multistage evolution of the neoarchean ca 2 7 ga igarape cinzento gt 46 iron oxide copper gold deposit cinzento shear zone carajas province brazil

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

rapid generation of both high and low δ18o large volume silicic magmas at the timber mountain oasis valley caldera complex nevada we present an oxygen isotope and pet rologic study of four voluminous zoned ash flow sheets of the southwestern nevada volcanic field swnvf topopah spring ts 1200 km 3 12 8 ma tiva canyon tc 1000 km in the basin and range province and it fa vors nearly total evacuation of the magma chamber during a single eruption each si licic magma unit was generated by a shal low influx of new mafic magma that melted 18 o 16 o depleted as in the case of at or 18 o 16 o enriched rm ts rocks the at tuff and associated pre and post caldera lavas are 2 5 lower in d 18 o than in the rm tuff and represent the largest known low d 18 o magma we find that all units of the at cycle contain isotopically zoned zir cons that have up to 2 core to rim zon ing and correspondingly smaller out of equilibrium quartz zircon and melt zircon fractionations air abraded cores of quartz and sphene do not preserve any d 18 o zon ing the higher d 18 o zircon cores in low d

timing and thermochemical constraints on multi element mineralisation at the nori ra cu mo u prospect great bear magmatic zone northwest territories canada the timing of cu mo u mineralisation at the nori ra prospect in the paleoproterozoic great bear magmatic zone has been investigated using re os molybdenite and 40ar 39ar biotite geochronology the re os molybdenite ages presented are the first robust sulphide mineralisation ages derived from the great bear magmatic zone cu mo u mineralisation is hosted in early to syn deformational hydrothermal veins consisting of quartz and k feldspar or more commonly tourmaline biotite quartz k feldspar with associated wall rock alteration assemblages being predominantly biotite sulphide and oxide minerals consist of chalcopyrite molybdenite and uraninite with lesser pyrite and magnetite elevated light rare earth elements and tungsten concentrations associated with the cu mo u mineralisation have also been reported at the prospect by previous workers molybdenite and uraninite occur intimately in dravitic tourmaline growth zones and at grain margins attesting to their syngenetic nature with respect to hydrothermal veining two molybdenite separates yield re os model ages of 1 874 4 8 7 2 and 1 872 4 8 8 ma 2 with a weighted average model age of 1 873 4 6 1 ma 2 laser step heating of biotite from the marginal alteration of the wall rock adjacent to the veins yields a 40ar 39ar maximum cooling age of 1 875 8 ma mswd 3 8 2 indistinguishable from the re os molybdenite model age and a previously dated syn tectonic aplitic dyke in the region dravitic tourmaline hosts abundant primary liquid vapour solid bearing fluid inclusions analytical results indicate liquid vapour homogenisation at 260 c constraining the minimum temperature of mineralisation the solids which are possibly trapped did not homogenise with the liquid vapour by 400 c salinities in the inclusions are variable raman spectra identify that at least some of the solids are calcite and anhydrite raman spectra also confirm the vapour phases contain some co2 whereas clathrates or ch4 was not observed or detected quartz grains only host secondary fluid inclusions which fluoresce under ultraviolet light indicating trapped hydrocarbons we speculate that these resulted from phanerozoic fluid circulation through the proterozoic basement the collective interpretation of the age hydrothermal character and associated metals high temperature and variable salinity suggests that the nori ra cu mo u mineralisation can be linked with the earliest stages of plutonism in the great bear magmatic zone from a regional perspective the mineralisation may pre date the extensive multi element mineralisation now recognised as part of the iron oxide copper gold iocg spectrum of deposits as iocg provinces generally contain a variety of mineralisation styles we interpret this as the earliest phase of the extensive mineralising system

shrimp u pb geochronology of detrital zircons from the treasure lake group new evidence for paleoproterozoic collisional tectonics in the southern hottah terrane northwestern canadian shield the recently defined treasure lake group is a platformal sedimentary sequence more than 3 km thick that was deformed and metamorphosed prior to formation of the great bear continental arc during 1875 1845 ma the group is exposed as remnants in the southern part of the arc its basement is not exposed but is interpreted to comprise the paleoproterozoic hottah terrane which is exposed at hottah lake in the northern part of the arc studies in that region show that the hottah terrane collided with the archean slave craton to the east ca 1883 ma following reversal of subduction polarity from west to east continued eastward subduction under the collision zone formed the great bear arc the present study was undertaken to determine the age and provenance of the treasure lake group thirty nine detrital zircon grains from a sample of a quartz arenite unit in the middle part of the treasure lake group were analyzed by shrimp sensitive high resolution ion microprobe they show an age range from ca 2729 to

the candelaria punta del cobre iron oxide cu au zn ag deposits chile several iron oxide rich cu au zn ag deposits define an approximately 5 km wide and at least 20 km long belt along the eastern margin of the coastal batholith near copiapo chile this belt includes the large candelaria mine and a group of middle and small sized mines in the punta del cobre district which is located about 3 km northeast of the candelaria deposit estimated geologic resources of the belt are on the order of 700 to 800 million metric tons mt at 1 0 percent cu the ore occurs in veins breccia and stringer bodies and in replacement bodies that are roughly concordant to bedding the orebodies are hosted mainly by volcanic and volcaniclastic rocks of the punta del cobre formation and in places also occur in volcaniclastic intercalations in the lower part of the overlying early cretaceous chanarcillo group most of the larger orebodies in the belt are located where northwest trending brittle faults intersect the contact between massive volcanic and volcaniclastic rocks these northwest faults and a major northeast trending ductile shear zone control portions of the ore of the candelaria deposit chalcopyrite is the only hypogene cu mineral the cu au ore is characterized by abundant magnetite and or hematite and by locally elevated concentrations of ag zn mo and light rare earth elements the ore is hosted mainly in zones with biotite potassium feldspar calcic amphibole epidote alteration at candelaria in the punta del cobre district ore in the deeper parts of the deposits is similarly associated whereas at shallow levels it occurs in zones of biotite potassium feldspar or albite chlorite calcite alteration mineralization at candelaria punta del cobre took place under relatively oxidized conditions manifested by the formation of specular hematite in parts of the district the pseudomorphic replacement of early specular hematite by magnetite during the main iron oxide formation marks a shift toward more reduced conditions or higher temperatures the bulk of the magnetite probably formed at temperatures of about 500 to 600 c the main sulfide stage followed with formation of pyrite and chalcopyrite at temperatures of 470 to 328 c subsequent martitization of the magnetite points to a temperature decrease cooling of the hydrothermal system is also indicated by the homogenization temperatures of 236 c of saline fluid inclusions in late stage calcite oxygen isotope combined with microthermometric data suggest that magmatic fluids or nonmagmatic fluids equilibrated with magmatic silicates were dominant during the main copper mineralization relatively light oxygen isotope signatures of fluids in equilibrium with late calcite suggest mixing with a nonmagmatic fluid e g basinal brines or meteoric waters during the late stages of hydrothermal activity sulfur isotope ratios of chalcopyrite pyrite pyrrhotite and sphalerite from the bronce candelaria las pintadas santos and socavon rampa deposits range from 34scdt values of 0 7 to 3 1 per mil this narrow range of sulfur isotope ratios near 0 per mil is consistent with sulfur of magmatic origin anhydrite from the candelaria mine paragenetically overlaps with chalcopyrite fluid inclusions in this anhydrite homogenize between 340 and 470 c and it has 34scdt values between 14 5 and 17 5 per mil a sulfate sulfide value of 13 4 per mil for a sample with coexisting anhydrite and chalcopyrite is consistent with sulfide sulfate fractionation at temperatures on the order of 400 c ore lead isotope signatures are homogeneous and similar to those of least altered volcanic host rocks and nearby intrusive rocks radiometric ages including new 40ar 39ar ages for hydrothermal alteration at candelaria point to a main cu au mineralization event at candelaria punta del cobre at around 115 ma the ages indicate that ore formation was broadly coeval with batholithic granitoid intrusions and with regional uplift they further imply that the cu au zn ag deposits formed at shallow crustal levels 3 km the stable isotope data the presence of previously reported hypersaline co2 rich fluid inclusions in the main cu ore stage and saline fluid inclusions in late stage calcite the oxidized character of the first ore fluid pulses and the mineralization age coeval with nearby intrusive activity are consistent with but not unequivocally evidence of magmatic fluid contribution into the hydrothermal system

numerical model to determine the composition of h2o nacl cacl2 fluid inclusions based on microthermometric and microanalytical data natural fluids approximated by the h2o nacl cacl2 system are common in a wide range of geologic environments including sedimentary basins associated with hydrocarbon occurrences and mvt deposits submarine hydrothermal systems and other metamorphic magmatic and hydrothermal environments we present a comprehensive numerical model and microsoft excel based computer program to determine the compositions of fluid inclusions in the h2o nacl cacl2 system based on microthermometric and microanalytical data the model consists of six polynomial correlation equations that describe liquid salinity as a function of nacl cacl2 ratio and melting temperature on each of the ice hydrohalite halite antarcticite cacl2 4h2o and cacl2 2h2o vapor saturated liquidus surfaces the cotectic and peritectic boundaries are determined from the intersections of the liquidus surfaces the model is implicitly internally consistent and topologically correct the model expands upon the compositional range of applicability and the data types that can be used for compositional determination it reproduces experimental data for all compositions that lie within the h2o nacl cacl2 4h2o compositional triangle in the h2o nacl cacl2 system and yields accurate reproductions of the h2o nacl and h2o cacl2 binaries furthermore in comparison to previously published models the one presented here eliminates systematic errors wavy isotherms and cotectic and peritectic curves with local bumps

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

magmatic hydrothermal processes within an evolving earth iron oxide copper gold and porphyry cu mo au deposits iron oxide copper gold iocg deposits formed by magmatic hydrothermal fluids mh iocg share many similarities with but have important differences from porphyry cu mo au porphyry deposits mh iocg deposits predominantly occur in precambrian rocks are fe oxide rich and have volumetrically extensive high temperature alteration zones whereas porphyry deposits occur almost exclusively in phanerozoic rocks are fe sulfide rich and have narrower high temperature alteration zones we propose that these deposit types are linked by common subduction modified magmatic sources but that secular changes in oceanic sulfate content and geothermal gradients at the end of the precambrian caused a transition from the predominance of s poor arc magmas and associated s poor mh iocg systems to s rich arc magmas and associated s rich porphyry deposits in the phanerozoic phanerozoic mh iocg and rare precambrian porphyry deposits are explained by local or periodic fluctuations in oceanic oxidation state and sulfate content or remobilization of previously subduction modified lithosphere in post subduction tectonic settings

double vein ore bearing vs ore free structures in the xitian ore field south china implications for fluid evolution and mineral exploration abstract two types of spatially and temporally associated hydrothermal veins ore bearing and ore free occur as double vein structures in the xitian ore field south china here we investigated the genetic relationship material provenance fluid properties and prospecting significance of the co developed veins in the granite related fault controlled vein type xiangdong w goudanlan w chaling pb zn and xinggao fluorite deposits of the xitian ore field focusing on two common minerals quartz and zircon petrographic cl microthermometric and h o isotopic analyses of stage i and stage ii quartz in both the ore bearing and ore free veins of these deposits document two episodes of hydrothermal activity the first episode involved a high temperature up to 370 high salinity up to 28 nacl equiv high 18o 2 1 to 7 2 and high 2h 82 to 54 mean 68 n 13 fluid that corresponds to the main magmatic hydrothermal stage of the deposits and the second episode involved a low temperature down to 90 low salinity down to 1 nacl equiv low 18o 8 8 to 5 3 and low 2h 83 to 60 mean 73 n 6 fluid that represents an accessary mineralization stage variations in fluid inclusion types and compositions indicate that the processes of fluid mixing boiling cooling and degassing played important roles in mineralization and that magmatic fluid became less important than meteoric water along during the formation of ore free veins la icpms u pb dating reveals totally different age populations for these veins zircons from the ore bearing quartz veins yield older and more variable ages pre devonian 400 ma n 68 that are similar to that of the local basement sedimentary rocks whereas zircons from the ore free quartz veins yield younger and less variable ages triassic 250 200 ma n 31 and jurassic 170 140 ma n 13 that are identical to those of the host granite zircons in addition the ore free veins show lower temperatures salinities and total rees as recorded by quartz but greater alteration as reflected by cl and trace element features of hydrothermally altered zircons and wider vein size than the ore bearing veins we propose a mineral water separation model to explain the formation of the double vein structures in this model ore bearing veins and ore free veins formed in succession during faulting events ore bearing veins formed first in the smaller fractures and were the dominant sites of mineralization whereas the ore free veins served as dissipating passageways for residual water and silica in larger fractures we suggest that the mineral water separation model has good application prospects in mineral exploration and that zircons can be applied as an indicative mineral to differentiate the mineralization potential of different types of quartz veins

