multiple sulfur isotopes monitor fluid evolution of an archean orogenic gold deposit abstract the evolution of a gold bearing hydrothermal fluid from its source to the locus of gold deposition is complex as it experiences rapid changes in thermochemical conditions during ascent through the crust although it is well established that orogenic gold deposits are generated during time periods of abundant crustal growth and or reworking the source of fluid and the thermochemical processes that control gold precipitation remain poorly understood in situ analyses of multiple sulfur isotopes offer a new window into the relationship between source reservoirs of au bearing fluids and the thermochemical processes that occur along their pathway to the final site of mineralisation whereas 34s is able to track changes in the evolution of the thermodynamic conditions of ore forming fluids 33s is virtually indelible and can uniquely fingerprint an archean sedimentary reservoir that has undergone mass independent fractionation of sulfur mif s we combine these two tracers 34s and 33s to characterise a gold bearing laminated quartz breccia ore zone and its sulfide bearing alteration halo in the 6 moz au structurally controlled archean waroonga deposit located in the eastern goldfields superterrane of the yilgarn craton western australia over 250 analyses of gold associated sulfides yield a 34s shift from what is interpreted as an early pre mineralisation phase with chalcopyrite pyrrhotite 34s 0 7 to 2 9 and arsenopyrite cores 34s 0 5 to a syn mineralisation stage reflected in au bearing arsenopyrite rims 34s 7 6 to 1 5 this shift coincides with an unchanging 33s value 33s 0 3 both temporally throughout the au hosting hydrothermal sulfide paragenesis and spatially across the au ore zone these results indicate that sulfur is at least partially recycled from mif s bearing archean sediments further the invariant nature of the observed mif s signature demonstrates that sulfur is derived from a homogeneous mif s bearing fluid reservoir at depth rather than being locally sourced at the site of au precipitation finally by constraining the mif s bearing sulfur source to a fixed reservoir we are able to display the thermochemical evolution of the ore fluid in 34s space and capture the abrupt change in oxidation state that causes au precipitation our results highlight the importance of constraining multiple sulfur isotopes in space and time in order to elucidate the source and evolution of any given au bearing fluid

oxidized and reduced mineral assemblages in greenstone belt rocks of the st ives gold camp western australia vectors to high grade ore bodies in archaean gold deposits hydrothermal sulfide oxide gold mineral assemblages in gold deposits in the archaean st ives gold camp in western australia indicate extremely variable redox conditions during hydrothermal alteration and gold mineralization in space and time reduced alteration assemblages pyrrhotite pyrite occur in deposits in the southwest of the camp e g argo junction deposits and moderately to strongly oxidized assemblages magnetite pyrite hematite pyrite occur in deposits in the central corridor in the northeast e g north orchin revenge deposits reduced mineral assemblages flank the central corridor of oxidized deposits and locally cut across it along e w trending faults oxidized mineral assemblages in the central corridor are focused on gravity lows which are interpreted to reflect abundant felsic porphyritic intrusions at about 1 000 m below present surface hydrothermal magnetite predates and is synchronous with early phases of gold associated albite carbonate pyrite biotite chlorite hydrothermal alteration later stage gold associated pyrite is in equilibrium with hematite the spatial distribution and temporal sequence of iron sulfides and oxides with gold indicate the presence of at least two spatially restricted but broadly synchronous hydrothermal fluids with contrasting redox states sulfur isotope constraints support the argument that the different mineral assemblages reflect differences in redox conditions the 34s values for pyrite for the st ives gold camp range between 8 4 and 5 1 with the negative values occurring in oxidized magnetite rich domains and slightly negative or positive values occurring in reduced pyrrhotitic domains preliminary spatial and paragenetic analysis of the distribution of iron sulfides and oxides in the st ives camp suggests that gold grades are highest where the redox state of the hydrothermal alteration assemblages switches from relatively reduced pyrrhotite pyrite to relatively oxidized magnetite pyrite and hematite pyrite both in space and time gold deposition is inferred to have occurred where fluids of contrasting redox state mixed

mapping of monthly soil erosion risk of mainland mauritius and its aggregation with delineated basins this paper reports the mapping of monthly soil erosion risk on mauritius which was carried out using gis decision rules and readily available data namely monthly rainfall depth soil types slope and land cover slope and soil were first combined to produce soil erosion susceptibility followed by land cover to produce erosion sensitivity and then rainfall to produce erosion risk the high erosion areas of the island have been extracted from the soil erosion risk maps whereby these areas can face land degradation problems and can be responsible for sediment discharge into wetlands located at the outlet of drainage basins as such drainage basins have been delineated using automatic catchment delineation tools and their percentage of high erosion areas computed basins with the greatest percentage of high erosion areas and particularly those that directly have a wetland at the outlet can be given priority for soil and water conservation efforts the mapping reported in this paper can be adapted to other countries which need an erosion assessment for the identification of high erosion areas and priority action areas

modeling the role of sodic alteration in the genesis of iron oxide copper gold deposits eastern mount isa block australia liberation of iron and potassium by widespread postmetamorphic albitization of country rocks was one of the likely contributing processes in the formation of both barren and mineralized magnetite chalcopyrite biotite gold hematite clinopyroxene actinolite apatite ironstones in the cloncurry district of the proterozoic mount isa block whole rock geochemical data indicate nearly immobile al ga ti zr during transformation of a variety of least altered rocks toward albitite the data indicate that the addition of na from a brine to the rock accompanied the loss of fe k ba rb ca sr co v mn pb and zn from those altered rocks and enrichment in the brine but that cu was not systematically stripped from a variety of wall rocks during albitization conversely the formation of metasomatic ironstones the immediate hosts to some cu au ores involves addition of most of the same elements that were lost during albitization the correlation between intensity of alteration its distribution and timing e g in breccias cored by ca 1530 1500 ma granitoids and convergence of all rock types toward magmatic stable isotope values provides strong evidence for a substantial component of igneous derived fluid simulations of the albitization process were carried out isothermally at 550 c and 350 mpa and polythermally from these conditions down to 400 c and 200 mpa using the gibbs minimization method with hch software and the unitherm database both simple fluid rock mixing models and more complicated reactor style algorithms used a range of input fluids from fluids equilibrated fluid with two feldspar granite through to na k ratios consistent with fluid inclusion results and geochemical data for initially unaltered wall rocks the apparent paradox of widespread albitization resulting from fluid released by two feldspar granites can be explained by relatively small shifts away from the k feldspar albite equilibrium curve and even isothermal model fluids derived from two feldspar granites produce albitites in calc silicate rocks outboard of granite proximal k feldspar clinopyroxene skarns matching field patterns those models with fluid na k ratios similar to those of fluid inclusions produced the most realistic alteration assemblages dominated by albite for both isothermal conditions and decreasing temperature which approximate those observed in the field pixe and microthermometric data on fluid inclusions from quartz in two feldspar quartz monzonite and pegmatite at the top of the mount angelay pluton indicate bulk na k molar ratios in the fluid of between 10 and 20 considerably higher than our thermodynamically calculated values for fluid in equilibrium with two feldspar granite of around 3 at 550 c 350 mpa such shifts may have been attained by admixture of magmatic hydrothermal fluid with small amounts of nacl brines trapped along grain boundaries in scapolite bearing calc silicate wall rocks by fluid immiscibility due to high initial co2 contents in the felsic intrusions by contributions from mafic magmas or from dissolution of salt rich layers into the intrusions prior to crystallization and fluid release with increasing amounts of fluid rock interaction in the models the fluids were enriched in k fe and ca approaching compositions observed in fluid inclusions in the ore deposits these fluids reacted with pelitic rocks which are common ore hosts would produce magnetite clinopyroxene biotite actinolite alteration at high temperature similar to the proximal alteration around ore deposits we infer that precipitation of sulfides in the cu au deposits was the result of mixing of cu bearing brine of ultimately magmatic origin but modified extensively via albitization with sulfur bearing fluids or reaction of the brine with sulfur bearing rocks when cu was absent from the initial magmatic fluid barren ironstones may have been the result

ephemeral gully erosion in southern navarra spain abstract ephemeral gully erosion is a common type of water erosion in the loamy soils formed on the miocene marls of southern navarra a study has been conducted to describe the different types of ephemeral gullies and to determine their origin evolution and importance as sediment sources three main types were identified according to their origin classic ephemeral gullies formed within the same field where runoff starts drainage ephemeral gullies collecting the runoff from another upstream area and discontinuity ephemeral gullies due to some abrupt slope change in the landscape during 2 consecutive years of normal precipitation ephemeral gully erosion has caused significant soil losses locally over 2 66 kg m 2 yr 1 which is above tolerable levels in each year one short and intense rainfall event has been responsible for almost all soil losses from ephemeral gullies the main cause of gully formation is the lack of any proper waterway for conveying water excess conventional tillage practices provoked gully occurrence while the maintenance of stubble or vegetation cover completely prevented their formation although farmers refill gullies every year they reappear at the same position if no preventive measures are taken early winter has been the most critical period for gully formation when the soil is wetter and the surface cover is scarce but a strong summer thunderstorm also caused severe gully erosion in unprotected soils two simple topographic indices based on the area and the slope of the watersheds are useful to describe total soil losses

