摘要
古—中元古代时期是地质历史上最重要的铅-锌硫化物爆发性成矿阶段,位于我国华北板块北缘西段的狼山多金属成矿带是这个时期的典型案例。作为成矿带内最大的硫化物矿床,炭窑口和东升庙矿床赋矿层位和矿化特征一致,被认为是同一个次级盆地内一次大型热液成矿事件的产物。这两个矿床所赋存的数亿吨硫化物矿石均不同程度富集重S同位素(平均值>+30‰),而对于矿石硫的具体来源及34S富集机制尚存争议。本文对炭窑口围岩和矿石中黄铁矿进行了Fe-S同位素研究,对其沉积-成岩环境和矿化过程给出综合制约。炭窑口碳质板岩中粗粒黄铁矿δ^56FeIRMM值在−0.51‰~−0.15‰之间,δ^34SV-CDT值在+35.0‰^+39.6‰之间,相比东升庙矿床围岩中不规则黄铁矿明显富集轻Fe同位素且更加富集重S同位素,可能是盆地边缘、相对氧化的浅水环境下Fe以氧化态几乎完全沉淀后成岩期转化的产物。该认识与炭窑口矿区常见层状重晶石所指示的比东升庙更加氧化的盆地边缘浅水环境一致。本文认为炭窑口矿石与围岩中硫化物均强烈富集重S同位素(δ^34SV-CDT值均在+26.6‰^+41.0‰之间)表明成岩环境同样是矿石硫化物结晶沉淀的主战场,其中半局限环境中浓缩海水硫酸盐在较封闭成岩条件下被甲烷为主的还原剂几乎完全还原。“局限盆地+成岩环境”模式可以解释炭窑口—东升庙盆地围岩和矿石均异常富集重S同位素的特征。前人报道的与成矿关系密切的碳酸盐相对富集轻C同位素所指示的有机碳参与成矿为成岩环境成矿提供了可靠证据。尽管不能完全排除细菌还原硫酸盐的作用,炭窑口—东升庙盆地各个层位围岩与矿石轻S同位素完全缺失表明开放体系细菌还原作用导致的硫化水体环境主导成矿的可能性较小。
Mid–Proterozoic massive sulfides represent the most important Zn metallogenic event in geologic history and are characterized by common occurrence of 34S enrichments.Significantly,several giant Zn-Pb deposits in the Langshan polymetallic ore district of Inner Mongolia,especially the Tanyaokou and adjacent Dongshengmiao deposits seem to be the most heavy sulfide ore deposits in the world.Detailed Fe-S isotope analyses of the Tanyaokou deposit have demonstrated that the origin of disseminated pyrite and the generally highδ^34S values represent closed-system pyrite production in sediment porewater,whereas roughly negativeδ^56Fe values represent diagenetic conversion of ferric(oxyhydr)oxides,which were totally oxidized by the Fe2+in the seawater.Given that there is a 13C-depletion anomaly in the carbonate associated with sulfide mineralization,strong TSR,or SR-AOM involving organic matter in closed-system conditions in the sediment pile,together with basin restriction,provides a plausible explanation for the world’s most heavy ore sulfur.Although the role of BSR cannot be completely ruled out,the complete lack of light S isotopes in sediments and ores of the Tanyaokou—Dongshengmiao basin indicates that the open system BSR is unlikely the dominant metallogenic mechanism.
作者
高兆富
朱祥坤
孙剑
周子龙
GAO Zhao-fu;ZHU Xiang-kun;SUN Jian;ZHOU Zi-long(MNR Key Laboratory of Isotope Geology,MNR Key Laboratory of Deep-Earth Dynamics,Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037)
出处
《地球学报》
EI
CAS
CSCD
北大核心
2020年第5期675-685,共11页
Acta Geoscientica Sinica
基金
国家重点研发计划项目(编号:2019YFA0708400)
国家自然科学基金项目(编号:41803025,41873027)
中国地质科学院基本科研业务费专项经费(编号:J2012)
自然资源部深地动力学重点实验室自主研究课题(编号:J1901-20-1)联合资助。
