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It is impossible that the African admixture in Portugal come from the slave trade because the biggest part of the African admixture is Northwest African (=Maghreban) then East African (Ethiopia, Somalia, etc.). Only a tiny fraction is Neo-African or Paleo-African.
Another reason it is impossible is the timeframe. The African admixture is present in virtually every individual from Western Iberia, and the vast majority of all Iberians. It means that this is a quite ancient admixture, not one from the Renaissance, and probably not from the medieval Moorish period either.
That's why I think we have to look at a much older migration, any time between 20,000 and 4,000 years ago.