evidence for interaction of a spreading ridge with the outer california borderland abstract the southward passage of the rivera triple junction and its effect on the north american plate are primary controls on the miocene tectonic evolution of the outer borderland of california detrital modes of sand shed off the patton ridge and cored by the deep sea drilling project provide evidence of progressive tectonic erosion of the patton accretionary prism and near trench volcanism volcanic glass in the sediment is predominantly calcalkaline rhyolite and andesite typical of subduction related volcanism but also includes minor low k2o tholeiitic basalt we attribute these compositional features to interaction with a spreading ridge associated with a possible trench ridge trench triple junction along the patton escarpment from 18 to 16 ma this study suggests that evidence of ridge trench interaction may be commonly preserved along submerged plate margins in contrast to its more limited recognition and discussion in the literature based on exposed examples in chile japan and alaska

relations between fluid inclusion trails and regional stress field a tool for fluid chronology an example of an intragranitic uranium ore deposit northwest massif central france abstract episyenites within the uranium ore bearing le bernardan granite northwest french massif central were formed through the dissolution of quartz by hydrothermal solutions these solutions may now be found fossilized as fluid inclusions f i the f i form rectilinear trails along microcracks which trapped the percolating fluids simultaneous study of the orientations of the trails and the chemistry of the inclusions shows that 1 1 the trails may be grouped according to direction each related to successive regional stress fields 2 2 the fluids of each group are compositionally distinct marking different stages of the hydrothermal event this example shows that the geometric relations between secondary f i trails and regional stress fields may be used to establish the chronology of both during a region s geological evolution

slope and deep sea fan facies and paleogeography of upper cretaceous chatsworth formation simi hills california the upper cretaceous chatsworth formation is a sand rich deep sea fan deposit consisting of thick channelized sequences and interchannel deposits typical of middle fan facies it has poorly developed outer fan lobes and slope facies petrographic and x ray diffraction xrd data indicate the arkosic deposits of the chatsworth formation were derived from an uplifted granitic terrane paleocurrent studies indicate northward sediment transport and an east trending slope regional comparisons of the simi hills with the santa monica and santa ana mountains suggest these terranes were once contiguous and covered part of a deep sea fan system that comprised much of the southern california area subsequently these terranes have been offset along the malibu cucamonga and elsinore faults and rotated in response to the opening of the los angeles basin and to other adjustments in the tectonic framework of the area the chatsworth formation is thought to be part of a forearc basin deposit similar to the great valley sequence and not part of the salinian borderland block the original geographic position of the forearc basin was to the south of the sierra nevada mountains and the great valley sequence

structural controls on fe mineralization at thabazimbi mine south africa abstract chemical sedimentary rocks of the transvaal supergroup along the northern margin of the transvaal basin were imbricated during formation of the mohlapitsi fold and thrust belt between ca 2 55 and 1 93 ga high grade 63 fe stratiform fe ore bodies occur at or close to the base of the banded ironstone dominated penge formation which forms a distinct e w trending mountain range in the area the penge formation is separated from underlying malmani dolostone by a thin laterally continuous rheologically weak carbonaceous shale layer that has been intensely sheared this horizon denotes the position of a regional top to n low angle detachment that has been variably deformed during progressive development of the mohlapitsi belt several of kumba s thabazimbi iron ore deposits including kwaggashoek east donkerpoort west kumba donkerpoort west and buffelshoek west are confined to a zone directly above this detachment recent studies of ore petrography fluid alteration fluid composition and stable isotope geochemistry link economic iron ore mineralization to hydrothermal or hypothermal processes along with localized high grade mineralization a structural control on fluid flow has been suggested although a lack of structural data has hampered more concise mineralization models thus far in this study we present and analyse data from detailed structural mapping over a protracted period of mining across four opencast pits this mapping is combined with drillhole data to produce accurate intergrated 3d models of these deposits which reveal the true geometry of these ore bodies data and resultant models indicate that enhanced hydrothermal fluid flow associated with significant iron mineralization is linked to thrusting shearing in response to 1 fold amplification and flexural flow along the e w trending detachment between the malmani dolostone and banded iron formation 2 subsidiary fold accommodation structures such as conjugate shear joints normal and reverse faults and thrusts associated with inner and outer arc shortening and extension respectively 3 localization of fe at the intersections of major reverse faults the detachment and stratigraphically higher low angle thrusts which are preferentially developed at the contacts between diabase and banded iron formation collectively these features both aided and compartmentalized regional pervasive fluid flow along the detachment and in stratigraphically higher sites subsequent ore preservation was enhanced by reactivation of nw to nnw trending mesozoic normal faults and concomitant downthrow of fault bounded blocks structural relationships and timing of high grade iron ore mineralization further suggest that associated top to n kinematic fold and thrust formation of the mohlapitsi belt and subsequently the main detachment between malmani dolostone and banded iron formation of the penge formation could have been active during and immediately after intrusion of the bushveld complex at ca 2 05 ga

