TORONTO and MONTREAL • The increasingly bitter legal fight between Osisko Mining Corp. and Goldcorp Inc. centres on a deal for which there is no documentation: a gentlemen’s agreement.

According to Osisko, Goldcorp did not honour this arrangement between the two companies, which was reached at the start of a crucial meeting in Calgary last October. And that was the catalyst for a hostile bid that it believes is illegal.

The allegations appear in a statement of claim and affidavit that Montreal-based Osisko filed in the Quebec Superior Court this week to try to block the $2.6-billion hostile offer from Goldcorp. It provides details on why Osisko thinks Goldcorp misused confidential information when it made the offer.

The two miners have a history of on-and-off merger negotiations dating back to 2008, but the talks leading up to this bid began in late 2012, when they signed a confidentiality agreement.

Negotiations continued in 2013, and on Oct. 21, they held a critical meeting in Calgary in which Osisko provided a detailed review of its Quebec-based Canadian Malartic mine.

According to an affidavit from Osisko chief financial officer Bryan Coates, the company planned to reveal a lot of private data in that meeting. But a standstill agreement between the two firms was expired at that point, meaning Goldcorp was free to make a hostile bid.

Osisko was concerned about handing over confidential data that could be used against it in a hostile offer. So at the start of the meeting, it asked Goldcorp to extend the standstill to June 30 of this year. Mr. Coates said Goldcorp accepted the verbal agreement.

Four Goldcorp executives were at the meeting, along with two from Osisko. (There was also one outside consultant, acting for Goldcorp). According to Mr. Coates, Osisko revealed to Goldcorp that it expected to receive permits to use land adjacent to the Canadian Malartic mine. That was confidential information and very important, since it meant Osisko’s reserves and production could increase.

“It was additional strategic and competitive information that helped Goldcorp prepare its hostile offer,” Mr. Coates said in his affidavit.

He said that Osisko assumed the extension to the standstill would work under the same terms as the existing confidentiality agreement. But then Goldcorp drafted amendments that made the standstill conditional on Osisko providing even more documents.

“Osisko would never have [participated] in the Calgary meeting and would not have shared, as exhaustively as it did, its confidential information at that meeting had it known that Goldcorp did not have the intention to honour its commitment to extend the standstill protections … in similar ways to that of the confidentiality agreement in effect at the time,” Mr. Coates said.

He added that Goldcorp appeared to start planning its bid right after the Calgary meeting, as it wanted to move ahead while it still had an “informational advantage” over shareholders and other gold companies.

The $2.6-billion hostile offer was finally announced Jan. 13.

Osisko believes the bid is illegal because several key players involved in it had access to this confidential information, which they were not permitted to act on.

Even though the standstill was no longer in place by January, Goldcorp had to use a “non-contaminated” team of people (who did not have the data) to mount the bid, according to Osisko. Otherwise, it violates the terms of the confidentiality agreement between the companies, which remains in effect (only the standstill portion is expired).

Most notably, Osisko claims that Goldcorp wrongly shared the confidential data with the Bank of Nova Scotia, which is one of the key players in this hostile offer. The bank is lending Goldcorp US$1.25-billion for the bid and is also acting as its financial advisor.

Lawyers for both companies appeared in court on Friday before Judge Louis Gouin to argue the case.

Goldcorp’s lawyers claimed they had just received the lawsuit and needed more time to review it.

The judge took some time to discuss the details of the case. He was clearly disturbed by some of Osisko’s allegations, but acknowledged that he has not yet heard Goldcorp’s side of the story.

“This meeting in Calgary is what bothers me the most,” he said in French. “There’s something that troubles me about that meeting. You get the impression that there was a gentlemen’s agreement.”

With files from Drew Hasselback