Spectroscopy of Carlin Trend Deposits

The Carlin Trend is located in north eastern Nevada and is
one of the world's richest gold mining districts.
It is a belt of sediment-hosted disseminated gold deposits about 5 miles wide and 40 miles long, extending in a northnorthwest direction through the town of Carlin, Nevada.
The gold deposits occur primarily in Paleozoic limy
sediments and are characterized by invisible (typically
microscopic and/or dissolved) gold in pyrite and
arsenopyrite.
Gold was first discovered in the area in the 1870s, but there
was very little production until 1909, and only about 22,000
ounces was produced through 1964. By 2008, mines in the
Carlin Trend had produced over 70 million ounces of gold,
worth around US$85 billion at 2010 prices.
1
Gold in Carlin Trend deposits is micron in grain size and is
disseminated through carbonate replaced host rocks,
mudstones and siltstones. Some of the deposits are close to
intrusives. Many of them carry carbonaceous material.
Origins of the deposits are not completely understood.
The alteration is subtle with the gold associated with clays in
the oxidized zones and sulfides in the refractory zones.
Common clays found include illite, kaolinite, smectite and
dickite. All except the smectite can be found with gold.
There is also minor alunite, but this can be a good target for
remote sensing. Ammonia is also associated with the illites
and can be used to vector the gold.
2
Landsat image of the Carlin Trend.
Note some of the open pit mines lined up along the red
arrow.
3
The Carlin mine is owned by Newmont Gold Company and is
located in the northern part of Eureka County, Nevada. It
commenced production in April 1965.
It was the first major modern gold discovery in the district and
started a gold boom in the region.
Ore reserves at the beginning of operations were 11 million
tons averaging 0.32 ounces of gold per ton. The Carlin mine
had produced a total of 3.2 million ounces of gold when
mining was suspended in 1986.
Photo: www.nevada-gems.com
4
From www.resourceinvestor.com:
The mechanics of a Carlin-style deposit are outlined here. A fault or dyke is
created in the earth’s crust by tectonic movement such as by strike/slip or
wrench faulting. Such a fault has lateral displacement but very little vertical
displacement of the fault blocks. Volcanic activity deposits an impermeable
“cap rock” over the area which seals off the upper aperture of the fault. At
some later stage magma squeezes into the fault. If an aquifer is present then
a hydrothermal process commences and hot mineral enriched fluids
percolate in the fault. If the faulted block contains a permeable layer, as
pressure builds in the cap-rock sealed fault the mineral enriched fluids will
flow into the permeable layer or layers. Such layers are typically quartz
bearing calcitic siltstones. The hot and corrosive hydrothermal fluids dissolve
the inter-granular calcite which increases the siltstone permeability and
porosity while the quartz grains prevent the formation from collapsing. Over
millions of years the formation is invaded with rich mineralized fluids which
eventually crystallize into gold rich minerals that fill the void between the
quartz grains. Unlike vein style deposits the gold is distributed throughout the
rock in a micro-crystalline form and is often not visible to the naked eye and
is only identified by laboratory assays.
5
This diagram shows a list of infrared (SWIR) active minerals
that can be identified in sediment hosted gold deposits.
6
The Dee Fault strikes north-south and bisects the pit. It overprints a set of
NW structures. There is an anticlinal feature in the Bootstrap Limestone with
an EW strike.
Lithology includes Bootstrap Limestone, Rodeo Creek and Vinnini.
Ore is proximal to the fault zone. There is also a flat, tabular ore zone
between the Bootstrap and lower Rodeo Creek. This is 500’ x 300’ thick
Four of the five cross sections were taken through the mineralized fault
zone. They all show similar associations within the fault zone of ammoniabearing illite, most of which are also high in aluminum.
It is potentially possible to differentiate the Bootstrap Limestone and silicified
Bootstrap by the spectral signature which has alternating layers of calcite
and illite with silica. The unmineralized Bootstrap shows mostly calcite with
some aluminum deficient illite.
The top of the system has an acid sulfate alteration suite including alunite,
jarosite, dickite and silica. Dickite is also seen either at the edge of the fault
or just into the fault on Sections 1, 4 and 5.
