Geochronology and geology of late Oligocene through Miocene volcanism and mineralization in the western San Juan Mountains, Colorado

Twenty-five new 40Ar/39Ar ages from volcanic rocks and veins in the western San Juan
Mountains clarify relationships between volcanism and mineralization in this classic area. Five
calc-alkaline ash-flow sheets erupted from caldera sources (Ute Ridge, Blue Mesa, Dillon Mesa,
Sapinero Mesa, and Crystal Lake Tuffs) from 28.6 to 27.6 Ma. This is a much more restricted
time interval than previously thought and indicates that the underlying batholith rose and evolved
very rapidly beneath the western San Juan Mountains. The new ages and geologic relations
constrain the timing of joint resurgence of the Uncompahgre and San Juan calderas to between
28.2 and 27.6 Ma. The collapse of the Silverton caldera produced a set of strong ring fractures
that intersected with graben faults on the earlier resurgent dome to produce the complex set of
structures that localized the mid-Miocene epithermal gold veins.
Later calc-alkaline monzonitic to quartz monzontic plutons solidified at 26.5-26.0 Ma as
the underlying batholith rose through its volcanic cover. A new age from lavas near
Uncompahgre Peak supports earlier interpretations that these lavas were fed by nearby 26 Ma
monzonite intrusions. Nearly all of these intrusions are associated with subeconomic Mo and
Cu mineralization and associated alteration, and new ages of 26.40 and 25.29 Ma from the
Ute-Ulay and Lilly veins in the Lake City region show that some of the most important silver and base-metal veins were temporally and possibly genetically connected to these plutons. In
addition, the Golden Fleece telluride vein cuts all of the post-Uncompahgre caldera volcanics in
the area and is probably temporally related to this cycle, though its age of 27.5 ? 0.3 Ma was
determined by less precise U/Pb methods.
The 22.9 Ma Lake City caldera collapsed within the older Uncompahgre caldera structure
but is petrologically unrelated to the older calc-alkaline activity. The distinctive suite of
high-silica rhyolite tuff and alkaline resurgent intrusions indicates that it is closely related to the
early stages of bimodal high-silica rhyolite-alkali basalt volcanism that accompanied the onset of
extensional tectonism in the region. Both 40Ar/39Ar ages and paleomagnetic data confirm that the
entire caldera sequence formed in less than 330,000 years. Only weak quartz vein mineralization
is present in the center of the caldera, and it appears to be related to leaching of metals from the
intracaldera tuffs above the resurgent intrusion. Massive alunitization and weak Mo and Cu
mineralization along the eastern ring fracture are associated with calc-alkaline lavas and stocks
related to late stages of the caldera cycle. These calc-alkaline stocks also appear to be genetically
and temporally linked to a radial pattern of barite-precious metal veins on the northeastern
margin of the Lake City caldera.