oxygenation episodes on the continental shelf of central peru remote forcing and benthic ecosystem response abstract the interplay between the oxygen minimum zone and remotely forced oxygenation episodes determines the fate of the benthic subsystem off the central peruvian coast we analyzed a 12 year monthly time series of oceanographic and benthic parameters at 94 m depth off callao central peru 12 s to analyze i near bottom oxygen level on the continental shelf in relation to dynamic height on the equator 095 w and ii benthic ecosystem responses to oxygen change macrobiotic infauna meiofauna and sulphide oxidizing bacteria thioploca spp shelf oxygenation episodes occurred after equatorial dynamic height increases one month before consistent with the propagation of coastal trapped waves several but not all of these episodes occurred during el ninos the benthic biota responded to oxygenation episodes by undergoing succession through three major ecological states under strong oxygen deficiency or anoxia the sediments were nearly defaunated of macro invertebrates and thioploca was scarce such that nematode biomass dominated the macro and meiobiotas when frequency of oxygenation events reduced the periods of anoxia but the prevailing oxygen range was 10 20 mol l 1 mats of thioploca formed and dominated the biomass finally with frequent and intense 40 mol l 1 oxygenation the sediments were colonized by macrofauna which then dominated biomass the thioploca state evolved during the 2002 2003 weak en while the macrofauna state was developed during the onset of the strong1997 1998 en repeated episodes of strong oxygen deficiency during the summer of 2004 in parallel with the occurrence of red tides in surface waters resulted in the collapse of thioploca mats and development of the nematode state ecological interactions may affect persistence or the transition between benthic ecosystem states

trace element signatures of chilean upwelling sediments at 36 s abstract with this contribution we continue the discussion on trace element te cycling in chilean upwelling sediments from within and below the oxygen minimum zone omz boning et al 2005 marine geology 220 1 21 low authigenic i e non lithogenic contents of te ag cd mo re and se were interpreted to result from the distinct seasonality in productivity induced organic flux bottom water oxygenation as well as dilution by terrigenous matter while all sediments were reducing enough to become depleted in mn highest enrichments in mo cd and re occur in sediments in contact with the omz here for a meaningful discussion total contents of as bi co cr cu ni pb sb tl v u and zn were corrected for the lithogenic background by determining te al ratios from local river samples the latter showed significantly lower te al ratios than average shale crust or andesite which are classically used for background correction hence the precise assessment of the lithogenic te contribution for the calculation of authigenic te contents allows to examine the source sink relationship of individual authigenic te for chilean sediments four te groups may be distinguished i mo re u and sb likely originate from sea water and accumulate via diffusive supply across the swi ii cd cr cu ni and zn require biologically induced pre concentration within biogenic material iii ag as se tl and v show an intermediate behaviour and iv bi and pb are contributed by anthropogenic activity highest enrichments in ni cu u as and v occur in sediments in contact with the omz the distinct depletion in co along with mn indicates the overall reducing but non sulfidic character of the chilean omz the degree of sedimentary sulfate reduction seems to be the most important control on chilean and peruvian near coastal enrichment of cd mo and u

the genesis of gold mineralisation hosted by orogenic belts a lead isotope investigation of irish gold deposits abstract lead isotope analyses have been performed on 109 gold and 23 sulphide samples from 34 irish gold occurrences including 27 placers and used to shed light on the sources of mineralising fluids and metals associated with gold mineralisation hosted by orogenic belts the pb isotope ratios of lode and placer gold range from 206pb 204pb 17 287 18 679 207pb 204pb 15 382 15 661 and 208pb 204pb 37 517 38 635 consistent with the pb isotopic data on previously reported irish sulphide mineralisation there is no evidence that gold mineralisation is associated with distinctive source regions and it appears to have been derived from similar sources to those responsible for the widespread sulphide mineralisation in ireland it is inferred that the principal controls on the au mineralisation are structural and not related to the distribution of au in their source rocks the range of pb isotope ratios favours the interaction of multiple source reservoirs predominantly during the caledonian orogeny c 475 380 ma underlying basement was the primary control on two key sources of pb gold occurrences located to the south east of the iapetus suture are characterised by pb compositions that derive predominantly from the late proterozoic crustal basement or overlying lower palaeozoic sediments whilst those located north west of the iapetus suture are characterised by less radiogenic pb signatures derived predominantly from late proterozoic or older crustal basement a third source relatively enriched in radiogenic pb also played a role in the formation of a number of irish gold occurrences and may have been associated with syn to post orogenic intrusives magmatic processes may therefore have played an important role in the formation of some orogenic gold occurrences

styles of lode gold mineralization contributing to the placers of the indian river and black hills creek yukon territory canada as deduced from microchemical characterization of placer gold grains between 1978 and 2009 approximately 430 000 oz of placer gold were obtained from the indian river and black hills creek which equates to roughly 20 of the production for the entire yukon territory during that period the area is unglaciated exposure is poor and there are few known lode gold occurrences present the technique of microchemical characterization of placer gold grains has been applied to illuminate the style s of source mineralization and their relationship to placer gold from the klondike gold district immediately to the north a total of 2 613 placer gold grains from 22 localities were characterised in terms of the au ag cu and hg content of their alloy and associated suite of opaque mineral inclusions a combination of alloy and inclusion mineralogy was used to define gold signatures which augmented the previous classification of orogenic gold in the klondike gold type 3b 8 25 ag is the main component of the placers in lower dominion creek but is augmented and eventually replaced by type 3a gold 10 40 ag in placers in the main indian river valley probably through erosion of gold bearing veins in the valley floor type 4 gold exhibits highly variable ag which may contain hg to a maximum of 11 wt this gold type also hosts a distinctive inclusion assemblage of complex polymetallic sulphides tellurides sulfotellurides and sulfosalts and has previously been ascribed to local low sulfidation epithermal mineralization placer gold in drainages radiating from eureka dome exhibits various proportions of types 3 and 4 gold depending on location but type 3 gold forms the major component in black hills creek and northerly flowing tributaries of the indian river with the exception of eureka and montana creeks type 5 gold is found only in placers in the middle and lower indian river it is distinguished by slightly elevated 0 05 0 17 cu in the gold alloy together with low 5 9 ag contents inclusions of bi minerals cr bearing magnetite and molybdenite within type 5 gold suggest derivation from an intrusion related source candidates for such a source include undiscovered lode occurrences associated with cretaceous age intrusions to the south of the indian river or deformed cu au mo porphyry occurrences which are known to be present in the same area this analysis of placer gold has indicated that the contribution of low sulfidation epithermal gold from eureka creek to the larger placers of the indian river is minor consequently the placer gold inventory of the indian river is primarily orogenic in origin similarly the characterization of placer gold in blackhills creek strongly suggests an orogenic source this study has demonstrated for the first time that orogenic lode gold mineralization extends a considerable distance to the south of the southern klondike goldfield this information contributes to the regional models of gold mineralization in an area which is currently the focus of intensive exploration

chemical and mineralogical signatures of gold formed in oxidizing chloride hydrothermal systems and their significance within populations of placer gold grains collected during reconnaissance the close association of pd with au has been previously reported in highly oxidized chloride hydrothermal systems from several regions including australia brazil poland and the united kingdom the distinctive mineralization precipitated usually as a consequence of contact with reducing environments is characterized by au alloys containing low ag pd hg cu associated with selenides tellurides antimonides arsenides placer gold grains derived from this style of mineralization inherit these chemical and physical characteristics which are shown to be distinct from those placer grains derived from other styles of mineralization the discovery of placer gold grains of this type in areas remote from known in situ mineralization initiated the present project and provided a methodology for the study of a relatively large number of occurrences data describing alloy chemistry and mineralogy of polished sections of 957 placer grains from 19 localities in the united kingdom and 1 locality in the czech republic have augmented previously published data for 303 gold grains from 10 localities in the united kingdom six generic gold alloy types have been identified based on the relative proportions of au ag pd and hg mineralogical descriptions of similar gold alloys from localities worldwide usually conform to this classification but slight deviations are noted where p t conditions differ from those previously identified in devon alloy types may occur either as discrete grains or as phases within heterogeneous grains type 1 gold defined as 99 mass au type 2 gold au pd and type 3 gold au ag alloys with selenide inclusions are the most common compositions and occur at most localities although in different proportions type 4 gold is also an au ag alloy but contains sulfide and sulfarsenide inclusions gold alloys or intermetallics containing pd hg and or pd sb alloys type 5 and cu type 6 are found only at some localities as is a variety of type 3 gold selenide inclusions which contains hg the spatial arrangement of the main alloy types within heterogeneous gold grains shows a consistent precipitation order of type 1 type 2 and type 3 independent of locality interpretation of the chemical controls on alloy precipitation indicates that o2 controls whether type 1 or type 2 gold precipitates initially the formation of type 4 gold au ag alloys with sulfide inclusions is ascribed to a fall in o2 to around the hematite magnetite buffer where hs aq replaces cl aq as the aqueous gold complex types 3 and 5 gold commonly mantle earlier alloys and show evidence for aggressive replacement of the core this texture is interpreted as evidence for influx of a more oxidizing fluid which also accounts for the abundance of selenide and telluride mineral inclusions within this gold type this approach provides a plausible explanation of the complex heterogeneity within and between gold grains from the localities and accounts for the common sequence of alloy precipitation without invoking multiple episodes of hydrothermal activity to account for different alloys however the absence of thermodynamic data describing the au cu au ag cu and au ag pd hg systems prevents interpretation of the significance of some alloy types this study has demonstrated that gold mineralization associated with oxidizing strata is more widespread than currently appreciated the methodology described here not only permits identification of gold from this style of mineralization but allows initial evaluation of the economic potential of the source through the study of the alloy types this information may be gained at an early stage in the exploration process from gold grains collected during routine reconnaissance

