Extreme fractionation of selenium isotopes and possible deep biospheric origin of platinum nuggets from Minas Gerais, Brazil

Platinum-rich nuggets offer an opportunity for understanding how precious metals accumulate. We analyzed the selenium (Se) isotopic composition of Se-rich (102–103 μg g–1) platinum-palladium (Pt-Pd) nuggets from a recent placer deposit in Minas Gerais, Brazil, for which a biogenic origin has been inferred. We obtained Se isotopic values with a relatively narrow range (δ82/76SeNIST3149 = –17.4‰ to –15.4‰ ± 0.2‰, two standard deviations [2 SD]). The Pt-Os age of the nuggets is 181 ± 6 Ma (2 SD). The data indicate that the nuggets did not form in the recent placer deposit, but by replacement of hydrothermal vein minerals at ~70 °C and at least 800 m below the surface. The high abundance and extreme isotopic composition of Se as well as the presence of other biophilic elements like iodine, organic carbon, and nitrogen within the nugget matrix are consistent with a microbial origin. Although abiogenic reduction of Se oxyanions cannot be ruled out, the nuggets plausibly record Se-supported microbial activity in the deep biosphere.

Cosmogenic 3He in detrital Pt-alloy grains: tracing the accumulation of critical metals

<p>The application of cosmogenic noble gases (<sup>3</sup>He and <sup>21</sup>Ne) in detrital grains to quantify sediment transport rates and storage timescales is largely undeveloped. We have previously shown that cosmogenic He can be measured in single grains (Yakubovich et al. 2019). The low He diffusion rate and the chemical and/or physical robustness of metal alloys (eg Au, Ag, Cu, Pt, Fe) means the technique has potential to determine how critical metals accumulate at the Earth surface.  In an effort to determine how long detrital Pt-alloy grains spend travelling to placer deposits we have measured cosmogenic<sup>3</sup>He concentrations in 60 (0.5–7 mg) grains of isoferroplatinum (Pt<sub>3</sub>Fe) from the world largest alluvial Pt placer deposits in the Kondyor-Uorgalan rivers in Khabarovsk region, and the Is-Turinsk and Nyas’ma river systems in Middle Urals, Russia.</p><p>In both systems, there are no significant cosmogenic <sup>3</sup>He in the grains from the low order streams that drain the source rock. <sup>3</sup>He<sub>cos</sub>concentrations in Pt grains from distant placers (30 km) varies in order of magnitude from 0.3 to 30 x 10<sup>7</sup>at g<sup>-1</sup>in grains in Uorgalan river, 5 to 40 x 10<sup>7</sup>at g<sup>-1</sup>in Glubokinskoe placer deposit (Is river) and 20 to 70 x 10<sup>7</sup>at g<sup>-1</sup>in the Generalka deposit (Nyas’ma river). Converting this to surface residence times (P = 25 at g y<sup>-1</sup>, Yakubovich et al., 2019) yields model exposure durations of0.1­–10, 3–20 and 13–38 Myr for grains from the Uorgalan, Glubokinskoe and Generalka deposits respectively. Assuming that all grains were extracted from shielded locations this exposure ages indicate the total time of transport and residence of the grains on river bed surface prior to final deposition.</p><p>Pt grains are added to the river system constantly. If we assume that the average transport conditions are essentially identical for all grains, then the difference between the maximum and minimum exposure ages within the same location indicates the total duration of placer supply from the source rock. For the Urals it is equal for 17–25 Myr, while for Kondyor-Uorgalan placer it is 10 Myr. This is in a good agreement with geological observations. Within Kondyor-Uorgalan placer deposit the age of the alluvium varies from Neogene to Quaternary, while in the Urals Pt-bearing Jurassic, Neogene and Quaternary fluvial sediments are distinguished within the placers.</p><p>Paleo peneplain surfaces are established nearby the Kondyor massif and pre-date the most productive platinum sands. Several stages of peneplenisation are evident in the Urals from thick weathering crusts. The exposure ages indicate10’s million-year scale of transport/deposition histories of detrital Pt alloys, which might result of multiple redeposition of material during long term accumulation of cosmogenic <sup>3</sup>He during peneplenisation.  </p><p>Yakubovich O., Stuart F.M., Nesterenok A. and Carracedo A. (2019). Chem. Geol. 517, 22-33.</p>