radiogenic and stable isotope constraints on the genesis of the eloise cu au deposit cloncurry district northwest queensland new radiogenic 40ar 39ar and stable oxygen hydrogen and sulfur isotope analyses of metamorphic and metasomatic minerals constrain the age metasomatic evolution and genesis of the eloise cu au deposit 3 1 mt 5 5 cu 1 4 g t au and 16 g t ag biotite from a pre to syn d2 stage vein has an 40ar 39ar age of 1555 4 ma which is interpreted to coincide with a regional metamorphic event synchronous with d2 six later stages of alteration and mineralization are recognized all of which postdate peak metamorphism and d2 stages i to iii are volumetrically the most significant and comprise early albitization stage i quartz hornblende biotite veins and alteration stage ii and cu au mineralization stage iii stages iv v and vi are localized vein events and postdate the main cu au mineralization stage ii vein and alteration hornblendes have 40ar 39ar ages of 1530 3 ma biotite from the same stage has an 40ar 39ar age of 1521 3 ma muscovite from a postore shear zone has an 40ar 39ar age of 1514 3 ma these results provide a maximum ca 1530 ma and minimum ca 1514 ma age for the mineralizing event however the intimate relationship between stage ii mafic silicate veins and alteration and the stage iii cu au event combined with fluid inclusion results which indicate that a cooling evolving high salinity fluid was responsible for both stages suggests that the older age is likely to be closer to the age of mineralization the stage ii biotite age of ca 1521 ma is interpreted to record thermal resetting during postore ductile deformation the results suggest that micas in the cloncurry district are more susceptible to later thermal resetting than amphiboles which may have significant implications for deposits which have only been dated by micas quartz from stages ii iii and iv have 18oquartz values ranging between 10 1 and 11 9 per mil stage ii biotite has a lower 18o composition 5 6 than biotite from a pre d2vein 6 9 but both have identical d compositions 84 stage ii hornblende has lower 18o 6 8 and 7 4 and d 88 and 90 values than does stage iii actinolite 8 0 and 84 respectively the 34s values for chalcopyrite pyrrhotite and pyrite fall in a narrow range between 0 0 and 2 3 per mil a distinct trend can be recognized with 34s values becoming progressively greater from south to north thus reflecting the zoned alteration system magnetite pyrite rich in the south through to pyrrhotite rich in the north the change in mineralogy and sulfur isotope values may be due to cooling and sulfidation processes which resulted in changes in oxygen and sulfur fugacities the oxygen hydrogen and sulfur isotope data combined with high temperature and high salinity fluid inclusion data indicate a predominantly magmatic origin for the ore forming fluids the deposit is interpreted to have formed from magmatic hydrothermal fluids which were tapped by deep seated crustal structures

thermochronology of isotopically zoned micas insights from the southwestern usa proterozoic orogen abstract we have used three different 40 ar 39 ar laser microprobe methods to explore the distribution of radiogenic 40 ar in 1 0 1 5 mm biotite crystals from the ca 1680 ma horse mountain monzogranite of central arizona incremental heating of two single crystals with a defocused laser beam produced flat age spectra with near plateau ages of 1190 ma showing no indication of intracrystalline 40 ar inhomogeneity in contrast total fusion of twenty five biotite fragments 100 pm yielded apparent ages ranging from 1006 7 to 1212 0 ma detailed age mapping in the 001 plane of two crystals with the laser focused to a minimum spot size confirms that the age dispersion in the fragment data reflects the existence of 200 m y age gradients in single crystals the two mapped crystals display very different age distribution patterns that suggest radiogenic 40 ar loss through two mechanisms volume diffusion on a scale comparable to that of the grain radius and more rapid diffusion along discrete zones of high crystal defect density simple inverse modeling of the overall age dispersion in the two mapped crystals and the fragment population is consistent with the development of the observed age gradients by slow cooling at an average rate of 0 5 k m y the horse mountain results as well as previously published studies indicate that conventional incremental heating of hydrous phases can homogenize intracrystalline gradients in 40 ar thereby masking important details of the thermal history of analyzed samples in contrast detailed isotopic mapping studies offer a wealth of information and will become more powerful with continued improvement in the spatial resolution of 40 ar 39 ar laser microprobes total fusion studies of crystal fragment populations can be readily automated making them less labor intensive than mapping studies our preliminary experiment on a limited horse mountain fragment population suggests that this procedure has great potential as a reconnaissance tool for thermal history research

carbon and oxygen isotope profiles across meso neoproterozoic limestones from central brazil bambui and paranoa groups we present carbon and oxygen isotopic data on carbonates along three profiles of the neoproterozoic bambui group in central brazil this unit covers an area of more than 300 000 km2 and comprises carbonate silicliclastic sediments at the base that grade into siliciclastic sediments towards the top the bambui group overlies by unconformity the paranoa group which consists mostly of siltstone quartzite and minor limestone the data presented here improve the stratigraphic correlation within the bambui basin and show that it evolved in an environment significantly different from that of the paranoa basin our data show large fluctuations of 13cpdb in limestones from the bambui group from 0 8 to 13 5 in all the three studied areas some of these fluctuations represent stratigraphic markers that can be used as a chronostratigraphic tool within a basin scale this observation is relevant considering the lack of fossil record and other stratigraphic markers in neoproterozoic sequences we also present the first isotopic profiles along the paranoa bambui transition which shows that the 13cpdb values grade from 1 0 in the paranoa group to 2 6 in the lower portion of the bambui group increasing up to 12 in the upper part of this unit based on our carbon isotope data as well as other geological mineralogical and nd isotope studies we argue that the sediments of the paranoa group were deposited on an open platform that was fully connected to the ocean on the other hand the sediments of the bambui group were deposited in an epicontinental sea and during a tectonic inversion in a foreland basin at about 790 600 ma this unit displays an increased amount of clastic sediments upwards we argue that the high carbon isotope values observed in limestones and marlstones from the bambui group are correlated to worldwide high carbon isotope values reported for the neoproterozoic however we also point out that novel marine conditions induced by the tectonic inversion of the basin may also have contributed to increase the carbon isotopic composition of the bambui carbonates

tectonomagmatic evolution of the southern great bear magmatic zone northwest territories canada implications for the genesis of iron oxide alkali altered hydrothermal systems the great bear magmatic zone in northwestern canada is a paleoproterozoic volcano plutonic belt of high k calc alkaline to shoshonitic affinity interpreted as a continental arc that formed between 1 87 and 1 85 ga following the short lived calderian orogeny tectonomagmatic evolution of this magmatic zone favored the formation of multiple iron oxide and alkali alteration systems within a time frame of 10 m y as constrained geochronologically within error between 1875 and 1865 ma this illustrates a temporal and genetic relationship between shoshonitic to high k calc alkaline continental arc magmatism and the formation of iron oxide rich deposits and alkali alteration associated with iron oxide copper gold iocg mineralization early rhyolitic magmatism formed the basal sequence of the lou assemblage under a compressive or transpressive tectonic regime between 1872 and 1867 ma an apparent higher magmatic production is linked to a marked change in composition of the volcanic and plutonic rocks from rhyolite to intermediate felsic and locally mafic compositional homogeneity of the intrusive and volcanic rocks of the mazenod and bea assemblages termination of ductile to brittle ductile deformation along the main deformation zones and transition to widespread brittle fracturing and breccia formation are interpreted to reflect a change in the regional stress regime from compressional transpressional to extensional transtensional this change in stress regimes is associated with iron oxide rich mineralization that initially formed the nico au bi co deposit followed by iocg mineralization in the nico and sue dianne deposits as well as albitite hosted u mineralization in the southern breccia

an intrusion related origin for cu au mineralization in iron oxide copper gold iocg provinces major cu au deposits of iron oxide copper gold iocg style are temporally associated with oxidized potassic granitoids similar to those linked to major porphyry cu au deposits stable and radiogenic isotope evidence indicates fluids and ore components were likely sourced from the intrusions iocg deposits form over a range of crustal levels because co2 rich fluids separate from the magmas at higher pressures than in co2 poor systems thereby promoting partitioning of h2o cl and metals to the fluid phase at deep levels the magma fluid system cannot generate sufficient mechanical energy to fracture the host rocks as in porphyry systems and the iocg deposits therefore form in a variety of fault related structural traps where the magmatic fluids may mix with other fluids to promote ore formation at shallow levels the iocg deposits form breccia and fracture hosted mineralization styles similar to the hydrothermal intrusive breccias and sulphide vein systems that characterize many porphyry cu au deposits the fluids associated with iocg deposits are typically h2o co2 salt fluids that evolve by unmixing of the carbonic phase and by mixing with fluids from other sources in contrast fluids in porphyry systems typically evolve by boiling of moderate salinity fluid to produce high salinity brine and a vapor phase commonly with input of externally derived fluids these different fluid compositions and mechanisms of evolution lead to different alteration types and parageneses in porphyry and iocg deposits porphyry cu au deposits typically evolve through potassic sericitic and intermediate and or advanced argillic stages while iocg deposits typically evolve through sodic calcic potassic and carbonate rich stages and at deeper levels generally lack sericitic and argillic alteration the common association of porphyry and iocg cu au deposits with potassic oxidized intermediate to felsic granitoids together with their contrasting fluid compositions alteration styles and parageneses suggest that they should be considered as part of the broad family of intrusion related systems but that they are typically not directly related to each other

re os geochronology and systematics in molybdenite from the endako porphyry molybdenum deposit british columbia canada precise re os molybdenite ages for the endako porphyry molybdenum deposit and the nithi mountain occurrence british columbia canada yield new information on the timing of molybdenite deposition three episodes of mineralization are indicated 1 154 ma 2 148 to 146 ma and 3 145 ma re os dates agree well with new u pb and 40 ar 39 ar data from granitoids of the nithi phase 155 2 1 5 ma the endako phase 148 4 1 5 ma and a preore dike 147 4 0 6 ma and the casey phase 145 1 0 2 ma plus hydrothermal biotite 144 5 0 75 ma villeneuve et al 2001 the excellent correlation between the re os dates from molybdenite and the ages of granitoids from the area indicates a direct genetic relationship between these plutons and molybdenite mineralization these re os data also provide further insight into the systematics and robustness of re os in molybdenite the endako molybdenum deposit is the product of two episodes of mineralization both associated with moderate salinity 5 15 wt nacl equiv and high temperature 440 c fluids a third low temperature 190 300 c hydrothermal event is associated with eocene faulting selby et al 2000 the clear distinction between two of the re os molybdenite dates in the deposit and the close agreement of these dates to the ages of granitoid intrusions suggest that the re os system in molybdenite is not reset by high temperature 440 c moderate salinity hydrothermal fluids in contrast magmatic biotite in a 147 ma preore dike has a 40 ar 39 ar age of 145 ma identical to the age of the hydrothermal biotite indicating that 440 c fluids reset the ar ar system in biotite hence we interpret that the re os isotope system in molybdenite is resistant to resetting by high temperature fluids 440 c in contrast to the 40 ar 39 ar system in biotite the re os system in molybdenite is also unaffected by postore hydrothermal fluids these data show that molybdenite can provide critical information in understanding sulfide mineral deposition in ore systems which is not recorded by other isotopic systems in alteration minerals