the lightning creek sill complex cloncurry district northwest queensland a source of fluids for fe oxide cu au mineralization and sodic calcic alteration the lightning creek cu au prospect is hosted by a cogenetic suite of plutonic i type granitoids the dominant rock type is a porphyritic quartz monzodiorite that is intruded by more fractionated rocks including monzogranite and alkali feldspar granite a series of flat lying sills are interpreted to be late stage differentiates based on their timing mineralogy and chemistry in parts of the prospect there is pervasive sodic calcic alteration pyroxene after amphibole albite after k feldspar and oligoclase of the plutonic rocks this alteration predates sill emplacement and is unrelated to veining or fracturing of any kind the presence of small amounts of carbonate in the altered rocks suggests that the fluids were co2 bearing quartz and feldspar separates from these altered rocks have oxygen isotope compositions similar to those from fresh quartz monzodiorite suggesting that the fluids were hot and of magmatic composition sodium and ca were added and k fe cl and cu were stripped during what is interpreted as an autometasomatic event the sills display considerable textural and mineralogical complexity and evolved from equigranular quartzofeldspathic rocks aplites with magmatic chemistry to unusual fe rich rocks albite magnetite quartz that exhibit a range of bizarre spherulitic textures some of the albite and magnetite in the sills is secondary albite forms pseudomorphs after k feldspar na fe ca alteration along sill margins and within sills at the contacts between different textural zones halos of disseminated magnetite clinopyroxene fe ca na alteration are developed adjacent to early magnetite veins fluid inclusion studies indicate that these rocks crystallized at temperatures in excess of 500 c and at pressures in excess of 1 5 kbar the range of spherulitic textures is taken to indicate crystallization under hydrous conditions with the episodic release of a fluid phase this magmatic fluid phase was dominated by h2o co2 and chlorine and underwent phase separation into a co2 rich vapor and a hypersaline brine 33 55 wt nacl equiv the hypersaline fluid was enriched in fe 10 wt and cu 1 wt pixe analysis in addition to na k and ca where this fluid was retained within fe rich portions of the sills it caused ca fe na alteration pyroxene albite magnetite growth at the expense of quartz where the fluid was expelled from the sills it produced quartz magnetite clinopyroxene albite veins broadly coeval with the early magnetite veins although rich in cu these granitoid derived magmatic fluids did not generate significant cu au mineralization perhaps because of the high temperatures involved and or a lack of reduced sulfur in the fluids or host rock however the amount of iron present is estimated from the aeromagnetic anomaly to be in excess of 2 000 million tonnes mt a later generation of calcite chlorite pyrite chalcopyrite veins contain traces of cu au mineralization fluid inclusion and stable isotope work indicate that these veins probably crystallized from cooler 200 c more dilute 15 28 wt nacl equiv fluids perhaps generated by the admixture of a meteoric component the conclusions reached in this study have implications for understanding the genesis of fe oxide cu au deposits and related sodic calcic alteration the study indicates the potential for co2 rich granitoid magmas to evolve hypersaline fe and cu rich fluids capable of causing intense magnetite veining and cu au mineralization autometasomatic sodic calcic alteration of the granitoids may be an important precursor to mineralization contributing fe k cu and cl to the magmatic fluids

the nature of sulfidic metasediments at kambalda and their broad relationships with associated ultramafic rocks and nickel ores sedimentary horizons at kambalda occur at the base of and at interflow boundaries within the same lower one third of the ultramafic sequence as do the major ni cu orebodies although sediments occur on all the same stratigraphic horizons as the ores they are almost entirely absent from within the volume containing the ores being separated by zones free of both sediments and ni cu sulfides generally they are broadly lenticular are not very extensive in area and correlate poorly laterally mineralogically and chemically the sediments are very variable but much of this is attributable simply to a variable iron sulfide content geographic and stratigraphic location and lithology are other significant causes of these variations proximity to the ni cu ores has at best an exceedingly subtle control on chemistry or mineralogy the kambalda sediments are not compositionally very different from other archean sediments in broadly similar environments however their sulfur and chalcophile element levels are high and au is especially enriched sulfur isotopes have a narrow spread near zero range 1 5 5 and the s se is around 15 000 to 20 000 indicating a little modified magmatic source for the sulfur apart from transfer of sulfur due to pyrite breakdown at peak metamorphism to the ultramafics there was negligible chemical exchange across sediment ultramafic interfaces virtually no ni was extracted from the ultramafic silicates during this sulfurization and migration of ni into the sediments was negligible nickel sulfide ore formation by sulfurization or sedimentary replacement is rejected ores intimately involved with sediments make up only 3 to 5 percent of the total ni reserve there is a strong inverse relationship between the quality of ore environments and the amount of their ore which is in contact with sediments the four largest ore locations accounting for about 70 of the reserve each has considerably less than 1 percent of ore with sediments extensive ore sediment contact occurs locally and can result from either 1 physical injection of preexisting ni cu ore into previously barren sediments 2 a synsedimentary intermixing of ores and sediments or 3 a passive and largely coincidental relationship the volcanic exhalative model of lusk 1976 for the ni ores is rejected the sediments are interpreted to be a mixture of local mafic ultramafic and distant felsic granitic detritus s pb zn fe and cu from ultramafic exhalations s co ni and cu from local sea floor leaching of ultramafics a biological carbonaceous component and probably minor chemical silica sea floor alteration affecting na k and mn diagenesis and metasomatic k rb ba sr and co 2 introductions are evident

bedrock incised gully erosion phenomena on round island mauritius abstract round island is a 219 ha volcanic island that lies 22 km north of mauritius reaching almost 300 m in altitude the small island has steep slopes thin soils and a rocky coast that dips steeply into the sea in the past the island has been subjected to human influence through the introduction of alien species including goats rabbits and plants severely degrading the floral and faunal ecology since being declared a nature reserve in 1957 the rehabilitation of the island has received international praise however extensive erosion remains problematic unusually the island exhibits gully forms that extend deeply into the tuff bedrock a modified version of the southern african regional commission for the conservation and utilisation of the soil sarccus erosion classification system was used to describe two such bedrock incised gully networks camp and big gully based on morphometric parameters and erosion severity rock hardness was also assessed to determine lithological controls on bedrock incised erosion features the gullies have their starting points on the mid upslope regions as rills these increase in width and depth downslope to a very severe bedrock gully erosion classification and an exit at sea level the predominant rock type on round island is tuff which is a relatively weak volcanic rock as indicated by low mean schmidt hammer r values implying higher expected bedrock erosion rates during periods of intense rainfall the bedrock incised gullies act as transport channels for sediment which is ultimately lost to sea rehabilitation efforts entail planting of indigenous species and mechanical soil traps on the slopes which has been effective although localised natural re vegetation has been occurring in the gully channels particularly in the mid sections of the gullies the most appropriate steps toward rehabilitation should thus focus on the wide scale establishment of vegetation as it would appear that due to the extensive nature of gullies on round island and its steep topography erosion is a natural phenomenon that is sensitive to human influence

formation of high frequency sequences and their bounding surfaces case study of the eocene yegua formation texas gulf coast usa abstract short term variations in accommodation and sediment supply on the continental shelf cause imbalances within longer term dynamic equilibrium conditions these imbalances result in the deposition of high frequency sequences the eocene yegua formation of the texas gulf coast is made up of six such sequences which have average durations of 0 8 million years or less they formed in response to distinct variations in the rate and quantity of sediment supply increases and decreases in the average grain size carried into the basin relative sea level position and the capacity of basinal energy regimes to transport and rework sediments each sequence may be characterized by the mix of particular regime conditions which were dominant during its deposition if the rate of sediment supply and the average grain size were more significant factors than the rate of relative sea level change and the capacity for basinal transport then the sequence was supply dominated and was characterized by fluvio deltaic deposition progradation of parasequences and river mouth bypassing onto the shelf and slope if instead relative sea level rise and basinal transport were the dominant factors the sequence was accommodation dominated and was characterized by estuarine and wave dominated deposition retrogradation of parasequences and shoreface bypassing onto the shelf when the accommodation and supply factors were in equilibrium sequences containing both fluvial and estuarine deposits characterized by aggradational parasequence stacking were deposited no two consecutive sequences in the yegua were dominated by the same mix of regime conditions therefore the sequence boundaries are important indicators of changes in regime conditions all of the boundaries are highly correlatable well constrained regional marine flooding surfaces flooding of the shelf thus apparently either causes or occurs as a result of a readjustment between sediment supply and accommodation continuous readjustments between these factors result in long term equilibrium conditions

river incision into bedrock mechanics and relative efficacy of plucking abrasion and cavitation improved formulation of bedrock erosion laws requires knowledge of the actual processes operative at the bed we present qualitative field evidence from a wide range of settings that the relative efficacy of the various processes of fluvial erosion e g plucking abrasion cavitation solution is a strong function of substrate lithology and that joint spacing fractures and bedding planes exert the most direct control the relative importance of the various processes and the nature of the interplay between them are inferred from detailed observations of the morphology of erosional forms on channel bed and banks and their spatial distributions we find that plucking dominates wherever rocks are well jointed on a submeter scale hydraulic wedging of small clasts into cracks bashing and abrasion by bedload and chemical and physical weathering all contribute to the loosening and removal of joint blocks in more massive rocks abrasion by suspended sand appears to be rate limiting in the systems studied here concentration of erosion on downstream sides of obstacles and tight coupling between fluid flow patterns and fine scale morphology of erosion forms testify to the importance of abrasion by suspended load rather than bedload particles mechanical analyses indicate that erosion by suspended load abrasion is considerably more nonlinear in shear stress than erosion by plucking in addition a new analysis indicates that cavitation is more likely to occur in natural systems than previously argued cavitation must be considered a viable process in many actively incising bedrock channels and may contribute to the fluting and potholing of massive unjointed rocks that is otherwise attributed to suspended load abrasion direct field evidence of cavitation erosion is however lacking in terms of the well known shear stress or stream power erosion law erosion by plucking is consistent with a slope exponent n of 2 3 to 1 whereas erosion by suspended load abrasion is more consistent with a slope exponent of 5 3 given that substrate lithology appears to dictate the dominant erosion process this finding has important implications for long term landscape evolution and the models used to study it

