Chemical (EPMA) and boron isotope (SIMS) analyses on tourmaline breccias from the Río Blanco-Los Bronces porphyry copper district, Chile

Tourmaline-cemented magmatic-hydrothermal breccias are a major host to sulphide mineralization in the supergiant Río Blanco–Los Bronces (RB–LB) porphyry Cu-Mo district in central Chile. We made an extensive study of the chemical and boron isotopic composition of tourmaline from this district to shed light on the composition and origin of mineralizing fluids and to test the utility of tourmaline as an indicator mineral by comparing compositions from mineralized and barren breccias. Río Blanco-Los Bronces is a world-class porphyry-type Cu-Mo district of late Miocene age hosted in a granodioritic batholith and related porphyry intrusions in central Chile (33°9’ S latitude, 70°17’W longitude). The porphyry intrusions and related orebodies are distributed along a structurally-controlled NW-SE zone. Mineralization comprises quartz-sulfide veins, disseminated sulfide miner-alization in altered porphyry host rocks and disseminated sulfides in hydrothermal breccias. See Toro et al. (2012) for an overview of the geology, geochronology and mineralization in the district. Descriptions of the mineralized tourmaline breccias are given by Frikken et al. (2005) and Skewes et al. (2003). The data set provided here comprises in-situ chemical analyses of tourmaline by electron microprobe (EPMA) as well as in-situ boron-isotope analyses of tourmaline in the same samples by SIMS. Tourmaline was analysed in 12 samples including 8 from mineralized breccia bodies (Sur-Sur: 4, La Americana: 4), and 2 samples each from barren breccia and nearby granite-hosted tourmaline nodules in the Diamante area. We also give results of mass balance calculations testing the hypoth-esis of a magmatic-hydrothermal origin of the boron.

Electron microprobe (EPMA) Most samples were analyzed at the Technical University, Bergakademie in Freiberg while samples RB304 and RB306 were at the German Research Centre for Geosciences (GFZ) in Potsdam. All analyses were done on carbon-coated polished thin sections with wavelength-dispersive spetrometers, see details below. All mineral formulae were calculated by normalizing to 15 cations on the Y, Z, or T sites as implemented in the software WinTcalc (Yavuz et al., 2014). EPMA analyses in Freiberg employed a JEOL JXA-8900R instrument set at 20 kV accelerating voltage, a beam current of 12 nA, and a beam diameter of 5 microns on the sample surface. The counting times on peak were 15 s for Na, 20 s for Al, Si; 30 s for Fe, Mg; 60 s for Ti, K, V, Ca, Mn, Cr; and 90 s for F. Background counting times were half of those on the respective peak. Calibration standards used: wollastonite (Ca, Si), diopside (Mg), hematite (Fe), albite (Na), rutile (Ti), orthoclase (K, Al), fluorite (F), rhodonite (Mn), V-metal (V). The analyses at the GFZ Potsdam were carried out with a JEOL JXA 8230 electron microprobe equipped with a LaB6-cathode. The accelerating voltage was 15 kV, beam current 10 nA and the beam diameter on sample was 5 microns. Counting times on peak were 10 s for Si, K, and Na; 20 s for Al, F, Ti, Ca, Cl, Mg, Fe and Mn; background counting times were half of those on the respective peaks. Fluorine was not analysed in the GFZ session. Calibration employed the following mineral standards: orthoclase (Si, Al, K), rutile (Ti), diopside (Ca, Mg), synthetic albite (Na), hematite (Fe) and rhodonite (Mn). SIMS Boron isotope analyses employed the Cameca 1280-HR instrument at the GFZ Potsdam operated in multicollection mode with Faraday cups. Analyses were made on polished thin sections after cleaning in pure ethanol and gold coating in vacuum. The 16O- primary beam of approx. 5nA current and 13 kV energy was focused to about 5 microns on the sample surface. Secondary ions were extracted with a 10 kV potential and no offset voltage. Each analysis was preceded by a 90 s sputtering followed by 20 cycles of 10B (4 s integration time) and 11B (2 s integration) per cycle. The mass resolution M/ΔM of 2000 was more than enough to separate 11B and 10B1H masses. Instrumental mass fractionation and analytical quality were determined by repeated analyses of reference materials dravite (Harvard #108796) and schorl (Harvard #112566) described by Dyar et al. (2001) during each analytical session (see Table 3). The internal precision of each analysis (1 standard deviation / mean of 20 cycles) was typically around 0.1‰. Repeatability on the individual reference tourmalines was between 0.1 and 0.2%, and the combined 1 s.d. variability of all analyses from both reference materials was <0.8‰. This variability includes any matrix effect resulting from different chemical composition of the tourmalines and is taken as an estimate for the overall uncertainty. After correction for instrument mass fractionation, the 11B/10B ratios were converted to δ11B values relative to NIST SRM 951 (11B/10B = 4.04362 according to Catanzaro et al., 1970).

Acknowledgements: The study was supported by the Chilean Commission for Scientific and Technological Research (CONICYT) and the German Academic Exchange Service (DAAD). We thank Thomas Seifert and Lothar Ratschbacher for support in Freiberg including expenses for field visits, sample preparation and analyses. The EPMA analyses were conducted with the help of Bernhard Schulz and Joachim Krause in Freiberg and Oona Appelt in Potsdam. The SIMS analyses in Potsdam were done with the expert help of Frederic Couffignal.