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WOOD II U.S. Bureau of Mines U.S. GEOLOGICAL SURVEY BULLETIN 1702 MINERAL RESOURCES OF WILDERNESS STUDY AREAS: SOUTHWESTERN AND SOUTH-CENTRAL ARIZONA DEPARTMENT OF THE INTERIOR MANUEL LUJAN, JR.·, Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director 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 UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1989 For sale by the Books and Open-File Reports Section, U.S. Geological Survey Federal Center, Box 25425 Denver, CO 80225 Ubrary of Congress Cataloging-In-Publication Data Mineral resources of the Trigo Mountains Wilderness Study Area, La Paz County, Arizona I by David R. Sherrod .. : [et al.]. p. em. - (U.S. Geological Survey bulletin; 1702) (Mineral resources of wilderness study areas-southwestern and south-central Arizona ; ch. J.) Includes bibliographical references. Supt. of Docs. no.: I 19.3:1702-J 1. Mines and mineral resources-Arizona-Trigo Mountains Wilderness. 2. Trigo Mountains Wilderness (Ariz.) I. Sherrod, David R. II. Series. Ill. Series: Mineral resources of wilderness study areas-southwestern and south-central Arizona ; ch. J. QE75.89 no. 1702-J 557.3 s-dc20 89--600283 [TN24.A6] [553' .09791 '72] CIP STUDIES RELATED TO WILDERNESS Bureau of Land Management Wilderness Study Areas The Federal Land Policy and Management Act (Public Law 94-579, October 21, 1976) requires the U.S. Geological Survey and the U.S. Bureau of Mines to conduct mineral surveys of certain areas to determine the mineral values, if any, that may be present. Results must be made available to the public and submitted to the President and the Congress. This report presents the results of a mineral survey of part of the Trigo Mountains Wilderness Study Area (AZ-050-023B), La Paz County, Arizona. CONTENTS Summary Jl Abstract Jl Character and setting Jl Identified resources Jl Mineral resource potential J2 Introduction J2 Location, physiography, and access J2 Previous and present investigations J2 Acknowledgments J3 Appraisal of identified resources J3 Methods of investigation J3 Production history and mining and leasing activity J4 Mineral deposits J4 Gold J4 Manganese J4 Silver, lead, and zinc JS Industrial minerals JS Assessment of mineral resource potential JS Geology JS Geochemical studies JS Methods of investigation JS Comparison with nearby mining districts JS Summary of results J9 Geophysical studies J9 Mineral and energy resources J9 Gold and silver J9 Manganese JlO Other commodities JlO References cited JlO Appendixes Definition of levels of mineral resource potential and certainty of assessment J14 Resource/reserve classification JlS Geologic time chart J16 FIGURES 1. Index map showing location of Trigo Mountains Wilderness Study Area, La Paz County, Arizona J3 2. Map showing mineral resource potential and generalized geology of Trigo Mountains Wilderness Study Area, La Paz County, Arizona J6 Contents V Mineral Resources of the Trigo Mountains Wilderness Study Area, La Paz County, Arizona By David R. Sherrod, Richard M. Tosdal, Robert B. Vaughn, David B. Smith, and M. Dean Kleinkopf U.S. Geological Survey Robert H. Wood II U.S. Bureau of Mines SUMMARY north of Yuma, Ariz. Relief in the Trigo Mountains is locally as great as 2,300 ft, although it is less than about Abstract 1,000 ft in the study area. The mountains are underlain by Proterozoic and Mesozoic gneiss and granitic rocks, The Trigo Mountains Wilderness Study Area (AZ-050-023B) is located in La Paz County, southwestern and by Tertiary volcanic rocks that underwent a major Arizona. At the request of the U.S. Bureau of Land Manage­ episode of extensional deformation about 22 to 20 mil­ ment, 29,095 acres of the Trigo Mountains Wilderness Study lion years ago (Ma) (see geologic time chart in "Appen­ Area were evaluated for mineral resources (known) and dixes"). Sedimentary basins that bound the range were mineral resource potential (undiscovered). Throughout this created during this time and subsequently filled by coa­ report, reference to the "Trigo Mountains Wilderness Study lescing alluvial fans. An arm of the early Gulf of Califor­ Area• or to the "study area• refers only to that part of the nia briefly inundated the Palo Verde Valley northwest of wilderness study area for which mineral surveys were re­ the study area about 5 Ma, resulting in sandstone, quested by the U.S. Bureau of Land Management. siltstone, and limestone that abut the northwest edge of An inferred subeconomic manganese resource of the Trigo Mountains. Faulting since that time enabled 210,000 long tons, having a grade of 12.7 percent, is located the Colorado River to establish its modern course in a in the central part of the study area. Placer gold occurs along the north margin of the study area, downstream from the trough along the west margin of the Trigo Mountains. nearby Hart mine. Common rock and sand and gravel in the Mines that have produced gold, silver, lead, and study area have no unique qualities that make them more (or) zinc are located outside the northeast and southeast valuable than vast quantities in the surrounding areas. There boundaries of the study area .. The associated mineralized is low mineral resource potential for gold and silver associ­ rock, however, whose distribution is largely fault con­ ated with quartz-bearing veins near the northeast corner of trolled, does not extend into the study area. Manganese the study area and in the three areas underlain by crystalline has been mined in the study area and is limited to veins rocks in the south-central part. The north-central part of the and fault breccia in Tertiary volcanic rocks. study area has a high resource potential for manganese in deposits of limited extent. There is no potential for geother­ mal energy or for oil resources; the potential for natural gas is Identified Resources low along the northwest border of the study area. Manganese oxides occur as fissure fillings in faults and fractures that cut Tertiary volcanic rocks in the Trigo Character and Setting Mountains Wilderness Study Area. Inferred subeco­ nomic manganese resources of about 100,000 long tons The Trigo Mountains Wilderness Study Area, (It) of nearly 13 percent manganese and about 110,000 It which includes a large part of the Trigo Mountains, is in of about 9 percent manganese were calculated for the the southwestern part of La Paz1 County, about 23 mi Lopez Wash area.