lab scale ash production by abrasion and collision experiments of porous volcanic samples in the course of explosive eruptions magma is fragmented into smaller pieces by a plethora of processes before and during deposition volcanic ash fragments smaller than 2 mm has near volcano effects e g increasing mobility of pdcs threat to human infrastructure but may also cause various problems over long duration and or far away from the source human health and aviation matters we quantify the efficiency of ash generation during experimental fracturing of pumiceous and scoriaceous samples subjected to shear and normal stress fields experiments were designed to produce ash by overcoming the yield strength of samples from tenerife canary islands spain sicily and lipari islands italy with this study having particular interest in the 355 m fraction fracturing within volcanic conduits plumes and pyroclastic density currents pdcs was simulated through a series of abrasion shear and collision normal experiments an understanding of these processes is crucial as they are capable of producing very fine ash 10 m these particles can remain in the atmosphere for several days and may travel large distances 1000s of km this poses a threat to the aviation industry and human health from the experiments we establish that abrasion produced the finest grained material and up to 50 of the generated ash was smaller than 10 m in comparison the collision experiments that applied mainly normal stress fields produced coarser grain sizes results were compared to established grain size distributions for natural fall and pdc deposits and good correlation was found energies involved in collision and abrasion experiments were calculated and showed an exponential correlation with ash production rate projecting these experimental results into the volcanic environment the greatest amounts of ash are produced in the most energetic and turbulent regions of volcanic flows which are proximal to the vent finest grain sizes are produced in pdcs and can be observed as co ignimbrite clouds above density currents finally a significant dependency was found between material density and the mass of fines produced also observable in the total particle size distribution higher values of open porosity promote the generation of finer grained particles and overall greater ratios of ash while this paper draws on numerous previous studies of particle comminution processes it is the first to analyze and compare results of several comminution experiments with each other in order to characterize these mechanisms

chronology of deformation within the turbidite dominated lachlan orogen implications for the tectonic evolution of eastern australia and gondwana ar ar data from fabric forming white mica in slates syntectonic quartz veins and granitic mylonites constrain the timing of metamorphism deformation and exhumation in the lachlan orogen australia these data also help define the tectonic evolution of the tasmanides during paleozoic time the lachlan orogen formed by the progressive accretion of a thick turbidite fan sequence and volcanic terrains to gondwana during the closing of a small marginal ocean or back arc basin this tectonic setting was similar to the present western and southwestern pacific region accretion of the lachlan orogen to gondwana occurred by closing of the basin system by subduction accretion processes and some translation the process is typified in the western lachlan orogen by a major eastward migrating deformation involving chevron folding and faulting over an eastward propagating decollement melange zone and is recorded by ar ar mica ages ranging from 455 ma in the western part to 390 ma in the eastern part in the central lachlan orogen deformation migrated southwestward from 440 430 to 405 ma away from the high temperature wagga omeo metamorphic complex where deformation metamorphism occurred between 440 and 400 ma in the north 400 ma mica ages record deformation and inversion of structures in the cobar basin in the eastern lachlan orogen ar ar mica dates range from 450 to 340 ma ages of 455 445 ma are yielded by the narooma accretionary complex 405 390 ma ages are found along the major thrust faults bounding high grade metamorphic complexes and 360 340 ma cooling ages are found in the inverted extensional basins e g hill end and related structural zones the ar ar results also document periods of reactivation on early formed structures during later deformation elsewhere in the orogen

patterns of deposition from experimental turbidity currents with reversing buoyancy turbidity currents are turbulent sediment laden gravity currents which can be generated in relatively shallow shelf settings and travel downslope before spreading out across deep water abyssal plains because of the natural stratification of the oceans and or fresh water river inputs to the source area the interstitial fluid within which the particles are suspended will often be less dense than the deep water ambient fluid consequently a turbidity current may initially be denser than the ambient sea water and propagate as a ground hugging flow but later reverse in buoyancy as its bulk density decreases through sedimentation to become lower than that of the ambient sea water when this occurs all or part of the turbidity current lofts to form a buoyant sediment laden cloud from which further deposition occurs deposition from such lofting turbidity currents containing a mixture of fine and coarse sediment suspended in light interstitial fluid is explored through analogue laboratory experiments complemented by theoretical analysis using a box and cloud model particular attention is paid to the overall deposit geometry and to the distributions of fine and coarse material within the deposit a range of beds can be deposited by bimodal lofting turbidity currents lofting may encourage the formation of tabular beds with a rapid pinch out rather than the gradually tapering beds more typical of waning turbidity currents lofting may also decouple the fates of the finer and coarser sediment depending on the initial flow composition the coarse fraction can be deposited prior to or during buoyancy reversal while the fine fraction can be swept upwards and away by the lofting cloud an important feature of the results is the non uniqueness of the deposit architecture different initial current compositions can generate deposits with very similar bed profiles and grading characteristics highlighting the difficulty of reconstructing the nature of the parent flow from field data it is proposed that deposit emplacement by lofting turbidity currents is common in the geological record and may explain a range of features observed in deep water massive sands thinly bedded turbidite sequences and linked debrites depending on the parent flow and its subsequent development for example a lofting flow may lead to a well sorted largely ungraded or weakly graded bed if the fines are transported away by the cloud however a poorly sorted largely ungraded region may form if during buoyancy reversal high local concentrations and associated hindered settling effects develop at the base of the cloud

toward an understanding of the metallogeny of the tasman fold belt system the tasman fold belt system which makes up the eastern third of the australian continent is a composite of five paleozoic orogenic belts the major mineral provinces of the tasman fold belt system include provinces of volcanic hosted massive sulfide deposits porphyry granitoid associated cu au and au deposits and granitoid associated sn and w deposits as well as provinces of structurally controlled au and cu pb zn deposits the localization of these provinces is discussed in terms of the nature of the magmatism which is a source of heat and in many cases of fluids and metals the fracture systems which facilitate the transfer of heat magmas and or aqueous fluids through the crust and the timing and nature of the stress regimes that permit these fracture systems to be open to fluid flow the available data indicate that the magmatism of the cu au province of central west new south wales originated in the mantle magma systems linked with the deposits of the volcanic hosted massive sulfide province of western tasmania the sn w provinces of the tasman fold belt system and the porphyry associated au deposits of north queensland had their roots in crust mantle interactions crystal fractionation was an important process in the magma systems related to the development of these provinces a feature of the tasman fold belt system is the prevalence of major north south trending fault systems more subtle west northwest structural trends can be appreciated at the continent scale and these proterozoic structural trends appear to have exerted a significant control on the distribution of major mineral provinces within the tasman fold belt system extension reactivation of basement structures late in tectonic cycles was a major mechanism for creating permeable domains in the upper crust and focusing fluid flow and hydrothermal mineralization it is suggested that the permo carboniferous au and sn provinces of north queensland developed

recognizing unsteadiness in the transport systems of dilute pyroclastic density currents laboratory density currents generated with unsteady source conditions provide insight into the distances and timescales over which unsteadiness at the vent should persist in natural dilute pyroclastic density currents pdcs the laboratory experiments comprise heated 20 m talc particles turbulently suspended in air and introduced as density currents into an 8 5 6 2 6 m air filled chamber the densimetric and thermal richardson froude stokes and settling numbers are all similar to those of natural dilute pdcs although the experiments reynolds numbers are several orders of magnitude less than natural pdcs the experiments are fully turbulent and are thus dynamically similar to some dilute natural pdcs unsteadiness in the experiments is generated by adding two pauses of duration t to the eruption e g a 100 s experiment comprising three 33 s pulses separated by two pauses of t 10 s propagation distance of the leading head is proportional to the square root of time positions of the trailing pulses have similar time dependence but they generally travel slightly faster than and thus catch and merge with the leading current trailing pulses are more easily distinguished from the body of the preceding current when t is large analysis of turbulent structures through space and time shows that unsteadiness at the eruption source can be distinguished from turbulent fluctuations in the currents when t is greater than the integral turbulent timescale of the current body body a statistical measure of the characteristic timescale of unsteadiness within a turbulent flow the distances over which unsteadiness in the transport system persist scale with t body and the ratio of the thermal richardson numbers of the trailing and leading pulses as t body increases from 2 to 4 trailing pulses remain distinct over an increasing fraction of the lead runout distance when subsequent pulses have higher rit than the leading current they coalesce with the leading pulse at more proximal distances but when rit is much lower they remain distinct over distances similar to that of the leading head even when t is similar to body for currents that have depositional timescales comparable to or shorter than body deposits should be expected to preserve a record of unsteadiness over the distances that unsteady pulses persist

martian landslides in valles marineris wet or dry the objective of this paper is to determine whether martian landslides in valles marineris were wet or dry and place constraints on the availability of liquid water in valles marineris during the amazonian when the landslides occurred we thus statistically compare the power law relationship between the volume and runout distance of landslides on earth with those in valles marineris mars the exponent of the power law for martian landslides is similar to that for dry landslides and volcanic flows on earth and differs significantly from wet debris flows on earth the constant of proportionality in the observed power law relationship for martian flows is linearly proportional to gravity as predicted from physical modeling of dry flows in which the dissipation occurs in a layer of uniform thickness conversion of gravitational potential energy to heat is insufficient to generate more than a few weight percent of liquid water in the landslide we thus conclude that water did not significantly influence the dynamics of landslides in valles marineris this implies predominantly dry conditions in valles marineris during the amazonian

end permian to mid triassic termination of the accretionary processes of the southern altaids implications for the geodynamic evolution phanerozoic continental growth and metallogeny of central asia the altaids is one of the largest accretionary orogenic collages in the world with the highest rate of phanerozoic continental growth and significant metallogenic importance it is widely accepted that subduction related orogenesis of the altaids started in the late precambrian and gradually migrated southward present coordinates however it is uncertain when and how the building of the altaids was finally completed based on structural geology geochemical geochronological and paleomagnetic data this paper presents late paleozoic to early mesozoic accretionary tectonics of two key areas north xinjiang in the west and inner mongolia in the east together with neighboring mongolia the late paleozoic tectonics of north xinjiang and adjacent areas were characterized by continuous southward accretion along the wide southern active margin of siberia and its final amalgamation with the passive margin of tarim which may have lasted to the end permian to early mid triassic in contrast in inner mongolia and adjacent areas two wide accretionary wedges developed along the southern active margin of siberia and the northern active margin of the north china craton which may have lasted to the mid triassic the final products of the long lived accretionary processes in the southern altaids include late paleozoic to permian arcs late paleozoic to mid triassic accretionary wedges composed of radiolarian cherts pillow lavas and ophiolitic fragments and high pressure ultrahigh pressure metamorphic rocks permian alaskan type zoned mafic ultramafic complexes intruded along some major faults of the tien shan we define a new tarim suture zone immediately north of the tarim craton that is probably now buried below the tien shan as a result of northward subduction of the tarim block in the cenozoic the docking of the tarim and north china cratons against the southern active margin of siberia in the end permian to mid triassic resulted in the final closure of the paleoasian ocean and terminated the accretionary orogenesis of the southern altaids in this part of central asia this complex geodynamic evolution led to formation of giant metal deposits in central asia and to substantial continental growth

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

a crucial role for slab break off in the generation of major mineral deposits insights from central and eastern australia in the lachlan fold belt of southeastern australia major orogenic gold and porphyry gold copper deposits formed simultaneously within distinct tectonic settings during a very short time interval at ca 440 ma the driving mechanism that controlled the temporal coincidence of these deposits remains largely unexplained a review of contemporaneous metallogenic tectonic magmatic and sedimentological events in central and eastern australia reveals that a change in subduction dynamics along the australian sector of the early palaeozoic circum gondwana mega subduction system could have influenced lithospheric stress conditions far inboard of the subduction margin the magnitude of ore formation and the spatial extent of related events are proposed in this paper to have been controlled by the interplay of mantle processes and lithospheric changes that followed slab break off along a portion of the mega subduction system surrounding gondwana at that time slab break off after subduction lock up caused mantle upwelling that in turn provided an instantaneous heat supply for magmatic and hydrothermal events coincident reorganisation of lithospheric stress conditions far inboard of the proto pacific margin of australia controlled reactivation of deep lithospheric fault structures these fault systems provided a pathway for fluids and heat fuelled by mantle upwelling into the upper lithosphere and caused the deposition of 440 ma gold deposits in the lachlan fold belt as well as a range of metallogenic tectonic and sedimentary changes elsewhere in central and eastern australia

