magmatic evolution of a volcano studied by 230th 238u disequilibrium and trace elements systematics the etna case age determinations of several lava flows from etna through 230th 238u disequilibrium internal isochrons yield a precise chronology of the volcano s history for the last 200 000 years and emphasize the main episodes in the formation of this huge complex strato volcano study of 230th232th 0 initial ratios of lavas together with their trace element compositions yields a consistent model of magmatic evolution implying the existence for 200 000 years of a deep reservoir of alkalic magma periodically mixed with magmas of tholeiitic affinity these short periods of mixing appear to be related to the formation of the large calderas of etna in addition to these processes affecting the deep reservoir fractional crystallization also occurred in more superficial levels of the volcanic edifice thereby yielding several series of differentiation of relatively short duration as for its geochemistry etna s volcanism is of oceanic type but with its own characteristics and in a peculiar geodynamic context at the edge of the african plate

fissure fills along faults variscan examples from gower south wales the extent to which persistent rather than transient fissures wide planar voids can exist along upper crustal faults is important in assessing fault permeability to mineral and hydrocarbon bearing fluids variscan late carboniferous faults cutting dinantian lower carboniferous limestones on the gower peninsula south wales host clear evidence for fissures up to several metres wide evidence includes dendritic hematite growth and elongate calcite growth into open voids spar ball and cockade breccia formation laminated sediment infill and void collapse breccias detailed mapping reveals cross cutting geometries and brecciation of earlier fissure fills showing that fissures were formed during rather than after active faulting fissures therefore probably formed by geometric mismatch between displaced fault walls rather than by solution widening along inactive faults

on the reliability of the 230th 238u dating method applied to young volcanic rocks abstract hemond and condomines 1985 claim that our results on zero age volcanic materials of vesuvius and etna are incorrect that no disequilibrium exists in those materials and that the validity of the 230 th 238 u dating method is thus confirmed we show that other researchers obtained similar results in zero age rocks the validity of the 230 th 238 u dating method is simply ruled out by a comparison of the ages obtained by this method with those obtained on the same samples by different methods

geology and geochemistry of gunung subang gold deposit tanggeung cianjur west java indonesia abstract the gunung subang gold deposit is located in the sunda arc volcanic belt in west java indonesia the ore bodies are associated with volcanic rocks from the miocene koleberes and bentang formation to constrain the significance of the ore genesis and mineralization this study was carried out with the aim of examining the deposit using methods such as detailed field observation petrography petrochemistry zircon u pb dating hf isotope fluid inclusion analysis and sulfur and in situ pb isotope analyses the results suggest the following 1 the volcanic rocks in gunung subang belong to the basalt andesite group with 48 66 79 17 average 61 04 sio2 the trace element patterns of the volcanic rocks exhibit relatively low hree content flat hree patterns and weak negative or positive eu anomalies which are similar to those of typical arc magmatic rocks 2 most zircons selected from the volcanic rocks are magmatic and only a few are inherited the magmatic zircons have a weighted mean u pb age of 17 0 0 4 ma and the inherited zircons are from the precambrian dated at 887 2379 ma the magmatic zircons have ehf t values ranging from 1 24 to 13 59 indicating that the magmatism may be contaminated predominantly with juvenile crust but also with a small percentage of ancient crust 3 the sulfur isotopic compositions are fairly uniform and the 34s values of pyrite galena and sphalerite range from 0 53 to 2 97 suggesting a magmatic origin 4 the pb isotopic compositions of pyrite and galena are clustered and some include relatively high amounts of radiogenic lead the mean ratios of 206pb 204pb 207pb 204pb and 208pb 204pb are 18 59 0 007 15 65 0 007 and 38 89 0 017 respectively indicating that the ore forming material is highly related to miocene to pliocene volcanic rocks 5 fluid inclusions in the quartz from the ores are liquid and vapor liquid phases with components of the nacl h2o system with homogenization temperatures of 240 320 c and salinities of 0 2 6 2 wt nacl the coexistence of vapor and liquid rich fluid inclusions suggests that boiling occurred throughout the deposition of the ore 6 the results of electron probe microanalysis epma indicate that pyrite is the major au bearing mineral combined with the observation of scanning electron microscope sem gold is primarily present as au ag telluride minerals and there may also be native gold au0 finally we consider the gunung subang is an epithermal gold deposit related to the mid miocene magmatism

the geochemistry of potassic lavas from vulsini central italy and implications for mantle enrichment processes beneath the roman region major and trace element and 143nd 144nd 0 51209 0 51216 and 87sr 86sr 0 70879 0 71105 isotope analyses are presented on a representative group of lavas from the vulsini district of the roman magmatic province three distinct series are identified the high k and low k series are similar to those described from other italian volcanoes while the third is represented by a group of relative ly undifferentiated leucite basanites which are thought to be near primary mantle melts major and trace element variations within the high k series are consistent with fractional crystallisation from a parental magma similar to the most magnesian leucitites crustal contamination resulted in an increase in 87sr 86sr with increasing fractionation but it was superimposed on magmas which had already inherited a range of incompatible element and isotope ratios from enrichment processes in the sub continental mantle these are reviewed using the available results from vulsini roccamonfina and ernici transition element abundances and ta yb ratios indicate that the pre enrichment mantle was similar to that of e type morb and that these elements were not mobilised by the enrichment process mixing calculations suggest that three components were involved in the enrichment process mantle comparable with the source of morb and two other components rich in trace elements one the low k component had high sr nd th ta and ba nb and no europium anomaly while the second had lower sr nd a negative europium anomaly and very high th ta it was also characterised by low nb ba and high rb ba ratios similar to those reported from phlogopite rich peridotite xenoliths the trace element enrichment processes are therefore thought to have occurred in the mantle wedge above a subduction zone with the trace element characteristics of the high k end member reflecting the subduction of sediments and the stabilisation of mantle phlogopite

