Time, space, and composition relations among northern Nevada intrusive rocks and their metallogenic implications

Northern Nevada represents an extraordinary metallogenic province that contains a wide array of mineral-deposit types of variable age. (The term mineral deposit is used throughout this paper to include ore mineral occurrences as well as productive mineral deposits.) While these mineral occurrences are distributed throughout the region, many of the largest and most economically important deposits (sediment-hosted Au, Paleozoic sedimentary exhalative barite, Jurassic-Cretaceous-Tertiary porphyry Cu-MoW-Au, and epithermal Ag-Au deposits) occur along mineral-deposit “trends,” first recognized by Roberts (1960), that appear to follow regional structural fabrics and igneous intrusions. With subsequent discovery of Carlin-type gold deposits, these same trends became synonymous with the alignment of giant gold deposits in the Carlin, Battle Mountain-Eureka, and Getchell trends that, together, have made this region the third largest gold-producing province in the world (Hofstra and Cline, 2000). Decades of geologic investigations in northern Nevada demonstrate that many mineral deposits, representing diverse deposit types, are spatially, and probably temporally and genetically, associated with igneous intrusions. Most recently, Ressel et al. (2000) and Ressel and Henry (2006) highlighted the association between Eocene magmatism and sediment-hosted Au deposits in north-central Nevada. Similarly, Theodore (2000) defined the association between igneous intrusions and mineral deposits in the Battle Mountain area. A direct relationship between magmatism and Carlin-type deposits, however, has been hotly debated (Sillitoe and Bonham 1990; Seedorf, 1991; Ilchik and Barton, 1997; Hofstra et al., 1999; Hofstra and Cline, 2000; Ressel et al., 2000; Emsbo et al., 2003; Muntean et al., 2004; Cline et al., 2005). Despite the number and importance of igneous intrusions in this metallogenic province, neither comprehensive characterization of these rocks nor synthesis of their metallogeny has been undertaken. The objective of this study is to synthesize the petrologic and petrochemical features of these rocks and to identify distinctive characteristics that define productive intrusions. Intrusions on and near the well-defined northern Nevada mineral-deposit trends are emphasized, given the potential importance of large, associated mineral (especially gold) deposits.

The geographic area addressed in this study, north-central and northeastern Nevada, is approximately bounded by latitudes 38.5° and 42° N, longitude 118.5° W, and the Nevada-Utah border (Fig. 1). The area, hereafter referred to as northern Nevada, contains numerous large plutons and smaller stocks, but also contains many small, shallowly emplaced intrusive bodies, including dikes, sills, and partially intrusive lava dome complexes that combine to form an important component of the regional geologic framework (Stewart and Carlson, 1978). Abundant Jurassic, Cretaceous, and middle Tertiary intrusions represent byproducts of protracted subduction-related processes, including continental-rifting backarc magmatism, that prevailed along the west edge of the North American plate during this interval (Ludington et al., 1996; John, 2001; Dickinson, 2006). Information available for a small group (n = 11) of volumetrically minor intrusions considered to be Triassic (Johnson, 1977) is limited. No interpretation of these intrusions is included in this study because of the paucity of available geochemical and geochronologic data.

Previous data compilations for Nevada's intrusions pertain to restricted areas and (or) do not include sufficient data to adequately interpret them in a regional context. Maldonado et al. (1988) compiled the distribution and some main characteristics of intrusive rocks throughout Nevada. Lee (1984), John (1983, 1987, 1992), Christiansen and Lee (1986), John et al. (1994), Radebaugh (1999), and Ressel (2005) have compiled data that characterize intrusions in various parts of Nevada. Barton (1990, 1996) completed early syntheses of geochemical, geochronologic, and mineral-deposit distribution data for intrusions in the Great Basin and proposed correlations between their ages, compositions, and associated mineral-deposit types. Smith et al. (1971) and Silberman and McKee (1971) presented potassium-argon geochronologic data for intrusions in northern Nevada. More recently, Mortensen et al. (2000) presented uranium-lead geochronology for a small number of intrusions in northern Nevada. Sloan et al. (2003) and Henry and Sloan (2003) have compiled geochronologic data for igneous rocks of Nevada. The geologic evolution of the Great Basin, including northern Nevada, is reasonably well understood and has been synthesized by Dickinson (2006). As part of the U.S. Geological Survey Metallogeny of the Great Basin project, du Bray and Crafford (2007) compiled modal, age, and geographic distribution data for northern Nevada intrusions. Simultaneously, du Bray et al. (2007) compiled a geochemical database for these same rocks. Now, using these recently developed databases, the time, space, and compositional evolution of magmatism in northern Nevada can be evaluated, and associations between magmatism and mineral deposits can be considered.

Some care must be taken in using and interpreting the data compiled by du Bray and Crafford (2007). Specifically, only known intrusions, large enough to be shown on Nevada Bureau of Mines and Geology county (1:250,000-scale) maps, were included in the synthesis described here; rare, very small or undocumented intrusions may have been overlooked. Similarly, data presented by du Bray et al. (2007) must be carefully considered. Much of that data pertains to phaneritic intrusive rocks, which may represent cumulates present in a plutonic mush zone from which various amounts of silicate liquid have been extracted, as suggested by Hildreth and Wilson (2007). As such, geochemical abundances for analyses of northern Nevada phaneritic intrusions do not precisely correspond to magmatic liquid compositions; associated geochemical interpretations are accordingly approximate. Data synthesized in this study are available as Excel files: NoNvintrgx1.xls; NoNvintrgx2.xls; and mrdsNASHmodels.xls¹.

The database compiled by du Bray et al. (2007) contains analyses for more than 2800 rock samples that represent ∼360 intrusions. Some of these samples were collected near mineral deposits where interaction with associated hydrothermal fluids modified their compositions. Abundance versus frequency histograms (du Bray et al., 2007) helped identify thresholds, in the tails of frequency distributions, used to identify analyses of altered rocks or inaccurate analyses. Samples with (in weight percent) SiO₂ >79, Al₂O₃ <9 or >21, analytical totals <94 or >103, Na₂O/K₂O <0.15 or >6, total volatile constituents >4, CO₂ >0.5, S >0.2, total iron (as FeO) >15, or CaO >17, were considered altered or inaccurate and were deleted from the data set, leaving ∼2100 samples. These measures of alteration are approximate and have probably allowed retention of some altered samples.

Geochronologic data compilations for northern Nevada igneous rocks (Henry and Sloan, 2003; Sloan et al., 2003) were synthesized (du Bray and Crafford, 2007) to yield age distributions of northern Nevada intrusions. Age determinations are available for 164 of the nearly 360 intrusions in northern Nevada. The age versus frequency distribution of these data (Fig. 2) includes three major magmatic pulses: (1) 15–40 Ma (32 percent of dated intrusions), most at 35–40 Ma, (2) 55–120 Ma (42 percent), principally 65–110 Ma, most at 90–95 Ma, and (3) 130–190 Ma (26 percent), principally 145–175 Ma, most at 155–160 Ma. Relative abundances of the three intrusion age groups were corroborated by du Bray and Crafford (2007); their geochronologic and qualitative geologic data compilation indicates that the relative abundances of the three intrusion age groups are 36.5, 41.0, and 22.5 percent for Tertiary, Cretaceous, and Jurassic intrusions, respectively.

Previously, Ressel and Henry (2006) suggested that magmatism in northern Nevada occurred in four pulses, three of which correspond to the pulses of intrusive magmatism described here, and a Miocene pulse at 15 Ma. However, the distribution of northern Nevada intrusion ages (Fig. 2) suggests that magmatism in northern Nevada was continuous between 15 and 40 Ma. Miocene intrusions are rare in most of northern Nevada; most are in a geographically restricted area in the southwest part of the study area. The small size, geographic isolation, and small number (∼20) of these Miocene intrusions suggest that they are associated with rocks of the western andesite assemblage (Ludington et al., 1996; John, 2001) that represent the southern extension of the ancestral Cascades magmatic arc (Christiansen and Yeats, 1992). In addition, a voluminous episode of Miocene volcanism, the bimodal basalt-rhyolite assemblage, has also been documented (Ludington et al., 1996; John, 2001) in northern Nevada. Associated intrusions (including Buckskin, National, and Canyon Creek) are probably rare in this region because of the prevalence of volcanic cover and shallow depth of exposure. Consequently, evaluating the distribution of Miocene magmatism using only intrusion ages, yields a biased outcome. Voluminous Miocene volcanic deposits in northern Nevada substantiate a fourth magmatic pulse, associated with backarc, continental-rifting (Christiansen and Yeats, 1992; Ludington et al., 1996). The volume of Miocene volcanic rocks notwithstanding, Eocene intrusions, which correspond to coeval volcanic rocks included in the interior andesite-rhyolite assemblage (Ludington et al., 1996; John, 2001), are the volumetrically dominant manifestation of Tertiary intrusive activity in northern Nevada. Given the small number and volume of Miocene intrusions in northern Nevada and their lack of petrologic distinctiveness relative to the Eocene intrusions, they are included in the Tertiary age group in the synthesis that follows.

