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