GEOCHEMISTRY OF MIOCENE TRAVERTINE AS A REFLECTION OF GROUNDWATER AQUIFERS, ATACAMA DESERT, CHILE

The hyperarid Atacama Desert in northern Chile is ideal for the preservation of limestone. Travertine, predominantly chemically precipitated limestone, is associated with springs where groundwater, saturated with carbonate, breaches the surface. Geochemical and petrographic analysis of Miocene travertine from two sites, Chintoraste and El Sifon, in the Atacama Desert, Chile, indicate variation in source waters that likely reflect differences in regional hydrology. At El Sifon and Chintoraste, local faults are mapped close to the travertine deposits, and may have channeled water sourced from different aquifers to spring sites where degassing produced calcium carbonate. U/Pb dating yields a 7 Ma age for the El Sifon travertine, with rapid precipitation in <0.5 million years. Travertine at Chintoraste occurs directly above Oligo-Miocene conglomerates, suggesting a Miocene or younger age.

X-Ray Diffraction (XRD) results for individually drilled laminations show that El Sifon carbonates are predominantly aragonite, with secondary calcite, whereas, the Chintoraste samples are largely composed of calcite, including Mg-calcite, with minor aragonite. Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) results show that the aragonite-dominated carbonate is significantly enriched in Sr (average 12,556 ppm), relative to the calcite +/-Mg-calcite samples. Mn and Fe are low in the aragonite from El Sifon, but are enriched in calcite from Chintoraste (Mn average 860 ppm; Fe average 2,318 ppm). Although aragonite typically accommodates the large Sr ion more readily than does calcite, the variation in carbonate polymorphs and trace element chemistry between the locations could reflect different source waters, rates of precipitation, and/or spring water carbonate saturation. ⁸⁷Sr/⁸⁶Sr variation between the sites suggests that different groundwater sources were primarily responsible for the different chemistries. Acicular aragonite with cm-scale fans and botryoids suggests relatively rapid precipitation rates at El Sifon in a largely inorganic spring setting, whereas the calcite-dominated laminations at Chintoraste are interbedded with small stromatolites and detrital sands, suggesting slower precipitation rates in a more complex spring environment.