GEOELECTRIC SIGNATURES OF CENOZOIC THROUGH PRESENT-DAY TECTONOMAGMATISM AND METALLOGENY IN CENTRAL COLORADO (Invited Presentation)

We present new three-dimensional, crustal-scale electrical conductivity images of central Colorado that provide novel insights into both Cenozoic metallogeny and post-Laramide tectonomagmatism in this region. These conductivity images are derived from broadband (~10^-3-10³ s period) magnetotelluric data that were collected with <20 km station spacing. Conductive (>0.01 S/m) features generally appear at either upper crustal (<5 km) or lower crustal (>20 km) levels; much of the mid crust (5-20 km) is electrically resistive (<0.001 S/m), as would be expected for the ancient crystalline rocks that comprise much of the central Colorado crust. Signatures of early-mid Cenozoic metallogenic systems are largely confined to upper crustal levels and are predominantly laterally discontinuous; we see no through-going, large-scale electrical conductivity features that could be associated with the Colorado “Mineral Belt.” At lower crustal levels, we resolve two major subparallel, north-south-trending, steeply dipping high-conductivity (~1 S/m) lineaments. Extending vertically upward into the mid crust (~20 km), these zones split into individual conductive (0.01-0.1 S/m) pipe-like “fingers” that rise sub-vertically into the upper-mid crust (~10 km). The western of these two zones, generally beneath the upper Arkansas River Valley, aligns with the strike of major extensional faults and elongated domains of late-Cenozoic cooling ages that are characteristic of Rio Grande rift processes in this portion of Colorado. However, the eastern zone, beneath the Front and Rampart Ranges, does not clearly relate to mapped recent extension or exhumation. Regardless, we propose that these major conductive zones represent a wider spatial footprint of Rio Grande rift-related extension in Colorado than has been previously recognized. Given this interpretation, the conductive tracks likely postdate and overprint the majority of Colorado “Mineral Belt” metallogenic systems; however, these conductors may be the present-day manifestation of a tectonic setting that initially produced the types of systems classically exemplified at Climax and Cripple Creek. The magnitude and geometry of these observed high conductivity values highlight the importance of post-Laramide geologic processes that are still active today.