Suggested areas for prospecting in the central Koyukuk River region, Alaska

By Thomas P. Miller and Oscar J. Ferrians, Jr.

STEWART L. UDALL, Secretary

Geological Survey

Abstract- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Introduction - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Geologic setting- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Mineralized areas - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Introductory statement - - - - - - - - - - - - - - - - - - - - - - - - - - Indian Mountain- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Sun Mountain - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Oakli ----------------------------------- Clear Creek - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Caribou Mountain- - - - - - - - - - - - - - - - - - - - - - - - - - - - - Hawk River- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Purcell Mountain - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Summary - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - References cited - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Figure 1. Map of central Koyukuk River region showing location of areas described in text - - - - - - - - - - - - - - - - - - - - 2. Geologic map of the Indian Mountain area - - - - - - - - - - - 3. Geologic map of the Zane Hills area - - - - - - - - - - - - - - 4. Geologic map of \the Clear Creek area - - - - - - - - - - - - - 5. Geologic map of the Purcell Mountains area - - - - - - - - - -

Tables 1-3. Analyses of selected grab samples fromareas described in text - - - - - - - - - - - - - - - - - - - -

2. Geologic map of the Indian Mountain area - - - - - - - - - - -
3. Geologic map of the Zane Hills area - - - - - - - - - - - - - -
4. Geologic map of \the Clear Creek area - - - - - - - - - - - - -
5. Geologic map of the Purcell Mountains area - - - - - - - - - -
6. Copper and lead content of stream-sediment samples,
7. Analyses of selected grab samples from the Hawk River
8. Utopia Creek placer mine and altered fine-grained felsic intrusives, Indian Mountain area-- - - - - - -
9. Head of Black Creek, Indian Mountain area - - - - - -
10. Zane Hills area - - - - - - - - - - - - - - - - - - - - -

Clear Creek area - - - - - - - - - - - - - - - - - - - - -

locality near Purcell Mountain - - - - - - - - - - - - - - CONfENTS

Page Reconnaissance investigation of the m in era 1 resources of the central Koyukuk River region, westcentral Alaska, for the Heavy Metals program, U.S. Geological Survey, suggests seven localities that are particularly interesting for mineral prospecting (fig. 1). Although the region currently has a productive placer gold deposit, much of the region is relatively inaccessible and has received little attention from prospectors.

The central Koyukuk River region includes 6,600 square miles covering portions of the Hughes, Melozitna, and Shungnak 1:250,000-scale quadrangle maps. The only town in the district is Hughes, a small village on the Koyukuk River (fig. 1), which is serviced several times a week by scheduled airlines from Fairbanks.

Areas with known mineral deposits and other areas with geologic environments favorable for the occurrence of mineral deposits were investigated in the summer of 1967. Over 400 samples were collected and analyzed by semiquantitative spectrographic procedures and by atomic-absorption methods.

The authors were assisted in the field by Vernon DeRuyter, geologic field assistant. Semiquantitative spec t r o graph i c analyses were done by Arnold Farley, Jr., J. M. Motooka, G. W. Sears, Jr., and Chris Heropoulos, and atomic-absorption analyses were done by W. L. Campbell, R. L. Miller, T. A. Roemer, A. L.

By Thomas P.. Miller and Oscar J. Ferrians, Jr.

INTRODUCTION Meier, and M. S. Rickard. C. W. Maeser assisted on the fission- track analysis. 0 ff i cia 1 s of the United States Smelting Refining and Mining Co., particularly Jack Fisher, manager of the Hogatza placer mine, were very helpful in permitting theinvestigationofthe mine.

The bedrock of the central Koyukuk River region consists chiefly of granitic rocks intruded during two late Mesozoic plutonic episodes into marine andesitic volcanic rocks, volcanic graywacke, and mudstone of older Mesozoic age. Quartz latite exposed near Purcell Mountain (fig. 1) was extruded between plutonic episodes, and fine-grained felsic intrusives of Late Cretaceous or Early Tertiary age occur near Indian Mountain (figs. 1, 2). Mineralization in the region appears to have been associated with the granitic intrusions.

