Martin Reich

“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.

Giant Kiruna-type deposits form by efficient flotation of magmatic magnetite suspensions

Kiruna-type iron oxide-apatite (IOA) deposits are an important source of Fe ore, and two radically different processes are being actively investigated for their origin. One hypothesis invokes direct crystallization of immiscible Fe-rich melt that separated from a parent silicate magma, while the other hypothesis invokes deposition of Fe-oxides from hydrothermal fluids of either magmatic or crustal origin. Here, we present a new model based on Fe and O stable isotopes and trace and major element geochemistry data of magnetite from the ~350 Mt Fe Los Colorados IOA deposit in the Chilean iron belt that merges these divergent processes into a single sequence of events that explains all characteristic features of these curious deposits. We propose that concentration of magnetite takes place by the preferred wetting of magnetite, followed by buoyant segregation of these earlyformed magmatic magnetite-bubble pairs, which become a rising magnetite suspension that deposits massive magnetite in regionalscale transcurrent faults. Our data demonstrate an unambiguous magmatic origin, consistent with the namesake IOA analogue in the Kiruna district, Sweden. Further, our model explains the observed coexisting purely magmatic and hydrothermal-magmatic features and allows a genetic connection between Kiruna-type IOA and iron oxide-copper-gold deposits, contributing to a global understanding valuable to exploration efforts.

Thermal behavior of metal nanoparticles in geologic materials

Although significant progress has been made in understanding the behavior of natural nanoparticles in Earths critical zone (i.e., surface and near-surface environment), little is known about nanoparticle stability in higher-temperature environments where they are increasingly being found. Here we report the first direct observations of the thermal behavior of natural nanoparticles at near atomic scale, revealing that their thermal stability is dependent on particle size and on the surrounding host mineral. Native Au nanoparticles (mean diameter ∼4 nm) incorporated in arsenian pyrite from refractory Au ores were observed under the transmission electron microscope during in situ heating to 650 °C. While isolated Au nanoparticles melt, with the melting point being a function of size, we show that when incorporated in arsenian pyrite, Au nanoparticles become unstable unless the nanoparticle size distribution coarsens by diffusion-driven, solid-state Ostwald ripening. This change in nanoparticle stability starts above 370 °C, setting an upper temperature and size limit for the occurrence of nanoparticulate Au in refractory sulfides. These findings not only provide new insights into the behavior of nanoparticulate Au and other metals during geological processes and throughout their metallurgical recovery from refractory ores, but also provide a new tool to define the thermal history of nanoparticle-bearing geologic and planetary materials.

Formation of cristobalite nanofibers during explosive volcanic eruptions

High-resolution transmission electron microscopy (HRTEM) observations of unaltered volcanic air-fall deposits from the ongoing lava dome explosive eruption at Chaiten Volcano, Chilean Patagonia, revealed the presence of highly crystalline silica nanofibers in the respirable fraction of the volcanic ash ( 240 °C), beta form of cristobalite, with average lengths of hundreds of nanometers and widths on the order of tens of nanometers. We propose that the beta-cristobalite nanofibers are formed during explosive eruptions by the reduction of amorphous silica by carbon monoxide to its reactive suboxide SiO, which is later oxidized to form one-dimensional crystalline silica nanostructures. Nucleation and growth of the nanofibers are enhanced by the high surface area of the micrometer- to nanometer-sized fragments of silica glass in the volcanic column. The formation of nanocrystalline cristobalite fibers during explosive lava dome eruptions poses new challenges for the assessment of the short- and long-term health hazards associated with the respirable nanofibrous components of volcanic ash.

