Native Gold in the Chudnoe Au-Pd-REE Deposit (Subpolar Urals, Russia): Composition, Minerals in Intergrowth and Genesis

Abstract

Composition of native gold and minerals in intergrowth of the Chudnoe Au-Pd-REE deposit (Subpolar Urals, Russia) was studied using optical microscopy, scanning electron microscopy, and electron microprobe analysis. Five varieties of native gold have been identified, based on the set of impurity elements and their quantities, and on intergrown minerals. Native gold in rhyolites from the Ludnaya ore zone is homogeneous and contains only Ag (fineness 720‰, type I). It is in intergrowth with fuchsite or allanite and mertieite-II. In rhyolites from the Slavnaya ore zone, native gold is heterogeneous, has a higher fineness, different sets and contents of elements: Ag, Cu, 840–860‰ (type II); Ag, Cu, Pd, 830–890‰ (III); Ag, Pd, Cu, Hg, 840–870‰ (IV). It occurs in intergrowth with fuchsite, albite, and mertieite-II (type II), or albite, quartz, and atheneite (III), or quartz, albite, K-feldspar, and mertieite-II (IV). High-fineness gold (930–1000‰, type V) with low contents of Ag, Cu, and Pd or their absence occurs in the form as microveins, fringes and microinclusions in native gold II–IV. Tetra-auricupride (AuCu) is presented as isometric inclusions in native gold II and platelets in the decay structures in native gold III and IV. The preliminary data of a fluid inclusions study showed that gold mineralization at the Chudnoe deposit could have been formed by chloride fluids of low and medium salinity at temperatures from 105 to 230 °C and pressures from 5 to 115 MPa. The formation of native gold I is probably related to fuchsitization and allanitization of rhyolites. The formation of native gold II-V is also associated with the same processes, but it is more complicated and occurred later with a significant role of Na-, Si-, and K-metasomatism. The presence of Pd and Cu in the ores and Cr in fuchsite indicates the important role of mafic-ultramafic magmatism.

Palladian Gold: Chemical Composition, Minerals in Association, and Physicochemical Conditions of Formation at Different Types of Gold Deposits

“…Since that time, much data on new deposits with palladian gold have been published ( [11], and cited references). New data on the compositions of palladian gold and its minerals in the association have been recently obtained by the authors of this paper for such deposits as Chudnoe [29], Volkovskoe [30], Ozernoe [31] (Urals, Russia), and Bleida Far West (Morocco) [32], and for the placers and ore occurrences of the mafic-ultramafic Itchayvayam massif (Kamchatka, Russia) [33], which differ from other deposits. In this paper, we present a review of the composition of palladian gold of the systems Au-Pd, Au-Pd-Ag, Au-Pd-Hg, and Au-Pd-Cu and the more complex systems Au-Pd-Ag-Cu, Au-Pd-Ag-Hg, Au-Pd-Ag-Cu-Hg, etc.…”

“…Variations in the concentrations of impurities in native gold range from LOI up to 26 for Pd, <50 Ag, <18 Cu, and <6 Hg (in wt.%). According to Figure 10, the Ag,Cu,Hg-bearing palladian gold depending on the Pd, Ag, and Cu contents can be divided into several varieties: 6) (filled marker-endogenous, empty-exogenous): 1-Chudnoe (Subpolar Urals, Russia) [29,53,[112][113][114]; 2, 3-Serra Pelada (Brazil) [97]; 4-Gongo Soco (Brazil) 6) (filled marker-endogenous, empty-exogenous): 1-Chudnoe (Subpolar Urals, Russia) [29,53,[112][113][114]; 2, 3-Serra Pelada (Brazil) [97]; 4-Gongo Soco (Brazil) [25,78]; 5-Zechstein (Poland) [63]; 6, 7-r.Mayat and r. Bol'shaya Kuonamka, Anabar river basin (Russia) [47,116]; 8-Dziwiszow (Poland) [65]; 9-Whipsaw Creek (Canada) [71]; 10-Similkameen river (Canada) [71]; 11-Friday Creek (Canada) [71]; 12-theoretical compositions of Au-Pd intermetallic compounds.…”

“…The salinity of fluids varies from 20.1 to 0.2 wt.% NaCl eq. [29] (Table 9). Au-Pd and Au deposits of the Permian-Triassic basins in SW England (Thorverton) formed in the temperature range from 60 to 155 • C, and the salinity of fluids varied from 2 to 30 wt.% NaCl eq.…”

“…Figure 10. Dependencies of the concentrations of Pd on Au (a), Pd on Cu (b), and Ag on Hg (c) in Pd-bearing gold and other Pd-bearing minerals of the Au-Pd-Ag-Cu-Hg system (in at.%) in various deposits and ore occurrences (Table6) (filled marker-endogenous, empty-exogenous): 1-Chudnoe (Subpolar Urals, Russia)[29,53,[112][113][114]; 2, 3-Serra Pelada (Brazil)[97]; 4-Gongo Soco (Brazil)…”

“…Figure 10. Dependencies of the concentrations of Pd on Au (a), Pd on Cu (b), and Ag on Hg (c) in Pd-bearing gold and other Pd-bearing minerals of the Au-Pd-Ag-Cu-Hg system (in at.%) in various deposits and ore occurrences (Table6) (filled marker-endogenous, empty-exogenous): 1-Chudnoe (Subpolar Urals, Russia)[29,53,[112][113][114]; 2, 3-Serra Pelada (Brazil)[97]; 4-Gongo Soco (Brazil)[25,78]; 5-Zechstein (Poland)[63]; 6, 7-r.Mayat and r. Bol'shaya Kuonamka, Anabar river basin (Russia)[47,116]; 8-Dziwiszow (Poland)[65]; 9-Whipsaw Creek (Canada)[71]; 10-Similkameen river (Canada)[71]; 11-Friday Creek (Canada)[71]; 12-theoretical compositions of Au-Pd intermetallic compounds.…”