the seamounts are stronger and less deformable than the inner slope material and are not offscraped onto inner trench slopes two other examples of deformed inner trench slopes around japan which can be explained by subduction of topographic highs are presented one example is a depression on the foot of the inner trench slope northeast of the junction between the kyushu palau ridge and the nankai trough another one is an area of complex morphology of the inner trench slope along the japan trench around the daiichi kashima seamount

the inverse problem in microtectonics and the separation of tectonic phases abstract the algorithm of inverse problems is applied to three cases in microtectonics a collection of striations measured on pre existing faults may be processed automatically and interpreted in terms of tectonic stress a comparison is made between two different methods formerly proposed by the authors both methods are tested with field data the theoretical results agree with independent field criteria for a separation of tectonic phases

detailed paleomagnetic records during the brunhes matuyama geomagnetic reversal and a direct determination of depth lag for magnetization in marine sediments abstract detailed paleomagnetic records were studied from homoclinal bathyal siltstone deposited during the brunhes matuyama geomagnetic reversal outcropping in the boso peninsula central japan the records are based on three continuous sediment cores from three sections within a distance of 14 km the stratigraphic horizons were correlated using a volcanic ash bed which occurs about 2 m above the brunhes matuyama reversal the sedimentation rate according to the oxygen isotope stratigraphy for the three sections decreases eastward and is 3 70 2 20 and 1 83 m ka 1 the depth lag of magnetization in the sediments was calculated directly as 42 cm based on the differences in the relative position of the key ash layer in the correlated virtual geomagnetic pole vgp paths for the three sections vgp movement during the geomagnetic reversal is characterized by short intervals of very rapid motion between periods of relative stability the last swing of the vgp from the southern hemisphere to the northern hemisphere in the brunhes matuyama geomagnetic reversal took only 38 yr intensities of the remanent magnetization normalized by anhysteretic remanent magnetization indicate that the intensity of the geomagnetic field decreased to one fifth for about 5 4 ka including the geomagnetic reversal the movement of the vgp longitude is 0 36 yr 1 westward and this westward drift can be related to the rotation of the non dipole field

phase relationships and p t conditions of coexisting eclogite blueschists and their transformation to greenschist facies rocks in the nerkau complex northern urals abstract eclogites and blueschists coexist in the same lithologic sequence in the nerkau complex one of the metamorphic complexes in the northern urals russia two types of eclogites can be distinguished in this complex 1 glaucophane free eclogites consisting of garnet omphacite barroisite calcic amphibole albite epidote rutile titanite quartz in these rocks phase compatibilities indicate that barroisite was stable at eclogite conditions together with garnet and omphacite this assemblage was overprinted by another consisting of barroisite mg hornblende albite epidote which developed under albite bearing greenschist conditions 2 paragonite glaucophane bearing eclogites consist of omphacite garnet glaucophane barroisite actinolite paragonite albite epidote titanite rutile quartz textural and chemographic analyses indicate that two main metamorphic episodes took place as well represented by the assemblages omphacite garnet glaucophane and barroisite albite chlorite respectively the blueschists surrounding the eclogites consist of garnet glaucophane barroisite phengite paragonite chlorite albite quartz titanite rutile ilmenite phase relationships indicate that no reaction relates eclogites with blueschists and that both mineral assemblages can be stable at the same metamorphic conditions the occurrence of these assemblages side by side could thus be controlled by the bulk rock composition metamorphic conditions have been calculated for each lithology with different but complementary methods the two types of eclogites were stable between 450 and 490 c or 500 and 537 c depending on the thermometer used and minimum pressures of 10 to 11 kbar these values compare favourably with 530 20 c and 14 5 1 kbar obtained with a p t diagram calculation and a forward modelling approach in the case of blueschists coexisting glaucophane and garnet are stable at similar metamorphic conditions the final retrogressive transition to greenschist took place at p below 9 kbar and t around 480 c this results in an almost isothermal decompressional path

