Jonathan G. Price

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.

Geochemical modeling approach to predicting arsenic concentrations in a mine pit lake

Abstract Between 1968 and 1983, the North pit at the Getchell Mine, Humboldt County, NV, filled with water to form a lake. In 1983, water quality data were collected with the following results: As concentrations of 0.29 to 0.59 mg/L, pH of 7.1 to 7.9, SO 4 concentrations of 1490 to 1640 mg/L, and TDS of 2394 to 2500 mg/L. Using geochemical modeling techniques presented here, pit lake waters have been theoretically allowed to react for 8.5 a, the approximate time that the North pit had been completely full by 1983. Modeling results predict pH of 7.9 to 8.2, SO 4 concentrations of 1503 to 1644 mg/L, TDS of 2054 to 2366 mg/L, and As concentrations ranging from 0.57 in the hypolimnion to 96 mg/L in the epilimnion. In the epilimnion, model results do not match observed As concentrations, suggesting that mechanisms, such as precipitation of arsenate salts or adsorption to mineral surfaces, may control As levels in an actual pit lake system. Adsorption to Fe oxyhydroxide surfaces is questioned by the authors because of the low Fe content in the Getchell system, but adsorption to Al(OH) 3 (gibbsite) and clay mineral surfaces may be important in controlling natural As concentrations.

Stress orientations during Oligocene volcanism in Trans-Pecos Texas: Timing the transition from Laramide compression to Basin and Range tension

Orientations of dikes and hydrothermal veins indicate that east-northeast compression occurred during the main pulse of volcanism in west Texas 39 to 32 m.y. ago. Direction of compression was essentially the same as that during Eocene Laramide deformation. The observations support a continental-arc tectonic setting. The beginning of extension between 32 and 30 m.y. ago coincides with a regional change in style of magmatism from intrusions and eruptions spanning a wide range in chemical composition to basaltic volcanism in Texas and to bimodal but dominantly rhyolitic volcanism in northern Mexico.

Mid‐Cenozoic stress evolution and magmatism in the Southern Cordillera, Texas and Mexico: Transition from continental arc to intraplate extension

Orientations of dikes, veins, faults, and slickenlines reveal the evolution of stress during Eocene to Miocene magmatism in the southern Cordillera. Where most thoroughly studied by us in Trans-Pecos Texas, magmatism began at about 48 Ma shortly after the cessation of Laramide folding. Dikes and veins that formed from then until about 32 Ma strike dominantly east-northeast. This indicates that the least principal stress (σ3) was north-northwest; additional data suggest that the maximum principal stress (σ1) was east-northeast. The stress field changed to σ1 vertical and σ3 east-northeast (i.e., east-northeast extension) at least by 28 Ma and probably by 31 Ma. Dikes and veins that formed between 31 and 17 Ma, when all magmatism ceased in Texas, strike north-northwest. This change marks the beginning of regional, Basin and Range extension; however, major normal faulting, exclusively of high-angle type, did not begin until about 24 Ma. A similar stress change, marked by a similar change in dike and vein orientations, occurred throughout the southwestern United States and northern Mexico. The time of change is not well constrained in Texas, but available information allows it to have occurred at the same time thoughout the southern Cordillera. We suggest the earlier stress field is related to east-northeast convergence between the Farallon and North American plates. The change in stress is approximately coincident with collision of the East Pacific Rise and paleotrench. Extension may be related to the change from a convergent to a transform margin along the western edge of North America. The changes in the stress field are accompanied by changes in the sources and compositions of magmas erupted in Texas. Contemporaneity of the changes in stress and magmatism indicates that they are related. Combined with regional age patterns, paleostress and geochemical data indicate that pre-31 Ma magmatism in the southern Cordillera occurred in a subduction-related, continental volcanic arc. Subsequent magmatism occurred in an environment of intraplate extension of the Basin and Range province.

