This field course examined active mines and exploration projects, geochemical and geophysical data, and tectonic setting to address a series of questions about lithium and gold deposits. The course was held over four days in conjunction with the SEG 2018 Conference and started and ended in Reno, Nevada. The detailed road log describes stops at Angel Island, Clayton Valley, Eastside Project, Tonopah, Rhyolite Ridge, Round Mountain, Montezuma Range, and Hasbrouck Mountain.

This field course examined active mines and exploration projects, geochemical and geophysical data, and tectonic setting to address a series of questions about lithium and gold deposits. The course was held over four days in conjunction with the SEG 2018 Conference and started and ended in Reno, Nevada. The detailed road log describes stops at Angel Island, Clayton Valley, Eastside Project, Tonopah, Rhyolite Ridge, Round Mountain, Montezuma Range, and Hasbrouck Mountain.

Trends in global mineral production and expanding uses of mineral resources foretell a bright future, though one with significant challenges, for exploration and development. Demand for mineral resources is likely to remain high and grow to meet increases in world population and standards of living. Significant challenges include meeting future demand with new discoveries and developing the resources in environmentally, socially, and economically sustainable ways. A historical perspective from the past 50 years on finding new mineral districts, discovering new types of ore deposits, and using new technologies in exploration suggests that the world will not run out of mineral resources. It is likely that substitution and recycling will play increasingly major roles in meeting global mineral demand. New technologies for ocean mining will help add to the resource base. Historical perspectives also suggest that mining scams will continue, and environmental, health, and safety concerns will be major factors in deciding where future mines will be located and how they will be operated.

Abstract Beryllium ore is hosted by fluorspar deposits at Round Top, one of five rhyolite laccoliths exposed approximately 120 km southeast of El Paso, just north of Interstate Highway 10 and just west of the town of Sierra Blanca, in Trans-Pecos Texas (Fig. 1). The fluorspar deposits occur within the mid-Cretaceous Buda Limestone, which is intruded by the rhyolite, and weakly extend into the rhyolite as well (Fig. 2). Cyprus Beryllium Corporation, sole operator of the property, lists reserves and resources of 850,000 tons of ore averaging 1.5 wt% BeO (25 million pounds BeO; H. Harlan, written comm., 1990). The magma that formed Round Top was highly enriched in Li, Be, F, Zn, Rb, Y, Zr, Nb, Sn, Cs, rare earth elements (REEs, especially heavy REEs- [HREEs]), Pb, Th, and U (Rubin et al., 1987; Price et al., in press). In fact; the Round Top intrusion, the most rareelementenriched of the five laccoliths, could be considered a high-tonnage, low-grade resource for several rare metals. For example, the average Th content of the Round Top rhyolite (180 ppm) is almost four times that of the Conway Granite (New Hampshire), which has been considered a hightonnage resource of Th (Finch et al., 1973). The fluoritized samples show complementary enrichment in many of the abovementioned incompatible elements as well as Na, AI, Ti, V, and As. The enriched elements, including Be, are present either in discrete minerals or, particularly in the case of the REEs, in solid solution in fluorite. McAnulty (1974, 1980) first

Peraluminous rhyolites that are chemically somewhat similar to topaz rhyolites and anorogenic granites occur in an orogenic setting near Sierra Blanca in the Tertiary Trans-Pecos magmatic province. The Sierra Blanca rhyolites are even more enriched in most incompatible trace elements than are topaz rhyolites. Some of the extreme enrichments may in part be the result of chemical modification by crystallization from an F-rich vapor phase. The rhyolites were intruded as laccoliths at 36 Ma, during the main phase of Trans-Pecos igneous activity, which is characterized by ash-flow eruptions from numerous calderas and widespread mafic, intermediate, and silicic intrusions. A dominant east-northeast orientation of dikes and veins throughout the region indicates mild compression that was residual from Laramide deformation. This compressive tectonic setting, coupled with concurrent volcanism in Mexico and the east-northeast change in magma chemistry from calc-alkalic in western Mexico through alkali-calcic to alkalic in Texas, suggests that the rhyolites were emplaced in a continental arc. Extension did not begin in Trans-Pecos Texas until after 32 Ma; 31- to 17-Ma dikes are dominantly oriented north-northwest, perpendicular to the direction of extension during early Basin and Range deformation. Thus, the tectonic setting of the Sierra Blanca rhyolites contrasts with that of typical topaz rhyolites, most of which were emplaced during periods of crustal extension. The Sierra Blanca rhyolites are chemical and mineralogic oddities for the region, where most rhyolites are peralkaline or metaluminous. The rhyolites are depleted in the same elements as topaz rhyolites (Mg, Ca, Ti, Sr, Ba) but are more highly evolved than topaz rhyolites. Extreme trace-element enrichments (Li, F, Zn, Rb, Y, Zr, Nb, Sn, Ta, Pb, HREE, Th, U) are accommodated in Li-rich white mica, Zn-rich biotite, Rb-rich feldspars, and numerous trace minerals, including cassiterite, changbaiite, columbite, thorite, xenotime, yttrium- and REE–rich fluorides, and zircon. The rhyolites are large-tonnage, low-grade resources of several rare metals. Also enriched in Be (as much as 180 ppm), the rhyolites are the sources of Be and F in beryllium deposits in fluoritized limestones along the contacts with the laccoliths. Interaction with the limestones probably locally elevated the Ca, Mg, and Sr contents of the rhyolites. Vapor-phase crystallization has modified the original magmatic chemistry of the rocks. Evidence of vapor-phase crystallization includes the presence of minerals typical of pegmatites: cryolite (from 0 to 3 volume percent), alkali feldspars with nearly end-member compositions, polylithionite-zinnwaldite mica, and rutilated quartz, plus fluid inclusions defining quartz overgrowths on magmatic grains. Extreme HREE enrichments (Yb to 72 ppm; chondrite-normalized REE patterns with positive slopes) may also be the result of vapor-phase crystallization.

