Valcambi SA has resigned from the Swiss gold refining association due to “irreconcilable differences” between the two.

The world’s top gold refinery left the Swiss Association of Precious Metal Manufacturers on Thursday, according to a statement on Friday from the industry group. Following discussions between Valcambi and the ASFCMP, it became clear their were irreconcilable differences and continued collaboration was no longer an option, the statement said.

Valcambi didn’t immediately respond to an email seeking comment. The refinery has come under increased scrutiny in recent years over the sourcing of its gold.