sulfur isotopic studies of the dunka road cu ni deposit duluth complex minnesota the dunka road deposit is one of a series of cu ni occurrences located within troctolitic rocks of the southern portion of the duluth complex minnesota sulfide minerals principally pyrrhotite chalcopyrite cubanite and minor pentlandite occur interstitially with silicate minerals and average about 2 volume percent in ore grade horizons areas of sulfide concentrations in excess of 1 to 2 percent occur erratically throughout the troctolitic sequence but are in general concentrated near the basal contact with rocks of the metasedimentary virginia formation the delta 34 s values of sulfides in the troctolites range from 0 2 to 15 8 per mil with an average value near 7 5 per mil pyrrhotite in the underlying virginia formation hornfels and pyrite from unmetamorphosed virginia formation are characterized by a similar range in delta 34 s values the high positive delta 34 s values and similarity to values found in the virginia formation strongly suggest that most of the sulfur in the dunka road deposit was derived from the breakdown of pyrite in the virginia formation the sulfur isotopic compositions of minerals in the troctolites show no consistent lateral or vertical trends and are in general characterized by a high degree of variability such fluctuations in delta 34 s values indicate that sulfur isotopic homogenization was not attained throughout the troctolitic sequence several mechanisms may have contributed to the variations in delta 34 s values found within the troctolites including mixing of sedimentary and igneous sulfur assimilation of sedimentary sulfur characterized by variable delta 34 s values fractionation caused by fluctuations in oxidation states during sulfur volatilization and assimilation and periodic introduction of isotopically distinct magma batches in light of the erratic sulfide mineral and delta 34 s distributions the predominant mechanism is thought to be the introduction of both isotopically and spatially variable country rock sulfur into crystallizing troctolitic magma

carbonate periplatform sedimentation by density flows a mechanism for rapid off bank and vertical transport of shallow water fines existing theories of off bank sediment transport cannot account for rapid rates of sedimentation observed in bahama bank and florida shelf periplatform environments analysis of the physical processes operating during winter cold fronts suggests that accelerated off bank transport of shallow water mud may be achieved by sinking off bank flows of sediment charged hyperpycnal super dense platform waters

precious and base metal geochemistry and mineralogy of the grasvally norite pyroxenite anorthosite gnpa member northern bushveld complex south africa implications for a multistage emplacement the grasvally norite pyroxenite anorthosite gnpa member within the northern limb of the bushveld complex is a mineralized layered package of mafic cumulates developed to the south of the town of mokopane at a similar stratigraphic position to the platreef the concentration of platinum group elements pge in base metal sulfides bms has been determined by laser ablation inductively coupled plasma mass spectrometry these data coupled with whole rock pge concentrations and a detailed account of the platinum group mineralogy pgm provide an insight into the distribution of pge and chalcophile elements within the gnpa member during both primary magmatic and secondary hydrothermal alteration processes within the most unaltered sulfides containing pyrrhotite pentlandite and chalcopyrite only the majority of ipge rh and some pd occur in solid solution within pyrrhotite and pentlandite with an associated pt as and pd bi te dominated pgm assemblage these observations in conjunction with the presence of good correlations between all bulk pge and base metals throughout the gnpa member indicate the presence and subsequent fractionation of a single pge rich sulfide liquid which has not been significantly altered in places the primary sulfides have been replaced to varying degrees by a low temperature assemblage of pyrite millerite and chalcopyrite these sulfides are associated with a pgm assemblage characterized by the presence of pd antimonides and pd arsenides which are indicative of hydrothermal assemblages the presence of appreciable quantities of ipge pd and rh within pyrite and to a lesser extent millerite suggests these phases directly inherited pge contents from the pyrrhotite and pentlandite that they replaced the replacement of both the sulfides and pgm occurred in situ thus preserving the originally strong spatial association between pgm and bms but altering the mineralogy precious metal geochemistry indicates that fluid redistribution of pge is minimal with only pd au and cu being partially remobilized and decoupled from bms this is also indicated by the lower concentrations of pd evident in both pyrite and millerite compared with the pentlandite being replaced the observations that the gnpa member was mineralized prior to intrusion of the main zone and that there was no local footwall control over the development of sulfide mineralization are inconsistent with genetic models involving the in situ development of a sulfide liquid through either depletion of an overlying magma column or in situ contamination of crustal s we therefore believe that our observations are more compatible with a multistage emplacement model where preformed pge rich sulfides were emplaced into the gnpa member such a model explains the development and distribution of a single sulfide liquid throughout the entire 400 800 m thick succession it is therefore envisaged that the gnpa member formed in a similar manner to its nearest analogue the platreef notable differences however in pge tenors indicate that the ore forming process may have differed slightly within the staging chambers that supplied the platreef and gnpa member

periplatform drift the combined result of contour current and off bank transport along carbonate platforms hydroacoustic and sedimentological data from the western leeward flank of the great bahama bank document the interplay of off bank sediment export along slope transport and erosion which together shape facies and thickness distribution of slope carbonates the integrated data set depicts the combined product of these processes and allows formulation of a comprehensive model of a periplatform drift that significantly amends established models of carbonate platform slope facies distribution and geometry the basinward thinning wedge of the periplatform drift at the foot of the bank escarpment displays along slope and downslope variations in sedimentary architecture sediments are muddy carbonate sands that coarsen basinward the drift wedge has a pervasive cover of cyclic steps in zones of lower contour current speed depth related facies belts develop whereas strike discontinuous sediment lobes scarps and gullies characterize areas with higher current speed this understanding of the impact of currents on carbonate slope sedimentation has wider implications for seismic and sequence stratigraphic interpretation of carbonate platforms and for applied aspects such as hydrocarbon exploration

sulfur degassing due to contact metamorphism during flood basalt eruptions abstract we present a study aimed at quantifying the potential for generating sulfur rich gas emissions from the devolatilization of sediments accompanying sill emplacement during flood basalt eruptions the potential contribution of sulfur rich gases from sediments might augment substantially the magma derived sulfur gases and hence impact regional and global climate we demonstrate from a detailed outcrop scale study that sulfur and total organic carbon have been devolatilized from shales immediately surrounding a 3 m thick dolerite sill on the isle of skye scotland localized partial melting occurred within a few centimetres of the contact in the shale generating melt filled cracks pyrite decomposed on heating within 80 cm of the contact generating sulfur rich gases a mixture of h2s and so2 and pyrrhotite the pyrrhotite shows 32s enrichment due to loss of 34s enriched so2 further decomposition and oxidation of pyrrhotite resulted in hematite and or magnetite within a few cm of the contact iron sulfates were produced during retrogressive cooling and oxidation within 20 cm of the contact decarbonation of the sediments due to heating is also observed particularly along the upper contact of the sill where increasing i 13c is consistent with loss of methane gas the geochemical and mineralogical features observed in the shales are consistent with a short lived intrusion emplaced in 5 h the dolerite magma contains pervasive pyrite and localized sulfur concentrations greater than the sulfur concentration at sulfide liquid saturation consistent with addition of sulfur perhaps from sediments at a late stage our study provides evidence for desulfurization as well as decarbonation of shales adjacent to an igneous intrusion the liberated fluids rich in sulfur and carbon are likely to be focused along regions of low pore fluid pressure along the margins of the sill the sulfur gases liberated from the sediments would have augmented the sulfur dioxide and hydrogen sulfide yield of the eruption substantially had they reached the surface this enhancement of the magmatic sulfur budget has important implications for the climate impact of large flood basalt eruptions that erupt through thick volatile rich sedimentary sequences

sulfur isotope exchange and metal enrichment in the formation of magmatic cu ni pge deposits sulfur isotope measurements provide a useful means of evaluating the source of sulfur in magmatic cu ni pge deposits although low sulfur pge rich deposits e g j m reef of the stillwater complex merensky reef of the bushveld complex show little or no isotopic evidence for the involvement of crustally derived sulfur several high sulfur deposits e g noril sk duluth complex show strong evidence for contamination of mafic magmas by sulfur of crustal origin processes such as magma degassing or redox changes accompanying crystallization are ineffective in significantly modifying sulfur isotope signatures of sulfides that crystallize from immiscible liquids that exsolve from the mafic magmas for this reason 34s values are primarily dependent on sulfur source characteristics and variations can often be treated using a two component mixing approach however deposits such as those at noril sk and voisey s bay that appear to have formed in magma conduit systems present another possibility for modification of sulfur isotope signatures these deposits are thought to have been upgraded in metal tenor by reaction with uncontaminated mantle derived melts that traversed the accumulated sulfide on their way to the surface our calculations illustrate the efficiency of reactions that involve the exchange of fe for cu ni or pge in producing increases in the metal tenor of the accumulated sulfide sulfur isotope exchange between the crustally contaminated sulfide ore accumulation and sulfur of mantle origin may accompany chalcophile element exchange and is capable of masking or limiting the degree of isotopic evidence for the initial stage of ore genesis that involved crustal sulfur sulfur isotope exchange is distinct from the addition of sulfur in the form of immiscible sulfide droplets from a sulfide saturated magma to the accumulated sulfide although both additive mixing and exchange can lead to similar isotopic effects stable isotopic exchange provides a mechanism in addition to additive mixing that may lead to decoupling of different isotopic systems

seismic architecture and seismic geomorphology of heterozoan carbonates eocene oligocene browse basin northwest shelf australia abstract seismic characterization of eocene oligocene heterozoan carbonate strata from the browse basin northwest shelf of australia defines marked progradation of nearly 10 km stratal terminations and stacking subdivide the succession into mappable seismic units stratal architecture and seismic geomorphology varies systematically through the succession individual surfaces discerned by toplap onlap and truncation outline sigmoidal to tangential oblique clinoforms with heights of ranging from 350 to 650 m and maximum gradients between 8 and 18 sigmoidal clinoforms can include aggradation in excess of 200 m prograde more than 500 m and have slopes characterized by inclined wavy to discontinuous reflectors that represent ubiquitous gullies and channels in contrast the overlying tangential oblique clinoforms include downstepped shelf margins limited on shelf aggradation the associations of shelf aggradation shelf margin progradation and slope channeling within sigmoidal seismic units and the less marked progradation and channeling within tangential oblique seismic units contrast with the classic sequence model in which sediment delivery to the slope and pronounced progradation is favored by limited shelf accommodation this distinct divergence is interpreted to reflect the prolific heterozoan production across the shelf during periods of rising and high base level when the shelf is flooded perhaps enhanced by downwelling comparison with purely photozoan systems reveals similarities and contrasts in seismic stratigraphic heterogeneity and architecture interpreted to be driven by distinct characteristics of heterozoan sedimentary systems

fluid flow through carbonate platforms as evidence for deep seated reservoirs in northwest australia fluid flow features in carbonate platforms of the outer browse basin northwest australia are investigated using 3d seismic and borehole data during the cenozoic the study area evolved from a carbonate ramp to a rimmed platform with isolated carbonate build ups as such it played a key role in focusing fluid on their buoyant flow to the surface statistical analyses of direct hydrocarbon indicators show fluid flow to be focused in elevated areas i e the greater is the focusing of fluid the larger is the carbonate platform or isolated build up locally karst systems created regions of enhanced permeability through the evolving carbonate stratigraphy karstified horizons are located on the top of carbonate build ups and platform clinoforms as fluid buoyantly concentrates and diffuses up into topographic highs in turn the putative migration of gas and fluid generated hypogenic karst systems enhancing permeability in otherwise lower permeability rock based on the interpreted data we suggest a pivotal relationship between the presence of carbonate build ups and karsts in the browse basin and hydrocarbon accumulations at depth areas of elevated topography focus sub surface fluid enhancing the permeability of carbonate successions in turn focused fluid flow can lead to the generation of methanogenic carbonates promoting the growth of isolated build ups and leading to further generation of elevated features this grants the identification of similar features on seismic data from northwest australia and other equatorial margins in the world as a valid proxy for the recognition of hydrocarbon accumulations below thick carbonate successions