a fluid inclusion study of au bearing quartz vein systems in the central and north deborah deposits of the bendigo gold field central victoria australia the central and north deborah gold deposits with a production of about 7 tons au are located in one of 15 domes in the bendigo gold field central victoria australia an important mesothermal lode gold province the gold bearing quartz vein systems are hosted predominantly by reverse faults in lower ordovician turbidites which underwent lower greenschist facies metamorphism in the central and north deborah mines there are six stages of quartz veins that developed in the following sequence 1 laminated veins with a metal association of au as sb pb zn ni 2 spurs au 3 massive barren veins main dilation stage 4 brecciated veins with au as 5 quartz ankerite veinlets and 6 late pure quartz or calcite veinlets of the three mineralized vein stages 1 2 and 4 only the first one predated or was coeval with the main deformation event whereas stages 2 and 4 postdated the main deformation the vein structures indicate that vein development was controlled by repeated fluctuations of fluid pressure inducing hydraulic fracturing fluid inclusions in vein quartz contain c o h fluids of variable compositions three main types of fluid inclusions are recognized at room temperature type i two phase primary ch4 v h2o l fluid inclusions type ii two or three phase primary pseudosecondary co2 v or l ch4 v or l h2o l fluid inclusions including iia ch4 co2 h2o and iib co2 h2o fluid inclusions and type iii h2o v h2o l fluid inclusions type iii fluid inclusions are further divided into three groups iiia primary pseudosecondary vapor rich iiib primary pseudosecondary liquid rich and iiic secondary liquid rich data from fluid inclusion distribution microthermometry and raman spectroscopy indicate that fluids associated with au mineralized quartz veins stages 1 2 and 4 have moderate salinity ranging from 0 to 12 wt percent nacl equiv modeled salinities around 7 8 wt nacl equiv these veins formatted at temperatures from 325 to 375 c and pressures of 200 to 300 mpa with emplacement depths of about 8 to 12 km fluids associated with barren quartz veins stages 3 5 and 6 have a low salinity of about 0 to 3 wt percent nacl equiv modeled salinities about 2 3 wt nacl equiv and lower temperatures there is evidence of fluid immiscibility in all vein stages and mixing between fluids of the same souce but differing fo2 may also have influenced gold deposition cox et al 1995 hydrothermal ore forming fluids responsible for gold mineralization are in the ch2 co2 h2o nacl system whereas those of massive barren veins stage 3 and veinlets stages 5 and 6 are in the co2 h2o nacl and h2o nacl systems respectively the carbonic phases of type i and ii fluid inclusions range from pure ch4 through mixed ch4 and co2 to pure co2 gold ore forming fluids were weakly acidic with f o2 estimated to have been 10 25 to 10 37 bars and generally above the co2 ch4 and below the so2 h2s buffer boundaries throughout gold mineralization the relatively reduced mineralizing fluids are similar to those of other turbidite hosted lode gold deposits and counterparts in volcanic plutonic terranes

halogen chemistry as an indicator of metamorphic fluid interaction with the ponder pluton coast plutonic complex british columbia canada during hydrothermal fluid activity associated with the intrusion of the ponder pluton coast plutonic complex british columbia canada the halogen composition of biotite amphibole and apatite is controlled by the composition of the metamorphic fluid the core of the pluton was not affected by infiltrating fluids and the fluorine composition of biotite was controlled by the biotite fe mg ratio the edge of the cooling crystallized pluton interacted with a chlorine rich fluorine poor fluid which changed the halogen composition of biotite and amphibole the composition of the metamorphic fluid derived from biotite and apatite compositions was relatively high in chlorine and low in fluorine the concentration of chlorine in the metamorphic fluid increases towards the pluton whereas fluorine remains constant this suggests that the metamorphic fluid near the pluton had the greatest potential for masstransfer of metals

structural geology of the dugald river znpbag deposit mount isa inlier australia abstract the sediment hosted zn pb ag deposit at dugald river is situated 87 km northeast of mount isa nw queensland it is a mid scale base metal accumulation restricted to a black slate sequence of low metamorphic grade the orebody is tabular and consists of fine to medium grained sulphides with a dominant mineralogy of sphalerite pyrrhotite pyrite galena quartz and muscovite three different ore types have been recognized based on mineralization textures laminated banded and brecciated the present reserve stands at 38 million tons of ore averaging 13 0 zn 2 1 pb and 42 g t ag a structural investigation has revealed that six stages of deformation have affected the metasediments in the dugald river area the first four d 1 d 2 d 3 and d 4 are characterized by the extensive development of folds and associated axial plane cleavage they were all generated in a ductile regime and are of considerable significance for the structural evolution of this region as well as for the emplacement and localization of the sulphide mineralization d 5 provides a transition towards brittle deformation developing strong kink folds with subhorizontal axial planes d 6 was a brittle event producing e w trending open folds and major ne and nw strike slip faults crosscutting all the pre existing structural elements plus segmenting the orebody correlation between the development of deformation and the formation of mineralization can be observed from macro to microscales relationships of mineralization with folds and cleavage indicate a post d 2 dominant deformation event and probably syn d 4 deformation timing for the zn pb ag mineralization at dugald river as suggested by the ubiquitous truncations of d 2 fabrics by ore mineral assemblages throughout the deposit

multiple foliations defined by different sillimanite crystal habits partial melting and the late metamorphic development of the cannington ag pb zn deposit northeast australia a succession of foliations defined by different sillimanite bearing structural fabrics suggests that the macroscopic isoclinal synform that dominates the geometry of the cannington ag pb zn deposit northwest queensland australia formed during d2 the five foliations in this succession s1 to s5 are defined by aligned sillimanite with habits ranging from individual crystals in s1 through s4 to clusters of fibrolite in s5 in both the matrix and as inclusion trails within garnet and gahnite zn rich spinel porphyroblasts s1 s3 and s5 formed as sub horizontal foliations whereas s1a s2 and s4 formed sub vertically foliation intersection inflexion axes fias within garnet and gahnite porphyroblasts preserve a succession of trends that shifts from w e to n s this succession indicates that this region experienced n s followed by w e bulk crustal shortening n s shortening occurred during d1and d1a and w e shortening occurred from d2 to d5 prismatic and rhombic sillimanite produced during d1 d4 accompanied prograde metamorphism to ca 634 62 c and 4 8 1 3 kbar the coexistence of fibrous prismatic and rhombic sillimanite resulted from post peak metamorphic reactivation of the early foliations during d5 the synformal d2 fold was intensified during d4 by w e bulk shortening local partial melting occurred after d1 in the appropriate bulk rock compositions based on calculation of p t pseudosections in the chemical systems kfmash kfmashto nckfmash and mnnckfmash zn mineralization related to gahnite growth occurred during d3 through d4 and was redistributed by partial melting into structural and rheological sites during d4 and d5shearing

