On September 6, 1999, at 0830 mountain daylight time, a Bell UH-1H helicopter, N4085L, was substantially damaged during impact with terrain following an autorotation near Silverton, Colorado. The commercial pilot, the sole occupant on board, received minor injuries. The helicopter was being operated by High Country Helicopters, Inc., of Montrose, Colorado, under Title 14 CFR Part 133. Visual meteorological conditions prevailed for the local long-line cargo flight which originated from a staging area approximately 20 minutes before the accident. No flight plan had been filed. The pilot reported that this was his first trip of the morning, and he was transporting approximately 300 pounds of construction materials which were to be used to close a mine entrance. He said that the load was on the ground and he was hovering while ground personnel were disconnecting the cargo net. He said that one of the ground team radioed him and reported that "fire and smoke was coming from your tail pipe." The pilot said that he noticed that the rotor rpm warning light was on, and the engine out lights and horns had been activated. He lowered the collective and autorotated to the ground, but the main rotors impacted terrain. The tail rotor and its associated gear boxe separated from the aircraft. The tail boom was bent and pushed forward into the transmission. Aircraft records indicated that the engine was overhauled by the U.S. Army 791 hours before the accident. The maintenance records further indicated that the engine came from a civilian overhaul facility and was installed in N4085L approximately 21 hours prior to the accident. The Colorado Department of Minerals and Mines reported the mine entrance elevation was 11,500 feet. The density altitude was calculated to be 13,000 feet.