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and thus represent the physical and chemical conditions in the system at the time

trapping. FIAs may be composed of primary inclusions trapped during precipitation of

the host phase, or may contain secondary inclusions that are trapped along fractures in the

host phase at some time after the mineral has formed. FIAs in samples from the Veta

Madre were further classified as containing either only liquid-rich inclusions with

consistent liquid-to-vapor ratios, or containing coexisting liquid-rich and vapor-rich

inclusions with a broad range in liquid-to-vapor ratios (Fig. 6). Finally, the

homogenization temperature of the fluid inclusions was estimated based on the liquid-to-

vapor ratio in the fluid inclusions at room temperature (Bodnar, 1983) (Fig. 6).

Fluid inclusions in the Veta Madre, and in all epithermal deposits, must be

interpreted with caution because much of the host material was originally deposited as an

amorphous silica phase or as fine-grained chalcedony and has since (re-) crystallized to

produce coarse-grained quartz. Primary-appearing inclusions in such samples are unlikely

to record the original formation conditions (Bodnar et al., 1985; Sander and Black, 1988).

In this study, only secondary inclusions that clearly crosscut quartz crystal boundaries,

and therefore were trapped after the quartz (re-) crystallized, were studied in quartz that

showed textures indicative of original precipitation as an amorphous or fine-grained

phase, as described below. Similar arguments apply to fluid inclusions in replacement

minerals, such as quartz replacing lattice-bladed calcite.

Silica and calcite textures:

Previous workers have shown that silica and carbonate phases in the epithermal

environment can have highly variable and sometimes diagnostic textures (Figs. 7 and 8)

16

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