Ore Deposits of the Western United States in Relation to Mass Distribution in the Crust and Mantle

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.

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.).

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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. Another acidic intrusions of the Colorado mineral belt stone volcanic area and is more prominent in the drawback of residual gravity maps involves the penetrating some uplifts of the Rocky Moun- 1,000-km map. removal of long-wavelength anomalies caused tains. Crustal roots beneath high topography A major area of scattered gravity lows ex- by broad shallow features. The reader is referred also contribute to the amplitude of gravity lows tends across south-central Nevada in the 1,000- to Kane and Godson (1985) for more detailed over mountainous regions like the Sierra km map. This trend of anomalies terminates on discussions of the limitations of interpreting Nevada and Rocky Mountains. the south as a rather sharp east-west boundary wavelength-filtered gravity maps. Gravity highs reflecting mafic rocks correlate, which extends across southern Nevada and The effects of deeper sources are enhanced by for instance, with the Snake River Plain in southwestern Utah. The steepness of the east- increasing the cutoff wavelength. If many of the southern Idaho and with the Great Valley of west gradient suggests that the source is structur- broad gravity anomalies in the residual 625-km California. ally controlled, possibly faulted at least locally. (Fig. 3) and 1,000-km (Fig. 2) wavelength maps Broadfields of gravity highs reflecting updom- The coincident sharp change in the course of the are compared with regional terrain features, it ing of the upper mantle occur, in particular, in Colorado River, from north-south to east-west can be seen that they are very similar. The sim- northwestern Nevada and northwestern Utah. In (Fig. 2), may be related to upward propagation ilarities substantiate the theory of isostasy, which both of these areas, the gravity highs correlate of a parallel, east-west-trending basement struc- states that anomalous mass loads such as moun- with thinning of the crust, apparent in the con- ture. Mabey and others (1978) recognized, in tain ranges (for example, the Rockies and Cas- tours of crustal thickness shown by Smith their magnetic study of the crust, indications of a cades) are supported by nearly equal mass (1978). (Compare the shapes of the positively morphological step on the Precambrian base- deficiencies at depth, whereas relative mass defi- anomalous areas of northwestern Nevada and ment in the same region (for data on gravity ciencies in crustal material such as those of sed- northwestern Utah, shown in red in Fig. 2, with gradients, see Eaton and others, 1978, p. 76). imentary accumulations in basins (for example, the contours shown in Fig. 4.) Kutina (1983b) As shown above, most of the prominent grav- in northwestern Nevada and northwestern suggested that the thinning of the crust in ity lows of the 1,000-km residual map reflect Utah) are compensated for by relatively denser northwestern Nevada and northwestern Utah, areas where magmatic fluids penetrated the lith- mass at depth. Some broad shallow features which is associated with mantle uplift, occurred, ospheric plate and produced a major pattern (for example, mafic sources of the Snake River in each case, in an area of intersection of deep- of compositional heterogeneity. We use this Plain in southern Idaho) are also enhanced in seated fracture zones which delineate major pattern for comparison with the distribution of the residual 625- and 1,000-km wavelength- lithospheric blocks. endogenic ore deposits that provide records of filtered maps. The major gravity lows, if they reflect masses the metallogenic processes in this vast region. The residual gravity maps (Figs. 1-3) of the of acidic rocks, indicate, to some degree, the The residual Bouguer gravity map at a 625- western United States exhibit a wide variety of zones of tectonic weakness along which magmas km wavelength cutoff (Fig. 3), which has been anomaly patterns and trends. Eaton and others or magmatic fluids penetrated. The large size of prepared in addition to the 250- and 1,000-km (1978) pointed out that gravity values over the these anomalies masks in some cases the respec- residual maps (Figs. 1 and 2) is very similar to western Cordillera are complex functions of tive deep-seated structural boundaries. A clearer the 1,000-km map but greatly enhances a major temperature, composition, and thickness of the picture of the size of the bodies can be obtained zone of negative anomalies in eastern Nevada. crust and the lithosphere. by comparing the gravity fields produced with Eaton and others (1978) suggested that this Comparison of the main fields of gravity different wavelength filtering. For instance, the gravity low reflects a major density contrast at anomalies of the 625-km map (Fig. 3) with geo- gravity lows which reflect the Sierra Nevada depth (which also produces a regional topo- logic and tectonic maps of the western United batholith and the batholithic masses of southern graphic high). To the east in western Utah, States indicates that usually the gravity lows re- California (1 and 2 in Fig. 3) are more pro- seismic-refraction data (Smith, 1978) indicate flect either low-density sedimentary rocks, thick nounced in the 1,000-km map (Fig. 2) than in crustal thinning and low upper mantle velocities. sections of low-density volcanic rocks, or low- the 250-km map (Fig. 1). Over the Idaho batho- The transition between the Great Basin and the density granitic intrusions. The gravity highs are lith, the areal extent of gravity lows in the 1,000- Wasatch Front, therefore, may represent a usually associated with mafic rocks, sometimes km map is much larger than that in the 250-km change in crustal and mantle composition and in reflecting areas of crustal thinning in which a map. Also, major gravity lows which reflect the crustal thicknesses and is manifested as a steep more mafic mantle material lies closer to the acidic intrusions of the Colorado mineral belt gravity gradient separating a regional low in Earth's surface. are more pronounced in the 1,000-km map than eastern Nevada and a regional gravity high in Gravity lows corresponding to low-density in the 250-km map. western Utah. sedimentary rocks occur, for instance, in the An area of prominent gravity lows near Cape Rocky Mountains, especially in Wyoming, re- Mendocino in northern California appears in the RELATIONSHIPS BETWEEN THE flecting intermontane basins such as the Powder 1,000-km map. The northern part of these grav- DISTRIBUTION OF ORE DEPOSITS River basin, Bighorn basin, Wind River basin, ity lows geographically correlates with high- OF THE WESTERN UNITED STATES and others. density Jurassic granitic rocks of the Klamath AND MASS DISTRIBUTION IN THE Gravity lows reflecting low-density granitic in- Mountains (Jachens and Griscom, 1983). A CRUST AND UPPER MANTLE trusions are common in the western United question arises of whether a hidden low-density States. Actually, most of the prominent negative granitic body can be expected at a greater depth Figures 1 and 2 show the distribution of the anomalies of Figure 3 reflect areas where mag- in the vicinity of Cape Mendocino, controlled main endogenic ore deposits of the western matic fluids penetrated the lithospheric plate and by the junction of lithospheric plates at Cape United States on the basis of Kutina's (1969) produced a pattern of major compositional het- Mendocino. compilation of 149 locations, revised and ex-

