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At least 1,045 records · Page 58Linked to original sources
USGS1998

USGS Mineral Resources Program: A National Perspective

Minerals are chemical compounds abundant in the rocks, soil, and water around us and they have a profound impact on the lives of all beings. Naturally occurring minerals define the landscape in which we live. They affect our ecosystems, influence the availability of nutrients that support biota, impact the distribution of vegetation, and may also contribute to contamination of the environment. Minerals are used in fertilizers for farming, in concrete and building materials for construction, in aggregate for roads, in steel for cars and all manner of transportation, and in materials crucial to the communications industry.

Fact Sheet↗
USGS1968

Stratigraphy of Slick Rock district and vicinity, San Miguel and Dolores Counties, Colorado

The Slick Rock district covers about 570 square miles in western San Miguel and Dolores Counties, in southwestern Colorado. It is at the south edge of the salt-anticline region of southwestern Colorado and southeastern Utah and of the Uravan mineral belt. Deposition of Paleozoic sedimentary rocks in the district and vicinity was principally controlled by development of the Paradox Basin, and of Mesozoic rocks by development of a depositional basin farther west. The Paleozoic rocks generally are thickest at the northeast side of the Paradox Basin in a northwest- trending trough which seems to be a wide graben in Precambrian igneous and metamorphic basement rocks; Mesozoic rocks generally thicken westward and southwestward from the district. Sedimentary rocks rest on a Precambrian basement consisting of a variety of rocks, including granite and amphibolite. The surface of the Precambrian rocks is irregular and generally more than 2,000 feet below sea level and 7,000-11,000 feet below the ground surface. In the northern part of the district the Precambrian surface plunges abruptly northeastward into the trough occupying the northeast side of the Paradox Basin, and in the southern part it sags in a narrow northeasterly oriented trough. Deepening of both troughs, or crustal deformation in their vicinity, influenced sedimentation during much of late Paleozoic and Mesozoic time. The maximum total thickness of sedimentary rocks underlying the district is 13,000 feet, and prior to extensive erosion in the late Tertiary and the Quaternary it may have been as much as about 18,000 feet. The lower 5,000 feet or more of the sequence of sedimentary rocks consists of arenaceous strata of early Paleozoic age overlain by dominantly marine carbonate rocks and evaporite beds interbedded with lesser amounts of clastic sediments of late Paleozoic age. Overlying these rocks is about 4,500 feet of terrestrial clastic sediments, dominantly sandstone with lesser amounts of shale, mudstone, siltstone, and conglomerate, of late Paleozoic and Mesozoic age. Above these rocks is as much as 2,300 feet of marine shale of late Mesozoic age. Perhaps about 5,000 feet of clastic sedimentary rocks, dominantly sandstone and in part shale, of late Mesozoic and early Cenozoic age, overlay the older rocks of the district prior to late Cenozoic erosion... Outside the Slick Rock district the Mancos Shale is overlain by dominantly terrestrial sandstone, mudstone, and coaly beds of the Mesaverde Group of Late Cretaceous age, and younger units such as the Wasatch and Green River Formations of Tertiary age, which once may have extended across the district. These units, totaling possibly 5,000 feet in thickness, were removed by erosion following middle Tertiary uplift of the Colorado Plateau. Igneous rocks of Tertiary age crop out in only one small area in the district, but they are intruded extensively in the Mancos Shale east of the district, and, as shown by deep oil test wells, appear to be intruded widely in the Paradox Member of the Hermosa Formation in the southern part of the district and southeast of the district. Andesite porphyry occurs in a dike on Glade Mountain, microgranogabbro and microgranodiorite occur in thin sills east of the district, and rocks of similar composition form thick sills in the subsurface. All are similar chemically to igneous rocks in the San Juan Mountains southeast of the district and probably were the result of a specific igneous episode. They were intruded most likely during the Miocene. Surficial deposits of Quaternary age include glacial till, terrace gravels, alluvial fans, landslide debris, loess, other soil, alluvium, colluvium, and talus. On Glade Mountain, glacial till of probable early Pleistocene age merges westward with terrace gravels that are correlative with terrace gravels which lie on an old weathered surface of Mancos Shale farther west on the rim of the Dolores River Canyon.

