U.S. Department of the Interior

U.S. Geological Survey

Professional Paper 1776–A

Studies by the U.S. Geological Survey in Alaska, 2008–2009

Reconnaissance Study of the Taylor Mountains Pluton, 

Southwestern Alaska
FRONT COVER

Rubble of tourmaline and quartz replaced granite at the Whitewater occurrence,

Taylor Mountains, southwestern Alaska.
Reconnaissance Study of the Taylor Mountains Pluton, 

Southwestern Alaska

By Travis L. Hudson, Marti L. Miller, Edward P. Klimasauskas, and Paul W. Layer

Professional Paper 1776–A

U.S. Department of the Interior

U.S. Geological Survey 

Studies by the U.S. Geological Survey in Alaska, 2008–2009
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

This report and any updates to it are available online at:

http://pubs.usgs.gov/pp/1776/a/

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Suggested Citation

Hudson, T.L., Miller, M.L., Klimasauskas, E.P., and Layer, P.W., 2010, Reconnaissance study of the Taylor Mountains pluton, 

southwestern Alaska, in Dumoulin, J.A., and Galloway, J.P., eds., Studies by the U.S. Geological Survey in Alaska, 2008–2009: 

U.S. Geological Survey Professional Paper 1776–A, 13 p.
 III

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Taylor Mountains Pluton...............................................................................................................................2

Porphyritic Granite Containing Coarse-Grained Groundmass......................................................3

Porphyritic Granite Containing Fine-Grained Groundmass ...........................................................3

Fine-Grained Granite............................................................................................................................5

Chemistry.........................................................................................................................................................5

Age of the Taylor Mountains Pluton ...........................................................................................................7

Mineral Occurrences....................................................................................................................................8

Discussion.....................................................................................................................................................11

Conclusion.....................................................................................................................................................11

References Cited..........................................................................................................................................11

Appendix—Summary of Analytical Method for 40Ar/39Ar Analysis......................................................13

Figures

1. Map showing the location of the Taylor Mountains and the Taylor Mountains 

1:250,000-scale quadrangle, southwestern Alaska ....................................................................1

2. Geologic map of the Taylor Mountains pluton, southwestern Alaska ........................................2

3. Photograph of a hand specimen of porphyritic granite containing coarse-grained 

groundmass, Taylor Mountains, southwestern Alaska..............................................................3

4. Photographs showing (A) a field exposure of the porphyritic granite containing 

fine-grained groundmass, and (B) a hand specimen of same rock, 

Taylor Mountains, southwestern Alaska......................................................................................5

5. Photograph of a hand specimen of fine-grained granite, Taylor Mountains, 

southwestern Alaska.......................................................................................................................6

6. Harker diagrams showing variation of major oxides with silica content for samples 

of the Taylor Mountains igneous rocks, southwestern Alaska ................................................6

7. Trace-element variation diagram for Taylor Mountains igneous rocks (chondrite 

normalized), southwestern Alaska................................................................................................7

8. 40Ar/39Ar age spectra for sample 05AH19 from the Taylor Mountains, 

southwestern Alaska.......................................................................................................................8

9. Diagram comparing four previously reported K/Ar ages and the new Ar/Ar age (sample 

AH19, this report) , Taylor Mountains, southwestern Alaska....................................................8

10. Rubble of tourmaline and quartz replaced granite at the Whitewater occurrence 

(locality 2, fig. 2) , Taylor Mountains, southwestern Alaska....................................................10
IV 

Tables

1. Major-oxide and trace-element data for samples of the Taylor Mountains pluton, 

southwestern Alaska.......................................................................................................................4

2. 40Ar/39Ar data for biotite from sample 05AH19, Taylor Mountains, southwestern Alaska .......9
Studies by the U.S. Geological Survey in Alaska, 2008–2009

Edited by Julie A. Dumoulin and John P. Galloway

U.S. Geological Survey Professional Paper 1776–A

Reconnaissance Study of the Taylor Mountains Pluton, 

Southwestern Alaska

By Travis L. Hudson1, Marti L. Miller2, Edward P. Klimasauskas2, and Paul W. Layer3

Abstract

The Taylor Mountains pluton is a Late Cretaceous to 

early Tertiary (median age 65+2 Ma) epizonal, composite 

biotite granite stock located about 235 km (145 mi) northeast 

of Dillingham in southwestern Alaska. This 30 km2 (12 mi2) 

pluton has sharp and discordant contacts with hornfels that 

developed in Upper Cretaceous clastic sedimentary rocks 

of the Kuskokwim Group. The three intrusive phases in the 

Taylor Mountains pluton, in order of emplacement, are (1) 

porphyritic granite containing large K-feldspar phenocrysts in 

a coarse-grained groundmass, (2) porphyritic granite containing large K-feldspar and smaller, but still coarse, plagioclase, 

quartz, and biotite phenocrysts in a fine-grained groundmass, 

and (3) fine-grained, leucocratic, equigranular granite. The 

porphyritic granites have different emplacement histories, but 

similar compositions; averages are 69.43 percent SiO2, 1.62 

percent CaO, 5.23 percent FeO+MgO, 3.11 percent Na2O, and 

4.50 percent K2O. The fine-grained, equigranular granite is 

distinctly felsic compared to porphyritic granite; it averages 

75.3 percent SiO2, 0.49 percent CaO, 1.52 percent FeO+MgO, 

3.31 percent Na2O, and 4.87 percent K2O. Many trace elements including Ni, Cr, Sc, V, Ba, Sr, Zr, Y, Nb, La, Ce, Th, 

and Nd are strongly depleted in fine-grained equigranular 

granite. Trace elements are not highly enriched in any of the 

granites. Known hydrothermal alteration is limited to one tourmaline-quartz replacement zone in porphyritic granite. Mineral deposits in the Taylor Mountains area are primarily placer 

gold (plus wolframite, cassiterite, and cinnabar); sources for 

these likely include scattered veins in hornfels peripheral to 

the Taylor Mountain pluton. The granite magmas that formed 

the Taylor Mountains pluton are thought to represent melted 

continental crust that possibly formed in response to high 

heat flow in the waning stage of Late Cretaceous subduction 

beneath interior Alaska.

