CLAY FORMATION DEPENDENT ON BRINE CHEMISTRY IN SURFICIAL HYDROTHERMAL SYSTEMS

The geochemical composition of surficial hydrothermal brines can be divided into two end-members: neutral-chloride and acid-sulfate (+/-chloride). Neutral-chloride systems are dominated by siliceous sinters, while acid-sulfate systems are dominated by clays and Fe-oxides. These differences have been variably explained as reflecting the results of water flux through the systems, microbial activity, and authigenic mineral formation at depth beneath the spring. We examined authigenic clay mineral formation in hydrothermal systems using an experimental batch reactor system based on the dry-season aqueous geochemistry of Las Pailas hot springs (pH 2.6, T=80^oC, [SO₄^2-]=38.38 mM, [Fe²⁺]=22-38.38 mM, [Al³⁺]=0.1-15.27 mM, [F^-]=0-0.11 mM, & [Ca²⁺]= 0-0.55mM), Costa Rica, which are known for their clay rich hot springs, mud pots, & fumaroles. In one half of trials, sulfate was replaced with chloride. X-ray diffraction analysis of precipitates shows mixed-layer smectites, nontronite, illite, kaolinite/halloysite, and Fe-oxides form within 7 days in experiments that had sulfate as the predominant anion.. Fewer precipitates formed in chloride experiments, however, the mineralogy included mixed-layer smectite and amorphous silica. PhreeqC modeling predicted equivalent moles of nontronite should form in both experimental scenarios, however, chloride solutions produced 90% less clay after 18 mos. Our results demonstrate that Al-complexation by fluoride significantly influences clay mineral formation and crystallinity in hydrothermal systems. However, it’s inclusion in chloride solutions was in sufficient to produce substantial authigenic clays. Al-complexation by sulfate, a ligand with high affinity for Al, enhances kaolinite & nontronite formation. The sulfate ion, therefore, mediates clay mineral formation in two ways: (1) acid attack of primary mineral phases to create ion rich solutions, and (2) by mediating the formation of authigenic clay minerals through Al-complexation. This study indicates brine geochemistry influences authigenic mineral formation in hydrothermal systems and that this effect may have more control on mineral precipitation than simply solution pH. Indeed, mineralogy may be an important indicator of past brine geochemical composition on other planets, including Mars.