Reconnaissance for Radioactive Deposits in the Darby Mountains, Seward Peninsula, Alaska, 1948

UNITED STATES DEPARTMENT OF THE INTERIOR Douglas McKay, Secretary GEOLOGICAL SURVEY W. E. Wrather, Director GEOLOGICAL SURVEY CIRCULAR 300 RECONNAISSANCE FOR RADIOACTIVE DEPOSITS IN THE DARBY MOUNTAINS, SEWARD PENINSULA, ALASKA, 1948 By Walter S. West This report concerns work done on behalf of the U. S. Atomic Energy Commission and is publhhed with the psnnission of the Commission. Washington. D. C., 1963 Frec on application to the Geolo~icalSurvey, Washington 26, D. C. RECONNAISSANCE FOR RADIOACTIVE DEPOSITS IN THE DARBY MOUNTAINS, SEWARD PENINSULA, ALASKA, 1948 CONTENTS Page Page Abstract ..................................... 1 Geology-Continued Introduction .................................. 1 Mineral deposits.. ...................... 3 Geology ..................................... 2 Radioactivity investigations. .................. 3 Undivided metamorphic rocks. ............ 2 Kachauik Creek-Cheenik Rocks of Paleozoic age.. ................. 2 Creekarea ........................... 4 Greenstone and greenstone schist ......... 2 McKinley Creek-Bryan Creek area ....... 4 Undivided igneous complex ............... 2 Portage Creek-Cape Darby area.. ......... 4 Granite ................................. 2 Norton Bay coastal area between Ungalik conglomerate. ................... 2 Cape Darby and Kwiniuk River ......... 5 Basalt .................................. 3 Kwiniuk River. ......................... 5 Unconsolidated deposits.. ................ 3 'Tubutulik River area.. ................... 6 Hot springs ............................. 3 Summary and conclusions.. ................... 7 Veins .................................. 3 Literature cited .......... ., ................. 7 ILLUSTRATION Page Pla.te 1. Geologic map of the southern and eastern parts of the Darby Mountains, Seward Peninsula, Alaska ...........................................................In pocket TABLES Page Table 1. ' Mineralogic composition of selected radioactive heavy-mineral subfractions (specific gravity greater than 3.3) of concentrates from the Portage Creek-Ca.pe Darby area.. ................. 5 2. Mineralogic composition uf heavy-mineral subfractions (specific gravity greater than 3.3) of concentrates from the Tubutulik River area ............................................. 7 ABSTRACT from the Clear Creek area and on Golovnin Bay is due chiefly to an unidentified ura.nium-titanium niobate, Radioactivity in the southern and eastern parts whereas the higher radioactivity at the headwaters of of the Darby Mountains, Seward Peninsula, Alaska, the Kwiniuk River is due to thorianite. appears to be directly related to the occurrence of granite. Concentrates from placers derived from areas containing granite are more radioactive than concen- IN'I'RODUCTION trates from placers not derived from the granite and, generally, contain from 0.01 to 0.05 percent equivalent 'The Darby Mountains are situated in the south- uranium. The radioactivity of these concentrates is eastern part of the Seward Peninsula, Alaska (pl. 1). largely due to radioactive elements in common acces- The range has a general north-south trend and extends sory minerals in granite, such as sphene, allanite, from Cape Darby on the south to the southeastern zircon, and, locally, monazite. Locally, in lhe Clear portion of the Bendeleben Mountains on the north, a Creek-Vulcan Creek area, the headwaters of the distance of approximately 75 miles. The area is Kwiniuk River, and on Golovnin Bay near McKinley bounded on the west by Golovnin Lagoon, Golovnin Bay, Creek, concentrates from placers derived frorn granitic and the Fish River and in general on the east by Norton terrain contain as much as 0.1 percent equivalent Ray and the Tubutulik Rlver, although the upper half of uranium. The higher radioactivity of the concentrates the latter river dissects the eastern flankof the mountains. The area is accessible from Norne, approxima.tely and possibly pre-Cambrian age (pl. 1). These rocks 70 miles to the west, by a.ir to two sma.11 airstrips, one consist of quartzose, ca.lcareous, biotitic, chloritic, located at Golovin on the western side of the Darby feldspathic, and graphitic schist and thin limestones. Peninsula and the other at White Mountain on the Fish The schist has been cut by greenstone, granite, diorite, River (outside of area shown on pl. 1) and the Moses a.nd other igneous rocks which have had a share in the Point Civil Aeronautics Authority a.irfield near the metamorphism and obliteration of the original char- mouth of the Kwiniuk River, 46 miles northeast of a.cters of the schist. Golovin. There are no good harbors along the entire coast. Transportation of supplies within the area must be accomplished by tractor or in part by small boats, Rocks of