569

GEOLOGICAL SURVEY CIRCULAR 569

Geochemical Anomalies and 

Metalliferous Deposits Between 

Windy Fork and Post River 

Southern Alaska Range

Geochemical Anomalies and 

Metalliferous Deposits Between 

Windy Fork and Post River 

Southern Alaska Range

By Bruce L. Reed and Raymond L. Elliott 

GEOLOGICAL SURVEY CIRCULAR 569

Washington 1968
United States Department of the Interior

STEWART L UDALL, Secretary

Geological Survey

William T. Pecora, Director

Free on application to the U.S. Geological Survey, Washington, D.C. 20242
CONTENTS

Page

Abstract- ------------______ 1

Introduction ----------------- 1

Geologic environment- ----------- 1

Sedimentary rocks ----------- 1

Igneous rocks -------------- 2

Structure ---------------- 2

Geochemical data -------------- 2

Areas of anomalous metal concentrations - 3

Area l-----------__--___ 3

Area 2------------------ 4

Areas of anomalous metal 

concentrations- -Continued 

Area 3----------

Area 4----------

Area 5----------

Area 6----------

Area 1--. -------

Age of mineralization - - - 

Summary and conclusions - 

References cited ------

Page

ILLUSTRATIONS

Figure 1. Index map showing report area and location of McGrath A-2, A-3, B-2, and B-3, 

quadrangles ------------------------------------

2. Reconnaissance map showing generalized geology between Windy Fork and Post 

River, southern Alaska Range --------------------------

3. Map showing location of stream-sediment samples between Windy Fork and Post 

River, southern Alaska Range --------------------------

4-9. Maps of 

4. Copper distribution showing location of samples with 100 ppm (parts per

million) or greater -----------------------------

5. Lead distribution showing location of samples with 50 ppm or greater - - -

6. Molybdenum distribution showing location of samples with 5 ppm or greater

7. Nickel distribution showing location of samples with 100 ppm or greater - -

8. Silver distribution showing location of samples with 1 ppm or greater - - -

9. Zinc distribution showing location of samples with 200 ppm or greater - - -

10. Map showing areas with anomalous metal concentrations in stream-sediment

samples --------------------------------------

11. Map of Bowser Creek showing location of stream-sediment samples and sulfide 

mineral deposits ---------------------------------

Page

11

12

14

15

16

17

18

19

20

21

22

TABLES

Table 1. Semiquantitative spectrographic analyses and gold analyses of bedrock samples from 

the Windy Fork-Post River area ----------------------------

2. Partial analyses of bedrock samples from areas 3, 4, and 5 --------------

Page 

6

III

Geochemical Anomalies and Metalliferous Deposits

Between Windy Fork and Post River

Southern Alaska Range

By Bruce L. Reed and Raymond L. Elliott

Abstract

Geochemical sampling between Windy Fork and Post River, 

southern Alaska Range, defines seven areas with anomalous 

concentrations of lead, zinc, silver, copper, or molybdenum in 

stream-sediment samples. Five of these areas contain metalliferous deposits that are spatially related to centers of hypabyssal 

igneous bodies. The deposits occur as replacement bodies in limestone and as replacement bodies and fracture fillings in intrusive 

rocks. Most of the deposits that crop out are small and discontinuous, but the number of occurrences with locally high silver 

content, their possible continuity beneath surficial deposits, and a 

favorable geologic environment suggest that this region has an 

economic potential for base and precious metals.

INTRODUCTION

A new region of lead, zinc, and silver mineralization 

was identified in the summer of 1967 during field 

geologic evaluation of the southern Alaska Range as 

part of the U.S. Geological Survey's Heavy Metals 

Program. One lead^zinc-silver deposit in this region 

has already been described (Reed and Elliott, 1968). 

The seven geochemical maps accompanying this report 

delineate other areas of anomalous concentrations of 

lead, zinc, silver, copper, molybdenum, and nickel in 

stream-sediment samples. Known mineral occurrences in these areas are briefly described.

The region described in this report is about 150 

miles northwest of Anchorage, on the west side of the 

upper reaches of the South Fork of the Kuskokwim 

River. The region covers about 560 square miles and 

includes parts of the McGrath A-2, A-3, B-2, and 

B-3 quadrangles (fig. 1). It lies south of Farewell, a 

Federal Aviation Agency Station that maintains a 

5,000-foot gravel runway. McGrath, about 62 air miles 

northwest of Farewell, is the nearest source of gasoline 

and supplies.

The region described herein is best reached by helicopter. The only road in the region is a jeep trail 

which extends south from the Farewell airfield along 

Sheep Creek for about Smiles. From this point a fourwheel-drive vehicle with large tires can continue south 

along the creek for an additional 6 miles. This, however, necessitates crossing the creek several times, 

and crossings can be accompli shed only during periods 

of low water. Travel by tracked vehicle is possible in

the major north-south valleys (Kuskokwim River, Post 

River, Sheep Creek, and Windy Fork) in the winter 

and early spring. Small, properly equipped aircraft can 

land on gravel bars along the Post and Kuskokwim 

Rivers and Windy Fork, and on Smith, Post, and 

Valeska Lakes.

The region is in rugged terrain. Relief increases 

from north to south and averages between 2,000 and 

3,000 feet; peaks in the southern part of the region 

crest between 6,000 and 7,700 feet in height. Timber 

is sparse and consists chiefly of spruce and scattered 

cottonwood that are restricted to the lower altitudes 

along the major north-south valleys.

The only previous geological investigation in this 

region is that made by Brooks (1911) who traversed 

the Alaska Range via the South Fork of the Kuskokwim 

River in 1902. The present report describes the mineral occurrences and results of stream-sediment 

sampling in part of the region that was covered during 

the 1967 field season. Between 15 minutes and an hour 

were spent examining each of the metalliferous 

deposits described in this report; additional mapping 

and physical exploration are necessary to evaluate 

these occurrences.

