GEOLOGICAL SURVEY CIRCULAR 565 

Cassiterite in Gold Placers 

at Humboldt Creek 

Serpentine-Kougarok area 

Seward Peninsula, Alaska 

Cassiterite in Gold Placers 

at Humboldt Creek 

Serpentine-Kougarok area 

Seward Peninsula, Alaska 

By C. L. Sainsbury, Reuben Kachadoorian 

Thomas E~ Smith, and William C. Todd 

G E 0 L 0 G I C A L S U R V E Y C I R C U L A R 565 

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 

General geology ----------------------------- 2 

Relation of geologic structure and mineralization ---------------------------- 4 

Cassiterite on Humboldt Creek____________ 4 

Conclusions --------------------------------- 6 

References cited ---------------------------- 7 

ILLlJSTRATIONS 

Page 

FIGURE 1. Index map of Seward Peninsula, showing location of Serpentine-Kougarok area _____________ 2 

2. Reconnaissance geologic map of Serpentine-Kougarok area -------------------------------- 3 

3. Photograph of cassiterite nuggets from Humboldt Creek, Serpentine-Kougarok area ________ 5 

TABLE 

Page 

TABLE 1. Semiquantitative spectographic analyses of a cassiterite nugget and of a placer concentrate, 

Humboldt Creek ___________________________ ----------------------·----------------------- 6 

III 

CASSITERITE IN GOLD PLACERS AT HUMBOLDT CREEK 

SERPENTINE-KOUGAROK AREA 

SEWARD PENINSULA, ALASKA 

By C. L. SAINSBURY, REUBEN KACHADOORIAN, THOMAS E. SMITH, and WILLIAM C. TODD 

Abstract 

Large amounts of cassiterite accompany placer gold 

in Humboldt Creek in the Serpentine-Kougarok area, 

Seward Peninsula, Alaska. Cassiterite has also been 

reported from other placer-gold workings nearby. To 

date, no cassiterite has been recovered commercially; 

chemical analyses of concentrates from Humboldt 

Creek show a tin content of 60 percent--a commercially 

saleable product. Recovery of a saleable tin product 

would increase the possibility for profitable operation 

of the gold placers. 

The cassiterite is intergrown with vein quartz without any noticeable sulfide minerals; the cassiterite 

grains occur in size and quantity that suggest a 

nearby lode source worth the effort of a search to 

find it. 

INTRODUCTION 

The occurrence of cassiterite in placer-gold 

concentrates from Humboldt Creek, in the 

Serpentine-Kougarok area (fig. 1), was reported by Knopf in 1908; traces of cassiterite 

in placer-gold deposits elsewhere in the region 

1 

were reported by Knopf (1908, p. 63) and 

Moxham and West (1953). In 1967, during a 

helicopter reconnaissance of the streams in 

the granite area west of Humboldt Creek, 

Kachadoorian sampled concentrates present in 

substantial tonnages at old placer-gold workings on Humboldt Creek; the concentrates 

proved to be composed principally of cassiterite 

in the form of nuggets as much as 3 inches in 

diameter. The amount, size, and composition 

of the nuggets all suggest that an economic 

placer operation could be established on Humboldt Creek provided the cassiterite is recovered during placer-gold operations. Moreover, a nearby lode source of cassiterite must 

exist. 

This brief report includes a new geologic 

map of the Serpentine-Kougarok area prepared 

by the authors. Earlier work includes geologic 

reconnaissance and mineral-deposit investigations by Collier, Hess, Smith, and Brooks 

( 1908), Smith ( 1908) , and Moxham and West 

(1953). Moxham and West also investigated 

radioactive deposits in the Serpentine Hot 

Springs area. The present work was done as 

part of the Heavy Metals program of the U.S. 

Geological Survey. 
Fig 1 

50 0 50 MILES 

FIGURE 1. Index map of Seward Peninsula, showing location of Serpentine-Kougarok area. 

