Manganese Can Cause ProblemsThe Angry Geologist points to the Athena Langtry project for comparison with Sandra and postulates that the poor silver recovery there is caused by small silver grain size, which he claims is also the case at Sandra.
My review of the literature indicates that the presence of manganese can also impede cyanide leaching and that manganese is present at Athena Langtry: See P1
https://athenasilver.com/SRK_43-101-TR_Langtry_348600-010_20120501_FNL_AVM.pdf
But if this is also the case at Sandra, it is not necessarily terminal. I found many patents and other documented work for various pre-treatment options to raise silver recovery above 90%.
Just google " pretreating manganese silver ores"