Introduction

This study reports field relations, petrology, major- and 

trace-element data, and one new Ar-Ar age for the granite 

pluton in the Taylor Mountains, southwestern Alaska. This 

pluton is part of a regionally extensive suite of Late Cretaceous and early Tertiary igneous rocks widely developed in 

Alaska (Moll-Stalcup and others, 1994; Bundtzen and Miller, 

1997). Reifenstuhl and others (1985) compiled field observations and reported some major-element data and K-Ar ages for 

the Taylor Mountain pluton.

The Taylor Mountains are a 200 km2 (77 mi2) isolated 

glaciated upland in the northwestern Taylor Mountains D-4 

quadrangle, about 235 km (145 mi) northeast of Dillingham 

(fig. 1) in southwestern Alaska. Valley glaciers flowed outward 

from the central uplands during the Last Glacial Maximum. 

The northerly flowing glaciers cut longer and deeper valleys 

and coalesced along the north flanks of the mountains. The 

mountains reach an elevation of 1,091 m (3,581 ft). Maximum 

vertical relief is about 700 m (2,300 ft), and outcrops are scattered along the rubble-mantled ridges left between the glacial 

valleys.

1

Applied Geology, Inc., 902 Vista del Mar, Sequim, WA 98382.

2

U.S. Geological Survey, Alaska Science Center, 4210 University Drive, 

Anchorage, AK 99508-4667.

3

University of Alaska, Fairbanks, Geophysical Institute, 903 Koyukuk 

Drive, Fairbanks, AK 99775.

N

Dillingham 500 km

Taylor

Mtns.

quad.

ALASKA

Taylor

Mtns.

Figure 1. Map showing the location of the Taylor Mountains and the 

Taylor Mountains 1:250,000-scale quadrangle, southwestern Alaska.
2 Reconnaissance Study of the Taylor Mountains Pluton, Southwestern Alaska

The Taylor Mountains are an area of resistant hornfels 

intruded by a 30 km2 (12 mi2) granite pluton. The hornfels is 

developed in clastic sedimentary rocks of the regionally extensive Upper Cretaceous Kuskokwim Group (Cady and others, 

1955; Decker and others, 1994; Miller and others, 2006). 

Cordierite bearing spotted hornfels is found locally near the 

contact; the hornfels aureole extends for several kilometers 

and gradually decreases in grade away from the contact.

The Kuskokwim Group was deposited mainly as turbidites in a basin underlain by a variety of older basement 

terranes (Decker and others, 1994). The nearest such basement 

rock lies about 25 km (16 mi) to the northeast of the Taylor 

Mountains in an area where the Kuskokwim Group appears 

to depositionally overlie platform facies sedimentary rocks of 

the Neoproterozoic to Jurassic Farewell terrane and to have 

been involved in thrusting that imbricated the Farewell rocks 

Unconsolidated sediments, undivided (Quaternary)

Fine-grained granite (Tertiary-Cretaceous)

Porphyritic granite containing fine-grained groundmass (Tertiary-Cretaceous)

Porphyritic granite containing coarse-grained groundmass (Tertiary-Cretaceous)

Kuskokwim Group, largely hornfels sandstone, siltstone, and shale (Cretaceous)

Mapped dikes, fine-grained granite (Tertiary-Cretaceous)

Strike and dip of bedding

Strike and dip of cleavage

Locality referenced in text

Geochronology sample

Kk

Kk

Kk

Kk

Kk

Kk Qs

Qs

Qs

Qs

Qs

TKpf Qs

TKf

TKpc

TKpc

TKpc

3

2 1

2

62

77

v

v v

v

v

v v v

v v

v

v v

Qs

TKf

TKpf

TKpc

Kk

3

0 0.5 1 Mile 0 0.5 1 Kilometer

(Miller and others, 2006). In the area of Figure 1, the nonhornfels Kuskokwim Group south of Taylor Creek is a thick 

section of mudstone, siltstone, and fine- to medium-grained 

lithic sandstone, moderately deformed into broad, open folds.

Taylor Mountains Pluton

The Taylor Mountains pluton is a 5 by 6.5 km (3 by 4 mi) 

granite stock, slightly elongate in a northwest-southeast direction 

(fig. 2). The contacts with surrounding hornfels are sharp, discordant, and appear to be steep to moderately steep, dipping away 

from the pluton; however, the northwestern contact, near benchmark “Spike” (northwest corner, fig. 2), dips shallowly north, just 

below the ridgeline, for about 3 km (2 mi). A small roof pendant 

Figure 2. Geologic 

map of the Taylor 

Mountains pluton, 

southwestern 

Alaska.
 Taylor Mountains Pluton 3

of hornfels (unit Kk) is present on the north side of elevation 

3,581, the highest peak in the Taylor Mountains (fig. 2).

The composite Taylor Mountains pluton contains three 

intrusive phases (fig. 2). The earliest phase is porphyritic biotite granite containing large K-feldspar phenocrysts scattered 

through a medium- to coarse-grained groundmass. This phase 

lies on the north-northeast side and makes up approximately 

40 percent of the exposed pluton (fig. 2). Most of the remainder of the pluton is porphyritic biotite granite containing large 

K-feldspar phenocrysts in a fine-grained groundmass. The latest intrusive phase is a small body of fine-grained, equigranular biotite-muscovite granite in the pluton’s interior (fig. 2).

The pluton’s internal contacts all appear to be sharp and 

intrusive, although they were not directly observed in outcrop. 

The porphyritic granite containing a coarse-grained groundmass 

is interpreted as the oldest phase because it is cut by dikes (fig. 

2) that have textures similar to the other two phases. These dikes 

are primarily fine-grained granite that is texturally and chemically similar to the fine-grained granite phase (fig. 2). One dike 

of porphyritic granite containing a fine-grained groundmass 

was observed to cut porphyritic granite containing a coarsegrained groundmass (locality 1, fig. 2). The fine-grained granite is interpreted as the youngest phase because of its location 

within the pluton and its fine-grained equigranular texture.

