PRELIMINARY STRUCTURAL ANALYSIS OF ALKALINE DIKES AND MINERALIZED VEINS IN THE WET MOUNTAINS, SOUTH-CENTRAL COLORADO: CONNECTING RARE EARTH ELEMENT MINERALIZATION TO CAMBRIAN RIFTING

The Wet Mountains of south-central Colorado, expose Precambrian basement rocks east of the Sangre de Cristo Range and contain numerous Cambrian, failed-rift-related, alkaline magmatic rocks and associated thorium- and especially rare earth element (REE)-bearing alkaline dikes and veins. There are three main alkaline complexes sequentially cross-cut by lamprophyre, syenite, and carbonatite dikes followed by mineralized quartz-barite-thorite veins. The syenite dikes show hydrothermal alteration that is temporally associated with the mineralized veins. Carbonatite dikes can be subdivided into two types: primary dikes that intrude Precambrian country rock and replacement dikes that primarily replace lamprophyre dikes. The purpose of this study is to investigate the structural controls on, and kinematics during emplacement of these dikes and veins to establish rift orientation(s) during the Cambrian. Preliminary data show that all types of dikes/veins predominately strike northwest, and dip steeply. Four principal styles of emplacement are interpreted: (1) fault-controlled emplacement indicated by adjacent brecciated and gouged wall rock, (2) tensile, fracture-controlled emplacement based on no associated wall rock deformation, (3) emplacement along reopened pegmatite intrusions of unknown age, and (4) local emplacement along Precambrian metamorphic foliation. These preliminary observations are consistent with emplacement along northwest-trending tensile fractures and high-angle faults formed in a northeast-southwest-directed extensional regime. It is hypothesized that this extensional regime may be a continuation of the northwest-trending, Cambrian southern Oklahoma aulacogen, the only nearby Cambrian rift system in that orientation.