mother lode

Reader's Digest July/Aug号の記事、"Take Me Out to the Blackberry Patch"、野球少年だった筆者が思い出すのは野球の試合の中身ではなく、試合の途中で見つけたブラックベリーを食べたことでした。
As it turns out, the ball had come to rest in plain sight about ten feet past the fence. Out two missing outfielders had seen it. But they had also discovered a blackberry bramble. It was filled with a mother lode of ripe and apparently delicious blackberries.
...
It's always the blackberry moments that stand out when I stop to think about the wide variety of sports I've had the chance to be part of in the winding course of my career.
"mother lode" は以前取り上げているはずと思ったのですが、記録にはありません。そこで改めて辞書の説明を引用します。
・Oxford English Dictionary: A rich source of something.: your portfolio holds a mother lode of opportunities
・Macmillan Dictionary: supply of something that you can use: His vast experience gives him a mother lode of political stories to tell.
元々は鉱脈を示していた表現ですね。
Reader's Digest is a mother lode of English expressions for me.