The afternoon light was fading and the malarial mosquitos had come out into the camp, whining down toward arms and necks damp with sweat. Inside the office, with its bare light bulb hanging and its shelves stacked with bagged rice and prospectors' pans, the thin assayer's hands worked quickly over his balance -- the 20-gram weight, the 10-gram weight, the light metal wafers to even it off.

The miner watching him wore old rolled blue jeans and was missing his front teeth. He had walked in the way all the miners do, unfolding a crumpled paper to lay its contents on the assayer's long wood counter, and now his strike gleamed on the dusty balance: a small, brilliant pile of Amazonian gold.

"Forty-two grams," the assayer said. He made some notes on a government form, pulled out a stack of bills, and counted out 59 crisp 1,000-cruzeiro notes, plus change -- $550 worth. The miner counted them again and then smiled very broadly. "This has been a good day," he said.

Of all the massive mineral wealth of the Amazon basin, none has stirred such feverish excitement as the extraordinarily pure and bountiful gold that has made overnight legends of the scruffy prospectors Brazilians call "garimpeiros." At Serra Pelada, the celebrated northern Amazonian open mine that has produced 20,218 pounds since gold was discovered there last year, one prospector found a single 15-pound rock of gold that sold for $84,000. Near Itaituba, in central Amazonia, there are shootouts over river claims, and at the Itaituba Airport a light plane takes off every three minutes as dredge owners and gold buyers fly in and out of their campsites.

And at the Rio Madeira, the wide brown river that flows past Mutum-Parana, miners last month motored their dredges in hasty floating caravans to a spot where the river was said to have yielded 220 pounds of gold in 10 days.

"It's hard to find people to do construction work here," complained an architect in Porto Velho, the dusty territorial capital four hours' drive from Mutum-Parana. "Everybody's off looking for gold."

The garimpeiro is as well-worn a tradition as the old prospector of California or the Klondike, but the new high price of gold and the sheer extravagance of the first Serra Pelada strikes have sent thousands of Brazilian men scrambling for the first time to Amazonian mining camps. They come from places like Ceara State, where the rural poor struggle with cyclical drought, or Parana State, where growing agribusiness is gradually pushing small farmers off the land.

They descend on riverside camps like Mutum-Parana, by the bus and truckful, carrying battered Samsonite bags they plan to stuff with money and gold. They live in malaria-infested clusters of thatched shacks and cotton hammocks and tarp-covered lean-tos, their wallets full of pin-up pictures to keep them company when the prostitutes are too scarce or too costly. They pay outrageous prices for food and electricity, buy women for the week or young men for the night, take dubious medications to cure their venereal diseases and get up with the heat of daybreak to start working their claims again -- bent over the shallow riverbed with a pan to swirl the sand or working a motorized dredge for a small cut of the day's take.

On the Rio Madeira, the dredge owner takes 60 percent of the gold and divides the rest among his workers, usually four or five men per dredge. The gold buyers fly in and out from Porto Velho, paying three quarters of the international gold price -- the rest is supposed to go to the government -- and flashing breathtaking amounts of cash and bagged gold as they do business on the dirt floors of palm-frond huts.

"Ninety-four percent purity," said Simon Frederico Carmelo, the Mutum-Parana gold buyer who also makes what looks to be a handsome income by running the camp's supply shop and grocery stores. "This is the purest gold in the world." Carmelo is a placid-looking man with a neat gray moustache, and as he casually stuffed a brick-sized sack of gold into his shoulder bag, he was asked if carrying such things around did not perhaps make him a little nervous.

Carmelo smiled. "No," he said. He opened the back section of his vinyl bag, pushed aside a few large rolls of money, and pulled out a black, 32-caliber pistol. "It's a little dangerous, and I've got pistoleiros behind me to watch out for," Carmelo said, meaning the famous gunmen who still pull off an occasional shootout around here. "But a man doesn't die before his day is due." He stuck his gun into his pants, the butt hanging out over his belt, and walked down the dirt street of Mutum-Parana with two very large and unpleasant-looking gentlemen flanking him all the way.

From one end of Amazonia to the other, tales of sudden riches sweep the camps. "One guy got three pounds in two days, with one dredge -- right over there," said a broad-backed woman up to her knees in the muddy river water of a crowded Rio Madeira mining settlement. There is talk of the Serra Pelada man who made $2 million and hired 10 men to work for him, and the Mutum-Parana miner who started prodigiously drinking beer one night and ended up handing out a half pound of gold nuggets to women he wanted to impress.

"We've got people here who've already pulled out 30, 45, 60 pounds of gold," said Joao Lucena Leal, the graying-haired local public security chief whose job is keeping passion, liquor and women under control at the Rio Madeira camp called Tambourete. "But they don't say anything. There are 10 or 20 here who've got their fortunes -- solid."

Even after allowances for wild embellishment and wishful thinking, the last few years' astonishing quantities of gold yields -- the 1981 production is estimated at 41 tons -- have pushed Brazil ahead of the United States as the third largest gold producer in the world. Only South Africa and the Soviet Union, with their massive mines, produce more gold than the garimpeiros of Brazil. In Sao Paulo, South America's first gold futures market went into action four months ago.

"The garimpeiro knows the price of gold in London every day. He tunes into the BBC -- and he listens to it in English," said Gerobal Guimaraes, the geologist who supervises gold production at the National Ministry of Mines and Energy in Brasilia. "The great importance of this run of gold is that the government now sees that we can produce gold in quantity in a short time -- we can double, triple, quadruple this production in a few years if the government wants."

The emerging wealth of the four richest Amazonian areas, in addition to other parts of Brazil where smaller strikes have turned up in recent years, is a considerable comfort to a country so deeply in debt. Although the government has so far left the mining mostly to the garimpeiros -- "like the gold they found in California," said another Brazilian geologist -- officials at Serra Pelada moved in to make sure the gold goes straight to the Central Bank. And in many of the mining areas, government authorities have arrived simply to keep people from killing themselves in accidents or shooting each other all the time.

"We had 13 deaths here in a month and a half before I got here in August," said Lucena as he mopped sweat from his forehead in a palm-thatch shack in Tambourete that served as a makeshift luncheonette. "We had men exploiting other men. There were 200 prostitutes living here . . . and almost everybody had a .38."

Tambourete, in short, was a standard Amazonian mining camp -- 1,500 men living together in the kind of atmosphere that makes local doctors wince and one priest throw up his hands in despair. A mining company found substantial gold deposits in the Rio Madeira last year, and when the garimpeiros finally received permission this year to bring motorized dredges onto the river, they threw together the palm thatch settlement of Tambourete in a matter of weeks, hacking a bumpy airstrip out of the jungle for the gold buyers.

Like the government police who were sent last year to get the brawling Serra Pelada mine under control, Lucena was dispatched to Tambourete to look into the miners' deaths and clean up the town as best he could. What he found, he said, was so much liquor and shoddy equipment that most of the dead miners were divers who had drowned while working near the dredges.

Lucena landed on Tambourete with a vengeance. "After nine at night nobody circulates here," he said. "After six at night women can only go out if they're accompanied. I outlawed prostitutes permanently. And I outlawed alcohol and the use of any kind of arms."

At Serra Pelada, the major in charge banned women completely. There are about 40 women living now at Tambourete, Lucena said -- many of them, as he delicately put it, "temporary wives." Any miner who wants to bring such a woman to camp must sign a contract taking full responsibility for her.

And if she takes on certain extra work opportunities among the miners? "She has 24 hours to get out of camp," he said.