Yes it's encased in a sulfide it's not encased in black sand. Heat in the presence of oxygen burns off the sulfides.
Sulfides can ruin your whole day. They coat almost everything with surface of sulfide that will prevent you from amalgamating, reacting with cyanide, or dissolving in solutions of halides. If you have a little piece of silver, a little hydrogen sulfide from the local volcano, you will have a piece of silver with a coat of silver sulfide on it. This coat will prevent you from dissolving it in cyanide or any other.
Fortunately for us sulfides are relatively unstable. Want to destroy a sulfide? It’s not too difficult but one that I am afraid most folks ignore. HEAT IT! Almost all sulfides will dissociate with heat. That is, if you heat a sulfide in the presence of oxygen you will boil off the sulfur as either sulfur dioxide or as hydrogen sulfide. If you heat some ore that you suspect of having sulfides present you will smell a rather unique odor...
Sulfides
Although having nothing to do with acids and bases I think it is in order to discuss the problem of sulfides in your concentrates and ore. Since we are talking about cleaning of ores of unwanted or interfering salts we should consider the case of ores/concentrates that contain sulfides. Sulfides (salts of sulfur) are one of the main reasons why sometimes you just cant get the gold to amalgamate with mercury, dissolve in cyanide, or react to any other extraction method. Sulfides are very resistant to chemical conversion to soluble salts. So, we have to do something different. One more way to skin the cat. Unfortunately we will have to resort to heat. The time-honored way to remove sulfides from ore is simply to get it very hot.
The way to do this on a small scale is simply to get a piece of sheet metal. Old corrugated roofing, a piece of "valley tin", any old kind of thin steel. Find a few rocks or other equal put the metal on them. Now build a roaring fire underneath. Put your ore/concentrates on the metal.
Spread it out so that it will heat rapidly and go back to your camper for a cold brew. When it starts to get hot, if sulfides are the problem, you will start to smell sulfur, the odor of rotten eggs. Does anyone but me remember when you got a rotten egg once in awhile? If not, it’s the odor of a catalytic converter that isn’t working just right. Heat the ore/concentrates until the odor is no longer noticeable
