Abstract Lithium is a critical and technologically important element that has widespread use, particularly in batteries for hybrid cars and portable electronic devices. Global demand for lithium has been on the rise since the mid-1900s and is projected to continue to increase. Lithium is found in three main deposit types: (1) pegmatites, (2) continental brines, and (3) hydrothermally altered clays. Continental brines provide approximately three-fourths of the world’s Li production due to their relatively low production cost. The Li-rich brine systems addressed here share six common characteristics that provide clues to deposit genesis while also serving as exploration guidelines. These are as follows: (1) arid climate; (2) closed basin containing a salar (salt crust), a salt lake, or both; (3) associated igneous and/or geothermal activity; (4) tectonically driven subsidence; (5) suitable lithium sources; and (6) sufficient time to concentrate brine. Two detailed case studies of Li-rich brines are presented; one on the longest produced lithium brine at Clayton Valley, Nevada, and the other on the world’s largest producing lithium brine at the Salar de Atacama, Chile.

Transtensional deformation was concentrated in a zone adjacent to the Tintina strike-slip fault system in Alaska during the early Tertiary. The deformation occurred along the Victoria Creek fault, the trace of the Tintina system that connects it with the Kaltag fault; together the Tintina and Kaltag fault systems girdle Alaska from east to west. Over an area of ∼25 by 70 km between the Victoria Creek and Tozitna faults, bimodal volcanics erupted; lacustrine and fluvial rocks were deposited; plutons were emplaced and deformed; and metamorphic rocks cooled, all at about the same time. Plutonic and volcanic rocks in this zone yield U-Pb zircon ages of ca. 60 Ma; 40 Ar/ 39 Ar cooling ages from those plutons and adjacent metamorphic rocks are also ca. 60 Ma. Although early Tertiary magmatism occurred over a broad area in central Alaska, metamorphism and ductile deformation accompanied that magmatism in this one zone only. Within the zone of deformation, pluton aureoles and metamorphic rocks display consistent NE-SW–stretching lineations parallel to the Victoria Creek fault, suggesting that deformation processes involved subhorizontal elongation of the package. The most deeply buried metamorphic rocks, kyanite-bearing metapelites, occur as lenses adjacent to the fault, which cuts the crust to the Moho (Beaudoin et al., 1997). Geochronologic data and field relationships suggest that the amount of early Tertiary exhumation was greatest adjacent to the Victoria Creek fault. The early Tertiary crustal-scale events that may have operated to produce transtension in this area are (1) increased heat flux and related bimodal within-plate magmatism, (2) movement on a releasing stepover within the Tintina fault system or on a regional scale involving both the Tintina and the Kobuk fault systems, and (3) oroclinal bending of the Tintina-Kaltag fault system with counterclockwise rotation of western Alaska.

We report 777 U-Pb SHRIMP detrital zircon ages from thirteen sandstones and metasandstones in interior Alaska. About sixty grains per sample were analyzed; typically, half to three-fourths of these were concordant within ± 10%. Farewell terrane . Two quartzites were collected from Ruby quadrangle and a third from Taylor Mountains quadrangle. All three are interpreted to represent a low stratigraphic level in the Nixon Fork platform succession; the samples from Ruby quadrangle are probably late Neoproterozoic, and the sample from Taylor Mountains quadrangle is probably Cambrian in age. The youngest detrital zircon in any of the three is 851 Ma. The two Ruby quadrangle samples area almost identical: one has a major age cluster at 1980–2087 and minor age clusters at 944–974 and 1366–1383 Ma; the other has a major age cluster at 1993–2095 Ma and minor age clusters at 912–946 and 1366–1395 Ma. The Taylor Mountains sample shows one dominant peak at 1914–2057 Ma. Notably absent are zircons in the range 1800–1900 Ma, which are typical of North American sources. The detrital zircon populations are consistent with paleontological evidence for a peri-Siberian position of the Farewell terrane during the early Paleozoic. Mystic subterrane of the Farewell terrane. Three graywackes from flysch of the Mystic