source and partitioning of b and sn in the cornubian batholith of southwest england abstract emplacement of the variscan s type cornubian batholith initiated the formation of the historical world class sn and cu deposits and extensive tourmalinisation of southwest england the hypothesis is tested that the sn and tourmaline mineralisation were wholly formed by magmatically derived hydrothermal fluids with no requirement for a contribution from the stripping of country rocks by circulating hydrothermal fluids from simple geochemical modelling sufficient sn concentrations to match those in the main 90 of granite outcrop biotite granites average of 23 mg kg and to produce the large sn deposits can be derived by partial melting of pre granite southwest england crust at geologically realistic levels of partial melting 30 fractionation 10 to 50 and melt aqueous fluid partitioning assumed to be 6 wt of the mass of the granite batholith in contrast insufficient b 700 mg kg or b concentrations found in many of the granites we conclude that tourmaline in the biotite granites was formed from magmatically derived hydrothermal fluids which emanated as a result of first and possibly second boiling an additional source of b for the hydrothermal fluids may have been the devolatilisation of stoped blocks of country rock mudstones these findings provide fundamental constraints on the evolution of the cornubian batholith which is used as a model for a large number of sn provinces worldwide

numerical simulations of co2 migration during charnockite genesis charnockite is considered to be generated either through the dehydration of granitic magma by co2 purging or by solid state dehydration through co2 metasomatism during granulite facies metamorphism to understand the extent of dehydration co2 migration is quantitatively modeled in silicate melt and metasomatic fluid as a function of temperature h2o wt pressure basal co2 flux and dynamic viscosity numerical simulations show that co2 advection through porous and permeable high grade metamorphic rocks can generate dehydrated patches close to the co2 flow path as illustrated by the occurrences of incipient charnockites co2 reaction front velocity constrained by field observations is 0 69 km m y a reasonable value which matches well with other studies on the other hand temperature rate of cooling and basal co2 flux are the critical parameters affecting co2 diffusion through a silicate melt co2 diffusion through silicate melt can only occur at temperature greater than 840 degrees c and during slow cooling 3 7 x 10 5 degrees c yr features that are typical of magma emplacement in the lower crust stalling of co2 fluxing at similar to 840 degrees c explains why some deep level plutons contain both hydrous and anhydrous charnockitic mineral assemblages co2 diffusion through silicate melt is virtually insensitive to pressure addition of co2 basal flux facilitates episodic dehydrated melt migration by generating fracture pathways

primary lithological variation in the kaolinized st austell granite cornwall england geological mapping of existing and redundant kaolin workings within the st austell granite has identified a suite of granitic rocks which show evidence of complex late stage magmatic and hydrothermal processes coarse porphyritic biotite granites like those which predominate in southwest england occur much more widely than previously acknowledged and are intruded by an apparently cogenetic suite of lithium mica granites and tourmaline granites the tourmaline granites characteristically exhibit very variable textures with coarse quartz grains set within a fine grained tourmaline rich matrix a highly evolved fine grained tourmaline granite represents the most evolved of this suite topaz granites intrude the earlier granite varieties and all are intruded by rhyolite porphyry dykes elvans major and trace element chemical data suggest that the biotite granite lithium mica granite tourmaline granite suite represents the product of crystallization of a granitic magma within which b but not f became progressively enriched until water saturation was achieved water exsolution effectively quenched any remaining granitic melt resulting in the very variable textures shown by the tourmaline granites the topaz granites are chemically distinct from their predecessors showing marked enrichment in f li and p 2 o 5 but not b instead of being products of differentiation of biotite granite magma the topaz granite melts may have been derived separately in a later episode of partial melting of the same source kaolinization is widespread throughout the western part of the st austell granite and deposits worked at present tend to be located in granite varieties other than biotite granite the geochemical parameters used to distinguish the primary granite types particularly nb v zr and ga nb zr plots are sufficiently robust to permit the parent granitic rock type to be identified for heavily kaolinized material

