I

RI 9145Bureau of Mines Report of Investigations/1987

Characterization of Ketchem DomeTin Prospect, East-Central Alaska

By D. C. Dahlin, L. L. Brown, and J. D. Warner

UNITED STATES DEPARTMENT OF THE INTERIOR

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4·lf

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Report of Investigations 9145

Characterization of Ketchem DomeTin Prospect, East-Central Alaska

By D. C. Dahlin, L. L. Brown, and J. D. Warner

UNITED STATES DEPARTMENT OF THE INTERIORDonald Paul Hodel, Secretary

BUREAU OF MINESDavid S. Brown, Acting Director

Library of Congress Cataloging in Publication Data:

Dahlin, D. C. (David Clifford), 1951-Characterization of Ketchem Dome tin prospect, east-central Alaska.

(Report of investigations ; 9145)

Bibliography: p. 11.

Supt. of Docs. no.: I 28.23: 9145.

1. Tin ores-Alaska-Ketchem Dome. I. Brown, L. L. (Lawrence L.), 1928- . II.Warner, J. Dean. III. Title. IV. Series: Report of investigations (United States. Bureau ofMines); 9145.

TN23.U43 [QE390.2.T48] 622 s [553.4'53'09798] 87-600264

CONTENTS

Page

Abstract...... .. ................... .............................................. 1Introduction ........ ... ...................................................... .. 2Location and prospect history.................................................. 3Geology........ .. ................... ............................................. 4Mineralogy ................... . ............................................... . 6

Bulk sample................................... ............................. 6Tin concentrate ...................................................... ... 8

Concentration results................................................................................... 9Conclusions ................................................................... 11References ...................................................................... . 11

ILLUSTRATIONS

1. Location of the Ketchem Dome tin prospect ................................ 32. Generalized outline of a portion of the Circle intrusion showing location

of tin occurrences and proposed regional trend ............................ 43. Ketchem Dome prospect with detail of trench ................................ 54. SEM photomicrographs of some of the accessory minerals in the rock matrix.. 75. SEM photomicrograph and element distribution maps of the tin concentrate... 86. Cassiterite (light) locked with iron and silicate gangue ................... 97. Procedure used to concentrate the bulk sample .............................. 10

TABLES

1. Size distribution of the rougher table feed ................................ 92. Table concentration of the Ketchem Dome greisen bulk sample ................ 103. Size distribution of the rougher table slime tailings ...................... 10

ft

in

lb

pm

m.y.

UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

foot oz/st troy ounce per short ton

inch pct percent

pound ppm part per million

micrometer wt pct weight percent

million years

CHARACTERIZATION OF KETCHEM DOME TINEAST-CENTRAL ALASKA

PROSPECT,

By D. C. Dahlin, L. L. Brown,2 and J. D. Warner 3

ABSTRACT

The Bureau of Mines investigated the Ketchem Dome tin prospect ineast-central Alaska to evaluate the tin resource. The studies includedfield investigations, mineralogical characterization, and concentrationof the tin-bearing minerals. The tin mineralization consists of cassit-erite (SnO2), which occurs as grains disseminated in a greisen (quartz-mica) matrix. A bulk sample from the largest and best-developed greisenvein exposed at Ketchem Dome contained 0.51 pct Sn. Concentration ofthe cassiterite by gravity table methods resulted in a product that con-tained 56 pet of the tin at a grade of 41.4 pet Sn. The mineraliza-tion exposed at Ketchem Dome is typical of greisen found in the area andelsewhere in interior Alaska.

1 1Metallurgist, Albany Research Center, Bureau of Mines, Albany, OR.2Geologist, Albany Research Center (retired).3Physical scientist, Alaska Field Operations Center, Bureau of Mines, Fairbanks, AK

(now with NERCO Minerals, Inc., Vancouver, WA).

2

Tin is a relatively scarce element inthe Earth's crust, but it is an indis-pensable material in many industrial ap-plications. It is used as a protectivecoating for metals such as tinplate andas an alloying element in solders,bronze, babbitt, and other metals. Theprimary end-uses of tin in the UnitedStates are cans and other containers,chemicals, machinery, electrical equip-ment, and applications in transportationand construction. Tin has strategic im-portance in solders for sophisticated de-fense electronics equipment and computersand in military vehicles, containers,parts, and fittings (1).4

Although the United States is con-sidered to be the world's major consumerof tin and the largest producer of sec-ondary tin, domestic primary tin produc-tion is almost entirely from importedore, primarily from Southeast Asia andSouth America (2). Only a small fractionof the U.S. tin requirement is met by do-mestic mines, and domestic reserves aresmall.

