焦家式蚀变岩型金矿床热液蚀变成因: 磷灰石矿物学、年代学和地球化学约束

胶东是我国最大的金矿集中区, 累计探明金资源量5000余吨, 焦家式蚀变岩型金矿床提供了80%以上的金资源量, 钾长石化和黄铁绢英岩化蚀变是该类矿床的重要找矿标志, 但对其成因与形成时限缺乏有效约束。为探究两类蚀变的成因并限定其形成年龄, 本文对典型蚀变岩型金矿床中钾长石化花岗岩与黄铁绢英岩化碎裂岩进行锆石和磷灰石LA-ICP-MS U-Pb地质年代学、磷灰石微区原位微量元素地球化学测试。结果表明, 钾长石化玲珑花岗岩的锆石U-Pb年龄为155.0±1.1Ma~155.8±1.3Ma, 代表了其岩浆侵位年龄; 钾长石化玲珑花岗岩蚀变弱的A1型磷灰石没能得到有效年龄, 而蚀变较强的A2型磷灰石U-Pb年龄为146±7Ma~147±6Ma, 代表了钾长石化蚀变作用发生的时间; 黄铁绢英岩化碎裂岩蚀变较弱的B1型磷灰石数量较少未形成协和年龄, 强蚀变B2型磷灰石U-Pb年龄为125±6Ma, 代表了黄铁绢英岩化蚀变作用发生的时间。A1型磷灰石的稀土配分曲线与未蚀变玲珑花岗岩较为一致, A2型磷灰石显示更高的轻稀土含量和更明显的Eu负异常, 随着钾长石化蚀变程度增强, La/Yb比值逐渐增大, Sr含量和Sr/Y比值同步降低, 暗示蚀变流体相对富轻稀土, 蚀变过程磷灰石的Sr被活化迁移; B1型磷灰石的稀土配分曲线与未蚀变玲珑花岗岩相似, B2型磷灰石和未蚀变郭家岭花岗岩较为一致, 轻重稀土分异明显, 且Eu异常不明显, 在黄铁绢英岩化蚀变过程中Sr含量、Sr/Y比值和La/Yb比值显著增高。本文认为与焦家式蚀变岩型金矿成矿相关的蚀变作用与区域岩浆作用有关, 晚侏罗世玲珑花岗岩的自交代作用形成了钾长石化蚀变, 早白垩世郭家岭花岗岩分异的热液沿区域性断裂迁移, 导致断裂带内发生黄铁绢英岩化。

Hydrothermal origin of the Jiaojia altered rock type gold deposit: Constrains from mineralogy,geochronology and geochemistry of apatite

Jiaodong Peninsula is the largest gold province of China. The Jiaojia type (altered rock type) gold deposits account for more than 80% of the gold resources. Previous studies have indicated that K-feldspar alteration and beresitization are closely associated with gold mineralization, and are important prospecting indicators. However, the timing and genesis of the two altered types lack of effective restrictions. In this study, we use in-situ LA-ICP-MS to obtain the U-Pb age of zircon and apatite, and trace elements of apatite from K-feldspar alteration granite and pyrite-sericite-quartz cataclasite in gold deposits. The K-feldspar alteration granite has zircon U-Pb ages varying from 155.0±1.1Ma to 155.8±1.3Ma which represent the emplacement age of the rock, while A2-type apatite U-Pb ages in the K-feldspar alteration granite range from 146±7Ma to 147±6Ma, which represent the timing of K-feldspar alteration. The pyrite-sericite-quartz cataclasite has a B2-type apatite U-Pb age of 125±6Ma, representing the formation time of phyllic. The rare earth element (REE) pattern of A1-type apatite is consistent with that of unaltered Linglong granite, while A2-type apatite has higher REE contents, more negative Eu anomaly, gradually increased La/Yb ratio, and decreased Sr content and Sr/Y ratio with altered intensity of K-feldspar alteration, suggesting that the alteration fluid is relatively rich in light rare earth element (LREE), and Sr in apatite is activated and migrated during the alteration. The REE pattern of B1-type apatite is consistent with that of unaltered Linglong granite, while the REE pattern of B2-type apatite is consistent with that of unaltered Guojialing granite, showing an obvious differentiation of LREE and HREE and no obvious negative Eu anomaly, and higher Sr content, Sr/Y and La/Yb ratios. Thus, our results suggest that the alteration related to the mineralization of Jiaojia type (altered rock type) gold deposit is related to regional magmatism. The self-metasomatism of Linglong granite leads to K-feldspar alteration, and the metasomatism of rocks in the regional faults by magmatic hydrothermal fluid differentiated from Guojialing granite leads to beresitization.