subterrane, Talkeetna quadrangle, were sampled with the expectation that all three were Pennsylvanian. Asample from Pingston Creek is Triassic (as revealed by an interbedded ash dated at ca. 223 Ma) and is dominated by age clusters of 341–359 and 1804–1866 Ma, both consistent with a sediment source in the Yukon-Tanana terrane. Minor age clusters at 848–869 and 1992–2018 Ma could have been sourced in the older part of the Farewell terrane. Still other minor age clusters at 432–461, 620–657, 1509–1536, and 1627–1653 Ma are not readily linked to sources that are now nearby. A sample from Surprise Glacier is mid-Mississippian or younger. A dominant age cluster at 1855–1883 and a minor one at 361–367 Ma could have been sourced in the Yukon-Tanana terrane. Other age clusters at 335–336, 457–472, 510–583, and 1902–1930 have no obvious nearby source. A sample from Ripsnorter Creek is Silurian or younger. The dominant age cluster at 937–981 Ma and a minor one at 2047–2077 Ma could have been sourced in the Farewell terrane. Minor age clusters at 1885–1900 and 2719–2770 Ma could have been sourced in the Yukon-Tanana terrane. Other age clusters at 429–490, 524–555, 644–652, 1023–1057, 1131–1185, and 1436–1445 Ma have no obvious nearby source. The so-called Mystic subterrane is structurally complex and would appear to include more than one Phanerozoic turbidite succession; more mapping and detrital zircon geochronology are needed. Wickersham and Yukon-Tanana terranes. A grit from Wickersham terrane in Tanana quadrangle and a grit from Yukon-Tanana terrane in Talkeetna quadrangle have similar, exclusively Precambrian detrital zircon populations, supporting previous correlations. The Wickersham sample has major age clusters at 1776–1851 and 1930–1964 Ma, and the youngest grain is 1198 Ma. The Yukon-Tanana grit has a major age cluster at 1834–1867 Ma, and the youngest grain is 1789 Ma. A North American source has been previously proposed, and this seems likely based on detrital zircon data. Ruby terrane and Minook Complex. Detrital zircons from quartzites in the Ruby terrane show two quite different age patterns. A sample from the Bear Creek area of Tanana quadrangle has detrital zircon ages that are similar to those from the Wickersham and Yukon-Tanana grits. The dominant age clusters are 1823–1856 and 1887–1931 Ma. In contrast, a quartzite from nearby Senatis Mountain (Tanana quadrangle) yielded a completely different detrital zircon age spectrum, featuring a broad peak with no significant gaps from 1024 to 1499 Ma and a minor age cluster at 1671–1695 Ma. The youngest concordant zircon is 1024 ± 6 Ma. A quartzite from the Minook Complex, a sliver along the Victoria Creek strike-slip fault in Tanana quadrangle, is similar to the Senatis Mountain sample. Its detrital zircon population is dominated by grains between 1103 and 1499 Ma, with peaks within that range at 1161–1234 and 1410–1490 Ma; minorolderage clusters are at 1643–1676, 1765–1781, and 1840–1874 Ma. The youngest concordant grain is 1103 ± 6 Ma. Finally, a quartzite from Illinois Creek (Nulato quadrangle) at the extreme west end of the Ruby geanticline, previously assigned to the Ruby terrane, also has a detrital zircon age spectrum like that at Senatis Mountain. Mesoproterozoic zircons are predominant, with main age groups at 1329–1391 and 1439–1493 Ma and lesser ones at 1058–1072, 1184–1193, 1681–1692, and 1852–1879 Ma. The youngest concordant grain is 1058 ± 33 Ma. These barcodes are dominated by Mesoproterozoic zircons that are strikingly similar in age to detrital zircons in Neoproterozoic Sequence B in northwestern Canada (and easternmost Alaska, where it equates to the lower Tindir Group). Among other rocks, the Ruby geanticline thus might include a shortened, metamorphosed, and offset continuation of this ancient North American basin, which was sourced in the Grenville orogen. Rampart Group, Angayucham-Tozitna terrane. The Rampart Group is thought to have been deposited in an ocean basin that closed during the Brookian Orogeny. Detrital zircons from graywacke of the Rampart Group are dominated by an age cluster at 380–404 Ma, with lesser ones at 351–364, 426–440, 484–504, 909–920, 1001–1020, 1127–1128, 1211–1217, and 1912–1953 Ma. The youngest grain is 260 ± 1 Ma. The dominant 380–404 Ma age cluster can be reasonably linked to sources in Devonian plutons of the now-adjacent Brooks Range and Ruby terrane.

