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

Content Provider	Springer Nature Link
Author	Ciobanu, C. L. Cook, N. J. Pring, A. Damian, G. Căpraru, N.
Copyright Year	2007
Abstract	Extensive compositional heterogeneity is displayed by Pb-Sb-Au tellurides from the type locality at S $\check{\rm{a}}$ c $\check{\rm{a}}$ rîmb. These phases are collectively considered as varieties of nagyágite in the absence of crystal chemical data confirming the presence of distinct, but topologically closely related compounds. Chemical heterogeneity is seen relative to ‘normal’ nagyágite, 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,Te b )1+x (S3−y Te y )]Σ6(Au1−z−w Te2+z S w ), 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 nagyágite, and by Sb+As+Te b 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 fault-system responsible for vein formation. Replacement is by coupled dissolution-reprecipitation reactions, as indicated by pseudomorphism of one nagyágite 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 nagyágite 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 nagyágite lamellae is considered to be a diffusion driven Te for Au substitution in the presence of Te-bearing fluid.Replacement of ‘normal’ 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. Nagyágite 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 nagyágite will map and assist coupling between dissolution and precipitation during such reactions.
Starting Page	273
Ending Page	307
Page Count	35
File Format	PDF
ISSN	09300708
Journal	Mineralogy and Petrology
Volume Number	93
Issue Number	3-4
e-ISSN	14381168
Language	English
Publisher	Springer-Verlag
Publisher Date	2007-12-06
Publisher Place	Vienna
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Mineralogy Geochemistry
Content Type	Text
Resource Type	Article
Subject	Geophysics Geochemistry and Petrology

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