Using the intrusion age distribution defined in this paper, and correlating individual analyzed samples with known intrusions (du Bray and Crafford, 2007), geochemical analyses (du Bray et al., 2007) were assigned to one of the three age groups, hereafter referred to as the Tertiary, Cretaceous, and Jurassic age groups. Of the ∼2100 analyses of unaltered samples, 793 represent Tertiary intrusions, 582 are Cretaceous, and 392 are Jurassic. An additional 346 samples represent intrusions whose ages are not well defined; these samples were assigned to the “unknown” intrusion age group. Seven samples represent intrusions (not depicted on the age distribution histogram) considered to be Triassic (Johnson, 1977).

The compilation of du Bray and Crafford (2007), which employed available modal data and published intrusion lithologic designations, was used to compile rock names of 343 reasonably well characterized northern Nevada intrusions into categories that correspond to rock name fields (hereafter referred to as modal compositions) defined by Streckeisen (1973). That compilation includes many intrusions with hyphenated composition names, e.g., monzogranite-granodiorite. Assuming that the first-listed composition is volumetrically dominant, each hybrid name was simplified to its first-listed composition, e.g., monzogranite in the example above. Limited numbers of lithologic designations that diverge somewhat from those defined by Streckeisen (1973) were recast to the appropriate composition as follows: intrusions named granite or leucogranite were compiled as monzogranite, and similarly, those named leucogranodiorite were compiled as granodiorite. For simplicity, the compositions of one intrusion each composed of monzodiorite and monzogabbro were compiled as diorite or gabbro, respectively. The compositions of one intrusion composed of quartz porphyry and another of quartz-feldspar porphyry were compiled as rhyolite. The composition of two intrusions composed of andesite, two of basalt, and one of diabase were compiled as miscellaneous. Intrusions identified as granitic or intrusive rock were not included in the modal composition synthesis because these compositional terms are too generalized to be useful.

Nearly two-thirds of northern Nevada intrusions are classified as either granodiorite or monzogranite; granodiorite intrusions are slightly more abundant than those composed of monzogranite (Fig. 3). The shallow intrusive equivalents of granodiorite and monzogranite, dacite and rhyolite, respectively, form the second most abundant group of intrusions. Dacite and rhyolite, however, account for less than 18 percent of northern Nevada intrusions; in a reversal of the granodiorite-monzogranite abundance relation, rhyolite intrusions are slightly more abundant than those composed of dacite. The number of northern Nevada intrusions that have other compositions is small. Of these, diorite intrusions are most abundant (∼6 percent); intrusions that have other compositions vary between 1 and 4 percent of the total. Northern Nevada intrusions are dominated by felsic compositions, and compositional diversity is relatively restricted.

Modal composition versus age for northern Nevada intrusions was synthesized from the compilation of du Bray and Crafford (2007). Reliable modal and age data (helpful for assignment of intrusions to one of the three age groups) are available for 284 of the intrusions (Fig. 4). With several significant exceptions, intrusion age group-composition characteristics are similar to those described for all of the intrusions. Specifically, the numbers of granodiorite and monzogranite intrusions are vastly greater in each of the three age groups than intrusions of other compositions. Granodiorite intrusions are more common than monzogranite among Cretaceous and Jurassic intrusions; the converse holds for Tertiary intrusions. The number of intrusions of most other compositions, including intermediate to mafic compositions, is uniformly low, regardless of intrusion age group. However, relative to the other two age groups, Jurassic intrusions contain a larger proportion of diorite and are further distinguished by gabbro intrusions (including the voluminous Humboldt complex).

A significant feature of the northern Nevada intrusion age-composition distribution is the large number of Tertiary rhyolite and dacite intrusions. The Tertiary monzogranite-granodiorite abundance ratio is approximately equal to the Tertiary rhyolite-dacite ratio and the total number of Tertiary monzogranite and granodiorite intrusions is about the same as the number of Tertiary rhyolite and dacite intrusions. Jurassic rhyolite intrusions are rare, Jurassic dacite intrusions are absent, and Cretaceous rhyolite and dacite intrusions are scarce. The apparent scarcity of shallow, aphanitic intrusions of Cretaceous or Jurassic age is likely due to a lack-of-preservation effect, as small, near-surface aphanitic intrusions may have been completely removed by prolonged erosion, experienced to a lesser degree by younger, Tertiary equivalents. Otherwise, time versus composition relations for the Tertiary, Cretaceous, and Jurassic intrusion age groups are quite similar.

Basic statistics, including mean, standard deviation, median, maximum, and minimum, were computed to provide basic statistical characterization for each of the age groups ¹⁰¹¹⁰². These measures do not identify statistically meaningful compositional characteristics among the intrusion age groups. Given the lack of age context for samples of unknown age intrusions, their compositional characteristics were not interpreted. As a group, compositional features of the unknown–intrusion-age samples are statistically indistinguishable from those of the other age groups and consequently do not provide additional insights. The lack of distinctiveness probably indicates that this group of samples may be a representative sampling of northern Nevada intrusions. Much of the discussion that follows derives from analysis of variation diagrams on which the compositions of samples representing the Tertiary, Cretaceous, and Jurassic intrusion age groups are portrayed.

Major-oxide analyses of northern Nevada intrusions were normalized to 100 percent on a volatile-free basis (du Bray and Crafford, 2007). Several important major-oxide compositional features are common to most Tertiary, Cretaceous, and Jurassic intrusions of northern Nevada (Fig. 5). Most obviously, the vast majority of the analyzed samples contains between 60 and 77 weight percent SiO₂; the relative proportion of Jurassic samples with SiO₂ less than 60 weight percent is slightly higher than for the other two intrusion age groups. With the exception of Al₂O₃, Na₂O, and K₂O, major-oxide abundance content, form single, variations, relative to SiO₂ well-defined, linear to curvilinear compositional variation arrays (Fig. 5). These arrays are most well defined among samples with more than 60 weight percent SiO₂. Abundance variations of the major oxides in the relatively small number of samples with less than 60 weight percent SiO₂ form lower silica extensions of the higher silica arrays; however, these lower silica array extensions are more diffuse and less well defined. In samples that contain less than 60 weight percent and decrease SiO₂, abundances of Al₂O₃ SiO₂ simultaneously, causing a significant bend in the Al₂O₃ SiO₂ versus compositional array. Abundances of Na₂O, and K₂O relative to silica exhibit considerable scatter.

As is the case for most intrusive rock suites throughout the world, abundances of TiO₂, Al₂O₃, FeO* (total iron in the ferrous state), MnO, MgO, CaO, and P₂O₅ in intrusive rocks of northern Nevada decrease with increasing SiO₂. Abundances of Na₂O, with a median of ∼3.5 weight percent, are essentially uniform, regardless of SiO₂ content, whereas K₂O abundances increase significantly with increasing SiO₂. Relative ferric and ferrous iron abundances of northern Nevada intrusions have been inconsistently determined (du Bray et al., 2007); furthermore, published data often do not represent magmatic values because of oxidation during deuteric and post-magmatic hydrothermal alteration and weathering. Consequently, although redox conditions are critically important to understanding mineral-deposit formation, information presented in this paper does not characterize the oxidation state of northern Nevada intrusions. Abundance variations of all major oxides, for samples of all three intrusion age groups, are continuous (lack composition gaps) between 60 and 77 percent SiO₂. With the following exceptions, compositional arrays depicted by the three intrusion age groups are indistinguishable (Fig. 5): (1) at a given silica content, many analyses of Cretaceous intrusions contain very slightly lower TiO₂, FeO*, and MgO and higher Al₂O₃ abundances than the other two intrusion age groups; and (2) at a given silica content, many analyses of Tertiary (and to a lesser extent, Cretaceous) intrusions contain very slightly lower Na₂O abundances than the Jurassic intrusions. However, these compositional distinctions are subtle and probably neither diagnostic nor petrologically significant.

Several additional major-oxide composition parameters provide further geochemical characterization of northern Nevada intrusive rocks. Ewart (1982) and Le Maitre (1989) defined four compositional fields based on the relative (Fig. 5). The pre-abundances of K₂O and SiO₂ ponderance of northern Nevada intrusive rocks compositions plot in the high-K field; compositions for some samples scatter into the adjacent shoshonitic and medium-K fields. However, given the ease with which K₂O is mobilized by hydrothermal fluids, some of this scatter may reflect nonmagmatic K₂O losses or gains related to hydrothermal alteration of some samples.

Alkalinity provides another igneous rock compositional characterization. Irvine and Baragar (1971) divided igneous rock compositions into alkaline and subalkaline series on the basis of SiO₂ contents. Total alkali and total alkali versus SiO₂ abundances for samples of the three northern Nevada intrusion age groups form a single, well-clustered array within the subalkaline field (Fig. 6). Compositions of some samples stray into the alkaline field, but as described for K₂O abundance relations, it is probable versus SiO₂ that some of these samples are somewhat hydrothermally altered.

The modified alkali-lime (MALI) index of Frost et al. (2001) provides a basic measure of the balance between calcium and the alkali elements in igneous rocks with various silica contents. The majority of computed MALI indices for northern Nevada intrusions are consistent with a calc-alkalic affinity though some analyses scatter into the adjacent calcic and alkali-calcic fields (see supplemental Excel files; ¹). As is true for most other geochemical parameters for the northern Nevada intrusions, the three age groups are indistinguishable in terms of their MALI indices.

