**REPUBLISHED WITH PERMISSION FROM "tHE ALASKA MINER" VOLUME 34, NUMBER
2, FEBRUARY, 2006**

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height="4.493055555555555in"}

Cover photo: View looking southwest toward the Shotgun gold prospect, 5
miles away (see arrow). Boulders in the foreground are part of the
Shotgun Hills \~70 Ma granodiorite pluton. The main Shotgun deposit is
hosted by quartz-feldspar porphyry, a phase thought to be related to the
granodiorite. (USGS photo)

**USGS mineral resource studies in the Taylor Mountains quadrangle,
southwestern Alaska---new "AMRAP"**

**By: M.L. Miller^1^, E.A. Bailey^1^, Beischer, G.A.^2^, D.C.
Bradley^1^, T.K. Bundtzen^3^, R.J. Goldfarb^4^, S.M. Karl^1^, R.W.
Saltus^4^, and B. Wang^1^**

^1^ U.S. Geological Survey, Anchorage

^2^ Bristol Environmental & Engineering Services Corporation

^3^ Pacific Rim Geological Consulting

^4^ U.S. Geological Survey, Denver

Introduction

The Taylor Mountains 1:250,000-scale quadrangle (Fig. 1) has long been
recognized as prospective ground for yet-to-be discovered mineral
resources based on the presence of a few known prospects and generally
favorable geology, combined with knowledge that a large part of the
region remained mostly unexplored. The lack of exploration is not
surprising given the difficult logistical access. In 2004, the U.S.
Geological Survey (USGS) began a new project to evaluate the potential
for undiscovered mineral resources in southwestern Alaska, starting in
the Taylor Mountains quadrangle, the largest data gap in the region.
Adapting the approach of a 1970s to mid 1990s USGS Program that was
called AMRAP (Alaska Mineral Resource Assessment Program), this new
project's goals were to provide basic geologic mapping, regional
geochemical sampling, mineral deposit studies, and airborne geophysics.

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Figure 1. The Taylor Mountains quadrangle is located in central
southwestern Alaska.

The project is funded under the USGS Mineral Resources Program (MRP). In
2004 an aeromagnetic survey at 1-mile spacing was flown for the entire
quadrangle (Saltus and Milicevic, 2004), geologic mapping was completed
for 700 square miles (\~10% of the quadrangle), and stream-sediment,
heavy mineral concentrate, and water samples were collected at 86 sites.
As part of a Cooperative Research and Development Agreement (CRADA) with
USGS, Bristol Bay Native Corporation (BBNC) collected an additional 111
stream-sediment and concentrate samples.

In 2005, project funding nearly doubled largely due to a special
one-year Congressional appropriation, championed by AMA, for the "Alaska
Mineral Resources Assessment program" (AMRAP). The greater funding
allowed us to vastly increase our field studies in the Taylor Mountains
quadrangle, effectively quadrupling the area covered. In 2005 we
completed reconnaissance geologic mapping for about 3,300 square miles
(\~50% of the quadrangle) and collected stream sediment, concentrate,
and water samples from more than 400 sites. We conducted detailed
economic geology studies in several locations and we ran magnetometer
ground traverses near TNR Gold's Shotgun Hills property. We also
contracted for an aeromagnetic survey of the Dillingham quadrangle, just
south of the Taylor Mountains quadrangle; resulting data are expected to
be released in April, 2006. The additional data will improve the ability
of both USGS researchers and our customers to assess the potential for
undiscovered mineral deposits throughout a large part of southwestern
Alaska. A brief summary of results to date follows.

Geology

The Taylor Mountains quadrangle is largely underlain by Mesozoic flysch
sequences that overlap several older terranes (Fig. 2). The northeastern
corner of the quadrangle contains rocks of the Neoproterozoic to
Jurassic Farewell terrane---a microcontinental fragment. The
southwestern part of the quadrangle contains rocks assigned to the
Goodnews terrane, a Paleozoic to Mesozoic oceanic assemblage. The
northwestern corner is underlain by rocks that Decker and others (1994)
included as part of the Togiak terrane, a Mesozoic arc. A suture zone
must exist between the Farewell and these other terranes, but it is
obscured and covered by deformed Mesozoic flysch. The right-lateral
Denali fault splays into the region. The Mulchatna fault, which is
likely another strike-slip feature, cuts through the southeastern corner
of the quadrangle.

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Figure 2. Generalized summary of the 2004-2005 geologic mapping in the
Taylor Mountains quadrangle. Mineral localities mentioned in the text
are also indicated.