ekman drift in homogeneous water measurements made with satellite tracked buoys drogued in different layers between the sea surface and 30 m depth under homogeneous winter conditions in the north sea allow analysis of the ekman currents under a large variety of wind conditions the experiment lasted from november 20 1991 until february 29 1992 the first 4 weeks of this period during which the buoys stayed close together are used to determine the ekman stresses the total current field is a superposition of barotropic currents due to sea level variations and ekman currents the classical ekman theory is not able to describe properly the observed deflection of the currents to the right of the wind direction and their decay with depth this deflection is 10 near the sea surface and increases to approximately 50 in 25 m depth the relation between wind stress and the stress field in the interior of the water is given by a tensor which describes the rotation and the variation of the stress with increasing depth the concept of eddy viscosity is applicable if a viscosity tensor is used to relate stress and vertical shear the viscosity tensor is a function of the vertical coordinate only and is independent from the wind stress it shows maximum values in 15 to 20 m depth and may be due to langmuir circulation cells further studies are needed to determine the physics of this tensor its magnitude in the interior of the mixed layer exceeds 1000 cgs units consequently ekman currents are weak and may not be the dominant currents within the mixed layer

indium mineralisation in sw england host parageneses and mineralogical relations abstract the sw england ore region contains significant amounts of indium in in early permian granite related skarn and lode parageneses and to a lesser extent triassic epithermal crosscourse veins ore parageneses that predate granite emplacement devonian and lower carboniferous sedimentary exhalative and vein parageneses are largely devoid of in cadmium cd and gallium ga occur widely in all sulphide bearing parageneses across the region with sphalerite concentrations locally reaching 1 74 wt cd and 1750 ppm ga indium displays a strong affinity to sulphide bearing magmatic hydrothermal parageneses it occurs in silicate sulphide skarns polymetallic sulphide lodes and sulphide bearing portions of greisen bordered sheeted vein systems and quartz tourmaline lodes across the region magnetite silicate skarns and quartz tourmaline lodes that are devoid of sulphide in contrast were comparatively unfavourable for in precipitation the highest in concentrations are found in mineral lodes associated with the carnmenellis and st agnes granites which are the districts that had the highest historical production of sn total in concentrations in these areas locally exceed 430 ppm while concentrations elsewhere are systematically below 200 ppm the principal in hosts are chalcopyrite sphalerite and stannite group minerals with local concentrations within cassiterite and tennantite surprisingly chalcopyrite accounts for the majority of the in budget throughout the region although concentrations are highest in sphalerite and stannite group minerals sphalerite locally contains up to 1 42 wt in chalcopyrite has up to 2200 ppm and stannite group minerals up to 6800 ppm no in was detected in lollingite arsenopyrite rutile haematite magnetite tourmaline biotite chlorite galena bornite chalcocite or pyrrhotite scattered concentrations in pyrite relate to impurities rather than incorporation by solid solution roquesite and possibly dzhalindite or native in formed locally where in bearing chalcopyrite or sphalerite has been replaced by bornite and quartz the in partitioning between sphalerite and chalcopyrite varies broadly between 1 1 and 10 1 sporadic in was included in triassic crosscourse veins as a result of interactions between migrating cacl2 rich basinal brines and earlier formed granite related parageneses the interactions involved at least two distinct components 1 incorporation of clasts of magmatic hydrothermal veins in crosscourse veins during faulting and 2 dissolution and re precipitation of magmatic hydrothermal vein minerals in crosscourse fluids local concentrations reach 140 ppm in the magmatic hydrothermal parageneses in sw england are comparable to the south china tin belt mount pleasant as well as erzgebirge krusne hory magmatic hydrothermal fluids associated with peraluminous granites have developed a variety of skarn greisen lode and veins parageneses by interactions with their host rocks and contained fluids crosscourse epithermal mineralisation occurred as in was transported by cacl2 rich basinal brines in a similar manner as in bearing veins in the west shropshire ore field

