The operator reported that the helicopter was being used to sling construction equipment to a remote mine site located in an area of steep mountainous terrain. After the pilot placed the external load at a prearranged site next to the drill pad, he maneuvered the helicopter to a landing site in an area of rock-covered terrain to disconnect and stow the lightweight Kevlar long line that was still attached to the helicopter’s cargo hook. After a successful landing, the pilot reduced the engine throttle to ground idle, locked the flight controls, then exited the helicopter with the engine and rotor still operating. As the pilot coiled the Kevlar long line into the helicopter, a portion of the line became entangled in the rock covered terrain, and the pilot pulled hard to free it. As the Kevlar long line suddenly released from the rocks, the recoiling line became entangled in the operating main rotor blades which resulted in a severe main rotor imbalance. As a result, the main rotor gearbox and entire main rotor system were subsequently ripped from the helicopter. The helicopter sustained substantial damage to the fuselage, tail boom assembly, and main rotor drive system. The pilot reported that there were no preaccident mechanical failures or malfunctions with the helicopter that would have precluded normal operation.