(Adds details, update prices to mid-morning)
* TSX down 50.58 points, or 0.36 percent, at 13,812.58
* Eight of the TSX's 10 main groups fall
TORONTO, Oct 29 (Reuters) - Canada's main stock index fell on Thursday, with Bombardier Inc plunging after reporting a massive quarterly loss and Potash Corp also retreating as it trimmed its earnings and sales forecasts.
The losses were offset by gains in the heavyweight energy sector amid volatile oil market trade.
Bombardier shed 15.5 percent to C$1.36 after the Montreal-base plane and train maker said the Quebec government will invest $1 billion in its long-delayed CSeries jet program and announced a $4.6 billion third-quarter loss.
Potash Corp fell 3.8 percent to C$27.07 as the world's biggest fertilizer company by capacity said it was cutting production as economic pressures weigh on demand for potash, a crop nutrient.
The Toronto Stock Exchange's S&P/TSX composite index was down 50.58 points, or 0.36 percent, at 13,812.58 by 10:20 a.m. ET (1420 GMT).
Eight of its 10 main sectors were in negative territory, although decliners outnumbered advancing issues by only 133 to 101, for a 1.32-to-1 ratio on the downside.
On the positive side, Suncor Energy Inc gained 2.3 percent to C$38.39 and Cenovus Energy added 4.3 percent to C$20.07, while the overall energy group rose 1.8 percent.
U.S. crude prices were up 1.4 percent to $46.56 a barrel, while Brent added 0.3 percent to $49.20.
The financial sector lost 1 percent, with Toronto-Dominion Bank down 1.3 percent to C$54.65 and insurer Manulife Financial off 1 percent to C$22.32.
Reporting by Alastair Sharp; Editing by Dan Grebler