The U.N. Law of the Sea Conference ended its latest session yesterday with mixed results and little agreement on rules on the use and economic exploitation of the oceans' vast mineral treasures.

Virtually no discernable progress was made here on how to define the limits of a nation's continental shelf, which often contains valuable oil and gas supplies.

There was not enough of a consensus, however, to allow new provisions to be included into the Law of the Sea treaty draft text, although there will be committee reports and an addendum which will be used for another round of talks scheduled to begin in New York Aug. 21.

Fundamental differences remain on the major stumbling block to an agreement, an international authority to regulate deep seabed mining and distribute a share of the profits to the international community.

In addition, the conference, following the disastrous Amoco Cadiz oil spill onto coast of France, has further strengthened the power of nations to protect their coastlines against accidents and pollution.

Many delegates to the long-running conference now feel that the ideological postures of the past are beginning to give way to serious bargaining on subjects which combine vital economic and security interests. It could be years, however, before a treaty emerges, and some key delegates wonder if the most disputed items can ever be resolved.

The conference faces its most difficult dispute over the principles involved in setting up a seabed authority council to regulate national and private industries' deep sea mining of an estimated 3 trillion potato-sized nodules rich with copper, nickle, manganese and cobalt. Difficult jargon and legal complexities have kept the subject from wide public attention. As one negotiator from a developing country put it, "This is the most important negotiation of the centurty."

At stake, as U.S. Special Ambassador to the conference Elliot Richardson noted, is a precedent which will influence negotiations between rich and poor nations over common resources.

In the seabed case, the Americans and other industrial countries which will be mining the nodules and sharing the profits with the international community under the treaty are insisting that they have an effective veto over decisions of the treaty's proposed deep Seabed Authority. Developing countries insist on a majority rule which would favor them.

There was no progress on this key question during the session, and many delegates from developing countries are angry that the Carter administration is now supporting a bill in Congress that will allow American mining companies to begin deep seabed operations.

Although the bill contains revenue-sharing provisions and is designed as an interim measure until a treaty is agreed to, many delegates here feel that unilateral U.S. action directly violates the principles of the conference and will make developing countries entrench their opposition to U.S. views on the Seabed Authority. The United States hopes that after initial outcries the conference will feel moved to negotiate more seriously in the face of unilateral alternatives.

Ironically, the minerals, oil and gas in the most lucrative part of the sea, at least in the short term, are not up for negotiation under the "common heritage" idea that the sea belongs to all mankind. That area, extending 200 miles from the coastline has been claimed exclusively by coastal states except for allowing access to surplus fish and other living resources.

A 1970 U.S. proposal to have international revenue sharing for these resources failed and now the Americans say that it is politically impossible to move the conference to adopt a similar scheme recently proposed by Nepal.

"It's the luck of the draw," said a diplomat, reflecting a common attitude. "Bolivia's got tin and we've got a coastline."