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Geology, alteration, mineralization and geochemistry of tourmaline breccia complexes in the Andes : Rio Blanco-Los Bronces, Chile and San Francisco de los Andes, Argentina

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Testa, FJ 2020 , 'Geology, alteration, mineralization and geochemistry of tourmaline breccia complexes in the Andes : Rio Blanco-Los Bronces, Chile and San Francisco de los Andes, Argentina', PhD thesis, University of Tasmania.

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Abstract

San Francisco de los Andes is a small, tourmaline breccia-hosted Bi–Cu–Au deposit in the Frontal Cordillera, Argentina. The San Francisco de los Andes district is spatially and temporally related to the Permian Tocota Pluton, part of the Colangüil Batholith. The Tocota Pluton is a post-orogenic intrusive complex formed after partial melting of continental crust, triggered by a mafic underplate. Its high-K, calc-alkaline signature relates to minimum melting at relatively low P\(_{H2O}\). The youngest granitoid, the Rosados Granite, has features common of hypersolvus granite including a leucogranite composition with biotite (but no hornblende), accessory muscovite, and orthoclase with perthitic textures. The geochemical evolution of the granitoids documents a transition between an arc-related to an extensional geodynamic setting. Distinctive geochemical signatures of post-orogenic magmatism in the Tocota Pluton include depletion in Eu, Sr and La (i.e., non-adakitic compositions), and absence of strongly fractionated rocks (i.e., strong LREE enrichment and HREE depletion), which relate to the presence of plagioclase (± clinopyroxene – amphibole) as residual phases in restite during partial melting of a felsic source rock. Low water contents in the Tocota Pluton derive from partial melting of biotite from the original felsic source. Younger diorite and andesitic dikes represent admixtures of mafic magma with felsic melt, whereas the very scarce basaltic dikes material derived from the mafic underplate intruded along joints. New U–Pb zircon ages show that the Tocota Pluton was the first intrusion of the post-orogenic Permo-Triassic magmatic cycle in the Colangüil Batholith. The La Fragüita Granodiorite has U–Pb zircon ages of 282.5 ± 2.8 to 275.2 ± 2.8 Ma, older than previously documented ages for the Tocota Pluton. The Rosado Granite yielded U–Pb zircon ages of 272.8 ± 1.6 to 271.2 ± 2.2 Ma, whereas diorite and andesitic dikes are 268.0 ± 3.4 and 265.3 ± 1.9 Ma, respectively. The new U–Pb zircon ages are to some extent comparable to those from the San Rafael block, Cordillera del Viento, Elqui–Limarí and Chollay batholiths in Chile and the Central Peruvian batholiths. Tourmaline is an accessory magmatic mineral in the Leoncito Tonalite and Rosados Granite, and hydrothermal cement in breccias and veins. U–Pb zircon age from a coarse-grained tourmaline vein yielded 266.0 ± 3.5 Ma. The Middle-upper Permian tourmaline-cemented breccias and veins at San Francisco de los Andes are genetically related with granitoids of the Tocota Pluton, and represent mineralization formed prior to the rupture of Gondwana.
The San Francisco de los Andes breccia complex crops out in a region characterized by Paleozoic sedimentary and igneous rocks, and unconsolidated Quaternary sediments. It has a magmatic-hydrothermal origin, based on breccia facies, their internal organization, and moderate- to high-temperature cement. Stage 1 quartz has a late-magmatic origin based on high Ti, low Ge, and TitaniQ temperatures between 524 and 648°C at 3 kbar. Stage 1-rim quartz is the hydrothermal overgrowth with high Ga, low Ti, lower TitaniQ temperatures; comparable to hydrothermal quartz from stages 2, 3a and 4a breccia cement and veins. Likewise, B isotopic compositions from stage 1 tourmaline from granite-hosted, tourmaline – quartz aggregates (-26.5 to -25.5 ‰) contrast with stage 1-rim hydrothermal tourmaline overgrowth (-15.4 ‰), which is equivalent to stage 2, 3a and 4a hydrothermal tourmaline breccia cement and veins (-14.9 to -9.7‰). The large δ\(^{11}\)B variation (i.e., -26.5 to -9.7‰) reflects mixing of different boron sources and isotope fractionation during magmatic degassing and crystallization of the host Rosados Granite. Non-marine evaporates were likely present during magma generation due to partial melting of preexisting sialic crust, or were assimilated during magma ascent. The coalescence of two breccia pipes is inferred based on the ‘figure 8’ geometry of the breccia complex, and major differences in mineral assemblages, metal assays, mineral composition and elements substitution mechanisms in mineral phases in each domain. During the final stage of crystallization of the Tocota Pluton, a magmatic-hydrothermal fluid was exsolved and trapped under a granitic carapace. Hydrostatic pressure built up under the impermeable barrier until it exceeded the lithostatic pressure and the tensile strength of the surrounding host rocks; leading to gas expansion, causing brecciation resulting in a column of broken sedimentary and igneous rocks (SE domain). Violent release of pressure triggered phase separation into a vapor phase and a dense brine. The vapor was enriched in light lithophile elements (particularly B), steam and HCl. Due to having low relative density, vapor upwelling occurred right after brecciation, whereas the dense brine remained in the base of the breccia column, close to the magmatic source. This weakly acidic fluid was responsible for quartz – illite – tourmaline alteration, and tourmaline – quartz cementation. The brine precipitated sulfides and sulfosalts. A deeper seated blast reamed out the NW conduit soon after the SE pipe had already been cemented. Similar moderate-temperature minerals and additional late-stage, low-temperature galena and sphalerite cemented the NW breccia pipe.
