post apocalyptic fantasy - female heroine

Hey guys! Hoping someone out there can help me find what I am looking for. It is a book I read when I was 12 or so, so a good 13 years aago. It was set in a post apocalyptic earth where the sun was covered by clouds and the world was dark and.cold. The 'slave' class was forced to mine for some type of rocks that could be burned to keep people warm. They are all branded with clan symbols, and I remember the heroine having a lion head / sun symbol on her forehead. Somehow or another she becomes the apprentice to one of the holy men of the novel and, in the end, discovers the ability to warm herself.from the inside, negating use of the rocks? Theres a lot of drama surrounding her ascension because females are considered far bekow.men but theres no denying her ability to 'warm' herself. In the end theres some type of battle, between 'her' forces and her opponents. Also at the bery very end there is a.single sunbeam that comes out. Hope someone knows the title!