on the reliability of the 230th 238u dating method applied to young volcanic rocks discussion abstract the reliability of the 230 th 238 u dating method applied to young volcanic rocks was recently questioned by capaldi and pece 1981 on the basis of th isotopic disequilibria found in historic lava flows from etna vesuvius and stromboli in contrast with their results measurements carried out in our laboratory on lava samples from the same eruptions etna and vesuvius do not show any th isotopic disequilibrium between minerals and whole rock the general validity of this dating method is thus confirmed by our results

genesis of cockade breccias in the tectonic evolution of the cirotan epithermal gold system west java the pliocene cirotan gold deposit provides an exceptional example of mineralized breccias and in particular cockade breccias analysis of these breccias approached through both field observation and image analysis on photographs made it possible to determine their method of graded formation the mineralized structure as much a 25 m thick in places is a right lateral strike slip fault that evolved to a normal fault at the end of the system s development it consists of juxtaposed breccia bodies separated by major slip or septa which formed at a major inflection of the host fault the breccia bodies all show a similar internal organization from footwall to hanging wall one passes from a siliceous breccia with angular clasts to a thick infill of cockade breccia showing a reverse grading with small centimetre size cockades aspect ratios of 1 10 passing to larger up to 1 m more equant cockades aspect ratios 1 3 in contact with the hanging wall the different stages of cockade formation demonst

a practical procedure for the back analysis of slope failures in closely jointed rock masses abstract where closely jointed rock masses are encountered in slopes failure can occur both through the rock mass as a result of combination of macro and micro jointing and through the rock substance determination of the strength of this category of rock mass is extraordinarily difficult since the size of representative specimens is too large for laboratory testing this difficulty can be overcome by using a non linear rock mass failure criterion or by back analysis of such slopes to estimate the rock mass strength in this paper a practical procedure and a computer program are presented for the back determination of shear strength parameters mobilized in slopes cut in closely jointed rock masses which obey a non linear failure criterion rather than a linear one the procedure shows that the constants to derive normal stress dependent shear strength parameters of the failed rock masses can be determined by utilizing a main cross section and without a pre determined value of rock mass rating rmr trials are made for different rmr m and rmr s values corresponding to various possible combinations of the constant m and s which are used in the hoek brown failure criterion satisfying the limit equilibrium condition it is also noted that the procedure provides a quick check for the rock mass rating obtained from the site investigations the method is used in conjunction with the bishop s method of analysis based on circular slip surfaces the procedure outlined in this paper has also been satisfactorily applied to documented slope failure case histories in three open pit mines in turkey

chert in the palaeoproterozoic bartle member killara formation yerrida basin western australia a rift related playa lake and thermal spring environment abstract palaeoproterozoic chert in the bartle member of the killara formation yerrida group yerrida basin of western australia contains an anomalous association of crystal structures and rock fabrics that are difficult to interpret the association appears to indicate an evaporitic pyroclastic thermal spring environment associated with rifting at about 2 2 ga the chert member contains silica pseudomorphs after evaporite minerals that in places enclose relict isolated crystals and aggregates of crystals of gypsum and anhydrite the evaporite minerals are associated with minerals such as barite and analcime the association of these minerals together with palaeoenvironmental evidence invites comparison with rocks of the afar region and lake magadi in the east african rift system the bartle member locally contains anomalous gold up to twelve times the average crustal values of 4 ppb and barium it also contains finely disseminated kerogen together with numerous spheroids classified here as microdubiofossils and some curious structures showing organisation into complex petal like structures classified here as putative microfossils the bartle member chert has many characteristics of playa lake and thermal spring deposits and may host epithermal precious metal mineralisation

the rare alkalies in hydrothermal alteration at wairakei and broadlands geothermal fields n z abstract drillcores and waters from wairakei and broadlands geothermal areas new zealand have been analyzed for li rb cs na k mg ca al ti mn and be the drillcores were altered to various degrees at temperatures below 300 c in slightly alkaline chloride water probably derived from rock water interaction in untapped horizons at higher temperature it changes its composition as it leaches ca and na from the rock and adds k rb cs and li evaluation of these changes in relation to the dimensions of the altered zone under observation suggests that a high mass ratio of water to rock e g 100 and a period of up to 1 million years are responsible for the present stage of alteration increase of k and rb in the altered rocks is a result of the formation of abundant adularia in addition to illite the k rb ratio of the rock decreases during alteration but remains higher than that of the fluid only clay materials and zeolites that preferentially absorb rb give slightly lower k rb ratios than the fluid the mineral phases responsible for the uptake of lithium during alteration are chlorite 300 ppm li and quartz up to 430 ppm li li uptake in quartz is considered to be the mechanism by which al rich quartz crystallises from alumino silicates li al atomic ratios of 0 3 0 57 and al concentrations up to 3000 ppm have been observed relatively small concentrations of cs are found in potassic minerals e g 10 ppm cs in adularia 44 ppm cs in illite however 240 ppm cs are found in wairakite from wairakei equilibrated at 235 c lower equilibration temperatures may lead to higher cesium concentrations this effect in conjunction with a more concentrated hydrothermal fluid could explain a content of 4500 ppm cs in wairakite extracted from a drillcore taken in the el tatio geothermal field in chile

