SANTIAGO, Oct 30 (Reuters) - Copper output tumbled 10.3 percent in Chile in September from a year earlier, falling for a third consecutive month year-on-year amid less concentrate production from the nation's largest mines.

Copper output for September was 429,309 tonnes, compared to 425,524 tonnes in August when output was down 1.2 percent versus the year ago period, and to 477,828 tonnes in September 2007.

Output from January through September this year fell 3.1 percent compared to the same period last year to 3,984,965 tonnes.

"Once again, the fall in production of copper concentrates was what had the greatest impact on overall national copper output," the National Statistics Institute (INE) said in a statement.

"The fall was due mainly to lower output at big companies."

The INE said molybdenum output in the month of September was 2,437 tonnes, or 40.7 percent less than in the year ago period. For the Jan-Sept period, molybdenum output was down 25.7 percent to 24,501 tonnes.

Chile is the world's largest copper producer and second-largest molybdenum producer.

Output in both metals has fallen off as ore grades fell in the nation's biggest mines.