* TSX falls 66.04 points, or 0.48 percent, to 13,681.48 * Eight of 10 main index sectors decline * Goldcorp down 1 percent, Osisko up 20.7 percent * Magna climbs after Barron's report By John Tilak TORONTO, Jan 13 (Reuters) - Canada's main stock index dropped on Monday as shares of Goldcorp Inc declined after the miner made a $2.6 billion ($2.4 billion) offer to acquire a smaller rival and shares of energy producers fell on weaker oil prices. Goldcorp launched an unsolicited cash-and-stock bid to buy Osisko Mining Corp to gain control of Osisko's Malartic gold mine in Quebec. Further dampening the mood, the price of oil sold off on news of a deal between Western nations and Iran to curb the country's nuclear program. Investors were also cautious about what the U.S. and Canadian earnings seasons, just getting underway, will bring. The Canadian benchmark index, which rose 9.6 percent last year, eased on Monday after four straight days of gains. The market will be volatile this year, said Marcus Xu, portfolio manager at MY Capital Management Corp in Vancouver, but he predicted the TSX benchmark will perform more strongly than it did last year. "The sentiment is starting to improve. A lot of people are starting to warm up to (Canadian equities)," he said. The Toronto Stock Exchange's S&P/TSX composite index closed down 66.04 points, or 0.48 percent, at 13,681.48, after reaching as high as 13,771.13 earlier in the session, its highest level since mid-2011. Eight of the 10 main sectors on the index were in the red. Energy shares gave back 1.3 percent. In the group, Suncor Energy Inc lost 1.8 percent to C$37.27, and Canadian Natural Resources Ltd declined 0.9 percent to C$35.85. Financials, the index's most heavily weighted sector, were down 0.4 percent, with Manulife Financial Corp dropping 2.3 percent to C$21.59. But auto-parts maker Magna International advanced 0.5 percent to C$90.73 after Barron's, in a report on Sunday, said a possible recovery in the European car market could lift Magna shares in 2014. The gold-mining sector ended in positive territory, though Goldcorp weakened 1 percent to C$25.04. Osisko shot up 20.7 percent to C$6.24.