the mokrsko west gold deposit bohemian massif czech republic mineralogy deposit setting and classification abstract the mokrsko west deposit is unique among european variscan gold deposits from the points of view of both the structure an approx 200 m thick complex of sheeted several mm thick densely spaced quartz veins and the economic viability gold reserves of about 100 t the deposit is hosted mainly by tonalite of the calc alkaline sazava tonalite suite ca 354 ma of the central bohemian plutonic complex mineralization is characterized by quartz dominated gangue no visible hydrothermal alteration low sulfide content high fineness native gold accompanied by maldonite aurostibite native bismuth and numerous bi te s phases five mineralogical stages are described in great detail arsenopyrite and chlorite thermometers mineral phase stabilities and published isotope and fluid inclusion data are used to reconstruct the temperature and compositional evolution of the system the role of liquid bismuth in the sequestration of gold is also discussed the deposit shares the features of both orogenic gold org and intrusion related gold irg deposits the irg model is advocated by close spatial association between the ore zone and the tonalite host intrusion by the absence scarcity of hydrothermal alteration by the au bi te as elemental association and by marked thermal gradients from the early to late mineralization stages the org model is advocated by an approx 15 10 ma gap between the intrusion of the tonalite host and the ore formation by isotope and geochemical evidence for a key role of metamorphic fluids in the mobilization and transfer of many elements species inclusive s and au the apparently ambiguous classification of the deposit can most probably be attributed to deposit formation at a depth of 9 km and to setting of the deposit at inside a large scale plutonic complex with multiple and prolonged tectonic and intrusive activity

a simple multicloud parameterization for convectively coupled tropical waves part i linear analysis abstract recent observational analysis reveals the central role of three multicloud types congestus stratiform and deep convective cumulus clouds in the dynamics of large scale convectively coupled kelvin waves westward propagating two day waves and the madden julian oscillation a systematic model convective parameterization highlighting the dynamic role of the three cloud types is developed here through two baroclinic modes of vertical structure a deep convective heating mode and a second mode with low level heating and cooling corresponding respectively to congestus and stratiform clouds a systematic moisture equation is developed where the lower troposphere moisture increases through detrainment of shallow cumulus clouds evaporation of stratiform rain and moisture convergence and decreases through deep convective precipitation a nonlinear switch is developed that favors either deep or congestus convection depending on the relative dryness of the troposphere in particular a dry troposphere

hydrothermal alteration of basalts beneath the bent hill massive sulfide deposit middle valley juan de fuca ridge burial of midocean ridges by clastic sediments particularly at continental margins profoundly affects the geometry and chemistry of hydrothermal circulation and mineralization in the upper crust middle valley the sediment covered northern extension of the bare rock endeavour segment of the juan de fuca ridge is host to the base metal rich cu zn bent hill massive sulfide deposit at a water depth of 2 400 m the 9 mt bent hill deposit is a steep sided body 200 m across and 100 m thick ocean drilling program odp hole 856h penetrates through the massive sulfide and underlying feeder zone extending to a total depth of 500 m below sea floor through the base of the strongly recrystallized quartz chlorite sediment pile and into the uppermost volcanic basement the basaltic rocks beneath the bent hill deposit include narrow sills intruded into indurated sediments a volcanic flow erupted on top of sediments and pillow lavas below the lowermost sediments recovered similar styles of alteration are present in both the sills and flows and alteration is dominated by the effects of large scale hydrothermal upflow rather than hydrothermal activity associated with individual eruptions or intrusions the basalts are slightly to completely altered to greenschist facies secondary minerals principally quartz chlorite and titanite with subsidiary epidote cu fe sulfides and rare actinolite there are steep mineralogical chemical and isotopic alteration gradients from the highly altered basalt sediment interfaces down to the less altered flow interiors suggesting the channeling of hydrothermal fluids along the basalt sediment boundaries alteration is reflected in intense metasomatic changes in the basalts assuming immobile tio 2 the most intensely altered basalts have undergone about 20 percent mass loss during recrystallization to chlorite quartz rocks with depletions in silica alumina and alkali alkali earth and base metals chloritized pillow margins with strongly light rare earth element enriched chondrite normalized patterns la sm n 1 5 cf fresh basalts 0 7 that mimic profiles for midocean ridge hydrothermal fluids require fluid rock exchange with large quantities of hydrothermal fluid w r 27 000 oxygen isotope compositions of chlorite quartz rocks 18 o 1 8 2 4 suggest that alteration occurred between 320 and 370 c strontium isotope compositions of the altered basalts and the chlorite quartz rocks are not homogeneous and range from 87 sr 86 sr ratio 0 7037 to 0 7046 there is a strong mode in 87 sr 86 sr ratio at 0 7038 suggesting that much of the alteration occurred by isotopic exchange with a hydrothermal fluid of that composition this ratio is significantly lower than that measured for 265 c fluids venting from the nearby odp mound 87 sr 86 sr 0 7044 the occurrence of epidote and isocubanite in the chloritized glassy pillow margins suggests that these rocks may retain a record of the high temperature 350 c hydrothermal fluid responsible for the formation of the overlying bent hill massive sulfide deposit the strontium isotope composition of the chloritized glassy pillow margins and hence the mineralizing fluid is slightly more radiogenic 87 sr 86 sr 0 7046 this composition could result from the addition of 15 percent of a pelagic or sedimentary component to the 87 sr 86 sr ratio 0 7038 fluid responsible for most of the sr isotope exchange with the upper basement the sediments beneath the bent hill deposit are also strongly recrystallized to quartz and chlorite although their strontium isotope compositions are much lower than those in pelagic or terrigenous sediments in the region 87 sr 86 sr 0 709 0 720 the range of compositions 87 sr 86 sr 0 7046 0 7060 has little overlap with that of the altered basalts and chlorite quartz rocks from the sills and uppermost basement this lack of overlap suggests that the sediments either retain some of their original sedimentary strontium or that there is a range of fluid compositions in the sediment pile beneath the bent hill deposit sediments from the margins of middle valley far from zones of active black smoker venting odp site 855 have 87 sr 86 sr ratios with a significant hydrothermal component 0 7059 0 7086 these ratios indicate that the subsurface hydrology of middle valley is dominated by evolved fluids rather than seawater and that recharge into this system is not through boundary faults or through the sedimentary blanket as suggested by previous models rather exposed basement rocks that form the flanks of middle valley are the most likely zones of regional seawater recharge to the deep high temperature hydrothermal systems

mineralogy and mineral chemistry of bi minerals constraints on ore genesis of the beiya giant porphyry skarn gold deposit southwestern china abstract the beiya deposit located in the sanjiang tethyan tectonic domain sw china is the third largest au deposit in china 323 t au 2 47 g t as a porphyry skarn deposit beiya is related to cenozoic himalayan alkaline porphyries abundant bi minerals have been recognized from both the porphyry and skarn ores comprising bismuthinite bi cu sulfosalts emplectite wittichenite bi pb sulfosalts galenobismutite cosalite bi ag sulfosalt matildite bi cu pb sulfosalts bismuthinite derivatives bi pb ag sulfosalts lillianite homologs galena matildite series and bi chalcogenides tsumoite the unnamed bi 2 te the unnamed ag 4 bi 3 te 3 tetradymite and the unnamed bi pb 3 te s 4 native bismuth and maldonite are also found in the skarn ores the arsenopyrite geothermometer reveals that the porphyry au mineralization took place at temperatures in the range of 350 450 c and at log f s 2 in the range of 8 0 to 5 5 respectively in contrast the beiya bi mineral assemblages indicate that the skarn ore forming fluids had minimum temperatures of 230 175 c prevailing temperatures exceeding 271 c and fluctuating f s 2 f te 2 conditions we also model a prolonged skarn au mineralization history at beiya including at least two episodes of bi melts scavenging au we thus suggest that this process was among the most effective au enrichment mechanisms at beiya

madden julian oscillation analog and intraseasonal variability in a multicloud model above the equator the madden julian oscillation mjo is the dominant component of tropical intraseasonal variability and a theory explaining its structure and successful numerical simulation remains a major challenge a successful model for the mjo should have a propagation speed of 4 7 m s predicted by theory a wavenumber 2 or 3 structure for the planetary scale low frequency envelope with distinct active and inactive phases of deep convection an intermittent turbulent chaotic multiscale structure within the planetary envelope involving embedded westward and eastward propagating deep convection events and qualitative features of the low frequency envelope from the observational record regarding e g its zonal flow structure and heating here such an mjo analog is produced by using the recent multicloud model of khouider and majda in an appropriate intraseasonal parameter regime for flows above the equator so that rotation is ignored key features of the multicloud model are i systematic low level moisture convergence with retained conservation of vertically integrated moist static energy and ii the use of three cumulus cloud types congestus stratiform and deep convective together with their differing vertical heating structures besides all of the above structure in the mjo analog waves there are accurate predictions of the phase speed from linear theory and transitions from weak regular mjo analog waves to strong multiscale mjo analog waves as climatological parameters vary with all of this structure in a simplified context these models should be useful for mjo predictability studies in a fashion akin to the lorenz 96 model for the midlatitude atmosphere

a multiscale model for tropical intraseasonal oscillations the tropical intraseasonal 40 to 50 day oscillation tio is the dominant component of variability in the tropical atmosphere with remarkable planetary scale circulation generated as envelopes of complex multiscale processes a new multiscale model is developed here that clearly demonstrates the fashion in which planetary scale circulations sharing many features in common with the observational record for the tio are generated on intraseasonal time scales through the upscale transfer of kinetic and thermal energy generated by wave trains of organized synoptic scale circulations having features in common with observed superclusters the appeal of the multiscale models developed below is their firm mathematical underpinnings simplicity and analytic tractability while remaining self consistent with key features of the observational record the results below demonstrate in a transparent fashion the central role of organized vertically tilted synoptic scale circulations in generating a planetary circulation resembling the tio

a new multiscale model for the madden julian oscillation abstract a multiscale model of the mjo is developed here that accounts in a simplified fashion for both the upscale transfer from synoptic to planetary scales of momentum and temperature from wave trains of thermally driven equatorial synoptic scale circulations in a moving convective envelope as well as direct mean heating on planetary scales this model involves idealized thermally driven congestus synoptic scale fluctuations in the eastern part of the moving wave envelope and convective superclusters in the western part of the envelope the model self consistently reproduces qualitatively many of the detailed structural features of the planetary circulation in the observations of the mjo including the vertical structure in both the westerly onset region and the strong westerly wind burst region as well as the horizontal quadrupole planetary vortex structure the westerly midlevel inflow in the strong westerly region and the quadrupole vortex are largely produced in the model by the upscale transpor

