The Teotihucanos, the Toltecs, and surely the Aztecs as well, also had access to copper mines, in Michoacán in southwestern Mexico, at the ancient city of Xiuhquilan:
These metal-working Indians were extremely skilled. It is well known that it is much harder to work and shape copper than it is gold. The Indians not only knew to melt and hammer native metals into shape, they also knew how to locate ores, mine them (by open pit or shaft mining as appropriate) and smelt them. By 900 AD they knew how to reduce both carbonates (a relatively easy process) and sulfates (a much harder one) in order to extract the metals.

Metallurgical tests of artifacts and slag from waste tips have shown that the Indians of Michoacán even produced several alloys of copper, and were able to color them. These alloys included various bronzes and tumbaga, a mix of gold with copper and usually some silver. Tumbaga looks just like gold, leading the Spaniards to believe that there was far more gold than there really was.

http://www.mexconnect.com/articles/1238-did-you-know-lots-of-real-aztec-gold-was-only-tumbaga
Indeed:
The P'urhépecha were skilled weavers and became known for their feathered mosaics made from hummingbird plumage and precious stones. With time, these gifted people also became skilled craftsmen in metalworking, pottery, and lapidary work. In the Michoacán of this pre-Hispanic period, gold, copper, salt, obsidian, cacao, cotton, cinnabar, seashells, fine feathers, wax and honey were abundant and quickly became highly prized products to the Spaniards.

http://en.wikipedia.org/wiki/Michoac%C3%A1n

After the Aztecs settled in central Mexico, in the 13th century AD, the rise of the Aztec Empire corresponds completely to the rise of the Mississippian Culture, including Cahokia*.

* http://www.ohiohistorycentral.org/entry.php?rec=1282&nm=Late-Prehistoric-Period

This seemingly suggests, that the Aztec trade routes reached as far afield as northwestern Mexico, southwestern USA, and even southeastern USA, so that the rise of the Aztecs probably powered the resulting rise of the Mississippian Mound Builders. This perhaps represents some sort of cultural recovery, after the global climate collapse of the 13th century AD*.

* http://www.archaeologyfieldwork.com/forums/viewtopic.php?t=6156

[color=gray:]Why would the Aztecs rise, even as Europe & Viking Vinland began plunging into the Little Ice Age ??

You last set of assumptions is making connections that may not be valid (i.e. Aztec development pushed Mississippian). It is also quite likely that either or both could be independent responses to increased times of stress - but not related to each other.

[quote:="Widdekind"]Why would the Aztecs rise, even as Europe & Viking Vinland began plunging into the Little Ice Age ??

Funny thing the climate, its changes are not uniform. A drying event along a certain latitude, say around the Great Basin/Four Corners region of the US, could also include a deep cooling trend along northern lats of Europe, such as england, and increased rainfall in lower lats, say along the valley of Mexico.

A classic example to model would be the ENSO modulation. When we have an El Niño event on the West Coast the SoCal area often receives much more rain than average, while the PacNorthwest receives a below average amount. Just imagine what would happen when this change sets up for decades rather than once every decade or so. Just for starters electricity in the west would be much more expensive, and water supplies would be just about non-existent. The West Coast (esp. the southern portions) would see dramatic changes, if not a certain amount of collapse.