* New projects, not existing mines, would be hit * Government to outline plan to Congress on July 9 * Canadian mining shares hit, but later recover some losses By Mike McDonald GUATEMALA CITY, June 28 (Reuters) - Guatemala's president is planning a constitutional amendment to give the government as much as a 40 percent stake in new mining projects, a government spokesman said on Thursday. Initial news of the plan hit Canadian mining companies working in Guatemala, especially Tahoe Resources Inc which is developing a $500 million Escobal silver project southeast of Guatemala City. Tahoe later recovered some losses, saying the government had guaranteed its operations would not be affected by any new law. A spokesman for President Otto Perez Molina told Reuters the changes would only apply to future projects that have not begun the licensing process. "It is an executive proposal for non-renewable resources. The government would become a partner in 40 percent of the properties," said government spokesman, Francisco Cuevas. "This is a result of looking at mining policies in Chile, Bolivia and other countries who have used a similar model." The plan will be formalized early next month and should be presented to Congress, along with other proposed constitutional changes, including judicial and legislative reforms, on July 9. It takes a two-thirds vote in Congress to pass a constitutional change and a public referendum to ratify. Perez Molina, a conservative retired general, took office in January promising to boost government revenues from mining projects in the country. Resource nationalization is a growing concern in South America. Argentina nationalized energy company YPF in April, sending shock waves through the Latin American investment community. While Bolivia took control of a tin and zinc mine owned by global commodities firm Glencore in June, after already seizing the country's natural gas and electricity industries. SHARES TUMBLE Shares of Canada's Tahoe Resources dived 39 percent after the initial media reports of Guatemala's plans. It recovered to be down more than 18 percent at C$13.16 in Thursday afternoon trading on the Toronto Stock Exchange. Tahoe's New York-listed shares fell nearly 20 percent to $12.70. "The government has no intention of acquiring an interest in the Escobal project or other mining projects in the country," the company said. Tahoe said Escobal construction was on track and it expects to have the project fully permitted in the second half of 2012. Tahoe Resources is 40 percent-owned by Canadian gold miner Goldcorp Inc, which owns the Marlin gold mine in Guatemala, which produced 382,400 ounces of gold in 2011 and generated revenue of over $900 million. A spokesman for Goldcorp said the company had also been told by Guatemalan officials that the proposed mining law changes would not impact its Marlin mine operations. Shares of Goldcorp dropped 4.2 pct to C$36.78 on the Toronto Stock Exchange. Radius Gold, which is developing the Holly-Banderas project in Guatemala, also fell in early trading before snapping back.