The formation process of the Mengyejing potash deposit, Yunnan, China: evidence from geochemical and petrological charac
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Geosciences Journal
GJ
The formation process of the Mengyejing potash deposit, Yunnan, China: evidence from geochemical and petrological characteristics Lijian Shen1*, Chenglin Liu1, Jian-xin Zhao2, Yuexing Feng2, and Licheng Wang3 1
MNR Key Laboratory Metallogeny and Mineral Assessment, Institute of Minerals Resources, Chinese Academy of Geological Sciences, Bejing 100037, China 2 Radiogenic Isotope Facility, School of Earth Sciences, The University of Queensland, Brisbane QLD 4072, Australia 3 CAS Center for Excellence in Tibetan Plateau Earth Sciences, Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China
ABSTRACT: Two types of potash ores were sampled from the Mengyejing potash deposit (the MPD) which is the only ancient potash deposit ever found in China. Geochemical analyses, including Br, K, Mg and some heavy metals, were performed on those samples. The relatively consistent Br contents (141.42 to 763.67 ppm) in both bedded potash and veined potash ores show that the potashforming brine was likely derived from seawater. K contents and Br/Cl ratios of the veined potash ores are relatively high in contrast to bedded potash ores. The K/Br ratios of veined potash ores exhibit a positive correlation with Br contents, which is opposite to that of the bedded potash ore of the MPD and potassium-salts from the evaporation of modern seawater. The contents of heavy metals in veined potash ores are much higher than those of bedded potash ores in the MPD and potash in other evaporite basins of the world. Sr and S isotopic compositions of these two types of potash ores suggest that the brines for precipitating veined potash were formed by mixing of connate waters (seawater or/and continental freshwater) and hydrothermal fluids. The geochemical and petrological characteristics of the veined potash ores indicate that the brine for forming veined potash orebodies caused by dissolution of previous bedded potash orebodies by hydrothermal fluids. The veined potash orebodies were formed through a cooling process by which sylvite precipitated first prior to halite. Key words: The Mengyejing potash deposit, Br geochemistry, heavy metals, petrography, ore-forming process Manuscript received February 2, 2020; Manuscript accepted May 25, 2020
1. INTRODUCTION Ancient potash deposits (Pre-Quaternary) are basically the bittern stages of marine-fed basin-wide deposits (Warren, 2010). Naturally, the most commonly occurring potash minerals are sylvite and carnallite, and sylvite is the economically most important mineral. Other less common potassium minerals include kainite, kieserite, polyhalite, langbeinite and leonite (Warren, 2006, 2010). Two types of potash deposits, divided into MgSO4-rich and *Corresponding author: Lijian Shen MNR Key Laboratory Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Street, Xicheng District, Beijing 100037, China Tel: +86-1302000114
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