The Coles Hill uranium deposit, with an indicated resource of about 130 Mlb of U3O8, is the largest unmined uranium deposit in the United States. The deposit is hosted in the Taconian (approx. 480–450 Ma) Martinsville igneous complex, which consists of the Ordovician Leatherwood Granite (granodiorite) and the Silurian Rich Acres Formation (diorite). The host rock was metamorphosed to orthogneiss during the Alleghanian orogeny (approx. 325–260 Ma), when it also underwent dextral strike-slip movement along the Brookneal shear zone. During the Triassic, extensional tectonics led to the development of the Dan River Basin that lies east of Coles Hill. The mineralized zone is hosted in brittle structures in the footwall of the Triassic Chatham fault that forms the western edge of the basin. Within brittle fracture zones, uranium silicate and uranium-bearing fluorapatite with traces of brannerite form veins and breccia-fill with chlorite, quartz, titanium oxide, pyrite, and calcite. Uranium silicates also coat and replace primary titanite, zircon, ilmenite, and sulfides. Sodium metasomatism preceded and accompanied uranium mineralization, pervasively altering host rock and forming albite from primary feldspar, depositing limpid albite rims on igneous feldspar, altering titanite to titanium oxide and calcite, and forming riebeckite. Various geothermometers indicate temperatures of less than ~200°C during mineralization. In situ U-Pb analyses of titanite, Ti-oxide, and apatite, along with Rb/Sr and U/Pb isotope systematics of whole-rock samples, resolve the timing of geologic processes affecting Coles Hill. The host Leatherwood Granite containing primary euhedral titanite is dated at 450 to 445 Ma, in agreement with previously obtained ages from zircon in the Martinsville igneous complex. A regional metamorphic event at 330 to 310 Ma formed anhedral titanite and some apatite, reequilibrated whole-rock Rb/Sr and U-Pb isotopes, and is interpreted to have coincided with movement along the Brookneal shear zone. During shearing and metamorphism, primary refractory uranium-bearing minerals including titanite, zircon, and uranothorite were recrystallized, and uranium was liberated and mixed locally with hematite, clay, and other fine-grained minerals. Uranium mineralization was accompanied by a metasomatic episode between 250 and 200 Ma that reset the Rb-Sr and U-Pb isotope systems and formed titanium oxide and apatite that are associated and, in places, intimately intergrown with uranium silicate dating mineralization. This event coincides with rifting that formed the Dan River Basin and was a precursor to the breakup of Pangea. The orientation of late-stage tectonic stylolites is compatible with their formation during Late Triassic to Early Jurassic basin inversion, postdating the main stage of uranium mineralization and effectively dating mineralization as Mesozoic. Based on the close spatial and temporal association of uranium with apatite, we propose that uranium was carried as a uranyl-phosphate complex. Uranium was locally reduced by coupled redox reactions with ferrous iron and sulfide minerals in the host rock, forming uranium silicates. The release of calcium during sodium metasomatic alteration of primary calcic feldspar and titanite in the host rock initiated successive reactions in which uranium and phosphate in mineralizing fluids combined with calcium to form U-enriched fluorapatite. Based on the deposit mineralogy, oxygen isotope geochemistry, and trace element characteristics of uranium silicate and gangue minerals, the primary mineralizing fluids likely included connate and/or meteoric water sourced from the adjacent Dan River Basin. High heat flow related to Mesozoic rifting may have driven these (P-Na-F-rich) fluids through local aquifers and into basin margin faults, transporting uranium from the basin or mobilizing uranium from previously formed U minerals in the Brookneal shear zone, or from U-enriched older basement rock.

