PITINGA, BRAZIL -- In the late 1960s, Octavio Cavalcanti Lacombe had a mad notion. The Sao Paulo engineer, who started his career building highways in the civilized south of Brazil, bought out a small mining company and went looking for tin in Amazonia.

Back then, no Brazilian company had any experience in such mining. In all of the Amazon, only one other mechanized mining venture had ever been attempted, and that was comfortably backed by the American metals giant, Bethlehem Steel.

"Amazonia was virgin then. We were called adventurers," Lacombe recalled recently. "And adventurers always got swallowed up by the great green jungle."

Lacombe decided to buck the long odds and went ahead with his jungle folly. In the late 1970s, after a brush with bankruptcy at a small mine in western Amazonia, he acquired mineral rights to Pitinga, a little known area 200 miles north of the Amazon River where government geologists had stumbled on significant deposits of cassiterite, or tin ore.

Today he occupies the penthouse suite of an 11-floor glass and steel office tower in the heart of Sao Paulo. The building belongs to the leading Brazilian mining corporation, Paranapanema, which belongs to Lacombe. Call him the new Simon Patino, the legendary early 20th century Bolivian magnate who made that tiny Andean country the world's leading tin supplier.

Half a century after Patino, Octavio Lacombe and his partner Jose Carlos Araujo founded Paranapanema, and in 25 years turned the modest Sao Paulo construction firm into the world's single largest tin producer.

With nine mines in Brazil, Paranapanema produced just over 20,000 tons of tin last year -- 13 percent of the world supply, and three times that of Bolivia, whose antiquated mines are shutting down due to inefficiency and rocketing costs.

Propelled by the giant Pitinga mine in the northern state of Amazonas, the Lacombe production has catapulted Brazil from a lowly seventh place among tin producers to number two last year. Brazil may soon overtake the leading producer, Malaysia.

A sturdy man of 61, whose father once raised pigs in a small country town, Lacombe flies all over the globe to meet with executives. But what he really likes, he said, is tramping about his projects in the Amazon.

"He's our jungle man," said Samuel Hanan, a company vice president.

When Pitinga was discovered, Lacombe began prospecting in a land where there were no roads or air strips. Geologists landed hydroplanes on the Uatama River and trekked or canoed for up to 20 days back down the Pitinga River. Pack mules had to be tranquilized and flown in to haul equipment.

The advance teams hacked a camp out of the jungle, slept under tarps, scraped away at the forest floor, and finally hit pay dirt. "We found some cassiterite, then more cassiterite, and still more," said one of the geologists who discovered Pitinga. "It was fantastic."

Pitinga is now known to harbor 500,000 tons of high-grade cassiterite, the largest reserves ever discovered of tin ore.

Tin is not the only treasure. Buried in the region's soils are vast deposits of other valuable minerals such as niobium, used to temper steel for rockets and airplanes, and tantalum, a rare, corrosion-resistant metal used in microelectronics.

The rain forest was not the company's only obstacle.

Paranapanema has also met stiff opposition from the Indigenous Missionary Council (CIMI), an arm of the influential Brazilian Bishops Conference.

The Waimiri-Atroari Indians, numbering roughly 370, lost thousands of acres to the Pitinga project when the federal government redrew the limits of their reserve. CIMI claimed the truncated reserve was the work of Paranapanema. Paranapanema responded that the new maps simply corrected "geographical errors."

Lacombe insists today, "We are good neighbors to many Amazon tribes." A number of Indians have in fact taken jobs with Paranapanema or sought assistance in mining their own reserves, and 10 Tucano Indians are studying at the company's mining school in the port city of Manaus.

CIMI fears, however, that among the relatively weak and impoverished tribes, indigenous "leaders" can be easily enchanted by powerful companies -- or worse, by outright swindlers -- into making bad deals for token prizes.

But perhaps the company's sternest opposition has come from what Lacombe calls "the ferocious tribes of tin production." A powerful tin cartel charged that his refusal to join the producer's association coupled with Pitinga's unbridled production provoked a drastic plunge in tin prices in 1985 and the subsequent collapse of the International Tin Council.

Lacombe replied that the tin crisis, in which prices dropped from $16,000 per ton in 1982 to below $6,000 last year, had started long before Pitinga began producing. Although world demand for metals and other raw materials shrank after the 1982 recession, the tin price was artificially buoyed by the cartel's manipulation of supply through enormous buffer stocks.

The tin producers, who have buffed their image under a new association, are now courting Paranapanema again. After a series of meetings -- including one in Rio de Janeiro in 1986 -- Lacombe agreed to cooperate. Though he declined to join the new association, he has consented to contain the company's production -- but only to a point.

"Cartels don't work. It didn't work with petroleum, and it will never work with tin," he said.

Brazil's tin man flashes a smile behind his cigarillo. "Many have tried, but there's no way of escaping the law of supply and demand."