what is this?

whilst driving my rover 216 1999 t reg i noticed steam coming out of my bonnet, under further inspection the coolant tank level was low so i topped this up using antifreeze/water mix. i then drove the car for 10 mins when i heard a noise like a kettle boiling and brown water/steam was leaking from the cap on the coolant tank, any ideas how to solve this problem/ is it cheap to repair? many thanks

matt
April 2009