Hydrothermal vents, either from volcanoes or meteoritic impact, are often associated with high concentrations of minerals that may be of value to Martian settlements.
With hydrothermal vents, hot water heating through volcanism or asteroid impact circulates through a volume of rock. the water dissolves minerals, that condense out at the exit points, the hydrothermal vents. This provides a natural concentration mechanism for minerals and creates deposits that may be economically mined.
There appears to be a number of areas on Mars where hydrothermal vents may have existed.
Areas where hydrothermal vents may exist: