evidence for two stages of mineralization in west africa s largest gold deposit obuasi ghana the supergiant obuasi gold deposit is the largest deposit in the paleoproterozoic birimian terranes of west africa with 62 moz of gold past production resources the deposit is hosted in the paleoproterozoic kumasi group sedimentary rocks composed of carbonaceous phyllites slates psammites and volcaniclastic rocks intruded by different generations of felsic dikes and granites a three stage deformation history is defined for the district the d1 ob stage is weakly recorded in the sedimentary rocks as a layer parallel fabric and indicates that bedding parallel shearing occurred during the early stage of deformation at obuasi the d2 ob is the main deformation stage affecting the obuasi district and corresponds to a nw se shortening tight to isoclinal folding as well as intense subhorizontal stretching occurred during d2 ob parallel with the plane of a pervasive ne striking subvertical foliation s2 ob finally a n s shortening event d3 ob refolded previously formed structures and formed a distinct ene striking variably dipping s3 ob cleavage that is domainal in nature throughout the deposit two economic styles of mineralization occur at obuasi and contribute equally to the gold budget these are 1 gold bearing sulfides dominantly arsenopyrite mainly disseminated in metasedimentary rocks and 2 native gold hosted in quartz veins that are as much as 25 m wide microstructural evidence such as strain shadows surrounding gold bearing arsenopyrite parallel with s2 ob but folded by s3 ob indicates that the sulfides were formed during d2 ob concentrations of as much as 700 ppm au are present in the epitaxial growth zones of the arsenopyrite grains although the large mineralized quartz veins are boudinaged and refolded indicating their formation during d2 ob field and microanalytical observations demonstrate that the gold in the veins is hosted in microcracks controlled by d3 ob where the s3 ob cleavage crosscuts the quartz veins in the main ore zones thus these observations constitute the first evidence for multiple stages of gold deposition at the obuasi deposit futhermore three dimensional modeling of stratigraphy structure and gold orebodies highlights three major controls on oreshoot location which are 1 contacts between volcaniclastic units and pre d 1 felsic dikes 2 fault intersections and 3 f3 ob fold hinges the maximum age for the older disseminated gold event is given by the age of the granites at 2105 2 ma which is within error of hydrothermal rutile in the granites of 2098 7 ma the absolute age of the younger gold event is not known

gold speciation and transport in geological fluids insights from experiments and physical chemical modelling this contribution provides an overview of available experimental thermodynamic and molecular data on au aqueous speciation solubility and partitioning in major types of geological fluids in the earth s crust from low temperature aqueous solution to supercritical hydrothermal magmatic fluids vapours and silicate melts critical revisions of these data allow generation of a set of thermodynamic properties of the auoh aucl 2 auhs and au hs 2 complexes dominant in aqueous hydrothermal solutions however other complexes involving different sulphur forms chloride and alkali metals may operate in high temperature sulphur rich fluids vapours and melts the large affinity of au for reduced sulphur is responsible for au enrichment in s rich vapours and sulphide melts which are important gold sources for hydrothermal deposits thermo dynamic speciation and partitioning data and their comparison with au and s contents in natural fluid inclusions from magmatic hydrothermal gold deposits provide new constraints on the major physical chemical parameters temperature pressure salinity acidity redox and ubiquitous fluid components sulphur carbon dioxide arsenic affecting au concentration transport precipitation and fractionation from other metals in the crust the availability and speciation of sulphur and their changes with the fluid and melt evolution are the key factors controlling gold behaviour in most geological situations

