The remaking of the Mengyejing potash deposit in Yunnan, China: Evidence from Rb-Sr isotopic systematics

The Mengyejing potash deposit (MPD) is the first significant ancient potash deposit ever found in China, and it was considered to have formed during the middle to late Cretaceous. The Sr-87/Sr-86 ratios of potash bulk samples suggest that both bedded and veined potash orebodies of the MPD have been affected by igneous activity resulting from decompression melting of a metasomatically altered, depleted mantle from 16 Ma to present. The Rb-Sr isotopic systematics of the potash bulk samples define several apparent "isochrons" ranging from 0.609 +/- 0.026 to 14.23 +/- 0.63 Ma, with the maximum age being consistent with previous K-Ar ages of the K bearing minerals from the MPD. Such isotopic signatures imply that the MPD may have been affected by multiple-stage fluid flow events since the mid-Miocene, consisting of hydrothermal fluids, continental fresh water, and/or hot spring water. It is very likely that the MPD is still affected by fluid flow-events in modern times. The mineral assemblage of halite + sylvite with minor amounts of camallite was formed due to the recrystallization and diagenetic modification by hydrothermal fluids. Some characteristics of halite photomicrographs and inclusions and the lack of original halite chevron patterns are also supportive of the hypothesis that the deposit has undergone a diagenetic modification and dissolution redrystallization processes.