Austin Pluton and Reese River Mining District, Central Nevada: Magmatism, Alteration, Ore Formation, and Post-Ore Dismemberment and Tilting

New rock-type and alteration mapping, geochemical analyses, and structural reconstructions of the Reese River mining district, Lander County, Nevada, improve
understanding of ore-forming and deformational processes in the northern Toiyabe
Range (39.5° N, 117.0° W). Approximately 20 Moz of Ag were produced from silverbearing veins in the district from 1865 to 1887, with minor contributions of Au and
base metals.
The Austin pluton (~159 Ma) is an equigranular biotite-hornblende granodiorite that intruded lower Paleozoic rocks of the upper and lower plates of the Roberts Mountains thrust. New mapping and U-Pb zircon geochronology revealed two
additional, previously undistinguished intrusive units in and around the Austin pluton: the Grass Valley leucogranite (162.7 ± 1.6 Ma) and the Emigrant Pass porphyry (161.7 ± 1.6 Ma). A K-Ar age of 96 Ma on vein adularia indicates that the veins
probably formed during the early Late Cretaceous. Oligocene rhyolitic ash-flow tuffs
(31.4–25.4 Ma) unconformably overlie the Austin pluton and Paleozoic wall rocks and
dip ~35° east.
The district contains evidence for at least three hydrothermal systems: (1) The
equigranular Austin pluton contains quartz + tourmaline + biotite + K-feldspar and
albite-epidote veins, similar to veins that typically form at relatively deep levels of
porphyry copper systems. Newly mapped phases of the Jurassic intrusion, locally
a porphyry, are pervasively altered with kaolinite but weakly mineralized. (2) The
Austin pluton near Lander Hill contains the silver-bearing veins of the Reese River
district. The veins, commonly only a few centimeters wide, contain quartz, pyrite,
argentite, proustite-pyrargyrite, additional sulfosalts and sulfides, and inclusions of
sericite. Alteration envelopes contain sericite, chlorite, and limonite. Veins generally
strike northwest and dip moderately northeast. The origin of the veins remains uncertain, but they apparently are products of a Late Cretaceous, Au-poor, epithermal
system. (3) Breccias cemented by amorphous silica are hosted by Cenozoic volcanic
rocks northeast of Austin Summit; they are anomalous in Au and Ag and constitute
an undated epithermal system.
Gently to moderately east-dipping Oligocene volcanic rocks record large magnitude extension that initiated in the early Miocene, culminating in 3.8 km (45%) of
extension accommodated by at least eight gently- to moderately-west-dipping normal faults that divide the pluton into two major fault blocks. The faults have strike
lengths of 1.5 to > 35 km and offsets of 20–2,600 m. Emplacement pressures estimated
via amphibole-plagioclase paleobarometry suggest that the pluton is exposed from
depths of 5.2–7.0 km paleodepth in the western fault block and from 10.4–12.6 km
in the eastern fault block. Structural reconstructions suggest that the pluton and its
wall rocks were tilted ~20° to the west prior to Oligocene volcanism, then tilted ~35°
east following deposition of ash-flow tuffs. The origin of earlier inferred westward
tilting is poorly understood but may have been caused by concealed thrust ramps or basement-cored uplifts associated with east-vergent shortening of the Eureka and Sevier belts synchronous with Late Cretaceous plutonism. Eastward tilting is associated
with Basin and Range faulting that began at ~15 Ma. Restoration of Oligocene volcanic rocks suggests that cumulative tilting is < 10° to the east, i.e., Mesozoic intrusive rocks and associated hydrothermal systems are nearly upright at the modern surface. Key Words: Reese River mining district, Austin pluton, Low-sulfidation, Epithermal, Orogenic, Silver