ammonium in granites and its petrogenetic significance abstract ammonium is present as a trace constituent in all granites with an average concentration of 45 ppm nh4 equivalent to 35 ppm of elemental n it shows wide variations related to petrography and region but the only significant correlation between ammonium and other geochemical parameters is that it is most abundant in peraluminous granites and least abundant in peralkaline granites these variations can be related to a the amount of sedimentary material in the magmatic source region and b redox conditions in the source region the ammonium content of granitic magmas can also be modified by fractionation or contamination hydrothermal alteration has a major effect on the ammonium content of granitic rocks and variation due to this cause may exceed the magmatic variation most hydrothermally altered granites are enriched in ammonium as a result of the transfer of ammonium from sedimentary country rocks by the hydrothermal fluids

nitrogen and carbon isotopic composition in the diamonds of mbuji mayi zaire the concentration and isotopic composition of nitrogen measured in large diamonds gram size from the mbuji mayi kimberlite district zaire show a large range of variation 100 n 2100 ppm 11 2 15n 6 0 the15n depleted nitrogen is associated with the higher nitrogen concentrations the large diamonds are individually rather homogeneous in13c range of 13c 0 9 while variations occur within small octahedral diamonds from the same district range up to 5 8 the total range of 13c variation is about the same for the large diamonds 10 5 13c 5 5 the small octahedral diamonds 10 13c 4 6 and the carbonates from local kimberlites 11 8 13c 5 5 the diamond carbon isotopic data could indicate a complex story of crystallization within a deep seated system fractionating its carbon isotopes the nitrogen results indicate that nitrogen in diamonds is on the average markedly depleted in15n weighted average 5 15 relative to atmosphere sediments and upper mantle

carbon isotope systematics of archean au ag vein deposits in the superior province abundant carbonate is a characteristic feature of most archean mesothermal au ag vein deposits but the source of the c is controversial for superior province deposits collectively the maximum variation of average 13c values is from 9 0 0 7 1 n 19 darius to 0 6 1 6 1 n 7 cochenour willians and limiting 13c values are 13 6 and 1 3 at the deposit scale fe dolomites in nongraphitic lithologies are for the most part isotopically uniform where 13c 3 4 0 4 1 hollinger 3 2 0 3 mcintyre 4 7 1 7 dome 2 8 0 6 buffalo ankerite 3 6 0 5 macassa 3 2 0 3 bousquet 5 4 0 9 lamaque and 5 3 0 5 hasaga the restricted individual ranges of 13c values imply a corresponding uniformity to the ambient temperature and 13c c of the ore forming fluids within individual deposits small systematic variations of 13c carbonate arise from i interaction of hydrothermal fluids with carbonaceous rocks ii immiscible separation of co2 ch4 or iii

nitrogen isotope geochemistry of organic matter and minerals during diagenesis and hydrocarbon migration the magnitude of isotopic variations between organic and inorganic nitrogen was examined in samples from three stacked hydrocarbon reservoirs in the fordoche field louisiana gulf coast basin usa measurements were made of 15n in kerogen bitumen oil formation water and fixed nh4 extracted from mudstones nonproductive sandstones and productive sandstones nitrogen isotope fractionation occurs because 14n is released preferentially to 15n from organic molecules during thermal maturation released 14n goes into solution or may be adsorbed by minerals leaving crude oil enriched in 15n diagenetic clay minerals e g illite commonly form in the temperature range of hydrocarbon generation and nh4 may be fixed in clay interlayers with an isotopic ratio similar to that of the migrating fluids results indicate that the influence of organic matter on mineral 15n depends on the timing of authigenic mineral formation relative to fluid migration the average 15n of kerogen 3 2 0 3 and fixed nh4 from mudstones 3 0 1 4 is similar while bitumen increases from 3 5 to 5 1 with depth in deep reservoir sandstones 100 c the 15n of crude oil averages 5 2 0 4 similar to the 15n of bitumen in the proposed source rocks formation waters are 14n enriched with an average 15n of 2 2 2 6 fixed nh4 15n values lie between that of the oil and water the average 15n of fixed nh4 is 3 0 1 2 in productive sandstones and 0 2 2 4 innonproductive sandstones in the shallower reservoir sandstones 90 c fixed nh4 is apparently not influenced by the presently associated fluids productive and nonproductive sandstones have distinctly low average 15n values 1 2 0 8 yet crude oil 11 1 0 3 and water 3 8 0 1 have been 15n enriched by 6 relative to the deeper reservoirs this suggests that the present fluids migrated into the reservoir after authigenic illite had formed fluids become enriched in 15n during migration and the amount of enrichment may be a function of the amount of interaction with argillaceous sediments

lower tertiary and upper cretaceous source rocks in louisiana and mississippi implications to gulf of mexico crude oil the lower tertiary sparta formation wilcox group and the midway group in southern louisiana include organic rich source facies that generate crude oil at relatively high thermal maturities the wilcox group is an important source of wilcox crude oil and regional kerogen variations explain two crude oil subfamilies wilcox crude oils in downdip areas of southern louisiana migrated short distances but long range lateral migration about 150 km best explains wilcox crude oils far updip from mature source rocks crude oils in oligocene and younger reservoirs in southern louisiana migrated vertically from deep lower tertiary source rocks some crude oils in upper cretaceous tuscaloosa reservoirs were emplaced by long range lateral migration from tuscaloosa source rocks g ven little evidence of upper tertiary source rocks and the overmaturity problems of mesozoic source rocks most crude oils in upper tertiary and pleistocene reservoirs of the gulf continental shelf are best explained by vertical migration from deep lower tertiary source rocks even so it is simplistic to assume an exclusive lower tertiary origin many tertiary and pleistocene crude oils of this study probably include an overprint of high maturity hydrocarbons from mesozoic sources

channel trenching and climatic change in the southern u s great plains fifteen alluvial sequences in texas and oklahoma exhibit the same late holocene record of channel trenching at 1 ka the erosion was preceded by slow alluvial sedimentation in most stream valleys resulting in the formation of a cumulic organic rich flood plain soil previously named the copan soil the soil formed during a period of regionally moister climate as indicated by pollen spectra molluscan faunas vertebrate faunas sedimentary structures and high alluvial water tables at 1 ka the regional climate changed from moist to dry coinciding with an episode of channel incision of valley floors throughout the southern great plains channel trenching occurred simultaneously in both small and large streams in drainage basins of the arkansas red trinity brazos and colorado rivers the sequences are the first documented example of widespread holocene incision accompanied by firm evidence for a synchronous change in regional climate