Porphyritic Granite Containing Coarse-Grained 

Groundmass

Large, white, euhedral K-feldspar phenocrysts—commonly 6 cm (2.5 in) long, but ranging in length from 3 to 12 

cm (1 to 5 in)—make up 10 to 15 percent of this biotite granite 

(fig. 3). The phenocrysts are scattered through a medium- to 

Figure 3. Photograph of a hand specimen of porphyritic granite 

containing coarse-grained groundmass, Taylor Mountains, 

southwestern Alaska.

coarse-grained hypidiomorphic equigranular groundmass 

of biotite, quartz, plagioclase, and anhedral K-feldspar. The 

K-feldspar phenocrysts are weakly to moderately aligned in 

some places. Aplitic segregations and rare tourmaline clots are 

locally present.

The K-feldspar phenocrysts are perthitic and have inclusions of plagioclase, quartz, and biotite. Biotite, which constitutes 5 to 10 percent of this granite, is as much as 0.5 cm (0.2 

in) across, dark red brown, and scattered through the groundmass as discrete intergranular grains. Plagioclase is subhedral 

and not zoned. Zircon is the accessory mineral in this phase; 

it occurs as subhedral to euhedral intergranular grains and as 

small inclusions in biotite. Zircon in biotite lacks well-developed pleochroic halos. Minor muscovite incompletely replaces 

biotite and some plagioclase in places, and it also forms a few 

intergranular grains associated with small, opaque grains that 

may represent completely replaced biotite.

Granitic dikes locally intrude this phase (fig. 2). These 

dikes are primarily fine-grained equigranular granite (for 

example, sample AH-21, table 1) measuring as much as several 

meters (tens of feet) wide. A thicker (6 m, 20 ft) dike (locality 1, 

fig. 2) is porphyritic granite containing a fine-grained groundmass. Small tourmaline-bearing quartz veins are associated with 

some fine-grained granite dikes. Xenoliths, other than hazy, 

biotite-rich patches, were not observed in the dikes.

Porphyritic Granite Containing Fine-Grained 

Groundmass

K-feldspar phenocrysts in this phase of the Taylor 

Mountains pluton are similar to those in the porphyritic granite 

containing coarse-grained groundmass. The K-feldspar phenocrysts are large, white, and euhedral; they are commonly 

6 to 8 cm (2.4 to 3 in) long, but locally exceed 9 cm (3.5 in). 

However, these phenocrysts are slightly less abundant than 

those in the other porphyritic phase (5 to 10 percent), and they 

are not generally aligned. The large K-feldspar phenocrysts, 

accompanied by phenocrysts of much smaller, but still coarse 

plagioclase, quartz, and biotite (to 0.5 cm across) are scattered through a fine-grained, hypidiomorphic, equigranular 

groundmass (fig. 4). Small biotite-rich clots and scarce, thin 

tourmaline-bearing quartz veins are locally present. The jointing in this phase appears to be more closely spaced than that in 

the coarse-grained-groundmass porphyritic granite.

The K-feldspar phenocrysts are perthitic and include 

some plagioclase, quartz, and biotite. The biotite is dark red 

brown and occurs as both coarser, subhedral grains and as 

smaller, somewhat shreddy grains in the groundmass. The 

coarser plagioclase grains display some growth bands along 

their outer margins, but are not strongly zoned. The K-feldspar 

in the groundmass is mostly anhedral. Zircon occurs as inclusions in biotite (without strong pleochroic halos) and as intergranular, subhedral to euhedral grains. Anhedral and intergranular sphene is present in trace amounts. Muscovite irregularly 

replaces biotite and clouds parts of some plagioclase crystals.
4 Reconnaissance Study of the Taylor Mountains Pluton, Southwestern Alaska

Granite

unit

Porphyritic, coarse-grained

groundmass

Porphyritic, fine-grained

groundmass Fine-grained (AH-21 is a dike)

Sample No. AH-19 AH-22 AH-31 AH-24 AH-25A AH-33 AH-25B AH-32 AH-36 AH-21

Unnormalized major elements (weight, in percent)

 SiO2 68.22 68.35 69.36 66.98 68.06 67.89 73.61 74.76 73.07 73.72

 TiO2 0.629 0.543 0.617 0.735 0.685 0.675 0.094 0.070 0.229 0.179

 Al2O3 14.95 14.82 14.36 15.14 14.59 14.78 13.90 13.46 14.20 13.69

 FeO* 4.04 3.52 3.90 4.30 4.10 4.11 0.92 0.74 1.71 1.57

 MnO 0.080 0.096 0.086 0.097 0.088 0.095 0.038 0.033 0.063 0.063

 MgO 1.07 0.94 1.05 1.26 1.26 1.23 0.17 0.15 0.41 0.32

 CaO 1.57 1.42 1.55 1.91 1.58 1.51 0.39 0.34 0.63 0.57

 Na2O 3.10 3.11 3.19 3.08 2.86 3.01 3.51 3.20 3.26 3.00

 K2O 4.51 4.66 4.01 4.39 4.48 4.42 4.47 4.79 4.93 4.90

 P2O5 0.302 0.305 0.271 0.349 0.304 0.297 0.261 0.243 0.280 0.226

 Sum 98.47 97.76 98.40 98.25 98.01 98.03 97.36 97.78 98.79 98.24

 LOI (%) 0.73 0.79 0.72 0.77 0.75 0.97 0.59 0.34 0.81 0.82

Normalized major elements (weight, in percent)

