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Thacker Pass, Kings Valley |
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Nevada, USA |
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Li
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Thacker Pass, previously Kings Valley, Lithium Project, is located in Humboldt County in northern Nevada, ~100 km north-northwest of Winnemucca, ~33 km west-northwest of Orovada, Nevada and 33 km due south of the Oregon border (#Location: 41° 42' 42"N, 118° 4' 26"W).
Precis - The Thacker Pass Project is located within the McDermitt Caldera, the remnant of an extinct 40 x 30 km supervolcano, straddling the Oregon-Nevada border that formed at ~16.3 Ma over the migrating hotspot that currently is situated beneath the Yellowstone Plateau of Wyoming, Idaho and Montana. Following an initial eruption of ignimbrite and concurrent collapse of the McDermitt Caldera, a large 'moat' lake was formed in the caldera basin. The lake water became strongly enriched in lithium as a result of extensive hydrothermal activity and the leaching of lithium-rich volcanic rocks associated with caldera volcanism. This hydrothermal activity, accompanied by erosion and sedimentation, led to the accumulation of a thick sequence of lithium-rich muddy lacustrine clays at the bottom of the lake. Renewed volcanic activity caused uplift in the centre of the caldera, altering some of the smectite clays to illite, draining the lake and bringing the lithium-rich moat sediments to the surface. The result is a high-grade, large and near-surface lithium-clay deposit.
History - In 1975, Chevron Resources began an exploration program aimed at uranium within the sediments of the McDermitt Caldera. During the early stages of this program, the USGS, which had been investigating lithium sources, alerted Chevron to the presence of anomalous concentrations of lithium associated with the caldera. While continuing to focus on uranium, including extensive airborne gamma ray spectrometry, Chevron added lithium to its assays in 1978. Exploration work also included mapping to delineate the McDermitt Caldera moat sedimentary rocks. In 1979, a clay analysis program was commenced, confirming the high lithium concentrations within the clays. From 1980 to 1987, a 24 hole rotary and 1 diamond core drilling program was undertaken that focused on determining the grade and extent of lithium mineralisation, as well as extensive metallurgical testing of the clays to determine the viability of lithium extraction. However, in 1986, Chevron USA decided to discontinue this program and to lease out many of the lithium project claims to the J. M. Huber Corporation, and in 1991 sold its remaining interest in the claims to Cyprus Gold Exploration Corporation. In 1992, Huber terminated their leases, and Cyprus Gold allowed the claims to lapse, whilst passing on much of the exploration data to Jim LaBret, one of the other owners of claims that had been leased as part of the project. In 2005, Western Energy Development Corporation (WEDC), a Nevada corporation, leased LaBret’s claims, and in doing so gained access to the Chevron data, drill core and sample pulps. In December 2007, WEDC signed an agreement with Western Lithium USA Corp. to explore, develop, and mine or otherwise process any and all lithium deposits discovered on the claims - lithium had become the prime target of the project. Both companies were subsidiaries of Western Uranium Corporation. In July 2008, Western Lithium USA Corp. became an independent publicly traded company, which entered into an agreement for the purchase from WEDC of the royalties and titles for the then renamed Kings Valley mineral property, giving it complete control. Between 2007 and 2010, 235 x HQ and 7 x PQ diamond core holes were drilled. In March 2016, Western Lithium USA Corp. announced a name change to Lithium Americas Corp. and the name of its Nevada-based wholly owned subsidiary was changed from Western Lithium Corp. to Lithium Nevada Corp. In 2017-2018, a further 144 HQ diamond core holes were drilled and a feasibility study was completed in 2022. Construction at Thacker Pass commenced in early 2023, with production expected to commence in 2028 after resolution of permitting issues and litigation by Nevada first nations and conservation groups. Mining is planned for two phases, the first to produce 40 000 tonnes of battery grade lithium carbonate per annum over years 1 to 3, followed by phase 2 from years 4 to 40, with a capacity of 80 000 tonnes of the same product. An Offtake Agreement has been signed with General Motors for 100% of production volumes from Phase 1 for 20 years, plus 38% of Phase 2 production volumes for 20 years, and a right of first offer on the remaining Phase 2 production volumes.
