Earth Science Frontiers ›› 2019, Vol. 26 ›› Issue (5): 222-242.DOI: 10.13745/j.esf.sf.2019.8.1

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Polymetallic droplets in a quartz diorite porphyry from the GangchaKemo gold deposit, West Qinling Belt, China: implications in petrogenesis and prospecting

SHEN Junfeng,WANG Shuhao,XU Liwei,LUO Zhaohua,LI Jinchun,LIU Haiming, NIE Xiao,QIN Yuliang,PENG Zidong,NIU Gang,DU Baisong,LIU Jiajun   

  1. 1. School of Earth Sciences and Resources, China University of Geosciences(Beijing), Beijing 100083, China
    2. Shandong Earthquake Agency, Jinan 250014, China
    3. Jinyou Geological Exploration Co. Ltd., Beijing 100011, China
    4. Heli Mining Co. Ltd., Lanzhou 747000, China
    5. Département de géologie et de génie géologique, Université Laval, Québec, QCG1V0A6, Canada
    6. Dingfeng Mining Co. Ltd., Lanzhou 730000, China
    7. Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
  • Received:2019-01-20 Revised:2019-08-30 Online:2019-09-28 Published:2019-09-28
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Abstract: The metallogenic belt at the northern margin of the West Qinling Belt in China has potential for porphyry-type Cu-Au deposits. The Dewulu intrusion and its contact zone in the region are important targets for ore prospecting. Recent field investigations revealed that the quartz diorite porphyry, at the southeastern edge of the Dewulu intrusion, features abundance of ellipsoidal and sub-spheroidal polymetallic droplets, and their mineral assemblages are very similar to that of orebodies in the GangchaKemo gold deposit near the Dewulu intrusion. The droplets contained variety of ore minerals such as pyrite, arsenopyrite, chalcopyrite, galena, sphalerite, pyrrhotite, ilmenite and cobaltite, as well as complex non-metallic mineral assemblages of chlorite, epidote, calcite, K-feldspar, albitic plagioclase, quartz, sericite, tourmaline, prehnite, apatite, sphene and rutile. Detailed mineralogical study and geochemical comparison showed that the droplets and the host rock are co-genetic. Chlorite and two-feldspar geothermometric measurements suggest the droplets are formed at temperatures ranging from 297 to 435 ℃. The host rock is formed at pressures between 1.02 and 2.28 kbar as indicated by geobarometer readings of total aluminum in amphibole. Accordingly, we infer that the droplets are formed from deep metal-rich fluid bubbles rising through the magma by the flotation mechanism, i.e., as bubbles migrate into the shallow domains of the magma, they are quickly disaggregated to allow fluid phase to escape and droplets are ultimately formed and trapped by the cooling magma. Eventually, as we propose here, rising bubbles transport the ore-forming elements into the quartz diorite porphyry, where some bubbles unload some of metals at shallower levels, but most of metals may be stored in the deeper domains of the quartz diorite porphyry. Accordingly, the polymetallic droplets can be used as an indicator of ore prospecting at depths.

 

Key words: polymetallic droplets, mineral assemblages, mineral thermometry, ore prospecting, Dewulu intrusion, West Qinling Belt

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