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

<p>The application of cosmogenic noble gases (³He and ²¹Ne) 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. &#160;In an effort to determine how long detrital Pt-alloy grains spend travelling to placer deposits we have measured cosmogenic³He concentrations in 60 (0.5&#8211;7 mg) grains of isoferroplatinum (Pt₃Fe) from the world largest alluvial Pt placer deposits in the Kondyor-Uorgalan rivers in Khabarovsk region, and the Is-Turinsk and Nyas&#8217;ma river systems in Middle Urals, Russia.</p><p>In both systems, there are no significant cosmogenic ³He in the grains from the low order streams that drain the source rock. ³He_cosconcentrations in Pt grains from distant placers (30 km) varies in order of magnitude from 0.3 to 30 x 10⁷at g^-1in grains in Uorgalan river, 5 to 40 x 10⁷at g^-1in Glubokinskoe placer deposit (Is river) and 20 to 70 x 10⁷at g^-1in the Generalka deposit (Nyas&#8217;ma river). Converting this to surface residence times (P = 25 at g y^-1, Yakubovich et al., 2019) yields model exposure durations of0.1&#173;&#8211;10, 3&#8211;20 and 13&#8211;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&#8211;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&#8217;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 ³He during peneplenisation. &#160;</p><p>Yakubovich O., Stuart F.M., Nesterenok A. and Carracedo A. (2019). Chem. Geol. 517, 22-33.</p>