the characteristics and origin of the big lake mafic ultramafic hosted volcanogenic massive sulfide occurrence marathon ontario canada the big lake volcanogenic massive sulfide vms occurrence located in the schreiber hemlo belt in the wawa subprovince of the superior province is hosted in an archean mafic ultramafic metavolcanic sequence lacking felsic volcanic or volcaniclastic rocks discovered in 2006 the occurrence is a subeconomic assemblage of pyrrhotite chalcopyrite sphalerite cobaltite and galena currently defined over an area of 0 5 0 5 km in plan view the mineralized zone is less than 10 m thick with the highest grade intersection containing 4 0 m of 7 5 cu 2 2 zn 138 0 gpt ag and 9 2 gpt au mineralization is hosted in metavolcanic rocks of basaltic to komatiitic basalt compositions the sulfides and spatially associated hydrothermal alteration regionally metamorphosed to chlorite actinolite hornblende biotite phlogopite and anthophyllite are located 5 to 50 m structurally below a thick ultramafic cumulate complex the host sequence to the occurrence is overturned with vms sulfides stratigraphically overlying the ultramafic cumulates rocks stratigraphically below the vms mineralization are transitional to alkaline basalts and komatiitic basalts with convex upward lree and fractionated hree when normalized to primitive mantle basalts stratigraphically overlying the vms mineralization have flat normalized ree patterns la ybcn 1 2 typical of tholeiitic plateau basalts found elsewhere in the wawa belt the lithostratigraphic assemblage at big lake is consistent with an oceanic plateau setting and is interpreted to have formed by eruption of plateau basalts and thick ultramafic flows from a mantle plume unlike vms systems that formed above subvolcanic intrusions 0 5 km below sea floor the big lake vms occurrence may have formed as a result of cooling of the stratigraphically underlying ultramafic cumulates and demonstrates that similar styles of ponded flow driven vms systems albeit small ones may be preserved elsewhere in the superior province or in other archean cratons

controls on the siting and style of volcanogenic massive sulphide deposits abstract volcanogenic massive sulphide vms deposits form in subaqueous environments from circulating hydrothermal fluids heated by volcanic activity these deposits form as sulphide mounds stratiform exhalative and or replacive bodies and commonly have stockwork vein mineralization in their immediate footwall these various styles are essentially facies of mineralization each one being the product of a particular set of conditions that control the ore forming processes and the consequent geometry and architecture style of the deposits these controls include the physical and or chemical nature of the host rocks the temperature and composition of the hydrothermal fluids and the redox state of the depositional environment the style of exhalative deposits is controlled by the salinity of the vented fluids and the redox state at the seafloor hydrothermal fluids with salinities less than twice that of seawater that vented into open oxic oceanic environments typically formed small mound and chimney complexes unless they were rapidly covered by sediments or volcanic rocks the massive sulphides were rapidly oxidized and partly dissolved by seawater in contrast stratiform sheet like deposits are typically formed in anoxic bottom waters anoxic marine conditions were periodically of global extent particularly prior to 2 4 ga or of a regional nature local anoxic conditions can also be self induced by the exhalation of saline and reducing hydrothermal fluids that ponded in bathymetric depressions such as second or third order basins to form a brine pool these exhalative systems may have been initiated as chimney vent complexes and subsequently overlain by stratiform sulphides formed under the self induced anoxic conditions deposits formed in anoxic environments can be significantly larger than those in oxic settings and this is attributed to several factors that include longer lived hydrothermal circulation more efficient sulphide precipitation and reduced or inhibited oxidation thereof replacement of volcanic and sedimentary strata by sulphide typically occurs within the feeder zones beneath the exhalative mineralization however successions with abundant porous permeable and or reactive rocks such as glassy and or pumiceous volcaniclastic rocks and in some cases limestone favoured the development of large replacive deposits that may have had little surficial expression on the sea floor vms deposits at spreading centres within oceanic crust formed almost exclusively as mounds most of them have not been preserved likely due to oxidation of the sulphides in the prevailing oxic environment and or destruction of oceanic crust during subsequent subduction intra continental rifts arc rifts and back arc rifts commonly have more complexity in their structure and facies architecture and environments and can host all styles of vms mineralization in these settings early extension favoured the formation of restricted basins with ideal conditions for the onset of hydrothermal activity and development of anoxic bottom waters whereas in mature rifts the conditions were less conducive for the formation of regionally extensive anoxic environments formation of replacive deposits was permissible in all settings with porous or reactive subsea floor strata replacive mineralization is the most likely to be preserved in the geological record due to the sulphides being physically shielded from oxidative weathering and mechanical erosion at the seafloor the various styles of vms mineralization can rarely be distinguished using a single criterion in most cases multiple criteria are required mound style mineralization is distinguished by a mound or lens shaped morphology b presence of chimney fragments c presence of abundant sedimented sulphide breccias d location on a stratigraphic boundary ore horizon and e association with a thin horizon or thicker stratigraphic interval of fine grained clastic rocks e g shale mudstone that accumulated at slow sedimentation rates stratiform exhalative mineralization is distinguished by a sheet like morphology prior to deformation b presence of fine grained clastic host rocks that accumulated at relatively slow rates e g mudstone c presence of local or extensive planar stratification replacive mineralization is characterized by a irregular geometry and distribution of sulphide bodies b gradation from massive sulphides to semi massive sulphides and disseminated mineralization with relict textures of the host rock and c originally pumiceous glassy or reactive host rocks emplaced at high depositional rates mass flow deposits lavas carbonate altered mass flow deposits or limestone one deposit or district may comprise two or more of these main styles of mineralization in many cases the main styles of vms mineralization present in a particular region can be predicted from examination of the facies architecture and depositional environments of the host succession recognition of the style s of mineralization that occur in a particular basin or mineral belt enables exploration models to be improved and should influence the strategy of exploration for vms deposits

geochemical and oxygen isotope signature of sea floor alteration associated with a polydeformed and highly metamorphosed massive sulfide deposit ruostesuo central finland massive sulfide deposits in the vihanti pyhasalmi district central finland are hosted by a highly deformed and metamorphosed bimodal volcanic sequence alteration zones that envelop these deposits are composed of cordierite orthoamphibole garnet bearing rocks referred to as cor rocks belonging to upper amphibolite to granulite facies outcrops of mafic and felsic cor rocks mcor and fcor in the vicinity of the ruostesuo zn cu deposit define mappable layers with distinctive metamorphic mineral assemblages orthoamphibole cordierite plagioclase quartz m1cor garnet orthoamphibole cordierite plagioclase quartz m2cor quartz cordierite biotite orthoamphibole cummingtonite garnet plagioclase f1cor and quartz cordierite garnet sillimanite f2cor these rocks are interlayered with or grade into typical amphibolites i e hornblende plagioclase and quartz plagioclase bearing rocks that are interpreted as least altered mafic and felsic metavolcanic rocks respectively whole rock geochemical data for least altered metavolcanic rocks and cor rocks at ruostesuo define three mafic suites primitive basalts basaltic andesites and ferrobasalts and two felsic suites r1 and r2 rhyolites using a combination of high field strength element ratios and rare earth element profiles cor rocks can be assigned to all five volcanic suites calculated mass changes for the majority of exposed cor rocks at ruostesuo are similar to those reported for chlorite quartz assemblages found within the inner core of alteration pipes located below some sea floor massive sulfide deposits relative to least altered precursors mafic cor rocks have gained fe mg s and lost variable amounts of si ca na k rb ba sr and eu which resulted in an average net mass loss of about 10 percent the bulk composition of most altered mafic cor rocks near total loss of ca na k reflects the proportion of chlorite to quartz within the premetamorphic alteration assemblages and the composition of the chlorite mass change results for altered mafic rocks indicate that basalts at ruostesuo were altered by an evolved fe rich hydrothermal fluid at high water rock ratios mass changes for cor rocks of the r2 felsic volcanic suite are similar to those of the mafic rocks enriched in fe mg and depleted in ca na k rb ba sr and eu except for significant additions of si that resulted in net mass gains of 20 to 30 percent felsic cor rocks of the r1 volcanic suite show gains in k rb and ba that are attributed to the formation of premetamorphic sericite during an alteration stage dominated by hydrothermal seawater in comparison to least altered metavolcanic rocks mafic and felsic cor rocks are depleted in 18o with delta 18o values 2 to 5 per mil lower than precursor rocks estimated temperatures of alteration were 270 to 350 c these data indicate that the signature of a high temperature 300 c sea floor hydrothermal upflow zone is preserved within upper amphibolite to granulite facies rocks at ruostesuo the stratigraphic boundary between strongly chloritized mafic volcanic rocks and silicified felsic volcanic rocks marks a transition from net mass losses associated with hydrothermal fluid dominated alteration to net mass gains associated with seawater dominated alteration silicification at or near the boundary between mafic and felsic volcanic rocks may have been the result of conductive cooling or mixing with seawater from above in the absence of textural or crosscutting relationships indicative of a younging direction a stratigraphic model is proposed on the basis of the above geochemical constraints altered basalts at ruostesuo represent the exposed base of a chloritic stockwork zone that passes upsection into altered felsic volcanic rocks that host massive zn cu sulfides this stratigraphic model has significance for the interpretation of other massive sulfide deposits in the vihanti pyhasalmi district and exploration strategies that couple chemostratigraphy with mass change results more significantly the preservation of premetamorphic oxygen isotope values for strongly metamorphosed and locally migmatitic cor rocks indicate that oxygen isotope mapping is a viable exploration tool in the district and in other mineralized upper amphibolite to granulite facies terranes worldwide

mobilism and plate tectonics in the ussr abstract the history of development of mobilistic ideas in russia ussr is reviewed after the first burst of interest stimulated by wegener s works a long period of domination of fixist reaction followed the revival of mobilism in the sixties was expressed mainly in the discovery of the very important role of nappes and wrench faults in the structure of all the folded systems of the ussr territory the plate tectonics concept met at the beginning a very strong resistance but later it gained a fast growing number of supporters and in the last years soviet geologists made a seizable contribution to the development of this concept

caldera formation in the rum central igneous complex scotland the northern marginal zone of the rum central igneous complex in nw scotland represents part of the early felsic phase of the volcano the marginal zone is a relic of the early caldera floor and the infilling of sedimentary and igneous rocks its formation has been explored through field examination of the ring fracture system of the complex and its pyroclastic and epiclastic intracaldera facies a sequence of magmatic tumescence and chamber growth caused initial doming followed by the formation of a collapse structure without accompanying volcanism this collapse structure circular in plan is akin in origin to a salt basin formed by crustal stretching above a rising diapir we call this the proto caldera collapse breccias which represent the slumping and sliding of megablocks blocks and boulders of the torridonian sandstones which form the walls of the basin were the original infilling logs of these deposits reveal considerable variation in thickness of the breccias from 80 170 m in the complex indicating an uneven floor to the proto caldera consistent with piecemeal collapse following accumulation of up to 70 m thickness of breccia thin interbedded rhyodacitic crystal tuffs 10 30 cm record the earliest eruptions of felsic magma in the caldera caldera formation was then interrupted by a period of quiescence recorded by the presence of an epiclastic sandstone of locally several metres thickness formed by washout of fines from the breccias subsequent resurgence created a fracture pattern characteristic of doming along which rhyodacite magma rose in dykes and erupted up to perhaps 10 km3 of rhyodacitic intracaldera ignimbrites this major eruption caused further incremental subsidence of the caldera floor into a now partly emptied magma chamber mafic inclusions in the ignimbrites point to the eruption being triggered by multiple injections of basic magma into a chamber occupied by felsic magma

