The minimum cash deposit for gold futures will rise 21 percent to $11,475 per 100-ounce contract in the speculative Tier 1 category at the close of trading on Sept. 26, Chicago- based CME said in a statement. For silver, the minimum cash deposit was raised to $24,975 from $21,600.

Comex is making it more expensive for speculators to trade after silver plunged the most since October 1979 and gold had its biggest two-day slide in 28 years. Open interest for gold options climbed to a record 1.46 million contracts on Sept. 21, CME said.

“It’s likely that more speculators will exit the market, adding to selling pressure in the near term,” Marshall Berol, a co-portfolio manager of the Encompass Fund in San Francisco, said in a telephone interview.

The CME last raised gold margins on Aug. 24, when prices fell 5.6 percent, the biggest slump since March 28.

Gold futures for December delivery plunged $101.90, or 5.9 percent, to settle at $1,639.80. In two days, the metal dropped 9.3 percent, the most since February 1983. The weekly decline of 9.6 percent also was the most since that year.

The 10-day historical volatility for gold jumped to 42.84 percent, the highest since March 2009, Bloomberg data show.

Silver futures for December delivery fell $6.477, or 18 percent, to $30.101 an ounce on the Comex, the biggest drop since October 1979.