UPDATE 1-Miner Fresnillo trims silver output guidance

(Adds details on silver and gold production, forecast)

Oct 24 (Reuters) - Precious metals miner Fresnillo Plc has cut its guidance for silver production this year for a second time, after posting lower-than-expected production for the metal in the third quarter, primarily due to lower than expected ore grades at two of its mines.

The Mexico-focused miner said on Wednesday it was trimming its output guidance to between 62 million ounces and 64.5 million ounces from a range of 64.5 million to 67.5 million earlier.

Silver production rose 6.3 percent to 15.5 million ounces in the third quarter due to the contribution of a new plant, higher ore grades at some of its mines and higher volumes at its expanded San Julián facility in Northwestern Mexico.

However, the third quarter silver production was not as high as anticipated due to lower than expected ore grade at its Saucito mine and slightly lower than expected ore throughput and ore grade at its namesake Fresnillo mine, both in southern Mexico.

Gold production fell 3.5 percent to 225,202 ounces in the third quarter, partly due to lower volume of ore processed at its Herradura mine in Northwest Mexico and lower ore grade and recovery rate at San Julián Veins.

However, the miner raised its full-year gold production forecast for the second time this year. It now expects gold output between 920,000 ounces and 940,000 ounces from its prior forecast of 900,000 ounces and 930,000 ounces, as volumes at its Saucito and Noche Buena mines exceeded targets.

Fresnillo, which listed its shares on LSE about 10 years ago, also said its development project - the Pyrites plant for increasing silver and gold recovery rates is expected to be completed by the end of 2019. (Reporting by Samantha Machado; editing by Patrick Graham, Amrutha Gayathri)