Last spring something odd happened to a host of small US gold miners. Their stocks declined unexpectedly, despite the price of the lustrous metal hitting a five-month high at the time.

The dips were not caused by any of the usual reasons such as poor earnings statements, or signs of a waning appetite for gold among New York socialites, Indian brides or Chinese hoarders. Rather, they fell because of the swelling importance of exchange traded funds.

These odd ructions in the stock price of a few gold miners is an under-appreciated but potent example of how ETFs are having a mounting and sometimes unintended impact on markets, even as they save investors around the world billions of dollars worth of fees that would otherwise go to traditional fund managers.