云南镇雄羊场磷矿早寒武世无机碳、有机碳同位素研究

Inorganic and organic carbon isotopes from Early Cambrian Yangchang phosphate deposit in Zhenxiong County,Yunnan Province

近年来云南省昭通市镇雄县发现了超大型隐伏磷矿(羊场磷矿),目前针对该磷矿的研究主要集中在矿床学方面,相关的沉积地球化学及古环境研究较少,而且国内外对寒武纪早期纽芬兰世无机碳同位素(δ^(13)C_(carb))、有机碳同位素(δ^(13)C_(org))及其二者间关系的研究也鲜有报道。本文选取羊场磷矿3个钻孔(ZK001、ZK0701、ZK1512)开展高分辨率δ^(13)C_(carb)和δ^(13)C_(org)研究,取得以下主要认识:从灯影组顶部至朱家箐组底部,δ^(13)C_(carb)和δ^(13)C_(org)出现明显负漂移,这与前人在华南及世界多地报道的Basal Cambrian Carbon Isotope Excursion(BACE)负漂移事件一致,可能是由有机碳库氧化造成的;3个钻孔中均未能发现Zhujiaqing Carbon Isotope Excursion(ZHUCE)正漂移事件,这可能是因为磷矿矿体产出层位存在短暂的沉积间断。尽管如此,通过3个钻孔的无机碳同位素与前人报道的华南地区无机碳同位素综合曲线对比,认为该磷矿产出时代属于寒武纪纽芬兰世幸运期;对钻孔ZK001和ZK0701的无机碳、有机碳同位素对比研究,发现二者差值(ΔC_(carb-org)=δ^(13)C_(carb)-δ^(13)C_(org))的平均值在灯影组中分别为28.8‰和22.2‰,在朱家箐组下部分别上升到31.7‰和31.7‰(解耦),朱家箐组中部开始往上分别减小至29.6‰和29.8‰;朱家箐组中出现δ^(13)C_(carb)与δ^(13)C_(org)解耦现象,可能是初级生产力或碳埋藏的增加所导致。本研究初步揭示了羊场磷矿的产出时代和古环境演变过程,同时也为区域地层对比提供了佐证。

The recently discovered Yangchang deposit in Zhenxiong County,Zhaotong City,Yunnan Province is an ultra-large-scaled buried phosphate deposit.Previous research on this deposit mainly focused on the metallogeny,while the study of sedimentary geochemistry and the paleoenvironment is lacked.More importantly,the inorganic(δ^(13)C_(carb))and organic(δ^(13)C_(org))carbon isotopes as well as their relationships in the early Cambrian were rarely reported.In this study,three drill cores(ZK001,ZK0701,ZK1512)in Yangchang phosphate deposit were selected for high-resolution investigations ofδ^(13)C_(carb)andδ^(13)C_(org).Bothδ^(13)C_(carb)andδ^(13)C_(org)show negative shifts in the top Pre-cambrian Dengying Formation and the bottom of its overlying Zhujiaqing Formation,which is consistent with the global Basal Cambrian Carbon Isotopic Excursion(BACE),likely due to the oxidation of organic carbon reservoir.However,we did not find significant positive shifts in the overlying strata(i.e.,Zhujiaqing Carbon Isotopic Excur-sion,ZHUCE),which may be caused by sedimentary hiatus given that the Yangchang phosphate deposit is mostly intra-clastic phosphorus-rich deposits.Nevertheless,we propose that this phosphate deposit was mainly formed dur-ing the Fortunian according to the comparison between theδ^(13)C_(carb)in the three drill cores and the composite profile ofδ^(13)C_(carb)in South China.The comparisons for theδ^(13)C_(carb)andδ^(13)C_(org)values in the drill cores ZK001 and ZK0701,show that the average value of theΔC _(carb-org)(difference betweenδ^(13)C_(carb)andδ^(13)C_(org))is 28.8‰and 22.2‰in the Dengying Formation,and these values change to 31.7‰and 31.7‰at the bottom of the Zhujiaqing Formation,and 29.6‰and 29.8‰,in the middle and upper of the Zhujiaqing Formation,respectively.We propose that the decoupledδ^(13)C_(carb)andδ^(13)C_(org)occurring in the middle of the Zhujiaqing Formation may be caused by the increased primary productivity or buried organic carbon.In summary,the high-resolution paired inorganic and organic carbon isotopes in this work indicate the formation time of Yangchang phosphate deposit and shed light on associated paleo-environmental changes,which also support the regional stratigraphic correlation.