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Proterozoic continental sedimentary basins contain a unique record of the evolving Earth in their sedimentology and stratigraphy and in the large-scale, redox-sensitive mineral deposits they host. The Paleoproterozoic (Stratherian) Kombolgie Basin, located on the Arnhem Land Plateau, Northern Territory, is an exceptionally well preserved, early part of the larger McArthur Basin in northern Australia. This intracratonic basin is filled with 1 to 2 km-thick, relatively undeformed, nearly flat-lying, siliciclastic rocks of the Kombolgie Subgroup. Numerous drill cores and outcrop exposures from across the basin allow sedimentary fabrics, structures, and stratigraphic relationships to be studied in great detail, providing an extensive stratigraphic framework and record of basin development and evolution. Tectonic events controlled the internal stratigraphic architecture of the basin and led to the formation of three unconformity-bounded sequences that are punctuated by volcanic events. The first sequence records the onset of basin formation and is comprised of coarse-grained sandstone and polymict lithic conglomerate deposited in proximal braided rivers that transported sediment away from basin margins and intra-basin paleohighs associated with major uranium mineralization. Paleo-currents in the upper half of this lower sequence, as well as those of overlying sequences, are directed southward and indicate that the major intra-basin topographic highs no longer existed. The middle sequence has a similar pattern of coarse-grained fluvial facies, followed by distal fluvial facies, and finally interbedded marine and eolian facies. An interval marked by mud-rich, fine-grained sandstones and mud-cracked siltstones representing tidal deposition tops this sequence. The uppermost sequence is dominated by distal fluvial and marine facies that contain halite casts, gypsum nodules, stromatolites, phosphate, and “glauconite” (a blue-green mica group mineral), indicating a marine transgression. The repeating pattern of stratigraphic sequences initiated by regional tectonic events produced well-defined coarse-grained diagenetic aquifers capped by intensely cemented distal fluvial, shoreface, eolian, and even volcanic units, and led to a well-defined heterogenous hydrostratigraphy. Basinal brines migrated within this hydrostratigraphy and, combined with paleotopography, dolerite intrusion, faulting, and intense burial diagenesis, led to the economically important uranium deposits the Kombolgie Basin hosts. Proterozoic sedimentary basins host many of Earth's largest high-grade iron and uranium deposits that formed in response to the initial oxygenation of the hydrosphere and atmosphere following the Great Oxygenation Event. Unconformity-related uranium mineralization like that found in the Kombolgie Basin highlights the interconnected role that oxygenation of the Earth, sedimentology, stratigraphy, and diagenesis played in creating these deposits.

Different types of sediment-hosted whole-rock Pb isotope (206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb) compositions were determined from phyllites, carbonaceous phyllites (>1% TOC), and meta-litharenites belonging to the Serra do Garrote Formation, which is part of the Proterozoic Vazante Group, Brazil. Results were integrated with lithogeochemistry in order to identify the Pb isotopic signature of Zn enrichment (up to 0.24 wt.% Zn) associated with meta-siliciclastic-hosted sulfide mineralization that formed prior to the Brasiliano Orogeny (850 to 550 Ma) in order to (1) understand the nature of siliciclastic sediment sources, (2) identify possible metal sources in pre-orogenic meta-siliciclastic-hosted Zn mineralization, and (3) evaluate the genetic links between the Zn enrichment in the relatively reduced phyllite package, and different styles of syn-orogenic Zn ± Pb mineralization (hypogene Zn-silicate and Zn-Pb sulfide) in overlying dolomitic carbonates throughout the Vazante-Paracatu Zn District, Brazil. The whole-rock 206Pb/204Pb and 207Pb/204Pb isotope ratios of meta-siliciclastic rocks plot as positively sloping, sub-parallel arrays with radiogenic, upper continental crust compositions, which could represent a detrital contribution from at least two upper continental crust sources. However, the 206Pb/204Pb versus207Pb/204Pb isotope system does not distinguish between Zn-enriched samples and un-mineralized samples. In the whole-rock 206Pb/204Pb–208Pb/204Pb plot, Zn-enriched samples form a flat trend of lower 208Pb/204Pb values (38.3 to 39.5) compared to the Zn-poor ones that follow common upper crustal trends. Zinc-enriched samples have low whole-rock Th/U values (

Combined structural, thermochronological and geochronological studies were carried out to unravel the complex late Paleozoic – Mesozoic tectonic evolution of the eastern Taimyr – Severnaya Zemlya Fold and Thrust Belt, along with the adjoining Yenisey-Khatanga Depression and Olenek Fold Zone. New detailed field mapping, a fault and fold geometry and kinematic study, and paleostress reconstruction were undertaken for key areas within the Central and Southern Taimyr domains and the Olenek Fold Zone. Thirty seven samples were used for apatite fission track (AFT) analysis along with two samples for U-Pb zircon granite dating, and three samples for Ar-Ar metamorphic muscovite dating. The late Paleozoic (Early Carboniferous – Permian) tectonic event is recognized only in a relatively narrow zone including the Northern Taimyr Domain and part of the Central Taimyr Domain. It started in the Early Carboniferous (Visean) and is synchronous with the earliest stages of collision in the Uralian Orogen. Our results reveal a widespread distribution of latest Triassic – earliest Jurassic and Cretaceous deformations. The latest Triassic – earliest Jurassic tectonic event significantly overprinted the Northern and Central Taimyr domains and original unaltered late Paleozoic AFT ages are identified only locally. New completely reset AFT cooling ages group at 195-180 Ma, in reasonable agreement with structural data. Older Neoproterozoic and late Paleozoic Ar-Ar ages of metamorphic muscovite are retained, demonstrating that the dated rocks were not heated above 300 °C. An Early Cretaceous tectonic event is inferred from both the structural study and ca. 115–125 Ma AFT cooling ages, and is most widespread in the Southern Taimyr Domain, Olenek Fold Zone and adjoining parts of the Yenisey-Khatanga Depression, modifying older geological structures. A younger Late Cretaceous tectonic event at ca. 100-60 Ma is also inferred from AFT cooling ages, but its distribution is poorly constrained. Our study reveals that the distribution of late Mesozoic tectonic events has been significantly underestimated in previous structural studies of the region. The latest Triassic – earliest Jurassic and Cretaceous tectonic events are easily correlated with those in the Pai-Khoi – Novaya Zemlya and northern Verkhoyansk fold and thrust belts.

Smelting of base metal sulfide rich ore in Rouyn-Noranda, Québec, has led to the formation of black rock coatings on exposed rocks to a maximum distance of 6 km from the smelter centre. This study has shown that these coatings are excellent mineralogical and chemical fingerprints of smelter emissions, ore types, and elemental partitioning into mineral phases. The black coatings are composed of a silica-rich matrix that formed due to the intense chemical weathering of exposed silicate rocks interacting with acidic meteoric waters. They contain metal sulfate rich layers along the atmosphere-coating interface (ACI) and rock-coating interface (RCI) formed by the in situ dissolution and precipitation of metal(loid)-bearing phases. Entombed within the silica matrix are spherical particulates and particles composed of Cu- and Zn-bearing Fe oxides (e.g., spinels), Fe oxides (e.g., hematite), Pb silicates (e.g., alamosite), sulfates (anglesite (PbSO4) and minerals of the jarosite group), amphiboles, pyroxenes, micas, Na feldspar, and clinochlore. Concentrations of elements are low in proximity to the smelter but drastically increase ∼2 km from the stack, most likely the result of a shadow effect of the smelter. This shadow effect is more pronounced if an element is highly compatible with minerals of the jarosite and spinel groups; it is called the smelter-compatibility effect. Elements displaying a high smelter-compatibility effect are Ag, Cu, Se, and As, whereas elements such as Hg, which is incompatible with the jarosite and spinel groups, show a low smelter-compatibility effect. High δ34S (5.5‰) values in proximity to the smelter and their decrease with distance is the result of mixing processes between primary and secondary sulfates in the atmosphere. The relative enrichments of metal(loid)s in coatings at Rouyn-Noranda and Sudbury, Ontario, when normalised to the MUd standard from Queensland, Australia, (MUQ) reflect the composition of the smelter emissions, ore, and lithologies. Black rock coatings of the Rouyn-Noranda and Sudbury study areas are enriched, for example, in Pb and Fe, respectively, reflecting higher abundances of galena and Fe-bearing minerals in the respective ore, emissions, and rocks in the region.

The Sokoman Formation is a ca. 100-m-thick succession of interbedded iron formation and fine-grained siliciclastics deposited at 1.88 Ga. Accumulation occurred on a dynamic paleoshelf where oxygen stratification, coastal upwelling of hydrothermally derived Fe and Si, microbial processes, tide and storm currents, diagenesis, and low-grade prehnite–pumpellyite metamorphism controlled lithofacies character and produced complex associations of multigenerational chert, hematite, magnetite, greenalite, stilpnomelane and Fe carbonate. Hematite-rich facies were deposited along suboxic segments of the coastline where photosynthetic oxygen oases impinged on the seafloor. Hematitic, cross-stratified grainstones were formed by winnowing and reworking of freshly precipitated Fe-(oxyhydr)oxide and opal-A by waves and currents into subaqueous dunes. Magnetite-rich facies contain varying proportions of greenalite and stilpnomelane and record deposition in anoxic middle shelf environments beneath an oxygen chemocline. Minor negative Ce anomalies in hematitic facies, but prominent positive Ce and Eu anomalies and high LREE/HREE ratios in magnetite-rich facies imply the existence of a weakly oxygenated surface ocean above anoxic bottom waters. The Fe isotopic composition of 31 whole rock (−0.46 ⩽ δ 56 Fe ⩽ 0.47‰) and 21 magnetite samples (−0.29 ⩽ δ 56 Fe ⩽ 0.22‰) from suboxic and anoxic lithofacies was controlled primarily by the physical oceanography of the paleoshelf. Despite low-grade metamorphism recorded by the δ 18 O values of paragenetically related quartz and magnetite, the Sokoman Formation preserves a robust primary Fe isotopic signal. Coastal upwelling is interpreted to have affected the isotopic equilibria between Fe 2+ aq and Fe-(oxyhydr)oxide in open marine versus coastal environments, which controlled the Fe isotopic composition of lithofacies. Unlike previous work that focuses on microbial and abiotic fractionation processes with little regard for paleoenvironment, our work demonstrates that depositional setting is paramount in governing the Fe isotopic composition of iron formations irrespective of what Fe-bearing minerals precipitated.

