Have you forgotten about Duval Corporation and it's Mineral Park open pit copper mining property that operated for over 40 years or is your reference strictly referring to primary precious metal mines? I'm guessing the latter. Mineral park concentrates also contained payable quantities/credits of Ag and if I recall correctly Au too. Those payments probably put a healthy dent in the smelting fees and deducts.
For those not in the know:
https://en.wikipedia.org/wiki/Mineral_Park_mine
I, my Mother and my Wife had a long history with Duval. My Mother was the first female employed to work at a Duval owned operating mine site. That was back in the early 60's and she did office work at the Carlsbad, NM potash operation until her retirement.