Distribution of gold, copper, and some other metals in the McCarthy B-4 and B-5 quadrangles, Alaska

By E. M. MacKevett, Jr., and James G. Smith

Geological Survey

Abstract-------------------------------- Introduction ----------------------------- Geology--------------------------------- Geologic setting ----------------------- Summary of economic geology----------- Distribution of metals------------------ Gold-------------------------------- Copper ----------------------------- Silver ------------------------------ Figure 1. Index map showing location of the

Figure 1. Index map showing location of the ~cCarthy B-4 and B-5 quadrangles ------------------------------------------- 2 2. Generalized geologic map showing sample locations in the ~cCarthy B-4 and B-5 quadrangles -------------------- 4

2. Generalized geologic map showing sample locations
3. Analyses of stream-sediment samples from the ILLUSTRATIONS

in the B-4 and B-5 quadrangles --------------------

~cCarthy B-4 and B-5 quadrangles--------------------------

~cCarthy B-4 and B-5 quadrangles-------------------------- CONTENTS

TABLES Page Geology- -Continued Distribution of metals- -Continued Antimony ---------------------------

Arsenic----------------------------- Other meta~s ------------------------ Summary and suggestions for prospecting -- References cited-------------------------

Geochemical investigations were conducted during the summer of 1967 in the McCarthy B-4 and B-5 IS-minute quadrangles, Alaska, and in small adjacent parts of the McCarthy A-4 and A-5 quadrangles, as part of the U.S. G eo 1 o g i c a 1 Survey's Heavy Metals Program. The B-4 and B-5 quadrangles, which are centered about 250 miles east of Anchorage (fig. 1), include parts of the southern Wrangell Mountains and the contiguous Chitina Valley. The landforms of the B-4 and B-5 quadrangles have been strongly modified by glaciation and range in altitude fr<.>m about 1, 400 to 9, 265 feet. M o s t of the mountainous regions are sparsely covered by vegetation and have good outcrops, but bedrock in the lowlands is generally obscured by diverse vegetation and extensive surficial deposits. The investigators utilized helicopter support concomitantly with other geologic studies in the region. The fieldwork consisted of collecting 105 stream-sediment samples and 146 samples from veins, altered zones, and rocks. G. R. Winkler ably assisted in the sampling. The samples were sieved to -180 mesh and analyzed in U.S. Geological Survey laboratories for gold by atomic absorption methods and for 33 other elements by semiquantitative spectroscopy. In addition to sampling virgin outcrops, numerous samples were collected from mines and prospects to augment the study of the region's mineral deposits and the interpretation of the general sampling. Results of previous geologic studies in the B-4 andB-5 quadrangles are mainly incorporated in reports by Moffit and Capps (1911), Moffit (1938), andMacKevett(1965).

The McCarthy B-4 and B-5 quadrangles are underlain largely by thick sequences of sedimentary and

