Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 25-9
Presentation Time: 9:00 AM-6:00 PM


PIERCE, Margaret C., California State University, Chico, Department of Geological and Environmental Sciences, 400 West 1st St, Chico, CA 95929 and AIRD, Hannah M., Department of Geological and Environmental Science, CSU Chico, 400 W 1st Street, Chico, CA 95929

The Lights Creek Stock (178-148 Ma) is a small 31 km2 pluton originally mapped as five distinct compositions, and is associated with three IOCG deposits. The pluton was last mapped by J.D. Juilland in 1970 who noted variations within the pluton lithology and contoured the potassium feldspar to plagioclase ratio defining five distinct compositions of the granitoid. He attributed the variations in composition to a felsic magma that assimilated a preexisting mafic igneous body as evident by the mafic inclusions. The purpose of this project is to reevaluate the magmatic evolution of the Lights Creek Stock by analyzing bulk geochemistry, lithology, and petrography for each plutonic unit. Whole rock geochemical data was collected using XRF and ICP-MS, and was plotted using Excel and GCDkit. Geochemical results show that with increasing SiO2: CaO and Al2O3 decreased, K2O increased, and Na2O remained constant. On an AFM plot, nine of the twelve samples are tholeiitic and three are calc-alkaline. Preliminary results indicate that the map of the Light Creek Stock needs to be reassessed. For example, petrography and geochemistry of two samples (LC-16 and LC-17) classified as the same unit reveal different proportions of plagioclase and potassium feldspar: LC-16 has 35% potassium feldspar and 28% plagioclase; and LC-17 has 20% potassium feldspar and 40% plagioclase. The geochemistry of the same two samples shows differences in weight percent of SiO2, CaO, and K2O: LC-16 has 66.4 wt% SiO2, 1.97 wt% CaO, and 4.68 wt% K2O; and LC-17 has 64.3 wt% SiO2, 3.74 wt% CaO, and 2.97 wt% K2O. The geochemistry also indicates that the pluton likely underwent potassic and sodic alteration. Future work includes remapping the Lights Creek Stock and quantifying the degree of potassic and sodic alteration.