(Bloomberg) -- Silver vaulted above $21 for the first time in two years and gold advanced for a fourth day on speculation of more central bank stimulus in the wake of the U.K.'s vote to leave the European Union. Mining shares surged.

Spot silver jumped as much as 7% to $21.1377 an ounce and settled at $20.3246 in New York on Monday, according to Bloomberg generic pricing. Spot gold rose as much as 1.2% to $1,357.63 an ounce, near a two-year high, and settled at $1,350.79.

Bullion has benefited as turbulence in financial markets after the Brexit vote added to speculation that central banks will step up stimulus, with interest rates in the U.S. set to remain low. Holdings in silver-backed ETFs expanded to a record last month, and assets in gold ETFs are now at the highest since August 2013.

Investors are buying more silver than gold. Daniel Marburger, a CoinInvest.com director, said that 90% of the company's orders in the past three days had been for silver, compared with more typical demand of 70% to 80% for gold.

"The U.K. demand for Silver Britannia coins is very strong," he said by e-mail. "Three times more than average."

Gold bought as few as 64.2 ounces of silver on Monday, the least since August 2014, after purchasing as much as 83.8 ounces in February, the most since the global financial crisis in 2008. Silver has jumped 47% this year, outpacing gold's 27% advance. Funds have boosted their bullish positions in the two metals to the highest since the data began in 2006.

"The outperformance was due for a long time and it's recently accelerated," said Georgette Boele, an analyst at ABN Amro Bank NV. "Uncertainty surrounding Brexit is providing safe-haven demand. At the same time, markets are pricing out Fed rate hikes, which is also supportive for gold and silver."

Boele said the long-term trend in the gold-silver ratio has broken to the downside and that silver could continue to outperform gold in the current market environment. "Moves in silver are often more substantial, both up and down," she said.

Global gold holdings in ETFs have expanded by more than 500 metric tons to 1,959.1 tons since bottoming in January, while silver assets climbed to a record 20,232.1 tons in June, according to data compiled by Bloomberg.

Fresnillo Plc, the world's biggest primary-silver producer, rose 7.7% in London. It's up 168% this year, the best performer in the U.K.'s benchmark FTSE 100 Index. Randgold Resources Ltd. gained 4.4% to close at a record. In Toronto, Kinross Gold Corp., Eldorado Gold Corp. and Yamana Gold Inc. all rose more than 6%.

Still, not all investors are convinced by the rally in precious metals.

"I regard gold and silver as both part of the commodities complex," James Bevan, chief investment officer at CCLA Investment Management, said in a Bloomberg TV interview. "I worry that commodities generally are now relatively overpriced in the context of slowing industrial activity on a global basis. The floor for these metals is really very low."

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