Share prices of gold-producing companies have lagged the huge run-up in the price of gold, as well as the broad stock market, for years. Investors, instead, have poured billions of dollars into exchange-traded funds backed by actual bullion, believing that would provide purer exposure to gold prices and avoid unrelated stock-market risk.

This year, the biggest gold ETF, SPDR Gold Shares, is up 7.6%, easily outperforming the 12% fall in the Dow Jones US Gold Mining Index. Over five years, the disparity is even starker. The ETF has more than doubled, while gold-mining shares are up a paltry 12%.