tectonic record of convergence changes in a collision area the boso and miura peninsulas central japan abstract late cenozoic formations in boso and miura have been affected by several tectonic events tectonic analysis enables us to reconstruct six different paleostress types 1 early extension affecting the oligocene early miocene mineoka group 2 and 3 relatively minor compressional and extensional events probably early pliocene in age 4 major nne ssw compression dominating prior to 2 or 3 ma ago and 5 and 6 more recent major nnw sse compression to the west and wnw ese extension to the east both types affecting the pleistocene and prevailing since 1 2 ma ago the counterclockwise change from nne ssw to nnw sse compression is not accurately dated but very likely occurred between 2 and 3 ma ago it is compared to similar evolutions in other areas of the izu collision zone we conclude that it corresponds to a major counterclockwise change in the direction of plate convergence from sse nnw to se nw the relationships between the directions of convergence and the distributions of plio quaternary compressional paleostresses in and around the collision zone are described through a simple analogy for the two stages of plio quaternary collision this counterclockwise change in stress fields and relative motions also described in the taiwan collision zone along the same philippine sea plate eurasia boundary is interpreted as a major event at the scale of the plate the possible significances of the other paleostress types identified in boso are discussed we conclude that tectonic analysis in and along collision boundaries provides a key for understanding kinematic evolution

late kinematic timing of orogenic gold deposits and significance for computer based exploration techniques with emphasis on the yilgarn block western australia abstract orogenic gold deposits are a widespread coherent group of epigenetic ore deposits that are sited in accretionary or collisional orogens they formed over a large crustal depth range from deep seated low salinity h2o co2 ch4 n2 ore fluids and with au transported as thio complexes regional structures provide the main control on deposit distribution in many terranes first order faults or shear zones appear to have controlled regional fluid flow with greatest ore fluid fluxes in and adjacent to lower order faults shear zones and or large folds highly competent and or chemically reactive rocks are the most common hosts to the larger deposits focusing of supralithostatic ore fluids into dilatant zones appears to occur late during the evolutionary history of the host terranes normally within d3 or d4 in a d1 d4 deformation sequence reactivation of suitably oriented pre existing structures during a change in far field stress orientation is a factor common to many deposits and repeated reactivation may account for multiple mineralization episodes in some larger deposits absolute robust ages of mineralization support their late kinematic timing and in general suggest that deposits formed diachronously towards the end of the 100 to 200 m y long evolutionary history of hosting orogens for example in the yilgarn block a region specifically emphasised in this study orogenic gold deposits formed in the time interval between 40 and 90 m y with most about 60 to 70 m y after the youngest widespread basic ultrabasic volcanism and towards the end of felsic magmatism the late timing of orogenic gold deposits is pivotal to geologically based exploration methodologies this is because the present structural geometries of i the deposits ii the hosting goldfields and iii the enclosing terranes are all essentially similar to those during gold mineralization at least in their relative position to each other thus interpretation of geological maps and cross sections and three dimensional models can be used to accurately simulate the physical conditions that existed at the time of ore deposition it is particularly significant that the deposits are commonly related to repetitive and predictable geometries such as structural heterogeneities within or adjacent to first order structures around rigid granitoid bodies or in specific locked up fold thrust structures importantly the two giant greenstone hosted goldfields kalgoorlie and timmins show a remarkably similar geometry at the regional scale computer based stress mapping and gis based prospectivity mapping are two computer based quantitative methodologies that can utilize and take advantage of the late timing aspect of this deposit type to provide important geological aids in exploration both in broad regions and more localized goldfields both require an accurate and consistent solid geology map stress mapping requires knowledge of the far field stresses during mineralization and the empirical prospectivity mapping requires data from a significant number of known deposits in the terrane the kalgoorlie terrane in the yilgarn block meets these criteria and illustrates the potential of these methodologies in the exploration for orogenic gold deposits low minimum stress anomalies interpreted to represent dilational zones during gold related deformation coincide well with the positions of known goldfields rather than individual gold deposits in the terrane and there are additional as yet unexplained anomalies the prospectivity analysis confirms that predictable and repetitive factors controlling the siting of deposits are i proximity to and orientation and curvature of granitoid greenstone contacts ii proximity to segments of crustal faults which strike in a preferred direction iii proximity to specific lithological contacts which have similar preferred strike iv proximity to anticlinal structures and v the presence of preferred reactive host rocks e g dolerite the prospectivity map defines a series of anomalous areas which broadly conform to those of the stress map 78 correspondence the most prospective category on this map covers less than 0 3 of the greenstone belts and yet hosts 16 of the known deposits which have produced 80 of known gold thus it discriminates in favour of the larger economically more attractive deposits in the terrane the successful application of stress mapping and prospectivity mapping to geology based exploration for orogenic gold deposits indicates that more quantitative analysis of geological map data is a profitable line of research the computer based nature of these methodologies is ideal for the production of an ultimate integrated deposit target map which can be compared to other more conventional targeting parameters such as geophysical and geochemical anomalies such an integrated strategy appears the way forward in the increasingly difficult task of cost effective global exploration for orogenic gold deposits in poorly exposed terranes