cumulative deformation and original geometry of the bushveld complex abstract a structural review and re interpretation of the bushveld complex based on 254 georeferenced maps from research technical reports and geophysical surveys suggests that its current geometry differs markedly from its original geometry broadly n s flattening left lateral transpressional shearing along an interconnected fault network steepening of cumulate layering and 10 km of uplift to the north of the northern margin of the complex s main chamber were accompanied by an eastward verging escape structure burgersfort bulge and syn cooling gap areas on its western limb southward verging deformation was transmitted from the villa nora fragment via thrusting in the rooiberg area generating 50 70 km of southward displacement uplift and erosional loss of the northern limb s western portion a string of anomalous occurrences of bushveld complex units and dense areas outlined by unconstrained gravity isoshells suggest a further 170 km southward displacement through the center of the main chamber the southern limb underwent predominantly right lateral ductile offset from the western limb along the crocodile river and rustenburg faults expressed by an in line series of nnw trending magnetic highs to the south of their mapped terminations left lateral shearing and progressive splaying of a nne trending dyke swarm to the west of an inflection point provides strain markers for this offset and 35 anticlockwise rotation of the burgersfort bulge and central parts of the main chamber although recent work suggests that the complex cooled within 1 myr late intrusion syn cooling sse wards movement of its deeper ductile central parts within at least 5 10 myr after its initial intrusion the transvaalide fold and thrust event and the expulsion of fluids to form the high ti igneous suite hitis and related bodies appear to have closely post dated the complex s intrusion removal of the effects of cumulative strain produces a more equant complex shortly after its intrusion occupying an orthogonal network of normal faults and perpendicular strike slip transverse faults

flow bearing structures of fractured rocks insights from hydraulic property scalings revealed by a pumping test abstract a long duration pumping test conducted over 151 days in a fractured sandstone and shale formation displays a nonstandard drawdown response and anomalous pressure diffusion which cannot be properly interpreted using existing frameworks e g homogeneous double porosity boundary conditions and fractal models an alternative framework with simple geometry and more complex hydraulic properties is thus proposed to interpret such kind of drawdown responses the analytical development allows first to demonstrate all scaling relations in this interpretation framework then and most importantly the multi scale hydraulic test provides consistent scalings of transmissivity t to storativity s over distances ranging from 83 to 383 m in a faulted area drawdown analysis in several monitoring wells shows persistent decrease of transmissivity in highly channelized fracture flow structures in one structure the cubic dependency of transmissivity to storativity identifies a well defined fault and also demonstrates the validity of poiseuille flow at a scale rarely investigated in the other structure the linear dependency of transmissivity to storativity indicates that the flow bearing structure is the surrounding fracture network well designed pumping tests combined with scaling analysis driven by geological evidence thus provide essential information on flow bearing structures for site characterization and modeling tasks at least for moderate to low permeable fractured rocks the scaling of transmissivity to storativity appears to be more informative than any separate interpretation of hydraulic property scaling exponents

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

new geochronological results and structural evolution of the pataz gold mining district implications for the timing and origin of the batholith hosted veins abstract the paleozoic pataz parcoy gold mining area is located in a right stepping jog on the regional cordillera blanca fault in northern peru most of the 8 million ounces of gold production from this area has come from quartz carbonate sulfide veins hosted by the pataz batholith despite a subduction zone setting since at least the cambrian the area records several periods of extension and its present structure is that of a rift and graben terrain the pataz district the northern part of the pataz parcoy area is dominated structurally by northwest to north northwest striking nw nnw faults and northeast to east northeast striking ne ene lineaments both of which have been active periodically since at least the mississippian early carboniferous nw nnw faults control the margins of a central horst that exposes basement schist and the pataz batholith and step across ne ene lineaments the lavasen graben to the east of the central horst contains the lavasen volcanics and the chagual graben to the west contains an allochthonous sedimentary sequence derived from the western andean cordillera new shrimp zircon geochronological data indicate emplacement of the pataz batholith during the middle mississippian at around 338 336 ma approximately 10 ma earlier than previous estimates based on 40 ar 39 ar geochronology the calc alkaline i type batholith comprises diorite and granodiorite the latter being the major component of the batholith and was emplaced as a sill complex within the moderately ne dipping sequence of the eastern andean cordillera moderate to high temperature ductile deformation took place on the batholith contacts during or shortly after emplacement following emplacement of the batholith differential uplift occurred along nw nnw faults forming the lavasen graben into which the lavasen volcanics were deposited shrimp u pb in zircon ages for the lavasen volcanics and the esperanza subvolcanic complex which was intruded into the western margin of the graben are within error of one another at ca 334 ma the ductile batholith contacts were cut by renewed movement on nw nnw faults such that the margins of the batholith are now controlled by these steep brittle ductile faults the nw nnw faults were oriented normal to the principal axis of regional shortening ene wsw during formation of the batholith hosted gold bearing quartz carbonate sulfide veins the misoriented faults were unable to accommodate significant displacement leading to high fluid pressures vertical extension in the competent batholith and formation of gold bearing veins brittle failure of the batholith was most extensive in the northern pataz district where the fault controlled western contact of the batholith is offset by a swarm of ne ene lineaments the timing of vein formation is not established despite published 40 ar 39 ar ages of 312 to 314 ma for metasomatic white mica which are interpreted as minimum ages of formation gold bearing veins formed during or shortly after uplift of the pataz batholith and formation of the lavasen graben they were therefore broadly coeval with deposition of the lavasen volcanics and emplacement of the esperanza subvolcanic complex these k rich weakly alkalic ferroan a type magmas may provide a viable source for the ore fluid that deposited gold in the pataz batholith

