The rocks, the crust of the Earth (which strictly speaking geologists call lithosphere), are not a continuum. Moreover, after a certain depth they don't even behave exactly like a solid, though this aspect deserves a separate article. Each rock type has more or less porosity, both by the nature of the rock itself or due to the effect of fractures and dissolution phenomena. These characteristics make the lithosphere behave as a store of water and other liquids and gases, which of course are subject to extreme pressure and temperature we can imagine for the solid part of the planet's outer layer. Well, that part in liquid (or rather, fluid) state is which the study of hydrothermalism is focused on.
The phenomenon of hydrothermal circulation is simply the movement of these generally hot fluids within the rocks that form the lithosphere. Of course, since it is not pure water, it can change the composition of the surrounding rocks, sometimes very intensely. We call this "hydrothermal alteration." There are different types of hydrothermal circulation. One of them has already been advanced, and is associated with volcanic regions. Volcanic phenomena have a limited life, ie volcanoes die out. But they do not suddenly but gradually do so, and even if the eruptions have ceased, there remain magma and a high temperature underground. As the magma cools and solidifies, the remaining fluid will be enriched progressively in substances that hardly can get into the structures of the minerals that make up rocks. They are what geologists call "incompatible" elements and substances, many of which are fluid. These hot fluids of magmatic origin tend to rise through cracks and fractures within the rocks, many of them previously formed by volcanic eruptions. In places where these magmatic fluids reach the surface, geysers, fumaroles and thermal fields could be formed.
In Cabo de Gata there is currently no active hydrothermal process, but there were very intense ones in the final stages of volcanism. In fact, the hydrothermal alteration is largely responsible for the Rodalquilar and surrounding areas mineral wealth, forming precious metals deposits like gold, other of economic interest such as lead, and even industrial minerals like the bentonite clays, with applications ranging from the oil industry until the pastry, through construction, pharmacy, and soil decontamination. In fact, many large mineral deposits all over the world are associated to a greater or lesser extent to hydrothermal phenomena.