Elemental concentrations and stable isotopic compositions of 41 sweeteners (syrups, honeys, and sugars) and 43 edible oils were determined to evaluate their potential as parameters for food authentication. The addition of as little as 10% of corn syrup to pure maple syrup can be detected using molar ratios of Na/(Na + K) and Na/(Na + Ca) for maple syrups. The detection of more than 15% adulterant in maple syrup and honey is also possible due to the 13‰ difference in carbon isotopic composition of C3 plants for authentic honeys and maple syrups (25.1 ± 1.1‰) relative to the carbon isotopic composition of C4 plants for corn syrups and cane sugars (−11.4 ± 0.8‰) as major adulterant agents. Nearly constant net oxygen (1.028 ± 0.002) as well as hydrogen (0.974 ± 0.005) isotope fractionation factors between honeys and average local meteoric waters from two areas, Kingston and Costa Rica, suggests that these isotope ratios of honeys are good candidates to authenticate their geographic locations. The net oxygen (1.03 ± 0.001) as well as hydrogen (0.894 ± 0.003) isotope fractionation factors between olive oils and local meteoric waters from different parts of Italy suggests that these isotope ratios are independent of the oil processing method and therefore can be used to trace geographic locations.

Nitrogen isotopic compositions and concentrations in micas from kimberlite-hosted MARID (mica-amphibole-rutile-ilmenite-diopside) suite of xenoliths from the Kimberley area of the Kaapvaal Craton, South Africa, were measured using continuous-flow isotope-ratio mass spectrometry and Fourier transform infrared spectroscopy. Fourier transform infrared spectroscopy data suggest that the nitrogen in MARID phlogopite is from ammonium in the mica structure. The replacement of potassium by ammonium is the major mechanism for the storage of nitrogen within these phlogopites. The δ15N values of micas from MARID xenoliths range from − 11 to + 9‰, and nitrogen contents range between 113 and 272 ppm. The 15N enrichment of most samples relative to the average upper mantle δ15N value of − 5‰ suggests that the nitrogen reflects a recycled reservoir from subducted material within their source region. The presence of nitrogen from a recycled reservoir indicates that sedimentary organic nitrogen in the sub-continental lithosphere is not effectively cycled to the surface resulting in a geochemically heterogeneous upper mantle.

Black limestone clasts are present across the globe at modern and ancient subaerial exposure surfaces, and in intertidal and supratidal environments. These problematic clasts are particularly abundant at multiple subaerial exposure surfaces across southern Australia. The genesis of such exceptionally preserved clasts can be interpreted through universal fabrics across wide geographic regions and geologic ages. Regardless of location or age, all south Australian blackened clasts studied have a similar microstructure that is dominated by porous micritic laminae and chambered tubules. Such laminae are interpreted to represent calcified root cells and occur in two different varieties: 1) micrite-walled tubules with hollow centers that are thought to represent calcification in the rhizosphere or outer surfaces of the root, and 2) tubules consistent with calcification of both the medulla and cortex, which preserved medulla, cortex, and parenchymatic cells along with xylem vessels. Preserved root fabrics are present only in the blackest clasts and are consequently thought to be the main mechanism driving limestone blackening. The dark coloration in southern Australian clasts is derived from the impregnation of partially decayed terrestrial organic substances. These organic substances were trapped in the cellular structures of roots as they calcified and were subsequently prevented from further decay and oxidation. A threshold of approximately 7-weight % organic carbon is needed in the insoluble residue to produce the distinct black coloration in limestone. This process occurs at the base of slightly organic-rich humic soil profiles where they interact with underlying carbonate bedrock (B–C soil horizon). Roots both physically and chemically (through acid secretion) corrode the underlying limestone and provide the calcium bicarbonate needed in the root calcification process. Specific semiarid flora have the ability to calcify their cellular structures, which could explain why blackened clasts occur at only some unconformities. This result means that the formation of blackened clasts via root calcification and entrapment of organics relies upon specific conditions of semiarid plants and climate along with organic soils. Such conditions would be restricted to a specific climate regime and when combined with the new proposed blackening mechanism herein could help explain their limited distribution and occurrences worldwide.

Black silica- and sulfate-bearing rock coatings in the Greater Sudbury area, Canada provide a record of atmospheric processes and emitted particulate matter associated with historical smelting operations in this area. Coating samples collected over the Greater Sudbury region are characterized with scanning electron microscopy, electron microprobe analysis, laser ablation inductively coupled mass-spectrometry, S-stable isotope measurements and micro X-ray fluorescence. On the micrometer scale, Cu, Pb, As, Se and S occur in close association within metal–sulfate rich layers composed of Fe- and Cu-sulfates. The concentrations of these and other elements do not represent their chemical proportions in the smelter plumes due to dissolution–precipitation processes, element substitutions and the stability of various phases involved in the coating formation. On the regional scale, the atomic ratios of Pb:Ni, As:Ni and Se:Ni decrease in the coatings with increasing distance from the smelting centers. This observation is consistent with higher wet deposition rates of small diameter Pb, As and Se-bearing primary sulfate aerosols ( 2.5 μm) Ni-bearing particulate matter. The mixing of primary (higher δ34S values) and secondary (lower δ34S values) sulfates explains the δ34S values of sulfates within the coatings close to smelting centers and the decrease in these values is attributed to the decrease in the ratio of primary to secondary sulfates with distance from the smelting centers. The information preserved in mineral surface-coatings together with an understanding of stoichiometry, geochemical processes and former environmental conditions provide a valuable record of atmospheric compositions, mixing, scavenging, deposition rates and oxidation processes, and the nature and source of anthropogenic releases to the atmosphere.

Chert and iron formation from the Ferriman Group (ca. 1.88 Ga), Labrador Trough, Canada, contain an exceptional assemblage of fossil bacteria and biofilms preserved within a well-defined stratigraphic framework. Lithofacies associations suggest that microbes were restricted to suboxic, shallow-water environments through three sea-level cycles. Microfossils are preserved as chert or sedimentary apatite (francolite) in hematite-rich, peritidal facies. Nearshore evaporation and Fe-redox pumping of silica drove rapid penecontemporaneous silicification of microbial mats. In some communities, breakdown of organic matter and Fe-redox pumping of pore water phosphate caused their syndepositional phosphatization. Preservation of bacteria was precluded in deeper settings because these processes could not operate at an anoxic seafloor. Filamentous forms dominate microbial morphologies. Secondary electron imaging of freshly broken surfaces shows filaments that are similar in size and shape to modern bacteria; filaments vary from 0.5 and 1 μm wide and reach tens of μm in length. They commonly envelop chert and iron oxide grains, which stabilized the seafloor and contributed to firmground development. The filamentous morphology, similar mat-forming behavior, and suboxic paleoenvironments where these fossils lived are consistent to those of modern Fe-oxidizing bacteria and some cyanobacteria. Associated organic matter with low δ 13 C values is also present. Most values vary between ca. −38 and −20‰, which is consistent with isotopic fractionation by such bacteria. Unlike modern settings, however, their distribution was restricted to shallow marine and peritidal environments where photosynthetically produced oxygen oases likely impinged on the seafloor. Although Fe-oxidizing bacteria were possibly important in forming some iron formation facies in the Ferriman Group, physiochemical precipitation of Fe-(oxyhydr)oxides caused by photosynthetic oxygen was also probably widespread in the photic zone across the shelf. This study demonstrates that a combined microanalytical, sedimentologic, and stratigraphic analysis of fossil bacteria not only yields a more detailed picture of the Precambrian biosphere, but also highlights the importance of taphonomy in the search for early life.

Both uranous and uranyl minerals are present in the Centennial unconformity-type U deposit situated in the SW Athabasca Basin, Canada. At least two generations of uraninite are present (disseminated and massive), often strongly altered to coffinite, followed by minor fibrous coffinite forming in veins. Uranyl minerals, mostly uranophane with minor haiweeite precipitating in veins and in hand sample-scaled breccias, are present in significant amounts ( ca . 5% of the ore) not observed elsewhere in the Athabasca Basin. The multi-stage evolution of the Centennial deposit is explained by the existence of high permeability conduits, promoting the recurrent circulation of P-rich fluids that remobilized U locally. Stable isotopes composition of uranophane (δD of ca . –130 ‰ and δ 18 O of ca . 6 ‰) suggests that recent oxidizing meteoric fluids penetrated to the unconformity (at a depth of ca . 800 m) via the major Dufferin lake fault.

The Livingstone Range anticlinorium marks a hanging-wall ramp across which the Livingstone thrust cuts up eastward approximately 1000 m (3280 ft) between regional decollements in the Devonian and the Jurassic strata. It is well exposed and provides actualistic models for exploration of analogous subsurface structures. More than 30 km (19 mi) of detailed mapping along strike reveals generally gradual changes, punctuated by abrupt terminations or offsets of structures at northeast- to east-trending tear faults, interpreted to be reactivated basement faults. Some folds plunge out at tear faults, forming domal culminations between them. Hinge zones of chevron-style, flexural-slip thrust-propagation folds display a distinctive pattern of ramp-flat thrusting comprising stacked detachment thrusts, each of which emerges from a different zone of interbed slip in the backlimb and deflects the hinge zone eastward. Each successively lower detachment thrust dies out in the hinge zone just below an overlying one. Displacements on the detachments, rotation of fold limbs, and interbed flexural slip were integrated kinematically. Thrust-propagation folding involved a form of cataclastic flow: individual blocks of rock delimited by faults, joints, and sheared bedding surfaces underwent minor relative displacements with little or no internal deformation, despite large translations and rotations of the thrust sheet. Pressure solution and vein formation were widespread but were minor components of the deformation. Complex fracturing within the anticlines is dominated by conspicuous widely spaced (150 m [490 ft]) transverse (east-northeast–striking) zones of intense fracturing that transect hinge zones and limbs. These permeable zones likely originated by reactivation of basement faults, and their significance may not be predictable by curvature analysis.

The Livingstone Range anticlinorium (LRA) marks a major hanging-wall ramp where the Livingstone thrust cuts approximately 1000 m (3281 ft) between regional decollement in the upper part of the Devonian Palliser Formation and the Jurassic Fernie Formation. Prethrusting and folding jasper fluorite sphalerite veins with halos of altered dolomitic host rock with high 87Sr/86Sr ratios (0.7094–0.7101) relative to most Paleozoic carbonate rocks (0.7081–0.7091) record percolation of fluids along basement faults that may also have contributed anomalously radiogenic strontium to diagenetically altered Paleozoic carbonate rocks throughout the Western Canada sedimentary basin. Fluid flow that occurred during thrust-propagation folding is recorded by dolomite calcite veins, with 18O values that are similar to those of host rocks (–7.92 to –1.08 Peedee belemnite). Anomalously high equilibrium temperatures (250 50C) as determined by oxygen-isotope thermometry and slightly higher 87Sr/86Sr ratios relative to adjacent host rocks indicate that they formed from formation fluids and hot basement fluids in a rock-dominated system. Calcite veins with very low 18O values (–18 to –9) precipitated along faults that were active while the LRA was transported eastward by underlying thrust faults, uplifted, and rapidly cooled by infiltrating meteoric water. Thrusting created heating in the foreland basin ahead of the deformation because of the influx of thick insulating foreland basin sediments, causing thermal maturation of hydrocarbons. As thrusting deformation advanced through the rocks, infiltrating meteoric waters cooled the rocks and hydrocarbon maturation stopped. Structural traps accumulated hydrocarbons only if they were juxtaposed over both thermally favorable and hydrocarbon-favorable source rocks.