 SiO2 69.28 69.91 70.49 68.18 69.44 69.25 75.60 76.46 73.97 75.05

 TiO2 0.639 0.555 0.627 0.748 0.699 0.688 0.097 0.072 0.232 0.182

 Al2O3 15.19 15.15 14.60 15.41 14.89 15.08 14.28 13.77 14.37 13.93

 FeO* 4.10 3.60 3.96 4.38 4.18 4.20 0.95 0.75 1.73 1.60

 MnO 0.081 0.098 0.088 0.099 0.090 0.097 0.039 0.034 0.064 0.064

 MgO 1.08 0.96 1.07 1.28 1.29 1.26 0.17 0.15 0.42 0.32

 CaO 1.59 1.46 1.58 1.94 1.61 1.54 0.40 0.35 0.64 0.58

 Na2O 3.14 3.18 3.24 3.13 2.92 3.07 3.60 3.27 3.30 3.05

 K2O 4.58 4.77 4.07 4.47 4.57 4.51 4.59 4.90 4.99 4.99

 P2O5 0.307 0.312 0.275 0.356 0.310 0.303 0.268 0.249 0.284 0.230

Total 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

Trace elements (in parts per million)

Ni 19 23 19 17 22 24 9 6 10 8

Cr 29 26 28 31 37 36 5 4 10 9

Sc 9 8 9 9 10 9 4 4 6 4

V 63 54 60 71 76 71 5 2 24 16

Ba 692 837 565 861 752 720 58 31 222 107

Rb 215 206 207 189 193 192 322 265 280 253

Sr 157 163 157 202 166 156 14 9 50 34

Zr 257 226 238 226 207 223 37 32 77 68

Y 23 22 19 22 22 26 8 7 12 13

Nb 26.1 23.2 25.7 23.8 22.5 23.6 11.1 5.1 16.7 12.8

Ga 25 22 23 24 21 22 21 19 20 19

Cu 3 29 4 11 15 19 9 1 2 0

Zn 103 103 93 103 93 96 53 30 50 50

Pb 18 19 16 19 18 19 13 15 17 17

La 37 34 36 35 31 36 5 3 12 11

Ce 77 65 66 70 64 69 9 7 28 25

Th 11 10 11 11 10 10 2 1 3 3

Nd 34 29 28 31 29 32 3 4 11 11

U 5 7 4 4 4 4 6 5 3 4

Cs 28 22 28 16 19 14 58 14 38 33

As 6 15 16 4 20 13 16 2 20 11

Table 1. Major-oxide and trace-element data for samples of the Taylor Mountains pluton, southwestern Alaska.

[Analyzed at the GeoAnalytical Laboratory, Washington State University, Pullman. Trace-element contents of V, Cr, Ni, Cu, Zn, Ga, and Zr determined by 

wavelength-dispersive X-ray-fluorescence analysis and of other elements by inductively coupled plasma mass-spectrometric analysis. LOI, weight loss on ignition]
 Chemistry 5

Figure 4. Photographs 

showing (A) a field exposure 

of the porphyritic granite 

containing fine-grained 

groundmass and (B) a 

hand specimen of same 

rock, Taylor Mountains, 

southwestern Alaska.

B

A

Fine-Grained Granite

Fine-grained, hypidiomorphic, equigranular granite (fig. 5), 

in places verging on medium-grained and seriate, makes up a 

small (0.6 km2; 0.24 mi2), slightly elongate body within the porphyritic granite containing fine-grained groundmass (fig. 2). The 

fine-grained granite has 1 to 2 percent biotite, minor, disseminated tourmaline, and some feldspar that weathers light rosy 

red. Hydrothermal alteration that significantly modifies original 

textures and mineralogy is not developed in this phase.

The subhedral to shreddy biotite is yellowish brown, 

but it is largely replaced by muscovite. Muscovite, as much 

as 1 or 2 percent, also forms intergranular grains. Small, 

opaque grains commonly accompany muscovite. Plagioclase 

is subhedral and unzoned. A few grains of zircon are included 

in biotite. Traces of tourmaline occur as anhedral to euhedral 

interstitial and intergranular grains.

Chemistry

Major- and trace-element data for three samples of 

porphyritic granite containing coarse-grained groundmass, 

three samples of porphyritic granite containing fined-grained 

groundmass, and four samples of fine-grained granite 

(including a dike, sample AH-21) are listed in table 1. The 

least felsic sample (AH-24, 68.18 percent SiO2, normalized) 

is from the fine-grained-groundmass porphyritic granite 

unit. Sample AH-24 was collected from the central part of 

the pluton, near the contact with coarse-grained-groundmass 

porphyritic granite; a small, biotite-rich clot was observed in 

outcrop here, and this sample contains slightly more biotite 

than the others. The major-oxide composition of the porphyritic-granite samples is generally similar (fig. 6); SiO2

 ranges 

from 68.18 to 70.49 percent (averages 69.43 percent), CaO 

ranges from 1.46 to 1.94 percent (averages 1.62 percent), 
6 Reconnaissance Study of the Taylor Mountains Pluton, Southwestern Alaska

Figure 6. Harker diagrams showing variation of major oxides 

with silica content for samples of the Taylor Mountains igneous 

rocks, southwestern Alaska.

Figure 5. Photograph of a hand specimen of fine-grained granite, 

Taylor Mountains, southwestern Alaska.

FeO+MgO ranges from 4.56 to 5.66 percent (averages 5.23 

percent), Na2O ranges from 2.92 to 3.24 percent (averages 3.11 

percent), and K2O ranges from 4.07 to 4.77 percent (averages 

4.50 percent). The major-oxide data reported by Reifenstuhl 

and others (1985) for three samples from the Taylor Mountains 

pluton are similar to that for the porphyritic-granite samples 

reported here.

The fine-grained granite is distinctly more felsic than the porphyritic granites (table 1; fig. 6). SiO2

 ranges from 73.97 to 76.46 

percent (averages 75.3 percent), CaO ranges from 0.35 to 0.64 percent (averages 0.49 percent), FeO+MgO ranges from 0.90 to 2.15 

percent (averages 1.52 percent), Na2

O ranges from 3.05 to 3.60 

percent (averages 3.31 percent), and K2O ranges from 4.59 to 4.99 

percent (averages 4.87 percent). The principal major-oxide differences between the fine-grained granite and the porphyritic granites 

are in their CaO, FeO, MgO, and TiO2 contents. On average, these 

oxides are much less abundant in the fine-grained granite than in 

the porphyritic granite. SiO2, Na2O, and K2O are relatively similar 

between the fine-grained granite and the porphyritic granites, 

although there is a 3-percent SiO2 gap between the most felsic 

porphyry and the least felsic fine-grained granite (fig. 6).

