A MADDENING THING happened at the United Nations the other day on the world's way to a fairly promising Law of the Sea treaty. A draft text on the key issue of seabed mining, one that was potentially acceptable to a broad concensus of both developed and developing nations, had been negotiated painstakingly in a committee led informally by Norway's Jens Evensen. The draft then quite literally disappeared into the office of the committee's formal chairman. Paul Engo of Cameroon. The U.S. delegation was denied access to it. That is to say, the country financially and technologically most ready to mine and to enable others to mine was not permitted to offer its suggestions or even learn of the suggestions of others. Mr. Engo wrote in changes suggested by a few delegates seemingly committed less to negotiating an agreed text than to organizing seabed mining on radical Third World lines.

The American delegate, Elliot Richardson, who has a precise way of putting things, was compelled to pronounce the result "fundamentally unacceptable." Under its terms, neither American nor - here's the irony - Third World mining would seem to have a prayer. Whether the Law of the Sea Confererence can stay afloat now becomes a chancy thing.

Those American most committed to a treaty (virtually any treaty) as a model of international problem-solving argue that the mining hangup doesn't cross out the conference's achievements in ensuring free transit through the new 200-mile national economic zones and through straits, and in providing for research, control of pollution and settlement of ocean disputes.Mr. Richardson acknowledged all of these pluses. Our guess is, nonetheless, that few Americans are ready to "pay" in foregone mining opportunities for a treaty that bestows in the future transit rights that, after all, the United States has enjoyed in the past without a treaty.

Even if the economic and strategic value of seabed minerals falls substantially short of heady industry estimates, that proposal is a political non-starter. Moreover, the administration will have a tough time stopping Congress from unilaterally legislating some kind of legal license and financial-safety net for American mining consortia. Such legislation, and the mining plans it will propel, will in turn arouse the Third World and render compromise that much more difficult.

More in sorrow than anger, Mr. Richardson is recommending to the President that the United States review not only its interest in the negotiations but also the wisdom of sticking with negotiations that allow procedures as unfair as Mr. Engo's. He could have done no less. That kind of stand fires up Third World confrontationists and scares their American apologists. But it is quite justified. In no other way can the administration hope either to induce the Congress to proceed, if it must, tentatively and carefully on mining legislation, or to convince the Third World that the United States is not so eager to curry its favor that it will accept just about any treaty. Only when the Third World majority believes that to be the case will it make an effort to wrest control of its own interests back from its radical fringe.