A large mining company has left an agreement that would have made it a major funding partner for the embattled Pebble copper and gold prospect, raising questions about the project's future.

First Quantum Minerals and project owner Northern Dynasty Minerals could not finalize a proposed agreement announced in December, Northern Dynasty said Friday.

The deal could have pumped more than $1 billion into the Southwest Alaska project, the subject of strong nationwide opposition from critics who say it would devastate the Bristol Bay salmon fishery.

Northern Dynasty said in a short statement Friday that the proposed framework agreement between the two companies "has been terminated."

Tom Collier, chief executive of Pebble Limited Partnership, the project developer owned by Northern Dynasty, said Friday the project continues to "press forward" in the effort to receive state and federal approval.

"I believe we will secure the necessary funding to continue the permitting and review process for Pebble under the National Environmental Policy Act," Collier said.

Northern Dynasty calls Pebble the world's largest undeveloped copper and gold resource.

"Our project is well defined and we are going to continue communicating with Alaskans about why we believe in the opportunity it represents," Collier said.

Canadian company First Quantum is one of the world's top 10 copper mining companies, with projects around the world. The company's involvement was considered a major boost for Pebble when the possible long-term deal was announced.

But Pebble has faced continued headwinds. In January, big California pension funds invested in First Quantum called on the mining company to back out. Also that month, Scott Pruitt, administrator of the Environmental Protection Agency, said "any mining projects in the region likely pose a risk to the abundant natural resources that exist there."

In 2013 and 2014, the project lost other major mining partners when Anglo American and Rio Tinto backed out.

Northern Dynasty, a Canadian mining company whose main asset is Pebble, has pushed the project forward on its own. It launched efforts in December to win approval from federal regulators.

Shares of Northern Dynasty stock plunged Friday, dropping 33 percent to less than 50 cents apiece.

The proposed deal ultimately would have given First Quantum the option of investing $1.35 billion to become a 50 percent project partner, after it had invested $150 million to support the permitting process.

Financial analysts who track Northern Dynasty gave various takes on the unraveled deal.

Daniel McConvey owns Rossport Investments, a New York firm with investments in First Quantum and other mining stocks.

McConvey said relatives of Bristol Bay fishermen spoke against Pebble at First Quantum's annual meeting in Toronto early this month. At least one of the family members said they were a First Quantum shareholder, McConvey said.

"The arguments of these representatives against developing the project resonated with me and may have resonated with many others in the meeting," McConvey said in an email. "I was taken by their apparent sincerity. They convinced me that they were fully committed to fighting the Pebble joint venture in the courts through the permitting process."

Chris Mancini, a New York analyst with Gabelli Funds, said he believes First Quantum wanted to focus on its major copper project in Panama. First Quantum needs cash flow from that "make or break" project to pay down debt, he said.

"They have bigger things to focus on at this point," he said.

Mancini said that without First Quantum, Northern Dynasty could have less money to pay for data collection and manpower in the coming years, potentially slowing the permitting process.

The U.S. Army Corps of Engineers plans to issue a final report on the project's environmental impact in late 2019. The Corps and other federal agencies would use the report to determine whether to approve the project, or not, perhaps in 2020.

"This is going to make it harder for Pebble to get permitted in the time frame proposed by the Army Corps, unless Northern Dynasty can secure another source of funding," Mancini said.

John Tumazos, a mining securities analyst at John Tumazos Very Independent Research in New Jersey, said mining agreements are complex. The deal may have fallen apart simply because the companies couldn't agree on just one of numerous details.

Pebble is by no means dead, he said.

First Quantum invested $37.5 million in the project last year, as it considered a larger investment, he said. That about a year's worth of funding for the project, he said.

Tumazos said other mining companies have expressed an interest in Pebble. He suspects at least one of those will step in as a partner.

First Quantum shares fell 4 percent Friday, to under $20 apiece, a sizable drop.

"Its shareholders clearly were disappointed it lost 50 percent of Pebble potentially, and thought Pebble would be a good asset for the long-term," Tumazos said in an email.

Conservation groups celebrated Friday's news. They said it underscored the project as a major risk, economically and environmentally.