Identification and discussion on the origin of glauberite pseudocrystals in the Pingchau formation of the Pingchau Island, Hongkong

Abstract

Based on outcrop observation, microscopic identification and EPMA, it is determined that the mineral association complexes in the Pingchau Formation of the Pingchau Island, Hong Kong, are glauberite pseudocrystals replaced by analcime, acmite and calcite, which show bipyramid shapes and scattered occurrence in mud beds. It is proposed that hydrothermal alteration and replacement processes played a main role for the glauberite to have been changed to analcime, acmite and calcite, as the primary glauberite provided Na 2O and CaO and the hydrothermal activity produced SiO 2-bearing fluid. This result suggests that the Pingchau Formation is a salt-bearing sequence, and that the Mirs Bay was once a highly saline evaporate basin during the late Cretaceous to Eogene.

本文通过野外地质调查、显微鉴定分析、EPMA测试和 X射线分析 ,确定香港坪洲岛坪洲组地层中的方沸石、锥辉石和次生方解石等矿物部分是交代原始盐类矿物钙芒硝形成的 ,它们多以集合体方式构成钙芒硝假晶 ,具有钙芒硝的棱面体形态 ,并可以和区域上含钙芒硝的盐系地层进行对比。本文认为 ,原始钙芒硝为上述矿物的形成提供了 Na2 O和 Ca O,热液活动带来了部分 Si O2 ,通过交代蚀变作用形成了丰富的钙芒硝假晶。这一研究成果表明 ,坪洲组是一盐系地层 ,大鹏湾盆地在晚白垩世—古近纪时期是一演化程度较高的硫酸盐型蒸发岩盆地。