安基山和铜山铜(钼)矿床中辉钼矿的铼-锇同位素年龄及其意义

Re-Os Isotope Ages of Molybdenite from the Anjishan Copper Deposit and the Tongshan Copper Molybdenum Deposit and Their Implications

安基山和铜山铜（钼）矿床为分别位于宁镇山脉弧形构造拐转处和弧形构造中段的两个中型矿床。前者为与燕山晚期花岗闪长斑岩石英闪长玢岩有关的、以斑岩型和矽卡岩型为主的铜（钼）矿床，岩体的单矿物Ｋ－Ａｒ同位素年龄为１２３．１×１０６—１０６．０×１０６ａ，Ｋ－Ａｒ等时线值为１２３．０×１０６ａ；后者为与燕山晚期石英闪长玢岩有关的矽卡岩型铜（钼）矿床，主岩体的Ｋ－Ａｒ同位素年龄为１１７．０×１０６ａ。从安基山铜（钼）矿床的细脉浸染型钼矿石和铜山铜（钼）矿床的浸染状钼矿石中选取了辉钼矿纯样并对其进行了Ｘ射线衍射分析，结果表明：安基山矿床的辉钼矿为２Ｈ１＋３Ｒ型，铜山矿床的辉钼矿为２Ｈ１型。采用同位素稀释－等离子体质谱法测定了辉钼矿中的１８７Ｒｅ和１８７Ｏｓ，并以衰变常数λ（１８７Ｒｅ）＝１．６４×１０－１１ａ－１计算出两矿床内辉钼矿的成矿年龄分别为（１０８±２）×１０６ａ和（１０６±３）×１０６ａ。

The studied copper(molybdenum) deposits include the Anjishan copper deposit and the Tongshan copper molybdenum deposit, of which the former lies in the inflexion section of Nanjing Zhenjiang Mountain arcuate structure, and the latter lies in the middle part of the same mountain chain. Both are medium size ones. Associated with granodiorite porphyry and quartz diorite porphyrite, the Anjishan copper deposit is mainly of porphyry and skarn type. The single mineral (biotite and hornblende) K Ar isotope ages of Anjishan rock bodies are between 123.1×10 6a and 106.0×10 6a,and K Ar isochron dating gives an age of 123.0×10 6a. The Tongshan copper molybdenum deposit is mainly of skarn type associated with quartz diorite porphyrite, whose K Ar isotope age is 117.0×10 6a. Two pure molybdenite samples were collected from fine grained disseminated ores of Anjishan and Tongshan deposits, with the former determined as 2 H 1+3 R polytype and the latter as 2 H 1 by means of XRD analysis. 187 Re and 187 Os contents of molybdenite from the two deposits are determined by Inductively Couple Plasma Mass Spectrometry (ICP MS), with Re Os ages calculated by using the formula λ ( 187 Re)=1.64×10 -11 a -1 . The results are 108×10 6a for the Anjishan deposit and 106×10 6a for the Tongshan deposit, which are both younger than the ages of the associated magmatic rock bodies, with the differences being 15×10 6a (Anjishan) and 11×10 6a (Tongshan) respectively.