The oldest bedrock unit in the province is a thick sequence mainly of andesitic volcanic rocks of Early Cretaceous and possibly Late Jurassic age. It consists of pyroxene andesite and trachyandesite flows, pyroclastic and volcaniclastic rocks, and sub ordinate fossiliferous limestones. The unit crops out over large areas of west- central Alaska and has been assigned to the Koyukuk Group by Smith (1913, p. 80).

Overlying this unit is a thick sequence of interbedded volcanic graywacke, mudstone, and quartz conglomerate of late Early Cretaceous and perhaps younger age (Patton and Miller, 1966). The graywacke and mudstone are composed of angular detritus derived primarily from the underlying volcanic rocks and consist chiefly of plagioclase and volcanic rock fragments in an argillaceous matrix. Locally, this unit contains marine fossils, plant fragments, and coal beds. Crystal lithic tuffs and volcaniclastic rocks locally occur at the base of this unit.

Plutons and dikes of medium-grained granodiorite, quartz monzonite, and syenite intruded the volcanic and sedimentary rocks during two Late Cretaceous plutonic episodes. Three intrusive igneous bodies in the western part of the region that are tentatively assigned an age of 100 m.y. (million years) based on potassium-argon age determinations (M i 11 e r and others, 1966) were intruded during the earlier episode. These bodies are the Purcell Mountain pluton composed chiefly of porphyritic quartz monzo:tite, the Shiniliaok Creek pluton composed of monzonite and syenite, and the Hawk River pluton composed chiefly of monzonite. Other main igneous bodies of the region were intruded during the younger episode and are assigned an age of 80 m.y. (Miller and others, 1966). They consist chiefly of granodiorite and quartz monzonite with subordinate alaskite. Late-stage aplite and alaskite dikes cut both episodes of plutonic rocks. A few quartz veins occur near the borders of the main igneous bodies and in the nearby wallrock. A volcanic unit composed of quartz latite, latite, and rhyolite separates the two plutonic episodes in the vicinity of Purcell Mountain; it is dated at 86 m. y. (Patton and others, 1968) and is assigned a Late Cretaceous age.

The gran it i c rocks of both suites are generally' massive and medium grained, and contacts between the igneous bodies and country rock are sharp, relatively straight, steeply dipping, and everywhere appear to be discordant. Rocks of the hornblende-hornfels facies form a narrow zone in country rock adjacent to the intrusive and grade into a broader zone of a 1 biteepidote-hornfels facies rocks away from the contacts. Elsewhere, the volcanic and sedimentary units are unaltered. The hornfelsic contact rocks are generally more resistant than the granitic rocks and form topographic ridges surrounding parts of the igneous bodies.

The lowland areas are underlain by unconsolidated deposits in contrast to the mountainous areas and adjacent uplands, which are underlain by bedrock. Most of the Koyukuk Flats, in the southwestern part of the region, is underlain by fine-grained, water-laid, and windbome sediments of Pleistocene and Holocene age. The lowlands in the northern part of the region surrounding the Purcell Mountains, Zane Hills, lsahultila Mountains, Indian Mountains, and adjacent uplands are underlain by glacial drift of Pleistocene age (Coulter and others, 1965), which was deposited by large piedmont glaciers that came from the Brooks Range to the north. Also, small local glaciers emanated from the higher parts of the Purcell Mountains, Zane Hills, and Indian Mountains during Pleistocene time and deposited glacial drift in some of the larger valleys within these mountains.

Most mineralized areas in the region occur along or near the contacts between the late Mesozoic plutons and country rock, indicating a close relatioPship between the igneous intrusive rocks and the mineralization.

