豫西银家沟硫铁多金属矿床Re-Os和40Ar-39Ar年龄及其地质意义

河南省银家沟硫铁多金属矿床位于华北克拉通南缘的华熊地块内,是东秦岭地区最大的硫铁多金属矿床,以其硫铁矿储量大及共、伴生元素复杂区别于东秦岭其他以钼为主的矿床.矿化在空间上呈规律性的带状分布,从岩体内向外,依次出现斑岩型钼矿体→斑岩型硫铁矿体→矽卡岩型铁矿体、钼矿体→矽卡岩型硫铁、铜、锌、金矿体→脉型铅、锌、银矿体.选取5件接触带矽卡岩型钼矿体中的辉钼矿样品进行Re-Os同位素定年,获得(142.9±2.1) Ma～(143.7±2.3) Ma的模式年龄,加权平均值为(143.4±0.9) Ma(MSWD=0.071),等时线年龄为(140.0±18.0) Ma(2σ,MSWD=0.095),将(143.4±0.9) Ma认作辉钼矿的结晶年龄,表明银家沟矿床矽卡岩型矿体形成于约143 Ma前；选取1件硅化、绢云母化、黄铁矿化、辉钼矿化钾长花岗斑岩中的绢云母样品定年,获得40Ar-39Ar坪年龄为(143.6±1.4) Ma,相应的39 Ar/36 Ar-40 Ar/39Ar等时线年龄为(143.0±2.0) Ma(MSWD=0.13),将(143.0±2.0)Ma认作绢云母的Ar封闭年龄,表明银家沟矿床斑岩型矿化亦发生在约143Ma前.本次辉钼矿Re-Os和绢云母40Ar-39A定年结果表明,银家沟矽卡岩型和斑岩型矿体均形成于早白垩世初期.银家沟矿床辉钼矿的ω(Re)在38.5×10-6～43.2×10-6之间,成矿物质主要来自由火成物质组成的宽坪群和二郎坪群,成矿与矿区内的钾长花岗斑岩有关.结合前人对东秦岭造山带中生代期间地球动力学背景的研究成果,笔者认为银家沟矿床形成于EW向构造体制向NNE向构造体制大转换阶段,即形成于挤压体制向伸展体制转换的背景.

Molybdenite Re-Os and sericite 40Ar-39Ar ages of Yinjiagou pyrite-polymetallic deposit in western Henan Province, and their geological significance

Located in the Huashan-Xiong'ershan block on the southern margin of the North China craton, the Yin-jiagou pyrite-polymetallic deposit in Henan Province is the largest pyrite-polymetallic deposit in the East Qinling orogenic belt. The deposit is characterized by abundant pyrite resources and complex paragenetic and associated elements, which is different from most molybdenum deposits marked by the assemblage of molybdenum and tungsten in the East Qinling area. The mineralization types of the Yinjiagou deposit include porphyry Mo and pyrite ore bodies in the inner part of the intrusion, skarn Fe, Mo, pyrite, Cu, Zn and Au ore bodies along the contact zone between K-feldspar granite porphyry and dolostone, and vein-type Pb, Zn and Ag ore bodies in dolostone. Five molybdenite samples from skarn-type Mo ore within the contact zone were selected for Re-Os dating, which yielded apparent ages of (142.9±2.1) Ma to (143.7±2.3) Ma, with a weighted mean age of (143.4±0.9) Ma (MSWD=0.071) and an isochron age of (140.0±18.0) Ma (MSWD=0.095). The authors interpret the weighted mean age of (143.4±0.9) Ma as the crystallization age of the molybdenite, indicating that the skarn ore bodies in the Yinjiagou deposit formed around 143 Ma ago. A sericite sample from the altered K-feldspar granite porphyry was selected for ⁴⁰Ar-³⁹Ar dating, which yielded a ⁴⁰Ar-³⁹Ar plateau age of (143.6±1.4) Ma and an isochron age of (143.0±2.0) Ma with an MSWD value of 0.13, suggesting that the porphyry ore bodies in the Yinjiagou deposit were developed approximately 143 Ma ago. The dating data obtained in this study indicate that both skarn- and porphyry-type ore bodies of the Yinjiagou deposit formed approximately 143 Ma ago during the Early Cretaceous. The Re content of the molybdenite in the Yinjiagou deposit varies from 38.5×10^-6 to 43.2×10^-6, implying that the ore-forming material was mainly derived from the Kuanping and Erlangpin Groups, composed of poor differential igneous material, and the formation of the deposit was related to the K-feldspar granite porphyry in the Yinjiagou ore district. Combined with the previous studies of the geodymanic settings of the East Qinling orogenic belt during the Mesozoic period, the authors hold that the Yinjiagou pyrite-polymetalilic deposit was most likely formed in a transitional tectonic setting from nearly EW-trending to nearly NNE-trending tectonic regimes, namely, from a compressional to extensional transitional setting.