thorium uranium disequilibrium in a geothermal discharge zone at yellowstone whole rock samples of hydrothermally altered biscuit basin rhyolite from yellowstone drill cores y 7 and y 8 were analyzed for sup 230 th sup 234 u sup 238 u and sup 232 th extreme disequilibrium was found with sup 230 th sup 234 u ranging from 0 30 to 1 27 values of sup 230 th sup 232 th and sup 234 u sup 232 th define a linear correlation with a slope of 0 16 0 01 which corresponds to a sup 230 th sup 234 age of approximately 19 ka the sup 230 th sup 234 u disequilibrium was apparently caused by u redistribution which occurred mostly at about 19 ka and is not related simply to the relative degree of hydrothermal alteration and self sealing of the rhyolite mass balance of u requires a large flux of u bearing ground water through the rhyolite at the time of u redistribution rough estimates of minimum water rock ratio range from 10 sup 2 to 10 sup 4 for a range of possible ground water u concentrations conservative hydraulic calculations indicate that the required ground water flux could have occurred within a period of hundreds of years prior to self sealing the disequilibrium data are consistent with a model involving u redistribution during the initial stages of development of a geothermal discharge zone that formed in response tomore the hydrogeologic effects of glacial melting and unloading during the decline of the pinedale glaciation less

cirotan west java indonesia a 1 7 ma hybrid epithermal au ag sn w deposit the cirotan gold deposit dated at 1 7 or 0 1 ma is a mineralized fault with ore shoots contained within a system of fractures cutting a miocene volcano sedimentary series 9 5 ma intruded by pliocene microdiorite 4 5 ma the mineralization hosted by a right lateral strike slip fault that evolved into a normal fault has been partly eroded the deposit is of the adularia sericite type with strong vertical ore zoning ag au 64 at the top and only 7 in depth and resulted from a single mineralizing event with five stages of deposition silicification stage a siliceous breccias stage b cockade breccias with polymetallic sulfide ores and local rhodochrosite rhodonite stage c precious brecciated ore including bonanzas stage d and late vuggy quartz stage e this evolution was accompanied by an increase in w sn au and bi reflected in the late appearance of abundant ferberite scheelite gold cassiterite and rare minerals such as te bearing canfieldite and lillianite fluid inclusion studies indicate a slight increase in temperature 235 degrees 255 degrees c and salinity 2 89 7 15 wt nacl equiv from the initial silicification to the bonanza ore and an absence of volatiles other than h 2 o that this could reflect an increased participation of magmatic fluids is supported by the sulfur isotope compositions of pyrite and galena avg delta 34 s 2 2 and 1 0ppm respectively close to the 0 per mil isotopic value of magmatic sulfur lead isotope results establish a common origin for both the cirotan gold deposit 206 pb 204 pb 18 77 and 207 pb 204 pb 15 67 and the pliocene andesitic dacitic magmas to which the gold is related and indicate a remobilization of precambrian continental basement although cirotan shows strong affinities with classic epithermal gold deposits the late geochemical and mineralogical evolution with w sn and bi is extremely unusual if not unique in the context of the malaysia sumatra tin belt this is considered to reflect a genetic link with porphyry tin deposition and is explained by a progressive upward migration of pliocene magma favored by strike slip faulting the occurrence of the two types of mineralization within a single structure thus makes cirotan a hybrid deposit

a new general empirical approach for the prediction of rock mass strengths of soft to hard rock masses abstract it is almost impossible to prepare representative cores of rock masses including discontinuities patterns for laboratory studies to overcome these difficulties researchers have focused on developing empirical equations for estimating of the stress strain behavior of a rock mass including measurements of the discontinuity patterns as can be seen in the literature the uniaxial compressive strength value of rock mass ucs rm can be estimated by reducing the uniaxial compressive strength of intact rock material ucs i based on the quality of a rock mass represented by variables such as rock mass rating rmr geological strength index gsi and q value for this reason a unique reducing curve form empirical equation has limited application and generally cannot be applied to all kind of rock masses from particularly soft to hard rock masses in this study a new general empirical approach is constructed to estimate the strength of rock masses of varying hardness the new empirical equations have been calibrated using data from five slope failures and four sets of uniaxial compressive strength data of rock masses in the new empirical equations the ucs i is considered not only to be a scale parameter used in the strength reduction but also used to adjust the degree of strength reduction in conjunction with elastic modulus of the rock material e i the disturbance factor on the rock mass is taken into consideration by two separate reduction factors applied to the structure rating sr to capture increasing joint density and to the s and m b parameters of the hoek brown criterion to decrease the degree of interlocking hence non interlocked cohesionless under zero normal stress rock masses such as spoil piles can also be modeled in the new empirical approach

discussion on the paper by m orhan n s isik t topal m ozer effect of weathering on the geomechanical properties of andesite ankara turkey environmental geology 50 1 85 100

seismic cycle recorded in cockade bearing faults col de teghime alpine corsica abstract few fault rocks are known to be associated undoubtedly with seismic faulting here we investigated the formation mechanism of cockade breccias found in transtensional faults cutting marbles and quartzites from the col de teghime area alpine corsica france field surveys coupled with detailed microanalytical investigations indicated that i the core clasts of the cockades are composed of host rock fragments 310 m in size that are suspended in the slipping zones and arranged in inverse grading ii the concentric rims of the cockades show a cyclic zoning made of saddle dolomite mg calcite goethite anatase iii the cockade bearing veins are associated with minor fault veins filled with fine fragments we propose that the cockade bearing faults formed at shallow crustal depths

