the calamine nonsulfide zn pb deposits of belgium petrographical mineralogical and geochemical characterization abstract the belgian nonsulfide zn deposits represent the historical basis for the zinc mining and smelting industry in europe the zinc ores were called calamines and consist of a mixture of zn carbonates smithsonite hydrozincite and zn silicates hemimorphite willemite and zn clays with a variable content of pb minerals the calamines are considered to be oxidation products of primary sulfide ores the most important was the la calamine orebody more than 600 000 tons of zn metal that was continuously exploited for several centuries nonsulfide ores were also exploited at schmalgraf engis dickenbusch fossey rocheux oneux welkenraedt and in other smaller deposits all no longer accessible for study where the calamines extended from the surface to an average depth of 40 to 50 m the primary zn pb sulfide mineralization consists of post variscan hydrothermal veins and replacement bodies mostly occurring in the dinantian visean limestones the sulfides intersect the paleozoic rocks and are truncated and unconformably covered by late cretaceous sediments santonian aachen fm the same setting has been observed for the nonsulfide ores suggesting an early to mid cretaceous weathering consistent with other geological and geochronological data on paleoweathering in europe the extensive occurrence of willemite that appears to be the first deposited nonsulfide mineral is a striking particularity of the belgian calamines most fluid inclusions in willemite 80 and all inclusions in smithsonite are monophase aqueous ice melting temperatures suggest salinities between 0 and 5 wt nacl equiv homogenization temperatures of rare two phase inclusions liquid vapor in willemite display a large variation between 80 and 190 an interval matching the temperature ranges of other willemite ores throughout the world so far considered of hydrothermal origin the stable isotope variation of belgian smithsonites and cerussites is very similar to the range measured in other supergene nonsulfide deposits similar to those of sw sardinia belgian smithsonites exhibit a limited range of 18o values from 27 1 to 30 6 averaging 28 4 0 8 v smow this limited range points to a relatively uniform isotope composition of the oxidizing waters and constant temperatures of smithsonite crystallization carbon isotope values even from the same mine show a considerable range from 11 6 to 1 6 this extensive range indicates at least two carbon sources 13c depleted carbon derived from the organic matter in the soils and 13c enriched carbon originating from marine carbonate host rocks cerussites have oxygen isotope values ranging from 16 8 to 19 3 averaging 17 8 1 0 and carbon isotope values of 18 4 to 14 7 the metallic deposits of eastern belgium have undergone a polyphase history which started with a possibly jurassic sulfide mineralization followed by oxidation under high silica activities and consequently by supergene weathering owing to structure and mineralogical association we do not consider the high homogenization temperatures as reliable minimum temperature estimates of willemite formation however it is possible that the willemite concentrations could have been derived from low temperature localized hydrothermal processes caused by deeply reaching oxidizing fluids smithsonite and hemimorphite often developed at the expenses of willemite have all the characteristics of supergene oxidation products

thallium nickel cobalt and other trace elements in iron sulfides from belgian lead zinc vein deposits fe sulfides associated with belgian pb zn vein deposits have been analysed by dc arc spectrometry and xrf for tl ni co mn zn pb ge cd and as they show high tl contents from 30 ppm upt to 6800 ppm and co ni ratios 1 these features permit to distinguish them from fe sulfides of sedimentary and high temperature occurrences the tl distribution in a botryoidal marcasite of vedrin average tl content of 0 68 has been investigated by ion microprobe it displays a pattern similar to colloform texture principal component analysis shows that the behaviour of tl is partly controlled by a factor opposing tl to all other elements this is interpreted as reflecting the influence of sorption mechanisms by contrast with direct precipitation tl and the other elements are not considered of magmatic origin because no acidic magmatism is known during or after variscan times extraction from sedimentary rocks by hydrothermal brines is favoured

the structural development of the wessex basin the mesozoic and cenozoic structures of southern england form part of a system of disturbances which extends across the channel into northern france they are reviewed in the light of published data from south dorset basin development started with permo triassic rifting and is believed to have continued through the jurassic and early cretaceous with intermittent growth on deep seated listric normal faults probably associated with roll over anticlines this regime had ceased by the aptian and the basin passed through a period of relative stability in the late cretaceous followed by tectonic inversion and in the miocene by northwards compressive movement along the former normal faults this partially restored the earlier separative displacement at depth and probably accentuated pre existing roll over anticlines it was expressed in the surficial upper cretaceous palaeogene sequence as monoclinal flexuring the early rifting and the change from overall n s crustal stretching to compression may reflect plate tectonic events in the north atlantic region

