This is a wonderful find, very much like the oak leaf crown on the head of  of Napoleon in the coronation painting in the Louvre museum, very much in the ancient Roman Caesar style..
 
It has probably come from a statue and fits well on the head form it is displayed on in some of the images. 
 
Made of a heavy weight gilded metal band with oak leaves and acorns attached. This is solidly and well made, the leaves have some flexibility but are stiff and well attached which is probably why ot has lasted so long. The gilding is bright, suggesting it has been well looked after or in a display case.
 
At the back is a rotating metal bar with a nut on the end, it has been painted gold to blend in, which could have been used to attach the couronne behind the head for security. This could easilt be removed.
 
It measures 6" in diameter and 1 1/4" deep