Mineral occurrences in outcrop or anomalous concentrations of metals in stream-sediment and rock samples indicate seven areas that are worthy of further investigation and prospecting. These areas are: Indian Mountain, Sun Mountain, Dakli, Clear Creek, Caribou Mountain, Hawk River, and Purcell Mountain (fig. 1). They are described in the following sections of the report and all the areas except Sun Mountain are shown on small-scale geologic maps (figs. 2, 3, 5); a more detailed map is provided for the Clear Creek area (fig. 4). The geologic maps also show the lead and copper content of stream-sediment samples. Lead and copper values above 50 parts per million are anomalous for streams draining the bedrock of the region.

Mineralized rock was found in the Indian Mountain area (fig. 2) near old placer mines at Utopia and Black Creeks and along the east-northeast-trending fa u It zone south of Utopia Creek.

Abundant boulders of massive white barite as much as 2 feet in diameter were found in the tailings near the head of an abandoned placer gold mine along Utopia Creek. Some of the barite boulders contain fine- grained tetrahedrite, galena,· and sphalerite. Analyses of two samples of barite, one barren of visible metallic minerals and the other containing visible but minor amounts of tetrahedrite and other metallic minerals, are given in table I (samples lA and 1B). Both samples contain silver and anomalous amounts of lead, zinc, copper, antimony, and gold; the tetrahedrite-bearing sample (lB) contains about 21 ounces ofsilverper ton and the principal silver-bearing mineral is probably tetrahedrite. A stream-sediment sample(table 1, sample 1C), taken from a tributary on the south side of the valley .near the upstream end of the tailings, contains the highest values of lead, zinc, and silver found in any stream-sediment sample in the region. The lead value of 2,000 ppm and the zinc v a 1 u e of 700 ppm are extremely high. Contamination by the placer mining operation seems unlikely because the sample was col-MINERALIZED AREAS Introductory statement

e ~ }

lected away from the operation and because of the high silver value. Copper, gold, and barium also are anomalously high in the sample.

The country rock at Utopia Creek is slightly metamorphosed andesite of Late Jurassic(?) and Ear 1 y Cretaceous age cut locally by fine-grained, felsic intrusives. The Indian Mountain granodiorite pluton is about 2 miles away, and Utopia Creek does not drain the granodiorite-andesite contact as do the other streams in the region that contain gold-bearing gravel deposits. The lack of granitic rocks in the tailings, tog e the r with the abundance and size of the barite boulders, suggests that the boulders were derived from tetrahedrite-sphalerite-galena-bearing veins in the andesite volcanic rocks. Veins of this sort may also have been the source of the placer gold found here. Also, the location of the tailings suggests that the pay streak did not lie in the main part of the valley in this area but instead continued up the south slope of the valley toward the area drained by the tributary stream containing sediments rich in lead and zinc. Because of inclement weather near the end of the field investigation, little time was available to look for the bedrock source of the barite. The drainage basin is not large and therefore should be amenable to prospecting.

An east-northeast-trendingbelt ofpyritiferous, silicified, fine-grained intrusive rocks occurs along a well-marked fault zo.ae a few miles southofthe Indian Mountain pluton and immediately south of the Utopia Creek barite occurrence (fig. 2). The age of these intrusives is unknown, but they maybe as young as early Tertiary. Oxidation of disseminated pyrite in the intrusives has resulted in the formation of conspicuous orange-red-yellow gossans visible from many miles away. Typical intrusive rocks from. these gossans contain small but anomalo:Js amounts of lead, copper, silver, and gold (table 1, samples 2-5).

Highly altered, fine-grained, pyritiferous intrusive rocks, similar to the intrusives mentioned above, occur on Indian River at locality 6 (fig. 2). Bo::h the intrusive rocks and the intruded and e site are much oxidized and altered; however, some of the fresher specimens of intrusive rock contain disseminated pyrite and sphalerite. Both the altered andesite and intrusive show anomalous amounts oflead, zinc, copper, molybdenum, silver, and gold (table 1, samples 6A-6F). Sample 6E contains about 6 p;>m of gold, equivalent to approximately 0.16 ounce per ton.