jurassic mesothermal gold mineralization of the samhwanghak mine youngdong area republic of korea constraints on hydrothermal fluid geochemistry jurassic 166 or 2 ma gold vein deposits of the samhwanghak mine youngdong area are examples of mesothermal gold deposits in the republic of korea the deposits are composed of gold bearing but relatively sulfide poor massive quartz veins which occupy shear zones in graphite bearing paragneiss of precambrian age vein quartz was deposited mostly prior to deposition of sulfides and gold and contains three main types of fluid inclusions i co 2 ch 4 fluids ii low salinity 2 kbars initial hydrothermal fluids were near their critical states at temperatures of 410 degrees to 480 degrees c and were homogeneous compositionally with x sub co 2 ch 4 of 0 45 x sub ch 4 0 14 and salinity of asymp 9 wt percent nacl type iii inclusions appear to be secondary and represent later fluids which formed through extensive fluid unmixing of type ii fluids with progressive decreases in temperature below 370 degrees c and pressure down to asymp 1 kbar but may also include deep circulating meteoric waters thermochemical calculations indicate the following evolution of hydrothermal fluids fugacity of sulfur decreased progressively with decreasing temperature near the pyrite pyrrhotite sulfidation curve fugacity of oxygen was controlled largely through graphite fluid interaction and evolved near the qfm buffer ph was maintained near neutrality temperature versus f sub o 2 relationships indicate that hydrothermal fluids were derived originally from a nearby s type magma the middle jurassic kimcheon granite gold occurs as gold rich avg 72 at au electrum intimately associated with sulfide minerals and precipitated from later aqueous type iii fluids at temperatures of 340 degrees to 240 degrees c as a combined result of cooling and decrease of sulfur activity by sulfide precipitation and or h 2 s loss accompanying fluid unmixing calculated sulfur isotope compositions of hydrothermal fluids delta 34 s sub sigma s near 2 per mil indicate that ore sulfur was derived from an ilmenite series s type granitic melt measured and calculated oxygen and hydrogen isotope compositions of ore fluids delta 18 o water 6 1 8 4ppm delta d water 62 to 72ppm are consistent with derivation from and interaction with magmatic rocks mesothermal gold vein deposits of the youngdong area were formed by extensive fracturing and veining during the jurassic daebo orogeny and developed from fluids that may have included an important component of magmatic fluids mineralogy and ore fluid chemistry of the deposits of the youngdong area are similar to those of archcan mesothermal gold vein deposits possibly validating a more general application of a magmatic hydrothermal model for the formation of mesothermal gold vein deposits

sea level changes an integrated approach sea level changes an integrated approach in october 1985 sepm sponsored a four day conference entitled sea level changes an integrated approach the purpose of the conference was to provide a forum for an interdisciplinary exchange of ideas on sea level changes and to provide an opportunity for integrating various types of evidence in approaching unresolved issues the conference was successful in bringing together scientists from industry academia and government representing all of the major geosciences disciplines presentations of many new papers plus significant releases of data that were previously held proprietary provided fertile ground for discussion this much cited volume represents the best of the material presented at the conference includes the early vail chart

depositional tectonic and eustatic controls on hydrocarbon distribution in divergent margin basins cenozoic gulf of mexico case history abstract the evolution of continental margins and related distribution of hydrocarbon reservoirs in divergent margin basins reflects the primary interaction between sediment supply and process regime within the basin and secondary modifications by sea level change and structural history cenozoic sequences of the northwestern gulf coast basin contain numerous highly explored hydrocarbon fairways that reflect typical productive styles resulting from progressively reduced influences of tectonic events in the north american plate and progressively increased influences of eustasy most paleogene and lower neogene sequences record moderate to high rates of sediment supply locally enhanced load or salt induced subsidence and infrequent relatively low amplitude eustatic change accommodation space was created by crustal loading and gravity driven continental margin extension or salt evacuation along with progradation into the deep water gulf shorezone deposits are sand rich because slow or limited eustatic shoreline shift promoted effective reworking and shelfal storage of coastal facies progradational depocenters were characterized by large long lived wave reworked shelf edge delta systems sands were largely stored close to the delta front and flanking shorezone systems prolonged periods of aggradation and retrogradation preserved thick vertically stacked parasequences of deltaic and interdeltaic barrier strandplain facies hydrocarbons accumulated in multiple superimposed reservoirs primarily in mixed structurally and depositionally defined plays in contrast upper neogene and quaternary sequences are products of frequent high amplitude eustatic cycles rapid subsidence and locally very high rates of sedimentation accommodation space was created by load induced subsidence modified by salt evacuation and intermittent relative rises in sea level shorezone deposits are heterolithic because rapid subsidence and frequent eustatic shoreline shifts prevented extensive reworking progradational episodes were characterized by mud rich river dominated shelf edge deltas of moderate size rapid lowering of sea level promoted gravity remobilization of deltaic sediments and deposition of sand rich lobes that created broad structurally and bathymetrically complex submarine fans and slope aprons some sand rich submarine fan lobes extend more than 160 km basinward from the paleoshelf margin in contrast strike reworking and sand storage on the shelf were minor combination structural and stratigraphic traps define most upper neogene and quaternary hydrocarbon plays correlating observed patterns of cenozoic basin fill with contemporaneous plate interactions and eustatic fluctuations reveals the relative control of these variables on rates of sediment supply characteristics of depositional sequences and styles of basin fill deformation

the nature and dimensions of regional and local gold related hydrothermal alteration in tholeiitic metabasalts in the norseman goldfields the missing link in a crustal continuum of gold deposits the archaean lode gold deposits at norseman western australia consist of auriferous quartz veins in dextral reverse ductile brittle shear zones within tholeiitic metabasalts of upper greenschist to amphibolite facies metamorphic grade three types of deposits northern central southern are delineated on the basis of their spatial distribution veining style alteration mineraloty and metamorphic grade of host rocks northern deposits hosted in upper greenschist to lower amphibolite facies rocks comprise massive to laminated quartz veins with selvedges of quartz chlorite calcite biotite plagioclase assemblages central deposits hosted in lower amphibolite facies rocks consist of laminated to massive quartz veins with selvedges of quartz actinolite biotite plagioclasecalcite assemblages southern deposits hosted in middleamphibolite facies metabasalts consist of banded quartz diopside calcite microcline zoisite veins all deposits exhibit variable ductile deformation of veins and contiguous alteration haloes consistent with a syn deformational genesis at high temperatures from northern to southern deposits the alteration assemblages are indicative of higher temperatures of formation and there are progressively greater degrees of dynamically recovered textures in alteration and gangue minerals these observations imply that a thermal variation of gold related hydrothermal alteration exists within the norseman terrane over a distance of 40 km with tnorthern tcentral tsouthern this thermal zonation is corroborated by t xco 2 phase relations between vein selvedge assemblages which signify formation temperatures of approximately 420 475 c 470 495 c and 500 c for northern central and southern deposits respectively the sum of structural petrographic and mineral chemistry data indicates that the alteration assemblages formed in high temperature open hydrothermal systems and have not been subsequently metamorphosed the thermal differences between the deposit groups may reflect 1 a temperature gradient at relatively constant p corresponding to the proximity of the deposits to regional granitoid complexes or 2 formation of the deposits at progressively deeper crustal levels from north to south in either case the deposits represent a continuum of gold deposition from upper greenschist to amphibolite facies now exposed in an oblique section through the archaean crust at norseman

