CHARACTERIZING CHLORITE ASSOCIATED WITH THE SUPERIOR IOCG DEPOSIT, LIGHTS CREEK STOCK, PLUMAS COUNTY, CA

The Lights Creek Stock (LCS) in Northern CA hosts three potentially economically viable iron-oxide copper gold (IOCG) deposits, including the Superior deposit. Petrography of samples from the Superior IOCG deposit shows different types of chlorite present. This study aims to characterize the different chlorite generations and their formation temperatures to learn more about the evolution of the altering fluid. Chlorite group minerals are hydrous phyllosilicates whose composition is influenced by the chemistry and temperature of the fluids from which they form. Chlorite-bearing mineral assemblages can develop under a range of conditions, including sodic-calcic, potassic, and hydrolytic hydrothermal alteration. Chlorite is commonly seen throughout the Superior IOCG deposit and throughout the broader LCS, establishing its potential significance as a geothermometer in this study. Petrography of Superior samples reveals distinct mineral assemblages for two generations of chlorite. A biaxial negative chlorite type with anomalous blue birefringence is associated with magnetite, sericite, and epidote. Additionally, biaxial positive chlorite with anomalous brown birefringence is associated with weathered feldspars, magnetite, amphibole, rutile, and tourmaline, typical of sodic-calcic alteration. Geochemical analysis using EPMA and SEM-EDS, along with WinCcac software, were used to calculate the chlorite structural formulae based on 14 oxygens. Eight different chlorite geothermometers, based primarily on tetrahedral aluminum occupancy and octahedral vacancy, were then used to calculate the temperature of formation for the different chlorite types. Preliminary data indicates that the blue chlorites exhibit higher ratios of Fe/(Fe + Mg) when compared to the brown chlorites. Characterizing the chlorite generations in the Superior IOCG deposit provides insight into the thermal evolution of the altering fluids and will be used in comparison to chlorite chemistry from the other two IOCG deposits hosted by the LCS to better understand the broader hydrothermal system.