5.4. Differences of pyrites in nuclei and cortex
Framboidal pyrites distributed in both cortex and nuclei of the ooids. In contrast, euhedral pyrites were found only in the nuclei composed of carbonate fragments. Euhedral pyrites show concentric fabrics caused by elements concentration indicating multi-stage growth of these minerals, which could lead to significantly larger size of them than framboid pyrites. The early postsedimentary fluids would lead to pyrite overgrowths, and generate chemical zones in pyrites (Agangi, Hofmann, & Wohlgemuth-Ueberwasser, 2013). Previous studies documented that later pyrite often overgrows earlier generations of pyrite resulting in zoned pyrite with different trace elements patterns during hydrothermal fluid-flow processes (Thomas et al., 2011). However, there are not any Jurassic volcanic rocks or other clues of hydrothermal activities recorded in the southern zone of Tibetan Himalayas. Nevertheless, framboid aggregates can transfer to euhedral crystals through continuous growth of the constituent microcrystals (Ostwald & England, 1979; Bailey et al., 2010; Soliman & El Goresy, 2012; Wacey et al., 2015). The Nuclei are also a more open system relative to tightly packed cortex which is more appropriate for the formation of framboids in a close system (Merinero, Lunar, Somoza, Díaz-del-Río, & Martínez-Frías, 2009). This study therefore suggests that in the Nieniexiongla grainstones, the euhedral pyrites in the ooid nuclei were transferred from original framboids.