zircon la icpms geochronology of the cornubian batholith sw england abstract available u pb age data for the cornubian batholith of sw england is based almost entirely on monazite and xenotime and very little zircon u pb age data has been published as a result no zircon inheritance data is available for the batholith by which the nature of the unexposed basement of the rhenohercynian zone in sw england might be constrained zircon la icpms data for the cornubian batholith provides concordia ages bodmin moor granite 316 4 ma carnmenellis granite 313 3 ma dartmoor granite 310 ma st austell granite 305 5 ma and land s end granite 300 5 ma that are consistently 20 30 ma older than previously published emplacement ages for the batholith and unrealistic in terms of geologic relative age relationships with respect to the country rock this discrepancy is likely as a consequence of minor pre granitic pb inheritance several of the batholith s granite plutons contain a component of late devonian inheritance that may record rift related lower crustal melting or arc related magmatism associated with subduction of the rheic ocean in addition the older granites likely contain mesoproterozoic inheritance although the highly discordant nature of the mesoproterozoic ages precludes their use in assigning an affinity to the rhenohercynian basement in sw england

tectonic record magmatic history and hydrothermal alteration in the hercynian guerande leucogranite armorican massif france abstract the guerande peraluminous leucogranite was emplaced at the end of the carboniferous in the southern part of the armorican massif at the scale of the intrusion this granite displays structural heterogeneities with a weak deformation in the southwestern part whereas the northwestern part is marked by the occurrence of s c and mylonitic extensional fabrics quartz veins and pegmatite dykes orientations as well as lineations directions in the granite and its country rocks demonstrate both e w and n s stretching therefore during its emplacement in an extensional tectonic regime the syntectonic guerande granite has probably experienced some partitioning of the deformation the southwestern part is characterized by a muscovite biotite assemblage the presence of restites and migmatitic enclaves and a low abundance of quartz veins compared to pegmatite dykes in contrast the northwestern part is characterized by a muscovite tourmaline assemblage evidence of albitization and gresenization and a larger amount of quartz veins the southwestern part is thus interpreted as the feeding zone of the intrusion whereas the northwestern part corresponds to its apical zone the granite samples display continuous compositional evolutions in the range of 69 8 75 3 wt sio 2 high initial 87 sr 86 sr ratios and low e nd t values suggest that the peraluminous guerande granite a cnk 1 1 was formed by partial melting of metasedimentary formations magmatic evolution was controlled primarily by fractional crystallization of k feldspar biotite and plagioclase an20 the samples from the apical zone show evidence of secondary muscovitization they are also characterized by a high content in incompatible elements such as cs and sn as well as low nb ta and k rb ratios the apical zone of the guerande granite underwent a pervasive hydrothermal alteration during or soon after its emplacement u th pb dating on zircon and monazite revealed that the guerande granite was emplaced 309 7 1 3 ma ago and that a late magmatic activity synchronous with hydrothermal circulation occurred at ca 303 ma these new structural petrological and geochronological data presented for the guerande leucogranite highlight the interplay between the emplacement in an extensional tectonic regime magmatic differentiation and hydrothermal alteration and provide a general background for the understanding of the processes controlling some mineralization in the western european hercynian belt

