GEOCHEMICAL AND GEOCHRONOLOGICAL ANALYSIS OF THE ENGELS GABBRO AND DIORITE IN THE LIGHTS CREEK AREA, PLUMAS COUNTY, NORTHERN CALIFORNIA

The Lights Creek Stock (LCS) granitoid in Northern California hosts three IOCG copper ore deposits. The three ore deposits (Moonlight, Superior, and Engels), all have differing copper mineralization and hydrothermal alteration. The Engels ore deposit is hosted by diorite and gabbro units on the Eastern side of the Lights Creek Stock granitoid. The gabbro and diorite units are texturally variable including local strong pegmatite and porphyritic textures. It is currently understood that the diorite and gabbro units were the first intrusions into the area and were later intruded by the LCS granitoids, and this is supported by core samples showing granitoid dikes intruding the gabbro. The LCS granitoids have been previously dated using U-Pb isotopes at 178.1+/- 3.9 Ma to 184 +/- 15 Ma (Dilles and Stephens, 2011), but an unpublished date at Engels is 110 Ma using K-Ar isotope methods (Judith Hannah pers. comm). This dating would change the interpreted timeline considerably as it would make the gabbro much younger than the granitoids. The purpose of this research project is to provide an absolute age for the diorite and gabbro, as well as further refine the genetic and temporal relationship between them and the granitoids that intruded them. Preliminary geochemical data suggests that the gabbroic rocks originated from a separate magma than the granitoid rocks. Specifically, when looking at a TiO2 vs Al2O3/TiO2 graph (Woodhead et al 1998), the granitoid and gabbroic rocks follow different differentiation trends, suggesting they are genetically unrelated. Nine samples of the gabbro and diorite units have been collected in a representative area of the Engels deposit for this project. These will be analyzed geochemically and five were selected for zircon mineral separation which will be used for geochronology. Geochemical data will be collected using in-house ICP-MS and XRF. Geochronological data will be collected at CSU Northridge using U-Pb age dating methods in their laser ablation lab.