High-U hydrothermal apatite with complex U-Pb systematics is closely spatially associated with mineralization at the Coles Hill deposit, the largest unmined uranium deposit known in the United States. The deposit is hosted in metasomatized rocks of the 450- to 430-Ma-old Martinsville Intrusive Complex in south-central Virginia. Direct dating of metamict uranium-ore minerals, mostly coffinite, is not possible due to open-system radon loss. Instead, U-Pb isotopes in cogenetic apatite were investigated as a means of evaluating the age of mineralization. Here we report in situ electron probe microanalyses (EPMA) of coffinite, isotope-dilution thermal-ionization mass spectrometry (ID-TIMS) U-Pb data for mineralized whole rock samples, and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) U-Pb isotope data for apatite in both unmineralized and U-mineralized host rocks. Massive deficits in radiogenic Pb preclude reliable U-Pb "chemical ages" calculated from EPMA data obtained from coffinite. In contrast, LA-ICPMS data for secondary apatite in unmineralized rocks indicate low-U concentrations (10(6)-10(2) ppm), "normal" (consistent with models of terrestrial Pb isotopic evolution) initial Pb isotope compositions, and U-Pb age estimates of similar to 330 Ma, which is consistent with dates previously proposed for the regional Paleozoic shear zone that hosts the deposit. Ore-stage apatite associated with coffinite has high-U concentrations (typically 10(2)-10(3) ppm but up to 2.4 wt% U) and large excesses of Pb-206 (Pb-207/Pb-206 < 0.01) unsupported by in situ U decay. Data show that initial Pb had variable isotopic compositions including both "normal" Pb derived from host rocks and Pb-206-enriched Pb introduced by secondary metasomatic fluids. Evaluation of the complex evolution and mixing of Pb sources has broader implications for UPb dating of hydrothermal apatite. Excess Pb-206 in apatite is derived from decay products of Rn-222 lost from coffinite and mobilized by Na-, P-, and U-enriched metasomatic fluids during the main mineralizing event at similar to 230 Ma. Ore-stage alteration did not uniformly reset the U-Pb systematics in host rocks precluding a well-constrained whole-rock isochron age. However, whole-rock isotope data imply U mobility at similar to 200-220 Ma and support a Triassic age for the final stages of mineralization. Results also indicate that apatite with up to several weight percent uranium is able to retain U and its decay products for hundreds of millions of years; an important consideration when assessing this mineral as a potential matrix for long-term storage of radioactive waste.

Contact metamorphism associated with mafic intrusives is one of several mechanisms that has been invoked to produce extensive high‐temperature (HT) metamorphism and associated partial melting of the crust. Indisputable evidence for polymetamorphism in these settings can be difficult to decipher because both melt loss and retrogression (i.e. rehydration) can erase or obscure the records of earlier HT metamorphism by modifying HT mineral parageneses and compositions. Here, a combination of detailed field and petrographical observations, inverse mineral thermometry, and thermodynamic forward modelling is used to delineate the polymetamorphic history of migmatites from the Smith River Allochthon (SRA) in the central Appalachians. Bulk rock geochemical data suggest that some metapelitic samples lost a significant amount of melt during interpreted contact metamorphism with the Rich Acres gabbro, resulting in a residual bulk composition (<50 wt% SiO2, ~30 wt% Al2O3). Garnet cores (Grt1) in SiO2‐depleted samples are interpreted to grow during this HT contact metamorphism, with Fe‐Ti oxide thermometry on spinel inclusions in Grt1, cordierite–garnet thermometry, and thermodynamic forward modelling constraining peak P–T conditions during contact heating of the migmatites to ~800ºC and ∼0.5 GPa. This is associated with an inferred peak assemblage prior to melt loss of crd+kfs+pl+grt+bt+spl (mag+usp+hc)+ilm+sil+qtz+melt. Garnet in SiO2‐depleted samples has a distinct high‐Ca rim (Grt2), which appears to record a younger metamorphic event. A combination of substantial melt loss and later rehydration appears to be a major control on the ability of SiO2‐depleted samples to faithfully record evidence for this polymetamorphism. The tectonic implications of this younger metamorphic event are not entirely clear, but it appears to record renewed burial and heating of the SRA sometime after the Taconic orogeny, which may be related to either the neo‐Acadian or Alleghanian orogenies.

