43-101 2008 Technical Report On The La Fortuna Project, Durango ...

4.0 GEOLOGICAL SETTING
4.1 REGIONAL GEOLOGY
The Property lies within the Altiplano Subprovince of the Sierra Madre Occidental (“SMO”) (Consejo de
Recursos Minerales, 1993). The SMO is a regionally extensive Eocene to Miocene volcanic field, which
extends southeast from the U.S-Mexico border to Central Mexico. The Altiplano Subprovince is on the
east flank the SMO and is comprised of Jurassic to Late Tertiary sedimentary and volcanic rocks.
The oldest rocks in the El Castillo region are Late Cretaceous siltstones, sandstones and limestones of the
Indidura and Caracol formations (Table 6), which, crop out in the incised stream valleys at El Castillo.
These sedimentary rocks commonly occur as small windows in the younger volcanics and further east as
prominent northwest-trending ranges. This sedimentary sequence is unconformably overlain by a thick
package of Tertiary volcanic rocks, which characterizes the SMO. The package is comprised of an older
andesite-dominated series and a younger pyroclastic-dominated rhyolite series. The traditional
nomenclature refers to these as the “Serie Volcanica Inferior” (Lower Volcanic Series), and the “Serie
Volcanica Superior” (Upper Volcanic Series) respectively.
The Lower Series in the SMO reaches a thickness of 1,000 m and is dominated by Paleocene and Eocene
andesitic lava and pyroclastic rocks with interbedded volcaniclastic strata. Extensive outcroppings of this
andesite unit are located near San Lucas de Ocampo, approximately 20 km southwest of El Castillo. The
Lower Series is cut by calc-alkaline intrusives that typically occur as dacite-rhyodacite domes, sills and
dykes. These intrusive units are exposed in the road cuts at El Castillo and in a large dome field near the
village of San Agustín. The Upper Series unconformably overlies the Lower Series and is up to 1,000 m
thick. The Upper Series forms a regionally extensive cap throughout the SMO and is dominated by
Oligocene and Early Miocene dacite-rhyolite pyroclastic strata. Precious metal deposits are abundant in
the Lower Series, however, the Upper Series is mostly devoid of precious metal mineralization with the
exception of the Cienaga Mine, operated by Peñoles, in northwestern Durango.
Table 6. General stratigraphy of the El Castillo area.
ERA PERIOD EPOCH ROCK TYPE FORMATION
Alluvium
RECENT
QUATERNARY
Volcanic agglomerate
PLEISTOCENE Basalt Metates
PLIOCENE
Upper Series
TERTIARY MIOCENE Rhyolite, ignimbrites
Volcanics
OLIGOCENE Rhyolite tuff
Upper Series
Volcanic
EOCENE
Andesite, porphyritic with Lower Series
hornblende
Volcanics
CENOZIOC
PALEOCENE
Andesite
Lower Series
Volcanic
MESOZOIC
CRETACEOUS
Siltstone, sandstone,
limestone
Indidura, Caracol
The prominent structural trend in the region is northwest and is delineated by northwest-trending basin
and range (horst and graben) topography with ranges of both sedimentary and volcanic rocks. The ranges
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