* TSX down 104.02 points, or 0.78 percent, to 13,205.78
* Five of the TSX's 10 main groups were lower
TORONTO, Dec 29 (Reuters) - Canada's main stock index fell on Tuesday amid weakness in resource stocks and a tumble in shares of Valeant Pharmaceuticals International Inc as the market reopened following an extended Christmas break.
The market was closed Friday for Christmas and Monday in lieu of Boxing Day on Saturday. It rose 2.2 percent last week.
Valeant Pharmaceuticals International Inc fell nearly 12 percent to C$138.81. On Monday, the company said a group of executives would take over for its chief executive officer until he returns from medical leave. CEO Michael Pearson, 56, is being treated for severe pneumonia. The stock saw a more moderate loss in New York after falling sharply on Monday.
Goldcorp Inc fell 4.6 percent to C$16.12 and Barrick Gold Corp declined 4.3 percent to C$10.38.
The overall materials group fell 2.8 percent.
At 11:12 a.m. EST (1612 GMT), the Toronto Stock Exchange's S&P/TSX composite index fell 104.02 points, or 0.78 percent, to 13,205.78, with five of the index's 10 main groups in negative territory.
Energy stocks fell 1.2 percent, including a 2.9 percent drop in Encana Corporation to C$6.78.
Crude oil prices rallied in thin trade on the prospect of colder weather, but only after falling more than 3 percent on Monday.
U.S. crude prices were up 2.2 percent to $37.63 a barrel, while Brent crude added 2.4 percent to $37.5.
Financial sector stocks clung to a modest gain, helped by a 0.4 percent advance for Bank of Montreal to C$79.06.
Loblaw Companies Ltd rose 1.2 percent to C$67.08. (Reporting by Fergal Smith; Editing by Bernadette Baum)