IOCG ALTERATION AND PETROGENESIS OF LIGHTS CREEK STOCK, PLUMAS COUNTY, CA

The Lights Creek Stock (LCS) is a 184-178 Ma pluton originally mapped by J.D. Juilland in 1970 as five distinct compositions based on the ratio of potassium feldspar to plagioclase. LCS is also host to three economically viable Iron-Oxide ± Cu ± Au (IOCG) deposits and their associated alteration. It is possible that Juilland’s mapped zonations are representative of hydrothermal alteration rather than igneous processes. Common types of alteration associated with IOCG deposits are: sodic(-calcic), potassic, and hydrolytic. Sodic(-calcic) alteration is defined by the addition of Na(-Ca) and yields plagioclase. High temperature potassic alteration is defined by the addition of K and yields K-feldspar. Hydrolytic alteration is defined by the presence of sericite ± chlorite and is typically associated with upper portions of IOCG deposits. The purpose of this project is to classify alteration types throughout the main “unaltered” body of the LCS granitoids, and compare observed alteration patterns to Juilland’s findings. Preliminary petrographic and geochemical data from 38 samples suggest that high temperature K alteration assemblages are present in rocks outside of the three deposits that were previously described as “unaltered”, and occasionally occur with Na alteration assemblages. These alteration zones could be used as an exploration tool for undiscovered Cu resources. Future work includes using SEM analysis to more accurately classify alteration and using U-Pb geochronology to better constrain the emplacement history of LCS.