evolution of the cornubian ore field southwest england part i batholith modeling and ore distribution the cornubian ore field of southwest england is associated with an s type ilmenite series high heat production hhp two mica tourmaline bearing monzogranite batholith which formed between 270 to 295 ma base metal deposits are mainly in the form of steeply dipping lodes emplaced in the batholith roof and a hornfelsic sequence of upper paleozoic mud rocks sandstones and mafic volcanics the steep sided batholith has a gently sloping base 10 to 15 km below the present surface three dimensional modeling of gravity data indicates that it has a total volume of about 68 000 km 3 and is divisible into distinct western and eastern parts computer aided evaluation of mineral production statistics shows that most of the sn w cu and as production is located within the axial trace of the batholith with the western end enriched in sn cu and zn relative to the eastern end by contrast most of the fe as sb f ba and china clay production is located in the eastern end while pb and ag production is concentrated in the country rock filled south facing embayments on the flank of the batholith all significant wolframite production is from within the granite or within a 700 m distance of an observed or projected granite contact whereas 90 to 100 percent of the sn cu and as production occurs within 1 500 m of a contact by contrast 30 percent of the zn production and over 80 percent of the pb and ag production is from distances of more than 1 500 m from a granite contact pre syn and postbatholith faults and extensional fractures exert a fundamental control on ore distribution with most of the sn w cu as zn ores hosted by fractures trending parallel to the axial trace of the batholith and most of the pb sb ag ba and fluorite mineralization hosted by faults trending approximately normal to the axial trace of the batholith the nature of the sedimentary sequence also influenced metal distribution with most of the sn cu as production coming from areas underlain by devonian argillaceous rocks an important feature of the granite which has a bearing on its metallogenic potential is its high internal heat production due to its high total content of u th and k modeling of the cooling history of the granites reveals that temperatures in the deeper parts of the batholith were maintained close to or above the solidus for 10 to 20 ma after initial emplacement at 280 to 295 ma residual volatile and metal enriched magmas trapped within the batholith provided source materials for younger magmatism 270 280 ma and the main episode of lode formation which was related to the late magmatic events continued high heat flow throughout the mesozoic and cainozoic promoted thermally driven ground water convection and epithermal mineralization

in situ la icpms u pb dating of cassiterite without a known age matrix matched reference material examples from worldwide tin deposits spanning the proterozoic to the tertiary abstract cassiterite sno2 a main ore mineral in tin deposits is suitable for u pb isotopic dating because of its relatively high u pb ratios and typically low common pb we report a la icpms analytical procedure for u pb dating of this mineral with no need for an independently dated matrix matched cassiterite standard la icpms u th pb data were acquired while using nist 612 glass as a primary non matrix matched standard raw data are reduced using a combination of iolite and other off line data reduction methods cassiterite is extremely difficult to digest so traditional approaches in la icpms u pb geochronology that utilize well characterized matrix matched reference materials e g age values determined by id tims cannot be easily implemented we propose a new approach for in situ la icpms dating of cassiterite which benefits from the unique chemistry of cassiterite with extremely low th concentrations th u ratio of 10 4 or lower in some cassiterite samples accordingly it is assumed that 208pb measured in cassiterite is mostly of non radiogenic origin it was initially incorporated in cassiterite during mineral formation and can be used as a proxy for common pb using 208pb as a common pb proxy instead of 204pb is preferred as 204pb is much less abundant and is also compromised by 204hg interference during the la icpms analyses our procedure relies on 208pb 206pb vs 207pb 206pb pb pb and tera wasserburg 207pb 206pb vs 238u 206pb u pb isochron dates that are calculated for a 1 54 ga low th cassiterite reference material with varying amounts of common pb that we assume remained a closed u pb system the difference between the nist 612 glass normalized biased u pb date and the pb pb age of the reference material is used to calculate a correction factor f for instrumental u pb fractionation the correction factor f is then applied to measured u pb ratios and tera wasserburg isochron dates are obtained for the unknown cassiterite analyzed in the same analytical session this allows for u pb dating of cassiterite of any age with no need for an independently dated matrix matched reference material nor assumptions about the isotopic composition of common pb results for cassiterite from tin deposits in bolivia brazil china russia saudi arabia south africa spain and the united kingdom with ages ranging from 20 ma to 2060 ma demonstrate the applicability of this approach across a broad range of geologic time these ages are in good agreement with published geochronology of the host rocks associated with the tin deposits and with previously published u pb ages of some cassiterites from the same deposits thus our in situ la icpms methodology verifies the use of cassiterite as a reliable u pb mineral geochronometer with the advantages of fast and relatively low cost in situ analyses with moderate spatial resolution