tectonothermal history of the western lachlan fold belt australia insights from white mica studies detailed b lattice parameter and illite crystallinity ic studies of k white micas in slates from the stawell and ballarat bendigo zones sz bbz in the western lachlan fold belt of victoria australia reveal a metamorphic pattern characterized by regional metamorphism associated with crustal thickening and younger contact metamorphism accompanied by deformation the ic data indicate that rocks regionally metamorphosed prior to the intrusion of the early and late devonian granitoids vary in grade from epizonal greenschist facies to diagenetic zeolite facies and that most are of epizonal to anchizonal prehnite pumpellyite facies grade in the bbz a decrease in grade from west to east occurs across fault zones ic values show little change indicating that limited vertical displacement has occurred this is in accord with the thin skinned deformation model proposed for the western lachlan fold belt the b lattice parameters x 9 022 a n 137 s n 0 009 indicate baric conditions intermediate between those of new hampshire p al 2 sio 5 triple point and otago intermediate p thus a moderately low geother mal gradient existed 450 430 ma ago when these rocks were deformed k d fe mg actinolite fe mg chlorite values 0 52 0 70 obtained from coexisting actinolite and chlorite in metabasites from fault zones support the moderately high p c 4 kbar metamorphism suggested by the b cell parameter values the metamorphic conditions indicated by these data are contrary to the low p high t conditions pro posed by previous authors who inferred an intimate association between deformation granitoid intrusion and gold mineralization the b lattice parameter of white micas in slates adjacent to early devonian c 400 ma granitoids with schist bearing aureoles in the north eastern part of the bbz x 9 002 a n 27 s n 0 007 indicate pressures in the order of c 2 5 kbar which are in accord with those obtained from andalusite cordierite and zoisite garnet bearing assemblages observed in the higher grade metapelitic and calcareous rocks this contrasts with the higher pressure c 4 kbar existing during regional metamor phism and implies that c 6 5 8 km of metasedimentary rocks in the bbz were removed before the emplacement of the early devonian granitoids metamorphic assemblages in hornfelses associated with late devonian granitoids indicate a further 5 6 km of metasediment were removed in the next 40 ma prior to their emplacement this study shows the value of white mica studies in elucidating the tectono thermal history of a low grade metamorphic terrane dominated by metapelitic rocks

late collisional granites in the variscan erzgebirge germany the late collisional erzgebirge granites 325 318 ma were introduction emplaced at shallow crustal levels in the variscan metamorphic the erzgebirge is a ne sw trending antiformal structure basement shortly after large scale extension caused by orogenic exposing variscan crystalline basement rocks at the northcollapse these granites comprise mildly peraluminous transitional ern margin of the bohemian massif the region is famous i s types and strongly peraluminous s type rocks which can be among economic geologists for its extensive and highly subdivided into three major groups low f biotite granites low f variable metallic ore deposits which have been mined two mica granites and high f high p2o5 li mica granites the since the middle ages e g stemprok seltmann 1994 highest degree of differentiation is reached in the li mica granites tischendorf forster 1994 and for the classic studies which exhibit strongly elevated concentrations of p f li rb cs in the fields of economic geology mining and metallurgy ta sn w and u but very low ti mg co ni sr ba y zr chemistry and mineralogy e g werner 1791 breithaupt hf th and rare earth elements crystal melt fractionation is the 1849 agricola 1974 dominant process controlling the bulk composition of all groups of as in many of the other variscan provinces in central granites however metasomatic processes involving late stage residual europe granitic rocks make up a large proportion of the melts and high t orthomagmatic fluids became increasingly more exposed rocks in the erzgebirge and a great variety of important in highly evolved units and have modified the abundances compositional and textural types occur fig 1 knowof mobile elements p f li rb cs ba sr in the li mica ledge of the processes involved in the generation and granites particularly isotopic and geochemical characteristics suggest evolution of these silicic magmas is essential for underthat the three granite groups cannot be derived from a common standing heat and mass transport during the variscan precursor magma their discrete compositions are source related orogeny and also for the formation of metallic ore and are attributed to melting of quartzo feldspathic and pelitic deposits several lines of evidence indicate that early crustal lithologies in different proportions granites are common in high temperature sn w deposits are associated with the the central european variscides but the erzgebirge is unusual for emplacement of highly evolved volatile rich granitic the predominance of evolved li mica granites associated with melts tischendorf 1986 forster tischendorf 1992 economically important sn w and u deposits the abundance of stemprok 1993 the role of the granites in formation of li mica granites is attributed to a combination of favourable factors later permian u pb zn and younger ore mineralization 1 low degrees of anatectic melting of crustal protoliths 2 wide f ba is controversial but good evidence exists that the distribution of fertile lithologies rich in large ion lithophile elements vein type u deposits owe their uranium to leaching from and ore elements 3 extended magmatic differentiation by crystal the uraninite rich granites e g tischendorf forster melt fractionation and subsequent autometasomatism 1994

eruption fed particle plumes and volcaniclastic deposits at a submarine volcano nw rota 1 mariana arc 1 nw rota 1 is an active submarine volcano in the mariana arc with a summit depth of 517 m and an explosively erupting volcanic vent southwest of the summit at a depth of 530 560 m during a period of ongoing explosive eruptions particle plumes surrounded the volcano and at least 3 3 107 m3 of volcaniclastic material was deposited on the southern flank particle plumes over the summit were magmatic hydrothermal in origin characterized by 3he enrichment hydrothermal precipitates and low ph values plumes at multiple depths below the summit surrounded the volcano and were composed overwhelmingly of fresh glassy shards of basalt rare anhydrite particles were present but there was a complete absence of other hydrothermal components in the deep plume samples these short lived anhydrite particles indicate the source of the deep plumes is from within or very near the eruptive vent and the mechanism for transport down the flanks of the volcano must be far faster than settling of individual particles the deep plumes most likely originated from sediment gravity flows generated by explosive eruptions or slope failure and landslides of unstable materials that had accumulated near the eruptive vent suspended sediments detach from the volcano slopes at multiple depths and are transported laterally up to tens of kilometers where they contribute to fall out deposits in distal sediments these observations link mechanisms for the transport of volcanic ash in the submarine environment to the types of deposits common in volcaniclastic aprons and fine ash layers in distal sediments

a box model for non entraining suspension driven gravity surges on horizontal surfaces the propagation of and the deposition from a turbulent gravity current generated by the release of a finite volume of a dense particle suspension is described by a box model the approximate model consists of a set of simple equations a predetermined depth dependent leading boundary condition and one experimentally determined parameter describing the trailing boundary condition it yields predictions that agree well with existing laboratory observations and more complex theoretical models of non eroding non entraining suspension driven flows on horizontal surfaces the essential features of gravity surge behaviour have been observed and are captured accurately by the box model these include the increased rate of downstream loss of flow momentum with increased particle setting velocity the existence of maxima in the thickness of proximal deposits and the downstream thinning of distal deposits our approximation for the final run out distance xr of a surge in deep water is given by xr3 g oq3o w2s 1 5 where g o is the initial reduced gravity of the surge qo the initial two dimensional volume and ws the average settling velocity of the particles in the suspension a characteristic thickness of the resulting deposit is given by oqo xr where oo is the initial volumetric fraction of sediment suspended in the surge our analysis provides additional insight into other features of gravity surge dynamics and deposits including the potential for the thickening of currents with time the maintenance of inertial conditions and the potential for strong feedback in the sorting of particle sizes in the downstream direction at travel distances approaching xr box model approximations for the evolution of gravity surges thus provide a useful starting point for analyses of some naturally occurring turbidity surges and their deposits

porphyry gold copper mineralisation in the cadia district eastern lachlan fold belt new south wales and its relationship to shoshonitic magmatism the cadia porphyry gold copper district is the largest hydrothermal intrusion related gold deposit in eastern australia discovered in 1992 by newcrest mining limited pre mine resources in the district were in excess of 585 t au and 2 35 mt cu the cadia district lies within shoshonitic subaqueous volcanic rocks of the late ordovician molong volcanic belt in the eastern lachlan fold belt lfb of new south wales an island arc tectonic setting is envisaged for the formation of these volcanic rocks but the nature of the arc basement and geometry of the subduction zone s is debated mineralisation occurs in four principal porphyry deposits cadia hill cadia ridgeway cadia east cadia far east and cadia quarry as sheeted and stockwork quartz sulfide veins and locally as broadly stratabound disseminated mineralisation cadia east and skarn big cadia and little cadia all of the porphyry deposits show a close spatial association with shoshonitic monzodiorite to quartz monzonite dykes and stocks of the cadia intrusive complex cic gold copper mineralisation is hosted by these intrusions and also by the enclosing forest reefs volcanics frv wall rocks hydrothermal alteration associated with mineralisation is potassic which is overprinted by selectively pervasive propylitic and silica albite assemblages petrological studies and major and trace element analysis of rocks of the cic and frv as well as of other intrusions in the cadia district that appear unrelated to mineralisation have been conducted unaltered samples from the cic are characterised by high k2o contents up to 6 5 wt and molecular k na ratios consistently 1 confirming the alkalic and shoshonitic nature of the complex the cic samples also fall just within the shoshonite field on ce yb ta yb and th yb ta yb plots although immediately adjacent to the calc alkaline field the frv are geochemically similar to the cic although lower in p2o5 and ce lree dacitic and hornblende porphyry intrusions from within the district but spatially unrelated to mineralisation have a distinctive geochemical signature and are not shoshonitic in composition the close spatial association between gold copper mineralisation and the shoshonitic monzonitic cic argues strongly for a genetic link between the two a similar association occurs at the economic goonumbla porphyry gold copper deposits in the eastern lfb while sub economic gold copper mineralisation at cargo and copper hill is associated with calc alkaline quartz diorite and dacitic intrusions the cadia district is considered to be an excellent example of the association of alkaline potassic magmas and gold copper porphyry style mineralisation

the wyoming gold deposits volcanic hosted lode type gold mineralisation in the eastern lachlan orogen australia the eastern lachlan orogen in southeastern australia is noted for its major porphyry epithermal skarn copper gold deposits of late ordovician age whilst many small quartz vein hosted or orogenic lode type gold deposits are known in the region the discovery of the wyoming gold deposits has demonstrated the potential for significant lode type mineralisation hosted within the same ordovician volcanic stratigraphy outcrop in the wyoming area is limited with the ordovician sequence largely obscured by clay rich cover of probable quaternary to cretaceous age with depths up to 50 m regional aeromagnetic data define a north south trending linear belt interpreted to represent the ordovician andesitic volcanic rock sequence within probable ordo silurian pelitic metasedimentary rocks drilling through the cover sequence in 2001 to follow up the trend of historically reported mineralisation discovered extensive alteration and gold mineralisation within an andesitic feldspar porphyry intrusion and adjacent volcaniclastic sandstones and siltstones subsequent detailed resource definition drilling has identified a substantial mineralised body associated with sericite carbonate albite quartz chlorite pyrite arsenopyrite alteration the wyoming deposits appear to have formed as the result of a rheological contrast between the porphyry host and the surrounding volcaniclastic rocks with the porphyry showing brittle fracture and the metasedimentary rocks ductile deformation the mineralisation at wyoming bears many petrological and structural similarities to orogenic lode style gold deposits although the timing of alteration and mineralisation in the wyoming deposits remain problematic a relationship with possible early to middle devonian deformation is considered likely

rare earth element and neodymium isotope tracing of sedimentary rock weathering abstract chemical weathering plays an important role in sequestering atmospheric co2 but its potential influence on global climate over geological timescales remains debated to some extent this uncertainty arises from the difficulty in separating the respective contribution of sedimentary and crystalline silicate rocks to past weathering rates in the geological record two types of rocks having presumably different impact on the long term carbon cycle in this study we investigate the use of rare earth element ree and neodymium isotopes end in leached iron oxide fractions of river sediments for tracing the origin of weathered rocks on continents a new index called concavity index ci is defined for measuring the degree of mid ree enrichment in geological samples which enables the determination of the source of iron oxides in sediments such as seawater derived fe oxyhydroxide phases ancient marine fe oxides derived from the erosion of sedimentary rocks and recent secondary oxides formed in soils via alteration of crystalline silicate rocks or pyrite oxidation using this index we demonstrate that the end difference between paired fe oxide and detrital fractions in river sediments defined here as end feox det directly reflects the relative contribution of sedimentary versus crystalline silicate rocks during weathering while rivers draining old cratons and volcanic provinces display near zero end feox det values indicative of dominant silicate weathering 0 5 1 1 n 30 multi lithological catchments hosting sedimentary formations yield systematically higher values 2 7 1 2 n 44 showing that sedimentary rock weathering can be traced by the occurrence of riverine fe oxides having more radiogenic nd isotope signatures compared to detrital fractions this assumption is reinforced by the evidence that calculated end feox det values agree well with previous estimates for carbonate and silicate weathering rates in large river basins examining the influence of climate and tectonics on measured nd isotopic compositions we find that end feox det is strongly dependent on temperature in lowlands following an arrhenius like relationship that reflects enhanced alteration of silicate rocks and formation of secondary fe oxides in warmer climates in contrast in high elevation catchments end feox det defines striking correlation with maximum basin elevation which we also interpret as reflecting the intensification of silicate weathering and associated fe oxide formation as elevation decreases due to the combined effects of thicker soils and warmer temperature overall our new findings are consistent with previous assertions that the alteration of sedimentary rocks prevails in high elevation environments while silicate weathering dominates in floodplains this novel approach combining ree and nd isotopes opens new perspectives for disentangling the weathering signals of sedimentary and crystalline silicate rocks in the geologic record which could be used in future studies to reassess the causal relationships between mountain uplift erosion and climate throughout earth s history

