Practically on the eve of his departure to a last-gasp session of the marathon United Nations Law of the Sea Conference, Special Ambassador Elliot L. Richardson warned yesterday that the United States will not be a party to a "simplistie, ideological platform of one nation, one vote" just to obtain an international imprimatur for lucrative undersea mining rights.

Although denying emphatically that he was issuing an ultimatum, Richardson warned in no uncertain terms that intransigent positions taken by developing nations on the issue of dividing vast storehouses of minerals on the oceans' floors could lead to a debacle at the U.N. conference, scheduled to resume March 28 in Geneva.

In a State Department briefing, Richardson complained that the current text of the Law of the Sea treaty, which led to a chaotic breakup of the conferenc last summer, is unacceptable to the United States and other developed nations.

"The text imposes onerous financial conditions, dictates mandatory transfer of technology . . . and contains several other features which would combine ot deter entrepreneurs from investing the $700 million to $900 million required to bring a single mining site into operation," Richardson said.

"We are still convinced that a new magnitude of global order can emanate from (the negotiations) . . . But commitment alone carries no guarantee of success," Richardson warned.

The tone of Richardson's remarks accorded with the current U.S. negotiating poture, which is that failure to adopt a new Law of the Sea treaty might not be absolutely calamitous, and that U.S. mining companies are fully prepared to begin unilaterally mining the trillions of dollars worth of cobalt, nickel manganese and copper on the sea bottom.

Congress bas been moving along on a bill that would authorize and encourage unilateral U.S. undersia mining operations, and Richardson is known to regard House Passage of the bill sometime during the conference as a valuable negotiating instrument.

For a decade - largely out of view of a disinterested public - nearly 150 nations have been engaged in a global tug-of-war on the issue of deep ocean mining, trying to reach agreement on a treaty whose principle is that all resources under the sea are a common heritage of manking.

Developing nations, promoting a new world economic order, and industril nations, defending entrepreneurship and protecting their technological advantage, have been attempting to create a system to govern harvesting the valuable mineral nodules.

Richardson said the U.S. position in Geneva will reject the concept of "rigid state centralism projected on a global scale," and favor instead a "parallel" system in which nations and private companies would share seabed mining operations with an international "enterprise" created by way of a treaty.

Squabbling over the treaty repeatedly has centered on the division of power between the "enterprise," and private companies, as well as such issues as division of profits among nations, production limitations, price controls and the transfer of technology from developed nations to developing nations.