petrology geochemistry and oxygen isotope interpretation of felsic volcanic and related rocks hosting the brunswick 6 and 12 massive sulfide deposits brunswick belt bathurst mining camp new brunswick canada the large brunswick massive sulfide deposits brunswick 6 and 12 and austin brook are hosted in the lower part of the middle ordovician bimodal volcano sedimentary tetagouche group along the autochthonous eastern margin of the bathurst mining camp the footwall rocks of the nepisiguit falls formation are relatively homogeneous quartz and alkali feldspar porphyritic crystal rich and matrix supported tuffs with a cryptocrystalline glassy to granular microcrystalline groundmass the fine to coarse grained crystal rich tuffs and tuffaceous sedimentary rocks are thickest 500 m beneath the deposits possibly due to a basinal accumulation of pyroclastic flow material the brunswick horizon massive sulfide deposits and associated exhalite algoma type iron formation formed after a period of felsic pyroclastic volcanism nepisiguit falls formation and before the onset of the overlying thick succession of intercalated rhyolite flows tuffs tuffites and hyalotuffs flat landing brook formation the footwall and hanging wall felsic volcano sedimentary rocks are variably altered which occurred mainly during the syngenetic mineralizing event rocks with minimal evidence of alteration and or tectonic fabric development however allow some interpretation of the major element variations in the least altered suite the sio 2 contents range from 68 to 72 wt percent with intermediate alkali element contents and low fe and mg contents typical of calc alkaline rhyolites weak regional silicification is irregularly distributed in the upper parts of the nepisiguit falls rocks but also in the flat landing brook rocks however there is considerable mobility of na k fe and mg as well as numerous other minor and trace elements as a result of albitization sericitization chloritization and carbonatization reactions associated with vent related and local seawater related alteration processes as observed in earlier alteration studies in the camp there is an antipathetic relationship between sericite and chlorite with chlorite predominant near the deposits

trace element and isotope constraints on crustal anatexis by upwelling mantle melts in the north atlantic igneous province an example from the isle of rum nw scotland sr and nd isotope ratios together with lithophile trace elements have been measured in a representative set of igneous rocks and lewisian gneisses from the isle of rum in order to unravel the petrogenesis of the felsic rocks that erupted in the early stages of palaeogene magmatism in the north atlantic igneous province naip the rum rhyodacites appear to be the products of large amounts of melting of lewisian amphibolite gneiss the sr and nd isotopic composition of the magmas can be explained without invoking an additional granulitic crustal component concentrations of the trace element cs in the rhyodacites strongly suggests that the gneiss parent rock had experienced cs and rb loss prior to palaeogene times possibly during a caledonian event this depletion caused heterogeneity with respect to 87sr 86sr in the crustal source of silicic melts other igneous rock types on rum dacites early gabbros are mixtures of crustal melts and and primary mantle melts forward rare earth element modelling shows that late stage picritic melts on rum are close analogues for the parent melts of the rum layered suite and for the mantle melts that caused crustal anatexis of the lewisian gneiss these primary mantle melts have close affinities to mid oceanic ridge basalts morb whose trace element content varies from slightly depleted to slightly enriched crustal anatexis is a common process in the rift to drift evolution during continental break up and the formation of volcanic rifted margins systems the early felsic later mafic volcanic rock associations from rum are compared to similar associations recovered from the now drowned seaward dipping wedges on the shelf of se greenland and on the voring plateau norwegian sea these three regions show geochemical differences that result from variations in the regional crustal composition and the depth at which crustal anatexis took place

coupled caldera subsidence and stirring inferred from analogue models analogue modelling of caldera forming eruptions suggests that sinking of the magma chamber roof is variable in space and time leading to substantial stirring and mixing of magma this can explain the common occurrence of geochemical zonation and magma mingling in deposits erupted from calderas

rifting evolution in geological history abstract rift and rift like zones global linear extensional structures of the crust have appeared during most periods of the earth s history rifting as a geological process has undergone a complex evolution with its historical roots in the early precambrian the development of the oldest linear mobile tectonic zones archean greenstone belts and granulite belts combined the features of both rift and geosynclinal processes in the following epochs early proterozoic mobile zones protogeosynclinal regions and protorift zones evolved under similar tectonic regimes of crustal destruction in the early stages and compression in the late stages of their development but different in size scale and intensity of the deformations and thermal processes experienced typical rift structures have existed since the late proterozoic in the late proterozoic and paleozoic continental intracratonic rifting in the form of aulacogen and aulacogeosynclinal zone development was a process related and subordinated to a geosynclinal process in the adjacent mobile belts in the paleozoic the first epiorogenic postorogenic rift zones appeared within the epigeosynclinal folded belts during the meso cenozoic epoch of the earth s history rifting increased considerably and became independent of geosynclinal processes it is believed to be one of the main forms of continental crust destruction and formation of the oceanic basins and probably occurred under conditions of some general expansion of the earth

the tertiary lavas and sediments of northwest rhum inner hebrides several small outliers of tertiary lavas and sediments rest with strong unconformity on a buried landscape eroded from torridonian sediments and tertiary granophyre erosion continued during the period of sediment and lava accumulation four formations are recognized these are in order of increasing age the orval formation hawaiite and basaltic hawaiite lavas the guirdil formation icelandite lavas interbedded conglomerates the upper fionchra formation tholeiitic basaltic andesite lavas hyaloclastite deposits basal conglomerate and the lower fionchra formation alkali and transitional basalt basaltic hawaiite and hawaiite lava flows basal conglomerate each is separated by an erosional interval clasts in the conglomerates reveal a history of erosion of a terrain exposing gneisses torridonian sediments igneous rocks derived from the rhum tertiary central complex including allivalites and tertiary lavas of local origin but also including in the oldest conglomerates tholeiitic basalts not now preserved on or near rhum prior to and during lava and sediment accumulation erosion on rhum had cut down to a level similar to that of the present day although not to the extent that high grade thermally altered rocks which are a marked feature of the central complex were being eroded in any quantity a sequence of east west trending valleys possibly initiated on the line of the earlier main ring fault drained the area of the central complex which then as now must have been high ground small lakes occasionally formed in the valleys allowing the accumulation of fine grained sediment with plant remains and promoting the formation of hyaloclastite deposits when buried by later flows no source for any of the lava formations is preserved on rhum they are thought to have come from feeders north of rhum possibly near canna and to have ponded against the hills and valleys near and in the central complex the oldest tholeiitic lavas not now found in situ were followed by alkali and transitional flows compositionally similar to the skye main lava series but characteristically feldsparphyric the most mafic also contain phenocrysts of magnesian olivine with included cr al rich spinels and aluminous spinel both the early alkalic transitional basalts and the youngest hawaiites and basaltic hawaiites equilibrated at pressures apart from a few n s to nw se trending basalt dykes the lava formations represent the youngest tertiary igneous event on rhum

volcaniclastic gravity flow deposits in the dezadeash formation jura cretaceous yukon canada implications regarding the tectonomagmatic evolution of the chitina arc in the northern cordillera of north america abstract the chitina arc in the northern cordillera of north america evolved during the accretion of the wrangellia composite terrane to the western margin of north america in the jurassic and cretaceous the dezadeash formation a 3000 m thick sequence of deepwater turbidites in southwestern yukon was deposited as a submarine fan in a backarc basin to the chitina arc only limited geochemistry of altered volcanic rocks associated with the arc are available from southern alaska however three thick volcaniclastic beds occur in the dezadeash formation these consist of fine to medium grained vitric to crystal tuffs that are interpreted as resedimented syn eruptive volcaniclastics a u pb zircon age 149 4 0 3 ma indicates they are contemporaneous with the chitina arc petrographic examination shows the volcaniclastic rocks are altered mainly by albite and locally calcite with a chemical index of alteration suggesting the rocks are relatively unweathered or have undergone only incipient weathering harker diagrams indicate that cao k 2 o and rb were mobile based on the th vs co compositional diagram the volcaniclastic rocks are classified as dacites and according to various trace element plots they are calk alkaline and adakite like in composition the volcaniclastic rocks plot in the active continental margin field on a variety of tectonic discriminant diagrams and chondrite normalized multi element plots display parallel listric shaped profiles with significant light rare earth element enrichment and minor heavy rare earth element enrichment sm nd systematics indicate the volcaniclastic rocks represent mixing of a depleted mantle source and an older crustal source these data suggest mainly slab dehydration with low pressure fractionation controlled by amphibole with possibly minor input by slab melting the chitina arc has previously been interpreted as an oceanic island arc and the continental arc signature of volcaniclastic rocks in the dezadeash formation together with a continental arc signature of altered volcanic rocks in southern alaska is attributed to arc magmas erupting through paleozoic volcanic sedimentary and plutonic rocks of the wrangellia composite terrane proxying for continental crust

major stages of rifting evolution in the earth s history abstract linear extensional rift and rift like structures have been generated many times during various periods of the earth s history since early times these structures have varied with time however in scale morphology formations paragenesis of tectonic features regularity of distribution and evolution i distinguish and consider five major stages of rifting they are katarchean archean early proterozoic late proterozoic paleozoic and mesozoic cenozoic linear structures of the oldest stage combine features of development and structure similar to those typical of the rift and geosynclinal zones in the later periods of the earth s history at the second stage proto rift and proto geosynclinal structures can be distinguished for the first time though differences are not distinct paleo rift zones of the third and fourth stages differ from geosynclinal belts and zones their structural and genetic evolution however is related to the development of the latter in contrast the distribution and evolution of the rift zones and belts during the fifth stage are connected mainly with the destruction of the continents and formation of the secondary oceanic basins they are complementary to the mesozoic cenozoic geosynclinal orogenic belts periods of the greatest development of rift and rift like structures in the archean riphean and mesozoic cenozoic probably correspond to the major phases of the earth s expansion which can be distinguished in its general pulsating evolution

