Isotopic Geochronology of the Leadville 1°X2° Quadrangle, West-Central Colorado­ Summary and Discussion U.S. GEOLOGICAL SURVEY BULLETIN 2104 Isotopic Geochronology of the Leadville 1°X2° Quadrangle, West-Central Colorado­ Summary and Discussion By ALAN R. WALLACE U.S. GEOLOGICAL SURVEY BULLETIN 2104 UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1995 U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY Gordon P. Eaton, Director For sale by U.S. Geological Survey, Information Services Box 25286, Federal Center Denver, CO 80225 Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government Library of Congress Cataloging-in-Publication Data Wallace, Alan R. Isotopic geochronology of the Leadville 1°X2° quadrangle, west-central Colorado : summary and discussion I by Alan R. Wallace. p. em.- (U.S. Geological Survey bulletin; 2104) Includes bibliographical references. Supt. of Docs. no.: I 19.3:2104 1. Radioactive dating-Colorado-Leadville Region. 2. Geology, Stratigraphic. 3. Geology-Colorado-Leadville Region. I. Title. II. Series. QE75.B9 no. 2104 [QE508] 557.3s-dc20 [551.7'009788] 94-31625 CIP CONTENTS Abstract................................................................................................................................. 1 Introduction . .. .. ... .. .. .. .. .. .. .. .. .. .. .. .. ... .. .. ... .. .. 1 Acknowledgments . .. .. .. .. ... .. .. .. ... .. .. .. .. .. .. .. .. ... .. .. .. 3 Geologic Summary............................................................................................................... 4 Proterozoic Geology..................................................................................................... 4 Metasedimentary and Metavolcanic Rocks.......................................................... 4 Intrusive Rocks..................................................................................................... 4 Metamorphism and Structure .. ... .... ... ........... ... ......... .. ......... ........... ......... .... 4 Paleozoic and Mesozoic Geology................................................................................. 4 Cenozoic Geology . .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. .. .. .. .. 4 Laramide Structure and Stratigraphy.................................................................... 4 Laramide and Oligocene Igneous Activity........................................................... 5 Late Cenozoic Structure and Sedimentation .. .. .................... .............. ........ .......... 5 Late Cenozoic Igneous Activity ... ....................... .......... ............. ... ....... ............ .. 5 Discussion of Isotopic Dates . .. .. .. .. .. .. .. ... .. .. .. .. .. .. .. .. .. ... .. 5 Proterozoic Dates.......................................................................................................... 5 Paleozoic and Mesozoic Dates . .. .. .. .. .. .. .. ... .. .. .. ... .. .. .. 6 Late Cretaceous and Cenozoic Dates .. .. .. .. .. .. .. .. .. ... ... ... .. 7 Igneous Systems . ........ ... ........... ........... ... ......... ..................... ............. ... ...... .. ....... 7 Mineral Deposits................................................................................................... 10 Tectonic Events.................................................................................................... 12 Undated Units and Events............................................................................................ 13 References Cited........................................................................................................... 16 Appendix A. Summary of isotopic geochronology and related information, Leadville 1°X2 ° quadrangle . .. ... .. .. .. .. .. ... .. .. .. .. .. .. .. .. .. ............. .. 20 Appendix B. Geochronologic data listed in order of increasing age................................. 34 Appendix C. Geochronologic data listed by unit name..................................................... 39 Appendix D. Geochronologic data listed by geochronologic method............................... 44 Appendix E. Geochronologic data listed by mining district .. .......... ............. .......... .. ... .... 49 PLATE [Plate is in pocket] 1. Map showing locations of isotopic geochronology samples, Leadville 1°X2 o quadrangle, Colorado. III IV CONTENTS FIGURES 1. Map showing major geographic features of the Leadville 1°X2° quadrangle, west-central Colorado........................ 2 2. Map showing major geologic features of the Leadville 1°X2° quadrangle.................................................................. 3 3. Plot of Cretaceous and Cenozoic apatite fission-track dates for rocks in the Leadville 1°X2° quadrangle................. 