Another look at nagyágite from the type locality, S $\check{\rm{a}}$ c $\check{\rm{a}}$ rîmb, Romania: Replacement, chemical variation and petrogenetic implications

Abstract

Extensive compositional heterogeneity is displayed by Pb-Sb-Au tellurides from the type locality at S\(\check{\rm{a}}\)c\(\check{\rm{a}}\)rimb. These phases are collectively considered as varieties of nagyagite in the absence of crystal chemical data confirming the presence of distinct, but topologically closely related compounds. Chemical heterogeneity is seen relative to ‘normal’ nagyagite, with close to the ideal composition Pb3[Pb1.8(Sb1.1,As0.1)1.2]Σ3S6 (AuTe2), which is the primary and common type in the deposit. A modified formula, (Pb3S3)[(Pb2−x)(Sb,As,Teb)1+x(S3−yTey)]Σ6(Au1−z−wTe2+zSw), accounts for the chemical variation observed. Values of x (0.2 to 1.15) express substitution of Pb by Sb+As for Me2 in sulfosalt modules in the case of Au-depleted and low-Au nagyagite, and by Sb+As+Teb in high-As and low-Pb varieties (b = x+1−(Sb+As) = 0.24 to 0.29). Excess Te compensates for Au deficiency in the telluride layer, with substitution by S also observed; empirical values of z and w are 0 to 0.45 and 0 to 0.32, respectively. Minor substitution of Te for S (y < 0.17) is noted in all varieties except low-Au.