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tended to 223 deposits, using Guild's (1981a, 5. Cretaceous (except latest) (12), 6. Laramide When comparing the individual age groups of 1981b, 1981c) map and list of ore deposits. The (latest Cretaceous-Eocene) (50), 7. Oligocene- ore deposits with the mass distribution in the ore deposits listed by state in Table 1 include all Pliocene (84), 8. post-Tertiary (12). An addi- lithosphere, we must realize that the gross pic- of the age groups1 distinguished by Guild, each tional 36 deposits of uncertain age are included. ture of the compositional heterogeneity, shown group having the following number of deposits: Figures 5 and 6 show the two most important in Figures 1 through 6, has apparently originated 1. Precambrian (9 deposits), 2. Cambrian- age groups of ore deposits of the western United as late as the Mesozoic and Cenozoic. This is Middle Devonian (6), 3. Late Devonian-Early States, the Laramide (latest Cretaceous-Eocene) supported by a number of observations, espe- Triassic (1), 4. Middle Triassic-Jurassic (13), and the Oligocene-Pliocene deposits, superim- cially by the occurrence of major Mesozoic posed on a black-and-white version of the 625- bathol.iths (such as the Sierra Nevada batholith) 'Possible ranges in age are given in Table 1. km residual map. and of areas rich in Cenozoic magmatism (such

Figure 3. Simplified residual Bouguer gravity anomaly map of the western United States at a wavelength cutoff of 625 km. The main structural and rock units of surface geology, prevailing in the fields of some positive and negative anomalies, are identified by numbers. Sources of the geologic data: Cohee and others (1962), King and Beikman (1974), King (1977), Bayer (1983), and, for Wyoming and parts of Colorado, Geologic Atlas of the Rocky Mountain Region (1972). Arizona. 1—Coconino Plateau, San Francisco volcanic area (Tertiary and Quaternary); 2—White Mountains volcanic area (essentially Tertiary); 3—parts of Colorado Plateau. California. 1—Sierra Nevada batholith (Lower and Upper Cretaceous granitic rocks); 2—batholithic masses in southern California, Peninsu- lar batholith (Upper Cretaceous); 3—areas of Transverse Ranges, metamorphic and plutonic rocks; 4—Franciscan complex (mainly in the southern and northwestern parts of the area) and crystalline rocks of Klamath Mountains in the north (the latter with ultrabasic and younger, Jurassic, granitic rocks); 5—Great Valley sequence with oceanic crust or ophiolites at the base. Colorado. 1—Front Range uplifts and other units; 2—San Juan volcanic area (lower Tertiary volcanics with strong participation of felsic rocks); 3—parts of Colorado Plateau; 4—Uncompahgre uplift; 5—White River uplift. Idaho. 1—Idaho batholith (Cretaceous granitic rocks; Tertiary volcanic and intrusive rocks in eastern part of the area); 2—Snake River Plain (Quaternary volcanics). Montana. 1—Boulder batholith (Late Cretaceous, granitic rocks prevailing). Nevada. 1—Basin and Range province above crustal thinning, manifested by a broad field of positive gravity anomalies; 2—Basin and Range province within a negative gravity field, occurring between two areas of crustal thinning (one in western Nevada, and one in western Utah). New Mexico. 1—Datil volcanic area (Tertiary); 2—portions of Rio Grande rift valley; 3—parts of Colorado Plateau. Oregon. 1—Cascade Range, volcanics of Cascade Range in places invaded by felsic intrusives (Neogene volcanic arc); 2—Coastal Ranges of the Paleogene forearc basin; 3—Columbia Plateau basalts (Neogene). South Dakota. 1—Black Hills uplift. Utah. 1—Uinta uplift; 2—Uinta basin; 3—parts of Colorado Plateau; 4—High Plateaus volcanic area; 5—Wasatch fault block; 6—structural block with uplifted basement, within the Intermountain seismic belt; 7—Basin and Range province above a crustal thinning, manifested by a field of positive gravity anomalies. Washington. 1—Cascade Range (Paleogene volcanic arc), andesitic volcanics of Tertiary age invaded by masses of Tertiary granodiorite and quartz diorite; 2—Coastal Ranges (Paleogene forearc basin); 3—Columbia Plateau basalts (Neogene); 4—areas of batholithic masses of Cretaceous granitic rocks and Tertiary intrusives; 5—Snoqualmie batholith (Tertiary). Wyoming. 1—Bighorn uplift; 2a, 2b—parts of Powder River basin; 3—Bighorn basin; 4—Wind River basin; 5—Hartville uplift; 6—Laramie uplift; 7—Wind River uplift; 8—Green River basin; 9—Sweetwater uplift; 10—Yellowstone volcanic area; 11—Great Divide basin and Washakie basin; 12—Hanna basin; 13—Ventre uplift.

as eastern Nevada) in the regions of negative anomalies. As indicated by more detailed studies, however, at least some of the assumed zones of tectonic weakness which are supposed to have guided the Mesozoic and Cenozoic magmatism are very old, probably Precambrian (see, for example, Tweto and Sims, 1963). The main reactivation of these old structural breaks occurred in the Mesozoic and Cenozoic, in connec- tion with the motion of the North American plate. With regard to this long history, it is not surprising to find that igneous rocks and mineralization of different ages occur along the same zones of tectonic weakness. The main observations coming from the comparison between the distribution of endogenic ore deposits of Guild's eight groups and the Bouguer anomaly data of wavelengths less than 250, 625, and 1,000 km are summarized below. Figure 4. A part of the map of crustal thickness of the western United States (from Smith, 1. Nearly all of the deposits of Colorado 1978), with the Mendocino latitude (M) introduced by Kutina (1983b). Contours in kilometres occur in the regions of gravity lows of the 1,000- below surface. km residual map. The main exception is the gold The circular or oval-shaped areas of thinner crust as shown in northwestern Nevada and district of Cripple Creek (deposit 10). The grav- northwestern Utah reflect updoming of the upper mantle. These areas correlate with the fields ity lows partly coincide with and partly extend of positive anomalies shown in the residual Bouguer gravity anomaly map at a wavelength beyond the broad, northeast-trending zone of cutoff of 1,000 km (Fig. 2). The ore cluster of north-central Utah, including the districts of the Colorado lineament as defined by Warner Bigham, Tintic, Park City, and others (shown in Fig. 2), occurs along the eastern flank of the (1978). The shape of these anomalies reflects crustal thinning and mantle updoming as shown in this illustration.

