A proposal to build a branch of George Mason University in Leesburg, which would be the fourth campus of the fast-growing and ambitious state-supported Northern Virginia school, is under discussion by town and university officials.

Each side said yesterday that although the talks are very preliminary, all parties involved are interested in making a proposal to the General Assembly, with a possible opening in 1990.

But the idea is likely to face hard questions from the state Council for Higher Education, which advises the governor and the legislature. The council's director, Gordon K. Davies, recommended a new state university in Northern Virginia last week and specifically rejected the idea of branch campuses as inferior.

Council spokesman Barry Dorsey said yesterday that council officials could not comment on the Leesburg idea because they had not seen a formal proposal.

University Senior Vice President J. Wade Gilley said yesterday there is no reason a George Mason satellite campus could not become that new university. He said there is a long tradition in Virginia of satellite campuses that grow into major universities -- among them George Mason itself, which began 30 years ago as a branch of the University of Virginia.

For George Mason University, a Leesburg campus would become part of an expanding educational empire that includes a main campus in Fairfax, a law school in Arlington and a proposed branch in Manassas. The main campus in Fairfax is the fastest-growing of the state universities in Virginia.

For rapidly developing Leesburg, 35 miles northwest of Washington, a university campus would be a bragging point as it transforms itself, through economic development, from a historic village into a high-priced suburb. "It's something else we could sit back and show," said council member Brian Kelley.

Kelley said that he has spoken with the owners of several large tracts near the site of a proposed extension of the Dulles Toll Road east of town and that "there's potential" for donations of the 35 to 40 acres needed for a campus. George Mason officials have told town officials that they could not afford to buy the land.

Gilley said a Leesburg campus probably would begin like the proposed Manassas branch, with a few graduate and undergraduate programs in engineering, business and education -- subjects with particular appeal to older students.

Among those who have been in on the discussions are Leesburg's mayor, vice mayor, town manager, two council members and the town's planning chief. They had lunch last week with University President George Johnson. Gilley said the local General Assembly delegation has been briefed but told that the proposal is preliminary.

The idea got launched about a year ago, although Kelley and Gilley disagreed on who approached whom. Gilley said GMU was contacted by an engineering firm and later by town officials. Kelley said the university went to Leesburg.

George Mason has 11,655 full-time students and is predicted to peak at 20,000 by the early part of next century.

Its proposed Manassas campus, to be on the grounds of Northern Virginia Community College, also must be approved by the legislature.