湘东南锡田辉钼矿Re-Os同位素定年及其地质意义

湘东南锡田是近年来新发现的一个具有大型规模的钨锡多金属矿田,该矿田位于湘赣边界,南岭成矿带与钦杭成矿带的交汇部位,扬子地块与华夏地块的拼合带。目前对于矿体与成矿岩体之间的关系以及矿体形成时代的问题尚存争议,钨锡矿化究竟是与印支期还是与燕山期花岗岩有关,矿田中众多矿体是否同期形成,这些问题仍待进一步确定。本文选取了两个矿床,即山田云英岩-石英脉型锡多金属矿床和桐木山破碎带蚀变岩型锡多金属矿床,分别对来自这两个矿床的辉钼矿样品进行了Re-Os同位素定年,获得的ReOs模式年龄分别为(158.9±2.2)Ma(2SD)和(160.2±3.2)Ma(2SD),表明这两个矿床形成于晚侏罗世早期。高精度的云母Ar-Ar和辉钼矿Re-Os年龄数据表明锡田钨锡多金属矿田的垄上、荷树下、山田、桐木山矿床均形成于150~160 Ma,即南岭与花岗岩有关钨锡多金属矿大规模成矿作用的高峰期。上述两个辉钼矿样品的铼含量分别为12.44×10-6和2.367×10-6,指示成矿物质分别为壳-幔混合来源和地壳来源,为准确认识该矿田的成矿物质来源提供了进一步的制约。本文还对南岭地区晚侏罗世与花岗岩有关的钨锡多金属矿中90个辉钼矿的铼含量数据进行了统计,结果表明钨锡多金属矿的成矿物质绝大多数为地壳来源,少数为壳-幔混合来源。

Molybdenite Re-Os Isotopic Dating of Xitian Deposit in Hunan Province and Its Geological Significance

The Xitian large-scale W-Sn polymetallic ore field has been explored in recent years. It is found in jointed adjoining areas of the Nanling and Qinhang metallogenic belts which are located on the boundary between Hunan and Jiangxi provinces, and the junction of the Yangtze and Cathaysia blocks. At present, there are several debates about the formation ages of the ore bodies and nearby granitic intrusions. For example, is W-Sn mineralization related to either Indosinian or Yanshanian granites? Were numerous different types of ore bodies formed in the corresponding period? The separation of molybdenite minerals from two tin polymetallic deposits, Shantian greisen-quartz vein ore deposit and Tongmushan crushed zone-alterated rock ore deposit was undertaken and is reported in this paper. Re-Os model ages of molybdenite in these two deposits yielded (158.9±2.2) Ma (2SD) and (160.2±3.2) Ma (2SD), respectively, which show they were generated in the early stage of the late Jurassic period. According to statistical data of mica Ar-Ar and molybdenite Re-Os ages, Longshang, Heshuxia, Shantian and Tongmushan ore deposits were all formed during 150-160 Ma, which is the peak period of W-Sn large-scale mineralization related to granitic intrusions. Rhenium contents of the two molybdenite samples are 12.44×10^-6 and 2.367×10^-6, respectively, which suggests that metallogenic materials were derived from crustal-mantle mixture and crust. Combining with ninety data of rhenium contents in molybdenites of W-Sn polymetallic deposits closely related to granitoids in Nanling area in late Jurassic, the suggestion that a majority of metallogenic materials were derived from crust is given, with only a few of them derived from crust-mantle mixture.