Later igneous intrusions or the ordinary ground-waters may bring in minerals which locally enrich ores under anamorphic conditions, but these are relatively minor effects.
"The Economic Aspect of Geology"
C. K. Leith
On the other hand, the same general kind of anamorphic processes, when applied to coal, result in concentration and improvement of grade.
"The Economic Aspect of Geology"
C. K. Leith