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 TABLE I. LISTING C'F ENDOGENIC ORE DEPOSITS OF Colorado 21. Jarbridgo district: Au Ag. Age 7. THE WESTERN UNITED STATES AS SHOWN IN 1. Alma district: AuAgPb Zn Cu. Age 6. 22. Jerrit (Marlboro) Canyon: Au. Age 7. FIGURES 1 AND 2 AND SELECTIVELY (TWO AGE GROUPS) 2. Aspen district: Ag Pb; Zn Cu. Age 6. 23. Lynn district-Carlin mine: Au. Age 7. IN FIGURES 5 AND 6 3. Bonanza district: Ag Pb Zn Cu Au. Age 7. 24. Manhattan (Gold Hill district): Au Ag. Age 7. 4. Breckenridge district: Au Ag Pb Zn. Age 6. 25. McDermitt caldera (Cordero et al.) district: Hg U. Age 7. 5. Browns Canyon-Poncha Springs district: F. Age 7. 26. Mill City district: W. Age 6-. 6. Central City-Idaho Springs-Trail Creek: Au Ag; Cu Pb Zn, Age 6. 27. Minerva district: W. Age? Some placer deposits of endogenic minerals, transported from a near source 7. Climax mine: Mo; W Sn. Age 7. 28. Mountain City Copper (Rio Tinto) mine: Cu. Age 2. (with regard to the scale of the maps used in this paper), have been included and 8. Cochetopa Creek area: U. Age 7. 29. Nevada «heeliteand Leonard mines: W. Age? are distinguished in this table. 9. Creed district: Ag Pb Zn Au Cu. Age 7. 30. Pioche district: Zn Pb Ag Cu Mn. Age 5. The names of the deposits • deleting some of the alternative names or names 10. Cripple Creek district: Au; Ag F. Age 7. 31. Quinn Canyon Range: F; Ag Au. Age 7. of individual mines), the metals contained, and the age groups are after Guild 11. Eureka district: Au Ag Pb Zn Cu: Mn W F Ba. Age 7. (not distinguished; 32. Reese River (Austin) district: AgAu: U. Age 6+. (1981b) unless a reference to mother source is given. coincides with deposit 27) 33. Rochester district: Ag Au; Pb Cu. Age? The system of age groups a > used by Guild (1981 b): 12. Georgetown-Silver Plume: AuAgPb. Age? (plotted after Marsh and 34. Round Mountain district: Au Ag. Age 7. 1. Precambrian Queen, 1974) 35. Silver Peak (Clayton Valley): Li. Age 8. 2. Cambrian-Middle Devonian 13. Gilman district: Zn Ag Cu Pb Au; Mn. Age 6. 36. Tem Piute district: W Ag F. Age? 3. Late Devonian-Early Tria sic 14. Jamestown district: FAu Ag; Pb Zn. Age 6. 37. Tonopah district: AgAu Pb; U W Se. Age 7. 4. Middle Triassic-Jurassic 15. Lake City district: AgPbAu Cu Zn. Age 7. 38. Tuscarora district: AgAu. Age 7. 5. Cretaceous (except latest) 16. La Plata: Au Ag; Pb Cu. Age 6. 39. Tybo district: AgAu Pb Zn. Age 7. 6. Latest Cceiaceous-Eocene (Laramide) 17. Leadville district-. Ag Zn Pb Au Cu: Mn Fe Bi W. Age 6. 40. White F ine (Hamilton district): Ag Pb; Zn. Age 5. 7. Oligocene-Pliocene 18. Marshall Pass area: U. Age 7. (not plotted; should be between 3 and 19) 41. Yellow Pine (Goodsprings) district: Zn Pb AgAu Cu. Age 6+. 8. Post-Tertiary 19. Monarch-Tomichi-White Pine district: Zn Ag Pb Cu Au. Age 6. 42. Yerington district: Cu; Ag Fe. Age 4. Guild's system has been modified to code all deposits of Laramide age as 6 20. Montezuma district: Ag Pb Zn; Au Cu Bi. Age 6. (Guild, 1981b, p. 5). 21. Mount Emmons (Red Lady basin): Mo. Age 7. New Mexico A plus sign (+) after the age number means "or younger" (such as 5+); a 22. Nederland (Boulder County) district (Caribou in the map by Marsh and 1. Central (Santa Rita) district (Chino et al.): Cu Zn Pb Mo minus sign (-) means "or older." Two digits separated by a hyphen indicate the Queen, 1974): W. Age 7. Ag; Au Sb Fe. Age 6. possible range in ages (following the system used by Guild). 23. Northgate district: F. Age 7. Burro Mountain district (Tyron area): Cu Mo. Age 6. I plotted jointly The numbers given below .n the alphabetical listing of deposits of each state 24. Ouray (Uncompahgre): Ag Au Pb Cu Zn; Ba Sb. Age 6. (name "Ouray" Boston Hill mine. Silver City area: Fe Mn Ag. Age 6. J are used in Figures I, 2, 5, and 6. The symbols of the main metals are following Marsh and Queen, 1974) Fierro-Hannover district: Fe; Cu Zn. Age 6. in italics. 25. Powderhorn Complex: Th REE Nb Fe Ti. Age 1. (following Laznicka, 2. Gila district: F. Age 7. 1983; in Guild, 1981b, given as Iron HiU-Cebola-Powerhouse districts) 3. Mogollon district: Ag Au; Cu Pb Zn. Age 7. 26. Rico district: Ag Zn Pb Au Cu. Age 7. 4. Lordsburg district: Cu; Au Ag Pb Zn Mo. Age 6. 