bedrock channel incision along piccaninny creek australia piccaninny creek in northwestern australia drains approximately 60 km 2 of high relief sandstone and conglomerate terrain the drainage basin lies within the seasonal tropics and is characterized by ephemeral high magnitude flows flood flows of 50 to 100 m 3 s 1 are probably primarily responsible for the spectacular erosional features along the bedrock channel segments of the creek these erosional features occur in two basic forms as parallel longitudinal grooves and as a downstream sequence that begins with shallow linear depressions parallel to flow and ends in a deep narrow inner channel the longitudinal grooves are assumed to result from longitudinal vortices while the erosional sequences which closely resemble von karman vortex streets are interpreted to result from turbulent vortices shed off irregularities of the channel bed the longitudinal grooves and the inner channel sequence alternate with each other and with gravel floored depositional reaches downstream along the channe

quantified spatial relationships between gold mineralisation and key ore genesis controlling factors and predictive mineralisation mapping st ives goldfield western australia abstract determining the spatial relationships between various geological features and mineralisation is not only important for understanding the ore genesis of mineral deposits but can also help to guide mineral exploration by providing predictive mineral maps in this gis based study we quantify the spatial relationships between gold mineralisation and plausible controlling factors in the central part of the st ives area western australia we weigh plausible control factors including rock types lithological boundaries and faults using gold occurrences in drill cores and then apply the weights to the geological data to generate a predictive map for the entire st ives area the three major findings of this study are 1 all major gold deposits are controlled by faults and small scale fault systems have a stronger correlation with gold mineralisation than large faults 2 felsic to intermediate intrusive rocks show strong correlations with gold mineralisation whereas metamorphic mafic rocks greenstones possibly acted as part of the broad regional environment for the mineral province rather than as a factor controlling the exact locations of the deposits and 3 our predictive mapping indicates that the southeast part of the st ives goldfield has a high potential for discovering new gold mineralisation

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

fluid wall rock interaction in an archean hydrothermal gold deposit a thermodynamic model for the hunt mine kambalda gold mineralization at the hunt mine kambalda is situated in a 20 m thick steeply dipping shear zone within a sequence of rather uniform metabasalts zonal alteration of the host rocks occurred around fractures which acted as conduits during gold mineralization limited major element mobility ensured assemblages of low variance over most of the alteration zone providing a suitable environment for the application of thermodynamics in conjunction with detailed petrographic geochemical electron microprobe analysis of mineral compositions and fluid inclusion studies microthermometry equilibrium equations involving fluid components and alteration minerals are used to determine conditions during the alteration event and in particular activity gradients of components profiles across the schist zone show increasing f sub h 2 s around auriferous quartz veins log f sub h 2 s 0 6 but constant f sub o 2 log f sub o 2 29 7 these results are near the magnetite pyrrhotite pyrite triple point for the inferred temperature of 350 degrees c and explain the pyrite pyrrhotite magnetite zonation sequence outward from veins the profile of x sub co 2 is very close to the value of 0 25 determined independently from fluid inclusions the k super h super profile shows increases adjacent to auriferous veins corresponding to the zone of biotite stability using a value for fluid salinity of 2 equiv wt percent nacl ph was calculated to be around 6 9 the solubility of gold in solution was estimated for the calculated conditions for the complex au hs super 2 the maximum solubility is around 0 5 mg kg super 1 i e 0 5 ppm for au cl super 2 it is close to 10 super 6 mg kg super 1 indicating the efficacy of sulfide transport of gold compared to chloride transport under these conditions the calculated decrease in f sub h 2 s away from the auriferous quartz veins results in an 80 percent or greater decrease in gold solubility suggesting hat the sulfidation of fe rich wall rocks is a geologically reasonable and very efficient mechanism for precipitating gold from solution at elevated temperatures

shelf sequence and characteristic bounding surfaces in a wave dominated setting latest pleistocene holocene examples from northeast japan abstract this paper presents modern models for coastal to shelf sequences in a microtidal and wave dominated setting based on detailed analyses of latest pleistocene holocene sequences on the pacific side of northeast japan the latest pleistocene holocene sequences on the pacific side of northeast japan created in response to sea level changes for the last 30 kyr can be assigned to lowstand transgressive and highstand systems tracts related to sea level changes and show characteristic bounding surfaces and depositional systems these basically support the sequence architecture model of vail however transgression and regression occurred almost simultaneously with rise and fall of sea level in the study areas and so inflection points of sea level curves where the absolute slope or the rate of change is greatest are less significant for these high frequency sequences the characteristic bounding surfaces are a sequence boundary a transgressive surface which is the boundary between the lowstand and the transgressive systems tracts a ravinement surface which is found mainly in deposits of the transgressive systems tract and a maximum flooding surface which is the boundary between the transgressive and the highstand systems tracts the sequence boundary was created in response to falling sea level fluvial entrenchment shifted seaward with falling sea level therefore the boundary is a time transgressive unconformity and is not an isochronous boundary the transgressive surface is a nearby isochronous boundary that occurred when the shoreline had started to move landward from its maximum seaward excursion the ravinement surface is a diachronous surface created by shoreface erosion during transgression the shoreface erosion shifted landward with rising sea level and so the erosion surface created is time transgressive this surface is the most distinct bounding surface in the shelf sequence these characteristics of the bounding surfaces support and clarify the model of thorne and swift 1991 the transgressive systems tract on the shelf can be divided into two backstepping wedges separated by the ravinement surface the lower wedge consists of coastal plain and fluvial systems and the upper wedge is composed of transgressive sand and or sand ridges both the wedges show a backstepping depositional pattern which can be recognized in seismic records and cores these wedges correspond to the backstep backbarrier wedge and backstep shelf wedge of thorne and swift 1991 respectively

geological controls on seismic sequence resolution a seismic sequence analysis of the canterbury basin eastern south island of new zealand has illustrated the roles of subsidence sediment supply and current activity as controls on sequence resolution and architecture during basin evolution the rates of sediment supply and subsidence determine the background depositional regime transgressive or regressive and effectively determine the frequency response of the continental margin sedimentary section to input signals with a broad range of frequencies including eustasy a regressive progradational depositional regime and minimal current erosion favor the preservation of high frequency sequences particularly at fourth order level under less favorable conditions the record of sequences is incomplete or ambiguous such frequency response characteristics must be considered when inverting sequence records to derive the frequency of the input cyclicity and when making global comparisons of regional sequence stratigraphic studies clastic basins with simple subsidence histories and uniform or increasing rates of sediment supply develop from a transgressive phase characterized by ramplike major sequence boundaries to a mature progradational shelf phase with clinoform sequences and optimum sequence resolution the mature phase constitutes the preferred setting for sequence stratigraphic analyses

influence of rock strength on the valley morphometry of big creek central idaho usa abstract analysis of valley morphometry and bedrock strength along big creek central idaho shows that valley floor width is strongly controlled by bedrock we performed statistical analysis of schmidt hammer rock strength as a function of lithology and aspect and of valley morphometry as a function of rock strength rock strength is significantly greater on the south side of the valley and in eocene granodiorites rock strength is weakest in eocene volcanic tuffs valley floor width depends negatively on weakest valley side rock strength and hillslope gradient on the north side of the valley depends positively on rock strength stream gradient does not depend on rock strength valley floor width appears to be controlled by bedrock strength on the weaker side of the valley which was generally the north south facing side we speculate that a higher degree of weathering via freeze thaw cycles contributes to lower strength on the north side the positive dependence of hillslope gradient on rock strength on the north side provides evidence that differential weathering across lithologies determines the gradient that can be maintained as lateral migration of the stream erodes valley walls these results suggest that in situ rock strength exerts strong influences on some measures of valley morphometry by modulating hillslope mass wasting processes and limiting lateral erosion

fluid inclusion and stable isotope o h c constraints on the genesis of the pedra branca gold deposit troia massif borborema province ne brazil an example of hypozonal orogenic gold mineralization abstract at the archean paleoproterozoic troia massif ne brazil two major paleoproterozoic greenstone belts are documented algodoes and serra das pipocas they share similar lithostratigraphic characteristics with other 2 2 2 1 ga greenstone belts of the surrounding cratonic domains e g guiana shield and west africa craton gold mineralization in the serra das pipocas greenstone belt is associated with a regional ne trending shear zone in the mineralized area the pedra branca gold deposit the main gold mineralization is found in association with quartz veins calc silicate alteration e g diopside k feldspar amphibole titanite biotite pyrite albite magnetite carbonates and albitization pyrrhotite is commonly found as inclusions in pyrite and late magnetite replaces pyrite suggesting progressive oxidation of the ore forming fluid free milling gold commonly precipitates at the late stages of alteration in close association with magnetite and tellurides the main tellurides found are hessite altaite sylvanite calaverite and tellurobismuthite two fluid inclusion assemblages were identified from mineralized quartz veins assemblage 1 is characterized by pseudo secondary trails that show the coexistence of co2 rich and low salinity 0 8 wt nacl equiv co2 h2o nacl and h2o nacl inclusions suggesting formation during phase separation fluid immiscibility assemblage 2 is represented by late secondary low temperature th

implications of pb isotope signatures of rocks and iron oxide cu au ores in the candelaria punta del cobre district chile lead isotope ratios of ores of the candelaria punta del cobre iron oxide cu au deposits and associated early cretaceous volcanic and batholithic rocks have been determined for the igneous rocks a whole rock acid attack technique based on the separate analyses of a leachate and the residual fraction of a sample was used the lead isotope systematics of leachate residue pairs are significantly different for unaltered and altered igneous rocks of the candelaria punta del cobre district residues of unaltered igneous rocks likely represent the common lead in contrast residues of all the altered igneous rocks except two samples have higher pb isotope ratios than those of unaltered magmatic rocks and cannot represent common lead we suggest that this is a result of the hydrothermal alteration suffered by these rocks and that the common lead composition of the altered igneous volcanic and plutonic rocks must have been similar to that of the unaltered batholith rocks the conclusion that the altered volcanic rocks originally had a similar common lead isotope composition as the batholith is consistent with geological and geochemical arguments e g setting regional geologic evolution ages and relative distribution of volcanic and intrusive rocks magmatic affinities which indicate that these rocks were derived from similar early cretaceous parent magmas the modification of the leachate residue pair lead isotope systematics of most altered igneous rocks is consistent with a selective removal of lead and uranium from these rocks by an oxidized hydrothermal fluid the result of the hydrothermal leaching has been to alter magmatic rocks in a way that 1 their leachable fraction is presently a mix of common lead similar to that of the ore event and of radiogenic lead evolved from a source with a consistently high th u and that 2 their residual fraction has less common lead than unaltered rocks the outcrop area with altered volcanic rocks displaying anomalously high lead isotope ratios extends over 25 km along the eastern margin of the batholith since lead of the ores in the candelaria punta del cobre district has the same isotopic composition as the common lead of unaltered magmatic rocks of the area the lead isotope data are consistent with a derivation of the ore lead and by inference of other metals like cu both directly from a magmatic fluid exsolved during crystallization of the batholith and or from hydrothermal leaching of the volcanic rocks originally having similar isotopic compositions as the batholith

