The first Metallogenic Chart of Mexico has been prepared in compliance with the Metallogenic Chart of the World Project and in keeping with the North America Subcommittee commitments.

It portrays the location of mines and/or mineral districts, and through special symbols and colors from an ample and explicit legend, shows the type of ore deposits, their age, and depositional environment, etc.

Thus, on the basis of mineral-deposit environment, the author tentatively proposes to divide the Mexican territory into six metallogenic provinces. (These later may be subdivided into subprovinces and smaller units.)

1. The Baja California province, on the north, with an approximate surface area of 92,000 sq km, is made up of one or more very large granodioritic batholiths, vast areas of metamorphic rocks of undetermined age, and restricted volcanic-neck areas. The southern part shows extensive andesitic and ignimbrite flows as well as silicic and mafic intrusive rocks.

2. The Sierra Madre Occidental province is a 266,000-sq km area of volcanic rock intruded by granites and granodiorites and subordinate mafic rocks. Mineral deposits are present as veins in the intrusive

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and extrusive rocks. It is assumed that the intrusives were the origin of the mineral deposits in the volcanic rocks. Some mineralization occurs in Cretaceous limestone by contact metasomatism.

3. The Sierra Madre Oriental province, a 379,000-sq km area, occupies the great Mexican geosynclinal folded belt of Laramide age. Intrusive rocks and some volcanoes establish the metallogenic processes, which are mostly by contact metasomatism and vein filling. Metallogenesis appears to be of late Tertiary age.

4. The Sierra Madre del Sur area extends for 114,000 sq km from the State of Michoacan ESE toward the State of Oaxaca. Geologically it is very similar to the Sierra Madre Occidental province, but seems to constitute a different block of generally lower topography and with more sedimentary Cretaceous limestone. Limestone remnants overlie extensive intrusive rocks. Some are mineralized. Large areas of metamorphic rocks seem to carry mineralization of Precambrian or pre-Paleozoic age. Mineralization in the volcanic and sedimentary rocks seems to be of late Tertiary age.

5. The Mesa Central province extends over an area of 105,000 sq km from the northern edge of the volcanic axis on the south to northern Zacatecas and to part of the Durango on the Central Plateau. The geology is made up of very vast flows of andesite in the southern part, and predominantly rhyolite in the northern part. The thick sequence of volcanic rocks shows low-temperature and pressure mineralization. This is specially noticeable where intrusives, as in Guanajuato and Pachuca, affect the extrusive rocks. Mercury and fluorite deposits are abundant.

6. Eje Neo-Volcanico, a 190,000-sq km volcanic zone or volcanic chain crosses the continent from Bahia Banderas, in the vicinity of Puerto Vallarta, on the Pacific coast, to the Sierra of San Andres Tuxtla on the Gulf of Mexico.

Some authors have postulated the thesis that a large transverse fault crosses the continent, as a continental expression of the Clarion fault. The author's recent paper on ERTS-1 image interpretation does not show evidence of this effect.

The famous "Taxco," "Pachuca," "Angangueo," "El Oro and Tlalpujagua" silver deposits are along this volcanic belt. Much more research on their origin is needed, and the Metallogenic Chart of Mexico will enhance this research.

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