You don't need flux to melt, you need flux to smelt. That is the difference between Melting and Smelting.

A good Flux lowers melting point and removes impurities. Otherwise melting without flux, you end up with a block of AgAuZn

See http://koldun.tripod.com/bcftp/docs/melting-smelting.htm

There is a whole bunch of difference between melting and smelting. We are gonna try to put this subject to rest. It is simple but I seem to have a big problem trying to explain the difference. I think we all understand pretty well what it means to melt something. That is what happens when you put a glob of hog lard in a frying pan and stick heat to it. It is what happens when you touch a piece of solder to a hot soldering iron. It is the changing of a solid to a liquid by the application of heat. When the heat is removed the melted material returns to it's solid state. It's chemical content has not been changed. O.K., maybe if it was a powder when you stuck the heat to it, when it cools, it will probably be just a lump of something that is not finely divided. It is still the same stuff with the same composition. If it was 20% Zinc when in a powder form, it is still 20% Zinc after it cools to a solid lump. Are ya with me? Yeah, I think so. That ain't too hard to grasp, even for us miner types.

Now, there is a thing called "smelting". That is a metallurgical term. It really has nothing to do with "melting" other than the fact that both things have to be done at high temperatures where the material in question is probably a liquid. Just a coincidence.
Lets try explaining the difference with a "fer instance". Fer instance, if I had 10 grams of Gold. Hell, we all know it ain't all gold. So lets say it is 20% copper and 80% Gold. If it happens to be in a powder form and we melt it down to a nice lump of metal, it is still 20% Copper and 80% Gold. Right? Don't want to lose anyone this early.

O.K., so you are a purist and don't like the idea that your beautiful lump of metal has all that nasty Copper in it. Won't do no good at all to melt it again. You just end up with the same stuff. So, being a Basement Chemist, how are you gonna skin that cat? Well, one way is to smelt it. So how does that differ from melting? Physically, not much but chemically it is like being on a different planet. So what is this great difference that we are going to utilize? We are simply going to subject the molten metal to a highly "oxidizing" environment. Hey, you know about redox. You also know that it is very difficult to "oxidize" Gold but pretty easy to "oxidize" Copper. That is why Gold stays nice and shiny but Copper turns green pretty quick. Well, that is what we Are gonna use to clean up our gold a little. We only have to do two things. First we have to convert the Copper to some salt that will not form an amalgam with Gold. Second, we have to give the Copper salt some way to "escape" from the Gold. Hey, that ain't no big problem for a Basement Chemist.

O.K., now an easy, cheap, way to this is simply mix the Gold/Copper with an "oxidizing agent" such as Sodium Nitrate. That will get the job done. However, the Copper is sort of "locked up" inside the Gold so the Nitrate can't really get at it. Hey, we solve that problem by melting the whole mess. Now the nitrate can contact the Copper and oxidize it to Cupric Nitrate. Yeah, yeah, I know you all knew that but I had to say it.

Well, that solves the problem, right? Remember, we still have to get the Gold and Copper Nitrate separated. Otherwise, it will just be sort of a mixed mess. Hey, we can do that too. What we gotta do is provide a second liquid that will dissolve the Copper Nitrate but not the Gold. And it has to be a liquid at the temperature of molten Gold. Water don't last long at that temperature. So what we are gonna do is use a hi-temp liquid. Molten glass works great. So you just got to toss in some white, quartz construction sand, a smashed up disposable beer bottle, or a smashed piece of yr wife's best crystal (if you got the balls). Any of that stuff will work.

So now we got the nitrate, we got the silica (beer bottle). We heat that up with the Gold/Copper until it all melts. About 1,100 deg C or 1,800 deg F. Now all we have to do is pour it into some sort of mold and let it cool, (you can use that cast iron breadstick pan that yr wife has stashed away somewhere), bust off the glass containing the Copper and we got a button of pretty good Gold.

But, wouldn't ya know it. Sumpin went to hell. The molten goo was so thick it just wouldn't pour out of the crucible. Maybe you should use a different brand of beer bottle. Well, before we go to that extreme, lets see if the wife has some ol 20 Mule Team Borax lurking around the kitchen/utility room somewhere. Now if you just toss a teaspoon of that stuff right in the mix with her smashed crystal, you will find that when it melts, it will be much thinner and you can pour it much easier.

Now you just bash off the glass and you will find that your Gold is much cleaner than when you started. Now it is probably 95% Gold and 5% Copper. I can't guarantee the numbers, but something like that.
So, fellow scientists, that is the difference between Melting and Smelting.

You probably can melt gold concentrate if it is reasonably pure. Typically a flux is added to even re-melting gold bullion to reduce volatilisation and if it is in a finely divided form the lower melting point flux as a liquid provides better heat transfer to the solids. In its most basic form a borax cover is often all that is used. Other flux reagents are added to prevent oxidised compounds fuming off by combining with these in a liquid from. You mentioned the concentrate had low lead levels but by not using a flux there is potential for most of what lead is there to fume off causing possible safety issues. The other thing fluxes do is remove troublesome elements that can interfere with producing a "clean" bullion bar and  they also reduce gold losses.  An example of this can be production a matte or speiss that forms a separate layer on top of bullion bars and contain significant amounts of gold. The aim of smelting on site with fluxes is to produce a bullion product with minimum precious metal losses that is in a form that can be readily sold to the refiner. The flux use is to ensure this can be done with minimal risk and produces an acceptable produce and is not to refine the gold as such. It may do this by default by removing deleterious components in the concentrate.