crystalline placer gold from the rio neuquen argentina implications for the gold budget in placer gold formation recent fluvial sediments of the rio neuquen argentina contain variable amounts of placer gold in addition to rounded detrital placer gold the sediments also contain pristine crystalline gold type 1 gold which occurs as overgrowths on detrital placer gold cores type 1 gold is locally rimmed by gold with a wormlike or crystalline texture type 2 gold the chemistry of the primary cores indicates variable intragrain and intergrain compositional heterogeneity this is interpreted to indicate a multisource origin type 1 gold is composed of virtually pure gold 98 at au and type 2 gold is an au hg amalgam 4 9 at hg both varieties are authigenic in origin the formation of the virtually pure type 1 gold is interpreted to be due to the addition of au from an external source rather than by the removal of ag and other metals the auhgag type 2 gold is interpreted to be due to hg contamination from historic mining type 1 gold is interpreted to be analogous to the ag poor rims present on many placer gold grains the evidence presented in this study supports the view that the ag poor rims on placer gold grains are the result of au addition and not ag removal this study also has implications for both quantitative morphological and paragenetic studies of placer gold grains

biogeochemical cycling in an organic rich coastal marine basin 6 temporal and spatial variations in sulfate reduction rates rates of sulfate reduction were measured over a 3 year period in the anoxic nearshore sediments of cape lookout bight north carolina using both a tube incubation method and a 35s sulfate direct injection technique the methods yielded similar depth integrated rates over the upper 30 cm ranging from less than 10 mol so 4 m 2 y 1 in winter to greater than 50 mol so 4 m 2 y 1 in summer there were also seasonal changes in the arrhenius activation energies for the sulfate reduction rates indicating that the assumption that ea is constant with temperature is not always valid the time averaged annual turnover rate for all three years was 20 4 11 4 mol so 4 m 2 y 1 surface rates ranged seasonally from less than 0 01 to over 3 mm so 4 d 1 between winter and summer respectively a subsurface rate maximum was observed to develop during the summer months which accounted for 28 percent of the annual depth integrated sulfate reduction rate subsurface rate maxima are the result of changes in the chemistry substrate type and or concentration and the microbiology in the sediments the possibility of the subsurface maximum being an artifact of the 35s method is also discussed however the sulfate reduction rates compare well with previous measurements of the carbon sediment water plus burial fluxes and with a depth integrated co2 production rate modelled from a co2 concentration profile from the same site

assessing redox zones and seawater intrusion in a coastal aquifer in south korea using hydrogeological chemical and isotopic approaches a shallow 25 m coastal alluvial groundwater system underneath a paddy field in the yangyang area of south korea was investigated to examine the occurrence of redox processes the aquifer is affected by seawater intrusion and is characterized by a highly reducing environment facilitated by high organic matter in the sediments hydrochemical data with 34s and 18o of sulfate were examined for depth specific groundwater from two multilevel samplers that were installed at seaward yy2 and landward yy1 locations shallow groundwater showed distinct patterns of redox zoning evidence of significant bacterial sulfate reduction bsr was observed throughout the nearly entire depths of the two boreholes while at the depths of active seawater intrusion in yy2 conditions suitable for methanogenesis were never reached thus at yy2 the deep zone of intense bsr was overlain by a zone in which methanogenesis occurred in a low sulfate environment in contrast concurrent bsr and methanogenesis in yy1 occurred at depths with high sedimentary organic matter and low dissolved sulfate due to intensive bsr considerable bsr in the groundwater representing trapped seawater in a clay layer had resulted in a very strong increase of 34ssulfate up to 99 9 the inferred sulfur isotopic enrichment factor e for bsr in the lower part of yy2 was 12 3 while e at yy1 was much higher 45 9 in addition the observed trends of 18osulfate at yy1 indicated significant oxygen isotope exchange of sulfate oxygen with ambient water likely because of lower cell specific rates of bsr and higher sulfur isotope fractionation as indicated by the 34s in contrast there was little evidence of oxygen isotope exchange between water and so42 at yy2 this study indicates that in coastal aquifers with sulfate reducing activity 34s and 18o of sulfate can reveal zones of active seawater intrusion and of trapped seawater this study provides an example of the application of sulfur and oxygen isotope data with hydrochemical and hydrogeologic data to interpret complex redox zonation in an organic rich coastal environment

control of sulfate pore water profiles by sedimentary events and the significance of anaerobic oxidation of methane for the burial of sulfur in marine sediments abstract gravity driven mass flow deposits proven by sedimentary and digital echosounder data are indicative for prevailing dynamic sedimentary conditions along the continental margin of the western argentine basin in this study we present geochemical data from a total of 23 gravity cores pore water so 4 is generally depleted within a few meters below the sediment surface by anaerobic oxidation of methane aom the different shapes of so 4 profiles concave kink and s type can be consistently explained by sedimentary slides possibly in combination with changes in the ch 4 flux from below thus mostly representing transient pore water conditions since slides may keep their original sedimentary signature a combined analysis and numerical modeling of geochemical physical properties and hydro acoustic data could be applied in order to reconstruct the sedimentary history we present first order estimates of the dating of sedimentary events for an area where conventional stratigraphic methods failed to this day the results of the investigated sites suggest that present day conditions are the result of events that occurred decades to thousands of years ago and promote a persisting mass transport from the shelf into the deep sea depositing high amounts of reactive compounds the high abundance of reactive iron phases in this region maintains low hydrogen sulfide levels in the sediments by a nearly quantitative precipitation of all reduced sulfate by aom for the total region we estimate a so 4 or ch 4 flux of 6 6 10 10 moles per year into the zone of aom projected to the global continental slope and rise area this may sum up to about 2 6 10 12 moles per year provided that the sulfur is completely fixed in the sediments it is about twice the global value of the recent global sulfur burial in marine sediments of 1 2 10 12 moles per year as previously estimated thus aom obviously contributes very significantly to the regulation of global sulfur reservoirs which is hitherto not sufficiently recognized this finding may have implications for global geochemical models as sulfur burial is an important control factor in the development of atmospheric oxygen levels over time

silver isotope analysis of gold nuggets an appraisal of instrumental isotope fractionation effects and potential for high resolution tracing of placer gold abstract we describe a technique for high precision analysis of 109ag 107ag ratios in natural and archaeological gold 1 0 120 mg g ag with particular emphasis on the control of ag yield and effects of instrumental mass bias after dissolution of mg sized gold samples in aqua regia and conversion to chlorides a miniature column filled with the anion exchange resin ag 1 x8 was used for removal of au from the sample solutions in a second miniature column made of the triskem tbp resin ag was converted from the chloride to the nitrate form isotopic analyses on the mc icp ms employed the combination of pd doping and standard bracketing and considering the measurements of replicate dissolutions and chromatographic separations for srm978a and cezag reference solutions the combined analytical uncertainty 2 s of the analytical method is better than 0 016 a solution prepared from several gold nuggets was characterised isotopically cezag with 109ag 0 043 0 015 for use as a silver in gold reference material in ag isotope studies of natural and processed gold results for gold nuggets from three continents indicate a wide variation in ag concentrations 5 to 123 mg g and this is matched by an equally wide range in isotopic compositions 109ag 0 58 to 0 83 this is larger than the variation found so far in in native gold from primary deposits 0 42 to 0 5 although small isotopic effects to lower 109ag during strong ag loss are possible within a single grain most analysed nuggets are internally isotopically homogeneous this suggests preservation of the primary 109ag in the supergene environment and that gold was sourced from several distinct gold deposits in the catchment recent studies however indicate that even single primary deposits may be isotopically heterogeneous implying that ag isotope fractionation is caused by numerous deposition dissolution cycles in hydrothermal systems fine grained detrital gold from three placers along the rhine river in germany show only small differences in 109ag 0 005 0 047 to 0 124 0 007 despite being distributed along 480 km of river length this may indicate thorough mixing of gold grain populations during transport the small sample size required for ag isotope work on gold opens the way for detailed micro sampling approaches these may be used to further examine the potential for within grain isotopic variability related to fluviatile processing and can be used to correlate the composition of the placers with that of grains from primary sources

