Ba-rich minerals are frequently observed in epithermal environments and include characteristic phases such as barite and alunite supergroup minerals. At Yellowstone, electron microprobe analysis shows that Ba in the unaltered third-cycle Tuff of Sulphur Creek is largely contained within sanidine phenocrysts (mean 1.60 wt% BaO) with lesser concentrations in plagioclase (mean 0.22 wt% BaO) and volcanic glass (mean 0.05 wt% BaO). Whole-rock XRD analyses of rocks hydrothermally altered by alkaline-chloride fluids at Ridge 7741 in Seven Mile Hole, Yellowstone National Park, show they are dominated by illite + quartz +/- hydrothermal feldspar, primarily adularia. In this alteration zone, adularia is the principal phase that contains significant Ba (mean 0.43 wt% BaO). In shallower alteration, dominated by acid-sulfate assemblages, such as kaolinite + opaline silica +/- alunite supergroup minerals (alunite, walthierite, huangite) +/- barite, Ba is sequestered in the sulfate minerals. Alunite supergroup minerals (mean 1.12 wt% BaO) are more prevalent than barite and are largely found from the modern valley rim to about 60 m below the modern surface, especially around the South Fork of Sulphur Creek. However, nearly 80 m below the modern rim of the Grand Canyon of the Yellowstone River, in areas previously altered by alkaline-chloride fluids, adularia altered to alunite supergroup minerals may contain similar to slightly elevated Ba concentrations relative to the replaced grain. Barite is primarily found sporadically in altered rocks along the valley rim of the South Fork of Sulphur Creek, with rare occurrences along the rim of the Grand Canyon. Despite the hydrothermal alteration, whole-rock XRF and ICP-MS analyses show similar mean concentrations between unaltered (0.11 wt% BaO) and altered (0.09 wt% BaO) Tuff of Sulphur Creek samples. Hydrothermally altered rocks are important sources of Yellowstone low-delta 18O rhyolites, like the Tuff of Sulphur Creek, which inherits their low delta 18O signal from them. Cenozoic rhyolites throughout the North American Cordillera tend to exhibit high Ba concentrations, including the low-delta 18O Yellowstone rhyolites. This work shows that hydrothermal alteration mobilizes Ba in volcanic units with minimal dispersion of Ba out of that unit. The genesis of similar silicic volcanic rocks with elevated Ba, relative to mean upper crustal concentrations, may be the result of partial melting of hydrothermally altered rock.

Conductive heat transfer dominates surface heat flux in non-steaming geothermal areas. Despite this, many scientists have hypothesized that conductive heat flux is negligible when compared to other mechanisms such as advection from surface manifestations. Perhaps this misconception lies in the fact that conductive heat flux is difficult to measure without thermal gradients acquired by costly drilling of gradient wells and is, therefore, not well constrained in many areas. In places such as Yellowstone National Park where drilling is prohibited, it can be virtually impossible to measure directly. By employing techniques of dimensional analysis applied to specific boundary conditions, we constrain the conductive heat flux in the Morning Mist Springs area of Lower Geyser Basin, Yellowstone National Park and show this flux to be non-negligible as compared to advective heat flux from the nearby springs.

The “PLUTONS: Investigating the Relationship between Pluton Growth and Volcanism in the Central Andes” themed issue of Geosphere is dedicated to the memory of Todd Christian Feeley, our friend, colleague, and mentor. He distinguished his career with a long string of insightful papers that integrated the geology and petrology of volcanic rocks. His work was always soundly based in field geology, and his coworkers remember him for his outstanding skills in the field.

