CHARACTERIZATION AND COMPARISON OF CHLORITE IN THE ENGELS, MOONLIGHT, AND SUPERIOR IOCG DEPOSITS, LIGHTS CREEK STOCK, PLUMAS COUNTY, CA

The Lights Creek Stock (LCS) in Northern CA hosts three iron-oxide copper gold (IOCG) deposits whose petrogenesis is not well understood. Several minerals are being used as geothermometers to characterize the igneous and hydrothermal history of the pluton and IOCG deposits. Chlorite group minerals are hydrous phyllosilicates whose composition is influenced by the chemistry and temperature of the fluids from which they form and chlorite-bearing assemblages can develop under various conditions. Chlorite is commonly seen throughout the IOCG deposits, establishing its potential as a useful geothermometer. This study aims to characterize chlorite in the Engels and Moonlight deposits and compare findings with the previously-analyzed Superior deposit, to gain insights into the evolution of the altering fluid across the LCS. In all deposits, petrography reveals several types of chlorite, including biaxial negative chlorite with anomalous blue birefringence and biaxial positive chlorite with anomalous brown birefringence. Geochemical analysis with EPMA and SEM-EDS were used to calculate the chlorite structural formulae based on 14 oxygens. Different chlorite geothermometers, based on tetrahedral aluminum occupancy and octahedral vacancy, were then applied to calculate the temperature of formation for the different chlorite types. Data indicates that blue chlorite typically exhibits higher ratios of Fe/(Fe + Mg) when compared to brown chlorite. In Superior, blue chlorites show the highest temperatures, 425-375℃, and assemblages indicate local potassic alteration. In Moonlight, preliminary temperatures of brown chlorite range from 350-375℃, similar to brown chlorites in Superior which range from 275-300℃. Both indicate sodic-calcic alteration. Blue chlorites in Moonlight range from 290-325℃ and exhibit evidence of hydrolytic alteration. No definitive trends between the two types of chlorite are seen in the preliminary data of the Engels deposit, showing overlapping temperature ranges of 315-345℃ and 330-345℃. Characterizing and comparing the chlorite generations between the different IOCG deposits of the LCS provides insight into the thermal evolution of the altering fluids, enhancing our understanding of the hydrothermal conditions necessary for mineralization.