Search USGS⌕ Search

Geology topics

Charles N. Alpers

Publications and source records attributed to Charles N. Alpers.

118 records · Page 7Linked to original sources
USGS1992

Chemical, crystallographic and stable isotopic properties of alunite and jarosite from acid-Hypersaline Australian lakes

Chemical, crystallographic and isotopic analyses were made on samples containing alunite and jarosite from the sediments of four acid, hypersaline lakes in southeastern and southwestern Australia. The alunite and jarosite are K-rich with relatively low Na contents based on chemical analysis and determination of unit cell dimensions by powder X-ray diffraction. Correcting the chemical analyses of fine-grained mineral concentrates from Lake Tyrrell, Victoria, for the presence of halite, silica and poorly crystalline aluminosilicates, the following formulas indicate best estimates for solid-solution compositions: for alunite, K 0.87 Na 0.04 (H 3 O) 0.09 (Al 0.92 Fe 0.08 ) 3 (SO 4 ) 2 (OH) 6 and for jarosite, K 0.89 Na 0.07 (H 3 O) 0.04 (Fe 0.80 Al 0.20 ) 3 (SO 4 ) 2 (OH) 6 . The δD-values of alunite are notably larger than those for jarosite from Lake Tyrrell and it appears that the minerals have closely approached hydrogen isotope equilibrium with the acidic regional groundwaters. The δD results are consistent with a fractionation ∼60–70‰ between alunite and jarosite observed in other areas. However, interpretation of δD results is complicated by large variability in fluid δD H2O from evaporation, mixing and possible ion hydration effects in the brine. δD-values of water derived from jarosite by step-wise heating tend to be smaller at 250°C, at which temperature hydronium and other non-hydroxyl water is liberated, than at 550°C, where water is derived from the hydroxyl site, but the differences are not sufficiently different to invalidate measurements of total δD obtained by conventional, single-step heating methods. δ 34 S-values for alunite and jarosite from the four lakes (+19.7 to +21.2‰ CDT) and for aqueous sulfate from Lake Tyrrell (+18.3 to +19.8‰) are close to the values for modern evaporites ( +21.5 ±0.3‰ ">‰ +21.5 ±0.3‰ ) and seawater ( +20±0.5‰ ">‰ +20±0.5‰ ) and are probably typical of seawater-derived aerosols in arid coastal environments. δ 34 -S-values slightly smaller than that for seawater may reflect a minor contribution of sulfate from pyrite oxidation in the Parilla Sand or a reservoir effect from removal of gypsum enriched in 34 S. δ 18 O SO4 -values for alunite from three Western Australia lakes (+17.8 to +18.3‰ V-SMOW), for alunite and jarosite from Lake Tyrrell (+22.6 to +24.9‰) and for aqueous sulfate from Lake Tyrrell (+17.3 to +19.0‰) are much larger than the average value for seawater (+9.6‰). The data suggest an approach to 18 O- 16 O equilibrium between aqueous sulfate and groundwater, which is known from experimental studies to be possible at low pH and low temperatures, but has not been previously documented in nature. A residence time of ∼0.1–1 kyr for sulfate in acidic water (pH 3–4) is needed to achieve the apparent partial oxygen exchange, using previously published data of R.M. Lloyd.

Chemical Geology↗
USGS1990

Stable isotopes of carbon dioxide in soil gas over massive sulfide mineralization at Crandon, Wisconsin

Stable isotope ratios of oxygen and carbon were determined for CO 2 in soil gas in the vicinity of the massive sulfide deposit at Crandon, Wisconsin with the objective of determining the source of anomalously high CO 2 concentrations detected previously by McCarthy et al. (1986). Values of δ 13 C in soil gas CO 2 from depths between 0.5 and 1.0 m were found to range from −12.68‰ to −20.03‰ (PDB). Organic carbon from the uppermost meter of soil has δ 13 C between −24.1 and −25.8‰ (PDB), indicating derivation from plant species with the C 3 (Calvin) type of photosynthetic pathway. Microbial decomposition of the organic carbon and root respiration from C 3 and C 4 (Hatch-Slack) plants, together with atmospheric CO 2 are the likely sources of carbon in soil gas CO 2 . Values of δ 18 O in soil-gas CO 2 range from 32 to 38‰ (SMOW). These δ 18 O values are intermediate between that calculated for CO 2 gas in isotopic equilibrium with local groundwaters and that for atmospheric CO 2 . The δ 18 O data indicate that atmospheric CO 2 has been incorporated by mixing or diffusion. Any CO 2 generated by microbial oxidation of organic matter has equilibrated its oxygen isotopes with the local groundwaters.

