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At least 1,117 records · Page 62Linked to original sources
USGS1989

Constraints from fluid inclusions on sulfide precipitation mechanisms and ore fluid migration in the Viburnum Trend lead district, Missouri

Measurements on fluid inclusions in hydrothermal dolomite cements place constraints on sulfide precipitation mechanisms and on the thermal-hydrologic processes which formed the Viburnum Trend Mississippi Valley-type lead district. Homogenization temperatures and freezing point depressions were determined for fluid inclusions in Bonneterre Dolomite-hosted dolomite cements in mine samples, as well as drill core from up to 13 km outside of the district. A well-defined cathodoluminescent zonation distinguishes dolomite growth zones in the Vi-burnurn Trend as older or younger than main-stage mineralization (octahedral galena) and facilitates correlation with other dolomites outside the Viburnum Trend.Homogenization temperatures and salinities in samples from mines are not systematically different from those of samples outside of the district. Medians of homogenization temperature distributions differ by not more than 25 degrees C, so that a temperature gradient, if present, should not have exceeded approximately 25 degrees C within the study area. These observations are interpreted to indicate that the Viburnum Trend was not strongly thermally anomalous with respect to surrounding country rock and that fluid flow occurred on a broad scale through not only the Lamotte Sandstone but through the overlying Cambrian carbonates as well.The absence of a significant, recognizable decrease in temperature either vertically within the section or east-west across the district, coupled with the minor amount of silica in the district, argues against cooling as a primary cause of sulfide precipitation. Fluids whose primary aquifer was the Lamotte Sandstone, predominantly a quartz arenite, should have been in equilibrium with quartz. Quartz in the Viburnum Trend occurs as a minor, drusy, vug-lining phase, but the district lacks the intense silicification found in other Mississippi Valley-type districts such as Tri-State (Oklahoma, Kansas, Missouri). Quartz solubility is strongly temperature dependent and, under equilibrium conditions, a decrease of 10 degrees C or more should have precipitated at least as many moles of silica as galena (assuming a galena solubility of between 1 and 10 ppm). Clearly this is not the case, as galena is far more abundant than quartz in the Viburnum Trend.Ice final-melting temperatures (T m ) in fluid inclusions generally range from -14 degrees to -27 degrees C for primary dolomite-hosted inclusions. Using these T m values and cation ratios for the inclusion fluids, absolute concentrations for the individual cations and chloride were calculated using the thermochemical model of Spencer et al. (1990). The corresponding high but variable salinities, 3.9 to 5.9 chloride molality, are evidence for the presence of more than one distinct fluid during mineralization.In a reduced sulfur mineralization model with Pb carried as chloride complexes, dilution is also a possible sulfide precipitation mechanism. The difference in Pb solubility (for an equal quantity of reduced sulfur) in the extremes of the chloride concentration range, 3.9 vs. 5.9 molal, reaches 1 ppm only for pH values below approximately 4.5. Accepting 1 ppm as a minimum metal concentration for a viable ore-forming fluid, dilution only appears capable of precipitating sulfides in a fluid with pH near the lower limit of values considered geologically reasonable or attainable.Dolomite cements hosting warm (approximately 105 degrees -125 degrees C) saline fluid inclusions are ubiquitous in the porous dolomitic facies of the Bonneterre Dolomite. Based on stratigraphic reconstructions, however, it is unlikely that the Bonneterre was buried deeper than 1.5 km. The distribution of warm inclusions beyond the Viburnum Trend district implies that fluid migration was regional in scale. Fluid inclusion temperatures inconsistent with typical basement heat-flow-controlled geothermal gradients (25 degrees -35 degrees C/km) may be explained by long-distance migration of warm, basin-derived brines. Elevated temperatures observed in fluid inclusions at shallow stratigraphic depths are consistent with a gravity flow hydrologic system characterized by rapid flow rates and the capacity for advective heat transport.

Economic Geology↗
USGS1992

Hydrothermal ore-forming processes in the light of studies in rock- buffered systems: II. Some general geologic applications

The experimental metal solubilities for rock-buffered hydrothermal systems, reported by Hemley et al. (1992), provide important insights into the acquisition, transport, and deposition of metals in real hydrothermal systems that produced base metal ore deposits. Water-rock reactions that determine pH, together with total chloride and changes in temperature and fluid pressure, play significant roles in controlling the solubility of metals and determining where metals are fixed to form ore deposits.Hydrothermal systems circulate fluids and heat, and the transport path of a hydrothermal fluid is likely to lie somewhere between an adiabatic (no heat loss to adjacent rocks) and a geothermal (complete thermal equilibrium with adjacent rock) path. The transport path of the hydrothermal fluids emanating from, or circulating near, deep-seated crystallizing plutons can be approximated by a quasi-adiabatic pressure-temperature path. In such a quasi-adiabatic setting, the pressure effect on rock-buffered metal solubilities is significant and allows metal transport over long distances because the trend of decreasing metal solubility with decreasing temperature is compensated by the trend of increasing metal solubility with decreasing pressure. The high-temperature portion of a quasi-adiabatic hydrothermal system will tend to leach metals from the rock and fix K and Na in feldspars. The source of the extracted metals may be late-stage magmatic melt, trace metals distributed in the lattice of silicate minerals destroyed during rock metasomatism, and/or small amounts of base metal sulfides disseminated throughout a given rock.Deposition of metals in hydrothermal systems occurs where changes such as cooling, pH increase due to rock alteration, boiling, or fluid mixing cause the aqueous metal concentration to exceed saturation. Relative metal transport concentrations, the availability of sulfur, the disposition of the saturation surfaces relative to each other, and the interplay of these variables through time are the major factors controlling the pattern of metal deposition (and nondeposition).Metal zoning results from deposition occurring at successive saturation surfaces. Zoning is not a reflection simply of relative solubility but of the manner of intersection of transport concentration paths with those surfaces. The experimental results are consistent with the typical outward zonation of Cu-Zn-Pb observed in porphyry coppers, Butte-type base metal vein deposits, skarns, and massive sulfides. Implications to mineralization patterns in Mississippi Valley-type, sedimentary Cu, and other low-temperature deposits are also of interest, with due recognition of the greater uncertainty regarding speciation and attainment of equilibrium in those environments. In such deposits, a probable outward zoning of Cu-Zn-Pb-Fe is suggested from the results.Saturation surfaces will tend to migrate outward and inward in prograde and retrograde time, respectively, controlled by either temperature or chemical variables. This, in turn, gives rise to zone migration and, where one zone encroaches on another, the appropriate apparent paragenetic relations. Such textural implications are incorrect, however, unless viewed within the context of the overall mineralization process. Additional controls bearing on metal precipitation sequence and coprecipitation, the presence or absence of zoning, reversals in zoning, sulfidation state, and timing relations between alteration and metallization are implicit in the results and are discussed.

