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J.J. Hemley

Publications and source records attributed to J.J. Hemley.

11 recordsLinked to original sources
USGS2024

Argillization by descending acid at Steamboat Springs, Nevada

Steamboat Springs, Nevada, an area of present-day hot springs, clearly illustrates the genetic dependence of some kaolin deposits on hot-spring activity. Andesite, granodiorite and arkosic sediments are locally altered at the land surface to siliceous residues consisting of primary quartz and anatase, plus opal from primary silicates. These siliceous residues commonly exhibit the textural and structural features of their unaltered equivalents. Beneath the siliceous residues, kaolin and alunite replace primary silicates and fill open spaces, forming a blanketlike deposit. Beneath the kaolin-alunite zone, montmorillonite, commonly accompanied by pyrite, replaces the primary silicates. On the ground surface, the same alteration mineral zones can be traced outward from the siliceous residue; however, hematite rather than pyrite accompanies montmorillonite. Chemical analysis indicates that sulfuric acid is the active altering agent. The acid forms from hydrogen sulfide that exsolves from deep thermal water, rises above the water table and is oxidized by sulfur-oxidizing bacteria living near the ground surface. This acid dissolves in precipitation or condensed water vapor and percolates downward destroying most of the primary minerals producing a siliceous residue. Coincidence of the water table with the downward transition from siliceous residue to kaolin-alunite signifies decreasing hydrogen metasomatism because of dilution of descending acid by ground water. In hot-spring areas, beds of siliceous sinter deposited at the surface by hypogene thermal water look, superficially, like areas of surficial acid alteration. Features diagnostic of a surficial alteration are the relict rock structures of a siliceous residue and a kaolin-alunite zone immediately beneath.

Nevada↗
USGS1994

An experimental study of zinc chloride speciation from 300 to 600 °C and 0.5 to 2.0 kbar in buffered hydrothermal solutions

The solubility of sphalerite (ZnS) was measured in KCl-HCl-H 2 O solutions at 300–600°C and 0.5–2.0 kbar. The silicate assemblage K-feldspar-muscovite (or andalusite)-quartz was used to buffer the solution to acid conditions, resulting in the total solubility reaction 2 K + + KAl 2 AlSi 3 O 10 ( OH ) 2 + 6 SiO 2 + ZnS + nCl − = ZnCl n (2− n ) + 3 KAlSi 3 O 8 + H 2 S . (muscovite) (quartz) (sphalerite) (K-feldspar) A computer retrieval technique was used to derive average chloride ligand numbers for chlorozinc species at 0.25–2.0 molal total chloride. This technique mathematically solves for the average ligand number using a series of pertinent chemical relations at P and T . Mono- and di-chlorozinc species were found to predominate throughout the pressure-temperature-composition range investigated. The logarithms of the first and second dissociation constants for ZnCl 2 0 were evaluated over the P - T range; for example, at 1 kbar, the values −0.41 and −1.42 were computed for the logarithm of the first dissociation constant, while −7.62 and −10.57 were computed for the logarithm of the second dissociation constant, for 400 and 500°C, respectively. Results are compared to past studies conducted at subcritical conditions and differ in that we find no evidence for more highly coordinated chloro-zinc species except possibly for ZnCl 3 − at 600°C, 1 and 2 kbar. Our results are consistent with electrostatic theory, which favors lower charged to neutral molecules in low dielectric-constant media.

Geochimica et Cosmochimica Acta↗
USGS1992

Hydrothermal ore-forming processes in the light of studies in rock- buffered systems: I. Iron-copper-zinc-lead sulfide solubility relations

