Earth Science Frontiers ›› 2020, Vol. 27 ›› Issue (5): 116-125.DOI: 10.13745/j.esf.sf.2020.5.48

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The genesis of color zonation of emerald from Dayakou, Yunnan Province: implication for multi-stage mineralization

YU Xiaoyan1(), ZHENG Yuyu1, ZHANG Tingya1, GUO Hongshu1, LONG Zhengyu1, WAN Jiaxin1, ZHANG Cun2   

  1. 1. School of Gemology, China University of Geosciences(Beijing), Beijing 100083, China
    2. School of Earth Sciences and Resources, China University of Geosciences(Beijing), Beijing 100083, China
  • Received:2020-04-12 Revised:2020-05-22 Online:2020-09-25 Published:2020-09-25

Abstract:

Color zonation with obvious composition variation often appear in emeralds from Dayakou, Yunnan Province, China. However, the origin of color zonation is still controversial, and there has been no reliable evidence for the related multistage mineralization hypothesis. In this study, we analyzed the main and trace elemental signatures of the Dayakou emerald with obvious color zonation by electron microprobe (EMP) and laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). We also carried out microthermal analysis of fluid inclusions hosted within emerald and other associate minerals from different types of veins. The composition results showed that the trace elemental contents in emerald increased sharply from the light greenish white core to the green rim, yielding V((970-10077)×10-6), Rb((9.6-27.4)×10-6), Cs((535-3108)×10-6), Fe((1376-2199)×10-6) and Ga((4-14.7)×10-6); and the syngenetic black inclusions which only occur at the green crystalline rim were proved to be V-rich intermediate oxy-schorl-dravite. The above evidence strongly suggests that color zonation are formed from multistage crystalline mineralization. Furthermore, the microthermal analysis results showed that salinities of fluid inclusions hosted within minerals from the feldspar-calcite and quartz veins and pegmatite-granofel contact zones were 3.39%-10.36%, 5.71%-12.29% and 8%-18.72%(NaCleq), respectively, again indicating different mineralization stages in pegmatite veins and suggesting increasing parent fluid salinity with evolution of the mineralization stage. And the considerable variation of V concentration is also caused by increasing salinity at the greisenization stage.

Key words: Yunnan emerald, major and trace elemental analysis, fluid inclusions, origin of color zonation, multistage mineralization

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