Many people think biotite is only black, but it can be black, dark brown, and even green yet! ) It's got a lot of iron in
it, the green cast can be from chromium, and of course biotite mica itself is a silicate. Logically, iron and silicates are often associated with gold.
In prospecting field work, and being a persistent type, I always looked at it this way, gold is where you find it, and it's most likely to be found in contact areas. Where there are contacts showing slickensides and schistose rocks which are often associated with gold ( and/or perhaps sulphide mineralization with shearing) , there most certainly can be biotite in those structures, so why not use the reverse association as well? It's a good indicator.
I would guess that direct gold/biotite association is more likely found where there is physical metamorphism caused by shearing, as opposed to gold solutions being injected into biotitic structures after-the-fact ? Maybe in the long-term scheme of things-- D-2 and even D3 events ( is there such a thing as a D3?) , maybe that could happen too ?
One of my old low-grade gold finds had a band or dike of rotten biotite (maybe a foot across) between silicate/quartz gold-bearing rocks and a structure, (I interpreted it as foot rock) that had sections full of garnet ( the garnet-bearing layers were metamorphic rock). The adjacent biotite-bearing rock did carry some gold.
That's about all I know about gold and biotite, I've probably forgotten more than that I imagine..LOL
It's interesting that prospectors might overlook biotite as a barren mineral , it's not really very exciting stuff, but if you happened to find a big enough deposit, the vermiculite industry would be interested in it.
That's about it, how's the weather down there? It quieted down here, about -10 C,,,,stopped snowing, not bad for Nov. 27th.