From Melissa Pistilli comes a brief sketch of what makes for a world-class deposit and a list of several of them. The largest gold mine in the world is the Grasberg in Paupa, Indonesia. That mine opened twenty years ago and has 38.5 million ounces of proven and probable gold reserves remaining.
World-class deposits tend to be found in greenstone, which contains gold-copper porphyries. The most easily extractable deposits have oxide skins, which make heap leaching easier. One statistic of note: good-quality underground mines have grades of 8-10 g/t gold. Good-quality open-pit mines tend to have 1-4 g/t. It's possible to mine a less concentrated deposit, but the economics become iffier.