glacial versus interglacial sedimentation rates and turbidite frequency in the bahamas the southern tongue of the ocean is a 1300 m deep flat floored basin in the bahamas that receives large amounts of sediment from the carbonate platforms surrounding it on three sides we have examined five 8 13 m long piston cores and determined bulk sedimentation rates turbidite frequency and turbidite accumulation rates for the past two glacial and interglacial periods the mean of bulk sedimentation rates is four to six times higher in interglacial periods average accumulation rates of recognizable turbidites are higher by a factor of 21 to 45 and interglacial turbidite frequency is higher by a factor of 6 to 14 sediment composition indicates that increased interglacial rates are due to higher accumulation of platform derived material additional data from other bahamian basins as well as published material from the caribbean strongly suggest that highstand shedding is a general trend in pure carbonate depositional systems carbonate platforms without a siliciclastic component export more material during highstands of sea level when the platform tops are flooded and produce sediment the response of carbonate platforms to quaternary sea level cycles is opposed to that of siliciclastic ocean margins where sediment is stored on the inner shelf during highstands and passed on to continental rises and abyssal plains during lowstands of sea level

new insights into the morphology and sedimentary processes along the western slope of great bahama bank new high quality multibeam and seismic data image the western slope of the great bahama bank and the adjacent floor of the straits of florida the extensive survey reveals several unexpected large and small scale morphologies these include bypass areas channel leveelobe systems gullied slopes and products of slope instabilities at various scales including long slump scars at the lower slope and mass transport complexes that extend 30 km into the adjacent basin floor the toe of the slope is irregularly covered with deep water carbonate mounds the abundance of the individual morphological features varies from north to south from 26 00 n to 25 20 n the slope is dissected by numerous deep canyons that abruptly end southward where the slope is characterized by a smooth lower portion and small regularly spaced furrows in its upper part further south two long 25 50 km scars document instability at the lower slope one of these scars is the source area of a large mass transport complex in addition to this large scale feature several types of gravity induced sedimentary processes are revealed most of the morphologies and inferred processes of this carbonate system are similar to those observed in siliciclastic systems including mass transport complexes gravity currents initiated by density cascading and overspilling channeled turbidity currents for the first time a clear asymmetric channel levee system has been identified along the slope suggesting similitude in sorting processes between carbonate and siliciclastic systems and enhancing the reservoir bearing potential of carbonate slopes notable differences with siliciclastic systems include the lack of connection with the shallow and emerged part of the system i e bank top and the small size of the sedimentary system 2012 geological society of america

canyon morphology on a modern carbonate slope of the bahamas evidence of regional tectonic tilting new high quality multibeam data presented here depict the northern slope of the little bahama bank bahamas the survey reveals the details of large and small scale morphologies that look like siliciclastic systems at a smaller scale including large scale slope failure scars and canyon morphologies previously interpreted as gullies and creep lobes the slope exhibits mature turbidite systems built by mass fl ow events and turbidity currents the sediment transport processes are probably more complex than expected slope failures show sinuous head scarps with various sizes and most of the scars are fi lled with recent sediment canyons have amphitheater shaped heads resulting from coalescing slump scars and are fl oored by terraces that are interpreted as slump deposits canyons rapidly open on a short channel and a depositional fan shaped lobe the entire system extends for 40 km the development of these small turbidite systems similar to siliciclastic systems is due to the lack of cementation related to alongshore current energy forcing the transport of fi ne particles and fl ow differentiation detailed analyses of bathymetric data show that the canyon and failurescar morphology and geometry vary following a west east trend along the bank slope the changing parameters are canyon length and width depth of incision and canyon and channel sinuosity accordingly failure scars are larger and deeper eastward these observations are consistent with a westward tectonic tilt of the bank during the cenozoic

architecture and growth history of a miocene carbonate platform from 3d seismic reflection data luconia province offshore sarawak malaysia using 3d seismic reflection data and wireline logs we reconstruct in detail the architecture and growth history of a miocene carbonate platform in the luconia province offshore sarawak malaysia platform growth started in the late oligocene to early miocene by coalescence of isolated patch reefs the growth history includes phases of progradation backstepping and occasional collapsing of platform flanks guided by syndepositional faulting the most pronounced seismic reflections in the platform correspond to flooding events thin transgressive systems tracts subaerial exposure preceding the flooding could be demonstrated in only one case platform growth was terminated by gradual submergence drowning indicated by smooth concentric seismic reflectors forming a convex mound

a laser ablation inductively coupled plasma mass spectrometry study of the distribution of chalcophile elements among sulfide phases in sedimentary and magmatic rocks of the duluth complex minnesota usa abstract nickel copper sulfide deposits occur in the basal unit of the partridge river intrusion duluth complex minnesota usa many lines of evidence suggest that these sulfides are formed after assimilation of the proterozoic s rich black shales known as the bedded pyrrhotite unit in addition to s black shales are enriched in te as bi sb and sn tabs and the basaltic magma of the intrusion is contaminated by the partial melt of the black shales the tabs are chalcophile and together with the platinum group elements ni and cu partitioned into the magmatic sulfide liquid that segregated from the duluth magma the tabs are important for the formation of platinum group minerals pgm thus their role during crystallization of the base metal sulfide minerals could affect the distribution of the pge however the concentrations of tabs in magmatic ni cu pge deposits and their distribution among base metal sulfide minerals are poorly documented in order to investigate whether the base metal sulfide minerals host tabs in magmatic ni cu pge deposits a petrographic and laser ablation inductively coupled plasma mass spectrometry la icp ms study has been carried out on base metal sulfide and silicate phases of the partridge river intrusion duluth complex petrographic observations showed that the proportions of the base metal sulfide minerals vary with rock type the sulfide assemblage of the least metamorphosed bedded pyrrhotite unit from outside the contact metamorphic aureole consists of pyrite with minor pyrrhotite plus chalcopyrite 95 with small amount of chalcopyrite our la icp ms study shows that sulfides of the bedded pyrrhotite unit are rich in tabs consistent with these s rich black shales being the source of tabs that contaminated the mafic magma most of the tabs are associated with sulfides and platinum group minerals in the rocks of the bedded pyrrhotite unit from the contact aureole the bedded pyrrhotite unit xenoliths and the mafic rocks of the duluth complex in addition to these phases the laser maps show that silicate phases i e orthopyroxene and plagioclase contain sn and pb respectively in contrast in the least metamorphosed samples of the bedded pyrrhotite unit from outside the contact aureole although the pyrite contains some tabs mass balance calculations indicates that most the tabs are contained in other phases in these rocks galena hosts significant amounts of te bi sb sn and ag and few very small grains of sb rich phases were also observed the host phases for as were not established but possibly organic compounds may have contributed

trace element variations in olivine from the eastern deeps intrusion at voisey s bay labrador as a monitor of assimilation and sulfide saturation processes ni cu co sulfide mineralization in the mesoproterozoic voisey s bay intrusion is spatially and genetically associated with magmatic breccia zones within an extensive dike system and in particular with the entry point of this system into a larger intrusion eastern deeps intrusion the massive sulfide and breccia bearing inner basal margin of this upper intrusion is enveloped by weakly mineralized variable textured troctolite that progressively decreases in sulfide content contains fewer paragneiss fragments and eventually grades into a largely sulfide barren homogeneous plagioclase and olivine meso cumulate termed normal troctolite toward the top of the intrusion olivine from the troctolite lithologies the mineralized basal breccia sequence and the uppermost olivine gabbro in the eastern deeps intrusion was analyzed with secondary ion mass spectrometry sims we present multitrace element data including petrogenetically important elements v cr mn co ni zn sr and y which indicate that four petrological processes important to ore formation are recorded by the compositional diversity of olivine the heterogeneous ni co fo contents of olivine in the eastern deeps intrusion co ni ratios in olivine gabbro 0 18 1 1 normal troctolite 0 07 0 26 variable textured troctolite 0 11 0 29 basal breccia sequence 0 20 0 34 are due to variable modification of original compositions by reequilibration with trapped silicate and sulfide liquids the abrupt changes of fe fo79 fo66 and ni 2 500 to 80 ppm contents and some trace elements up to 1 5x more sr up to 3x more y than average normal troctolite olivine in olivine from intervals in the barren upper normal troctolite indicate that olivine crystallized from separate pulses of magma with variable degrees of differentiation the severe ni depletion of one troctolite melt batch 10 ppm ni vs 290 ppm ni for average normal troctolite magma is likely a result of sulfide saturation and segregation either in a deeper staging chamber or in the conduit prior or contemporary to olivine precipitation and emplacement the gradational increase of fe from fo80 to fo62 mn from 4 000 to 10 000 ppm and zn from 400 to 620 ppm in olivine from the weakly mineralized and contaminated lower part of the variable textured troctolite downward into the massive sulfide bearing and strongly contaminated basal breccia sequence is consistent with olivine fractionation from an increasingly country rock contaminated sulfide saturated silicate magma olivine here crystallized prior to or contemporaneously with the segregation of an immiscible sulfide liquid olivine grains that are enclosed by sulfide display homogeneous forsterite contents fo62 with pronounced intragrain trace element zonation mn ni co zn diffusion profiles mn ni indicate a relatively short immersion time 1 000 years of these olivine grains in sulfide liquid which likely resulted in the partial retention of ni in the olivine olivine in the vicinity of massive sulfides in the eastern deeps intrusion from variable textured troctolite to basal breccia sequence has a distinct minor and trace element signature v cr mn co ni zn compared to olivine from the barren troctolite and olivine gabbro a multiple regression analysis using these data yields an index that describes the relative vertical proximity of olivine to massive sulfides in conjunction with the stratigraphic context of the host lithology this information could potentially be used as a vector toward massive sulfide mineralization in the eastern deeps intrusion and also support ni exploration in other olivine bearing mafic intrusions with similar style of sulfide mineralization

segregation of magmatic fluids and their potential in the mobilization of platinum group elements in the south kawishiwi intrusion duluth complex minnesota evidence from petrography apatite geochemistry and coexisting fluid and melt inclusions abstract pegmatitic and other felsic rock pockets and dike like intrusions are abundant in the south kawishiwi intrusion of the duluth complex including the basal cu ni pge mineralized units these occurrences are found as pockets pods or as veins and contain abundant accessory apatite and quartz quartz hosts primary fluid inclusions as well as silicate melt inclusions combined microthermometry and raman spectroscopy helped to determine the bulk composition of primary fluid inclusions that are co 2 rich 95 mol and contain small amounts of h 2 o 4 5 mol ch 4 0 4 mol and trace n 2 respectively this combined technique also made it possible to measure total homogenization temperatures of the inclusions th tot 225 10 c otherwise not detectable during microthermometry silicate melt inclusions have been quenched to produce homogeneous glasses corresponding to the original melt composition of the entrapped melt is granitoid peraluminous and is very poor in mafic components we interpret the melt as a product of partial melting of the footwall rocks due to the contact effect of the south kawishiwi intrusion the presence of co 2 in the vapor bubbles of the quenched melt inclusions and petrographic evidence suggest that the fluid and melt inclusion assemblages are coeval the composition of the fluid and melt phase implies that the fluid originates from the mafic magma of the south kawishiwi intrusion and the fluid and melt phases coexisted as a heterogeneous melt fluid system until entrapment of the inclusions coexistence of primary fluid and melt inclusions makes it possible to calculate a minimum entrapment pressure 1 7 kbar and thus estimate formation depth 5 8 km for the inclusions chlorine is suggested to behave compatibly in the silicate melt phase in the fluid melt system represented by the inclusions indicated by the high up to 0 3 cl concentrations of the silicate melt and co 2 rich nature of the fluid apatite halogen contents provide further details on the behavior of cl apatite in pegmatitic pockets often has elevated cl concentrations compared to troctolitic rocks suggesting enrichment of cl with progressive crystallization systematic trends of cl loss at some differentiated melt pockets suggest that in some places cl exsolved into a fluid phase and migrated away from its source the segregation of cl from the melt is probably inhibited by the presence of co 2 rich fluids until the last stages of crystallization increasing the potential for the development of late stage saline brines platinum group minerals are often present in microcracks in silicate minerals in late stage differentiated sulfide veinlets and in association with chlorapatite indicating the potential role of cl bearing fluids in the final distribution of pges

origin of seismic reflections in a carbonate gas field lower miocene offshore myanmar abstract the geological origin of seismic reflections within a carbonate gas reservoir has been determined on the basis of an integrative analysis of cores well logs pre production and 4d time lapse seismic data the gas reservoir is subdivided into meter to decameter thick diagenetic units which are characterized by distinct acoustic properties low impedance units are dominant and consist of highly microporous and vuggy foraminiferal coralline algal floatstones intervals whose porosity has been significantly enhanced in burial diagenetic environments high to moderate acoustic impedance units thickness 1 20 m are related to various depositional and diagenetic fabrics 1 early lithified coral floatstones 2 foraminiferal rudstone whose intergranular space is occluded by shallow burial sparry calcite and 3 carbonate breccia with neomorphosed corals occurring below an intra formational subaerial exposure surface within the gas zone seismic reflection horizons may form at the boundary between two diagenetic units with contrasted acoustic impedance or may result from the interference between reflection at base and top of such intervals a stratigraphy cross cutting reflection horizon has been shown to form at a major and laterally continuous shift in porosity which is related to a differential diagenetic evolution between the gas and the water zone during or after hydrocarbon emplacement finally the analysis of the geological origin of seismic reflections has been proved to help predicting and mapping the major heterogeneities with significant impact on flow during production

