Lots of Geology enthusiasts out there. Thats good to see.
Someone just asked me for the link to the SGM Geological maps and it made me realize that I should have posted it earlier.
You have to click on the product that you want first, then it will give you a map index (Map of Mexico) to locate the area you want. They download in .PDF format. The Geological maps are free. They charge for other data like geochemical surveys.
This data has not always been this well organized. Only in the last year or so has it been this easy to access this data. There is a beautiful geological map of all of Mexico. I'd like to put it in my GPS as a basemap.