27. Silverton district: Ag Au Pb Zn Cu; Mn W Sb Bi F. Age 7. (plotted jointly 5. Taylor Creek (Black Range) district: Sn. Age 7. Arizona with 11 Eureka) 6. Chloride district (Silver Monument et al.): Ag Au Cu. Age 7. 1. Ajo district: Cu Au Ag; J'.n, Mo. Age 6. 28. Sneffels-Red Mountain-Telluride districts: Au Ag Pb Cu Zn. Age 7. 7. Hillsbo'o district: Au. Age 7. 5 2. Bagdad: Cu Mo; Au Ag b Zn. Age 6. 29. Summitville district: Au; Ag Cu Pb. Age 7. 8. Caballo Mountains area: Fe. Age 3. 3. Big Bug district: Zn Pb Cu Ag Au. Age 1. 30. Tarryal Springs district: Be W. Age 1. 9. Lake Valley district: Ag Pb Mn; Mo V. 4. Bisbee (Warren) district: Cu Ag Au Pb Zn. Age 4. 31. Urad-Henderson: Mo. Age 7. 10. Cooke'» Peak district: Ag Pb Zru Mn F. Age 6. 5. Casa Grande: Cu Mo. A|;e 6. 32. West Cliff-Silver Cliff: Ag Au Pb; Cu Zn. Age 6. 11. Organ district: Pb Ag Cu Zn; Mn Mo F. Age 6. 6. Christmas and other mines: Cu: Fe Mo W Zn Pb. Age 6. 12. Tortugiis district: F. 7. Clifton-Morenci district: Cu Mo; Ag Au Zn Pb. Age 6. Idaho 13. White Oaks district: Au W. Age 6. 8. Copper Basin district: Cu Mo Au Ag Pb; Zn F. Age 6. 1. Atlanta (Middle Boise) district: Au Ag. Age? 14. Hansonburg (Carthage) district: Pb Ag Ba. Age 6. 9. Crown King-Tiger-Brae shaw districts: Au Ag; Zn Pb Cu. Age 6+. 2. Bayhorse district: Ag Pb Zn. Age 5+. 15. Magdaiena district: Zn Pb Ag CuAu. Age 7. (0. Esperanza and Sterrita n ines: Cu Mo; Ag Au. Age 6. 3. Bear Valley: TiNb U REE Age 8. (Racer). 16. Zuni Mountains district: F. 11. Globe-Miami district: Cu MoAgAu. Age 6. 4. Blackbird: Co Cu; Ni Au. Age? 17. Old Placers district (San Pedro and other mines): Cu Au; W. Age 7. 12. Jerome district: Cu Ag Au Zn; Pb. Age 1. 5. Blue Wing district: W; Ag Pb Cu. Age? 18. New Placers district (Ortiz, Dolores mines): Pb Zn AgAu Cu. 13. Johnson Camp (Republic el al.): Cu Zn Ag; W Ti. Age 6. 6. Boise basin district: Au; Ti. Age 8. (Placer). 19. Willow Creek district (Pecos mine): Zn PbAgAu Cu. Age 1. 14. Kofa district: Au; Ag. A|$e 7. 7. Cascade region: Ti Th REE Nb. Age 8. (Placer). 20. Picuris district: W. 15. Lakeshore: Cu Mo. Age 6. 8. Coeur d'Alene region West: Pb Ag Zn; Cu Au Sb. Age? \ plotted jointly 21. Questa mine: Mo. Age 7. 16. Martinez district (Congress mine): Au. Age? Coeur d'Alene region East: Pb Ag Zn; Cu Au Sb. Age? J 22. Eliza bethtown-Baldy Mountain district: Au Ag Cu; Fe Pb W. Age 7. 17. Mission, Pima, Daisy, Piilo Verde: Cu Mo; Zn Pb. Age 6. 9. Hall Mountain area: Th REE Age? 18. Patagonia district: Pb Zr Ag Cu Au; V Mo Mn. Age? 10. Hermes mine: Hg. Age? Oregon 19. Planet district: Cu Fe. Age? 11. ldaho-Almaden (Weiser) mine: Hg. Age 7. 1. Black Butte: Hg. Age 7. 20. Ray district: Cu: Ag Au Pb. Age 6. 12. Idaho County gold districts: Au; Th Ti REE. Age 7-8. (Placer). 2. Blue Mountain region (Eastern): Au; Ag. Age 8. (Placer). 21. SafTord: Cu Mo. Age 6. 13. Mineral Hill (Wood River) district: Ag Pb Zn; Sb. Age? 3. Bonanza-Nonpareil mines: Hg. Age 7. 22. San Francisco: Au Ag: Cu Pb F. Age 7. 14. Seven Devils deposits: Cu; Au Ag Zn. Age 4. 4. Bretz Opalite: Hg. Age 7. 23. San Manuel-Kalamazoo: Cu Mo: Pb Zn Au Ag. Age 6. 15. Silver City and Delamar districts: Ag Au Sb; Se. Age 7. (Se added by 5. Horse Heaven mine: Hg. Age 7. 24. Silver Bell: Cu Mo; Zn Pb. Age 6. J. Kutina) 6. White King-Lucky Lass mines: U; Sb Hg. Age 7+. 25. Superior (Pioneer) district: Cu Zn; Pb Ag. Age 6. 16. South Mountain: Zn Cu Fe PbAg; Cd Bi Au. Age? (added by J. Kutina) 26. Tombstone district: Ag."b Zn Au Cu. Age 7. 17. Thompson Creek prospect: Mo. Age 5. South Dakota 27. Turquoise district: Ag Pb Cu Au Zn; Mo Mn. Age? 18. Warm Springs district: Zn PbAgAu Cu. Age? 1. Homcitake mine: Au; Ag Pb. Age 1. 28. Twin Buttes: Cu Mo. A|je 6. 19. White Cloud-Little Boulder: Mo. Age 6. 29. Union Pass district: Au 4g. Age 7. 20. Wildhorse district: W. Age? Utah 30. Vulture mine: Au Ag. Age? 21. Yellow Pine deposit: W Au Sb. Age? 1. Bingham (West Mountain) district, Carr Fork et al.: Cu Mo Au Ag Pb; 31. Weaver district: Au. Age 7. Zn. Aj»e 7. 2. Camp Floyd (Mercur) district: Au; Ag Hg TI. Age 7. (TI added after Montana Lazntcka, 1983) 1. Argenta district: Ag Pb Au; Zn Cu. Age 6. California 3. Cottonwood (Little and Big) districts: Pb Zn Ag Cu Au. Age 7. 2. Barker (Hughesville) district: AgPb Zn Au; Cu. Age 6. 1. Alleghany-Downieville district: Au. Age4+. 4. Gold Hill district (and Clifton area): Ag Au Pb As Bi; W. Age 7. 