fluid evolution and transport during metamorphism evidence from the llano uplift texas a field petrologic and stable isotopic investigation of the marbles and calc silicates of the 1 15 b y valley spring gneiss documents the dilution of internally evolved co2 rich fluids by externally derived aqueous fluids introduced along channelways reaction textures within calcsilicates record the evolution through time of initially co2 rich fluids toward increasingly more aqueous compositions assemblage zonations within calc silicates require equilibration within local gradients of the mole fraction of co2 in the fluid and suggest that the infiltration of aqueous fluids was largely channelized along more permeable lithologies localized depletions in 13c and 18o corroborate petrologic evidence for channelized infiltration isotopic compositions reflect both devolatilization and the introduction of low 18o fluids estimated minimum oxygen equivalent fluid to rock ratios are near unity both mineralogical and stable isotopic systematics document the essential role of infiltration in driving decarbonation reactions during calc silicate formation the calc silicate assemblages which equilibrated with fluids of the lowest mole fraction of co2 record isotopic exchange equilibrium with fluids of 18o typical of those derived from normal granites as do the granitic aplites and pegmatites which transect most calcsilicate occurrences thus the infiltrating fluids are believed to be genetically linked to the intrusion of a suite of granitic plutons emplaced after the peak of regional metamorphism

the measurement of δ15n in crustal rocks by static vacuum mass spectrometry application to the origin of the ammonium in the cornubian batholith southwest england the release of ammoniacal nitrogen from muscovite biotite orthoclase microcline and a whole rock granite by vacuum pyrolysis has been investigated under the heating regimes employed it was found that micas lose most of their nitrogen below the temperature at which the powders begin to sinter ca 1100 c the peak release of n occurring in either the 900 c or 1000 c heating step it is possible that the loss of ammonium is linked to the loss of h2o and perhaps f for the feldspars there was insignificant release of n below the melting point of the minerals about 1150 c suggesting that diffusion of ammonium or ammonia is the principal control for both micas and feldspars there was significant kinetic fractionation of the nitrogen isotopes during stepped heating from these results a technique has been devised for the routine determination of 15n and ammonium concentration in whole rock granites by static vacuum mass spectrometry the ammonium concentrations and 15n values have been determined in whole rock samples from the cornubian batholith southwest england the ammonium concentrations were in the range 8 to 187 ppm and the 15n values were in the range 5 1 to 10 2 most of the granite samples had 15n values between 8 4 and 10 2 and are within the range that have been obtained from amphibolite facies metasediments these granites are also typified by high 87sr86srinit 0 7130 to 0 7166 and high normative corundum 2 31 to 2 55 taken together the data suggest that the ammonium present within these granites is of biological origin and that it was incorporated via the anatexis of metasediments which originally contained organic material the samples were collected over a distance of 120 km and the limited range in 15n suggests either 1 1 the 15n values within the metasedimentary protolith were rather homogeneous or 2 2 the nitrogen isotopes were homogenised during the incorporation of ammonium into the granite magmas two of the granites had lower 15n values of 5 1 and 7 0 these granites also have lower ammonium concentrations 8 and 15 ppm lower 87sr86srinit 0 7094 0 7095 and lower normative corundum 1 44 and 1 63 the preferred explanation is that they were derived from or have had a contribution from a second crustal source containing sedimentary or metasedimentary material with a lower proportion of pelitic to other constituents

genetic stratigraphic sequences in basin analysis ii application to northwest gulf of mexico cenozoic basin the northwest gulf of mexico cenozoic sedimentary wedge illustrates the application of genetic stratigraphic sequence analysis and documents several general conclusions 1 sequences defined by regional marine flooding are the principal genetic stratigraphic units of the basin fill continental margins are characterized by repetitive episodes of basin margin offlap punctuated by periods of transgression and marine flooding of the depositional platform 2 continental margin outbuilding is concentrated at one or more shelf edge deltaic depocenters separated by interdeltaic bights depocenters remain fixed during a depositional episode but commonly relocate during transgression and flooding 3 a distinct syndepositional structural style in prograding continental margins results in sporadic uplift of a basin fringing peripheral bulge and accentuates preservation of shelf margin facies along zones of extensional normal faulting and enhanced subsidence 4 genetic stratigraphic sequences commonly reflect an evolving interplay among two or even three variables for example early cenozoic gulf sequences are most closely related to tectonic events of the intraplate source terrane which in turn affect rate and location of sediment supply and basin margin response to loading late cenozoic sequences more closely reflect proposed eustatic cycles

classification of offshore gulf of mexico oils and gas condensates five groups of petroleums can be recognized in the offshore gulf of mexico on the basis of carbon isotope and sulfur isotope ratios sulfur content and relative abundances of nickel and vanadium of 355 oils analyzed 341 96 1 are evidently closely related and cannot be subdivided on the basis of carbon or sulfur isotopes we suggest that these oils represent a single broad superfamily however two subgroups within this set are distinguishable on the basis of sulfur content and relative abundances of nickel and vanadium group 1a sigma 13 c 26 5 0 4 parts per mil sigma 34 s 5 6 2 4 parts per mil s 0 25 0 15 v ni v 0 27 0 10 and group 1b sigma 13 c 26 5 0 3 parts per mil sigma 34 s 5 9 1 2 parts per mil s 0 73 0 43 v ni v 0 67 0 07 group 1a oils are broadly distributed across the louisiana and adjacent texas continental shelf whereas group 1b oils are found along the louisiana shelf edge and continental slope group 1b oils have substantial genetic affinities to those from the campeche shelf offshore mexico of mesozoic origin the remaining 14 analyzed petroleums include supermature condensates from offshore texas and oils from offshore southern florida believed to be cogenetic with those from the jurassic smackover formation a three dimensional appraisal of geohistory suggests that petroleum generation began in the eocene in the region of the rio grande delta progressed coastwise to the mississippi delta area by the miocene and then occurred in regions of progres ively deeper water during the pliocene pleistocene

c o h n fluids in quartz segregations from a major ductile shear zone the berzosa fault spanish central system the berzosa fault is a major ductile shear zone the berzosa shear zone bsz which separates the ollo de sapo anticline from the inner higher grade crystalline axis of the iberian hercynian belt this shear zone is the site of abundant early kinematic quartz al silicates segregations rich in fluid inclusions host rocks are medium grade staurolite schists and sillimanite gneisses fluid inclusions in selected quartz segregations across the berzosa shear zone have been studied by microthermometric methods as well as in some instances by raman analysis the recorded fluid inclusion history begins at the end of an intense secondary recrystallization period during late peak metamorphic conditions and lasts until late in the uplift history of the zone three types of inclusions have been found which in a time sequence are co2 h2o h2o salt b type and n2 ch4 three types of b inclusion may be distinguished in turn depending on whether they were trapped during an earlier dynamic recovery phase b1 type formed later as intergranular trails b2 type or were trapped apparently along with n2 ch4 in clusions from a heterogeneous fluid b3 type considerations from isochores confirm that co2 h2o inclusions were trapped during late peak and high t retrograde metamorphic conditions in the range 650 500 c and 5 2 kbar whilst n2 ch4 inclusions along with the b3 type of inclusions formed at low pressures 1 kbar and temperatures 300 c b2 type inclusions were trapped chronologically between these two in a period in which strong inverse lateral thermal gradients developed in the zone inferred p t paths for the area are convex to the t axis

