OF-23-09 Geologic Map of the Montezuma Quadrangle, Summit, Clear Creek, and Park Counties, Colorado

The purpose of this publication is to describe the bedrock and surficial deposits, structural and metamorphic history, and mineral deposits of the Montezuma 7.5-minute quadrangle. It is a continuation of field mapping carried out by Colorado School of Mines Associate Professor Yvette Kuiper and four previous MS students in the central Colorado Front Range, focusing primarily on investigating the effects of the ~1.4 Ga Picuris orogeny, and separating those from the earlier Paleoproterozoic events. It is a direct continuation of previous mapping in the adjacent Mt. Blue Sky (formerly Mt. Evans) 7.5-minute quadrangle to the east, published in 2022. Mapping of the Montezuma 7.5-minute quadrangle was carried out by Colorado School of Mines MS students Erick Bora, Ariel Borsook, and Dustin Shockley, and guided by Yvette Kuiper and USGS geologist Cal Ruleman. Digital ZIP download. OF-23-09D

From the abstract:

A 1:24,000 bedrock and surficial geological map was produced for the Montezuma 7.5’ quadrangle in the central Colorado Front Range. The bedrock consists of primarily of Proterozoic rock, and the Paleogene Montezuma stock in the northwestern part of the quadrangle. Bedrock mapping focused on rock types and their structural and metamorphic histories. Three main generations of folds affected the Montezuma 7.5’ quadrangle. Isoclinal F1 folds in various orientations are overprinted by shallowly to moderately northwest-plunging tight to open F2 folds in the southern half of the quadrangle, and shallowly to moderately southwest- and northeast-plunging tight to open F2 folds in the northern half of the quadrangle. These are folded by tight to open shallowly to moderately east- and west-plunging F3 folds, primarily exposed in the east-central part of the quadrangle. The Mount Blue Sky batholith contains a pervasive moderately north- to northwest-dipping tectonic foliation, suggesting north- to northwest-directed shortening after ~1443 Ma, probably during F3 folding. Jointing in the Montezuma stock may have been related to the late Mesozoic – early Cenozoic Laramide orogeny, or to late Cenozoic extension associated with the Rio Grande Rift. Surface deposits consists mainly of till of the Upper Pleistocene Pinedale glaciation, and also mass-wasting and alluvial and colluvial deposits. Over 100 historic mines are present, primarily in the northern and western areas of the quadrangle. Copper, gold, lead, gold, silver, and zinc are the primary commodities.