3. Bryant (Hecla) district: Ag Pb Au. Age 6. 2. Altocna: Hg. Age 7. 5. House Range deposits: W. Age? 4. Butte (Summit Valley) district: Cu Zn Pb AgAu; Mn Mo. Age 6. 3. Atolia district: W. Age 4+. 6. Iron Springs (Cedar City) district: Fe. Age 7. 5. Crystal Mountain mine: F; REE Sc. Age 5. 4. Bodie district: Au Ag. Age 7. 7. Mystery Sniffer and other mines: U. Age 7. 1 plotted jointly 6. Helena-Last Chance district: Au; Cu Pb. Age 6. 5. Calico district: Ag Ba. Age 7. Marys vale district: K AIV; F. Age 7. f 7. Lemhi Pass: Th REE Age? 6. Cerro Gordo district: Ag Pb Zn. Age 5. 8. Ophir district: Pb Zn Ag. Age 7. 8. Marysville district: Au Ag; Pb Zn Cu W. Age 6. 7. Cleat Lake district (Sulphur Bank mine): Hg. Age 8. 9. Park City district: Pb Zn Ag Cu Au. Age 7. 9. Neihart district: Ag Pb Zn Au Mo; Cu. Age 6. 8. Darwin district: Pb Ag Zn Cu Au. Age 5. 10. Pine Grove prospect: Mo; W. Age? 10. Philipsburg district: Ag Pb Zn Mn; Au Cu. Age 6. 9. Eagle Mountain district: Fe. Age 4+. 11. San Francisco and Preuss districts: Pb Ag Zn Au; Cu Mo Ba. Age 7. 11. Pryor Mountain (Old Glory mine): U. Age? 10. Folsom district: Au. Age 8. 12. Spor Mountain: Be F. Age 7. 12. Spar Lake deposit: Cu: Ag. Age 1-*-. 11. Foothill Belt district: Ci Zn; FeS.* Age 4. 13. Stockton (Rush Valley) district: Pb Zn Ag. Age 7. 13. Stillwater Complex: Cr Pi; Ni Cu. Age 1. 12. Grass Valley- Nevada City district: A u Ag; W. Age 4+. 14. Tintic district: Ag Pb Zn Au; Cu. Age 7. 13. Guerneville district (Sonoma mine): Hg. Age 7+. 14. Virginia City (Alder Gulch) district: Au; Ag. Age 6. 15. Wickes district: Au Ag Pb Zn. Age 6. 14. Genesee district: Cu. A ?e 4+. Washington 15. Hammonton district: Au. Age 8. (Placer). 1. Chelan Lake district: Cu Zn Au Ag; Fe. Age 2+. 16. Island Mountain district: Cu; Ag Au. Age 4+. Nevada 2. Earl (Margaret) deposit: Cu; Au. Age 7. 17. Julian district: Au. Ag«? 1. Alligator Ridge: Au. Age? (added by J. Kutina) 3. Germania mine and district: W. Age? 18. Knoxville district: Hg. \ge 7+. 2. Aurora district: Au Ag. Age 7. 4. Glaci:r Peak (Miners Ridge) deposit: Cu Mo. Age 7. 19. Mayacmas district: Hg. Age 7+. 3. Battle Mountain district (Copper basin, Copper Canyon): Cu ZnPbAu Ag. 5. Meta'ine district: Zn Pb; Ag Cu. Age 2+. 20. Mother Lode district, Amador, Calaveras: Au. Age 4+. Age 7. 6. Midnite mine area: U. Age 6. 21. Mountain Pass area: REE; Ba. Age 1. 4. Buena Vista Hills district: Fe. Age 4+. 7. Mount Tolman: Cu Mo. Age 6. 22. New Almaden district: Hg. Age 7. 5. Candelaria district: Ag Au Pb: Sb Cu Zn. Age 5. 8. North Fork deposit: Cu. Age 7. 23. New Idria: Hg. Age 7. 6. Cherry Creek district: W. Age? 9. Northpon district: Zn Pb Ag. Age? 24. Oroville district: Au. Age 8. (Placer). 7. Comstock district: AgAu; Cu Pb. Age 7. 10. Northwest Uranium (Peter's Lease): U. Age 7. 25. Pine Creek area (Bishcp district): WMo; Au Cu. Age 4. 8. Cortez district: Au Ag Pb. Age 7. 11. Republic district: Au Ag. Age 7. 26. Randsburg district: Au- Ag. Age? 9. Cucomungo: Mo. Age? 12. Sultan district: Cu Mo; Au W. Age 7. 27. Resting Springs (Tecojia) district: Pb Zn Ag; Au Cu. Age? 10. Ely (Robinson) district: Cu Mo; Au Ag Pb Zn. Age 5. 28. Rosamond-Mojave district: Au Ag. Age 7. 11. Eureka district: AgAu Pb Zn. Age 5. Wyoming 29. Salmon River district: Au. Age 8. (Placer). 12. Ferguson district: Au Ag. Age? 1. Gibbs Creek deposits: Ti Th REE Age 5 (Placer) 30. San Gabriel Mountain»: Fe Ti. Age 1. 13. Fluorine district: F. Age 7. 2. Copper Mountain deposit: W. Age 1. 31. Shasta district West: Cu Zn; Ag. Age 3+. 14. Garnet Tungsten: W. Age? 3. Rambler mine: Cu; Pt Pd Au Ag. Age 1. 32. Slate Creek (La Porte) district: Au. Age 6+. (Placer). 15. Getchell mine: AuAsHg. Age 5+. 4. State-line kimberlite diatremes with diamond, Albany County, Wyoming, 33. Sloughhouse district: Au. Age 7+. (Placer). 16. Golconda mine: W Mn. Age 8. and Larimer County, Colorado (McCallum and Mabarak, 1976). 34. Stedman district: Au/g; Cu. Age? 17. Gold Circle (Midas) district: Au Ag. Age 7. 35. Trinity River basin: Ait Age 8. (Placer). 18. Goldfield district: Au; Ag Cu. Age 7. 36. Wild Rose district: Au. Age? I plotted jointly 19. Hall prospect (Liberty): Mo: Cu. Age 5. •FeS and FeS2 not differentiated in the table. Wildrose Canyon: Sb. Age 7. J 20. Hawthorne district: Ag Au Pb. Age 4.

