地学前缘 ›› 2020, Vol. 27 ›› Issue (5): 126-135.DOI: 10.13745/j.esf.sf.2020.5.47

• 地质过程与成矿作用示踪的成因矿物学 • 上一篇    下一篇

山西省五台白云叶蜡石矿地质特征及其对深部找矿的启示

张华锋,张少颖   

  1. 中国地质大学(北京) 地球科学与资源学院, 北京 100083
  • 收稿日期:2020-03-11 修回日期:2020-05-12 出版日期:2020-09-25 发布日期:2020-09-25
  • 作者简介:张华锋(1971—),男,博士,副教授,主要从事成因矿物学与找矿矿物学研究。E-mail:nightyzhf@cugb.edu.cn
  • 基金资助:

    国家深部探测研究计划项目(2016YFC0600106)

Geological signatures of the Baiyun pyrophyllite deposit in Wutai, Shanxi Province: implication for prospecting of Cu-Au ores in deep

ZHANG Huafeng, ZHANG Shaoying   

  1. School of Earth Sciences and Resources, China University of Geosciences(Beijing), Beijing 100083, China
  • Received:2020-03-11 Revised:2020-05-12 Online:2020-09-25 Published:2020-09-25

摘要:

山西五台县北约15 km的白云村一带出露有叶蜡石矿及金矿化点,其形成特点与关系对区域深部找矿具有重要启示意义。该叶蜡石矿体产于经历古元古代绿片岩相变质的新太古代酸性火山岩之中(绢英片岩),切割区域片理并被晚期的辉绿玢岩截切。矿体总体呈透镜状产出,倾向北西,近东西向延伸大于5 km,最厚处约1 km,未见底。该叶蜡石矿形成于中生代早白垩世。矿石矿物以叶蜡石和石英为主,含少量绢云母、伊利石、硬水铝石和赤铁矿等,反映出蚀变流体酸性及氧化的特征。叶蜡石矿体外围发育黄铁绢英岩化带,强度由近及远逐渐减弱。在靠近叶蜡石矿的黄铁绢英岩中,硫化物颗粒边缘常见赤铁矿化现象,应与叶蜡石化流体叠加有关。以上蚀变特征与叶蜡石化细脉灌入黄铁绢英岩相吻合。叶蜡石矿体上盘的黄铁绢英岩化带宽约0.4 km,其内发育一小型石英脉型金矿(岭底金矿),下盘的黄铁绢英岩化带最宽约1.5 km,局部发育金矿化现象((1~18)×10-6)。黄铁绢英岩中硫化物以黄铁矿为主,可见少量黄铜矿、斑铜矿和辉铜矿。金矿化黄铁绢英岩以发育大量浸染型黄铁矿为特征,该类黄铁矿无韧性变形,未见其他共生硫化物,SEM-EDS分析结果显示其内发育有明显的不可见金。黄铁绢英岩化可依据矿物变形与否分为两期。早期的蚀变分布局限,见于新太古代糜棱岩化奥长花岗岩与酸性火山岩(绢英片岩)接触带,其中的黄铁矿、绢云母和石英等矿物发育韧性变形特征;晚期的蚀变带内硫化物、绢云母和石英均无韧性变形特征。依据样品的岩相学及全岩Au、As含量特征,可以确认金矿化与晚期黄铁绢英岩化有关,与早期蚀变无关。叶蜡石矿石的H-O同位素组成(δ18OV-SMOW=11.2‰~13.9‰;δDV-SMOW=-34‰~-20‰)显示流体以气相为主,且缺乏大气水的混合。上述特征说明白云叶蜡石矿是深部岩浆侵位固结后出溶的以气相为主的热液与浅部新太古代变质火山岩反应的结果,类似于斑岩成矿系统中普遍发育的高级泥化带(岩帽)的底部蚀变特征。

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关键词: 叶蜡石化, 金矿作用, 热液蚀变, 五台山, 深部找矿

Abstract:

The Baiyun pyrophyllite deposit is ~15 km away to north of Wutai county, Shanxi Province of China, where the basement rocks belong to the Wutai Complex, north-central of the North China Craton. The main orebody occurs as a lens in fracture of the late Archean meta-volcanic rocks, crosscut by the Mesozoic mafic dikes. The wall rocks exhibit phyllic alteration halos which are weakened outwardly from the pyrophyllite orebody. Auriferous quartz sulfide vein and disseminated gold mineralization occur in the phyllic alteration zone. Mineral assemblages of pyrophyllite ores are dominated by pyrophyllite and quartz, with minor of diaspore, kaolinite, illite, sericite and hematite. Accordingly, the lgf(O2)-pH diagram is constructed that the pyrophyllic alteration fluids were intensively acidic (pH=23) with relatively high oxygen fugacity (>HM buffer). Altered zircon yielded a lower intercepted U-Pb age of (136±7) Ma, similar to those of Cu-Au deposits regionally. Whole-rock H-O isotopic data (δDSMOW=-34‰--20‰; δ18OSMOW=11.2‰-13.9‰) of pyrophyllite ores suggest that the fluids for pyrophyllite alteration are cooling magmatic without pronounced effects of meteoric waters. As to the abundances of metals, the Archean wall rocks are markedly higher than those of the pyrophyllite ores. The concentrations of Au and As in the pyrophyllite ores are below the detected limit, suggesting that the physicochemical conditions of fluids for pyrophyllite alteration are not conductive to gold mineralization, inversely, they can scavenge some metals from wall rocks and transfer them upwardly. Our results suggest that pyrophyllite alteration formed by acid and oxidized fluids might be an early stage of evolving fluids for high sulfidation epithermal deposit. Consequently, a prospective porphyry style or skarn type of Cu-Au deposit in deep is suggested.

Key words: pyrophyllite alteration, gold mineralization, hydrothermal alteration, Wutai complex, ore prospecting

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