chlorine isotopes in fluid inclusions determination of the origins of salinity in magmatic fluids abstract thermal ionisation mass spectrometry tims of the cs 2 cl ion has been used to determine the 37 cl value of paleofluids trapped in fluid inclusions modifications to the existing published method have lowered the precision by a factor of 2 0 09 1 thereby allowing real differences to be measured with greater certainty the method has been applied to determine 37 cl of two sets of magmatic fluids trapped in fluid inclusions samples were studied from the capitan pluton in new mexico and the sw england batholith both of which have high temperature fluids whose d and 18 o signatures are distinctly magmatic relative to standard mean ocean chloride smoc the 37 cl values of the capitan samples cluster around 0 whereas those from sw england cluster around 1 8 the br cl ratios of the capitan fluid inclusions are low indicative of halite dissolution and are comparable to local high salinity waters derived from leaching permian evaporites the br cl ratios of the sw england fluid inclusions are much higher and within the range of values normally considered to be magmatic we conclude that the 37 cl value of the capitan magmatic fluids confirms the evidence from br cl ratios that cl was derived from an evaporite source whereas the 37 cl value of the sw england fluids appears to be more representative of a deep magmatic source

a fluid inclusion and stable isotope study of 200 ma of fluid evolution in the galway granite connemara ireland fluid inclusions in granite quartz and three generations of veins indicate that three fluids have affected the caledonian galway granite these fluids were examined by petrography microthermometry chlorite thermometry fluid chemistry and stable isotope studies the earliest fluid was a h2o co2 nacl fluid of moderate salinity 4 10 wt nacl eq that deposited late magmatic molybdenite mineralised quartz veins v1 and formed the earliest secondary inclusions in granite quartz this fluid is more abundant in the west of the batholith corresponding to a decrease in emplacement depth within veins and to the east this fluid was trapped homogeneously but in granite quartz in the west it unmixed at 305 390 c and 0 7 1 8 kbar homogeneous quartz 18o across the batholith 9 5 0 4 n 12 suggests v1 precipitation at high temperatures perhaps 600 c and pressures 1 3 kbar from magmatic fluids microthermometric data for v1 indicate lower temperatures suggesting inclusion volumes re equilibrated during cooling the second fluid was a h2o nacl kcl low moderate salinity 0 10 wt nacl eq moderate temperature 270 340 c high d 18 2 low 18o 0 5 2 0 fluid of meteoric origin this fluid penetrated the batholith via quartz veins v2 which infill faults active during post consolidation uplift of the batholith it forms the most common inclusion type in granite quartz throughout the batholith and is responsible for widespread retrograde alteration involving chloritization of biotite and hornblende sericitization and saussuritization of plagioclase and reddening of k feldspar the salinity was generated by fluid rock interactions within the granite within granite quartz this fluid was trapped at 0 5 2 3 kbar having become overpressured this fluid probably infiltrated the granite in a meteoric convection system during cooling after intrusion but a later age cannot be ruled out the final fluid to enter the granite and its host rocks was a h2o nacl cacl2 kcl fluid with variable salinity 8 28 wt nacl eq temperature 125 205 c d 17 to 45 18o 3 to 1 2 13cco2 19 to 0 and 34ssulphate 13 23 that deposited veins containing quartz fluorite calcite barite galena chalcopyrite sphalerite and pyrite v3 correlations of salinity temperature d and 18o are interpreted as the result of mixing of two fluid end members one a high d 17 to 8 moderate 18o 1 2 2 5 high 13cco2 4 low 34ssulphate 13 high temperature 205 230 c moderate salinity 8 12 wt nacl eq fluid the other a low d 61 to 45 low 18o 5 4 to 3 low 13c 10 high 34ssulphate 20 23 low temperature 80 125 c high salinity 21 28 wt nacl eq fluid geochronological evidence suggests v3 veins are late triassic the high d end member is interpreted as a contemporaneous surface fluid probably mixed meteoric water and evaporated seawater and or dissolved evaporites whereas the low d end member is interpreted as a basinal brine derived from the adjacent carboniferous sequence this study demonstrates that the galway granite was a locus for repeated fluid events for a variety of reasons from expulsion of magmatic fluids during the final stages of crystallisation through a meteoric convection system probably driven by waning magmatic heat to much later mineralisation concentrated in its vicinity due to thermal tectonic and compositional properties of granite batholiths which encourage mineralisation long after magmatic heat has abated

