Gaua, a little known volcano in the northern part of Vanuatu archipelago, went through a long term eruptive activity between September 2009 and July 2010. The eruption started by a phreatic to phreatomagmatic activity which progressively shifted into a magmatic discharge. The first eruptive phase involved the hydrothermal system in place. The latter was likely influenced by seawater seepage, leading to the formation of anhydrite. Magma involved hereafter this opening phase is of basaltic andesite and basaltic trachyandesite composition (high K calc-alkaline series), typical of the northern part of the Vanuatu archipelago. The 2009–2010 activity discharged at least 184 kt of SO2 and a significant amount of halogens (72 kt Cl and 217 kt F). Such halogen releases indicate that Gaua is a strong source of halogens into the atmosphere. High and sustained amount of F discharges are known to induce health issues and should not be ignored on Gaua island. During this eruption the quiescent and voluminous Lake Letas was slightly affected by the eruption. Nevertheless the hydrothermal discharge point into the lake, situated on the southeastern part of Mt. Garet appeared to be relatively active. At this particular location rock forming elements, leached out from volcanic rock by acid fluids released by the new intrusion of magma, were discharged along with anions into Lake Letas. This release has triggered localized chemistry changes in the lake. We speculate that this discharge has also disturbed the bottom water in a limited perimeter, remobilizing reduced Fe to the surface and subsequently triggering the change in the water color by Fe oxidation.