Experimental studies, using cold-seal and extraction vessel techniques, were conducted on Fe, Pb, Zn, and Cu sulfide solubilities in chloride solutions at temperatures from 300 degrees to 700 degrees C and pressures from 0.5 to 2 kbars. The solutions were buffered in pH by a quartz monzonite and the pure potassium feldspar-muscovite-quartz assemblage and in f (sub S 2 ) - f (sub O 2 ) largely by the assemblage pyrite-pyrrhotite-magnetite. Solubilities increase with increasing temperature and total chloride, and decrease with increasing pressure. The rise in solubility is particularly steep between 300 degrees and 500 degrees C and between 1,000 and 500 bars. With increasing temperature at any given pressure, or with decreasing pressure at any given temperature, metal solubility eventually passes through a maximum due to increasing competition for chloride by the alkali, hydrogen, and base metal ions and because intersection with a two-fluid region eventually occurs. In that portion of the two-fluid region encountered in the study, metal solubilities in the brine were very high, but solubilities in the gas phase also were significant. In a system controlled by the potassium feldspar-muscovite-quartz buffer, 1-m total Cl (super -) , and the assemblage pyrite-pyrrhotite-magnetite-sphalerite-galena-chalcopyrite, solubilities in ppm at 1 kbar and 300 degrees , 400 degrees , and 500 degrees C were 237, 1,216, and 5,636, for Fe; 51, 613, and 3,105 for Pb; 36, 423, and 2,649 for Zn; and 11, 40, and 113 for Cu, respectively. At 400 degrees C, 0.5 and 2 kbars, the values were 2,627 and 500 for Fe; 1,262 and 194 for Pb; 983 and 120 for Zn; and 60 and 29 for Cu, respectively. All of the above were in the single-fluid region. Single-metal solubilities also were investigated to assess the influence of iron on the solubility of the other metals and to corroborate preliminary dissociation constants for the metal chloride complexes involved. The effect of increasing chloride concentration on solubility reflects primarily a shift to lower pH via the silicate buffer reactions. The effect of decreasing pressure reflects primarily the relative change in the dissociation constants of the chloride complexes involved. Increasing sulfur fugacity lowers solubility, but in systems controlled at relatively low values by an f (sub S 2 ) buffer or wall-rock sulfidation reactions, solutions of high metal content relative to reduced sulfur will tend to develop at high chloride concentrations. Similarity in behavior with respect to the temperature and pressure of Fe, Zn, and Pb sulfide solubilities points to similarity in chloride speciation, and the neutral species appear to be dominant in the high-temperature region. At 500 degrees C and 1 kbar, the log K D values for FeCl degrees 2 , PbCl degrees 2 , ZnCl degrees 2 , and CuCl degrees are, respectively, -8.76, -9.14, -10.86, and -6.22.

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

Experimental study of iron-chloride complexing in hydrothermal fluids

Mineral assemblage solubilities were measured in cold-seal pressure vessels as a function of pressure, temperature, and potassium chloride concentration in order to determine the nature and thermodynamic properties of iron-chloride complexes under hydrothermal conditions. The assemblage pyritepyrrhotite-magnetite was used to buffer ƒS 2 "> ƒS 2 and ƒO 2 "> ƒO 2 , and K + H + "> K + H + ratios were buffered at reasonable geologic values using the assemblage potassium feldspar-muscovite (or andalusite)-quartz. The pressure-temperature ranges were 0.5-2.0 kbar and 300–600°C, and initial fluid compositions ranged from 0.01–2.0 molal KCl. With all other factors constant, the concentration of iron in solution increases with increasing temperature, with decreasing pressure, and with increasing total potassium chloride concentration. Changes in iron concentrations as a function of KCl concentration, in conjunction with charge balance, mass action, and mass balance constraints on the system, place constraints on the stoichiometry of the important iron-chloride complexes under each of the experimental conditions. Using least-squared linear regression fits to determine these slopes, the calculations yield values for the average ligand numbers that are in the range 1.2-1.9, with uncertainties ranging from ±0.1-0.6 at the several PT conditions considered. The slopes of the regressed fits to the data suggest that both FeCl + and FeCl 2 0 are important in the experimental fluids, with FeCl 2 0 becoming dominant at the higher temperatures. Theoretical calculations, however, indicate that FeCl + does not contribute significantly to the solubility. Because of the large uncertainties associated with some of the calculated average ligand numbers, we base our data analysis on the theoretical calculations. A statistical analysis is applied to the solubility data in order to determine the values and uncertainties of the dissociation constant for FeCl 2 0 that best fit the data at each of the experimental pressures and temperatures. The calculated stability of FeCl 2 0 increases with increasing temperature and total chloride concentration, and with decreasing pressure. The values of the dissociation constant of FeCl 2 0 that are calculated in this study are in moderately good agreement with FeCl 2 0 dissociation constants from other studies of iron-chloride complexing in supercritical fluids. Differences are likely due to different assumptions made concerning activity coefficients of aqueous species. Log k d values for full dissociation of FeCl 2 0 at 0.5 kbar—300°C—and at 1 kbar—400, 500, and 600°C, respectively—are −3.75 ± 0.40, −6.25 ± 0.10, −9.19 ± 0.44, and −13.29 ± 0.09.

