Low-temperature Hydrothermal Systems Research Articles

Chemical and isotopic investigation of warm springs associated with normal faults in Utah

Thermal springs associated with normal faults in Utah have been analyzed for major cations and anions, and oxygen and hydrogen isotopes. Springs with measured temperatures averaging greater than 40°C are characterized by Na + K- and SO 4 + Cl-rich waters containing 10 3 to 10 4 mg/l of dissolved solids. Lower temperature springs, averaging less than 40°C, are more enriched in Ca + Mg relative to Na + K. Chemical variations monitored through time in selected thermal springs are probably produced by mixing with non-thermal waters. During the summer months at times of maximum flow, selected hot springs exhibit their highest temperatures and maximum enrichments in most chemical constituents. Cation ratios and silica concentrations remain relatively constant through time for selected Utah thermal springs assuring the applicability of the geothermometer calculations regardless of the time of year. Geothermometer calculations utilizing either the quartz (no steam loss), chalcedony or Mg-corrected Na/K/Ca methods indicate that most thermal springs in Utah associated with normal faults have subsurface temperatures in the range of 25 to less than 120°C. This temperature range suggests fluid circulation is restricted to depths less than about three kilometers assuming an average thermal gradient of about 40°C/km. Thermodynamic calculations suggest that most thermal springs are oversaturated with respect to calcite, quartz, pyrophyllite, (Fe, Mg)-montmorillonite, microcline and hematite, and undersaturated with respect to anhydrite, gypsum, fluorite and anorthite. Chalcedony and cristobalite appear to be the only phases consistently at or near saturation in most waters. Theoretical evaluation of mixing on mineral saturation trends indicates that anhydrite and calcite become increasingly more undersaturated as cold, dilute groundwater mixes with a hot (150°C), NaCl-rich fluid. The evolution of these thermal waters issuing from faults appears to be one involving the dissolution of silicates such as feldspars and micas by CO 2-enriched groundwaters that become more reactive with increasing temperature and/or time. Solution compositions plotted on mineral equilibrium diagrams trend from product phases such as kaolinite or montmorillonite toward reactant phases dominated by alkali feldspars. Isotopic compositions indicate that these springs are of local surface origin, either meteoric (low TDS, < 5000 mg/l) or connate ground water (high TDS, > 5000 mg/ l). Deviations from the meteoric water line are the result of rock-water isotopic exchange, mixing or evaporation. Fluid source regions and residence times of selected thermal spring systems (Red Hill, Thermo) have been evaluated through the use of a σ D-contour map of central and western Utah. Ages for waters in these areas range from about 13 years to over 500 years. These estimates are comparable to those made for low-temperature hydrothermal systems in Iceland.

Post-Kombolgie metasomatism at Jabiluka, Northern Territory, Australia, and its significance in the formation of high-grade uranium mineralization in lower Proterozoic rocks

Diagenetic lithification of Kombolgie sandstones followed by a metasomatic event which produced veining and replacement. Metasomatism was controlled by brecciation and bedding within the sandstones. The high-grade uranium ores are interpreted to be the product of a low-temperature (100 degrees -200 degrees C) hydrothermal system of ground water which was driven by a regional heating event also manifested by phonolite and/or diabase intrusions. The Kombolgie sandstones were an essential part of the hydrologic system which formed the high-grade orebodies, after lithification of the lower part of the thick clastic section but possibly contemporaneous with diagenetic events in the upper part. The processes operating in the Alligator Rivers uranium field were essentially similar to those in the Athabasca District of northern Saskatchewan, the primary difference probably being that no organic material was remobilized into the overlying sandstone to act as a reductant in the Australian case.--Modified journal abstract.

An evaluation of exploration methods for low-temperature geothermal systems in the Artesian City area, Idaho

Numerous wells and a few springs around the perimeter of the Rock Creek Hills (South Hills) south of Artesian City, Idaho, produce water as warm as 49 °C. Warm-water samples throughout the study area share similar chemical characteristics, suggesting a common source of recharge and similar flow systems. Precipitation in the Rock Creek Hills probably infiltrates to depths in excess of 650 m along numerous normal faults and within permeable horizons of the Idavada Volcanics. Underlying Paleozoic rocks are relatively impermeable, except where fractured by recent tectonism, and probably direct the infiltrating ground water toward the lowland of the Snake River Plain. The volcanics have buried a Miocene topographic high that at present separates the warm ground water into two flow systems. Heavy pumping at the edge of the Snake River Plain at present captures the warm-water flow at depth and encourages the infiltration of irrigation water through the Quaternary alluvium and basalt flows of the plain into the underlying Idavada Volcanics. Several common geotechnical methods proved effective for the reconnaissance exploration of this low-temperature hydrothermal system. Measured temperatures of well and spring water located areas appropriate for study. The measurement of ground-water levels and hydrothermal gradients as defined herein enable the characterization of the warm-water flow systems and the identification of zones of warm-water production. A geochemical survey distinguishes waters with unique chemistries and recharge areas. Reconnaissance geologic mapping and drillers-log interpretation identify the stratigraphic and structural framework controlling the flow systems.