the role of fluid pressure in the formation of bedding parallel thrust hosted gold deposits sabie pilgrim s rest goldfield eastern transvaal abstract shallowly dipping thrust hosted gold veins occur within the paleoproterozoic transvaal supergroup rocks in the eastern transvaal of south africa internally the veins record episodic events of fluid infiltration and subsequent mineral deposition within discrete local scale thrust faults evidence also exists for subvertical dilation within the individual veins and textures which record open space filling are preserved the probable depth of formation suggests lithostatic pressures in excess of 2 2 kbar and the vein textures indicate numerous reactivation events associated with individual thrust faults occurring in a terrain characterised by low levels of preserved strain fault reactivation may have occurred under conditions of low differential stress in the presence of high pressure fluids with p fluid approaching or exceeding lithostatic levels

structural and statistical characterization of joints and multi scale faults in an alternating sandstone and shale turbidite sequence at the santa susana field laboratory implications for their effects on groundwater flow and contaminant transport abstract this paper describes the properties of faults and fractures in the upper cretaceous chatsworth formation exposed at santa susana field laboratory and its surroundings simi hills california where groundwater flow and contamination have been studied for over three decades the complex depositional architecture of this turbidite consisting of alternating sandstones and shales interacting with formative stress conditions are responsible for multi scale fault hierarchies and permeable fractures in which nearly all groundwater flow occurs intensity and distribution of background fractures and their relation to bedding thickness are established for sandstones the dominant lithology the architecture of faults with increasing displacement is described and relationships among fault dimensional parameters captured data from 400 boreholes and piezometers reveal the effect of faults and fractures on groundwater flow large hydraulic head differences observed across fault zones with shale rich cores indicate these structures as cross flow barriers moreover hydraulic head profiles under ambient conditions and pumping tests suggest strong hydraulic connectivity in all directions to depth of hundreds of meters this outcrop based structural characterization relates the horizontal hydraulic conductivity to the observed well connected fracture network and explains the strong vertical connectivity across low hydraulic conductivity shales as faults and sheared fractures provide flow pathways

compaction corrected paleomagnetic paleolatitudes for late cretaceous rudists along the cretaceous california margin evidence for less than 1500 km of post late cretaceous offset for baja british columbia the paleolatitudinal distribution of bivalve rudists has important significance for the baja british columbia baja bc hypothesis that western canadian superterranes from british columbia have been displaced 3000 km since cretaceous time rudists are not observed in baja bc sedimentary rocks yet they are common in late cretaceous strata in california and baja california which have the same paleomagnetically determined paleolatitudes approximately 25 n as baja bc rocks of late cretaceous age in order to resolve this contradiction and to delimit more exactly the southern paleolatitudes of baja bc paleomagnetic inclinations corrected for the effects of burial compaction were used to determine the paleolatitudinal distribution of rudists along the california margin compaction corrected paleomagnetic data from the peninsular ranges and salinia terranes indicate that rudists were restricted to paleolatitudes between 34 and 40 n evidence of coastal upwelling in the latest cretaceous marca shale may explain the northern limit of the rudist distribution these data suggest that baja bc was no farther south than 40 n in the late cretaceous thus limiting its post cretaceous displacement to less than 1500 km and that burial compaction has also affected the paleomagnetism of nanaimo group sedimentary rocks from vancouver island this result also helps resolve the conflict between paleomagnetic results which show 1500 km of post late cretaceous offset between the insular coast plutonic complex superterrane and the intermontane superterrane and geologic observations which can allow only tens of kilometers of offset between these terranes in the methow tyaughton basin

