FAILED RIFT-RELATED ALKALINE MAGMATISM AND RARE EARTH ELEMENT DEPOSITS IN THE WET MOUNTAINS, COLORADO: THE STRUCTURAL, GEOPHYSICAL, AND GEOCHRONOLOGICAL STORY

The Wet Mountains of south-central Colorado contain Ediacaran-Ordovician, failed rift-related, alkaline intrusive rocks crosscut by thorium- and rare earth element (REE)-enriched alkaline dikes and veins. Three bimodal alkaline intrusive complexes (McClure Mountain, Gem Park, and Democrat Creek) are sequentially cross-cut by lamprophyre, syenite, and carbonatite dikes and mineralized quartz-barite-thorite veins. REE mineralization occurs in carbonatite dikes and quartz-barite-thorite veins, and to a lesser extent in hydrothermally altered syenite dikes. The stress field during emplacement and ages of the alkaline complexes and associated dikes/veins were analyzed to interpret rift history. Mapped structural orientations of dikes/veins show that alkaline magma and REE fluids utilized migration pathways along northwest-trending tension fractures and high angle faults that share an extension direction towards 045°. The geophysical anomaly orientations over the alkaline complexes indicate tectonic elongation of the intrusive bodies approximately parallel to the 045° extension direction shared by dikes/veins. A magnetic anomaly was identified in the southeast mapping area outlining an interpreted buried mafic-ultramafic alkaline complex with a similar elongated orientation. Th-Pb LA-ICP-MS analysis of low-U zircon in carbonatite and U-Pb LA-ICP-MS analysis of monazite in mineralized dikes/veins give a weighted mean age of 465 ± 18 Ma and a lower intercept age of 465 ± 44 Ma, respectively. While these ages are within error of the previously determined 551±30 to 483.0±1.8 Ma age of alkaline magmatism, they suggest that REE mineralization is younger and may represent the last phase of failed rifting. The proposed extension direction for, and ages of Ediacaran-Ordovician, failed rift-related, alkaline intrusive rocks and veins are similar to those in the along-strike southern Oklahoma aulacogen, suggesting that the failed rift of the Wet Mountains is a continuation of the northwest-trending, Ediacaran-Cambrian southern Oklahoma aulacogen.