by Rob Rice
The Colorado Mineral Belt (CMB) is a ribbon of metalliferous hydrothermal deposits, resulting from Laramide-aged magmatic activity at the forefront of the Rocky Mountain orogenic event (Tweto and Sims, 1963). The placement of the magmatic bodies and their resulting interaction with groundwater to produce the ore deposits has long been something of a mystery. At first glance, the NE-SW trending narrow belt seems to cross-cut the N-S orogenic trend. Initially, this was seen as a response to plate-scale structural inheritance (Caine et al., 2010).
Location of the CMB in relation to Laramide Rocky Mountain uplift, and post-Laramide
magmatism. Image obtained from GSA Geosphere.
The CMB can trace its roots deep into the past of the North American continent. The NE trendline of ore deposits lies sub-parallel to the suture zone where the Mazatzal province was created by a series of island arcs colliding with the Yavapai province, 1.4-1.8 billion years ago (Karlstrom and Crossey, 2012). This Proterozoic tectonic event was long thought to be the easiest explanation. Existing shear zones and weak spots would be natural conduits for plutonic emplacement during the Laramide reactivation (Ye, 1996).
Terranes that comprise North America, and the timing of their accretions, with suture zones
and overplating direction. Adapted from Kansas Geological Survey by Dr. Jack Shore
Other evidence has led researchers to dispute the structural inheritance as the primary control of magmatic emplacement. While the CMB is sub-parallel to the sutures, it is not, in fact, aligned with them. Laramide plutons occur in both brittle as well as ductile shear zones, seemingly irrespective of the easiest path upward (Cain e et al., 2010). Volcanics along the belt, as well as both north and south of it, display a variety of chemistries, and also ages. Some volcanics began occuring 70 MYA, while others have continued until as recently as 10 MYA (there is even a small volcano in western Colorado that is only 5k years old!). Chemical analysis of the Twin Lakes batholith in central Colorado reveals at least 7 distinct plutonic intrusions, between 70 MYA and 30 MYA, with a gap in acivity between 55-43 MYA (Feldman, 2012).
Pattern of magmatism across Colorado during Laramide, and post-Laramide tectonism.
Image obtained from University of Colorado.
The pattern of volcanics can be seen to advance west to east, pause, and then roll back westward with time. The Farralon plate subducting along the west coast is not thought to have reversed direction, since North America is still pushing westward. Additionally, the chemistry of the magmatics changes with time. Front Range plutons from around 65 MYA are mostly alkaline. Volcanics in the central Rockies consisted of alkaline granodiorites (Chapin, 2012). San Juan volcanism begins as calc-alkic, silica rich cinder cones 28.6-26.0 MYA, followed by rhyolite-dacite plutons 23-10 MYA, and ended with mafic extrusions 21-16 MYA (Bove et al., 2000). 10 MY old volcanics in western Colorado are basaltic, some of them exhibiting a potassium enriched MORB-like chemistry (Livaccari, 2014)(Cole, 2011).
The Laramide Orogeny occupies a geospatial gap in an arc of magmatism that co-occurred (Chapin, 2012). The arc traces the forefront of the subducted Farralon plate as it is overrun by the North American plate. While the magmatic arc was erupting, the area that is now Colorado was rising. In fact, volcanoes north of the CMB from this time are few. The gap coincides roughly with the Colorado Plateau. Why did this section of the plate rise, instead of crack?
Laramide magmatism across western North America, 70-50 MYA. Location of
CMB trending NE across a gap in magmatism. Image from GSA Geosphere
Gravity surveys compared with tomography at various wavelengths have indicated multiple geophysical phenomena in the region.
Bouger gravity anomaly map of Colorado. Note low density beneath the Rocky Mountains,
trending sub-parallel to the CMB. Image from USGS Publications
Seismic velocity differences beneath the North American plate indicate the upwelling of hot, liquid-like
material beneath the Colorado Plateau,
with a piece of colder denser Farallon plate remaining somewhat stuck beneath the eastern edge. Adapted from GSA Geology
Developments on either side of the CMB point to differing conditions during a region-wide tectonic event.
The geologic clues taken together seem to indicate that the Colorado Mineral Belt occurs where it does due to a combination of control factors. However, one possible control factor often overlooked is the structural inheritence of the Farallon plate. Subsurface investigation reveals a picture that may be described as a piece of the Farallon slab remaining relatively stuck to the bottom of the North American crustal plate, effectively thickening the crust and insulating the surface from the magmatism occurring in neighboring provinces. This section of Farallon crust seems to be heterogenous, and to have retained some of its topography. Perhaps a ridge or plateau is what became enmeshed with the overriding plate (Chapin, 2012). The interaction of the two plates at depth created the chemistry characteristic of the early post-Laramide volcanics. After colliding with the much thicker cratonic interior, overrun paused. The Farallon plate fragment "peeled" off and reversed direction, subsiding. Its subsidence drew in hotter upper mantle material to take its place, which interacted with the continental crust above it creating the magmatic chemistry seen in later Oligocene volcanics. The structure of the Farallon fragment, interacting with the North American crust above it, would create not simply the conduits for plutonic emplacement, but the raw material and opportunity. Surface expression may not be indicative of basement-level structural interactions. The lineament of the CMB is a structural remnant of the path traced by the Farallon plate fragment prior to getting "stuck" as it moved NE relative to North America. Its near-parallel alignment with the Proterozoic suture zone seems to be merely coincidental.