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