Colorado↗
USGS2022

A channel sampling strategy for measurement of mineral modal and chemical composition of drill cores: Application to lower oceanic crustal rocks from IODP Expedition 345 to the Hess Deep rift

We report a new sampling strategy for collecting representative samples of drill core. By splitting the core with a diamond saw into working and archive halves, the saw cuttings constitute a “channel” sample, the best subsample from which to obtain an average mineralogical and geochemical composition of a core. We apply this procedure to sampling core of the lower oceanic crust in the Hess Deep obtained during Expedition 345 of the Integrated Ocean Drilling Program (now International Ocean Discovery Program). Our results show that particles produced by sawing range from sand to clay sizes. Sand- and silt-sized cuttings can be sampled with a spatula, whereas clay-sized particles remained in suspension after 12 h and could be collected only by settling, aided by centrifuge. X-ray diffraction (XRD) analysis and Rietveld refinement show that phyllosilicates were fractionated into the clay-sized fraction. Thus, collection of both the sedimented fraction and the clay-sized suspended fraction (commonly > 15 wt % of the total) is necessary to capture the whole sample. The strong positive correlation between the recovered sample mass (in grams) and length of core cut demonstrates that this sampling protocol was uniform and systematic, with almost 1.4 g sediment produced per centimeter of core cut. We show that major-element concentrations of our channel samples compare favorably with the compositions of billet-sized samples analyzed aboard the JOIDES Resolution , but the results show that individual billet analyses are rarely representative of the whole core recovered. A final test of the validity of our methods comes from the strong positive correlation between the loss on ignition (LOI) values of our channel samples and the H 2 O contents calculated from the modal mineralogy obtained by X-ray diffraction and Rietveld refinement. This sampling procedure shows that grain-sized fractionation modifies both mineralogical and chemical compositions; nevertheless, this channel sampling method is a reliable method of obtaining representative samples of bulk cores. With the ever-increasing precision offered by modern analytical instrumentation, this sampling protocol allows the accuracy of the analytical results to keep pace.

Scientific Drilling↗
USGS2004

Hydrothermal alteration, ore fluid characteristics, and gold depositional processes along a trondhjemite-komatiite contact at Tarmoola, Western Australia

Tarmoola is a structurally controlled Archean orogenic gold deposit hosted in greenschist facies metamorphosed komatiite and trondhjemite in the Leonora district of the Eastern Goldfields province, Yilgarn craton. High-grade (>1 g/t Au) orebodies are located in komatiite wall rock adjacent to the eastern and northeastern margins of the asymmetrical, north-south-striking, Tarmoola trondhjemite intrusion. Gold-bearing veins post-date trondhjemite emplacement (ca. 2700 Ma), quartz diorite dikes (ca. 2667 Ma), and regional greenschist facies metamorphism. Textures and crosscutting relationships in gold-bearing veins indicate two stages of hydrothermal fluid infiltration associated with a single gold-related hydrothermal event: a volumetrically dominant, but gold-poor, stage I fluid and a gold-rich stage II fluid. Gold-bearing veins contain stage I milky quartz and pyrite that are overprinted by stage II quartz-ankerite-muscovite-chalcopyrite-sphalerite-galena-gold-tellurides ?? albite ?? chlorite ?? fuchsite ?? epidote ?? scheelite. Stage I hydrothermal alteration assemblages are different in trondhjemite and komatiite due to contrasting reactions between a common ore fluid and disparate wall-rock chemistry. Stage II fluid-wall rock interaction was minor compared to stage I and is indicated by the overprinting of stage I mineral assemblages by stage II microveins. Wall-rock alteration proximal to veins in trondhjemite is characterized by replacement of igneous plagioclase, amphibole, biotite, and metamorphic chlorite by hydrothermal quartz, muscovite, ankerite, calcite, pyrite, chalcopyrite, sphalerite, galena, tellurides, and gold, whereas in proximal alteration in komatiite, metamorphic chlorite and talc are replaced by ankerite, quartz, muscovite, albite, chlorite, fuchsite, pyrite, chalcopyrite, sphalerite, galena, tellurides, and gold. The stage II fluid was enriched in H2O, CO2, Si, Ca, K, Na, S, Au, Ag, Cu, Pb, W, Bi, As, Mo, Zn, and Te. Based on fluid inclusion studies and stage II mineral equilibria, gold deposited from a homogeneous, neutral to slightly alkaline (pH 5.1-5.5), reduced, low-salinity (<5.5 wt % NaCl equiv) fluid that had a bulk composition of 78 mole percent H2O and 21 mole percent CO2, and trace amounts of CH4, C2H6, H2, Ar, H2S, and He. Gold deposition occurred at 300?? ?? 50??C and 0.5 to 3.0 kbars. Assuming lithostatic fluid pressures, gold precipitated at a 2- to 10-km depth. Stage II gray quartz ??18Ofluid values range from 5.9 to 7.5 per mil, whereas ??Dfluid values calculated from the dehydration of muscovite grains and measured directly from bulk fluid inclusion analyses of stage II gray quartz have ranges of -9 to -35 and -27 to -28 per mil, respectively. Hydrothermal ore fluids were transported from greater crustal depths to the site of gold deposition during the district-scale D3 event by shallowly W dipping, reverse brittle-ductile shear zones in supracrustal rock and along the steeply east dipping trondhjemite contact. Associated subhorizontal east-west shortening caused the reactivation of the eastern trondhjemite margin and subparallel foliation, which facilitated the transport of hydrothermal fluids and the generation of gold-bearing veins and hydrothermal alteration zones in komatiite. East-west-striking fractures in trondhjemite aided the lateral migration of ore fluids away from trondhjemite margins and the formation of east-west-striking gold-bearing veins and broad alteration zones. Gold was most likely transported in the stage II fluid as bisulfide complexes. The sulfidation of trondhjemite and komatiite wall rock by the stage II fluid caused the destabilization of An bisulfide complexes and gold deposition. Potassium, Ca, and CO2 metasomatism of komatiite wall rock may have enhanced gold deposition via the acidification of the stage II fluid. The physicochemical characteristics of the Tarmoola ore fluid and relative timing of gold mineralization are consistent with the Yilgarn-wide,