estimating abundances of volatile and other mobile components in evolved silicic melts through mineral melt equilibria to variable degrees of accuracy from source partitioning between and earths residual minerals and anatectic melts through melt fractionation partitioning between igneous minerals and residual melts to the end stages of magma solidification where higher concentrations of normally trace elements may promote saturation in their crystalline phases the magmatic abundance of rare alkalis and alkaline earths

pb pb and u pb zircon dating of subvolcanic rhyolite as a time marker for hercynian granite magmatism and sn mineralisation in the eibenstock granite erzgebirge germany considering effects of zircon alteration tin mineralisation is closely related to rhyolite stocks and dykes which occur in the endo and exocontact of the eibenstock granite erzgebirge germany the same structures which cut the granite control the rhyolite emplacement and the location of ore bearing greisen bodies albitisation and greisenisation related to tin mineralisation as well as sericitisation and argillic alteration may be traced by changes in chemical and mineralogical composition of both rhyolite and granite wall rock comprehensive zircon studies by scanning electron microscopy secondary and backscattered electron as well as cathodoluminescence imaging edx measurements reveal that zircon from rhyolite and from the enclosing granite shows significant changes in chemical composition and crystallinity including distortion of the u pb isotope system when affected by greisenisation single evaporation analysis of zircon from rhyolite with little or no greisenisation gave a 207pb 206pb age of 290 5 ma whereas zircon from little altered granite gave a 207pb 206pb age of 320 8 ma the single evaporation age of zircon from rhyolite is confirmed by 238u 206pb shrimp ages of 297 8 ma the significant time gap of at least 20 ma between granite intrusion and rhyolite formation suggests that the late magmatic evolution of the eibenstock granite cannot be regarded as a source for tin ore forming fluids as previously assumed

diachronous and independent histories of plutonism and mineralization in the cornubian batholith southwest england concordant u pb dates of magmatic monazites in the dominant coarse grained biotite muscovite granites of the late variscan cornubian batholith demonstrate that the emplacement of the six main plutons was diachronous ranging from 293 1 1 3 ma carnmenellis to 274 5 1 4 ma land9s end southern lobe 40 ar 39 ar age spectra for magmatic muscovites confirm that cooling to about 320 c at the present level of exposure was accomplished within 4 to 5 ma in each intrusion moreover age spectra for hydrothermal muscovites show that development of the major sn cu lodes of the south crofty mine commenced before 286 ma only 2 to 3 ma after cooling of the hosting cam brea granite a satellite of the carnmenellis pluton the older plutons had therefore approached ambient temperatures and had even experienced intense hydrothermal activity before emplacement of the youngest

geochronological evidence for post variscan cooling and uplift of the carnmenellis granite sw england apatite fission track aft dates for granitic samples from surface exposures and deep boreholes in the 293 3 1 2 ma carnmenellis granite west cornwall decrease radically from 130 28 2 ma at surface to 26 6 ma at c 2500 m depth a trend accompanied by a decrease in mean track length from 12 99 0 80 m to 9 83 0 62 m in contrast 40 ar 39 ar apparent age plateaux for muscovite decrease only from 288 5 1 0 ma at surface to 286 1 1 0 ma at c 2500 m depth the small difference in the muscovite ages may record the presence of a slightly younger intrusive phase at depth but it is apparent that the preserved upper 2 5 km of the pluton cooled rapidly through c 320 c however thermal modelling of the aft data delimits a distinct fast cooling event at c 137 155 ma exhibiting a satisfactory correlation with a major regional uplift episode in late jurassic early cretaceous times recorded by the sedimentary basins flanking the cornubian massif

