Christopher D. Henry

The mobility of zirconium and other immobile' elements during hydrothermal alteration

Abstract Development of zircon and other Zr phases in hydrothermal deposits indicates that Zr can be highly mobile in these systems. Mobility is most common in, but not restricted to, F-rich hydrothermal systems related to alkalic, F-rich igneous suites; these suites can range from peralkaline through metaluminous to peraluminous. A few examples are neither alkalic nor F rich. Three locations in the Trans-Pecos Magmatic Province, Texas, U.S.A., demonstrate this hydrothermal Zr mobility. All three igneous systems are alkalic and F rich but vary in alkali/Al ratios. Peralkaline rhyolites and trachytes in the Christmas Mountains contain as much as 2100 ppm Zr, mostly in aegirine or arfvedsonite; zircon is rare or absent. Fluorspar replacement deposits in limestone at contacts with the rhyolites contain as much as 38,000 ppm Zr, occurring as small, disseminated zircons. The deposits also are enriched in a variety of incompatible elements, including Be, rare-earth elements (REE), Y, Nb, Mo, Hf, Pb, Th and U. The Sierra Blanca intrusions, a series of mildly peraluminous, F-rich rhyolite laccoliths, contain as much as 1000 ppm Zr, mostly as zircon. Hydrothermal zircon occurs as overgrowths on magmatic grains, as veinlets connected to overgrowths, and in fluorspar replacement bodies in adjacent limestone. The highest Zr concentrations in fluorspar are ∼200 ppm. Metaluminous quartz monzonite from the Infiernito caldera contains 400–600 ppm Zr, mostly as zircon. Euhedral zircon in quartz-fluorite veins in the quartz monzonite indicates mobility of Zr. Zirconium concentrations in the veins are unknown, but the paucity of zircon suggests little Zr enrichment relative to the host. Zircon and, more rarely, zirconolite, occur in skarn in the Ertsberg District of Irian Jaya, Indonesia. Unlike in Texas, related igneous rocks are metaluminous, and the hydrothermal system was F poor. Worldwide, hydrothermal zircon, other Zr phases, and Ti- and Al-bearing phases occur in skarn, epithermal precious metal veins, volcanogenic massive-sulfide deposits and mylonites. We propose that differences in Zr mineralogy of igneous source rocks is an important factor in determining the availability of Zr to hydrothermal fluids. Although Zr concentrations in the Sierra Blanca and Christmas Mountains rhyolites are similar, Zr enrichment in fluorspar was much greater in the Christmas Mountains. We suggest that hydrothermal solutions could easily break down aegirine and arfvedsonite to release Zr, but that zircon was only moderately attacked. Therefore, far more Zr was available for transport and subsequent deposition in the Christmas Mountains than at Sierra Blanca. Availability of other trace elements probably is also governed by their mineral host. Although Zr mobility is most common in F-rich hydrothermal systems related to alkalic and F-rich igneous systems, mobility at Ertsberg may have been promoted by sulfate complexing.

Kinematics of the northern Walker Lane: An incipient transform fault along the Pacific–North American plate boundary

In the western Great Basin of North America, a system of dextral faults accommodates 15%–25% of the Pacific–North American plate motion. The northern Walker Lane in northwest Nevada and northeast California occupies the northern terminus of this system. This young evolving part of the plate boundary offers insight into how strike-slip fault systems develop and may reflect the birth of a transform fault. A belt of overlapping, left-stepping dextral faults dominates the northern Walker Lane. Offset segments of a W-trending Oligocene paleovalley suggest ∼20–30 km of cumulative dextral slip beginning ca. 9–3 Ma. The inferred long-term slip rate of ∼2–10 mm/yr is compatible with global positioning system observations of the current strain field. We interpret the left-stepping faults as macroscopic Riedel shears developing above a nascent lithospheric-scale transform fault. The strike-slip faults end in arrays of ∼N-striking normal faults, suggesting that dextral shear diffuses into extension in the Great Basin. Coeval extension and dextral shear have induced slight counterclockwise fault-block rotations, which may ultimately rotate Riedel shears toward the main shear zone at depth, thus facilitating development of a throughgoing strike-slip fault.