gold and tin mineralisation in the ultramafic hosted cheoeum vent field central indian ridge the cheoeum vent field cvf is the first example of an inactive ultramafic hosted seafloor massive sulphide sms deposit identified in the middle part of the central indian ridge here we report on the detailed mineralogy and geochemistry of ultramafic hosted sulphide sample atop a chimney together with a few small fragments hydrothermal chimneys are characterised by high concentrations of au up to 17 8 ppm and sn up to 1720 ppm the sulphide mineralisation in the cvf shows 1 early precipitation of anhedral sphalerite and pyrite marcasite aggregates under relatively low temperature 250 c fluid conditions 2 intensive deposition of subhedral pyrrhotite isocubanite chalcopyrite fe rich sphalerite sp iii and electrum from high temperature 250 365 c and reduced fluids in the main mineralisation stage and 3 a seawater alteration stage distinguished by the mineral assemblage of marcasite pseudomorphs altered isocubanite phase covellite amorphous silica and fe oxyhydroxides electrum 2 m in size is the principal form of au mineralisation and is mainly associated with the main mineralisation stage the consistently high fineness of electrum 801 to 909 is indicative of the selective saturation of au over ag in the fluid during high temperature mineralisation which differs from the au mineralisation associated with typical basaltic hosted hydrothermal systems on mid ocean ridges tin is mainly substituted in structures of sphalerite isocubanite and chalcopyrite as a solid solution and not as mineral inclusions the continuously ascending hydrothermal fluids enable the early formed sn bearing sulphide to be dissolved and reprecipitated producing significantly sn enriched replacement boundaries between isocubanite and sp iii this study suggests that au sn mineralisation could be facilitated by the low redox potential of ultramafic hosted hydrothermal systems such as in the cvf which may be a common occurrence along slow spreading mid ocean ridges

stratigraphy structure and lode gold system at the central manitoba mine trend rice lake greenstone belt archean superior province manitoba canada abstract the central manitoba mine trend is one of the most important lode gold districts in the rice lake greenstone granitoid belt of the western uchi subprovince within western superior province manitoba canada neoarchean host rocks consist of a south facing metavolcano sedimentary succession 2 75 2 73 ga intruded by voluminous gabbroic sills and tonalitic granodioritic plutons 2 73 2 72 ga as well as late aplite dikes 2 73 2 72 ga and quartz feldspar porphyry dikes 2 73 2 71 ga five generations g1 g5 of deformation structures have been recognized through detailed geological mapping the entire succession was folded during g1 and g2 prior to rare late aplite dike emplacement all fault fill veins and extension veins cut all lithologic units and are structurally governed by late g3 and g4 conjugate shear zones main gold mineralization occurs within fault fill veins hosted by west trending steeply dipping g4 dextral brittle ductile and ductile shear zones which occur along or across contacts of metabasalt metagreywacke and metagabbro or entirely within metagabbro microstructural and paragenetic analyses on main gold bearing veins have revealed that gold is intimately associated with quartz pyrrhotite and tellurobismuthite main gold introduction is interpreted to have taken place contemporaneously with pyrrhotite and tellurobismuthite deposition early during g4 shearing these results in structural and mineralogical aspects at the central manitoba mine trend provide new constraints on structural evolution and gold mineralization of the southeastern rice lake belt and can be used to improve exploration models for gold deposits in archean terranes

peculiarities of some mafic ultramafic and ultramafic hosted massive sulfide deposits from the main uralian fault zone southern urals abstract some cu rich mafic ultramafic and ultramafic hosted massive sulfide deposits from the southern segment of the main uralian fault zone ivanovka and ishkinino deposits southern urals show unusual characteristics their major features include i relatively high co ni au very low zn and negligible pb grades ii a pyrrhotite dominated mineralization locally characterized by the presence of open latticework aggregates of lamellar pyrrhotite with mg saponite mg chlorite and carbonate matrix iii hydrothermal alteration of ultramafic host rocks into talc carbonate quartz chlorite and of mafic host rocks into chloritites iv the presence of clastic facies with reworked sulfide and ultramafic or mafic components v the widespread occurrence of sulfide associated chromite vi the specific mineralogy of co ni fe and as including sulfoarsenides mono and diarsenides and co rich pentlandite and pyrite vii the supra subduction zone geochemical signature of the host serpentinites and volcanic rocks although some of these features have been separately reported in certain modern ocean seafloor and ophiolite hosted fossil deposits a true equivalent has yet to be found based on recognized partial analogies with a few modern seafloor examples the arc tholeiitic boninitic geochemical signature of sulfide associated volcanic rocks and the highly refractory compositions of sulfide hosted chromite relicts the studied deposits are believed to have formed by seafloor subseafloor hydrothermal processes in an oceanic island arc setting possible tectonostratigraphic correlation of sulfide associated units with infant non accretionary arc volcanic units of the adjacent magnitogorsk oceanic island arc system suggests formation of the studied deposits during the earliest stages of devonian subduction related volcanism

petrogenesis of bismuth minerals in the dabaoshan pb zn polymetallic massive sulfide deposit northern guangdong province china abstract located in the northern guangdong province the dabaoshan pb zn polymetallic massive sulfide deposit is one of the most important regions to have produced fe cu pb zn and s in southern china while much progress has been made with respect to the geology and geochemistry of the deposit a better understanding of ore genesis is warranted this ore body contains abundant bismuth element only few studies exist on the distribution characteristics and existing states of bismuth electron microprobe study yields that native bismuth tellurides e g hedleyite and sulfide minerals are the main forms in which this element is found the occurrence and characteristics of native bismuth and its mineral assemblages are different with high t gold bearing deposits but are more similar to those of mid low temperature hydrothermal deposits our research shows that the bi te at dabaoshan derived from the late yanshanian dacite porphyry the bi te rich ore forming fluid developed during intrusion of the dacite and mineralized along fractures and overprint the earlier pb zn mineralization during the early overprinting event the ore forming fluid was rich in bi and te and poor in s under uniform mid temperature conditions as the hydrothermal fluids evolved they became enriched with ag and pb sulfur was rich in the fluid during the late overprinting event this may have been related to dissolution of sulfide the low abundances of bi te and ag in sulfide minerals e g galena and sphalerite indicate that ore genesis and the ore forming materials were different between the pb zn and bi te ag stages thus new geological and geochemical data were used to delineate the sequence ore genesis of dabaoshan pb zn mineralization

boiling induced formation of colloidal gold in black smoker hydrothermal fluids gold colloids occur in black smoker fluids from the niua south hydrothermal vent field lau basin south pacific ocean confirming the long standing hypothesis that gold may undergo colloidal transport in hydrothermal fluids six black smoker vents varying in temperature from 250 c to 325 c were sampled the 325 c vent was boiling at the time of sampling and the 250 c fluids were diffusely venting native gold particles ranging from 50 nm to 2 m were identified in 4 of the fluid samples and were also observed to precipitate on the sampler during collection from the boiling vent total gold concentrations dissolved and particulate in the fluid samples range from 1 6 to 5 4 nm in the high temperature focused flow vents although the gold concentrations in the focused flow fluids are relatively high they are lower than potential solubilities prior to boiling and indicate that precipitation was boiling induced with sulfide lost upon boiling to exsolution and metal sulfide formation gold concentrations reach 26 7 nm in the 250 c diffuse flow sample and abundant native gold particles were also found in the fluids and associated sulfide chimney and are interpreted to be a product of colloid accumulation and growth following initial precipitation upon boiling these results indicate that colloid driven precipitation as a result of boiling the persistence of colloids after boiling and the accumulation of colloids in diffuse flow fluids are important mechanisms for the enrichment of gold in seafloor hydrothermal systems

occurrences and distribution of invisible precious metals in sulfide deposits from the edmond hydrothermal field central indian ridge abstract here we report the first documented occurrences of invisible gold and silver in seafloor sulfide deposits from an active hydrothermal system on the central indian ridge a detailed mineralogical and geochemical study of polymetallic sulfides from the edmond vent field was conducted in order to identify controls on the distribution of precious metals bulk samples n 18 contain up to 18 7 ppm au and 1450 ppm ag with average concentrations of 2 3 ppm au and 218 9 ppm ag among them several zn rich chimney fragments and anhydrite dominated ore samples have higher contents of precious metals than fe cu rich massive sulfides and silica rich hydrothermal precipitates native gold grains are mainly associated with sphalerite anhydrite barite and fe oxyhydroxides abundant submicroscopic au ag alloys tend to occur along grain boundaries between cu fe sulfides and tennantite or close to the rims of fe poor sphalerite in contrast to primary electrum with high ag au ratios the absence of detectable silver in high purity gold indicates that secondary au enrichment has probably occurred after a direct co precipitation with zn rich mineral assemblages upon cooling and mixing of vent fluids with cold seawater a suite of late stage ag rich phases including argentotennantite pearceite and acanthite occur as crack filling veinlets and patches in low temperature fahlores or as tiny inclusions enclosed by pyrite chalcopyrite and colloform sphalerite by using hrtem combined with haadf stem imaging we have found out that silver is also present in significant quantities as discrete colloidal nanoparticles in tennantite minor native copper is closely associated with altered chalcopyrite sphalerite and covellite exhibiting signs of dissolution recrystallization and reprecipitation extensive hydrothermal reworking resulted from a long history of high temperature venting in this field together with post depositional supergene replacement processes involving oxidation leaching or coupled dissolution reprecipitation mechanisms facilitated by a permeable porosity generated in primary cu fe sulfides are considered to be important for the remobilization and local reconcentration of early formed precious metals and may have been responsible for the formation of relatively coarse grained native gold or silver within recrystallized massive sulfides and chimney debris

la icpms and epma studies of pyrite arsenopyrite and loellingite from the bhukia jagpura gold prospect southern rajasthan india implications for ore genesis and gold remobilization abstract the bhukia jagpura gold prospect is hosted by a paleoproterozoic metasedimentary rock sequence that constitutes the lowermost part of the aravalli supergroup lying in close proximity to the basement cover contact a combined ore petrographic la icpms and epma studies of iron sulfides and sulfarsenides from the prospect has revealed four varieties of pyrite and two varieties of arsenopyrite with different trace element compositions the data indicates a sedimentary diagenetic origin for the earliest pyrites py i and ii which are devoid of any gold in contrast to hydrothermal py iii and iv with a minor but significant content of invisible gold 0 3 1 6 ppm associated loellingite 35 51 ppm and arsenopyrite aspy i 0 28 10 ppm contain the maximum concentration of invisible gold while it is substantially lower in aspy ii 0 06 1 5 ppm the compositions of the gold bearing pyrites low ni co ratios arsenopyrites and loellingite rich in co and the presence of au bi te mineral phases within arsenopyrite support a magmatic hydrothermal model for the origin of gold sulfide mineralization which appears to have taken place during the peak of regional metamorphism or at an earlier stage the different stages of deformation and metamorphism released and remobilized gold in invisible and discrete forms from the sulfarsenides during prograde metamorphism of aspy i gold was partially incorporated and relatively enriched within the structure of loellingite later during retrograde metamorphism gold was exsolved as visible grains at the interface of loellingite and aspy ii and or within loellingite

