One way I would try to go about this problem.
Metallic powder containing Au,Ag,Pd, and Cu,Sn, Pb, possibly some other base metals.
If in powder form already it is easier to work with than an alloy.
I would do this in a ceramic casserole dish, leaving the powders in the dish, removing warm or hot liquids as needed after powders settle well, trying to complete whole process as much as possible with powders remaining in dish throughout the process.
Incineration can help, it can help to oxidize some of these base metal powders, making some easier to dissolve and others less likely to be dissolved by acids.
After incineration, a boil in HCl will dissolve many of the base metal oxides, boiling water washes will help to remove soluble salts of many of these base metal chlorides.
(Test the HCl washes with SnCl2 and DMG for value, precipitate them with copper if needed).
Notes: The lead chloride is soluble in boiling hot water, but crystallizes out in cold water as a white precipitant. silver chloride is an insoluble white powder, but the cottage cheese can take time to settle (giving it time to settle in the hot solution you can leave it behind as you decant the dissolved lead in the hot water washes), you may pick up some Pd in these washes (test for them with DMG).
Wash remaining powders with a caustic solution of sodium hydroxide, just strong enough to neutralize the powders and convert any base metals to oxides and hydroxides,(also converting silver chlorides to silver oxides), good stirring is important, crushing lumps of powders, letting the powders settle and washing with boiling hot water, to remove NaCl salts that form, from the chlorides that were involved previously in the powders, repeat washes till clear, but not losing the valuable powders as you decant the salt water solutions.
With the washed powders in the dish to dry on low heat, raising the heat only as needed to dry, checking on them and adjusting heat as needed to prevent bubbling and splashing of powders or values, note even after powders are dry they can sometimes convert to a fuse syrup again and splash as bubbles of the syrup pop, if this happens lower heat so as not to boil this syrup, until powders dry again, you can raise heat as needed as powders dry, they may need crushed again if the fuse into lumps or a cake, a ceramic pestle can crush them easily if done before the cake is cooked hard, when dry on high heat, and with a torch, roast the crushed fine powders to a glowing red hot, to drive off the previous acids as gas. keep stirred well to expose to air or oxygen during the roasting process, when red hot you can remove heat and stir a little more to give them plenty of oxygen.
Lower heat to cool; now these powders can be treated in a dilute nitric acid solution without problems of the tin.
I think you can figure out the process from here.
Now lets say you do not have nitric acid, so lets look at proceeding without it:
we can use HCl and 3% H2O2 to remove some more copper or any remaining base metals, solutions color is a good indicator we are dissolving them, we will be converting silver back to insoluble chloride that will need to settle, we may pick up some pd (but much of our pd may be insoluble after forming oxidized Pd in the roast above (always a good idea to check solutions anyway).
After removing base metals with our copper II Chloride leach, copper or any soluble base metals iron or other base metals, we can wash remaining powders well, (note: sometimes if we use heat in this process we can form insoluble iron hydroxide red rouge powders, these are not a problem as they become insoluble to acids), good hot water washes will help remove lead chlorides in the hot wash.
Now we can be left with our gold powders, mixed with some silver chlorides, possibly Pd or Pd Oxides (which may be hard to get to dissolve into solution), from here we can go to our HCl/ NaClO (sodium hypochlorite, bleach) leach to dissolve gold, any palladium that will, and leave most of the silver chloride and insoluble Pd oxides or insoluble PGM with our sand in the dish...
again you can figure it out from here.