geochemistry of fine grained clastic sediments in the archean abitibi greenstone belt canada implications for provenance and tectonic setting abstract archean sandstone and shale samples from turbiditic tf and alluvial facies af in the southern volcanic zone of the abitibi greenstone belt canada have been analyzed for major and trace elements the sandstones from tf are mainly greywackes whereas those from af are lithic arenite sio 2 ranges from 51 to 68 geochemically the clastic sediments cluster into mafic element mep and low mafic element lmep populations the mep sediments which include turbiditic facies sediment samples from the larder lake group in the kirkland lake area are characterized by elevated abundances of mgo 5 ni 200 ppm cr 400 ppm sc 22 ppm and feo 7 5 abundances lower contents of incompatible elements such as th u ta hf zr and total rees n ratios are 4 5 with eu eu 1 the lmep sediments which include samples from the beatty dome and three nations lake formations in timmins the kewagama group noranda area and timiskaming group in the kirkland lake area however show reverse abundance distributions the geochemical gaps shown by the major and trace elements define two populations of sediments suggesting contrasting provenances for them the mep sediments require a dominantly mafic and ultramafic 90 source with 10 felsic contribution whereas the lmep sediments require a mixture of felsic 75 and mafic 25 rocks this compositional distribution of the sediments is closely related to their tectonic environments the mep was formed in basins proximal to or within an active oceanic rift consequently having ultramafic and mafic rocks as the major source for the lmep calc alkaline volcanic sequences formed in an island arc like setting were the major source these two contrasting volcanic plutonic terranes were tectonically juxtaposed during late archean collisional tectonics

distribution of ammonium in minerals of metamorphic and granitic rocks abstract sixty nine analyses are given for nh4 in minerals of metamorphic and granitic rocks mostly from the ryoke belt japan the distribution of nh4 in coexisting minerals is quite systematic suggesting that nh4 is one of the stable geochemical components in high temperature processes biotite has the highest content of nh4 followed by muscovite k feldspar and plagioclase pure quartz is almost free from nh4 calcic plagioclase contains less nh4 than does sodic plagioclase the partition coefficients d pl bi d kf bi and d kf bi are on the average 0 11 0 38 and 0 43 respectively the fractionation of nh4 in these minerals is quite similar to that of rb but much smaller than that of cs distribution of nh4 as well as those of rb and cs appears to be explained by its ionic radius and the shortest cation o distances in alkali positions of minerals

c and n isotopic composition and the infrared absorption spectra of coated diamonds evidence for the regional uniformity of co2h2o rich fluids in lithospheric mantle abstract the 13 c and 15 n values nitrogen abundances and nitrogen aggregation states of coated diamonds from botswana angola sierra leone and siberia have been determined a single cubic micro diamond from the northern territory australia was also analysed the cubic coats of the diamonds and the cubic diamond from australia had a restricted range in isotope composition 13 c 7 2 to 4 1 15 n 8 7 to 1 7 in contrast the cores of the coated diamonds were found to be highly variable 13 c 21 1 to 1 9 and 15 n 2 8 to 12 1 with the majority being positive all of the coats gave the type iaa absorption spectra together with others due to micro inclusions dominated by h 2 o and co 2 whereas the cores contained nitrogen that was more highly aggregated the results suggest that coated diamonds were formed following the influx of co 2 h 2 o rich fluids into diamond bearing lithosphere pre existing diamonds acted as seeds for renewed growth and these are now the cores of the diamonds these cores may be very variable in terms of morphology isotopic composition age nitrogen aggregation state and crystallinity depending on the particular history of the source regions and the conditions of diamond growth it is believed that the influx of volatiles and the growth of the coats was linked to kimberlite magmatism the 13 c and 15 n results for the coats indicate that in terms of c and n isotope composition the source of co 2 h 2 o rich fluids is globally quite homogeneous it is probably located beneath continental lithosphere and in addition may have characteristics similar to the source of the ocean island basalts

a record of appalachian denudation in postrift mesozoic and cenozoic sedimentary deposits of the u s middle atlantic continental margin abstract the complex interplay between source terrain uplift basin subsidence paleoclimatic shifts and sea level change left an extensive sedimentary record in the contiguous offshore basins of the u s middle atlantic margin salisbury embayment baltimore canyon trough and hatteras basin isopach maps of 23 postrift lower jurassic to quaternary a allostratigraphic units coupled with a revised stratigraphic framework reveal that tectonism by regulating sediment supply accumulation rate dominated the interplay of forcing mechanisms tectonic pulses are evidenced by abruptly accelerated sediment accumulation marked latitudinal shifts in the location of depocenters and regional changes in lithofacies relatively rapid tectonic subsidence during the early and middle jurassic history of the basins may have enhanced sediment accumulation rates beginning in the late jurassic however subsidence rates decreased significantly though occasional short pulses of subsidence may have effected relative sea level rises sea level change heavily influenced the distribution and redistribution of sediments one they reached the basins and paleoclimate regulated the relative abundance of carbonates and evaporites in the basins we conclude that source terrains of the central appalachian highlands were tectonically uplifted intensely weathered and rapidly eroded three times since the late triassic 1 early to middle jurassic aalenian to callovian 2 mid early cretaceous barremian and 3 late cenozoic middle miocene intervals of tectonic quiescence following these three tectonic pulses provided conditions suitable for the formation of regional erosion surfaces geomorphic features commonly reported to characterize the central appalachian highlands this series of three irregularly spaced tectonic quiescent cycles does not however match the traditional four cycle concept of post triassic appalachian peneplanation

isotopic variations of the fixed nitrogen in metamorphic rocks the content and isotopic composition of fixed nitrogen nh 4 n in metasedimentary rocks phyllites mica schists and gneisses from prograde metamorphic zones in the erzgebirge saxon ore mountains g d r were measured and show that the nitrogen content decreases and 15n increases with increasing metamorphic grade the geochemical and isotopic behaviour of nitrogen during contact metamorphism was investigated in a mica schist profile through the contact aureole of a granitic pluton the nitrogen concentrations of the wall rocks decrease towards the contact whereas the 15n values become higher various model experiments were carried out in the laboratory to explain the changes observed in the rock samples the nitrogen isotope fractionation factor was determined for the isotopic exchange in the system ammonium sulphate ammonia at 250 c 1 0143 0 0012 and 350 c 1 0128 0 0006 in accordance with the model experiments the conclusion seems warranted that the change in nitrogen content and its isotopic composition in rocks during metamorphic processes depend mainly on the metamorphic temperature

archaean carbon reservoirs and their relevance to the fluid source for gold deposits it is commonly assumed1 2 that seawater derived carbon in altered volcanic rocks is the only major pre metamorphic carbon in carbonate reservoir in archaean greenstone belts thus carbonates from archaean gold deposits with stable isotope ratios too negative median 13c 3 to have been derived from such carbon 13c usually assumed to be near 0 have been interpreted as being derived from a local felsic magmatic source1 barley and groves3 however recognize two carbonate alteration styles that predate regional metamorphism and gold mineralization in the norseman wiluna greenstone belt of western australia evidence is summarized below these are seafloor alteration and fault controlled regional alteration which as shown below have completely different carbon isotope compositions the latter median 13c 4 8 implies a major juvenile carbon co2 flux from the mantle during greenstone belt evolution and is thus of fundamental importance in itself however here we show why the mere existence of two carbon in carbonate reservoirs and variation in carbon isotope ratios of carbonates between individual gold deposits in one area negate some of the fundamental assumptions used to support magmatic fluid models1

