F. Goff et Jn. Gardner, EVOLUTION OF A MINERALIZED GEOTHERMAL SYSTEM, VALLES CALDERA, NEW-MEXICO, Economic geology and the bulletin of the Society of Economic Geologists, 89(8), 1994, pp. 1803-1832
The 20-km-diam Valles caldera formed at 1.13 Ma and had continuous pos
tcaldera rhyolitic eruptions until 0.13 Ma. Hot springs and fumaroles
are surface manifestations of a hydrothermal reservoir (210 degrees-30
0 degrees C; 2-10 X 10(3) mg/kg Cl) that is most extensive in fracture
d, caldera fill tuffs and associated sedimentary rocks, located in spe
cific structural zones. Fluids are composed of deeply circulating wate
r of (primarily) meteoric origin that have a mean residence time in th
e reservoir of 3 to 10 k.y. The only component of clearly magmatic ori
gin is anomalous He-3 although it is possible that some magmatic water
, carbon, and sulfur is contributed to present hydrothermal fluids. Ho
st rocks show intense isotopic exchange with hydrothermal fluids. Alte
ration assemblages are controlled by temperature, permeability, fluid
composition, host-rock type, and depth. A generalized distribution fro
m top to bottom of the system consists of argillic, phyllic, propyliti
c, and calc-silicate assemblages. Typical alteration minerals in phyll
ic and propylitic zones are quartz, calcite, illite, chlorite, epidote
, and pyrite, whereas common vein minerals consist of the above minera
ls plus fluorite, adularia, and wairakite. Argentiferous pyrite, pyrar
gyrite, molybdenite, sphalerite, galena, chalcopyrite, arsenopyrite, s
tibnite, barite, and tetradymite (?) have been found at various depths
in the Valles system. Fluid inclusion studies show that these mineral
assemblages formed from liquid water (0-5.5 wt % NaCl equiv) at tempe
ratures ranging from 175 degrees to 310 degrees C, depending on locati
on in the system. Fluid inclusion studies also show that the top of th
e liquid-dominated zone has descended, leaving behind a low-pressure v
apor cap. Dating of spring deposits, core samples of vein minerals, an
d altered host rocks by K-Ar, U-Th, U-U, and paleomagnetic methods ind
icates that the hydrothermal system was created at about 1.0 Ma and ha
s been continuously active to the present. However, these dates in com
bination with geologic and fluid inclusion evidence suggest that the v
apor zone formed at about 0.5 Ma, after a breach of the southwest cald
era wall drained widespread intracaldera lakes and lowered the hydraul
ic head on the hydrothermal reservoir. Although petrologic and geophys
ical evidence indicates that residual pockets of melt still reside in
the pluton beneath the caldera, the size of the hydrothermal system ha
s shrunk since initial formation. Thus, the Valles caldera contains a
mature hydrothermal system that remains hot, that contains a classic g
eothermal configuration, and that displays many analogies to epitherma
l mineral deposits exposed in older volcanic environments.