科研之友
微信
新浪微博
Facebook
分享链接
摘要
Extensive compositional heterogeneity is displayed by Pb-Sb-Au tellurides from the type locality at Sacarimb. These phases are collectively considered as varieties of nagyaigite in the absence of crystal chemical data confirming the presence of distinct, but topologically closely related compounds. Chemical heterogeneity is seen relative to 'normal' nagyaigite, with close to the ideal composition Pb-3[Pb-1.8(Sb-1.1,As-0.1)(1.2)](Sigma 3)S-6 (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) )](Sigma 6)(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 Me-2 in sulfosalt modules in the case of Au-depleted and low-Au nagyaigite, 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 nagyaigite 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 nagyaigite 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 nagyaigite lamellae is considered to be a diffusion driven Te for Au substitution in the presence of Te-bearing fluid.
Replacement of 'normal' nagyaigite by other varieties can be linked to high fluid acidity, whereas replacement by galena-altaite symplectites relates to changes in the fTe(2)/fS(2) within a narrow domain defined by coexistence of these two minerals. Nagyaigite 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 nagyaigite will map and assist coupling between dissolution and precipitation during such reactions.
- 出版日期2008-7