submarine gold mineralization near lihir island new ireland fore arc papua new guinea gold rich siliceous veins with disseminated polymetallic sulfides and pyritic stockwork mineralization have been recovered from the top of conical seamount a shallow 1 050 m water depth submarine volcano located about 10 km south of lihir island papua new guinea grab samples from the summit of conical seamount contain the highest concentration of gold yet reported from the modern sea floor max 230 ppm au avg 26 ppm n 40 the gold occurs in sulfide rich veins of black amorphous silica hosted by intensely altered high k calc alkaline basalts sulfides in the veins consist of sphalerite galena pyrite chalcopyrite marcasite and a variety of cu pb as sb sulfosalts the gold occurs as native gold and electrum in the amorphous silica and as inclusions in the sulfides the highest gold concentrations are associated with high ag as sb and hg zoned alteration adjacent to the veins consists of illite smectite amorphous silica k feldspar secondary plagioclase minor chlorite and trace carbonate the association of gold with illite smectite amorphous silica and k feldspar indicates deposition from near neutral ph hydrothermal fluids however the auriferous polymetallic sulfide veins and the associated alteration are overprinted on stockwork pyrite mineralization that is associated with earlier acid alteration containing alunite aluminum phosphate sulfates kaolinite and other clay minerals the platy habit of the alunite in this assemblage the presence of alumium posphate sulfate minerals and the sulfur isotope ratios of the crystalline pyrite 8 6 to 0 2 34 s n 28 and alunite 7 5 and 6 4 34 s are consistent with a contribution of magmatic volatiles in the earliest stages of the hydrothermal system framboidal pyrite within and at the margins of the mineralized zone has 34 s values suggesting involvement of biogenic activity 11 6 to 13 9 34 s the gold rich veins at conical seamount are distinct from sea floor massive sulfide deposits and represent a new style of mineralization on the modern sea floor the mineralogy alteration geochemistry and texture of the veins resemble those of some subaerial epithermal gold deposits and indicate that features long considered to define a subaerial setting can also form in a submarine environment the proximity of conical seamount to the giant ladolam epithermal gold deposit on nearby lihir island also raises the possibility that both subaerial and submarine gold mineralization in the region may be related to the same district scale magmatic events

on the initiation of metamorphic sulfide anatexis mineral assemblages in common sulfide ore deposits are examined together with phase relations to 1 investigate the pressure temperature conditions required for the onset of metamorphically induced partial melting involving economic minerals and 2 place constraints on the amount of melt produced deposits that contain sulfosalt or telluride minerals may start to melt at conditions ranging from lowest greenschist facies to amphibolite facies deposits lacking sulfosalt and or telluride minerals may begin to melt once p t conditions reach the upper amphibolite facies if galena is present or well into the granulite facies if galena is absent the result is two broad melting domains a lowto medium temperature low melt volume domain involving melting of volumetrically minor sulfosalt and or telluride minerals and a high temperature potentially higher melt volume domain involving partial melting of the major sulfide minerals epithermal gold deposits which are especially rich in sulfosalt minerals are predicted to commence melting at the lowest temperatures of all sulfide deposit types massive pb zn cu deposits may start to melt in the lower to middle amphibolite facies if pyrite and arsenopyrite coexist at these conditions and in the upper amphibolite facies if they do not excepting sulfosalt bearing occurrences massive ni cu pge platinum group element deposits will show little to no melting under common crustal metamorphic conditions whereas disseminated cu deposits are typically incapable of generating melt until the granulite facies is reached when partial melting commences in bornite bearing rocks the volume of polymetallic melt that can be generated in most deposit types is therefore largely a function of the abundance of sulfosalt minerals even at granulite facies conditions this volume is usually less than 0 5 the exception is massive pb zn deposits where melt volumes significantly exceeding 0 5 vol may be segregated into sulfide magma dykes allowing mobilization over large distances

geochemistry of basalt from escanaba trough evidence for sediment contamination mid ocean ridge basalts morbs erupted in escanaba trough introduction the southernmost segment of the gorda ridge have greater isotopic crustal contamination of basaltic magmas is generally and geochemical variability than those from the remainder of the recognized for continental volcanoes e g taylor 1980 ridge samples from the sediment free northern sites in the trough leeman hawkesworth 1986 glazner et al 1991 the are depleted normal morb similar to those from the northern composition thickness and low melting temperatures of gorda ridge however samples from the sediment covered portion continental crustal rocks facilitate assimilation that can of the trough are significantly enriched in incompatible elements be easily identified however the extent to which basaltic and sr nd and pb isotopic ratios are shifted toward the composition magmas are contaminated by oceanic crust is a topic of of local sediments pb isotopic ratios in particular lie on a mixing intense debate e g bohrson reid 1995 clague et al line with local sediments assimilation fractional crystallization 1995 eiler et al 1996 bohrson et al 1996 and references afc calculations suggest that 25 40 fractional crystallization therein thirlwall et al 1997 the debate has focused coupled with assimilation of 1 2 of local sediments can produce on distinguishing the chemical characteristics magmas the enrichment observed for some escanaba basalts variations in acquire during ascent crustal storage and emplacement the amount of enrichment of elements with similar incompatibility from those that have been used to identify heterogeneous for comparably differentiated basalts and uncorrelated isotopic mantle sources for most ocean island volcanoes the ratios suggest variable amounts of assimilation of compositionally crustal components available to serve as contaminants heterogeneous assimilants by parental magmas assimilation of are poorly characterized but include the underlying sediment by morb magmas probably resulted when laccolithic altered oceanic crust pelagic to hemipelagic sediment intrusions ponded at the sediment basement interface beneath the and the hydrothermally altered volcanic edifice thick sediment cover in the southern part of escanaba trough mid ocean ridge basalts morbs are relatively unigeochemical and isotopic evidence for contamination can be detected form geochemically with depleted incompatible element in these lavas because of the strong compositional contrast between and radiogenic isotope compositions compared with lavas the parental magma and assimilant the depleted morb magma from ocean island or continental volcanoes however and highly enriched sediment assimilant are end member comexcess chlorine content and cl k ratios in fresh morb positions that provide a sensitive indicator for the smallest amount glasses support the idea that morb magmas may also of contamination of oceanic magma by crustal processes be affected by assimilation of hydrothermally altered oceanic crust e g michael schilling 1988 michael cornell 1996 chlorine like other incompatible elements shows the greatest enrichment in strongly frac

electron back scattered diffraction and la icp ms analysis of pyrite from the dahu lodegold deposit southern north china craton insights into geochemistry and distribution of trace element connection to microstructure of pyrite abstract the dahu lodegold deposit is situated in the xiaoqinling district hosted in the east west trending shear zones and characterized by auriferous quartz vein type based on the morphology and paragenetic assemblage three generations of pyrite termed as first generation pyrite py1 py2 and py3 can be classified the py2 can be further identified as brittle fractured pyrite py2 b and ductile deformed pyrite py2 d the different generations of pyrites feature distinct contents and distribution patterns of au and other trace elements the coarse grained euhedral py1 disseminating in the early formed pyrite poor quartz veins contained negligible to extremely low levels of au and associated trace elements probably indicating a slow growth rate from a hydrothermal fluid unsaturated with respect to fe s and au and other trace elements such as cu pb bi and te during the early orestage py2 occurring as laminations in the auriferous quartz veins consistently contained significant amounts of au bi and te with a strong positive correlation between these elements probably indicating a rapid growth rate and concentrated precipitation process of pyrite due to a hydrothermal fluid supersaturated with respect to fe s au and other trace elements during the pyrite rich auriferous quartz veining py3 which precipitated in the polymetallic sulfide orestage contained a small amounts of ag and a dramatically decreased content of au bi te and ratios of bi ag the results of electron back scattered diffraction analysis on the py2 revealed that py2 b was characterized by predominantly brittle deformation with a limited local plastic deformation by dislocation glide on a simple slide system whereas the py2 d displayed a significant crystal plastic deformation through a lattice rotation on two separate 1 0 0 axes and local dislocation creep microscopic petrography and laser ablation inductively coupled plasma mass spectrometry analyses revealed that the brittle fracturing dominated py2 b generally displayed homogenous distribution patterns of au and other trace elements and scarce visible gold grains whereas the plastically deformed py2 d generally exhibited an inhomogeneous distribution pattern of trace element and commonly contained fine grained irregular shaped visible native gold and other sulfide inclusions in the low angle boundaries and dislocation sites within py2 d possibly suggesting the significant influence of ductile deformation on remobilization and reprecipitation of au and associated trace elements once locked in py2 the corrosion of au te bi bearing py2 by the late polymetallic sulfide assemblage led to a liberation of invisible au and other trace elements and reprecipitation of visible native gold and other bi pb minerals the results indicate that brittle deformation exhibited no essential effects on the crystal structure and chemical component of pyrite whereas the plastic deformation can cause the readjustment of crystal lattices and liberation of impurities e g au te bi and cu from the crystal lattices of pyrite the plastic deformation and dissolution remobilization of pyrite are the important mechanisms responsible for the visible discrete gold grain formation in auriferous quartz veins macroscopically a preferred constant e w extending orientation and typical lamination structure of the auriferous quartz veins suggest a channeled fluid flowing along the closed and connected fractures under a syn mineralization s n oriented extensional tectonic regime related to the xiaoqinling metamorphic core complex here we report that the microstructures of ductile deformation and brittle fracturing of the auriferous quartz veins are probably indicative of the continuous shearing activation of the ore hosting normal fault