GEOLOGIC ENVIRONMENT

The part of the Alaska Range considered here is 

made up of Paleozoic sedimentary rocks intruded by 

granitic stocks, hypabyssal breccia bodies, and many 

felsic and mafic dikes. Tertiary extrusive rocks, including mafic flows and pyroclastic deposits with 

interbedded sedimentary units, locally overlie the 

Paleozoic rocks. Mineral deposits are spatially related 

to the intrusive bodies and occur as replacement deposits in limestone or in the intrusive rocks.

Sedimentary Rocks

The Paleozoic rocks consist predominantly of 

interbedded shale, siltstone, and argillite, but t;hey 

include prominent units of thin to massively bedded 

gray limestone. These rocks show little or no effect 

of regional metamorphisnru Because of the stratigraphic and structural complexity of the region, the 

sedimentary rocks are shown in figure 2 as one unit, 

with the exception of prominent limestone units. The
limestone units shown locally in the southeast part 

of figure 2 may be Ordovician in age, whereas the 

limestone north of the Farewell fault is believed to 

be of Devonian age. Graptolites, including species of 

Glyptograptus and Climacograptus of Middle or Late 

Ordovician age (Michael Churkin, oral commun., 1968), 

were found in carbonaceous shale at localities F-1 and 

F-2 in figure 2. At Saint Johns Hill, north of the Farewell fault (locality F-3, fig. 2) several rugose corals, 

including Cylindrophyllum and Disphyllum of probable 

Middle Devonian age (CharlesMerriam, oral commun., 

1968), occur in thick to massive gray limestone.

Igneous Rocks

The intrusive rocks are typically porphyritic and 

include quartz monzonite, granodiorite, and minor 

amounts of quartz diorite. Most of the intrusive bodies 

shown in figure 2 have nearly vertical contacts and 

include varying amounts of igneous breccia. Many of 

the breccia bodies are roughly circular in plan and 

probably are volcanic breccia pipes that were feeders 

for explosive eruptions. Parts of the igneous bodies 

are hydrothermally altered. Narrow contact aureoles 

are locally developed, and in places the bordering 

limestone is bleached and recrystallized to hard, finegrained, silicated marble or calc-silicate hornfels. 

The pelitic rocks are baked and converted to hornfels. 

All the known mineral deposits are spatially related 

to intrusive rocks.

The dike swarm shown diagrammatically in the 

southwest part of figure 2 appears to be related to a 

large intrusive body west of Windy Fork. Other dikes, 

too small to be shown at the scale of figure 2, are also 

spatially related to the intrusive bodies shown in figure 

2. They are intermediate to felsic in composition, and, 

where they intrude limestone, skarn zones of potential 

economic significance may be developed. Ore minerals 

in the skarn are chiefly pyrrhotite and sphalerite 

(marmatite) with associated calc-silicate minerals, 

chiefly epidote and clinopyroxene.

Extrusive volcanic rock consists of flows of intermediate composition and pyroclastic deposits that are 

interbedded with local units of tuffaceous sandstone, 

siltstone, and shale. These rocks are spatially related 

to intrusive centers and probably belong to the same 

period of igneous activity. Fossil plants from sediments 

interbedded with the volcanics (F-4, fig. 2, LJSGS 

Paleobotany locality 11126) include Metasequoia 

cf. occidentalis (Newb.) Chan. and Cocculus cf. flabella 

(Newb.) Wolfe, of early Tertiary, probablyPaleocene, 

age (J. A. Wolfe, oral commun., 1968).

Structure

The dominant structural grain of the Alaska Range 

within the region described in this report is approximately north to northeast. The region is characterized 

by tightly compressed north-trending folds that are 

locally overturned to the west. Near the mountain front, 

west of the area shown in figure 2, broad open folds 

that trend east-northeast are superimposed on the 

north-trending folds.

The north- to northeast-trending structural features 

are truncated to the north by the Farewell fault zone, 

a major right-lateral strike-slip crustal feature that 

is part of the Denali fault system (Grantz, 1966). 

Immediately south of the fault zone, folds are dragged 

to the east-northeast, and the fault zone is broken into 

numerous small segments. Small, hydrothermally 

altered, slightly serpentinized bodies of ultramafic 

rock (picrite?) occur locally along segments of the 

Farewell fault zone in this part of the Alaska Range.

The faults that trend north-northeast as shown in 

the southern part of figure 2 may have formed during 

the same period of deformation as the north- to 

northeast-trending folds. Parts of the broad northeasttrending valleys in the -southern part of the map area 

probably follow large fault zones. Many small cross 

faults, too small to show at the scale of figure 2, border 

the intrusive bodies. These cross faults and related 

shear zones locally contain sulfide mineral deposits.

Emplacement of the igneous bodies may have been 

controlled by the preexisting north- to northeasttrending structural grain. In particular, a fault west 

of Post River that has been traced for more than 20 

miles seems to have guided the intrusion of several 

igneous masses. From south to north these masses 

include a small stock with associated pyrrhotite and 

chalcopyrite on the West Fork of Post River (fig. 10, 

samples 24, 25); intrusive breccias, with associated 

deposits of lead, zinc, and silver, at Bowser Creek 

(fig. 10, area 1); and the large elongate north-trending 

igneous body between Sheep Creek and Post River, 

where tributary streams contain sediments with 

anomalous amounts of lead and zinc (fig. 10, area 5). 

This fault also appears to have been a locus of mineralization as suggested by high molybdenum values 

along the southern 10 miles of its length (figs. 2 and 

6), by a small shear zone that contains pyrite and 

argentiferous galena (fig. 10, sample 22) on the south 

tributary of Hippie Creek, and by the vein of calcite, 

argentiferous galena, and pyrite on Hippie Creek (fig. 

10, south part of area 5).

GEOCHEMICAL DATA

The locations of the stream-sediment samples are 

shown in figure 3. Analytical data for the 34 elements 

determined in these 556 samples have been released 

in an open-file report (Elliott and Reed, 1968).

Standard procedures were followed in the collection 

and preparation of the stream-sediment samples. 

Where possible, the sample was collected from the 

active stream channel; where this was not possible, 

the sample was collected from higher level stream 

deposits adjacent to the active channel. The samples 

were dried, sieved, and the minus 80 mesh fractions 

were analyzed for 34 elements by the six-step 

semiquantitative spectrographic method. The spectrographic analyses were reported in percentage or parts 

per million (ppm) to the nearest number in the series 

0.5, 0.7, 1.0, 1.5, 3, 5, 7, 10, 15, and so on. The precision of a reported value is approximately plus 100
percent or minus 50 percent. The analysts were W. L. 

Campbell, N. M. Conklin, Arnold Parley, Jr., J. L. 

Finley, D. J. Grimes, J. C. Hamilton, A. L. Meier, 

R. L. Miller, H. G. Neiman, and T. A. Roemer.

Limits of detection for the six elements discussed 

in this report are: 1 ppm for silver, 2 ppmfor copper, 

5 ppm for molybdenum, 2 ppm for nickel, 10 ppm for 

lead, and 200 ppm for zinc. Three of the six elements 

considered (Ag, Mo, and Zn) either were not detected 

or were detected but below the limit of determination 

in the majority of samples. Lead was not detected in 

3 percent of the samples submitted for analysis.

Six monoelemental geochemical maps (figs. 4-9) 

show the distribution of possibly anomalous concentrations of each of the metals. Histograms are given 

for each of the geochemical maps. The histograms 

represent the distribution of analyses of the element 

for the 556 stream-sediment samples shown in figure 

3. The cutoff point for concentrations represented on 

each map was selected largely on the basis of these 

histograms; the selection of anomalous levels is subjective and interpretive on the part of the writers. The 

assignment of symbols to represent certain concentrations or ranges of concentrations is arbitrary; the 

same symbols on different maps do not imply an equal 

degree of anomaly.

Analytical results are summarized below and 

anomalous values are suggested for each element.

Copper. Concentrations of copper range from 10 to 

500 ppm in the stream-sediment samples. The median 

copper content is 50 ppm. Samples which contain 100 

ppm or greater, about 14 percent of the total number 

of samples, are shown in figure 4. Values of 150 ppm 

or greater, 7 percent of the total, are considered 

anomalous.

Lead. In 97 percent of the samples, lead was reported in concentrations ranging from 10 to 700 ppm. 

The median value is 20 ppm. Samples with a lead 

content of 50 ppm or greater, about 24 percent of the 

total number, are shown in figure 5. Lead contents of 

100 ppm or greater, about 10 percent of the total, are 

considered anomalous.

Molybdenum. In the majority of analyses, molybdenum was not detected. The median value is less than 

5 ppm the limit ofdetectability for the spectrographic 

method. About 18 percent of the samples contain 5 ppm 

or more of molybdenum; these are shown in figure 6. 

Molybdenum concentrations of 10ppm or greater, about 

7 percent of the total, are considered anomalous.

Nickel. The content of nickel in the samples ranges 

from 2 to 300 ppm, and the median value is 50 ppm. 

About 6 percent of the samples had contents of 100 ppm 

or greater; all are shown in figure 7. It is questionable 

whether any of these amounts should be considered 

truly anomalous; rather, they may represent an area 

of locally higher background concentrations related to 

small mafic or ultramafic bodies.

Silver. In the majority of samples, silver was not 

detected. The median value falls below the 1 -ppm limit 

ofdetectability for the analytical method. Samples with 

a silver content of 1 ppm or greater, or about 8 percent 

of the total number, are considered anomalous and are 

shown in figure 8.

Zinc. In the majority of stream-sediment samples, 

zinc was not reported. The median value is probably 

well below the 200-ppm minimum detectability of the 

analytical method. Samples which contain 200 ppm or 

greater, about 15 percent of the total, are shown in 

figure 9. Amounts of 300 ppm and above, 10 percent 

of the total, are considered anomalous.

AREAS OF ANOMALOUS METAL 

CONCENTRATIONS

The six monoelemental geochemical drainage maps 

show seven areas with anomalous concentrations of 

two or more metals. These areas, shown in figure 10, 

are:

Area and location Anomalous metals 

1. Bowser Creek -------- - Pb, Zn, Ag, Cu

2. Between Rat Fork and

Ozzna Creek ---------- Pb, Zn, Ag, Cu

3. South of Ozzna Creek- ------ Pb, Zn, Ag

4. Southwest of Smith Lake ----- Pb, Zn, Cu

5. Between Sheep Creek and

and Post River --------- Pb, Zn

6. Southwest of Saint Johns

Hill --------------- Zn, Cu, Mo, Ni

7. Jay Creek area ---------- Mo, Pb

Areas 1-5 contain deposits of sulfide minerals, and 

a brief description of these deposits is given below. As 

yet, no metal deposits have been found in areas 6 and 7; 

therefore, the reason for the anomalous metal concentrations in these areas is uncertain. The location 

of bedrock samples described below is also shown in 

figure 10.

Area 1

Bowser Creek drains an area of known lead, zinc, 

and silver deposits previously described (Reed and 

Elliott, 1968). The deposits in this area are (1) argentiferous galena-sphalerite or sphalerite-pyrrhotite 

deposits in limestone, and (2) pyrrhotite-sphalerite 

fracture fillings in igneous breccia. The deposits in 

limestone are replacement bodies and fissure veins; 

some chip samples show as much as 14.7 percent zinc 

and 52 ounces of silver per ton. Stream-sediment 

samples from this creek serve as a basis of reference 

in interpreting lead-zinc-silver values of other areas 

sampled.

The location of stream-sediment samples, areas of 

sulfide mineralization, and sampling profiles along 

Bowser Creek are shown in figure 11. The sampling 

profiles show, in general, an increase in Pb, Zn, Ag, 

and Cu as the mineralized areas are approached. The
sampling profiles also show that metal concentrations 

diminish rapidly downstream from the known sulfide 

deposits. Within 1% miles from the deposits all metal 

concentrations fall below levels considered anomalous 

for the region in general. One possible explanation for 

this short metal train is that Bowser Creek is in an 

area of high relief, and colluvium from valley slopes 

is constantly being supplied to the creek. In many 

places, steep talus slopes feed directly into the creek. 

