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

spatial and temporal variation of summer precipitation over the eastern tibetan plateau and the north atlantic oscillation abstract the interannual variability of summer precipitation over the eastern tibetan plateau etp was examined in relation to the northern hemisphere macroscale circulation patterns during the period 1961 90 summer precipitation data for 66 stations located above 2000 m msl are used in the analysis using principal component analysis it is found that the dominant spatial pattern of interannual variability of the summer precipitation is a seesaw structure between the southern and northern parts of etp correlation analysis shows that this pattern of precipitation anomalies is closely associated with the north atlantic oscillation nao further analysis based on midtropospheric geopotential height and wind data suggests the upstream zonal flow variation associated with the nao pattern as the major mechanism linking the regional precipitation fluctuation to macroscale circulation conditions during the summers of low nao index values the westerly winds between 40 and 50 n from the eastern atlantic to

the puguang gas field new giant discovery in the mature sichuan basin southwest china the giant puguang gas field with a proven original in place gas volume of 350 109 m3 12 36 tcfg was discovered in 2003 in the eastern sichuan fold thrust belt of the mature sichuan basin southwest china the field is a combination structural stratigraphic trap closed by lateral depositional change and fault closure the trap evolved from a paleo oil reservoir originating in the triassic jurassic the entrapment of thermal gas which was derived from lower middle silurian and permian source rocks occurred during deep burial in the jurassic cretaceous tertiary quaternary compression transformed the paleotrap into the present gas reservoir gas is contained in the lower triassic feixianguan and the upper permian changxing reservoirs which consist predominantly of dolomitized oolites deposited in shelf and platform margin shoal and backreef environments reservoir quality is characterized by porosity of 1 29 and permeability of 0 01 9664 md with buried depth greater than 5000 m 16 400 ft the discovery of the puguang field exemplifies the successful application of a new play concept and new technology in a mature basin the discovery resulted from a shift in exploration strategy from structures to stratigraphic traps in reef and shoal dolomites and benefited from advanced high resolution seismic techniques the discovery will not only broaden the exploration scope in the sichuan basin but also provide an excellent analog for exploration in other fold thrust belts worldwide

the structure and geological development of the erickson gold mine cassiar district british columbia with implications for the origin of mother lode type gold deposits the erickson gold mine is a typical gold quartz vein deposit the veins are hosted by a thrust imbricated gently dipping synformal allochthon of low grade metamorphic devonian to upper triassic basalts argillites and peridotites of oceancrustal origin belonging to the sylvester group part of the slide mountain assemblage the sylvester allochthon lies concordantly on devonian miogeoclinal sedimentary rocks of the north american continental margin and was emplaced in the middle jurassic as a result of the collision of the quesnel arc with north america the veins in the mine are hosted mainly by a moderately dipping system of shear zones with approximately orthorhombic symmetry indicating a triaxial bulk inhomogeneous strain pattern superimposed on the earlier formed gently dipping thrusts steeply dipping extension veinlets rotation of schistosity and downdip slickenlines indicate the maximum shortening axis was subvertical the veins display complex superimposed ribbon and breccia textures in

measurements of the cretaceous paleolatitude of vancouver island consistent with the baja british columbia hypothesis a previously unsampled outcrop of gently dipping or flat lying upper cretaceous sedimentary strata in the vancouver island region which contains unaltered aragonitic mollusk fossils yielded a stable remanent magnetization that is biostratigraphically consistent with cretaceous magnetochrons 33r 33n and 32r these results characterized by shallow inclinations indicate an upper cretaceous paleolatitude of about 25 3 degrees north which is equivalent to that of modern day baja california these findings are consistent with the baja british columbia hypothesis which puts the insular superterrane well south of the oregon california border in the late cretaceous

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

constraints of fluid inclusions and in situ s pb isotopic compositions on the origin of the north kostobe sediment hosted gold deposit eastern kazakhstan abstract the north kostobe gold deposit in the kalba gold province eastern kazakhstan is tectonically located in the chara shear zone of the western altaids the chara shear zone separates the kazakhstan microcontinent and siberia craton which collided in the late carboniferous in the north kostobe deposit au mineralization is distributed along an e w striking shear zone linked to the nw se trending regional faults and is mainly concentrated in quartz sulfide carbonate veins or in disseminated sulfides hosted in carbonaceous metasedimentary rocks sulfide minerals are mainly pyrite and arsenopyrite and were formed in three generations the first generation is dominated by pyrite py1 occurring as microcrystal aggregates followed by the second generation including euhedral arsenopyrite and compact pyrite py2 locally overgrowing early py1 grains as rims third generation of pyrite py3 is present in barren micro fractures crosscutting early mineralized rocks native gold is present in cracks of brecciated arsenopyrite grains and as inclusions in py2 investigations on fluid inclusions in auriferous quartz veins indicate that the ore related fluids are co 2 bearing with homogenization temperatures of 288 c and low salinity 1 42 to 8 03 wt nacl equiv the fluids have calculated 18 o and d values ranging from 9 96 to 11 86 and from 75 to 97 1 respectively suggesting that they were most likely metamorphic in origin the first and second generations of ore sulfides have similar 34 s values 6 to 2 6 which possibly indicating a common sulfur source as the hosting sedimentary rocks in contrast py3 grains have a huge range of 34 s values 40 to 54 5 indicating a biogenic source for the sulfur on the other hand in situ pb isotopic compositions of different generations of sulfides show a similarly mixed lead source possibly involved orogen mantle and lower crust pb reservoirs we propose that the gold bearing fluids of the north kostobe gold deposit were most likely derived from gold rich pyritic carbonaceous sedimentary rocks through dehydration during the metamorphism that was related to the collision between the kazakhstan microcontinent and siberia craton in the late carboniferous it is thus concluded that the north kostobe gold deposit shares many characteristics with typical orogenic gold deposits in terms of tectonic settings mineralization styles fluid compositions and source of fluids and gold our new findings thus have important implications for regional exploration in the kalba gold province

paleomagnetism of the spences bridge group and northward displacement of the intermontane belt british columbia a second look the spences bridge group is a mid cretaceous 104 ma volcanic succession in the southern intermontane belt of the canadian cordillera 50 5 n 121 w it comprises the pimainus formation mafic to felsic lava volcaniclastic and interbedded epiclastic rocks and the overlying spius formation andesitic lava flows including previous work we have 55 sites distributed among 15 localities representing most of the 3000 m thickness forty seven sites 286 oriented cores 457 specimens mainly andesites yielded acceptable data the beds are gently to moderately tilted partly due to synvolcanic deformation samples taken from a sequence of flows at any one locality have with one exception well grouped magnetization directions polarities are all normal as expected for rocks laid down in the cretaceous normal superchron declinations always are clockwise of that expected of cratonic north america indicating 60 rotation of the spences bridge group as a whole however declinations differ from locality to locality implying relative interlocality rotations about vertical axes hence inclination only analysis has been used to estimate dispersion mean inclination and paleolatitude minimum dispersion for the spius formation 27 sites was achieved after 80 untilting but the changes between 80 and 100 untilting are insignificant indicating that magnetization was acquired predominantly before tilting by contrast minimum dispersion for the pimainus formation 20 sites was achieved after 50 untilting indicating that magnetization was acquired after synvolcanic tilting when buried beneath the overlying spius formation polished thin section studies show that magnetite in the pimainus formation has undergone extensive low temperature hydrothermal alteration whereas magnetite in the spius formation shows little alteration throughout the range of tilt correction from 0 to 100 the mean inclinations of both formations were less than expected from observations obtained from mid cretaceous rocks of cratonic north america the best estimate of paleolatitude from the spius formation is 50 8 5 0 p 0 05 which is 9 5 5 7 less than would be expected had the rocks had been rigidly attached to north america this corresponds to displacement from the south of 1100 600 km displacement between the northern intermontane belt and craton probably was accommodated along major strike slip faults northern rocky mountain trench finlay pinchi etc in the south our results require a major dextral fault the intra quesnellia fault to be situated during late cretaceous or paleocene time within or marginal to the omineca belt along which about 1000 km of dextral motion occurred this could be a southern extension of the pinchi fault whose trace is now obscured by eocene extension and tectonic denudation the results also indicate that the largest tectonic discontinuity in the canadian cordillera occurs not to the east of the intermontane belt as commonly assumed but to the west because the displacements relative to cratonic north america observed from the intermontane belt are only about one third of those observed from the coast belt

formation of gold deposits review and evaluation of the continuum model abstract the continuum model has been one of the more widely advocated genetic models for gold only deposits such as those found in archaean greenstone belts it postulates that hydrothermal gold deposits were formed throughout a 20 25 km vertical crustal profile from temperatures above 700 c to below 180 c and that the deposition occurred synchronous with the peak of metamorphism the continuum model is reviewed at this stage because we believe that it does not successfully account for many aspects of gold deposit formation the most obvious shortcoming is in considering ore deposits found in rocks of uppermost amphibolite and granulite facies domains where temperatures were appropriate for partial melting the main chemical condition that favours melting is access to h2o from either aqueous fluid or the breakdown of hydrous minerals a major gold forming hydrothermal event at the peak of high grade metamorphism as implied by the continuum model is incompatible with partial melting of wallrocks instead of forming a hydrothermal gold deposit an aqueous fluid introduced during partial melting would be consumed to produce further melt five gold deposits are documented from high metamorphic grade domains within four separate archaean cratons one is the type example used in the continuum model the other four have been significant producers partial melting has been recorded in the wallrocks adjacent to gold mineralisation at big bell hemlo challenger and renco gold deposits in the sulphide rich ore itself at challenger and hemlo deposits and as cross cutting dykes and migmatites at griffins find for these five deposits and indeed in general the continuum model does not easily account for aspects of the fluid source fluid composition gold transport and metal deposition the evidence that has been used to support deposit formation at the peak of high grade metamorphism as required by the continuum model is generally compatible with one or more alternative genetic models textural and structural siting of the gold high variance mineral assemblages proportion of various gold host rocks and relative element enrichments at high metamorphic grade can be accounted for by gold formation at greenschist facies conditions followed by progress of the deposit to high metamorphic grade metamorphic model moreover the type examples in the continuum model of deposits in sub greenschist facies domains appear to be incorrectly ascribed and are likely to be in greenschist facies domains the continuum model does not adequately explain archaean gold deposits

structural aspects of suspect terranes and accretionary tectonics in western north america abstract the regional structural relationships between the cordilleran suspect terranes and cratonic north america and within the suspect terranes themselves are reviewed for eastern alaska western canada and united states and northern mexico a distinctive characteristic of the relationships between the suspect terranes and the north american craton is that although much modified by post accretionary strike slip faulting thrust faulting and extensional detachment faults the generally more distal and or oceanic terranes are frequently found as rootless nappes or thin thrust sheets sitting upon more inboard terranes or upon the craton itself the amount of tectonic transport implied is large strike slip faulting has been a very significant aspect of the structural evolution of the north american cordillera with total distributed displacements of over 2000 km documented on geologic relationships alone and much more implied by paleomagnetic evidence at least since later mesozoic time to the present movements were such that the terranes moved northward post accretionary intraplate telescoping along thrust faults transpressional strike slip faulting and delamination at mid to upper crustal levels were important processes in cordilleran tectonic evolution

metamorphic origin of ore forming fluids for orogenic gold bearingquartz vein systems in the north american cordillera constraints froma reconnaissance study of δ15n δd and δ18o the western north american cordillera hosts a large number of gold bearing quartz vein systems from the mother lode of southern california through counterparts in british columbia and southeastern alaska to the klondike district in central yukon these vein systems are structurally controlled by major fault zones which are often reactivated terrane bounding sutures that formed in orogens built during accretion and subduction of terranes along the continental margin of north america mineralization ages span mid jurassic to early tertiary and encompass much of the evolution of the cordilleran orogen nitrogen contents and 15n values of hydrothermal micas from veins are between 130 and 3 500 ppm and 1 7 to 5 5 per mil respectively these values are consistent with fluids derived from metamorphic dehydration reactions within the phanerozoic accretion subduction complexes which have 15n values of 1 to 6 per mil the 18o values of gold bearing vein quartz from different locations in the cordillera are between 14 6 and 22 2 per mil but are uniform for individual vein systems the d values of hydrothermal micas are between 110 and 60 per mil ore fluids have calculated 18o values of 8 to 16 per mil and d values of 65 to 10 per mil at an estimated temperature of 300 c d values of ore fluids do not show any latitudinal control these results indicate a deep crustal source for the ore forming fluids most likely of metamorphic origin low dh2o values of 120 to 130 per mil for a hydrous muscovite from the sheba vein in the klondike district reflect secondary exchange between recrystallizing mica and meteoric waters collectively the n h and o isotope compositions of ore related hydrothermal minerals indicate that the formation of these gold bearing veins involved dilute aqueous carbonic and nitrogen bearing fluids that were generated from metamorphic dehydration reactions at deep crustal levels these data are not consistent with either mantle derived fluids or granitoid related magmatic fluids nor do they support a model involving deeply circulated meteoric water