Another important petrologic parameter pertains to the abundances of alumina versus those of the alkalis and CaO (Fig. 7). Igneous rocks are peraluminous when the alumina saturation index (ASI), the molar ratio of Al₂O₃/(Na₂O + K₂O + CaO), is greater than 1.0; rocks with ASI <1.0 are metaluminous. Similarly, igneous rocks are peralkaline when the agpaitic index (AI), molar ratio of Al₂O₃/(Na₂O + K₂O), is less than 1.0. Fewer than ten samples included in this study have peralkaline compositions. Given their scarcity and the fact that the data set may still include a few that are altered, likely few to none of the northern Nevada intrusions are truly peralkaline. In contrast, a large number (as many as half) of northern Nevada intrusions are peraluminous. Zen (1988) suggests that peraluminous igneous rocks have one of three origins: (1) partial melting involving peraluminous sedimentary rocks, (2) differentiation of metaluminous magma, or (3) late-stage metasomatic alkali loss. About the same proportion of samples from each of the intrusion age groups has either 1.0 <ASI ≤1.1 or ASI >1.1. Among igneous rocks of the world, samples with ASI >1.0 are not especially rare; weak alteration or weathering can cause rocks with a primary metaluminous affinity to have ASI >1.0. However, igneous rocks with ASI >1.1 (vertical dashed line on Fig. 7) are compositionally noteworthy; compositions such as these often have some metallogenic implications, especially with regard to potential for associated Sn, W, Mo, rare-metal mineral deposits (Taylor and Strong, 1988).

Given that easily identifiable altered/mineralized samples have been deleted from the interpreted data set, remaining volatile constituent abundances (including loss on ignition [LOI]), H₂O⁺ (bound), H₂O⁻ (nonessential moisture) CO₂, Cl, F, S, and total volatile content) provide the best available estimates of volatile contents of the three northern Nevada intrusion age groups. These abundances do not precisely replicate primary magmatic volatile contents, often modified by diverse post-magmatic processes, but potentially identify differences between various suites of intrusions. Abundances for specific volatile constituents are part of (not in addition to) LOI values, which represent the summation of all volatile constituents. Unfortunately, many analyses compiled by du Bray et al. (2007) do not include volatile constituent abundances. Because volatile constituents are the principal mediators of mineralizing processes among hydrothermal mineral deposits, knowing whether volatile abundances in northern Nevada intrusions are distinct and whether particular volatile abundances correlate with associated hydrothermal mineral deposits is significant.

A series of variation diagrams was used to characterize northern Nevada intrusion volatile constituent abundances (Fig. 8). LOI, H₂O⁺, H₂O⁻, CO₂, Cl, F, S, and total volatile contents are essentially uncorrelated with SiO₂ abundances. Median volatile constituent abundances of northern Nevada intrusions, in weight percent, are LOI (0.67), H₂O⁺ (0.67), H₂O⁻ (0.14), CO₂ (0.06), Cl (0.01), F (0.06), S (0.003), and total volatiles (0.74). With exceptions noted below, neither graphical portrayal nor descriptive statistics ¹⁰¹¹⁰² suggest that volatile constituent abundances for any of the northern Nevada intrusion age groups are distinctive relative to one another or igneous rocks of the world. Cretaceous intrusions with more than 71 weight percent SiO₂ have slightly greater F content than similar Tertiary or Jurassic intrusions. Of samples with greater than 0.1 weight percent F, most are from Cretaceous intrusions (Round Mountain and Robinson) and a few are from Tertiary intrusions (Horse Canyon, as well as dikes and sills). A small group of samples, about half Tertiary and half Cretaceous intrusions, which contain greater than 64 weight percent SiO₂, contain elevated S abundances. Otherwise, volatile constituent abundances for most of the three intrusion age groups form single arrays coincident with their median abundances; volatile contents of some samples, representing all three intrusion age groups, scatter to higher abundances, at all SiO₂ contents. Data for all northern Nevada intrusions form an array that depicts very slightly decreasing total volatile content with increasing SiO₂. Tertiary intrusions display a weak tendency to contain slightly higher total volatile contents at any particular silica content than samples of the other two intrusion age groups.

Petrologists routinely employ trace-element data to classify, characterize, and interpret the compositions of igneous rocks. The diverse geochemical behavior of trace elements suggests that they could be very useful in distinguishing northern Nevada intrusive rock age groups. A particular advantage related to studying trace elements is that many are relatively immobile during alteration or mineralization events and therefore maintain primary magmatic signatures. Several different patterns of trace-element abundance variation relative to silica abundances were identified. The abundance variations of many trace elements, including La, Ce, Zr, Th, Ga, Ni, and Cr, are not correlated with SiO₂ content (see supplemental Excel files; ¹); abundances of these elements relative to silica scatter on variation diagrams forming nonsystematic data clouds. Subtle, but faintly distinguishable geochemical relations among these trace elements and the intrusion age groups, are as follows (at any given silica content): (1) La and Ce contents of Cretaceous intrusions are slightly less than those for Tertiary and Jurassic intrusions; (2) Zr contents of all three intrusion age groups are similar except very few samples of Jurassic intrusions contain less than 50 ppm Zr. With increasing silica content, Zr abundances decrease slightly; (3) Th contents of Tertiary intrusions that contain 73–77 weight percent SiO₂ are distinctive; this small group of ∼20 samples (representing, in particular, the White Cloud Canyon and Cocomongo intrusions, several dikes and sills, and intrusions in the Ruby Range) contains elevated Th abundances (30–70 ppm); (4) although the Cr content of the three intrusion age groups is similar, a significant subset of the Tertiary samples with silica contents greater than 65 weight percent contain distinctly elevated Cr content.

Abundances of four trace elements, Co, Sc, V, and, in a less systematic fashion, Sr, decrease systematically with increasing SiO₂ content. These compositional arrays on silica variation diagrams are essentially linear; compositions of samples from the three age groups overlap and are essentially indistinguishable. One distinctive characteristic of the Jurassic intrusions, in contrast to the Cretaceous and Tertiary intrusions, is that very few samples of these contain less than ∼150 ppm Sr. In samples of all three intrusion age groups that contain less than ∼60 weight percent SiO₂, identified composition arrays become somewhat more diffuse but generally maintain their linearity.

Abundance variations of Ba and Rb, relative to silica content, form poorly defined (especially Ba), curvilinear patterns. Ba abundances define a concave downward pattern such that abundances generally decrease with increasing silica content. In contrast, Rb abundances increase with increasing silica content. For the three intrusion age groups, distributions of Ba and Rb abundances relative to silica content are mutually overlapping and indistinguishable. With the exception of the features noted below, the three northern Nevada intrusion age groups define similar abundance variation arrays for Ba and Rb: (1) few samples of the Jurassic intrusions contain less than ∼500 ppm Ba, and (2) a subset of Tertiary intrusions samples, those that contain between ∼73 and 77 weight percent SiO₂ and have elevated Th, Nb, and Y abundances, have distinctly elevated Rb contents (250 to almost 500 ppm).

Abundance variation arrays for Nb and Y are somewhat unique relative to those for other trace elements. In most samples of all three intrusion age groups, Y and, less systematically, Nb abundances decrease slightly with increasing silica content. With the minor exceptions noted below, abundance variation arrays for Nb and Y among the three northern Nevada intrusion age groups are very similar: (1) samples of Cretaceous intrusions with the highest SiO₂ abundances contain less Nb and Y (less than 5 and 10 ppm, respectively) than is generally true for either Jurassic or Tertiary intrusions, and (2) samples of Tertiary age intrusions (including White Cloud Canyon, Granite Peak East Range, Steptoe Warm Springs, and especially those in the Ruby Range) that contain 73–77 weight percent SiO₂ have distinctly elevated abundances of Nb and Y (40–80 and 30–90 ppm, respectively).

The common spatial relationship between mineral deposits and northern Nevada intrusions suggest that it is important to establish whether these intrusive rocks have intrinsically elevated metallic ore-element contents. If so, their metallic ore-element contents might be useful in identification of other intrusions with potential for associated metallic mineral deposits. Elevated ore-metal abundances are well known in some unmineralized intrusive rock associated with metal deposits, which suggests that data of this sort could be diagnostic of intrusions with potential for associated mineral deposits (du Bray et al., 1988; Borrok et al., 1999). Unfortunately, metallic ore-element abundances, especially those of Ag and Au, are not determined for most intrusive rock samples. In addition, metallic ore-element abundance data (du Bray et al., 2007) are most likely to have been obtained only for northern Nevada intrusions known or suspected to have an association with mineral deposits. As a consequence, northern Nevada intrusive rock, metallic ore-element contents are not well characterized, may not be representative of primary magmatic abundances, and are probably upward biased due to the emphasis on sampling near mineralized systems. As a basis for comparison, metallic ore-element abundances of northern Nevada intrusions can be compared to those of high-calcium granitic rock, for which average abundances (in parts per million) are: Ag, 0.51; Au, 0.004, Cu, 30; Mo, 1.0; Pb, 15; and Zn, 60 (Turekian and Wedepohl, 1961).

