Geology of the Antler and Basalt gold deposits, Glamis-Marigold mine, Humboldt County, Nevada

Glamis Marigold Mining Company operates a multiple
open pit gold mine in extreme southeastern Humboldt County,
Nevada, a few miles south of Interstate Highway 80 near Valmy
(Fig. 1). The Marigold mine has been in operation since stripping
of the covered 8 South ore zone began during September,
1988. The first doré bar was produced nearly a year later in late
August, 1989. The property first operated as a multiple open pit
mine with combined run-of-mine heap leach and cyanide-inleach
mill processing, but in recent years all ore has been
treated by heap leaching. McGibbon and Wallace (2000)
described the general geology of the immediate mine area and
the individual ore zones then under development.
Pre-mining reserves at $400/oz Au totaled approximately
658,000 oz Au. About two-thirds of this total was in the 8 South
pit (Fig. 2), the first to be mined, with the rest distributed
among the Top Zone, East Hill, and Red Rock deposits. The
latter three deposits are now combined in a larger operation
called the Terry Zone. Through the end of July, 2004, approximately
1.6 M oz Au have been mined, yielding about 1.2 M oz
for sale, and at year-end 2003 the proven and probable reserves
were 93,315,300 short tons at an average grade of 0.024 oz
Au/ton, and 2,211,450 contained oz Au. Presently Glamis Gold,
Ltd. owns two-thirds interest, with the remaining one-third
interest held by Barrick Gold.
A comprehensive study named the “Millennium” Project
was designed and conducted by Marigold to calculate reserves
and resources, and to optimize equipment size, fleet size, and
mine production rates. The new additions to reserves and
resources along with economies of scale from bigger truck and
equipment fleets that were justified by increased reserves have
transformed Marigold from a modest operation producing
70–90,000 oz Au/year to a major mine with a planned rate of
well over 200,000 oz/year for the next several years. Significant
exploration potential still remains on the property.
Present (late 2004) mining activities are situated in the Terry
Zone pit complex (Fig. 2) in Sections 18 and 19, T33N, R43E.
One new pit known as Target 1 is presently being opened in Section
30. This pit lies between the present Terry Zone pit and the
planned Basalt and Antler pits that will be discussed in more
detail below. Two other zones, one northeast of Target 1, and the
other further south are also planned to enter the mining sequence;
these are known as the Mackay and Target 2 zones respectively.
The two deposits known as the Antler and Basalt zones
(Figs. 2 and 3) are located near the southern end of the Marigold
property in Section 31, T33N, R43E and Section 6, T32N,
R43E, and are scheduled for production beginning in early 2005.
Combined preliminary measured, indicated, and inferred
resources for the Basalt and Antler zones are 1.48 M oz Au.
Exposed mineralization that became part of the Antler zone
was discovered in 1986 by Cordex Exploration personnel in surface
samples taken from a small ore-grade jasperoid body
exposed by erosion along a minor structure in a gulley. The
Antler zone is so-named because it is hosted primarily by altered
rocks of the Antler sequence. The Antler deposit is located
mostly in the southwestern part of Section 31, T33N, R43E, with
part of the zone extending southward into northwestern Section
6, T32N, R43E (Fig. 4). The recently discovered (1999) Basalt
zone (Figs. 3 and 4) was encountered in the first hole drilled in
an exploration drilling program designed to evaluate a covered
area underlain by potential host rock of the Valmy Formation
and believed to contain a north-striking high-angle structure.
This blind gold zone is less than 300 meters northeast of the
present-design Antler pit. The ore zone lies below an inferred
intra-formational thrust sheet, composed in part of metabasalt,
which is responsible for the name of the deposit.