quartz vein fabrics coupled to elevated fluid pressures in the stawell gold deposit south eastern australia shear and extensional veins formed during the reactivation of the magdala shear system at stawell in western victoria australia contribute to the formation of the auriferous central and basalt contact lodes within this shear system is a range of fault rocks accompanied by steep dipping 65 quartz rich laminated shear veins and relatively flat lying extensional veins both vein sets appear to have been a primary source for the host rock permeability during fluid flow in a regime of significant deviatoric stresses the macro and microstructures suggest that the dilatancy that produced mineralized veins formed under conditions of overpressure generated by fluid infiltration late in a tectonic regime a new microfabric analysis technique is used to investigate the quartz rich veins which allows rapid integration of the microstructure with the crystallographic preferred orientations cpos both the shear and extensional quartz veins have a random cpo with 120 dihedral angles between the quartz quartz grains which is typical of a metamorphic equilibrium microfabric the microstructures indicate that the quartz has undergone extensive grain adjustment in the solid state with grain shape and size affected by interfacial solution pressure solution effects these features are consistent with inferences from experimental rock deformation studies where grain boundary migration is enhanced in a water rich environment the onset of solution transfer processes pressure solution developed as the quartz microfabric stabilized and continued to modify the cpo and microstructure significantly it is concluded that grain growth and pressure solution are coupled diffusive mass transfer processes related to fluctuations in pore fluid pressures in a region undergoing deformation at near lithostatic pressures

gold remobilisation and formation of high grade ore shoots driven by dissolution reprecipitation replacement and ni substitution into auriferous arsenopyrite abstract both gold rich sulphides and ultra high grade native gold oreshoots are common but poorly understood phenomenon in orogenic type mineral systems partly because fluids in these systems are considered to have relatively low gold solubilities and are unlikely to generate high gold concentrations the world class obuasi gold deposit ghana has gold rich arsenopyrite spatially associated with quartz veins which have extremely high localised concentrations of native gold contained in microcrack networks within the quartz veins where they are folded here we examine selected samples from obuasi using a novel combination of quantitative electron backscatter diffraction analysis ion microprobe imaging synchrotron xfm mapping and geochemical modelling to investigate the origin of the unusually high gold concentrations the auriferous arsenopyrites are shown to have undergone partial replacement 15 by au poor nickeliferous arsenopyrite during localised crystal plastic deformation intragranular microfracture and metamorphism 340 460 c 2 kbars our results show the dominant replacement mechanism was pseudomorphic dissolution reprecipitation driven by small volumes of an infiltrating fluid that had relatively low s 2 and carried aqueous nicl 2 we find that arsenopyrite replacement produced strong chemical gradients at crystal fluid interfaces due to an increase in s 2 during reaction which enabled efficient removal of gold to the fluid phase and development of anomalously gold rich fluid potentially 10 ppm or more depending on sulphur concentration this process was facilitated by precipitation of ankerite which removed co 2 from the fluid increasing the relative proportion of sulphur for gold complexation and inhibited additional quartz precipitation gold re precipitation occurred over distances of 10 m to several tens of metres and was likely a result of sulphur activity reduction through precipitation of pyrite and other sulphides we suggest this late remobilisation process may be relatively common in orogenic belts containing abundant mafic ultramafic rocks which act as a source of ni and co scavenged by chloride bearing fluids both the preference of the arsenopyrite crystal structure for ni and co rather than gold and the release of sulphur during reaction can drive gold remobilisation in many deposits across broad regions

hydrothermal mineralising systems as chemical reactors wavelet analysis multifractals and correlations abstract hydrothermal mineralising systems are discussed as large open flow chemical reactors held far from equilibrium during their life time by deformation and the influx of heat fluid and dissolved chemical species as such they are nonlinear dynamical systems and need to be analysed using the tools that have been developed for such systems hydrothermal systems undergo a number of phase transitions during their evolution and this paper focuses on methods for characterising these transitions in a quantitative manner and establishing whether they resemble either abrupt or continuous critical phase transitions or whether they have some other kind of nature critical phase transitions are characterised by long range correlations for some parameter characteristic of the system power law probability distributions so that there is no characteristic length scale and a high sensitivity to perturbations the transitions undergone in mineralised hydrothermal systems are i widespread non localised mineral alteration involving exothermic mineral reactions that produce hydrous silicate phases carbonates and iron oxides ii strongly localised veining brecciation and or stock work formation iii a series of localised endothermic mineral reactions involving the formation of non hydrous silicates sulphides and metals such as gold iv multiple overprinting repetitions of transitions ii and iii we quantify aspects of these transitions in some gold deposits from the yilgarn craton of western australia using wavelet transforms this technique is convenient and fast it enables one to establish if the transition is multifractal and if so quantify the multifractal or singularity spectrum and to determine the scale dependence of long range correlations or anti correlations other aspects of the spectrum enable quantitative distinctions between sub critical critical and super critical systems the availability of long drill holes with detailed chemical analyses and mineral abundances derived from hyperspectral data enables individual ore bodies to be characterised rapidly in a quantitative manner and constraints placed on whether the various transitions are possibly critical or of some other form we also present some simple nonlinear models including numerical simulation self organised branching and multiplicative cascade processes that produce the multifractal character and correlation scaling relations observed in these data sets distinctions between systems that are strongly and weakly mineralised are discussed