Regional Setting
The Thacker Pass Project lies within the McDermitt Volcanic Field, a magmatic complex composed of four large, middle Miocene, rhyolitic calderas (Benson et al., 2017). Magmatic activity within this volcanic field was coeval with the voluminous, earliest stages, of the ~16.6 to 15 Ma Columbia River flood basalt lavas to the north. This magmatism is interpreted to be related to the impingement of the Yellowstone mantle plume head on the continental crust, resulting in crustal melting and volcanism along four distinct radial swarms centred around Steens Mountain, Oregon (Coble and Mahood, 2012; Benson et al., 2017). The McDermitt Volcanic Field is located within the southeast-propagating swarm of volcanism, extending from Steens Mountain into north-central Nevada (Benson et al., 2017). The Thacker Pass Project is located within the largest and southeastern most caldera of the volcanic field, the north-south elongate, 60 x 35 to 25 km, McDermitt Caldera.
Geology
The geologic history of the McDermitt Caldera can be summarised as follows (after Roth et al. 2022):
Pre-Caldera Volcanism - Prior to the McDermitt Caldera collapse at 16.33 Ma, small amounts of trachytic to rhyolitic lavas were erupted near the present-day Oregon-Nevada border in the Trout Creek and Oregon Canyon Mountains. These lavas, which are dated at 16.5 to 16.3 Ma, as well as the flood basalts, are exposed along the walls of the McDermitt Caldera (Benson et al., 2017; Henry et al., 2017).
Syn-Caldera Volcanism and Collapse - commencing with eruption of the trachytic to rhyolitic Tuff of Long Ridge at ~16.33 Ma, to form the keyhole-shaped, McDermitt Caldera that straddles the Oregon-Nevada border. Although, this volcanic event had originally been interpreted to represent the eruption of four different ignimbrites from a single magma chamber (e.g., Rytuba and McKee, 1984 and Conrad 1984), regional reconnaissance indicates that there was a single large, laterally extensive, and crystal-poor (<3% feldspar) caldera-forming eruption (Benson et al., 2017). While this event dominated, other less voluminous tuffs are exposed close to the vent and their eruptions and concomitant collapses may have contributed to the peculiar shape of the caldera. Some 500 km3 of ignimbrite is estimated to have been ponded prior to the eruption, after which it was spread out for up to 60 km from the resultant caldera collapse structure (Benson et al., 2017; Henry et al., 2017).
Post-Caldera Activity - subsequent to eruption of the Tuff of Long Ridge, a large lake formed in the caldera depression, into which detrital sediments and subordinate volcanic rock (tephra, basaltic lava, rhyolitic tuff) were accumulated. Sedimentation is estimated to have persisted for several hundreds of thousands of years, as has been demonstrated in other nearby Miocene caldera lakes (e.g., Coble and Mahood, 2012; Benson et al., 2017). 40Ar/39Ar dating of primary tephra and authigenic feldspar from the sedimentary sequence are as young as ~14.9 Ma, suggesting sedimentation and sporadic volcanism continued for at least ~1.5 m.y. (Castor and Henry, 2020). During this interval, the caldera underwent a period of resurgence at ~16.2 Ma (Castor and Henry, 2020), resulting in the uplift of a large volume of intra-caldera ignimbrite and caldera lake sediments that form the present-day Montana Mountains in the core of the caldera. This magmatic resurgence was accompanied by the introduction of hot, acidic fluids, rich in Li, K, F, Mo, Cs, Rb and other elements associated with hydrothermal systems (Ingraffia et al., 2020). This fluid is interpreted to have altered much of the smectite-bearing clays in the vicinity of Thacker Pass to lithium-bearing illite, localised around intracaldera normal faults. From ~12 Ma, Basin and Range normal faulting associated with extension of North American lithosphere (Colgan et al., 2006; Lerch et al., 2008) resulted in uplift of the western half of the McDermitt Caldera, and subsidence of the Kings River Valley to the west of Thacker Pass. Faults were formed along reactivated ring fractures in the western McDermitt Caldera, and in the Tuff of Thacker Creek, facilitating the weathering and erosion of rocks within the caldera.
