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, that hosts three IOCG (iron oxide copper gold) deposits historically mined for copper. The three deposits have differing alteration sequences and mineralization, but tourmaline is found throughout the LCS and deposits in various textures and assemblages. 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 hydrothermal fluids. Insight on the hydrothermal fluids within the system will help to determine the genesis of the IOCG deposits. This project aims to characterize the generations of tourmaline and the surrounding assemblages in all three of the deposits using petrographic, SEM-EDS, EPMA, and isotopic analysis. Samples were taken from outcrops and available drill cores across the stock. Preliminary work suggests that potassic, sodic-calcic, and hydrolytic alteration are present in varying proportions across the three IOCG deposits, and that tourmaline observed in all deposits is associated with sodic-calcic alteration. Preliminary EPMA analyses within the Moonlight deposit have shown multiple generations of tourmaline growth with the chemistry of schorl, dravite, feruvite, and uvite. Having observed a transition in tourmaline composition from brown schorl to blue dravite tourmaline overgrowths indicates a chemical transition in the hydrothermal fluid that grew these crystals. The fluid must have evolved from relatively Fe-rich to Mg-rich, which is consistent with magnetite crystallization which draws Fe from the hydrothermal fluid. Tourmaline occurs in blobs, nests, and veins, as well as in tabular and acicular morphologies throughout the three deposits and further characterization will aim to decipher the connections between compositions, textures, and their associated mineral assemblages seen throughout the deposits.