A series of metallic ore-element abundance versus silica content variation diagrams (Fig. 9) is used to compare metallic ore-element contents for the three age groups. Silver abundances (median of all samples, 0.19 ppm) are not systematically correlated with SiO₂ abundance variations. Many samples contain ∼0.1 ppm Ag, although an equal number of samples contain abundances that scatter to as much as 4 ppm. Notably, a subset of samples of Cretaceous intrusions contains distinctly elevated (1.5–3.5 ppm) Ag abundances. Cu abundances in samples of all three intrusion age groups scatter nonsystematically relative to silica content and most are less than 100 ppm (median of all samples, 7.0 ppm). In contrast to samples of the Tertiary and Cretaceous intrusions, very few samples of Jurassic intrusions contain more than 50 ppm Cu. Most samples of northern Nevada intrusions contain less than 10 ppm Cu. The distribution of Mo abundances (median of all samples, 2.6 ppm) among samples of northern Nevada intrusive rocks is distinctive. Almost all samples of Jurassic and Cretaceous intrusions, as well as about a third of the samples of Tertiary intrusions, contain less than 3 ppm Mo. However, about two-thirds of the samples of Tertiary intrusions (including Cherry Creek, Cocomongo, Copper Canyon, Elder Creek, Granite Mountain, Hilltop, Jersey Canyon, Lone Tree Hill, Mason Creek, Modoc, Nannies Peak, Post, Railroad Bullion, Swales Mountain, and Tenabo), contain distinctly elevated Mo abundances that range from 5 to 25 ppm. Molybdenum abundances are uncorrelated with silica content. Lead abundances (median of all samples, 18.9 ppm) generally increase with increasing silica and are indistinguishable among the three age groups. Among samples in all three age groups, Zn abundances decrease slightly and fairly coherently with increasing SiO₂ content. The abundance distributions of Zn in all three age groups are very similar; a small number of Tertiary samples contain elevated (120–180 ppm) Zn abundances. Data for the Au content of the three age groups are insufficient for interpretation. Scant available data (median of all samples, 0.10 ppm) do not suggest that any of the three age groups is characterized by distinctive Au contents.

Compositional features, including major-oxide, volatile-constituent, trace-element, and metallic ore-element abundances, of northern Nevada intrusions do not suggest that any of the intrusion age groups is geochemically distinct. Given the diversity of geologic environments across northern Nevada, in the crust and subjacent mantle, as well as the likelihood that diverse petrogenetic processes contributed to compositional evolution of intrusions emplaced in this region between the Jurassic and mid Tertiary, considerable variation among the evolutionary major-oxide–abundance pathways defined by the three northern Nevada intrusion age groups might be predicted. Abundance data for a diverse set of trace elements, subsets of which are known to exhibit distinctive behavior with different magmatic environments, should also delineate and emphasize distinctions among the northern Nevada intrusion age groups. One might also expect that volatile constituents and metallic ore elements, important in mineral-deposit formation, would be distinctive among northern Nevada intrusions associated with mineral deposits. However, with exception of a few relatively subtle features, geochemical signatures for the three intrusion age groups are mutually overlapping and indistinguishable, include no meaningful compositional gaps or clusters, and appear to provide no basis for distinguishing age or mineral-deposit association.

An initial hypothesis of this investigation held that intrusions along the major northern Nevada mineral-deposit trends might be geochemically distinct relative to other intrusions in this region. To evaluate this hypothesis, the geochemical compilation of du Bray et al. (2007) was culled to a subset of ∼300 analyses for samples of intrusions (about half from phaneritic masses and the remainder representing dikes, sills, and plugs) along the Carlin, Battle Mountain-Eureka, and Getchell trends. Geochemical data for these samples were directly compared to compositional arrays formed by the full suite of northern Nevada intrusions (Figs. 5–9). Significantly, the geochemistry of intrusions from along the trends forms a fully congruent subset that is completely indistinguishable from compositional data for the full population. Compositions of the trend intrusions overlap the parent population, include no meaningful compositional gaps or clusters, and appear to provide no basis for distinguishing productive from unproductive intrusions. Hence, the initial hypothesis was not substantiated.

The compositions of magmatic arc rocks have long been known to vary systematically inboard from subduction zones (Dickinson and Hatherton, 1967; Lipman et al., 1972; Feeley, 1993). Since at least the Jurassic, the principal tectonic features, especially magmatic arcs and associated subduction zones, have been essentially parallel to the present west coast of North America (Dickinson, 2006). Thus, the extent to which subduction processes contributed to the nature and extent of northern Nevada magmatism (and attendant geochemical variation), can be evaluated by examining geochemical variation across longitude, and generally perpendicular to inferred subduction zones. Calculated Rb-Sr ratios provide a simple yet significant characteristic of intrusion geochemistry because Rb and Sr are incompatible and compatible trace elements, respectively (Hanson, 1980). In particular, Rb-Sr ratios characterize the degree of magmatic evolution as well as the scope and character of crustal contaminant assimilation. Consequently, considering Rb-Sr ratios of northern Nevada intrusions relative to longitude places any variation in a magmatotectonic context (Fig. 10).

The most obvious relationship pertaining to position and geochemistry of the three northern Nevada intrusion age groups is that the vast majority of these intrusions, from across all of northern Nevada (Fig. 10A), have Rb-Sr ratios between 0.2 and 0.3 (median value, 0.26); small numbers of samples from each age group have Rb-Sr ratios that scatter up to values as high as ∼10. For comparison, average Rb-Sr ratios of within-plate granites of the Arabian Shield and Andean magmatic arc rocks are 10–100 and 0.23, respectively (du Bray et al., 1988; du Bray et al., 1995). In addition to the uniformity of Rb-Sr ratios among the three age groups, it is also striking that Rb-Sr ratios do not vary systematically with respect to longitude. The most distinctive divergence from these relations pertains to samples, most derived from Tertiary intrusions east of longitude 115.5° W (particularly unnamed intrusions in the Lamoille Canyon area of the Ruby Mountains), but locally in other ranges to the east (Coffeepot, Bearpaw Mountain, Franklin Lake, and Terraces intrusions). About 50 samples of Tertiary intrusions from the Ruby Mountains have Rb-Sr ratios that range from ∼1 to as high as 10; five samples of Cretaceous intrusions from near this same area have Rb-Sr ratios that range from 6 to 8. A smaller group of samples from several small, unnamed Tertiary intrusions located at about longitude 115.0° W on the east side of the East Humboldt Range has Rb-Sr ratios that range from ∼1–6.5 and two small groups of samples, representing Cretaceous intrusions (Toano Springs and Smith Creek Snake) at about longitude 114.2° W, have Rb-Sr ratios that range from ∼1–4.5. A group of 11 samples from the Granite Peak intrusion in the East Range at about longitude 117.8° W has Rb-Sr ratios that range from ∼2–5. Each of these areas that contain intrusions with elevated Rb-Sr ratios is geographically restricted; longitude ranges that encompass these samples span less than 0.2°–0.3°. An evaluation of this sort must account for Cenozoic tectonic extension that has affected much of northern Nevada. As well summarized by John (2001), extension in northern Nevada, largely along east-west axes, was spatially very heterogeneous, causing some areas to be extended 100–400 percent, and others to be unaffected. Removing the effects of extension would decrease the east-west extent of already narrow geochemical features.

To evaluate whether magmatotectonic processes produced any subduction-zone–perdindicular geochemical variation, north to south across northern Nevada, Rb-Sr ratio variation was also evaluated relative to latitude (Fig. 10B). Given the nearly east-directed Tertiary Farallon slab-foundering process suggested by Best and Christiansen (1991) and Humphreys (1995), such an evaluation seems warranted. Rb-Sr ratios for samples of northern Nevada intrusions are obviously the same as those considered above, but the distribution of elevated values relative to latitude is somewhat more systematic than is the case for longitude. Areas that contain intrusions with elevated Rb-Sr seem to be located in the central part of northern Nevada between about latitude 39.5° and 41.0° N; intrusions with Rb-Sr ratios greater than 2 are rare north of latitude 41° N and south of latitude 39.5° N. However, Rb-Sr ratio variations are uncorrelated with the mineral trends.

The spatial distribution of intrusions of the three age groups across northern Nevada can also be gleaned from an evaluation of geochemistry versus location (Fig. 10). Considered as a group, northern Nevada intrusions extend continuously, with no breaks, across the full longitude extent of northern Nevada. In a longitude and latitude context, several minor distribution discontinuities were identified within each of the three age groups. In a longitude context, Tertiary intrusions seem to be almost continuously distributed across northern Nevada. Likewise, Jurassic intrusions are nearly continuously distributed but are essentially absent west of longitude 117.9° W and scarce in the limited range between longitudes 115.5° and 116.0° W. Cretaceous intrusions are almost absent between longitudes 116.1° and 116.9° W and 114.3° and 114.9° W, but are otherwise present across all of northern Nevada. Intrusions also extend continuously through the full latitude extent of northern Nevada. Tertiary intrusions are present from latitude 38.5° N to about latitude 41.3° N, but seem to be absent further north. Jurassic intrusions are absent between latitudes 41.0° and 41.5° N and essentially absent south of latitude 38.9° N. The distribution of Cretaceous intrusions is essentially continuous throughout the latitude range of northern Nevada, though their abundance near latitudes 41.5° and 40.0° N is minimal (Figs. 1 and 10).