The trace-element data (table 1) show strong differences 

between fine-grained granite and porphyritic granite. Compared 

to the porphyritic granites, fine-grained granite is strongly 

depleted in most trace elements, including Ni, Cr, Sc, V, Ba, 

Sr, Zr, Y, Nb, La, Ce, Th, and Nd (fig. 7). Overall, base-metal 

concentrations in all of the granites are low; Cu is generally 

lower (0 to 9 ppm) and Zn is distinctly lower (30 to 53 ppm) in 

the fine-grained granite versus the porphyritic granite. Cesium 

has variable concentrations (14 to 58 ppm), and uranium has 

low concentrations (3 to 7 ppm) in all the granites. The Taylor 

Mountains granites, even the very felsic fine-grained granite, 

are not highly evolved in their trace-element contents. 
 Age of the Taylor Mountains Pluton 7

Figure 7. Trace-element variation diagram for Taylor Mountains igneous rocks (chondrite normalized), southwestern Alaska.

For example, compared to tin granites of Seward 

Peninsula, Alaska (Hudson and Arth, 1983), the Taylor 

Mountains granites have low average concentrations of U 

(5 versus 17 ppm), Th (7 versus 46 ppm), Cs (27 versus 40 

ppm), and Rb (230 versus 570 ppm).

Age of the Taylor Mountains Pluton

A sample of the coarse-grained-groundmass porphyritic 

granite was dated by the 40Ar/39Ar method at the Geochronology Laboratory, University of Alaska, Fairbanks. A single biotite crystal was hand picked for analysis after the sample was 

crushed, washed, and sieved. Analytical methods are described 

in detail in appendix 1. This biotite yielded a plateau age of 

63.4+0.3 Ma (fig. 8, table 2).

Reifenstuhl and others (1985) earlier reported four 

conventional K-Ar ages (three biotite and one muscovite) 

Chondrite Normalized Values

Cs Rb Ba Th U Ta Nb La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Sr Hf Zr Y

AH-19

AH-22

AH-31

AH-24

AH-25A

AH-33

AH-25B

AH-32

AH-36

AH-21

1,000

100

10

1

for the Taylor Mountains pluton. These ages are 64.5+1.9, 

65.0+2.0, 65.3+2.0, and 67.6+2.0 Ma (fig. 9). All four of these 

ages, and our one new Ar/Ar age, are from what we interpret 

to be the oldest phase—the porphyritic granite containing a 

coarse-grained groundmass (fig. 2). By using all five ages, the 

median age for the Taylor Mountains pluton is calculated to be 

65+2 Ma. The pluton apparently was emplaced in the earliest 

Tertiary or the latest Cretaceous. It is slightly younger than the 

hypabyssal felsic intrusive rocks and related gold mineralization at the Shotgun prospect (70 to 68 Ma; Hudson, 2001), 

70 km (44 mi) to the southwest, and slightly older than the 

epizonal granite and related tin mineralization at the Sleitat 

prospect (56.8±2.8 Ma; Burleigh, 1991), 103 km (64 mi) to the 

south-southeast of the Taylor Mountains.

The Taylor Mountains pluton was emplaced in several 

phases between 67 and 63 Ma, putting it near the middle of 

the Late Cretaceous and early Tertiary episode of magmatism 

(75 to 50 Ma; Moll-Stalcup, 1994) widely distributed through 

southern and interior Alaska. Emplacement of the Taylor Moun-
8 Reconnaissance Study of the Taylor Mountains Pluton, Southwestern Alaska

Figure 8. 40Ar/39Ar age spectra for sample 05AH19 

from the Taylor Mountains, southwestern Alaska. 

Shown are age, Ca/K, and Cl/K spectra.

Figure 9. Diagram comparing four previously reported 

(Reifenstuhl and others, 1985) K/Ar ages and the new Ar/Ar age 

(sample AH19, this report) , Taylor Mountains, southwestern 

Alaska. Each age bar is labeled with the sample number and the 

mineral dated. The calculated median for these ages is shown. 

Sample locations are shown in figure 2.

tains pluton also was during the peak magmatism (72 to 65 Ma) 

in the Kuskokwim mineral belt (Bundtzen and Miller, 1997).

AGE

234c box heights are 2σ

biotite 

309

biotite 235

biotite 235

muscovite

AH19

biotite 

Map No.

in fig. 2

Sample

number

Age ± 2σ

error, in Ma Method Mineral

1

2

2

2

3

82MR309

83MR234c

83MR235

83MR235

05AH19

65.3 ± 2 

67.6 ± 2

65.0 ± 2

64.5 ± 1.9

63.4 ± 0.3 

K/Ar

K/Ar

K/Ar

K/Ar

40Ar/39Ar

biotite

biotite

biotite

muscovite

biotite

Median age 65.0 +2.6 / -1.6 Ma

70

68

66

64

62

Plateau age = 63.4±0.3 Ma

Fraction of 39Ar Released

05AH19 Biotite

Age in Ma Ca/K Cl/K

100

80

60

40

20

0

0.5

0.4

0.3

0.2

0.1

0

.05

.04

.03

.02

.01

0

0 0.2 0.4 0.6 0.8 1.0

Mineral Occurrences

The Taylor Mountains pluton has one area of known 

hydrothermal alteration. The Whitewater tourmaline occurrence (locality 2, fig. 2) is a fault- or joint-controlled zone of 

strong tourmaline and quartz replacement of the fine-grainedgroundmass porphyritic granite. Rubble crop of massive 

blocks of black tourmaline and quartz as much as 2 m (6 ft) 

across are distributed over 30 to 60 m (100 to 200 ft) of a 

northwest-trending linear zone across the ridge crest (fig. 10). 

Two samples of this alteration, collected and analyzed by the 

Bureau of Land Management, have low metal contents; for 

example, 100 and 300 ppm As, 106 and 107 ppm Cu, 5 and 

15 ppm Sn, and 3 to 15 ppm U (Ellefson and others, 2005). 

