THE APPLICATION OF MULTISPECTRAL IMAGING TO GEOLOGICAL MAPPING OF THE SIERRA DE GUANAJUATO, MÉXICO

The application of multispectral imaging to remote regions such as the Sierra de Guanajuato, makes the construction of detailed geologic maps possible and economical.

The excellent exposure of the Mesozoic pre-cretaceous volcanic-sedimentary basement and the Tertiary volcanic and intrusive rocks made the use of Landsat ETM7 data desirable in this study, this sensor captures panchromatic images with 15 meters resolution and two thermal images, covering an approximate area of 7500 Km².

Landsat ETM7 data were used to create a spectral ratio images of bands ¾, 4 and 1, displayed as RGB and combination of bands 741, 731, 457, 531, with algorithms created with ERMapper 6.4, these images were interpreted in the context of available geological maps, field work, lithostratigraphic and new age data. Thanks to the combinations of bands and the algorithms created we can highlight variations of color, texture, tonality and differentiate the different types of rocks and structures that exist on the surface. Band 7 allows us to map hydrothermal alterations associated with Ag-Au deposits.

Geological setting of one of the world-famous silver-gold deposits type is also presented. The silver-gold vein system occurs in the southeastern portion of the Sierra de Guanajuato, which is a 100 km long, N45°W-trending mountain range in Central Mexico.

The rocks exposed in the Sierra de Guanajuato and its surroundings can be divided into four broad stratigraphic successions: (1) the “Basal complex” which includes Mesozoic rocks and an early Eocene granitic batholith, (2) a thick (> 1000 m) red bed sequence resting unconformably atop the basal complex, which was deposited in the lapse between 49 and 36 Ma, (3) a thick, faulted and tilted, Paleogene subaerial volcanic succession, and (4) Immediately around of Sierra de Guanajuato are exposed a widespread, flat lying, sand and gravel deposits accumulated in a continental basin.

These sediments (4) contain fossil vertebrate faunas with ages between late Clarendonian and Blancan. Southeast of El Bajío fault system sand and gravel deposits, partially covered by basaltic andesites crop out. A similar relation between gravel deposits and mafic lava flows is seen at the top of Cerro El Cubilete, which is one of the highest point in Sierra de Guanajuato.

Succession (1) experienced several shortening events during the Cretaceous and Paleocene, whereas all successions were extended in several deformation pulses during the Cenozoic.