The Lone Ranger wouldn't necessarily need a steam-driven six-stamp mill, mind; you -can- smelt silver in small quantities "by hand" with a great deal of hammer work; there's a work by a fellow named Georgius Agricola called "De Re Metallica"--translated by, of all people, Herbert Hoover, who you may recognize as a former US President--that describes medieval german methods of performing ore extractions and the like.

Bullets don't weigh all -that- much, so it's theoretically plausible that he and Tonto could spend a couple weeks on a particularly rich seam and get enough materials for a few dozen bullets, but that's still going to have some pretty obvious signs, what with having to take care of the tailings and all the smoke from the smelting operation.

See, the ancient method of silver smelting didn't necessarily involve mercury, but involved heating it with lead ore, to give a lead/silver admixture, which could then be refined to get the silver out. Needless to say, that's a health hazard right there--but it's a relatively low-temperature "artisinal" method of silver extraction that one or two people could manage relatively easily.

From "De Re Metallica"
The smelter extracts the slags from the forehearth with a hooked bar; if the ore to be smelted is rich in gold or silver he puts into the forehearth a centumpondium of lead, or half as much if the ore is poor, because the former requires much lead, the latter little; he immediately throws burning firebrands on to the lead so that it melts. Afterward he performs everything according to the usual manner and order, whereby he first throws into the furnace as many cakes melted from pyrites, as he requires to smelt the ore; then he puts in two wicker baskets full of ore with litharge and hearth-lead, and stones which fuse easily by fire of the second order, all mixed together; then one wicker basket full of charcoal, and lastly the slags. The furnace now being filled with all the things I have mentioned, the ore is slowly smelted; he does not put too much of it against the back wall of the furnace, lest sows should form around the nozzles of the bellows and the blast be impeded and the fire burn less fiercely.

Fancy words aside, that's the bit after you light the furnace and get a good solid blast going--stick a big pool of lead on the hearth for the metals to melt into, and then charge the heated furnace with your ore load. "Sows" are, I think, bits of hardened slag--slag being kind of a mixed blessing, as it's required as a flux to extract the ore, but too much will quash your operation entirely.