The real southern Basin and Range: Mid- to late Cenozoic extension in Mexico

Much of northern and central Mexico underwent east-northeast extension in the mid- to late Cenozoic; this area constitutes a major but little-recognized part of the Basin and Range province. The extended region is bounded on the east by the Laramide thrust front of the Sierra Madre Oriental. On the west, the relatively unfaulted Sierra Madre Occidental separates the extended area in central Mexico from that around the Gulf of California. Extension occurred as far south as what is now the Trans-Mexican volcanic belt, and in Oaxaca, south of the belt. The Basin and Range province in Mexico constitutes approximately half of the 19 x 10 5 km 2 of western North America that underwent mid- to late Cenozoic extension. North-northwest orientations of numerous epithermal vein systems indicate that east-northeast extension began as early as 30 Ma in areas north of the volcanic belt. Major episodes of faulting began at about 23 to 24 Ma and 12 to 13 Ma, both in Mexico and in the southwestern United States. Faulting was commonly accompanied by eruption of alkali basalts typical of intraplate rifting. Widespread Quaternary fault scarps and alkali basalts indicate that extension continues to the present in the region north of the Trans-Mexican volcanic belt. In contrast, the tectonics in and south of the belt are now probably related to subduction of the Rivera plate. Contemporaneity of the 12 Ma episode with early extension around the Gulf of California attributed to Pacific-North American plate boundary reorganization suggests that extension is related predominantly to plate boundary effects. The beginning of extension at ∼30 Ma may be related to initial encroachment of the East Pacific Rise upon the trench that lay off western North America.

Ash-flow tuffs and paleovalleys in northeastern Nevada: Implications for Eocene paleogeography and extension in the Sevier hinterland, northern Great Basin

Northeastern Nevada is generally interpreted as an area of large-magnitude Eocene extension possibly due to gravitational collapse of crust thickened during the Sevier orogeny. The extensional interpretation is based in part on the presence of widespread Eocene conglomerates and lacustrine basins, as well as on thermochronology-based evidence of major Eocene cooling and uplift of the Ruby Mountains–East Humboldt Range core complex. The distribution of 45–40 Ma ash-flow tuffs and interbedded coarse conglomerates and lacustrine deposits, however, indicates they were predominantly deposited in a system of east-draining paleovalleys incised into a plateau or moderate-relief upland. A large, contiguous sedimentary basin probably was never present. Paleovalleys were as much as 10 km wide and 500 m to possibly as much as 1.6 km deep, based on the thickness of intra-valley deposits. Ash-flow tuffs are widely distributed near source calderas but are almost entirely confined to the paleovalleys as little as 20 km from their source. Basal, mostly pre-volcanic conglomerates contain clasts up to 6 m in diameter. The clasts are well rounded, indicating significant fluvial transport, not derivation from nearby fault scarps. Lacustrine deposits also are restricted to paleovalleys and accumulated during two periods that are interpreted to coincide with episodes of minor, northwest-directed extension, one before 41 Ma and possibly as old as 46 Ma, and another between 40 and 38 Ma. Extension formed small displacement, northeast-striking, mostly down-to-the-northwest faults that temporarily dammed the paleovalleys to form lakes. Lakes probably also formed where volcanic rocks or landslides dammed paleovalleys, a common process both in the Eocene and historically in the western United States. The absence of major Eocene extension suggests that gravitational collapse of overthickened crust, even assisted by thermal weakening of lithosphere by intense magmatism, was not sufficient to generate major extension. Absolute elevation of the high plateau is uncertain, but it was high enough to have paleovalleys as much as 1.6 km deep. Based on published paleoflora data, interfluves could have been at elevations of ∼4 km. The Eocene paleovalleys in northeastern Nevada most likely drained eastward to remnants of the Uinta basin. An approximately north-south paleodivide through northeastern Nevada separated these east-draining paleovalleys from paleovalleys that drained westward to the Pacific Ocean.