Pyrite and sparse chalcopyrite are disseminated in metavolcanic rocks and late-stage aplites near the pluton a.~ the head of Black Creek (fig. 2), upstream from gold placer operations. The coarse, angular nature of the gold panned here, together with the very small drainage- basin area, indicates a local and nearby source. Several bedrock grab samples from this area contain disseminated sulfides and show anomalous copper values, but only two contain gold (table 2). No other large zones of alteration or obvious source rock

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for the placer gold of the Indian River gold mine were found. It is possible that larger zones of mineralization are buried beneath the colluvium and talus of the steep-walled canyon.

The area in the vicinity of the Indian Mountain pluton and, in particular, near Utopia Creek warrants further investigation. The fine-grained pyritiferous intrusive rocks found here are strongly altered and slightly mineralized. Although the rocks are not nearly of commercial grade, they may indicate deeper or hidden mineralization, and it seems p o s sib 1 e that the fa u 1 t controlling the main zone of intrusives could be mineralized. The general area is only 15 miles from the Koyukuk River, which is navigable by barge during the summer months and which provides access to shipping points farther south.

Sun Mountain Scattered occurrences of chalcopyrite, malachite, and pyrite were observed along the Koyukuk River in the southern Isahultila Mountains near Sun Mountain (fig. 1). In addition, a 2-foot-long angular float block of altered silicified rock, w~1ich contained disseminated cerussite, galena, and malachite, was found on a cutbank at the south end ofBatzaSlough(fig. 1). The float block contains 5 percent oflead, 0.5percentof copper, and 3 ounces of silver per ton. The numerous quartz latite porphyry dikes throughout the Sun Mountain area, together with a widespreadzoneofthermallymetamorphosed andesite, suggest a buried pluton at s h a 11 ow depth. Several massive fossiliferous limestones, as much as 65 feet thick in this area, are favorable hosts for ore deposits. Prospecting will be handicapped by the heavy forest cover and by the thick covering of loeas that· mantles most of the area and prevents reliable stream-sediment sampling.

Dakli The Dakli area is just east of the Dakli VABM in the northwest part of the Zane Hills (fig. 3) near the northern edge of the Zane Hills pluton. The contact between the hornblende-biotite granodiorite pluton and andesite wallrock dips gently to the north in contrast to the steep contact around the restofthe pluton. This contrast in dip is shown by the occurrence of granodiorite in the valleys and of andesite hornfels on the ridgetops, as well as by a broad zone (not shown on the map) of th·ermally metamorphosed rocks north of the pluton. The abundance of hornblende in the northern half of the pluton, in contrast to the hornblende-poor southern half, suggests contamination of the pluton near its roof. Roof areas such as this are known to be favorable zones for mineralization.

Malachite and limonitic chalcopyrite occur in fractures in frost-riven blocks of brecciated quartz, andesite, and altered granodiorite along a belt 100 feet wide on the north side of the saddle separating the andesite from the gneissic granodiorite at locality 2 (fig. 3). The mineralized andesite and granodiorite here contain as much as 30 ppm of silver, 0.09 ppm of gold, 700 ppm of zinc, and from 1,000 to 20,000 ppm of copper (table 3). Similar rocks occur at the granodiorite-andesite contact at locality 3.

Nearby, at locality 4 (fig. 3), rusty-colored molybdenite-bearing quartz veins as much as 2 feet thick

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....._... cut the andesite, again near the granodiorite contact. A grab sample from one vein (table 3, sample 4) contains 0. 2 P·=rcent of molybdenum.

Sediment in streams draining both east and west from the Dakli contact area contains anomalous copper values ranging from 100 to 300 ppm over an area of about 20 square miles (fig. 3). Hence, copper deposits are probably more widespread than indicated by this reconnaissance investigation. Pyrite- and chalcopyrite-bearing quartz veins as much as 3 feet thick cut meta-andesite north of the Zane Hills pluton at localities 5, 6, and 7 (fig. 3). Analyses of grab samples are given in table 3 (samples 5, 6, 7).

