I often find with multi metal resources is that in maximizing processing of primary metal a secondary metal is reduced in extraction percentage. Even a further reduction occurs if there is a third metal involved.
Simplified example:

Nickel alone might be processed with a 90% efficiency.
Same goes for copper, platinum and gold, etc.
If found together in an ore, nickel is still processed to that 90% efficiency BUT because of added chemistry used in extracting the nickel, the copper extraction percentage is compromised to 85%. Further, platinum to 80% and last gold to 75%.
All the added chemicals at each stage interfere with next metal's extraction efficiency.