By E. M. MacKevett, Jr., and James G. Smith

GEOLOGY Geologic Setting volcanic rocks that range in age from Permian and Permian(?) to Cretaceous and by diverse Quaternary surficial deposits (fig. 2). The older rocks are cut by Tertiary intrusive rocks, and near the northeast corner of the B-4 quadrangle are overlain by Tertiary continental sedimentary rocks that contain intercalated lava flows. The pre-Cretaceous rocks mainly occupy a northwest-trending belt along the south flank of the Wrangell Mountains. The southern part of this belt is overlapped by Cretaceous marine sedimentary rocks and by surficial deposits, the northern part by volcanic rocks of the extensive Wrangell Lava. The oldest rocks in the quadrangles are submarine lava flows and their derivative volcaniclastic rocks, which together constitute the Station Creek Formation of the Sko1ai Group (Smith and MacKevett, 1968). These rocks are slightly metamorphosed and are probably Permian in age. They are conformably overlain by slightly metamorphosed fossiliferous Permian rocks, the Hasen Creek Formation of the Skolai Group (Smith and MacKevett, 1968). Triassic rocks are represented by a local thin remnant of unnamed Middle Triassic sedimentary rocks; by extensive basalt flows, the Nikolai Greenstone of late Middle and (or) early Late Triassic age; by Late Triassic carbonate rocks, the Chi tis tone and Nizina Limestones; and by carbonaceous shales, cherts, and impure limestones of the Late Triassic part of the McCarthy Formation. Early Jurassic rocks, chiefly spiculites and shales, of the upper member of the McCarthy Formation are sparsely distributed in thenorthernpartsoftheB-4 and B-5 quadrangles. The pre-Cretaceous rocks were folded and faulted during the major orogeny of the region, near the close of the Jurassic or in the Early Cretaceous; most major structures are related to this period of deformation. Cretaceous clastic sedimentary rocks, largely of shallow marine origin, overlie the older rocks with a strong an g u 1 a r unconformity and are widely distributed throughout the southern parts of the B-4 and B-5 quadrangles (Jones and MacKevett, 1968). Quaternary surficial deposits, largelyoffluvioglacial derivation, cover most low parts of the quadrangles.

Intrusive rocks in the quadrangles include gabbro, granodiorite, and altered felsic hypabyssal rocks. The gabbro is Permian or Triassic in age, and is con~ fined to terrane underlain by Skolai Group rocks; it Figure 1.--Index map showing location of the McCarthy B-4 and B-5 quadrangles.

generally forms small discordant or concordant plutons. The granodiorite is Tertiary in age and forms small discordant stocks near Williams, Andrus, and Pyramid Peaks (fig. 2). The hypabyssal rocks, the youngest intrusives in the quadrangles, are also Tertiary. They form small stocks and numerous dikes and sills that commonly cut Cretaceous rocks. Most of the hypabyssal rocks are porphyritic rhyodacite and dacite that in places have been extensively altered to clay minerals, sericite, and chlorite. Intruded rocks near the gabbro, in particular those adjacent to the granodiorite, have been metamorphosed, chiefly to hornfels.

Hydrothermally altered zones, a few inches to 250 feet wide, are common in the eastern part of the B-4 quadrangle, where they are localized mainly along faults. The altered zones consist largely of copiously iron-stained yellowish- or reddish-brown leached or gougy material.

For the purpose of this report, formations that contain similar rock types have been grouped as follows in figure 2 regardless of their geologic ages:

1. All surficial deposits, such as alluvium, talus, rock glaciers, and diverse fluvioglacial deposits, and areas covered by glacier, snowfields, and water.
2. Volcanic and volcaniclastic rocks that include rocks of the Nikolai Greenstone and Station Creek Formation and basal flows of Wrangell Lava.
3. Detrital clastic rocks, mainly sandstone, shale, and mudstone. These rocks are largely Cretaceous, but they include subordinate Permian, Triassic, Jurassic, and Tertiary rocks.
4. Carbonate rocks of the Chitistone and Nizina Limestones.
5. Gabbro.
6. Intermediate intrusives, mainly granodiorite.
7. Felsic intrusive rocks of Tertiary age.

Summary of Economic Geology

The general region is best known for the massive copper sulfide lodes at the Kennecott mines, which are mainly in the nearby McCarthy C...:.5 quadrangle. For many years before 1938, when large-scale mining ceased at Kennecott, these mines were among the world's premier copper producers with a total production of about 1.2 billion pounds of copper and a significant byproduct of silver. The Kennecott lodes formed near the base of the Chi tis tone Limestone within a few hundred feet of its contact with the subjacent Nikolai Greenstone. Several small copper mines and prospects are localized in the basal Chi tis tone strata in the B-4 and B-5 quadrangles (fig. 2), but none have yielded significant production. Other copper deposits in these quadrangles consist of chalcopyrite, bornite, or chalcocite-rich veins in the Nikolai Greenstone, as exemplified by the Nikolai mine and the Radovan greenstone prospect, and of native copper and tenorite lodes localized between Nikolai G r e en s ton e flows and in amygdules at the Erickson mine (fig. 2). Production from these deposits has been negligible. Native copper is sparsely distributed in a few of the Nikolai Greenstone amygdaloids, and copper sulfides are scattered along a fault zone that cuts Chitistone-Limestone and Nikolai Greenstone at the Radovan low-contact prospect (fig. 2). Chalcopyrite is a minor to trace constituent at the Taylor, Porphyry Mountain, and Crumb Gulch prospects (fig. 2).