annual rainfall and nitrogen isotope correlation in macropod collagen application as a palaeoprecipitation indicator abstract collagen has been extracted and analysed for 15 n from modern kangaroos macropods macropus eugenii m antilopinus m fuliginosus m rufus m robustus in western australia and south australia comparison of nitrogen isotope ratios with annual rainfall for the respective areas produced a good negative correlation for macropods but not for other marsupials this may be related to metabolic adaptations in macropods to compete against water loss hence the correlation between 15 n and annual rainfall may be species specific nitrogen isotope analyses on collagenic material extracted from fossil macropods macropus spp have been determined from two late pleistocene deposits in south australia corroboration with spore pollen and sedimentological data confirm that 15 n of collagenic material can be used as a proxy for palaeoprecipitation levels in australia this has great potential for understanding palaeoclimatic changes especially drought periods in australia during the pleistocene

p t composition and evolution of paleofluids in the paleoproterozoic mag hill iocg system contact lake belt northwest territories canada the echo bay stratovolcano complex and contact lake belt of the great bear magmatic zone northwest territories host a series of coalescing paleoproterozoic hydrothermal systems that affected an area of several hundred square kilometers they were caused by intrusion of synvolcanic diorite monzodioritic plutons into andesitic host rocks producing several characteristic hydrothermal assemblages they include early and proximal albite magnetite actinolite apatite and potassic k feldspar alteration followed by more distal hematite phyllic quartz sericite pyrite and propylitic chlorite epidote carbonate sericite albite quartz alteration and finally by late stage polymetallic epithermal veins these alteration types are characteristic of iron oxide copper gold deposits however with distal and lower temperature assemblages similar to porphyry cu systems magnetite actinolite apatite alteration formed from high temperature up to 560 c fluids with average salinity of 12 8 wt nacl equivalent the prograde propylitic and phyllic alteration stages are associated with fluids with temperatures varying from 80 to 430 c and a wide salinity range 0 5 45 6 wt nacl equivalent similarly wide fluid temperature 104 450 c and salinity 4 2 46 1 wt nacl equivalent ranges are recorded for the phyllic alteration this was followed by cu ag u zn co pb sulfarsenide mineralization in late stage epithermal veins formed at shallow depths and temperatures from 270 c to as low as 105 c the polymetallic veins precipitated from high salinity mean 30 wt nacl equivalent dense fluids 1 14 g cm3 with a vapor pressure of 3 8 bars typical of epithermal conditions fluid inclusion evidence indicates that mixed fluids with evolving physicochemical properties were responsible for the formation of the alteration assemblages and mineralization at mag hill an early high temperature moderate salinity and magmatic fluid was subsequently modified variably by boiling mixing with cooler low salinity meteoric water and simple cooling the evidence is consistent with emplacement of the source plutons and stocks into an epithermal environment within 1 km of surface this generated near surface high temperature alteration in a dynamic hydrothermal system that collapsed telescoped resulting in widespread evidence of boiling and epithermal mineralization superimposed on earlier stages of alteration

the remarkable re os chronometer in molybdenite how and why it works the re os rhenium osmium chronometer applied to molybdenite mos2 is now demonstrated to be remarkably robust surviving intense deformation and high grade thermal metamorphism successful dating of molybdenite is dependent on proper preparation of the mineral separate and analysis of a critical quantity of molybdenite unique to each sample such that recognized spatial decoupling of 187re parent and 187os daughter within individual molybdenite crystals is overcome highly precise accurate and reproducible age results are derived through isotope dilution and negative thermal ion mass spectrometry id ntims spatial decoupling of parent daughter precludes use of the laser ablation icp ms microanalytical technique for re os dating of molybdenite the use of a reference or control sample is necessary to establish laboratory credibility and for interlaboratory comparisons the rb sr k ar and 40ar 39ar chronometers are susceptible to chemical and thermal disturbance particularly in terranes that have experienced subsequent episodes of hydrothermal magmatic activity and therefore should not be used as a basis for establishing accuracy in re os dating of molybdenite as has been done in the past re os ages for molybdenite are almost always in agreement with observed geological relationships and when available with zircon and titanite u pb ages for terranes experiencing multiple episodes of metamorphism and deformation molybdenite is not complicated by overgrowths as is common for some minerals used in u pb dating e g zircon monazite xenotime nor are re and os mobilized beyond the margins of individual crystals during solid state recrystallization moreover inheritance of older molybdenite cores incorporation of common os and radiogenic os loss are exceedingly rare whereas inheritance common pb and pb loss are common complications in u pb dating techniques therefore molybdenite ages may serve as point in time markers for age comparisons

antiquity of aridity in the chilean atacama desert the atacama desert of northern chile and southern peru has one of the longest histories of arid conditions known although most studies have focused on the hyper aridity prevalent since the miocene all terrestrial sediments in the region from the late triassic onwards also record evaporitic and thus arid climates supergene mineralization in the region did not develop under a more humid climate conducive to deep weathering but under arid conditions these processes may have been facilitated by hydrological changes during miocene uplift and drainage incision but were operational prior to the uplift similarly global cooling and changes to oceanic circulation in the post miocene period only accentuated existing conditions a whole regolith perspective is vital to understanding the history of aridity in the atacama desert and its relevance to arid zone morphogenesis regolith formation and supergene mineralization in particular the long history of aridity raises the possibility that supergene mineralisation under the appropriate conditions form in arid environments instead of requiring humid conditions

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

stable isotope s o h and c studies of the phyllic and potassic phyllic alteration zones of the porphyry copper deposit at sungun east azarbaidjan iran abstract the porphyry copper deposit pcd at sungun is located in east azarbaidjan nw of iran the magmatic suites in the sungun area are a part of the nw se trending cenozoic magmatic belt of iran the sungun porphyries occur as stocks and dikes the stocks are divided into two groups i and ii porphyry stock ii ranging in composition from quartz monzonite through granodiorite to granite hosts the sungun pcd four distinct types of hypogene alterations were recognized at sungun 1 potassic 2 potassic phyllic 3 phyllic and 4 propylitic stable isotope s o h and c studies were restricted to within the phyllic and potassic phyllic alteration zones where numerous cross cutting quartz sulfides carbonates and sulfate veinlets are present the objective of these studies was to determine the origin of the ore forming solutions and their important components e g sulfur and carbon twenty sulfide and four sulfate samples were taken from sulfide and gypsum veinlets within porphyry stock ii and the associated skarn zone for sulfur isotopic analyses the 34 s values of sulfides pyrite chalcopyrite molybdenite galena sphalerite and sulfate gypsum range from 4 6 to 0 2 mean of 1 5 and from 10 9 to 14 4 mean of 12 9 respectively these values are almost analogous to those from el salvador chile and ajo arizona and twin buttes arizona and strongly suggest a magmatic source for the sulfur at sungun twenty eight fluid inclusion rich quartz samples from quartz veinlets beneath the supergene zones of the porphyry stock ii were chosen for o and h isotopic analyses the 18 o of quartz and d of fluid inclusions in quartz values range from 8 3 to 10 2 mean of 9 2 and 58 to 75 mean of 66 relative to standard mean ocean water smow respectively the calculated 18 o values of the fluids range from 4 4 t 375 c to 7 6 t 570 c with a mean of 6 4 the 18 o and d values of the fluids lie within the typical range of magmatic waters based upon the lowest obtained d values the calculated maximum exchangeable meteoric water that might be involved in the phyllic and potassic phyllic alteration processes was 20 fourteen calcite samples from carbonate veinlets in the porphyry stock ii and two samples from associated marble aureole were collected for carbon and oxygen isotopic analyses the 13 c calcite from porphyry stock ii and associated marble aureole varies from 0 4 to 2 9 and 0 06 to 2 relative to the pdb standard of the university of chicago respectively the corresponding 18 o calcite values range from 6 to 13 2 relative to smow the 13 c values lie within the sedimentary range suggesting that there was little or almost no magmatic carbon in the hydrothermal system in contrast the 18 o calcite values are far lower than that of cretaceous sedimentary carbonates which suggest that the values were influenced by those of magmatic fluids through isotopic re equilibration these isotopic investigations demonstrated that 1 in contrast to some well known pcds about 80 of the fluids involved in the phyllic and potassic phyllic alterations at sungun had a magmatic o and h isotopic signature 2 the upper cretaceous limestone was the potential source of c and 3 the sulfur had a magmatic origin

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

a comparison of mineral deposits at the gecko and white devil deposits implications for ore genesis in the tennant creek district northern territory australia detailed geologic metal zonation paragenetic fluid inclusion and stable isotope studies have been undertaken of the cu rich gecko k44 and the au rich white devil deeps zone orebodies in the proterozoic tennant creek district of the northern territory australia despite differences in the grade and mineral abundances in the ores three paragenetic stages are present in both deposits 1 an early synkinematic oxide stage at which time the ironstones formed 2 a syn to postkinematic sulfide stage at which time cu au and bi were introduced and 3 a late minor carbonate stage with minor pb and zn the oxide stage involved precipitation of magnetite hematite quartz and chlorite this stage was subsequently foliated the sulfide stage involved the precipitation of gold chalcopyrite bi minerals chlorite and quartz and the hematization of magnetite the carbonate stage involved precipitation of carbonate talc galena and sphalerite and the alteration of chalcopyrite to bornite and covellite fluid inclusion studies indicate that oxide stage minerals formed from low temperature 250 degrees c high salinity 15 20 wt nacl equiv fluids whereas the sulfide stage formed from high temperature 350 degrees c high salinity 20 wt nacl equiv fluids that show evidence of phase separation sulfur isotope studies indicate that lower delta 34 s values occur in au rich pods whereas higher delta 34 s values occur in cu rich halos metal zonation studies indicate that in the white devil deeps zone orebody a basal au rich zone is surrounded by a bi rich shell this zonation is broadly similar to zonation at the juno deposit au bi cu pyrite large 1975 owing to its very low grades the distribution of cu in the white devil deeps orebody is erratic conversely the gecko k44 orebody is only weakly zoned with au enrichment at the top of the orebody in the sulfide tongue mineral deposition occurred in well zoned au rich e g juno and white devil and weakly zoned chalcopyrite hematite rich e g gecko deposits owing to the reduction of highly oxidized sulfide stage fluids through hematization of magnetite in ironstones the fluids evolved through the hematite field in f sub o 2 ph space to the hematite pyrite join and then followed the hematite pyrite join toward the hematite pyrite magnetite triple point this fluid evolution caused early gold precipitation in the hematite field but chalcopyrite precipitated only along the hematite pyrite join which accounts for metal zonation in well zoned au rich deposits in chalcopyrite hematite rich deposits fluid evolution was restricted to the hematite pyrite join and minor gold precipitated with chalcopyrite the k44 sulfide tongue is inferred to be a small zoned au rich system adjacent to the main k44 orebody differences in metal abundance and zonation can be interpreted in terms of varying proportions of highly oxidized magmatic hydrothermal fluids relative to more reduced connate basinal fluids introduced into the ore forming systems a large component resulted in well zoned au rich deposits an intermediate component resulted in chalcopyrite hematite rich au poor deposits and a small component resulted in unzoned chalcopyrite rich pyrrhotite bearing deposits with intermediate au grades e g peko

