CONTRIBUTION OF ABIOTIC AND BIOTIC PROCESSES INVOLVED IN TWO TRAVERTINES DEPOSITION FROM PUNA REGION, CENTRAL ANDES - ARGENTINA.

The study of travertine systems has gained relevance in recent years due to their potential as world-class petroleum reservoirs and analogs of ancient microbial carbonates. It is, therefore, necessary to understand the contribution of abiotic and biotic processes involved in their deposition. In this work, we analyzed two active travertine systems, one subaqueous from El Peinado Lake (PND) situated at 3757 meters above sea level (masl), and another one subaerial, Vega Verde- Botijuela (VV-BOT) situated at 3600 masl. Both are located on the southern border of the Puna Plateau (North-West Argentina), at a high-altitude volcano-tectonic setting. Samples from both sites were analyzed by X-ray diffraction (XRD), X-ray fluorescence (XRF), stable isotopes of δ¹³C and δ¹⁸O (V-DPB), petrography and SEM-EDS analysis. The deposits from PND were mainly composed of calcite and magnesium calcite, and to a lesser extent, gypsum and aragonite. The δ¹⁸O evolution from -5,59 to 10.59‰ and high positive δ¹³C values from 7 to 10.01‰ of these samples suggest a highly evaporitic process involved. On the other hand, VV-BOT showed that calcite is the predominant mineral, and to a lesser extent, gypsum, halides, and detrital components like diopside and quartz.  This system presented a wide data dispersion with δ¹⁸O values from -8.91 to -0.67‰ and δ¹³C values from 1.65 to 10.89‰. These data suggest that the isotopic fractionation in this place may be due to an interplay between degassing by flow in a high-slope setting and evaporation with water recharge cycle processes. Geochemically, both systems showed high values of S (PND~7477 ppm and VV-BOT~2794 ppm), As-Pb (PND~1582 ppm and VV-BOT ~4543 ppm), Fe₂O₃ (~1,61% PND and ~4.3% VV-BOT) and SiO₂ (PND~7,32 % and VV-BOT~2%), which evidences a volcanic-hydrothermal influence in the formation of these deposits. Petrographic descriptions and SEM images showed a clear microbial contribution to the sedimentological textures in PND and VV-BOT rock samples. In VV-BOT, the mineral morphology related to microbial mediation was associated with an unconservated exopolymeric substance (EPS), and cyanobacterial sheets remain.

On the contrary, in PND, the diversity and abundance of microbes were more evident; in these samples, diatoms occur in large numbers, forming microfacies composed of almost agglutinated frustules in the carbonate sediment. Filamentous and coccoid cyanobacteria and EPS were also observed. It should be noted that in the rock samples analyzed and in fresh in situ samples, the photosynthetic organisms are in close spatial relationship with the carbonate sediments, which could also contribute to the isotopic signature. The δ¹³C presented higher values in the proximal zone samples from both places, where the microbial microfacies were more evident. The results presented in this work increase the knowledge and understanding of how different geochemical and biological processes interact at macro and micro scales, forming complex carbonate systems such as the Puna travertines.