archean geodynamics and the abitibi pontiac collision implications for advection of fluids at transpressive collisional boundaries and the origin of giant quartz vein systems abstract giant quartz vein systems of all ages from the archean to cenozoic are characterised by similarities of geodynamic setting local structure paragenesis metal budget p t t paths and chemical fluid dynamic and isotopic properties that collectively imply a singular hydrothermal process such quartz vein provinces form in regional brittle ductile shear zones that define terrane boundaries including closure of back arc basins and arc continent or continent continent collisions examples include the svz of the archean abitibi belt the norseman wiluna belt yilgarn block kolar schist belt india foothills metamorphic belt california and coast ranges megalineament in the cordillera and the cenozoic monte rosa district insubric line evidence for an archean collisional environment comes from the abitibi and pontiac subprovince tectonic boundary which hosts giant quartz vein systems in this region the amphibolite facies lacorne tectonic block in the archean abitibi greenstone belt is anomalous with respect to the prevalent low grade supracrustal sequences in neighbouring blocks the lacorne block has mature clastic sediments with a zircon provenance age spectrum from 3040 2691 ma and two granitic magma series a late syntectonic monzodiorite monzonite granodiorite syenite series formed over 2685 2670 ma similar to phanerozoic volcanic arc granites and post tectonic garnet muscovite granites emplaced at 2650 2630 ma which compositionally resemble phanerozic collisional s type granites the pontiac subprovince to the south of the abitibi greenstone belt shares all the above features with the lacorne block and collided with and was locally thrust under the abitibi belt with differential uplift of the lacorne block following collision to generate a tectonic window following collision of allochthonous terranes the conjuction of large volumes of subcreted oceanic crust and sediments between accreted terranes displaced isotherms rise and metamorphic dehydration are all necessary conditions for forming giant quartz vein systems metamorphic fluids are expelled along the terrane boundary structures at deep levels and focussed into second and higher order splays at mid crustal levels where quartz and au precipitation occurs hydrothermal fluids that formed the giant quartz veins are remarkably uniform in h o c and sr isotopic compositions albeit with small provincial variations the veins and gold precipitated at 270 360 c from fluids with low salinity and moderate co2 at 2 3 kbar in the brittle ductile transition

the carbon isotopic composition and nitrogen content of lithospheric and asthenospheric diamonds from the jagersfontein and koffiefontein kimberlite south africa asthenospheric diamonds from the jagersfontein kimberlite south africa have a 13c range from 8 9 to 24 4 vs pdb deep diamonds from the nearby koffiefontein mine have 13c values between 2 9 and 6 1 at these two locations asthenospheric diamonds are more frequently nitrogen type ii than type i if they are type i the nitrogen content is low and its state of aggregation is high diamonds from the jagersfontein kimberlite have a bimodal 13c distribution on the basis of the carbon isotopic composition of the hosts and their mineral inclusion composition one can suggest that as many as six distinct diamond source regions were sampled by the kimberlite in the mantle beneath jagersfontein the relative abundance of low 13c diamonds may be higher in the asthenosphere than in the lithosphere in the koffiefontein sample suite asthenospheric and lithospheric diamonds are indistinguishable in 13c the differences in the nature of the relationships between isotope and inclusion chemistry demonstrated by deep diamonds from two diatremes which are very closely associated in space and time indicate a high degree of complexity in mantle carbon geochemistry even within the asthenosphere

high ammonium contents in the 3800 ma isua supracrustal rocks central west greenland abstract ammonium contents of whole rock and biotite samples from the metasedimentary sequence of the 3800 ma isua supracrustal belt central greenland have been determined and compared with the ammonium contents of volcanic sedimentary metamorphic and granitic rocks from the late archaean early proterozoic a sample of the amitsoq gneiss has an nh4 content whole rock 9 ppm biotite 29 ppm that is similar to many younger igneous rocks ammonium contents of the supracrustal rocks vary with chemistry and mineralogy of the rock and most likely were inherited from the original sediments a series of carbonate bearing and basic plagioclase bearing biotite rich schists from sequence a have varying nh4 contents whole rock 6 24 ppm that increase with an increasing chemical clastic component and decreasing volcanogenic component biotites in these rocks together with biotites in mica schists of the same sequence have high nh4 contents which vary over a small range of 54 95 ppm suggesting that the nh4 in these rocks was contained mostly in authigenic clays of the original sediments all these data imply that clay minerals were the major sink of nh4 or other nitrogen compounds on the earth s surface at 3800 ma two samples of graphite bearing garnet staurolite mica schist from sequence b have remarkably high nh4 contents whole rock 68 and 86 ppm biotite 234 and 253 ppm which are as high as or even higher than those of common late archaean early proterozoic argillites it appears almost certain that the original thick clay rich bottom sediments contained a large amount of nh4 or other nitrogen compounds the most likely major source of nitrogen in these nh4 enriched graphite bearing mica schists might be organic nitrogen

rapid dewatering of the crust deduced from ages of mesothermal gold deposits the large scale migration of fluids through the continental crust has been well documented but there is no consensus regarding the timing of fluid migration relative to orogenic episodes or rates of crustal dewatering1 here we present40ar 39ar dates for muscovites from quartz veins along a major shear zone in southeast alaska which show that the veins were emplaced in the early eocene during the late stages of orogenic deformation hydrother mal activity took place for only about 1 myr and along a distance of at least 200 km the fluids were generated by metamorphic reactions in subducted crust along the north american plate margin and were apparently trapped in the crust by the low permeabilities accompanying a convergent tectonic regime until 56 myr ago the rapid dewatering event coincided with a change in plate motion at 56 55 myr which caused a shift from convergent to partly transcurrent tectonics we suggest that this change in tectonic regime led to increased crustal permeabilities and hence the possibility of large scale fluid migration

a preliminary study of 15n14n in octahedral growth form diamonds abstract the 13 c 15 n and nitrogen concentrations have been determined in 43 octahedral diamonds from southern africa australia and north america the sample set included locations such as finsch south africa a mine which produces diamonds having 13c values close to 5 and argyle australia a mine which produces 13c depleted diamonds the nitrogen within high 13c diamonds 13 c 6 4 to 2 9 was generally depleted in 15n relative to atmospheric nitrogen 15n 12 3 to 5 9 mode 4 similar to the results which have been obtained previously from fibrous diamond it is believed that the volatiles from which these diamonds grew are primitive being derived from a source located beneath the continental lithosphere which may have changed little in terms of 13c and 15n since the mid archean the low 13c diamonds 13c 19 4 to 9 5 contained nitrogen generally enriched in 15n relative to air 15n 3 0 to 16 4 mode 5 it is suggested that the major carbon isotope heterogeneity within the mantle as represented by diamonds such as these is located in deep mechanically unstable regions of the continental lithosphere the isotopic compositions of both c and n are consistent with this heterogeneity resulting from the subduction of crustal material however isotope fractionation related directly to diamond growth cannot be ruled out