Widespread, lavalike silicic volcanic rocks of Trans-Pecos Texas

Several silicic units of the Trans-Pecos volcanic field have outcrop and thin-section scale features of lava flows but areal extents and aspect ratios of ignimbrites. These voluminous rocks (up to hundreds of cubic kilometres per unit) are quartz trachytes to low-silica rhyolites (68% to 72%SiO 2 ). Lava flow features include flow banding and folding, elongated vesicles, and autobreccias and vitrophyres at the base and top of units. Pyroclastic flow features include sheetlike geometry, lateral extents up to 70 km, aspect ratios as low as 1:700, and areal extents up to 3000 km 2 . A few of these units are clearly rheomorphic ignimbrites, but others show no unambiguous evidence of a primary pyroclastic origin. Although no adequate explanation currently exists for the origin of the latter, we evaluate two end-member hypotheses: (1) they are ignimbrites in which extreme rheomorphism has obliterated primary internal features, and (2) they are highly viscous lavas with unusually high heat retention or effusion rates that allowed them to spread over great areas. Either origin requires a rock type and eruptive mechanism not commonly recognized.

Case study of an extensive silicic lava: the Bracks Rhyolite, Trans-Pecos Texas

Abstract Field, petrographic, and chemical data indicate that the Bracks Rhyolite of western Trans-Pecos Texas is a single lava flow that traveled as much as 35 km from a source in the north-central part of its outcrop. With a minimum original extent of 1000 km2 and volume of 75 km3, the Bracks is far more extensive and voluminous than typical silicic lavas. The Bracks crops out in a 55 × 16 km north-trending belt. It thins radially from a maximum thickness of about 120 m. However, except at flow margins where it thins abruptly, it is everywhere at least 25 m thick. Clusters of vitrophyric domes that intrude and are otherwise identical to the dominantly crystalline lava may represent diapiric rise of hotter, less dense, lower parts of the flow. Some domes may overlie fissure vents that fed the flow. The source area is in the north-central part of the flow, on the basis of thickness and flow patterns and distribution of vitrophyric domes. The Bracks is a slightly peralkaline low-silica rhyolite or trachydacite (68–69% SiO2). Whole-rock analyses for major oxides and rare earth elements and microprobe analyses of alkali feldspar, Fe-rich clinopyroxene, fayalite, and magnetite phenocrysts show that the Bracks, including vitrophyric domes, is chemically and mineralogically homogeneous, both laterally and vertically. Evidence that the Bracks was emplaced as a lava flow includes autobreccia at the base and top, steep flow fronts, abundant flow bands and folds, elongate vesicles, trachytic texture, and groundmass textures that indicate crystallization from a liquid. Basal breccia that occurs throughout the extent of the flow, is uniformly coarse, and varies in thickness laterally can only form from a lava that flowed over its entire extent. Evidence of a pyroclastic origin, such as shards, pumice, lithics, or welding zonation, is absent. The low aspect ratio of the Bracks (approximately 1 : 500), although in a range typical of many ash-flow tuffs, is considerably higher than that of unequivocal tuffs in Texas, which have comparable outcrop areas but are much thinner. The great extent and sheet-like geometry of the Bracks probably reflects high eruption temperature (≥ 900°C), low volatile content, moderately low viscosity, rapid eruption, and slow cooling. Unusually low viscosity, on the order of basaltic lavas, was not a factor because, despite peralkalinity and high temperature, the Bracks probably had a low volatile content.

Radon in Outdoor Air in Nevada

Measurements of radon at 50 sites with varying geology indicate that outdoor air in Nevada is comparable to that measured nationwide by Hopper et al. (1991). The statewide median of 15 Bq m-3 (0.4 pCi L-1) is essentially the same as the nationwide median. The range is considerable: from 2.6-52 Bq m-3 (0.07-1.40 pCi L-1). Variations in these measurements can generally be correlated with different concentrations of radon in soils and uranium and its progeny in rocks. Silica-rich igneous rocks (rhyolites and granites) appear to be the main sources of high levels of radon in outdoor air in Nevada. Concentrations of radon in outdoor air generally correlate with levels of radon in soil gas. Measurements taken from heights of 0.5, 1.0, and 2.0 m above the ground suggest that radon in outdoor air reflects the local geology throughout this range of heights. Towns for which > 20% of the homes have indoor-air radon concentrations > 48 Bq m-3 (4 pCi L-1) generally have relatively high soil-gas radon, relatively high outdoor-air radon, or both.

