Iron Oxide Coatings Research Articles

818 酸化鉄皮膜の摩擦・摩耗特性

818 酸化鉄皮膜の摩擦・摩耗特性

The laser-induced combustion synthesis of iron-oxide nanocomposite coatings on aluminum

The potential of synthesizing nano-structure coatings on aluminum cast alloys during laser surface engineering has been explored based on combustion synthesis and underlying rapid-solidification principles. The roles of cooling rate, heterogeneous nucleation site density, and grain multiplication in structure refinements are well known. By combining combustion synthesis and laser surface engineering, refinement to nanostructure is envisioned.

Bacterial transport in heterogeneous porous media: Observations from laboratory experiments

Transport of bacteria through heterogeneous porous media was investigated in small‐scale columns packed with sand and in a tank designed to allow the hydraulic conductivity to vary as a two‐dimensional, lognormally distributed, second‐order stationary, exponentially correlated random field. The bacteria were Pseudomonas ftuorescens R8, a strain demonstrating appreciable attachment to surfaces, and strain Ml, a transposon mutant of strain R8 with reduced attachment ability. In bench top, sand‐filled columns, transport was determined by measuring intensity of fluorescence of stained cells in the effluent or by measuring radiolabeled cells that were retained in the sand columns. Results demonstrated that strain Ml was transported more efficiently than strain R8 through columns packed with either a homogeneous silica sand or a more heterogeneous sand with iron oxide coatings. Two experiments conducted in the tank involved monitoring transport of bacteria to wells via sampling from wells and sample ports in the tank. Bacterial numbers were determined by direct plate count. At the end of the first experiment, the distribution of the bacteria in the sediment was determined by destructive sampling and plating. The two experiments produced bacterial breakthrough curves that were quite similar even though the similarity between the two porous media was limited to first‐ and second‐order statistical moments. This result appears consistent with the concept of large‐scale, average behavior such as has been observed for the transport of conservative chemical tracers. The transported bacteria arrived simultaneously with a conservative chemical tracer (although at significantly lower normalized concentration than the tracer). However, the bacterial breakthrough curves showed significant late time tailing. The concentrations of bacteria attached to the sediment surfaces showed considerably more spatial variation than did the concentrations of bacteria in the fluid phase. This contrast between behavior in the fluid phase and on the solids is consistent with field observations by other authors and initial modeling of these heterogeneous media.

Effect of ultrasound on surface cleaning of silica particles

One substantial benefit of ultrasonics in mineral processing can be the removal of surface coatings of clay and iron oxides from mineral surfaces. This is mainly achieved through the large, but very localised, forces produced by cavitation. The application of sonication to the reduction of iron oxide in a silica sand from 0.025% to less than 0.012% Fe 2O 3 — the difference between material suitable for clear glass container ware and that suitable for table ware — is presented. The ultimate reduction in iron oxide contamination is dependent on the sonication power, whereas increasing the concentration of Na 2CO 3 in the suspension, up to 0.2%, shortens the sonication time to approach this limit.

Bioavailability of particle‐associated silver, cadmium, and zinc to the estuarine amphipod Leptocheirus plumulosus through dietary ingestion

We conducted experiments to determine effects of particle type on assimilatory metal bioavailability to Leptocheirus plumulosus, an infaunal, estuarine amphipod that is commonly used in sediment toxicity tests. The following particles were used to represent natural food items encountered by this surface‐deposit and suspension‐feeding amphipod: bacterial exopolymeric sediment coatings, polymeric coatings made from Spartina alterniflora extract, amorphous iron oxide coatings, the diatom Phaeodactylum tricornutum, the chlorophyte Dunaliella tertiolecta, processed estuarine sediment, and fresh estuarine sediment. Bioavailability of the gamma‐emitting radioisotopes 110mAg, 109Cd, and 65Zn was measured as the efficiency with which L. plumulosus assimilated metals from particles using pulse‐chase methods. Ag and Cd assimilation efficiencies were highest from bacterial exopolymeric coatings. Zn assimilation efficiency exhibited considerable interexperimental variation; the highest Zn assimilation efficiencies were measured from phytoplankton and processed sediment. In general, Ag and Cd assimilation efficiencies from phytoplankton were low and not related to the proportion of metal associated with cell cytosol or cytoplasm, a phenomenon reported for other particle‐ingesting invertebrates. Amphipod digestive processes explain differences in Ag and Cd assimilation efficiencies between exopolymeric coatings and phytoplankton. Results highlight the importance of labile polymeric organic carbon sediment coatings in dietary metals uptake by this benthic invertebrate, rather than recalcitrant organic carbon, mineralogical features such as iron oxides, or phytoplankton.

