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Mine (390 and 414 levels), south Valenciana Mine (320 level), Guanajuatito Mine and

San Vicente Mine (190, 220, 270, 275 and 345 levels) (Fig. 5, bottom). Levels for all

mines indicate depth beneath the surface in meters, relative to the elevation at the top of

the Rayas shaft, which is at an elevation of 1,999 m above sea level. Drill core samples

were obtained from the underground and surface drilling program and historical drilling

(Fig. 5, bottom). At each location one “representative” sample was collected, although it

should be noted that recognizable heterogeneity occurred over short distances along each

level.

The hand samples were cut perpendicular to the vein to produce a section that

extended from the vein wall towards the center of the vein. Then a thin section-sized

billet approximately 13/16 inches x 1 inch (21 mm x 35 mm) was cut from the larger slab

and a “thick” section approximately 75 µm thick was prepared. If the vein thickness was

greater then twice the long dimension of the billet, the billet was cut to include one of the

vein walls. The logic of this approach is that the material closest to the vein wall is the

oldest material in the vein, because these are open-space filling veins and not crack-seal

veins. Thus, even if mineralization at that location occurred later than the time

represented by the sample, the sample is still likely to contain secondary fluid inclusions

containing fluids responsible for precipitating later material. This approach increases the

likelihood of sampling all fluid events associated with vein formation while at the same

time minimizing the size and/or number of thick sections needed.

Thick section preparation was done by an outside vendor and at Virginia Tech.

This study involved only petrographic examination of the samples, thus doubly polished

sections were not required. “Quick plates” were prepared (Goldstein and Reynolds, 1994)

14

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