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Hot springs in the Alvord/Pueblo valleys in southeastern Oregon are analogous to Basin-and-Range hydrothermal systems where heat source and permeable pathways are met through crustal thinning. Silica sinter deposition at Mickey Springs, Alvord Valley, predates the late Pleistocene high stand of pluvial Lake Alvord. At Borax Lake, Pueblo Valley, sinter deposition occurred during the Holocene. This study examines the evolution of springs at Mickey Springs, where three morphologies of sinter are present: (1) basalt clasts surrounded by sinter in interbedded conglomerate and sandstone, (2) pool-edge and aprons of sinter surrounding depressions (12–32 m diameter), and (3) quaquaversal sinter mounds with pool-edge sinter. The oldest sinter occurs in silica-cemented conglomerate and sandstone, where deposition occurred prior to 30 kya. Deposition around broad depressions and mounds occurred after 30 kya but before water levels began to rise in pluvial Lake Alvord. Thermoluminescence dates suggest sinter deposition ceased before 18 kya when silt and clay filled inactive vents and buried aprons. A few mounds hosted active springs after sinter deposition ceased but while submerged in pluvial Lake Alvord. Now, high-temperature springs, steam vents, and mud pots are concentrated in a 50 × 50 m area near the southern edge of the spring area.

Molybdenum isotopes in three deep and well-characterized weathering profiles – a saprolite formed on meta-diabase from South Carolina, USA, and two ferruginous bauxites formed on Columbia River Basalts in Oregon and Washington, USA – elucidate Mo isotope behavior during continental weathering. The saprolite records an overall loss of Mo relative to the fresh bedrock, as indicated by negative τMoTi, which defines the loss of Mo relative to the relatively immobile element Ti. The saprolites are also isotopically light: δ98Mo values range from −0.89‰ (relative to NIST 3134) to −0.05‰, mean δ98Mo = −0.40‰, compared to +0.55‰ NIST3134 for the underlying unweathered bedrock. By contrast, the ferruginous bauxites generally record addition of Mo relative to the fresh bedrock (zero to positive τMoTi) and generally have higher δ98Mo values than the parental basalts: δ98Mo of the bauxites range from −0.14‰ to +0.38‰ compared to −0.33‰ and +0.02‰ for the unweathered parental basalt. Low δ98Mo values in the saprolites likely reflect preferential retention of isotopically light Mo adsorbed onto accessory Fe-oxy-hydroxides and clays during weathering, whereas the high δ98Mo values in the bauxites reflect the addition of isotopically heavy Mo from groundwater. When the three profiles are combined, there is a positive correlation between τMoTi and δ98Mo, suggesting that when Mo is lost during continental weathering, the resulting regolith is isotopically light, whereas groundwater addition can shift the regolith to heavier values. Because saprolites are a more common weathering product than bauxites, we conclude that, in general, continental weathering fractionates Mo isotopes such that the weathered upper crust retains isotopically light Mo. In contrast, the groundwater that leaches Mo from the weathered crust is isotopically heavy. Thus, chemical weathering of continents generates the isotopically heavy riverine signature observed globally, and partially contributes to the isotopically heavy seawater signature. Finally, these data, in conjunction with previously published data for glacial diamictites, can be used to assess changes in the crustal Mo isotope signature over the last 2.9 Ga.

Magnesium isotopes have been shown to fractionate significantly during continental weathering, however, the degree and direction of fractionation varies from one study to the next, and the main factors that control Mg isotope fractionation during weathering have yet to be delineated. Here, we report Mg contents and isotopic compositions for two ∼10 m deep drill cores through bauxites developed on Columbia River Basalts (CRBs). Samples from these two drill cores have very low MgO contents (0.12–0.25 wt.%) and variable but high δ26Mg values (from −0.1 to up to +1.8, the heaviest isotopic composition ever reported for Mg) relative to the average values of fresh CRBs (δ26Mg = −0.23 ± 0.07 and MgO = 5.9 wt.%). These features reflect the near complete loss of Mg in the isotopically heavy regolith. The most highly weathered bauxites at the tops of the profiles have the lowest δ26Mg values, reflecting the addition of isotopically light eolian dust. Excluding these samples, δ26Mg in bauxites displays a weak, positive correlation with gibbsite abundance, suggesting that gibbsite preferentially retains 26Mg in the bauxites. The integrated Mg isotopic fractionation factors between bauxite and fluid (Δ26Mgbauxite-fluid), inferred from Rayleigh fractionation calculations, vary from 0.05‰ to 0.4‰, which is similar to the range inferred from previous studies of igneous rock weathering profiles, though none of these previous studies found the extremely heavy δ26Mg and massive Mg depletion seen here. Collectively, studies of weathering profiles suggest loss of isotopically light Mg from the continents, which must contribute to the very light δ26Mg seen in river water and seawater.