a fluid inclusion and stable isotope study of au quartz veins in the klondike district yukon territory canada a section through a mesothermal vein system gold bearing quartz veins in the klondike district of yukon territory are of earliest cretaceous age and are hosted by permian middle to upper greenschist facies metasedimentary and meta igneous lithologies belonging to the yukon tanana terrane the veins occur in brittle extensional structures which may have developed during extensional deformation during uplift of the klondike region in the late jurassic to early to middle cretaceous following thrust imbrication in the early jurassic the quartz veins are massive and unbanded with localized carbonate flooding and or pyritization of the wall rocks fluid inclusion studies have delineated significant regional p t x trends in primary inclusion fluids from the au quartz veins the fluids have low salinities 6 wt nacl equiv but variable co 2 concentrations decreasing from approximately 0 13 to 0 18 mole fraction co 2 in the southeastern klondike through variable concentrations in the central klondike x sub co 2 0 01 to asymp 1 00 to dominantly aqueous fluids x sub co 2 0 01 in veins from the northwest portion of the region estimated trapping pressures decrease from 2 300 or 700 bars to between 300 to 900 bars from southeast to northwest estimated fluid temperatures range from 300 degrees to 350 degrees c in central and southern lodes to 200 degrees to 250 degrees c in the northwest klondike the p x trends are interpreted as evidence for 1 effervescence of co 2 from the mineralizing fluid most likely in response to regional uplift during the life of the hydrothermal system and 2 that the au quartz vein system represents a section through a mesothermal vein system with deeper structural levels exposed in the southeastern klondike convection of the hydrothermal fluid may have been driven by an elevated geothermal gradient resulting from the rapid uplift the calculated isotopic composition of the au quartz vein fluids delta 18 o fluid 8 3 or 1 2ppmsmow delta d fluid asymp 150 or 18ppmsmow indicates that the mineralizing fluid was meteoric in origin but underwent considerable isotopic evolution prior to mineralization additionally the delta d fluid values of extracted inclusion fluids 166 or 11ppm and from hydrothermal muscovites 174 or 3ppm from the sheba vein system are in good agreement and indicate that the deuterium signature reflects that of the primary mesothermal fluid

mineralogic fluid inclusion and stable isotope studies of several gold mines in the mother lode tuolumne and mariposa counties california gold bearing quartz veins in the southern part of the mother lode california formed from fluids which contained si k co 2 ca s and metals and which were focused into permeable fault zones in the coulterville district of the mother lode these fluids altered serpentinite to a talc chlorite magnesite quartz assemblage which is crosscut by veins containing quartz dolomite sericite sulfides tellurides and gold textural relationships among the vein filling minerals reveal that galena tellurides pb ni ag au ag and native gold precipitated last among the vein minerals observations from the oro rico and mcalpine mines coulterville district indicate that these deposits were subjected to deformation during and after their formation fluid inclusions in the milky vein quartz exhibit only a narrow range in t sub m ice but a large range in t h which probably reflects posttrapping deformation microthermometry of fluid inclusions from vug filling quartz in the alleghany district in the northern portion of the mother lode reveals low salinity 2 h 2 o fluids with t sub mco 2 56 7 degrees c t sub m clathrate 9 5 degrees c and t sub h total 150 degrees to 200 degrees c visual estimates of co 2 content indicate a maximum of 10 mole percent co 2 in contrast inclusions from vug filling quartz in the coulterville district are filled with low salinity low t h 110 degrees 140 degrees c h 2 o rich fluids which were probably later than and unrelated to the gold mineralizing fluids oxygen isotope values of vein quartz from the southern mother lode indicate formation by 18 o enriched fluids in the range 6 to 11 per mil vs smow these values are similar to estimates of 8 to 15 per mil from vein quartz of the alleghany district taylor 1981 boehlke and kistler 1986 the average delta 13 c values for magnesite and dolomite of 4 5 or 0 6 per mil vs pdb from the southern mother lode are identical to results from the alleghany district 200 km to the north rosenbaum and taylor 1984 these data suggest either a single or well mixed source of oxidized carbon for the fluids oxygen isotope compositions of magnesite calculated using an experimentally derived phosphoric acid fractionation factor at 50 degrees c of 1 0096 are enriched in 18 o relative to coexisting dolomite as predicted for isotopic equilibrium the chemistry of the mother lode ore forming fluids is characterized by several approaches with an x sub co 2 estimate of 0 1 from fluid inclusions quartz magnesite assemblages in the coulterville serpentinite alteration zones limit the temperature of formation to below 325 degrees c for p 2 h 2 o fluid coupled with the assumption of trapping temperatures of 300 degrees c suggests minimum trapping pressures of 1 000 to 2 000 bars assuming equilibrium the assemblage of daughter minerals described by coveney 1981 indicates neutral ph conditions of 5 5 to 6 0 at the prevailing p t conditions sulfide mineral stabilities imply that h 2 s was the dominant sulfur species and that a sub o 2 of the mineralizing fluids was in the range 10 super 32 to 10 super 35 our preferred model for the formation of the mother lode vein system involves magmatic activity associated with a subducting slab in this model generation of ore forming fluids is possible by a variety of mechanisms including metamorphic devolatilization reactions in rocks adjacent to the plutons direct magmatic input and deep circulation of meteoric water any of these mechanisms is possible given the available data on the mother lode deposits

nitrogen isotope fractionations during progressive metamorphism a case study from the paleozoic cooma metasedimentary complex southeastern australia abstract the well studied paleozoic cooma metamorphic complex in southeastern australia is characterized by a uniform siliciclastic protolith of uniform age with a continuous range of metamorphic grade from subgreenschist to upper amphibolite facies and migmatite grade in an annular pattern around the cooma granodiorite those conditions are optimal for investigating variations of n concentrations and 15 n values during progressive metamorphism nitrogen concentrations decrease and 15 n increases with increasing metamorphic grade sub chlorite zone 120 ppm n 15 n 2 3 chlorite zone 110 ppm n 15 n 3 0 biotite and andalusite zone 85 ppm n 15 n 3 8 sillimanite and migmatite zones 40 ppm n 15 n 10 7 covariation of k and n contents is consistent with n substituting for k as nh 4 in micas observed trends of increasing 15 n values with decreasing nitrogen concentrations can be explained by a continuous release of nitrogen depleted in 15 n with progressive metamorphism which causes an enrichment of 15 n in the residual nitrogen of the rock equilibrium models for rayleigh distillation and batch volatilisation for data of the greenschist and amphibolite facies metasedimentary rocks can be explained by n 2 nh 4 exchange at temperatures of 300 600 c whereas observed large fractionations for the upper amphibolite facies and melt products in the migmatite grade samples may be interpreted as nh 3 nh 4 exchanges at temperature of 650 730 c lower values in the highest grade zones may also stem in part from input of 15 n depleted fluids from the granodiorite the magnitude of isotope fractionation of nitrogen is about 1 2 during progressive metamorphism of metasedimentary rocks from sub chlorite zone to biotite andalusite zone which is consistent with previous studies consequently the large spread of 15 n values in archean greenschist facies metasedimentary rocks of 6 to 30 can be accounted for by variable mixtures of mantle plume dominated volatiles with a 15 n of 5 and a 15 n enriched marine sedimentary kerogen component inherited from a ci chondrite veneer having 15 n of 30 to 42

timing of mineralization and source of fluids in a slate belt auriferous vein system hill end goldfield nsw australia evidence from 40ar39ar dating and o and h isotopes abstract 40 ar 39 ar dating of metamorphic biotite and alteration muscovite from the auriferous veins and host rocks at the hill end goldfield n s w australia has distinguished four major geological events including the timing of gold mineralization the earliest hydrothermal event occurred during the middle devonian tabberabberan orogeny 370 380 ma and resulted in the formation of quartz veins barren of au a second and major episode of vein emplacement occurred in the early carboniferous during the principal phase of metamorphism and deformation at 359 363 ma this was followed by au accumulation in two stages 1 after the major phases of quartz deposition and 2 during and after the development of conspicuous internal vein laminations 357 ma and 343 ma respectively two sources of fluid are proposed for vein and ore formation the first is a local metamorphic fluid characterized by 18 o h 2 o values of 8 9 to 12 5 per mil and d h 2 o values of 87 to 90 per mil the second is a mixed ore fluid with 18 o and d values in the range of 18 o h 2 o 8 4 to 11 per mil and d h 2 o of 49 to 36 per mil progressive entry of this second fluid sourced from trough fill or deeper crustal rocks is linked closely to cycles of gold precipitation at hill end

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

nitrogen isotope systematics of mesothermal lode gold deposits metamorphic granitic meteoric water or mantle origin archean gold bearing quartz vein systems constitute one of the most important classes of precious metal deposits however their origin remains controversial mantle granitoid meteoric water and metamorphic related models do not resolve this controversy new n and nitrogen isotope data on hydrothermal muscovite from six gold camps from the late archean superior province of canada and biotite from nine au mineralized locations from the norseman wiluna greenstone belt western australia are reported the majority of muscovites and biotites have n contents of 20 190 ppm and 15 n of 10 20 and 10 70 ppm and 10 24 respectively the 15 n of the mantle is 5 and of granitoids is 0 10 both with generally low n contents organic n is 0 10 meteoric water is 1 9 9 4 and metamorphic rocks are 3 18 increasing with higher grade accordingly the data appear to rule out a mantle hypothesis granitoid related magmatic processes or meteoric water for the deposits but are consistent with ore fluids derived by metamorphic dehydration reactions

geology hydrothermal fluids h o s pb isotopes and rb sr geochronology of the daxintun orogenic gold deposit in heilongjiang province ne china abstract the newly discovered daxintun sb au deposit heilongjiang province ne china is located at the north eastern margin of the central asian orogenic belt and it is hosted by neoproterozoic to early cambrian two mica schist and marble the antimony bearing minerals include stibnite famatinite tetrahedrite boulangerite freibergite and minor andorite gold occurs largely as native gold microparticles 0 1 20 m with high fineness 974 983 accompanied by quartz stibnite calcite and pyrite fluid inclusions in ore bearing quartz and calcite phases mainly comprise liquid vapor and minor co2 bearing three phase types and the gas phases contain co2 ch4 and h2s the daxintun fluid inclusions homogenize at 111 8 353 4 c and are of low salinity 0 53 12 51 wt nacl equiv the 18osmow of the daxintun quartz falls into a wide range 4 2 16 9 yet the dsmow falls into a narrow range 108 0 to 87 3 suggesting that the ore forming fluids formed from metamorphic fluids that mixed with meteoric water the stibnite 34s ranges between 0 9 and 5 1 mean 2 8 the stibnite has 206pb 204pb of 18 234 18 306 207pb 204pb of 15 537 15 561 and 208pb 204pb of 38 103 38 227 implying a metamorphic fluid source the conditions of gold and antimony precipitation are comprised of neutral ph and fluid mixing with decreasing fs2 and fo2 zircon u pb dating of the fine grained daxintun granite yields a late jurassic age 154 0 2 0 ma and the daxintun diorite dike yields a weighted mean age of 122 1 1 3 ma stibnite rb sr dating suggests that the sb au mineralization has an age of approximately 152 8 2 4 ma the observed ore deposit geology as well as new data on the ore forming fluids h o s pb isotopes and rb sr geochronology suggest that the daxintun sb au deposit is best described as an orogenic type gold deposit that formed during subduction of the palaeo pacific and the mongolia okhotsk oceans during the late jurassic