Economic Geology↗
USGS1992

The stable isotope geochemistry of acid sulfate alteration

Acid sulfate wall-rock alteration, characterized by the assemblage alunite + kaolinite + quartz + or - pyrite, results from base leaching by fluids concentrated in H 2 SO 4 . Requisite amounts of H 2 SO 4 can be generated by different mechanisms in three principal geologic environments: (1) by atmospheric oxidation of sulfides in the supergene environment, (2) by atmospheric oxidation at the water table in the steam-heated environment of H 2 S released by deeper, boiling fluids, and (3) by the disproportionation of magmatic SO 2 to H 2 S and H 2 SO 4 during condensation of a magmatic vapor plume at intermediate depths in magmatic hydrothermal environments in silicic and andesitic volcanic terranes. In addition, coarse vein alunite may form in a magmatic steam environment from rapid release of an SO 2 -rich magmatic vapor phase at high temperature and low pressure or from the oxidation of a more reduced magmatic vapor by entrained atmospheric oxygen in the carapace of a volcanic edifice.Alunite [KAl 3 (SO 4 ) 2 (OH) 6 ] contains four stable isotope sites and complete analyses (delta D, delta 18 O (sub SO 4 ) , delta 18 O OH , and delta 34 S) are now possible. Except for delta 18 O OH in magmatic hydrothermal alunites, primary values are usually retained. In cooperation with many colleagues, over 500 measurements have been made on nearly 200 samples of alunite and associated minerals from 23 localities, and 55 additional analyses have been taken from the literature. This survey confirms that kinetic factors play an important role in the stable isotope systematics of alunite and acid sulfate alteration. To a very large extent they form the isotopic basis for distinguishing between environments of acid sulfate alteration, and they provide important insights into attendant processes. Stable isotope analyses of alunite, often in combination with those on associated sulfides and kaolinite, permit recognition of environments of formation and provide information on origins of components, processes (including rates), physical-chemical environments, and temperatures of formation.Supergene acid sulfate alteration may form over any sulfide zone when it is raised above the water table by tectonics or exposed by erosion. It may overprint earlier acid sulfate assemblages, particularly the magmatic hydrothermal assemblages which are pyrite rich such as at El Salvador, Chile; Rodalquilar, Spain; and Goldfield, Nevada. Supergene alunite normally has delta 34 S values virtually identical to precursor sulfides unless bacteriogenic reduction of aqueous sulfate takes place in standing pools of water. delta D values are close to that of local meteoric water unless extensive evaporation occurs. delta D and delta 18 O OH values of supergene alunites from a range of latitudes fall in a broad zone parallel to the meteoric water line much the way delta D and delta 18 O values of associated halloysite-kaolinite fall near the kaolinite line of Savin and Epstein (1970). delta 18 O (sub SO 4 ) values are kinetically controlled and will reflect the hydro-geochemistry of the environment. delta 18 O (sub SO 4 -OH) alues are grossly out of equilibrium and large negative values are definitive of a supergene origin.In steam-heated environments, such as those at the Tolfa district, Italy, and Marysvale, Utah, and numerous modern geothermal systems, acid sulfate alteration zones are characterized by pronounced vertical zoning. Such acid sulfate alteration may occur over adularia-sericite-type base and precious metal ore deposits such as the one at Buckskin, Nevada. Initial delta 18 O (sub SO 4 ) and delta 34 S values are kinetically controlled, but delta 18 O (sub SO 4 ) values usually reach equilibrium with fluids, and even delta 34 S values may reflect partial exchange with H 2 S where the residence time of aqueous sulfate is sufficient. Most alunites of steam-heated origin have delta 34 S values the same as those of precursor H 2 S (and as related sulfides, if present) and delta D values similar to that of local meteoric water. In the samples analyzed, most delta 18 O (sub SO 4 -OH) values give reasonable temperatures of 90 degrees to 160 degrees C, indicating that delta 18 O (sub SO 4 ) and delta 18 O OH values reflect a close approach to equilibrium with the fluid. The delta 18 O (sub SO 4 ) and delta 18 O OH values also reflect the degree of exchange of the meteoric fluids with wall rock. Coeval kaolinites typically have delta 18 O and delta D values to the left of the kaolinitc line.Magmatic hydrothermal, acid sulfate alteration zones in near-surface epithermal deposits such as Summitville, Colorado. Julcani, Peru, and Red Mountain and Lake City, Colorado, are characterized by vertical orientation and horizontal zoning, the presence of coeval pyrite, PO 4 analogues of alunite, zunyite, and later gold, pyrite and enargite, and often other Cu-As-Sb-S minerals. Acid sulfate alteration assemblages also occur as late stages in the porphyry-copper deposit at E1 Salvador, Chile. In the examples studied, magmatic hydrothermal alunites have delta D values close to those for magmatic water. delta 34 S values are 16 to 28 per mil larger than those for associated pyrite, reflecting equilibrium between aqueous H 2 S and SO 4 formed by the disproportionation of magmatically derived SO 2 . delta 18 O (sub SO 4 ) values are usually 8 to 18 per mil and vary systematically with delta 34 S values, reflecting variations in temperature and/or H 2 S/SO 4 fluid ratios. Further variation in delta 18 O (sub SO 4 ) values may result if SO 2 condenses in mixed magmatic meteoric water fluids. delta 18 O (sub SO (sub 4-) OH) values of magmatic hydrothermal alunites are generally unsuitable for temperature determinations because of retrograde exchange in the OH site, but delta 34 S (sub alunite-pyrite) values provide reliable temperature estimates.Magmatic steam environments appear to occur over a range of depths and are characterized by monomineralic veins of coarse alunite in variably alunitized and kaolinized wall rocks containing minor pyrite. Alunite formed in the magmatic steam environment can usually be recognized by delta 34 S near delta 34 S (sub Sigma S) values and delta D and delta 18 O (sub SO 4 ) values near magmatic values. Magmatic steam alunite differs from magmatic hydrothermal alunite by having delta 34 S close to delta 34 S (sub Sigma S) values of the system. delta 18 O (sub SO (sub 4-) OH) values of most magmatic steam alunite give temperatures ranging from 90 degrees to 210 degrees C but, for reasons which are not understood, some temperatures as well as calculated delta 18 O (sub H 2 O) values are too low for presumed precipitation from a magmatic vapor phase. Magmatic steam environments may occur over porphyry-type mineralization as at Red Mountain, Colorado, and Alunite Ridge, Utah, and over or adjacent to adularia-sericite-type deposits in volcanic domes as at Cactus, California.

