COBALT CONTENT IN SAMPLES FROM THE OMAR COPPER PROSPECT, BAIRD MOUNTAINS, ALASKA by: Uldis Jansons Alaska Field Operations Center, Anchorage, Alaska

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Open File Report

MLA 109-82
UNITED STATES DEPARTMENT OF THE INTERIOR James G. Watt, Secretary BUREAU OF MINES Robert C. Horton, Director

CONTENTS Page

Abstract.... 1 Introduction.,. -oooo. o..1 Acknowledgements-. Location and access.

.o-o-o-o. 2 2

Physical setting-o-o.. -. Geological setting. o. Local geological setting. Mineralizationo. oo Polished section study.. Sampling.

4.-..oo. oo

o-o. o.-.. 6 ---. 8 9

Data presentations.

Interpretation.. Summary.-oo..

.. 10 15.. 16

. o..-.

References.,.o oo. -o.

TABLES 1. Chemical Analyses of Rock, Soil, and Stream Silt Samples from the Omar Copper Prospect Area, Baird Mountains,
Alaska. o-oo. o. o. 12-14
ILLUSTRATIONS Figure 1. 2. 3. Location of Omar prospect area in Alaska. Location of the Omar prospect, Baird Mountains, Alaska.. Cobalt content in Omar prospect area samples, Alaska. Page 7
COBALT CONTENT IN SAMPLES FROM THE OMAR COPPER PROSPECT, BAIRD MOUNTAINS, ALASKA

by 1/ Uldis Jansons

ABSTRACT The cobalt content of rock, soil, and stream silt was determined for samples from the Omar copper prospect, located near the Omar River in the Baird Mountains, northwestern Alaska. Anomalously high values were
detected in all three types of materials collected near the area of copper mineralization. The highest cobalt values were: rock - 545 ppm, soil
66 ppm, and stream silt - 47 ppm.
Samples with high cobalt content genCobalt values in stream silt near copper mineralization A cobalt association
erally also had high base metal contents. samples were highest in materials

collected

and lowest in sediments away from mineralization. with the copper mineralization is indicated.

INTRODUCTION

The presence of cobalt in carbonate-hosted copper deposits, principally Ruby Creek, in the Western Brooks Range in Alaska, has been recognized since at least the early 1960's. High grade (1%+) 2/ Bornite, Alaska (1). tenths of one percent cobalt Recent zones occur work at in the Ruby Creek deposit shows that at

that deposit

several
cobalt may be associated with large tonnages of The mineralization at Ruby Creek is underindustry. Copper in the Omar River
this copper mineralization.
going re-evaluation by the mineral
area was recognized in the early 1960's. 1/ 2/
The property was subsequently
Supervisory Physical Scientist, Alaska Field Operation Center, Anchorage Underlined numbers in parentheses refer to references at back of report.

staked and

a limited

amount

drilling

was done

following

various

surface studies.
Subsequently the claims were allowed to lapse. similarities between the Ruby Creek and Omar
Because of geological deposits, the potential
of a cobalt resource at Omar seemed possible. soil, and stream silt samples from the Omar content. The analytical
Previously collected rock, area were
analysed to determine their cobalt
results are presented in this report. ACKNOWLEDGEMENTS The description of the Omar area and samples from the Omar prospect used in this report were provided by WGM Inc. of Anchorage, Alaska, as part of a contract mineral resource study of the western Brooks Range for the Bureau of Mines (2). Samples were analysed for cobalt at a commercial
laboratory at Spokane, Washington using standard analytical techniques. LOCATION AND ACCESS The Omar prospect is located in T24N, RIOW, Kateel River Base-

line and Meridian, in the Baird Mountains, approximately 65 miles northeast of Kotzebue (figures 1 and 2). Access to the area is by helicopter. Fixed wing aircraft have been
landed at extensive terraces located about 8 miles to the southwest from the property. PHYSICAL SETTING The Omar prospect is in the Baird Mountains Physiographic Division of Alaska (3). The general topography of this division consists of
moderately rugged mountains having rounded to sharp summits which rise 2

