Hydrothermal system associated to volcanic activity. On Mars, they were generated by volcano or crater formation, sill intrusion, or other plutonic intrusions into permeable, liquid water or ice-rich subsurface environments. They generated local thermal anomalies which induced density perturbations in the ground water as heat is dissipated out into the surrounding rock. Water near these anomalies migrates upward in response to buoyancy forces. This action results in the flow of ground water towards the thermal anomaly.
Source: Virginia Gulick, NASA Ames