Thus, a rapid dilution of the sediment anomaly begins 

as soon as the stream leaves the source area. Other 

factors such as the pH of the stream (limestone forms 

a large percentage of rocks in the drainage area) may 

also influence the extent of the anomaly.

Most of the high metal values in Bowser Creek are 

related to areas of known sulfide deposits (fig. 11; and 

Reed and Elliott, 1968). However, the highest metal 

contents (Cu, 500 ppm; Pb, 700 ppm; Ag, 7 ppm; and 

Zn, 2000 ppm) are from the northeasternmost tributary 

to Bowser Creek, an area not examined. This drainage 

area should be investigated.

Area 2

Stream-sediment samples from the headwaters of 

Rat Fork and Ozzna Creek show anomalous contents 

of copper, lead, zinc, and silver. The headwaters of 

these two creeks are separated by the southern part 

of a dumbbell-shaped intrusive (fig. 2) that forms a 

prominent peak at an altitude of 6,657 feet. Because 

of the extremely rugged terrain, this intrusive was 

not examined, but it is probably quartz diorite inasmuch 

as quartz diorite porphyry dikes cut limestone and 

argillite in the nearly vertical 1,500-foot-high cirque 

walls at the headwaters of Rat Fork. Skarn zones 2 to 

10 feet wide that contain pyrrhotite, sphalerite, and 

chalcopyrite are associated with some of the quartz 

diorite porphyry dikes (samples 1 and 2, fig. 10, table 

1). The intrusive cuts limestone, siltstone, and argillite. Rocks near the north and west contacts of the 

southern part of the intrusive are intensely altered and 

dark reddish and yellowish brown. Boulders of 

pyrrhotite and sphalerite, p y r i t e-phrrhotite-galenasphalerite-arsenopyrite, or pyrite and bluish-gray 

magnetite are abundant near the headwaters of Ozzna 

Creek and Rat Fork and in the talus slopes adjacent 

to the intrusive (samples 3-5, fig. 10, table 1).

One of the boulders (sample 3, table 1), about 14 

inches in diameter, assayed 9.6 ounces of silver per 

ton (300 ppm), more than 10 percent of lead, and 7 

percent of zinc.

The anomalously high metal content of the streamsediment samples, the intense alteration along the 

intrusive contacts, and the locally derived sulfide 

boulders with high metal content indicate that this area 

merits further investigation.

Area 3

Sediment samples from the main northwest-flowing 

tributary to Ozzna Creek show high values for lead,

zinc, and silver. This stream drains the northern part 

of a roughly circular igneous body about 2 miles in 

diameter (fig. 2). The body forms a prominent 7,205- 

foot peak which is surrounded by complexly folded 

argillite, limestone, and siltstone. Talus from adjacent 

slopes is chiefly coarse igneous breccia with clasts 

ranging from less than 1 mm (millimeter) to more than 

18 inches in diameter. The clasts are angular to subangular and consist of fine-grained dioritic rock, 

argillite, silicated limestone, and quartz-feldspar 

fragments. The clasts are embedded in an aphanitic 

greenish-gray matrix. Other lithologies in the talus 

are greenish-gray lithic tuff and yellowish-gray felsic 

tuff. The composition and texture of the talus material 

adjacent to the body, the shape of the body, and its 

nearly vertical contacts suggest that it may be a 

breccia pipe. The nearly flat-lying horseshoe-shaped 

outcrop of flows and lithic tuffs 2 miles southwest of 

the peak (fig. 2) is probably related to this igneous 

center. The center also may contain pyroclastic material. Felsic dikes, chiefly quartz monzonite porphyry, 

and mafic dikes cut the sedimentary rocks. Relations 

between the dikes and the main igneous body are not 

known.

Sulfide mineral deposits are exposed at an altitude 

of about 4,750 feet on the valley wall south of the 

northwest-flowing tributary. The area is intensely 

stained by iron oxide. The deposits are small pods 

and narrow lenses of pyrrhotite (which contains 

euhedral pyrite crystals), sphalerite, and argentiferous galena. Galena appears to replace pyrrhotite and 

sphalerite. Veinlets of calcite cut all the sulfide minerals. The deposits are along shear zones in a felsic 

dike. The shear zones are subvertical and strike 

N. 55° E. Sporadically distributed sulfide minerals 

extend updip along the shears for about 150feet; their 

downdip distribution is obscured by talus. The pods, 

although locally quite rich (samples 7 and 8, table 2), 

are small, ranging from 0.5 foot to 2 feet in width and 

from 1 foot to 3 feet in length. Some pods are connected 

by sulfide veins 1 to 3 inches thick. A selected sample 

of galena contained 52.2 ounces of silver per ton 

(sample 8, table 2), but a 10-foot chip sample across 

two pods in the dike yielded less than half an ounce of 

silver per ton, 1.5 percent of lead, and 1 percent of 

zinc (sample 6, table 1).

Another lens of sulfide minerals, about 200 feet west 

of where samples 6-8 were collected, is along an 

east-striking fault zone that cuts igneous breccia and 

a series of felsic and mafic dikes (samples 9 and 10, 

fig. 10, table 2). The fault zone is 3 to 15 feet wide, 

dips vertically, and shows intensive hydrothermal 

alteration. The sulfide lens has a maximum exposed 

width of 4M feet. It pinches out within 20 feet upward 

and to the west. Possible extensions below and east of 

the outcrop are covered by talus. Ore minerals in the 

lens are pyrrhotite, argentiferous galena, pyrite, and 

sphalerite. The galena appears to be late and cuts the 

pyrrhotite. A selected sample of galena, sphalerite, 

and pyrrhotite from this lens (sample 10, table 2) con-
tains 70.4 ounces of silver per ton, 11.3 percent of 

lead, and 7 percent of zinc. The chip sample across 

the lens (sample 9, table 2) shows moderate amounts 

of lead, zinc, and silver.