bedding parallel fibrous veins beef and cone in cone worldwide occurrence and possible significance in terms of fluid overpressure hydrocarbon generation and mineralization abstract bedding parallel fibrous veins are common worldwide in sedimentary basins especially within strata of low permeability the term beef refers to bedding parallel veins of fibrous minerals where the fibres are mutually parallel and have formed quasi vertically more complex on a smaller scale are cone in cone structures yet these are also common within bedding parallel veins for both beef and cone in cone we have compiled a worldwide catalogue 157 localities typically the veins consist mainly of white gangue minerals for example calcite gypsum or quartz but may also contain accessory minerals of economic interest for example bitumen sulphides emerald pitchblende or gold fluid inclusions may contain oil or gas calcite beef 110 localities is common in organic rich shale of marine carbonate origin especially of 1 cambrian ordovician 2 devonian carboniferous 3 early jurassic or 4 cretaceous to palaeogene ages gypsum beef 30 localities is common in evaporitic or lacustrine strata of continental origin especially of triassic or neogene ages quartz beef 17 localities is common within meta turbidite sequences especially of ordovician or proterozoic ages because these modal ages seem to reflect climatic controls we infer that the fibre forming mineral species have not travelled far vertically the same conclusion holds for accessory minerals typical temperatures of formation are 1 up to 60 c for gypsum beef 2 70 c to 120 c for calcite beef and 3 200 c to 350 c for quartz beef hydrocarbon bearing calcite beef may be a good indicator of a petroleum system in which oil or gas migrate together with aqueous solutions we argue that beef and cone in cone layers result from tensile fracturing and vertical dilation coeval with fibre growth possible causes are either 1 force of crystallization or 2 seepage forces due to fluid overpressure for layers that form at depths of several km fluid overpressure is the more likely cause

flexural slip structures in the bushveld complex south africa this paper investigates structures in the layered critical zone of the bushveld complex in south africa that have detrimentally affected mining operations the common structures are layer parallel faults with reverse dip slip and ramp faults with curved slip planes and prominent striations geometric variations include duplex ramps contained within floor and roof layer parallel faults and linking ramps connecting separate layer parallel faults hidden in the footwall and hangingwall the orientations geometries displacements and shear senses of the layer parallel faults and ramp faults are interpreted to be flexural slip structures formed during bending of the originally horizontal bushveld complex into a basin fold geometry during crustal loading

the terminology of structures in thrust belts abstract a review of structures and geometric relationships recognized in thrust belts is presented a thrust is defined as any contractional fault a corollary being that thrusts must cut up section in their transport direction flats are those portions of a thrust surface which were parallel to an arbitrary datum surface at the time of displacement and ramps are those portions of thrusts which cut across datum surfaces ramps are classified on the basis of their orientation relative to the thrust transport direction and whether they are cut offs in the hangingwall or footwall of the thrust lateral variations in the form of staircase trajectories are joined by oblique or lateral ramps which have a component of strike slip movement an array of thrusts which diverge in their transport direction may form by either of two propagation models these are termed piggy back propagation which is foreland directed and overstep propagation which is opposed to the thrust transport direction an array of thrust surfaces is termed an imbricate stack and should these surfaces anastamose upwards a duplex will result the fault bounded blocks are termed horses a duplex is bounded by a higher roof thrust and a lower floor thrust the intersection of any two thrust planes is termed a branch line thrusts can be classified on the basis of their relationship to asymmetric fold limbs which they cut a further classification arises from whether a particular thrust lies in the hangingwall or footwall of another one the movement of thrust sheets over corrugated surfaces or the local development of thrust structures beneath will fold higher thrust sheets these folds are termed culminations and their limbs are termed culmination walls accommodation of this folding may require movement on surfaces within the hangingwall of the active thrust these accommodation surfaces are termed hangingwall detachments and they need not root down into the active thrust this category of detachment includes dip slip hangingwall drop faults which are developed by differential uplift of duplex roofs and out of the syncline thrusts which develop from overtightened fold hinges back thrusts as well as forming as hangingwall detachments may also form due to layer parallel shortening above a sticking thrust or by rotation of the hangingwall above a ramp