An additional two samples collected by the USGS show more 

elevated metal contents of 89 to 789 ppb Au, 2 ppm Ag, 224-

770 ppm As, 209 to 359 Cu, 40 to 103 ppm Sb, and 5 to 28 

ppm U (Klimasauskas and others, 2006).

Other mineralization in the Taylor Mountains area is 

evidenced primarily by placer deposits. A lode occurrence of 

wolframite in a quartz vein cutting hornfels is reported east 

of the pluton (Hudson, 2001), but the principal wolframite 

occurrence is in a small, placer gold deposit on an unnamed 
Laser 

(mW)

Cum.

39Ar

40Ar/39Ar 

measured ±

37Ar/39Ar 

measured ± measured ± % Atm. 40Ar Ca/K ± Cl/K ± 40Ar*/ 39ArK 

300 0.0011 152.3549 8.0614 0.13886 0.04655 0.54915 0.03979 106.5 0.2548 0.0854 0.01595 0.00478 -9.938 

500 0.0042 36.1855 0.6046 0.11142 0.01324 0.09873 0.00953 80.7 0.2045 0.0243 0.01163 0.00221 6.992 750 0.0176 20.3620 0.1540 0.04115 0.00779 0.04193 0.00325 60.9 0.0755 0.0143 0.01379 0.00061 7.945 

1,000 0.0538 13.8512 0.1195 0.00968 0.00136 0.01105 0.00112 23.6 0.0178 0.0025 0.01758 0.00041 10.557 1,200 0.1116 11.4905 0.0605 0.00375 0.00067 0.00159 0.00063 4.1 0.0069 0.0012 0.01859 0.00036 10.991 1,400 0.1827 11.2337 0.0660 0.00405 0.00066 0.00205 0.00075 5.4 0.0074 0.0012 0.01873 0.00024 10.600 

1,600 0.2740 11.1672 0.0755 0.00262 0.00055 0.00160 0.00058 4.2 0.0048 0.0010 0.01895 0.00027 10.666 1,800 0.3671 11.2070 0.1150 0.00212 0.00045 0.00092 0.00040 2.4 0.0039 0.0008 0.01895 0.00027 10.907 2,000 0.4248 11.3113 0.0718 0.00620 0.00132 0.00117 0.00113 3.1 0.0114 0.0024 0.01854 0.00038 10.937 2,400 0.4855 11.3376 0.0831 0.01015 0.00139 0.00224 0.00054 5.8 0.0186 0.0025 0.01844 0.00037 10.649 2,800 0.5405 11.4007 0.0413 0.01136 0.00102 0.00110 0.00039 2.9 0.0208 0.0019 0.01884 0.00021 11.047 3,500 0.6049 11.2907 0.0461 0.01212 0.00091 0.00095 0.00044 2.5 0.0222 0.0017 0.01887 0.00021 10.984 5,000 0.6590 11.3097 0.0556 0.01296 0.00108 0.00081 0.00051 2.1 0.0238 0.0020 0.01847 0.00029 11.042 9,000 0.7432 11.1162 0.0738 0.01871 0.00074 0.00049 0.00037 1.3 0.0343 0.0014 0.01837 0.00025 10.945 

9,001 1.0000 11.0135 0.0645 0.08951 0.00084 0.00042 0.00010 1.1 0.1642 0.0015 0.01694 0.00014 10.868 

Integrated 11.6457 0.0250 0.02998 0.00030 0.00284 0.00015 7.2 0.0550 0.0006 0.01811 0.00008 10.781 

Table 2. 40Ar/39Ar data for biotite from sample 05AH19, Taylor Mountains, southwestern Alaska.[Weighted average of J from standards = 3.298e-03±8.605e-06. mW, megawatts. Cum., cumulative]
10 Reconnaissance Study of the Taylor Mountains Pluton, Southwestern Alaska

Figure 10. Rubble of tourmaline and quartz replaced granite at the Whitewater occurrence (locality 2, 

fig. 2) , Taylor Mountains, southwestern Alaska.
 References Cited 11

north tributary to Kiknik Creek (section 16, T9N, R46W of 

the Seward Meridian). This placer is 4.8 km (3 mi) south of 

the southern contact of the Taylor Mountain pluton (south of 

the area in fig. 2). A pan concentrate sample collected from 

the placer contained 2,350 ppm Sn and 820 ppm W; a sluice 

concentrate sample contained 2,330 ppm Sn and 36 percent 

W (Ellefson and others, 2005). Cassiterite and cinnabar are 

reported from a gold placer along Taylor Creek, just to the 

east (about 2 km or 1.2 mi) of the wolframite-rich placer. 

In general, data from samples taken from within the granite 

pluton and from peripheral mineralization suggest that a large 

mineralizing system did not evolve during crystallization of 

the Taylor Mountain pluton. Veins scattered through hornfels 

and nearby sedimentary rocks of the Kuskokwim Group could 

be contributing to the placer deposits.

Discussion

The initially emplaced coarse-grained-groundmass 

porphyritic granite represents a magma that may have 

partially crystallized at depth to develop its large K-feldspar 

phenocrysts, but upon emplacement at epizonal levels, 

gradually cooled and crystallized in place. The similarity 

in chemical composition and coarse-grained mineralogy 

between the two porphyritic granite phases shows that their 

principal differences are textural. The magma that formed 

the fine-grained-groundmass porphyritic granite crystallized 

coarse to very coarse biotite, plagioclase, quartz, and 

K-feldspar at depth. Mobilization and upward movement 

of this part of the magma system, along with, and perhaps 

to slightly shallower levels than, the previously emplaced 

porphyritic granite, appears to have initiated crystallization 

of its fine-grained groundmass. The parent magma for both 

porphyritic granites was very similar.

