Stable isotope compositions of fossil mollusks from southern California: Evidence for a cool last interglacial ocean

Stable isotope compositions have been determined for modem mullusks and fossil mollusks collected from uplied marine terraces at three l d t i e s in southern California. By using a paleocliatic model that decouples the temperature and ice-volume signals in ocean water, ocean-water temperatures off southern California are estimated to have been -3.8 °C at ~85 ka, -3.0 °C at ~107 ka, and -2.2 °C at ~125 ka relative to present temperature. These results indicate rather cool conditions during the peak of the last interglacial stage at 125 ka and conflict with results from terrace faunal studies that suggest water temperatures were warm or warmer than at present

Trace element and stable isotope dispersion halo around a high sulfidation epithermal Au-Ag deposit emplaced in Quarzitic Sandstone: The Lagunas Norte Deposit, Northern Peru

The Lagunas Norte high sulfidation epithermal Au-Ag deposit is, with 11.2 Moz of total resources, the most important ore deposit discovered recently in Peru. It is largely hosted by folded, Cretaceous quartzitic sandstone of the Chimu Formation, but ~30% of the ore was emplaced in the Miocene Calipuy Group volcanic rocks overlying the quartzites, as well as in two phreatomagmatic diatreme breccia bodies cutting the Cretaceous units. Due to the low reactivity of the quartzites, the alteration halo around the deposit is obscure so that the deposit eluded discovery until 2001. However, stable-oxygen isotope geochemistry combined with multi element data can vector to ore. The oxygen isotopic composition of quartz reflects hydrothermal alteration beyond the halo visible in the relatively pure (mostly >95% SiO2) sandstones. Regional background δ18O values of unaltered sandstones are between 13.2‰ and 13.8‰ whereas much higher v18O values between 16.5‰ and 16.8‰ in finegrained yellowish quartz veins in the sandstones (silice parda) and vuggy quartz in the volcanic rocks record the influence of the mineralizing fluid. The quartzitic sandstones affected by hydrothermal fluids near the deposit show δ18O values between 14‰ and 15‰ and similar elevated values are observed near faults to the north of the deposit. The highest δ18O values in the silica parda correlate with high Ti and Te contents but low Ga and Pb contents relative to those of the unaltered quartzites. Mobile elements like Na and K are commonly below detection limit (ALS Chemex - ICP-MS ME-ICP06 & ME-MS61), even in the unaltered sandstones. We conclude that stable isotope geochemistry is a potentially powerful tool when defining the extent of hydrothermal fluid flow in non-reactive host rocks

Oxygen isotopes and trace elements in the Tiva Canyon Tuff, Yucca Mountain and vicinity, Nye County, Nevada

Yucca Mountain is being studied as a potential site for an underground repository for high-level radioactive waste. Because Yucca Mountain is located in a resource-rich geologic setting, one aspect of the site characterization studies is an evaluation of the resource potential at Yucca Mountain. The Tiva Canyon Tuff (TCT) is a widespread felsic ash-flow sheet that is well exposed in the Yucca Mountain area. Samples of the upper part of the TCT were selected to evaluate the potential for economic mineral deposits within the Miocene volcanic section. These samples of the upper cliff and caprock subunits have been analyzed for oxygen isotopes and a large suite of elements. Oxygen isotope compositions ({delta}{sup 18}O) of the TCT are typical of felsic igneous rocks but range from 6.9 to 11.8 permil, indicating some post-depositional alteration. There is no evidence of the low {delta}{sup 18}O values (less than 6 permil) that are typical of epithermal precious-metal deposits in the region. The variation in oxygen isotope ratios is probably the result of deuteric alteration during late-stage crystallization of silica and low-temperature hydration of glassy horizons; these processes are also recorded by the chemical compositions of the rocks. However, most elemental contents in the TCT reflect igneous processes, and the effects of alteration are observed only in some of the more mobile elements. These studies indicate that the TCT at Yucca Mountain has not been affected by large-scale meteoric-water hydrothermal circulation. The chemical compositions of the TCT, especially the low concentrations of most trace elements including typical pathfinder elements, show no evidence for epithermal metal deposits. Together, these data indicate that the potential for economic mineralization in this part of the volcanic section at Yucca Mountain is small

Redox Conditions of the Late Cretaceous Western Interior Seaway Recorded by REEs of Bearpaw Molluscan Fossils

Understanding the causes of the low biodiversity faunas in the Western Interior Seaway (WIS), overwhelmingly dominated by ammonite and bivalve molluscs, will help us to understand paleoenvironmental conditions in the WIS. In this study, we examined rare earth elements (REEs) in well-preserved molluscs from the Bearpaw Formation to reconstruct WIS redox conditions. Both nektonic and benthic molluscs share similar REE patterns with enrichment in light REEs. There is only a slight Ce depletion in both types of molluscs, indicating no significant fractionation of Ce from the other REEs. A lack of significant Ce anomalies in molluscs points to oxygen-deficient (probably dysoxic) conditions in the middle to bottom part of the water column where the molluscs lived. Given the general lack of significant Ce anomalies also in molluscs from older formations, oxygen deficiency was likely prevalent in the late Cretaceous in the WIS. A warmer climate in the Cretaceous is probably a driver of such conditions, as predicted by ocean models. Long-term oxygen deficiency and stratification, inferred from the heterogeneity in δ18O and δ13C values of molluscs, may also imply weak circulation, reducing the exchange of surface water and subsurface water, and the transport of oxygen into the WIS. The oxygen-deficiency and weak circulation of the seaway is also expressed in the general characteristics of faunas and possibly Bearpaw sedimentary rocks. Oxygen deficiency rather than the brackish-water conditions may have been largely responsible, therefore, for the low diversity of the WIS fauna.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

The Black Rock Coatings in Rouyn-Noranda, Québec: Fingerprints of Historical Smelter Emissions and the Local Ore

Smelting of base-metal-sulfide rich ore in Rouyn-Noranda, Québec have resulted in 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 around 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, and is thus called the smelter-compatibility effect. Elements displaying a high smelter-compatibility effect are Ag, Cu, Se and As whereas elements such as Hg, incompatible with the jarosite and spinel structure, show a low smelter-compatibility effect. High δThe accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author