I am a bit unsure about how the word "motherload" should be seen – as a common mistake or slang word or both.
I've encountered the word the first time in a recent Guardian article, but to my surprise it was later changed to "mother lode":
This article was amended on 17 May 2017. An earlier version spelled
“mother lode” as “motherload”.
After having seen something like that in a book
Since the dramatic emergence of Prince, however, Minneapolis has yeilded
(sic !)
a rich motherload of new soul music talent.
I am not sure if many uses of "motherload" are actually because of ignorance. The word often appears in a context, even scientific one, which does not really "allow" the usage of slang. The only online dictionary that lists "motherload" seems to be Wiktionary. So I wonder if the intentional use of "motherload" as a slang word is actually rare?
Best Answer
"Motherload" is a non standard spellings of the more common "motherlode".
As suggested by the following source load and lode were originally the same term. Through the centuries they developed different connotations and lode was more commonly used from the 17th century to refer to a rich vein of mineral ore, from which the more common spelling "motherlode" from the 1920s especially in a figurative sense:
.......
(www.word-detective.com)