The company said Tobin, whose departure was immediate, would become vice chair of investment and banking group BMO Capital Markets.
He initially joined the board in February.
Making an immediate transition into his responsibilities is Veniez, whose two decades of experience also includes coal and forest products.
He is president of DDV Enterprises and chairman of Pink Lotus Infrastructure.
Cline said Veniez, who is to be based in Vancouver, had overseen the restructuring of several companies and also had extensive public policy experience at both the federal and provincial levels.
The appointment is still subject to Toronto Stock Exchange approval.
“The board and senior management … would like to thank Brian for his invaluable advice and counsel,” chairman Bill McKnight said.
“We wish him well as he takes on a role where he will no doubt once again make a positive contribution to the broader Canadian economy.”
McKnight added that Cline was looking forward to the benefit of Veniez’s insight and experience.
Cline’s primary coal interests are in British Columbia and in Colorado, the latter of which is home to its 100%-held New Elk coking coal mine.