introduction from the british tertiary into the future modern perspectives on the british palaeogene and north atlantic igneous provinces the study of volcanic rocks and igneous centres has long been a classic part of geological research despite the lack of active volcanism the british isles have been a key centre for the study of igneous rocks ever since ancient lava flows and excavated igneous centres were recognized there in the 18th century hutton 1788 this led to some of the earliest detailed studies of petrology the starting point for many of these studies was the british palaeogene igneous province bpip formerly known as the british tertiary judd 1889 and still recognized by this name by many geologists around the globe this collection of lavas volcanic centres and sill dyke swarms covers much of the west of scotland and the antrim plateau of northern ireland and together with similar rocks in the faroe islands iceland and greenland forms a world class large igneous province this north atlantic igneous province naip began to form through continental rifting above a mantle plume at c 60 ma and subsequently evolved as north america separated from europe creating the north atlantic ocean

sims oxygen isotopes indicate phanerozoic fluids permeated a precambrian gold deposit abstract the reef deposit is an anomalous au cu occurrence in the paleoproterozoic terranes of northern wisconsin better known as host to significant cu zn volcanogenic massive sulfide vms deposits previous work using lead isotopes and fluid inclusions has identified a protracted development of the mineralization from initial formation as the root zone veins of a vms deposit during the penokean orogeny 1 9 1 8 ga with the most recent mineralization remobilization activity associated with late paleozoic mississippi valley type mvt fluid flow haroldson et al 2018a 2018b here we use the oxygen isotope history of the reef deposit to verify and further examine the deposit s protracted development laser fluorination oxygen isotope measurements of primary mineralized quartz veins range in 18o from 6 8 to 10 0 vsmow and a trend is observed of increasing 18o values of parallel vein zones along a 400 m traverse from northwest to southeast likely from a temperature gradient during initial formation temperature estimates for a vms deposit setting 230 to 400 c are consistent with a hydrothermally shifted formation fluid using seawater 18o estimates during the penokean orogeny and evolving to higher 18o by incorporation of magmatic fluids or interaction with local crust in situ oxygen isotope measurement by secondary ion mass spectrometer sims of cross cutting quartz and carbonate range in 18o from 19 4 to 28 4 for quartz 25 3 to 28 1 for dolomite and 9 6 to 29 3 vsmow for calcite high 18o values 19 are measured in a crustiform textured quartz stockwork microveinlet in dolomite observed in late carbonate microveinlets and calcite in settings associated with the late quartz and dolomite and a separate calcite only setting directly linked with gold mineralization remobilization

geochemistry and paleotectonic setting of felsic volcanic rocks in the finlayson lake volcanic hosted massive sulfide district yukon canada the finlayson lake volcanic hosted massive sulfide vhms district represents one of canada s most recent vhms discovery regions with 34 million metric tons mt of massive sulfide mineralization found since the mid 1990s felsic volcanic rocks are associated with three units the fire lake unit the kudz ze kayah unit and the wolverine succession significant accumulations of polymetallic felsic vhms deposits kudz ze kayah gp4f and wolverine have only been discovered in the kudz ze kayah unit and wolverine succession to date in the hanging wall of the money creek thrust felsic volcanic and high level intrusive rocks in the fire lake unit have calc alkalic and tholeiitic affinities with low high field strength element hfse contents and intermediate zr sc 9 4 43 4 and zr tio2 254 864 ratios these rocks are interlayered with mafic rocks with arc geochemical signatures to date devoid of significant vhms mineralization and represent bimodal magmatism within an evolving devonian mississippian continental arc system the kudz ze kayah unit in the footwall of the money creek thrust stratigraphically overlies the fire lake unit and consists of felsic volcanic rocks with high hfse contents within plate a type signatures and high zr sc 15 3 190 3 and zr tio2 630 2 185 ratios the kudz ze kayah unit felsic rocks are crosscut and overlain by alkalic mafic rocks are associated with abundant carbonaceous sedimentary rocks and represent magmatism within a devonian mississippian ensialic back arc rift basin environment the wolverine succession unconformably overlies the kudz ze kayah unit felsic rocks below the wolverine deposit have geochemical attributes similar to the kudz ze kayah unit with high hfse contents within plate a type signatures and high zr sc 29 9 84 2 and zr tio2 391 1 220 ratios in contrast aphyric rhyolite flows in the hanging wall of the deposit have much lower hfse contents and the lowest zr sc 3 5 27 7 and zr tio2 181 591 ratios in the district all the felsic rocks of the wolverine succession are interlayered with abundant carbonaceous sedimentary rocks and are overlain by midocean ridge basalt morb like basaltic rocks the wolverine succession is interpreted to have formed within an early mississippian ensialic back arc basin environment that eventually evolved to sea floor spreading the variation in the hfse budgets of the felsic rocks of the finlayson lake district likely reflects variations in the source and or temperature of crustal melting in particular felsic rocks of the fire lake unit have higher nb ta and lower ti sc ratios than other volcanic rocks in the district suggesting possible derivation from mafic crustal sources and or lower crustal fusion temperatures the kudz ze kayah unit and footwall rocks to the wolverine deposit are inferred to have formed from high temperature partial melting of continental crust the hanging wall aphyric rhyolites from the wolverine deposit may have formed from either lower temperature continental crustal melting or may have been derived from the mixing of hfse depleted n morb mafic magmas and evolved continental crust polymetallic felsic volcanic associated vhms deposits within the finlayson lake district are preferentially associated with hfse enriched felsic rocks with high zr sc 15 3 190 3 and zr tio2 391 2 105 ratios the hfse and rare earth element ree systematics of vhms associated felsic rocks of the finlayson lake district are different from prospective felsic rocks from archean vhms environments in the superior province and are displaced toward higher zr y and la ybn ratios their hfse and ree systematics are similar to many phanerozoic vhms environments in particular those at least partially to fully underlain by evolved continental crust the geochemical differences between the felsic rocks of the finlayson lake district and those from archean vhms environments most likely reflect differences in the substrates from which the felsic rocks were derived e g evolved versus juvenile

absolute reconstructions of the paleozoic oceans abstract the orientation of three intraplate magmatic belts the mongolian from 130 to 280 m y south siberian from 320 to 400 m y and baltic from 365 to 400 m y belts which can be considered as hot spot tracks are used together with apparent paleomagnetic pole wander paths to reconstruct true motions of continents during paleozoic times the reconstructions obtained show that the old late precambrian pangea continued to exist in the early paleozoic with an inner arrangement of constituent continents strongly different from that of the late paleozoic pangea the continents were constantly located in the eastern hemisphere only the western hemisphere was occupied by the paleo pacific oceans four oceans existed in paleozoic times between the continents in the eastern hemisphere 1 the iapetus which originated in the late precambrian and closed before devonian times 400 m y 2 the asiatic paleo ocean which originated in the late precambrian and closed 450 m y ago 3 the uralian paleo ocean which opened around 500 m y ago and closed in the latest permian 240 230 m y 4 the paleo tethys which opened in the ordovician 480 450 m y and became the mesozoic tethys life duration of the oceans was 200 400 m y the main trend in the earth s evolution during the paleozoic was a break up of the old precambrian supercontinent and construction of the new late paleozoic pangea

early mafic magmatism and crustal anatexis on the isle of rum evidence from the am mam intrusion breccia the rum igneous centre comprises two early marginal felsic complexes the northern marginal zone and the southern mountains zone along with the later central ultrabasic basic layered intrusions these marginal complexes represent the remnants of near surface to eruptive felsic magmatism associated with caldera collapse examples of which are rare in the north atlantic igneous province rock units include intra caldera collapse breccias rhyolitic ignimbrite deposits and shallow level felsic intrusions as well the enigmatic am mam intrusion breccia the latter comprises a dacitic matrix enclosing lobate basaltic inclusions 1 15 cm and a variety of clasts ranging from millimetres to tens of metres in diameter these clasts comprise lewisian gneiss torridonian sandstone and coarse gabbro detailed re mapping of the am mam intrusion breccia has shown its timing of emplacement as syn caldera rather than pre caldera as previously thought textural analysis of entrained clasts and adjacent uplifted country rocks has revealed their thermal metamorphism by early mafic intrusions at greater depth than their present structural position these findings provide a window into the evolution of the early mafic magmas responsible for driving felsic magmatism on rum our data help constrain some of the physical parameters of this early magma crust interaction and place it within the geochemical evolution of the rum centre

silurian devonian active margin deep marine systems and palaeogeography alai range southern tien shan central asia analysis of mid palaeozoic successions in the northern part of the alai range kyrgyzstan and bordering uzbekistan southern tien shan central asia has identified a silurian devonian deep marine depositional system of basin slope facies associations here we document the stratigraphy and sedimentology of a region in central asia that through conflict has become inaccessible for geological research the turbidite dominated pul gon formation silurian accumulated in sea floor depressions and within the inferred basin axis the large scale coarse clastic lenses of the dzhidala formation mostly devonian represent the fills of submarine channels canyons or gullies of the palaeoslope other slope apron processes include sediment slides debris flows and olistoliths the partly time equivalent condensed sequences of the mid to late silurian kursala formation and early devonian tamasha formation represent graptolitic mudstone and chert accumulation respectively together with thin dolomitic limestones that accumulated over c 10 15 ma probably on seamounts in the turkestan fergana ocean between the continental margin of kazakhstania and the alai microcontinent the graptolitic shale rich chakush formation early silurian is geographically and petrographically different from the pul gon and dzhidala formations and suggests a different provenance opposing continental margin or seamount talus

methods of determining the rheological properties of magmas at sub liquidus temperatures non vesicular basaltic melts behave as newtonian fluids at temperatures above their liquidus and their viscosities can be calculated using a method developed by shaw 1972 and bottinga and weill 1972 because many igneous processes involve the flow of silicates at sub liquidus temperatures numerous attempts have been made to calculate the interactive effects of the crystal phase however current methods are appropriate only for relatively low crystal concentrations and they assume newtonian behaviour we argue that this assumption is invalid when bubble or crystal concentrations exceed 30 at higher concentrations factors other than concentration become increasingly important these include particle shape and size distribution a method which takes these factors into account is tested on a range of suspensions including magmas at sub liquidus temperatures and rheological properties calculated using this method agree closely with the measured values we also demonstrate that an equation which was introduced to explain large differences in measured apparent viscosities during the cooling and crystallisation of mount st helens dacite murase et al 1985 and which is currently used to calculate the rheological properties of crystallising lavas generates viscosities which may be in error by several orders of magnitude this difference is argued to be caused by a combination of factors including the ten orders of magnitude range in the strain rates utilised during the mount st helens measurements causing orders of magnitude difference in the resulting apparent viscosities rheological data on crystallising silicic melts are reinterpreted taking into account the non newtonian rheology of the magmas and changes in activation energy of flow during crystallisation

