PRESIDENT Fernando Collor de Mello appears to be having some success in an attempt to isolate Brazil's approximately 9,000 Yanomami Indians, who say disease and pollution have been brought to their homeland by gold miners.Mr. Collor's next and bigger challenge is to keep his promise to demarcate their homeland. A 1985 study that included anthropological factors recommended setting aside a continuous chunk of land covering 36,000 square miles of northern Brazil. Now, the Yanomami are waiting for Collor to make a final decision on the size and form of their territory. Ministry sources in Brasilia say the president could announce the demarcation this week or next, as part of a series of measures to improve the situation for Brazil's indigenous peoples. Claudia Andujar, coordinator of the not-for-profit Commission for the Creation of the Yanomami Park (CCPY) says the Yanomami issue is a "test case" that will show how Brazil intends to treat its indigenous peoples and their environments. Brazil is home to 180 different tribes and an estimated 200,000 to 300,000 Indians, who speak 170 languages. Another 9,000 or so Yanomami live in Venezuela. When the government's Operating Plan for the Preservation of Yanomami Life began July 19, there were about 4,650 gold miners in the region, according to Funai, the National Indian Foundation. This week, Funai said that about 4,400 had left of their own accord. The remaining 250 miners have been involved in shootouts with government agents. About 120 Brazilian miners are expected to enter the area in the next several days, coming over the border from Venezuela, where the Venezulean army has occupied their landing strip in an affort to remove them, Funai said. In the huge, sparsely populated Amazon region, where access is difficult, it isn't easy to obtain independent information on the Indians and the gold miners. A Sept. 11 flying visit by congressmen, journalists, and armed-forces officials spotted more than 200 active gold-miner airstrips, according to the daily newspaper Jornal do Brasil. But a Funai spokesman says the fact that the regional gold market has lost 90 percent of its former volume is evidence that most miners have stopped flying. Davi Kopenawa Yanomami, a Yanomami leader, told an anthropologist in 1990 of tribal lore that when the white men mine gold, they release a mythical kind of "smoke," Xawara, that causes disease. Until the miners came, the Yanomami god, Omame, supposedly kept the Xawara under the earth. Mining has been a source of trouble for the Yanomami, the largest indigenous group in the Americas that still follow traditional, primitive ways. Many of them were attracted to the money, food, alcohol, clothing, and other enticements brought by miners, beginning in 1987. But contact with the outside is also thought to have caused a multiplicity of diseases against which health authorities say the Indians have little natural defense. The injuries, some contend, include psychosocial damage. About 1,500 h ave died of illness or violence during the last five years. "The arrival of the miners was violent and rapid," says Marcos Terena, an activist member of the Terena tribe. "The Yanomami will never be the same." The government plan to isolate the tribe began with a radio and newspaper campaign in the Yanomami's Amazon homeland, asking the miners to leave the area. Next, police took over 14 illegal air strips near Boa Vista, the capital of the northern state of Roraima. AT this small staging city, miners loaded themselves, their equipment, and supplies onto small planes to fly out to the Amazon rivers where gold lies. The mercury used to sift it from the silt has polluted rivers, poisoned fish, and those who eat them. Now the government is using rented helicopters to airlift miners out of areas that are unapproachable by plane. Later, new observation stations, 17 of them, will be set up to make sure miners do not return. "We have to occupy the territory," says Sydney Possuelo, the new president of Funai. To remove miners and demarcate the land, the government has set aside $8 million. Funai also hopes to raise $120 million from activists abroad to help demarcate the Yanomami and other indigenous peoples' lands. The Yanomami preservation operation comes after years of neglect and halfway measures. Just after his March 1990 inauguration, Collor made headlines by personally helping to dynamite miners' airstrips. Months later, however, the airstrips were back in operation and more Yanomami were falling ill. Fabio Feldmann, an environmentalist and federal deputy, says the government's latest effort to stop what he calls "genocide" has come as a result of pressure on Collor during the last few months, in meetings abroad with US and Latin American officials. "We have an opportunity now that the [Indian] has appeared on the Brazilian scene as a central figure," Funai's Mr. Possuelo says. "This is happening by determination of the president. If this were not the case, the economic interests in the indigenous areas would be much stronger than Funai." Human occupation of Brazil's vast Amazon region has been an issue for decades. The military, which ruled Brazil from 1964 to 1985, is still concerned with how the jungle's use might affect national security. Some military officials and Amazonian politicians fear that foreigners could invade the region to exploit its natural resources or to take over the job of protecting the environment. Possuelo notes that Collor's commitment to the Yanomami comes as more Brazilians shed the old mentality of "development at any cost." Brazil's record on such issues will also be highly visible when the country hosts a United Nations environmental conference in June 1992. Just one of many pro-Yanomami activist groups, CCPY welcomes Collor's new resolution, but worries that he may give in to local political pressures to reduce the size of the tribe's park. The Justice Ministry has been delaying on a recommendation, a CCPY spokeswoman says, "allowing the opposition to give their opinions and pressure Collor... ." Collor, whose efforts these days are mostly focused on garnering support for his plan to reform the Constitution and improve the government's failing finances, recently signaled his continuing commitment when he decreed the demarcation of a homeland for the Enawene-Nawe tribe, in the state of Mato Grosso. "There is undoubtedly a great will to get the miners out and demarcate the area ... which is a tremendous change," says Ms. Andujar. But getting "the Yanomami case through will depend on continued pressure on a very high level, international pressure and a kind of reward, so Collor will feel that it's worthwhile to keep this promise."