Sulfur isotope analysis of authigenic pyrite in the Creede Formation documents its precipitation by the reaction between iron in the volcaniclastic sediments and H 2 S formed through bacteriogenic reduction of sulfate added to the lake during and immediately following repeated volcanic eruptions during sedimentation. Pyrite veinlets in the underlying Snowshoe Mountain Tuff were formed by the percolation of H 2 S-bearing pore waters into fractures in the tuff. Conventional analyses of bulk samples of authigenic pyrite range from -20.4‰ to 34.5‰, essentially equivalent to the range of -30‰ to 40‰ determined using SHRIMP microprobe techniques. Conventional analyses of bulk samples of pyrite from veinlets in the Snowshoe Mountain Tuff range from -3.5‰ to 17.6‰, much more limited than the ranges of -23‰ to 111‰ and -15.6‰ to 67.0‰ determined by SHRIMP and laser ablation microbeam techniques, respectively. The extreme range of δ 3 4 S for the veinlets is interpreted to be the result of continued fractionation of the already 3 4 S-depleted pore waters. Oxygen isotope analysis of authigenic smectite, kaolinite, and K-feldspar together with fluid-inclusion temperatures and oxygen isotope analysis of calcite coexisting with kaolinite indicate that the smectites formed early during burial diagenesis, in accord with the petrographic observations. The 4 0 Ar/ 3 9 Ar dating of K-feldspar, concordance of K-feldspar, kaolinite, and calcite δ 1 8 O values, and fluid-inclusion temperatures in calcite, indicate that the sediments at core hole CCM-1 were subjected to a hydrothermal event at 17.6 Ma. The minerals formed from oxygen-shifted meteoric waters with δ 1 8 O values of ∼-9‰. Smectites at CCM-1 at least partially exchanged with these waters. Carbon and oxygen isotope analysis of authigenic calcites in the Creede Formation show that they formed over a wide range of temperatures from fluids having a wide range of isotopic composition, presumably over an extended period of time. Some of the cements apparently formed very late from unexchanged meteoric water. Concretions and possibly some cements at CCM-1 appear to have exchanged with the 17.6 Ma oxygen-shifted hydrothermal fluids. Such exchange is consistent with evidence that lacustrine carbonates at CCM-1 exchanged with low 1 8 O waters, whereas those at CCM-2 underwent little, if any, exchange. The δ 1 3 C-δ 1 8 O values for calcite veinlets in the Creede Formation are similar to those for authigenic calcites. Fluid-inclusion temperatures and δ 1 8 O values indicate that some were deposited during the 17.6 Ma hydrothermal event and others from unexchanged meteoric water at a later date. The isotope studies confirm that part of the model of Rye et al., proposing that the barites in the southern end of the Creede Mining District were formed by mixing of the Creede hydrothermal system with Lake Creede pore or lake waters. The silicate and carbonate isotope data indicate that the pores of the Creede Formation were occupied by at least three isotopically distinct waters since the time of deposition. The original pore fluids probably shifted to lower δ 1 8 O values during burial diagenesis as a result of the hydrolysis of the volcanic glass to form smectites and other hydrous silicates. During or prior to a 17.6 Ma hydrothermal event in the vicinity of CCM-1, the Creede Formation was flushed with oxygen-shifted meteoric water, possibly related to the breaching of the east side of the caldera wall sometime between 20 and 22 Ma.
Abstract In the context of exploration for epithermal deposits, why study geothermal systems at all? After all, not one exploited system to date has been shown by drilling to harbor any economically significant metal resource--but then until recently not one had been drilled for other than geothermal energy exploration.* The latter involves drilling to depths of 500-3000 meters in search of high temperatures and zones of high permeability which may sustain fluid flow to production wells for steam separation and electricity generation. In many cases such exploration wells have discovered disseminated base-metal sulfides with some silver and argillic-propylitic alteration equivalent to that commonly associated with ore-bearing epithermal systems (Browne, 1978; Henley and Ellis, 1983; Hayba et al., 1985, this volume). In general, however, geothermal drilling ignores the upper few hundred meters of the active systems and drill sites are situated well away from natural features such as hot springs or geysers, the very features whose characteristics (silica sinter, hydrothermal breccias) are recognizable in a number of epithermal precious-metal deposits (see, for example, White, 1955; Henley and Ellis, 1983; White, 1981; Berger and Eimon, 1983; Hedenquist and Henley, 1985; and earlier workers such as Lindgren, 1933). Knowledge of the upper few hundred meters of active geothermal systems is scant and largely based on interpretation of hot-spring chemistry. Tantalizingly, in a number of hot springs, transitory red-orange precipitates occur which are found to be ore grade in gold and silver and which carry a suite of elements (As, Sb, Hg, Tl) now recognized as characteristic of epithermal gold deposits (Weissberg, 1969).