The distribution of pegmatitic rocks in the zoned gabbro-diorite plutons of the late Jurassic Smartville Complex is counter-intuitive. Whereas pegmatitic gabbros are common in mafic cumulate olivine gabbros within the zoned plutons, they appear to be absent from the more evolved gabbros and diorites. We argue that this paradox is resolved by examining the crystallization and temperature history of the rocks in question. The evolved rocks in the plutons consist of twopyroxene hornblende biotite gabbro and diorite. The amphibole in these rocks occurs as a partial replacement of pyroxene that forms as a consequence of the incongruent crystallization reaction generalized as amphibole6quartz6plag1 1/4 pyroxenethornhydrous melt6plag2 [reaction (1)]. This is a vapor-absent reaction that can buffer melt water content during crystallization and conceivably preclude water saturation altogether. Experimental evidence places the onset of the incongruent formation of amphibole at c. 900 degrees C. In contrast, amphibole and other hydrous phases occur only in trace amounts in the cumulate olivine gabbros. The water-buffering reaction is not encountered during crystallization, the intercumulus melt proceeds to water saturation, and high-temperature plagioclase-pyroxene pegmatites form. We argue that the primary role for undercooling favored for the formation of granitic pegmatites is not likely to be applicable to the Smartville gabbro pegmatites. As crystallization of the gabbro pegmatite proceeds, the amphibole-in boundary is eventually reached and incongruent crystallization of amphibole occurs. Nevertheless, once water saturation occurs, it is irreversible, and is eventually manifested by the low-temperature deuteric alteration associated with the pegmatites. In short, we argue that pegmatite formation in the Smartville gabbros, and probably many other gabbroic plutons, reflects water saturation of a magma at high temperature, estimated here at > 900 degrees C. In systems where reaction (1) is attained prior to water saturation, pegmatite formation may be delayed until late in the crystallization history or precluded altogether. MELTS modeling of basalt crystallization suggests a solution to the paradox, indicating that the difference between the two systems can reflect equilibrium versus fractional crystallization. Equilibrium crystallization of trapped, intercumulus melt favors hightemperature crystallization and water saturation and, hence, pegmatite formation. Fractional crystallization, for example in an open-system magma chamber, favors lower-temperature crystallization and permits reaction (1) to proceed, inhibiting water saturation and preventing pegmatite formation. The open-system nature of the fractionating magma chamber might also allow escape of volatiles at its margins, with the lower volatile content then communicated to the magma as a whole as the system convects or otherwise mixes.

Given that secondary magnetite is common in serpentinites, it is clear that serpentinites are oxidized rocks. Questions remain, however, concerning the distribution of ferric iron among magnetite and serpentine minerals and the role of ferric iron-rich serpentine in the formation of secondary magnetite. Direct determination of ferric iron in serpentine is not possible using an electron microprobe. We show, however, that the stoichiometic effects of ferric iron substitutions are detectable, although not quantifiable, by microprobe. First, we demonstrate that for studies that provide both microprobe analyses of major elements of serpentine and Mossbauer analysis of ferric iron, substitution effects are obvious. Next, it is equally clear that the early veins forming at the onset of olivine hydration (type 1 veins) show no indication of the presence of ferric serpentine, although a small amount of ferric brucite' may occur. Finally, we show that secondary (type 2) veins, which form as the system becomes open to fluids in equilibrium with plagioclase or pyroxene, contain, in addition to significant alumina, stoichiometric indications of ferric iron substitution. The serpentine in these veins is magnesian, usually with Mg#s around 96-98. Thus, even if a significant proportion of this iron is ferric, it comprises only a small fraction of the total ferric iron budget of the rock. Given that reduced iron is known to be abundant in early-formed brucite and early-formed serpentine and given that brucite, in particular, is absent from evolved serpentine veins, we propose that most magnetite in serpentinites forms as a tertiary product via oxidation of brucite.

