扬子地块东南缘寒武纪早期黑色页岩容矿的镍钼多金属矿床中黄铁矿原位硫同位素组成及其地质意义^*

In-situ sulfur isotope compositions of pyrites from the Early Cambrian black shale-hosted polymetallic Ni-Mo deposits in southeastern margin of Yangtze Block and its geological significance

摘要

扬子地块东南缘寒武系下部黑色页岩容矿的镍钼多金属矿在全球范围内颇具特色,但其成矿过程长期存在争议。本研究通过显微镜和扫描电镜观察识别出3种不同类型的黄铁矿。沉积—成岩早期形成的草莓状黄铁矿(Py1)与钼硫碳集合体(MoSC)和胶状针镍矿共生,黄铁矿原位硫同位素组成{\delta }^{34}{\mathrm{S}}_{\mathrm{V}-\mathrm{CDT}}在-12.55‰~10.53‰之间(平均值为-3.38‰,n=25),其硫同位素组成主要受控于海水硫酸盐细菌还原作用。成岩晚期形成的团块状细粒黄铁矿(Py2)集合体硫同位素组成的变化范围在1.98‰~23.54‰之间(平均值为11.88‰,n=16),硫同位素分馏受到受限制沉积盆地的局部硫化环境的影响。热液期形成的粗粒黄铁矿(Py3)与少量脉状针镍矿、脉状闪锌矿、黄铜矿和重晶石共生,说明早期形成的镍钼矿经历的热液叠加成矿作用,黄铁矿硫同位素组成在-21.67‰~-9.73‰之间(平均值为-16.87‰,n=10),反映了热液流体的硫同位素组成特征。沉积—成岩早期形成的黄铁矿与MoSC集合体和胶状针镍矿共生,说明虽然受到热液流体叠加的影响,但是镍和钼主要形成于沉积—成岩早期,寒武纪早期特殊的古海洋环境和海水化学性质依旧是控制镍钼多金属矿形成的主要因素。

Abstract

The polymetallic Ni-Mo deposits hosted in the Early Cambrian black shales are unique worldwide. However,their mineralization processes have long been controversial. Based on detailed microscopic and scanning electronic microscopic observations,we identified three types of pyrites in the sulfide-rich ores. The framboidal pyrites(Py1)formed during the sedimentary-early diagenesis epoch are intergrowth with MoSC compounds and colloidal millerite. In-situ sulfur isotope compositions({\delta }^{34}{\mathrm{S}}_{\mathrm{V}-\mathrm{CDT}})of Py1 vary from-12.55‰ to 10.53‰(-3.38‰ on average,n=25). Fractionation of sulfur isotope of Py1 was mainly controlled by bacterial sulfate reduction. The late diagenesis is characterized by fine-grained pyrite aggregates(Py2);their sulfur isotope compositions vary from 1.98‰ to 23.54‰(11.88‰ on average,n=16). Strong sulfur isotope fractionation may be caused by locally sulfidic environments in restricted basins. The coarse-grained pyrites(Py3)formed during the hydrothermal overprinting epoch are intergrowth with millerite and sphalerite veins,chalcopyrite,and barite. Sulfur isotope compositions of Py3 vary from-21.67‰ to-9.73‰(-16.87‰ on average,n=10),which are dominantly a reflection of sulfur isotope compositions of hydrothermal source. The MSC compounds coexist with sedimentary-early diagenesis formed Py1,suggesting that although affected by hydrothermal fluids overprinting,the formation of the polymetallic Ni-Mo sulfide ores was mainly a consequence of unique redox sedimentary environments and chemical compositions during the Early Cambrian.

关键词

镍钼多金属矿 / 原位硫同位素 / 黄铁矿 / 寒武纪早期 / 黑色页岩 / 扬子地块

Key words

polymetallic Ni-Mo deposits / in-situ sulfur isotope / pyrite / Early Cambrian / black shales / Yangtzs Block

基金资助

*国家自然科学基金项目(编号: 41972072,42172080)和黔科合平台项目(编号: KXJZ[2024]016)联合资助