gold mineralization in the ashanti belt of ghana genetic constraints of the stable isotope geochemistry the ashanti belt of ghana is the key district of gold mineralization in the paleoproterozoic terrane of west africa the area considered in southwest ghana is covered by lithologies of the volcanic sedimentary birimian supergroup and the overlying elastic sedimentary tarkwaian group which were jointly folded and metamor phosed under greenschist facies conditions during the eburnean teetonothermal event at about 9 1 ga regional fotiation md subparallel shear zones hosting mesothermal gold mineralization developed during deformation coeval with metamorphism four major tsoes of primary gold mineralization are present in the ashanti belt 1 mesothermal generally steeply dipping quartz veins in shear zones mainly in birimian sedimentary rocks 9 sulfide ores with auriferous arsenopyrite and pyrite spatialty dosely associated with the quartz veins 3 sulfide disseminations and stockworks in granitoids and 4 paleoplaeers of the tarkwaian group this study concentrates on types 1 and 9 of the hydrotherlnal gold mineralization stable isotope analyses of host rock and ore components were performed with the ailn of obtaining parameters relevant to the origin and evolution of the fluids that produced gold mineralization carbonaeeous matter in the birimian metasediments displays 93c values ranging from 11 4 to 9 8 3 per mil relative to pdb indicating an organogenie origin carbonates display a unimodal distribution of 9ac values ranging from 9 9 to 17 0 per rail relative to pdb coz extracted from fluid inclusions in the auriferous quartz veins has 93c values ranging from 9 5 to 15 7 per mil relative to pdb it is proposed that these carbon isotope compositions of carbonates and coz reflect extensive interaction of the co 2 rieh hydrothermal fluids with reduced carbon in birimian sediments in the deeper parts of the hydrothermal systems carbonates and auriferous vein quartz have 6 so values ranging from 19 9 to 9 9 9 and 19 8 to 15 6 per mil relative to smow respectively carbonates and quartz were deposited in near isotopic equilibrium with respect to 6 so indicating fluid dominated conditions during ore formation from fluids of metamorphic or magmatie origin such an origin is corroborated bysd values of water extracted from fluid inclusions in vein quartz 37 to 53 relative to smoxv pyrite of synsedimentary diagenetie origin in birimian schists displays sulfur isotope compositions ranging from 7 3 to 9 0 9 per rail median ca 10 relative to cdt similar compositions and wide ranges are usually attributed to sulfide generation by bacterial sulfate reduction from seawater arsenopyrite and eogenetie pyrite from the sulfide ores generally have 534s values in the range 5 3 to 10 9 per rail relative to cdt the tight unimodal distribution of 534s values indicates a large homogeneous fluid reservoir the low 534s values are interpreted as source inherited not related to unusual ph eh temperature or depositional conditions sulfides in birimian sediments represent the most likely sulfur reservoir tapped by the fluid systems the c o h and s isotope compositions of ore related hydrothermal minerals and fluid inclusion compo nents indicate that the mineralizing fluids interacted extensively with the paleoproterozoie rocks especially birimian sediments at deeper crustal levels and at high temperatures the isotopic compositions are most compatible with the formation of fluids from devolatilization reactions invomng birimian strata during pro grade metamorphism at depth metamorphic fluids

h o s and pb isotope geochemistry of the awanda gold deposit in southern tianshan central asian orogenic belt implications for fluid regime and metallogeny abstract the awanda gold deposit is located within the southern tianshan accretionary complex a late paleozoic orogenic belt in the northern margin of tarim block in nw china the gold mineralization here occurs in primarily within shear zones and secondary s faults with quartz mica schist as the principal host rock the sulfide mineral assemblage is dominated by arsenopyrite and pyrrhotite with minor pyrite and gudmundite alteration includes silicification sericitization and rare carbonatization the measured 18 o values for the gold bearing quartz range from 15 9 to 20 3 and the estimated 18 o fluid values range from 7 8 to 12 2 the d values for fluid inclusions from quartz range from 116 3 to 87 6 the 34 s values of gold associated sulfide minerals range from 3 6 to 6 2 and the calculated 34 s h2s values of h 2 s for the hydrothermal fluids range from 4 8 to 5 8 with an average of 0 4 lead isotopic compositions of sulfides from gold bearing ores show 206 pb 204 pb ratios ranging from 17 4625 to 18 5356 207 pb 204 pb ratios from 15 4943 to 15 6468 and 208 pb 204 pb ratios from 37 8825 to 38 8238 respectively collectively the isotope h o s pb characteristics suggest that the ore forming fluids were of metamorphic origin derived through dehydration of carbonaceous black shale with the sulfur sourced from crustal components and the lead from a mixture of host rocks and ore forming metamorphic fluids this deposit can be classified as orogenic gold deposit and share many major characteristics with those of muruntau type gold deposits during late paleozoic the southern tianshan accretionary complex was intensely deformed into an orogenic belt by folding and thrusting generating regional thrust faults at different scales the ore controlling subsidiary faults also formed subsequently with the ore forming fluids derived from dehydration of deeply buried carbonaceous black shale and the ore fluids channeled into shear zones leading to gold precipitation

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

electron microprobe analyses of ore minerals and h o s isotope geochemistry of the yuerya gold deposit eastern hebei china implications for ore genesis and mineralization abstract the yuerya gold deposit in eastern hebei province china is located on the eastern margin of the north china craton and is hosted by mesozoic yanshanian granitoid rocks and adjacent mesoproterozoic gaoyuzhuang formation carbonates the auriferous quartz veins in this deposit are dominated by pyrite with subordinate sphalerite chalcopyrite and galena in a quartz dominated gangue that also contains calcite dolomite barite apatite and fluorite gold is present as native gold and electrum which are generally present as micron size infillings in microfissures within pyrite and less commonly as tiny inclusions within pyrite quartz and tellurobismuthite the pyrite in this deposit has high co ni ratios and contains elevated concentrations of both of these elements suggesting that the yuerya gold deposit has a magmato hydrothermal origin and that the ore forming fluids that formed the deposit leached trace elements such as co ni as and au during passage through archean metamorphic rocks mesoproterozoic carbonates and the yanshanian yuerya granitoid pyrite in the study area has s se ratios and s isotopic compositions that suggest that the sulfur and by inference the gold within the deposit was sourced from magmato hydrothermal fluids that were probably originally derived from archean metamorphic rocks and yanshanian granitoids tellurobismuthite in the study area is closely intergrown with gold and was the single telluride phase identified during this study the fineness of gold associated with tellurobismuthite is greater than the fineness of gold associated with pyrite although the fine particle size of the gold surrounded by tellurobismuthite means that the recovery of this gold is difficult in turn meaning that the tellurobismuthite has little significance to the economics of the yuerya gold deposit only trace amounts of sulfides are associated with the tellurobismuthite within the yuerya gold deposit suggesting that this mineral was deposited under conditions of low fs2 and or high fte2 in addition the presence of tellurides within the yuerya gold deposit reflects a genetic relationship between the deposit and magmatism quartz from mineralized veins in the study area has 18o values of 11 2 12 9 and the fluids that formed these veins have d values of 78 3 to 72 1 the 34s values of pyrite within the deposit are rather restricted 2 3 3 5 these data combined with the trace element geochemistry of sulfides within the deposit suggest that the formation of the yuerya gold deposit was closely related to both archean metamorphic rocks and the yanshanian yuerya granitoid

geochemistry of mineralizing fluids in the bralorne pioneer mesothermal gold vein deposit british columbia canada gold quartz veins at the bralorne mesothermal vein deposit have extensive quartz ankeritic carbonate muscovite hydrothermal alteration envelopes that grade outward to chlorite calcite albite minor pyrite with traces of sphalerite galena and tetrahedrite are found in the veins with native gold more abundant pyrite and arsenopyrite with lesser pyrrhotite and chalcopyrite occur in altered wall rock adjacent to the veins envelopes are characterized by addition of k 2 o co 2 s as and au with corresponding depletion of na 2 o feo total and mgo sio 2 and cao are locally depleted and reconcentrated fluid inclusions suggest that deposition of gold was from dilute aqueous solutions 2 ch 4 at 350 degrees c and 1 75 kbars progressive dilution of this fluid by cooler meteoric water led to fluids at 250 degrees c and 0 5 to 1 kbar with 1 wt percent or less nacl equivalent and no detectable carbonic component sulfur isotope ratios of sulfides associated with gold mineralization range from 7 to 9 per mil clustering about a magmatic signature of 0 per mil the ore fluid had a delta 18 o value of 13 or 1 and delta 13 c of 11 or 2 per mil based on measurements in coexisting vein quartz carbonate and muscovite temperature of the mineralizing fluids appears to have increased with depth at a normal geothermal gradient of approximately 30 degrees c km gold sulfides and the associated alteration assemblages are modeled using observed mineralogy and thermodynamic data for chloride complexes as having been deposited from a slightly acid solution ph 4 5 with an na k ratio of at least 8 1 and a high content of dissolved co 2 log fugacity 2 5 conditions were strongly reducing as suggested by the log fugacity of ch 4 0 5 with f sub o 2 about 10 super 30 bars and f sub s 2 about 10 super 7 bars precipitation of gold in the immediately adjacent highly quartz sericite ankerite pyrite altered wall rock was due to reaction with carbonation of the wall rock that caused the ph of the ore fluid to rise destabilizing aurous chloride complexes such a shift is in the direction of slightly increasing or constant stability of gold thiosulfide ions and at a relatively constant f sub o 2 the most significant result of this modeling is that in detail the actual precipitation of gold is by reduction of aurous ions by electrons donated through concurrent oxidation of s super 2 in h 2 s to s super in pyrite fes 2 this explains the empirically observed correlation between pyrite and gold in this type of deposit and in the model between consumption of fe super 2 and production of pyrite on the one hand with precipitation of gold on the other

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

stable isotope o h s c and n systematics of quartz vein systems in the turbidite hosted central and north deborah gold deposits of the bendigo gold field central victoria australia constraints on the origin of ore forming fluids the central and north deborah gold deposits are located in one of 15 mining centers of the bendigo gold field central victoria australia a major world class gold province developed in a turbidite terrane gold mineralization occurs primarily within domes and reverse fault systems in multiply deformed lower ordovician metaturbidites deformation and thrusting occurred between 450 and 420 ma coevally with regional metamorphism at zeolite to greenschist facies in the central and north deborah deposits six stages of quartz veining each with characteristic geometry and mineral paragenesis developed sequentially stages 1 2 and 4 are gold bearing whereas stages 3 5 and 6 are barren gold related quartz veins were deposited at temperatures of 350 25 c based on oxygen isotope geothermometry of silicate phases quartz muscovite and sulfur isotope compositions of coexisting sulfide mineral pairs sphalerite galena this result is consistent with temperatures inferred from fluid inclusions the 18o values of gold bearing vein quartz and coexisting hydrothermal muscovite are uniform within and between the six vein stages ranging from 15 9 to 17 2 per mil and 11 6 to 12 8 per mil respectively the d values of hydrothermal muscovites are also uniform at 68 to 55 per mil ore fluids have calculated 18o values of 8 to 11 per mil and d values of 37 to 17 per mil pyrite of synsedimentary diagenetic origin in slate and sandstone displays sulfur isotope compositions of 23 5 to 8 0 per mil and 0 1 to 11 9 per mil respectively sulfides from gold bearing quartz veins have a range of 7 4 to 8 1 per mil median ca 2 5 the relatively uniform vein pyrites indicate a large uniform sulfur source most likely from desulfidation reactions of sulfide minerals interaction of the ore forming fluid with depleted sulfides at the site of deposition may have shifted the sulfur isotope signatures of source sulfides to more negative values carbon isotope compositions of carbonates range from 5 1 to 9 0 per mil median ca 6 5 nitrogen concentrations and nitrogen isotope compositions of hydrothermal muscovite from quartz veins are 652 to 895 ppm and 2 84 to 4 49 per mil respectively collectively the data define a band in n content vegh rsus 15n coordinate space from higher n and lower 15n in the phanerozoic metasedimenary hosted central and north deborah gold deposits to lower n and higher 15n in predominantly meta igneous hosted neoarchean gold provinces collectively the h o s c and n isotope compositions of ore related hydrothermal minerals indicate that the dilute aqueous carbonic and n bearing ore forming fluids were generated from metamorphic dehydration reactions at deeper crustal levels and were variably influenced by exchange with wall rocks during transport to the site of deposition this genetic model for the central and north deborah gold deposits is consistent with other turbidite hosted lode gold deposits and counterparts in volcanic plutonic terranes in contrast these data are not consistent with either mantle derived fluids or magmatic fluids nor do they support a deeply circulated meteoric surface water model for vein formation

