FCA has said Manley will work to ensure a "strong and independent" future for the group.
Marchionne attended the University of Windsor and called his years there among the best of life.
Marchionne will be replaced as FCA boss by Briton Mike Manley, head of the iconic Jeep brand.
It was Marchionne's success in turning around a pair of Swiss businesses that drew the attention of the Agnelli family.
Marchionne, who brought Fiat back from the brink of bankruptcy, has been lauded by the Italian press since his serious condition was made known over the weekend. It is now the world's seventh-largest carmaker and is debt-free. Around 80 percent of FCA's second-quarter profit came from the US market and the automaker said its new trucks should help lift its North American pre-tax adjusted margin to 10 percent in the second half of 2018.
"We at General Motors offer our condolences to Sergio Marchionne's family and friends".
Elkann's office declined to comment.
Some experts have cast doubt on the ability of Marchionne's successors to maintain the momentum he had built up over the years.
FCA said it has fixed-price contracts for most raw steel through 2018, but would see increases in 2019 at current prices.
"Those of us watching have received an education in management, finance, politics, oratory and what's possible via sheer force of will", Bernstein analyst Max Warburton said.
Altavilla, 54, quit after being told of Manley's appointment on July 21, one of the sources said. He brought in other outsiders to key positions and relaunched the iconic 500, which became one of the new Fiat's calling cards as it expanded overseas. Yet even as he garnered criticism from politicians and unions for slashing jobs and cutting costs, Marchionne argued that moving slowly could be even more risky.
"For Altavilla it was too important - he wants to be CEO at least once in his career", one of the sources said.
His death is believed to have resulted from complications of recent right shoulder surgery he underwent. And many of us have lost a very dear friend: "Sergio Marchionne".
Fiat Chrysler Automobiles declined to comment, citing privacy issues.
Marchionne joined Fiat in 2004 and led the Turin-based company's merger with bankrupt USA carmaker Chrysler. When he took over both Fiat and Chrysler, he always maintained, the companies needed radical change in order to survive. That ultimate deal eluded him. As chief executive of Alusuisse, he hived off its drug business into Lonza group, and in three years had tripled profits. Marchionne is credited with revamping the company, Italy's premier private enterprise, from top to bottom.
Here's what industry leaders have said in tribute to Marchionne. The company's new chief officer was the former head of the Ram and Jeep brands, two of FCA's most profitable and valuable product lines.
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