planar defects as ar traps in trioctahedral micas a mechanism for increased ar retentivity in phlogopite abstract the effects of planar defects and composition on ar mobility in trioctahedral micas have been investigated in samples from a small marble outcrop 500 m2 in the frontenac terrane grenville province ontario these micas crystallized during amphibolite facies metamorphism at 1170 ma and experienced a thermal pulse 100 ma later at shallow crustal levels associated with the emplacement of plutons 87rb 86sr ages of the phlogopites range from 950 to 1050 ma consistent with resetting during the later thermal event the same phlogopites however give 40ar 39ar ages between 950 and 1160 ma spanning the age range of the two thermal events this result is intriguing because these micas have undergone the same thermal history and were not deformed after peak metamorphic conditions in order to understand this phenomenon the chemical crystallographical and microstructural nature of four mica samples has been characterized in detail using a wide range of analytical techniques the scanning electron microscope sem electron microprobe emp and laser ablation inductively coupled plasma mass spectrometry la icp ms data show that the micas are chemically homogeneous with the exception of ba and similar in composition the fourier transform infrared spectroscopy and mossbauer results show that the m sites for three of the micas are dominated by divalent cations and the fe3 fe2 fe3 ratio for all four phlogopites ranges from 0 10 to 0 25 the stable isotopic data for calcite indicate that this outcrop was not affected by hydrothermal fluids after peak metamorphism no correlation between chemical composition and 87rb 86sr and 40ar 39ar age or between crystal size and 40ar 39ar age is observed the only major difference among all of the micas was revealed through transmitted electron microscope tem which shows that the older 1m micas contain significantly more layer stacking defects associated with crystallization than the younger micas we propose that these defect structures which are enclosed entirely within the mineral grain may serve as ar traps and effectively increase the ar retentivity of the mineral as this phenomenon has not been previously documented in micas this may have significant implications for the interpretation of 40ar 39ar ages of minerals which have similar defect structures

sims analyses of silicon and oxygen isotope ratios for quartz from archean and paleoproterozoic banded iron formations banded iron formations bifs are chemical marine sediments dominantly composed of alternating iron rich oxide carbonate sulfide and silicon rich chert jasper layers isotope ratios of iron carbon and sulfur in bif iron bearing minerals are biosignatures that reflect microbial cycling for these elements in bifs while much attention has focused on iron banded iron formations are equally banded silica formations thus silicon isotope ratios for quartz can provide insight on the sources and cycling of silicon in bifs bifs are banded by definition and microlaminae or sub mm banding are characteristic of many bifs in situ microanalysis including secondary ion mass spectrometry is well suited for analyzing such small features in this study we used a cameca ims 1280 ion microprobe to obtain highly accurate 0 3 and spatially resolved 10 lm spot size analyses of silicon and oxygen isotope ratios for quartz from several well known bifs isua southwest greenland 3 8 ga hamersley group western australia 2 5 ga transvaal group south africa 2 5 ga and biwabik iron formation minnesota usa 1 9 ga values of d 18 o range from 7 9 to 27 5 and include the highest reported d 18 o values for bif quartz values of d 30 si have a range of 5 from 3 7 to 1 2 and extend to the lowest d 30 si values for precambrian cherts isua bif samples are homogeneous in d 18 o to 0 3 at mm to cm scale but are heterogeneous in d 30 si up to 3 similar to the range in d 30 si found in bifs that have not experienced high temperature metamorphism up to 300 c values of d 30 si for quartz are homogeneous to 0 3 in individual sub mm laminae but vary by up to 3 between multiple laminae over mm to cm of vertical banding the scale of exchange for si in quartz in bifs is thus limited to the size of microlaminae or less than 1 mm we interpret differences in d 30 si between microlaminae as preserved from primary deposition silicon in bif quartz is mostly of marine hydrothermal origin d 30 si 0 5 but silicon from continental weathering d 30 si 1 was an important source as early as 3 8 ga

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

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

genesis of the paleoproterozoic nico iron oxide cobalt gold bismuth deposit northwest territories canada evidence from isotope geochemistry and fluid inclusions abstract the nico deposit is located in the southern portion of the paleoproterozoic great bear magmatic zone northwest territories canada the majority of the mineralization lies within the bowl zone hosted by the treasure lake group tlg where heavily altered precursor rocks are interpreted to be carbonate rich wackes and siltstones these rocks are crosscut by a set of pre ore quartz calcite amphibole k feldspar veins s1 the co au bi w cu mo mineralization at nico is contained in stratabound ore lenses within the bowl zone and comprises a prograde assemblage of co rich arsenopyrite arsenopyrite i and loellingite cobaltite pyrite actinolite ferrohornblende biotite and rare scheelite molybdenite along with minor magnetite and amphibole retrograde assemblages resulted from re crystallization of the co bearing phases to form arsenopyrite ii and iii along with precipitation of marcasite pyrite hastingsite native bi au te and minor bismuthinite and magnetite the latest stage of retrograde mineralization comprises chalcopyrite hastingsite chlorite and hematite emplectite two sets of barren quartz dolomite amphibole k feldspar chalcopyrite veins s2 and s3 post date the mineralization the southern breccia zone hosts minor u cu mo mineralization and is interpreted to be the deepest portion of the nico system two molybdenite samples from the bowl zone and the southern breccia yielded re os ages of 1865 9 and 1877 8 ma respectively consistent with the interpreted ca 1870 ma age of the nico deposit this age is also synchronous with the onset of magmatism in the great bear magmatic arc ca 1875 1850 ma ore mineral 34s values 3 3 6 7 sulfides indicate that crustal sulfur was assimilated by upwelling felsic magmatic melts the 18o values of the fluids precipitating magnetite and co rich arsenopyrite 6 and 8 and from pre and syn mineralization magnetite 0 8 to 1 5 support a magmatic hydrothermal origin of the fluids two out of three calcite samples from pre s1 and post ore s3 veins have also 13c values consistent with a magmatic origin 5 5 to 3 6 however one calcite sample from the s3 veins has a value that indicates a reduced sulfur source 15 6 this value is similar to those of the much younger secondary trails of native bi in s1 vein quartz are associated with liquid vapor lv and liquid vapor halite lvs inclusions which indicates that bi and possibly au were transported in saline to hyper saline brines lv bi 2 16 wt nacl equiv 8 22 wt cacl2 equiv lvs bi 37 wt nacl equiv with homogenization temperatures of 137 216 c and 192 to 350 c for lv bi thl v l and lvs bi thl s l respectively the presence of calcium rich fluids might indicate extensive equilibration of those solutions with the host tlg rocks if a pressure correction is applied to the lv inclusions using a minimum entrapment temperature of 271 4 c the bi melting point a minimum crystallization depth of between approximately 5 and 8 km is indicated trace element analyses carried out in this study and compiled from acosta gongora et al 2014 show that the least altered metasedimentary tlg rocks contain up to six times more as carbonate unit 30 5 ppm than the average upper continental crust conversely concentrations of au

microscale oxygen isotope variations in 1 9 ga gunflint cherts assessments of diagenesis effects and implications for oceanic paleotemperature reconstructions abstract variations in the oxygen isotope composition 18 o of five cherts from the 1 9 ga gunflint iron formation canada were studied at the micrometer scale by ion microprobe to try to better understand the processes that control 18 o values in cherts and to improve seawater paleotemperature reconstructions gunflint cherts show clearly different 18 o values for different types of silica with for instance a difference of 15 between detrital quartz and microquartz microquartz in the five samples is characterized by large intra sample variations in 18 o values 18 o of quartz varies from 4 6 to 6 6 at the 20 m scale and from 12 to 14 at 2 m scale isotopic profiles in microquartz adjacent to hydrothermal quartz veins demonstrate that microquartz more than 200 m away from the veins has preserved its original 18 o value at the micrometer spatial resolution of the ion probe data reveal that microquartz has preserved a considerable 18 o heterogeneity that must be regarded as a signature inherited from its diagenetic history modelling of the 18 o variations produced during the diagenetic transformation of sedimentary amorphous silica precursors into microquartz allows us to calculate seawater temperature t sw at which the amorphous silica precipitated and diagenesis temperature t diagenesis at which microquartz formed that reproduce the 18 o distributions mean range and shape measured at micrometer scale in microquartz the two critical parameters in this modelling are the 18 o value and the mass fraction of the diagenetic fluid under these assumptions the most likely ranges for t sw and t diagenesis are from 37 to 52 c and from 130 to 170 c respectively

magmatic evolution of the southern great bear continental arc northwestern canadian shield geochronological constraints the calc alkaline great bear continental arc in the wopmay orogen developed after a collision ca 1890 ma of the archean slave craton with the paleoproterozoic hottah terrane to the west u pb zircon dating of three volcanic and six intrusive rocks from the southern part of the arc shows four stages of development i intrusion of a few small sodic leucogranite plutons at 1873 2 ma into a previously folded metasedimentary sequence ii abundant calc alkaline felsic volcanism and subvolcanic intrusions during the period 1870 1866 ma bracketed by five ages iii intrusion of large calc alkaline granitic plutons including the marian river batholith dated by zircon and titanite from two samples at 1866 2 3 ma and iv emplacement of the potassic faber lake rapakivi granite at 1856 2 3 ma the arc was developed on the hottah terrane due to easterly subduction of an oceanic plate under the amalgamated slave craton hottah terrane the oldest exposed rocks in the southern part of the arc are remnants

late and mid cretaceous mineralization in the northern canadian cordillera constraints from re os molybdenite dates new re os molybdenite dates are presented from three deposits casino cash and mt nansen in the dawson range yukon northern canadian cordillera the re os molybdenite dates provide constraints on the timing of sulfide mineralzation and together with 40 ar 39 ar and k ar mineral dates yield estimates for the longevity of the porphyry hydrothermal system at the casino deposit re os molybdenite dates associated with the potassic 75 05 0 28 74 69 0 28 ma alteration are nominally older to overlapping with re os molybdenite dates 74 4 0 28 ma n 2 with phyllic alteration and suggest that it is likely that potassic and phyllic mineralization alteration was contemporaneous the re os molybdenite dates are significantly older than hydrothermal k feldspar 40 ar 39 ar and k ar biotite dates 72 ma which most likely reflect the cessation of the porphyry hydrothermal system the molybdenite age data combined with silicate hydrothermal mineral dates suggest a porphyry hydrothermal system longevity of 4 16 to 1 94 casino 8 66 to 6 44 cash and 2 47 to 1 48 ma mt nansen these longevity estimates for porphyry hydrothermal systems are greater than previously proposed 50 c km may control the disparate re os molybdenite and 40 ar 39 ar k feldspar dates of the porphyry systems of this study the contrasting re os and 40 ar 39 ar dates clearly show the re os isotope system in molybdenite is resistant to prolonged 1 ma interaction with a high temperature 400 500 c hydrothermal fluid and that the re os isotope systematics in molybdenite unambiguously record directly the timing of sulfide mineralization this ability to directly date sulfide mineralization has identified mid pattison idaho creek 102 104 ma and late cretaceous zappa koffee 74 5 ma mineralization which is hosted by the mid cretaceous 104 102 ma casino plutonic suite the late cretaceous date is very similar to that for molybdenite mineralization at casino and we suggest that the zappa koffee mineralization is an area of late cretaceous porphyry mineralization for which the host pluton has not yet been discovered

