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Fang Ma, Peking University (China)
Shaoyong Jiang, Nanjing University (China)
Yaohui Jiang, Nanjing University (China)
Rucheng Wang, Nanjing University (China)
Hongfei Ling, Nanjing University (China)
Pei Ni, Nanjing University (China)
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The Washan and Dongshan iron deposits are two of the most important volcanic hosted iron deposits in China, which occur in the Mesozoic Ningwu volcanic basin of the lower Yangtze River district of eastern China. In the Ningwu basin, the volcanic sequence includes four members, namely, the Longwangshan, Dawangshan, Gushan, and Niangniangshan members from bottom upward. The orebody developed in the gabbro-diorite porphyry, which intruded into the Dawanshan member of trachyte-andesite and trachyte-andesitic tuff.
The iron ores shows a typical magnetite-apatite-actinolite mineral assemblage. In this study, we analyzed the major, trace and rare earth element concentrations and Sr isotopic compositions of apatite from the Washan and Dongshan iron deposits in the Ningwu volcanic basin. The apatites from the iron deposits belong to fluorapatite with large variations of F contents (1.68 to 3.51 wt%). Cl contents in apatites also vary significantly, and can divided into a low-Cl apatite with Cl contents <0.24 wt%, and a high-Cl apatite with Cl contents of 0.80 wt% up to 1.19 wt%. Variable amounts of S are also presented in the apatites with SO3 contents from 0.02 to 0.95 wt%, which are similar to the apatites (SO3= <0.04 to 0.63 wt%) from volcanic rocks worldwide. The apatites are characterized by low Mn (162-226 ppm), high Zn (25-72 ppm), Cu (6-114 ppm), V (27-38 ppm), and Th (146-194 ppm), and high REE contents (4453-5734 ppm), which are quite similar to apatites from the mafic I-type granites worldwide.
The REE distribution patterns of apatites are characterized by LREE-enriched patterns with high (La/Yb)CN ratios of 7.7∼12.7 and a strong negative Eu anomaly (Eu/Eu*=0.28-0.34), but without any significant Ce anomaly (Ce/Ce*=1.02-1.07). The apatite REE patterns are similar to the host volcanic rocks in the Washan and Dongshan iron deposits, indicating a genetic link between the iron orebody and the host volcanic rocks. The major, trace, and rare earth element characteristics in apatites also indicate a high fO2 environment for the crystallization of apatites and associated iron ores, possibly in an open system. Sr isotopic compositions of apatites from the Washan and Dongshan iron deposits are homogeneous with 87Sr/86Sr =0.7069, which are close to the initial Sr isotopic compositions of the volcanic rocks (0.7040-0.7070) and the apatites (0.7071-0.7072) from other iron deposits in the basin, and may suggest a unique magmatic-hydrothermal fluids and ore materials in the basin scale.
We suggest that the ore-forming fluid may have derived from the evolution of the same parental magma of grbbro-dioritie porphyries, being magmatic-hydrothermal transitional fluid with relatively high fO2. The Washan and Dongshan iron deposits in the Ningwu volcanic basin share many similarities with the Kiruna type iron deposits and they may have similar ore-forming mechanism.
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