carbonate ramp depositional systems abstract the classification tectonic settings stratigraphy and early diagenesis of carbonate ramp systems are reviewed carbonate ramps are common in all geological periods but were dominant at times when reef constructing organisms were absent or inhibited ramps can be subdivided into inner mid and outer ramp environments the mid ramp zone extends from fair weather wave base to normal storm wave base although the water depths which these boundaries represent vary an additional outer slope environment occurs on distally steepened ramps as with siliciclastic shelves a range of wave storm and tide dominated ramps can be recognised and this forms the most convenient basis for ramp classification the carbonate productivity profile of ramps differs from that of rimmed shelves with the inner ramp showing lower production rates than comparable shallow water facies on rimmed shelves the zone of greatest organic carbonate sediment production appears to have shifted from the mid ramp to the inner ramp since the late jurassic carbonate ramps occur in most types of sedimentary basin but are best developed where subsidence is flexural and gradients are slight over large areas as in foreland and cratonic interior basins and along passive margins the featureless depositional profiles of many ramps means that sequence geometries are best observed on regional seismic lines some show low angle sigmoidal or shingled clinoforms suggesting that ramps may seldom be homoclinal but possess subtle slope geometries which reflect depositional environments because of their low angle slopes ramps respond differently to rimmed shelves during relative sea level changes although results seem to be strongly dependent on the rate of relative sea level change during a minor fall shallow ramp facies belts will simply shift basinwards in a forced regression in contrast the whole surface of a steep sloped flat topped rimmed shelf may be exposed so that sediment production ceases or is drastically reduced during a major fall shallow ramp bounded basins may empty completely conversely ramps also flood gradually whereas rimmed shelves do so more rapidly homoclinal ramps develop no resedimented lowstand deposits rimmed shelves and distally steepened ramps in contrast may develop lowstand talus or turbiditic wedges distally steepened ramps may behave more like homoclinal ramps during minor base level falls and like rimmed shelves during major base level falls many ramps consist of layered successions of several ramp sequences stacked one upon the other ramp stacks of this sort may show gross vertical accretion but individual ramp sequences seldom appear to develop in a keep up style apart from minor organic buildups as with many rimmed shelves steepening of the outer ramp margin due to tectonism slope inheritance or differential sedimentation may promote the development of a distally steepened ramp or rimmed shelf a wide variety of organisms have constructed buildups in mid and outer ramp environments isolated buildups may seed early in ramp development accrete to wave base or sea level and continue growth by stacking through successive ramp sequences so that depositional and diagenetic features within them are in concert with those of the shallow ramp the location of isolated buildups on ramps is governed by tectonism halokinesis antecedent topography or by the subtle slope geometry of the previous ramp sequence diagenesis on ramps shows some major variations compared with diagenesis on steep sloped flat topped carbonate platforms ramp bounded basins may form prolific petroleum sourcing and reservoiring systems and offer a range of subtle stratigraphic play types and lateral facies variations which determine reservoir quality and distribution isolated buildups in the mid and outer ramp environments represent one of the commonest petroleum reservoirs in ramp systems and tend to have their foundations in transgressive systems tracts grainstone and packstone reservoirs are widespread and range from shoreline carbonate sandbodies to major detached shoal complexes or shoals over offshore highs grainstone sandbodies occur in both highstand systems tracts and in prograding lowstand wedges

highstand versus lowstand deposition on carbonate platform margins insight from quaternary foreslopes in the bahamas abstract examples of both highstand and lowstand deposition have been identified on the upper platform slopes of the tongue of the ocean bahamas highstand deposits consist of a thick wedge of fine grained skeletal and non skeletal sands and carbonate muds derived predominantly from the platform and deposited on slopes of 25 28 these sediments were deposited subsequent to platform flooding and may be more than 30 m thick along open leeward margins in contrast lowstand deposits consists of coarse skeletal sands gravels and boulders deposited on primary depositional slopes of 35 45 these sediments were derived from fringing reefs along the marginal escarpment and deposited during the initial rise of sea level following the last lowstand these variations in depositional character result in a distinctive signature of highstand versus lowstand deposition for steep sided platforms and further elucidate the response of carbonate platforms to high magnitude 5th order scale 20 000 yrs fluctuations in sea level

boiling and vertical mineralization zoning a case study from the apacheta low sulfidation epithermal gold silver deposit southern peru the au ag pb zn apacheta deposit is located in the shila district 600 km southeast of lima in the cordillera occidental of arequipa province southern peru the vein mineralization is found in early to middle miocene calc alkaline lava flows and volcanic breccias both gangue and sulfide mineralization express a typical low sulfidation system assay data show element zoning with base metals enriched at depth and higher concentrations of precious metals in the upper part of the veins three main deposition stages are observed 1 early pyrite and base metal sulfides with minor electrum 1 and acanthite 2 brecciation of this mineral assemblage and cross cutting veinlets with subhedral quartz crystals mn bearing calcite and rhombic adularia crystals and finally 3 veinlets and geodal filling of an assemblage of tennantite tetrahedrite colorless sphalerite 2 galena chalcopyrite electrum 2 fluid inclusions in the mineralized veins display two distinct types aqueous carbonic liquid rich lw c inclusions and aqueous carbonic vapor rich vw c inclusions microthermometric data indicate that the ore minerals were deposited between 300 and 225 c from relatively dilute hydrothermal fluids 0 6 3 4 wt nacl the physical and chemical characteristics of the hydrothermal fluids show a vertical evolution with in particular a drop in temperature and a loss of h2s the presence of adularia and platy calcite and of co existing liquid rich and vapor rich inclusions in the ore stage indicates a boiling event strong h2s enrichment in the vw c inclusions observed at 200 m the abundance of platy calcite and the occurrence of hydrothermal breccia at this level may indicate a zone of intense boiling the vertical element zoning observed in the apacheta deposit thus seems to be directly related to the vertical evolution of hydrothermal fluid characteristics precious metal deposition mainly occurred above the 200 m level below the present day surface in response to a liquid vapor phase separation due to an upward boiling front

hydrothermal alteration and cu ni pge mobilization in the charnockitic rocks of the footwall of the south kawishiwi intrusion duluth complex usa in the neoarchean 2 7 ga contact metamorphosed charnockitic footwall of the mesoproterosoic 1 1 ga south kawishiwi intrusion of the duluth complex the primary metamorphic mineral assemblage and cu ni pge sulfide mineralization is overprinted by an actinolite chlorite cummingtonite prehnite pumpellyite quartz calcite hydrothermal mineral assemblage along 2 3 cm thick veins in calcite hosted by the hydrothermal alteration zones and in a single recrystallized quartz porphyroblast four different fluid inclusion assemblages are documented the composition of these fluid inclusions provide p t conditions of the fluid flow and helps to define the origin of the fluids and evaluate their role in the remobilization and reprecipitation of the primary metamorphic sulfide assemblage pure co2 fluid inclusions were found as early inclusions in recrystallized quartz porphyroblast these inclusions may have been trapped during the recrystallization of the quartz during the contact metamorphism of the footwall charnockite in the footwall of the ski the estimated trapping pressure 1 6 2 0 kbar and temperature 810 920 c conditions correspond to estimates based on felsic veins in the basal zones of the south kawishiwi intrusion fluid inclusion assemblages with co2 h2o nacl and ch4 n2 h2o nacl compositions found in this study along healed microfractures in the recrystallized quartz porphyroblast establish the heterogeneous state of the fluids during entrapment the estimated trapping pressure and temperature conditions 240 650 bar and 120 150 c for co2 h2o nacl inclusions and 315 360 bar and 145 165 c for ch4 n2 h2o nacl inclusions are significantly lower than the p t conditions 700 c and 1 6 2 kbar during the contact metamorphism indicating that this fluid flow might not be related to the cooling of the duluth complex and its contact aureole the presence of chalcopyrite inclusions in these fluid inclusions and in the trails of these fluid inclusion assemblages confirms that at least on local scale these fluids played a role in base metal remobilization no evidences have been observed for pge remobilization and transport in the samples the source of the carbonic phase in the carbonic assemblages co2 ch4 could be the graphite present in the metasedimentary hornfelsed inclusions in the basal zones of the south kawishiwi intrusion the hydrothermal veins in the charnockite can be characterized by an actinolite cummingtonite chlorite prehnite pumpellyite calcite i ii quartz mineral assemblage chlorite thermometry yields temperatures around 276 308 c during the earliest phase of the fluid flow in the late calcite ii phase high salinity 21 6 28 8 nacl cacl2 equiv wt low temperature 90 160 c primary aqueous inclusions were found chalcopyrite sphalerite millerite replacing and intersecting the early hydrothermal phases are associated to the late calcite ii phase the composition of the formational fluids in the canadian shield is comparable with the composition of the studied fluid inclusions this suggests that the composition of the fluids did not change in the past 2 ga and base metal remobilization by formational fluids could have taken place any time after the formation of the south kawishiwi intrusion sulfur isotope studies carried out on the primary metamorphic 34s 7 4 8 9 and the hydrothermal sulfide mineral assemblage 34s 5 5 5 7 proves that during the hydrothermal fluid flow the primary metamorphic ores were remobilized

roles of xenomelts xenoliths xenocrysts xenovolatiles residues and skarns in the genesis transport and localization of magmatic fe ni cu pge sulfides and chromite abstract most sulfide rich magmatic ni cu pge deposits form in dynamic magmatic systems by partial melting s bearing wall rocks with variable degrees of assimilation of miscible silicate and volatile components and generation of barren to weakly mineralized immiscible fe sulfide xenomelts into which ni cu co pge partition from the magma some exceptionally thick magmatic cr deposits may form by partial melting oxide bearing wall rocks with variable degrees of assimilation of the miscible silicate and volatile components and generation of barren fe ti oxide xenocrysts into which cr mg v ti partition from the magma the products of these processes are variably preserved as skarns residues xenoliths xenocrysts xenomelts and xenovolatiles which play important to critical roles in ore genesis transport localization and or modification incorporation of barren xenoliths autoliths may induce small amounts of sulfide chromite to segregate but incorporation of sulfide xenomelts or oxide xenocrysts with dynamic upgrading of metal tenors pge cu ni co and cr v ti respectively is required to make significant ore deposits silicate xenomelts are only rarely preserved but will be variably depleted in chalcophile and ferrous metals less dense felsic xenoliths may aid upward sulfide transport by increasing the effective viscosity and decreasing the bulk density of the magma denser mafic or metamorphosed xenoliths may also increase the effective viscosity of the magma but may aid downward sulfide transport by increasing the bulk density of the magma sulfide wets olivine so olivine xenocrysts may act as filter beds to collect advected finely dispersed sulfide droplets but other silicates and xenoliths may not be wetted by sulfides xenovolatiles may retard settling of or in some cases float dense sulfide droplets reactions of sulfide melts with felsic country rocks may generate fe rich skarns that may allow sulfide melts to fractionate to more extreme cu ni rich compositions xenoliths xenocrysts xenomelts and xenovolatiles are more likely to be preserved in cooler basaltic magmas than in hotter komatiitic magmas and are more likely to be preserved in less dynamic less turbulent systems domain phases than in more dynamic more turbulent systems domains phases massive to semi massive ni cu pge and cr mineralization and xenoliths are often localized within footwall embayments dilations jogs in dikes throats of magma conduits and the horizontal segments of dike chonolith and dike sill complexes which represent fluid dynamic traps for both ascending and descending sulfides oxides if skarns residues xenoliths xenocrysts xenomelts and or xenovolatiles are present they provide important constraints on ore genesis and they are valuable exploration indicators but they must be included in elemental and isotopic mass balance calculations