anomalous subsidence history of the west siberian basin as an indicator for episodes of mantle induced dynamic topography abstract vertical intraplate motions are not easily related to plate tectonics understanding their underlying mechanism is further complicated by poor constraints on the magnitude and timing of these motions western siberia is a prominent intraplate area affected by vertical tectonics shortly after the assemblage of pangaea the region experienced substantial uplift followed by a long period of subsidence that led to formation of the world s largest sedimentary basin the west siberian basin its sedimentary cover has been extensively explored and described in the soviet literature here we digitise compilations of stratigraphic data to calculate the basin s subsidence history by backstripping analysis our results confirm the prolonged and high amplitude regional subsidence that has been noted before and constitutes the basin s first order vertical motion however our backstripping results also reveal a secondary mode of shorter spatial and temporal scales induced by the migration of the maximum subsidence across the basin importantly the generally slow subsidence of the basin was interrupted by uplift events in the early and late cretaceous and in the middle oligocene while the secondary mode of vertical motion is not easily understood in terms of traditional subsidence models restricted to lithospheric cooling after stretching it is consistent with rapid uplift events that have been reported for other locations such as elevated passive continental margins we discuss the geodynamic implications and conclude that the basin may hold important constraints on dynamic topography induced by sublithospheric mantle flow processes

petrology and u pb geochronology of footwall porphyritic rhyolites from the wolverine volcanogenic massive sulfide deposit yukon canada implications for the genesis of massive sulfide deposits in continental margin environments porphyritic rhyolite sills form an important component of the footwall of the wolverine volcanogenic massive sulfide vms deposit yukon canada and occur proximal to mineralization in the immediate deposit area wolverine lynx zone and at similar stratigraphic levels along strike fisher puck and sable zones porphyritic rhyolites are of two types an older quartz feldspar porphyritic qfp rhyolite suite and a younger feldspar porphyritic fp suite both the qfp and fp suites of intrusions are semiconcordant suggesting a silllike morphology and are altered and crosscut by veinlet mineralization suggesting that they are pre to synmineralization the margins of qfp suite of intrusions contain minor xenoliths of surrounding shales and poorly developed chilled margins suggesting emplacement into partially consolidated sedimentary rocks whereas the fp suite of intrusions shows well developed chilled margins indicative of emplacement into fully solidified sedimentary rock these features suggest that the qfp suite of intrusions represents an older phase of rhyolitic magmatism whereas the fp suite represents a younger event this is supported by u pb zircon ages which indicate a 352 4 1 5 ma emplacement age for the fp suite and a 347 to 346 ma emplacement for the fp suite two ages at 347 8 1 3 and 346 0 2 2 ma both suites of porphyries have inherited proterozoic zircon and have ratios of la smucn 1 and nb thucn 1 ucn upper continental crust normalized indicating derivation from and or extensive interaction with ancient upper continental crustal materials the fp suite however has elevated high field strength element hfse and rare earth element ree contents high zircon saturation temperatures and higher nb ta ratios and lower ti sc ratios than the qfp suite these features are interpreted to reflect that the fp suite of magmas was hotter 900 c melts with a greater mantle component in their genesis both suites however are interpreted to have formed due to basaltic upwelling crustal melting and crust mantle mixing during ensialic back arc basin activity the presence of mantle heat within the wolverine basin from 352 to 347 to 346 ma a minimum of 5 m y suggests that sustained mantle heat flow was critical to the genesis of the wolverine porphyries it is also suggested that this sustained mantle heat was responsible for the wolverine hydrothermal system and that upwelling mantle may be essential in providing the heat to drive hydrothermal systems even in continental margin type vms environments e g bathurst iberian pyrite belt the wolverine porphyries are among the most hfse and ree enriched felsic rocks associated with vms mineralization globally the high hfse and ree concentrations in these rocks are interpreted to be due to

partially melted lithic megablocks in the yardea dacite gawler range volcanics australia implications for eruption and emplacement mechanisms lithic megablocks ranging from 200 m felsic volcanic unit in the mesoproterozoic gawler range volcanic province grv of south australia throughout its vast extent the yardea dacite shows typical lava like features in that it is massive columnar jointed and evenly porphyritic with 30 40 crystals in a spherulitic and granophyric groundmass in addition flow banding is present at many locations the megablocks are abundant at two sites 50 km apart but isolated megablocks and smaller 6 cm lithic clasts are also scattered throughout the unit at both sites the megablocks are matrix supported non graded randomly oriented and show no evidence of being confined to a particular stratigraphic level in the dacite the most abundant and largest megablocks are granitoids derived from older basement and from early crystallised plutons of the hiltaba suite which is broadly coeval and comagmatic with the grv the granitoid megablocks have been partially melted most likely prior to eruption when resident in the thermal aureole of the yardea dacite magma chamber the lithic megablock occurrences are unlike coarse pyroclastic breccias but are similar in distribution and abundance to xenoliths in lavas consistent with the lava like character of the host dacite using reasonable estimates of megablock density magma density and magma viscosity we show that the rise rate of the dacitic magma exceeded the settling velocity of the megablocks implying that they could have been entrained and erupted effusively all but the largest and least melted megablocks would have remained suspended or else settled very slowly in the dacitic lava during outflow the rapid rate of magma withdrawal required to produce such an extensive felsic sheet could have also triggered disintegration of the thermally stressed wallrock surrounding the magma chamber dislodging megablocks that were later entrained and effusively erupted

the diagenesis of the late dinantian derbyshire east midland carbonate shelf central england carbonate cements in late dinantian asbian and brigantian limestones of the derbyshire carbonate platform record a diagenetic history starting with early vadose meteoric cementation and finishing with burial and localized mineral and oil emplacement the sequence is documented using cement petrography cathodoluminescence trace element geochemistry and c and o isotopes the earliest cements pre zone 1 are locally developed non luminescent brown sparry calcite below intrastratal palaeokarsts and calcretes they contain negligible fe mn and sr but up to 1000 ppm mg their isotopic compositions centre around 18o 8 5 13c 5 0 calcretes contain less 13c subsequent cements are widespread as inclusion free low mg low fe crinoid overgrowths and are described as having a dead bright dull cathodoluminescence the dead cements zone 1 are mostly non luminescent but contain dissolution hiatuses overlain by finely detailed bright subzones that correlate over several kilometres across dead bright subzones there is a clear trend in mg 500 900 ppm mn 100 450 ppm and fe 80 230 ppm zone 1 cements have isotopic compositions centred around 18o 8 0 and 13c 2 5 zone 2 cement is bright thin and complexly subzoned it is geochemically similar to bright subzones of zone 1 cements dull zone 3 cement pre dates pressure dissolution and fills 70 or more of the pore space it generally contains little mn fe and sr but can have more than 1000 ppm mg increasing stratigraphically upwards the 18o compositions range from 5 5 to 15 and the 13c range is 1 to 3 20 00 zone 4 fills veins and stylolite seams in addition to pores it is synchronous with pb ba f ore mineralization and oil migration zone 4 is ferroan with around 500 ppm fe up to 2500 ppm mg and up to 1500 ppm mn isotopic compositions range widely 15o 2 7 to 9 and 13c 3 8 to 2 50 unaltered marine brachiopods suggest a dinantian seawater composition around 15o 0 smow but vital isotopic effects probably mask the original 13c pdb value pre zone 1 calcites are meteoric vadose cements with light soil derived 13c and light meteoric 18o an unusually fractionated pluvial 15o smow value of around 6 is indicated for local dinantian meteoric water calcrete 18o values are heavier through evaporation zone 1 textures and geochemistry indicate a meteoric phreatic environment fe and mn trends in the bright subzones indicate stagnation and precipitation occurred in increments from widespread cyclically developed shallow meteoric water bodies meteoric alteration of the rock body was pervasive by the end of zone 1 with a general resetting of isotopic values zone 3 is volumetrically important and external sources of water and carbonate are required emplacement was during the namurian early westphalian by meteoric water sourced at a karst landscape on the uplifted eastern edge of the derbyshire east midland shelf the light 18o values mainly reflect burial temperatures and an unusually high local heat flow but an input of highly fractionated hinterland derived meteoric water at the unconformity is also likely relatively heavy 13c values reflect the less altered state of the source carbonate and aquifer zone 4 is partly vein fed and spans burial down to 2000 m and the onset of tectonism light organic matter derived 13c and heavy 18o values suggest basin derived formation water combined with textural evidence of geopressures this relates to local high temperature ore mineralization and oil migration low water to rock ratios with host rock buffering probably affected the final isotopic compositions of zone 4 masking extremes both of temperature and organic matter derived co2

geochemistry of surface waters associated with an undisturbed zn pb massive sulfide deposit water rock reactions solute sources and the role of trace carbonate abstract we report results of a detailed geochemical study of surface waters from several streams in a small catchment in northern new brunswick canada rocks in the catchment represent metamorphosed greenschist facies felsic volcanic rocks metavolcaniclastic sediments and mafic volcanic rocks no massive carbonate or evaporite lithologies are present thus providing an excellent opportunity to investigate the relative influence of silicate weathering compared to trace carbonate vein and disseminated dissolution and the influence of volcanogenic massive sulfide vms mineralization on surface water chemistry surface waters catchment lithologies and stream sediments were analyzed for a full suite of major and trace elements surface waters are dilute typically 3 type most waters have ca na molar 1 the major ion chemistry of the waters is consistent with binary mixing between silicate weathering and dissolution of trace calcite si ca molar relationships suggest that trace calcic silicates are insignificant compared to calcite as a ca source and po 4 concentrations of waters are too low for apatite to be a major source of ca host rocks have highly variable ca na mg na and k na values complicating the assignment of a silicate end member geochemical modeling indicates that surface waters range from essentially 100 of the na and ca being derived by silicate weathering to being dominantly controlled by trace calcite also consistent with mass balance calculations the cl ca ratios are consistent with the host felsic and mafic metavolcanic rocks as being an important source of cl in addition to precipitation the host lithologies have much larger variations in mg na ca na k na and sr na than the stream sediments and waters and the sediments are shifted to higher mg na and k na and lower ca na than the waters it is remarkable that the waters have such small variations in major ion ratios relative to the host rocks indicating that the controls on solute loads of these streams is more a function of relative elemental solubility ca mg na secondary mineral formation e g mg and k rich clays incongruent dissolution and water rock reactions than end member rock compositions oxidation of massive and disseminated sulfide mineralization accounts for on average 60 of the dissolved sulfate calculations indicate that sulfide oxidation sulfuric acid weathering accounts for around 20 of the cation flux in the watershed on average 12 of the ca and 72 of the mg are derived from silicate weathering although cationic silicate denudation rates are only 1 88 tonnes km 2 year compared to 4 16 tonnes km 2 year for trace carbonate dissolution the total cation denudation rate and co 2 consumption rate are similar to watersheds draining volcanic rocks in the western canadian cordillera

phanerozoic shallow water diversity driven by changes in sea level shallow water 50 m marine diversity curves for the phanerozoic based on area changes due to sea level variation and continental movements are presented it is shown that a change in the degree of endemism can alter diversity more than the effect of sea level change or change in the number of faunal provinces the pattern produced for a constant degree of endemism shows some rapid but not periodic falls in diversity these occur in the middle cambrian at the end of the ordovician devonian permian cretaceous and eocene and in the messinian there is no recurring single cause for these falls

