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amorphous silica. As such, these fluid inclusions are useful monitors of the fluid

conditions during the later stages of the hydrothermal history of the system.

The next most abundant texture (549 samples; Fig. 9) is plumose quartz texture

(Sander and Black, 1988) that shows variable extinction positions when observed under

crossed polars (Figs. 7C,D). This silica texture has also been referred to as “feathery”

(Fig. 7B) or “flamboyant” (Fig. 7C) by Adams (1920) and Dong et al. (1995). This

recrystallization texture is thought to develop from aggregates of fibrous chalcedony with

rounded external surfaces which originate as silica gel (Dong et al., 1995). The silica gel

is originally precipitated when silica supersaturation occurs in response to rapid cooling

and concomitant pressure decrease followed by precipitation of amorphous silica (Henley

and Hughes, 2000). As with the jigsaw texture described earlier, primary fluid inclusions

in this type of quartz do not record the original depositional conditions, but secondary

inclusions provide information concerning later conditions.

Rhombic calcite crystals (Fig. 7E) are observed in 190 out 855 samples from the

Veta Madre. While bladed calcite is thought to be characteristic of deposition from a

boiling solution (see below), an association between rhombic calcite and boiling is less

clear. Fluid inclusions were observed in this mineral but were not recorded because of the

ease with which fluid inclusions in calcite, and other soft minerals with perfect cleavage,

reequilibrate, both in nature and during sample preparation and analysis in the laboratory

(Ulrich and Bodnar, 1988; Bodnar, 2003b).

Colloform texture silica is observed in 193 out of 855 samples from the Veta

Madre. Rogers (1918) introduced the term “colloform” to describe silica with a rounded

or botryoidal form that occurs in continuous bands (Fig. 7G, 8J). When observed under


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