accommodation and supply a dual control on stratigraphic sequences abstract it is widely accepted that both eustatic and tectonically controlled regional changes of sea level have contributed to the record of stratigraphic sequences i suggest that environmental change be added as a third autonomous control sedimentologic principles clearly indicate that sequences and their systems tracts are controlled by the interplay of two rates the rate of change in accommodation space available for sedimentation and the rate of sediment supply sea level has direct control on accommodation but its influence on sediment supply is remote and easily overshadowed by environmental factors for instance the record of the most recent sea level rise is a transgressive systems tract where supply is low it is a prograding highstand systems tract in deltas where the supply is high examples of sequence boundaries generated by changes in sediment supply include tectonically driven shifts in sediment input into basins changes in ocean currents pulsating supply from failure of submarine slopes and drowning of carbonate platforms by environmental stress furthermore the stratigraphic sequences in fluviatile continental basins are physically removed from sea level induced changes in accommodation and must have formed by changes in the rate and pattern of supply subaerial exposure of marine sediments at the sequence boundary is a most important criterion for recognizing sea level cycles as opposed to supply cycles other criteria include downstepping of shelf breaks and characteristic patterns in the spacing of time lines within sequences some third order cycles ca 0 5 3 ma duration meet these criteria others do not cycle stacking patterns and the shifting facies belts on cratons indicate that many second and third order cycles lack pronounced exposure unconformities and represent gradual changes superimposed on more rapid shorter oscillations seismic data yield poor images of these gradational changes because they lack resolution seismic models of outcrops show that near the limits of resolution the seismic tool tends to portray rapid changes in facies and bed thickness i e lateral variations in supply as unconformities seismic unconformities do not necessarily correspond to unconformities in outcrop the succession of stratigraphic sequences is best considered a composite record of sea level and environmental change separating the various controls requires carefully planned experiments work on the oxygen isotope curve another proxy for sea level has set an example in this respect

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

geochemistry of post 1540 ma granites in the cloncurry district northwest queensland proterozoic fe oxide cu au co mineralization in the cloncuriy district is spatially and temporally related to intrusion of voluminous postpeak metamorphic granites that form most of the williams and naraku batholiths detailed mapping and geochemical study of these granites in the southern part of the district have identified two groups of intrusions that are referred to here as the cloncurry supersuite and eureka supersuite the cloncurry supersuite is composed mainly of monzogranite and syenogranite which consist of quartz plagioclase alkali feldspar hornblende biotite with accessory magnetite titanite apatite and zircon the timing of intrusion of the wiley igneous complex is interpreted to be pre or syn d 3 based on overprinting relationships while hornblende geobarometry suggests an emplacement depth of approximately 14 km the granites are subalkaline high k metaluminous rocks and range from 59 to 74 wt percent sio 2 increasing sio 2 is accompanied by increasing k 2 o rb and th and decreasing mgo fe 2 o 3 feo tio 2 cao p 2 o 5 sr eu zr cu zn ni v and sc this is consistent with fractionation of observed phases including plagioclase amphibole biotite magnetite titanite and apatite the eureka supersuite ranges from diorite to syenogranite in composition more mafic varieties contain pyroxene and hornblende while more felsic varieties contain biotite as the only ferromagnesian silicate sodiccalcic alteration events are closely linked in space and time to intrusion of eureka supersuite magmas the timing of intrusion is interpreted to be pre and post d 3 based on overprinting relationships between different plutons and mylonitic fabrics hornblende geobarometry suggests an emplacement depth of approximately 10 km for the mount angelay igneous complex eureka supersuite granites in the mount angelay igneous complex range from 51 to 76 wt percent sio 2 the granites are alkaline and k 2 o rich leading to their classification as a shoshonite series with increasing sio 2 content tio 2 al 2 o 3 fe 2 o 3 t mno mgo cao p 2 o 5 sr eu cu zn ni v and sc decrease indicating fractionation of observed minerals including apatite magnetite pyroxene hornblende plagioclase and biotite some samples are characterized by very high abundances of k 2 o p 2 o 5 sr ba and light rare earth elements lree compared to other eureka supersuite samples this may reflect their derivation from a source that was variably enriched in incompatible elements compared with many other shoshonitic rocks high k eureka supersuite samples are strongly depleted in sr and to a lesser extent eu and enriched in y and heavy rare earth elements hree this suggests that garnet was not a stable phase in the source region that the source contained plagioclase as a stable phase and or the granites evolved by extensive plagioclase fractionation partial melting in the lower crust to generate post 1540 ma granites was most likely caused by intrusion of basic magma into the crust eureka supersuite magmas were probably derived from partial melting of variably incompatible element enriched gabbroic rocks while cloncurry supersuite magmas were most likely derived from partial melting of more felsic material of dioritic to tonalitic composition

biotite and apatite as tools for tracking pathways of oxidized fluids in the archean east repulse gold deposit australia fluid pathways in hydrothermal ore deposits can potentially be traced through alteration zoning patterns and the chemistry of mineral phases i e major trace elements and isotopic composition which provide insight into the physico chemical gradients within and adjacent to pathways world class neoarchean gold deposits of the eastern yilgarn can be zoned with respect to alteration assemblages but in many cases fluid pathways are poorly understood which makes it difficult to identify the nature and source of fluids this study conducted detailed core logging sem mineral mapping of alteration assemblages and microprobe analysis of biotite and apatite in order to identify chemical gradients across the east repulse gold deposit of the st ives gold camp eastern yilgarn western australia results show that the east repulse deposit is vertically zoned with relatively oxidized sulfate rich alteration in the footwall of the deposit anhydrite celestine and or barite sulfide rich alteration in the core of the deposit pyrite magnetite with minor barite and celestine and relatively reduced sulfide only assemblages pyrite pyrrhotite with minor pentlandite millerite and cobaltite in the hanging wall of the deposit gold was identified as inclusions in pyrite and associated biotite implying a link between biotite pyrite alteration and gold mineralization the abundance of f in biotite and apatite progressively decreases from the footwall of the deposit into the mineralized core this trend of decreasing f in biotite and apatite with decreasing depth is consistent with an hypothesis that oxidized f and k bearing fluids were focused along subvertical pathways beneath the deposit within or subjacent to granitoid dikes the biotite apatite geothermometer shows that these fluids were relatively hot 480 60 c at the time of alteration these relatively high temperature oxidized f and k bearing fluids may have been derived from a proximal magmatic source

occurrence and origin of marialitic scapolite in the humboldt lopolith n w nevada the occurrence and origin of marialitic scapolite in the humboldt lopolith was investigated in the field and in the laboratory using petrographic and experimental techniques scapolite occurs in three modes as a pervasive replacement of plagioclase and other minerals in gabbro diorite and extrusive rocks as a poikiloblastic mineral in scapolitite dikes and as a fracture filling mineral with analcime albite and sphene in scapolite veins additional secondary minerals associated with scapolite include epidote prehnite hornblende and diopside salite clinopyroxene relations with these minerals suggest that most marialitic scapolite grew at temperatures around 400 c scapolite composition varies from eqan12 to eqan37 containing from 72 to 96 atomic cl in the r position experiments on systems of similar compositions indicate that nacl h2o fluid having more than 40 mol nacl is needed to stabilize the scapolite

stable isotope evidence for the origin of the mesoproterozoic starra au cu deposit cloncurry district northwest queensland new stable isotope data are consistent with a metasomatic origin for the controversial mesoproterozoic ironstone hosted au cu deposit at starra in the cloncurry district of northwest queensland this supports textural and mineralogical evidence that the ore bearing ironstones formed after the peak of metamorphism and were strongly controlled by brittle ductile deformation three dominant paragenetic stages are recognized these are 1 early widespread na ca metasomatism quartz albite scapolite actinolite 2 localized k fe metasomatism biotite magnetite hematite quartz pyrite and 3 mineralization quartz anhydrite barite hematite calcite gold pyrite chalcopyrite bornite chalcocite chlorite muscovite crosscutting anhydrite veins associated with carbonate or hematite are rare in the upper parts of the system but more common at depths greater than 700 m stage 3 minerals demonstrate that the ore fluid was highly oxidized a restricted range of fe oxide delta 18 o implies isotopically similar fluids were responsible for the magnetite ironstones and later hematite alteration associated with gold and sulfides between 0 2 to 3 3ppm except for one magnetite at 5 4ppm sulfur isotope compositions suggest that pyrite 0 2 to 3 9ppm chalcopyrite 5 8 to 0 7 and 14 6ppm bornite 4 7ppm chalcocite 0 1ppm and anhydrite 1 1 2 4ppm all formed from the same sulfur source anhydrite delta 34 s falls within the sulfide range suggesting that the sulfate inherited its delta 34 s through hydrolytic processes similar to those of some porphyry environments delta 13 c of ore stage carbonates range from 7 3 to 2 2 per mil temperatures based on stage 2 quartz magnetite pairs imply 400 degrees to 500 degrees c for ironstone formation and stage 3 calcite hematite pairs indicate 180 degrees to 330 degrees c for mineralization stage 2 fluid inclusions have homogenization temperatures between 345 degrees and 615 degrees c and salinities of 34 to 52 wt percent nacl equiv stage 3 fluid inclusions homogenized between 225 degrees and 360 degrees c and have salinities of 30 to 42 wt percent nacl equiv calculated delta 18 o fluid compositions for magnetite quartz and hematite calcite average 7 8 per mil and 9 5 per mil respectively fluid compositions for stages 2 and 3 fall within the range for a magmatic or metamorphic fluid however combined stable isotope and fluid inclusion data and thermodynamic considerations suggest a magmatic metasomatic origin for both the magnetite ironstones and the highly oxidized mineral assemblage associated with high grade gold deposition