Paleozoic age a.s there are no roads. Carboniferous, Devonian, and Silurian rocks are The Darby Mountains a.re a relatively low range best represented along the eastern flank of the Darby with the highest peaks seldom rising to more than Mountains and are well exposed along the sea coast 3,000 feet above sea level. In the heart of the range, from the mouth of Iron Creek southwest nearly to glaciation has produced cirques and steep-sided, Carson Creek. This group of rocks, composed mainly U-shaped valleys. Some streams ha.ve cut deep valleys. of limestone with subordinate dolomite, black slate, Wave action along the south coast of the Darby Peninsula and schist, is complexly folded, faulted, metamorphosed, has produced a, rugged coast with high cliffs and a beach and cut by intrusives of greenstone, granite, and diorite. so narrow that it is impossible to walk along the shore in many places. Greenstone and greenstone schist The greater part of the Darby Mountains district is ma.de up of well-rounded tundra- or vegetation-covered The post-Carboniferous greenstone and 'greenstone hills a.nd mountains, Rock pinnacles, talus, and rock schist, which occur along the eastern flank of the Darby fragments occur on a.nd near the tops of ma.ny moun- Mountains, were not observed in place, although green- tains, but bedrock outcrops are surprisingly scarce. stone cobbles and pebbles were seen on the bars a.long the lower course of the Tubutulik River. Previous investigations (Gault, Killeen, West, and others, 1953) in the northeastern part of the Seward Peninsula to the northeast of the Darby Mountains re- Undivided igneous complex vealed the presence of uranothorianite and several other less radioactive minerals widely distributed, but An undivided igneous complex and the granite locally concentrated, in areas underlain by felsic make up the central core of the Darby Mountains. The igneo. s rocks. The purpose of the 1948 reconnaissance undivided igneous complex is believed to belong to one in the Darby Mountains was to determine whether sim- geologic body of pre-Cretaceous age. It is composed ilar radioactive minerals also occur in the felsic igneous mainly of granite, which has undergone varying degrees rocks of this district, and, if present, attempt to dis- of metamorphism, but also includes some diorite, cover any sites of high concentration and the bedrock greenstone, and several other types of igneous rocks source of such minerals. The reconnaissance was whose characteristics are not sufficiently clear to allow conducted during the summer of 1948 by a field party their identification and subd'ivision. consisting of W. S. West, geologist, and A. E. Nessett, J. J. Otoyuk, and I. M. Ikleasik, Camp assistants. Time permitted only the investigation of the southern Granite and eastern parts of the district. The route followed is indicated by the location of camps, numbered in The granite, also pre-Cretaceous in age, chronologic order starting at Golovin, on plate 1. ranges from even-grained to porphyritic in texture and exhibits considerable variation in mineralogical composition. In the area studied the porphyritic granite is much more abundant than the even- grained. Outcrops usually take the form of knobs The only work of the Geological Survey in the a.nd pinnacles on the tops and sides of the moun- Darby Mountains prior to this investigation was done tains. Inclusions of diorite in the porphyritic by two parties; that of Mendenhall (1901)1, who in 1900 granite noted especially along the east coa.st of mapped both topographically and geologically the Koyuk the Darby Peninsula indicate tha.t the granite is River valley and parts of the Darby Mountains on an younger tha.n some of the diorites in the undivided exploration-reconnaissa.ncescale; and that of Smith igneous complex. and Eakin (1911), who in 1909 mapped the area between Nula.to and Council on a similar scale. Ungalik conglomerate The geologic fea.tures of the Darby Mountains district as described by Mendenhall (1901) and Smith The Ungalik conglomerate, the oldest formation and. Eakin (l911), are discussed briefly below. of the Cretaceous sedimentary sequence in west-central Alaska, is made up of rounded debris derived from older formations including limestone, schist, igneous Undivided metamorphic rocks rocks, quartz veins, and sandstone. It forms the first main divide east of the Tubutulik River on the extreme The oldest rocks in the district, which occur eastern flank of the Warby Mountains. One outcrop of mainly a.long the western flanks of the Darby Mountains, this conglomerate was noted along the bank of the lower a.re folded a.nd shea.red metamorphic rocks,of pre-Silurian Tubutulik River. Basalt headwater fork of the left branch of Grouse Creek, a tributary to the Tubutulik River.

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