The Kola Superdeep Borehole, the deepest ever drilled into the Earth's crust with a depth of 12.2 km, is located in the Kola Peninsula, Russia, and consists of high-grade metasedimentary and metaigneous lithologies ranging in age from Archean to Proterozoic. Metabasic rocks and amphibolites from the borehole have δ18O values in the range from +4.4‰ to +9.2‰ and δD values in the range from −73‰ to −40‰, gneisses and a migmatite have δ18O values in the range from +7.8‰ to +9.4‰ and δD values in the range from −63‰ to −44‰, and metasedimentary rocks show large variations in δ18O values from +6.3‰ to +15‰, and have δD values in the range from −57‰ to −33‰. The isotopic compositions of the deep borehole samples are identical with their surface equivalents, indicating that the processes of emplacement and exposure of deep crustal rocks have not affected their H and O isotopic compositions in marked contrast to what has been observed for crust in tectonically active areas. Fluid compositions calculated from the δ18O and δD values of coexisting amphibole, feldspar, biotite and garnet indicate that two isotopically distinct fluids have interacted with the Kola Superdeep Borehole rocks. One fluid has isotopic compositions consistent with locally rock-buffered regional metamorphic fluids, and the other fluid has compositions expected for Paleoproterozoic seawater. The δ18O values of co-existing amphibole–feldspar and biotite–garnet pairs in amphibolites and quartzite have apparent equilibration temperatures from ca. 500 °C to 800 °C, which are in good agreement with temperatures determined by cation exchange geothermometry and co-existing assemblages. Only rocks associated with two major faults and the thrust zone that marks the Proterozoic–Archean boundary indicate isotopic exchange with regional metamorphic fluids circulating along these structures. The stable isotopic compositions of the Kola Superdeep Borehole rocks indicate that there was interaction of the rocks with channelized fluids moving along major faults, but there was no pervasive fluid flow during metamorphism.

The Marcona–Mina Justa deposit cluster, hosted by Lower Paleozoic metaclastic rocks and Middle Jurassic shallow marine andesites, incorporates the most important known magnetite mineralization in the Andes at Marcona (1.9 Gt at 55.4% Fe and 0.12% Cu) and one of the few major iron oxide–copper–gold (IOCG) deposits with economic Cu grades (346.6 Mt at 0.71% Cu, 3.8 g/t Ag and 0.03 g/t Au) at Mina Justa. The Middle Jurassic Marcona deposit is centred in Ica Department, Peru, and the Lower Cretaceous Mina Justa Cu (Ag, Au) prospect is located 3–4 km to the northeast. New fluid inclusion studies, including laser ablation time-of-flight inductively coupled plasma mass spectrometry (LA-TOF-ICPMS) analysis, integrated with sulphur, oxygen, hydrogen and carbon isotope analyses of minerals with well-defined paragenetic relationships, clarify the nature and origin of the hydrothermal fluid responsible for these contiguous but genetically contrasted deposits. At Marcona, early, sulphide-free stage M-III magnetite–biotite–calcic amphibole assemblages are inferred to have crystallized from a 700–800°C Fe oxide melt with a δ18O value from +5.2‰ to +7.7‰. Stage M-IV magnetite–phlogopite–calcic amphibole–sulphide assemblages were subsequently precipitated from 430–600°C aqueous fluids with dominantly magmatic isotopic compositions (δ34S = +0.8‰ to +5.9‰; δ18O = +9.6‰ to +12.2‰; δD = −73‰ to −43‰; and δ13C = −3.3‰). Stages M-III and M-IV account for over 95% of the magnetite mineralization at Marcona. Subsequent non-economic, lower temperature sulphide–calcite–amphibole assemblages (stage M-V) were deposited from fluids with similar δ34S (+1.8‰ to +5.0‰), δ18O (+10.1‰ to +12.5‰) and δ13C (−3.4‰), but higher δD values (average −8‰). Several groups of lower ( 200°C) fluids can be recognized in the main polymetallic (Cu, Zn, Pb) sulphide stage M-V and may record the involvement of modified seawater. At Mina Justa, early magnetite–pyrite assemblages precipitated from a magmatic fluid (δ34S = +0.8‰ to +3.9‰; δ18O = +9.5‰ to +11.5‰) at 540–600°C, whereas ensuing chalcopyrite–bornite–digenite–chalcocite–hematite–calcite mineralization was the product of non-magmatic, probably evaporite-sourced, brines with δ34S ≥ +29‰, δ18O = 0.1‰ and δ13C = −8.3‰. Two groups of fluids were involved in the Cu mineralization stage: (1) Ca-rich, low-temperature (approx. 140°C) and high-salinity, plausibly a basinal brine and (2) Na (–K)-dominant with a low-temperature (approx. 140°C) and low-salinity probably meteoric water. LA-TOF-ICPMS analyses show that fluids at the magnetite–pyrite stage were Cu-barren, but that those associated with external fluids in later stages were enriched in Cu and Zn, suggesting such fluids could have been critical for the economic Cu mineralization in Andean IOCG deposits.

Marcona, the preeminent Andean magnetite deposit (1.9 Gt @ 55.4% Fe and 0.12% Cu), is located in the iron oxide copper-gold (IOCG) subprovince of littoral south-central Peru. Fe oxide and Cu (-Zn-Pb) sulfide mineralization was controlled by northeast-striking faults transecting a Middle Jurassic (Aalenian-to-Oxfordian) andesitic, shallow-marine arc and a succession of contiguous, plate boundary-parallel, Late Jurassic to mid-Cretaceous volcanosedimentary basins. At Marcona, hydrothermal activity was initiated in the earliest Middle Jurassic (161–177 Ma) by high-temperature Mg-Fe metasomatism represented by cummingtonite and phlogopite-magnetite assemblages. Subsequently, during the terminal eruptions (156–162 Ma) of the arc, widespread albite-marialite alteration (Na-Cl metasomatism) was followed by the emplacement of an en echelon swarm of massive magnetite ore-bodies with subordinate, overprinted magnetite-sulfide assemblages, hosted largely by Paleozoic metasilici-clastics. The magnetite orebodies exhibit abrupt, smoothly curving contacts, dike-like to tubular apophyses, and intricate, amoeboid interfingering with dacite porphyry intrusions. There is no convincing megascopic or microscopic evidence for large-scale Fe metasomatism associated with the main, sulfide-poor mineralization. The largest, 400 Mt Minas 2-3-4 orebody is interpreted as a bimodal magnetite-dacite intrusion comprising commingled immiscible melts generated through the dissolution of metasedimentary quartz in parental andesitic magma. Oxygen and sulfur stable-isotope geothermometry indicates that the evolution at ca. 159 Ma from magnetite-biotite-calcic amphibole ± phlogopite ± fluorapatite to magnetite-phlogopite-calcic amphi-bole-pyrrhotite-pyrite assemblages coincided with quenching from above 800° C to below 450°C and the concomitant exsolution of dilute aqueous brines. Subsequently, chalcopyrite-pyrite-calcite ± pyrrhotite ± sphalerite ± galena assemblages, in part metasomatic, were deposited from lower temperature (≤360°C) brines. The Cu-poor Marcona (“Kiruna-type”) magnetite and Cu-rich IOCG deposits in the district, therefore, although spatially contiguous, represent contrasting ore deposit types. The former are interpreted as the product of Fe oxide melt coexisting with dacite magma within an andesitic arc which failed during the closure of a back-arc basin. The weak associated magmatic-hydrothermal Cu sulfide mineralization at Marcona was generated through melt vesiculation and contrasts with the considerably higher grade Cu- and Ag-rich orebodies of the major Cu-rich IOCG deposits in the Central Andes, e.g., La Candelaria-Punta del Cobre, Mantoverde, Raul-Condestable, and Mina Justa, which were the products of cool, oxidized, hydrothermal fluids plausibly expelled from the adjacent basins during tectonic inversion.

The Camaguey district, Cuba, is known for its epithermal precious metal deposits in a Cretaceous volcanic arc setting. Recently, the La Union prospect was discovered in the southern part of the district, containing gold and minor copper mineralization interpreted as porphyry type. Mineralization is hosted in a 73.0 ± 1.5 Ma calc–alkaline I-type oxidized porphyry quartz diorite intrusive within volcanic and volcaniclastic rocks of the early Cretaceous Guaimaro Formation. The porphyry is affected by propylitic alteration and crosscut by a network of quartz and carbonate veinlets and veins. Chlorite, epidote, sericite, quartz, and pyrite are the main minerals in the early veins which are cut by late carbonate and zeolite veins. Late barite pseudomorphously replaces pyrite. Gold is associated with pyrite as disseminations in the altered quartz diorite and in the veins, occurring as inclusions or filling fractures in pyrite with 4 g/t Au in bulk samples, and up to 900 ppm Au in in pyrite. Fluid inclusion and oxygen isotope data are consistent with a H2O–NaCl–(KCl) mineralizing fluid, derived from the quartz diorite magma, and trapped at least at 425°C and 1.2 kbar. This primary fluid unmixed into two fluid phases, a hypersaline aqueous fluid and a low-salinity vapor-rich fluid. Boiling during cooling may have played an important role in metal precipitation. Pyrite δ34S values for the La Union prospect range between 0.71‰ and 1.31‰, consistent with a homogeneous magmatic sulfur source. The fluids in equilibrium with the mineralized rocks have estimated δ18O values from 8‰ to 11.8‰, calculated for a temperature range of 480–505°C. The tectonic environment of the La Union prospect, its high gold and low copper contents, the physical–chemical characteristics of the mineralizing fluids and the isotopic signature of the alteration minerals and fluids indicate that the La Union gold mineralization is similar to the porphyry gold type, even though the ore-related epidote–chlorite alteration can be classified as propylitic and not the classic potassic and/or phyllic alteration. The low copper contents in the prospect could be due to a mineralizing fluid previously saturated in copper, which is indicated by trapped chalcopyrite crystals in high-temperature fluid inclusions. The low-temperature paragenesis, represented by carbonate, zeolite and barite, indicates epithermal overprint. The study shows the potential for other gold porphyry-type deposits in the Cretaceous volcanoplutonic arc of Cuba.