pyrite trace element geochemistry of mafic granulite xenoliths from xikeer implications for the source of cu in the sediment hosted mineralization in the northwestern tarim basin northwest china the xikeer mafic granulite xenoliths are hosted by a cenozoic basanite sill 20 ma in the northwest tarim basin northwest china sulfides identified in these xenoliths consist mainly of pyrite two groups of pyrite types a and b can be distinguished based on petrography and trace element geochemistry as determined by laser ablation icp ms analysis type a pyrite is subhedral and in general has lower trace element abundance than type b pyrite type b pyrite is fractured with ragged anhedral morphology and has extremely high cu tl ni and co abundance the low co and ni contents combined with a positive correlation between cu cr and pb all indicate that the type a pyrite may have formed from a high temperature magmatic hydrothermal fluid in contrast the cu tl ni and co rich type b pyrite was probably formed in a relatively low temperature basinal environment the cu mineralization at xikeer can be explained when cu tl ni and co were leached from the intruded basanites and subsequently enriched in circulating basin brines such enrichment of cu in basin brines could provide an important cu source for the cu mineralization in the xikeer district and other cenozoic sub basins in the tarim basin

strike slip orogens an explanation for the hercynides the nature of oblique plate interactions is not yet well understood the oblique interaction of pacific and n american plates has resulted in large scale migration rotation and accretion of microplates intervening basins are ephemeral individually filled and deformed oblique motion results in zones of both compression and extension the latter may be sufficient to generate new oceanic crust and the former to obduct it both subduction and collision are restricted in time and space the resulting orogen is extraordinarily complex in detail the far more complicated hercynides are the result of dextral interaction between continental plates europe and africa the lateral variability of the orogen the major and long lived strike slip faults the bimodal volcanism and related ore deposits of the european trough the lizard ophiolite and its melanges the nw spanish ophiolites and the iberian pyrite belt are all explained using an oblique collision model in particular the generation deformation and obduction of ophiolites in oblique slip mobile zones is discussed

mesozoic reactivation of variscan thrusting in the bristol channel area uk new seismic reflection data from the bristol channel area provide direct evidence of a major variscan thrust in the palaeozoic basement beneath the mesozoic bristol channel syncline the thrust has an average southerly dip of about 25 and it rises into the major fault the central bristol channel fault zone cbcfz at the northern margin of the bristol channel syncline the cbcfz is attributed to reactivation in extension of the variscan thrust probably during the early cretaceous there is no evidence for major synsedimentary movement on the cbcfz and the bristol channel syncline does not represent a rollover structure section restoration suggests that the cbcfz has cut up through a previously tilted sequence of triassic and jurassic strata pervasive nnw sse cross faults are interpreted as original variscan transfer faults that have been reactivated in the mesozoic and tertiary

the sticklepath lustleigh fault zone tertiary sinistral reactivation of a variscan dextral strike slip fault dextral strike slip movement on the sticklepath lustleigh fault zone slfz is indicated by displacements of permian and older rocks previous authors have inferred that the main dextral movement which caused these displacements was post permian and noting the presence of small tertiary pull apart basins along the fault zone probably of tertiary age however the geometry of these early tertiary pull apart basins indicates sinistral rather than dextral strike slip movement we present an alternative model for the history of the sticklepath lustleigh fault zone summarized below 1 late variscan strike slip movement with a total displacement of up to 10 km produced the slfz and offset dextrally an earlier variscan thrust 2 extensional reactivation of this thrust led to rapid permo triassic subsidence in the crediton trough and a dextrally offset neighbour the hatherleigh outlier 3 approximately 6 km of early tertiary sinistral movement on the slfz produced small pull apart sedimentary basins and reduced the net dextral offset across the fault 4 in mid tertiary times minor dextral movements on the slfz may have produced reverse faulting on the margins of the tertiary basins

the variscan fold thrust belt of southeast ireland and its control on early mesozoic extension and deposition a method to predict the sherwood sandstone