microchemical characterisation of natural gold and artefact gold as a tool for provenancing prehistoric gold artefacts a case study in ireland the importance of au to ancient societies has encouraged many archaeologists to search for the sources exploited in antiquity these projects generally involve detailed studies of artefacts and comparison of their chemical characteristics with those reported for natural au however descriptions of natural au are frequently inadequate for provenancing studies and the compositional variability of the material is not widely recognised the present study describes a new approach to gold provenancing using the technique of microchemical characterisation in which populations of gold grains are classified according to the alloy compositions and the assemblages of microinclusions of other minerals this technique originally developed to identify sources of alluvial gold during au exploration has proved applicable to provenancing studies in four main areas firstly microchemical characterisation of artefacts grouped according to archaeological criteria can indicate the number of sources exploited in relation to time and artefact taxonomy secondly knowledge of the total variation in chemical characteristics of natural au from a particular region provides an excellent database for provenancing and reduces the need for exhaustive sampling thirdly it is possible to predict how au alloys were modified during fabrication as a consequence of assimilation of mineral inclusions finally identification of inclusion phases in artefact au can provide information on metallurgical practices these principles have been applied to the search for the source of au used for the unique traditions of prehistoric irish metalworking studies of 180 irish au artefacts belonging to four major metalworking traditions dating from the early bronze age 2400 bc to the iron age 150 bc show that each group exhibits distinctive ag and cu contents parallel studies of 2267 natural au grains from 58 alluvial localities and four bedrock localities throughout ireland reveal a broad pattern of alloy compositions consistent with style of mineralisation and host geology the ranges of ag contents of early bronze age and middle bronze age artefacts suggests that the au source lies within lower palaeozoic sedimentary rocks of the southern uplands terrane and significantly that the same source or sources were used in both periods a different source of relatively ag rich au most probably at croagh patrick co mayo was exploited in the late bronze age iron age artefacts have ag contents higher than natural irish au evidence for evolution of metallurgical practice during the bronze age is provided by the increasing cu content of the gold alloys to levels far in excess of natural gold and the nature of inclusions in artefacts of different ages elevated sn in cu rich alloys suggests deliberate or accidental alloying with bronze this approach has provided the first clear indication that only a few individual indigenous irish sources of au were used during the bronze age and that their relative importance changed over time future archaeological investigations may adopt a geographical focus that was not previously possible

remineralization and accumulation of organic carbon and nitrogen in marine sediments of eutrophic bays the case of the bay of concepcion chile abstract the bay of concepcion 36 40 s 73 02 w is a semi enclosed and shallow embayment in which biogeochemical processes are seasonally coupled to coastal upwelling during the austral spring and summer the nutrient cycle in the bay is complex due to the combined effects of a pronounced o2 minimum layer and high nutrient concentrations both originating from subsurface equatorial water during coastal upwelling and a rapid rate of sediment nutrient recycling the sediments are characterized by a high content of organic matter mainly due to the extremely high rates of phytoplankton production and deposition during the upwelling period a black flocculent layer frequently covers the sediment water interface in the inner part of the bay where an extensive mat of beggiatoa spp develops three approaches are used to analyse the extent to which the benthic system recycles or retains nutrients at two stations located at the centre station c st c and mouth station b st b of the bay for a 1 year period march 1996 1997 1 estimation of c and n remineralization rates based on so42 reduction measurements 2 calculation of c and n turnover rates using a diagenetic model applied to total organic carbon and total nitrogen vertical distributions and 3 construction of c and n budgets from direct measurements of sedimentation from a sediment trap and estimates of the c and n burial rates depth integrated so42 reduction rates varied between 3 4 winter and 25 5 summer mmol m 2 d 1 estimated c and n oxidation rates ranged between 7 9 and 87 8 mol c m 2 yr 1 and between 0 9 and 6 9 mol n m 2 yr 1 respectively each approach yielded minor differences in the c and n remineralization rates and also minor differences between both studied stations except when the kinetic model was applied to c and n distribution without including the presence of the flocculent layer the rates of carbon oxidation and sulphate reduction were considerably higher than in other coastal sediments with similar depositional regime the c and n burial rates were 2 23 and 0 21 st c and 1 30 and 0 09 st b mol m 2 yr 1 respectively the c n ratio of the buried fraction was ca 10 6 at st c and 14 4 at st b because the observed differences in burial rates could not be ascribed to distinctive depositional both stations have similar sediment accumulation rates and oceanographic similar o2 concentration and hydrography conditions differences may be due to in part spatial heterogeneity in the supply of organic matter the degree of preservation of organic matter as plankton detritus and nitrogen accumulating bacterial biomass associated with beggiatoa spp at st c may also be involved

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

characterization of gold mineralization in the northern cariboo gold district british columbia canada through integration of compositional studies of lode and detrital gold with historical placer production a template for evaluation of orogenic gold districts there is a strong association between regions containing orogenic gold mineralization and exploitation of placer gold although in many cases the nature of the source mineralization for these placer deposits remains unclear this study describes a novel approach to evaluating the economic potential of in situ orogenic gold mineralization through characterization of both lode and placer gold mineralogy followed by synthesis of this information with records of both mineral occurrences and historical placer mining the northern cariboo gold district in east central british columbia canada was chosen as a location for the study because of the gold endowment of the area 1 2 million ounces moz of lode gold and between 0 5 and 3 moz of placer gold and the information available from both placer and lode gold mining compositional analysis of 533 gold grains from 21 lode localities and 1 914 gold grains from 30 placer localities from throughout the cariboo gold district has identified four main compositional types in terms of their alloy compositions and associated suite of mineral inclusions revealed in polished section a distinctive low 4 7 ag gold that exhibits a strong bi association in the mineral inclusion suite is geographically limited to the wells area where it is recorded in both lode mineralization and its placer expression regionally pervasive mineralization yields gold of binary au ag alloy and a simple inclusion suite of sulfides and sulfarsenides gold in most large placers in trunk drainages was derived from multiple mostly small occurrences of this type the nature of compositional variation between gold grains liberated from hypogene ore has informed the history of episodic mineralization and suggests multiple stages of gold emplacement at some localities while others are dominated by gold deposited in a single stage the new information from gold grain analysis has been considered in the context of other information classification of a placer as either allocthonous or autocthonous both informs interpretation of compositional characteristics of the detrital gold grains and provides information on distance to the hypogene source mineral inclusion assemblages observed in sample populations of placer gold grains have been correlated with reports of hypogene vein mineralogy described in mineral occurrence records to clarify the geographical extent of specific mineralization types the compositional range of alloys of different gold types has been compared to historical records of gold production and gold fineness au ag ratio to reconstruct the size and distribution of hypogene sources prior to erosion synthesis of gold compositional studies with other publicly accessible data sets provides a new generic approach capable of evaluating the most attractive targets for future exploration and highlighting compositional signatures of placer gold which relate to undiscovered in situ sources

integration of detrital gold microchemistry heavy mineral distribution and sediment geochemistry to clarify regional metallogeny in glaciated terrains application in the caledonides of southeast ireland regional exploration for rare and precious metal enrichment traditionally uses multielement geochemical analysis of the fines 150 m fraction of stream sediments however these data can be misleading in glaciated regions with complex geology here we compare the spatial distributions of data from sediment fines with distributions of heavy mineral concentrates including detrital gold in the same region gold grains are characterized according to abundance and morphology plus a microchemical signature from the combination of alloy analysis and systematic identification of mineral inclusions revealed in polished sections these inclusions survive indefinitely and are indicative of hypogene mineralogy whereas the metal loadings of fines may be affected by weathering or anthropogenic activity all three approaches are mutually supportive sediment fines analyses provide a basis for more labor intensive targeted gold grain studies which in turn highlight specific gold element associations useful for interpretation of geochemical data sets spatial distributions of resistate heavy mineral suites constrain the directions and extents of glacial transport which facilitates more confident interpretations of placer lode relationships from gold grain studies characterization of 2 160 gold grains from 40 localities in the auriferous region of southeast ireland provided a clear indication of proximity of gold to source and identified gold derived from different episodes of mineralization a distinction is apparent between gold in the south of the region wexford likely derived from widespread stratabound au as fe s mineralization and that in the north wicklow where the historical placer mining district of the goldmines river yielded gold with inclusions exhibiting a distinctive pb bi as association the goldmines river placers formed by the efficient accumulation and preservation of detrital gold derived from several discrete intravalley sources we recommend that a combination of classic stream sediment geochemistry heavy mineral analysis and gold grain studies is used more widely to provide additional insights on the location and nature of gold mineralization and regional metallogeny in regions of poor exposure and complex geology

biogeochemical cycling in an organic rich coastal marine basin 7 sulfur mass balance oxygen uptake and sulfide retention sulfur and oxygen fluxes were quantified in the seasonally varying anoxic marine sedimentary system of cape lookout bight n c u s a over the three year study period 1981 1983 the mean annual sulfate reduction rate was determined to be 18 2 1 6 moles m 2 y 1 this value added to the estimate of the detrital sulfur input of 1 2 4 4 gave a total sulfur input of 19 4 4 7 moles m 2 y 1 the sulfide flux to the sediment water interface measured in anaerobic benthic chambers was 4 6 0 5 moles m 2 y 1 and represented 37 of the annual oxygen uptake rate of 25 2 2 8 moles m 2 y 1 the sulfide burial rate determined to be 15 5 3 1 moles m 2 y 1 was within 5 of the value predicted by summing the fluxes at the sediment water interface the cs weight ratio of sediment below the depth of diagenetic reaction was determined to be 2 75 the sulfide retention rate in these rapidly accumulating sediments 10 cm yr was 77 19 comparison of this result with previous studies shows that rapid sediment accumulation and the lack of bioturbation control this unusually high degree of sulfide retention

