I've read this thread, and want to say up front, In so far as the Aztecs, I know squat - except what the movies tell me.
However, I don't believe you need to know the Aztecs, to know where all that gold is. Several thoughts come to mind;
The picture of natives collecting gold from the river is interesting. It would however be very tedious and not net you a whole heck of a lot. Most if not all that would be placer type powder and flakes.
The picture would be consistent with my next thought, that I disagree with some here on one point - You do need some level of understanding of metallurgy, to mine and process gold and silver. Hard-rock mining? how are you going to accomplish this, without iron or bronze? Even bronze is fairly soft. Certainly wood won't break rock very well. When you smelt and homogenize the gold or silver, how the heck are you going to do that without a bronze or iron cauldron? A stone cauldron you say? How are you going to carve that without strong metal? A natural rock bowl for a cauldron? That would be very tedious, my friend. IMO to process these metals, you need to first have a grasp on iron and bronze craft with which to make tools. BTW, gold melts at 1850 F. The Aztecs stored their gold in places like AZ, UT or NM? That's a stretch. How many people would it take to carry tons of gold and silver that distance? Way too many, IMO.
Here's the conclusion I came to. The Spaniards were explorers, commissioned by the king/queen of Spain. That was their
livelihood. What was their first thought, when they reached the shores of central America and were met by people wearing deer-skin thongs and living in adobe huts? Oh crap - these folks are pretty poor! Of course they're going to make up some spectacular story about how the pyramids were shod with gold! everyone wore gold, even the commoner. Sure, perhaps most had ornamental gold, but as one previous poster said, it was whatever they found in the rivers or in the caves. It was nothing substantial. "Oh! Yes Sir! More gold than you can imagine!" "Of course we need to go back, this is but a sample of what we found. There was too much to bring back all in one trip!" Wouldn't YOU want to keep your well financed job and your reputation? Of course you would! Then, upon hearing the initial stories, more explorers came, and failed to find these magical pyramids clad in gold, so the locals were tortured to get an appropriate answer. Wasn't this about the same time as the Spanish inquisitions? Some how, the locals striped all this gold off the buildings, and now there is none to be found. How much gold have we mined commercially in the US?
Roughly 4,600 tons since 1970.
http://www.goldsheetlinks.com/production.htm
How much is that?
In my estimation, that is a solid block of gold the size of the USS Nimitz.
1x 25 ton stone from Stonehenge is average 13 x 7 x 4 feet <times> 2 (for 50 ton stone) = 26 x 14 x 8. 50 tons per stone is roughly 1.1% of 4600 tons. so I multiplied that org. dimensions by 100 and came to 1300 x 700 x 400 or a better estimation would be roughly 1100 x 450 x 300. A solid piece of gold, the size of the USS Nimitz.
But that's not too much gold, right? think about industrial applications of gold. Computers and electronics, cell phones, dental prosthetics and more. Since 1970. In 40 years with modern technologies and a quantum leap in extraction and processing technology experienced in the 1980s, we've only managed to extract this much gold, in the US. Also it's important to note, the US averages between 10% and 25% of total gold production, year to year.
I digress though. I would think, there is no sudden jump in gold content hidden in the ground, once you cross the border into MX. The border is merely a geopolitical line. I would think the land would generally be as homogenized in Mexico as it is in America, with localized veins of pockets of gold, here and there. How much gold could the Aztecs really extracted from the ground? Not much, IMO. I would hazard a guess that most of it was found laying around, and pounded into crude devices for wear. Yes, the building and buildings and structures could have been shod with gold. I know gold can be extruded into an extremely thin sheet, as it is in many other instances, but even that would require more than rudimentary tools. Again, as another poster pointed out, I believe most of the gold from Mexico and central America is currently on display in European museums as components of other artifacts and crown jewels pieces. If we want to find the gold in Mexico and central America, we better start digging.
Not to try and downplay anyone's dreams, it's just my 2 cents...
,but sometimes it's pretty entertaining for those in the audience. 
cubic feet, so 4600 tons of gold would only fill around 30% of my house which is not all that large.
