皖南东至兆吉口铅锌矿床成矿时代的厘定及其找矿指示意义

皖南东至兆吉口铅锌矿床是近年来在江南过渡带西段新发现的一个规模达中型的中-低温热液型矿床。目前揭露的铅锌矿体均赋存于东至断裂带及其西侧次级张(扭)性断裂及裂隙中。文章选取了该矿床主要矿石矿物闪锌矿以及与主成矿阶段金属硫化物紧密共生的石英,分别采用流体包裹体Rb-Sr法和40Ar-39Ar法进行同位素地质年龄测定。4件闪锌矿样品获得流体包裹体Rb-Sr等时线年龄为(128±1)Ma;2件石英样品获得流体包裹体40Ar-39Ar坪年龄分别为(128.74±3.02)Ma和(128.19±1.98)Ma,等时线年龄分别为(128.81±5.25)Ma和(128.30±3.47)Ma,反等时线年龄分别为(128.82±5.24)Ma和(128.31±3.47)Ma。同位素地质年龄测试结果基本一致,表明兆吉口铅锌矿床形成于128 Ma左右,成矿时代为早白垩世。该成矿年龄与赋矿闪长玢岩的形成时间((129.0±2.3)Ma~(128.4±2.7)Ma)一致,也与东至断裂燕山晚期的活动年龄((126.3±2.2)Ma)相近,表明该矿床成矿与燕山晚期的构造-岩浆活动密切相关,此时区域构造背景为强烈挤压后的伸展环境。这一矿化事件在成矿时代上明显晚于长江中下游成矿带中的断隆区(如铜陵、安庆-贵池等矿集区)的铜金多金属矿床和皖南成矿带中的钨钼多金属矿床(136 Ma),而与长江中下游成矿带断坳区的玢岩铁矿床的成矿时间(135~127 Ma)相近,指示长江中下游成矿带与皖南成矿带之间的江南过渡带中发育有燕山晚期较晚阶段(128 Ma左右)的铅锌矿化事件,为今后在该区开展铅锌多金属矿床找矿勘探提供了重要依据。

Tentative temporal constraint on Zhaojikou Pb-Zn deposit in Dongzhi County of South Anhui Province and its prospecting significance

Located in Dongzhi County of south Anhui Province in the western part of Jiangnan transition belt, the Zhaoji-kou Pb-Zn deposit is a medium-size meso-epithermal ore deposit, whose Pb-Zn orebody is hosted in the Dongzhi fault and its secondary extensional-torsional fractures. The fluid inclusion Rb-Sr isotope dating was conducted for the mineral sphalerite and ⁴⁰Ar-³⁹Ar isotope dating was carried out for the mineral quartz associated with metal sulfides from the Zhaojikou Pb-Zn deposit,respectively. Four sphalerite samples yielded a Rb-Sr isochron age of (128±1) Ma,and two quartz samples yielded ⁴⁰Ar-³⁹Ar plateau ages of (128.74±3.02) Ma and (128.19±1.98) Ma,with isochron ages of (128.81±5.25) Ma and (128.30 ±3.47) Ma,inverse isochron ages of (128.82±5.24) Ma and (128.31±3.47) Ma,respectively. These identical ages obtained by different methods show that the Zaojikou Pb-Zn deposit was formed in Cretaceous. The age of the deposit is consistent with the age of the ore-bearing diorite porphyry ((129.0±2.3)Ma~(128.4±2.7)Ma) and the age of the Dongzhi fault which was active during late Yanshanian ((126.3±2.2)Ma),indicating that the ore-forming process of the Zhaojikou Pb-Zn deposit was closely related to the tectono-magmatic activity during late Yanshanian,and the regional tectonic setting at that time was an extension environment after intense compression. The mineralization event of the Zhaojikou Pb-Zn deposit occurred later than Cu-Au polymetallic mineralization event (>136 Ma) in the block uplift of the Middle-Lower Yangtze River metallogenic belt (e.g.,Tongling,Anqing-Guichi ore concentration area) and W-Mo polymetallic mineralization events in the South Anhui Province metallogenic belt,but corresponded to those (135~127 Ma) of porphyritic iron deposits in the basin of Middle-Lower Yangtze River Metallogenic Belt. The result of this research indicates that the magmatism and mineralization in the Jiangnan transition belt had transitional and compound characteristics between the Middle-Lower Yangtze River metallogenic belt and the South Anhui Province metallogenic belt. The result obtained by the authors provides fresh evidence for the regional mineral exploration.