Abstract The U.S. Geological Survey (USGS) operates the Core Research Center (CRC) in Denver, Colorado, USA, a public access repository of rock cores from over 9800 wells and drill cuttings from over 53 000 wells, primarily from states in or adjacent to the Rocky Mountain Region. Annually, approximately 1400 visitors use the collection for traditional and innovative research. The CRC has an online, searchable database which includes downloadable core photos, analytical data, and thin-section images. When visitors sample for analyses, the results must be returned to the CRC for public dissemination providing immediate, free access to users while sparing the finite, irreplaceable collection from redundant testing. A representative quantity of every core depth is preserved in perpetuity. Studies on CRC materials, paired with new extraction methods, have unlocked new productive deposits. Materials drilled and curated decades ago remain in high demand while materials receiving little attention today may be crucial for future research. The collection provides immediate, inexpensive access to subsurface materials at a fraction of the cost of new drilling, sparing money, time and environmental impacts.

ABSTRACT Two models have been proposed to explain continental crust generation in accretionary orogens. One model suggests that accretionary orogens are formed by the successive collision of juvenile arcs. The second model invokes tectonic switching, which is the repeated cycles of slab rollback and extensional backarc basin formation followed by basin collapse caused by collision, shallow subduction, and/or increased convergence rate. The northern Colorado Front Range, specifically in and around the Big Thompson, Rist, and Poudre Canyons, offers excellent exposures of Paleoproterozoic rocks to test which accretionary model best explains crust generation for a portion of the Yavapai Province. In this contribution we have two goals: The first is to provide a field-trip guide that augments Mahan et al.’s (2013) field guide, which uses many stops that have become inaccessible or have changed because of catastrophic flooding that occurred in September 2013. This more current guide focuses on a variety of mostly Paleoproterozoic rocks within what some call the Poudre Basin. These rocks include clastic metasedimentary rocks, amphibolite, the Big Thompson Canyon tonalite suite, the northern Front Range granodiorite, granitic pegmatites, and Mesoproterozoic Silver Plume granite. The second goal is to present and synthesize new and existing geochemistry, geochronology, and isotopic data, and then discuss the origins, age, deformation, and metamorphism of these rocks in the context of the proposed tectonic models. These data were synthesized into the following tectonic model for the Poudre Basin. At ca. 1780 Ma, the juvenile Green Mountain arc, located today along the Colorado-Wyoming border, formed and extended shortly thereafter during slab rollback, resulting in the extensional backarc Poudre basin between the diverging arc fragments. Sedimentation within the basin began at inception and continued to ca. 1735 Ma when basin rocks were intruded by the Big Thompson Canyon tonalite suite and the northern Front Range granodiorite, all of which were subsequently metamorphosed and deformed at ca. 1725 Ma. Felsic magmatism and deformation within the basin were perhaps driven by the northward shallow subduction of an oceanic plateau or seamount. This suggests that following accretion of the Green Mountain Arc, tectonic switching explains formation and collapse of the Poudre Basin and creation of some of northern Colorado’s crust.

Abstract Alkaline igneous rock-related gold deposits, primarily of Mesozoic to Neogene age, are among the largest epithermal gold deposits in the world. These deposits are a subset of low-sulfidation epithermal deposits and are spatially and genetically linked to small stocks or clusters of intrusions possessing high alkali-element contents. Critical-, near-critical, or energy-critical elements associated with these deposits are F, platinum-group elements (PGEs), rare earth elements (REEs), Te, V, and W. Fluorine and tungsten have been locally recovered in the past, and some other elements could be considered as future by-products depending on trends in demand and supply. The Jamestown district in Boulder County, Colorado, historically produced F from large lenticular fluoritebearing breccia bodies and Au-Te veins in and adjacent to the Jamestown monzonite stock. Several hundred thousand metric tons (t) of fluorspar were produced. Some alkalic epithermal gold deposits contain tungstenbearing minerals, such as scheelite, ferberite, or wolframite. Small tungsten orebodies adjacent to and/or overlapping the belt of Au telluride epithermal deposits in Boulder County were mined historically, but it is unclear in all cases how the tungsten mineralization is related genetically to the Au-Te stage. Micron-sized gold within deposits in the Ortiz Mountains in New Mexico contain scheelite but no record of tungsten production from these deposits exists. The most common critical element in alkaline igneous-rock related gold deposits is tellurium, which is enriched (>0.5%) in many deposits and could be considered a future commodity as global demand increases and if developments are made in the processing of Au-Te ores. It occurs as precious metal telluride minerals, although native Te and tetradymite (Bi2Te2S) have been reported in a few localities. Assuming that the Dashigou and adjacent Majiagou deposits in Sichuan province, China, are correctly classified as alkalic-related epithermal gold deposits (exact origin remains unclear), they represent the only primary producers of Te (as tetradymite) from this deposit type. It is worth noting that some epithermal veins (and spatially or genetically related porphyry deposits) contain high contents of Pt or Pd, or both. The Mount Milligan deposit typically contains >100 ppb Pd, and some values exceed 1,000 ppb. However, owing to the presence of other large known PGE resources in deposits in which PGEs are the primary commodities, it is unlikely that alkaline-related epithermal gold deposits will become a major source of PGEs. Similarly, many epithermal gold deposits related to alkaline rocks have high vanadium contents, but are unlikely to be considered vanadium resources in the future. Roscoelite (V-rich mica) is a characteristic mineral of alkalic-related epithermal deposits and is particularly abundant in deposits in Fiji where it occurs with other V-rich minerals, such as karelianite, Ti-free nolanite, vanadium rutile, schreyerite, and an unnamed vanadium silicate. A few alkaline intrusive complexes that contain anomalous concentrations of gold or were prospected for gold in the past are also host to REE occurrences.The best examples are the Bear Lodge Mountains in Wyoming and Cu-REE-F (±Ag, Au) vein deposits in the Gallinas Mountains in New Mexico, which have REE contents ranging up to 5.6% in addition to anomalous Au.