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

stable isotope chemistry population histories and late prehistoric subsistence change in the aleutian islands aleut population history has been a topic of debate since the earliest archaeological investigations in the region in this paper we use stable isotope chemistry to evaluate the hypothesis that two distinct groups of people paleo and neo aleut occupied the eastern aleutians after 1000 bp this study focuses on 80 sets of directly dated eastern aleutian burial assemblages from chaluka midden shiprock island and kagamil island we use a linear mixing model informed by isotopic analysis of two large aleut faunal assemblages to address temporal and spatial variation in human carbon and nitrogen stable isotope data from these sites the patterning we report addresses both aleut demographic and economic prehistory illustrating a transition in both at ca 1000 bp our results suggests that the chaluka diet dominated by paleo aleut inhumations differed in both trophic level and foraging location from the other two sites for much of the past 4000 years trends in our data also suggest that individuals from shiprock and kagamil burial caves primarily neo aleuts had enough access to higher trophic level foods to differentiate their bone chemistries from those buried in chaluka midden these trends in diet recently reported genetic differences as well as the introduction of novel mortuary practices at ca 1000 bp suggest that neo aleuts do represent a population new to the eastern aleutians

fingernails and diet stable isotope signatures of a marine hunting community from modern uummannaq north greenland c n ad s stable isotope compositions of fossil human bones are frequently used in palaeodiet studies but results from strictly marine communities are scarce modern hunting and fishing inuit greenlanders in the uummannaq district in north greenland are known to live from a diet high in local marine food seal whale halibut combined with basic foods bread cereals dairy products imported from denmark and thus provide an excellent test population here were report diet anamnesis data and stable isotope compositions of fingernails from 82 uummannaq district inuit individuals and a control group of 32 danes living in denmark the uummannaq fingernails are significantly enriched in 13c 15n 34s compared to the danish group t test 99 conf level calculated from isotope modelling the uummannaq inuit get on average 50 of their diet range 28 to 68 from marine food placed high in the trophic system reflecting high proportions of carnivorous marine mammals seal in the diet and the most 15n enriched individuals have trophic positions close to polar bears comparisons to other human hair and fingernail studies reveal the uummannaq inuit to be the most heavy isotope enriched modern community analysed so far the consistency of our data lead us to conclude that the fingernail isotope signatures are reliable measures of modern human diet composition and that fingernail isotope analyses can provide an easy quantitative supplement to other diet studies sulphur isotope analyses in special may provide unambiguous results in terrestrial to marine mixing systems

sulfur carbon hydrogen and oxygen isotope geochemistry of the idaho cobalt belt cobalt copper gold deposits of the idaho cobalt belt including the deposits of the blackbird district have been analyzed for their sulfur carbon hydrogen and oxygen isotope compositions to improve the understanding of ore formation previous genetic hypotheses have ranged widely linking the ores to the sedimentary or diagenetic history of the host mesoproterozoic sedimentary rocks to mesoproterozoic or cretaceous magmatism or to metamorphic shearing the 34 s values are nearly uniform throughout the blackbird district with a mean value for cobaltite coass the main cobalt mineral of 8 0 0 4 n 19 the data suggest that 1 sulfur was derived at least partly from sedimentary sources 2 redox reactions involving sulfur were probably unimportant for ore deposition and 3 the sulfur was probably transported to sites of ore formation as h 2 s hydrogen and oxygen isotope compositions of the ore forming fluid which are calculated from analyses of biotite rich wall rocks and tourmaline do not uniquely identify the source of the fluid plausible sources include formation waters metamorphic waters and mixtures of magmatic and isotopically heavy meteoric waters the calculated compositions are a poor match for the modified seawaters that form volcanogenic massive sulfide vms deposits carbon and oxygen isotope compositions of siderite a mineral that is widespread although sparse at blackbird suggest formation from mixtures of sedimentary organic carbon and magmatic metamorphic carbon the isotopic compositions of calcite in alkaline dike rocks of uncertain age are consistent with a magmatic origin several lines of evidence suggest that siderite postdated the emplacement of cobalt and copper so its significance for the ore forming event is uncertain from the stable isotope perspective the mineral deposits of the idaho cobalt belt contrast with typical vms and sedimentary exhalative deposits they show characteristics of deposit types that form in deeper environments and could be related to metamorphic processes or magmatic processes although the isotopic evidence for magmatic components is relatively weak

geochemistry of hydrothermal tourmaline from iocg occurrences in the great bear magmatic zone implications for fluid source s and fluid composition evolution abstract tourmaline crystallized during iron oxide and alkali calcic alteration and iocg mineralization in the great bear magmatic zone canada within the samples analyzed tourmaline occurs as small crystals in the groundmass of hydrothermal breccias southern breccia and contact lake or as large prismatic crystals in quartz ore mineralogy veins contact lake devries lake and nico the tourmaline from these localities is predominantly shorlitic in composition with minor dravitic uvitic and feruvitic components the boron isotopic composition ranges from 15 to 5 with the majority of values between 15 and 10 the lack of variation among samples supports a common fluid source across the belt for the metasomatic hydrothermal systems these results are isotopically similar to global magmatic and non marine evaporitic reservoirs and lighter than seawater and marine sedimentary rocks the 11b results support a single fluid iocg deposit model for the systems studied wherein metasomatic hydrothermal fluids derived by magmatism ascend and evolve over time through fluid rock interactions

strata bound fe co cu au bi y ree deposits of the idaho cobalt belt multistage hydrothermal mineralization in a magmatic related iron oxide copper gold system mineralogical and geochemical studies of strata bound fe co cu au bi y rare earth element ree deposits of the idaho cobalt belt in east central idaho provide evidence of multistage epigenetic mineralization by magmatic hydrothermal processes in an iron oxide copper gold iocg system deposits of the idaho cobalt belt comprise three types 1 strata bound sulfide lenses in the blackbird district which are cobaltite and less commonly chalcopyrite rich with locally abundant gold native bismuth bismuthinite xenotime allanite monazite and the be rich silicate gadolinite y with sparse uraninite stannite and bi tellurides in a gangue of quartz chlorite biotite muscovite garnet tourmaline chloritoid and or siderite with locally abundant fluorapatite or magnetite 2 discordant tourmalinized breccias in the blackbird district that in places have concentrations of cobaltite chalcopyrite gold and xenotime and 3 strata bound magnetite rich lenses in the iron creek area which contain cobaltiferous pyrite and locally sparse chalcopyrite or xenotime most sulfide rich deposits in the blackbird district are enclosed by strata bound lenses composed mainly of cl rich fe biotite some deposits have quartz rich envelopes whole rock analyses of 48 co and or cu rich samples show high concentrations of au up to 26 8 ppm bi up to 9 16 wt y up to 0 83 wt rees up to 2 56 wt ni up to 6 780 ppm and be up to 1 180 ppm with locally elevated u up to 124 ppm and sn up to 133 ppm zn and pb contents are uniformly low 821 and 61 ppm respectively varimax factor analysis of bulk compositions of these samples reveals geochemically distinct element groupings that reflect statistical associations of monazite allanite and xenotime biotite and gold detrital minerals chalcopyrite and sparse stannite quartz and cobaltite with sparse selenides and tellurides significantly cu is statistically separate from co and as consistent with the general lack of abundant chalcopyrite in cobaltite rich samples paragenetic relations determined by scanning electron microscopy indicate that the earliest y ree be mineralization preceded deposition of co cu au and bi allanite xenotime and gadolinite y commonly occur as intergrowths with and inclusions in cobaltite the opposite texture is rare monazite in contrast forms a poikiloblastic matrix to cobaltite and thin rims on allanite and xenotime reflecting a later metamorphic paragenesis allanite and xenotime also show evidence of late dissolution and reprecipitation forming discordant rims on older anhedral allanite and xenotime and separate euhedral crystals of each mineral textural data suggest extensive deformation of the deposits by folding and shearing and by pervasive recrystallization all during cretaceous metamorphism sensitive high resolution ion microprobe u pb geochronology by aleinikoff et al 2012 supports these paragenetic interpretations documenting contemporaneous mesoproterozoic growth of early xenotime and crystallization of megacrystic a type granite on the northern border of the district these ages are used together with mineralogical and geochemical data from the present study to support an epigenetic iocg model for fe co cu au bi y ree deposits of the idaho cobalt belt a sulfide facies variant of iocg deposits is proposed for the blackbird district in which reducing hydrothermal conditions favored deposition of sulfide minerals over iron oxides this new model includes mesoproterozoic vein mineralization and related fe cl metasomatism that formed the biotite rich lenses a predominantly felsic magmatic source for metals in the deposits given their local abundance of y rees and be and a major sedimentary component in the hydrothermal fluids based on independent sulfur isotope and boron isotope data for sulfides and ore related tourmaline respectively

formation of albitite hosted uranium within iocg systems the southern breccia great bear magmatic zone northwest territories canada uranium and polymetallic u mineralization hosted within brecciated albitites occurs one kilometer south of the magnetite rich au co bi cu nico deposit in the southern great bear magmatic zone gbmz canada concentrations up to 1 wt u are distributed throughout a 3 by 0 5 km albitization corridor defined as the southern breccia zone two distinct u mineralization events are observed primary uraninite precipitated with or without pyrite chalcopyrite molybdenite within magnetite ilmenite biotite k feldspar altered breccias during high temperature potassic iron alteration subsequently pitchblende precipitated in earthy hematite specular hematite chlorite veins associated with a low temperature iron magnesium alteration the uraninite bearing mineralization postdates sodic albite and more localized high temperature potassic iron biotite magnetite k feldspar alteration yet predates potassic k feldspar boron tourmaline and potassic iron magnesium hematite k feldspar chlorite alteration the southern breccia zone shares attributes of the valhalla australia and lagoa real brazil albitite hosted u deposits but contains greater iron oxide contents and lower contents of riebeckite and carbonates potassium ni and th are also enriched whereas zr and sr are depleted with respect to the aforementioned albitite hosted u deposits field relationships geochemical signatures and available u pb dates on pre syn and post mineralization intrusions place the development of the southern breccia and the nico deposit as part of a single iron oxide alkali altered ioaa system in addition this case example illustrates that albitite hosted u deposits can form in albitization zones that predate base and precious metal ore zones in a single ioaa system and become traps for u and multiple metals once the tectonic regime favors fluid mixing and oxidation reduction reactions

paleodiet in northern chile through the holocene extremely heavy δ15n values in dental calculus suggest a guano derived signature abstract dental calculus extracted from the teeth of 28 prehistoric human skeletons from coastal and valley archaeological sites in northern chile that date from the archaic period 2300 bc to the late intermediate period ad 1476 was analyzed for stable isotope 15 n 13 c compositions 15 n compositions were extremely heavy 17 8 to 33 1 but comparable to some studies using conventional human biomaterials hair nail muscle at other prehistoric sites in northern chile there was a negative correlation between 15 n and 13 c for coastal sites but a positive correlation for valley sites results for the valley sites point to a diet that was influenced by marine resources throughout all time periods the unusually heavy 15 n values for the coastal sites require a dietary component with a 15 n composition significantly heavier than that of marine resources the hyper aridity at the study area mean annual rainfall of 0 5 0 6 mm year is a likely contributing factor to the production of heavy 15 n values but is unlikely to account for the heaviest 15 n values 30 one possible explanation for the heaviest 15 n values is that dietary components were impacted by isotopically heavy guano which is abundant in the region guano may have been used as a fertilizer during crop cultivation at the onset of the formative period continuing through the historic period after spanish contact the indirect impact of guano from abundant bird habitats in the region may have influenced wild foodstuffs harvested throughout the pre agricultural period results provide support for the utility of dental calculus as a proxy for obtaining stable carbon and nitrogen isotope signatures for use in paleodietary studies

