The helicopter was being used on a seismic data acquisition operation. While landing by a road in mountainous terrain at an elevation of approximately 8,400 ft, the helicopter was observed to approach to a hover over the roadway. A considerable amount of dust was blown from the rotor downwash causing personnel on the ground to cover their eyes and look away. The helicopter was next observed to pull up and away from the ground personnel, then move up the hill (beside the road) and turn right to face the hillside. Ground personnel reported that it then made an unusual noise, spun around, began to whip back and forth, then descended into trees in an uncontrolled manner. After impact, it rolled down the hillside and came to rest at the road. The pilot was injured and could not remember the occurrence. Ground personnel thought the engine lost power, but no preimpact failure was found. Several circuit breakers were 'out' but two of them (boost pump) would normally have been pulled after landing. Wind was from the south at 10 knots. A witness said a gust blew dust back toward the aircraft while hovering.