carbonate platform flanks slope angle and sediment fabric more than 20 examples of fossil carbonate platform systems were compared for slope angle and sediment fabric plots of slope angle versus sediment fabric show that grainy non cohesive mud free sediments build steeper slopes than muddy cohesive sediments examples near the end members of grainy and muddy carbonate platform flanks are found in the triassic of the dolomites in northern italy and in the bahamas respectively they document the flank geometry and the processes readjusting the slope profile once the limiting slope angle is exceeded the grainy flank sediments in the dolomites modified by shearing and avalanching produce straight slope profiles with declivities up to 35 whereas the muddy bahamian flank sediments modified by large scale creep and rotational to translational sliding and slumping produce a concave upwards slope profile inclined at less than 4 the comparison between slope angle and sediment fabric indicates that the physical behaviour of sediments in the gravity field angle of shearing and mode of readjustment processes is linked to the composition of the slope sediment among the variables such as sea level subsidence climate plate motion and oceanographic setting windward leeward sediment fabric is suggested to be a major if not the major control on slope angle and slope curvature of carbonate platform flanks besides the recently documented tendency of carbonate sediments to build steeper slopes than siliciclastics this proposed relation sheds new light on the analysis and quantification of the variables influencing the geometry and depositional evolution of carbonate systems furthermore it provides an opportunity to deduce sediment composition from seismic lines and predict lithology prior to drilling

biogeographic impact of the leeuwin current in southern australia since the late middle eocene abstract the leeuwin current can be tracked from the western margin of the australian continent to the southern margin by the record of fossilized organisms typical of warm water marine environments this transport smudges the normal latitudinal asymmetry in the biotas of opposing oceanic and continental margins in which the eastern margins of oceans are cooler than the western margins and warmer biotas are restricted to lower latitudes in the east the most comprehensive record is in the large benthic foraminifera although fossils of benthic invertebrates nektonic nautiloids and planktonic protists are also informative in addition organic biomarker hydrocarbons in stranded bitumens and resins demonstrate that they have travelled the same route from their ultimate sources in cenozoic sedimentary basins and modern tropical rainforests of the indonesian archipelago the earliest spoor of the current is in the later middle eocene at which time the part deflection of counter gyral circulation in the indian ocean to the southeast was stimulated by the accelerated opening of the oceanic gap between australia and antarctica thus the origin of the leeuwin current is twice the age of the previously suggested origin in the miocene the current turns on and turns off in the great australian bight during the late quaternary in concert with the interglacials and the glacials at scales of 10 5 yr the switch can be seen in the faunal succession of planktonic foraminifera which are consistent with the neritic benthic faunas of the central gulfs both communities show that at the warm peak of the last interglacial the current transported biota across the bight more strongly than it has during the holocene the cenozoic record of the past 40 ma is in the same mode at 10 6 yr scales the relevant fossils are found concurrently at major marine transgressions and warming reversals of the overall fall in global temperature however the fossil pattern is due to transport on the activated leeuwin current not merely to general warming and the spread of friendly environments to higher latitudes

sources of periplatform carbonates northwest providence channel bahamas abstract sources and quantities of periplatform carbonate sediments have been determined in northwest providence channel bahamas by an end member method employing geochemical textural petrographic and sem data the holocene sediment over most of this open seaway 200 to 2 000 m deep between great bahama bank and little bahama bank is a drape of largely fine grained lime mud 50 cm thick shallow platform sources contribute 75 90 percent the remainder is planktonic foraminifera pteropods and coccoliths mud 62 m from each bank and the planktonic source are each geochemically distinct and can be quantified the two shallow platform sources dominate sedimentation in the deep basin trend surface maps of mud derived from each bank reveal no windward leeward asymmetry in cont ast to reported sand transport patterns this indicates that the fines are swept off both platforms by storm and tidal exchange concentric distribution of fines relative to source indicates no offset by currents suggesting that fecal pelleting must rapidly remove fine sediment from the water column canyon valleys and intercanyon highs have equal thicknesses of holocene sediment which indicates that pelagic processes of deposition presently dominate over gravity flow processes confirmation of this depositional process is that the deposition rate increased from 2 cm 1 000 years to 10 cm 1 000 years when postglacial sea level flooded the bank tops thus 80 percent of the present sedimentation rate results from bank top contribution furthermore the calculated mass of mud traceable to little bahama bank agrees excellently with the independent estimate of overproduction there in cores bank sediments resulting from the most recent sea level highstand sharply overlie planktonic sediments writing a sea level history in alternating stable and unstable mineral suites

petrogenesis of pelitic xenoliths at the babbitt cuni deposit duluth complex minnesota u s a abstract the babbitt deposit consists of disseminated cu fe ni sulfides found within mafic rocks of the duluth complex generally near contacts with underlying metasedimentary rock types host rocks for the deposit include troctolites olivine gabbros gabbronorites norites and occasionally country rock hornfels xenoliths of country rocks are abundant in the deposit and suggest a relationship between sulfide mineralization and country rock contamination country rocks in the babbitt area include those of the middle precambrian biwabik iron formation and both calcareous and non calcareous pelites of the virginia formation xenoliths contain the assemblage cordierite plagioclase biotite orthopyroxene and are thought to have been derived from virginia formation protoliths comparison of protoliths and xenoliths using composition volume element ratio and mass balance techniques suggests that xenoliths have been strongly depleted in volatiles alkalis and si footwall rocks show only a depletion in volatiles neither fluid phase transport nor diffusion through an intergranular fluid can account for the mass of material transferred extensive partial melting of xenoliths with residual enrichment of feo mgo and al2o3 is the most viable transfer process the lack of sio2 concentration gradients around xenoliths and anomalous igneous rock compositions suggest that desilicification occurred at a time when physical mixing of extracted partial melt and host magma was possible and prior to final emplacement sulfide saturation may have been initiated due to si assimilation with an auxiliary magma chamber however the composition of ores in the babbitt deposit is consistent with saturation being achieved by addition of sediment derived volatile sulfur independent of major element assimilation

holocene sediment dynamics on a cool water carbonate shelf otway southeastern australia the otway shelf is covered by cool ocean waters and veneered by bryozoan dominated carbonate sediments radiocarbon dating and stratigraphy of shelf vibracores and slope gravity cores document late pleistocene holocene deposition shelf sediments of the late pleistocene highstand 60 26 ky are rare either never having been deposited or having been removed during the following sea level fall 26 17 ky during the subsequent lowstand the shelf was exposed facies shifted basinward and beach dune complexes were constructed near the shelf edge the deep shelf was characterized by nondeposition and hardground formation and the shelf margin became locally erosional upper slope bryozoan sponge assemblages continued to grow actively and lower slope foraminifera and nannofossil ooze was increasingly enriched in hemipelagic terrigenous mud swept off the wide shelf coarse shelf debris and lowstand dune sands were erosively reworked and transported onto the upper slope and redistributed to deep slope aprons during early transgression 17 10 ky this event locally terminated shelf edge bryozoan and sponge growth continued erosive shoreface retreat formed a bored and cemented ravinement surface mantled by bivalves and lithoclasts across the open shelf base level rise reduced the amount of terrigenous material reaching the continental slope slower transgressive flooding from 10 to 6 5 ky allowed infilling of shallow embayments on the shelf and outer shelf sediment accumulation began by 7 ky sea level stabilized near its present level by 6 5 ky and the area is now partitioned into three zones by an energetic wave and swell regimen 1 inboard protected embayments and or beach dune complexes with high sediment accumulation rates about 100 cm ky 2 an open shelf where although sediment production is active accumulation is patchy 3 5 cm ky to nonexistent within the zone of wave abrasion and minimal about 23 cm ky between maximum abrasion depth and swell wave base and 3 a deep shelf edge and upper slope where sediment is accumulating at rates of 2 50 cm ky interpreted deep water 100 m bryozoan sponge mounds are growing at rates of 105 cm ky accumulation decreases progressively down slope with deep slope pelagic carbonate sedimentation at rates of 5 cm ky the late quaternary shelf record resembles that of flat topped warm water platforms with holocene sediment overlying pleistocene tertiary limestone but for different reasons much of this high energy shelf is still within lag depth more than 10 ky after flooding the steady state deep water shelf margin bryozoan factory has been largely unaffected by 50 100 m eustatic changes in sea level and has contributed to slow but continuous platform progradation the slow growth potential uniform profile of sediment production and distribution and inability of constituent organisms to construct rigid frameworks favor maintenance of a shallow ramp profile and makes the cool water carbonate system an excellent modern analog for interpretation of many ancient ramp successions

sonic velocity submarine canyons and burial diagenesis in oligocene holocene cool water carbonates gippsland basin southeast australia burial diagenetic processes largely control sonic velocity and porosity destruction in the oligocene holocene seaspray group carbonates of the gippsland basin extensive cementation of the carbonates begins at around 300 500 m burial depth and most macroporosity is filled by calcite cement at a burial depth of around 1 5 km sonic velocity data from well logs are strongly correlated with the burial diagenetic processes operating in the carbonates above 300 500 m burial depth sonic velocity increases rapidly with depth with porosity loss being dominated by mechanical compaction below 500 m burial depth the sonic velocity increases at a lesser rate with porosity loss being dominated by pressure solution and calcite cementation at these greater depths sonic velocity and therefore porosity is a function of burial depth carbonate content and to a lesser extent sediment age lateral sonic velocity variations within the seaspray group are associated with submarine canyon facies the submarine canyons are filled by sediments that have a higher carbonate content than the surrounding succession during burial these carbonate rich canyon fill sediments are more rapidly cemented by calcite producing higher sonic velocities than the surrounding strata the resultant velocity variations cause problems in time depth conversion

sulfide saturation in mafic magmas is external sulfur required for magmatic ni cu pge ore genesis the importance of externally derived sulfur in the genesis of sulfide rich magmatic ni cu platinum group element pge deposits remains a key yet unresolved issue calculations utilizing a variety of mafic magma types indicate that in general sulfide saturation by fractional crystallization occurs after ni has been depleted due to olivine crystallization cu and pge rich layers may form during relatively later stages of closed system crystallization but unless the collection of the cotectic proportion of immiscible sulfide is extremely efficient the mass of sulfide is too small to produce an economic deposit we show that there are numerous processes that may lead to early sulfide saturation in mafic ultramafic magmas contamination of mantle derived magmas by siliceous country rocks or their partial melts will lower the sulfur content needed to induce sulfide liquid saturation typically by amounts ranging from 200 to 700 ppm the mixing of magmas particularly if the result is to lower the liquidus temperature of the mixed magma may also lower the sulfur content needed to attain sulfide saturation by similar amounts an increase in magma fo2 related to the addition of volatiles such as h2o and co2 is less effective in decreasing the sulfur concentration needed to achieve sulfide liquid saturation contamination processes that lead to an increase in the activity of sio2 in the melt and hence may promote orthopyroxene rather than olivine crystallization aid in generating relative ni enrichment in remaining liquid as a result of the lower dni mineral melt value of orthopyroxene relative to olivine although contamination and magma mixing may produce early sulfide saturation without the addition of externally derived sulfur ni rich sulfide deposits can form in such cases only from large volume open systems where the efficiency of sulfide collection is high no matter what the liquidus minerals may be without the addition of country rock derived sulfur the mass of sulfide necessary to generate economic ni cu pge concentrations requires efficient sulfide collection from large but not necessarily unrealistic volumes of magma small deposits 4 30 mt of sulfide may form from the collection of cotectic proportions of sulfide from less than 50 km3 of magma larger deposits such as those at noril sk could involve more than 200 km3 of magma this volume of magma is not unreasonable particularly in rift plume related settings despite such possibilities sulfur isotope data clearly indicate that externally derived sulfur has been involved in the formation of many large deposits and that collection of mantle derived sulfide in sufficient quantities to produce orebodies is a rare process we propose that magmatic ni cu pge sulfide ore formation normally requires significant sulfide supersaturation and that the addition of sulfur derived from xenoliths is the most viable mechanism for producing sulfide well above the cotectic proportion in mafic ultramafic magmas

biocalcarenite and mixed cool water prograding bodies of the mediterranean pliocene and pleistocene architecture depositional setting and forcing factors