chaotic breccia along the dent fault nw england implosion or collapse of a fault void a body of chaotic breccia along the reverse oblique dent fault zone is ascribed to hanging wall collapse into persistent voids created by geometric mismatch of fault walls although some implosion into transient voids is a possibility the breccia comprises a 20 m wide body of hanging wall lithologies with a chaotic clast supported fabric that contrasts with the fitted fabric breccias typical of the dent fault damage zone the breccia body has crude bedding defined by clast shape and size contrasts the void fill is cut by variscan fault strands which together with its ferroan calcite and barite cement prove its late carboniferous rather than recent age it is shown that any fault void transient or persistent had a smaller aperture than the final width of the breccia body and no more than 5 m a span that can be supported to depths of 2 or 3 km however cement zonation in the breccia fill suggests that the void opened in multiple increments each of an aperture compatible with the maximum displacement in any one event along the dent fault the dent fault example highlights the possible general importance of fault void collapse but also the problems in distinguishing it from implosion processes

indirect determination of the modulus of deformation of rock masses based on the gsi system today as a result of the increase in the number of large tunnel constructions and underground excavations required for civil engineering and waste disposal and the operation of deep open pit mines determination of rock mass properties has become one of the most important stages in design and construction for rock engineering when rocks and rock masses are classified for geotechnical purposes this needs to be on the basis of strength or static modulus of deformation to give an indication of their stability or deformational response in addition the strength and modulus of deformation are the necessary input parameters of the rock masses to be utilized by numerical methods the modulus of elasticity of the rock material is undoubtly the geomechanical parameter that best represents the mechanical behaviour of rock material a similar conclusion can also be made for rock masses however the modulus of deformation of rock masses should be determined in situ by carrying out special tests which normally implies considerable costs and operational difficulties because of such difficulties the determination of the modulus of deformation of the rock masses continues to attract the attention of rock engineers and engineering geologists given the importance of having the value of the modulus of deformation available and at the same time the operational and economical difficulties mentioned above many researchers mentioned below were led to consider several cost effective and not too time consuming indirect procedures that can provide information on the modulus of deformation these attempts

cockade breccia product of mineralisation along dilational faults cockade breccias are fault fills in which individual clasts are completely surrounded by concentric layers of cement they occur particularly in low temperature near surface hydrothermal veins at least six mechanisms have been proposed for the formation of cockade breccia like textures but only two repeated rotation accretion and partial metasomatic replacement of clast minerals have been supported by detailed evidence a typical example of cockade breccia from the gower peninsula south wales shows clear evidence for the rotation accretion mechanism in particular overgrown breakage points in cement layers where cockades were previously touching each other and rotated geopetal infills of haematitic sediment based on the available evidence it is proposed that cockade textures result from low rates of cement growth compared to high rates of dilational fault slip seven criteria are given for the correct identification of cockade breccias

deformation and structural evolution of mantle peridotites during exhumation on transform faults a forced transition from ductile to brittle regime abstract the exhumation of peridotite rocks in oceanic transform zones passes by the rheological transition between the ductile and brittle deformation until the complete emplacement in the oceanic lithosphere the sao pedro and sao paulo archipelago spspa in the equatorial atlantic records the deformational products of ductile brittle and the rocks fluid interaction generating specific structures in each domain the deformational stages are related to the transpressional and transtensional geodynamics of sao paulo transform fault firstly during transpression exhumation occurs associated with the ductile domain causing intense mylonitization in temperatures between 700 and 800 c defined by olivine and orthopyroxene recrystallization the interaction with fluids initially originated from the mantle generates amphibole and oxide rich layers marking the passage to a semi brittle deformation the continuation of peridotite exhumation associated with an nw se shortening and transpressional led to a higher availability of hydrothermal fluids as a consequence four serpentinization episodes are recorded which are associated with semi brittle to brittle transition under temperatures between 300 and 400 c finally the complete exhumation and establishment of brittle mechanisms led to carbonatation phase near the surface with temperatures ranging from 300 to 150 c the active nw se tectonic stress generated e w strike slip faults that were filled by carbonates recording the final exhumation stage

chaotic breccia zones on the pembroke peninsula south wales evidence for collapse into voids along dilational faults chaotic breccias and megabreccias locally called gash breccias hosted within the pembroke limestone group visean mississippian lower carboniferous of southwest wales are re mapped along with spatially related crackle and mosaic breccias of thirteen studied megabreccia bodies seven lie along steep nnw or nne striking strike slip faults originating during north south variscan late carboniferous shortening though reactivated during later extension four bodies are conformable with e w striking steeply dipping bedding and two have irregular or indeterminate margins the bedding parallel zones are interpreted as the dilational tips of listric normal faults and the cross strike faults as transtensional transfer zones sub horizontal clast fabrics suggest brecciation by gravitational collapse into opening fissures rather than by cataclasis along the faults most fissures have geometrically matched margins produced by this dilational faulting and only locally have the indented margins indicating solutional processes the most likely age for the main fissure extension and fill is late triassic based on analogous dated fills at the eastern end of the bristol channel basin the pembroke megabreccias blur the distinction between fault rocks formed by deformation and those formed by redeposition along fault zones