the setting style and role of magmatism in the formation of volcanogenic massive sulfide deposits throughout earth s history all volcanogenic massive sulfide vms hosting environments are associated with specific assemblages of mafic and felsic rocks with distinct petrochemistry petrochemical assemblages indicative of formation at anomalously high temperatures within extensional geodynamic environments in mafic dominated juvenile ophiolitic vms environments there is a preferential association with mafic rocks with boninite and low ti tholeiite mid ocean ridge basalt morb and or back arc basin basalt affinities representing forearc rifting or back arc initiation mid ocean ridges or back arc basin spreading or back arc basins respectively felsic rocks in juvenile oceanic arc environments in archean terrains are high field strength element hfse and rare earth element ree enriched in post archean juvenile oceanic arc terrains felsic rocks are commonly hfse and ree depleted and have boninite like to tholeiitic signatures in vms environments that are associated with continental crust i e continental arc and back arc and dominated by felsic volcanic and or sedimentary rocks evolved environments felsic rocks are the dominant hosts to mineralization and are generally hfse and ree enriched with calc alkalic a type and or peralkalic affinities representing continental arc rifts continental back arcs and continental back arcs to continental rifts respectively coeval mafic rocks in evolved environments have alkalic within plate ocean island basalt like and morb signatures that represent arc to back arc rift versus back arc spreading respectively the high temperature magmatic activity in vms environments is directly related to the upwelling of mafic magma beneath rifts in extensional geodynamic environments e g mid ocean ridges back arc basins and intra arc rifts underplated basaltic magma provides the heat required to drive hydrothermal circulation extensional geodynamic activity also provides accommodation space at the base of the lithosphere that allows for the underplated basalt to drive hydrothermal circulation and induce crustal melting the latter leading to the formation of vms associated rhyolites in felsic dominated and bimodal vms environments rifts also provide extensional faults and the permeability and porosity required for recharge and discharge of vms related hydrothermal fluids rifts are also critical in creating environments conducive to preservation of vms mineralization either through shielding massive sulfides from seafloor weathering and mass wasting or by creating environments conducive to the precipitation of subseafloor replacement style mineralization in sedimented rifts subvolcanic intrusions are also products of the elevated heat flow regime common to vms forming environments shallow level intrusive complexes i e within 1 3 km of the seafloor may not be the main drivers of vms related hydrothermal circulation but are likely the manifestation of deeper seated mantle derived heat i e 3 10 km depth that drives hydrothermal circulation these shallower intrusive complexes are commonly long lived i e millions of years and reflect a sustained thermally anomalous geodynamic environment such a thermally anomalous environment has the potential to drive significant hydrothermal circulation and therefore multi phase long lived subvolcanic intrusive complexes are excellent indicators of a potentially fertile vms environment the absence of intrusive complexes however does not indicate an area of low potential as they may have been moved or removed due to post vms tectonic activity in some cases shallow level intrusive systems contribute metals to the vms hydrothermal system

on the pseudomorphing of melt filled pores during the crystallization of migmatites pseudomorphs of melt filled pores recognized by their generally cuspate shape are used as diagnostic for the former presence of partial melt they are commonly observed in migmatites from the mid to deep crust although they occur in the smaller pores in migmatites from shallower levels 1 2 kbar the pseudomorphing of meltfilled pores is controlled by the kinetics of nucleation and is a consequence of the greater supersaturation required for nucleation in a small pore compared with a larger one we examine three migmatites in detail a contact metamorphosed cherty band from an iron formation an archaean regional granulite from an accretionary prism and an amphibolite facies sample from the roots of an archaean mountain chain the greater undercooling required for nucleation in progressively smaller pores is recorded by the composition of plagioclase pseudomorphs a study of dihedral angles at the corners of pseudomorphed pores demonstrates that melt solid textural equilibrium was probably attained only in the contact aureole the regional granulite preserves an almost unmodified reaction controlled melt distribution with little evidence for either melt solid textural equilibration or solid solid re equilibration whereas the reactioncontrolled melt distribution in the regional amphibolite facies example has been modified by a partial approach to solid solid textural equilibrium it is not clear whether the differences in dihedral angle population are due to differences in uplift and exhumation rates or due to the presence of h2o on grain boundaries

4 transpressional tectonics lower crust decoupling and intrusion of deep mafic sills a model for the unusual metallogenesis of sw iberia abstract sw iberia is interpreted as an accretionary magmatic belt resulting from the collision between the south portuguese zone and the autochthonous iberian terrane in variscan times 350 to 330 ma in the south portuguese zone pull apart basins were filled with a thick sequence of siliciclastic sediments and bimodal volcanic rocks that host the giant massive sulphides of the iberian pyrite belt massive sulphides precipitated in highly efficient geochemical traps where metal rich but sulphur depleted fluids of dominant basinal derivation mixed with sulphide rich modified seawater massive sulphides formed either in porous reactive volcanic rocks by sub seafloor replacement or in dark shale by replacement of mud or by exhalation within confined basins with high biogenic activity crustal thinning and magma intrusion were responsible for thermal maturation and dehydration of sedimentary rocks while magmatic fluids probably had a minor influence on the observed geochemical signatures the ossa morena zone was a coeval calc alkaline magmatic arc it was the site for unusual mineralization particularly magmatic ni cu and hydrothermal fe oxide cu au ores iocg most magmatism and mineralization took place at local extensional zones along first order strike slip faults and thrusts the source of magmas and iocg and ni cu deposits probably lay in a large mafic ultramafic layered complex intruded along a detachment at the boundary between the upper and lower crust here juvenile melts extensively interacted with low grade metamorphic rocks inducing widespread anatexis magma contamination and further exsolution of hydrothermal fluids hypersaline fluids 18 o fluid 5 4 to 12 were focused upward into thrusts and faults leading to early magnetite mineralization associated with a high temperature 500 c albite actinolite salite alteration and subsequent copper gold bearing vein mineralization at somewhat lower temperatures assimilation of sediments by magmas led in turn to the formation of immiscible sulphide and silicate melts that accumulated in the footwall of the layered igneous complex further injection of both basic and sulphide rich magmas into the upper crust led to the formation of ni cu rich breccia pipes younger 330 to 280 ma peraluminous granitoids probably reflect the slow ascent of relatively dry and viscous magmas formed by contact anatexis these granitoids have w sn and pb zn related mineralization that also shows geochemical evidence of major mantle crust interaction late epithermal hg cu sb and pb zn ag mineralization was driven by convective hydrothermal cells resulting from the high geothermal gradients that were set up in the zone by intrusion of the layered igneous complex in all cases most of the sulphur seems to have been derived from leaching of the host sedimentary rocks 34 s 7 to 20 with only limited mixing with sulphur of magmatic derivation the metallogenic characteristics of the two terranes are quite different in the ossa morena zone juvenile magmatism played a major role as the source of metals and controlled the styles of mineralization in the south portuguese zone magmas only acted as heat sources but seem to have had no major influence as sources of metals and fluids which are dominated by crustal signatures most of the magmatic and tectonic features related to the variscan subduction and collision seem to be masked by those resulting from transpressional deformation and deep mafic intrusion which led to the development of a metallogenic belt with little resemblance to other accretionary magmatic arcs

the shapes of dikes evidence for the influence of cooling and inelastic deformation we document the shape of dikes from well exposed field locations in the isle of rum scotland 14 and helam mine south africa the basaltic rum dikes crop out on a smaller scale than the 15 helam kimberlite dikes and have a smaller length to thickness ratio 100 1 isle of rum 16 1000 1 helam mine we compare dike thickness field measurements with the geometry 17 predicted by elastic theory finding best fit models to estimate magma overpressure and regional 18 stress gradients at the time of dike emplacement most of the dike shapes fit poorly with elastic 19 theory being too thick at the dike ends and too narrow in the middle our calculated 20 overpressures and stress gradients are much larger than independent estimates based on rock 21 strength dike shape can be explained by a combination of host rock inelastic deformation and 22 magma chilling at the dike s tapering edges preventing its closure as magma pressure declines 23 during emplacement the permanent wedging of the dike edges due to chilling has implications 24 for crustal magma transport and strain response in the crust due to dike emplacement

effect of host rock rheology on dyke shape thickness and magma overpressure the size and thickness of dykes is of fundamental importance for volcano dynamics because dykes are the primary path for magma transport and because large numbers of dykes often comprise a major p

a robust least squares gondwanan apparent polar wander path and the question of palaeomagnetic assessment of gondwanan reconstruction abstract two hundred and nineteen palaeomagnetic results are combined in calculating gondwanan apparent polar wander paths their confidence intervals and the residual mean square errors about the paths for six reconstructions a quantitative assessment of the six reconstructions of gondwanaland is made on the basis of the consistency of palaeomagnetic pole positions using a new statistical technique smith and hallam s 1 gondwanan reconstruction is the most effective in accounting for the distribution of the palaeomagnetic data and is used in calculating our preferred apparent polar wander path

the southern mountains zone isle of rum scotland volcanic and sedimentary processes upon an uplifted and subsided magma chamber roof the southern mountains zone of the rum central complex lies inside a major ring fault and comprises an intricate association of country rock outcrops breccias and rhyodacite the breccias and rhyodacite were long thought to be products of subterranean explosion and intrusion respectively here we report new observations that support re interpretation of these units as mass movement deposits and ignimbrites the most abundant breccias coire dubh type consist mainly of country rock clasts

application of high field strength elements to discriminate tectonic settings in vms environments high field strength element ratios in felsic volcanic rocks reflect the tectonomagmatic evolution of a region th ta th hf ta hf th yb and ta yb ratios collectively help to define the tectonic environment of volcanism separating oceanic arcs active continental margins and within plate volcanic zones on the basis of the above empirical observations the tectonic environments of selected archean and post archean volcanogenic massive sulfide deposits were reexamined published data sets plotted on discriminant diagrams represent twelve volcanogenic massive sulfide vms deposits from two distinct tectonic environments the volcanic rocks of the kuroko deposits japan san nicolas mexico and bathurst new brunswick canada plot within the active continental margin field on all diagrams whereas volcanic rocks from archean deposits of the superior province canada kidd creek kamiskotia geco winston lake mattabi lyon lake noranda selbaie and south bay plot in the within plate volcanic zone field the discriminant diagrams demonstrate that although the chemical character of rhyolites has not changed significantly since the archean the range of geotectonic setting of vms deposits probably has it is suggested that archean deposits most likely formed in an ensimatic rift environment whereas post archean deposits formed in subduction related rift zones of arc environments subduction related rhyolites of vms deposits are variably enriched in th and the light rare earth elements with respect to ta nb and the heavy rare earth elements