a post deformational post peak metamorphic timing for mineralization at the archaean big bell gold deposit western australia abstract previous workers were unable to find any suitable criteria to time gold mineralization at big bell mine an enigmatic low grade large tonnage deposit hosted in an upper amphibolite facies greenstone belt sequence in the murchison province of the archaean yilgarn craton western australia earlier studies suggested that the gold mineralization occurred by hydrothermal alteration either synchronous with volcanism or prior to peak metamorphism the following conclusions have been drawn from a detailed meso and microstructural analysis of the mine sequence and greenstone belt hosting the orebody 1 1 prior to mineralizaton the big bell area underwent five phases of deformation and m1 regional metamorphism to the staurolite andalusite zone of the amphibolite facies for metasediments 2 2 granite emplacement post dated d5 folding causing local m2 thermal metamorphism this peaked at the ms qtz breakdown reaction for pelites on or close to the and sil transition but below the onset of partial melting characterized by the narrow non degenerate divariant field involving seven kfmash phases qtz ms bt crd kfs als h2o this hornfelsing consumed cross cut and post dated s5 cleavage prograde qtz and sil veins replaced this hornfels assemblage and s5 cleavage and were caused by volatile escape at depth and hydraulic fracturing coincident with a reduction in bulk rock permeability resulting from post deformational hornfelsing 3 3 retrograde m3 assemblages and two stages of gold related mineralization cross cut and replaced all previous structures the retrograde gold bearing assemblages are both underformed stage 1 mineralization is typically found with widespread retrograde muscovite plates and consists of the following mineral associations py po ccp ilm mag au tur sch rt spn zrn ms bt ep czo qtz the second stage of mineralization is characterized by pervasive sericitic alteration that is locally developed to an extent where all pre existing mineralogies including quartz are totally destroyed within sericitic domains the stage 2 mineral associations are apy sbt sb mo sp au py po ilm mag ccp sericite chl carbonate cal ep czo qtz 4 4 gold mineralization at big bell was post deformational and post peak metamorphic unlike many archaean gold deposits there are no obvious shear zones controlling the formation of the big bell gold deposit at the structural level exposed either on the surface or in drill core big bell mine was in a low strain domain during the formation of late stage sinistral sub vertical d6 shear zones that are typically concentrated at the granitoid greenstone contacts the retrograde character of the mineralization has been deduced from detailed meso and microstructural observations and demonstrates the emplacement of this gold deposit was very late in the overall orogenic history of the murchison province

tectonic analysis of fault slip data sets using data that include the direction and the sense of motion on individual fault surfaces determined by slickenside lineations it is possible to reconstruct reduced stress tensors that correspond to the orientation of stress axes and to the ratio 2 3 1 3 between principal stress values 1 2 3 compression being positive no assumption is made concerning the orientation of fault planes relative to stress axes so that reactived faults are taken into account as well as newly created ones qualitative and quantitative methods for analysis of fault slip data were developed during the last 10 years the practical limitation of the methods and the necessity for critical field observations are emphasized these methods can be applied to focal mechanisms of earthquakes a more complex analysis of heterogeneous data sets involving an iterative separation of different stress systems is also discussed this analysis enables one to distinguish successive faulting events careful qualitative study in the field is in all cases essential

structural constraints and numerical simulation of strain localization in the bendigo goldfield victoria australia strain localization and fluid flow patterns adjacent to the massive quartz hosted reefs of the bendigo gold field have been investigated using new structural observations three dimensional geometry of fold fault relationships and gold distribution quartz reef and vein formation in the bendigo goldfield is associated with an initial e w compression event that produced a penetrative upright cleavage and n s trending fold hinges in the immediate hanging wall of quartz reefs with high gold grades the s1 cleavage is locally deformed by an e w trending s2 cleavage that is attributed to a low strain n s oriented compressive event further evidence of this low strain event is provided by gently s plunging quartz carbonate slickenline lineations on s1 cleavage surfaces and the reef margin surfaces and visible gold within the outer portions of the reef the reefs are commonly situated on or immediately to the south of a left flex sinistral change in strike of the axial surface of the folds and the flexure occurs close to an anticlinal culmination defined by the double plunging n s striking folds the folds become deformed with meter to decimeter wavelengths which generally affect the stiff units in a disharmonic style accompanying this is a reactivation of reef margins by oblique slip and the formation of dilational sites and tensional vein arrays that provide the conduits for fluid flow and gold deposition numerical models that best simulate the observed distribution of strain based upon the location of quartz reefs involve nw se oriented contraction with a significant sinistral strike slip component this contraction event has an overall n s sense orientation and is therefore consistent with deformation features in the immediate hanging wall of quartz reefs the models also demonstrate a geometrical dependence on the localization of strain and thus the potential localization of fluid flow volume and shear strain increase with the flex angle and variations in the magnitude of strain are also associated with variations in the strike angle of the flexure this analysis highlights that the modification of preexisting fold structures and quartz veins by a low strain event may enhance the localization of dilation sites within individual quartz reefs where there are subtle changes in the orientation of the reef margins such numerical modeling can also improve ranking of exploration targets based on fold geometry and provide a better understanding of fluid flow in hydrothermal mineral systems