Economic Geology↗
USGS2003

Crustal structure in the Elko-Carlin Region, Nevada, during Eocene gold mineralization: Ruby-East Humboldt metamorphic core complex as a guide to the deep crust

The deep crustal rocks exposed in the Ruby - East Humboldt metamorphic core complex , northeastern Nevada , provide a guide for reconstructing Eocene crustal structure ∼50 km to the west near the Carlin trend of gold deposits. The deep crustal rocks, in the footwall of a west-dipping normal-sense shear system, may have underlain the Piñon and Adobe Ranges about 50 km to the west before Tertiary extension, close to or under part of the Carlin trend. Eocene lakes formed on the hanging wall of the fault system during an early phase of extension and may have been linked to a fluid reservoir for hydrothermal circulation. The magnitude and timing of Paleogene extension remain indistinct, but dikes and tilt axes in the upper crust indicate that spreading was east -west to northwest-southeast, perpendicular to a Paleozoic and Mesozoic orogen that the spreading overprinted. High geothermal gradients associated with Eocene or older crustal thinning may have contributed to hydrothermal circulation in the upper crust . Late Eocene eruptions, upper crustal dike intrusion, and gold mineralization approximately coincided temporally with deep intrusion of Eocene sills of granite and quartz diorite and shallower intrusion of the Harrison Pass pluton into the core - complex rocks. Stacked Mesozoic nappes of metamorphosed Paleozoic and Precambrian rocks in the core complex lay at least 13 to 20 km deep in Eocene time, on the basis of geobarometry studies. In the northern part of the complex , the presently exposed rocks had been even deeper in the late Mesozoic, to >30 km depths, before losing part of their cover by Eocene time. Nappes in the core plunge northward beneath the originally thicker Mesozoic tectonic cover in the north part of the core complex . Mesozoic nappes and tectonic wedging likely occupied the thickened midlevel crustal section between the deep crustal core - complex intrusions and nappes and the overlying upper crust . These structures, as well as the subsequent large-displacement Cenozoic extensional faulting and flow in the deep crust , would be expected to blur the expression of any regional structural roots that could correlate with mineral belts. Structural mismatch of the mineralized upper crust and the tectonically complex middle crust suggests that the Carlin trend relates not to subjacent deeply penetrating rooted structures but to favorable upper crustal host rocks aligned within a relatively coherent regional block of upper crust .

Nevada↗
USGS2025

Pre-eruptive characteristics of “suspect” silicic magmas in Carlin-type Au-forming systems