geochemistry and u pb dating of felsic volcanic rocks in the riotinto nerva unit iberian pyrite belt spain crustal thinning progressive crustal melting and massive sulphide genesis abstract we present new geochemical sm nd and u pb data on the felsic volcanic rocks containing the world class volcanic hosted massive sulphide deposits in the riotinto nerva unit iberian pyrite belt spain three new u pb ages from older plagioclase quartz phyric dacites and plagioclase phyric rhyolites to youngest plagioclase quartz phyric rhyolites indicate a time span for felsic volcanism ranging from 351 5 0 4 to 345 7 0 6 ma the youngest felsic rocks exhibit lower e nd as well as contrasting ti sr zr hf and eu eu values we interpret that in the riotinto nerva unit crustal melting successively affected shallower more evolved horizons in the crust progressive crustal melting is consistent with current interpretations of the iberian pyrite belt in terms of a late devonian early carboniferous transtensional setting coupled with underplating by basic magma we suggest that low e nd evolved crustal magmatism could be used as a proxy in studies of the genesis of these and possibly other phanerozoic massive sulphide deposits supplementary material details of the analytical methods and whole rock data for basalts are available at http www geolsoc org uk sup18452

new paleomagnetic results from northern china collision and suturing with siberia and kazakhstan abstract we report new results from our paleomagnetic studies of the inner mongolia autonomous region in northeastern china and the xinjiang autonomous region in northwestern china primary magnetizations are isolated in tertiary cretaceous late jurassic and late permian rocks and secondary components of magnetization of late jurassic and recent ages are seen in strata of inner mongolia western junggar basin and part of the tarim block suffered widespread complete tectono thermal remagnetization at about 280 ma permian the paleomagnetic evidence suggests that the sampled areas in inner mongolia have been part of the north china block at least since late permian time and the junggar block collided with the tarim block by late permian time our own paleomagnetic results together with available data from other parts of eastern eurasia suggest a tectonic model in which kazakhstan europe and siberia collided with each other and formed the europe siberia kazakhstan plate eur sib kaz in late permian time to the east mongolia outer mongolia inner mongolia and the north china block were also wedged together forming the mongolia north china plate mon nc by this time these two composite continental plates were not a stable unit in the late permian the geometrical relationship of the paleomagnetic poles for these two plates suggests that collision initially occurred in the late permian near the western end 50 n 75 e of the southern siberian fold belt and progressed eastward as the mon nc plate rotated and clockwise about 117 relative to the eur sib kaz plate during the early mesozoic suturing and rotation were probably complete by the late jurassic significant relative motion between the tarim and siberia blocks continued from early triassic to late jurassic as tarim moved northeastward 1400 km relative to siberia

cordierite gedrite rocks from the central metasedimentary belt boundary thrust zone grenville province ontario mesoproterozoic metavolcanic rocks with affinities to the composite arc belt cordierite gedrite rocks in the southern grenville province occur near the base of the central metasedimentary belt boundary thrust zone interpreted by some as a crustal suture between the 1 29 1 24 ga composite arc belt and 1 4 ga rocks of laurentia major and trace element compositions of these rocks are consistent with volcanic protoliths that range in composition from basalt to dacite these cordierite gedrite rocks have low cao average 1 2 wt and major element and oxygen isotope ratios suggestive of hydrothermal alteration before metamorphism rare earth element ree compositions also indicate igneous protoliths although some ree patterns have been modified by local melt extraction the trace element compositions of cordierite gedrite rocks and neodymium isotope systematics are similar to those of metavolcanic rocks in the composite arc belt and are consistent with the extension of the composite arc belt to the base of the boundary thrust zone

hotspots in the southern oceans an absolute frame of reference for motion of the gondwana continents abstract the geometry and geochronology of aseismic ridges and oceanic islands in the southern oceans provide a good test of the proposition that hotspots remain fixed over long periods of time that is motion of an order of magnitude less than the relative motion between plate pairs in most cases it is concluded that inter hotspot movement cannot be discerned for the period 100 m y to present and that widely distributed hotspots in the atlantic and indian oceans provide a frame of reference for plate motions following the disintegration of gondwanaland which is independent of paleomagnetism this frame of reference is absolute in that it gives the motion of the lithosphere with respect to the mantle hotspots the absolute motion model indicates that africa and antarctica are now moving only very slowly that there has been significant relative movement between east and west antarctica since the cretaceous and prescribes the relative motion between the somali and african plates

inverting airborne geophysical data for mega cell and giga cell 3d earth models today s mineral exploration is driven by the simple fact that discovery rates have not kept pace with the depletion of existing reserves to improve discovery rates there is an industry wide consensus on the need to increase the discovery space by exploring under cover and to greater depths this attracts increased risks which may be mitigated by improved targeting to do this mining geophysics needs to shift toward 3d geological models founded upon improved petrophysical understanding and geophysical inversion regardless of the inversion methodology used all geological constraints manifest themselves in the user s prejudice of an a priori model upper and lower bounds and choice of regularization however the practice of geologically constrained inversion is not the major problem needing to be addressed it is known and accepted that geology is inherently 3d and is a result of complex overlapping processes related to genesis metamorphism deformation alteration and or weathering yet the mining geophysics community to date has not fully accepted that geophysics should also be 3d and most often relies on qualitative analysis 1d inversion and depositscale 2d or 3d inversion there are many reasons for this unfortunate deficiency not the least of which has been the lack of capacity of existing 3d inversion algorithms to date these have not been able to invert entire surveys with sufficient resolution in sufficient time to practically affect exploration decisions this problem is most critical for airborne geophysical sur

petrogenetic evolution of felsic volcanic sequences associated with phanerozoic volcanic hosted massive sulphide systems the role of extensional geodynamics abstract felsic volcanic hosted massive sulphide deposits form in many different geotectonic environments usually associated with episodes of extension however empirical geochemical links between felsic volcanism geotectonic environment and extensional geodynamics have not been adequately documented so as to distill the fundamental relationships between these features and formation of volcanic hosted massive sulphide vhms mineralization which is important considering most vhms deposits are hosted in felsic volcanic bearing sequences using felsic geochemical data from six different vhms mineralized phanerozoic geotectonic settings it seems that the felsic geochemical data are consistent with the overall geotectonic interpretations for these areas which in turn helps in stratigraphic and geotectonic interpretations for detailed vhms exploration in extensional geodynamic settings bimodal volcanism is common compared to the continuum of compositions observed under compressional to neutral high stress regimes in calc alkaline arc building environments the empirical geochemical metallogenic vhms relationships evident in the archean felsic volcanic rocks in particular the low zr y n yb n 2 o conditions in different source areas and geotectonic environments they seem to be the predominant controls on the compositional evolution of felsic magmas in extensional settings because of the limited degree of assimilation and fractional crystallization afc affecting their compositions with rapid intrusion into the uppermost crust to positions were they achieve neutral buoyancy in the volcanic to subvolcanic environment within plate felsic or a type anorogenic type and anomalous ocean ridge or type felsic magmas that have tholeiitic affinities relatively low alkali elements are common within parts of the volcanic sequences hosting massive sulphide deposits reflecting in part a relationship to extension the high contents of high field strength elements zr ga lree hree y nb etc and other lines of evidence indicate they are relatively high temperature relatively reduced magmas probably derived from previously dehydrated granulitic or melted oceanic simatic or continental sialic crust low p granulites or amphibolites but also in some cases possibly mantle rocks such that they are significantly water undersaturated during melting i e a h 2 o 1 reduced viscosities typical of high temperature melts enable rapid intrusion to higher levels diking vs diapirism higher rates of heat advection from mantle mafic magmas into the thickened oceanic and or continental crust during lithospheric extension are needed to facilitate melting of these rock to form a type and or or type partial melts tholeiitic felsic this usually results in bimodal magmatism as magma mixing is not favored in extensional settings especially at high magma production intrusion and extrusion rates i e high heat flow this would inhibit ferromagnesian phase in particular hornblende and magnetite fractionation especially at low f o 2 in the felsic magmas that would deplete the melt in y hree as well as ore related elements particularly zn which is 2 to 3 times higher in a type and or type rocks compared to other magma types therefore the empirical felsic igneous geochemical metallogenic relationships observed by others low zr y n yb n

pre eruptive magma mixing in ash flow deposits of the tertiary rum igneous centre scotland the northern marginal zone of the rum igneous centre is a remnant of an early caldera and its infill it is composed of intra caldera breccias and various small volume pyroclastic deposits overlain by prominent rhyodacite ash flow sheets of up to 100 m thickness the ash flows were fed from a feeder system near the caldera ring fault and intrusive rhyodacite can locally be seen grading into extrusive deposits a variety of features suggest that the ash flows were erupted from a magma chamber that contemporaneously hosted felsic and mafic magmas i chilled basaltic inclusions in rhyodacite ii formerly glassy basaltic to andesitic enclaves with fluid fluid relationships iii feldspars with thick reaction rims enclosed in the basaltic to andesitic inclusions yet with cores chemically resembling those of the rhyodacite iv trace element compositions of the rhyodacite and the mafic enclaves form a mixing line between the end member rhyodacite and basalt compositions additionally textural and chemical features in the rhyodacite feldspar phenocrysts are consistent with magma mixing v feldspars with resorption embayments cutting through internal zonation of the crystals vi complexly zoned crystals with sieve textured zones that are overgrown with euhedral zones vii oscillatory zonation of feldspar phenocrysts in the rhyodacite showing sharp increases in anorthite an 10 followed by gradual decrease in an content an 4 this evidence points to eruption of ash flows from a felsic magma chamber that was periodically replenished by injection of mafic magma diffusional mixing between the two magmas was permitted by temperature and compositional differences but was slow due to the contrast in viscosities and densities the fe ti p enriched basic magma that replenished the chamber was degassing on entering the lower temperature environment and soon equilibrated thermally followed by chemical exchange between the two end member magmas this process formed hybrid andesite enclaves enriched in trace elements beyond that caused by simple mixing implying trace element diffusion in addition to bulk mixing eruption was caused by replenishment with and degassing of the basic magma and the chamber partially evacuated while the process of hybridisation was underway the erupted products record magma mixing by chamber replenishment blending of two magmas and elemental exchange in the magma chamber and also physical mingling in the eruptive conduit

rapid formation of the small isles tertiary centre constrained by precise 40ar 39ar and u pb ages the relative chronology of magmatic and tectonic events is key to an understanding of the influence of the iceland plume on the north atlantic in particular the location and duration of magmatism is of fundamental importance initial widespread flood basalt formation occurred in baffin island greenland and britain before complete plate break up at 56 ma after which time magmatism became concentrated in the active rift zone historically the british tertiary igneous province btip has been instrumental in advancing many concepts of igneous petrology however the absolute age and duration of the province remains unresolved here we present new internally consistent 40ar 39ar ages that help to constrain the volcanic activity in the small isles centre to within 2 my this short duration has implications for the onset of magmatism in the larger north atlantic province the rapid unroofing of the rum volcano and more significantly some of the evidence used to propose pulsing of the iceland plume