oxygen isotope ratio of quartz veins from the auriferous ramagiri penakacherla schist belt and surrounding granitoids in the eastern dharwar craton a case for a possible link between gold mineralization and granite magmatism abstract in the eastern dharwar craton among the many shear zone hosted lode gold deposits those at ramagiri and penakacherla are located near the western margin of the craton mineralized quartz sulfide carbonate veins are hosted by the schistose metavolcanic and carbonaceous metasedimentary rocks in close spatial association with granitoids having quartz and quartzofeldspathic veins representing hydrothermal activities associated with them mineralized quartz veins from the ore zones in ramagiri and penakacherla regions and quartz or pegmatitic veins from the surrounding granitic terrane were chosen for 18 o analysis samples from the schistose and granitic domains show 18 o quartz values in the range of 10 4 14 9 and 9 3 10 9 respectively the ore zone fluids from the ramagiri and penakacherla regions give 18 o values of 7 9 1 5 and 5 1 0 8 calculated at pressure corrected temperatures obtained from fluid inclusion microthermometry the late magmatic fluid is relatively 18 o poor with 18 o values estimated at 4 5 0 7 and the value is closer to what is obtained for the ore zones based on the 18 o values reported and a possible magmatic contribution to ore fluid deciphered from fluid inclusion characteristics a genetic relationship between granitic magmatism and gold mineralization is surmised the observed increase in the 18 o 16 o ratio from the magmatic fluid to ore fluid in the shear zone is attributed to interaction of the magmatic fluid with host metasediments that agrees well with the variation in the co 2 ch 4 ratio of carbonic component in such fluids

multistage gold mineralization events in the archean tati greenstone belt northeast botswana constraints from integrative white mica ar ar garnet u pb and sulfides pb pb geochronology abstract the tati greenstone belt tgb in northeastern botswana hosts numerous au deposits shashe mupane and signal hill associated with sulfides garnet and white mica in this study integrative white mica ar ar garnet u pb and sulfides pb pb dating techniques were combined with whole rock and sulfide pb isotope characteristics to track the source s of gold and constrain the timeframes of gold mineralization events all sulfides and arsenopyrite samples from the tgb yielded overlapping pb pb errorchron ages of 2227 66 ma and 2220 73 ma respectively which coincide with the shashe sulfides pb pb errorchron age of 2250 110 ma these relatively imprecise pb pb dates suggest heterogeneity in the initial pb isotope ratios of sulfides at mupane whereas au mineralization associated hydrothermal almandine garnet yielded overlapping tera wasserburg lower intercept 206pb 238u age and a concordia age of 2119 18 ma mswd 1 03 and of 2105 24 ma mswd of concordance 1 10 respectively sulfides produced an errorchron pb pb age of 2873 140 ma which despite the large error remains geologically meaningful as it coincides within error with the first neoarchean limpopo liberian orogeny 2 70 2 65 ga granitoids intrusion emplacement 2 65 2 73 ga and deposition of banded iron formation 2 73 0 15 ga ore related white mica from signal hill yielded an overlapping ar ar plateau age of 1987 24 ma and a weighted mean ar ar age of 1987 13 ma n 15 mswd 4 3 which coincide within error with the 2 05 1 95 ga second limpopo liberian tectonic cycle herein considered to have possibly triggered the au mineralization in this area the obtained radiometric dates point to at least three well constrained gold mineralization events with two of them possibly triggered by two different regional tectonic events lead isotope compositions of most of the sulfides overlap with those of spatially associated schists and granitoids thus suggesting these units possibly represent pb and by inference au rock sources the genetic model of the tgb au deposits is consistent with many greenstone hosted gold deposits worldwide suggesting our results are not solely of local regional interest but can be used to characterize greenstone hosted gold deposits worldwide

metamorphism in archaean greenstone belts calculated fluid compositions and implications for gold mineralization an inescapable consequence of the metamorphism of greenstone belt sequences is the release of a large volume of metamorphic fluid of low salinity with chemical characteristics controlled by the mineral assemblages involved in the devolatilization reactions for mafic and ultramafic sequences the composition of fluids released at upper greenschist to lower amphibolite facies conditions for the necessary relatively hot geotherm corresponds to those inferred for greenstone gold deposits xco2 0 2 0 3 this result follows from the calculation of mineral equilibria in the model system cao mgo feo al2o3 sio2 h2o co2 using a new expanded internally consistent dataset greenstone metamorphism cannot have involved much crustal over thickening because very shallow levels of greenstone belts are preserved such orogeny can be accounted for if compressive deformation of the crust is accompanied by thinning of the mantle lithosphere in this case the observed metamorphism which was contemporaneous with deformation is of the low p high t type for this type of metamorphism the metamorphic peak should have occurred earlier at deeper levels in the crust i e the piezothermal array should be of the deeper earlier type however at shallow crustal levels the piezothermal array is likely to have been of deeper later type as a consequence of erosion thus while the lower crust reached maximum temperatures and partially melted to produce the observed granites mid crustal levels were releasing fluids prograde into shallow crustal levels that were already retrograde we propose that these fluids are responsible for the gold mineralization thus the contemporaneity of igneous activity and gold mineralization is a natural consequence of the thermal evolution and does not mean that the mineralization has to be a consequence of igneous processes upward migration of metamorphic fluid via appropriate structurally controlled pathways will bring the fluid into contact with mineral assemblages that have equilibrated with a fluid with significantly lower xco2 these assemblages are therefore grossly out of equilibrium with the fluid in the case of infiltrated metabasic rocks intense carbonation and sulphidation is predicted if as seems reasonable gold is mobilized by the fluid generated by devolatilization then the combination of processes proposed most of which are an inevitable consequence of the metamorphism leads to the formation of greenstone gold deposits predominantly from metamorphic fluids

genetic implications of stable isotope characteristics of mesothermal au deposits and related sb and hg deposits in the canadian cordillera the canadian cordillera hosts a large number of mesothermal au or ag vein deposits the deposits are scattered throughout accreted and suspect terranes of the cordillera and often have a distinct spatial association with strike slip faults these vein deposits possess a striking degree of homogeneity in delta 18 o values on vein district and regional scales in most districts delta 18 o values for mineralized quartz veins vary only by 2 to 3 per mil across the entire district over the canadian cordillera as a whole delta 18 o values for such veins range from 13 to 18 per mil smow in contrast to the high degree of homogeneity evident in the delta 18 o values of the veins alteration zones in adjoining wall rocks possess distinctive zoning patterns indicating isotopic exchange between ore fluids and host units deposits of hg and sb are also closely associated with strike slip faults in the canadian cordillera area the delta 18 o studies of quartz from these deposits indicate average values of 29 or 2 0 per mil smow for quartz from the hg deposits and 21 0 or 1 5 per mil for quartz from the sb deposits when corrected for isotopic fractionation assuming equilibrium conditions average delta 18 o values for ore forming fluids of the various deposit types all fall within a 5 per mil range indicating a probable common fluid source for the three deposit types all three deposit types have delta 13 c values for carbonates in the range of 10 to 2 per mil pdb which are believed to reflect a heterogeneous source for the carbon the delta d values from serpentinites associated with au deposits and inclusion fluids from all three deposit types are lighter than 100 per mil and latitudinally dependent indicating the involvement of chemically evolved meteoric water in the ore forming process a model for formation of these deposits is presented involving the convection of meteoric water in the brittle crust to depths of 12 to 15 km and temperatures of 300 degrees to 400 degrees c during descent the fluid evolves in delta 18 o and acquires constituents involved in the later ore forming process fluids ascend in highly permeable zones associated with strike slip faults depositing quartz sulfides and au at a depth of approximately 10 km and a temperature of 300 degrees or 50 degrees c at shallower levels and cooler temperatures sb quartz or au veins are formed and near the surface cinnabar and carbonate are deposited at temperatures near 150 degrees c similarities in geology and geochemistry of mesothermal au deposits of the canadian cordillera and deposits of the mother lode california and the archecan cratons suggest that the model proposed herein may also be applicable to those deposits

genesis of the shangxu orogenic gold deposit bangong nujiang suture belt central tibet china constraints from h o c si he and ar isotopes abstract shangxu is an orogenic gold deposit in the bangong nujiang suture zone central tibet china formed in the early cretaceous orogene related to convergence and collision between the qiangtang and lhasa terranes the mineralization at shangxu is hosted by jurassic turbidite sedimentary rocks of the mugagangri group and is associated with a regional fault system hydrothermal minerals develop muscovite carbonate sulfides and chlorite hydrothermal fluids record three main hydrothermal stages based on mineral paragenesis the earliest is barren quartz stage h1 which is pre ore during the early mineralization quartz pyrite stage h2a defined by massive quartz veins with minor euhedral pyrite and gold hydrothermal fluids had a 18ofluid of 6 1 6 4 d of 74 to 116 13cco2 of 5 4 to 7 6 and 30si of 0 1 in the quartz pyrite sulfides stage h2b characterized by abundant quartz granular pyrite muscovite with minor chalcopyrite galena sphalerite and gold fluids had a 18ofluid of 7 8 2 d of 109 to 120 13cco2 of 9 6 and 30si of 0 1 to 0 2 during the ankerite sulfide stage h3a distinguished by abundant ankerite muscovite sulfides with minor quartz and chlorite hydrothermal system had a fluid with 18ofluid of 4 9 5 3 d of 125 3 30si of 0 1 13cco2 of 12 4 in quartz inclusion fluid and 13cco2 of 2 7 in ankerite the calcite sulfide stage h3b is characterized by calcite sulfides with minor quartz and chlorite quartz formed earlier than calcite from a fluid having a 18ofluid of 6 4 d of 112 6 30si of 0 1 and 13cco2 of 7 4 after which calcite precipitated from a hydrothermal fluid with 18ofluid of 5 5 9 13cco2 of 0 5 to 2 8 hydrothermal fluids in h2b pyrite have 3he 4he ratios of 0 27 0 42ra and 40ar 36ar ratios of 313 372 the stable isotope composition of hydrothermal fluids from the shangxu gold deposit is similar to that of typical orogenic gold deposits the early stage h1 methane bearing fluids were probably sourced from the basin sediments leading to precipitation of early barren quartz veins and siderite alteration from the quartz sulfide stage h2 to carbonate sulfide stage h3 hydrothermal fluids were likely derived from dissolution of the marine carbonate cement from sedimentary host rocks during metamorphism at depth the decreasing d and 13c of the ore forming fluids from early to late suggests mixing with 13c depleted oxidized graphite in sedimentary rocks meteoric waters or reation between d depleted organic matters generally these fluids were likely generated at depth through prograde metamorphic devolatilization of hydrous minerals in the deeper equivalents of the sedimentary rocks of the mugagangri group sulfur and gold by inference likely originated from sedimentary diagenetic pyrite or the metasedimentary rocks between the greenschist and amphibolite facies and migrated with the metamorphic fluids during transportation to the site of deposition gold bearing fluids variably reacted with country rocks

