GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 198-21
Presentation Time: 9:00 AM-6:30 PM


AYALA, Morelia, Geologic and Environmental Sciences, California State University, Chico, 400 W. First St, Chico, CA 95929, LOPEZ, Alexis R., Geological and Environmental Sciences, California State University, Chico, 400 W 1st Street, Chico, CA 95929 and AIRD, Hannah M., Department of Geological and Environmental Science, CSU Chico, 400 W 1st Street, Chico, CA 95929

The Moonlight deposit was emplaced 178 Ma and is hosted in the quartz monzonite intrusion found in the Lights Creek Stock (LCS) (Dilles et al., 2016). The Moonlight deposit is considered an unusual iron oxide copper-gold (IOCG) deposit because it contains tourmaline as the only mafic mineral in a quartz monzonite composition. From lithologic analyses of the drill core, tourmaline occurs as blebs, veinlets, and nest accumulations, which indicates two generations of tourmaline growth (Julliand, 1970). Tourmaline is an excellent petrogenetic indicator because it is mechanically and chemically stable due to its slow diffusion rate. In the deposit, tourmaline, oxides, and sulfides are associated in every mineralization stage. This research will investigate the history of fluid evolution in the Moonlight deposit through petrogenetic and geochemical analyses of tourmaline to characterize its variation in trace element composition and crystal habits. The goal is to investigate the fluid origins of tourmaline and its association with IOCG mineralization assemblages of ± chalcopyrite, hematite, chalcocite, quartz, alkali-feldspar, sericite, chlorite, bornite, pyrite, and malachite. The mineral assemblages associated with tourmaline are present in veins along fractures and occurs as disseminated blebs in and away from fractures, which also indicate hydrothermal fluid transportation. Methods used in the study to characterize tourmaline include lithologic, petrologic, geochemical, EPMA and SEM-EDS analysis. The analyses will determine the fluid origin of trace element chemical composition and textures of tourmaline. In thin section, tourmaline ranges from Mg-rich (dravite) to Fe-rich (schorl), suggesting a compositional change in fluids that altered the host rock. The LCS aplitic and granitic dikes are a source of heat and fluids that altered sedimentary and igneous rocks and produced the mineralization phases in the deposit (Stephens, 2011). Understanding the role in primary mineralization and evolution of tourmaline will expand current knowledge on the petrogenesis and alteration of the Moonlight deposit, and is applicable to interpreting the formation of similar IOCG deposits.