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Metallogenetic Maps of the Ophiolite Belts of the Western United States

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Metallogenetic Maps of the Ophiolite Belts of the Western United States DEPARTMENT OF THE INTERIOR UNITED STATES GEOLOGICAL SURVEY METALLOGENETIC MAPS OF THE OPHIOLITE BELTS OF THE WESTERN UNITED STATES By Jocelyn A. Peterson MISCELLANEOUS INVESTIGATIONS SERIES Published by the U.S. Geological Survey, 1984 G DEPARTMENT OF THE INTERIOR TO ACCOMPANY MAP 1-1505 UNITED STATES GEOLOGICAL SURVEY METALLOGENETIC MAPS OF THE OPHIOLITE BELTS OF THE WESTERN UNITED STATES By Jocelyn A. Peterson INTRODUCTION commodities shown on the metallogenetic maps was developed by geologists who are involved with IGCP Ophiolite is a distinctive assemblage of mafic and Project 39 and is similar to the symbology developed by ultramafic rocks which form a sequence, from bottom to Guild (1968). Table 1 indicates the criteria used in top, of usually serpentinized ultramafic rocks, gabbroic determining deposit size. Preliminary versions of these rocks frequently exhibiting cumulus textures, a mafic metallogenetic maps were presented at the meeting of sheeted-dike complex, and mafic volcanic rocks. Otlier Project 39 held in Athens, Greece, October 9-11, 1980 associated rocks include chromitite bodies, sodic felsic (Peterson, 1980). igneous rocks, and overlying sedimentary rocks composed of ribbon chert, thin shale, and minor limestone (Coleman, Table 1.-Criteria for deposit size in tons of commodity 1977). The term ophiolite has been in use since the beginning of this century, but the definition and genetic Commodity Large Small Occurrence models for ophiolite have been varied. However, it is now widely accepted that ophiolites represent oceanic crust that Asbestos -- > 10,000,000 <10,000,000 Non-producer has somehow been .emplaced onto the continents, Talc------ > 10,000,000 <10,000,000 Do. although there is still much discussion about the Magnesite-- > 10,000,000 <10,000,000 Do. mechanisms of emplacement. Iron------ >100,000,000 <100,000,000 Do. Certain mineral commodities and types of ore deposits Copper --- > 1,000,000 <1,000,000 Do. that are known to be associated with ophiolites have Chromite-- > 1,000,000 <1,000,000 Do. received widespread interest by mining companies and Manganese- > 10,000,000 <10,000,000 Do. geologists in recent years. Some of these deposits, such as Nickel ---- >500,000 <500,000 Do. the chromitite pods in the ultramafic complex and massive Platinum -- * * Do. sulfide bodies within the pillow lavas, are probably related to volcanic processes at spreading centers (Sillitoe, 1973), * Undefined by Project 39 group whereas other deposits, including lateritic nickel, some iron deposits, and asbestos deposits within serpentinized METHODS ultramafic complexes, result from weathering and emplacement of ophiolites (Coleman, 1977). Most of the initial data for the metallogenetic maps for In 1974 the Board of the International Geological this study were obtained from the CRIB (Computerized Correlation Program (IGCP) approved a project for the Resources Information Bank) files stored on the USGS 1 study of "Ophiolites of Continents and Comparable (U.S. Geological Survey) Amdahl Computer • CRIB is a set Oceanic Rocks." Thus Project 39 Ophiolites under the of variable-length records on the mineral resources of the sponsorship of UNESCO (United Nations Educational, Sci­ United States and other countries, and any record may entific, and Cultural Organization) came into being with a contain information about names and location, facts of dis­ general program having the following goals: (1) organize an covery and exploration, data concerning mining methods, ophiolite information center, (2) prepare an international production and reserve figures where available, and geolog­ ophiolite atlas, (3) organize field trips to outstanding ic and commodity data (Calkins and others, 1978). Records ophiolites, and (4) publish reports on ophiolite areas. One may be for metallic or nonmetallic commodities, but the important aspect of this program is the investigation of ore empnasis so far has been on gathering data for metallic deposits in ophiolites. Geologic maps have been published commodities. The GIPSY (General Information Processing at a scale of 1:2,500,000 for the Mediterranean region System) program (Office of Information Systems Programs, (Dietrich, 1979), the Himalayas (Gansser, 1979), California 1977) processes and maintains the data files and allows and Oregon (Irwin, 1979), and the Ural Mountains highly selective searching procedures. Output is available (Knipper, 1979). One group within the project is producing in several forms including map plotting capabilities when metallogenetic maps of these same areas to include data interfaced with other software packages such as CAM about