reduced and oxidized au cu bi iron oxide deposits of the tennant creek inlier australia an integrated geologic and chemical model iron oxide associated au cu bi deposits of the tennant creek gold field range from magnetite pyrrhotite pyrite reduced cu rich deposits through sulfide poor magnetite rich high grade au bi intermediate f o2 deposits to hematite rich high grade au oxidized deposits discordant magnetite hematite chlorite quartz ironstone bodies that host the ore formed hydrothermally during the 1840 to 1860 ma barramundi orogeny at temperatures up to 350 to 400 c and 2 5 to 5 kbars deposition of ironstone minerals was localized by dilatency that was generated during d1 reverse shearing and folding a separate au cu bi hydrothermal stage overprinted selected ironstones during d2 at or before 1825 to 1830 ma where favorable d1 structures and fluid pathways were reactivated the extremes of the reduced to oxidized deposit spectrum represent divergent chemical conditions that cannot be reconciled by the action of a single fluid we have identified both reducing and oxidizing fluids that helped to form the au cu bi deposits at the west peko deposit which represents the reduced end member subtype constraints from oxide silicate sulfide mineral stabilities fluid inclusions and chlorite thermochemistry indicate that au cu bi ore was introduced into ironstone at 300 to 340 c by a reducing pyrrhotite stable weakly acidic sulfur rich and n2 ch4 rich aqueous fluid containing 3 to 10 wt percent nacl equiv an ore fluid source outside the relatively oxidized metasedimentary host sequence warramunga formation is implied we suggest similar ore fluids for other reduced cu and sulfide rich deposits in the district chemical mass transfer modeling of reactions between reducing fluid and ironstone incorporating estimated thermodynamic properties for aqueous bismuth complexes indicates that highly efficient gold and native bi deposition was predominantly in response to desulfidation and oxidation of the fluid chalcopyrite precipitated mainly in response to increased ph gold bismuth ore in the eldorado deposit an oxidized and hematitic end member is interpreted to have involved mixing of an oxidizing hematite stable ca na cl brine with an intermediate f o2 low to moderate salinity ore fluid as well as reaction of both fluids with ironstone furthermore the mass transfer modeling suggests that mixing of reducing and oxidizing fluids in the absence of ironstones could have resulted in high grade au deposits lacking fe oxides a subtype that has received comparatively little exploration attention in the tennant creek district

metal sources in the middle valley massive sulphide deposit northern juan de fuca ridge pb isotope constraints abstract massive sulphides drilled from the bent hill deposit during ocean drilling program leg 139 at middle valley northern juan de fuca ridge have 206 pb 204 pb 18 43 18 86 207 pb 204 pb 15 43 15 63 208 pb 204 pb 38 08 38 60 these span 35 of the total pb isotope range measured in hydrothermal sulphides from the east pacific ocean 206 pb 204 pb ratios overlap the range of local basalts but 208 pb 204 pb and 207 pb 204 pb require a contribution from a more radiogenic source that is similar to the overlying sedimentary sequence three samples have lower 207 pb 204 pb ratios than the mean basalt value which may reflect the preservation of heterogeneities in the local basalts the pb isotopic compositions of primary pyrrhotites are indistinguishable from the secondary pyrite dominated assemblage which recrystallised from low temperature hydrothermal fluids this may be explained by the importance of the sulphosalts as a host for pb in the deposit clastic sulphides from sediments at bent hill are at the radiogenic end of the massive sulphide range suggesting that surficial sulphides may not provide an unbiased sample of the past hydrothermal circulation responsible for massive sulphide deposition if the pb content of sediment is 100 times greater than the basalts the sulphide pb isotopes imply that the hydrothermal fluids have extracted pb from 60 to 900 times more basalt than sediment lead in the massive sulphide deposit has been extracted from 2 10 8 3 10 8 m 3 of oceanic crust demonstrating the local nature of hydrothermal circulation at mid ocean ridges the 3 he 4 he of hydrothermal fluids in the sulphides 6 2 10 6 7 8 10 6 show only half the variation between the sediment and basalt end members displayed by pb isotopes this is consistent with volatile acquisition from magma degassing while the pb is leached during hydrothermal fluid interaction with crystallised oceanic crust

pb isotopes in sulfides from mid ocean ridge hydrothermal sites the authors report pb isotope ratios of sulfides deposited at seven recently active mid ocean ridge mor hydrothermal vents sulfides from three sediment starved sites on the juan de fuca ridge contain pb with isotope ratios identical to their local basaltic sources lead in two deposits from the sediment covered escanaba trough gorda ridge is derived from the sediments and does not appear to contain any basaltic component there is a range of isotope ratios in a guaymas basin deposit consistent with a mixture of sediment and mor basalt pb lead in a galapagos deposit differs slightly from known galapagos basalt pb isotope values the faithful record of pb isotope signatures of local sources in mor sulfides indicates that isotope ratios from ancient analogues ca be used as accurate reflections of ancient oceanic crustal values in ophiolite hosted deposits and continental crustal averages in sediment hosted deposits the preservation of primary ophiolitic or continental crustal pb isotope signatures in ancient mor sulfides provides a powerful tool for investigation of crustal evolution and for fingerprinting ancient terranes

petrogenetic significance of au bi te s associations the example of maldon central victorian gold province australia abstract mineralization at maldon victorian goldfields australia is part of the western lachlan foldbelt and is hosted by deformed lower ordovician metasediments intruded by the late devonian harcourt granite gold bi te s associations in the deposit include four gold bearing minerals maldonite native gold jonassonite and aurostibite and a range of sulphotellurides from the tetradymite group all of which are paragenetically tied to native bismuth the simpler phase associations involving bismuth maldonite and one or the other bi sulpho tellurides resemble the equivalents of eutectics in the system au bi te and hint at crystallisation from melts the complex associations include symplectites of gold and bismuth resulting from replacement of maldonite and are dominated by co existing sulphur bearing species joseite a joseite b an unnamed phase with bulk composition approximating to bi 3 te s 2 and jonassonite the complexity of such assemblages is also underpinned by chemical structural inhomogeneity within the unnamed phase i e nanoscale domains of bi 14 te s 9 and bi 5 te s 3 as documented from electron diffractions obtained from a fib tem foil although such lattice scale intergrowths reveal polysomatic disorder that can be associated with replacement of one telluride by another the structural modules in the overall stacking sequence indicate formation under local equilibrium conditions the mineralogical complexity of all au bi te s associations can be modelled in terms of interaction between simpler assemblages and sulphur bearing fluids three stages all involving gold minerals are recognised 1 bismuth maldonite hedleyite 2 bi sulphotellurides jonassonite and decomposition of maldonite gold bismuth and 3 bismuthinite gold from decomposition of maldonite or jonassonite the last stage is coincident with chloritisation and occurs during or following granite emplacement thermodynamic modelling of maldonite replacement by bismuthinite and gold indicates conditions to overlap with the pyrite hematite buffer at 258 c if neutral fluids are involved the general replacement model for the deposit shows that local scale reworking of maldonite and resultant gold remobilisation contributed to the apparent abundance of native gold this interpretation is consistent with the protracted geological history at maldon where granite intrusion post dates an orogenic event initial sulphur poor assemblages crystallised from melts predating granite emplacement the latest sulphidation event is attributed to retrograde fluids in the contact aureole whereas the earlier one could also have been produced during the multi stage deformation although observed assemblages and textures are the product of a complex sequence of overprinting there is no need to invoke input of gold from more than a single fluid generation

antecedents and self induction of active break south asian monsoon unraveled by satellites 1 understanding the origin and perpetuation of the monsoon cycle has eluded scientists for decades using a suite of unprecedented satellite measurements we show that the antecedents of the active break monsoon emerge in the western equatorial indian ocean eio the initiation of a new rainy phase in the western eio is preceded by in situ surface wind convergence and central eio warming both are induced by the subdued condition over the eastern eio set up in the previous cycle thus a self induction mechanism appears to be operating in maintenance of the indian monsoon intraseasonal oscillation the finding here offers a focus for prediction of the active break indian monsoon with potential predictability about four weeks in advance

native antimony in the baogutu gold deposit west junggar nw china its occurrence and origin abstract the baogutu gold deposit west junggar xinjiang nw china is composed of quartz sulfide veins and their stockworks and is hosted within an early carboniferous volcanic sedimentary sequence three ore forming paragenetic stages can be identified coarse grained quartz sulfide vein stage i gold bearing fine grained quartz sulfide vein stage ii and native antimony bearing calcite sulfide veinlets stage iii the estimated formation temperatures approximately 360 to 220 c f s 2 7 to 15 log units and f o 2 26 to 43 log units decrease from stage i through stage ii to stage iii the nature of the hydrothermal fluid changed from weakly acidic ph 5 to 6 at stage i to alkaline ph 7 at stage iii two different occurrences of native antimony could be identified one coexists with chalcopyrite and pyrrhotite whereas the other occurs as isolate grains within calcite veins native antimony up to 100 m in size contains minor amounts of as 2 0 to 2 7 wt empirical formula sb 0 95 0 96 as 0 03 0 04 decrease of temperature and f o 2 at high ph and low f s 2 conditions favored co precipitation of native antimony ullmannite assemblages and the decomposition of tetrahedrite native antimony crystallized later than the native gold suggesting fractionation between au and sb during the hydrothermal evolution of the deposit

organization of convection within the madden julian oscillation convection associated with the madden julian oscillation mjo is organized over a broad range of time and space scales within a large scale envelope of activity the large scale envelope of convection is characterized as an eastward propagating disturbance predominantly of zonal wavenumber 2 which is confined over the warmest waters of the indian and western pacific oceans large scale convection is shifted toward the disturbance s temperature anomaly which appears to result from frictional effects in the boundary layer synoptic and mesoscale convective activity is enhanced within the wet phase of the large scale envelope the increase at synoptic scales occurs uniformly over all scales but little difference in the character of the variability is observed between dry and wet phases the enhanced mesoscale activity within the wet phases exhibits a marked tendency to be organized into westward propagating clusters with periods near 2 days and half wavelengths of about 1000 km the role of these modes for the evolution of the mjo is discussed

gold refining by bismuth melts in the iron oxide dominated nico au co bi cu w deposit nwt canada the nico au co bi cu w deposit is located in the great bear magmatic zone nwt canada where numerous polymetallic iron oxide dominated mineralized systems have been recognized petrographic electron microprobe analysis empa and laser ablation inductively coupled plasma mass spectrometry la icp ms studies of host rock alteration and ore mineralogy together with sulfarsenide geothermometry have been carried out to constrain the nature of alteration and or mineralization assemblages in this deposit metasedimentary rocks of the treasure lake group host nico and are pervasively altered to an assemblage of ferrohornblende i actinolite i biotite i magnetite i orthoclase which is cut by barren veins composed of quartz ferrohornblende orthoclase calcite set 1 these alteration events are overprinted by metasomatic prograde and retrograde mineralized assemblages and both brittle and ductile deformation accompanied the metasomatism the prograde assemblage 400 c consists of cobaltite co rich loellingite and co rich arsenopyrite stage i magnetite ii ferrohornblende ii actinolite ii biotite ii pyrite and minor scheelite and orthoclase the earliest retrograde mineralization consists of arsenopyrite stages ii and iii which contains variable amounts of co together with native bi bismuthinite and au with lesser magnetite marcasite pyrite hastingsite and minor quartz the preservation of solidified native bi droplets suggests a temperature range of 270 to 400 c for precipitation of this assemblage the final stage of retrograde mineralization consists of a chalcopyrite bismuthinite hematite chlorite assemblage together with hastingsite emplectite which formed at temperatures of less than 270 c textural and trace element evidence indicates that the au and bi present within arsenides and sulfarsenides in the nico system resulted from the initial partitioning of structurally bound au and or invisible nanometer sized particles of au and bi into the prograde sulfarsenide and arsenide phases which contain up to 81 ppm au the au and bi were remobilized following retrograde alteration of those minerals to arsenopyrite ii molten bi droplets are interpreted to have scavenged au insitu resulting in the formation of the bi au inclusions observed in arsenopyrite ii the second mechanism of gold refining is explained by the occurrence of contemporaneous bi te melt and hydrothermal fluids that also could have fractionated gold during transport in solution and deposited it in fractures interstitially to earlier mineral grains and as disseminations within ca fe amphibole magnetite biotite altered rocks overall the gold upgrading at nico is consistent with the liquid bismuth collector model suggesting that this process was an important control on gold concentration in this and potentially other au bi te fe as s rich iron oxide copper gold iocg deposits

