MAGMATISM DURING FLAT SLAB SUBDUCTION – METALLOGENIC IMPLICATIONS FOR UTAH’S GREAT BASIN

The Thompson Knolls exploration project is located in the Kings Canyon District, west-central Millard County, 60 miles southwest of Delta, Utah, and 4 miles south of State Route 6. The area has been known for sediment-hosted gold potential since the 1980s, having grades of up to 0.63 oz/t Au in a drill hole by Centurion Resources in 1996. Despite a drill-indicated resource of ~200,000 oz Au at the Kings Canyon deposit, this district has never been in mine production. Based on a subtle magnetic anomaly interpreted from historical USGS aeromagnetic survey data, Inland Exploration and BCM Resources began developing Bingham-style porphyry copper and sediment-hosted gold targets in 2015 and 2016. Using modern geophysics, geochemistry, and nearby sediment-hosted gold and skarn mineralization to vector, the target was refined to a 3-square-mile area under pediment cover to the west of the Confusion Range. One hole was drilled in 2018 that intercepted quartz monzonite porphyry (QMP) under 595 ft of fanglomerate cover. The QMP continued until the hole was lost at 1108 ft, and exhibited argillic and sericitic alteration and zones of weak pyritic copper mineralization. An initial U-Pb age based on 36 zircon grains indicates that this intrusion is early Paleocene (62.2 +- 0.7 Ma) and also has an inherited Jurassic population (n = 11). Not only is this age older than the assumed Eocene (Bingham) age, it represents the only known Paleocene and Laramide intrusive age in Utah, and the only known Paleocene age in the Utah-Nevada Great Basin (the Cortez district in Nevada contains rare Cretaceous dikes). The Laramide period is well known for prolific porphyry copper mineralization in Arizona, but magmatism during this time in Utah has been believed to be inactive due to the flat slab subduction of the Farallon plate. This new data calls for a more nuanced understanding of magmatism during the Laramide in Utah and an evaluation of implications for the metallogenic evolution of the eastern Great Basin.