Duration of Hydrothermal Activity at Steamboat Springs, Nevada, from Ages of Spatially Associated Volcanic Rocks
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Duration of Hydrothermal Activity at Steamboat Springs, Nevada, From Ages of Spatially Associated Volcanic Rocks GEOLOGIC AiL SURVEY PROFESSIONAL PAPER 458-D Duration of Hydrothermal Activity at Steamboat Springs, Nevada, From Ages of Spatially Associated Volcanic Rocks By M. L. SILBERMAN, D. E. WHITE, T. E. C. KEITH, and R. D. DOCKTER GEOLOGY AND GEOCHEMISTRY OF THE STEAMBOAT SPRINGS AREA, NEVADA GEOLOGICAL SURVEY PROFESSIONAL PAPER 458-D UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1979 UNITED STATES DEPARTMENT OF THE INTERIOR CECIL D. ANDRUS, Secretary GEOLOGICAL SURVEY H. William Menard, Director Library of Congress Cataloging in Publication Data Main entry under title: Duration of hydrothermal activity at Steamboat Springs, Nevada, from ages of spatially associated volcanic rocks. (Geology and geochemistry of the Steamboat Springs area, Nevada) (United States. Geological Survey. Professional paper ; 45 8-D) Bibliography: p. D13-D14. 1. Geothermal resources-Nevada-Steamboat Springs. 2. Geology- Nevada Steamboat Springs. 3. Potassium-argon dating. I. Silberman, Miles L. II. Series. III. Series: United States. Geological Survey. Professional paper ; 45 8-D. QE75.P9no. 458-D [GB1199.7.N3] 557.3'08s [553] 79-16870 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 Stock Number 024-001-03215-5 CONTENTS Page Abstract __________ _______.._____________ Dl Potassium-argon ages Continued Introduction ______________________________________________ 1 Rhyolite domes______________________ _ D8 Regional geologic relations ________________________________ 1 Altered basaltic andesite _____________ ______________ 10 Geology of the area near Steamboat Springs ___________ 3 Interpretation ____________________________________________ 10 Structural control of hot-spring activity ________________ 3 Implications of the age of the Steamboat Springs thermal Alteration and ore minerals___________ _______________ 5 system __________________________________________________ 10 Ore minerals, trace-metals, and isotopic compositions____ 5 Steamboat Springs as related to other thermal systems and Geologic limits on the time span of hydrothermal activity _ 6 hydrothermal ore deposits ____________________________ 11 Potassium-argon ages______________________________________ 7 Methods of sample preparation ____________________________ 12 Samples ______________________________________________ 7 References cited ___________________________________ 13 Basaltic andesite _____________________________________ 8 ILLUSTRATIONS Page FIGURE 1. Generalized geologic map of Storey and southern Washoe Counties, Nevada ___________ ____________________________ D2 2. Generalized geologic map of the region near Steamboat Springs, Nevada __________ _____ ________-----__ 4 3. Diagram showing composite stratigraphic relations at Steamboat Springs, Nevada ______________ 6 TABLES Page TABLE 1. Analytical data and radiometric ages for volcanic rocks from the Steamboat Springs region Dl 2. Timespans of geothermal and ore-forming epithermal systems _________________________ . 2 III GEOLOGY AND GEOCHEMISTRY OF THE STEAMBOAT SPRINGS AREA, NEVADA DURATION OF HYDROTHERMAL ACTIVITY AT STEAMBOAT SPRINGS, NEVADA, FROM AGES OF SPATIALLY ASSOCIATED VOLCANIC ROCKS By M. L. SILBERMAN, D. E. WHITE, T. E. C. KEITH, and R. D. DOCKTER ABSTRACT ature, and hydrothermal alteration to epithermal Steamboat Springs is a presently active equivalent of epithermal gold-silver-depositing systems (White, 1955, 1974). gold-silver ore-forming systems. Hot-spring sinter deposits contain Because some active hydrothermal systems, including small amounts of gold, silver, mercury, antimony, and arsenic. Steamboat Springs, seem to be the present-day equiv Hot-spring activity probably started before extrusion of the basaltic andesite of Steamboat Springs. Old sinter from the Steamboat alents of the ore-forming systems, they help to en Springs system occurs in gravels above and below the basaltic hance our understanding of ore-forming processes. andesite. Intense hydrothermal alteration, including almost com Few direct data on the limits of age, duration, and plete replacement by hydrothermal potassium-feldspar, has affected continuity of activity are yet available either for ac the basaltic andesite. Three plagioclase separates of differing potas tive hot-spring systems or for epithermal ore-forming sium content from fresh basaltic andesite yielded potassium-argon ages of 2.52-2.55 m.y. Basaltic andesite almost completely replaced systems of the past. by potassium-feldspar yielded an age of 1.1 m.y. Our study provides much age data for a notable ac The thermal area lies approximately