The exposed deposits in this area are small but 

locally rich. Additional prospecting, mapping, and 

physical exploration is necessary to further define 

their extent and tenor.

Area 4

Stream-sediment samples in the northern half of 

area 4 are characterized by moderately high concentrations of copper, lead, and zinc, and sediment from 

a stream in the southern part of the area contains 10 

and 15 ppm of molybdenum. A small stock of quartz 

monzonite or granodiorite porphyry cuts limestone, 

siltstone, and argillite in the southern part of this area 

(fig. 2). Shear zones in dikes near the north side of the 

stock contain sphalerite and pyrrhotite in small lenses 

(tables 1 and 2, samples 11-13). Contacts of the stock 

were not investigated, and only scattered streamsediment samples were obtained from the south and 

west drainages of the stock.

Area 5

Streams that drain the eastern margin of the elongate 

north-trending igneous complex in area 5 are characterized by anomalous lead and zinc values. Other 

scattered anomalous lead and zinc values occur north 

and south of the igneous body.

This large igneous complex consists of porphyritic 

felsic intrusives in the form of dike swarms and small 

stocklike bodies, with local capping flows and pyroclastic units. The bordering rock is chiefly limestone 

with subordinate amounts of argillite and siltstone.

Sulfide minerals occur in small veinlets and along 

shear zones in this area (table 1, samples 14-17). The 

largest deposit seen is a calcite vein in limestone on 

Hippie Creek (samples 18-21, fig. 10, tables land 2). 

The vein contains pyrite and argentiferous galena in 

scattered grains and as discontinuous bands 1 to 10 

inches wide. It also contains minor amounts of arsenopyrite. The vein strikes east-northeast, dips vertically, 

and is exposed discontinuously for about 100 feet along 

the creek where it appears to be along a fault. The 

vein has a maximum width of 6 feet and narrows eastward to about 6 inches. To the west the vein is covered 

by surficial deposits. Lead values downstream from the 

vein are low, but slightly anomalous lead values upstream, and a similar occurrence of calcite, pyrite, 

and galena in a narrow shear (sample 22, fig. 10, table 

1), suggest that other lead-silver occurrences maybe 

present.

Viewed individually, these small deposits have no 

apparent economic significance, but they might justify 

some prospecting for larger concealed deposits.

Many anomalous metal concentrations are from 

stream-sediment samples taken from large streams

where dilution would be at a maximum. Small cobbles 

containing lead and zinc sulfides were found in most 

streams draining the igneous mass. Stream-sediment 

sampling should be undertaken in all the small streams 

to delineate the areas of anomalous metal concentrations.

Area 6

Stream-sediment samples from the creek draining 

the mountain front southwest of Saint Johns Hill show 

anomalous concentrations of copper, molybdenum, 

zinc, and nickel. Only a geochemical reconnaissance 

was made in this area, and bedrock data are not available.

Aerial reconnaissance and photointerpretation suggest that the area consists of limestone, overlain 

locally by Tertiary lava flows and interbedded sediments. Area 6 is structurally complex and is cut by 

segments of the Farewell fault zone. The anomalous 

nickel values may be related to mafic or ultramafic 

bodies that are emplaced locally along segments of the 

Farewell fault in this part of the Alaska Range. The 

source of the copper, molybdenum, and zinc in the 

sediments is not known.

Area 7

Several stream-sediment samples from Jay Creek 

show anomalous values for molybdenum and one high 

value for lead. Sulfide deposits are not known to occur 

in this drainage an area underlain chiefly by siltstone, shale, and limestone.

The lead anomaly, which is in a creek that drains 

the west side of the intrusive breccia at Bowser Creek, 

probably reflects lead deposits farther upstream near 

the intrusive contact.

The anomalous molybdenum values at Jay Creek and 

in the southern part of the map area (fig. 6) cannot be 

readily explained. Their distribution does not correspond to exposed igneous rocks, but the highest 

values (10-30 ppm) trend in a northeast direction and 

show a strong correlation with the faults shown in 

figure 2. South of the map area stocks and batholiths 

are present; it is possible that the molybdenum values 

represent "leakage" of metal-bearing fluids that have 

migrated upward along the faults from buried intrusive 

rocks. Metal values in stream-sediment samples 

downstream from the trace of the faults would be 

expected to show the effects of sediment dilution; such 

is the case at Jay Creek.

AGE OF MINERALIZATION

Most of the geochemical metal anomalies and all the 

known mineral deposits in the region are spatially related to the igneous centers. Many of the deposits are 

skarn deposits in limestone near granitic bodies or 

felsic dikes, and others occur within the dikes or 

granitic bodies themselves. The ore-mineral assemblages include two or more of the following: Pyrrhotite,
Table 1. Semiquantitative spectrographic analyses and gold

[Analysts: Arnold Parley, Jr., J. Finley, D. J. Grimes, J. C. Hamilton, H. G, 

ported in the series 0.1, 0.15, 0.2, 0.3, 0.5, 0.7, 1.0, 1.5, and so on, 

shown; >, greater than value shown; M, greater than 10 percent; ND, not 

Limit of detectability given by symbol defined at bottom of table. Results 

percent. Locations and sample numbers are shown in figure 10J

Limit of Sample Lab. Field

Location dectability No. No. No.

Area 2

3

4

5

South

tributary

to Hippie

Creek

Northeast

of area 2

West ForkPost River

Sample

1 ------ 

 2 ----- -

 j

4 ------

5 ------

6 ------

** 1 ACJ523 67AMa413

** 2 ACJ517 Ma407

f 3 ACG886 Ma272B

** 4 ACJ527 Ma415

** 5 ACJ136 Er317

** 6 ACJ433 67ARa468A

t 11 ACG021 67AEr359A

t 13 ACJ028 R392

t 14 ACG659 67AR291A

t 15 ACG665 R294B

** 16 ACJ448 R480

* 17 ACG549 Erl87

t 18 ACG638 R266A

t 19 ACG640 R266C

** 20 ACG758 Mal59B

* 22 ACG831 67AMa235 >1

t 23 ACJ106 67AEr260

t 24 ACJ574 67AEr220A

t 25 ACJ575 Er270B

t Limit of detectability

A M MM f\n

&& Hn UO

Description

- Chalcopyrite-pyrrhotite and sphalerite in 

4-foot-wide skarn adjacent to quartz diorite 

dike.

