GEOLOGY AmNIIXrnRAIL MINING DISTRICT, IDITAROD B-4 AND EASTERN B-5 QUADRANGLES, SOUTHWESTERN ALASKA By Thomas K. Bundtzen, Marti L. Miller, Gregory M. Laird, and Katherine F. Bull Professional Report 97 Prepared in cooperation with U.S, Geological Survey and Doyon Limited Published by STATE OF ALASKA DEPARTMENT OF NATURAL RESOURCES DIVISION OF GEOLOGICAL & GEOPHYSICAL SURVEYS Alaska Department of NATURAL Spring 1992 RESOURCES GEOLOGY AND MINERAL RESOURCES OF IDITAROD MINING DISTRICT, IDITAROD IB-4 AND EASTERN B-5 QUADRANGLES, SOUTHWESTERN ALASKA By Thomas K. Bundtzen, Marti L. Miller, Gregory M. Laird, and Katherine F. Bull Prepared in cooperation with U.S. Geological Survey and Doyon Limited Dbcovery Camp, Otter Creek Valley, circa 1913. Photo courtesy ofilorothy Loftus Collection, University ofAlaska Fairbanks Archives. Cover: Beaton and Donnelly (left) and Riley Investment Company (light) Professional Report 97 dredges in operation on adjoining claims near the mouth ofBlack Creek Division of Geological & during the 1916 season. The two companies could not initially re,rolve a dkpure concerning the lacation of a claim boundaiy. At onepoitlt the Geophysical Surveys bucket lines were only 6fr apart. The Beaton and Donnelly D~edge beganmining up Black Creekluter in theseason. Photocourtesy of John Fairbanks, Alaska Mbcovich. Spring 1992 Richard E. Fullerton was born on May 17, 1921, in the Iditarod-Flat district and passed away in Anchorage July 29, 1990 after a brief battle with cancer. Both he and his brother John placer mined for a half century in southwest Alaska. Like many family mining enterprises, the Fullterton operation began small. They started on Flat Creek with limited mechanized equipment consisting of second-hand tractors and simple sluicing devices. METRIC CONVERSION FACTORS Factors for converting U.S. customary units to international metric units are as follows: To convert from to multiply by Mass Ounce, troy (oz tr) kilogram (kg) Ounce, avoirdupois (oz avdp) kilogram (kg) Pound, avoirdupois (Ib) kilogram (kg) Ton, short (2,000 Ib) tonne (mg) Tonne (mg) ton (2,000 Ib) Length Foot (ft) meter (m) Mile (mi) kilometer (km) Area Mile2(mi2) Acre Volume Yard3 (yd') meter3 (m3) Gallon liter STATE OF ALASKA Walter J. Hickel, Governor DEPARTMENT OF NATURAL RESOURCES Harold C. Heinze, Commissioner DIVISION OF GEOLOGICAL & GEOPHYSICAL SURVEYS Thomas E. Smith, Director and State Geologist Address mail orders to the Fairbanks office. DGGS publications may be inspected at the following locations. Alaska Division of Geological & Geophysical Surveys 794 University Avenue, Suite 200 Fairbanks, Alaska 99709-3645 U.S. Geological Survey Earth Science Information Center Grace Hall, Alaska Pacific University Campus 4230 University Drive, Room 101 Anchorage, Alaska 99508-4664 This publication, released by the Division of Geological & Geophysical Surveys, was produced and printed in Fairbanks, Alaska, at a cost of $10 per copy. Publication is required by Alaska Statute 41, "to determine the potential of Alaskan land for production of metals, minerals, fuels, and geothermal resources; the location and supplies of groundwater and construction materials; the potential geologic hazards to buildmgs, roads, bridges, and other installations and structures; and shall conduct such other surveys and investigations as will advance knowledge of the geology of Alaska." CONTENTS Page Abstract ............................................................................................ Introdtrction and geography ................................................................... Bedrock geology .................................................................................. Introduction .................................................................................. Innoko Terrane .............................................................................. Kuskokwim Group .......................................................................... Volcanic and plutonic rocks ............................................................... Volcanic rocks ............................................................................ Plutonic rocks ............................................................................ Dikes and small sills ..................................................................... Petrogenesis and age ........................................................................ Hornfels ...................................................................................... Late Tertiary(?)-Quaternarygeology ........................................................ Geologic units ............................................................................... Colluvial deposits ........................................................................ Alluvial deposits ......................................................................... Gwmorphology ............................................................................. Structural geology ................................................................................ Economic geology ................................................................................ Introduction .................................................................................. Mining history ............................................................................... Placer mining methods ..................................................................... Lode deposits ................................................................................ Introduction ............................................................................... Golden Horn deposit .................................................................... Glenn Gulch or Mohawk lode ......................................................... Minnie Gulch mineralized area ........................................................ Chicken Mountain lodes ................................................................ Paragenesis of lode deposits ............................................................ Prospects north of Flat .................................................................. Other prospects ........................................................................... Mineralized dikes ........................................................................ Coal ........................................................................................... Placer deposits ............................................................................... Chicken Mountain placers .............................................................. Placer deposits of Otter Creek drainage .............................................. Classification of lode and placer deposits ............................................... Acknowledgments ................................................................................ Bibliography ....................................................................................... ILLUSTRATIONS Figure 1 . Location map of Iditarod B-4 and eastern B-5 Quadrangles. Iditarod mining district. Alaska ....................................Facing page 1 2 . Schematic section of Cretaceous sedimentary rock units in Iditarod B-4 and eastern B-5 Quadrangles showing interfingering relationships of lithologic units ............................................. 3 3 . Photo of Ir~oceramussp . pelecypod recovered from bedrock cut in Ksh unit. Prince Creek drainage near sample site 84BT97 .................. 4 vii Photo micrograph of basaltic andesite from north of the summit of Chicken Mountain ........................................................... Photomicrograph of olivine alkali gabbro found on Chicken Mountain, Black Creek, Boulder Creek, and Swinging Dome .................... Photo of a xenolith of picrite, encased in monzodiorite along eastern edge of Chicken Mountain pluton ........................................ Plot of Chicken Mountain and Boulder Creek volcanic rocks, Iditarod district, on YO-SiO, diagram ............................................. Nonnative plot of plutonic and dike rocks from Iditarod district ......... AFM plot of Late Cretaceous-early Tertiary igneous rocks, Iditarod district .............................................................. Chondrite normalized logarithmic diagram of rare-earth elements in selected volcanic, plutonic, and dike samples, Iditarod mining district ......................................................................... Graphic showing classification of volcanic and plutonic rocks in Iditarod district using the alkali-lime index ............................. Photo of ice-rich palsa in eolian loess and sphagnum peat overlying auriferous paystreak on ancestral channel of Willow Creek .......... Photo of Otter and Black Creek valleys looking south-southwest Chicken Mountain in the left foreground ................................ Aerial photo of Ruby Creek area south of Iditarod-Nixon Fork fault showing anticline of "upper" sands plunging over Ks section; inferred thrust fault plane shown. ......................................... Photo with trace of Iditarod-Nixon Fork fault looking southwest down Bonanza Creek drainage through study area ........ Photo at city of Flat as seen in 1912, looking from north to south, with Flat Creek and Otter Creek ......................................... Photo of the city of Flat as seen in 1986 showing similar perspective to that depicted in figure 16 ..............................................