Acacia Mining is facing increased pressure to resolve human-rights concerns at its North Mara goldmine.
The firm has been accused by the Tanzanian government of tax evasion, breach of environmental regulations, money laundering and corruption, among other infringements.
In 2017, the government slapped a $190 billion tax bill on Acacia for allegedly under-declaring exports and banned exports of unprocessed metal.
Last week, the firm, which is 63.9 per cent owned by Canada's Barrick Gold Corporation, was asked by British charity RAID, which exposes corporate human-rights violations, to review its use of the Tanzanian police in providing security at the North Mara mine. RAID alleged that the police had been involved in dozens of deaths, rapes and scores of injuries on or near the mine.
Tanzanian police have been providing security at the North Mara mine since 2010.
"Acacia Mining appears to have transformed parts of the Tanzanian police into a brutal and unaccountable private security force at its North Mara mine," said RAID executive director Anneke van Woudenberg in a public letter to the Acacia board.
"It has gone beyond the point where alarm bells should be ringing. The deployment of the police to provide on-site security for its gold mining operations raises the risk the company could be seen as complicit in serious crimes," she added.
Acacia is being accused of regularly using the police to deal with "intruders" inside the mine's perimeter. In return for the security services, the firm provides the police with per diems, vehicles, fuel, accommodation, food and other benefits.
According to RAID and MiningWatch Canada, at least 22 people have been killed and 69 injured, many by bullets, at or near the North Mara mine, between 2014 and 2016.
A Tanzanian parliamentary inquiry into the abuses in North Mara in 2016 received reports of 65 killings and 270 people injured by police jointly with the mine security.
Acacia did not respond to the allegations.