stratigraphic and structural controls on gold mineralization in the pilgrim s rest goldfield eastern transvaal south africa abstract since 1872 approximately 185 metric tons of gold and a significant amount of silver have been won from numerous stratiform ore bodies within the pilgrim s rest goldfield the epigenetic stratiform ore bodies are found primarily in the malmani dolomite subgroup a laterally extensive lower proterozoic carbonate platform succession a strong relationship exists between the presence of stratiform ore bodies and environment of deposition and structural deformation analysis of the host sediments indicates deposition in a retrogradational followed by an aggradational sequence that comprises supratidal sabkha siliciclastic sediments to midshelf carbonates the retrogradational and aggradational sequences are characterized by numerous higher frequency transgressive and regressive cycles mineralization is with few exceptions restricted to shallow water siliciclastic sediments deeper water carbonates are barren more importantly siliciclastic mudstones and sandstones have acted as zones of detachment for thin skinned thrust fault deformation resulting in the development of conformable passageways in which fluids concentrated their auriferous lodes evidence of shallow hinterland dipping duplex antiformal stack and imbricate thrust systems abounds in several underground and field exposures the gold silver emplacement model for the pilgrim s rest goldfield infers fluids derived from a deep seated magmatic source oxygen isotopic compositions of the mineralizing fluids recalculated from 18o quartz 12 1 to 19 5 point to a magmatic source and subsequent mixing with saline evolved formation water a magmatic origin is also supported by the high homogenization temperature gradient displayed by the fluid inclusions 100 c km the bushveld igneous complex rustenburg layered suite to the west of pilgrim s rest is thought to be the source of the auriferous hydrothermal solutions and its emplacement was the driving mechanism of thrust fault deformation in the pilgrim s rest goldfield

intrusion hosted mineralization in the charters towers goldfield north queensland new isotopic and fluid inclusion constraints on the timing and origin of the auriferous veins auriferous quartz sulfide veins of the charters towers goldfield are mainly hosted in oxidized i type granitoids of the ravenswood batholith k ar and ar ar isotope age data of hydrothermal muscovite from alteration envelopes of veins at charters towers and the hadleigh castle mine 40 km east of charters towers are indistinguishable within error suggesting broadly synchronous gold deposition between 410 and 404 ma and across a significant segment of the ravenswood batholith published geochronological data indicate that several granitoid bodies were emplaced into the ravenswood batholith at the time of gold mineralization despite their association in space and time with igneous rocks the gold deposits lack an obvious causative intrusion moreover published lead isotope studies indicate that the lead in the ore was not acquired from any of the exposed intrusions a distal origin of the ore forming fluids also may be inferred from nitrogen isotope values of hydrothermal sericite suggesting wall rock interaction with metamorphic fluids or fluids that were in equilibrium with metamorphic rocks during the paragenetic stage of pyrite and arsenopyrite deposition stage ii veins of the charters towers goldfield contain three different types of fluid inclusions which are distinguishable by petrography and microthermometry this study is the first to report saline fluid inclusions in sphalerite 18 9 28 3 wt nacl equiv and vein quartz 20 9 24 7 wt nacl equiv trapped during the stage of gold deposition stage iii the range of temperatures and salinities particularly of the saline inclusions in sphalerite could indicate mixing between hotter more saline fluids e g deep seated magmatic and cooler more dilute solutions e g modified meteoric as the cause of gold deposition the geological and geochemical data are not compatible with derivation of fluids metals and ligands from individual plutons similarity of host rocks ore element associations alteration assemblages structural controls and tectonic settings strongly suggest that the auriferous veins of the charters towers goldfield belong to a group of granitoid hosted lode gold deposits that are generally classified as orogenic

thrust controlled gold mineralisation at the elandshoogte mine sabie pilgrim s rest goldfield south africa stratiform quartz sulphide gold veins locally termed reefs are hosted within the proterozoic transvaal sequence sedimentary succession in the sabie pilgrim s rest goldfield eastern transvaal these deposits have produced about 180 tonnes of gold and share many characteristics with those of telfer western australia detailed examination of the elandshoogte mine shows that gold deposition occurred in two stages both linked to bedding parallel thrust faulting within the sedimentary pile deformation being concentrated within incompetent shale beds interlayered within more competent units the majority of gold was introduced in the second stage of mineralisation and occurs within fractures in early formed sulphide minerals deposition of competent quartz veins accompanying early sulphide and gold mineralisation resulted in a change in deformation style within the reef zone from early shearing in shales to later duplex faulting of the quartz reef fluids accompanying faulting are implied to have transported gold and a magmatic source of mineralisation is suggested

water sill hypothesis for the origin of certain veins in the meguma group nova scotia canada certain bedding parallel quartz veins some gold bearing in the cambrian ordovician meguma group nova scotia are believed to indicate the former presence of water sills the veins contain primary subhedral quartz crystals which tend to be oriented normal to the vein walls and display brazil twins twinned crystals commonly show hexagonal sections with dark solid inclusion rich centers and white fluid inclusion rich rims viewed in a plane perpendicular to bedding the dark centers of crystals exhibit millimetre thick quartz plates bounded by bedding parallel micrometre thick inclusion bands of white micas the inclusion bands are aligned parallel to the vein walls and terminate within tapered crystals that have an outer zone of milky quartz the quartz veins are on limbs and hinges of folds particularly anticlines from these observations we conclude that 1 the solid inclusion banded quartz resulted from episodic hydraulic extension fracturing and incremental growth of dispersed quartz bridges across water sills 2 the milky quartz filled the vein by growing in optical continuity with the solid inclusion banded quartz 3 localization of certain bedding parallel quartz veins is due to ponding of water beneath aquacludes in very broad domes formed by differential compaction in the meguma group 4 during tectonic shortening these intensely veined rocks localized some of the major anticlines