7 4. Plot of potassium-argon dates for Cretaceous and Cenozoic igneous rocks in the Leadville 1°X2° quadrangle......... 9 5. Map showing locations of major mining districts and mineral deposits in the Leadville 1°X2° quadrangle.............. 11 6. Plots of apatite and zircon fission-track data for the Gilman, Tennessee Pass, Aspen, and Leadville mining districts in the Leadville 1°X2° quadrangle.................................................................................................................. 14 TABLES 1. Summary of ages of major Late Cretaceous and Tertiary igneous rock units, Leadville 1°X2° quadrangle............... 8 2. Summary of ages of ore deposits, Leadville 1°X2° quadrangle................................................................................... 10 ISOTOPIC GEOCHRONOLOGY OF THE LEADVILLE 1°X2° QUADRANGLE, WEST-CENTRAL COLORADO-SUMMARY AND DISCUSSION By Alan R. Wallace ABSTRACT INTRODUCTION More than 360 isotopic dates have been obtained by The rocks and structures of the Leadville 1°X2° quad­ numerous workers from rocks in the Leadville 1°X2° quad­ rangle, in the mountains of west-central Colorado (figs. 1, 2), rangle in west-central Colorado. The dates provide important reflect a complex geologic history that spans the past 1.8 bil­ information regarding the age of emplacement of Protero­ lion years. The quadrangle also contains a rich endowment zoic, Late Cretaceous, and Tertiary igneous rocks. Some of of metallic mineral deposits. As a result, the geology and the dates on the Proterozoic rocks in the quadrangle provide mineral deposits of the quadrangle have been the focus of evidence regarding the actual age of Early and Middle Prot­ numerous geologic studies related to regional geologic erozoic intrusive activity, but most of the dates on those research and mineral exploration. rocks reflect resetting by Phanerozoic tectonic and thermal During the course of these and other investigations, events. Due largely to the absence of pre-Laramide Phaner­ many samples of a wide variety of rocks have been collected ozoic igneous activity, what little is known about the ages of and dated, using various geochronologic techniques, to aug­ tectonic events during that time is derived from dated strati­ ment the broader research. Through mid-1994, more than graphic correlations from outside the quadrangle. 360 separate dates have been reported from rocks from the The majority of the dates are Late Cretaceous and quadrangle. Other dates undoubtedly have been determined younger, reflecting extensive igneous, hydrothermal, and but not reported, and, although every effort was made to tectonic events during that time. Igneous activity spanned identify and include those dates in this compilation, some much of the Tertiary but was most intense during the Lara­ probably have been missed. The reported dates unfortunately mide (about 73-52 Ma), middle Tertiary (44-30 Ma), and are scattered throughout the literature, and only rarely has late Tertiary (28-1.6 Ma). In mineralized areas, such as the one data set been compared with others. Richard F. Marvin, world-class Climax, Gilman, and Leadville deposits, dates of the U.S. Geological Survey, periodically compiled and show that many mineral deposits in the quadrangle are of published dates that had been reported in the literature or had middle Tertiary age and are related to specific intrusive sys­ been determined in the geochronology facilities of the USGS tems. Fission-track dates on apatite and zircon have been (Marvin and others, 1974; Marvin and Cole, 1978; Marvin used to determine the timing of Tertiary uplift of major and Dobson, 1979; Marvin and others, 1989), but many other horsts such as the Sawatch Range. Dates on basalt flows dates were not included in these summaries. Also, many demonstrate major post-10-Ma tectonism and formation of potassium-argon dates reported before 1977 do not reflect a the Colorado River drainage system. change in isotopic constants (Steiger and Jager, 1977) and Many of the dates on rocks from the quadrangle reflect therefore are too young by as much as several percent. partial to complete resetting by one or more subsequent ther­ In an effort to enhance future studies in the quadrangle, mal events. Some of these dates, when incorporated into a all of the isotopic dates for the Leadville 1°X2° quadrangle larger data base and placed into a geologic context, can pro­ that had been made available through mid-1994 have been vide useful information. However, many reset dates, espe­ compiled into the present data set. The data are presented in cially when considered alone, have been used to arrive at appendix A along with information regarding location,