Hydrothermally altered basaltic lava-units in the northern Kerguelen Archipelago contain a wide variety of secondary silicate and carbonate minerals, including zeolites, hydrothermal calcite, dolomite and magnesite, as well as celadonite, orthoclase (adularia) and quartz. Petrography, fluid-inclusion microthermometry, trace-elements geochemistry, Sr isotopes and stable-isotope compositions indicate hydrothermal fluid cells derived from meteoric water interacting with basalts and Rb-rich subvolcanic peralkaline rocks at temperatures ranging from 50 to 200 °C associated with the cooling of the lava pile. The calculated δ 18 O values of meteoric-hydrothermal waters in fossil hydrothermal systems are identical to those in present-day hot springs, suggesting that meteoric recharge was continuous throughout the igneous cooling cycles of the 28–23 Ma older host basalts and the younger 15–5 Ma old peralkaline intrusions. The Kerguelen northern coastline hydrothermal system in the basaltic pile demonstrates that much of the silicate mineralogy and almost all carbonate secondary minerals in altered basalts were derived from meteoric-hydrothermal fluids, rather than products of seawater interaction, or even magmatic-hydrothermal fluids associated with peralkaline intrusions.

The Bahamian Archipelago is one of the few locations in the world where ooid formation is actively occurring. Ooid cortices from six locations in the region were incrementally dissolved and analyzed for 14C, δ18O, δ13C, Mg/Ca, and Sr/Ca ratios. Ooids were examined under SEM after each step in the incremental analyses to characterize the nature of dissolution. Radiocarbon dating indicates that ooids presently at the sediment–water interface began forming across the Archipelago between 1000 and 2800 yr BP and continue to form today. The ooids have the same pattern of microboring alteration across the region. The surface and outer cortex of the ooids are punctuated with unfilled microborings, whereas the inner cortex contains two morphologies of aragonite cement filling the microborings. The two morphologies of cement form in association with two different species of cyanobacteria, one is Solentia sp. the other is interpreted to be Hyella sp. The chemistry of ooids from across the region is remarkably similar. δ18O and δ13C values for all samples vary directly, having a slope of approximately 1. The outer cortex has low δ18O and δ13C values of −3.4‰ and 0.2‰ respectively, whereas the δ18O and δ13C values of the inner cortex are high with values of 1.9‰ and 6.8‰ respectively. The presence of aragonite cement in microborings in the inner cortex increases the overall isotopic composition of both oxygen and carbon in the ooid, causing it to appear close to equilibrium with seawater. The isotopic variation in δ18O and δ13C within the cortex can be characterized as a mixing line between the low values in the unaltered ooid laminae and the aragonite cement in the microborings. The most exterior portion of the ooid has very high Mg/Ca values and is interpreted as an amorphous calcium carbonate (ACC) coating. There are two other phases in the cortex, both being aragonite. The outer cortex has a higher Mg/Ca ratio and lower Sr/Ca ratio than the inner cortex. This difference in chemistry is a result of the presence of aragonite cement in the inner cortex. Stable isotopic and trace element results coupled with SEM investigations indicate that microbes do not play a role in ooid formation, but instead alter the texture and chemistry of ooids after they have formed. A new model of ooid formation is proposed whereby a veneer of ACC precipitates on an ooid while it is at the sediment–water interface (the active phase). This veneer of ACC later recrystallizes to aragonite needles and a new cortex layer is formed (the stationary phase). Observations from this study lead to a deeper understanding of the chemical processes involved in ooid genesis, which allow a better understanding of paleoenvironments hosting ooid formation.

The Mesozoic iron oxide-copper-gold (IOCG) subprovince of littoral south-central Peru, centered at latitude 15°11′ S, longitude 75°6′ W, incorporates Marcona, the preeminent central Andean iron oxide deposit (1.9 Gt@ 55.4% Fe), and Mina Justa, one of the few major Andean IOCG deposits with economic copper grades (346.6 Mt @ 0.71% Cu). The emplacement of magnetite orebodies with uneconomic Cu grades (avg 0.12%) at Marcona was controlled by northeast-striking faults transecting an active andesitic-dacitic, shallow-marine Middle Jurassic (Aalenian to Oxfordian) arc. In contrast, hypogene Cu sulfide (~15 g/t Ag, 0.12 g/t Au) mineralization at Mina Justa was emplaced along reactivated listric-normal detachment faults during the mid-Cretaceous inversion of the contiguous, plate boundary-parallel, Aptian to Albian Canete basin, accompanied by the earliest, largely granodioritic-dioritic, stocks of the Coastal batholith. Alteration and mineralization assemblages, supported by 40Ar/39Ar geochronology of biotite, phlogopite, actinolite, cummingtonite, and K-feldspars, reveal a history of magmatic and hydrothermal processes extending episodically for at least 80 m.y., from ca. 177 to 95 Ma, wherein metal-rich mineralization events were preceded and separated by episodes of barren alteration. At Marcona, precursor, subocean-floor hydrothermal activity in the Aalenian (177 Ma) and Bajocian (171 Ma) generated, respectively, cummingtonite and phlogopite-magnetite assemblages through high-temperature Mg-Fe metasomatism of previously metamorphosed Lower Paleozoic Marcona Formation siliciclastic rocks and minor carbonate units underlying the nascent Rio Grande Formation arc. Subsequent areally widespread, albite-marialite alteration (Na-Cl metasomatism) largely predated but overlapped with the emplacement of an en echelon swarm of massive magnetite orebodies, in turn overprinted by subordinate magnetite-sulfide assemblages. Magnetite and weak Cu and Zn sulfide mineralization coincided with a 156 to 162 Ma episode of andesitic eruption and dacitic intrusion which terminated the growth of the arc, but was hosted largely by quartz-rich metaclastic rocks. From 162 to 159 Ma, iron oxide mineralization evolved from magnetite-biotite-calcic amphibole ± phlogopite ± fluorapatite to magnetite-phlogopite-calcic amphibole-pyrrhotite-pyrite assemblages. These were overprinted at 156 to 159 Ma by chalcopyrite-pyrite-calcite ± pyrrhotite ± sphalerite ± galena assemblages, locally resulting in grades of 0.45 percent Cu and 0.5 percent Zn. Hydrothermal activity was thereafter focused in the Mina Justa area, 3 to 4 km to the northeast of Marcona, where Middle Jurassic andesites experienced intense albite-actinolite alteration at ca. 157 Ma, i.e., contemporaneous with sulfide mineralization at Marcona, and magnetite-microcline alteration (K-Fe metasomatism) at ca. 142 Ma. Development of the Mina Justa Cu (-Ag) deposit proper, however, began much later, with, successively, actinolitization at ca. 109 Ma, the deposition of calcite and specular hematite, now entirely pseudo-morphed by magnetite, and the metasomatic emplacement of bodies of barren, massive magnetite and pyrite at 101 to 104 Ma. Finally, at 95 to 99 Ma, chalcopyrite-bornite-digenite-chalcocite mineralization, with abundant calcite and hematite, was emplaced as two ~400-m-long, ~200-m-wide, gently dipping, tabular arrays of breccia and stockwork, cored by preexisting magnetite-pyrite lenses. Supergene oxidation generated a chryso-colla-atacamite-covellite blanket, hosting ~40 percent of the Cu reserve, prior to the eruption of a 9.13 ± 0.25 Ma rhyodacitic ignimbrite flow. Although areally contiguous, the major magnetite and copper-rich centers of the Marcona district record independent metallogenic episodes widely separated in age. Further, whereas the Cu-poor magnetite mineralization at Marcona was integral to the terminal eruptions of the Middle Jurassic arc, representing a shallow-marine analog of the Pliocene El Laco magnetite deposits of northern Chile, the Mina Justa Cu sulfide orebodies—like the other economic, mid-Cretaceous, Cu-rich IOCG deposits of the central Andes, e.g., Can-delaria-Punta del Cobre, Mantoverde, and Raul-Condestable—was the product of brines released during the inversion of back-arc volcanosedimentary basins. The latter environment recurred episodically in the Mesozoic Andes, as in comparable orogenic settings elsewhere, and extended histories of hydrothermal alteration and mineralization, incorporating numerous barren events, may therefore represent a salient feature of the IOCG deposit clan.

The 395 Ma Brazil Lake granitic pegmatite, a rare occurrence of an LCT-type (Li–Cs–Ta) pegmatite in the Meguma Terrane of Nova Scotia, Canada, is hosted by metasedimentary and metavolcanic rocks of the Silurian White Rock Group. Previous mineralogical studies showed the pegmatite formed in two distinct stages, an early spodumene – K-feldspar – quartz ± muscovite stage that was followed by a pervasive metasomatic stage in which albite and secondary muscovite grew at the expense of earlier K-feldspar. The economically important oxide phases (Ta–Nb) are intimately associated with secondary albite. Determination of the stable isotopic (δ18O, D) values for silicate phases (quartz, K-feldspar, spodumene, albite, muscovite, garnet, tourmaline) provide insight into the origin of the pegmatite-forming melt and its subsequent internal evolution. A primary δ18O signature for the melt of +8‰ is calculated using the δ18Ovalues of quartz and Δquartz – whole-rock values from studies on other granitic suites. Interpreted in the context of the geological setting, the δ18O data preclude a dominantly metasedimentary source and suggest, instead, that the pegmatite-forming melt originated via partial melting of a mixed igneous ( i.e., volcanic)- sedimentary source. The calculated δ18OH2O–δDH2O values, at 500°C, correspond to the field for magmatic waters, but some isotopic data, including δD for fluid-inclusion extracts, suggest late-stage incursion of metamorphic water that had equilibrated with the surrounding metasedimentary rocks. Mineral–mineral fractionation ( e.g., ΔQtz–Ms) does not reflect equilibrium within the pegmatite, which may have resulted from several processes, including non-equilibrium crystallization, variable degrees of isotopic exchange during protracted cooling, or interaction with an external reservoir(s). The data for albite (δ18O = +9.1‰, n = 6) are consistent with a magmatic origin, albeit late in the evolution of the pegmatite, on the basis of textural arguments. A model is proposed for the origin of the extensive albite-enriched zones, which involves introduction of a sodic melt derived at depth after protracted fractionation of the same parental melt from which the Li-rich parts of the pegmatite originated.

