for rock to make it to powder I use my cement mixer filling it with different sizes of rock up to larger than a fist add some water keep wet so poder doesnt stick to rock and cake on sides then run it, fine screen mud slush to bucket, returning gravel to mixer adding more rock and water,
get some of that coragated black plastic sewer pipe split it in two
Half longwise set in wood frame to make a sluice angle it run your mud with water through sluice box. trapping heavy's, roast them >700deg F, coal fire and tin works about an hour to remove sulfides or acid from rock Caution dont breath gasses, then you can process them as you choose,
what makes you think this gravel is woth processing?
activated charcoal in bags will collect gold from streams or rivers as will charcoal filters collect gold from wells, or solutions, (if its there), mining timbers are Known to suck up gold from the acidic water in mines, old timbers usually are burnt to recover it,carbon dead wood is one theory why rivers have nuggets and the mountain that the gold came from (in hard rock) the gold is microscopic not visible to the eye.
when running a sluice I dont care about what small floating gold that throughnot to enough to fiddle with would have more in black sand recoverd , unless you were only processing surface sand then would need someway of slowing flow from the dredge for a charcoal bag to do any good (side trough sluice) but if that was my problem then i would take copper sheets clean with nitric acid wipe on distilled mercury to plate copper wipe off excess now a slow flow of your gold sand will almagam and stick to the copper Hg plates, but with miners moss and a baffle in the header box and propper set up of dredge your gonna get most all gold, and there are alot of other options long toms,ect ect,