Clear Creek The Clear Creek gold area is on the east side of the Zane Hills pluton (figs. 3, 4) and includes the Hogatza placer gold mine. The granitic rocks of the pluton in this area are gneissic monzonite and quartz monzonite (kmq, figs. 3, 4) (discussed in greater detail in the section on Caribou Mountain) and massive, medium-grained granodiorite typical of the rest of the pluton (kgq, figs. 3, 4). The country rock is andesitic volcanic rock of Early Cretaceous and Late Jurassic age. The contact between granitic rock and andesite is in colluvium-filled, heavily vegetated saddles, w hi c h separate the subdued low hills of andesite from the much higher mountains of granitic rock. This abrupt scarp-like feature is indicative of a fault contact which would be a favorable site for mineralization.

Sediment samples from streams draining the andesite-monzonite contact on both sides of Clear Creek contain anomalous amounts of lead and copper (fig. 4 and table 4). A sediment sample from a spring near the contact high on the south slope of Clear C r e e k valley (fig. 4, and table 4, sample 1 lA) contained 1. 74 ppm of gold-the highest value of gold found in any sediment sample from the entire region. Brecciated quartz float from the contact zone on the· ridge above this spring (fig. 4) contains anomalous am o u n t s of antimony (table 3,·sample 8)~ andsoil samples from this same area eontain. lead and anomalous amounts of copper, as shown below in parts per million:

Lab.

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. nomic potential of this zone.

Pb Cu Mineralized bedrock is found in several places in this general area (table 3, locs. 8-11) and contains as much as 2.4 ppm of gold (0.07 ozper ton) as well as anomalous amounts of silver, bismuth, copper, and molybdenum.

The occurrence of these an om a 1o us values in stream-sediment and rock sam:Jles indicates that mineralization may have taken ·pl~ce along and near the monzonite-andesite contact in this area. This possibility is further enhanced by the fact that the mining operations of the Hogatza gold mine on Bear C reek began only a mile downstream. The auriferous alluvial gravels in this valley are de r i v e d from o u twas h streams emanating from a lobe of the glacier which moved down Clear Creek valley in early Pleistocene time. This lobe crossed the ridge between Clear and Bear Creeks near the monzonite-andesite contact, eventually covered the entire ridge, and terminated in Bear Creek valley. Glaciers are not efficient sorting or mixing agents and rock material incorporated into the ice on one side of the glacier would tend to remain 0.:1 that side. Since the bedrock exposed above the point where the Clear Creek glacier crossedoverinto Bear Creek is unaltered granitic rock, which is not a likely host rock for lode gold, the source of the placer gold is thought to be in or near the concealed andesitemonzonite contact. The location of the tailings at Bear Creek indicates that the pay streak extended to very near this contact. · The monzonite-andesite contact zone on either side of Clear Creek is an attractive site for lode gold prospecting. A small trenching or drilling program would give considerable information in e v a I u at in g the eco-

Caribou Mountain Border phases of the Zane Hills pluton in two areas alQng the southeastern marginofthepluton(fig. 3, map unit kmq) show anomalous radioactivity-five to ten times the background radioactivity of the rest of the pluton. These border phases are composedofmediumto coarse-grained, trachytoid to gneissic, hornblende-biotite quartz monzonite and monzonite readily distinguishable in the fie I d from the typical m as s i v e, granitic-textured granodiorite of the restofthepluton. Tourmaline;.. bearing aplite dikes are common in the border phase as are dark-colored hybrid ·rocks indicative of contamination of granitic magma by ande-

(as long as 3 inches) also are common in these border phases and locally in the adjacent metamorphosed andesite.

An !analysis of porphyritic quartz monzonite from this border phase shows 20 ppm of uranium. This is five to six times more than the published averages for rocks of this composition (Smith, 1963, p. 402). Examination of thin sections of this rock shows that biotite and hornblende. contain numerous inclusions surrounded by SD

pleochroic halos indicative of radioactivity. Some of these halos are obviously around zircon crystals, but other much more intense halos are around grains of a colorless to faintly yellow, isotropic mineral of high relief. A thin section of this porphyritic quartz monzonite was exposed to a thermal neutron beam in a reactor in order to cause the fission of u235. The fission events were recorded in a piece of lexan w h i c h covered the section. Later, etching of the lexan showed the anomalously occurring uranium in the sample to be associated with the colorless isotropic mineral.