pluton shapes in the cornubian batholith new perspectives from gravity modelling gravity modelling of the dartmoor bodmin st austell and carnmenellis plutons of the cornubian batholith of sw england strongly supports a tabular form for the exposed granites modelled thicknesses for the individual bodies with the exception of dartmoor are consistent with those derived from empirical relationships for plutons and laccoliths in the case of the dartmoor bodmin and st austell plutons the deepest parts are located near their southern margins suggesting a steep conduit with northerly and outward direct flows modelling of the carnmenellis granite suggests a more centrally located feeder in the case of dartmoor at least the granite appears to have exploited the devonian carboniferous interface during its emplacement

volatile production during contact metamorphism the role of organic matter in pelites elemental analyses of palaeozoic mudrocks from south cornwall indicate the presence of 0 04 0 74 wt carbon primarily in an elemental and or organic form in addition to 0 04 0 10 wt nitrogen as nh 4 in micas and in organic material both elements are depleted in samples from the lithologically homogeneous mylor slate formation next to the tregonning granite an offshoot of the cornubian batholith the depletions are due to an h 2 o co 2 n 2 ch 4 fluid formed during contact metamorphic devolatilization of the pelitic country rocks fluids of this type have been reported from a number of hercynian sn w provinces and may play an important role in mobilizing metals in pelitic rocks during granite related contact metamorphism and hence in the metallogenic evolution of such regions

geology and mineralization of the uchkoshkon tin deposit associated with a breccia pipe eastern kyrgyzstan abstract the uchkoshkon tin deposit is one of the largest deposits in the sn w saryjaz ore district of eastern kyrgyzstan the district coincides spatially with the saryjaz syntaxis which is the narrowest portion of the south tien shan collisional belt the syntaxis is distinguished by collisional thrust tectonics and postcollisional a type granite magmatism of permian age the uchkoshkon deposit is in the exocontact zone of a 260 km 2 273 279 ma ilmenite series high heat production biotite granite pluton a satellite granite stock 400 250 m and an explosive breccia pipe 450 150 m crosscutting the stock are the central structural features of the deposit area the main stock accompanied by li f felsite dykes controlled early stage mo w and sn w greisen stockwork mineralization that surrounds the apical portion of the stock the breccia pipe postdates the emplacement of the li f felsite dykes and greisen stockwork mineralization the pipe is rooted in main pluton and has been traced by a borehole to a depth of 800 m the breccia was injected by a swarm of tree like branching dykes with diffuse contours consisting of k and b rich rhyolite porphyry cementing the breccia fragments the main breccia body has been intruded by a much smaller 150 100 m hydrothermal breccia pipe with quartz tourmaline matrix which immediately preceded main stage lode mineralization the majority of the lodes occur outside the breccia pipe within a zone of pervasive scapolitization that surrounds the main pluton the main stage lodes are represented by quartz tourmaline cassiterite veins with minor sulfides which occupy a series of steeply dipping near parallel north northeast trending shear fractures that are related to postcollisional strike slip tectonics where quartz tourmaline cassiterite veins crosscut moderately dipping skarn deposits the latter are commercially mineralized with cassiterite nordenskioldine and borosilicate minerals the formation of both explosive breccia and main stage lode mineralization was related to the evolution of b and h 2 o rich residual magma deep within the main pluton the suggested formation model of the uchkoshkon tin deposit is generally applicable to other major sn and sn w deposits in the saryjaz ore district