timing of gold mineralisation in western and central victoria australia new constraints from shrimp ii analysis of zircon grains from felsic intrusive rocks abstract age constraints provided by felsic intrusive rocks indicate that gold mineralisation in western and central victoria australia occurred over a prolonged period of time ranging from prior to the late silurian to the late devonian an early phase of gold mineralisation associated with regional deformation pre dated the emplacement of late silurian felsic porphyry dykes at 413 3 ma in the central lode system of the magdala deposit stawell gold mineralisation associated with the final stages of the main regional deformation also pre dated the intrusion of early ca 400 ma to late devonian ca 370 ma plutons at stawell wonga deposit mt bute tarnagulla tallangalook maldon and heathcote in both the stawell pyrenees and ballarat metallogenic provinces consistent with preliminary dating studies of hydrothermal micas from major gold deposits later phases of gold mineralisation throughout western and central victoria were either temporally associated with or post dated the intrusion of late silurian to late devonian felsic dykes examples of this style of mineralisation include the wonga deposit at stawell mafeking fosterville maryborough minor occurrences near linton and the woods point region preliminary constraints from ballarat west indicate that late devonian dykes intruded at ca 386 ma were pervasively altered by interaction with an auriferous fluid similar to that responsible for gold deposition within the guiding star lode but that this was probably related to remobilisation during emplacement of the dykes into a pre existing mineralised structure with the exception of the woods point region and ballarat west these later phases often have a magmatic affiliation indicated by an association with cu mo bi sb and w these complex timing relationships reflect the diachronous nature of deformation metamorphism and magmatism in the western lachlan fold belt of southeast australia and suggest at least two discrete periods of gold mineralisation in the region

timing of gold and crustal evolution of the palaeozoic south central andes nw argentina implications for the endowment of orogenic belts abstractmajor and trace element concentrations and nd isotope compositions of ordovician volcano intrusive rocks in the puna regionof northwestern argentina suggest formation in a pericratonic setting above thickened and evolved continental crust input fromjuvenile sources was limited and there is no evidence for oceanic crust or the presence of a terrane bounding suture in the studyregion geochronological constraints from 40 ar 39 ar sericite rich whole rock and re os arsenopyrite pyrite dating indicateinitial deposition of gold in the turbidite dominated sierra de rinconada during orogenic events in the late ordovician this periodcorresponds to an epoch of major continental growth that includes development of abundant gold deposits along the long lived andextensive accretionary margins of gondwana the ca 440 ma mineralisation ages represent the oldest recorded timing of goldemplacement in the central andes these ages also coincide with ca 440 ma ages obtained for the timing of mineralisation in eachof the four largest orogenic gold deposits in the western lachlan orogen of eastern australia in contrast to analogous terraneselsewhere in the central andes e g peru and bolivia however the palaeozoic succession in northwest argentina hosts relativelyminor gold mineralisation possible explanations for the lack of a more substantial gold endowment include the i specific tectonicevolution of this region ii absence of hydrated oceanic substrate in the northern puna iii lack of first order trans crustalconduits iv restriction of possible asthenospheric upwelling to a short period in the early ordovician and v relatively limitedextent of crustal shortening and thickening during ordovician and subsequent orogenic events when compared to analogouspalaeozoic metamorphic terrains elsewhere that are characterised by substantial orogenic gold endowment 2006 elsevier b v all rights reserved

possible intrusion related gold systems in the western lachlan orogen southeast australia several gold deposits occurring in the western lachlan orogen have geological geochemical and geochronological characteristics that distinguish them from typical vein hosted orogenic gold deposits of the central victorian gold province the latter are responsible for more than 90 percent of primary hard rock gold production from this region and are generally considered to represent the only economically significant type of gold deposit in the western lachlan orogen atypical gold occurrences at malmsbury myrtle creek mount piper and the wonga deposit in the stawell goldfield are characterized by a close spatial and temporal association with posttectonic felsic intrusions disseminated to stockwork style mineralization alteration dominated by sericitization sulfidation silicification carbonatization and tourmalinization and associated complex au mo w bi te cu the deposits have a number of features in common with intrusion related gold deposits elsewhere in phanerozoic orogenic belts although production from this type of gold mineralization in the western lachlan orogen has been small compared to orogenic gold deposits the possible existence of intrusion related gold deposits has potentially important implications for exploration in this region and also provides significant clues to the tectonic framework and paleozoic metallogeny of eastern australia

types geological features and geodynamic significances of gold copper deposits in the kanggurtag metallogenic belt eastern tianshan nw china occupying the middle of the central asia paleozoic accretionary and collisional orogenic belt the eastern tianshan area has a great economic potential due to au cu mineralization during syn and post orogenic events in the kanggurtag au cu metallogenic belt three major types of gold deposits have been recognized ductile shear zone hosted gold deposits magmatic hydrothermal gold deposits and epithermal gold deposits four kinds of copper deposits have also been identified recently the porphyry type the skarn type the magmatic type and volcanic sedimentary type tectonically the development of these late paleozoic gold and copper deposits was closely associated with the subduction and collision of the ancient tianshan ocean that intervened between the tarim craton and the siberian block in the early to mid carboniferous n dipping subduction beneath the dananhu arc generated magmatic intrusions leading to formation of the porphyry cu deposits the magmatic front migrated southward to form the yamansu arc upon the kanggurtag accretionary wedge in the latest carboniferous to early permian during the closure of the ancient tianshan ocean large mafic ultramafic complexes were emplaced resulting in several magmatic copper nickel deposits gold deposits of the shear zone type are controlled by the kanggurtag ductile shear zone which is related to collisional orogenesis the epithermal gold deposits are associated with extensional tectonics and post tectonic volcanic activity the tectonic settings geological features and temporal and spatial distributions of these different types of gold and copper deposits reflect to a great extent the accretionary and collisional tectonics that occurred between the northern margin of the tarim block and the southern margin of the siberian block

magmatic and hydrothermal history of the porphyry style deposits of the ardlethan tin field new south wales australia the cassiterite sulfide deposits of the ardlethan tin field are classified as porphyry style sn deposits on the basis of the intimate spatial and temporal relationships of the breccia pipes and sn deposits with the ardlethan granite and in particular the mine porphyry the continuum of salinity and temperature conditions from high temperature and high salinity fluids in the ardlethan granite to lower temperature and less saline fluids in the breccia pipes in the mine granite and the isotopic character of the ore fluids which is consistent with a magmatic fluid source characterized by primitive isotopic signatures for carbon and sulfur the deposits occur in breccia pipes in the siluro devonian mine granite 417 or 2 5 ma close to its contacts with the ardlethan granite 410 or 2 5 ma and ordovician metasediments the ardlethan granite and related phases are moderately to strongly fractionated and enriched in rb th u nb y li as ta w f and sn tourmaline nodules occur within the ardlethan granite and halite bearing fluid inclusions found in quartz from the nodules homogenized between 250 degrees and 485 degrees c one of the most fractionated phases of the ardlethan granite the mine porphyry was emplaced into the major breccia pipe of the mine granite following the onset of brecciation brecciation and mineralization are linked to the release of magmatic volatiles and saline brines from the ardlethan granite the vug filling minerals in all the deposits show a paragenetic sequence from cassiterite wolframite arsenopyrite milky quartz and tourmaline to pyrite chalcopyrite sphalerite galena tourmaline and clear quartz to late stage toothy quartz fluorite and cookeite fluid inclusion data from quartz show that the deposition of milky quartz and cassiterite occurred between 300 degrees and 370 degrees c sulfide and clear quartz between 210 degrees and 2800c and toothy quartz fluorite and cookeite between 105 degrees and 245 degrees c fluid salinities ranged

surface runoff features on mars testing the carbon dioxide formation hypothesis 1 fresh gully like features on mars strongly suggest that fluid flowed on the surface in the recent past here we consider the possibility that co2 vapor supported flows formed the gullies we find that neither condensed co2 nor co2 clathrate hydrate are likely to accumulate in significant quantities in the martian crust in addition if condensed co2 were present under lithostatic pressures exposure to the atmosphere would produce features analogous to terrestrial pyroclastic flows not surface runoff finally the source volume of co2 required to support a flow excludes clustered or episodic gully formation therefore we conclude that co2 cannot have formed the gullies in light of these results liquid water flow remains the preferred formation mechanism for the recent surface runoff features

tectonic and economic implications of trace element 40ar 39ar and sm nd data from mafic dykes associated with orogenic gold mineralisation in central victoria australia abstract mafic to intermediate dykes are spatially and temporally closely associated with major post tectonic granitic complexes in the western lachlan orogen of se australia these dykes which range petrographically from basaltic to andesitic are concentrated within several north to northwest trending zones and were emplaced during two broad intervals of extensive magmatic activity during the silurian devonian period geochemical and sm nd isotopic characteristics of these mafic intrusives are consistent with their formation in a complex subduction related tectonic setting interaction between mantle wedge material sinking oceanic crust and input from the overlying continental crust resulted in the petrological and geochemical variations displayed by these and more felsic dykes throughout the study region field evidence and 40 ar 39 ar data show that in the eastern part of the stawell zone and in the northwest portion of the bendigo zone mafic dyke were intruded between 410 and 400 ma late silurian early devonian further emplacement in the bendigo zone and the eastern part of the melbourne zone took place at between 375 and 365 ma middle to late devonian episodic mantle derived magmatism was possibly related to step wise rollback slab detachment or changes in the angle and rate of westward subduction in response to periodically occurring accretionary pulses a close spatial and temporal relationship also exists between the dykes and orogenic gold mineralisation in the central victorian gold province mafic to intermediate dykes both crosscut and are host to mineralisation in a number of goldfields although there is little evidence for a direct genetic association the two processes are linked by the common utilisation of translithospheric structures which facilitated the rapid ascent into shallow crustal levels of both mantle derived magma and crustal scale ore forming fluid systems previous studies have suggested that transfer of heat into the crust via ascending mafic mantle magmas can provide a thermal engine which triggers and sustains extensive crustal melting thus explaining the commonly observed close association of mafic to intermediate and felsic intrusive suites this study supports the viability of this mechanism and in addition indicates that a causal link exists between the formation of mantle magmas in collisional zones and the generation of orogenic gold deposits

wall rock petrology and geochemistry in alteration halos associated with mesothermal gold mineralization central victoria australia integrated petrological and geochemical examination of metasedimentary assemblages associated with mesothermal lode gold mineralization in five major centers of past and present production ballarat percydale tarnagulla maldon fosterville in central victoria australia reveal that hydrothermal alteration in the hosting cambro ordovician turbidites is much more pronounced and extensive than has previously been recognized characteristically the auriferous reef structures are surrounded by bleached zones up to several tens of meters wide sericitization carbonatization and carbonate spotting and pyrite and arsenopyrite porphyroblasts within and beyond zones of visible bleaching are the most obvious features of alteration quantitative xrd analysis illustrates that these marked bleached halos are due to the replacement of albitized plagioclase clasts by white mica and the breakdown of metamorphic chlorite to form secondary carbonates and to a lesser extent kaolin the negative correlation between metamorphic chlorite and hydrothermal carbonate phases provides a reliable mineralogical alteration index useful to exploration geochemical reconnaissance profiles also indicate the development of systematic trends in response to hydrothermal alteration involving mainly sio2 k2o na2o co2 as au and s at constant bulk rock volume calculation of a geochemical alteration index given by the ratio k2o co2 k2o co2 na2o al2o3 suggests that this can be a reliable indicator of alteration in particular where concentrations of individual invariable oxides such as al2o3 vary too much in response to lithological variations saturation indices that are based on ratios of 3k al and co2 mg fe ca are less reliable because muscovite and carbonate are not always the dominant alteration mineral species of relevance the alteration assemblages as well as mineralogical and geochemical trends at the five ore systems investigated herein are common to many slate belt hosted gold deposits the results of this investigation show that although the detection of such systematic spatial variations relative to mineralization can be complicated for example by postore modification of the wall rocks due to contact metamorphism e g at maldon or where mineralization is not associated with massive to laminated quartz veins e g at fosterville recognition of extensive though not always conspicuous alteration halos around mesothermal lode gold mineralization represents a potentially powerful tool for exploration in slate belts

tectonic constraints on the timing relationships of metamorphism fluid production and gold bearing quartz vein emplacement abstract there is an ongoing discussion about the genetic significance of the postmetamorphic emplacement of many gold mineralizations as a contribution to this discussion this paper reviews the timing relationships between metamorphic devolatilization and metamorphism at different crustal levels for different tectonic processes for example denudation accompanying or following thickening causes metamorphism to occur later at depth than at shallow levels thus many internally heated terrains e g barrovian terrains are of deep later type that is peak metamorphism and devolatilization at deeper levels occurs at later times metamorphic processes of the deep later type may produce and release metamorphic fluids during prograde metamorphism at depth and precipitate them in the form of postmetamorphic quartz veins at shallow levels magmatic heating from below on the other hand is a deep earlier process causing inverse timing relationships in many metamorphic terrains deep later and deep earlier processes may compete for example if denudation at the surface accompanies heating from below if denudation and heating from below are of similar duration deep later characteristics prevail and a strongly bi modal age distribution of peak metamorphism in different crustal levels may arise the upper crust will experience metamorphism early but largely contemporaneously and the lower crust will experience metamorphism much later this process is particularly favourable for substantial simultaneous devolatilization of large sections of the lower crust to postdate metamorphism at shallow levels thus such processes may result in late infiltration events in cooling rocks of low metamorphic grade the common observation of postmetamorphic fluid infiltration and mineralization may therefore indicate but does not require that the mineralising fluids are related to the same metamorphic event