New mapping suggests that terrane definitions and boundaries in the
Taylor Mountains quadrangle need revision. In the southwestern corner,
the "Tikchik subterrane" of the Goodnews terrane was previously thought
to be a chaotic mélange (Decker and others, 1994), but new mapping shows
instead a complicated, yet largely coherent sequence of repeated thrust
panels juxtaposing Permian, Triassic, and Early Cretaceous rocks. The
northwestern corner of the quadrangle includes rocks traditionally
considered to be part of the Togiak terrane. These rocks, part of the
newly "restricted" Gemuk Group, form a coherent deep marine
stratigraphic succession. They include Triassic intermediate pillow
lavas interbedded with siltstone, chert, and rare limestone; Jurassic
chert and siltstone; and Lower Cretaceous graywacke and tuff. We believe
these rocks formed in a back-arc, intra-arc, or forearc basin setting,
thus distinct from but perhaps associated with, the Togiak volcanic arc.
The Farewell terrane in the northeastern part of the quadrangle is
represented by an imbricated fold-thrust belt that juxtaposes Cambrian
to Silurian limestone with Triassic to Lower Jurassic mixed chert,
limestone, and clastic rocks. The contact with the overlapping
Cretaceous flysch was previously understood to be a simple thrust fault,
but the complex map pattern suggests that some of the thrusting involved
the Cretaceous rocks as well. We hypothesize that the Farewell terrane
exists here as a thin flap thrust westward several tens of kilometers
over rocks of the Kuskokwim Group.

Mesozoic flysch overlaps all of the older terranes. These rocks, which
are largely marine turbidites, have traditionally been mapped as part of
the Upper Cretaceous Kuskokwim Group. Most of the flysch in the Taylor
Mountains quadrangle is probably indeed part of the Kuskokwim Group, but
based on our new mapping and on the mineralogy of the heavy mineral
concentrates, we think that these monotonous expanses of flysch may
include units genetically unrelated to rocks of the Kuskokwim Group. One
suspect area lies near the eastern edge of the quadrangle, where a minor
amount of chert is found in association with sandstone and shale. Chert
is not a component of Kuskokwim Group lithologies; in addition, a
Jurassic age on radiolarians from one sample implies a relatively older
age for this flysch section. The sandstone here is also distinctly
different from Kuskokwim sandstone in that it contains detrital
potassium feldspar. Further, the non-magnetic fractions of the
concentrate samples in this area contain dark monazite, which is not
present in concentrates from other parts of the quadrangle. Another
suspect area lies on the western edge of the quadrangle. Sedimentary
rocks here are not typical of the Kuskokwim Group in that they contain
breccia beds and local calcareous sections that are so limey they form
karst. We anticipate that thin section studies and detrital zircon
analyses will help us distinguish Kuskokwim Group from other
similar-looking flysch units.

Late Cretaceous to early Tertiary igneous rocks intrude both the
amalgamated terranes and the overlying flysch. Large granitic plutons
include those exposed in the Shotgun Hills (70-69 Ma), in the Taylor
Mountains (\~68 Ma), and at Sleitat Mt. (\~60 Ma). Dike and sill
complexes of similar \~70 Ma age are widely distributed, but they are
not obviously genetically connected to the large intrusions. At least
two compositions of dike and sill complexes are found: 1) several
generally east-west trending granitic suites, including a swarm in the
Little Taylor Mountains dated at about 68 Ma (U-Pb), and 2) a
northwest-trending intermediate to mafic suite (Late Cretaceous?)
emplaced into rocks of the Togiak terrane near Cinnabar Creek. Neogene
basalt flows are exposed in two places: 1) a large known exposure
covering \~6 mi^2^ north of Cinnabar Creek, and 2) a small exposure,
discovered in 2005, located west of Taylor Mountains at Kazik Hill. This
newly identified olivine basalt represents the easternmost exposure of
Neogene volcanic rocks in southwestern Alaska.

All the stratified rocks that predate the \~70-Ma plutons have been
folded, and thrust faults have been mapped in the Farewell, Togiak, and
Goodnews terranes. Metamorphism did not exceed prehnite-pumpellyite
facies except in contact aureoles of the larger plutons. Strands of the
Denali dextral strike-slip fault cut rocks in the northwestern corner of
the quadrangle; some folding in this area may be related to strike-slip
movement, rather than to regional shortening.

Geophysics

During the summer of 2004, an airborne magnetic survey of the Taylor
Mountains quadrangle was conducted; the technical specifications and
details may be found in Saltus and Milicevic (2004). In the context of
the entire state, the Taylor Mountains quadrangle has a fairly neutral
magnetic character, indicative of generally continental crust, but in
detail it exhibits a number of local anomalies. Numerous
northeast-southwest trends are consistent with the known regional
structures. The basement framework is punctuated by several circular to
ovoid features (both highs and lows) that indicate shallow igneous
intrusions. Some of these intrusions are exposed in outcrop, such as the
Taylor Mountains and Shotgun Hills bodies; others are buried and were
not previously recognized---for example, one lies below Finn Mt. In the
Shotgun Hills, three ground magnetic traverses and scattered spot
measurements of magnetic susceptibility were made to investigate the
airborne magnetic anomalies and relate them to the local geology.
Preliminary analysis of the ground profiles suggests that significantly
magnetic material is present at fairly regular intervals in the shallow
subsurface within the hornfels margin of the Shotgun Hills granodiorite.
Interpretation of these shallow magnetic anomalies is problematic
because no rocks of sufficiently magnetic character were found at the
surface.