seismicity of the atlantis massif detachment fault 30 n at the mid atlantic ridge 1 at the oceanic core complex that forms the atlantis massif at 30 n on the mid atlantic ridge slip along the detachment fault for the last 1 5 2 ma has brought lower crust and mantle rocks to the seafloor hydroacoustic data collected between 1999 and 2003 suggest that seismicity occurred near the top of the massif mostly on the southeastern section while detected seismicity along the adjacent ridge axis was sparse in 2005 five short period ocean bottom seismographs obs were deployed on and around the massif as a pilot experiment to help constrain the distribution of seismicity in this region analysis of six months of obs data indicates that in contrast to the results of the earlier hydroacoustic study the vast majority of the seismicity is located within the axial valley during the obs deployment and within the array seismicity was primarily composed of a relatively constant background rate and two large aftershock sequences that included 5 teleseismic events with magnitudes between 4 0 and 4 5 the aftershock sequences were located on the western side of the axial valley adjacent to the atlantis massif and close to the ridge transform intersection they follow omori s law and constitute more than half of the detected earthquakes the obs data also indicate a low but persistent level of seismicity associated with active faulting within the atlantis massif in the same region as the hydroacoustically detected seismicity within the massif the data indicate a north south striking normal fault and a left lateral strike slip fault near a prominent transform parallel north facing scarp both features could be explained by changes in the stress field at the inside corner associated with weak coupling on the atlantis transform alternatively the normal faulting within the massif might indicate deformation of the detachment surface as it rolls over to near horizontal from an initial dip of about 60 beneath the axis and the strike slip events may indicate transform parallel movement on adjacent detachment surfaces

the transition from granite to banded aplite pegmatite sheet complexes an example from megiliggar rocks tregonning topaz granite cornwall abstract the genetic relationship between a granite pluton and adjacent complex of rare metal pegmatite aplite banded sheets megiliggar sheet complex msc has been studied at the border of the tregonning topaz granite at megiliggar rocks cornwall sw england similarities in whole rock chemical and mineralogical compositions together with a gradual change in textures away from the granite margin provide strong evidence for a genetic link between the tregonning granite and msc the sheets are likely to represent apophyses of residual melt which escaped from the largely crystallized roof of the granite pluton the escaping melt was peraluminous had a composition near the f b li slightly enriched granite minimum and in comparison with other cornish granites was enriched in f li rb cs sn w nb ta and u and depleted in fe mg ca sr th zr and ree with increasing distance from the tregonning granite the silicate melt crystallized as homogeneous leucogranite sheets and banded complex sheets i e combinations of bands with granitic aplitic and pegmatitic textures then layered aplite pegmatites this sequence becoming progressively more depleted in the fluxing and volatile elements f li rb and cs but showing no change in zr hf ratios the fixed zr hf ratio is interpreted as indicating a direct genetic link parental melt between all rock types however the melt progressively lost fluxing and volatile elements with distance from the granite pluton probably due to wall rock reaction or fluid exsolution and migration via fractures differentiation of the primary melt into na li f rich and separate k b rich domains was the dominant chemical process responsible for the textural and mineral diversity of the msc on a large cliff section scale the proximal na li f rich leucogranite passes through complex sheets into k b rich aplite pegmatites whilst at a smaller

late magmatic immiscibility during batholith formation assessment of b isotopes and trace elements in tourmaline from the land s end granite sw england quartz tourmaline orbicules are unevenly distributed in the roof segment of the land s end granite sw england this study shows that the orbicules formed from an immiscible hydrous borosilicate melt produced during the late stages of crystallization and differentiates tourmaline formed by dominantly magmatic and dominantly hydrothermal processes trace elements and boron isotope fractionation can be tracked in tourmaline and create a timeline for crystallization tourmaline from the granite matrix has higher v cr and mg content and is isotopically heavier than the later crystallizing inner orbicule tourmaline overgrowths of blue tourmaline occurring together with quartz showing hydrothermal cathodoluminescence textures crystallized from an aqueous fluid during the very last crystallization and are significantly higher in sr and sn and isotopically heavier tourmaline associated with sn mineralization is also high in sr and sn but has boron isotopic compositions close to that of the magmatic tourmaline and is not formed by the same fluids responsible for the blue overgrowths the ore forming fluids precipitating tourmaline and cassiterite are likely derived from the same magma source as the granite but exsolved deeper in the magma chamber and at a later stage than orbicule formation tourmaline from massive quartz tourmaline rocks is concentrically zoned with major and trace element compositions indicating crystallization from a similar melt as for the orbicules but shows a more evolved signature