The Bache Group Inc. revealed yesterday that it put up silver as collateral for millions of dollars of bank loans and now must come up with additional collateral because of the fall of silver prices.

Bache said its bankers are not demanding immediate payment and have given it more time to make good on the loans.

Bache's borrowings add to the hundreds of millions of dollars to debts that must be repaid after silver prices plunged from $50 an ounce in January to $10.80 last week.

Bache's silver transactions and its dealings with Nelson Bunker Hunt and his brother Herbert are being investigated by the Securities and Exchange Commission, the Commodity Futures Trading Commission and a congressional committee.

SEC lawyers are looking into possible conflicts of interest because the Hunts owned more than 5 percent of Bache and also apparently were one of its biggest customers. For a time last week it was feared the Hunts' losses of silver speculation might bankrupt Bache.

The Securities and Exchange Commission yesterday agreed to let trading in Bache stock resume today, several days earlier than anticipated. Last Thursday the SEC stopped trading of the stock for 10 days because of the collapse of silver prices.

The SEC said it was letting the share's trade again because Bache has disclosed to investers the extent of its possible losses on silver dealings with the Hunt brothers of Texas.

In another development in silver yesterday, The Washington Post learned that Nelson Bunker Hunt made a secret trip from Paris to a Boca Raton, Fla., bankers convention last weekend.

Hunt flew to Florida for a meeting of the Association of Reserve City Banks, a group of big banks from the major cities that have Federal Reserve banks.

Federal Reserve Board Chairman Paul Volcker and Treasury Secretary G. William Miller attended the Boca Raton session. Press aides to both Miller and Volcker said yesterday that neither of them had met with Hunt.

It had been reported in Paris over the weekend that Hunt and his brother Herbert were desperately trying to raise additional cash to finance their crumbling silver empire, and were seeking help from the Middle Eastern investors.

Neither of the billionaire brothers had appeared in public since Thursday's dramatic drop in silver prices.

A family representative in Texas yesterday said the Hunts are far from bankrupt and intend to repay all the money they have lost speculating in silver.

The brothers have "a short-term liquidity crunch," the spokesman told United Press International, but they have never reneged on "any lawful and binding olbigation and they have no intention of doing so now."

On paper the brothers have lost billions of dollars since the price of silver plummeted. On Monday they were forced to give $400 million worth of canadian oil and natural gas wells and 8.5 million ounces of silver to Englehard Minerals and Chemicals to settle a silver debt.

After recovering on Friday and Monday, silver prices fell again yesterday by 24.7 cents an ounce to $14.

The Hunts did much of their silver business with Bache Group's Bache Halsey Stuart Shields brokerage subsidiary and also bought silver from the Bache metals subsidiaries in New York and London.

Until this week it had been reported that most of the Hunts' losses had occured in the silver futures market, where contracts for delivery of the metal at some future date are bought and sold.

But the Hunts' dealings with Englehard were for millions of ounces of silver bullion and they also bought undisclosed quantities of the metal from the Bache subsidiaries.

Bache officials said Bache, Halsey Stuart Metal Co. of New York loaned money to the Hunts to buy silver bullion and let the Hunts put up silver as collateral.

Instead of keeping the Hunts' silver, however, Bache used the same silver as collateral to borrow more money from banks, the company disclosed for the first time.

When the price of silver collapsed, not only did the Hunts have to come up with additional collateral to satisfy Bache, Bache also found itself short of collateral for its own loans.