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).
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)