ORES, MAGMATISM AND TECTONICS OF THE NORTHEASTERN SIERRA NEVADA MOUNTAINS—RECORD FROM THE LIGHTS CREEK COPPER DISTRICT, CALIFORNIA

The Taylorsville, CA, area in the northern Sierra Nevada contains a nearly continuous record of Jurassic (Kettle Rock and Mount Jura sequences) through mid Cretaceous (Evans Peak sequence) magmatism related to the Mesozoic Cordilleran arc. The Jurassic rocks host the granitic intrusions of the Plumas Copper Belt and are unconformably overlain by the Cretatecous (ca 141 - 127 Ma) clastic sedimentary rocks of Evans Peak, which is cut by a 117 Ma stock.

The 178±2 Ma Lights Creek stock (LCS) is the host for copper sulfide ores at the Superior and Engels Mines and Moonlight Valley deposit. The LCS has depletions in Ti, P, Ta, and Nb consistent with arc origin and a flat REE pattern. The hypabyssal stock intrudes island arc basaltic andesites of the Kettle Rock, which are extensively Na-altered to albite-rich assemblages near the contact. Na-alteration is widespread but weaker within the LCS, which principally is hydrothermally altered to biotite ± K-feldspar associated with tourmaline, biotite, magnetite and Cu-Fe sulfide veins. The LCS is cut by the China Gulch (CG) stock (148 Ma). Porphyry dikes with sericitic alteration (146 Ma) and local pyrite-chalcopyrite mineralization cut the Mount Jura sequence and post-date LCS magmatic and hydrothermal events. The LCS has both a zircon Eu/Eu* anomaly ~0.02-0.25 and whole rock Sr/Y (2-7) dissimilar to porphyry copper intrusions, whereas those of CG stock are similar (~0.5, 40-55, respectively).

Lead isotopic compositions of igneous and hydrothermal feldspar and sulfides from these intrusions [206Pb/204Pb vs 207Pb/204Pb plots] illustrate that K-feldspar from the LCS and CG are similar to 146 Ma sulfides (18.65; 15.60), whereas the sulfide and hydrothermal K-feldspar in the LCS ranges widely (18.50-19.60; 15.45-15.80, respectively; Stephens, 2011). The hydrothermal lead in the LCS stock is therefore partly derived from the stock, but also contains a large component of more radiogenic lead. These data support origin of the copper deposits in the Taylorsville area as IOCG type, similar to the iron oxide and copper deposits associated with Na-alteration and early Jurassic evaporites in the Pine Nut terrane of western Nevada. In contrast, the CG ages of 146-148 Ma are unusual regionally but similar to some igneous ages south along the Melones Fault zone and in the northern Klamath Mountains.