湘南大义山花岗岩地质地球化学特征及其与成矿的关系

通过岩体地质地球化学等特征的调查研究,对大义山花岗岩体进行了岩石谱系单位划分,认为它是由印支期和燕山期3期次多阶段岩浆活动形成的复式岩体。成矿作用主要与中侏罗世—晚侏罗世的花岗质岩浆活动相关。并对含锡多金属矿花岗岩提出了如下认识:①同一超单元内花岗质岩浆演化从早至晚次单元有由酸性向(贫钙富碱)酸碱性演化的规律;即晚期次更具弱钙碱性—偏碱性岩系,成矿专属性明显。②斜长石的An值常为4～15;以钠长石为主;黑云母Mf值<0.12,属铁叶云母—铁质黑云母。具高硅SiO2>73×10-2,富碱Na2O+K2O>7.5×10-2、K2O/Na2O>1.20,贫钙CaO<0.80×10-2;过铝指数AKNC>1.10,碱度指数AR>2.5,分异指数DI>90,氧原子数与分子总数比大于1.98及富F、B的特征。轻稀土相对重稀土富集,Eu亏损值小于0.30。③岩石中Sn、W、F、As、Sb、Pb、Zn、Cu、U等有高的综合异常和重砂有锡石、黑钨矿、锑矿物、辰砂及铅、锌、铜矿物异常。④印支—燕山期北西向郴州—邵阳断裂有早期为左行压扭剪切,晚期为张扭的复合多次转换特征,与断裂同活动的花岗质岩浆有沿北西向构造带从南东向北西斜上方先后3期次顺序侵位的规律。岩体内部构造及接触带围岩的构造位态显示为“被动”侵位特征,岩体定位方式属剪切扩张型。锡多金属矿床(点)有围绕岩体展布的规律;矿床类型主要有蚀变花岗岩体型、云英岩脉型、断裂破碎带型等,并总结分析了成矿条件和找矿标志。

Geological and geochemical characteristics of the Dayishan granitoids in southern Hunan and their relations to mineralization

Hierarchical units of the Dayishan granite are determined through geological and geochemical survey and study of the rock body. The authors think that the Dayishan granite is a composite intrusion formed by three phases and multiple stages of Indosinian and Yanshanian magmatic activities. Mineralization is mainly related to Mid-Late Jurassic granitic magmatism. The authors give the following points of understanding of tin-polymetallic ore-bearing granite: (1) Granitic magma in the same super-unit evolved from acid to acid-alkaline (calcium-poor and alkali-rich) composition from early to late times, i.e. the later-stage acid-alkaline intrusions show more marked metallogenic specialization. (2) Plagioclase commonly has An=4 to 15, with albite predominating; biotite has Mf<1.20, belonging to siderophyllite-ferruginous biotite; the granite has high silica with SiO2>73%, higher alkalis with Na2O+K2O>7.5% and K2O / Na2O>1.20 and lower calcium with CaO<0.80%, and the peralumina index AKNC>1.10, alkalinity AR>2.5 and differentiation index DI>50; the ratio of the number of oxygen atoms to the total number of molecules is >1.98 and is rich in F and B; LREE is enriched relative to HREE and the value of Eu depletion is <0.30. (3) The rock shows higher Sn, W, F, As, Sb, Pb, Zn, Cu and U anomalies and the placer minerals are cassiterite, wolframite, antimony minerals and cinnabar, as well as lead, zinc and copper minerals. (4) The Indosinian-Yanshanian NW-trending Chenzhou-Shaoyang fault is of sinistral compresso-shear origin in the early stage and shows tenso-shear, compounding and multiple transition features; the syntectonic granitic magma was emplaced sequentially in three phases obliquely along the NW-trending tectonic belt from southeast to northwest. Tin-polymetallic deposits (occurrences) are distributed around the intrusion. The deposit types mainly include the altered granite type, greisen dike type and fractured type. The metallogenic conditions and ore indications are also summarized and analyzed.