** Copper miners' shares fall as prices plummet to lowest in 6-1/2 years over concerns of slowing growth and demand in China
** Benchmark copper hit low of $4,381, levels not seen since May 2009
** Freeport-McMoran Inc's shares down 15 pct at $4.58 in morning trading; hit 15-year low of $4.55
** Fitch Ratings downgrades Freeport's Issuer Default Rating to "BBB-" from "BBB" along with FCX's senior unsecured debt
** Fitch also cuts rating outlook to "negative" from "stable"
** U.S.-listed miners: Cliffs Natural Resources Inc down 17 pct, Teck Resources Ltd 7.2 pct
** Toronto-listed miners also fall: First Quantum Minerals Ltd 10 pct, HudBay Minerals Inc 9.7 pct, Lundin Mining Corp 5 pct