metasomatic albitites and related biotite rich schists from a low pressure polymetamorphic terrane snake creek anticline mount isa inlier north eastern australia microstructures and p t d paths rocks of the snake creek anticline are mainly pelitic schists psammitic schists and quartzites that were metamorphosed during multiple high t low p events extending from d1 to d5 with the metamorphic peak occurring late to post d3 albitites are widespread but are concentrated in five areas they are typically fine to medium grained and consist of albite with or without combinations of quartz biotite staurolite cordierite garnet andalusite sillimanite kyanite gedrite and tourmaline from the presence or absence of albite inclusions in porphyroblasts the albitites are interpreted as forming early in the d3 event as a result of infiltration of external fluids psammitic schists and quartzites were preferentially altered but pelitic schists were also albitized in localities where the alteration was more extreme with the replacement of muscovite total and the replacement of quartz and biotite variable structural controls on albitization include fracturing and syn d3 shear zones in fold hinges biotite schists with abundant porphyroblasts combinations of staurolite garnet andalusite and cordierite occur adjacent to albitites and it is argued that they formed by the addition of fe and mg sourced from the albitites in several albitite rich areas cordierite grew early in d3 and was partly or entirely replaced during or after d3 by combinations of biotite andalusite tourmaline staurolite and sillimanite a postulated p t d path involved an increase in pressure with or without a decrease in temperature subsequent to early d3 albitization followed by an increase in temperature up to the metamorphic peak late d3 to early d4 the metamorphism was contemporary in part with the emplacement of the williams batholith c 1550 1500 ma which probably supplied the na rich fluids

a test of the viability of fluid wall rock interaction mechanisms for changes in opaque phase assemblage in metasedimentary rocks in the kambalda st ives goldfield western australia transitions from pyrrhotite magnetite to pyrite magnetite and pyrite hematite bearing assemblages in metasedimentary rocks in the kambalda st ives goldfield have been shown to be spatially associated with economic gold grades fluid mixing fluid rock interaction and phase separation have been proposed previously as causes for this association textural mineralogical and isotopic evidence is reviewed and thermodynamic calculations are used to investigate the mineralogical consequences of progressive fluid rock interaction in interflow metasediments fluid rock interactions in response to fluid infiltration and or bulk composition variation are plausible mechanisms for production of the observed features

two sulphur isotope provinces deduced from ores in the mount isa eastern succession australia the sulphur isotopic characteristics of ore deposits in the australian mount isa eastern succession are not well known unlike those of the western succession in this study new detailed analyses are provided for recently discovered eastern succession mineralisation such as the starra and osborne bif hosted cu au ores the dugald river sediment hosted pb zn prospect and four vein hosted cloncurry style cu au deposits hampden mt elliot swan mt cobalt and the answer mine all of the deposits of the eastern succession have 34ssulphide between 8 and 9 regardless of their genesis empirically a moderate 34s range averaging close to 0 characterises starra style cu au and pegmont pb zn bif ores whereas shear and vein style cu mineralisation populations are tighter and do not average close to 0 this is a particularly surprising result for dugald river where a larger isotopic variation more typical of stratiform sediment hosted pb zn ores in the region might have been expected by comparison western succession stratiform pb zn and vein style cu deposits span a huge range of 15 to 51 large sulphur isotope ranges typify sulphate evaporite or organic sulphur rich sedimentary successions the lack of such variation in the eastern succession in turn suggests that primary evaporite sequences there were halite dominated but sulphate poor and or contained only limited volumes of organic sulphur rich sediment eastern succession sequences were therefore less likely hosts for giant stratiform pb zn deposits because of their paucity of sulphur although local sulphur sources permitted small deposits such as dugald river to develop sedimentary conditions were more favourable for the development of sulphur poor synsedimentary hydrothermal systems such as starra osborne and pegmont although sulphur isotope evidence is equivocal about the origin of these epigenetic deposits close to the williams batholith mt dore hampden owe their clustering around 0 to their granitic fluid source

onlap and downlap patterns in plio pleistocene forearc and backarc basin fill successions japan abstract discontinuities such as onlap toplap and downlap terminations are important key structures in seismic stratigraphy these patterns characterize the plio pleistocene kakegawa group in shizuoka the middle pleistocene kasamori formation in chiba and the plio pleistocene uonuma group in niigata the patterns within these successions can be detected by tracing intercalated pyroclastic layers tephra beds which provide the same information as the synchronous reflection lines in seismic data these patterns therefore allow the stratigraphic sequence to be divided into forearc and backarc basin fill successions the depositional sequences of the kakegawa and uonuma groups are third order ranging from ca 2 6 ma to 1 0 ma the kasamori formation has high frequency sequences dating from 0 60 ma to 0 45 ma all these sequences are believed to have been produced by glacio eustatic sea level changes

influence of vegetation distribution on sediment yield in forested marly gullies abstract the cover provided by forest vegetation makes it possible to fight erosion efficiently furthermore vegetation barriers can also play a major role by trapping eroded sediments and thus vegetation located downslope of a gully can be important the objective of this study is to highlight the role of vegetation distribution in a marly gully in reducing sediment loss at the outlet to do this sediment yield in gullies with similar geological and geomorphological components but with different vegetation cover and distribution was studied for 2 years sediment traps installed at gully outlets permitted the distinction of active gullies from inactive ones the results show that gully activity is not correlated with the percentage of total vegetation cover but with the percentage of cover of low vegetation in the gully floor this is due to sediment trapping processes and highlights the importance of the spatial distribution of forest vegetation in reducing sediment yield at the gully outlet thus gullies with similar total vegetation cover can have very different activities and inactive gullies exist that are only partially covered with vegetation inactive gullies with as low as 33 vegetation cover occur where vegetation is mainly present in the gully floor the results also show that above 50 cover of low vegetation in the gully floor as a percentage of the gully floor surface gullies are generally inactive

contrasting fluid chemistries alteration characteristics and metamorphic timing relationships recorded in hybridized orebodies of the missanabie renabie gold district archean wawa subprovince ontario canada

rippled scour depressions on the inner continental shelf off central california abstract side scan sonar records taken during the recent coastal ocean dynamics experiment code show elongate shore normal rippled depressions of low relief on the inner continental shelf off central california between bodega bay and point arena these features extend up to 2 km from the coast into water depths of up to 65 m the proposed mechanism for their generation is storm generated bottom currents associated with coastal downwelling during the late fall and winter which scour the surficial fine sand sediment and expose the coarser sand substrate in the depressions the zones of most intense erosion and the irregular spacing of the features may be controlled by submerged rock ledges and other prominent coastal features the large straight crested ripples within the depressions heights to 40 cm wavelengths to 1 7 m are probably formed by large amplitude long period surface waves generated by winter storms

magmatic hydrothermal albite actinolite apatite rich rocks from the cloncurry district nw queensland australia abstract albite quartz actinolite apatite rich rocks with accessory titanite form a carapace that caps a small dome like intrusion of roxmere pluton and provide evidence of the accumulation and release of magmatic fluids that may have contributed to regionally extensive na ca alteration in the proterozoic cloncurry district australia the roxmere pluton was emplaced after the peak amphibolite facies metamorphism and into psammitic metasedimentary rocks that are stratigraphically younger and overlie the calc silicate rich mary kathleen group na ca hydrothermal assemblages closely spatially related to the roxmere pluton suggest that na ca alteration in this case is inconsistent with excepted up temperature metamorphic fluid circulation models the carapace rocks comprise two variants aplite and pegmatite both of which were affected by alteration and brecciation aplitic material predominates and the carapace has a brecciated top and a distinct textural zonation from an aplite rich pegmatite poor base to a relatively aplite poor pegmatite rich top this zonation is interpreted to represent the progressive accumulation of volatile phases toward the top of the carapace which culminated in fluid overpressuring and brecciation of the granitoid stock the convoluted and ptygmatic habit of the aplitic and pegmatitic material within the carapace is similar in nature to the comb quartz unidirectional solidification textures that are developed in the roof zones of some porphyry stocks these complex textures are interpreted to have formed from a single phase of fluid saturation the roxmere pluton or a deeper equivalent has a na mg and ca rich composition and a clinopyroxene amphibole rich mineralogy that is consistent with the formation of the contained albite actinolite apatite rich phases however the source of the rocks within the carapace is equivocal given that their contact relationships are obscured the 18 o composition of minerals within the carapace has the following ranges albite 7 1 8 7 actinolite 8 0 8 5 and quartz 8 9 11 4 albite and quartz 18 o equilibria suggest that these minerals in the pegmatitic and aplitic components formed at temperatures of 510 c to 540 c the calculated 18 o composition of a fluid in equilibrium with these minerals 6 0 to 8 0 at temperatures between 450 c and 550 c is consistent with a magmatic origin these estimated temperatures and 18 o fluid composition are comparable to estimates for regional na ca alteration in contrast d analysis of actinolite 136 to 150 within the carapace demonstrates that these fluids have a significantly lower d signature compared to na ca alteration assemblages associated with regional alteration 70 to 90 the low values in the carapace could either be interpreted as being produced from meteoric fluids or from degassed magmatic fluids although a meteoric fluid model is deemed improbable as meteoric fluids in the district have a documented d composition of ca 0 0 as a consequence an open system degassed magmatic hydrothermal fluid origin is considered to be the most plausible explanation for the low isotopic signatures and is also consistent with the complex textural relationships described within the carapace

