remember that when the spanish colonized the new world, they focused on finding gold (seven cities of gold in the southwest, etc.) when gold was located, the most efficient means of refining it from the gold containing ore was a method called amalgamation. it consisted of combining the ore with mercury which would leach the gold out of the matrix of the ore by combining and semi-liquifying the gold. then the mercury could be separated from the gold in a second step. the spanish sent ships heavily laden with mercury which was mined in portugal, and got back ships heavily laden with gold. lead is another "heavy metal" which will amalgamate with mercury.