Mineralisation
The Thacker Pass Deposit has a sub-horizontal attitude, masked by a thin cover of alluvium, and is only partially exposed at surface, which is at elevations of between 1500 and 1300 m asl. It is hosted within caldera lake 'moat' sedimentary rocks that have been separated from topographically higher correlates to the north by post-caldera resurgence and Basin and Range normal faulting. As such, exposure of the host sedimentary rocks is limited to a few drainages and isolated road cuts, and as a consequence, the stratigraphic sequence and mineralisation in the deposit is principally derived from drill core.
The host moat sedimentary sequence has a maximum drilled thickness of ~160 m, and is composed of alternating layers of claystone and volcanic ash, with intermittent bands of basaltic lava. Claystone comprises 40% to 90% of the section, and in many intervals is intimately intermixed with ash. These claystones are variably brown, tan, grey, bluish-grey and black, whilst the ash is generally white or very light grey. Individual claystone-rich units may laterally persist for strike lengths of >150 m, although their thickness can vary by as much as 20%. Ash-rich layers are more variable, and display some textures that imply reworking. All units exhibit finely graded bedding and laminar textures that suggest lacustrine deposition.
The host moat sedimentary rocks have been oxidised to depths of 15 to 30 m, underlain by a transition to fresh rock over an interval of as much as 4.5 m in thickness. The claystones oxidise to brown, tan or light greenish-tan colours and contain iron oxide, whereas ash is white, with some orange-brown iron oxide. This host sedimentary sequence directly overlies the hard, dense, indurated intra-caldera Tuff of Long Ridge. The base of the sedimentary sequence, is occupied by a zone of weakly to strongly silicified sedimentary rock, the Hot Pond Zone (HPZ). Both the underlying Tuff of Long Ridge and HPZ are generally oxidised.
The bulk of the sedimentary rocks in the 'moat' basin at Thacker Pass contain strongly anomalous levels of Li, averaging >1000 ppm, with Intervals that are predominantly composed of ash or volcanic rock carrying <800 ppm whilst those dominated by claystone contain >1000 ppm Li. Many of the drilled intervals have very high contents of >4000 ppm Li. Zones with extreme lithium contents of >8000 ppm are found sporadically throughout the deposit.
There is no obvious variation in lithium content across the transition from oxidised to un-oxidised rocks. The highest lithium grades generally occur in the deeper illite-rich middle to lower and basal sections of the sedimentary sequence, overlain by the lower grade, shallower, smectite-rich portion of the sequence, as detailed below. The lateral Lithium grade continuity is good, being continuous over strike lengths of >1 km. Past and more recent drill holes have intersected lithium grades ranging from 2000 to 8000 ppm Li over significant lateral extents. There is a relatively continuous high-grade sub-horizontal clay horizon carrying >5000 ppm Li across the Project area. This horizon averages 1.47 m in thickness at an average depth of 56 m down hole. The lithium grade for several metres above and below the high-grade horizon typically ranges from 3000 to 5000 ppm Li. The bottom of the deposit is well defined by the hydrothermally altered oxidised ash and sediments of the Hot Pond Zone (HPZ) that contains <500 ppm, and often <100 ppm Li.
The lithium grade within the host claystone can generally be correlated with the colour and texture of the rock, as well as the amount of intercalated ash. Intervals with high, >4000 ppm Li, usually comprise grey to dark-grey or black claystone with <10% ash. Bluish-grey claystone with low ash content correlates with moderate lithium contents of 2500 to 3000 ppm, whilst light-coloured, tan, light grey, greenish-tan claystone have lower grades of generally 1500 to 2500 ppm Li. Intervals of mixed claystone and ash are common and carry variable contents, generally 1500 to 3000 ppm Li depending on the type of claystone and proportion of ash present.