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

petrogenesis of the cretaceous cassiar batholith yukon british columbia canada implications for magmatism in the north american cordilleran interior the ca 100 ma cassiar batholith is the single largest plutonic body in the hinterland of the canadian cordillera and is part of widespread middle cretaceous to eocene magmatism that occurred in the omineca crystalline belt the cassiar batholith is dominated by muscovite biotite granite and biotite muscovite granodiorite along with subordinate biotite hornblende granodiorite quartz monzodiorite and quartz monzonite the batholith has a wide range of strontium neodymium and oxygen isotope compositions 87 sr 86 sr i 0 706 to 0 734 nd 2 7 to 17 1 t dm 2000 to 980 ma 18 o 7 6 to 11 6 but the batholith9s lead isotope compositions are limited 206 pb 204 pb 19 14 to 19 24 207 pb 204 pb 15 70 to 15 72 208 pb 204 pb 39 14 to 39 26 these data require that the cassiar magmas have been generated from at least two distinct sources an isotopically evolved felsic source and an isotopically more primitive source of mafic to intermediate composition we propose that the felsic source comprised mesoproterozoic to neoproterozoic metasedimentary rocks and paleoproterozoic basement rocks of the north american cratonic margin determining the mafic source is more problematic but most likely it was proterozoic basement rocks of mafic to intermediate composition such as have been reported from various parts of the omineca crystalline belt the cassiar batholith is lithologically geochemically and isotopically similar to other major batholiths in the cordilleran interior including the batholiths of southeastern british columbia and the idaho batholith we interpret these similarities to reflect melting of common crustal source materials namely proterozoic basement and cover rocks of western laurentia the cassiar batholith and other cordilleran interior granitoids are distinct from granitoids generated in known subduction zone environments in terms of their mineralogical geochemical and isotopic characteristics these differences reflect a predominantly or exclusively crustal source region for the cordilleran interior granitoids in contrast subduction zone magmatism contains a significant mantle input even in cases where subduction takes place beneath thick continental crust these results strongly suggest that cordilleran interior granitoids did not form directly from a subduction setting rather an intracontinental collisional setting seems more appropriate in which magma generation occurred in response to crustal thickening followed in some cases by extension

molecular structure and origin of solid bitumen from northern sichuan basin abstract four natural solid bitumen samples were collected from the guangyuan area northwest sichuan basin southwest china to characterize their structure samples were ground to power 120 mesh purified by znbr2 solution to remove mineral and soluble impurities and analyzed using rock eval pyrolysis elemental analysis raman spectroscopy solid bitumen reflectance and solid state 13c nuclear magnetic resonance to determine the maturity degree of samples and obtain structural information results showed that guangyuan solid bitumen samples are in low maturity and three solid bitumen maturity detection methods are compared in this research with respect to the solid bitumen molecular structure less than 10 carbon atoms were determined in aliphatic chains and the solid bitumen aromatic clusters ranged from two to four rings which indicates that the solid bitumen comprises complex geological organic matter such as kerogen four molecular models were established in this research to reflect the basic molecular configuration of natural solid bitumen samples were classified into two types based on the structural information and models and the results imply that the solid bitumen samples may have been derived via different evolutionary processes this phenomenon indicates that structural studies could play an important role in the research of oxidation and carbon isotope changes in solid bitumen in summary this investigation provides an understanding of the evolutionary stages of guangyuan bitumen establishes associated molecular structure models and researches the role of structural information in bitumen source studies as the focus of this study the four molecular models were used to restore the structural characteristics of natural solid bitumen as possible and this study tried to find out the relationship between the solid bitumen structure information and it geology evolution as the carbon isotope may not accurately determine the source of solid bitumen because of oxidation molecular information could be used to research the solid bitumen source and associated evolution

intrusion hosted mineralization in the charters towers goldfield north queensland new isotopic and fluid inclusion constraints on the timing and origin of the auriferous veins auriferous quartz sulfide veins of the charters towers goldfield are mainly hosted in oxidized i type granitoids of the ravenswood batholith k ar and ar ar isotope age data of hydrothermal muscovite from alteration envelopes of veins at charters towers and the hadleigh castle mine 40 km east of charters towers are indistinguishable within error suggesting broadly synchronous gold deposition between 410 and 404 ma and across a significant segment of the ravenswood batholith published geochronological data indicate that several granitoid bodies were emplaced into the ravenswood batholith at the time of gold mineralization despite their association in space and time with igneous rocks the gold deposits lack an obvious causative intrusion moreover published lead isotope studies indicate that the lead in the ore was not acquired from any of the exposed intrusions a distal origin of the ore forming fluids also may be inferred from nitrogen isotope values of hydrothermal sericite suggesting wall rock interaction with metamorphic fluids or fluids that were in equilibrium with metamorphic rocks during the paragenetic stage of pyrite and arsenopyrite deposition stage ii veins of the charters towers goldfield contain three different types of fluid inclusions which are distinguishable by petrography and microthermometry this study is the first to report saline fluid inclusions in sphalerite 18 9 28 3 wt nacl equiv and vein quartz 20 9 24 7 wt nacl equiv trapped during the stage of gold deposition stage iii the range of temperatures and salinities particularly of the saline inclusions in sphalerite could indicate mixing between hotter more saline fluids e g deep seated magmatic and cooler more dilute solutions e g modified meteoric as the cause of gold deposition the geological and geochemical data are not compatible with derivation of fluids metals and ligands from individual plutons similarity of host rocks ore element associations alteration assemblages structural controls and tectonic settings strongly suggest that the auriferous veins of the charters towers goldfield belong to a group of granitoid hosted lode gold deposits that are generally classified as orogenic

cascadia subducting plate fluids channelled to fore arc mantle corner ets and silica deposition in this study we first summarize the constraints that on the cascadia subduction thrust there is a 70 km gap downdip between the megathrust seismogenic zone and the episodic tremor and slip ets that lies further landward there is not a continuous transition from unstable to conditionally stable sliding seismic rupture occurs mainly offshore for this hot subduction zone ets lies onshore we then suggest what does control the downdip position of ets we conclude that fluids from dehydration of the downgoing plate focused to rise above the fore arc mantle corner are responsible for ets there is a remarkable correspondence between the position of ets and this corner along the whole margin hydrated mineral assemblages in the subducting oceanic crust and uppermost mantle are dehydrated with downdip increasing temperature and seismic tomography data indicate that these fluids have strongly serpentinized the overlying fore arc mantle laboratory data show that such fore arc mantle serpentinite has low permeability and likely blocks vertical expulsion and restricts flow updip within the underlying permeable oceanic crust and subduction shear zone at the fore arc mantle corner these fluids are released upward into the more permeable overlying fore arc crust an indication of this fluid flux comes from low poisson s ratios and vp vs found above the corner that may be explained by a concentration of quartz which has exceptionally low poisson s ratio the rising fluids should be silica saturated and precipitate quartz with decreasing temperature and pressure as they rise above the corner

early late cretaceous early tertiary gold mineralization a galena lead isotope study of the bridge river mining camp southwestern british columbia canada the bridge river gold camp 180 km north of vancouver canada produced more gold than any other camp in british columbia over its 70 years of operation 130 metric tons or 4 million oz mainly from the bralorne and pioneer deposits a compilation of new and existing lead isotope ratios from 20 widespread and diverse deposits in the bridge river camp shows a distinct clustering of data this is interpreted to be the expression of a single protracted but episodic mineralizing event coinciding with the emplacement of the coast plutonic complex during early late cretaceous to early tertiary time 90 45 ma the timing of this event is constrained at bralorne by a premineral albitite dike set 91 4 or 1 4 ma by u pb on zircon and a late intramineral hornblende porphyry dike set 85 7 or 3 ma by k ar on hornblende late lamprophyre dikes that crosscut the mineralized veins dated at 43 5 or 1 5 ma by k ar on biotite set a lower age limit for the mineralization the host rocks the bralorne intrusions have no genetic relation to the mineralization since they are early permian 270 or 5 ma by u pb on zircon the conclusion that all mineralization is related to a coherent event is in accord with recognition of a districtwide mineral zoning scheme by woodsworth pearson and sinclair in this scheme a sequence from high temperature au as w mo through intermediate sb ag au as to low temperature sb hg vein assemblages as generated from southwest to northeast within the thermal aureole of the coast plutonic complex stable isotope and temperature data from other workers support this northeastly decreasing gradient in mineralization temperature clustered lead isotope data define a trend that plots between model pb isotope curves of doe and zartman for the upper crust and the mantle this trend is interpreted to be a mixing line between primitive mantle type lead and more radiogenic lead by one or a combination of two postulated processes between two groups of rocks more primitive oceanic volcanics and more radiogenic upper crustal volcanic arc clastic rocks by a meteoric thermal system generated by the emplacement of the coast plutonic complex or between less and more radiogenic magmas during the emplacement of the mesozoic cenozoic volcanics and intrusion hosting the deposits the accreted nature of the host oceanic and arc terranes is emphasized by the differences between the model proposed and the upper crustal shale curve model of godwin and sinclair which represents much of the locally adjacent autochthonous portion of the canadian cordillera

the geology and mineralogy of the loulo mining district mali west africa evidence for two distinct styles of orogenic gold mineralization loulo is a world class orogenic gold mining district in the birimian terrane of western mali orebodies are located along second or higher order shears associated with the senegal mali shear zone with gold mineralization largely linked to a transtensional event the mine camp is divided into two distinct styles of gold deposit on the basis of differing geologic characteristics one group is typified by the gara deposit whereas the other by the yalea deposit both deposit styles are hosted by similar rock types calcareous graywackes and calciticdolomitic marbles gara style orebodies occur as sulfide disseminations or ankerite rich shear vein stockworks hosted in folded tourmalinized sandstones and breccias mainly within 2 km of the senegal mali shear zone these deposits are characterized by intense multi stage albitization and tourmalinization pre syn and postmineralization gold lodes are fe rich dominated by nickeloan pyrite contain cu ni co minor and trace sulfides e g chalcopyrite gersdorffite pentlandite cobaltite millerite and show consistently high levels of p ree w bearing phases apatite monazite xenotime and scheelite base metal concentrations show a marked increase in marble host rocks with the formation of nickeloan pyrite cobaltite clausthalite ores in contrast yalea style deposits are associated with quartz ankerite vein lodes and disseminated sulfide stringer zones mineralization occurs along highly altered tourmaline absent brittle ductile shears up to 8 km away from the senegal mali shear zone wall rock alteration is characterized by addition of k 2 o cao co 2 h 2 o and sio 2 with mineral assemblages consisting of chlorite sericite carbonate quartz albite ore paragenesis is enriched in as mainly as multistage growth of arsenopyrite and arsenian pyrite base metal sulfides scheelite and ree phosphates are extremely rare the diversity in the ore paragenesis is controlled by a dynamic hydrothermal system that sourced fluids and metals from different reservoirs within the region the as rich yalea style deposits have characteristics typical of birimian gold mineralization in ghana and auriferous fluids are likely derived from the dewatering of sedimentary rocks during regional metamorphism on the other hand the polymetallic fe b rich gara style orebodies show atypical features for birimian gold mineralization instead field relations and the mineralogy and geochemistry of the gara style lodes indicate a strong hydrothermal influence from surrounding intrusive stocks with possible links between gold mineralization and iron oxide skarn development in the region the data collected at loulo highlight the diverse nature of orogenic gold deposits especially in west africa this style of ore deposit can form from a variety of fluid sources both metamorphic and magmatic

paleomagnetism of the upper cretaceous strata of mount tatlow evidence for 3000 km of northward displacement of the eastern coast belt british columbia this study contributes the first paleomagnetic data from dated cretaceous units of the coast belt 51 3 n 123 8 w in which paleohorizontal bedding surfaces can be identified with certainty we sampled 38 sites through a 1 5 km stratigraphic interval of cenomanian silverquick sedimentary strata and cenomanian to campanian powell creek volcanics in the tatlow syncline in the tyaughton methow basin magnetizations at 21 sites have high unblocking temperatures tub and pass the tilt test precision k 5 before untilting maximum k 48 after 100 untilting magnetic minerals of volcanics show original textures mean declinations in volcanic and sedimentary rocks are statistically identical and polarity is consistently normal the mean inclination 55 4 3 4 probability p 0 05 of this pretilting magnetization is 20 1 4 0 shallower than that expected if the rocks were deposited in their present position relative to cratonic north america field and laboratory observations rule out other causes for these anomalous shallow inclinations these rocks therefore have been displaced northward by 3000 500 km relative to cratonic north america since mid cretaceous time this displacement is almost 3 times greater than that estimated from mid cretaceous volcanics in the now adjacent intermontane belt evidently the two belts were not amalgamated until latest cretaceous or early tertiary time these results are inconsistent with all previous geologically based tectonic reconstructions of the cordillera magnetizations at eight sites in silverquick and powell creek beds have low tub and directional characteristics similar to those in three younger diorite intrusions reversed and normal polarities occur the rocks are commonly highly altered and maximum precision occurs after 40 untilting the magnetizations therefore are interpreted as overprints of latest cretaceous age acquired during folding

