I just found this thread and sorry I didn't find it sooner. It's all very interesting and exciting. Anyway, just thought that I'd offer a suggestion. I saw a post earlier that suggested using mercury to collect the silver, gold and pt groups. The problem with this is, you would have an amalgm of everything. Then, it costs to have it all separated.
Why not take your clay (somewhat dry,) put it in a tank. The tank would have to be either stainless steel or plastic. (Come to think of it, they make plastic water holding tanks,) It shouldn't be hard to build a slow agitator for the tank, then use Nitric Acid to collect the silver. Then, it's relatively inexpensive to electrowin the silver. (I use to help a friend with this process.) Once you drain the acid off to win the silver out of it, the residue in the tank would be, (for the most part,) gold, platinum, the black sand, and mica/silica particals. Since I've never dealt with platinum, I'm not sure if it is attracted to mercury, but I know that gold is.
Obviously, there's no problem in washing the mica/silica away, and the black sand can be collected and saved. If it's magnatite, (and I'd almost be willing to guarantee you that most of it will be,) use a magnet to get it out of the gold and platinum, but still save it. Many years ago, I had a metallurgist tell me; "magnatite forms around a core metal, most often gold or platinum". This is the reason that there are companies that pay good money for "black sand".
Anyway, I hope that this helps a little.
Eagle
Depending on the comparitive amounts, the mixture of gold and silver would be little or no problem, as both are high on the "desirable" list.
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