Hard to see any dateable clothing, but the pose suggests 1870s. I suspect she is actually dressed in a style from a decade earlier because that skirt looks a bit too full. But it's an 1870s pose.
If you can open the case, try a small magnet on the back of the photo...that will tell you if it's a tintype or not. If the case doesn't open, try it on the front. Either way, don't let it touch the surface in case it's an ambrotype which could easily be damaged by anything touching the back.
Cheers,
China