the influence of ambient fault temperature on flash heating phenomena 1 recent high velocity friction experiments on room temperature rocks reveal a dramatic weakening caused by extreme frictional heating and thermal degradation of the strength of microscopic asperity contacts on a slip surface i e flash heating here we explore the effects of elevated ambient fault temperature up to that at the base of the earth s seismogenic zone on the flash heating behavior of quartzite westerly granite and india gabbro the experiments demonstrate that elevated ambient fault temperature causes 1 an increase in the characteristic weakening velocity for flash heating and 2 an increase in the friction coefficient at a given velocity above this characteristic weakening velocity these unexpected results are reconciled with flash heating theory when temperature specific values of various material parameters are incorporated in existing constitutive descriptions of flash heating our results suggest that constitutive equations for flash heating can be extrapolated with confidence to the elevated ambient temperatures of the seismogenic zone

fluid pressure dominated orogenic gold mineralization under low differential stress case of the yaoure gold camp cote d ivoire west africa the 1 5 moz yaoure gold deposit which is located within the birimian greenstone belts of central cote d ivoire west africa is associated with two stages of economic gold mineralization both ore forming stages occurred during protracted e w directed shortening associated with the eburnean orogeny the first mineralization stage is hosted by fault fill veins along conjugate ene striking dextral and wnw striking sinistral strike slip fault arrays this ore stage is associated with a polymetallic assemblage including mo bi pb w te ag and au and cryptic alteration in wall rocks the second and main gold mineralization stage is associated with a low displacement fault fracture vein mesh formed during a reverse faulting episode this second gold phase is associated with a polymetallic assemblage including w and au pb and is characterized by pronounced albite ankerite pyrite hydrothermal alteration in wall rocks shear veins associated with this second mineralization episode formed during episodic fluid pressure variations under low differential stress within pre existing low angle reverse fault planes permeability destruction by albite ankerite pyrite hydrothermal sealing of these low angle reverse faults resulted in fluid pressure build up and ultimately in the development of extension veins under low differential stress in the late stages of the structural evolution of the deposit this study presents the structural evolution of a polyphase orogenic gold deposit and highlights that the interplay between fault zone activity hydrothermal alteration and fluid pressure fluctuations at low differential and deviatoric stresses were key in controlling fluid flow and the formation of ore within optimally oriented faults

flash vaporization during earthquakes evidenced by gold deposits fluids flowing through cavities in earth s crust can deposit gold thermo mechanical modelling of a fluid filled cavity that expands suddenly during an earthquake shows that the drop in pressure would cause the fluid to vaporize and deposit the gold almost instantaneously

