It could be a number of things. It could be some type of fungal infection. It could be some type of dermatitis. I am not trying to scare you on this but you may want to go to a dermatologist to make sure it is not cancerous.
Dark, dry patches are most likely a result of hormonal issues. Often a woman will develop these during pregnancy, it is then called "the mask of pregnancy". Wear sunscreen.
Hormonal changes in a woman's body can cause this problem. Also, the amount of time spent in the sun can greatly exaggerate the color of the patches on the skin.
causes for dark dry patches on face is sun, bathing in pools, lack of nourished diet, using soaps, deodorants, cosmetics, pills. remedies - apply coconut oil, lemon and carrot juices.
These dark patches are called melasma, this has alot to do with your hormones. Exposure to sun can cause it to become darker. Heredity,race,medication is also a factor in melasma. Protect your skin with a sunscreen.