Although the uranium-bearing mineral is only a minor constituent in the samples studied, it may be more abundant elsewhere in the radioactive border phase-possibly in amounts large enough to be important economically, or other uranium minerals may be present. A panned concentrate collected in 1964 from Caribou Creek on the southeastern side of the Zane Hills contained 200 ppm of thorium, which was probably derived from this more radioactive border phase of the pluton.

Other mineralized rock occurrences in this area are: (I) a pyritiferous fine-grained intrusive cutting the border phase monzonite at locality 12 (fig. 3); a sample

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of this material (table 3, sample 12) contains anomalous amounts of silver, gold, lead, bismuth, and tungsten, and (2) a chalcopyrite-bearing quartz vein 1 to 3 feet wide cutting monzonite at locality 13 (fig. 3); a sample of this material (table 3, sample 13) contains anomalous amounts of tungsten in addition to copper.

Hawk River Sediment from streams in a large area south of Purcell Mountain, which includes the headwaters of the Hawk River, contains anomalous amounts oflead, copper, and silver (fig. 5, table 5). These streams drain an area in which quartz veins containing pyrite, and locally, argentiferous galena are abundant.

The bedrock south of Purcell Mountain (fig. 5) consists of andesitic volcanic rocks of Late Jurassic(?) and Early Cretaceous age and quartz latite volcanic rocks of Late Cretaceous age which are intruded by alaskite and quartz monzonite of Late Cretaceous age (Patton and others, 1968). The older andesitic volcanic rocks are thermally metamorphosed in the vicinity of the Wheeler Creek pluton, Purcell Mountain pluton, and Hawk River pluton. The Upper Cretaceous quartz latite is t h e r m a 11 y metamorphosed only near the

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Figure 5.--Geoloaic aap of the Purcell Mountains area.

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Wheeler Creek pluton. Numerous fine-grainedpyritiferous felsic dikes as much as tens of feet wide cut both the andesitic and quartz latite volcanic rocks throughout this area.

Pyritiferous milky-white vuggy quartz veins showing well-developed comb structure cut both volcanic units in a northwest-trending belt 6M by IM miles in areal extent (fig. 5). This belt is parallel to a zone of prominent northwest- trending lineations and faults. The veins are mostly narrow, ranging in width from 1 inch to 2 feet, and they appear to strike northeast and dip steeply to the north. They seem to occur as clusters of veins 2 to 20 feet apart; the most extensive concentrations of quartz veins are at localities 2 and 3 (fig. 5). Locally, they contain irregular segregations of argentiferous g a I en a with I e s s e r amounts of chalcopyrite and malachite. Pyrite occurs in cubes up to 1 inch on a side and is almost completely oxidized to limonite; galena is fresh and fills vugs and fissures along the edge of the veins. Chemical analyses of grab samples representative of these deposits are given in table 5. No galena or chalcopyrite was found in the andesitic or latitic volcanic rocks; however, the andesitic wallrock locally contains abundant dis seminated pyrite.

The mineralized quartz veins cut the Upper Cretaceous quartz latite, dated at 86 m. y. (Patton and others, 1968), and may be related to the intrusion of the Wheeler Creek pluton dated at 80 m.y. (Miller and others, 1966) that is just east of this area (fig. 5).