metallogenic relationships to tectonic evolution the lachlan orogen australia abstract placing ore formation within the overall tectonic framework of an evolving orogenic system provides important constraints for the development of plate tectonic models distinct metallogenic associations across the palaeozoic lachlan orogen in se australia are interpreted to be the manifestation of interactions between several microplates and three accretionary complexes in an oceanic back arc setting in the ordovician significant orogenic gold deposits formed within a developing accretionary wedge along the pacific margin of gondwana at the same time major porphyry cu au systems formed in an oceanic island arc outboard of an evolved magmatic arc that in turn gave rise to granite related sn w deposits in the early silurian during the ongoing evolution of the orogen in the late silurian to early devonian sediment hosted cu au and pb zn deposits formed in short lived intra arc basins whereas a developing fore arc system provided the conditions for the formation of several volcanogenic massive sulphide deposits inversion of these basins and accretion to the australian continental margin triggered another pulse of orogenic gold mineralisation during the final consolidation of the orogenic belt in the middle to late devonian

neoarchaean metamorphic evolution of the yilgarn craton a record of subduction accretion extension and lithospheric delamination abstract the greater part of the metamorphic record of the yilgarn craton formed during a protracted middle neoarchaean orogenic cycle spanning from about 2750 to 2620 ma at least six distinct metamorphic events are defined by parageneses in different tectono stratigraphic settings this review characterizes the middle neoarchaean metamorphic response of the entire craton using large databases rock descriptions from 60 000 sites in government databases and 1040 pt determinations from all sources are used to construct metamorphic map patterns for each of these metamorphic events p t paths and timing of each metamorphic event are characterized using petrology and the well constrained pt determinations from this study n 287 combined with metamorphic age determinations from all sources n 114 the spatial patterns and metamorphic conditions of each event provides critical constraints on tectonic settings and changes in tectonics over 130 million years the record of metamorphism in conjunction with stratigraphic kinematic and magmatic constraints shows that the neoarchaean craton evolved from an accretionary crustal growth phase ma and m1 to thermal reworking of the crust during massive influx of granitic intrusions m2 and lithospheric extension after the termination of subduction m3 these events were followed by lithospheric delamination resulting in a craton wide diffusion delayed thermal pulse that coincided with renewed contraction m4 m1 metamorphism at 2748 19 2727 8 and 2706 10 ma experienced high p moderate t hairpin clockwise p t paths during burial of magmatic arc margins during terrane accretion events m2 regional contact type metamorphism at 2671 6 ma tracked low p moderate t clockwise p t paths in back arc settings during voluminous felsic magmatism related to shallow subduction m3 metamorphism at 2656 5 ma experienced low p moderate t anticlockwise p t paths in post volcanic rift basins formed during lithospheric extension m4 metamorphic peaks at 2644 4 and 2629 7 ma experienced low p high t clockwise p t paths during regional scale thermal pulses resulting from lower crust and mantle lithosphere delamination at 2665 2668 ma

in situ production of ash in pyroclastic flows 1 abrasion and comminution of pumice clasts during the propagation of pyroclastic flows have long been recognized as a potential source for the enhanced production of volcanic ash however their relative importance has eluded quantification the amount of ash produced in situ can potentially affect runout distance deposit sorting the volume of ash introduced in the upper atmosphere and internal pore pressure we conduct a series of laboratory experiments on the collisional and frictional production of ash that may occur during different regimes of pyroclastic flow transport ash produced in these experiments is predominately 10 100 microns in size and has similar morphology to tephra fall ash from plinian events we find that collisional ash production rates are proportional to the square of impact velocity frictional ash production rates are a linear function of the velocity of the basal particle enriched bed load region of these flows using these laboratory experiments we develop a subgrid model for ash production that can be included in analytical and multiphase numerical procedures to estimate the total volume of ash produced during transport we find that for most flow conditions 10 20 of the initial clasts comminute into ash with the percentage increasing as a function of initial flow energy most of the ash is produced in the high energy regions near the flow inlet although flow acceleration on steep slopes can produce ash far from the vent on level terrain collisionally and frictionally produced ash generates gravity currents that detach from the main flow and can more than double the effective runout distance of these flows ash produced at the frictional base of the flow and in the collisional upper regions of the flow can be redistributed through the entirety of the flow although frictionally produced ash accumulates preferentially near its source in the bed load flows that descend steep slopes produce the majority of their ash in the collisonally dominant flow head and flow snouts likely develop subangular to rounded pumice during this process

how coriolis forces can limit the spatial extent of sediment deposition of a large scale turbidity current abstract the rotation rate of the earth controls the deposition scale of turbidity currents when their flow transit time is comparable to one day using laboratory experiments it is shown that the maximum length scale l of deposition is set by the scale where the rossby number of the current is equal to one the dimensionless rossby number is defined as ro u fl where u is a depth averaged velocity of the turbidity current and f is the coriolis parameter in these new laboratory experiments the coriolis parameter is varied by changing the platform rotation rate and there is good agreement between experimental observations and the prediction that the extent of the deposition scales as l 4 1 4 g v f 1 4 where g is the reduced gravity and v the initial volume of the sediment laden material this scaling implies that the depositional patterns will vary with latitude and that at higher latitudes one would expect smaller thicker deposits using this scaling implies that the 200 400 km spatial extent of the turbidite arising during the 1929 grand banks earthquake had ro 1 and so coriolis forces controlled the length scale of the turbidite deposition

acid neutralization mechanisms and metal release in mine tailings a laboratory column experiment mining and milling of base metal ore deposits can result in the release of metals to the environment when sulfide minerals contained in mine tailings are exposed to oxygen and water they oxidize and dissolve two principal antagonistic geochemical processes affect the migration of dissolved metals in tailings impoundments sulfide oxidation and acid neutralization this study focuses on acid neutralization reactions occurring in the saturated zone of tailings impoundments to simulate conditions prevailing in many tailings impoundments 0 1 mol l sulfuric acid was passed continuously through columns containing fresh unoxidized tailings collected at kidd creek metallurgical site the results of this column experiment represent a detailed temporal observation of ph eh and metal concentrations the results are consistent with previous field observations which suggest that a series of mineral dissolution precipitation reactions control ph and metal mobility typically the series consists of carbonate minerals al and fe iii hydroxides and aluminosilicates in the case of kidd creek tailings the dissolution series consists of ankerite dolomite siderite gibbsite and aluminosilicates in the column experiment three distinct ph plateaus were observed 5 7 4 0 and 1 3 the releases of trace elements such as cd co cr cu li ni pb v and zn were observed to be related to the ph buffering zones high concentrations of zn ni and co were observed at the first ph plateau ph 5 7 whereas cd cr pb as v and al were released as the ph of the pore water decreased to 4 0 or less

factors influencing the deposit geometry of experimental turbidity currents implications for sand body architecture in confined basins two sets of scaled laboratory experiments were performed to examine the effect of flow volume flow density and grain size distribution on the transport efficiency of turbidity currents the experiments employed two sediment analogues ballotini and silica flour intended to model medium to coarse grained sand and mud respectively in the first set of experiments each parameter was varied to examine its effect upon deposit geometry increases in the initial flow density volume and proportion of fines had the effect of increas ing the amount of sediment that was transferred to the floor of the experimental tank by the turbidity currents increase of each of these parameters has a characteristic effect on the three dimensional of the deposit the deposits of large volume flows are elongate and those of fines rich flows are broad increase of flow density increases the initial potential energy of the flow thus increasing the runout distance increase of the initial density beyond a sediment concentration of 13 by mass results however in a reverse of the geometrical trend of deposit elongation possibly because of turbulence suppression at high densities increase of flow volume also increases the initial potential energy and reduces the rate of velocity decrease due to gravitational spreading increase in the proportion of fines leads to main tenance of negative buoyancy as the fine fraction remains suspended until the flow has virtually come to rest it also decreases the settling velocity of the coarser fraction and thus delays its sedimentation the second set of experiments was performed to investigate the influence of flow efficiency on the interaction of turbidity currents with topography a single arcuate obstacle was placed in the path of the flows in successive experiments flow efficiency was increased by progressively increasing the proportion of fines silica flour both the proportion of sediment reaching the obstructing topography and the proportion of it able to surmount the topography increased as flow efficiency increased thus flow effi ciency may determine whether or not an enclosed basin hosts deposits whose geometry has been affected by the confinement and may also determine the relative effectiveness of the topography in confining inbound turbidity currents and thus trapping their sediment load

predicting the geometry of channelized deep sea turbidites analytical relationships between the dynamic properties of a deposit forming turbidity current propagating across a sea floor with small slope and the geometry of the resulting deposit have been confirmed experimentally for a wide range of particle sizes initial concentrations and volumes of the driving suspension these simple expressions provide a basis for inferring the dynamics of natural flows from their deposits we surmise that the turbidity current responsible for depositing the black shell turbidite in the northwestern atlantic ocean had an initial sediment concentration on the order of 100 g of silt per litre and a volume on the order of 1000 km 3

the spectrum of ore deposit types volcanic environments alteration halos and related exploration vectors in submarine volcanic successions some examples from australia variations in shape metal content alteration mineralogy and volcanic host rocks of the ore deposits in the two major volcanic hosted massive sulfide vhms districts of eastern australia the cambrian mount read volcanics and the cambro ordovician mount windsor subprovince strongly reflect their volcanic environment conditions of ore formation and hydrothermal alteration processes lens and sheet style polymetallic zinc rich deposits such as rosebery hellyer que river and thalanga are considered to have formed in moderate to relatively deep water environments 500 1 000 m these deposits probably formed either on the sea floor e g hellyer que river or by replacement of porous volcaniclastic units directly below the sea floor e g rosebery the footwall alteration associated with these polymetallic vhms deposits was controlled by host rock permeability and porosity which are in turn related to volcanic facies type degree of fracturing and synvolcanic structural architecture focusing of hydrothermal fluids along synvolcanic structures has resulted in well zoned chlorite sericite footwall alteration pipes within footwall lavas at hellyer on the other hand diffuse fluid flow through very thick pumice breccia at rosebery and hercules has resulted in strata bound sericite dominated footwall alteration zones parallel to the paleosea floor and the ore lenses massive and disseminated pyritic cu au deposits such as those in the mount lyell field and at highway reward formed by subsea floor replacement and are associated with only minor zinc lead massive sulfide ore these deposits formed from higher temperature fluids 300 degrees c in which copper transport is enhanced and are commonly located in felsic volcanic centers dominated by shallow porphyritic intrusions e g highway reward the cu au ore lenses may be strata bound e g mount lyell or crosscutting pipes e g highway reward depending on the structure and permeability characteristics of the felsic volcanic host rocks the presence of high sulfidation alteration minerals e g pyrophyllite zunyite in some of the cu au deposits e g mount lyell field indicates that fluids were relatively acidic and suggests the possibility of magmatic fluid input into the hydrothermal system alteration zonation associated with the cu au vhms deposits is more symmetrical than that of the zn rich deposits with sericite rich alteration extending into the hanging wall in keeping with the subsurface replacement origin of these deposits synvolcanic gold rich deposits with high gold base metal ratios are less common than the cu au and zn rich vhms ore types the gold rich ores e g henty south hercules are strata bound in nature have low sulfide contents and are associated with central zones of intense silicification surrounded by envelopes of sericite pyrite and carbonate alteration volcanological and geochemical studies at henty indicate the gold rich ore formed by the replacement of particular volcanic units deposited in a relatively shallow water environment dominated by volcaniclastic facies lavas and limestones this spectrum of cu au zn rich and au only deposits in the mount read volcanics and the mount windsor subprovince is interpreted to represent a continuum from classic sea floor vhms ores toward those with features more akin to porphyry cu au and epithermal au ag deposits this spectrum relates to the interplay between factors in the submarine volcanic environment and the character of the hydrothermal fluid as follows 1 proportions of volcaniclastic lava and subvolcanic intrusive facies 2 depth of seawater 3 permeability and porosity of volcanic host rocks 4 balance between magmatic components and seawater components in the ore fluid and 5 temperature and acidity of the ore fluid mineralogical lithogeochemical and isotopic studies have revealed a range of alteration vectors useful in exploration for both the zn rich and cu au vhms deposits carbonate and white mica compositional variations are highlighted as important mineralogical vectors thallium and antimony halos may be useful trace element vectors and oxygen and sulfur provide important isotope vectors toward the center of the hydrothermal system