mechanics of formation of gold bearing quartz veins nova scotia canada abstract the meguma terrane of nova scotia canada is composed largely of low grade clastic metasediments swarms of structurally complex bedding concordant quartz veins are found across the whole terrane partly or wholly outlining anticlinal domes these veins exhibit spectacular features due to intermittent hydraulic fracturing and healing crack seal such as phyllosilicate inclusion bands columnar quartz grains and included sheets of host rock overprinting all the precipitation features are a suite of deformation structures which include pinch and swell tectonic stylolites and cross cutting extension and hybrid fractures the veins were formed syntectonically in folded and foliated strata the vein precipitating fluids pooling beneath less permeable beds in anticlinal domes fluid migration paths were controlled by the two fundamental mechanical anisotropies in the host rocks bedding and foliation a simple two dimensional vein model is considered using fracture propagation and quartz precipitation rates estimates of rates of vein formation are determined for example it seems possible that a quartz vein 100 m long and 175 cm wide could form in less than 10 yrs such times are effectively geologically instantaneous and have considerable implications with respect to physical and chemical processes in deformation and metamorphism

structural controls on development and localization of syntectonic copper mineralization at mount isa queensland three major deformation events were responsible for the structural relationships in the mount isa mine where steeply west dipping siliceous and dolomitic sediments of the mount isa group which host the ag pb zn ores as well as the epigenetic cu orebodies are truncated against the underlying greenschists the cu orebodies and their alteration envelope the silica dolomite developed during the third deformation d 3 within the dolomitic urquhart shale adjacent to this greenschist contact this contact is a fault that formed during thrusting in d 1 and was rotated and overturned by the combined effects of d 1 and d 2 into a shallowly dipping attitude on the western limb of a regional scale upright d 2 antiform consequently it lay at a high angle to the steeply plunging stretching direction established during the third deformation d 3 this fault was a plane of considerable anisotropy and competency contrast because the rocks above were dolomitic siltstones whereas those below were greenschists with bedding respectively at a high angle and subparallel to the fault folding during d 3 generated opposed senses of shear and hence differential movement across the fault especially at fold hinges this in combination with the differential anisotropy and competency contrast across the fault and extension at a high angle to it resulted in localized fracture and opening of the contact and consequently a rapid drop in fluid pressure possibly on several occasions in different locations along the greenschist contact the large imbalance in fluid pressure across the walls of these localized openings caused explosive fracturing and local brecciation especially of the dolomitic rocks on the fault walls into the space created this provided access for the syndeformation hydrothermal fluid to all faces of the jostled blocks of dolomitic shales enabling their alteration to form the silica dolomite and the cu ores

c o h s fluid composition and oxygen fugacity in graphitic metapelites c 0 h fluid produced by the equilibration of hzo and excess graphite must maintain the atomic h o ratio of water 2 1 this constraint implies that all thermodynamic properties of the fluid are uniquely determined at isobaric isothermal conditions the o h20 and co fugacities j fhto and fc of such fluids have been estimated from equations of state and fit as a function of pressure and temperature these fugacities can be taken as characteristic for graphitic metamorphic systems in which the dominant fluid source is dehydration e g pelitic lithologies because there are no compositional degrees of freedom for graphite saturated fluids produced entirely by dehydration the variance of the dehydration process is not increased in comparison with that in non graphitic systems thus compositional buffering of c 0 h fluids by dehydration equilibria a common petrological model requires that redox reactions decarbonation reactions or external h o 2 fluid sources perturb the evolution of the metamorphic system such perturbations are not likely to be significant in metapelitic environments but their tendency will be to increase the fo of the fluid phase at high metamorphic grades pyrite desulphidation reactions may cause a substantial reduction of fhz0 and slight increases info and fcs relative to sulphur free fluid at low metamorphic grade sulphur solubility in h o s 2 fluids is so low that pyrite decomposition must occur by sulphurconserving reactions that cause iron depletion in silicates a common feature of sulphidic pelites with increasing temperature and sulphur solubility pyrite desulphidation may be driven by dehydration reactions or infiltration of h o rich fluids the absence of magnetite and the assemblages carbonate aluminosilicate or pyrite pyrrhotite ilmenite from most graphitic metapelites is consistent with an h o 2 model for gcoh s fluid for graphitic rocks in which such a model is inapplicable a phase diagram variable that defines the h o ratio of gcoh s fluid is more useful than the conventional

