Ore deposits of the western United States in relation to mass distribution in the crust and mantle JAN KUTINA Laboratory of Global Tectonics and Metallogeny, c/o Department of Chemistry, American University, Washington, D.C. 20016 T. G. HILDENBRAND Branch of Geophysics, U.S. Geological Survey, Federal Center, Denver, Colorado 80225 ABSTRACT comes broadest in the western United States boundary and extends down to the base of the (Guild, 1978). lithosphere. Advances in geophysical investiga- Colored, digital residual Bouguer gravity The reason for the concentration of metals in tions of the upper mantle, especially by seismic maps of the conterminous United States at this region lies in its complex geologic evolution, and gravity methods, have brought the above different cutoff wavelengths reveal significant characterized by Burchfiel (1979) as "an exam- topic to the forefront of metallogenic studies. compositional heterogeneities in the litho- ple of plate boundary and intraplate activity that sphère, providing a new tool for the studies of extends over more than 2.5 billion years of earth COMPOSITIONAL HETEROGENEITY controls of mineralization. Examples are history and is still active." In general, the metal- IN DEEPER PARTS OF THE presented showing correlations between logenic processes took place in the western LITHOSPHERE OF THE WESTERN main endogenic ore deposits of the western United States at different times, starting in the UNITED STATES—REVEALED BY United States and gravity anomalies in the Precambrian and reaching maximum intensity FILTERING OF GRAVITY FIELDS residual Bouguer gravity maps at 250-, 625-, during the Mesozoic and Cenozoic. The metal- and 1,000-km wavelength cutoffs. Some of logenic evolution of the area has been compre- Bouguer gravity anomaly maps provide in- the clusters of ore deposits are in or adjacent hensively treated, in the light of plate tectonics, formation about mass distribution beneath the to regions of gravity lows (for example, the by Guild (1978) and Proffett (1979). Earth's surface. The terms "residual gravity" and majority of deposits of the Colorado mineral A number of scientists have described the role "regional gravity" are generally used to make a belt). Most of i:he high-amplitude gravity of deep-seated fracture zones (or zones of tec- distinction between gravity anomalies arising lows reflect low-density igneous masses em- tonic weakness) in the localization of major ore from local, near-surface masses and those arising placed along zones of tectonic weakness deposits, ore districts, or mineral belts of the from larger and usually deeper features, respec- which guided the ascent of magmatic fluids western United States (Billingsley and Locke, tively. The process of wavelength filtering, to and caused major geochemical changes in the 1935; Mayo, 1958; Wisser, 1959; Badgley, remove the masking effects of broad (that is, lithosphere. Some clusters of ore deposits (for 1962; Jerome and Cook, 1967; Landwehr, long-wavelength) regional anomalies, enhances example, the group of Tertiary deposits of the 1967; Kutina, 1969, 1980, 1983a, 1983b; the appearance of small (that is, short-wave- Salt Lake City urea, Utah, including Bing- Wertz, 1974,1976; and others). One of the most length) residual anomaly trends that give a first ham, Tintic, Park City, and others) are along prominent examples is the Colorado mineral order indication of structure and compositional the flanks of major zones of gravity highs. In belt, controlled by a broad, northeast-trending heterogeneities. For example, Hildenbrand and the Salt Lake City area, the major concentra- belt of fracturing of Precambrian ancestry others (1982) have used, in their colored, digital tions of metals occur near the eastern edge of (Tweto and Sims, 1963) which was reactivated Bouguer gravity anomaly maps of the contermi- a broad gravity high that correlates with in later geologic history. According to Warner nous United States, two wavelength cutoffs of crustal thinning. These relations (and others) (1978, as well as Discussion and reply by 250 and 1,000 km to produce regional and re- between the location of major ore deposits Dutch, 1979, and Warner, 1979), this belt rep- sidual maps. and mass distribution in the crust and upper resents a middle Precambrian wrench fault sys- The 250-km wavelength cutoff residual map mantle may serve as general guidelines in tem that extends northeast beyond the Rocky shown in Figure 1 contains gravity anomalies of mineral exploration on a regional scale. Mountain Front. wavelengths of 250 km or less. Essentially, the Some of the geophysical, geological, and geo- masking effects of broad regional gravity lows BACKGROUND chemical data, however, indicate that the genesis over the Rocky Mountains, Basin and Range of some major clusters of endogenic ore deposits province, and Idaho batholith have been re- The region of the western United States, ex- was guided by structural