trace metals in holocene coastal peats and their relation to pyrite formation nw germany abstract three drill cores from the marshlands of nw germany which cover the entire holocene were analyzed at high resolution for bulk composition al fe selected trace metals and stable sulfur isotopes the drill cores contain two lithological types of peat i basal peats overlying pleistocene sands and ii intercalated peats situated between clastic sediments of predominantly marine origin the peat layers are characterized by distinct enrichments in pyrite due to microbial sulfate reduction under almost open system conditions with respect to seawater sulfate as shown by sulfur isotope partitioning the main fe source seems to be the freshwater environment the determination of dissolved and particulate fe of channels and small rivers close to the study area revealed a 50 fold higher fe content of the freshwater environment when compared with north sea water pyrite enrichments are explained by two scenarios i pyrite formation coincides with fen reed peat growth basal and intercalated under the influence of a brackish water zone salinity app 5 15 and ii pyrite was formed after peat growth in the lowest limnic basal peat intervals maximum pyrite accumulation ts 28 occurs in latter peats that contain thin clastic layers as a result of tidal channel activities after peat formation the occurrence of clastic layers may have favoured the inflow of saline groundwater the peat layers are also characterized by enrichments in redox sensitive trace metals as mo re u and cd whereas co cr cu mn ni pb tl and zn reflect the geogenic background leaching experiments have shown that as co cu mo re and tl are predominantly fixed as sulfides and or incorporated into pyrite the remaining trace metals show no distinct trends only cr reveals a strong relation to the lithogenic detritus seawater is the dominating source for as cd mo re and u the remaining trace elements seem to have a freshwater source similar to fe in contrast to the distribution of pyrite highest amounts of redox sensitive trace metals are seen in fen reed peats basal and intercalated that were formed under a direct influence of seawater and brackish water respectively therefore we suggest that saline groundwater entering the basal peats was probably depleted in redox sensitive trace metals e g owing to microbially induced reduction of trace metals and subsequent precipitation as sulfides or fixation by organic matter

assessment of pyrite composition by la icp ms techniques from massive sulfide deposits of the bathurst mining camp canada from textural and chemical evolution to its application as a vectoring tool for the exploration of vms deposits abstract pyrite is ubiquitous throughout the massive sulfide deposits of the bathurst mining camp canada pyrite locally preserves primary pseudo primary textures as well as secondary textures that formed in response to the hydrothermal metamorphic and deformation processes to which the host sequence was exposed in the bathurst mining camp most forms of pyrite are arsenian up to 3 13 wt as with considerable enrichment in tl sb sn bi ag se and to a lesser extent cd ga te hg ge in and au la icp ms analyses of primary pyrite show significant enrichment in fluid mobile elements in particular tl sb as and hg as well as bi pb and sn in contrast secondary pyrite syn to post deformation varieties is not enriched in these elements superimposed metamorphism typically causes loss of fluid mobile elements to different degrees except for se pyrite chemical variations have the potential to monitor ore petrogenesis at inter and intra deposit scales pyrite from the cu zone is enriched in se relative to pyrite from zn pb zone in which tl is elevated throughout the host sequence fluid mobile element concentrations of pyrite vary by several orders magnitude with the highest fluid mobile element abundances in footwall rocks immediately proximal to the deposit moreover elevated fluid mobile element concentrations of pyrite are recognized in footwall felsic volcanic and sedimentary rocks as much as several hundred metres away from the deposit hanging wall zones contain pyrite with fluid mobile element contents comparable to pyrite from sulfide horizon the fluid mobile element concentrations of pyrite are higher in exhalative sedimentary rocks than pyrite from the sulfide mineralization itself the spatial variation in fluid mobile element concentrations in pyrite as identified using la icp ms can be used to identify otherwise cryptic deposit related haloes ultimately this technique when combined with traditional exploration tools can be used to vector toward vms mineralization in the bathurst mining camp and possibly a useful indicator in a prospect scale base metal exploration