palaeozoic arc growth deformation and migration across the lachlan fold belt southeastern australia abstract the palaeozoic lachlan fold belt lfb is a low pressure metamorphic belt characterized by greenschist facies rocks and infolded volcanic sequences which indicate that the belt has not undergone substantial uplift at any stage porphyroblast matrix relationships in one of the highest grade metamorphic zones of the lfb the omeo complex indicate that the thermal peak was reached before or was synchronous with the earliest deformation similar to that in high grade metamorphic zones of the northern arunta inlier central australia in both areas the metamorphism is of low p high t type centred on major granite intrusions as regional aureoles and characterized by anti clockwise p t t paths however in the outer lower grade parts of the aureoles peak metamorphic conditions post date the earliest deformation this diachronous relationship indicates that 1 outward migration of a thermal front was centred on major granitoid intrusions batholiths 2 deformation propagated outward more rapidly than migration of the thermal front and 3 plastic and ductile deformation began only after heating around the batholiths thus orogeny was initiated after and localized by heat focusing in the mid crust associated with batholith emplacement therefore the early deformation is a result of thermal softening it is typically subhorizontal ductile shear at mid crustal levels but is characterized by upright to inclined folds at upper crustal levels in the latter environment the typically discordant contact aureole plutons are often interpreted as post tectonic granitoids but they were part of the ongoing mid crustal thermal perturbation that induced regional greenschist facies metamorphism which overprinted the early formed upright folds as it migrated to upper crustal levels orogeny migrated generally eastward through the lfb it began with the early silurian benambran orogeny centred on the wagga metamorphic belt wmb and terminated with the carboniferous kanimblan orogeny which was most intense in the hill end trough eastward migration of orogeny across central victoria part of the west lfb began in the ballarat bendigo zone during the early devonian and terminated in the melbourne zone in the middle devonian orogeny was centred on the meridional batholiths of the lfb each of which are considered to represent the transitory axis of an ancient magmatic arc stepwise but generally eastward arc migration caused the eastward migration of orogeny a tectonic model based on the modern southwest pacific arc system can be applied to the lfb following cambrian intra oceanic arc growth associated with west dipping subduction an ordovician marginal sea developed partly on stretched cambrian crust and was flooded by an extensive turbiditic wedge closure of the eastern part the east lfb began in the early silurian after a magmatic arc developed over the wmb possibly associated with an east dipping subduction zone after re establishment of west dipping subduction in the middle silurian absolute motion retreat of the upper australian plate caused by oblique plate convergence resulted in dextral transtension and the generation of siluro devonian inter arc and back arc basins transient compression resulted in deformation along the magmatic arc and induced formation of a new outboard eastward arc system periodic compression within the evolving arc and back arc system resulted in a series of east younging east verging linear siluro devonian fold and thrust belts which were localized adjacent to elongate batholiths that represent the relict arc systems local westward jumps in deformation and plutonism occurred the most significant is the 600 km westward jump into the west lfb caused by closure of the remainder of the passive ordovician back arc basin melbourne and bendigo ballarat zones west dipping subduction beneath the basin produced a low pressure fold thrust belt similar to the east lfb though the granitoids have a more primitive isotopic character indicating closure of thin dominantly oceanic crust in the early to middle devonian localized deformation induced by thermal softening of the crust within magmatic arcs that have developed on earlier passive margins is likely to produce anticlockwise p 7 t paths and may be a typical feature of southwest pacific style tectonism

endeavour 26 north a porphyry copper gold deposit in the late ordovician shoshonitic goonumbla volcanic complex new south wales australia endeavour 26 north is the largest of 12 pipelike porphyry cu au deposits found near parkes in central west new south wales within the late ordovician goonumbla volcanic complex the dominant volcanic rocks of the complex the goonumbla volcanics are latitic flows and volcanic breccias which are overlain by the trachytic wombin volcanics the volcanic rocks are characterized by high k 2 o and high k 2 o na 2 o ratios which are typical of the shoshonite association as well as an enrichment in large ion lithophile elements and a low concentration of high field strength elements the nelungaloo volcanics form an early ordovician nonshoshonitic volcanic substrate to the complex which initially developed in a submarine and subsequently subaerial setting the 87 sr 66 sr epsilon nd and pb isotope data indicate the magmas were derived from mantle sources the shoshonitic magmas may be products of high pressure fractionation in the subcontinental lithosphere the porphyry deposits are associated with a gravity low which is taken to reflect a subvolcanic chamber beneath the center of the complex all deposits are located within a pronounced circular feature in the aeromagnetic data that is interpreted as the margin of a collapsed caldera dioritic to monzonitic stocks which in part define the caldera rim represent a resurgent phase of magmatism the endeavour 31 stock which was part of this phase is ringed by six porphyry cu au deposits including endeavour 26 north the deposits are centered on fingerlike quartz monzonite porphyries which cut the endeavour 31 stock at depth geochemical anomalies of cu and zn in the bedrock define the size of the alteration system associated with the stock in part the distribution of the cu au deposits is controlled by the endeavour lineament in comparison to most other volcanic and intrusive rocks of the goonumbla volcanic complex the endeavour

australian volcanic hosted massive sulfide deposits features styles and genetic models the cambro ordovician period has yielded the major development of volcanic hosted massive sulfide vhms deposits in australia resulting in about 12 million metric tons of contained metal copper lead and zinc concentrated in the mount read volcanics tasmania and the mount windsor volcanics queensland the archean 4 million metric tons of metal and the silurian 3 5 million metric tons of metal constitute the next important episodes while the devonian and permian have produced isolated deposits the deposits range from cu type to zn cu type to zn pb cu type the zn cu type deposits are restricted to the archean and silurian whereas the cu type and zn pb cu type occur sporadically throughout the time span from early archean to permian in general terms the major vhms bearing districts are calc alkaline in character and display a thick basal pile of rhyolitic volcanics 1 3 km thick including lavas epiclastics and subvolcanic intrusions which is overlain by a polymodal sequence containing various proportions of rhyolite dacite andesite basalt and sediments the major deposits are commonly located at the top contact of the rhyolite pile or within the lower part of the overlying polymodal sequence there is a wide range in variability of styles of australian vhms deposits including mounds pipes sheets layered deposits stacked deposits stockwork and disseminated deposits distal reworked deposits and cyclic layered deposits although these various styles show a range of features there is a consistent theme across the spectrum of metal zonation alteration mineralogy alteration chemistry sulfur isotopes macrotextures microtextures and host volcanic relationships which strongly suggests that they all belong to the one genetic group of ore deposit the classic mound style deposits such as hellyer have a series of key features subsets of which are represented in the other deposit styles the mound deposits are considered to form from deposition of metal sulfides on the sea floor immediately around the hydrothermal vent growth of the mound occurs by upward replacement of sulfide assemblages stable at higher fluid temperatures leading to zone refining and lead zinc silver gold enrichment in the outer and upper parts of the mound departure from the classic mound style of deposit is related principally to three key factors the chemistry of the ore fluid salinity temperature f02 and ah2s the nature permeability and chemistry of the volcanic pile and the sea hoor environment sea hoor topography and seawater depth these factors control the aspect ratio of the deposit the extent of stringer zone development the degree of subsea floor replacement mineralization the nature and spatial development of footwall alteration and the development and style of related distal mineralization sulfur isotope studies of australian vhms deposits indicate that reduced seawater sulfate is a major source of sulfur in the deposit whereas lead and strontium isotope results are compatible with the metals being derived by seawater convection and leaching from the volcanic pile and basement rocks this conclusion is supported by the application of various leaching models which demonstrate the availability of an adequate source of both base and precious metals in the footwall volcanic sequences the input of metals directly from volcanic magma chambers is not precluded by the available data and one likely scenario involves the contribution of gold and copper from a magmatic vapor plume rising to mix with convective seawater fluids which contribute lead zinc silver and gold leached from the footwall volcanics and basement rocks the relative importance of the magmatic input compared with the seawater convective input may help to explain the spectrum of deposit styles arid their spatial relationship to volcanic centers or adjacent sedimentary basins

character and kinematics of faults within the turbidite dominated lachlan orogen implications for tectonic evolution of eastern australia reply fault zones within turbidite dominated orogenic systems typified by the lachlan orogen of eastern australia are characterised by higher than average strain and intense mica fabrics transposition foliation and isoclinal folds poly deformation with overprinting crenulation cleavages and steeply to moderately plunging meso and micro folds they have a different character compared to the brittle ductile fault zones of classic foreland fold and thrust belts such as the appalachians and the canadian rocky mountains multiple cleavages and transposition layering record a progressive shear related deformation history an intense mica fabric evolves initially during shortening of the overlying sedimentary wedge but is progressively modified during rotation and emplacement to higher structural levels along the steep parts of inferred listric faults the deformed wedge outside the fault zones generally undergoes one phase of deformation shown by a weak to moderately developed slaty cleavage which is parallel to the axial surface of upright subhorizontally plunging chevron folds other faults within the turbidites of the lachlan orogen include the steep zones of ductile strike slip deformation that bound a centrally located high t low p metamorphic complex characterised by s c mylonites these ductile shear zones indicate a southward passage of the metamorphic complex as a crustal wedge with emplacement to higher structural levels along a leading edge ductile thrust fault ar ar dating constrains the timing of regional deformation to be mostly late ordovician through silurian across the lachlan orogen faults in the low grade turbidite sequences record the kinematic evolution of the developing lachlan orogen and indicate progressive deformation associated with simultaneous eastward propagating and migrating deformation fronts in both the western and eastern parts of the fold belt these deformation fronts are related to accretionary style deformation at the leading edges of overriding plates in an inferred southwest pacific type subduction setting from the late ordovician to the mid devonian along the former gondwana margin the fault zones effectively accommodate and preserve movements within the structurally thickening migrating and prograding accretionary wedge

carbonate mineral water interactions in sulfide rich mine tailings the chemical composition and mineralogy of coatings on carbonate minerals from mine tailings have been studied using aqueous geochemical methods time of flight laser ionization mass spectrometry tof lims and transmission electron microscopy tem the goal is to study major and trace element partitioning between the aqueous and solid phase and to infer mechanisms that control the concentrations of elements in the pore water of sulfide rich mine tailings pore water samples and carbonate mineral grains were collected from four geochemically distinct zones within the tailings oxidation of sulfide minerals near the surface results in a large range in pore water ph 3 85 to 6 98 and aqueous concentrations of metals and sulfate with increasing depth in the tailings mineral water interactions lead to increasing ph and decreasing concentrations of metals and sulfate calculated mineral saturation indices trends in the abundance of ca fe mg and mn in tof lims profiles through the secondary coatings and electron diffraction patterns obtained from the coatings suggest that precipitation dissolution of jarosite group minerals gypsum goethite akaganeite amorphous fe oxyhydroxides and siderite control the aqueous ca fe na k and so4 concentrations the occurrence of secondary coatings on primary minerals is widespread and reactions with the secondary minerals rather than the primary mineral substrate probably represent the principal controls on trace element distributions in the pore water the data indicate that adsorption surface complexation and co precipitation reactions are important controls on the concentrations of trace elements in the pore water the occurrence of siderite coatings on the surface of ankerite grains suggests that fe bearing dolomite structure carbonate minerals dissolve incongruently this corroborates inferences made by previous workers that solubility differences between calcite and siderite lead to calcite dissolution and siderite precipitation during acid neutralization the accumulation of secondary coatings will diminish the rate of mineral dissolution reactions involving the primary minerals consequently the use of kinetic rate constants obtained from the literature for simulating the dissolution of primary mineral phases such as the carbonates and alumino silicates will likely overestimate the actual rates

the contribution of experimental volcanology to the study of the physics of eruptive processes and related scaling issues a review abstract the experimental approach has become a major tool increasingly used by volcanologists in recent decades to investigate the physics of eruptive processes in complement to field and theoretical works researchers have developed various methodologies to study volcanic phenomena at reduced length scale the works involve natural or analogue materials and their types range from first order tests to identify fundamental processes and make qualitative comparison with field observations to more sophisticated experiments in which precise data obtained in a controlled environment can be used to validate outputs of theoretical models scaling is a central issue to ensure dynamic similarity between the small scale experiments and the large scale volcanic phenomena when natural conditions cannot be simulated due to inherent length scale difference and or technical limitations in this respect dimensionless numbers are used to map physical regimes and to define scaling laws which allow experimental results to be extrapolated to natural scale this review presents the variety of experimental studies conducted to investigate subterraneous and aerial volcanic phenomena involving in particular fluid particle mixtures we focus on the major scaling issues and the physical regimes investigated and we also highlight in a historical perspective some of the major advances achieved through experimental studies finally we conclude on some perspectives for future works

a review of the metallogeny and tectonics of the lachlan orogen future mineral exploration within eastern australia will be enhanced by resolving the tectonic evolution of the lachlan orogen to establish the spatial and temporal terrane distribution of the various mineral deposits the lachlan orogen from north eastern tasmania through to central and eastern new south wales is host to a number of major mineral deposit styles including orogenic gold e g stawell ballarat bendigo volcanic hosted massive sulphide e g woodlawn currawong sediment hosted cu au e g cobar basin deposits porphyry au cu e g cadia parkes cowal and granite related sn e g ardlethan beechworth each of these mineral deposit styles is a sensitive and diagnostic indicator of the prevalent tectonic environment during their formation in this review we briefly summarise the deposit to large scale factors that define the diverse metallogenic evolution of the lachlan orogen this overview is intended to set the scene for subsequent specialist papers published in this thematic issue on the metallogeny and tectonics of the lachlan orogen in south east australia

