The French had a big advantage in their explorations being at higher latitudes and having rivers to follow. This gave them easy access to water, lots of game to hunt, and significant additional natural food choices (nuts, berries, greens, etc) at certain areas along their routes, not to mention abundant forage for their livestock. Feeding the troops was critical when moving lots of men, animals and supplies through unsettled country. They did get starved out and had to retreat from Des Moines, but that was a military punitive operation in the dead of winter, with heavy resistance from the natives - not an organized steady journey. Compare these conditions with those of the Spanish mining ventures in the Southwest (mostly alleged, by the way, not documented), who had scant water, less game, fewer natural edibles and scarcer grass for the horses. When you look at it this way, the alleged Treasure Mountain expedition in the 1700s seems quite feasible. It seems to me that it would have been easier to make it work with, say, 50 men rather than 300. Think about those French trapping parties who worked the streams all over the Rockies. They traveled light, lived off the land (with the help of natives in some cases), amassed hundreds of pounds of fur pelts (which they cached in numerous locations to be recovered later). A placer mining operation would have been quite similar, it seems to me.