Pyrrhotite-sphalente-chalcopyrite skarn, 2 

feet wide.

Pyrrhotite-pynte-sphalerite-ga 1 en a-arsenopyrite boulder, 14 inches in diameter. 

- Magnetite-pyrite boulder.

Pynte-sphalente-galena in silicated limestone.

Sample across altered felsite dike containing 

two pyrrhotite-sphalerite-galena pods, 1 to 2 

feet wide and localized along shear zones.

Ag

50

3

As

N

N

300 100,000

1.5

30

15

2

7

70

1.5

30

7

200

700

150

,000

1

50

10

1

.5

.5

Sample

11 - -

13

N

500

N

N

N

N

N

N

<200

3,000

5,000

1,500

1,000

N

30,000

N

2,000

200

200

Au Ba Bi Cd Co Cr

0.03 7 15 N 150 <5

.04 5 N 300 20 30

.04 7 N 500 N 2

.06 N N N 50 15

ND 30 N 50 70 <5

.08 50 10 200 10 15

.05 1,000 N N 15 50

.04 30 N N 150 2

.04 15 70 200 30 30

.05 15 N N 20 7

.06 1,500 N 100 30 7

.03 <100 <10 <20 70 N

.05 20 N 500 15 3

.04 15 N 300 N 1.5

<.02 10 N 500 N 7

.06 <100 <10 <20 <10 10

<.02 20 100 700 7 20

.1 20 150 N 100 5

.1 7 N N 150 10

.02 2 10 50 31

.02 100 10 20 10 10

.02 5 10 20 55

Description

Several light-gray sheared, fractured, and altered felsite porphyry dikes that cutargillite 

and limestone. Sulfides, chiefly pyrrhotite and 

sphalerite, occur sporadically as small pods

and lenses along shear zones in dikes. Sample 

11, across a 2 -foot -wide pod that pinches out 

upward in 3 feet; lower extension covered,

includes 3 feet of dike. 

Discontinuous veins and pods of pyrrhotite in

argillite and limestone. Size of bodies variable: 1 to 4 feet wide, as muchas 20 feet long; 

locally adjacent to quartz-feldspar porphyry 

dikes. Intrusive to south is granodiorite or

quartz monzonite porphyry; several oxidized

pods seen from a distance in north-facing

cirque walls.
analyses of bedrock samples from the Windy Fork - Post River area

Neiman. Analyses unless noted, are semiquantitative spectographic and are reor by the following symbols: N, not detected; <, detected but below value 

determined; I, interference. Analyses for gold are by atomic absorption, 

are given in parts per million except for Fe, Mg, Ca, Ti, which are given in

Cu

>20,000

1,000

3,000

1,500

1,000

150

1,000

3,000

7,000

7,000

1,500

2,000

700

3,000

1,500

Mo

N

N

N

N

N

N

N

N

N

N

N

I

N

N

N

Ha

700

700

1,500

300

70

500

200

70

1,500

3,000

700

1,500

300

300

5,000

Ni

30

30

N

70

30

10

20

150

N

15

2

15

3

3

N

Pb

50

50

M

N

20,000

15,000

100

70

5,000

200

20,000

70

20,000

70,000

>20,000

Sb

N

N

700

N

<100

N

N

N

N

N

N

N

<200

300

200

Sn

70

10

15

100

N

N

N

N

30

15

N

I

30

150

200

Zn

1,500

> 10, 000

70,000

200

10,000

10,000

50,000

3,000

10,000

1,500

> 10, 000

3,000

100,000

70,000

> 10, 000

Fe

20

15

M

20

15

7

10

M

M

7

10

>20

M

M

15

Mg

3

1

.05

2

.07

.7

2

.07

.7

3

.7

.7

.03

<.005

.5

Ca

3

1

3

2

<.05

1

.5

.07

M

M

.15

2

3

.3

5

Ti

0.01

.1

.003

.05

.0015

.1

.3

.007

.015

.15

.2

.03

.015

.0015

.015

Sample 

type

Selected.

Do.

Stream boulder.

Float in talus.

Stream cobble.

Chip;--/ 10 1.

Chip;i/ 5 1.

Grab.

Stream gobble.

Selected.

Do.

Stream cobble.

Selected.

Do.

Selected.

300 1,500 <2 >5,000 3,000 >10,000 10 .3 .03 Selected.

10,000 1,500 5 10,000 N 70,000 10 Selected.

5,000

15,000

1

2

2

N

N

3

5

5

30

100

1

20

20

7

50

3

2

2

20

10

10

10

10

N

N

200

100

100

N

N

10

10

10

700

700

200

200

200

M

M

.001

.05

.05

.7

1.5

.005

.02

.02

.02

2

.005

.05

.05

.02 Selected

.03 Selected.

.0002

.002

.002

i/For chip samples: First number is length of chip in feet, second number is interval between chips; that is, 

5 1 is a 5-foot-chip sample with chips collected every foot.

Sample Description Description

14 _______ pyrrhotite with minor sphalerite and chalcopyrite.

15 _______ Narrow malachite-stained shear (strike north,

dip 80° E.) in limestone. Limestone is altered yellowish gray within 2 to 5 feet of 

shear. Sparse sulfides (chiefly chalcopyrite) 

localized within 1 foot of shear for about 20 

feet along dip.

16 ------- 1-inch-wide seam of pyrite and galena in altered

light-gray porphyritic dike.

17 ---_-__ pyrrhotite with a trace ofchalcopyrite and 

sphalerite.

18 ------- pyrite with some galena.

19 ------- pyrite, galena, and arsenopyrite from 10-inchwide sulfide band in calcite vein. 

20------- Calcite with pyrite and galena.

