湖北省恩施地区二叠系低丰度草莓状黄铁矿的两种成因

Two Causes for the Low Abundance of Framboidal Pyrite in the Permian in Enshi Area in Hubei Province

湖北省西部恩施地区恩施剖面中二叠统栖霞组氧化环境与赵家坝剖面上二叠统大隆组还原环境中草莓状黄铁矿丰度均较低,甚至缺失。通过对比这两个剖面中富草莓状黄铁矿与贫草莓状黄铁矿层位的总有机碳(TOC)含量以及总铁含量,结果展示在恩施剖面栖霞组缺乏草莓状黄铁矿的氧化碳酸盐岩层段中TOC含量较低而富草莓状黄铁矿层位TOC含量较高;在赵家坝剖面大隆组缺乏草莓状黄铁矿层段的还原的硅质岩中总铁含量较低而富草莓状黄铁矿层段总铁含量较高。分析认为氧化层段中缺乏草莓状黄铁矿是因为较低的有机质导致细菌硫酸盐还原反应形成的硫化氢浓度较低,从而形成的草莓状黄铁矿丰度较低;还原层段中缺乏草莓状黄铁矿是因为较低的总铁浓度导致细菌硫酸盐还原反应生成的硫化氢与铁直接生成黄铁矿单晶,没有胶黄铁矿的生成,从而由于较弱的磁性没有聚合形成草莓状黄铁矿。

Low abundance of framboidal pyrites occurred in both oxic middle Permian Chihsia Formation of Enshi section and reducing upper Permian Dalong Formation of Zhaojiaba section,in Enshi area,Hubei province,South China. We compared the geochemical proxies total organic carbon（ TOC） content and total iron（ Fe） between framboidal pyrite-rich succession and framboidal pyrite-depleted succession of these two sections,displaying that the oxic framboidal pyrite-depleted carbonate succession had low TOC content while the framboidal pyrite-rich marl shale succession had high TOC content for the Enshi section,and that the dysoxic or even anoxic framboidal pyrite-depleted chert succession had low Fe content while the framboidal pyrite-rich chert succession had high Fe content. We suggest that the framboidal pyrite depletion in oxic environments probably due to the low TOC,and thus the low concentration of hydrogen sulfur from the bacterial sulfate reduction,resulting in the low abundance of framboidal pyrite,and that the framboidal pyrite depletion in reducing environments probably due to the low Fe content,and thus the direct formation of pyrite without the formation of greigite,resulting in the absence of framboidal pyrite because of the weak magnetism of pyrite.