Geochemical features of igneous rocks have long been used to deduce the tectonic regime that prevailed during their genesis. One of the earliest attempts to use geochemistry in this way employs the relative abundances of the alkalis, FeO*, and MgO on an AFM diagram. Irvine and Baragar (1971) summarized criteria for distinguishing tholeiitic and calc-alkaline compositions and proposed a dividing line to separate the two types (Fig. 11). Delineation of the two compositional fields and definition of the Cascades calc-alkaline trend were strongly influenced by their compilation of compositions for samples of the Cascades volcanic arc. Comparing (Fig. 11) northern Nevada intrusion compositions with the Cascade calc-alkaline trend (Irvine and Baragar, 1971) indicates a strong similarity between archetypal, subduction-related, Cascade magmatic arc rocks and intrusions of northern Nevada. The calc-alkaline character of most northern Nevada intrusions is confirmed by FeO*/MgO versus silica relations (see supplemental Excel files; ¹). In terms of the relative abundances of the alkalis, FeO*, and MgO, the three northern Nevada age groups are indistinguishable although a small subset of samples of Cretaceous age is distinguished by a tholeiitic affinity. These relations strongly suggest a similarity of large-scale processes responsible for genesis of the magmas represented by the three northern Nevada intrusion age groups and subduction-related rocks of the Cascades arc.

Pearce et al. (1984) developed several classification diagrams used to deduce the tectonic setting from which diverse suites of igneous rocks were derived. The diagrams are based on relative trace-element abundances and emphasize those trace elements whose abundances are least affected by post-magmatic processes. In particular, the diagram that uses relative abundances of Rb, Y, and Nb is especially useful because abundances of these elements are well known for many northern Nevada intrusions. According to the classification diagrams (Fig. 12) of Pearce et al. (1984), the vast majority of northern Nevada intrusions have compositions that are consistent with an origin in a subduction-related, magmatic arc environment. Most samples for each of the three age groups form a mutually overlapping data cloud within the volcanic arc field. A number of samples, especially a subset of samples from previously identified Tertiary intrusions, scatter into the (Y + Nb enriched) within-plate field. Intrusions with these characteristics probably reflect derivation from a more evolved source, a genesis that involved better developed continental crust (consistent with their age and geotectonic evolution of northern Nevada), and (or) greater compositional evolution via fractional crystallization. Data for several dozen samples of Cretaceous intrusions (Bearpaw Mountain, Birch, Franklin Lake, Illinois, Lamoille Canyon, Millett north, Terraces, Toano Springs, and Trenton Canyon), extend into the syncollisional field and have low Y + Nb and high Rb abundances relative to those for samples of either Tertiary of Jurassic intrusions). Whether these analyses are indicative of primary magmatic compositions, result from alteration, or represent inaccurate data is not known. The petrogenetic implications of this small group of samples are indeterminate, but low (relative) Nb abundances are diagnostic of a genesis that involved contamination by continental crust (Pearce et al., 1984).

A subduction-related tectonic affinity for northern Nevada intrusions can be further corroborated by considering the values of certain trace-element ratios. Gill (1981) determined that the Ba-Nb and La-Nb ratios for modern arc rocks are >26 and 2–7, respectively, whereas Pearce et al. (1984) indicated that values for these ratios are <<26 and <3, respectively, for within-plate igneous rocks. The Ba-Nb and La-Nb ratios for all intrusions of northern Nevada have median values of 64 and 2.7, respectively; values for the intrusion age groups are statistically indistinguishable. Although these ratios become less useful among more evolved rocks because of fractionation effects (see plots of these ratios, relative to silica contents, in supplemental Excel files; see ¹), abundances of Ba, Nb, and La in northern Nevada intrusions, relative to the Ba-Nb and La-Nb criteria of Gill (1981) and Ba-Nb criterion of Pearce et al. (1984) are consistent with a magmatic arc affinity. However, relative to the La-Nb criterion of Pearce et al. (1984), an arc affinity for northern Nevada intrusions is less well substantiated. La-Nb ratios indicate a weak within-plate affinity among northern Nevada intrusions. This affinity may reflect a backarc setting in which magmas represented by some northern Nevada intrusions were slightly more contaminated by interaction within thicker, better developed continental crust during their genesis and emplacement (Feeley, 1993).

In a large-scale spatial context, the setting of magmatic activity represented by northern Nevada intrusions is somewhat distinct relative to that typical of continental margin magmatic arcs. Even accounting for locally significant extension (as summarized by John, 2001) associated with middle to late Tertiary tectonic development of northern Nevada, magmatism represented by Jurassic intrusions in northern Nevada was well inboard of the middle Jurassic continental-margin magmatic arc. Dickinson (2006) relates this pulse of backarc magmatism to renewed upwelling of asthenospheric mantle as a consequence of subducted, oceanic slab (the Mezcalera plate) breakoff and foundering. As such, middle Jurassic intrusions in northern Nevada represent a type of slab-window magmatism. The end of this magmatism coincides with the arrival of the leading edge of the subducted Farallon plate beneath northern Nevada and slab-window closure. The onset of Cretaceous magmatism in northern Nevada is best ascribed to a late Mesozoic period of flat subduction associated with rapid plate convergence along western North America (Coney and Reynolds, 1977). With flattened subduction, the asthenospheric mantle wedge moved well east of the Sierra Nevada and promoted magmatism beneath northern Nevada. Slowing convergence at the end of the Cretaceous caused the hinge line of the subducted oceanic plate to migrate west; subsequent slab rollback caused subduction-related magmatism to retreat westward, bringing another cessation of magmatism beneath northern Nevada. Renewed magmatism in northern Nevada ca.40 Ma has been associated with yet another large-scale, plate tectonic process. Christiansen and Yeats (1992) documented a dramatic north-to-south sweep of magmatic activity, starting in the Pacific Northwest and arriving in northern Nevada by ca.45–40 Ma. A similar south-to-north magmatic sweep, starting near the U.S.-Mexico border is less well documented. Humphreys (1995) attributed these patterns as deriving from asymmetric foundering of the Farallon slab beneath the Great Basin. His model invokes downward buckling of the Farallon slab along an east-northeast–trending axis with concomitant tearing of the slab away from its northern and southern boundaries, approximately coincident with the northern and southern U.S. boundaries. The buckling-tearing process once again promoted development of a pair of asthenospheric mantle wedges (a northern wedge migrated to the south with time, while a southern wedge migrated northward) beneath western North American. As this process ensued, renewed, subduction-related magmatism migrated southward and northward from the two subducted–oceanicplate tear zones. In northern Nevada, these processes caused renewed, southward sweeping magmatism that reached central Nevada ca. 30 Ma (Dickinson, 2006). Subsequently, primarily subduction-related magmatism in northeast Nevada waned. Ongoing arc magmatism migrated west to near the present-day California-Nevada border, where Miocene arc magmatism, represented by a cluster of Miocene intrusions in the extreme southwest part of the study area characterizes the southern segment of the ancestral Cascades arc (Christiansen and Yeats, 1992). Cenozoic magmatism in northern-central Nevada culminated with a voluminous episode of bimodal volcanism that began ca.16.5 Ma and continues to the present (Ludington et al., 1996; John, 2001). This magmatic event is represented by very few known intrusions, including Buckskin, National, and Canyon Creek (du Bray and Crafford, 2007), and is associated with the onset of backarc continental rifting (Ludington et al., 1996; John, 2001). As suggested by John (2001), the middle Tertiary igneous history of central Nevada is complicated by the temporal and spatial convergence of extrusive and intrusive magmatism associated with subduction (western andesite assemblage of the ancestral Cascades arc), continental rifting (bimodal basaltrhyolite assemblage), and Farallon slab foundering (interior andesite-rhyolite assemblage).

Major-oxide contents of the three northern Nevada age groups provide some insights with regard to primary petrogenetic controls on their compositional diversity. Perhaps the most obvious characteristic, one consistent with the importance of fractional crystallization, of northern Nevada intrusion major-oxide compositions is the linear to curvilinear nature of variation diagram abundance distributions (Fig. 5). However, abundances of many major oxides and trace elements in samples that contain less than 64 percent silica deviate from linear trends and scatter significantly. These relations are consistent with mixing a primitive (mantle) component with differing amounts of compositionally diverse (crustal) components. Given that the origin of most northern Nevada intrusive rocks is consistent with magmatic arc processes, a subduction-related, asthenospheric mantle component is probably one of the end members involved in genesis of northern Nevada intrusions. The second component is very likely a collection of crustal contaminants. Mixing various fractions of these two components is consistent with much of the compositional diversity depicted by mafic northern Nevada intrusions. Compositions of intermediate to felsic intrusions (>64 percent silica) follow trends consistent with fractional crystallization. In particular, abundances of many trace elements in samples with greater than 73 percent silica diverge dramatically from trends defined by lower silica samples in ways characteristic of extensive fractional crystallization. Geochemical trends (especially major-oxide abundance variations) indicative of fractional crystallization within individual intrusive systems are concealed within age-group data arrays.