Uplifted during the 1964 Alaskan earthquake, extensive intertidal flats around Middleton Island expose 1300 m of late Cenozoic (Early Pleistocene) Yakataga Formation glaciomarine sediments. These outcrops provide a unique window intooutershelfandupperslopestratathatareotherwiseburiedwithinthesoutheast Alaska continental shelf prism. The rocks consist of five principal facies in descending order of thickness: (i) extensive pebbly mudstone diamictite containing sparse marine fossils; (ii) proglacial submarine channel conglomerates; (iii)burrowedmudstoneswithdiscretedropstonelayers;(iv)boulderpavements whose upper surfaces are truncated, faceted and striated by ice; and (v) carbonates rich in molluscs, bryozoans and brachiopods. The carbonates are decimetre scale in thickness, typically channellized conglomeratic event beds interpreted as resedimented deposits on the palaeoshelf edge and upper slope. Biogenic components originated in a moderately shallow (ca 80 m), relatively sediment-free,mesotrophic,sub-photicsetting.Thesecomponentsareamixture of parautochthonous large pectenids or smaller brachiopods with locally important serpulid worm tubes and robust gastropods augmented by sand-size bryozoan and echinoderm fragments. Ice-rafted debris is present throughout these cold-water carbonates that are thought to have formed during glacial periods of lowered sea-level that allowed coastal ice margins to advance near to the shelf edge. Such carbonates were then stranded during subsequent sea-level rise. Productivity was enabled by attenuation of terrigenous mud deposition during these cold periods via reduced sedimentation together with active wave and tidal-current winnowing near the ice front. Redeposition was the result of intense storms and possibly tsunamis. These sub-arctic mixed siliciclasticcarbonate sediments are an end-member of the Phanerozoic global carbonate depositional realm whose skeletal attributes first appeared during late Palaeozoic southern hemisphere deglaciation.

Selenium concentrations of transition metal sulphides, albeit low, may be an important tool in exploration for economic ore deposits. These low concentrations necessitate the use of hydride generation for sample introduction, a very sensitive means of pre-concentration that results in low interference. However, transition metals interfere with the production of selenium hydride so that their removal from solutions made from dissolution of transition metal sulphides is necessary for hydride generation of selenium to be effective. We have devised a two-step process for the determination of Se concentrations in small samples ( c. 50 mg) of sulphide minerals wherein dissolved transition metals are removed by precipitation as metal hydroxides under alkaline conditions (pH c . 12) to prevent sorption of Se, followed by further metal removal by chelating resin. Determinations made by hydride generation inductively coupled plasma mass spectrometry on the CCU-1c certified reference material (copper concentrate) and concentration standards showed quantitative recoveries (100 ± 5%) of Se. Using this technique we find that sulphide minerals from the Horne volcanic-hosted massive sulphide deposit give high Se concentrations ranging from 250 to 750 μg g −1 . Average precision, expressed as relative standard deviation, is 10% and the detection limit is 4 μg g −1 of Se in a sulphide mineral. The procedure offers a method for Se determination of sulphides that is operationally simpler than many other methods.

Elemental and nitrogen isotopic compositions of tree-rings adjacent to a fire-scar in a white birch (Betula papyrifera) are compared to those away from the scar in the same tree, and to those of nearby non-scarred yellow birches (Betula alleghaniensis) and white birches. The δ15N value at the fire-scar was 1.5 ‰ lower than non-fire-scarred trees and had elevated Ba, Ca, Mg, Sr, and Mn concentrations relative to elsewhere around the bole and to non-scarred trees. Variations in tree-ring chemistry may be useful in mapping the extent and assessing the magnitude of historic fire events.

A method for the fabrication of solid multi-element calibration standards by a sol–gel process, suitable for laser ablation ICPMS, is described. The addition of an analyte (Se) and an internal standard (S) to a normal sol–gel method does not impair the production of the glass-like discs (xerogels). Heterogeneity of Se and S concentrations in the xerogels is less than that of NIST SRM 610 and 612 glass standards. Small differences in slopes of calibrations based on sulfide standards and those based on xerogel standards reflect differential matrix effects. The calculated S contents in the NIST SRM 610 (634 ± 151 µg g−1) and 612 (133 ± 17 µg g−1), using the xerogels as standards, are comparable to available S concentration data. Multiple element standards of Se, S and transition metals are hampered by the formation of inclusions resulting in visible heterogeneity, limiting the concentrations of transition metals with Se and S in the xerogels. The xerogels are potential standards for a variety of elements in glasses, minerals and other materials

Laser-ablation high-resolution inductively coupled plasma mass spectrometry (LA-HR-ICP-MS) is a rapid, accurate, and inexpensive technique for making in situ U-Pb isotopic measurements of uraninite and davidite. The advantages of this method include: (1) mineral separation and chemical digestion are not required; (2) measurements on complex samples are feasible because significant isobaric interferences can be resolved; and (3) accurate U-Pb and 207Pb/206Pb dates on 10–25 μm spots can be obtained rapidly. The LA-HR-ICP-MS method is applied to U oxide minerals from four deposits and prospects from northern Australia, and the new dates are compared to previously published conventional thermal ionization mass spectroscopy (TIMS) dates, and to known ages of geologically important events. These comparisons permit us to assess elemental fractionation of U and Pb for uraninite and davidite of the new method, relative to zircon, as well as its geochronological accuracy and precision. U-Pb apparent ages measured previously agree well with our measurements for El Sherena, Palette, and Mt. Isa. Additionally, the upper-intercept 207Pb/206Pb dates for Adelaide River (701 ± 190 Ma, 1σ) and Palette (841 ± 94 Ma, 1σ) uraninite, measured here, are similar to those previously obtained for Palette (730 Ma), Nabarlek (920 Ma), and Koongarra (870 Ma), and the upper-intercept date for El Sherena uraninite (1573 ± 160 Ma) is within error of that previously determined for Ranger (1550 ± 15 and 1472 ± 40 Ma). Such apparent-age agreement for uraninite (and similarly for davidite) indicates that U and Pb fractionations are within error of that for zircon, whereas the inherent imprecision of our dates and their associated MSWD values greater than 2.5 probably indicate that multiple resetting events affected our samples. Analytically, these results demonstrate that the LA-HR-ICP-MS technique provides excellent spatial resolution while also removing argide, phosphide, sulfide, and halide interferences that can otherwise lead to erroneous data when using quadrupole-ICP-MS. Geologically, the individual 207Pb/206Pb and upper-intercept U-Pb dates of uraninite from Adelaide River and Palette are ca. 800 Ma, possibly reflecting recrystallization of uraninite during the break-up of Rodinia.

Quartz cement microstratigraphy and high precision in situ δ 18 O values obtained by secondary ion mass spectrometry (SIMS) from μm-size quartz cement zones have been used here to determine the timing of cementation and to evaluate precipitation mechanisms within the basal sandstones of three economically significant Paleoproterozoic basins, the Athabasca and Thelon basins, Canada, and the McArthur Basin, Australia. In these examples, the earliest quartz cements have the highest δ 18 O values (up to 33‰; mean=26.0‰, V-SMOW) indicative of low temperature precipitation at depths of 0 to 1.1 km. Some paragenetically early cement phases, however, have lighter isotopic values that suggest either precipitation from isotopically depleted water or precipitation at higher temperatures. Subsequent generations of quartz cements have progressively lower δ 18 O values (mean=+16.1±3‰) consistent with precipitation at higher temperatures. These data combined with petrographic observations indicate quartz cementation began in near-surface vadose and phreatic zones; the next stage of cementation is characterized by syntaxial burial cement overgrowths. Regionally and stratigraphically, well-sorted sandstone facies that were initially aquifers preferentially experienced early quartz cementation, which resulted in cement-bridged pore throats at relatively shallow depths of 0–2 km and very early in their burial history transforming these units into diagenetic aquitards. As a result, these units did not experience mineralization. Compositionally and texturally immature braided fluvial facies did not experience early addition of quartz cement, but are instead marked by minor syntaxial overgrowth cement that make upb5% of the intergranular volume, formed at depths of 3–5 km, and were variably replaced by illite and chlorite during burial. These lithologies were still open to fluid movement at critical times in the basin when U and

The hydrogen isotope system is used extensively to provide information on the genesis of minerals (e.g., source of fluids and mechanisms of precipitation). The copper isotopic system is less well understood, but has the potential to provide valuable insight on mineral precipitation, particularly supergene Cu-rich minerals. Here we present a rapid and precise method for measuring hydrogen and copper isotopes in semi-precious gem-quality turquoise (Cu(Al,Fe3 +)6(PO4)4(OH)8 · 4H2O) by secondary ion mass spectrometry (SIMS). The suitability of standards for instrumental mass fractionation (IMF) calibration was assessed by external precision of SIMS measurements for each standard (1–4‰ for δDIMF and 0.1–0.4‰ for δ65CuIMF). IMF in turquoise was correlated with H and Fe contents for D/H measurements and Fe content for 65Cu/63Cu measurements. Based on these correlations, IMF can be corrected to enable δD and δ65Cu analyses by SIMS with accuracies of ± 5‰ and ± 0.5‰, respectively. The precision and accuracy of SIMS thus rivals those of other mass spectrometric methods for H and Cu isotopes and demonstrates the potential of SIMS applications in identifying gemstones provenance and understanding the genesis of turquoise deposits.

