Interior Secretary James G. Watt agreed yesterday to halt his most sweeping deregulation drive--the proposed relaxation of dozens of strip-mining rules--until his department prepares a detailed study of how such changes would affect the environment.

The agreement settled a lawsuit challenging Watt's strip-mining program, which he cites as the centerpiece of his effort to reduce "excessive, burdensome and counterproductive" federal regulations.

The suit, filed by the National Wildlife Federation, alleged that Watt was effectively undoing portions of the 1977 federal strip-mine reclamation law through 46 regulatory changes.

Watt did not admit or deny the charge in yesterday's settlement, and refused to comment on it. The agreement will delay at least until October the sweeping revision of strip-mining controls that Watt at first planned to complete last summer. The deadline was later moved to last January, and then to mid-summer this year.

Greeted coolly by the coal industry, the settlement was termed a "major victory" by environmentalists.

In it, Watt agreed to prepare a detailed environmental impact statement on proposals to relax 33 strip-mining control regulations, including rules designed to protect water supplies, prevent landslides and ensure reclamation of mine sites. Those rules, scheduled to take effect this summer, will be held up until the impact statement is complete, the settlement says.

Interior Department officials said the delay should not last beyond October, but environmentalists predicted it could take more than a year. Citizen groups could also use the environmental impact statement as a basis for taking the department back to court in hopes of blocking certain regulatory changes, wildlife federation attorney Norman Dean said.

The lawsuit characterized the 46 regulations as one "major federal action," which under the National Environmental Policy Act could not proceed without an environmental impact statement. The Interior Department had conducted limited environmental studies on certain proposed regulations, but not a full impact statement.

Under the settlement, the department will have to hold two public hearings to determine the scope of the environmental study. It must then solicit public comment for 60 days and revise the study in final form based on those comments.

The settlement also allows the department to proceed with 23 of its proposed rule changes, including regulations that would relax restrictions on bonding of mine operations and mining on federal lands. But the Interior Department must complete more limited environmental studies on those rules as well, the settlement says.

Yesterday's agreement did not affect the rule change that both industry and environmental groups consider the most significant: one that allows vastly increased flexibility for states to design their own strip-mining rules.

The original rule required state regulations to be "as stringent as" federal rules; the new rule says they must be only "as effective," releasing state officials from following detailed design criteria set out in many federal regulations. That change is being challenged in a separate lawsuit by the Sierra Club.

It also does not affect major features of Watt's strip-mining program not associated with rule writing, such as staff cuts and the reorganization of the Office of Surface Mining, changes hailed by industry as "reasonable" but attacked by environmentalists as a "gutting" of the federal reclamation law.

The feature of the settlement that most angered the coal industry was the Interior Department's guarantee that citizens can accompany state strip-mining regulators on mine inspections. The department deleted that provision in one of the rules proposed during the past year, but agreed yesterday to reinstate it.

For that reason, two industry groups that had intervened in the lawsuit did not join in yesterday's settlement, according to National Coal Association president Carl Bagge. NCA and the American Mining Congress gave qualified support to the settlement, however, in a joint statement that said: "The coal industry believes the regulatory reform process should go forward expeditiously and therefore generally supports . . . portions of this settlement."

Privately, industry lobbyists said they were upset by the delay that the environmental review is likely to bring to the deregulation drive. The industry considers present strip-mining regulations overly burdensome and expensive, Bagge said, and those rules will remain in effect until Watt's proposals can proceed.

Watt targeted strip mining early in his tenure as the most overregulated area in his department, and called for a speedy shift of power to the states. The 1977 strip-mining control law provided for a transfer of regulatory authority to the states, but only after state laws and programs were brought up to federal standards.

The premise behind the law was that it was impossible for certain states, particularly those where the coal industry was most powerful, to force adequate reclamation of strip-mined lands. Uniform federal standards were considered the solution.

Watt has said his strip-mining program will prove as effective as the original one, but will reduce costs and burdens for states and the industry. Environmental groups have charged, however, that he is giving states too much flexibility too fast, undermining the 1977 law. They hailed yesterday's agreement in part because it slows the momentum behind those changes.

"This is a major first step," said Ed Grandis of the Citizens Mining Project, which monitors enforcement of the 1977 law. "We have managed to bring some order to a process that seemed to be part of an irrational crusade."