22 ------- Narrow 1- to 3-inch shear containing sulfides

in massive gray limestone. Shear is 2 feet 

from contact and parallel to a 20-foot-thick 

brownish-gray pyrite-bear ing felsite porphyry sill (strike N. 35° W., dip 75° SW.). 

Shear discontinuously mineralized; chiefly 

calcite, pyrite, and galena.

23 ------- Highly oxidized northeast-striking fault(?)zone

about 8 to 10 feet wide between argillite and 

limestone. Selected sample of oxidized material from zone includes pyrrhotite with subordinate sphalerite, galena, and chalcopyrite 

that replace silicated limestone.

24 and 25 - - - Scattered small lenses 6 to 8 inches wide and 

disseminations of pyrrhotite in mafic dikes 

that cut argillite adjacent to a small stock. 

Samples include minor chalcopyrite and 

sphalerite.
Table 2. Partial analyses of bedrock samples from areas 3, 4, and 5

[Analysts: W. D. Goss, Claude Huffman, Jr., L. B. Riley, V. E. Shaw, and E. J. Fennelly. Copper, lead, and zinc 

determined by atomic absorption, gold, by fire assay, and silver by gravimetric fire assay]

Area (shown 

in fig. 10)

3

4

5

Sample 

No.

7

8

9

10

12

21

Lab. 

No.

ACJ434

ACJ436

ACJ437

ACJ438

ACJ023

ACG849

Field 

No.

67AR468B

AR469B

AR470A

AR470B

AR3590

67AMa247A

Au 

(ppm)

0.03

.1

<.02

<,02

<.02

<.02

Ag 

(oz/ton)

17.3

52.2

17.9

70.4

i/.4

1/2.2

Cu

0.05

.01

.08

.1

.15

.03

Pb

(percent)

24.4

60.0

5.5

11.3

.04

4.8

Zn

11.2

1.9

5.0

7.2

7.6

.63

Sample type

Selected.

Do.

Chip;!/ 7 1

Selected.

Do.

Chip;!/ 7 0.5

 For chip samples: First number is length of chip in feet, second number is interval between samples; that 

is» 7 0,5 is a 7-foot chip sample with chips collected every 0.5 foot. 

2_/Analyses by atomic absorption.

Sample No. and description of sample

(minerals listed in order of abundance)

 Pyrrhotite-sphalerite-galena-pyrite from 

sulfide lens in dike.

 Galena-pyrrhotite-sphalerite and pyrite 

from sulfide lens in dike.

 Across 4 1/2-foot-thick lens of sulfides, and 2 1/2 feet of altered, 

brecciated felsite dike.

pyrite, sphalerite (marmatite), and argentiferous 

galena-which indicates, perhaps, a general contomppoporaneity of the metallization. If the extrusive 

volcanics and interbedded sedimentary rocks of early 

Tertiary age are indeed coeval with the period of 

hypabyssal igneous activity, as suggested by their 

proximity to the intrusive bodies, then a Tertiary age 

is indicated for the metallization as well. In other parts 

of the southern Alaska Range, including the Iliamna 

area, sulfide mineral deposits are associated with the 

granitic plutons of Late Cretaceous and Tertiary age 

(Reed, 1967; B. L. Reed and M. A. Lanphere, written 

commun., 1968).

SUMMARY AND CONCLUSIONS

Studies of stream-sediment samples have defined 

seven areas of anomalous metal concentrations in a 

part of the southern Alaska Range. In these areas, 

concentrations of copper, lead, and zinc in streamsediment samples fall off rapidly within a mile of 

sulfide occurrences, and values considered to be 

anomalous for these elements are not present more 

than 1M miles from the deposits. Sediment dilution 

due to the addition of large amounts of colluvium from 

steep valley slopes is probably an important factor 

controlling the length of observed metal trains.

Five of the geochemically anomalous areas contain 

metalliferous deposits that are related spatially and 

perhaps genetically to Tertiary(?) igneous centers. 

The deposits occur as replacement bodies and fracture 

fillings in intrusive rocks. Many of the deposits in 

limestone are narrow skarn zones associated with 

felsite dikes. The skarn typically contains the ore minerals pyrrhotite and sphalerite (marmatite). Deposits

Sample No. and description of sample

(minerals listed in order of abundance) Continued

10 Pyrrhotite-galena-sphalerite-pyrite from 

same lens as chip sample 9 above.

12 From pod of pyrrhotite and sphalerite; description of occurrence in table 1, 

sample 11.

21 Across calcite dike containing 2- to 6- 

inch-thick bands of pyrite and galena.

in intrusive rocks occur as fracture fillings in igneous 

breccia, as at Bowser Creek (Reed and Elliott, 1968) 

and, more commonly, as narrow discontinuous sulfide 

bodies along shear zones in felsite dikes.

Further examination of the region should include 

study of the contacts of the igneous bodies shown in 

figure 2, with special emphasis on associated dikes. 

Limestone, where it is in contact with the hypabyssal 

bodies and associated dikes, is a favorable host for 

lead, zinc, and silver deposits in skarn. In addition, 

sampling of sediments in all the smaller streams 

draining intrusive contacts should be done.

Aerial reconnaissance south of the region described 

in this report indicates the presence of sedimentary 

rocks, including limestone, that are intruded by stocks 

and batholiths. The local development of contact 

aureoles and zones of alteration suggests that this 

region also merits consideration by the prospector.

Although the exposed mineral deposits described in 

this report are small and sporadic, they are locally 

rich in lead, zinc, or silver, and they occur in a geologic environment that offers some promise for the 

occurrence of larger deposits. Additional geologic 

mapping, geochemical studies, and prospecting or exploration will be required to fully evaluate the mineral 

potential of this region.

REFERENCES CITED

Brooks, A. H., 1911, The Mount McKinley region, 

Alaska, with descriptions of the igneous rocks and 

of the Bonnifield and Kantishna districts, by L. M. 