GENERAL GEOLOGY 

The bedrock in the Humboldt Creek area 

(fig. 2) consists of two dissimilar groups of 

rocks. The older, of pre-Ordovician age, consists of slate, schist, and schistose limestone 

intruded by bosses of gabbro and related mafic 

rocks, in part altered to glaucophane-garnet 

rock. The younger group consists enti~rely of 

carbonate rocks of Paleozoic .age; its contact 

with the older rocks is everywhere thrust 

faulted. These faults form part of an extensive 

belt of thrust faults that extend from near the 

western tip of the Seward Peninsula to beyond 

the Humboldt Creek area. The belt is not yet 

completely delineated, but has been mapped in 

detail in the York Mountains some 70 miles 

west of the area of this report (Sainsbury, 

1965). 

2 

The bedrock near Humboldt Creek consists 

dominantly of slate, quartzitic slate, graphitic 

siltstone, and hornfels encircling a stock of 

biotite granite; to the east, calcareous schist 

and marble prevail. The granite crops out over 

a roughly oval area about 8 by 6 miles; it extends over a low divide into a creek which joins 

Humboldt Creek from the west below the 

placer cuts from which the coarse cassiterite 

was obtained. The granite is cdarse grained 

and is composed of quartz, orthoclase, and 

biotite; zircon, sphene, and allanite are common accessories (Moxham and West, 1953, p. 

8). Local variants include aplite, quartzmuscovite pegmatite, and biotite-rich facies. 

Neither these variants nor the granite itself 

has been studied in detail. Megascopically, the 

granite is similar to some of the other granites 
I pOg I 

Gabbro and alt ered maf· IC rocks · 

I pOs I 

Slate ' graywacke ' h ornfels 

L" I pOls I 

Imestone ' miCa-calcit . e schist . 

pOe 

pOsl 

S . pOe chist and l" 

pOe, chlorit. Imestone 

pOsl sch. u: schist ' UJtose limestone 

FIGURE 2. Reconnaissance geologic 

EXPLANATION 

TKg I 

Granite 

I Limestone Dl '}~ ~ and > 

dolomite ~ 

L" I Pzu I 

Imestone and miCa-cal . . . undifferent· t cite schist Ia ed ' 

)

(/) >- ::::>0::: 

Q<( 

w- ul-

<(0::: 

1-w wlo:::o::: 

uo 

u 

6 

N 

0 

w 

...J 

<( 

0.. 

~s 

Silicified r Imestone . copper . with mmerals 

Dashed whe Contact - re gradat' wnal or iriferred 

--?-?-

Dashed Fault where appro . queried whe m:nately located· re ~riferred ' 

T _._?_.._?...JSawteeth hrust fault on up-p_er plate; que . ~riferred ned where 

map of the 

northeast 

Serpentine-Kou corner of area. garok area. Humboldt Creek P 1 acer deposit is in 

3 
of the western Seward Peninsula with which 

lode or placer tin is associated (Knopf, 1908); 

the tin unquestionably is genetically related to 

the granite (Hosking, 1967; Sainsbury and 

Hamilton, 1967). 

RELATION OF GEOLOGIC STRUCTURE AND MINERALIZATION 

The structure of the area covered by figure 

2, although not shown in detail on the map, is 

similar to that found elsewhere on the Seward 

Peninsula (Sainsbury, 1965). After thrusting, 

granite stocks pierced the thrust plates, p·robably during Late Cretaceous or Tertiary time. 

Still later, several sets of faults, including a 

strong set of normal faul.ts striking about 

north to N. 15°-20° E., cut the thrust plates 

and, locally, the granites. Hydrothermal alte.ration took place along many of these northtrending faults, and most of the main placergold deposits are spatially related to them. 

