* Gecamines copper output has plunged since 1980s
* Carter Center calls Gecamines "financial black box"
* Company chairman declined comment
ABIDJAN, Nov 3 (Reuters) - Democratic Republic of Congo's state mining company failed to internally register $740 million in income between 2011-2014, much of which is now untraceable, the Carter Center said in a report on Friday.
The amount makes up two-thirds of the $1.1 billion that the copper producer was entitled to collect from international mining partners during that period and there is no evidence how half of it was spent, the U.S.-based democracy watchdog said.
Albert Yuma, the chairman of Gecamines' board, declined to comment on the report's accusations. Valery Mukasa, chief of staff to Congo's mines minister, said before the report's publication that he could not comment since he had not seen it.
Yuma, Mukasa and Gecamines' interim director-general Jacques Kamenga did not respond to additional requests for comment on Friday after the report was released publicly.
Gecamines and the mines ministry have previously rejected accusations that company funds are misused, insisting that all of the company's revenues are properly accounted for and are not used for political purposes, as its critics have alleged.
The report did not suggest that any of Gecamines' partners had acted improperly in relation to the missing funds.
"Gecamines continues to be a financial black box," said the report, which was based on 200 interviews and a review of thousands of mining contracts and other documents. "Poor governance has allowed ... Gecamines to engage in opaque mining deals that fail to serve the public interest," the report said.
Democratic Republic of Congo is Africa's biggest copper producer and also mines significant quantities of gold, diamonds and cobalt but remains one of the world's least developed countries, with an annual budget of roughly $5 billion.
It is also in the throes of a political crisis prompted by repeated delays to an election to replace President Joseph Kabila, originally scheduled for 2016. Authorities have cited the more than $1 billion price tag for the presidential and other elections as one of the main obstacles.
Gecamines was once one of Africa's largest copper producers, with annual output peaking at about 500,000 tonnes in the late 1980s. But output has since tumbled due to political upheaval, mismanagement and the sell-off of assets to private investors like Freeport McMoRan and Glencore.
Last year, it mined just 10,000 tonnes of copper despite repeated promises to dramatically scale up production.
Gecamines collects significant revenues in the form of signing bonuses, royalties and dividends from its joint ventures with private investors.
But when the Carter Center compared public disclosures of payments to Gecamines - including financial reports by partners and reports to an industry transparency group - to records obtained from an internal database maintained by Gecamines, it found a $740 million discrepancy in income between 2011-14.
The Carter Center said the database it obtained was maintained by Gecamines partnership department, where all revenues from its more than 20 partnerships should be recorded.
The Carter Center was able to trace about half of the money to specific expenditures, including investments in new equipment and asset purchases.
However, the remaining half could not be traced and sources inside Gecamines told the Carter Center that much of the money was used on political expenditures like financing campaigns.
Gecamines has also failed to publish dozens of mining contracts, amendments and annexes, as required by the law, the report said.
Gecamines said in September that the company's partnerships with foreign investors have not been managed in the best interest of Gecamines and that it intended to implement new controls to "hold its partners accountable". (Reporting By Aaron Ross; Editing By Edward McAllister and Edmund Blair)