Fluvial travertines, mound travertines, and perched spring travertines: deposition and geothermal implications

Travertines are common carbonate deposits in hot springs and have attracted increasing attention recently for their potential applications in palaeotectonic, palaeoenvironmental, and palaeoclimatic reconstruction, abiotic versus biotic carbonate precipitation analyses, and pre-salt petroleum system exploration. However, to best understand and interpret travertines, the deposition mechanism(s) of travertines must be firstly identified. Travertine depositional environments are highly diverse, but despite many studies having been performed on travertines, few of them focus on travertines in fluvial systems, mound systems, and perched spring systems. Furthermore, travertines, as typical surficial geothermal manifestations, might provide some geothermal information, especially reservoir information, but studies on this topic are also limited. In order to better determine the depositional mechanism(s) and geothermal implications of travertines in fluvial systems, mound systems, and perched spring systems, fluvial travertines at Bagni San Filippo (Italy), mound travertines at Heinitang (China), and perched spring travertines at Shihuadong were carefully investigated in this dissertation. Travertine deposition in the three studied systems is found to be the result of interplay of many factors. The Bagni San Filippo fluvial system generated mixed carbonate and siliciclastic deposits and its travertine deposition was influenced by local topography, relative contribution of spring water to original river water, original river bed geometry, and fluvial erosion. The Heinitang mound spring complex system formed many small travertine mounds whose formation was controlled by topography, substrate, and fault activity. The extinction of the Heinitang mounds was likely caused by tectonic activity and climate fluctuations, while their lateral development was considered to be the result of low water discharges and relatively low Ca2+ concentrations in vent waters. The Shihuadong perched spring system mainly formed travertines in topographically steep areas away from the orifice(s) and its travertine deposition was possibly an outcome of special headspring waters (low Ca2+ concentrations and low initial CaCO3 saturation states) and flat topography near the orifice(s). Geochemical features of the studied travertines revealed the genesis of the travertines and the reservoirs of the studied geothermal systems. Calculated (palaeo-)temperatures show that the studied travertines were precipitated in hot spring environments (>20°C). Calculated δ13Cparent-CO2 of the travertines shows that all the travertines received carbon from magmatic CO2, but the Bagni San Filippo travertines and the Shihuadong travertines also attained carbon from marine carbonates and soil CO2, respectively. 87Sr/86Sr ratios of the Bagni San Filippo fluvial/non-fluvial travertines and REY (rare-earth element and yttrium) patterns of the Bagni San Filippo non-fluvial travertines have great similarities to local Tuscan Nappe carbonates and evaporites, reflecting that the Tuscan Nappe carbonates and evaporites might be the main reservoir rocks. 87Sr/86Sr analyses of the Heinitang and Shihuadong travertines and the distribution of different rocks exposed in Tengchong indicate that the Late Cretaceous granitoids in Tengchong are the most possible Sr source rocks and reservoir rocks. However, the Heinitang and Shihuadong travertines show HREE enrichment relative to the Late Cretaceous granitoids, and such HREE enrichment is interpreted to be caused by the preferential dissolution of LREE-enriched minerals, solution complexation, or surface adsorption. Furthermore, Eu (europium) and Y (yttrium) anomalies relative to potential reservoir rocks were observed in the studied travertines, showing the non-conservative behaviors of some elements during water-rock interaction. Additionally, detrital contamination on 87Sr/86Sr and REY compositions of travertines was present and was found to be environmentally controlled, indicating the importance of careful geochemical interpretations in travertine studies. A systematic comparative analysis shows that evaluating geothermal reservoirs based on travertine investigations is achievable and can at least be used to differentiate geothermal systems dominated by marine carbonate reservoirs from those dominated by granitoid reservoirs. The lateral extent and geochemical signatures (especially δ13C, 87Sr/86Sr, and REY) of travertines were found to be potential geothermal reservoir indicators. However, more studies focusing on both epigean and hypogean travertines formed in different environments and in different geological settings are still required to decipher the relationship between geothermal reservoirs and travertines and to better interpret geothermal reservoir indicators. Overall, the above results and findings provided useful information for the characterization, interpretation, and geothermal implications of travertines.