Sonochemical Synthesis and Characterization of Iron Oxide Coated on Submicrospherical Alumina: A Direct Observation of Interaction between Iron Oxide and Alumina

Coating of iron oxides on submicrospheres of alumina was carried out by the sonochemical method. Three kinds of alumina were used, namely, the as-prepared, 700 and 1000 °C heated-alumina. TEM results reveal various coating effects on different heat-treated substrates. The optimum coating is obtained on as-prepared amorphous alumina, in which most of the iron oxide particles are adhered to alumina spheres tightly, while in the sample coated on crystallized alumina heated at 1000 °C, most of the iron oxide particles remain separate from the alumina spheres. The strong interaction between iron oxides and alumina substrate was directly observed by XRD, TEM, and IR and magnetic measurements. Owing to the strong interaction between adhered iron particles and alumina substrate, the complete transformation of γ-Fe2O3 to α-Fe2O3 was retarded to higher than 700 °C; conversely, the presence of haematite can induce the formation of α-Al2O3 at the high temperatures. TEM images clearly show the changes of particle size...

Bacteriophage PRD1 and Silica Colloid Transport and Recovery in an Iron Oxide-Coated Sand Aquifer

Bacteriophage PRD1 and silica colloids were co-injected into sewage-contaminated and uncontaminated zones of an iron oxide-coated sand aquifer on Cape Cod, MA, and their transport was monitored over distances up to 6 m in three arrays. After deposition, the attached PRD1 and silica colloids were mobilized by three different chemical perturbations (elevated pH, anionic surfactant, and reductant). PRD1 and silica colloids experienced less attenuation in the contaminated zone where adsorbed organic matter and phosphate may be hindering attachment of PRD1 and silica colloids to the iron oxide coatings. The PRD1 collision efficiencies agree well with collision efficiencies predicted by assuming favorable PRD1 deposition on iron oxide coatings for which the surface area coverage was measured by microprobe analysis of sediment thin sections. ζ potentials of the PRD1, silica colloids, and aquifer grains corroborated the transport results, indicating that electrostatic forces dominated the attachment of PRD1 and silica colloids. Elevated pH was the chemical perturbation most effective at mobilizing the attached PRD1 and silica colloids. Elevated surfactant concentration mobilized the attached PRD1 and silica colloids more effectively in the contaminated zone than in the uncontaminated zone.

Fluvio-pedogenic processes in an ephemeral stream channel, Nahal Yael, Southern Negev, Israel

A detailed stratigraphic survey conducted in the alluvial fill of the channel of Nahal Yael—a small catchment in the Southern Negev Desert—indicates the existence of a continuous, compacted, red-colored unit at an average depth of 50 cm beneath the surficial grey non-cohesive alluvium. Granulometric distinction between the grey and the red alluvium is evident only within the fine (<0.063 mm) fractions; the clay content in the red alluvium, though less than 5%, is more than double in the grey alluvium. Cementation by calcium carbonate causes the hardening of the `red' unit. Reddening is due to small amounts of iron oxide coatings. While the upper contact of the `red' unit is abrupt, the compaction and coloring diminish gradually downward, forming a diffuse lower boundary. The structure controlled infiltration rates into the upper grey alluvium exceed by an order of magnitude those into the `red' unit, but are two orders of magnitude less than the velocity of the flood front, indicating that the flood fronts progress over unsaturated alluvium. The proposed model is one of a dual component flow within the alluvial fill: a longitudinal flow within the upper active layer, proceeding mainly along structure determined paths, and a vertical flow which affects the whole alluvial fill. The formation of the `red' unit is tied to the cumulative influence of persistent differences in water availability to various parts of the channel before and during flood events. The `red' unit forms at the lower limit of the contemporary scour and fill processes, and therefore, its depth reflects the contemporary hydroclimatic regime. Its identification as a partial buffer to floodwater infiltration offers new insights into the problem of the generation of desert floods and the mechanism of transmission losses in ephemeral streams.