geology sulfide mineralogy and petrogenesis of the angstberg ni cu pge sulfide mineralization lausitz block bohemian massif germany a potential ni cu exploration target in central europe abstract germany s largest known magmatic ni cu pge sulfide enrichment is associated with the gabbroic angstberg intrusion that is one of several small scaled ni cu sulfide bearing intrusions in the northern bohemian massif central european variscan belt the intrusions are part of a middle devonian to early carboniferous mafic ultramafic magmatic system that emplaced within cadomian granodiorites of the lausitz block the angstberg intrusion represents an exposed sub vertical dike like body 320 60 m in plan view and comprises olivine gabbronorite olivine gabbro gabbro olivine hornblende gabbro and hornblende pyroxene gabbro the hornblende bearing and hornblende free lithologies originate from at least two variably fractionated basaltic parental magma batches which differ in their ti lree hree and volatile contents fractionation probably took place in deeper seated sections of the magmatic system possibly within staging magma chamber s overall enriched incompatible trace element patterns suggest that the angstberg lithologies are linked with intra plate magmatism likely associated with a transtensional setting in the course of the variscan orogeny the previously suggested devonian age of the intrusion is confirmed by an 40ar 39ar biotite age of 373 4 0 9 ma for the olivine gabbro the magmatic sulfide mineralization is predominated by disseminated sulfides that are hosted in the olivine richest parts of the olivine gabbronorite and olivine hornblende gabbro lithologies in addition some sulfide matrix breccias up to 70 modal sulfides occur directly on the footwall contact of the intrusion to granodiorite both mineralization types are characterized by a predominance of pyrrhotite over pentlandite and chalcopyrite with minor secondary violarite and pyrite and accessory pge au and ag phases nickel rich bismuthotellurides of the melonite merenskyite solid solution series and froodite are the most common platinum group minerals disseminated sulfides are characterized by high ni moderate cu and low pt and pd tenors 4 9 wt ni 2 6 wt cu 200 800 ppb pt 200 1000 ppb pd sulfide matrix breccias feature comparable ni but lower cu pt and pd tenors 4 6 wt ni 0 3 1 wt cu 10 100 ppb pt 40 200 ppb pd fractional crystallization and wall rock assimilation of granodiorite are regarded as key factors for sulfide saturation in the magmas associated with the angstberg intrusion sulfur isotope compositions of pyrrhotite pentlandite chalcopyrite assemblages 1 4 to 0 2 vcdt propose a mantle origin of s whereas addition of crustal s cannot be completely ruled out as the s isotope values of the sulfides overlap with those of the granodiorite 7 to 4 vcdt olivine and sulfide textures suggest that disseminated sulfides likely originate from sulfide melts that were entrained in an olivine rich crystal mush during magma rise to upper levels the sulfide matrix breccias also resulted from entrained sulfide melts or from gravity driven downward percolation of sulfide melts from stratigraphically higher now eroded levels in both cases the formation of sulfide matrix breccias most likely was amplified by in situ thermomechanical erosion of the granodioritic wall rock caused by flash boiling of thermally dehydrating biotite despite spatial limitations of the mineralized zones and limited information about the architecture of the intrusions in the northern bohemian massif the angstberg intrusion as well as other neighboring magmatic sulfide occurrences may represent a new exploration target for ni and cu in central europe

petrogenesis of cenozoic temperate water calcarenites south australia a model for meteoric shallow burial diagenesis of shallow water calcite sediments abstract the gambler limestone is one of several extensive shallow water shelf carbonates of eocene to miocene age exposed along the southern margin of australia it is a muddy to grainy bryozoan calcarenite with accessory benthonic foraminifers and echinoids the sediments originally composed almost entirely of calcite or mg calcite have been in vadose and phreatic environments for over 10 ma yet are virtually unlithified the only cements of any consequence are epitaxial on echinoids numerous karst features dolines caves speleothems and surface karren attest to prolonged residence in the meteoric zone the gambler limestone is presently one of the best fresh water aquuifers in australia most flow occurs through intergranular pores in sediments with over 30 porosity cementation is by minor intergranular pressure solution which has developed under overburden of less than 100 m the overlying naracoorte limestone miocene a calcarenite of warmer water aspect contains numerous aragonite molds and is well cemented we propose that such cool water limestones are a better model for the meteoric diagenesis of calcite sediments of all ages than are aragonite rich tropical sediments it is probable that many similar early and middle paleozoic calcite limestones may have been in the meteoric zone for prolonged periods yet contain little or no petrographic or geochemical record of such exposure

re os sm nd and pb isotopic constraints on mantle and crustal contributions to magmatic sulfide mineralization in the duluth complex abstract previous petrologic and stable isotopic studies of sulfide mineralization in the duluth complex have led to the premise that sulfide genesis is strongly linked to the interaction between mantle derived magmas and sedimentary country rocks in a rift zone environment in order to more fully evaluate the nature of this interaction and to gain an insight into the possible importance of externally derived metals in the ore forming process pb sm nd and re os isotopic studies of the babbitt cu ni deposit were initiated rock types examined include low sulfide troctolite that show very little petrologic evidence for contamination disseminated sulfide bearing troctolitic to gabbroic rocks that occur close to the basal contact with country rocks and contain metasedimentary xenoliths and massive sulfide lead isotopic values of whole rocks plagioclase mineral separates and massive sulfides show only subtle differences and are compatible with 3 to 5 contamination of a mantle derived melt with a proterozoic crustal contaminant e nd 1 1 ga values of the troctolite and gabbro samples are chondritic and only the massive sulfides show strong evidence for contamination based on sm nd isotopic values massive sulfides tend to be more sensitive indicators of contamination in the sm nd system because of late stage incorporation of a light rare earth element rich fluid into a ca po 4 apatite component of the immiscible sulfide liquid os 1 1 ga values are also strongly anomalous and range from 500 to 1200 in disseminated sulfide bearing troctolites and massive sulfides these values are also consistent with from 1 to 3 contamination by c rich and strongly radiogenic proterozoic sedimentary rocks elemental and isotopic mass balance calculations suggest that up to 50 of the pb and at least 35 of the os in the sulfide mineralization have been derived from external sources a corollary is that other metals may also be in part derived from external sources which would aid in explaining the compositional diversity of the sulfide mineralization selective assimilation of country rocks by mantle derived magmas may occur in crustal staging chambers during ascent to shallower levels or in situ as a result of devolatilization reactions and partial melting

origins of calcite in a boiling geothermal system the formation of hydrothermal calcite relates to the movement of carbon dioxide in a geothermal system as governed by boiling dilution and condensation in this paper we show how these processes control the occurrence distribution and stable isotope composition of calcite based on a study at broadlands ohaaki the two principal calcite occurrences in the broadlands ohaaki geothermal system are 1 as replacement of rock forming minerals and volcanic glass and 2 as platy crystals infilling voids both are stable over a broad temperature range from 160 degrees c to 300 degrees c replacement calcite is widespread and forms through hydrolysis reactions involving calcium alumino silicates and sub boiling liquids that contain 0 3 to 0 75 m co 2 platy calcite in contrast forms over a restricted vertical interval of a few hundred meters within the upflow zone it precipitates from boiling fluids through exsolution of carbon dioxide as indicated by coeval liquid rich and vapor rich fluid inclusions and its formation in the two phase zone fluid inclusion data help to define the boiling paths of fluids from which platy calcite formed homogenization temperatures range from 160 degrees c to 310 degrees c and are consistent within the present geothermal regime ice melting temperatures range from 0 0 to 1 0 degrees c and indicate the presence of up to 0 5 m dissolved carbon dioxide model boiling curves calculated to match these data show how the concentration of dissolved carbon dioxide in the preboiled fluid dictates the depth of first boiling most fluid inclusion data lie along a model boiling path characteristic of the centre of the upflow zone in which the rising fluid initially containing 0 75 m co 2 begins to boil at approximately 320 degrees c and approximately 2000 m depth data from well br 18 instead matches a curve in which the rising fluid initially containing 0 53 m co 2 begins boiling at approximately 245 degrees c and approximately 900 m depth the shallowing of the depth of first boiling likely results from dilution of dissolved carbon dioxide in the parent chloride water as it rises and mixes with marginal waters calcite precipitates from both shallow formed steam headed groundwater and deeply derived chloride water and these waters are isotopically distinct at broadlands ohaaki the delta 18 0 values of calcite at 200 degrees c range from 0 5 to 7 5 per mil whereas delta 18 0 values of calcite at 200 degrees c range from 4 to 10 per mil taking appropriate temperature dependent fractionation factors into account these data indicate equilibration with chloride water delta 18 0 h2o 4 5 per mil and steam heated groundwater delta 18 o h2o 7 0 per mil respectively oxygen isotopes of hydrothermal calcites in the nearby wairakei and waiotapu geothermal systems show similar patterns consistent with the occurrence of both chloride and steam heated waters there calcite formation is explained by a model that describes the distribution of two phase conditions and aqueous carbon dioxide concentrations in a column of hydrothermal fluid rising through a rock matrix of isotropic permeability in this ideal situation platy calcite forms along the inner margin of the two phase zone having the shape of an inverted cone whereas replacement calcite mostly forms in the surrounding one phase liquid only zone the sparse occurrence of calcite at less than or equal to 800 m depth in the central upflow of the ohaaki sector at broadlands ohaaki is compatible with this model and appears related to the exsolution of dissolved carbon dioxide through boiling deeper in the system

underground fumaroles excess heat effects in vein formation we examine the heat extracted from a fractured hydrothermal system following the sudden dilation of fractures in the host rock this dilation increases the permeability of the system and veins form along irregular fractures where the opening is localized for simplicity we will consider one such vein in this paper fluid in the system is assumed to be initially close to vapor saturation and in thermal equilibrium with the surrounding host rock the rapid opening of the vein leads to approximately isenthalpic vapor separation and causes a pressure drop to propagate through the system as the two phase fluid in the fracture system is constrained to the vapor pressure curve the depressurization is accompanied by a rapid cooling of the fluid fluid flow is also induced from a hydrothermal reservoir toward the dilated discharge vein the temperature difference between the host rock and the two phase fluid in the fracture system drives heat from the rock into the fluid the excess heat 1 1 derived from the host rock in this manner evaporates additional liquid in the two phase flow which develops substantially larger vapor fractions than those due only to the rapid initial adiabatic depressurization of the fluid such additional vapor separation therefore leads to high solute concentrations in the liquid phase and potentially results in faster deposition of minerals in the fracture system and local veins a model is developed here to describe the minimum excess heat transfer i e the worst case scenario into the two phase saturated liquid vapor mixture as it is drawn horizontally from the hydrothermal reservoir toward the discharge vein excess heat effects are shown to be effective within tens to hundreds of meters of actively forming veins and a principal cause of oscillatory banding and lamination fabrics this transient heat extraction process applies to a wide range of shallow crustal deposit styles associated with brittle and brittle ductile deformation these deposit styles range from epithermal bonanza veins sea floor massive sulfide deposits mesothermal veins in shear zones to the higher temperature veins and stockworks of hypothermal and porphyry mineralization 1 fn 1

fluid inclusion petrography abstract a procedure of fluid inclusion studies is proposed with emphasis on the criteria of selecting fluid inclusions for detailed microthermometry and spectroscopic analysis an overview of descriptive and genetic classifications of fluid inclusions in single crystals and in massive rocks is given with the intention of further differentiating the commonly used terms primary and secondary fluid inclusions some principles of fluid inclusion modification are explained cathodoluminescence cl studies of quartz with the optical high power cl microscope and the electron microprobe provided with a cl detector are an important help in fluid petrography cl textures are subdivided in primary growth textures and a wide variety of secondary microtextures which are in part induced by fluid inclusions the latter is grouped in textures indicative of local lower crystal order increasing defect structures and microtextures indicative of local quartz healing reduction of the defect structures microtextures showing the genetic relationship between fluid inclusions and the host mineral provide information about the possible post entrapment changes of fluid inclusions and therewith testify their geological relevance

offbank transport of carbonate sands along open leeward bank margins northern bahamas abstract vigorous offbank transport of carbonate sands occurs along the west facing leeward open margins of little and great bahama bank large offbank oriented sand waves thick sand bodies covering reefs and shallow water sands mostly non skeletal obtained from adjacent deep flanks all demonstrate the existence of this shallow to deep pathway of sediment transport a variety of data suggests that transport occurs during storms and not during normal tidal current fluctuations an abundance of composite grains as well as vertical sequences of submarine cemented horizons seen via seismic profiling within the marginal sand bodies indicate cyclic periods of relative quiescence followed by moments of intense sediment movement current meter data obtained during six weeks of low wind activity show that critical threshold velocities for sand transport are only barely exceeded during portions of the tidal cycle these flows coupled with higher frequency bottom currents generated by surface gravity waves winnow the sands but do not provide significant net offbank transport offbank sand transport provides material to the shallow and deep bank margins enabling them to expand during sea level highstands each sea level fluctuation is likely to produce within the shallow margins a vertical sequence of basal reefs covered by a thick non skeletal calcarenite capped by a subaerially exposed diagenetically altered surface shallow water sands carried off the marginal escarpment help to construct deep flanks and large sediment drifts

