Geology of the Skagway B-3 and B-4 Quadrangles, Southeastern Alaska GEOLOGICAL SURVEY PROFESSIONAL PAPER 832 Geology of the Skagway B-3 and B-4 Quadrangles, Southeastern Alaska By E. M. MAcKEVETT, JR., E. C. ROBERTSON, and G. R. WINKLER GEOLOGICAL SURVEY PROFESSIONAL PAPER 832 UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1974 UNITED STATES DEPARTMENT OF THE INTERIOR ROGERS C. B. MORTON, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress Catalog-card No. 74-600017 For sale by the Superintendent of Documents, U.S. Government Printing Office ,,Vashington, D.C. 20402 - Price $2.05 (paper cover) Stock No. 2401-02472 CONTENTS Page Page Abstract 1 Geology-Continued Introduction 1 Rocks east of the Chilkat River-Continued Previous investigations -----------------------------------------------------­ 2 Pyroxenite -------------------------------------------------------------------- 15 Present investigations -------------------------------------------------------­ 2 Quartz diorite -------------------------------------------------------------- 15 Acknowledgments -------------------------------------------------------------- 3 Leucogranodiorite ------------------------------------------------------ 16 Geology ---------------------------------------------------------------------------------------- 3 Dikes ---------------------------------------------------------------------------- 16 General geologic setting ---------------------------------------------------- 3 Origin, age, and correlation-------------------------------------- 16 Rocks west of the Chilkat River _______________________________________ _ 3 Surficial deposits -------------------------·--------------·----------------------- 17 Metamorphic rocks ---------------------------------------------------- 3 Structure ------------------------------------------------------------------------------ 19 Gneiss -------------------------------------------------------------------­ 4 Faults ---------------------------------------------------------------------------- 19 Marble ------------------------------------------------------------------ 4 Other structural features____________________________________________ 20 Amphibolite -------------------------------------------------------- 6 Speculations and implications ---------------------------------- 20 Schist and phyllite ---------------------------------------------- 6 Economic geology---------------------------------------------------------------- 20 Slate ---------------------------------------------------------------------- 7 Gold ------------------------------------------------------------------------------ 20 Undivided metamorphic rocks -------------------------­ 8 Placer deposits ---------------------------------------------------- 21 Age and correlation -------------------------------------------- 8 Lode deposits -----------------------------------------------------· 21 Intrusive rocks ------------------------------------------------------------ 9 Iron and titanium________________________________________________________ 24 Dioritic rocks ------------------------------------------------------ 9 Placer deposits .................................................... 24 Quartz diorite and granodiorite _______________________ _ 10 Lode deposits ...................................................... 25 Cretaceous quartz diorite and Barite, silver, and base metals.................................. 25 granodiorite ---------------------------------------------­ 10 Other commodities ...................................................... 29 Tertiary quartz diorite and Geochemical sampling ................................................ 30 granodiorite ---------------------------------------------- 10 Precious metals .................................................... 30 Dikes and sills______________________________________________________ 12 Base and related metals...................................... 30 Rocks east of the Chilkat River________________________________________ 13 Iron and ferroalloy metals ................................ 31 Metabasalt ------------------------------------------------------------------ 14 Miscellaneousmetals .......................................... 32 Gabbro and diorite------------------------------------------------------ 14 References cited .......................................................................... 33 ILLUSTRATIONS PLATE 1. Reconnaissance geologic map of the Skagway B-3 and B-4 quadrangles, Alaska.............................................. In pocket Page FIGURE 1. Index map showing location of Skagway B-3 and B-4 quadrangles ···········································--------------------------------- 2 2. Photograph of the environs of Porcupine Creek and terrain bordering the Klehini River........ -------------------------------· 2 3-8. Photographs: 3. Sawn slabs of marble showing organic remnants, probably stromatoporoids.................................................. 6 4. Rocks of the strongly foliated schist and phyllite unit exposed in roadcut west of Muncaster Creek._____ 7 5. View of Jarvis Glacier showing extensive outwash near junction of Klehini River and Jarvis Creek and moraines associated with Jarvis Glacier....................................................................................................... 18 6. Stream-cut bank in the Klukwan fan........................................................................................................................ 18 7. Remnants of the town of Porcupine, the main placer gold camp in the study area........................................ 21 8. Altered zone at main barite-rich prospect................................................................................................................ 26 9. Geologic sketch map showing sample locations at main barite-rich deposit north of Glacier Creek.......................... 27 10. Photomicrographs of barite-rich samples from main deposit............................................................................................ 28 11. Photograph of altered zone exposed in roadcut west of Muncaster Creek.................................................................... 32 III IV CONTENTS TABLES Page TABLE 1. Semiquantitative spectrographic analyses of the metamorphic rocks.... -------------------------------------------------------------------------· 5 2. Chemical analyses of the marble·-----------------------------------------------------------------------------------------------------------------·····-···-·-·············· 6 3. Semiquantitative spectrographic analyses of the intrusive rocks.......... -------------------------------------------------------------------------- 11 4. Potassium-argon age determinations from intrusive rocks west of the Chilkat River________________________________________________ 12 5. Estimated modes of the metabasalt............................ ----------------------··----···--··-----·--···--·-------·---··----------·---···-·---··---··-··-···-········· 14 6. Estimated modes of the gabbro and diorite and quartz diorite.......... ------------------------------··---------------------------------------------· 15 7. Estimated gold productionfrom the Porcupine placer district, 1898-1955.... ------------------·--------------------------------------------- 21 8. Semiquantitative spectrographic analyses of veins, lodes, and altered zones________________________________________________________________ 22 9. Gravimetric analyses for barium·-------------------------------------·---------------·-----··----···---··-·-··--····--·---······-------------------------------············ 29 GEOLOGY OF THE SKAGWAY B-3 AND B-4 QUADRANGLES, SOUTHEASTERN ALASKA By E. M. MACKEVETT, JR., E. C. ROBERTSON, and G. R. WINKLER ABSTRACT is the major access artery. This highway links Haines, The Skagway B-3 and B-4 quadrangles are near the northern a port on the Alaska Ferry System that also is served extremity of southeastern Alaska in a mountainous region by a commercial airline, with the Alaska Highway. The characterized by abundant glacier-related erosional and deposi­ Dalton Trail of Klondike gold-rush lore formerly tional features. The Chilkat River fault, a segment of the extended across the quadrangles following the south­ Chatham Strait fault which is a major tectonic element in southeastern Alaska, underlies the markedly linear Chilkat west sides of the Chilkat and Klehini Rivers; only River valley and separates the quadrangles into two distinctive vestiges of the trail are preserved. Additional access geologic terranes. The terrane east of the fault is dominated by is provided by local logging roads and by a few land­ igneous rocks of Cretaceous and early Tertiary age. These rocks ing sites suitable for small fixed-wing aircraft, but consist of an older mafic and ultramafic assemblage that most parts of the quadrangles are accessible only by includes metamorphosed lavas, gabbro and diorite, and pyroxe­ nite and the younger quartz diorite and related rocks of the helicopter or by foot. The latter is a tedious method Coast Range batholith. Structural and topographic trends east of travel because of rugged topography, dense brush, of the fault have a markedly linear northwestward orientation, and swift-flowing streams that are dangerous to cross. which is typical of large parts of southeastern Alaska. The small population of the two quadrangles is The terrane west of the fault is characterized by Paleozoic localized at Klukwan, an Indian village