New data from an Upper Triassic to Lower Cretaceous deep marine succession—the herein reinstated and restricted Gemuk Group—provide a vital piece of the puzzle for unraveling southwestern Alaska's tectonic history. First defined by Cady et al. in 1955 , the Gemuk Group soon became a regional catchall unit that ended up as part of at least four different terranes. In this paper we provide the first new data in nearly half a century from the Gemuk Group in the original type area in Taylor Mountains quadrangle and from contiguous rocks to the north in Sleetmute quadrangle. Discontinuous exposure, hints of complex structure, the reconnaissance level of our mapping, and spotty age constraints together permit definition of only a rough stratigraphy. The restricted Gemuk Group is at least 2250 m thick, and could easily be at least twice as thick. The age range of the restricted Gemuk Group is tightened on the basis of ten radiolarian ages, two new bivalve ages, one conodont age, two U-Pb zircon ages on tuff, and U-Pb ages of 110 detrital zircons from two sandstones. The Triassic part of the restricted Gemuk Group, which consists of intermediate pillow lavas interbedded with siltstone, chert, and rare limestone, produced radiolarians, bivalves, and conodonts of Carnian and Norian ages. The Jurassic part appears to be mostly siltstone and chert, and yielded radiolarians of Hettangian-Sinemurian, Pliensbachian-Toarcian, and Oxfordian ages. Two tuffs near the Jurassic-Cretaceous boundary record nearby arc volcanism: one at 146 Ma is interbedded with red and green siltstone, and a second at ca. 137 Ma is interbedded with graywacke turbidites. Graywacke appears to be the dominant rock type in the Lower Cretaceous part of the restricted Gemuk Group. Detrital zircon analyses were performed on two sandstone samples using SHRIMP. One sandstone yielded a dominant age cluster of 133–180 Ma; the oldest grain is only 316 Ma. The second sample is dominated by zircons of 130–154 Ma; the oldest grain is 292 Ma. The youngest zircons are probably not much older than the sandstone itself. Point counts of restricted Gemuk Group sandstones yield average ratios of 24/29/47 for Q/F/L, 15/83/2 for Ls/Lv/Lm, and 41/48/11 for Qm/P/K. In the field, sandstones of the restricted Gemuk Group are not easily distinguished from sandstones of the overlying Upper Cretaceous turbidite-dominated Kuskokwim Group. Petrographically, however, the restricted Gemuk Group has modal K-feldspar, whereas the Kuskokwim Group generally does not (average Qm/P/K of 64/36/0). Some K-feldspar-bearing graywacke that was previously mapped as Kuskokwim Group (Cady et al., 1955) is here reassigned to the restricted Gemuk Group. Major- and trace-element geochemistry of shales from the restricted Gemuk Group and the Kuskokwim Group show distinct differences. The chemical index of alteration (CIA) is distinctly higherforshales of the Kuskokwim Group than forthose of the restricted Gemuk Group, suggesting more intense weathering during deposition of the Kuskokwim Group. The restricted Gemuk Group represents an estimated 90–100 m.y. of deep-water sedimentation, first accompanied by submarine volcanism and later by nearby explosive arc activity. Two hypotheses are presented for the tectonic setting. One model that needs additional testing is that the restricted Gemuk Group consists of imbricated oceanic plate stratigraphy. Based on available information, our preferred model is that it was deposited in a back-arc, intra-arc, or forearc basin that was subsequently deformed. The terrane affinity of the restricted Gemuk Group is uncertain. The rocks of this area were formerly assigned to the Hagemeister subterrane of the Togiak terrane—a Late Triassic to Early Cretaceous arc—but our data show this to be a poor match. None of the other possibilities (e.g., Nukluk and Tikchik subterranes of the Goodnews terrane) is viable; hence, the terrane subdivision and distribution in southwestern Alaska may need to be revisited. The geologic history revealed by our study of the restricted Gemuk Group gives us a solid toehold in unraveling the Mesozoic paleogeography of this part of the northern Cordillera.

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Full article available in PDF version.