element redistribution during hydrothermal alteration of rhyolite in an active geothermal system yellowstone drill cores y 7 and y 8 abstract hydrothermal alteration of the 0 54 ma biscuit basin rhyolite flow in yellowstone drill cores y 7 and y 8 has involved significant changes in ti normalized whole rock concentrations of li mg ca na k mn rb sr sb cs ba and u whereas such changes have generally not occurred for al fe sc co y zr ree hf ta and th the relatively mobile elements have been redistributed over distances from 2 5 cm to more than 10 100 m and are associated mainly with zeolites and clays in thoroughly altered samples average whole rock changes in ti normalized mass range from about 0 5 17 g kg 9 49 for major elements and 0 1 280 mg kg 19 4260 for minor and trace elements compositional changes correlate with hydrothermal mineralogy the relatively immobile elements have been redistributed over a distance of at least 10 100 m but less than 2 5 cm and are associated mainly with clays in thoroughly altered samples addition of sio2 ranges from 110 890 g kg in samples where porosity has been thoroughly sealed by silica deposition thermal water is not in oxygen isotopic equilibrium with any primary or secondary phases analyzed except calcite modest depletions of 18o 1 2 due to exchange with thermal water apparently have occurred in y 8 plagioclase and obsidian oxygen isotope ratios in obsidian replacing smectite and in veinlet celadonite and cristobalite suggest that these phases precipitated from water locally enriched in 18o up to 8 heavier than present thermal water the minimum integrated water rock mass ratio in y 7 and y 8 is 103 104 assuming advective transport

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

modelling the evolution of columnar joints abstract cooling cracks developing from the margins of solidifying lava flows or intrusive dykes or sills often propagate inward and divide the rock into prismatic columns it has been shown that 1 the initial crack pattern that forms at the surface of the body is not as regular as the pseudo hexagonal pattern which evolves later 2 columnar structures develop by repeated step wise crack advances forming cm to dm transverse bands on the column faces 3 even within well developed colonnades the polygonal outlines continue to shift slightly from one crack advance to the next we propose that each new crack should propagate parallel to the highest thermal gradient ahead of the current crack tip when adjacent columns are of unequal size the local asymmetry of the isotherms drives the new crack toward the biggest and hottest column we present a geometrical model and algorithm that follows this rule and mimics the successive crack advances as the cooling front migrates into the lava sheet or dyke the model polygonal pattern obtained consists of convex irregular polygons with a variable number of sides but in which pentagons and hexagons predominate the algorithm successfully reproduces the rapid evolution from an initial immature crack pattern to a pseudo hexagonal mature one in which the average number of sides approaches six it predicts also the predominance of y type crack junctions the rapid evolution toward columns of approximately equal sizes and the persistent changes in length and diverging orientation of growth steps observed along the sides of columns the model polygonal patterns exhibit geometrical properties which are very similar to those observed in well developed columnar jointed flows such as the giant s causeway

sedimentary fluids fault interaction during syn rift burial of the lodeve permian basin herault france an example of seismic valve mechanism in active extensional faults abstract during basin burial interstitial fluids initially trapped within the sedimentary pile easily move under thermal and pressure gradients as the main mechanism is linked to fluid overpressure such fluids play a significant role on frictional mechanics for fault reactivation and sediment deformation the lodeve permian basin herault france is an exhumed half graben with exceptional outcrop conditions providing access to barite sulfide mineralized systems and hydrocarbon trapped into syn rift roll over faults architectural studies show a cyclic infilling of fault zone and associated bedding parallel veins according to three main fluid events during dextral normal faulting contrasting fluid entrapment conditions are deduced from textural analysis fluid inclusion microthermometry and sulfur isotope geothermometer we conclude that a polyphase history of trapping occurred during permian syn rift formation of the basin the first stage is characterized by an implosion breccia cemented by silicifications and barite during an abrupt pressure drop within fault zone this mechanism is linked to the dextral strike slip motion on faults and leads to a first sealing of the fault zone by basinal fluid mineralization the second stage consists of a succession of barite ribbons precipitated under overpressure fluctuations derived from fault valve action this corresponds to periodic reactivations of fault planes and bedding controlled opening localized at sulphide rich micro shearing structures showing a normal movement this process formed the main mineralized ore bodies by the single action of fluid overpressure fluctuations undergoing changes in local stress distribution the last stage is associated with the formation of dextral strike slip pull apart infilled by large barite and contemporaneous hydrocarbons under suprahydrostatic pressure values this final tectonic activation of fault is linked to late basinal fluids and hydrocarbon migration during which shear stress restoration on the fault plane is faster than fluid pressure build up this integrated study shows the interplay action between tectonic stress and fluid overpressure in fault reactivation during basin burial that clearly impact potential economic reservoirs

the geochemistry and petrogenesis of the lavas of the vulsinian district roman province central italy the vulsinian lavas are dominated by a suite of undersaturated leucite bearing basic to intermediate compositions the remaining lavas are mainly oversaturated and have shoshonitic affinities one hundred and thirty five samples have been analysed for major elements and most for 20 trace elements twenty seven lavas have been analysed for ree they are all perpotassic for the undersaturated lavas k2o na2o 2 8 and have very high lil element concentrations e g rb 400 800 ppm th 25 150 ppm ree reecho c 200 lree hree cho c 20 even in the most basic rocks