Current methods of technogenic gold placer deposit mining with small size gold extraction technology

The purpose of this article is to conduct industrial tests of spiral separation technology on technogenic refractory raw materials. The object of the study is the Yurskoye gold placer deposit in the Republic of Sakha (Yakutia). An OKV-100 benefication complex with the productive capacity of 100 m3 /h by initial sands was set up in order to conduct pilot mining of the fine-grained tailings dumps of the Yurskoye deposit with the maximum extraction of small size gold. Sands supplied from the different sections of the technogenic deposit had different gold content, granulometric characteristics, and mineralogical composition. The study included experiments carried out at different initial feed; equipment was adjusted for specific raw material depending on productive capacity, liquid/solid ratio, product yield of the small size gold recovery module. The complex productivity was from 90 to 150 m3 /h. The yield and extraction of gold-bearing concentrate, as well as its gold content were determined. The pilot washing with the use of small size gold recovery technology resulted in the average additional gold recovery from sluice tailings of 19.07 %, which is a high indicator, given the low content of gold in technogenic placers. The technology under consideration can be used for the development of both technogenic and operated gold placer deposits.

Garnet sand reveals rock recycling processes in the youngest exhumed high- and ultrahigh-pressure terrane on Earth

Rock recycling within the forearcs of subduction zones involves subduction of sediments and hydrated lithosphere into the upper mantle, exhumation of rocks to the surface, and erosion to form new sediment. The compositions of, and inclusions within detrital minerals revealed by electron microprobe analysis and Raman spectroscopy preserve petrogenetic clues that can be related to transit through the rock cycle. We report the discovery of the ultrahigh-pressure (UHP) indicator mineral coesite as inclusions in detrital garnet from a modern placer deposit in the actively exhuming Late Miocene–Recent high- and ultrahigh-pressure ((U)HP) metamorphic terrane of eastern Papua New Guinea. Garnet compositions indicate the coesite-bearing detrital garnets are sourced from felsic protoliths. Carbonate, graphite, and CO2 inclusions also provide observational constraints for geochemical cycling of carbon and volatiles during subduction. Additional discoveries include polyphase inclusions of metastable polymorphs of SiO2 (cristobalite) and K-feldspar (kokchetavite) that we interpret as rapidly cooled former melt inclusions. Application of elastic thermobarometry on coexisting quartz and zircon inclusions in six detrital garnets indicates elastic equilibration during exhumation at granulite and amphibolite facies conditions. The garnet placer deposit preserves a record of the complete rock cycle, operative on <10-My geologic timescales, including subduction of sedimentary protoliths to UHP conditions, rapid exhumation, surface uplift, and erosion. Detrital garnet geochemistry and inclusion suites from both modern sediments and stratigraphic sections can be used to decipher the petrologic evolution of plate boundary zones and reveal recycling processes throughout Earth’s history.

Platinum and platinoids in the Bolotisty gold-placer deposit (Khabarovsky Krai, Russia)

The article shows the results of studies the platinum mineralization (within the framework of studying the complexity of deposits) in the waste of heavy concentrate in the southeastern flank of the Bolotisty gold-placer deposit, which the located in the Khabarovsky Krai (Russia). The deposit is localized in the “erosion window” among the covers of Miocene basalts and Eocene andesites, where Cretaceous sedimentary, tuffaceous-sedimentary rocks of the basement of the volcano structure, broken through by an Eocene intrusive body of intermediate and basic composition, come to the surface. An abbreviated mineralogical analysis for monofraction of precious metals was carried out for a concentrate fraction weighing 1.813 kg with a grain size of -3.0 + 0.1 mm. Gold and platinum were found - 1.38 mg and 1.08 mg. Platinum has an iron impurity up to 3-5 % Fe and Cu, Ir, Rh impurities are variable. There are plate-like inclusions of native osmium with an admixture of ruthenium in 43 % of the studied platinum grains. There are single inclusions of ehrlichmanite. A large amount of high-temperature dipyramidal quartz, along with magnetite, chromite, tourmaline and other minerals is a characteristic feature of the concentrate and platinum grains of this area.