HYDROLOGIC CONSTRAINTS ON LISBON VALLEY COPPER MINERALIZATION WITHIN THE PARADOX BASIN, UTAH

We constructed a suite of NW-SE cross-sectional hydrothermal models between Lisbon Valley and the La Sal Mountains within the Paradox Basin, Utah. Today, a shallow, topography-driven flow system extends down into the Cutler Formation. This shallow flow system is underlain by dense brines associated with the dissolution of Paradox Formation evaporites. Groundwater migrates from the La Sal Mountains discharging about 30 km to the southeast within Lisbon Valley. The La Sal Mountains act as a “water tower” capturing orographic recharge. We were able to match the range of groundwater residence time and oxygen-18 data from groundwater samples collected from wells screened within the Navajo and Burrow Canyon formations within the Lisbon Valley. We represented magmatic intrusions beneath the La Sal Mountain region during Oligocene times. We set nodal temperatures to be 600 ^oC in order to represent multiple episodes of laccolith emplacement. The intrusive rocks cooled over a period of about 1 million years. The deposition of 2-km of Mancos Shale prior to La Sal volcanism effectively shut off topography-driven flow in our model. Thermal convection cells which formed within the La Sal intrusive rocks were largely confined to the volcanic stock beneath the mountains. Tilting of the sedimentary strata associated with laccolith intrusion induced thermo-haline convection cells within the Cutler and shallower formations. These convection cells were long lived and probably only dissipated following the erosion of the Mancos Shale at 6 Ma. Lateral flow rates within the Cutler Formation during the Oligocene were on the order of 0.1 m/yr. These convection cells may have facilitated copper mineralization in Lisbon Valley over time scales of tens of millions of years. Deeper thermal convection cells also formed within the Leadville Limestone with lateral flow rates of 0.02 m/yr. We speculate that the hydraulic “lid” associated with the deposition of the Mancos Shale may have promoted thermo-haline convection transporting relatively saline fluids over long lateral distances (~ 20km) contributing to ore formation.