The ancient treatise is a relic that may be received from breaking open barronite deposits. A relic from the Second Age, it belonged to the inhabitants of Lassar, who were destroyed during the Zarosian Extermination by Saradominist armies, which included the Imcando dwarves.
The treatise can be handed to Curator Haig Halen at the Varrock Museum for 1 Kudos and 200 coins. Players will receive 100 coins for additional ancient treatises. When given to the curator for the first time, he will comment "I'm not even familiar with the glyphs in this Ancient treatise. It's the first time I've seen them."
For an exhaustive list of all known sources for this item, see here