I came across one of these (I think) in Lake Park. I ended up using historic=tomb and tomb=tumulus. Take a look in the wiki to see if the tomb type is the same. :-)

If there's a marker you might add historic=memorial and memorial=plaque and memorial:type=plaque.

https://www.openstreetmap.org/node/4067090340
https://wiki.openstreetmap.org/wiki/Tag:historic=tomb?uselang=en