Plate interactions control middle-late Miocene, proto-Gulf and Basin and Range extension in the southern Basin and Range

Middle‐late Miocene (proto-Gulf; ~12‐6 Ma) extension around the Gulf of California (Gulf Extensional Province) is commonly interpreted as resulting from partitioning of oblique Pacific‐North American plate motion into strike‐ slip displacement along the margin and east‐northeast extension perpendicular to the margin within the North American plate. We propose that this mechanism also applies to kinematically similar, predominantly east‐northeast extension that occurred at the same time throughout the southern Basin and Range province, from southern Arizona and New Mexico to the Trans-Mexican Volcanic Belt. New field and 40Ar/39Ar data in Sinaloa and Durango confirm that this episode of extension occurred on the mainland side of the Gulf and in the Basin and Range east of the Sierra Madre Occidental, which is generally considered the eastern margin of the Gulf Extensional Province. Published data indicate the middle‐late Miocene episode also occurred across the northern and southern ends of the Sierra Madre where the Gulf Extensional Province connects with the Basin and Range: (1) from central Sonora into southern Arizona and New Mexico, and (2) from Nayarit into central Mexico north of the Trans-Mexican Volcanic Belt. This episode appears to have aVected an area that continues to the eastern edge of the Basin and Range province in Texas and San Luis Potosi. Recognition that this episode of extension aVected the entire southern Basin and Range resolves the discrepancy between the amount of extension calculated based on plate reconstructions and that based on field data within the Gulf Extensional Province alone. Published plate reconstructions require 160 to 110 km of east‐northeast extension between ~12 and 6 Ma. If taken up solely within the Gulf Extensional Province, this would have generated 66 to 78% extension, which is much greater than observed. Spread across the entire southern Basin and Range it requires only ~20% total extension, which is more consistent with observations of cumulative extension between 12 and 6 Ma. Extension was partitioned into the Gulf Extensional Province because (1) it lies between two stable batholith belts (Mesozoic Peninsular Ranges on the west and mid-Tertiary Sierra Madre Occidental on the east) that resisted extension and (2) the Gulf was thermally weakened by immediately preceding arc magmatism. Extension in the main Basin and Range province in part probably avoided the relatively strong, batholithic crust of the Sierra Madre Occidental.

