I wouldn't completely rule out that it could be Haleigh, but Mandarin is a "historical" area (I am from Jacksonville originally and lived there most of my life). Mandarin, Fruit Cove and across the river, Fleming Island were all huge plantations at one time. If you were to look the legal descriptions for some of the properties they start out by saying ....a part of the (insert a name here) Spanish Land Grant, etc. They go waaaay back. I wouldn't be surprised if this was could have been some type of family gravesite that was unearthed. I would love to get some closure for Haleigh though.
OT - The Mandarin area is beautiful. Many years ago, I read a Eugenia Price book, "Margaret's Story" and it was based in the Fleming Island and Julington Creek areas. I loved it. If you aren't familiar with Price, she took facts about real people in history and created stories based on those facts. I guess she was a creative sleuther herself.
Thanks for your interesting post. I'm going to try and find that book because I love history and it sounds really great.