Crustal deformation effects on the chemical evolution of geothermal systems: the intra-arc Liquiñe–Ofqui fault system, Southern Andes

A better understanding of the chemical evolution of fluids in geothermal and hydrothermal systems requires data-based knowledge regarding the interplay between active tectonics and fluid flow. The Southern Andes volcanic zone is one of the best natural laboratories to address this issue because of the occurrence of numerous geothermal areas, recent seismic activity generated by regional fault systems, and intense volcanic activity. Geothermal systems have been understudied in this area, and limited scientific information exists about the role of local kinematic conditions on fluid flow and mineralization during the development and evolution of geothermal reservoirs. In this study, we provide data for a 1:200,000 scale geological and structural map of the Villarrica–Chihuio area as a setting in which to perform a structural analysis of active geothermal areas. This structural analysis, combined with geochemical modelling of hot spring data, allows the identification of two magmatic-tectonic-geothermal domains based on fault systems, volcanic activity, and lithologies. The Liquiñe–Ofqui fault system (LOFS) domain encompasses geothermal areas located either along the master or subsidiary faults. These are favourably orientated for shear and extension, respectively. In the LOFS domain, the geochemistry of hot spring discharges is controlled by interaction with the crystalline basement, and is characterized by low B/Cl conservative element ratios and high pH. In marked contrast, the arc-oblique long-lived fault systems (ALFS) domain includes geothermal occurrences located on the flanks of volcanoes forming WNW-trending alignments; these systems are built over faults that promote the development of crustal magma reservoirs. Unlike the first domain, the fluid chemistry of these geothermal discharges is strongly controlled by volcanic host rocks, and is typified by lower pH and higher B/Cl ratios. Reaction path modelling supports our model: chemical evolution of geothermal fluids in the Villarrica–Chihuio area is strongly dependent on structurally controlled mechanisms of heat transfer. Within this framework, heat transfer by conduction is responsible for the LOFS domain, whereas magmatically enhanced advective transport dominates heat flow in the ALFS domain. Although more studies are needed to constrain the complex interplay between tectonics and fluid flow, results from this study provide new insights towards efficient exploration strategies of geothermal resources in Southern Chile.

Climate change and tectonic uplift triggered the formation of the Atacama Desert’s giant nitrate deposits

The giant nitrate deposits of the hyperarid Atacama Desert (Chile) are one of the most extraordinary, yet enigmatic, mineral occurrences on Earth. These deposits are complex assemblages of highly soluble nitrates, chlorides, sulfates, perchlorates, iodates, and chromates, and their preservation is the result of prevalent hyperarid climate conditions in the Atacama Desert since the late Miocene, with average rainfall rates of <10 mm/yr in the past ~3 m.y. Although several hypotheses have been proposed since the mid-1800s, the formation of these extensive deposits still remains highly controversial despite the fact that recent studies have argued toward an atmospheric source for the nitrate, sulfate, and perchlorate components. In this report, we focus on the often overlooked and poorly studied iodine and chromium components of Atacama’s nitrates. We present the fi rst cosmogenic iodine ( 129 I) and stable chromium (δ 53/52 Cr) isotope data of nitrates showing that groundwater has played an unforeseen role in the formation of these massive deposits. The isotopic signature of I in the nitrates ( 129 I/I ~150–600 × 10 –15 ) share similarities with deep sedimentary (marine) pore waters and shales, deviating signifi cantly from atmospheric iodine ( 129 I/I ~1500 × 10 –15 ), while the positive and highly fractionated δ 53/52 Cr SRM979 values (+0.7‰ to +3‰) are indicative of intense Cr redox cycling due to groundwater transport. Our evidence points toward a multi-source genetic model for the Atacama Desert nitrate deposits, where these extensive accumulations were the result of long-lived, near-surface mineral precipitation driven by groundwater (i.e., chromates, iodates) coupled with dry atmospheric deposition (i.e., nitrates, perchlorates) and sea spray inputs (i.e., sulfates, chlorides), triggered by increasing aridity and tectonic uplift.

