* TSX up 0.89 percent at 12,341.23, fresh 4-month high * Investors cheer Fed decision, commodity stocks jump * Energy companies gain as Mideast tensions boost crude By Alastair Sharp TORONTO, Sept 13 (Reuters) - Canada's mining and energy companies jumped on Thursday, boosting the key Toronto index to a fresh four-month high, after the U.S. Federal Reserve said it would buy more bonds until the jobs outlook rebounds for Canada's biggest trading partner. The Fed said it will buy $40 billion of mortgage-related debt per month as long as inflation remains contained, and indicated interest rates would not be raised from current rock-bottom lows until at least mid-2015. "Some investors were definitely surprised with how accommodative a lot of the statements were," said Youssef Zohny, portfolio manager at Stenner Investment Partners, part of Richardson GMP in Vancouver. "It was the open-ended language that markets are responding to." Gold miners jumped sharply after the news, which boosted the price of bullion, and energy stocks also advanced. Goldcorp Inc jumped 4.2 percent to C$43.77 and Barrick Gold Corp added 2.6 percent to C$39.76. Diversified miner Teck Resources Ltd moved 2.8 percent higher to C$31.47 and Suncor Energy was up 1.1 percent at C$32.98. "Commodities are responding as they typically do when you see further inflation in the economy," Zohny said. Previous stimulus programs encouraged investors to push money into riskier asset classes, including commodities and equities, lifting prices. At 1:36 p.m. (1736 GMT), the Toronto Stock Exchange's S&P/TSX composite index was up 108.61 points, or 0.89 percent, at 12,341.23, its highest since May 1. It had been trading lower before the Fed announcement. "We could actually see - which would be a change from what we've seen year to date - the TSX perhaps lead the S&P 500 and outperform it over the near term on rising commodity prices," said Philip Petursson, director of the portfolio advisory group at Manulife Asset Management. The broad U.S. index is up more than 14 percent so far this year, while the TSX has advanced just 2.3 percent. Among individual companies, Transat A.T. Inc shares jumped almost 22 percent to C$4.50 after the travel operator swung to a profit as cost-cutting bore fruit, and said it expects margins to continue to improve. Shares of medical isotope provider Nordion Inc pared some recent losses, gaining 3.5 percent to $C7.11 on news it will reorganize its business into two distinct units and appoint a private equity executive to its board. The stock had slumped earlier this week as Nordion suspended its dividend after an arbitration panel rejected its claim for damages from its main supplier. Macdonald, Dettwiler and Associates shares fell 5.7 percent to C$50.74 after the surveillance and satellite company said the U.S. Department of Justice was seeking more information about its planned acquisition of Space Systems/Loral Inc.