gold enrichment in hydrothermal sulfides from the okinawa trough an in situ la icp ms study abstract we assess the distribution of gold and investigate the physicochemical controls on gold enrichment in the okinawa trough an immature back arc basin we conducted a systematic study of gold and other trace element concentrations using in situ laser ablation inductively coupled plasma mass spectrometry la icp ms for various minerals in hydrothermal sulfide samples from the iheya north knoll and the yonaguni knoll iv hydrothermal fields orpiment colloform pyrite and colloform sphalerite are relatively enriched in au mean contents of 30 60 17 48 and 8 68 ppm respectively compared with gold poor massive chalcopyrite sphalerite and pyrite based on gold budget estimation colloform pyrite is the most important au host followed by colloform sphalerite and orpiment with these three minerals containing the majority of gold in hydrothermal sulfides gold occurs as invisible in host minerals as lattice bound and submicron inclusions in colloform pyrite as homogeneously distributed lattice bound or nano inclusions in colloform sphalerite and generally as lattice bound in orpiment and tetrahedrite three sets of gold host minerals are identified with distinct ore formation temperatures colloform sulfides and orpiment incorporate the majority of gold at low temperatures whereas tetrahedrite and massive sulfides incorporate only minor or trace amounts of gold at medium and high temperatures respectively our results show that gold enrichment in si ba fe rich hydrothermal sulfides at low temperatures is a significant contributor to gold enrichment in hydrothermal sulfide deposits in the okinawa trough along with the more common zn and cu rich hydrothermal sulfides thermodynamic modeling calculations suggest that the near neutral ph of hydrothermal fluids and the dominant transport species namely au hs complexes rather than au cl complexes are likely responsible for gold enrichment in low temperature mineral assemblages in hydrothermal precipitates from the okinawa trough

mineralogy of bi sulfosalts and tellurides from the yaoan gold deposit southwest china metallogenic implications abstract bismuth sulfosalts and tellurides are commonly present in many hydrothermal gold deposits and can provide important genetic and physicochemical constraints on the metallogenesis the abundant auriferous specular hematite veins in the yaoan gold deposit of the jinshajiang red river alkaline porphyry metallogenic belt southwest china contain an assemblage of bi sulfosalts and tellurides together with hematite pyrite and chalcopyrite bismuth sulfosalts are characterized by replacement textures resulting from changes of fluid composition e g pb bi cu and ag and mainly include bismuthinite derivatives e g bismuthinite paarite krupkaite lindstromite and friedrichite aikinite pad raite cuprobismutite homologues probably submicroscopic intergrowths of hodrushite and cuprobismutite and emplectite tellurides are tiny comprising tsumoite melonite hessite and calaverite which coexist with bismuthinite derivatives these bi sulfosalts and tellurides have direct contact with gold indicating that the yaoan gold metallogenesis was likely linked to the syenite porphyry magmatism that was originally sourced from a regional lithospheric mantle probably metasomatized by subduction of te rich sediments the telluride assemblage also reveals that the gold was precipitated under oxidizing conditions that has a high tellurium fugacity at the beginning fte2 10 7 9 and fs2 10 8 5 at 300 c

highly variable nutrient concentrations in the northern gulf of mexico abstract the distribution of surface nutrients along the salinity gradient in the mississippi atchafalaya river outflow region was examined during four cruises including two simultaneous cruises conducted in the northern gulf during the summer of 2010 and 2011 and in late spring of 2012 the new extensive data set covers the salinity gradient from 11 to 37 psu practical salinity unit in a year of extraordinarily high river discharge 2011 with few samples from a year of average 2010 and below average 2012 river outflow the overall surface concentrations of nitrate nitrite orthophosphate and silicate are compared to those recorded in cruises spanning the 1985 2009 interval using monte carlo simulations to test the statistical significance we found that surface orthophosphate and nitrate nitrite concentrations are approximately three and two fold smaller respectively in the 2010 2012 period compared to the previous years changes in silicate concentrations were in most cases not significant and their assessment complicated by different measurement techniques and potential preservation artifacts the weighted river loading of these nutrients was on the other hand very high in the latest period when samples mostly covered 2011 the well known negative correlation between nutrient concentrations and salinity at the ocean surface is confirmed in the most recent data the area surrounding the mississippi river mouth is characterized by inorganic n p ratios greater than 30 1 that decrease to values typically less than 10 1 at about 100 km from of the mouth overall our analysis suggests that surface nutrient concentrations in the northern gulf of mexico cannot be described with any good accuracy by a linear model based on river discharge alone

a gold copper association in ultramafic hosted hydrothermal sulfides from the mid atlantic ridge sulfide samples from the logatchev hydrothermal field 14 degrees 459 n mid atlantic ridge which occurs on an ultramafic substrate show a bimodal distribution of copper and zinc rich mineralogies both of which have elevated gold contents the copper rich vents show an unusual depression hosted morphology with few chimney structures copper rich samples show evidence for a primary gold copper association as well as gold concentration during small scale replacement and reworking processes evidence from vent morphologies au ag ratios and sulfidation state suggest that in the copper rich samples gold was not transported as au hs super 2

bi melt formation and gold scavenging from hydrothermal fluids an experimental study abstract the role of polymetallic melts in scavenging ore components has recently been highlighted in the context of fluid poor metamorphosed ore deposits in contrast the role of polymetallic melts in systems dominated by hydrothermal fluids remains poorly understood using a simple au bi model system we explored experimentally whether such polymetallic melts can precipitate directly from a hydrothermal fluid and investigated the ability of these melts to scavenge au from the solution the experiments were conducted in custom built flow through reactors designed to reproduce a hydrothermal system where melt components are dissolved at one stage along the flow path e g bi was dissolved by placing bi minerals along the fluid path whereas melt precipitation was caused further along the flow path by fluid rock interaction bi rich melts were readily obtained by reaction with pyrrhotite graphite or amorphous fes when au was added to the system bi au melts with compositions consistent with the au bi phase diagram were obtained in the case of fluid reaction with pyrrhotite epitaxial replacement of pyrrhotite by magnetite was observed with textures consistent with an interface coupled dissolution reprecipitation reaction icdrr in this case the metallic melt precipitated as blebs that were localized at the replacement front or within the porous magnetite direct fractionation of bi au melts from a hydrothermal fluid or precipitation of a bi melt followed by partitioning of au from ambient fluid offer new pathways to the enrichment of minor ore components such as au without requiring fluid saturation with respect to a au mineral this mechanism can explain the strong geochemical affinity recognized between au and low melting point chalcophile elements such as bi in many gold deposits examples of deposits where such a model may be applicable include orogenic gold deposits and gold skarns contrary to models involving migration of polymetallic melts to explain element remobilization only small quantities ppm of polymetallic melts are required to affect the au endowment of a deposit via interaction with a hydrothermal fluid the experiments also show that micro environments can play a critical role in controlling melt occurrences for example reaction fronts developing via icdr reactions can promote melt formation as observed during the replacement of pyrrhotite by magnetite the associated transient porosity creates space for the melt and promotes melt fluid exchanges whereas the reaction front provides local geochemical conditions favorable to melt precipitation e g reduced low ah2s aq and catalytic surface

deep sea sedimentary record of the late wisconsin cataclysmic floods from the columbia river new results from ocean drilling program site 1037 and u s geological survey high resolution seismic reflection profiles confirm the great thickness fast deposition rate distant source and convolute path of turbidites that fill the escanaba trough the rift valley of the southernmost segment of the gorda ridge accelerator mass spectrometry 14 c measurements provide the first direct dating of the escanaba trough turbidites demonstrating an average deposition rate faster than 10 m k y between 32 and 11 ka and as fast as 15 m k y during the oxygen isotope stage 2 lowstand in the upper 60 m of sediment the petrology of turbidite sand beds which are as much as 12 m thick show that the dominant source for the turbidites is from the columbia river which is more than 800 km to the north rather than from the much closer rivers of northern california new high resolution seismic reflection profiles show that except for areas of very recent volcanism the entire escanaba trough below 3200 m water depth is floored by the turbidite sequence that was cored in the upper 60 m at site 1037b the ages of the upper 120 m of turbidites correspond with the ages of channeled scabland deposits associated with latest quaternary jokulhlaups from glacial lake missoula the age and source characteristics suggest that these megaturbidite beds in escanaba trough are most likely deposits formed by hyperpycnally generated turbidity currents as the largest of the lake missoula floods entered the sea

divining gold in seafloor polymetallic massive sulfide systems hydrothermal fluids on the modern seafloor are important carriers of base and precious metals in a wide range of volcanic and tectonic settings the concentrations and distribution especially of gold and silver in associated seafloor massive sulfide sms deposits are strongly influenced by variable source rocks fluid chemistry and precipitation mechanisms compositional data of 130 sms deposits around the world show a large range of gold and silver grades in part reflecting strong buffering of the hydrothermal fluids by their host rocks geochemical reaction path modeling shows that in most cases the investigated hydrothermal fluids are undersaturated with gold and silver and solubilities can be orders of magnitude higher than the au and ag concentrations measured in the corresponding fluids precipitation of gold during conductive cooling of mid ocean ridge black smoker mor fluids occurs at low temperatures but can be very rapid with 90 of the gold deposited in the first 25 c of cooling below 150 c the result is a zn au polymetallic assemblage with au and ag deposited at the same time together with pb and sulfosalts in ultramafic dominated um systems the strongly reduced hydrothermal fluids promote the deposition of gold at higher temperatures and explain the correlation between gold and copper in these deposits in this case the lower stability of the auhs complex at low o2 buffered by fayalite magnetite and quartz results in gold deposition at 250 c with early bornite and chalcopyrite and before sphalerite and silver producing a high temperature cu au assemblage in sediment hosted sed systems the much higher ph stabilizes au hs 2 and keeps gold in solution to very low temperatures after the precipitation of chalcopyrite sphalerite and galena resulting in au poor polymetallic sulfides and very late stage deposition of gold commonly with amorphous silica in arc related arc systems gold deposition occurs at somewhat higher temperatures than in the mor case in part because the fluids start with higher gold concentrations this can be explained by probable direct magmatic contributions and the high o2 of the fluids which promotes the solubility of gold at the source during cooling gold precipitates at about 160 c with sphalerite tennantite silver and galena resulting in an au rich polymetallic sulfide assemblage the mixing of hydrothermal fluids with seawater generally causes oxidation and eventually a decrease in the ph at a mixing ratio of 1 1 causing an initial increase in the solubility of gold and silver this can delay gold deposition from aqueous species to very low temperatures these complex systematics make prediction of au and ag grades difficult however important new data are coming to light on the actual concentrations of the precious metals in hydrothermal fluids in particular the input of magmatic volatiles and leaching of pre existing gold can lead to significant increases in the au and ag concentrations of the venting fluids and earlier deposition in several cases it appears that at least part of the gold load is present as nanoparticles in suspension allowing bulk gold concentrations that may be far in excess of liquid saturation boiling at the seafloor is now widely observed even at great water depths close to the critical point of seawater model calculations of phase separation during boiling show the competing effects on gold solubility of h2 h2s and co2 partitioning into the vapor which can result in highly variable gold to base metal ratios in the deposits flashing of the vent fluids into steam at high temperatures is also commonly observed and can lead to spectacular au grades with a strong cu au association in the deepest and hottest vents