crustal scale fluid pathways and source rocks in the victorian gold province australia insights from deep seismic reflection profiles deep seismic reflection data collected across the western lachlan orogen of southeast australia have provided important insights into crustal scale fluid pathways and possible source rocks in one of the richest orogenic gold provinces in the world the profiles span three of the most productive structural zones in victoria the stawell bendigo and melbourne zones zone scale variations in the age and style of gold deposits correspond with differences in crustal structure and composition the bilateral distribution of gold production in the stawell and bendigo zones is related to the v shaped crustal scale geometry of the two zones in cross section major first order faults like the east dipping moyston fault and a set of west dipping listric faults within the bendigo zone were probably major fluid conduits in the lower to middle crust during gold mineralization first order faults in the stawell and bendigo structural zones appear to have accommodated large scale thickening down to the lower crust the faults converge in a region beneath the western bendigo zone where a thick section of mafic volcanic rocks and lesser sedimentary rocks are identified as a likely common source of gold bearing fluid that was largely generated by late ordovician to early silurian metamorphism the areas with the greatest gold endowment lie above lower crustal regions that have preserved the thickest succession of fertile mafic igneous rocks up to about 25 km thick they also correspond to a region of thin or absent precambrian lithosphere mafic rocks in the stawell zone and far western bendigo zone were probably partly consumed by cambrian west dipping subduction similar rocks in the rest of the bendigo zone lay outside the influence of cambrian subduction accretion and were deformed later probably beginning in the late ordovician early silurian benambran orogeny during crustal scale imbrication this imbrication preserved much of the mafic rocks to form the lower to middle crust first order listric faults in the bendigo zone are interpreted as major controls on the locations of goldfields even though they are largely unmineralized near the present surface the shallow dipping segments of first order listric faults were favorably oriented for reactivation at the time of gold mineralization and acted as major fluid conduits in the lower to middle crust in contrast the upper steeply dipping segments of first order listric faults were unfavorably oriented for reactivation and were poor fluid conduits the seismic data show that the transition from predominantly shallow to steeply dipping fault segments occurs in the middle to upper crust near the boundary between thick imbricated metavolcanic rocks that lie immediately below 6 to 15 km of folded metasedimentary rocks this transition may have coincided with fluid escape zones that aided the transfer of permeability away from first order faults and into the overlying fold dominated turbidites this transfer of permeability was enhanced by the growth of subvertical fold related fault and fracture meshes in the upper crustal turbidites the fault and fracture meshes consisted of bedding parallel faults limb thrusts and tension vein arrays that developed along fold hinges in the bendigo zone individual fold hinges and regional fold culminations were important controls on the distribution of fluid flow fluid flow was partly syndeformational but overlapped into the immediate postdeformational period of the benambran orogeny later reactivation of first order faults in the late silurian to early devonian and again in the late devonian led to further although less important mineralizing events as fluids exploited the preexisting fault architecture deeply penetrating north dipping listric faults in the less gold rich melbourne zone cut into inferred proterozoic basement and may have been fluid conduits during a late devonian mineralizing event

the late orogenic timing of mineralisation in some slate belt gold deposits victoria australia gold mineralisation in classic australian slate belt gold deposits at ballarat bendigo st arnaud and inglewood occurred very late in the orogenic history of these rocks rather than during formation of the main slaty cleavage this has been revealed through the examination of microstructural relationships in gold bearing quartz veins and their host rocks from these deposits which has established a d1 to d4 deformation stage history and consistent timing for gold mineralisation over a wide area the gold was deposited synorogenically but during the fourth deformation stage d4 of the orogeny a relatively weak event occurring two deformations after the main slaty cleavage producing event d2 previously d2 had been regarded as both the source and control of gold mineralisation as most of the quartz veins that occur in these deposits formed before or during this deformation event however most gold is hosted in breccia veins that formed during d4 the wallrock clasts within these breccia veins contain a young rotated foliation and the breccia veins are spatially associated with a paragenetically consistent alteration of the host rocks in the deposits this alteration both crosscuts and preferentially mineralises wallrock s4 allowing the timing of the breccia veins alteration and gold deposition to be defined as syn d4 in age

magnetic mineralogy and metamorphic zonation in the ardennes massif france belgium abstract in the ardennes area pelitic schists of cambrian to early devonian age have experienced a low grade polyphase metamorphism which can be characterised by changes in magnetic mineralogy using mainly rock magnetic tools we have defined various iron oxides and sulphide assemblages allowing a magnetic mineralogy zoneography of the ardennes metamorphism to be constructed low temperature and three axial irm measurements identify pyrrhotite in the southern part of the massif this mineral had not been previously recognised in the ardennes area in the devonian cover toward the metamorphic apex the appearance of coarse grained haematite is characterised by low coercivity values the trends of low field and anhysteretic susceptibilities allow a discrimination between the lithologic and metamorphic effects to be made additionally the relative contributions of an early metamorphic event and the classical metamorphism linked to the variscan shortening are discussed

