THE LIGHTS CREEK DISTRICT COPPER DEPOSITS: PORPHYRY COPPER OR EVAPORITIC-SOURCE COPPER IRON-OXIDE MINERALIZATION?

The Lights Creek Stock is an 18 km2 copper-bearing granitoid intrusion associated with the Plumas copper belt in the northern California Sierra Nevada. Preliminary U-Pb zircon geochronology suggests that the stock is Late Jurassic in age. Although the Lights Creek copper deposits have been previously considered a porphyry system on the basis of stockwork veinlets and veins in a granitic host, the mineral alteration assemblages and zonation in the stock differ significantly from literature descriptions of classic porphyry systems. The association of granitic host rock with hydrothermal tourmaline and muscovite associated with disseminated and vein Cu-sulfides (bn, cc, cp) in the Lights Creek Stock are suggestive of porphyry style mineralization. However, many of the features reported are similar to the class of ores known as iron oxide-copper-gold (IOCG) deposits.

In the early 20th century, ore was extracted from two zones of multiple copper-sulfide veins within the southeastern granodiorite to gabbro part of the Lights Creek Stock: the Engels mine and the Superior mine with an average recovery grade of 1.79% copper from approximately 20 Mt of ore. Placer Dome discovered and extensively drilled the Moonlight Valley deposit in the 1960s, and Sheffield Resources has recently commenced a drilling project in Moonlight Valley to improve the orebody assessment, which is currently estimated to be ~200 Mt of rocks containing ~0.4 wt. % copper.

Geoanalytical, petrographic, and field studies are being used collectively to formulate a viable genetic hypothesis or geologic model for the copper-bearing igneous-related hydrothermal system and to provide insights into the source and composition of mineralizing fluids. The Lights Creek Stock will be particularly evaluated with respect to the iron oxide-copper-gold hydrothermal models of Barton and Johnson (1996) and Pollard (2006). If Lights Creek represents an IOCG type ore system, it represents one of the largest of its type in the USA. Geologic and geochemical characterization of the mineralization in the Lights Creek Stock could encourage not only further exploration and identification of additional sources of copper ore in the district but also contribute to a better understanding of copper mineralization both in the Lights Creek Stock and in other global occurrences.