The iron oolitic deposits of the lower Devonian Yangmaba formation in the Longmenshan area, Sichuan Basin

• Published in: Marine and petroleum geology 2021-08, Vol.130, p.105137, Article 105137
• Main Authors: Li, Fengjie, Zhang, Pengfei, Ma, Xiaokang, Yuan, Guangyao
• Format: Article
• Language: English
• Subjects: Chamosite and hematite ooids; Lower devonian; Mixed deposit; Ningxiang-type iron ore
• ISSN: 0264-8172; 1873-4073
• DOI: 10.1016/j.marpetgeo.2021.105137

Oolitic ironstones are widely distributed in South China, particularly in Middle and Upper Devonian strata, and are referred to as Ningxiang-type iron deposits. The hybrid siliciclastic-carbonate deposits of the Lower Devonian Yangmaba Formation iron-enriched interval (dominated by hematite ooids and chamosite ooids) are recognized in the Shigouli section of the Longmenshan area, Sichuan Basin. The Yangmaba Formation ironstones differ from other Devonian Ningxiang-type oolitic ironstones: The Ninxiang-type ironstone deposits are commonly interpreted to have been deposited in a narrow coastal zone or inland restricted bays and lagoons with clastic and carbonate sedimentary sequences; by contrast, the studied oolitic ironstones are interpreted to have been deposited in a shoreface zone to inner shelf based upon field and petrographic investigations. Our study shows that this oolitic ironstone consists of dominantly hematite ooids and chamosite ooids. Major element and trace element discrimination diagrams, PAAS (Post-Archaean average Australian sedimentary rock) normalized REE (Rear earth element) patterns, and positive Ce/Ce* anomalies show evidence of a hydrogenous origin. The weathering of adjacent terrestrial rocks from the formation of the palaeo-land surface is interpreted to be the main Fe source for the oolitic ironstones. Environmental energy and redox conditions are the important factors controlling ironstone formation and distribution of the hybrid deposits of the Yangmaba Formation in the Shigouli section of the Longmenshan area. Oolitic ironstones formed in oxidizing conditions of the upper shoreface zone include the carbonate concentric ooids with slightly hematite ooids occurring in higher energy upper oxidizing shoreface subzone, moderately hematite concentric ooids deposited in middle oxidizing shoreface subzone of fluctuant environmental energy, and hematite ooids present in lower oxidizing shoreface subzone of lower-energy conditions or weakly agitated environment. The chamosite concentric ooids and superficial ooids formed in reducing conditions of the lower shoreface and upper offshore transition. The former occurred in upper reducing shoreface subzone near the fair weather wave base, and the chamosite superficial ooids were deposited in lower reducing shoreface subzone and upper offshore transition that was affected by episodic high energy events. •Chamositic and hematitic ooids were deposited in the early Devonian.•The distribute model