effects of devolatilization on the hydrogen isotopic composition of pelitic rocks in the contact aureole of the duluth complex northeastern minnesota u s a abstract d values of unmetamorphosed chlorite plus muscovite bearing pelitic rocks of the virginia formation in northeastern minnesota range from 68 to 54 with h2o contents between 3 66 and 4 41 wt contact metamorphism by mafic magmas of the duluth complex has led to devolatilization and loss of 2 5 3 5 wt h2o accompanied by a decrease in whole rock d values minerals produced during metamorphism include cordierite biotite and locally orthopyroxene footwall hornfels at the dunka road cu 1bni deposit are characterized by d values between 91 and 59 with h2o contents between 2 52 and 1 10 wt xenoliths and footwall rocks at the babbitt cu 1bni deposit show a wide range in d 131 to 69 and h2o 0 50 2 63 values isotopic modelling of chlorite muscovite and biotite consuming reactions suggest that rayleigh fractionation during devolatilization may account for the trend of decreasing d and h2o content the computations indicate that the d and h2o data are consistent with the liberation of a fluid during contact metamorphism that contained up to 15 methane results are in excellent agreement with fluid compositions estimated from mineral equilibria and carbon isotopic data the average isotopic composition of the evolved fluid is 53 d values that are recorded in some xenoliths and footwall rocks at babbitt may be a result of retrograde hydration and isotopic exchange with a d depleted fluid of magmatic origin or may reflect variations in parent rock d value or chlorite muscovite ratio

high production and highstand shedding from deeply submerged carbonate banks northern nicaragua rise abstract because the shallow isolated carbonate banks on the northern nicaragua rise the nicaragua honduras and southern jamaica carbonate shelves and many other modern carbonate banks worldwide are covered by an average of 20 to 30 m of water and by a thin blanket of coarse carbonate sediments other carbonate sedimentologists have considered these banks good examples of incipiently drowned or even drowned carbonate banks however based on recent research on the northern nicaragua rise we can demonstrate that these banks currently are healthy producers of large volumes of periplatform sediments fine aragonite and magnesian calcite which are exported almost entirely to the deep surrounding slopes these sediments which were deposited during the past 9000 10 000 years form peripl tform wedges on the middle and upper slopes which are seen clearly on 3 5 khz seismic profiles radiocarbon ages of the wedge surface sediment range between 230 and 610 ybp and provide clear evidence for the contemporaneous production of sediments on the shallow bank and shelf and their instantaneous export to the upper slopes for the last 5000 years sedimentation rates ranged from 2000 mm ky off pedro bank to 1300 mm ky off jamaica these rates are somewhat lower than but of the same order of magnitude as sedimentation rates on the western leeward slopes of great bahama bank deposition of the metastable carbonate sediments discussed in this paper occurred as soon as sea level rose to flood the bank and shelf which flank walton basin following the last glacial lowstand they com rise a wedge shaped package of transgressive and highstand sediments on the upper and middle slopes of pedro bank and the southern jamaica shelf

multiple sulfur and iron isotope composition of magmatic ni cu pge sulfide mineralization from eastern botswana we report here high precision multiple sulfur and iron isotope compositions for a series of mineralized samples from ni cu pge sulfide deposits in the archean tati greenstone belt and the phikwe complex of eastern botswana mineralized samples from the phoenix and selkirk ni cu pge deposits in the tati greenstone belt display slightly positive 34s isotope values ranging from 0 2 to 0 8 v cdt 33s values of sulfides at phoenix and selkirk are 0 01 to 0 08 v cdt suggesting either a dominantly mantle sulfur source or effective eradication of a crustal 33s anomaly through equilibration with large amounts of silicate melt in the selebi phikwe belt a granite gneiss terrane with abundant amphibolite lenses of either volcanic and or intrusive nature mineralized lower grade samples from the phikwe phokoje and dikoloti ni cu pge deposits have more variable 34s values ranging from 3 1 to 0 3 and display significant mass independent anomalies 33s values ranging from 0 89 to 0 27 suggesting that barren sulfides associated with distal or low temperature sea floor hydrothermal activity contributed sulfur to these deposits iron isotopes of sulfides from these deposits show a relatively small range of negative 56fe values 0 29 to 0 04 consistent with high temperature fractionations in magmatic systems with the exception of one sample from the dikoloti ni cu pge deposit of the selebi phikwe greenstone belt which shows a more negative 56fe value of 0 61 consistent with assimilation of sedimentary or hydrothermal sulfides rather than fractionations in high temperature magmatic systems data from this study highlight the complexity and variability that characterize ore forming processes in magmatic systems we suggest that the presence of sulfur bearing lithologic units in host rocks of mafic and ultramafic intrusions may not be essential toward the assessment of the prospectivity of a province to host orthomagmatic nickel sulfides geologic settings without any or little sulfur in the stratigraphy which have been traditionally neglected in terms of their prospectivity should thus be revisited and possibly reassessed considering the potential importance of external source of sulfur to generate ore deposits

a method for restoring sedimentary sequence original structural profiles a case study of miocene strata from the northern continental slope of the south china sea abstract most sequence stratigraphy models have been derived from the observation of seismic profiles which still retain the original form of the sedimentary stage for folded and fractured formations there is no way to establish a full view of the basin wide continuous stratal configuration this paper presents an approach to restore the sedimentary sequence original structural profile through a process of segmented line drawing of seismic events reassembly and modification we use a case study from the pearl river mouth basin in the south china sea to demonstrate the principle and method of this technology after restoration we obtained a basin wide sedimentary stage form and continuous sequence stratigraphic structure profiles and thus further investigated the sedimentary landforms palaeo water depth sedimentary systems stacking pattern stratal configuration parasequence sets and systems tract which may be ambiguous in conventional seismic profiles this approach may be applied to the study of basin and sequence stratigraphy and basin sedimentary process

deciphering magmatic and metasomatic processes recorded by fluid inclusions and apatite within the cu ni pge sulfide mineralized bathtub intrusion of the duluth complex ne minnesota usa

seismic geomorphology and growth architecture of a miocene barrier reef browse basin nw australia abstract the cenozoic succession of browse basin is characterized by a carbonate system that developed from a non tropical ramp in eocene lower miocene times to a tropical rimmed platform in the middle miocene the evolution of the platform was unraveled through the interpretation of the seismic geomorphology and borehole data of the middle miocene tropical reef system the first reef structures developed during the early middle miocene as narrow linear reef belts with an oblique orientation with respect to shelf strike direction subsequently they prograded toward the platform margin to form a barrier reef with a minimum length of 40 km the barrier reef itself comprises three distinct ridges separated by progradational steps the second and third step are separated by a karstified horizon which is interpreted to represent the global sea level fall shortly before the serravallian tortonian boundary the following third ridge formed in a slightly downstepped position during the sea level lowstand and initial transgressive phase further sea level rise during the early tortonian first drowned the barrier reef system and subsequently also the patch reefs and relic atolls that had established in a backstepped position in the platform interior the similar evolution of the browse basin reef system and other contemporaneous carbonate systems indicates a strong impact of eustatic sea level changes relatively large subsidence rates in the study area possibly augmented the eustatic sea level rise in the tortonian and hence contributed to the drowning of the reef system however the initiation and final demise of the reef system was also governed by global and regional climate variations the first seismically defined reefs developed simultaneous to a maximum in the transport capacity of the indonesian throughflow which brings warm low salinity waters to the north west shelf reef drowning followed the restriction of this seaway close to the middle to early miocene boundary this near closure of the indonesian seaway possibly led to a regional amplification of the global middle to late miocene cooling trend and hampered the potential of the reef system to keep up with the rising sea level

benthic carbonate factories of the phanerozoic marine carbonate precipitation occurs in three basic modes abiotic or quasi abiotic biotically induced and biotically controlled on a geologic scale these precipitation modes combine to form three carbonate production systems or factories in the benthic environment 1 tropical shallow water factory dominated by biotically controlled mainly photo autotrophic and abiotic precipitates 2 cool water factory dominated by biotically controlled mainly heterotrophic precipitates and 3 mud mound factory dominated by biotically induced mainly microbial and abiotic precipitates sediment accumulations of the factories differ in composition geometry and facies patterns and some of these differences appear prominently in seismic data thus facilitating subsurface prediction the characteristic accumulation of the tropical factory is the flat topped often reef rimmed platform in cool water systems reefs in high energy settings are scarce and hydrodynamic influence dominates producing seaward sloping shelves and deep water sediment drifts often armored by skeletal framework the typical accumulation of the mud mound factory is groups of mounds in deeper water where the mud mound factory expands into shallow water it forms rimmed platforms similar to the tropical factory the tropical factory is most productive the mud mound factory reaches 80 90 and the cool water factory 20 30 of the tropical growth rate the three factories represent end members connected by transitions in space transitions in time are linked to biotic evolution

anatomy of a modern open ocean carbonate slope northern little bahama bank the open ocean carbonate slope north of little bahama bank consists of a relatively steep 4 upper slope between water depths of 200 and 900 m and a more gentle 1 2 lower slope between depths of 900 and 1300 m the upper slope is dissected by numerous small submarine canyons 50 150 m in relief that act as a line source for the downslope transport of coarse grained carbonate debris the lower slope is devoid of any well defined canyons but does contain numerous small 1 5 m hummocks of uncertain origin and numerous larger 5 40 m patchily distributed ahermatypic coral mounds sediments along the upper slope have prograded seaward during the cenozoic as a slope front fill seismic facies of fine grained peri platform ooze surface sediments show lateral gradation of both grain size and carbonate mineralogy with the fine fraction derived largely from the adjacent shallow water platform near surface sedimentary facies along the upper slope display a gradual downslope decrease in the degree of submarine cementation from well lithified hardgrounds to patchily cemented nodular ooze to unlithified peri platform ooze controlled by lateral variations in diagenetic potential and or winnowing by bottom currents submarine cementation stabilizes the upper part of the slope allowing upbuilding of the platform margin and controls the distribution of submarine slides as well as the headward extent of submarine canyons where unlithified sediments are heavily bioturbated and are locally undergoing dolomitization upper slope sediments are also conditioned eustatically resulting in vertical cyclic sequences of diagenetically unstable aragonite and magnesian calcite rich and stable calcite rich carbonates that may explain the well bedded nature of ancient peri platform ooze sequences lower slope sediments have prograded seaward during the cenozoic as a chaotic fill seismic facies of coarse grained carbonate turbidites and debris flow deposits with subordinate amounts of peri platform ooze coarse clasts are internally derived from fine grained upper slope sediments via incipient cementation submarine sliding and the generation of sediment gravity flows gravity flows bypass the upper slope via a multitude of canyons and are deposited along the lower slope as a wedge shaped apron of debris parallel to the adjacent shelf edge consisting of a complex spatial arrangement of localized turbidites and debris flow deposits a proximal apron facies of thick mud supported debris flow deposits plus thick coarse grained ta turbidites grades seaward into a distal apron facies of thinner grain supported debris flow deposits and thinner finer grained ta b turbidites with increasing proportions of peri platform ooze both the geomorphology and sedimentary facies relationships of the carbonate apron north of little bahama bank differ significantly from the classic submarine fan model as such a carbonate apron model offers an alternative to the fan model for palaeoenvironmental analysis of ancient open ocean carbonate slope sequences

