(Adds details on market activity throughout; updates prices)
By Fergal Smith
TORONTO, March 7 (Reuters) - Canada's main stock index fellon Wednesday as a drop in commodity prices weighed on resourceshares, while steel producer Stelco Holdings Incrallied on prospects for Canada to be exempted from proposedU.S. metals tariffs.
* The White House said Canada and Mexico, and possibly othercountries, may be exempted from planned U.S. import tariffs onsteel and aluminum on the basis of national security.
* Stelco rose 3.9 percent to C$24.63. The stock had hit anearly seven-week low on Tuesday at C$23.00.
* The Toronto Stock Exchange's S&P/TSX composite indexclosed down 72.58 points, or 0.47 percent, at15,472.61.
* The resignation of Gary Cohn, top economic adviser to U.S.President Donald Trump, intensified investor worries over aglobal trade war, while the Bank of Canada said trade policy isan "important and growing source of uncertainty."
* The materials group, which includes precious and basemetals miners and fertilizer companies, lost 1.4 percent.
* Agnico Eagle Mines Ltd fell 2.0 percent toC$49.47 after Credit Suisse cut its price target on the stock.
* Gold futures fell 0.2 percent to $1,326 an ounce,while U.S. crude oil futures settled 2.3 percent lower at$61.15 a barrel.
* The energy group fell nearly 1 percent, with TransCanadaCorp down 2.3 percent at C$55.65.
* The shares of trade-sensitive auto parts and railroadcompanies lost ground. Magna International Inc declined0.4 percent to C$67.66 and Canadian National Railway Cowas down 0.7 percent at C$94.09.
* Great Canadian Gaming Corp rose 12.4 percent toC$38.15 after reporting fourth-quarter results after the bell onTuesday.(Reporting by Fergal SmithEditing by Bernadette Baum and James Dalgleish)