Scientific American provides a detailed, hypothetical response to the question which has no doubt crossed the mind of anyone involved with the mining sector – how long before the Earth’s bounty of resources runs out?
See http://www.mining.com/how-long-before-the-earths-bounty-runs-out-a-future-history-61042/?utm_source=digest-en-mining-120905&utm_medium=email&utm_campaign=digest.