Economic Geology↗
USGS1994

Evolution of volcanic rocks and associated ore deposits in the Marysvale volcanic field, Utah

A geological account on the igneous activity and associated mineral deposition in the volcanic field of Marysvale in Utah is presented. Three episodes (34-22 Ma, 22-14 Ma and 9-5 Ma) involved in the volcanic rock eruption and associated mineralization are described. The first episode is believed to have occurred during the time of tectonic convergence when two contrasting suites of rocks, Mount Dutton Formation and Bullion Canyon Volcanics, erupted concurrently. Mineralization during this period was sparse. In the second episode, change from intermediate to bimodal volcanism occurred. During the third episode, basaltic compositions did not change. Although major element constituent had rhyolites similar to that of the second episode, rhyolites had a marked radiogenic isotope characteristic difference.

Utah↗
USGS2012

The Kanuti ophiolite, Alaska

The Kanuti ophiolite is a mafic-ultramafic thrust sheet of probable Jurassic age, formerly considered to be the upper part of the Yukon-Koyukuk ophiolite belt (Angayucham terrane). It is here called the Kanuti ophiolite after the Kanuti River region on the southeastern flank of the Yukon-Koyukuk Basin. The thrust sheet crops out discontinuously for a distance of more than 900 km along the northern and southeastern margins of the basin. It is probably correlative with similar ophiolite thrust sheets to the north in the western Brooks Range and to the south in the Ruby geanticline. Technically, the ophiolite is considered to be the Kanuti thrust panel of the Angayucham-Totzitna terrane. The Kanuti consistently overlies another extensive thrust sheet, consisting mostly of pillow basalt and radiolarian chert of Devonian to Jurassic age (Narvak thrust panel). This sheet is thrust over a third sheet consisting of probable Devonian phyllite and metagraywacke, which is in turn thrust over older metamorphic rocks (Slate Creek thrust panel). The Kanuti ophiolite is a partial ophiolite that consists of a lower residual mantle suite and an upper magmatic suite, but dikes, extrusives, and sediments are absent. The residual mantle suite is composed of harzburgite and dunite with refractory mineral compositions. The harzburgite is attributed to partial melting and extraction of basaltic magma; residual dunite is attributed to partial melting or to reaction of orthopyroxene out of harzburgite in contact with ascending melt diapirs. The magmatic suite consists of layered ultramafic and gabbroic rocks, containing minerals having limited iron enrichment. The absence of large volumes of magmatic rocks intermediate in composition between cumulus ultramafics and evolved gabbros favors periodic introduction of magma, rather than closed system fractional crystallization. The ultramafic rocks of both the residual mantle and magmatic suites are tectonites, which have undergone high-temperature deformation involving isoclinal folding on all scales and related syntectonic recrystallization. The olivine fabric is consistent with the glide system {0kl} [100], which has been produced experimentally at 800°–1190°C at 20 kbar. Olivine Z axes and subparallel isoclinal fold axes have consistent, northeast trends throughout the Kanuti region (>100 km NE-SW) and may be close to the original upper mantle flow direction, despite later low-angle thrust faulting. The order of crystallization in the cumulus ultramafic rocks of the magmatic suite is olivine, clinopyroxene, plagioclase, and orthopyroxene. The high Mg numbers of clinopyroxene (0.85–0.93) coexisting with olivine suggest that the cumulus ultramafic rocks crystallized at relatively high pressures (>10 kbar). The effects of parental magma composition cannot be evaluated, but the small difference in Mg numbers of coexisting olivine and clinopyroxene in the cumulus ultramafic rocks and in residual harzburgite suggests that regardless of absolute pressures, the pressure difference between the melting that produced the basalt magma and the initial fractional crystallization of the magma is small. Because of the limited range in rock types in the ophiolite, the tectonic environment cannot be interpreted unambiguously. However, the structural and petrological data are best reconciled with an origin in a volcanic arc tectonic setting.

