I need a solder that melts at the lowest possible heat because I am trying to solder a thin sheet of gold to silver and don't want the gold to melt. 

There is easy-easy but I'm not sure it is the best choice for gold. Heck it isn't even the best choice for silver as it has so much zinc in it. I don't work gold but I think that there are gold based solders made for this application.

 perhaps think about keum boo instead of soldering

Aspin, check out the first soldering thread, there is a product from Brownell's gunsmithing that I use sometimes when heat is an issue, It flows @ 475. all the info is in the thread.

mtlctr