sedimentary and volcano tectonic processes in the british paleocene igneous province a review research on the british paleocene igneous province bpip has historically focused on the emplacement chemistry and chronology of its elaborate central intrusive complexes and lava fields however the bpip has also been dramatically shaped by numerous erosion sedimentation and volcano tectonic events the significance of which becomes ever clearer as localities in the bpip are re investigated and our understanding of volcano sedimentary processes advances the resultant deposits provide important palaeo environmental palaeo geographical and stratigraphical information and highlight the wide range of processes and events that occur in ancient volcanic settings such as the bpip in this paper we review the sedimentary and volcano tectonic processes that can be distinguished in the bpip and conceptualize them within a generalized framework model we identify and describe the sedimentary responses to four broadly chronological stages in the history of the bpip volcanoes 1 the development of the lava fields 2 early intrusion induced uplift 3 caldera collapse and 4 post volcano denudation and exhumation of central complexes we highlight and illustrate the range of sedimentary processes that were active in the bpip these operated on and helped shape a dynamic landscape of uplands and lowlands of alluvial fans braided rivers lakes and swamps and of volcanoes torn apart by catastrophic mass wasting events and or caldera collapse

sirte basin north central libya cretaceous rifting above a fixed mantle hotspot the complex pattern of horsts and grabens in the sirte basin may have developed when mesozoic drift of the african plate put north central libya over a fixed mantle hotspot in early cretaceous time 140 to 100 m y ago significant change in the motion of the plate during the prolonged residence above a hypothetical cameroon plume may have produced stress that fragmented thinned and weakened lithosphere successive uplift and subsidence along a reconstructed track of the plume as well as in the sirte basin are compatible with predicted effects of the drift of northern africa over a fixed mantle hotspot this speculation suggests a plausible alternative to the possibility that rifting throughout northern africa in early cretaceous time may have been produced along a wide zone of extension between two african plates when they were at rest relative to underlying plumes

geochemistry and architecture of the host sequence of the massive sulfides in the northern iberian pyrite belt abstract the northernmost iberian pyrite belt hosts several small felsic volcanogenic massive sulfide orebodies geochemically the volcanic sedimentary complex is dominated by andesitic to rhyolitic calc alkaline magmatic rocks with a significant proportion of dacite and andesite the lower felsic and upper felsic units are geochemically identical and probably represent dismembered parts of a single volcanic felsic complex if that holds true massive sulfide deposits of the northern ipb are hosted by volcaniclastic rocks located in the hanging wall of a single unit dominated by dacite rhyolite dome complexes this volcanic unit is characterized by zr contents structural analysis and the geochemistry suggest that the volcanic sequence is tectonically imbricated with the youngest units located at the lowest structural position the proposed reconstructed stratigraphic sequence comprises an andesitic footwall overlain by the felsic complexes of the middle felsic and lower upper felsic units thus the massive sulfide deposits formed toward the top of the whole sequence and associated with the youngest felsic rocks having a specific geochemistry

low temperature alteration at the world class laronde penna archean au rich volcanogenic massive sulfide deposit abitibi subprovince quebec canada evidence from whole rock oxygen isotopes the oxygen isotope composition of 169 whole rock samples from the world class laronde penna au rich volcanogenic massive sulfide deposit range from 6 5 to 22 0 fractional crystallization from basalt to rhyolite accounts for a variation of approximately 2 in primary whole rock 18o values the remaining variance in whole rock 18o values found at laronde penna must therefore be ascribed to water rock exchange with hydrothermal fluids the metamorphosed hydrothermally altered host rocks have been subdivided according to mineralogical assemblages which display weak covariance with 18o values and whole rock geochemical alteration indices water rock equilibrium exchange models indicate that the high 18o values that characterize the laronde penna deposit are compatible with low temperature 150oc hydrothermal alteration at high water rock oxygen atomic ratios 1 50 zones of lower 18o values stratigraphically beneath the principal ore lens 20 north indicate local zones of higher temperature hydrothermal alteration the oxygen isotope composition of the hydrothermal fluid is estimated to have been 5 as a consequence of mixing between seawater 18o 0 and a component of magmatic water degassed from the volcanic and intrusive rocks associated with the laronde penna deposit

spatial constraints on magma chamber replenishment events from textural observations of cumulates the rum layered intrusion scotland the eastern layered series of the rum layered suite is formed of 16 macro units each comprising a lower peridotite and an overlying feldspar rich layer the local term is allivalite the origin of the peridotite layers is unresolved with two contrasting models the earlier of the two is based on repeated replenishment of an opensystem magma chamber with deposition of fractionated material on the chamber floor the second is based on the early formation of a troctolitic complex which is then repeatedly intruded by sills of replenishing picritic magma to form the peridotite horizons the lack of resolution of this fundamental problem is a consequence of the reliance of previously published studies on field observations i present evidence to show that the clinopyroxene in the allivalites preserves information about the distribution of the last melt to solidify permitting determination of not only the extent of supersolidus textural equilibration but also the sub solidus history of the allivalite horizons comparison of profiles of clinopyroxene plagioclase plagioclase median dihedral angle across allivalite units demonstrates that it is possible to distinguish between those that were intruded by later picrite sills and those adjacent to peridotite horizons formed by replenishment and subsequent deposition of fractionated crystals above the pre existing pile in the region studied only the main peridotite body of unit 9 was intruded into a pre existing allivalitic mush

constraining the post emplacement evolution of the hebridean igneous province hip using low temperature thermochronology how long has the hip been cool abstract the thermal history of the hebridean igneous province has been determined through the application of low temperature thermochronology to the four central complexes in the province the zircon u th he age 59 4 3 3 ma 1 n 18 and the ages from each complex 60 7 2 3 ma for skye 58 0 0 4 ma for mull 55 9 3 2 ma for ardnamurchan 55 6 2 7 ma for rum are indistinguishable from the crystallization ages apatite fission track ages 61 2 57 2 ma mean of 59 3 3 4 ma from the major plutonic units also overlap crystallization ages implying that on a regional scale the hebridean igneous province cooled rapidly to near surface temperatures immediately after emplacement however apatite fission track ages and track lengths and apatite u th he ages from some small volume intrusions in the skye and rum central complexes identify localized mid eocene 45 47 ma cooling forward and inverse modelling suggests a discrete heating cooling event at c 47 ma which may have been caused by structurally controlled localized advection of heat above shallow emplacement this is the first suggestion of eocene magmatism in the hebridean igneous province

geology alteration and lithogeochemistry of the hood volcanogenic massive sulfide vms deposits nunavut canada the hood volcanogenic massive sulfide vms deposits are hosted by the 2 68 ga amooga booga volcanic belt abvb in the northwestern archaen slave craton and consist of three deposits hood 10 41 and 41a and three occurrences 46 461 and 462 the mineralized zones consist of massive to semi massive pyrrhotite pyrite chalcopyrite sphalerite and galena hosted predominantly by felsic volcanic flows within the predominantly mafic abvb the mineralized lenses occur at different stratigraphic levels and have textural alteration and stratigraphic features consistent with formation via subseafloor replacement the felsic volcanic rocks in the hood deposits can be subdivided into groups based on immobile trace element geochemistry the main felsic types a and b are petrographically indistinguishable type a has higher high field strength element hsfe and rare earth element ree contents than type b suggesting a higher temperature of formation type a rocks also have higher nb ta values indicative of a greater mantle input in their genesis compared to type b rocks mineralization is more closely associated with type a than type b rocks the two mafic volcanic rock types previously identified in the abvb type i and type ii both occur within the hood deposits the type ii mafic group is interpreted to be the result of variable crustal contamination of type i magma the volcanic rocks of the abvb are interpreted to have formed in a continental margin arc back arc setting the genesis of these magmatic suites involved magmatic underplating and emplacement through pre existing sialic basement that resulted in crustal melting mantle crust mixing and contamination leading to the aforementioned geochemical features in both mafic and felsic suites this type of extensional tectonic environment was likely associated with high heat flow and is similar to global vms environments proximal to extending continental margins e g sturgeon lake bathurst and finlayson lake the similarities of the abvb to other slave craton greenstone belts further highlights the overall potential for greenstone hosted vms mineralization in the slave craton chlorite sericite quartz is the dominant hydrothermal alteration assemblage in the hood deposits and is typical of vms style mineralization mass change calculations illustrate that elemental changes are typical of vms environments with gains in fe2o3 mgo and base metals associated with chlorite alteration near mineralized zones k2o gains associated with sericite alteration and losses of na2o in both alteration types

pyroclastic rocks in the cnapan breaca felsite rhum the cnapan breaca felsite sheet contains strongly devitrified pyroclastic rocks including welded tuffs these indicate formation in a high level surficial or sub volcanic environment the gross structure and lithological associations of the sheet are similar to those of some ignimbrites these observations support a resurgent caldera model for the evolution of the rhum centre and raise some doubts about previous interpretations of the felsite breccia association on the island

global comparisons of volcanic associated massive sulphide districts although volcanic associated massive sulphide vms deposits have been studied extensively the geodynamic processes that control their genesis location and timing remain poorly understood compar

petrochemistry hydrothermal alteration mineralogy and sulfur isotope geochemistry of the turgeon cu zn volcanogenic massive sulfide deposit northern new brunswick canada the turgeon deposit is a mafic type cu zn volcanogenic massive sulfide vms deposit it is hosted by middle ordovician pillow basalts of the devereaux formation of the fournier group within the elmtree belledune inlier near the bathurst mining camp bmc in northern new brunswick canada the turgeon deposit consists of two cu zn massive sulfide lenses 100m zn 48 49 composed of pyrite chalcopyrite pyrrhotite and sphalerite which are underlain by chalcopyrite pyrite stockwork veins pyrite is overprinted and replaced by chalcopyrite in the stockwork and vent complex sulfide facies where both minerals are enriched in se and co relative to pyrite and chalcopyrite in the massive pyrite and breccia sulfide facies in se and co display a positive covariation with cu whereas zn displays a positive covariation with cd trace element geochemistry indicates that the host rocks are primarily tholeiitic basalts and andesites that have signatures between that of mid ocean ridge basalt and island arc

neodymium isotope geochemistry of felsic volcanic and intrusive rocks from the yukon tanana terrane in the finlayson lake region yukon canada devonian mississippian felsic rocks from the finlayson lake region have variable geochemical and nd isotopic characteristics that provide insights into the tectonic and metallogenic evolution of the yukon tanana terrane ytt and the northern cordillera late devonian 365 360 ma calc alkaline and tholeiitic arc felsic rocks in the mafic dominated fire lake unit yield end350 4 8 and 0 1 respectively and have 1 49 1 94 ga depleted mantle model ages tdm devonian mississippian 360 356 ma felsic volcanic kudz ze kayah unit wolverine succession and intrusive rocks grass lakes suite associated with volcanogenic massive sulphide vms deposits have end350 7 8 to 9 5 with tdm 1 59 2 25 ga a granitoid sample from the early mississippian 350 345 ma simpson range plutonic suite has end350 12 9 and tdm 2 01 ga similar to previously reported values for this suite the vms associated grass lakes suite of granitoids has higher high field strength element hfse and rare earth element ree contents and higher zr sc zr tio2 nb la and zr la values relative to the simpson range plutonic suite these geochemical features are similar to coeval vms associated felsic volcanic rocks in the kudz ze kayah unit the identification of similar hfse ree enriched felsic volcanic and subvolcanic intrusive rocks may aid in delineating prospective regions for vms mineralization in the ytt and other continental margin arc to back arc environments the geochemical and nd isotopic data for these ytt felsic rocks suggest that they reflect episodic mid paleozoic arc fire lake unit simpson range plutonic suite and back arc magmatism kudz ze kudz unit wolverine succession built upon a transitional basement with variable but significant influence from evolved proterozoic crustal materials

