Stephen E. Kesler

Global Lithium Availability

There is disagreement on whether the supply of lithium is adequate to support a future global fleet of electric vehicles. We report a comprehensive analysis of the global lithium resources and compare it to an assessment of global lithium demand from 2010 to 2100 that assumes rapid and widespread adoption of electric vehicles. Recent estimates of global lithium resources have reached very different conclusions. We compiled data on 103 deposits containing lithium, with an emphasis on the 32 deposits that have a lithium resource of more than 100,000 tonnes each. For each deposit, data were compiled on its location, geologic type, dimensions, and content of lithium as well as current status of production where appropriate. Lithium demand was estimated under the assumption of two different growth scenarios for electric vehicles and other current battery and nonbattery applications. The global lithium resource is estimated to be about 39 Mt (million tonnes), whereas the highest demand scenario does not exceed 20 Mt for the period 2010 to 2100. We conclude that even with a rapid and widespread adoption of electric vehicles powered by lithium‐ion batteries, lithium resources are sufficient to support demand until at least the end of this century.

Oxidation state of gold and arsenic in gold-bearing arsenian pyrite

Abstract XANES measurements on gold-bearing arsenian pyrite from the Twin Creeks Carlin-type gold deposits show that gold is present as both Au0 and AuI+ and arsenic is present at AsI-. Au0 is attributed to sub-micrometer size inclusions of free gold, whereas AuI+ is attributed to gold in the lattice of the arsenian pyrite. STEM observations suggest that AsI- is probably concentrated in angstrom-scale, randomly distributed layers with a marcasite or arsenopyrite structure. Ionic gold (AuI+) could be concentrated in these layers as well, and is present in both twofold- and fourfold-coordinated forms, with fourfold-coordinated AuI+ more abundant. Twofold-coordinated AuI+ is similar to gold in Au2S in which it is linearly coordinated to two sulfur atoms. The nature of fourfold-coordinated AuI+ is not well understood, although it might be present as an Au-As-S compound where gold is bonded in fourfold coordination to sulfur and arsenic atoms, or in vacancy positions on a cation site in the arsenian pyrite. AuI+ was probably incorporated into arsenian pyrite by adsorption onto pyrite surfaces during crystal growth. The most likely compound in the case of twofold-coordinated AuI+ was probably a tri-atomic surface complex such as Spyrite-AuI+-Sbi-sulfideH or AuI+-S-AuI+. The correlation between gold and arsenic might be related to the role of arsenic in enhancing the adsorption of gold complexes of this type on pyrite surfaces, possibly through semiconductor effects.

“Invisible” gold revealed: Direct imaging of gold nanoparticles in a Carlin-type deposit

Abstract Although As-rich, hydrothermal overgrowths on pyrite have been recognized as the primary host phase for Au in Carlin-type deposits in Nevada, the chemical and structural state of the Au has remained unresolved. Spectroscopic and electron imaging techniques have suggested that Au is either structurally bound (e.g., Au1+) or occurs as particles of native Au (Au0), but the latter has never been observed directly. We have determined that Au is present in significant quantities as discrete nanoparticles of native Au (~5.10 nm) in As-rich overgrowths on pyrite from the Screamer deposit in the Carlin trend, Nevada, using analytical and high-resolution TEM and high-angle annular dark-field (HAADF) imaging in STEM mode. Electron microprobe and secondary ion mass spectrometry (SIMS) analyses of the As-rich rims containing the Au-particles reveal that these rims (1.20 µm) contain up to 0.8 wt% Au, among the highest Au-contents ever reported for arsenian pyrite. These observations suggest two possible mechanisms for nanoparticle formation: that Au exceeded its solubility limit in arsenian pyrite causing it to be deposited as nanoparticles of native metal; or that exsolution of native metal from metastable arsenian pyrite was caused by a later event in the history of the deposit.

Mercury isotope fractionation in fossil hydrothermal systems

The Hg isotopic compositions of samples throughout the vertical extent of two fossil hydrothermal systems were analyzed by multicollector inductively coupled plasma-mass spectrometry. Results show >5‰ (δ 2 0 2 Hg/ 1 9 8 Hg; relative to NIST 3133) fractionation, more than 50 times greater than the 0.1‰ (2σ) external reproducibility of the analyses. The Hg isotope compositions from both hydrothermal systems can be grouped by dominant mineralogy and position; δ 2 0 2 Hg/ 1 9 8 Hg values at the tops of the systems are -3.5‰ to -0.4‰ in cinnabar-dominant sinter and -0.2‰ to +2.1‰ in metacinnabar-dominant sinter, and the underlying veins have δ 2 0 2 Hg/ 1 9 8 Hg values of -1.4‰ to +1.3‰. These differences probably resulted from the combination of boiling of the hydrothermal fluid, oxidation near the surface, and kinetic effects associated with mineral precipitation. The natural variation in Hg isotopic compositions observed in this study is higher than that expected from the trend of decreasing mass-dependent fractionation with increasing mass extrapolated from stable isotope systems up to Z = 26 (Fe), confirming that even the heaviest elements undergo significant stable isotope fractionation in hydrothermal systems.