The carbonatite dike swarm and vein stockwork at the center of the Paleogene Bear Lodge alkaline complex (BLAC), Wyoming, USA, is host to diverse REE mineral assemblages that are largely a result of subsolidus modification and REE redistribution. Pseudomorphic replacement of primary burbankite by an assemblage of ancylite, strontianite, and barite is the result of interaction with late-stage hydrothermal fluids that added Sr, Ba, S, F, and REE, analogous to the replacement processes described for some carbonatite complexes of Russia's Kola Peninsula. Carbon and oxygen stable isotope ratios indicate that the primary carbonatite mineralogy experienced degassing/pneumatolysis and alteration by fluids of variable temperature, CO2/H2O ratios, and/or meteoric water content. Isotopic differences of matrix calcite between Group 1 carbonatites (avg.delta C-13 = -7.3 parts per thousand;delta O-18 = 9.1 parts per thousand) and Group 2 carbonatites (avg.delta C-13 = -9.9 parts per thousand;delta O-18 = 10.2 parts per thousand) are consistent with loss of CO2 during degassing. The open-system alteration of burbankite caused a pronounced positive delta O-18-shift in bulk ancylite pseudomorphs (delta O-1(8): 14.3-25.7 parts per thousand) relative to matrix calcite (delta O-1(8): 8.7-11.2 parts per thousand) Oxygen isotope compositions of biotite (delta O-1(8): 4.5-5.9 parts per thousand) and K-feldspar (delta O-1(8): 7.3-7.9 parts per thousand) in unoxidized carbonatite are typical of primary magmatic silicates and suggest that fluids responsible for the burbankite-to-ancylite conversion remained predominantly magmatic (carbohydrothermal). Concomitant increases toward the surface in C-13 and O-1(8), oxidation, matrix carbonate dissolution, and the replacement of REE carbonates (ancylite, carbocernaite, and burbankite) by Ca-REE fluorocarbonates (basnasite, parisite, synchysite) suggest interaction with late-stage, low temperature (<250 degrees C) fluids characterized by lower CO2/H2O ratios, and an increasing meteoric water component. The first 40Ar/39Ar ages from carbonatite-hosted biotite and K-feldspar at the BLAC are between 51.45 +/- 0.08 and 51.89 +/- 0.14 Ma. Although carbonatite is commonly observed as the final intrusive phase in alkaline igneous complexes, relative-age relationships and previously published geochronology for Bear Lodge rocks indicate that alkaline silicate magmatism both preceded and followed carbonatite emplacement. Published by Elsevier B.V.

Actual measurements of advective heat flux from Yellowstone hot springs (Wyoming, USA) are seldom made, due to the difficulty of obtaining mass flow rates to support such measurements. Yet such measurements would provide important information that can be used to help evaluate the total thermal heat transport associated with the Yellowstone Caldera. Typically, discharge from thermal springs migrates through the shallow subsurface, making accurate measurement problematic. Here we present direct measurements of mass and thermal discharge from hot springs in the Lower Geyser Basin of Yellowstone National Park, USA. We added small amounts of nearly pure D2O to four springs in the Morning Mist Springs area that ranged in temperature from 74 to 95 degrees C and analyzed time-series delta D samples to determine the volumes and discharge rates of the test springs. D2O was chosen to limit the ecological and/or visual impacts of other common tracers, such as NaCI or fluorescein dyes. We calculated spring volumes to range between 560 and 27,400 L and estimated mass and heat discharge as 0.08-1.25 L/s and 0.0189-0.312 MW, respectively. The volumes calculated by deuterium doping were larger in every case than those estimated by field inspection, suggesting that the volume participating in shallow fluid circulation is generally larger than is apparent from the surface. The heat flow data, when paired with conductive heat loss estimates in the vicinity of the springs, suggest that current estimates of thermal discharge at Yellowstone may underestimate heat loss from the caldera and offer insights on the rate of magma supplied by the mantle. Thermal flux estimates suggest that a minimum of 3.2-6.3 km(3) x 10(-2) of basalt magma enters the base of the crust annually.