Chemical Weathering in a Tropical Watershed, Luquillo Mountains, Puerto Rico: II. Rate and Mechanism of Biotite Weathering

Samples of soil, saprolite, bedrock, and porewater from a lower montane wet forest, the Luquillo Experimental Forest (LEF) in Puerto Rico, were studied to investigate the rates and mechanisms of biotite weathering. The soil profile, at the top of a ridge in the Rio Icacos watershed, consists of a 50–100-cm thick layer of unstructured soil above a 600–800 cm thick saprolite developed on quartz diorite. The only minerals present in significant concentration within the soil and saprolite are biotite, quartz, kaolinite, and iron oxides. Biotite is the only primary silicate releasing significant K and Mg to porewaters. Although biotite in samples of the quartz diorite bedrock is extensively chloritized, chlorite is almost entirely absent in the saprolite phyllosilicates. Phyllosilicate grains are present as 200–1000 μm wide books below about 50 cm depth. X-ray diffraction (XRD) and electron microprobe analyses indicate that the phyllosilicate grains contain a core of biotite surrounded by variable amounts of kaolinite. Lattice fringe images under transmission electron microscope (TEM) show single layers of biotite altering to two layers of kaolinite, suggesting dissolution of biotite and precipitation of kaolinite at discrete boundaries. Some single 14-Å layers are also observed in the biotite under TEM. The degree of kaolinitization of individual phyllosilicate grains as observed by TEM decreases with depth in the saprolite. This TEM work is the first such microstructural evidence of epitaxial growth of kaolinite onto biotite during alteration in low-temperature environments. The rate of release of Mg in the profile, calculated as a flux through the soil normalized per watershed land area, is approximately 500 mol hectare −1 yr −1 (1.6 × 10 −9 mol Mg m soil −2 s −1). This rate is similar to the flux estimated from Mg discharge out the Rio Icacos (1000 mol hectare −1 yr −1, or 3.5 × 10 −9 mol Mg m soil −2 s −1), indicating that scaling up from the soil to the watershed is possible for Mg release. The rate of Mg release from biotite, normalized to Brunauer-Emmett-Teller (BET) surface area, is calculated using a mass balance equation which includes the density and volume of phyllosilicate grains, porewater chemistry and flux, and soil porosity. The mean rates of biotite weathering calculated from K and Mg release rates are approximately 6 and 11 × 10 −16 mol biotite m biotite −2 s −1 respectively, significantly slower than laboratory rates (10 −12 to 10 −11 mol biotite m biotite −2 s −1). The discrepancy in scaling down from the soil to the laboratory is probably explained by (1) differences in weathering mechanism between the two environments, (2) higher solute concentrations in soil porewaters, (3) loss of reactive surface area of biotite in the saprolite due to kaolinite and iron oxide coatings, and/or (4) unaccounted-for heterogeneities in flow path through the soil.

The Effect of Chemical Pretreatments on the Composition of Natural Illite

In order to remove impurities, natural illite is often subjected to chemical pretreatments before analysis, the assumption being that such pretreatments have little or no effect on the composition of illite. Bulk analyses are sometimes reported before and after pretreatment (Aja et al. 1991) but, because the contribution of the impurities to the bulk composition is unknown, the possible effects of chemical pretreatments on the composition of illite cannot be assessed. Recently, Rosenberg and Hooper (1996) have shown that analytical transmission electron microscopy (ATEM) techniques, which exclude impurities, can be used to determine the bulk compositions of relatively pure illite by averaging the analyses of at least 50 crystals per sample. This approach can be applied to impure illite in order to determine the effect of chemical pretreatments on the composition of natural illite.

Occurrence of talc in soils with high iron content from the south-west of Spain

The mineralogy of the clay fractions of 2 soil profiles classified as an Alfic Dystric Eutrochrept (Soil I) and a Xerochreptic Haploxeralf (Soil II) has been determined. A notable feature of these soils is the presence of talc and the high iron content as goethite, hematite, lepidocrocite, and maghemite. Talc was the most abundant non-iron mineral of the clay fraction of Soil II. In Soil I, talc occurs as a residual phase from the parent material with its persistence in the profile aided by coatings of iron oxides, which inhibit further weathering. In these soils there is a high proportion of goethite accompanied by hematite. In Soil I the proportion of iron oxides changes within the profile. Another notable feature of this profile is the presence of maghemite in the upper horizons where this is the predominant iron oxide mineral.

