海南主要金矿类型的铅同位素特征及其地质意义

LEAD ISOTOPIC CHARACTERISTICS OF MAJOR TYPES OF GOLD DEPOSITS IN HAINAN PROVINCE AND THEIR GEOLOGICAL IMPLICATIONS

对琼西抱板和土外山两矿区典型矿体剖面样品的Ｐｂ同位素比值和Ｐｂ浓度作了测定。这两个金矿的矿体及其两侧蚀变岩和混合岩围岩具有一个共同的初始Ｐｂ同位素比值（（２０６）Ｐｂ／（２０４）Ｐｂ＝１８．６８３±０．０２），同时，它们具有各自明显不同的Ｐｂ同位素变化范围，其富金矿体和含金蚀变岩具有比混合岩围岩均一的Ｐｂ同位素变化，尤其是抱板矿区Ｖ１矿体，其富矿核部分具有最均一的和最低的（２０６）Ｐｂ／（２０４）Ｐｂ比值，暗示在这个混合岩地区，有矿部分与无矿部分、富矿部分与贫矿部分的Ｐｂ同位素比值可以区别。发现北牛和二甲矿体具有比抱板和土外山矿体更低的初始Ｐｂ同位素比值（（２０６）Ｐｂ／（２０４）Ｐｂ＝１８．５３８±０．０１８）。上述两组矿体都产于同一混合岩田中，且与戈枕断裂关系密切，这表明该混合岩地区存在两个成矿期，其成矿年龄分别为１７０±１１Ｍａ和８４±１２Ｍａ，即为燕山运动的早晚期。还研究了王下矿区的浅变质层控型金矿，获得该矿（２０６）Ｐｂ／（２０４）Ｐｂ初始值为１８．３５１±０．０１８，其成矿模式年龄为２８０±１０Ｍａ。

The Pb isotopic ratios and Pb concentrations were obtained for the samples from the ore bodies in the Baoban and the Tuwaishan gold deposits. The ore bodies,altered rocks and migmatitic wall rocks have the same initial Pb isotopic ratio (206Pb/204Pb=18.683±0.020), but differ in the present Pb isotopic ratio. The ore bodies and their associated hydrothermally altered rocks are more homogeneous than the migmatitic wall rocks in Pb isotopic ratio. Especially the fertile ore core from the VI ore body in the Baoban gold field has the most homogeneous and lowest 206Pb/204Pb ratio. The Pb isotopic signature is different between the big fertile and the small barren ore bodies. The Beiniu and the Erjia gold ore bodies have a lower initial Pb isotopic ratio (206Pb/204Pb=18.538±0.018) than the Baoban and Tuwaishan ore bodies. Both these two groups of ore bodies are hosted in migmatite, and closely related to the Gezhen fault. It was concluded that there are two metallogenic stages in the migmatite area. The mineralization ages are 170±11 Ma and 84±12 Ma for them, respectively. The Pb isotopic characteristics of strata-bound gold deposits hosted in low grade metamorphic rocks in the Wangxia gold field were also discussed.