In October 1985, the economics of pro-ducing and marketing tin on the worldmarket dramatically changed. At thattime, the International Tin Council,formed by major producers and consumersof tin to stabilize supply and prices,ran out of funds to support the price oftin, and, consequently, the price droppedto approximately half its former value(3). The lower prices had a significantnegative effect on high-cost oper-ations such as the old, underground minesin Bolivia and the gravel-pump operationsthat have accounted for about two-thirds

of the production from Malaysia. Allproducers reevaluated their tin miningindustries, and mines were closed in

INTRODUCTION

Australia, Bolivia, Malaysia, Thailand,and the United Kingdom (4-5). Lower tinprices would benefit the United States,and U.S. consumption is expected to showslower growth between 1986 and 1993.However, the uncertainty of foreignsources dictates that domestic resourcesbe evaluated for their potential.

The Bureau of Mines has a continuingprogram for investigating critical andstrategic mineral resources in the UnitedStates. Warner (6) provided a detailedsummary of the Bureau's efforts to iden-tify tin, tantalum, and columbiumresources in Alaska and presented anextensive list of references. Alaskacontains both placer and lode tinoccurrences. Historically, most of theState's tin production has been fromplacer deposits on the Seward Peninsula,but lode deposits account for most of thereserves and resources. The lode occur-rences can be categorized in five groups,as follows: greisen, vein, skarn, pegma-tite, and volcanogenic massive sulfide.The Ketchem Dome prospect is a greisenoccurrence.5

This report describes a preliminaryevaluation of the tin resource at KetchemDome, an established tin prospect withgood exposure of the mineralization. Thestudy was done because the prospect isthought to be typical of similar occur-rences in the area and in other parts ofinterior Alaska. Also, the form, altera-tion mineralogy, geologic setting, andthe tin head grade indicate that it issimilar to greisen veins mined elsewherein the world.

5"Greisen" refers to hydrothermallyaltered granitic rock composed primarilyof quartz, mica, and topaz. The mica isusually muscovite or chlorite; among thecommon accessory minerals are tourmaline,fluorite, rutile, cassiterite, andwolframite.

4Underlined numbers in parentheses re-fer to items in the list of references atthe end of this report.

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LOCATION AND PROSPECT HISTORY

The Ketchem Dome tin prospect is lo-cated approximately 5 miles southwest ofCircle Hot Springs in the north-centralportion of the Circle (B-2) Quadrangle,in east-central Alaska (fig. 1). Theprospect lies 1 mile to the west of, andapproximately 1,300 ft above, a gravelroad the leads north to a year-round roadbetween Central (90 miles northeast ofFairbanks on the Steese Highway) andCircle Hot Springs.The Ketchem Dome area was originally

staked in 1977 by Resource Associates ofAlaska, Inc., Fairbanks, AK (7), which

optioned the property to Houston Oiland Minerals Co. (now Tenneco MineralsCo., Lakewood, CO). Geologists withHouston Oil and Minerals Co. located tin-bearing greisen vein mineralization on asmall ridge extending northeastward fromthe northern flank of Ketchem Dome(figs. 2-3). The option on the propertywas subsequently dropped by Houston andacquired by an individual placer goldminer from Fairbanks. Bureau geologistsmapped and sampled the Ketchem Dome pros-pect in 1984.

FIGURE 1.-Location of Ketchem Dome tin prospect.

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FIGURE 2.-Generalized outline of a portion of the Circle intrusion showing location of tin occurrences and proposed regionaltrend.

GEOLOGY

Tin-bearing greisen vein mineralizationnear Ketchem Dome cuts a leucocratic(light-colored) seriate phase of the Cir-cle Hot Springs biotite granite plutonnear its contact with hornfelsed sedi-mentary rocks of Paleozoic and/orPrecambrian (?) age (8, figs. 2-3). Thepluton is a deeply eroded composite in-trusion that forms a crudely horseshoe-shaped outcrop pattern that extends ap-proximately 8 miles to both the northwestand southeast from Ketchem Dome and hasbeen dated at 60.5±1.8 m.y. by thepotassium-argon method (9). One thinsection of the leucocratic phase of thepluton collected near Ketchem Dome con-tained approximately 36 pet quartz,33 pet orthoclase, 26 pct plagioclase(oligoclase), and 5 pct biotite. Minoramounts of accessory muscovite and pos-sibly topaz, as well as fluorite, zircon,and opaque minerals were also observed.

Four hand-excavated trenches expose thegreisen vein near Ketchem Dome (fig. 3).The greisen is mineralogically zoned,with a selvage of muscovite-altered

granite that grades into a core of dense,black, chlorite- and sericite-alteredgranite with traces of limonite afterpyrite and veinlets of quartz. Purplefluorite occurs on fractures, and topazis locally associated with more pervasivealteration.

