1. it wont happen because africa doesnt have the military force necessary to defend itself, see Japan
Milestones: 1830–1860 - Office of the Historian
2. This notion of african resources needs to be put in perspective, a lot of black people get this wrong
a) Those minerals are only worth something now because there are used in technology. The real problem with minerals is that africa doesnt have the infrastructure to make the technologies, until that happens it would be pointless to close off the continent
b) resources is the worst way to make money in the global economy because the global economy is based on knowledge. if you have knowledge to make a cell phone (as an example) you will always be richer than the person selling you the parts. Even if africa raised its praises (for example 100 dollars per pound instead of 1 dollar per pound) the cell phone makers would simply increase the prices to 2000 dollars. (I'm just throwing out numbers, but I hope people get the point). Minerals are a small part of making a cell phone. Right now africa doesnt have the scientists, engineers, trained workers, factories and transportation system to deliver a cell phone to the masses, thats why Africa is poor, its not because cell phone makers are robbing africa.
So to answer the question, closing africa would leave Africa behind because Africa doesnt have the infrastructure to provide a modern technologies to itself
3. if countries like the US and China closed themselves off they would suffer an economic collapse, im not seeing what would africa gain from closing itself of. It would immediately make all its resources worthless since they are valuable only in terms of the global economy. the pennies it makes from its resources would be gone and there would be nothing else