Paul,
I think it may be more of a difference about the definition of halfbreed. Personally, I think of them as the "nuggets" with more than about 25% silver for a "classic" halfbreed. Specimens from the underground I would classify as a silver and copper specimen, not a halfbreed. I think that classifying a copper specimen with a spec of silver on it as a halfbreed is just marketing hype.
Silver is later in the paragenetic sequence as evidenced by the short range of diffusion of elements where the two metals are in contact.
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