Motherlode was an inhabited planet of the Arcadian system.
In 2269, Motherlode didn't recognize Federation law. In that same year, Harry Mudd attempted to exploit the heavy metal miners on the planet by selling them a fake love potion. (TAS: "Mudd's Passion")
In the novelization of "Mudd's Passion" (as published in Star Trek Log 3), "Motherlode" was said to be the unofficial name for Arcadia III – the third planet in the Arcadia system.