Geochimica et Cosmochimica Acta↗
USGS1986

Effect of pressure on ore mineral solubilities under hydrothermal conditions

Experimental studies were conducted at elevated pressures and temperatures on the combined solubilities of iron, zinc, and lead sulfides in chloride solutions buffered in pH by a silicate assemblage of quartz monzonite composition plus added muscovite, and buffered in/s2 and/o2 by the assemblage pyrite-pyrrhotite-magnetite. Major controls on base metal concentration are temperature, total chloride, and pressure. Higher temperature and higher chloride concentration favor higher metal solubilities as expected, but the pressure effect is opposite to that generally expected and is of considerable importance to the problem of ore mineral transport. At 500 °C, 0.5 kbar, and 1 m total chloride, Fe, Zn, and Pb solubilities were 8500, 4300, and 8700 ppm, respectively, whereas at 1 kbar they were 4200, 2400, and 2600 ppm, and at 2 kbar, 1700, 800, and 1200 ppm. The experimental results thus indicate that the metals could be carried over long distances on a decreasing pressure gradient so long as the temperature decreases were not sufficient to significantly offset the pressure effect. Such a condition could be approximated by a near-adiabatic transport cooling path. Such a condition is probably common geologically, especially for hydrothermal processes involving fairly deep-seated sources of heat and mineral components.

Geology↗
USGS1985

Some mineral stability relations in the system CaO MgO SiO2 H2O HCl

Mineral-aqueous solution equilibria for the assemblages talc-quartz, tremolite-talc-quartz, diopside-tremolite-quartz, wollastonite-diopside-quartz and wollastonite-quartz have been studied at 2 kb total pressure, 500° to 700°C and chloride concentrations from 0.03 to 6.0 molal. Most work was at 1 m chloride. Both buffered and unbuffered data were obtained and a recalibration of the Ag-AgCl buffer is presented. Log equilibrium quotients at 500°, 600° and 700°C are respectively: Ta-Qz ( m MgCl 2 m HCl 2 "> mMgCl2mHCl2 ) 2.57, 1.71, 0.73; Tr-Ta-Qz and Di-Tr-Qz ( m CaCl 2 m MgCl 2 m HCl 2 "> mCaCl2mMgCl2mHCl2 ) 4.98, 3.99, 2.21 and 7.29, 5.30, 3.56; WoDi-Qz ( m CaCl 2 m MgCl 2 "> mCaCl2mMgCl2 ) 3.30, 3.00, 2.79: Wo-Qz ( m CaCl 2 m HCl 2 "> mCaCl2mHCl2 ) 5.15, 3.95, 2.68. Mineral stability fields plotted in terms of these concentration data more tangibly represent the compositional character of real systems and the mass transfer capabilities of their fluids than do the analogous theoretical activity diagrams. Overall dissociation constants of MgCl 2 and CaCl 2 were calculated from the experimental data using the calculated ionic activity constants for the reactions and the established dissociation constants of HCl. The negative log values are respectively: 3.88. 6.63, 9.20 for CaCl 2 and 4.60, 7.54, 10.37 for MgCl 2 at 500°, 600° and 700°C, 2 kb. The Ca values are about an order of magnitude more positive than the conductance-derived values by Frantz and Marshall (1982). The phase relations developed in this study have application to the genesis of talc, tremolite, and diopside-bearing assemblages in some regional metamorphic rocks, but more specifically to the calcsilicate skarn assemblages of many metasomatic aureoles. The equilibrium fluids are characterized by high concentrations of Ca relative to Mg and increasing Ca Mg "> CaMg ratios with decreasing temperatures. The stability fields of talc, tremolite, and quartz expand relative to those of diopside and wollastonite with decreasing temperature, hence their more common appearance as retrograde products in skarn systems.

Geochimica et Cosmochimica Acta↗
USGS1967

Sulfide solubilities in alteration-controlled systems

Solubilities of sphalerite (ZnS) and galena (PbS) were determined at 300° to 500°C and 1000 bars total pressure in a chemical environment buffered by silicate mineral equilibria. Chloride solutions and muscovite-bearing assemblages characteristic of hydrothermal wall-rock alteration were used; weak acidities at temperature were therefore involved. The metal concentrations encountered tended to be higher than those observed in high bisulfide-H 2 S systems at neutral to weakly basic pH used in most previous experimentation; the chemical conditions of the work, although not completely satisfactory, are geologically more realistic than previous experimentation done in the basic-pH region.

Science↗
USGS1964

Chemical aspects of hydrothermal alteration with emphasis on hydrogen metasomatism

The ratio of cation to hydrogen-ion concentration is used as a basis for coordinating many observed varieties of silicate rock alteration. Low ratios drive alteration from feldspar through sericite, montmorillonite, or paragonite, toward kaolinite or pyrophyllite. Principal controls of ratio are reactions with wall rock, pressure-temperature changes, mixing of solutions, and oxidation of H 2 S; their interactions govern the assemblage attained.

Economic Geology↗