the development of a regional scale intraplate strike slip fault system alpine deformation in the north of ireland abstract a paucity of displacement markers can make constraining the kinematics and interaction of regional scale strike slip faults difficult in this study a high resolution aeromagnetic survey allows quantitative analysis of kilometre scale paleogene faulting in the north of ireland mapping offset dykes and igneous centres reveals the presence of two orientations of cenozoic strike slip faults attributed to broadly n s alpine shortening arising from the convergence of africa and europe reactivated ne sw striking faults accommodated up to 2 3 km of sinistral displacement mainly during the paleocene following tectonic quiescence up to 2 5 km of oligocene aged dextral movement was accommodated on newly formed nnw sse striking faults displacement analysis suggests that the faults were kinematically linked and experienced a switch in polarity of the predominantly active fault orientation during the paleogene forming an apparent conjugate system with asymmetric displacement distributions the switch in fault polarity is attributed to a rotation in the regional maximum horizontal stress orientation and the locus of deformation moving from pre existing reactivated structures to newly formed faults the analysis also illustrates that despite low strain rates the interaction of the principal components of the fault system controlled the formation of sedimentary basins and defined the present day geography of the region far from an active plate boundary

active aeolian processes on mars a regional study in arabia and meridiani terrae 1 we present evidence of widespread aeolian activity in the arabia terra meridiani region mars where different kinds of aeolian modifications have been detected and classified passing from the regional to the local scale we describe one particular dune field in meridiani planum where two ripple populations are distinguished by means of different migration rates moreover a consistent change in the ripple pattern is accompanied by significant dune advancement between 0 4 1 meter in one martian year that is locally triggered by large avalanche features this suggests that dune advancement may be common throughout the martian tropics

object based dune analysis automated dune mapping and pattern characterization for ganges chasma and gale crater mars abstract a method that enables the automated mapping and characterization of dune fields on mars is described using ctx image mosaics the introduced object based dune analysis obda technique produces an objective and reproducible mapping of dune morphologies over extensive areas the data set thus obtained integrates a large variety of data allowing a simple cross analysis of dune patterns spectral and morphometric information and mesoscale wind models two dune fields located in gale crater and ganges chasma were used to test and validate the methodology the segmentation of dune related morphologies is highly efficient reaching overall accuracies of 95 in addition we show that the automated segmentation of slipface traces is also possible with expected accuracies of 85 90 a qualitative and quantitative comparison of the final outputs with photointerpretations is performed and the precision of the directional characterization of the dune patterns is evaluated we demonstrate a good agreement between the obda outputs and the photointerpreted dune morphologies with local trend deviations below 45 for 80 95 of the mapped areas because the developed algorithm is tuned for the recognition of linear features from the imagery the slipfaces of small barchans can be preferentially overlooked owing to their small extent at the spatial resolution of the ctx mosaics dune types composed of longer linear morphologies are much better represented including correct mapping of secondary structures having proved the effectiveness and accuracy of the mapping procedure we discuss its future applications for the improvement of dune catalogs on mars

the post variscan development of the british isles within a regional transfer zone influenced by orogenesis abstract the break up of pangaea after the variscan orogeny included rifting extending southwards from the barents sea via the norwegian greenland rift and into the north sea and northwards from the central atlantic these two major rift systems interacted to form an approximately 1200 km wide transfer zone across the british isles where a complex network of basins developed during the mesozoic fault patterns were commonly controlled by reactivation of precambrian caledonian and variscan structures the two main rift systems were unable to breach this regional transfer zone where the crust had been thickened by the caledonian and variscan orogenies until the eocene breaching did not occur down the north sea and through the english channel because of alpine contraction in nw europe instead breaching occurred around the west of ireland and nw scotland so the british isles remained connected to europe rather than to the north american plate

