NEW YORK/LONDON, Oct 21 (Reuters) - Copper ended a volatile week on firmer footing Friday, rallying 6 percent in its largest one-day advance since early 2010, as opportunistic buyers stepped up purchases on the eve of a European Union debt-crisis summit. A day removed from its worst one-day collapse in four weeks, copper bounced back strongly to lead all gainers in the Reuters-Jefferies CRB index , as investors grew hopeful for a near-term solution to the euro zone debt crisis. The metal was also helped by Chinese buying signals.