Reduction of Substituted Nitrobenzenes by Fe(II) in Aqueous Mineral Suspensions

The kinetics of the reduction of 10 monosubstituted nitrobenzenes (NBs) by Fe(II) has been investigated under various experimental conditions in aqueous suspensions of minerals commonly present in soils and sediments. Aqueous solutions of Fe(II) were unreactive. In suspensions of Fe(III)-containing minerals (magnetite, goethite, and lepidocrocite), Fe(II) readily reduced the NBs to the corresponding anilines in a strongly pH-dependent reaction. Our results suggest that on other mineral surfaces (γ-aluminum oxide, amorphous silica, titanium dioxide, and kaolinite) iron (hydr)oxide coatings are indispensable to promote the reduction of NBs by adsorbed Fe(ll). Apparent pseudo-first-order rate constants, k obs , were used to describe the initial kinetics of the NB reduction, covering several half-lives of the compounds. The distinct effect of substituents on k obs and the observed pronounced competition between different NBs indicate that precursor complex formation as well as the (re)generation of reactive surface sites are rate-determining steps in the overall reduction of the NBs. The results of this study demonstrate that Fe(ll) adsorbed on iron (hydr)oxide surfaces or surface coatings may play an important role in the reductive transformation of organic pollutants in subsurface environments. Our findings may also contribute to a better understanding of the various redox processes involved in groundwater remediation techniques based on Fe(0) as the bulk reductant.

Improving the Iron Oxide Sink Method for Extracting Soil Phosphorus

AbstractIron oxide coated filter paper can estimate labile P in soil by acting as a P sink. Variations in procedures and contamination from soil adhering to the sinks limit effective data interpretation, however. We evaluated two papers with pores &lt;5.0 µm and one with pores 20 to 25 µm. Iron oxide coatings were formed by exposing FeCl3‐treated papers to NH3 vapor or 2.7 M NH4OH. Coated papers were either fixed in a screen assembly or allowed to move freely during shaking. The screen assembly eliminated abrasion loss of the FeO coating. Paper circles (5.5 cm) proved satisfactory and eliminated the need to cut 2 by 10 cm strips from coated papers. Fixed, small‐pore circles had less soil contamination (0.8–1.9 mg) than fixed, large‐pore circles (4.7–8.1 mg), a condition confirmed by x‐ray diffraction. Similarly, free, small‐pore circles retained less soil contamination (3.5–9.0 mg) than free, large‐pore circles (16.1–21.7 mg). Two sources of acid‐extractable P inflated the labile P pool. First, acid‐extractable P from soil adhering to papers increased sink‐extractable P by 7.5 to 24.1 mg P kg−1, an error of 28 to 65%. Also, paper prepared in NH3 vapor acidified the extracting solution with Kahola (fine‐silty, mixed, mesic Cumulic Hapludoll) and Haynie (coarse‐silty, mixed [calcareous], mesic Typic Udifluvent) soils and increased their extractable P an average of 13 mg P kg−1, or 56%. To eliminate some analytical problems associated with the FeO‐sink extraction of soil P, we recommend preparing small‐pore circles (&lt;5 µm) in NH4OH solution and securing them during shaking.

Possible effects of iron oxide coating in the recovery of particulate gold from stream sediments

The influence of iron hydroxide/oxide coatings on the recovery of gold was studied using 13 stream sediment samples collected from an area of known gold mineralisation. Magnetic products collected from a Frantz isodynamic separator showed higher gold values than did non-magnetic fractions. An acid-treated magnetic product when run through the isodynamic separator yielded a significant amount of non-magnetic component which, when viewed by binocular microscope, showed the presence of gold grains in both bulk sample and in bromoform-separated heavy mineral concentrate. It is suggested that concentrates are treated with 10% HCl prior to magnetic separation where iron oxide coatings are conspicuous.

Vertical Distribution and Partitioning of Chromium in a Glaciof luvial Aquifer

AbstractThe vertical distribution and partitioning (between the solid and aqueous phase) of chromium in a glaciofluvial aquifer in northeastern Connecticut were assessed. Most of the chromium (99 percent of its mass) is bound to the soil. Retardation is primarily the result of binding to organic matter and adsorption to iron oxide coatings. However, other attenuation mechanisms also appear to be significant. If the degree of chromium binding observed here is representative of other chromium contaminated sites, pump‐and‐treat remediation will not remove the vast amount of chromium from the subsurface. However, most of the chromium may be immobile, and removal may not be required following the initial pumping to remove the mobile fraction. Further knowledge of the mechanisms that bind chromium to the soil, their reversibility, and their kinetics is essential to developing effective remediation strategies.