depositional bias and environmental change important factors in sequence stratigraphy abstract differences among depositional systems here called depositional bias strongly influence sequence patterns siliciclastics and shallow water carbonates for instance shed most of their sediment during opposite phases of a sea level cycle lowstand shedding and highstand shedding respectively furthermore the two systems generate their own system specific relief on the sea floor disperse their sediment load along different avenues and differ in the way they are deactivated reefs and carbonate platforms can be drowned whereas siliciclastic deposition can be shut off and renewed at any depth as a consequence of these differences pronounced unconformities drowning unconformities develop where carbonate platforms are terminated and buried by siliciclastics the siliciclastic to carbonate transition tends to be more gradual drowned platforms and drowning unconformities appeared world wide in great abundance in the miocene cretaceous valanginian turonian jurassic toarcian and devonian frasnian famennian examples of drowning unconformities interpreted as sequence boundaries include those of the early cretaceous platforms off new jersey and off morocco the mid cretaceous unconformity in the gulf of mexico and miocene unconformities on top of reefs in the far east the eustatic cycles postulated from sequence stratigraphy are a very unlikely cause for the mass drownings of reefs and platforms either the rates of rise are an order of magnitude lower than the growth potential of platforms or cycle amplitudes are too small or the cycles are too short and subsidence too slow to remove platforms permanently from the photic zone the critical element in the mass drowning seems to be environmental stress that reduces the growth potential of carbonate systems thus drowning unconformities demonstrate the importance of environmental change as a control on sequence development other examples of dominantly environment controlled sequences include slope deposits shaped by shifting currents e g florida and blake plateau and basin fills that were controlled by changes in sediment input related to tectonic alteration of the drainage in the hinterland e g gulf of mexico environmental change must be considered as a third independent factor that competes with eustatic and tectonically driven regional changes of sea level for control of sequences the definition of sequences and sequence boundaries should be broad enough to include the possibility of non sea level controls

density cascading off shelf sediment transport evidence and implications bahama banks abstract suspended sediment concentrations in shelf waters are often appreciable but sediments on their own rarely contribute enough density to cause suspension currents to transport them offshelf however sediment charged hyperpycnal high density waters are often generated over continental shelves because of the same set of air sea interactions intense heat and moisture flux responsible for thermohaline production and circulation of deep ocean water masses we propose the term density cascading to describe the processes by which hyperpycnal waters are released from shelf areas and together with entrained sediments sink to their compensation level in the adjacent ocean density cascading probably involves a range of physical processes from pelagic draping of slopes and basins with shelf muds to initiating turbidity currents capable of scouring canyons and plunge pools sediment bypassing of slopes and deposition of sands in deep basins thermodynamic air sea interactions can generate hyperpycnal water at all latitudes therefore density cascading has important implications not only to the sedimentology of modern carbonate platforms such as the florida shelf and bahama banks and stratigraphy in ancient limestones but also to offshelf sediment flux in siliciclastic environments

seismic geomorphology and evolution of early mid miocene isolated carbonate build ups in the timor sea north west shelf of australia abstract analysis of extensive 2d and 3d seismic data reveals the presence of about sixty isolated carbonate build ups icbs of early mid miocene age spreading over a wide area of the northwestern bonaparte basin individual build ups are 100 m thick with an average diameter of 3 km integration of full volume 3d seismic attribute analysis with extensive biostratigraphic and lithological data from exploration wells allows examining the evolution of icbs and their associated morphologies at high resolution the development of icbs took place above a seismically flat surface the stratigraphic architecture of the build ups typically consists of 1 mid burdigalian initiation tf1 cn2 2 lateral expansion during the late burdigalian cn3 and 3 backstepping and drowning during the langhian tf2 cn4 this is followed by sub aerial exposure that probably corresponds to a major sea level fall during the serravallian only small patch reefs pinnacles developed afterwards during the late miocene the observed growth phases of the icbs correlate with global sea level fluctuations and major changes in global climate and oceanography the role of local tectonics in the initiation and demise of these build ups appears as minimal growth of the northwestern bonaparte basin icbs is synchronous with the establishment of tropical carbonate factories elsewhere along the australia s north west shelf that suggest an acme of reef development during the early to middle miocene

pinnacle features at the base of isolated carbonate buildups marking point sources of fluid offshore northwest australia we investigated pinnacle features at the base of late oligocene miocene isolated carbonate buildups using three dimensional seismic and borehole data from the browse basin northwest australia brightened seismic reflections dim spots and other evidence of fluid accumulation occur below most pinnacle features an important observation is that all pinnacles generated topography on successive late oligocene miocene paleoseafloors therefore forming preferential zones for the settlement of reef building organisms by raising the paleo seafloor into the photic zone their height ranges from 31 m to 174 m for a volume varying from 33 km3 to 11 105 km3 most of the pinnacles however are less than 2000 km3 in volume and present heights of 61 80 m as a result of this work pinnacles are explained as the first patch reefs formed in association with mud volcanoes or methanogenic carbonates and they are considered as precluding the growth of the larger isolated carbonate buildups we postulate that pinnacle features above fluidflow conduits demonstrate a valid seep reef relationship and we propose them to be refined diagnostic features for understanding fluid flow through geological time

the influence of country rock assimilation and silicate to sulfide ratios r factor on the genesis of the dunka road cu ni platinum group element deposit duluth complex minnesota the dunka road deposit is one of several cu ni platinum group element pge sulfide occurrences found along the northwestern margin of the duluth complex where the host troctolitic rocks are in contact with metasedimentary rocks of the animikie group magma contamination through assimilation of sulfidic argillaceous country rocks is generally recognized as having played a key role in the genesis of the mineralization three main types of disseminated sulfide mineralization have been identified within the dunka road deposit i norite hosted sulfides ii troctolite hosted sulfides and iii pge rich sulfide horizons the norite hosted sulfides are found either adjacent to country rock xenoliths or near the basal contact the troctolite hosted sulfides form the bulk of the deposit and occur throughout the lower 250 m of the intrusion the pge rich sulfide horizons are typically localized directly beneath ultramafic layers the composition of the different types of sulfide occurrences is modelled using cuipd ratios it is shown that each type results from the interplay of two main parameters namely the degree of magma contamination and the silicate magma to sulfide melt ratio r factor the norite hosted sulfides formed at low r factors and high degrees of contamination as expressed by their pge depleted nature low seis ratios and elevated content in pyrrhotite and arsenide minerals the troctolite hosted sulfides formed at moderate r factors and small degrees of contamination as shown by their moderate pge content and mantle like seis ratios finally the pge rich sulfide horizons are modelled using elevated r factors from an uncontaminated parental magma which is substantiated by their elevated noble metal content and seis ratios and low pyrrhotite to precious metal sulfide ratio le gite de dunka road est un parmi plusieurs gites de sulfures de cu ni elements du groupe du platine egp trouves le long de la bordure nord ouest du complexe de duluth ou les roches hotes troctolitiques sont en contact avec les roches metasedimentaires du groupe d animikie il est generalement reconnu que la contamination du magma par l assimilation de roches argileuses sulfurees encaissantes a joue un role capital dans la genese de la mineralisation trois types principaux de mineralisation de sulfures dissemines ont ete identifies dans le gite de dunka road i sulfures dans la norite hote ii sulfures dans la troctolite hote et iii horizons de sulfures enrichis en egp les sulfures dans la norite hote apparaissent soit dans les zones adjacentes aux xenolites encaissants ou pres du contact basal les sulfures dans la troctolite hote forment le plus gros du gisement et ils apparaissent dans les 250m inferieurs de l intrusion la localisation typique des horizons de sulfures enrichis en egp se trouve directement sous les couches ultramafiques les rapports cu pd ont ete utilises pour elaborer un modele de la composition des differents types de mineralisation de sulfures il est demontre que chaque type resulte de l interaction de deux parametres dominants a savoir le degre de contamination du magma et le rapport du magma silicate au liquide sulfure facteur r les sulfures dans la norite hote ont ete formes sous des conditions de faibles facteurs r et de fortes contaminations comme en temoignent la teneur appauvrie en egp les faibles rapports se s et les concentrations elevees de la pyrrhotite et des mineraux arsenieux les sulfures dans la troctolite hote ont ete formes sous des conditions de facteurs r moderes et de faibles contaminations comme le revelent leur teneur modere en egp et les rapports se s analogues a ceux du manteau finalement les horizons de sulfures enrichis en egp sont modelises en utilisant des facteurs r eleves et un magma parental non contamine ce qui est appuye par leur forte teneur en elements nobles et les rapports se s eleves et par le rapport faible de la pyrrhotite sur les sulfures de metaux precieux

re os isotopic systematics of the voisey s bay ni cu co magmatic ore system labrador canada abstract re and os concentrations and os isotopic compositions have been obtained for massive matrix and disseminated sulphide ores from three environments within the voisey s bay intrusion the ovoid eastern deeps and discovery hill zone in order to assess the role of crustal contamination in the genesis of this large cu ni co deposit these samples have high re concentrations 148 to 288 ppb in 100 sulphide for their common os concentrations 4 8 to 24 ppb in 100 sulphide yielding high re os ratios 12 to 33 these data confirm that the magma parental to the voisey s bay ore system was broadly basaltic in major element chemistry rather than picritic consistent with the low ni cu ratio of the ores 1 5 re os isotopic data exhibit a limited spread in 187 re 188 os 57 to 157 and define an imprecise 1323 135 ma model 3 isochron likely the result of small r factor variations within the ore system the re os isochron age is within error of 1334 ma u pb ages obtained for baddeleyite from the ore bearing troctolites demonstrating that whole rock re os isotopic systematics have remained closed since crystallisation the initial os isotopic composition of the isochron os 1040 200 implies significant magma interactions with radiogenic os that most likely resides in the nain churchill province crust these data are therefore consistent with the parental magma achieving sulphide saturation as a result of contamination by radiogenic crustal components with further addition of base and precious metals as a function of r factor analyses of sulphide separates from the proterozoic tasiuyak churchill and archaean nain gneisses confirm that both units contained significant os 8 6 ppb and 0 38 ppb respectively that was very radiogenic at 1334 ma os 1908 and 5202 respectively yielding crustal residence tchur model ages of 2200 to 2400 ma however these model ages may have been affected by re and or os mobility during the 1 85 ga torngat and 1 1 ga grenville orogeny re os isotopic modelling suggests that a reasonable fit to the ore data may be obtained via bulk crustal contamination of magmas parental to the nain plutonic suite nps with either tasiuyak 2 contamination or nain 16 contamination gneiss followed by an r factor process 200 to 1000 which improved the tenor of the sulphide liquid during transport in active magma conduits or after deposition in an active replenished voisey s bay intrusion magma chamber however the dominance of plagioclase rich high al2o3 magmas in the mafic members of the nps troctolites and anorthosites may indicate that other deeper forms of lithospheric interaction potentially involving mafic lower crust may be crucial to understanding this style of magmatic sulphide ore system

syn sedimentary deformation as a mechanism for the initiation of submarine gullies on a carbonate platform to slope transition browse basin australian north west shelf abstract syn sedimentary deformation and submarine channel initiation are both important processes influencing the stratigraphic development and reservoir properties of carbonate platforms based on a high resolution 3d seismic dataset our study shows that platform margin parallel syn sedimentary faulting in the vicinity of the clinoform breakpoint can lead to the indention of channel heads that subsequently grow and expand in headward and downslope direction the carbonate dominated miocene succession of the browse basin on the australian north west shelf reveals a distinct submarine channel system during a time of overall platform progradation the progradation occurs over a drowned eocene to lower miocene carbonate ramp which formed a large scale inflection previous studies investigated the architecture of the carbonate platform containing a large neogene barrier reef system we present a high resolution 3d seismic study of the contemporaneous platform to slope transition landward of first significant gullies a distinct network of margin parallel small scale arcuate faults is observed active faulting during appearance of the first gullies is exhibited by growth strata which are located directly basinward of the faults these steep 65 85 small scale faults are characterized by throws of 5 25 ms twt and show a tendency to merge laterally and vertically with adjacent faults syn sedimentary deformation associated with these faults likely triggered slope failure and the generation of turbidity currents finally leading to the growth of the gullies by headward and downslope erosion

bladed quartz and its relationship to gold mineralization in the hishikari low sulfidation epithermal gold deposit japan bladed quartz frequently is observed in low sulfidation epithermal gold deposits bladed calcite also has been documented in boiling zones of some active epithermal systems and boiling in these systems has been directly linked to gold mineralization in the hishikari low sulfidation epithermal gold deposit the distribution and texture of fluid inclusions within bladed quartz reveal a similar relationship to gold precipitation the formation of bladed quartz at hishikari involves several stages 1 deposition of bladed calcite 2 precipitation of fine grained adularia and quartz on the surface of calcite blades 3 dissolution of calcite blades leaving cavities in the interstices between aggregates of adularia and quartz and 4 infilling of the cavities by later quartz i e pseudomorphs of the original bladed calcite bladed quartz is present largely in the deeper part of the vein system beneath the high grade gold ore zone at hishikari this distribution may be explained by the fact that the original bladed calcite formed at depth in the system where boiling and loss of co 2 initially caused the precipitation of the calcite and quartz formed as pseudomorphs of the original calcite blades

