拉拉铁铜矿床成因:来自隐爆角砾岩结构定量化和锆石U-Pb年代学的证据

Petrogenesis of the Lala iron-copper deposit:Evidence by cryptoexplosive breccia CSD data and their zircon U-Pb data

拉拉铁铜矿床被认为是中国西南地区规模最大的元古代内生金属矿床。该矿床主要赋存于古元古代河口群地层中,传统上被归属为火山喷流-沉积成因。近年来,一些学者陆续发现拉拉铁铜矿床中有Au、REE、Mo等金属的富集,因而将其归属为IOCG矿床。但是,对该区出露的隐爆角砾岩及其与成矿作用的关系缺乏详细报道,因而对矿床成因和成矿时代的判断尚显依据不足。作者对拉拉铁铜矿床的小露天采坑及其邻近地区进行了详细野外地质观察和剖面测量,发现采坑内出露的角砾岩具有隐爆角砾岩的特征。角砾主要来自下伏河口群的细碧-角斑岩建造,呈棱角状-次圆状,由热液物质胶结,且胶结物中见有矿化现象。定量化结构分析表明,隐爆角砾岩中的角砾具有机械破碎成因的特点,其分形特征与火山碎屑岩类似,定向程度和圆度明显不同于断层角砾岩。对角斑岩和隐爆角砾岩分别进行了锆石U-Pb年代学分析。来自角斑岩中的锆石晶体具有典型的岩浆锆石特征,产生了1720±11Ma的锆石U-Pb年龄;而来自隐爆角砾岩的锆石晶体展示了较显著的后期热液改造特征。岩相学观察表明,细碧岩和角斑岩在成矿作用之前经受过强烈构造变形,表现为岩石的片理化,而沿片理面充填的含矿细脉则没有任何变形的迹象。据此可以得出结论:(1)拉拉铁铜矿床的形成时间明显晚于细碧-角斑岩建造,不支持火山喷流-沉积成因的观点;(2)矿区内产有与成矿作用紧密相关的隐爆角砾岩,后者被认为是IOCG型矿床的重要标志之一。前人报道的成矿元素组合特征以及本文提供的新证据支持该矿床为IOCG型矿床的认识。其成矿过程为成矿流体在流体库中膨胀后并由下部上侵,下部流体遇薄弱带侵入成矿,上部流体在密闭空间内形成超压进而发生爆破,并侵入细碧岩的片理中成矿,这种成因过程类似于透岩浆流体成矿模型。

Lala iron-copper deposit,one of the largest deposits in SW China,is hosted in the Late Paleoproterozoic of lower strata of the Hekou Group. However,the petrogenesis and the type of the deposit is still lack of significant evidence. Recently,it is continuously proved that Lala deposit is Iron Oxide Copper-Gold（ IOCG） deposit in contrast to the classical opinions that it is attributed to submarine volcanic sedimentary exhalative processes,but few detailed reports of the cryptoexplosive breccia. In this article,we used the example of Xiaolutian pit and studied on the basis of geology and petrography of the crytoexplosive breccia and their wallrocks. With the application of Quantitative Textural Analysis theories and methods,and the LA-ICP-MS zircon U-Pb age,we attempt to interpret the genesis and types of the Lala iron-copper-gold deposit. Fractal dimension of the breccia illustrates the cause of its mechanical fragmentation in comparison with fault breccia. The spatial density of population reveals that it is mechanically compressed and can be distinguished from the fault breccia in the alignment factor and roundness data. Petrographic showed that minerals in the rock have obvious orientation,the magnetite vein intruded into the schistosity or weak zone and the chalcopyrite occurred in breccia cements. This explained the wallrocks have undergone constructed shear deformation before mineralization. So the age of the wallrocks can ＇t representative the age of mineralization. It was the fluid rich in iron or copper that added into the fluid reservoir,which resulted fluid overpressure internally,then caused crytoexplosion. After that,fluid in the lower part intruded into the weak zone of strata,which forms the main hosts of the ores. The spilite,keratophyre and crytoexplosive breccia where infilled by hydrothermal minerals derived from the upper fluid. The process was like the transmagmatic fluid model. Experiments data yielded an age of 1720 ± 11 Ma of spilitekeratophyre formation. Zircon crystals of crytoexplosive breccia showed significant characteristics of late hydrothermal alteration,nevertheless,it still deny the theory of submarine volcanic sedimentary mineralizion. The discovery of crytoexplosive breccia provides an important basis for which Lala deposit is IOCG deposit.