"Cousin Jack" by Steve Knightley of Show of Hands isn't a traditional song, but I've a feeling it mentions South Africa as one of the places emigrant Cornish miners went.

Even if it doesn't, it was one of the places they went - a Google search threw up a locally published book, Cornish Pioneers in South Africa, by Richard Dawe, published in 1998 by Cornish Hillside: "The story of the Cornish men and women who went to South Africa in the nineteenth century and played a pioneering role in the gold and diamond mines of Kimberley and the Rand (Mentioned on this bookshop page.) So there could perhaps be some Cornish songs, back in Cornwall, or somewhere out in South Africa.