Acid-sulfate wall-rock alteration, characterized by the assemblage alunite+kaolinite+ quartz±pyrite, results from base-leaching by fluids concentrated in H2SO4.Requisite amounts of H2SO4 can be generated by different mechanisms in 3 principal geological environments: 1) by atmospheric oxidation of sulfides in the super gene environment, 2) by atmospheric oxidation at the water table in the steam-heated environment, of H2S released by deeper, boiling fluids, and 3) by the disproportionation of magmatic SC>2 to F^S and H2SO4 during condensation of a magmatic vapor plume at intermediate depths in magmatic-hydrothermal environments in silicic and andesitic volcanic terrains.In addition, coarse vein alunite may form in a magmatic-steam environment from rapid release of a SO2-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 [KAh(SO4)2(OH)fil contains four stable isotope sites and complete analyses (SD, 818Oso4» 8 °OQH» and 5^4S) are now possible.Except for 818OoH in magmatichydrothermal 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 20 localities and 55 additional analyses have been taken from the literature.These complete stable isotope analyses 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 acidsulfate assemblages, particularly the magmatic-hydrothermal assemblages which are pyriterich such as at El Salvador, Chile; Rodalquilar, Spain and Goldfield, Nevada.Supergene alunite has S^S virtually identical to precursor sulfides unless bacteriogenic reduction of aqueous sulfate takes place in standing pools of water.SD values will be close to that of local meteoric water unless extensive evaporation occurs, and 8D-818OoH of supergene alunites from a range of latitudes fall in a broad zone parallel to the meteoric water line much the way SD and S18O values of associated halloysite/kaolinite fall near the kaolinite line of Savin and Epstein (1970).S18Oso4 values are kinetically controlled and will reflect the hydrogeochemistry of the environment.A 18OsO4-OH values are grossly out of equilibrium and negative values are usually 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 at Buckskin, Nevada.Initial 8 18OsO4 and 834S values are kinetically controlled, but S18Oso4 values usually reach equilibrium with fluids, and even 834S values may reflect partial exchange with H^S where the residence time of aqueous sulfate are sufficient.Most alunites of steam-heated origin have S^S the same as precursor H2S (and as related sulfides, if present) and SD the same as local meteoric water.In the samples analyzed, 8 18Osot and 818OoH appear to reflect a close approach to equilibrium with the fluid, and A 18Oso4-OH values give depositional temperatures of 90° to 160°C.The 818Oso4 and 818OoH values reflect the degree of exchange of the meteoric fluids with wall rock.Coeval kaolinites typically have 818Oso4 and SD to the left of the kaolinite line.Magmatic-hydrothermal acid-sulfate environments in near-surface epithermal deposits such as Summitville, Colorado; Julcani, Peru and Red Mountain, Lake City, Colorado are characterized by vertical aspect and horizontal zoning, the presence of coeval pyrite and possibly PO4 analogs of alunite, and zunyite, and later gold, pyrite and enargite.Acidsulfate alteration assemblages also occur as late stages in the porphyry-copper deposit at El Salvador.Chile.In the examples studied, magmatic-hydrothermal alunites have SD and initial 518OoH close to values for magmatic water.S34S values are 16 to 28 %c larger than those for associated pyrite, reflecting equilibrium between aqueous H2S and SO4 formed by the disproportionation of magmatically derived SO2.818Oso4 values are 10 to 15 %o and vary systematically with S^S reflecting variations in temperature and/or H2S/SO4 fluid ratios.Further variation of 8 18Oso4 may result if SC>2 condenses in mixed magmaticmeteoric water fluids.A 18Oso4-OH values of magmatic-hydrothermal alunites are generally unsuitable for temperature determinations because of retrograde exchange in the OH site but A^Saiun-py 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 S^S near 834S£S and 8D and 8 18Oso4 near magmatic values.Magmatic-steam alunite differs from magmatic-hydrothermal alunite by having S^S close to 834Sss of the system.A 18Oso4-OH values of most magmatic-steam alunite give temperatures ranging from 90 to 200°C but, for reasons which are not understood, calculated 518On2O values are often too low for presumed precipitation from a magmatic vapor phase.Magmatic-steam environments may occur over porphyry type mineralization as Red Mountain, CO and Alunite Ridge, UT and over adularia-sericite type deposits as at Cactus, CA.As indicated above, this study has been conducted