Gold posted the longest run of weekly gains in almost two years and silver surged to the highest since 2014 as Britain’s vote to quit the European Union fueled speculation that central banks will boost economic stimulus.

Havens such as gold and silver are in demand on prospects for weaker economies. Treasury yields fell to record lows along with sovereign rates from Spain to Japan as policy makers signaled their readiness to shore up economies. Governor Mark Carney said Thursday that the Bank of England could cut interest rates within months.

Gold and silver had the biggest first half gains in about four decades amid mounting speculation that interest rates in the U.S. will remain low, which is a boon to precious metals because they don’t offer interest. Expectations for U.S. rate increases have been wound back since the Brexit vote, while investors continue to pile into exchange-traded funds backed by gold.