Gravity and Magnetic Features as Related to Geology in the Leadville 30-Minute Quadrangle, Colorado GEOLOGICAL SURVEY PROFESSIONAL PAPER 726-C Gravity and Magnetic Features as Related to Geology in the Leadville 30-Minute Quadrangle, Colorado By OGDEN TWETO and J. E. CASE GEOPHYSICAL FIELD INVESTIGATIONS GEOLOGICAL SURVEY PROFESSIONAL PAPER 726-C A summary and correlation of geologic and geophysical data on the most productive part of the Colorado mineral belt UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1972 UNITED STATES DEPARTMENT OF THE INTERIOR ROGERS C. B. MORTON, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress catalog-card No. 72-600032 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 Stock Number 2401-2145 CONTENTS Page Page Abstract...... ------------------------------------------------------------------------------- C1 Gravity features .... ·------------------------------------------------------------------ C16 Introduction .............................................................................. 1 Gravity surveY------------------------------------------------------------------ 16 · Geology ____________________________________________________________________________________ : __ 3 Errors in the Bouguer anomalies _________________________ _ 17 Major features .......... -------------------------------------------------------- 3 Interpretation of the gravity anomaly map _______________________ _ 18 Precambrian rocks __________________________________________________________ _ 3 ..,. Mineral-belt gravity low________________________________________________ _ 18 Sedimentary rocks ___________________________________________________________ _ 4 Underlying batholith as source ............................. 18 Upper Cretaceous and Tertiary intrusive rocks ........ 4 Deep crustal or subcrustal source........................ 20 Tertiary volcanic rocks __________________________________________________ _ 7 Local anomalies along the mineral-belt low _______________ _ 20 Structure... ------------------------------------------------------------------------- 8 Gravity low over the Arkansas valley graben ............ 22 Precambrian Homestake shear zone ____________________ _ 8 Other bedrock anomalies ________________________________________________ _ 22 Laramide and younger structure .......................... 8 Aeromagnetic survey______________________________________________________________ _ 24 Geology of the Arkansas valley bottom....................... 9 Interpretation of the magnetic map .... --------------------·------------ 24 Ore deposits----------------------------------------------------------------------- 11 General features of the magnetic map ________________________ _ 24 Densities and magnetic properties of the Anomalies over the Arkansas valleY----------------------------· 25 major rock units----------------------------------------------------------- 12 Anomalies related to the Homestake shear zone ........ 25 Densities ____________________________________________________________________________ . 13 Anomalies along the Sawatch Range ........................... 25 Magnetic properties--------------------------------------------------------- 14 Anomalies in the southern Gore Range ....................... 26 Precambrian gneiss ........... --------------------------------------· 15 Anomalies along the Mosquito-Tenmile Range __________ _ 26 Precambrian granitic rocks·----------------------------------· 15 Anomalies in the eastern part of the quadrangle ...... 27 Paleozoic sedimentary rocks·---------------------------------· 15 Summary and conclusions .... --------------------------------------------------· 27 Upper Cretaceous and Tertiary intrusive rocks. 16 References cited .. ---------------------------------------------------------------------- 29 ILLUSTRATIONS Page PLATE 1. Geologic and Bouguer anomaly map and sections of the Leadville 30-minute quadrangle, central Colorado.... -----------------------------------------------------------------------------------------------------------------------------------------------------In pocket 2. Aeromagnetic map and profiles of the Leadville 30-minute quadrangle ________________________________________________________________ ln pocket FIGURE 1. Index map of Colorado __________________________________________________ -------------------------------------------------------------------------------------------------------· C2 2. Sketch map showing major structural features ........ -------------------------------------------------------------------------------------------------------· 6 3. Histogram showing densities of major rock units·----------------------------------------------------------------------------------------------------------· 12 4. Histogram showing magnetic susceptibilities of major rock units ...... --------------------------------------------------------------------------· 13 5. Regional Bouguer anomaly maP----------------------------------------------------------------------------------------------------··---------------------·---·---------· 17 6. Section showing interpretation of regional gravity anomalY------------------------------------------·--·------------------------------------------ 19 7. Section showing gravitation effects of a deep crustal-mass deficiency beneath the Colorado mineral belt............ 21 8. Map and section showing residual gravity anomaly over the Arkansas valley graben and its interpretation.... 23 TABLES Page TABLE 1. Generalized geologic column, Leadville 30-minute quadrangle ........... --------------------------------·-----------------------------------------· C5 2. Summary of magnetic properties of the major rock units--------------------------------------------------------------------------------------------- 15 III GEOPHYSICAL FIELD INVESTIGATIONS GRAVITY AND MAGNETIC FEATURES AS RELATED TO GEOLOGY IN THE LEADVILLE 30-MINUTE QUADRANGLE, COLORADO By OGDEN TWETO and J. E. CASE ABSTRACT stock of Tertiary porphyry. Magnetic highs and lows occur Regional gravity and aeromagnetic surveys were made in both over Precambrian granitic and gneissic rocks and over the Leadville 30-minute quadrangle, Colorado, to supplement porphyry intrusives. The magnetic susceptibilities of these and extend geologic studies in a highly productive segment of rocks have overlapping ranges; thus, many of the anomalies the Colorado mineral belt. The quadrangle lies at the inter­ can be confidently interpreted only where the sources are section of the mineral belt with the Arkansas valley graben, exposed. Many magnetic highs correlate with topographic a northern extension of the Rio Grande and San Luis Valley highs developed in crystalline rocks, but other highs are graben system of New Mexico and southern Colorado. nearly independent of topography, and reflect strong mag­ Almost the entire quadrangle is a part of the eastern flank netization of the rocks. of the huge Sawatch anticline. The Sawatch Range, in the The Precambrian Homestake shear zone, which trends western part of the area, is carved in the Precambrian rocks northeast in the northwestern part of the quadrangle, is ex­ in the core of this anticline. The Arkansas River valley, at the pressed by a strong magnetic gradient that reflects magnetic foot of the range, is an elongate downfaulted block, or graben, biotite gneiss and migmatite within, and northwest of, the high on the eastern flank of the anticline. The Mosquito and shear zone, and less magnetic amphibole and quartz gneiss Tenmile Ranges, which occupy a broad central belt through southeast of the shear zone. The gradient indicates both con­ the length of the quadrangle, constitute an upthrown fault tinuity and location of the shear zone beneath areas covered block east of the Arkansas valley graben. They consist of by Paleozoic sedimentary rocks. Precambrian rocks capped by Paleozoic sedimentary rocks. Magnetic and gravity highs are found over a fault block The sedimentary rocks dip eastward into South Park, a broad near Sheep Mountain and Round Hill, reflecting dense and structural basin. magnetic rocks, possibly mafic intrusives, in the block. Gravity The Precambrian rocks and the Paleozoic sedimentary rocks and magnetic lows are found in the southeastern part of the are cut by numerous bodies of intrusive porphyry of Late quadrangle, where Paleozoic rocks are several thousand feet Cretaceous and Tertiary age. These porphyry bodies and re­ thick. lated mineral deposits- such as those of Climax, Leadville, Neither the Climax nor the Leadville mining district has a and several other mining districts - characterize the Colorado gravity or total-intensity-magnetic expression, but both are mineral belt, which here forms a broad northeast-trending belt marked by weak residual magnetic lows, probably reflecting that occupies much of the quadrangle. A deep fill of upper alteration of the crystalline rocks. Many of the mining dis­ Tertiary alluvial deposits and mantling Pleistocene glacial tricts are located near the axis of the main gravity low, and, deposits in the Arkansas valley covers the bedrock along the thus, it may be postulated that mineralization arose from the axial trend of the mineral belt for several miles southwest of apical region of the batholith at depth. Leadville. The dominant gravity anomaly is a huge low of 30-50 INTRODUCTION milligals which trends northeast across the quadrangle. From analysis of steep gradients on the flanks