in cooperation with a number of colleagues engaged in more comprehensive studies of several of the examples chosen for investigation, particularly mineralized areas.Antonio Arribas, Jr., currently studying the Rodalquilar district in Spain as part his Ph.D. program, and Jeff Deen, who recently completed a Ph.D. dissertation on Julcani, Peru each conducted the stable isotope portion of his research in Rye's laboratory, and provided a number of analyses herein reported, in addition to those related to his thesis studies.Each provided geological and mineralogical information and insights that were critical to the interpretation of the stable isotope data.Roger Stoffregen, whose recently published study of the Summitville, Colorado Cu-Au-Ag deposit (Stoffregen, 1987) provided the mineralogical and paragenetic basis for the interpretation of that deposit, supplied samples and cooperated in the interpretation of the stable isotope data for Summitville.Dana Bove worked out the complex paragenesis of acid-sulfate alteration in the Red Mountain district near Lake City, Colorado, and supplied samples based on that paragenesis.In addition, a number of other colleagues supplied samples from various localities.Foremost among these was Cy Field who supplied all the samples from Tolfa, Italy and samples from El Salvador, Chile; Santa Rita, New Mexico; and Mineral Park, Arizona.Rich Fifarek supplied most of the samples on Round Mountain, Nevada.Peter Vikre and Roger Ashley provided samples and shared their knowledge of several districts in Nevada as noted at appropriate places in the text.CoCa Mines permitted access to their Cactus, California gold deposit and Jim Brady shared his knowledge of the district and guided the sampling of the deposit.The stable isotope analysis of all four sites in alunite is a formidable task.Considerable effort was expended in developing mineralogical and chemical separation techniques as well as isotope analytical methods.A summary of the methodology is currently being prepared for publication.The development of these techniques was undertaken by Wasserman with the assistance of Rye on isotope analyses and Arribas and Bethke on mineralogical separation and J. A. Goss and Bethke on the chemical separation of sulfate from alunite.A number of analyses were made by Carol Gent.In addition to those colleagues whose contributions to this study have been cited above, a number of others have contributed suggestions, information and other support.These include
Introduction Sample preparation and documentation Components of the system used to study fluid inclusions Magni fyi ng/i11uminati ng system Gas-flow system Electrical system Temperature measurement and recording system Measuring procedures Advantages of system References cited ILLUSTRATIONS Page Q 12 (Figure captions appear on page 13; figures follow on pages 14-18) Figure 1.Photograph showing the components of the system used to analyze fluid inclusions.2. Close-up photograph of the heating/freezing stage.3. Schematic diagram of the air/nitrogen flowcontrol manifold.4. Photograph showing the coils that carry nitrogen gas into the Dewar of liquid nitrogen.5. Photograph showing the tube assembly to prevent frosting on the windows of the stage.TABLE Table 1.Equipment used in the fluid inclusion laboratory AbstractThe supplementary components and operating procedures for the U.S. Geological Survey gas-flow heating/freezing stage have heen developed to enhance the rapid, accurate measurement of fluid inclusions homogenizing between -150°C and FiOn 0 C. Pouhly polished mineral slabs as thick as ?mm are carefully photographed at several magnifications to provide the optimum samples for large-scale growth history-fluid inclusion studies such as those pursued in our laboratory.Long-focal-length condensers and fiber-optic illuminators provide optimal viewing conditions and the large sample chamber volume saves both documentation and samplechangeover time.Nitrogen gas, chilled by passage through a tank of liquid nitrogen, and air, heated by passage through a glass-encased, nichrome-wire heating coil, are used for freezing and heating runs, respectively.Precise control of temperatures and heating rates, as well as careful stage and thermocouple calibration, make it possible to achieve accuracies of +_2.0°C for heating runs and +_0.2°C for freezing runs.The use of gas to control temperature (1) permits rapid accumulation of abundant, accurate data, (2) ameliorates thermal gradient problems so often encountered in stages using convection and/or conduction for heat transfer, and (3) permits the use of a cyclical procedure to measure temperatures that would otherwise be unmeasurable.
