Literal or figurative source of great wealth, like a mine that yields gold. Can refer to a business, investment, or opportunity that generates large profits.
This new venture is truly a gold mine.
financial backer; patron; source of funds
A person or organization that provides money, like a sponsor or angel investor. Often used in contexts where someone is the main source of financial support.
rich person; moneybags
A wealthy individual, often with a nuance of being a source of money for others. Can be used somewhat colloquially or jokingly.
He is the financial backer of our project.
I hear that person is the richest in town.
Compound of 金 (gold, money) and 穴 (hole, pit). The image is of a hole from which gold or money can be extracted, hence a source of wealth. The exact historical origin is uncertain, but the word is used figuratively in modern Japanese.