推荐回答（1个）
1.红岩矿床
据方铅矿、黄铁矿和全岩样共12个样品的同位素组成分析结果（表4-4），红岩矿床的铅同位素组成除了一个样品为J型铅外，其余均为正常普通铅（高μ值单阶段铅）。206 Pb/204Pb，207Pb/204Pb，208Pb/204Pb比值变化范围很小，在207Pb/204Pb—206Pb/204Pb坐标图上投影点非常靠近，没有显示线性关系（图4-17）。根据多伊（Doe，1974）的单阶段演化模式，计算出模式年龄除了一个方铅矿为187 Ma外，其余10个都接近泥盆系地层年龄，表明沉积与成矿来源具同源性。按斯塔西（Stacey，1975）混合模式计算铅的来源大约1/3为幔源，2/3为壳源。模式年龄偏低的那个方铅矿，可能与后期岩浆岩侵入（燕山期岩体）有关；桂头组千枚岩样品为J型的原因，可能是由于碎屑岩中锆石所含的Th和U使千枚岩明显富含206Pb和208Pb的结果。
2.凡口矿床
凡口矿体中方铅矿、黄铁矿以及灰岩全岩、辉绿岩全岩的铅同位素组成如（表4-5）所列。矿体中四个采自同一层位（D3tb）方铅矿的同位素组成十分接近，三个黄铁矿矿石样品的铅同位素组成也较接近，三个天子岭组灰岩的铅同位素组成差别也不大。矿石与围岩铅同位素组成相近似，模式年龄多在3.1亿～3.4亿年之间，灰岩铅模式年龄略大于矿石模式年龄，反映矿石和灰岩中成矿金属元素来源有密切联系，成矿作用与灰岩沉积作用大致同时进行。μ值在9.73～9.90之间，方铅矿中Th/U比值较低（3.11），反映铅的来源具壳源性质，源区可能富含铀铅。
表4-4 红岩黄铁矿床铅同位素组成表
表4-5 凡口矿区矿石与岩石铅同位素组成一览表
根据前人测试结果，部分矿石铅同位素组成中钍铅含量较高，同位素组成具中等程度变化（变化范围3%～4%）。这可能是具复杂来源的地层铅的反映，也可能与主要矿化作用后沿断裂发生的溶液活动有关。少数采自小脉中的方铅矿样品模式年龄为1.78 亿年，说明局部有燕山期成矿作用的叠加。少量样品的铅模式年龄在4亿～5亿年间，表明下伏岩层（桂头组和寒武系）的成矿物质参入了成矿活动。
图4-17 红岩黄铁矿铅同位素单阶段演化模式图
图中数字代表样品为：1—32；2—Hpb-2；3—Hpb-3、Hpb-1、172、92；4—265、83、红同Pb-1、红同Pb-2；5—红同Pb-3；6—Hpb-4
3.大宝山矿床
大宝山矿区硫化物中铅同位素组成较特殊，206Pb/204Pb为18.997～18.565，207Pb/204Pb为15.931～15.461，208Pb/204Pb为39.706～38.516。属于多阶段的放射性较高的铅。μ值为10.07～9.17，V值为0.073～0.067，ω值为42.69～36.13，K1值为4.24～3.75，K2值为584.30～525.73，K3值为4.10～3.63。以上特点表明大宝山矿区铅为多来源和多阶段成因的铅。在207Pb/204Pb-206Pb/204Pb坐标图（图4-18）上，大宝山矿区铅源最复杂，既有壳源铅，也有岛弧铅和大洋火山岩铅，比红岩矿区和凡口矿区的铅源更为复杂。
图4-18 粤北泥盆系层控矿床铅同位素组成坐标图
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