a frictional skeleton model for the madden julian oscillation abstractthe madden julian oscillation mjo is a multiscale system a skeleton model developed by majda and stechmann can capture some of planetary scale aspects of observed features such as slow eastward propagation nondispersive behavior and quadrupole vortex structure however the majda stechmann model cannot explain the source of instability and the preferred planetary scale of the mjo since the mjo major convection region is leaded by its planetary boundary layer pbl moisture convergence here a frictional skeleton model is built by implementing a slab pbl into the neutral skeleton model as a skeleton model allowing the scale interaction this model is only valid for large scale waves this study shows that the pbl frictional convergence provides a strong instability source for the long eastward modes although it also destabilizes very short westward modes for the long waves wavenumber less than 5 the pbl ekman pumping moistens the low troposphere to the east of the mjo convective envel

geochemistry mineralogy and mineral chemistry of the welatam cu mo bi au prospect northeast myanmar abstract the welatam prospect located in kachin state northeast myanmar is the first discovered bismuth gold locality within the tengchong lianghe tengliang metallogenic district two mineralization types identified in this area are related to three hydrothermal events the massive sulfide mineralization was formed during the hydrothermal activities of stage i and ii producing mainly iron sulfides e g pyrrhotite chalcopyrite pyrite arsenopyrite whereas the activity of stage iii resulted in quartz molybdenite mineralization which is characterized by the deposition of molybdenite galena and bi te au bearing phases the bi mineral assemblages suggest that at stage iii the prevailing temperatures were above 271 c fluid rock interaction that changed the ph fo2 fs2 and in particular temperature in hydrothermal fluids is important in causing the precipitation of liquid bismuth from bi3 the high correlation between bi and au r 0 782 demonstrates that liquid bismuth may have played a crucial role in gold enrichment here bi au bearing minerals commonly precipitate along the cleavages of molybdenite forming a distinctive mineral assemblage it may indicate that the cleavages besides providing spaces for the precipitation of liquid bismuth create local chemical conditions e g reducing and catalytic surfaces favorable for bi melt to scavenge au from ambient fluid the abundant bi minerals metal association e g cu bi mo au te w the restricted 34s values 7 3 to 1 9 of sulfides and the proximity with the welatam monzogranites suggest a magmatic contribution to the ore forming fluids

mineralogy and mineral chemistry of the metamorphosed and precious metal bearing ming deposit canada abstract the ming deposit in the northwestern canadian appalachians is a metamorphosed bimodal mafic precious metal au ag bearing and cu rich volcanogenic massive sulfide vms deposit consisting of several spatially proximal lenses in a sericite to chlorite altered rhyodacitic footwall the various lenses 1807 zone 1806 zone ming south up plunge ming south down plunge and the lower footwall zone have variations in cu au ag and zn grades that reflect varying physico chemical conditions of ore formation this study describes the complex ore mineralogy of the orebodies and constrains the genesis of the deposit using field methods mineralogy whole rock sulfide geochemistry and micro analytical methods the orebodies are predominantly pyrite chalcopyrite with lesser sphalerite pyrrhotite and trace galena and arsenopyrite with the exception of the lower footwall zone which consists of a high temperature 320 c chalcopyrite pyrrhotite pyrite cubanite assemblage the other orebodies 1806 1807 ming south up plunge and down plunge contain trace amounts of tellurides hessite altaite tsumoite unnamed bismuthtelluride sulfosalts ag poor and ag rich tennantite tetrahedrite meneghinite agsb phases stannite and precious metal phases electrum aghg au alloys the 1807 zone is enriched in te bi and se whereas the 1806 zone is telluride free and contains as sb hg au and ag mineral chemistry of sphalerite shows strong variations in fe content 1 12 11 04 wt intermediate fe 4 33 6 33 wt fe and fe rich 7 327 11 04 wt fe sphalerite are common in all orebodies whereas fe poor sphalerite 1 12 3 57 wt fe occurs exclusively in the 1807 and 1806 zones tennantite tetrahedrite is typically ag poor 0 25 2 19 wt in the 1807 zone but is significantly enriched in ag up to 29 3 wt in the 1806 zone galena in the 1807 zone and ming south orebodies is commonly myrmekitically intergrown with tellurides and has high concentrations of te bi se and ag in contrast galena in the 1806 zone is less enriched in te bi and se but high in ag variations in mineralogy epithermal elements as bi hg sb se sn te and precious metal ag au content and mineral chemistry between the different orebodies indicate that they were formed from predominantly reduced acidic hydrothermal fluids with varying te2 s2 se2 s2 and mbi msb ratios as temperatures steadily decreased from 300 c to

multiple scales of hydrothermal circulation in middle valley northern juan de fuca ridge physical constraints and geologic models a synthesis of data including seafloor heat flow water column thermal anomalies shallow pore pressures pore water chemical profiles paired borehole observations and 3 5 khz records help to characterize distinct hydrothermal circulation systems within and surrounding two active vent areas in middle valley a sedimented rift on northern juan de fuca ridge we estimate the total heat and fluid budgets for a 260 km 2 area encompassing 17 km of ridge to be 274 mw 16 mw km 1 of ridge and 0 157 m 3 s 1 respectively about half of the seafloor heat loss is conductive requiring efficient lateral fluid and heat transport in basement lateral head gradients fluid driving forces in basement between the primary fluid discharge site and the surrounding formation are very low simple mass balance and flow calculations indicate that the majority of the recharge to the primary hydrothermal reservoir cannot occur through sediments immediately around the vent area recharge is probably focused along valley bounding normal 10 12 2 faults crustal scale lateral permeability in basement must be 10 to 10 m in order to allow rapid flow and maintain nearly isothermal temperatures over many kilometers on a smaller scale heat flow patterns within the main vent area are controlled by the depth to the primary hydrothermal reservoir proximity to active vents and local recharge of a secondary circulation system shallow underpressures and isolated zones of very low heat flow result from vigorous secondary circulation driven by pressure gradients associated with venting models of secondary circulation suggest that most of the overpressure observed in hydrothermal basement is lost when the fluid enters the vents and that cooler sediment pore fluids flow toward and into the vents as a result of shallow underpressuring secondary circulation results in significant heat flow reduction at the seafloor but adds minor sediment pore fluid to vent discharge

modeling of gold scavenging by bismuth melts coexisting with hydrothermal fluids the effect of gold scavenging by bismuth melts is investigated using equilibrium thermodynamic modeling of an aqueous solution mineral melt system the calculations for the au bi na cl s h o system performed at temperatures between 300 and 450 c demonstrate that au concentrations in the melt are several orders of magnitude higher than in the coexisting fluid indicating the possible formation of economic gold deposits from undersaturated aqueous fluids in which mineralization would not be expected in the absence of a bismuth melt the model applies to any deposit where a bi melt is stable and coexists with a hydrothermal fluid examples of such deposits are known from skarn intrusion related orogenic and volcanogenic massive sulfide vms gold systems in sulfur poor systems the partitioning curves presented here can be used directly to correlate the gold concentration in the fluid and the au grade in the ore e g escanaba trough vms deposit these results also illustrate important principles generally applicable to understanding magmatic hydrothermal and metamorphic deposits that may have contained significant volumes of more complex polymetallic melts

gold distribution in as deficient pyrite and telluride mineralogy of the yangzhaiyu gold deposit xiaoqinling district southern north china craton the mesozoic yangzhaiyu lode gold deposit is situated in the southern edge of the north china craton gold mineralization is hosted in archean amphibolite facies metamorphic rocks and consists mainly of auriferous quartz veins pyrite is the predominant sulfide mineral with minor amounts of chalcopyrite sphalerite and galena based on morphology and paragenesis there are three generations of pyrite termed as first generation g1 second generation g2 and third generation g3 they have distinct contents occurrences and distribution patterns of gold the coarse grained euhedral g1 pyrite contains negligible to low levels of gold whereas both invisible and visible gold are present in the fine to medium grained g2 pyrite that is characterized by abundance of microfractures and porosities forming a foam like texture laser ablation inductively coupled plasma mass spectrometry la icp ms depth profiles indicate that invisible gold occurs either as solid solution or as nanoparticles of gold bearing tellurides in the g2 pyrite visible gold is widespread and present as irregular grains and stringers of native gold mostly along grain boundaries or filling microfractures of pyrite likely resulting from remobilization of invisible gold once locked in the g2 pyrite the g3 pyrite invariably intergrown with chalcopyrite sphalerite and galena contains the highest levels of invisible gold there is a positive correlation between au ag and te indicating that gold occurs as submicroscopic au bearing telluride inclusions in the host minerals whenever gold either invisible or visible is present as is always below or only marginally higher than the detection limit of la icp ms this indicates that as played an insignificant role in gold mineralization tellurides are widespread in the auriferous quartz veins consisting mainly of petzite calaverite hessite altaite and tellurobismuthite native gold commonly occurs as intergrowths with tellurides textural evidence indicates a precipitation sequence in a temporal order of calcaverite petzite altaite tellurobismuthite and hessite little amount of sulfide phases has been found in association with the tellurides indicating that tellurides were deposited under low s fugacity fs2 and or high te fugacity fte2 conditions the textural relationships when combined with fluid inclusion microthermometric data of auriferous quartz veins and tellurides thermodynamic data permit estimation for logfte2 during telluride formation which are 6 8 to 10 8 at 300 c and 9 6 to 17 6 at 250 c available geochronological and geochemical data suggest that te was most likely derived from the late mesozoic magmatic rocks widespread in the xiaoqinling district and other parts of the southern north china craton which were emplaced broadly contemporaneous with gold mineralization at yangzhaiyu this study highlights the role of te and tellurides as important gold scavengers in as deficient ore fluids