I I I I i I

FIGURE 1. -
Location of Omar prospect area in Alaska

16030'

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h i q _r. l ^

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1 3Xr j

jz' dot

.- I I

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67'30' T24N

,.Ma-l

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Scale 9

Mi les

Kiiomewters
Contour interval 1000 feet

FIGURE 2. -

Location of the Omar prospect, Baird lMountai ns, A1laska
abruptly from lowlands. altitudes.
The summits generally have 2,500 to 3,000-foot
The Baird Mountains are drained by streams that flow north River and south to the Kobuk River. The south-flowing

to the Noatak

streams head in narrow ravines having steep headwalls, several hundred feet high. The Omar River is one of these south-flowing streams. by two of southerly-flowing the forks of the The Omar River
prospect area is drained River, which (figure 2).

is a tributary

easterly-flowing

Squirrel

GEOLOGICAL SETTING The following discussion is abstracted from the geologic map on the Omar prospect and an accompanying geologic report on mineral evaluations of the Western Brooks Range provided by a contractor to the Bureau of Mines (2). The preliminary geologic (4-5) makes general way. it possible to The prospect work in the Baird Mountains regional setting Ouadrangle only in a rocks

describe the is underlain

by Devonian

carbonate
which cover an area of about 550 square miles. inately of limestone and/or marble, and
The strata consist predomof the Baird Group.
Local Geological Setting Carbonate sequences, both dolomite and limestone, at the Omar
prospect have been folded into broad anticlines and synclines of variable attitudes. Detailed mapping indicates the presence of a series of steepThe fracture
lydipping north northwest-trending fracture systems.
zones are not always apparent in individual exposures although the presence of highly fractured rocks along a NNW alignment strongly suggests their existence. 5

Most of

the dolomites
fine-grained dark-gray massive

bodies

that display very few sedimentary features.

A Devonian age has been

assigned to these rocks based on the abundance and variety of favositid corals. The distribution and general character of the fossils suggests formed in a paleo-reef environment. Associated local

they probably

breccia-cones may represent clastic reef breccia aprons. Limestone units, which are widespread along the western portion of the study area, are typically finely banded and commonly interbedded with light or dark-gray dolomites. Fracturing has resulted in a stockwork of very narrow, white-colored, dolomite-filled fractures in dark-gray, generally massive, dolomite host rocks. Dolomite breccias commonly feature rounded or angular clasts in A tectonic origin for the breccias is suggested.

a finer-grained matrix.

Exposures at the prospect display many examples of fracturing that are gradational between the two extremes. The variation in intensity of
fracturing apparently reflects structural extremes between mild, incipient fracturing and complete rupturing and dislocation. Mineralization Mineralized bedrock exposures are of limited areal extent, crop
out in windows in pervasive talus in some areas, and are more or less surrounded by barren to very weakly mineralized rock in other areas. Occurrences of copper and iron mineralization at the prospect are shown on figure 3. Surface copper trending fractures mineralization occurs locally along several NNW
over a distance of about 9,000 feet.

The fracture

zones, which are up to 100 feet wide, are confined to a 1,000 to 3,5006

\ "(/2'.

- "

,,326(16)

C 3_.7I;'

577(16)

, 327(-7)

"/

8t3C-,, 2(13)

>>,

2000 F.M

346C2&;(,,

Scale Contow interlol 00 tfo

' 329(16)

$344(18)

-34612

LEGEND
- , 595Cr5)7 x 3 (80), ' 596II 6) /' '316( 17) j 39(1)-' 339(16)j-1 K, |331(13)4375(68(1!6338(6

~'"

.- 320 (41) /370(14) --x 316(17)

Trmncft wnpI

(,ooit vokw)
SI rltn srft somp *ite (cobalt

Rock swnnp* (cobol site

o 329(17) sal
ompw it* (co"ot -kie)
CuorlfCro.,s float or oitcrmp
Irw, onide, %to.t iarq gor n OroinoWe

20(11)

804(13),

'806 02) \

J /817(13) 816(12)

~80701.

