论河北丰宁牛圈银(金)矿床的成矿时限问题

对河北牛圈银(金)矿区细粒钾长花岗岩株和含矿隐爆角砾岩墙钾长石样品进行了详细的40Ar/39Ar同位素年龄测定。细粒钾长花岗岩株钾长石样品主要加热阶段所获得数据构成一条相关性很好的直线,其对应的同位素等时线年龄值为(216.6±3.2)Ma(2σ),40Ar/39Ar初始比值为297±28。相比之下,含矿隐爆角砾岩墙钾长石样品特定加热阶段(1200～1300℃和1360～1450℃)所获数据同样也构成两条直线,其对应的同位素等到时线年龄值分别为(227.9±4.8)Ma(2σ)和(216.7±6.1)Ma(2σ),40Ar/39Ar初始比值分别为307±41和269±39。考虑到所测样品均采自未受到明显后期构造-岩浆活动或其他热事件影响的岩(矿)体,因此,细粒钾长花岗岩株和含矿隐爆角砾岩墙的形成时代为216Ma,与印支期富钾岩浆作用具有密切成因联系。根据含矿隐爆角砾岩墙的野外地质证据,结合矿物共生组合关系和钾长石同位素年龄数值,可以推测牛圈银(金)矿区早期成矿作用发生在(227.9±4.8)Ma,而主要成矿期为(216.7±6.1)Ma,是印支期多阶段富碱质成矿流体活动的产物。牛圈银(金)矿区成矿作用时间的厘定对于提高该区成矿理论研究水平和寻找隐伏金属矿床均具重要意义。

Discussion on the time limitation of silver (gold) mineralization in the Niujuan deposit, Northern Hebei Province

The Niujuan deposit is one of the largest silver(gold)deposits in north China and is located in the central part of the Inner Mongolian Axis,in the middle-eastern section of the northern margin of the North China Craton.This region is considered as one of the key metallogenic belt of northern China.Silver(gold)mineralization in the Niujuan deposit is hosted within a Paleozoic coarse-grained K-feldspar granite batholith.The mineralization is characterized by veinlets or vein stockworks,within altered-fractured blocks and lenses,and shows an intimate spatial relationship with a Mesozoic cryptoexplosive breccia dyke and fine-grained K-feldspar granite stock.40Ar/39Ar isotope data for K-feldspar separated from both the fine-grained K-feldspar granite stock and the ore-bearing cryptoexplosive breccia dyke are presented in this paper.Step heating Ar data of the K-feldspar give an 40Ar/39Ar isochron age of(216.6±3.2)Ma(2σ)and 40Ar/36Ar initial value of(297±28)(2σ),which is close to Nier's value.This age may represent the crystallization time of the K-feldspar in the K-feldspar granite stock,which is considered the peak of the Mesozoic tectonic-magmatic events occurring within the Niujuan mineralized district.For the ore-bearing cryptoexplosive breccia dyke,one K-feldspar sample has two 40Ar/39Ar isochron ages,under different stepwise heating conditions.Three-step heating Ar data at 1 200-1 300℃ constitute an 40Ar/39Ar isochron line,which shows an age of(216.7±6.1)Ma(2σ)and 40Ar/36Ar initial value of(268±39)(2σ),again similar to Nier's value.Meanwhile,three-step heating Ar data at 1 360-1 450℃ give an 40Ar/39Ar isochron age of(227.9±4.8)Ma(2σ)and 40Ar/36Ar initial value of(307±41)(2σ)(close to Nier's value).Considering that(1)the K-feldspar separated from the ore-bearing cryptoexplosive breccia dyke shows clear co-existing textures with silver(gold)-bearing sulfides and tellurides,and(2)both the K-feldspar granite stock and the ore-bearing cryptoexplosive breccia dyke have the same K-feldspar 40Ar/39Ar isochron ages((216.6±3.2)Ma and(216.7±6.1)Ma),we suggest that the isotopic age of(227.9±4.8)Ma(2σ)may be regarded as the time of the earliest silver(gold)mineralization event,and that the isotopic age of(216.7±6.1)Ma may represent the main ore-forming time of the Niujuan deposit.Isotopic age data and field observations indicate that the ore-forming processes consisted of two stages:(1)an early event at(227.9±4.8)Ma(2σ)and(2)a late(main)event at(216.7±6.1)Ma.Therefore,the Niujuan silver(gold)deposit was the rseult of tectonic movements and granitoid magmatic activity,during the Indosinian orogeny.