motherlodenoun [ C usually singular ]
also mother lode /ˈmʌð.ə.ləʊd//-ɚ.loʊd/specialized geology a place that has very large supplies of a mineral or other natural resource:
The Pacific Northwest has been the motherlode of the nation's aluminum output for 60 years.
Cairn Energy recently struck the motherlode of oil fields in India.
something that supplies a very large amount of a thing, quality, etc. that you want:
The flight data recorder had documented about 200 pieces of information, a motherlode for crash investigators.
With this market, vintage clothes shoppers have hit the motherlode.