microbial mn iv and fe iii reduction in northern barents sea sediments under different conditions of ice cover and organic carbon deposition carbon oxidation rates and pathways were determined in two sediments at latitude 751 and 771n southeast of svalbard in the northern barents sea seasonal ice cover restricts primary production to few months a year which determines the sedimentation rate of organic material to the seafloor at one station with seasonally extended ice cover low organic carbon content and sedimentation rate combined with relatively high concentrations of mn and fe iii oxides favored dissimilatory fe and mn reduction 98 of anaerobic carbon oxidation over sulfate reduction in the top 12 cm of the sediment in contrast in a sediment that had not been ice covered for at least 12 months and with more organic carbon and a higher sedimentation rate sulfate reduction was the most important anaerobic electronaccepting process 480 of anaerobic carbon oxidation in the upper 3 cm microbial fe and sulfate reduction occurred simultaneously and sulfate reduction dominated at 3 12 cm oxygen consumption rates 1 9 and 3 7 mmol m 2 d 1 and anaerobic co2 production rates 1 3 and 6 4 mmol m 2 d 1 of both

phase relations among tellurides sulfides and oxides pt ii applications to telluride bearing ore deposits data on telluride bearing mineral assemblages from various types of ore deposits are compiled and interpreted in terms of the prevailing fugacities of te 2 s 2 and the temperature during deposition in the majority of hydrothermal deposits the appearance of tellurides after initial deposition of sulfides reflects an increase in the f sub te 2 f sub s 2 ratio most likely due to input of h 2 te from a magmatic source in some vein deposits such as boulder county colorado initial deposition of native tellurium is followed by deposition of minerals with progressively lower tellurium content indicating a decrease in f sub te 2 over the course of telluride deposition this trend may reflect local isolation of early formed minerals from the hydrothermal fluids and or progressive depletion of tellurium at the source ore minerals in other deposits such as mahd adh dhahab saudi arabia reveal systematic temporal or spatial variations in mineralogy that are attributed to introduction of a tellurium rich fluid into a preexisting sulfide assemblage and subsequent buffering of f sub te 2 and f sub s 2 by reactions such as 2pbs te 2 2pbte s 2 this buffering may exert considerable control on vein minerals such as the presence and relative abundance of gold and calaverite and the presence or absence of native tellurium

microchemical studies of placer and lode gold in the klondike district yukon canada 2 constraints on the nature and location of regional lode sources alloy compositions and associated suites of opaque mineral inclusions have been used to define the micro chemical signature of 3 050 placer gold grains from 34 localities and 561 gold grains from six lode localities from the northeastern central and southern klondike gold district yukon canada these data augment those from a detailed parallel study of placer lode gold relationships in the western part of the klondike goldfield see chapman et al 2010 gold compositions have been considered in association with chemical controls on alloy compositions to establish whether variation between signatures represents zonation within a single mineralizing event or discrete episodes of mineralization three main gold types have been identified each of which exhibits subtypes type 1 gold is generally a simple au ag alloy with low ag 12 25 and an inclusion suite dominated by pyrite and simple base metal sulfides type 2 gold is generally ag rich 18 55 and contains hg 9 type 3 gold 10 40 ag hg 4 is prevalent in the south of the region analysis of large sample sets has allowed us to delineate the geographic extent of each gold type and their relative importance within the placer populations type 1 gold is the major component of placer deposits in bonanza hunker dominion and upper sulphur creeks type 2 gold is relatively trivial but contributes to placer gold in bear creek and some of the paleoplacers in white channel gravel in the north of the study area type 3 gold is actively augmenting the placer inventory in the south of the region this study builds upon a detailed parallel study in the western klondike which concluded that types 1 and 2 gold were emplaced by a single hydrothermal system of limited geographic extent centered on the lone star ridge here we propose that small rich hydrothermal systems were also present in the upper reaches of bear and last chance creeks and hunker dome if correct this model shows that economically important orogenic gold mineralization can be formed in smaller hydrothermal systems than previously recognized in addition type 3 gold is widespread in the south of the study area where it appears to be present at the surface consideration of the abundance of the various gold types contributing to the placers has established type 1 gold to be the most important and originally present in geographically constrained vertically persistent mineralization we propose that this type offers the best target for exploration in the northern and central klondike in the southern klondike the large placers containing only type 3b gold suggest a potentially important undiscovered source in the catchment of gold run creek

stable isotope patterns of coexisting pyrite and gypsum indicating variable methane flow at a seep site of the shenhu area south china sea aggregates of authigenic pyrite and gypsum were found at site hs373 in the shenhu seepage area south china sea most of the pyrite occurs as tubes consisting of framboids gypsum aggregates are mainly represented by rosettes and some of them grew over pyrite revealing that gypsum precipitation postdated pyrite formation at seeps extensive sulfate driven anaerobic oxidation of methane aom commonly leads to the accumulation of 34s enriched sulfide resulting in the formation of 32s depleted pyrite the sulfur isotopic composition of pyrite at site hs373 reveals a great variability of 75 5 in 34s values spanning from 37 6 to 37 9 v cdt with a distinct trend towards higher values with increasing depth this trend indicates an increase of the effect of sulfate driven aom with depth whereas pyrite at shallow depth apparently resulted mostly from organiclastic sulfate reduction to date reports on coexisting pyrite and gypsum in seep environments are limited and therefore the mechanism of gypsum formation is insufficiently understood this is the first time that the sulfur and oxygen stable isotope composition of gypsum has been analyzed to understand its formation at seeps the 34s values of gypsum vary from 24 5 to 0 9 revealing a similar trend with depth like pyrite the 18ogypsum values range from 5 3 to 12 3 v smow with a scattered trend to lower values with depth the combined evidence reveals that gypsum formation was linked to the establishment of more oxidizing conditions probably caused by waning seepage this led to the oxidation of pyrite and the associated increase in pore water sulfate and acidity which favored carbonate mineral dissolution providing the calcium ions required for gypsum formation the observed isotopic variability of gypsum suggests that oxidation occurred by different mechanisms at different depth whereby pyrite oxidation was dominated by aerobic processes at shallow depth involving dissolved molecular oxygen and by anaerobic processes at greater depth involving electron acceptors like ferric iron taken together the stable isotope compositions of pyrite and gypsum suggest that the redox conditions within the studied sediments were variable such variability was controlled by complex factors among which changes in seepage activity have been instrumental for the vertical displacement of the sulfate methane transition zone

typomorphic features of placer gold of vagran cluster the northern urals and search indicators for primary bedrock gold deposits abstract the vagran placer cluster is located on the eastern slope of northern urals during 100 years of gold mining history approximately 40 tons of gold have been extracted from the placer deposits bedrocks of the region consist of high metamorphic upper proterozoic and paleozoic terrigeneous terrigeneous volcanogenic and igneous rocks gold placer deposits are mostly alluvial genesis deposits and of quaternary to oligocene age the alluvial deposits consist of gravel with pebbles boulders and sandy clay covered by sandy silt and a soil layer the thickness of the alluvial sequence is usually 5 10 m and reaches 18 m in the main watercourses of the third order nearly all of the alluvial sediments are gold bearing but concentrations of economic importance prevail in the bottom part of the sequence above the bedrock there are four different types of gold particles i rounded and well rounded particles of high fineness and homogeneous inner structure ii rounded to sub rounded high fineness particles with a pure gold rim developed over a core iii crystallomorphic idiomorphic high fineness with a homogeneous inner structure and iv irregular angular and subangular particles of medium fineness with a significant content of ag 10 40 wt and elevated hg up to 1 15 wt the first type is prevalent and comprises up to 65 of the total gold particles it is uniformly distributed throughout the territory there are features with initially complicated dendritic and laminar shaped particles which were rounded during transportation the second and third types have a propensity for zones of the inherited erosion tectonic depressions apparently types i ii and iii are related with orogenic mesothermal gold sulfide quartz mineralization the differences of these types depend on the primary zonation of ore bodies and supergenic transformation of the alloys they were connected with middle depth ore bodies of an orogenic gold sulfide quartz formation the fourth type is evident of nearby transportation from primary sources and a short duration of supergenic influence it is controlled by a zone of nw se orientation diagonal to the main structures of ural fold belt the plot of au content vs coefficient of heterogeneity ratio of the au content in the core and in the rim of the grains is the distinguishing factor between the four types of gold grains both by primary hypogenetic characteristics and supergenetic features no corresponding lode occurrence of gold sulfide quartz mineralization has been identified to date in this region placer gold concentrations are related to the intermediate hosts of the mesozoic cenozoic surfaces of the ural peneplain uplift in the oligocene and eroded in miocene quaternary time this factor determines the widespread distribution of placer gold in the territory of the vagran cluster the large carlin type vorontsovsk gold deposit is located 60 km south east from the vagran area it has a shallow erosional level small size of native gold and its distal location from the placer deposits makes it an unlikely primary source for the vagran placers however mineralization of this type of deposit is noted within the cluster gold of the fourth type nearly resembles the gold of the vorontsovsk deposit and apparently the source is related to the same hydrothermal mineralization event icp ms analyses of the quartz sulfide lodes in the floor of gold bearing valleys revealed a gold content of 2 0 6 9 g t in the zone of type iv distribution therefore gold of the fourth type can be used as an indicator for the exploration of primary bedrock mineralization the geological setting and typomorphic features of this placer gold shows that the primary gold mineralization is similar to the vorontsovsk deposit and within the zone of distribution of the placer gold of the fourth type