provenance of the late ediacaran rocha formation cuchilla dionisio terrane uruguay tectonic implications on the assembly of gondwana abstract a provenance study on the rocha formation cuchilla dionisio terrane uruguay based on elemental and isotope geochemistry and geochronology revealed characteristics typical of turbidites linked to orogenic arcs textural and mineralogical immaturity and low weathering and sedimentary recycling indicated by low cia values th u and zr sc ratios we separate data into two groups namely northern and southern sectors even though the whole unit shows a provenance of upper continental crust denoted by th sc zr sc th u ratios around ucc averages ree patterns parallel to paas and negative eu anomalies a more depleted component was detected mainly in the northern sector ns showing lower th sc and th u ratios negative eu anomalies of up to 0 9 high abundances of compatible elements and lower end t and 87sr 86sr ratios than the southern sector ss u pb detrital zircon ages indicate 569 10 ma as the maximum sedimentation age the most prominent age peak is neoproterozoic and while the ns shows dominance of tonian and a broad data spread for paleoproterozoic ages the southern sector comprises both the youngest and oldest zircon and displays a pattern with a more pronounced stenian population whereas paleoproterozoic ages are restrained to orosirian a k s test indicates that the change in provenance shown by detrital zircon patterns is reflecting variations of the sediment supply linked to basinal geomorphic heterogeneities within an active margin given detailed whole rock lithogeochemistry nd pb and sr isotopic analyses as well as detrital zircon age patterns correlations with the oranjemund group of the marmora terrane and with the tygerberg formation and associated units of the malmesbury terrane western saldania belt raised from previous interpretations are reassessed source areas were most likely located within the cuchilla dionisio terrane and kalahari craton and therefore deposition within a foredeep basin occurred before tangential accretion to the rio de la plata craton along the sierra ballena shear zone

hydrothermal transport and depositional processes in archean lode gold systems a review abstract compelling evidence now exists for the formation of archean lode gold deposits over a substantial range in metamorphic conditions and crustal depth the commonality of ore fluid isotopic and geochemical characteristics the extreme fluid rock ratios within actual ore zones and the observed depth continuity within individual orebodies all suggest that these deposits formed within vertically extensive crustal scale hydrothermal systems calculations based on new thermodynamic data and estimated ore fluid conditions suggest that gold would have been transported as au hs 2 over most of this range gold transport in the ore fluids that formed some diopside and diopside k feldspar class deposits t 550 c would have been as aucl2 whereas the au hs complex may have been significant in some of the lower temperature 270 c less alkaline ore fluids the high solubility of gold in lode gold ore fluids at amphibolite facies conditions 10 s to 1000 s ppb suggests that fluids may have been undersaturated in gold within their source regions promoting the ore fluid s capacity to retain gold in solution over the large transport distances involved ore depositional mechanisms were also likely to vary between crustal levels fluid rock interactions and wallrock sulphidation in particular appear to be the most important type of precipitation mechanism however with decreasing depth of formation the scope of other depositional mechanisms notably phase separation and fluid mixing becomes greater

shear heating of a fluid saturated slip weakening dilatant fault zone 2 quasi drained regime 1 this paper analyzes slip on a fluid infiltrated dilatant fault for imposed tectonic strain rates much slower than the rate of fluid exchange between the gouge zone and the surroundings and for exchange of heat slower than of fluid typical of interseismic or most laboratory loading conditions the limiting solution corresponding to the infinitely rapid drainage rate developed in the companion paper by garagash and rudnicki 2003 yielded unbounded slip acceleration for sufficiently large slip weakening of the fault analysis for rapid but finite drainage rates quasi drained condition reveals two types of possible instability the first is inertial instability a seismic event characterized by an unbounded slip rate and attributed to the destabilizing effects of shear heating and fault slip weakening the second corresponds to loss of uniqueness and arises from the small pressure increase caused by shear heating for rapid but finite rates of drainage this instability emerges for even small amounts of thermomechanical coupling and a large range of dilatancy and slip weakening its resolution probably requires a more elaborate frictional description e g rate and state but within the slip weakening framework here leads to either slip arrest or inertial instability the response is always unstable in one of these two ways for sufficiently large thermal coupling or initial stress regardless of the amount of slip weakening and dilatancy the counterintuitive stabilizing effect of increased slip weakening or decreased dilatancy similar to the effect in the companion paper also occurs for nearly drained slip

