SUN Guo-tao^{1, 2}, SHEN Neng-ping^1*, SU Wen-chao¹, FENG Yue-xing³, ZHAO Jian-xin³, PENG Jian-tang¹, DONG Wen-dou^{1, 2}, ZHAO Hai^{1, 2}

(1. State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China;
2. University of Chinese Academy of Sciences, Beijing 100049, China;
3. Radiogenic Isotope Facility, School of Earth Sciences, The University of Queensland, Brisbane, QLD 4072, Australia)

Abstract: Miaolong Au-Sb deposit is hosted in limestone of Upper Cambrian Sandu Formation, located at the Sandu-Danzhai district, Guizhou Province, China. Trace elements, strontium and neodymium isotopic compositions of ore-stage calcites from the deposit were studied. Results show that contents of rare earth elements in calcites range widely (5.98～139 μg/g). In chondrite-normalized rare earth patterns, most calcites show LREE enrichment ((La/Yb)N = 1.84～9.18, (Gd/Yb)N = 2.55～6.14), while some are enriched with MREE ((La/Yb)N=0.33～1.39, (Gd/Yb)N=1.29～2.24). The calcites do not display Ce anomalies (δCe = 0.97～1.19), but show Eu anomalies (δEu = 0.63～1.80), suggesting that calcites are deposited in reductive condition. The 87Sr/86Sr ratios for calcite vary from 0.7108 to 0.7144, with a mean ratio of 0.7119, which are higher than 87Sr/86Sr ratios (0.7090) of the host rocks. It indicates that radioactive strontium in hydrothermal fluids was likely to be transported from Precambrian basement which has high 87Sr/86Sr ratios rather than mobilized from limestone of Sandu Formation. The study of Sm-Nd isotope system shows that ore-stage calcites form two isochrons, corresponding to isochron ages of 273±14 Ma and 272±43 Ma, respectively. However the isochron ages are not coherent geological setting, suggesting that Sm-Nd system in this set of samples cannot be used to date the formation of Miaolong deposit radiometrically. The pseudoisochrons indicate that the calcites may have formed from fluid with heterogeneous initial 143Nd/144Nd. Negative εNd(0) (-14.02～-9.48) compositions of calcites indicate a crustal source for the Sm and Nd found in the fluid.

Keywords: calcite; trace element; strontium and neodymium isotopic composition; Miaolong Au-Sb deposit

E-mail: shennengping@vip.gyig.ac.cn

ACTA MINERALOGICA SINICA Vol. 36, No. 3, 2016, Page 404-412