Journal of Geophysical Research Solid Earth↗
USGS2009

Airborne gamma-ray and magnetic anomaly signatures of serpentinite in relation to soil geochemistry, northern California

Serpentinized ultramafic rocks and associated soils in northern California are characterized by high concentrations of Cr and Ni, low levels of radioelements (K, Th, and U) and high amounts of ferrimagnetic minerals (primarily magnetite). Geophysical attributes over ultramafic rocks, which include airborne gamma-ray and magnetic anomaly data, are quantified and provide indirect measurements on the relative abundance of radioelements and magnetic minerals, respectively. Attributes are defined through a statistical modeling approach and the results are portrayed as probabilities in chart and map form. Two predictive models are presented, including one derived from the aeromagnetic anomaly data and one from a combination of the airborne K, Th and U gamma-ray data. Both models distinguish preferential values within the aerogeophysical data that coincide with mapped and potentially unmapped ultramafic rocks. The magnetic predictive model shows positive probabilities associated with magnetic anomaly highs and, to a lesser degree, anomaly lows, which accurately locate many known ultramafic outcrops, but more interestingly, locate potentially unmapped ultramafic rocks, possible extensions of ultramafic bodies that dip into the shallow subsurface, as well as prospective buried ultramafic rocks. The airborne radiometric model shows positive probabilities in association with anomalously low gamma radiation measurements over ultramafic rock, which is similar to that produced by gabbro, metavolcanic rock, and water bodies. All of these features share the characteristic of being depleted in K, Th and U. Gabbro is the only rock type in the study area that shares similar magnetic properties with the ultramafic rock. The aerogeophysical model results are compared to the distribution of ultramafic outcrops and to Cr, Ni, K, Th and U concentrations and magnetic susceptibility measurements from soil samples. Analysis of the soil data indicates high positive correlation between magnetic susceptibilities and concentration of Cr and Ni. Although the study focused on characterizing the geophysical properties of ultramafic rocks and associated soils, it has also yielded information on other rock types in addition to ultramafic rocks, which can also locally host naturally-occurring asbestos; specifically, gabbro and metavolcanic rocks.

Applied Geochemistry↗
USGS1996

Summary of the Oahu, Hawaii, regional aquifer-system analysis

Oahu, the third largest of the Hawaiian islands, is formed by the eroded remnants of two elongated shield volcanoes with broad, low profiles. Weathering and erosion have modified the original domed surfaces of the volcanoes, leaving a landscape of deep valleys and steep interfluvial ridges in the interior highlands. The Koolau Range in eastern Oahu and the Waianae Range in western Oahu are the eroded remnants of the Koolau and Waianae Volcanoes. The origin, mode of emplacement, texture, and composition of the rocks of Oahu affect their ability to store and transmit water. The volcanic rocks are divided into four groups: (1) lava flows, (2) dikes, (3) pyroclastic deposits, and (4) saprolite and weathered basalt. Stratified sequences of thin-bedded lava flows form the most productive aquifers in Hawaii. Dikes are near-vertical sheets of massive intrusive rock that typically contain only fracture permeability. Pyroclastic deposits include ash, cinder, and spatter; they are essentially granular, with porosity and permeability similar to those of granular sediments. Weathering of basaltic rocks in the humid, subtropical climate of Oahu alters igneous minerals to clays and oxides, reducing the permeability of the parent rock. Saprolite is weathered material that has retained textural features of the parent rock. Estimates of hydraulic conductivity along the plane of dike-free lava flows tend to fall within about one order of magnitude, from about 500 to about 5,000 feet per day. Estimates of specific yield range from about 1 to 20 percent; most of the values lie within a narrow range of about 5 to 10 percent. The occurrence of ground water on Oahu is determined by the type and character of the rocks and by the presence of geohydrologic barriers. The primary modes of freshwater occurrence on Oahu are as a basal lens of fresh ground water floating on saltwater, as dike-impounded ground water, and as perched ground water. Saltwater occurs at depth throughout much of the island. A regional aquifer system composed of the Waianae aquifer in the Waianae Volcanics and the Koolau aquifer in the Koolau Basalt is subdivided into well-defined areas by geohydrologic barriers. The aquifers are separated by the Waianae confining unit formed by weathering along the Waianae-Koolau unconformity. In some coastal areas, a caprock of sedimentary deposits overlies and confines the aquifers. The island of Oahu has been divided into seven major ground-water areas delineated by deep-seated structural geohydrologic barriers; these areas are further subdivided by shallower internal barriers to ground-water flow. The Koolau rift zone along the eastern (windward) side of the island and the Waianae rift zone to the west (Waianae area) constitute two of the major ground-water areas. North-central Oahu is divided into three smaller ground-water areas, Mokuleia, Waialua, and Kawailoa. The Schofield ground-water area encompasses much of the Schofield Plateau of central Oahu. Southern Oahu is divided into six areas, Ewa, Pearl Harbor, Moanalua, Kalihi, Beretania, and Kaimuki. Southeastern Oahu is divided into the Waialae and Wailupe-Hawaii Kai areas. Along the northeast coast of windward Oahu is the Kahuku ground-water area. The aquifers of Oahu contain shallow freshwater and deeper saltwater flow systems. There are five fresh ground-water flow systems: meteoric freshwater flow diverges from ground-water divides that lie somewhere within the Waianae and Koolau rift zones, forming an interior flow system in central Oahu (which is divided into the northern and southern Oahu flow systems) and exterior flow systems in western (Waianae area) Oahu, eastern (windward) Oahu, and southeastern Oahu. Development of the ground-water resources on Oahu began when the first well was drilled near Honouliuli in the summer of 1879. By 1890, 86 wells had been drilled on the island. From about 1891 to about 1910, development increased rapidly with the drilling of a