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Figure S. Ore deposits of Laramide age (latest Cretaceous-Eocene) superimposed on a simplified residual Bouguer gravity anomaly map of the western United States at a wavelength cutoff of 62S km. Note the particular concentration of Laramide ore deposits (mainly porphyry coppers) in southern Arizona. Most of the Laramide deposits of Colorado occur in the region of negative anomalies, where an intensive metallogenesis continued in Oligocene-Pliocene time (Fig. 6). More details are given in the text. Names of the batholithic masses of the surface area, partly corresponding to some of the broad zones of negative anomalies, are given in Figure 3.

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 6. Ore deposits of the Oligocene-Pliocene age group superimposed on a simplified residual Bouguer gravity anomaly map of the western United States at a wavelength cutoff of 625 km. Comparison of Figures 5 and 6 shows that the metallogenesis spread, in Oligocene-Pliocene time, over a vast region of the western United States but essentially did not continue in southern Arizona. Note, in particular, the continuation of metallogenic processes in the region of negative anomalies in Colorado, including the genesis of the major molybdenum deposits of Climax and Urad-Henderson. An important cluster of ore deposits originated in north-central Utah, close to the western edge of major negative anomalies and close to the eastern border of a major field of positive sinomalies, which in western Utah, correlate with an area of crustal thinning and mantle updoming.

some structural directions oriented transversely 6. Some groups of ore deposits do not occur structural intersection which guided magmas of to the northeast trend (Kutina, 1986). in the regions of negative anomalies of the 625- granodioritic, monzonitic, rhyolitic, and other 2. Most of the large- and medium-sized de- and 1,000-km residual maps. The most note- compositions (Kutina and Bowes, 1982). posits of Idaho and the adjacent parts of worthy example is the group of the Laramide Montana coincide with a very broad zone of porphyry copper deposits of southern and TARGET AREAS FOR MINERAL gravity lows, which occupy, in the 625- and the southeastern Arizona, including the districts of EXPLORATION 1,000-km residual maps (Figs. 2 and 3), an area Ajo (1), Casa Grande (5), Lakeshore (15), larger than that of the outcrops of the Idaho and Globe-Miami (11), Ray (20), San Manuel- The relationships between major ore deposits Boulder batholiths. The main exception is the Kalamazoo (23), and others. The impression and mass distribution in the crust and upper Coeur d'Alene district, possibly owing to a Pre- from Figure 2 is that these deposits correlate mantle, described in this paper, may stimulate cambrian age of the main stage of mineralization with borders of a zone of gravity highs, but the similar studies in other parts of the world. Al- (see arguments in Hobbs and Fryklund, 1970, 250-km residual map (Fig. 1) indicates that the though every territory, depending on its nature p. 1435). structural control is much more complicated. and geotectonic setting, requires a special ap- 3. Several large- and medium-sized deposits This latter map reveals belts of gravity lows proach, the western United States provides an are along the northeastern and eastern margins trending northwest and northeast, away from a opportunity to demonstrate the use of the resid- of gravity lows representing the Sierra Nevada "core" of gravity highs between deposits 5 and ual gravity maps in derivation of target areas for batholith. A particular concentration of metallic 30. mineral exploration. A few examples will be deposits occurs on the Nevada side of the anom- 7. The major cluster of ore deposits of the given and a methodical approach described. alous field, along a system of fractures of the Salt Lake City area in Utah, including the min- The first feature of special importance is the northwest-southeast-trending Walker Lane, ing districts of Bingham (1), Tintic (14), Park compositional heterogeneity of the crust and which is postulated to extend parallel to the City (9), and others, has, from a broad regional upper mantle, revealed by anomalies in the re- Nevada-California border. Some of these depos- point of view, a very special position. It occurs, sidual gravity maps (Figs. 1 and 2). One task is its or districts belong to the Middle Triassic- in the 250-km residual map of Figure 1 (pri- to distinguish which gravity lows reflect batholi- Jurassic group (42, Yerington; 20, Hawthorne), marily reflecting structures of the crust), near the thic masses of lower density, primarily acidic some are Cretaceous (5, Candelaria), and some junction or intersection of several gravity anom- (felsic) rocks, with which many metal deposits are Oligocene-Pliocene (7, Comstock; 2, alies: east-west-trending anomalies (positive and may be associated. The three largest areas of Aurora). negative, reflecting the Uinta uplift and Uinta gravity lows of the western United States 4. A number of deposits occur in south- basin, respectively), a north-south gravity low (Fig. 2) are of this type. That in east-central central and east-central Nevada, in the region of (partly developed in northern Utah), and a California corresponds to the Sierra Nevada a broad zone of gravity lows scattered in the northeast-trending gravity low (extending to batholith, well exposed at the surface; that in 1,000-km residual map (Fig. 2). These deposits central Utah from the southwestern part of the Idaho and Montana correlates with the Idaho are controlled by fracture zones of different state). A northwest-trend of gravity lows extend- and Boulder batholiths, respectively, also ex- trends (Fig. 18 in Kutina, 1980), among which ing into Utah from southwestern Colorado does posed at the surface. The gravity lows in Colo- the east-west direction plays an important role. not reach the ore deposit cluster in the residual rado, each extending toward the west, do not Stewart and others (1977) have described an 250-km map. correlate with major areas of intrusive rocks at east-west pattern of outcrops of igneous rocks of In the 625-km map (Figs. 3, 5, and 6), the the surface, but a number of stocks and other Cretaceous age in this region and southward mi- north-south trend of negative anomalies be- intrusive bodies exposed at the surface indicate gration of igneous activity through time. Many comes very prominent in Utah, correlating with the presence of more continuous batholithic deposits of this area are associated with Ceno- or adjoining the north-south-trending Inter- masses at depth. zoic magmatism, but some are older, such as the mountain seismic belt. The next task is to locate, in the regions of Cretaceous deposit of Ely (10). The gravity field of the 1,000-km residual gravity lows, zones of deep-seated fracturing 