boundaries and com- moved. The resulting residual gravity map tending from the Rocky Mountain Front west of positional changes occurring at still greater shows with clarity the expressions of sources re- Denver, Colorado, to the Pacific coast, is one of depths, beneath the Mohorovicic discontinuity siding primarily in the Earth's crust (Kane and the world's richest areas of metallic ore deposits. (see, for example, Kutina, 1983b, and Shcheg- Godson, 1985). It is part of the Cordillera, which extends along lov, 1983). Therefore, our attention is focused Kane and Godson (1985) investigated limits the western continental margin of the American on the upper mantle, in particular that part of source depths of residual anomalies and con- plates from Alaska to southern Chile and be- which occurs immediately beneath the Moho cluded that anomalies in the 250-km wave- Geological Society cif America Bulletin, v. 99, p. 30-41, 6 figs., ] table, July 1987. 30 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/99/1/30/3952304/i0016-7606-99-1-30.pdf by guest on 28 September 2021 Figure 1. The main ore deposits of endogenic origin, western United States (black dots with numbers, explained in Table 1), are superimposed on Hildenbrand and others' (1982) colored, digital residual Bouguer gravity map, which has a wavelength cutoff of 250 km. Explanation is given in the text. The distribution of ore deposits is based on Kutina's (1969) compilation, revised and extended to include all of the large- and the medium- sized deposits of Guild (1981a), as long as their endogenic nature was obvious. Some placer deposits of endogenic minerals, transported from a close source (in the sense of distances in this map), have been included and are distinguished in Table 1. The number of iron deposits is incomplete. Guild's deposits of medium size are within the category shown by small dots. The large-sized deposits of Guild are shown by medium and large dots, distinguished on the basis of a global comparison by Laffitte and Rouveyrol (1964). The sizes of Rochester in Nevada (no. 33), Mount Emmons (no. 21) and Powderhorn (no. 25) in Colorado, and Mt. Tolman (no. 7) in Washington have been upgraded to the medium size category, following Vikre (1981) for Rochester and following Laznicka (1983) for the other deposits. The Mount Emmons deposit, Colorado (no. 21), may belong to the same size category as does Climax (no. 7) (A. V. Heyl, 1986, personal commun.). Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/99/1/30/3952304/i0016-7606-99-1-30.pdf by guest on 28 September 2021 MGALS we < »0 • us • «St • «ID • »9 • »1 • »9 - in - ID! - It • 00 ' -SI' -10« ' -MO • ISO • K0 • »0 • -•mo- 4SI • -MO H -«0 - 4J(0 • Figure 2. Residual Bouguer gravity anomaly map of the western United States at a wavelength cutoff of 1,000 km (Hildenbrand and others, 1982), revealing major compositional heterogeneities in the crust and the upper mantle. Most of the large zones of negative anomalies (having lowest values in dark blue) reflect masses of less dense, more acidic rocks, which developed along ;cones of tectonic weakness penetrating the more dense and more basic rock complexes. The latter appear as the positively anomalous background (having the highest values in dark orange red) hosting the negative anomalies. Some, mostly smaller, areas of negative anomalies reflect suites of porous sediments in some sedimentary basins, especially in Wyoming. The names of the main batholithic masses and of some sedimentary basins are given in Figure 3. The main ore deposits of endogenic origin (black dots with numbers, explained in Table 1) are superimposed on the 1,000-km residual map. Some clusters of deposits occur in the region of negative anomalies (shown in blue), especially in Colorado and Idaho. Some occur near the borders of negative (blue) and positive (orange-red) anomalies, especially in Utah. A more detailed explanation is given in the text. For the size of the ore deposits, see explanations in caption of Figure 1. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/99/1/30/3952304/i0016-7606-99-1-30.pdf by guest on 28 September 2021 ORE DEPOSITS OF WESTERN UNITED STATES: MASS DISTRIBUTION IN CRUST 33 length residual map represent sources principally erogeneity, with strong participation of granitic In Wyoming, some of the gravity lows reflect residing in the crust and give a clearer indication rocks: the Sierra Nevada batholith in California sedimentary basins (shown by numbers in Fig. of source geometry than does the unfiltered and adjoining parts of Nevada, the batholithic 3): anomaly 2A, Powder River basin; 4, Wind Bouguer gravity map. They pointed out, how- masses in southern California, the Idaho batho- River basin; 11, Great Divide basin and Washa- ever, that the residual anomalies are distorted in lith, the Boulder batholith in Montana, and the kie basin. Gravity low 10 reflects the Yellow- shape and exaggerated in amplitude.