Hawaii↗
USGS1956

Uranium deposits of the northern part of the Boulder Batholith, Montana

Uranium minerals and radioactivity anomalies occur in many silverlead veins and chalcedony veins and vein zones in the Boulder batholith of southwestern Montana. Pitchblende has been identified in a few silverlead veins. These silver-lead veins occupy shear zones along which there is no evidence of large-scale lateral displacement. The wall rock adjacent to the veins is intensely silicified and sericitized quartz monzonite and granodiorite. The veins have yielded substantial quantities of lead, silver, zinc, and gold. The silver-lead veins consist principally of galena, sphalerite, tetrahedrite, chalcopyrite, and pyrite in a gangue of light- to dark-gray quartz, altered rock, gouge, and subordinate chalcedony and carbonate minerals. No anomalous radioactivity nor uranium minerals have been found in similar veins in prebatholithic rocks of the area.Chalcedony veins and vein zones, some of which are uraniferous, are distinctly different from the silver-lead veins and, with a single exception, are known only in the batholith. The chalcedony vein zones consist of one or more discontinuous stringers or veins of chalcedony and microcrystalline quartz in silicified and sericitized quartz monzonite and granodiorite, and in less strongly altered alaskite. Only small amounts of silver ore have been produced from these chalcedony veins and vein zones. All of the veins are early Tertiary in age, but the silver-lead veins probably are older than the chalcedony veins. Uranium is closely associated with chalcedony and microcrystalline quartz in both types of veins. This association suggests that all of the uranium in the area is of the same age. If so, some of the silver-lead veins must have been re-opened during the period of chalcedony vein formation.

Montana↗
USGS2011

U-Pb (zircon) and geochemical constraints on the age, origin, and evolution of Paleozoic arc magmas in the Oyu Tolgoi porphyry Cu-Au district, southern Mongolia

Uranium–Pb (zircon) ages are linked with geochemical data for porphyry intrusions associated with giant porphyry Cu–Au systems at Oyu Tolgoi to place those rocks within the petrochemical framework of Devonian and Carboniferous rocks of southern Mongolia. In this part of the Gurvansayhan terrane within the Central Asian Orogenic Belt , the transition from Devonian tholeiitic marine rocks to unconformably overlying Carboniferous calc-alkaline subaerial to shallow marine volcanic rocks reflects volcanic arc thickening and maturation. Radiogenic Nd and Pb isotopic compositions ( ε Nd(t) range from + 3.1 to + 7.5 and 206 Pb/ 204 Pb values for feldspars range from 17.97 to 18.72), as well as low high-field strength element (HFSE) contents of most rocks (mafic rocks typically have < 1.5% TiO 2 ) are consistent with magma derivation from depleted mantle in an intra-oceanic volcanic arc. The Late Devonian and Carboniferous felsic rocks are dominantly medium- to high-K calc-alkaline and characterized by a decrease in Sr/Y ratios through time, with the Carboniferous rocks being more felsic than those of Devonian age. Porphyry Cu–Au related intrusions were emplaced in the Late Devonian during the transition from tholeiitic to calc-alkaline arc magmatism . Uranium–Pb (zircon) geochronology indicates that the Late Devonian pre- to syn-mineral quartz monzodiorite intrusions associated with the porphyry Cu–Au deposits are ~ 372 Ma, whereas granodiorite intrusions that post-date major shortening and are associated with less well-developed porphyry Cu–Au mineralization are ~ 366 Ma. Trace element geochemistry of zircons in the Late Devonian intrusions associated with the porphyry Cu–Au systems contain distinct Th/U and Yb/Gd ratios, as well as Hf and Y concentrations that reflect mixing of magma of distinct compositions. These characteristics are missing in the unmineralized Carboniferous intrusions. High Sr/Y and evidence for magma mixing in syn- to late-mineral intrusions distinguish the Late Devonian rocks associated with giant Cu–Au deposits from younger magmatic suites in the district.