Prindle: U.S. Geol. Survey Prof. Paper 70, 234 p.
Elliott, R. L., and Reed, B. L., 1968, Results of the Iliamna quadrangle, and at Kasna Creek, Lake 

stream-sediment sampling between Windy Fork and Clark quadrangle, Alaska: U.S. Geol. Survey openPost River, southern Alaska Range: U.S. Geol. Sur- file rept., 10 p. 

vey open-file rept. Reed, B. L., and Elliott, R. L., 1968, Lead, zinc, and

Grantz, Arthur, 1966, Strike-slip faults in Alaska: U.S. silver deposits at Bowser Creek, McGrath A-2

Geol. Survey open-file rept., 82 p. quadrangle, Alaska: U.S. Geol. Survey Circ. 559,

Reed, B. L., 1967, Results of stream-sediment sam- 17 p. 

pling and bedrock analyses in the eastern part of

155 153°

10 20 MILES

Figure 1. Index map showing report area and location of McGrath A-2, 

A-3, B-2, and B-3 quadrangles.

11
154°00' 153*

62"00'

5 MILES

Figure 2. Reconnaissance map showing generalized geology between Windy Fork and Post

River, southern Alaska Range.

12
EXPLANATION

Surficial deposits

Includes alluvium, talus, rock glaciers, and 

various mineral deposits

IGNEOUS ROCKS

Intrusive rocks

Quartz monzonite, granodiorite, and quartz 

diorite; includes intrusive breccia and, 

locaily, extrusive equivalents. May include some Mesozoic intrusive rocks

Extrusive volcanic rocks and 

interbedded sedimentary rocks

Flows, tuffs, and interbedded tuffaceous 

sandstone, siltstone and shale

SEDIMENTARY ROCKS

Dikes

Intermediate to ftlsic. Major dike swarm 

diagramatically shown

Shale, siltstone, argillite, and 

limestone

May include some Mesozoic sedimentary 

rocks. Short dashed lines represent trace 

of beds

Is, prominent thin- to massively-bedded 

gray limestone units, shown locally

Contact

Dashed where approximately located; 

queried where doubtful

Fault

Long dashed where approximately located; 

queried where doubtful; dotted where 

concealed

Inclined Vertical 

Strike and dip of beds

Dashed symbol indicates altitude estimated 

from distant field observations

Jeep road

Anticline

Showing trace of axial plane; dashed 

where approximately located

F-l

Fossil locality

Figure 2. Continued

13
62*30' 154'00' 153*3(y

62°00'

5 MILES

j___i i

Figure 3. Map showing location of stream-sediment samples (dots) between Windy Fork 

and Post River, southern Alaska Range.

14
62*30' 154*00' 153*30'

EXPLANATION

 100 ppm

O 150 ppm

Q 200 ppm

62*00'

5 0

I i . . i I

5 MILES

Figure 4. Copper distribution showing location of samples with 100 ppm or greater. Histogram shows percentage frequency distribution of copper in 556 stream-sediment samples collected in map area.

15
62*30

154*00' 153-30'

EXPLANATION

 50 ppm

O 70-100 ppm

Q 150-200 ppm

 300-700 ppm 

(value shown)

i i i i i i i i i i

Z O O">O O O O OOOOOO *- rt rt^csi «ior~ Oiooooo <-ir-i cvim in rv

62*00

5 MILES

Figure 5. Lead distribution showing location of samples with 50 ppm or greater. Histogram shows percentage 

frequency distribution of lead in 556 stream-sediment samples collected in map area.

If
62°30' 154°00' 153°30'

EXPLANATION

 5-7 ppm

O 10 ppm

Q 15 ppm

 20-30 ppm 

(value shown)

78 

14-

12-

8- 

6- 

4- 

2-

PARTS PER

MILLION 

N = not detected

62°00'

5 MILES

Figure 6. Molybdenum distribution showing location of samples with 5 ppm or greater. Histogram shows 

percentage frequency distribution of molybdenum in 556 stream-sediment samples collected in map^area.

17
62*30

154'00' 153'3Cr

EXPLANATION

 100 ppm 

O 150 ppm

 200-300 ppm 

(value shown)

35- 

30-

020-

10-

5-

1 T I I I I I I I I I II 

(MC\KT>ir>r-~OinpOOOQOQQ .-H'-ieQcoinr^oinoo V

PARTS PER MILLION

62°00

5 MILES

Figure 7. Nickel distribution showing location of samples with 100 ppm or greater. Histogram shows percentage frequency distribution of nickel in 556 stream-sediment samples collected in map area.

18
62"30' 154"00' 153-30'

EXPLANATION

 1-1.5 ppm

O 2-3 ppm

 1? 7 ppm (value shown)

PARTS PER

MILLION 

N = not detected

62"00

5 MILES

Figure 8.-Silver distribution showing location of samples with 1 ppm or greater. Histogram shows percentage 

frequency distribution of silver in 556 stream-sediment samples collected in map area.

19
62°30

EXPLANATION

 200 ppm 

O 300 ppm 

O 500-700 ppm

 1500-2000 ppm 

(value shown)

153°30'

PARTS PER MILLION 

N = not detected

62*00

5 MILES

Figure 9. Zinc distribution showing location of samples with 200 ppm or greater. Histogram shows percentage 

frequency distribution of zinc in 556 stream-sediment samples collected in map area.

20
62' 154'00 153'

62°00'

Figure 10. Map showing areas with anomalous metal concentrations in stream-sediment samples. 

Circled numbers identify areas discussed in text. Small numbers show location of bedrock samples; analyses of these samples are given in tables 1 and 2.

21
 -/M i n e ra I i zed a rea s 

fi

X"Mineralized areas

WEST 

500 

400 

300 

200 

100 

0

700

z

§ 400

Sample localities 

X \ . h-H-1 I EAST

Bowser Creek

200

2

0

2000

1500

1000

500

0

Ag

Zn

Figure 11. Map of Bowser Creek showing location of stream-sediment samples and 

sulfide mineral deposits. Sample profiles showing distribution of copper, lead, 

silver, and zinc in stream-sediment samples. The closed circles on the map and 

profiles represent sediment samples from tributaries to Bowser Creek. The 

dashed line on the sample profiles represents values that are considered anomalous for the area discussed in this report. Stream flows to right (east).

22