the application of microchemical analysis of alluvial gold grains to the understanding of complex local and regional gold mineralization a case study in the irish and scottish caledonides terranes of the appalachian caledonian orogen north of the iapetus suture host gold mineralization in north america scotland and ireland to date commercial exploitation has been confined to the appalachian terrane of canada in scotland and ireland regional analysis of the types of gold mineralization has been handicapped by the relatively small number of bedrock gold occurrences available for study however there are a large number of alluvial gold localities a technique of microchemical characterization of gold grains has been applied to facilitate the classification of alluvial gold localities in terms of the style or styles of source mineralization thereby permitting a more complete interpretation of regional gold mineralization the technique of microchemical characterization used in this study classifies populations of gold grains from each locality in terms of the assemblage of opaque mineral inclusions enclosed within the gold grains and the concentration of minor alloying elements in the gold a total of 2 400 gold grains collected from 44 sites in ireland and scotland have been studied the microchemical signatures of populations of gold grains extracted from bedrock samples correlate closely with those from adjacent alluvial populations and furthermore our data are compatible with the descriptions of the vein mineralogy of bedrock mineralization reported by other workers this result has facilitated informed speculation on the type of source mineralization of other alluvial occurrences where the bedrock source remains undiscovered populations of alluvial gold from individual localities have been grouped into larger regional data sets on the basis of similarities in microchemical signature the signatures of these data sets were analyzed using the same criteria and 10 classes of gold were identified regional analysis of the distribution of the gold classes showed that the occurrence of the four major classes correlates strongly with host terrane irrespective of geographical location gold from the southern uplands terrane is characterized by inclusions of base metal sulfides and sulfarsenides with the silver content of the gold being less than 15 percent in contrast populations of alluvial gold from the grampian and highland terranes commonly exhibit median silver contents of over 20 percent and frequently contain silver telluride and bismuth telluride inclusions which are extremely rare in gold from the southern uplands terrane however in other cases the style of mineralization is controlled by specific host or source rocks and is independent of the terrane two examples are gold mineralization within porphyry systems and gold associated with red bed sediments the study has confirmed the value of the technique of gold grain characterization using information from both concentrations of alloying elements and inclusion assemblages as a powerful tool in interpreting complex local and regional gold mineralization

geochemical characteristics of chilean upwelling sediments at 36 s abstract we examined the cycling of c s fe and the trace elements te ag cd mo mn re and se in chilean upwelling sediments from water depths of 127 2744 m at 36 s the sediments are clay rich with moderate total organic carbon toc and low biogenic caco3 contents for the shallow site within oxygen minimum zone omz which shows some thioploca biomass highest observed toc contents correspond to highest productivity induced toc flux and or enhanced preservation under o2 deficient conditions pyrite formation may be restricted by the concurrent presence of elemental s in thioploca and or the availability of reactive iron in the other sediments 370 m water depth pyrite formation is strongly restricted due to the low availability of metabolizable toc for sulfate reduction calculations of the degree of bacterially mediated remineralization suggest that more nitrogen than toc was remineralized by sulfate reduction than expected from redfield c n relationship manganese is significantly depleted due to reductive dissolution of mn hydr oxides within all sediments mo is only significantly enriched vs the lithogenic background in the shallow core implying anoxic conditions contents of re and mo as well as re mo ratios indicate suboxic conditions for the other cores cd and re contents decrease with increasing water depth reflecting decreasing reducing conditions in contrast authigenic ag contents increase with water depth suggesting enrichment controlled by seawater availability and or opaline input and not redox conditions elevated se contents in all cores may be either due to bio detrital input or diffusion from seawater accumulation rates of toc ts and te off chile are in the same order as those from the gulf of california and lower than those off peru distinct seasonality in upwelling and dilution of chilean sediments with terrigenous material lead to rather low metabolizable toc contents and lower te trapping capacity

the relationship between placer gold particle shape rimming and distance of fluvial transport as exemplified by gold from the klondike district yukon territory canada a semiquantitative study of various physical features of approximately 2 700 gold particles from 21 lode deposits and 36 placer deposits in the klondike district yukon territory canada was undertaken the shape parameters of roundness and flatness were determined using a classification method there appears to be a systematic variation between these parameters and between the parameters and the distance of transport of the particle these relationships were confirmed by quantitative measurements on critical samples rim characteristics were also estimated they show a systematic relationship between one another with the particle shape and less clearly with the distance of transport gold particle shape data shows a smooth well defined relationship to distance of transport from the lode source of the gold both roundness and flatness show a rapid increase within the first 5 km from the lode source and a slower more linear increase beyond this relationship seems to apply worldwide the relationship between the shape of a gold particle and the rim characteristic supports the conclusion that rims are formed by the removal of ag hg and cu and not by the precipitation of au thus rim thickness and the associated high fineness is probably the result of the dynamic process of ag removal in stagnant sediments to create the rim and abrasion in active sediments to remove the rim the composition of the rims represents the stable composition of gold in normal surficial environments together these observations place limits on the occurrence of new gold hammering is the main cause of shape change in fluvially transported gold particles its principal effect is to flatten particles abrasion has its most marked effect on surface texture hammering and abrasion both decrease the mass size and increase the roundness of the gold particles it is therefore possible to make a connection between hydraulic conditions during gold particle transport and its shape change

a revised seawater sulfate oxygen isotope record for the last 4 myr abstract using a new method to purify barium sulfate from marine barite we produce a highly resolved seawater sulfate oxygen isotope data set for the past 4 million years we show that the overall magnitude of the 18o shift is smaller than previously thought 1 1 5 vs 6 expanding on previous work we explore the effects of quaternary sea level variations on the global sulfur cycle our modeling results confirm the earlier hypothesis that the increased duration and amplitude of sea level lowstands during the quaternary favors direct sulfide oxidation over microbial disproportionation we show that oxygen isotope ratios of core top barite are up to 2 5 lower than the seawater sulfate 18o value we hypothesize that this offset is caused by incorporation of sulfate from oxidized organic sulfur compounds during precipitation of barite our results provide another puzzle piece in the attempt to understand the origin of marine barite

anaerobic methane oxidation by bacteria employing 14c methane uncontaminated with 14c carbon monoxide 14c labelled methane biologically prepared by methanobacterium thermoautotrophicum is widely used to determine methane oxidation rates however m thermoautotrophicum synthesizes carbon monoxide as a by product during methanogenesis in this study sulfate reducing bacteria utilizing the acetyl coa carbon monoxide dehydrogenase pathway were able to form 14co2 from 14ch4 containing 14co 14c labelled carbon monoxide was removed from 14ch4 by oxidation over hopcalite to carbon dioxide and fixation in sodium hydroxide solution measurable formation of 14co2 from purified 14c labelled methane by sulfate reducing bacteria was not observed therefore reported anaerobic methane oxidation rates in marine habitats measured with 14ch4 from m thermoautotrophicum are inclined to include carbon monoxide oxidation rates anaerobic oxidation of 14ch4 by sulfate reducing and acetogenic bacteria and methanogenic archaebacteria was tested only methanogenic species produced up to 900 ppm 14co2 from 14ch4 applied this observation and the absence of methane oxidation by sulfate reducing bacteria sustain the hypothesis that methanogenic archaebacteria in a syntrophic community might be responsible for the oxidation of methane in anaerobic habitats

a distinctive pd hg signature in detrital gold derived from alkalic cu au porphyry systems abstract this study comprises the first systematic classification of native gold geochemistry within alkalic porphyry cu au systems and the placer expression of such systems the geochemistry and mineral associations of gold from four alkalic porphyry deposits in british columbia canada afton mount milligan mount polley and copper mountain have been compared to comment on the likely detrital gold expression of similar systems globally populations of gold grains collected from in situ hypogene mineralization as well as fluvial deposits downstream of these deposits have been characterized in terms of their alloy composition au ag cu hg and pd and associated mineral inclusions these data are combined to generate a microchemical signature gold compositions vary according to the alteration zone within a porphyry system previous compositional studies of gold in porphyry systems have focused on the most economically important ore associated with potassic alteration wherein native gold hosted either by bornite or chalcopyrite comprises tiny blebs typically 5 20 m containing detectable cu in the gold alloy to a maximum of around 5 but quite variable 2 30 wt ag the presence of such grains have been confirmed in hypogene ore from the four systems studied but they have been shown to be compositionally distinct from detrital gold collected from nearby fluvial placers which exhibit a strong pd and hg signature both in the alloy and as mineral inclusions several workers have described late stage veins associated with alkalic porphyries which contain distinctive pd hg bearing minerals in association with other sulphides and sulphosalts this unusual mineralogy has been observed in the mineral inclusion suites of populations of detrital gold grains collected in the environs of the porphyry systems we conclude that whilst the micron scale cu rich gold grains formed in potassically altered rocks are in general too small to be recovered during routine sediment sampling those formed in later stage hydrothermal systems are larger and exhibit a distinctive microchemical signature which may be differentiated from those of gold formed in other mineralizing systems consequently compositional studies of detrital gold could underpin a mineral indicator methodology in the exploration for alkalic porphyry cu au deposits

