内蒙古兴和县曹四夭超大型斑岩钼铅锌金成矿系统年代学及其地质意义

曹四夭斑岩钼矿床位于内蒙古兴和县,是华北克拉通北缘最大的钼矿床。该矿床中部发育斑岩型钼矿体,外围和上部产出热液脉型铅锌金矿体。文章选取1件与斑岩钼矿化共生的绢云母样品进行了40Ar-39Ar定年,获得40Ar-39Ar坪年龄为(144.4±1.2)Ma,相应的39Ar/36Ar-40Ar/39Ar等时线年龄为(146.4±2.2)Ma(MSWD=0.31),将等时线年龄认作绢云母的Ar封闭年龄,表明曹四夭斑岩钼矿化发生在约146 Ma前。选取14件脉型铅锌矿石中的闪锌矿、黄铁矿和磁黄铁矿样品开展了Rb-Sr定年,获得4件闪锌矿的Rb-Sr等时线年龄为(145.1±3.0)Ma(MSWD=0.63);5件黄铁矿的Rb-Sr等时线年龄为(145.2±1.3)Ma(MSWD=0.53);4件闪锌矿、5件黄铁矿和1件磁黄铁矿的Rb-Sr等时线年龄为(145.3±1.0)Ma(MSWD=0.43)。硫化物Rb-Sr定年结果表明曹四夭矿床脉型铅锌矿化形成于约145 Ma前。本次绢云母40Ar-39Ar和硫化物Rb-Sr定年结果表明,曹四夭矿床斑岩型钼矿化和脉型铅锌金矿化为同期产物,两者均形成于晚侏罗世末期,属于同一个斑岩成矿系统。曹四夭矿床硫化物的w(Rb)和w(Sr)分别为0.1867×10~(-6)~1.305×10~(-6)和0.3175×10~(-6)~6.935×10~(-6),Sr同位素初始比值(87Sr/86Sr)i介于0.709 919~0.711 951之间,平均值0.710 952,结合前人获得的辉钼矿Re含量,认为曹四夭矿床的成矿物质主要来源于地壳。

Chronology of giant Caosiyao porphyry Mo-Pb-Zn-Au ore-forming system and their geological significance in Xinghe County, Central Inner Mongolia

The giant Caosiyao porphyry Mo deposit, located in Xinghe County of Inner Mongolia, is the largest Mo deposit on the northern margin of the North China craton. The mineralization types of the deposit include porphyry Mo orebodies in the central part of the ore district and vein-type Pb-Zn-Au orebodies in outer and upper parts of the porphyry Mo orebodies. A sericite sample from the Mo ore was selected for ⁴⁰Ar-³⁹Ar dating, and it yielded a ⁴⁰Ar-³⁹Ar plateau age of (144.4±1.2) Ma and an isochron age of (146.4±2.2) Ma with an MSWD value of 0.31. The isochron age is closer to the Ar closure age of sericite; therefore, the porphyry Mo orebodies in the Caosiyao deposit must have been formed around 146 Ma ago. Fourteen sulfide samples consisting of six sphalerites, seven pyrites, and one pyrrhotite from the vein-type Pb-Zn ore were selected for Rb-Sr dating, of which four sphalerites yielded an isochron age of (145.1±3.0) Ma (MSWD=0.63), five pyrites yielded an isochron age of (145.2±1.3) Ma (MSWD=0.63), and four sphalerites, five pyrites, and one pyrrhotite yielded an isochron age of (145.3±1.0) Ma (MSWD=0.43), indicating that the vein-type Pb-Zn orebodies in the Caosiyao deposit were formed approximately 145 Ma ago. The dating data obtained in this study indicate that both porphyry-and vein-type orebodies of the Caosiyao deposit were formed in the same period and that they both belong to a porphyry metallogenic system occurring at the end of the late Jurassic (146~145 Ma). The Rb and Sr values range from 0.1867×10^-6 to 1.305×10^-6 and 0.3175×10^-6 to 6.935×10^-6, respectively, and the initial Sr isotope ratios (⁸⁷Sr/⁸⁶Sr)_i range from 0.709 919 to 0.711 951, with an average of 0.710 952. Considering the Sr isotopic composition obtained by the authors in this study and Re content in molybdenite obtained by previous researchers, the authors conclude that ore-forming materials of the Caosiyao deposit mainly originated from the earth''s crust.