thermo poro mechanics of chemically active creeping faults 3 the role of serpentinite in episodic tremor and slip sequences and transition to chaos during the last decade knowledge over episodic tremor and slip ets events has increased dramatically owing to the widespread installation of gps and seismic networks the most puzzling observations are i the periodic nature of slow seismic events ii their localization at intermediate depths estimated 15 40 km and iii the origin of the nonvolcanic fluids that are responsible for the tremor activity we reconcile these observations using a first principles approach relying on physics continuum mechanics and chemistry of serpentinite in the megathrust interface the approach reproduces the gps sequences of 17 years of recording in cascadia north america as well as over 10 years in the hikurangi trench of new zealand we show that strongly endothermic reactions such as serpentinite dehydration are required for ets events we report that in this tectonic setting it is its chemical reaction kinetics not the low friction that marks serpentinite as a key mineral for stable self sustained oscillations we find that the subduction zone instabilities are driven from the ductile realm rather than the brittle cover even when earthquakes in the cover perturb the oscillator it relaxes to its fundamental mode such a transition from stable oscillations to chaos is witnessed in the ets signal of nz following the m6 8 2007 seismic event which triggered a secondary mode of oscillations lasting for a few years we consequently suggest that the rich dynamics of ductile modes of failure may be used to decipher the chaotic time sequences underpinning seismic events

thermal pressurization and slip weakening distance of a fault an example of the hanaore fault southwest japan a controversy over the last decade is the large difference in the slip weakening distance d c between laboratory measured and seismically estimated values our numerical analyses of the thermal pressurization process based on measured transport properties of the hanaore fault zone in japan reveal that the process is effective for this fault and that d c becomes of the order of several decimeters to about 1 m when the width of the deformation zone is less than 10 20 mm this d c value is of exactly the same order as that determined seismically so thermal pressurization can be a mechanism for solving the d c paradox the thermal pressurization process effectively reduces the heat production rate and the temperature rise expected for this fault zone is at most about 600 c which is not high enough a temperature to melt rocks

mineral precipitation in the quartz reefs of the bendigo gold deposit victoria australia gold precipitation at bendigo postdates cleavage development and was initiated during folding increased during a reverse faulting stage in bedding parallel quartz veins and culminated in quartz reefs associated with a strike slip faulting event some gold was remobilized along carbonate rich cataclasites associated with later brittle faults understanding the paragenesis of the sulfide and gangue minerals associated with the auriferous quartz reefs is critical to unraveling the cycle of ore genesis from source to sink in this study a petrographic approach closely tied to macroscopic structural observations along with stable isotope and fluid inclusion analyses places constraints on the relative timing of gold mineralization with respect to deformation early sulfide assemblages in bedding and cleavage parallel veins are dominated by pyrite pyrrhotite siderite and are free of visible gold later assemblages in reactivated bedding parallel veins and other fold related veins are characterized by the presence of arsenopyrite ankerite gold fault related veins and the massive quartz reefs are rich in sphalerite and galena with associated gold but lack pyrrhotite small amounts of late stage antimony bearing minerals occur in many vein types and postdate the precipitation of gold decreasing temperature and increasing sulfur activity controlled successively younger sulfide assemblages in the quartz veins stable oxygen isotope data show that quartz from all vein types 18 o 15 9 19 0 homogenized with the host rocks whereas carbon 13 c 14 0 to 4 1 and oxygen 18 o 4 5 24 0 in carbonate have a wider range in values that is interpreted to be a function of decreasing temperature this corresponds with early siderite pyrrhotite and anhedral pyrite and with later ankerite and ferroan dolomite associated with arsenopyrite galena and euhedral pyrite that are crosscut by calcite veins fluid inclusions in quartz veins are predominantly composed of water with carbon dioxide with smaller proportions of nitrogen and methane particularly in the later strike slip faults the sporadic occurrence of methane suggests that there was an open fluid system with incoming fluids from an external source mixing with those in the host rocks although several recent studies have argued that synsedimentary preenrichment may be a significant factor determining the size and distribution of gold in the bendigo orogenic gold deposits we believe there is little direct or deposit scale evidence for a relationship between original metal content in the host rock metasedimentary rocks and the distribution of the gold mineralization in specific structural sites it is suggested that deep seated faults act as conduits for fluid flow and the source for the gold at bendigo the gold is externally derived from deeply sourced auriferous metamorphic fluids that have been focused into discrete structural sites

