@Kaetchi I am not very knowledgeable about the Congo Copper Belt. Congo has very old copper-rich rocks (the Katanga Supergroup, 700-800 million year-old sediments). Malachite forms when underground water rich in CO2 reacts with existing copper minerals. Congo has not been glaciated for a long time, so that shallow-formed malachite is still preserved.
Others who know more may pipe up or correct what I've written.