The Christmas Mountains caldera complex, Trans-Pecos Texas

The Christmas Mountains caldera complex developed approximately 42 Ma ago over an elliptical (8×5 km) laccolithic dome that formed during emplacement of the caldera magma body. Rocks of the caldera complex consist of tuffs, lavas, and volcaniclastic deposits, divided into five sequences. Three of the sequences contain major ash-flow tuffs whose eruption led to collapse of four calderas, all 1–1.5 km in diameter, over the dome. The oldest caldera-related rocks are sparsely porphyritic, rhyolitic, air-fall and ash-flow tuffs that record formation and collapse of a Plinian-type eruption column. Eruption of these tuffs induced collapse of a wedge along the western margin of the dome. A second, more abundantly porphyritic tuff led to collapse of a second caldera that partly overlapped the first. The last major eruptions were abundantly porphyritic, peralkaline quartz-trachyte ash-flow tuffs that ponded within two calderas over the crest of the dome. The tuffs are interbedded with coarse breccias that resulted from failure of the caldera walls. The Christmas Mountains caldera complex and two similar structures in Trans-Pecos Texas constitute a newly recognized caldera type, here termed a laccocaldera. They differ from more conventional calderas by having developed over thin laccolithic magma chambers rather than more deep-seated bodies, by their extreme precaldera doming and by their small size. However, they are similar to other calderas in having initial Plinian-type air-fall eruption followed by column collapse and ash-flow generation, multiple cycles of eruption, contemporaneous eruption and collapse, apparent pistonlike subsidence of the calderas, and compositional zoning within the magma chamber. Laccocalderas could occur else-where, particularly in alkalic magma belts in areas of undeformed sedimentary rocks.

Chemical and thermal zonation in a mildly alkaline magma system Infiernito Caldera, Trans-Pecos Texas

Postcollapse lavas of the Infiernito caldera grade stratigraphically upward from nearly aphyric, high-silica rhyolite (76% SiO2) to highly prophyritic trachyte (62% SiO2). Plagioclase, clinopyroxene, orthopyroxene, magnetite, ilmenite, and apatite occur as phenocrysts throughout the sequence. Sanidine, biotite, and zircon are present in rocks with more than about 67% SiO2. Major and trace elements show continuous variations from 62 to 76% SiO2. Modeling supports fractional crystallization of the observed phenocrysts as the dominant process in generating the chemical variation.Temperatures calculated from coexisting feldspars, pyroxenes, and Fe-Ti oxides agree and indicate crystallization from slightly more than 1100° C in the most mafic trachyte to 800° C in high-silica rhyolite. The compositional zonation probably arose through crystallization against the chilled margin of the magma chamber and consequent rise of more evolved and therefore less dense liquid.Mineral compositions vary regularly with rock composition, but also suggest minor mixing and assimilation of wall rock or fluids derived from wall rock. Mixing between liquids of slightly different compositions is indicated by different compositions of individual pyroxene phenocrysts in single samples. Liquid-solid mixing is indicated by mineral compositions of glomerocrysts and some phenocrysts that apparently crystallized in generally more evolved liquids at lower temperature and higher oxygen fugacity than represented by the rocks in which they now reside. Glomerocrysts probably crystallized against the chilled margin of the magma chamber and were torn from the wall as the liquid rose during progressive stages of eruption. Assimilation is indicated by rise of oxygen fugacity relative to a buffer from more mafic to more silicic rocks.Calculation of density and viscosity from the compositional and mineralogical data indicates that the magma chamber was stably stratified; lower temperature but more evolved, thus less dense, rhyolite overlay higher temperature, less evolved, and therefore more dense, progressively more mafic liquids. The continuity in rock and mineral compositions and calculated temperature, viscosity, and density indicate that compositional gradation in the magma chamber was smoothly continuous; any compositional gaps must have been no greater than about 2% SiO2.