大兴安岭北部甲乌拉铅锌银矿床Rb-Sr同位素测年及其地质意义

甲乌拉铅锌银矿床位于内蒙古自治区满洲里市南西150km。矿床产于中蒙古-额尔古纳兴凯造山带南东缘之得尔布干断裂北西侧。本文在甲乌拉矿床选取7件闪锌矿和6件黄铁矿样品开展了Rb-Sr定年。获得闪锌矿的Rb-Sr等时线年龄为143.0±2.0Ma(MSWD=3.2),锶同位素初始值I Sr=0.71265;黄铁矿的Rb-Sr等时线年龄为142.0±3.0Ma(MSWD=5.7),锶同位素初始值ISr=0.71267;闪锌矿与黄铁矿的Rb-Sr等时线年龄为142.7±1.3Ma(MSWD=3.8),锶同位素初始值ISr=0.71266。上述定年结果表明,甲乌拉矿床形成于早白垩世初期。甲乌拉矿床硫化物的Rb和Sr含量分别介于0.1034×10-6～7.367×10-6和1.301×10-6～7.148×10-6之间,Sr同位素初始比值(87Sr/86Sr)i介于0.71238～0.71277之间,平均值为0.71264,暗示甲乌拉矿床的成矿物质主要来源于地壳。甲乌拉矿床形成于蒙古-鄂霍茨克造山过程的后碰撞阶段。

Rb-Sr isochron age of the Jiawula Pb-Zn-Ag deposit in the Manzhouli area and its geological significance

The Jiawula Pb-Zn-Ag deposit, situated 150km southwest of Manzhouli City, Inner Mongolia, is located in the northwest of the Derbugan fault, which is located on the southeast edge of the Central Mongolia-Argun Khanka orogenic belt. Seven sphalerite samples and six pyrite samples was selected for the Rb-Sr isochron dating method to determine the mineralization age of the Jiawula Pb-Zn-Ag deposit. A Rb-Sr isochron defined by seven sphalerite samples yields an age of 143.0±2.0Ma (MSWD=3.2), with initial Sr isotopic composition I_Sr=0.71265; six pyrite samples yield a Rb-Sr isochron age of 142.0±3.0Ma (MSWD=5.7), with initial Sr isotope composition I_Sr=0.71267; the Rb-Sr isochron age from sphalerite and pyrite is 142.7±1.3Ma (MSWD=3.8), with initial Sr isotope composition I_Sr=0.71266. These dating results indicate that the Jiawula deposit formed approximately 143Ma ago during the Early Cretaceous. The Rb contents range from 0.1034×10^-6 to 7.367×10^-6, the Sr contents range from 1.301×10^-6 to 7.148×10^-6, and the initial Sr isotope ratios (⁸⁷Sr/⁸⁶Sr)_i range from 0.71238 to 0.71277, with an average of 0.71264. Both Rb and Sr geochemical characters imply that ore-forming minerals of the Jiawula deposit mainly originated from the earth's crust, but also mixed with small amount of mantle material. The origin of the Jiawula deposit may be related to Mesozoic post-collisional orogeny of the Mongolia-Okhotsk orogen.