I think that we don't get enough translation in this direction. Anyone want to have a go at this? 'The mother lode' I use it idiomatically to mean: the thing/ or means producing the greatest return for the effort put in. It is however, not an idiom in British English but simply the main vein of an ore, I can look that up and will later, but it would be good if anyone knows if there is a Thai idiomatic equivalent; and if there aint one, then perhaps we can just tranlate it and there will be.

Try these from Lexitron as a start:

ขุมสมบัติ [N] treasure trove; cache; financial resources; trove

Syn. ขุมทรัพย์, กรุสมบัติ

Sample:"ยังมีคนไทยกลุ่มเล็กๆ ที่ยังเชื่อตามคำบอกเล่าเรื่องลายแทงขุมสมบัติ"

This is a small group of Thais who still believe in a story of clues to a treasure trove.

ขุมทรัพย์ [N] treasure

Class. ขุม

Syn. ขุมสมบัติ, กรุสมบัติ

Def. "แหล่งที่มีทรัพย์สินมากมายหรือเป็นที่รวมทรัพย์สิน."

"A place where there is great treasure or where there is a storage place for wealth.

Sample:"ในนิทาน คนดีมักจะได้พบขุมทรัพย์โดยบังเอิญ"

"(Only) in fables will the worthy will find great wealth accidentally."

In a word: ลาภ

(Or maybe that was an excuse to post a favourite Thai proverb: โลภมากลาภหาย)