We show that a norite at the contact between a mafic diorite intrusion and a pelitic granulite gneiss formed via a reaction similar to that first postulated by N.L. Bowen in 1928, .e., clinopyroxene (Cpx) + sillimanite = anorthite (An) + orthopyroxene (Opx). Major element, compatible trace element, and isotope chemistry are consistent with a mixed pelite-diorite origin for the norite. The norite consists of bytownite (An(71)) and Opx with minor accessory apatite and oxide minerals. The bytownite contains abundant, optically continuous quartz inclusions. The bytownite and the Opx are interpreted as having formed by reaction between pelite and diorite at the contact. The norite feldspar is notably calcic compared to plagioclase in the local diorite and pelite. Except for a slight Al enrichment, the Opx in the norite has major element chemistry similar to that of the Opx in the local diorite. However, norite Opx and plagioclase (Pl) are significantly enriched in Ga. The Ga and major element compositions of the Opx and bytownite are decoupled from the bulk composition of the norite, suggesting reaction rather than bulk compositional control on phase chemistry. Ga enrichment requires input from an aluminous phase. The presence of cocrystallizing quartz (Qz) and bytownite in the norite suggests that the aluminum source is cordierite in the gneiss via the reaction 5 Pl (An(40)) + 5 Cpx + 2.5 cordierite = 10 Pl (An(72)) + 10 Opx + 2.5 Qz. Enrichments in Y, Sc, and heavy rare earth elements in the Opx are consistent with garnet involvement. However, these elements exist in sufficient quantities in the precursor Cpx to account for abundances in most of the Opx. Although one can postulate a mixing origin for the norite on the basis of whole-rock chemistry, it is only through the analysis of minerals that the decoupling of bulk and phase chemistry can be identified and the phases involved in the assimilation reaction be constrained. The documentation of this classic reaction highlights the importance of reactive processes during assimilation.

Silica-rich granites and rhyolites are components of igneous rock suites found in many tectonic environments, both continental and oceanic. Silica-rich magmas may arise by a range of processes including partial melting, magma mixing, melt extraction from a crystal mush, and fractional crystallization. These processes may result in rocks dominated by quartz and feldspars. Even though their mineralogies are similar, silica-rich rocks retain in their major and trace element geochemical compositions evidence of their petrogenesis. In this paper we examine silica-rich rocks from various tectonic settings, and from their geochemical compositions we identify six groups with distinct origins. Three groups form by differentiation: ferroan alkali-calcic magmas arise by differentiation of tholeiite, magnesian calc-alkalic or calcic magmas form by differentiation of high-Al basalt or andesite, and ferroan peralkaline magmas derive from transitional or alkali basalt. Peraluminous leucogranites form by partial melting of pelitic rocks, and ferroan calc-alkalic rocks by partial melting of tonalite or granodiorite. The final group, the trondhjemites, is derived from basaltic rocks. Trondhjemites include Archean trondhjemites, peraluminous trondhjemites, and oceanic plagiogranites, each with distinct geochemical signatures reflecting their different origins. Volcanic and plutonic silica-rich rocks rarely are exposed together in a single magmatic center. Therefore, in relating extrusive complements to intrusive silica-rich rocks and determining whether they are geochemically identical, comparing rocks formed from the same source rocks by the same process is important; this classification aids in that undertaking.

The magnetization of zircons from sedimentary rocks of the Jack Hills (Yilgarn Craton, Western Australia) provide evidence for a Hadean to Paleoarchean geodynamo, 4.0 to 4.2 billion years old. These magnetizations pass a microconglomerate test, attesting to the fidelity of Jack Hills zircons as recorders of these most ancient magnetic signals. The lack of pervasive remagnetization of the Jack Hills is also documented through a positive conglomerate test conducted on cobble-sized clasts. A key element of the latter test is the preservation of a high unblocking temperature magnetization that can survive peak metamorphic temperatures. Rock magnetic studies suggest the mineral carrier is magnetite. Herein, we investigate the magnetic mineral carriers in cobble samples through scanning electron microscope and microprobe analyses, conduct an inter-laboratory paleomagnetic study to evaluate sensitivities required to evaluate the weak magnetizations carried by the Jack Hills sediments, and assess provenance information constrained by the opaque minerals. These data confirm magnetite as a detrital phase and the presence of high unblocking temperature magnetizations, further supporting the posit that the Jack Hills sediments can preserve primary magnetic signatures. We note that some of these magnetizations are near the measurement resolution of standard cryogenic magnetometers and thus exacting laboratory procedures are required to uncover these signals. In addition to magnetite, the cobbles contain an assemblage of Mg poor Cr–Fe chromites, Ni-sulfides and pyrrhotite that suggest a source in a layered intrusion different from the granitoid source of the zircons. Any Hadean rock fragment in these sediments, if present, remains elusive.

