Short answer. Not as usufructuary. Article 566 gives him all the natural, industrial and civil fruits of the property, but says that as to hidden treasure he is considered a stranger. Treasure is not a fruit of the land, so his right to enjoy the property does not reach it.
What the law says
The usufructuary shall be entitled to all the natural, industrial and civil fruits of the property in usufruct. With respect to hidden treasure which may be found on the land or tenement, he shall be considered a stranger.
Civil Code, Article 566 — Right to All the Fruits. Read the full provision →
A usufruct is a right to what the property produces
The first sentence is generous and worth noting for its breadth: natural fruits, industrial fruits, and civil fruits — so the crops, the young of animals, whatever the land yields through cultivation, and the rent if the property is leased out. That is what enjoyment of another's property means. Hidden treasure fails that description entirely. It is not produced by the land, it is merely concealed in it, and it would have been there whether the property was farmed, leased or left idle. Nothing about the usufructuary's right explains why it should fall to him.
What being "a stranger" gets him
It is a deliberate positioning rather than an exclusion. Treated as a stranger, the usufructuary neither gains from his relationship to the property nor loses by it — he stands where any outsider would stand. So the share the Code allots to the owner of the land goes to the naked owner, whose thing the treasure is part of. And where the usufructuary himself is the one who came upon it by chance, he is not disqualified from the finder's share simply because he happens to hold the usufruct.
The condition attached to a finder's share
This is where these claims are usually decided. A finder's share belongs to someone who discovers treasure by chance. A person who went looking for it — who dug because he suspected it was there, or who obtained access in order to search — is not a chance finder, and the point bites hardest against a usufructuary because he is on the land continuously and controls what work is done. Excavation carried out deliberately in the hope of a discovery is a very different fact from a discovery made in the course of ordinary cultivation or repair.
What is worth establishing at once
Two things: whether the find really is hidden treasure, and the circumstances of the finding. Not every buried object qualifies — the Code's notion is of concealed money, jewellery or other precious objects whose owner cannot be identified, so a find with a traceable owner belongs to that owner and none of this applies. Record where and how it was found, who was present, and what work was underway at the time, before the account has been told several times. Items of possible cultural or historical character carry their own restrictions and should not be moved or sold on assumption.