WARSAW, March 17 (Reuters) - Poland's KGHM, one of the world's biggest copper and silver producers, said on Friday it had put on hold the second phase of expansion of its Sierra Gorda project in Chile.
* State-run KGHM gained control over Sierra Gorda in 2011 when it bought Canada's Quadra FNX, for C$2.87 billion ($2.14 billion) in the largest-ever foreign acquisition by a Polish company
* But Sierra Gorda, which KGHM co-owns with Japan's Sumitomo Metal Mining Co Ltd, has struggled to master complex ores
* Asset impairment charges related to Sierra Gorda sent KGHM's financial results into the red in 2015 and 2016.
* "We have put Sierra Gorda phase two on hold due to changing conditions," CEO Radoslaw Domagalski-Labedzki said.
* "We believe in the Sierra Gorda project, we want to stay on it. I'm an optimist when it comes to the effectiveness of the project," he said. (Reporting by Agnieszka Barteczko; editing by Lidia Kelly and Jason Neely)