Electrocatalysis of cathodic hydrogen and anodic oxygen evolution in alkaline water electrolysis by in situ activation procedures

The possibility of in situ activation of oxygen evolving anodes and hydrogen evolving cathodes in alkaline water electrolysers has been investigated. It is possible to activate anodes by depositing on them cobalt oxide or iron oxide or an even more active mixture of both. In all three cases the oxygen evolution overpotential at 1 A cm −2 which amounts under standard conditions to approximately 350 mV improves by approximately 80–100 mV. However, activation by iron oxide is not stable as the solubility of Fe 2O 3 in the electrolyte is too high, the coating dissolves and iron is precipitated on the cathode. Also cobalt oxide coatings are subject to slow deterioration—though to a much slower rate than the deterioration of iron oxide coatings. Most interesting is the fact that combined deposition of iron and cobalt oxide leads to a much more stable catalyst, which shows undiminished activity over at least 4000 h. Furthermore it is shown, that it is possible to precipitate in situ metallic ruthenium on hydrogen evolving cathodes in alkaline water electrolysers. The cathodic overpotential is thus reduced to less than −100 mV at 1 A cm −2 and 120°C. The use of complexing agents yields in rather smooth and compact ruthenium deposits with enhanced long-term stability. Copper substrates may be used instead of nickel resulting in relatively low costs.

Groundwater Flow, Late Cementation, and Petroleum Accumulation in the Permian Lyons Sandstone, Denver Basin

The gray diagenetic facies of the Permian Lyons Sandstone, which is associated with all known petroleum accumulations in the formation, formed late in the history of the Denver basin as an alteration product of the formation's red facies. The red facies that makes up most of the sandstone contains iron oxide coatings, quartz overgrowths and calcite cements. The gray facies, which occurs locally in the deep basin, is distinguished by pore-filling dolomite and anhydrite cements and by a lack of iron oxide and calcite. The dolomite and anhydrite cements overlie bitumen that was deposited by migrating oil, and hence formed after oil was first generated in the basin, late in the Cretaceous or early in the Tertiary. The isotopic composition of oxygen in the dolomite ranges to s ch light values that the cement must have formed deep in the basin in the presence of meteoric water. The gray facies likely formed in a regime of groundwater flow resulting from Laramide uplift of the Front Range during the Tertiary. In our model, saline groundwater flowed eastward through the Pennsylvanian Fountain Formation and then upwelled along the basin axis, where it discharged into the Lyons Sandstone. The saline water mixed with more dilute groundwater in the Lyons, driving a reaction that dissolved calcite and, by a common-ion effect, precipitated dolomite and anhydrite. The facies' gray color resulted from reduction of ferric oxide in the presence of migrating oil or the Fountain brine. Underlying source beds by this time had begun to generate petroleum, which migrated by buoyancy into the Lyons. The association of the gray facies with petroleum accumulations can be explai ed if the Fountain brines discharged across aquitards along the same fractures that transmitted oil. As petroleum accumulated in the Lyons, the newly formed cements prevented continued migration, as is observed in shallower strata, by sealing oil into the reservoirs from which it is produced today.

Ferrihydrite, Lepidocrocite, and Goethite in Coatings from East Texas Vertic Soils

AbstractIron oxide coatings from ped surfaces and pores of three rice paddy soils and one non‐paddy soil near Beaumont, TX, provided samples for studying Fe oxide mineralogy and the relationship of extractable Fe to extractable Al, Si, and P. Analyses of Fe oxide coatings, concentrated by a combination of scraping, sonication, centrifugation, and magnetic separation, showed that these Fe oxide coatings cement clay minerals and quartz particles together. Iron oxide coatings in all soils contained lepidocrocite and smaller amounts of goethite. An improved differential x‐ray diffraction (DXRD) method, in situ DXRD on a nonreflecting quartz plate, allowed the identification of ferrihydrite, which is otherwise difficult to identify because of its poor crystallinity and low concentration. Most of the ferrihydrite was dissolved by 10 min in pH 3 ammonium oxalate in the dark (AOD). Sequential 10‐, 50‐, and 180‐min AOD treatments extracted lepidocrocite of progressively increasing particle size. The 180‐min AOD treatment extracted minimal amounts of goethite. The DXRD and chemical data indicate that the paddy soils have more ferrihydrite in relation to total Fe oxides than the non‐paddy soil. The P/Fe ratio decreased with each AOD treatment step for all soils. This study demonstrates that physical separation and in situ AOD treatment can be used with the DXRD method to identify ferrihydrite, lepidocrocite, and goethite in soil clays containing extractable Fe concentrations as low as 10 to 44 g/kg. We concluded that a short incremental AOD treatment (≈10–30 min) is a better approach to DXRD of ferrihydrite in &lt;2‐µm fractions of seasonally reduced soils than the much longer treatment recommended in the past for soils.

