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”.
Since the dramatic emergence of Prince, however, Minneapolis has yeilded
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?