Excerpts from Books and Wikipedia
"Modern Colombia became the center of a third viceroyalty (called New Granada, 1717) partly because of its gold. Eventually, another jurisdiction was created to stop Potosí silver from escaping untaxed through the area of modern Argentina. This became the fourth viceroyalty (the Río de la Plata, 1776), with its capital at the Atlantic port of Buenos Aires. Despite the two new viceroyalties, however, Peru and Mexico remained the core areas of Spanish colonization." [Born in Blood & Fire: A Concise History of Latin America, 4th Ed., p. 58]