Vorontsovka Carlin-style gold deposit in the North Urals: Mineralogy, fluid inclusion and isotope data for genetic model

Abstract

The Vorontsovka gold deposit in the Tagil zone, North Urals, hosts 101 t Au, averaging 7 g/t Au and including 30 t Au in the regolith, and 120 t Ag, averaging 8 g/t Ag. Early skarn-magnetite mineralisation related to a dioritic intrusion of the shoshonitic series was formed in the Late Emsian. Water and volatiles (mainly CO2) released from host metapelites and limestones were mixed with the metalliferous magmatic fluid during catagenesis to contact metamorphism. The modified fluid extracted ore components from host rocks and syn-volcanic stratiform mineralisation and became enriched in Fe, Mn, Ba, Zn, Pb, Au, Ag and S to deposit these components as sulphide dissemination in the footwall of siliciclastic unit and underlying limestone both brecciated along the trust. At the final Early Eifelian stage, there was an additional inflow of deep mantle/magmatic fluid, enriched in Au, Hg, As, Sb and Tl. The mixing of this fluid with metamorphic water resulted in Carlin-style gold-realgar-stibnite (+Tl minerals) mineralisation mainly in carbonate breccia. The main gold endowment associates with gold‑arsenic ores located in brecciated silicified limestone (jasperoid) and argillised tuff-siltstone. Early ore assemblages were formed at 510–240 °C (including magnetite skarn), whereas overprinting Carlin-style gold-(Fe, As, Sb, Hg, Tl) sulphide mineralisation was deposited at decreasing temperatures mainly from 260 to 110 °C. Fluid inclusion data revealed aqueous‑carbonic, Ca-, Na-, Mg- and K-chloride compositions of ore-bearing fluid of 4.8–9.3 wt% eq. NaCl. Changing fluid pressure corresponded to near lithostatic (0.5–0.6 kb) to hydrostatic (0.15–0.2 kb) conditions. Based on the δ34S ranges of sulphides determined for siliciclastic rocks (−2.3 … +1.8‰) and skarns (−2 … +1.6‰) it is assumed that most of the sulphur derived from magmatic reservoir associated with the mantle. The lead isotopic compositions of sulphides show a relatively narrow range and continue the linear trend of the Early Silurian VMS deposits corresponding to the mixing line of mantle and crustal lead sources. Model PbPb ages for the Vorontsovka sulphides are grouped into two clusters: 425–416 and 398–388 Ma. The second period is close to the UPb isotope age of the final stage of the Auerbakh pluton as well as in a good agreement with the ArAr isotope age of 391.1 ± 4.9 Ma of hydromica from the late gold-arsenopyrite assemblage. The specific features of the Vorontsovka deposit indicative of the Carlin-style mineralisation are as follows: the paleo-continental margin and trust-related position; causing high-K/shoshonitic igneous activity; the fine-disseminated sulphide mineralisation in the highly brecciated carbonate-clastic sequence; jasperoid and argillic alteration accompanying gold orebodies with assay boundaries; multiple fluid source; Au-As-Hg-Tl geochemical association; the abundance of As- and As-Fe-sulphides and AsSb sulphosalts; the occurrence of Hg-, Tl-sulphides and sulphosalts; the association of gold with arsenian pyrite and arsenopyrite overgrowing previously forming pyrite; and the predominance of submicroscopic gold in the ores. A LA-ICP-MS analysis of pyrite confirms that elevated concentrations of Au regularly correlate with higher contents of As, Ag, Sb or Tl.