Cassiterite in gold placers at Humboldt Creek, Serpentine-Kougarok area, Seward Peninsula, Alaska
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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 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 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 Introduction General geology Relation of geologic structure and mineralization Cassiterite on Humboldt Creek Conclusions References cited ILLlJSTRATIONS Page FIGURE 1. Index map of Seward Peninsula, showing location of Serpentine-Kougarok area 2. Reconnaissance geologic map of Serpentine-Kougarok area 3. Photograph of cassiterite nuggets from Humboldt Creek, Serpentine-Kougarok area TABLE Page TABLE 1. Semiquantitative spectographic analyses of a cassiterite nugget and of a placer concentrate, Humboldt Creek
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 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 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). 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 coarse 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 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 Dl
Limestone and
dolomite
L" Pzu Imestone and miCa-cal . indifferent· t cite schist Ia ed ' ) Q<( wul- 1-w wlo:::o::: uo u N 0 w
Silicified r Imestone copper with mmerals Dashed whe Contact re gradat' wnal or iriferred Dashed Fault where appro . queried whe m:nately located· re ~referred ' T ?...JSawteeth trust fault on up-p_er plate; que . ~referred ned where map of the northeast Serpentine-Kou corner of area. garok area. Humboldt Creek 1 P acer deposit is in
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 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.
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 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 CONCLUSIONS Cassiterite nugget 67-ASn652 ACB 293 A A A M A 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 Tin-bearing stripped concentrate 67-AS.n492 D129728 A A A M Detection limit 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. 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.