significant role of organic sulfur in supporting sedimentary sulfate reduction in low sulfate environments abstract dissimilatory sulfate reduction dsr is a major carbon mineralization pathway in aquatic sediments soils and groundwater which regulates the production of hydrogen sulfide and the mobilization rates of biologically important elements such as phosphorus and mercury it has been widely assumed that water column sulfate is the main sulfur source to fuel this reaction in sediments while this assumption may be justified in high sulfate environments such as modern seawater we argue that in low sulfate environments mineralization of organic sulfur compounds can be an important source of sulfate using a reaction transport model we investigate the production of sulfate from sulfur containing organic matter for a range of environments the results show that in low sulfate environments 50 of sulfate reduction in well oxygenated systems porewater sulfate profiles often exhibit sub interface peaks so that sulfate fluxes are directed out of the sediment our measurements in lake superior the world s largest lake corroborate this conclusion offshore sediments act as sources rather than sinks of sulfate for the water column and sediment dsr is supported entirely by the in sediment production of sulfate sulfate reduction rates are correlated to the depth of oxygen penetration and strongly regulated by the supply of reactive organic matter rate co regulation by sulfate availability becomes appreciable below 500 m level the results indicate the need to consider the mineralization of organic sulfur in the biogeochemical cycling in low sulfate environments including several of the world s largest freshwater bodies deep subsurface and possibly the sulfate poor oceans of the early earth

variation in placer style gold morphology and gold particle behavior down gravel bed load rivers an example from the shotover arrow kawarau clutha river system otago new zealand gold eroded from quartz veins on the slopes of actively uplifting and physically weathering tributary catchments is transported to the fluvial system with little modification by mass movement mechanisms gold concentrates initially in the bed load component of spatially limited high grade primitive placers in moderate to high gradient incised juvenile valleys in which the rivers flow on or near bedrock interglacial and or uplift induced downcutting events and major floods periodically reconcentrate gold into lags on bedrock maximum gold particle size increases for the first few kilometers of primitive placers then decreases progressively downstream coarse gold 3 mm 0 5 g and gold with low flatness index 10 and is well rounded and commonly folded flattening progressively increases both the surface area to volume ratio and the entrainment potential of gold particles a reliable relationship between fluvial transport distance and flatness index maxima throughout the river system defines a critical flatness index above which gold is entrained and below which gold is not transported at given values of river gradient velocity and bed roughness both entrainment and retention of gold in primitive placer valleys are controlled by shape and mass characteristics that are largely inherited from the primary sources in transition and trunk placer valleys gold entrainment is controlled by flattening of particles to a critical flatness index state whereas gold retention is controlled by flatness index reduction to sub critical state by particle folding as well as input of relatively proximal locally derived primary or paleoplacer gold with subcritical flatness index or mass

microbial mediated pre salt carbonate deposition during the messinian salinity crisis calcare di base fm southern italy abstract a multi scale analysis of sedimentary carbonate facies and post sedimentary diagenetic features of the calcare di base formation the precursor to evaporites in upper messinian successions of northern calabria and central sicily has revealed their microbial bio mediated origin massive to laminated microbial boundstones represent the most common sedimentary facies forming flat to low relief cm to m scale stromatolitic and thrombolitic bodies the fabric of the micrite varies from peloidal to aphanitic and almost always preserves filamentous bacteria which characterized the original microbial mat the mat was dominated by sulphur oxidizing bacteria belonging to the thiotrichaceae but there is evidence for a more complex community with sulphate and or nitrate reducing bacteria all being responsible for the mediation of the carbonate precipitation microbial boundstones are rich in pseudomorphs of ca sulphate and halite which formed during the deposition of the microbial carbonate layers of primary gypsum are interbedded locally with carbonates suggesting the presence of restricted marine conditions the stable o and c isotopic composition of the carbonates that vary from dolomite to aragonite and calcite suggests a complex interplay between arid to humid climatic changes expressed cyclic interbedding of the carbonate with marl marlstone later diagenetic events mainly consist of phreatic meteoric recrystallization and cementation although considered as diachronous the microbial carbonates can be mapped out over a distance of more than 500 km across southern italy this indicates near constant environmental conditions upon the central mediterranean shelf at the beginning of the salinity crisis deposition of the extensive subaqueous microbial deposit that largely comprises the calcare di base is envisaged to have taken place across a shallow to moderate depth platform with local slopes into deeper water areas where some resedimentation occurred

the origin and evolution of the klondike goldfields yukon canada abstract the world famous klondike goldfields are located in the unglaciated part of west central yukon canada since their discovery over 100 years ago they have produced an estimated 311 tonnes of gold primarily from bench and creek placers that are fluvial in origin and range from pliocene to holocene in age historically the placers are classified into three levels of gravel with four main units these include the high level white channel gravel pliocene presently the most important gold bearing unit which sits nonconformably on an erosional bedrock surface i e the white channel strath and is overlain and interbedded with the glaciofluvial klondike gravel pliocene the intermediate level gravel pleistocene the least important economically and the low level gravel pleistocene holocene historically the most important gold bearing unit but it has been mined three or four times now the goldfields originated from the weathering and erosion of early cretaceous discordant mesothermal quartz veins and the light grey color of the matrix of the white channel gravel is due mainly to weathering and diagenetic alteration by groundwater flow the concentration of placer gold is related to a hierarchy of physical scales at the lithofacies scale metres bed roughness determined sites of gold deposition at the element scale tens of metres gravel bars were preferentially enriched in gold at the reach scale hundreds of metres stream gradient was an important factor at the system scale hundreds of km braided river environments transported large amounts of gold and at the sequence scale thousands of km economic placers formed initially in the high level white channel gravel and later in the intermediate level and low level gravel the white channel strath is interpreted as an erosional tectonic terrace that formed during isostatic uplift and under conditions of dynamic equilibrium the high level white channel gravel and klondike gravel are interpreted as a depositional climatic terrace that formed during a reversal in the tectonically induced downcutting which is attributed to the initial and most extensive of the pre reid glaciations 3 ma in the yukon the intermediate level gravel is interpreted as minor erosional complex response terraces that formed during static equilibrium when there were pauses in valley floor degradation which are attributed to the subsequent and less extensive pre reid glaciations the low level gravel formed also during valley floor degradation and may represent a return to dynamic equilibrium conditions hence the dominant forcing mechanisms controlling the evolution of the goldfields were isostatically compensated exhumation and climatic change related to the repeated glaciation of the yukon in addition the lowering of baselevel from high level to intermediate level and finally to low level gravel was accompanied by a decrease in accommodation space as indicated by a decrease in gravel thickness which resulted in an increase in the concentration of the placer gold

lode and placer gold composition in the klondike district yukon territory canada implications for the nature and genesis of klondike placer and lode gold deposits we have determined the major and trace element composition of approximately 2 700 gold particles from 21 gold bearing mesothermal quartz veins and 35 placer gold samples from the klondike district in western yukon territory measured au ag cu and hg contents were used to define a characteristic geochemical signature or fingerprint for each of the vein samples these signatures were then compared with the various compositional populations that we have distinguished within each of the placer samples preliminary conclusions derived from the study include 1 placer gold in both recent stream deposits and in the pliocene to pleistocene white channel gravels is detrital in origin 2 the placer gold is mainly if not entirely derived from mesothermal quartz veins 3 all lode sources for the placer gold have not yet been located and 4 gold composition data can be used to identify the lode signature for placer gold and if it has not been removed by erosion help locate and link placer gold to its specific lode source

macrobenthic zonation caused by the oxygen minimum zone on the shelf and slope off central chile abstract the relationship between macrobenthic 300 m zonation and the oxygen minimum zone omz o2

native gold from the kamenka barabanovsky and kharuzovka alluvial placers northwest salair ridge western siberia russia typomorphic features and possible bedrock sources abstract northwest salair ridge is one of the oldest alluvial gold mining regions it is located in the western part of the altai sayan folded area the placers localized along the kharuzovka and kamenka barabanovsky creeks the right branch of the suenga river were chosen for investigation the only bedrock deposit in the study area is the novolushnikovsky gold deposit located in the suenginsky placer field of the egoryevsky district the source of gold supply for the kharuzovka and kamenka barabanovsky placers spatially related to the hypothetical continuation of the type ii ore zones of the novolushnikovsky deposit the composition of gold grains derived from placers was determined based on a representative collection a distinctive feature of native gold from type ii ores is unusually high content of hg up to 20 wt the composition of gold grains from studied placers shows that the average hg content is 1 5 5 wt however there is a group of hg rich placers