-I 0 '

81.304)

I II I , I\-\

1, I. I

FIGURE 3. -
Cobalt content in Omar prospect area samples, Alaska 7

foot wide

north-south

trending

Surface

indications

copper
mineralization occur only along the fracture zones confined to the west side of the prospect area. The individual surface shows located along
the NNW fractures are separated by barren rubble zones that are up to several hundred along the more than feet long. Samples collected length, samples at occasional exposures

intervening 2% copper.

9,000-foot High-grade

contain were

from 0.1% copper to collected from talus.
Numerous small exploration in the
pits and trenches, dug at the time of the initial southern and central area of the property, contain
highly leached gossan showing minor to conspicuous amounts of malachite. Samples BM 450, BM 451, and BM 452 which contains 1,070 ppm, >20,000
ppm, and 13,400 ppm copper respectively, were collected from gossan in three 30 to 40-foot long trenches across the width of a zone of fractured dolomite. Hand specimens of mineralized samples often consist of medium gray, massive dolomite displaying considerable malachite, as well as substantial amounts of bornite, chalcopyrite, and covellite. The sulfides occur in
irregular masses from 3 to 7 inches long, and as fracture fillings less than 2 mm wide. copper. Polished Section Study Study of the Omar polished surfaces a of mineralized sulfide rock specimens dominated from by Randomly sampled rubble often contains several percent

prospect

reveals

simple

mineralogy
bornite, with considerable chalcopyrite and covellite. tite are present in minor amounts. masses in well-defined fractures Bornite

Pyrite and tennan-

occurs as large irregular fine-grained (<0.1

and as extremely

mm) intergranular blebs.
Chalcopyrite commonly occurs as large, anhedral
inclusions (usually between 0.1-2.0 mm) in the bornite masses, although a considerable amount occurs also as tiny needle-like inclusions (usually <0.2 mm long) in the bornite. Covellite typically occurs as very narrow, of bornite grains and around also been masses observed and as as larger
inconspicuous envelopes along the margins the tiny chalcopyrite needles. in coarser It has bornite

irregular inclusions

fine-grained
disseminations (<O.lmm long) in dolomite. Inclusions of pyrite, usually both chalcopyrite and bornite. less than 1 mm long, are common in
Most of the observed pyrite occurs as very
fine-grained (approximately 0.01 to 0.05 mm) framboids in carbonate host rock. The framboidal pyrite grains probably formed syngenetically, and only coincidently of tennantite related to observed the copper mineralization. in both chalcopyrite and
appear to be A few grains
bornite masses. The textural features of the coarse bornite and chalcopyrite grains suggest the minerals very are likely probably the coeval. of The chalcopyrite needle

structures are

result

exsolution,

although local
concentration adjacent to fractures suggests that they may be related to secondary (supergene?) processes. The relationship between the sulfides
and dolomite host rock suggests that much of the sulfide was probably deposited in secondary fractures and primary pore spaces rather than by extensive replacement of host material. SAMPLING Rock, mineral, Inc. (2). soil, and stream silt samples were collected by WGM
The sample site locations are shown on figure 3. Rock samples, 9
some including sulfide minerals, were collected from various locations. Extensive rubble cover and rapid downslope migration of the rubble on
steep slopes militate against development of soil horizons at the prospect. The so-called soil samples are probably fine-grain sand and perhaps Silt samples were collected silt samples may be
represent an incipient "C" soil horizon.
from streams which drain the prospect area.
enriched by physically transported copper sulfide and oxide minerals. Malachite is visible in the silt fraction of the stream sediments.
DATA PRESENTATION The cobalt content of the samples as well as the previously
determined base and precious metal contents are shown on table 1. Only the cobalt values are shown on figure 3. Cobalt values in 17 rock samples range from 6 ppm to 545 ppm, and average 82 ppm. Twelve soil samples contain from 15 ppm to 66 ppm cobalt Stream silt samples from

and have an average value of 28.4 ppm cobalt.
various parts of the study area contain from 2 ppm to 47 ppm cobalt and average 15.7 ppm cobalt.