The Late Miocene to Early Pliocene Rio Blanco-Los Bronces district of Central Chile is the largest accumulation of copper on planet Earth, with over 200 Mt of copper contained within a series of tourmaline- and biotite-cemented breccias and related vein stockworks. The La Americana, Sur-Sur, Cerro Negro and Las Areneras sectors exhibit classic alteration and mineralization assemblages typical of porphyry systems. Deep stage 1 potassic (± sodic) alteration grade upwards and laterally outwards to stage 2 propylitic alteration, both stages occur at the district-scale. Stage 3 phyllic alteration is intimately related with the formation of tourmaline ± quartz-cemented breccias.
Stage 4 mineral assemblages have cemented breccias and infilled C veins. Stage 4 fluids evolved from Fe-oxide stable and reducing (i.e., H\(_2\)S predominant; La Americana and Areneras) to more oxidized fluids that precipitated anhydrite (i.e., HSO\(_4\)\(^-\), SO\(_4\)\(^{2-}\) or SO\(_2\) predominant; La Americana, Cerro Negro and Areneras), and finally back to reduced conditions that stabilized sulfides (i.e., H\(_2\)S predominant; La Americana, Cerro Negro and Areneras). Stage 5 (i.e., tourmaline-altered, matrix-rich breccias and veins), and stage 6 (D veins) caused sericite and intermediate argillic alteration. Stage 7 produced carbonate-bearing E veins, which formed from low temperature, epithermal fluids. La Americana, Sur-Sur and Cerro Negro are well-mineralized areas, whereas Las Areneras is a weakly mineralized to barren sector. Hydrothermal tourmaline breccia cement and veins yielded δ\(^{11}\)B values between -5.6 and 6.2‰, comparable to nearby Cenozoic volcanic rocks and data from other porphyry Cu deposits. Boron is interpreted to be extracted from upwelling fluids related to the subducting slab and overlying sediments, ultimately derived by fluids from oceanic crust altered by seawater. These interpretations are consistent with adakitic compositions for the subduction-related San Francisco Batholith responsible for the formation of the giant Rio Blanco-Los Bronces district.
At the regional scale, processed ASTER imagery has proven highly effective at mapping lithologies, hydrothermal alteration assemblages and individual hydrothermal minerals in a large area surrounding San Francisco de los Andes in the Argentinean Frontal Cordillera. Argillic and phyllic alteration assemblages were identified in areas spatially and genetically related with epithermal and porphyry deposits. Furthermore, ASTER imagery highlighted potential exploration targets for economic mineralization to the west of the already identified mineral deposits.
At the district scale, tourmaline chemistry show potential as an innovative tool to aid mineral exploration. It has the potential to both vector towards the ore center and assess the fertility of an area. Tourmaline chemistry from the San Francisco de los Andes district is a significantly better guide to mineralization than whole rock geochemistry, providing proximal and distal indicators of mineralization. In the Rio Blanco-Los Bronces district, tourmaline chemistry varies as a function of depth. At higher elevations, tourmalines are enriched in Fe, Ca, As, Sr, Mn, Zn, Ba and Pb in mineralized areas (e.g., La Americana and Sur-Sur). A povondraite component was identified in tourmalines from both districts. Oxidized and highly saline fluids are required to form povondraite compositions, which are the type of hydrothermal fluids responsible for the formation of many mineralized systems. Contrasting tourmaline compositions from mineralized and barren systems are explained here by preferential partition in the Y-site based on elements availability from the original hydrothermal fluids. The Y-site in tourmaline is mainly filled by R\(^{2+}\) cations (i.e., Fe\(^{2+}\), Mg\(^{2+}\), Mn\(^{2+}\)). If there is still available space, transition metals (e.g., Cu\(^{2+}\), Zn\(^{2+}\), Ni\(^{2+}\), Co\(^{2+}\)) partition into the Y-site. Those ore-bearing hydrothermal fluids are highly enriched in Cu (and Zn), thus preferentially partition into the Y-site leaving little to no space for Co or Ni. Conversely, tourmalines from barren hydrothermal fluids lack Cu and Zn, thus enriched in Ni and Co. Furthermore, if there is available space in the Y-site after Ni and Co replacement, Ti\(^{4+}\) may also partition into the tourmaline structure (only tourmalines from Las Areneras are enriched in Ni, Co and Ti). Antimony and As are pathfinder elements enriched in ore-related tourmalines. Some of the most effective fertility indicators proposed in this study are: Sb vs As, Cu / Co vs As, and Cu / Ti vs As. They efficiently distinguish the barren Las Areneras area from the mineralized Sur-Sur, La Americana, and San Francisco de los Andes deposits, and also from the Cadia East porphyry Cu–Au deposit in Australia.

Item Type: Thesis - PhD
Authors/Creators:Testa, FJ
Keywords: Ore deposits, Hydrothermal alteration, geochemistry, magmatic-hydrothermal breccias, porphyry, remote sensing, mineral chemistry, tourmaline.
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