TIMING THE ONSET OF VOLATILE-RICH, HIGH-SILICA MAGMATISM IN THE CENTRAL COLORADO MINERAL BELT: NEW INSIGHTS FROM CHEMICAL ABRASION ID-TIMS U/PB ZIRCON GEOCHRONOLOGY

The central Colorado Mineral Belt (CCMB) is a continental magma system that was active during Laramide flat-slab subduction and subsequent extension that occurred throughout the western United States. The cessation of subduction in the CCMB has been related to a switch from dominantly intermediate to more volatile-rich, high silica magmatism, and the formation of historically productive ore districts. However, there exists a great deal of uncertainty regarding the absolute timing of these events in the CCMB, largely due to the use of geochronometers that are prone to thermal disturbance (i.e., zircon fission-track; K-Ar ages). We use chemical abrasion ID-TIMS U/Pb zircon geochronology to overcome this problem and to better evaluate the temporal framework of magmatism and mineralization in the CCMB.

Seven new U/Pb zircon ages document the shift from early, intermediate magmatism through younger high-silica, volatile-rich magmatism, which accompanies a shift in metal deposits. A pre-mineral intermediate intrusion near the Leadville district yields an age of 43.56 ± 0.11 Ma. The Montezuma pluton is quartz monzonite and is variably cut by mesothermal Ag-Pb-Zn veins. Two samples of different phases of the pluton are indistinguishable in age: the eastern megacrystic phase of the pluton is 38.843 ± 0.078 Ma and the western phase is 38.811 ± 0.058 Ma. The Webster Pass rhyolite (38.720 ± 0.050 Ma) is anomalously enriched in Pb-Zn-Cu, and likely represents a volcanic conduit above a southern phase of the Montezuma pluton. Three new ages for high-silica, Mo-F-related intrusions include: the Middle Mountain porphyry (36.449 ± 0.048 Ma), Turquoise Lake porphyry (~ 36.0 Ma), and the Chalk Mountain rhyolite (26.305 ± 0.025 Ma). These data document a temporal shift from dominantly Ag-Pb-Zn-rich, quartz monzonite-associated magmatism at ~ 38.7 Ma to Mo-F-rich magmatism beginning at ~ 36.4 Ma within the CCMB. The time period between 38.7 and 36.4 Ma corresponds to the onset of explosive silicic volcanism that blanketed a regional erosional surface, and suggests that the emplacement of volatile-rich mineralizing intrusions could be closely related in space and time to assembly of ignimbrite-forming magma bodies.