LATERAL IOCG FORMATION IN THE PLUMAS COPPER BELT: INSIGHTS FROM COPPER ISOTOPES

The Plumas Copper Belt, located in Plumas County, California, is a historically significant copper producer that began operations in the 19th Century. Some of the major Cu bodies are hosted by the Lights Creek Stock (LCS), a quartz monzonite that hosts 3 iron oxide–copper–gold (IOCG) deposits. Two IOCG deposits of interest are Superior, a magnetite-dominated system, and Moonlight, a hematite-dominated system. These deposits exhibit similar alteration patterns and are approximately 2 km from each other, suggesting a single ore-forming system. Although both Moonlight and Superior display sodic-calcic alteration, Moonlight also exhibits extensive hydrolytic and supergene alteration, whereas Superior has localized potassic alteration and limited to no hydrolytic and supergene alteration.

Copper isotope studies may be used to explore the source of Cu in a deposit, with different Cu reservoirs having distinct isotopic signatures. For example, magmatic chalcopyrite typically falls within a δ65Cu of +1.0 to -1.0 while supergene copper minerals are enriched in δ65Cu, and leached minerals are depleted. Variations between +1.0 and -1.0 indicate fractionation between lighter to heavier magmatic chalcopyrite, though mixing fluids can influence the fractionation of copper. Superior has an average δ65Cu value of +0.51, while Moonlight has a value of -0.28, indicating Superior is isotopically heavier than Moonlight with combined data showing a linear trend and normal distribution at 0. Mineralogy coupled with copper isotopes suggest Moonlight was subjected to a higher degree of magmatic and brine fluid mixing compared to Superior allowing the crystallization of lighter copper. This is supported by previous hydrothermal and igneous lead isotope data indicating that Moonlight has a higher radiogenic signature showing significant mixing of non-magmatic and magmatic hydrothermal fluids while Superior shows mostly magmatic signatures.

The copper isotope data support the hypothesis that Superior and Moonlight are part of a single ore-forming system. The magmatic fluid source is likely proximal to Superior, and the non-magmatic brine closer to Moonlight, suggesting the IOCG system formed laterally, since no evidence of significant tilting is documented in the region.