Rb-Sr dating of sphalerites from Mississippi Valley-type (MVT) ore deposits

Abstract Mississippi Valley-type (MVT) ore deposits are epigenetic carbonate-hosted Pb-Zn deposits that contain galena, sphalerite, fluorite, barite, dolomite, calcite, and quartz. Although they are thought to form from basinal brines, their exact origins are still unclear, partly because of the scarcity of reliable geochronological data. Rb-Sr dating of sphalerites has recently been shown to be a promising technique for the direct dating of ore minerals in MVT deposits. This paper reports the results of a reconnaissance study of sphalerites, their fluid inclusions, and associated minerals from MVT deposits of North America. Sphalerites from Immel mine, Mascot-Jefferson City district, east Tennessee, define a Rb-Sr age of 347 ± 20 Ma consistent with a Rb-Sr age of 377 ± 29 Ma for sphalerites from Coy mine in the same district, but inconsistent with models that ascribe their genesis to the effects of the late Paleozoic Alleghenian orogeny. Rb-Sr isotopic analyses of K-feldspar from Immel mine preclude the possibility that the Rb-Sr data reflect feldspar inclusions. Sphalerites from the main ore zone of Daniels Harbour mine, New foundland, do not form a linear isochron and open behavior of the Rb-Sr system is suspected. Sphalerites from the Pine Point district, Northwest Territories, Canada, define a Rb-Sr age of 361 ± 13 Ma, indicating that the mineralization took place shortly after the deposition of the middle Devonian host carbonate rocks. These results are not compatible with mineralization models based on regional fluid migration related to early Tertiary Cordilleran deformation. Sphalerites from northern Arkansas have very low Rb and Sr concentrations (less than 0.1 ppm). The Rb-Sr data do not form isochrons and the sphalerites have higher 87 Sr 86 Sr ratios than expected, given their Rb Sr ratios and reasonable constraints on their ages. The sphalerites are suspected to contain clay inclusions; and it is likely that the Sr isotopic compositions of these sphalerites, which have very low Sr concentrations, were affected by small amounts of inherited inclusions. Except for sphalerite from northern Arkansas, SEM studies and isotope dilution trace element measurements have so far failed to identify any suitable phases other than sphalerite that might be a host for the Sr.

Na-Cl-Br systematics of mineralizing brines in Mississippi Valley–type deposits

New and published analyses of fluid-inclusion leachates from Mississippi Valley–type (MVT) deposits can be divided into two groups on the basis of Na/Br and Cl/Br ratios. MVT leachates from the Illinois-Kentucky and Cincinnati arch districts and from cubic galena in the Viburnum Trend have Na/Br and Cl/Br ratios that extend to values significantly above that of seawater, which are characteristic of evaporite-dissolution brines. MVT leachates from Polaris and octahedral galena in the Viburnum Trend have Na/Br and Cl/Br ratios that plot below seawater and along the compositional trend formed by evaporation. Solubility-volume constraints require that brines formed by seawater evaporation had high dissolved metal contents. Preliminary correlation of leachate compositions allows delineation of two brine provinces in the midcontinent United States: (1) an early, high-Br province found only in southeastern Missouri, and (2) a later, low-Br province of probable Permian age that extended from the Cincinnati arch to the Tri-State district. These observations, along with Na-Cl-Br data for modern brines from the Illinois basin, argue against models for single-stage midcontinent MVT brine flow based on recharge from late Paleozoic (Arkoma) foreland basins to the south.

NA-CL-BR SYSTEMATICS OF FLUID INCLUSIONS FROM MISSISSIPPI VALLEY-TYPE DEPOSITS, APPALACHIAN BASIN : CONSTRAINTS ON SOLUTE ORIGIN AND MIGRATION PATHS

Abstract This study evaluated Na-Cl-Br systematics of fluid inclusion-hosted brines in Mississippi Valley-type (MVT) deposits from the Appalachian Basin. Unlike other geochemical tracers such as lead and strontium isotopes which constrain metal sources, Na-Cl-Br systematics identify sources of brine salinity. Saline formation waters can vary systematically within and between basins with regard to their Na-Cl-Br compositions depending on the importance of halite dissolution relative to retention of subaerially evaporated seawater for the halogen budget. Oil field brine compositions from the Illinois and Appalachian basins are quite distinct in their Na-Cl-Br systematics. Compositions of saline fluid inclusions in MVT deposits generally are consistent with these regional differences. These results shed new light on the extent of regional flow systems and on the geochemical evolution of saline fluids responsible for mineralization. Nearly all fluid inclusions analyzed from the Appalachian MVT deposits have Na/Br and Cl/Br ratios less than modern seawater, consistent with ratios observed in marine brines involved in halite precipitation. The Na-Cl-Br systematics of the brines responsible for Appalachian MVT deposits may be inherited from original marine brines refluxed into the porous carbonate shelf sediments that host these deposits. The Cl/Br and Na/Br ratios of most fluid inclusion-hosted brines from Appalachian MVT sphalerites and fluorites fall into two compositional groups, one from the Lower Cambrian paleoaquifer and another from the Lower Ordovician paleoaquifer. Leachates from most MVT barite deposits form a third compositional group having lower Na/Br and Cl/Br ratios than the other two. Appalachian MVT leachate compositions differ significantly from those in MVT deposits in the Cincinnati arch-midcontinent region suggesting that these two MVT provinces formed from brines of different origin or flow path.