The Oregon-Idaho graben is a newly identified north-south–trending synvolcanic graben in southeastern Oregon and southwestern Idaho within the middle Miocene backarc rift system that extends 1100 km from southern Nevada to southeastern Washington. The graben formed along the western margin of the North American craton shortly after the largest volumes of tholeiitic flood basalt erupted (Columbia River Basalt Group, Steens-Pueblo Basalt, basalt of Malheur Gorge, basalt and latite unit of [Ekren et al., 1981][1]). Rhyolite flows and ash-flow tuffs (16.1–14.0 Ma) erupted from northeastern Oregon (Dooley Volcanics) to northern Nevada (McDermitt volcanic field) shortly after the flood basalt was emplaced. Subsidence of the Oregon-Idaho graben (15.5–15.3 Ma) coincides with eruption of rhyolite flows and caldera-related ash-flow tuffs from vents along the margins and within the graben. Mafic and silicic intragraben volcanism accompanied sedimentation from about 15.3 to 10.5 Ma. Sedimentary and volcanic rocks from extrabasinal sources, especially southwestern Idaho, were introduced periodically. After initial subsidence, the evolution of the Oregon-Idaho graben is divided into three stages. Stage 1 (15.3–14.3 Ma) followed intragraben caldera collapse and was marked by deposition of fluvial and lacustrine sediment across the graben. Stage 2 (14.3–12.6 Ma) movement on intragraben fault zones divided the graben into distinct subbasins and marked the onset of calc-alkalic volcanism. Fine-grained tuffaceous sediment derived from glassy rhyolite and pyroclastic deposits and basalt tuff cones interbedded with rhyolite ash and lapilli-fall deposits and locally erupted basalt hydrovolcanic deposits predominated during synvolcanic subsidence. Synsedimentary hot-spring alteration and precious-metals mineralization of graben fill were controlled by the same intragraben fault zones that served as magmatic conduits. During stage 3 (12.6–10.5 Ma) the subbasins were filled, and graben-wide fluviatile and lacustrine sedimentation resumed. At about the same time, renewed rhyolitic volcanism occurred on both flanks, and tholeiitic volcanism resumed within the Oregon-Idaho graben. Subsidence in the Oregon-Idaho graben ceased as west-northwest–striking faults related to the formation of the western Snake River plain became active. [1]: #ref-28

Mineralogical, chemical, and Li and Nd isotopic compositions of two drill cores (8-9m deep) through bauxites developed on the Miocene Columbia River Basalts document the changes associated with basalt weathering, provide insights into the processes involved, and allow us to examine the overall influence of chemical weathering on juvenile (basaltic) crust. Gibbsite, hematite, ±kaolinite, halloysite, goethite, and maghemite are the weathering products in the bauxites. Quartz is observed near the tops of the cores and its abundance decreases progressively with depth; no quartz is observed below five meters depth in either core. Most major and trace elements, including "mobile" and some "immobile" elements are severely depleted in the bauxites. Niobium is less mobile relative to the rare earth elements, thus chemical weathering attenuates the negative Nb anomaly in the continental crust. Li and Nd are strongly depleted relative to fresh basalt, and both increase systematically towards the surface in the quartz-bearing samples while δ7Li and εNd values decrease systematically towards the surface in these same samples. Both Li and Nd were likely lost from the bauxites through leaching. The systematic enrichment of Li, Nd, and quartz, as well as the less radiogenic Nd isotopic composition at the tops of both profiles reflects 20-60wt.% addition of an eolian component to the soils. The eolian dust is unlikely to have experienced significant post-depositional weathering due to the relatively high Li contents near the tops of the profiles, and, therefore, the low δ7Li and εNd values suggest that the dust came from an old, weathered region of the continent. Our results demonstrate that lithium isotopes are sensitive tracers of chemical weathering, particularly in extreme weathering settings, and support the hypothesis that chemical weathering influences the mass and composition of the continental crust. © 2013 Elsevier Ltd.

