* TSX down 28.64 points, or 0.2 pct, at 11,492.54 * Materials lead losses on lower gold, copper prices * BoC holds interest rates, maintains hawkish tone By Jon Cook TORONTO, July 17 (Reuters) - Canada's main stock index slid in volatile trade on Tuesday, led lower by mining shares, after Federal Reserve Chairman Ben Bernanke dampened expectations of further stimulus measures and the Bank of Canada left interest rates unchanged. Bernanke said the Fed stands ready to offer ad