Millions of U.S. dollars and gold were seized and a suspected American smuggler arrested in Democratic Republic of Congo, according to officials.
Provincial Governor Julien Paluku said four foreigners —the U.S. man, two Nigerians and a Frenchman — were detained after a car chase from the Goma airport.
Officials said they had arrived aboard a Nigerian aircraft on Thursday.
Paluku said the suspects were found with a large sum of money to buy gold.
The government banned mining and mineral trade in eastern Congo in September, trying to regulate an industry controlled by armed groups including former rebels in the national army.
"Initial inquiries have revealed that these people arrived in Goma with a large sum of money, in millions of dollars, for a gold transaction," Paluku said.
The money was seized by the authorities and the men were being held while investigations continue, he added.
Despite the official end to Congo's 1998-2003 war, much of the east remains littered with rebel groups, local militia and ill-disciplined army units.
The area is rich in tin, coltan and gold and gunmen on all sides, who are frequently accused of widespread abuses, frequently clash over access to the resources.
A U.N. panel of experts on Congo said last year that almost every mining deposit in the east was controlled by an armed group, and demilitarizing Congo's mine sites was crucial to cleaning up the trade.
Campaign group Enough said last week that rebel groups and senior Congolese commanders were still profiting from the trade despite the ban.
Neighboring Rwanda and Uganda were also accused of involvement, it added.
New U.S. legislation forcing companies who source minerals from the region to prove they are not fuelling conflict, is due to come into force in April.
However, there are concerns about how effective the scheme will be. Some analysts believe the ban has forced legal traders out of business, only for them to be replaced by more criminal elements.