numerical simulation of block and ash flows for different eruptive scenarios of the tacana volcanic complex mexico guatemala abstract the tacana volcanic complex tvc shared by mexico and guatemala is considered an active volcano that resumed phreatic explosions in 1949 and 1986 the tvc has recorded at least eight eruptions emplacing block and ash flows bafs around the volcanic edifice during the past 42 000 years the largest event generated the mixcun deposit some 1950 yr b p from san antonio volcano the youngest structure of the complex based on the eight mapped baf deposits of the complex we perform numerical simulations with the titan2d code to recreate the distribution and extent of these deposits the results were used to prepare the zonation of the tvc based on the surface bed friction angles which are related to the different morphologies of the volcano from this zonation and the features of the past baf deposits we define three new eruptive impact scenarios low medium and high to perform a hazard assessment of this phenomenon but considering the present morphology of the complex of the four eruptive centers next we ranged the input parameters for each case scenario to analyze the dispersal thickness and maximum runout of simulated flows from the numerical simulations we found that the most influential parameter in the model outputs is the basal friction angle which varied from 23 to 12 then we merged the eruptive scenarios simulated to construct new hazard maps for the different mass volumes simulated for each scenario these maps clearly evidenced that the mexican territory would be seriously affected in case of a renewed explosive activity of the tvc in fact even the partial collapse of any of the tvc domes would produce bafs capable to flow over 70 communities affecting ca 20 000 inhabitants as well as promote the generation of syn and post eruptive lahars if they flow into the main rivers that drain the volcano therefore these maps are a great advance to evaluate the destructive potential of the tvc and must serve as the basis for the construction of a new risk map by the local authorities to update the operative and evacuation plans of the region these actions would help to the 350 000 inhabitants living in a 35 km radius from the volcano s summit to be aware of the perils posed by the volcano and how to act and evacuate in case of a volcanic crisis

chronology and tectonic framework of turbidite hosted gold deposits in the western lachlan fold belt victoria results abstract 40 ar 39 ar data constrain the history of metamorphism deformation and mineralization in the western subprovince of the lachlan fold belt rocks in the mount stavely volcanic complex and the bushy creek pluton give 40 ar 39 ar ages of about 500 ma for hornblende and biotite the dates from the mount stavely volcanic complex limit the age of mineralization in the metavolcanic rocks of the stavely belt whereas the date for the bushy creek pluton extends the area of cambrian ordovician plutons of the delamerian orogeny to east of the grampians mountains and helps define the surface expression of the western border of the paleozoic lachlan fold belt 40 ar 39 ar dating of metamorphic mica growth phengite in cleaved slates and phyllites within the landsborough fault avoca fault zone heathcote fault zone and mount wellington fault zone gives ages of 453 2 440 2 426 4 and 410 390 ma respectively these data indicate a progression of deformation from west to east starting in late ordovician early silurian time in the stawell and bendigo ballarat structural zones the major phase of deformation in the western subprovince of the lachlan fold belt is therefore silurian and not devonian sericites from major gold deposits in the stawell zone and bendigo ballarat zone give 40 ar 39 ar dates of about 440 ma that are coincident with folding and thrusting a second episode of mineralization and fault reactivation at about 420 to 410 ma is found in some deposits in these zones based on data from a previous study in the melbourne zone including the woods point dike swarm and locally in the bendigo ballarat zone some gold deposits give sericite dates of about 380 360 ma these ages are consistent with the close association of many melbourne zone gold deposits with devonian dikes and plutons the results indicate that gold mineralization in the stawell zone bendigo ballarat zone and melbourne zone was episodic and both temporally and spatially associated with crustal heating linked to regional metamorphism and plutonism initial stages of mineralization in the province comprising many of the large deposits in western and central victoria e g bendigo ballarat stawell wattle gully tarnagulla accompanied metamorphism folding and thrusting subsequent mineralization is widespread but produced mainly smaller deposits

metallogenic episodes of the tasman fold belt system eastern australia precise dating of alteration minerals and host rocks to au and base metal deposits in the tasman fold belt system eastern australia has established a number of distinct metallogenic epochs which correspond to episodes of magmatism or to deformation events base and precious metal mineralization was emplaced in the mount read volcanics tasmania at about 500 ma and other hydrothermal or deformational events may have occurred in the hellyer deposit between approximately 480 and 440 ma copper and au mineralization was emplaced in the lachlan fold belt new south wales at about 440 ma and appears to be related to the end stages of ordovician magmatism that spans a period from approximately 480 through to 440 ma gold deposits or base metals which formed in new south wales and tasmania at about 420 to 380 ma are essentially coeval with deformation and or are broadly related to siluro devonian magmatism that largely occurred from about 420 to 390 ma gold mineralization in the anakie inlier northeast drummond basin north queensland was essentially coeval with the emplacement of the host drummond sequence at about 350 ma

thrusts and thrust associated mineralization in the lachlan orogen the lachlan orogen is characterized by a thin skinned deformational style with major thrusts and strike slip faults soling into detachments in the upper and middle crust thrusts are divided into three types type 1 thrusts that formed in apparently unconstrained sediment packages such as early ordovician turbidites type 2 thrusts formed during inversion of mid silurian to mid devonian constrained sedimentary or volcanic basins and type 3 major thrusts that bound the edges of rare upthrusted middle crustal complexes all three types of thrusts are associated with mineral deposits mineralized type 1 thrusts occur in the gold fields of central victoria mineralized type 2 thrusts occur both on the edges of deformed sedimentary and sedimentary or volcanic basins e g in synrift packages at cobar and captains flat respectively as well as within deformed sedimentary and sedimentary or volcanic basins e g at quidong and hill end and in synrift rocks at woodlawn respectively base metal deposits in synrift rocks at benambra on the edge of the deformed sedimentary or volcanic cowombat rift are the best examples of mineralization associated with a type 3 thrust the spatial and geometrical associations between deposits and thrusts in these and other examples lead to the recognition of a class of thrust associated mineral deposits which constitute a major resource within the lachlan orogen thrust associated deposits include deposits localized on thrusts deposits above or below thrusts and deposits in anticlines above inferred blind thrusts some thrust associated deposits are syntectonic with mineralization genetically related to thrusting and deformation such deposits are thrust related and formed during and after deformation other thrust associated deposits predate deformation and are called thrust modified deposits the orebody and wall rock contacts of these deposits have been modified by

particulate gravity currents along v shaped valleys this paper extends previous studies of saline gravity currents at high reynolds number flowing along a tank with a v shaped valley we use experiments and a box model to determine the primary features of the flow the particulate gravity currents were initiated by releasing a fixed volume of fluid consisting of pure water mixed with silicon carbide particles from a lock at one end of the tank the resulting motion and deposit pattern differ significantly from those for the propagation of a particulate gravity current along a flat bottomed tank the front of the current seen from above is approximately parabolic with axis parallel to the flow direction in contrast to the current in a flat bottomed tank where it is nearly a straight line perpendicular to the flow this feature mimics the results for pure saline currents when seen in profile the currents do not have a clearly defined raised head which is a feature of the flat bottomed currents the mass deposited per unit area varies nearly monotonically with respect to distance down the tank again in contrast to the case of the flat bottomed tank the exceptions to this are the two experiments which have the highest ratio of lock height to length the mass deposited per unit area across the v shaped valley is much larger in the central part of the valley than it is on the flanks for any position along the valley we find that the results can be described with remarkable accuracy by a box model using a generalization of the equation for sedimentation from a turbulent medium due to martin and nokes our results further show that the factor used in the deposition rate equation which is commonly assumed to be 1 should be smaller typically 0 7

heavy metal retention in secondary precipitates from a mine rock dump and underlying soil dalarna sweden this study investigates the retention of heavy metals in secondary precipitates from a sulfidic mine rock dump and underlying podzolic soils by means of mineralogical and chemical extraction methods the rock dump which is at least 50 years old consists of a 5 10 cm thick leached zone and an underlying 110 115 cm thick accumulation zone optical microscopy and electron microprobe analyses confirm that pyrrhotite weathering has proceeded much further in the leached horizon relative to the accumulation horizon the weathering of sulfides in the leached zone has resulted in the migration of most heavy metals to the accumulation zone or underlying soils where they are retained in more stable phases such as secondary ferric minerals including goethite and jarosite some metals are temporarily retained in hydrated ferrous sulfates e g melanterite rozenite

a comparison of orogenic gold mineralisation in central victoria aus western south island nz and nova scotia can implications for variations in the endowment of palaeozoic metamorphic terrains abstract the palaeozoic western lachlan orogen in victoria se australia the buller terrane western south island new zealand and the meguma terrane nova scotia canada are remarkably similar in regard to their geological age and history structural make up and the setting and formation of orogenic lode and disseminated style gold deposits within these fold belts a comprehensive review of the principal characteristics of orogenic gold deposits in each of the three historically important gold provinces illustrates the many similarities but it also reveals several major geological structural tectonic geochemical and geochronological differences that may account for the disparity in contained gold and overall endowment between the three fold belts aspects that stand out as potentially playing a critical role in the generation of a phanerozoic world class orogenic gold province include 1 the presence of a hydrated oceanic character substrate that can provide a fertile source rock for both fluids and metals 2 asthenospheric thermal input to trigger and sustain crustal devolatilisation and melting 3 the existence of a number of near vertical deep seated faults 4 substantial transcurrent movement and 5 evolution of an accretionary subduction system that promotes development of an extensive fore arc system with prolonged fluid generation and circulation quantification of these controlling factors provide a significantly improved understanding of the processes that control the genesis of orogenic gold deposits and also have the potential to enable discrimination between more and less gold favourable orogens and parts of orogens

tracing metal sources in peribatholitic hydrothermal w deposits based on the chemical composition of wolframite the example of the variscan french massif central abstract we present a complete dataset for major minor and trace elements in wolframite fe mn wo4 based on the example of several peribatholitic hydrothermal w deposits located in the variscan french massif central fmc the wolframite samples were characterized down to the micrometer scale by combining scanning electron microscopy observations and in situ chemical analysis by electron probe microanalyzer and laser ablation inductively coupled plasma mass spectrometry most samples are fe dominated wolframites and are characterized by variable fe fe mn values for the studied deposits from 0 36 to 0 86 without significant alteration features wolframites present intracrystalline variations for concentrations of several minor and trace elements principally for nb ta and sn which reflect growth zoning the minor and trace element compositions of the different wolframite samples show at first order similar enrichments in ta nb sn zn and in as well as depletions in lree li th ti zr hf pb v co and mg compared to the upper continental crust second order compositional variations are observed between the studied deposits which can be separated in four main regional geochemical groups this work highlights that the chemical composition of wolframite is controlled by both the crystallochemical parameters and the composition of the primary hydrothermal fluid the regional variations observed in the geochemical signatures of wolframite reflect local variations in the composition of the hydrothermal fluid i e variable metal sources therefore this suggests that the minor and trace element contents in wolframite likely represent direct markers of the source of the hydrothermal fluid and ultimately of the metals the comparison between the trace element compositions of wolframite with the compositions of regional granitic rocks and worldwide shales suggests that evolved peraluminous granites represent the main source of metals in peribatholitic hydrothermal w deposits from the fmc whereas metasedimentary rocks have a limited to no influence on the metal budget

a geochronological framework for orogenic gold mineralisation in central victoria australia new 40ar 39ar geochronological data support and significantly expand upon preliminary age data that were interpreted to suggest an episodic and diachronous emplacement of gold across the western lachlan fold belt australia these geochronological data indicate that mineralisation in the central victorian gold province occurred in response to episodic eastward progressing deformation metamorphism and exhumation associated with the formation of the western lachlan fold belt initial gold formation throughout the stawell and the bendigo structural zones can be constrained to a broad interval of time between 455 and 435 ma with remobilisation of metals into new structures and or new pulses of mineralisation occurring between 420 and 400 ma and again between 380 and 370 ma linked to episodic variations in the regional stress field and during intrusion of felsic dykes and plutons this separation of ages is incompatible with the view that gold emplacement in the western lachlan fold belt was the result of a single orogen wide event during the devonian a distinct phase of gold mineralisation characterised by elevated cu mo sb or w is associated with both late silurian to early devonian 420 to 400 ma and middle to late devonian 380 to 370 ma magmatism when crustal thickening and shortening during the ongoing consolidation of the western lachlan fold belt led to extensive melt development in the lower crust and resulted in widespread magmatism throughout central victoria these 420 to 400 ma and 380 to 370 ma occurrences best exemplified by the wonga deposit in the stawell structural zone and many of the woods point deposits in the melbourne structural zone but also evidenced by occurrences at fosterville and maldon in the bendigo structural zone clearly formed synchronous with or post date the emplacement of plutons and dykes and thus are spatially if not genetically related to melt generation at depth this later magmatic associated and polymetallic type of gold mineralisation is economically subordinate to the earlier metamorphic associated type of gold deposition in the stawell and bendigo structural zones but tends to be the dominant style in the melbourne zone these new geochronological constraints together with zircon u pb data from felsic intrusive rocks of known relationship to gold mineralisation demonstrate that initial hydrothermal alteration associated with gold emplacement in the western lachlan fold belt was metamorphic related predating the emplacement of granite plutons by as much as 80 million years this timing differs from other important orogenic gold districts where gold deposition is closely associated spatially with felsic magmatism the early introduction of metamorphically derived fluids well before magmatism may reflect variations in the timing of peak metamorphic conditions at different crustal levels in an accretionary prism undergoing simultaneous deformation and erosion consequently no genetic link exists between the main phase s of gold mineralisation and magmatism in the central victorian gold province with the exception of formation of a minor magmatism related and geochemically distinct mineralisation style at about 420 to 400 ma and again at about 380 to 370 ma the apparent spatial relationship between gold mineralisation and felsic intrusions is merely the result of melts and fluids being channelised along the same structures

pyrite oxidation in shales implications for palaeo redox proxies based on geochemical and sem edx evidence abstract pyrite oxidation in marine shales during weathering has been widely studied however the effects of this process on geochemical proxies commonly used to reconstruct ocean redox conditions or the details of pyrite oxidation at the mineral grain scale have not received much scientific attention we conducted a four week laboratory experiment designed to stimulate pyrite oxidation at shale outcrops and to assess effects on the chemical phase of iron and sulphur in the samples both in bulk sediments useful for comparison to palaeo environmental geochemistry and at a finer scale to gain a better understanding of the nature of these changes geochemical and scanning electron microscopy sem techniques provide evidence for pyrite oxidation carbonate dissolution and iron oxyhydr oxide formation during the experiment the net effect of the experiment on the ratio of highly reactive iron phases fehr to total iron fet is minimal 0 03 difference suggesting that this redox proxy behaves relatively conservatively during weathering the effect of weathering on the ratio of pyrite bound iron to highly reactive iron fepy fehr used to investigate the availability of sulphur in contrast is pronounced up to 32 5 difference due to the oxidation of pyrite and the precipitation of iron oxyhydr oxides in the shale samples electron microscopy provides evidence that iron oxyhydr oxides precipitated in situ as rims around cores of pyrite particles passivating and protecting them from further oxidation the quantification of these partly oxidised pyrite particles is now possible using a novel automated particle analysis method coupled to chemical mapping developed in this study we conclude that this method can be of wide use both to quantify pyrite oxidation and assess the significance of fepy fehr at measured in shale outcrop samples