The Paleo- to Mesoproterozoic McArthur basin in the Northern Territory, Australia, is filled with mixed carbonate-siliciclastic successions and felsic and mafic volcanic units that contain authigenic minerals that formed during burial diagenesis. Petrographic investigations indicate that some of these lithologic units were cemented early by quartz overgrowths and became diagenetic aquitards, whereas secondary porosity was developed in others that became diagenetic aquifers for basinal brines. The authigenic minerals in both lithology types preserve geochemical information about the types of fluids that occurred in the basin, with those that resided in the diagenetic aquifers having similar geochemical composition to those indicated by alteration minerals in the McArthur River Pb-Zn deposit and the Westmoreland U deposits. Quartz overgrowths formed early during shallow burial in all clastic lithologic units and are particularly common in the well-sorted, marine sediments; these became diagenetic aquitards due to the porosity-occluding cement. In the calcareous units, early dolomite replaced calcite and aragonite at shallow burial depths. Fluid inclusion microthermometry indicates that most quartz overgrowths formed between 80° and 125°C from a fluid with a salinity between 0 and 13.1 wt percent NaCl equiv. The isotopic composition of dolomite indicates δ18Ofluid of −4.5 ± 1.5 per mil and δ13Cfluid of −6.9 ± 1.0 per mil, suggesting formation from a mixed marine and meteoric fluid. Illite cemented by early quartz has a 40Ar/39Ar age of 1738 ± 10 Ma in one diagenetic aquitard, which reflects the age when the host sediment was deposited and confirms that quartz cementation occurred early during burial. With increased burial, the poorly sorted sandstones and conglomerates that were initially poor aquifers at or near the surface experienced framework grain dissolution and the creation of secondary porosity at depth. As a result, the sediments of the Westmoreland Conglomerate, the lower Yiyintyi Sandstone, significant proportions of the Warramana Sandstone, and most of the sandstone interbeds of the Gold Creek Volcanics became diagenetic aquifers and conduits for basinal brines. Illite, dolomite, and chlorite formed late in the paragenesis and filled the secondary porosity. Quartz veins formed last and primarily in faults and fractures. The dolomite and quartz veins contain fluid inclusions that have low eutectic temperatures indicative of Ca2+- and Na+-dominated brines and salinities between 18.5 and 30.9 wt percent NaCl equiv. Illite crystallinity data and chlorite chemical compositions indicate formation between 150° and 250°C. The isotopic compositions of illite and chlorite indicate δ18Ofluid values between 2.9 and 9.1 per mil and δDfluid values between −61 and −25 per mil, which suggest formation from a mixed meteoric and marine fluid. These δ18Ofluid values are indistinguishable from those determined from the isotopic composition of alteration minerals at the McArthur River Pb-Zn deposit (5 ± 5‰) or synore illite at the Westmoreland uranium deposit (4 ± 2‰), suggesting that the diagenetic aquifers and the deposits may be linked. Argon ages of illite from the diagenetic aquifers indicate that fluid migration began as early as 1680 Ma and continued to approximately 1541 Ma, coinciding with and extending past the time when the McArthur River Pb-Zn deposit (1640 Ma) and the Westmoreland U deposits (1655–1606 Ma) formed. The results presented in this study show that clastic sediments dominated by poorly sorted sandstone and conglomerate facies evolved through burial diagenesis to become aquifer lithologic units that hosted basinal brines with chemical characteristics and ages similar to those that are reported from the Pb-Zn and U deposits of the southern McArthur basin.

The western succession of the Mount Isa basin in northwest Queensland hosts four supergiant Zn-Pb ± Cu deposits and numerous smaller Cu and Zn-Pb deposits. Mineralization is primarily hosted in carbonaceous and calcareous shales and siltstones belonging to the 1670 to 1575 Ma Isa superbasin, but little is known about the source of metals that formed these deposits. The underlying clastic and volcanic successions belonging to the 1800 to 1750 Ma Leichhardt superbasin and the 1735 to 1690 Ma Calvert superbasin are potential metal source rocks and host a variety of diagenetic minerals that preserve geochemical information about the evolution of brines in the basin. Quartz overgrowths and pressure solution features formed during shallow burial in all clastic lithologic units but are particularly common in the well-sorted, marine-dominated units that became aquitards due to the porosity-occluding diagenetic cement. Microthermometry on fluid inclusions in the quartz overgrowths indicates formation between 100° and 174°C from a low-salinity, 2.7 to 9.1 wt percent NaCl equiv fluid (fluid 1). These data together with sequence stratigraphic mapping, basin reconstruction, and stable isotope values from the quartz overgrowths show that the diagenetic aquitards formed at 18 O fluid and δ D fluid values of 4.5 ± 4.2 and −34 ± 14 per mil, respectively, indicating evolution from a seawater-dominated source. Silicate dissolution and the widespread formation of diagenetic illite and chlorite occurred late, during deep burial diagenesis and primarily in the proximal fluvial lithologic units. These units are recognized as diagenetic aquifers and they occur adjacent to and within the Eastern Creek and Fiery Creek Volcanics where metals could have been sourced. Basin reconstruction shows that the diagenetic aquifers formed at depths between 5 and 10 km. Illite and chlorite extracted from the diagenetic aquifers have distinct δ 18 O fluid and δ D fluid values of 4.5 ± 2.8 and −63 ± 11 per mil, respectively, indicating evolution from a meteoric fluid with a variable marine contribution. These isotopic values cannot be differentiated from published isotopic values of fluid inclusion water in quartz-dolomite-chalcopyrite veins at Mount Isa or sphalerite and illite from the Century Zn deposit and the Zn lodes from the Burketown mineral field. This suggests that the diagenetic aquifers were likely source rocks for metals in the deposits in the Mount Isa basin. In contrast to the phyllosilicates in the diagenetic aquifers, regional dolomitic grainstones and dolomudstones in the Lawn Hill platform precipitated from fluids with δ 18 O fluid between −2.6 and 1.1 per mil and δ 13 C fluid between −8.6 and −3.9 per mil. This suggests that these units did not contribute to the ore-forming brines. Quartz veins formed during the later diagenetic history in the Mount Isa basin from a low-salinity brine, between 2.7 and 10.4 wt percent NaCl equiv (fluid 2). Oxygen isotope geothermometry on quartz-hematite pairs indicates that these veins formed at approximately 230°C. Crosscutting relationships reveal that a subsequent generation of quartz veins host fluid inclusions with a distinctly saline brine, with compositions between 11.9 and 23.2 wt percent NaCl equiv (fluid 3), indistinguishable from fluid inclusion compositions recorded in sphalerite from the Century and Walford Creek Zn deposits in the Lawn Hill platform and the quartz-dolomite-chalcopyrite veins at Mount Isa. Some of these quartz veins formed at ca. 400°C, based on quartz-hematite geothermometry, but fluid inclusion homogenization temperatures between 86° and 260°C suggest that another set of quartz veins, also containing high-salinity fluid inclusions, is preserved in the basin but formed at lower temperatures. Irrespective of formation temperature or timing, it is noteworthy that quartz veins hosting high-salinity fluid inclusions have δ 18 O fluid and δ D fluid values that are indistinguishable from those recorded by (1) illite and chlorite in the diagenetic aquifers, (2) fluid inclusion water in sphalerite and synore quartz and illite in the Zn deposits in the Lawn Hill platform, and (3) various alteration minerals from the Mount Isa Cu deposit. Collectively, this suggests that the quartz veins represent fluids that migrated along faults from the diagenetic aquifers during late diagenesis to form the low-temperature Zn-Pb deposits between 1650 and 1575 Ma and later during the Isan orogeny to form the high-temperature Cu deposits. Metamorphic quartz-hematite ± feldspar ± chlorite veins exist in the basin and formed between 325° and 450°C during the Isan orogeny from a brine having salinities between 12.6 and 23.2 wt percent NaCl equiv (fluid 4). These veins have distinct δ 18 O fluid and δ D fluid values of 11.8 ± 2.0 and 30 ± 2 per mil, respectively, consistent with formation from fluids derived from graywackes and arkoses during greenschist facies metamorphism. The last recognized fluid identified in the basin (fluid 5) is only found in secondary fluid inclusions that form trails across earlier formed quartz veins. This fluid was trapped after the Isan orogeny, has a low salinity, between 0.0 and 8.1 wt percent NaCl equiv, records temperatures between 131° and 256°C, and is indistinguishable from postore fluids that have been reported in the Mount Isa Cu deposit and the Zn lodes in the Burketown mineral field.

The Paleoproterozoic Sioux Basin and the Baraboo, Barron, Flambeau, McCaslin, and Waterloo quartzites in northcentral USA are all supermature quartz arenites that are among the world's oldest redbeds. These quartzites were deposited on a stable craton at ca. 1750 Ma, following exhumation and uplift of Penokean crust and major orogenic activity related to the amalgamation of Archean microcontinents at ca. 2.0–1.8 Ga to form Laurentia. 40Ar/39Ar ages of 1615–1543 Ma from diagenetic muscovite in the Sioux Basin provide the first geochronological evidence for Mazatzal-related deformation and low-grade metamorphism of the quartzites during subsequent convergence on the southern margin of Laurentia. One muscovite sample from the Sioux Basin has a 40Ar/39Ar age of 1465 Ma that likely corresponds with fluid and thermal resetting due to felsic magmatism in the region and is similar to 40Ar/39Ar ages for muscovite from the Baraboo and Waterloo quartzites. Thus, the Sioux Basin and other quartzites in the region have experienced a similar tectonic and fluid history. The Sioux Basin has similar sedimentologic characteristics as other Paleoproterozoic basins in North America, including the uranium-rich Athabasca Basin, Canada, that is host to some of the world's largest and highest-grade unconformity-type uranium deposits. We compared key characteristics, such as the stratigraphy, character of diagenetic fluids, paleoaquifer characteristics, and composition of leachable Pb in the sandstones from the Sioux Basin with the Athabasca Basin in order to evaluate the fluid history of the basin and the potential of the Sioux Basin to host unconformity-type uranium mineralization. Based on the hydrogen and oxygen isotopic composition of muscovite (M1) that formed during diagenesis at 150–200 °C, the pore fluids that were present during peak diagenesis in the Sioux Basin had δ18O and δD values of −0.5 to +5.2‰ and −32 to −46‰, respectively, similar to those of the fluids associated with uranium mineralization and alteration in the Athabasca Basin. However, fluid flow in the Sioux Basin was likely restricted to relatively thin and discontinous proximal fluvial facies that could have been diagenetic paleoaquifers, in contrast to the extensive paleoaquifers in the Athabasca Basin that promoted large-scale diagenetic reactions and alteration that led to the formation of unconformity-type uranium mineralization. In addition, the non-radiogenic isotopic composition of leachable Pb in the Sioux Basin sandstones indicates that the majority of the Pb is from a normal crustal source, inconsistent with mobilization of radiogenic Pb from a uranium-rich source, such as a uranium deposit. Although the Sioux Basin shares similar general characteristics to the Athabasca Basin, the results of this study suggest that it has a lower potential than the Athabasca Basin to host significant high-grade unconformity-type deposits.

Early and middle Miocene cool-water carbonates from the Murray Basin, South Australia, preserve an excellent stable-isotope record of ocean-climate change. These variably fossiliferous heterozoan deposits accumulated on a low-energy, mesotrophic, centripetal epeiric ramp during a gradual shift in climate from cool, wet conditions and abundant continent-derived nutrients to a seasonal, arid climate with reduced delivery of trophic resources to the marine realm. Temporal trends in δ13C and δ18O from unaltered brachiopods record an epeiric sea response to this warming. The globally recognized middle Miocene Monterey Event (~ 17 to 13.5 Ma) dominates the carbon isotope record, albeit with higher (~ 0.5‰) than open-ocean δ13C values. Such higher δ13C values are attributed to an increase in benthic carbonate production that accompanied climate change and the relatively short seawater mixing times characteristic of epeiric-sea systems. The Murray Basin oxygen isotope curve contains lower δ18O values (~ 2.0‰ lower) than those of the deep-sea record. This difference is ascribed to the warmer seawater temperatures (~ 17 to 22°C) that prevailed across the Miocene Murray Basin. These results show that the isotope chemistry of epeiric-sea brachiopods can be a reliable gauge of regional and global environmental evolution. Although diagenetic overprinting from meteoric cement-filled punctae and local forcing factors introduce "noise" that mutes isotopic signals, the open-ocean secular record is clearly discernible.