5. Deposits of northeastern Nevada belong to map (Fig. 2), to which the mass distribution of which could guide the ascent of ore-bearing so- different types and age groups. Of special inter- the upper mantle probably contributes, shows, lutions. In the case of the Sierra Nevada batho- est are the disseminated, replacement deposits of in western Utah, a major zone of gravity highs, lith, a major zone of fracturing extends in a gold and silver (of the Carlin type), some of correlating with an oval-shaped area of crustal northwest-southeast direction, along the north- which are bound on the northwest-trending thinning distinguished by Smith (1978, Fig. 6-2) ern border of the gravity low, parallel to the zones of tectonic weakness (Roberts, 1966; and referred to in the preceding section (Fig. 4). California-Nevada border. Several deposits oc- Roberts and others, 1971) that control location The cluster of ore deposits of the Salt Lake City cur in this zone (7,42,35, and others in Fig. 2). of a number of small intrusive bodies, well ex- area occurs along the eastern margin of the zone If major concentration of metals occurred in the pressed in the aeromagnetic map by Zietz and of gravity highs, along the edge of the area of roof above the batholith, the respective deposits others (1978). Some of these gold-silver deposits crustal thinning and mantle updoming. were mostly removed by erosion. The location occur above the western, peripheral part of the Another zone of gravity highs, occurring of deposit 25 (Pine Creek, Bishop district; major zone of negative anomalies of eastern within the same latitudes in northwest Nevada, W, Mo, Au, and Cu) suggests that this could Nevada, manifested in the 625-km residual map also correlates with an area of crustal thinning happen. (Fig. 6; Carlin or Lynn, 23; Jerrit Canyon, 22; and mantle updoming (Figs. 2 and 4). Deposits The two major gravity lows of Colorado 2 2 Getchell, 15 ; Alligator Ridge l ); some are 26 (Mill City district, tungsten) and 33 (Roches- (Fig. 2) correlate with a zone of fracturing farther west (Cortez, 8; and others). ter district, Ag-Au), which are in the central part which trends northeast-southwest and is the of the positive anomalous zone of the 1,000-km primary structural element controlling the Colo- 2Shown in Figure 2. map (Fig. 2), occur near the center of a major rado mineral belt. The nose-like westward pro-

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jection of these two gravity lows has been to Kutina's (1983a, 1983b) model of block ascent of magmas and magmatic fluids and explained by Kutina (1986) as caused by inter- structure of the western United States, this man- caused major geochemical changes in the section of the northeast-trending Colorado lin- tle updoming occurred near corners of major lithosphere. eament with two deep-seated fracture zones of lithospheric blocks of orthogonal shape, delin- The resulting picture of compositional heter- an east-west trend, along which the ascending eated by deep-seated structural boundaries ogeneity resembles that of products of progres- magma protruded toward the west. The two trending east-west and north-south. One of the sive metasomatism. Such a metasomatism is gravity lows and their immediate vicinity thus east-west boundaries, on the basis of the con- known at different mega- and micro-scales, in represent a large-sized target area for mineral tours of Pn velocities and of other data, is many cases starting at and spreading away from exploration. marked by the M in Figure 4. The cluster of intersecting fractures or other zones of tectonic Delineation of more specific targets within major ore deposits of the Salt Lake City area weakness, finally consuming the palasome and the above large areas required analysis on a occurs in places essentially correlating with the masking the original pathway along which the more detailed scale. The gravity patterns shown intersection of the above east-west-trending fluids penetrated. in Figures 1 ancl 2 were enlarged from the structural boundary and a north-south-trending Inevitably, some of the main clusters of endo- l:7,500,000-scale map of Hildenbrand and oth- zone of tectonic weakness, the latter manifested genic ore deposits of the western United States ers (1982) to ma tch the 1:500,000-scale geo- by the Intermountain seismic belt and the Wa- are preferentially located in or adjacent to the logic map of Colorado by Tweto (1979). The satch fault system. No comparable intersection regions of gravity lows of the 625- and 1,000- gravity contours of the residual maps were com- occurs on the western side of the same gravity km residual maps (for example, most of the pared with geology and with distribution of 89 highs; consequently, no major ore cluster is major ore deposits of Colorado, the clustering of mining districts ot Colorado, the latter adopted known along the western flanks of the respective deposits in a large area in Idaho and the adjacent from Marsh and Queen's map (1974). The re- mantle updoming. parts of Montana, and others). sults of this analysis are given separately (Ku- A search for a structural intersection in Some clusters of deposits, especially the Lar- tina, 1986). One of the most prominent features northwestern Nevada which would be compar- amide porphyry coppers of southern and south- is the association of some important ore depos- able to that in northwestern Utah led to the eastern Arizona, are along the flanks of a major its, both of the Laramide age (such as Leadville recognition of the north-south-trending Buena zone of gravity highs in the 1,000-km map and Breckenridge) and of the post-Laramide Vista lineament (Kutina, 1980). The deposits 26 (Fig. 2). The mass distribution revealed by the Tertiary age (such as Climax and Urad- (Mill City, tungsten) and 33 (Rochester, Ag, 250-km residual map (Fig. 1), however, indi- Henderson), with a relatively narrow zone of the Au) are the biggest ones close to the intersection cates that the above areas of mineralization are lowest gravity values (lower than -35 mgal of of the Buena Vista lineament with the east- characterized by the presence of belts of rela- the 250-km wavelength map, roughly corre- west-trending structural boundary extending at tively more felsic rocks, indicative of penetration sponding with areas having values of -320 mgal a latitude close to 40°N (M in Fig. 4). A new of intermediate or acidic magmas toward the and less of the complete Bouguer gravity map by discovery of a porphyry-type Cu-Mo minerali- Earth's surface. Cordell and others, 1982). This zone, extending zation, made on the Granite Mountain east of Some major gravity lows reflect batholithic as a narrow "ridge" inside each of the two grav- Rochester (Kutina and Bowes, 1982; Bowes and masses, partly exposed on the Earth's surface ity lows, does not in many cases exceed 20 km others, 1982), indicates that the region of the (Idaho