The expression “Finding the motherlode” is usually used figuratively for the discovery of an abundance of almost anything.
The phrase comes from the mining camps of the late nineteenth century.
A lode is a mining term for a vein of metal ore, the discovery of which would be exciting enough, but add “mother” and you’ve come across the origin of all the veins in the region.
The motherlode is, literally, an abundant source of supply or the discovery of riches.
