赣北石门寺钨多金属矿床花岗闪长岩蚀变带元素、流体迁移规律及其对成矿作用的制约

Elements and Fluids Migration Regularity of Granodiorite Alteration Zones in the Shimensi Tungsten Polymetallic Deposit in Northern Jiangxi and Their Constrain on Mineralization

石门寺钨多金属矿床大地构造位置处于扬子地块内江南地体的九岭隆起中段构造区。本文采用Isocon图解法对各蚀变带的元素质量平衡进行了定量计算。云英岩化阶段与绢云母化阶段的∑REE、LREE和HREE都表现出不同程度的富集,HREE富集程度都大于LREE,结合分散元素Cd和In的高度富集,暗示了其岩浆成因的特点。云英岩化花岗闪长岩与绢云母化花岗闪长岩的Y/Ho比值分别为22.77～25.64（平均为24.72）和25.35～26.48（平均为25.98）,均小于28,说明两个阶段的蚀变流体都富含HCO-3。Eu的质量得失率分别为-29%和1%,云英岩化阶段的流体具有较高的温度和较高的氧逸度。W与FeO呈正相关关系,说明低氧逸度有利于钨的富集沉淀。云英岩化花岗闪长岩中CaO质量得失率为-12%,亏损的Ca2＋进入成矿流体,成矿流体中Ca2＋与WO2-4的结合导致了钨的沉淀而形成白钨矿。花岗闪长岩发生云英岩化的过程也是FeO和CO2亏损、P富集、水化脱钙的一个演化过程,某种程度上也是成矿流体与之作用并导致WO2-4卸载沉淀的化学响应。云英岩化花岗闪长岩微裂隙发育,有利于成矿流体长时间的溶浸,成矿流体受控于热动力和压力差的驱动而不断向外接触带浸入相当长的一段距离,赋存的大量斜长石一经蚀变即释放Ca2＋,节约了成矿流体对Ca2＋的投放量,于外接触带形成厚大连续的似层状细脉浸染型白钨矿体。

The Shimensi tungsten polymetallic deposit is tectonically located in mid-segment of Jiulin uplift,Jiangnan terrane,Yangtze block.The mass balance quantitative calculation of alteration zones elements was conducted using Isocon graphic method in this paper.∑REE,LREE and HREE were enriched with different degrees at the stages of greisenization and sericitization,with HREE enriched higher than LREE.All this geochemical features,along with highly enrichment of Cd and In,show its characteristic of magmatic origin.The Y/Ho ratios of greisenized granodiorite and sericitized granodiorite are 22.77～25.64（averaging 24.72）and 25.35～26.48（averaging 25.98）,respectively.The both values are lower than 28,indicating alteration fluids in two processes were enriched in HCO-3.Mass gain and loss rates of Eu are-29% and 1%,respectively,suggesting the fluids of greisenization stage be characterized by high temperature and high oxygen fugacity.There is a positive correlation between W and FeO,which indicates that low oxygen fugacity is conductive to enrichment and precipitation of wolfram.The CaO mass gain and loss rate of greisenized granodiorite is-12%,with depleted Ca2＋into metallogenic fluid.The combination of Ca2＋and WO2-4in metallogenic fluid results in deposition of tungsten and formation of scheelite.Greisenization of granodiorite is an evolution process of FeO and CO2 decrease P,hydration and decalcification.To some extent,the change is chemical interaction between metallogenic fluid and granodiorite,resulting in discharge and precipitation of WO2-4.Greisenized granodiorite is characterized by rich microfissures,which is conductive to permeating of metallogenic fluid for a long time.Metallogenic fluid permeates the exocontact zone for quite a long distance,with the drive of thermo-motive and pressure difference.A large quantity of plagioclases released Ca2＋when alteration occurred,which saved circulation quantity of Ca2＋from metallogenic fluid.Therefore,large,thick continuous scheelite body with stratoid veinlet-disseminated type formed in the exocontact zone.