THE USE OF POTENTIAL FIELD DATA IN CHARACTERIZING BASEMENT ROCKS AND IGNEOUS SUITES NEAR THE TELLURIUM-RICH CRIPPLE CREEK ALKALIC EPITHERMAL DEPOSIT, COLORADO

The rapidly growing solar energy field has increased the demand for tellurium. Alkalic epithermal systems such as Cripple Creek, Colorado, contain extremely high concentrations of tellurium. At Cripple Creek, the tellurium enrichment has been hypothesized to be related to the composition of the Proterozoic crust and/or greater assimilation of crustal material as the Tertiary ore-related magmas traveled through the anomalously thick crust beneath Colorado. In this on-going study we use magnetic susceptibility and density measurements, along with geochemical data, to understand observed magnetic and gravity anomalies over both the Proterozoic basement rocks and Tertiary igneous suites.

Aeromagnetic and gravity data show highs and lows over the four outcropping Proterozoic units that surround the Cripple Creek deposit. The oldest rocks consist of biotite gneiss that exhibit magnetic and gravity highs and have magnetic susceptibility values around 18 10^-3 SI. The 1.7 Ga granodiorite shows magnetic highs but has moderate amplitude gravity highs. Measured magnetic susceptibilities are around 19 10^-3 SI. The 1.5 Ga Cripple Creek quartz monzonite produces both magnetic highs and lows, along with a prominent gravity high. A limited number of samples indicate magnetic susceptibility around 0.057 10^-3 SI. The 1.0 Ga Pikes Peak granite exhibits a distinct magnetic low, but has a moderate gravity anomaly high with magnetic susceptibility and density values around 0.050 10^-3 SI and 2.57 g/cm³, respectively.

Whole-rock geochemistry of the ore-related Tertiary alkalic rocks indicates compositions ranging from tephrite to phonolite having strongly oxidized redox classification. Tellurium concentrations are up to 0.08 ppm in phonolite and as much as 0.5 ppm in lamprophyre rocks. Magnetic susceptibility values range from 0.458 to 56 10^-3 SI and generally increase with higher oxidation state. Measured density values for these rocks are between 2.56 to 2.76 g/cm³ with the more dense samples having higher magnetic susceptibility. The findings help characterize the geophysical response over concealed basement rocks at Cripple Creek and thus may be extrapolated to other basement regions that may serve to enrich tellurium in shallow epithermal systems.