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

lithospheric controls on the formation of provinces hosting giant orogenic gold deposits ages of giant gold systems 500 t gold cluster within well defined periods of lithospheric growth at continental margins and it is the orogen scale processes during these mainly late archaean palaeoproterozoic and phanerozoic times that ultimately determine gold endowment of a province in an orogen a critical factor for giant orogenic gold provinces appears to be thickness of the subcontinental lithospheric mantle sclm beneath a province at the time of gold mineralisation as giant gold deposits are much more likely to develop in orogens with subducted oceanic or thin continental lithosphere a proxy for the latter is a short pre mineralisation crustal history such that thick sclm was not developed before gold deposition in constrast orogens with protracted pre mineralisation crustal histories are more likely to be characterised by a thick sclm that is difficult to delaminate and hence such provinces will normally be poorly endowed the nature of the lithosphere also influences the intrinsic gold concentrations of potential source rocks with back arc basalts transitional basalts and basanites enriched in gold relative to other rock sequences thus segments of orogens with thin lithosphere may enjoy the conjunction of giant scale fluid flux through gold enriched sequences although the nature of the lithosphere plays the crucial role in dictating which orogenic gold provinces will contain one or more giant deposits the precise siting of those giants depends on the critical conjunction of a number of province scale factors such features control plumbing systems traps and seals in tectonically and lithospherically suitable terranes within orogens

orogenic gold mineral systems of the western lachlan orogen victoria and the hodgkinson province queensland crustal metal sources and cryptic zones of regional fluid flow abstract orogenic gold mineral systems in the western lachlan orogen victoria and the hodgkinson province queensland produced gold provinces characterised by vastly different scales of gold endowment and strongly uneven distribution of gold mineralisation within each province the volume of hydrous pyrite bearing rocks undergoing metamorphic devolatilisation during thermo tectonic events driving orogenic gold mineral systems represents a fundamental first order constraint on the total gold endowment and its broad spatial distribution both between and within the provinces most of the largest gold deposits in both regions occur in linear richly endowed metallogenic zones oblique to the dominant regional structures and related to deep crustal domain boundaries these boundaries with only subtle surface expressions were the major regional structural controls which promoted focused near vertical flow of mineralising metamorphic fluids above the outer margins of cratonic blocks in the lower crust recognised major faults represented only more local scale and often indirect controls on the focused fluid flow particularly effective above the deep cratonic block boundaries overlain by relatively thick crustal source rocks

the emplacement of low aspect ratio ignimbrites by turbulent parent flows 1 a quantitative model for a radially spreading suspension driven gravity current is used to interpret parent flow conditions of a volumetrically wide range of pyroclastic deposits known from the recent and ancient geologic records of explosive volcanism the analysis which requires no freely adjustable parameters yields values of the solids concentration time of duration or equivalently a minimal estimate of eruptive flux of solids and average speed for the ground hugging deposit forming gravity currents a consistent picture emerges in which the initial volumetric concentration of pyroclastic solids in the parent flows is of the order 10 2 or less and the expanded flows travel with average speeds in the range 25 250 m s 1 to reach their farthest extent in a matter of only minutes further application of the model suggests that depositional structures in a widespread and thin ignimbrite are likely to reflect the intensity of mass flux in the parent flow and thus the intensity and overall volume of the parent eruption conditions in small magnitude pyroclastic currents of recent history can result in cross stratified deposits primary sedimentary structures or their absence in some regionally widespread and massive deposits known only in the geologic record in contrast were probably emplaced under conditions of intense sheet flow transport driven by the parent gravity currents

geochemistry of early middle palaeozoic basalts in the hodgkinson province a key to tectono magmatic evolution of the tasman fold belt system in northeastern queensland australia the palaeozoic hodgkinson province in northeastern queensland australia is host to late ordovician to devonian rock assemblages that contain tholeiitic to calc alkaline basalts these basalts occur as massive fault bounded units interspersed with marine sedimentary rocks and limestones that are metamorphosed to lower greenschist facies in the ordovician mulgrave silurian chillagoe and devonian hodgkinson formations respectively the petrogenetic and sm nd isotope characteristics of these mafic volcanic rocks were investigated to constrain the tectonic setting in which they erupted major trace and rare earth element analyses were carried out on samples from these formations and intrusive dolerites the mafic rocks can be classified as basalts and basaltic andesites with distinct morb characteristics furthermore the basalts are characterized by a slight to moderate enrichment in th and concomitant depletion in nb both of which become less pronounced with basalt evolution through time these features are consistent with decreasing volcanic arc affinity of silurian and devonian morb type basalts in the hodgkinson province sm nd isotope characteristics of these basalts indicate a change in source region from dominantly sub continental lithospheric mantle in the silurian to asthenospheric input in the devonian collectively the geochemical and isotopic characteristics of the hodgkinson province basalts are interpreted to reflect deposition in an evolving back arc basin setting the onset of basin extension was initiated in the silurian accelerated basin subsidence occurred throughout the devonian and was halted by basin inversion in the late devonian basin evolution was controlled by an eastward stepping subduction zone outboard of the australian craton

soil alteration by continued oxidation of pyrite tailings abstract this work examines the alteration processes triggered after the oxidation of pyrite tailings deposited for 3 years over a carbonate soil the infiltration of the acidic solution into the soil is causing important morphological compositional and mineralogical changes in the profile after 3 years of continued action of such alteration a considerable degradation of the main soil properties was evident the most notable being the decline in the cation exchange capacity caused by the decreases in clay and organic matter content texture variation greater electrical conductivity 10 fold greater than in unaffected soil and the appearance of horizons with colorations strongly differing from those of the original soil a discoloured layer with greyish tonalities in the first 5 mm followed by a reddish brown layer to a depth of 65 70 mm at the same time the carbonates have weathered disappearing completely from the upper 35 mm and partially to 80 mm in depth there has also been an intense acidification of the soil with ph values close to 2 0 within the greyish layer as well as a partial hydrolysis of the primary silicates mainly feldspars and phyllosilicates causing extreme infertility of the soil the resulting products in this process give rise to intense neoformation of gypsum and hydroxysulphates of fe and al which together with the acidic conditions of the medium determine the distribution of the main elements of the soil both in their total and soluble forms

runout and fine sediment deposits of axisymmetric turbidity currents we develop a model that describes the runout behavior and resulting deposit of a radially spreading suspension driven gravity current on a surface of negligible slope our analysis considers the separate cases of constant volume and constant flux sources it incorporates expressions for the conservation of volume a froude number condition at the current front and the evolution of the driving suspension due to settling of particles to the underlying bed the model captures the key features of a range of experimental observations the analysis also provides important scaling relationships between the geometry of a deposit and the source conditions for the deposit forming flow as well as explicit expressions for flow speed and deposit thickness as functions of radial distance from the source among the results of our study we find that in the absence of information regarding flow history the geometries of relatively well sorted deposits generated by flows with source conditions of constant volume or constant flux are virtually indistinguishable the results of our analysis can be used by geologists in the interpretation of some geologically important gravity surge deposits using our analytical results we consider three previously studied radially symmetric turbidites of the hispaniola caicos basin in the western atlantic ocean from gross geometry and grain size of the turbidites alone we estimate for the respective deposit forming events that upon entry into the basin the initial sediment concentrations were approximately 3 by volume and the total volumes were roughly between 30 km 3 and 100 kn1 3 each of the suspension driven flows is inferred to have spread into the basin with a characteristic speed of 3 5 m s 1 and reached its ultimate runout length of about 60 75 km while laying down a deposit over a period of about 10 12 hours

ordovician convergent margin volcanism and tectonism in the lachlan sector of east gondwana the enigmatic early paleozoic tectonic setting of the lachlan orogen in australia has obscured reconstructions of east gondwana recognition of an intraoceanic mafic island arc in this paper establishes the presence of a convergent plate boundary in the tasmanides from the early ordovician to the early silurian this in turn facilitates the updating of supercontinent correlations

mediterranean neogene stratigraphy development and evolution through the centuries the mediterranean area in general and italy in particular played an important role in developing the principles of stratigraphy since the early days of geological thinking in the middle of the 19th century there was a rush to name all the intervals of the stratigraphic column at that time when invertebrate palaeontology was the principal source of information on the relative age of the rock units dozens of stage names were created for the neogene only a few of these stage names survive and are accepted worldwide as langhian serravallian tortonian messinian zanclean and piacenzian in the later part of the 19th century and in the first decades of the 20th century a wealth of new data was made available by extensive palaeontological studies but the correlation potential of the fossil faunas was constrained poorly and understood less leading to the development of a floating stratigraphy the situation improved after world war ii when micropalaeontology underwent a fast development the creation of common stratigraphic guidelines the introduction of the stratotype concept followed by the golden spike concept global stratotype section and point combined with the application of multiple stratigraphic methodologies especially palaeomagnetic stratigraphy or magnetic polarity time scale astrocyclostratigraphy or astronomical time scale resulted in new scenarios a turning point may be located around 1970 soon after the beginning of the deep sea drilling project which greatly contributed to the development of a new approach to neogene stratigraphy now all the global stratotype sections and points for the neogene stages are being defined by multiple criteria with precise definitions in terms of magnetic polarity time scale and astronomical time scale the rossello composite section in southern sicily is the template for the late neogene stratigraphy with the best time resolution provided by astronomical forcing to complete the planned study the same criteria have to be applied downwards and upwards

crustal scale fluid pathways and source rocks in the victorian gold province australia insights from deep seismic reflection profiles deep seismic reflection data collected across the western lachlan orogen of southeast australia have provided important insights into crustal scale fluid pathways and possible source rocks in one of the richest orogenic gold provinces in the world the profiles span three of the most productive structural zones in victoria the stawell bendigo and melbourne zones zone scale variations in the age and style of gold deposits correspond with differences in crustal structure and composition the bilateral distribution of gold production in the stawell and bendigo zones is related to the v shaped crustal scale geometry of the two zones in cross section major first order faults like the east dipping moyston fault and a set of west dipping listric faults within the bendigo zone were probably major fluid conduits in the lower to middle crust during gold mineralization first order faults in the stawell and bendigo structural zones appear to have accommodated large scale thickening down to the lower crust the faults converge in a region beneath the western bendigo zone where a thick section of mafic volcanic rocks and lesser sedimentary rocks are identified as a likely common source of gold bearing fluid that was largely generated by late ordovician to early silurian metamorphism the areas with the greatest gold endowment lie above lower crustal regions that have preserved the thickest succession of fertile mafic igneous rocks up to about 25 km thick they also correspond to a region of thin or absent precambrian lithosphere mafic rocks in the stawell zone and far western bendigo zone were probably partly consumed by cambrian west dipping subduction similar rocks in the rest of the bendigo zone lay outside the influence of cambrian subduction accretion and were deformed later probably beginning in the late ordovician early silurian benambran orogeny during crustal scale imbrication this imbrication preserved much of the mafic rocks to form the lower to middle crust first order listric faults in the bendigo zone are interpreted as major controls on the locations of goldfields even though they are largely unmineralized near the present surface the shallow dipping segments of first order listric faults were favorably oriented for reactivation at the time of gold mineralization and acted as major fluid conduits in the lower to middle crust in contrast the upper steeply dipping segments of first order listric faults were unfavorably oriented for reactivation and were poor fluid conduits the seismic data show that the transition from predominantly shallow to steeply dipping fault segments occurs in the middle to upper crust near the boundary between thick imbricated metavolcanic rocks that lie immediately below 6 to 15 km of folded metasedimentary rocks this transition may have coincided with fluid escape zones that aided the transfer of permeability away from first order faults and into the overlying fold dominated turbidites this transfer of permeability was enhanced by the growth of subvertical fold related fault and fracture meshes in the upper crustal turbidites the fault and fracture meshes consisted of bedding parallel faults limb thrusts and tension vein arrays that developed along fold hinges in the bendigo zone individual fold hinges and regional fold culminations were important controls on the distribution of fluid flow fluid flow was partly syndeformational but overlapped into the immediate postdeformational period of the benambran orogeny later reactivation of first order faults in the late silurian to early devonian and again in the late devonian led to further although less important mineralizing events as fluids exploited the preexisting fault architecture deeply penetrating north dipping listric faults in the less gold rich melbourne zone cut into inferred proterozoic basement and may have been fluid conduits during a late devonian mineralizing event