tellurium magmatic fluids and orogenic gold an early magmatic fluid pulse at cononish gold deposit scotland significant tellurium enrichment occurs in many orogenic gold deposits but the factors causing this are little understood some authors suggest this demands a magmatic input whereas others suggest it need not fractionation of te from se and s could offer insight into source pathway processes of auriferous fluids the metasedimentary hosted cononish vein gold deposit scotland is unusually te rich compared to many orogenic gold deposits with te au 2 4 whereas most orogenic deposits have te au 1 here ag in au ag alloy increases from 10 to 90 wt through the paragenesis correlating with decreasing hessite ag2te abundance this suggests the au ag alloy composition was controlled by the fluid te activity and that this decreased through time this is coupled to an increase in pyrite 34s from 2 0 to 11 4 through the paragenesis thus the deposit formed from a primary fluid with a low 34s and high te au ag that evolved to a high 34s low te pb cu bearing fluid the high 34s of the later fluid suggests it can only be sourced from specific nearby metamorphosed sedex horizons the early fluid that deposited most of the gold could be sourced from other metasedimentary units in the stratigraphy or be magmatic in origin we argue that two observations taken together suggest it is most likely that this fluid was magmatic the age of the mineralisation is identical to the last stage of crystallization of nearby granite batholiths and the fluid has a s isotope signature consistent with a magmatic source gold deposits in orogenic belts are almost certainly polygenetic and this study demonstrates evidence for te rich orogenic deposits having a significant magmatic component

topographic gradients and lode gold sourcing recorded by placer gold morphology geochemistry and mineral inclusions in the east fork san gabriel river california u s a abstract topography plays a major role in the morphology geochemistry and mineral inclusion profiles of placer gold from the east fork of the san gabriel river these trends are consistent with a significant change in topographic gradient over a short distance and coincident with the break in slope along the range front of the san gabriel mountains placer gold from above this topographic transition is more rounded and 3 dimensional with elevated silver in the alloy of the native gold little evidence of gold enrichment rims and abundant inclusions of brittle and reactive minerals below the topographic transition placer gold assumes a distinctly flattened form with lower silver alloy content thin gold enrichment rims and rare inclusions of relatively inert minerals this relationship is observed for placer gold shed from two inferred lode sources within the river catchment with placer gold from each of these sources being traceable as a unique population over at least 8 km of the river significantly these placer gold properties are distinctive enough to fingerprint gold from specific portions of the placer deposit these identifying characteristics with virtually identical morphology and chemical trends are observed in the large nuggets of the mel fisher collection of purported san gabriel gold these similarities appear to confirm the provenance of the collection including identification of the specific locations where each nugget originated

gold nugget morphology and geochemical environments of nugget formation southern new zealand abstract gold nuggets centimetre scale have formed in a supergene alteration zone on hydrothermal gold deposits and occur intergrown with quartz and iron oxyhydroxide pseudomorphs after sulphide minerals and along fractures in quartz and host rocks the supergene alteration was driven by groundwater driven water rock interaction near to a regional unconformity beneath fluvial sediments and involved clay alteration and oxidation that extended up to 50 m below the unconformity oxidation of pyrite and arsenopyrite produced temporary thiosulphate ligands that mobilised microparticulate gold encapsulated in the sulphide minerals the nuggets have some crystalline form and internally they consist of anhedral grains elongated gold plates and intimate intergrowths of gold and iron oxyhydroxide nugget surfaces have further micron scale overgrowths of microparticulate gold gold plates and gold crystals nuggets were eroded and recycled into nearby proximal miocene quartz pebble conglomerates where they concentrated in placers near the basal unconformity later recycling transferred gold into pleistocene fluvial channels gold dissolution and redeposition as plates and crystals occurred on the exterior surfaces of placer gold particles with little change in mass all groundwater maintained high ph throughout the geological history because there was sufficient calcite in the basement rocks to neutralise any acid generated by pyrite oxidation hence gold mobility in sediments was driven by thiosulphate complexes as for the in situ nuggets albeit with lower dissolved sulphur concentrations despite aridification of the climate in the late cenozoic with resulting localised high dissolved chloride concentrations chloride complexation did not contribute to gold mobility

geologic and geochemical controls on the mineralogy and grain size of gold bearing phases eastern australian volcanic hosted massive sulfide deposits the mineralogy and occurrence of gold varies in a systematic manner in volcanic hosted massive sulfide deposits of eastern australia in deposits where it is concentrated with zinc at or near the top of the massive sulfide deposit gold occurs in auriferous arsenopyrite and pyrite in the center of the lens and electrum of variable fineness 300 900 in the baritic top of the lens the former occurrence results from precipitation of the gold into the lattice of arsenopyrite and coprecipitation of submicroscopic gold grains in arsenic poor pyrite the latter occurrence results from direct precipitation of electrum from thio complexes owing to the decrease in a sub h 2 s caused by dilution and oxidation as hydrothermal fluids mix with seawater in deposits where gold is concentrated with copper at the base of the sulfide lens and in the stringer zones the gold occurs as electrum of high fineness 750 1 000 or as gold tellurides that are commonly associated with chalcopyrite and or pyrite this occurrence is caused by the direct precipitation of gold from chloro complexes owing to an increase in ph or a decrease in temperature as the hydrothermal fluids move up through the sulfide body gold in auriferous pyrite is readily mobilized during deformation as the pyrite fractures gold migrates via solution precipitation mechanisms to cracks and grain boundaries to form electrum by combination with silver which is released from sulfide minerals in a similar manner with increasing stress the electrum is released from pyrite to recrystallize with other easily remobilized minerals such as galena during this process electrum grain size increases thermodynamic modeling of the variation in electrum fineness shows good agreement with variations measured in massive sulfide ores for gold transported as a thio complex the predicted fineness of 190 to 990 for temperatures of 200 degrees to 300 degrees c and typical volcanogenic fluids matches the observed variation in deposits of the zn au association the very high finenesses observed in deposits of the cu au association can be modeled assuming gold chloro complex transport and high ag au ratios in the hydrothermal fluids

geochemical processes and chemosynthetic primary production in different thiotrophic mats of the hakon mosby mud volcano barents sea we have investigated if in a cold seep methane or sulfide is used for chemosynthetic primary production and if significant amounts of the sulfide produced by anaerobic oxidation of methane are oxidized geochemically and hence are not available for chemosynthetic production geochemically controlled redox reactions and biological turnover were compared in different habitats of the ha kon mosby mud volcano the center of the mud volcano is characterized by the highest fluid flow and most primary production by the microbial community depends on oxidation of methane the small amount of sulfide produced is oxidized geochemically with oxygen or is precipitated with dissolved iron in the medium flow peripheral beggiatoa habitat sulfide is largely oxidized biologically the oxygen and nitrate supply is high enough that beggiatoa can oxidize the sulfide completely and chemical sulfide oxidation or precipitation is not important an internally stored nitrate reservoir with average concentrations of 110 mmol l21 enables the beggiatoa to oxidize sulfide anaerobically the ph profile indicates sequential sulfide oxidation with elemental sulfur as an intermediate gray thiotrophic mats associated with perturbed sediments showed a high heterogeneity in sulfate turnover and high sulfide fluxes balanced by the opposing oxygen and nitrate fluxes so that biological oxidation dominates over geochemical sulfide removal processes the three habitats indicate substantial small scale variability in carbon fixation pathways either through direct biological use of methane or through indirect carbon fixation of methane derived carbon dioxide by chemolithotrophic sulfide oxidation

determination of organic degradation rates in 100 my old sediments application to cretaceous black shale intervals from demerara rise odp leg 207 abstract thick organic rich albian to santonian black shale sequences were recovered from ocean drilling program sites 1261 and 1257 on the demerara rise offshore surinam at depths of 564 and 174 m below seafloor bsf respectively total organic carbon toc concentration varies between 3 9 and 17 3 wt rock eval pyrolysis parameters and elemental analysis indicate that the kerogen samples are of type ii thermally immature and with well preserved organic matter the hydrogen index hi values fluctuate between 608 and 765 mg hydrocarbon hc g toc and correlate well with c n and the sorg c atomic ratios indicating varying early diagenesis conditions between samples higher c n and sorg c values for kerogen from site 1261 suggest more reducing depositional conditions than at site 1257 long term organic diagenesis is marked by a linear decrease in the oxygen index oi at site 1261 from 34 to 22 mg co2 g toc ester and carboxyl functional groups are the main target of the degradation as inferred from the decrease in c o and o c c fourier transform infrared spectroscopy ftir bands as well as in the ch oh and the o ch3 chx n bands in 13c nuclear magnetic resonance nmr spectra degradation rate at site 1261 determined combining quantitative nmr spectra and bulk elemental characterization is between 1 7 and 12 1 mol m3 yr per 1 toc at the lower end of reported rates in a similar context kerogen samples from site 1257 do not show any decrease in oxygen content with depth indicating significantly lower degradation rate we propose that the long term exposure to moderate temperature 30 c at site 1261 activated the organic matter thereby sustaining microbial diagenesis