Abundance variations of Al₂O₃, however, are distinctly nonlinear and amplify the importance of fractional crystallization. In samples with less than 64 weight percent SiO₂, Al₂O₃ abundances increase with increasing silica content. This trend results from fractionation of a mineral assemblage, primarily mafic silicates and Fe-Ti oxides, which contained relatively low alumina contents. The bend in the Al₂O₃ versus SiO₂ compositional array is probably coincident with the onset of significant (relatively alumina-rich) plagioclase crystallization and fractionation. Plagioclase crystallization caused residual silicate liquid to contain progressively less alumina at higher silica contents and reversed the alumina enrichment trend characteristic of samples with less than 60 weight percent SiO₂.

The significant role of fractional crystallization, especially during late-stage processes, is most clearly demonstrated by abundance variations of incompatible trace elements, including Rb, as depicted on a plot of Rb-Sr ratio ver sus SiO₂(Fig. 13). The vast majority of Rb-Sr ratios for northern Nevada intrusions increase slightly, in a linear fashion, from less than 0.1 to an average of ∼0.5, between 50 and 73 weight percent SiO₂. A significant number of samples with 73–77.5 weight percent have Rb-Sr SiO₂ ratios that lie along the linear trend of increasing Rb-Sr. However, superimposed on this trend is a group of samples, with more than ∼70 weight percent SiO₂, that have Rb-Sr ratios that increase dramatically from 1.5 to as much as 10 with increasing silica content. These samples are mostly from Tertiary intrusions but also include several from Cretaceous and Jurassic intrusions (Angel Lake, Bearpaw Mountain, Clipper Gap, Cocomongo, Coffeepot, Franklin Lake, Gabbs Valley Range, Garden Pass, Granite Peak East Range, Harrison-Green Mountain Creek, Horse Creek, Jakes Valley, King Peak, Millett north, New York Canyon, Pearl Creek, Smith Creek Snake, Steptoe Warm Springs, Tenabo, Terraces, Toano Springs, White Cloud Canyon, several unnamed intrusions in the Ruby Range, and many dikes). Data for these samples form a curvilinear array that depicts an exponential Rb-Sr ratio increase. These samples identify the point at which extensive fractional crystallization dominates processes involved in solidification and geochemical evolution of the most highly evolved northern Nevada intrusions.

A final insight into the petrogenetic evolution of northern Nevada intrusions derives from their relative abundances of Sr, K, and Rb. In igneous systems, abundances of Sr, K, and Rb are principally controlled by feldspar and biotite (Hanson, 1980). Abundances of Sr and Rb are highest and lowest in rocks that contain the highest and lowest plagioclase abundances, respectively. Compositions plotting nearest the Sr apex of the Rb-K/100-Sr ternary diagram (Fig. 14) represent the least evolved compositions, those principally associated with unevolved melts neither significantly contaminated by assimilated crust nor subjected to much fractional crystallization. With progressively greater amounts of fractional crystallization and crustal assimilation, involving assimilants with less plagioclase than the primary magma, K abundances increased relative to those of Sr. Because fractional crystallization appears to have had a significant role in the compositional evolution of northern Nevada intrusions, plagioclase crystallization and fractionation may be principally responsible for the observed Sr depletion. The preponderance of samples from northern Nevada intrusions form a compositional array that extends from primitive compositions near the Sr apex toward the K/100 apex; compositions with progressively greater relative abundances of K resulted from progressively greater amounts of plagioclase fractionation. The resulting evolutionary compositional array becomes indistinct as it approaches the K/100-Rb sideline on the Sr-K/100-Rb ternary diagram, where, as represented by compositions of a small subset of samples (Fig. 14), it turns toward the Rb apex. This dramatic trend change denotes the point at which potassium feldspar and biotite crystallization and fractionation become important to compositional evolution of northern Nevada intrusions. The compositional array is composed of overlapping, indistinguishable arrays of data for the three intrusion age groups. More evolved (Rb-enriched) samples are dominated by samples of Tertiary and Cretaceous intrusions; Rb-enriched samples of Jurassic intrusions are rare.

Relative Sr, K, and Rb abundances among northern Nevada intrusions also support a magmatic arc affinity. Abundances of these trace elements in samples of Andean arc rocks (du Bray et al., 1995), the archetypal continental, subduction-related magmatic arc, form a relative abundance array entirely coincident with that of the northern Nevada intrusions (Fig. 14). In contrast, within-plate intrusions represented by highly evolved granites of the Arabian Shield (du Bray et al., 1988) form an array that extends toward relatively Rb-enriched compositions along the Rb-K/100 sideline on the Sr-K/100-Rb ternary diagram. By these criteria, relative abundances of Sr, K, and Rb in the vast majority of samples of northern Nevada intrusions are consistent with an origin associated with subduction-related, arc magmatism. In contrast, compositions of the remaining samples indicate a weak similarity to within-plate magmatism.

A principal goal of this study was to evaluate associations between hydrothermal mineral deposits and intrusive rocks in a time, space, and compositional framework and develop descriptive and (or) genetic parameters that can help clarify relations between intrusions and mineral-deposit formation. Data compiled by du Bray and Crafford (2007) and du Bray et al. (2007) were combined with information (U.S. Geological Survey, 2005) contained in the Mineral Resources Database System (MRDS) to evaluate relations between mineral deposits/occurrences (hereafter referred to as deposits) and intrusive rocks. Data, representing almost 2800 mineral deposits in northern Nevada, were extracted from MRDS and classified (J.T. Nash and others, 2006, personal commun.) using the mineral-deposit model nomenclature of Cox and Singer (1986). The resulting data were culled, and deposit types not known to be associated with intrusive rocks (including massive sulfide, Almaden mercury, sedimentary exhalative, low-sulfide gold, and placer gold) were eliminated; the remaining deposits (n = 2561) were tabulated (see supplemental Excel files; ¹) and plotted (Fig. 15). As highlighted by Barton (1990, 1996), Ressel and Henry (2006), and Theodore (2000), many intrusions have a close spatial, and presumably a genetic association with a wide array of deposit types. Intrusions (du Bray and Crafford, 2007) with mineral deposit(s) within 2 km (in map view) were identified to tabulate the association of intrusions having specific ages and compositions with various mineral-deposit types. In the discussion that follows, mineral deposits and intrusions within 2 km are inferred to be genetically related, but that relationship, especially where contemporaneity is difficult to establish, is rarely fully demonstrable. Intrusions with several different nearby mineral-deposit types were annotated accordingly. This form of analysis portrays the propensity for intrusions of defined age and composition to be associated with particular mineral-deposit types. Small, incompletely characterized intrusions, identified only as granitic (n = 8) or intrusive rock (n = 9), were not included in the metallogenic synthesis; most of these intrusions do not have associated mineral deposits. However, a few of these intrusions (n = 7) have proximal polymetallic vein deposits, and two have associated polymetallic replacement deposits. The geologic map used to establish relations between intrusions and mineral deposits in northern Nevada limits the precision of this assessment. Geology was compiled (du Bray and Crafford, 2007) from 1:250,000-scale geologic maps, and some small or unknown intrusions may not have been included in the synthesis. More recent, larger scale geologic mapping, as well as subsurface information and geophysical data, can be used to refine subsequent site-specific interpretations.

Several relationships are common to mineral deposits and northern Nevada intrusions of all ages. Most obviously, mineral deposits in northern Nevada are dominantly associated with intrusions composed of monzogranite or granodiorite, regardless of age (Fig. 15). However, it is equally noteworthy that one-third to one-half of all monzogranite and granodiorite intrusions, of all ages, are not associated with mineral deposits of any type. Monzogranite or granodiorite intrusions are associated with several different mineral-deposit types. In contrast, the small number of intrusions that have other compositions is associated with deposits of limited mineral-deposit–type diversity. Polymetallic vein deposits (22C; mineral-deposit model numbers, used hereafter, of Cox and Singer, 1986) are the most common deposit type in northern Nevada. Both monzogranite and granodiorite intrusions, except those of Cretaceous age, are most commonly associated with polymetallic vein deposits (22C). Cretaceous granodiorite intrusions are more commonly associated with tungsten skarn deposits (14A) than intrusions of other ages and compositions (a relationship also identified by John and Bliss, 1993) and secondarily with polymetallic vein deposits. A moderate number of monzogranite and granodiorite intrusions, about equally distributed among the three intrusion age groups, are associated with tungsten vein deposits (15A). Despite the large number of polymetallic vein and tungsten vein deposits in northern Nevada, they are of little economic significance. Within the current and foreseeable economic framework, resources associated with few, if any, of these deposits represent viable, large-scale modern-mining targets, although the presence of these deposits may suggest other, more significant mineral-deposit types nearby. A few northern Nevada intrusions, especially those having intermediate to mafic compositions, are associated with various types of skarn deposits (14A, 18A, 18B, 18C, 18D, and 18F).