In contrast to the alteration along the 

normal faults, during which gold (and tin?) 

was introduced, material brought in along the 

thrus~t faults consists of large masses of 

dolomite, sideritic carbonate, or silica wirbh 

but minor amounts of ore minerals, the type of 

material depending on the composition of the 

underlying rocks. Where carbonate rocks of the 

thrust plates overlie dolomitic limestone, barren dolomite has extensively replaced the upper 

carbonate rocks; where chloritic schist is overridden, masses of sideritic carbonate and minor 

amounts of quartz, with traces of gold, have 

migrated upward; where thrust plates ha:ve 

overridden slates, large amounts of quartz 

have replaced the carbonate rocks of the upper 

-plate, bringing anywhere f;rom a trace to considerable amounts of copper (as at the Ward 

Mountain copper prospects) without detectruble 

gold or tin. Hence, although some ore minerals accompanied the materials migrating 

upward from the underlying rocks during 

thrust faulting, most of the economically interesting deposits of gold and tin seem to be 

related to .a distinctly later period of mineralization which followed intrusion of granite and 

normal faulting. 

Significantly, several high-angle faults 

similar in orientation to those which controlled 

the gold deposits of the Kougarok River drainage cross Humboldt Creek or its tributaries 

4 

above the placer cuts from which the cassiterite was recovered. These faults were plotted 

from aerial photographs; they were not examined on the ground, but might be a source 

of the cassiterite. 

CASSITERITE ON HUMBOLDT CREEK 

Humboldt Creek (fig. 2) flows northeastward from the north peak of ·Midnight Mountain, and joins the Goodhope River some 18 

miles from the cassiterite-bearing placer cuts. 

The placer cuts that yielded the cassiterite 

begin some 3 miles from the headwaters of 

Humboldt Creek and extend about 2 miles 

downstream. According to one of the former 

miners (Harold Tweet, oral commun., 1967), 

the cassiterite, not then identified, was so 

plentiful that during placer mining it clogged 

the sluice riffles in a few hours and very 

serious,ly hampered the recovery of gold. This 

problem, in conjunction with a royalty, rendered the operation unprofitable. A dredge 

operation is now being considered and would 

very likely be profitable if both cassiterite and 

gold were recovered. 

Figure 3 is a photograph of some cassiterite 

nuggets sel·ected from a can of concentrate 

taken randomly from one of the many barrels 

of stripped concentrate found at the goldplacer cuts. ("Stripped concentrate" is one 

from which all gold has been removed except 

that which could be recovered by grinding in 

an amalgamation barrel.) The nuggets are angular to subrounded; many are intergrown 

with vein quartz. Nuggets as much as 4 inches 

across were seen in the barrels; however, the 

sand fraction also· contains abundant cassiterite, with a few visible grains of bright yellow 

gold. At least three distinctly different types 

of cassiterite occur in the nuggets. Most nuggets larger than half an inch in diameter consist of brown brecciated cassiterite intergrown 

with broken quartz, some consist of very dark 

brown cassiterite with a limonitic-brown 

streak , and a few consist of black crystalline 

cassiterite, still exhibiting crystal faces, intergrown with quartz. The brecciated cassiterite 

apparently is very unusual, as it has not been 

observed elsewhere by Sainsbury in the tin 

deposits of the Seward Peninsula. 
FIGURE 3. Cassiterite nuggets from Humboldt Creek. Shiny nuggets at lower left are coated with tin by the 

action of nascent hydrogen in the laboratory. Nuggets as much as 4 inches in diameter were found in the 

placer concentrates. Photograph by E. P. Krier. 

5 
In addition to cassiterite, the finer grained 

concentrates contain numerous tabular striated 

crystals of black hematite, noticeable sulfide 

minerals (principally pyrite), bits of bleached 

slate and schist, and unidentified mineral 

grains, together with rounded gold nuggets 

heavily coated with iron oxides. Some cassiterite occurs as euhedral doubly terminated 

pyramids as much as a quarter of an inch in 

diameter. To check for other elements often 

associated with tin deposits-notably tantalum 

and niobium-semiquantitative spectrographic 

analyses were made of a cassiterite nugget and 

of a sample of tin-bearing s;tripped concentrate. The results are given in table 1. As 

elsewhere in the Seward Peninsula, tantalum 

is .absent, and niobium is but a minor constituent, in contrast to some other placer tin 

deposits of the world (Nigeria, for example) 

that contain economic amounts of columbite 

and tantalite. 