biogenic and thermogenic sulfate reduction in the sullivan pb zn ag deposit british columbia canada evidence from micro isotopic analysis of carbonate and sulfide in bedded ores abstract the layered clastic hosted sedex type proterozoic ca 1475 ma sullivan pb zn ag deposit was metamorphosed by several episodes of metamorphic recrystallization that have extensively modified or erased primary grain to grain textural relationships yet genetically significant variations of sulfur and carbon isotope ratios have been preserved at the millimeter and greater scale whereas sulfur isotope fractionations on a grain to grain scale cannot distinguish between likely formation temperatures and post peak metamorphic cooling the preserved fine scale and deposit scale stratigraphic variations of 34 s and 13 c bear on the origin and formational history of this deposit high spatial resolution documentation of accurate grain to grain and stratigraphic sulfur isotope variations in selected portions of bedded ore from the sullivan deposit was made possible by use of a cold stage during in situ laser assisted isotopic analysis pyrrhotite formed from biogenic sulfate reduction in the water column can be distinguished from sulfides e g sphalerite with higher 34 s values that formed in response to hydrothermal fluids in a brine pool isotopic analysis of carbonates in the presence of abundant sulfides was facilitated by development of a gas chromatographic carbonate extraction system with a high efficiency cryogenic trap calcite present in interlayered massive sulfide and siliciclastic layers in the bedded ore zone in trace amounts to a few weight percent records a carbon isotopic stratigraphy that suggests variable thermogenic sulfate reduction stratigraphic curves illustrating the variation in carbon and sulfur isotopes from the base of the main band to above the hu bands mine stratigraphic names in the bedded ore zone are similar in shape values of 13 c decrease abruptly at the base of the main band decrease slightly to the c band then increase to pre ore values the values of 34 s decrease from the base of the main band to the c band and then increase to pre ore values this similarity suggests some linkage between carbon and sulfur isotopic systematics and a sulfur budget dominated in the early stages of massive sulfide precipitation by hydrothermal processes involving thermogenic sulfate reduction giving way to bacteriogenic sulfate reduction in a closed then finally open marine basin

alteration facies linkages among iron oxide copper gold iron oxide apatite and affiliated deposits in the great bear magmatic zone northwest territories canada high temperature metasomatism driven by ascent of voluminous saline fluid columns in the upper crust plays a major role in the genesis of iron oxide alkali alteration ore systems but fundamental questions remain on genetic linkages among iron oxide copper gold iocg iron oxide apatite ioa albitite hosted uranium and skarn deposits that they produce excellent surface exposures of such systems in the great bear magmatic zone of northernwestern canada record the depth to paleosurface prograde evolution of iron oxide alkali alteration facies and mineralization across the belt albitite corridors that are tens of kilometers in length record the earliest reactions between highly saline fluids and host rocks along fault zones and subvolcanic intrusions pervasive albitization partitioned metals from the host rocks into the ascending fluid column leaving behind structurally weakened corridors of porous albitite these corridors were cut replaced and overprinted by amphibole and magnetite bearing calcic iron alteration assemblages in extreme cases the discharge of calcium iron and specialized metals formed iron oxide apatite deposits vanadium rare earth elements while recharging the outgoing fluids in sodium potassium and base and precious metals as temperatures declined and fluid chemistry evolved through fluid rock reactions the formation of potassic iron alteration assemblages breccias and sulfides resulted in magnetite and hematite group iocg mineralization within carbonate units skarns formed prior to are replaced by and evolved to calcic iron alteration facies skarns can locally host base metal mineralization tectonically uplifted albitite breccias replaced by potassic iron alteration assemblages became a preferential host for uranium mineralization the results of this study also illustrate that permutations and cyclical build up of alteration products can arise from a combination of faulting differential uplift and renewed magmatism framed within an alteration facies deposit model alteration zones and mineral occurrences play a pivotal role in predicting the mineral potential of iron oxide and alkali altered systems at district to deposit scales

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

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

trace element geochemistry of magnetite and its relationship to cu bi co au ag u w mineralization in the great bear magmatic zone nwt canada the paleoproterozoic great bear magmatic zone is the focus of ongoing exploration for iron oxide copper gold iocg deposits and also hosts iron oxide apatite occurrences examples of iocg deposits in the great bear magmatic zone include sue dianne and nico and other smaller prospects including damp fab and nori ra the past producing terra mine property hosts significant iocg like alteration that contains dome shaped iron oxide apatite bodies petrographic study has identified multiple generations of magnetite at nico fab and nori ra and for the most part a single generation of magnetite at sue dianne damp and terra laser ablation inductively coupled plasma mass spectrometry la icp ms documents important geochemical differences in v ni cr and co concentrations within the magnetite variations of trace elements in magnetite from the great bear magmatic zone could be a result of 1 host rock fluid equilibration during regional metamorphism 2 postmetamorphic hydrothermal metasomatism of treasure lake group metasedimentary rocks 3 preferential solubility of co over ni within the fe rich fluids 4 changes in oxygen fugacity f o2 and 5 partitioning of elements into coprecipitating sulfides regionally the cr co ratio is higher in barren and pre ore magnetite compared to magnetite coprecipitated with ore minerals and or present in ore rich veins and breccias locally at the nori ra prospect the v ni ratio in magnetite differentiates between barren and ore related magnetite and at damp and sue dianne the co ni ratio is extremely high and clearly different from that of other great bear magmatic zone magnetite samples these results provide the first database for geochemically characterized magnetite from different stages of iocg alteration and illustrate the potential use of magnetite as an indicator mineral in the exploration for iocg deposits

the raul condestable iron oxide copper gold deposit central coast of peru ore and related hydrothermal alteration sulfur isotopes and thermodynamic constraints the iron oxide copper gold iocg raul condestable deposit is located 90 km south of lima peru and approximately 5 km from the pacific coast mineralization consists mainly of replacement mantos and disseminations within permeable volcaniclastic and carbonate rich rocks and structurally controlled veins surrounding a coeval and apparently causative intrusion of tonalitic composition emplaced in the core of a dacitic volcano potassic biotite grading upward to sericite chlorite alteration and a poorly developed almost sulfide free quartz stockwork closely border the tonalite affecting the basaltic to dacitic lower cretaceous volcano sedimentary host sequence ore is associated with a hydrated calc silicate mainly amphiboles alteration that surrounds the biotite alteration a hematite chlorite albite epidote calcite alteration affects the periphery of the system the main ore stage is characterized by two end member mineral associations that were formed according to 1 an oxidized deposition sequence hematite magnetite pyrite chalcopyrite occurring in and near feeder structures and 2 a reduced deposition sequence pyrrhotite pyrite chalcopyrite found in volcaniclastic rocks and veins early specular hematite of the oxidized sequence is transformed to magnetite mushketovite the main ore stage mineralization is cut by minor late stage calcite sulfide veins main ore stage sulfides have 34s values asymmetrically distributed from 1 0 to 26 3 per mil with a median at 6 6 per mil n 51 similar values are observed for pyrrhotite pyrite and chalcopyrite the 34s values depend on the stratigraphic position with deep seated vein samples normally distributed between 1 0 and 6 3 per mil avg about 3 5 n 13 and shallower samples from 2 7 to 26 3 per mil median around 7 5 n 39 sulfides found in late stage calcite sulfide veins show strongly negative 34s values ranging between 32 7 and 22 9 per mil n 6 indicating a possible biogenic source because no rock unit is known to occur in the internal parts of the deposit that could have oxidized fluids to the point of hematite stability the oxidized mineral sequence is best explained by magmatic brines following the so2 h2s gas buffer at high temperature 350 c and fluid rock ratio this is supported by the close to magmatic 34s values of sulfides from the deep parts of feeder veins mass balance calculation based on sulfur isotope data suggests that at the deposit scale the bulk of the sulfides is dominated by magmatic sulfur with sulfides of the oxidized minerals association having a larger component of magmatic sulfur than those of the reduced mineral association the deposition sequence from hematite to chalcopyrite reflects the cooling of the magmatic fluid and redox and ph buffering by the basaltic andesitic volcano sedimentary host rocks thus the occurrence of magnetite pseudomorphous after early hematite mushketovite paragenetically followed by iron bearing sulfides is interpreted to be direct field evidence for precipitation from oxidized magmatic brines the same sequence has been described in many iocg skarn and some porphyry copper deposits worldwide 34s values of sulfides ranging up to 26 3 per mil are found in what corresponded to a relatively shallow aquifer filled with evolved reduced seawater heavy sulfur in h2s was produced through thermochemical reduction of aptian seawater sulfate 34s 14 in the recharge zone which is interpreted to correspond to the hematitechlorite albite epidote calcite alteration present at the upper flanks of the hydrothermal system adjacent to the causative intrusion hematitization through oxidation resulted from the high fo2 of seawater and from the reduction of its sulfate to h2s by the fe2 contained in the rock in the core of the system the seawater derived fluids reached near chemical equilibrium with their actinolitized host rock at about 300 to 350 c in reduced rock dominated conditions mixing of these fluids with magmatic brines already partially or totally reduced through reaction with wall rock at medium to low magmatic fluid rock ratio can explain the large positive 34s scatter observed in sulfides of the reduced mineral association at stratigraphically shallow positions

on the relationship between alteration facies and metal endowment of iron oxide alkali altered systems southern great bear magmatic zone canada the great bear magmatic zone in northern canada hosts large iron oxide and alkali altered systems that developed within a 10 m y period during the paleoproterozoic in the eastern treasure lake and duke sectors and at the nico deposit early albitization associated with magmatism was followed by tungsten mineralization that was spatially associated with extensive skarn and fe skarn alteration subsequent fluid rock reactions led to substantial iron enrichment that culminated in iron oxide apatite mineralization at the transition from compressional transpressional to extensional transtensional stress regimes changes in magma composition increased the metal budget of exsolved magmatic hydrothermal fluids and played a key role in the formation of au co bi mineralization at the nico deposit and its satellite showings in the duke zone this process happened as the systems regionally transitioned to magnetite bearing potassic iron alteration the continued evolution of magmatism and fluid chemistry then contributed to the formation of iron oxide copper gold iocg and variant mineralized zones in the sue dianne and nico deposits and other showings and albite hosted uranium mineralized zones elsewhere the great bear magmatic zone illustrates that the evolution of magmatically derived fluids during a transition between different stress regimes can play a significant role in metal endowment and the ability of metasomatic systems to form skarn iron oxide apatite iocg and albitite hosted uranium mineralization

oxygen isotope compositions of iron oxides from high grade bif hosted iron ore deposits of the central hamersley province western australia constraints on the evolution of hydrothermal fluids the hamersley province of northwest western australia is one of the world s premier iron ore regions with high grade martite microplaty hematite iron ore deposits mostly hosted within banded iron formation bif sequences of the brockman iron formations of the hamersley group these high grade iron ores contain between 60 and 68 wt percent fe and formed by the multistage interaction of hydrothermal fluids with the host bif formation the oxygen isotope compositions of magnetite and hematite from bif hydrothermal alteration assemblages and high grade iron ore were analyzed from the mount tom price paraburdoo and channar iron ore deposits the 18o values of magnetite and hematite from hydrothermal alteration assemblages and high grade iron ore range from 9 0 to 2 9 per mil a depletion of 5 to 15 per mil relative to the host bif the 18o values are spatially controlled by faults within the deposits a response to higher fluid flux and larger influence on the isotopic compositions by the hydrothermal fluids the oxygen isotope composition of hydrothermal fluids 18ofluid indicates that the decrease in the 18o content of iron oxides was due to the interaction of both basinal brines and meteoric fluids with the original bif late stage talc bearing ore at the mount tom price deposit formed in the presence of a pulse of 18o enriched basinal brine indicating that hydrothermal fluids may have repeatedly interacted with the bifs during the paleoproterozoic

