EXTENT OF ALTERATION WITHIN THE SUPERIOR MINE AT THE LIGHTS CREEK STOCK (IOCG), PLUMAS COUNTY, CA

The Lights Creek Stock (LCS) is a quartz monzonite granitoid that hosts multiple iron oxide copper-gold (IOCG) deposits which have been mined for copper since the 19th century. These deposits exhibit diverse degrees and types of hypogene and supergene alteration. A notable contrast between two mines close in proximity, Superior and Moonlight, is the type of iron oxide that dominates, with Superior a magnetite-dominated system, while Moonlight a hematite-dominated system.

Magnetite, which can form both magmatically and hydrothermally, typically forms deeper than hematite in IOCGs, at higher temperatures and lower oxygen fugacity, so Superior is thought to have been proximal to the fluid source in comparison to Moonlight, though currently they reside at approximately the same elevation. Copper mineralization at Superior is dominantly vein-hosted with minor disseminated chalcopyrite and trace amounts of bornite. The copper sulfides hosted within the Moonlight deposit have been characterized as mostly disseminated, with a larger proportion of bornite (25%) and chalcocite (15%). This suggests Superior had little supergene alteration in relation to the Moonlight deposit which would occur at shallower depths mobilizing and upgrading the copper ore grade. Utilizing both a reflective light microscope and a scanning electron microscope to identify alteration sequences, the presence of titanite was used to identify potassic alteration, and sodic-calcic alteration was identified by tourmaline and rutile. Both hydrothermal and primary magnetite are interpreted to be present within Superior: magmatic magnetite contains >100 ppm Ti; hydrothermal magnetite contains <100 ppm Ti and a higher Ni/Cr ratio than magmatic.

Characterizing the IOCG deposits associated with Lights Creek Stock will not only further understanding of mineralization and alteration sequences in this very broad class of deposits. With Moonlight and Superior being in such close proximity geographically, this gives a unique perspective to understand the differences between magnetite and hematite-dominated IOCGs, in particular, insight into deeper magnetite-dominated higher temperature hydrothermal alteration that is closer to the fluid source compared to a hematite-dominated system that shows more supergene alteration.