If your trying to smelt black sands your dealing with mostly Iron ore, some magnetic and some nonmagnetic, the iron is in various states, roasting will help to improve the ore somewhat.
driving off acids and putting the ore in a better state for oxidation (or reduction).
The flux would need to be able to get the iron and other base metals to form a slag glass, the gold would need to be able to melt and combine into one mass of molten gold, fluxing has to be chosen to do several jobs at once, oxidize base metals to form slag glass of those metals, reduce or not oxidize the metals you wish to keep elemental and to recover as a molten mass or button, to increase the fluidity of the melt so that the fine gold particles can combine and melt together, and not just become trapped in the glass slag as small beads.
As I see it, smelting is a mechanical and chemical process that is using high temperature and the chemical reactions of the flux and ore to accomplish a task. To oxidize and form a metal-silica glass out of the unwanted metals and to reduce the wanted metals into a molten lump of metal.
Like other chemical reactions the type and state of the ore, the chemicals used (flux) and processes used, will make a big difference in what happens with the chemical reaction.
Will smelting be a good option to recover your gold from black sands, this is a hard thing to say if it will work or not, (basically impossible to judge from here, where I cannot even see what you have), there are many variables here, pretreatment like roasting, how much gold to the amount of black sands, would you need a collector metal involved in with your flux, to collect the gold in the melt. or would the gold to iron ratio be such that you can flux the gold to melt a molten combined lump of metal, and oxidize the iron into the silica glass.
What I can say without seeing what you have is that the better you can concentrate your gold, (separate the gold from the unwanted material or ore), the better you will have success with any type of recovery method you choose.
I would concentrate my time and energy on concentrating the gold, using the gravity methods, to separate the black sand as much as possible from the gold first before using any method of recovery either with chemicals or furnace.
The less trash you have to deal with, the easier the recovery of your gold will be, I would eliminate as much trash as possible mechanically as I could, before trying to use the furnace or chemicals to attempt at removing more trash. Or to recover the gold in proceeding steps, So your gold is not just lost in the melt of the trash, (or leaching process of the trash whatever the case may be).
If you melt gold with iron you can form an alloy, although the iron would fairly easy oxidize.
In the smelt with flux you can reduce or oxidize the iron from the black sand, the flux chosen and the properties of the ore involved, will make a big difference in what happens in the smelting process.
If you melt a lot of iron with very little gold your gold could be lost in the iron and the slag of the melt.
Smelting uses chemicals (flux), to oxidize or reduce metals, the chemicals (ingredients of the flux) have to be chosen for the chemical reaction you need to achieve, a collector metal may be needed to collect the gold from the melt, or you may be able to use flux to oxidize the metal into the slag away from the gold, the flux also has to improve the Flow of the melt, so that the metals can move or flow to combine in the melt and separate from the slag which is lighter.
Smelting like assaying ( or other forms of recovery) is a science and art, like with other chemical reactions you cannot just put a bunch of junk in a crucible with some chemical flux, or pot of acids and get out what you want without having a fairly good idea of how to do it, pretreatments can be very important, the higher the gold to trash ratio the better, the better you understand the reactions involved, and chemistry needed, the better you know how to do tests to see if changing the chemistry of the reactions, which may be needed to recover the gold from the process, the more skill you have in this...
You may get lucky by buying some ready made flux recipe, (from say action mining) and dumping it in a crucible with your black sand, and forming a gold button, but it may or may not work, you may or may not get all of the gold from the sand, or you could lose it in the slag, there are many variables involved.
Concentrate the gold first, separate it from the black sands mechanically. This would be my first goal, with less trash involved, what ever method you choose after that would make later recovery and refining processes of the gold much easier.
