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Nicholas Wilson

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Ph. D. Thesis

The Role of Petroleum in the Formation of the El Soldado Copper Deposit, Chile: Hydrothermal Replacement of a Biodegraded Petroleum Reservoir.

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Bitumen (solidified petroleum) is found intimately associated with copper sulphide ores within several strata-bound (manto-type) copper deposits in central Chile. One of the largest deposits is the El Soldado copper deposit, which is of high-grade (100 Mt. @ 2-3% Cu) and is hosted by the Lower Cretaceous volcano-sedimentary Lo Prado Fm. Bitumen occurs intimately associated with the copper ores and the question that has been the focus of this thesis is what is the role of organic matter, if any, in the formation of the El Soldado deposit?

To answer this question samples were collected in order to study both the organic and inorganic phases: extensive petrology was performed on the textural relationships between bitumen and the sulphide minerals, which was complemented by microprobe and stable isotope analysis (S, C, and O) of sulphide, organic, carbonate and silicate phases. Reflectance (BRo) and Rock-Eval pyrolysis were used to study the bitumen and K-feldspar, associated with copper ores, was dating using 40Ar/39Ar geochronology. The integration of the above data suggests the following model for the El Soldado deposit: .

As a result of basinal burial, petroleum was generated from organic-rich shales in the lower Lo Prado Fm. and migrated into primary and structural porosity (N-S basinal faults) in the overlying upper Lo Prado Fm. Low-temperature biodegradation of the liquid petroleum formed an early assemblage (Stage I assemblage) of pyrite (± sphalerite ± chalcopyrite) within the liquid petroleum and as basinal burial continued the petroleum degassed and solidified, forming solid bitumen.

Around 20 Ma later (~103 Ma) copper was introduced (Stage II assemblage) by high-temperature (200-300°C) metamorphic fluids that had equilibrated with the basin rocks. Copper was transported as chloride complexes (CuCl), precipitating where the CuCl-rich solutions reacted with bitumen and pyrite from Stage I. The copper was most likely derived from the overlying oxidized Veta Negra Fm. and no addition sulphur was needed as sulphur was recycled from the Stage I assemblage. Mineral zonation suggests that the provenance of the mineralizing fluid was from the south towards the north with focussing of the fluids provided by pre-existing normal north-south faults and by the formation of NNW and E-W trending faults. Ore bodies were formed in the proximity and in the intersection of the above faults and hot fluids circulated through these conduits thermally altering the bitumen, increasing reflectance up to ca. 5% in the core of the orebodies, produced anisotropy, and locally created graphite.

In the case of El Soldado, copper ores developed preferentially where an abundant pyrite precursor had developed and the bitumen interacted with the mineralizing fluid, destabilizing copper complexes, and precipitated copper.

Two other smaller manto-type deposits were studied in this thesis and are hosted in equivalent aged strata 500km north of the El Soldado deposit. In both these deposits bitumen is intimately associated with the copper ores and although the specific parageneses of the deposits are different the process appears to be the same: early petroleum migration, solidification into bitumen, with later copper mineralization.

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Supervisor: Marcos Zentilli

Dr. Nicholas S.F.Wilson
Geoscience Department
University of Nevada, Las Vegas
4505 Maryland Parkway
Box 454010
Las Vegas Nevada
89154-4010
Tel: (702) 895-3250
Fax: (702) 895-4064