Southern Mongolia↗
USGS1982

Tertiary volcanic rocks and uranium in the Thomas Range and northern Drum Mountains, Juab County, Utah

The Thomas Range and northern Drum Mountains have a history of volcanism, faulting, and mineralization that began about 42 m.y. (million years) ago. Volcanic activity and mineralization in the area can be divided into three stages according to the time-related occurrence of rock types, trace-element associations, and chemical composition of mineral deposits. Compositions of volcanic rocks changed abruptly from rhyodacite-quartz latite (42-39 m.y. ago) to rhyolite (38-32 m.y. ago) to alkali rhyolite (21 and 6-7 m.y. ago); these stages correspond to periods of chalcophile and siderophile metal mineralization, no mineralization(?), and lithophile metal mineralization, respectively. Angular unconformities record episodes of cauldron collapse and block faulting between the stages of volcanic activity and mineralization. The youngest angular unconformity formed between 21 and 7 m.y. ago during basin-and-range faulting. Early rhyodacite-quartz latite volcanism from composite volcanoes and fissures produced flows, breccias, and ash-flow tuff of the Drum Mountains Rhyodacite and Mt. Laird Tuff. Eruption of the Mt. Laird Tuff about 39 m.y. ago from an area north of Joy townsite was accompanied by collapse of the Thomas caldera. Part of the roof of the magma chamber did not collapse, or the magma was resurgent, as is indicated by porphyry dikes and plugs in the Drum Mountains. Chalcophile and siderophile metal mineralization, resulting in deposits of copper, gold, and manganese, accompanied early volcanism. Te middle stage of volcanic activity was characterized by explosive eruption of rhyolitic ash-flow tuffs and collapse of the Dugway Valley cauldron. Eruption of the Joy Tuff 38 m.y. ago was accompanied by subsidence of this cauldron and was followed by collapse and sliding of Paleozoic rocks from the west wall of the cauldron. Landslides in The Dell were covered by the Dell Tuff, erupted 32 m.y. ago from an unknown source to the east. An ash flow of the Needles Range(?) Formation was erupted 30-31 m.y. ago from an unknown source. Mineralization probably did not occur during the rhyolitic stage of volcanism. The last stage of volcanism was contemporaneous with basin-and-range faulting and was characterized by explosive eruption of ash and pumice, forming stratified tuff, and by quiet eruption of alkali rhyolite as viscous flows and domes. The first episode of alkali rhyolite volcanism deposited the beryllium tuff and porphyritic rhyolite members of the Spor Mountain Formation 21 m.y. ago. After a period of block faulting, the stratified tuff and alkali rhyolite of the Topaz Mountain Rhyolite were erupted 6-7 m.y. ago along faults and fault intersections. Erosion of Spor Mountain, as well as explosive eruptions through dolomite, provided abundant dolomite detritus to the beryllium tuff member. The alkali rhyolite of both formations is fluorine rich, as is evident from abundant topaz, and contains anomalous amounts of lithophile metals. Alkali rhyolite volcanism was accompanied by lithophile metal mineralization which deposited fluorite, beryllium, and uranium. The structure of the area is dominated by the Thomas caldera and the younger Dugway Valley cauldron, which is nested within the Thomas caldera; the Thomas caldera is surrounded by a rim of Paleozoic rocks at Spor Mountain and Paleozoic to Precambrian rocks in the Drum Mountains. The Joy fault and Dell fault system mark the ring-fracture zone of the Thomas caldera. These structural features began to form about 39 m.y. ago during eruption of the Mt. Laird Tuff and caldera subsidence. The Dugway Valley cauldron sank along a series of steplike normal faults southeast of Topaz Mountain in response to collapse of the magma chamber of the Joy Tuff. Caldera structure was modified by block faulting between 21 and 7 m.y. ago, the time of widespread extensional faulting in the Basin and Range Province. Vents erupted alkali rhyolite 6-7 m.y. ago along basin-and-range faults.

Professional Paper↗
USGS2023

Porosity, strength, and alteration – Towards a new volcano stability assessment tool using VNIR-SWIR reflectance spectroscopy