The rich, fossiliferous Triassic sediments exposed in the Virginia Solite Quarry include a 34mm-thick ‘‘insect layer’’ that is notable for detailed preservation of soft-bodied invertebrate and vertebrate remains. We describe this unique KonservatLagerstätte and use sedimentologic and geochemical analyses to interpret the environmental conditions necessary to preserve such delicate fossils. This work is among the first attempts to apply detailed geochemical/stratigraphic analysis to the study of Lagerstätten and we report on a 332-mm-thick section that includes the insect layer and the rocks immediately below and above it. Our analysis successfully constrains various aspects of the depositional and diagenetic history of the Lagerstätte and permits a detailed analysis of changing conditions prior to, during, and after deposition. Geochemical and sedimentologic analyses of the insect layer and P. C. Ragland—deceased. C. M. Liutkus (&) Department of Geology, Appalachian State University, Boone, NC 28607, USA e-mail: liutkuscm@appstate.edu J. S. Beard · N. C. Fraser · P. C. Ragland Virginia Museum of Natural History, Martinsville, VA 24112, USA N. C. Fraser National Museums Scotland, Edinburgh EH1 1JF, UK surrounding lithologies reveal a change from siliciclastic-dominated layers (Unit 1) to dolomite-siliciclastic laminites above (Unit 2 and the insect layer), separated by a boundary dolostone layer that is traceable for over 200 m. We interpret this sedimentary shift as the initial stages in the transgression of a shallow, saline, alkaline rift-basin lake over lake margin deposits. The absence of bioturbation by plants and benthic organisms, as well as a lack of predation on the insects, is not explained by significant water depth, but is instead more reasonably considered a result of the chemistry of the water at the lake margin, affected by groundwater seeps, which provided F-, Mg-, and Ca-rich fluids. Although the initial conditions of preservation are remarkable, it is equally impressive that the fossils survived extensive diagenesis, e.g. dissolution of quartz and coarsening of dolomite.

Serpentinization is an important geochemical process that affects the chemistry and petrophysical properties of the oceanic lithosphere and supports life through abiogenic formation of hydrogen. Here, we document through detailed mineralogical evidence and equilibrium thermodynamic models the importance of water (H2O) and silica (SiO2) activities on mineral assemblages produced during progressive serpentinization of a harzburgite. We describe a harzburgite from the Santa Elena Ophiolite in Costa Rica that is ~30 % serpentinized. Serpentine + brucite ± magnetite veins occur in olivine, Al-rich serpentine + talc veins occur in orthopyroxene, and Al-rich serpentine ± talc ± brucite veins occur at the boundary of orthopyroxene and olivine. Bulk vein chemistry and element distribution maps demonstrate distinct chemical zonations within veins and chemical gradients between orthopyroxene- and olivine-dominated areas. Specifically, the sample records (1) varying brucite composition depending on whether or not it is associated with magnetite, (2) formation of magnetite from Fe-rich brucite (±Fe-rich serpentine) during olivine hydration, where magnetite coexists with brucite Mg#96 and serpentine Mg#99, (3) chemical gradients in Si, Al, Cr, and Ca within and between orthopyroxene- and olivine-hosted veins, and 4) local (different) equilibrium assemblages within different zones of veins. The studied sample preserves rarely observed textures documenting continuous replacement of olivine, rather than individual vein generations and overprinting that is typically observed in more intensely serpentinized peridotites. Furthermore, the presence of a discrete sequence of vein textures and mineralogy allows direct comparison between mineral textures and equilibrium thermodynamic models and permits new insights into mineral reactions during serpentinization.

