Episodic copper mineralization in sedimentary basins: evidence from the Paradox Basin, USA

Episodicity is common in mineral systems – it is responsible for mineral parageneses that form over timescales from 10 s of kyr to 10 s of Myr. New U-Pb dating of vein carbonate minerals reveals an unexpected epoch of fluid flow and sediment-hosted copper (Cu) mineralization between 18 and 3 Ma across large parts of the evaporite-dominated Paradox Basin. Sulfide-bearing carbonate veins from 9 locations with a mix of geologic settings and metal traps were analyzed by laser ablation inductively coupled mass spectrometry (LA-ICP-MS); of 37 samples examined, 13 from 9 mineral deposits had suitable U-Pb contents. Our results add to previously reported mid-Cretaceous to Oligocene ages for Cu mineralization in some of the same areas, highlighting the long-term and episodic nature of these systems. Cu mineralization requires appreciable fluid flux, yet the new ages mostly precede flow related to rapid exhumation (<6 Ma) and post-date inferred flux maxima in the Mesozoic and early Cenozoic. Miocene flow of mineralizing fluids (oxidized brines) implies a change in hydrologic regime stimulated by some combination of partial removal of Paleogene and Late Cretaceous strata evidenced by new detrital zircon data, previously undocumented salt movement, and far-field influence by extension and magmatism beyond the Colorado Plateau. The new data imply a newly identified early stage of local exhumation, that in turn drove novel ore-forming hydrologic systems. These complexities illustrate the challenges in evaluating many sediment-hosted Cu deposits where hard-to-date mineralization can occur episodically from diagenesis to late-stage basin inversion, but usually happens over 10 s of Myr.