The engineering geology of the Lower Lias Clay at Blockley, Gloucestershire, UK

The Lower Lias Clay at Blockley, 20 km due south of Stratford-upon-Avon, UK, is thought to be representative of this clay formation over much of the Severn Basin. As far as the mineralogy of the clay is concerned, illite is the dominant clay mineral, kaolinite being subsidiary, with quartz, calcite, pyrite and chlorite/vermiculite present in subsidiary or accessory amounts. Weathering changes the mineralogy, with illite being degraded and calcite and pyrite being removed. Furthermore free iron oxide coatings become important as a result of weathering, with the maximum concentrations being present in the highly weathered material. The unweathered clay possesses a preferred orientation associated with turbostratic structures. At certain horizons microfolding and remoulding occurs in the unweathered clay and silty layers are displaced. Weathering has progressively destroyed the micro-structures present in the weathered clay horizons. The fissures in the Lower Lias Clay occur in five uniplanar sets. In the unweathered clay the intensity is fairly constant but it increases in a narrow zone just below the weathered horizons. The number of curved fissures also increases at this depth. This intense fissuring continues into the lowest zone of weathered clay where weathering is confined to the fissure surfaces. Fissures become less apparent in the more weathered horizons as more and more degradation has taken place. However, new smaller fissure systems are developed in the weathered clay, as are desiccation cracks. Engineering index properties and values of shear strength are given. Relationships between moisture content and strength, liquid limit and iron (Fe) content were observed, and a relationship between weathering zones and the shear strength-depth curve was established.

Lead isotopes in iron and manganese oxide coatings and their use as an exploration guide for concealed mineralization

Lead isotopes from Fe and Mn oxides that coat stream pebbles from around the Mount Emmons porphyry molybdenum deposit in Colorado were studied to assess the feasibility of using Pb isotopes to detect concealed mineral deposits. The Fe/Mn oxide coatings were analyzed to determine their elemental concentrations using ICP-AES. The Pb isotope compositions of solutions from a selected suite of samples were measured, using both thermal ionization and ICP mass spectrometry, to compare results determined by the two analytical methods. Heavy mineral concentrates from the same sites were also analyzed to compare the Pb isotope compositions of the Fe/Mn coatings with those found in panned concentrates. The Fe/Mn and 206Pb/ 204Pb ratios of the oxide coatings are related to the lithology of the host rocks; Fe/Mn oxide coatings on pebbles of black shale have higher Fe/Mn values than do the coatings on either sandstone or igneous rocks. The shale host rocks have a more radiogenic signature (e.g. higher 206Pb/ 204Pb) than the sandstone or igneous host rocks. The Pb isotope data from sandstone and igneous hosts can detect concealed mineralized rock on both a regional and local scale, even though there are contributions from: (1) metals from the main-stage molybdenite ore deposit; (2) metals from the phyllic alteration zone which has a more radiogenic Pb isotope signature reflecting hydrothermal leaching of Pb from the Mancos Shale; (3) Pb-rich base metal veins with a highly variable Pb isotope signature; and (4) sedimentary country rocks which have a more radiogenic Pb isotope signature. An investigation of within-stream variation shows that the Pb isotope signature of the molybdenite ore zone is retained in the Fe/Mn oxide coatings and is not camouflaged by contributions from Pb-rich base-metal veins that crop out upstream. In another traverse, the Pb isotope data from Fe/Mn oxide coatings reflect a complex mixing of Pb from the molybdenite ore zone and its hornfels margin, Pb-rich base-metal veins, and sedimentary country rocks. Stream-sediment anomalies detected using oxalic acid leaches can be evaluated using Pb isotope analysesof selected geochemical anomalies. Such an evaluation procedure, given regional target Pb isotope signatures for concealed mineralization, can greatly reduce the cost of exploration for undiscovered ore deposits concealed beneath barren overburden. Lead isotope measurements on aliquots of the same solutions showed that ICP-MS determinations are of low precision and vary non-systematically when compared with the Pb isotope values of the higher precision thermal ionization method. These variations and lower precision of the ICP-MS measurements are attributed to matrix effects.