President Reagan yesterday sent Congress a policy statement intended to stimulate the domestic mining industry and reduce U.S. dependence on "potentially unstable foreign sources" for minerals essential to the national defense.

The statement, which declares mineral development a key to a strong national defense and economic recovery, reflects Secretary of State Alexander M. Haig Jr.'s warning that the Soviet Union is waging a "resource war" to imperil key U.S. mineral supply lines.

It also reflects Interior Secretary James G. Watt's controversial commitment to open public lands to development, despite stern opposition from across the political spectrum. The report signed yesterday was drafted largely by Reagan's Cabinet council on natural resources, headed by Watt.

To decrease U.S. vulnerability, the policy calls for opening vast areas of protected public lands to mineral development, and for the purchase of $12.5 billion of strategic minerals such as bauxite, chromium, cobalt and tungsten for the national stockpile--enough to sustain the country for three years during a national emergency, as the stockpile act recommends.

"The United States must implement materials and minerals policy programs to ensure that America's capacity to field and sustain fighting forces in the event of war or national emergency is not curtailed by a shortage of critical raw materials," the National Materials and Minerals Program Plan states. The plan to reduce minerals dependence is billed as "the first such presidential commitment in nearly three decades."

The 33-page statement, anxiously awaited for months by the mining industry and conservation groups, was distributed without comment by the White House on a day when official attention was focused on the president's proposal for a summit with Soviet President Leonid I. Brezhnev and on the administration's budget crisis.

It proposed no legislation or appropriations, instead invoking mineral development as a reason for continued pursuit of existing administration policies, including regulatory relief, business tax cuts and elimination of barriers to deep-sea ocean mining. It also called for targeting government-financed research toward projects to increase mineral production.

Despite its lack of specifics, the minerals policy was immediately greeted with strong praise by industry spokesmen and denunciations from conservation groups.

"Overall, it amounts to fabrication of a crisis to justify further attack on the nation's public land heritage and to attempt to justify sweetheart deals for the mineral industry," said Terry Sopher, who studies and lobbies on public lands issues for the Wilderness Society.

"It hits the right areas," said Richard Seibert of the National Association of Manufacturers. "Our nonfuel mineral resources have been treated much like petroleum prior to 1972. If we learn anything we should be wary of growing overly dependent on raw materials from foreign sources."

In addition to calling for increased stockpile purchases, the plan states that the administration will seek congressional approval to sell an estimated $4.92 billion of stockpiled minerals that now exceed three-year levels. This includes tin, the main stockpiled mineral now being sold.

The policy statement directs the Federal Emergency Management Agency to decide which minerals to purchase and in what quantities, timing transactions to avoid disruption of world markets. Past stockpile sales have depressed world prices, and have been opposed by countries whose economies depend on mineral exports.

The proposed $12 billion of stockpile purchases would be deferred until later years when "the budget isn't in such terrible shape," according to Interior Department official Will Dare, who participated in drafting the policy. The purchases would be made on the open market, which likely would mean from foreign sources in many cases, Dare said.

"The policy is a sign of awareness. By God, environmental policy isn't going to be the only concern when it affects minerals," Dare said. "Nobody says we should mine everything, but this is an extremely important industry and we cannot keep preventing mineral exploration."

The report notes that the United States now imports more than half of its total supplies of 20 strategic minerals--a figure that conservation groups contend is a distortion of U.S. vulnerability. Past Defense Department reports have pegged import dependence at more than 90 percent for chromium, cobalt, manganese, vanadium, columbium and other minerals essential to the aerospace and defense industry. Southern Africa and the Soviet Union control most present supplies of these minerals, the reports say.

Reagan last year called for $100 million in stockpile purchases, including $78 million for cobalt, the first major additions to the stockpile in more than 20 years, according to the report. Last November, Reagan directed FEMA to buy 1.6 million tons of Jamaican type metal grade bauxite for the stockpile, according to the policy statement.