What a fitting subject for October~Nov (Day of the Dead) but spirits guarding treasures! There are so many tales of this, one wonders why if there is so much "smoke" then is there not some "fire" behind it?
The idea of spirits killing anyone who steals the treasure may be based on sudden deaths that occurred with the finding of treasures, like has been mentioned - some psychosomatic, some from the shock of the find, others perhaps from the very real death traps that have been left for the unwary. Some however might have been a case of being frightened to death, this is a medical rarity (thankfully fainting usually occurs instead of death) and then we have to wonder just what the dead person experienced that led to such fear? All in their heads?
I had a few experiences with what seemed to be a spirit protecting a treasure, wrote up an article for LT on it some years ago; this was a very nice little old man when he was alive (a shoe maker actually) but his "ghost" certainly wasn't. My brother experienced more harsh incidents than I did, including having china dishes smashed to bits, a window slammed (almost) on his hand, and others that no one would believe here; what seemed odd to me about it was that this "ghost" had hidden a treasure of perhaps several thousand dollars in a cave, and while we WERE looking for it, I am pretty confident that we were not anywhere near finding it. One instance I got to hear dis-embodied laughter while searching another empty cave, that sounded exactly like the old shoemaker's laughter (I had known him in life) which really gave me the creeps after the initial looking around to find that there was NO ONE else anywhere around.
If there are ghosts, I would think that some would indeed "haunt" "their" treasure; if I were dead and had a treasure to guard, I would sure try to have fun spooking folks. I would not let a story of spirits guarding a treasure prevent looking for it, but would sure keep an eye open for traps and deadfalls. Heck those spirits might be trying to guide you to help FIND it, for that matter - (another long winded theory, not my own but has to do with a ghost might appear different depending on who is looking at it whether it is frightening or not) so who knows?
HOLA Tropical Tramp! The old copper coin and the rolling coin sounds, boy I don't know that doesn't sound like it could have been the same coin rolling over and over again, you know the rules of physics - it would have to have some thing to set it into motion as an object at rest tends to remain at rest. Sounds like an unseen hand, and perhaps there was some meaning intended to be picked up besides the coin?
On a clear sunny day we never run into such things, but get out in the tall tules for a few months or even a few weeks and you will very likely see and hear things that
don't fit with what is SUPPOSED to be "reality"!

Oroblanco