1. a rich or abundant source of something valuable or desired
Usage: Originally a mining term; now used figuratively in everyday speech
2. the main vein of ore in a mine
Usage: technical; mining

Examples

• The library's archive turned out to be a motherlode of historical documents.
• Finding that old record collection was a motherlode for the music historian.
• The miners finally struck the motherlode after months of digging.
• Social media has become a motherlode of personal data for advertisers.
• That vintage store is a motherlode of unique clothing and accessories.
• The discovery of the fossil site was a motherlode for paleontologists.