Distinguishing strongly rheomorphic tuffs from extensive silicic lavas

High-temperature silicic volcanic rocks, including strongly rheomorphic tuffs and extensive silicic lavas, have recently been recognized to be abundant in the geologic record. However, their mechanisms of eruption and emplacement are still controversial, and traditional criteria used to distinguish conventional ash-flow tuffs from silicic lavas largely fail to distinguish the high-temperature versions. We suggest the following criteria, ordered in decreasing ease of identification, to distinguish strongly rheomorphic tuffs from extensive silicic lavas: (1) the character of basal deposits; (2) the nature of distal parts of flows; (3) the relationship of units to pre-existing topography; and (4) the type of source. As a result of quenching against the ground, basal deposits best preserve primary features, can be observed in single outcrops, and do not require knowing the full extent of a unit. Lavas commonly develop basal breccias composed of a variety of textural types of the flow in a finer clastic matrix; such deposits are unique to lavas. Because the chilled base of an ashflow tuff generally does not participate in secondary flow, primary pyroclastic features are best preserved there. Massive, flow-banded bases are more consistent with a lava than a pyroclastic origin. Lavas are thick to their margins and have steep, abrupt flow fronts. Ashflow tuffs thin to no more than a few meters at their distal ends, where they generally do not show any secondary flow features. Lavas are stopped by topographic barriers unless the flow is much thicker than the barrier. Ash-flow tuffs moving at even relatively slow velocities can climb over barriers much higher than the resulting deposit. Lavas dominantly erupt from fissures and maintain fairly uniform thicknesses throughout their extents. Tuffs commonly erupt from calderas where they can pond to thicknesses many times those of their outflow deposits. These criteria may also prove effective in distinguishing extensive silicic lavas from a postulated rock type termed lava-like ignimbrite. The latter have characteristics of lavas except for great areal extents, up to many tens of kilometers. These rocks have been interpreted as ash-flow tuffs that formed from low, boiling-over eruption columns, based almost entirely on their great extents and the belief that silicic lavas could not flow such distances. However, we interpret the best known examples of lava-like ignimbrites to be lavas. This interpretation should be tested through additional documentation of their characteristics and research on the boiling-over eruption mechanism and the kinds of deposits it can produce. Flow bands, flow folds, ramps, elongate vesicles, and probably upper breccias occur in both lavas and strongly rheomorphic tuffs and are therefore not diagnostic. Pumice and shards also occur in both tuffs and lavas, although they occur throughout ash-flow tuffs and generally only in marginal breccias of lavas. Dense welding, secondary flow, and intense alteration accompanying crystallization at high temperature commonly obliterate primary textures in both thick, rheomorphic tuffs and thick lavas. High-temperature silicic volcanic rocks are dominantly associated with tholeiitic flood basalts. Extensive silicic lavas could be appropriately termed flood rhyolites.

Sierra Nevada–Basin and Range transition near Reno, Nevada: Two-stage development at 12 and 3 Ma

Relative and absolute elevations of the Sierra Nevada and adjacent Basin and Range province, timing of their differentiation, and location, amount, and timing of strike-slip movement between them are controversial. The provincial boundary near Reno developed in two stages. (1) At ca. 12 Ma, the ≥700 km 2 Verdi-Boca sedimentary basin formed across what was to become the boundary, probably as a result of a small-magnitude but regional extensional episode that affected much of the western Basin and Range. (2) At 3 Ma, the basin was complexly faulted and folded during a larger magnitude extensional episode that established the modern Sierran structural and topographic boundary in this area. The boundary is really a transition zone with a western edge along the Donner Pass, California, fault zone, which is farther west than previously placed. Both episodes appear to have resulted from east-west extension only, which suggests that northwest motion of the Sierra Nevada relative to the Basin and Range shown by geodetic data began after 3 Ma or was taken up farther east.

Eocene–Early Miocene paleotopography of the Sierra Nevada–Great Basin–Nevadaplano based on widespread ash-flow tuffs and paleovalleys