integrating 3 d seismic data field analogs and mechanical models in the analysis of segmented normal faults in the wytch farm oil field southern england united kingdom we propose a methodology for the analysis of normal fault geometries in three dimensional 3 d seismic data sets to provide insights into the evolution of segmented normal fault systems and to improve recovery efforts in fault controlled oil fields limited seismic resolution can obscure subtle fault characteristics such as segmentation and gaps in fault continuity that are significant for oil migration and thus accurate reservoir characterization detailed seismic data analyses that incorporate principles of normal fault mechanics however can reveal evidence of fault segmentation we integrate seismic attribute analyses outcrop analog observations and numerical models of fault slip and displacement fields to augment the use of 3 d seismic data for fault interpretation we applied these techniques to the wytch farm oil field in southern england resulting in the recognition of significant lateral and to a lesser extent vertical segmentation of reservoir scale faults slip maxima on fault surfaces indicate two unambiguous segment nucleation depths controlled by the lithological heterogeneity of the faulted section faults initiated preferentially in brittle sandstone and limestone units subsequent growth and linkage of segments predominantly in the lateral direction resulted in composite fault surfaces that have long lateral dimensions and multiple slip maxima reservoir compartmentalization is greatest at the level of prevalent segment linkages which corresponds at wytch farm with the predominant hydrocarbon producing unit the sherwood sandstone at relatively shallower depths fault segments are younger and less evolved resulting in a greater degree of segmentation with intact relay zones

genesis of natural hydraulic fractures as an indicator of basin inversion abstract satin spar fibrous gypsum veins which occur in evaporite basins worldwide provide significant insights into host rock deformation and fluid flow although the genetic mechanism remains obscure satin spar veins in the red marls of the triassic mercia mudstone of the bristol channel basin were characterized in the context of regional and local setting the vein network in the keuper marl lower mercia exhibits a lack of systematic cross cutting between three distinct vein sets two sets of veins are observed in the overlying tea green marl with one set clearly crossing the other the gypsum veins commonly contain a blocky median zone of multiple thin bands of host rock inclusions and alabastrine gypsum crystals exhibiting crack seal patterns fibrous zones on either side of the median zone consist of pure parallel aligned gypsum fibres that are oblique to vein walls indicating a hybrid shear extensional mode of vein widening veins developed within reverse reactivated faults contain fibre lineations in the median zones and also on vein surfaces suggesting a minimum of two phases of fault slip the veins are interpreted to have formed as a result of overpressure in the low permeability mudstones by tectonic compression during basin inversion giving rise to the median zone growth of gypsum fibres commenced when the initial fractures were completely sealed the timing of vein formation is suggested to be miocene based on cross cutting relationships with folds formed during the latest phase of basin inversion because the fractures initiated in response to fluid overpressures they provide a useful analog to subsurface hydraulic fracture systems in low permeability rocks subjected to tectonic compression the fibrous widening of the veins post dates their initiation and led to continued propagation and increased connectivity

the thermal and mechanical development of the wessex basin southern england summary the wessex basin of southern england is an e w trending late palaeozoic to tertiary extensional basin formed on early palaeozoic variscan crust extension occurred over a protracted period from late carboniferous to cretaceous times resulting in the evolution of four main depocentres or sub basins the channel basin winterborne kingston trough vale of pewsey and weald basins each sub basin was controlled fundamentally by the normal reactivation of basement thrust faults and wrench or transfer faults the basement faults represent primary structures associated with the fold and thrust terrain of the devonian carboniferous variscan orogeny subsequent but synchronous reactivation of both thrust and transfer faults tended to compartmentalize the basement leading to discrete depocentres and the production of general rhomboidal shaped sub basins characteristic of pull apart basins in strike slip terrains to investigate the thermo mechanical properties of the lithosphere during basin formation 50 boreholes within the wessex basin were back stripped to isolate the driving tectonic subsidence responsible for basin formation the resultant driving subsidence is characterized by a gentle sometimes negative exponential form with superimposed rapid discrete subsidence events as the geological development of the wessex basin was characterized by polyphase extension and the utilization of crustal detachments the lithospheric model used to determine crustal extension above the detachment and stretching below the detachment included the effects of depth dependent stretching and finite re rifting from the back stripping analysis the basin wide averaged upper crustal extension and lower crustal sub crustal lithospheric stretching were 11 and 5 respectively the distribution of is strongly influenced by the location of growth faults and hence basement thrust fault reactivation suggesting that is reflecting the brittle failure and collapse of hanging wall blocks repeated slip and block collapse resulted in the polyphase extension responsible for renewed basin subsidence in contrast is uniformly distributed across the basin suggestive of a ductile failure of the lower crust and responsible for the low magnitude thermal subsidence strain compatibility between the upper crust and lower crust sub crustal lithosphere is maintained partly within the basin 5 and partly external to the basin as the variscan basal thrust was the fundamental relay in the sense of wernicke responsible for the wessex basin formation we postulate that strain balancing induced a cumulative essentially thermal isostatic uplift of 60 m under the northern margin of the armorican massif

