广东天堂铜铅锌多金属矿床Rb-Sr等时线年龄及其地质意义

Rb-Sr isochron age of Tiantang Cu-Pb-Zn polymetallic deposit in Guangdong Province and its geological significance

文章选取8件闪锌矿、3件方铅矿和1件黄铁矿样品,采用Rb-Sr等时线定年方法测定天堂铜铅锌多金属矿床的成矿时代。获得闪锌矿等时线年龄为(98.1±1.6)Ma,闪锌矿+方铅矿等时线年龄为(99±2)Ma,闪锌矿+方铅矿+黄铁矿等时线年龄为(98.2±1.3)Ma,闪锌矿+黄铁矿等时线年龄为(97.87±0.96)Ma,方铅矿+黄铁矿等时线年龄为(98.6±4.2)Ma。Rb-Sr定年结果表明,天堂铜铅锌多金属矿床的成矿时代为98Ma左右,矿床形成于晚白垩世早期,可能与135Ma之后太平洋板块的运动方向发生转向,使得中国大陆包括华南板块在内均处于持续伸展阶段有关。硫化物矿石Rb-Sr所得的Sr同位素初始比(87Sr/86Sr)i平均值为0.7117,小于陆源硅酸盐的值(0.720),高于地幔Sr的初始值0.707,结合笔者对该矿床所做的S、Pb等同位素组成特征研究,显示成矿物质来源于壳幔混合。研究表明,利用特定矿床的主要矿石矿物,采用Rb-Sr等时线定年方法通过共生矿物组合和单矿物相互约束,可以有效地确定成矿时代,这对了解矿床的成矿背景等具有一定的指示意义。

The ages of the Tiantang Cu-Pb-Zn polymetallic deposit were determined by the Rb-Sr isochron method through dating samples of sphalerite and three groups of intergrown mineral assemblage （pyrite and galena）. The selected samples included eight pieces of sphalerite, three pieces of galena and one piece of pyrite. In contrast with sphalerite age of（98.1±1.6） Ma and sphalerite ＋ galena pyrite assemblage age of （99±2） Ma, the authors also obtained sphalerite ＋ galena ＋ pyrite assemblage age of （98.2±1.3） Ma, sphalerite ＋ pyrite age of （97.87±0.96） Ma and galena ＋ pyrite assemblage age of （98.6±4.2） Ma by the same means. These data convince the authors that the metallogenic epoch is about 98 Ma, and that the Late Cretaceous is the main time for the formation of the Tiantang Cu-Pb-Zn polymetallic deposit. The ore-forming process was probably related to the fact that China＇s mainland including South China Plate was in a continuous stretching phase caused by the change of the movement direction of the Pacific Plate after 135 Ma. The （ 87Sr/86Sr）i of sulfides is equal to 0.7117 and it is lower than that of the terrigenous silicates （0.720） and higher than that of the mantle （0.707）. In combination with S and Pb isotope data avialable, the authors consider that the ore-forming material of the ore bodies in the Tiantang Cu-Pb-Zn polymetallic deposit was derived from the crust-mantle mixture. It is thus held that direct dating of the ages of metal minerals by using Rb-Sr isochron method for single minerals and mineral assemblages could serve as a routine chronological method, and that the means is of importance for indicating and understanding metallogenic background of the ore deposit.