Hydrothermal fluid flow evidenced by mineral alteration assemblages andchemistry of metamorphic rocks, sediments and volcanics on top of thesouthernmost Río de la Plata craton, eastern Argentina

MARTÍNEZ JUAN C.; DRISTAS A. JORGE; MASSONNE, H-J,

Viena, Austria

Congreso; European Geoscience Union. General Assembly 2011; 2011

Copernicus Meetings

Two horizontally widespread alteration zones of the Barker-Villa Cacique area, Tandilia ranges, share common alteration features recognized by petrography, X-ray diffractometry, electron microprobe analysis, bulk-rock geochemistry and K-Ar ages. An asymmetric alteration pattern with development of an argillic clay mineral assemblage is seen at the Tandilia Late-Proterozoic unconformity zone between the migmatitic and meta-igneous basement, mainly of granodioritic composition, and a quartz-rich epiclastic overlying succession. The most pervasively altered rocks, presented at the unconformity, usually have Na-bearing dioctahedral K-white micas, Na and K-bearing pyrophyllite, LREE-bearing aluminum phosphate sulphate (APS) minerals florencite-type and tourmaline. Transitional zones to unaltered basement rocks are dominated by dioctahedral K-white mica, chlorite and calcite.  Ti-oxides are obvious in the alteration profiles. In the middle upper section the overlying sedimentary succession shows lenses of silicified breccias, reworked and altered pyroclastic material recognized as reddish and whitish clays and an alternating succession of reworked tuff material and epiclastic quartz arenites (Frisicale and Dristas, 2000). These rocks are usually cut by subhorizontal and vertical breccias (Dristas and Frisicale, 1992). The mineral paragenesis of the reddish and whitish clays corresponds to an advanced argillic alteration (AAA) where isolated quartz grains are corroded by pyrophyllite-sericite.  This alteration includes pyrophyllite + sericite + kaolinite ± tourmaline ± rutile ± alumino phosphate sulphate (APS) minerals ± diaspore ± alunite. Pseudocubic Na-bearing alunite crystals are present either as veins or lenses mainly in the whitish clays. X-ray compositional maps on alunite reveal tabular-like cores with significant amounts of P, Ba, Sr, REE, Ca and Fe pointing to APS minerals florencite-type (Martínez et al., 2006). Intensity of alteration diminished upwards the succession. Bulk-rock chemistry of altered rocks show rapid changes from non-altered basement to the most altered basement and sedimentary rocks at the unconformity with a negative Eu anomaly (Eu/Eu* =0.24), strongly positive Eu anomaly (Eu/Eu* =11.7) and positive Ce anomaly (Martínez et al., 2010). In the same way, wall-rock of alunite veins shows positive Ce anomalies when pyroclastic component is not diluted by epiclastic components. K-Ar ages determined on altered rock rich in secondary K-white micas of the basement and the interbeded altered tuff and quartz-rich layers points to minimum alteration ages ca. 600 Ma (Bonhomme and Cingolani, 1980; Dristas & Martínez, 2007) in accordance with K-Ar age in pure alunite veins (650 ± 20 Ma). Similarities in chemistry, mineralogy, alteration textures and ages allow interpreting both alteration zones as related to fluid flow migration probably linked either to hidden igneous rocks, which are common in the portion of the Uruguayan Rio de la Plata craton, or to metamorphic fluids expelled out during basement uplift.