gold mine; treasure trove; mountain of treasures
Idiomatic expression meaning a place or source of great wealth, valuable resources, or abundant opportunities. Can be used literally or figuratively.
This library is a treasure trove of knowledge.
His ideas were a gold mine for the business.
Compound of 宝 (treasure) + の (possessive particle) + 山 (mountain), literally 'mountain of treasures'. The figurative meaning is a natural extension of the literal image.