ENGELS GABBRO AND GRANITOID DIKE RELATIONSHIPS WITHIN THE LIGHTS CREEK STOCK

Around 178 million years ago during the Jurassic period, the Lights Creek Stock (LCS) granitoid complex was emplaced beneath what is now Plumas County, California (Stephens 2011). The LCS is a granitoid host to three iron-oxide-copper-gold (IOCG) deposits, one of which is called the Engels deposit that is dominantly hosted by mafic gabbro and diorite. IOCG deposits are typically exploited for their copper and/or gold content and are associated with widespread silicate alteration from the mixing of magmatic and basinal fluids. The Engels gabbro-diorite deposit of the LCS is intruded at depth by several cross-cutting quartz monzonite dikes, and the goal of this project is to determine if there is a relationship between the Engels host rock, the cross-cutting granitoid dikes and/or the LCS granitoid.

To achieve this goal, the analysis of field and drill core relations, petrography, X-ray fluorescence, and bulk rock geochemistry via in-house ICP-MS methods were used. Petrographic analysis showed evidence of potassic hydrothermal alteration, indicated by the presence of large amounts of biotite and potassium feldspar, particularly in the dike samples, whereas the gabbro samples showed high amounts of amphibole and pyroxene but also included some small amounts of biotite.

Though more ICP-MS and XRF testing is still needed, it is expected that the quartz monzonite dikes relate significantly to the LCS granitoid and share the same magmatic source, and are altered from this source. Since the Engels gabbro-diorite has a drastically different bulk composition, it is expected that it is unrelated to the LCS granitoid and the dikes, and has a different magmatic source.