A federal grand jury is investigating a convicted blackmailer's allegation that Texas millionaire Nelson Bunker Hunt made it possible for the wife of Rep. George Hansen (R-Idaho) to realize a quick $87,000 profit from a 1979 silver transaction.

The allegations were first made by Arthur G. Emens III, 35, of Winter Park, Fla., in a blackmail letter to Hunt in March, 1981, in which he said he would go to the FBI with charges that Hunt had bribed Hansen unless Hunt gave him a $440,000 loan, which Emens said he would repay in 120 days. Hunt's lawyer and Hansen turned the letter over to the Justice Department.

Emens pleaded guilty in U.S. District Court here to a misdemeanor blackmail charge and was sentenced Monday to three years' probation. He is now cooperating with the grand jury investigation.

In his letter to Hunt, Emens accused Hunt of making the payoff to Hansen to get the congressman's support for a bid on an Idaho silver mine. But in the letter, Emens also characterized his own allegation about the reason for the alleged payoff as "guessing" and further admitted that other information he had was based on "only secondhand proof."

Hunt's lawyer, Fred M. Vinson Jr., said yesterday that Hunt "explicitly and emphatically denies any wrongdoing whatsoever."

Hansen said that a story in yesterday's Wall Street Journal about Emens' allegations and the Hansens' finances was "a slanted rehash of old garbage which appears in the press each election year."

Hansen said in the statement that he and his wife had voluntarily provided federal prosecutors with "all our records to show that this blackmail was based on a lie, a fact known to the Justice Department and conceded by the blackmailer himself."

In the blackmail letter, now part of the federal court record, Emens alleged that on Jan. 16, 1979, Hunt purchased 125 silver contracts through Ming Commodity Services, Hunt's private broker in Oklahoma City, valued at the time at almost $4 million. Emens alleged that Hunt subsequently placed the trade in an account in the name of Hansen's wife, Connie, paying the initial $125,000 down payment on the contracts for her.

Emens alleged that the contracts were sold two days later, making a profit of $87,475 for Mrs. Hansen. The Hansens have reportedly told prosecutors that she bought the contracts on a tip.

According to court records, Emens acquired knowledge of the silver transaction while employed at Ming as an account executive in the spring of 1981 and, at the time of the 1979 silver trade, as an assistant office manager at Cargill Investor Services working on the New York City commodities exchange.

In the blackmail letter, which was unsigned, Emens directed Hunt to wire a $440,000 "loan" to a bank account in the Cayman Islands under the name "Gordon Warner." Otherwise, Emens said in the letter, "we shall contact the FBI . . . , " which he said was "the last thing in this world we wish to do."

Emens later sent a second letter to Hunt, signed "Gordon Warner," in which he said that "due to a change in plans we no longer need these funds" and apologized for "any problems" the situation may have caused Hunt, according to court records.

Emens' lawyer said in court that Emens, who has no prior criminal record, received no money as a result of the scheme. U.S. Magistrate Jean F. Dwyer gave Emens a suspended sentence of one year in jail and a $2,000 fine, placed him on three years' probation and ordered him to complete 75 hours of free community service.