Welcome to Treasurenet NEWN,
I had suspicions just like you - that Waltz had really just stolen the ore, but as our mutual friend Cactusjumper pointed out, the ores do not match. Gold ores are a bit like fingerprints, in that they are unique, the ore from any mine will not match the ore from any other mine. An expert who examined ore from Waltz said it did not match any known source - it is from an unknown source. This fact really is the only way that anyone can ever prove that they found the Lost Dutchman mine, by having a piece of ore examined by a geologist who could state with authority whether it is a match or not.
Several other mines have also been proposed as the true source of Waltz's ore, including the Mammoth, the Bulldog etc but again the ores do not match. Is it so hard to believe that a gold mine, with an entrance no larger than a barrel and deliberately covered up, could remain undiscovered? I am convinced this is the case.
Highgraders (men who steal rich ore from a mine) were customarily hung at the Vulture, supposedly at least 18 men were hanged there. Waltz himself never worked at the Vulture, so would not likely have had the opportunity to highgrade ore.
Oroblanco