Sedimentological and geochemical aspects of the Botijuela travertine systems , Antofalla Salar- Central Andes, Argentina.

Carbonate accumulation processes in modern non-marine systems producing tufa and travertine deposits have gained relevance in recent years due to their importance as possible analogues to the hydrocarbon reservoirs of the Brazilian and African Pre-Salt. The biotic/abiotic genesis of these carbonates is related to the fact that these rocks create favorable situations for the proliferation of benthic microbial communities, which can influence the genesis of travertine and/or its final form. The aim of this study is to characterize the carbonate deposits and the biotic influence in the Botijuela travertine systems, located on the western margin of the Salar de Antofalla in the Puna region of Argentina. Descriptions of morphological features and sedimentary facies in the outcrops, thin-slice petrography, mineralogical analysis by X-ray diffraction (XRD), chemical analysis by X-ray fluorescence (XRF), and stable isotopes of C and O (V-DPB) were done. The Botijuela was divided into two active systems: Vega Verde and Vega Blanca. The vent of the first one is a conical mound (~15m) with a length of 530 m, and the second one is a fissure ridge with fractures with a length of 325m, also with two fossilized travertine systems, named I and II. At the proximal depositional zone of Vega Verde, the system presents carbonate facies of mudstones with bubbles, shrub-boundstones, and oncoidal (~0.5cm diameter) rudstones. The fractures of Vega Blanca in the proximal zone present halite and subsequent development of carbonate pools (transversal 2–7 cm) and big oncoids (~2 cm in diameter). Dams and barrages in the pools present botryoids in areas that show subaerial exposure. Oncoids and dendriform shrubs are well developed in the proximal-intermediate depositional zones in low-slope areas. Intermediate zones of both systems are characterized by higher-energy carbonate facies: crenulated mudstones, oncoidal grainstones, and rudstones. The distal area of Vega Verde presents rudstones with detrital fragments, diatoms, and ostracods that interbed with siliciclastic sediments of the Antofalla Salar basin. Vega Blanca presents bigger (~15cm) transversal carbonate pools with oncoids and botryoids at the dams. Mineralogically, the systems are mainly calcite, and calcite low in Mg. Geochemically, Vega Verde shows a content of CaO of ~51.05% and Vega Blanca ~46.63%, with average loss of ignition values of ~40% for both. Vega Verde proximal zone is characterized by a high Fe-As-Pb content (Fe₂O₃ ~4.3%, Pb-As 0.5 to 1.7%) with an isotopic signature of δ¹³C (1.97-10.84) and δ¹⁸O (-0.67-8.91). In comparation, Vega Blanca systems show averagely higher values of SiO₂ (I~10.79% -, II~5.17%) and Na₂O (I~ 0.32; II~1.47%), with isotopical signatures for system I of δ¹³C (5.31~14.78) and δ¹⁸O (-4.86~ -0.24), for system II of δ¹³C (6.34~15.36) and δ¹⁸O (-3.20~ -0.71). Signatures indicate a water hypogene origin with δ¹³C fractionation enrichment towards the Antofalla basin. Geochemical data allows us to infer that the systems have a different water origin. Overall, high-resolution facies analysis of travertine systems and their integration with their geochemical framework and tectonic setting constitute a step forward regarding the environmental distribution of microbially related deposits and the comprehension of their main constraints.