underground fumaroles excess heat effects in vein formation we examine the heat extracted from a fractured hydrothermal system following the sudden dilation of fractures in the host rock this dilation increases the permeability of the system and veins form along irregular fractures where the opening is localized for simplicity we will consider one such vein in this paper fluid in the system is assumed to be initially close to vapor saturation and in thermal equilibrium with the surrounding host rock the rapid opening of the vein leads to approximately isenthalpic vapor separation and causes a pressure drop to propagate through the system as the two phase fluid in the fracture system is constrained to the vapor pressure curve the depressurization is accompanied by a rapid cooling of the fluid fluid flow is also induced from a hydrothermal reservoir toward the dilated discharge vein the temperature difference between the host rock and the two phase fluid in the fracture system drives heat from the rock into the fluid the excess heat 1 1 derived from the host rock in this manner evaporates additional liquid in the two phase flow which develops substantially larger vapor fractions than those due only to the rapid initial adiabatic depressurization of the fluid such additional vapor separation therefore leads to high solute concentrations in the liquid phase and potentially results in faster deposition of minerals in the fracture system and local veins a model is developed here to describe the minimum excess heat transfer i e the worst case scenario into the two phase saturated liquid vapor mixture as it is drawn horizontally from the hydrothermal reservoir toward the discharge vein excess heat effects are shown to be effective within tens to hundreds of meters of actively forming veins and a principal cause of oscillatory banding and lamination fabrics this transient heat extraction process applies to a wide range of shallow crustal deposit styles associated with brittle and brittle ductile deformation these deposit styles range from epithermal bonanza veins sea floor massive sulfide deposits mesothermal veins in shear zones to the higher temperature veins and stockworks of hypothermal and porphyry mineralization 1 fn 1

the strontium isotopic composition of brigantian late dinantian seawater abstract strontium isotopic determinations of different fossiliferous biominerals from brigantian late dinantian carbonates in northern england reveal that many are affected by diagenetic overprinting of their original 87 sr 86 sr ratios only pristine non luminescent articulate brachiopod shell material can be used to gain reliable estimates of carboniferous and by deduction palaeozoic ocean chemistries conodonts are found to be the only type of phosphatic biomineral capable of preserving original palaeoseawater chemistries however this only applies to specimens subjected to minimal burial temperatures other phosphatic biominerals are conclusively shown to be readily susceptible to diagenetic overprinting of original sr isotopic ratios a brigantian strontium curve compiled from pristine brachiopod calcite shows a relatively complex evolution with time but it can be resolved into two separate components operating on different time scales a cyclic sinusoidally shaped component with a frequency of 1 ma appears to fluctuate about a longer term more gradual linear increase in the brigantian seawater isotopic composition

timing of multiple magma events and duration of the hydrothermal system at the yu erya gold deposit eastern hebei province china constraints from u pb and ar ar dating abstract the yu erya deposit is a large granite hosted gold deposit in the eastern hebei province classified as an intrusion related gold system the yu erya complex contains the causative intrusion which was the source of hydrothermal metal bearing fluids this composite granite complex contains miarolitic pegmatite like cavities and pegmatite veins a combination of detailed field observations and u pb dating by shrimp and la icp ms is used to document multiple magmatic events which occurred at 210 220 175 173 171 170 167 166 and 161 ma formed the yu erya granite complex the initiation of the hydrothermal system as determined via ar ar dating is closely related to the exsolution of a magmatic volatile phase from the g1 granite at ca 176 ma the hydrothermal system operated for 16 my 176 160 ma and produced multiple mineralising events 176 172 168 and 162 160 ma linked to emplacement of repeated magma pulses from 175 ma to 161 ma our results suggest that multiple magmatic hydrothermal events over a time period of several million years may be a fundamental factor responsible for the enrichment process during formation of intrusion related gold deposits and thus may be a requirement for forming economic orebodies

