ALBERT

Description: To evaluate whether anatectic and/or highly fractionated lithophile element-enriched rhyolite tuffs deposited in arid lacustrine basins lose enough lithium during eruption, lithification, and weathering to generate significant Li brine resources, pre-eruptive melt compositions, preserved in inclusions, and the magnitude of post-eruptive Li depletions, evident in host rhyolites, were documented at six sites in the western United States. Each rhyolite is a member of the bimodal basalt-rhyolite assemblage associated with extensional tectonics that produced the Basin and Range province and Rio Grande rift, an evolving pattern of closed drainage basins, and geothermal energy or mineral resources. Results from the 0.8 Ma Bishop tuff (geothermal) in California, 1.3 to 1.6 Ma Cerro Toledo and Upper Bandelier tephra (geothermal) and 27.9 Ma Taylor Creek rhyolite (Sn) in New Mexico, 21.7 Ma Spor Mountain tuff (Be, U, F) and 24.6 Ma Pine Grove tuff (Mo) in Utah, and 27.6 Ma Hideaway Park tuff (Mo) in Colorado support the following conclusions. Melt inclusions in quartz phenocrysts from rhyolite tuffs associated with hydrothermal deposits of Sn, Mo, and Be are extremely enriched in Li (1,000s of ppm); those from Spor Mountain have the highest Li abundance yet recorded (max 5,200 ppm, median 3,750 ppm). Forty-five to 98% of the Li present in pre-eruptive magma was lost to the environment from these rhyolite tuffs. The amount of Li lost from the small volumes (1–10 km 3 ) of Li-enriched rhyolite deposited in closed basins is sufficient to produce world-class Li brine resources. After each eruption, meteoric water leaches Li from tuff, which drains into playas, where it is concentrated by evaporation. The localized occurrence of Li-enriched rhyolites may explain why brines in arid lacustrine basins seldom have economic concentrations of Li. Considering that hydrothermal deposits of Sn, Mo, Be, U, and F may indicate potential for Li brines in nearby basins, we surmise that the world’s largest Li brine resource in the Salar de Uyuni (10 Mt) received Li from nearby rhyolite tuffs in the Bolivian tin belt.

Description: The Hideaway Park tuff is the only preserved extrusive volcanic unit related to the Red Mountain intrusive complex, which produced the world-class Henderson porphyry Mo deposit. Located within the Colorado Mineral Belt, USA, Henderson is the second largest Climax-type Mo deposit in the world, and is therefore an excellent location to investigate magmatic processes leading to Climax-type Mo mineralization. We combine an extensive dataset of major element, volatile, and trace element abundances in quartz-hosted melt inclusions and pumice matrix glass with major element geochemistry from phenocrysts to reconstruct the pre-eruptive conditions and the source and evolution of metals within the magma. Melt inclusions are slightly peraluminous topaz rhyolitic in composition and are volatile-charged (≤6 wt % H 2 O, ≤600 ppm CO 2 , ~0·3–1·0 wt % F, ~2300–3500 ppm Cl) and metal-rich (~7–24 ppm Mo, ~4–14 ppm W, ~21–52 ppm Pb, ~28–2700 ppm Zn, 〈0·1–29 ppm Cu, ~0·3–1·8 ppm Bi, ~40–760 ppb Ag, ~690–1400 ppm Mn). Melt inclusion and pumice matrix glass chemistry reveal that the Hideaway Park magma evolved by large degrees of fractional crystallization (≤60–70%) during quartz crystallization and melt inclusion entrapment at pressures of ≤300 MPa (≤8 km depth), with little to no crystallization upon shallow ascent and eruption. Filter pressing, crystal settling, magma recharge and mixing of less evolved rhyolite melt, and volatile exsolution were important processes during magma evolution; the low estimated viscosities (~10 5 –10 10 Pa s) of these H 2 O- and F-rich melts probably enhanced these processes. A noteworthy discrepancy between the metal contents in the pumice matrix glass and in the melt inclusions suggests that after quartz crystallization ceased upon shallow magma ascent and eruption, the Hideaway Park magma exsolved an aqueous fluid into which Mo, Bi, Ag, Zn, Mn, Cs, and Y strongly partitioned. Given that the Henderson deposit contains anomalous abundances of not only Mo, but also W, Pb, Zn, Cu, Bi, Ag, and Mn, we suggest that these metals were sourced from similar fluids exsolved from unerupted portions of the same magmatic system. Trace element ratios imply that Mo was sourced deep, from either the lower crust or metasomatized mantle. The origin of sulfur remains unresolved; however, given the extremely low S solubility of rhyolite melts in the shallow crust we favor the possibility that another source of S might supplement or account for that present in the ore deposit, probably the comagmatic, mantle-derived lamprophyres that occur in minor quantities with the voluminous topaz rhyolites in the area. To account for the 437 Mt of MoS 2 (~1·0 x 10 6 t Mo) present in the Henderson ore deposit, a volume of ~45 km 3 of Hideaway Park rhyolite magma would have been necessary to supply the Mo (a cylindrical pluton measuring 3·1 km x 6·0 km) along with sparging of ~6·8 x 10 5 t of S from ~0·05 km 3 of lamprophyre magma. Based on a weighted mean 40 Ar/ 39 Ar age of 27·58 ± 0·24 Ma, similar melt geochemistry, and characteristically F-rich biotite phenocrysts, we conclude that the Hideaway Park tuff was cogenetic with the intrusions at Red Mountain that formed the Henderson deposit.

