کد مقاله | کد نشریه | سال انتشار | مقاله انگلیسی | نسخه تمام متن |
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6437286 | 1637971 | 2016 | 19 صفحه PDF | دانلود رایگان |
The first globally significant phosphorous-rich deposits appear in the Paleoproterozoic at around 2Â Ga, however, the specific triggers leading to apatite precipitation are debated. We examine phosphorous-rich rocks (up to 8Â wt% P2O5) in 1.98-1.92Â Ga old Pilgujärvi Sedimentary Formation, Pechenga Greenstone Belt, Russia. Phosphates in these rocks occur as locally derived and resedimented sand-to-gravel/pebble sized grains consisting of apatite-cemented muddy sediments. Phosphatic grains can be subdivided into four petrographic types (A-D), each has a distinct REE signature reflecting different early-to-late diagenetic conditions and/or metamorphic overprint. Pyrite containing petrographic type D, which typically has a flat REE pattern, negative Ce anomaly and positive Eu anomaly, is the best preserved of the four types and best records conditions present during apatite precipitation. Type D phosphatic grains precipitated under (sub)oxic basinal conditions with a significant hydrothermal influence. These characteristics are similar to Zaonega Formation phosphates of NW Russia's Onega Basin, and consistent with phosphogenesis triggered by the development of anoxic(sulfidic)-(sub)oxic redoxclines at shallow sediment depth during the Paleoproterozoic.
Journal: Geochimica et Cosmochimica Acta - Volume 186, 1 August 2016, Pages 135-153