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Cassiterite in Gold Placers at Humboldt Creek Serpentine-Kougarok Area Seward Peninsula, Alaska

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Cassiterite in Gold Placers at Humboldt Creek Serpentine-Kougarok Area Seward Peninsula, Alaska 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 min- eralization ---------------------------- 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 were reported by Knopf (1908, p. 63) and Moxham and West (1953). In 1967, during a Large amounts of cassiterite accompany placer gold helicopter reconnaissance of the streams in in Humboldt Creek in the Serpentine-Kougarok area, the granite area west of Humboldt Creek, Seward Peninsula, Alaska. Cassiterite has also been Kachadoorian sampled concentrates present in reported from other placer-gold workings nearby. To date, no cassiterite has been recovered commercially; substantial tonnages at old placer-gold work­ chemical analyses of concentrates from Humboldt ings on Humboldt Creek; the concentrates Creek show a tin content of 60 percent--a commercially proved to be composed principally of cassiterite saleable product. Recovery of a saleable tin product in the form of nuggets as much as 3 inches in would increase the possibility for profitable operation diameter. The amount, size, and composition of the gold placers. of the nuggets all suggest that an economic The cassiterite is intergrown with vein quartz with­ out any noticeable sulfide minerals; the cassiterite placer operation could be established on Hum­ grains occur in size and quantity that suggest a boldt Creek provided the cassiterite is re­ nearby lode source worth the effort of a search to covered during placer-gold operations. More­ find it. over, 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 INTRODUCTION reconnaissance and mineral-deposit investiga­ tions by Collier, Hess, Smith, and Brooks ( 1908), Smith ( 1908) , and Moxham and West The occurrence of cassiterite in placer-gold (1953). Moxham and West also investigated concentrates from Humboldt Creek, in the radioactive deposits in the Serpentine Hot Serpentine-Kougarok area (fig. 1), was re­ Springs area. The present work was done as ported by Knopf in 1908; traces of cassiterite part of the Heavy Metals program of the U.S. in placer-gold deposits elsewhere in the region Geological Survey. 1 Fig 1 50 0 50 MILES FIGURE 1. Index map of Seward Peninsula, showing location of Serpentine-Kougarok area. GENERAL GEOLOGY The bedrock near Humboldt Creek consists The bedrock in the Humboldt Creek area dominantly of slate, quartzitic slate, graphitic (fig. 2) consists of two dissimilar groups of siltstone, and hornfels encircling a stock of rocks. The older, of pre-Ordovician age, con­ biotite granite; to the east, calcareous schist sists of slate, schist, and schistose limestone and marble prevail. The granite crops out over intruded by bosses of gabbro and related mafic a roughly oval area about 8 by 6 miles; it ex­ rocks, in part altered to glaucophane-garnet tends over a low divide into a creek which joins Humboldt Creek from the west below the rock. The younger group consists enti~rely of carbonate rocks of Paleozoic .age; its contact placer cuts from which the coarse cassiterite with the older rocks is everywhere thrust was obtained. The granite is cdarse grained faulted. These faults form part of an extensive and is composed of quartz, orthoclase, and belt of thrust faults that extend from near the biotite; zircon, sphene, and allanite are com­ western tip of the Seward Peninsula to beyond mon accessories (Moxham and West, 1953, p. the Humboldt Creek area. The belt is not yet 8). Local variants include aplite, quartz­ completely delineated, but has been mapped in muscovite pegmatite, and biotite-rich facies. detail in the York Mountains some 70 miles Neither these variants nor the granite itself west of the area of this report (Sainsbury, has been studied in detail. Megascopically, the 1965). granite is similar to some of the other granites 2 EXPLANATION (/) >- ~s ::::>0::: Q<( I pOg I w­ Silicified rImestone . TKg ul­ copper . with Gabbro and altered maf·IC rocks· I <(0::: mmerals Granite 1-w ) wl- I pOs I o:::o::: uo Dashed whe Contact - Slate' graywacke' h ornfels re gradat'wnal or iriferred I Dl --?-?- Limestone'}~ ~ u L" I pOls I and > Imestone ' miCa-calcit. e schist. 