fluid evolution and origins of iron oxide cu au prospects in the olympic dam district gawler craton south australia geologic observations suggest two stages of hydrothermal activity at a number of presently subeconomic iron oxide copper gold systems in the olympic dam district eastern gawler craton they contain high and moderate to low temperature fe oxide rich hydrothermal alteration the mineral assemblages include magnetite calc silicate alkali feldspar fe cu sulfides and hematite sericite chlorite carbonate fe cu sulfides u ree minerals in all documented prospects the minerals of the hematitic assemblages replace the minerals of the magnetite rich assemblages the bulk of the subeconomic cu au mineralization is associated with the hematitic alteration assemblages microanalysis by proton ion probe pixe of hypersaline fluid inclusions in magnetite rich assemblages however demonstrates that significant amounts of copper 500 ppm were transported by the early stage high temperature 400 c fluids responsible for the magnetite rich alteration these brine inclusions contain multiple solid phases liquid vapor multiple solids including chalcopyrite in some cases in comparison inclusions of the hematitic stage are relatively simple liquid vapor types with homogenization temperatures of 200 to 300 c and containing 1 to 8 wt percent nacl equiv the br cl ratios of the magnetite forming fluids measured by pixe lie beyond the range of typical magmatic and or mantle values allowing for the possibility that the fluids originated as brines from a sedimentary basin or the crystalline basement sulfur isotope compositions of chalcopyrite and pyrite demonstrate that sulfur in both alteration assemblages was derived either from cooling magmas and or crystalline igneous rocks carried by relatively oxidized fluids so42 h2s 34ssulfides from 5 to 2 or from crustal sedimentary rocks 34ssulfides from 5 to 10 oxygen and hydrogen isotope compositions of waters calculated for minerals of the magnetite rich assemblage have 18o values of 7 7 to 12 8 per mil and d values of 15 to 21 per mil the only available 18o and dfluid values for the hematitic assemblage are 4 7 and 9 per mil respectively the isotopic compositions of both fluids coupled with the available literature data can be explained in terms of fluid reequilibration with felsic gawler range volcanics or other felsic igneous rocks in the region and with metasedimentary rocks of the wallaroo group at low water to rock ratios prior to their arrival at the mineralization sites the lack of significant copper mineralization associated with magnetite forming fluids that carried copper suggests that there was no effective mechanism of saturation of copper minerals or the quantity of these fluids was not sufficient to produce appreciable copper mineralization association of the copper gold mineralization with the hematitic alteration in the subeconomic prospects can be explained by a two stage model in which preexisting hydrothermal magnetite with minor associated copper gold mineralization was flushed by late stage oxidized brines that had extensively reacted with sedimentary or metamorphic rocks the reduction of these brines driven by conversion of magnetite to hematite resulted in precipitation of copper and gold the oxidized brines may have contributed additional copper and gold to the system in addition to upgrading preexisting subeconomic cu au mineralization when compared to published models for the olympic dam deposit the new data for fluids in subeconomic fe oxide cu au prospects of the olympic dam district indicate the diversity of origins of iron oxide copper gold systems even within the same geologic region

controls on hydrothermal fe oxide cu au co mineralization at the guelb moghrein deposit akjoujt area mauritania the guelb moghrein fe oxide cu au co deposit with a total resource of 23 6 mt at 1 88 cu 1 41 g t au and 143 g t co is hosted by an extensive metacarbonate body however it is restricted to up to 30 m wide tabular breccia zones developed parallel to discrete shear zones that transect the host metacarbonates the fe mg clinoamphibole chlorite schists represent up to 1 m thick interlayer metasediments and localized viscous shearing in these shear zones siderite of the metacarbonate body was deformed into a breccia and was replaced by an ore and alteration assemblage comprised of fe mg clinoamphibole magnetite pyrrhotite chalcopyrite graphite fe co ni arsenides arsenopyrite cobaltite uraninite and bi au ag te minerals in contact with wall rock amphibolites the metacarbonate body is enveloped by an alteration halo up to 40 m wide consisting of biotite actinolite grunerite chlorite calcite albite and quartz the guelb moghrein ore body is structurally controlled by shear zones that developed in the footwall of a regional thrust zone this thrust separates greenschist facies quartz sericite schists and biotite garnet quartz schists of the sainte barbe volcanic unit in the hanging wall from amphibolite facies metavolcanic rocks metacarbonates and the guelb moghrein ore body of the akjoujt metabasalt unit in the footwall peak temperatures of the latter unit are estimated by hornblende plagioclase thermometry at 580 40 c thrusting was retrograde for the akjoujt metabasalt unit but prograde for the sainte barbe volcanic unit at p t conditions of about 410 30 c and 2 3 kbar garnet biotite thermometry structural and petrological evidences suggest that the ore fluids migrated along the shear zones and reacted with the siderite in the metacarbonate this evolution and the setting of guelb moghrein in the fold and thrust belt of the pan african to variscan mauritanides mauritania west africa resemble proterozoic fe oxide cu au co deposits such as examples from the tennant creek and mount isa inliers australia

origin of sulfur and crustal recycling of copper in polymetallic cu au co bi u ag iron oxide dominated systems of the great bear magmatic zone nwt canada the great bear magmatic zone in northwest canada contains numerous polymetallic mineral occurrences prospects and deposits of the iron oxide copper gold deposit iocg family the mineralization is hosted by the treasure lake group and igneous rocks of the great bear arc and was deposited concomitantly with the arc magmatism ca 1 88 to 1 87 ga in situ 34s n 48 and 65cu n 79 analyses were carried out on ore related sulfides from a number of these systems the 34s values mainly vary between 0 and 5 consistent with derivation of sulfur from the mantle lower 34s values 7 7 to 1 4 from the sue dianne breccia may indicate so2 disproportionation of a magmatic hydrothermal fluid the 65cu values vary between 1 2 and 0 3 and are lower than the igneous 65cu range of values 0 0 0 27 the s and cu isotopic data are decoupled which suggests that cu and possibly some s was dissolved and remobilized from supracrustal rocks during early stages of alteration e g sodic alteration and then precipitated by lower temperature more oxidizing fluids e g ca fe k alteration a limited fluid inclusion dataset and 13c and 18o values are also presented the 18ofluid values are consistent with a magmatic origin or a host rock equilibrated meteoric water source whereas the 13cfluid values support a marine carbonate source combined the s and cu isotopic data indicate that while the emplacement of the great bear magmatic bodies may have driven fluid convection and may be the source of fluids and sulfur metals such as cu could have been recycled from crustal sources

evaporitic source model for igneous related fe oxide ree cu au u mineralization we propose that many igneous related fe oxide rich ree cu au u bearing deposits form by hydrothermal processes involving evaporitic ligand sources either coeval salars or older evaporites these deposits are abundant in both phanerozoic and proterozoic extensional continental and continent margin settings they commonly form in global arid zones but they also occur where magmatism is superimposed upon older evaporites magmatic compositions exert only second order control mainly on alteration mineralogy and on element abundances hot s poor brines generated by interaction with evaporitic materials are consistent with geologic settings and help rationalize the distinctive element enrichments siderophile lithophile and hydrothermal alteration sodic locally alkaline found in these systems this model contrasts with immiscible oxide melt and magmatic hydrothermal origins commonly proposed for these deposits although all three mechanisms can occur 31 refs 3 figs 1 tab

alteration of 15n14n and 13c12c ratios of plant matter during the initial stages of diagenesis studies utilizing archaeological specimens from peru abstract the stable carbon and nitrogen isotope ratios of plants extracted from peruvian archaeological sites ranging in age from 400 to 4000 years b p have been measured the 13 c and 15 n values of prehistoric plants that were carbonized prior to deposition are similar to those of modern plants which have the same carbon dioxide fixation or nitrogen assimilation mechanisms in contrast the 15 n values of prehistoric plants that were not carbonized are generally 10 20 and as much as 35 more positive than those of their modern counterparts the 13 c and 15 n values of different parts of prehistoric uncarbonized plants differ by as much as 8 and 21 respectively whereas the same parts of modern plants have 13 c or 15 n values that lie within 2 of one another sem analysis etiminated the possibility that diagenetic alteration of the isotope ratios of the prehistoric uncarbonized plants was caused by the adsorption of particulate soil matter isotopic analysis of the organic residues that remained after the prehistoric uncarbonized plants were treated to remove fractions that might have been added in the depositional environment indicated that the alteration of the 13 c values resulted from the adsorption of humic and fulvic acids whereas the diagenetic shifts of the nitrogen isotope ratios were not caused by adsorption of 15 n enriched organic or inorganic forms of nitrogen from the soil our observations do not permit us to speculate on a mechanism that accounts for the 13 n enrichment of the prehistoric uncarbonized plant remains although the excellent state of preservation of these samples suggests that isotopic fractionation during microbial decomposition was not the cause the observation that the isotope ratios of the prehistoric carbonized plants are similar to those of their modern counterparts indicates that it will be possible to separate carbonized plants residues into three groups legumes non leguminous c 3 plants or c 4 and cam plants based on their 13 c and 15 n values this method should provide useful information to archaeologists working in environments in which carbonized plant remains often burned beyond morphological recognition are the only archaeobotanical remains which are recovered

guelb moghrein an unusual carbonate hosted iron oxide copper gold deposit in mauritania northwest africa the guelb moghrein cu au deposit occurs within metacarbonate bodies derived from sedimentary carbonate beds of probable late archean age the metacarbonate lenses are enclosed within mafic meta igneous rocks that display a coarse grained amphibolite facies hornblende albite assemblage that is mineralogically anomalous relative to the greenschist facies metamorphic rocks present regionally within the district and may represent a zone of synmetamorphic hydrothermal metasomatism crosscutting relationships and textural evidence for progressive replacement of different mineral assemblages demonstrate that the deposit had a complex multiphase alteration mineralization history early sodic alteration at guelb moghrein resulted in the formation of albitized zones along moderately to shallowly dipping structures sodic alteration was followed by syndeformational ferroan potassic alteration that resulted in the growth of biotite and grunerite cummingtonite in meta igneous rocks and conversion of calcite dolomite in metacarbonate bodies to ankerite and siderite rich assemblages progressive alteration resulted in breakdown of siderite to magnetite and graphite at approximately 400 to 450 c sulfide mineralization took place at ca 2 5 ga concurrently with or shortly after ferroan potassic alteration and magnetite graphite formation an early assemblage of co ni and as rich sulfide minerals was followed by precipitation of pyrrhotite and chalcopyrite sulfide mineralization was focused within brittle deformation zones along the margins of the metacarbonate bodies which represent high permeability zones adjacent to more ductile structures the nucleation of sulfides on graphite during siderite breakdown suggests redox reactions may have largely controlled sulfide precipitation a late magnesian alteration event at ca 1 74 ga resulted in the formation of chlorite rich shear zones and precipitation and or recrystallization of sulfides the guelb moghrein deposit is a member of the iron oxide cu au class of deposits but is unusual in being hosted in carbonate strata