Gold-silver mineralization is hosted by quartz-carbonate veins that cut sericitized and pyritized volcaniclastic rocks. Six stages of veining are defined, with Au-Ag mineralization (electrum) focused in stages III-V: stage I-III veins consist of quartz with minor carbonate, chlorite, sericite, and sulfide minerals; stage IV quartz veins contain more abundant base metal sulfides (pyrite, sphalerite, and galena with minor sulfosalts); stage V veins are dominantly calcitic; late post-mineralization stage VI veins are quartz-calcite and contain sparse pyrite and chlorite, but no electrum. The veins are interpreted to be syn- to late-tectonic, with deformation decreasing from locally penetrative in the host-rocks prior to veining (resulting in local foliation of sericite and pressure shadows around pyrite), to dismemberment and shearing of early stage I-II quartz veins, brittle disruption of stage II-IV quartz and stage V carbonate veins, and minimal deformation of post-mineralization stage VI quartz-carbonate veins. A much younger set of extensional muscovite veins locally cuts the deposit with Late Cretaceous apparent ages, and appears to be associated with a weak thermal overprint that has reset K-Ar and 40Ar/39Ar ages in sericite throughout the district. Gold-silver mineralization is hosted by quartz-carbonate veins that cut sericitized and pyritized volcaniclastic rocks. Six stages of veining are defined, with Au-Ag mineralization (electrum) focused in stages III-V: stage I-III veins consist of quartz with minor carbonate, chlorite, sericite, and sulfide minerals; stage IV quartz veins contain more abundant base metal sulfides (pyrite, sphalerite, and galena with minor sulfosalts); stage V veins are dominantly calcitic; late post-mineralization stage VI veins are quartz-calcite and contain sparse pyrite and chlorite, but no electrum. The veins are interpreted to be syn- to late-tectonic, with deformation decreasing from locally penetrative in the host-rocks prior to veining (resulting in local foliation of sericite and pressure shadows around pyrite), to dismemberment and shearing of early stage I-II quartz veins, brittle disruption of stage II-IV quartz and stage V carbonate veins, and minimal deformation of post-mineralization stage VI quartz-carbonate veins. A much younger set of extensional muscovite veins locally cuts the deposit with Late Cretaceous apparent ages, and appears to be associated with a weak thermal overprint that has reset K-Ar and 40Ar/39Ar ages in sericite throughout the district. Gold-silver mineralization is hosted by quartz-carbonate veins that cut sericitized and pyritized volcaniclastic rocks. Six stages of veining are defined, with Au-Ag mineralization (electrum) focused in stages III-V: stage I-III veins consist of quartz with minor carbonate, chlorite, sericite, and sulfide minerals; stage IV quartz veins contain more abundant base metal sulfides (pyrite, sphalerite, and galena with minor sulfosalts); stage V veins are dominantly calcitic; late post-mineralization stage VI veins are quartz-calcite and contain sparse pyrite and chlorite, but no electrum. The veins are interpreted to be syn- to late-tectonic, with deformation decreasing from locally penetrative in the host-rocks prior to veining (resulting in local foliation of sericite and pressure shadows around pyrite), to dismemberment and shearing of early stage I-II quartz veins, brittle disruption of stage II-IV quartz and stage V carbonate veins, and minimal deformation of post-mineralization stage VI quartz-carbonate veins. A much younger set of extensional muscovite veins locally cuts the deposit with Late Cretaceous apparent ages, and appears to be associated with a weak thermal overprint that has reset K-Ar and 40Ar/39Ar ages in sericite throughout the district. Gold-silver mineralization is hosted by quartz-carbonate veins that cut sericitized and pyritized volcaniclastic rocks. Six stages of veining are defined, with Au-Ag mineralization (electrum) focused in stages III-V: stage I-III veins consist of quartz with minor carbonate, chlorite, sericite, and sulfide minerals; stage IV quartz veins contain more abundant base metal sulfides (pyrite, sphalerite, and galena with minor sulfosalts); stage V veins are dominantly calcitic; late post-mineralization stage VI veins are quartz-calcite and contain sparse pyrite and chlorite, but no electrum. The veins are interpreted to be syn- to late-tectonic, with deformation decreasing from locally penetrative in the host-rocks prior to veining (resulting in local foliation of sericite and pressure shadows around pyrite), to dismemberment and shearing of early stage I-II quartz veins, brittle disruption of stage II-IV quartz and stage V carbonate veins, and minimal deformation of post-mineralization stage VI quartz-carbonate veins. A much younger set of extensional muscovite veins locally cuts the deposit with Late Cretaceous apparent ages, and appears to be associated with a weak thermal overprint that has reset K-Ar and 40Ar/39Ar ages in sericite throughout the district. Gold-silver mineralization is hosted by quartz-carbonate veins that cut sericitized and pyritized volcaniclastic rocks. Six stages of veining are defined, with Au-Ag mineralization (electrum) focused in stages III-V: stage I-III veins consist of quartz with minor carbonate, chlorite, sericite, and sulfide minerals; stage IV quartz veins contain more abundant base metal sulfides (pyrite, sphalerite, and galena with minor sulfosalts); stage V veins are dominantly calcitic; late post-mineralization stage VI veins are quartz-calcite and contain sparse pyrite and chlorite, but no electrum. The veins are interpreted to be syn- to late-tectonic, with deformation decreasing from locally penetrative in the host-rocks prior to veining (resulting in local foliation of sericite and pressure shadows around pyrite), to dismemberment and shearing of early stage I-II quartz veins, brittle disruption of stage II-IV quartz and stage V carbonate veins, and minimal deformation of post-mineralization stage VI quartz-carbonate veins. A much younger set of extensional muscovite veins locally cuts the deposit with Late Cretaceous apparent ages, and appears to be associated with a weak thermal overprint that has reset K-Ar and 40Ar/39Ar ages in sericite throughout the district.