Of 129 well-characterized Tertiary intrusions, 47 (36 percent) have no known associated mineral deposits (Fig. 15A). Deposits proximal to Tertiary intrusions, other than polymetallic replacement and vein deposits (19A, 22C), are of relatively limited mineral-deposit type diversity and number (n = 38). A distinctive number of Tertiary monzogranite and, to a lesser extent, granodiorite intrusions, are associated with polymetallic replacement deposits (19A). A moderate number of northern Nevada Tertiary rhyolite and dacite intrusions are associated with several mineral-deposit types; among these, epithermal precious-metal deposits (25A, 25C, and 27A) are most common. Simmons et al. (2005) highlight the association between epithermal deposits and the subaerial parts of young, subduction-related volcanic centers. In northern Nevada, the greater abundance of epithermal deposits associated with Tertiary rhyolite and dacite compared to those associated with either Cretaceous or Jurassic subvolcanic intrusions probably reflects a relative lack of preservation among older, more deeply eroded intrusions. Given the Eocene age (Hofstra et al., 1999; Ressel et al., 2000; Cline et al., 2005; Ressel and Henry, 2006) accepted for Carlin-type gold deposits (26A), it is noteworthy that none of these deposits are within 2 km of volumetrically significant, presently exposed Tertiary intrusions (du Bray and Crafford, 2007); however, Tertiary dikes, possibly derived from larger, shallow intrusions, are associated with some of these deposits. Significantly, several Tertiary northern Nevada intrusions, which have a range of compositions, are associated with various important mineral-deposit types, including distal disseminated Au-Ag (19C, n = 2), porphyry copper (21A, n = 2), gold skarn (18F, n = 1), low-fluorine porphyry molybdenum (21B, n = 1), and Climax molybdenum (16, n = 1) deposits; these are large deposits that have (or may) contribute significantly to regional metal production (see supplemental Excel files; ¹).

Of 168 well-characterized northern Nevada Cretaceous intrusions, 39 (23 percent) have no associated mineral deposits. Many (n = 78) Cretaceous intrusions are associated with a limited diversity of mineral-deposit types other than polymetallic replacement and vein (19A, 22C) deposits. The most distinctive metallo-genic feature of Cretaceous intrusions is the large number of granodiorite, and to a lesser extent, monzogranite intrusions associated with W skarn (14A) deposits (John and Bliss, 1993). A moderate number of Cretaceous granodiorite and monzogranite intrusions are also associated with W vein (15A) deposits. Given the association of Tertiary and Cretaceous intrusions in western North America with porphyry copper deposits (21A) (Seedorf et al., 2005), the small number (n = 4) of Cretaceous intrusions in northern Nevada known to host known porphyry copper deposits is noteworthy. Four other northern Nevada Cretaceous intrusions (Birch, Mineral Point, Todd Mountain, and Osgood) are spatially associated with Carlin-type gold deposits (26A). Deposits associated with these intrusions are not typical of Carlin-type deposits along the Battle Mountain-Eureka and Carlin trends (Cline et al., 2005) with respect to size and (or) geologic character; in addition, those associated with the Mineral Point and Osgood intrusions may represent superimposition of several deposit types. The spatial association between Carlin-type gold deposits and pre-Eocene intrusions is a manifestation of both being localized by fault zones coincident with major northern Nevada mineral belts. In addition, the rheological contrast between intrusive rocks and their hosts create structural features that focus hydrothermal fluid flow (Hofstra and Cline, 2000; Cline et al. 2005). Consequently, spatial associations between these Cretaceous intrusions and Carlin-type gold deposits are not meaningful, especially in light of the Eocene age of most Carlin-type gold deposits (Hofstra et al., 1999; Ressel et al., 2000; Cline et al., 2005; Ressel and Henry, 2006). Finally, several Cretaceous northern Nevada intrusions, most composed of monzogranite or granodiorite, are associated with low-fluorine porphyry molybdenum (21B, n = 3), distal disseminated Au-Ag (19C, n = 2), and generally small, epithermal vein (25A, 25C, and 27A, n = 10) deposits.

Of 85 well-characterized northern Nevada Jurassic intrusions, 19 (22 percent) have no associated mineral deposits. The Jurassic intrusions are the least distinctive of the three intrusion age groups with respect to associated mineral deposits. A moderate number (n = 42) of Jurassic intrusions are associated with a limited number of mineral-deposit types, other than polymetallic replacement and vein (19A, 22C) deposits, which are especially common near intrusions composed of monzogranite and granodiorite. Several Jurassic intrusions are also associated with W vein (15A) and or W skarn (14A) deposits. The number (n = 5) of granodiorite Jurassic intrusion-associated Cu skarn deposits (18B) is also noteworthy. Four Jurassic intrusions, composed of granodiorite or quartz monzonite, are spatially associated with Carlin-type gold deposits (26A); however, the age of these intrusions is inconsistent with a genetic relationship. One intrusion (Bald Mountain) is spatially and probably genetically associated with a distal disseminated (19C) deposit. Given the economic importance of this deposit type, its association with a Jurassic intrusion is noteworthy. Seven intrusions, composed of monzogranite or granodiorite, are spatially associated with epithermal vein deposits (25A and 25C) and three intrusions are associated with antimony deposits (27D). Finally, three northern Nevada Jurassic intrusions are associated with iron deposits (25I) that may be iron-oxide–copper gold deposits (Johnson and Barton, 2000). The size and diversity of metals potentially associated with deposits of this type make them attractive exploration targets (Barton and Johnson, 1996; Williams et al., 2005). The association of Jurassic northern Nevada intrusions with known iron deposits could be significant relative to undiscovered iron-oxide–copper gold deposit potential.

Granodiorite and monzogranite compositions are most common among nearly 360 northern Nevada intrusions. The Tertiary age group includes almost 20 percent rhyolite and dacite intrusions; otherwise, northern Nevada contains few intrusions composed of anything other than granodiorite, monzogranite, rhyolite, and dacite. Mesozoic and Cenozoic intrusive magmatism in northern Nevada occurred in three discrete pulses: Jurassic (175–145 Ma), Cretaceous (115–65 Ma), and middle Tertiary (40–15 Ma). These three pulses are represented by subequal numbers of intrusions with relative abundances as follows: Cretaceous > Tertiary > Jurassic. The relative distributions of modal compositions among the three intrusion age groups are essentially indistinguishable except for a large number of rhyolite and dacite intrusions restricted to the Tertiary. The relative abundance of Tertiary intrusions composed of rhyolite and dacite probably reflects their preferential preservation rather than an actual time-composition distribution disparity. The majority of northern Nevada intrusions are composed of subalkaline, high-K (Ewart, 1982; Le Maitre, 1989) calc-alkalic (Frost et al., 2001) rock that is metaluminous to weakly peraluminous. About 20 percent of analyzed samples have ASI values greater than 1.10.

Major-oxide and trace-element characteristics of northern Nevada intrusions age groups are remarkably similar, and with minor noted exceptions form compositional arrays that are continuous, completely overlapping, and indistinguishable. The similar geochemical composition of these many intrusions suggests that the basic petrologic and tectonic processes responsible for their genesis, during ∼150 million years, were essentially unchanging. This conclusion is supported by lead and strontium isotopic data, summarized by Tosdal et al. (2000), which indicates that intrusions across most of northern Nevada bear a mantle imprint that has been modified by isotopically distinct crust that underlies this region. Furthermore, they note that across much of northern Nevada, isotopic data for intrusions “… have the same trend regardless of pluton age …” and appear consistent with a genesis involving thinned, transitional continental crust. However, Tosdal et al. (2000) point out that the lead and strontium isotopic characteristics of intrusions in eastern Nevada, especially large plutons in the Kern Mountains and the Snake Range, and to a lesser extent intrusions in the Ruby Mountains, are distinctive and probably related to partial melting that involved Proterozoic crust. Many intrusions in easternmost northern Nevada are also consistently more peraluminous and have higher Rb-Sr ratios than most northern Nevada intrusions. Miller and Bradfish (1980) suggest that the genesis of strongly peraluminous intrusions (including those in easternmost Nevada) in the inner Cordillera of North America, involved a significant contribution from older pelitic or quartzofeldspathic sedimentary sources. Lee et al. (2003) point out that Neoproterozoic rocks of the McCoy Creek Group, pelitic and quartzofeldspathic sedimentary deposits, crop out extensively in the eastern Great Basin and would be fertile sources for generation of aluminous, Rb-enriched magmas. Consequently, distinctive isotopic compositions (Tosdal et al., 2000) and peraluminous compositions of intrusions exposed in the Kern and Ruby Mountains (the latter also distinguished by elevated Rb-Sr ratios) and the Snake Range are consistent with a genesis involving rocks of the McCoy Creek Group. Otherwise, isotopic diversity of the crust beneath most of northern Nevada is relatively minor, and processes responsible for magma genesis were sufficiently consistent that, in terms of major-oxide, trace-element, and isotopic compositions, the resulting intrusions are essentially indistinguishable within the Jurassic-middle Tertiary time-space framework.

A variety of indicators suggest that magmas represented by most northern Nevada intrusions are subduction-related products of arc magmatism. Relative abundances of FeO*, total alkalis, and MgO (Fig. 11); Rb, Y, and Nb (Fig. 12); and various trace-element abundance ratios are all consistent with most intrusions of each northern Nevada age group having origins related to subduction and arc magmatism. None of these indicators imply a within-plate origin for more than a small percentage of northern Nevada intrusions. Limited data indicative of a within-plate origin are for evolved Tertiary intrusions, which by virtue of their age and locus within an evolving tectonic framework, interacted with thicker, more completely developed continental lithosphere; these intrusions probably experienced greater degrees of petrogenetic evolution and assimilated relatively greater amounts of continental crust, both of which imparted a weakly developed within-plate geochemical signature. Given the inferred orientation and configuration of mid-Mesozoic through Cenozoic subduction along western North America (Dickinson, 2006), it seems plausible that intrusion geochemical signatures might vary systematically across northern Nevada as has been demonstrated for other magmatic arcs (Feeley, 1993). Although lead and strontium isotope data vary subtly but systematically across northern Nevada (Tosdal et al., 2000), other geochemical metrics, such as Rb-Sr ratio, imply no such systematic variation with respect to either longitude or latitude. In addition, intrusions of all three age groups are present, without significant gaps, across northern Nevada, which suggests that subduction-related processes responsible for magma genesis across this region must have been relatively consistent through time and space. Accounting for Tertiary tectonic extension results in even more pronounced intrusion continuity across northern Nevada.