Gold valued at close to a mHlion dollars has been recovered from the placers of Dan and Chititu Creeks and their tributaries. The few known gold lodes are in Tertiary intrusive ro:ks or in adjacent hornfelsed Cretaceous rocks.

Antimony has not been recovered from either the B-4 or the B-5 quadrangles, but stibnite is the major ore mineral of the Crumb Gulch lodes and a minor constituent of the Radovan low-contact prospect.

Molybdenite occurs in a lenticular quartz vein cutting Tertiary intrusive rocks at the Porphyry Mountain prospect (fig. 2) and as a sparse constituent of a few small quartz veins that cut Cretaceous rocks south of Dan Creek. Molybdenite-quartz float is widespread in the moraines of Canyon Creek Glacier, but its lode source probably is in the McCarthy B- 3 quadrangle.

Silver is associated with the copper-sulfide and copper-iron-sulfide lodes and with the native copper and gold deposits. No commodities other than the aforementioned are known to exist in potentially recoverable quantities in the mines and prospects of the Mc-Carthy B-4 and B-5 quadrangles. Distribution of Metals The significant metals in the quadrangles, gold, copper, silver, antimony, molybdenum, and arsenic, are discussed with regard to their distribution in (1) previously known deposits, (2) virgin outcrops, including veins, altered zones, and rocks, and (3) stream sediments. Analytical data for the rock, veins, and altered zone samples are given in table 1; similar data for the stream-sediment samples are given in table 2. The sample locations are shown in figure 2.

Samples from the known deposits do not necessarily represent the highest grade material available from the deposits. Several of these samples were taken to determine the extent of mineralized rock away from the lodes or to test structures that are not obviously mineralized. Most samples of rocks, altered zones, and veins were grab samples, although several chip or channel samples were taken at the mines and prospects and from the altered zones. The stream-sediment samples represent the finest grained material available at the sample site. Not enough sample information is available to determine adequately the background distribution of metals in the diverse rocks of the quadrangles. Consequently, the designation of anomalous values is somewhat arbitrary and is based on limited analytical data for the local rocks and on information obtained from the literature.

Gold The highest concentration of gold, 66.0 ppm (parts per million), was detected in a channel sample (16).!/ from the prospect north of Crumb Gulch. A few other samples from this prospect also contained anomalous amounts of gold. Samples from other known deposits that contained abnormal concentrations of gold are from the Taylor prospect (as much as 15.4ppm (74)), and the Porphyry Mountain prospect.

The richest sample from a deposit not previously known is from slightly altered Station Creek volcaniclastic rock collected northeast of Canyon Creek. This sample contained 5.2 ppm gold (103). Samples from altered zones near Canyon Creek (87, 108) and from stibnite-quartz veins north of Eagle Creek (63) contained gold in excess of 1 ppm. Several lesser gold anomalies, reflected by samples that contained between 0.1 and 1 ppm gold, are from altered zones elsewhere in the quadrangles.

Stream-sediment sampling detected small anomalous concentrations of placer gold in most of the main drainages. Samples from several of the streams that have been worked for placer gold and from which gold can be panned fairly readily yielded surprisingly low gold values. This is probably attributable to the vagaries of sampling and possibly to the size distribution of the placer gold.

