LA-ICPMS原位微区面扫描分析技术及其矿床学应用实例

Laser Ablation-Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) elemental mapping and its applications in ore geology

LA-ICPMS原位微区面扫描技术的发展对解析具有包裹体、环带结构的矿物或受多期岩浆/热液活动影响形成的溶蚀再结晶的矿物具有重要的地质意义。黄铁矿作为最普遍的硫化物种类之一,可形成于各种类型矿床中,其微量元素组份记录了矿床形成过程的重要信息。通过LA-ICPMS原位微区面扫描技术获得的黄铁矿晶体内部结构组份信息可用来限定成矿流体的性质、厘定成矿流体的演化规律、示踪成矿物质来源、约束变形变质作用过程中元素的活化和迁移行为,以及约束矿床成因等。本次研究选取了内蒙古二连盆地中巴彦乌拉大型铀矿床、辽宁青城子地区榛子沟铅锌矿、加拿大Pardo砂金矿中的黄铁矿作为研究对象,运用原位微区LA-ICPMS元素面扫描技术揭示其内部组份及结构信息,探索该技术在不同类型矿床中的应用价值。巴彦乌拉大型砂岩型铀矿床中的黄铁矿的内部组构信息显示成矿体系的p H和Eh对硫酸盐还原细菌活动均有影响,并控制与之相关的铀矿化;且成矿体系的p H和Eh受到外界不断供给的地下水的影响,呈振荡性变化。辽宁榛子沟铅锌矿中的黄铁矿的内部组构信息显示矿化过程受到了至少两期流体活动的叠加影响,包括早期同沉积时期的流体以及晚期与燕山期岩浆活动有关的热液流体。加拿大Pardo金矿中黄铁矿的内部组构信息显示矿床中半自形-自形的黄铁矿实际包含碎屑成因的磨圆状核部及后期热液活动形成的增生边;金主要呈浸染状分布在碎屑成因的黄铁矿核部,后期热液活动对金矿化影响微弱。

With the development of two-dimensional trace element mapping by laser-ablation inductively-coupled-plasma mass spectrometry( LA-ICP-MS),it is now possible to reveal in much greater detail any compositional interior textures of the pyrites,thus permitting the investigation of the history of pyrite crystals and providing new insight into the genesis and evolution of mineralisation.Exploiting the advantages of high spatial resolution,low detection limits,and capability of full quantification,LA-ICP-MS elemental mapping is being widely applied to imaging sulphides from various types of deposits. In this study,we applied the LA-ICP-MS elemental mapping technique on pyrites from different types of deposits from China and Canada,for the purpose of monitoring the changes of physio-chemical condition,defining ore genesis,and revealing the distribution patterns of precious metals. The interior compositional textures of pyrites show that the mineralization system of the Bayinwula roll-front U deposit in Inner Mongolia was open and continuously affected by the re-charge of oxidized groundwater,along with a periodic change of p H and Eh that affected the activities of sulphate bacteria and the associated U mineralization. The trace element maps of pyrites from the Zhenzigou Pb-Zn deposit in Liaoning Province indicate that several episodes of fluids were involved and responsible for the mineralisation,namely,an early stage of syn-sedimentary origin and a late hydrothermal mineralisation related to the Yanshanian magmatic activity. The trace element distribution patterns of pyrites from the Palaeoproterozoic placer Au deposit in Canada showed that the subeuhedral-euhedral pyrite,which is a dominant type of pyrite in the conglomerates,is detrital origin and later modified by hydrothermal( or metamorphic) fluids,and Au is mostly associated with the detrital pyrite grains/cores,and disseminated in the porosity on the surface.