new geologic fluid inclusion and stable isotope studies on the controversial igarape bahia cu au deposit carajas province brazil the igarape bahia cu au deposit in the carajas province brazil is hosted by steeply dipping metavolcano sedimentary rocks of the igarape bahia group this group consists of a low greenschist grade unit of the archean 2 750 ma itacaiunas supergroup in which other important cu au and iron ore deposits of the carajas region are also hosted the orebody at igarape bahia is a fragmental rock unit situated between chloritized basalt with associated hyaloclastite banded iron formation bif and chert in the footwall and mainly coarse to fine grained turbidites in the hanging wall the fragmental rock unit is a nearly concordant 2 km long and 30 250 m thick orebody made up of heterolithic usually matrix supported rocks composed mainly of coarse basalt bif and chert clasts derived from the footwall unit mineralization is confined to the fine grained matrix and comprises disseminated to massive chalcopyrite accompanied by magnetite gold u and light rare earth element lree minerals and minor other sulfides like bornite molybdenite cobaltite digenite and pyrite gangue minerals include siderite chlorite amphibole tourmaline quartz stilpnomelane epidote and apatite a less important mineralization style at igarape bahia is represented by late quartz chalcopyrite calcite veins that crosscut all rocks in the deposit area fluid inclusions trapped in a quartz cavity in the ore unit indicate that saline aqueous fluids 5 to 45 wt nacl cacl2 equiv together with carbonic co2 ch4 and low salinity aqueous carbonic 6 wt nacl equiv fluids were involved in the mineralization process carbonates from the fragmental layer have 13c values from 6 7 to 13 4 per mil that indicate their origin from organic and possibly also from magmatic carbon the 34s values for chalcopyrite range from 1 1 to 5 6 per mil with an outlier at 10 8 per mil implying that most sulfur is magmatic or leached from magmatic rocks whereas a limited contribution of reduced and oxydized sulfur is also evident oxygen isotopic ratios in magnetite quartz and siderite yield calculated temperatures of 400 c and 18o enriched compositions 5 to 16 5 per mil for the ore forming fluids that suggest a magmatic input and or an interaction with 18o rich probably sedimentary rocks the late veins of the igarape bahia deposit area were formed from saline aqueous fluids 2 to 60 wt nacl cacl2 equiv with 18ofluid compositions around 0 per mil that indicate contribution from meteoric fluids with respect to geological features igarape bahia bears similarity with syngenetic volcanic hosted massive sulfide vhms type deposits as indicated by the volcano sedimentary geological context stratabound character and association with submarine volcanic flows hyaloclastite and exhalative beds such as bif and chert on the other hand the highly saline ore fluids and the mineral assemblage dominated by magnetite and chalcopyrite with associated gold u and lree minerals and scarce pyrite indicate that igarape bahia belongs to the fe oxide cu au iocg group of deposits the available geochronologic data used to attest syngenetic or epigenetic origins for the mineralization are either imprecise or may not represent the main mineralization episode but a later superimposed event the c s and o isotopic results obtained in this study do not clearly discriminate between fluid sources however recent b isotope data obtained on tourmaline from the matrix of the fragmental rock ore unit xavier wiedenbeck dreher rhede monteiro araujo chemical and boron isotopic composition of tourmaline from archean and paleoproterozoic cu au deposits in the carajas mineral province 1 simposio brasileiro de metalogenia gramado brazil extended abstracts cd rom 2005 provide strong evidence of the involvement of a marine evaporitic source in the hydrothermal system of igarape bahia evaporite derived fluids may explain the high salinities and the low reduced sulfur mineral paragenesis observed in the deposit evaporite derived fluids also exclude a significant participation of magmatic or mantle derived fluids reinforcing the role of nonmagmatic brines in the genesis of igarape bahia considering this aspect and the geological features the possibility that the deposit was generated by a hydrothermal submarine system whose elevated salinity was acquired by leaching of ancient evaporite beds should be evaluated

retention of hunter gatherer economies among maritime foragers from caleta vitor northern chile during the late holocene evidence from stable carbon and nitrogen isotopic analysis of skeletal remains abstract on the basis of stable carbon and nitrogen isotope analysis of human remains this paper provides evidence for the retention of hunter gatherer economies among coastal inhabitants in northern chile during the late holocene at the same time that inland populations were adopting agricultural economies coastal diets from the caleta vitor region of the atacama desert were dominated by marine based foods predominantly from upper trophic levels the focus on reliable marine food resources is interpreted as a risk minimisation strategy in this marginal arid environment although these coastal hunter gatherers adopted other goods and traditions from agricultural populations their participation in this larger interregional exchange network did not affect their basic subsistence economies associated new radiocarbon dates from the site are also discussed skeletal remains and artefacts associated with the caleta vitor shell middens generally originate from sites with dates ranging from c 9000 476 bp however the human remains that are the subject of this research cover the time period c 4000 476 bp

hydrothermal fluid evolution and metal transport in the kiruna district sweden contrasting metal behaviour in aqueous and aqueous carbonic brines abstract iron oxide copper gold iocg deposits and fe oxide apatite deposits from norrbotten sweden formed in similar settings and in some cases iocg mineralisation overprinted fe oxide apatite mineralisation fluid inclusions in quartz veins cutting fe oxide apatite deposits range in salinity from 33 to 37 wt nacl eq and those in iocg type deposits from 41 to 54 wt nacl eq minimum trapping conditions for these inclusions are 200 300 mpa and 200 300 c in the fe oxide apatite bodies and 250 to 300 mpa and 300 500 c in the iocg deposits deformed cu au deposits have similar early fluid characteristics but contain complex secondary fluid inclusion assemblages including halite saturated 20 30 wt nacl eq aqueous carbonic 3 13 wt nacl eq x co 2 0 17 0 29 and co 2 rich fluids the aqueous carbonic and carbonic inclusions are consistent with aqueous carbonic fluid immiscibility at 150 mpa a secondary population with a high ca content occurs in all deposit types the chemical composition of these inclusions has been determined by crush leach analysis and la icpms halogen contents indicate a range of salinity sources with possible inputs from both magmatic and halite dissolution brines element ratios suggest the alkali content of the fluid exceeded the buffer capacity of the host rocks iron and other transition metal contents correlate strongly with cl concentrations with secondary controls on solubility from ph redox and temperature copper and ag contents are higher in lower salinity aqueous carbonic brines up to 5000 ppm cu 900 ppm ag than in the most saline brines up to 2297 ppm cu 837 ppm ag this may reflect differences in metal source between deposit types but is also consistent with the complexation of cu by bisulphide in the lower salinity fluids late stage aqueous carbonic fluid flux through the deformed deposits either introduced additional copper to the deposits remobilised pre existing copper or both

metabasalts as sources of metals in orogenic gold deposits although metabasaltic rocks have been suggested to be important source rocks for orogenic gold deposits the mobility of au and related elements as sb se and hg from these rocks during alteration and metamorphism is poorly constrained we investigate the effects of increasing metamorphic grade on the concentrations of au and related elements in a suite of metabasaltic rocks from the otago and alpine schists new zealand the metabasaltic rocks in the otago and alpine schists are of morb and wpb affinity and are interpreted to be fragments accreted from subducting oceanic crust gold concentrations are systematically lower in the higher metamorphic grade rocks average au concentrations vary little between sub greenschist 0 9 0 5 ppb and upper greenschist facies 1 0 0 5 ppb but decrease significantly in amphibolite facies samples 0 21 0 07 ppb the amount of au depleted from metabasaltic rocks during metamorphism is on a similar scale to that removed from metasedimentary rocks in otago arsenic concentrations increase with metamorphic grade with the metabasaltic rocks acting as a sink rather than a source of this element the concentrations of sb and hg decrease between sub greenschist and amphibolite facies but concentration in amphibolite facies rocks are similar to those in unaltered morb protoliths and therefore unaltered oceanic crust cannot be a net source of sb and hg in a metamorphic environment the concentrations of au as sb and hg in oceanic basalts that have become integrated into the metamorphic environment may be heavily influenced by the degree of seafloor alteration that occurred prior to metamorphism we suggest that metasedimentary rocks are much more suitable source rocks for fluids and metals in orogenic gold deposits than metabasaltic rocks as they show mobility during metamorphism of all elements commonly enriched in this style of deposit

fluid inclusion characteristics of hydrothermal cu ni pge veins in granitic and metavolcanic rocks at the contact of the little stobie deposit sudbury canada abstract cu and precious metal enriched massive ore and veins occur at contacts of two fe ni cu sulfide orebodies of the little stobie mine sudbury with metavolcanic rocks of the elsie mountain formation and the murray granite veins contain chalcopyrite pyrrhotite pentlandite platinum group minerals quartz carbonate chlorite amphibole and other minerals a granitic dike resulting from partial re melting of the murray granite cuts back into the sudbury igneous complex near little stobie mine and contains barren veinlets with similar mineralogy but no sulfides quartz from ore veins contains fluid inclusions that were trapped during several stages early high temperature fluids at least 180 270 c in orebody 1 and 280 350 c in orebody 2 were extremely saline and occur as polyphase isolated and fracture controlled inclusions with halite sylvite fe mn as well as pb ba chloride daughter minerals and other unknown solids late secondary aqueous inclusions are not chloride saturated they were trapped at a minimum of 80 150 c their microthermometric behavior may be modeled in the cacl 2 nacl h 2 o system with salinity of 21 27 cacl 2 equiv wt very late secondary inclusions have either ca rich saline or very dilute about 1 equiv wt nacl compositions and homogenization temperatures of 200 300 c with the exception of these very late secondary inclusions the association of co 2 ch 4 rich inclusions with aqueous ones was usually observed the density of these carbonic fluids decreased from early to late stages microthermometric data from barren veins are fundamentally different from those of early inclusions from orebodies this implies that these heavy metal rich fluids were responsible for ore deposition in veins the minimum pressure of entrapment for early fluids was 1800 2200 bars late ca rich brines were trapped at lower minimum pressure 200 900 bars high pressure data are in agreement with the estimated minimum paleodepth of crystallization of south range ores later fluids were probably trapped during the uplift of orebodies and their host rocks comparison of data to other cu pge au rich ore of the sudbury structure suggests that the presence of co 2 rich fluids in south range deposits and their absence in north range deposits may be related to different metamorphic histories the high temperature hydrothermal fluids were driven by the heat of the sudbury igneous complex these very saline fluids interacted with primary magmatic ores remobilized metals and redeposited them along convenient structures such as fracture zones and breccias in and along various units near the footwall contact the identification of such highly saline fluid inclusions with high heavy metal content may be useful in the exploration for cu pge au enriched footwall vein type ores in the sudbury structure

experimental evidence for the alteration of the fe3 σfe of silicate melt caused by the degassing of chlorine bearing aqueous volatiles the temporal evolution of the redox conditions undergone by degassing magmas has petrologic implications for magmatic phase equilibria we present new experimental data that demonstrate that a cl bearing magmatic volatile phase may affect the concentration and activity of ferrous iron in silicate liquid via the preferential mass transfer of fe 2 into the volatile phase the data are consistent with a model wherein the exsolution of a cl bearing volatile phase from the melt modifies the mgo feo ratio of the residual degassed melt increasing the fe 3 fe ratio of the melt degassing induced alteration of the melt may cryptically affect the phases that constitute the hypersolidus crystallizing assemblage the new results indicate that a low to moderate salinity magmatic volatile phase may contain several weight percent iron present as fecl 2 as such passive degassing over geologically plausible protracted periods has the potential to extract significant quantities of fe 2 resulting in a residual melt depleted in feo relative to fe 2 o 3 notably preferential mass transfer of fe 2 from the melt to the fluid results in self oxidation of the melt driving the oxygen fugacity of the melt across the phase boundary that demarcates the stability fields of sulfide and sulfate phases stabilizing sulfate at the expense of sulfide such a process has significant implications for the remobilization of chalcophile metals during the evolution of magmatic hydrothermal systems

the sources and evolution of mineralising fluids in iron oxide copper gold systems norrbotten sweden constraints from br cl ratios and stable cl isotopes of fluid inclusion leachates we have analysed the halogen concentrations and chlorine stable isotope composition of fluid inclusion leachates from three spatially associated fe oxide cu au mineralising systems in norrbotten sweden fluid inclusions in late stage veins in fe oxide apatite deposits contain saline brines and have a wide range of br cl molar ratios from 0 2 to 1 1 10 3 and 37cl values from 3 1 to 1 0 leachates from saline fluid inclusions from the greenstone and porphyry hosted cu au prospects have br cl ratios that range from 0 2 to 0 5 10 3 and 37cl values from 5 6 to 1 3 finally the cu au deposits hosted by the nautanen deformation zone ndz have br cl molar ratios from 0 4 to 1 1 10 3 and 37cl values that range from 2 4 to 0 5 although the bulk of the data fall within 0 0 5 the br cl ratios of leachates are consistent with the derivation of salinity from magmatic sources or from the dissolution of halite most of the isotopic data from the fe oxide apatite and greenstone deposits are consistent with a mantle derived source of the chlorine with the exception of the four samples with the most negative values the origin of the low 37cl values in these samples is unknown but we suggest that there may have been some modification of the cl isotope signature due to fractionation between the mineralising fluids and cl rich silicate assemblages found in the alteration haloes around the deposits if such a process has occurred then a modified crustal source of the chlorine for all the samples cannot be ruled out although the amount of fractionation necessary to generate the low 37cl values would be significantly larger the source of cl in the ndz deposits has a crustal signature which suggests the cl in this system may be derived from meta evaporites or from input from crustal melts such as granitic pegmatites of the lina suite