structural control of greenstone hosted gold mineralization by transcurrent shearing a new interpretation of the kalgoorlie mining district western australia abstract the kalgoorlie mining district encloses the golden mile and mt charlotte mining areas and has produced 1200 metric tons of gold it is classed as one of the largest archaean gold deposits in the world the district is located in the norseman wiluna greenstone belt of the yilgarn craton western australia this greenstone belt is characterized by nnw trending transcurrent shear zones some of which are traceable over more than 500 km strike length mineralization in the district is controlled by second third and fourth order structures located between principal shear zones of regional extent an early deformation phase of regional folding d1 associated with prograde regional metamorphism of low grade preceded two phases of wrenching the geometry of mineralized shear zones in both mining areas as well as field and microstructural studies allow us to distinguish an early phase of transpressional sinistral shearing d2 from a later phase of transcurrent dextral shearing d3 the earlier sinistral shear system in the golden mile area was in part reactivated but its gross original geometry was preserved these observations are consistent with clay model experiments carried out by the authors hydrothermal carbonate sericite alteration and gold mineralization commenced during the sinistral shear event and continued into the dextral event mineralization within individual shear zones forms tabular orebodies whereas pipe shaped ore shoots occur at the intersection of shear zones the ore shoots contain high grade telluride mineralization locally relative timing relationships show that mineralization postdates the peak of regional metamorphism hydrothermal activity in the district was preceded by the intrusion of calc alkaline porphyry dykes of granodioritic to tonalitic composition and accompanied by the intrusion of lamprophyre dykes

sequential porphyroblast growth and crenulation cleavage development during progressive deformation abstract six stages of crenulation cleavage development during a single deformation event d2 can be recognised in both matrix and porphyroblast inclusion trails in the robertson river formation n e australia these stages progress from undeformed to crenulation and differentiated crenulation of s1 through development of differentiated crenulation cleavage s2 and differentiated schistosity s2 to homogeneous foliation s2 these rocks underwent prograde metamorphism during d2 and chloritoid garnet staurolite andalusite and sillimanite isograds have been mapped most rocks are rich in porphyroblasts which commonly contain well defined inclusion trails the geometry of these trails varies from one mineral type to the next depending on the timing of porphyroblast growth relative to the stage of crenulation cleavage development in the schistose matrix the deformation history involved progressive bulk inhomogeneous shortening and the strain is very heterogeneous on all scales partly as a consequence of this hence each of the six stages of crenulation cleavage development formed locally at various times during d2 therefore if a porphyroblast grew late in the deformation it could overgrow all stages of crenulation cleavage development including the very early ones the temperature increased during metamorphism in any one locality and consequently some porphyroblasts overgrew others that were unstable in the new conditions preserving them from destruction hence the precise timing of mineralogical reactions deduced from isograds and mineral chemistry relative to the stages in schistosity development can also be determined in the andalusite zone the sequence of porphyroblasts in decreasing order of age is biotite garnet biotite staurolite andalusite dissolution of garnet occurred during growth of staurolite and especially andalusite and dissolution of staurolite accompanied andalusite growth

late structural control of mesothermal vein hosted gold deposits in central victoria australia mineralization mechanisms and exploration potential abstract comparison of the structural relationships between five gold deposits in ordovician rocks of the central victorian slate belt reveals a similar history and structural control of gold mineralization large quartz veins spatially associated with gold mineralization such as the famous saddle reefs of bendigo formed during the second deformation earlier than the gold mineralization itself where they were overprinted by the effects of the fourth deformation they provided structural sites for gold deposition on their boundaries with country rock where brecciation jostling and fracture were favoured access of fluids resulting from metamorphism and deformation to the brecciated fractured and jostled blocks of country rock created conditions suitable for the precipitation of gold through a sequence of alteration replacement and infill the dominant structural relationships that resulted in the generation of these sites were a the high angle of the axial plane of the fourth deformation to earlier structures and b the presence during mineralization of large pre existing quartz veins that acted as structural discontinuities on which brecciation could occur these factors allow a predictive capacity for exploration in the region