World-class Carlin-type Au deposits hosted in sedimentary rock were formed when profuse Eocene silicic magmatism swept across northern Nevada in response to arc migration. Carlin-type Au deposits formed along with porphyry/skarn Cu-Mo-W-Au deposits, epithermal Ag-Au deposits, and distal disseminated Ag-Au deposits. But unlike these other Au-bearing deposits that have clear associations with igneous intrusions, Carlin-type ore deposits appear to have formed distant from concealed plutons, and their origin remains controversial. Despite decades of abundant geophysical, geochronological, and geochemical studies suggesting the involvement of magmas, concrete evidence for magmatic involvement is still lacking. Consequently, the involvement of contemporaneous igneous systems remains inferred based on age, proximity, and variable isotopic, geochemical, and geophysical clues. A recent synthesis of deposit models postulates that Carlin-type Au deposits are intrusion-related, but that the causative magmas reside deeper (∼6–12 km) than in typical porphyry and peripheral systems (∼3–5 km), meaning that Carlin-type deposits are perhaps more distal expressions of igneous intrusions. We investigate a collection of “suspect” magmatic systems over a ∼7 m.y. timespan (∼41–34 Ma) that are contemporaneous with and near known Carlin-type ore deposits. We report results of a multifaceted array of in situ geochemical analyses (FTIR, EMP, SHRIMP-RG, LA-ICP-MS) of quartz-hosted melt inclusions, biotite, and quartz to better characterize the pre-eruptive characteristics of these magmas. We also report results of thermobarometry and thermodynamic phase equilibria modeling to help place constraints on magmatic reservoir depths and processes. Rather than a single “flavor” of silicic magma, we observe a surprisingly broad compositional spectrum of rhyolites, with one end of the spectrum exhibiting more arc-like (I-type) characteristics and the other end displaying more post-subduction, thick-crust extensional (A-type) characteristics. This broad compositional spectrum suggests a more complex picture of silicic crustal magmatism operating over a narrow span of time during slab rollback. Despite this spectrum, magmatic systems in this study are consistently ferroan and generally peraluminous, which we interpret as an expression of the relatively elevated geotherm at the time and incorporation of variable amounts of highly peraluminous metasedimentary crustal components. The silicic magma spectrum encompasses a range of mineralization associations, including subduction-related Cu-Mo-W-Au-Ag and post-subduction, thick-crust extensional rare-metal Mo-Sn-W-F-Be-Ag-Au, consistent with the prolific and diverse array of ore deposits that formed during this time. Carlin-type Au deposition appears to be associated with nearly the entire magmatic spectrum. This apparent indifference to silicic magma “flavor” would seem to imply that if magmas are involved in Carlin-type Au deposit genesis, they perhaps do not need to be compositionally specialized and/or possibly are only relevant as heat sources driving circulation to remobilize and redistribute metals.

Nevada↗
USGS2000

Stable isotope systematics of sulfate minerals

Stable isotope studies of sulfate minerals are especially useful for unraveling the geochemical history of geological systems. All sulfate minerals can yield sulfur and oxygen isotope data. Hydrous sulfate minerals, such as gypsum, also yield oxygen and hydrogen isotope data for the water of hydration, and more complex sulfate minerals, such as alunite and jarosite also yield oxygen and hydrogen isotope data from hydroxyl sites. Applications of stable isotope data can be divided into two broad categories: geothermometry and tracer studies. The equilibrium partitioning of stable isotopes between two substances, such as the isotopes of sulfur between barite and pyrite, is a function of temperature. Studies can also use stable isotopes as a tracer to fingerprint various sources of hydrogen, oxygen, and sulfur, and to identify physical and chemical processes such as evaporation of water, mixing of waters, and reduction of sulfate to sulfide. Studies of sulfate minerals range from low-temperature surficial processes associated with the evaporation of seawater to form evaporite deposits to high-temperature magmatic-hydrothermal processes associated with the formation of base-and precious-metal deposits. Studies have been conducted on scales from submicroscopic chemical processes associated with the weathering of pyrite to global processes affecting the sulfur budget of the oceans. Sulfate isotope studies provide important information to investigations of energy and mineral resources, environmental geochemistry, paleoclimates, oceanography (past and present), sedimentary, igneous, and metamorphic processes, Earth systems, geomicrobiology, and hydrology. One of the most important aspects of understanding and interpreting the stable isotope characteristics of sulfate minerals is the complex interplay between equilibrium and kinetic chemical and isotopic processes. With few exceptions, sulfate minerals are precipitated from water or have extensively interacted with water at some time in their history. Because of this nearly ubiquitous association with water, the kinetics of isotopic exchange reactions among dissolved species and solids are fundamental in dictating the isotopic composition of sulfate minerals. In general, the heavier isotope of sulfur is enriched in the higher oxidation state, such that under equilibrium conditions, sulfate minerals (e.g. barite, anhydrite) are expected to be enriched in the heavy isotope relative to disulfide minerals (e.g. pyrite, marcasite), which in turn are expected to be enriched relative to monosulfide minerals (e.g. pyrrhotite, sphalerite, galena) ( Sakai 1968 , Bachinski 1969 ). The kinetics of isotopic exchange among minerals with sulfur at the same oxidation state, such as sphalerite, and galena, are such that equilibrium is commonly observed. In contrast, isotopic equilibrium for exchange reactions between minerals of different oxidation states depends on factors such as the pH, time and temperature of reaction, the direction of reaction, fluid composition, and the presence or absence of catalysts ( Ohmoto and Lasaga 1982 ). The kinetics of oxygen isotope exchange between dissolved sulfate and water are extremely sluggish. Extrapolation of the high-temperature (100 to 300°C) isotopic exchange kinetic data of Chiba and Sakai (1985) to ambient temperatures suggests that it would take several billions of years for dissolved sulfate and seawater to reach oxygen isotopic equilibrium. In contrast, the residence time of sulfate in the oceans is only 7.9 million years ( Holland 1978 ). However, at higher temperatures (>200°C), oxygen isotopic exchange is sufficiently rapid to permit application of sulfate isotope geothermometry to geothermal systems and hydrothermal mineral deposits. In general, equilibrium prevails at low pH and high temperatures, whereas kinetic factors preclude equilibrium at low temperatures even at low pH. Thus, the sluggish kinetics of sulfur and oxygen isotope exchange reaction at low temperatures impair the use of these isotopes to understand the conditions of formation of sulfate minerals in these environments. However, because of these slow kinetics, the oxygen and sulfur isotopic compositions of sulfate minerals may preserve a record of the sources and processes that initially produced the dissolved sulfate, because the isotope ratios may not re-equilibrate during fluid transport and mineral precipitation. The first part of this chapter is designed to provide the reader with a basic understanding of the principles that form the foundations of stable isotope geochemistry. Next, an overview of analytical methods used to determine the stable isotope composition of sulfate minerals is presented. This overview is followed by a discussion of geochemical processes that determine the stable isotope characteristics of sulfate minerals and related compounds. The chapter then concludes with an examination of the stable isotope systematics of sulfate minerals in a variety of geochemical environments.