Mineral deposits

Precious-metal and polymetallic lode deposits and occurrences that are
spatially (and, in many cases, genetically) associated with the Late
Cretaceous to early Tertiary igneous rocks, are distributed widely
across the Taylor Mountains quadrangle. These include lode gold (and
related placer), tin, mercury, and local polymetallic skarn.

As in other parts of the Kuskokwim Mineral Belt (Bundtzen and Miller,
1997), lode gold occurs in two associations: (1) within and adjacent to
stocks that are part of volcanic-plutonic complexes, and (2) hosted by
dike and sill swarms. The Shotgun Hills property, an important example
of the volcanic-plutonic type gold deposit, is being explored by TNR
Gold and is presently the most promising lode gold target in the
quadrangle. The main Shotgun deposit is hosted by a quartz-feldspar
porphyry---a phase thought to be related to the adjacent main
granodiorite stock. The porphyry is cut by \~70 Ma gold-bearing quartz
veins, breccias, and stockworks, which contain arsenopyrite, pyrite,
pyrrhotite and chalcopyrite (see also Rombach and Newberry, 2001).
Petrological and geochemical studies are in progress to understand the
evolution of the magmatic system that produced the metal-bearing fluids.
The second lode gold association---silicified, commonly granitic, dike
and sill complexes---is typical of the giant gold system at Donlin
Creek, \~100 miles to the north. The dike and sill complexes in
southwestern Alaska appear to define structurally favorable traps for
hydrothermal fluids convecting through permeable zones. An example of
this gold occurrence type is found in the Little Taylor Mountains where
an E-W trending, granite porphyry dike swarm, about 2 miles wide can be
traced for at least 6 miles of strike length. Sulfides associated with a
silicified zone include arsenopyrite, scorodite, and covellite. This
swarm can be traced westward almost to a placer gold mine on the south
side of the Taylor Mountains. New mapping indicates the presence of
similar granite porphyry dike swarms, locally associated with sheeted
quartz veins, in the far eastern and far western parts of the
quadrangle.

Tin-bearing occurrences, some associated with volcanic-plutonic
complexes, are widely distributed in the Kuskokwim Mineral Belt
(Bundtzen and Miller, 1997). The Sleitat prospect, in the southern part
of the Taylor Mountains quadrangle where we have not yet mapped, was
discovered and explored in the mid-1980's by Cominco. It is described as
a tin-tungsten-silver-bearing sheeted greisen system. Interestingly,
nearly all of the reconnaissance heavy mineral concentrate samples
collected near Sleitat contain visible gold (Fig. 3), suggesting the
presence of an as-yet-unrecognized gold lode source, perhaps associated
with the zones of greisenization. In the northern part of the
quadrangle, the Taylor Mountains intrusion has potential for lode tin
based on the widespread occurrence of tourmaline in the upper parts of
the pluton and placer cassiterite in the streams to the south. A newly
identified third tin occurrence in the central part of the quadrangle is
potentially significant. At Finn Mt, hornfels sedimentary rock is cut by
thin felsic to intermediate dikes above a buried pluton. Veins of
axinite occur over a wide zone. This is a relatively rare boron-bearing
mineral that is also found in the Beaver Mountains, a volcanic-plutonic
complex near McGrath, and in association with auriferous occurrences in
the Shotgun Hills. We are awaiting 2005 assay results from Finn Mt., but
one 2004 sample contains 300 ppm Sn along with minor silver (up to 4
ppm) and cadmium (up to 14 ppm).

Southwestern Alaska is widely known as a mercury province and the Taylor
Mountains quadrangle contains the past producing Cinnabar Creek deposit.
Mercury ore from this property is notable because it contains native
mercury as well as cinnabar. Our new work in this part of the quadrangle
has revealed a previously unmapped northwest-trending suite of mafic
sills and dikes. These dikes are possibly the structural host for
epizonal mineralization and deeper parts to the hydrothermal system
could potentially be gold rich.

