Couillard is projecting total investments of about $50 billion by the year 2035.
Of that, $2.7 billion will come from the government for infrastructure purposes, while Hydro-Quebec will contribute more than $20 billion.
The rest is mainly expected to come from private enterprise.
Couillard estimates 10,000 jobs will be created over the 20-year period.
In 2011, then-premier Jean Charest projected investments of $80 billion and 20,000 jobs over 25 years.
Couillard said falling metal prices had a major impact on his decision to scale back the investment plans.
"Markets change," he told a news conference. "That's what has led us to being more conservative in our investment projections."
The premier said it is important to go ahead with the so-called Plan nord in order to create a climate favourable to investors.
"The main thing is to invite people by telling them there is a plan," he said. "In the minds of investors I have met abroad, there has been no plan over the last two years."
Couillard said one difference between his plan and the previous one is a clearer emphasis on sustainable development.
"We're talking not only about the economy but also about social development and protecting the environment and putting a lot of attention to the people who have been living there for centuries before us," he said.
The Plan nord will cover 1.2 million square kilometres.
Much of northern Quebec will remain protected from industrial activity.
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