Brazil, boxed in economically by mounting oil bills and tightening foreign credit, is looking to optimistic reports of abundant gold reserves as a potential escape -- harking to the time-worn proverb here that "gold opens all locks."

"Brazil has the largest gold reserves in the world," proclaimed a recent headline in O Globo, the nation's largest circulation newspaper and one with strong government ties. The newspaper published a chart showing that Brazil's ballooning foreign debt -- the world's largest at $56.5 billion -- has actually shrunk by 25 percent over the last decade by virtue of the great increase in the price of gold.

Shigeaki Ueki, president of Petrobas, the government petroleum monopoly, argued last week in an article in O Globo that Brazil's gold deposits are now "more than sufficient collateral" to guarantee the continued flow of international loans to Brazil.

The source of much of the optimism about gold is the state-controlled Mineral Resources Research Co., which late last year produced studies pegging Brazil's potential gold deposits at 33,000 tons -- allegedly twice those of South Africa, which is the world's top producer.

The reports predicted that Brazil's 1980 gold production -- generously estimated at 40 tons or third worldwide -- could quintuple to 200 tons a year by 1985. Other officials have talked of another "golden century" -- like the 1700s when mines in colonial Brazil produced half of world's gold output.

But bankers and independent geologists here are skeptical.

"It's not a bonanza," said the top economist for a major American bank here that is one of Brazil's largest creditors. "Even if Brazil is producing $3 billion of gold by 1985, that will be less than 5 percent of the debt -- it's not a solution."

Five independent geologists with long experience working in Brazil also challenged the research company's forecasts in interviews last week.

"Brazil does have a lot of gold, but it is totally impossible to say how large the reserves are -- the U.S. Geological Survey can't say how much gold is in the States," said Patrick Delaney, a consulting geologist who has worked here for 23 years. "We all know God is a Brazilian," Delaney added, repeating another popular saying here, "but when he put the gold here, he didn't tell anyone where it was."

A high official at the Anglo-American Corp. of Brazil, a South African-based minerals consortium that runs one of Brazil's largest gold mines, curtly termed the government predictions as "pretty optimistic."

He cited production obstacles such as the three-year minimum needed to open a new mine and the necessity to process 20 million tons of ore body to produce 200 tons of gold.

"You need a big capital program -- it's not just a question of digging holes," he said.

"These [Mineral Co.] numbers should be ignored," said Joao Gilberto Lyrio, coordinator of gold projects for Decegeo, a government-run mining company. He criticized the company study for using radar maps to estimate depths of gold fields, which he said is virtually impossible to do accurately.

Meanwhile, the government's enthusiastic scenarios have diverted attention away from a real-life gold rush that has been sweeping Brazil's Amazonian frontier since early last year.

Last February, Serra Pelada in the northern state of Para was an unexplored mountain covered with rain forests. Today, renamed "Babylon," it is a mile-wide open pit mine where 20,000 men work from dawn to dusk extracting gold.

Serra Pelada is the largest of dozens of smaller gold strikes that have lured at least 60,000 persons into the area. Mahagony production, the traditional industry, has declined and ranchers complain that they cannot find hired hands to work their cattle herds. Conflicts have flared between eager prospectors and Indian tribes.

To cool Serra Pelada's Wild West atmosphere, the federal police took control of the area last summer, banning guns, gambling and women on the grounds that all three provoke fights.

Officials at the research company in Rio admit that with gold selling at $600 an ounce their reconnaissance efforts in the field are often hamstrung by desertions.

"In Porto Velho this month we lost the head of the department and five men -- it happens all the time," said Gaston Pereira Bascope, a company official.