CANADA STOCKS-TSX closes lower on weakness in mining stocks

(Adds details, activity, updates prices to close) TORONTO, Feb 20 (Reuters) - Canada's main stock index closed lower on Tuesday, following a mixed session, with weakness among gold mining stocks offsetting gains for the energy group. The Toronto Stock Exchange's S&P/TSX composite index .GSPTSE closed down 13.2 points, or 0.1 percent at 15,439.44. Just three of the index's 10 main groups ended lower. * The materials group, which includes precious and base metals miners and ferti