Geochemical anomalies in northern Chile as a surface expression of the extended supergene metallogenesis of buried copper deposits

ABSTRACT Supergene enrichment played a critical role in making northern Chile the most productive copper province of the world. This occurred over a long period from 45 to 9 Ma in a semi-arid climate. Subsequently, many deposits were covered by thick alluvial gravels, such that exploration is now focused on deposits buried beneath these gravels. Well-defined saline+metal anomalies are present at the gravel surface above deposits. Work, starting in 1999, characterized these anomalies and ascribed their origin to pumping of mineralized saline waters to the surface during earthquakes. Atacamite, a copper hydroxychloride, and an important ore mineral, had previously been considered to be of primary supergene origin, i.e. 45 to 9 Ma. However, it is readily soluble in meteoric water and could not have been stable either during supergene enrichment by meteoric water, or later, when stream waters carrying alluvial gravels penetrated oxide zones. Atacamite must be younger than 9 Ma. Studies at the University of Chile have established that: (a) atacamite has been found at the surface along faults, 300 m above copper mineralization; (b) salinities of fluid inclusions in atacamite are the same as local saline groundwater; (c) 36Cl analyses of atacamite show that it could not have formed prior to 1.5 Ma; (d) U-Th disequilibrium dating of gypsum from atacamite-gypsum intergrowths give Pleistocene ages that range from 237 ka for Chuquicamata in the east to 75 ka for Michilla on the Pacific coast. We propose that anomaly formation at the surface and saline metasomatism of supergene oxides were parts of the same process, which occurred after the climate became hyper-arid. Tectonic de-watering of the forearc basin sent saline waters through oxide zones to replace pre-existing minerals with atacamite and continued to the surface to create geochemical anomalies; these waters, having first modified the deposits, were rich in indicator elements. Thus, although perceptions are that geochemical anomalies above thick gravel sequences should be weak, selective leach anomalies in northern Chile have clear anomaly/background contrast.

Constraints on the solid solubility of Hg, Tl, and Cd in arsenian pyrite

Abstract Arsenic-rich (arsenian) pyrite can contain up to tens of thousands of parts per million (ppm) of toxic heavy metals such as Hg, Tl, and Cd, although few data are available on their solid solubility behavior. When a compilation of Hg, Tl, and Cd analyses from different environments are plotted along with As in a M(Hg, Tl, and Cd)-As log-log space, the resulting wedge-shaped distribution of data points suggests that the solid solubility of the aforementioned metals is strongly dependent on the As concentration of pyrite. The solid solubility limits of Hg in arsenian pyrite—i.e., the upper limit of the wedge-shaped zone in compositional space—are similar to the one previously defined for Au by Reich et al. (2005) (CHg,Au = 0.02CAs + 4 × 10–5), whereas the solubility limit of Tl in arsenian pyrite is approximated by a ratio of Tl/As = 1. In contrast, and despite a wedge-shaped distribution of Cd-As data points for pyrite in Cd-As log-log space, the majority of Cd analyses reflect the presence of mineral particles of Cd-rich sphalerite and/or CdS. Based on these data, we show that arsenian pyrite with M/As ratios above the solubility limit of Hg and Tl contain nanoparticles of HgS, and multimetallic Tl-Hg mineral nanoparticles. These results indicate that the uptake of Hg and Tl in pyrite is strongly dependent on As contents, as it has been previously documented for metals such as Au and Cu. Cadmium, on the other hand, follows a different behavior and its incorporation into the pyrite structure is most likely limited by the precipitation of Cd-rich nanoparticulate sphalerite. The distribution of metal concentrations below the solubility limit suggests that hydrothermal fluids from which pyrite precipitate are dominantly undersaturated with respect to species of Hg and Tl, favoring the incorporation of these metals into the pyrite structure as solid solution. In contrast, the formation of metallic aggregates of Hg and Tl or mineral nanoparticles in the pyrite matrix occurs when Hg and Tl locally oversaturate with respect to their solid phases at constant temperature. This process can be kinetically enhanced by high-to-medium temperature metamorphism and thermal processing or combustion, which demonstrates a retrograde solubility for these metals in pyrite.

