低U方解石微区原位U-Pb定年技术及其在卡林型金矿中的应用研究

LA-ICP-MS方解石U-Pb定年是国际上近年来新兴的定年技术，对中低温含碳酸盐矿物地质体的高精度定年有巨大的应用潜力，在矿床学领域应用较少，国内处于起步阶段。该方法的标样均一程度和储备、U极低且分布不均导致的样品定年可行性评价和靶区优选技术、年龄解译技术等亟待完善和发展。申请者以国际标样WC-1和LA-(SF-)ICP-MS初步建立该方法，拟以贵州惠水方解石矿、贵州黄色文石、湘南芙蓉石炭系灰岩和烂泥沟、水银洞超大型卡林型金矿为研究对象。通过LA-ICP-MS元素mapping、预筛选和单点分析等方法的优化组合，揭示目标方解石的元素含量，尤其U和Pb的分布特征，实现样品定年可行性和靶区的快速评价和优选；通过目标方解石的LA-ICP-MS U-Pb定年，尝试开发有潜力的第一或第二标样；将完善后的方法应用并精确厘定典型卡林型金矿的成矿年龄，为解决含碳酸盐矿物低温矿床的年代学问题提供重要手段。

LA-ICP-MS U-Pb dating of calcite is an emerging but rapidly growing field of application in geochronology with great potential in mid-low temperature geological bodies which are composed by carbonate minerals and difficult to date otherwise. Currently, this method has few applications in mineral deposits and is still at the starting stage in China. In particular, in order to applicate it widely, the quality and source of calcite reference material, the method of how to quickly and accurately evaluate the feasibility of in-situ U-Pb dating, target the higher U/Pb ratio zone and interpret U-Pb age of the unknown (due to commonly containing very low and variable U contents, as well as easily undergoing alteration and recrystallization), need to be improved or established as soon as possible. Using the international calcite reference material of WC-1, the simple method of in-situ U-Pb dating of calcite has been established at the preliminary stage in our laboratory utilized LA-(SF-)ICP-MS systems. This study will focus on the Huishui calcite deposit, the yellow aragonite deposit in Guizhou Province, carbonate (limestone) of the Carboniferous system in the Furong district, Hunan Province, and two super-large Lannigou and Shuiyindong Carlin-type gold deposits in Yunnan-Guizhou-Guangxi region. On the basis of detailed field investigations and observations on the thin sections, we use the most advanced and/or newly developed in-situ analytical methods, such as LA-ICP-MS mapping, pre-screened and single spot analysis, to reveal the contents and relationships among the different elements of objective calcite, especially the distribution characteristics of U and Pb. After comparison among mapping, pre-screened and single spot analysis, one or a combination of them was selected to quickly and accurately evaluate the feasibility of in-situ U-Pb dating and target the higher U/Pb ratio zones of unknown calcite. Based on all analysis in this study, the samples that show the characteristics of high U, low Pb and mid-high degree of isotopic homogeneity or plotting as a scatter line with a smaller MSWD in Tera-Wasserburg Concordia diagram relatively to WC-1 (207Pb/206Pb vs 238U/206Pb) will be introduced as potential U-Pb reference material and helpful to a wide application of this dating method. A systematic method of in-situ U-Pb dating of calcite will be tried to exactly constrain the epochs of two typical Carlin-type gold deposits and also use as an important one to solve the geochronological problem on low-temperature and bearing-carbonate mineral hydrothermal deposits.