Yankuang ceshi (Jul 2019)
The Petrogeochemistry and Zircon U-Pb Age of Nephrite Placer Deposit in Xiuyan, Liaoning
Abstract
BACKGROUND The Xiuyan placer nephrite is located in the Xiyu gully, Baisha river valley and terrace, located in Xiuyan, Liaoning Province. Currently, the primary nephrite in the Xiyu gully is regarded as the Xiuyan placer nephrite's primary deposit according to location. However, the mineral composition, geochemical characteristics and age of the placer nephrite is unknown; whether they are consistent with primary nephrite needs to be studied further. OBJECTIVES To confirm whether the Xiuyan placer nephrite is from primary nephrite by investigating the mineral composition, geochemical characteristics and age of the Xiuyan placer nephrite. METHODS Electron microprobe analysis (EMPA), inductively coupled plasma-mass spectrometry (ICP-MS), X-ray fluorescence spectrometry (XRF), stable isotope mass spectrometry and zircon SHRIMP U-Pb dating were employed. RESULTS Primary minerals of the Xiuyan placer nephrite were tremolite, actinolite, whereas secondary minerals were diopside, apatite, chlorite, epidote, quartz, titanite, zircon, magnetite, and smithsonite. The Xiuyan placer nephrite was dolomite-related nephrite. Its total rare earth element (REE) was low with LREE enrichment and positive Eu anomaly. The δ18O and δD isotope compositions of nephrite were from 8.00‰ to 10.60‰ and from -94.75‰ to -75.20‰, respectively. Water in ore-forming fluids was mainly magmatic water. Zircon U-Pb age of the placer nephrite was 220.8±7.6Ma, which suggested that the placer nephrite formed after this time. CONCLUSIONS The Xiuyan placer nephrite is different from primary nephrite in bulk-rock REE pattern, hydrogen and oxygen isotope compositions, and source of ore-forming fluids and age, which may indicate that the placer nephrite is not derived from the original nephrite in the Xiyu gully.
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