on a line connecting four tive hot-spring system. Steamboat Springs and the rhyolite domes, the largest of which is 3 km southwest of the surrounding region have been thoroughly studied springs and the others are to the northeast. Several domes occupy by geologic, geochemical, and hydrologic methods vents from which tephra of pre- and post-basaltic andesite age was (Thompson and White, 1964; White and others, 1964; erupted, as indicated by pumice blocks up to 5 cm diameter in gravel underlying the andesite, and isolated pumice fragments White, 1968). The hot-spring activity has occurred lying on the andesite. The source of energy for the thermal convec over a relatively long period of time. Uplift and ero tion system is probably a large rhyolitic magma chamber that sion of the thermal area alternated with burial by vol supplied the pumice and from which the rhyolite domes were canic rocks, alluvium, and hot-spring deposits, thus emplaced. Sanidine and obsidian from four of the rhyolite domes preserving much geologic evidence that permits re yielded potassium-argon ages of 1.15-1.52 m.y. and obsidian from one of the northeastern domes yielded apparent ages of 2.97 and construction of its history. Potassium-argon ages, now 3.03 m.y. available for many of the fresh and altered young vol The data indicate that hydrothermal activity has occurred at canic rocks in and near the thermal area, permit pre Steamboat Springs, possibly intermittently, for more than 2Vz m.y. cise estimates to be made for the total time span of These data agree with other radiogenic age studies indicating 1- hydrothermal activity. However, we do not yet know and 2-m.y. lifetimes for the hydrothermal systems that generate epithermal gold-silver deposits. whether the activity has been continuous or intermit tent through this timespan. INTRODUCTION REGIONAL GEOLOGIC RELATIONS The timespan and continuity of hydrothermal sys tems are important to establish for two reasons. First, The Steamboat Springs area is located near the the geothermal energy that may be recoverable from northeast end of Steamboat Hills, a small northeast- active systems has been related to the age of hot- trending range that lies between the north-trending spring activity and its volcanic heat source, through Carson Range on the west and the Virginia Range on Smith and Shaw's (1975) magmatic model. Second, a the east (figs. 1 and 2). The springs are noted for major group of active hydrothermal systems, closely transport and deposition of mercury, antimony, gold, associated with large rhyolitic systems, is similar in silver, and other metals and gangue minerals (White water composition, metal content, subsurface temper and others, 1964). Dl D2 GEOLOGY AND GEOCHEMISTRY OF THE STEAMBOAT SPRINGS AREA, NEVADA 120°00' EXPLANATION Gravel, alluvium, sinter deposits 39°30' - Basalt, andesite, and dacite Includes McCellan Peak Olivine Basalt (Pleistocene), Mustang Andesite (Pleistocene?), Lousetown Formation (Pleistocene to Miocene), and basaltic andesite of Steamboat Springs (Pliocene) Rhyolite Includes Steamboat Hills Rhyolite (Pleistocene and late Pliocene?) 39°15' and Washington Hill Rhyolite (Pliocene) 120 00' Volcanic rocks,, mostly intermediate Includes Alta and Kate Peak 15 MILES Formations, and Hartford Hill Rhyolite (Miocene) Includes Davidson Granodiorite (Miocene) and other Tertiary granitic rocks s oo 00 Metamorphic rocks 2 H Contact Approximate location of K-Ar sample. Numbers refer to numbers in table 1 FIGURE 1. Generalized geologic map of Storey and southern Washoe Counties, Nevada. Geology modified from Bonham (1969). In the region surrounding Steamboat Springs (fig. on figure 2 include the Alta and the Kate Peak For 2), pre-Tertiary Triassic (?) metamorphic rocks and mations of Miocene age, the Lousetown formation, in Cretaceous granitic rocks that form the basement are cluding local basaltic andesite of Steamboat Springs of overlain by a sequence of middle Tertiary to Pleis Pliocene age, the Steamboat Hills Rhyolite of late tocene volcanic rocks that has been discussed in detail Pliocene (?) and Pleistocene age, and alluvium of late by Thompson and White (1964) and Bonham (1969). Pliocene to Holocene age. The geologic relations in the Rock units considered in this report and generalized vicinity of Steamboat Springs are summarized here DURATION OF HYDROTHERMAL ACTIVITY FROM AGES OF VOLCANIC ROCKS D3 from earlier papers in this series (Thompson and Alteration has affected nearly all of the rock units White, 1964; White and others, 1964). Ages of the vol exposed in the thermal area. Veins and veinlets of canic rocks in the region are based on potassium- opal, chalcedonic quartz, quartz, and calcite cut the argon dates, largely summarized by Silberman and granitic and volcanic rocks and sediments, as shown McKee