The largest and best-developed greisenvein is exposed in the two northerntrenches (fig. 3). The vein is 3 ft wideand strikes west-northwest for approxi-mately 20 ft between the two trenchexposures and is then lost in rubble onboth sides of the ridge. Similar greisenvein material was found in float onstrike of the greisen vein to the east,near the gravel road that parallelsKetchem Creek. This suggests that thegreisen vein may extend eastward at least5,000 ft along strike.The tin mineralization exposed near

Ketchem Dome is typical of greisen foundelsewhere in the Circle Hot Springs plu-ton and in interior Alaska. The vein atKetchem Dome occurs along a proposedregional trend of tin occurrences and

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Approx. scale, ft

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Approx. scale, ftContour interval, 200 ft

LEGENDQuartz-chlorite veins

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Geologic contact

Granite altered tochlorite and sericite

Granite altered to sericite6-i~

Tin -mineralized zone

Trench

t ' Seriate biotite granite Sample locationM5 Hornfels

FIGURE 3.-Ketchem Dome prospect with detail of trench.

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prospects that is associated with theCircle Hot Springs pluton and extendsfrom Boulder Creek easterly to beyondPortage Creek (fig. 2). This trend in-cludes both tin lode and placer mineral-ization. Tin lode mineralization alongthe trend and elsewhere in interior Alas-ka is typically iron- and chlorite-richand generally occurs in veins and alongfaults or shear zones within moderatelydeeply eroded composite biotite-granite

plutons. The mineralized veins or faultsare invariably 3 ft or less in width andcan usually only be traced along strikefor a few tens of feet or less. In someinstances, however, several closelyspaced greisen veins occur in widthsranging from 10 to 100 ft and may be dis-continuously traceable in rubble forseveral hundred to several thousand feetalong strike.

MINERALOGY

BULK SAMPLE

A 120-lb channel sample was collectedfrom the 3-ft-wide greisen vein exposedin the northeastern trench at KetchemDome (fig. 3). Representative hand spec-imens were selected from the bulk sam-ple for mineralogical characterizationstudies. The remainder was crushed andsplit for chemical analysis and physicalconcentration tests. The head analysisof the bulk sample was, in percent,0.51 Sn, 72.9 SiO 2, 9.80 A1203, 7.80 Fe,2.29 K20, 0.15 total rare-earth oxides,0.02 S,

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ark * ** _ . \ F , - -.-

uric X: -

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FIGURE 4.-SEM photomicrographs of some of the accessoryminerals in the rock matrix. A, Cassiterite (circled) with feldspar(light), muscovite (gray), and quartz (black) (X 250); B, cassiter-ite in quartz (X 400); C, two multiple-oxide minerals that con-tain uranium in quartz (X 500); D, yttrium-cerium phosphate(light) on apatite (light gray) in feldspar (dark gray) and quartz(black) (X 500); E, columbium-tantalum minerals (circled) withfeldspar (light), muscovite (bladed gray), and quartz (black)(X 200).

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order of abundance couldately established becauseamounts involved, so thelisted alphabetically.)

not be accur-of the smallminerals are

15 quartz, 4 iron-oxide minerals, 2 sul-

fides, and 2 apatite, and trace amountsof garnet, barite, ilmenite, magnetite,and fluorite. Other trace minerals werepresent but were not identified. Fig-ure 5 shows an SEM photomicrograph of thetin concentrate with corresponding tin,iron, and silicon distribution maps.Bright areas in each map indicate elementconcentrations.

TIN CONCENTRATE

The tin concentrate prepared fromthe bulk greisen sample consisted of,in percent, 55 cassiterite, 20 chlorite,

FIGURE 5.-SEM photomicrograph (A) and element distribution maps (B tin; C iron; D silicon) of the tin concentrate (X 120).

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EDXA of six ranerite grains inprepared from thefollowing results:

Cassiterite Sngrain

1............. 982............. 973............. 974............. 985............. 976............. 97

domly selected cassit-n a polished surface

concentrate gave the

02 Cb205 Fe 20 3 A1203

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L. 2.1.1.5.6.3

0.891.21.57ND

1.42.34

0.13ND.25.50ND.64

0.791.672.091.00

.991.41

ND Not detected.a

Although one would expect the cassiteritecomposition to be relatively uniformwithin a deposit, the grains showed arelatively wide range in the amounts ofsubstituted or included elements.Although liberation was good at

100 mesh, some of the cassiterite waslocked with other minerals present in theconcentrate. SEM images of polished sur-face mounts prepared from the concentrate

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FIGURE 6.-Cassiterite (light) locked with iron and silicategangue (X 180).

showed cassiterite locked in particlesless than 1 pm in size. Figure 6 showscassiterite grains intimately locked withgangue.