batholith, Boulder batholith, Sierra in width and represents, in agreement with Case mantle updoming in northwestern Nevada can Nevada batholith, and others). (1967), an important target area for mineral ex- be understood as a large-sized target area for Some major structural boundaries, manifested ploration. Within the above zone, a number of mineral exploration and deserves further study. in the surface geology or detected by other still more specific targets can be distinguished. methods, are indicated in some of the filtered Some of these targets are defined by intersec- SUMMARY AND CONCLUSIONS residual maps, depending on the presence of tions of the zone of the lowest gravity values masses with contrasting gravity properties. For with regional zones of fracturing of a north- The use of filtered gravity fields provides a instance:, a north-south-trending boundary ex- northwest trend. Some of the Laramide intru- tool to study the controls of mineralization. The tending through Utah (best manifested in the sions and associated metal deposits occur within mass distribution in the crust and upper mantle 625-km map) correlates with the course of the these areas of intei'section. which is revealed by the residual Bouguer grav- Wasatch Front, a zone of complex faulting, and As for zones of positive residual anomalies, ity maps at 1,000 and 625-km wavelength filters the Intermountain seismic belt. Also, the east- their interpretation and use in mineral explora- shows a pattern of broad gravity lows, distrib- west structures of the Uinta uplift and Uinta tion usually requires additional geophysical uted in the background of positive anomalous basin are reflected in the 250-, 625-, and even in data. An example is given below. values (Figs. 2 and 3). Some of the gravity lows the 1,000-km residual map. Two major gravity highs, one in northwestern reflect suites of porous sedimentary rocks such The relationships between the location of Utah and the other in northwestern Nevada as those of the intermontane basins in Wyo- major ore deposits of the western United States (Fig. 2), correlate with areas of crustal thinning ming. Most of the high-amplitude gravity lows and the mass distribution i.i the crust and upper shown in a map by Smith (1978; Fig. 4). These in Figures 2 and 3, however, reflect masses of mantle may serve as general guidelines in min- areas, interpreted as updoming of the upper igneous rocks (relatively less dense and more eral exploration on a very broad regional scale. mantle, with denser rock material closer to the felsic than those responsible for the positive It should be understood, however, that an appli- Earth's surface (K utina, 1983b), are of a special anomalous background) which originated along cation cf these observations to actual predicting importance for mi neral exploration. According the zones of tectonic weakness which guided the of new targets for mineral exploration requires

an analysis of the pathways which guided heat Dutch, S. I., and Warner, L. A., 1979, The Colorado lineament: A middle Laznicka, P., 1983, Giant ore deposits. A quantitative approach: Global Tec- Precambrian wrench fault system: Discussion and reply: Geological tonics and Metallogeny, v. 2, p. 41-63. and ore-forming fluids from deeper levels to the Society of American Bulletin, v. 90, p. 313-316. Mabey, D. R., Zietz, I., Eaton, G. P., and Kleinkopf, M. D., 1978, Regional Eaton, G. P., Wahl, R. R„ Prostka, H. J., Mabey, D. R., and Kleinkopf, M. D., magnetic patterns in part of the Cordillera in the western United States, places where the metals could be deposited. This 1978, Regional gravity and tectonic patterns: Their relation to late in Smith, R. B., and Eaton, G. P., eds., Cenozoic tectonics and regional analysis involves incorporation of additional Cenozoic epeirogeny and lateral spreading in the western Cordillera, in geophysics of the western Cordillera: Geological Society of America Smith, R. B., and Eaton, G. P., eds., Cenozoic tectonics and regional Memoir 152, p. 93-106. methods, especially seismic and magnetic, as geophysics of the western Cordillera: Geological Society of America Marsh, W. R., and Queen, R. W„ 1974, Map showing localities and amounts Memoir 152, p. 51-91. of metallic mineral production of Colorado: U.S. Geological Survey well as a careful analysis of geological and geo- Geologic atlas of the Rocky Mountain region, United States of America, 1972: Mineral Investigations Resource Map MR-58, scale 1:500,000. chemical data. A few examples of the utility of Denver, Colorado, Rocky Mountain Association of Geologists. Mayo, E. B., 1958, Lineament tectonics and some ore districts of the southwest: Guild, P. W., 1978, Metallogenesis in the western United States: Geological Mining Engineering, v. 10, pt. 3, p. 1169-1175. residual gravity maps in derivation of target Society of London Journal, v. 135, pt. 4, p. 355-376, McCallum, M. J., and Mabarak, Z. D., 1976, Diamond in state-line kimberlite chief compiler, 1981a, Preliminary metallogenic map of North Amer- diatreme, Albany County, Wyoming and Larimer County, Colorado: areas for mineral exploration are presented in ica; U.S. Geological Survey, scale 1:5,000,000. Geological Survey of Wyoming Report on Investigation 12. the preceding section. 1981b, Preliminary metallogenic map of North America: a numerical Proffett, J. M., 1979, Ore deposits of the western United States: A summary, in listing of deposits: U.S. Geological Survey Circular 858-A. Ridge, J. D., ed.. Papers on mineral deposits of western North America: 1981c, Preliminary metallogenic map of North America: An alphabeti- Nevada Bureau of Mines and Geology Report 33, p. 13-32. cal listing of deposits: U.S. Geologial Survey Circular 858-B. Roberts, R. J., 1966, Metallogenic provinces and mineral belts in Nevada: ACKNOWLEDGMENTS Hildenbrand, T. G., Simpson, R. W., Godson, R. H„ and Kane, M. F., 1982, Nevada Bureau of Mines and Geology Publication 13-A, p. 47-72. Digital colored residual and regional Bouguer gravity maps of the con- Roberts, R. J., Radtke, A. S., and Coats, R. R., 1971, Gold-bearing deposits in terminous United States with cut-off wavelengths of 250 km and north-central Nevada and southwestern Idaho: Economic Geology, The work of one of the authors (Kutina) has 1000 km: U.S. Geological Survey Geophysical Investigations Map v. 66, p. 14-33. GP-953-A. Shcheglov, A. D, 1983, Nonlinear metallogeny (in Russian): Academy of received support from W. A. Bowes, Inc., Hobbs, S. 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MANUSCRIPT RECEIVED BY THE SOCIETY MAY 19,1986 U.S. Geological Survey Geophysical Investigations Map GP-949, scale Landwehr, W. R., 1967, Belts of major mineralization in the western United REVISED MANUSCRIPT RECEIVED JANUARY 6, 1987 1:1,000,000. States: Economic Geology, v. 62, p. 494-501. MANUSCRIPT ACCEPTED JANUARY 21, 1987

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