textures paragenesis and wall rock alteration of lode gold deposits in the charters towers district north queensland implications for the conditions of ore formation ore deposits of the charters towers goldfield ctgf are mainly hosted by fault fill veins extensional 8 of all veins and stockwork like 3 veins are less common and of little economic significance crosscutting relationships and published structural and geochronological data indicate a late silurian to early devonian timing of gold mineralization coincident with regional shortening d4 and i type magmatism paragenetic relationships which are uniform in veins everywhere within the ctgf suggest that vein formation commenced with the deposition of large volumes of buck quartz stage i followed by buck and comb quartz and significant pyrite and arsenopyrite precipitation stage ii gold was introduced during stage iii after earlier sphalerite and coincident with galena and chalcopyrite narrow discontinuous calcite veins of stage iv mark the waning of gold related hydrothermal activity or a later unrelated episode ore zones within the veins are everywhere composed of comb and or gray quartz calcite and or ankerite and bands or clusters of fractured pyrite that are spatially associated with galena sphalerite or chalcopyrite low grade or barren vein sections on the other hand are mainly composed of milky buck quartz with little evidence for modification overprinting or interaction with later fluids gold related hydrothermal wall rock alteration is symmetrically zoned displaying proximal sericite ankerite and distal epidote chlorite hematite assemblages that may be taken to imply wall rock interaction with near neutral fluids ph 5 6 isocon plots assuming immobile al p ti y and zr consistently indicate as k pb s and zn enrichment and na si and sr depletion in altered wall rock specimens relative to the least altered rocks alteration assemblages quartz textures fault rocks and published fluid inclusion and stable isotope data imply that the veins were formed under conditions of episodic fluid overpressuring 0 9 3 8 kbar at a depth of 7 km and a temperature of 310 c the published fluid inclusion data also imply that gold precipitation may have been brought about by fluid mixing however physi and chemisorption of gold complexes onto sulfide surfaces may have been important depositional processes and controls on gold enrichment at the millimeter to centimeter scale given that most gold particles are attached to the surfaces of pyrite crystals of stage ii or to etch pits and fracture surfaces within the earlier pyrite

deep sea fan deposits in the macigno formation middle upper oligocene of the gordana valley northern apennines italy abstract facies characteristics are described for a 2300 m thick turbidite succession of the macigno formation middle upper oligocene which crops out as a continuously exposed section in the gordana valley northern apennines in the study area the macigno formation consists of two sandy members separated by a shaly member the sandy members contain lithic associations characteristic of submarine fan deposits and form distinct fan complexes the middle shaly member represents a period of basin starvation three main facies associations are recognized in the macigno fan deposits fan fringe outer fan lobe and braided midfan facies associations the fan fringe deposits consist of monotonous or small scale cyclically arranged thin bedded turbidites the outer fan lobe deposits are composed of medium to very thick bedded classical turbidites internally organized in thickening upward cycles with minor intercalations of thin bedded sandstone beds two distinct types of outer fan lobe deposits distal and proximal are recognized on the basis of rock type internal organization and vertical facies relationships the midfan deposits consist of amalgamated massive sandstone and pebbly sandstone which display repeated thinning upward cycles the three main facies associations are superposed t define major thickening and coarsening upward cycles reflecting distinct episodes of fan progradation the relationships between the facies associations of the macigno deposits and the existing models of submarine fan sedimentation are discussed after the deposition of the fan complexes fan sedimentation in this part of the basin was replaced by movements of allochthonous units heralded by large olistostromes

book review the techniques of modern structural geology volume 2 folds and fractures john g ramsay and martin i huber academic press london 1987 xi 309 398 pp price stg17 50 us 34 50 paperback

