ABSTRACT

Three crater lakes occur on top of the dormant stratovolcano Keli Mutu in Flores (Indonesia). The lakes contain cool (20–30°C) acid-sulfate-chloride brines and have high TDS with relatively high Zn and Pb contents. Two lakes show plume-like upwellings, probably fed by subaqueous fumaroles. A river on the mountain slope contains fluids that escaped from the lakes through seepage. The fluid compositions have varied strongly over time for S and the halogens (the volcanic-gas derived elements), whereas many of the major cations (rock-derived elements) show a net increase in the lakes over this century. Speciation and mineral saturation modeling, bulk sediment analyses, and micro-probe sediment analyses indicate that the lakes are reducing at depth, and all three lakes are close to saturation with gypsum/anhydrite. One lake is also saturated with Fe-oxides, while the other two are thought to be saturated with a suite of Cu and As minerals. The most acid lake is also saturated with native sulfur and possibly pyrite. Mass balance estimates based on fresh volcanic rocks and ashes, the dissolved element load and the lake sediment chemistry strongly suggest that the lakes are partly fed by underlying geothermal circulation systems. The lakes are thus not closed reaction vessels with dissolution of local rock debris by fluids acidified by volcanic gases, but they draw a rock-derived element flux from deeper levels. Mass transfer is large in these dynamic systems, including the transport of tonnes of ore metals over a period of centuries.