The Spanish enforced the Encomienda System on the native peoples, which was basically slavery— they were forced to work long hours to search for gold, and often died from harsh working conditions or torture
Because almost 90% of the native populations were killed, Spain began to send enslaved people from Africa to work in the colonies
This led to the beginning of the Transatlantic Slave Trade (Triangle Trade), in which Europe would sell textiles, brandy, and weapons to Africans in return for slaves who would be sent to the Spanish colonies to produce sugar, coffee, and tobacco for the Europeans
Two opposing viewpoints on the matter: Bartolome de Las Casas believed that the harsh treatment of the native peoples needed to be stopped (was against the orders of both the monarchs and God), while Juan Ginés de Sepúlveda thought that it should continue (the natives were an inferior people and deserved to be treated as such)