Abstract Dikes in Big Bend National Park illustrate the petrologic variety and tectonic setting of Tertiary magmatism in Trans-Pecos Texas. Most igneous activity occurred between two major episodes in the tectonic history of western North America: (1) Late Cretaceus to early Eocene Laramide compression and (2) late Oligocene to recent Basin and Range extension. Volumetrically minor magmatism occurred during the initial stage of Basin and Range extension and perhaps during the final stage of Laramide deformation. The general geology of the park, including aspects of stratigraphy, structure, petrology, and geomorphology, is best described in the treatise by Maxwell and others ( 1967). The 1:62,500-scale geologic map in Maxwell and others’ (1967) volume is also available at many stores in the Big Bend region as part of a nontechnical geologic guide to Big Bend by Maxwell (1968). The 1:250,000-scale geologic map by Barnes (1979) covers the southern part of the Trans–Pecos region. A field trip guidebook by Maxwell and Dietrich (1972) complements these publications. More recent studies have added considerably to our understanding of the tectonics, volcanic stratigraphy, and petrology of the region. Research results are summarized in guidebooks edited by Dickerson and Hoffer (1980), Dickerson and Muehlberger (1985), and Price and others (1986). In addition, details of the tectonic and magmatic history of the Trans–Pecos region are provided by Henry and Price (1984, 1985, 1986), Price and Henry (1984), and Price and others (1987). This chapter describes localities for examining dikes in Big Bend National Park (Fig. 1). These dikes represent

Alkalic rocks in Trans-Pecos Texas were emplaced in two distinctly different tectonic environments: one compressional (contractional) and one extensional. Rocks (Eocene and early Oligocene) of the older compressional environment can be divided into a western alkali-calcic belt and an eastern alkalic belt. The boundary between the two belts is parallel to the paleotrench that used to lie off the west coast of Mexico. The alkalic rocks were the most inland expression of subduction-generated volcanism. The predominance of east-striking dikes and veins and the orientation of en echelon dikes indicate igneous activity during residual compression remaining from Laramide deformation. As the dip of the subducting slab became gentler with time, calc-alkaline magmatism of Laramide age in Mexico graded eastward into the alkaline magmatism in Texas. Widespread extension and normal faulting began about 24 Ma in the Texas portion of the Basin and Range province. Between 24 and 17 Ma, alkalic basalts were extruded and intruded at several localities, dominantly as north-northwest-striking dikes. Both nepheline- and hypersthene-normative basalts occur in the extensional environment. Rocks of the compressional environment follow two major lines of differentiation: hypersthene-normative basalt to rhyolite and nepheline-normative basalt to phonolite. In contrast, rocks of the extensional environment are apparently limited to basalts. During contraction, magmas rising from the mantle probably formed chambers in which differentiation could occur. During extension, less differentiation occurred, either because tectonically dilated fractures permitted more direct rise of magma to the surface or because the volume of magma was too small. Basalts of the two contrasting tectonic settings are broadly similar in alkalinity and silica saturation. The basalts of the extensional environment are, however, generally richer in magnesium than are the basalts of the compressional environment. This difference is not simply a matter of degree of differentiation but is probably related to the different pressure-temperature regimes of the mantle from which the basalts originated.