Longqiao iron deposit is a buried iron-rich deposit, located at the northern margin of the Lujiang-Zongyang ore concentrated area in the Middle and Lower Reaches of Yangtze River. Orebodies occur mainly in argillaceous siltstone and carbonate between volcanic rocks of Lower Cretaceous Longmenyuan Formation and quartz syenite and monzonite. This study conducted Ar-Ar isotopic dating for biotites associated with pyrite orebodies and calcites (sample Lz370-l) and altered trachyandesites of country rocks (sample Lz370-2) in middle section of No. 370 line underground in the deposits. The biotite 40Ar-39Ar plateau age and isochron age for the two samples are 133. 4 1. 5Ma(2σ), 133. 1 ±4. 8Ma(MSWD = 0. 32) and 135. 4± 1. 4Ma(2σ), 135. 5 ± 1. 8Ma(MSWD= 1. 3), respectively, which represent the ages of hydrothermal alteration and mineralization of the Longqiao iron deposit. The characteristics of regional magmatism and metallogenic epochs of typical deposits show that the metallogenic epoch of volcanic iron deposits in the Middle and Lower Reaches of Yangtze River is less than 135Ma.%龙桥铁矿床是长江中下游地区隐伏的富铁矿床,位于庐江-枞阳火山岩矿集区的北缘,铁矿体主要赋存于下白垩统龙门院组火山岩之下和石英正长岩、二长岩之上的泥质粉砂岩和碳酸盐之中;本文分别采集了龙桥铁矿床井下370中段与黄铁矿矿体、方解石共生的黑云母样品(Lz370-1)和矿体围岩蚀变粗安岩中的黑云母样品(Lz370-2),进行Ar-Ar同位素测年,得到两个样品黑云母40Ar-39 Ar坪年龄和等时线年龄分别为133.4±1.5Ma(2σ)、133.1±4.8Ma(MSWD＝0.32)和135.4±1.4Ma(2o)、135.5±1.8Ma(MSWD=1.3),它们代表了龙桥铁矿床的热液蚀变和矿化年龄.区域岩浆活动和典型矿床的成矿时代特征表明,长江中下游地区火山岩型铁矿床主要成矿时代在135Ma之后.
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