South of the Montana Mountains in the centre of the caldera complex, the caldera lake/moat sediments dip slightly away from the centre of resurgence. As a result, the preserved package thickens to the south, and the lithium enriched package is at a lower altitude. Consequently, it has not been as deeply eroded during uplift compared to the up-dip sections further to the north. The proposed open pit is located where the lithium enriched interval has not been eroded, but is close to the surface with minimal overburden, due to uplift and erosion during both caldera resurgence and Basin and Range faulting.
Clays in the Thacker Pass Deposit include two distinctly different types, smectite and illite. Smectite occurs at relatively shallow levels of the deposit (Castor and Henry, 2020), generally carrying 2000 to 4000 ppm Li. The chemistry and structure of the smectite at McDermitt is most similar to the sub-type hectorite [NaO,3(Mg,Li)3Si4O10(OH)2], although chemically the clay is actually intermediate between hectorite and two other smectites, stevensite and saponite (Morissette, 2012).
Drill intersections with higher lithium contents, generally of >4000 ppm Li, contain clay with compositions typical of illite (Morissette, 2012; Castor and Henry, 2020). This illitic clay is found at relatively moderate to deep levels in the moat sedimentary sequence, and sporadically occurs in intervals that contain values approaching 9000 ppm Li, higher than a hectorite crystal can accommodate. This Li-rich illite is similar in character to tainiolite, a subtype of illite [K2(Mg4,Li2)Si8O20(OH,F)4] (Morissette, 2012; Castor and Henry, 2020). A relatively thin zone of interstratified smectite-illite clay is found separating the smectite and underlying illite-type clays (Castor and Henry, 2020).
The observations above are based on standard 1.5 m sample intervals of finely laminated sediments, and as such may contain a variety of lithologies and clay types. Consequently, clay separates were prepared from different sections of the deposit and analysed (Morissette, 2012). The illite clay concentrates averaged 1.2 wt.% Li, whilst the smectite clay concentrates had an average composition of 0.5 wt.% Li .
Other minerals in the Thacker Pass Deposit claystone include calcite, quartz, K feldspar, plagioclase, dolomite and fluorite. Pyrite and bitumen are found in the claystone below near-surface oxidised rock. Ash beds in the deposit contain quartz and feldspar with local analcime, and minor clay and pyrite. Zeolite minerals are typically present in the northern part of the caldera, but analcime is the only zeolite present within the deposit (Glanzman and Rytuba, 1979; Castor and Henry, 2020). Carbonates (calcite and dolomite) are present throughout the deposit as primary sedimentary beds, and as rosettes and masses (Castor and Henry, 2020). Fluorite occurs in the mixed smectite/illite and illite zones and is interpreted by Castor and Henry (2020) to be the product of a secondary fluid. Fluorite often replaces calcite in the illitic portion of the sedimentary sequence, further supporting its genesis as a secondary fluid.
Formation of the Deposit
The parent rhyolitic magmas of the McDermitt Volcanic Field have been shown to be enriched in lithium related to the assimilation of ~50% continental crust during magma formation (Benson et al., 2017). The same authors suggest the eruption of the Tuff of Long Ridge and the collapse of the McDermitt Caldera resulted in a large volume of Li-enriched glass, pumice and ash being accumulated at the surface proximal to the caldera. Subsequent weathering and erosion would have transported much of this lithium-rich detritus into the caldera lake, which acted as a structurally controlled depositional basin. Immediately following collapse, this same accumulation, proximal to the caldera margin, would have had a relatively large surface area from which Li could be leached by meteoric, and possibly hydrothermal fluids, and chemically deposited within the caldera lake.
The presence of sedimentary carbonate minerals and Mg-smectite (hectorite) throughout the lake sediments, is taken to indicate that the clays formed in a basic, alkaline, closed hydrologic system (Roth, et al., 2022). The same authors suggest that these conditions would enable the direct precipitation of clays from solution (neoformation). The composition of these clays would be dependent on the chemistry of the lake water (e.g., Tosca and Masterson, 2014). Roth, et al. (2022) also suggest that because the McDermitt Caldera lake water was rich in Li and F, the primary smectite to precipitate would be the Li-Mg variety, hectorite. They also conclude that the relatively low aluminum content of the clays supports a non-detrital, authigenic origin for the smectites. Ingraffia et al. (2020) hypothesise that the bulk of the Li mass within the caldera lake sediments is sourced from devitrification and degassing of glassy intra-caldera tuff as sediments were accumulating in the caldera basin.
