Did Spain conquered the Incas and the Aztecs for their gold and silver?

Yes, Spain wanted gold and silver, they also had heard that the Inca and the Aztec owned many of these desired riche, so they set off to establish a trade with them, only to discover the many inhumane practices both the Aztec and Incas had (cannibalism, human sacrifices, and other cruelties). After this discovery, the Spanish decided, the only way to eliminate these gory practices was to conquer those responsible, the Aztec's and Incas.