Just about everything in your question is wrong. They were not looing for gold, they were looking for a route to the isles of Spices that would bypass both the Silk Road and the Portuguese maritime routes. Africa was an entirely portuguese matter, first in the Treaty of Alcaçovas of 1479, which I proceed to quote:

They shall not disturb, bother, or discomfort, neither de facto nor de iure, consciously or unconsciously, those lords King and Prince of Portugal, nor any Kings of Portugal for the time being, or their kingdoms, the possession or quasi-possession that they had, on any deals, lands, or commercial agreements of Guinea, with its gold mines or any islands whatsoever discovered or to be discovered, isles of Madeira, Porto Santo, Deserto, and all the islands of Azores, and the islands of Flores near Cape Verde, and all the lands that are now discovered, and any whatsoever islands to be discovered or conquered, from the Islands of Canary southwards against Guinea, because everything that has been discovered or may be discovered, conquered or discovered shall be from those terms onwards be for the Kings and Princes of Portugal and their kingdoms, excepting the islands of Canary, those being Lanzarote, La Palma, Fuerteventura, La Gomera, el Hierro, la Graciosa, la Gran Canaria, Tenerife and all the other islands of Canary won or to be won, that are left for the kingdoms of Castile and León

This division changed with the Treaty of Tordesillas in 1494, where the line of demarcation went from being a parallel to being a meridian. This treaty was absolutely necessary, as the lands discovered by Columbus were not under the scope of the Alcaçovas treaty. They were indeed south of the Canary Islands parallel, but they were not "against Guinea". So, even if Spain had wanted to explore and conquer lands in Africa, they could not, as there were treaties barring that. Here is what the treaty of Tordesillas said:

That, whereas a certain controversy exists between the said lords, their constituents, as to what lands, of all those discovered in the ocean sea up to the present day, the date of this treaty, pertain to each one of the said parts respectively; therefore, for the sake of peace and concord, and for the preservation of the relationship and love of the said King of Portugal for the said King and Queen of Castile, Aragon, etc., it being the pleasure of their Highnesses, they, their said representatives, acting in their name and by virtue of their powers herein described, covenanted and agreed that a boundary or straight line be determined and drawn north and south, from pole to pole, on the said ocean sea, from the Arctic to the Antarctic pole. This boundary or line shall be drawn straight, as aforesaid, at a distance of three hundred and seventy leagues west of the Cape Verde Islands, being calculated by degrees, or by any other manner as may be considered the best and readiest, provided the distance shall be no greater than abovesaid. And all lands, both islands and mainlands, found and discovered already, or to be found and discovered hereafter, by the said King of Portugal and by his vessels on this side of the said line and bound determined as above, toward the east, in either north or south latitude, on the eastern side of the said bound provided the said bound is not crossed, shall belong to, and remain in the possession of, and pertain forever to, the said King of Portugal and his successors. And all other lands, both islands and mainlands, found or to be found hereafter, discovered or to be discovered hereafter, which have been discovered or shall be discovered by the said King and Queen of Castile, Aragon, etc., and by their vessels, on the western side of the said bound, determined as above, after having passed the said bound toward the west, in either its north or south latitude, shall belong to, and remain in the possession of, and pertain forever to, the said King and Queen of Castile, Leon, etc., and to their successors.