The Eocene (ca. 55-38 Ma) Bear Lodge alkaline complex in the northern Black Hills region of northeastern Wyoming (USA) is host to stockwork-style carbonatite dikes and veins with high concentrations of rare earth elements (e.g., La: 4140-21000 ppm, Ce: 9220-35800 ppm, Nd: 4800-13900 ppm). The central carbonatite dike swarm is characterized by zones of variable REE content, with peripheral zones enriched in HREE including yttrium. The principle REE-bearing phases in unoxidized carbonatite are ancylite and carbocernaite, with subordinate monazite, fluorapatite, burbankite, and Ca-REE fluorocarbonates. In oxidized carbonatite, REE are hosted primarily by Ca-REE fluorocarbonates (bastnasite, parisite, synchysite, and mixed varieties), with lesser REE phosphates (rhabdophane and monazite); fluorapatite, and cerianite. REE abundances were substantially upgraded (e.g., La: 54500-66800 ppm, Ce: 11500-92100 ppm, Nd: 4740-31200 ppm) in carbonatite that was altered by oxidizing hydrothermal and supergene processes. Vertical, near surface increases in REE concentrations correlate with replacement of REE( Sr,Ca,Na,Ba) carbonate minerals by Ca-REE fluorocarbonate minerals, dissolution of matrix calcite, development of Fe- and Mn-rich gossan, crystallization of cerianite and accompanying negative Ce anomalies in secondary fluorocarbonates and phosphates, and increasing 8180 values. These vertical changes demonstrate the importance of oxidizing meteoric water during the most recent modifications to the carbonatite stockwork. Scanning electron microscopy, energy dispersive spectroscopy, and electron probe microanalysis were used to investigate variations in mineral chemistry controlling the lateral complex-wide geochemical heterogeneity. HREE-enrichment in some peripheral zones can be attributed to an increase in the abundance of secondary REE phosphates (rhabdophane group, monazite, and fluorapatite), while HREEenrichment in other zones is a result of HREE substitution in the otherwise LREE-selective fluorocarbonate minerals. Microprobe analyses show that HREE substitution is most pronounced in Ca-rich fluorocarbonates (parisite, synchysite, and mixed syntaxial varieties). Peripheral, late-stage HREE-enrichment is attributed to: 1) fractionation during early crystallization of LREE selective minerals, such as ancylite, carbocernaite, and Ca-REE fluorocarbonates in the central Bull Hill dike swarm, 2) REE liberated during breakdown of primary calcite and apatite with higher HREE/LREE ratios, and 3) differential transport of REE in fluids with higher PO43-/CO32-- and F-/CO32- ratios, leading to phosphate and pseudomorphic fluorocarbonate mineralization. Supergene weathering processes were important at the stratigraphically highest peripheral REE occurrence, which consists of fine, acicular monazite, jarosite, rutile/pseudorutile, barite, and plumbopyrochlore, an assemblage mineralogically similar to carbonatite laterites in tropical regions. (C) 2017 Elsevier B.V. All rights reserved.

The Morrison porphyry Cu–Au–Mo deposit is genetically and spatially related to Eocene plagioclase–hornblende–biotite porphyry intrusions. One porphyry intrusion yielded a U–Pb age of 52.54 ± 1.05 Ma. Mineralization occurs in three stages: (1) vein-type and disseminated chalcopyrite and minor bornite (associated with potassic alteration and gold mineralization); (2) vein-type molybdenite (associated with weak phyllic alteration); and (3) polymetallic sulfide–carbonate veins (dolomite ± quartz–sphalerite–galena–arsenopyrite–chalcopyrite, associated with weak sericite–carbonate alteration). Re–Os dating of molybdenite yielded ages of 52.54 ± 0.22 and 53.06 ± 0.22 Ma, similar to the age of the host porphyry intrusion. Stage 1 vein fluids were predominantly of magmatic origin: Th = 400–526 °C; salinity = 39.8–47.8 wt.% NaCl equiv.; δ18Ofluid = 3.7‰–6.3‰; disseminated chalcopyrite–pyrite δ34SCDT = 0.2‰ and −0.8‰ (CDT, Canyon Diablo Troilite). Stage 2 fluids were a mixture of magmatic and meteoric water: Th = 320–421 °C; salinity = 37.0–43.1 wt.% NaCl equiv.; δ18Ofluid values range from 0.3‰ to 3.4‰; molybdenite and pyrite δ34SCDT = −2.1‰ and −1.2‰. Stage 3 fluids were predominantly of meteoric water origin: Th = 163–218 °C; salinity = 3.1–3.9 wt.% NaCl equiv.; δ18Ofluid = −2.3‰ to 3.9‰ for early vein quartz, and 1.1‰ to 6.1‰ for late vein dolomite; sphalerite and pyrite δ34SCDT = −7.1‰ to −5.6‰. Morrison is interpreted to be a typical porphyry Cu–Au–Mo deposit related to a calc-alkaline to a high-K calc-alkaline diorite to granodiorite intrusive suite, generated in a continental arc in response to early Eocene subduction of the Kula–Farallon plate beneath North America.