Reviews in Mineralogy and Geochemistry↗
USGS2000

Metal-sulfate salts from sulfide mineral oxidation

The observation of “efflorescences,” or the flowering of salts, associated with periods of dryness in soils, in closed-basin lakes, in rock outcrops, and in mines and mine wastes has been noted since early antiquity. The formation of metal-sulfate salts, in connection with the mining of metals, was a phenomenon well known to the early Greek and Roman civilizations. Alum, most commonly potash alum KAl(SO 4 ) 2 · 12H 2 O, which is from the Latin alumen , was extensively mined and used by goldsmiths, dyers, paper manufacturers, and physicians in ancient civilizations. It forms from the oxidation of pyrite in shales and slates and from oxidation of sulfurous gases in geothermal areas. The Greeks and the Romans described stalactites of atramentum (soluble metal-sulfate salts) that formed within mines and along rock faces (Agricola 1546, 1556). Furthermore, the toxic effects of these salts on animals were also noted. For example, in De Natura Fossilium , Agricola (1546) stated “….I mention the congealed acid juice which usually produces cadmia . It is white, hard, and so acrid that it can eat away walls, grills and even destroy all living matter.” Cadmia is thought to be derived from the oxidation of zinc, cobalt, and arsenic sulfides, such as cobaltite. He goes on to say that “Pyrite, unless it contains sulphates, is either a golden or silver color, rarely any other, while cadmia is black, yellow brown, or gray. The former will cure gatherings while the latter is a deadly poison and will destroy any living substance. It is used to kill grasshoppers, mice and flies.” These descriptions suggest the presence of arsenic compounds. The range of colors from white to black commonly is caused by different amounts of admixed pyrite with sulfate minerals. From the days of the Greek philosopher Theophrastus ( ca 325 BCE) and the Greek physician Dioscorides (first century CE), the efflorescent salts atramentum sutorium virida or melanterite (also called melanteria ) and atramentum sutorium caeruleum or chalcanthite were well known to form from the corrosion of pyrite and chalcopyrite by moisture (Agricola 1546, footnotes on p. 47–51). By the time of Pliny the Second (Caius Plinius Secundus, 23–79 CE), the names “green vitriol” for melanterite and “blue vitriol” for chalcanthite were in common use and continued to be used from the Middle Ages to the 20th century.

Reviews in Mineralogy and Geochemistry↗
USGS2024

Terrebonne Basin northern Gulf of Mexico, 30 July-28 September 2023

In the summer and fall of 2023, the Gulf of Mexico Deepwater Hydrate Coring Expedition (UT-GOM2-2) drilled, cored, made downhole measurements, and analyzed samples from the seafloor to the base of the gas hydrate stability zone in one location (Site H, WR313) in the Terrebonne basin, deepwater Gulf of Mexico. Analyses of data and samples from the expedition will inform biological, geochemical, and geomechanical models to constrain the role of gas hydrates in the carbon cycle and the potential for gas hydrates as an energy resource. Pressure and conventional cores were collected continuously to a depth of 155.1 meters below the seafloor (mbsf). At deeper depths, cores were taken periodically from hydrate-bearing sands and their bounding muds to a total depth of 861.3 mbsf. 162.6 m of conventional core and 54.8 m of pressure core were obtained. Twelve temperature measurements were made between 27.1 and 144.5 mbsf to determine the geothermal gradient. At the seafloor, more than 4 m of sandy silt of unknown origin was encountered. Beneath this sand, to a depth of ~200 mbsf, the section was composed of interbedded mud and biogenic carbonate ooze. The biogenic ooze correlated to low density and high porosity intervals observed in the previously acquired logging while drilling (LWD) data and as measured. Calcareous nannofossil biostratigraphy constrains the entire record to the Pleistocene (< 0.91 million years) with a pronounced increase in sedimentation rate with depth. Beneath 200 mbsf, the section was predominantly composed of mud with two thicker, hydrate-bearing coarse-grained intervals, which are commonly known as the Blue and Orange sands. The dissolved gas concentration was quantified from pressure cores. In the shallow section, dissolved methane concentration increased below the sulfate-methane transition zone (SMTZ) and reaches saturation (the limit of solubility for methane) at 147 mbsf. Gas expansion was very common in conventional and depressurized pressure (conventionalized) cores below the SMTZ. At deeper depths, the methane concentration within muds bounding the Blue and Orange reservoirs was generally found to be less than saturation. The dissolved and hydrate gas composition is consistent with a microbial source, containing greater than 99.99% methane and only trace concentrations of ethane, propane, and butane. The methane to ethane ratio (C 1 /C 2 ) and the methane to ethane plus propane (C 1 /(C 2 +C 3 )) decrease with depth down to at least 678 mbsf, mainly driven by the increase in ethane with depth. It is unclear if this trend continues through the Orange sand interval. The δ 13 C isotopic signature of methane ranges between -69.9 and -78.5 ‰ Vienna Pee Dee Belemnite (VPDB). Pressure core recovery of all sandy intervals was poor. However, pressure core logs of the Orange sand show intervals of low density and high velocity, which are indicative of high hydrate saturation. One core from within the Orange sand was composed of interbedded graded sandy silt and mud. The sandy silts from this core are composed of mainly quartz and feldspar with some lithics. Most of the recovered pressure core samples are maintained at near in-situ pressure and temperature (within the hydrate stability field) at the University of Texas Pressure Core Center awaiting analysis. In the shallow section, samples will be used to determine the flux of organic carbon through the basin system, find the rate at which that carbon was consumed, and understand the microbial population responsible for these processes. In the deeper section, samples from in and around the hydrate reservoirs will be used to determine the petrophysical properties of the reservoir and bounding seals in these systems.