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

nitrogen 15 enriched precambrian kerogen and hydrothermal systems 1 nitrogen isotope data are reported for kerogen from precambrian black shales at prehnite pumpellyite to greenschist facies and for micas from postmetamorphic hydrothermal quartz vein systems nitrogen in micas was acquired from n bearing aqueous metamorphic hydrothermal fluids generated from breakdown of sedimentary kerogen and dehydration of k silicates at deeper crustal levels the 2 7 ga kerogens and hydrothermal k micas yield 15n values of 15 24 compared to existing data of 2 6 in phanerozoic counterparts paleoproterozoic equivalents have intermediate 15n of 7 12 implying a secular decrease in 15n of shale kerogen the 15n enriched nitrogen in archean shales and hydrothermal vein systems cannot be caused either by n isotopic shifts accompanying metamorphism or rayleigh fractionation because premetamorphic and postmetamorphic samples from the same terrane are both enriched and lack covariation of 15n with n c n ratios or metamorphic grade the magnitude of shifts during progressive metamorphism of sedimentary rocks in previous studies is constrained at 1 to greenschist and about 3 to amphibolite facies furthermore 15n enriched values cannot stem from long term preferential diffusional loss of 14n as samples were selected from terranes where 40ar 39ar ages are within a few million years of concordant u pb ages nitrogen is structurally bound in micas whereas ar is not it is possible that the 15n enriched values stem from a different n cycle in the archean with large biologically mediated fractionations yet the magnitude of the fractionations observed exceeds any presently known and chemoautotrophic communities tend to depleted values earlier results on archean cherts show a range from 6 to 30 given the temporal association of chert banded iron formation with mantle plumes the range is consistent with mixing between mantle n2 of 5 and the enriched marine reservoir identified in this study we attribute the secular trend from archean kerogen of 15 24 to phanerozoic kerogen averaging 3 4 to a secondary atmosphere derived from ci chondrite like material and comets with 15n of 30 42 shifts of 15n to its present values of 0 can be accounted for by a combination of sequestration of atmospheric n2 into sedimentary rocks and mantle degassing of 5 mantle n2

a rock physics model for characterizing the total porosity and velocity of shale a case study in fuling area china abstract the total porosity in a shale gas reservoir is the key parameter to evaluate its potential production capability petroleum geologists have been pursuing an accurate way of predicting total porosity in shale gas reservoirs from logging curves in the fuling area near chongqing in china we use core test data and well logging data in order to establish a logging response model for porosity through the analysis of the data we found that the porosity and the p sonic p velocity have a strong correlation by combining our results with previous research i e physical rock data we establish a new model which better predicts the total porosity of a given a shale reservoir than the models established in previous research in addition to total porosity mineral composition pore pressure pore fluid type and confining pressure affect p velocity by taking a series of artificial cores as test objects and systematic analysis of total porosity mineral composition and content pore fluid type and pressure on the p velocity we establish response templates between every individual parameter and p velocity to this end we further establish a multi parameter rock physical characterization equation for the total porosity and p velocity and analyze its applicable conditions using the rich geological and geophysical data of the lower silurian series in the fuling area we gradually improve the model for the total porosity and verify the results using well logging data finally a set of accurate methods are established to predict the total porosity in shale gas reservoirs by using well logging data

special paper major gold deposits and belts of the north and south american cordillera distribution tectonomagmatic settings and metallogenic considerations a compilation of economically viable gold concentrations containing 10 moz in the north and south american cordillera reveals the existence of 22 discrete belts in addition to five major isolated deposits most formed over the last 150 m y the gold concentrations are attributed to eight widely recognized deposit types of which porphyry sediment hosted and high sulfidation epithermal are economically the most important individual gold belts are typically several tens to hundreds of kilometers long dominated by single deposit types and metallogenically active for relatively brief periods 5 20 m y many of the gold belts and major isolated deposits were generated under extensional or transtensional tectonic conditions in either arc or back arc settings nevertheless the two main high sulfidation epithermal gold belts were generated in thickening or already thickened crust during low angle subduction eight other gold belts or districts also accompanied compression or transpression with two of them the main orogenic gold belts occupying fore arc sites there is a strong suggestion that the preeminent cordilleran gold concentrations formed either during or immediately following prolonged contraction the major gold deposits and belts occur along the craton edge as well as in adjoining accreted terranes but almost all are of postaccretionary timing many of the gold belts and isolated deposits were localized by crustal scale faults or lineaments which may be either parallel or transverse to the cordilleran margin the gold concentrations accumulated during active subduction commonly in close spatial and temporal association with intermediate to felsic medium to high k calc alkaline igneous rocks by contrast the low sulfidation epithermal gold deposits accompany bimodal volcanic pulses of calc alkaline tholeiitic or alkaline affinity however a gold alkaline rock association is uncommon a genetic link between gold mineralization and coeval magmatism is widely accepted for most of the deposit types the exceptions being the sediment hosted and orogenic gold deposits notwithstanding the small cumulative extent of the gold concentrations relative to the entire cordilleran margin there is a marked tendency for two or more belts or isolated deposits of different ages and genetic types to occur in close proximity within relatively restricted arc including fore and back arc segments in the case of the western united states for example six belts and four isolated major deposits make up a particularly prominent cluster if fortuity is discounted this clustering or pairing of gold concentrations must imply a predisposition of certain arc segments to gold mineralization an analogous situation is evident for other metals particularly copper and tin the reason for the recurrent generation of major deposits and belts dominated by one or more metals remains uncertain although heterogeneously distributed metal preconcentrations favorable redox conditions or other parameters somewhere above the subducted slab between the mantle wedge and upper crust are widely contemplated possibilities elucidation of the reason s for this metallogenic inheritance at the scale of limited arc segments poses an important and challenging series of research questions as well as being critical to the planning of potentially successful greenfield exploration programs

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

the behavior of nitrogen and nitrogen isotopes during metamorphism and mineralization evidence from the otago and alpine schists new zealand abstract metamorphism is a major mechanism for the re distribution of fluids and mass in the earth s crust with these processes most prominently highlighted by the occurrence of major gold resources within these terranes however although orogenic gold deposits have contributed over 20 of the global gold production their origins remain controversial the nitrogen concentration and isotopic composition of rocks and minerals are potentially powerful tracers of crustal metamorphism and mineralization but there have been few detailed applications of this approach to date although nitrogen isotopes have recently been used to elucidate the source of fluids in some neoarchean orogenic gold deposits and proterozoic to paleozoic mountain belts due to their age and geological complexity of these terranes major uncertainties as to the behavior of nitrogen remain the otago and alpine schists in the south island of new zealand comprise a large comparatively young the range of 15n values for mica and whole rock samples from the schists spans 0 2 to 7 0 and the nitrogen concentration from 23 to 3483 ppm sample provenance and rock type have minimal influence on the nitrogen concentration and isotopic value which appears to have been inherited from the original sedimentary kerogen there is no systematic variation between metamorphic temperature and 15n or n concentration in micas suggesting that there has been little discernible loss of 15n depleted fluids from silicates with increasing metamorphic temperature comparison of fluid mobile alkali element k rb and cs carbon and nitrogen whole rock concentrations indicates that for the otago and alpine schists in rocks up to upper greenschist facies significant nitrogen remains hosted in phases other than micas most likely poorly matured carbonaceous material samples from macraes flat a major gold producer have a similar range of 15n values to the host terrane but show distinctly higher nitrogen concentrations relative to unmineralized samples due to the incorporation of nitrogen from the mineralizing hydrothermal fluid this suggests that there is only a subtle metamorphic re distribution of nitrogen during mineralization albeit with minimal isotopic fractionation in the case of nitrogen at least the isotopic signatures of mineralized rocks support a metamorphic fluid source and are inconsistent with mantle or meteoric fluid reservoirs however due to the high relative abundance of nitrogen in sedimentary rocks compared to other potential reservoirs unless fluid fluxes are very large and well channeled nitrogen signatures are not sensitive recorders of fluid inputs from mantle magmatic or meteoric reservoirs conversely the absence of a sedimentary source nitrogen isotope signature similar to the host rock in an orogenic deposit would be a very strong indicator of an external exotic source for the mineralizing fluids

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

canadian cordilleran tectonics from geosynclines to crustal collage the major contributions by the geological survey of canada in the canadian cordillera systematic mapping and definition of the regional geological framework led directly to tectonic syntheses that attempted to explain its origin from the late 1800 s until the 1960 s cordilleran mountain building was viewed as the end result of geosynclinal deposition early workers felt that a connection existed between mountain building and the pacific basin but its nature was never clear because little was known about ocean basins and their relationships to continental margins in the 1950 s the nature of the oceanic lithosphere and ocean continent relationships became better known the knowledge led to formulation of the plate tectonic hypothesis in the 1960 s a time fortuitously coinciding with completion by the survey of most of the regional geological mapping scale 1 250 000 geosynclinal rock units were reinterpreted in terms of their possible modern analogues oceanic island arc continental shelf slo

fluid inclusion and stable isotope study of the shagou ag pb zn deposit luoning henan province china implications for the genesis of an orogenic lode ag pb zn system abstract the shagou deposit located in the eastern qinling orogenic belt central china and hosted by the taihua supergroup is a large sized ag pb zn deposit and has proven reserves of 1 54 million tons of ore with grades of 767 g t ag 13 24 pb and 4 31 zn the ore forming fluids have been previously interpreted as products of direct exsolution of a crystallizing magma or sudden decompression of the fluid exsolved from magma defining the shagou deposit as an intrusion related system in this study the shagou deposit has been identified as an orogenic type mineralization system the hydrothermal ore forming process can be divided into the early quartz pyrite middle quartz polymetallic sulfides and late quartz carbonate stages four types of fluid inclusions in quartz can be distinguished between pc c w and pw types the early and middle stage quartz contains the pc c and w type fluid inclusions while only w and pw type fluid inclusions can be observed in the late stage quartz in the early stage the w type fluid inclusions homogenized at 173 369 c concentrating at 200 240 c with salinities ranging from 5 4 to 8 3 wt nacl equiv but in the middle stage the c and w type fluid inclusions homogenized at 112 313 c concentrating at 180 220 c with salinities ranging from 0 7 to 13 6 wt nacl equiv in the late stage the w type fluid inclusions homogenized at 108 253 c concentrating at 160 200 c with salinities ranging from 2 1 to 8 1 wt nacl equiv the fluids may have experienced a boiling process or fluid mixing in the middle stage the estimated trapping pressures of fluid inclusions in the middle stage ranged from 33 to 211 mpa corresponding to a mineralization depth from 3 3 to 7 7 km for the early stage fluids the calculated 18 o h 2 o and d h 2 o values are 7 5 9 1 96 2 to 110 5 respectively for the middle stage fluids these two values are 1 5 5 3 75 6 to 110 7 but change to 1 0 and 87 for the late stage suggesting an evolution from metamorphic to meteoric in origin in general the shagou ag pb zn deposit is similar in many respects to typical orogenic type mineralization systems

lead and oxygen isotope evidence for the origin of the monte rosa gold lode deposits western alps italy a comparison with archean lode deposits in the monte rosa area of the northwestern italian alps swarms of epigenetic quartz sulfide gold bearing veins occur within a terrane affected by tertiary regional metamorphism of greenschist to amphibolite grade they are preferentially associated with the felsic rocks of the upper penninic nappes field evidence and available geochronologic data imply emplacement ages postdating the metamorphic peak 38 or 2 m y the spatial distribution of the veins is related to late metamorphic large scale tectonic disturbances which displace truncate or fold the isograds lead isotope analyses of galenas and other sulfides were compared with those of feldspars clinozoisites and whole rocks from various unaltered host rocks the data suggest derivation of the ore lead from caledonian metapelites with a possible contribution from hercynian granitoids but they rule out significant contributions from the neighboring ophiolitic greenstones and associated stratiform massive sulfide deposits the separation of the ore leads into two distinct data fields is probably the result of ore deposition from at least two hydrothermal systems which were separated in space and possibly also in time the hypothesis of two distinct mineralization ages is also supported by geochronologic data oxygen isotope data indicate that the ore fluids had isotopic compositions consistent with a metamorphic origin with the notable exception of the val toppa mine where lighter possibly surface derived water was involved oxygen isotope fractionations between mineral pairs in metasediments and granites indicate metamorphic peak temperatures of 530 degrees to 605 degrees c in agreement with those inferred from petrologic evidence these temperatures and preliminary fluid inclusion data were used to predict oxygen isotope compositions of gangue quartz the comparison of predicted with measured delta 18 o values shows consistency with the assumption that the fluids were generated by metamorphic devolatilization within the metapelites and granitoids in agreement with the lead isotope data the results suggest that the gold bearing fluids and the lead were supplied by rocks derived from the continental crust in opposition to the data from many greenstone hosted archean gold lode deposits which indicate a mantle source this implies that metamorphic hosted quartz gold lodes do not necessarily derive metals and fluids from source beds within a greenstone sequence therefore the assumption that the archean gold was derived from greenstones should be checked by isotope tracing studies the common lead method appears to be particularly promising in distinguishing greenstone metamorphic from granitic sources in precambrian terranes in view of the contrasting u pb ratios usually stated in these lithologies

