CONTROLS ON PORPHYRY COPPER, MOLYBDENUM, AND RELATED SYSTEMS IN UTAH

In addition to the giant Bingham porphyry copper-gold-molybdenum-silver system, Utah hosts over a dozen other porphyry or porphyry-related hydrothermal systems. The porphyry copper and related systems are all associated with latest Eocene to Oligocene (40 to 30 Ma), calc-alkaline, intermediate intrusive centers. A later period (25 to 14 Ma) of more felsic volcanism produced the Pine Grove porphyry molybdenum deposit and several other known and suspected molybdenum occurrences. Mineralization in the Bingham district is centered on a high-K, calc-alkaline, quartz monzonite porphyry stock (38 Ma). Past production from the Bingham district includes nearly 2.5 billion metric tonnes of porphyry ore, 35 million tonnes of Pb-Zn-Ag-Au replacement ore, and an additional 28 million tonnes of distal disseminated gold ore. The remaining open pit porphyry reserves are roughly 700 million tonnes. Several other known sub-economic porphyry copper systems in Utah are related to important base metal replacement camps, also typically with lead dominant over zinc. These systems include the greater Park City Pb-Ag-Au-Zn mining district, Tintic Ag-Au-Pb-Zn district, and Stockton Pb-Zn-Ag-Au district. The Drum mining district hosts a failed porphyry copper system and a distal disseminated gold deposit. Pine Grove is a giant Climax-type porphyry molybdenum deposit related to a sub-volcanic, high-silica rhyolite porphyry plug (22.5 Ma). The deposit hosts an estimated resource of about 110 million tonnes of 0.3% MoS2 at depth. The surface expression of this system is a weakly argillized quartz eye rhyolite porphyry plug associated with a very small Zn-Pb-Ag replacement district. The other molybdenum systems in Utah show a similar propensity for minor, zinc-rich past production. Utah's porphyry systems general lie along one or more of three recognized lineament directions. The most important and widely recognized of these trends are the east-west-trending mineral belts, which contain nearly all of the deposits. The secondary control is the intersection of these mineral belts with the north-south-trending Wasatch line and/or northwest-trending lineaments. All three of these structural trends are believed to be long-lived features with geologic activity dating back at least into the Paleozoic. The Bingham mining district lies near the intersection of all three of these features.