Two volcanic sequences bounded by erosional unconformities compose the stratigraphy of the North Santiam mining district, Western Cascade Range, Oregon. Diorite, grandodiorite, and leucocratic quartz porphyry dikes, stocks, and sills intrude the breccias, flows, and tuffs of a volcanic center in the older Sardine Formation. Tourmaline-bearing breccia pipes are associated with the porphyritic granodiorite intrusions. An erosional unconformity separates the Sardine Formation from the overlying Elk Lake formation. The alteration patterns in the two formations are consistent with the development of hydrothermal systems during the eruption of each formation. However, the development of the two hydrothermal systems is separated by a period of erosion of the older volcanic pile. Early formation of mineralization that resembles porphyry copper deposits occurred within the Sardine Formation, and later, after eruption of the Elk Lake formation, epithermal veins and alteration developed along faults, fractures, and the margins of dikes in the Sardine Formation.

Opal-CT-rich fillings in tree molds and interstitial to breccia clasts in flow-top breccia reduce porosity and contain textures indicative of colloidal deposition. The opal-CT occurs in uniformly textured layers and porous laminated layers of particles and agglutinates cemented by chalcedony. Celadonite, smectite, clinoptilolite, quartz, chalcedony and minor calcite are associated with the opal-CT. These minerals occur in two settings: (1) within interstitial spaces between particles of opal-CT; and (2) within particles and agglutinates of opal-CT. Concentrations of trace and major elements are closely related to the textural setting of the secondary minerals within the layers of opal-CT. Layers of opal-CT that are enriched relative to other layers in REE and Th are red-brown in color, uniformly cryptocrystalline with some microcrystallinity, and lack secondary minerals interstitial to opal-CT particles.

The Anderson and Canyon coal seams are the main seams at the Antelope coal field in the south-central Powder River Basin. Each seam splits into multiple and thinner coal seams to the southwest in the study area. The strata underlying the Canyon coal seam suggest that the area of the Antelope coal field was a poorly drained swamp that developed into a well-drained swamp with minor small channels. The area further changed to a poorly drained swamp which initiated peat accumulation in the Canyon coal swamp. The lithologies, sedimentary structures, and isopach maps of the Canyon seam partings indicate that crevasse splaying, lacustrine conditions, and small crevasse-channel development produced the observed splits in the Canyon coal seam. Distal overbank deposits occur at the top of the C nyon seam and at the base of the Anderson seam; well-drained swamp deposits and crevasse splay, lake, lacustrine delta, and small channel-fill deposits occur between the coal seams. The Anderson coal seam represents another development of a poorly drained swamp where peat accumulation was significant. The rocks overlying the Anderson seam represent a combination of the environments mentioned above, with deposits from lakes and well-drained swamp environments dominating. The coal-beating sediments were deposited in paludal and tributary subsystems along the northern margin of the Wright delta. Deposition of the Canyon and Anderson coal seams and associated parting lithologies may have occurred when a river system or parts of that system underwent anastomosis. The coal swamp in which the Anderson seam was deposited was destroyed when the anastomosed character of the river system ended or the anastomosed river system shifted westward. This may reflect a relative change in the rate of subsidence within the basin.

The Eau Claire River complex is a metamorphosed differentiated mafic intrusion within the Chippewa amphibolite complex of western Wisconsin. The intrusion is layered and contains ultramafic, mafic, and feldspathic rocks; however, the internal stratigraphy and size of the complex are not known. Primary igneous structures, other than compositional layering, are rarely preserved, although autoliths and possible slump structures are exposed at Big Falls County Park. Primary igneous mineral textures are not preserved. Three metamorphic episodes are recognized and are characterized by distinct structures and mineral textures. Coarse garnet porphyroblasts, as much as 6 cm in diameter, are preserved from the mineral assemblage that formed under conditions of the upper amphibolite facies during the first metamorphism. Alternating hornblende-rich and plagioclase-rich compositional layers were isoclinally folded by associated deformation. These folded compositional layers are displaced across faults that were recrystallized and healed during the second metamorphism. During the second metamorphism, the complex was recrystallized to medium- to coarse-grained plagioclase and hornblende-bearing assemblages under conditions of the amphibolite facies. Compression perpendicular to compositional layering wrapped layering about older garnet porphyroblasts and produced zones parallel to compositional layering of rootless intrafolial folds and transposed foliation. The third metamorphism, under conditions of the green-schist to lower amphibolite facies, resulted in a partial recrystallization. A weak schistosity developed oblique to compositional layering. The age of the complex is uncertain. A uranium-lead age determination of 1,835 m.y. for a rutile sample is believed to be the age of the second metamorphism, the Penokean orogeny. The first deformation and metamorphism and magmatic crystallization may have occurred during the late Archean or very early Proterozoic.