cenozoic mineral deposits and subduction related magmatic arcs in mexico spatial distribution of isotopic ages of igneous rocks in northern mexico shows that a magmatic arc commenced at 140 m y b p near a paleotrench progressed 1 000 km eastward by 40 m y b p and then regressed to within 200 km of the continental margin by 18 m y b p during progression a calcic and calc alkaline continental margin volcano plutonic regime developed but farther east high k calc alkaline and alkaline facies formed in northeastern mexico and west texas following regression calc alkaline volcanism was renewed on the pacific margin at 18 m y b p and continued until cessation of subduction by initiation of transform movements between pacific and american plates benioff zone depth estimates from k 57 5 relationships suggest decreasing dip during progression and a variable dip variable depth regime from 140 to 16 m y b p both the dip angle convergence rate relationship and independent convergence rates require increasing convergence but decreasing sinking rates of subducted oceanic slab from 80 to 42 m y b p followed by decreasing convergence and sinking rates from 42 to 16 m y b p changes in subduction dynamics are attributed to plate reorganization at 44 m y b p and the approach of the east pacific rise differing hydrothermal ore assemblages in zones subparallel to the paleotrench are constrained by magma distribution in space and time cu mo w ores 106 to 40 m y old in calc alkaline magmas formed during progression whereas mo and caf 2 deposits coincide with the alkaline belt during arc stillstand above the deepest subduction depths and pb zn ag deposits 49 26 m y b p were formed during late arc progression and early regression these geodynamic variations plus sr rb and s isotopic characteristics of the arc terrane suggest that processes associated with subduction and some crustal contamination rather than local compositional anomalies in crust or mantle account for different magma and mineral deposit suites southern mexico exhibits fe and cu zones whereas the overlapping calc alkaline trans mexican chiapanecan arc of late miocene to holocene age contains several base and precious metal deposits

solubility of copper in a sulfur free mafic melt the solubility of cu in s free mafic melts has been measured at a series of o2 values and temperatures of 1245 and 1300 c at constant temperature cu solubility increases from 0 04 wt at log o2 11 9 to 1 10 wt at log o2 7 4 copper solubilities were in excess of 8 wt in two runs controlled at very high o2 conditions of 10 1 4 and 10 22 partitioning of cu between metal and glass shows a strong o2 dependence with dcumetgl ranging from 90 at log o2 7 4 to 2190 at log o2 11 9 slopes of cu solubility and dcumet gl vs log o2 suggest that cu dissolves predominantly in the 1 valence state copper solubility decreases with increasing temperature at constant o2 similar to experimental results for ni co and mo dingwell et al 1994 holzheid et al 1994 the data are consistent with cu dissolution as an oxide represented by cuo0 5 and suggest that changes in o2 fe2 fe3 variations and cu1 complexation with fe3 may have large effects on the distribution of cu between silicate and sulfide magmas results also suggest that the extraction of oxide bonded cu in mafic magmas by externally derived s may be an important mechanism in the generation of cu rich sulfide ores

sulfide mineralogy and sulfur isotope geochemistry of layered sills in the deer lake complex minnesota the deer lake complex located in north central minnesota consists of a series of layered peridotite pyroxenite gabbro sills sulfide minerals occur as fine disseminations throughout pyroxenite and gabbro units and occur more sporadically in peridotite and basal chilled margin units sulfide volume percentage rarely exceeds 0 5 a distinct zonation in sulfide mineralogy and sulfur isotopic composition characterizes the sills cobaltian pentlandite is the dominant sulfide mineral in peridotite pd units with ni enrichment most likely linked to the serpentinization process 34spd values are variable ranging from 3 5 to 2 8 sulfide assemblages in pyroxenite px and lower gabbro units consist of chalcopyrite pyrrhotite and minor pentlandite 34spx values range from 1 to 1 pyrite is the principal sulfide mineral in upper gabbro g units its origin may be related to increased f02 conditions of the remaining melt and to reaction between a s bearing volatile phase and mafic silicates 34sug values range from 1 to 3 5 sulfur isotopic values of chilled margin 2 9 and peridotite units together with the erratic spatial distribution of sulfide minerals in these zones suggests that the parent magma was not initially saturated with sulfur and that local sulfide concentrations are the result of incorporation of sulfur derived from metasedimentary country rocks sulfide saturation was more uniformly reached during pyroxenite formation with contained sulfur being of magmatic origin enrichment in 34s of pyrite from upper gabbro may be explained by buildup of isotopically heavy sulfur following a rayleigh process coupled with possible involvement of a so2 rich fluid phase during hydrothermal alteration

from the great bahama bank into the straits of florida a margin architecture controlled by sea level fluctuations and ocean currents high resolution multichannel seismic data collected across the great bahama bank margin and the adjacent straits of florida indicate that the deposition of neogene quaternary strata in this transect are controlled by two sedimentation mechanisms 1 west dipping layers of the platform margin which are a product of sea level controlled platform derived downslope sedimentation and 2 east or north dipping drift deposits in the basinal areas which are deposited by ocean currents these two sediment systems are active simultaneously and interfinger at the toe of slope the prograding system consists of sigmoidal clinoforms that advanced the margin some 25 km into the straits of florida the foresets of the clinoforms are approximately 600 m high with variable slope angles that steepen significantly in the pleistocene section the seismic facies of the prograding clinoforms on the slope is characterized by dominant partly chaotic cut and fill geometries caused by submarine canyons that are oriented downslope in the basin axis seismic geometries and facies document deposition from and by currents most impressive is an 800 m thick drift deposit at the confluence of the santaren channel and the straits of florida this santaren drift is slightly asymmetric thinning to the north the drift displays a highly coherent seismic facies characterized by a continuous succession of reflections indicating very regular sedimentation leg 166 of the ocean drilling program odp drilled a transect of five deep holes between 2 and 30 km from the modern platform margin and retrieved the sediments from both the slope and basin systems the neogene slope sediments consist of peri platform oozes intercalated with turbidites whereas the basinal drift deposits consist of more homogeneous fine grained carbonates that were deposited without major hiatuses by the florida current starting at approximately 12 4 ma sea level fluctuations which controlled the carbonate production on great bahama bank by repeated exposure of the platform top controlled lithologic alternations and hiatuses in sedimentation across the transect both sedimentary systems are contained in 17 seismic sequences that were identified in the neogene quaternary section seismic sequence boundaries were identified based on geometric unconformities beneath the great bahama bank all the sequence boundaries could be traced across the entire transect into the straits of florida biostratigraphic age determinations of seismic reflections indicate that the seismic reflections of sequence boundaries have chronostratigraphic significance across both depositional environments

precious metal vein systems in the national district humboldt county nevada the national district 20 miles 33 km southeast of mcdermitt nevada produced 200 000 ounces of gold and 750 000 ounces of silver during the period 1906 1941 precious metal quartz sulfide silicate veins near the townsite of national and on buckskin mountain occur within a 3 000 ft thick 910 m sequence of miocene volcanic and intrusive rocks which vary in composition from basalt to rhyolite veins formed about 15 6 million years ago and associated rhyolites are approximately 18 to 15 m y old the volcanic pile at national is highly faulted and deposits are localized within orthogonal fault sets that coincide with major upper crustal structures in the northern great basin intrusive rhyolite and eruptive centers spatially related to veins at national partly define the goosey lake depression a shallow basin situated on the western edge of the owyhee volcanic plateau wall rock in the district is progressively hydrolyzed and sulfidized to assemblages dominated by chlorite calcite illite montmorillonite quartz pyrite and kaolinite quartz pyrite as veins are approached alteration assemblages exposed by exploration on buckskin mountain include the paleosurface at buckskin planar open space filled veins disperse upward into structurally unconfined flat lying zones of kaolinite quartz quartz alunite and chalcedonic silica sinter quartz altered rhyolite the latter being the paleosurface assemblage metallic minerals in four nested and coalescing vein stages include cinnabar stibnite electrum silver sulfosalts and selenides and base metal sulfides vein silicates are quartz kaolinite illite and at depth muscovite the spatial distribution of hydrothermal sulfides and silicates throughout buckskin mountain is strongly influenced by physical properties of premineralization volcanic rocks porous and permeable rhyolite tuff capping buckskin mountain is pervasively hydrolyzed to laterally extensive silica alunite and kaolinite bearing assemblages which lack tabular zones of open space filling underlying dense and brittle rhyolite flows confine hydrothermal assemblages to a relatively few planar fractures flanked by parallel selvedges of progressively altered wall rock at buckskin mountain two distinct but synchronous thermal events are documented by fluid inclusion studies the earlier event involved temperatures ranging from approximately 295 degrees to 250 degrees c and culminated in rupture of overlying rhyolite producing an inverted conical mass of bilithic breccia fluids associated with the second postbrecciation event boiled at 100 degrees to 255 degrees c from the paleosurface to depths exceeding 2 000 ft 606 m near surface hydrothermal assemblages were deposited from low salinity 1 2 wt nacl solutions containing 2 weight percent dissolved gases precious metal precipitation occurred at a temperature of about 250 degrees c and a pressure of about 40 bars largely from boiling solution periodic restrictions to convective heat loss resulting from silica deposition and episodic fluid circulation cause observed temperature and pressure data to deviate somewhat from an ideal hydrostatic model the vertical temperature gradient is consistent with a water saturated melt of granitic composition about 10 000 ft 3 030 m below the paleosurface during mineralization close spacing of isotherms below 200 degrees c reflects paleosurface proximity cooling by conduction and steam loss as well as accelerated convective cooling of fluid circulating through permeable rhyolite tuff integration of hydrothermal assemblages thermal measurements fluid mineral equilibria and active hot springs data suggests that as temperatures in buckskin mountain decreased from 250 degrees c at depth to 100 degrees c at the surface ph increased from approximately 3 7 to near neutral sigma s increased from 10 super 4 5 m to approximately 10 super 2 6 m and f sub o 2 decreased from 10 super 39 to 10 super 48 the distribution of alteration assemblages precious metal minerals stibnite and cinnabar was apparently controlled by boiling a sulfldation gradient temperature decreases and near surface hydrology

metallogeny of the midcontinent rift system of north america abstract the 1 1 ga midcontinent rift system of north america is one of the world s major continental rifts and hosts a variety of mineral deposits the rocks and mineral deposits of this 2000 km long rift are exposed only in the lake superior region in the lake superior region the rift cuts across precambrian basement terranes ranging in age from 1850 ma to more than 3500 ma where exposed the rift consists of widespread tholeiitic basalt flows with local interlayered rhyolite and clastic sedimentary rocks beneath the center of lake superior the volcanic and sedimentary rocks are more than 30 km deep as shown by recent seismic reflection profiles this region hosts two major classes of mineral deposits magmatic and hydrothermal all important mineral production in this region has come from hydrothermal deposits rift related hydrothermal deposits include four main types 1 native copper deposits in basalts and interflow sediments 2 sediment hosted copper sulfide and native copper 3 copper sulfide veins and lodes hosted by rift related volcanic and sedimentary rocks and 4 polymetallic five element veins in the surrounding archean country rocks the scarcity of sulfur within the rift rocks resulted in the formation of very large deposits of native metals where hydrothermal sulfides occur i e shale hosted copper sulfides the source of sulfur was local sedimentary rocks magmatic deposits have locally supported exploration and minor production but most are subeconomic presently these deposits occur in intrusions exposed near the margins of the rift and include cu ni pge and ti fe v in the duluth complex u ree nb in small carbonatites and breccia pipes resulting from local hydrothermal activity around small felsic intrusions mineralization associated with some magmatic bodies resulted from the concentration of incompatible elements during fractional crystallization most of the sulfide deposits in intrusions however contain sulfur derived from country rocks the interaction between magma and country rocks was important in generation of the magmatic cu ni sulfide deposits a mantle plume origin has been proposed for the formation of the midcontinent rift more than 1 million km3 of mafic magma was erupted in the rift and a comparable volume of mafic intrusions are inferred beneath the rift providing a ready and structurally confined supply of mafic source rocks that were available for leaching of metals by basinal brines these brines were heated by a steep geothermal gradient that resulted from the melting and underplating of magma derived from the plume hydrothermal deposits were emplaced for at least 30 40 m y after rift magmatism and extension ceased this time lag may reflect either the time required to heat deeply buried rocks and fluids within the rift or may be due to the timing of post rift compression that may have provided the driving mechanism for expulsion of hydrothermal fluids from deep portions of the rift