Spatio-Temporal Shifts in Magmatism and Mineralization in Northern Colorado Beginning in the Late Eocene

Abstract

Magmatism in northern Colorado beginning in the late Eocene is associated with the formation of Pb-Zn-Ag carbonate-replacement and polymetallic vein deposits, the onset of caldera-forming magmatism, and eventually, the formation of rift-related, F-rich Mo porphyries (“Climax-type” intrusions). We use high-precision U/Pb zircon geochronology to better evaluate the temporal framework of magmatism and mineralization in the region. Our results demonstrate that mineralization in the Leadville area occurred between 43.5 and 39.7 Ma and was followed by mesothermal mineralization in the Montezuma area at approximately 38.7 Ma. Mineralization is associated with a suite of approximately 43 to 39 Ma intermediate magmatic centers that extended from Twin Lakes through Montezuma. The oldest porphyries associated with F-rich Mo prospects and deposits (Middle Mountain; 36.45 Ma) intruded 900 kyr after the start of the ignimbrite flare-up in the region. Spatiotemporal analyses reveal that the pattern of magmatism shifted in orientation between 40 and 35 Ma. We propose a model wherein magmatism before 39 Ma was the result of fluids evolved from the subducted Farallon slab being focused through weak zones in the lithospheric mantle and into the lower crust. This was followed by a more diffuse and higher power melting event that corresponds to a distinct change in the spatial patterns of magmatism. Our data suggest that low-grade Mo porphyry deposits can form close in time to calderas. We hypothesize that the transition from subduction to extensional tectonics in the region was responsible for this more widespread melting and a distinct shift in the style of magmatic-hydrothermal mineralization.

Timescales and thermal evolution of large silicic magma reservoirs during an ignimbrite flare-up: perspectives from zircon

“…Scenarios explaining zircon age dispersion include: (1) unmitigated Pb loss, (2) acquisition from surrounding country rock that is significantly older (xenocrysts), (3) acquisition of older zircons from previous magmatic episodes or pulses (antecrysts), and (4) continuous and prolonged crystallization in the same magma reservoir (autocrysts). Unmitigated Pb loss after thermal annealing and chemical abrasion has been demonstrated previously (Ovtcharova et al 2015;Gaynor et al 2019b;Rosera et al 2021), but there is little to no evidence for unmitigated Pb loss in this study because the youngest zircon ages are not younger than the 40 Ar/ 39 Ar sanidine ages for this eruptive sequence (Fig. 2).…”

“…1). The beginning of mid-Cenozoic volcanism in this region coincides approximately with the tectonic transition from the Sevier-Laramide Orogeny to the extensional environment that formed the Basin and Range province in the western U.S. (Humphreys et al 2003;Rosera et al 2021). Volcanism in the San Juan Mountains began in the late Eocene (35 Ma) with the eruption of the Conejos Formation, approximately 25,000 km 3 of mostly basaltic andesite to dacite lavas (Fig.…”

“…2). Numerous recent studies show dispersions of zircon ages in both plutonic and volcanic environments (e.g., Coleman et al 2004;Bachmann et al 2007;Schaltegger et al 2009;Schoene et al 2012;Wotzlaw et al 2013Wotzlaw et al , 2015Rivera et al 2014Rivera et al , 2016Singer et al 2014; Colón et al 2018), leading to revisions of the interpretation of U-Pb zircon geochronology of magmatic bodies (e.g., Schaltegger and Davies, 2017) and their relation to economic mineralization events (e.g., Tapster et al 2016;Gaynor et al 2019b;Large et al 2020;Rosera et al 2021). Here, we add to this discussion, providing an interpretation of U-Pb zircon ages for a magmatic system that produced one of the world's most rapid and voluminous ignimbrite flare-ups.…”

“…Magmatism during this transition is typically thought to be caused by hydration of lithospheric mantle during low-angle subduction followed by influx of hotter mantle during slab foundering or steepening (Dickinson and Snyder 1978;Farmer et al 2008). While subduction likely played some role in the generation of these magmas, though not typical arc magmatism, the relative timing of ignimbrite to onset of extension is not always clear (Best et al 2016;Ricketts et al 2016;Abbey and Niemi 2020;Rosera et al 2021).…”

Karst bauxite formation during Miocene Climatic Optimum (central Dalmatia, Croatia): mineralogical, compositional and geochronological perspectives

“…Individual zircon dates from the volcaniclastic horizon from the SB range over 2 Ma, and yield an MSWD in excess of that expected for a single population at the level of precision, and therefore require interpretation to determine the eruption age of these volcaniclastic rocks. It is common for high silica rocks to incorporate a moderate to high amount of antecrystic and xenocrystic zircon, which may host autocrystic overgrowths and therefore have euhedral appearances because of their recent magmatic history (e.g., Lipman and Bachman 2015;Gonzales 2015;Samperton et al 2015;Rosera et al 2021). While Pb-loss could artificially extend the age spectra by yielding young ages, a 12-h chemical abrasion has been demonstrated as a robust measure to limit Pb-loss, and was used in this work (Widmann et al 2019).…”