The fine-grained granite is the late-evolved phase of 

the Taylor Mountains pluton. Its small size, interior location, 

texture, and distinctly felsic composition identify it as the 

residual granite magma in this system. Its composition can 

be explained by removal of biotite, zircon, and sphene from 

the magma that formed the porphyritic granites. Fractionation 

appears to have produced small amounts of residual granite 

elsewhere in the subsurface of the system as evidenced by 

fine-grained granite dikes that locally intrude porphyritic 

granite (fig. 2).

Although fractionation evolved distinctly felsic residual 

magma in the Taylor Mountains pluton, this magma was not 

highly enriched in fugitive or volatile components. Muscovite, 

some intergranular and some replacing biotite and plagioclase, is 

the common late mineral phase in the granites; it is the principal 

hydrous phase in the fine-grained equigranular granite. Minor 

tourmaline is the other late-crystallizing mineral in the granites.

The Late Cretaceous and early Tertiary regional 

magmatism that includes the Taylor Mountains pluton is 

scattered across interior Alaska from the Alaska Range north 

to the Yukon-Koyukuk region and from the Bering Sea east 

to Canada. Magmas were emplaced across this vast 600 by 

1,200 km (370 by 750 mi) province throughout the interval 

between 76 and 50 Ma (Moll-Stalcup, 1994). The plutonic and 

volcanic rocks in this magmatic province range from calcic to 

alkalic, but are mostly calc-alkalic (for example, fig. 5 from 

Bundtzen and Miller, 1997). Magmas in this province have 

both mantle and crustal sources (Bergman and others, 1987; 

Moll-Stalcup, 1994; Arth, 1994). There is an apparent shift in 

general compositional character at about 63+3 Ma; younger 

rocks appear to reflect melting of a larger proportion of 

lithosphere, including continental crust (Bergman and others, 

1987). The Late Cretaceous magmatism has been interpreted 

as a magmatic arc developed above a fast-converging, shallowdipping subduction zone (Bergman and others, 1987; MollStalcup, 1994). The early Tertiary rocks in the province may 

have developed in a post-subduction setting (Bergman and 

others, 1987).

The Late Cretaceous tectonic setting produced high heat 

flow in addition to extensive magmatism in interior Alaska 

(Daggett, 1987; Miller and others, 2007). The crust in the part 

of southwestern Alaska that includes the Taylor Mountains is 

continental in character (Arth, 1994; Saltus and others, 1999). 

The overall felsic composition of the Taylor Mountains pluton 

suggests that its parental magmas were derived from melting 

of this continental crust in response to subduction-related high 

heat flow.

Conclusion

The 68 to 63 Ma Taylor Mountains pluton is an 

epizonal, composite biotite-granite stock. The stock 

is composed primarily of two compositionally similar 

porphyritic biotite-granite phases having textural differences 

developed in response to different emplacement histories. 

Fractional crystallization of the magmas produced a small 

amount of fine-grained residual granite. The residual 

granite, although distinctly felsic, is not strongly traceelement or volatile enriched. Mineral deposits in the area 

are primarily placers whose sources were probably veins in 

country rocks to the Taylor Mountains pluton rather than 

hydrothermal systems related to granite crystallization. The 

Taylor Mountains pluton is thought to represent melted 

continental crust; its parent magmas developed in a high heat 

flow setting during the waning stages of Late Cretaceous 

subduction beneath interior Alaska.

References Cited

Arth, J.G., 1994, Isotopic composition of igneous rocks of 

Alaska, in Plafker, George, and Berg, H.C., eds., The geology of Alaska: Geological Society of America, DNAG 

Series, v. G-1, p. 781-796.
12 Reconnaissance Study of the Taylor Mountains Pluton, Southwestern Alaska

Bergman, S.C., Hudson, T.L., and Doherty, D.J., 1987, Magmatic rock evidence for a Paleocene change in the tectonic 

setting of Alaska [abs.]: Geological Society of America, 

Abstracts with Programs, v. 19, no. 7, p. 586-587.

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.

Burleigh, R.E., 1991, Geology and geochemistry of the Sleitat 

Mountain tin deposit, southwestern Alaska, in Reger, R.D., 

ed., Short notes on Alaskan geology 1991: Alaska Division of Geological and Geophysical Surveys, Professional 

Report 111, p. 29-39.

Cady, W.M., Wallace, R.E., Hoare, J.M., and Webber, E.J., 

1955, The central Kuskokwim region, Alaska: U.S. Geological Survey Professional Paper 268, 132 p.

Daggett, P.H., 1987, Late Cretaceous thermal regime of southern Alaska [abs.]: Geological Society of America, Abstracts 

with Programs, v. 19, no. 7, p. 634-635.

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, 

The geology of southwestern Alaska, in Plafker, George, and 

Berg, H.C., eds., The geology of Alaska: Geological Society of 

America, DNAG Series, v. G-1, p. 285-310.

Ellefson, R.M., Hoppe, J.E., Kurtak, J.M., and Meyer, M.P., 

2005, Mineral investigations in the Aniak mining district, 

southwestern Alaska, 2004 field season: Bureau of Land 

Management, Open File Report 100, 44 p.

Hudson, Travis, 2001, Alaska resource data file for the Taylor 

Mountains quadrangle: U.S. Geological Survey, Open-File 

Report 01-200, 51 p.

Hudson, Travis, and Arth, J.G., 1983, Tin granites of Seward 

Peninsula, Alaska: Geological Society of America Bulletin, 

v. 94, p. 768-790.

Klimasauskas, E.P., Miller, M.L., Bradley, D.C., Bundtzen, 

T.K., and Hudson, T.L., 2006, Major- and trace-element 

concentrations in rock samples collected in 2005 from the 

Taylor Mountains 1:250,000-scale quadrangle, Alaska: U.S. 

Geological Survey Open-File Report 2006-1306 accessed 

April 10, 2010, [http://pubs.usgs.gov/of/2006/1306/].

Lanphere, M.A., and Dalrymple, G.B., 2000, First-principles 

calibration of 38Ar tracers: Implications for the ages of 40Ar/39Ar fluence monitors: U.S. Geological Survey Professional Paper 1621, 10 p.

