GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 184-2
Presentation Time: 9:00 AM-6:30 PM


VELAZQUEZ, Mario G., Geological and Environmental Science, California State University Chico, 400 W. First Street, Chico, CA 95929; Department of Geological and Environmental Science, CSU 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 Engels deposit, located in Plumas County, CA, is an iron oxide copper-gold (IOCG) deposit hosted by a mafic intrusion composed of gabbro, quartz diorite, and diorite. The gabbro unit within the Engels deposit contains layering. Previous mapping shows that the layered gabbro is located near the contact of the metavolcanics and gabbro. The goal of this project is to evaluate the gabbro’s magmatic history and to identify if the source of the layering is magmatically derived or driven by late-stage fluids. Past research on the Engels deposit has only focused on characterizing the mineralization of the gabbro unit rather than the textural features present within the gabbro (Juilland, 1970). The gabbro contains a high amount of amphibole, indicating an abundant source of water either before or after emplacement (early vs. late stage crystallization). During field observations, 10 gabbroic outcrops were observed that ranged in composition from hornblendite (hornblende rich gabbro, >80%) to plagioclase-rich gabbro (>55%). Observations show gabbro layering at five separate outcrops, displaying comb layering, and centimeter-scale layering (white layer average 0.7 cm, black layer average 1.4 cm). Comb layers contain alternating sequences of at least one of the following textures: (1) phenocrysts of anhedral hornblende, (2) veins of hornblendite, (3) plagioclase dominated veins with acicular hornblende crystals. The outcrops with no layering display textural variations ranging from equigranular crystals to poikilitic hornblende crystals enclosing the plagioclase. Observations from 10 thin sections, SEM-EDS data, and bulk-rock geochemical data are used to characterize the textural and compositional changes within the variety of gabbro textures, and to explain the gabbro magmatic history and source of the layering. Studying the formation of the gabbro from the Engels deposit can give insight into the formation and implication of hornblende-rich gabbros, increasing our understanding of the emplacement processes for similar mafic intrusions within the Sierra Nevada Batholith.