Motherlode was an inhabited planet. It was located in 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 an illegal love potion. (TAS: "Mudd's Passion")
Presumably, Motherlode was located in the Alpha Quadrant.
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.