a climatology of waves in the equatorial region abstract propagating anomalies of moisture and moist deep convection in the tropics are organized into a variety of large scale modes these include but are not limited to the so called intraseasonal oscillations convectively coupled waves similar to those predicted by shallow water theory on the equatorial beta plane and tropical depression type disturbances along with the annual and diurnal cycles these modes act and interact to control much of the variance of tropical convection analyses of 10 yr of outgoing longwave radiation olr and precipitable water pw data are carried out to develop comparative climatologies of these wavelike modes the analysis relaxes the commonly used cross equatorial symmetry constraints which allows study of the portions of the wavelike processes that are asymmetric across the equator mean background states are found for olr and for pw as functions of day of the year examination of anomalies together with the background reveals much about how the waves are affec

trace element mapping of copper and zinc rich black smoker chimneys from brothers volcano kermadec arc using synchrotron radiation xfm and la icp ms

turbidite megabeds in an oceanic rift valley recording jokulhlaups of late pleistocene glacial lakes of the western united states abstract escanaba trough is the southernmost segment of the gorda ridge and is filled by sandy turbidites locally exceeding 500 m in thickness new results from ocean drilling program odp sites 1037 and 1038 that include accelerator mass spectrometry ams 14c dates and revised petrographic evaluation of the sediment provenance combined with high resolution seismic reflection profiles provide a lithostratigraphic framework for the turbidite deposits three fining upward units of sandy turbidites from the upper 365 m at odp site 1037 can be correlated with sediment recovered at odp site 1038 and deep sea drilling program dsdp site 35 six ams 14c ages in the upper 317 m of the sequence at site 1037 indicate that average deposition rates exceeded 10 m k yr between 32 and 11 ka with nearly instantaneous deposition of one 60 m interval of sand petrography of the sand beds is consistent with a columbia river source for the entire sedimentary sequence in escanaba trough high resolution acoustic strat

volcanism and massive sulfide formation at a sedimented spreading center escanaba trough gorda ridge northeast pacific ocean seismic reflection profiles over the sediment filled escanaba trough at the southern gorda ridge reveal a series of volcanic centers that pierce the sediment the volcanic edifices are 3 to 6 km in diameter and are spaced at 15 to 20 km intervals along the axis of the trough the volcanic intrusions have uplifted the sediment as much as 100 m and in places fresh basaltic flows are exposed at the sea floor unoxidized pyrrhotite rich massive sulfide samples were obtained from the flanks of one of these volcanic centers the composition and form of the sulfide samples suggest significant interaction between hydrothermal fluids and sediment at depth and deposition of sulfide within the sediment pile

insights into the liquid bismuth collector model through analysis of the bi au stormont skarn prospect northwest tasmania the liquid bismuth collector model has previously been proposed to explain the common association between gold and bismuth seen in many ore deposits and involves the scavenging of gold from hydrothermal fluid by liquid bismuth here textural relationships and temperature and chemical conditions of mineralization at the stormont bi au skarn prospect in northwestern tasmania are analyzed to investigate whether the liquid bismuth collector model can explain gold accumulation at this prospect the calc silicate skarn assemblage containing grandite hedenbergite actinolite magnetite epidote quartz and calcite suggests that initial metasomatism involved temperatures 400 to 500 c visible gold is associated with native bismuth bismuthinite maldonite galena and an unknown bi te s mineral native bismuth is directly associated with grandite which formed during initial stages of metasomatism this implies that bismuth would have precipitated from a hydrothermal fluid as a liquid heating experiments showed that bismuth alloy inclusions in garnet melt between temperatures of 250 and 288 c the mineralization setting at stormont and textural relationships suggest that gold was scavenged from hydrothermal fluids by liquid bismuth rhythmically zoned grandite crystals provide evidence for fluctuating hydrothermal fluid conditions which may have contributed to dissolution and reprecipitation of gold thus facilitating an ore zone refining process within the prospect this refining process is made possible by the tendency of liquid bismuth to scavenge gold from significantly under saturated fluids infiltrating gold undersaturated fluids can dissolve gold not in contact with bismuth which can then be preferentially redeposited upon encountering a bismuth droplet enhancing the correlation between the two elements a long lived and focused fluid flow system would maximize the efficiency of this ore zone refining process which is likely to be relevant to gold deposition at a range of deposit types

convectively coupled equatorial waves 1 convectively coupled equatorial waves ccews control a substantial fraction of tropical rainfall variability their horizontal structures and dispersion characteristics correspond to matsuno s 1966 solutions of the shallow water equations on an equatorial beta plane namely kelvin equatorial rossby mixed rossby gravity and inertio gravity waves because of moist processes the tilted vertical structures of ccews are complex and their scales do not correspond to that expected from the linear theory of dry waves the dynamical structures and cloud morphology of ccews display a large degree of self similarity over a surprisingly wide range of scales with shallow convection at their leading edge followed by deep convection and then stratiform precipitation mirroring that of individual mesoscale convective complexes ccews have broad impacts within the tropics and their simulation in general circulation models is still problematic although progress has been made using simpler models a complete understanding of ccews remains a challenge in tropical meteorology

analytic representation of the large scale organization of tropical convection the pivotal role of mesoscale organization on the large scale coherence of tropical convection is represented by a nonlinear dynamical model the general model consists of two interlocked systems a mesoscale parameterization of organized convection and a two layer model of large scale equatorial dynamics the lower layer dynamics is rossby gyre like whereas outflow from organized convection maintains the upper layer circulation the transports of zonal momentum in the vertical and meridional directions are key processes an archetype of the general model in spite of being brutally simplified represents the convective organization momentum transport and equatorial superrotation realized by the cloud resolving convection parameterization or superparameterization explicit approach developed by grabowski the mesoscale parameterization is an analytic equivalent of the cloud system resolving models used in this computational approach finally issues in parameterizing convective organization are discussed

development characteristics and dynamic structure of tropical intraseasonal convection anomalies abstract the development and dynamical structure of intraseasonal low frequency convection anomalies in the equatorial region are investigated using 10 years 1975 85 of outgoing longwave radiation olr and 7 years 1979 85 of 200 and 850 mb wind data the composite olr anomalies for 36 cases show a four stage development process initiation over equatorial africa rapid intensification when passing through the indian ocean mature evolution characterized by a weakening in the maritime continent and redevelopment over the western pacific and dissipation near the date line in moderate events or emanation from the equator toward north america and southeastern pacific in strong events a noticeable feature in vertical structure is that the 850 mb convergence leads convection and midtropospheric upward motion by about 30 degrees longitude in both developing and mature phases equatorial upper lower level easterly westerly anomalies and associated twin anomalous anticyclonic cyclonic circulation an

a new indicator mineral methodology based on a generic bi pb te s mineral inclusion signature in detrital gold from porphyry and low intermediate sulfidation epithermal environments in yukon territory canada porphyry epithermal and orogenic gold are two of the most important styles of gold bearing mineralization within orogenic belts populations of detrital gold resulting from bulk erosion of such regions may exhibit a compositional continuum wherein ag cu and hg in the gold alloy may vary across the full range exhibited by natural gold this paper describes a new methodology whereby orogenic and porphyry epithermal gold may be distinguished according to the mineralogy of microscopic inclusions observed within detrital gold particles a total of 1459 gold grains from hypogene eluvial and placer environments around calc alkaline porphyry deposits in yukon nucleus revenue casino sonora gulch and cyprus klaza have been characterized in terms of their alloy compositions au ag cu and hg and their inclusion mineralogy despite differences in the evolution of the different magmatic hydrothermal systems the gold exhibits a clear bi pb te s mineralogy in the inclusion suite a signature which is either extremely weak or most commonly absent in both yukon orogenic gold and gold from orogenic settings worldwide generic systematic compositional changes in ore mineralogy previously identified across the porphyry epithermal transition have been identified in the corresponding inclusion suites observed in samples from yukon however the bi te association repeatedly observed in gold from the porphyry mineralization persists into the epithermal environment ranges of p t x conditions are replicated in the geological environments which define generic styles of mineralization these parameters influence both gold alloy composition and ore mineralogy of which inclusion suites are a manifestation consequently we propose that this methodology approach can underpin a widely applicable indicator methodology based on detrital gold

on the stability of gold silver alloys in the weathering environment thermodynamic analysis of the stability conditions for gold silver solid solutions reveals that the silver component is unstable with respect to chlorargyrite and silver sulfate in most weathering environments as a consequence supergene secondary gold and gold found in modern river placers detrital and authigenic tends to be pure impure detrital placer gold is subject to natural purification processes as evidenced by silver depleted spongey grain margins in contrast under hydrothermal conditions the silver content of primary gold can be controlled by equilibrium reactions with argentite

gold and native copper in supergene sulphides from the mid atlantic ridge in 1985 black smokers and massive sulphide deposits were discovered in the tag hydrothermal field in the rift valley of the mid atlantic ridge near latitude 26 08 n ref 1 sulphide samples from chimneys and mounds were recovered by dredging at depths between 3 620 and 3 670 m mineralogical and chemical analyses of the samples reveal 1 primary unaltered zn fe cu sulphides containing 0 8 4 0 p p m au 0 02 0 12 oz per ton and 2 secondary cu rich sulphides with native copper formed by supergene alteration of primary cu fe sulphides containing up to 16 4 p p m au 0 48 oz per ton as free native gold this is the first reported occurrence of secondary enrichment of gold and copper in recent submarine sulphides the high gold grades and native copper associated with secondary cu sulphides resemble occurrences in some supergene gossans overlying ancient massive sulphide deposits on land2

siliceous sediments of the guaymas basin the effect of high thermal gradients on diagenesis recent siliceous sediments of the guaymas basin a rapid spreading center in the gulf of california are intruded by basaltic sills which induced significant diagenetic and metamorphic changes in the sediments the transformation from opal a to opal ct opal ct to quartz and opal a directly to quartz in these sediments cored on dsdp leg 64 can be used to infer the temperature history and order of emplacement of intrusives at or near contacts with sills of 30 m or greater average thickness opal a inverts directly to authigenic quartz but there is less quartz than would be expected from the amount of opal a dissolution opal ct forms in sediments sandwiched between adjacent sills based on high rates of quartz nucleation and growth at high temperatures 150 c and on considerations of convective solution transfer opal ct is thought to form only where temperatures were lower or at positions between sills an environment which prevents rapid convective dissipation of silica in solution where temper