7
This is a very good example of changes in alteration types through a
mineralized zone. The Bootstrap samples show alternating zones of calcite
and silica with a silica illite signature in the jasperoid at the boundary with the
Dee Fault. Dickite occurs next to the jasperoid in the fault in a potentially
mineralized zone. The argillized fault zone contains silica and high
aluminum illite with ammonia. In the Rodeo Creek, the illite is mixed with
calcite.
It is hypothesized that the ammonia source lies in the black carbonaceous
pods which occur in the Rodeo Creek part of the Cross Section.
The illites within the fault zone are fairly consistent in composition and are all
high aluminum phases. Illites in other cross sections actually showed more
variations, some with lower aluminum contents. The compositional shift
seen to exist in hydrothermal illites associated with mineralization appears to
also exist at Dee. This must be verified with assay data.
The cross section is at the southern end at the bottom of the pit. The
pictures are dark.
8
The section starts in Rodeo Creek and goes from west to east through the
Dee Fault into silicified, mineralized Bootstrap limestone.
The fault zone has a diagnostic signature with NH-4 bearing, high aluminum
illites clustered in the zone. This is similar to DT-1, DEE-2 and DT-5
The zone in the Rodeo Creek, just before the Fault, is also similar to DT-1,
with high aluminum illites and this also has dickite and silica. There is
potential for mineralization here especially in the siliceous layers right next to
the fault zone.
In this section, the Bootstrap is silicified, mostly decalcified and mineralized.
Moderate to high aluminum illites appear with the silicification. These illites
have a different profile then those observed in the unmineralized, Bootstrap
samples from DT-3.
9
Unmineralized Bootstrap limestone - bench east side of pit – control
samples.
Calcite dominates the section with illite and some minor kaolinite. Illite is
well ordered implying a hydrothermal origin. It has moderate to low
aluminum content, which would be expected for this environment - proximal
to the main mineralized zones.
10
This is a diverse section. It lies in the Vinnini formation, which is thick
bedded chert in Upper Plate rocks.
On the west side of the Dee Fault Zone, the section goes through silicified
layers alternating with illite/silica/carbonate. Remnant carbonate implies
incomplete decalcification.
On the east side of the fault, the first 125’ contain dickite, alunite, jarosite,
silica. This could represent remnants of an advanced argillic cap, a steam
heated environment at the top of the fault. Since the precursor formation is
chert, the abundance of silica is not unusual. Some of the silica has an
almost opaline appearance.
The alunites are poorly crystalline and not a dominate phase and mixed with
jarosite and silica.
The NH4-illites could indicate a later stage of alteration, possibly venting up
the fault structure and overprinting the previous stage.
11
The Leeville Mine includes three deep, high-grade refractory
gold deposits (West Leeville, Turf and Four Corners) located
1.5 miles north of the Carlin Mine, Eureka County, Nevada.
Production commenced at West Leeville in 2006.
The deposits are dominantly hosted within flat-lying, silty
limestone of the upper Siluro-Devonian Roberts Mountains
Formation.
12
During the Society of Economic Geologists Carlin Type Deposits Field Conference, one of the
afternoon field visits involved looking at core from several deposits and drill holes. These
included West Leeville, the High Desert Joint Venture between Newmont and Barrick. Since
both Chief Geologists for the companies were present on the trip, it was possible to obtain
permission to look at selected samples of core with the PIMA InfraRed spectrometer.
The gold values were extracted from a plot provided by the High Desert Venture geologists,
Helen Robinson and Brad Leach in the Field Trip Guide.
The rocks in this section are very dark, actually black with some white clay and silica stringers.
They have very low reflectance, most less then 5%. The spectra taken with PIMA are plotted in
Figures A and B. Because of the low reflectance, there is a high degree of noise, especially
when enhanced, as in Figure B.
There does not seem to be an exact match of assay values to samples provided by the Newmont
geologist, however, the spatial relationship appears to be close enough to see a possible
correlation between gold and mineralogy.
Dickite and silica are the main InfraRed active minerals seen in this short section. The dickite is
mixed with kaolinite. The implication is that in this ore zone, the temperatures were borderline
for the formation of dickite. This produces such spectra as seen in Figure B, sample 622c where
absorption minima for both phases are seen.
This same type of relationship has been seen in the unoxidized rocks of other sediment hosted,
disseminated gold deposits such as the Betze Deposit, Genesis-Blue Star and Lone Tree.
Although it is not possible to make definitive conclusions on such selective data sets, the
relationships seen here merit further investigations.
13