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��� U.S. Department of the Interior Bureau of Land Management BLM Alaska Open File Report 103 BLM/AK/ST-06/001+3090+942 November 2005 An Investigation of the Geology and Gold Mineralization in the Nyac District, Southwest Alaska Zachary John Wenz Alaska U.S. DEPARTMENT OF THE INTERIOR BUREAU OF LAND MANAGEMMENT Mission Statement The Bureau of Land Management (BLM) sustains the health, diversity and productivity of the public lands for the use and enjoyment of present and future generations. Author Zachary John Wenz This report is the outcome of a thesis completed by the author in partial fulfillment of the requirements for a Masters of Science degree at the University of Alaska, Fairbanks. The project was supported in part by the BLM as part of the Aniak Mining District study. Cover Geologists Jeff Foley (left) and Zach Wenz examine a gold-bearing sericite-altered quartz-calcite vein zone on Bonanza Ridge, NYAC District, Southwest, Alaska (photo by Nick Enos). Open File Reports Open file reports present the results of inventories or other investigations published outside the formal BLM-Alaska technical publication series. These reports can include preliminary or incomplete data and are not published and distributed in quantity. Reports are available while supplies last from BLM External Affairs, 222 West 7th Avenue, #13, Anchorage, Alaska 99513 (907) 271-5555 and from the Juneau Minerals Information Center, 100 Savikko Road, Mayflower Island, Douglas, AK 99824, (907) 364-1553. Copies are also available for inspection at the Alaska Resource Library and Information Service (Anchorage), the United States Department of the Interior Resources Library in Washington D.C., various libraries of the University of Alaska, the BLM National Business Center Library (Denver), and other selected locations. A complete bibliography of all BLM-Alaska scientific reports can be found on the Internet at: http://www.ak.blm.gov/affairs/sci_rpts.html Related publications are also listed at: http://juneau.ak.blm.gov. An Investigation of the Geology and Gold Mineralization in the Nyac District, Southwest Alaska By Zachary John Wenz BLM Open File Report 103 November 2005 U.S. Department of the Interior Bureau of Land Management Alaska State Office 222 W. 7th Ave., #13 Anchorage AK 99513 i Abstract The Nyac district, southwest Alaska, contains multiple felsic to mafic plutons and dikes intruding the volcano-sedimentary package of the Nyac terrane. Dated plutons and dikes in the Nyac terrane record Early Cretaceous ages; other plutons in southwest Alaska are Late Cretaceous and Tertiary. The Nyac district contains high-temperature and low-temperature gold mineralization. The age of high-temperature and low-temperature mineralization is concordant with plutonism. The high-temperature mineralization occurs in the Bonanza pluton and associated gray granodiorite porphyry dikes. Bonanza pluton mineralization consists of gold-bearing quartz veins with pyrite-chalcopyrite-magnetite-bismuthinite- molybdenite. Fluid inclusions from mineralized quartz veins record trapping temperatures up to 560o C and salinities up to 60 wt% NaCl. Mineralization in the grey granodiorite porphyry dikes consists of gold- bearing quartz veins with tellurobismuthite-tetradymite-chalcopyrite. Fluid inclusions from mineralized o quartz veins record trapping temperatures up to 370 C, salinities up to 5 wt% NaCl, and CO2 concentrations up to 80 vol%. Both types display sericite-chlorite-albite alteration, and the Bonanza Pluton also displays potassic alteration. The low-temperature mineralization occurs at the Bonanza Creek Color Anomaly and along high-angle faults. This type contains anomalous Au and Hg. The alteration assemblage is sericite-kaolinite, indicating low temperature (<240o C) and acidic fluids. ii TABLE OF CONTENTS Abstract ............................................................................................................................................................i Table of Contents ............................................................................................................................................ii List of Figures .................................................................................................................................................v List of Tables.................................................................................................................................................vii List of Appendices ........................................................................................................................................vii Chapter 1: Introduction................................................................................................................................1 1.1 Regional Geology.....................................................................................................................................1 1.2 Previous Exploration................................................................................................................................4 1.3 Purpose of Study ......................................................................................................................................6 1.4 Methods....................................................................................................................................................6 1.4.1 Chemical Analysis ..........................................................................................................................6 1.4.2 Petrography .....................................................................................................................................7 1.4.3 Magnetic Susceptibilities ................................................................................................................7 1.4.4 Feldspar Staining.............................................................................................................................8 1.4.5 Microprobe Analysis.......................................................................................................................8 1.4.6 Fluid Inclusion Microthermometry .................................................................................................8 1.4.7 Radiometric 40Ar/39Ar Dating..........................................................................................................8 Chapter 2: Geology and Geophysics..........................................................................................................10 2.1 Jurassic Volcano-Sedimentary Rocks .....................................................................................................10 2.1.1Metavolcanic Rocks................................................................................................................15 2.1.2 Metasedimentary Rocks.........................................................................................................18 2.2 Introduction to Plutonic Rocks................................................................................................................20 2.3 Jurassic (?) Plutonic Rocks......................................................................................................................20 2.3.1 Rex Creek Pluton ...................................................................................................................21 2.3.2 Altered Granodiorite ..............................................................................................................26 2.3.3 Diorite Hornfels .....................................................................................................................29 2.3.4 Gabbro....................................................................................................................................29 2.4 Early Cretaceous Plutonic Rocks ............................................................................................................29 2.4.1 Nyac Batholith .......................................................................................................................30 2.4.2 Bonanza Pluton ......................................................................................................................32 2.4.3 Sawpit Pluton .........................................................................................................................33 2.4.4 Gray Dikes .............................................................................................................................33 iii 2.4.5 Red Dikes...............................................................................................................................34 2.5 Tertiary (?) Dikes ....................................................................................................................................34 2.5.1 Mafic Dikes............................................................................................................................34 2.5.2 Pink Dikes..............................................................................................................................35 2.5.3 Undifferentiated Dikes...........................................................................................................35 2.6 Regional Geophysics and Structural Geology.........................................................................................35 Chapter 3: Gold Mineralization.................................................................................................................41
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