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

manganese iron and sulphur cycling on the pakistan margin abstract benthic biogeochemical processes in the oceans oxygen minimum zones omzs are of global significance yet they remain poorly defined here sedimentary manganese iron and sulphur cycling are studied at sites above within and below the omz on the pakistan margin of the arabian sea before and immediately after the southwest monsoon mechanisms and rates of sedimentary mn fe and s cycling demonstrated down slope variation according to the position of the omz no significant seasonal change was evident except at one station on the upper margin where bottom waters became hypoxic during the late to post monsoon sampling due to the shoaling of the upper omz boundary within the omz mn 4 reduction within the water column limited supply of reactive mn to the underlying sediments the majority of reactive mn reaching the sedimentary environment is reduced abiotically via reaction with fe 2 sediments within the omz act as a source of mn 2 to the overlying water below the omz mn 2 is re oxidised within the water column and is deposited on the surface of the underlying sediments manganese is then reduced microbially and re oxidised within the oxygenated surface sediments uniform surface sediment fe al ratios across the margin suggest that fe 3 reduction is lacking within the water column omz in contrast fe is actively cycled within the sediments with fe 3 reduction occurring via dissimilatory pathways an absence of potential fe 2 oxidants within the omz results in an efflux of fe 2 from the sediments to the water column below the omz greater oxidant availability ensures that fe is retained within the sediments measured sulphate reduction rates at all sites across the margin were surprisingly low 0 0 45 mmol m 2 d 1 relative to rates observed on other omz margins the abundance of fe oxides in the sedimentary environment appears to suppress sulphate reduction

microchemical studies of placer and lode gold in the klondike district yukon canada 1 evidence for a small gold rich orogenic hydrothermal system in the bonanza and eldorado creek area orogenic gold camps are commonly associated with placer deposits although in many cases the relationship between placer and lode source may be unclear placer gold deposits in bonanza and eldorado creeks in the northern klondike district goldfield yukon canada probably yielded more than 2 million ounces moz from a small geographic area but known occurrences of gold bearing orogenic style veins from which the placers were derived accounted for only a few thousand ounces the alloy chemistry and mineralogy of opaque mineral microinclusions microchemical signatures within these gold grains were compared to that of samples collected during a detailed placer gold sampling program collection of placer samples from the headwaters of watercourses in this unglaciated area allowed identification of microchemical signatures within gold populations from proximal sources which augmented information from lode gold sources to facilitate characterization of local gold mineralization a total of 1 163 gold grains from 18 lode sources and 2 777 grains from 27 placer localities within the bonanza and eldorado creek drainages were characterized in terms of the ag and hg content of the alloy together with the associated inclusion suite and two main compositional ranges along with several subtypes were identified type 1 gold is a simple au ag alloy but various subtypes have been identified on the basis of different ranges of ag and or specific inclusion assemblages type 2 gold is an au ag hg alloy composition the spatial arrangement of the gold types shows a systematic change in microchemical signature the lowest ag gold occurs in the lone star ridge area and the ag content increases both in the northeast and southwest the appearance of te bearing mineral inclusions in populations coincides with increasing ag and argentite inclusions are present only in high ag gold toward the periphery of study area type 2 gold is absent at lone star ridge and all localities to the east but is progressively more abundant in sample localities to the west the geographic distribution of gold types correlates well with placer abundance inferred from the production from previous mining activity and the study area is bounded by creeks and gulches in which there was insufficient placer gold for economic extraction we conclude that the lone star ridge area represents the epicenter of a single hydrothermal system in which spatial and temporal fluid evolution has generated compositional zonation variation in ag is ascribed to decrease in au ag aq following preferential au deposition in the center of the system controls on hg contents within au alloy are unclear but may be a consequence of mineralization temperature the complex but consistent signatures of placer gold samples from the eldorado creek drainage represents liberation of types 1 and 2 gold grains which coexist in some lode occurrences the different proportions of types 1 and 2 gold observed in placer and lode samples from west of lone star ridge reflect both temporal and spatial evolution of the hydrothermal system the main eldorado creek placers exhibit a signature most similar to lode gold from the buckland and nugget zones on lone star ridge placer gold in upper bonanza creek is distinguished by a relatively narrow ag range and the small but consistent presence of hessite in the inclusion suite no corresponding lode occurrence for this gold type has been identified to date this is the first time that the compositional signatures of major placer gold deposits derived from orogenic gold vein systems has been studied in this detail and the application of basic chemical principles to fluid evolution and electrum precipitation has allowed demarcation of an area in which extremely rich gold mineralization was formerly present while simultaneously identifying the most promising areas for further exploration for undiscovered lode gold

geological and geochemical controls on the silver content fineness of gold in gold silver deposits abstract a compilation of microprobe fineness data and a re evaluation of bullion production fineness data suggest that the gold fineness model of fisher 1945 can be updated and applied to a wide range of gold mineralised environments the major deposit classes are characterised by the overall average or range of deposits averages and total range of gold fineness values as follows archaean 940 780 1000 slate belt 920 800 1000 plutonic 825 650 970 porphyry 700 1000 650 1000 volcanogenic 650 850 520 870 epithermal 440 1000 0 1000 the archaean including witwatersrand slate belt and plutonic classes are characterised by a high and consistent average fineness and a narrow fineness range there is no clear subdivision within these classes according to deposit style or inferred genesis the porphyry volcanogenic and epithermal classes have a variable average deposit fineness and a wide total range of fineness values within these three classes higher fineness is typical of deposits in andesitic rather than rhyolitic volcanic settings deposits with cu au rather than cu mo or pb zn element associations and deposits with telluride or selenide minerals solubility calculations indicate that in all these environments gold and silver form the same aqueous species with agcl 2 dominating for silver and au hs 2 dominating for gold in all the environments the fluids transport an equal amount of gold and silver in the form of their bisulphide complexes therefore the observed gold fineness values can not be explained by the transporting mechanism only low silver in the ores and higher gold fineness values observed in the archaean plutonic and slate belt environments can be attributed to sulphidation as the dominant mechanism of ore deposition sulphidation of wall rocks destabilises only the bisulphide complexes of gold and silver in contrast epithermal volcanogenic and porphyry environments are characterised by a diversity of ore depositing mechanisms cooling boiling mixing which act alone or in combination and destabilise chloro and bisulphide complexes of gold and silver the high silver concentration in the ores low fineness values and large variation in the fineness values in these environments can be attributed to the complexities in the ore depositing mechanisms

structural controls on orogenic gold mineralisation in the klondike goldfield canada the klondike schist that forms the basement rocks for the famous klondike placer goldfield was emplaced as km scale thrust slices in early jurassic time along with some thin 10 to 30 m scale slices of greenstone and ultramafic rocks permian metamorphic fabrics in the schists were deformed during thrust emplacement by structures formed as the rocks passed through the brittle ductile transition early formed thrust related structures were almost pervasive recumbent folds that affected both the schist and greenstone ultramafic slices and imposed a spaced cleavage with minor recrystallisation of micas these structures gave way to shallow dipping phacoidal cleavage near within 100 m of thrust structures thrust related structures have been overprinted locally by well defined steeply dipping reverse fault fold zones and associated upright folding on regional km to mesoscopic m scales the fold fault zones occur as two orthogonal sets of structures oriented nw to n and ne to e some of these steeply dipping fault zones have been reactivated by late cretaceous normal faulting orogenic mesothermal gold bearing veins were emplaced in local sites of extension during or after formation of the compressional fault fold zones and before normal fault reactivation over 400 veins m to cm scale observed in this study imply a general nw strike for mineralised structures w to n but with a broad scatter of orientations vein emplacement was controlled principally by fold axial surfaces of kink folds of the fault fold generation however some other local extension sites have opened along preexisting structures to host veins locally including metamorphic foliation and spaced cleavage planes in addition irregular extensional fractures with no obvious structural control host some veins the klondike mineralised veins formed as swarms with broad regional structural control but represent relatively diffuse mineralised zones with numerous scattered small veins compared to most orogenic vein systems these diffuse vein swarms appear to be sufficient sources for the rich and geographically localised placer gold deposits that formed in overlying gravels during erosion of the klondike schist basement

sulfate reduction and methanogenesis in a thioploca dominated sediment off the coast of chile continental shelf sediments of the central chile upwelling area are dominated by the presence of dense mats of the filamentous sulfur depositing bacterium thioploca spp we examined rates and pathways of s and methane cycling in these sediments along a transect from the bay of concepcion to the continental slope sulfate reduction rates 170 4670 nmol cm 3 d 1 were equal to or exceeded rates reported for other subtidal marine sediments elemental s and pyrite were the dominant end products of sulfate reduction in thioploca mats on the continental shelf whereas in the highly reducing beggiatoa dominated sediments of the nearby bay of concepcion acid volatile s was the principal end product dissolved organic c values were lowest at the stations with the highest sulfate reduction rates and increased offshore sediment porewater methane concentrations in all surface sediments were low 12 nmol cm 3 and methane production rates at the station most dominated by thioploca were extremely low 0 5 nmol cm 3 d 1 low methane production rates and concentrations were matched by low methane oxidation rates 0 1 nmol cm 3 d 1 radio tracer studies showed that methane production was almost exclusively from methylamines substrates which are noncompetitive with sulfate reduction rather than from acetate or co2 h2 bacterial mpn most probable number counts also indicated the presence of a methylotrophic population of methanogens surprisingly high numbers of autotrophic acetogenic bacteria were found suggesting that the bacterial population involved in anaerobic doc degradation is more complex than expected in spite of the high sulfate reduction rates sulfide concentrations in the shelf and slope were low or undetectable 0 5 m and sulfate concentrations were never depleted below bottom water levels down to depths of 25 30 cm calculations suggest that thioploca were oxidizing a maximum of 35 of sulfide production not enough to prevent sulfate depletion either other sulfide oxidizers were also present or transient hydrodynamic conditions coupled with bioturbation resulted in oxidation of the sediments