Some drainages not previously known to contain placer gold, such as those of Canyon and Toby Creeks and the Chitistone River, showedminorgoldanomalies in their stream sediments. The highest gold concentration detected in the stream-sediment samples, 9.6 ppm (86), was from a stream along the north side of the Glacier Creek Glacier west of the Erickson mine. Copper Copper is the major valuable metal at most mines and prospects in the quadrangles. Many samples from copper deposits contained copper in excess of 20,000 ppm, the upper limit reported in the analytical results. Undoubtedly, samples that contain similar quantities of copper could be obtained by selective sampling at any of the known copper deposits. Minor and probably insignificant amounts of copper were detectedin.samples from some of the prospects for other met a 1 s.

Native copper is concentrated in some of the gold placers of the region, generally in the form of small nuggets, but rarely as large slabs weighing from several tens to several hundred pounds. The placer deposits, however, probably do not contain enough copper to be exploited.

Many of the stream- sediment samples contained anomalous amounts of copper in concentrations· of as much as 300 ppm. Most of the anomalous samples were from Toby Creek and from Glacier Creek and its tributaries.

Four of the stream-sediment samples contained anomalous amounts of silver. The highest silver concentration, 5 ppm, was in a sample from G 1 a c i e r Creek below the Erickson mine(85). Samples that contained 0. 7 ppm silver were collected from the Chitistone River near the northern border of the B-4 quadrangle (73), from Toby Creek (79), and from Rader Gulch, a tributary of Dan Creek (105).

Several subparallel stibnite-quartz veins as much as 2 feet thick and a few antimony-bearing altered zones as much as 6 feet thick were found cutting Nizina Limestone north of Eagle Creek. The stibnite forms scattered needlelike cry s t a 1 s or local high-grade masses in the veins. The extent of the veins along their strikes could not be determined because of surficial cover. Samples representative of the higher grade parts of these deposits contained more than 10,000 ppm antimony (62, 65, 66). A sample of altered material from a fault zone, 10 feet wide, cutting Nizina Limestone north of Texas Creek contained 1,500 ppm antimony (55). Samples from several other altered zones and from a few mines and prospects contained between 100 and 200 ppm antimony. A sample of mineralized breccia float collected from Texas Creek contained more than 10,000 ppm antimony (59). No anomalous concentrations of antimony were found in any of the stream-sediment samples.

Five of the stream-sediment samples contained 5 or 10 ppm molybdenum. Two of these samples are from tributaries of the Chitis tone River near the western boundary of the B-4 quadrangle (60, 64). One is from a tributary of Canyon Creek (136), and the two others are from the headwaters of Young Creek (141, 142).

The richest arsenic sample from a deposit notpreviously known contained 7,000 ppm arsenic; this sample was from a copper deposit in sheared N i k o 1 a i Greenstone (91). Only two of the stream- sediment samples revealed anomalous amounts of arsenic. One, from Toby Creek near its junction with the Chitistone River, contained 700 ppm arsenic (72); the other, from Rader Gulch, contained 300 ppm arsenic.

A selected sample from a narrow altered zone in basal Chi tis tone Limestone at a copper prospect south of Nikolai Butte yielded 15,000 ppm lead, 10,000 ppm zinc, and 300 ppm cadmium (52).

SUMMARY AND SUGGESTIONS FOR PROSPECTING The McCarthy B-4 and B-5 quadrangles contain numerous copper deposits, many of which carry subordinate amounts of silver, and a few deposits of gold, antimony, molybdenum, and arsenic. The geochemical and related geological investigations in the quadrangles provide additional information on the distribution of metals in known deposits as well as data on the newly found deposits. The investigations also indicate a few areas that probably merit additional examination.