the geochemistry of black shales from the chuniespoort group transvaal sequence eastern transvaal south africa geochemical and mineralogical data have been obtained from black carbonaceous shales interbedded with dolomite in the chuniespoort group of the late archean early proterozoic transvaal sequence the black shales are fine grained and generally argillaceous and contain on average 1 2 percent total c organic 0 7 percent s 7 5 ppb au 60 ppm cu 330 ppm cr and 125 ppm ni these metal concentrations are similar to or slightly higher than the values given for the u s geological survey black shale standard sdo 1 all other heavy and transition metals however are depleted in the chuniespoort shales relative to sdo 1 it is well known that high concentrations of cr and ni occur in archcan shales the enhanced cr and ni values found in the chuniespoort shales are a reflection of their antiquity and provenance and are not linked to preferential uptake of these metals during sedimentation and or diagenesis the chuniespoort shales are characterized by excess fe with respect to that required to form pyrite and have extremely variable c s ratios as well this together with the wide ranging s isotope ratios is taken as an indication that biogenically produced sulfide was augmented by sulfide introduced from an extraneous source whole rock s isotopes further indicate that s could have been derived from biogenic and inorganic reduction of sulfate as well as from hydrothermal fluids interelement correlations suggest that organic carbon does not play a role in hosting significant concentrations of base and transition metals rather element associations indicate that v and cr are preferentially contained in illite hydromuscovite and that ni co zn cu fe pb and mo probably reside in pyrite and or chalcopyrite gold is only weakly correlated with other elements which may indicate that its distribution is not solely determined by the presence of sulfide minerals the fact that au cu and s c ratios show an increase in samples with delta 34 s values around 0 per mil suggests that these elements are likely to have an epigenetic hydrothermal origin the limited syngenetic metal accumulation can be explained by the stable cratonic environment in which the sediments were deposited and the absence of submarine hydrothermal springs venting metalliferous fluids into the basin other factors such as the low metal potential of the provenance the rate of sedimentation and the degree of oxygenation of the bottom waters of the basin may have been additional factors limiting the concentration in and extraction of metals from seawater the fact that the chuniespoort shales host significant epigenetic gold occurrences particularly in the sabie pilgrim9s rest gold field is largely related to their chemical characteristics which promoted permeability and high absorption capacities for metals in hydrothermal fluids because of their low metal concentrations elsewhere in the basin the shales cannot however be considered the source rocks for gold mineralization

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

origin and paleoenvironmental interpretation of aluminum phosphate sulfate minerals in a neoproterozoic baltic paleosol abstract aluminum phosphate sulfate aps mineral solid solutions occur as accessory phases in different sedimentary and hydrothermal deposits their composition is a sensitive environmental indicator recording changes in ph temperature and chemical composition of the weathering diagenetic or hydrothermal fluids in this contribution we studied aps mineralization in a neoproterozoic paleotropical paleosol developed on paleo mesoproterozoic crystalline basement in the baltic basin small and disseminated aps minerals occur in high abundance up to 4 wt of crystalline phases in the weathering profile developed on gabbroic rocks rich in magmatic apatite aps minerals belonging to a goyazite florencite svanbergite woodhouseite solid solution series occur in the uppermost part of the weathering profile and are replaced down profile with secondary apatite the change from aps minerals to secondary apatite precipitates reflects a paleo ph gradient in the weathering profile from acidic ph

using earth tide induced water pressure changes to measure in situ permeability a comparison with long term pumping tests good constraints on hydrogeological properties are an important first step in any quantitative model of groundwater flow field estimation of permeability is difficult as it varies over orders of magnitude in natural systems and is scale dependent here we directly compare permeabilities inferred from tidal responses with conventional large scale long term pumping tests at the same site tidally induced water pressure changes recorded in wells are used to infer permeability at ten locations in a densely fractured sandstone unit each location is either an open hole well or a port in a multilevel monitoring well tidal response is compared at each location to the results of two conventional long term and large scale pumping tests performed at the same site we obtained consistent values between the methods for a range of site specific permeabilities varying from 10 15 m2 to 10 13 m2 for both open wells with large open intervals and multilevel monitoring well we conclude that the tidal analysis is able to capture passive and accurate estimates of permeability

structural aspects of fluid rock interactions in detachment zones metamorphic core complexes and associated detachment faults of the north american cordillera represent gently dipping normal displacement shear zones detachment zones along which hot deeper levels of the crust were transported upward and outward from underneath a brittlely distended upper plate structures that formed during the ductile to brittle evolution of detachment zones can be used to reconstruct the relative magnitudes of fluid pressure and deviatoric stress at different levels within the shear zones mylonitization which occurred along deeper segments of detachment zones below the brittle ductile transition was locally accompanied by tensile failure indicative of low deviatoric stress and high fluid pressure this condition persisted into the earliest phases of brittle deformation after which shear fractures formed due to both a reduction in fluid pressure and an increase in deviatoric stress brittle deformation of upper plate rocks occurred largely under conditions of low fluid pressure structural and geochemical data suggest that normal displacement on detachment zones results in establishment of two fluid systems 1 an upper plate system driven by convection and dominated by meteoric and connate fluids at near hydrostatic pressures and 2 a system within deeper levels of the shear zone where fluids are largely derived from igneous sources and fluid migration is aided by dilatancy pumping the late phases of normal displacement on detachment zones structurally juxtaposed rocks affected by the two fluid systems and locally caused the shear zone rocks to be overprinted by mineralization related to the upper plate fluid system

