A U.S. appeals court said yesterday it had found strong evidence that Fairfax County had discriminated against women and black in its employment practices and directed a lower court to reopen a Justice Department lawsuit on the issue.

In a strongly worded opinion, the 4th U.S. Circuit Court of Appeals overturned a District Court order substantially clearing the county of discrimination charges and suggested that the suburban government should, in fact, be barred by a court order from any future discrimination.

County officials reacted angrily to the appeals court decision, announced in Richmond, and some promised to take the issue to the Supreme Court. "I think we're being descriminated against," snapped Fairfax Board Chairman John F. Herrity, one of the defendants in the lawsuit.

"This is one of those statistical federal government mumbo-jumbo suits," he said yesterday. "I guess we have to spend more of the taxpayers' money, putting up with federal meddling in Fairfax County affairs."

The appeals court yesterday directed U.S. District Judge Albert V. Bryan Jr. to reconsider claims that the county government had violated federal civil rights laws by clustering women and blacks in low-paying, undesirable jobs.

Last year, in a decision that was hailed by Fairfax officials, Bryan had sided with the county, rejecting most of the discrimination charges and praising the county's affirmative-action program as "impressive and persuasive." He did find that the county had discriminated against a relatively small number of women in the service and maintenance job categories. r

In the 27-page appeals court opinion, however, Judge Harrison L. Winter systematically rejected Bryan's reason. Winter found:

That Bryan failed to consider employment figures that include many blacks and women who live in the Washington metropolitan area, but outside the county.

That the judge failed to order Fairfax to comply with federal record-keeping requirements concerning job applications. The county "stored applicant records in a shoe box and routinely destroyed them," the appeals court said.

That the county used invalid tests to determine the qualifications of black applicants. "The (federal) government proved a prima facie case of disparate treatment of blacks in the county's use of hiring and promotion tests" employed for certain jobs, Winter wrote.

The three-judge appeals panel also directed that back pay or retroactive seniority, or both, should be granted blacks and women "if proof is offered of identifiable economic injury" and can be proved by Justice Department lawyers.

In his decision last year, Bryan had said that prior to the county's affirmative action program, narrowly adopted in 1978, there had been "purposeful discrimination" against blacks in hiring of police officers, firefighters and service and maintenance workers.

The appeals court said yesterday that because of Bryan's finding of past discrimination, he should have issued an injunction barring future discrimination. "It is commendable that the county is continuing its affirmative action programs, although there was some evidence that the goals may soon be released," the appeals court said.

One of the key issues in the dispute has been the size of the work force from which Fairfax County has recruited its 5,000 public employes. Although the Justice Department said that the Washington area's labor force was 24 percent black and 40 percent female in 1970, the county contended that it had to draw its workers from a much smaller labor pool that included fewer blacks and women.

At the time the lawsuit was filed, the Justice Department said that only 7.5 percent of the county's workers were blacks and only 26 percent were women. Fairfax officials had argued that their methods of calculating the number of available workers were accurate and Bryan had agreed. But the appeals court noted that Fairfax County relied on 1970 census data that was "suspect and probably an inaccurate reflection of actual conditions in 1974-78" -- years that were under contention in the lawsuit.

"In addition," the appeal court noted, Fairfax had included" . . . areas of distant Fauquier and Rappanhannock counties" in its labor force, an action the appeals court found questionable.

During the trial the Justice Department produced data showing that 59 percent of Fairfax's female employes were in clerical jobs as compared with less than 2 percent of male employes. Of blacks hired from 1974-78, 31 percent were assigned to public service and custodial jobs as compared to less than 5 percent of whites, the department said.

In 1978 before the Justice Department filed suit, assistant Attorney General for Civil Rights Drew Days said in a letter that Fairfax's "already small black work force" actually had decreased.

The board angrily rejected a federal request that the county adopt a five point plan of corrective measures and the Justice Department subsequently filed suit in Federal District Court in Alexandria.

At that time Days acknowledged that the suit was not based on complaints by individual employes. Instead, officials said the Justice Department annually examines employment practices of about seven communities picked at random.

"We're close to Washington and one of the biggest jurisdictions so I guess the Carter administration thinks it can get some mileage out of this," said Republican board chairman Herrity yesterday. "I guess that's partly why they did not go after Arlington or some other county."