Crater lakes of andesitic volcanoes commonly contain effluents of volcano-hydrothermal systems that provide direct chemical samples and insight into the dynamics of such active hydrothermal systems. Crater lake fluids are characterized by high concentrations of rock-forming elements and chloride, several % of sulfate, pH values below 1, and high metal concentrations. Examples of such brine lakes are Poas (Costa Rica), Keli Mutu and Ijen (Indonesia) and Copahue (Argentina). Some of these lakes have up to 75 % of a magmatic brine contribution. The Copahue system discharges into a river, and river flux and composition monitoring provides estimates of mass transfer for ore metals and rock forming elements. Water-rock interaction was close to congruent with subsequent precipitation of silica. Disproportionation of volcanic SO2 creates heavy bi-sulfate (+14 per mille) and relatively light elemental sulfur (-8 per mille). The rate of rock removal in this system is on the order of 25,000 m3/yr, but most of that void space is filled with a mixture of silica and elemental sulfur. Annual measured and modelled metal fluxes for Copahue are 10 tonnes Zn, 5 tonnes Pb, 3 tonnes Ni, 7 tonnes Cr and 15 tonnes As. Copper levels tend to be highly depleted in these fluids, suggesting precipitation of Cu minerals in the underlying hydrothermal system or crater lake environment. Comparison of fresh rocks and fluid compositions suggests that the metals enter the hydrothermal system with high-temperature volcanic gases and are not derived from water-rock interaction. Dramatic compositional changes are observed after volcanic eruptions, which we interpret as the result of new magma that intruded into the hydrothermal system. Acid attack on this new magma led to strongly increased element fluxes in the effluents.