isotope study on organic nitrogen of westphalian anthracites from the western middle field of pennsylvania u s a and from the bramsche massif germany abstract the objective of this study was to examine an aspect of the thermal cycling of organic nitrogen in sediments and metasediments the cycling of organic nitrogen is important because sedimentary organic matter is a shuttle of nitrogen from the atmosphere to the lower crust and thermal decomposition of organic matter is a critical step in the recycling of nitrogen between the different nitrogen pools abundance and isotopic composition of organic nitrogen were determined in the particular case of two low sulfur westphalian anthracites series from pennsylvania and bramsche massif they represent good examples of euramerica coals spanning the whole range of anthractization in single fields gold cell experimental simulation of the denitrogenation process was conducted at moderate pressure to show that both suites make ideal metamorphic profiles without any shift due to change of facies or to hydrothermal disturbance during anthracitization organic nitrogen content decreases rapidly while organic nitrogen isotopic composition does not change with rank increase the preservation of the isotopic signature implies that organic nitrogen isotopes could be used as indicators for the paleoecological and paleodepositional history reconstruction of the basins the striking contrast between the rapid and sharp decrease of nitrogen organic content and the invariance of its isotopic composition during the whole anthracitization suggests that ammonia is an important product of the denitrogenation process

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

pathways for nitrogen cycling in earth s crust and upper mantle a review and new results for microporous beryl and cordierite figure 1 earth s atmosphere contains 27 30 of the planet s nitrogen and recent estimates are that about one half that amount 11 16 is located in the continental and oceanic crust combined the percentage of n in the mantle is more difficult to estimate but it is thought to be near 60 at very low concentrations knowledge of the behavior of n in various fluid melt rock settings is key to understanding pathways for its transfer among the major solid earth reservoirs nitrogen initially bound into various organic materials is transferred into silicate minerals during burial and metamorphism often as nh4 substituting for k in layer silicates clays and micas and feldspars low grade metamorphic rocks appear to retain much of this initial organic n signature in both concentrations and isotopic compositions thus in some cases providing a relatively un or little modified record of ancient biogeochemical cycling devolatilization can release significant fractions of the n initially fixed in crustal rocks through organic diagenesis during progressive metamorphism at temperatures of 350 550 c depending on pressure loss of fractionated n during devolatilization can impart an appreciable isotopic signature on the residual rocks producing shifts in 15n values mostly in the range of 2 to 5 these rocks then retain large fractions of the remaining n largely as nh4 despite further heating and ultimately partial melting with little additional change in 15n this retention leads to the storage of relatively large amounts of n largely as nh4 in the continental crust nitrogen can serve as a tracer of the mobility of organic sedimentary components into and within the upper mantle this contribution focuses on our growing but still fragmentary knowledge of the n pathways into shallow to deep continental crustal settings and the upper mantle we discuss the factors controlling the return of deeply subducted n to shallower reservoirs including the atmosphere via metamorphic devolatilization and arc magmatism we discuss observations from natural rock suites providing tests of calculated mineral fluid fractionation factors for n building on our discussion of n behavior in continental crust we present new measurements on the n concentrations and isotopic compositions of microporous beryl and cordierite from medium and high grade metamorphic rocks and pegmatites both phases containing molecular n2 and nh4 bearing micas coexisting with them we suggest some avenues of investigation that could be particularly fruitful toward obtaining a better understanding of the key n reservoirs and the more important pathways for n cycling in the solid earth 1 pending yes

tectonic control and fluid evolution in the quartz hill california lode gold deposits at quartz hill in the klamath mountains of northern california gold bearing quartz carbonate pyrite chlorite sericite veins occupy reactivated moderately southeast dipping faults that originally formed synchronously with the transcrustal soap creek ridge fault geophysical evidence suggests that the soap creek ridge fault dips shallowly eastward to a depth of approximately 14 km where it intersects a major crustal suture zone that joins layered accreted oceanic crust to crystalline continental crust gold mineralization occurs in the footwall along the linear trace of this fault co 2 rich fluids reacted with wall rock to form alteration zones containing pyrite siderite sericite ankerite and chlorite calcite microthermometric studies reveal coexisting high salinity 11 16 wt salt aqueous and co 2 rich 13 51 mole co 2 inclusions that homogenize at temperatures greater than 285 degrees c these inclusions probably represent the unmixing of a parent fluid that was metamorphic in origin low salinity 0 6 wt salt inclusions with variable co 2 concentrations 0 9 mole co 2 homogenize at temperatures from 170 degrees to 270 degrees c these inclusions are likely to have formed from the mixing of hot saline co 2 rich solutions with cooler less saline fluids of meteoric origin k ar analyses of two size fractions of hydrothermal sericite from a quartz hill mine yielded concordant dates of 145 8 or 3 0 and 147 7 or 2 8 ma and probably represent the age of mineralization similarities in tectonic setting local structural control host rocks fluid inclusion properties and alteration and vein mineralogy suggest that the early cretaceous gold mineralization in the sierra nevada foothills was equivalent to the ore forming episode at quartz hill and throughout the klamath mountains a tectonic model for ore genesis consistent with thermochemical geophysical structural and geochronological data is that hydrothermal solutions were derived from subducting oceanic plates by devolatilization reactions associated with prograde metamorphism these co 2 rich fluids migrated upward through a crustal suture zone and were channeled along a transcrustal fault deposition occurred in response to pressure fluctuations within brittle fault zones important changes in farallon north american convergent plate motion occurred at 150 and 133 ma the synchronism of these plate motion changes with regional tectonic activity and gold mineralization suggests a genetic relationship these plate motion changes produced crustal scale tectonic responses which generated fluids and created pathways for ore fluid transport

a reinterpretation of the crustal n isotope record evidence for a n 15 enriched archean atmosphere a new compilation of n isotope and abundance data for metasedimentary rocks and hyrdothermal micas that proxy for bulk crust show systematic patterns 1 15n values of kerogen in precambrian cherts are more negative relative to siliciclastic counterparts probably due to a mantle hydrothermal component 2 there is a secular trend from average 15n 15 3 1 8 in archean shales through intermediate values in the proterozoic to phanerozoic counterparts where 15n averages 3 5 3 hydrothermal micas in metamorphic hydrothermal systems of palaeozoic and mesozoic age that proxy for crust have 15n within the range of contemporaneous sedimentary rocks 4 hydrothermal micas track the secular trend of 15n for kerogen from 2 7 ga to the phanerozoic 5 within precambrian datasets 15n does not increase with decreasing n content accordingly high 15n values cannot stem either from metamorphism or form rayleigh fractionation 6 previous studies show isotopic shifts during metamorphism are only 1 to 3 up to amphibolite facies values of 10 24 are attributed to a high 15n archean atmosphere a residual signature of ci carbonaceous chondrites where 15n is 30 to 42

multistage mineralization at the hypozonal sao sebastiao gold deposit pitangui greenstone belt minas gerais brazil abstract sao sebastiao is a recently discovered archean gold deposit at the northwesternmost part of the quadrilatero ferrifero qf southern sao francisco craton brazil the gold ore is strata confined in two levels of banded iron formation bif of the basal stratigraphic unit in the pitangui greenstone belt 2 8 ga pitangui is correlated to the widely known rio das velhas greenstone belt which hosts several orogenic gold deposits such as the world class cuiaba the gold mineralization at sao sebastiao is associated to replacement style sulfidation over magnetite where the main developed phases are pyrrhotite chalcopyrite arsenopyrite pyrite and arsenian pyrite native au occurs as inclusions in pyrrhotite formed at dilatation domains e g breccias fold hinges during compressional events rio das velhas orogeny at 2 8 2 75 ga but mainly as inclusions in late to post kinematic pyrite and arsenopyrite as fracture infills in arsenopyrite and in contact with gangue minerals electron microprobe analyses in arsenopyrite reveal a 465 560 c precipitation temperature interpreted from as atomic percentage and indicate that high temperature fluids were active after the deformational event the temperature increase caused the incorporation of la icp ms detected lattice bound ni and co in arsenian pyrite and arsenopyrite related to the mafic ultramafic signature of boundary rocks high temperature conditions are supported by pyrite and iss intermediate solid solution in the cu fe s system formation at 600 c and melting derived textures resulting from the precipitation of bi and as melts typical of amphibolite facies environments with high fs2 whole rock geochemical assays of drill core samples display two clear trends on ag bi cu vs au binary plots and on principal component analysis results the trends are a product of the heterogeneous spatial distribution of melts and high temperature fluids melting features and superposition of mineralization styles are common in hypozonal gold deposits experiencing higher heat influx at sao sebastiao the thermal effect is likely related to the 2 7 ga granitic intrusions that surround the pitangui belt sao sebastiao shows distinct characteristics than those described for the mesozonal gold deposits in the rio das velhas greenstone belt and may depict the deeper roots of the qf gold system

early cretaceous gold silver mineralization in the sylvester allochthon near cassiar north central british columbia k ar dates on sericite from several gold silver bearing white quartz veins in the cassiar area indicate that mineralization occurred in the early cretaceous at about 130 ma thus these veins predate the mid cretaceous cassiar batholith and late cretaceous and early tertiary plutons in the immediate area the early cretaceous date probably represents either a thermal precursor to emplacement of the cassiar batholith or a structurally related event associated with allochthonous emplacement of the sylvester group either of these events may have caused circulation of the meteoric fluids responsible for the veins

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

gold mineralization in the okanagan valley southern british columbia fluid inclusion and stable isotope studies two styles of precious metal vein mineralization epithermal and mesothermal occur in the okanagan valley of southern british columbia the epithermal deposit dusty mac is composed of quartz stockwork mineralization in eocene volcanic and volcaniclastic rocks the mesothermal deposits fairview and oro fino consist of thick 1 10 m quartz veins in paleozoic and mesozoic metamorphic rocks fluid inclusion and stable isotope studies indicate that two distinct hydrothermal fluids were responsible for the mineralization events at dusty mac the epithermal fluid had a relatively low temperature 230 degrees 250 degrees c extremely low salinity 18 o 7 to 9ppm and delta d 133ppm values at fairview and oro fino the mesothermal fluids had higher temperatures 280 degrees 330 degrees c significant co 2 contents and higher salinities 3 6 equiv wt nacl stable isotope data show that the mesothermal fluids had low delta d 121 to 148ppm and delta 13 c 8 5ppm values but high delta 18 o 4 to 6ppm values the data indicate that the fluids involved in both styles of mineralization are 18 o shifted meteoric water with shallow circulation responsible for the epithermal deposits and deep circulation involved in the formation of the mesothermal deposits although both styles of mineralization occur in the same tectonic regime it appears that they are not temporally or genetically related to each other

