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Recent advances in understanding the similarities and differences of Colombian euclases

Pignatelli, I. et al., The Canadian Mineralogist

Recent advances in understanding the similarities and differences of Colombian euclases

Pignatelli, I. ; Giuliani, G. ; Morlot, C. ; Rouer, O. ; Claiser, N. ; Chatagnier, P.Y. ; Goubert, D.

The Canadian Mineralogist, 2017, 55, 799-820

Abstract :

Colombian euclase is rare and associated with emerald in medium-temperature hydrothermal veins hosted by Lower Cretaceous black shales (BS). The original sources of euclase production were the mining districts of Gachala´ and Chivor in the eastern emerald belt, but in 2016 euclases were also found at the La Marina mine in the western emerald belt. The present study is centered on a chemical and mineralogical examination of zoned Colombian euclase sold on the gem market as ‘‘trapiche’’. Its texture is characterized by growth bands and sectors distinguished by the presence of numerous inclusions (mainly pyrite, carbonates, and organic matter) which represent around 0.2% of the total volume of the crystals. Xray computed tomography showed that the largest inclusions are randomly located, whereas the small inclusions are concentrated in the center of the crystals, along the crystallographic b axis, between neighboring growth sectors and between growth bands in each sector. The texture cannot be defined as ‘‘trapiche’’, like that of Colombian emeralds, because there is no matrix material from the surrounding BS trapped between the growth sectors and accumulated as dendrites. Three-phase fluid inclusions (FI) containing halite, liquid, and vapor phases are also observed in the euclase, and their volume is identical to that of the inclusions in emerald. Chromium and vanadium are the main chromophores, and the highest concentrations (1240 and 400 ppm, respectively) were found in deep blue-colored zones. Surprisingly, the euclase crystals have high Ge contents, between 230 and 530 ppm. The Rare Earth Element (REE) patterns of euclase are inherited from the enclosed BS or albitized and carbonatized BS. Euclase has the same REE pattern as emerald from the Gachala´ mines with an Eu anomaly (Eu/Eu* 0.40) and a depletion in Heavy Rare Earth Elements (HREE). The present study allows for the reconstruction of the formation conditions of ‘‘trapiche’’ euclase and discussion about its probable geographic origin, i.e., the eastern emerald belt.

Voir en ligne : http://dx.doi.org/doi:10.3749/canmi...

publié lundi 21 août 2017