One of the reasons it is significant in the book Cry, the Beloved Country that Johannesburg's growth is attributed to the gold minds, is because it underlines the basic racial discrimination found throughout the book.
It is the black, native Africans who work in the mines. It is difficult and dangerous work, however they do not yield any of the profit. Their wages are extremely low, as evidenced by the extreme poverty of the blacks living in Johannesburg. The whites, however, do receive the benefits from the gold mines. These mines really are one of the big indicators of the racial disparity in Johannesburg, and all of South Africa during this time.