Au-Ag-Se-Te-S Mineralization in the Maletoyvayam High-Sulfidation Epithermal Deposit, Kamchatka Peninsula

The Maletoyvayam high-sulfidation (HS) epithermal Au-Ag deposit is one of the numerous hydrothermal deposits of the Kamchatka volcanogenic belt, consisting of two main associations: Au-rich (Ag-free) and Ag-bearing. The first one derived from acidic solutions, whereas the second assemblage crystallized from moderately dilute solutions, with both occurring at high oxygen fugacity. The Au-rich association contains the most atypical gold chalcogenides of the Au-Se-Te-S system, which are characterized by Se-S and Te-Se substitutions, e.g., a complete series from maletoyvayamite to tolstykhite Au-3(Se,S)(4)Te-6; a series of auroselenide Au(Se1.00-0.64S0.36-0.00); a combined series of gachingite Au(Te,Se) and unnamed Au(Se,Te): Au(Te0.80-0.40Se0.20-0.60). Meanwhile, in the second Ag-bearing assemblage, sulfides of the Au-Ag type prevails, e.g., petrovskaite AuAgS, miargyrite (Ag,Au)(Sb,As)S-2, uytenbogaardtite Ag3AuS2, fischesserite Ag3AuSe2 with Au-Ag substitution, and tolstykhite. The Se/S ratio, of the second association, decreases while increasing the Ag concentration in the ore-forming system, including Au-Ag substitutions. The Au content in miargyrite (Au,Ag)SbS3 reaches up to 0.48 apfu, suggesting the existence of a new mineral phase of composition AgAuSb2S6. Au oxide complexes, in both associations, are represented by either a mixture of redeposited gold and Fe-Sb oxide or a homogeneous (Au,Sb,Fe)(2)O-3 composition. These oxides are formed by replacement of calaverite. The ore mineralization of this HS deposit is considered unique due to the special conditions of the ore-forming environment, such as acidic solutions, high oxygen fugacity, and log fSe(2) above -5.7; all contributed to the formation of AuSe phases.