Research Article| February 01, 1981 Silicic volcanism Peter W. Lipman; Peter W. Lipman Search for other works by this author on: GSW Google Scholar Philip M. Bethke; Philip M. Bethke Search for other works by this author on: GSW Google Scholar Hugh P. Taylor Hugh P. Taylor Search for other works by this author on: GSW Google Scholar Author and Article Information Peter W. Lipman Philip M. Bethke Hugh P. Taylor Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1981) 9 (2): 94–96. https://doi.org/10.1130/0091-7613(1981)9<94:SV>2.0.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Peter W. Lipman, Philip M. Bethke, Hugh P. Taylor; Silicic volcanism. Geology 1981;; 9 (2): 94–96. doi: https://doi.org/10.1130/0091-7613(1981)9<94:SV>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract No Abstract Available. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Galenas from the major Creede veins and their northern extensions are remarkably homogeneous in Pb-isotopic composition and are too radiogenic to have been derived from any magma comparable in composition to the principal volcanic rocks. This pattern was identified by Doe et al. in 1979 who proposed that the lead was derived from the Precambrian basement. The homogeneity of the ore leads, however, requires a uniform reservoir; an unlikely prospect for lead from the Precambrian basement. We report on 16 new analyses of geographically and paragenetically dispersed galenas from the Creede district and other areas as far as 11 km to the north. The lead values range from 18.972 to 19.060 for 206Pb/204Pb, from 15.591 to 15.671 for 207Pb/204Pb, and from 37.781 to 37.921 for 208Pb/204Pb. These ranges overlap those previously reported for the main ore zone. Recent work allows us to extend the results of Doe et al. and to consider alternative processes to explain the widespread homogeneity and radiogenic nature of the ore lead: 1) David Matty (pers. commun., 1986) has shown that some minor volcanic units in the area have unusually radiogneic lead values; magmas comparable in composition to the units are a possible, though improbable, source of the ore lead. 2) The uniformity of the isotopic values of galenas may have resulted from homogenization during an extensive potassium-metasomatic event that predated the ores; this possibility is being tested in an on-going study of feldspars from metasomatized and unmetasomatized rocks. 3) Recent regional studies suggest the possibility of a prevolcanic, NNW-trending graben system filled by clastic sediments derived from the Precambrian basement, a process that would have an homogenizing effect on the lead isotopes. This interpretation implies importation, deep within the Creede hydrologic system, of fluids from remote sources. These alternatives show that the Pbisotope systematics may have a profound impact on the interpretation of the Creede hydrothermal system, and that further study is warranted.
More than 1,300 measurements on fluid inclusions in fluorite and sphalerite indicate that stretching proceeds systematically and predictably. In order to generate internal pressures that are sufficiently high to cause stretching, most inclusions must be heated beyond their initial homogenization temperatures, i.e., overheated. The amount of overheating necessary to initiate stretching depends on the P-V-T-X properties of the inclusion fluid, the inclusion size and shape, physical properties of the host mineral, and the confining pressure. In the range of homogenization temperatures and salinities examined in this study, the amount of overheating necessary to initiate stretching of inclusions in fluorite is inversely related to inclusion volume. The results of this study show that the measured homogenization temperature of a fluid inclusion may be considerably higher than the true homogenization temperature if the internal pressure reached sufficiently high values during previous testing in the laboratory or, less likely, during postentrapment thermal events in nature. The systematic relationship between the internal pressure necessary to initiate stretching and the inclusion volume provides a means of recognizing previously stretched inclusions and estimating the magnitude of postentrapment thermal events. Furthermore, reproducibility of homogenization temperature measurements, lack of microscopically observable fractures in the inclusion walls, and the lack of a noticeable increase in the vapor bubble size after heating are insufficient to prove that an inclusion has not stretched.--Modified journal abstract.