CONCENTRATION RESULTS

A gravity concentration test was con-ducted on the bulk sample of the tin-bearing greisen. The sample was stage-ground in rod mills to pass 100 mesh, butelaborate steps were not taken to preventovergrinding. The size distribution ofthe ground sample is shown in table 1.The ground sample was then tabled on aslime deck of a wet shaking table to pro-duce a rougher concentrate, coarse table

TABLE 1. - Size distributionof the rougher table feed

Size, mesh wt pct Cumulative wt pctretained

Plus 150 ...... 27.9 27.9150 by 200.... 15.4 43.3200 by 270.... 6.5 49.8270 by 400.... 16.9 66.7400 by 500.... 6.8 73.5Minus 500..... 26.5 100.0

tailings (those that settled and bandedon the table), and slime tailings (thosethat washed off the table without set-tling). Recovery of concentrate wasemphasized over concentrate grade. Therougher concentrate was tabled again in acleaner step to produce the final tinconcentrate. Concentrate grade was em-phasized over recovery in the cleanerstep. A flowsheet of the test procedureis shown in figure 7, and the results ofthe test are shown in table 2.

The cleaner table concentrate containednearly 56 pct of the tin at a grade of41.4 pct Sn. These values compare favor-ably to those obtained from lode depositsin commercial operations. Althoughplacer concentrates may be nearly purecassiterite (70 to 75 pet Sn), concen-trates from lode deposits are substan-tially lower in grade and may containonly 15 to 20 pct Sn. Recoveries range

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TABLE 2. - Table concentration of the Ketchem Domegreisen bulk sample

Product wt pct Sn, petAnalysis Distribution

Rougher concentrate:Cleaner concentrate 1..... 0.7 41.4 55.9Cleaner tailings ......... 5.9 1.63 18.5

Rougher coarse tailings.... 62.6 .08 9.6Rougher slime tailings..... 30.8 .27 16.0

Composite or total ...... 100.0 .52 100.0'Additional analyses, in pct:

A1 203, 1.30 S, 0.40 F, 0.21 W,ides, 0.06 Cb, 0.05 Ce,

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CONCLUSIONS

The Bureau of Mines completed a prelim-inary evaluation of the tin resource atKetchem Dome in east-central Alaska. Tinoccurs as cassiterite in greisen mineral-ization that contains a large number ofaccessory minerals in small amounts. Thegreisen at Ketchem Dome is typical ofgreisen found elsewhere in the area andin interior Alaska.

A two-stage gravity concentration testrecovered 56 pct of the tin at a grade of41.4 pct Sn. These values compare favor-ably with results from commercial lodemining operations. Although optimum con-ditions and processes were not investi-gated, the potential exists for higherconcentrate grade and better recovery.

REFERENCES

1. Carlin, J. F., Jr. Tin. Ch. in 1983; available upon request fromMineral Facts and Problems, 1985 Edition.BuMines B 675, 1985, pp. 847-858.

2. . Tin. BuMines Minerals

Yearbook 1985, v. 1, 12 pp. (preprint).3. Spooner, J. Tin--Trial of Errors.

Min. Mag., v. 154, 1986, pp. 475-477.4. U.S. Bureau of Mines. Minerals

and Materials. A Bimonthly Survey.Feb./Mar. 1986, pp. 5-7.

5. . Minerals and Materials. ABimonthly Survey. Apr./May 1986, p. 18.

6. Warner, J. D. Critical and Strate-gic Minerals in Alaska. Tin, Tantalum,and Columbium. BuMines IC 9037, 1985,19 pp.7. Freeman, L. (Resource Associates

of Alaska, Inc.). Private communication,

J. D. Warner, BuMines, Fairbanks, AK.8. Foster, H. L., J. Laird, T. E. C.

Keith, G. W. Cushing, and W. D. Menzie.Preliminary Geologic Map of the CircleQuadrangle, Alaska. U.S. Geol. Surv.OFR 83-170A, 1983, 30 pp.

9. Wilson, F. H., and N. Shew. Mapand Tables Showing Preliminary Results ofPotassium-Argon Age Studies in the CircleQuadrangle, Alaska, With a Compilation ofPrevious Dating Work. U.S. Geol. Surv.OFR 81-889, 1981, 1 p.

10. Aplan, F. F. Tin. Sec. in SMEMineral Processing Handbook, ed. byN. L. Weiss. Soc. Min. Eng. AIME, 1985,pp. 27-10 to 27-14.

INT.-BU.OF MINES,PGH.,PA. 28613