6 Dashed Fault dolomite ~ N queriedwhere appro whe m:nately . located· 0 re ' pOe w ~riferred ...J pOsl <( L" I Pzu I 0.. T _._?_.._?...J- S . pOe Imestone and miCa-cal. chist and l" . undifferent· t cite schist Sawteeth hrust fault pOe, chlorit. Imestone Ia ed ' on up-p_er plate; que . pOsl sch. u: schist ~riferred ned where ' UJtose limestone Creek P1 acer deposit is in FIGURE 2. Reconnaissance geologic map of thecorner Serpentine-Kou of area. garok area. Humboldt northeast 3 of the western Seward Peninsula with which above the placer cuts from which the cassit­ lode or placer tin is associated (Knopf, 1908); erite was recovered. These faults were plotted the tin unquestionably is genetically related to from aerial photographs; they were not ex­ the granite (Hosking, 1967; Sainsbury and amined on the ground, but might be a source Hamilton, 1967). of the cassiterite. RELATION OF GEOLOGIC STRUCTURE AND MINERALIZATION CASSITERITE ON HUMBOLDT CREEK The structure of the area covered by figure Humboldt Creek (fig. 2) flows northeast­ 2, although not shown in detail on the map, is ward from the north peak of ·Midnight Moun­ similar to that found elsewhere on the Seward tain, and joins the Goodhope River some 18 Peninsula (Sainsbury, 1965). After thrusting, miles from the cassiterite-bearing placer cuts. granite stocks pierced the thrust plates, p·rob­ The placer cuts that yielded the cassiterite ably during Late Cretaceous or Tertiary time. begin some 3 miles from the headwaters of Still later, several sets of faults, including a Humboldt Creek and extend about 2 miles strong set of normal faul.ts striking about downstream. According to one of the former north to N. 15°-20° E., cut the thrust plates miners (Harold Tweet, oral commun., 1967), and, locally, the granites. Hydrothermal alte.ra­ the cassiterite, not then identified, was so tion took place along many of these north­ plentiful that during placer mining it clogged trending faults, and most of the main placer­ the sluice riffles in a few hours and very gold deposits are spatially related to them. serious,ly hampered the recovery of gold. This In contrast to the alteration along the problem, in conjunction with a royalty, ren­ normal faults, during which gold (and tin?) dered the operation unprofitable. A dredge was introduced, material brought in along the operation is now being considered and would thrus~t faults consists of large masses of very likely be profitable if both cassiterite and dolomite, sideritic carbonate, or silica wirbh gold were recovered. but minor amounts of ore minerals, the type of Figure 3 is a photograph of some cassiterite material depending on the composition of the nuggets sel·ected from a can of concentrate underlying rocks. Where carbonate rocks of the taken randomly from one of the many barrels thrust plates overlie dolomitic limestone, bar­ of stripped concentrate found at the gold­ ren dolomite has extensively replaced the upper placer cuts. ("Stripped concentrate" is one carbonate rocks; where chloritic schist is over­ from which all gold has been removed except ridden, masses of sideritic carbonate and minor that which could be recovered by grinding in amounts of quartz, with traces of gold, have an amalgamation barrel.) The nuggets are an­ migrated upward; where thrust plates ha:ve gular to subrounded; many are intergrown overridden slates, large amounts of quartz with vein quartz. Nuggets as much as 4 inches have replaced the carbonate rocks of the upper across were seen in the barrels; however, the -plate, bringing anywhere f;rom a trace to con­ sand fraction also· contains abundant cassit­ siderable amounts of copper (as at the Ward erite, with a few visible grains of bright yellow Mountain copper prospects) without detectruble gold. At least three distinctly different types gold or tin. Hence, although some ore min­ of cassiterite occur in the nuggets. Most nug­ erals accompanied the materials migrating gets larger than half an inch in diameter con­ upward from the underlying rocks during sist of brown brecciated cassiterite intergrown thrust faulting, most of the economically in­ teresting deposits of gold and tin seem to be with broken quartz, some consist of very dark related to .a distinctly later period of minerali­ brown cassiterite with a limonitic-brown zation which followed intrusion of granite and streak , and a few consist of black crystalline normal faulting.
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