* OZ Minerals lenders seen likely to extend debt deadline
* Patience of bidder Minmetals, copper price outlook seen key
* Shares drop as much as 15 percent on fresh uncertainty (Adds lender, OZ spokesman, Australian trade minister comments)
By Sonali Paul
MELBOURNE, March 24 (Reuters) - Lenders to OZ Minerals Ltd (OZL.AX) are likely to extend a March 31 debt deadline, a lender said on Tuesday, after the Australia government decided it needed more time to assess a $1.8 billion rescue bid for the group from Chinese state-owned Minmetals.
The world's second-biggest zinc miner was left at the mercy of its lenders this week when Australia's Foreign Investment Review Board extended its probe into Minmetals' takeover to June 22, well past the deadline for OZ to repay $1.3 billion.
That knocked its shares down as much as 15 percent on Tuesday as investors worried about the fate of the deal and the patience of the lenders.
"OZ Minerals lenders will extend the deadline. Most of the banks are currently in the midst of getting their internal approvals," said a lender to OZ who asked not to be named due to the sensitive stage of the talks.
The company said its banks have another week to make their decision.
"Talks with lenders are progressing," said OZ Minerals spokesman Matthew Foran.
OZ shares fell 10 percent to A$0.53 on Tuesday, about 36 percent below the A$0.825 a share offered by Minmetals, reflecting the uncertainty.
Two key factors would make the banks more willing to extend OZ's lifeline: Minmetals' commitment to wait for a final decision from the Australian government and improving commodity prices, said the lender.
"It really depends on how certain the lenders are of Minmetals' position in 90 days, which is a long time," the source said.
Minmetals said on Monday it respected the government's review process, but declined to comment further.
Copper prices touched a 4-½ month high this week on signs of improved demand from China and hopes for an economic recovery in the United States.
"With copper prices going up, OZ is making money, and this is a positive thing for the company," the lender added.
Observers, including investment analysts, lobbyists and academics, expect the deal to be approved by Australia's treasurer eventually, possibly with conditions as it involves a full takeover by a state-owned Chinese company.
The Minmetals deal is considered less controversial than Chinese state-owned Chinalco's $19.5 billion tie-up with global miner Rio Tinto (RIO.AX)(RIO.L), also being reviewed by the Australian government, as OZ's assets are not as strategic as Rio Tinto's iron ore.
Australian Trade Minister Simon Crean said the extension of the probe by the Foreign Investment Review Board showed it needed more time to weigh the full impact of the proposal.
"All proposals, whether they come from China or any other part of the world, have to be considered against national interest guidelines," Crean told reporters in Canberra.
"It is just a demonstration that more time is needed to consider those facts."
The government has said it welcomes Chinese investment, but wants to ensure that any state-owned companies investing operate at arm's length from the government and on commercial terms. (Additional reporting by Sharon Klyne and Rob Taylor in Canberra; Editing by James Thornhill)