Description: Eruptions and magma evolution of monogenetic volcanoes are thought to be controlled by rapid ascent of magmas over a short period of time. Volatiles degassing from magmas control the ascent velocity and therefore eruption intensity. Complex feedbacks exist between the rate and extent of volatile exsolution at shallow levels and groundmass crystallization, affecting the magma rheology, extent of fragmentation, resulting eruptive style and related hazards. Melt inclusions record the volatile contents and compositions of melts at various stages during their evolution, providing insights into degassing–crystallization processes at shallow crustal levels. Here we present new volatile and major element data from olivine-hosted melt inclusions from Pelagatos scoria cone, Mexico. These new data, combined with recent geochemical and textural data on Pelagatos eruptive products, allow us to propose a model for shallow magmatic processes at this volcano. Discharge of volatile-poor, low-viscosity magma drives early effusive activity along the fissure. Decreasing magma fluxes lead to the clogging of the fissure and the formation of a shallow magma reservoir where degassing and fractional crystallization take place. Subsequent explosive cone-forming activity is triggered by influx of deeper ( c. 5 km), less evolved, more volatile-rich magma into this shallow ( c. 1 km) reservoir. Supplementary material: New whole rock and matrix glass date are available at www.geolsoc.org.uk/SUP18782

Description: The systematic distribution of vein and alteration mineral assemblages in porphyry Cu deposits largely arises from changes in the temperature and pressure of fluids that traversed fractures throughout the hydrothermal system. Magmatic and hydrothermal minerals record the complex history of the fluctuating temperature and pressure regime as hydrothermal fluids transfer heat from their magmatic source to cold wall rock in response to lithostatic-to-hydrostatic pressure variations. We examine the thermal profile of the porphyry Cu-Mo deposit in Butte, Montana, by determining formation temperatures for magmatic and hydrothermal samples representing different time frames and depths within the deposit, in context with sample pressure estimates. We use three independent mineral thermobarometers: Ti in quartz, Zr in rutile, and X Mg -Ti in biotite, from which we estimate that final dike injection temperature, and hence the initial magmatic-hydrothermal fluid temperature, was ~700°C while the ambient host-rock temperature was ~450° to 500°C. We find a magmatic-hydrothermal continuum represented in hydrothermal veins, ranging from ~710° to 〈440°C. Distinct mineral generations within vein samples consistently display large temperature ranges, spanning 50° to 250°C, capturing the transient thermal condition of the ascending aqueous fluids. Mineral precipitation temperatures within veins show the same range as those in accompanying envelopes, indicating at least partly contemporaneous formation of veins and envelopes. Hydrothermal veins of a single type show no systematic relationship between temperature and depth within the deposit, although different vein types show systematic temperature ranges as a function of depth. We observe anomalous crosscutting relationships indicating that porphyry vein formation temperatures fluctuated significantly within a single cubic centimeter parcel of rock from one vein-forming episode to another. We suggest that the thermal profile does not mimic domical isograds based on alteration mineral zones, but rather it mimics an irregular pattern following active fractures at any given time and evolves by discrete cycles of dynamic, transitory, high-temperature hydrofracturing, fluid release, and vein formation that overprints cooler host-rock temperatures.

Description: Porphyry dikes and hydrothermal veins from the porphyry Cu-Mo deposit at Butte, Montana, contain multiple generations of quartz that are distinct in scanning electron microscope-cathodoluminescence (SEM-CL) images and in Ti concentrations. A comparison of microprobe trace element profiles and maps to SEM-CL images shows that the concentration of Ti in quartz correlates positively with CL brightness but Al, K, and Fe do not. After calibrating CL brightness in relation to Ti concentration, we use the brightness gradient between different quartz generations as a proxy for Ti gradients that we model to determine time scales of quartz formation and cooling. Model results indicate that time scales of porphyry magma residence are ~1,000s of years and time scales from porphyry quartz phenocryst rim formation to porphyry dike injection and cooling are ~10s of years. Time scales for the formation and cooling of various generations of hydrothermal vein quartz range from 10s to 10,000s of years. These time scales are considerably shorter than the ~0.6 m.y. overall time frame for each porphyry-style mineralization pulse determined from isotopic studies at Butte, Montana. Simple heat conduction models provide a temporal reference point to compare chemical diffusion time scales, and we find that they support short dike and vein formation time scales. We interpret these relatively short time scales to indicate that the Butte porphyry deposit formed by short-lived episodes of hydrofracturing, dike injection, and vein formation, each with discrete thermal pulses, which repeated over the ~3 m.y. generation of the deposit.

Description: Mapping of Venus Nepthys Mons Quadrangle (V54, 300-330 E, 25-50 S) has been proceeding for the last 21 months. Discussed here are several intriguing findings and a report on the use of the pseudostereo data set. Additional information is contained in the original extended abstract.