a fluid inclusion study of au bearing quartz vein systems in the central and north deborah deposits of the bendigo gold field central victoria australia the central and north deborah gold deposits with a production of about 7 tons au are located in one of 15 domes in the bendigo gold field central victoria australia an important mesothermal lode gold province the gold bearing quartz vein systems are hosted predominantly by reverse faults in lower ordovician turbidites which underwent lower greenschist facies metamorphism in the central and north deborah mines there are six stages of quartz veins that developed in the following sequence 1 laminated veins with a metal association of au as sb pb zn ni 2 spurs au 3 massive barren veins main dilation stage 4 brecciated veins with au as 5 quartz ankerite veinlets and 6 late pure quartz or calcite veinlets of the three mineralized vein stages 1 2 and 4 only the first one predated or was coeval with the main deformation event whereas stages 2 and 4 postdated the main deformation the vein structures indicate that vein development was controlled by repeated fluctuations of fluid pressure inducing hydraulic fracturing fluid inclusions in vein quartz contain c o h fluids of variable compositions three main types of fluid inclusions are recognized at room temperature type i two phase primary ch4 v h2o l fluid inclusions type ii two or three phase primary pseudosecondary co2 v or l ch4 v or l h2o l fluid inclusions including iia ch4 co2 h2o and iib co2 h2o fluid inclusions and type iii h2o v h2o l fluid inclusions type iii fluid inclusions are further divided into three groups iiia primary pseudosecondary vapor rich iiib primary pseudosecondary liquid rich and iiic secondary liquid rich data from fluid inclusion distribution microthermometry and raman spectroscopy indicate that fluids associated with au mineralized quartz veins stages 1 2 and 4 have moderate salinity ranging from 0 to 12 wt percent nacl equiv modeled salinities around 7 8 wt nacl equiv these veins formatted at temperatures from 325 to 375 c and pressures of 200 to 300 mpa with emplacement depths of about 8 to 12 km fluids associated with barren quartz veins stages 3 5 and 6 have a low salinity of about 0 to 3 wt percent nacl equiv modeled salinities about 2 3 wt nacl equiv and lower temperatures there is evidence of fluid immiscibility in all vein stages and mixing between fluids of the same souce but differing fo2 may also have influenced gold deposition cox et al 1995 hydrothermal ore forming fluids responsible for gold mineralization are in the ch2 co2 h2o nacl system whereas those of massive barren veins stage 3 and veinlets stages 5 and 6 are in the co2 h2o nacl and h2o nacl systems respectively the carbonic phases of type i and ii fluid inclusions range from pure ch4 through mixed ch4 and co2 to pure co2 gold ore forming fluids were weakly acidic with f o2 estimated to have been 10 25 to 10 37 bars and generally above the co2 ch4 and below the so2 h2s buffer boundaries throughout gold mineralization the relatively reduced mineralizing fluids are similar to those of other turbidite hosted lode gold deposits and counterparts in volcanic plutonic terranes

cenozoic sedimentation and paleotectonics of north central new mexico implications for initiation and evolution of the rio grande rift the rio grande rift is one of the major late cenozoic continental rifts of the world sharing most geophysical geochemical and geological characteristics with other rifts cenozoic evolution of the rift was synchronous with lithospheric plate interactions along and under the western north american margin paleocene eocene laramide primarily amagmatic compression related to flat slab subduction oligocene intermediate to silicic volcanism related to collapse of the slab miocene to present rifting related to complex plate interactions overprinted on the previous history cenozoic paleogeographic and paleotectonic characteristics are consistent with a passive mantle mode of rifting north central new mexico provides a unique opportunity to constrain models for rift initiation and evolution it is one of the few locations within any rift where excellently exposed pre rift and syn rift basin fill has been studied thoroughly enough to allow detailed paleogeographic reconstruction for almost the entire cenozoic cenozoic paleogeography for the study area is summarized as follows 1 eocene 58 37 ma a single amagmatic sedimentary basin el rito galisteo trended northwest southeast with laramide basement uplifts on three sides 2 early to late oligocene 37 28 ma intermediate magmatism with volcaniclastic aprons derived from the san juan and ortiz cerrillos volcanic fields and residual laramide uplifts 3 late oligocene early miocene 28 21 ma initiation of bimodal volcanism with widely dispersed volcaniclastic aprons derived from the predominantly silicic san juan and latir volcanic fields 4 early to middle miocene 21 15 ma continued volcaniclastic dispersal from silicic volcanic centers concurrent with initiation of block faulting to form half grabens internal drainage and erosion of phanerozoic strata and precambrian basement 5 middle to late miocene 15 8 ma continued deepening of half grabens widespread exposure of precambrian terranes formation of complex depositional environments in basin centers and continued bimodal volcanism 6 late miocene to present 8 0 ma concentration of extension in central grabens linked by accommodation zones major bimodal volcanism for example taos plateau and jemez mountains regional uplift and integration of rio grande drainage dispersal patterns petrofacies analysis k ar ages and chemical analyses of volcanic clasts provide details concerning three primary volcanic centers 1 san juan mountains 27 29 m y high k andesite and rhyodacite 2 latir volcanic field 25 28 m y high k andesite and rhyolite and 3 the previously unrecognized servilleta plaza center 22 23 m y latite high k andesite and rhyodacite which may have been a southern extension of the latir field these three volcanic centers provided detritus to the following units respectively 1 esquibel mbr of los pinos fm upper abiquiu fm middle picuris fm bishops lodge mbr of tesuque fm and volcaniclastic units in the northern albuquerque basin zia and abiquiu 2 cordito mbr of los pinos fm and uppermost abiquiu fm and 3 chama el rito mbr of tesuque fm and upper picuris fm use of petrofacies 1 esquibel 2 cordito and 3 plaza simplifies the chaos of stratigraphic nomenclature and promotes regional correlations of poorly exposed units

the late archean hemlo gold deposit ontario canada a review and synthesis abstract the hemlo gold deposit northwestern ontario canada is situated within the late archean hemlo heron bay greenstone belt of the wawa subprovince of the superior province the greenstone belt represents an ancient volcanic island arc and has subsequently been subjected to a complex history of evolution including two episodes of regional metamorphism three main pulses of granitoid plutonism and four generations of structures related to two major deformational events the main second regional metamorphism was a low p t event and generally belonged to greenschist amphibolite transition facies but climaxed at middle amphibolite facies along both margins and the central axis of the greenstone belt the hemlo gold deposit is located within the central metamorphic zone of middle amphibolite facies the main orebody contains approximately 80 mt of ore averaging 7 7 g t au and is 2900 m along strike 1300 m down dip and 3 to 45 m thick additional gold mineralization extends sporadically and discontinuously westward and includes the c zone of the williams mine and the north south and highway zones of the golden sceptre resources limited property the main orebody and north zone are contained within a major shear zone proximal to a lithological contact between structurally underlying felsic rocks and overlying metasedimentary rocks the c and south zones are entirely within the felsic rocks and commonly are present as intensively sheared and altered lenses the hemlo deposit is enriched in as ba hg mo s sb te tl and v its economic gold concentrations correlate with the isotopic compositions of sulphide minerals but not their total abundances the hemlo gold deposit is characterized by intensive and extensive hydrothermal alteration including potassic alteration calc silicate alteration sulphidation and less significant silicification tourmalinization and carbonatization field occurrence textural relationships quantitative p t x t data from mineral equilibria oxygen isotope geothermometry fluid inclusion microthermometry and u pb and 40 ar 39 ar geochronology suggest a protracted history of hydrothermal activity which commenced with an earlier calc silicate alteration 550 c and 3 4 kbar 2678 2676 ma immediately after the peak of the second regional metamorphism followed by pervasive potassic alteration mica schists and microcline rich rocks 450 500 c 2672 2670 ma and terminated with a later low to very low grade calc silicate alteration 200 400 c 1 2 kbar and 2643 2632 ma there remains uncertainty among investigators regarding the timing and style of gold mineralization of the hemlo deposit consequently many ore forming processes have been proposed for its origin including syngenetic metamorphic replacement and porphyry models suggesting gold mineralization either pre dating or during the second regional metamorphism and also structural and hot spring epithermal models indicating mineralization post dating the second regional metamorphism we suggest that the deposition of metals and sulphides in the hemlo deposit is most consistent with a multistage late replacement model the hemlo shear zone was activated as a regional planar structure during the second regional metamorphism and became a conduit for regional fluids during periods of dilatancy orogenic activity was protracted and fluids focused along the conduit could have been variously of metamorphic and magmatic origin at different stages in the development of the hemlo deposit gold mineralization most likely occurred during the late calc silicate alteration event about 40 ma after the second regional metamorphism