“Invisible” silver in chalcopyrite and bornite from the Mantos Blancos Cu deposit, northern Chile

Relatively little is known about the mineralogical occurrence and geochemical controls on the incorporation of “invisible” (refractory) silver and gold in hydrothermal sulfide minerals. Secondary ion mass spectrometry (SIMS) analysis reveals that bornite (81–649 ppm Ag) and chalcopyrite (0.61–2211 ppm Ag) are major hosts for silver in the Mantos Blancos deposit (500 Mt, @1 wt% Cu), the largest Jurassic stratabound Cu-(±Ag) deposit in the Costal Range of northern Chile. Gold concentrations are generally two orders of magnitude lower, ranging from 0.05 to 1.66 ppm Au in chalcopyrite, and 0.08 to 2.38 ppm Au in bornite. In addition to precious metals, SIMS analysis shows significant concentrations of As (~100 ppm in chalcopyrite, <10 ppm in bornite), whereas other metalloids and chalcogens, such as Sb, Se, and Te, have highly variable concentrations ranging from tens of ppb to ppm levels. These microanalytical results are consistent with a two-stage hydrothermal evolution model, as recently proposed for the Mantos Blancos deposit. Within this context, Ag, Au, As, and base metals were most likely sourced from a Late Jurassic (~155 Ma) rhyolitic dome, and partitioned into bornite and chalcopyrite in quartz-sericite veins after cooling below ~430°C. This first hypogene Cu-Ag ± Au event was followed by a second, higher-temperature alteration phase (400–600 °C) related to the emplacement of diorite and granodiorite stocks (~141–152 Ma), in which Ag and Au were partitioned into fine-grained, porous chalcopyrite in potassic alteration vein assemblages. When coupled with recent studies in the area, results presented here confirm that the high Ag endowment of Mantos Blancos is the result of multiple pulses of hypogene mineralization followed by supergene enrichment of metals.

Geodynamic implications of ophiolitic chromitites in the La Cabaña ultramafic bodies, Central Chile

Chromitites (>80% volume chromite) hosted in two ultramafic bodies (Lavanderos and Centinela Bajo) from the Palaeozoic metamorphic basement of the Chilean Coastal Cordillera were studied in terms of their chromite composition, platinum-group element (PGE) abundances, and Re-Os isotopic systematics. Primary chromite (Cr# = 0.64–0.66; Mg# = 48.71–51.81) is only preserved in some massive chromitites from the Centinela Bajo ultramafic body. This chemical fingerprint is similar to other high-Cr chromitites from ophiolite complexes, suggesting that they crystallized from arc-type melt similar to high-Mg island-arc tholeiites (IAT) and boninites in supra-subduction mantle. The chromitites display enrichment in IPGE (Os, Ir, Ru) over PPGE (Rh, Pt, Pd), with PGE concentrations between 180 and 347 ppb, as is typical of chromitites hosted in the mantle of supra-subduction zone (SSZ) ophiolites. Laurite (RuS2)-erlichmanite (OsS2) phases are the most abundant inclusions of platinum-group minerals (PGM) in chromite, indicating crystallization from S-undersaturated melts in the sub-arc mantle. The metamorphism associated with the emplacement of the ultramafic bodies in the La Cabaña has been determined to be ca. 300 Ma, based on K-Ar dating of fuchsite. Initial 187Os/188Os ratios for four chromitite samples, calculated for this age, range from 0.1248 to 0.1271. These isotopic compositions are well within the range of chromitites hosted in the mantle section of other Phanaerozoic ophiolites. Collectively, these mineralogical and geochemical features are interpreted in terms of chromite crystallization in dunite channels beneath a spreading centre that opened a marginal basin above a supra-subduction zone. This implies that chromitite-bearing serpentinites in the metamorphic basement of the Coastal Cordillera are of oceanic-mantle origin and not oceanic crust as previously suggested. We suggest that old subcontinental mantle underlying the hypothetical Chilenia micro-continent was unroofed and later altered during the opening of the marginal basin. This defined the compositional and structural framework in which the protoliths of the meta-igneous and meta-sedimentary rocks of the Eastern and Western Series of the Chilean Coastal Cordillera basement were formed.