INTERPRETATION

High cobalt values are present in rock, soil, and stream silt samples collected from a generally N-S trending zone that is closely related to areas of copper mineralization (figure 3). Observation of the analyti-
cal results (table 1) suggests that the highest cobalt values are from samples that contain high concentrations of other base metals. The lateral dispersion of cobalt away from the main zone of copper mineralization is shown on figure 3. The cobalt content in stream silt
samples in the area of the Omar prospect are rather specific to the area of mineralization. The highest cobalt values in stream silt samples
are found in drainages that appear to be collecting materials from the area of base metal mineralization. The stream silt samples furthest
away from out-and sub-cropping mineralization contain generally less than 14 ppm cobalt. The cobalt values seem to decrease progressively away
from the zone of known copper mineralization.
TABLE 1. - Chemical analyses of rock, soil, and stream silt samples from the Omar copper prospect area, Baird Mountains, Alaska
(Values in parts per million)
Rock Samples Sample No. BM 362 Co 13 Cu 180 18,500 4,>20,000 1,580 12,900 16,108 >20,000 9,Pb 1,56 1,160 1,440 1,Zn 109 1,200 5,10 9,400 9,500 2,0.6 2.6 2.4 0.2 2.0 0.3 0.2 2.8 10.0 3.4 2.4 2.6 3.1 0.5 0.3 0.5 0.4
398 Soil Samples BM 450 451

1,070 >20,000 13,18

1,12 62

1. 5 0.8

TADI C

IftDL-

r, L' --. - , II - I I I - - -4 wIr I1I acl a II aIy UI

- - -11 I UVIN

, - sUII,

3LrIaai

, + , -1 alu II IIIF
the Omar copper prospect area, Baird Mountains, Alaska.- Continued
Stream Silt Samples Sample No. Co Cu Pb Zn

369 370

448 449

80 660

4.2 4.2 4.1 4.0 4.0 3.6 1.8 2.0 4.0 4.1 3.4 3.7

596 597

198 140

TABLE 1. -

Chemical analyses of rock, soil, and stream silt samples from the Omar copper prospect area, Baird Mountains, Alaska.- Continued (Values in parts per million)

Stream Silt Samples Sample No. BM - = not analyzed > = greater than 14 Co Cu 2,1,Pb Zn 320 62
2.0 1.4 0.2 3.0 3.0 3.4 2.6 2.9 0.2 0.3 0.2 0.4 0.2 0.4 1.0 0.4 1.1 0.8 0.6 0.6 0.5 0.5 0.3 0.3 0.3 0.2

SUMMARY

Minor amounts of cobalt occur in rock, soil, and stream silt samples from the area of the Omar prospect in the Baird Mountains of the western Brooks Range. The areal distribution of high cobalt values in the area
of the Omar prospect in general coincides with the copper distribution. Up to 545 ppm highest cobalt contents. cobalt values was detected in a mineralized rock were found in samples having high sample. base The metal

The cobalt

source is probably in the sulfide mineralization.

REFERENCES

1. Runnels, D. D. The Copper Deposits of Ruby Creek, Cosmos Hills, Alaska. PhD Dissertation, Harvard University, 1963, 274 pp. 2. Degenhart, C. E. Mineral Studies of the Western Brooks Range Performed Under Contract to the U.S. Bureau of Mines, Contract # J0155089, OFR 10378, WGM Inc., Consultants. by C. E. Degenhart and others. 1978, 529 pp., 32 figs. Physiographic Divisions 3. Wahrhaftig, Clyde. Survey Professional Paper 482, 1965, 52 pp. of Alaska. U.S. Geol.
4. Brosge, W. P., H. N. Reiser, I. L. Tailleur. Selected Samples, Southwest Brooks Range, Alaska. OFR 274, 1967, 1 sheet.
Copper Analysis of U.S. Geol. Survey
Preliminary Geologic Map of 5. Beikman, H. M., and E. H. Lathram. Northern Alaska. U.S. Geol. Survey, Miscellaneous Field Studies, Map MF 789, 1976, 2 sheets.