influence of structural setting on sulphur isotopes in archean orogenic gold deposits eastern goldfields province yilgarn western australia the published mean 34s values of ore related pyrites from orogenic gold deposits of the eastern goldfields province yilgarn craton lie between 4 and 4 as for orogenic gold deposits worldwide most deposits have positive means and a restricted range of 34s values but some have negative means and wider ranges of 34s values wall rock carbonation and back mixing of similar source fluids with different fluid pathways can explain some of the more negative 34s signatures however structural setting appears to be the most important factor controlling ore fluid oxidation state and hence the distribution of 34s values in gold related pyrites shear hosted deposits appear to have experienced fluid dominated processes such as phase separation whereas stockwork vein hosted or disseminated deposits formed under conditions of greater rock buffering at victory defiance in particular negative 34s values are more common in gently dipping dilational structures compared to more compressional steeply dipping structures it appears most likely that fluid pressure fluctuations during fault valve cycles establish different fluid flow regimes in structures with different orientations rapid fluid pressure fluctuations in dilational structures during seismic activity can cause partitioning of reduced gas phases from the ore fluid during extreme phase separation and hence are an effective method of ore fluid oxidation leading to large local fluctuations in oxidation state it is thus not necessary to invoke mixing with oxidised magmatic fluids to explain 34s signatures indicative of oxidation in any case available robust geochronology in the eastern goldfields province does not support the direct involvement of oxidised magmatic fluids from adjacent granitic intrusions in orogenic gold genesis thus negative mean 34s values and large variations in 34s values of ore related pyrites in world class orogenic gold deposits are interpreted to result from multiple mechanisms of gold precipitation from a single ubiquitous ore fluid in varying structural settings rather than from the involvement of oxidised ore fluids from a different source such signatures are indicative but not diagnostic of anomalously large orogenic gold systems

origin of hydrothermal fluids at olympic dam preliminary results from fluid inclusions and stable isotopes fluid inclusions and stable isotope data provide evidence of the temperature composition and sources of hydrothermal fluids associated with formation of the fe rich breccias that host cu u au ag ore at olympic dam early magnetite associated with pyrite and siderite precipitated from fluids of high delta 18 o content approx 10ppm at temperatures near 400 degrees c and magnetite in k feldspar apatite veins at the nearby acropolis deposit probably formed from similar fluids in contrast hematite associated with ore bearing breccias formed at lower temperatures 200 degrees 400 degrees c from fluids of lower delta 18 o content 9ppm fluid inclusion homogenization temperatures in quartz fragments within hematite breccias overlap with and are lower than those indicated by isotope geothermometry two phase liquid plus vapor inclusions mainly homogenize at temperatures between 130 degrees and 280 degrees c with a mode at 170 degrees to 190 degrees c quartz fragments within rare laminated barite clasts display a larger range of homogenization temperatures from 110 degrees to 420 degrees c and may have preserved evidence of early high temperature hydrothermal events measurement of freezing point depressions in fluid inclusions provides evidence of widely varying salinities and complex solute chemistry indicated salinities vary from a minimum of 7 3 wt percent nacl equiv 1 3 m to a maximum of 23 7 wt percent nacl equiv 4 2 m in two phase fluid inclusions and up to 42 wt percent nacl equiv in halite saturated three phase inclusions all the fluid inclusions studied displayed evidence of additional solutes in addition to nacl all required a high degree of supercooling to induce freezing and displayed initial ice melting at temperatures below 23 degrees c and many formed distinctive brown granular ice in addition several inclusions also formed co 2 clathrates and other metastable solids qualitative assessment of these characteristics indicates fluids carrying multiple solutes probably including nacl kcl cacl 2 fecl 2 caf and co 2 these preliminary results suggest that at least two sources of fluids of contrasting temperature composition and oxygen isotope characteristics were involved in the formation of olympic dam early magnetite was deposited from hydrothermal fluids of deep seated possibly magmatic origin but magnetite bearing rocks are not significant hosts for copper uranium mineralization later hematite associated with ore bearing breccias was deposited by fluids of significantly lower temperature and lower delta 18 o values the geologic setting of the deposit a fault bounded depression at or near the palcosurface implies that waters of surficial origin seawater closed basin water or ground water may have been involved in the later stages of hydrothermal activity that formed the hematite breccias although our preliminary isotopic data do not require such waters they are consistent with this conclusion

trace element content of pyrite from the kapai slate st ives gold district western australia the kapai slate is a continuous pyrite rich carbonaceous shale horizon within the st ives au district that is spatially related to high grade au mineralization in situ laser ablation inductively coupled mass spectrometry la icpms trace element analyses in situ sensitive high resolution ion microprobe stable isotope shrimp si s isotope analyses and optical microscopy pyrite texture analyses were used to examine the different pyrite types in the kapai slate and au deposits these data were also used to confirm that the trace element signature of sedimentary pyrite can be preserved in rocks that underwent upper to mid greenschist facies metamorphism and significant hydrothermal overprint the data were further utilized to gain a more detailed understanding of the ocean conditions during deposition of the kapai slate and determine whether some of the au and s in the st ives district could have been sourced from the kapai slate seven different types of pyrite were identified fine grained sedimentary pyrite py 1 nodular sedimentary pyrite py 2 remobilized sedimentary pyrite py 3 coarse grained inclusions poor late pyrite py 4 inclusion rich magnetite series pyrite py 5 ore stage pyrite py 6 and pyrite associated with the mafic units py 7 each type of pyrite was found to have distinctive trace element compositions and s isotope signatures the results of the la icpms analyses provide evidence for early trace element enrichment in the kapai slate sedimentary pyrite median values of 158 ppm ni 387 ppm co 82 ppm cu 727 ppm as 1 91 ppm mo 13 ppm se 0 25 ppm au 7 72 ppm te and 3 36 ppm ag for py 1 and 223 ppm ni 158 ppm co 99 ppm cu 856 ppm as 1 27 ppm mo 10 2 ppm se 0 57 ppm au 10 09 ppm te and 6 62 ppm ag for py 2 concentrations of ni and co are low relative to other late archean sedimentary pyrite median of 813 and 465 ppm respectively and mo levels are near that of the euxinic shales of the similar aged jeerinah formation in the hamersley basin western australia these data suggest that the kapai slate was deposited in an anoxic to euxinic basin with relatively low biological productivity the 33 s and 34 s signatures of the sedimentary pyrite suggest two different sources of s positive 34 s and negative 33 s signatures indicate bacterial reduction of so 4 2 from seawater whereas positive 34 s and positive 33 s signatures indicate an elemental s 8 source indicating the pyrite formed later during diagenesis this s isotope signature is consistent with a transition between a near sediment environment to a more distal environment source analyses of the ore phase pyrite yield weakly positive 33 s values this suggests there was a minor contribution of sedimentary s to the more significant oxidized ore forming fluids which is consistent with a small contribution of au from a sedimentary source approximations of the degree of sedimentary pyrite destruction in the pyrrhotite pyrite dominated zones and pyrrhotite magnetite pyrite zones of the northern part of the st ives district were used to calculate the amount of au released from the early sedimentary pyrite the calculation suggests that a minor though possibly locally significant amount of au could have been sourced from the kapai slate

the relationship between glacio eustatic parasequences and a third order sequence in the kakegawa group central japan abstract the plio pleistocene kakegawa group central japan consists of a third order depositional sequence 2 6 1 0 ma the northwestern part of the kakegawa sequence consists of up to 500 m of alluvial shoreface shelf slope and submarine channel facies it contains at least sixteen upward shallowing cycles parasequences the deposition of which was affected by high frequency eustatic sea level cycles the lower part of the sequence is characterized by a retrogradational parasequence set which formed a transgressive systems tract 2 2 1 75 ma followed by a progradational parasequence set comprising a highstand systems tract 1 75 1 4 ma subsidence analysis and evaluation of changes in the shelf sedimentation rate estimated from cross sections suggest that formation of the third order sequence was controlled by tectonic subsidence and variation in the sedimentation rate rapid subsidence and a high rate of sedimentation during 2 2 2 0 ma resulted in deposition of the lower part of the transgressive systems tract characterized by thick backstepping successions the rate of subsidence decreased in the period 2 0 1 75 ma the sedimentation rate also decreased due to a high rate of sediment bypassing however subsidence was still the dominant factor leading to the formation of thin backstepping successions the 1 75 1 4 ma progradational succession resulted from a combination of a low rate of subsidence and moderate sedimentation the progradational units become thicker basinward owing to faster subsidence in the basin center the maximum flooding surface was formed around 1 75 ma even though subsidence was slow at this time