A hydrothermal model for the North Canning uranium deposit, Owl Creek Mountains, Wyoming

Analyses of six drill cores from and surrounding the North Canning uranium deposit in the Copper Mountain uranium district, Wyoming, suggest that a low-temperature hydrothermal system may have transported and precipitated uranium. Uranium is found in highly altered breccia zones and fractures in Archean granite and in overlying Tertiary sediment. Na montmorillonite invariably forms the matrix around stable and secondary K-feldspar and quartz in the breccias. Uraninite and amorphous pitchblende occur as disseminated fracture fillings and as encrustations on lithic fragments in the breccias. Hematite staining commonly occurs around uranium mineralization, and pyrite is disseminated in fractures and is intimately associated with uraninite. In the granite host rocks, sericite replaces plagioclase, epidote replaces plagioclase and biotite, and chlorite sporadically replaces biotite. Homogenization temperatures of fluid inclusions in quartz microveinlets average 136 degrees C.Anomalous uranium concentrations occur in the Copper Mountain granite (5 ppm regionally, but more than 20 ppm in a central core). Isotope systematics show 30 to 50 percent uranium losses in Tertiary time, from which preloss values greater than 35 ppm are calculated. The heat-generating capacity of this granite, using the back-calculated U value and averages of 28 ppm Th and 4.5 percent K 2 O is 11.2 mu W/m 3 . Steady state conductive temperatures greater than 150 degrees C would have been generated under 4.5 km of Cambrian to Cretaceous sediments. Laramide faulting opened the heated rock to convective water circulation, allowing silicate alteration, mobilization of uranium within the granite, and then deposition of uranium in the breccia zones.Activity diagrams show solution reaction paths associated with alteration and stability relationships for uraninite. Unaltered K-feldspar and plagioclase altered to sericite and Na montmorillonite suggest constant a (sub K (super +) ) /a (sub H (super +) ) and increasing a (sub Na (super +) ) /a (sub H (super +) ) ratios. The assemblage uraninite-pyrite-hematite is stable within f (sub o 2 ) values of 10 (super -44) to 10 (super -35) and f (sub s 2 ) values of 10 (super -20) to 10 (super -6 5) atm at an assumed P (sub CO 2 ) = 10 (super -2) atm and Sigma U = 10 ppb in the hydrothermal fluid. Oxidation of magnitite and sulfide solution species to hematite and hydrothermal pyrite caused reduction and precipitation of uranium from solution.

Mountkeithite, a new pyroaurite-related mineral with an expanded interlayer containing exchangeable MgSO4

AbstractMountkeithite [(Mg,Ni)9(Fe3+,Cr,Al)3(OH)24]3+[(CO{in3,SO4)1.5(Mg,Ni)2(SO4)2(H2O)11]3− occurs with pyrite, magnetite, magnesite, hexahydrite, and morenosite in a low-temperature hydrothermal vein system in the Mount Keith disseminated nickel sulphide deposit, Western Australia. Electron-microprobe analyses gave the following average composition: MgO 31.7, NiO 6.1, CuO 0.1, Cr2O3 6.1, Fe2O3 8.3, Al2O3 2.6, SO3 14.7; microanalysis for C and H gave CO2 3.9, H2O 30.6; total 104.1. The X-ray powder diffraction pattern has the following strongest lines: 11.30(10), 5.63(8), 4.63(2), 3.765(6), 2.645(4), 2.545(4), 1.554(5), and 1.505(3); these can be indexed on a hexagonal cell with a = 10.698 Å, c = 22.545 Å.Mountkeithite occurs as friable aggregates and rosettes composed of soft, pearly to translucent, pale pink to white flakes with perfect basal cleavage. Optical properties are uniaxial negative with ɛ = 1.51 and ω = 1.52; weakly dichroic from colourless to palest pink. Specific gravity is 2.12 (calc. 1.95). Mountkeithite is soluble, with efferves-cence, in dilute HCl. Mountkeithite changes to a pyroaurite-like phase, with a 7.8.Å basal spacing when immersed in water, but can be changed back to an 11.3 Å phase by immersion in 1M MgSO4; mountkeithite also collapses to a 7.8 Å basal spacing under vacuum. Phases similar to mountkeithite have been synthesized by immersing grains of stichtite in 1M MgSO4, at 20, 50, 105, and 200°C.The structure of mountkeithite is interpreted as consisting of positively charged brucite-like hydroxyl layers, containing both divalent and trivalent cations, separated by about 7 Å-thick, negatively charged interlayers containing carbonate and sulphate anions, magnesium sulphate, and water. Carrboydite, motukoreaite, and hydrohonessite are minerals with similarly large basal spacings of 10–11 Å, and are also believed to have pyroaurite-related structures with expanded interlayers.

Altered basaltic glass: A terrestrial analog to the soil of Mars

Analyses of Martian surface soil by Viking and Earth-based telescopes have been interpreted as indicating a regolith dominated by the weathering products of mafic or ultramafic rocks. Basaltic glass has previously been proposed as a more likely precursor than crystalline rock, given the low efficiency of surface weathering under present Martian conditions. On Earth large volumes of basaltic glass formed by quenching of magma by water. A similar interaction, between magma and ground ice, may have been a common occurrence on Mars. On the basis of this scenario palagonite, the alteration product of basaltic sideromelane glass, was studied as a possible analog to Martian soil. Samples from Iceland, Alaska, Antarctica, Hawaii, and the desert of New Mexico and Mexico were examined by optical and scanning electron microscopy, electron microprobe analysis, X-ray diffraction, spectrophotometry, and magnetic and thermogravimetric analysis. We suggest that palagonite is a good analog to the surface soil of Mars in chemical composition, particle size, spectral signature, and magnetic properties. Our model for the formation of fine-grained Martian surface soil begins with eruptions of basaltic magma through ground ice, forming deposits of glassy tuff. Individual glass shards are then altered by low-temperature hydrothermal systems to palagonitic material. Dehydration and aeolian abrasion strip the alteration rinds from the glass, and wind storms distribute the silt-sized palagonitic fragments in a planet-wide deposit.