Gulf of Mexico, Terrebonne Basin↗
USGS2001

Strontium isotope evolution of pore water and calcite in the Topopah Spring Tuff, Yucca Mountain, Nevada

Yucca Mountain, a ridge of Miocene volcanic rocks in southwest Nevada, is being characterized as a site for a potential high-level radioactive waste repository. One issue of concern for the future performance of the potential repository is the movement of water in and around the potential repository horizon. Past water movement in this unsaturated zone is indicated by fluid inclusions trapped in calcite coatings on fracture footwall surfaces and in some lithophysal cavities. Some of the fluid inclusions have homogenization temperatures above the present-day geotherm (J.F. Whelan, written communication), so determining the ages of the calcite associated with those fluid inclusions is important in understanding the thermal history of the potential repository site. Calcite ages have been constrained by uranium-lead dating of silica polymorphs (opal and chalcedony) that are present in most coatings. The opal and chalcedony ages indicate that deposition of the calcite and opal coatings in the welded part of the Topopah Spring Tuff (TSw hydrogeologic unit) spanned nearly the entire history of the 12.8-million-year-old rock mass at fairly uniform overall long-term rates of deposition (within a factor of five). Constraining the age of a layer of calcite associated with specific fluid inclusions is complicated. Calcite is commonly bladed with complex textural relations, and datable opal or chalcedony may be millions of years older or younger than the calcite layer or may be absent from the coating entirely. Therefore, a more direct method of dating the calcite is presented in this paper by developing a model for strontium evolution in pore water in the TSw as recorded by the strontium coprecipitated with calcium in the calcite. Although the water that precipitated the calcite in fractures and cavities may not have been in local isotopic equilibrium with the pore water, the strontium isotope composition of all water in the TSw is primarily controlled by water-rock interaction in the overlying nonwelded and essentially unfractured Paintbrush Group tuffs (PTn). The method of dating secondary minerals from known strontium evolution rates in rocks cannot be used in this study because it assumes the water that deposited the minerals was in isotopic equilibrium with the rock, which is not the case for the pore water in the TSw. Therefore, the evolution of the strontium isotope composition of the water that deposited the calcite, as recorded by the strontium coprecipitated with calcium in the calcite, was used to develop a model for determining the age of the calcite.

Nevada↗
USGS2025

Geochemistry and Soils of the Big Smoky Valley Fens, Nevada

Fens are groundwater-fed wetlands that can provide habitat for plants and animals. Due to anthropogenic activities and climate change, many fens around the world are at risk. This paper presents the results of a study of the hydrology and geochemistry of fens in Big Smoky Valley, central Nevada to support the Bureau of Land Management’s activities in the area. A water sample from the largest fen in the study area was analyzed for its water chemistry and compared to a nearby alluvial aquifer and hot spring. The high SiO 2 concentration of the fen sample implies that the fen water may originate from geothermal water. A soil core was taken to analyze radiocarbon age and soil type. A majority of the core was composed of silt and clay interlayered with water-filled voids. Changes in the character of the clay with depth suggest that there may have been changes in the depositional environment over time. Radiocarbon dating of Ruppia seeds showed longevity of the fen, with the minimum 14 C age of the core as 4,375±40 years. This paper provides reconnaissance-level information on the Big Smoky Valley fens, but further information would be needed to better understand the source of water to the fens or how the fen environment has changed over time with climate.

Nevada↗
USGS2014

Telescoping metamorphic isograds: Evidence from 40Ar/39A dating in the Orange-Milford belt, southern Connecticut

New 40 Ar/ 39 Ar ages for hornblende and muscovite from the Orange-Milford belt in southern Connecticut reflect cooling from Acadian amphibolite facies metamorphism between ∼380 to 360 Ma followed by retrograde recrystallization of fabric-forming muscovite and chlorite during lower greenschist facies Alleghanian transpression at ∼280 Ma. Reported field temperature and pressure gradients are improbably high for these rocks and a NW metamorphic field gradient climbing from chlorite-grade to staurolite-grade occurs over less than 5 km. Simple tilting cannot account for this compressed isograd spacing given the geothermal gradient of ∼20 °C/km present at the time of regional metamorphism. However, post-metamorphic transpression could effectively telescope the isograds by stretching the belt at an oblique angle to the isograd traces. Textures in the field and in thin section reveal several older prograde schistosities overprinted by lower greenschist facies fabrics. The late cleavages commonly occur at the scale of ∼100 μm and these samples contain multiple age populations of white mica. 40 Ar/ 39 Ar analysis of these poly-metamorphic samples with mixed muscovite populations yield climbing or U-shaped age spectra. The ages of the low temperature steps are late Paleozoic, while the ages of the older steps are late Devonian. These results support our petrologic interpretation that the younger cleavage developed under metamorphic conditions below the closure temperature for Ar diffusion in muscovite, that is, in the lower greenschist facies. The correlation of a younger regionally reproducible age population with a pervasive retrograde muscovite ± chlorite cleavage reveals an Alleghanian (∼280 Ma) overprint on the Acadian metamorphic gradient (∼380 Ma). Outcrop-scale structures including drag folds and imbricate boudins suggest that Alleghanian deformation and cleavage development occurred in response to dextral transpression along a northeast striking boundary. Alleghanian oblique collision of accreting terranes from the northeast would have resulted in northeast-southwest dextral transpression against the New York promontory. This deformation was responsible for crystallization of pervasive retrograde muscovite + chlorite cleavages and associated telescoping of the Acadian metamorphic isograds in southern Connecticut at ∼280 Ma.

