UPDATE 1-Mexico silver output up 18.6 pct in May

* May silver output 420,629 kg * May copper output up 4.2 pct to 35,911 tonnes July 31 (Reuters) - Mexican silver output jumped 18.6 percent in May from a year earlier to 420,629 kg, the National Statistics Institute said on Tuesday, as strong prices continued to spur production. Mexican copper output rose by 4.2 percent to 35,911 tonnes in May as production increased at the massive Buenavista mine -- formerly known as Cananea -- following a three-year strike that ended in 2010,