20 Economic Geology (Posters)

Sunday, 23 March 2014
1:30 PM - 4:15 PM
Freedom Hall A

AGE AND ORIGIN OF A GOLD-BEARING BRECCIA FROM THE HILLTOP GOLD DEPOSIT, LANDER COUNTY, NEVADA

Paper number: 20-1
Program: Technical Sessions
Time: 1:30 PM – 4:15 PM

Authors

Abstract

This study focuses on determining the age and origin of a gold-bearing breccia within the Hilltop gold deposit, northern Shoshone Range, Lander County, Nevada. The breccia consists of angular fragments of bleached and recrystallized Ordovician Valmy Formation (?) chert, argillite, siltstone, and quartzite and altered Tertiary feldspar-quartz porphyry within a matrix of clear, euhedral quartz + sulfides (py>asp>gal,stib), jarosite, and native gold/electrum.

Analysis of primary fluid inclusions (n=4) within the quartz yield salinity (2.7-4.0 equiv wt % NaCl) and Th (180.6-192.4oC) data. The calculated δ18OVSMOWsignature of the quartz matrix source fluid (-7.5 to -6.5‰) suggests the breccia formed from a mixture of meteoric and magmatic components.

Argon/argon age determination of a three-phase, K-bearing clay (65% illite, 22% quartz, 13% scorodite) within the quartz matrix yields a plateau age of 46.1±0.4 Ma, indicating a breccia age ~10 Ma older than a previously-reported age of 35.9±0.4 for the same breccia (illite/smectite; Kelson, 2008) and ~6 Ma older than ages of igneous rocks and molybdenites proximal to the breccia studied here (Kelson, 2008).

Fluid inclusion and stable isotope data support an epithermal origin for the breccia.