* TSX down 139.83 points, or 1.1 pct, at 12,274.03 * All of index's 10 main sectors lower * Bank, energy, materials issues lead way down By Jennifer Kwan TORONTO, March 29 (Reuters) - Toronto's main stock index fell to a three-week low on Thursday, as recent mixed U.S. economic data and worries about a slowdown in China raised investor concern about the global economic outlook. New U.S. jobless claims fell last week to their lowest level since April 2008, but missed forecasts. That followed a U.S. durable goods report on Wednesday that was slightly softer than expected, said Ian Nakamoto, director of research at MacDougall, MacDougall & MacTier. "It's a bit of a mixed message out here," he said. Big stocks on the downside included Potash Corp, down 1.5 percent at C$45.36, and Suncor Energy, which fell 1.6 percent to C$31.97. Royal Bank of Canada sank 1.2 percent to C$58.08. The key financial, energy and materials sectors, which comprise some 70 percent of the broader index, were all lower by at least 1 percent. At around 10:20 a.m. EDT (1420 GMT), the Toronto Stock Exchange's S&P/TSX composite index sank 139.83 points, or 1.1 percent, to 12,274.03, with all of its 10 main sectors lower. The index touched 12,258.39, its weakest level since Mar. 7. Concerns about a slowdown in Asia were also a major factor driving the TSX index lower, said Nakamoto. "Canada has been out of favor for a while. Until people have confidence in what's going on in China, it'll probably continue to lag the U.S. markets and other markets," he said. Resource stocks make up some 40 percent of the broader TSX index. Profit taking ahead of the end of the first quarter has added to the pullback in stocks, analysts have said. In company news, asset manager Sprott Inc, down 3.8 percent at C$5.80, said its quarterly profit plunged 96 percent mainly due to weak precious metals prices that dragged down investments. Ithaca Energy Inc gained 0.7 percent to C$3.07 and posted a slight decline in profit. The oil and gas producer said it continued to be in discussions with parties interested in acquiring it.