fluid sources and the role of abiogenic ch4 in archean gold mineralization constraints from noble gases and halogens abstract the st ives goldfield of western australia hosts high grade gold mineralization in a variety of lithologies within 2 7 ga greenstone belts the low salinity co 2 bearing fluids responsible for gold mineralization could have been generated either by metamorphic processes within the greenstones or from external magmatic sources and independently sourced abiogenic ch 4 has been suggested as an important control on gold mineralization in order to better constrain possible fluid sources we applied a combination of he ne ar kr xe isotope and cl br i analyses to quartz and carbonate veins containing h 2 o co 2 fluid inclusions pyrite magnetite hematite and quartz veins containing ch 4 rich fluid inclusions pyrrhotite pyrite samples containing h 2 o co 2 fluid inclusions have higher br cl and i cl values and less radiogenic noble gas isotope signatures than suggested for ch 4 rich fluid inclusions the samples containing ch 4 fluid inclusions have maximum 40 ar 36 ar of 50 000 20 ne 22 ne of 9 8 and maximum 21 ne 22 ne of 0 56 in contrast h 2 o co 2 fluid inclusions have maximum 40 ar 36 ar of only 20 000 and maximum 21 ne 22 ne of 0 14 the use of 40 ar 39 ar methodology to measure k simultaneously with 40 ar 36 ar enables us to show the highest 40 ar 36 ar ratios are representative of the fluids trapped at 2 65 ga and have not been significantly modified by post entrapment 40 ar r production 40 ar r radiogenic 40 ar furthermore the 40 ar 36 ar ratio is correlated with the 21 ne 22 ne and 136 xe 130 xe ratios demonstrating that other heavy noble gas isotope ratios are also close to their original composition the highly radiogenic noble gas isotope signatures are unlikely to have been generated within the greenstone host rocks because seawater altered volcanic rocks have high abundances of non radiogenic atmospheric noble gas isotopes in contrast the strongly radiogenic noble gas isotope signatures are likely to have been generated in ancient basement rocks underlying the greenstones at 2 65 ga the combined noble gas and halogen data are consistent with models in which redox reactions triggered by interaction of independently and distally sourced h 2 o co 2 and ch 4 localized high grade gold mineralization we suggest that the fluids were intimately associated with crustal magmatism in the st ives goldfield and that abiogenic ch 4 could be an important moderator of redox processes in other crustal environments

erosion and sediment transport on steep marly hillslopes draix haute provence france an experimental field study abstract despite a thin regolith layer measured sediment yields indicate denudation rates of more than 10 to 20 mm yr 1 on strongly denuded slopes in black marl terres noires near draix alpes de haute provence overland flow and rainfall simulation experiments were conducted on three slopes in the draix erosional research basin to determine the nature and intensity of hillslope erosion processes the experiments showed that during a single storm sediment transport includes water solid mixtures with a wide range of concentrations from highly charged rill flow up to 400 g l 1 to hyperconcentrated flow up to 800 g l 1 to debris flows up to 1400 g l 1 miniature debris flows mdfs strongly controlled sediment transport as long as the sediment store was not depleted slope surfaces were completely reworked in the winter and the regolith layer was rebuilt temperature records from nine recent winter periods indicate that conditions are favourable for soil frost processes

alteration of oceanic crust and geologic cycling of chlorine and water we report new estimates of transport rates for h2o and cl between the mantle and surface reservoirs our estimates take into consideration alteration of oceanic crust especially that of plutonic rocks and possible subduction of sediments the effect of hydrothermal alteration on the cl budget seems to be negligible but the effect on the h2o budget is significant altered oceanic crust excluding sediments contains about 10 times as much h2o as the unaltered crust and its subduction may result in a net transport of h2o to the upper mantle in subduction zones however the rate of expulsion of h2o from the mantle by subduction zone magmatism is comparable to the amount released by ridge magmatism and is only about 10 of the amount subducted therefore about 90 of the subducted h2o must be returned to the mantle or returned to the crust by other processes in addition subduction of oceanic sediments to mantle depths will result in 1 1 a further increase in the return rate of h2o to the mantle reservoir 2 2 possible net transfer of cl to the mantle depending on the rate of pore water expulsion

the influence of rock resistance on coastal morphology around lord howe island southwest pacific an erratum has been published for this article in earth surface processes and landforms 27 7 2004 931 lord howe island in the northern tasman sea is a remnant of a much larger late miocene basaltic shield volcano much of the island s coastline is exposed to waves that have unlimited fetch but a marked contrast is provided by a fringing coral reef and lagoon that very effectively attenuate wave energy along a portion of the western coastline the geology of the island is varied with hard and resistant basalt lavas breccias and tuffs of intermediate resistance and highly erodible eolianites this variability provides an excellent opportunity to examine the in uence of rock resistance on the development of the spectacular rock coast landforms that occur around the island the hardness of rocks and the extent of weathering around the coastline were assessed using a schmidt hammer and statistical analysis was undertaken to remove outlying values on all but one occasion higher mean rebound values were returned from fresh surfaces than weathered surfaces but only half of these differences were statistically signi cant shore platforms with two distinct levels are juxtaposed along two stretches of coastline and schmidt hammer results lend support to hypotheses that the raised surfaces may be inherited features relative rock resistance was assessed through a combination of schmidt hammer data and measurements of joint density and constrained on the basis of morphological data this approach formed a basis for examining threshold conditions for sea cliff erosion at lord howe island in the context of the distribution of resistant plunging cliffs and erosional shore platforms copyright 2004 john wiley sons ltd

medium term evolution of a gully developed in a loess derived soil field surveys in the belgian loess belt revealed the presence in many forested areas of large permanent gully systems most of which are currently inactive in cultivated areas such gullies can only be observed in cross sectional soil profiles through hollows as virtually all such large gullies are currently infilled with colluvium little is known about the spatial distribution initiation and temporal evolution of these large permanent gully systems on loess derived soils therefore the medium term evolution of a gully initiated in a cultivated area on loess derived soils southwest of leuven belgium in may june 1986 was studied over 13 years two intense rainfall events created this ephemeral gully which was not erased by subsequent tillage between june 1986 and the december 1999 eight field surveys were conducted to measure gully dimensions during two surveys topographic indices e g slope and drainage area were also measured daily rainfall for the measuring period were obtained from a rainfall station located some 10 km southwest of the gully analysis of rainfall data showed that no extreme rainfall event was required to initiate such large permanent gullies as observed in forested areas and through cross sectional profiles in cultivated fields in the belgian loess belt return periods of the event that caused the gully varied between 1 year and 25 years depending on the assumptions used for defining event rain intensity once established length surface area and volume of the studied gully evolved with time cumulative rainfall or cumulative runoff following a negative exponential relation this accords with observations reported for gullies in australia and the usa this study shows that a degressive increase of gully extension can be largely explained by the evolution of a slope drainage area factor s a which is proportional to stream power with time while gully length and gully surface area asymptotically evolve towards a final value gully volume decreased at a given point in time from this it is inferred that sediment deposition will potentially infill the gully to such an extent that the farmer can drive across it from this moment on the combined effect of water and tillage erosion in the gully drainage area will lead towards rapid infilling this expected evolution of a gully in cultivated fields accords with observations of large infilled gully systems in cultivated areas in eastern belgium the permanent gullies observed under forest are attributed to the fact that after severe gully erosion this area was reforested or abandoned therefore the sediment source was cut off and the gully was not filled in by sediment deposition

gully erosion impacts factors and control gully erosion attracts increasing attention from scientists as reflected by two recent international meetings poesen and valentin eds catena 50 2 4 87 564 li et al 2004 gully erosion under global change sichuan science technology press chengu china 354 pp this growing interest is associated with the increasing concern over off site impacts caused by soil erosion at larger spatial scales than the cultivated plots the objective of this paper is to review recent studies on impacts factors and control of gully erosion and update the review on dgully erosion and environmental change importance and research needst poesen et al 2003 catena 50 2 4 91 134 for the farmers the development of gullies leads to a loss of crop yields and available land as well as an increase of workload i e labour necessary to cultivate the land gullies can also change the mosaic patterns between fallow and cultivated fields enhancing hillslope erosion in a feedback loop in addition gullies tend to enhance drainage and accelerate aridification processes in the semi arid zones fingerprinting the origin of sediments within catchments to determine the relative contributions of potential sediment sources has become essential to identify sources of potential pollution and to develop management strategies to combat soil erosion in this respect tracers such as carbon nitrogen the nuclear bomb derived radionuclide 137 cs magnetics and the strontium isotopic ratio are increasingly used to fingerprint sediment recent studies conducted in australia china ethiopia and usa showed that the major part of the sediment in reservoirs might have come from gully erosion gullies not only occur in marly badlands and mountainous or hilly regions but also more globally in soils subjected to soil crusting such as loess european belt chinese loess plateau north america and sandy soils sahelian zone north east thailand or in soils prone to piping and tunnelling such as dispersive soils most of the time the gullying processes are triggered by

alteration patterns linked to high grade gold mineralization at the wattle dam deposit western australia abstract sharp physio chemical gradients occur in archean gold systems yet they are often poorly defined these physio chemical gradients can provide windows into fluids that deposit high grade gold mineralization recognizing different alteration styles and fluid types in mineral systems helps define processes that led to the deposition of high grade gold here we report on the architecture and chemical gradients across the komatiite hosted wattle dam gold deposit in the eastern yilgarn craton western australia which is one of australia s highest grade gold deposits the deposit notably contains millimeter to centimeter thick gold only veins with an absence of quartz whole rock geochemistry shows that high grade gold veins are hosted in komatiite flow margins facies with olivine orthocumulate komatiites in the footwall interbedded graphitic shale close to the lode and komatiite basalt in the hangingwall the komatiite flow margin facies that hosts high grade gold veins have been pervasively altered by stage 1 biotite actinolite stage 2 gold veins cross cut the biotite actinolite altered komatiite and these veins contain thin