the palaeozoic tectono metallogenic evolution of the northern tasman fold belt system australia interplay of subduction rollback and accretion abstract the tasman fold belt system in eastern australia provides a record of the palaeozoic geological history and growth of the australian continent along the proto pacific margin of gondwana inboard of an extensive and long lived subduction system the hodgkinson and broken river provinces represent prominent geological elements of this system and together form the northern tasman fold belt system geochronological age dating of the timing of gold formation in the amanda bel goldfield in the broken river province and the hodgkinson goldfield in the hodgkinson province provides constraints on the occurrence of a deformation and mineralisation episode in the late devonian early carboniferous integration of these newly obtained data with petrogenetic constraints and a time space evaluation of the geological evolution of the hodgkinson and broken river provinces as well as other terranes in the northern tasman fold belt system allows for the development of a geodynamic model for the palaeozoic evolution of the northern tasman fold belt system our model indicates that three cycles of extension contraction occurred during the palaeozoic evolution of the northern tasman fold belt system episodes of extension were controlled by rollback of the subduction system along the proto pacific margin of gondwana whereas episodes of contraction resulted from accretion following the arrival of positively buoyant segments i e micro continental blocks oceanic plateaus at the subducting trench our composite interpretative model on the geodynamic evolution of the northern tasman fold belt system integrates the timing of the development of mineral deposits throughout this part of the system and provides a significant advancement in the understanding of palaeozoic geodynamics along the margin of gondwana in northeast australia and allows comparison with the southern part of the tasman fold belt system

saline and particle driven interfacial intrusions this paper presents a theoretical and experimental investigation into saline and particle driven intrusions along the interface between two layers of dierent densities the conditions at the nose of an intrusion are described in an analysis similar to that applied by benjamin 1968 to boundary gravity currents equations for propagation velocity and front position as functions of relative density are derived these are used in an integral model for intrusions which also includes the eects of sedimentation of particles and detrainment of interstitial fluid the model describes the time evolution of the length of the intrusion and the sediment distribution it produces laboratory experiments were carried out with lock releases of a xed volume of saline or particleladen fluid into a two layer stratication measurements were taken of the intrusion propagation intrusion position and sediment distribution and are found to be in good agreement with the solutions of the integral model horizontal density gradients in a fluid may give rise to gravity currents sometimes called density currents the density gradients can be due to dierences in fluid composition phase temperature or concentration of suspended particulate in a suciently quiescent environment a sharp front may be formed across which a density dierence exists between the current and the surroundings this density dierence drives the mainly horizontal motion of the gravity current when the surrounding fluid is vertically stratied the gravity current can enter it at intermediate levels where its density is comparable with that of the ambient and is then called a gravity intrusion gravity intrusions occur frequently in the atmosphere when rising buoyant air reaches levels where its density matches the ambient density and spreads out horizontally a striking example is the outflow above volcanic plumes which carry ash high into the atmosphere and then spread laterally over many thousands of kilometres gravity intrusions are also an important phenomenon in oceanography a much quoted example is the warm but saline outflow from the mediterranean in the eastern atlantic ocean which produces a saline intrusion at about 1000 m depth particle driven intrusions in the ocean can occur when material is suspended by a river outflow by an underwater landslide from a shelf edge or even articially

fine sediment deposition from gravity surges on uniform slopes abstract the propagation of and the deposition from a noneroding turbulent gravity surge are described by a simple model for a two dimensional well mixed buoyant cloud of suspended particles moving down an inclined surface the model includes the effects of entrainment of ambient seawater deposition of suspended sediment seafloor friction and slope our results are applicable to large decelerating turbidity currents and their distal deposits on uniform slopes in lakes and the sea the scaling arguments that emerge from our analysis moreover have important ramifications for the design and interpretation of laboratory analogs of these phenomena general solutions are obtained to the coupled equations that describe the evolution of momentum total mass and particulate mass of a surge the solutions vary on two horizontal length scales xo beyond which the behavior of the surge is independent of the initial momentum and shape and xr beyond which the driving negative buoyancy of the surge is lost due to particle settling for fine particles whose settling velocity is much less than the forward propagation speed of the surge the suspension is well mixed and xo xr the deposit thickness diminishes as the inverse square root of the downstream distance x when xo x xr and then diminishes exponentially with downstream distance as x approaches and exceeds xr the length of a surge deposit scales with xr kbosin aws cos 2 where k is the assumed constant aspect ratio of the surge bo is the initial buoyancy per unit width at the point of issue onto a slope of constant angle a is the ambient density ws is the average settling velocity of the suspended particles and 6 8cd incorporates the ratio of the constant coefficients of drag cd and fluid entrainment extension of our model to the case of two particle sizes indicates that even for very poorly sorted suspensions the estimate for the length of a surge deposit xr is valid if ws is defined as the volume averaged settling velocity of the initial suspension at xo the ratio of coarse to fine material in model deposits generated from initially poorly sorted suspensions can diminish dramatically in the downstream direction however due to differential rates of gravitational settling

emplacement of the taupo ignimbrite by a dilute turbulent flow an ignimbrite is a pumice rich deposit that records the passage of a ground hugging ash flow a pyroclastic flow generated by the collapse of a volcanic eruption column1 geologists study such deposits to reconstruct the parent eruptions and to predict the consequences of future eruptions the 1 800 yr old taupo ignimbrite of new zealand has been interpreted to represent en masse emplacement by an avalanche like flow with a volumetric solids concentration in excess of 30 ref 2 the evidence for this is equivocal however and here we propose the alternative view that the deposit was emplaced by a relatively dilute and turbulent density current3 we present an isothermal hydraulic model the results of which taken together with existing observations suggest that the total flow rate of solids and gas was about 40km3s 1 for around 15 minutes this intense flux resulted in a flow which had a near vent solids concentration of 0 3 by volume was about 1 km thick and travelled outward from the vent with a typical speed of 200 ms 1 in view of the good agreement between predicted and observed radial trends in the taupo deposit we suggest that the origin of other ignimbrites with similar characteristics should be reconsidered

an experimental study of iron sulfides weathering under simulated calcareous soil conditions in calcareous sites hard rock mining activities release pyrite fes2 pyrrhotite fe1 xs and other sulfides to soils the sulfides then undergo weathering processes generating acid rock drainage and secondary compounds despite the potentially important environmental impacts very few studies have considered the mechanisms of pyrite and pyrrhotite weathering and the transformation of secondary compounds under neutral alkaline carbonated conditions in this study we used an experimental approach combining electrochemical microscopic and spectroscopic techniques to examine the interfacial processes involved in pyrite and pyrrhotite weathering under simulated calcareous soil conditions the results showed an initial oxidation step with the formation of variable amounts of surface sulfur compounds e g polysulfides sn2 and elementary sulfur s0 and acid generation leading to significant modification of the oxidative behavior of the minerals the surface changes that occurred as a result of mineral weathering provoked transient enhancement of pyrite reactivity and progressive passivation in the pyrrhotite system iron sulfides weathering was found to involve the formation of an intermediate siderite feco3 like compound preceding the predominant formation of k jarosite k fe3 so4 2 oh 6 and or ferric oxyhydroxide feooh compounds depending on the surface acid condition reached in the systems mechanisms of pyrite and pyrrhotite weathering in calcareous soils are suggested on the basis of surface characterization and chemical analysis of the leachates generated and the environmental implications are discussed

geodynamic settings of mineral deposit systems mineral deposits represent extraordinary metal concentrations that form by magmatic magmatic hydrothermal or hydrothermal processes in geodynamic environments typified by anomalously high thermal and or mechanical energy near plate boundaries as they require the conjunction of specific environmental conditions to form particular mineral deposit types tend to occupy specific geodynamic niches the temporal distributions of mineral deposit types reflect both formational and preservational processes in the archaean and palaeoproterozoic these were linked because of preservation in continental crust connected to thick buoyant subcontinental lithospheric mantle sclm but were decoupled by the neoproterozoic and phanerozoic as a result of evolution to thinner increasingly dense sclm the transition marks a change from mantle plume influenced plate tectonics to modern style plate tectonics with broadly coincident environmental changes and a major impact on the nature and abundance of preserved mineral deposit types as mineral deposits represent an integral part of tectonic process they are essential indicators of that process and geodynamic settings and should be incorporated into any holistic tectonic terrane analysis their distribution also provides a particularly critical test on ancient continental reconstructions derived from palaeomagnetic data conversely such reconstructions provide a first order targeting tool for the conceptual exploration required to discover new mineral provinces and deposits under cover

convergence and intraplate deformation in the lachlan fold belt of southeastern australia abstract the lachlan fold belt of southeastern australia is dominated by a widespread oceanic association including cambrian submarine mafic volcanics and an overlying widespread ordovician quartz rich turbidite and black shale succession with scattered mafic to andesitic volcanic centres these rocks are overlain by or in fault contact with deep marine to continental silurian to early carboniferous successions with mafic to silicic volcanics and abundant plutonic rocks major deformation achieved by thrusting and folding affected the fold belt during the silurian to middle devonian and the early carboniferous shortening estimates vary throughout the belt but the widespread ordovician quartz turbidite succession usually has at least 60 shortening which implies an original width of the fold belt of about 1700 km strike slip faulting was important on a local to regional scale but large scale strike slip displacements appear unlikely the large values of shortening are consistent with development of the belt in a convergent margin setting at least for the silurian to carboniferous the nature of the lower crust of the lachlan fold belt has always presented a problem of interpretation but we favour a late proterozoic quasi continental lower crustal layer that has deformed independently of the upper crust by either homogeneous flattening or crustal duplexing during silurian carboniferous convergence and provided a source for silurian devonian granites

stress controls on intrusion related gold lodes wonga gold mine stawell western lachlan fold belt southeastern australia the wonga gold lodes 300 000 oz au in the victorian gold fields of eastern australia formed at a depth of less than 7 km within the seismogenic zone the wonga gold deposit is hosted by mudstone dominant lithofacies that define the core of the regional synform these are underlain by sandstone dominant lithofacies that are interlayered with mafic schist the lodes occur within the contact aureole of a late stage granitic body that intrudes an earlier phase of the fractionated pluton in contrast to the nearby 440 ma magdala orogenic gold lodes 5 moz au this smaller deposit formed at ca 400 ma and has been classified as intrusion related the orogenic and intrusion related gold deposits formed at different stages in the structural and magmatic evolution of the fold belt and at different crustal levels compared to the magdala lodes the wonga lodes have a close temporal and spatial relationship with the intrusive rocks and a distinct structural style and mineralogy including acicular arsenopyrite stibnite gold telluride alloys bismuth silver and late stage molybdenite the wonga lodes formed between two distinct phases of regional faulting and the kinematics of the mineralized structures cannot be correlated with any preserved far field regional paleostresses instead they are inferred to reflect the local state of stress at the point that magmatic related fluid overpressure occurred pervasive fluid overpressure drove deformation across a broad zone producing massive mineralized quartz vein structures that included angular clasts of wall rock a discrete overpressure event is suggested rather than repeated crack seal behavior the lodes represent an early stage in the development of a fault zone characterized by a series of discontinuous discrete shear failures the geometry of the lodes is strongly dependent on structural position and rheology northeast dipping preexisting weaknesses such as dike margins and variably developed crenulation cleavages produced localized shear failure defined by dextral reverse slip structures the hanging wall structures 30 to 60 se dipping extension veins and extensional shear veins the link structures diverge from these creating en echelon lode geometries there is a sudden change over the top of a regional plunge reversal defined by the dominant fold axes from shear failure the hanging wall structures to extensional shear and then extensional failure the links this occurs along strike in the same structural corridor and within the same stratigraphic unit the explicit finite difference code flac was used to model the local state of stress at the point where an influx of magmatic fluids raised the pore fluid pressure the model predicts that a zone of slightly lower differential stress was associated with the plunge reversal and this variation in differential stress is interpreted to have caused the drastic change in lode style the modeling highlights that only subtle differences in the elastic moduli of the rocks are required to produce the observed stress geometry

geochemical transport modelling of drainage from experimental mine tailings cells covered by capillary barriers the use of covers with capillary barrier effects ccbes for reducing acid mine drainage amd from sulphidic mine tailings is simulated using the min3p finite volume model for coupled groundwater flow o2 diffusion and multi component reactive transport the model is applied to simulate five pilot scale in situ test cells containing reactive tailings from the manitou mine site val d or que canada four of the cells were constructed with ccbes over the tailings while the fifth tailings cell was left uncovered observed and simulated discharge from the base of each cell showed that the capillary barrier covers significantly reduced sulphide oxidation and amd compared to acidic discharge from the uncovered cell discharge from the four ccbe covered cells had neutral ph levels and 1 7 orders of magnitude lower concentrations of so4 fe zn cu and al the simulations showed that the moisture retaining layer of the ccbes reduced amd by inhibiting o2 diffusion into the underlying reactive wastes provided the moisture retention layer of the ccbe remains close to saturation its thickness had a relatively minor effect under such near saturated conditions o2 availability is limited by its diffusion rate through the bulk porous medium and not by the diffusion rate through the oxidized grain shells the model is providing important new insights for comparing design alternatives for reducing or controlling amd