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Another look at nagyágite from the type locality, Sãcãrîmb, Romania: Replacement, chemical variation and petrogenetic implications

Domenii publicaţii > Ştiinţele pământului şi planetare + Tipuri publicaţii > Articol în revistã ştiinţificã

Autori: Ciobanu,C. L.,Cook, N.J., Pring A, Damian Gheorghe & Cãpraru N.

Editorial: Mineralogy and Petrology, Vol 93, nr. 3-4/July, p.273-307, 2008.

Rezumat:

Extensive compositional heterogeneity is displayed by Pb-Sb-Au tellurides from the type locality at Sacaramb. 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]∑3S6(AuTe2), which is the primary and common type in the deposit. A modified formula, (Pb3S3)[(Pb2-x)(Sb,As,Teb)1+x(S3-yTe-y)]∑6(Au1-z-wTe2+zSw), accounts for the chemical variation observed. Values of x (0.2 to 1.15) express substitution of Pb by SbAs 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. These varieties are formed during replacement of the 'normal' type as seen in overprinting relationships in those veins reactivated during rotation of the duplex faultsystem responsible for vein formation. Replacement is by coupled dissolution-reprecipitation reactions, as indicated by pseudomorphism of one nagya´gite type by another in all cases. Variable rates of both molar-excess and -deficit reaction are invoked to explain the observed chemical and textural modifications. Low-Pb nagyagite is also present in zoned platelets where it grows over resorbed cores of ideal composition. Such platelets are instead interpreted as products of self-patterning in a residual precipitate. A marked depletion in the Au content of some nagya´gite lamellae is considered to be a diffusion driven Te for Au substitution in the presence of Te-bearing fluid. Replacement of 'normal' nagyagite 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. Nagyagite is a mineral with modular crystal chemistry and is able to adjust to variable rates of fluid infiltration by subtle chemical substitutions. The behavior of nagyagite will map and assist coupling between dissolution and precipitation during such reactions.

Cuvinte cheie: nagyagite, crystal chemical, heterogeneity, low-Au nagyagite, high-As and low-Pb varieties