Tectonics, Geochronolgy and Mining History of the Jarbidge Mountains Volcanic Field, Elko County, Nevada: An Introduction

The Jarbidge Mining District in northern Elko County, Nevada (Figure 1) hosts
low-sulfidation epithermal quartz-adularia veins in mid-Miocene rhyolitic volcanic
rocks. Greater than 355,000 ounces of gold and 1,600,000 of silver were produced from
approximately 800,000 tons of ore between 1918 and 1932 (LaPointe et. al., 1991). The
silver to gold ratio of the ore was approximately 10:1. The district was the largest
producer of gold in Nevada for a brief period prior to the success of the Nevada Consolidated
Copper Co. operations at Ely, Nevada (Schrader, 1923). All production was
from underground workings, typically utilizing the shrinkage-stoping method which
renders modern access into the mines very dangerous. Complex faulting and vein
development in conjunction with multiple laterally discontinuous but very similar
rhyolite eruptive units have hindered historic and modern exploration efforts.
Exposures of productive veins in the district are extremely rare and descriptions
of veins are primarily based on examinations conducted during the main period
of mining, primarily by Frank Schrader in 1920 (Schrader, 1923). Primary ore minerals
are native gold, electrum, argentite (Ag2S), and naumannite (Ag2Se) (Figure 2).
They occur in a variety of styles including comminuted margins of veins, hydrothermal
breccia lenses associated with veins and within finely crystalline quartz veins and
veinlets that typically contain late adularia along the margins (Figure 3). Multiple
vein deposition events and contemporaneous minor faulting with gouge development
are common. Where visible, electrum is typically associated with or adjacent to small
(< 2 mm) blebs of argentite or naumannite. Plate-texture of quartz and adularia after
calcite, named “fish-scale quartz” by early miners, is common in the district and
is compelling evidence for multiple boiling events. Within a specific vein formation
event adularia is typically deposited later than quartz, however, this later-deposited
adularia is often encapsulated by quartz deposited during a subsequent depositional
pulse. Adularia dominant veins typically have low gold content. Throughout the Jarbidge
District gold in the veins typically occurs as small particles which although
commonly free-milling is not suitable for recovery by gravity methods or amalgamation.
This, in conjunction with high stream gradients and very few exposed bedrock
traps, is the reason there was very little placer gold recovered in the district and that
significant lode mineralization was discovered very late relative to other epithermal
vein districts in Nevada.
Key Words: epithermal, adularia, Miocene, rhyolite, gold