Fishstick, as you and I both know, the "halo effect" is in regards to iron which breaks down and "scatters", dissipates, or leaches out minute particles of iron oxide from an iron target so easily and quickly, and does not become a target of other metallic bases in a general sense, because other metals take more than decades to break down that far, save for copper which breaks down a tiny bit more than other precious or semi-precious metals. With those other metals like gold, silver, brass, copper, aluminum, etc, it would take up to 400 years for enough trace particles to register on even a super-sensitive detector like the Gold Bug Pro or the Goldscanner Pro, and that would be so minimal that it may not register at all anyway. That said, the Compasses would "read" the conductivity of any of the trace iron a bit better, but the iron "halo effect" is never as great as most say it is anyway, so therein lies your answer. A target is a target, is a target, and the more metal it gets to read, the better the signal, regardless of which metal it is..