Skarn mineralization is known in a few localities in the Kuskokwim
Mineral Belt including one new occurrence in the northeastern part of
the Taylor Mountains quadrangle, where the \~68 Ma (Iriondo and others,
2003) Titnuk pluton (Fig. 2) intrudes Paleozoic carbonate rocks of the
Farewell terrane. Strongly iron-oxide-stained grab samples of skarn from
the contact aureole contain elevated values of arsenic (up to 1,500
ppm), molybdenum (up to 550 ppm), and zinc (up to 1,900 ppm). Minerals
identified in the non-magnetic fraction of heavy mineral concentrate
samples collected from nearby streams include abundant ilmenite, yellow
monazite, and scheelite. The U.S. Bureau of Land Management, one of our
partners in mineral investigations in this part of Alaska, performed
more detailed sampling of this locality in 2005 to better delineate the
potential resource.

Stream sediment, heavy mineral CONCENTRATE, and STREAM water
geochemistry

The geochemical sampling program is a regional-scale effort focusing on
first- and second-order drainages at a sampling density of approximately
1 sample per 10 mi^2^. To date the USGS and BBNC have collected
stream-sediment and concentrate samples at more than 600 sites (Fig. 3)
covering about 5,000 mi^2^ or about 75% of the Taylor Mountains
quadrangle, and stream-water samples at about half of the locations. All
samples were analyzed for a variety of minor and trace elements through
sensitive analytical techniques; the concentrate samples were examined
microscopically. The chemical and mineralogical data obtained from the
sediment and water samples will help define elemental background
concentrations, elemental anomalies, and possibly deposit type
fingerprints. Geochemical data and mineralogical information from the
2004 sampling effort will be published in digital format on the Internet
by early spring. We are awaiting the analytical data from the 2005
effort, but more rapid lab turnaround is expected and these data will
likely be published before summer.

Visible gold identified in the heavy mineral concentrate samples has
allowed us to outline several new targets that warrant further study
(Fig. 3). The recent geologic mapping indicates a dike and sill swarm in
the eastern part of the quadrangle (Fig. 2) and this coincides with
gold-bearing concentrate samples (area 1, Fig. 3), suggesting these
intrusive rocks are a likely lode source for this gold. Further studies
are required to determine potential deposit types and source areas
associated with other detrital gold concentrations. It is likely that
the stream-water, stream-sediment, and mineralogical data will identify
favorable areas for a variety of potential deposit types.

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Figure 3. Map showing the distribution of gold grains in the
non-magnetic (C3) fraction of the heavy mineral concentrate samples
collected in the Taylor Mountains quadrangle in 2004 and 2005. The
numbered areas outlined in black are new target areas we think warrant
further investigation; some of these are discussed in the text.

conclusion

The AMA, as well as the Alaskan exploration community, has long
recognized the value of regional geologic mapping, geochemical sampling,
geophysical surveys, and ore deposit research for evaluating the
potential for undiscovered resources. The AMA's efforts were
instrumental in bringing AMRAP funding to the USGS in 2005 and
increasing the amount of basic data that we could collect. The increased
funding allowed the USGS to quadruple the area covered compared to 2004,
making basic data available in this period of increasing interest and
exploration activity in southwestern Alaska. Field work in the Taylor
Mountains quadrangle will continue in 2006, but will cover a smaller
area relying entirely on USGS-MRP base funds. Priority for the geologic
mapping will be the southeastern corner of the Taylor Mountains
quadrangle; stream-sediment sampling will concentrate in the western
quarter of the quadrangle.

**References**

Bundtzen, T.K., and Miller, M.L., 1997, Precious metals associated with
Late Cretaceous-early Tertiary igneous rocks of southwestern Alaska,
*in* Goldfarb, R.J., and Miller, L.D., eds., Mineral deposits of Alaska:
Economic Geology Monograph 9, p. 242-286.

Decker, John, Bergman, S.C., Blodgett, R.B., Box, S.E., Bundtzen, T.K.,
Clough, J.G., Coonrad, W.L., Gilbert, W.G., Miller, M.L., Murphy, J.M.,
Robinson, M.S., and Wallace, W.K., 1994, Geology of southwestern Alaska,
*in* Plafker, George, and Berg, H.C., eds., The geology of Alaska, *v.
G-1 of* The geology of North America: Boulder, Colo., Geological Society
of America, p. 285-310.

Iriondo, Alexander, Kunk, M.J., and Wilson, F.H., 2003, 40Ar/39Ar
geochronology of igneous rocks in the Taylor Mountains and Dillingham
quadrangles in SW Alaska: U.S. Geological Survey Open-File Report
03-421, 32 p.

Rombach, C. S., and Newberry, R.J., 2001, Shotgun deposit: granite
porphyry-hosted gold-arsenic mineralization in southwestern Alaska, USA:
Mineralium Deposita, v. 36, no. 6, p. 607-621.

Saltus, R.W., and Milicevic, B., 2004, Preliminary grid data and maps
for an aeromagnetic survey of the Taylor Mountains quadrangle and a
portion of the Bethel quadrangle, Alaska: USGS Open-File Report
2004-1293, <http://pubs.usgs.gov/of/2004/1293/>.