pressure conditions for the solidification of the skaergaard intrusion eruption of east greenland flood basalts in less than 300 000 years abstract primary granophyres are differentiated from olivine tholeiitic magma and occur interstitially throughout the cumulus stratigraphy of the skaergaard intrusion east greenland samples from the lower zones a c lza c the middle zone mz and the sandwich horizon sh are included in the present study together with granophyric accumulations in gabbroic pegmatite from lza c fluid inclusions in quartz and the mineral assemblage in the granophyres record the pressure under which the skaergaard intrusion crystallised pegmatitic granophyre from lza c consists mainly of quartz plagioclase an 4 7 and alkali feldspar or 40 80 enclosing an earlier formed assemblage of ferrohastingsitic and ferroedenitic hornblende fayalite fo 4 5 titanite biotite and fluor apatite granophyric quartz albite and alkali feldspar crystallised from water saturated granitic melts near eutectic minimum conditions between 680 and 660 c the pressure of granophyre crystallisation was modelled by the intercept between fluid inclusion isochores and the minimum melt solidus for granitic compositions pressures recalculated to the roof pendant of the intrusion are 0 7 0 5 for lza 2 0 0 2 for lzb c 2 3 0 8 for mz the triple group level and 3 3 1 3 kb for sh amphibole geobarometry independently confirm the pressure estimates for pegmatitic granophyres in lza c the granophyres formed as the intrusion cooled through the minimum melt solidus in lza lzb lzc mz and sh respectively the pressure increase from lza to sh granophyres is explained by progressive burial during cooling of the intrusion and contemporaneous outpouring of 5 3 6 3 2 7 km of flood basalts during the initial opening of the northeast atlantic ocean accordingly the skaergaard intrusion evolved from a subvolcanic magma chamber at emplacement to a more deep seated igneous system during terminal crystallization when the majority of the intercumulus phases formed the present cooling history suggests the intrusion cooled through the temperature of granophyre formation in less than 300 000 years the increase in the pressure of granophyre formation therefore implies that 5 3 6 3 2 7 km of flood basalts were emplaced within this time window

on the use of changes in dihedral angle to decode late stage textural evolution in cumulates the melt filled pore structure in the final stages of solidification of cumulates must lie somewhere between the two end members of impingement in which pore topology is controlled entirely by the juxtaposition of growth faces of adjacent grains and textural equilibrium in which pore topology is controlled by the minimization of internal energies the exact position between these two end members is controlled by the relative rates of crystal growth and textural equilibration for samples in which growth has stopped or is very slow textural equilibrium will prevail a close examination of dihedral angles in natural examples demonstrates that these two end member textures can be distinguished the impingement end member results in a population of apparent solid melt dihedral angles with a median of 60 and a standard deviation of 25 30 whereas the texturally equilibrated end member population has a median of 28 and a standard deviation of 14 for the specific case of cumulates in the rum layered intrusion residual porosity in troctolitic cumulates was close to the impingement end member whereas that in peridotites was close to melt bearing textural equilibrium suites of glass bearing samples from small or frequently disturbed magma systems show modification of initial impingement textures these modifications may be a consequence of textural equilibration or of diffusion limited growth during quenching distinction can be made between these two processes by a consideration of grain shape the geometry of interstitial phases in suites of fully solidified cumulates from the rum layered intrusion shows variable approach to sub solidus textural equilibrium from an initial state inherited by pseudmorphing of the last melt textural equilibration at pore corners occurs as a continuous process with a gradual movement of the entire dihedral angle population towards the equilibrium final state if the initial pseudomorphed state is one of disequilibrium i e a melt present impingement texture this change is accompanied by a reduction in the spread of the population if it is one of equilibrium the change is accompanied by an initial increase in the spread of the population followed by a decrease these observations demonstrate that previously published models of dihedral angle change involving the instantaneous establishment of the equilibrium angle in the immediate vicinity of the pore corner are incorrect

gold in the brunswick no 12 volcanogenic massive sulfide deposit bathurst mining camp canada evidence from bulk ore analysis and laser ablation icp ms data on sulfide phases the 329 mt brunswick no 12 volcanogenic massive sulfide deposit total resource of 163 mt at 10 4 zn 4 2 pb 0 34 cu and 115 g t ag is hosted within a middle ordovician bimodal volcanic and sedimentary sequence massive sulfides are for the most part syngenetic and the bulk of the sulfide ore occurs as a zn pb rich banded sulfide facies that forms an intimate relationship with a laterally extensive algoma type iron formation and defines the brunswick horizon zone refining of stratiform sulfides is considered to have resulted in the development of a large replacement style cu rich basal sulfide facies which is generally confined between the banded sulfide facies and an underlying stringer sulfide zone complex polyphase deformation and associated lower to upper greenschist facies regional metamorphism is responsible for the present geometry of the deposit textural modification has resulted in a general increase in grain size through the development of pyrite and arsenopyrite porphyroblasts which tend to overprint primary mineral assemblages despite the heterogeneous ductile deformation primary features have locally been preserved such as fine grained colloform pyrite and base and precious metal zonation within the main zone base metal and trace element abundances in massive sulfides from the brunswick no 12 deposit indicate two distinct geochemical associations the basal sulfide facies characterized by a proximal high temperature hydrothermal signature cu co bi se contains generally low au contents averaging 0 39 ppm n 34 conversely au is enriched in the banded sulfide facies averaging 1 1 ppm au n 21 and is associated with an exhalative suite of elements zn pb as sb ag sn finely laminated sulfide lenses hosted by iron formation at the north end of the main zone are further enriched in au averaging 1 7 ppm n 41 and ranging up to 8 2 ppm laser ablation inductively coupled plasma mass spectrometry icp ms analyses of pyrite n 97 from the north end of the main zone average 2 6 ppm au and range from the detection limit 0 015 ppm to 21 ppm overall these analyses reveal a distinct au sb as ag hg mn association within pyrite grains gold is strongly enriched in large pseudo primary masses of pyrite that exhibit relict banding and fine grained cores smaller euhedral pyrite porphyroblasts and euhedral rims of metamorphic origin surrounding the pyrite masses contain much less au sb ag as and sn arsenopyrite occurring chiefly as late porphyroblasts contains less au averaging 1 0 ppm and ranging from the detection limit 0 027 ppm to 6 9 ppm depth profiles for single spot laser ablation icp ms analyses of pyrite and arsenopyrite display uniform values of au and an absence of discrete microscopic inclusions of au bearing minerals which is consistent with chemically bonded au in the sulfide structure the pervasive correlation of au with sn in the zn pb rich banded sulfide facies suggests similar hydrothermal behavior during the waxing stages of deposition on the seafloor under high temperature 350oc and moderate to low ph conditions au and sn in hydrothermal fluids would be transported as chlorocomplexes an abrupt decrease in temperature and ah2s accompanied by an increase in fo2 and ph during mixing with seawater would lead to the simultaneous destabilization of both au and sn chlorocomplexes the enrichment of au in fine grained laminated sulfides on the periphery of the deposit accompanied by sporadic occurrences of barite and fe poor sphalerite supports lower hydrothermal fluid temperatures analogous to white smoker activity on the flanks of a large volcanogenic massive sulfide system in lower temperature 350oc and mildly acidic hydrothermal fluids au would be transported by thiocomplexes which exhibit multifunctional retrograde prograde solubility and a capacity to mobilize au to the outer parts of the sulfide mound the sluggish nature of this low temperature venting together with larger variations in ambient fo2 could lead to a sharp enrichment of au towards the stratigraphic hanging wall of massive sulfide deposits

dating the onset of volcanism at the rum igneous centre nw scotland major volcanic activity on the isle of rum commenced with the eruption of thick 100 m intra caldera rhyodacite ash flow sheets fed from steep sided feeder conduits in the proximity of the main ring fault twenty plagioclase phenocrysts of the rhyodacite were analysed using single crystal 40 ar 39 ar laser dating yielding a mean apparent age of 60 83 0 27 ma mswd 3 65 on an age v probability plot the feldspars do not however show a simple gaussian distribution but a major peak at 60 33 0 21 ma and two smaller shoulders at c 61 4 ma and 63 ma these older ages are interpreted to represent recycled and largely re equilibrated feldspars the age peak at 60 33 0 21 ma is interpreted to represent the intrusion and eruption age of the rhyodacites this new age constraint overlaps with that for the ultrabasic intrusion implying that the latter was already forming at depth and supplying necessary heat during the early felsic activity phase and quickly thereafter migrated upwards to shallow structural levels and intruded the volcano9s earlier deposits combined with previously published ages these new age data highlight an extremely rapid succession of events at the rum centre the whole sequence occurring in potentially

significance of pre 1860 ma granitoid magmatism for crustal evolution and exploration targeting in the flin flon assemblage trans hudson orogen canada granitoid plutonism a significant contributor to orogenic evolution is a commonly overlooked vector in exploration targeting using the flin flon glennie complex of the trans hudson orogen as a template we show that the geochemistry of granitoid rocks can be a useful parameter in differentiating prospective and nonprospective areas for volcanic hosted massive sulfide vhms deposits the flin flon glennie complex includes 1910 to 1860 ma plutonic rocks that can be subdivided into three suites 1 juvenile arc related rocks of ferroan and magnesian subtypes 2 contaminated arc related rocks and 3 successor arc related rocks of both low and high sr y ratio petrogenesis of ferroan juvenile and low sr y successor arc plutons indicates they formed in regions of thinner crust and hence would have been associated with higher heat flow and arc rifting conditions that favor vhms endowment regions of the orogen with both ferroan and low sr y plutonic subtypes were long lived zones of thin crust and preserve vhms mineralization related to both juvenile and successor arc magmatism in contrast volcanic belts dominated by contaminated arc or high sr y successor arc plutons formed on thicker substrates with contaminated arc rocks built on archean paleoproterozoic pericratonic slivers and high sr y successor arc rocks associated with submarine to subaerial arc systems these regions of thicker crust demonstrably lack vhms deposits and were also the locus for late unroofing our results suggest that pluton geochemistry regionally correlates with mineral occurrence and could represent a useful regional exploration tool in buried or poorly exposed parts of the flin flon glennie complex and other paleoproterozoic orogens