Stable isotopic composition of soil calcite (O,C) and gypsum (S) overlying Cu deposits in the Atacama Desert,Chile: Implications for mineral exploration,salt sources,and paleoenvironmental reconstruction |
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| Institution: | 1. ALS Geochemistry, 2103 Dollarton Highway, North Vancouver, British Columbia, Canada V7H 0A7;2. Eion Cameron Geochemical Inc., 865 Spruce Ridge Road, Carp, Ontario, Canada K0A 1L0;3. Departamento de Geología, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago, Chile;4. Andean Geothermal Center of Excellence (CEGA), Universidad de Chile, Santiago, Chile;5. GNS Science, P.O. Box 30-368, Lower Hutt, New Zealand;6. Department of Earth and Environmental Sciences, Tulane University, New Orleans, LA 70118-5698, USA;1. International Laboratory for Air Quality and Health, Institute of Health and Biomedical Innovation, Queensland University of Technology, Brisbane, QLD 4001, Australia;2. Joint Mass Spectrometry Centre – Comprehensive Molecular Analytics, Helmholtz Zentrum München, Ingolstädter Landstraße 1, D-85764 Neuherberg, Germany;3. Institute of Chemistry, University of Rostock, Dr.-Lorenz-Weg 1, D-18051 Rostock, Germany;1. College of Information System and Management, National University of Defense Technology, 410073, Changsha, China;2. Aeronautical University of China People Liberation Airforce, 130012, Changchun, China;1. Applied Research Center, Florida International University, 10555 W. Flagler Street, Miami, FL 33174, USA;2. Pacific Northwest National Laboratory, PO Box 999, K3-62, Richland, WA 99352, USA;1. Faculty of Civil and Environmental Engineering, Technion-Israel Institute of Technology, Haifa, Israel;2. Environmental Wind Tunnel Laboratory, Department of Applied Mathematics, Division of Environmental Sciences, Israel Institute of Biological Research, Ness-Ziona, Israel |
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| Abstract: | Soils overlying two porphyry Cu deposits (Spence, Gaby Sur) and the Pampa del Tamarugal, Atacama Desert, Northern Chile were collected in order to investigate the extent to which saline groundwaters influence “soil” chemistry in regions with thick Miocene and younger sediment cover. Soil carbonate (calcite) was analyzed for C and O isotopes and pedogenic gypsum for S isotopes. Soil calcite is present in all soils at the Spence deposit, but increases volumetrically above two fracture zones that cut the Miocene gravels, including gravels that overlie the deposit. The C isotope composition of carbonate from the soils overlying fracture zones is indistinguishable from pedogenic carbonate elsewhere at the Spence deposit; all δ13CVPDB values fall within a narrow range (1.40–4.23‰), consistent with the carbonate having formed in equilibrium with atmospheric CO2. However, δ18OVPDB for carbonate over both fracture zones is statistically different from carbonate elsewhere (average δ18OVPDB = 0.82‰ vs. ?2.23‰, respectively), suggesting involvement of groundwater in their formation. The composition of soils at the Tamarugal anomaly has been most strongly affected by earthquake-related surface flooding and evaporation of groundwater; δ13CVPDB values (?4.28‰ to ?2.04‰) are interpreted to be a mixture of dissolved inorganic C (DIC) from groundwater and atmospheric CO2. At the Spence deposit, soils only rarely contain sufficient SO4 for S isotope analysis; the SO4-bearing soils occur only above the fracture zones in the gravel. Results are uniform (3.7–4.9‰ δ34SCDT), which is near the middle of the range for SO4 in groundwater (0.9–7.3‰). Sulfur in soils at the Gaby Sur deposit (3.8–6.1‰ δ34SCDT) is dominated by gypsum, which primarily occurs on the flanks and tops of hills, suggesting deposition from SO4-rich fogs. Sulfate in Gaby Sur deposit gypsum is possibly derived by condensation of airborne SO4 from volcanic SO2 from the nearby Andes. At the Gaby Sur deposit and Tamarugal anomaly, pedogenic stable isotopes cannot distinguish between S from porphyry or redeposited SO4 from interior salars.The three sites studied have had different histories of salt accumulation and display variable influence of groundwater, which is interpreted to have been forced to the surface during earthquakes. The clear accumulation of salts associated with fractures at the Spence deposit, and shifts in the isotopic composition of carbonate and sulfate in the fractures despite clear evidence of relatively recent removal of salts indicates that transfer from groundwater is an ongoing process. The interpretation that groundwaters can influence the isotopic composition of pedogenic calcrete and gypsum has important implications for previous studies that have not considered this mechanism. |
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