Rich source of anything

• Bonanza
• Gold seeker's target
• In the ___, there's the greatest concentration of ___
• Gold miner's bonanza
• Prospector's dream
• Major supply
• Jackpot
• Miner's dream

Wiktionary Word definitions in Wiktionary
n. 1 (context literally English) The main, central lode of a natural resource, near which smaller deposits of the same ore etc. exist. 2 (context figuratively English) The main prize, best cut or pick etc. by far, dwarfing all others.

Wikipedia Word definitions in Wikipedia
Motherlode was a Canadian pop rock group formed in 1969 in London, Ontario . The group scored some success in the US with their single , " When I Die ", which hit #18 on the U.S. Billboard Hot 100 chart in 1969. The act remains a one-hit wonder .