PETROGRAPHIC ANALYSIS OF THE TOURMALINES FROM THREE IOCG DEPOSITS WITHIN THE LIGHTS CREEK STOCK

The Lights Creek Stock (LCS) is a granitoid pluton located in the Northern Sierra Nevada, CA. The LCS hosts three IOCG deposits that were historically mined for Cu. These formed through a long history of magmatic activity, hydrothermal fluid, and surficial water interactions. The three deposits have varying alteration sequences and mineralization due to differences in host rock, depth, and interaction with hydrothermal fluids, but tourmaline is found in various textures and assemblages throughout the LCS. Tourmaline may form under a variety of conditions, from magmatic to hydrothermal, and the chemistry of hydrothermal tourmaline may be used to understand the chemistry of the altering fluids. Insight on the hydrothermal fluids within the system will help to determine the genesis of the IOCGs. This project aims to characterize the generations of tourmaline and the surrounding assemblages in all three of the deposits using petrographic and SEM-EDS analysis to prepare for future isotopic analysis of the tourmaline throughout the stock. Samples were taken from outcrops and available drill cores across the stock. Preliminary work within the Moonlight deposit has shown multiple generations of tourmaline growth with the chemistry of schorl, dravite, feruvite, and uvite. Preliminary work also details tourmaline observed in the Moonlight and Engels deposits occur largely due to sodic-calcic alteration. Currently, potassic, sodic-calcic, and hydrolytic alteration have been characterized in the IOCG deposits of the LCS. Tourmaline occurs in blobs, nests, and veins, as well as in tabular and acicular complexes throughout the three deposits and further examination will aim to constrain and connect textures to associated mineral assemblages seen throughout the deposits.