CARAJAS NATIONAL FOREST, Brazil » Archaeologists must climb tiers of orchid-encrusted rain forest, where jaguars roam and anacondas slither, to arrive at one of the Amazon’s most stunning sights: a series of caves and rock shelters guarding the secrets of human beings who lived here more than 8,000 years ago.

Almost anywhere else, these caves would be preserved as an invaluable source of knowledge into prehistoric human history. But not in this remote corner of the Amazon, where Vale, the Brazilian mining giant, is pushing forward with the expansion of one of the world’s largest iron-ore mining complexes — a project that will destroy dozens of the caves treasured by scholars.

The caves, and the spectacular mineral wealth in their midst, have presented Brazil with a dilemma. The iron ore from Carajas, exported largely to China where it is used to make steel, is a linchpin of Brazil’s ambitions of reviving a sluggish economy, yet archaeologists and other researchers contend that the emphasis on short-term financial gains imperils an unrivaled window into a nebulous past.

"This is a crucial moment to learn about the human history of the Amazon, and by extension the peopling of the Americas," said Genival Crescencio, a caver and historian in Para state, which includes Carajas. "We should be preserving this unique place for science, but we are destroying it so the Chinese can open a few more car factories."

As Brazil embarks on a frenzied effort to increase mining and improve infrastructure, work crews in the Amazon and beyond are unearthing one startling discovery after another. In Rio de Janeiro, archaeologists are examining a slave market and cemetery where thousands of Africans were buried. The discoveries have complicated the upgrade of the harbor and public transportation network before the 2016 Olympic Games.

Brazilian courts can require companies to preserve archaeological sites, or at least transfer archaeological material to universities or museums where it can be studied, before work continues. In some cases, rulings have stalled huge projects, as Anglo American, the mining giant, discovered this year when prosecutors halted work on a large mining project in Minas Gerais state over concerns that an archaeologically significant cave could be damaged.

Scholars say that the caves of Carajas, which archaeologists began exploring in the 1980s, offer coveted insight into what may be the earliest known stages of human settlement in the world’s largest tropical rain forest, helping to piece together the puzzle of how the Americas came to be inhabited.

Pieces of ceramic vessels and tools made of amethyst and quartz are among the signs of human occupation from thousands of years ago. Such artifacts, along with the abundance of the caves and rock shelters themselves, make Carajas one of the Amazon’s most important places for the study of prehistoric humans.

The Amazon is already a hotbed of archaeological investigation, as researchers find evidence that far more people might have lived in the region than once considered possible. While the Amazon was once thought incapable of supporting large, sophisticated societies, researchers now contend that the region might have been home to thriving urban centers before the arrival of Columbus.

Before those cities were carved out of the forest, people lived in the Amazon’s caves. At Pedra Pintada, a cave that, like those in Carajas, is also in Para, Anna C. Roosevelt, an American archaeologist, has shown that hunter-gatherers moved to the region 10,900 to 11,200 years ago, far earlier than once thought, about the same time people in North America were hunting mammoths.

Outside the Amazon, remarkable discoveries have been announced in recent months at other Brazilian sites. At Lapa do Santo, a rock shelter near the city of Belo Horizonte, archaeologists said this year that they found the New World’s oldest known figurative petroglyph. The rock art, a drawing of a man with an oversize phallus, is thought to have been made 10,500 to 12,000 years ago.

To reach the caves of Carajas, researchers must drive hours along washboard roads cut through the jungle, before scaling escarpments with spectacular views of the Carajas Mountains, a range of canopied peaks rising out of the forest. Macaws fly overhead and bats swirl inside the earth cavities in which hunting tribes once found shelter.

Some of the caves, substantially cooler inside their openings than the surrounding forest, are large enough for more than a dozen people; others might have provided just enough space for two or three people.

Vale, then a state-owned company, began developing the iron ore deposits here after they were discovered in 1967 by a Brazilian geologist on assignment to find manganese for the U.S. Steel Corp. Vale has since been privatized, but the government still controls big equity stakes.

Thanks largely to its Carajas complex, where thousands of workers labor 24 hours a day amid the clamor of digging machines, Vale accounts for 16 percent of Brazil’s total exports. As Vale grapples with a sharp decline in profits this year and delays at projects outside Brazil, Carajas is expected to become more important.

Vale has said it plans to create 30,000 jobs in the expansion of iron-ore mining at Carajas, a $20 billion project called Serra Sul, which is already luring thousands of migrants from around Brazil to this frenetic part of the Amazon.

To comply with regulations governing archaeological sites, Vale executives said, the company hired archaeologists and a team of speleologists, or cavers, to survey the caves, which are clustered around the open-pit Carajas mine. Vale also adapted its construction proposal to preserve some caves while planning to destroy dozens of others. While Vale acknowledged that at least 24 of the caves to be destroyed are of "high relevance," it said it would also preserve caves in another part of Para to compensate for their loss.

"For us there is just one procedure, and that is being transparent," said Gleuza Josue, Vale’s environmental director. Describing the expansion of Carajas as a project of "paramount importance," she said that Vale had rigorously complied with environmental and archaeological legislation in order to move forward with its plans.

Regulatory officials said they have won concessions from Vale but have not been able to stop the mine expansion. Despite archaeological concerns, the government granted the company a crucial environmental license in June, allowing the expansion to move forward. The company still needs another installation license, expected to be granted in 2013, to go ahead with Serra Sul. Archaeologists and cavers familiar with Carajas seem resigned to the possibility that Vale will get its way.

Frederico Drumond Martins, a government biologist who oversees the Carajas National Forest, said he remained concerned that mine expansions here in the decades ahead could eventually destroy every last cave in Carajas.

Renato Kipnis, a respected archaeologist in Sao Paulo whom Vale hired to survey the caves of Carajas, said that Vale had prohibited him from discussing their archaeological significance, because of a confidentiality agreement Vale had required him to sign. Later, a Vale spokeswoman allowed Kipnis to be interviewed by email, but only if the company was allowed to vet his replies.

In written replies screened by Vale, he marveled at the importance of the caves.

"The great challenge," he said, "is finding middle ground between preservation and development."

© 2012 The New York Times Company