multiple growth events during diamond genesis an integrated study of carbon and nitrogen isotopes and nitrogen aggregation state in coated stones the internal structure and growth history of diamonds have been investigated by the study of carbon isotopes and the abundance isotope composition and aggregation state of nitrogen polished plates of zoned diamonds coated stones were laser sectioned for infrared mapping and isotope profiling nine small cubic and three small octahedral diamonds from zaire and two coated stones and five small cubic diamonds specifically from the mbuji mayi area of that country were also investigated the isotope composition and infrared characteristics of the diamonds of cubic habit including coats were found to be indistinguishable with 13c in the range 5 9 to 7 5 and all the nitrogen being present as the paired structure iaa with a 15n of between 3 1 and 8 0 the diamonds of octahedral habit including the cores of the coated stones were more variable 13c varied from 4 9 to 9 8 infrared spectra of the octahedral diamond exhibited either strong iaa characteristics or a combination of iaa and iab platelets were present in most cases 15n values from octahedral diamonds varied from 5 8 to 13 4 the data suggest that the octahedral diamonds represent a variety of growth events separated in time and or space and that the diamonds resided in the lithosphere for a period sufficient for thermal maturation in the case of the coated stones the octahedral diamonds were later involved in an event which added a component from a single relatively homogeneous reservoir cubic habit diamonds were formed at the same time this latter event is believed to be related to the kimberlite eruption comparison of the data with carbon isotope analyses of cubic diamond from other sources suggests that this reservoir could be widespread

apatite fission track results from ireland and the porcupine basin and their significance for the evolution of the north atlantic abstract apatite fission track analysis was applied to samples from the irish mainland and exploration boreholes 13 3 1 26 26 1 and 26 28 1 in the northern porcupine and donegal basins to study the regional thermal and tectonic history of the irish continental margin samples from the irish mainland have apatite ages ranging from 301 25 to 56 5 ma with mean track lengths in the range 13 89 0 25 to 11 19 0 22 m and indicate higher temperatures during the middle jurassic about 160 180 ma and during the late cretaceous to early tertiary about 50 80 ma these two periods of increased palaeotemperatures correlate with major reorganizations in plate movements and suggest that uplift and erosion after the maximum palaeotemperatures was achieved as a consequence of a variation in the regional pattern of crustal stress the results from borehole 26 28 1 indicate that within the resolution of the data all samples are presently at their maximum post depositional burial temperatures to produce the mean track length results in the range 11 24 0 49 to 11 81 0 20 m from borehole 26 26 1 it is necessary to invoke post palaeocene geothermal gradients in excess of 50 c km increased relative to the present day gradient 28 c km similar increased geothermal gradients are recognized during the early tertiary in adjacent sedimentary basins and may be explained as a result of heat advection by gravity driven fluid flow established in response to tectonic uplift of the basin margins

jupiter s cloud structure from galileo imaging data the vertical structure of aerosols on jupiter is inferred from data obtained by the nasa galileo solid state imaging system during the first six orbits of the spacecraft images at 889 nm a strong methane band 727 nm a weaker methane band and 756 nm continuum taken at a variety of lighting and viewing angles are used the images are displayed and described in the companion paper by vasavada et al 1998 icarus 135 265 275 conservative scattering cloud particles with laterally uniform single scattering properties are assumed in the analysis and are shown to be consistent with the data at these wavelengths particles are bright and the darkest locations on jupiter correspond to the smallest optical thickness of aerosols optical depths and vertical positions of aerosol layers vary from place to place and are the retrieved quantities in the analysis only mid and low latitudes are sampled in this data set a stratospheric haze with an optical depth of roughly a tenth and an upper tropospheric haze with an optical depth of 2 to 6 exist over all regions both are consistent with previous conclusions based on data of lower spatial resolution e g west et al 1986 icarus 65 161 217 the new data show that these layers contain little lateral structure on scales smaller than the planetary jets on scales of the jets and ovals the top and bottom of the upper tropospheric haze vary in elevation the concentration of particles optical depth per pressure interval varies less than does the total optical depth near the base of the upper tropospheric haze is a third cloud component usually at pressure p 750 200 mb which is less than a scale height in geometric thickness its optical depth varies from zero to about 20 on regional scales and often varies by 50 on scales of a few tens of kilometers optical depth variations in this cloud are the principal cause of the features in jupiter s atmosphere seen at red and longer wavelengths it is probably composed of ammonia the expected nh 4sh cloud has not been identified in this work perhaps because it exists only at locations where it is concealed beneath higher clouds our retrievals also cannot rule out a pervasive deep haze without small scale structure finally in one region northwest of the great red spot a deeper cloud is identified parts of it lie at a pressure greater than four bars it is associated with a rapidly changing storm system with optical depth of several tens or more and a range of cloud heights between p 4 bars to p 400 mb it is probably composed of water

nitrogen isotope compositions of metasedimentary rocks in the catalina schist california implications for metamorphic devolatilization history abstract in the catalina schist subduction zone metamorphic complex california metasedimentary rocks show a decrease in n concentration and an increase in 15nair with increasing metamorphic grade lowest grade lawsonite albite rocks contain 632 185 ppm n with 15 n 1 9 0 6 whereas highgrade amphibolite equivalents contain 138 76 ppm n with 15 n 4 3 0 8 loss of n accompanied devolatilization reactions that evolved h2o rich c o h s n fluids through consumption of chlorite and phengitic white mica and production of mineral assemblages containing muscovite biotite garnet and kyanite whole rock n concentrations of up to 200 ppm in veins and pegmatites produced during high p t metamorphism reflect the redistribution of n during devolatilization and partial melting of the metasedimentary rocks bulk fluid rock n isotope fractionations 15n 15nfluid 15nrock of 1 5 1 were calculated with the rayleigh distillation equation taking into account variability in rock composition by comparison of samples with similar k2o concentrations these fractionations are similar to but slightly lower than published calculated fractionations for n2 nh4 exchange at the temperature range of 350 600 c over which most of the devolatilization occurred in the catalina schist approximately 3 4 to 2 25 the n systematics appear to be explained by n2 nh4 exchange and a devolatilization process intermediate in behavior to batch volatilization and rayleigh distillation the observed shifts in n concentration and 15n cannot be explained by nh3 nh4 exchange at these temperatures using the equilibrium models the distillation devolatilization process implicated in this study may govern the behavior of other trace elements partitioned into hydrous fluids during devolatilization e g b cs u similarity of the calculated fluid 15n 1 5 to 5 5 with compositions of natural gases inferred to be derived from metasedimentary sources indicates the possibility of using n as a tracer of large scale volatile transport

