Mother lode is the standard spelling of the phrase meaning an abundant or rich source. Lode here means a vein of mineral ore, and mother is a figurative adjective meaning giving abundantly. Mother lode was originally an American mining term, referring to the main vein of ore in an area, but it’s now usually used figuratively for any abundant source of something. Mother load, a common misspelling, sort of makes sense, but it’s not the conventional term, and many readers will view it as incorrect.
Mother lode is usually two unhyphenated words, although a few publications spell it motherlode. Both are more common than the hyphenated mother-lode.
He hit the mother lode, but not once did Josh Ferrin even think of laying claim on the more than $45,000 cash that he found in his garage. [Deseret News]
It is the newest find in a mother lode of prehistoric bones uncovered during a reservoir expansion project last fall. [Real Vail]
Mining coal is a dirty business: dirty enough for a mother lode of filth in the latest corruption case to touch India’s Congress-led government. [The Economist]
But there’s a mother lode of original Mother’s Day gifts to be had, many at local places. [Seattle Times]