geologic volcanologic and tectonic setting of the vico cimino area italy abstract although the cimino and vico volcanic complexes are located close to each other in a structural low they belong to two different provinces cimino is connected to the tuscan roman anatectic magmatic province and vico to the roman campanian potassic alkaline province barberi et al 1971 marinelli 1975 the two volcanic centers lie on a basement formed by clays and sands of a miocene pliocene aged sedimentary cycle the mount cimino complex forms a central volcano composed of a series of rhyodacitic domes it was emplaced along a nw se trending fracture zone which can be followed for about 16 km cimino volcano was active from 1 4 m y to 0 95 m y ago eruption of rhyodacitic ignimbrites were preceded and followed by the emplacement of domes of the same composition fluid lavas of latitic and olivine latitic composition were emitted from the central volcano during the final phase of activity vico is a central volcano mainly composed of lava with a composite summit caldera it was emplaced along a ne sw trending fracture system the vico stratovolcano is formed by lavas of various compositions including phonolitic tephrites tephritic phonolites and slightly undersaturated trachytes the construction of the stratovolcano the most ancient exposed products of which have an age of 0 4 m y was followed by the eruption of four pyroclastic flows totalling several cubic kilometers which are dated at 0 3 0 15 m y the collapse of the caldera is most likely connected to the eruption of these pyroclastic flows the ring fractures of the caldera are associated with the emplacement of a pyroclastic unit most of which erupted along the inner northeast and north rim of the caldera post caldera activity is characterized by the emission of tephritic phonolitic and tephritic products that form the intra caldera stratovolcano named mount venere the vico cimino area northern latium located between the apennine chain and the tyrrhenian sea lies along the circum tyrrhenian belt that extends from northern tuscany to campania this zone has experienced post orogenic tensional movements since the middle miocene the teconic evolution of the tyrrhenian basin is closely associated with acid anatectic and potassic alkaline volcanism along the tyrrhenian margin of the apennines the age composition and geographic position of the volcanoes are controlled by different magnitudes of tensional tectonics and crustal thinning processes of the western margin of the apennine chain

uth fractionation by fluids in k rich magma genesis the vico volcano central italy abstract the 230th 238u disequilibrium method has been applied to the potassic volcanic series of vico latium central italy the 3 main events of the recent volcanic activity have been dated using whole rock isochrons the lava flows of the main cone were erupted between 260 and 180 ky pyroclastic flows are dated around 90 ky and the final effusive activity mte venere around 60 ky these results are in good agreement with previous k ar ages the intense th u fractionation th u varying from 3 2 to 11 is apparently not related to processes of partial melting and crystal fractionation the th u fractionation is the most intense in basic terms of the first episode and is due to the percolation of a deep hydrothermal fluid which impoverishes u in different batches of primary magmas more limited th u fractionations in highly differentiated magma are due to u accompanied by sb and alkalies enrichments in the shallow magma chambers a time integrated model is proposed for the evolution of the 230th 232th ratio in the mantle source of vico it shows that th u and th ta fractionations are probably related to the bulk k u and th enrichment of the mantle of the roman province around 300 ky ago a metasomatic enrichment by a silicate melt is proposed to explain the anomalously high th u and th ta fractionations

engineering characteristics of ankara greywacke under the foundation of the beytepe open air theatre ankara turkey the construction of a large open air theatre having with an audience capacity of 7 000 is planned at the beytepe campus of hacettepe university located near ankara turkey in the heart of anatolia the foundation of the open air theatre is composed of weak and heavily jointed ankara greywacke an estimation of the strength and deformation of the ankara greywacke was required for assessments of short and long term stability accordingly rock mass characterization of the ankara greywacke was investigated in detail by scan line surveys and back analyses of in situ tests performed on the excavated surface of the rock mass the final version of the hoek and brown criterion hoek et al in proceedings of the north american rock mechanics society meeting toronto canada pp 1 6 2002 together with and improvements proposed by sonmez and gokceoglu int j rock mech min sci 43 671 676 2006 were considered together for the back analyses of small slope benches subjected to plate loading test loaded to failure in addition the plate loading test data were used with some well known empirical equations for predicting of deformation modulus of rock masses to calculate ranges of values of the geological strength index gsi the static and pseudostatic slope stabilities of the audience seating structures to be supported by benches excavated into the rock were also analyzed the stability assessments revealed the unlikelihood of large rock mass failures for the short term construction stages or for the long term as constructed conditions under static or pseudo static conditions additionally the analyses showed that there was no need for anchors between the concrete seating structures and greywacke rock mass

a coupled micro and macrostructural approach to the analysis of fluid induced brecciation curnamona province south australia conditions during the formation of breccias in the curnamona province eastern south australia have been investigated through a detailed fractal microstructural structural and fluid inclusion study of the breccias syn tectonic quartz veins and surrounding rocks fractal analysis of clast shapes and clast size distributions were able to distinguish two styles of breccia and indicate that fluid pressure fluctuations played a significant role though to differing degrees in the initiation of the brecciation process approximate palaeostress orientations prevalent during brecciation and their relationship to the deformational history of the terrane have been reconstructed by combining the orientations of microfractures sealed microfractures and fluid inclusion planes with structural field data fluid pressure fluctuations of 80 mpa during the brecciation process have been quantified by structurally constrained microthermometric analysis of syn tectonic fluid inclusions coupled with an analysis of the metamorphic history of the area results from this study allow brecciation within the curnamona province to be placed within a structural framework and relate the process of brecciation to the interaction between regional hydrothermal alteration fluid flow systems shear zone formation and lithological contrasts within the upper crust during the palaeoproterozoic olarian orogeny these methods and findings provide a template for the study of brecciation processes in other terranes where fluid induced brittle deformation is a controlling factor in the localisation of major ore deposits

