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by polishing the cut surface with 320 micron grit to eliminate the macroscopic

imperfections produced by cutting the sample. Next, the sample was mounted to a glass

slide with epoxy adhesive. The final process consisted of cutting and grinding the

sections to 75-100 micron thickness (Van den Kerkhof and Hein, 2001) in order to study

the fluid inclusions and mineral textures.

In addition to preparing thick sections from the samples, one half of each cut

sample was assayed for gold (Au), silver (Ag), copper (Cu), lead (Pb), zinc (Zn), arsenic

(As) and antimony (Sb).

Petrography

Goldstein and Reynolds (1994) describe the technique for observing unpolished

thick sections (“quick sections”) under the microscope that involves covering the surface

with immersion oil to improve the optical properties of the sample for viewing fluid

inclusions. The immersion oil with an index of refraction equivalent to that of the mineral

fills all cracks and imperfections. Because our samples consisted mostly of quartz we

used oil with an index of refraction of 1.515.

Samples were examined using a petrographic microscope, starting at low

magnification and preceding to higher magnification. The first task for each sample was

to identify the minerals and to classify the textures of quartz and calcite if present (every

sample contained quartz and about one-quarter also contained calcite). Next, the sample

was examined systematically to identify fluid inclusion assemblages (FIAs) and the types

of fluid inclusions in each FIA were noted. An FIA represents a group of fluid inclusions

that were all trapped at the same time (Goldstein and Reynolds, 1994; Bodnar, 2003a)

15

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