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People in the south used slaves because their economy was focussed on things like cash crops (tobacco, cotton, etc.) These crops required a lot of work so farmers and plantation owners could not do it by themselves. So by using enslave persons they could harvest more crops, sell more, make more money, and in return boost the southern economy.
Southern blacks lost rights in the years after the thirteenth fourteenth and fifteenth amendments because the North withdrew many of the troops that had been enforcing these new laws, leaving southerners to pass a series of Jim Crow laws that prohibited blacks from voting, gaining jobs, and a variety of other things in the South.
Answer: gold, copper, and iron ore.