* TSX falls 108.4 points, or 0.91 percent, to 11,821.39 * Materials sector falls nearly 2 percent * Poseidon Concepts down 55 percent on results, forecast By John Tilak TORONTO, Nov 15 (Reuters) - Canada's main stock index fell for the fourth straight session and tumbled to a three-month low on Thursday, dragged down by the material and financial sectors, as soft U.S. and European economic data dampened investor sentiment. The materials sector, home to mining companies, dropped nearly 2 percent, with Goldcorp Inc down 3.32 percent at C$39.91 and Barrick Gold falling 1.66 percent to C$33.43. Gold prices fell more than 1 percent to a one-week low. Financial markets have also been hit by ongoing fears about budget negotiations in Washington, the failure of which could trigger tax hikes and spending cuts that could push the U.S. economy, the world's biggest, into recession. "The biggest issue for investors is still the 'fiscal cliff' in the United States and the entrenched political positions on both sides," said Julie Brough, vice president at Morgan Meighen and Associates. "We're in a period of heightened uncertainty after what was a quiet period when people lulled themselves into a complacent state." Adding to the selling pressure, data on Thursday showed the United States reported a spike in new claims for jobless benefits last week and the euro zone slipped back into recession in the third quarter, its second recession since 2009. At midafternoon, the Toronto Stock Exchange's S&P/TSX composite index was down 108.4 points, or 0.91 percent, at 11,821.39. Earlier in the session, the index fell to 11,761.34, its lowest point since Aug. 7. "We're bouncing up and down like a yo-yo on a very long string. This is a very long, emotion-fueled week for the market," said Barry Schwartz, portfolio manager at Baskin Financial Services. POSEIDON PLUNGES Energy services company Poseidon Concepts Corp lost more than half its value and was one of the largest drags on the Toronto market. Its shares fell 55 percent to C$5.91 after it posted weak results and cut its outlook. The financial sector declined 0.61 percent as the macroeconomic concerns hit markets. Bank of Montreal fell 1.09 percent to C$57.08. Fairfax Financial Holdings Ltd dropped as much as 9.6 percent to a three-year low on Thursday after the stock was unexpectedly deleted from the MSCI Canada index. Research in Motion shares rose 3.17 percent to C$8.78, a day after its chief executive expressed confidence that a new line of BlackBerry 10 devices would offer long-term value for unhappy shareholders.