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The thickness of this unit is highly variable and ranges from 100 – 240 m in the district

(Gross, 1975; Stewart, 2006).

The Chichindaro Fm. (Fig. 4) is the youngest formation in the area and has been

dated at 32 +/- 1 Ma by K- Ar (Gross, 1975; Randall et al., 1994). It unconformably

overlies the Cedros Formation and is composed of felsic ash and flow breccias (Taylor,

1971). Buchanan (1979) reports that the Chichindaro Fm. is pre-mineralization.

Mineralization in the GMD is associated with three parallel, northwest trending

fault systems. The La Luz system is to the north and west of the city of Guanajuato, the

Sierra system is to the east and south of the city of Guanajuato, and the intermediate Veta

Madre system passes through city of Guanajuato (Fig. 2). All samples studied here were

collected from the Veta Madre and along a traverse perpendicular to the vein.

In the study area the Veta Madre dips 35-55° to the SW and varies from hairline

to several tens of m in thickness (Buchanan, 1979). The Esperanza Fm., composed of

dull-black carbonaceous and calcareous mudstones, shales, limestones and andesitic to

basaltic flows that have all been metamorphosed to phyllite, slate and marble, serves as

the footwall of the Veta Madre (Guiza, 1949; Edwards 1955; Cornelius, 1964). The

Guanajuato Fm., composed of red, poor- to well-sorted conglomerate, sandstone and

shale, forms the hanging wall of the Veta Madre (Edwards 1955) (Fig. 5).

The Veta Madre mineralization consists mostly of quartz and silica with lesser

calcite, fluorite, barite and adularia. The Veta Madre is composed of multiple generations

of mostly silica and carbonate deposition and brecciation to produce veins and

stockworks (Stewart, 2006) that are dominated by banded quartz (silica) (Wandke and

Martinez, 1928). The banding structures in the veins are closely associated with

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