广东河台金矿假玄武玻璃地球化学和年代学特征及其地质意义

前人对河台金矿中的假玄武玻璃早有报道,但是因为未认识到其与成矿的关系而被忽视。在前人发现的糜棱岩中假玄武玻璃的基础上,本次研究又发现了三种其他岩性中的假玄武玻璃。河台金矿中的假玄武玻璃呈黑色脉状、网脉状和树枝状贯穿于糜棱岩化片岩、糜棱岩、混合岩和矿体中。本研究结合野外宏观特征和室内的岩相学微观特征,运用粉晶X射线衍射、全岩地球化学和Ar-Ar同位素等实验分析手段,比较了河台金矿不同地质体中假玄武玻璃岩脉的特征。研究发现假玄武玻璃脉体中存在树枝状微晶结构和碎屑颗粒的港湾结构,同时也含有碎裂成因的石英、长石,表明其成因以同震断层的摩擦熔融为主,并伴随有少量碎裂化。通过计算得出假玄武玻璃样品中结晶态的各类矿物含量为石英53.3%、伊利石20.5%、高岭石17.3%、钾长石8.9%。假玄武玻璃与围岩都具有富SiO_2和Al_2O_3,富K_2O贫Na_2O,低MgO和Fe_2O_3~T的地球化学特征。两者的稀土元素配分型式也非常相似,都显示轻稀土富集和中等程度的负Eu异常(δEu=0.51～0.71)。假玄武玻璃的显微构造和地球化学特征指示其主要是由围岩原地熔融形成的,形成深度在6～18 km。我们认为河台金矿的假玄武玻璃可以指示云开大山抬升过程中伴随着地震的发生,同时,假玄武玻璃脉体在空间上切穿矿体,因此假玄武玻璃的⁴⁰Ar/³⁹Ar年龄123.3±1.0 Ma可以作为河台金矿的成矿年龄上限。

Geochronology,Geochemistry and Geological Significance of Pseudotachylites in Hetai Goldfield,Guangdong Province

Pseudotachylites in the Hetai goldfield have been depicted in previous papers,however,the relationship between the pseudotachylite and ore mineralization is still not clear and no integrated and detailed researches have been reported yet.Pseudotachylite vein was reported to have been observed in the mylonite,in this study we found that pseudotachylite veins occur in all types of rocks.Pseudotachylites are dark in color and occur as dykes or veins in the mylonitic schist,mylonite,migmatite and orebody.Pseudotachylite veins commonly show microlitic structure and embayed structure,and contain cataclastic quartz and feldspar.The structures suggest that the pseudotachylites were mainly frictional melting of coseismic faults accompanied by a small amount of fragmentation.XRD analysis shows that the pseudotachylite consists of minerals including quartz 53.3%,illite 20.5%,kaolinite 17.3%,orthoclase 8.9%.The pseudotachylites are geochemically similar to the country rocks,e.g,high SiO 2(>65.94%),Al2O3(>15.46%),rich K2O(K2O>Na 2O)and lower MgO(<2.14%)and Fe2O3 T(<5.42%).The pseudotachylites and country rocks have similar REE patterns that characterized by LREE-enrichment and moderate negative Eu anomalies(δEu=0.51-0.71).The microscopic and geochemical features show that the pseudotachylites may have been formed through melting of the country rocks at depth of 6 to 18 km.We suggest that the pseudotachylites in the Hetai goldfield were results of earthquakes during uplift of the Yunkaidashan.The crosscut relationship between the orebody and the pseudotachylite indicates that the gold mineralization in the Hetai goldfield postdate the pseudotachylite,which means that the gold mineralization is no younger than 123.33±1.00 Ma(40 Ar/39 Ar method),the age of the pseudotachylites.