possible variscan thrusting beneath the somerton anticline somerset vibroseis seismic data were acquired by goal petroleum plc during 1982 in the vicinity of the somerton anticline somerset these data show the simple form of the anticline but in addition show a southward dipping discontinuous seismic event beneath gravity modelling demonstrates that this event may be correlated with an e w trending gravity gradient which can be followed for approximately 150 km the dipping seismic event may represent a variscan thrust plane which dips to the ssw in contrast to similar planes seen to the east which dip to the sse the variation in dip seen between these postulated thrust planes may explain the curvature of structural trends seen within and to the east of the mendip hills the polden hills fault runs sub parallel to the proposed thrust plane beneath the somerton anticline and may be linked to a mesozoic reactivation of the postulated thrust

variscan structure of the inner bristol channel uk major variscan structures under the bristol channel are concealed beneath a mesozoic and quaternary cover mapping of reflectors on offshore seismic sections reveals two major wnw ese striking thrusts within the underlying palaeozoic sequence the more southerly with a moderate ssw dip has previously been identified as the bristol channel thrust another thrust the previously undescribed gravel margin thrust has been identified in the eastern part of the inner bristol channel and lies in the footwall of the bristol channel thrust it has a similar strike but steeper dip and extends to anticipated precambrian basement depths the bristol channel and gravel margin thrusts together juxtaposed the contrasting palaeozoic successions of south wales and sw england before mesozoic reactivation reflection packages identified on the offshore seismic sections are tentatively correlated with specific onshore palaeozoic sequences interpretation of the offshore seismic sections in conjunction with geological mapping along the north devon coast shows that the bristol channel and gravel margin thrusts have orientations and geometries typical of several mesoscale thrusts observed and measured at foreland point north devon along strike structural changes beneath the inner bristol channel seen on seismic sections provide evidence additional to earlier refraction and structural studies for the offshore continuation of the nw se trending watchet cothelstone hatch fault and its linkage with the previously described central bristol channel fault zone the bristol channel and gravel margin thrusts are interpreted to be offset from the cannington park thrust in north somerset by a 14 16 km variscan dextral strike slip displacement across the watchet cothelstone hatch fault the bristol channel gravel margin thrust and the cannington park thrust are interpreted as syngenetic structures linked by their lateral ramp the watchet cothelstone hatch fault using all the above information the overall structure of the variscides of the inner bristol channel is re evaluated and a new tectonic model is presented the structures discussed in this paper at a scale portrayed by commercial seismic data and published geological maps may represent merely the tips of linked thrusts that have previously been mapped down to mid crustal depths on birps seismic sections from the south west approaches traverse we show that enough data now exist to resolve outstanding problems of the concealed variscan structure in the inner bristol channel providing local structural detail that can be incorporated into the wider tectonic framework of nw europe

quantifying vein attributes in massive mudstones triassic sw england implications for progressive evolution of opening mode fracture networks abstract attributes of gypsum vein arrays in the triassic massive mudstones of sw england were analyzed to infer the process of vein network evolution field data demonstrate that there is no clear correlation between vein orientation and thickness however sub horizontal veins have a higher average thickness than the inclined veins vein thickness distribution is best characterized by a log normal distribution the fractal dimensions d of vein networks analyzed range from 1 44 to 1 71 vein networks which contain abundant bridging veins between neighbouring sub horizontal veins to form an anastomosing pattern exhibit a higher d value the fraction of vein volume derived from image analysis ranges from 6 75 to 17 94 a positive correlation is revealed by plotting the fractal dimensions of the vein networks versus the number of vein intersection points suggesting that an anti clustering distribution of veins could contribute to more interactions and also intersections of the gypsum veins the vein attribute data presented indicate that mineralization in the massive mudstones is mainly localized to some of the largest veins as a result of mechanical interaction of neighbouring veins during vein propagation hindering the nucleation of new veins as a consequence accommodation of the bulk rock strain could be facilitated mainly by expansion of the large backbone veins which follow a positive feedback mechanism of growth this study reveals a strong link between the geometric parameters of opening mode fractures with the history of their formation propagation and intersection