Power Dividend, a private company registered in Britain, said it already owned pounds 280m of RTZ shares and had worldwide backing to buy stock at 20 per cent above 'face value'.
RTZ said it had informed the Stock Exchange and Securities and Investments Board.
Paul Galloway, an RTZ spokesman, said: 'It is all rather bizarre, not least because Power Dividend does not seem to have complied with the necessary legal requirements. It really does sound quite extraordinary.
He added: 'At this stage we have no reason to feel under threat.' He said RTZ had never before heard of Power Dividend, nor of plans to buy up shares.
The Stock Exchange surveillance unit said the fax breached takeover rules that require any intentions to buy a company to be announced to the exchange. 'We will be investigating it,' the unit's Chris Yeo said. The surveillance unit had contacted the Takeover Panel over the faxed release, he added.
Raymond Grant, who signed the fax on behalf of Power Dividend, told the Reuters news agency that his group had individual, company and country members around the world who felt mineral wealth should not be controlled by companies.
'All we do is co-ordinate together for peace, for development in the developing world and we believe that the mineral wealth is the birthright of each nation,' he said.
'And we do not believe that the birthright of each nation should be held within a corporate structure. We have been basically asked to structure a purchase of the shares of RTZ through our membership.'
Asked if that meant the group would then try to buy all RTZ shares, Mr Grant said: 'If possible, yes.' He said Power Dividend was doing nothing illegal.
Companies House, the agency that registers British businesses, said a Power Dividend Ltd was incorporated in Britain in May 1992, but it had no details of its activities. Last night there was no one at the company to explain which individuals or companies had asked Power Dividend to make the RTZ approach.
Last week RTZ was subject to a stinging attack from the Prime Minister of Papua New Guinea, who claimed the company was trying to delay development of the Lihir gold mine, the largest find outside South Africa this century.
The government said it was seizing a 50 per cent stake in the venture, which has thrown into doubt a plan by RTZ to raise pounds 73m by selling part of its interest.Reuse content