Proterozoic structural history of the Montezuma mining district in the central Colorado Front Range

Abstract

The central Front Range of Colorado has experienced at least three distinct periods of deformation, associated with the Yavapai and Mazatzal orogenies in the Paleoproterozoic, and the Picuris orogeny in the Mesoproterozoic. However, the Picuris orogeny in Colorado has only recently been recognized and the nature and extent of it are poorly constrained. The deformation history of the three orogenies is recorded in shear zone fabrics of the Colorado Shear Zone System and in regional folds. The purpose of this study was to investigate Proterozoic ductile deformation in the metamorphosed Front Range basement of the Montezuma Mining District, to determine if a shear zone known as the Montezuma Shear Zone, existed and had any similarities to other Colorado Shear Zone System shear zones, or helped to localize the Colorado Mineral Belt-related Montezuma Stock and associated vein mineralization. Geochronology analyses suggest that the study area has undergone two main tectonic events, at ~1.68 and ~1.43 – 1.42 Ga based on U-Pb analysis of in situ monazite. The oldest event is interpreted to be the cause of the main S0-1 foliation in the study area, local centimeter- to meter-scale, F1 isoclinal folds of variable orientation, and overprinting centimeter- to kilometer-scale mostly upright originally northwest-plunging F2 folds. The earliest tectonism including F2 folding is constrained between the ~1.75 Ga age of a nearby quartzite and ~1.68 Ga metamorphic in situ monazite, but is mostly interpreted as having occurred at ~1.68 Ga. This tectonism may have taken place in an arc-related convergent or back-arc setting, based on the fact that the ~1.75 Ga quartzite contains a large detrital zircon grain population with an age close to its interpreted depositional age, which is typical for such settings. F2 folds are overprinted by moderately west-plunging, centimeter- to kilometer-scale F3 folds, and in situ monazite geochronology shows evidence of ~1.43 – 1.42 Ga metamorphism associated with this deformation. F3 folds are therefore interpreted as related to the Picuris orogeny and are of similar orientation to those in the Picuris Mountains in New Mexico, where the event was defined. In contrast to previous interpretations of the Montezuma Shear Zone, the zone does not have evidence of significant ductile shearing, and is better classified as a brittle fracture zone. Isolated zones of shearing within the previously mapped boundaries of the zone are not continuous, show inconsistent shear sense across the study area, and are more likely to have formed during flexural slip along megascopic F3 folds near Santa Fe Peak. They may, however, explain why the Montezuma Shear Zone was previously interpreted in the district. Mineralized veins in the Harold D. Roberts Tunnel are not parallel to the Montezuma Shear Zone, but instead to Laramide maximum compressive stress directions (068), suggesting that the Cordilleran-style vein mineralization may have taken advantage of Laramide brittle structures. Veins otherwise crosscut Proterozoic ductile structures including S0-1 foliation, as does the Montezuma Stock, which was shallowly emplaced. Although mineralization in the Montezuma Mining District may be partially explained as a product of brittle fracturing along pre-existing weaknesses associated with F2 axial planes in Domain 1, there is no evidence that Proterozoic ductile deformation and shearing along the Montezuma Shear Zone, which are associated with F3 and no younger than ~1.41 Ga based on the age of crosscutting Silver Plume Granite, are a major control for later mineralization. The North Fork Fault Zone, a previously recognized Proterozoic structure, in contrast to its name and the Montezuma Shear Zone, only shows ductile shear structures and is thus a ductile shear zone. In situ monazite analyses indicate it originated in the Paleoproterozoic, and may have been reactivated at ~1.43 Ga. In contrast to the Montezuma Shear Zone, it may be a part of the Colorado Shear Zone System.