Rocks of the Bravo Lake Formation on Turtle Back Island and Pillow Island off the western coast of central Baffin Island, Nunavut, represent a well-exposed example of the mafic volcanic and intrusive sequences commonly preserved near the base of Paleoproterozoic intracratonic basins in the Archean Rae and Hearne provinces of the Canadian Shield. The Bravo Lake Formation is composed of relatively undeformed amygdaloidal pillowed flows, radial columnar and tortoise-shell jointed pillows, hydroclastic breccia, extensional partially sheeted dyke swarms, laminated mafic sediments, massive and fragmental flows, and layered and megacrystic intrusions. Volcanic structures and textures imply emplacement in a low-energy shallow (

The Sigma-Lamaque mine (>100 metric tons (t) of gold) is an example of an Archean greenstone belt gold deposit formed by processes related to multistage hydrothermal fluid circulation. The earliest recognized vein systems are steeply to moderately dipping fault-fill veins within shear zones and contemporaneous subhorizontal veins, which were overprinted by the late North Dipper vein system. The latter strikes east-west, dips moderately to the north, and exhibits complex internal geometry with features similar to fault-fill and extensional veins, and therefore is interpreted as extensional shear veins. This late vein system was considered one of the major sources of gold in the shallow underground and open-pit ore reserve estimations. These late North Dipper veins were studied in detail in the Sigma pit where they contain ore-grade gold and are hosted by the calc-alkaline tuffs of the Val d’Or Formation and in the North zone where they are barren and hosted by the tholeiitic volcanic rocks of the Jacola Formation. In the Sigma pit, these veins have apparent thicknesses of 0.8 to 3 m and very irregular contacts with the vein walls. They comprise irregular zones of milky quartz filling, disrupted blocks of layered quartz and tourmaline and slabs of foliated pyrite-rich wall rock. Layers with tourmaline and quartz crystals oriented perpendicular to the vein walls are observed close to the vein walls. The veins contain minor amounts of calcite, pyrite, rutile, and traces of chlorite, muscovite, pyrrhotite, chalcopyrite, galena, bismuth tellurides, and gold. Locally, the sulfide content can attain 10 modal percent of the vein mineralogy. In the veins, native gold (with 6 wt % Ag) occurs rarely associated with the main vein-filling minerals, commonly as inclusions in pyrite (main vein-filling stage), and most commonly filling fractures in pyrite, tourmaline, and quartz, where it is alloyed with 8 to 17 wt percent Ag and is associated with calcite and Bi tellurides (late auriferous stage). The proximal wall-rock alteration related to the auriferous North Dipper veins overprints the metamorphic assemblage and is characterized by abundant fine-grained plagioclase, calcite, quartz and minor chlorite, tourmaline, and pyrite in the tuffs and more abundant muscovite in the intrusive feldspar porphyry. In the wall rock, gold with 6 to 28 wt percent Ag is found locally filling fractures in pyrite, where it is associated with a late generation of calcite. The barren North Dipper vein also comprises mainly quartz and tourmaline but lacks calcite and gold, has a very low content of sulfides ( Aqueous-carbonic fluid inclusions with salinities less than 8 wt percent (NaCl equiv) and homogenization temperatures between 228° and 440°C, and carbonic fluid inclusions with less than 0.2 mol percent CH 4 coexisting in the same healed fractures, are common in the auriferous North Dipper veins but very rare in the barren vein where aqueous fluid inclusions have lower temperatures (dominantly 125°–225°C). Crush leaching and laser ablation time-of-flight inductively coupled mass spectrometry (LA-TOF-ICPMS) of fluids in the inclusions indicate elevated concentrations of Ni, Cu, Sb, Pb, and Ag in the fluids in the auriferous veins compared to the aqueous fluids in the barren veins. This suggests different sources for the metals in the fluids associated with the gold, possibly including both igneous and sedimentary rocks. Calculated oxygen isotope equilibration temperatures for quartz-tourmaline pairs from the auriferous main vein-filling stage are 323° to 370°C. This is within the range documented in fluid inclusions related to the late auriferous stage, indicating that the temperature of the auriferous hydrothermal fluids did not change appreciably during the evolution of the system. Calculated δ 18 O H 2 O values for the mineralizing fluids are 9.5 per mil, consistent with either low water/rock ratio systems associated with high-grade metamorphism or a magmatic origin for the fluids. Gold included in pyrite may have precipitated during the main vein-filling stage due to the decrease of H 2 Sconcentration in the hydrothermal fluid following the deposition of sulfides. However, gold in fractures in the main vein-filling minerals precipitated later, due probably to dilution of the auriferous aqueous-carbonic fluids by mixing with aqueous fluids. The absence of ore-grade gold in the barren North zone vein is probably due to the lack of circulation of auriferous aqueous-carbonic fluids in the North zone, located distal to the first-order Larder-Lake Cadillac fault, which may have served as the main regional conduit for the auriferous aqueous carbonic fluids.

The Bravo Lake Formation on central Baffin Island, Canada formed in a Palaeoproterozoic (∼1.9 Ga) intrabasinal submarine volcanic rift, the rocks of which preserve mineral assemblages formed during submarine hydrothermal alteration, amphibolite facies regional metamorphism, and retrograde metamorphism. Both stable isotope and cation exchange geothermometry of co-existing mineral assemblages indicate preservation of hydrothermal alteration at ∼300 °C, amphibolite facies metamorphism at ∼590 °C, and retrograde metamorphism at ∼300 °C. Fluid isotopic compositions calculated from δ18O and δD values measured from minerals suggest that three isotopically distinct fluids interacted with the Bravo Lake Formation during its evolution: (1) Palaeoproterozoic seawater infiltration during hydrothermal alteration; (2) regional metamorphic fluids at low water–rock ratios; and (3) retrograde fluids indicating increasing water–rock ratios. Hydrothermal fluids have δ18O and δD values near 0‰, similar to modern seawater values. The similarity in the isotopic composition of Palaeoproterozoic and modern seawater is consistent with the buffering of the isotopic composition of ocean water by hydrothermal circulation at mid-ocean ridges by similar processes as operate today. The results of this study show that the isotopic composition of prior fluid events can be preserved in amphibolite facies submarine mafic volcanic rocks if there is minimal penetrative structural deformation and dry prograde metamorphism.

Accumulation of carbonate mud is minor on the Northwest Shelf (Western Australia), a broad (< 200–300 km wide), distally steepened, tropical carbonate ramp. It is negligible along most of the inner ramp (0 to 50 m water depth) owing to siliciclastic (riverine) input. It is also negligible along the mid (from 50 to 120 m) ramp, and where the entire ramp narrows to < 100 km, owing to constant reworking by storm waves and ocean swell. In these areas, carbonate mud is derived mostly from mechanical degradation of skeletal fragments, but this does not include calcified remains of calcareous green algae (e.g., Halimeda) although these plants grow locally. Only along the outer (120 to 200 m water depth) ramp and slope is there any relatively abundant accumulation of carbonate mud. Here the sediment consists of modern pelagic (foraminifer, pteropod, and nannoplantkon) ooze and aragonitic needle-rich (< 2 μm) micrite, the latter having an age (based on AMS 14C) of ~ 19 ka. Distribution of aragonite micrite defines an extensive (5.0 × 104 km2), yet relatively narrow (< 40 km), tract running ~ 600 km along the platform margin. Crystal morphology, mineralogy, and isotope (C, O) composition identify the micrite to be a seawater precipitate, its production and seaward export having occurred during the Last Glacial Maximum, when lowered sea level transformed the otherwise broad platform into a narrow epicontinental shelf margin, now the modern distal slope. Today, this sediment body lies stranded in deep water, about 200–300 km seaward of the shallow inner ramp. The volume of micrite produced appears equal to the present-day production potential of the Great Bahama Bank. Rapid postglacial rise in sea level, and change in platform geometry, climate, and oceanography, were very influential factors, in addition to water depth and area of submergence, in the production and distribution of aragonite micrite along the Northwest Shelf. In general: (1) epicontinental carbonate platforms can lack substantial micrite accumulation when oceanic current and wave energies sweep unimpeded across a platform, and coastal clastics and absence of shoal barriers prevent development of protective lagoons and carbonate tidal flats; (2) not all highstand carbonate systems are associated with significant amounts of micrite in shallow water environments, nor do they necessarily export large volumes of micrite to the periplatform realm; and (3) some lowstand systems can produce and export volumes of carbonate mud that rival highstand systems.

Surficial carbonate sediments on the southern continental shelf of Australia are cool-water in aspect and composed of biogenic particles produced largely during the late Quaternary. Current understanding is that such sediments are calcite-dominated, as were their older Cenozoic counterparts. The Holocene fraction of these sediments in modern open-shelf, neritic environments between 30 and 350 meters water depth is, however, 50% to 80% aragonite. Scant evidence of significant former aragonite in many cool-water carbonate sedimentary rocks implies that most aragonite is lost before such sediments exit the marine diagenetic environment. Although marine dissolution must be taking place in such settings, the conundrum is exacerbated because seawater over the shelf in southern Australia is saturated with respect to aragonite. It is proposed that the aragonite, from skeletons of gastropods, infaunal bivalves, and certain bryozoans, is dissolved in the shallow subsurface, probably as the byproduct of bacterial degradation of sedimentary organic matter. As a consequence, the geological and paleontological record of many cool-water carbonates is strongly biased, and the inferred original calcitic composition of such sediments is the product of early diagenetic taphonomic loss, not selective biogenic productivity. The net result is not only dissolution of aragonite but also neomorphism of Mg-calcite to calcite with a marine geochemical signature. Synsedimentary aragonite loss, by removing CaCO3 that is usually available for calcite cementation during meteoric diagenesis, leads to retarded lithification of these cool-water carbonates until deeply buried. Such removal of a significant carbonate fraction during deposition likely contributes to the low rates of cool-water sediment accumulation.

