Geology of the high-sulfidation Graben gold deposit, Borealis District, Mineral County, Nevada

The Borealis hydrothermal system is one of the largest areas of epithermal alteration
and mineralization in the state of Nevada, estimated at more than 20 square
miles. The high-sulfidation Graben deposit is located in the Borealis mining district
among a cluster of small to medium-size high-sulfidation gold deposits. High-grade
gold in the Graben deposit occurs with pyrite and marcasite in hydrothermal breccias
within Miocene andesite flows, lahars, and breccias beneath a combination of at least
200 feet of alluvium and 300–400 feet of post-mineralization (?) sedimentary rocks in
a pediment environment. Mineralization extends along a northerly striking corridor
that is characterized by a subhorizontal low-grade gold aureole surrounding steeply
dipping high-grade zones. Gold mineralization is closely associated with at least two
types of brecciation, multiple phases of silicification, and quartz and quartz-sulfide
veining. Hydrothermal alteration is consistently zoned outward from the gold deposit.
Immediately outward from gold bearing silicified zones, and in places overlapping it,
is a shell of argillization. The inner portion of the argillic zone is characterized by
advanced argillic mineralogy. Peripheral to the argillic zone, and partly overlapping
it, is an area of opal replacement. The outermost assemblage is characteristic of
propylitic alteration. The Graben deposit, containing a 700,000-oz resource, is the
largest gold deposit found to date in the Borealis district.
Several holes drilled in 1981–82 by Houston Oil and Minerals/Tenneco intercepted
anomalous gold in the area near the Graben deposit. Discovery of ore-grade
mineralization in the deposit was accomplished in 1986 by Tenneco/Echo Bay as stepoff
holes from the Freedom Flats deposit, and sporadic drilling continued through
1989. Subsequent exploration in the 1990’s by Santa Fe Pacific and Cambior provided
further definition of the deposit. Gryphon Gold, the current project operator, following
the planned drilling and expansion of near-surface oxidized gold resources, will
then focus on the Graben trend and other pediment targets in the company’s exploration
efforts to substantially increase additional reserves in the district.