a note on fault reactivation reactivation of existing faults whose normal lies in the 1 3 plane of a stress field with effective principal compressive stresses 1 2 3 is considered for the simplest frictional failure criterion n n p where and n are respectively the shear and normal stresses to the existing fault p is the fluid pressure and is the static friction for a plane oriented at to 1 the stress ratio for reactivation is 1 3 1 cot 1 tan this ratio has a minimum positive value at the optimum angle for reactivation given by 12tan 1 1 but reaches infinity when 2 beyond which 3 0 is a necessary condition for reactivation an important consequence is that for typical rock friction coefficients it is unlikely that normal faults will be reactivated as high angle reverse faults or thrusts as low angle normal faults unless the effective least principal stress is tensile

mechanisms of recharge in a fractured porous rock aquifer in a semi arid region abstract eleven porewater profiles in rock core from an upland exposed sandstone vadose zone in southern california with thickness varying between 10 and 62 m were analyzed for chloride cl concentration to examine recharge mechanisms estimate travel times in the vadose zone assess spatial and temporal variability of recharge and determine effects of land use changes on recharge as a function of their location and the local terrain the profiles were classified into four groups reflecting the range of site characteristics century to millennium average recharge varied from 4 to 23 mm y 1 corresponding to 1 moreover the temporal variability of recharge was estimated and along each profile past recharge rates calculated based on the sequence of cl concentrations in the vadose zone recharge rates increased at specific times coincident with historical changes in land use the consistency between the timing of land use modifications and changes in cl concentration and the match between observed and simulated cl concentration values in the vadose zone provide confidence in porewater age estimates travel times recharge estimates and reconstruction of recharge histories this study represents an advancement of the application of the chloride mass balance method to simultaneously determine recharge mechanisms and reconstruct location specific recharge histories in fractured porous rock aquifers the proposed approach can be applied worldwide at sites with similar climatic and geologic characteristics

microcracks in rocks a review abstract in the past decade the number of studies about microcracks in rocks has rapidly increased this review of recent work concentrates on microcracks in rock as separate entities emphasizing microcrack morphogenesis kinematics dynamics population statistics and observational techniques cracks are produced when the local stress exceeds the local strength the local stress may be augmented by twin lamellae interactions kink bands and deformation lamellae stress concentrations at grain boundary contacts and around intracrystalline cavities local strength may be reduced along cleavage planes along grain boundaries and along any internal surface as a result of corrosion by chemically active fluids dislocations appear not to be a significant factor for crack nucleation below about 500 c in silicates spatial and temporal changes in temperature can also induce microcracking as a result of differential thermal expansion between grains with different thermoelastic moduli and thermal conductivities the amount of quartz in the rock has a significant effect on thermally induced microcracks because of its large and variable thermal expansivity the application of hydrostatic pressure between 100 and 200 mpa effectively closes most cracks but the closure may not be uniform if crack wall asperities exist hydrostatic pressure appears to stabilize cracks and make crack growth more difficult the number and average size of mechanically induced microcracks is greater in rock deformed at higher pressures the application of a deviatoric stress field on the boundaries of a rock mass results on a microscopic scale in a very complex stress system which greatly affects nucleation and propagation paths the relative amount of intragranular and intergranular cracking appears to depend upon mineralogy rock type and stress state the vast majority of stress induced microcracks in rocks appear to be extensional statistically they are predominantly oriented within 30 of the macroscopic maximum stress direction crack densities increase as macroscopic deviatoric stress increases above a threshold level crack size distributions may be either lognormal or exponential fracture in rock under compressive boundary loads is a result of the coalescence of many microcracks not the growth of a single crack some crack configurations are more favorable for coalescence than others as deviatoric stress increases and rock failure is approached the microcrack population changes spatially from random to locally intense zones of cracking away from the fault the crack density dies off rapidly to the background level a few grains away under lesser deviatoric stresses slow subcritical microcrack growth can occur as a result of stress aided corrosion at the crack tip the rate governing mechanism may be either the chemical reaction rate or the rate at which water can get to the crack tip important details still remain to be worked out

the structure of shear related vein type gold deposits a review abstract the structure of vein type gold deposits is defined by the shapes and geometrical relationships of mineralized bodies the form of the mineralization making up these bodies and the sequence of vein forming events most mineralized zones occur within or are spatially associated with shear zones especially shear zones in larger systems of intersecting shear zone sets they range in shape from tabular to linear and in form from disseminated to breccia to stockwork or sheeted veinlet zones to single veins there typically is a complex history of mineral deposition which overlaps and is genetically related to the deformation that generated the host structural zone the purpose of this paper is to consider the structural features of vein type deposits in relation to the geometrical properties and evolution of veinhosting fractures particularly shear zones the evidence indicates that veins are localized by tectonically generated dilatancy in an environment of low mean stress caused by high fluid pressure it is proposed that a major cause of tectonic dilation of shear zones is the interference between intersecting shears during bulk inhomogeneous flattening by movement on systems of intersecting shear zone sets