chromium, platinum, nickel, copper, manganese, (Cartographic Automated Mapping) (Central Intelligence iron and titanium, talc, asbestos, and magnesite. The Agency, 1975). metallogenetic maps published with this paper complement Irwin's (1979) geologic map of the ophiolites for the tAny use of trade names is for descriptive purpose only and does not California-Oregon region. The explanation for the imply endorsement by the USGS. 1 The records of interest were first transferred to a working Feather River body, the Kings-Kaweah belt, and the file, where the data were rearranged slightly to facilitate Calaveras Formation. The remaining ophiolitic terranes are easy generation of maps by the computer. New -records considered to be Lower or Middle Jurassic (Irwin, 1977). were added for some of the commodities, notably talc, Duffield and SharjJ (1975) and Clark (1964, 1976) asbestos, magnesite, and chromite, from a literature discuss in detail the lithologies and structure of the melange search. Because much of the literature for the commodities belts within the Sierra Nevada which include the more discussed in this paper is old (pre-1970), data pertinent to extensive melanges: the Calaveras, Logtown Ridge, and the present study were not always reported. Therefore, Mariposa Formations, which are composed of sequences incomplete data on the map indicates that the information of metasedimentary and metavolcanic rocks. was not available in the literature. The Feather River ultramafic body is a metamorphosed When the information in the CRIB file was in the desired alpine peridotite and gabbro (Weisenberg and Ave format, I plotted each parameter on a separate map using Lallemant, 1977), and the entire ophiolitic assemblage the CAM program and a Zeta 3600s plotter. Since the contains metasedimentary and metavolcanic units related computer could not plot the symbols required, it was to island-arc activity (Ehrenberg, 1975; Hietanen, 1977). necessary to redraft the maps with the correct symbols. In Saleeby (1977) believes the Kings-Kaweah ophiolite belt this redrafting process it was also necessary for clarity to is a southward extension of the Paleozoic ophiolites and omit many points, particularly of chromite, manganese, are exposures of the bottom part of the ophiolite. The area and copper, where several or many mines clustered within contains the most complete Paleozoic ophiolite sequence a small geographic location. For these locations I retained in the Sierra Nevada. Most of the Kings-Kaweah belt the mines with the most available information assuming consists of discontinuous pendants or remnant inclusions of that other mines within the area would have similar ophiolite in the Sierra Nevada batholith. Saleeby (1974, geologic characteristics. 1975a, 1975b, 1976, 1977) has described this ophiolite region in great detail. A small Jurassic ophiolitic body termed the Tuolumne GEOLOGY OF THE WESTERN UNITED STATES River ophiolite (equivalent to the Tuolumne ophiolite of OPHIOLITES Irwin, 1977) occurs near Sonora (Morgan, 1973). The mafic and ultramafic sequence includes dunite, wehrlite, In the western United States, ophiolite occurs in a series gabbro dikes, gabbroic plutons, and pillow lavas. A of nearly parallel belts within the Sierra Nevada, California metamorphosed melange lies at the base of the complex. Coast Ranges, and Klamath Mountains (Irwin, 1979). The youngest of the ophiolites in the Sierra Nevada is Other ophiolite assemblages occur in the Ochoco-Biue the Smartsville complex consisting of pillow lavas, sheeted Mountains of east-central Oregon (near Baker). The dikes, trondhjemite, diorite, and gabbro (Moores, 1975; ophiolite belts have had a complex tectonic history which Buer, 1977) representative of upper levels of the JurassiC commonly resulted in deformation and dismemberment oceanic crust. and in the preservation of few complete sequences. The ophiolitic terranes as outlined on Irwin's (1979) map also KLAMATH MOUNTAINS include arc-related volcanic rocks and flysch. The volcanic The Klamath Mountains northwest of the Sierra Nevada rocks, flysch, and ophiolites may occur as melanges or are believed by some to be a northward extension of the coherent blocks. Isotopic and radiolarian age dates have Sierra Nevada foothills belt. Davis (1969) and Burch fie! shown that the belts become younger from east to west, and Davis (1981) present evidence for the structural and although there is uncertainty as to whether some ages lithologic similarities of the two regions. They correlate the represent time of formation or time of emplacement (Irwin, major Melones and Trinity faults and the ophiolite terranes 1977). Nevertheless, the reported ages provide a relative in the Sierra Nevada and Klamath Mountains. The time frame in which to work. distribution of platinum-group elements in placers suggests that geochemical evidence may not support all of
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