The distribution of Cenozoic ash-flow tuffs in the Great Basin and the Sierra Nevada of eastern California (United States) demonstrates that the region, commonly referred to as the Nevadaplano, was an erosional highland that was drained by major west- and east-trending rivers, with a north-south paleodivide through eastern Nevada. The 28.9 Ma tuff of Campbell Creek is a voluminous (possibly as much as 3000 km 3 ), petrographically and compositionally distinctive ash-flow tuff that erupted from a caldera in north-central Nevada and spread widely through paleovalleys across northern Nevada and the Sierra Nevada. The tuff can be correlated over a modern area of at least 55,000 km 2 , from the western foothills of the Sierra Nevada to the Ruby Mountains in northeastern Nevada, present-day distances of ∼280 km west and 300 km northeast of its source caldera. Corrected for later extension, the tuff flowed ∼200 km to the west, downvalley and across what is now the Basin and Range–Sierra Nevada structural and topographic boundary, and ∼215 km to the northeast, partly upvalley, across the inferred paleodivide, and downvalley to the east. The tuff also flowed as much as 100 km to the north and 60 km to the south, crossing several east-west divides between major paleovalleys. The tuff of Campbell Creek flowed through, and was deposited in, at least five major paleovalleys in western Nevada and the eastern Sierra Nevada. These characteristics are unusual compared to most other ash-flow tuffs in Nevada that also flowed great distances downvalley, but far less east and north-south; most tuffs were restricted to one or two major paleovalleys. Important factors in this greater distribution may be the great volume of erupted tuff and its eruption after ∼3 Ma of nearly continuous, major pyroclastic eruptions near its caldera that probably filled in nearby topography. Distribution of the tuff of Campbell Creek and other ash-flow tuffs and continuity of paleovalleys demonstrates that (1) the Basin and Range–Sierra Nevada structural and topographic boundary did not exist before 23 Ma; (2) the Sierra Nevada was a lower, western ramp to the Nevadaplano; and (3) any faulting before 23 Ma in western Nevada, including in what is now the Walker Lane, and before 29 Ma in northern Nevada as far east as what is now the Ruby Mountains metamorphic core complex, was insufficient to disrupt the paleodrainages. These data are further evidence that major extension in Nevada occurred predominantly in the late Cenozoic. Characteristics of paleovalleys and tuff distributions suggest that the valleys resulted from prolonged erosion, probably aided by the warm, wet Eocene climate, but do not resolve the question of the absolute elevation of the Nevadaplano. Paleovalleys existed at least by ca. 50 Ma in the Sierra Nevada and by 46 Ma in northeastern Nevada, based on the age of the oldest paleovalley-filling sedimentary or tuff deposits. Paleovalleys were much wider (5–10 km) than they were deep (to 1.2 km; greatest in western Nevada and decreasing toward the paleo–Pacific Ocean) and typically had broad, flat bottoms and low-relief interfluves. Interfluves in Nevada had elevations of at least 1.2 km because paleovalleys were that deep. The gradient from the caldera eastward to the inferred paleodivide had to be sufficiently low so that the tuff could flow upstream more than 100 km. Two Quaternary ash-flow tuffs where topography is nearly unchanged since eruption flowed similar distances as the mid-Cenozoic tuffs at average gradients of ∼2.5–8 m/km. Extrapolated 200–300 km (pre-extension) from the Pacific Ocean to the central Nevada caldera belt, the lower gradient would require elevations of only 0.5 km for valley floors and 1.5 km for interfluves. The great eastward, upvalley flow is consistent with recent stable isotope data that indicate low Oligocene topographic gradients in the Nevadaplano east of the Sierra Nevada, but the minimum elevations required for central Nevada are significantly less than indicated by the same stable isotope data. Although best recognized in the northern and central Sierra Nevada, early to middle Cenozoic paleodrainages may have crossed the southern Sierra Nevada. Similar early to middle Cenozoic paleodrainages existed from central Idaho to northern Sonora, Mexico, and persisted over most of that region until disrupted by major Middle Miocene extension. Therefore, the Nevadaplano was the middle part of an erosional highland that extended along at least this length. The timing of origin and location of this more all-encompassing highland indicates that uplift was predominantly a result of Late Cretaceous (Sevier) contraction in the north and a combination of Late Cretaceous–early Cenozoic (Sevier and Laramide) contraction in the south.

Late Cenozoic Basin and Range structure in western Mexico adjacent to the Gulf of California