Gold in porphyry copper deposits: its abundance and fate

Abstract Porphyry copper deposits are among the largest reservoirs of gold in the upper crust and are important potential sources for gold in lower temperature epithermal deposits. Whether gold remains in porphyry copper deposits is important both to their economic attractiveness and to the distribution of gold in the upper crust. Cu/Au atomic ratios of porphyry copper ore deposits form a continuous range from about 5000 to 5,000,000 with a median near 40,000, which separates gold-rich and gold-poor deposits. Gold is found in porphyry copper deposits in solid solution in Cu–Fe and Cu sulfides and as small grains of native gold, usually along boundaries of bornite. SIMS (ion probe) analyses of ore minerals from the gold-rich Batu Hijau, Kingking and Skouries porphyry copper deposits show that bornite contains about 1 ppm Au, whereas chalcopyrite contains about an order of magnitude less. Chalcocite and covellite contain 10–20 ppm Au, but are not abundant enough to account for a significant part of the gold endowment in many porphyry copper deposits. The amount of gold presently in solid solution in Cu–Fe sulfides is not adequate to account for all the gold in porphyry copper deposits, and the remainder is present as micron-scale grains of native gold. Experiments in the Cu–Fe–S–Au system show that bornite and chalcopyrite can contain about 1000 ppm gold at typical porphyry copper formation temperatures of 600–700°C, and indicate that bornite and chalcopyrite in porphyry copper deposits were saturated with respect to gold at temperatures of only 200–300°C. In contrast, Cu/Au ratios of bulk ore in porphyry copper deposits would require bornite and chalcopyrite to be saturated with respect to gold at temperatures similar to those at which primary (potassic) ore and alteration are thought to form. This indicates that the maximum gold endowment of porphyry copper deposits is probably fixed by the amount of gold that will go into solid solution in Cu–Fe sulfides when the deposit forms at high temperature, and that gold is not commonly added later from other sources, although it can be redistributed during cooling or later events. The experimental data also suggest that high-temperature (600–700°C) vapors can extract considerably more gold from porphyry copper systems than can low-temperature (300°C) alteration. Comparison to Cu/Au ratios of volcanic emissions suggests further that high-temperature processes remove copper (relative to gold) from porphyry copper systems, whereas low-temperature processes remove gold preferentially and that this can account for deposits with extremely low and high Cu/Au ratios, respectively. Deposits with Cu/Au ratios between about 20,000 and 100,000, however, probably reflect different degrees of removal of gold or copper by immiscible sulfides.

Direct dating of sulfides by RbSr: A critical test using the Polaris Mississippi Valley-type ZnPb deposit

Abstract The RbSr dating of sphalerites is a powerful method for directly determining the age of base metal deposits and testing models for large-scale fluid flow. However, the uncertainty over the exact host phases for the trace amounts of Rb and Sr and the causes of variability in Rb Sr have caused concern over the reliability of the method. Here we show that the Polaris MVT deposit, with a geologically well-constrained age of formation, confirmed by paleomagnetic measurements, and hosted in significantly older sedimentary rocks, yields a consistent RbSr age of 366 ± 15 Ma, providing the first unequivocal vindication of the reliability of the method.

Unusually Cu-rich magmas associated with giant porphyry copper deposits: Evidence from Bingham, Utah

Mass-balance constraints indicate that formation of giant porphyry copper deposits (PCDs) requires either highly efficient collection of Cu from large volumes of magma or unusually Cu-rich parent magmas. Support for the second of these possibilities has been discovered in the form of mafic enclaves with abundant bornite and chalcopyrite in the Last Chance stock, one of the parent intrusions of the giant Bingham PCD, Utah, United States. Mineral assemblages and compositions indicate that the Last Chance enclaves are autoliths consisting of phases that crystallized from the intrusion, and that the intrusion was unusually enriched in Cu. One possible mechanism for generating Cu-rich magmas is fractional crystallization during pyrrhotite undersaturated conditions. The high fO 2 conditions observed for the Last Chance stock may have allowed such an evolution. Alternatively, if the magma has not undergone significant fractionation, early crystallization of chalcopyrite and bornite from the magma would indicate that the lower-crustal source region for the magma probably contained Cu-Fe sulfides. Possible Cu-rich source regions are a subcrustal mafic intrusion with sulfide cumulates, or a deeply buried metamorphic terrane containing Cu deposits such as those in the Curaca Valley (Brazil) or Okiep (South Africa). Heterogeneous distribution of Cu-Fe sulfides in an Okiep-type source terrane would produce local PCDs such as Bingham, or large accumulations of Cu-Fe sulfides, possibly in the form of cumulates in subcrustal intrusions at convergent margins, could produce giant PCD provinces such as those in Indonesia, Papua New Guinea, and central Chile.