thorium uranium ratio of the earth through the study of komatiites and ophiolites remnants of the old mantle we review the evolution of the lead system in the depleted mantle through time using the 230th232th vs 208pb 206pb diagram 208pb 206pb radiogenic lead ratio we constrain the th u ratio of the bulk silicate earth to 4 2 this value is also in full agreement with ratios inferred from the study of the various types of meteorites using this value in the 208pb 206pb 143nd144nd diagram bulk earth plots on the oceanic basalt correlation line as previously guessed a comparison with sr and nd data shows that the th u fractionation correlates with that of rb sr and nd sm whereas the u pb fractionation is independent of other ratios

internal vein texture and vein evolution of the epithermal shila paula district southern peru the epithermal shila paula au ag district is characterized by numerous veins hosted in tertiary volcanic rocks of the western cordillera southern peru field studies of the ore bodies reveal a systematic association of a main e w vein with secondary n55 60 w veins two directions that are also reflected by the orientation of fluid inclusion planes in quartz crystals of the host rock in areas where this pattern is not recognized such as the apacheta sector vein emplacement seems to have been guided by regional n40 e and n40 w fractures two main vein filling stages are identified stage 1 is a quartz adularia pyrite galena sphalerite chalcopyrite electrum mn silicate carbonate assemblage that fills the main e w veins stage 2 which contains most of the precious metal mineralization is divided into pre bonanza and bonanza substages the pre bonanza substage consists of a quartz adularia carbonate assemblage that is observed within the secondary n45 60 w veins in veinlets that cut the stage 1 assemblage and in final open space fillings the two latter structures are finally filled by the bonanza substage characterized by a fe poor sphalerite chalcopyrite pyrite galena tennantite tetrahedrite polybasite pearceite electrum assemblage the ore in the main veins is systematically brecciated whereas the ore in the secondary veins and geodes is characteristic of open space crystallization microthermometric measurements on sphalerite from both stages and on quartz and calcite from stage 2 indicate a salinity range of 0 to 15 5 wt nacl equivalent and homogenization temperatures bracketed between 200 and 330 c secondary co2 n2 and h2s bearing fluid inclusions are also identified the age of vein emplacement based on 40ar 39ar ages obtained on adularia of different veins is estimated at around 11 ma with some overlap between adularia of stage 1 11 4 0 4 ma and of stage 2 10 8 0 3 ma a three phase tectonic model has been constructed to explain the vein formation phase 1 corresponds to the assumed development of e w sinistral shear zones and associated n60 w cleavages under the effects of a ne sw shortening direction that is recognized at andean scale these structures contain the stage 1 ore assemblage that was brecciated during ongoing deformation phase 2 is a reactivation of earlier structures under a nw se shortening direction that allowed the reopening of the preexisting schistosity and the formation of scarce n50 e striking s2 cleavage planes filled by the stage 2 pre bonanza minerals phase 3 coincides with the bonanza ore emplacement in the secondary n45 60 w veins and also in open space in the core of the main e w veins our combined tectonic textural mineralogical fluid inclusion and geochronological study presents a complete model of vein formation in which the reactivation of previously formed tectonic structures plays a significant role in ore formation

the development of a coal measure classification cmc and its use for prediction of geomechanical parameters abstract many studies have been performed to predict the strength and deformation moduli for both rock masses and discontinuities however in most empirical equations in the literature that have been proposed for the estimation of the strengths and deformation moduli the rock mass is assumed isotropic although it is known that the strength and deformation of a stratified rock mass varies depending on the loading direction in relation to the orientation of the lamination planes there exists at present no commonly accepted method for characterising this anisotropy this paper outlines the development of a rock mass rating system for coal measure rock masses that can be used to empirically predict the engineering properties of stratified rock masses the output from the classification system is two numerical ratings representing the different engineering properties of the strata in directions parallel and perpendicular to stratification a detailed description on the methodology used to develop the classification system has been given and the derivation and rating of the various rock mass parameters is outlined the methodology of deriving the in situ stratified rock mass input parameters and geomechanical computational analysis is illustrated for the case of two rock bolted roadways within a representative deep uk coal mine the model predictions were validated against the actual in situ monitoring data of displacements within the immediate roof of the roadway

nature and origin of collapse breccias in the zechstein of ne england local observations with cross border petroleum exploration and production significance across the north sea