Magma mixing is a widespread petrogenetic process. It has long been suspected to operate in concert with fractional crystallization and assimilation to produce chemical and temperature gradients in magmas. In particular, the injection of mafic magmas into felsic magma chambers is widely regarded as a key driver in the sudden triggering of what often become highly explosive volcanic eruptions. Understanding the mechanistic event chain leading to such hazardous events is a scientific goal of high priority. Here we investigate a mingling event via the evidence preserved in mingled lavas using a combination of X-ray computed microtomographic and electron microprobe analyses, to unravel the complex textures and attendant chemical heterogeneities of the mixed basaltic and rhyolitic eruption of Grizzly Lake in the Norris-Mammoth corridor of the Yellowstone Plateau volcanic field (YVF). We observe evidence that both magmatic viscous inter-fingering of magmas and disequilibrium crystallization/dissolution processes occur. Furthermore, these processes constrain the timescale of interaction between the two magmatic components prior to their eruption. X-ray microtomography images show variegated textural features, involving vesicle and crystal distributions, filament morphology, the distribution of enclaves, and further textural features otherwise obscured in conventional 2D observations and analyses. Although our central effort was applied to the determination of mixing end members, analysis of the hybrid portion has led to the discovery that mixing in the Grizzly Lake system was also characterized by the disintegration and dissolution of mafic crystals in the rhyolitic magma. The presence of mineral phases in both end member, for example, forsteritic olivine, sanidine, and quartz and their transport throughout the magmatic mass, by a combination of both mixing dynamics and flow imposed by ascent of the magmatic mass and its eruption, might have acted as a “geometric perturbation” of flow fields further fuelling mass exchange between magmas in terms of both chemical diffusion and crystal transfer. These results illuminate the complexity of mixing in natural magmatic systems, identifying several reaction-related textural factors that must be understood more deeply in order to advance our understanding of this igneous process.

The Bear Lodge alkaline complex (BLAC) in northeast Wyoming has received considerable attention for its potential as an economic source of rare earth elements (REEs) hosted in carbonatite. Prior to interest in REEs, the BLAC was considered a potential source of Au, Ag, Cu, Mo, Th, U, Mn, and fluorspar. Gold mineralization and alteration styles (potassic, ferric-iron metasomatism or fenitization) have been compared to other epithermal Au-Te or Au-rich porphyry systems related to alkaline magmatism (e.g., Cripple Creek, Colorado; Porgera, Papua New Guinea; Emperor, Fiji). Recent exploration reveals Au enrichment in some shallow, oxidized portions of the carbonatite stockwork. This demonstrates the importance of supergene processes to the concentration of Au, and suggests sulfide-rich carbonatite may be a suitable proto-ore for Au in addition to REEs. Gold concentrations as high as 10 ppm occur in Bear Lodge carbonatite, and the more typical Au concentrations of ~50-100 ppb are some of the highest reported for carbonatites worldwide. Macroscopic observations of drill core and examination by reflected light microscopy and SEM-BSE imaging suggest multiple stages of sulfide mineralization prior to oxidation. Sulfides in carbonatite are typically attributed to late-stage hydrothermal processes. However, in Bear Lodge carbonatites, early magmatic-stage sulfides are distinguished by, 1) an association with early REE-bearing phases (burbankite and carbocernaite), 2) massive pyrrhotite+pyrite+chalcopyrite segregations associated with spinifex-like platy calcite textures, 3) pyrrhotite and euhedral pyrite in close association with magmatic K-feldspar, biotite, and ilmenite phenocrysts and 4) the presence of pyrrhotite in carbonatite with primary igneous or mantle derived C and O isotope values unaffected by hydrothermal processes. Sphalerite contains drop-like, crystallographically-controlled segregations of galena similar to “chalcopyrite disease.” Preliminary electron microprobe analyses of galena reveal 0.4-0.7 wt. % Ag, 2.60-2.78 wt. % Bi, and a positive correlation between Ag and Te. Gold concentrations in sphalerite, galena, and pyrite are below detection (20 ppm) by microprobe, but native gold is found in cubic hematite pseudomorphs suggesting it is initially substitutional in pyrite.