Volcano slope stability analysis is a critical component of volcanic hazard assessments and monitoring. However, traditional methods for assessing rock strength require physical samples of rock which may be difficult to obtain or characterize in bulk. Here, visible to shortwave infrared (350–2500 nm; VNIR–SWIR) reflected light spectroscopy on laboratory-tested rock samples from Ruapehu, Ohakuri, Whakaari, and Banks Peninsula (New Zealand), Merapi (Indonesia), Chaos Crags (USA), Styrian Basin (Austria) and La Soufrière de Guadeloupe (Eastern Caribbean) volcanoes was used to design a novel rapid chemometric-based method to estimate uniaxial compressive strength (UCS) and porosity. Our Partial Least Squares Regression models return moderate accuracies for both UCS and porosity, with R 2 of 0.43–0.49 and Mean Absolute Percentage Error (MAPE) of 0.2–0.4. When laboratory-measured porosity is included with spectral data, UCS prediction reaches an R 2 of 0.82 and MAPE of 0.11. Our models highlight that the observed changes in the UCS are coupled with subtle mineralogical changes due to hydrothermal alteration at wavelengths of 360–438, 532–597, 1405–1455, 2179–2272, 2332–2386, and 2460–2490 nm. These mineralogical changes include mineral replacement, precipitation hydrothermal alteration processes which impact the strength of volcanic rocks, such as mineral replacement, precipitation, and/or silicification. Our approach highlights that spectroscopy can provide a first order assessment of rock strength and/or porosity or be used to complement laboratory porosity-based predictive models. VNIR-SWIR spectroscopy therefore provides an accurate non-destructive way of assessing rock strength and alteration mineralogy, even from remote sensing platforms.

Earth and Planetary Science Letters↗
USGS2022

The geochemical and textural transition between the Reef Package and its hanging wall, Stillwater Complex, Montana, USA

The highest grade Pd-Pt deposit on Earth, the J-M Reef, is hosted in coarse-grained to pegmatoidal cumulates called the Reef Package. Decades of mine development of the J-M Reef have revealed that a distinct discontinuity in rock fabric marks the top of the rock unit that hosts economic-grade sulfide mineralization. Mine geologists refer to this discontinuity as the hanging wall contact. This contact is the top of the Reef Package and is always locatable—either by the change in rock fabric or by distinctive hanging wall textures of silicate minerals—even when the reef sulfide mineralization is absent. This rather subtle textural feature is used reliably by mine geologists to follow the Reef during exploration and mine development. Although some high tenor sulfides (>1000 ppm Pd in 100% sulfide) are found sporadically in the hanging wall cumulates, these accumulations are too small to be economically viable. We present quantitative rock fabric data for four Reef Package and hanging wall intersections collected by electron back-scattered diffraction (EBSD). Plagioclase fabrics in the hanging wall are characterized by low variance in grain sizes and a strong point maximum concentration of (010) and a perpendicular girdle distribution of [100] consistent with an axial B-type fabric. These fabrics are indicative of either compaction of the crystal mush or crystal settling of nucleated crystals, the bulk magma in a chamber. Conversely, the fabrics of the Reef Package show higher variance grain in size distributions and weak to undeveloped preferred orientation of plagioclase crystals that did not undergo significant alignment or textural equilibration of plagioclase grains. The absence of foliation in the Reef Package stands in contrast both to hanging wall fabrics and to other reported EBSD datasets of plagioclase crystals orientations from the Bushveld Complex, the Skaergaard Intrusion, and the Rum Intrusion. Furthermore, plagioclase crystal size distributions for the Reef Package show flatter slopes and convex profiles with fewer crystals at small size fractions indicating the dissolution of small crystals during partial melting and textural coarsening (i.e . Ostwald ripening) and crystal growth. Crystal growth was favored over the nucleation of new crystals during prolonged interaction with a hot infiltrating melt into the resident mush resulting in the coarse-grained textures of the Reef Package cumulates. The hanging wall contact represents a boundary between partially remelted crystal mush of the Reef Package, where sulfide mineralization formed and accumulated, and an overlying essentially barren cumulate pile. The hanging wall cumulates formed following the cessation of footwall erosion and the resumption of crystal accumulation by normal magma chamber processes.

Montana↗
USGS2008

Geologic Map of the Eaton Reservoir Quadrangle, Larimer County, Colorado and Albany County, Wyoming

New geologic mapping of the Eaton Reservoir 7.5' quadrangle defines geologic relationships in the northern Front Range along the Colorado/Wyoming border approximately 35 km south of Laramie, Wyo. Previous mapping within the quadrangle was limited to regional reconnaissance mapping (Tweto, 1979; Camp, 1979; Burch, 1983) and some minor site-specific studies (Carlson and Marsh, 1986; W. Braddock, unpub. mapping, 1982). Braddock and others (1989) mapped the Diamond Peak 7.5' quadrangle to the east, Burch (1983) mapped rocks of the Rawah batholith to the south, W. Braddock (unpub. mapping, 1981) mapped the Sand Creek Pass 7.5' quadrangle to the west, and Ver Ploeg and Boyd (2000) mapped the Laramie 30' x 60' quadrangle to the north. Field work was completed during 2005 and 2006 and the mapping was compiled at a scale of 1:24,000. Minimal petrographic work and isotope dating was done in connection with the present mapping, but detailed petrographic and isotope studies were carried out on correlative map units in surrounding areas as part of a related regional study of the northern Front Range. Classification of Proterozoic rocks is primarily based upon field observation of bulk mineral composition, macroscopic textural features, and field relationships that allow for correlation with rocks studied in greater detail outside of the map area.