ore related breccias in volcanoplutonic arcs an overview of breccias related to a variety of base metal precious metal and lithophile element deposits in volcanoplutonic arcs permits definition of six possible mechanisms for subsurface brecciation 1 release of magmatic hydrothermal fluids from high level hydrous magma chambers during second boiling and subsequent decompression generates a spectrum of breccia types in which fragments may suffer collapse and or ascent single or multiple intrusion related breccia pipes and pre and intermineral breccias in porphyry copper deposits provide widespread examples 2 magmatic heating and expansion of meteoric pore fluids may lead to brecciation commonly of late or postmineral age and including pebble dikes in porphyry type and related deposits magmatic heating of rocks saturated with seawater may cause submarine hydrothermal eruptions late in the emplacement histories of kuroko type massive sulfide deposits many of the resultant breccias underwent limited sedimentary transport overpressuring of heated fluids beneath semipermeable partly self sealed cap rocks may lead to brecciation and subaerial hydrothermal eruptions in shallow epithermal precious metal settings magmatic heating or tectonic disturbance may have triggered brecciation 3 interaction of cool ground waters with subsurface magma can generate phreatomagmatic explosions postmineral phreatomagmatic diatremes associated with porphyry systems and premineral diatremes with epithermal precious or base metal deposits were generated in this manner these attained the palcosurface to produce pyroclastic base surge and fall deposits that accumulated as tuff rings around maar craters 4 magmatic hydrothermal brecciation may lead to disruption of rocks through to the paleosurface decompression and fragmentation and eruption of the top part of an underlying magma chamber pre and postmineral magmatic diatremes of this sort are inferred to accompany a few porphyry type and other base and precious metal systems they were manifested at the palcosurface by accumulations of pyroclastic fall and flow deposits 5 breccias may result from mechanical disruption of wall rocks during subsurface movement of magma any intrusion related deposit may include such intrusion breccias 6 tectonic breccias resulting from fault displacement may accompany any type of ore deposit a continuum exists between many of these breccia types and it is difficult to identify unique criteria for their unambiguous distinction

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

breccia formation by particle fluidization in fault zones implications for transitory rupture controlled fluid flow regimes in hydrothermal systems breccias in the rusey fault cornwall uk and the roamane fault porgera gold deposit papua new guinea provide insights about the dynamics of fluid flow and flow velocities when fault ruptures breach overpressured reservoirs of hydrothermal fluid these faults contain cockade like breccias in which rock fragments are mantled by spheroidal overgrowths of hydrothermal minerals at rusey the overgrowths are quartz dominant whereas in the roamane fault overgrowths are composed of calcite quartz or gold rich pyrite although none of the rock fragment cores of accretionary spheroids are in contact with their neighbours the spheroidal overgrowths do contact each other and are at least partially cemented together the hydrothermal overgrowths mostly comprise either outwards coarsening crystals that radiate from the surface of the core rock fragment or finer grained inequigranular to mesh like intergrowths concentric textural banding and oscillatory growth zones are present in some hydrothermal mantles the breccias occur as fault parallel layers and lenses each up to several tens of centimeters thick adjacent layers are characterized by texturally distinct ranges of clast sizes and different proportions of clasts to hydrothermal overgrowths in the rusey fault many texturally distinct breccia layers are present within a 3m wide fault core some layers truncate others and many breccia layers exhibit grainsize grading or banding clasts in the breccias include fragments of wall rock veins and various fault damage products including fragments of earlier generations of cemented breccia brecciation was episodic and separated by periods of cementation the distinctive textures of the breccias are interpreted to have formed by fluidization of fault damage products in a high fluid flux regime hydrothermal coatings developed while clasts were in a suspended state during fluid ascent through dilatant fault segments layered breccias record multiple episodes of particle fluidization and indicate that the faults provided conduits for repeated transitory fluid upflow particle size distributions indicate that fluid velocities during fluidization were in the range 0 1 ms 1 to 1 ms 1 the maximum flow rates correspond to fluid fluxes of 10 to 100 l s 1 per meter strike length of fault through dilatant fault apertures up to several tens of centimeters wide such high flow rates characteristically induce intense swarm seismicity rather than mainshock aftershock seismicity and have implications for the dynamics and rates of formation of fault related hydrothermal ore deposits

evidence for mantle metasomatism an oxygen and strontium isotope study of the vulsinian district central italy this paper presents new o and sr isotope data for lavas from the northern part of the roman perpotassic province the samples comprise the tephritic leucititic to leucite phonolitic lavas and the saturated lavas from the vulsinian district the olivine leucite melilitite of san venanzo and the kalsilite diopside melilitite of cupaello previous oxygen isotope work on the lavas of the vulsinian district suggested crustal contamination of normal mantle derived magmas the new data cover the ranges previously found o and sr isotope ratios of evolved lavas of the undersaturated suite indicate assimilation in variable amounts of up to ca 10 of continental crustal material the saturated lavas probably assimilated large amounts up to ca 50 of crust lavas chemically identified as corresponding to little modified mantle derived liquids are high in both87sr 86sr and 18o 0 7103 0 7107 7 8 to 9 4 vulsini 0 7104 12 3 san venanzo and 0 7112 14 4 cupaello these high values are interpreted to have been inherited from a metasomatized parental mantle hydrous fluids enriched in large ion lithophile elements and high in 18o and87sr 86sr are thought to have mixed with mantle of normal 18o and87sr 86sr the fluids probably origi dehydration of continent derived sediments which were subducted beneath a mantle wedge in the continent continent collision of the corsica sardinia block and the adriatic italian plate this hypothesis is supported by pb and nd isotopic evidence and is probably valid for the entire roman province

empirical estimation of rock mass modulus the deformation modulus of a rock mass is an important input parameter in any analysis of rock mass behaviour that includes deformations field tests to determine this parameter directly are time consuming expensive and the reliability of the results of these tests is sometimes questionable consequently several authors have proposed empirical relationships for estimating the value of rock mass deformation modulus on the basis of classification schemes these relationships are reviewed and their limitations are discussed based on data from a large number of in situ measurements from china and taiwan a new relationship based upon a sigmoid function is proposed the properties of the intact rock as well as the effects of disturbance due to blast damage and or stress relaxation are also included in this new relationship