A random grab sample of bulk concentrate 

was analyzed by Claude Huffman, Jr., using a 

wet chemical method. The sample was found 

to contain slightly more than 60 percent tin, 

and thus meets the requirements for a highgrade saleable tin concentrate. 

TABLE !.-Semiquantitative spectrographic analyses of a cassiterite nugget and of a placer concentrate from 

Humboldt Creek 

[A indicates absent or below detection limit for that element; M, major <>10 percent). Detection limits are given only for elements listed as A. Quantities of Si through Ti are in percent, all others are in parts per million. Analysts: J. L. Finlay (nugget), R. H. Heidel 

(concentrate)] 

Element 

Field No. __________________________ _ 

Lab. No. ----------------------------

Si ------------------------------ AI ---------------------------- Fe -----------------------------· Mg ----------------------------- Ca -----------------------------· N a ----------------------------- Ti ------------------------------ Mn ----------------------------- Ag --------- -------------------- B ---- -------------------------- Ba -----------------------------· Co ------------------------------ Cr -------------------------------

Cu -----------------------------· Ga ____ -----------------·--------· 

Nb ----------------------------- Ni -·----------------------------· 

Pb ----------------------------- Sc ------------------------------

Sn -----------------------------· Sr ------------------------------ V ------------------------------ y ------------------------------

Yb ----------------------------- Zr --------------------------- __ _ 

CONCLUSIONS 

Cassiterite 

nugget 

67-ASn652 

ACB 293 

0.3 

.15 

5.0 

.005 

.02 

A 

100 

A 

A 

10 

7 

5 

15 

.07 

5 

50 

20 

50 

20 

M 

A 

100 

20 

5 200 

Cassiterite accompanies gold in placers in 

the Serpentine Hot Springs area. In at least 

one creek (Humboldt Creek), cassiterite is 

present in large amounts. In the past, the 

6 

Tin-bearing 

stripped 

concentrate 

67-AS.n492 

D129728 

7.0 

.5 10.0 .1 

.15 

.3 

.5 500 1 

50 

70 

50 

30 

7 

A 

A 

50 

300 

A 

M 

10 

150 

20 

3 

30 

Detection 

limit 

0.05 

1 

20 

5 

10 

5 

5 

5 

cassiterite hindered the recovery of gold, but 

if, in future placer operations, the cassiterite 

could be recovered and sold, the economic potential would be considerably enhanced, and 

marginal gold ground might become profitable 

to mine. 
REFERENCES CITED 

Collier, A. J., Hess, F. L., Smith, P. S., and 

Brooks, A. H., 1908, The gold placers of 

parts of the Seward Peninsula, Alaska, 

including the Nome, Council, Kougarok, 

Port Clarence, and Goodhope precincts: 

U.S. Geol. Survey Bull. 328, 343 p. 

Hosking, K. F. G., 1967, The relationship between primary tin deposits and granitic 

rocks: Internat. Tin Council, First Tech. 

Conf. on Tin, London, Mar. 14-17, 1967, 

36p. 

Knopf, Adolph, 1908, Geology of the Seward 

Peninsula tin deposits, Alaska: U.S. Geol. 

Survey Bull. 358, 71 p. 

7 

Moxham, .R. M., and West, W. S., 1953, Radioactivity investigations in the SerpentineKougarok area, Seward Peninsula, Alaska, 

1946: U.S. Geol. Survey Circ. 265, 11 p. 

Sainsbury, C. L., 1965, Geology and ore deposits of the central York Mountains, 

western Seward Peninsula, Alaska: U.S. 

Geol. Survey open-file report, Aug. 23, 

1965, 150 p. 

Sainsbury, C. L. and Hamilton, J. C., 1967, 

Geology of lode tin deposits: Internat. 

Tin Council, First Tech. Conf. on Tin, 

London, Mar. 14-17, 1967,33 p. 

Smith, P. S., 1908, Investigations of the mineral deposits of Seward Peninsula: U.S. 

Geol. Survey Bull. 345-E, p. 206-250.