Layer, P.W., 2000, Argon-40/argon-39 age of the El’gygytgyn 

impact event, Chukotka, Russia: Meteroitics and Planatary 

Science, v. 35, p. 591-599.

Layer, P.W., Hall, C.M., and York, D., 1987, The derivation 

of 40Ar/39Ar age spectra of single grains of hornblende 

and biotite by laser step heating: Geophysical Research 

Letters, v. 14, p. 757-760.

McDougall, I., and Harrison, T.M., 1999, Geochronology 

and thermochronology by the 40Ar/39Ar method (2d ed.): 

New York, Oxford University Press, 269 p. 

Miller, M.L., Bailey, E.A., Beischer, G.A., Bradley, D.C., 

Bundtzen, T.K., Goldfarb, R.J., Karl, S.M., Saltus, R.W., 

and Wang, B., 2006, USGS mineral resource studies 

in the Taylor Mountains quadrangle, southwestern 

Alaska—New “AMRAP”: Journal of the Alaska Miners 

Association, v. 34, no. 2, p. 1, 6-7, 10, 12-15.

Miller, M.L., Bradley, D.C., Goldfarb, R.J., and 

Bundtzen, T.K., 2007, Tectonic setting of Late 

Cretaceous gold and mercury metallogenesis, 

Kuskokwim Mineral Belt, southwestern Alaska, in

Andrew, C.J., and others, eds., Digging deeper: Irish 

Association for Economic Geology, Proceedings of 

the Ninth Biennial Meeting of the Society for Geology 

Applied to Mineral Deposits, Dublin, Ireland, August 

20-23, 2007, v. 1, p. 683-686.

Moll-Stalcup, E.J., 1994, Late Cretaceous and Cenozoic 

magmatism in mainland Alaska, in Plafker, George, and 

Berg, H.C., eds., The geology of Alaska: Geological 

Society of America, DNAG Series, v. G-1, p. 589-619.

Moll-Stalcup, E.J., Brew, D.A., and Vallier, T.L., 1994, 

Latest Cretaceous and Cenozoic magmatic rocks of 

Alaska, in Plafker, George, and Berg, H.C., eds., The 

geology of Alaska: Geological Society of America, 

DNAG Series, v. G-1, Plate 5, scale 1:2,500,000.

Reifenstuhl, R.R., Decker, J.E., and Coonrad, W.L., 1985, 

Compilation of geologic data from the Taylor Mountains 

D-4 quadrangle, Southwestern Alaska: Alaska Division 

of Geological and Geophysical Surveys, Report of 

Investigations 85-2, scale 1:63,360.

Saltus, R.W., Hudson, T.L., and Connard, G.G., 1999,

 A new magnetic view of Alaska: GSA Today, v. 9, 

no. 3, p. 106.

Samson, S.D., and Alexander, E.C., 1987, Calibration of 

the interlaboratory 40Ar/39Ar dating standard, MMhb1: 

Chemical Geology, v. 66, p. 27-34.

Steiger, R.H., and Jaeger, E., 1977, Subcommission 

on geochronology: Convention on the use of decay 

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Science Letters, v. 36, p. 359-362.

York, D., Hall, C.M., Yanase, Y., Hanes, J.A., and Kenyon, 

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p. 1136-1138.
 Appendix 13

Appendix—Summary of Analytical 

Method for 40Ar/39Ar Analysis

One sample was submitted to the Geochronology 

Laboratory at the University of Alaska, Fairbanks for 40Ar/39Ar 

analysis. The sample was crushed, washed, and sieved to 

either 100–250 or 250–500 micron-size fractions, and hand 

picked for biotite. The monitor mineral MMhb-1 (Samson and 

Alexander, 1987), which has an age of 513.9 Ma (Lanphere 

and Dalrymple, 2000), was used to monitor neutron flux 

(and to calculate the irradiation parameter, J). The samples 

and standards were wrapped in aluminum foil and loaded 

into aluminum cans measuring 2.5 cm in diameter and 6 cm 

in height. The samples were irradiated in position 5c of the 

uranium-enriched research reactor of McMaster University in 

Hamilton, Ontario, Canada for 20 megawatt-hours.

Upon their return from the reactor, the samples and 

monitors were loaded into 2-mm diameter holes in a copper 

tray that was then loaded into an ultra-high vacuum extraction 

line. The monitors were fused, and the samples heated, 

by using a 6-watt argon-ion laser following the technique 

described in York and others (1981), Layer and others (1987), 

and Layer (2000). Argon purification was achieved by using 

a liquid nitrogen cold trap and a SAES Zr-Al getter at 400°C. 

The samples were analyzed in a VG-3600 mass spectrometer 

at the Geophysical Institute, University of Alaska, Fairbanks. 

The argon isotopes measured were corrected for system 

blank and mass discrimination, as well as for calcium, 

potassium, and chlorine interference reactions following 

procedures outlined in McDougall and Harrison (1999). 

System blanks generally were 2×10-16 mol 40Ar and 2×10-

18 mol 36Ar, which are 10 to 50 times smaller than fraction 

volumes. Mass discrimination was monitored by running 

both calibrated air shots and a zero-age glass sample. 

Measurements were made on a weekly to monthly basis to 

check for changes in mass discrimination.

The age, Ca/K, and Cl/K spectra plots are shown in 

figure 8, and detailed analyses are given in table 2. All ages 

are quoted to the +1 sigma level and calculated by using the 

constants of Steiger and Jaeger (1977). The integrated age is 

the age given by the total gas measured and is equivalent to a 

potassium-argon (K-Ar) age. The spectrum provides a plateau 

age if three or more consecutive gas fractions represent at least 

50 percent of the total gas release and are within two standard 

deviations of each other (Mean Square Weighted Deviation 

less than ~2.7). Sample 05AH19 meets the plateau criteria.

Produced in the Western Region, Menlo Park, California

Manuscript approved for publication, June 11, 2010

Text edited by Tracey Suzuki

Layout and design by Jeanne S. DiLeo
Dumoulin and Galloway, eds.—Studies by the U.S. Geological Survey in Alaska, 2008–2009—Professional Paper 1776–A