西秦岭岗岔—克莫金矿区石英闪长玢岩多金属“珠滴构造”及其找矿意义

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

西秦岭造山带北缘之金属成矿带是我国重要的斑岩型Cu-Au远景区,该带的德乌鲁岩体及其接触带是重要找矿潜力区。最新的野外调查发现,德乌鲁岩体东南缘接触带的石英闪长玢岩中发育大量含有多种金属矿物的椭圆形或近球形"珠滴构造",且其矿物组合非常类似于临近的岗岔-克莫金矿床矿石矿物组合,即"珠滴"内含有大量金属矿物黄铁矿、毒砂、黄铜矿、方铅矿、闪锌矿、磁黄铁矿、钛铁矿和辉砷钴矿等,此外还发育有绿泥石、绿帘石、方解石、钾长石、钠长石、石英、绢云母、电气石、葡萄石、磷灰石、榍石和金红石等复杂的非金属矿物组合。详细的矿物学和地球化学研究结果表明,"珠滴"与寄主岩石属同源产物。绿泥石温度计和二长石温度计显示"珠滴"形成温度范围在297~435℃,角闪石全铝压力计显示寄主岩石形成压力为1.02~2.28kbar。因此推断,"珠滴"形成于深部富含金属的流体,金属物质依附气泡按照"浮选"机制随岩浆侵位上升。当载有金属物质的气泡随着岩浆侵位到浅部时便快速解耦,其流体相逃逸,"珠滴"则被冷凝固结在岩浆圈闭之中。依据上述分析结果,认为携带金属物质的气泡把成矿元素运移至石英闪长玢岩体内,其中一些气泡把金属物质卸载到浅部,而大量的金属物质可能仍然留在深部石英闪长玢岩内。"珠滴构造"可能是深部成矿的重要指示标志。

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 Gangcha-Kemo 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.