| Abstract |
- The Lights Creek Stock is an 18 square kilometer copper-bearing granitoid
intrusion within the Plumas Copper Belt in the northern California Sierra Nevada. Engels
Mine, Superior Mine, and Moonlight Valley represent the main copper-mineralization in
the Lights Creek district and small prospects include the Ruby Mine, and Moonlight
Creek. The Moonlight Valley deposit within the Lights Creek Stock has been previously
considered a porphyry copper deposit on the basis of stockwork veinlets in a granitic
host. The granitic host rock of the Lights Creek Stock contains hydrothermal tourmaline
and biotite as well as disseminated and vein copper-sulfides (bornite-chalcocite-chalcopyrite),
and these features are supportive of porphyry-style mineralization.
Nonetheless, the mineral alteration assemblages and zonation observed at Moonlight
Valley and the Superior and Engels Mines differ significantly from literature descriptions
of porphyry systems. Ores in the Lights Creek district are notably sulfur-poor and
copper-rich. Mineralization includes copper-sulfide veins with pink potassium-feldspar
selvages, tourmaline veins with albitic-chlorite selvages, and actinolite-apatite veins, as
well as albite-magnetite alteration in the Kettle Rock volcanic sequence intruded by the
Lights Creek Stock. These features are similar to the class of ores known as iron oxidecopper-
gold (IOCG) deposits.
Lights Creek represents one of several copper- and iron -bearing ore districts
associated with early Mesozoic intrusions in northern Nevada and California, including
both porphyry copper-skarn ores and IOCG deposits. This region is underlain by an early
Mesozoic marine province of volcanic and sedimentary rocks, including evaporites,
intruded by shallowly emplaced Jurassic, and in some cases, Triassic plutons (Stewart et
al., 1997; Proffett and Dilles, 2008). This scenario is ideal for formation of IOCG
deposits produced by advection of sedimentary brines by the heat of the intrusions
(Barton and Johnson, 1996).
A uranium-lead zircon age of 178.1 ± 3.9 (2s) Ma was observed from two
samples of hypabyssal quartz monzonite on the east side of the Lights Creek Stock where
it intrudes the Early Jurassic volcanics of the Kettle Rock sequence. The Lights Creek
Stock originally contained igneous hornblende, which exhibits hydrothermal alteration to
tourmaline, chlorite and local biotite associated with narrow sulfide (pyrite, chalcopyrite)
and tourmaline veins. Apatite-actinolite veins and albite-alteration zones also cut the
Lights Creek Stock. In the Superior Mine, east-dipping ore-bearing veins (<10 cm to 2 m
wide) are zoned from central copper-rich zones (bornite, chalcopyrite, magnetite)
associated with hydrothermal tourmaline and biotite alteration to outer sulfide-rich zones
(chalcopyrite, pyrite, sphalerite) with sericite-chlorite alteration. Gently dipping
intermineral lamprophyre dikes cut the bornite-bearing veins, but are in turn cut by the
pyrite-chalcopyrite-rich veins. Similar ore zonation was observed by Storey (1981) in the
northwest part of the Lights Creek Stock at the large Moonlight Valley deposit. In the
Moonlight Valley, Engels Mine, and Superior Mine deposits porphyry dikes are absent.
The observations are consistent with an IOCG association of copper mineralization
broadly synchronous with the emplacement of the 178 Ma Lights Creek Stock.
The light-colored, relatively fresh and unmineralized China Gulch Granite
intrudes the east side of the Lights Creek Stock, and yielded a uranium-lead zircon age of
148.1±1.3 (2s) Ma. Quartz porphyry dikes intrude Middle Jurassic volcaniclastic and
volcanic rocks of the Mount Jura sequence (Christe, 2011) west of the Lights Creek
Stock along Moonlight Creek, and are here cut by quartz-tourmaline sulfide (pyrite,
chalcopyrite) veins associated with strong sericitic alteration. Muscovite from this
alteration zone yielded an ⁴⁰Ar-³⁹Ar plateau age of 146.05±0.88 (2s) Ma, the same age as
the China Gulch Granite to the east, and suggest latest Late Jurassic age of porphyry
copper mineralization.
Copper-sulfides from Moonlight Valley and Superior are characterized by lead
isotope values of ²⁰⁶Pb/²⁰⁴Pb ~19.4, while lead isotope values of copper-sulfides from
Moonlight Creek and Engels are close to the igneous values of the Lights Creek Stock at
²⁰⁶Pb/²⁰⁴Pb ~18.6. The lead isotope data support the hypothesis that the fluids that
formed the Moonlight Creek deposits and a portion of the ore of the Engels deposit were
magmatic-hydrothermal, while the Moonlight Valley and Superior Mine ores are in part
derived from non-magmatic fluid source reflecting an older lead source.
Copper mineralization in the Lights Creek district likely included components of
both porphyry-type magmatic-hydrothermal and IOCG-like non-magmatic sedimentary
brine fluids. The Lights Creek Stock was emplaced in the late Early Jurassic (~178 Ma)
and was in turn intruded by the younger late Late Jurassic China Gulch Granite (148 Ma).
Magmatic fluids derived from the Lights Creek Stock produced tourmaline ± biotite ±
magnetite ± chalcopyrite ± bornite veins in potassic alteration containing zones of
hydrothermal potassium feldspar + biotite. Contemporaneously, the intrusion of the
Lights Creek Stock drove advection of sedimentary brines through the adjacent
metavolcanic rocks and margins of the stock. These sedimentary brines produced zones
of albitic alteration and ores characterized by apatite + actinolite + titanite + magnetite +
ilmenite ± chalcopyrite ± bornite ± epidote observed at Sulfide Ridge and the Superior
Mine. Mixed fluids then produced zones of of albite ± chalcocite ± chalcopyrite ±
tourmaline occuring at Sulfide Ridge, Superior Mine, and Moonlight Valley. In the late
Late Jurassic (148 Ma), renewed magmatic activity resulted in the intrusion of the China
Gulch Granite and granite porphyry dikes. The latter produced small-scale porphyrystyle
mineralization along Moonlight Creek and Ruby Mine west of the Lights Creek
Stock.
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