Epithermal Deposits of Northern Nevada

Northern Nevada. Epithermal deposits of the low-sulfidation class are an important group of
precious-metal deposits in northern Nevada. Many of these deposits in Humboldt, Elko, Lander,
Eureka, and Washoe counties were developed into productive mines, from the turn of the century
to the present. Others are newly discovered or re-discovered, and are important to the exploration
and mining companies that are developing them into tomorrow’s mines.
These deposits display a familiar suite of characteristics. They occur along the Northern Nevada Rift
(NNR), parallel rifts, or splays. They developed during a major extensional event from 17 Ma to 13
Ma, with the largest number forming between 15-16 Ma. Bonanza gold and silver were deposited in
a variety of local settings and host rocks, commonly related to episodic boiling and the precipitation
of banded quartz and adularia. Some deposits formed in the Tertiary-age volcanic rocks (Midas,
Mule Canyon, Fire Creek, Goldbanks, and Sleeper), whereas others show the bonanza zone in the
underlying metasedimentary rocks (Goldbanks, Hollister, Jake Creek) beneath the Tertiary volcanic
pile. On a local scale, structural features such as dilatant jogs, cymoid loops, and open-space fissures
control the geometries of the high-grade ore shoots. In recent years, there has been a recognition
that the formation of the true bonanza ore grades occurred due to unique conditions in the
epithermal environment, such as deposition of gold and silver minerals via colloids. Lastly, in the
epithermal deposits along the NNR and related rifts, the role of bimodal volcanism is an important
aspect in the genesis of the host rocks. Extrusive and intrusive felsic and mafic host rocks behave
differently during mineralizing events, and control the various morphology and characteristics of the
gold and silver deposits in the region.
This three-day field trip is similar to the successful trip in 2010 that visited Wind Mountain,
Hollister, Hycroft, Sandman, and Midas. This year, we will leave Reno on Thursday morning, May
14, to visit three major deposits, two of which are new (Fire Creek and Goldbanks) and short
overviews for three other deposits. For convenience we will spend both evenings in Winnemucca.
Our first visit will be to the Goldbanks project, where startling new drill results in the last few years
require a major revision of the district’s exploration and genetic models. Our Thursday evening
entertainment will be provided by Don Harris, who will provide the group with an update on Allied
Nevada’s Hycroft gold-silver mine. On Friday morning, we will drive to the edge of the NNR for a
brief overview of the rift, the Midas trough, and two exploration projects, Jake Creek and Clover
Hill, before continuing to the Gold Circle (Midas) district. Stops in the Midas district will show us
the unique characteristics of the productive bonanza veins in the Ken Snyder mine, and will reveal
the activities of the new mine owner, Klondex Gold and Silver Mining Co. Lastly, we will drive to
Hollister (Ivanhoe district) for a roadside overview of the characteristics of the Hollister gold-silver
deposit. Friday evening we will hear from Justin Milliard as he describes the geology and ore
deposits at an advanced project in the region, Fire Creek. On Saturday morning, we will stop for roadside overviews at Mule Canyon and Beowawe, before continuing on to Fire Creek, where we
will see the relationship of truly rich bonanza ore shoots hosted in mafic flows and dikes. Following
the stops at Fire Creek, we will return to Reno via I-80.