地学前缘 ›› 2020, Vol. 27 ›› Issue (2): 197-217.DOI: 10.13745/j.esf.sf.2020.3.20

• 特色地域成矿背景与成矿作用 • 上一篇    下一篇

长江中下游成矿带玢岩铁矿研究新进展及对矿床成因的启示

赵新福, 曾丽平,廖旺, 李婉婷,胡浩, 李建威   

  1. 1. 中国地质大学(武汉)  地质过程和矿产资源国家重点实验室, 湖北 武汉 430074
    2. 中国地质大学(武汉) 资源学院, 湖北 武汉 430074
    3. 云南大学 资源环境与地球科学学院, 云南 昆明 650500
  • 收稿日期:2019-12-18 修回日期:2020-02-22 出版日期:2020-03-25 发布日期:2020-03-25
  • 作者简介:赵新福 (1982—),男,教授,主要从事岩浆热液铁铜金稀土成矿系统研究。E-mail:xfzhao@cug.edu.cn
  • 基金资助:

    国家自然科学基金项目(41822203,41972074,41602082);国家重点基础研究发展计划“973”项目(2012CB416802);中央高校基本科研业务费专项资金资助项目(CUG140618,CUGCJ1711);国家留学基金委资助项目(CSC No.201706410056)

An overview of recent advances in porphyrite iron (iron oxideapatite, IOA) deposits in the MiddleLower Yangtze River Valley Metallogenic Belt and its implication for ore genesis

ZHAO Xinfu, ZENG Liping,LIAO Wang,LI Wanting,HU Hao,LI Jianwei   

  1. 1. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences(Wuhan), Wuhan 430074, China
    2. School of Earth Resources, China University of Geosciences(Wuhan), Wuhan 430074, China
    3. School of Resource Environment and Earth Science, Yunnan University, Kunming 650500, China
  • Received:2019-12-18 Revised:2020-02-22 Online:2020-03-25 Published:2020-03-25

摘要:

长江中下游成矿带的宁芜和庐枞火山岩盆地中发育了大量与早白垩世(约130 Ma)陆相火山侵入岩有关的玢岩铁矿。这类矿床的特征为具有磁铁矿磷灰石阳起石(透辉石)矿物组合,在国际上一般被称为铁氧化物磷灰石型(Iron OxideApatite, IOA)或基鲁纳型(Kirunatype)矿床。玢岩铁矿的概念自20世纪70年代提出以来,其成因就一直存在争议,主要有矿浆、岩浆热液及矿浆热液过渡的观点。近年来的高精度年代学揭示出宁芜和庐枞盆地内玢岩铁矿在约130 Ma集中爆发成矿。矿物学、岩石学及地球化学的综合研究表明成矿物质主要来源于次火山岩体,且成矿早期流体具有高温(550~780 ℃)和超高盐度(可达90% NaCleq)的特点。这些特点与成矿岩体及周围火山岩在成矿早阶段发育大规模钠质蚀变相吻合;但同时S-Sr等同位素和流体包裹体成分分析表明在铁成矿过程中还有外来壳源(如膏盐层物质)流体的加入。一些研究工作还表明玢岩铁矿与夕卡岩型铁矿具有相似的热液蚀变演化过程,暗示两者或许存在某些成因联系,很可能是相似流体与不同性质围岩及在不同温度下水岩交代产物。这些新的证据为探讨玢岩铁矿的成矿作用过程和成因机制提供了新的制约,也带来了新问题。本文从成岩成矿年代学、成矿物质来源、成矿早期流体性质、玢岩铁矿与夕卡岩铁矿及其外围新发现的金铜矿化的成因联系等角度,对近年来长江中下游成矿带玢岩铁矿研究的主要新进展进行初步总结。当前IOA型矿床的成因研究成为国际上矿床学研究的一个热点,除了长期争论的矿浆成因和岩浆热液成因,最近提出多个了岩浆热液复合成矿模型,如岩浆磁铁矿气泡悬浮模型及富水铁熔体的上升、脱气和侵位成因模型。将IOA型矿床成因争论的焦点逐渐聚焦在岩浆到岩浆后(岩浆热液)阶段,铁质究竟是以含铁岩浆热液、铁矿浆 (Fe-O或P-Ca-Fe-O),还是岩浆磁铁矿微晶或其他未知的形式来富集成矿的,还有待进一步研究,文章对以上的新模型进行简要介绍和评述,并与长江中下游的矿床进行对比。

关键词: 长江中下游成矿带, 玢岩铁矿, 铁氧化物磷灰石矿床, 夕卡岩型铁矿床, 矿床成因

Abstract:

“Porphyrite iron deposits” within the Ningwu and Luzong volcanic basins in the MiddleLower Yangtze River Valley Metallogenic Belt are hosted in the early Cretaceous(~130 Ma) volcanicintrusive rocks. They are characterized by mineral assemblages of magnetiteapatiteactinolite(diopside)and have geological characteristics similar to other iron oxideapatite(IOA) or Kirunatype deposits worldwide. Since the concept of porphyrite iron deposit was proposed in the 1970s, there are several hypotheses for ore genesis, that include immiscible ironoxide melts of magmatic origin, magmatichydrothermal replacement, and a combination of magmatic and hydrothermal processes. This study presents an overview of recent advances in porphyrite iron deposits in China, including geochronologies of iron mineralization and the associated orerelated intrusions, sources of oreforming materials, nature of earlystage oreforming fluids, and possible genetic relationship between porphyrite iron deposits and skarn iron deposits as well as the newly discovered goldcopper mineralization at the periphery. Finally, we also make a comparison of those with global IOA deposits and comment on the potential problems of current genetic models. Recently, a large highprecision geochronological dataset has revealed that the IOA deposit in the Ningwu and Luzong basins intensively formed at ~130 Ma. It has been suggested that iron is dominantly derived from the subvolcanic intrusions, and the early stage ore fluids are characterized by hightemperature (550780 ℃), ultrahigh salinity (~90 wt% NaCleq). Such fluids are consistent with field evidence of the earliest, depositscale sodic alteration. On the other hand, S-Sr isotopes and insitu analyses of the fluid inclusions indicate that external evaporitederived fluids were involved in the iron mineralization processes, although it has not been known precisely, the role of evaporite during the mineralization. It should be also highlighted that the paragenetic sequence of hydrothermal alteration in the IOA deposit is similar to that of iron skarn deposits, implying a cryptic genetic link between the two types. We believe that the presence of different alteration phases possibly resulted from the interaction between similar oreforming fluids and different country rocks at different conditions. During the past decade, IOA deposits have been attracted much more global attention and quite a few papers had been published in high ranked international journals. Except for traditional hypotheses involving Feoxide melts and magmatichydrothermal replacement, other models combined with magmatic and magmatichydrothermal processes have also been proposed. These new models include a magmatic magnetitebubble suspension model and the model of ascent, degassing, and emplacement of hydrous immiscible Fe-Ca-P melts. Nevertheless, the current debate regarding the genesis of the IOA deposit still focuses on the transitional processes between magmatic and hydrothermal stage, and how iron and phosphorous separated from the magma or the intrusion, transported and concentrated. The proposed models, including iron oxide, melts (Fe-O or P-Ca-Fe-), magmatic magnetite microlites (Fe3O4), Ferich hydrothermal fluids, or other enigmatic processes, remain open to be testified through further research.

Key words: MiddleLower Yangtze River Valley Metallogenic Belt, porphyrite Fe deposit, Iron OxideApatite deposit, iron skarn deposit, genetic model

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