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

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.