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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Leybourne, Matthew I.
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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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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
d13CVPDB values fall within a narrow range (1.40–4.23‰), consistent with the carbonate having formed
in equilibrium with atmospheric CO2. However, d18OVPDB for carbonate over both fracture zones is statistically
different from carbonate elsewhere (average d18OVPDB = 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;
d13CVPDB 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‰ d34SCDT), 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‰ d34SCDT) 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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URI: https://repositorio.uchile.cl/handle/2250/126022
DOI: DOI: 10.1016/j.apgeochem.2012.10.030
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APPLIED GEOCHEMISTRY Volume: 29 Pages: 55-72 Published: FEB 2013
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