New U-Pb zircon SHRIMP geochronology confirms that the Coles Hill uranium deposit in Pittsylvania County, Virginia, is hosted within the Late Ordovician to Silurian Martinsville Intrusive Complex. The meta-igneous host rocks at Coles Hill consist of two units of the Martinsville Intrusive Complex: the felsic Leatherwood Granite and the mafic Rich Acres Formation. Two samples of unmineralized Leatherwood Granite orthogneiss yield 206Pb/238U ages between 444.5 ± 2.5 and 447.5 ± 1.9 Ma. A third sample of unmineralized Leatherwood Granite orthogneiss shows a wider range in 206Pb/238U ages, possibly due to Pb loss, and a 206Pb/207Pb age of 452 ± 18 Ma. Unmineralized Rich Acres Formation amphibolite that cuts the Leatherwood gives a mean 206Pb/238U age (426.2 ± 7.0 Ma), slightly younger than the Leatherwood age. Samples of mineralized orthogneiss and mineralized amphibolite give similar 206Pb/207Pb ages of 419 ± 19 and 426 ± 21 Ma, respectively. A biotite gneiss unit that underlies the mineralized zone yields a 206Pb/207Pb age of 415 ± 21 Ma, indicating that it is part of the Martinsville Intrusive Complex and not a member of the early Cambrian Fork Mountain Schist, as has been previously reported. A genetic model for the Coles Hill uranium deposit has not yet been developed, although age constraints indicate that mineralization is either late or postmagmatic, and this is consistent with the epigenetic, fracture-controlled nature of the mineralization. Results obtained here do not preclude either the igneous host rocks (or similar rocks at depth) or the sedimentary units in the adjacent Triassic basin as possible sources for the uranium.

Dunite from New Caledonia displays three types of serpentine-dominated veins. The earliest, type 1 veins are narrow (50–100μm wide) and rarely extend across more than a single olivine grain. They are lizardite, contain abundant brucite and never contain magnetite. Type 2 veins are 0.01 to 0.1mm wide, extend across several olivine grains and cut across the type 1 veins. They are lizardite, contain magnetite, often in vein interiors, and contain less brucite than type 1 veins. Type 3 veins are dominantly chrysotile, cm-scale, have a magnetite-rich core, and extend for meters or more. Analyses of two representative samples indicate that the type 1 veins have relatively Fe-rich serpentine (XMg=0.92) and brucite (XMg=0.82). These minerals are less magnesian than those in the type 2 veins; serpentine has XMg=0.93–0.94 and brucite has XMg=0.84. In the magnetite-rich core to the type 3 vein both serpentine (XMg=0.94–0.97) and one of the two brucite populations (XMg=0.94) are Mg-rich. Opx in harzburgite layers in these samples is cut by serpentine veins that are on the order of 0.05mm wide. The serpentine veins after Opx lack talc or magnetite and, as with veins cutting olivine, the older veins are more Fe rich (XMg=0.84) than the younger veins (XMg=0.90). We conclude that the formation of magnetite was accompanied by the extraction of iron from the early-formed serpentine and brucite.Thermodynamic calculations suggest that the type 1 veins formed in a rock-dominated system where the activities of FeO, MgO, and SiO2 were dictated by the compositions of olivine and orthopyroxene. In contrast the type 2 veins were formed in a more fluid-dominated system where the infiltrating fluid was relatively oxidizing and out of equilibrium with the original brucite–serpentine assemblage. Reduction of this fluid was accompanied by reaction of brucite and serpentine to magnetite and hydrogen. By producing magnetite, this reaction extracted iron from brucite and serpentine, making them both more magnesian. This would drive the brucite–serpentine–magnetite assemblage to higher oxygen fugacity, progressively decreasing the efficiency of the magnetite-forming reactions.