The Elkhorn mining district is located 22 miles southeast of Helena, Montana, on
the southeast edge of the Boulder Batholith. The district is composed of a sequence of
Precambrian to Mississippian carbonate rocks and mudstones that have been intruded
by the Cretaceous Boulder Batholith and related stocks, dikes and sills. The sedimentary
rocks are locally thermally metamorphosed in the vicinity of these intrusive rocks.
Numerous gold skarn occurrences associated with calc-silicate hornfels,
endoskarn-altered intrusive rocks and, to a lesser degree, marble were the targets of
exploration drilling by Gold Fields Mining Corporation between 1984 and 1993, as
well as Santa Fe Pacific Gold from 1993 to 1997. These programs delineated an open
pit geologic resource of 1,935,750 ounces at a grade of 0.073 opt (ounces per short ton)
gold and 20,568,000 pounds of copper including an underground resource of 244,350
ounces at a grade of 0.225 opt gold.
In general, the skarns appear to have undergone the following genesis: (1) initial
isochemical contact metamorphism related to emplacement of intrusive rocks,
(2) prograde metasomatism that caused formation of pyroxene > garnet exoskarn and
pyroxene-plagioclase endoskarn in diorite, and, (3) retrograde alteration where prograde
pyroxene and garnet were replaced by amphibole, idocrase, epidote, plagioclase,
K-feldspar and calcite. Sulfide mineral associations are dominantly pyrrhotite >
chalcopyrite with lesser arsenopyrite, pyrite, and marcasite. Gold is present late in
the paragenesis and is strongly associated with bismuth and tellurium minerals.
Gold skarns in the Elkhorn district are similar to the gold skarns at Fortitude,
Nevada and Hedley, British Columbia. In contrast, they have minor similarities to
and some major differences with