沥青Re-Os定年对卡林型金矿床成矿时代的约束：以右江盆地水银洞和高龙金矿床为例

Carlin-type gold deposits in the Youjiang basin represent the world’s largest concentrations of this type deposits outside Nevada. Due to the lack of suitable minerals that are unequivocally associated with gold precipitation, the ages for mineralization remain undetermined. Organic matter such as bitumen is commonly present as a migrated hydrocarbon product in high grade ore of Carlin-type gold deposits, providing an optimal candidate for Re-Os isochron dating to constrain the time of gold mineralization. This proposed research involves detailed field observations and intensive investigation on the mineralogy, geochemistry, and raman spectroscopy to identify the ore-related bitumen from the typical strata-bound Shuiyindong deposit in platform facies area and the typical fault-controlled Gaolong deposit in basinal facies area for Re-Os isochron dating of bitumen to precisely constrain the timing of gold mineralization. These results, combined with the regional geology, geochemistry, and geochronology data of the gold deposits in and around the Youjiang basin, are used to provide new insights into geodynamic setting under which the Carlin-type gold deposits formed. The expected results are not only helpful to complete the age framework of Carlin-type gold deposits in the Youjiang basin, but also provide new approach to date Carlin-type gold genesis elsewhere.

右江盆地是目前除美国内华达地区以外全球最大的卡林型金矿矿集区，由于缺乏合适的定年矿物，这类矿床的成矿时代一直存在较大争议。该区卡林型金矿床中普遍发育与金矿体时空关系密切的沥青，这为通过沥青Re-Os定年来限定金成矿时代提供了极好的条件。本项目拟选取右江盆地赋存于台地相区以“层控型”矿化为主的水银洞金矿床和赋存于盆地相区以“断控型”矿化为主的高龙金矿床为研究对象，通过详细的野外地质调查、矿物学、元素地球化学和拉曼光谱分析，查明金矿床中沥青的产状分布、矿物共生关系、主微量元素组成和拉曼光谱特征，识别和分选出与成矿作用相关的沥青，在此基础上开展沥青的Re-Os等时线定年，以准确限定金矿床的形成时代。最后结合区域地质、地球化学和年代学资料，进一步探讨该区卡林型金矿床成矿地球动力学背景。预期成果不仅有助于补充和完善右江盆地卡林型金矿床的年代学框架，而且为卡林型金矿床成矿年代学研究提供新的思路和借鉴。

我国华南右江盆地卡林型金矿床中普遍发育沥青等有机质，这些沥青与卡林型金矿床成矿作用的关系以及能否用于约束成矿年龄尚不清楚。本次研究选取我国右江盆地赋存于台地相区以“层控型”矿化为主的水银洞金矿床和盆地相区以“断控型”为主的高龙金矿床为研究对象，通过详细的野外地质调查和岩/矿相学研究，发现沥青主要呈浸染状、短脉状分布在高品位矿石中或以对称条带状分布在切穿矿体的热液方解石或石英脉中，同时与成矿期载金硫化物、热液伊利石、方解石或石英具有紧密的空间共生关系，显示随成矿流体迁移后裂解成因的特征。激光拉曼光谱分析显示这些沥青具有几乎一致的拉曼光谱特征，说明金矿床中的这些产状的沥青具有相同成因。“火试金”化学全分析和电子探针微区分析显示沥青中金含量高达14 ppm，进一步确认其成因与金矿化密切相关，为与成矿流体共同迁移的有机质裂解成因。通过高精度负离子热电离质谱仪（N-TIMS）沥青Re-Os同位素分析获得水银洞金矿床与成矿有关沥青得Re-Os等时线年龄为160 ± 12 Ma 和高龙金矿床与成矿有关沥青的Re-Os等时线年龄为209 ± 27 Ma，结合区域上已发表的同位素年代学数据和区域构造演化，认为本次研究所获得的水银洞和高龙金矿床的沥青Re-Os等时线年龄可能分别反映了右江盆地西北侧台地相区和东南侧盆地相区卡林型金矿床分别形成于侏罗纪太平洋板块俯冲作用和晚三叠世印支造山作用相关的伸展构造背景。这项研究显示沥青Re-Os等时线测年法可用于约束卡林型金矿床的成矿时代。