Connecticut↗
USGS1989

Liquid-vapor relations for the system NaCl-H2O: Summary of the P-T- x surface from 300° to 500°C

Experimental data on the vapor-liquid equilibrium relations for the system NaCl-H 2 O were compiled and compared in order to provide an improved estimate of the P-T-x surface between 300 &deg; to 500 &deg; C, a range for which the system changes from subcritical to critical behavior. Data for the three-phase curve (halite + liquid + vapor) and the NaCl-H 2 O critical curve were evaluated, and the best fits for these extrema then were used to guide selection of best fit for isothermal plots for the vapor-liquid region in-between. Smoothing was carried out in an iterative procedure by replotting the best-fit data as isobars and then as isopleths, until an internally consistent set of data was obtained. The results are presented in table form that will have application to theoretical modelling and to the understanding of two-phase behavior in saline geothermal systems.

American Journal of Science↗
USGS1991

Quartz solubility in hydrothermal seawater: An experimental study and equation describing quartz solubility for up to 0.5 M NaCl solutions

Experimental investigations confirm an increase of quartz solubility in sea-water relative to distilled water. Combination of the experimental data with published data, most of which related to distilled water, permitted construction of a database for calculating an equation that fits all the data. Application of the equation indicates a shallower depth of circulation for sea- floor hydrothermal waters than has previously been calculated. Use of the equation as a quartz geothermometer-geobarometer has applications not only for hydrothermal sea-water systems, but also for other geothermal and geological systems.

American Journal of Science↗
USGS1990

Diagenesis and interstitial-water chemistry at the Peruvian continental margin; major constituents and strontium isotopes

Two distinct hydrogeochemical regimes currently dominate the Peruvian continental margin. One, in shallower water (150-450 m) shelf to upper-slope regions, is characterized by interstitial waters with strong positive chloride gradients with depth. The maximum measured value of 1043 mM chloride at Site 680 at ITS corresponds to a degree of seawater evaporation of ~2 times. Major ion chemistry and strontioum isotopic composition of the interstitial waters suggest that a subsurface brine that has a marine origin and is of pre-early Miocene "age," profoundly influences the chemistry and diagenesis of this shelf environment. Site 684 at ~9°S must be closest to the source of this brine, which becomes diluted with seawater and/or interstitial water as it flows southward toward Site 686 at ~13°S (and probably beyond) at a rate of approximately 3 to 4 cm/yr, since early Miocene time. The other regime, in deep water (3000-5000 m) middle to lower-slope regions, is characterized by interstitial waters with steep negative and nonsteady-state chloride gradients with depth. The minimum measured value of 454 mM chloride, at Site 683 at ITS, corresponds to —20% dilution of seawater chloride The most probably sources of these low-chloride fluids are gas hydrate dissociation and mineral (particularly clay) dehydration reactions. Fluid advection is consistent with (1) the extent of dilution shown in the chloride profiles, (2) the striking nonsteady-state depth profiles of chlorides at Sites 683 and 688 and of 87Sr/86Sr ratios at Site 685, and (3) the temperatures resulting from an average geothermal gradient of 50°C/km and required for clay mineral dehydration reactions. Strontium isotope data reveal two separate fluid regimes in this slope region: a more northerly one at Sites 683 and 685 that is influenced by fluids with a radiogenic continental strontium signature, and a southerly one at Sites 682 and 688 that is influenced by fluids with a nonradiogenic oceanic signatures. Stratigraphically controlled fluid migration seems to prevail in this margin. Because of its special tectonic setting, Site 679 at ITS is geochemically distinct. The interstitial waters are characterized by seawater chloride concentrations to —200 mbsf and deeper by a significantly lower chloride concentration of about two-thirds of the value in seawater, suggesting mixing with a meteoric water source. Regardless of the hydrogeochemical regime, the chemistry and isotopic compositions of the interstitial waters at all sites are markedly modified by diagenesis, particularly by calcite and dolomite crystallization.

Peruvian Continental Margin↗
USGS1990

Gas hydrates of the Peruvian outer continental margin

Gas hydrates were recovered during coring by Ocean Drilling Program (ODP) Leg 112 at Sites 685 and 688 on the Peruvian outer continental margin at latitudes of 9° and 11.5°S, where water depths are 5070 and 3820 m, respectively. In addition, nearby Sites 682 and 683 yielded compelling evidence that gas hydrates are present, but gas hydrates were not directly observed there. Anomalous acoustic reflectors, known as bottom-simulating reflectors (BSRs), on marine seismic profiles from the region also provided inferential evidence that gas hydrates are present. Geothermal gradients of about 43 and 49°C/km were calculated on the basis of relations between depths to BSRs, bottom-water temperatures, and the pressure-temperature stability field of gas hydrates. Geochemical studies revealed that methane concentrations increase rapidly with depth after pore-water sulfate concentrations have been depleted. The relationship between methane and sulfate suggests that microbial processes account for the generation of methane, and the relationship between the carbon isotopic composition of methane and dissolved carbon dioxide supports this suggestion. We believe that decreasing chlorinity in pore water from squeezed sediment at the four sites results mainly from the decomposition of gas hydrates and is a dilution artifact observed as a result of the squeezing procedure. Maximum chlorinity values at or near the surface result from excess salt that comes from the formation of gas hydrates composed of freshwater. Record alkalinity attests to the intensity of diagenetic processes and has significant effects on salinity profiles at these sites. Gas hydrates were recovered at 99 and 166 meters below the seafloor (mbsf) at Site 685, and at 141 mbsf at Site 688 in Pleistocene diatomaceous mud. Methane constitutes more than 99% of the hydrocarbon gas mixture in the gas hydrates. The volumetric ratio of methane to water in the sample from Site 685 is 100, indicating that the sampled gas hydrate is either undersaturated with respect to methane or had partially decomposed during core recovery or both. The discovery of gas hydrates in lower slope deposits of the Peruvian outer continental margin extends our knowledge of gas-hydrate formation and occurrence in the Circum-Pacific region.