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

the massawa gold deposit eastern senegal west africa an orogenic gold deposit sourced from magmatically derived fluids the massawa gold project is situated on the senegalese side of the highly prospective productive palaeo proterozoic birimian kedougou kenieba inlier which hosts several world class orogenic gold deposits districts in western mali e g loulo and sadiola the massawa ore body has a strike length of at least 4 km and a current resource of 3 61 moz at a grade of 2 8 g t 1 the ore body is structurally controlled and located within a package of low grade regionally metamorphosed volcaniclastic sediments agglomerates tuffs and ash tuffs quartz feldspar and lithic wackes carbonaceous shales hydrothermal breccias and gabbro and porphyry sills these rocks have undergone pervasive silica alteration followed by a sericite ankerite chlorite alteration event related to mineralization two major styles of mineralization are recognized at massawa from field and laboratory studies the first stage of sulphide au mineralization is associated with disseminated arsenopyrite pyrite which follows shear zones in the sedimentary and volcano sedimentary host rocks the second stage consists of quartz stibnite tetrahedrite veining distinguished by coarse visible gold and represents a late stage overprint on the primary mineralization the two stages of gold mineralization are separated by a phase of quartz molybdenite veining a distinctive base metal trace assemblage is linked to stibnite formation including multiple sb phases such as chalcostibite zinkenite roshchinite aurostibite jamesonite and robinsonite secondary ion mass spectroscopy based gold deportment data indicate that up to 90 of stage 1 gold is held as a solid solution within either arsenopyrite or arsenian pyrite stable isotope data yield 34ssulphide values of between 0 and 4 1 and 18oh2o values of 5 5 10 9 for all stages of mineralization suggesting a magmatic fluid influence this is consistent with field data that suggest that mineralization is synchronous with emplacement of a sequence of concordant felsic sheets that mineralization occurred at shallow 6 km depths is suggested both by the presence of stibnite and by fluid inclusion studies low temperature homogenization temperatures between 150 c and 230 c h2o nacl fluids 6 wt nacl equiv and coeval co2 ch4 inclusions observed in both phases of mineralization indicate trapping conditions of 220 315 c at 1 1 65 kbar a combination of phase petrology fluid inclusion and stable isotope data suggests deposition of gold from low salinity magmatic fluids most probably released from felsic rocks similar to those emplaced into the massawa sequence during mineralization

the first precambrian gold deposit in north xinjiang nw china geochronology metallogenic character and ore genesis of the dajingou gold deposit abstract the dajingou gold deposit is one of the typical gold deposits in the quruqtagh metamorphic terrane north of the tarim craton the deposit mainly comprises gold bearing quartz veins hosted in precambrian metamorphic rocks and structurally controlled by the subsidiary faults and ductile shear zone to determine the age of gold mineralization in the dajingou deposit a single sericite 40ar 39ar age of 829 4 4 8 ma mswd 0 57 and hydrothermal zircon weighted average u pb ages 812 15 ma mswd 3 2 from auriferous quartz veins were obtained in this study indicating that mineralization occurred in the mid neoproterozoic ca 0 83 ga corresponding to regional metamorphism of the tarim craton termed the tarim orogeny the dajingou deposit is the first precambrian gold deposit identified in north xinjiang nw china the 18o and d of quartz ranges from 14 7 to 15 9 and 104 to 75 respectively the calculated 18oh2o of the fluids ranges from 0 9 to 5 0 this suggests that the ore forming fluids in the dajingou gold deposit were derived from dehydration of metamorphic rocks during regional metamorphism with a subsequent input of meteoric water based on its similarities with typical orogenic gold deposits the dajingou deposit is proposed to be an orogenic gold deposit formed during the mid neoproterozoic tarim orogeny in the tarim craton

geodynamic setting of mesothermal gold deposits an association with accretionary tectonic regimes mesothermal gold provinces of phanerozoic age are characteristically associated with regional structures along which allochthonous terranes have been accreted onto continental margins or arcs a recurring sequence of transpressive deformation uplift late kinematic mineralization and shoshonitic magmatism is consistent with thermal reequilibration of tectonically thickened crust mesothermal gold camps in the superior province are spatially associated with large scale structures that have been interpreted as zones of transpressive accretion of individual subprovinces or allochthonous terranes these boundary structures are characterized by the sequence of significant horizontal shortening uplift late kinematic mineralization and shoshonitic lamprophyres and therefore may have the same geodynamic significance as phanerozoic counterparts in this model thermal re equilibration of underplated and subducted oceanic lithosphere and sediments in a transpressive regime over time scales of 10 to 40 m y is a necessary precursor to gold mineralization hydrothermal fluids are released along boundary faults and their splays during uplift the uniform temperature low salinity and mole co2 signify uniform source conditions whereas the variable o c sr and pb isotopic compositions of fluids reflect lithological complexity of the source regions and conduits ou the basis of this model it is suggested that mesothermal lode gold deposits are the product of subduction related crustal underplating and deep late metamorphism rather than magmatic or metamorphic events in the supracrustal rocks secular variations in the generation of archean proterozoic and phanerozoic mesothermal au provinces reflect the timing of collisional orogenies within terranes of these eras

orogenic gold common or evolving fluid and metal sources through time abstract orogenic gold deposits of all ages from paleoarchean to tertiary show consistency in chemical composition they are the products of aqueous carbonic fluids with typically 5 20 mol co 2 although unmixing during extreme pressure fluctuation can lead to entrapment of much more co 2 rich fluid inclusions in some cases ore fluids are typically characterized by significant concentrations of ch 4 and or n 2 common estimates of 0 01 0 36 mol h 2 s a near neutral ph of 5 5 and salinities of 3 7 wt nacl equiv with na k ca mg this fluid composition consistency favors an ore fluid produced from a single source area and rules out mixing of fluids from multiple sources as significant in orogenic gold formation nevertheless there are broad ranges in more robust fluid inclusion trapping temperatures and pressures between deposits that support a model where this specific fluid may deposit ore over a broad window of upper to middle crustal depths much of the reported isotopic and noble gas data is inconsistent between deposits leading to the common equivocal interpretations from studies that have attempted to define fluid and metal source areas for various orogenic gold provinces fluid stable isotope values are commonly characterized by the following ranges 1 18 o for precambrian ores of 6 to 11 and for phanerozoic ores of 7 to 13 2 d and 34 s values that are extremely variable 3 13 c values that range from 11 to 2 and 4 15 n of 10 to 24 for the neoarchean 6 5 to 12 for the paleoproterozoic and 1 5 to 10 for the phanerozoic secular variations in large scale earth processes appear to best explain some of the broad ranges in the o s and n data fluid rock interaction particularly in ore trap areas may cause important local shifts in the o s and c ratios the extreme variations in d mainly reflect measurements of hydrogen isotopes by bulk extraction of waters from numerous fluid inclusion generations many which are not related to ore formation radiogenic isotopes such as those of pb sr nd sm and os measured on hydrothermal minerals are even more difficult to interpret for defining metal source particularly as the low salinity ore fluids transport limited amounts of these elements and significant amounts of these may be locally added to the minerals during alteration reactions at the sites of gold deposition noble gas and halogen data are equally equivocal fluid exsolution from granitoids emplaced into the upper and middle crust metamorphism of the crust or fluids entering trans crustal fault zones from below the crust all remain as permissive scenarios associated with orogenic gold formation as the abundant geochemical data are equivocal however geological and geochronological data weigh heavily against a magmatic hydrothermal model in the upper to middle crust there is no universal temporal association between orogenic gold and magmatism and where there is an overlap in age there is no specific type of magmatism consistently associated with gold formation nor element zonation around any specific pluton a crustal metamorphic model for fluid and metal sources is very consistent with geological geochronological and geochemical data although metamorphism on a regional scale that releases these components into major fault zones can be associated with many processes along active continental margins these can include crustal thickening and radiogenic heating slab rollback and heating during crustal extension or subduction of a spreading ridge heating the base of an accretionary prism in rare examples where phanerozoic orogenic gold deposits are hosted in precambrian high grade metamorphic terranes fluids and metals must however enter a transcrustal fault system from a sub crustal source this could either be a devolatilized subducted relatively flat perhaps stalled slab and its overlying sediment or the corner of the fertilized mantle wedge that releases a fluid during a thermal event

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

nitrogen isotope tracers of high temperature fluid rock interactions case study of the catalina schist california abstract nitrogen isotope data for metasomatized rocks veins and pegmatites in the catalina schist subduction zone metamorphic complex allow futher characterization of complex high p t metasomatic proceses and evaluation of the scales of isotopic equilibration and fluid transport during subduction zone metamorphism throughout the catalina schist n resides predominantly as nh4 in white mica which occurs in nearly all bulk compositions i e metasedimentary metamafic and to a lesser extent metaultramafic melange at all grades within each metamorphic unit of the catalina schist ranging in grade from lawsonite albite to amphibolite facies 15n values of mica in metasomatized metamafic and metaultramafic rocks are consistent with the metasomatic addition of n from nearby devolatilizing metasedimentary rocks into the initially n poor mafic and ultramafic rocks within each unit uniformity of mica 15n in metasomatized rocks relative to the 15n of metasedimentary rocks in the same unit implies mixing of n from nearby heterogeneous metasedimentary sources perhaps producing fluids with unifrom 15n at up to the kilometer scale however the trend in 15n of metasedimentary sources with increasing metamorphic grade is inconsistent with larger scale up temperature transfer of fluid in this case n2 bearing in the catalina schist paleosubduction zone such flow at scales of up to tens of kilometers has been inferred through previous oxygen isotope study nitrogen isotope compositions are instead believed to have been controlled at a more local scale than the o isotope systematics due to the more rock dominated fluid rock mass balance for n the 15n of muscovite in leucosomes and pegmatites in amphibolite grade metasedimentary exposures matches that of muscovite in metasedimentary hosts implying minimal n isotope fractionation during migmatization processes and possible transfer of metasedimentary n isotope signatures in silicate melts these results illustrate the potential of the n isotope system to yield valuable information regarding fluid rock interactions in the crust and mantle the data for the catalina schist demonstrate the ability of n isotopes to trace the transfer of sediment derived c o h s n fluids and silicate melts and show the expected benefit of the n isotope system in having a differing fluid rock mass balance relative to the more commonly used stable isotope systems that can yield unique constraints in quantitative models of crustal fluid processes

rb sr k ar and stable isotope evidence for the ages and sources of fluid components of gold bearing quartz veins in the northern sierra nevada foothills metamorphic belt california gold bearing quartz veins occur in and near major fault zones in deformed oceanic and island arc rocks west of the main outcrop of the sierra nevada composite batholith veins typically occupy minor reverse faults that crosscut blueschist to amphibolite grade metamorphic rocks whose metamorphic ages range from early paleozoic to jurassic vein micas and carbonate quartz mica assemblages that formed by hydrothermal metasomatism of ultramafic wall rocks in the alleghany grass valley washington and mother lode districts yield concordant k ar and rb sr ages the dated veins are significantly younger than prograde metamorphism penetrative deformation and accretion of their host rocks to the continental margin new and previously published mineralization ages from 13 localities in the sierra foothills range from about 140 to 110 m y ago with mean and median between 120 and 115 m y the age relations suggest that mineralizing fluids were set in motion by deep magmatic activity related to the resumption of east dipping subduction along the western margin of north america following the late jurassic nevadan collision event co 2 bearing fluids responsible for metasomatism and much of the vein mica carbonate albite and quartz deposition in several northern mines were isotopically heavy delta 18 o asymp 8 14ppm delta d between about 10 and 50ppm and do not resemble seawater magmatic or meteoric waters metasomatic and vein filling mica dolomite magnesite and quartz in altered ultramafic rocks generally formed from fluids with similar sr and o isotope ratios at a given locality consistent quartz mica delta 18 o fractionations delta 18 o sub q m 4 5 4 9ppm from various localities imply uniform equilibration temperatures probably between 300 degrees and 350 degrees c on a local mine scale fluids responsible for both carbonate alteration of mafic and ultramafic wall rocks and albitic alteration of felsic and pelitic rocks had similar sr isotope ratios samples from three veins in the central alleghany district fit a 115 7 or 3 m y rb sr isochron with a 87 sr 86 sr i value of approximately 0 7119 inferred 87 sr 86 sr ratios of metasomatic fluids from mines in different parts of the foothills region vary considerably 0 704 0 718 suggesting that sr was derived from sources ranging from western assemblage mesozoic ophiolitic or arc volcanic rocks to early paleozoic continent derived clastic rocks of the shoo fly complex systematic geographic variations in both sr and o isotopes can be rationalized by assuming extensive fluid interaction with rocks similar to the ones that are exposed within a few kilometers of the veins but the ultimate sources of the fluids and of au and other constituents may be independent of these isotopically lighter meteoric fluids deposited some late quartz overgrowths and occupied secondary fluid inclusions in earlier vein quartz