Purcell Mountain A small "inactive placer mine is on a stream draining to the west from the Purcell Mountain pluton (fig. 5). The stream crosses the contact between the quartz monzonite and andesite. Although no large zones of altered rock were observed in the area, abundant pebbles and cobbles of black tourmaline as much as 3 inches in diameter and of albitizedquartzmonzonite cut by black tourmaline veinlets were found in the tailings a.-,d gravels of this stream and the stream to the north. ·The tourmaline is associated with quartz and sparse disseminated oxidized sulfide, and it occurs in the veinlets as radiating sunbursts of crystals with individual crystal lengths of a few millimeters. The quartz monzonite appears to be altered adjacent to the veinlets. Analyses of two tourmaline-rich altered quartz monzonite cobbles show 700 and 1,000 ppm of copper but no detectable gold. The abundance of tourmaline, the small drainage basin, and the lack of any exposed large alteration zones suggest that the source of the placer gold here may be quartztourmaline-sulfide veins which cut and altered the quartz monzonite near the andesite contact. If this is so, the presence of tourmaline veinlets in place or in material in the alluvium may serve as a guide to possible gold mineralization elsewhere in this part of the region; similar tourmaline-rich cobbles are found within the Bear Creek placer-mine area on the east side of the Zane Hills and in a stream ::lraining the Dakli copper prospect.

SUMMARY Investigation of the central Koyukuk River region located seven areas containing mineral occurrences thought to be worthy of further prospecting. Because of the size of the district and the short time available for detailed investigation, individual areas were not stud~ed in detail, and more concentrated surfaceprospectmg and c I o s e r spaced geochemical stream- sediment sampling is needed to determine the extent and amount of mineralized rock. These mineralized areas are all near granitic plutons, and future prospecting elsewhere in the region should be conducted with this in mind.

REFERENCES CITED Coulter, H. W., Hopkins, D. M., Karlstrom, T. N. v., P~w~. T. L., Wahrhaftig, Clyde, and Williams, J. R., 1965, Map showing extent of glaciations in Alaska: U.S. Geol. Survey Misc. Geol. Inv. Map 1-415, scale 1:2,500,000. Eakin, H. M., 1916, The Yukon-Koyukuk region, Alaska:

U.S. Geol. Survey Bull. 631, 88 p. Fernald, A. T., 1964, Surficial geology of the central Kobuk River valley, northwestern Alaska: U.S. Geol. Survey Bull. 1181 -K, 31 p. Miller, D. J., Payne, T. G., and Gryc, George, 1959, G~ology of possible petroleum provinces in Alaska, with an annotated bibliography by E. H. Cobb: U.S. Geol. Survey Bull. I 094, 131 p. Miller, T. P., Patton, W. W., Jr., and Lanphere, M. A., 1966, Preliminary report on a plutonic belt in westcentral Alaska, ill. Geological Survey research 1966: U.S. Geol. Survey Prof. Paper 550-D, p. D158- DI62. Patton, W. W., Jr., and Miller, T. P., 1966, Regional geologic map of the Hughes quadrangle, A 1 ask a: U.S. Geol. Survey Misc. Geol. Inv. Mapl-459, scale 1:250,000. Patton, W. W., Jr., and Miller, T. P., 1968, Regional geologic map of the Selawik and southeastern Baird Mountain quadrangles, Alaska: U.S. Geol. Survey Misc. Geol. Inv. Map I-530, scale 1:250,000. Patton, W. W., Jr., Miller, T. P., and Tailleur, I. L., I968, Regional geologic map of the Shungnak and southern part of the Ambler River quadrangles, Alaska: U.S. Geol. Survey Misc. Geol. Inv. Map I-554, scale I :250,000. (In press.) Schrader, F. C., 1900, Preliminary report on a reconnaissance along the Chandlar and Koyukuk Rivers, Alaska, in 1899: U.S. Geol. Survey 21st Ann. Rept., pt. 2, p. 44] -486. Schrader, F. C., 1904, A reconnaissance inn or the r n Alaska across the Rocky Mountains, along Koyukuk, John, Anaktuvuk, and Colville Rivers and the Arctic coast to Cape Lisburne, in 1901: U.S. Geol. Survey Prof. Paper 20, 139 p. Smith, F. G., 1963, Physical geochemistry: Reading, Mass., Addison-Wesley Publishing Co., Inc., 624p. Smith, P. S., I913, The Noatak- Kobuk region, Alaska:

U.S. Geol. Survey Bull. 536, 160 p.

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