Minerals of the beudantite–segnitite series from the oxidation zone of the Berezovskoe gold deposit, middle Urals: Chemical variations, behavior of admixtures, and antimonian varieties

Abstract

In the oxidation zone of the Berezovskoe gold deposit in the middle Urals, Russia, minerals of the beudantite–segnitite series (idealized formulas PbFe₃ ³⁺ AsO₄)(SO₄)(OH)₆ and PbFe₃ ³⁺ AsO₄)(AsO₃OH)(OH)₆, respectively) form a multicomponent solid solution system with wide variations in the As, S, Fe, Cu, and Sb contents and less variable P, Cr, Zn, Pb, and contents K. The found minerals of this system correspond to series from beudantite with 1.25 S apfu to S-free segnitite, with segnitite lacking between 1.57 and 1.79 As apfu. Segnitite at the Berezovskoe deposit contains presumably pentavalent Sb (up to 15.2 wt % Sb₂O₅ = 0.76 Sb apfu, the highest Sb content in the alunite supergroup minerals), which replaces Fe³⁺. The Sb content increases with increasing As/S value. On the contrary, beudantite is free of or very poor in Sb (0.00–0.03 Sb apfu). Many samples of segnitite are enriched in Cu (up to 8.2 wt% CuO = 0.83 Cu apfu, uncommonly high Cu content for this mineral) and/or in Zn (up to 2.0 wt% ZnO = 0.19 Zn apfu). Both Cu and Zn replace Fe. The generalized formula of a hypothetic end member of the segnitite series with 1 Sb apfu is Pb(Fe³⁺ M ²⁺Sb⁵⁺)(AsO₄)₂(OH)₆, where M = Cu, Zn, Fe²⁺. The chemical evolution of beudantite–segnitite series minerals at the Berezovskoe deposit is characterized by an increase in the S/As value with a decrease in the Sb content from early to late generations.