archean gold bearing quartz veins at the sigma mine abitibi greenstone belt quebec part ii vein paragenesis and hydrothermal alteration the sigma deposit consists of an archean gold bearing vein system in deformed volcanic rocks intruded by two generations of porphyries the veins cut all rock types and wall rock alteration is superposed on greenschist facies metamorphic assemblages there are two related vein sets subvertical veins in ductile shears and subhorizontal veins in extensional fractures between shear zones this paper reports on the paragenesis of extensional veins and related hydrothermal alteration tension veins are composed chiefly of quartz and tourmaline and subordinate amounts of carbonates chlorite pyrite pyrrhotite scheelite and free gold the veins formed by one or more episodes of open space filling the paragenetic sequence consists essentially of successive deposition of carbonate scheelite tourmaline pyrite pyrrhotite carbonate chlorite and quartz most of the gold and tellurides was introduced into fractures and along grain boundaries of recrystallized vein minerals during a postfilling episode of deformation veins are surrounded by zoned alteration envelopes consisting of an outer zone of cryptic alteration characterized by a chlorite carbonate white mica assemblage and an inner zone of visible alteration characterized by a carbonate white mica outer subzone and a carbonate albite inner subzone alteration zones represent reaction fronts advancing into the wall rocks and resulted from progressive interaction of wall rocks with a single hydrothermal fluid the chlorite carbonate white mica zone is characterized by replacement of epidote by calcite and white mica accompanied by fixation of co 2 and k and by release of small amounts of si in carbonate white mica and carbonate albite subzones of visible alteration chlorite biotite and white mica were replaced by calcite albite and quartz variable amounts of gold tourmaline pyrite pyrrhotite and ilmenite were also introduced visible alteration zones are characterized by additions of ca co 2 na b s p and au and by important losses of mg fe al and h 2 o mass balance calculations show that ca na si co 2 s and b in vein minerals were contributed from the hydrothermal fluid whereas fe mg and al were derived from the walls the development of alteration envelopes began with the infiltration of hydrothermal fluids into open extensional fractures and it ceased before complete filling of the veins when continuous mineral coatings formed along vein walls deposition of carbonate accompanied development of cryptic alteration including initial co 2 additions to the wall rocks subsequent deposition of tourmaline pyrite and pyrrhotite carbonate chlorite accompanied the progressive development of visible alteration which encroached on cryptically altered rocks visible alteration is characterized by additions of ca and co 2 to the wall rocks and by release of mg fe and al later bulk quartz deposition which filled the vein openings generally took place after complete development of the alteration envelopes except for this late quartz deposition of vein minerals is attributed to chemical changes in the fluid resulting from progressive wall rock alteration bulk quartz deposition is attributed to progressive cooling of the hydrothermal fluid

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

the elura ag pb zn mine in australia ore genesis in a slate belt by syndeformational metasomatism along hydrothermal fluid conduits the ag pb zn ore deposit at the elura mine new south wales australia is situated in a devonian slate belt and essentially consists of two vertically elongated ellipsoidal bodies of massive sulfide these are located above a zone of subeconomic vein mineralization from the boundary of each orebody toward its core the host sediment is progressively incorporated as inclusions in a deformed matrix of massive sulfide but continuity of bedding attitudes is retained across separate inclusions preserved bedding patterns and the sulfide deformation show that each orebody formed by metasomatism in the hinge of a developing anticline the two anticlines are doubly plunging structures with superposed smaller folds in which four generations of fold axial plane cleavages s 1 s 4 represent polyphase deformation d 1 d 4 the sulfide distribution in bedding parallel outliers of the orebodies is controlled by the s 2 cleavage but the style of d 2 folding within such horizons has been influenced by the presence of sulfide also there are mesoscopic replicas of the orebodies with a shape and orientation that reflect the vertically elongated d 2 strain ellipsoid most replicas lie across hinges of d 2 folds but locally their presence has affected the fold pattern these and other complex relationships are explained as a consequence of relatively rapid metasomatism by sulfide during d 2 deformation synkinematic syn d 2 porphyroblasts of siderite mark an alteration halo around the ore alteration patterns indicate a coeval origin for the ore and its halo which is further marked by vein hosted breccias within a swarm of tension veins crack seal microstructures in the latter indicate repeated microcracking during vertical stretching of the dome and basin folds at lithostatic pore fluid pressures structural relationships between the crack seal veins and breccia veins are complex but suggest periodic release of the overpressured fluids by brecciation of sediment the drops in fluid pressure triggered metasomatism by carbonates and sulfides in the two largest fold domes the brecciation is attributed to dome collapse following dilational rupture due to faulting in a system defined by shear zones and domains of bedding disruption

magnetic susceptibilities of rocks of the antarctic peninsula implications for the redox state of the batholith and the extent of metamorphic zones a combination of 1555 magnetic susceptibility measurements from the antarctic peninsula between 65 s to 72 s for which the raw data are made available and petrological structural and metamorphic data has been used to improve understanding of 1 magma sources for the antarctic peninsula batholith 2 genetic relationships between granitoids and metamorphic rocks and 3 to provide a semi quantitative measure of the extent and grade of metamorphic zones the redox state of jurassic plutonic rocks indicates that these had continental magma sources in the tectonostratigraphic terranes of the central and eastern domains triassic and cretaceous plutons on the other hand had subduction related sources in the central domain of the western antarctic peninsula and continental sources in the gondwana margin eastern domain circum pacific comparisons of magnetite ilmenite belt geometries identify the antarctic peninsula belt as a natural continuation of comparable south american belts susceptibility trends are generally retained by orthogneisses suggesting that they formed by deformation of plutonic rocks mostly at or below upper amphibolite facies conditions metasedimentary rocks are weakly magnetic across the peninsula and are spatially associated with continentally derived or crustally contaminated plutons ilmenite series orthogneisses and metabasites from strongly magnetic plutonic environments show areas of very low magnetic susceptibilities that may be linked to fluid enhanced tectonometamorphism induced reduction of magnetic susceptibility the distribution of reduced magnetic susceptibility areas follows the surface traces of major shear and fault zones in palmer land our results are consistent with tectonic interpretations of oblique compression and uplift of an arc block on the pacific margin of gondwana during the palmer land event

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