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Mineralogical and Isotopic Zonation in the Sur-Sur Tourmaline Breccia, Rio Blanco-Los Bronces Cu-Mo Deposit, Chile: Implications for Ore Genesis

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Frikken, P and Cooke, DR and Walshe, JL and Archibald, DA and Skarmeta, J and Serrano, L and Vargas, R (2005) Mineralogical and Isotopic Zonation in the Sur-Sur Tourmaline Breccia, Rio Blanco-Los Bronces Cu-Mo Deposit, Chile: Implications for Ore Genesis. Economic Geology, 100 (5). pp. 935-961. ISSN 0361-0128

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Abstract

The Sur-Sur tourmaline breccia is located in the southeast part of the Rio Blanco-Los Bronces porphyry copper-
molybdenum deposit, central Chile. The breccia hosts approximately one-quarter of the total resource of
57 Mt of fine copper at Rio Blanco. The breccia is hosted within, and contains altered clasts of, granodiorite
from the 12 to 8 Ma San Francisco batholith, which intruded a sequence of Miocene volcanic and volcaniclastic
rocks. A series of weakly mineralized to barren felsic porphyries cut the breccia and indicate a minimum
age of approximately 6 Ma for mineralization at Sur-Sur.
The Sur-Sur breccia dike is at least 3 km long, 0.2 km wide, and has a vertical extent of at least 1 km. The
breccia has been cemented by early biotite and anhydrite at depth and by tourmaline and specularite at higher
altitudes. These early-formed cements have been overgrown and in some cases replaced by chalcopyrite, magnetite,
pyrite, and quartz. Mineralogical zonation in the breccia includes a transition from biotite cement and
related biotite alteration upward to tourmaline cement and quartz-sericite-tourmaline alteration at approximately
3,000-m elevation. Iron-oxide minerals are also zoned, with a transition upward from a magnetite-dominated
zone below 3,330 m to a specularite-dominated zone above 3,600 m. Pyrite is the dominant sulfide at
altitudes above 4,000 m.
Secondary liquid-rich, vapor-rich, and hypersaline fluid inclusions are preserved in quartz and tourmaline
cement. Measured homogenization temperatures are mostly between 300 degrees and 450 degrees C, and salinities range from 0 to 69 wt percent NaCl equiv. Sulfur isotope compositions of sulfide cement range from -4.1 to +2.7 per mil.
The lowest delta 34S(sulfide) values are in samples from between 3,700- and 4,000-m elevation, where they correspond
to the highest copper grades in the tourmaline breccia. This high-grade zone also contains abundant specularite
(locally replaced by magnetite). Modeling of sulfate-sulfide equilibrium indicate that approximately 150 degrees C of cooling over a vertical interval of 100 m would be required to account for the zonation of sulfide isotope
compositions at Sur-Sur, making conductive cooling an unlikely ore-forming mechanism.
Measured 206Pb/204Pb values of lead in anhydrite cement in the Sur-Sur tourmaline breccia and the Rio
Blanco magmatic breccia range from 17.558 to 18.479. 207Pb/204Pb values range from 15.534 to 15.623, and
208Pb/204Pb values range from 37.341 to 38.412. The lead in anhydrite is considerably less radiogenic than that
indicated by values obtained previously for lead in sulfide ores and igneous host rocks at Rio Blanco-Los
Bronces. The source of lead in anhydrite must have been from rocks external to the main magmatichydrothermal
system, probably the Precordilleran basement.
A magmatic-hydrothermal explosion from a deep-seated crystallizing intrusion triggered breccia formation
at Sur-Sur. Hydrostatic pressures catastrophically exceeded lithostatic load plus the tensile strength of the confining
granodiorite, leading to widespread brecciation and subsequent invasion by large volumes of magmatic
gas and hypersaline brine. The low-density gas phase (carrying H2O, SO2, HCl, and B2O3) separated physically
from the dense copper-bearing brine and flushed through the breccia column first, where it condensed into
ground waters of uncertain derivation. Anhydrite, specularite, and tourmaline were deposited from this lowsalinity,
acidic, oxidized hybrid solution. Subsequent upwelling of magmatic-hydrothermal brine resulted in
sulfide deposition. High-grade copper deposition is interpreted to have occurred in response to mixing of the
oxidized, acidic water with the copper-bearing magmatic-hydrothermal brine.

Item Type: Article
Keywords: porphyry copper radiogenic isotopes fluid mixing
Journal or Publication Title: Economic Geology
Page Range: pp. 935-961
ISSN: 0361-0128
Identification Number - DOI: 10.2113/100.5.935
Date Deposited: 26 Sep 2007
Last Modified: 18 Nov 2014 03:22
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