The Pierina high-sulfidation epithermal Au–Ag deposit (9°26.5′S; 77°35′W) was emplaced in the Middle Miocene into a hypabyssal-to-extrusive “pumice-tuff” and an underlying, older, dacitic flow-dome complex, both of which are cut by hydrothermal breccias and small dacitic domes. Stage I advanced argillic alteration generated a core of vuggy silica, focused in the tuff, and surrounded successively by zones of quartz–alunite, dickite±kaolinite±pyrophyllite, and illite–montmorillonite±kaolinite. Laser-ablation ICP-MS analysis of the sulfide minerals of the succeeding, Stage II, Cu (–Pb, Bi, Sb, Zn, As)–barite mineralization, largely confined to the vuggy-silica zone, reveals that both Au and Ag were introduced at that time. This assemblage was almost entirely obliterated during Stage III, when oxidation by low-temperature meteoric waters generated botryoidal hematite–goethite assemblages, which are now the main precious-metal hosts. Stage IV barite–acanthite mineralization shows limited temporal overlap with Stage III, but dominantly overprints the hematite–goethite assemblage. The deposit incorporates alunite exhibiting a wide range of modes of occurrence, grain size, and morphology. Disseminated alunite dominates Stage I alteration, which replaced phenocrysts and fragments, and locally hosts corroded, ≤40 μm alumino-phosphate-sulfate (APS) inclusions. The alunite shows limited Na substitution [molar Na/(Na+K)=

The North West Shelf is an ocean-facing carbonate ramp that lies in a warm-water setting adjacent to an arid hinterland of moderate to low relief. The sea floor is strongly affected by cyclonic storms, long-period swells and large internal tides, resulting in preferentially accumulating coarse-grained sediments. Circulation is dominated by the south-flowing, low-salinity Leeuwin Current, upwelling associated with the Indian Ocean Gyre, seaward-flowing saline bottom waters generated by seasonal evaporation, and flashy fluvial discharge. Sediments are palimpsest, a variable mixture of relict, stranded and Holocene grains. Relict intraclasts, both skeletal and lithic, interpreted as having formed during sea-level highstands of Marine Isotope Stages (MIS) 3 and 4, are now localized to the mid-ramp. The most conspicuous stranded particles are ooids and peloids, which 14C dating shows formed at 15·4–12·7 Ka, in somewhat saline waters during initial stages of post-Last Glacial Maximum (LGM) sea-level rise. It appears that initiation of Leeuwin Current flow with its relatively less saline, but oceanic waters arrested ooid formation such that subsequent benthic Holocene sediment is principally biofragmental, with sedimentation localized to the inner ramp and a ridge of planktic foraminifera offshore. Inner-ramp deposits are a mixture of heterozoan and photozoan elements. Depositional facies reflect episodic environmental perturbation by riverine-derived sediments and nutrients, resulting in a mixed habitat of oligotrophic (coral reefs and large benthic foraminifera) and mesotrophic (macroalgae and bryozoans) indicators. Holocene mid-ramp sediment is heterozoan in character, but sparse, most probably because of the periodic seaward flow of saline bottom waters generated by coastal evaporation. Holocene outer-ramp sediment is mainly pelagic, veneering shallow-water sediments of Marine Isotope Stage 2, including LGM deposits. Phosphate accumulations at ≈ 200 m water depth suggest periodic upwelling or Fe-redox pumping, whereas enhanced near-surface productivity, probably associated with the interaction between the Leeuwin Current and Indian Ocean surface water, results in a linear ridge of pelagic sediment at ≈ 140 m water depth. This ramp depositional system in an arid climate has important applications for the geological record: inner-ramp sediments can contain important heterozoan elements, mid-ramp sediments with bedforms created by internal tides can form in water depths exceeding 50 m, saline outflow can arrest or dramatically slow mid-ramp sedimentation mimicking maximum flooding intervals, and outer-ramp planktic productivity can generate locally important fine-grained carbonate sediment bodies. Changing oceanography during sea-level rise can profoundly affect sediment composition, sedimentation rate and packaging.

The Early Devonian Lake George granodiorite stock, New Brunswick, Canada, is spatially and temporally associated with early W‐Mo‐Au and later Au‐Sbmineralization. The former mine was the largest antimony producer in North America until the mid-1990s. On the basis of fluid-inclusion analyses, multiple generations of fluids evidently interacted with the granodiorite, with the dominant fluids approximated by the system H2O‐NaCl‐CO2‐CH4. Additional components of the fluid, S‐Ca‐Fe‐K‐Mg‐ Mn, reflect fluid‐rock reaction and partial re-equilibration among the rock-forming minerals (i.e., plagioclase, orthoclase, biotite, and hornblende) in the granodiorite. The CO2-bearing inclusions (Type 1) have a relatively low salinity, ranging from 0 to 15 wt.% equiv. NaCl (ave. 4 wt.%), and most homogenize to the CO2 phase at temperatures of 200 to 470°C. Inclusions that homogenize to the aqueous phase do so over a narrower range of temperatures (250‐350°C). Fluid pressures were probably 740 to 2210 bars for Type-1 inclusions. Aqueous liquid + vapor (Type 2) and aqueous liquid + vapor + halite or sylvite (Type 3) inclusions are recorded as well. Type-2 inclusions show a broader range of temperatures of homogenization (mostly to the aqueous phase), from 150 to 580°C, with a salinity between 2 and 19 wt.% equiv. NaCl. The stable isotope compositions of the rock-forming minerals in the granodiorite exhibit a disequilibrium relationship, e.g., either � 18 OK-feldspar > � 18 Oquartz, � 18 Oplagioclase > � 18 Oquartz, or � 18 Oplagioclase > � 18 OK-feldspar. The whole-rock � 18 O values range from 9.4 to 10.7‰, consistent with derivation by

Granitic rocks in the New Ross area, Nova Scotia, host polymetallic (Cu, Mo, Mn, Sn, U, W), aplite‐pegmatite, greisen, and vein-type mineral deposits. The host granitic rocks belong to the Late Devonian (380 Ma) peraluminous South Mountain Batholith (SMB), which consists of several coalesced plutons. Among these, the New Ross Pluton, composed of monzogranite, leucomonzogranite, and leucogranite, is one of the youngest. White mica separates from aplite‐pegmatite, greisen, and veins have 18 O values between +4.0 and +10.0‰, and D values between ‐42 and ‐108‰. Fluid-inclusion extracts from quartz in samples from the area of the same deposits have D values between ‐42 and ‐97‰. The isotopic composition of fluids in equilibrium with the white mica and isotopic data for the fluid-inclusion extracts record the transition from an early, orthomagmatic stage, in which magmatic fluid dominated (i.e., aplites, pegmatites), to a subsequent hydrothermal stage (i.e., greisens, veins) where an increasing amount of another fluid, inferred to be meteoric water, infiltrated the systems and mixed with the magmatic fluid. In addition, the low D values for fluid-inclusion extracts compared with the Dfluid values calculated for coexisting white mica samples are considered to record infiltration of a second meteoric water much later in the evolution of the SMB. Integration of the new stable isotope data with our previous results on fluid inclusions suggests the following history of fluid evolution within the New Ross area: (1) exsolution of a magmatic fluid at ~600°C, (2) incursion of a low- 18 O, high-D meteoric fluid at ca. 380 Ma through faults, and subsequent mixing with magmatic and metamorphic fluids through circulation by convection before cooling to ~400‐500°C, and (3) incursion at a much later time of a low-D meteoric fluid, mostly affecting the composition of fluid-inclusion extracts.

Lead isotope ratios and associated trace element concentrations (U, Th and Pb) extracted by partial-leaching with 2% nitric acid from Proterozoic sandstones and basement rocks reveal much about the fluid evolution of sedimentary basins hosting unconformity-type uranium deposits. In addition, these techniques have great potential as a guide for exploration of uranium and other types of deposits in basins of any age. Isotope ratios of Pb in Proterozoic sandstones from basins known to contain high-grade uranium deposits are radiogenic at key geological localities and settings distal to known mineralization and particularly in altered zones proximal to mineralization. Sandstones completely cemented by quartz overgrowths typically have non-radiogenic Pb isotope ratios, indicating early closure of porosity and isolation of these rocks from later fluid events. Alternatively, the unconformity served as both a source of uranium and radiogenic Pb as well as an avenue for late-stage (

207Pb/204Pb versus 206Pb/204Pb model ages using Shonkin Sag data and published analyses for magmas of the Cenozoic Wyoming-Montana alkaline province (WYMAP) provide evidence of an Archean age for the subcontinental lithospheric mantle (SLM) associated with the Wyoming craton. The SLM imprint on magmas is expressed as Ba, Ta, Nb and Ti "anomalies" which correlate with radiogenic isotopic data, and it resembles a subduction imprint on Cenozoic south-western USA basalts (SWUSAB). However the latter give Proterozoic Pb isotope model ages. Although the Archean and Proterozic model ages may represent mixing lines, the fact that they resemble the ages for continental crust cut by WYMAP and SWUSAB respectively indicates that the age of the underlying SLM helped control the "isochron" slopes and inferred "ages". Lower 143Nd/144Nd and 206Pb/204Pb but comparable 87Sr/86Sr for WYMAP suggest that SLM associated with Archean cratons has lower Sm/Nd, U/Pb and Rb/Sr ratios than SLM associated with SWUSAB Proterozic terranes, regardless of when the subduction imprint or imprints developed. WYMAP magmas have high Pb/Zr ratios indicating that Archean SLM, like Archean continental crust, is enriched in Pb compared to Proterozoic SLM. If the enrichment was Archean, it implies that higher Archean heat flow enhanced Pb transfer from the subducting slab to overlying lithospheric mantle and crust. A subducted sediment imprint on the SLM is also consistent with high δ18O values for the Shonkin Sag. Low TiO2 in WYMAP may reflect a residual mantle TiO2 phase. If so, the Nb "missing" from crustal and oceanic mantle reservoirs may reside in rutile of Archean SLM. Isotopic similarities between WYMAP and EM1 oceanic island basalts may reflect the presence of delaminated, Archean SLM in the oceanic mantle, although low Pb/Zr ratios and a lack of Ti, Nb and Ta anomalies in oceanic island basalts deserve further investigation.

A method for in situ U–Pb isotopic analyses by secondary ion mass spectrometry (SIMS) has been developed for uranium minerals with a range of chemical compositions. This method combines the advantages of conventional U–Pb dating (i.e., use of concordia) and in situ analysis, and therefore is ideally suited for the study of chemically complex and fine-grained uranium oxides associated with uranium deposits. An ion-yield normalizing coefficient (� SIMS) that accounts for variation in relative ion-yields with chemical composition of the mineral of interest was calculated using uraninite standards that cover a range of U