Roth, et al. (2022) note that geochemical and field evidence suggests the intra-caldera Tuff of Long Ridge was emplaced at high temperatures of >850°C, atypical of continental rhyolitic ignimbrites, resulting in intense welding and rheomorphism (Hargrove and Sheridan, 1984; Henry et al., 2017). They also suggest the cooling and degassing of this hot ignimbrite likely took place over much of the life of the caldera lake, which would add significant Li mass to the meteoric water system via hydrothermal fluids. They further suggest, these high-temperature fluids at >100°C likely mixed with the lake and groundwater to lead to a basin-wide warm hydrologic system.
The high-Li (>4000 ppm) illitic portions of the sedimentary sequence near Thacker Pass, are interpreted to have formed when a hot, low-pH, Li- and F-rich fluid altered smectite to illite and dissolved the disseminated carbonates within the sediments (Roth, et al., 2022). Geologic evidence for the interaction of sediments with this fluid include replacement of analcime by authigenic K feldspar (Castor and Henry, 2020), the presence of the siliceous Hot Pond Zone (HPZ) below the illite sediments, and high concentrations of Li, Rb, Cs, As, Mo, Sb and other trace metals (Castor and Henry, 2020) in the illite-rich portion of the deposit. This is taken to support a genetic model in which the initial neoformation of smectite in a closed hydrologic system was followed by hydrothermal alteration to illite in the vicinity of Thacker Pass. This explains why the illite in the Thacker Pass Deposit reaches whole-rock assay values up to 9000 ppm Li, whereas the smectite intervals rarely exceed 4000 ppm Li (Roth, et al., 2022).
Resources and Reserves
Ore Reserves as at 2 November, 2022 (Roth et al., 2022) were:
Proved Reserve - 192.9 Mt @ 3180 ppm Li for 3.3 Mt of Lithium Carbonate Equivalent (LCE);
Probable Reserve - 24.4 Mt @ 3010 ppm Li for 0.4 Mt of LCE;
Proved + Probable Reserves - 217.3 Mt @ 3160 ppm Li for 3.7 Mt of LCE.
NOTE: This estimate applied a maximum ash cutoff of 85%, and a cutoff grade of 1.533 kg of lithium extracted per tonne of ROM feed. Additionally, a 95% mining recovery factor is applied.
Mineral Resources as at 2 November, 2022 (Roth et al., 2022) were:
Measured Resource - 534.7 Mt @ 2450 ppm Li for 7.0 Mt of Lithium Carbonate Equivalent (LCE);
Indicated Resource - 922.5 Mt @ 1850 ppm Li for 9.1 Mt of LCE;
Measured + Indicated Resources - 1457.2 Mt @ 2070 ppm Li for 16.1 Mt of LCE;
Inferred Resource - 297.2 Mt @ 1870 ppm Li for 3.0 Mt of LCE;
TOTAL Resources - 1754.4 Mt @ 2036 ppm Li for 19.1 Mt of LCE;
NOTE: This resource estimate uses a cutoff grade of 1047 ppm lithium. Mineral Resources are inclusive of Ore Reserves.
The information within this summary has been largely drawn from Roth, D., Tahija, L., Iasillo, E., Martina, K., Chow, B., Mutler, W., Bahe, K., Kaplan, T., Cluff, T. and Shannon, B., 2022 - Feasibility Study Technical Report for the Thacker Pass Project, Humboldt County, Nevada, USA; an NI 43-101 Technical Report prepared by M3 Engineering & Technology Corporation, for Lithium Americas Corp., 350p.
The most recent source geological information used to prepare this decription was dated: 2022.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Thacker Pass Lithium
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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