A cervelleite-like mineral and other Ag-Cu-Te-S minerals [Ag_2CuTeS and (Ag,Cu)_2TeS] in gold-bearing veins in metamorphic rocks of the Cycladic Blueschist Unit, Kallianou, Evia Island, Greece

摘要

A cervelleite-like mineral, two unnamed silver sulfotellurides in the system Ag-Cu-Te-S [Ag_2CuTeS, (Ag,Cu)_2TeS], Te-rich polybasite and cadmian tetrahedrite occur in gold-bearing quartz veins in metapelites and faults within brecciated marbles of the Cycladic Blueschist Unit in the Kallianou area (southern Evia Island, Greece). The quartz veins and faults are discordant to syn-metamorphic structures and formed during ductile to brittle deformation in the final stages of exhumation of the Styra Nappe extrusion wedge (～21 Ma). Te-rich polybasite (up to 7.4 wt. % Te), cadmian tetrahedrite (up to 12.4 wt. % Cd), together with electrum (23-54 wt. % Ag) and the sulfotellurides, are the main silver carriers in the mineralization. The two unnamed sulfotellurides, Ag_2CuTeS and (Ag,Cu)_2TeS are believed to be new quaternary minerals in the system Ag-Cu-Te-S. These minerals and the cervelleite-like phase could have exsolved from galena during cooling (below 200°C). Initial temperatures for the formation of the sulfotellurides, in the form of hessite-intermediate solid solution, at Kallianou may be up to 300°C under log/S_2 values between-11.5 to -8.3, and log/Te_2 from～ -14.8 to -7.8. The values of log/Te_2 and log/S_2 during re-equilibration (at ~200°C) were constrained to -19.5 to -15.2 and to -15.8 to -11.5 respectively.