The area that surrounds the Gulf of California was intensely faulted during the late Cenozoic prior to opening of the gulf. In a representative 10,000-km 2 area of southern Sinaloa, the faults consist of a predominant north-northwest-striking set having normal displacement and a subsidiary east-northeast set having largely strike-slip displacement. Two domains are recognized: one in which both fault trends are abundant and one in which east-northeast faults are minor. In the former, north-northwest faults form a series of mostly southwest-tilted half-grabens filled with upper Tertiary sediments. Displacement on individual faults is commonly as much as several kilometers. East-northeast faults probably represent accommodation zones between areas of differential extension. In the second domain, north-northwest faults form an extensive graben system. Major faults, dipping 40° to 70° into the graben and having several kilometers of cumulative displacement, are spaced every 5 to 10 km. Bedding attitudes indicate that fault blocks are rotated as much as 65°. Dependent upon assumptions about subsurface geometry of the faults, total extension may range from 20% to 50%. Fault geometry and stress orientations calculated from fault-slip data indicate that the least principal stress was east-northeast. K-Ar ages of three north-northwest-striking dikes indicate that east-northeast extension began as early as 32 Ma. Ages of tilted volcanic rocks, however, indicate that measurable tilting, and therefore most faulting, began after about 17 Ma. Faulting having similar timing, style, and orientation occurred throughout the area surrounding the Gulf of California, as far south as Nayarit on the Mexican mainland and as far north as areas in Sonora that are unequivocally within the Basin and Range province. This continuity and similarity in characteristics of faulting around the gulf to those of early Basin and Range faulting in the United States indicate that the area around the gulf is part of the Basin and Range province. This Basin and Range faulting probably created the widely recognized but debated proto-Gulf of California. The present Gulf of California opened by ocean-floor spreading and transform faulting in a zone already weakened by Basin and Range extension. The lack of correspondence between areas of extension and likely areas of crustal thickening related either to Laramide contraction or magmatism argues against a model of Basin and Range extension related solely to spreading of overthickened crust.

Geology, geochemistry, and origin of volcanic rock-hosted uranium deposits in northwestern Nevada and southeastern Oregon, USA

Abstract Northwestern Nevada and southeastern Oregon have the largest uranium (U) deposits in Tertiary volcanic rocks in the US. Most deposits are in or adjacent to calderas or rhyolite lava-dome fields and are hydrothermal. Almost all are associated with rhyolites that have high primary U concentrations (9 to 20 ppm), but the rhyolites range from peralkaline to peraluminous. Caldera-related occurrences are hosted by peralkaline rocks in the McDermitt and Virgin Valley calderas. At McDermitt, major deposits are along or just inboard of the southwestern (Kings River) and northern ring fractures (Bretz–Opalite) of the caldera complex. The Kings River deposits are strongly enriched in trace elements associated with epithermal precious-metal deposits (e.g., As, Au, Ag, Mo, and Sb). Much of the U occurs as hydrothermal uraniferous zircon, and whole rock samples contain as much as 5% Zr. Gangue minerals include quartz, adularia, fluorite, and numerous sulfides. 40 Ar/ 39 Ar dating of adjacent volcanic rocks and adularia indicates that mineralization was contemporaneous with igneous activity. The Bretz–Opalite deposits, which include the large Aurora resource (7.5×106 kg U3O8), are less enriched in epithermal trace elements and contain uraninite, coffinite, chalcedony, opal, and numerous sulfides. At Virgin Valley, deposits are mostly in tuffaceous sediments adjacent to ring-fracture rhyolites. Trace-element enrichment is minor, U is hexavalent in all identified U minerals, and associated phases include opal, organic material, and pyrite. Deposits in the Lakeview U district, Oregon are hosted by weakly peraluminous rhyolite domes and tuffs and are highly enriched in epithermal trace elements; the mineral assemblage includes coffinite, opal, and numerous sulfides. Minor U occurrences at Buff Peak, Nevada, the newly recognized, westernmost topaz (peraluminous) rhyolite in the US, are also trace element enriched; gangue minerals include quartz, adularia, and sulfides. On the basis of their geologic setting, geochemistry, age, and mineralogy, the McDermitt, Lakeview, and Buff Peak deposits are hydrothermal; sparse, published fluid inclusion data indicate temperatures between 200°C and 330°C. Virgin Valley deposits most likely are hydrothermal. Minor deposits in outflow ash-flow tuffs are distant from any intrusive heat source and probably formed from low-temperature groundwater.