Of the additional 80% gold I'd guess about half represents profit for gold miners or tax or mining royalties. So 40% of the gold supply has come at zero marginal cost.
Also keep in mind that a technological breakthrough could produce a sudden glut in gold supply, as we have seen recently with oil and gas fracking.
The main reason the price has gone up is that the world population is in aggregate far richer than in 1980. What's more, Indians and Chinese (who are culturally inclined to invest in gold) have seen their wealth grow particularly rapidly, so the proportion of world assets stored in gold has likely increased.
Also keep in mind that a technological breakthrough could produce a sudden glut in gold supply, as we have seen recently with oil and gas fracking.
The main reason the price has gone up is that the world population is in aggregate far richer than in 1980. What's more, Indians and Chinese (who are culturally inclined to invest in gold) have seen their wealth grow particularly rapidly, so the proportion of world assets stored in gold has likely increased.