广西大气降水热液矿床稳定同位素特征及其找矿评价

STABLE AIOTOPES OF METEORIC WATER-HYDROTHERMAL DEPOSITS IN GUANGXI AND EVALUATION ON PROSPECTING WITH THEM

广西大气降水热液矿床有泗顶、北山、老厂等铅锌矿；寺村等重晶石矿；益兰等汞矿；金牙、高龙等超微粒型金矿；金山等银矿。这类矿床稳定同位素主要特征是：成矿流体的氢同位素组成严格受地理位置制约，δＤ＿（Ｈ＿２Ｏ）值显示比较稳定；而蚀变岩石的δ￣（１８）Ｏ值低于未蚀变岩石的δ￣（１８）Ｏ值；在δ￣（１８）Ｄ＿（Ｈ＿２Ｏ）－δ￣（１８）Ｏ＿（Ｈ＿２Ｏ）关系图上，其投影点绝大部分位于大气降水线上或附近。碳同位素在空间上表现为从围岩→蚀变围岩→矿体，δ￣（１３）Ｃ值呈逐渐降低的趋势。硫同位素组成变化范围往往很大，既有正值、也有负值，甚至出现正大伍和负大伍。铅同位素组成变化也较大，￣（２０６）ｐｂ／￣（２０４）ｐｂ为１７．７３７－１９．９７９、￣（２０７）ｐｂ／￣（２０４）ｐｂ为１５．１３０－１６．１５１、￣（２０６）ｐｂ／￣（２０４）ｐｂ为３７．４６０－４０．１８７。寻找和评价这类矿床必须注意δ￣（１８）Ｏ和δ￣（３４）Ｓ值和分带性，特别是在成矿区中，当蚀变岩石δ￣（１８）Ｏ值（一般低于未蚀变岩石）愈降低，则暗示成矿作用愈强烈，其低δ￣（１８）Ｏ值的中心，将是成矿中心，这是寻找和评价这类矿床、尤其是隐伏矿床、矿体的有效标志?

Meteoric－water hydrothermal deposits in Guangxi are exampled by lead－zinc depositsin Siding，Beishan，Laochang，etc．，barite deposits in Sicun ，ete.，mercury deposits inYilan，etc.，micrograined gold deposits in Jinya，Gaolong，etc., and silver deposits inJinshan，etc.. The stable isotopes of these deposits possess the following characteristics：the composition of hydrogen isotope of ore－forming solution is strictly controllde bygeographical locations，δD_(H_2O) Value is rather stable；while the δ^(18)O value of altered rocksis less than of non-altered rocks；On the δD_(H_2O)－δ^(18)O_(H_2O)relationship map，their projecteddots are most situated in or near the line of meteoric water. The δ^(13)C value of carbonisotope decreases gradually from enclusing rocks to altered enclosing rocks to ore bodies.The composition of sulphur isotope varies greatly in scope，there being not only positivevalue，but also negative value，even maximum positive value and negative yalue. Thecomposition of lead isotope also changes a lot，^(206)Pb／^(204)Pb being 17．737－19．979，^(207)Pb／^(204)Pb 15．13－16．151，^(206)Pb／^(204)Pb 37．46－40．187．The zonation of δ^(18)Oand δ^(34)S values mustbe paid attention to in prospecting and evaluating these deposits. Especially inmineralization province，lower δ^(18)O value of altered rocks（than that of non－alteredrocks）implies stronger mineralization，the center of the low δ^(18)Ovalue would be the centerof mineralization. That is the valid mark for prospecting and evaluating the deposits ofthese types，especially for concealed deposits and ore bodies.