T208 Gemological Research in the 21st Century: Gem Minerals and Localities

Thursday, 29 October 2020
1:30 PM - 5:30 PM
Meeting Rooms

THE STUDY OF HIGH-VANADIUM EMERALD FROM MALIPO，CHINA

Paper number: T208-16
Program: Technical Sessions
Time: 4:45 PM – 4:55 PM

Authors

Abstract

Emeralds are found in the Dayakou white tungsten deposit in Malipo county, southwestern China, which geologic origin is distinct for the abundant vanadium compositions. We have used EMPA to study the associated minerals and chemical fingerprint characters combined with LA-ICP-MS, studied the structure of emeralds by using XRD, and used the stable isotope geochemistry method to determine the 18O-compositions of emeralds which indicated the geological environment. We combined these techniques in an attempt to constrain the geographic origin determination for Malipo emeralds.

EMPA analysis displayed concentrations of trace metals Cr2O3 from 0.01 wt.% to 0.09 wt.%, averaging 0.06 wt.%, V2O3 from 0.11 wt.% to 0.82 wt.%, averaging 0.45 wt.%, FeO from 0.27 wt.% to 0.54 wt.%, averaging 0.36 wt.%, Na2O from 0.94 wt.% to 1.46 wt.%, averaging 0.95 wt.%, MgO from 0.58 wt.% to 1.62 wt.%, averaging 0.96 wt.%, Cs2O from 0.11 wt.% to 0.28 wt.%, averaging 0.19 wt.%. The samples contained relatively high V ,Fe than Cr. In addition, Sc (20-33 ppm), Rb (41-44 ppm), Zn (24-28 ppm), and Ga (averaged 14 ppm) were present in much lower concentrations. Cs ranged up to 3200 ppm. The relatively abundant trace elements coincided with lower Al2O3 (14.88~16.83 wt.%) indicated metals substitute for aluminum in the crystal structure. Also, the XRD results displayed the unit cell parameters a (averaged 9.233 Å) and c (averaged 9.197 Å) of samples. The range of the c/a ratios were 0.995~0.996, indicating that substitution in Malipo emerald attributed to [AlO6] octahedron replacement.

The associated minerals were mainly feldspar, biotite, arsenopryite, pyrite, scheelite, fluorite and tourmaline.Biotite showed vanadium-rich (1.18 wt.%). Arsenopryite and scheelite were typical associated minerals which related closely with the white tungsten deposit. Therefore, the categories of accompanying minerals provided beneficial information for identifying the provenance of emeralds.

We obtained oxygen isotope values of samples: δ18O values ranged from 10.60 to 10.82‰. The 18O-compositions indicated the mixture source of magmatic water and meteoric water (Giuliani et al. 1998, Xue et al. 2010, Giuliani et al. 2019).It indicated that the mineralization was affected by hydrothermal and magmatic activity. These results suggested that the Malipo emeralds with the contents of relatively higher V and Cs, poor Cr were compositionally distinct. Also, arsenopryite and scheelite could be the important indicators for identifying the origin compared to other emerald deposits.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-351564

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