Almost all compositional arrays formed by analyses of northern Nevada intrusions are linear to curvilinear and, therefore, are consistent with fractional crystallization. However, compositions of the most mafic intrusions may reflect mixing involving a subduction-zone–derived melt and various amounts and types of crustal contaminants related to the path traced by unevolved melts traversing upward through the crust during emplacement. Lead and strontium isotopic data for northern Nevada intrusions synthesized by Tosdal et al. (2000) are also largely consistent with a genesis involving mantle-derived magma mixed with continental crust in varying proportions. Although fractional crystallization played an important role in the petrogenetic evolution of northern Nevada magmatism, the geochemistry of only the most evolved (mostly Tertiary) intrusions is dominated by fractional crystallization. Trace-element abundance arrays for some intrusions with more than ∼73 weight percent silica deviate from linearity as a consequence of significant incompatible element behavior. This nonlinearity is an important feature that defines intrusions or parts thereof in which fractional crystallization became the dominant petrogenetic process. In summary, genesis of magmas represented by northern Nevada intrusions is akin to processes described by Hildreth and Moorbath (1988) for magma systems of the modern Andean arc, and represents the complex interplay between subduction processes, including generation of mantle-derived basalt, magma ascent and consequent mixing/assimilation of crustal contaminants, and protracted fractional crystallization in crustal reservoirs.

Intrusion age and composition data, combined with mineral-deposit occurrence information, constrain the metallogenic contributions of northern Nevada intrusions. Aside from distinctions outlined below, intrusion age and composition show little correlation to type, abundance, and size of associated mineral deposits (Fig. 16). Polymetallic vein and tungsten skarn/vein deposits, the most abundant types of mineral deposits in northern Nevada, are most commonly associated with granodiorite or monzogranite, but are not strongly correlated with intrusions of any particular age. Moreover, these deposit types are associated with intrusions representing the full compositional range (Fig. 16).

Although northern Nevada is renowned for Carlin-type gold, porphyry copper, distal-disseminated Au-Ag, and skarn deposits that are spatially associated with intrusions, a relatively small proportion of intrusions, of any age or composition are associated with significant mineral deposits (Fig. 16). The Battle Mountain area, for example, is well known (Theodore and Blake, 1975) for associations between Tertiary and Cretaceous granodiorite to monzogranite intrusions and porphyry copper, gold skarn, distal disseminated Au-Ag, and low-fluorine porphyry molybdenum deposits. However, the vast majority of northern Nevada intrusions of these ages and compositions do not have similar associated deposits. Similarly, although a spatial relationship exists between the Brown stock, gold-skarn deposits (Brooks et al., 1991), and distal disseminated Au-Ag (plus associated polymetallic base metal) deposits (Johnston, 2003), relatively few northern Nevada Eocene granodiorite intrusions are associated with deposits of this type (Fig. 16). Northern Nevada intrusions with associated mineral deposits are not characterized by distinctive volatile or metallic-element abundances, which further emphasizes the lack of a diagnostic relationship. Primary magmatic volatile constituent and metallic-element abundances of northern Nevada intrusions are not elevated relative to average intrusive rock abundances. Furthermore, volatile-constituent and metallic-element abundances among the three intrusion age groups are indistinguishable. Significantly, these observations imply that particular northern Nevada intrusions can be approximately coeval and spatially associated with mineral deposits without themselves containing unusual abundances of the ore metals or volatile constituents required to scavenge, transport, concentrate, and ultimately precipitate ore minerals in deposits. A strong relationship between the majority of large mineral deposits in northern Nevada and well-delineated mineral-deposit trends, including the Carlin, Battle Mountain-Eureka, and Getchell trends, has been well demonstrated. However, although intrusive igneous rocks are also abundant along these trends, data synthesized in this study indicate that neither the age, modal composition, nor geochemistry of these rocks is distinctive. Consequently, these features do not foster accurate identification of particular intrusions associated with the largest and most economically important types of deposits. A plethora of information indicates that in northern Nevada, most mineral deposits are spatially associated with intrusions, but the converse, that most intrusions are associated with mineral deposits, does not follow.

Given the lack of diagnostic petrologic features for northern Nevada intrusions, it is uncertain that presently available geochemical data can be of any use in identification of intrusions likely to have associated mineral deposits. These relations are in accord with the findings of Seedorf et al. (2005), who suggest, relative to the geochemical compositions of intrusions associated with porphyry deposits, “… compositions of most porphyry intrusions are not exceptional.” Unfortunately, available data suggest that the utility of intrusion geochemistry, modal composition, and age data as exploration guides is fairly limited. Nonetheless, this compilation of northern Nevada intrusive rock geochemical characteristics provides a framework for future comparison of intrusive rocks related to mineral deposits. Acquisition and application of different types of geochemical data, including more precise trace-element data, accessory mineral compositions, radiogenic and stable-isotope data, and systematic collection of data for an expanded set of trace elements, might yield criteria diagnostic of intrusions with associated mineral deposits.

It seems significant that Carlin-type gold deposits, the largest and economically most important deposits in northern Nevada, show little evidence of direct magmatic inputs to mineral-deposit formation (Ilchik and Barton, 1997; Hofstra et al., 1999; Hofstra and Cline, 2000; Emsbo et al., 2003; Cline et al., 2005; Emsbo et al., 2006). Locally, Eocene rhyolite and dacite dikes are the most obvious source of the thermal energy required to drive the fluid circulation essential to formation of Carlin-type gold deposits (Ressel and Henry 2006; Emsbo et al., 2006). However, the vast majority of compositionally indistinguishable Eocene dikes in northern Nevada are not associated with known deposits of this type.

The observation that neither intrusion composition nor age are critical to genesis of particular mineral-deposit types suggests that spatially superposed geologic factors might control mineral-deposit formation rather than particular petrologic characteristics of intrusions. As well summarized by Hofstra and Wallace (2006), the mineral deposits of this region developed and were localized by “… a wide range of structures, basins, magmas, and fluids …” Concatenation of the right type of ore metal-rich rock (source), hydrothermal fluids that have the required redox characteristics (transport medium), conductive structural features (conduit), and reactive/receptive host rocks (sink), may all have played a role in formation of Nevada's large mineral deposits. Perhaps, as suggested by Emsbo et al. (2006), northern Nevada metallogeny may result more from structural and chemical heritage than the operation of special magmatic or fluid-related processes. This hypothesis is consistent with the multiple periods of gold-deposit formation that define the mineral trends, and is in accord with the lack of distinguishable geochemical and petrographic characteristics of intrusive rocks along the mineral trends.

Although the geochemistry, age, and modal composition of northern Nevada intrusions are reasonably well characterized and a regional synthesis of these data is now available, considerably more work needs to be done to better understand the petrogenesis and metallogenesis of these intrusions. Promising research avenues include: (1) using data described in this paper and elsewhere, to systematically define the characteristics and evolutionary history of individual intrusions associated with mineral deposits in the continuing search for diagnostic identification criteria; (2) acquire additional, more precise geochemical data (particularly for intrusions associated with mineral deposits) for a broader spectrum of geochemical constituents; (3) use microanalytical techniques to determine the composition of potentially diagnostic accessory minerals; (4) conduct additional geochronologic investigations to determine ages of undated northern Nevada intrusions; (5) considerably increase the quantity of radiogenic and stable-isotope data available for intrusions associated with mineral deposits and those that are barren; (6) employ microanalytical techniques to analyze melt and fluid inclusions to further evaluate the links between intrusions (particularly their ore-metal and volatile contents) and mineral deposits; (7) conduct large-scale geologic mapping to characterize relations between intrusions and mineral deposits; (8) compare existing and new data for Miocene intrusive rocks with that for other middle Tertiary rocks to determine whether they are distinctive; and (9) establish intrusion size and depth of exposure and relate them to presence or absence of associated mineral deposits. Acquisition and synthesis of new data will certainly better define any genetic associations between northern Nevada intrusions and mineral deposits and contribute to development of diagnostic criteria.

A considerable number of individuals have helped make this study possible. Interaction with many members of the Metallogeny of the Great Basin project, particularly J.T. Nash, Poul Emsbo, A.H. Hofstra, and D.A. John, were critical and provided many of the insights and knowledge necessary to complete this study. Comments on an early version of the paper by Poul Emsbo, J.T. Nash, and C.J. Nutt are much appreciated and helped clarify and focus the study. Work by J.T. Nash, enhancing the content of the northern Nevada segment of the MRDS records, was critical to the metallogenic interpretation. The U.S. Geological Survey Mineral Resources Program provided the funding that made this study possible. Geosphere reviews by E.H. Christiansen and S.D. Ludington were very constructive and helped considerably improve this report.