Journal of Geochemical Exploration↗
USGS1990

Hydrogeochemistry and stable isotopes of ground and surface waters from two adjacent closed basins, Atacama Desert, northern Chile

The geochemistry and stable isotopes of groundwaters, surface waters, and precipitation indicate different sources of some dissolved constituents, but a common source of recharge and other constituents in two adjacent closed basins in the Atacama Desert region of northern Chile (24°15′–24°45′S). Waters from artesian wells, trenches, and ephemeral streams in the Punta Negra Basin are characterized by concentrations of Na>Ca>Mg and Cl≥SO 4 ,with TDS<10 g/l. Values of δD and δ 18 O for Punta Negra Basin waters follow an evaporitic trend typical of closed basin waters in northern Chile and elsewhere. In contrast, ground waters in the Hamburgo Basin, located about 25 km NW of the Punta Negra Basin, have concentrations of Na > Mg≥Ca and SO 4 >Cl, with TDS also <10 g/l. Aqueous speciation calculations indicate that Hamburgo Basin groundwaters are close to saturation with respect to gypsum. The relatively high SO 4 and low Ca in Hamburgo Basin waters result from SO 4 influx and subsequent gypsum precipitation related to weathering at La Escondida, a large porphyry copper deposit located near to the center of the basin. Deep mine waters from 130 m below the water table at La Escondida also have Na>Mg≥Ca and SO 4 >Cl, but with TDS up to 40 g/l. The deep mine waters have pH between 3.2 and 3.9, and are high in dissolved CO 2 (δ 13 C= −4.8%PDB), indicating probable interaction with oxidizing sulfides. The deep mine waters have δ 18 O values of ≈−1.8%.compared with values < −3.5‰ for other Hamburgo Basin waters; thus the mine waters may represent a mixture of meteoric waters with deeper “metamorphic” waters, which had interacted with rocks and exchanged oxygen isotopes at elevated temperatures. Alternatively, the deep mine waters may represent fossil meteoric waters which evolved isotopically along an evaporative trend starting from values quite depleted in δ 18 O and °Dd relative to either precipitation or shallow groundwaters. High I/Br ratios in the Hamburgo Basin waters and La Escondida mine waters are consistent with regionally high I in surficial deposits in the Atacama Desert region and may represent dissolution of a wind-blown evaporite component.

Atacama Desert↗
USGS1989

Paleohydrologic evolution and geochemical dynamics of cumulative supergene metal enrichment at La Escondida, Atacama Desert, northern Chile

Quantitative limonite mapping within the leached capping of the porphyry copper deposit at La Escondida, Chile, permits reconstruction of the paleohydrologic and chemical evolution of a well-developed supergene ore-forming system. The mineralogy, textures, and relative abundance of supergene limonite minerals (hematite, goethite, and jarosite) are used to reconstruct the former ratio of pyrite to chalcocite and the preoxidation copper grade based on empirical limonite sulfide correlations (after Locke, 1926; Blanchard, 1968; and Loghry, 1972). Estimates of preoxidation copper grades in surface exposures and tops of drill holes at La Escondida are significantly lower than actual copper grades in the underlying enrichment blanket at depth. This apparent inconsistency is explained by a progressive increase in the copper grade of the sulfide enrichment blanket as it descended to its present location in response to a descending water table. This systematic trend of reconstructed grades of supergene-enriched copper sulfide increasing with depth offers the first quantitative proof of cumulative downward enrichment in a supergene profile, as proposed by Locke (1926). The consistent trends of limonite mineralogy and abundance in vertical profiles indicate that water table descent at La Escondida was relatively continuous in space, although not necessarily in time.Evidence for cumulative downward enrichment in vertical profiles through leached capping allows quantitative analysis of chemical mass balance in dynamic supergene systems. Slopes of linear regressions for profiles of reconstructed enriched copper grades vs. depth indicate lateral fluxes into or out of a given vertical profile. This method provides independent verification of conclusions from a previous study of copper mass balance at La Escondida (Brimhall et al., 1985) which showed that lateral fluxes of copper were a significant factor during supergene leaching and enrichment.

Economic Geology↗
USGS1989

Solubility of jarosite solid solutions precipitated from acid mine waters, Iron Mountain, California

Because of the common occurrence of 15 to 25 mole percent hydronium substitution on the alkali site in jarosites, it is necessary to consider the hydronium content of jarosites in any attempt at rigorous evaluation of jarosite solubility or of the saturation state of natural waters with respect to jarosite. A Gibbs free energy of 3293.5±2.1 kJ mol-1 is recommended for a jarosite solid solution of composition K.77Na.03(H3O).20Fe3(SO4)2(OH)6. Solubility determinations for a wider range of natural and synthetic jarosite solid solutions will be necessary to quantify the binary and ternary mixing parameters in the (K-Na-H3O) system. In the absence of such studies, molar volume data for endmember minerals indicate that the K-H3O substitution in jarosite is probably closer to ideal mixing than either the Na-K or Na-H3O substitution.

California↗