ODP Leg 112↗
USGS1980

Geology and mineral resources of the North Absaroka Wilderness and vicinity, Park County, Wyoming, with sections on mineralization of the Sunlight mining region and geology and mineralization of the Cooke City mining district, and a section on aeromagnetic interpretation

SUMMARY The North Absaroka Wilderness is approximately 560 square miles (1,450 km 2 ) of rugged scenic mountainous terrain that adjoins the eastern boundary of Yellowstone National Park in northwestern Wyoming. The area was studied during 1970, 1971, and 1972 by personnel of the U. S. Geological Survey and the U. S. Bureau of Mines to evaluate its mineral-resource potential as required by the Wilderness Act of 1964. This evaluation is based on a search of the literature courthouse and production records, geologic field mapping, field inspection of claims and prospects, analyses of bedrock and stream-sediment samples, and an aeromagnetic survey. The North Absaroka Wilderness is underlain almost entirely by andesitic and basaltic volcanic rocks of Eocene age. These volcanics rest on deformed sedimentary rocks of Paleozoic and, locally, of Mesozoic age that are exposed at places along the northern and eastern edges of the wilderness. Dikes and other igneous intrusive bodies cut both the volcanic and sedimentary rocks. A nearly flat detachment fault, the Heart Mountain fault, and a related steep break-away fault have displaced middle and upper Paleozoic rocks and some of the older part of the volcanic sequence to the southeast. A much greater thickness of volcanic rocks was found to be involved in Heart Mountain faulting than had previously been recognized; however, most of the volcanic rocks and many of the intrusives were emplaced after Heart Mountain faulting. Local folding and high-angle faulting in mid-Eocene time have deformed all but the youngest part of the volcanic sequence in the southeastern part of the wilderness. This deformation is interpreted as the last pulse of Laramide orogeny. The results of this study indicate that the mineral-resource potential of the wilderness is minimal. Bentonite, petroleum, low-quality coal, and localized deposits of uranium and chromite have been produced in the surrounding region from rocks that underlie the volcanic rocks; but such deposits, if present in the wilderness, would be too deeply buried, too small, or too sporadically distributed to be profitably located and exploited. Copper and gold mines and prospects are present on the fringes of the wilderness, but otherwise the area seems to be devoid of economically valuable concentrations of metallic minerals. No surface evidence of geothermal-energy potential was found. Known mineral deposits in the vicinity of the North Absaroka Wilderness are associated with intrusive rocks. From the Cooke City mining district, just north of the wilderness, replacement deposits in Upper Cambrian carbonate rocks may extend a short distance into the north edge of the wilderness, In the Sunlight mining region, an enclave nearly surrounded by the wilderness, mineralization occurs in veins and is disseminated in volcanic and plutonic rocks. Richer concentrations of metallic minerals may occur in carbonate rocks adjacent to intrusive bodies at depth beneath the volcanic rocks in the Sunlight region. A few small intrusive bodies occur in the wilderness, but no significant associated mineralization was detected. Aeromagnetic data indicate that other intrusives not exposed by erosion may occur in the wilderness; however, no significant metamorphism or alteration is evident at the surface to indicate their presence. Although most of the rocks of the wilderness are of igneous origin, they are all so old (Eocene) that it is unlikely that they retain any original heat. The Pleistocene rhyolitic ash-flow tuffs in the southwestern part of the wilderness were erupted from sources in Yellowstone National Park just to the west; however, in the wilderness these tuffs are too thin to contain any residual heat.

Wyoming↗
USGS1987

Mineral resources of the East Fork High Rock Canyon Wilderness Study Area, Washoe and Humboldt counties, Nevada

The part of the East Fork High Rock Canyon Wilderness Study Area (CA-020-914/NV-020-006A) included in this study encompasses 33,460 acres in the northwestern part of Nevada. Throughout this report, "wilderness study area" and "study area" refertothe 33,460 acres for which mineral surveys were requested. The U.S. Geological Survey and the U.S. Bureau of Mines conducted geological, geophysical, and geochemical surveys to assess the mineral resources (known) and the mineral resource potential (undiscovered) of the study area. Fieldwork for this report was carried out in 1985 and 1986. No mines, significant prospects, or mining claims are located inside the study area, and no identified resources were found. The wilderness study area has moderate mineral resource potential for gold, silver, and mercury and for zeolite minerals. A low potential also exists for geothermal energy resources, and potential for oil and gas is unknown.

Nevada↗
USGS1987

Mineral resources of the Fort Piute Wilderness Study Area, San Bernardino County, California

The Fort Piute Wilderness Study Area (CDCA-267) is in northeastern San Bernardino County, California, near the boundary between California and Nevada. Mineral surveys were requested for 31,371 acres of the Fort Piute Wilderness Study Area. In this report the area studied is referred to as "the study area". Examination of mines and prospects in the area was accomplished by the U.S. Bureau of Mines in 1981 and 1982. Field investigations of the area were carried out by the U.S. Geological Survey in 1983 and 1985. No mines or prospects, few mining claims, and no identified resources are located within the wilderness study area. Moderate and low potential for gold resources appears limited to outcrops of gneiss and granite exposed along the eastern side of the Piute Range. Available information indicates that there is no potential for energy resources, including oil and gas, uranium, or geothermal, in the study area.

California↗