I have a dark circle in my pasture near and old buffalo trace were nothing grows. Can an ancient hut leave a dark circle on the ground?

No. That dark circle would be a fairy ring; it has nothing to do with that old buffalo trace. A fairy ring is basically a circle of mushrooms or a circle formed by fungi of the Basidiomyces class that live in the soil. These fungi release nitrogen which the grass takes up, and makes the grass in that ring turn a darker colour than the surrounding grass.