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.
Analyzing the chemical composition of rocks and minerals is an important tool for exploring and understanding mineral resources. Typically, hydrothermal ore deposits are characterized by primary alteration halos. At the world-class Panasqueira W-Sn-Cu deposit, the hydrothermal alteration of the wall rocks produced concentric zones with progressively greater distance from the veins, consisting of a proximal tourmaline-quartz-muscovite zone and a distal muscovite-quartz zone.
Panasqueira is a world-class W-Sn-Cu lode-type deposit located in the Castelo Branco district (Beira Baixa, central Portugal). The ore deposit consists of a swarm of sub-horizontal veins associated with a Late-Variscan S-type granite and enclosed by a metasedimentary unit of Late Ediacaran to Early Cambrian age (e.g., Kelly and Rye, 1979; Romão et al., 2013).
The veins are mainly composed of gangue quartz, muscovite and minor carbonates, apatite, topaz, topaz, fluorite, tourmaline, rutile, ilmenite, arsenopyrite, sphalerite, pyrite, marcasite, stannite, and pyrrhotite. Mineralization of wolframite, chalcopyrite, and cassiterite is predominantly hosted in veins with minor stringers and lenses of sulfide minerals in the wall rocks (e.g., Kelly and Rye, 1979; Polya, 1989; Polya et al., 2000). Although there is a strong variation in the vein mineralogy, typically, the quartz vein-filling is rimmed by a muscovite selvage up to 4-5 cm thick. The hydrothermal alteration produced a 2 to 30 cm thick tourmaline-rich alteration halo in the metasedimentary host rock (Bussink, 1984).
Tourmaline and mica are ubiquitous minerals at Panasqueira W-Sn-Cu and coexist in many other hydrothermal ore deposits worldwide. Both minerals are well-known to host variable amounts of trace elements and to have potential as pathfinder minerals as well as fluid monitors.
We analyzed major, minor and trace element contents of altered and unaltered metasediments from the Panasqueira by XRF and ICP-MS and tourmaline and white mica major, minor and trace element compositions by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) in previously well-characterized samples from different locations/setting in the mine (greisen, vein-selvages, wall-rock alteration zones, fault zone, and late vugs).
The analyzed samples are described by Codeço et al. (2017), Codeço et al. (2019), and Codeço et al. 2020). These studies discuss the chemical (major, minor, and trace elements) and boron-isotopic compositions of tourmaline and white mica, and whole-rock chemistry of altered and unaltered metasediments. Further details on sample description can be found in the folder "2020-002_Codeco-et-al_Samples" and the analytical methods are described in " 2020-002_Codeco-et-al_data-description.pdf".
Detailed information about the samples used, the location, and general geological background of the samples, and the analytical method is provided in the data description "2020-002_Codeco-et-al_data-description.pdf ".