as sb bi au mineralization in the baogutu gold deposit xinjiang nw china here we report the occurrence of some uncommon mineral assemblages including paakonenite aurostibite native arsenic native antimony and native bismuth found in the baogutu gold deposit in the western junggar xinjiang nw china the mineralization could be generally subdivided into two types the gold bearing quartz vein type mineralization and disseminated mineralization in the wall rocks the sulfide minerals in gold lodes commonly include pyrite arsenopyrite marcasite and stibnite however the l7 lode in no 4 orebody and the l1 lode in no 11 orebody of the baogutu gold deposit are quite different in terms of their mineral assemblages the l7 lode contains native arsenic quartz veins in shallow levels and stibnite quartz veins at depth gold bearing minerals electrum native gold and rarely aurostibite mainly coexist with paakonenite stibnite native arsenic and native antimony the crystallization of as and sb bearing minerals was likely to have consumed h2s from the hydrothermal fluid which probably triggered the precipitation of native gold the l1 lode consists of several discontinuous sulfide dominated lensoid orebodies the massive sulfide ores that produced most of the gold resource are characterized by an intimate association between native bismuth and native gold mineralization

the middle valley sulfide deposits northern juan de fuca ridge radiogenic isotope systematics the bent hill and the ore drilling program massive sulfide deposits in middle valley the sediment filled rift valley at the northern juan de fuca ridge are among the largest and richest sulfide deposits on the modern sea floor drilling during ocean drilling program leg 169 allowed a complete hydrothermal system to be sampled for the first time representative samples from this leg were analyzed for radiogenic isotopes sr nd and pb with the objectives of determining the sources of metals in the deposits and the nature of fluid circulation and evolution of the hydrothermal systems samples were chosen both from the sulfide deposits and host units the latter including sediments basaltic flows and sills sr nd and pb isotope compositions of the basalts and sills below the bent hill massive sulfide deposit confirm that both were highly altered by hydrothermal solutions the data also show that the sills are cogenetic petrologically with the underlying basaltic basement pb isotope compositions of the massive sulfides in both sulfide deposits vary within narrow ranges i e 206 pb 204 pb ranges of 18 83 18 89 and 18 77 18 89 for bent hill and ore drilling program respectively which could reflect remobilization replacement and recrystallization of sulfides during formation of the sulfide mounds but also mixing and homogenization during hydrothermal circulation of fluids below the sea floor data for the massive sulfides form elongate clusters between the basalts and the sediment clusters in pb isotope diagrams the most simple explanation for these patterns involves the mixing of pb from a basaltic source 206 pb 204 pb range of 18 64 18 74 with pb from isotopically heterogeneous sediments comprising a hemipelagic component with relatively higher 207 pb 204 pb and 208 pb 204 pb values and a clastic component turbidite with relatively higher 206 pb 204 pb values a third potential source is represented by hydrothermal sulfides present in both the sediments and basalts that have relatively unradiogenic pb isotope ratios 206 pb 204 pb range of 17 83 18 36 implying an old crustal source possibly older than 1 5 ga the overlapping pb isotope compositions of the massive sulfides in the two deposits suggest that the same sources were involved and in similar proportions to form the deposits ore drilling program massive sulfide is extremely enriched in zn compared to bent hill massive sulfide if the metal sources are the same for the two deposits other parameters must have produced the differences between them such as different fluid chemistry temperature or factors at the depositional site pb isotope data for sulfides of the bent hill massive sulfide feeder zone and the stratiform deep copper zone 206 pb 204 pb range of 18 59 18 82 are both much less radiogenic than the massive sulfides and overlap with the field for the underlying basalts the pb isotope ratios a high cu zn ratio and intense alteration of the basalts strongly suggest that the basalts were the main source for the metals in the deep copper zone and related feeder zones rather than a higher sediment influence in the overlying massive sulfides this conclusion is also supported by positive correlations between the pb isotope value and zn cu ratio contents of pb and mo a silicified zone immediately above the deep copper zone may have played a major role in the formation of the deposits by serving as an efficient seal preventing the deep seated fluids to escape to the sea floor as observed even today when the seal formed the fluids were forced to circulate in the lower part of the sedimentary sequence and in the basalts the seal would have insulated this hydrothermal system leading to higher temperatures in the fluids and subsequently to more efficient leaching of metals this model offers an explanation for differences in the pb isotope compositions of the massive sulfides and the underlying deep copper zone and feeder zones

sphalerite geobarometry in the cu fe zn s system phase relations have been determined among sphalerite pyrite pyrrhotite and intermediate solid solution iss from 439 degrees to 725 degrees c at 1 bar and 353 degrees to 760 degrees c at 5 kb the extents of solid solution in the four phase assemblage are described as functions of temperature in kelvins by the following equations log mole zns in iss 1 bar 2 795 1 632 t super 1 log mole zns in iss 5 kb 2 659 1 608 t super 1 log mole cus in pyrrhotite 1 bar 2 546 1 866 t super 1 log mole cus in sphalerite 1 bar 4 202 3 755 t super 1 and log mole cus in sphalerite 5 kb 4 084 3 791 t super 1 the pressure dependence in kb of the composition of sphalerite in equilibrium with pyrite and pyrrhotite with or without iss can be expressed for the temperature independent portion of the sphalerite geobarometer as p 42 30 32 10 log mole fes because of the low solubility of copper in sphalerite in the four phase assemblage below 650 degrees c the presence of chalcopyrite in natural ores should be of no consequence to sphalerite geobarometry over its temperature independent portion nevertheless sphalerites which are inhomogeneous in fes and which contain abundant blebs of chalcopyrite in excess of the amount that can be reasonably accounted for by an equilibrium process of solubility and subsequent exsolution are a common feature of many sulfide ores such sphalerite grains should be avoided in applying the geobarometer the chalcopyrite blebs may be a product of a premetamorphic sphalerite chalcopyrite intergrowth or represent true exsolution from sphalerite in any one of 1 a former sphalerite pyrrhotite iss equilibrium 2 a stable four phase assemblage at temperatures above the temperature independent portion of the geobarometer or 3 a metastable four phase assemblage in which pyrite was inert use of the sphalerite geobarometer requires a thorough understanding of the textural relationships and discrimination of equilibrium from disequilibrium or low temperature reequilibrated assemblages

gold enrichment and the bi au association in pyrrhotite rich massive sulfide deposits escanaba trough southern gorda ridge high gold contents to 10 1 ppm avg 1 4 ppm n 34 occur in pyrrhotite rich massive sulfide samples from the sediment covered floor of the escanaba trough the slow spreading southernmost segment of gorda ridge these concentrations reflect the presence of primary gold formed during high temperature hydrothermal activity in mounds and chimneys and secondary gold deposited during sea floor weathering of massive sulfide primary gold occurs as fine grained 2 m secondary gold grains have a porous flaky morphology and occur in samples in which pyrrhotite is oxidized and replaced by fe oxyhydroxides fe sulfate and sulfur mounds and chimneys dominated by pyrrhotite and containing lesser amounts of isocubanite chalcopyrite and fe rich sphalerite were formed by high temperature estimated range 325 275 c reduced low sulfur vent fluids the mineral and fluid compositions during this main stage of hydrothermal venting reflect subsurface interaction between circulating hydrothermal fluids and turbiditic sediment containing as much as 1 1 percent organic carbon as the deposition of pyrrhotite cu fe sulfides and sphalerite waned a volumetrically minor suite of sulfarsenide arsenide bi and au minerals was deposited from highly reduced late main stage fluids diffusing through mounds and chimneys the low solubility of au as a bisulfide complex and the absence of fluid mixing during this stage of hydrothermal activity apparently inhibited the precipitation of gold directly from solution instead gold precipitation is thought to be linked to elevated concentrations of bi in the late main stage fluids the textural relationships of au and bi minerals in pyrrhotite rich samples low melting point of native bismuth 271 4 c and recent experimental results on au and bi in hydrothermal fluids contribute to the hypothesis that gold was effectively scavenged from the escanaba trough vent fluids by coexisting droplets of liquid bismuth additional phase relationships of alloys in the au bi system indicate that deposition of native bismuth and maldonite occurred at temperatures as low as 241 c bismuth droplets trapped in void space between main stage mineral grains scavenged gold from ambient hydrothermal fluid to a greater extent than bismuth enclosed by late forming pyrrhotite the limited solid solution of au in bi can explain the apparent exsolution texture in which gold blebs are hosted by native bismuth the electrum native bismuth with gold inclusions and galena represent the last traces of gold mineralization from late main stage fluids during sea floor weathering and the oxidation of pyrrhotite in the mounds and chimneys secondary gold formed as aggregates of colloidal particles along ph gradients between acidic pore waters and ambient seawater gold was mobilized from earlier formed primary gold minerals and transported as aqueous chloride complexes the reduction of au iii by residual fe2 in partly altered pyrrhotite and adsorption of colloids by fe oxyhydroxides may have influenced the location of secondary gold grains within the alteration front solubility differences between gold and silver chloride complexes at low temperature account for the low ag content of secondary gold grains the high concentrations of bi and thus the association of au and bi minerals in pyrrhotite rich massive sulfide can be ascribed to the extensive interaction of hydrothermal fluids with sediment in the escanaba trough in contrast the absence of the au bi association in massive sulfides at other ridges including other sediment covered sites at middle valley and the guaymas basin reflects the predominantly basaltic source rocks

the mjo and convectively coupled waves in a coarse resolution gcm with a simple multicloud parameterization abstract the adequate representation of the dominant intraseasonal and synoptic scale variability in the tropics characterized by the madden julian oscillation mjo and convectively coupled waves is still problematic in current operational general circulation models gcms here results are presented using the next generation ncar gcm the high order methods modeling environment homme as a dry dynamical core at a coarse resolution of about 167 km coupled to a simple multicloud parameterization the coupling is performed through a judicious choice of heating vertical profiles for the three cloud types congestus deep and stratiform that characterize organized tropical convection important control parameters that affect the types of waves that emerge are the background vertical gradient of the moisture and the stratiform fraction in the multicloud parameterization which set the strength of large scale moisture convergence and unsaturated downdrafts in the wake of deep convection respectively three

mobility of gold and silver in lateritic weathering profiles some observations from western australia experimental evidence suggests that very acid chloride solutions generated in lateritic profiles by ferrolysis oxidation and hydrolysis of iron are responsible for the dissolution of gold and silver au is subsequently reprecipitated by reduction of the aucl 4 super ion with fe super 2 reduction of the agcl complex does not similarly occur because of the relative redox potentials of fe super 2 fe super 3 and ag agcl thus chemical refinement of gold occurs during lateritization the development of low ph and the redistribution of gold and silver in lateritic weathering profiles appear to be more common over granitic and gneissic basement and may be inhibited by the presence of carbonate in the weathering zone of basic rock sequences modified journal abstract