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

Abstract

Twenty-five new 40 Ar/ 39 Ar ages from volcanic rocks and veins in the western San JuanMountains 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 2 base-metal veins were temporally and poss ibly 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 40 Ar/ 39 Ar 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.

Ignimbrites to batholiths: Integrating perspectives from geological, geophysical, and geochronological data

“…The Capitol City intrusion at Uncompahgre has a 40 Ar/ 39 Ar age about a mil-lion years younger than the associated ignimbrite (Bove et al, 2001), similar to the situation at Silverton. A resurgent intracaldera granodiorite in Alpine Gulch at Lake City has 40 Ar/ 39 Ar ages (biotite, K-feldspar) within analytical uncertainties of the associated ignimbrite (M. Zimmerer, 2014, written commun.).…”

“…A 40 Ar/ 39 Ar age of 26.6 ± 0.03 Ma on biotite (Bove et al, 2001) and a similar LA-ICP-MS age on zircon (Gonzales and Pecha, 2015) from granodiorite indicated that, in contrast to results from Bonanza, final crystallization of the Sultan Mountain stock was about a million years after the ignimbrite eruption and caldera formation at Silverton.…”

“…Major concentrations of such batholith-margin intrusions include 23 to 20 Ma dikes and subvolcanic plugs of dacite and rhyolite along the north side of the Platoro caldera complex (Lipman, 1975;Lipman, W. McIntosh, and M. Zimmerer, 2013, written commun. ) and small intrusions of similar composition within and along the west margin of the Silverton caldera that have yielded ages of 26-10 Ma Bove et al, 2001). Distal sills and laccoliths of monzonite to granite and porphyritic rhyolite, including bodies near Ophir and Rico west and southwest of Silverton, have ages from 26 to 5 Ma (Naeser et al, 1980;Cunningham et al, 1994;Gonzales and Pecha, 2015).…”

A GIS and statistical approach to identify variables that control water quality in hydrothermally altered and mineralized watersheds, Silverton, Colorado, USA

“…1). Structures that developed coincident with caldera collapse served as flow paths for hydrothermal fluids during several episodes of hydrothermal alteration and base-and precious-metal mineralization (Casadevall and Ohmoto 1977;Bove et al 2001;Yager and Bove 2007). The northeasttrending Eureka graben served as an important flow path for mineralization and formed in response to caldera resurgence of the San Juan and Silverton caldera complex.…”