Belgium's privileged position as the predominant European investor in Zaire was apparently preserved yesterday when Belgian Premier Leo Tindemans slipped unannounced into Paris to meet with Zaire's President Mobutu Sese Seko.

The Zairian president had threatened to replace all 5,000 Belgians in mineral-rich Shaba Province with Frenchmen because, as he said, the Belgian paratroops were "the last to arrive and the first to leave." Mobutu instructed Zairian diplomats to avoid all contracts with Belgian Foreign Minister Henri Simonet.

Yesterday, however, according to a Belgian diplomat, Tindemans and Mobutu agreed "to forget the misunderstandings, and the burning words and to turn the page."

They also agreed, the diplomat said, that Belgian citizens working in and around the mines that provide Zaire with 70 percent of its foreign exchange have a right to security.

Tindemans has spoken publicly of providing such security with a West European force supported by U.S. logistics, along the lines of French and Belgian operations that were backed up by U.S. and some British transport planes.

Other European countries, notably West Germany, are said to be cool to the idea of having anything but an inter-African force to protect the region. It is generally agreed that the Zairian army is in no condition to do the job by itself.

American sources here say the Carter administration has become very sensitive to the weaknesses of Zaire's government, army, and economy revealed by the Shaba invasion and the dangers that poses for Western interests in Africa. The sources say they expect that to be one of the prime topics when French President Valery Giscrrd d'Estaing meets President Carter later this week.

Giscard, who met privately with Mobutu early yesterday, is generally thought here to have played a major role in arranging the Belgian-Zairian meeting and reconciliation. The French have been going out of their way to try to reassure the Belgians that contrary to the suspicions freely expressed in Brussels, they do not intend to try to displace Belgium in its former colony.

Mobutu met with Tindemans in a private residence the Zairian leader maintains on the most fashionable residential street in Paris, the Avenue Foch.

The two men sealed their reconciliation over lunch in a private dining room of a two-star restaurant not far away, La Maree (the Tide), where the cheapest fixed-price menu is $30 a head. With Mobutu inviting, the diplomatic friendship was reaffirmed, according to a Belgian source, over lobster, spring lamb, cheeses, Napoleaon pastries, Pouilly Montrachet 1976 for the white wine and Chateau Haut-Brion 1971, the red Bordeaux vintage of American millionaire-diplomat Douglas Dillion.

Mobutu was evidently in a generally friendly mood. Witnesses said that the Tindemans-Mobutu conversation was interrupted by the unexpected visit of Emperor Bokassa of the Central African Empire, coming to say farewell after the Franco-African summit that the two Africans attended here.

The two chiefs of state kissed, witnesses said.

A french presidential spokesman declined to say anything about Mobutu's unscheduled call on Giscard except that there is every reason to believe that they discussed the timing and conditions of the withdrawal of the French Foreign Legion paratroopers from Shaba.

The French government announced that its embassies in Zambia and Angola, where the invading force is retreating, had asked for help in recovering French hostages that the invaders took with them. There are growing number of accounts of hostages being seen with the rebels. The French say the have now counted 70 French citizens missing.

Addressing the Belgian parliament yesterday afternoon, Tindemans seemed to indicate that he is willing to settle for an African rather than a European force to protect the Belgians in Shaba.

"The events in Kolwezi," he said, "have shown us that we should be more preoccupied with the security of our compatriots in Zaire. We are going to discuss that question with the other countries concerned as well as with the Zairian government." But, he said, the job of protecting the Europeans should be given to the Organization of African Unity.

"Unless the security of the Belgians can be guaranteed soon, they will have to be called home," Tindemans told the parliament.

That would be a blow both to the Belgian and Zairian economies. The Belgian investment there is estimated at more than $1 billion.

Part of the reason that Mobutu has relented in his attitude toward Belgium is thought to be the realization that for France, with an investment of only $20 million in the country, to replace Belgium effectively might take years.

It seems doubtful after the flood of atrocity stories in the French media that there are many more Frenchmen than Belgians ready to go there.