sm nd in marine carbonates and phosphates implications for nd isotopes in seawater and crustal ages this study explores the possibility of establishing nd isotopic variations in seawater over geologic time calcite aragonite and apatite are examined as possible phases recording seawater values of nd modern biogenic and inorganically precipitated calcite and aragonite from marine environments were found to have nd concentrations of from 0 2 to 70 ppb showing that primary marine caco 3 contains little ree and that nd ca is not greatly enhanced relative to seawater during carbonate precipitation very young marine limestone and dolomite containing no continental detritus have 200 ppb nd all the carbonates are lree enriched 0 16 sm nd 0 45 modern and very young atlantic and pacific carbonates have nd in the range of shallow atlantic and pacific seawater respectively implying that they derive their ree from local seawater the nd in well preserved carbonate fossils is 4 10 4 ppb much greater than in their modern counterparts but like the high values found for carbonates in other studies we believe the high ree contents at the 500 ppb level in some detritusfree carbonates are due to ree rich fe hydroxide in on the carbonate in favorable cases such material may record seawater nd values however introduction of extraneous ree may obscure the original isotopic composition of pure caco 3 because of its very low intrinsic primary ree abundance modern biogenic apatite is also shown to have very low ree content 150 ppb nd but appears to quickly scavenge ree from seawater inorganically precipitated apatite from phosphorites has high concentrations of seawater derived ree young phosphorite apatite from the atlantic and pacific oceans has nd in the range of the seawater from these oceans older apatite samples of similar age from different localities bordering common oceans record similar values of nd t sedimentary apatite has sr t values in good agreement with the curves for 87 sr 86 sr of seawater as a function of time individual conodonts from a single formation yield the same sr t and nd t other workers have shown that sedimentary apatite preserves seawater ree patterns these characteristics suggest that sedimentary apatite can be used to determine nd t in ancient seawater the seawater values so inferred range between 1 7 and 8 9 over the last 700 my and lie in the range of modern seawater showing no evidence for drastic changes high values of seawater nd t in the triassic and latest precambrian may correlate with the breakup of large continental landmasses the initial nd t 15 0 of a 2 ae old phosphorite implies the presence of 1 5 ae old continental crust at 2 ae ago the approach outlined here can be used to constrain the age of the exposed crust as a function of time

self ordering and complexity in epizonal mineral deposits epizonal base and precious metal deposits makeup a range of familiar deposit styles including porphyry copper gold epithermal veins and stockworks carbonate replacement deposits and polymetallic volcanic rock hosted vhms deposits they occur along convergent plate margins and are invariably associated directly with active faults and volcanism they are complex in form variable in their characteristics at all scales and highly localized in the earth s crust more than a century of detailed research has provided an extensive base of obser vational data characterizing these deposits from their regional setting to the fluid and isotope chemistry of mineral deposition this has led to a broad understanding of the large scale hydrothermal systems within which they form low salinity vapor released by magma crystallization and dispersed into vigorously convecting ground water systems is recognized as a principal source of metals and the gases that control redox conditions within systems the temperature and pressure of the ambient fluid anywhere within these systems is close to its vapor liquid phase boundary and mineral deposition is a consequence of short timescale perturbations generated by localized release of crustal stress however a review of occurrence data raises questions about ore formation that are not addressed by traditional genetic models for example what are the origins of banding in epithermal veins and what controls the frequency of oscillatory lamina tion what controls where the phenomenon of mineralization occurs and why are some porphyry deposits for example so much larger than others the distinctive self organized characteristics of epizonal deposits are shown to be the result of repetitive coupling of fracture dilation consequent on brittle failure phase separation boiling and heat transfer between fluid and host rock process cou pling substantially increases solute concentrations and triggers fast far from equilib rium depositional processes since these coupled processes lead to localized transient changes in fluid characteristics paragenetic isotope and fluid inclusion data relate to conditions at the site of deposition and only indirectly to the characteristics of the larger scale hydrothermal system and its longer term behavior the metal concentra 1 the us government has the right to retain a nonexclusive royalty free license in and to any copyright covering this paper