Samples from some of the knovin deposits revealed concentrations of a few metals not previously reported, such as tungsten at the Crumb Gulch prospects and lead and zinc at a prospect south of Nikolai Butte. The apparently most significant of the previously unknown deposits are the copper lodes in the eastern part of the B-4 quadrangle and the stibnite lodes north of Eagle Creek. Our investigations do not provide adequate data on th~ size and grade of these deposits nor accurate appraisals of their potentials. Some of these deposits may justify further examinations. Other potential exploration targets in the B-4 quadrangle are the numerous altered zones, some of which are extensive, and the basal conglomerate of the Nikolai Greenstone, which contains abundant pyrite. Most of our samples from the altered zones and the conglomerate had low metal contents, but detailed sampling might disclose altered zones or parts of the conglomerate that have economic potential.

The minor gold anomalies in streams not previously known to contain placer gold, such as Toby and Canyon Creeks and the Chitistone River, and in altered rocks from near Canyon Creek, might warrant addition a I study as possible clues to large, low-'grade bedrock gold sources. Likewise, more thorough stream-sediment sampling of the drainages of Toby, Canyon, and Glacier Creeks may lead to the discovery of additional copper deposits. Additional sam p 1 in g and a thorough assimilation of all available sam p 1 e data could conceivably disclose pathfinder elements that would aid in prospecting the region.

Jones, D. L., and MacKevett, E. M., Jr., 1968, Summary of Cretaceous stratigraphy in pari: of the McCarthy quadrangle, Alaska: U.S. Geol. Survey Bull. 1274-K (in press). MacKevett, E. M., Jr., 1965, Preliminary geologic map of the McCarthy B-5 quadrangle, Alaska: U.S. Geol. Survey Misc. Geol. Inv. Map I-438, s c a I e 1:63,360. Moffit, F. H., 1938, Geology of the Chitina Valley and adjacent area, Alaska: U.S. Geol. Survey Bull. 894, 137 p. Moffit, F. H., and Capps, S. R., 1911, Geology and mineral resources of the Nizina district, Alaska: U.S. Geol. Survey Bull. 448, 111 p. Sainsbury, C. L., 1951, Geology of the Nelson and Radovan copper prospects, Glacier Creek, Alaska: U.S. Geol. Survey open-file report, Oct. 1951, 18p. Smith, J. G., and MacKevett, E. M., Jr., 1968, The Skolai Group in the McCarthy B-4, C-4, and C-5 quadrangles, Wrangell Mountains,· A 1 ask a: U.S. Geol. Survey Bull. 1274-Q (in press).

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See description of samples on pages 14-16.

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from the McCarthr B~4 and B~S guadrangles 2 Alaska

atomic absorption methods. Analyses reported to the nearest number in the series 1, 0.7, determination; >, greater than; <' less than. Semiquantitative spectrographic analyses by w. L. Campbell, A. L. Meier, R. L. Miller, M. s. Rickard, T. A. Roemer, and R. B. Tripp. sample 52; La, 50 ppm in samples 127, 132, and 138; 30 ppm in samples 63, 85, 87, 103, 125, detected in amounts between 5 and 30 ppm in most samples]

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Description of samples given in table 1

[Unless otherwise noted, all samples are grab samples. Sample locations are shown in fig. 2] Description of samples given in table !-Continued Description of samples given in table !-Continued

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the McCarthr B-4 and B-S guadrangles, Alaska

absorption. Analyses reported to the nearest number in the series 1, 0.7, O.S, 0.3, 0.2, O.lS, than. Semiquantitative spectrographic analyses by K. J. Curry, Arnold Farley, Jr., and C. L. but not detected: Ag, As, Bi, Cd, Hg, Mo, Pd, Pt, Sb, Sn, Ta, Te, W, and Zn; exceptions: Ag, sample lOS; Mo, 10 ppm in sample 64, S ppm in samples 60, 136, 141, and 142; W, 100 ppm in the limits of determination differ from those indicated at the end of the table, their values

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Table 2.--Analrses of stream-sediment samEles from the

Sample Field No. Lab. No. Au ACF-

0.02 Parts per million

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Limit of McCarthr B-4 and B-S guadrangles 2 Alaska--Continued

determination Parts per million--Continued

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