be aware of Carbonatite volcanics
rare on Earth, but in the African rift valleys there are volcanoes that spew lavas with a high concentration of washing soda (sodium carbonate). Clearly, these come from CO2-rich source regions and something similar may work on Mars, so the signature of carbonates in a volcanic area is not neccesarily a sign of water alteration - it could be far more interesting, more Martian, less Terran...