the geochemical behaviour of nitrogen compounds during the formation of black shale hosted quartz vein gold deposits north wales abstract previous fluid inclusion studies of the black shale hosted quartz vein au mineralisation at the clogau st davids mine have revealed an association between n 2 levels in the fluids and the grade of au mineralisation suggesting mobilisation and or involvement of n compounds during the mineralising event we have analysed the nh 4 contents of silicates in altered and unaltered rocks hosting the veins in an attempt to constrain the origin of the n ammonium levels in altered wall rocks differ from those in similar rocks remote from mineralisation implying n mobility but only in the zones of most intense alteration are wall rock shales severely depleted in nh 4 however fluids feeding the vein system are low in n and leaching of nh 4 from shales is the most likely source of n during mineralisation thermodynamic modelling of fluid phase equilibria shows that oxidation of nh 4 from silicates is a viable source of n in the fluid such a reaction will have a dramatic reducing effect on the redox state of the fluid and may in conjunction with reduction by graphite be an important trigger of ore deposition the results suggest that amine complexes are unlikely to have been important in metal transport as has been proposed for some epithermal volcanic au deposits however the deposits studied here are of metamorphic origin and the geochemical behaviour of n compounds appears to be markedly different in this case

frio formation of texas gulf coastal plain depositional systems structural framework and hydrocarbon distribution the frio formation oligocene miocene is one of the major tertiary progradational wedges of the texas gulf coastal plain and has yielded nearly 6 billion bbl of oil and 60 tcf of gas the frio and its updip equivalent the catahoula formation consists of deposits of two large fluvial and associated deltaic systems centered in the houston and rio grande embayments structural history in the houston embayment is dominated by syndepositional deformation of underlying jurassic salt mobilization of thick undercompacted prodelta and slope muds characterized the tectonic evolution of the deltaic sequence in the rio grande embayment these two major deltaic depocenters are separated by a vertically stacked strike parallel coastal barrier and strand plain system underlying interbedded and transgressive shelf prodelta and continental slope mudstone sequences provide principal source and sealing facies sparse organic geochemical data regional thermal gradients and distribution patterns of hydrocarbons show that large volumes of oil and gas have likely been generated within and effectively expulsed upward and landward from thermally mature normally to moderately undercompacted sequences of bounding mudstone facies all frio depositional systems contain major geologically defined hydrocarbon producing plays oil and gas field productivity data show a log normal frequency distribution for all but the largest fields per volume productivity production styles and types of hydrocarbons within each of the ten recognized plays reflect available source rock types differing thermal and compaction histories and variable reservoir and trap configurations that characterize each depositional system systematic distribution patterns of both physical and chemical properties of produced hydrocarbons can be related to source facies regional thermal regime and post generation modification by 1 continued maturation thermal cracking and deasphalting 2 migration and attendant chromatographic separatio and 3 bacterial alteration

paleogeographic evolution of early deep water gulf of mexico and margins jurassic to middle cretaceous comanchean the paleobathymetric configuration of the early gulf of mexico is inferred from 1 cretaceous carbonate shelf margins interpreted from seismic profiles and other stratigraphic data 2 distribution of jurassic and cretaceous platform and basinal facies and 3 hindcasting of subsidence history in the central basin substantial paleogeographic ambiguity results from uncertainty about 1 kinematics and timing of late triassic to jurassic extensional opening of the gulf basin which probably involved major strike slip faulting 2 the magnitude of subsequent compressive deformation on the western and southern basin margins and 3 possible accretion of allochthonous terranes the initial sub sea level topographic depression created by crustal extension is manifested by the distribution of callovian pre marine salt and oxfordian basinal marine facies late jurassic sea floor spreading split the main salt body and created a central deep water 1 km trough from the oxfordian to early neocomian ramp like platform margins generally underwent net retreat except in the northern gulf where major clastic progradation took place carbonate platform margins with sufficient paleorelief for geometric expression on seismic profiles developed early in the cretaceous these cretaceous carbonate margins exhibit a variety of architectural styles including 1 in the northwestern gulf two cycles of progradation coahuilan and comanchean terminated by drowning events 2 in the southwestern gulf a single cycle of progradation 3 continuous aggradation along the florida and campeche escarpments paleobathymetric relief across cretaceous carbonate margins ranges from high and steep to low relief to ramplike the close of comanchean drowning event greatly and permanently reduced the extent of circum gulf carbonate platforms and especially rimmed margins locations of cretaceous carbonate margins were influenced by basement hinge zones and paleohighs developed on attenuated continental crust by the distribution of salt and by clastic progradation eustacy is commonly invoked to explain behavior and architecture of carbonate margins in the gulf but other paleoenvironmental variables such as nutrient abundance are probably important as well

giant quartz vein systems in accretionary orogenic belts the evidence for a metamorphic fluid origin from δ 15 n and δ 13 c studies abstract nitrogen isotope compositions of hydrothermal micas in giant structurally hosted quartz vein systems place constraints on the origin of the hydrothermal fluids the vein systems are from neoarchean terranes in the superior province of canada and the norseman terrane western australia the four quartz vein systems studied in different terranes formed at metamorphic grades ranging from lower greenschist to lower amphibolite facies coevally with metamorphism of the host terrane nitrogen resides predominantly as nh 4 in hydrothermal mica for lower to upper greenschist facies quartz veins n contents and 15 n values of micas are between 40 and 200 ppm and 15 and 21 respectively whereas in quartz veins formed at the greenschist to amphibolite transition and lower amphibolite facies micas have n contents of 20 70 ppm and 15 n of 11 24 in contrast micas and k feldspars from granitoids in the neoarchean abitibi and red lake greenstone belts are characterized by systematically lower 15 n of 5 to 5 and generally lower n contents of 20 50 ppm comparable to other granitoids carbon and oxygen isotope compositions of hydrothermal ferroan dolomite and calcite show systematic depletions with increasing metamorphic grade the mean values range from 2 2 to 3 7 for 13 c and 13 8 to 12 3 for 18 o in veins formed at lower to upper greenschist facies but from 5 8 to 7 1 for 13 c and 11 4 to 9 8 for 18 o at higher metamorphic grades nitrogen isotope compositions rule out mantle 15 n mean 5 magmatic 15 n 5 to 15 n mean 4 4 2 0 for the quartz veins accordingly the results are consistent with fluids derived from metamorphic dehydration of oceanic crust and sediments within accretionary orogenic belts