Geology of the Moonlight Valley porphyry copper deposit, Lights Creek, Plumas County California

Abstract

The Moonlight Valley porphyry copper deposit contains 250 million tons of .35% copper and minor but important gold and silver, occurring within a cupola-like body adjacent to the quartz monzonitic Lights Creek stock. The principal ore mineral is chalcopyrite, but most ore zones are coincident with the presence of bornite and/or primary chalcocite. The sulfides are zoned from chalcocite-bearing cores outward to pyrite-bearing fringes. Pyrite is subordinate in quantity and extent of occurrence as compared to magnetite, indicating a low sulfur system. Potassic, sericitic, and propylitic alteration types are present, but are low in intensity and are telescoped over the same rock volume rather than being cylindrically arranged. The style of mineralization and alteration indicate that the deposit is more magmatic than hydrothermal in nature. The nature of the mineralization in Moonlight Valley, and the Lights Creek area as a whole, compares to mineralization in widely distributed areas throughout the world that are directly related to island arc magmatism and genesis. Because of the location within a Jurassic island arc terrain, and the existence of other known Jurassic deposits in this same terrain (e.g., Yerington), the Moonlight Valley deposit is considered to be Jurassic in age. It is not considered to be directly related to the adjacent Cretaceous Sierra Nevada batholith.