The marathon Law of the Seas Conference, which has been struggling for 10 years to write a treaty on the uses of the world's oceans, opened its seventh - and possibly last - session yesterday amidst procedural bickering over the election of a presiding officer.

Already, the chances for success of the conference are less than even, according to some delegates, and the leadership dispute in the first day threatened to push aside more substantive issues, such as control over the mining of vast mineral resources on the ocean's floors.

Informed sources at the United Nations session said a challenge to the conference's current president, H. Shirley Amerasinghe of Sri Lanka, was spearheaded by delegates from a group of coastal Latin American nations. These delegates feel Amerasinghe has been too sympathetic to land-locked nations that have sought to share in ocean resources harvested by bordering states.

As regional caucuses met on the leadership question, there was speculation by some delegates that a new president might be selected from Asia, perhaps Fiji or Singapore.

But far more difficult conflicts await the delegates from 158 nations who have convened here for a scheduled eight-week session.

For a decade, largely out of view of an uninterested public, the conference has been engaged in a global tug-of-war on the issue of deepsea mining, seeking agreement on a treaty whose foundation is based on the principle that all resources under the sea are the common heritage of mankind.

The object is to decide how to govern the harvesting of trillions of dollars worth of undersea potato-sized nodules rich with copper, nickle, manganese and cobalt.

Developing nations, promoting a new world economic order, on one hand, and the industrial nations, defending entrepreneurship, on the other, are still far apart on such issues as production limitation, revenue sharing and transfer of technology.

An informal draft negotiating text, prepared at the session last summer in New York, would establish an International Seabed Authority, which would control the mining. Each potential area of exploitation would be divided by the Authority into halves.

The Authority's operational arm, called the Enterprise, would conduct mining operations with revenues favoring the developing countries in one half of the area involved. In the other half, exclusive rights would be granted by the Authority to private or state-owned companies, which would share their profits with the Authority, which, in turn would distribute revenues to other nations.

The revenue sharing scheme is still under negotiation, but the U.S. delegation contends that the proposal involves, as special Ambassador Elliot Richardson put it, "onerous financial conditions" and mandatory transfer of technology previsions which are too open-ended.

The United States also fears that access to the mining areas will become politicized, since the authority will operate on a one-nation, one-vote basis to set policy rather than weighing voting in terms of production, investment and consumption.