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Scientific Communications |
Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia, B3H 3J5, Canada
Department of Mineralogy, The Natural History Museum, Cromwell Road, London, SW7 5BD, UK
Corresponding author: email,nwilson{at}nrcan.gc.ca
Strata-bound, manto-type copper deposits of Chile are a major source of copper, and some deposits (e.g., Mantos Blancos, El Soldado) contain as much metal as do large porphyry copper deposits elsewhere. El Soldado (200 Mt @ 1.35% Cu), the largest manto-type copper deposit in the Coastal Cordillera of central Chile, is hosted by the rhyodacite-basalt upper member of the marine Lower Cretaceous Lo Prado Formation. Paragenetic studies indicate that the deposit formed in two stages. The first was a low-temperature stage (stage 1; 130–125 Ma) in which liquid petroleum migrated from underlying carbonaceous shales into the overlying host volcanic rocks and abundant framboidal pyrite developed in a paleopetroleum reservoir. A later hydrothermal stage (stage 2; ca. 103 Ma) introduced copper and replaced earlier pyrite to form chalcopyrite, bornite, and chalcocite ores. This study presents isotopic data that support bacterial sulfur reduction as the process that formed the essential precursor stage 1 pyrite, and the participation of basinal fluids in the hydrothermal second stage.
A wide range of 
34S
values for early stage 1 pyrite (–11.1 to +28.0
) supports the hypothesis
that low-temperature, incomplete sulfate reduction from connate seawater sulfate
and Fe-enriched oil-field brines took place in a compartmentalized petroleum
reservoir by bacterial sulfate reduction. With the exception of a small
proportion of Cu-Fe sulfides with 34S
values between –2.0 and +2.0 per mil, the bulk of stage 2 Cu-Fe sulfides
replaced earlier pyrite and mimic the broad range in 34Svalues (–12.7 and +19.0) of stage 1 pyrite. Sulfur from early pyrite
was recycled with no significant homogenization, fractionation, or addition of
new sulfur, and indicates the essential role of hydrocarbons and bacteria in the
formation of the deposit. The 13C
values of gangue calcite vary between –4.2 and –20.2 per mil and reflect a
mixing of various proportions of oxidized, pyrobitumen-derived organic and
inorganic carbon from the mineralizing fluid. Pyrobitumen in the ores has 13C
values of –30 to –26 per mil, consistent with it being a residue of the
original liquid petroleum. 18O
values of K-feldspar in both ore and barren samples are similar (12.0–12.7)
and are compatible with the fluids being of metamorphic, rather than magmatic,
origin. The similar 18O
ranges of calcite (2.7) and K-feldspar (0.7) suggest that they formed from
the same hydrothermal fluid and that the fluid was in equilibrium with the host
rocks. The 87Sr/86Sr ratios in calcite of 0.7041 to 0.7051
reflect isotopic ratios inherited from the subaerial andesitic rocks that
overlay the deposit where the ore-forming fluids are thought to have originated.
This article has been cited by other articles:
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  A. Rieger, L. Schwark, M.-E. Cisternas, and H. Miller Genesis and Evolution of Bitumen in Lower Cretaceous Lavas and Implications for Strata-bound Copper Deposits, North Chile Economic Geology, March 1, 2008; 103(2): 387 - 404. [Abstract] [Full Text] [PDF]
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G. Southam and J. A. Saunders The Geomicrobiology of Ore Deposits Economic Geology, September 1, 2005; 100(6): 1067 - 1084. [Abstract] [Full Text] [PDF]
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