nature of ore forming fluid and formation conditions of bif hosted gold mineralization in the archean amalia greenstone belt south africa constraints from fluid inclusion and stable isotope studies abstract orogenic gold mineralization in the amalia greenstone belt is hosted by oxide facies banded iron formation bif hydrothermal alteration of the bif layers is characterized by chloritization carbonatization hematization and pyritization and quartz carbonate veins that cut across the layers the alteration mineral assemblages consist of ankerite ferroan dolomite minerals siderite chlorite hematite pyrite and subordinate amounts of arsenopyrite and chalcopyrite information on the physico chemical properties of the ore forming fluids and ambient conditions that promoted gold mineralization at amalia were deduced from sulfur oxygen and carbon isotopic ratios and fluid inclusions from quartz carbonate samples associated with the gold mineralization microthermometric and laser raman analyses indicated that the ore forming fluid was composed of low salinity h 2 o co 2 composition 3 wt nacl equiv the combination of microthermometric data and arsenopyrite pyrite geothermometry suggest that quartz carbonate vein formation gold mineralization and associated alteration of the proximal bif wall rock occurred at temperature pressure conditions of 300 30 c and 2 kbar thermodynamic calculations at 300 c suggest an increase in f o 2 10 32 10 30 bars and corresponding decrease in total sulfur concentration 0 002 0 001 m that overlapped the pyrite hematite magnetite boundary during gold mineralization although hematite in the alteration assemblage indicate oxidizing conditions at the deposit site the calculated low f o 2 values are consistent with previously determined high fe fe mg ratios 0 7 in associated chlorite absence of sulfates and restricted positive 34 s values in associated pyrite based on the fluid composition metal association and physico chemical conditions reported in the current study it is confirmed that gold in the amalia fluid was transported as reduced bisulfide complexes e g au hs 2 at amalia gold deposition was most likely a combined effect of increase in f o 2 corresponding to the magnetite hematite buffer and reduction in total sulfur contents due to sulfide precipitation during progressive fluid rock interaction the epigenetic features coupled with the isotopic compositions of the ore forming fluid 34 s s 1 8 to 2 3 18 o h2o 6 6 to 7 9 and 13 c c 6 0 to 7 7 at 300 330 c are consistent with an externally deep sourced fluid of igneous signature or and prograde metamorphism of mantle derived rocks

sediment and rock strength controls on river incision into bedrock recent theoretical investigations suggest that the rate of river incision into bedrock depends nonlinearly on sediment supply challenging the common assumption that incision rate is simply proportional to stream power our measurements from laboratory abrasion mills support the hypothesis that sediment promotes erosion at low supply rates by providing tools for abrasion but inhibits erosion at high supply rates by burying underlying bedrock beneath transient deposits maximum erosion rates occur at a critical level of coarse grained sediment supply where the bedrock is only partially exposed fine grained sediments provide poor abrasive tools for lowering bedrock river beds because they tend to travel in suspension experiments also reveal that rock resistance to fluvial erosion scales with the square of rock tensile strength our results suggest that spatial and temporal variations in the extent of bedrock exposure provide incising rivers with a previously unrecognized degree of freedom in adjusting to changes in rock uplift rate and climate furthermore we conclude that the grain size distribution of sediment supplied by hillslopes to the channel network is a fundamental control on bedrock channel gradients and topographic relief

volcanic ash layers facilitate high resolution sequence stratigraphy at convergent plate margins an example from the plio pleistocene forearc basin fill in the boso peninsula japan abstract in this paper a method is presented for the definition of high resolution siliciclastic depositional sequences in onshore outcrop belts in a forearc basin setting and the development of high frequency depositional sequences in the infill of the plio pleistocene kazusa forearc basin in the boso peninsula of japan in terms of the interaction between forearc tectonics and glacio eustasy is discussed volcanic ash layers are commonly intercalated in siliciclastic successions at convergent plate margins and provide physically defined chronostratigraphic markers the mapping of lateral termination of volcanic ash layers together with the interpretation of depositional facies discontinuities permits the identification of onlap offlap and downlap stratal termination patterns in onshore outcrop belts in particular this type of analysis is useful to identify bounding surfaces in offshore muddy sediments the relative abundance of intercalated volcanic ash layers and their associated pyroclastic volcaniclastic fragments in marine siliciclastic successions is interpreted to correspond to temporal variation in the relative rate of supply of coarse grained terrigenous sediments to a marine environment during a relative sea level cycle in particular transgressive stages are characterized by the reduction of active supply of coarse grained terrigenous sediments to a marine elastic environment and this results in the increase of relative abundance of intercalated volcanic ash layers and their associated volcanogenic lithofacies this kind of sediment is hierarchically identified in response to the development of marine flooding surfaces transgressive systems tracts and transgressive sequence sets in general boundaries of depositional sequences and their component building blocks are interpreted to provide a chronostratigraphic framework for local and regional correlations of siliciclastic successions the mapping of volcanic ash layers can identify diachronous evolution of bounding surfaces and component building blocks of a high frequency depositional sequence in relation to spatial variations in sedimentation rates and basin subsidence which are superimposed by eustatic sea level changes

sequence stratigraphy of the upper part of the plio pleistocene kakegawa group western shizuoka japan abstract the kakegawa group is exposed in shizuoka central japan it is the fill of a plio pleistocene fore arc basin and constitutes a third order sequence 2 6 1 0 m y the upper kakegawa group consists of a transgressive systems tract tst and a highstand systems tract hst the tst comprises parasequence sets that are retrogradational on the shelf and progradational on the slope the shelf parasequences consist of upward coarsening facies of tempestites bounded by ravinement surfaces the shelf parasequences can be traced downslope where they comprise upward thinning turbidite successions fourth or fifth order transgression containing backstepping sand dominated intervals and silt dominated upward thickening successions regression the hst consists of progradational parasequence sets the shelf parasequences are similar to those of the tst except for greater basinward distribution of shelf sand outer shelf to upper slope parasequences incorporate silt dominated upward thinning successions transgression and overlying upward thickening turbidite successions regression including sand dominated intervals at their tops the upward thinning successions are much thinner than those in the tst a condensed interval dividing these systems tracts consists of bioturbated silt and might include the maximum flooding surface in the tst parasequences slope sand packets are thicker are present on the lower slope and were formed during fourth or fifth order transgression in contrast the hst slope turbidite sands are thinner are present on the outer shelf to upper slope and were deposited during fourth or fifth order regression during transgression the large volumes of sediment derived from intense shoreface erosion bypassed the narrow shelf ca 10 km wide and contributed to channeling on the upper slope and accumulation of larger packets on the lower slope in the hst deposition of slope turbidites was induced by progradation of shelf sand during regression smaller rate supply of sandy sediment to the slope resulted in deposition of smaller packets the sand might have been deposited at the o ter edge of a wider shelf 15 20 km of the hst

development of badlands and gullies in the sneeuberg great karoo south africa abstract the study aims to examine the origin and development of land degradation with particular emphasis on badland and gully systems in the sneeuberg uplands of the great karoo this is an area of semiarid extensive stock farming where land degradation in the form of rill and gully erosion has accompanied the replacement of grassland by shrub vegetation species diversity has declined and ground cover has been reduced leading to a positive feedback loop which exacerbates the degradation many foot slopes developed in shales clays and colluvium have extensive incipient badland development with closely spaced gullying up to 1 5 m deep in valley bottom and valley side depression locations gullies up to 8 m deep have developed usually cut to bedrock through valley fills of mainly holocene colluvium both badlands and gullies appear to have developed since european settlement and to be part of the same hydrological system with extensive areas of bare ground badlands feeding water to incising gullies experiments using simulated rainfall throw some light on current processes badland areas are active under high frequency low magnitude rainfall events major gullies are likely to be the result of occasional high magnitude events but these have not been observed overgrazing in the past is the most likely cause of the degradation

gilt by association origins of pyritic gold ores in the victory mesothermal gold deposit western australia sulfur isotope data and calculated fluid rock reaction paths constrain the origin of pyritic gold ores in the victory mesothermal gold deposit of western australia pyrite 34s values range from 4 4 to 5 1 and are negatively correlated with gold tenor in mafic host rocks indicating a significant increase in fluid oxidation state accompanied mineralization geologic evidence argues strongly that mineralization occurred with wall rock sulfidation but this process alone cannot produce the degree of fluid oxidation necessary to explain the sulfur isotope range in contrast either carbonation of ferric iron bearing minerals or separation of immiscible fluid phases both of which are consistent with the available geologic data can drive fluid oxidation to the point of aqueous sulfate dominance coupled decreases in gold solubility and sulfide sulfur isotope values that are predicted by such a scenario provide an explanation for the trend between 34s and gold tenor observed at victory because both wall rock carbonation and fluid phase separation involve loss of aqueous sulfide but do not require precipitation of iron sulfides these processes may explain the poor correlation between gold tenor and pyrite abundance in many mesothermal deposits and the occurrence of some gold ores in iron poor rocks

formation of gold deposits a metamorphic devolatilization model a metamorphic devolatilization model can explain the enrichment segregation timing distribution and character of many goldfields such as those found in archean greenstone belts slate belts and other gold only provinces in this genetic model hydrated and carbonated greenschist facies rocks particularly metabasic rocks are devolatilized primarily across the greenschist amphibolite facies boundary in an orogenic setting devolatilization operates on the scale of individual mineral grains extracting not just h2o and co2 but also s and in turn au elevated gold in solution is achieved by complexing with reduced s and by h2co3 weak acid buffering near the optimal fluid ph for gold solubility the buffering is more important than being at the point of maximum gold solubility low salinity ensures low base metal concentrations in the auriferous metamorphic fluid migration of this fluid upwards is via shear zones and or into hydraulic fracture zones in rocks of low tensile strength the geometry of the shear zones dictates the kilometre scale fluid migration paths and the degree of fluid focusing into small enough volumes to form economic accumulations of gold deposition of gold from solution necessitates breakdown of the gold thiosulphide complex and is especially facilitated by fluid reduction in contact with reduced carbon bearing host rocks and or by sulphidation of wallrocks to generate iron bearing sulphide and precipitated gold as such black slate carbon seams banded iron formation tholeiitic basalt magnetite bearing diorite and differentiated tholeiitic dolerite sills are some of the important hosts to major goldfields gold deposition is accompanied by carbonation sulphidation and muscovite biotite alteration where the host rock is of suitable bulk composition the correlation of major gold deposits with rock type even when the gold is primarily in veins argues for rock dominated depositional systems not fluid dominated ones as a consequence a general role in gold deposition for fluid mixing temperature decrease and or fluid pressure decrease and boiling is unlikely although such effects may be involved locally several geological features that are recorded at gold only deposits today reflect subsequent modifications superimposed upon the products of this generic metamorphic devolatilization process overprinting by higher grade metamorphism and deformation and or palaeo weathering may provide many of the most obvious features of goldfields including their mineralogy geochemistry geometry small scale timing features geophysical response and even mesoscopic gold distribution

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