CONICET | Buscador de Institutos y Recursos Humanos

The southernmost part of the Rio de la Plata Craton, Tandilia Ranges, consists of an igneous-metamorphic basement, known as Buenos Aires Complex (Di Paola and Marchese, 1974), and a Neoproterozoic-Early Palaeozoic shallow marine succession. The former is mainly composed by granitoids, migmatites, amphibolites, gneisses, mylonitic rocks, metavolcanic rocks and marbles (Cingolani and Dalla Salda, 2000, Pankhurst et al., 2003, Delpino and Dristas, 2007). These rocks are largely related to the Transamazonia Orogenic cycle, ca. 2200-1800 M.a., (Cingolani and Dalla Salda, 2000). In the Olavarría area, northern Tandilia Ranges, the siliciclastic and carbonatic Sierras Bayas Group (Poire, 1987) usually overlies a large crystalline lithological variety of rocks. Hydrothermal fluid flow has previously been registered and characterized in altered rocks from Barker-Villa Cacique area (Martínez and Dristas, 2007), a hundred kilometres southwestwards from Olavarría, into the Tandilia Ranges. Field trip observations in the latter area, petrographic studies, electron microprobe, X-ray diffractometry and bulk-rock analyses of collected samples allowed to established different alteration patterns affecting three stratigraphic levels: (1) Buenos Aires Complex-Sierras Bayas Group unconformity. Altered samples from Villa Mónica quarry and drill core samples at El Polvorín quarry show a dominant chloritic-carbonatic and sericitic alteration. The basement from Villa Mónica quarry includes granitoids of granodioritic to granitic composition and amphibolitic migmatites. All these primary minerals display some degree of ductile deformation especially developed in quartz. Accessory minerals include zircon, apatite and allanite. In the eastern side of the quarry hydrothermal fluids generated chloritised tight fault planes of aptitude N50º/80ºNW as a result of a later fragile event which also affected the sedimentary cover. A few meters below the unconformity this chloritised basement is cross cut by veinlets. Whereas, basement is sericitised immediately below the well-known quartz arenites of the lower section of Sierras Bayas Group. In the same way, the sabulitic to coarse-grained arkose arenite from the lowermost part of sedimentary succession display a fine-grained sericitic alteration matrix. The same alteration pattern has been recognised in wacke sandstones from drill core sample at the unconformity in El Polvorín quarry. There, scarce K-feldspar clasts are replaced by sericite, apatite, calcite and chlorite. Tiny apatite LREE-bearing, anatase and barite crystals are scattered into the alteration matrix. Carbonates Mg-Fe and Sr-bearing pervasively altered the basement immediately underling wacke sandstones. Vermiculite and Fe-oxides alter and stain biotite, whereas crystals of K-feldspar are altered relics. Secondary apatite crystals usually content LREE (Ce, La, and Pr) and barite is commonly present. No fresh basement was reached in drill core, up to 5 m deep, below the contact with sedimentary rocks (75 m from surface). Altered sedimentary rocks and igneous-metamorphic basement in the unconformity zone exhibit similar REE patterns normalized to chondrite (Taylor and McLennan, 1985), where light REE are slightly enriched and heavy REE pattern are flat. (2) Quartz arenites, dolostones and associated pelitic rocks at El Polvorín and Piedra Amarilla quarries. Veins of goethite-hematite are commonly cutting and replacing quartz arenite levels. In dolostones previous recrystallised dolomite rombohedrons are partly to completely replaced by interstitial hydrothermal secondary quartz, with development of druses. Drill core samples of dolostone at el Polvorín quarry display breccia zones where fragments of clayey material, frequently with grains of deformed quartz basement-derived, are cemented by secondary carbonate and idiomorphic quartz. Anhedral anatase crystals (up to 0.5 mm) are scattered through this breccia rock. Lentiform clay levels, interlayered between dolostones and quartz arenites of the upper stratigraphic unit are originally composed by illite. These and the lowermost part of upper quartz arenites are commonly brecciated and altered by whitish clay assemblage (sericite + pyrophyllite + kaolinite). LREE-bearing APS minerals are commonly scattered into this clay secondary assemblage. Brecciated microcrystalline secondary quartz associated with pyrophyllite affected this zone. (3) The unconformity zone between the black limestones of upper Sierras Bayas Group level and the siltstones and marls of the Cerro Negro Fm. in El Polvorín, Puracal and Loma Negra quarries. Replacement of previous micrite, calcite and barite veining, dissolution and collapse of the upper sediments and breccia body intrusions are common features pointing to hydrothermal fluid migration, specially channelled through the unconformity surface. Veining and dykes of clayey and silty ascending material through the limestone and upper unit also point to hydrothermal fluid flow. Additional studies on isotopy, cathodoluminiscence and fluid inclusion are programmed in order to constrain fluids temperature, composition and further effects over altered rocks.