The United Mine Workers union and the Bituminous Coal Operators Association reached tentative agreement on a new contract yesterday, averting a threatened strike in the nation's coalfields.

Details of the new agreement were not immediately available, but UMW President Richard Trumka said the current contract has been extended until the membership has a chance to vote on the new contract. It is believed to be the first time the UMW has extended a national contract.

Trumka said all UMW members, including those who work for independent coal companies not covered by the agreement, should report to work Monday. The old contract had been set to expire at 12:01 a.m. Monday.

Trumka said the new contract terms "significantly expanded job opportunities and enhanced economic opportunity" for his members. He would not say how. He said the new agreement did not involve any "backward steps and no takeaways."

Under the union ratification procedures, members of the UMW international executive board and district presidents will come here Tuesday for a briefing on the contract. They will return to the coalfields, where district meetings will be conducted to explain the agreement. The ratification vote is scheduled for Feb. 8.

The contract covers 60,000 miners in more than a dozen states, primarily in the East and South. The biggest issue for the union was job security. The UMW has lost nearly a third of its membership since the last contract was signed in 1984.

The coal operators' association, which once counted more than 100 members, now represents 14 major coal companies. More than two dozen companies have quit the association since the last contract was signed, but most have signed what are called "me too" contracts in which they agree to accept the terms of any eventual association agreement. In exchange, the UMW agrees not to strike them.

Bituminous Coal Operators Association officials would not comment yesterday on the tentative accord. In a brief statement, the association acknowledged agreement had been reached and noted that both sides have to take it back to their membership for ratification. Announcement of the settlement was made by Trumka. There were no members of the management bargaining team present.

Although he would not give details of the agreement, Trumka has been seeking a job security pledge from coal operators' association members similar to one he signed last February with the Island Creek Coal Corp. At the time, Trumka called it a model for the industry. The Island Creek agreement, known as the Employment and Economic Security Pact, does not guarantee miners a job when a mine is shut down. But it guarantees they will have the first option on any new jobs at other company sites.

Management negotiators insisted from the start of negotiations that they must trim at least $1 from the hourly compensation paid to UMW members. The average UMW member earns about $30 an hour in wages and fringe benefits.

But UMW and industry negotiators say it appeared the industry might be able to trim needed costs from the contract by reducing prospective contributions to at least one of the union's four retirement and health and welfare funds without cutting existing compensation.