CAN TELLURIDES RECORD CHANGES IN FLUID CHARACTERISTICS? THE EXAMPLE OF NAGYÁGITE

The behavior of minerals with complex chemistry can record changes in fluid characteristics during single and/or overlapping events in a hydrothermal system. Chemical variation can be adjusted via substitutions and will also lead to structural modifications in minerals with crystal modularity. This is the case for Au-bearing tellurides and, in particular, nagyágite, a complex Pb-Sb-Au-sulfotelluride with a structure consisting of a (Au,Te) telluride layer stacked between 2x[Pb₂(Sb,As,Bi)₂S₂] sulfosalt modules. Nagyágite, with a composition close to ideal Pb₃(Pb₂,Sb)S₆(AuTe₂), is known from deposits of both epithermal and orogenic type.

The behavior of nagyágite from the epithermal Au-telluride deposit at Sacarimb (TL, Romania), where it is one of the main ore components, reflects overprinting by fluid-driven replacement reactions in veins reactivated during rotation of the duplex fault-system responsible for vein formation. Extensive substitutions of Pb by As+Sb and Au by Te are expressed by both chemical variation and structural modulations. Replacement of close to ideal nagyágite by other varieties can be linked to high fluid acidity, whereas replacement by galena-altaite symplectites relates to changes in the fTe₂/fS₂ within a narrow domain defined by coexistence of these two minerals at 300ºC. The modular crystal chemistry of nagyágite is able to adjust to variable rates of fluid infiltration by subtle chemical substitutions. The behavior of this mineral thus maps and assists coupling between dissolution and precipitation during such reactions.

Pseudomorphic replacement of nagyágite by galena-altaite symplectites is also observed at Sunrise Dam and Kalgoorlie, two world-class orogenic Au deposits from the Yilgarn Craton (Western Australia). The lack of any substitutional reactions between pervading fluids and parent nagyágite, even in the presence of As-rich fronts as observed at Sunrise Dam, indicates that the acidity of fluids generated during overlapping deformation events in these orogenic systems remain unchanged. Such fluids will assist local-scale remobilisation of ore components from the pre-existing assemblages and their redeposition during subsequent tectonic events. In particular, precipitation of native gold accompanies destabilization of nagyágite in such cases.