Ediacaran‐Ordovician Magmatism and REE Mineralization in the Wet Mountains, Colorado, USA: Implications for Failed Continental Rifting

• Published in: Tectonics (Washington, D.C.) Vol. 42; no. 4
• Main Authors: Magnin, Benjamin P., Kuiper, Yvette D., Anderson, Eric D.
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-04-2023
• Subjects: Ablation; airborne geophysics; Cambrian; Coastal inlets; Dikes; Direction; Earth; Earth crust; Embankments; Fault lines; Faults; faults and fractures; Fractures; geochronology; Geophysical data; Green energy; Isotopes; Lava; Magma; Magnetic data; Mass spectrometry; Mass spectroscopy; Mineral deposits; Mineralization; Minerals; Monazite; Mountains; Ordovician; Rare earth elements; Rifting; Southern Oklahoma Aulacogen; Thorium; Uranium; Veins (geology); Wet Mountains; United States > US; Oklahoma; Colorado; Wet Mountains
• DOI: 10.1029/2022TC007674
• EISSN: 1944-9194
• ISSN: 0278-7407

Structures associated with Ediacaran‐Ordovician alkaline magmatism and the timing of rare earth element (REE) mineralization in the Wet Mountains, CO, were analyzed using field, geophysical, and U‐Th‐Pb isotope methods to interpret their tectonic setting in the context of previously proposed rift models. The Wet Mountains are known for thorium and REE mineralization associated with failed rift‐related, Ediacaran‐Ordovician alkaline intrusions and veins. Structural field data indicate that alkaline dikes and mineralized veins are controlled by a system of northwest‐striking, high‐angle faults and tension fractures formed in a 040°‐directed extensional regime. Magnetic and surface expressions of Democrat Creek and McClure Mountain complexes show tectonic elongation toward ∼045°, consistent with NE‐directed extension. Magnetic data also suggest the existence of a fourth, previously unrecognized mafic‐ultramafic complex of inferred Cambrian age with a similar elongated orientation. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA‐ICP‐MS) 208Pb/232Th analysis of low‐uranium zircon from carbonatite dikes and in situ 206Pb/238U LA‐ICP‐MS analysis of monazite in mineralized dikes yielded 465 ± 18 Ma and 489 ± 33 Ma ages, respectively. These ages are consistent with the expected age based on slightly older, cross‐cut syenite dikes and the hypothesized Ordovician end to failed rift‐related magmatism. The Ediacaran‐Ordovician age of alkaline magmatic rocks and the associated northeast‐directed extension direction are similar to those of the along‐strike, Ediacaran‐Cambrian Southern Oklahoma Aulacogen. Therefore, the failed rift system in the Wet Mountains is interpreted to be a northwestern continuation of the Southern Oklahoma Aulacogen with carbonatite magmatism and thorium/REE mineralization representing late intrusive phases.
Plain Language Summary
The Wet Mountains of south‐central Colorado contain elements and minerals important to modern electronics and green energy technology. These minerals formed along an ancient tectonic system where the Earth's crust pulled apart, which was associated with uprise of magma from greater depths. The orientation of the system was previously unknown. This study used field and geophysical data to analyze the faults and fractures that controlled the migration of magma and emplacement of associated mineralization in order to understand the direction in which the Earth's crust pulled apart. Results of this study reveal that mineralization occurred along northwest‐striking faults and fractures, and that large magma bodies were elongated to the northeast during emplacement. This information suggests that the crust pulled apart by northeast‐directed extension along a northwest‐trending system. Additionally, isotopic analyses of mineralized features indicate that mineralization occurred at the end of this tectonic event. The orientation and age of the system is similar to a comparable system in southern Oklahoma, suggesting that the two are related. Based on this information, it is possible that additional mineral deposits formed between southern Colorado and southern Oklahoma.
Key Points
Ediacaran‐Ordovician failed rift‐related magmatism in the Wet Mountains formed along a northwest‐trending extensional regime
The failed‐rift system is likely related to the northwest‐trending, Ediacaran‐Cambrian Southern Oklahoma Aulacogen
Rare earth element mineralization occurred near the end of failed rifting