Scientific Investigations Map↗
USGS2010

Geologic map of the Sand Creek Pass quadrangle, Larimer County, Colorado, and Albany County, Wyoming

New geologic mapping within the Sand Creek Pass 7.5 minute quadrangle defines geologic relationships within the northern Front Range of Colorado along the Wyoming border approximately 35 km south of Laramie, Wyo. Previous mapping within the quadrangle was limited to regional reconnaissance mapping; Eaton Reservoir 7.5 minute quadrangle to the east (2008), granite of the Rawah batholith to the south (1983), Laramie River valley to the west (1979), and the Laramie 30' x 60' quadrangle to the north (2007). Fieldwork was completed during 1981 and 1982 and during 2007 and 2008. Mapping was compiled at 1:24,000-scale. Minimal petrographic work was done and no isotope work was done in the quadrangle area, but detailed petrographic and isotope studies were performed on correlative map units in surrounding areas as part of a related regional study of the northern Front Range. Stratigraphy of Proterozoic rocks is primarily based upon field observation of bulk mineral composition, macroscopic textural features, and field relationships that allow for correlation with rocks studied in greater detail outside of the map area. Stratigraphy of Phanerozoic rocks is primarily based upon correlation with similar rocks to the north in the Laramie Basin of Wyoming and to the east in the Front Range of Colorado.

Scientific Investigations Map↗
USGS1991

Mineral remains of early life on Earth? On Mars?

The oldest sedimentary rocks on Earth, the 3.8‐Ga Isua Iron‐Formation in southwestern Greenland, are metamorphosed past the point where organic‐walled fossils would remain. Acid residues and thin sections of these rocks reveal ferric microstructures that have filamentous, hollow rod, and spherical shapes not characteristic of crystalline minerals. Instead, they resemble ferric‐coated remains of bacteria. Modern so‐called iron bacteria were therefore studied to enhance a search image for oxide minerals precipitated by early bacteria. Iron bacteria become coated with ferrihydrite, a metastable mineral that converts to hematite, which is stable under high temperatures. If these unusual morphotypes are mineral remains of microfossils, then life must have evolved somewhat earlier than 3.8 Ga, and may have involved the interaction of sediments and molecular oxygen in water, with iron as a catalyst. Timing is constrained by the early in fall of planetary materials that would have heated the planet's surface. Because there are no earlier sedimentary rocks to study on Earth, it may be necessary to expand the search elsewhere in the solar system for clues to any biotic precursors or other types of early life. Evidence from Mars shows geophysical and geochemical differentiation at a very early stage, which makes it an important candidate for such a search if sedimentation is an important process in life's origins. Not only does Mars have iron oxide‐rich soils, but its oldest regions have river channels where surface water and sediment may have been carried, and seepage areas where groundwater may have discharged. Mars may have had an atmosphere and liquid water in the crucial time frame of 3.9–4.0 Ga. A study of morphologies of iron oxide minerals collected in the southern highlands during a Mars sample return mission may therefore help to fill in important gaps in the history of Earth's earliest biosphere.

Geomicrobiology Journal↗
USGS1972

Reconnaissance geologic map of the Solomon D-6 quadrangle, Seward Peninsula, Alaska

The Solomon D-6 15- by 30-minute quadrangle lies 20 miles northeast of Nome, Alaska, and adjoins on the east the Nome D-l quadrangle mapped by Hummel (1962b) and on the west the Solomon D-5 quadrangle mapped by Smith (1910). Placer gold has been produced from Iron Creek and many of its tributaries, and a small amount of copper was shipped from the Wheeler mine. Many lode prospects contain small amounts of copper in silicified limestone within and near the base of thrust plates of carbonate rocks. Other lodes consist of well-defined veins containing gold and base-metal sulfide minerals, but none has been commercially productive. Mapped rock units include the "York Slate," of Precambrian age, which consists of a highly deformed and thick sequence of graphitic calcareous siltite, which contains thin limestone beds and intercalated(?) volcaniclastic rocks, and Paleozoic carbonate rocks, which have been converted largely to marble and schistose marble. Along and near the Kigluaik Mountains in the north part of the quadrangle, quartzo-feldspathic gneisses of Precambrian age are intruded by fine-grained granite. A very thick sequence of schistose thin-bedded argillaceous and dolomitic limestone overlies the York Slate and is correlative with a similar rock originally mapped in the York Mountains. Surficial deposits, consisting principally of moraine and stream-reworked moraine, mantle large parts of the quadrangle.

Alaska↗